High-strength steel sheet and method for producing same

Patent metadata

Field	Value
Publication number	US-11946112-B2
Application number	US-201917270613-A
Country	US
Kind code	B2
Filing date	Aug 22, 2019
Priority date	Aug 31, 2018
Publication date	Apr 2, 2024
Grant date	Apr 2, 2024

Abstract

A high-strength steel sheet of the present invention has a specific chemical composition. Furthermore, in the steel sheet, a degree of Mn segregation in a specific region is 1.5 or less; a maximum P concentration in a specific region is 0.08 mass % or less; in a specific region, at least one specific MnS particle group is present, the number of specific MnS particle groups is 2.0 or fewer per 1 mm 2 , and the number of specific oxide-based inclusions is 8 or fewer per 1 mm 2 ; of all oxide-based inclusions, oxide-based inclusions having a specific composition are present in a number ratio of 80% or greater; the microstructure includes, in terms of a volume fraction, 30 to 95% martensite, 5 to 70% ferrite phase, less than 30% (and 0% or greater) bainite, and less than 2.0% (and 0% or greater) austenite phase; and a tensile strength is 980 MPa or greater.

First claim

The invention claimed is:

1. A high-strength steel sheet, the high-strength steel sheet having a chemical composition containing, in mass %,
C: 0.10 to 0.35%,
Si: 0.01 to 2.0%,
Mn: 1.5 to less than 2.2%,
P: 0.015% or less (and greater than 0%),
S: 0.0015% or less (and greater than 0%),
Sol. Al: 0.01 to 1.0%,
N: 0.0055% or less (and greater than 0%),
O: 0.0025% or less (and greater than 0%), and
Ca: 0.0005% or less (and 0% or greater), with a balance of Fe and incidental impurities, wherein
in a region within 100 μm of a surface of the high-strength steel sheet in a sheet thickness direction, a degree of Mn segregation is 1.5 or less,
in a region within 100 μm of the surface in the sheet thickness direction, a maximum P concentration is 0.08 mass % or less,
in a region within 100 μm of the surface in the sheet thickness direction, at least one MnS particle group formed of one or more MnS particles having a major axis of 0.3 μm or greater is present, the one or more MnS particles being elongated and/or distributed in a form of a sequence of dots in a rolling direction of the steel sheet, a distance between adjacent MnS particles being 40 μm or less in a case where the at least one MnS particle group is formed of two or more MnS particles, and a number of MnS particle groups having a longitudinal dimension of 150 μm or greater is 2.0 or fewer per 1 mm 2 , as viewed in a cross section in a sheet thickness direction and parallel to the rolling direction,
in a region within 100 μm of the surface in the sheet thickness direction, a number of oxide-based inclusions having a particle diameter of 5 μm or greater is 8 or fewer per 1 mm 2 as viewed in a plane parallel to a sheet surface,
of all the number of the oxide-based inclusions having a particle diameter of 5 μm or greater, oxide-based inclusions having a composition in which an alumina content is 50 mass % or greater, a silica content is 20 mass % or less, and a calcia content is 40 mass % or less are present in the number ratio of 80% or greater,
the high-strength steel sheet has a microstructure that includes, in terms of a volume fraction, 30 to 95% martensite, 5 to 70% ferrite phase, less than 30% (and 0% or greater) bainite, and less than 2.0% (and 0% or greater) austenite phase, and
the high-strength steel sheet has a tensile strength of 980 MPa or greater.

2. The high-strength steel sheet according to claim 1 , wherein the chemical composition further contains, in mass %, one or two or more selected from the following groups A to E:
Group A:
one or two or more selected from:
Ti: 0.003 to 0.05%,
Nb: 0.003 to 0.05%,
V: 0.001 to 0.1%, and
Zr: 0.001 to 0.1%.

Group B:
one or two or more selected from:
Cr: 0.01 to 1.0%,
Mo: 0.01 to 0.20%, and
B: 0.0001 to 0.0030%.

Group C:
one or two or more selected from:
Cu: 0.01 to 0.5%,
Ni: 0.01 to 0.5%, and
Sn: 0.001 to 0.1%.

Group D:
Sb: 0.001 to 0.1%.

Group E:
REMs and Mg in a total amount of 0.0002% or greater and 0.01% or less.

3. The high-strength steel sheet according to claim 1 , further comprising a galvanized layer on the surface.

4. The high-strength steel sheet according to claim 2 , further comprising a galvanized layer on the surface.

5. A method for producing a high-strength steel sheet, the high-strength steel sheet being the high-strength steel sheet according to claim 1 , the method comprising:
a casting step in which, after completion of refining, which is carried out in an RH vacuum degasser with a circulation time of 500 seconds or more, continuous casting is performed in a manner such that a difference between a casting temperature and a solidification temperature is 10° C. or greater and 35° C. or less, a flow rate of molten steel at a solidification interface near a mold meniscus is 0.5 to 1.5 m/min, and the steel is passed through a bending section and a straightening section at a temperature of 550° C. or higher and 1050° C. or lower;
a hot rolling step in which a steel starting material obtained in the casting step is heated directly after the casting step or after cooling to a temperature of 1220 to 1300° C. and held for 80 minutes or more; and hot rolling is performed in a manner such that an amount of reduction for a first pass of rough rolling is 10% or greater, and an amount of reduction for a first pass of finish rolling is 20% or greater;
a cold rolling step in which, after a hot-rolled steel sheet obtained in the hot rolling step is pickled, the hot-rolled steel sheet is subjected to cold rolling; and
an annealing step in which a cold-rolled steel sheet obtained in the cold rolling step is annealed.

6. A method for producing a high-strength steel sheet, the high-strength steel sheet being the high-strength steel sheet according to claim 2 , the method comprising:
a casting step in which, after completion of refining, which is carried out in an RH vacuum degasser with a circulation time of 500 seconds or more, continuous casting is performed in a manner such that a difference between a casting temperature and a solidification temperature is 10° C. or greater and 35° C. or less, a flow rate of molten steel at a solidification interface near a mold meniscus is 0.5 to 1.5 m/min, and the steel is passed through a bending section and a straightening section at a temperature of 550° C. or higher and 1050° C. or lower;
a hot rolling step in which a steel starting material obtained in the casting step is heated directly after the casting step or after cooling to a temperature of 1220 to 1300° C. and held for 80 minutes or more; and hot rolling is performed in a manner such that an amount of reduction for a first pass of rough rolling is 10% or greater, and an amount of reduction for a first pass of finish rolling is 20% or greater;
a cold rolling step in which, after a hot-rolled steel sheet obtained in the hot rolling step is pickled, the hot-rolled steel sheet is subjected to cold rolling; and
an annealing step in which a cold-rolled steel sheet obtained in the cold rolling step is annealed.

7. The method for producing a high-strength steel sheet according to claim 5 , wherein the annealing step is a step performed in a manner such that the cold-rolled steel sheet obtained in the cold rolling step is heated to a temperature range of 780 to 900° C.; thereafter, the steel sheet is soaked in the temperature range for 20 seconds or more; then, primary cooling is performed to cool the steel sheet from the soaking temperature to a rapid cooling start temperature of 580° C. or higher at an average primary cooling rate of 1 to 100° C./s; thereafter, secondary cooling is performed to cool the steel sheet from the rapid cooling start temperature to 300° C. or lower at an average secondary cooling rate of 80° C./sec or greater and less than 2000° C./sec, the average secondary cooling rate being an average cooling rate for a range from the rapid cooling start temperature to 300° C.; then, the steel sheet is held under conditions including a retention time for a temperature range of 450 to 130° C. of 10 to 1000 seconds; and then, tertiary cooling is performed to cool the steel sheet over a temperature range of 130 to 50° C. at an average tertiary cooling rate of 5° C./sec or greater.

8. The method for producing a high-strength steel sheet according to claim 6 , wherein the annealing step is a step performed in a manner such that the cold-rolled steel sheet obtained in the cold rolling step is heated to a temperature range of 780 to 900° C.; thereafter, the steel sheet is soaked in the temperature range for 20 seconds or more; then, primary cooling is performed to cool the steel sheet from the soaking temperature to a rapid cooling start temperature of 580° C. or higher at an average primary cool

Assignees

• Jfe Steel Corp

Inventors

• Ono Yoshihiko
• Honda Yuma
• Yoshioka Shimpei
• Taniguchi Koichi
• Matsuda Hiroshi
• Murai Takeshi
• Nakamura Nobuyuki

Classifications

• C21D8/02

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

• C21D9/46 Primary

  for sheet metals · CPC title

• C21D6/005

  containing Mn · CPC title

• C21D6/008

  containing Si · CPC title

• C21D8/0205

  Chemistry & Metallurgy · mapped topic