Hot-pressed member and method for manufacturing same, and cold-rolled steel sheet for hot pressing and method for manufacturing same

1. A cold-rolled steel sheet for hot pressing, comprising:

a chemical composition containing, by mass %, C: 0.28% or more and less than 0.42%, Si: 1.5% or less, Mn: 1.0% or more and 2.2% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.50% or less, N: 0.005% or less, and Ti: 0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities,

a microstructure in which at least 10 Ti-based precipitates having a grain size of 0.005 μm or more and 0.10 μm or less are present on average per 100 μm 2 of a cross section parallel to a thickness direction of the steel sheet within a range of 100 μm in the thickness direction from a surface of the steel sheet; and

a standard deviation of Vickers hardness of 40 or less, the Vickers hardness being measured every 200 μm in the thickness direction within a range from a depth position of 100 μm from the surface of the steel sheet to a mid-thickness part.

2. The cold-rolled steel sheet for hot pressing according to claim 1 , wherein the chemical composition further contains, by mass %, at least one selected from Nb: 0.15% or less, B: 0.0050% or less, Mo: 0.50% or less, Cr: 0.50% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less, V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less, and W: 0.10% or less.

3. The cold-rolled steel sheet for hot pressing according to claim 1 , wherein the steel sheet comprises on a surface thereof an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

4. The cold-rolled steel sheet for hot pressing according to claim 2 , wherein the steel sheet comprises on a surface thereof an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

5. A hot-pressed member comprising:

a steel chemical composition containing, by mass %, C: 0.28% or more and less than 0.42%, Si: 1.5% or less, Mn: 1.0% or more and 2.2% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.50% or less, N: 0.005% or less, and Ti: 0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities;

a microstructure in which

at least 5 Ti-based precipitates having a grain size of 0.005 μm or more and 0.10 μm or less are present on average per 100 μm 2 of a cross section parallel to a thickness direction of the member within a range of 100 μm in a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 μm to 100 μm in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 μm are present on average in a prior austenite grain; and

a tensile strength of 1780 MPa or more.

6. The hot-pressed member according to claim 5 , wherein the steel chemical composition further contains, by mass %, at least one selected from the group consisting of Nb: 0.15% or less, B: 0.0050% or less, Mo: 0.50% or less, Cr: 0.50% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less, V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less, and W: 0.10% or less.

7. The hot-pressed member according to claim 5 , comprising, on a surface layer thereof, an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

8. The hot-pressed member according to claim 6 comprising, on a surface layer thereof, an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

9. A method for manufacturing the cold-rolled steel sheet for hot pressing as recited in claim 1 , the method comprising:

preparing a steel raw material comprising a chemical composition containing, by mass %, C: 0.28% or more and less than 0.42%, Si: 1.5% or less, Mn: 1.0% or more and 2.2% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.50% or less, N: 0.005% or less, and Ti: 0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities;

hot rolling the steel raw material to obtain a hot-rolled steel sheet under a set of conditions including a rolling reduction of 12% or more at a final pass of finish rolling, a rolling reduction of 15% or more at a pass immediately before the final pass, and a finisher delivery temperature of 860° C. to 950° C.;

after the hot rolling, subjecting the hot-rolled steel sheet to primary cooling whereby the hot-rolled steel sheet is cooled to a cooling end temperature of 700° C. or lower at a first average cooling rate of 70° C./s or higher up to the cooling end temperature;

after the primary cooling, subjecting the steel sheet to secondary cooling whereby the steel sheet is coiled at a coiling temperature of 520° C. or lower at a second average cooling rate of 5° C./s to 50° C./s up to the coiling temperature;

then pickling the coiled hot-rolled steel sheet, and then subjecting the steel sheet to a first heat treatment whereby the steel sheet is heated in a temperature range of 830° C. to 930° C. for 600 seconds or shorter;

then cold rolling the steel sheet to obtain a cold-rolled steel sheet, then subjecting the cold-rolled steel sheet to a second heat treatment whereby the cold-rolled steel sheet is heated to a temperature range of 720° C. to 850° C. at an average heating rate of 5° C./s to 20° C./s and subjected to 15 seconds to 600 seconds of soaking in the temperature range of 720° C. to 850° C.; and

after the second heat treatment, subjecting the cold-rolled steel sheet to a tertiary cooling whereby the cold-rolled steel sheet is cooled to a cooling end temperature of 600° C. or lower at a third average cooling rate of 5° C./s or higher.

10. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 9 , wherein

the chemical composition further contains, by mass %, at least one selected from the group consisting of Nb: 0.15% or less, B: 0.0050% or less, Mo: 0.50% or less, Cr: 0.50% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less, V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less, and W: 0.10% or less.

11. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 9 , the method further comprising:

after the tertiary cooling, applying an Al or Al alloy coating or plating treatment or a Zn or Zn alloy coating or plating treatment to a surface of the cold-rolled steel sheet.

12. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 1 in a temperature range of an Ac 3 transformation temperature to 1000° C.; and

then hot pressing the steel sheet.

13. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 10 , the method further comprising:

after the tertiary cooling, applying an Al or Al alloy coating or plating treatment or a Zn or Zn alloy coating or plating treatment to a surface of the cold-rolled steel sheet.

14. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 2 in a temperature range of an Ac 3 transformation temperature to 1000° C.; and

then hot pressing the steel sheet.

15. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 3 in a temperature range of an Ac 3 transformation temperature to 1000° C.; and

then hot pressing the steel sheet.

16. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 4 in a temperature range of an Ac 3 transformation temperature to 1000° C.; and

then hot pressing the steel sheet.