US 2014/0212684 A1Published application

High-strength galvanized steel sheet excellent in bendability and manufacturing method thereof

AssigneeNippon Steel & Sumitomo Metal Corporation

InventorsHiroyuki Kawata, Naoki Maruyama, Akinobu Murasato, Akinobu Minami, Takeshi Yasui, Takuya Kuwayama, Shigeru Yonemura

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Abstract

The present invention provides a high-strength galvanized steel sheet with maximum tensile strength of 900 MPa or more. The high-strength galvanized steel sheet has an alloyed galvanized layer formed on a surface of a base steel sheet containing predetermined amounts of C, Si, Mn, P, S, Al, N, O with a balance being constituted of iron and inevitable impurities, in which in a structure of the base steel sheet, retained austenite is limited to 8% or less in volume fraction, kurtosis K* of the hardness distribution between 2% hardness and 98% hardness is −0.30 or less, a ratio between Vickers hardness of surface layer of the base steel sheet and Vickers hardness of ¼ thickness of the base steel sheet is 0.35 to 0.70, and a content of iron in the alloyed galvanized layer is 8 to 12% in mass %.

Claims11

Dependent claims

A high-strength galvanized steel sheet excellent in bendability with maximum tensile strength of 900 MPa or more, comprising an alloyed galvanized layer formed on a surface of a base steel sheet containing, in mass %,

C:
0.075 to 0.300%,
Si:
0.30 to 2.50%,
Mn:
1.30 to 3.50%,
P:
0.001 to 0.050%,
S:
0.0001 to 0.0100%,
Al:
0.005 to 1.500%,
N:
0.0001 to 0.0100%, and
O:
0.0001 to 0.0100% with a balance being constituted of iron and inevitable impurities, wherein:
retained austenite is limited to 8% or less in volume fraction in a range of ⅛ thickness to ⅜ thickness of the base steel sheet;
when plural measurement regions with a diameter of 1 μm or less are set in the range of ⅛ thickness to ⅜ thickness of the base steel sheet, measurement values of hardness in the plural measurement regions are arranged in an ascending order to obtain a hardness distribution, an integer N0.02 is obtained, which is a number obtained by multiplying a total number of measurement values of hardness by 0.02 and rounding up this number when this number includes a fraction, hardness of a measurement value which is N0.02-th largest from a measurement value of minimum hardness is taken as 2% hardness, an integer N0.98 is obtained, which is a number obtained by multiplying a total number of measurement values of hardness by 0.98 and rounding down this number when this number includes a fraction, and hardness of a measurement value which is N0.98-th largest from a measurement value of minimum hardness is taken as 98% hardness, kurtosis K* of the hardness distribution between the 2% hardness and the 98% hardness is −0.30 or less;
a ratio between Vickers hardness of surface layer of the base steel sheet and Vickers hardness of ¼ thickness of the base steel sheet is 0.35 to 0.70;
and
a content of iron in the alloyed galvanized layer is 8 to 12% in mass %.

18 more elements

A manufacturing method of a high-strength galvanized steel sheet excellent in bendability, the method comprising:

a hot-rolling step of heating to 1050° C. or more a slab containing, in mass %,
C:
0.075 to 0.300%,
Si:
0.30 to 2.50%,
Mn:
1.30 to 3.50%,
P:
0.001 to 0.050%,
S:
0.0001 to 0.0100%,
Al:
0.005 to 1.500%,
N:
0.0001 to 0.0100%, and
O:
0.0001 to 0.0100% with a balance being constituted of iron and inevitable impurities, completing hot rolling at a finish hot-rolling temperature of 880° C. or more, and coiling in a temperature region of 750° C. or less;
a continuous annealing step of heating the steel sheet in a temperature range between 600° C. and Ac1 transformation point at an average heating rate of 1° C. or more, retaining the steel sheet for 20 seconds to 600 seconds at an annealing temperature between (Ac1 transformation point+40)° C. and Ac3 transformation point and in an atmosphere in which log(water partial pressure/hydrogen partial pressure) is −3.0 to 0.0, performing bending-unbending deformation processing two or more times using a roll with a radius of 800 mm or less so as to make a difference in accumulated strain amount between a front and rear surface be 0.0050 or less, thereafter cooling the steel sheet in the temperature range of 740° C. to 650° C. at an average cooling rate of 1.0 to 5.0° C./second, and cooling the steel sheet in the temperature range of 650° C. to 500° C. at an average cooling rate of 5 to 200° C./second;
and
a plating alloying step of performing an alloying treatment including dipping the steel sheet after the continuous annealing step in a galvanizing bath, and then retaining the steel sheet at a temperature of 470 to 650° C. for 10 to 120 seconds.

17 more elements

Dependencies are inferred from the claim text.

Facts

Application: 14/234,826Filed 27 Jul 2012
Priority: JP 2011-16743629 Jul 2011
Published: 31 Jul 2014
Term ends: Not estimatedThe source records don’t support an estimate.

Classification35 codes

C23C 2/28
After-treatment: thermal after-treatment, e.g. treatment in oil bath
B32B 15/013
All layers being exclusively metallic: one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or…
C21D 8/0263
Characterised by the heat treatment: following hot rolling
C21D 8/041
To produce plates or strips for drawing, e.g. for deep-drawing: involving a particular fabrication or treatment of ingot or slab