JP2002038248A - Method for manufacturing galvanized steel sheet with high tensile strength and ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a galvanized steel sheet with a high tensile strength and ductility, which manufactures a steel sheet suitable for structural members of automobile, having a combination of high tensile strength, high ductility and superior corrosion resistance, by using a conventional facility. SOLUTION: This method comprises, (1) a cold rolled strip which contains 0.05-0.25% C, <=0.7% Si, 0.8-2.5% Mn, 0.4-2.0% sol. Al, and 1.0-2.5% Si+Al, (2) a continuous annealing which includes heating the strip at a two-phase region, cooling at 30 deg.C/second or more between 700-450 deg.C, and holding at 450-350 deg.C for 100-500 seconds, and (3) a hot-dip galvanizing after heating the strip at the two-phase region, cooling at 3 deg.C/second or more to 500 deg.C or lower, and holding at 460-500 deg.C for 15 seconds or longer. A galvannealed strip comprises alloying the galvanized strip by holding at 480-530 deg.C for 10-30 seconds. The steel strip may contain Ti, Nb, Cr, Mo, Cu, Ni, and Co.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength, high-ductility galvanized steel sheet which is suitable for a structural member of an automobile and has high strength, high ductility, and excellent rust prevention performance.

[0002]

2. Description of the Related Art Steel plates having higher strength than ever are required for structural members of automobiles and the like from the viewpoint of ensuring collision safety and reducing weight. In general, the strength of a steel sheet and the formability of a press or the like are contradictory, and the higher the strength of the steel sheet, the lower the ductility, the more difficult it is to press form, and the higher the strength, the more applicable parts are limited. Therefore, in order to expand the application of high-strength steel sheets, steel sheets having high strength and excellent formability are required.

It is also important that structural members of automobiles have excellent rustproofing properties, and therefore galvanized steel sheets that are inexpensive and have excellent rustproofing properties are widely used. Above all, alloyed hot-dip galvanized steel sheets in which a hot-dip galvanized steel sheet is heated to an alloying temperature and a Zn-plated layer is made of a Zn-Fe alloy are particularly widely used because of excellent slidability, weldability and paintability. Used.

[0004] To increase the strength of steel, C, Si, Mn
It is effective to include alloy elements such as. However, if a large amount of these alloying elements are contained in steel, the wettability of the surface of the base material before plating with molten zinc is reduced, non-plating is likely to occur, and the rate of reaction when performing alloying treatment is reduced. Become slow. For this reason, a hot-dip galvanized steel sheet or a galvannealed steel sheet with good surface and properties cannot be obtained by the method of including a large amount of alloying elements.

[0005] In austenitic stainless steel sheet, "transformation-induced plasticity (TRI)" shows a remarkable elongation due to transformation of austenite into martensite during forming.
P) "is well known. Several techniques for obtaining a high-strength steel sheet with improved ductility using this have already been proposed.

[0006] For example, in Japanese Patent Application Laid-Open No. Sho 60-43430, a cold rolled sheet containing a large amount of C, Si, and Mn is placed in a two-phase region of ferrite + austenite (hereinafter, also simply referred to as “two-phase region”). After heating and holding, it is cooled at a cooling rate of 50 ° C./sec or more to a temperature range of 450 ° C. to 650 ° C., kept at that temperature range for 10 to 50 seconds, and then cooled at a cooling rate of 30 ° C./sec or more. In the final product, each of ferrite and retained austenite has a volume ratio of 10% or more,
A method for producing a high-strength and high-workability composite structure steel sheet having a crystal structure composed of bainite and martensite is disclosed.

[0007] However, the above steel sheet has a high C content, so that the strength of the steel becomes too high and is not suitable for press forming applications. In addition, since the Si content of the steel is increased, there is a problem that hot-dip galvanizing is difficult and cannot be used as a hot-dip galvanized steel sheet. To solve these problems, several techniques have been proposed as follows.

[0008] Japanese Patent Application Laid-Open No. 5-70886 discloses a local ductility having retained austenite in which the C content is reduced and Si is replaced with Al to reduce the strength to the level of strength for press forming applications. An excellent high-strength thin steel sheet and a method for producing the same are disclosed. In the above-mentioned publication, as a manufacturing method, a cold-rolled sheet having a predetermined chemical composition is heated to a two-phase region and annealed (hereinafter simply referred to as “two-phase region annealing”). It is described that this can be achieved by holding at a low temperature for 30 seconds or more (preferably 2 minutes or more) in a temperature range of ° C. It is also described that the cooling rate from 700 ° C. to the holding temperature is preferably set to 50 ° C./sec or more in order to suppress the pearlite transformation.

[0009] Japanese Patent Application Laid-Open No. 5-247586 discloses a high-strength steel containing retained austenite in which the balance between plating adhesion and strength-ductility is improved by setting the content of Si and Al in steel to a specific range. A ductile hot-dip galvanized steel sheet is disclosed. However, the production method only discloses that the cooling rate after annealing in the two-phase region in the continuous hot-dip galvanizing line is set to 5 ° C./sec or more, and there is no description of other conditions. Poor utility in actual production.

[0010] Japanese Patent Application Laid-Open No. 11-222644 discloses a technique in which steel contains an appropriate amount of Ti to compensate for the deterioration of film adhesion and alloying treatment property due to Si. However, it is said that the production method may be any ordinary method, and the conditions of a hot-dip galvanizing line for optimizing mechanical properties and plating quality are not disclosed.

Japanese Patent Application Laid-Open No. H11-131145 discloses 2
After annealing in the phase region, it is cooled to 520 ° C. or lower, and then in a series of manufacturing processes such as hot-dip plating, adhesion control, alloying treatment, etc., at a temperature range of 520 ° C. to 400 ° C. for 90 seconds or more.
A method for producing a high-strength, high-ductility hot-dip galvanized steel sheet having retained austenite, which stays for 00 seconds or less and then cools to 200 ° C. or less, is disclosed.

JP-A-11-236621 discloses that a cold-rolled sheet having a specific chemical composition is pre-oxidized, then annealed in a two-phase region, and then maintained at a low temperature for at least 20 seconds. A method for producing a ductile galvanized steel sheet is disclosed.

JP-A-11-279691 discloses M
There is disclosed a technique in which the addition of n at least 15 times the C content significantly delays the progress of pearlite and bainite transformation by reheating for alloying treatment performed immediately after plating. However, the addition of a large amount of Mn increases the cost of the alloy and also has the disadvantage of impairing the plating wettability.

Conventionally, continuous annealing equipment used for continuous annealing of cold-rolled sheets is characterized by rapid cooling using gas-water mixed mist or jet water as a cooling medium as a cooling means from the annealing temperature, and rapid cooling. In many cases, a so-called overaging zone capable of performing an overaging treatment for a long time for precipitating the increased solid solution C is provided.

In order to manufacture a high-tensile steel sheet containing retained austenite, it is necessary to supercool untransformed austenite to a bainite transformation temperature range while suppressing pearlite transformation. Therefore, it is important to perform rapid cooling after annealing in the two-phase region. After that, in order to promote the bainite transformation without the formation of carbides and to further concentrate and stabilize carbon in austenite, it is important to perform a treatment of holding at a relatively low temperature for a long time.

Conventional continuous annealing equipment used for recrystallization annealing of a cold-rolled sheet generally has a rapid cooling device and an overaging zone. Therefore, it is extremely efficient to manufacture a high-strength steel sheet containing retained austenite in a continuous annealing facility for recrystallization annealing of a cold-rolled sheet, thereby relatively easily manufacturing a high-tensile steel sheet with excellent ductility. be able to.

On the other hand, in the conventional continuous hot-dip galvanizing equipment, it is difficult to use hot mist or jet water as a cooling medium because hot-dip coating must be performed after annealing. It's not easy. Also,
In continuous hot-dip galvanizing, in order to stabilize the temperature of a steel sheet before plating, the steel sheet before plating is held in a relatively low temperature range (hereinafter, also simply referred to as “low temperature holding”). However, if the temperature of the steel sheet before plating falls below the melting point of zinc, it is necessary to reheat the steel sheet before entering the plating bath, so it is difficult to maintain a low temperature for a long time in the conventional continuous galvanizing equipment. .

Accordingly, JP-A-5-70886, JP-A-11-131145 and JP-A-11-21-2
The rapid cooling or low-temperature holding for a long time disclosed in Japanese Patent No. 36621 cannot be easily performed due to the line configuration as long as a general continuous hot-dip galvanizing line is used.

The hot-dip plating temperature is about 460 ° C., and the alloying treatment is performed in a temperature range of 460 to 500 ° C. Maintaining a steel sheet having retained austenite in such a temperature range is because cementite precipitates in the retained austenite and consumes carbon, so that the volume ratio of the retained austenite is reduced, and the carbon concentration is reduced and the carbon concentration is reduced. There is a problem that the performance is reduced.

Japanese Patent Application Laid-Open No. 11-131145 discloses a technique in which austenite remains using a continuous galvanizing equipment. However, in general, hot-dip galvanizing lines have a short line length,
In some cases, it is difficult to maintain the temperature in the temperature range of about 400C to about 400C for a long time. Even if the line speed is reduced to increase the holding time, the cooling rate after annealing does not reach 6 ° C./sec.

Japanese Patent Application Laid-Open No. H11-236621 discloses the time and the alloying treatment temperature in detail, but it is not always easy to realize suitable conditions for the existing line.

In order to avoid these problems, even if improvement measures such as, for example, using Ti-containing steel are applied to the chemical composition of steel, the difference in performance due to the above equipment difference is dealt with only by improving the chemical composition. Is not enough as an effect.

As described above, various proposals have been made for hot-dip galvanized and alloyed hot-dip galvanized high-strength steel sheets having excellent formability utilizing the TRIP effect. However, it is not easy to stably and efficiently produce a galvanized steel sheet having high tensile strength, high ductility, and excellent corrosion resistance in an existing general continuous hot-dip galvanizing line.

[0024]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet which solves the above problems and has high strength, high ductility and excellent corrosion resistance suitable for structural members of automobiles. It is an object of the present invention to provide a method for producing a high-tension, high-ductility galvanized steel sheet that can be stably produced using the above-mentioned equipment.

[0025]

In order to manufacture a high-strength steel sheet excellent in ductility having a crystal structure having retained austenite, a cold-rolled sheet is annealed, and recrystallization and carbide dissolution are performed. The carbon is concentrated in the austenite by holding at the local temperature, and at the time of subsequent cooling, the cooling rate is set to several tens of degrees Celsius / second or more in order to suppress the pearlite transformation. At 370 ° C. in order to transform bainite into carbon and further enrich carbon contained in bainite in austenite.
As described above, it is preferable that the temperature is maintained for a long time in the temperature range of 450 ° C. or less.

From the above point of view, the problems at the time of manufacturing in a general hot-dip galvanizing line are summarized in the following three points. a. In a general hot-dip galvanizing line, the cooling rate between the two-phase annealing temperature and the intermediate temperature range is several degrees Celsius.
/ Slow. For this reason, austenite existing in the two-phase region is transformed into pearlite during the cooling process, and the amount of retained austenite in the final product decreases.

B. After cooling from the two-phase region annealing temperature to the intermediate temperature region, it is necessary to maintain the steel sheet at that temperature to transform austenite to bainite. However, due to equipment limitations in practice, the time from the completion of cooling after annealing in the two-phase region to the intrusion into the hot dip galvanizing bath (hereinafter, referred to as
The plating time must be shortened and the bainite transformation is insufficient. For this reason, untransformed austenite is not stabilized, and finally martensitic transformation occurs.

The bainite transformation has a nose near 400 ° C., but at a temperature lower than the plating bath temperature.
If the steel sheet is cooled to this temperature, solidification of zinc starts at the same time as entering the plating bath, and it becomes difficult to control the amount of adhesion. Therefore, the steel sheet must be maintained at a temperature higher than the nose. As described above, it is difficult to maintain the optimum bainite transformation temperature in the hot-dip galvanizing line, which is one of the reasons why it is not easy to obtain retained austenite in a hot-dip galvanized product.

C. In the alloying treatment, since the temperature is higher than the bainite transformation temperature, even if austenite remains immediately after immersion in the plating bath, bainite transformation does not proceed, and the retained austenite is easily decomposed into cementite. In particular, when a base material having a high Si content is plated, it is necessary to increase the alloying temperature in order to promote the alloying reaction at the base material interface, which further accelerates the decomposition of austenite. .

These factors make it difficult to secure the amount of retained austenite, and also reduce the carbon concentration of the retained austenite. Retained austenite having a low carbon concentration is unstable and easily transforms into martensite, transforms into martensite in the early stage of processing, and has the adverse effect of reducing the contribution of austenite to the TRIP effect.

The present inventors have conducted various studies to solve these problems, and as a result, have found that the above-mentioned problems can be solved very efficiently by performing annealing twice. Hereinafter, experimental results by the present inventors will be described.

Steel A having the composition shown in Table 1 was melted in a laboratory.
Hot forging was performed to obtain a steel slab having a thickness of 20 mm.

[0033]

[Table 1] This was heated at 1200 ° C. for 30 minutes, hot rolling was started from 1000 ° C. and completed at 850 ° C., then cooled to 550 ° C. by water spray, and charged in a lehr to 55 ° C.
After holding at 0 ° C. for 30 minutes, it was cooled to room temperature at a cooling rate of 20 ° C./hour to obtain a hot-rolled plate having a thickness of 3 mm. This is pickled to remove scale and cold-rolled to a thickness of 1.6.
mm cold-rolled plate.

The cold-rolled sheet is subjected to a heat treatment equivalent to a continuous annealing line (heat treatment A), a heat treatment equivalent to a continuous galvanizing line (heat treatment B), and a heat treatment equivalent to a continuous annealing line, followed by a continuous galvanizing line equivalent. (Heat treatment A + B), and then the mechanical properties of each steel plate were investigated.

FIG. 1 is a graph showing heat patterns when a heat treatment corresponding to a continuous annealing line and a heat treatment corresponding to a continuous hot-dip galvanizing line according to an embodiment of the present invention are continuously performed.

The left half of the diagram in FIG. 1 is an example of the heat pattern of the heat treatment A. The contents are as follows: heating at a heating rate of 10 ° C./sec to 830 ° C., which is a two-phase region, holding at this temperature for 60 seconds;
At a cooling rate of 50 ° C./sec.
After holding for 40 seconds, it is cooled to room temperature at a cooling rate of 20 ° C./second.

In FIG. 1, the right half of the diagram is an example of the heat pattern of the heat treatment B. The contents are performed under the same conditions as heat treatment A until the two-phase annealing, then cooled to 480 ° C. at a cooling rate of 8 ° C./sec, and held at that temperature for 20 seconds.
The cooling is performed at a cooling rate of 3 ° C./sec to 460 ° C. assuming a plating bath temperature.

When an alloying treatment is performed after the plating, the alloy is heated from 460 ° C. to an alloying temperature of 520 ° C. at a heating rate of 5 ° C./sec, and then cooled to room temperature at the illustrated cooling rate. When the alloying process is not performed, cooling is performed from the above plating temperature of 460 ° C. to a room temperature at a cooling rate shown by a broken line.

Next, the cold-rolled sheet after the heat treatment was subjected to temper rolling at an elongation of 1%, and a JIS-5 test piece was sampled in parallel with the rolling method to conduct a tensile test. FIG. 2 is a graph showing tensile strength, yield strength and elongation (total elongation) of each steel sheet. As can be seen from FIG. 2, when only heat treatment A was applied (marked with ■), the tensile strength was 610 MPa, the elongation was 39.0%, and the strength-ductility balance (tensile strength ×
(Elongation) is 23,000 MPa ·% or more, and has extremely excellent ductility.

On the other hand, when only heat treatment B was applied (marked with ○)
The elongation is significantly smaller than that of the case of only heat treatment A. In addition, the influence of the low-temperature holding temperature and the alloying temperature on ductility was slightly recognized.

However, when heat treatment A and heat treatment B are successively performed (indicated by a black circle, hereinafter also referred to as “heat treatment A + B”), heat treatment A is performed when no alloying treatment is performed.
Although the strength increases and the elongation decreases, the strength-ductility balance is remarkably improved as compared with the heat treatment B. As for the characteristics of the alloying material, when the alloying temperature was 530 ° C. or lower, good elongation at substantially the same level as that of the steel sheet not subjected to the alloying treatment was obtained.

That is, from the above experimental results, before continuous hot dip galvanizing, continuous cooling including rapid cooling after annealing and long-time low-temperature holding similar to that used for recrystallization annealing of a cold-rolled sheet. It has been clarified that by performing the annealing, a high-tensile-high-ductility galvanized steel sheet excellent in strength-ductility balance can be obtained stably.

This mechanism is not necessarily clear, but is presumed as follows. In the case of the heat treatment B, as in the case of the heat treatment A, recrystallization and dissolution of cementite (including cementite in pearlite) proceed simultaneously during heating to the two-phase region. Therefore, the distribution of austenite grains in the two-phase temperature range is random. However 2
In the cooling after maintaining at the phase region temperature, the austenite cannot be supercooled because the cooling rate is slower than in the case of heat treatment A, and pearlite precipitates during the cooling to reduce the amount of austenite.

In the heat treatment B, since the low-temperature holding temperature is higher than the optimum temperature of the bainite transformation, the concentration of carbon in the austenite becomes insufficient and the austenite cannot be stabilized. Therefore, it is considered that the final amount of retained austenite also decreased, and the strength-ductility balance decreased.

On the other hand, in the steel sheet annealed by heat treatment A, as is generally known, the retained austenite grains are distributed in a ferrite-based structure in isolation or in a film form between bainite layers. ing.

When the steel sheet having the crystal structure obtained by performing the heat treatment A as in the heat treatment A + B is heated again to the temperature in the two-phase region, the volume fraction of the austenite increases while carbon diffuses from the residual austenite to the surroundings. . At this time, the inside of the austenite grains does not have a uniform carbon distribution. That is, it is presumed that the portion of the retained austenite existing from the beginning has an extremely high carbon concentration, and the peripheral portion of the retained austenite has a low carbon concentration even within the same austenite grains.

When the austenite grains having such a C concentration distribution are cooled, even if the cooling rate is slow cooling, the peripheral portion becomes carbon-diluted and cannot be transformed into pearlite to become ferrite. On the other hand, it is considered that the center portion is supercooled to the bainite transformation temperature range while maintaining austenite because the carbon concentration is high, thereby suppressing the precipitation of pearlite.

Further, the supercooled austenite already has a high carbon concentration. From these, as in heat treatment B,
Even when the temperature is maintained at a temperature higher than the optimum temperature for bainite transformation, the same retained austenite as in heat treatment A is obtained, and as a result, it is considered that the strength-ductility balance is excellent.

The present invention has been completed on the basis of these findings, and the gist of the invention resides in the following method (1) to (5) for producing a high-tensile, high-ductility galvanized steel sheet. (1) In mass%, C: 0.05 to 0.25%, Si:
0.7% or less, Mn: 0.8 to 2.5%, P: 0.05
%, S: 0.01% or less, sol. Al: 0.4-
2.0%, N: 0.01% or less, and Si + Al:
A cold-rolled sheet having a chemical composition satisfying 1.0 to 2.5% is subjected to continuous annealing satisfying the following conditions, and then hot-dip galvanized satisfying the following conditions. Manufacturing method of ductile galvanized steel sheet; Continuous annealing condition: Heat steel in two-phase region of 780 ° C or more and 860 ° C or less, hold in the two-phase region for 30 seconds or more and 90 seconds or less, and then 700 ° C or less and 450 ° C or less. 30 ℃ over the temperature range above ℃
/ Second cooling rate, then 450 ° C or less,
After holding at a temperature range of 50 ° C. or more for 100 seconds or more and 500 seconds or less, it is cooled to room temperature. Hot-dip galvanizing condition: Heat the steel to a two-phase region of 780 ° C. or more and 860 ° C. After holding for not less than 90 seconds and not more than 500 seconds, it is cooled to not more than 500 ° C. at a cooling rate of not less than 3 ° C./sec.
After holding for more than 5 seconds, immerse in a molten zinc bath and plate
Cool to room temperature.

(2) The cold-rolled sheet having the chemical composition described in (1) above is subjected to continuous annealing and hot-dip galvanization satisfying the conditions described in (1) above, and then subjected to 480 ° C. or more and 530 Heat to a temperature range of less than or equal to
A method for producing a galvanized steel sheet having a high tensile strength and a high ductility, wherein a galvannealed steel sheet is produced by alloying a plating layer while holding the plating layer for at least 0 second and at most 30 seconds.

(3) The chemical composition of the steel further includes Ti and / or Nb in a total of 0.003 to 0.03% by mass.
The method for producing a high-tension, high-ductility galvanized steel sheet according to the above (1) or (2), wherein the galvanized steel sheet contains 5%.

(4) The chemical composition of the steel is further
And Cr: 0.05-1.0% and / or Mo:
The method for producing a high-tension, high-ductility galvanized steel sheet according to any one of the above (1) to (3), characterized by containing 0.01 to 0.2%.

(5) The chemical composition of the steel is further
And Cu: 0.1-1.0%, Ni: 0.05-0.5
%, Co: 1 in the group consisting of 0.0005 to 1.0%
The production of a high-tensile high-ductility galvanized steel sheet according to any one of the above (1) to (4), characterized in that the steel sheet contains at least 1.5% by mass of two or more kinds. Method.

[0054]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail. In the following description, the percentage of chemical composition means% by mass.

(1) Chemical composition of steel sheet C: C is the most powerful austenite stabilizing element,
This is one of the essential components of the present invention. In order to stabilize austenite at room temperature, it is necessary to concentrate to about 1% or more in austenite in the annealing step.
Therefore, the C content of steel is set to 0.05% or more. As the C content of the steel increases, the strength of the steel can be increased. However, if C is contained in excess of 0.25%, the strength of the steel becomes too high, which makes the steel unsuitable for plastic working and also deteriorates the weldability. Therefore, the C content is set to 0.25% or less. Preferably it is 0.20% or less.

Si and Al: Si and Al are both ferrite stabilizing elements. When these elements are contained, the volume fraction of ferrite increases during annealing in the two-phase region, and austenite which equilibrates with ferrite is obtained. The C concentration of the phase is increased, and the effect of stabilizing austenite is obtained. Furthermore, since Si has an effect of suppressing the precipitation of carbides, it has an effect of enriching C in austenite even during bainite transformation during cooling after annealing in the two-phase region. In order to obtain these effects, the total content of Si and Al is set to 1.0% or more.

When a large amount of these ferrite stabilizing elements is added, C is concentrated in austenite, but the volume ratio is reduced. Therefore, the total content of Si and Al is set to 2.5% or less.

On the other hand, Si has an effect of inhibiting wettability between the base material and the hot-dip galvanizing bath during hot-dip galvanizing.
If i is contained excessively, non-plating may occur. Si also has the effect of delaying alloying of the galvanized layer. Therefore, Si does not have to be contained,
Even if it is contained, in order to avoid the above-mentioned adverse effects, S
The i content is 0.7% or less. Preferably it is 0.6% or less.

Al has an effect of stabilizing ferrite as described above and does not inhibit plating wettability.
It is a more preferable element. A for stabilizing ferrite
1 is contained at least 0.4%. Further, Al is used as a deoxidizer during steelmaking, but if it is contained in excess of 2.0%, inclusions increase in the steel sheet and impair ductility, so the Al content is 2.0% or less.

Mn: Mn is an austenite stabilizing element and is one of the essential elements of the present invention. In order to allow austenite to remain at room temperature without transforming to martensite in the cooling process after annealing in the two-phase region, M
n is contained 0.8% or more. On the other hand, Mn tends to segregate when the steel is solidified. If Mn is excessively contained, a striped structure due to segregation occurs, and ductility is impaired. To avoid this, the Mn content is set to 2.5% or less. Preferably, it is 2.0% or less.

P: P tends to segregate during solidification of steel. If P is contained excessively, a striped structure is generated due to segregation, and ductility is impaired. Also, weldability is impaired. In order to avoid these harms, the P content is set to 0.05% or less. Preferably it is 0.02% or less. However, when P coexists with Cu, a stable film may be formed on the surface layer of the steel sheet to improve the corrosion resistance in some cases. Therefore, in steel containing Cu, it is preferable to contain P in an amount of 0.01% or more and 0.05% or less.

S: Inevitably contained in steel as an impurity element. If the S content is large, the amount of precipitation as MnS increases, ductility is inhibited, and Mn as an austenite stabilizing element is consumed. Therefore, the S content is 0.01%
The following is assumed.

N: Inevitably contained in steel as an impurity element, the lower the better. If the N content is high, it causes slab cracking due to AlN, and AlN also reduces ductility in products, so the upper limit is made 0.01%. When more emphasis is placed on workability, the content is desirably 0.005% or less.

Ti and Nb: These elements are carbide forming elements, and form fine precipitates in the steel to strengthen the steel sheet and promote the alloying of the plating layer by making the crystal structure fine. effective. In order to obtain these effects, the total content of both may be 0.003% or more. The content of these elements is 0.05% in total
If it exceeds 300, the decrease in ductility is more remarkable than the increase in strength, so that the total amount when contained is 0.05% or less.

Further, Ti is easily bonded to N in steel, and Al
The precipitation of TiN in preference to the precipitation of N leads to Al
This has the effect of preventing slab cracking due to the precipitation of N.
To ensure this effect, (Ti / 48) / (N / 1
4) It is preferable to include Ti so that ≧ 2.

Cr and Mo: These elements have the effect of suppressing the pearlite transformation, and have the effect of suppressing the pearlite transformation and the reduction of austenite during cooling after annealing in the two-phase region. Conversely, when these elements are excessively contained, the bainite transformation is delayed, and the carbon concentration in austenite becomes insufficient. Therefore, in order to obtain an appropriate retained austenite, Cr is contained in a range of 0.05% or more and 1.0% or less, and Mo is contained in a range of 0.01% or more and 0.2% or less. It does not matter.

Cu, Ni and Co: All of these elements are hardly soluble in iron carbide, and have the effect of suppressing the precipitation of carbides during bainite transformation and leaving austenite. To obtain this effect, Cu should be 0.1
%, Ni is 0.05% or more, Co is 0.0005%
One or more kinds may be contained within the above range. If these elements are contained excessively, bainite transformation becomes insufficient, and the overaging zone in the continuous annealing equipment,
Or, since carbon is not sufficiently concentrated in austenite during low-temperature holding in continuous hot-dip plating, even if it is contained, Cu is 1.0% or less, Ni is 0.5% or less,
Co is 1.0% or less, and 1.5% or less in total.

Further, Cu improves the corrosion resistance when coexisting with P, and may be contained for this purpose. Since Cu causes slab cracking, it is preferable to add Ni at the time of adding Cu so as to satisfy Ni ≧ Cu / 2.

The balance is Fe and inevitable impurities. (2) Rolling, Annealing and Plating Conditions The cold-rolled sheet having the above chemical composition is prepared by melting steel having a predetermined chemical composition in a converter or the like according to a conventional method, continuously casting to form a slab, and hot rolling. After that, it is obtained by a method such as cold rolling through a treatment such as pickling.

The hot rolling may be performed by a known method.
The cast slab may be directly hot-rolled, or may be re-heated and then hot-rolled. If the rolling temperature is too low,
Since the ferrite is transformed during rolling to form a processed ferrite structure, the finish-rolling exit temperature (finish temperature) is preferably set to 800 ° C. or higher. In order to secure the finishing temperature over the entire width and the entire length of the steel sheet, auxiliary heating may be performed between rough rolling and finish rolling. Cooling after finish rolling is not particularly limited, but it is preferable to perform water cooling immediately after finish rolling in order to make the structure of the hot-rolled sheet finer.

If the winding temperature is too low, bainite and martensite are formed as the second phase, making cold rolling difficult. Therefore, it is preferable to wind at 500 ° C. or higher. If the winding temperature is too high, the scale loss increases, and the steel sheet becomes too soft, so that the rolled form is likely to collapse.

Cold rolling: Cold rolling may be carried out by a conventional method. However, if the rolling reduction is too low, the rolling efficiency decreases, and if the rolling reduction is too high, the rolling load increases and rolling becomes difficult. As described above, the content is preferably in the range of 70% or less.

The cold-rolled sheet is subjected to continuous annealing (first annealing) and then to continuous galvanizing. In continuous galvanizing, annealing (second annealing) is performed before plating.

Continuous annealing: Continuous annealing (first annealing)
Known continuous annealing equipment used for recrystallization annealing of a cold-rolled sheet, that is, equipment provided with a heating zone, a soaking zone, a cooling zone, and an overaging zone in order from the entry side is desirable. Cold rolled sheet is
In the heating zone, 780 ° C. or higher, 860
The steel is annealed by heating to a temperature in a two-phase region of not more than ℃.

When the annealing temperature is lower than 780 ° C., recrystallization becomes insufficient and it takes too much time for cementite to form a solid solution again. Conversely, if the annealing temperature exceeds 860 ° C., the austenite volume fraction of the steel at the annealing temperature becomes excessive, and the C concentration in austenite decreases. Preferably it is 800 to 850 ° C.

The holding time (soaking time) in the two-phase region is 3
When the time is less than 0 second, the re-dissolution of cementite becomes insufficient, and when the time is more than 90 seconds, austenite grains become undesirably coarse. Therefore, the holding time is 30 seconds or more and 90 seconds or less.

After the two-phase annealing, the temperature range of 700 ° C. or lower and 450 ° C. or higher is cooled at a cooling rate of 30 ° C./sec or higher in order to suppress the pearlite transformation. Although the upper limit of the cooling rate is not particularly limited, if it is too fast, the effect of suppressing the pearlite transformation is saturated, and it may be difficult to control the cooling end temperature.
/ Sec or less.

The starting point of the above-mentioned cooling is not limited, but in order to increase the volume fraction of ferrite and to enrich C in austenite, the cooling is performed at a cooling rate of 10 ° C./sec or less from the annealing temperature to 700 ° C. Is preferred.

Following the above cooling, the temperature was lowered to 450 ° C.
Hold at a temperature range of 0 ° C. or more for 100 seconds or more and 500 seconds or less. The holding may be performed by maintaining the temperature at a constant value or by gradually lowering the temperature within the above temperature range. Beyond either the upper limit or the lower limit of the temperature range or the holding time range, bainite transformation becomes insufficient and the concentration of C in austenite hardly occurs. Preferably 4
200 seconds or more in a temperature range of 50 ° C. or less and 370 ° C. or more,
Hold for 400 seconds or less. The subsequent cooling is not limited, but forced cooling may be performed.

Hot-dip galvanizing: Hot-dip galvanizing (including the second annealing) is performed by known continuous hot-dip galvanizing equipment, that is, a heating zone, a soaking zone, a cooling zone, a low-temperature holding zone, and a hot-dip zinc bath in order from the entrance side. It is desirable to use equipment provided with an alloying zone. If alloying is not performed,
No alloying zone is required.

The steel sheet annealed continuously was heated at 780 ° C.
As described above, annealing is performed by heating to a two-phase region of 860 ° C. or lower (second annealing). The residual austenite formed in the first annealing is decomposed into ferrite and cementite during the heating in the above-mentioned annealing, but is transformed into austenite again at an Ac point or higher. If the annealing temperature is lower than 780 ° C., the retained austenite remains decomposed and the amount of austenite formed is small. Conversely, when the annealing temperature exceeds 860 ° C., the crystal structure of the steel sheet becomes close to the austenite single phase, and thus C enrichment in austenite does not proceed.

If the residence time (soaking time) in the two-phase region is less than 10 seconds, the dissolution of cementite becomes insufficient, so that the retention time in the above-mentioned temperature region is 10 seconds or more.
If the holding time is excessively long, the carbon concentration distribution in the austenite grains (the above-described high concentration in the center and low concentration in the periphery) disappears by diffusion, and a region having a high carbon concentration disappears, so the first annealing is performed. It is not good because it will be the same as when there is no. In order to avoid this, the holding time in the two-phase region is set to 90 seconds or less.

It is preferable to increase the cooling rate after annealing in order to make the austenite in a supercooled state. However, since a general hot-dip galvanizing line is not equipped with a powerful cooling facility, the hot-dip galvanizing line is not used. Rapid cooling such as continuous annealing equipment is difficult.

However, in the manufacturing method of the present invention, as described above, the residual austenite phase in which carbon is concentrated is formed by the first annealing, and the residual austenite phase is uniformly formed in the second annealing without breaking the carbon-enriched region. Because of heating, austenite in which C is concentrated at a higher concentration is formed than at the time of soaking in the first annealing. Therefore, in the second annealing, if the cooling rate from the annealing temperature to 500 ° C., which is lower than the nose of the pearlite transformation, can be set to 3 ° C./sec or more, the precipitation of pearlite is suppressed and the austenite amount is maintained. be able to.

Following the above cooling, 460 ° C. to 500 ° C.
Low-temperature holding is performed for 15 seconds or more in the above temperature range. By performing this low-temperature holding, the bainite transformation proceeds,
C is further concentrated and stabilized in austenite.
The temperature for maintaining the low temperature is higher than the overaging temperature used in ordinary continuous annealing. However, since the carbon concentration in the austenite is increased by performing the annealing twice as described above, it is considered that the bainite transformation sufficiently proceeds even in the low-temperature holding as described above.

If the low-temperature holding temperature exceeds 500 ° C., the pearlite transformation proceeds during the holding, and the desired retained austenite cannot be obtained. If the low-temperature holding temperature is lower than 460 ° C., since the temperature is lower than that of the plating bath, solidification of the plating bath starts at the same time as the steel sheet enters the plating bath, and it becomes difficult to control the amount of plating applied. If reheating is performed immediately before the plating bath, keep the low temperature at 460.
Although it is possible to perform in the temperature range below ℃,
Even in that case, the temperature is preferably 350 ° C. or more.

If the low-temperature holding time is less than 15 seconds, the concentration of C in austenite is insufficient and the stabilization of the retained austenite becomes insufficient, and the austenite is easily decomposed during the alloying treatment. Increasing the low-temperature holding time has the advantage that carbon is concentrated at a higher concentration and decomposition during the alloying treatment is suppressed. Therefore, the low-temperature holding time is preferably set to 20 seconds or more.

However, if the low-temperature holding time is excessively long, the stabilizing action of the retained austenite is saturated, and if the holding time is longer than this, the productivity is only impaired.

The hot-dip galvanizing may be performed by a conventional method. After pulling up the steel sheet from the plating bath, the amount of coating is adjusted by a conventional method, and if no alloying treatment is performed, the steel sheet is cooled to room temperature.

In the case of performing the alloying treatment, after adjusting the coating weight, the alloy is heated to an alloying temperature of 480 ° C. or more and 530 ° C. or less in an alloying furnace, and is heated for 10 seconds or more in the temperature range.
Hold for 30 seconds or less. When the alloying temperature is less than 480 ° C. or the holding time is less than 10 seconds, alloying cannot be performed sufficiently. When the alloying temperature exceeds 530 ° C., the retained austenite is decomposed into cementite, and the mechanical properties are deteriorated. If the holding time exceeds 30 seconds, alloying proceeds excessively, and a powdering defect in which a plating film peels off during processing is not good.

Except for the above conditions, it may be manufactured by a conventional method. For example, after continuous annealing or hot-dip galvanizing, flatness correction of the steel sheet, surface roughness adjustment, and temper rolling for reducing the yield point elongation may be performed.

[0092]

EXAMPLE Steel having the chemical composition shown in Table 2 was melted in a laboratory and hot forged to obtain a steel slab having a thickness of 20 mm.

[0093]

[Table 2] The slab was heated to 1200 ° C. and held for 30 minutes.
Hot rolling is started at 00 ° C. and completed at 850 ° C., then cooled to 550 ° C. by water spray, placed in a lehr, kept at 550 ° C. for 30 minutes, and then cooled at a cooling rate of 20 ° C./hour to room temperature. This was gradually cooled to obtain a hot-rolled plate having a thickness of 3 mm. These were pickled to remove scale and cold-rolled to obtain a cold-rolled plate having a thickness of 1.6 mm.

Using the sample obtained from the cold-rolled sheet, heat treatment A corresponding to the continuous annealing line in the heat pattern shown in FIG. 1 was performed as the first annealing, and then heat treatment corresponding to the continuous galvanizing line was performed. B was also used for the second annealing. Some steels were also tested and evaluated when only heat treatment B was applied as a conventional example. At that time, annealing conditions (annealing and heating conditions of heat treatment A: R _A1 , annealing and soaking conditions of heat treatment B: R _A2 , cooling rate: C _R2 , low temperature holding conditions T _LH
Was variously changed as shown in Table 3.

[0095]

[Table 3] The steel sheet after the above heat treatment is subjected to temper rolling at an elongation of 1%,
A JIS-5 test piece was sampled in parallel with the rolling method, and a tensile test was performed. Table 4 shows the obtained test results.

[0096]

[Table 4] Test No. 1 manufactured using steel B having a component in the chemical composition range specified by the present invention under the conditions specified in the present invention
-3 showed an excellent strength-ductility balance. Test No. 3 which was not subjected to the alloying treatment showed the same excellent strength-ductility balance as Test No. 1. Test number 4
Since heat treatment A was not performed, heat treatment B same as Test No. 1 was performed, but the elongation was small and the strength-ductility balance was not good.

Test No. 5 where the annealing temperature of heat treatment A was too low
Austenite is not formed during soaking in heat treatment A,
Since there is no residual austenite after heat treatment A,
Even when heat treatment B was performed under the conditions specified by the present invention, carbon could not be concentrated in austenite, and no residual austenite was formed, resulting in low elongation.
In Test No. 6, austenite was formed by heat treatment A, but the soaking time was too short to completely dissolve cementite, the concentration of C to austenite was insufficient, and the elongation was low. In Test No. 7, the retained austenite was formed in the heat treatment A, but the annealing temperature in the heat treatment B was too low, so that the retained austenite was decomposed into cementite, and the retained austenite was not obtained.
Growth was low. In Test No. 8, the annealing temperature in heat treatment B was too high, so that the carbon-enriched region formed in heat treatment A disappeared by diffusion, and the remaining austenite could not be left by the subsequent cooling. It was low. In Test No. 9, in the heat treatment B, the cooling rate after soaking was too low, so that pearlite was precipitated, the amount of austenite was reduced, and the elongation was low. In Test No. 10, pearlite was precipitated and elongation was low because the cooling holding temperature in heat treatment B was too high. In Test No. 11, the elongation was low because the retained austenite was decomposed because the alloying temperature among the alloying treatment conditions was too high. Test number 1
In No. 2, austenite was decomposed because the holding time of the alloying treatment conditions was too long, and as a result, good elongation was not obtained.

A steel C having the chemical composition defined by the present invention,
Test Nos. 13, 14, 17, 18, 19, and 20 produced using D, G, H, and I under the conditions specified by the present invention exhibited excellent strength-ductility balance. In Test No. 15, the Mn of the steel E used was too small, the austenite stabilizing effect was insufficient, no retained austenite was obtained, and the elongation was low. Test No. 16 is (Si + A) of the steel F used.
l) Since the content was too small, cementite was precipitated during the transformation of bainite, the effect of concentrating C in austenite was insufficient, and no residual austenite was obtained, resulting in low elongation.

[0099]

According to the production method of the present invention, a general existing continuous annealing equipment and continuous hot-dip galvanizing equipment are used, and a cold-rolled sheet is subjected to two heat treatments under specific conditions.
A hot-dip galvanized steel sheet having both high strength and high ductility or an alloyed hot-dip galvanized steel sheet can be easily manufactured. Therefore, the manufacturing method of the present invention facilitates the application of high-strength steel sheets to automobiles and various other structural members and products, and contributes greatly to improving the collision safety of automobiles and promoting the reduction of weight of steel products. The industrial merit obtained by the invention is great.

[Brief description of the drawings]

FIG. 1 is a graph showing a heat treatment cycle of a cold-rolled sheet according to an example of the present invention.

FIG. 2 is a graph showing the effect of heat treatment conditions on tensile properties.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/06 C22C 38/06 38/58 38/58 (72) Inventor Hiroyuki Nakagawa Omitsu, Kajimu, Ibaraki Pref. No. 3 Sumitomo Metal Industries, Ltd. Kashima Works, Ltd. F-term (reference) 4K027 AA02 AA05 AA23 AB02 AB42 AC12 AC32 AC73 AE12 AE22 4K037 EA01 EA05 EA06 EA10 EA11 EA13 EA15 EA16 EA17 EA18 F05 F03 F03 F03 F03 F05 GA07

Claims (5)

[Claims] 1. A mass% of C: 0.05 to 0.25%,
Si: 0.7% or less, Mn: 0.8 to 2.5%, P:
0.05% or less, S: 0.01% or less, sol. Al:
0.4 to 2.0%, N: 0.01% or less, and Si +
Al: A cold rolled sheet having a chemical composition satisfying 1.0 to 2.5% is subjected to continuous annealing satisfying the following conditions, and then hot-dip galvanized satisfying the following conditions. Method for producing high-tensile ductile galvanized steel sheet; Continuous annealing conditions: Heat steel in a two-phase region of 780 ° C or more and 860 ° C or less, hold it in the two-phase region for 30 seconds or more and 90 seconds or less, and then 700 ° C or less. Temperature range of 450 ° C or higher at 30 ° C
/ Second cooling rate, then 450 ° C or less,
After holding at a temperature range of 50 ° C. or more for 100 seconds or more and 500 seconds or less, it is cooled to room temperature. Hot-dip galvanizing condition: Heat the steel to a two-phase region of 780 ° C. or more and 860 ° C. After holding for not less than 90 seconds and not more than 500 seconds, it is cooled to not more than 500 ° C. at a cooling rate of not less than 3 ° C./sec.
After holding for more than 5 seconds, immerse in a molten zinc bath and plate
Cool to room temperature. 2. A cold-rolled sheet having the chemical composition described in claim 1 is subjected to continuous annealing and hot-dip galvanization satisfying the conditions described in claim 1, and then subjected to 480 ° C. or higher and 530 ° C.
Heating to a temperature range of not more than 10 ° C.
A method for producing a high-tension, high-ductility galvanized steel sheet, wherein the galvannealed steel sheet is manufactured by holding the alloy layer for 30 seconds or less to alloy the plating layer. 3. The steel of claim 1 wherein said chemical composition further comprises Ti and / or
Or Nb is 0.003 to 0.05% by mass in total.
3. The composition according to claim 1, wherein
The method for producing a high-tensile-high-ductility galvanized steel sheet according to item 1. 4. The steel according to claim 1, wherein the chemical composition is further
r: 0.05 to 1.0% and / or Mo: 0.01
The method for producing a high-tension, high-ductility galvanized steel sheet according to any one of claims 1 to 3, characterized in that the steel sheet contains 0.1 to 0.2%. 5. The steel according to claim 1, wherein said chemical composition further comprises:
u: 0.1 to 1.0%, Ni: 0.05 to 0.5%, C
o: One or two or more of the group consisting of 0.0005 to 1.0% are contained in a total of 1.5% or less by mass%. The method for producing a high-tension, high-ductility galvanized steel sheet according to any one of the above.