High strength steel sheet having excellent formability and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-9580779-B2
Application number	US-201214342550-A
Country	US
Kind code	B2
Filing date	Sep 10, 2012
Priority date	Sep 16, 2011
Publication date	Feb 28, 2017
Grant date	Feb 28, 2017

Abstract

A method for manufacturing the high strength steel sheet having excellent formability includes hot-rolling a steel slab having a chemical composition containing, by mass %, C: 0.03% or more and 0.35% or less, Si: 0.5% or more and 3.0% or less, Mn: 3.5% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.008% or less and the balance comprising Fe and inevitable impurities, coiling the hot-rolled steel sheet at a temperature range of the Ar 1 transformation point to the Ar 1 transformation point+(the Ar 3 transformation point−the Ar 1 transformation point)/2, cooling the coiled steel sheet down to 200° C. or lower, heating and holding the cooled steel sheet at a temperature range of the Ac 1 transformation point−200° C. to the Ac 1 transformation point for 30 minutes or more, pickling the heated steel sheet, cold-rolling the pickled steel sheet under the condition that the rolling reduction is 20% or more, and heating and holding the cold-rolled steel sheet at a temperature range of the Ac 1 transformation point to the Ac 1 transformation point+(the Ac 3 transformation point−the Ac 1 transformation point)/2 for 30 seconds or more.

First claim

The invention claimed is:

1. A method for manufacturing a high strength steel sheet having excellent formability, the method comprising:
hot-rolling a steel slab having a chemical composition containing, by mass %, C: 0.03% or more and 0.35% or less, Si: 0.5% or more and 3.0% or less, Mn: 3.8% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.008% or less and the balance comprising Fe and inevitable impurities;
coiling the hot-rolled steel sheet at a temperature range of the Ar 1 transformation point to the Ar 1 transformation point+ (the Ar 3 transformation point− the Ar 1 transformation point) / 2;
cooling the coiled steel sheet down to 200° C. or lower;
heating and holding the cooled steel sheet at a temperature range of the Ac 1 transformation point−200° C. to the Ac 1 transformation point for 30 minutes or more and 750 minutes or less;
pickling the heated steel sheet;
cold-rolling the pickled steel sheet under the condition that the rolling reduction is 20% or more; and
heating and holding the cold-rolled steel sheet at a temperature range of the Ac 1 transformation point to the Ac 1 transformation point+ (the Ac 3 transformation point− the Ac 1 transformation point) / 2 for 30 seconds or more and 360 minutes or less,
wherein the steel sheet includes a microstructure including: in terms of area fraction, 30.0% or more and 80.0% or less of ferrite, in which the ratio of the amount of Mn (mass %) in the ferrite divided by the amount of Mn (mass %) in the steel sheet is 0.20 or more and 0.80 or less; in terms of volume fraction, 10.0% or more of retained austenite, in which the amount of Mn in the retained austenite is 6.0% or more and 11.0% or less, and in which the average grain size of the retained austenite is 0.1 μm or more and 2.0 μm or less; and, in terms of area fraction, 3.0% or less of bainite, and an aspect ratio of the retained austenite is 2.0 or less.

2. A method for manufacturing a high strength steel sheet having excellent formability, the method comprising:
hot-rolling a steel slab having a chemical composition containing, by mass %, C: 0.03% or more and 0.35% or less, Si: 0.5% or more and 3.0% or less, Mn: 3.8% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.008% or less and the balance comprising Fe and inevitable impurities;
coiling the hot-rolled steel sheet at a temperature range of the Ar 1 transformation point to the Ar 1 transformation point+ (the Ar 3 transformation point− the Ar 1 transformation point) / 2,
holding the coiled steel sheet at a temperature range of the Ar 1 transformation point to the Ar 1 transformation point+ (the Ar 3 transformation point− the Ar 1 transformation point) / 2 for 5 hours or more and 12 hours or less;
cooling the coiled steel sheet down to 200° C. or lower;
heating and holding the cooled steel sheet at a temperature range of the Ac 1 transformation point−200° C. to the Ac 1 transformation point for 30 minutes or more and 750 minutes or less;
pickling the heated steel sheet;
cold-rolling the pickled steel sheet under the condition that the rolling reduction is 20% or more; and
heating and holding the cold-rolled steel sheet at a temperature range of the Ac 1 transformation point to the Ac 1 transformation point+(the Ac 3 transformation point−the Ac 1 transformation point) / 2 for 30 seconds or more and 360 minutes or less,
wherein the steel sheet includes a microstructure including: in terms of area fraction, 30.0% or more and 80.0% or less of ferrite, in which the ratio of the amount of Mn (mass %) in the ferrite divided by the amount of Mn (mass %) in the steel sheet is 0.20 or more and 0.80 or less; in terms of volume fraction, 10.0% or more of retained austenite, in which the amount of Mn in the retained austenite is 6.0% or more and 11.0% or less, and in which the average grain size of the retained austenite is 0.1 μm or more and 2.0 μm or less; and, in terms of area fraction, 3.0% or less of bainite, and an aspect ratio of the retained austenite is 2.0 or less.

3. A method for manufacturing a high strength steel sheet having excellent formability, the method comprising:
hot-rolling a steel slab having a chemical composition containing, by mass %, C: 0.03% or more and 0.35% or less, Si: 0.5% or more and 3.0% or less, Mn: 3.8% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.008% or less and the balance comprising Fe and inevitable impurities;
coiling the hot-rolled steel sheet at a temperature range of the Ar 1 transformation point to the Ar 1 transformation point+ (the Ar 3 transformation point− the Ar 1 transformation point) / 2;
optionally further holding the coiled steel sheet at a temperature range of the Ar 1 transformation point to the Ar 1 transformation point+ (the Ar 3 transformation point− the Ar 1 transformation point) / 2 for 5 hours or more and 12 hours or less;
cooling the coiled steel sheet down to 200° C. or lower;
heating and holding the cooled steel sheet at a temperature range of the Ac 1 transformation point−200° C. to the Ac 1 transformation point for 30 minutes or more and 750 minutes or less;
pickling the heated steel sheet;
cold-rolling the pickled steel sheet under the condition that the rolling reduction is 20% or more;
heating and holding the cold-rolled steel sheet at a temperature range of the Ac 1 transformation point to the Ac 1 transformation point+ (the Ac 3 transformation point− the Ac 1 transformation point) / 2 for 30 seconds or more and 360 minutes or less;
cooling the heated steel sheet down to a temperature of 200° C. or lower; and
heating and holding the cooled steel sheet at a temperature range of the Ac 1 transformation point to the Ac 1 transformation point+ (the Ac 3 transformation point− the Ac 1 transformation point) / 2 for 10 seconds or more,
wherein the steel sheet includes a microstructure including: in terms of area fraction, 30.0% or more and 80.0% or less of ferrite, in which the ratio of the amount of Mn (mass %) in the ferrite divided by the amount of Mn (mass %) in the steel sheet is 0.20 or more and 0.80 or less; in terms of volume fraction, 10.0% or more of retained austenite, in which the amount of Mn in the retained austenite is 6.0% or more and 11.0% or less, and in which the average grain size of the retained austenite is 0.1 μm or more and 2.0 μm or less; and, in terms of area fraction, 3.0% or less of bainite, and an aspect ratio of the retained austenite is 2.0 or less.

4. A method for manufacturing a high strength steel sheet having excellent formability, the method further comprising performing hot-dip galvanizing treatment on the high strength steel sheet manufactured by the method according to claim 1 .

5. The method for manufacturing a high strength steel sheet having excellent formability according to claim 4 , the method further comprising performing alloying treatment at a temperature range of 470° C. to 600° C. on the galvanized layer formed by the hot-dip galvanizing treatment.

6. The method for manufacturing a high strength steel sheet having excellent formability according to claim 1 , the steel slab having the chemical composition further containing, by mass %, Al: 0.01% or more and 2.5% or less.

7. The method for manufacturing a high strength steel sheet having excellent formability according to claim 1 , the steel slab having the chemical composition further containing, by mass %, at least one chemical element selected from Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less.

8. The method for manufacturing a high strength steel sheet having excellent formability accor

Assignees

• Kawasaki Yoshiyasu
• Hasegawa Hiroshi
• Nakagaito Tatsuya
• Kaneko Shinjiro
• Nagataki Yasunobu
• Jfe Steel Corp

Inventors

• Kawasaki Yoshiyasu
• Hasegawa Hiroshi
• Nakagaito Tatsuya
• Kaneko Shinjiro
• Nagataki Yasunobu

Classifications

• C21D8/02

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

• Y02P10/20

  Recycling · CPC title

• C21D2211/005

  Ferrite · CPC title

• C22C38/005

  containing rare earths, i.e. Sc, Y, Lanthanides · CPC title

• C22C38/60

  containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur · CPC title