High-strength steel sheet and galvanized steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, and methods of producing the steel sheets

A high-strength cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, according to claim 1, further comprising, in mass %, B: 0.0001 to less than 0.010%.

A high-strength cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, according to claim 1, further comprising, in mass %, one or two or more of:nCr: 0.01 to 1.0%,Ni: 0.01 to 1.0%,Cu: 0.01 to 1.0%, andMo: 0.01 to 1.0%.

A high-strength cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, according to claim 1, further comprising, in mass %, one or two or more of Nb, Ti and V in a total of 0.001 to 0.14%.

A high-strength cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, according to claim 1, further comprising, in mass %, one or two or more of Ca, Ce, Mg, and REM in a total of 0.0001 to 0.5%.

A high-strength galvanized cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, comprising a steel sheet in accordance with claim 1 having a zinc-based plating on its surface.

A method of producing a high-strength cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, characterized in heating a cast slab having a chemical composition in accordance with claim 1, directly or after once cooling, to 1,050° C. or greater; completing hot rolling at or above Ar3 transformation point; coiling in a temperature range of 400 to 670° C.; pickling followed by cold rolling reduction of 40 to 70%; during passage through a continuous annealing line, heating at a heating rate (HR1) of 2.5 to 15° C./sec between 200 and 600° C. and a heating rate (HR2) of (0.6×HR1)° C./sec or less between 600° C. and maximum heating temperature; annealing with the maximum heating temperature set at 760° C. to Ac3 transformation point; cooling between 630° C. and 570° C. at an average cooling rate of 3° C./sec or greater; and holding in a temperature range of 450° C. to 300° C. for 30 sec or greater.

A method of producing a high-strength hot-dip galvanized cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, characterized in heating a cast slab having a chemical composition in accordance with any of claim 1, directly or after once cooling, to 1,050° C. or greater; completing hot rolling at or above Ar3 transformation point; coiling in a temperature range of 400 to 670° C.; pickling followed by cold rolling reduction of 40 to 70%; during passage through a continuous hot-dip galvanizing line, heating at a heating rate (HR1) of 2.5 to 15° C./sec between 200 and 600° C. and a heating rate (HR2) of (0.6×HR1)° C./sec or less between 600° C. and maximum heating temperature; annealing with the maximum heating temperature set at 760° C. to Ac3 transformation point; cooling between 630° C. and 570° C. at an average cooling rate of 3° C./sec or greater to a temperature of (galvanizing bath temperature −40)° C. to (galvanizing bath temperature +50)° C.; and holding in a temperature range of (galvanizing bath temperature +50)° C. to 300° C. for 30 sec or greater either before or after or both before and after immersion in the galvanizing bath.

A method of producing a high-strength alloyed hot-dip galvanized cold rolled steel sheet having very good balance between hole expansibility and ductility, and also excellent in fatigue resistance, characterized in heating a cast slab having a chemical composition in accordance with claim 1, directly or after once cooling, to 1,050° C. or greater; completing hot rolling at or above Ar3 transformation point; coiling in a temperature range of 400 to 670° C.; pickling followed by cold rolling reduction of 40 to 70%; during passage through a continuous hot-dip galvanizing line, heating at a heating rate (HR1) of 2.5 to 15° C./sec between 200 and 600° C. and a heating rate (HR2) of (0.6×HR1)° C./sec or less between 600° C. and maximum heating temperature; annealing with the maximum heating temperature set at 760° C. to Ac3 transformation point; cooling between 630° C. and 570° C. at an average cooling rate of 3° C./sec or greater to a temperature of (galvanizing bath temperature −40)° C. to (galvanizing bath temperature +50)° C.; conducting alloying treatment at a temperature of 460 to 540° C. as required, and holding in a temperature range of (galvanizing bath temperature +50)° C. to 300° C. for 30 sec or greater before or after immersion in the galvanizing bath or after alloying treatment or in total.