High strength galvanized steel sheet and production method therefor

Patent metadata

Field	Value
Publication number	US-10329638-B2
Application number	US-201515329117-A
Country	US
Kind code	B2
Filing date	Jun 10, 2015
Priority date	Jul 25, 2014
Publication date	Jun 25, 2019
Grant date	Jun 25, 2019

Abstract

A high strength galvanized steel sheet that has a tensile strength (TS) of 780 MPa or higher and excellent bendability, fatigue resistance, and surface appearance quality and a method for producing the high strength galvanized steel sheet. The steel sheet includes, at a position ½ of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction.

First claim

The invention claimed is:

1. A high strength galvanized steel sheet having a composition comprising, in terms of % by mass, C: 0.05% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 2.2% or more and 3.5% or less, P: 0.001% or more and 0.050% or less, S: 0.010% or less, sol. Al: 0.005% or more and 0.100% or less, N: 0.0001% or more and 0.0060% or less, at least one element selected from the group consisting of Nb: 0.01% or more and 0.10% or less and Ti: 0.01% or more and 0.10% or less, and the balance being Fe and unavoidable impurities,
wherein a microstructure at a position ½ of a thickness of the steel sheet includes 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction,
a hardness at a position 5 μm from a surface layer of the steel sheet in a thickness direction is 80% or less of a hardness at the position ½ of the thickness,
a hardness at a position 15 μm from the surface layer of the steel sheet in the thickness direction is 90% or more of the hardness at the position ½ of the thickness, and
the high strength galvanized steel sheet has a tensile strength of 780 MPa or higher.

2. The high strength galvanized steel sheet according to claim 1 , wherein the composition further comprises, in terms of % by mass, at least one element selected from the group consisting of Mo: 0.05% or more and 1.00% or less, V: 0.02% or more and 0.50% or less, Cr: 0.05% or more and 1.00% or less, and B: 0.0001% or more and 0.0030% or less.

3. A method for producing a high strength galvanized steel sheet that has a tensile strength of 780 MPa or higher and includes, at a position ½ of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction, in which a hardness at a position 5 μm from a surface layer of the steel sheet in a thickness direction is 80% or less of a hardness at the position ½ of the thickness and a hardness at a position 15 μm from the surface layer of the steel sheet in the thickness direction is 90% or more of the hardness at the position ½ of the thickness, the method comprising:
hot rolling a steel slab having a composition comprising, in terms of % by mass, C: 0.05% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 2.2% or more and 3.5% or less, P: 0.001% or more and 0.050% or less, 0.010% or less, sol. Al: 0.005% or more and 0.100% or less, N: 0.0001% or more and 0.0060% or less, at east one element selected from the group consisting of Nb: 0.01% or more and 0.10% or less and Ti: 0.01% or more and 0.10% or less, and the balance being Fe and unavoidable impurities;
cold rolling the resulting hot rolled sheet;
performing primary annealing, soft reduction rolling, pickling, and secondary annealing; and
galvanizing the resulting steel sheet,
wherein in the primary annealing, heating is carried out at an average heating rate of 0.1° C./second or more and less than 3° C./second in a temperature range of 600° C. to 750° C., an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds, and cooling is carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature to a cooling stop temperature not higher than 600° C.,
in the soft reduction rolling, soft reduction is carried out at a rolling reduction of 0.3% or more and 2.0% or less,
in the pickling, a pickling weight loss of the steel sheet is adjusted to be in the range of 0.05 to 5 g/m 2 on an Fe basis,
in the secondary annealing, an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds and cooling is then carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature, and the resulting steel sheet is dipped in a zinc coating bath to galvanize the steel sheet, and
cooling is carried out to a temperature of 150° C. or lower at an average cooling rate in the range of 5 to 100° C./second after the galvanizing.

4. The method for producing a high strength galvanized steel sheet according to claim 3 , wherein in the hot rolling, cooling is started within 3 seconds after completion of finish rolling and the resulting steel sheet is cooled at an average cooling rate in the range of 5 to 200° C./second in a temperature range of a finish rolling temperature to 100° C. below the finish rolling temperature and is coiled at a coiling temperature in the range of 450° C. to 650° C., and
in the cold rolling, rolling is carried out at a rolling reduction of 30% or more.

5. The method for producing a high strength galvanized steel sheet according to claim 3 , wherein the method further comprises alloying treatment after the galvanizing.

6. The method for producing a high strength galvanized steel sheet according to claim 4 , wherein the method further comprises alloying treatment after the galvanizing.

7. A method for producing a high strength galvanized steel sheet that has a tensile strength of 780 MPa or higher and includes, at a position ½ of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction, in which a hardness at a position 5 μm from a surface layer of the steel sheet in a thickness direction is 80% or less of a hardness at the position ½ of the thickness and a hardness at a position 15 μm from the surface layer of the steel sheet in the thickness direction is 90% or more of the hardness at the position ½ of the thickness, the method comprising:
hot rolling a steel slab having a composition comprising, in terms of % by mass, C: 0.05% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 2.2% or more and 3.5% or less, P: 0.001% or more and 0.050% or less, S: 0.010% or less, sol. Al: 0.005% or more and 0.100% or less, N: 0.0001% or more and 0.0060% or less, at least one element selected from the group consisting of Nb: 0.01% or more and 0.10% or less and Ti: 0.01% or more and 0.10% or less, at least one element selected from the group consisting of Mo: 0.05% or more and 1.00% or less, V: 0.02% or more and 0.50% or less, Cr: 0.05% or more and 1.00% or less, and B: 0.0001% or more and 0.0030% or less, and the balance being Fe and unavoidable impurities;
cold rolling the resulting hot rolled sheet;
performing primary annealing, soft reduction rolling, pickling, and secondary annealing; and
galvanizing the resulting steel sheet,
wherein in the primary annealing, heating is carried out at an average heating rate of 0.1° C./second or more and less than 3° C./second in a temperature range of 600° C. to 750° C., an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds, and cooling is carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature to a cooling stop temperature not higher than 600° C.,
in the soft reduction rolling, soft reduction is carried out at a rolling reduction of 0.3% or more and 2.0% or less,
in the pickling, a pickling weight loss of the steel sheet is adjusted to be in the range of 0.05 to 5 g/m 2 on an Fe basis,
in the secondary annealing, an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds and cooling is then carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature, and the resulting steel sheet i

Assignees

• Jfe Steel Corp

Inventors

• Kimura Hideyuki
• Hasegawa Hiroshi
• Kohsaka Noriaki
• Aoyama Mai

Classifications

• B21B2001/225

  by hot-rolling · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C22C38/32

  with boron · CPC title

• C21D8/0263

  following hot rolling · CPC title

• C22C18/00

  Alloys based on zinc · CPC title