Steel sheet and method for producing same

A method of production of steel sheet, which method of production of steel sheet comprising

• a hot rolling step (a) of hot rolling a slab having a chemical composition comprising, by mass %,
• C:
• 0.15 to 0.35%,
• Si:
• 0.01 to 1.20%,
• Mn:
• 1.00 to 3.50%,
• Al:
• 0.300 to 1.500%,
• Ti:
• 0.001 to 0.100%,
• B:
• 0.0005 to 0.0050%,
• P:
• 0.050% or less,
• S:
• 0.0100% or less,
• N:
• 0.010% or less,
• O:
• 0.0100% or less,
• Cr:
• 0 to 1.00%,
• Mo:
• 0 to 1.00%,
• Cu:
• 0 to 1.00%,
• Ni:
• 0 to 1.00%,
• Co:
• 0 to 1.00%,
• W:
• 0 to 1.00%,
• Sn:
• 0 to 1.00%,
• Sb:
• 0 to 0.50%,
• Nb:
• 0 to 0.200%,
• V:
• 0 to 1.00%,
• As:
• 0 to 0.10%,
• Zn:
• 0 to 1.00%,
• Ca:
• 0 to 0.0100%,
• Mg:
• 0 to 0.0100%,
• Zr:
• 0 to 0.0100%,
• Hf:
• 0 to 0.0100%,
• Bi:
• 0 to 0.0100%,
• REM:
• 0 to 0.015%, and
• balance:
• Fe and impurities at a 850 to 950° C. finish rolling end temperature to obtain hot rolled steel sheet, then cooling the hot rolled steel sheet down to 450 to 680° C. and coiling the hot rolled steel sheet,
• a pickling step (b) of pickling the steel sheet obtained at the hot rolling step (a),
• a cold rolling step (c) of cold rolling the steel sheet obtained by the pickling step (b) by a 30 to 75% rolling reduction to obtain a cold rolled steel sheet,
• a heat treatment step (d) of heat treating the steel sheet obtained at the cold rolling step (c), and
• a grinding step (e), before or after the pickling step (b), of using a rotary type grinding brush containing an abrasive to grind front and back surfaces of the steel sheet obtained at the hot rolling step (a) or the steel sheet obtained at the pickling step (b),
• in the hot rolling step (a), finish rolling comprises three passes or more, a rolling reduction of respective passes of final three passes of the finish rolling is 20% or more, a time between passes is within 1 second, an entry side steel sheet temperature before the final three passes is 1000° C. or less, and a time from completion of a final pass to start of cooling is within 3 seconds,
• in the grinding step (e), a rotational speed R (rpm) of the grinding brush, a diameter D (m) of the grinding brush, and a running speed V (m/min) of the steel sheet satisfy the following formula (5),
• the heat treatment step (d) further provided with
• a step (d-1) of heating the steel sheet obtained at the cold rolling step (c) from 650° C. to a maximum heating temperature of Ac1+50° C. or more and 950° C. or less by a 0.5 to 500° C./s average heating speed,
• a step (d-2) of holding the steel sheet obtained at the cold rolling step (c) at the maximum heating temperature for 1 second to 300 seconds,
• a step (d-3) of cooling the steel sheet obtained at the cold rolling step (c) down to Ms point−30° C. or less, at which step, cooling from 700° C. to 500° C. by a 10° C./s or more average cooling speed, and
• a step (d-4) of holding the steel sheet obtained at the cold rolling step (c) at 300 to 450° C. for 100 to 600 seconds,
• at step (d-1), an atmosphere in surroundings of the steel sheet obtained at the cold rolling step (c) having a steam partial pressure pH2O and hydrogen partial pressure pH2 satisfying a following formula (6):
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