JP2001207236A - 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 galvanizes steel plat excellent in ductility and impact 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 and 1.0 to 3.0% Mn is subjected to a first stage in which the same is subjected to primary heating treatment at (the Ac3 transformation point-50 deg.C) or more and is rapidly cooled to the Ms point or less, a second stage in which the same is subjected to secondary heating treatment in a dual- phase region and is rapidly cooled to 500 deg.C or less and a third stage in which the same is applied with hot dip galvanizing treatment and is rapidly cooled to 300 deg.C in succession. In this way, the high tensile strength hot dip galvanized steel plate having a composite structure containing F of 30% or more, tempered M or 20% or more, retained γ of 2% or more and martensite of 5% or more as a low temperature transformed phase and an instant (n) value of 0.35 or more in elongation of 10% in a tensile test at a high strain rate (2×103/s) and excellent in ductility and impact resistance can be 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 hot-dip galvanized steel sheet, and more particularly to improvement of ductility and collision resistance of a high-tensile 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, and a high-tensile steel sheet excellent in ductility is required. In addition, recently, importance has been placed on the safety of automobiles, and for that purpose, there has been a demand for an improvement in collision resistance, which is a measure of safety in a collision. For this reason, it is strongly required that a steel sheet for automobile parts also have excellent ductility and impact resistance.

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 promote weight reduction and strengthening of an automobile body, a high-strength hot-dip galvanized steel sheet excellent in corrosion resistance, ductility and collision resistance is an indispensable material.

[0005] As a high tensile strength 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, 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.

In such a situation, a method for producing a high-strength hot-dip galvanized steel sheet using a continuous hot-dip galvanizing line includes a steel sheet having a composition containing a large amount of a hardenability improving element such as Cr or Mo. It is effective to use a method for facilitating the formation of a low-temperature transformation phase as a plating mother plate. However, there is a problem that the addition of a large amount of such an alloying element causes an increase in manufacturing cost.

[0007] To solve such a problem, for example, Japanese Patent Publication No. Sho 62
Japanese Patent No. -40405 discloses a cooling rate in a continuous hot-dip galvanizing line, from a heating temperature between the Ac _{1 to} Ac ₃ transformation points, until hot-dip galvanizing is performed, and a cooling step of cooling to 300 ° C. or less after the alloying treatment. There has been proposed a method for producing a structure-reinforced high-strength hot-dip galvanized steel sheet using a low-temperature transformation phase by setting the cooling rate to a predetermined critical cooling rate or more.
However, the structure-strengthened high-strength hot-dip galvanized steel sheet obtained by the technology described in Japanese Patent Publication No. 62-40405 is sufficiently satisfactory in ductility in terms of the current requirements for steel sheets for automobile parts and the like. I can't say. Furthermore, these manufacturing conditions have left operational problems for application in continuous galvanizing lines.

As a high-tensile steel sheet having excellent formability, a steel sheet utilizing a structure mainly composed of tempered martensite has been proposed. For example, Japanese Patent Application Laid-Open No. Hei 6-93340 discloses that after cold-rolling a hot-rolled steel sheet, it is heated and maintained at a temperature higher than a recrystallization temperature and at a temperature higher than an Ac ₁ transformation point, and then reaches an Ms point before reaching a molten zinc tank. A stretch flange, which is quenched below to form partially or entirely martensite in the steel sheet, and then heated to a temperature not lower than the Ms point and at least to the temperature of the molten zinc bath and the temperature of the alloying furnace to produce tempered martensite; A method for producing a high-strength galvannealed steel sheet having excellent heat resistance has been proposed.

Japanese Patent Application Laid-Open No. 6-108152 discloses a recrystallization annealing step including maintaining at a temperature of (Ac ₃ transformation point −50 ° C.) to 900 ° C. for at least 1 second after hot rolling and cold rolling. ,
A step of applying zinc plating and a step of performing reheating at a temperature of 250 ° C. or lower below the Ac ₁ transformation point, and after the recrystallization annealing step,
A high-strength molten zinc having a tempered martensitic structure and excellent in bending workability, characterized in that it is cooled from a temperature higher than the Ms point to at least the Ms point at a predetermined critical cooling rate or higher before the reheating treatment step. A method for producing a plated steel sheet has been proposed.

However, high-strength hot-dip galvanized steel sheets obtained by the techniques described in JP-A-6-93340 and JP-A-6-108152 are excellent in stretch flangeability and bending workability, but are not suitable for automobile parts. As a steel sheet widely used as a material for use, it was not sufficiently satisfactory in terms of ductility. On the other hand, as a high-tensile steel sheet having excellent impact resistance, for example, Japanese Patent Application Laid-Open No. Hei 9-111396 discloses that C: 0.05-0.
20% or less, Si: 0.01 to 1.50%, Mn: 0.5 to 3.0%, further contains one or two of Cr and Mo, and one or two of Ti and Nb, and has an average Martensite with a grain size of 3 μm or less and ferrite with an average grain size of 5 μm or less
A high-strength hot-rolled steel sheet for automobiles having a phase structure and containing 5 to 30% of the martensite and having excellent impact resistance has been proposed.

[0011]

SUMMARY OF THE INVENTION
Using the steel sheet described in 9-111396 publication, even as a hot-dip galvanized steel sheet in a normal continuous hot-dip galvanizing line,
The cooling rate in the continuous hot-dip galvanizing line is slow, it is difficult to contain martensite in the steel sheet after plating, and a hot-dip galvanized steel sheet having both ductility and impact resistance has not been obtained.

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 impact resistance, which is suitable as a material for automobile parts, and a method for producing the same. Aim. 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.

[0013]

Means for Solving the Problems First, 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 impact resistance. 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 was changed to ferrite, tempered martensite,
It has been found that excellent ductility can be exhibited by forming a composite structure composed of retained austenite and a low-temperature transformation phase and setting the volume ratio of each phase in the composite structure to a predetermined ratio. Further, by making the low-temperature transformation phase of the composite structure contain at least 5% or more by volume of martensite, the instantaneous n value (elongation at 10% elongation at the time of tensile deformation at a strain rate of 2 × 10 ³ / s) is obtained. In the present invention, the instant n value is also referred to as the dynamic n value) is 0.35 or more, and it has been found that the collision resistance is improved in addition to the ductility.

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. The steel sheet structure is subjected to reheating treatment and plating treatment to form a composite structure including ferrite, tempered martensite, retained austenite, and a low-temperature transformation phase within a predetermined volume ratio range. In particular, the low-temperature transformation phase has a volume ratio of at least 5%. % Or more of martensite,
It has been found that the dynamic n value becomes 0.35 or more, and it is possible to obtain a high-tensile hot-dip galvanized steel sheet having improved impact resistance in addition to ductility.

The dynamic n value referred to in the present invention is a value introduced by the present inventors as an index of the collision resistance of a steel sheet.
By using the dynamic n value, the collision safety of the steel sheet can be more accurately evaluated than before. Conventionally, crash safety has been considered in relation to strength, and it has been considered that the higher the strength, the higher the crash safety. However, strength and safety are not necessarily in a unique relationship. Then, the present inventors further examined the evaluation of collision safety.

As a result, in order to absorb more of the energy at the time of collision of the automobile with the steel plates constituting the vehicle body and the like, the elongation at the time of tensile deformation at a high strain rate (2 × 10 ³ / s) is 10%. It has been found that it is effective to increase the n value (instantaneous n value) of the steel sheet in the above. In the event of a car collision, the strain rate applied to the vehicle body increases to 2 × 10 ³ / s. The present inventors have found that the instantaneous n value at the elongation of 10% is a dynamic n value, and that the dynamic n value is 0.35 or more, the collision resistance is significantly improved.

The present invention has been made based on the above findings. That is, the first 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, and has an elongation of 10% when subjected to tensile deformation at a strain rate of 2 × 10 ³ / s. % Is a high tensile galvanized steel sheet excellent in ductility and impact resistance characterized by having an instantaneous n value of 0.35 or more, and in the first present invention, the steel sheet has a mass% of C : 0.05 ~ 0.20%,
Si: 0.3-1.8%, Mn: 1.0-3.0%, composition consisting of balance of Fe and unavoidable impurities, ferrite with volume ratio of 30% or more, tempered martensite with volume ratio of 20% or more, volume ratio It is preferred that the low-temperature transformation phase has a composite structure composed of 2% or more of retained austenite and a low-temperature transformation phase, and that the low-temperature transformation phase contains at least 5% or more by volume of martensite. In the first aspect of the present invention, in addition to the composition, one or two of the following (Group a) to (Group d) (Group a): Cr and Mo are added in a total amount of 0.1%.
05-1.0 mass%, (group b): 0.003 mass% or less of B, (group c): 0.01 mass% or less in total of one or two selected from Ca and REM (group d): It is preferable that one or more selected from Ti, Nb, and V are contained in total in one or more selected from 0.01 to 0.2 mass%.

In the second invention, the mass% and C: 0.
A steel sheet containing 05-0.20%, Si: 0.3-1.8%, Mn: 1.0-3.0%, and having a composition consisting of the balance of Fe and unavoidable impurities, has a temperature range of (Ac ₃ transformation point -50 ° C) or more for 5 seconds. After performing the primary heat treatment holding the above, a primary step of cooling to a temperature below the Ms point at a cooling rate of 10 ° C./s or more, and then a temperature range of (Ac ₁ transformation point to Ac ₃ transformation point). ~ 120sec
After performing a secondary heat treatment for holding for a while, 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 a galvanizing treatment is performed, and a hot-dip galvanized layer is formed on the surface layer of the steel sheet. Is formed and then cooled at a cooling rate of over 10 ° C / s.
And a tertiary step of cooling to 0 ° C. in order.
% Of tempered martensite, at least 2% by volume of retained austenite and at least 5% by volume of martensite at a low temperature transformation phase.
The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility and impact resistance having an instantaneous n value of 0.35 or more at an elongation of 10% when subjected to tensile deformation at ³ / s. The tertiary step is performed at a temperature of 450 ° C. after a hot-dip galvanizing process is performed to form a hot-dip galvanized layer on the surface layer of the steel sheet.
Preferably, the step is a step of reheating to a temperature range of up to 550 ° C. to perform an alloying treatment on the hot-dip galvanized layer, and thereafter cooling to 300 ° C. at a cooling rate of more than 10 ° C./s.
In the second aspect of the present invention, in addition to the composition, one or two of the following (Group a) to (Group d) (Group a): Cr and Mo are added in a total amount of 0.1%.
05-1.0 mass%, (group b): 0.003 mass% or less of B, (group c): 0.01 mass% or less in total of one or two selected from Ca and REM (group d): It is preferable that one or more selected from Ti, Nb, and V are contained in a total of 0.01 to 0.2 mass%, or one or two or more selected from Ti, Nb, and V.

[0019]

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 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 is degraded. For this reason, C was limited to the range of 0.05 to 0.20%.

Si: 0.3-1.8% 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 a 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.

Further, in the steel sheet of the present invention, if necessary, in addition to the chemical components described above, the following (group a) to
One or more of (d) groups can be contained. (Group a): One or two of Cr and Mo are added in a total amount of 0.
Each of 05 to 1.0% Cr and Mo is an element having an effect of improving the hardenability of steel and promoting the formation of a low-temperature transformation phase, and can be contained as necessary. Such an effect is observed when one or two of Cr and Mo are contained in a total amount of 0.05% or more. on the other hand,
Even if one or two of Cr and Mo exceed 1.0% in total, the effect saturates, an effect corresponding to the content cannot be expected, and it 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. A more preferred range is 0.05 to 0.5% in total of one or two of Cr and Mo.

(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 in total of two or more species Ca, REM has the effect of controlling the form of sulfide inclusions, thereby improving the stretch flangeability of the steel sheet, and is contained as necessary. it can. Such effects are Ca,
Saturation occurs when the content of one or two selected from 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. Note that a more preferable range is 0.00.
1 to 0.005%.

(D group): 0.01 to 0.2% in total of one or more selected from Ti, Nb, and V Ti, Nb, and V form carbonitrides in steel; These carbon nitrides have the effect of strengthening the steel by precipitation strengthening, and also have the effect of reducing the crystal grain size, and can be contained as necessary. These effects are due to Ti, Nb, V
One or two or more selected from the total of 0.01
% Or more. On the other hand, even if the content exceeds 0.2% in total, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, the content of one or more of Ti, Nb, and V is 0.01 to 0.2 in total.
% Is preferable.

In the steel sheet used in the present invention, the balance other than the above-mentioned chemical components consists of Fe and unavoidable impurities. As inevitable impurities, Al: 0.1% or less, P: 0.05% or less, and S: 0.02% or less are acceptable. Furthermore, the steel sheet of the present invention has the above composition and (1) ferrite, (2) tempered martensite, (3) retained austenite, and (4)
It is a steel sheet having a composite structure composed of a low-temperature transformation phase. By forming a composite structure in which these phases coexist, effects such as an 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, has 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. If the amount of ferrite exceeds 70%, it is difficult to obtain the advantages of the multi-phase composite structure, so 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. Tempered martensite is softened by tempering and has a sufficient plastic deformability, and thus 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 is limited to 20% or more. If the amount of tempered martensite exceeds 60%, it becomes difficult to obtain the advantage of the multi-phase composite structure, and thus the amount of tempered martensite is 60%.
% Is desirable.

(3) Retained austenite Retained austenite undergoes strain-induced transformation into 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 referred to in the present invention refers to untempered martensite or bainite. Martensite and bainite are both 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 generation of movable dislocations, it also has the effect of lowering the yield ratio of the steel sheet. In addition, in order to obtain the above effect sufficiently,
The low-temperature transformation phase is preferably martensite.

Further, the present inventors have newly found that martensite has an effect of increasing the dynamic n value. By containing at least 5% by volume of martensite as a low-temperature transformation phase, the dynamic n value can be made 0.35 or more, and the collision resistance is greatly improved. For this reason, it is preferable to limit the content of martensite to 5% or more by volume as the low-temperature transformation phase. As the impact resistance, the larger the amount of martensite, the better, but it is practically 20% or less.

On the other hand, as a low-temperature transformation phase, bainite is
Since it has a tendency to lower the dynamic n value, it is preferable that the amount is as small as possible for improving the collision resistance. 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, and is preferably 5 to 20% by volume. The content of bainite is preferably 0 to 5%.

The high-strength hot-dip galvanized steel sheet 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 the surface 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 the high-strength galvanized steel sheet of the present invention will be described. First, a steel having the above-described composition is melted, cast into a cast piece 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-described 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 hot-dip galvanizing line. Tempering of the lath martensite formed in the step, and a secondary step (a) for partially re-austenizing the steel sheet structure for generating retained austenite and a low-temperature transformation phase after the tertiary step, followed by galvanizing The steel is subjected to a tertiary step () for performing treatment and cooling to generate residual austenite and a low-temperature transformation phase, thereby obtaining a high-strength hot-dip galvanized steel sheet having excellent ductility and impact resistance.

Primary Step In the primary step, the steel sheet is subjected to a primary heat treatment for maintaining the steel sheet in a temperature range of not less than (Ac ₃ transformation point−50 ° C.), preferably not more than (Ac ₃ transformation point + 100 ° C.) for at least 5 seconds. The steel sheet is quenched at a cooling rate of 10 ° C / s or more to a temperature below the Ms point. By this primary step, lath martensite is generated in the steel sheet. Ferrite,
In order to obtain a uniformly fine composite structure of tempered martensite, retained austenite, and a low-temperature transformation phase, it is necessary that the steel sheet structure after the first step be a structure containing lath-like martensite.

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. In addition, the heating holding temperature of the primary heat treatment is preferably (Ac ₃ transformation point +10) from the viewpoint of miniaturization of lath martensite.
0 ° C) or less. Further, the holding time is preferably set to 120 sec or less.

The cooling rate after the primary heat treatment is 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. When a hot-rolled steel sheet in which the final rolling is performed at a temperature of (Ar ₃ transformation point −50 ° C.) or higher is used as the plating base plate, when cooling after the final rolling, the temperature is reduced to a temperature not higher than the Ms point by 10%. This primary step can be replaced by rapid cooling at a cooling rate of at least ° C / s.

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 the Ac ₁ transformation point to the 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.
On the other hand, when the holding temperature exceeds the Ac ₃ transformation point, the structure of the steel sheet becomes entirely austenite, 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 by the secondary heat treatment is transformed into ferrite or pearlite, and retained austenite or low-temperature transformation is performed. Not in phase. In order to increase the amount of martensite, the cooling rate after the secondary heat treatment is preferably 10 ° C / s or more, more preferably 20 ° C / s or more. The cooling rate after the secondary heat treatment is preferably 100 ° C./s or less in order to keep the shape of the steel sheet good.

This secondary step is preferably carried out 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 more than 10 ° C./s. 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. For this reason, the cooling rate in the temperature range from after plating to 300 ° C. is preferably more than 10 ° C./s, and more preferably limited to 20 ° C./s or more.
The temperature is preferably 100 ° C./s or less from the viewpoint of the shape of the steel sheet. Needless to say, after plating, wiping for adjusting the basis weight may be performed as necessary.

After the hot-dip galvanizing treatment, the plating layer may be subjected to alloying treatment. 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. 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.

After the alloying treatment, it is preferable to cool to 300 ° C. at a cooling rate of more than 10 ° C./s, more preferably 20 ° C./s or more. If the cooling rate after the alloying treatment is extremely low, it becomes difficult to secure the necessary amount of retained austenite or the amount of martensite as a low-temperature transformation phase. For this reason, the cooling rate in the temperature range from after the alloying treatment to 300 ° C. exceeds 10 ° C./s, more preferably 20 ° 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 and the like. 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 in a hot-dip galvanizing line in which annealing equipment, plating equipment and alloying treatment equipment are continuous, the secondary step and the tertiary step are performed continuously, but each step is performed by independent equipment. It is also possible.

[0046]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter and cast into pieces by continuous casting. 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.

[0047]

[Table 1]

Next, these cold-rolled steel sheets were subjected to a primary step of heating and holding under the primary step conditions shown in Table 2 and then cooling in a continuous annealing line. 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%.

[0050]

[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 low-temperature transformation phase such as lath martensite, ferrite, tempered martensite, martensite, etc. in a steel sheet, a 100 mm square set arbitrarily by image analysis using a cross-sectional structure photograph of 1000 times magnification The occupied area ratio of the relevant phase existing in the region was determined and defined as the volume ratio of the relevant phase. The amount of retained austenite was determined by polishing a steel sheet up to the center plane in the thickness direction and measuring the diffraction X-ray intensity at the center plane in the thickness direction. MoK α-rays were used for the incident X-rays, and the residual austenite phases {111}, {200},
The diffraction X-ray intensity ratio of each of the {220} and {311} surfaces was determined, and the average value thereof was defined as the volume fraction of retained austenite.

The mechanical properties of the steel sheet were examined by a tensile test. Tensile test was conducted using a JI sample taken from a steel plate in the direction perpendicular to the rolling direction.
Using JIS No. 5 test piece specified in S Z2204, JIS Z2241
The yield strength (YS), the tensile strength (TS) and the elongation at break (El) were measured in accordance with the rules of the above. The impact resistance of steel sheet is
It was investigated by a high strain rate tensile test. The high strain rate tensile test is performed using a Hopkinson pressure bar tester at a strain rate of 2 × 10 ³ / s to determine the instantaneous n value when the elongation is 10%, and to determine the dynamic n value. did. (Note that the strain amount ε
The instantaneous n value at (ε−2.5)% strain and (ε + 2.5)%
Calculated using stress and strain at strain. Table 3 shows the obtained results.

[0053]

[Table 3]

From Table 3, it can be seen 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 20000 MPa ·% or more, and a dynamic strength. It is a high tensile galvanized steel sheet having excellent ductility and excellent collision resistance with a target n value of 0.35 or more. On the other hand, in Comparative Examples outside the range of the present invention, there was no example satisfying both excellent ductility and excellent collision resistance at the same time. In particular, in Comparative Examples in which the amount of martensite as the low-temperature transformation phase is less than 5%, the dynamic n value is as low as less than 0.35, and the ductility and the impact resistance are not simultaneously excellent.

[0055]

As described above, according to the present invention,
A high-tensile galvanized steel sheet which has extremely excellent ductility and impact resistance and is actually suitable as a molded article material represented by an automobile part can be manufactured stably at low cost, and has a remarkable industrial effect.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takashi Sakata 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref. No. 1 town Kawasaki Steel Engineering Laboratory F term (reference) 4K037 EA02 EA05 EA06 EA09 EA11 EA15 EA16 EA17 EA19 EA27 EA28 EA31 EA32 EA36 GA05

Claims (6)

[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 an instant n at an elongation of 10% when subjected to tensile deformation at a strain rate of 2 × 10 ³ / s. A high-strength hot-dip galvanized steel sheet having excellent ductility and impact resistance, having a value of 0.35 or more. 2. The composition according to claim 1, wherein the steel sheet contains, by mass%, C: 0.05 to 0.20%, Si: 0.3 to 1.8%, Mn: 1.0 to 3.0%, and the balance being Fe and unavoidable impurities;
It has a composite structure consisting of 30% or more by volume of ferrite, 20% or more by volume of tempered martensite, 2% or more by volume of retained austenite and a low-temperature transformation phase, and
The high-strength hot-dip galvanized steel sheet according to claim 1, wherein the low-temperature transformation phase contains at least 5% or more by volume of martensite. 3. In addition to the above composition, the following (group a)
The high tensile galvanized steel sheet having excellent ductility and impact resistance according to claim 2, comprising one or more groups selected from (d). Note (Group a): One or two of Cr and Mo are added in a total of 0.
05-1.0 mass%, (group b): 0.003 mass% or less of B, (group c): 0.01 mass% or less in total of one or two selected from Ca and REM (group d): One or more selected from Ti, Nb, and V in total of 0.01 to 0.2 mass% 4. 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) (Point -50 ℃) 5se above temperature range
c After performing a primary heat treatment of holding at or above, a primary step of cooling at a cooling rate of 10 ° C./s or more to a temperature below the Ms point, and then in a temperature range of (Ac ₁ transformation point to Ac ₃ transformation point). 5-
After performing a secondary heat treatment for 120 seconds, a secondary process of cooling at a cooling rate of 5 ° C./s or more to a temperature of 500 ° C. or less, and then performing a galvanizing treatment and subjecting the surface layer of the steel sheet to galvanizing. After forming the layer, a tertiary step of sequentially cooling to 300 ° C. at a cooling rate of more than 10 ° C./s is sequentially performed, wherein the structure is ferrite having a volume fraction of 30% or more, and having a volume fraction of 20% or more. A composite structure consisting of tempered martensite, a low-temperature transformation phase containing 2% or more retained austenite by volume and 5% or more martensite by volume.
A method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility and impact resistance having an instantaneous n-value of 0.35 or more at an elongation of 10% when subjected to tensile deformation at 2 × 10 ³ / s. 5. The method according to claim 3, wherein the tertiary step is performed by 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 exceeding 10 ° C / s after the alloying process. The method for producing a high-strength hot-dip galvanized steel sheet having excellent ductility and impact resistance according to claim 4. 6. 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 and impact resistance according to claim 4 or 5, comprising at least one group selected from (d) and (d). . Note (Group a): One or two of Cr and Mo are added in a total of 0.
05-1.0 mass%, (group b): 0.003 mass% or less of B, (group c): 0.01 mass% or less in total of one or two selected from Ca and REM (group d): One or more selected from Ti, Nb and V in total of 0.01 to 0.2 mass%,