Hot-rolled steel sheet and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-12344913-B2
Application number	US-202117794672-A
Country	US
Kind code	B2
Filing date	Apr 21, 2021
Priority date	May 8, 2020
Publication date	Jul 1, 2025
Grant date	Jul 1, 2025

Abstract

Adopted are a hot-rolled steel sheet having a predetermined chemical composition, in which a region, where a rotation angle between a normal line of a surface and a (011) pole near the normal line becomes 5° or less, is 0.150 or less from the surface in terms of a sheet thickness direction position standardized by a sheet thickness, and a region, where the rotation angle becomes 20° or more, is 0.250 or more from the surface in terms of the sheet thickness direction position standardized by the sheet thickness and a manufacturing method thereof.

First claim

The invention claimed is:

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
C: 0.060% to 0.170%;
Si: 0.030% to 1.700%;
Mn: 1.20% to 3.00%;
Al: 0.010% to 0.700%;
Nb: 0.005% to 0.050%;
P: 0.0800% or less;
S: 0.0100% or less;
N: 0.0050% or less;
Ti: 0% to 0.1800%;
Mo: 0% to 0.150%;
V: 0% to 0.3000%;
Cr: 0% to 0.500%;
B: 0% to 0.0030%; and
a remainder consisting of Fe and an impurity,
wherein, in metallographic structures at a ¼ position in a sheet thickness direction from a surface and at a ½ position in the sheet thickness direction from the surface, by vol %,
bainite and martensite are a total of 80.0% or more,
ferrite is 20.0% or less, and
cementite and residual austenite are a total of 0% to 10.0%,
in a metallographic structure of a region from the surface to a 100 μm position in the sheet thickness direction from the surface,
an average grain diameter of prior austenite grains is less than 30.00 μm,
a region, where a rotation angle between a normal line of the surface and a (011) pole near the normal line is 5° or less, is 0.150 or less from the surface in terms of a sheet thickness direction position standardized by a sheet thickness,
a region, where the rotation angle between the normal line of the surface and the (011) pole near the normal line becomes 20° or more, is 0.250 or more from the surface in terms of the sheet thickness direction position standardized by the sheet thickness, and
a tensile strength is 880 MPa or more.

2. The hot-rolled steel sheet according to claim 1 , comprising, as the chemical composition, by mass %, one or more selected from the group of:
Ti: 0.0200% to 0.1800%;
Mo: 0.030% to 0.150%;
V: 0.0500% to 0.3000%,
Cr: 0.050% to 0.500%; and
B: 0.0001% to 0.0030%.

3. A manufacturing method of the hot-rolled steel sheet according to claim 1 , comprising:
a casting step of, in continuous casting of a slab having the chemical composition according to claim 1 , performing the continuous casting in a manner that an average surface temperature gradient in a region from a meniscus to 1.0 m from the meniscus becomes 300 to 650° C./m to obtain the slab;
a heating step of heating the slab to 1200° C. or higher and holding the slab for 30 minutes or longer;
a hot rolling step of performing rough rolling on the slab, and performing finish rolling in a manner that a total rolling reduction in a temperature range of 870° C. to 980° C. becomes 80% or larger, an elapsed time between rolling stands in the temperature range of 870° C. to 980° C. becomes 0.3 to 5.0 seconds, and a total rolling reduction in a temperature range of lower than 870° C. becomes smaller than 10%;
a cooling step of cooling for 30.0 seconds or shorter to cool to a temperature range of lower than 300° C. after the finish rolling; and
a coiling step of, coiling in a manner that a coiling temperature becomes lower than 300° C. after the cooling.

4. The manufacturing method of the hot-rolled steel sheet according to claim 3 , further comprising:
a heat treatment step of holding in a temperature range of 200° C. or higher and lower than 450° C. for 90 to 80000 seconds after the coiling.

5. A manufacturing method of the hot-rolled steel sheet according to claim 2 , comprising:
a casting step of, in continuous casting of a slab having the chemical composition, performing the continuous casting in a manner that an average surface temperature gradient in a region from a meniscus to 1.0 m from the meniscus becomes 300 to 650° C./m to obtain the slab;
a heating step of heating the slab to 1200° C. or higher and holding the slab for 30 minutes or longer;
a hot rolling step of performing rough rolling on the slab, and performing finish rolling in a manner that a total rolling reduction in a temperature range of 870° C. to 980° C. becomes 80% or larger, an elapsed time between rolling stands in the temperature range of 870° C. to 980° C. becomes 0.3 to 5.0 seconds, and a total rolling reduction in a temperature range of lower than 870° C. becomes smaller than 10%;
a cooling step of cooling for 30.0 seconds or shorter to cool to a temperature range of lower than 300° C. after the finish rolling; and
a coiling step of, coiling in a manner that a coiling temperature becomes lower than 300° C. after the cooling.

6. A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
C: 0.060% to 0.170%;
Si: 0.030% to 1.700%;
Mn: 1.20% to 3.00%;
Al: 0.010% to 0.700%;
Nb: 0.005% to 0.050%;
P: 0.0800% or less;
S: 0.0100% or less;
N: 0.0050% or less;
Ti: 0% to 0.1800%;
Mo: 0% to 0.150%;
V: 0% to 0.3000%;
Cr: 0% to 0.500%;
B: 0% to 0.0030%; and
a remainder comprising Fe and an impurity,
wherein, in metallographic structures at a ¼ position in a sheet thickness direction from a surface and at a ½ position in the sheet thickness direction from the surface, by vol %,
bainite and martensite are a total of 80.0% or more,
ferrite is 20.0% or less, and
cementite and residual austenite are a total of 0% to 10.0%,
in a metallographic structure of a region from the surface to a 100 μm position in the sheet thickness direction from the surface,
an average grain diameter of prior austenite grains is less than 30.00 μm,
a region, where a rotation angle between a normal line of the surface and a (011) pole near the normal line is 5° or less, is 0.150 or less from the surface in terms of a sheet thickness direction position standardized by a sheet thickness,
a region, where the rotation angle between the normal line of the surface and the (011) pole near the normal line becomes 20° or more, is 0.250 or more from the surface in terms of the sheet thickness direction position standardized by the sheet thickness, and
a tensile strength is 880 MPa or more.

Assignees

• Nippon Steel Corp

Inventors

• Sakurada Eisaku
• Yasutomi Takashi
• Abukawa Genki

Classifications

• C21D8/02

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

• C22C38/38 Primary

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

• C22C38/34

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

• C22C38/32

  with boron · CPC title

• C22C38/28

  with titanium or zirconium · CPC title