High-strength hot-rolled steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-12590344-B2
Application number	US-202117910421-A
Country	US
Kind code	B2
Filing date	Mar 17, 2021
Priority date	Mar 25, 2020
Publication date	Mar 31, 2026
Grant date	Mar 31, 2026

Abstract

After low-temperature finish rolling has been performed on a steel material having a certain chemical composition, cooling is performed at an average cooling rate of 10° C./s or higher to a temperature of 500° C., rapid cooling is further performed in a temperature range from a Ms temperature to a temperature of (Ms temperature-200° C.), coiling is thereafter performed in a low temperature range of 250° C. or lower, and the coiled steel sheet is uncoiled and further subjected to rolling with a certain amount or more of rolling load per unit width and the like. Consequently, it is possible to obtain a high-strength hot-rolled steel sheet having a microstructure including, in terms of area fraction, 95% or more of a martensite phase at a position located at ¼ of the thickness of the steel sheet, in which an average aspect ratio of prior austenite grains is 3.0 or more.

First claim

The invention claimed is:

1 . A high-strength hot-rolled steel sheet having
a chemical composition containing, by mass %,
C: 0.07% to 0.20%,
Si: 1.50% or less,
Mn: 1.0% to 4.0%,
P: 0.030% or less,
S: 0.0030% or less,
Al: 0.010% to 1.000%, and
a balance of Fe and incidental impurities,
a microstructure including, in terms of area fraction, 95% or more of a martensite phase at a position located at ¼ of a thickness of the steel sheet, in which an average aspect ratio of prior austenite grains is 3.0 or more, an amount of stress relaxation after a lapse of 5 min of 20 MPa or lower in a stress relaxation test with an applied stress of 400 MPa, and a tensile strength of 1180 MPa or higher.

2 . The high-strength hot-rolled steel sheet according to claim 1 , wherein the chemical composition further contains one, two, or more selected from Group A to Group E below:
Group A: by mass %, one, two, or more selected from Mo: 0.005% to 2.0%, V: 0.005% to 2.0%, Nb: 0.005% to 0.20%, and Ti: 0.005% to 0.20% Group B: by mass %, one, two, or more selected from Cr: 0.005% to 2.0%, Ni: 0.005% to 2.0%, and Cu: 0.005% to 2.0% Group C: by mass %, B: 0.0001% to 0.0050% Group D: by mass %, one or two selected from Ca: 0.0001% to 0.0050% and REM: 0.0001% to 0.0050% Group E: by mass %, one or two selected from Sb: 0.0010% to 0.10% and Sn: 0.0010% to 0.50%.

3 . The high-strength hot-rolled steel sheet according to claim 1 , wherein the microstructure further includes, in terms of area fraction, 5% or less of a retained austenite phase.

4 . A method for manufacturing the high-strength hot-rolled steel sheet of claim 1 , the method comprising performing heating, rough rolling, and finish rolling on a steel material, wherein the steel material is a steel material having the chemical composition according to claim 1 , wherein the finish rolling is performed with a finishing delivery temperature of 890° C. or lower, and wherein, after the finish rolling performed, cooling is performed at an average cooling rate of 10° C./s or higher to a temperature of 500° C. and at an average cooling rate of 100° C./s or higher in a temperature range from a Ms temperature to a temperature of (Ms temperature-200° C.), coiling is thereafter performed at a coiling temperature of 250° C. or lower, and the coiled steel sheet is subjected to at least one rolling pass with a rolling load per unit width of 0.20 ton/mm or more, or alternatively, after the finish rolling performed, the cooling is performed to a temperature of 250° C. or lower, and the cooled steel sheet, before being subjected to coiling, is subjected to at least one rolling pass with a rolling load per unit width of 0.20 ton/mm or more and then coiled; thereby producing the high-strength hot rolled steel sheet of claim 1 .

5 . The high-strength hot-rolled steel sheet according to claim 2 , wherein the microstructure further includes, in terms of area fraction, 5% or less of a retained austenite phase.

6 . A method for manufacturing the high-strength hot-rolled steel sheet of claim 2 , the method comprising performing heating, rough rolling, and finish rolling on a steel material, wherein the steel material is a steel material having the chemical composition according to claim 2 , wherein the finish rolling is performed with a finishing delivery temperature of 890° C. or lower, and wherein, after the finish rolling performed, cooling is performed at an average cooling rate of 10° C./s or higher to a temperature of 500° C. and at an average cooling rate of 100° C./s or higher in a temperature range from a Ms temperature to a temperature of (Ms temperature-200° C.), coiling is thereafter performed at a coiling temperature of 250° C. or lower, and the coiled steel sheet is subjected to at least one rolling pass with a rolling load per unit width of 0.20 ton/mm or more, or alternatively, after the finish rolling performed, the cooling is performed to a temperature of 250° C. or lower, and the cooled steel sheet, before being subjected to coiling, is subjected to at least one rolling pass with a rolling load per unit width of 0.20 ton/mm or more and then coiled; thereby producing the high-strength hot rolled steel sheet of claim 2 .

Assignees

• Jfe Steel Corp

Inventors

• Hasegawa Hiroshi
• Kimura Hideyuki
• Moriyasu Noriaki
• Goto Sota

Classifications

• C22C38/06

  containing aluminium · CPC title

• C22C38/04 Primary

  containing manganese · CPC title

• C22C38/02 Primary

  containing silicon · CPC title

• C22C38/002

  containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60 · CPC title

• C21D2211/008

  Martensite · CPC title