High-strength steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-10697039-B2
Application number	US-201515542272-A
Country	US
Kind code	B2
Filing date	Aug 28, 2015
Priority date	Jan 16, 2015
Publication date	Jun 30, 2020
Grant date	Jun 30, 2020

Abstract

Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 25% or less of a ferrite phase, 75% or more of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 5% to 20%, and a tensile strength is 1180 MPa or more.

First claim

The invention claimed is:

1. A high-strength steel sheet having
a chemical composition containing, by mass %, C: 0.100% to 0.150%, Si: 0.30% to 0.70%, Mn: 2.20% to 2.80%, P: 0.025% or less, S: 0.0020% or less, Al: 0.020% to 0.060%, N: 0.0050% or less, Nb: 0.010% to 0.060%, Ti: 0.010% to 0.030%, B: 0.0005% to 0.0030%, Sb: 0.005% to 0.015%, Ca: 0.0015% or less, and the balance being Fe and inevitable impurities,
a microstructure including, in terms of area ratio, 25% or less of a ferrite phase, 75% or more of a bainite phase and/or a martensite phase, and 5% or less of cementite,
wherein, in a surface layer that is a region within 50 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 5% to 20%,
a tensile strength being 1180 MPa or more, and
the chemical composition further contains at least one element selected from at least one group consisting of, by mass %,
Group I: one or more elements selected from Cr: 0.30% or less, V: 0.10% or less, Mo: 0.20% or less, Cu: 0.10% or less, and Ni: 0.10% or less, and
Group II: REM: 0.0010% to 0.0050%.

2. The high-strength steel sheet according to claim 1 , the steel sheet further having a YR of 0.85 or less.

3. A method for manufacturing a high-strength steel sheet according to claim 1 , the method comprising:
a hot rolling process in which finish rolling is performed on a steel material having a chemical composition containing, by mass %, C: 0.100% to 0.150%, Si: 0.30% to 0.70%, Mn: 2.20% to 2.80%, P: 0.025% or less, S: 0.0020% or less, Al: 0.020% to 0.060%, N: 0.0050% or less, Nb: 0.010% to 0.060%, Ti: 0.010% to 0.030%, B: 0.0005% to 0.0030%, Sb: 0.005% to 0.015%, and Ca: 0.0015% or less, with the balance being Fe and inevitable impurities, at a temperature equal to or higher than the Ara transformation temperature and in which coiling is performed at a temperature of 600° C. or lower;
a pickling process in which pickling is performed on the hot-rolled steel sheet after the hot rolling process; and
a continuous annealing process in which the steel sheet which has been pickled in the pickling process is heated to a temperature range of 570° C. or higher at an average heating rate of 2° C./s or more, in which a holding time during which the steel sheet is held in a temperature range equal to or higher than the Acs transformation temperature is 60 seconds or more, in which the held steel sheet is then cooled to a temperature range of 620° C. to 740° C. at an average cooling rate of 0.1° C./s to 8° C./s, in which a holding time during which the cooled steel sheet is held in the temperature range is 10 seconds to 50 seconds, in which the held steel sheet is then cooled to a temperature range of 400° C. or lower at an average cooling rate of 5° C./s to 50° C./s, and in which a holding time during which the cooled steel sheet is held in a temperature range of 150° C. or higher and 400° C. or lower is 200 seconds to 800 seconds.

4. The method for manufacturing a high-strength steel sheet according to claim 1 , the method further comprising a cold rolling process in which cold rolling is performed on the pickled steel sheet after the pickling process and before the continuous annealing process.

5. The high-strength steel sheet according to claim 1 , wherein the microstructure includes, in terms of area ratio, 11% or more and 25% or less of the ferrite phase.

Assignees

• Jfe Steel Corp

Inventors

• Kariya Nobusuke
• Ono Yoshihiko
• Funakawa Yoshimasa
• Mori Kazuma
• Sugihara Reiko
• Kawamura Kenji

Classifications

• C21D8/02

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

• C22C38/40

  with nickel · CPC title

• B21B3/00

  Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences {; Rolling of aluminium, copper, zinc or other non-ferrous metals}(altering special metallurgical properties of alloys, other than structure consolidation or mechanical properties resulting therefrom C21D, C22F) · CPC title

• C22C38/58

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

• C22C38/14

  containing titanium or zirconium · CPC title