GALVANIZED STEEL SHEET HAVING MAXIMUM
TENSILE STRENGTH OF 980 MPa OR MORE, EXCELLENT DELAYED
FRACTURE RESISTANCE, PLATING ADHESION, ELONGATION, AND HOLE
EXPANSIBILITY AND METHOD OF MANUFACTURING THE SAME
[Technical Field]
[0001]
The present invention relates to a galvanized steel sheet which has maximum tensile strength (TS) of 980 MPa or more and is excellent in delayed fracture resistance, plating adhesion, elongation, and hole expansibility. The galvanized steel sheet according to the present invention is particularly suitable for structural members, reinforcing members, and suspension members for automobiles. Here, the coated steel sheet according to the present invention is a hot-dip galvanized steel sheet or a galvannealed steel sheet. In the coated layer, in addition to pure zinc, it may be contained Fe, Al, Mg, Mn, Si, Cr, Ni, Cu or the like. [Backgroimd Art] [0002]
In members such as cross members and side members for automobiles, a weight reduction has been investigated to respond to a recent trend toward a reduction in fuel consumption, and it has been attempted to increase the strength of a steel sheet jfrom the viewpoint of ensuring the strength and collision safety of automobiles even when a thinner steel sheet is used for the members. However, since increasing the strength of the steel sheet leads to a deterioration of the formability of materials, in order to realize the weight reduction of the members, it is necessary to manufacture a steel sheet which satisfies both press formability and high strength. [0003]
Particularly, when the steel sheet is formed as structural members or reinforcing members for automobiles which have a complex shape, the steel sheet having excellent ductility is required. In recent years, a steel sheet having a maximum tensile strength of 440 MPa class or 590 MPa class has been mainly used for frameworks of automobiles, and development of a steel sheet having a tensile strength of 980 MPa or more is desired in the future to achieve a further weight reduction. [0004]
When a steel sheet of 590 MPa class is replaced with a steel sheet of 980 MPa class, the same elongation as the elongation of the steel sheet of 590 MPa class is required in the steel sheet of 980 MPa class. Thus, development of a steel sheet which has a tensile strength of 980 MPa or more and has excellent elongation is desired. [0005]

As a steel sheet excellent in total elongation (El) in a tensile test, there is a multi-phase structure steel sheet which has a microstructure in which residual austenite as a secondary phase is dispersed in soft ferrite that is a primary phase. In the multi-phase structure steel sheet, the ductility is ensured by the ferrite and the strength is ensured by the martensitic transformation of the residual austenite, and the residual austenite is transformed into martensite at plastic working. There is a steel, which is applied the transformation, such as a transformation induced plasticity (TRIP) steel and the applications of the TRIP steel have been expanded in recent years. [0006]
Since the TRIP steel has a particularly excellent elongation compared to precipitation strengthened steel and dual phase (DP) steel (steel is consisting of ferrite and martensite), the applications of the TRIP steel is strongly desired to be expanded. Although the TRIP steel shows excellent strength and ductility, the TRIP steel has a feature of low hole expansibility in general. [0007]
Further, in order to promote a weight reduction of an automobile body in the future, a usable strength level of a high strength steel sheet should be increased as compared with that of conventional one. For example, in order to use the high strength steel sheet for a hard-to-form member such as a suspension part, formability such as hole expansibility should be improved. [0008]
In addition, when a steel sheet of 980 MPa or more is applied to the member for an automobile, in addition to properties of strength and workability, delayed fracture resistance is required. The delayed fracture is caused by stress applied to steel or hydrogen brittleness and is a phenomenon in which a structure is firactured by accumulating diffused hydrogen in a stress concentration area of the steel used as the structure. [0009]
Specifically, examples of the delayed fracture include a suddenly fractured phenomenon that a member, such as a prestressed concrete (PC) steel wire or a bolt, is suffered high stress load under the usage condition. [0010]
It is known that delayed fracture is closely related to the hydrogen which penetrates into the steel from the environment. As the hydrogen which penetrates into the steel from the environment, there are various types of hydrogen such as hydrogen which is contained in the atmosphere, hydrogen generated in a corrosive environment. When the hydrogen penetrates into the steel from any of the hydrogen, the hydrogen may induce the delayed fracture.

[0011]
For this reason, as the usage environment of the steel, an environment in absence of hydrogen is desired. However, when a steel is applied to the structxire or the automobile, the steel is used outdoors and the penetration of hydrogen cannot be avoided. [0012]
As the stress which acts on the steel used as the structure, a stress which is loaded on the structure and a residual stress, that some of stress generated at the forming remains inside of the steel, are included. Particularly, in the steel used as a member after forming such as a thin steel sheet for an automobile or the like, the residual stress is a significant problem compared to a thick steel plate or a steel bar that is a product used as a bolt or a thick plate. Accordingly, when a steel sheet that the delayed fracture is a problem is formed, it is desirable to form a steel sheet such that the residual stress does not remain. [0013]
For example, in Patent Document 1, there is disclosed a method that a strength is increased by once heating a steel sheet at a high temperature and by processing the steel sheet and then by quenching the steel sheet using a die. In this method, since the steel sheet is processed at a high temperature, residual stress is alleviated by recovering dislocation which causes the residual stress and which is introduced at the processing, or by causing transformation after the processing. ITierefore, very little residual stress remains in a formed product. It is possible to improve the delayed fracture resistance of the steel sheet by strengthening the steel sheet using this method. [0014]
In addition, since the residual stress is present on a cutting surface in machining such as cutting or punching, there is a concern of causing delayed fracture. Thus, when a high strength steel sheet having a tensile strength of 980 MPa or more is processed, the steel sheet is cut by a method using a laser or the like which is not accompanied by machining, and the generation of residual stress is avoided. However, the laser cutting costs more compared to shear cutting or punching. [0015]
To solve these problems, a material capable of avoiding delayed fracture by improving hydrogen embrittlement resistance has been developed. For example, in Non-Patent Document 1, there is disclosed a high strength bolt having excellent hydrogen embrittlement resistance that the steel is quenched from austenite single phase at high temperature so as to obtain a martensite single phase microstructure and then the above fine precipitates of elements such as Cr, Mo, V and the like, which exhibit temper softening resistance, are coherently precipitated in the martensite by tempering. [0016]

In the method, the hydrogen penetrated into the steel is inhibited from being diffused or being concentrated on an area as a starting point of delayed fracture where stress is concentrated by using the hydrogen penetrated into the steel being trapped aroimd the fine precipitates such as VC and the like which are coherently precipitated in the martensite. Conventionally, steel having high strength and excellent in delayed fracture resistance has been developed by utilizing this method. [0017]
The improvement of the delayed fracture resistance due to utilizing the precipitates as hydrogen trap sites such as VC and the like is brought by coherently precipitating of the precipitates into the martensite structure. Therefore, it is necessary to coherently precipitate the precipitates in the structure. [0018]
However, several hours or more of heat treatment is necessary to precipitate the precipitates, and there is a problem in manufacturability. That is, in a steel sheet manufactured by using general manufacturing facilities for a thin steel sheet such as continuous annealing facilities or continuous hot dip galvanizing facilities, texture control is performed in a short period of time such as several tens of minutes at most. Thus, it is difficult to improve delayed fracture resistance by the precipitates. [0019]
In addition, when precipitates that are precipitated in a hot rolling process are utilized, even if the above precipitates are precipitated in the hot rolling process, an orientation relationship between the precipitates and a base structure such as ferrite and martensite is lost due to processing of the steel sheet at the subsequent cold rolling and due to recrystallization during continuous anneahng. That is, in this case, the precipitates are not coherent precipitates. As a result, the delayed fracture resistance of the obtained steel sheet is significantly deteriorated. [0020]
A high strength steel sheet in which there is concern of generation of delayed firacture usually has a microstructure mainly including martensite. The martensite is formed at a low temperature, and the precipitates including VC as the hydrogen trap sites cannot be precipitated in the low temperature region in which the martensite is fonned. [0021]
As a result, in the thin steel sheet, if it is intended that the coherent precipitates such as VC are participated in order to improve the delayed fracture resistance, it is necessary to precipitate the precipitates by additionally performing heat treatment after the microstructure of the steel is once formed by using the continuous annealing facilities or the continuous hot dip galvanizing facilities. This process brings about a significant increase in manufacturing cost.

[0022]
In addition, when the above heat treatment is additionally performed on the microstructure mainly including martensite, the problem that the martensite is drastically softened is generated. As a result, it is difficult to utilize the coherent precipitates such as VC in order to improve the delayed fracture resistance of the high strength thin steel sheet. [0023]
Since the steel described in Non-Patent Document 1 is steel including 0.4% or more of C and a large amoimt of alloy elements, the workability and the weldability which are required for the thin steel sheet are deteriorated. [0024]
In Patent Document 2, there is disclosed a technology in which hydrogen defects are prevented by oxides mainly including Ti, and Mg. However, in the technology disclosed in Patent Document 2, the target is thick steel plate, although hydrogen brittleness is considered after welding of high heat input, both the high formability and hydrogen brittleness resistance, which are required for a thin steel sheet, are not considered at all. [0025]
Conventionally, in a thin steel sheet, (1) since the sheet thickness is thin, even when hydrogen penetrates into the thin steel sheet, the hydrogen is released to the outside in a short period of time. Further, (2) since workability is prioritized, a steel sheet having a tensile strength of 900 MPa or more has not been used before. For this reason, problems of delayed fracture have been small. However, since a demand for using the high strength steel sheet is rapidly increasing, the development of a high strength steel sheet having excellent hydrogen brittleness resistance has been required. [0026]
The technologies for improving the hydrogen brittleness resistance that are mostly related to steel such as bolts, steel bars, and plate steel have been developed. The steel is not almost subjected to forming and is often used at proof stress or yield stress or less. That is, in the related art, both of the workability required for automobile members, such as cuttability or member formability (press formability), and the hydrogen embrittlement resistance after processing are not considered. [0027]
In a member after forming, a stress that is referred to as a residual stress remains the inside of the member. Although the residual stress is present in the local, the residual stress has a high value exceeding the yield stress of material in some cases. For this reason, it is required that hydrogen embrittlement not generate in the thin steel sheet under high residual stress.

[0028]
Regarding the hydrogen brittleness of the thin steel sheet, for example, Non-Patent Document 2 reports the aggravation of hydrogen brittleness due to strain induced transformation of residual austenite. Non-Patent Document 2 discloses that a formation of thin steel sheet has been considered, but that an amoimt of the residual austenite is regulated so as not to cause deterioration in the hydrogen brittleness resistance. [0029]
That is, the technology described in Non-Patent Document 2 relates to a high strength thin steel sheet having a particular microstructure, and is not a basic and enough countermeasure for improving hydrogen embrittlement resistance. In addition, in the steel sheet actively utilizing of the residual austenite as like as the steel sheet of the present invention, this method is not a countermeasure. As described above, it is very difficult to obtain a steel sheet which simultaneously demonstrates high corrosion resistance, high tensile strength, excellent delayed jSracture resistance and high ductility. [Prior Art Document] [Patent Document] [0030]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-18531
[Patent Document 2] J^anese Unexamined Patent Application, First Publication No. HI 1-293383
[Non-Patent Documents] [0031]
pSTon-Patent Document 1 ] "New Developments in Elucidation of Hydrogen Embrittlement" (the Iron and Steel Institute of Japan, January 1997) [Non-Patent Document 2] CAMP-ISU, Vol. 5, No. 6, Pages 1839 to 1842, Yamazaki et al., October 1992, issued by the Iron and Steel Institute of Japan [Summary of the Invention] [Problem to be solved by the Invention] [0032]
An object of the present invention is to provide a galvanized steel sheet (including a galvannealed steel sheet) which has a tensile strength (TS) of 980 MPa or more emd which has excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility. [Means for Solving the Problem] [0033]
The inventors have been investigated. As a result, the inventors have found

that when plating capable of improving delayed fracture resistance is performed as means for improving the delayed fracture resistance without influence on steel quality, the delayed fracture resistance of the steel is improved. [0034]
Specifically, when the hydrogen which penetrates from the environment is trapped with the oxide by dispersing an oxide of Si, Mn, and AI in a plating layer, it has been found that diffusion of hydrogen into a stress concentration area and delayed fracture caused by the diffusion of hydrogen into the stress concentration area can be delayed. [0035]
In order to achieve both tensile strength (TS) of 980 MPa or more and excellent formability, it has been found that it is unportant to form a ferrite as a primary phase with a volume fraction of 40% or more and a residual austenite as a secondary phase with a volume fraction of 8% or more in a microstructure by ftiUy utilizing Si which is a strengthening element. [0036]
TTiat is, the present invention can provide a hot-dip galvanized steel sheet which has tensile strength (TS) of 980 MPa or more and has excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility, and the gist of the invention is as follows. [0037]
(1) A hot-dip galvanized steel sheet having maximum tensile strength of 980
MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high
elongation and excellent hole expansibility includes, by mass%, C: 0.05 to 0.40%, Si: 0.5
to 3.0%, Mn: 1.5 to 3.0%, P: limited to 0.04% or less, S: limited to 0.01% or less, O:
0.0001 to 0.01% or less, Al: 0.0005 to 2.0% or less, N: limited to 0.01% or less, and Si x
Al > 0.5 %, and a balance consisting of Fe and imavoidable impurities, a microstructure
of the steel sheet includes 40% or more of one or two of a tempered martensite and a
bainite by volume fraction as a primary phase and 8% or more of an austenite, and a
balance consisting of ferrite, includes a hot-dip galvanized layer havmg less than 7
mass% of Fe and the balance consisting of Zn, Al and unavoidable impurities on a
surface of the steel sheet in which 10% or less of a pearlite is included, the plating layer
includes one or two or more of an oxide of Si, Mn, or Al.
[0038]
(2) A hot-dip galvanized steel sheet having maximum tensile strength of 980
MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high
elongation and excellent hole expansibility includes, by mass%, C: 0.05 to 0.40%, Si: 0.5

to 3.0%, Mn: 1.5 to 3.0%, P: limited to 0.04% or less, S: limited to 0.01% or less, O: 0.0001 to 0.01% or less, Al: 0.0005 to 2.0% or less, N: limited to 0.01% or less, and Si x Al > 0.5 %, and a balance consisting of Fe and unavoidable impurities, a microstructure of the steel sheet includes 40% or more of one or two of a tempered martensite and a bainite by volume fraction as a primary phase and 8% or more of an austenite, and a balance consisting of ferrite, includes a galvannealed layer having 7 to 15 mass% of Fe and the balance consisting of Zn, Al and unavoidable impurities on a surface of the steel sheet in which 10% or less of a pearlite is included, the plating layer includes one or two or more of an oxide of Si, Mn, or Al. [0039]
(3) In the hot-dip galvanized steel sheet having maximum tensile strength of 980
MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high
elongation and excellent hole expansibility according to (1) or (2), the steel may further
include, by mass%, one or two or more of Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ni: 0.05 to
1.0%, and Cu: 0.05 to 1.0%.
[0040]
(4) In the hot-dip galvanized steel sheet having maximum tensile strength of 980
MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high
elongation and excellent hole expansibility according to any one of (1) to (3), the steel
may further include, by mass%, one or two or more of Nb: 0.005 to 0.3%, Ti: 0.005 to
0.3%, and V: 0.005 to 0.5%.
[0041]
(5) In the hot-dip galvanized steel sheet having maximum tensile strength of 980
MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high
elongation and excellent hole expansibility according to any one of (1) to (4), the steel
may fiirther include, by mass%, B: 0.0001 to 0.01%.
[0042]
(6) In the hot-dip galvanized steel sheet having maximum tensile strength of 980
MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high
elongation and excellent hole expansibility according to any one of (1) to (5), the steel
may further include, by mass%, atotal of one or two or more of Ca, Mg, and REM:
0.0005 to 0.04%.
[0043]
(7) In the hot-dip galvanized steel sheet having maximum tensile strength of 980
MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high
elongation and excellent hole expansibility according to any one of (1) to (6), the oxide in
the plating layer is membranous and shares 10% or more by area fraction.
[0044]

(8) The the hot-dip galvanized steel sheet having maximum tensile strength of
980 MPa or more and excellent delayed jfracture resistance, excellent plating adhesion,
high elongation and excellent hole expansibility, the steel sheet having the component
composition according to any one of (1) to (6) is dipped in molten zinc plating bath in
which a flow rate changes in a range of 10 m/min to 50 m/min and the hot-dip
galvanizing is performed to the steel.
[0045]
(9) The manufacturing method of hot-dip galvanized steel sheet having
maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance,
excellent plating adhesion, high elongation and excellent hole expansibility according to
any one of (1) to (6), heating at 1200°C or more the casting slab directly or after once
cooled, the hot rolling is completed at a temperature of an Axs transformation point or
higher, winding at 700°C or lower, pickling and cold rolling the steel, retaining the steel
at 550°C to 750°C for 20 seconds or more when the steel sheet pass through a continuous
hot-dip galvanizing line, and annealing the steel at 750°C to 900°C, cooling the steel to
an intermediate cooling temperature in a temperature range of 500°C or higher and lower
than 750°C at a first average cooling rate of 0.1 °C/s to 20 °C/s, fiirther cooling at that
temperature to 350°C or higher and lower than 500°C, and the hot-dip galvanizing is
performed to the steel under the condition according to (8).
[0046]
(10) The manufacturing method of hot-dip galvanized steel sheet havmg
maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance,
excellent plating adhesion, high elongation and excellent hole expansibility according to
any one of (1) to (6), heating at 1200°C or more the casting slab directly or after once
cooled, the hot rolling is completed at a temperature of an Ars transformation point or
higher, winding at 700°C or lower, pickling and cold rolling the steel, retaining the steel
at 550°C to 750°C for 20 seconds or more when the steel sheet pass through a continuous
hot-dip galvanizing line, and annealing the steel at 750°C to 900°C, cooling the steel to
an intermediate cooling temperature in a temperature range of 500°C or higher and lower
than 750°C at a first average cooling rate of 1 °C/s to 20 °C/s, fiirther cooling to room
temperature or higher and lower than 500°C with a cooling rate of 1 °C/s or more, and
reheating to 350°C to 500°C and holding for 20 seconds or more in the temperature range
of 350°C to 500°C, and the hot-dip galvanizing is performed to the steel under the
condition according to (8), and cooling to a room temperature.
[0047]
(11) The manufacturing method of hot-dip galvanized steel sheet having
maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance,
excellent plating adhesion, high elongation and excellent hole expansibility according to

10
(8) or (9), may further include heating the steel to 460°C to 600°C to perform alloy
treatment after the hot-dip galvanizing is performed to the steel and cooling to a room
temperature.
[Effects of the hivention]
[0048]
According to the above aspects of the present invention, it is possible to provide the a hot-dip galvanized steel sheet (including a galvannealed steel sheet) which is suitable for structural members, reinforcing members, and suspension members for automobiles and which has a tensile strength of 980 MPa or more, excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility, at a low cost. [Brief Description of the Drawings] [0049]
FIG. 1 is a view schematically illustrating a method of calculating an area fraction of an oxide according to the present invention.
FIG. 2 is a view illustrating an observation result (in which the oxide is dispersed in the plating layer) using FE-TEM at 50 000 times after processing by FIB on a cross section of galvannealed steel sheet according to the present invention. [Description of Embodiments] [0050]
The inventors have been investigated so as to solve the above problems. As a result, the inventors have found that when an oxide which includes at least one of Si, Mn, and Al is dispersed in a plating layer, the oxide can be utilized as a hydrogen trap site and the delayed fracture resistance of a steel sheet (galvanized steel sheet) is improved. In addition, the inventors also have found that when the steel sheet is held at 550X to 750°C during heating in annealing, and the oxide which includes at least one of Si, Mn, and Al is formed on the outermost layer of the steel sheet, it is possible to obtain a galvanized steel sheet having a plating layer that the oxide is dispersed by the subsequent plating or by the subsequent plating and alloy treatment. [0051]
Hereinafter, content of the present invention will be described in detail. [0052]
First, the plating layer 3 will be described. [0053]
In this present invention, the plating layer contains an oxide including one or two or more of Si, Mn, and Al. It is most important to disperse such an oxide in the plating layer. Particularly, when the oxide 3a is dispersed in a region of the plating layer within 5 fim from an interface between the steel sheet and the plating layer, a hydrogen trappmg

11
effect becomes remarkable. [0054]
Although a detailed mechanism is not clear, the oxide includes large number of defects, and thus, the oxide in the plating layer traps hydrogen which penetrates from the surface of the plated steel sheet, for example, hydrogen generated by corrosion reaction or hydrogen in the atmosphere, and the penetration of the hydrogen into the steel sheet is delayed. As a resuh, it is considered that delayed fracture resistance is improved. [0055]
In addition, since a steel sheet for an automobile is used in an environment where a wet environment and a dry environment are repeated, the hydrogen that is trapped by the oxide is released to the atmosphere in the dry environment. Therefore, in an actual environment where an automobile is used, it is possible to continuously use a hydrogen trapping effect by the above oxide, and it is considered that the above plated steel sheet exhibits a high effect due to delayed fracture resistance. [0056]
The effect is remarkably exhibited by dispersing the oxide including Si, Mn, and Al in plating layer. Particularly, an oxide of Si, an oxide of Mn, an oxide of Al have a high melting point compared to zinc and are easily dispersed in the plating layer as membranous oxides having a high hydrogen trapping effect. [0057]
These oxides can be dispersed in an inside of the plating layer by coating with formation of membranous oxides on the outer layer of the steel sheet or by alloying the coating. The reason why oxides on the surface of the steel sheet are utilized, it is capable of controlling a size or number density. On the other hand, it is difficult to perform coating to a steel sheet with molten zinc including the oxides. [0058]
For example, even when the oxide is dispersed in the molten zinc, the oxide forms a cluster by Van der Waals force and grows into a large oxide having a size of several to several hundreds of [xm. As a result, since the large oxide causes non-plating or flaws, it is not preferable to disperse the oxide in a plating bath. [0059]
However, in the hot dip galvanizing bath, there is an oxide of Zn or Al as an imavoidable oxide. It is preferable to remove the oxide as much as possible or it is desirable to control a reaction with a steel sheet. However, the oxide may be unavoidably included in the coating layer. [0060]
The oxide in the plating layer must be an oxide including Si, Mn, and Al independently or a combination of Si, Mn, and Al. Because it is possible to control the

12
oxides by controlling atmosphere at annealing and it is possible to add Si, Mn, or Al into
a steel sheet.
[0061]
On the other hand, since the addition of element which is difficult to be oxidized as Ni or the like causes the oxidation of Fe besides the additional elements, it is difficult to secure a size and an amount of the oxides in addition to coatability. Thus, it is necessary to be the oxides including Si, Mn, and Al independently or a combination of Si, Mn, and Al. [0062]
It is desirable that an area fraction of the oxide for the surface of the steel sheet is 10% or more. The area fraction is coverage of the surface of the steel sheet when seen the steel sheet from the surface of the steel sheet. In this present invention, since the purpose of the oxides is that hydrogen which penetrates from the surface of the plated steel sheet can be trapped in the plating layer, it is preferable that the oxide be present in the plating layer and on a surface parallel to the surface of the steel sheet as much as possible. Here, the area fraction is set to be 10% or more. The area fraction is preferably 15% or more and is more preferably 20% or more. [0063]
The area fraction of the oxide can be easily measured by observing the cross section of the plated steel sheet. For example, as shown in FIG. 1, it is capable of evaluating an area fraction by share of the length of the oxide which is vertically inhabited an interface of the steel sheet and the plating layer. In this present invention, the ratio of the length was measured at five visual fields at a magnification of 10000 times, and an average value thereof was defined as the area fraction. [0064]
The chemical composition and the area fraction of the oxide can be evaluated by observing the structure in the cross section of the plated steel sheet. For example, there is a method that after the plated steel sheet is processed to a flake using a focused ion beam working device (FIB working device) so as to include the plating layer, the surface of the flake is observed using a field emission type transmission electron microscope (FE-TEM) and the composition analysis is performed using an energy distributed X-ray detector (EDX). [0065]
For example, in FIG. 2, an observation sample was prepared using the FIB, and then the oxide was observed using the FE-TEM at a magnification of 50,000 times. In addition, it is possible to identify the oxide by analyzing the oxide using the EDX. [0066]
In order to include the oxide including Si, Mn, and Al independentiy or a

13
combination of Si, Mn, and Al in the plating layer, in CGL annealing process, after the oxide of oxidizable elements was formed on the surface of the steel sheet, it is necessary to plate to the steel sheet so as to include the oxide into the plating layer. [0067]
When an atmosphere in CGL annealing process is controlled, it is possible to flexibly control the oxide including Si, Mn, and Al formed on the surface of the steel sheet can be controlled. That is, when a H2 concentration and a dew point in the annealing atmosphere are managed, for example, the dew point may be set as -20°C or higher in the N2 atmosphere with a H2 concentration of 20 volume% or less, which is applied in usual annealing conditions. In this case, the amount and the shape of the oxide including Si, Mn, and Al can be more flexibly controlled. [0068]
The plating layer must be a galvanized layer including an amount of Fe is limited to 15 mass% or less. When the plating layer is a galvannealed layer due to alloying process, it is possible to increase spot weldability and paintability. In particular, after the steel sheet is immersed in the zinc plating bath, by performing alloying process to the plated steel sheet, Fe in the steel sheet can be incorporated in the plating layer, and thus, the galvannealed steel sheet having excellent spot weldability and paintability can be obtained. [0069]
When an amount of Fe in the plating layer is lower than 7 mass % after performing alloying process, spot weldability is insufficient. On the other hand, when an amount of Fe in the plating layer is more than 15 mass %, adhesion of the plated layer itself is impaired. Thus, it causes flaws at forming due to adhering to the mold because the plating layer is destroyed and fell off during working. Accordingly, when the alloy treatment is performed, the amount of Fe in the plating metal is 7 mass% to 15 mass%. [0070]
Even when the amount of Fe in the plating metal is less than 7 mass%, the corrosion resistance, formability, and hole expansibility of the plated steel sheet are satisfactory. [0071]
The amoimt of the plating layer per unit area of the surface of the steel sheet is not particularly limited, but the plating amount per surface at one side is preferably 5 g/m^ or more fi-om the viewpoint of increasing corrosion resistance. Here, for the purpose of fiirther improving properties such as paintability, weldability and the like, coating films formed by various coating film treatments, for example, an upper plating layer formed by electroplating or the like, a chromate coating film formed by chromate treatment, a phosphate coating film formed by phosphate treatment, a lubrication coating

14
film, and a coating film for improving v^reldability may be provided on the surface of the
plating layer. Therefore, this does not departing from the invention.
[0072]
In order to fiirther increase plating adhesion of the plated steel sheet, before annealing, fiirther Single or multiple plating of Ni, Cu, Co, Fe may be performed to the steel sheet. Therefore, this does not departing firom the invention. [0073]
Here, detailed methods fi-om a process of pickling the steel sheet to a process of immersing the steel sheet into a plating bath are not particularly limited as long as the above conditions are satisfied. For example, as such methods, the Sendzimir process of "After degreasing and pickling, heating in a non-oxidizing atmosphere, aimealing in the reducing atmosphere containing H2 and N2, then, cooling to near a plating bath temperature, and immersing in a plating bath," the total reduction fiimace method of "Regulating the atmosphere during annealing, once oxidizing a steel sheet surface, then performing reduction of the surface of steel sheet so as to perform cleaning of the surface of steel sheet, and thereafl:er immersing in a plating bath;" the flux process of "Degreasing and pickling a steel sheet, performing flux treatment using ammonium chloride or the like, and immersing in a plating bath." may be applied wdth changes according to each process of the embodiment as necessary. [0074]
The molten metal in the plating bath may be pure zinc or may contain chemical elements such as, Fe, Mg, Mn, Si, Cr, and the like. In this manner, when the alloying of the plating layer is performed, the plated steel sheet may be heated to 460°C or higher. When the temperature of the alloying treatment is lower than 460°C, the alloying is not effectively performed at a high alloying rate, and thus, it is impossible to sufficiently increase productivity. Meanwhile, the upper limit is not limited, when the alloying temperature is higher than 600°C, carbides are formed and the volume fraction of hard microstructure (martensite, bainite, and residual austenite) is decreased in the steel in a final product. Therefore, the substantive upper limit of the alloying temperature is set to 600°C because it is difficult to secure an excellent elongation. [0075]
When the galvannealed steel sheet is manufactured or when the alloying of the plating layer is performed, the amoimt of effective Al in the plating bath is preferably controlled to 0.05 mass% to 0.500 mass% so as to control the properties of the plating layer. Here, the amount of effective Al in the plating bath is a value obtained by subtracting the amount of Fe in the plating bath from the amount of Al in the plating bath. [0076]
The reason why the amount of the effective Al is 0.05 mass% to 0.500 mass%

15
is as follows. When the amount of the effective Al is lower than 0.05 mass%, the plating layer having a good appearance cannot be obtained and dross generation cannot be suppressed. That is, when the amount of the effective AI is more than 0.05 mass%, productivity cannot be also increased sufficiently and the delay of alloying can be occurred. Thus, the upper limit of the amount of the effective AI in the plating bath is preferable to be 0.500 mass% or less. [0077]
The amount of Fe and the amoimt of Al in the plating layer may be measured by dissolving the plating layer with an acid, removing an undissolved oxide and the like, and then performing chemical analysis of an obtained solution. With respect to the galvannealed steel sheet, for example, the plated steel sheet which is cut in a size of 30 mm X 40 mm is immersed into a 5% aqueous HCl solution to which an inhibitor is added, and while liquation of the chemical elements in the steel sheet are suppressed, it is possible to obtain a solution by dissolving only the plating layer. An undissolved oxide and the like are removed iBrom the obtained solution, and then, the amount of Fe and the amount of Al may be quantified fiom the signal intensity obtained by ICP emission analysis of the solution and a calibration curve prepared from a solution of known concentration. [0078]
La addition, in this case, measurement values of at least three samples cut fi-om the same galvannealed steel sheet in consideration of unevenness in measurement among respective samples may be averaged. [0079]
The plating amount of the plating layer per unit area of the surface of the steel sheet is not particularly limited, but the plating amount per surfece at one side is preferably 5 g/m^ or more from the viewpoint of increasing corrosion resistance. In addition, from the viewpoint of increasing plating adhesion, the plating amount per surface at one side is preferably 100 g/m^ or less. Here, for the purpose of fiirther improving properties such as paintability, weldability and the like, coating films formed by various coating film treatments, for example, an upper plating layer formed by electroplating or the like, a chromate coating film formed by chromate treatment, a phosphate coating film formed by phosphate treatment, a lubrication coating film, and a coating film for improving weldability, may be provided on the surface of the plating layer. Therefore, this does not departing from the invention. [0080]
Next, the microstructure of the steel sheet of the present invention will be described. [0081]

16
The microstructure of the steel sheet of the present invention consists of one or two or more of bainite, martensite, tempered martensite and residual austenite. 30 % or less of ferrite by volume fraction may be included in the steel sheet and lower than 10 % of perlite by volume fraction may be included in the steel sheet. [0082]
In order to achieve both ductility and hole expansibility after the tensile strength of 980 Mpa or more is achieved, it is necessary to set the microstructure of the steel sheet be bainite or martensite as primary phase. The reason vdiy bainite or martensite is to be primary phase so that the tensile strength of 980 Mpa or more is secured and excellent hole expansibility is secured. [0083]
Generally, the larger the hardness difference between the structures is, the lower the hole expansibility is. For example, in a steel including ferrite and martensite, since strain is concentrated in an interface between the ferrite and the martensite during deformation and voids are generated, hole expansibility is low. Then, since the microstructure of the steel sheet includes both or any one of bainite which is hard phase and martensite as primary phase, the generation of voids is suppressed during deformation so as to improve hole expansibility. Even tempered martensite including carbide in martensite by tempering or quenched martensite can be given in either martensite. As a result, the steel sheet having tensile strength of 980 MPa or more, excellent elongation and hole expansibility can be obtained. [0084]
In the present invention, the reason why the total of bainite and martensite is 40% or more by volume fraction so that tensile strength of 980 MPa or more is obtained. [0085]
Meanwhile, since the bainite and martensite includes a large number of dislocations, the bainite and martensite has high strength, but the ductility is deteriorated. Here, the ductility is improved using the transformation induced plasticity of the residual austenite. When the volume fraction of the residual austenite is less than 8%, a sufficient ductility cannot be obtained. Therefore, the lower limit of the amount of the residual austenite is set to 8%. On the other hand, since the microstructure includes 40% or more of bainite and martensite, the amount of the residual austenite may be less than 60% in terms of volume fraction. [0086]
In the present invention, the microstructure may include 30% of ferrite by volume fraction. Ferrite may be formed by cooling after dual phase annealing or single phase region annealing so as to stabilize the residual austenite. Here, the higher the volume fraction of the ferrite is, the lower the strength is. Therefore, the upper limit of

17
the volume fraction of the ferrite is 30% or less. [0087]
In addition, 10% or less of pearlite by the volume fraction may be included in the microstructure. The pearlite is formed by the transformation of austenite. For this reason, since the pearlite reduces the amount of austenite and the amount of C in the austenite, strength and ductility are deteriorated. Therefore, it is preferable that the microstructure does not contain the pearlite. However, when the volume fraction of the pearlite is limited to 10% or less, it is possible to ensure tensile strength of 980 MPa or more and ductility. Thus, the upper limit of the amount of the pearlite is set to 10%. [0088]
In addition, each phase, the bainite, the martensite, the tempered martensite, the residual austenite, the ferrite, and the pearlite, of the above-described microstructure and the structure of the reminder are identified and existence positions of each phase are observed so as to measure an area fraction of each phase corresponding to the volume fraction of each phase. In the measurement, a cross section of the steel sheet in a rolling direction or a cross section in the right angle direction of the rolling direction was etched using a nital reagent and a reagent disclosed in Japanese Unexamined Patent Application, First Publication No. S59-219473 and are observed using an optical microscope at a magnification of 1,000 times, or a scanning type or transmission type electron microscope at a magnification of 1000 to 100000 times so as to quantify each phase. In this case, the area fractions of each phase corresponding to the volume fraction of each phase can be obtained using a point count method or using image analysis by observing each 20 view fields or more. [0089]
As described above, by confroUing the chemical composition and the microstructure of the steel sheet, the plated steel sheet having tensile strength of 980 MPa, excellent ductility and excellent hole expansibility can be obtained. [0090]
The excellent elongation of the steel sheet in the present invention means as follows. An elongation index is obtained from a product of tensile strength TS (MPa) and total elongation El (%), when the product is 16000 (MPa x %) or more (TS x El > 16000 MPa x %, the elongation is evaluated to be excellent. When the elongation is emphasized, the product (TS x El) is preferably 18000 MPa x % or more, and more preferably 20000 MPa x % or more. [0091]
The excellent hole expansibility of the steel sheet in the present invention means as follows. A hole expansibility index is obtained from a product of the tensile strength TS (MPa) and hole expansion ratio X (%), when the product is 40000 (MPa x %)

18
or more (TS x ^ > 40000 MPa x %), the hole expansibility is evaluated to be excellent. When the hole expansibility is emphasized, the product (TS x X) is preferably 45000 MPa x % or more, and more preferably 50000 MPa x % or more. [0092]
Next, a manufacturing method of a galvanized steel sheet according to the present invention will be described in detail. [0093]
In the present invention, during heating before the above-mentioned annealing, since an oxide including Si, Mn and Al is formed on the surface of the steel sheet, non-plating (a defect in plating, an unplated area) or the delay of alloying easily occurs after the steel sheet is drawn up from the plating bath. Here, in the plating bath, the molten metal is flowed at a flow rate of 10 m/rain to 50 m/min. By allowing the molten metal to flow at such a flow rate, non-plating can be prevented. In addition, when the oxide is formed on the surface of the steel sheet, in a case where the plating layer is alloyed, the alloying is delayed. However, the alloying can be promoted by controlling the above flow rate of the molten metal. As a result, it is possible to disperse the oxide in the plating. [0094]
Generally, in the hot dip galvanizing bath, an oxide film of Zn or Al floats. Here, the oxide film of Zn or Al is called as scum and causes non-plating or alloying delay. The inventors have found that when the oxide is present on the surface of the steel sheet, it is easy that the scimi adheres onto the steel sheet during immersion in the bath, and thus non-plating is easily generated. [0095]
In addition, the inventors have found a problem that the scxrai adhering onto the steel sheet causes not only non-plating, but also alloying delay. This problem becomes significant in a steel sheet which includes a large amount of Si and Mn. Although a detailed mechanism is not clear, it has been considered that the oxides of Si and Mn formed on the surface of the steel sheet react with or interact with the scum that is as an oxide, to promote non-plating or alloying delay. [0096]
By setting the flow rate of the molten metal to 10 m/min to 50 m/min, it is possible to form a plating layer including an oxide while non-plating is prevented. When the flow rate of the molten metal is less than 10 m/min, a contact ratio of the molten metal in the plating bath caimot be increased by suppressing an adhesion of the oxide in the plating bath onto the surface of the steel sheet. Thus, non-plating cannot be prevented and the appearance of the plating layer is deteriorated. On the other hand, when the flow rate of the molten metal is more than 50 m/min, excessive facility

19
investment is required for obtaining such a flow rate. [0097]
Next, the reason wliy the chemical composition of the steel sheet is limited will be described. Here, % in the chemical composition of the steel sheet means mass%. [0098]
C: C is an element which increases the strength of the steel sheet. When the amount of C is less than 0.05%, it is difficult to achieve both tensile strength of 980 MPa or more and workability. In addition, when the amoimt of C is more than 0.40%, sufficient elongation and hole expansibility cannot be obtained. In addition, in this case, it is difficult to ensure spot weldabiUty. Therefore the C content is 0.05% to 0.40%. [0099]
Si; Si is an important element for improving the hydrogen brittleness resistance. When the amount of Si is less than 0.5%, the amount of the oxide in the plating layer is insufficient and delayed firactuie resistance is not improved. Therefore, the lower limit of the amount of Si is set to 0.5%. When the amount of Si is more than 3.0%, the microstructure cannot be controlled due to an excessive generation of ferrite, or workability is deteriorated. Therefore, the amount of Si is set to 0.5 to 3.0%. In addition, Si is an element which increases the strength of the steel sheet. [0100]
Mn: Mn is an element which forms an oxide, and is an element which increases the strength of the steel sheet. When the amount of Mn is less than 1.5%, it is difficult to obtain tensile strength of 980 MPa or more. When a large amount of Mn is included, a common segregation of Mn and P and Mn and S is promoted to deteriorate workability. Therefore, the upper limit of the amount of Mn is set to 3.0%. The Mn content is preferably 2.0% to 2.7%. [0101]
O: O forms oxides in steel and deteriorates elongation, bendability, and hole expansibility, and thus, it is necessary to suppress the amoimt of O in the steel. Particularly, the oxides are present as inclusions in many cases and when the oxides are present at a punched edge surface or a cutting surface, a notched flaw or a coarse dimple is formed on the end surface. The flaw or the dimple causes stress concentration during hole expansion or hard working and becomes a starting point of the generation of cracks, and thus, hole expansibility or bendability is significantly deteriorated. [0102]
When the amount of O is more than 0.01%, the above tendency becomes significant, and thus, the upper limit of the amount of O is set to 0.01%. The lower limit of the amount of O is not particularly limited, but when the amount of O is less than 0.0001%, costs increase excessively. Thus, the substantive lower limit of the amount of

20
O may be set to 0.0001%. [0103]
P: P is segregated in the center area of the steel sheet in the thickness direction and is an element which causes embrittlement of the welded zone. When P is more than 0.04%, the embrittlement of the welded zone becomes significant, so the upper limit is set to 0.04%. The amount of P is not particularly limited. However, when the amount of P is less than 0.0001 %, costs increase. Thus, the amount of P is preferably 0.0001 % or more. [0104]
S: S is an element which has a detrimental effect on the weldability and the manufacturability of the steel sheet at casting and at hot rolling. For this reason, the upper limit of the amount of S is set to 0.01%. The lower limit of the amount of S is not particularly limited. However, when the amoimt of S is less than 0.0001 %, costs increase, and thus, the amount of S was preferably 0.0001% or more. In addition, since S bonds with Mn to form coarse MnS and deteriorates bendability and hole expansibility, the amount of S has to be reduced as much as possible. [0105]
Al: Al is an element which can be utilized as an oxide to improve delayed fracture resistance. In addition, Al is an element which can be utilized as a deoxidizer. However, when an excessive amount of Al is added, the number of Al-based coarse inclusions are increased, a deterioration in hole expansibility and surface flaws are caused, and thus, the upper limit of the amount of Al is set to 2.0%. Although the lower limit of the amount of Al is not particularly limited, it is difficult to set the amount of Al to 0.0005% or less. Thus, the substantive lower limit of the amount of Al is 0.0005%. [0106]
Al+Si: Here, all Al and Si are elements which suppress formation of cementite. Therefore, when the total amount of Al and Si is controlled, it is advantageous to control the microstructure which will be described later. When the total amount thereof is 0.5% or more, it is possible to more easily suppress the formation of cementite. Thus, the total amount of Al and Si is preferably 0.5% or more. [0107]
N:N forms coarse nitrides and is an element which deteriorates bendability and hole expansibility. Therefore, the amount of N has to be suppressed. When the amount of N is more than 0.01%, the above tendency becomes significant, and thus, the upper limit of the amount of N is set to 0.01 %. In addition, a small amount of N is preferable since N generates a blowhole during welding. The lower limit of N is not particularly limited. However, when the amount of N is less than 0.0005%, the manufacturing cost increases remarkably, and thus, the substantive lower limit of the

21
amount of N is set to 0.0005%. [0108]
Mo: Mo is a strengthening element and is an important element for improving hardenability. In the case where Mo is added in the steel, when the amoimt of Mo is less than 0.01%, the effect of the addition cannot be obtained and thus, the lower limit of Mo may be 0.01 %. When the amoimt of Mo is more than 1.0%, the manufacturability of the steel sheet 2 is deteriorated at manufacturing and at hot rolling, and thus, the upper limit of the amoimt of Mo is set to 1.0%. [0109]
Cr: Cr is a strengthening element and is an important element for improving hardenability. In the case where Cr is added in the steel, when the amount of Cr is less than 0.05%, the effect of the addition cannot be obtained and thus, the lower limit of Cr may be 0.05%. When the amount of Cr is more than 1.0%, the manufacturability of the steel sheet 2 is deteriorated at manufacturing and at hot rolling, and thus, the upper limit of the amount of Cr is set to 1.0%. [0110]
Ni: Ni is a strengthening element and is an important element for improving hardenability. In the case where Ni is added in the steel, when the amount of Ni is less than 0.05%, the effect of the addition cannot be obtmned and thus, the lower limit of Ni may be 0.05%. When the amount of Ni is more than 1.0%, the manufacturability of the steel sheet is deteriorated at manufacturing and at hot rolling, and thus, the upper limit of the amount of Ni is set to 1.0%. In addition, Ni improves the wettability of the steel sheet or promotes alloying reaction. [0111]
Cu: Cu is a strengthening element and is an important element for improving hardenability. In the case where Cu is added in the steel, when the amount of Cu is less than 0.05%, the effect of the addition cannot be obtained and thus, the lower limit of Cu is 0.05%. When the amount of Cu is more than 1.0%, the manufacturability of the steel sheet is deteriorated at manufacturing and at hot rolling, and thus, the upper limit of the amount of Cu is set to 1.0%. In addition, Cu improves the wettability of the steel sheet or promotes alloying reaction. [0112]
B: B is effective element for strengthening a grain boundary and improving the strength of the steel sheet. In the case where B is added in the steel, when the amount of B is less than 0.0001%, the effect of the addition cannot be obtained and thus, the lower limit of B is 0.0001 %. On the other hand, when the amount of B is more than 0.01%, not only is the effect of the addition saturated, but also the manufacturability of the steel sheet is deteriorated at manufacturing and at hot rolling. Thus, the upper limit of the

22
amount of B is set to 0.01%. [0113]
Ti: Ti is a strengthening element. Ti contributes to an increase in the strength of the steel sheet through precipitate strengthening, grain-refining strengthening by siq>pressing ferrite grain growth, and dislocation strengthening through the suppression of recrystallization. In the case where Ti is added in the steel, when the amount of Ti is less than 0.005%, the effect of the addition cannot be obtained and thus, the lower limit of Ti is 0.005%. On the other hand, when the amount of Ti is more than 0.3%, heavy precipitation of carbonitrides are caused and formability is deteriorated. Thus, the upper limit of the amount of Ti is set to 0.3%. [0114]
Mb: Mb is a strengthening element. Nb contributes to an increase in the strength of the steel sheet through precipitate strengthening, grain-refining strengthening by suppressing ferrite grain growth, and dislocation strengthening through the suppression of recrystallization. In the case where Nb is added in the steel, when the amount of Nb is less than 0.005%, the effect of the addition cannot be obtained and thus, the lower limit of the amount of Nb is 0.005%. On the other hand, when the amount of Nb is more than 0.3%, heavy precipitation of carbonitrides are caused and formability is deteriorated. Thus, the upper limit of the amount of Nb is set to 0.3%. [0115]
V: V is a strengthening element. V contributes to an increase in the strength of the steel sheet through precipitate strengthening, grain-refining strengthening by suppressing ferrite grain groAvth, and dislocation strengthening through the suppression of recrystallization. In the case where V is added in the steel, when the amount of V is less than 0.005%, the effect of the addition cannot be obtained and thus, the lower limit of the amount of V is 0.005%. On the other hand, when the amount of V is more than 0.5%, heavy precipitation of carbonitrides is caused and formability is deteriorated. Thus, the upper limit of the amount of V is set to 0.5%. [0116]
One or two or more of Ca, Mg, and REM can be added to 0.0005% or more and 0.04% or less at most as a total content thereof. Ca, Mg, and REM are elements used for deoxidation, and one, two or more of Ca, Mg, and REM as the total content thereof is preferably contained 0.0005% or more in the steel. Here, REM means rare earth metal. [0117]
When the total amount of at least one selected from Ca, Mg, and REM is more than 0.04%, formability is deteriorated, and thus, the upper limit of the total amount is set to 0.04%. Here, REM is generally added in the steel as mischmetal. In addition to La

23
and Ce, at least one of lanthanoid series elements may be contained in some cases. The steel sheet 2 may contain lanthanoid series elements other than La and Ce as unavoidable impurities or metallic La and metallic Ce may be added in the steel. Thus, the effect of the present invention is expressed. [0118]
Next, the reason for limitation of the condition of a manufacturing method of a thin steel sheet according to the present invention will be described. [0119]
In the present invention, steel having the chemical composition described in the above is manufactured in the usual method and casted. The casting slab which is obtained is provided for hot rolling. In addition, after pickling, cold-rolling and annealing is performed, galvanizing and alloying heat treatment is performed. [0120]
After the casting slab is directly or once cooled, the steel is heated to 1200°C or higher and provided for hot rolling. The hot rolling is completed at the temperature of Ars transformation point or higher. [0121]
When the coiling temperature in the hot rolling is more than 700°C, the microstructure of the hot-rolled steel sheet is a coarse ferritepearlite structure and each phase of the microstructure of the final steel sheet after the subsequent processes, for example, cold rolling, aimealing, and galvanizing and alloying heat treatment, become the uneven microstructure. As a result, the excellent hole expansibility cannot be obtained. Thus, the upper limit of the coiling temperature is set to 700°C. The coiling temperature is preferably 550°C or lower so that the micro structure of the steel sheet is bainite as single phase. [0122]
Although the lower limit of the coiling temperature is not particularly defined, when the coiling temperature is lower than 300°C, it is caused a trouble during cold-rolling process due to that the intensity of the steel sheet become high. Therefore, the coiling temperature is preferably 300°C or higher. [0123]
The hot-rolled steel sheet manxifactured in this manner is subjected to pickling. Since the pickling removes oxides on the surface of the steel sheet, the pickling is important to improve plating properties. The steel sheet may be pickled once or may be pickled a plurality of times in a divided manner. The pickled hot-rolled steel sheet is cold-rolled and passes through a continuous hot-dip galvanizing line. The rolling reduction is preferable 40% or more and 80% or less. [0124]

24
When the rolling reduction is lower than 40%, it is difficult to maintain a shape becoming flat; ftirthermore ductility of final product of the steel sheet is deteriorated. Thus, the rolling reduction is preferably 40% or more. On the other hand, when the rolling reduction is higher than 70%, it is difficult to perform the cold rolling because the load during cold rolling becomes excessively large. Thus, the rolling reduction is preferably 70% or less. Moreover, the rolling reduction is more preferably in a range of 45% to 65%. The effect of the present invention can be expressed without that the number of rolling passes or the rolling reduction in the respective passes is particularly defined. [0125]
The effect of the present invention can be expressed without that the heating rate is particularly defined when the steel sheet passes through the plating line. When the heating rate is lower than 0.5 °C/s, it is impossible to ensure sufficient productivity, and thus, the heating rate is preferably 0.5 °C/s or more. When the heating rate is faster than 100 °C/s, it is economically undesirable because it leads to excessive capital investment, and thus, the substantive upper limit of the heating rate is preferably 100 °C/s fix)m a viewpoint of cost. [0126]
During the heating, it is necessary that the steel sheet is retained at 550°C to 750°C for 20 seconds or more. TTiis is because the membranous oxide can be dispersed by retaining the steel sheet in the temperature range. It is considered that the oxide formation is closely related to the recrystallization of the cold-worked ferrite. That is, the oxide which includes one of or a combination of Si, Mn, and Al tends to be formed in the grain boundary of the ferrite on the surface of the steel sheet. The fine ferrite grain boimdary formed by recrystallization as described above is utilized as an oxide forming site. [0127]
When retaining temperature is lower than 550°C, only the as-worked ferrite which is greatly extended is present. Thus, a grain boundary with a sufficient amoxmt for forming membranous oxides is not present. Since the oxide is preferentially formed in the ferrite grain boundary, the membranous oxide generally has a network structure. [0128]
The highest heating temperature is in a range of 750°C to 900°C. When the highest heating temperature is lower than 750°C, it takes a long time to perform re-solid solution of carbides or some of carbides formed during the hot rolling, and the carbides remain, and thus, hardenabihty of the steel sheet is deteriorated. Therefore, it is difficult to ensure tensile strength of 980 MPa or more. Therefore, the lower limit of the highest heating temperature is 750°C.

25
[0129]
Excessive high-temperature heating causes not only an increase of cost but also troubles such as deterioration of a sheet shape when the steel sheet passes through the plating line at a high temperature and a decrease in the life of the roll. As a result, it is economically undesirable. Therefore, the upper limit of the highest heating temperature is set to 900°C. A heat treatment time at the above temperature range is not particularly limited but is preferably 10 seconds or more for dissolution of carbide. [0130]
When the heat treatment time is over 600 seconds, it leads to increase the costs, and thus, it is economically undesirable. The effect of the present invention can be expressed under the condition that the steel sheet may be performed isothermal holding at the highest heating temperature or the steel sheet may be started cooling immediately after gradient heating is performed and the temperature reaches the highest heating temperature. [0131]
After above annealing is completed, the steel sheet is cooled to a temperature in a temperature range from 500°C or higher to 750°C or less at a cooling rate of 0.1 °C/s to 20 °C/s. When the cooling rate is slower than 0.1 °C/s, it is not economically desirable because the productivity is greatly damaged. When a cooling rate is excessively increased, it leads to increase manufacturing cost. Thus, the upper limit of a cooling rate is set to 20 °C/s. [0132]
When the cooling stop temperature is more than 750°C, it is not economically -desirable because it leads to increase manufacturing cost. When the cooling stop temperature is lower than 500°C, it is impossible to obtain a necessary amount of bainite by volume fraction due to increase of ferrite phase. Thus, the lower limit of the cooling stop temperature is set to 500°C. [0133]
After the cooling is completed, the steel sheet is preferably cooled to a temperature of 350°C or higher and 500°C or less at a cooling rate of 5 °C/s to 100 °C/s. Or, the steel sheet is re-heated to 350°C or higher and 500°C or less and is retained for 20 seconds or more after the steel sheet is cooled to a temperature in a range of room temperature or higher to 500°C or less. When the steel sheet is maintained at a low temperature or at a high temperature for a short period of time, bainite transformation is not sufficiently proceed. As a result, it is not possible to obtain good elongation and excellent hole expansibility. When the steel sheet is maintained at a high temperature such as higher than 500°C, pearlite is further formed in the steel sheet, and thus, hole expansibility of the steel sheet is deteriorated.

26
[0134]
After above retaining process is completed, the steel sheet is immersed into a hot dip galvanizing bath. When the sheet temperature is lower than the plating bath temperature by 40°C or higher, a temperature of molten zinc around the surface of the steel sheet at the time when the steel sheet is immersed into the plating bath decreases significantly and some of the molten zinc is solidified. The solidification deteriorates plating appearance and the sheet temperature by re-heating is restored to (plating bath temperature - 40°C). The plating bath may contain Fe, AI, Mg, Mn, Si, Cr, and the like in addition to pure zinc. [0135]
When the alloying of the plating layer is performed, the plated steel sheet is heated to 460°C or higher. When the temperature of the alloying treatment is lower than 460°C, an advance of the alloying is slow, and thus, the productivity is deteriorated. When the alloying temperature is higher than 600°C, carbides are formed and the volume fiaction of austenite is decreased in the steel in a final product. Thus, it is difficult to ensure maximum tensile strength of 980 MPa or more and excellent ductility of the steel sheet. Therefore, the upper limit of the temperature of the alloying treatment is set to 600°C. [0136]
After the heat treatment for the steel sheet is completed, the rolling reduction of the skin pass rolling is preferably 0.1 % to 1.5%. When the rolling reduction of the skin pass rolling is lower than 0.1%, the effect of the skin pass rolling is not suflSciently obtained and it is difficult to control of rolling. Thus, the rolling reduction of the skin pass rolling is preferably 0.1 % or more. When the rolling reduction of the skin pass rolling is higher than 0.1%, the productivity is significantly deteriorated. Thus, the rolling reduction of the skin pass rolling is preferably 1.5 % or less. The skin pass may be may be performed at in-line or may be performed at off-line. Li order to obtam a desired rolling reduction, the skin pass may be performed once or may be performed at divided into a plurality of times. [0137]
The galvanized steel sheet 1 according to the present invention has tensile strength TS of 980 MPa or more and is excellent in delayed fracture resistance, in plating adhesion, in elongation and hole expansibility. The galvanized steel sheet as a material) according to the present invention adopts a product manufactured through each process of smelting, steelmaking, refining, casting, hot rolling, and cold rolling which are common iron making processes used in principle and can be suitably obtained by the manufacturing method according to the embodiment which will be described later. However, even with a product that is manufactured by omitting a part of or all the iron

27
making processes, as long as the product satisfies the conditions of the present invention,
the effect described in the present invention can be obtained.
[Examples]
[0138]
Next, examples of the present invention will be described in detail. [0139]
The hot-rolled steel sheets obtained by hot-rolling the continuously cast slabs having chemical compositions shown in Table 1 under the conditions of hot rolling shown in Tables 2 and 3 (in Tables, slab heating temperature and finish rolling temperature) were water-cooled in a water-cooling zone, and then, coiled at the temperature shown in Tables 2 and 3 (in Tables, coiling temperature). The thickness of the hot rolled steel sheets was 2 mm to 4.5 mm.

40
[0143]
The hot-rolled steel sheets were pickled and then cold-rolled to have a thickness of 1.2 mm after cold rolling under the conditions of cold rolling shown in Tables 2 and 3, and thus, cold-rolled steel sheets were formed. Then, the cold-rolled steel sheets were subjected to various heat treatments and hot dip galvanizing treatment m a continuous galvannealing line under the conditions shown in Tables 2 and 3. [0144]
As shown in Table 2 and 3, the steel sheets were annealed at a temperature range of 750°C to 900°C after the steel sheets were retained at a temperature range of 550°C to 750°C for a predetermined time. From aimealing temperature to a temperature range of 500°C to 750°C, the steel sheets were cooled at a predetermined cooling rate, and from a temperature range of 500°C to 750°C to a temperature range of 350°C to 500°C, the steel sheets were cooled at cooling rate which are shown in Table 2 and 3. Or, from annealing temperature to a temperature range of a room temperature to 500°C, the steel sheets were cooled at cooling rate which are shown in Table 2 and 3, and the steel sheets were re-heated at a temperature range of 350°C to 500°C. [0145]
Then, after the steel sheets were held for 20 seconds or more at a temperature range of 350°C to 500°C, the steel sheet is immersed into a hot dip galvanizing bath which was controlled to a predetermined condition and then cooled at room temperature. In the plating bath, the amount of Al in the molten metal in the plating bath was 0.09 mass% to 0.17 mass%. Some of the steel sheets were subjected to alloying treatment under the respective conditions after being immersed into the hot dip galvanizing bath and the obtained steel sheets were cooled to room temperature. The plating amount (the amount of plating layer) on both surfaces at this time was about 35 g/m^. Finally, the obtained steel sheets were subjected to skin pass rolling under a rollmg reduction of 0.4%. [0146]
In the tensile test, JIS No. 5 test pieces were cut out from the steel sheets having a thickness of 1.2 mm in a right angle direction of the rolling direction and parallel to the rolling direction to evaluate tensile properties. Each of five test pieces was subjected to a tensile test and an average value of the respective values was obtained to calculate tensile strength (TS), total elongation (El), and the like from the average value. [0147]
Here, when a balance index (TS x EI) of the tensile strength (TS) and the total elongation (El) is more than 16000 (MPa x %), the elongation was evaluated to be excellent. When a balance index (TS x X) of the tensile strength (TS) and the hole

41
expansion ratio (X) is more than 40000 (MPa x %), the hole expansibility was evaluated
to be excellent.
[0148]
The oxides in the plating layer were observed at the cross sections of the plated steel sheets. Further, using a focused ion beam processing apparatus (FIB), the surfaces of the steel sheets in the thickness direction were processed into flakes so as to include the plating layers of the surfaces of the plated steel sheets, and then, oxides m the plating layers of the obtained flakes were observed by a field emission type transmission electron microscope (FE-TEM) to perform composition analysis (oxide identification) by an energy distributed X-ray detector (EDX). With the FE-TEM, five visual files were observed at a magnification of 10000 to 50000 times and the chemical composition (compound types) and the projection area firaction of the oxides were evaluated fix)m data obtained by the FE-TEM and the EDX. [0149]
A solution obtained by dissolving the plating layers of the plated steel sheets using a 5% aqueous HCl solution to which an inhibitor was added and removing a residue such as an undissolved oxide was subjected to ICP emission analysis to measure the amount of Fe in the plating layers. In the measurement, using three samples, an average value of the amount of Fe in the three samples was set to Fe% of the plating layers. [0150]
The evaluation of the composition of the oxides or the area fiaction of the oxides can be performed by observing the microstructure of at the cross section of the plating steel sheet. For example, using a focused ion beam processing apparatus (FIB), the surfaces of the steel sheets in the thickness direction are processed into flakes so as to include the plating layers of the surfaces of the plated steel sheets, the method of observation by a field emission type transmission electron microscope (FE-TEM) and the method of composition analysis by an energy distributed X-ray detector (EDX) are cited. [0151]
After a sample for observation is made by a focused ion beam processing apparatus (FIB), the oxides of the sample are observed by a field emission type transmission electron microscope (FE-TEM) at a magnification of 50000 times. In addition, the oxides of the sample are analyzed by an energy distributed X-ray detector (EDX) so that it is possible to identify the oxides. [0152]
In the annealing process at continuous hot-dip galvanizing line, the oxides of easily oxidizable element are formed on the surface of the steel sheet and then plating is performed, it is necessary to be included in the plating layer so that the oxides having one

42
or more of the oxides which include an oxide of Si, an oxide of Mn, or an oxide of Al independently, or a composite oxide of Si, Mn or Al are included in the plating layer. [0153]
Next, in order to evaluate delayed firacture resistance, test pieces were prepared by a U bending test and were subjected to a delayed firacture resistance test by electrolytic charge. The delayed fracture resistance of the plated steel sheets obtained by using the method according to the present invention was evaluated according to a method disclosed in "Materia (Bulletin of the Japan histitute of Metals) Vol. 44, No. 3 (2005), pp. 254 to 256". [0154]
Specifically, after the steel sheets were subjected to mechanical cutting, the cross sections were subjected to mechanical grinding, and then, the test pieces were subjected to the U bending test to have a lOR bend radius. A stain gauge was attached to the center of the surface of each obtained test piece, and the both ends of the test pieces were screwed by bolts to apply stress to the test pieces. The applied stress was calculated by the monitored stain gauge. The applied stress was 0.7 times of tensile strength TS (0.7 x TS). For exaixxple, the applied stress is 700 MPa with respect to a 980 MPa class steel sheet, the aqiplied stress is 840 MPa with respect to a 1180 MPa class steel sheet, and the applied stress is 925 MPa with respect to a 1320 MPa class steel sheet. [0155]
The reason is considered that the residual stress introduced to a steel sheet during formation increases as the tensile strength TS of the steel sheet increases. Each of the obtained U bending test pieces was immersed into an ammonium thiocyanate aqueous solution and a current flowed to an electrolytic charging apparatus at a current density of 0.1 mA/cm^ so that the steel sheet (U bending test piece) was used as a negative electrode and a platinum electrode was used as a positive electrode to conduct an electrolytic charge test for 2 hours. [0156]
The hydrogen generated in the electrolytic charge test penetrates into the steel sheet and may cause delayed firacture. After the electrolytic charge test, the test pieces were drawn up from the solution and the center area (bent area) of each U bending test piece was visually observed to inspect the presence of cracking. Since there is a large residual stress in the bent area, if cracks are generated in the bent area, a rapid progress is made. Therefore, when cracks are generated, there were large opening cracks in all the test pieces and the presence of cracks could be easily determined even visually. [0157]
Using a loupe and a stereomicroscope, the test pieces were careftilly observed to

43
the ends and the presence of cracks was confirmed again- When there were no opening
cracks, it was also confirmed that there were no fine cracks.
[0158]
The evaluation and observation results are shown in Table 4 and Table 5 (the continuation of Table 4). Here, in the result of the delayed fiiacture test (delayed fi-acture resistance) shown in Tables 4 and 5, "©" represents that cracks were not generated, and "x" represents that cracks were generated. [0159]
The plating properties (galvanizability) were evaluated as follows. Plating properties were, G>; there is no non-coating area, ^; there is some of non-coating area, x; there is many of non-coating areas. [0160]
The tensile strength, delayed fracture resistance, plating properties and the Fe content in the plating layer which were measured are together shown in Table 4 and Table 5. It was foimd that all of the steel sheets according to the present invention have the excellent delayed fracture resistance, the excellent formability and the excellent plating properties.

57
[hidustrial Applicability] [0163]
As described above, according to the present invention, it is possible to provide the galvanized steel sheet (including the hot-dip galvanized steel sheet and the galvannealed steel sheet) which is suitable for structural members, reinforcing members, and suspension members for automobiles, has tensile strength of 980 MPa or more, and is excellent in delayed fracture resistance, plating adhesion, elongation, and hole expansibility at low cost. Therefore, since the present invention greatly contributes to a weight reduction of an automobile, industrial applicability is high.

CLAIMS
A hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility, the hot-dip galvanized steel sheet comprising:
a steel sheet; and
a hot-dip galvanized layer on a surface of the steel sheet,
wherein the steel sheet includes, by mass%,
C: 0.05 to 0.40%,
Si: 0.5 to 3.0%,
Mn: 1.5 to 3.0%,
P: limited to 0.04% or less,
S: limited to 0.01% or less,
O: 0.0001 to 0.01% or less,
Al: 0:0005 to 2.0% or less,
N: limited to 0.01% or less, and
Si X Al > 0.5 %, and
a balance consisting of Fe and unavoidable impurities,
a microstructure of the steel sheet includes 40% or more of one or two of a tempered martensite and a bainite by volume jfraction as a primary phase and 8% or more of an austenite, and a balance consisting of ferrite,
the hot-dip galvanized layer having less than 7 mass% of Fe and the balance consisting of Zn, Al and unavoidable impurities on a surface of the steel sheet in which 10% or less of a pearlite is included,
the hot-dip galvanized layer includes one or two or more of an oxide of Si, Mn, orAl. [Claim 2]
A hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility, the hot-dip galvanized steel sheet comprising:
a steel sheet; and
a hot-dip galvannealed layer on a surface of the steel sheet
wherein the steel sheet includes, by mass%,
C: 0.05 to 0.40%,
Si: 0.5 to 3.0%,
Mn: 1.5 to 3.0%,

59
P: limited to 0.04% or less,
S: limited to 0.01% or less,
0:0.0001 to 0.01% or less,
Al: 0.0005 to 2.0% or less,
N: limited to 0.01% or less, and
Si X Al > 0.5 %, and a balance consisting of Fe and unavoidable impurities,
a microstructure of the steel sheet includes 40% or more of one or two of a tempered martensite and a bainite by volume fraction as a primary phase and 8% or more of an austenite, and a balance consisting of ferrite,
the galvannealed layer having 7 to 15 raass% of Fe and the balance consisting of Zn, Al and unavoidable impurities on a surface of the steel sheet in which 10% or less of a pearlite is included,
the galvannealed layer includes one or two or more of an oxide of Si, Mn, or Al. [Claim 3]
The hot-dip galvanized steel sheet having maxunum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility according to Claim 1 or 2,
wherein the steel further includes, by mass%, one or two or more of
Mo: 0.01 to 1.0%,
Cr: 0.05 to 1.0%,
Ni: 0.05 to 1.0%, and
Cu: 0.05 to 1.0%. [Claim 4]
The hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility according to any one of Claims 1 to 3,
wherein the steel further includes, by mass%, one or two or more of
Nb: 0.005 to 0.3%,
Ti: 0.005 to 0.3%, and
V: 0.005 to 0.5%. [Claim 5]
The hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility according to any one of Claims 1 to 4,
wherein the steel fixrther include, by mass%,
B: 0.0001 to 0.01%. [Claim 6]

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The hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility according to any one of Claims 1 to 5,
wherein the steel fiirther include, by mass%, a total of one or two or more of
Ca, Mg, and REM: 0.0005 to 0.04%. [Claim 7]
The hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility according to any one of Claims 1 to 6,
wherein the oxide in the plating layer is membranous and shares 10% or more by area fraction. [Claim 8]
The the hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility,
wherein the steel sheet having the component composition according to any one of Claims 1 to 6 is dipped in molten zinc plating bath in which a flow rate changes in a range of 10 m/min to 50 m/min and the hot-dip galvanizing is performed to the steel. [Claim 9]
The manufacturing method of hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility according to any one of Claims 1 to 6, the manufacturing method comprising;
heating at 1200°C or more the casting slab directly or after once cooled, the hot rolling is completed at a temperature of an Ax3 transformation point or higher,
winding at 700°C or lower,
pickling and cold rolling the steel,
retaming the steel at 550°C to 750°C for 20 seconds or more when the steel sheet pass through a continuous hot-dip galvanizing line, and
annealing the steel at 750°C to 900°C, cooling the steel to an intermediate cooling temperature in a temperature range of 500°C or higher and lower than 750°C at a first average cooling rate of 0.1 °C/s to 20 °C/s,
fiirther cooling at that temperature to 350°C or higher and lower than 500°C, and the hot-dip galvanizing is performed to the steel under the condition according to Claim 8. [Claim 10]
The manufacturing method of hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent

61
plating adhesion, high elongation and excellent hole expansibility according to any one of Claims 1 to 6, the manufacturing method comprising;
heating at 1200°C or more the casting slab directly or after once cooled, the hot rolling is completed at a temperature of an Ara transformation point or higher,
winding at 700°C or lower, pickling and cold rolling the steel,
retaining the steel at 550°C to 750°C for 20 seconds or more when the steel sheet pass through a continuous hot-dip galvanizing line, and
annealing the steel at 750°C to 900°C,
cooling the steel to an intermediate cooling temperature in a temperature range of 500°C or higher and lower than 750°C at a first average cooling rate of 1 °C/s to 20 "C/s,
further cooling to room temperature or higher and lower than 500°C with a cooling rate of 1 °C/s or more, and
reheating to 350°C to 500°C and holding for 20 seconds or more in the temperature range of 350°C to 500°C, and the hot-dip galvanizing is performed to the steel under the condition according to Claim 8, and
cooling to a room temperature. [Claim 11]
The manufacturing method of hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility according to Claim 8 or 9,
wherein heating the steel to 460°C to 600°C to perform alloy treatment after the hot-dip galvanizing is performed to the steel and cooling to a room temperature.

[Document Type] Abstract
[Abstract]
[Subject]
A high-strength hot-dip galvanized steel sheet having maximum tensile strength of 980 MPa or more and excellent delayed fracture resistance, excellent plating adhesion, high elongation and excellent hole expansibility is divided. [Solution]
A galvanized steel sheet includes a steel sheet and a plating layer on the surface of the steel sheet, the steel sheet includes the predetermined amount of C, Si, Mn, P, S, Al, N, Si+Al, and the balance consisting of Fe and unavoidable impurities, a microstructure of the steel sheet includes 40% or more of one or two of a tempered martensite and a bainite by volume fraction as a primary phase, 8% or more of an austenite and the balance of consisting a ferrite, may include a perlite of 10% or more. On the steel sheet, the galvanized steel sheet has the galvannealed layer in which 7 to 15 mass% of Fe and the balance consisting of Zn, Al and unavoidable impurities, the galvannealed layer includes one or two or more of an oxide of Si, Mn, or Al. [Selection Figure] Figure 1