JP2003342641A - Process for manufacturing hot-dip galvanized high tensile steel sheet showing excellent ductility and fatigue resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a process for manufacturing a hot-dip galvanized high tensile steel sheet having a ductility sufficient as a material for automobile members and showing an excellent fatigue resistance. <P>SOLUTION: A hot-rolled sheet used as a material in the manufacturing process has a component composition comprising, by mass, 0.05-0.20% C, 0.3-1.8% Si, 1.0-3.0% Mn and the balance being Fe and unavoidable impurities and a composite structure comprising a main ferrite phase and a second phase. Here, the second phase comprises 80 vol.% or more of bainite and the balance being at least one chosen from martensite, retained austenite and pearlite, and a ratio of the average length in the plate thickness direction to the average length of the second phase is ≥0.7. The hot-rolled sheet is cold-rolled and subsequently subjected to a predetermined heat treatment, hot-dip galvanizing treatment and cooling treatment. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength hot-dip galvanized steel sheet, and more particularly to a high-strength hot-dip galvanized steel sheet produced in a continuous hot-dip galvanizing line.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of conservation of the global environment,
There is a demand for improved fuel efficiency of automobiles, and also for the safety of automobile bodies to protect passengers in the event of a collision. Under these circumstances, the weight reduction of automobile bodies and the strengthening of automobile bodies are being actively promoted. To satisfy the weight reduction and strengthening of the car body at the same time,
Since it is effective to increase the strength of parts materials,
High-strength steel sheets are actively used for automobile parts.

Since most of automobile parts made of steel sheet are formed by press working, excellent press formability is required for the steel sheet for automobile parts. In order to realize excellent press formability, it is essential to secure high ductility, and therefore high tensile strength steel sheets for automobile parts are strongly required to have high ductility.

As a high-tensile steel sheet having excellent ductility, a structure-strengthening steel sheet having a composite structure of ferrite and a low temperature transformation phase has been proposed. A typical example of this microstructure-strength steel sheet is a dual-phase steel sheet having a composite structure of ferrite and martensite. Further, recently, a high ductility steel sheet utilizing transformation-induced plasticity due to retained austenite has also reached the stage of practical application.

On the other hand, automobile parts may also be required to have high corrosion resistance depending on the application site. A hot-dip galvanized steel sheet, which is a typical example of an alloyed hot-dip galvanized steel sheet, is suitable as a material for parts applied to such a portion. Therefore, in order to further promote the weight reduction and strengthening of an automobile body, a high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and excellent ductility is essential as a raw material.

Here, most of the hot-dip galvanized steel sheets are manufactured on a continuous hot-dip galvanizing line. In this continuous hot-dip galvanizing line, an annealing facility and a plating facility are often installed in series, and cooling after the annealing is interrupted by the plating treatment in order to perform the plating treatment after the annealing. For this reason, it is difficult to increase the average cooling rate in the entire process.

That is, in a high-strength hot-dip galvanized steel sheet produced by a continuous hot-dip galvanizing line, martensite and residual austenite, which are generally produced under cooling conditions with a high cooling rate, may be contained in the steel sheet after the plating treatment. It's difficult.

In a continuous hot-dip galvanizing line, a method for producing a structure-strengthened high-strength hot-dip galvanized steel sheet is to add a large amount of an alloying element such as Cr or Mo that enhances hardenability, and to use a low temperature transformation phase such as martensite. Is known to facilitate the generation of. However, there is a problem that the addition of a large amount of alloying elements causes an increase in manufacturing cost.

On the other hand, in Patent Document 1, a steel sheet containing C: 0.05 to 0.20% by mass, Mn: 1.0 to 3.0% by mass and Si: 0.3 to 1.8% by mass is rapidly cooled to below the Ms point after the primary heat treatment. After performing the step and the secondary step of quenching after the secondary heat treatment, by the tertiary step of subjecting to hot dip galvanizing and quenching, tempered martensite, retained austenite and a composite structure composed of ferrite and low temperature transformation phase, and ductility. A method for producing a high-strength hot-dip galvanized steel sheet excellent in heat resistance has been proposed. However, with the technique described in Patent Document 1, although it is possible to obtain an excellent ductility,
The obtained steel sheet had a problem in fatigue resistance.

[0010]

[Patent Document 1] Japanese Patent Laid-Open No. 2001-3150

[0011]

The present invention solves the above-mentioned problems of the prior art and is a method for producing a high-strength hot-dip galvanized steel sheet which has sufficient ductility as a material for automobile parts and is excellent in fatigue resistance. The purpose is to propose.

[0012]

[Means for Solving the Problems] The inventors of the present invention have solved the above-mentioned problems and have proposed a method for producing a high-ductility high-strength hot-dip galvanized steel sheet using a continuous hot-dip galvanizing line. We have earnestly studied from the viewpoint of organization. As a result, first, the chemical composition and structure of the hot-rolled sheet to be the raw material are regulated, cold rolling and heat treatment, and the high-strength hot-dip galvanized steel sheet obtained after the hot dip galvanizing treatment includes tempered martensite and residual austenite. It has been found that a steel sheet having excellent ductility and fatigue resistance can be exhibited by forming a composite structure with the balance being ferrite and a low temperature transformation phase.

The composite structure of tempered martensite and retained austenite of the steel sheet structure, and the balance of ferrite and low-temperature transformation phase, has a lath-like shape made from a hot-rolled sheet having a controlled chemical composition and structure. It was found that a structure containing martensite and a reheating treatment and a plating treatment under a predetermined condition in a continuous galvanizing line can be obtained.

Further, by initially forming the structure containing lath-like martensite, austenite formed during the subsequent tempering treatment can be finely dispersed,
Residual γ obtained by facilitating the enrichment of C in austenite and stabilizing austenite by refining
It was also found that the amount can be increased and a high-strength hot-dip galvanized steel sheet having excellent ductility and fatigue resistance can be obtained. The present invention was made based on the above findings.

That is, the gist of the present invention is as follows. (1) C: 0.05 to 0.20% by mass, Si: 0.3 to 1.8% by mass and Mn: 1.0 to 3.0% by mass, with the composition of the composition consisting of the balance Fe and unavoidable impurities and the main phase and second phase of ferrite. And the second phase is bainite:
The balance is 80 vol% or more and the balance is one or more of martensite, retained austenite, and pearlite, and the ratio of the average length in the plate thickness direction to the average length in the rolling direction of the second phase is 0.7. The above is the hot-rolled sheet as a raw material, the hot-rolled sheet is cold-rolled, and then held for 5 s or more in the temperature range of the Ac ₁ transformation point or higher, and after the primary heat treatment, 10 ° C./s or higher At a cooling rate of less than the Ms point, and further from the Ac ₁ transformation point to the primary heating temperature range of 5
After the secondary heat treatment of holding for 120 seconds, cool at a cooling rate of 5 ° C / s or more to a temperature of 500 ° C or less, then perform hot dip galvanizing, and then cool 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 and fatigue resistance, which is characterized by cooling to 300 ° C.

(2) C: 0.05 to 0.20 mass%, Si: 0.3 to 1.
A steel material containing 8% by mass and Mn: 1.0 to 3.0% by mass and having a composition composition of balance Fe and unavoidable impurities is subjected to hot rolling after heating, and subsequently at a cooling rate of 10 ° C / s or more. After cooling, it is wound up at a temperature of 300 ° C to 550 ° C and then 5s in the temperature range of Ac ₁ transformation point or higher.
After the above holding, after the primary heat treatment, it is cooled to a temperature below the Ms point at a cooling rate of 10 ° C / s or more, and further held for 5 to 120 seconds in the temperature range from the Ac ₁ transformation point to the primary heating temperature. After the secondary heat treatment, 50 at a cooling rate of 5 ℃ / s or more
High-strength hot-dip zinc with excellent ductility and fatigue resistance characterized by being cooled to a temperature of 0 ° C or lower, then subjected to hot dip galvanizing treatment, and then cooled to 300 ° C at a cooling rate of 5 ° C / s or higher. Manufacturing method of plated steel sheet.

(3) In the above (1) or (2), after forming a hot-dip galvanized film by hot-dip galvanizing treatment, 40
A method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility and fatigue resistance, which comprises reheating the hot-dip galvanized coating to a temperature range of 0 ° C to 550 ° C to alloy it.

(4) In the above (1), (2) or (3),
The steel material has a composition that further contains a component selected from any one or more of the following (a) to (e), and is characterized by a high tensile strength hot-dip galvanized steel sheet having excellent ductility and fatigue resistance. Manufacturing method. Note (a) Al: 0.2 to 1.5 mass% (b) Any one or two of Cr and Mo is 0.
05 to 1.0% by mass (c) B: 0.003% by mass or less (d) One or two selected from Ti, Nb and V
0.01 to 0.3% by mass in total for one or more kinds (e) One or two kinds of Ca and REM in total
0.01 mass% or less

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The manufacturing method of the present invention will be described below step by step. First, the reasons for limiting the component composition of the steel sheet used as the material will be described. C: 0.05 to 0.20% by mass C is an essential element for increasing the strength of the steel sheet, and is effective for generating retained austenite and a low temperature transformation phase, and is an essential element. However, if the C content is less than 0.05% by mass, the desired high strength cannot be obtained, while if it exceeds 0.20% by mass, the weldability is deteriorated. Therefore, the C content is limited to the range of 0.05 to 0.20% by mass.

Mn: 1.0 to 3.0 mass% Mn has the effects of strengthening the steel by solid solution strengthening, improving the hardenability of the steel, and further promoting the formation of retained austenite and the low temperature transformation phase. This kind of action
It is recognized when the Mn content is 1.0 mass% or more. On the other hand, if the content exceeds 3.0% by mass, the effect is saturated, and the effect commensurate with the content cannot be expected, resulting in an increase in cost. Therefore,
Mn was limited to the range of 1.0 to 3.0 mass%.

Si: 0.3 to 1.8 mass% Si has the effects of strengthening the steel by solid solution strengthening and suppressing the formation of carbides and promoting the formation of retained austenite phase. Such an effect is recognized when the Si content is 0.3% by mass or more. On the other hand, if the content exceeds 1.8% by mass, the plating property is significantly deteriorated. Therefore, Si is 0.3-1.
It was limited to the range of 8% by mass.

Further, in the present invention, in addition to the above chemical components, a component selected from any one or more of the following (a) to (e) can be further added, if necessary. (A): Al: 0.2 to 1.5 mass% Al has a function of suppressing the formation of carbides and promoting the formation of a retained austenite phase, like Si. Such an effect is recognized when the Al content is 0.2% by mass or more. On the other hand, 1.
If the content exceeds 5 mass%, the amount of inclusions in the steel increases and the ductility decreases. Therefore, the Al content is 0.2-1.5.
It is preferable to limit to the range of mass%.

(B): 0.05 to 1.0 mass% of one or two of Cr and Mo in total Cr and Mo have the action of improving the hardenability of steel and promoting the formation of a low temperature transformation phase. It is an element that has. Such an effect is obtained by adding one or two of Cr and Mo to a total of 0.05
It is recognized to contain more than mass%. On the other hand, the effect is saturated even if it is contained alone or in a total amount of more than 1.0% by mass, the effect commensurate with the content cannot be expected, and it is economically disadvantageous. Therefore, the total of one or two of Cr and Mo is 0.05.
It is desirable to limit the content to the range of 1.0 mass%.

(C): B: 0.003 mass% or less B is an element having an action of improving the hardenability of steel, and can be contained if necessary. However, the B content is 0.00
Since the effect is saturated when it exceeds 3% by mass, it is preferable to limit B to 0.003% by mass or less. More preferably, 0.
It is added in the range of 001 to 0.002 mass%.

(D): 0.01 to 0.3 mass% of one or more of Ti, Nb and V in total Ti, Nb and V form carbonitrides and have high strength due to precipitation strengthening of steel. It has a function to be converted, and can be added if necessary. Such an action is observed in a total amount of at least 0.01 mass% of one or more selected from Ti, Nb and V. On the other hand, if it is contained alone or in a total amount of more than 0.3% by mass, the strength becomes excessively high and the ductility decreases. Therefore, the content of one or more of Ti, Nb and V is preferably limited to the range of 0.01 to 0.3 mass%.

(E): 0.01% by mass or less of one or two of Ca and REM in total Ca and REM have the function of controlling the morphology of sulfide inclusions, and as a result, It has the effect of improving stretch flangeability. Such an effect is saturated when the total amount of one or two selected from Ca and REM exceeds 0.01% by mass. Therefore, the content of any one or more of Ca and REM is preferably limited to 0.01% by mass or less. More preferably, it is added in the range of 0.001 to 0.005 mass%.

The balance other than the above chemical components is Fe.
And inevitable impurities. As the unavoidable impurities, P: 0.05% by mass or less and S: 0.02% by mass or less are acceptable.

Next, the structure of the steel sheet will be described in detail. [Structure of hot-rolled sheet] A composite structure composed of a ferrite main phase and a second phase. The second phase contains bainite: 80 vol% or more, and the balance is martensite, retained austenite, and pearlite. It consists of more than one seed.
By using bainite as the second phase in the structure of the hot-rolled sheet as a material, the second phase is likely to be divided in the subsequent cold rolling, and the residual γ increases due to the refinement of the final structure, and It works effectively to improve the ductility. When the bainite in the second phase is less than 80 vol% and the martensite increases, the second phase is less likely to be divided during cold rolling, and the finally obtained second phase becomes coarse and ductility decreases. Further, when the amount of pearlite increases, the cementite is not re-dissolved during the primary heat treatment and remains undissolved, and C is consumed as cementite, so that the residual γ amount decreases and the ductility decreases. The main phase means a phase containing 50 vol% or more.

The ratio of the average length in the plate thickness direction to the average length of the second phase (hereinafter referred to as the aspect ratio) is 0.7 or more. The second phase of the hot-rolled sheet structure has an aspect ratio smaller than 0.7, that is, When the stretched shape is obtained, the finally obtained second phase is also formed in rows at positions along the original second phase, which is disadvantageous in the propagation of fatigue cracks and deteriorates fatigue resistance. Therefore, the aspect ratio of the second phase is set to 0.7 or more.
Although the upper limit of the aspect ratio is not particularly specified, it is actually unlikely that the average length of the second phase in the rolling direction becomes smaller than the average length in the plate thickness direction. The upper limit of is substantially 1.0.

[Final Structure] In the present invention, cold rolling using a hot-rolled sheet having the above component composition and structure as a raw material,
It is characterized in that a hot-dip galvanized steel sheet having a desired final structure is produced by performing heat treatment and plating treatment. As this final structure, tempered martensite,
It is essential to have a composite structure composed of retained austenite, ferrite and a low temperature transformation phase. This tempered martensite is a phase having a uniform and fine structure that inherits the form of lath martensite before tempering.
Since tempered martensite is softened by tempering and has sufficient plastic deformability, it is an effective phase for improving the ductility of high-strength steel sheets.

The hot-dip galvanized steel sheet obtained according to the production method of the present invention must contain such a tempered martensite phase in an amount of 20 vol% or more. This is because if the amount of tempered martensite is less than 20 vol%, a remarkable effect of improving ductility cannot be expected.

Further, the retained austenite has a function of performing strain-induced transformation into martensite during working, widely dispersing locally applied working strain, and improving the ductility of the steel sheet. The steel sheet obtained according to the manufacturing method of the present invention must contain such residual austenite in an amount of 3 vol% or more. That is, the amount of retained austenite is 3 vol%
If it is less than 1, the remarkable improvement in ductility cannot be expected.

The phases other than the tempered martensite and retained austenite in the final composite structure are ferrite and low temperature transformation phase. Ferrite is a soft phase containing no carbide, has a high deformability, and improves the ductility of the steel sheet. Therefore, the ferrite content in the final composite structure is preferably 30 vol% or more. On the other hand, the low temperature transformation phase referred to in the present invention refers to martensite or bainite that has not been tempered. Both martensite and bainite are hard phases and increase steel plate strength. Ferrite, which is a soft phase, and low temperature transformation phase, which is a hard phase, form a composite structure with tempered martensite and retained austenite, resulting in a microstructure in which the soft phase to the hard phase coexist, resulting in high ductility of the steel sheet. As a result of the realization of a high yield ratio and a low yield ratio, the formability of the steel sheet is significantly improved.

The basis weight of the hot-dip galvanized layer may be appropriately determined according to the corrosion resistance requirement or the like according to the purpose of use, and is not particularly limited. For example, in a steel sheet used for structural parts of automobiles, the thickness (area weight) of the hot dip galvanized layer is preferably 30 to 60 g / m ² .

Next, a method for producing a hot-dip galvanized steel sheet having the above-mentioned final structure by using a hot-rolled sheet having the above-mentioned composition and structure as a raw material will be described in detail for each step. First, molten steel having the above-described composition is melted and cast into a slab or the like by continuous casting or the like to obtain a steel material for rolling. Then, this steel material is heated according to the general method of this type of steel sheet, roughly rolled into a sheet bar, and finish rolled into a hot rolled sheet having a desired sheet thickness, followed by winding and hot rolling. In this hot rolling process, 10 ° C after rolling
By cooling at a cooling rate of / s or more and winding at a temperature of 300 ° C. or more and 550 ° C. or less, the hot rolled sheet can have the above-described structure.

That is, in the component system of the present invention containing a high proportion of alloying components such as Mn, segregation of these alloying elements occurs during casting. When the cooling rate is lower than 10 ° C / s and the coiling temperature is higher than 550 ° C, ferrite easily avoids these segregated portions and is easily generated.
The segregated portion will remain as the second phase. As a result, the second phase of the hot-rolled sheet becomes an expanded form, and the second phase finally obtained
The phases are also formed in rows at positions along the original second phase, and as a result, fatigue cracks are more likely to propagate and fatigue resistance is degraded. When the winding temperature is lower than 300 ° C, martensite in the second phase of the hot rolled sheet increases, the second phase is hard to be divided during cold rolling, and the finally obtained second phase is coarse. And the ductility decreases. Therefore, the cooling rate is limited to 10 ° C / s or more, and the winding temperature is limited to the range of 300 ° C to 550 ° C.

After the above hot rolling, cold rolling is carried out,
Use cold-rolled steel sheet. If necessary, pickling or annealing can be performed. After that, the steel sheet is cooled to a structure containing martensite after the primary heat treatment, the primary step, subjected to a secondary heat treatment in a continuous hot-dip galvanizing line, tempering of the martensite formed in the primary step, A secondary step for producing retained austenite and a low temperature transformation phase is performed, and then a tertiary step for galvanizing is performed to obtain a high-strength hot-dip galvanized steel sheet having excellent ductility. The three steps will be described in detail below.

[Primary Step] In the primary step, the steel sheet is subjected to a primary heat treatment in which the steel sheet is held at a temperature range of Ac ₁ transformation point or higher for at least 5 s, and then a temperature of 10 ° C./s or higher up to a temperature of Ms point or lower. To quench quickly. By this primary step, 20 vol% or more of lath martensite is formed in the steel sheet. That is, in order to obtain uniformly fine tempered martensite, it is necessary to have a structure containing lath-shaped martensite as a pre-structure.

Here, the heating and holding temperature of the primary heat treatment, that is, the primary heating temperature is less than Ac ₁ or the holding time is 5 s.
When the amount is less than the above, the amount of austenite generated during heating and holding is small, and the amount of lath martensite obtained after cooling is insufficient. If the primary heating temperature exceeds 950 ° C, the crystal grains are likely to coarsen, so the primary heating temperature is preferably 950 ° C or lower. Furthermore, if the cooling rate after the primary heat treatment is less than 10 ° C./s, the steel sheet structure after cooling cannot be made a structure containing lath martensite. The upper limit of the cooling rate after the primary heat treatment is preferably 100 ° C./s or less in order to keep the shape of the steel sheet good. Further, the holding time is preferably 5 s or more and 120 s or less.

[Secondary Step] In the secondary step, a steel plate on which 20 vol% or more of lath-like martensite was produced in the primary step was further added in the temperature range from the Ac ₁ transformation point to the primary heating temperature.
After the secondary heat treatment of holding for ~ 120s, 5 ℃ / s
Cool to a temperature below 500 ° C at the above cooling rate. By this secondary step, the lath-like martensite produced in the primary step is made into tempered martensite, and at the same time, partial re-austenitization of the steel sheet structure for finally producing retained austenite and low temperature transformation phase is aimed at.

Here, when the heating and holding temperature of the secondary heat treatment, that is, the secondary heating temperature is lower than the Ac ₁ transformation point, re-austenization does not occur. On the other hand, when the secondary heating temperature exceeds the primary heating temperature, the fine structure generated in the primary step becomes coarse and the elongation decreases.

If the heating and holding time in the secondary heat treatment is less than 5 s, re-austenization is insufficient, so that retained austenite cannot be obtained. On the other hand, if it exceeds 120 s, re-austenization proceeds, and it becomes difficult to obtain the necessary amount of tempered martensite.

Further, the cooling rate after the secondary heat treatment is 5 ° C.
If it is less than / s, the cooling rate is slow and the austenite produced in the secondary heat treatment is transformed into ferrite, pearlite, etc.,
It does not become retained austenite or low temperature transformation phase. The cooling rate after the secondary heat treatment should be 50 ° C / s or less,
It is preferable in order to keep the shape of the steel plate good.

Incidentally, it is preferable to carry out this secondary step in a continuous hot dip galvanizing line having both an annealing equipment and a hot dip galvanizing equipment. This is because the continuous hot dip galvanizing line allows the process to immediately shift to the tertiary process after the secondary process, which improves the productivity.

[Third Step] In the third step, the steel sheet subjected to the second step is hot dip galvanized and cooled to 300 ° C. at a cooling rate of 5 ° C./s or more. The hot dip galvanizing process is
Usually, the treatment conditions used in the continuous hot-dip galvanizing line may be used, and there is no particular limitation. However, it is difficult to secure a necessary amount of retained austenite by plating at an extremely high temperature. Therefore, it is preferable to carry out the plating treatment at 500 ° C. or less. Further, when the cooling rate after plating is extremely low, it becomes difficult to secure retained austenite. Therefore, it is preferable to limit the cooling rate in the temperature range from the plating treatment to 300 ° C. to 5 ° C./s or more. The upper limit of this cooling rate is preferably 50 ° C./s. Needless to say, wiping for adjusting the basis weight may be performed after the plating treatment, if necessary.

Further, alloying treatment may be performed after the hot dip galvanizing treatment. In the alloying treatment, after the hot dip galvanizing treatment, the hot dip galvanized coating is alloyed by reheating to a temperature range of 450 to 550 ° C. After the alloying treatment, it is preferable to cool to 300 ° C at a cooling rate of 5 ° C / s or more. When alloying at high temperature, it becomes difficult to secure the necessary amount of retained austenite, and the ductility of the steel sheet decreases. Therefore, the upper limit of the alloying temperature is preferably 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 lowered. Further, when the cooling rate after the alloying treatment is extremely low, it becomes difficult to secure the necessary retained austenite. Therefore, it is preferable to limit the cooling rate in the temperature range from the alloying treatment to 300 ° C to 5 ° C / s or more.

Further, the steel sheet after the plating treatment or the alloying treatment may be subjected to temper rolling for correcting the shape and adjusting the surface roughness. Further, there is no inconvenience even if the surface of the steel sheet after plating is treated with resin or oil coating or various coatings.

In the above description, the secondary heating of the steel sheet, the hot dip galvanization, and the alloying treatment are performed in the hot dip galvanizing line in which the annealing equipment, the plating equipment, and the alloying treatment equipment are continuous. However, it is also possible to carry out each process in an independent facility or process.

[0049]

EXAMPLE Steels having the composition of components shown in Table 1 were melted in a converter and made into cast pieces by a continuous casting method. The obtained slab was hot-rolled to a plate thickness of 2.6 mm under the conditions shown in Tables 2 and 3, then pickled, and then cold-rolled to obtain a steel plate having a plate thickness of 1.0 mm. Here, the structure after hot rolling is composed of 50 to 90 vol% ferrite (main phase) and the second phase, and the volume ratio of bainite in the second phase and the aspect ratio of the second phase are From the results of the structure observation with the optical microscope, it was confirmed that the results are shown in Tables 4 and 5. Next, these cold-rolled steel sheets were heated and cooled in a continuous annealing line according to the conditions of the primary steps shown in Tables 2 and 3. Further, the steel sheet that has undergone the primary process is processed by the continuous hot dip galvanizing line in Table 2
And, according to the conditions of the secondary step shown in Table 3, after performing heating, cooling and isothermal holding, hot dip galvanizing treatment is subsequently performed, and then, for some of them, an alloy of hot dip galvanizing coating is reheated after hot dip galvanizing treatment. After the chemical treatment, cooling was performed, and then a third step was performed.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

The hot dip galvanizing process is performed at a bath temperature of 475.
After immersing the steel plate in a plating bath at ℃, pull it up and perform gas wiping treatment so that the basis weight per side is 50 g / m ^2, and then raise the temperature to 500 ℃ at a heating rate of 10 ℃ / s, alloy Processed. The holding time during the alloying treatment was adjusted so that the iron content in the plating film was 9 to 11 mass%.

With respect to the hot-dip galvanized steel sheet thus obtained, the results of investigation on mechanical properties and structure were
It shows in Table 4 and Table 5. The amounts of retained austenite in Tables 4 and 5 were obtained by grinding a sample taken from a steel plate to the center plane in the plate thickness direction and measuring the diffraction X-ray intensity at the plate thickness center plane. MoKα rays are used as incident X-rays, and the diffracted X-ray intensity of each face of ferrite (200) (211) and the diffracted X-ray intensity of each face of austenite (200) (220) are calculated to obtain ferrite (marten). The ratio of the integrated intensity of (200) (211) (including sites) to the integrated intensity of (200) (220) of austenite was determined and used as the volume ratio of retained austenite.

As for the mechanical properties, the yield strength (YP), the tensile strength (TS) and the elongation (El) were measured using JIS No. 5 tensile test pieces taken from the steel sheet in the direction perpendicular to the rolling direction. In the fatigue test, the fatigue limit (FL) was measured by the double swing plane bending test method with a frequency of 20 Hz.

[0056]

[Table 4]

[0057]

[Table 5]

[0058]

According to the present invention, a high-strength hot-dip galvanized steel sheet having sufficient ductility as a material for automobile parts and excellent in fatigue resistance can be manufactured.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 2/06 C23C 2/06 2/28 2/28 2/28 (72) Inventor Takashi Sakata Chuo-ku, Chiba City, Chiba Prefecture Kawasaki-cho No. 1 Kawasaki Steel Co., Ltd. Technical Term F-term (reference) 4K027 AA05 AA22 AB42 AC72 AE12 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA15 EA16 EA17 EA19 EA27 EA27 FE28 FE01 F01 FD01 FD01 FD01 FD01 FD01 FD01 FD01 FD01 FD01 FD01 FD01 FD01

Claims (4)

[Claims] 1. C: 0.05 to 0.20 mass%, Si: 0.3 to 1.8
Mass% and Mn: 1.0 to 3.0% by mass, and has a composition of the balance Fe and unavoidable impurities, and a composite structure composed of a ferrite main phase and a second phase, and the second phase is bainite: 80 vol. %, The balance is martensite,
A hot-rolled sheet made of any one or more of retained austenite and pearlite, having a ratio of the average length in the plate thickness direction to the average length in the rolling direction of the second phase of 0.7 or more, as a raw material, The hot rolled sheet was cold rolled and then Ac ₁
After performing a primary heat treatment of maintaining the temperature range above the transformation point for 5 s or more, cool it to a temperature below the Ms point at a cooling rate of 10 ° C / s or more, and further change the temperature from the Ac ₁ transformation point to the primary heating temperature. After the secondary heat treatment of holding for 5 to 120 seconds in the area,
After cooling to a temperature of 500 ° C or less at a cooling rate of 5 ° C / s or more, and then performing hot dip galvanizing, 5 ° C / s
A method for producing a high-strength hot-dip galvanized steel sheet excellent in ductility and fatigue resistance, characterized by cooling to 300 ° C at the above cooling rate. 2. C: 0.05 to 0.20 mass%, Si: 0.3 to 1.8
%, And Mn: 1.0 to 3.0% by mass, and a steel material having a component composition consisting of balance Fe and unavoidable impurities, subjected to hot rolling after heating, and subsequently cooled at a cooling rate of 10 ° C./s or more. After that, it is wound up at a temperature of 300 ° C or higher and 550 ° C or lower, and then 5 seconds at a temperature range of the Ac ₁ transformation point or higher.
After the above holding, after the primary heat treatment, it is cooled to a temperature below the Ms point at a cooling rate of 10 ° C / s or more, and further held for 5 to 120 seconds in the temperature range from the Ac ₁ transformation point to the primary heating temperature. After the secondary heat treatment, 50 at a cooling rate of 5 ℃ / s or more
High-strength hot-dip zinc with excellent ductility and fatigue resistance, characterized by being cooled to a temperature of 0 ° C or lower, then subjected to hot dip galvanizing treatment, and then cooled to 300 ° C at a cooling rate of 5 ° C / s or higher. Manufacturing method of plated steel sheet. 3. The method according to claim 1, wherein after forming the hot-dip galvanized coating by hot-dip galvanizing, the temperature is 400 ° C.
A method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility and fatigue resistance, which comprises reheating to a temperature range of to 550 ° C to alloy the hot-dip galvanized coating. 4. The steel material according to claim 1, 2 or 3, wherein the steel material has a composition further containing a component selected from any one or more of the following (a) to (e). A method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility and fatigue resistance. Note (a) Al: 0.2 to 1.5 mass% (b) Any one or two of Cr and Mo is 0.
05 to 1.0% by mass (c) B: 0.003% by mass or less (d) One or two selected from Ti, Nb and V
0.01 to 0.3% by mass in total for one or more kinds (e) One or two kinds of Ca and REM in total
0.01 mass% or less