Low alloy steel and weld joint thereof excellent in corrosion resistance to hydrochloric acid and sulfuric acid

The present invention relates: to a low alloy steel and a weld joint thereof excellent in acid corrosion resistance; specifically, to a low alloy steel and a weld joint thereof excellent in resistance to hydrochloric acid and sulfuric acid; and, more specifically, to a low alloy steel and a weld joint thereof which secure an excellent corrosion resistance even in an environment of sulfuric acid or hydrochloric acid dew-point corrosion, the environment being produced in an atmosphere of combustion exhaust gas coming mainly from a boiler or a gasifying fusion furnace, or in an environment of a sulfuric acid or hydrochloric acid aqueous solution.

To be more precise, the present invention relates to a low alloy steel and a weld joint thereof which are used for smoke exhausting equipment of a boiler wherein fossil fuel such as heavy oil and coal, fuel gas such as liquefied natural gas, non-industrial waste such as municipal refuse, industrial waste such as wood chips, fiber chips, waste oil, plastics, waste tires and medical waste, and sewage sludge are incinerated, the smoke exhaustion equipment including a gas duct, a casing, a heat exchanger, a gas-to-gas heater, a wet or dry desulfurizer, an electrostatic precipitator, an induced draft fan, a basket material and a heat conduction element plate of a rotary regenerative air preheater, a cooling tower, a bag filter and a stack. To be still more precise, the present invention relates: to an acid dew-point corrosion resistant low alloy steel and a weld joint thereof excellent in economical efficiency, which show an excellent resistance to sulfuric acid or hydrochloric acid dew-point corrosion caused in smoke exhausting equipment and are applicable to a welded structure and also to a fin of a heat exchanger tube, a heat conduction element plate of an air preheater and an expansion joint of a flue, those requiring severe cold-workability; and to a low alloy steel and a weld joint thereof excellent in corrosion resistance to hydrochloric acid and sulfuric acid, which are applicable to a steel pickling tank for receiving pickling liquor composed of either hydrochloric acid or sulfuric acid or a mixture thereof.

A combustion exhaust gas coming from a thermoelectric power station, a municipal refuse incinerator or an industrial waste incinerator is mainly composed of moisture, sulfur oxide (sulfur dioxide, sulfur trioxide), hydrogen chloride, nitrogen oxide, carbon dioxide, nitrogen and oxygen. When sulfur trioxide is included in a combustion exhaust gas even at 1 ppm, in particular, the dew point of the exhaust gas reaches a temperature of 100° C. or higher and thus sulfuric acid dew-point corrosion is caused. To cope with the corrosion problem, a sulfuric acid dew-point corrosion resistant steel (for example, Japanese Examined Patent Publication No. S43-14585) and a highly corrosion resistant stainless steel (for example, Japanese Unexamined Patent Publication No. H7-316745) are used.

In the case of the flue gas system of a coal burning thermal power boiler or a non-industrial or industrial waste incinerator, as a considerable amount of hydrogen chloride is contained, besides the aforementioned sulfur trioxide, in combustion gas, both sulfuric acid condensation and hydrochloric acid condensation are caused. Further, a sulfuric acid dew-point temperature and a hydrochloric acid dew-point temperature vary in accordance with the composition of the combustion exhaust gas. In general, a sulfuric acid dew-point temperature is roughly in the range from 100° C. to 150° C. and a hydrochloric acid dew-point temperature is in the range from 50° C. to 80° C. Therefore, in a flue gas system, even though the configuration of the system is not changed and the temperature of a passing combustion gas is nearly constant, some portions are dominated by sulfuric acid dew-point corrosion and some other portions by hydrochloric acid dew-point corrosion because the temperatures of the walls of the portions vary in accordance with the structures and the portions. For that reason, the development of a material for a structural member excellent in both sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance has been a challenge.

As an example to cope with the challenge, Japanese Examined Patent Publication No. S46-34772 discloses a technology related to an acid resistant ultra-low carbon low alloy steel, wherein sulfuric acid resistance and hydrochloric acid resistance are enhanced by ultra-low carbon (not more than 0.03 mass % C) and the addition of Cu and Mo.

Further, Japanese Unexamined Patent Publication No. H9-25536 discloses a technology related to a sulfuric acid dew-point corrosion resistant steel, wherein hydrochloric acid dew-point corrosion resistance is improved, while sulfuric acid dew-point corrosion resistance is secured, by reducing an S amount and adding Sn and/or Sb. Furthermore, Japanese Unexamined Patent Publication No. H10-110237 discloses a steel excellent in sulfuric acid resistance, hydrochloric acid resistance and hot-workability, wherein Sn and/or Sb are added to a copper contained steel of ultra-low sulfur (not more than 0.005 mass % S) and further Mo and/or B are added thereto for recovering hot-workability that has been deteriorated by the addition of Sn and/or Sb.

In the case of the technology disclosed in Japanese Examined Patent Publication No. S46-34772, however, the steel is limited to an ultra-low carbon steel and therefore there have been the problems of requiring other expensive alloying elements for enhancing strength, limiting the application of the steel to a heat conduction element plate for an air heater or the like to which even a steel having the tensile strength of less than 400 N/mm 2 is applicable, and therefore being hardly used as a steel for a welded structure. Further, though the details are explained later, there have been the problems of: not only extremely hindering sulfuric acid resistance in the sulfuric acid concentration range of 10 to 40 mass % when Mo is added excessively within the component range specified in the publication; but also, under some conditions, deteriorating hydrochloric acid resistance to worse than that of a steel according to Japanese Examined Patent Publication No. S43-14585 or an S-TEN 1 steel (a Cu—Sb steel) (Nippon Steel Corporation Product Catalog, sulfuric acid dew-point corrosion resistant steel S-TEN, Cat. No. AC107, 1981, Sixth Edition).

In the case of a sulfuric acid dew-point corrosion resistant stainless steel disclosed in Japanese Unexamined Patent Publication No. H7-316745 and others, there have been the problems of: requiring securing resistance to stress corrosion cracking and crevice corrosion besides general corrosion resistance because stress corrosion cracking and crevice corrosion, together with dew-point corrosion, are caused by the concentration of chloride in a hydrochloric acid dew-point corrosion environment; therefore inevitably requiring an increase in alloying elements; causing the cost to increase further than that of a low alloy steel and strength to increase; and therefore making it difficult to secure both acid-resistance and cold-workability simultaneously.

Further, in the case of a steel disclosed in Japanese Unexamined Patent Publication No. H9-25536, there has been the problem of only showing sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance equal to or inferior to the hydrochloric acid dew-point corrosion resistance of aforementioned S-TEN 1.

Furthermore, in the case of a steel disclosed in Japanese Unexamined Patent Publication No. H10-110237, firstly, there has been the problem of incurring excessive increase of a steelmaking cost because the steel requires ultra-low sulfur. A second problem of the steel, which has newly been found by the present inventors, is that an excessive reduction of sulfur deteriorates sulfuric acid resistance, as mentioned later. A third problem thereof has been that, when the steel is used for a welded structure, though a weld metal also requires ultra-low sulfur at the same level as the steel, it is extremely difficult to attain such ultra-low sulfur when some other requirements such as weld operability are taken into consideration and, therefore, when the steel is used for a welded structure, namely a join, the corrosion resistance of the join is hardly secured. A fourth problem thereof has been that, though the addition of Sn and/or Sb is inevitably required, an excessive addition of Sn and/or Sb conspicuously deteriorates the toughness of a steel sheet as mentioned later and, therefore, the application of the steel to a hot-rolled steel sheet for a welded structure such as a duct or a stack is not practical.

As a result of the study by the present inventors, it has been clarified that the aforementioned S-TEN 1 steel designed by employing the composition of a Cu—Sb steel as the basic composition has the most excellent hydrochloric acid resistance among various sulfuric acid dew-point corrosion resistant steels that have ever been developed.

However, in some waste incinerators for burning vinyl chlorides and kitchen garbage, the concentration of hydrogen chloride in an exhaust gas may sometimes reach 4,000 ppm. For that reason, a novel sulfuric acid dew-point corrosion resistant steel, that: has outstandingly improved hydrochloric acid resistance while the sulfuric acid resistance of an S-TEN steel is maintained; can be produced as a steel for a welded structure and a cold-rolled steel sheet excellent in cold-workability; and is far more economical than a highly corrosion resistant stainless steel excellent in hydrochloric acid resistance, has strongly been desired.

Further, such a low alloy steel is often used for an ordinary welded structure.

In general, in the case where a welded structure is used in a corrosive environment, when the corrosion resistance of a weld differs from that of a base material, the one having inferior corrosion resistance is selectively corroded and the service life of the welded structure is extremely shortened. What is worse, when a weld is selectively corroded, stress concentration occurs at corroded pores and, in the extreme case, the destruction of a structure may be caused. Therefore, when such a low alloy steel is used for a welded structure that cannot ignore corrosion degradation, the corrosion resistance of not only a base material but also a weld must be secured sufficiently.

In smoke exhausting equipment such as a flue and a stack of a coal burning thermal power boiler or a refuse incinerator, sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion are caused by sulfur trioxide and hydrogen chloride in an exhaust gas. In such an environment, a sulfuric acid dew-point corrosion resistant steel (refer to, for example, Nippon Steel Corporation, S-TEN Product Catalog, Cat. No. AC107, 1981, Sixth Edition) is used. As a welding material, a welding material for a soft steel and a welding material exclusively used for a sulfuric acid dew-point corrosion resistant steel (refer to, for example, Nippon Steel Welding Products & Engineering Co., LTD., Nittetsu welding Material & Apparatus Handbook, p. 61, p. 164, p. 208 and p. 291) are used.

A welding material exclusively used for a sulfuric acid dew-point corrosion resistant steel has been a material containing Cu itself as a corrosion resistant element or a material containing components of a Cu and Cr. A weld joint formed by using such an existing welding material shows a sufficiently excellent corrosion resistance in a sulfuric acid dew-point corrosion environment caused by the smoke exhaustion equipment of a single-fuel fired boiler for heavy oil. However, the problem of such a weld joint has been that, in an environment of a coal burning boiler, a refuse incinerator, a gasifying fusion furnace or the like, both sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion are caused simultaneously, therefore the corrosion resistance of the weld metal at the weld is insufficient, and the weld metal that has corrosion resistance inferior to that of the base material is selectively corroded.

The present invention is intended to solve the above problems and the object thereof is to provide: a low alloy steel and a weld joint thereof excellent in acid dew-point corrosion resistance; specifically, a low alloy steel and a weld joint thereof excellent in resistance to hydrochloric acid and sulfuric acid; and more specifically, a low alloy steel and a weld joint thereof which secure an excellent resistance to sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion caused in an atmosphere of a combustion exhaust gas coming mainly from a boiler or a gasifying fusion furnace.

To be more precise, the object of the present invention is to provide a low alloy steel and a weld joint thereof which are used for smoke exhaustion equipment of a boiler wherein fossil fuel such as heavy oil and coal, fuel gas such as liquefied natural gas, non-industrial waste such as municipal refuse, industrial waste such as wood chips, fiber chips, waste oil, plastics, waste tires and medical waste, and sewage sludge are incinerated, the smoke exhausting equipment including a gas duct, a casing, a heat exchanger, a gas-to-gas heater, a wet or dry desulfurizer, an electrostatic precipitator, an induced draft fan, a basket material and a heat conduction element plate of a rotary regenerative air preheater, a cooling tower, a bag filter and a stack. To be still more precise, the object of the present invention is to economically provide: an acid dew-point corrosion resistant low alloy steel and a weld joint thereof excellent in economical efficiency, which show an excellent resistance to sulfuric acid or hydrochloric acid dew-point corrosion caused in smoke exhaustion equipment and are applicable to a welded structure and also to a fin of a heat exchanger tube, a heat conduction element plate of an air preheater and an expansion joint of a flue requiring severe cold-workability; and a sulfuric acid dew-point corrosion resistant low alloy steel and a weld joint thereof excellent in corrosion resistance to hydrochloric acid, which are applicable to a steel pickling tank for receiving pickling liquor composed of either hydrochloric acid or sulfuric acid or a mixture thereof.

The present inventors, as a result of studying metallurgical factors influencing sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance, have found that hydrochloric acid dew-point corrosion resistance can be improved remarkably, while an S amount is not regulated to an ultra-low amount that has been required before and sulfuric acid dew-point corrosion resistance superior to that of a sulfuric acid corrosion resistant S-TEN 1 steel (a Cu—Sb steel) is maintained, by adding a very small amount of Mo in such a range that the value of an after-mentioned acid corrosion resistance index AI is positive (zero or larger) to the aforementioned S-TEN 1 steel. The findings by the present inventors related to the present invention are hereunder described in detail. Note that an addition amount of a component element is expressed in terms of mass %.

It has so far been understood that Mo is an element that extremely deteriorates the sulfuric acid resistance of a steel or a copper contained low alloy steel (refer to, for example, Kowaka, Moroishi and Muroya: The Japan Institute of Metals, 1966 Spring Convention Lecture Outline, p. 84 (1966) and Teramae, Kado, Otoguro and Todoroki: Fuji Steel Corporation Technical Report No. 17, p. 103 (1968)). The present inventors focused their attention on the fact that the Mo addition amount was more than 0.1% in the past research of investigating the influence of Mo, and investigated precisely the influence of a minute addition amount of Mo on the behavior of the hydrochloric acid resistance, chloride containing acid corrosion resistance and sulfuric acid resistance of a Cu—Sb steel when the Mo addition amount was not more than 0.1%.

As the results, the following new findings have been obtained:

1) as shown in FIG. 1 , there exists a singular point where hydrochloric acid resistance dramatically improves when a very minute amount of Mo (less than 0.01%) is added to a Cu—Sb steel; 2) further, as it is understood from FIG. 2 , sulfuric acid resistance also improves by adding a very minute amount of Mo, that has so far been recognized as an element that deteriorates sulfuric acid resistance, to a Cu—Sb steel, in contrast to the past knowledge; and 3) furthermore, from FIG. 2 , sulfuric acid resistance deteriorates when Mo is added in excess of a certain amount.

On the basis of the above findings, the present inventors defined the threshold Mo addition amount that did not hinder sulfuric acid resistance (hereunder referred to as “sulfuric acid resistant Mo amount limit”) as follows. That was, steels containing Mo and those not containing Mo, both having identical components except for Mo, were compared with each other in terms of sulfuric acid resistance and the upper limit of the Mo amount in which sulfuric acid resistance started to deteriorate was defined as “sulfuric acid resistant Mo amount limit.”

A sulfuric acid resistant Mo amount limit has a very clear relationship with the amounts of C and Sb in a steel and is an Mo amount that satisfies AI=0 when an AI value (a sulfuric acid or hydrochloric acid corrosion resistance index) is defined by the following expression <1>,

AI/ 10,000=0.0005+0.045×Sb %−C %×Mo %  <1>.

Here, % means mass %.

In a steel according to the present invention, as shown in FIGS. 1 and 2 , though hydrochloric acid corrosion resistance improves with the addition of Mo, an excessive addition amount of Mo exceeding a certain amount significantly deteriorates sulfuric acid corrosion resistance and an Mo amount limit that allows sulfuric acid corrosion resistance to be maintained corresponds to the Mo amount calculated from the expression <1> when the value of AI is set at zero.

When an AI value is less than zero, and though excellent hydrochloric acid resistance is maintained, sulfuric acid resistance deteriorates as it has been reported in prior art. On the other hand, when an AI value is not less than zero, both sulfuric acid resistance and hydrochloric acid resistance improve.

FIG. 3 is a graph explaining the relationship between the sulfuric acid resistant Mo amount limit curve that satisfy AI=0 and sulfuric acid resistance in a Mo—C chart of a C—Cu—Mo steel. In the figure, the left region of the sulfuric acid resistant Mo amount limit curve represents a region wherein AI≧0 is satisfied and both sulfuric acid resistance and hydrochloric acid resistance improve. On the other hand, the right region of the sulfuric acid resistant Mo amount limit curve represents a region wherein an AI value is less than zero and sulfuric acid resistance deteriorates though a good hydrochloric acid resistance is maintained.

As can be understood from the definition of the AI value, when a C amount is fixed, the addition of Sb has the function of increasing the sulfuric acid resistant Mo amount limit. This knowledge has a great significance industrially. The significance is explained hereunder.

The same study as above was carried out on Cu—Mo steels not containing Sb and Cu—Sb—Mo steels. As a result, the hydrochloric acid resistance of the Cu—Mo steels was not so excellent as that of the Cu—Sb—Mo steels and the sulfuric acid resistant Mo amount limit beyond which Mo functions as an element to hinder sulfuric acid resistance was given by the expression C %×Mo %=0.0005%.

FIG. 4 shows the effect of Sb addition and the sulfuric acid resistant Mo amount limit curves of the Cu—Mo steels (not containing Sb) and the Cu—Sb—Mo steels (containing 0.1% Sb and 0.15% Sb). In the case of the Cu—Mo steels (not containing Sb), when a C amount is 0.1%, the sulfuric acid resistant Mo amount limit is 0.005% (reference numeral 1 in FIG. 4 ). Therefore, when a steel is required to contain around 0.1% C for securing a strength necessary for a steel for a welded structure, in a Cu—Mo steel not containing Sb, it is necessary to control an Mo amount to not more than 0.005% for securing excellent sulfuric acid resistance and hydrochloric acid resistance. However, as the industrially controllable range of an Mo amount in a steel is around ±0.02% as shown by the reference numeral 4 in FIG. 4 , it is extremely difficult to control an Mo amount in a steel in the above range industrially.

On the other hand, in the case of the Cu—Mo-0.1% Sb containing steels, when a C amount is 0.1%, the sulfuric acid resistant Mo amount limit is 0.05% (reference numeral 2 in FIG. 4 ). Therefore, it is enough if Mo of not more than 0.05% is added intentionally. Also, in the case of the Cu—Mo-0.15% Sb containing steels, when a C amount is 0.1%, the sulfuric acid resistant Mo amount limit is 0.075% (reference numeral 3 in FIG. 4 ). Therefore, it is enough if Mo of not more than 0.075% is added. Accordingly, in these cases, an Mo amount in a steel can be controlled easily in the industrially controllable range of ±0.02% (reference numeral 4 in FIG. 4 ).

The findings obtained by the precise investigations on the sulfuric acid resistance and hydrochloric acid resistance of the Cu—Mo steels and the Cu—Mo—Sb steels are summarized as follows:

1) A Cu—Mo—Sb steel is outstandingly excellent in hydrochloric acid resistance in comparison with a Cu—Mo steel or a Cu—Sb steel. 2) In a Cu—Mo—Sb steel or a Cu—Mo steel, when Mo is added to the steel in a range not exceeding a certain upper limit, namely in a range not exceeding a sulfuric acid resistant Mo amount limit, Mo which has so far been regarded as harmful functions as an element that improves sulfuric acid resistance. 3) The upper limit of a sulfuric acid resistant Mo amount limit can be expressed by the clear relation with the amounts of C and Sb in a steel, and is given by the expression C %×Mo %≦0.0005+0.045×Sb % in the case of a Cu—Mo—Sb steel and by the expression C %×Mo %≦0.0005 in the case of a Cu—Mo steel. The index that expresses the relation is the aforementioned AI, namely, AI/10,000=0.0005+0.045×Sb %−C %×Mo %, and, when an AI value is zero or more, both hydrochloric acid resistance and sulfuric acid resistance are excellent.

The present inventors, as a result of further studying metallurgical factors influencing the hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance of a weld joint when such a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance as described above is used for a welded structure, have found: that a weld joint excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance can be obtained by employing a low alloy steel composed of an aforementioned composition as a base material and making a weld metal composed of a specific composition on the basis of the knowledge on the improvement of the hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance of a base material, in other words, by 1) making a weld metal composed of a Cu—Mo—Sb containing composition and 2) controlling an AI value in a specific range; and that, preferably, a further excellent weld joint can be obtained by restricting the difference between the AI value of a base material and the AI value of a weld metal in a specific range.

The present invention has been established based on the above findings and the gist thereof is as follows:

(1) A low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, characterized in that: said steel contains, in mass, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less,

with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance index AI, which is calculated from the following expression <1>, of said steel is zero or positive,

AI/ 10,000=0.0005+0.045×Sb %−C %×Mo %  <1>,

where % means mass %.

(2) A low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, characterized in that: said steel contains, in mass,

C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less,

and further one or more of

Nb: 0.005 to 0.1%, Ta: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, and W: 0.05 to 1%,

with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance index EI, which is calculated from the following expression <2>, of said steel is zero or positive,

EI/ 10,000=0.0005+0.045×Sb %−C %×Mo eq   <2>,

where % means mass % and Mo eq (mass %) is defined by the following expression,

Mo eq =Mo %+5.1×(Nb %+Ta %)+4.2×V %+9.3×Ti %+0.5×W %.

(3) A low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (1), characterized in that said steel contains S: more than 0.005 to 0.025 mass %.

(4) A low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (2), characterized in that said steel contains S: more than 0.005 to 0.025 mass %.

(5) A low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to any one of the items (1) to (4), characterized in that said steel further contains, in mass, one or more of

Cr: 0.1 to 0.5%, Ni: 0.1 to 1%, N: 0.001 to 0.007%, Al: 0.005 to 0.1%, Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, REM: 0.0002 to 0.01%, B: 0.0002 to 0.005%, La: 0.0002 to 0.01%, Ce: 0.0002 to 0.01%, Sn: 0.01 to 0.3%, Pb: 0.01 to 0.3%, Se: 0.001 to 0.1%, Te: 0.001 to 0.1%, Bi: 0.001 to 0.1%, Ag: 0.001 to 0.5%, and Pd: 0.001 to 0.1%.

(6) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, characterized in that: said weld joint is composed of a base material containing, in mass,

C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less,

with the balance consisting of Fe and unavoidable impurities and a weld metal containing, in mass,

C: 0.005 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 0.5%, Sb: 0.01 to 0.2%, P: 0.03% or less, and S: 0.03% or less,

with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance indexes AI, which are calculated from the following expression <1>, of said base material and weld metal are zero or positive,

AI/ 10,000=0.0005+0.045×Sb %−C %×Mo %  <1>,

where % means mass %.

(7) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (6), characterized in that the absolute value of the difference |ΔAI| between the acid corrosion resistance indexes AI of said base material and weld metal is 20 or less,

where |ΔAI|=|AI of a base material−AI of a weld metal|.

(8) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (6), characterized in that: said base material contains S: more than 0.005 to 0.025 mass %; and said weld metal contains S: more than 0.005 to 0.02 mass %.

(9) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (7), characterized in that: said base material contains S: more than 0.005 to 0.025 mass %; and said weld metal contains S: more than 0.005 to 0.02 mass %.

(10) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (6), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(11) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (7), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(12) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (8), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(13) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (9), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(14) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, characterized in that: said weld joint is composed of a base material containing, in mass,

C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less,

and further one or more of

Nb: 0.005 to 0.1%, Ta: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, and W: 0.05 to 1%,

with the balance consisting of Fe and unavoidable impurities and a weld metal containing, in mass,

C: 0.005 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 0.5%, Sb: 0.01 to 0.2%, P: 0.03% or less, and S: 0.03% or less,

and further one or more of

Nb: 0.005 to 0.1%, Ta: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, and W: 0.05 to 1%,

with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance indexes EI, which are calculated from the following expression <2>, of said base metal and weld metal are zero or positive,

EI/ 10,000=0.0005+0.045×Sb %−C %×Mo eq   <2>,

where % means mass % and Mo eq (mass %) is defined by the following expression,

Mo eq =Mo %+5.1×(Nb %+Ta %)+4.2×V %+9.3×Ti %+0.5×W %.

(15) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (14), characterized in that the absolute value of the difference |ΔEI| between the acid corrosion resistance indexes EI of said base material and weld metal is 20 or less,

where |ΔEI|=|EI of a base material−EI of a weld metal|.

(16) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (14), characterized in that: said base material contains S: more than 0.005 to 0.025 mass %; and said weld metal contains S: more than 0.005 to 0.02 mass %.

(17) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (15), characterized in that: said base material contains S: more than 0.005 to 0.025 mass %; and said weld metal contains S: more than 0.005 to 0.02 mass %.

(18) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (14), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(19) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (15), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(20) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (16), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(21) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to the item (17), characterized in that: said base material further contains N: 0.001 to 0.007 mass %; and said weld metal further contains N: 0.001 to 0.02 mass %.

(22) A weld joint of a low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance according to any one of the items (6) to (21), characterized in that said base material and weld metal further contain, in mass, one or more of

Cr: 0.1 to 0.5%, Ni: 0.1 to 1%, Al: 0.005 to 0.1%, Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, REM: 0.0002 to 0.01%, B: 0.0002 to 0.005%, La: 0.0002 to 0.01%, Ce: 0.0002 to 0.01%, Sn: 0.01 to 0.3%, Pb: 0.01 to 0.3%, Se: 0.001 to 0.1%, Te: 0.001 to 0.1%, Bi: 0.001 to 0.1%, Ag: 0.001 to 0.5%, and Pd: 0.001 to 0.1%.

FIG. 1 is a graph showing the influence of Mo addition amounts on the hydrochloric acid resistance of Cu—Sb steels.

FIG. 2 is a graph showing the influence of Mo addition amounts on the sulfuric acid resistance of Cu—Sb steels.

FIG. 3 is a graph schematically showing the influence of the addition amounts of C and Mo on the sulfuric acid resistance of C—Cu—Mo steels.

FIG. 4 is a graph showing the influence of Sb amounts on the sulfuric acid resistant Mo amount limit curves of Cu—Mo steels.

FIG. 5 is a graph showing the influence of S addition amounts on the hydrochloric acid resistance of Cu—Mo—Sb steels.

FIG. 6 is a graph showing the relationship between the AI values of Cu—Mo—Sb steels and the corrosion rates thereof in sulfuric acid.

FIG. 7 is a graph showing the relationship between the EI values of Cu—Mo—Sb steels and the corrosion rates thereof in sulfuric acid.

FIG. 8 is a graph showing the thickness reduction amounts of invention steel sheets and comparative steel sheets with the passage of time in a hydrochloric acid pickling tank.

FIG. 9 is a graph showing the corrosion rates of invention steels and comparative steels in smoke exhausting equipment of a refuse incinerator.

FIG. 10 is a graph showing the relationship between the |ΔAI| values and the corrosion rates of the base steels and weld metals of weld joints according to the present invention in sulfuric acid.

FIG. 11 is a graph showing the relationship between the |ΔEI| values and the corrosion rates of the base steels and weld metals of weld joints according to the present invention in sulfuric acid.

FIG. 12( a ) is an illustration showing how to sample corrosion test pieces for evaluating the corrosion resistance of a weld metal.

FIG. 12( b ) is a cross-sectional illustration taken on line x-x′ of FIG. 12( a ) showing how to sample corrosion test pieces for evaluating the corrosion resistance of a weld metal.

FIG. 13 is an illustration showing how to sample a corrosion test piece for evaluating the corrosion resistance of a weld joint.

The present invention will be explained in detail. Firstly, the component elements and the addition amounts thereof related to a low alloy steel and a base material for a weld joint thereof according to the present invention (hereunder referred to as “invention steels”) and the weld metal of a weld joint are explained. Here, % means mass %.

(1) Invention Steels (Low Alloy Steels and Base Materials for Weld Joints Thereof)

C is added by not less than 0.001% since the reduction of a C amount to less than 0.001% significantly deteriorates economical efficiency industrially. A C addition amount of not less than 0.002% is desirable for securing strength. Though C has so far been regarded as an element that deteriorates sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance to some extent, according to the present invention, in the case of a Cu—Mo—Sb steel, the deterioration of hydrochloric acid resistance is small if a C amount is not more than 0.06%. Further, with regard to sulfuric acid resistance, though sulfuric acid resistance is not hindered when the expression C %×Mo %≦0.0005+0.045×Sb % is satisfied, sulfuric acid resistance drastically deteriorates when C is added in excess of the amount regulated by the expression. When a steel is used for an application wherein hydrochloric acid resistance is more important, the above-mentioned regulation is not required and a preferable C addition amount is 0.005% or more. On the other hand, when a C amount exceeds 0.2%, weldability, sulfuric acid resistance and hydrochloric acid resistance deteriorate. Therefore, a C amount is limited in the range from 0.001 to 0.2%, in particular, preferably, from 0.02 to 0.15% for the application of a welded structure such as a stack or a duct and from 0.002 to 0.03% for the application of cold-working.

Si is added by not less than 0.01% for deoxidization. An excessive Si addition causes hot-rolling scales to stick to a hot-rolled steel sheet (deterioration of descaling capability) and scale defects called arabesques and pockmarks to increase drastically. Therefore, the upper limit of an Si amount is set at 2.5%. Si has an effect of improving sulfuric acid resistance nearly in proportion to the addition amount thereof, but the effect of Si on hydrochloric acid resistance is hardly observed. Therefore, when hydrochloric acid resistance is regarded to be important, the preferable range of an Si amount is from 0.55 to 1.2%. When weldability, the toughness of a weld and productivity are concerned as a steel for a welded structure, the preferable range thereof is from 0.1 to 0.55%. Further, when scale defects formed during the production of a hot-rolled steel sheet are concerned, the most preferable range thereof is from 0.2 to 0.35%.

Mn is added by not less than 0.1% for adjusting the strength of a steel and the addition amount of 2% is sufficient for the adjustment. Therefore, an Mn amount is limited in the range from 0.1 to 2%. Though the increase of an Mn addition amount tends to deteriorate sulfuric acid resistance to some extent, that does not significantly deteriorate the excellent sulfuric acid resistance of a Cu—Mo—Sb steel according to the present invention and therefore Mn may be added in consideration of the balance between a C amount and an Mn amount conforming to the mechanical properties required of a steel for a welded structure. When corrosion resistance is desired to the maximum, the preferable range of an Mn amount is from 0.1 to 0.7%.

Cu must be added by not less than 0.1% for securing sulfuric acid resistance and hydrochloric acid resistance. However, even when Cu is added in excess of 1%, the effect is almost saturated, and rather strength increases excessively and productivity deteriorates. Therefore, a Cu amount is limited in the range from 0.1 to 1%. Preferably, a Cu addition amount in the range from 0.2 to 0.4% is very excellent in the balance between corrosion resistance and productivity.

Mo is an essential element that significantly improves hydrochloric acid resistance when it is added to a Cu—Sb steel by not less than 0.001%. In the case of producing a steel with a 300 ton converter, for example, when Mo oxide or Mo base metal of 30 kg or more is charged in the converter, the improvement effect appears even when the amount of Mo in the steel is lower than the analyzable lower limit (0.003%) of emission spectral analysis (Quantovac). Therefore, in commercial production, it is enough if the charge of Mo alloy in molten steel is secured regardless of the yield of Mo in the molten steel. Further, when an Mo amount is in the range wherein the expression C %×Mo %≦0.0005+0.045×Sb % is satisfied, Mo functions as an element that improves sulfuric acid resistance and thus not only hydrochloric acid resistance but also sulfuric acid resistance improves. A sulfuric acid resistant Mo amount limit under which sulfuric acid resistance is not hindered is mitigated (increased) as a C amount decreases or an Sb amount increases as it has been stated above. The upper limit of an Mo amount is set at 1% in consideration of the influence on mechanical properties.

Sb must be added by not less than 0.01% for securing sulfuric acid resistance, hydrochloric acid resistance and chloride containing acid corrosion resistance. Sulfuric acid resistance improves as an Sb amount increases. However, the effect is almost saturated with an Sb addition of 0.1%. On the other hand, when Sb is added in excess of 0.2%, hot-workability and the toughness of a steel sheet and a weld joint deteriorate. Therefore, an Sb amount is limited in the range from 0.01 to 0.2%. A preferable range of an Sb amount is from 0.05 to 0.15% in consideration of the balance among corrosion resistance, hot-workability and mechanical properties.

P is an inevitably included impurity element and significantly deteriorates sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance. Therefore, a P amount is limited to not more than 0.05%. Preferably, sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance are remarkably improved by limiting a P amount to not more than 0.01%. The most preferable range of a P amount is 0.005 to 0.01% in consideration of the burden of a dephosphorising process and economical efficiency.

S is, like P, an inevitably included impurity element and, when it is added in excess of 0.05%, hot-workability and mechanical properties deteriorate. Therefore, the upper limit of an S amount is set at 0.05%. However, as shown in FIG. 5 , in the case of a Cu—Mo—Sb steel according to the present invention, sulfuric acid resistance and hydrochloric acid resistance improves remarkably when S is contained in an appropriate amount in the range exceeding 0.005% and therefore it is preferable to contain S by more than 0.005%. However, the effect is saturated even if S is added in excess of 0.025%. Therefore, it is preferable to contain S in the range from more than 0.005 to 0.025%. In particular, the most preferable range of an S amount is from 0.01 to 0.025% in consideration of hydrochloric acid resistance, sulfuric acid resistance, mechanical properties (lamellar tear resistance, etc.) and hot-workability.

In a steel according to the present invention, when an acid corrosion resistance index AI defined by the following expression <1> satisfies AI≧0, extremely excellent hydrochloric acid resistance and sulfuric acid resistance are obtained,

AI/ 10,000=0.0005+0.045×Sb %−C %×Mo %  <1>.

That is, FIG. 6 shows the relationship between AI values and sulfuric acid corrosion rates and, as shown in the figure, it is understood that sulfuric acid resistance improves remarkably when an acid corrosion resistance index AI is zero or positive. Therefore, in the present invention, Sb, C and Mo are added so that an acid corrosion resistance index AI given by the above expression <1> may be zero or positive.

When AI≧0 is satisfied, in other words, C %×Mo %≦0.0005+0.045×Sb % is satisfied, in the case of a cold-rolled steel sheet requiring excellent workability for example, it is preferable that a C amount is as low as not more than 0.01%, and, in this case, when an Sb addition amount is 0.1%, excellent sulfuric acid resistance and hydrochloric acid resistance are obtained simultaneously with the Mo addition amount of not more than 0.5%.

On the other hand, when an aforementioned AI value exceeds 75, namely, when an Sb amount is excessive, not only the effect of improving sulfuric acid resistance and hydrochloric acid resistance is almost saturated but also hot-workability deteriorates. Therefore, it is preferable that the upper limit of an AI value is set at 75.

Further, in a steel according to the present invention, Nb, Ta, V, Ti and/or W are added as occasion demands for the purpose of improving the strength, toughness, weldability and high temperature properties of the steel.

Nb, Ta, V and Ti are elements effective for the improvement of strength and toughness caused by the fractionization of crystal grains and the improvement of high temperature strength when they are added by not less than 0.005%, respectively. Also, they have the effect of improving cold-workability. However, the effects are saturated when an amount of each exceeds 0.1%. Therefore, the addition amount of each of Nb, Ta, V and Ti is limited in the range from 0.005 to 0.1%.

W is effective for the improvement of high temperature strength and hydrochloric acid resistance when it is added by not less than 0.05% and the effects are saturated with a W addition amount of 1%. Therefore, a W addition amount is limited in the range from 0.05 to 1%.

Further, when carbonitride forming elements such as Nb, Ta, V, Ti and W are added excessively, the sulfuric acid corrosion resistance of a steel according to the present invention deteriorates. That is, those elements have the function of lowering a sulfuric acid resistant Mo amount limit and the degree of the lowering is expressed by the following Mo equivalent (Mo eq ),

Mo eq (mass %)=Mo %+5.1×(Nb %+Ta %)+4.2×V %+9.3×Ti %+0.5×W %.

When Nb, Ta, V, Ti and W are added, the combination of the components that determines the limit of not hindering sulfuric acid resistance is given by an expanded acid corrosion resistance index EI instead of the aforementioned index AI. An index EI is a function composed of Sb, C and Mo eq (mass %) and given by the following expression <2>,

EI/ 10,000=0.0005+0.045×Sb %−C %×Mo eq %  <2>.

When Nb, Ta, V, Ti and W are added in combination for a certain purpose, by satisfying the relation EI≧0, extremely excellent hydrochloric acid resistance and sulfuric acid resistance are obtained simultaneously.

That is, FIG. 7 shows the relationship between EI values and sulfuric acid corrosion rates and, as shown in the figure, it is understood that sulfuric acid resistance improves remarkably when an expanded acid corrosion resistance index EI is zero or positive. Therefore, in a steel according to the present invention, Sb, C and Mo are added so that an expanded acid corrosion resistance index EI given by the above expression <2> may be zero or positive.

On the other hand, when an aforementioned EI value exceeds 75, namely, when an Sb amount is excessive, not only the effect of improving sulfuric acid resistance and hydrochloric acid resistance is almost saturated but also hot-workability deteriorates. Therefore, it is preferable that the upper limit of an EI value is set at 75.

Cr is added by not less than 0.1% as occasion demands for improving weather resistance. However, when Cr is added in excess of 0.5%, particularly the sulfuric acid resistance of a steel according to the present invention deteriorates conspicuously. Therefore, when Cr is added as occasion demands, the upper limit is set at 0.5%. From the viewpoint of further improving sulfuric acid resistance and hydrochloric acid resistance, it is preferable to limit a Cr content to 0.1% or less, or most desirably not to add Cr at all.

Ni is an element that has the function of improving hydrochloric acid resistance and is added by not less than 0.1% as occasion demands for the purpose of preventing surface cracks from occurring during hot-working when Cu and Sb are added to a steel according to the present invention. However, the effect is saturated when Ni is added in excess of 1%. Therefore, an Ni amount is limited in the range from 0.1 to 1%. It is particularly preferable to limit an Ni amount to the range from 0.1% to Cu %×0.5.

N is an inevitably included impurity element that has the function of improving hydrochloric acid resistance and is added by not less than 0.001% as occasion demands. However, an N addition exceeding 0.007% causes surface cracks to occur during slab casting. Therefore, an N amount is limited in the range from 0.001 to 0.007%.

Al is added usually as a deoxidizing element. In a steel according to the present invention, Al is added by not less than 0.005% as occasion demands. However, when Al is added in excess of 0.1%, hydrochloric acid dew-point corrosion resistance and hot-workability deteriorate. Therefore, an Al amount is limited in the range from 0.005 to 0.1%.

Further, Ca, Mg, Ce, La, REM and B are added to a steel according to the present invention as occasion demands.

Ca, Mg, REM and B contribute to the enhancement of the toughness of a steel by the improvement of cleanliness and the fractionization of crystal grains, and are added by 0.0002% or more, respectively, as occasion demands since their addition up to certain upper limits does not influence corrosion resistance. However, when an amount of Ca, Mg, Ce, La or REM exceeds 0.01% or a B amount exceeds 0.005%, corrosion resistance begins to be badly influenced. Therefore, an amount of each of Ca, Mg, Ce, La and REM is limited in the range from 0.0002 to 0.01% and a B amount in the range from 0.0002 to 0.005%.

Further, Sn, Pb, Se, Te and Bi are added to a steel according to the present invention as occasion demands.

Sn and Pb have the function of improving a free-cutting property and must be added by not less than 0.01% respectively for securing the effect. However, when each of the contents of Sn and Pb exceeds 0.3%, hot-workability deteriorates. Therefore, the upper limit of each of their contents is set at 0.3%.

Se, Te and Bi have the function of further improving acid resistance and must be added by not less than 0.001% respectively for securing the effect. However, when each of the contents of Se, Te and Bi exceeds 0.1%, the deterioration of productivity and the increase of a production cost are caused. Therefore, the upper limit of each of their contents is set at 0.1%.

Furthermore, Ag and Pd are added to a steel according to the present invention as occasion demands.

Ag and Pd have the function of improving corrosion resistance in an environment of a high temperature and a high sulfuric acid concentration and must be added by not less than 0.001% respectively for securing the effect. However, when the contents of Ag and Pd exceed 0.5% and 0.1%, respectively, not only the increase of a production cost but also the deterioration of hot-workability is caused. Therefore, the upper limits of the contents of Ag and Pd are set at 0.5% and 0.1%, respectively.

(2) Weld Metal

C is added for securing the strength of a weld joint of a steel for a welded structure. However, when C is added in excess of 0.2%, weldability and sulfuric acid resistance deteriorate. Therefore, a C amount is limited to 0.2% or less. Though a lower C content is desirable from the viewpoint of sulfuric acid resistance and hydrochloric acid resistance, in consideration of strength and economical efficiency, a C amount is limited in the range from 0.005 to 0.2%. In particular, when C coexists with Mo, a C addition amount exceeding 0.2% causes sulfuric acid resistance to deteriorate conspicuously. When the balance between sulfuric acid resistance and hydrochloric acid resistance must be considered, a preferable C amount is 0.05% or less.

Si improves sulfuric acid resistance and hydrochloric acid resistance by the coexistence with Cu when Si is contained by 0.01% or more in a weld metal. Therefore, the lower limit of an Si amount is set at 0.01%. On the other hand, when an Si amount exceeds 2.5%, corrosion resistance does not improve any longer and the toughness of a weld metal deteriorates considerably. Therefore, the upper limit of an Si amount is set at 2.5%. When weld operability and the toughness of a weld must be seriously considered, a preferable range of an Si amount is from 0.1 to 1%.

Mn is added by 0.1% or more for deoxidization and the adjustment of strength and the effects are sufficiently secured with an addition of 2%. Therefore, an Mn amount is limited in the range from 0.1 to 2%.

Cu must be added by 0.1% or more for securing sulfuric acid resistance and hydrochloric acid resistance. However, corrosion resistance is almost saturated even with a Cu addition amount exceeding 1% and an excessive strength increase and solidification cracks are caused instead. Therefore, a Cu amount is limited in the range from 0.1 to 1%. Preferably, a Cu addition amount in the range from 0.25 to 0.75% makes the balance between corrosion resistance and production operability better.

Mo is an indispensable element that improves hydrochloric acid resistance considerably by adding an Mo amount of 0.001% or more to a Cu—Sb containing weld metal. Further, when an Mo amount satisfies the expression C %×Mo %≦0.0005+0.045×Sb %, Mo functions as an element that further improves sulfuric acid resistance and improves sulfuric acid resistance together with the aforementioned improvement of hydrochloric acid resistance. The upper limit of an Mo amount is set at 0.5% in consideration of the influence on the mechanical properties of a weld metal.

Sb is an element that further improves sulfuric acid resistance and hydrochloric acid resistance by the coexistence with Cu when it is added by 0.01% or more. Therefore, the lower limit of an Sb amount is set at 0.01%. A preferable addition amount of Sb is 0.05% or more for securing a sufficient corrosion resistance. On the other hand, the effect is almost saturated when Sb is added by 0.2%. Therefore, an Sb amount is limited in the range from 0.01 to 0.2%. A preferable range of an Sb amount is 0.05 to 0.15% because weld operability deteriorates when an Sb amount in a weld metal exceeds 0.15%.

P is an inevitably included impurity element and deteriorates sulfuric acid resistance and hydrochloric acid resistance conspicuously. Therefore, a P amount is limited to 0.03% or less. Preferably, sulfuric acid resistance and hydrochloric acid resistance are remarkably improved by limiting a P amount to 0.01% or less. A still more preferable upper limit of a P amount is in the range from 0.005 to 0.01% and even 0.005% or less.

S is, like P, an inevitably included impurity element and deteriorates sulfuric acid resistance when the amount exceeds 0.03%. Therefore, the upper limit of an S amount must be set at 0.03% for securing sulfuric acid resistance and hydrochloric acid resistance sufficiently. However, as shown FIG. 5 , in a Cu—Mo—Sb steel according to the present invention, sulfuric acid corrosion resistance and hydrochloric acid corrosion resistance improve conspicuously when S is added in excess of 0.005%. For that reason, it is preferable to make also a weld metal contain S by more than 0.005%. However, the effect is saturated when S is added in excess of 0.02%. Moreover, an S amount of 0.02% or less is desirable in consideration of securing the toughness of a weld metal. In particular, when sulfuric acid resistance, hydrochloric acid resistance and mechanical properties are taken into consideration, the most desirable range of an S amount is from more than 0.005 to 0.02%.

In a weld joint according to the present invention, very excellent hydrochloric acid resistance and sulfuric acid resistance are obtained when the acid corrosion resistance index AI, defined by the following expression <1>, of each of a base material and a weld metal satisfies the relation AI≧0,

AI/ 10,000=0.0005+0.045×Sb %−C %×Mo %  <1>.

FIG. 6 shows the relationship between AI values and sulfuric acid corrosion rates. Since, as seen in the figure, sulfuric acid resistance improves conspicuously when an acid corrosion resistance index AI is zero or positive, in the present invention, Sb, C and Mo are added also to a weld metal so that an acid corrosion resistance index AI given by the expression <1> may be zero or positive.

When the relation AI≧0 is satisfied, namely when the expression C %×Mo %≦0.0005+0.045×Sb % is satisfied, or for example, when a weld metal contains an Mo amount of 0.5% or less while containing a C amount of 0.01% or less and a Sb amount of 0.1%, excellent sulfuric acid resistance, hydrochloric acid resistance and chloride containing acid corrosion resistance are obtained.

Further, when an aforementioned AI value exceeds 75, namely, when an Sb amount is excessive, not only the effect of improving sulfuric acid resistance, hydrochloric acid resistance and chloride containing acid corrosion resistance is almost saturated but also hot-workability deteriorates. Therefore, it is preferable that the upper limit of an AI value is set at 75.

Further, in a weld joint according to the present invention, it is preferable to set a selective corrosion resistance index |ΔAI| of the weld joint at an appropriate value.

A selective corrosion resistance index |ΔAI| of a weld joint is defined by the following expression <3> composed of an acid corrosion resistance index AI of a base material and an acid corrosion resistance index AI of a weld metal,

|Δ AI|=|AI of a base material− AI of a weld metal|  <3>.

FIG. 10 is a graph showing the relationship between |ΔAI| values and the sulfuric acid corrosion rates of base materials and weld metals. As shown in FIG. 10 , when |ΔAI| exceeds 20, the corrosion rate of either of a base material or a weld metal having the lower AI value than the other increases and accelerated corrosion occurs selectively in a sulfuric acid environment. Therefore, a |ΔAI| value is set at 20 or less for preventing a weld joint from corroded selectively at an accelerating rate.

Further, in a weld joint according to the present invention, one or more of Nb, Ta, V, Ti and W are added to the weld metal of the weld joint as occasion demands for the purpose of improving the strength, toughness, high temperature property and the like of the weld joint.

Nb, Ta, V and Ti are elements effective for the improvement of strength and toughness caused by the fractionization of crystal grains and the improvement of high temperature strength when they are added by not less than 0.005%, respectively. However, the effects are saturated when an amount of each exceeds 0.1%. Therefore, the addition amount of each of Nb, Ta, V and Ti is limited in the range from 0.005 to 0.1%.

W is effective for the improvement of high temperature strength and hydrochloric acid resistance when it is added by not less than 0.05% and the effects are saturated with a W addition amount of 1%. Therefore, a W addition amount is limited in the range from 0.05 to 1%.

Further, when carbonitride forming elements such as Nb, Ta, V, Ti and W are added excessively in a weld metal, the sulfuric acid corrosion resistance of a weld joint according to the present invention deteriorates. That is, those elements have the function of lowering sulfuric acid resistant Mo amount limit and the degree of the lowering is expressed by the following Mo equivalent (Mo eq )

Mo eq (mass %)=Mo %+5.1×(Nb %+Ta %)+4.2×V %+9.3×Ti %+0.5×W %.

When Nb, Ta, V, Ti and W are added, the combination of the components that determines the limit of not hindering sulfuric acid resistance is given by an expanded acid corrosion resistance index EI instead of the aforementioned index AI. An index EI is a function composed of Sb, C and Mo eq (mass %) and is given by the following expression <2>,

EI/ 10,000=0.0005+0.045×Sb %−C %×Mo eq   %<2>.

When Nb, Ta, V, Ti and W are added in combination for a certain purpose, by satisfying the relation EI≧0, extremely excellent hydrochloric acid resistance and sulfuric acid resistance are obtained simultaneously.

That is, FIG. 7 shows the relationship between EI values and sulfuric acid corrosion rates and, as shown in the figure, it is understood that sulfuric acid resistance improves remarkably when an expanded acid corrosion resistance index EI is zero or positive. Therefore, in the weld metal of a weld joint according to the present invention, Sb, C and Mo are added so that an expanded acid corrosion resistance index EI given by the above expression <2> may be zero or positive.

Further, when one or more of Nb, Ta, V, Ti and W are added, an expanded selective corrosion resistance index of a weld joint is defined by employing an expanded acid corrosion resistance index EI, instead of the aforementioned index AI, and a more excellent weld joint is obtained by evaluating the weld joint with the expanded selective corrosion resistance index.

An expanded selective corrosion resistance index |ΔEI| of a weld joint is defined by the following expression <4> composed of an expanded acid corrosion resistance index EI of a base material and an expanded acid corrosion resistance index EI of a weld metal,

|Δ EI|=|EI of a base material− EI of a weld metal|  <4>.

FIG. 11 is a graph showing the relationship between |ΔEI| values and the sulfuric acid corrosion rates of base materials and weld metals. As shown in FIG. 11 , when |ΔEI| exceeds 20, the corrosion rate of either of a base material or a weld metal having the lower EI value than the other increases and accelerated corrosion occurs selectively in a sulfuric acid environment. Therefore, it is preferable that a |ΔEI| value is set at 20 or less for preventing a weld joint from corroded selectively at an accelerating rate.

Further, Cr, N, Ni and Al are added to the weld metal of a weld joint according to the present invention as occasion demands.

Cr is added by not less than 0.1% as occasion demands for improving weather resistance and the like. However, when Cr is added in excess of 0.5%, particularly the sulfuric acid resistance of a steel according to the present invention deteriorates conspicuously. Therefore, when Cr is added as occasion demands, the upper limit is set at 0.5%. From the viewpoint of further improving sulfuric acid resistance and hydrochloric acid resistance, it is preferable to limit a Cr content to 0.1% or less or, most desirably, not to add Cr at all.

Ni is an element that has the function of improving hydrochloric acid resistance and is added by not less than 0.1% as occasion demands for the purpose of preventing high temperature cracks of a weld metal from occurring when Cu and Sb are added to a steel according to the present invention. However, the effect is saturated when Ni is added in excess of 1%. Therefore, an Ni amount is limited in the range from 0.1 to 1%. It is particularly preferable to limit an Ni amount to the range from 0.1% to Cu %×0.5.

N is an inevitably included impurity element that has the function of improving hydrochloric acid resistance and is added by not less than 0.001% as occasion demands. However, an N addition amount exceeding 0.02% causes the generation of blowholes in a weld metal and the deterioration of the toughness of a weld metal. Therefore, an N amount is limited in the range from 0.001 to 0.02%.

Al is added by not less than 0.005% as occasion demands. However, when Al is added in excess of 0.1%, hydrochloric acid resistance and the toughness of a weld joint deteriorate. Therefore, an Al amount is limited in the range from 0.005 to 0.1%.

Further, Ca, Mg, Ce, La, REM and B are added to the weld metal of a weld joint according to the present invention as occasion demands.

Ca, Mg, REM and B contribute to the enhancement of the toughness of the weld metal of a weld joint by the improvement of cleanliness and the fractionization of crystal grains, and are added by 0.0002% or more, respectively, as occasion demands since their addition up to certain upper limits does not influence corrosion resistance. However, when an amount of Ca, Mg, Ce, La or REM exceeds 0.01% or a B amount exceeds 0.005%, corrosion resistance begins to be badly influenced. Therefore, an amount of each of Ca, Mg, Ce, La and REM is limited in the range from 0.0002 to 0.01% and a B amount in the range from 0.0002 to 0.005%.

Further, Sn, Pb, Se, Te and Bi are added to the weld metal of a weld joint according to the present invention as occasion demands.

Sn and Pb have the function of improving the operability of the grinder conditioning of a weld bead and must be added by not less than 0.01% respectively for securing the effect. However, when each of the contents of Sn and Pb exceeds 0.3%, the toughness of a weld metal deteriorates. Therefore, the upper limit of each of their contents is set at 0.3%.

Se, Te and Bi have the function of further improving acid resistance and must be added by not less than 0.001% respectively for securing the effect. However, when each of the contents of Se, Te and Bi exceeds 0.1%, the deterioration of the toughness of a weld metal is caused. Therefore, the upper limit of each of their contents is set at 0.1%.

Furthermore, Ag and Pd are added to the weld metal of a weld joint according to the present invention as occasion demands.

Ag and Pd have the function of improving corrosion resistance of the weld metal of a weld joint in an environment of a high temperature and a high sulfuric acid concentration and must be added by not less than 0.001% respectively for securing the effect. However, when the contents of Ag and Pd exceed 0.5% and 0.1%, respectively, not only the increase of a cost but also the deterioration of weld operability is caused. Therefore, the upper limits of the contents of Ag and Pd are set at 0.5% and 0.1%, respectively.

Next, preferable methods for producing steels according to the present invention are explained hereunder. A preferable reheating temperature after continuous casting or breakdown rolling is 1,000° C. or higher from the viewpoint of a rolling load and the like. However, when a steel is reheated to a temperature exceeding 1,300° C., the coarsening of crystal grains and the increase of decarburization and oxide scales are caused excessively. Therefore, a preferable reheating temperature is limited in the range from 1,000° C. to 1,300° C.

When a finishing temperature at hot rolling is lower than 800° C., the formation of unevenly mixed grains is unavoidable. On the other hand, when it exceeds 1,000° C., coarse grains are formed. Therefore, it is preferable that a hot-rolling finishing temperature is in the range from 800° C. to 1,000° C. Thereafter, the steel sheet is cooled in the air for making the microstructure thereof mainly composed of ferrite. Note that, in the hot rolling of the so-called thin steel sheet, since there is a concern that the cooling rate may be too high, the steel sheet is coiled in the temperature range from 600° C. to 750° C. and then cooled in the air or in a furnace. Further, in the case of the production of a cold-rolled steel sheet, it is preferable that a hot-rolled steel sheet is, after cooled in the air at hot rolling, subjected to cold rolling at a reduction ratio in the range from 30 to 90% and subsequently continuous annealing or box annealing in the temperature range from 700° C. to 900° C.

A steel according to the present invention, after being produced into a steel ingot, may be used by being formed into steel sheets, bars and wires, section steels, sheet piles or the like through hot rolling, forging, cold rolling, wire drawing or the like, for example. Further, they may be formed into prescribed shapes through pressing, etc. or subsequently formed into final products through processing and/or welding. Furthermore, a steel sheet may be firstly formed into a steel pipe, for example an electric resistance welded steel pipe or the like, and thereafter used for a final product through secondary processing and/or welding, etc. Still further, any optimum production processes including other processes may be selected in consideration of costs and the restrictions of existing equipment, and any processes may be selected as long as a steel according to the present invention may be produced.

Further, a weld joint according to the present invention shows excellent sulfuric acid corrosion resistance and hydrochloric acid corrosion resistance and is quite appropriate when the aforementioned steel according to the present invention is used in the weld joint by employing one of various welding methods. A weld joint according to the present invention is formed by welding steel materials formed into various shapes or steel materials formed into prescribed shapes through further processing the steel materials by employing one of various welding methods. In general, the aforementioned steel materials, after appropriate edge preparation if necessary, may be welded by employing any of various methods such as; arc welding in the atmosphere or an atmosphere containing shielding gas, arc welding such as submerged arc welding, plasma arc welding, electron beam welding, and the like. In that case, a weld joint according to the present invention may be formed by adjusting the composition of a weld metal by means of selecting; the composition of the core wire of a welding rod or a welding wire, the compositions of the coating material of a welding rod and flux, a welding atmosphere, and/or the like.

A steel or a weld joint according to the present invention may be processed into a steel material having the surfaces containing chemical components stipulated in the claims of the present invention by: applying an alloy containing necessary elements on the surface of an alloy having an appropriate composition by a plating method, cladding method or the like; and diffusing the elements by an appropriate treatment such as heat treatment. Further, when a steel or a weld joint according to the present invention is used, any anticorrosive measure may be employed, including surface treatment, coating, electric protection, or the combination thereof, and the application of an anticorrosive agent, and the employment of any of the above anticorrosive measures does not deviate from the scope of the present invention.

The present invention makes it possible to obtain a steel having extremely excellent resistance to hydrochloric acid dew-point corrosion and an acid-containing chloride and excellent sulfuric acid dew-point corrosion resistance. Therefore, the present invention makes it possible to economically provide a steel excellent in durability and capable of prolonging equipment service life, and reducing maintenance, as a material for a stack, a flue, a heat exchanger, a casing or an expansion joint, that is exposed to a combustion exhaust gas and causes harsh hydrochloric acid and/or sulfuric acid dew-point corrosion in a thermoelectric power station, a non-utility electric power generator, or a non-industrial or industrial waste treatment plant.

Further, the present invention makes it possible to economically provide a steel for a welded structure that shows excellent corrosion resistance in the application of the steel to a steel tank of a pickling apparatus containing hydrochloric acid or sulfuric acid.

Furthermore, the present invention makes it possible to provide a weld joint, having extremely excellent hydrochloric acid resistance and sulfuric acid resistance, easily and at a low cost.