JP2001192768A - High tensile strength hot dip galvanized steel plate and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength hot dip galvanized steel plate having excellent ductility, stretch-flanging properties and fatigue resistance and to provide a method for producing the same. SOLUTION: A steel plate having a composition containing 0.05 to 0.20% C, 0.3 to 1.8% Si, 1.0 to 3.0% Mn and <=0.005% S is subjected to a first stage in which the same is subjected to primary heating treatment in the temperature range of (the Ac3 transformation point -50 deg.C) to (the Ac3 transformation point + 100 deg.C) and is thereafter rapidly cooled to the Ms point or less, a second stage in which the same is subjected to secondary heating treatment in a two phase region and is rapidly cooled to 500 deg.C or less and a third stage in which the same is subjected to hot dip galvanizing treatment and is rapidly cooled to 300 deg.C in succession. In this way, a composite structure containing a low temperature transformed phase inclusive of, by volume, ferrite of 30% or more, tempered martensite of 20% or more, retained austenite of 2% or more and, preferably, martensite of 2 to 5%, and in which the average crystal grain size of ferrite and tempered martensite is 10 μm or less, preferably, the average grain size is 5 μm or less, is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength galvanized steel sheet, and more particularly to an improvement in ductility and stretch flangeability of a high-strength galvanized steel sheet manufactured in a continuous hot-dip galvanizing line, and further improvement in fatigue resistance. About.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment,
There is a demand for improved fuel efficiency of automobiles. In addition, in order to protect occupants in the event of a collision, there is a demand for improved safety of the vehicle body. For these reasons, reduction of the weight of the vehicle body and reinforcement of the vehicle body have been actively promoted. It is said that it is effective to increase the strength of the component material in order to satisfy the weight reduction and strengthening of an automobile body at the same time. Recently, high-tensile steel sheets have been actively used for automobile components.

[0003] Since many automotive parts made of steel sheets are formed by press working, steel sheets for automotive parts are required to have excellent press formability. In order to realize excellent press formability, it is essential to secure high ductility in the first place. Also, in press molding of automobile parts, stretch flange deformation is often used. In particular, in the case of a member constituting a skeletal member for ensuring the strength of an automobile body, a component constituting a reinforcement, or the like, component molding using stretch flange deformation is often performed. For this reason, there is a strong demand for steel sheets for automotive parts to have excellent ductility and stretch flangeability.

[0004] In addition, the members constituting the skeletal members and the reinforcement for ensuring the strength of the vehicle body need to have more excellent fatigue resistance in addition to the static strength described above. Therefore, steel sheets for automobile parts are required to have excellent fatigue resistance properties.

On the other hand, high corrosion resistance is required for automotive parts depending on the application site. A hot-dip galvanized steel sheet is suitable for a material of a component applied to a part where high corrosion resistance is required.

Therefore, in order to further reduce the weight and strengthen the automobile body, a high-strength hot-dip galvanized steel sheet which is excellent in corrosion resistance, ductility, stretch flangeability, and fatigue resistance is indispensable. It has become.

As a high-tensile steel sheet having excellent ductility, a dual-phase steel sheet having a composite structure of ferrite and martensite is typical. In recent years, a highly ductile steel sheet utilizing transformation induced plasticity caused by retained austenite has also reached the stage of practical use. However, such a structure strengthened steel sheet has low local elongation because hard martensite is used as a main strengthening factor. Therefore, there is a problem that stretch flangeability is poor. Further, there is a problem that fatigue resistance is deteriorated due to a difference in hardness between the hard martensite phase and the soft ferrite phase.

[0008] Many continuous hot-dip galvanizing lines are provided with continuous annealing equipment and plating equipment.
Due to the continuous plating process, cooling after annealing is interrupted at the plating temperature, and the average cooling rate throughout the process is necessarily reduced. Therefore, in a steel sheet manufactured by a continuous hot-dip galvanizing line, it is difficult to include martensite and residual austenite generated under cooling conditions with a high cooling rate in the steel sheet after plating. For this reason, it is generally difficult to produce a high-strength hot-dip galvanized steel sheet having these phases in a continuous hot-dip galvanizing line.

As a high-tensile steel sheet having excellent stretch flangeability, a steel sheet having a structure mainly composed of bainite or tempered martensite has been proposed. It is relatively easy to form bainite and tempered martensite even under cooling conditions in a continuous galvanizing line.

For example, Japanese Patent Application Laid-Open No. Hei 5-179356 discloses that cooling is started between 0.1 and 2 s after finish rolling, and the cooling is started at 50 to 200 ° C./s.
At a cooling rate of 450 ° C. or lower, and wound at a temperature of 350 ° C. to 450 ° C. to form a bainite + ferrite composite structure containing 50% or more bainite or a bainite single phase structure, and then (Ac ₁ + 20 ° C.) (Ac ₁ + 70 ° C) (α +
γ) After hot-soaking at two-phase coexistence temperature, hot-dip galvanizing is applied, then alloying treatment, cooling, and skin pass rolling are performed. Manufacturing methods have been proposed.

Japanese Patent Application Laid-Open No. 6-93340 discloses that a hot-rolled steel sheet is cold-rolled, heated to a recrystallization temperature or more and kept at _one point or more, and then heated to a molten zinc tank.
Quenched to a temperature below the melting point, to generate partially or completely martensite in the steel sheet, and then heated to a temperature not less than the Ms point and at least to the temperature of the molten zinc bath and the temperature of the alloying furnace, to transform the tempered martensite. A method for producing a high-tensile alloyed hot-dip galvanized steel sheet having excellent stretch flangeability has been proposed.

However, the high-strength hot-dip galvanized steel sheet obtained by the techniques described in JP-A-5-179356 and JP-A-6-93340 has excellent stretch flangeability, but has a sufficient ductility. It was not satisfactory.

On the other hand, as a method for producing a high-strength steel sheet excellent in both ductility and stretch flangeability, Japanese Patent No. 282481 discloses that a steel having a predetermined chemical composition is hot-rolled and cold-rolled, There has been proposed a method for producing a high-strength thin steel sheet having excellent local elongation by quenching and tempering at a temperature to obtain a steel sheet having a structure mainly composed of tempered martensite containing retained austenite of 1.0 to 6.0%.

Japanese Patent Application Laid-Open No. 6-322479 discloses that C: 0.
Not more than 06%, Mn: 0.2-3.0%, Si: 1.5% or less, and further contains one or more of V, Ti, and Nb in a total amount of 0.005-0.3%, and has a main phase of microstructure. Is a high-strength hot-dip galvanized steel sheet with excellent fatigue characteristics and local deformability, in which ferrite or bainite is used, the occupation ratio of iron carbide in grain boundaries is 0.1% or less, and the maximum particle size of iron carbide is 1 μm or less. Have been.

[0015]

However, Patent No. 282481
The ductility of a high-strength steel sheet obtained by the technique described in the above-mentioned publication is still at an insufficient level, and is not sufficiently satisfactory as a high-tensile steel sheet widely used as a steel sheet for automobiles. Further, the manufacturing method proposed in Japanese Patent No. 282481 does not completely match the thermal history of a steel sheet in a general continuous hot-dip galvanizing line. Therefore, it is impossible to apply the production method described in Japanese Patent No. 282481 to a continuous hot-dip galvanizing line, and it is difficult to obtain a high-strength hot-dip galvanized steel sheet having excellent ductility and stretch flangeability. . Further, although the steel sheet described in JP-A-6-322479 is excellent in fatigue properties,
There is a problem that the steel sheet does not have sufficient ductility as a steel sheet for automobiles.

The present invention solves the above-mentioned problems of the prior art and provides a high-strength hot-dip galvanized steel sheet having excellent ductility and stretch flangeability and excellent fatigue resistance, which is suitable as a material for automobile parts. It is an object of the present invention to provide a manufacturing method thereof. In addition, it is desirable that the high tension hot-dip galvanized steel sheet in the present invention is manufactured using a continuous hot-dip galvanizing line.

[0017]

Means for Solving the Problems First, in order to solve the above-mentioned problems using a continuous hot-dip galvanizing line, the present inventors examined the effects of the composition and microstructure of a steel sheet on ductility and stretch flangeability. We continued our research. As a result, the structure of the high-strength hot-dip galvanized steel sheet obtained after the hot-dip galvanizing process is a composite structure consisting of ferrite, tempered martensite, retained austenite, and a low-temperature transformation phase, and the volume ratio of each phase in the composite structure is It has been found that by setting the ratio to a predetermined ratio, it is possible to express excellent ductility. In addition, they have found that, by reducing the crystal grain size of ferrite and tempered martensite, which are the main components of the composite structure, excellent stretch flangeability as well as high ductility can be obtained.

Further, the present inventors have made a steel sheet having a chemical composition adjusted to a predetermined range into a structure containing lath-like martensite, and further, under a predetermined condition in a continuous hot-dip galvanizing line. By performing the reheating treatment and the plating treatment, the structure of the steel sheet has a ferrite within a predetermined volume ratio range, tempered martensite, retained austenite,
It has been found that a high-strength hot-dip galvanized steel sheet that has a composite structure composed of a low-temperature transformation phase and has fine grains of ferrite and tempered martensite, and has improved ductility and stretch flangeability can be obtained. .

Further, the present inventors have incorporated one or more of Ti, Nb and V and refined the crystal grains of each phase such as ferrite and tempered martensite to an average crystal grain size of 5 μm or less. In some cases, it has been found that the presence of an appropriate amount of martensite phase significantly improves fatigue resistance.

The present invention has been made based on the above findings.

That is, a first present invention is a hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a steel sheet, wherein the steel sheet has a mass%
C: 0.05 to 0.20%, Si: 0.3 to 1.8%, Mn: 1.0 to 3.0
%, S: 0.005% or less, having a composition consisting of the balance of Fe and unavoidable impurities, a composite structure consisting of ferrite, tempered martensite, retained austenite, and a low-temperature transformation phase. 30% or more, 20% or more by volume of the tempered martensite, 2% or more by volume of the retained austenite, and the average grain size of the ferrite and the tempered martensite is 10% or more.
It is a high tensile galvanized steel sheet excellent in ductility and stretch flangeability characterized by being not more than μm,
In the first invention, in addition to the above composition, the following (a)
Groups) to (d) (Group a): 0.05 to 1.0 mass% in total of one or more of Cr, Mo, and Cu (Group b): B: 0.003 mass% or less (Group c) : 0.01 mass% or less in total of one or two selected from Ca and REM (d group): 0.01 or more in total of one or two selected from Ti, Nb and V One or two or more groups selected from the group consisting of -0.2 mass%.

Further, the present invention provides a method of the present invention, wherein the mass% and the C: 0.
05-0.20%, Si: 0.3-1.8%, Mn: 1.0-3.0%,
S: A steel sheet containing 0.005% or less and having a balance of Fe and unavoidable impurities has a (Ac ₃ transformation point of −50 ° C.)
After performing the primary heat treatment for 5 seconds or more in the temperature range of (Ac ₃ transformation point + 100 ° C), M is cooled at a cooling rate of 10 ° C / s or more.
a primary step of cooling to a temperature below the s point and then (Ac ₁
A second step of performing a secondary heat treatment for 5 to 120 seconds in a temperature range between the transformation point and the Ac ₃ transformation point, followed by cooling to a temperature of 500 ° C. or less at a cooling rate of 5 ° C./s or more. A hot-dip galvanizing process, a hot-dip galvanized layer formed on the surface layer of the steel sheet, and a tertiary step of cooling to 300 ° C. at a cooling rate of 5 ° C./s or more, and then sequentially performing a ductility and an elongation. It is a method for producing a high-tensile hot-dip galvanized steel sheet having excellent flangeability, and in the second invention, the tertiary step is performed after hot-dip galvanizing treatment to form a hot-dip galvanized layer on the steel sheet surface. Re-heated to the temperature range of ℃ ~ 550 ℃ to perform alloying treatment of the hot-dip galvanized layer,
After the alloying treatment, it is preferable to perform a step of cooling to 300 ° C. at a cooling rate of 5 ° C./s or more. In the second invention, in addition to the above composition, the following (group a) to (a) d group) (a group): 0.05 to 1.0 mass% in total of one or more of Cr, Mo, and Cu (group b): B: 0.003 mass% or less (group c): Ca, REM 0.01 mass% or less in total of one or two selected from (d group): 0.01 to 0.2 mass% in total of one or more selected from Ti, Nb and V One or two or more groups selected from the above may be contained.

A third invention is a hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface layer of a steel sheet, wherein the steel sheet has a mass% of C:
0.05-0.20%, Si: 0.3-1.8%, Mn: 1.0-3.0%,
S: 0.005% or less, 1 selected from Ti, Nb, and V
Species or two or more species in total, containing 0.01-0.2%, balance
A composition consisting of Fe and unavoidable impurities, ferrite,
It has a composite structure of tempered martensite, retained austenite and a low-temperature transformation phase, and the ferrite has a volume ratio of 30% or more, and the tempered martensite has a volume ratio of
20% or more, 2% or more by volume of the retained austenite, and 2% by volume of martensite as the low-temperature transformation phase.
And high-tensile melting excellent in ductility, stretch flangeability and fatigue resistance, characterized in that the average crystal grain size of the ferrite, tempered martensite, retained austenite and low-temperature transformation phase is 5 μm or less. A galvanized steel sheet, and in the third aspect of the present invention, in addition to the above composition, one or more of the following (Group a) to (Group c) (Group a): Cr, Mo, Cu The total of the above is 0.05 to 1.0 mass%, (group b): 0.003 mass% of B
Hereinafter, (group c): one or two selected from Ca and REM in total are selected from 0.01 mass% or less.
It is preferable to contain a group or two or more groups.

Further, the fourth invention is characterized in that, in mass%, C: 0.
05-0.20%, Si: 0.3-1.8%, Mn: 1.0-3.0%,
S: 0.005% or less, one selected from Ti, Nb, and V
Species or two or more species in total, containing 0.01-0.2%, balance
For steel sheets having a composition consisting of Fe and unavoidable impurities,
(Ac ₃ transformation point -30 ° C) After performing primary heat treatment for 5 to 90 seconds in the temperature range of (Ac ₃ transformation point + 60 ° C), 10 ° C / s
A primary step of cooling to a temperature below the Ms point at the above cooling rate, and then (Ac ₁ transformation point + 30 ° C. to Ac ₃ transformation point −30 ° C.)
After performing a secondary heat treatment for 5 to 90 seconds at a temperature range of 5 ° C./s, a secondary step of cooling to a temperature of 500 ° C. or less at a cooling rate of 5 ° C./s or more, and then performing a galvanizing treatment, After forming a hot-dip galvanized layer on the steel sheet surface layer,
And a tertiary step of cooling to 300 ° C. at a cooling rate of not lower than 300 ° C./s, which is a method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility, stretch flangeability and fatigue resistance. In the present invention, the tertiary step comprises:
After performing a hot-dip galvanizing process to form a hot-dip galvanized layer on the surface layer of the steel sheet, re-heating to a temperature range of 450 ° C. to 550 ° C. to perform an alloying process on the hot-dip galvanized layer. Preferably, the process is a step of cooling to 300 ° C. at a cooling rate of not less than 300 ° C./s. In the fourth present invention, in addition to the above composition, the following (group a) to (group c) (group a) : 0.05 to 1.0 mass% in total of one or more of Cr, Mo and Cu, (group b): 0.003 mass% of B
Hereinafter, (group c): one or two selected from Ca and REM in total are selected from 0.01 mass% or less.
It is preferable to contain a group or two or more groups.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The high-strength hot-dip galvanized steel sheet of the present invention is a hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface layer of the steel sheet.

First, the reasons for limiting the composition of the steel sheet used in the present invention will be described. In addition, mass% is simply described as%.

C: 0.05 to 0.20% C is an essential element for increasing the strength of steel, and has an effect on generation of retained austenite and a low-temperature transformation phase, and is an essential element. However, if the C content is less than 0.05%, the desired high strength cannot be obtained, while if it exceeds 0.20%, the weldability is degraded. For this reason, C was limited to the range of 0.05 to 0.20%.

Si: 0.3 to 1.8% Si has the effect of strengthening steel by solid solution strengthening, stabilizing austenite, and promoting the formation of a retained austenite phase. This effect is due to the Si content of 0.3.
% Or more. On the other hand, when the content exceeds 1.8%, the plating property is remarkably deteriorated. Therefore, Si is 0.3 ~
Limited to the 1.8% range.

Mn: 1.0 to 3.0% Mn has the effect of strengthening the steel by solid solution strengthening, improving the hardenability of the steel, and promoting the generation of retained austenite and low-temperature transformation phase. This effect is due to the Mn content
Approved at 1.0% or more. On the other hand, if the content exceeds 3.0%, the effect saturates and the effect corresponding to the content cannot be expected, resulting in an increase in cost. Therefore, Mn is 1.0-3.0%
Limited to the range.

S: not more than 0.005% S is an impurity element that forms MnS in the steel and lowers the stretch flangeability and fatigue resistance of the steel sheet. For this reason, the content of S is limited to 0.005% or less.

Further, in the steel sheet of the present invention, the following (group a) to
One or more of (d) groups can be contained.

(Group a): One or more of Cr, Mo, and Cu in a total of 0.05 to 1.0% Cr, Mo, and Cu all improve the hardenability of steel and are transformed at low temperatures. It is an element that has the function of promoting the formation of a phase. Such an effect is recognized when one or more of Cr, Mo, and Cu are contained in a total amount of 0.05% or more. On the other hand, Cr, Mo, Cu
Even if one or more of them exceeds 1.0% in total, the effect saturates, an effect corresponding to the content cannot be expected, and it is economically disadvantageous. Therefore, one or more of Cr, Mo, and Cu are desirably limited to a total range of 0.05 to 1.0%. The more preferred range is Cr,
One or two or more of Mo and Cu in a total of 0.05 to 0.5
%.

(Group b): B: 0.003% or less B is an element having an effect of improving the hardenability of steel,
It can be contained as needed. However, if the B content exceeds 0.003%, the effect is saturated, so it is desirable to limit B to 0.003% or less. Note that a more preferable range is 0.00.
1 to 0.002%.

(Group c): 1 selected from Ca and REM
0.01% or less of the total of two or more kinds of Ca and REM have the effect of controlling the form of the sulfide-based inclusions, and thereby have the effect of improving the stretch flangeability and fatigue resistance of the steel sheet. These effects are due to Ca, RE
Saturation occurs when the content of one or two selected from M exceeds 0.01% in total. Therefore, the content of one or two of Ca and REM is preferably limited to 0.01% or less in total. Note that a more preferable range is 0.00.
1 to 0.005%.

(D group): one or more selected from Ti, Nb and V in a total amount of 0.01 to 0.2% Ti, Nb and V form carbonitrides in steel; Precipitation strengthening by these carbonitrides has the effect of strengthening the ferrite phase and increasing the strength of steel, and has the effect of reducing the crystal grain size, and can be contained as necessary. Such an effect is achieved by selecting one or two selected from Ti, Nb, and V.
More than 0.01% of species or more are found. On the other hand, if the content exceeds 0.2% in total, the effect saturates and the effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, the content of one or more of Ti, Nb, and V is preferably limited to a total of 0.01 to 0.2%. In the present invention, when particularly excellent fatigue resistance is required, these elements are essential.

In the steel sheet used in the present invention, the balance other than the above chemical components consists of Fe and inevitable impurities. As unavoidable impurities, Al: 0.1% or less and P: 0.05% or less are acceptable.

Further, the steel sheet of the present invention has the above composition and (1) ferrite, (2) tempered martensite, (3)
It is a steel sheet having a composite structure consisting of retained austenite and (4) a low-temperature transformation phase. By forming a composite structure in which these phases coexist, effects such as improvement in ductility of the steel sheet are exhibited. The tempered martensite in the present invention refers to a phase generated when lath-like martensite is heated.

(1) Ferrite Ferrite is a soft phase containing no iron carbide, has a high deformability, and improves the ductility of a steel sheet. The steel sheet of the present invention contains such ferrite in a volume ratio of 30% or more. If the amount of ferrite is less than 30%, a remarkable ductility improving effect cannot be expected. For this reason, the amount of ferrite in the composite structure is limited to 30% or more. The ferrite content is 70%
If it exceeds 50, it is difficult to obtain the advantage of the multi-phase composite structure, so that the amount of ferrite is desirably 70% or less.

(2) Tempered martensite Tempered martensite is characterized by having a fine internal structure that inherits the lath form of lath martensite before tempering, and has a stretch flangeability and fatigue resistance of a steel sheet. This is an effective phase for improving characteristics. In addition, tempered martensite
Since it is softened by tempering and has sufficient plastic deformability, it is an effective phase for improving the ductility of a steel sheet. The steel sheet of the present invention contains such tempered martensite in a volume ratio of 20% or more. If the amount of tempered martensite is less than 20%, the above effects cannot be sufficiently expected. For this reason,
The amount of tempered martensite in the composite structure was limited to 20% or more. When the amount of tempered martensite exceeds 60%,
Since it is difficult to obtain the advantage of the multi-phase composite structure, the amount of tempered martensite is desirably 60% or less.

(3) Retained austenite Retained austenite undergoes strain-induced transformation to martensite during processing, widely dispersing locally applied processing strain,
It has the effect of improving the ductility of the steel sheet. The steel sheet of the present invention contains such retained austenite in a volume ratio of 2% or more. If the amount of retained austenite is less than 2%, remarkable improvement in ductility cannot be expected. For this reason, the amount of retained austenite was limited to 2% or more. The amount of retained austenite is preferably at least 5%. The larger the amount of retained austenite is, the better, but it is practically 10% or less.

(4) Low-temperature transformation phase The low-temperature transformation phase in the present invention refers to martensite or bainite that has not been tempered.

Both martensite and bainite are hard phases and have the effect of increasing the strength of the steel sheet by strengthening the structure. In addition, since the generation of transformation involves the occurrence of mobile dislocations,
It also has the effect of lowering the yield ratio of the steel sheet. In order to sufficiently obtain the above-mentioned effects, it is preferable that the low-temperature transformation phase is martensite. Martensite also has the effect of generating compressive residual stress during transformation, suppressing the growth of initial cracks in fatigue, and has a great effect of improving fatigue resistance. This effect is particularly noticeable when martensite
It becomes significant when 5% is present.

On the other hand, bainite has a small effect of generating residual stress of compression during transformation, and has a small effect of suppressing the growth of initial cracks in fatigue. From the viewpoint of improving the fatigue resistance, the inclusion of bainite is not particularly required.
If the content of bainite exceeds 30%, it becomes difficult to obtain the advantages of the multi-phase composite structure, so that
% Is desirable.

In the present invention, the amount of the low-temperature transformation phase is not particularly limited, and may be appropriately distributed according to the strength of the steel sheet.
Preferably, the volume ratio is 5 to 30%. When excellent fatigue resistance is required, it is preferable to contain 2 to 5% by volume of martensite as a low-temperature transformation phase.

Further, in the steel sheet of the present invention, the crystal grain size of ferrite and tempered martensite in the above-mentioned composite structure is set to 10 μm or less in average grain size.

Refinement of the crystal grain size has the effect of improving the stretch flangeability and fatigue resistance of the steel sheet. In the steel sheet of the present invention, the average crystal grain size of ferrite and tempered martensite in the composite structure is set to 10 μm or less. Average grain size of ferrite and tempered martensite is 10μm
If it exceeds 3, a remarkable action of improving the stretch flangeability cannot be expected. For this reason, the average grain size of ferrite and tempered martensite is limited to 10 μm or less.

In the composite structure of the steel sheet of the present invention,
The ferrite and tempered martensite are the main phases, and the balance is retained austenite and a low-temperature transformation phase. Such a retained austenite and a low-temperature transformation phase exist in a dispersed state between grains of ferrite and tempered martensite as main phases or in tempered martensite grains. For this reason, the average particle size of the retained austenite and the low-temperature transformation phase is smaller than the average particle size of the ferrite and the tempered martensite, which are the main phases, and is not particularly limited in the present invention.

When particularly excellent fatigue resistance is required, the average crystal grain size of each phase is preferably limited to 5 μm or less. If the average crystal grain size of each phase exceeds 5 μm, remarkable improvement in fatigue resistance cannot be expected. Therefore, when particularly excellent fatigue resistance is required, the average crystal grain size of each phase is set to 5 μm or less.

The high-strength hot-dip galvanized steel sheet of the present invention is a coated steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on a surface layer of a steel sheet having the above-described composition and the above-described composite structure. The amount of coating (weight per unit area) of the plating layer may be appropriately determined according to the corrosion resistance requirement depending on the use site, and is not particularly specified. For steel sheets used for automotive parts, the coating weight of the hot-dip galvanized layer is 30-120 g / m ²
It is preferred that

Next, a method for producing a high-strength galvanized steel sheet according to the present invention will be described.

First, a slab having the above-described composition is melted and cast by an ordinary known method, and then hot-rolled or further cold-rolled by an ordinary known method to obtain a steel sheet. Further, if necessary, a step such as pickling or annealing can be added.

In the present invention, the steel sheet having the above-mentioned composition is subjected to a primary heat treatment followed by cooling to form a structure containing lath martensite, and then a secondary heat treatment in a continuous galvanizing line. Tempering of the martensite formed in the first step, and a second step () for partially re-austenizing the steel sheet structure for generating the retained austenite and the low-temperature transformation phase after the third step, and then performing zinc. A tertiary step (2) is performed to perform a plating treatment and to cool to produce a retained austenite and a low-temperature transformation phase, thereby obtaining a high-strength hot-dip galvanized steel sheet excellent in ductility and stretch flangeability, or even fatigue resistance.

Primary Step In the primary step, the steel sheet is subjected to a primary heat treatment in which the steel sheet is kept in a temperature range of (Ac ₃ transformation point−50 ° C.) to (Ac ₃ transformation point + 100 ° C.) for at least 5 seconds, and then subjected to a temperature below the Ms point. Up to 10 ℃
The steel sheet is quenched at a cooling rate of / s or more. By this primary step, lath martensite is generated in the steel sheet. In order to obtain a uniform and fine composite structure of ferrite, tempered martensite, retained austenite, and low-temperature transformation phase in the steel sheet after the tertiary step, the steel sheet structure after the primary step is to be changed to a structure containing lath martensite. It is necessary to.

If the heating holding temperature of the primary heat treatment is less than (Ac ₃ transformation point −50 ° C.) or the holding time is less than 5 seconds,
The amount of austenite generated during heating and holding is small, and the amount of lath martensite obtained after cooling is insufficient. On the other hand, the heating holding temperature of the primary heat treatment is (Ac ₃ transformation point + 100
C.), the austenite grain size becomes coarse during heating and holding. Therefore, the average crystal grain size of ferrite and tempered martensite obtained after the third step is 10 μm.
m or less. Further, the holding time is preferably set to 120 sec or less.

In order to reduce the average crystal grain size of each phase obtained after the third step to 5 μm or less in order to improve the fatigue resistance, the heating holding temperature of the primary heat treatment is set to (Ac ₃ transformation point −30).
° C.) ~ (preferably in the range of Ac ₃ transformation point + 60 ° C.).

The cooling rate after the primary heat treatment was 10 ° C. /
If it is less than s, the steel sheet structure after cooling cannot be a structure containing lath martensite. In addition, the cooling rate after the primary heat treatment is 100 to maintain the shape of the steel sheet well.
C./s or less is desirable.

The final rolling of the plating base plate was (Ar
₍₃ Transformation point -50 ℃) When using a hot-rolled steel sheet performed at a temperature of at least, at the time of cooling after the final rolling, quench at a cooling rate of at least 10 ℃ / s to a temperature below the Ms point. , This primary step can be substituted.

Secondary Step In the secondary step, the steel sheet which has produced lath martensite in the primary step is further subjected to a secondary heat treatment for 5 to 120 seconds in a temperature range from Ac ₁ transformation point to Ac ₃ transformation point. After giving
Cool at a cooling rate of 5 ° C / s or more to a temperature of 500 ° C or less. By this secondary process, the martensite formed in the primary process is turned into tempered martensite, and after the tertiary process, a part of the steel sheet structure for generating residual austenite and a low-temperature transformation phase is re-austenitized.

The heating holding temperature in the secondary heat treatment is Ac ₁
Below the transformation point, austenite is not regenerated, and no residual austenite or low-temperature transformation phase is obtained after the third step.
Further, when the holding temperature exceeds the Ac ₃ transformation point, the steel sheet structure is re-austenitized, and tempered martensite disappears. In addition, the heating holding time in the secondary heat treatment is 5 seconds.
If it is less than 1, the regeneration of austenite is insufficient.
A sufficient amount of retained austenite cannot be obtained after the third step. On the other hand, when the heating holding time exceeds 120 seconds, the re-austenitization of tempered martensite proceeds, and it becomes difficult to obtain a required amount of tempered martensite.

If the cooling rate in the temperature range up to 500 ° C. after the secondary heat treatment is less than 5 ° C./s, the austenite formed in the secondary heat treatment is transformed into ferrite or pearlite, and the residual austenite or low-temperature transformation Not in phase. The cooling rate after the secondary heat treatment is 5 ° C / s or more and 50 ° C / s.
It is preferable to set the following.

In order to improve the fatigue resistance of the low-temperature transformation phase, especially to a phase containing 2 to 5% martensite, the secondary heat treatment is carried out at (Ac ₁ transformation point + 30 ° C. to Ac ₃ transformation point). −
After maintaining the temperature for 5 to 90 seconds in a temperature range of 30 ° C.), it is preferable to cool to a temperature of 500 ° C. or less at a cooling rate of 5 ° C./s or more. When the heating holding temperature and the cooling rate are out of the above-mentioned ranges, a desired structure cannot be obtained, and remarkable improvement in fatigue resistance cannot be obtained.

This secondary step is preferably performed in a continuous hot-dip galvanizing line having both annealing equipment and hot-dip galvanizing equipment. By using such a continuous galvanizing line, the process can be shifted to the tertiary process immediately after the secondary process, and the productivity is improved.

Tertiary Step In the tertiary step, the steel sheet subjected to the secondary step is subjected to hot dip galvanizing, and cooled to 300 ° C. at a cooling rate of 5 ° C./s or more. The hot-dip galvanizing process may be performed under the processing conditions usually performed in a continuous hot-dip galvanizing line, and there is no particular limitation. However, plating at an extremely high temperature makes it difficult to secure a necessary amount of retained austenite. Therefore, 500
Preferably, the plating treatment is performed at a temperature of not more than ℃. When the cooling rate after the plating treatment is extremely low, it is difficult to secure the amount of retained austenite. Therefore, the cooling rate in a temperature range from after plating to 300 ° C. is preferably limited to 5 ° C./s or more. In addition, it is preferably 50 ° C./s or less. Needless to say, after plating, wiping for adjusting the basis weight may be performed as necessary.

After the hot-dip galvanizing treatment, an alloying treatment of the plating layer may be performed. The alloying treatment of the hot-dip galvanized layer is performed by reheating to a temperature range of 450 to 550 ° C. after the hot-dip galvanizing treatment. After the alloying treatment, it is preferable to cool to 300 ° C. at a cooling rate of 5 ° C./s or more. In the alloying treatment at a high temperature, it is difficult to secure a necessary amount of retained austenite, and the ductility of the steel sheet is reduced. For this reason, the upper limit of the alloying treatment temperature is limited to 550 ° C. On the other hand, if the alloying temperature is lower than 450 ° C., the progress of alloying is slow and the productivity is reduced. For this reason, the lower limit of the alloying treatment temperature is preferably set to 450 ° C. Further, when the cooling rate after the alloying treatment is extremely low, it becomes difficult to secure a necessary amount of retained austenite. Therefore, the cooling rate in the temperature range from after the alloying treatment to 300 ° C. is preferably limited to 5 ° C./s or more.

The steel sheet after plating or alloying may be subjected to temper rolling for shape correction and adjustment of surface roughness. In addition, there is no inconvenience even if a treatment such as resin or oil coating, various kinds of painting or electroplating is performed.

The present invention is based on the premise that the secondary step and the tertiary step are performed continuously in a hot-dip galvanizing line in which annealing equipment, plating equipment, and alloying treatment equipment are continuous. It is also possible to carry out.

[0067]

EXAMPLES Example 1 Steel having the composition shown in Table 1 was melted in a converter and cast into slabs by a continuous casting method. The obtained slab was hot-rolled to a sheet thickness of 2.6 mm, then pickled, and then cold-rolled to obtain a cold-rolled steel sheet having a sheet thickness of 1.4 mm. Table 1
The balance other than the chemical components shown in (1) is Fe and inevitable impurities.

[0068]

[Table 1]

Next, these cold-rolled steel sheets were subjected to a primary step of heating and holding them in a continuous annealing line under the primary step conditions shown in Table 2, followed by cooling. After the primary process, the microstructure of the steel sheet was examined, and the amount of lath martensite was measured.
Further, the steel sheet subjected to the primary step was subjected to a secondary step of heating and holding and then cooling under the secondary step conditions shown in Table 2 in a continuous hot-dip galvanizing line, followed by hot-dip galvanizing treatment. A tertiary step of performing an alloying process on the hot-dip galvanized layer, which is reheated after the hot-dip galvanizing process, and then cooling was performed.

The hot-dip galvanizing treatment was performed at a bath temperature of 475 ° C.
The steel sheet was immersed in a plating tank of No. 1, and after the immersed steel sheet was pulled up, the weight per unit area (adhesion amount) was adjusted by gas wiping such that the weight per unit area was 50 g / m ² . When performing the alloying treatment of the galvanized layer, after the wiping treatment, the temperature was increased to 500 ° C. at a heating rate of 10 ° C./s to perform the alloying treatment. The holding time during the alloying treatment was adjusted so that the iron content in the plating layer was 9 to 11%.

[0071]

[Table 2]

The microstructure of the steel sheet was investigated by observing the cross section in the rolling direction of the steel sheet with an optical microscope or a scanning electron microscope. Regarding the amount of lath martensite, ferrite and tempered martensite in the steel sheet, a magnification of 10
Using the photograph of the cross-sectional structure at a magnification of 00 times, the occupied area ratio of the relevant phase existing in a 100 mm square area arbitrarily set by image analysis was determined and defined as the volume ratio of the relevant phase. The amount of retained austenite was determined by polishing a steel plate to the center in the thickness direction and measuring the diffraction X-ray intensity at the center of the thickness. MoK α-rays are used as incident X-rays, and diffraction X on each of the {111}, {200}, {220}, and {311} faces of the retained austenite phase.
The linear intensity ratio was determined, and the average value was defined as the volume fraction of retained austenite.

The grain size of the ferrite was determined by measuring the grain size in accordance with the provisions of JIS Z0552 and converting the average grain size. Further, the tempered martensite particle size was determined by the same method as the ferrite particle size.

The mechanical properties of the steel sheet were examined by a tensile test and a hole expanding test.

The tensile test was carried out by using a JIS No. 5 test piece specified in JIS Z2204 sampled from a steel sheet in a direction perpendicular to the rolling direction.
The tensile strength (TS) and the elongation at break (El) were measured according to the rules of Z2241.

The hole expanding test was conducted according to the Japan Iron and Steel Federation standard JFS T10.
According to 01, a circular hole of 10mmφ (D ₀ ) is punched into a steel plate,
The punched hole was pushed and expanded with a conical punch having a vertex angle of 60 °, and the hole diameter D immediately after the crack penetrated in the thickness direction was determined. From D and D ₀ , the hole expansion ratio (λ) defined by λ = {(D−D ₀ ) / D ₀ } × 100 (%) was obtained and used as an index of stretch flangeability.

Table 3 shows the obtained results.

[0078]

[Table 3]

As shown in Table 3, the hot-dip galvanized steel sheet according to the present invention has a tensile strength (TS) of 590 MPa or more, a strength-elongation balance (TS × El) of 20,000 MPa ·% or more, and -High tension hot-dip galvanized steel sheet with excellent hole expansion ratio balance (TS × λ) of 60,000 MPa ·% or more and excellent ductility and stretch flangeability.

On the other hand, in a comparative example outside the scope of the present invention,
None shows high values in both the strength-elongation balance and the strength-hole expansion ratio balance, and none has excellent ductility and stretch flangeability at the same time.

In steel sheet No. 2, the heating temperature in the primary heat treatment was low, the amount of lath martensite obtained after cooling was small, the amount of tempered martensite and residual austenite after plating decreased, and the strength-elongation balance was reduced. And the strength-hole expansion ratio balance is reduced.

In steel sheet No. 4, the heating temperature in the primary heat treatment was high, the grain size of ferrite and tempered martensite after plating became large, and the strength-hole expansion ratio balance was lowered.

In steel sheet No. 5, since the cooling rate after the primary heat treatment was low and lath-like martensite was not formed after cooling, tempered martensite and residual austenite were not obtained after plating, and the strength-elongation balance and The strength-hole expansion ratio balance is reduced.

In steel sheet No. 6, since the heating temperature in the secondary heat treatment was too high, tempered martensite and retained austenite were not obtained after the plating treatment, and the strength-elongation balance and the strength-hole expansion ratio balance were reduced. ing.

In steel sheet No. 7, since the heating temperature in the secondary heat treatment was too low, no residual austenite and low-temperature transformation phase were obtained after plating, and the strength-elongation balance and the strength-hole expansion ratio balance were reduced. I have.

In steel sheets Nos. 13 to 15, the composition of the steel sheet was out of the range of the present invention, the amount of tempered martensite or retained austenite was reduced, and the strength-elongation balance and the strength-hole expansion ratio balance were reduced. I have. Also, steel sheet N
In o.16, the composition of the steel sheet was out of the range of the present invention, the sulfide in the steel was increased, and the strength-hole expansion ratio balance was lowered. (Example 2) Steel having a composition shown in Table 4 was melted in a converter and cast into a slab by a continuous casting method. The obtained slab is used for plate thickness 2.
After hot rolling to 6 mm and then pickling, a cold rolled steel sheet having a thickness of 1.4 mm was obtained by cold rolling. The balance other than the chemical components shown in Table 4 is Fe and inevitable impurities.

[0087]

[Table 4]

Next, these cold-rolled steel sheets were subjected to a primary step of heating and holding them in a continuous annealing line under the primary step conditions shown in Table 5, followed by cooling. After the primary process, the microstructure of the steel sheet was examined, and the amount of lath martensite was measured.
Furthermore, these steel sheets subjected to the primary step were subjected to a secondary step of heating and holding and then cooling under a secondary step condition shown in Table 5 in a continuous hot-dip galvanizing line, and subsequently subjected to a hot-dip galvanizing treatment. A tertiary step of performing an alloying process on the hot-dip galvanized layer, which is reheated after the hot-dip galvanizing process, and then cooling was performed.

The hot-dip galvanizing treatment was performed at a bath temperature of 475 ° C.
The steel sheet was immersed in a plating tank of No. 1, and after the immersed steel sheet was pulled up, the weight per unit area (adhesion amount) was adjusted by gas wiping such that the weight per unit area was 50 g / m ² . When performing the alloying treatment of the galvanized layer, after the wiping treatment, the temperature was increased to 500 ° C. at a heating rate of 10 ° C./s to perform the alloying treatment. The holding time during the alloying treatment was adjusted so that the iron content in the plating layer was 9 to 11%.

The microstructure, mechanical properties and fatigue properties of these steel sheets were investigated.

[0091]

[Table 5]

The microstructure of the steel sheet was examined by observing the cross section in the rolling direction of the steel sheet with an optical microscope or a scanning electron microscope. The amount of lath-like martensite, ferrite, tempered martensite, and low-temperature transformation phase in the steel sheet exists in a 100 mm square area arbitrarily set by image analysis using a 1000-fold cross-sectional structure photograph. Then, the occupied area ratio of the relevant phase was determined and defined as the volume ratio of the relevant phase.
The amount of retained austenite was determined by polishing a steel plate to the center in the thickness direction and measuring the diffraction X-ray intensity at the center of the thickness. MoK α-rays were used for the incident X-rays, and the diffracted X-ray intensity ratios of each of the {111}, {200}, {220}, and {311} faces of the retained austenite phase were determined, and the average of these was used as the residual austenite phase. Volume ratio.

The grain size of the ferrite was determined by measuring the grain size in accordance with JIS Z 0552 and converting it to the average grain size. Further, the tempered martensite particle size was determined by the same method as the ferrite particle size. The average particle diameter of the low-temperature transformation phase was converted from the area of each particle to an equivalent circle diameter by image analysis, and the average value thereof was used.

The mechanical properties of the steel sheet were examined by a tensile test and a hole expanding test.

The tensile test and the hole expanding test were the same as in Example 1.

As for the fatigue characteristics, the fatigue limit (FL) was measured by a double swing plane bending fatigue test in accordance with JIS Z 2275.

Table 6 shows the obtained results.

[0098]

[Table 6]

From Table 6, it is clear that the hot-dip galvanized steel sheet of the present invention has a tensile strength (TS) of 590 MPa or more, a strength-elongation balance (TS × El) of 20,000 MPa ·% or more, and -When the hole expansion ratio balance (TS x λ) is 60,000 MPa ·% or more,
It is a high-strength hot-dip galvanized steel sheet that has excellent ductility and stretch flangeability, and has a durability ratio FL / TS of 0.5 or more and excellent fatigue resistance.

On the other hand, in a comparative example outside the scope of the present invention,
None of the strength-elongation balance, the strength-hole expansion ratio balance and the durability ratio show high values, and none of them have excellent ductility, stretch flangeability and fatigue resistance at the same time.

The steel sheet No. 2-2 has a low heating temperature in the primary heat treatment, a small amount of lath martensite obtained after cooling, a decrease in the amount of tempered martensite and residual austenite after the plating treatment, and a reduction in strength. Elongation balance and strength-hole expansion ratio balance are reduced.

In steel sheet No. 2-3, the cooling rate after the primary heat treatment was low, the average grain size of ferrite and tempered martensite was large, and no retained austenite was obtained.
The strength-elongation balance and the strength-hole expansion ratio balance are reduced.

In steel sheet No. 2-4, since the heating temperature in the secondary heat treatment was too low, no residual austenite and low-temperature transformation phase (martensite) were obtained after the plating treatment, and the strength-elongation balance and the strength-hole expansion. Rate balance is falling.

In steel sheet No. 2-6, since the heating temperature in the primary heat treatment was too high, the grain size of ferrite and tempered martensite was large after the plating treatment, no retained austenite was obtained, and the strength-elongation balance and strength −
The hole expansion ratio balance and durability ratio have decreased.

In the steel sheet No. 2-7, tempering martensite and retained austenite were not obtained after the plating treatment because the heating temperature in the secondary heat treatment was too high, and the strength-elongation balance and the strength-hole expansion ratio balance were not improved. Is declining.

In steel sheets No. 2-12 to No. 2-14, the composition of the steel sheet was out of the range of the present invention, the amount of tempered martensite and / or retained austenite was reduced, and the strength-elongation balance and strength-hole expansion. Rate balance is falling.

[0107] In the steel sheet No. 2-15, the composition of the steel sheet was out of the range of the present invention, the sulfide in the steel was increased, and the strength-hole expansion ratio balance and durability ratio were lowered.

[0108]

As described above, according to the present invention,
High tensile strength galvanized steel sheet with extremely excellent ductility, stretch flangeability or even fatigue resistance, which is really suitable as a molded article material represented by automotive parts, can be manufactured stably at low cost, and it is industrially outstanding. Has the effect of

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takashi Sakata 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Kawasaki Steel Corporation Mizushima Works (72) Inventor Chikako Fujinaga 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Technical Research Institute (72) Inventor Osamu Furukun Chiba Prefecture 1 Kawasaki-cho, Chuo-ku, Chiba F-term (reference) in Kawasaki Steel Engineering Laboratory 4K037 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA19 EA25 EA27 EA28 EA31 EA32 EA36 EB11 FM04 GA05

Claims (10)

[Claims] 1. A hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a steel sheet, wherein the steel sheet is mass%, C: 0.05 to 0.20%, Si: 0.3 to 1.8%. , Mn: 1.0-3.0%, S: 0.005% or less, the composition comprising balance Fe and unavoidable impurities;
Ferrite, tempered martensite, has a composite structure consisting of retained austenite and low-temperature transformation phase, and the ferrite has a volume fraction of 30% or more, the tempered martensite has a volume fraction of 20% or more, and the retained austenite has a volume fraction of at least 20%. A high-strength hot-dip galvanized steel sheet having excellent ductility and stretch flangeability, comprising a volume fraction of 2% or more, and the ferrite and tempered martensite having an average crystal grain size of 10 μm or less. 2. In addition to the above composition, the following (group a)
The high-strength hot-dip galvanized steel sheet having excellent ductility and stretch flangeability according to claim 1, comprising one or more groups selected from (d). Note (Group a): 0.05 to 1.0 mass% of one or more of Cr, Mo, and Cu in total, (Group b): 0.003 mass% or less of B, (Group c): Ca, REM 0.01 mass% or less in total of one or two selected from (d group): 0.01 to 0.2 mass% in total of one or more selected from Ti, Nb and V , 3. A hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a steel sheet, wherein the steel sheet is mass%, C: 0.05 to 0.20%, Si: 0.3 to 1.8%. , Mn: 1.0 to 3.0%, S: 0.005% or less, one or two or more selected from Ti, Nb, V are contained in a total of 0.01 to 0.2%, the balance being Fe and unavoidable impurities. The composition has a composite structure consisting of ferrite, tempered martensite, retained austenite and a low-temperature transformation phase, and the ferrite has a volume fraction of 30% or more, and the tempered martensite has a volume fraction of 20% or more. Contains at least 2% by volume of retained austenite and 2 to 5% by volume of martensite as the low-temperature transformation phase, and further has an average crystal grain size of the ferrite, tempered martensite, retained austenite, and low-temperature transformation phase. High-tensile galvanized steel sheet having excellent ductility, stretch-flange formability and fatigue properties, characterized in that it is 5μm or less. 4. In addition to the above composition, the following (group a)
The high tensile galvanized steel sheet having excellent ductility, stretch flangeability and fatigue resistance according to claim 3, comprising one or more groups selected from (c). Note (Group a): 0.05 to 1.0 mass% of one or more of Cr, Mo, and Cu in total, (Group b): 0.003 mass% or less of B, (Group c): Ca, REM 0.01 mass% or less in total of one or two selected from 5. A steel sheet containing, by mass%, C: 0.05 to 0.20%, Si: 0.3 to 1.8%, Mn: 1.0 to 3.0%, and S: 0.005% or less, with the balance being Fe and unavoidable impurities. In addition, (Ac ₃ transformation point -50 ° C) ~ (Ac ₃ transformation point + 10
(0 ° C.) in a temperature range of 5 ° C. or more, followed by a primary step of cooling at a cooling rate of 10 ° C./s or more to a temperature below the Ms point, and then (Ac ₁ transformation point to Ac ₃ (Transformation point) in a temperature range of 5 to 120 seconds, followed by a secondary step of cooling at a cooling rate of 5 ° C./s or more to a temperature of 500 ° C. or less, and then a galvanizing treatment. After forming a hot-dip galvanized layer on the surface layer of the steel sheet,
A method of manufacturing a high tensile galvanized steel sheet having excellent ductility and stretch flangeability, wherein a tertiary step of cooling to 300 ° C. at a cooling rate of not lower than 300 ° C./s is sequentially performed. 6. The tertiary step comprises: performing a hot-dip galvanizing process to form a hot-dip galvanized layer on a surface layer of the steel sheet;
The process is characterized by reheating to a temperature range of 0 ° C to 550 ° C to perform an alloying process on the hot-dip galvanized layer, and then cooling to 300 ° C at a cooling rate of 5 ° C / s or more after the alloying process. The method for producing a high tensile galvanized steel sheet having excellent ductility and stretch flangeability according to claim 5. 7. In addition to the above composition, the following (group a)
The method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility and stretch flangeability according to claim 5 or 6, comprising one or more groups selected from (d). Note (Group a): 0.05 to 1.0 mass% of one or more of Cr, Mo, and Cu in total, (Group b): 0.003 mass% or less of B, (Group c): Ca, REM 0.01 mass% or less in total of one or two selected from (d group): 0.01 to 0.2 mass% in total of one or more selected from Ti, Nb and V , 8. In mass%, C: 0.05 to 0.20%, Si: 0.3 to 1.8%, Mn: 1.0 to 3.0%, S: 0.005% or less, one or more selected from Ti, Nb, and V A steel sheet containing a total of two or more of 0.01 to 0.2% and having a balance of Fe and unavoidable impurities has a (Ac ₃ transformation point −30).
℃) ~ (Ac ₃ transformation point + 60 ℃) in the temperature range of 5 to 90 s after performing a primary heat treatment, followed by cooling at a cooling rate of 10 ℃ / s to a temperature below the Ms point below the primary step, , (A
c 5 to 90 in the temperature range of ₁ transformation point + 30 ° C to Ac ₃ transformation point -30 ° C)
a second step of performing a secondary heat treatment for holding for s, and then cooling to a temperature of 500 ° C. or less at a cooling rate of 5 ° C./s or more;
Next, a hot-dip galvanizing treatment is performed to form a hot-dip galvanized layer on the surface layer of the steel sheet, and then at a cooling rate of 5 ° C./s or more.
A method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility, stretch flangeability and fatigue resistance, characterized by sequentially performing a tertiary step of cooling to 300 ° C. 9. The method according to claim 9, wherein the tertiary step performs a hot-dip galvanizing process to form a hot-dip galvanized layer on a surface layer of the steel sheet.
The process is characterized by reheating to a temperature range of 0 ° C. to 550 ° C. to perform alloying of the hot-dip galvanized layer, and then cooling to 300 ° C. at a cooling rate of 5 ° C./s or more after the alloying. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 8, which is excellent in ductility, stretch flangeability and fatigue resistance. 10. In addition to the above composition, the following (group a)
A high-strength galvanized steel sheet having excellent stretch flangeability and fatigue resistance according to claim 8 or 9, wherein the high-strength galvanized steel sheet has excellent stretch flangeability and fatigue resistance. Production method. Note (Group a): 0.05 to 1.0 mass% of one or more of Cr, Mo, and Cu in total, (Group b): 0.003 mass% or less of B, (Group c): Ca, REM 0.01 mass% or less in total of one or two selected from