JP2002129241A - Method for manufacturing high tensile hot-dip galvanized steel sheet having excellent ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a high tensile hot-dip galvanized steel sheet having sufficient ductility required for a stock for automotive parts by utilizing a continuous hot-dip galvanizing line. SOLUTION: A steel sheet having a composition consisting of, by mass, 0.05-0.20% C, 0.3-1.8% Si, 1.0-3.0% Mn and the balance Fe with inevitable impurities is subjected to: a primary step where the steel sheet is rapidly cooled to a temperature not higher than the MS point after primary heat treatment; a secondary step where the steel sheet is rapidly cooled to 470-350 deg.C after secondary heat treatment and held at a temperature ranging from cooling stop temperature to 350 deg.C for 10-500 s to undergo holding treatment; and a tertiary step where the steel sheet is subjected to hot-dip galvanizing treatment and rapid cooling. In this way, a composite structure consisting of tempered martensite, >=3% retained austenite, ferrite, and low-temperature transformation phase is formed, and also a hot-dip galvanizing layer is formed on the surface of the steel sheet. Moreover, alloying treatment can be applied after the hot-dip galvanizing treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-strength hot-dip galvanized steel sheet, and more particularly to an improvement in ductility of a high-strength hot-dip galvanized steel sheet manufactured by a continuous hot-dip galvanizing line.

[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. 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-reinforced steel sheet having a composite structure of ferrite and a low-temperature transformation phase has been proposed. A typical example of the structure strengthened steel sheet is a two-phase structure steel sheet having a composite structure of ferrite and martensite.
Recently, a highly ductile steel sheet utilizing transformation induced plasticity caused by retained austenite has also reached the stage of practical use.

On the other hand, high corrosion resistance is required for automotive parts depending on the application site. A hot-dip galvanized steel sheet mainly composed of an alloyed hot-dip galvanized steel sheet is suitable for a component material applied to such a part. Therefore, in order to further reduce the weight and strengthen the automobile body, a high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and excellent ductility is an indispensable material.

However, at present, most hot-dip galvanized steel sheets are manufactured in a continuous hot-dip galvanizing line. In these continuous hot-dip galvanizing lines, the annealing equipment and the plating equipment are often installed in a continuous manner, and the cooling after annealing is interrupted at the plating temperature due to the plating treatment after annealing. For this reason, it becomes difficult to increase the average cooling rate in the entire process.

[0006] Therefore, in a high-strength hot-dip galvanized steel sheet manufactured in a continuous hot-dip galvanizing line, martensite and residual austenite, which are generally generated under cooling conditions with a high cooling rate, do not exist in the steel sheet after the plating treatment. difficult. As a method of manufacturing a structure-reinforced high-strength galvanized steel sheet in a continuous hot-dip galvanizing line,
There is a method of adding a large amount of an alloying element such as Cr or Mo that enhances hardenability to facilitate generation of a low-temperature transformation phase such as martensite. However, there is a problem that the addition of a large amount of alloying elements causes an increase in manufacturing cost.

For example, Japanese Patent Publication No. Sho 62-40405 discloses that C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, Cr: 0.03%.
Alloying by heating a thin steel sheet containing 0.80.8% between the Ac ₁ transformation point and the Ac ₃ transformation point, performing hot-dip galvanizing during cooling, and further heating to a temperature between 500 ° C. and the Ac ₁ transformation point A method has been proposed for producing a structure-strengthened alloyed hot-dip galvanized high-strength steel sheet using a continuous hot-dip galvanizing line that is subjected to a treatment and then cooled to 300 ° C. In this method for producing an alloyed hot-dip galvanized high-strength steel sheet, cooling after heating between the Ac ₁ transformation point and the Ac ₃ transformation point and cooling to 300 ° C. after the alloying treatment are related to the amounts of Cr and Mn. It is characterized in that it is performed at a cooling rate higher than the critical cooling rate specified by the following formula, and it is a two-phase steel sheet containing a low-temperature transformation structure mainly composed of martensite in a ferrite base, and an alloyed galvanized layer is formed on the steel sheet. And a steel plate having

However, in the technique described in Japanese Patent Publication No. 62-40405, it is necessary to adjust the cooling conditions after annealing or plating in a continuous galvanizing line in accordance with the composition of each steel sheet. Adjustment of such cooling conditions
There was a problem due to restrictions on the equipment of the continuous galvanizing line. Further, the ductility of the steel sheet manufactured by the technique described in Japanese Patent Publication No. 62-40405 was not sufficient.

On the other hand, unlike the structure-strengthened hot-dip galvanized high-strength steel sheet described in Japanese Patent Publication No. 62-40405, the formability is improved using tempered martensite using a continuous hot-dip galvanizing line. A method for obtaining an excellent high tensile galvanized steel sheet has been proposed. For example, JP-A-6-9334
The 0 JP, in a continuous galvanizing line, and kept heated above the recrystallization temperature or higher and Ac ₁ transformation point, then rapidly cooled to below then M _S point, then at least molten zinc at a temperature of more than M _S point A method for producing a high-strength alloyed hot-dip galvanized steel sheet that is heated to a bath temperature and an alloying furnace temperature and then immersed in a hot-dip zinc bath has been proposed.

Japanese Patent Application Laid-Open No. 6-108152 discloses (A
c ₃ Transformation point -50 ° C) ~ 900 ° C for at least 1 sec
A recrystallization annealing step including holding the above, a step of applying galvanization, and after these steps, an Ac ₁ transformation point or less of 250 or less.
Having a step of performing a reheating treatment at a temperature of not less than ℃, after the recrystallization annealing step and before the reheating treatment step, from a temperature higher than the _MS point, a critical cooling rate or more depending on the alloy element amount or more A method for producing a high-strength alloyed hot-dip galvanized steel sheet that is excellent in bending workability and that is cooled to a temperature equal to or lower than the _MS point at a cooling rate has been proposed.

JP-A-6-93340 and JP-A-6-1081
In any of the techniques described in Japanese Patent Publication No. 52, the steel sheet is heated from the austenitic temperature range before plating or alloying treatment.
This is a method for producing a high-strength galvannealed steel sheet having a martensite structure by quenching the steel sheet to a temperature equal to or lower than the _S point and reheating the steel sheet to form tempered martensite. However, none of the steel sheets manufactured by the techniques described in JP-A-6-93340 and JP-A-6-108152 cannot sufficiently satisfy the ductility currently required for materials such as automobile parts. Further improvement in ductility has been desired.

[0012]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, and provides a method for producing a high-strength hot-dip galvanized steel sheet having sufficient ductility as a material for automobile parts and excellent strength-elongation balance. Is provided. In the method for producing a high-strength galvanized steel sheet of the present invention, it is desirable to use a continuous galvanizing line.

[0013]

Means for Solving the Problems The present inventors have conducted intensive studies from the viewpoint of the composition and microstructure of a steel sheet in order to manufacture a high ductility and high tensile galvanized steel sheet using a continuous galvanizing line. . As a result, the structure of the high-strength hot-dip galvanized steel sheet obtained after the hot-dip galvanizing treatment was tempered martensite, including retained austenite, and the balance being a composite structure composed of ferrite and a low-temperature transformation phase, thereby excelling in the steel sheet. It has been found that ductility can be expressed.

Further, in order to obtain a composite structure including tempered martensite and retained austenite and a balance of ferrite and a low-temperature transformation phase, the structure of the steel sheet in which the chemical composition is adjusted to a predetermined range is defined by: First, a structure containing lath-like martensite, and further subjected to reheating treatment and plating treatment under predetermined conditions in a continuous hot-dip galvanizing line, including tempered martensite and residual austenite, and the rest being ferrite and low-temperature transformation. It has been found that it is possible to obtain the above-mentioned composite structure composed of phases and to obtain a high tensile galvanized steel sheet having extremely excellent ductility.

Further, by first forming a structure containing lath-like martensite, austenite precipitated during the subsequent tempering treatment can be finely dispersed, so that enrichment of C in austenite is facilitated. Found that austenite was stabilized by the refinement of austenite, the amount of retained austenite was increased, and a high-tensile galvanized steel sheet having extremely excellent ductility could be obtained.

The present invention has been made based on the above findings. That is, the present invention is a method for producing a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface layer of the steel sheet, wherein, by mass%, C: 0.05 to 0.20%, and Si: 0.3 to 1.
8%, Mn: 1.0 to 3.0%, and having a composition comprising the balance of Fe and inevitable impurities, (Ac ₃ transformation point −50)
° C.) at temperatures above was subjected to primary heat treatment for holding more than 5 sec, a primary step of cooling to a temperature below M _S point 10 ° C. / sec or more cooling rate, then, (Ac ₁ transformation point to Ac ₃
After performing a secondary heat treatment for 5 to 120 seconds in a temperature range between (transformation point) and 470 to 35 at a cooling rate of 5 ° C./sec or more.
After cooling to a cooling stop temperature in a temperature range of 0 ° C., a secondary process of performing a staying process of staying at a temperature range of 350 ° C. or lower that is not higher than the cooling stop temperature for 10 to 500 seconds, and then performing a hot-dip galvanizing process on the steel sheet And forming a hot-dip galvanized film on the surface and then sequentially performing a tertiary step of cooling to 300 ° C. at a cooling rate of 5 ° C./sec or more. Further, in the present invention, after the tertiary step is subjected to hot-dip galvanizing to form a hot-dip galvanized film on the steel sheet surface, 450
Preferably, the method is a step of re-heating to a temperature range of from 5 ° C. to 550 ° C. to perform an alloying treatment of the hot-dip galvanized film, and cooling to 300 ° C. at a cooling rate of 5 ° C./sec or more after the alloying treatment.

In addition, the present invention further provides, in addition to the above-mentioned composition, the following (groups) to (e) (group a): 0.2 to 1.5% by mass of Al; One or two types in total of 0.
05 to 1.0% by mass, (group c): B: 0.003% by mass or less, (d): one or two selected from Ti, Nb, and V
(E group): 1 or 2 groups selected from among 0.01% by mass or less of one or two selected from Ca and REM in total. It is preferable to contain the above.

Further, in the present invention, the steel sheet is a hot-rolled steel sheet in which final hot rolling is performed at a temperature of (Ar ₃ transformation point−50 ° C.) or more, and the final hot rolling is performed in place of the primary step. The subsequent cooling can be a hot rolled steel sheet structure adjusting step of rapidly cooling to a temperature below the _MS point at a cooling rate of 10 ° C./sec or more.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a method for producing 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 the present invention,% in the composition means mass%.

C: 0.05 to 0.20% C is an essential element for increasing the strength of a 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%, the desired high strength cannot be obtained, while if it exceeds 0.20%, the weldability deteriorates. For this reason, C was limited to the range of 0.05 to 0.20%.

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 formation of retained austenite and a low-temperature transformation phase. Such an effect is observed when the Mn content is 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 to
Limited to the 3.0% range.

Si: 0.3 to 1.8% Si has the effect of strengthening the steel by solid solution strengthening, suppressing the formation of carbides, stabilizing austenite, and promoting the formation of a retained austenite phase. Such an effect is observed when the Si content is 0.3% or more. Meanwhile, 1.
If the content exceeds 8%, the plating property is significantly deteriorated.
For this reason, Si is limited to the range of 0.3 to 1.8%.

Further, in the steel sheet of the present invention, if necessary, in addition to the chemical components described above, the following (group a) to
It is possible to further contain one or more of (group e). (Group a): Al: 0.2 to 1.5%, Al has an effect of suppressing the formation of carbides and promoting the formation of a retained austenite phase, as in the case of Si, and can be contained as necessary in the present invention. Such an effect is observed at a content of 0.2% or more. On the other hand, when the content exceeds 1.5%, the amount of inclusions in the steel increases and the ductility decreases. For this reason, Al
Is preferably limited to the range of 0.2 to 1.5%.

(Group b): One or two of Cr and Mo in a total amount of 0.05 to 1.0% Cr and Mo improve the hardenability of steel and promote the formation of a low-temperature transformation phase. Is an element having This effect is achieved by adding one or two of Cr and Mo in a total of 0.05%.
Above is recognized. On the other hand, if the content exceeds 1.0% in total, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, it is desirable to limit one or two of Cr and Mo to a range of 0.05 to 1.0% in total.

(Group c): 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. In addition, 0.001 to 0.002% is more preferable. (D group): one or two selected from Ti, Nb and V
At least 0.01% to 0.3% of the species, Ti, Nb, and V form a carbonitride and have the effect of increasing the strength of the steel by precipitation strengthening, and can be added as necessary. Such an effect is recognized when one or more selected from Ti, Nb, and V are contained in a total amount of 0.01% or more. On the other hand, if the total content exceeds 0.3%, the strength becomes excessively high and the ductility decreases. Therefore, T
It is preferable that the content of one or more of i, Nb, and V is limited to a total range of 0.01 to 0.3%. In addition, more preferably, it is 0.01 to 0.15%.

(Group e): 1 selected from Ca and REM
Ca or REM of 0.01% or less in total of two or more species has the effect of controlling the form of sulfide-based inclusions, and thereby has the effect of improving the stretch flange properties of the steel sheet. In order to obtain such an effect, it is preferable that one or two of Ca and REM are contained in a total of 0.001% or more.
Such an effect is saturated when the content of one or two selected from Ca and REM 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.

The steel sheet used in the present invention comprises the balance Fe and unavoidable impurities other than the above-mentioned chemical components. As inevitable impurities, Al: 0.1% or less, P: 0.05% or less,
S: 0.02% or less is acceptable. Next, the reasons for limiting the manufacturing steps in the method of the present invention will be described. Melt the molten steel having the above composition, cast by a usual known method,
The steel sheet is hot-rolled or further cold-rolled by a known method. 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 martensite (), followed by a secondary heat treatment in a continuous hot-dip galvanizing line. Tempering of the martensite formed in step 2) and a secondary step (1) for generating retained austenite and a low-temperature transformation phase, and then a tertiary step (2) of galvanizing to obtain high-tensile hot-dip galvanizing with excellent ductility Obtain a steel plate.

[0029] In the primary process first reaction step is subjected to primary heat treatment for holding at least 5sec or more (Ac ₃ transformation point -50 ° C.) temperatures above the steel sheet, cooling over 10 ° C. / sec to a temperature below M _S point Quench at speed. By this primary process, 20% (volume ratio) of lath martensite is generated in the steel sheet. In order to obtain tempered martensite according to the present invention, it is necessary to have a structure containing lath martensite as a prestructure.

When the holding temperature of the primary heat treatment is (Ac ₃ transformation point −50)
C) or less than 5 sec, the amount of austenite generated during heating and holding is small, and the amount of lath martensite obtained after cooling is insufficient. If the cooling rate after the primary heat treatment is less than 10 ° C./sec, the steel sheet structure after cooling cannot be a structure containing lath martensite. The upper limit of the cooling rate after the primary heat treatment is preferably 100 ° C./sec or less in order to keep the shape of the steel sheet good. Further, the holding time is preferably set to 5 seconds or more and 120 seconds or less.

When a hot-rolled steel sheet whose final hot rolling is performed at a temperature of (Ar ₃ transformation point −50 ° C.) or higher is used as the plating base plate, this primary step is performed after the final rolling. The cooling can be replaced by rapid cooling to a temperature below the _MS point at a cooling rate of 10 ° C./sec or more. However, in order to homogenize the structure of the steel sheet after cooling, it is preferable that the primary step be performed as an independent step after hot rolling.

Secondary Step In the secondary step, the steel sheet which has produced lath martensite of 20% or more in the primary step is further subjected to (Ac ₁ transformation point to A
c After performing a secondary heat treatment for 5 to 120 seconds in the temperature range between ₃ transformation points), at a cooling rate of 5 ° C./sec or more, 470 to
It is cooled to a cooling stop temperature in a temperature range of 350 ° C., and is subjected to a staying process of staying at a temperature range of 350 ° C. or less below the cooling stop temperature for 10 to 500 seconds.

By this secondary step, the lath martensite generated in the primary step is converted into tempered martensite, and a part of the steel sheet structure is finally re-austenitized to finally generate retained austenite and a low-temperature transformation phase. Aim. When the heating holding temperature in the secondary heat treatment is lower than the Ac ₁ transformation point, austenite is not regenerated, and no residual austenite or low-temperature transformation phase is obtained after cooling. On the other hand, when the holding temperature exceeds the Ac ₃ transformation point, tempered martensite is re-austenitized.

If the heating holding time in the secondary heat treatment is less than 5 seconds, austenite is not sufficiently regenerated, so that a sufficient amount of retained austenite cannot be obtained after cooling. On the other hand, when the time exceeds 120 seconds, the re-austenitization of tempered martensite proceeds, and it becomes difficult to obtain a required amount of tempered martensite. In addition, 47
If the cooling rate to the temperature range of 0 to 350 ° C. is less than 5 ° C./sec, the cooling rate is low and the austenite formed by the secondary heat treatment transforms to ferrite, pearlite, etc., and does not become a retained austenite or a low-temperature transformation phase. Note that the cooling rate after the secondary heat treatment is preferably 5 ° C./sec or more and 50 ° C./sec or less.

In the temperature range between the cooling stop temperature and 350 ° C. (hereinafter also referred to as the “retention temperature range”), the retention treatment for maintaining the temperature isothermally or gradually cooling is performed to promote the generation of retained austenite. By performing this staying treatment, austenite is stabilized, and the amount of retained austenite increases. If the temperature range of the retention treatment is lower than 350 ° C, austenite may transform to martensite, while
If the temperature exceeds 0 ° C., the bainite transformation proceeds excessively, the precipitation of carbides becomes faster, and the amount of retained austenite decreases.

If the residence time in the retention temperature range is less than 10 seconds, the concentration of C in austenite is insufficient and austenite cannot be stabilized. On the other hand, if it exceeds 500 sec, bainite transformation proceeds excessively and the amount of retained austenite decreases, so that the ductility of the steel sheet decreases. For this reason,
The residence time in the residence processing is limited to 10 to 500 sec.
In addition, preferably, it is 10 to 100 seconds. Residence time 100s
Even if the length is longer than ec, the concentration of C in austenite is saturated, and the austenite is sufficiently stabilized, so that a longer residence time longer than that will lower the productivity.

This secondary step is preferably performed in a continuous hot-dip galvanizing line having both an annealing facility and a hot-dip galvanizing facility. By using the continuous hot-dip 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 a hot-dip galvanizing treatment and cooled to 300 ° C. at a cooling rate of 5 ° C./sec or more. In addition, the sheet temperature of the steel sheet after the secondary process is, for example,
If the temperature is less than 430 ° C, which is extremely low compared to the plating bath temperature and there is a possibility of solidification of zinc, before hot-dip galvanizing treatment, the sheet temperature of the steel sheet is adjusted to a temperature suitable for hot-dip galvanizing treatment by a heating means. It is preferable to perform a pre-plating heat treatment for raising the temperature. The temperature of the pre-plating heat treatment is preferably set to a plating bath temperature or higher from the viewpoint of plating adhesion. The heating means does not need to be particularly limited, but induction heating or the like is preferable.

Further, the hot-dip galvanizing treatment may be performed under the processing conditions usually performed in a continuous hot-dip galvanizing line.
There is no particular limitation. However, plating at an extremely high temperature makes it difficult to secure a necessary amount of retained austenite. For this reason, it is preferable to perform plating at 500 ° C. or lower. When the cooling rate after plating is extremely low, it is difficult to secure retained austenite. For this reason, it is preferable that the cooling rate in the temperature range from the plating treatment to 300 ° C. be limited to 5 ° C./sec or more. Preferably, it is 50 ° C./sec 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 may be performed. In the alloying treatment, after the hot-dip galvanizing treatment, reheating to a temperature range of 450 ° C. to 550 ° C. is performed to alloy the hot-dip galvanized film. After the alloying treatment, it is preferable to cool to 300 ° C. at a cooling rate of 5 ° C./sec or more. Alloying at a high temperature makes it difficult to secure the necessary amount of retained austenite, and reduces the ductility of the steel sheet. For this reason, the upper limit of the alloying temperature is preferably limited to 550 ° C. On the other hand, when the alloying temperature is lower than 450 ° C., the progress of alloying is slow, and the productivity is reduced. If the cooling rate after the alloying treatment is extremely low, it becomes difficult to secure necessary austenite. Therefore, it is preferable to limit the cooling rate in the temperature range from after the alloying treatment to 300 ° C. to 5 ° C./sec or more.

The steel sheet after the plating treatment or the alloying treatment may be subjected to temper rolling for shape correction and adjustment of surface roughness and the like. Further, there is no inconvenience even if a treatment such as resin or oil coating or various kinds of painting is performed.
The present invention is based on the premise that the steel sheet is subjected to secondary heating, hot-dip galvanizing, and alloying in a continuous hot-dip galvanizing line with annealing equipment, plating equipment, and alloying treatment equipment. Alternatively, it can be performed in a process.

The hot-dip galvanized steel sheet obtained by the manufacturing method of the present invention is a steel sheet having the above composition and a composite structure composed of tempered martensite, retained austenite, ferrite, and a low-temperature transformation phase. The tempered martensite in the present invention refers to a phase formed by precipitation of iron carbide when lath-like martensite is heated and maintained in a temperature range of (Ac ₁ transformation point to Ac ₃ transformation point) for a short time. Point.

Tempered martensite is a phase having a fine internal structure that inherits the form of lath martensite before tempering. Tempered martensite is a phase that is effective for improving the ductility of a high-strength steel sheet because it is softened by tempering and has sufficient plastic deformability. The galvanized steel sheet obtained by the production method of the present invention contains such a tempered martensite phase in a volume ratio of 20% or more. If the amount of tempered martensite is less than 20%, a remarkable ductility improving effect cannot be expected. On the other hand, if it exceeds 60%, it is difficult to increase the strength of the steel sheet.

The retained austenite has a function of transforming martensite into strain during processing, dispersing locally applied processing strain widely, and improving ductility of the steel sheet. The steel sheet obtained by the production method of the present invention contains such retained austenite in a volume ratio of 3% or more. If the amount of retained austenite is less than 3%, remarkable improvement in ductility cannot be expected. The amount of retained austenite is preferably 5%.
That is all. The larger the amount of retained austenite is, the better, but it is actually 15% or less in the manufacturing method according to the present invention which passes through the heat history of the continuous hot-dip galvanizing line.

In the composite structure of the steel sheet obtained by the production method of the present invention, except for the above-mentioned tempered martensite and retained austenite, there are ferrite and a low-temperature transformation phase. Ferrite is a soft phase that does not contain iron carbide, has a high deformability, and improves the ductility of a steel sheet. The steel sheet obtained by the production method of the present invention preferably contains ferrite in a volume ratio of 30% or more. If it is less than 30%, the improvement in ductility is small. On the other hand, if it exceeds 70%, it becomes difficult to increase the strength of the steel sheet, so it is preferably set to 70% or less.

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. These low-temperature transformation phases are generated during the cooling process after the second step in the production method of the present invention. Both martensite and bainite are hard phases and increase the strength of the steel sheet. In order to sufficiently increase the strength, it is preferable that the low-temperature transformation phase is martensite that has not been tempered. The amount of the low-temperature transformation phase is not particularly limited in the present invention. It may be appropriately distributed according to the strength of the steel sheet, but preferably 5 to 30% by volume.

The ferrite, which is a soft phase, and the low-temperature transformation phase, which is a hard phase, form a composite structure together with tempered martensite and retained austenite, resulting in a microstructure in which a soft phase to a hard phase are mixed. High ductility and a low yield ratio of the steel sheet are realized, and the formability of the steel sheet is remarkably improved. The high-strength hot-dip galvanized steel sheet obtained by the production method of the present invention is a coated steel sheet in which a hot-dip galvanized layer or an alloyed hot-dip galvanized layer is formed on a surface layer of a steel sheet having the above-described composition and the above-described composite structure. It is. The basis weight 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 automobile structural parts, the thickness of hot-dip galvanized layer (weight per unit area)
Is preferably 30 to 60 g / m ² .

[0047]

EXAMPLES (Example 1) Steel having the composition shown in Table 1 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 1.0 mm thick steel sheet was obtained by cold rolling.

Next, these cold-rolled steel sheets were subjected to a primary step of cooling after heating and holding under the primary step conditions shown in Table 2 in a continuous annealing line. After the first step, the structure was examined to measure the amount of lath martensite. Further, the steel sheet after the first step is subjected to a second step of heating and holding, then cooling, and subsequently performing a stagnation treatment in a continuous hot-dip galvanizing line under the second step conditions shown in Table 2, and subsequently, A tertiary step was performed in which a hot-dip galvanizing treatment was performed, a part of the hot-dip galvanizing treatment was re-heated after the hot-dip galvanizing treatment, and an alloying treatment was performed on the hot-dip galvanized film. The hot-dip galvanizing treatment was performed by immersing the steel sheet in a plating bath at a bath temperature of 475 ° C., and was lifted up to adjust the basis weight by gas wiping such that the basis weight per side was 50 g / m ² . In addition, when performing the alloying treatment of the plating film, after the wiping treatment,
The temperature was raised to 500 ° C. at a heating rate of 10 ° C./sec to perform an alloying treatment. The holding time during the alloying treatment was adjusted so that the iron content in the plating film was 9 to 11%. For steel sheets with a plate temperature below 430 ° C before infiltration of the plating bath, or for some of the steel sheets with a plate temperature of 430 ° C or higher before infiltration of the bath,
After the secondary process and before the hot-dip galvanizing treatment, a heat treatment for increasing the sheet temperature to a predetermined temperature was performed.

The microstructure and mechanical properties of the obtained steel sheet were investigated and are shown in Table 3. The microstructure of the steel sheet was observed with an optical microscope or a scanning electron microscope on the cross section of the steel sheet. The amount of lath martensite and the amount of tempered martensite in the microstructure are calculated as follows: the area occupied by the relevant phase in a 100 mm square area arbitrarily set by image analysis using a cross-sectional structure photograph at a magnification of 1000 Was determined as the volume ratio of the relevant phase. The amount of retained austenite was determined by polishing a specimen taken from a steel plate to the center in the plate thickness direction and measuring the diffraction X-ray intensity at the center in the plate thickness. MoKα rays were used for the incident X-rays, and the diffracted X-ray intensity ratios of the {111}, {200}, {220}, and {311} planes of the retained austenite phase in the specimen were obtained, and the average of these values was calculated. The volume ratio of retained austenite was used.

As for the mechanical properties, the yield strength (yield point) YP, the tensile strength TS, and the elongation El were measured using a JIS No. 5 tensile test piece taken from a steel sheet in a direction perpendicular to the rolling direction.
Table 3 shows the results.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

From Table 3, it can be seen that the inventive examples have a tensile strength TS of 59.
A high ductility and high tensile hot-dip galvanized steel sheet having excellent strength-elongation balance of 0 MPa or more, elongation El of 30% or more, and strength-elongation balance (TS × El) of 23000 MPa% or more. On the other hand, in the comparative examples out of the range of the present invention, the ductility is not sufficient, and the strength-elongation balance is lowered.

The steel sheet No. 2 has a low heat holding temperature in the primary heat treatment, a reduced amount of lath martensite obtained after cooling, a reduced amount of tempered martensite and an amount of retained austenite after plating, and a reduction in strength. Elongation balance is decreasing. In steel sheet No. 4, the holding time in the primary heat treatment was short, the amount of lath martensite obtained after cooling was small, the amount of tempered martensite after plating was reduced, and the strength-elongation balance was reduced. . Also, steel plate
In No. 5, since the cooling rate after the primary heat treatment was too slow, the amount of lath martensite was small, the amount of tempered martensite after plating was low, and the strength-elongation balance was low.

Further, in steel sheet No. 7, since the holding temperature in the secondary heat treatment was too low, the amount of retained austenite was small after plating, and the strength-elongation balance was lowered. Also,
In steel sheet No. 8, since the holding temperature in the secondary heat treatment was too high, the amount of tempered martensite after the plating treatment was small, and the strength-elongation balance was lowered. Further, in steel sheet No. 10, since the holding time in the secondary heat treatment was too short, the amount of retained austenite was reduced after the plating treatment, and the strength-elongation balance was lowered. Further, in steel sheet No. 11, since the holding time in the secondary heat treatment was too long than the preferred range, the amount of tempered martensite after the plating treatment was small, and the strength-elongation balance was slightly lowered.

Steel sheet No. 13 has a low cooling rate after the secondary heat treatment, steel sheet No. 20 has a low cooling rate to 300 ° C. after the alloying treatment, and the amount of retained austenite after the plating treatment is small. Strength-elongation balance is reduced.
For steel sheet No.15, the residence temperature range was too high, and for steel sheet No.16
In the sample, the retention temperature range is too low, the amount of retained austenite after the plating treatment is reduced, and the strength-elongation balance is lowered.

For steel sheet No. 18, the residence time was too short,
The amount of retained austenite after the plating treatment is reduced, and the strength-elongation balance is lowered. In steel sheet No. 24, the staying treatment was omitted, the amount of retained austenite after the plating treatment was reduced, and the strength-elongation balance was lowered. In steel sheets Nos. 21 to 23, the composition of the steel sheets was out of the range of the present invention, the amount of tempered martensite or retained austenite was reduced, and the strength-elongation balance was lowered.

It should be noted that a steel plate having a plate temperature equal to or higher than the plating bath temperature had better plating adhesion than a steel plate having a plate temperature lower than the plating bath temperature. (Example 2) Steel B having the composition shown in Table 1 was melted in a converter and cast into a slab by a continuous casting method. The obtained slab was subjected to a hot rolling step of hot rolling to a sheet thickness of 2.3 mm, and immediately after hot rolling, a quenching was immediately performed under the conditions shown in Table 4 and a hot rolled steel sheet structure adjusting step of winding into a coil was performed. . This hot rolled steel sheet structure adjusting step was used as an alternative to the primary step in the production method of the present invention. After the hot-rolled steel sheet structure adjusting step, the microstructure of the steel sheet was examined, and the amount of lath martensite was measured.

Next, the hot-rolled steel sheet was subjected to a secondary step of heating, holding and cooling under the secondary step conditions shown in Table 4 in a continuous hot-dip galvanizing line, and subsequently to a hot-dip galvanizing treatment. Then, a galvanizing treatment of the hot-dip galvanized film was performed, and then a tertiary step of cooling was performed. Hot-dip galvanizing was performed in the same manner as in Example 1. The microstructure and mechanical properties of the obtained steel sheet were examined in the same manner as in Example 1, and the results are shown in Table 5.

[0061]

[Table 4]

[0062]

[Table 5]

From Table 5, the hot-dip galvanized steel sheet of the present invention is a high-tensile hot-dip galvanized steel sheet having excellent ductility.

[0064]

According to the present invention, a high-strength hot-dip galvanized steel sheet having extremely excellent ductility and actually suitable as a molded article material typified by an automobile part can be manufactured stably at low cost. It has a remarkable industrial effect.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C23C 2/06 C23C 2/06 2/28 2/28 (72) Inventor Kazuhiro Seto Chiba City, Chiba Pref. 1 Kawasaki-cho, Ward, Kawasaki Steel Engineering Co., Ltd. (72) Inventor Tetsuo Shimizu 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.Technical Research Laboratory (72) Inventor Takashi Sakata Central, Chiba-shi, Chiba No. 1 Kawasaki-cho, Ward F-term in Kawasaki Steel Engineering Laboratory Co., Ltd. 4K027 AA05 AB02 AB42 AC12 AC72 AC73 AC87 AD25 4K037 EA05 EA06 EA15 EA16 EA27 EA28 EB09 FD03 FD04 FF02 FF03 GA05

Claims (4)

[Claims] 1. 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 having a composition consisting of balance Fe and unavoidable impurities, (Ac ₃ transformation) At a temperature of -50 ° C) or more and 5se
was subjected to a primary heat treatment of holding more than c, the primary step of cooling to a temperature below M _S point 10 ° C. / sec or more cooling rate, then the temperature between (Ac ₁ transformation point to Ac ₃ transformation point) After performing a secondary heat treatment for 5 to 120 seconds in the temperature range,
a second step of cooling at a cooling rate of not less than sec to a cooling stop temperature in a temperature range of 470 to 350 ° C., and then performing a staying process of staying for 10 to 500 sec in a temperature range of 350 ° C. or less below the cooling stop temperature; A hot-dip galvanizing treatment to form a hot-dip galvanized film on the surface of the steel sheet, and then sequentially performing a tertiary step of cooling to 300 ° C. at a cooling rate of 5 ° C./sec or more, and a high tensile strength excellent in ductility. Manufacturing method of hot-dip galvanized steel sheet. 2. The tertiary step is to perform a hot-dip galvanizing process to form a hot-dip galvanized film on the surface of the steel sheet, and then reheat the steel sheet to a temperature range of 450 ° C. to 550 ° C. to alloy the hot-dip galvanized film. And 5 ° C. /
2. The method for producing a high-tensile hot-dip galvanized steel sheet having excellent ductility according to claim 1, wherein the step is a step of cooling to 300 ° C. at a cooling rate of at least sec. 3. In addition to the above composition, the following (group a)
The method for producing a high-tensile hot-dip galvanized steel sheet having excellent ductility according to claim 1 or 2, comprising one or more groups selected from (e) to (e). (Group a): Al: 0.2 to 1.5% by mass, (Group b): One or two of Cr and Mo in total of 0.
05 to 1.0% by mass, (group 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 of at least one species, (group e): 0.01% by mass or less in total of one or two selected from Ca and REM 4. The steel sheet is subjected to final hot rolling (Ar_Threetransformation
Point -50 ° C) or higher.
Instead of the next step, cooling after the final hot rolling _SBelow the point
Hot rolled steel sheet that is rapidly cooled to a temperature at a cooling rate of 10 ° C / sec or more
4. An organization adjustment step.
High tensile galvanizing with excellent ductility according to any of the above
Steel plate manufacturing method.