US 11,078,552 B2 · High-strength steel sheet and method for manufacturing the same

Abstract

A high-strength steel sheet exhibiting excellent ductility and stretch-flangeability, and a method for manufacturing such a high-strength steel sheet. The high-strength steel sheet has a chemical composition including specific proportions of components in which C/Mn is 0.08 to 0.20, the balance being iron and inevitable impurities, and includes microstructures including, in terms of area fraction relative to all the microstructures, 40% to 70% total of ferrite and bainitic ferrite, 5% to 35% martensite and 5% to 30% retained austenite. The proportion of martensite (including retained austenite) adjacent to bainitic ferrite is not less than 60% of all martensite (including retained austenite). The proportion of 4.0 GPa and smaller differences in microhardness measured at 0.5 μm intervals is not less than 70%. The proportion of microstructures with 8.0 GPa or smaller microhardness is not less than 85% of all the microstructures.

Claims15

Dependent claims

A high-strength steel sheet having a chemical composition comprising:

C:
0.10% to 0.35%, by mass %,
Si:
0.5% to 2.0%, by mass %,
Mn:
1.5% to 3.0%, by mass %,
P:
not more than 0.050%, by mass %,
S:
not more than 0.0100%, by mass %,
Al:
0.001% to 1.00%, by mass %,
N:
0.0005% to 0.0200%, by mass %, and
iron and inevitable impurities,
wherein:
a ratio of C/Mn is from 0.08 to 0.20,
the steel sheet has microstructures including, in terms of area fraction relative to all the microstructures:
less than 10% ferrite,
40% to 70% total of ferrite and bainitic ferrite,
5% to 35% martensite, and
5% to 30% retained austenite,
a proportion of martensite including retained austenite adjacent to bainitic ferrite being not less than 60% of all martensite including retained austenite,
a proportion of 4.0 GPa and smaller differences in microhardness measured at 0.5 μm intervals being not less than 70% of all indentations, and
a proportion of microstructures with 8.0 GPa or smaller microhardness being not less than 85% of all the microstructures.

22 more elements

A method for manufacturing a high-strength steel sheet, the method comprising:

providing a steel sheet having a chemical composition comprising:
C:
0.10% to 0.35%, by mass %,
Si:
0.5% to 2.0%, by mass %,
Mn:
1.5% to 3.0%, by mass %,
P:
not more than 0.050%, by mass %,
S:
not more than 0.0100%, by mass %,
Al:
0.001% to 1.00%, by mass %,
N:
0.0005% to 0.0200%, by mass %, and
iron and inevitable impurities,
wherein:
a ratio of C/Mn is from 0.08 to 0.20, and
the steel sheet includes microstructures in which a total of bainite and martensite, both having a grain size of 1 μm to 25 μm and a block interval of not more than 3 μm, represents not less than 80% of all the microstructures,
heating the steel sheet to 700° C. at an average heating rate of not less than 15° C./sec,
holding the steel sheet at a temperature in the range of 740° C. to 860° C. for 60 seconds to 600 seconds,
cooling the steel sheet to a temperature in the range of 350° C. to 550° C. at an average cooling rate of not more than 50° C./sec, and
after cooling the steel sheet, subsequently holding the steel sheet at a temperature in the range of 350° C. to 550° C. for 30 seconds to 1200 seconds,
where the high-strength steel sheet has microstructures including less than 10% ferrite in terms of area fraction relative to all the microstructures.

21 more elements

Dependencies are inferred from the claim text.

High-strength steel sheet and method for manufacturing the same

Life of this patent

High-strength steel sheet and method for manufacturing the same

Life of this patent

Abstract

Claims15