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

Patent metadata

Field	Value
Publication number	US-12065711-B2
Application number	US-201917262884-A
Country	US
Kind code	B2
Filing date	Jun 10, 2019
Priority date	Jul 31, 2018
Publication date	Aug 20, 2024
Grant date	Aug 20, 2024

Abstract

A high-strength hot rolled steel sheet and a method for manufacturing the high-strength hot rolled steel sheet are disclosed. The high-strength hot rolled steel sheet includes a specific chemical composition, and a steel structure in which a lower bainite phase and/or a tempered martensite phase at 90% or more in terms of a total area fraction is contained as a dominant phase, an average grain size of the dominant phase is 10.0 μm or less, and an amount of Fe in Fe-based precipitates is 0.70% or less in mass %, in which an arithmetic average roughness (Ra) of a surface is 2.50 μm or less, and a tensile strength TS is 1180 MPa or more.

First claim

The invention claimed is:

1. A high-strength hot rolled steel sheet comprising :
a chemical composition containing, in mass %,
C: 0.07% or more and 0.20% or less,
Si: 0.10% or more and 2.0% or less,
Mn: 0.8% or more and 3.0% or less,
P: 0.100% or less (including 0%),
S: 0.0100% or less (including 0%),
Al: 0.010% or more and 2.00% or less,
N: 0.010% or less (including 0%),
Ti: 0.02% or more and less than 0.16%,
B: 0.0003% or more and 0.0100% or less, and the balance including Fe and incidental impurities; and
a steel structure in which a lower bainite phase and/or a tempered martensite phase at 90% or more in terms of a total area fraction is contained as a dominant phase, an average grain size of the dominant phase is 10.0 μm or less, and an amount of Fe in Fe-based precipitates is 0.70% or less in mass %,
wherein an arithmetic average roughness (Ra) of a surface is 2.50 μm or less, and
a tensile strength TS is 1180 MPa or more.

2. The high-strength hot rolled steel sheet according to claim 1 , wherein the chemical composition further contains at least one selected from the following groups A to D:
group A: one or two or more selected from, in mass %,
Cr: 0.01% or more and 2.0% or less,
Mo: 0.01% or more and 0.50% or less,
Cu: 0.01% or more and 0.50% or less, and Ni: 0.01% or more and 0.50% or less,

group B: one or two selected from, in mass %,
Nb: 0.001% or more and 0.060% or less, and V: 0.01% or more and 0.50% or less,

group C: in mass %,
Sb: 0.0005% or more and 0.0500% or less, and

group D: one or two or more selected from, in mass %,
Ca: 0.0005% or more and 0.0100% or less,
Mg: 0.0005% or more and 0.0100% or less, and REMs: 0.0005% or more and 0.0100% or less.

3. The high-strength hot rolled steel sheet according to claim 1 , comprising: a coating layer on a surface.

4. The high-strength hot rolled steel sheet according to claim 2 , comprising: a coating layer on a surface.

5. A method for manufacturing a high-strength hot rolled steel sheet that is a method for manufacturing the high-strength hot rolled steel sheet according to claim 1 , the method comprising:
heating a steel material to 1150° C. or more;
performing rough rolling on the steel material after the heating to form a steel sheet;
performing, before finish rolling to be performed after the rough rolling, high-pressure water descaling on the steel sheet under a condition of a collision pressure of 2.5 MPa or more;
performing finish rolling on the steel sheet after the high-pressure water descaling under a condition of a finisher delivery temperature of (RC −200° C.) or more and (RC +50° C.) or less, where an RC temperature is defined by Formula (1);
starting cooling after an end of the finish rolling, and performing cooling under conditions of a cooling stop temperature of 200° C. or more and an Ms temperature or less, where the Ms temperature is defined by Formula (2), and an average cooling rate of 20° C./s or more, and under a condition of a time from the end of the finish rolling to a start of the cooling being within 2.0 s in a case where the finisher delivery temperature is RC or more;
coiling the steel sheet after the cooling at the cooling stop temperature; and
cooling the steel sheet under conditions of an average cooling rate of less than 20° C./s and a cooling stop temperature of 100° C. or less after the coiling to produce the high-strength hot rolled steel sheet,
RC (° C.)=850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V   Formula (1)

Ms (° C.)=560−470×C−33×Mn−24×Cr−17×Ni−20×Mo   Formula (2)

where each of symbols of elements in Formula (1) and Formula (2) is a content amount (mass %) in steel of a respective element, and a symbol of an element in the formulae of a not-contained element is set to 0 for calculation.

6. A method for manufacturing a high-strength hot rolled steel sheet that is a method for manufacturing the high-strength hot rolled steel sheet according to claim 2 , the method comprising:
heating a steel material to 1150° C. or more;
performing rough rolling on the steel material after the heating to form a steel sheet;
performing, before finish rolling to be performed after the rough rolling, high-pressure water descaling on the steel sheet under a condition of a collision pressure of 2.5 MPa or more;
performing finish rolling on the steel sheet after the high-pressure water descaling under a condition of a finisher delivery temperature of (RC −200° C.) or more and (RC +50° C.) or less, where an RC temperature is defined by Formula (1);
starting cooling after an end of the finish rolling, and performing cooling under conditions of a cooling stop temperature of 200° C. or more and an Ms temperature or less, where the Ms temperature is defined by Formula (2), and an average cooling rate of 20° C./s or more, and under a condition of a time from the end of the finish rolling to a start of the cooling being within 2.0 s in a case where the finisher delivery temperature is RC or more;
coiling the steel sheet after the cooling at the cooling stop temperature; and
cooling the steel sheet under conditions of an average cooling rate of less than 20° C./s and a cooling stop temperature of 100° C. or less after the coiling to produce the high-strength hot rolled steel sheet,
RC (° C.)=850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V   Formula (1)

Ms (° C.)=560−470×C−33×Mn−24×Cr−17×Ni−20×Mo   Formula (2)

where each of symbols of elements in Formula (1) and Formula (2) is a content amount (mass %) in steel of a respective element, and a symbol of an element in the formulae of a not-contained element is set to 0 for calculation.

7. The method for manufacturing the high-strength hot rolled steel sheet according to claim 5 , further comprising: performing coating treatment on a surface of the steel sheet.

8. The method for manufacturing the high-strength hot rolled steel sheet according to claim 6 , further comprising: performing coating treatment on a surface of the steel sheet.

9. The high-strength hot rolled steel sheet according to claim 1 , wherein the arithmetic average roughness (Ra) of the surface is 2.50 μm or less and 1.05 μm or more.

Assignees

• Jfe Steel Corp

Inventors

• Yamazaki Kazuhiko
• Yokota Takeshi
• Kaiho Sumio
• Moriyasu Noriaki

Classifications

• 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

• 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

• C22C38/26

  with niobium or tantalum · CPC title