High-strength steel sheet and high-strength galvanized steel sheet excellent in shape fixability, and manufacturing method thereof

1. A steel sheet, comprising:

in mass %,

C: 0.075 to 0.300%;

Si: 0.30 to 2.5%;

Mn: 1.3 to 3.50%;

P: 0.001 to 0.030%;

S: 0.0001 to 0.0100%;

Al: 0.080 to 1.500%;

N: 0.0001 to 0.0100% ;

O: 0.0001 to 0.0100%; and

a balance composed of Fe and inevitable impurities, wherein:

a steel sheet structure contains a retained austenite phase of 5 to 20% in volume fraction in a range of ⅛ thickness to ⅜ thickness of the steel sheet;

an amount of solid-solution C contained in the retained austenite phase is 0.80 to 1.00% in mass %;

W Siγ defined as an amount of solid-solution Si contained in the retained austenite phase is 1.10 times or more W Si* defined as an average amount of Si in the range of ⅛ thickness to ⅜ thickness of the steel sheet;

W Mnγ defined as an amount of solid-solution Mn contained in the retained austenite phase is 1.10 times or more W Mm* defined as an average amount of Mn in the range of ⅛ thickness to ⅜ thickness of the steel sheet; and

when a frequency distribution is measured, by setting a plurality of measurement regions each having a diameter of 1 μm or less in the range of ⅛ thickness to ⅜ thickness of the steel sheet, with respect to a sum of a ratio between W Si defined as a measured value of an amount of Si in each of the plurality of measurement regions and W Si* being the average amount of Si and a ratio between W Al defined as a measured value of an amount of Al in each of the plurality of measurement regions and W Al* defined as an average amount of Al, a mode value of the frequency distribution is 1.95 to 2.05, and a kurtosis is 2.00 or more.

2. The steel sheet according to claim 1 , wherein:

the steel sheet structure further contains a ferrite phase of 10 to 75% in volume fraction, and either or both of a bainitic ferrite phase and a bainite phase of 10 to 50% in total; and

a tempered martensite phase is limited to less than 10% in volume fraction, and a fresh martensite phase is limited to 15% or less in volume fraction.

3. The steel sheet according to claim 1 , further comprising

in mass %,

one or two or more of

Ti: 0.005 to 0.150%,

Nb: 0.005 to 0.150%,

V: 0.005 to 0.150%,

B: 0.0001 to 0.0100%,

Mo: 0.01 to 1.00%,

W: 0.01 to 1.00%,

Cr: 0.01 to 2.00%,

Ni: 0.01 to 2.00%, and

Cu: 0.01 to 2.00%, and/or

one or two or more of Ca, Ce, Mg, Zr, Hf, and REM of 0.0001 to 0.5000% in total.

4. A galvanized steel sheet, comprising

the steel sheet according to claim 1 having a galvanized layer formed on a surface thereof.

5. The galvanized steel sheet according to claim 4 , wherein

a coating film made of a composite oxide containing a phosphorus oxide and/or phosphorus is formed on a surface of the galvanized layer.

6. A manufacturing method of the steel sheet according to claim 1 , comprising:

a hot-rolling step being a step of heating a slab containing:

in mass %,

C: 0.075 to 0.300%;

Si: 0.30 to 2.5%;

Mn: 1.3 to 3.50%;

P: 0.001 to 0.030%;

S: 0.0001 to 0.0100%;

Al: 0.080 to 1.500%;

N: 0.0001 to 0.0100%;

O: 0.0001 to 0.0100%; and

a balance composed of Fe and inevitable impurities to 1100° C. or more,

performing hot rolling on the slab in a temperature region in which a higher temperature between 850° C. and an Ar 3 temperature is set to a lower limit temperature,

performing first cooling of performing cooling in a range from a completion of rolling to a start of coiling at a rate of 10° C./second or more on average,

performing coiling in a range of coiling temperature of 600 to 750° C., and

performing second cooling of cooling the coiled steel sheet in a range of the coiling temperature to (the coiling temperature−100)° C. at a rate of 15° C./hour or less on average; and

a continuous annealing step of performing annealing on the steel sheet at a maximum heating temperature (Ac 1 +40)° C. to 1000° C. after the second cooling,

next performing third cooling at an average cooling rate of 1.0 to 10.0° C./second in a range of the maximum heating temperature to 700° C.,

next performing fourth cooling at an average cooling rate of 5.0 to 200.0° C./second in a range of 700° C. to 500° C., and

next performing retention process of retaining the steel sheet after being subjected to the fourth cooling for 30 to 1000 seconds in a range of 350 to 450° C.

7. The manufacturing method of the steel sheet according to claim 6 , further comprising

a cold-rolling step of performing pickling and then performing cold rolling at a reduction ratio of 30 to 75%, between the hot-rolling step and the continuous annealing step.

8. The manufacturing method of the steel sheet according to claim 6 , further comprising

a temper rolling step of performing rolling on the steel sheet at a reduction ratio of less than 10%, after the continuous annealing step.

9. A manufacturing method of a galvanized steel sheet, comprising

forming, after performing the retention process when manufacturing the steel sheet in the manufacturing method according to claim 6 , a galvanized layer on a surface of the steel sheet by conducting electrogalvanization.

10. A manufacturing method of a galvanized steel sheet, comprising

forming, between the fourth cooling and the retention process, or after the retention process when manufacturing the steel sheet in the manufacturing method according to claim 6 , a galvanized layer on a surface of the steel sheet by dipping the steel sheet in a galvanizing bath.

11. The manufacturing method of the galvanized steel sheet according to claim 10 , wherein

the steel sheet after being dipped in the galvanizing bath is reheated to 460 to 600° C., and retained for two seconds or more to make the galvanized layer to be alloyed.

12. The manufacturing method of the galvanized steel sheet according to claim 9 , wherein

after the galvanized layer is formed, a coating film made of a composite oxide containing either or both of a phosphorus oxide and phosphorus is given to a surface of the galvanized layer.

13. The manufacturing method of the galvanized steel sheet according to claim 11 , wherein

after the galvanized layer is alloyed, a coating film made of a composite oxide containing either or both of a phosphorus oxide and phosphorus is given to a surface of the alloyed galvanized layer.

14. The manufacturing method of a steel sheet according to claim 6 , wherein the slab comprises, in mass %,

one or two or more of

Ti: 0.005 to 0.150%,

Nb: 0.005 to 0.150%,

V: 0.005 to 0.150%,

B: 0.0001 to 0.0100%,

Mo: 0.01 to 1.00%,

W: 0.01 to 1.00%,

Cr: 0.01 to 2.00%,

Ni: 0.01 to 2.00%, and

Cu: 0.01 to 2.00%, and/or

one or two or more of Ca, Ce, Mg, Zr, Hf, and REM of 0.0001 to 0.5000% in total.