High-strength steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-12392006-B2
Application number	US-202218282299-A
Country	US
Kind code	B2
Filing date	Mar 15, 2022
Priority date	Mar 31, 2021
Publication date	Aug 19, 2025
Grant date	Aug 19, 2025

Abstract

The high-strength steel sheet has a chemical composition with MSC value in the range of 2.7% to 3.8% by mass defined by a specific formula, wherein the high-strength steel sheet has a microstructure including specific microstructures in a surface layer region extending from the surface of the steel sheet to a depth of 100 μm and in an inner region other than the surface layer region. The high-strength steel sheet has the maximum height of the surface roughness of 30 μm or less, a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio of 10 7 -cycle plane bending fatigue strength to tensile strength (fatigue limit ratio) of 0.45 or more.

First claim

The invention claimed is:

1. A high-strength steel sheet comprising:
a chemical composition containing, in mass %:
C: 0.05% to 0.20%,
Si: 0.6% to 1.2%,
Mn: 1.3% to 3.7%,
P: 0.10% or less,
S: 0.03% or less,
Al: 0.001% to 2.0%,
N: 0.01% or less,
O: 0.01% or less,
B: 0.0005% to 0.010%, the remainder being Fe and incidental impurities, and
MSC value defined by the following formula (1) in the range of 2.7% to 3.8% by mass;
a microstructure in a surface layer region extending from a surface of the steel sheet to a depth of 100 μm containing 70% by area or more of upper bainite and 2% by area or more of fresh martensite and/or retained austenite in total, the upper bainite having an average grain size of 7 μm or less, the fresh martensite and/or retained austenite having an average grain size of 4 μm or less, and the fresh martensite and/or retained austenite having a number density of 100/mm 2 or more; and
a microstructure in an inner region other than the surface layer region containing 70% by area or more of upper bainite and 3% by area or more of fresh martensite and/or retained austenite in total,
wherein the high-strength steel sheet has:
a maximum height of a surface roughness of 30 μm or less, and;
a tensile strength of 980 MPa or more, and a uniform elongation of 6% or more, and a ratio of 10 7 -cycle plane bending fatigue strength to tensile strength (fatigue limit ratio) of 0.45 or more,
MSC (% by mass)=Mn+0.2×Si+1.7×Cr+2.5×Mo  (1)

where each element symbol in the formula (1) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

2. The high-strength steel sheet according to claim 1 , wherein the area of fresh martensite and/or retained austenite in total in the surface layer region is smaller than the area of fresh martensite and/or retained austenite in total in the inner region.

3. The high-strength steel sheet according to claim 1 , wherein the chemical composition further contains at least one selected from following groups A to D consisting of:
Group A: in mass %, at least one of
Cr: 1.0% or less and
Mo: 1.0% or less;

Group B: in mass %, at least one of
Cu: 2.0% or less,
Ni: 2.0% or less,
Ti: 0.3% or less,
Nb: 0.3% or less, and
V: 0.3% or less;

Group C: in mass %,
Sb: 0.005% to 0.020%; and

Group D: in mass %, at least one of
Ca: 0.01% or less,
Mg: 0.01% or less, and
REM: 0.01% or less.

4. The high-strength steel sheet according to claim 2 , wherein the chemical composition further contains at least one selected from following groups A to D consisting of:
Group A: in mass %, at least one of
Cr: 1.0% or less and
Mo: 1.0% or less;

Group B: in mass %, at least one of
Cu: 2.0% or less,
Ni: 2.0% or less,
Ti: 0.3% or less,
Nb: 0.3% or less, and
V: 0.3% or less;

Group C: in mass %,
Sb: 0.005% to 0.020%; and

Group D: in mass %, at least one of
Ca: 0.01% or less,
Mg: 0.01% or less, and
REM: 0.01% or less.

5. A method for manufacturing the high-strength steel sheet according to claim 1 , comprising:
heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;
hot rolling including rough rolling and finish rolling the steel material after the heating into a hot-rolled steel sheet,
while performing descaling at least twice between start of the rough rolling and start of the finish rolling including performing the descaling with a water pressure of 15 MPa or more once or more within 5 seconds before the start of the finish rolling,
the finish rolling being performed under conditions of a finishing temperature: (RC2-50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;
cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate: 5° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;
coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and
cooling the hot-rolled steel sheet to 100° C. or less with an average cooling rate of 20° C./s or less,
wherein RC1, RC2, and Trs are represented by the following formulae (2), (3), and (4), respectively,
RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (2)

RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (3)

Trs(° C.)=500−450×C−35×Mn−15×Cr−10×Ni−20×Mo  (4)

where each element symbol in the formulae (2), (3), and (4) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

6. A method for manufacturing the high-strength steel sheet according to claim 2 , comprising:
heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;
hot rolling including rough rolling and finish rolling the steel material after the heating into a hot-rolled steel sheet while performing descaling at least twice between start of the rough rolling and start of the finish rolling including performing the descaling with a water pressure of 15 MPa or more once or more within 5 seconds before the start of the finish rolling, the finish rolling being performed under conditions of a finishing temperature: (RC2-50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;
cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate: 5° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;
coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and
cooling the hot-rolled steel sheet to 100° C. or less with an average cooling rate of 20° C./s or less,
wherein RC1, RC2, and Trs are represented by the following formulae (2), (3), and (4), respectively,
RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (2)

RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (3)

Trs(° C.)=500−450×C−35×Mn−15×Cr−10×Ni−20×Mo  (4)

where each element symbol in the formulae (2), (3), and (4) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

7. A method for manufacturing the high-strength steel sheet according to claim 3 , comprising:
heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;
hot rolling including rough rolling and finish rolling the steel material after the heating into a hot-rolled steel sheet while performing descaling at least twice between start of the rough rolling and start of the finish rolling including performing the descaling with a water pressure of 15 MPa or more once or more within 5 seconds before the start of the finish rolling, the finish rolling being performed under conditions of a finishing temperature: (RC2-50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;
cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate: 5° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;
coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and
cooling the hot-rolled steel sheet to 100° C. or less with an average cooling rate of 20° C./s or less,
wherein RC1, RC2, and Trs are represented by

Assignees

• Jfe Steel Corp

Inventors

• DOAN Thi Huyen
• Hasegawa Hiroshi
• Kimura Hideyuki

Classifications

• C21D8/02

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

• C22C38/60

  containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• C22C38/32

  with boron · CPC title

• C22C38/28

  with titanium or zirconium · CPC title