Hot-pressed member, cold-rolled steel sheet for hot-pressed member, and method for producing the same

1. A hot-pressed member comprising:

a steel sheet; and

a surface layer disposed on a surface of the steel sheet,

the steel sheet having a chemical composition containing, in mass%,

C: 0.28% or greater and less than 0.50%,

Si: 0.01% or greater and 1.5% or less,

Mn: 1.0% or greater and 2.2% or less,

P: 0.05% or less,

S: 0.005% or less,

Al: 0.01% or greater and 0.50% or less,

N: 0.01% or less,

Sb: 0.001% or greater and 0.020% or less, and

one or more selected from

Mo: 0.005% or greater and 0.35% or less,

Cr: 0.005% or greater and 0.35% or less,

Nb: 0.001% or greater and 0.05% or less,

Ti: 0.001% or greater and 0.05% or less,

B: 0.0002% or greater and 0.0050% or less,

Ca: 0.005% or less,

V: 0.05% or less,

Cu: 0.50% or less,

Ni: 0.50% or less, and

Sn: 0.50% or less,

with a balance being Fe and incidental impurities,

wherein, in the hot-pressed member,

the steel sheet has a microstructure in which a prior austenite average grain diameter is 8 μm or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 μm of the surface of the steel sheet of the hot-pressed member,

a Ni diffusion region having a thickness of 0.5 μm or greater exists in the surface layer,

a standard deviation of Vickers hardness values of the hot-pressed member is 35 or less,

Mndif, a degree of Mn segregation in a sheet thickness direction, obtained by the following equation, is 0.20 mass% or less:

Mndif (mass%) =Mnmax (mass%)−Mnmin (mass%) ≤0.20, where

Mnmax (mass%) is a maximum value of amounts of Mn measured by EPMA line analysis in a sheet thickness direction, and

Mnmin (mass%) is a minimum value of the amounts of Mn measured by EPMA line analysis in a sheet thickness direction, and

the hot-pressed member has a tensile strength of 1780 MPa or greater, and

the hot-pressed member has satisfactory delayed fracture resistance if a bolt-tightened test piece, having a bolt secured to the hole of the nut of a welded body, upon immersion in a solution of hydrochloric acid (pH=2.0) at room temperature for 100 hours or more, displayed no fracture under either of the following loads:

(i) a load of Fs at which the nut was peeled off from the bolt-tightened test piece when an indentation-peel test was conducted in accordance with JIS B 1196: 2001, or

iii) a load of 0.7xFs,

wherein the bolt-tightened test piece was prepared by welding an M8 bolt in a hole of 9 mm diameter in the center of a test piece measuring 50 mm×150 mm cut from the hot-pressed member, and

wherein welding was performed at welding conditions of an electrode force of 3200 N, a welding time of 5 cycles (50 Hz), a welding current of 12 kA, and a holding time of 10 cycles (50 Hz).

2. A cold-rolled steel sheet for the hot-pressed member of claim 1 ,

the cold-rolled steel sheet comprising a Ni-containing coating layer disposed on a surface of the steel sheet,

the steel sheet having a chemical composition containing, in mass%,

C: 0.28% or greater and less than 0.50%,

Si: 0.01% or greater and 1.5% or less,

Mn: 1.0% or greater and 2.2% or less,

P: 0.05% or less,

S: 0.005% or less,

Al: 0.01% or greater and 0.50% or less,

N: 0.01% or less,

Sb: 0.001% or greater and 0.020% or less, and

one or more selected from

Mo: 0.005% or greater and 0.35% or less,

Cr: 0.005% or greater and 0.35% or less,

Nb: 0.001% or greater and 0.05% or less,

Ti: 0.001% or greater and 0.05% or less,

B: 0.0002% or greater and 0.0050% or less,

Ca: 0.005% or less,

V: 0.05% or less,

Cu: 0.50% or less,

Ni: 0.50% or less, and

Sn: 0.50% or less,

with a balance being Fe and incidental impurities,

wherein

the steel sheet has a microstructure in which ferrite having an average grain aspect ratio of 2.5 or less is present in a volume fraction of 20% or greater,

a standard deviation of Vickers hardness values of the steel sheet is 30 or less,

Mndif (mass%) in a sheet thickness direction =Mnmax (mass%) - Mnmin (mass%) 0.20, where

Mndif (mass%) is a degree of Mn segregation,

Mnmax (mass%) is a maximum value of amounts of Mn measured by EPMA line analysis in a sheet thickness direction, and

Mnmin (mass%) is a minimum value of the amounts of Mn measured by EPMA line analysis in a sheet thickness direction, and

the Ni-containing coating layer has a thickness of 0.5 μm or greater.

3. A method for producing the hot-pressed member according to claim 1 , the method comprising:

heating the cold-rolled steel sheet for a hot-pressed member in a temperature range of an Ac 3 transformation temperature to 1100° C.; and

subsequently hot-pressing the steel sheet,

wherein the cold-rolled steel sheet comprises a Ni-containing coating layer disposed on a surface of the steel sheet,

the steel sheet having a chemical composition containing, in mass%,

C: 0.28% or greater and less than 0.50%,

Si: 0.01% or greater and 1.5% or less,

Mn: 1.0% or greater and 2.2% or less,

P: 0.05% or less,

S: 0.005% or less,

Al: 0.01% or greater and 0.50% or less,

N: 0.01% or less,

Sb: 0.001% or greater and 0.020% or less, and

one or more selected from

Mo: 0.005% or greater and 0.35% or less,

Cr: 0.005% or greater and 0.35% or less,

Nb: 0.001% or greater and 0.05% or less,

Ti: 0.001% or greater and 0.05% or less,

B: 0.0002% or greater and 0.0050% or less,

Ca: 0.005% or less,

V: 0.05% or less,

Cu: 0.50% or less,

Ni: 0.50% or less, and

Sn: 0.50% or less,

with a balance being Fe and incidental impurities,

wherein

the steel sheet has a microstructure in which ferrite having an average grain aspect ratio of 2.5 or less is present in a volume fraction of 20% or greater,

a standard deviation of Vickers hardness values of the steel sheet is 30 or less, the Ni-containing coating layer has a thickness of 0.5 μm or greater.

4. A method for producing the cold-rolled steel sheet for a hot-pressed member according to claim 2 , the method comprising:

forming a continuously cast slab from molten steel having the chemical composition according to claim 2 , cooling the continuously cast slab to 850° C. at an average cooling rate of 20° C/hr or greater, and subsequently cooling the continuously cast slab to 650° C. at an average cooling rate of 150° C/hr or less;

subsequently reheating the continuously cast slab and then hot-rolling the continuously cast slab under conditions in which a reduction ratio for a final pass of finish rolling is 10% or greater, a reduction ratio for a pass preceding the final pass is 12% or greater, and a finishing delivery temperature is 850 to 950° C.;

after the hot-rolling, performing primary cooling, in which a resulting steel sheet is cooled to a cooling stop temperature of 700° C. or less at a first average cooling rate of 55° C/s or greater, and after the primary cooling, performing a secondary cooling, in which the resulting steel sheet is cooled to a coiling temperature of 650° C. or less at a second average cooling rate of 5 to 50° C/s;

subsequently coiling and pickling the resulting steel sheet;

subsequently cold-rolling the resulting steel sheet;

subsequently heating the resulting steel sheet to a temperature range of 600 to 820° C. at an average heating rate of 30° C/s or less, then holding the resulting steel sheet in a soaking temperature range of 600 to 820° C. for 20 seconds or more and 30000 seconds or less, and subsequently cooling the resulting steel sheet to room temperature; and

subsequently subjecting the resulting steel sheet to a coating process for applying a Ni-containing coating layer to a surface of the steel sheet.