Steel sheet suitable for impact absorbing member and method for its manufacture

1. A method for manufacturing a steel sheet having a microstructure containing, by area %, bainite: more than 50%, martensite: at least 3% and at most 30%, and retained austenite: at least 3% and at most 15%, the remainder consisting of ferrite having an average grain diameter of less than 5 μm; and mechanical properties in which the product of uniform elongation and hole expansion ratio is at least 300% 2 , and an effective flow stress when 5% true strain is applied is at least 900 MPa, comprising following steps (A) to (C):

(A) a hot rolling step in which a slab having a chemical composition containing, by mass %, C: at least 0.08% and at most 0.30%, Mn: at least 1.5% and at most 3.5%, Si+Al: at least 0.50% and at most 3.0%, P: at most 0.10%, S: at most 0.010%, N: at most 0.010%, Cr: 0 to at most 0.5%, Mo: 0 to at most 0.5%, B: 0 to at most 0.01%, Ti: 0 to less than 0.04%, Nb: 0 to less than 0.030%, V: 0 to less than 0.5%, Ca: 0 to at most 0.010%, Mg: 0 to at most 0.010%, REM: 0 to at most 0.050%, and Bi: 0 to at most 0.050%, the remainder being Fe and impurities is subjected to multi-pass hot rolling in which rolling is completed at a temperature of at least Ar 3 point, the obtained steel sheet is cooled to a temperature range of at least 620° C. and at most 720° C. under a cooling condition in which cooling is started within 0.4 seconds after completion of rolling, and an average cooling rate is at least 600° C./sec, as well as a time required for cooling from completion of rolling in a rolling pass which is two passes before the last rolling pass to 720° C. is at most 4 seconds, and the steel sheet is held in the temperature range for at least 1 second and at most 10 seconds, thereafter being cooled to a temperature range of at least 300° C. and at most 610° C. at an average cooling rate of at least 10° C./sec and at most 100° C./sec, and being wound up to obtain a hot-rolled steel sheet;

(B) a cold rolling step in which the hot-rolled steel sheet obtained by the hot rolling step is subjected to cold rolling of a rolling reduction of at least 40% and at most 70% to be formed into a cold-rolled steel sheet; and

(C) an annealing step in which the cold-rolled steel sheet obtained by the cold rolling step is subjected to a heat treatment in which the steel sheet is held in a temperature range of at least (Ac 3 point−30° C.) and at most (Ac 3 point +100° C.) for at least 10 seconds and at most 300 seconds, and then is cooled at an average cooling rate of at least 15° C./sec in a temperature range of at least 500° C. and at most 650° C., thereafter being held in a temperature range of at least 300° C. and at most 500° C. for at least 30 seconds and at most 3000 seconds.

2. The method set forth in claim 1 , wherein the microstructure satisfies the following formulas (1) and (2):

1.2≦H M0 /H B0 ≦1.6  (1)

0.9≦{(H M10 /H M0 )/(H B10 /H B0 )}≦1.3  (2)

where,

H M0 : initial average nano hardness of the martensite,

H B0 : initial average nano hardness of the bainite,

H M10 : average nano hardness of the martensite after 10% tensile deformation,

H B10 : average nano hardness of the bainite after 10% tensile deformation.

3. The method set forth in claim 1 , wherein

the chemical composition contains one or more selected from Cr: at least 0.1% and at most 0.5%, Mo: at least 0.1% and at most 0.5%, and B: at least 0.0010% and at most 0.01%.

4. The method set forth in claim 1 , wherein

the chemical composition contains one or more selected from Ti: at least 0.01% and less than 0.04%, Nb: at least 0.005% and less than 0.030%, and V: at least 0.010% and less than 0.5%.

5. The method set forth in claim 1 , wherein

the chemical composition contains one or more selected from Ca: at least 0.0008% and at most 0.010%, Mg: at least 0.0008% and at most 0.010%, REM: at least 0.0008% and at most 0.050%, and Bi: at least 0.0010% and at most 0.050%.