High-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel sheet excellent in mechanical cutting property, and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-9708679-B2
Application number	US-201214348502-A
Country	US
Kind code	B2
Filing date	Sep 28, 2012
Priority date	Sep 30, 2011
Publication date	Jul 18, 2017
Grant date	Jul 18, 2017

Abstract

There is provided a high-strength hot-dip galvanized steel sheet and the like excellent in mechanical cutting property, which are capable of obtaining high ductility while ensuring high strength with maximum tensile strength of 900 MPa or more. The high-strength hot-dip galvanized steel sheet has a sheet thickness of 0.6 to 5.0 mm and has a plating layer on a surface of a steel sheet with component compositions being set in appropriate ranges, in which the steel sheet structure contains a 40 to 90% ferrite phase and a 3% or more retained austenite phase by volume fraction. In the retained austenite phase, a solid solution carbon amount is 0.70 to 1.00%, an average grain diameter is 2.0 μm or less, an average distance between grains is 0.1 to 5.0 μm, a thickness of a decarburized layer in a steel sheet surface layer portion is 0.01 to 10.0 μm, an average grain diameter of oxides contained in the steel, sheet surface layer portion is 30 to 120 nm and an average density thereof is 1.0×10 12 oxides/m 2 or more, and moreover, a work hardening coefficient (n value) during a 3 to 7% plastic deformation is 0.080 or more on average.

First claim

The invention claimed is:

1. A high-strength hot-dip galvanized steel sheet, having a sheet thickness of 0.6 to 5.0 mm and comprising a plating layer on a surface of a steel sheet containing, by mass %,
C: 0.075 to 0.400%,
Si: 0.01 to 2.00%,
Mn: 0.80 to 3.50%,
P: 0.0001 to 0.100%,
S: 0.0001 to 0.0100%,
Al: 0.001 to 2.00%,
N: 0.0001 to 0.0100%, and
O: 0.0001 to 0.0100% with a balance being constituted of iron and inevitable impurities, wherein:
in a range of ⅛ thickness to ⅜ thickness with ¼ of the sheet thickness being a center from the surface of the steel sheet, the steel sheet has a structure comprising a ferrite phase that occupies 40-90% of the steel sheet and a retained austenite phase that occupies 3% or more of the steel sheet by volume fraction;
in the retained austenite phase, a solid solution carbon amount in the phase is 0.70 to 1.00%, an average grain diameter is 2.0 μm or less, and an average distance between grains is 0.1 to 5.0 μm;
a thickness of a decarburized layer in a steel sheet surface layer portion is 0.01 to 10.0 μm, an average grain diameter of oxides contained in the steel sheet surface layer portion is 30 to 120 nm and an average density thereof is 1.0×10 12 oxides/m 2 or more; and
moreover, a work hardening coefficient (n value) during a 3 to 7% plastic deformation is 0.080 or more on average.

2. The high-strength hot-dip galvanized steel sheet according to claim 1 , further containing, by mass %, one or more of
Ti: 0.001 to 0.150%,
Nb: 0.001 to 0.100%, and
V: 0.001 to 0.300%.

3. The high-strength hot-dip galvanized steel sheet according to claim 1 , further containing, by mass %, one or more of
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Mo: 0.01 to 2.00%,
B: 0.0001 to 0.0100%, and
W: 0.01 to 2.00%.

4. The high-strength hot-dip galvanized steel sheet according to claim 1 , further containing, by mass %, 0.0001 to 0.0100% in total of one or more of Ca, Ce, Mg, Zr, La, and REM.

5. A high-strength alloyed hot-dip galvanized steel sheet, formed by alloying the plating layer of the high-strength hot-dip galvanized steel sheet according to claim 1 .

6. A manufacturing method of a high-strength hot-dip galvanized steel sheet according to claim 1 , the method comprising:
a hot rolling step of heating to 1180° C. or more a slab having chemical components containing, by mass %,
C: 0.075 to 0.400%,
Si: 0.01 to 2.00%,
Mn: 0.80 to 3.50%,
P: 0.0001 to 0.100%,
S: 0.0001 to 0.0100%,
Al: 0.001 to 2.00%,
N: 0.0001 to 0.0100%, and
O: 0.0001 to 0.0100% with a balance being iron and inevitable impurities, directly or after cooled once and performing hot rolling in which rolling completion temperature is 850 to 950° C., thereafter rapidly cooling to 500 to 650° C. at an average cooling rate of 10° C./s or more, and thereafter coiling on a coil and slowly cooling to 400° C. by taking 1.0 hour or more;
a cold rolling step of performing cold rolling so that a total reduction ratio is 30 to 75% after pickling subsequently to the hot rolling step;
an annealing step of heating the steel sheet after the cold rolling step to 750° C. or more with an average heating rate between 600 and 750° C. being 20° C/s or less, then cooling with an average cooling rate between 750 and 650° C. being 1.0 to 15.0° C/s and cooling with an average cooling rate from 650° C. being 3.0° C./s or more, and performing, while retaining for 20 to 1000 seconds in the temperature range of 300 to 470° C. and while applying a tension of 5 to 100 MPa in this temperature range, one or more times of bending with a bending radius of 800 mm or less;
a plating step of performing hot-dip galvanizing on surfaces of the steel sheet to form a plating layer by immersing after the annealing step the steel sheet in a galvanizing bath under the conditions of plating bath temperature: 450 to 470° C., steel sheet temperature at a time of entering the plating bath: 430 to 490° C., and effective Al amount in the plating bath: 0.01 to 0.18 mass %; and
a cooling step of cooling at an average cooling rate of 0.5° C./s or more to 150° C. or less after the plating step,
wherein the annealing step is such that in a preheating zone under the condition of an air ratio: 0.7 to 1.2 in a mixed gas of air and fuel gas used in a preheating burner, the air ratio being a ratio of a volume of air contained in the mixed gas in a unit volume and a volume of air logically needed for complete combustion of fuel gas contained in the mixed gas in the unit volume, oxides are generated in a steel sheet surface layer portion by passing through while heating to a steel sheet temperature of 400 to 800° C., then the oxides generated in the preheating zone are reduced by heating to 750° C. or more in a reduction zone with a partial pressure ratio P(H 2 O)/P(H 2 ) between water vapor (H 2 O) and hydrogen (H 2 ): 0.0001 to 2.0, and thereafter cooling is performed.

7. The manufacturing method of a high-strength alloyed hot-dip galvanized steel sheet according to claim 6 , the method comprising,
after the hot rolling step, the cold rolling step, the annealing step, and the plating step are performed and before the cooling step, performing an alloying treatment on a plating layer formed in the plating step at temperatures of 470 to 620° C.

Assignees

• Nippon Steel & Sumitomo Metal Corp

Inventors

• Kawata Hiroyuki
• Maruyama Naoki
• Murasato Akinobu
• Minami Akinobu
• Yasui Takeshi
• Kuwayama Takuya
• Ban Hiroyuki
• Hiramatsu Kaoru

Classifications

• C22C38/04

  containing manganese · CPC title

• C22C38/001 Primary

  containing N · CPC title

• C21D2211/001

  Austenite · CPC title

• C21D8/0226

  Hot rolling · CPC title

• C21D8/0263 Primary

  following hot rolling · CPC title