In a method for continuously casting an extremely low carbon steel using a continuous casting machine, by adjusting the chemical components of extremely low carbon steel within a specified range by taking into account an interface tension gradient in a concentration boundary layer on a front surface of a solidified shell, and also by optimizing intensities of the DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively corresponding to a slab width of a slab to be casted and a casting speed, it is possible to acquire the slab having high quality not only with the small number of defects caused by the entrainment of bubbles, non-metallic inclusion and a mold flux into the molten steel.

Description

27 parts

›CROSS REFERENCE TO RELATED APPLICATION

This application is the U.S. National Phase application of PCT International Application No. PCT/JP2011/056122, filed Mar. 9, 2011, and claims priority to Japanese Patent Application Nos. 2010-053869, filed Mar. 10, 2010, and 2010-053870, filed Mar. 10, 2010, the disclosure of each of which are incorporated herein by reference in their entireties for all purposes.

›FIELD OF THE INVENTION

The present invention relates to a continuous casting method for steel which produces a slab by casting molten steel while controlling the flow of the molten steel in a mold by an electromagnetic force, and a method for manufacturing a steel sheet by using the slab produced by casting using the continuous casting method.

›BACKGROUND OF THE INVENTION

In the continuous casting of steel, molten steel charged into a tundish is poured into a mold for continuous casting through an immersion nozzle connected to a bottom portion of the tundish. In this case, a molten steel flow discharged into the inside of the mold from discharge holes of the immersion nozzle includes non-metallic inclusions such as alumina clusters and bubbles of an inert gas which is blown off from an inner wall surface of an upper nozzle (an inert gas blown off for preventing the clogging of the nozzle caused by adhesion or stacking of alumina or the like). When these non-metallic inclusions and bubbles are caught in a solidified shell, product defects (inclusion-caused defects, bubble-caused defects) occur. Further, a mold flux (mold powder) is entrained into an upward molten steel flow which reaches a meniscus so that the mold flux is also caught in the solidified shell leading to defects of a product.

Conventionally, to prevent the non-metallic inclusions, mold flux and bubbles in the molten steel from causing a product defect being caught in the solidified shell, a magnetic field is applied to a molten steel flow in the mold and the flow of molten steel is controlled by making use of an electromagnetic force generated by a magnetic field. Many proposals have been made with respect to this technique.

For example, Patent Document 1 discloses a method where a molten steel flow is braked by a DC current magnetic field which is applied to a pair of upper magnetic poles which is arranged to face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged to face each other with the mold long-side portion sandwiched therebetween. In this method, out of the molten steel flow which is divided into an upward flow and a downward flow after being discharged from discharge openings of the immersion nozzle, the downward flow is braked by a lower DC magnetic field and the upward flow is braked by an upper DC magnetic field thus preventing the non-metallic inclusions and a mold flux included in the molten steel flow from being caught in the solidified shell.

Patent document 2 discloses a method where a molten steel flow is braked by a DC current magnetic field which is applied to a pair of upper magnetic poles which is arranged to face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged to face each other with the mold long-side portion sandwiched therebetween in the same manner as Patent Document 1, and an AC magnetic field is applied to the upper magnetic poles or the lower magnetic poles in a superimposed manner. This method provides the braking of the molten steel flow by the DC magnetic field in the same manner as Patent Document 1, and also aims at the acquisition of a cleaning effect of non-metallic inclusions or the like on an interface of a solidified shell due to stirring of molten steel by an AC magnetic field.

Further, Patent Document 3 discloses a method where a molten steel flow is braked by a DC magnetic field which is applied to a pair of upper magnetic poles which is arranged so that the magnetic poles face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged so that the magnetic poles face each other with the mold long-side portion sandwiched therebetween respectively. In this method, the intensity of DC magnetic field and an intensity ratio between a DC magnetic field of the upper magnetic poles and a DC magnetic field of the lower magnetic poles are set to values which fall within specific numerical ranges.

Further, Patent Document 4, 5 disclose a continuous casting method where the catching of bubbles in a solidified shell can be suppressed by controlling a surface tension due to concentration gradient of C, S, N, O in molten steel on a front surface of the solidified shell, that is, by adjusting the concentrations of C, S, N, O in molten steel such that the surface tension becomes equal to or below a predetermined value.

›PATENT LITERATURE

[Patent Document 1] JP-A-3-142049

[Patent Document 2] JP-A-10-305353

[Patent Document 3] JP-A-2008-200732

[Patent Document 4] JP-A-2003-205349

[Patent Document 5] JP-A-2003-251438

›SUMMARY OF THE INVENTION · 1 of 3

Along with the recent tendency that the requirement on quality of a steel sheet for a skin plate of an automobile becomes stricter, defects caused by entrainment of minute bubbles, minute non-metallic inclusions and a minute mold flux which had not been treated as problems are considered as problems now. The continuous casting method disclosed in Patent Documents 1 to 3 cannot sufficiently cope with such strict requirement on quality. Particularly, in manufacturing a hot-dip galvannealed steel sheet, after hot dipping, the steel sheet is heated so that an iron component in a base-material steel sheet is diffused into a galvanized layer and hence, a surface layer property of the base-material steel sheet largely influences quality of a hot-dip galvannealed layer. That is, when bubble-caused defects, inclusion-caused defects or mold-flux-caused defects are present in the surface layer of the base-material steel sheet and hence, irregularities occur in a thickness of a plated layer even when a defect is small, and these irregularities appear as stripe-like defects on a surface of the base-material steel sheet so that the steel sheet cannot be used in applications on which strict requirements of quality are imposed such as a skin plate of an automobile.

Further, in Patent Document 4 and in Patent Document 5, the catching of non-metallic inclusion such as alumina cluster by a solidified shell has not been studied at all. Further, although these documents suggest that the catching of the bubbles in the solidified shell is influenced corresponding to the composition of molten steel, the relationship between the catching of bubbles and flow velocity of molten steel at molten steel-solidified shell interface is not clarified and hence, the catching of the bubbles cannot be quantitatively grasped. This is because, in the inside of an actual mold, resistance attributed to flow velocity of molten steel works simultaneously with a surface tension (=catching force of the solidified shell) generated by a concentration distribution of C, S, N, O but also and hence, in studying the catching of bubbles and the non-metallic inclusions in the solidified shell, it is necessary to take into account the resistance by the flow velocity of molten steel at molten steel-solidified shell interface.

Accordingly, the present invention provides a continuous casting method for extremely low carbon steel which can overcome the above-mentioned drawbacks of the related art and can, by controlling a molten steel flow in a mold using an electromagnetic force, produce a slab having high quality not only with the small number of defects caused by non-metallic inclusions and a mold flux which have been considered as problems conventionally but also with the small number of defects caused by entrainment of minute bubbles, minute non-metallic inclusions and a minute mold flux.

Inventors of the present invention have studied various casting conditions at the time of controlling a flow of molten steel in a mold by making use of an electromagnetic force for overcoming the above-mentioned drawbacks. As a result of this study, the inventors have found that in a method for continuously casting extremely low carbon steel while braking a molten steel flow by a DC magnetic field applied to a pair of upper magnetic poles which is arranged to face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged to face each other with the mold long-side portion sandwiched therebetween respectively, the chemical component of extremely low carbon steel is adjusted within a specified range determined by taking into account an interface tension gradient in a concentration boundary layer on a front surface of a solidified shell, and intensities of DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively are optimized corresponding to a slab width of a slab to be casted and a casting speed so that molten steel in the mold can be brought into an appropriate flow state where non-metallic inclusions and bubbles are not caught in the solidified shell and no mold powder entrainment is generated whereby it is possible to produce a slab having high quality not only with small number of the defects caused by non-metallic inclusions and a mold flux which had been considered as problems conventionally but also with the small number of defects caused by minute bubbles, minute non-metallic inclusions and a minute mold flux. Further, the inventors have also found that to acquire the slab having high quality in such continuous casting, a nozzle immersing depth, optimum ranges exist with respect to a nozzle inner diameter of an immersion nozzle, a slab thickness and the like, and advantageous effects of the present invention can be realized most within the ranges.

By continuously casting the slab by controlling the flow of molten steel in the mold with DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively, mold-flux-caused defects caused by entrainment of a mold flux can be prevented, and also a defect caused by bubbles and non-metallic inclusions having a relatively large size (usually 1 mmφ or more) can be prevented. However, in this continuous casting method, it is difficult to surely prevent the catching of more minute bubbles and minute non-metallic inclusions by a solidified shell and hence, there exists a possibility that a surface defect caused by the entrainment of such minute bubbles and minute non-metallic inclusions arises. To the contrary, by adjusting the chemical component of extremely low carbon steel within the specified range by taking into account the interface surface gradient in the concentration boundary layer of the front surface of the solidified shell and also by optimizing the intensities of the DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles corresponding to the slab width to be cast and the casting speed, it is possible to suppress the catching of the minute bubbles and minute non-metallic inclusions in the solidified shell. Accordingly, the entrainment of the mold flux can be prevented, and also the catching of the bubbles and the non-metallic inclusions in the solidified shell can be prevented irrespective of sizes of the bubbles and the non-metallic inclusions and hence, it is possible to manufacture a steel sheet having high quality with extremely small number of surface defects caused by the entrainment of bubbles, the non-metallic inclusions and the mold flux.

›SUMMARY OF THE INVENTION · 2 of 3

It is also found that by applying pickling and cold rolling to a hot-rolled steel sheet obtained by rolling the slab produced by casting by the above-mentioned continuous casting method under specified conditions, a steel sheet having high quality with extremely small number of blisters can be manufactured.

The present invention has been made based on these findings, and the gist of the present invention according to exemplary embodiments is as follows.

[1] A continuous casting method for steel in which extremely low carbon steel containing 0.003 mass or less of C is continuously cast using a continuous casting machine where a pair of upper magnetic poles which is arranged so that the upper magnetic poles face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged so that the lower magnetic poles face each other with the mold long-side portion sandwiched therebetween are provided to an outer side of a mold, an immersion nozzle with a molten steel discharge angle of a molten steel discharge hole directing downward from a horizontal direction is set to 10° or more and less than 30° is provided, and the molten steel discharge hole is positioned between a peak position of a magnetic field of the upper magnetic poles and a peak position of a magnetic field of the lower magnetic poles, while a molten steel flow is braked by a DC magnetic field applied to the pair of upper magnetic poles and the pair of lower magnetic poles, wherein molten steel containing chemical components where an X value defined by a following formula (1) satisfies X≦5000 is continuously cast at a casting speed of 0.75 m/min or more and in accordance with following conditions (X), (Y).

X= 24989×[% Ti]+386147×[% S]+853354×[% O] (1)

Here, [% Ti]: Ti content in molten steel (mass %), [% S]: S content in molten steel (mass %), and [% O]: O content in molten steel (mass %).

Condition (X): When a slab width of a slab to be cast and a casting speed fall within following ranges (a) to (i), intensity of a DC magnetic field applied to the upper magnetic poles is set to 0.03 to 0.15 T and intensity of a DC magnetic field applied to the lower magnetic poles is set to 0.24 to 0.45 T.

(a) the slab width being less than 950 mm and the casting speed being less than 2.05 m/min,

(b) the slab width being 950 mm or more and less than 1050 mm and the casting speed being less than 2.25 m/min,

(d) the slab width being 1350 mm or more and less than 1450 mm and the casting speed being less than 2.25 m/min,

(e) the slab width being 1450 mm or more and less than 1650 mm and the casting speed being less than 2.15 m/min,

(f) the slab width being 1650 mm or more and less than 1750 mm and the casting speed being less than 2.05 m/min,

(g) the slab width being 1750 mm or more and less than 1850 mm and the casting speed being less than 1.95 m/min,

(h) the slab width being 1850 mm or more and less than 1950 mm and the casting speed being less than 1.85 m/min, and

(i) the slab width being 1950 mm or more and less than 2150 mm and the casting speed being less than 1.75 m/min.

Condition (Y): when a slab width of a slab to be cast and a casting speed fall within following ranges (j) to (s), intensity of a DC magnetic field applied to the upper magnetic poles is set to more than 0.15 to 0.30 T and intensity of a DC magnetic field applied to the lower magnetic poles is set to 0.24 to 0.45 T.

(j) the slab width being less than 950 mm and the casting speed being 2.05 m/min or more and 3.05 m/min or less,

(k) the slab width being 950 mm or more and less than 1050 mm and the casting speed being 2.25 m/min or more and 3.05 m/min or less,

(l) the slab width being 1050 mm or more and less than 1350 mm and the casting speed being 2.35 m/min or more and 3.05 m/min or less,

(m) the slab width being 1350 mm or more and less than 1450 mm and the casting speed being 2.25 m/min or more and 3.05 m/min or less,

(n) the slab width being 1450 mm or more and less than 1550 mm and the casting speed being 2.15 m/min or more and 3.05 m/min or less,

(o) the slab width being 1550 mm or more and less than 1650 mm and the casting speed being 2.15 m/min or more and 2.85 m/min or less,

(p) the slab width being 1650 mm or more and less than 1750 mm and the casting speed being 2.05 m/min or more and 2.65 m/min or less,

(q) the slab width being 1750 mm or more and less than 1850 mm and the casting speed being 1.95 m/min or more and 2.55 m/min or less,

(r) the slab width being 1850 mm or more and less than 1950 mm and the casting speed being 1.85 m/min or more and 2.55 m/min or less, and

(s) the slab width being 1950 mm or more and less than 2150 mm and the casting speed being 1.75 m/min or more and 2.55 m/min or less.

[2] A method for manufacturing a steel sheet where a hot-rolled steel sheet is obtained by hot rolling a slab produced by casting using the continuous casting method in the above-mentioned [1], the hot-rolled steel sheet is subject to pickling and, thereafter, in applying cold rolling to the hot-rolled steel sheet, time t and/or a maximum surface temperature T of the steel sheet is controlled so as to satisfy a following formula (1a).

Hc/Ho >exp{−0.002×( T+t/ 100)} (1a)

Here, Ho: hydrogen concentration (mass ppm) in steel sheet immediately after pickling is finished

Hc: critical hydrogen concentration (mass ppm) in steel sheet immediately before cold rolling at which surface quality defects occur by blister, the critical hydrogen concentration being determined based on cold rolling conditions

t: time until cold rolling starts after pickling is finished (seconds)

T: maximum surface temperature T (K) of steel sheet after pickling is finished and before cold rolling starts (the steel sheet surface temperature also includes steel sheet surface temperature when the steel sheet is heated after pickling is finished and before cold rolling)

[3] In the continuous casting method for steel or the method for manufacturing a steel sheet in the above-mentioned [1] or [2], a nozzle immersing depth of the immersion nozzle is set to a value which falls within a range from 230 to 290 mm.

›SUMMARY OF THE INVENTION · 3 of 3

[4] In the continuous casting method for steel or the method for manufacturing a steel sheet in any one of the above-mentioned [1] to [3], a nozzle inner diameter (the nozzle inner diameter at a position where the molten steel discharge hole is formed) of the immersion nozzle is set to a value which falls within a range from 70 to 90 mm.

[5] In the continuous casting method for steel or the method for manufacturing a steel sheet in any one of the above-mentioned [1] to [4], an opening area of each molten steel discharge hole of the immersion nozzle is set to a value which falls within a range from 3600 to 8100 mm 2 .

[6] In the continuous casting method for steel or the method for manufacturing a steel sheet in any one of the above-mentioned [1] to [5], with respect to the molten steel in the mold, turbulence energy of molten steel on top surface is set to a value which falls within a range from 0.0010 to 0.0015 m 2 /s 2 , flow velocity of molten steel on top surface is set to 0.30 m/s or less, and flow velocity of molten steel at molten steel-solidified shell interface is set to a value which falls within a range from 0.08 to 0.15 m/s.

[7] In the continuous casting method for steel or the method for manufacturing a steel sheet in the above-mentioned [6], with respect to the molten steel in the mold, the flow velocity of molten steel on top surface is set to a value which falls within a range from 0.05 to 0.30 m/s.

[8] In the continuous casting method for steel or the method for manufacturing a steel sheet in the above-mentioned [6] or [7], with respect to the molten steel in the mold, a ratio A/B between flow velocity of molten steel at molten steel-solidified shell interface A and flow velocity of molten steel on top surface B is set to a value which falls within a range from 1.0 to 2.0.

[9] In the continuous casting method for steel or the method for manufacturing a steel sheet in any one of the above-mentioned [6] to [8], with respect to the molten steel in the mold, concentration of bubbles at molten steel-solidified shell interface is set to 0.008 kg/m 3 or less.

[10] In the continuous casting method for steel or the method for manufacturing a steel sheet in the above-mentioned [9], a slab thickness of a slab to be cast is set to a value which falls within a range from 220 to 300 mm, and a blow-off amount of an inert gas from an inner wall surface of the immersion nozzle is set to a value which falls within a range from 3 to 25 NL/min.

[11] In the method for manufacturing a steel sheet in any one of the above-mentioned [2] to [10], a hot-rolled steel sheet after pickling and before cold rolling is heated to a temperature higher than a steel sheet temperature immediately after the pickling is finished.

According to the continuous casting method for steel according to embodiments of the present invention, by adjusting the chemical components of extremely low carbon steel within the specified range by taking into account the interface tension gradient in the concentration boundary layer on the front surface of the solidified shell, and also by optimizing intensities of the DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively corresponding to the slab width of the slab to be casted and a casting speed, it is possible to acquire the slab having high quality not only with the small number of defects caused by non-metallic inclusions and a mold flux which have been considered as problems conventionally but also with the small number of defects caused by minute bubbles and minute non-metallic inclusions.

Further, particularly, by optimizing the nozzle immersing depth and the nozzle inner diameter of the immersion nozzle and an opening area of a molten steel discharge hole, it is possible to produce a slab having higher quality.

Still further, according to the method for manufacturing a steel sheet, a steel sheet having high quality with extremely small number of blisters can be manufactured.

›BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a mold of a continuous casting machine and an immersion nozzle of an embodiment for carrying out the present invention.

FIG. 2 is a horizontal cross-sectional view of the mold and the immersion nozzle of the embodiment shown in FIG. 1 .

FIG. 3 is a graph showing the relationship between a molten steel discharge angle of the immersion nozzle and an occurrence rate of surface defects (defect index).

FIG. 4 is a graph showing the relationship between an X value of molten steel, flow velocity of molten steel at a molten-steel solidified shell interface, and a catch rate of non-metallic inclusions by a solidified shell.

FIG. 5 is a graph showing the influence of a nozzle immersing depth of the immersion nozzle (influence which is exerted on mold-flux-caused defects and bubble-caused defects) in a method of the present invention.

FIG. 6 is a graph showing the influence of a nozzle inner diameter of the immersion nozzle (influence which is exerted on mold-flux-caused defects) in an exemplary method of the present invention.

FIG. 7 is a graph showing the influence of an opening area of respective molten steel discharge holes of the immersion nozzle (influence which is exerted on mold-flux-caused defects and bubble-caused defects) in an exemplary method of the present invention.

FIG. 8 is a conceptual view for explaining turbulence energy of molten steel on top surface, flow velocity of molten steel at solidification interface (flow velocity of molten steel at molten steel-solidified shell interface), flow velocity of molten steel on top surface, and concentration of bubbles at solidification interface (concentration of bubbles at molten steel-solidified shell interface) of molten steel in a mold.

FIG. 9 is a graph showing the relationship between turbulence energy of molten steel on top surface and a surface defect rate (the number of defects) of molten steel in the mold.

FIG. 10 is a graph showing the relationship between flow velocity of molten steel on top surface and a surface defect rate (the number of defects) of molten steel in the mold.

FIG. 11 is a graph showing the relationship between flow velocity of molten steel at solidification interface (flow velocity of molten steel at molten steel-solidified shell interface) and a surface defect rate (the number of defects) of molten steel in the mold.

FIG. 12 is a graph showing the relationship between a ratio A/B between flow velocity of molten steel at solidification interface A and flow velocity of molten steel on top surface B and a surface defect rate (the number of defects) of molten steel in the mold.

FIG. 13 is a graph showing the relationship between concentration of bubbles at solidification interface (concentration of bubbles at molten steel-solidified shell interface) and a surface defect rate (the number of defects) of molten steel in the mold.

FIG. 14 is a graph showing the relationship between a weight reduction amount of a hot-rolled steel sheet by pickling and the hydrogen concentration H 0 in a steel sheet immediately after pickling is finished.

FIG. 15 is a graph showing the relationship between H 0 ·exp{−0.002×(T 0 +t 1 /100)} and the hydrogen concentration H 1 in a steel sheet at a point of time that a time t 1 elapses from finishing of pickling assuming hydrogen concentration in a hot-rolled steel sheet immediately after pickling is finished as H 0 and a steel sheet surface temperature as T 0 .

FIG. 16 is a graph showing the relationship between the hydrogen concentration H in a steel sheet immediately before cold rolling and the number of occurrence of blister defects as a graph in terms of a finished plate thickness of a cold-rolled plate.

›DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION · 1 of 15

In a continuous casting method for steel according to embodiments of the present invention, extremely low carbon steel is continuously cast using a continuous casting machine where a pair of upper magnetic poles which is arranged to face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged to face each other with the mold long-side portion sandwiched therebetween are provided to an outer side of the mold (a back surface of a side wall of the mold), an immersion nozzle with a molten steel discharge angle α of a molten steel discharge hole directing downward from a horizontal direction is set to 10° or more and less than 30° is provided, and the molten steel discharge hole is positioned between a peak position of a magnetic field of the upper magnetic poles and a peak position of a magnetic field of the lower magnetic poles. The extremely low carbon steel continuously casts a molten steel flow while braking the molten steel flow with a DC magnetic field applied to the pair of upper magnetic poles and the pair of lower magnetic poles.

The inventors of the present invention have studied the above-mentioned continuous casting method by performing numerical value simulations and the like and, as a result of the study, have found that, as factors relating to the occurrence of bubble-caused defects, inclusion-caused defects and mold-flux-caused defects (primary factors), turbulence energy of molten steel on top surface (relating to the occurrence of a vortex flow in the vicinity of the top surface), flow velocity of molten steel at molten steel-solidified shell interface (hereinafter may be also simply referred to as “solidification interface”) (hereinafter also may be simply referred to as “flow velocity of molten steel at solidification interface”) and flow velocity of molten steel on top surface are listed, and these factors influence the occurrence of defects. Particularly, the inventors have found that the flow velocity of molten steel on top surface and the turbulence energy of molten steel on top surface influence the entrainment of a mold flux, and the flow velocity of molten steel at solidification interface influences bubble-caused defects and inclusion-caused defects. The inventors of the present invention have studied various actions generated by an upper DC magnetic field and a lower DC magnetic field applied to a molten steel flow based on such findings, and the followings are clarified as a result of the study.

(1) When a DC magnetic field is applied to the upper magnetic poles, an upward flow of molten steel (an upward flow which is generated when a blow-off flow from the molten steel discharge hole impinges on a mold short side and is reversed) is braked so that flow velocity of molten steel on top surface and turbulence energy of molten steel on top surface can be reduced. With only such a DC magnetic field, however, the flow velocity of molten steel on top surface, the turbulence energy of molten steel on top surface and flow velocity of molten steel at solidification interface cannot be controlled in an ideal state.

(2) From the above-mentioned point, it is considered that the application of a DC magnetic field to the upper magnetic poles is effective for preventing both bubble-caused defects/inclusion-caused defects and mold-flux-caused defects. However, when only the DC magnetic field is applied to the upper magnetic poles, a sufficient advantageous effect in braking cannot be obtained, and casting conditions (a slab width of a slab to be cast, casting speed) and an application condition of a DC magnetic field applied to the upper magnetic poles and the lower magnetic poles respectively are mutually relevant to each other and optimum ranges exist with respect to these conditions.

(3) Particularly, to prevent minute non-metallic inclusions or the like from being caught in a solidified shell, it is necessary to adjust the chemical component of molten steel to a composition range where non-metallic inclusions or the like are hardly caught in the solidified shell at molten steel-solidified shell interface (that is, a range specified by taking into account an interface tension gradient in a concentration boundary layer on a front surface of the solidified shell), and based on such adjustment, it is necessary to set flow velocity of molten steel at solidification interface to an appropriate value by optimizing the intensity of the above-mentioned DC magnetic field thus acquiring a cleaning effect by a molten steel flow.

In the present invention, the continuous casting of extremely low carbon steel is preferably performed under the following conditions (A), (B) based on such finding, and such continuous casting can effectively suppress both the occurrence of bubble-caused defects/inclusion-caused defects and the occurrence of mold-flux-caused defects.

Condition (A): Chemical components of molten steel (extremely low carbon steel) are adjusted within a specified range determined by taking into account an interface tension gradient in a concentration boundary layer of a front surface of a solidified shell.

Condition (B): Intensities of DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively are optimized corresponding to a slab width of a slab to be casted and a casting speed.

FIG. 1 and FIG. 2 show a mold of a continuous casting machine and an immersion nozzle of one embodiment for carrying out the present invention, wherein FIG. 1 is a longitudinal cross-sectional view of the mold and the immersion nozzle, and FIG. 2 is a horizontal cross-sectional view of the mold and the immersion nozzle (cross-sectional view taken along a line II-II in FIG. 1 ).

In the drawing, numeral 1 indicates the mold, wherein the mold 1 is formed of mold long side portions 10 (mold side walls) and mold short side portions 11 (mold side walls) and has a rectangular shape as viewed in a horizontal cross section.

›DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION · 2 of 15

Numeral 2 indicates the immersion nozzle, and molten steel in a tundish (not shown in the drawing) arranged above the mold 1 is poured into the mold 1 through the immersion nozzle 2 . The immersion nozzle 2 has a bottom portion 21 at a lower end of a cylindrical nozzle body, and a pair of molten steel discharge holes 20 is formed in a side wall portion at a position right above the bottom portion 21 such that the molten steel discharge holes 20 face both mold short side portions 11 in an opposed manner respectively.

To prevent a case where non-metallic inclusions such as alumina in molten steel adhere to or are stacked on an inner wall surface of the immersion nozzle 2 thus clogging the nozzle, an inert gas such as an Ar gas is introduced into a gas passage formed in the inside of the nozzle body of the immersion nozzle 2 or in the inside of an upper nozzle (not shown in the drawing), and such an inert gas is blown into the nozzle through an inner wall surface.

The molten steel which is flown into the immersion nozzle 2 from the tundish is discharged into the mold 1 through the pair of molten steel discharge holes 20 formed in the immersion nozzle 2 . The discharged molten steel forms a solidified shell 5 by being cooled in the mold 1 , and the solidified shell 5 is continuously drawn to an area below the mold 1 so that a slab is formed. A mold flux is added to a meniscus 6 in the mold 1 as a heat insulation agent for molten steel and as a lubricant between the solidified shell 5 and the mold 1 .

Further, bubbles of the inert gas blown through the inner wall surface of the immersion nozzle 2 and through the inner portion of the upper nozzle are discharged into the inside of the mold 1 together with the molten steel from the molten steel discharge holes 20 .

On an outer side of the mold 1 (a back surface of the mold side wall), a pair of upper magnetic poles 3 a , 3 b which is arranged so that the upper magnetic poles 3 a , 3 b face each other with the long side portion of the mold sandwiched therebetween and a pair of lower magnetic poles 4 a , 4 b which is arranged so that the lower magnetic poles 4 a , 4 b face each other with the long side portion of the mold sandwiched therebetween are mounted. The upper magnetic poles 3 a , 3 b and the lower magnetic poles 4 a , 4 b are arranged so as to extend over the whole width of the mold long side portion 10 in the widthwise direction.

The upper magnetic poles 3 a , 3 b and the lower magnetic poles 4 a , 4 b are arranged such that, in the vertical direction of the mold 1 , the molten steel discharge holes 20 are positioned between a peak position of a magnetic field of the upper magnetic poles 3 a , 3 b (peak position in the vertical direction: usually, a center position of the upper magnetic poles 3 a , 3 b in the vertical direction) and a peak position of a magnetic field of the lower magnetic poles 4 a , 4 b (peak position in the vertical direction: usually, a center position of the lower magnetic poles 4 a , 4 b in the vertical direction). Further, the pair of upper magnetic poles 3 a , 3 b is usually arranged at a position where the upper magnetic poles 3 a , 3 b cover the meniscus 6 .

The molten steel discharged from the molten steel discharge holes 20 of the immersion nozzle 2 in the directions toward the mold short side portions 11 impinges on the solidified shell 5 formed on front surfaces of the mold short side portions 11 so that the molten steel is divided into an upward molten steel flow and a downward molten steel flow. A DC magnetic field is applied to the pair of upper magnetic poles 3 a , 3 b and the pair of lower magnetic poles 4 a , 4 b respectively. A basic action brought about these magnetic poles is that by making use of an electromagnetic force which acts on the molten steel moving in the DC magnetic field, the upward molten steel flow is braked (subject to the reduction of speed) by the DC magnetic field applied to the upper magnetic poles 3 a , 3 b , and the downward molten steel flow is braked (subject to reduction of speed) by the DC magnetic field applied to the lower magnetic poles 4 a , 4 b.

In an embodiment of the method of the present invention, used is an immersion nozzle where a molten steel discharge angle α at which the molten steel is discharged from the molten steel discharge hole 20 , that is, the molten steel discharge angle α directing downward from a horizontal direction is set to 10° or more and less than 30°. In case the molten steel discharge angle α is set to less than 10°, even when the upward molten steel flow is braked by the DC magnetic field of the upper magnetic poles 3 a , 3 b , the disturbance of a surface of the molten steel cannot be properly controlled and hence, the entrainment of the mold flux occurs. To the contrary, it is found that when the molten steel discharge angle α is set large, the non-metallic inclusion and bubbles are carried to a lower side of the mold by the downward molten steel flow so that the non-metallic inclusions and bubbles are liable to be caught in the solidified shell, while when the molten steel discharge angle α is set to less than 30°, the molten steel flow can be optimized using a DC magnetic field control according to an exemplary method of the present invention. Accordingly, the immersion nozzle 2 with the molten steel discharge angle α of less than 30° is preferably used in the present invention. Further, from the above-mentioned viewpoint, a lower limit of the molten steel discharge angle α is more preferably set to 15°, and an upper limit of the molten steel discharge angle α is more preferably set to 25°.

FIG. 3 shows the relationship between the molten steel discharge angle α of the immersion nozzle and an occurrence rate of surface defects (defect index). A continuous casting test is performed under various conditions where the molten steel composition and intensity of a magnetic field, a casting speed and a slab width under conditions (X), (Y) described later satisfy ranges defined by the present invention. In the test, the slab which is produced by continuous casting is formed into a steel sheet by hot rolling and cold rolling, hot-dip galvannealing treatment is applied to the steel sheet, and the influence of the molten steel discharge angle α exerted on the occurrence of such surface defects is examined. In this test, surface defects of a hot-dipped galvannealed steel sheet are continuously measured using an online surface defect meter. Out of the surface defects, steel-making-caused defects (mold-flux-caused defects, and bubble-caused defects/inclusion-caused defects) are determined by the defective appearance, an SEM analysis, an ICP analysis or the like, and the number of defects per 100 m of a coil length is evaluated on the basis of the following criteria and is set as a surface defect index.

›DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION · 3 of 15

3: the number of defects being 0.30 or less

2: the number of defects being 0.30 or more and 1.00 or less

1: the number of defects being 1.00 or more

Hereinafter, the conditions (A), (B) described above are explained in order.

Condition (A)

In an embodiment of the present invention, molten steel containing chemical component where C: 0.003 mass % or less, and an X value defined by the following formula (1) satisfies X≦5000 is set as an object of casting.

X= 24989×[% Ti]+386147×[% S]+853354×[% O] (1)

Here, [% Ti]: Ti content in molten steel (mass %),

[% S]: S content in molten steel (mass %), and

[% O]: O content in molten steel (mass %).

Extremely low carbon steel whose C content is 0.003 mass or less is produced by melting through decarburization refining at an atmospheric pressure in a steel converter and decarburization refining under a reduced pressure in vacuum degassing facility such as an RH vacuum degassing device (hereinafter, referred to as “vacuum decarburization refining”). The decarburization refining does not advance unless the concentration of resolved oxygen in molten steel reaches a certain level and hence, a large amount of resolved oxygen remains in molten steel at the time of finishing decarburization refining. The cleanliness of steel is degraded when a large amount of resolved oxygen remains in this manner and hence, in a melting step of extremely low carbon steel, after vacuum decarburization refining is finished, metal Al is added to molten steel thus performing deoxidization treatment of molten steel. Due to this deoxidization treatment, the concentration of dissolved oxygen in molten steel is rapidly lowered and alumina is formed as a deoxidized product.

Alumina formed in this manner is coagulated during a period before molten steel is poured into the inside of a mold for casting thus forming alumina cluster. Most of non-metallic inclusions (hereinafter, simply referred to as “inclusions”) contained in molten steel is formed of alumina cluster. When such inclusions are poured into the inside of the mold together with molten steel and are caught in a solidified shell of a slab, the inclusions become a surface defect of an extreme low carbon steel slab thus lowering quality of the slab.

The inventors of the present invention have studied in detail the influence of chemical components of molten steel and flow velocity of molten steel on a front surface of a solidified shell exerted on the catching of inclusions in the solidified shell, and as a result of the study, the inventors have found that the catching of the inclusions or the like in the solidified shell can be effectively suppressed by setting the chemical components of the molten steel (extremely low carbon steel whose C content is 0.003 mass % or less) to satisfy X value≦5000 and by controlling a flow state of molten steel by the condition (B) described later thus adjusting flow velocity of molten steel at solidification interface to an appropriate value.

The above-mentioned X value indicates a scale of an attracting force in the direction toward the solidified shell due to an interfacial tension gradient which acts on inclusions intruded into a concentration boundary layer of solute elements (Ti, S, O) formed on a front surface of the solidified shell in the mold.

Hereinafter, reasons that the X value is induced are explained.

As described in publication “Iron and Steel Vol. 80 (1994)” p. 527, a force F which the inclusions receive in the direction toward the solidified shell due to an interfacial tension gradient K, that is, dσ/dx (σ: interfacial tension, x: distance), in the concentration boundary layer formed on the front surface of the solidified shell is expressed by the following formula (2)

F =−(8/3)×π R 2 K (2)

Here, F: force which inclusion receives (N)

π: circle ratio

R: radius of inclusion (m)

K: interfacial tension gradient (N/m 2 )

The interfacial tension gradient K is, as expressed by the following formula (3), the product of a change in interfacial tension due to solute element concentration and a concentration gradient of a component.

K=dσ/dx =( dσ/dc )×( dc/dx ) (3)

Here, σ: interfacial tension between molten steel and inclusion (N/m)

x: distance from solidification interface (m)

dσ/dc: change in interfacial tension due to solute element concentration (N/m/mass %)

dc/dx: concentration gradient of component (mass %/m)

Based on the theory on solidification, the concentration gradient dc/dx of the component under a condition where flow velocity of molten steel is present as in the case of the inside of the mold is expressed by the following formula (4).

dc/dx=−C o ×(1− Ko )×( V s /D )×exp[−V s ×( x −δ)/ D] (4)

Here, C o : solute element concentration in molten steel before casting (mass %)

K o : distribution coefficient of solute element (−)

V s : solidification speed (m/s)

D: diffusion coefficient of solute element in molten steel (m 2 /s)

δ: thickness of concentration boundary layer (m)

In the above-mentioned formula (4), by assigning δ to x, the concentration gradient (dc/dx) when x is δ (x=δ) can be obtained by the following formula (5).

dc/dx=−C o ×(1− K o )×( V s /D ) (5)

By substituting the formula (5) for the formula (3), the interfacial tension gradient K indicating the scale of a force which acts immediately after the inclusions intrude into the concentration boundary layer can be obtained by the following formula (6).

K =( dσ/dc )×[− C o ×(1− K o )×( V s /D )] (6)

Here, dσ/dc in the above-mentioned formula (6) is described in publication “Manual on physical property of molten iron and molten slag” (edited by The Iron and Steel Institute of Japan, 1972), and it is found that, out of chemical constitutional elements in extremely low carbon steel, the elements which largely influence a value of interfacial tension gradient K are Ti (titanium), S (sulfur) and O (oxygen=dissolved oxygen), and there arises no problem even when the value of interfacial tension gradient K which is calculated based on these active elements are used in examining the catching of the inclusions in the solidified shell.

›DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION · 4 of 15

Further, although the distribution coefficients K o of the solute elements are described in publication “Basis of Manual on Iron and Steel, third version” (edited by The Iron and Steel Institute of Japan, 1981, p. 194 and the like, for example, with respect to the distribution coefficients K O of the respective solute elements, values of the distribution coefficients K O of the respective solute elements described in “Iron and Steel, Vol. 80 (1994)” p. 534 are used.

Although the diffusion coefficient D is described in publication “Manual on physical property of molten iron and molten slag” (edited by The Iron and Steel Institute of Japan, 1992) and the like, for example, with respect to O and S, values described in “Iron and Steel Vol. 80 (1994)” p. 534 are used, and with respect to Ti, a value described in “Iron and Steel Vol. 83 (1997)” p. 566 is used.

Further, the solidification speed V S can also be obtained by the heat-transfer calculation. V S is calculated using 0.0002 m/s.

Values shown in Table 1 are used as values of dσ/dc, K o , D, V S of Ti (titanium), S (sulfur) and O (oxygen=dissolved oxygen).

Accordingly, by substituting a change dσ/dc in an interfacial tension of each solute element based on the concentration of the solute element, a distribution coefficient K O , a diffusion coefficient D, and a solidification speed V s in the mold of the above-mentioned respective solute elements for the above-mentioned formula (6), 24989×[% Ti], 386147×[% S] and 853354×[% O] can be obtained as respective interfacial tension gradients K due to Ti, S and O which act on alumina clusters in a concentration boundary layer, and the total of these interfacial tension gradients K constitute the X value.

By carrying out a casting test using molten steels having various compositions, the relationship between the above-mentioned X value and a catch rate of inclusions in a solidified shell is examined. In this test, the relationship between the X value and the catch rate of inclusions at respective flow velocities of molten steel at solidification interface is examined with respect to cases where the flow velocity of molten steel at solidification interface in the mold is 0.01 m/s, 0.08 m/s, 0.10 m/s and 0.15 m/s respectively. Here, the catch rate of inclusions is, as shown in the above-mentioned formula (7), a value which is obtained by dividing an inclusion index in the solidified shell by an inclusion index in molten steel, and is a value which indicates the frequency of catching inclusions per unit inclusion concentration.

α= I/A (7)

Here, α: catch rate of inclusions (−)

I: inclusion index in solidified shell (−)

A: inclusion index in molten steel (−)

Here, the inclusion index is a value which is obtained such that a long axis and a short axis of the inclusion are measured by an optical microscope, an area of the inclusion as an ellipsoidal body is calculated, and a value obtained by summing areas of the observed inclusions is divided by the measured area, and is an index which indicates the number of inclusions included in a unit measured area. The inclusion index of the molten steel can be calculated by measuring inclusions in a specimen sampled from molten steel.

The above-mentioned test result is shown in FIG. 4 . It is understood that when the X value is 5000 or less (X value≦5000), the catching of the inclusions in the solidified shell can be suppressed by imparting a certain level of flow velocity of molten steel at solidification interface. Further, such an advantageous effect becomes large when the X value is 4000 or less (X value≦4000), particularly, when the X value is 3000 or less (X value≦3000). Accordingly, by setting the chemical components of the molten steel such that the X value becomes 5000 or less (X value≦5000) (preferably to 4000 or less (X value≦4000), more preferably to 3000 or less (X value≦3000)), and by imparting flow velocity of molten steel at solidification interface under conditions (B) described later, it is possible to properly prevent the catching of the inclusions (particularly, fine inclusions) and the like in the solidified shell. Here, due to the limitation imposed on the chemical components in molten steel (extremely low carbon steel), usually, a substantial lower limit of the X value is set to approximately 2000.

With respect to the chemical composition of the molten steel produced by casting according to the present invention, provided that C content is 0.003 mass % or less and the X value is less than 5000 (X value≦5000), contained elements are not particularly limited. However, from a viewpoint of particularly effectively obtaining the advantageous effects of the present invention, as chemical components other than C, the steel preferably contains Si: 0.05 mass % or less, Mn: 1.0 mass % or less, P: 0.05 mass % or less, S: 0.015 mass % or less, Al: 0.010 to 0.075 mass %, Ti: 0.005 to 0.05 mass %, and also contains one or more kinds of components selected from a group consisting of Nb: 0.005 to 0.05 mass % when necessary, and contains Fe and unavoidable impurities as a balance.

Hereinafter, the reason of limiting the above-described chemical components is explained.

C deteriorates workability of a thin steel sheet when C content becomes high. Accordingly, C content is set to 0.003 mass % or less so that a steel having excellent elongation and deep drawing property as an IF steel (Interstitial-Free steel) can be acquired when a carbide forming element such as Ti or Nb is added to the steel.

Si is a solid solution strengthening element, when Si content is large, workability of the thin steel sheet is deteriorated. Further, an upper limit of Si content is preferably set to 0.05 mass % also by taking the influence of Si exerted on the surface treatment into consideration.

Mn is a solid solution strengthening element. Although the addition of Mn increases the strength of the steel, the addition of Mn lowers workability of steel on the other hand. Accordingly, an upper limit of Mn content is preferably set to 1.0 mass %.

›DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION · 5 of 15

P is a solid solution strengthening element, and the addition of P increases the strength of steel. However, when P content exceeds 0.05 mass %, workability and weldability are deteriorated and hence, an upper limit of P content is preferably set to 0.05 mass %.

S cause cracks at the time of hot rolling, and forms an A-based inclusion which lowers workability of a thin steel sheet. Accordingly, S content is preferably decreased as much as possible. Accordingly, an upper limit of the content of S is preferably set to 0.015 mass %.

Al functions as a deoxidizing agent, and Al content is preferably set to 0.010 mass % or more for acquiring a deoxidizing effect. However, the addition of Al exceeding a necessary amount pushes up the manufacturing cost and hence, Al content is preferably set to a value which falls within a range from 0.010 to 0.075 mass %.

Ti fixes C, N, S in the steel as precipitates, and the addition of Ti enhances workability and deep drawing property of steel. However, when Ti content is less than 0.005 mass %, the sufficient workability and deep drawing property enhancing effect cannot be acquired. On the other hand, Ti is also a precipitation strengthening element and hence, when the content of Ti exceeds 0.05 mass %, a steel sheet is hardened and workability is deteriorated. Accordingly, Ti content is preferably set to a value which falls within a range from 0.005 to 0.05 mass %.

Nb fixes C, N, S in the steel as precipitates in the same manner as Ti, and the addition of Nb enhances workability and deep drawing property of the steel. However, when Nb content is less than 0.005 mass %, the sufficient workability and deep drawing property enhancing effect cannot be acquired. On the other hand, Nb is also a precipitation strengthening element and hence, when Nb content exceeds 0.05 mass %, the steel sheet is hardened so that the deterioration of workability occurs. Accordingly, Nb content is preferably set to a value which falls within a range from 0.005 to 0.05 mass %.

Condition (B)

It is found that, in casting molten steel containing the above-described chemical components (X value≦5000), it is sufficient to optimize intensities of DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles corresponding to a slab width of a slab to be casted and a casting speed as follows (I), (II) basically.

(I) A “slab width-casting speed” region where a casting speed which is set corresponding to each slab width is relatively small: A throughput amount is relatively small and hence, a blow-off speed of molten steel from the molten steel discharge hole of the immersion nozzle is relatively small. Accordingly, the upward flow (reversed flow) also becomes small and hence, intensity of a DC magnetic field of the upper magnetic poles for braking the upward flow is set relatively small. On the other hand, to suppress inclusions and bubbles which follow the downward flow from getting in the downward molten steel, and also to prevent inclusions and bubbles from being caught in the solidified shell by changing the downward flow of molten steel upwardly and increasing flow velocity of molten steel at an solidification interface in a region above the lower magnetic field, the intensity of a DC magnetic field of the lower magnetic poles is set sufficiently large. By applying the above-mentioned DC magnetic fields to the molten steel under the condition where chemical components of the molten steel satisfies the condition that X value≦5000, it is possible to control turbulence energy of molten steel on top surface, flow velocity of molten steel at solidification interface, and flow velocity of molten steel on top surface to appropriate ranges and hence, it is possible to prevent the occurrence of bubble-caused defects, the inclusion-caused defects, and mold-flux-caused defects.

(II) A “slab width-casting speed” region where a casting speed which is set corresponding to each slab width is relatively large: A throughput amount is relatively large and hence, a blow-off speed of molten steel from the molten steel discharge hole of the immersion nozzle is also relatively large. Accordingly, the upward flow (reversed flow) also becomes large and hence, the intensity of a DC magnetic field of upper magnetic poles for braking the upward flow is set relatively large. On the other hand, in the same manner as the above-mentioned (I), to suppress non-metallic inclusions and bubbles which follow the downward flow from getting in the downward molten steel, and also to prevent non-metallic inclusions and bubbles from being caught in the solidified shell by changing the downward flow of molten steel upwardly and increasing the flow velocity of molten steel at solidification interface in a region above the lower magnetic field, the intensity of a DC magnetic field of the lower magnetic poles is set sufficiently large. By applying the above-mentioned DC magnetic fields to the molten steel under the condition where chemical components of the molten steel satisfy a condition that X value≦5000, it is possible to control turbulence energy of molten steel on top surface, flow velocity of molten steel at solidification interface, and flow velocity of molten steel on top surface to appropriate ranges and hence, it is possible to prevent the occurrence of bubble-caused defects and mold-flux-caused defects.

According to an exemplary method of the present invention, in addition to setting a casting speed to 0.75 m/min or more from a viewpoint of productivity, by optimizing the intensities of DC magnetic fields which are applied to the upper magnetic poles 3 a , 3 b and the lower magnetic poles 4 a , 4 b respectively corresponding to a slab width of a slab to be casted and a casting speed under the following conditions (X), (Y), it is possible to suppress the entrainment an catching of a mold flux in the solidified shell 5 and, at the same time, the catching of the minute bubbles (mainly, bubbles of an inert gas blown from the inner wall surface of the immersion nozzle) and inclusions in the solidified shell 5 which cause the mold-flux-caused defects, the bubble-caused defects and inclusion-caused defects.

›DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION · 6 of 15