High-strength steel sheet and method for producing same

Patent metadata

Field	Value
Publication number	US-12258646-B2
Application number	US-202017593062-A
Country	US
Kind code	B2
Filing date	Feb 21, 2020
Priority date	Mar 11, 2019
Publication date	Mar 25, 2025
Grant date	Mar 25, 2025

Abstract

A high-strength steel sheet comprises: a chemical composition containing C, Si, Mn, P, S, Al, N, Mo, Cr, Ca, and Sb with a balance consisting of Fe and inevitable impurities, wherein [% Si], [% Mn], [% P], [% Mo], and [% Cr] satisfy a predetermined relationship; a steel microstructure that contains ferrite, hard phase, and retained austenite and in which a carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, an amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, a surface layer softening thickness is 5 μm or more and 150 μm or less, and a corresponding grain boundary frequency in a surface layer of the steel sheet after a high-temperature tensile test is 0.45 or less; and a tensile strength of 980 MPa or more.

First claim

The invention claimed is:

1. A high-strength steel sheet comprising:
a chemical composition containing, in mass %,
C: 0.120% or more and 0.250% or less,
Si: 0.80% or more and 2.00% or less,
Mn: 1.50% or more and 2.45% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 1.000% or less,
N: 0.0100% or less,
Mo: 0.500% or less,
Cr: 0.300% or less,
Ca: 0.0200% or less, and
Sb: 0.200% or less,
with a balance being Fe and inevitable impurities, wherein Mn eq calculated according to the following formula (1) is 2.40% or more and 3.40% or less,
Mn eq =0.26×[% Si]+[% Mn]+3.5×[% P]+2.68×[% Mo]+1.29×[% Cr]  (1)

where [% Si], [% Mn], [% P], [% Mo], and [% Cr] denotes content in mass % of Si, Mn, P, Mo, and Cr in steel, respectively, and is 0 in the case where each of Si, Mn, P, Mo, and Cr is not contained;
a steel microstructure in which an area ratio of ferrite is 15% or more and 55% or less, an area ratio of hard phase which is any of bainitic ferrite, bainite, tempered martensite, and quenched martensite is 40% or more and 85% or less, a volume fraction of retained austenite is 4% or more and 20% or less, a carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, an amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, a surface layer softening thickness is 5 μm or more and 150 μm or less, and a corresponding grain boundary frequency in a surface layer of the steel sheet after a high-temperature tensile test is 0.45 or less, where the high-temperature tensile test is performed such that after heating a test piece to 900° C. at 100° C./s, the test piece immediately gas-cooled at 40° C./s and after reaching 700° C., a tensile test immediately conducted at a crosshead rate of 50 mm/s until the test piece fractured; and
a tensile strength of 980 MPa or more.

2. The high-strength steel sheet according to claim 1 , wherein an average minor axis length of the retained austenite is 2.0 μm or less.

3. The high-strength steel sheet according to claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
Ti: 0.001% or more and 0.100% or less,
Nb: 0.001% or more and 0.100% or less,
V: 0.001% or more and 0.100% or less,
B: 0.0001% or more and 0.0100% or less,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 0.50% or less,
Sn: 0.001% or more and 0.200% or less,
Ta: 0.001% or more and 0.100% or less,
Mg: 0.0001% or more and 0.0200% or less,
Zn: 0.001% or more and 0.020% or less,
Co: 0.001% or more and 0.020% or less,
Zr: 0.001% or more and 0.020% or less, and
REM: 0.0001% or more and 0.0200% or less.

4. The high-strength steel sheet according to claim 1 , comprising
a coating or plating layer at a surface thereof.

5. The high-strength steel sheet according to claim 2 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
Ti: 0.001% or more and 0.100% or less,
Nb: 0.001% or more and 0.100% or less,
V: 0.001% or more and 0.100% or less,
B: 0.0001% or more and 0.0100% or less,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 0.50% or less,
Sn: 0.001% or more and 0.200% or less,
Ta: 0.001% or more and 0.100% or less,
Mg: 0.0001% or more and 0.0200% or less,
Zn: 0.001% or more and 0.020% or less,
Co: 0.001% or more and 0.020% or less,
Zr: 0.001% or more and 0.020% or less, and
REM: 0.0001% or more and 0.0200% or less.

6. The high-strength steel sheet according to claim 2 , comprising
a coating or plating layer at a surface thereof.

7. The high-strength steel sheet according to claim 3 , comprising
a coating or plating layer at a surface thereof.

8. The high-strength steel sheet according to claim 5 , comprising
a coating or plating layer at a surface thereof.

9. A method for producing the high-strength steel sheet of claim 1 , the method comprising:
subjecting a steel slab having the chemical composition according to claim 1 to hot rolling, to obtain a hot-rolled sheet;
thereafter subjecting the hot-rolled sheet to pickling;
thereafter subjecting the hot-rolled sheet to cold rolling under a condition that a rolling reduction in a final pass in the cold rolling is 1% or more and 5% or less and a rolling reduction in a pass immediately before the final pass is 5% or more and 30% or less, to obtain a cold-rolled sheet;
thereafter subjecting the cold-rolled sheet to annealing that involves heating to a heating temperature of 740° C. or more and 880° C. or less in an atmosphere with a dew point of −35° C. or more and thereafter cooling to a cooling stop temperature of 150° C. or more and 300° C. or less under a condition that an average cooling rate from the heating temperature to 500° C. is 10° C./s or more; and
thereafter reheating the cold-rolled sheet to a reheating temperature that is not less than a sum of the cooling stop temperature and 50° C. and is not more than 500° C., and holding the cold-rolled sheet at the reheating temperature for 10 s or more.

10. A method for producing the high-strength steel sheet of claim 3 , the method comprising:
subjecting a steel slab having the chemical composition according to claim 3 to hot rolling, to obtain a hot-rolled sheet;
thereafter subjecting the hot-rolled sheet to pickling;
thereafter subjecting the hot-rolled sheet to cold rolling under a condition that a rolling reduction in a final pass in the cold rolling is 1% or more and 5% or less and a rolling reduction in a pass immediately before the final pass is 5% or more and 30% or less, to obtain a cold-rolled sheet;
thereafter subjecting the cold-rolled sheet to annealing that involves heating to a heating temperature of 740° C. or more and 880° C. or less in an atmosphere with a dew point of −35° C. or more and thereafter cooling to a cooling stop temperature of 150° C. or more and 300° C. or less under a condition that an average cooling rate from the heating temperature to 500° C. is 10° C./s or more; and
thereafter reheating the cold-rolled sheet to a reheating temperature that is not less than a sum of the cooling stop temperature and 50° C. and is not more than 500° C., and holding the cold-rolled sheet at the reheating temperature for 10 s or more.

11. A method for producing high-strength steel sheet of claim 5 , the method comprising:
subjecting a steel slab having the chemical composition according to claim 8 to hot rolling, to obtain a hot-rolled sheet;
thereafter subjecting the hot-rolled sheet to pickling;
thereafter subjecting the hot-rolled sheet to cold rolling under a condition that a rolling reduction in a final pass in the cold rolling is 1% or more and 5% or less and a rolling reduction in a pass immediately before the final pass is 5% or more and 30% or less, to obtain a cold-rolled sheet;
thereafter subjecting the cold-rolled sheet to annealing that involves heating to a heating temperature of 740° C. or more and 880° C. or less in an atmosphere with a dew point of −35° C. or more and thereafter cooling to a cooling stop temperature of 150° C. or more and 300° C. or less under a condition that an average cooling rate from the heating temperature to 500° C. is 10° C./s or more; and
thereafter reheating the cold-rolled sheet to a reheating temperature that is not less than a sum of the cooling stop temperature and 50° C. and is not more than 500° C., and holding the cold-rolled sheet at the reheating temperature for 10 s or more.

12. The method for producing a high-strength steel sheet according to claim 9 , comprising
subjecting the cold-rolled sheet to

Assignees

• Jfe Steel Corp

Inventors

• Minami Hidekazu
• Tanaka Yuji
• Yokota Takeshi

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D8/00

  Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment (hardening articles or materials formed by forging or rolling with no further heating beyond that required for the formation C21D1/02) · CPC title

• C23C2/28

  Thermal after-treatment, e.g. treatment in oil bath · CPC title

• C23C2/0224

  Two or more thermal pretreatments · CPC title

• C23C2/024

  by cleaning or etching · CPC title