US 2014/0234659 A1Published application

High-strength hot-dip galvanized steel sheet having excellent delayed fracture resistance and manufacturing method thereof

AssigneeNippon Steel & Sumitomo Metal Corporation

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

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Abstract

A hot-dip galvanizing layer or an alloyed hot dip galvanizing layer is formed on the surface of a base steel sheet in which in volume fraction, 40 to 90% of a ferrite phase and 5% or less of a retained austenite phase are contained, and a ratio of non-recrystallized ferrite to the entire ferrite phase is 50% or less in volume fraction, and further a grain diameter ratio being a value of, of crystal grains in the ferrite phase, an average grain diameter in the rolling direction divided by an average grain diameter in the sheet width direction is 0.75 to 1.33, a length ratio being a value of, of hard structures dispersed in island shapes, an average length in the rolling direction divided by an average length in the sheet width direction is 0.75 to 1.33, and an average aspect ratio of inclusions is 5.0 or less.

Claims11

Dependent claims

A high-strength hot-dip galvanized steel sheet having excellent delayed fracture resistance, comprising:

a base steel sheet made of steel containing:
in 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%;
O:
0.0001 to 0.0100%;
N:
0.0001 to 0.0100%;
and
a balance being composed of Fe and inevitable impurities;
and
a hot-dip galvanizing layer formed on the surface of said base steel sheet, wherein
in a range of ⅛ thickness to ⅜ thickness with the position of ¼ thickness of the sheet thickness of said base steel sheet from the surface of said base steel sheet being the center, a structure of said base steel sheet is turned into a structure in which 40 to 90% in volume fraction of a ferrite phase is contained, a retained austenite phase is 5% or less in volume fraction, and further a ratio of non-recrystallized ferrite to the entire ferrite phase is 50% or less in volume fraction,
a grain diameter ratio of crystal grains in theferrite phase in said base steel sheet is 0.75 to 1.33, wherein said grain diameter ratio is defined as the ratio of an average grain diameter in the rolling direction divided by an average grain diameter in the sheet width direction of said phase, and a length ratio of hard structures dispersed in island shapes in the ferrite phase as 0.75 to 1.33, wherein said length ratio is defined as an average length in the rolling direction divided by an average length in the sheet width direction of said structures, and further an average aspect ratio of inclusions contained in said base steel sheet is 1.0 to 5.0, and
a surface layer of said base steel sheet is turned into a decarburized layer having a thickness of 0.01 to 10.0 μm, and further an average grain diameter of oxides in the decarburized layer is 30 to 500 nm, and an average density of the oxides in the decarburized layer is in a range of 1.0×1012 oxides/m2 to 1.0×1016 oxides/m2.

22 more elements

A manufacturing method of a high-strength hot-dip galvanized steel sheet having excellent delayed fracture resistance, comprising:

a hot rolling step in which a slab containing:
in 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%;
O:
0.0001 to 0.0100%;
N:
0.0001 to 0.0100%;
and
a balance being composed of Fe and inevitable impurities is heated to 1080° C. or higher, hot rolling is started, the total number of passes (−) from hot rolling start to hot rolling finish is set to N, a rolling temperature (° C.) at the ith pass is set to TPi, and a reduction ratio (−) at the ith passis set to ri, the hot rolling is performed in such a manner that N, TPi, and ri satisfy Expression A below, and the hot rolling is finished when the temperature of a base steel sheet is a temperature in the 850-to-980° C. range;
a primary cooling step in which an elapsed time from the finish of the hot rolling to start of cooling is set to 1.0 second or longer, the hot-rolled base steel sheet is primarily cooled at a cooling rate of not less than 5° C./second nor more than 50° C./second, and the primary cooling is stopped when the temperature of the base steel sheet is a temperature in the 500-to-650° C. range;
subsequently to said primary cooling step, a secondary cooling step in which the base steel sheet is slowly cooled in such a manner that an elapsed time until the temperature of the base steel sheet becomes 400° C. from the temperature at the time of the primary cooling being stopped becomes one hour or longer, and is secondarily cooled;
after the secondary cooling, a cold rolling step of cold rolling the base steel sheet by setting the total reduction ratio to 30 to 75%;
after the cold rolling, an annealing step in which the temperature is increased, in such, a manner that an average temperature increasing rate in the 600-to-750° C. range becomes 20° C./sec or less, the cold-rolled base steel sheet is heated to a temperature of 750° C. or higher, and subsequently the heated basesteel sheet is cooled in such a manner that an average cooling rate in the 750-to-650° C. range becomes 1.0 to 15.0° C./second;
and
a plating step of performing hot-dip galvanizing on the surface of the base steel sheet obtained after said annealing step.
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89 more elements

Dependencies are inferred from the claim text.

Facts

Application: 14/348,335Filed 28 Sep 2012
Priority: JP 2011-21877630 Sep 2011
Published: 21 Aug 2014
Term ends: Not estimatedThe source records don’t support an estimate.

Classification34 codes

B21B 1/26
In a continuous or semi-continuous process: by hot-rolling , e.g. Steckel hot mill
B21B 3/00
Rolling materials of special alloys so far as the composition of the
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 8/1255
Characterised by the heat treatment: with diffusion of elements, e.g. decarburising, nitriding