Hot-pressed member, cold-rolled steel sheet for hot pressing, and manufacturing methods therefor
Provided is a hot-pressed member that combines both high strength of 1850 MPa or more in TS and excellent delayed fracture resistance. A hot-pressed member comprises: a predetermined chemical composition; a microstructure in which a prior austenite average grain size is 8 μm or less, a volume fraction of martensite is 95% or more, and a volume fraction of granular carbide of 0.1 μm or more in grain size is 0.10% to 4.0%; a Ni diffusion region of 2.0 μm or more in a depth direction in a surface layer; and a tensile strength of 1850 MPa or more.
1. A hot-pressed member, comprising:
a chemical composition consisting of, in mass %,
C: 0.31% or more and less than 0.55%,
Si: 0.01% or more and 1.0% or less,
Mn: 1.0% or more and 2.5% or less,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01% or more and 0.50% or less,
N: 0.01% or less, and
Cu: 0.002% or more and 0.25% or less,
and optionally one or more one or more selected from the group consisting of
Mo: 0.005% or more and 0.35% or less,
Cr: 0.005% or more and 0.35% or less,
Nb: 0.001% or more and 0.05% or less,
Ti: 0.001% or more and 0.050% or less,
B: 0.0002% or more and 0.0050% or less,
Ca: 0.005% or less,
V: 0.05% or less,
Sb: 0.001% or more and 0.020% or less,
Ni: 0.50% or less, and
Sn: 0.50% or less,
with a balance consisting of Fe and inevitable impurities;
a microstructure in which a prior austenite average grain size is 8 μm or less, a volume fraction of martensite is 95% or more, and a volume fraction of granular carbide of 0.1 μm or more in grain size is 0.10% or more and 4.0% or less;
a Ni diffusion region of 2.0 μm or more in a depth direction in a surface layer; and
a tensile strength of 1850 MPa or more.
2. The hot-pressed member according to claim 1 , further comprising
a Ni-containing Zn or Zn alloy coating or plating layer at a surface thereof.
3. A cold-rolled steel sheet for hot pressing, comprising:
a chemical composition consisting of, in mass %,
C: 0.31% or more and less than 0.55%,
Si: 0.01% or more and 1.0% or less,
Mn: 1.0% or more and 2.5% or less,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01% or more and 0.50% or less,
N: 0.01% or less, and
Cu: 0.002% or more and 0.25% or less,
and optionally one or more selected from the group consisting of
Mo: 0.005% or more and 0.35% or less,
Cr: 0.005% or more and 0.35% or less,
Nb: 0.001% or more and 0.05% or less,
Ti: 0.001% or more and 0.050% or less,
B: 0.0002% or more and 0.0050% or less,
Ca: 0.005% or less,
V: 0.05% or less,
Sb: 0.001% or more and 0.020% or less,
Ni: 0.50% or less, and
Sn: 0.50% or less,
with a balance consisting of Fe and inevitable impurities; and
a microstructure in which a volume fraction of granular carbide of 0.1 μm or more in grain size is 1.0% or more and 25% or less.
4. The cold-rolled steel sheet for hot pressing according to claim 3 , further comprising
a Ni-containing coating or plating layer of 0.5 μm or more in thickness, at a surface thereof.
5. The cold-rolled steel sheet for hot pressing according to claim 4 , wherein the Ni-containing coating or plating layer is a Ni-containing Zn or Zn alloy coating or plating layer.
6. A manufacturing method for a cold-rolled steel sheet for hot pressing, comprising:
hot rolling a steel slab having the chemical composition according to claim 3 at a finisher delivery temperature of 840° C. or more and 950° C. or less, to obtain a hot-rolled steel sheet;
cooling the hot-rolled steel sheet to a cooling stop temperature of 700° C. or less at a primary average cooling rate of 55° C./s or more as primary cooling, thereafter cooling the hot-rolled steel sheet to a coiling start temperature of 650° C. or less at a secondary average cooling rate of 5° C./s or more and 60° C./s or less as secondary cooling, and thereafter subjecting the hot-rolled steel sheet to coiling and pickling;
cold rolling the hot-rolled steel sheet after the pickling, to obtain a cold-rolled steel sheet; and
heating the cold-rolled steel sheet to a soaking temperature of 550° C. or more and 680° C. or less at an average heating rate of 1° C./s or less, holding the cold-rolled steel sheet at the temperature for 60 min or more and 3000 min or less, and thereafter cooling the cold-rolled steel sheet to room temperature.
7. The manufacturing method for a cold-rolled steel sheet for hot pressing according to claim 6 , comprising
performing a coating or plating treatment on a surface of the cold-rolled steel sheet with a Ni-containing coating or plating.
8. A manufacturing method for a hot-pressed member, comprising
heating the cold-rolled steel sheet for hot pressing according to claim 3 to a temperature range of 850° C. or more and 1000° C. or less, and thereafter hot pressing the cold-rolled steel sheet.
9. A manufacturing method for a hot-pressed member, comprising
heating the cold-rolled steel sheet for hot pressing according to claim 4 to a temperature range of 850° C. or more and 1000° C. or less, and thereafter hot pressing the cold-rolled steel sheet.
10. A manufacturing method for a hot-pressed member, comprising
heating the cold-rolled steel sheet for hot pressing according to claim 5 to a temperature range of 850° C. or more and 1000° C. or less, and thereafter hot pressing the cold-rolled steel sheet.