Hot-rolled steel sheet and method for manufacturing same

Patent metadata

Field	Value
Publication number	US-12428694-B2
Application number	US-201917283476-A
Country	US
Kind code	B2
Filing date	Oct 21, 2019
Priority date	Oct 19, 2018
Publication date	Sep 30, 2025
Grant date	Sep 30, 2025

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, and in a case where the thickness is denoted by t, the metallographic structure at a t/4 position from the surface contains one or both of tempered martensite and lower bainite at a volume percentage of 90% or more, the tensile strength is 980 MPa or more, and the average Ni concentration on the surface is 7.0% or more.

First claim

What is claimed is:

1. A hot-rolled steel sheet comprising, as a chemical composition expressed by an average value in an entire sheet thickness direction, by mass %:
C: 0.050% or more and 0.200% or less;
Si: 0.05% or more and 3.00% or less;
Mn: 1.00% or more and 4.00% or less;
Al: 0.001% or more and 2.000% or less;
N: 0.0005% or more and 0.1000% or less;
Ni: 0.02% or more and 2.00% or less;
Nb: 0% or more and 0.300% or less;
Ti: 0% or more and 0.300% or less;
Cu: 0% or more and 2.00% or less;
Mo: 0% or more and 1.000% or less;
V: 0% or more and 0.300% or less;
Cr: 0% or more and 2.00% or less;
Mg: 0% or more and 0.0100% or less;
Ca: 0% or more and 0.0100% or less;
REM: 0% or more and 0.1000% or less;
B: 0% or more and 0.0100% or less;
one or two or more of Zr, Co, Zn, and W: 0% to 1.000% in total;
Sn: 0% to 0.050%;
P: 0.100% or less;
S: 0.0300% or less;
O: 0.0100% or less; and
a remainder including Fe and impurities,
wherein Expression (1) is satisfied,
PCM represented by Expression (2) is 0.20 or more,
Ms represented by Expression (3) is 400° C. or higher,
in a case where a thickness is denoted by t, a metallographic structure at a t/4 position from a surface contains one or both of tempered martensite and lower bainite at a total volume percentage of 90% or more,
a tensile strength is 980 MPa or more,
an average Ni concentration on the surface is 7.0% or more, and
a standard deviation of an arithmetic average roughness Ra of the surface of the hot-rolled steel sheet is 10.0 μm or more and 50.0 μm or less,
0.05%≤Si+Al≤2.50%  Expression (1)

PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Mo/15+Cr/20+V/10+5×B   Expression (2)

Ms=561−474×C-33×Mn-17×Ni-17×Cr-21×Mo  Expression (3)

where a symbol of each element shown in the expressions indicates mass % of the element contained in the hot-rolled steel sheet.

2. The hot-rolled steel sheet according to claim 1 ,
wherein the hot-rolled steel sheet contains, as the chemical composition, by mass %,
Ni: 0.02% or more, 0.05% or less.

3. The hot-rolled steel sheet according to claim 1 ,
wherein an average number density of iron-based carbides present in the tempered martensite and the lower bainite is 1.0×10 6 carbides/mm 2 or more.

4. The hot-rolled steel sheet according to claim 1 ,
wherein an internal oxide layer is present in the hot-rolled steel sheet, and
an average depth of the internal oxide layer is 5.0 μm or more and 20.0 μm or less from the surface of the hot-rolled steel sheet.

5. The hot-rolled steel sheet according to claim 1 ,
wherein the hot-rolled steel sheet contains, as the chemical composition, by mass %, one or both of
B: 0.0001% or more and 0.0100% or less, and
Ti: 0.015% or more and 0.300% or less.

6. The hot-rolled steel sheet according to claim 1 ,
wherein the hot-rolled steel sheet contains, as the chemical composition, by mass %, one or two or more of
Nb: 0.005% or more and 0.300% or less,
Cu: 0.010% or more and 2.00% or less,
Mo: 0.010% or more and 1.000% or less,
V: 0.010% or more and 0.300% or less, and
Cr: 0.01% or more and 2.00% or less.

7. The hot-rolled steel sheet according to claim 1 ,
wherein the hot-rolled steel sheet contains, as the chemical composition, by mass %, one or two or more of
Mg: 0.0005% or more and 0.0100% or less,
Ca: 0.0005% or more and 0.0100% or less, and
REM: 0.0005% or more and 0.1000% or less.

8. A method for manufacturing a hot-rolled steel sheet according to claim 1 , the method comprising:
casting a molten steel having the chemical composition according to claim 1 to obtain a slab;
heating the slab in a heating furnace which includes a regenerative-type burner and has at least a preheating zone, a heating zone, and a soaking zone;
hot-rolling the heated slab so that a finish rolling temperature is 850° C. or higher to obtain a hot-rolled steel sheet;
performing primary cooling on the hot-rolled steel sheet to a temperature range equal to or lower than a Ms point temperature calculated by Expression (4) so that an average cooling rate from the finish rolling temperature to the Ms point temperature is 50° C./sec or higher; and
coiling the hot-rolled steel sheet at a temperature of lower than 350° C.,
wherein in the heating of the slab, an air ratio in the preheating zone is 1.1 or more and 1.9 or less,
Ms=561−474×C-33×Mn-17×Ni-17×Cr-21×Mo  Expression (4).

9. The method for manufacturing a hot-rolled steel sheet according to claim 8 ,
wherein the primary cooling is stopped at a temperature lower than the Ms point temperature and 350° C. or higher, and the hot-rolled steel sheet after the primary cooling is cooled at a temperature of lower than 350° C. so that a maximum cooling rate is lower than 50° C./sec.

10. The method for manufacturing a hot-rolled steel sheet according to claim 8 ,
wherein in the heating of the slab, an air ratio in the heating zone is 0.9 or more and 1.3 or less.

11. The method for manufacturing a hot-rolled steel sheet according to claim 8 ,
wherein in the heating of the slab, an air ratio in the soaking zone is 0.9 or more and 1.9 or less.

12. The method for manufacturing a hot-rolled steel sheet according to claim 8 ,
wherein the air ratio in the preheating zone is higher than the air ratio in the heating zone.

13. The method for manufacturing a hot-rolled steel sheet according to claim 8 , further comprising:
pickling the hot-rolled steel sheet after the coiling of the hot-rolled steel sheet using a 1 to 10 wt % hydrochloric acid solution at a temperature of 20° C. to 95° C. under a condition of a pickling time of 30 seconds or more and less than 60 seconds.

14. The hot-rolled steel sheet according to claim 2 ,
wherein an average number density of iron-based carbides present in the tempered martensite and the lower bainite is 1.0×10 6 carbides/mm 2 or more.

15. The method for manufacturing a hot-rolled steel sheet according to claim 9 ,
wherein in the heating of the slab, an air ratio in the heating zone is 0.9 or more and 1.3 or less.

16. The method for manufacturing a hot-rolled steel sheet according to claim 9 ,
wherein in the heating of the slab, an air ratio in the soaking zone is 0.9 or more and 1.9 or less.

17. The method for manufacturing a hot-rolled steel sheet according to claim 9 ,
wherein the air ratio in the preheating zone is higher than the air ratio in the heating zone.

Assignees

• Nippon Steel Corp

Inventors

• Yokoi Tatsuo
• Hayashida Teruki
• Sakakibara Mutsumi
• Ando Jun
• Kai Shinsuke
• Shuto Hiroshi

Classifications

• C21D8/02

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

• C23G1/08

  Iron or steel · CPC title

• C22C38/58 Primary

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

• C22C38/16

  containing copper · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title