JPH1096064A - Galvannealed steel sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the powdering resistance, chipping resistance at a low temp., press formability and BH properties of a steel sheet, at the time of producing a steel sheet for an automotive body, by using an extra-low carbon steel contg. alloy elements having solid solution strengthening properties such as Si, P or the like for increasing its tensile strength, and furthermore, controlling the heating and cooling rates at the time of alloying treatment after plating. SOLUTION: The steel sheet contains, by weight, 0.004 to 0.008% C, 2.5×P(%)-0.2% Si, 0.10 to 0.40% Mn, 0.017 to 0.045% P, <=0.35% S, 0.003 to 0.008% sol.Al, <=0.004% N, 0.002 to 0.015% Ti and 0.010 to 0.030% Nb, also satisfying the conditions of Ti+Nb=0.012 to 0.035%, and the balance substantial Fe. Furthermore, at the time of alloying treatment after galvanizing, it is heated to a treating temp. at a rate of >=20 deg.C/sec, is treated and is thereafter cooled at a cooling rate of >=10 deg.C/sec. Moreover, the tensile strength of this steel plate is regulated to >=340MPa.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alloyed hot-dip galvanized steel sheet which is suitable as a high-strength steel sheet for automobiles and has excellent powdering resistance and low-temperature chipping resistance of a coating film, and has paint bake hardenability. It relates to the manufacturing method.

[0002]

2. Description of the Related Art In recent years, galvannealed steel sheets have been widely used as steel sheets for automobile bodies. Alloyed hot-dip galvanized steel sheet is used for hot-dip galvanized steel sheet.
It is manufactured by performing an alloying process of heating to 500 to 600 ° C. and holding for 3 to 60 seconds. By the alloying treatment, the Zn layer usually becomes an Fe—Zn alloy layer containing 8 to 15% by weight of Fe. When the plating film is an Fe-Zn alloy layer, the adhesion between the plating film and the paint is excellent as compared with ordinary hot-dip galvanization, the corrosion resistance after painting is remarkably improved, and the spot weldability is also improved. . If the galvanized galvanized steel sheet used for automobiles has an excessive amount of zinc attached, the powdering resistance deteriorates. Therefore, the amount of zinc attached is usually 20 to 70 g / m ² per one side of the steel sheet.

When such an alloyed hot-dip galvanized steel sheet is used as a steel sheet for an automobile body, the following performance is required with respect to the formability of the steel sheet and the corrosion resistance of the plating film.

(A) The plating film does not peel off during processing of a steel sheet (powdering resistance).

(B) The plating film has excellent adhesion in a low-temperature environment (low-temperature chipping resistance).

(C) The base material must have sufficient formability and strength to meet the demand for high strength.

(D) The steel sheet has paint bake hardenability.

[0008] Powdering is a phenomenon in which a plating film is finely crushed and peeled in a region where a steel sheet undergoes compression deformation during press working, and powdery plating film fragments are generated. Fe
Since the -Zn alloy is harder than normal metal Zn, the plating film itself tends to be powdered when compressed and deformed.
When this powdering occurs, not only does the corrosion resistance of that portion deteriorate, but also the powder that has peeled off and adhered to the press mold causes surface defects on the molded product.

[0009] Low-temperature chipping is a phenomenon in which when a coated alloyed hot-dip galvanized steel sheet is subjected to a shocking deformation in a low-temperature environment, a plating film peels off from an interface with a base material. Low-temperature chipping may occur when pebbles collide with a painted surface of a vehicle body of an automobile running in a cold region or the like, and its improvement is required.

Higher strength is demanded as a measure to reduce the weight of a vehicle body and to improve the dent resistance of exterior materials such as doors in order to promote improvement in fuel efficiency of automobiles. The strength of a steel sheet for vehicle body with excellent formability is usually 300MP in tensile strength.
a), but the tensile strength is 340 to reduce the body weight.
Approximately 400 MPa is demanded.

The term “paint bake hardenability” refers to a property that hardens age hardening at room temperature but hardens age hardening in a temperature range where a paint is baked (hereinafter, this property is simply referred to as “BH property”. The steel sheet having the same is simply referred to as “BH steel sheet”).
Since this steel sheet is pressed with a low yield point as it was when the steel sheet was manufactured, an accurate shape of the body part can be obtained during the pressing. When the paint is baked after pressing, the yield point of the steel sheet increases due to the age hardening phenomenon, so that the strength of the product after painting increases. In other words, if a BH steel sheet is used, it is easy to obtain the shape accuracy of the part during press working, and it is possible to obtain a part with high strength after painting.
This is a necessary function for a steel sheet for an automobile exterior that requires both an accurate shape and dent resistance (hereinafter referred to as dent resistance).

That is, in the case of an alloyed hot-dip galvanized steel sheet applied to an automobile outer panel, press formability, high strength, and
It is desired that the alloyed hot-dip galvanized steel sheet has BH properties and that the plating film has excellent adhesion, that is, excellent powdering resistance and low-temperature chipping resistance.

As methods for improving the powdering resistance, optimization of the amount of zinc attached, the amount of Al added to the plating bath, the degree of alloying, etc., and the alloying element in the base material that impairs the adhesion to the plating film are considered. Restrictions on the amount of addition have been proposed.

[0014] Both powdering and chipping are phenomena in which the plating film peels off. For this reason, conventionally, it has been considered that if the powdering resistance is improved, the low-temperature chipping resistance is also improved. However, actually, even if the powdering resistance is improved, the low-temperature chipping resistance is not always improved. It has been found that the low-temperature chipping resistance is mainly a problem of adhesion at the interface between the plating film and the base material surface, and a method for improving the adhesion of the plating film at the base material interface has been proposed.

Japanese Patent Application Laid-Open No. 2-97753 discloses an alloyed hot-dip galvanized steel sheet having excellent workability and a method for producing the same. This steel sheet has a feature that when a painted steel sheet is cut, the plating film is not easily peeled off at the cut end face. This steel sheet has a structure in which Zn has infiltrated and diffused into crystal grain boundaries on the base material surface, and the zinc exists in a wedge-like form at the crystal grain boundaries, so that the plating film is unlikely to peel off from the base material interface. Have been. This steel sheet is manufactured by plating with an Al concentration in a plating bath higher than usual and performing an alloying treatment at a higher temperature than usual for a long time.
However, when alloying at a high temperature, the powdering resistance tends to be impaired, and when the alloying treatment time is long, productivity is impaired. Further, although it is effective for peeling the plating film from the cut end surface, its relationship with the chemical composition of the base material and its effect on low-temperature chipping resistance generated in a low-temperature environment are not clear.

[0016] Si and P have the effect of increasing the strength of the steel sheet without significantly impairing the formability steel, and are inexpensive alloy elements. Therefore, Si and P are suitable elements for strengthening steel. However, Si and P impair the plating property. It is known that when too much Si is added to the base material, non-plating occurs and the alloying speed is reduced. P is known to be an element that slows down the alloying rate.
It is considered that P tends to segregate at the crystal grain boundaries of steel, so that in steels having a high P content, the reaction between the crystal grain boundaries of the base metal and Zn is suppressed. For this reason, it is not easy to achieve both high strength of a steel sheet and plating property using these elements.

JP-A-6-81099 discloses an alloyed hot-dip galvanized steel sheet having excellent adhesion between a plating film and a base material and suitable as a coated steel sheet for home appliances or a steel sheet for automobiles. In the present invention, in order to improve the adhesion, a very low C steel is made to contain Si, and a base material having a limited P content is used, and the steel at the interface between the plating film after the alloying treatment and the base material is used. By improving the surface of the film to a rough surface with severe irregularities, the adhesion is improved, and the chipping resistance after painting is improved. In the present invention, P contained in the base material is controlled as low as possible, and the C content is determined to be an extremely low C steel because it hinders the improvement of the adhesion, and C is fixed by adding Ti.

It has been reported that when Si is added to ultra-low carbon Ti-added steel, zinc infiltration into the crystal grain boundaries of the base material is promoted and the adhesion at the interface between the plating film and the base material steel is improved. Yes (W.van.Koesveld and others: GALVATEC'95 Conference Procee
dings p.343-353). However, the technology disclosed in this report is directed to a soft IF steel, and its base metal has no solid solution C, and mentions a high-strength P-added steel required for automotive steel sheets. Not.

In response to the demand for an alloyed hot-dip galvanized steel sheet having BH properties, for example, Japanese Unexamined Patent Publication No. 4-80349 discloses a baking hardenable high-strength galvannealed steel sheet having excellent powdering resistance and its production. A method is disclosed.
In the present invention, the content of P is limited to 0.03% or less in order to improve the adhesion of the plating film, and B is added for the purpose of suppressing powdering. further,
In order to obtain BH properties, Nb is contained in an amount of 2 to 7.5 times the amount of C. In the present invention, although the powdering resistance is improved, nothing is described about the low-temperature chipping resistance. Further, the P content is limited to a low value, and when B is added, the formability of the steel sheet may be impaired.

JP-A-5-195148 discloses a cold-rolled steel sheet and a hot-dip galvanized steel sheet having excellent paint bake hardenability and secondary workability, and a method for producing the same. In the present invention, Si and P are contained in an ultra-low carbon steel. The type of precipitate is controlled by setting the ratio of the contents of Ti and S to an appropriate range, thereby exhibiting BH properties and secondary workability. However, in the galvanized steel sheet described in the example, neither Si nor P was added. Further, the adhesion of the plating film is not evaluated, and no mention is made of the low-temperature chipping resistance.

[0021]

As described above, the alloyed hot-dip galvanized steel sheets have excellent individual performance such as those having BH properties, those having excellent powdering resistance or low-temperature chipping resistance. There have been several proposals for steel sheets and their manufacturing methods. However, a steel sheet in which all properties of strength, BH property, powdering resistance and low-temperature chipping resistance exceed predetermined levels has not yet been obtained.

An alloyed hot-dip galvanized steel sheet having excellent powdering resistance and chipping resistance in a low-temperature environment (hereinafter referred to as low-temperature chipping resistance), good press formability, high strength and BH property, and It is to provide a manufacturing method.

[0023]

The gist of the present invention is to provide an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and low-temperature chipping resistance described in the following (1) to (3); and (4) In its manufacturing method.

(1) The chemical composition of the base material is% by weight, and C:
0.004-0.008%, Si: 2.5 × P (%) ~
0.20%, Mn: 0.10 to 0.40%, P: 0.0
17-0.045%, S ≦ 0.015%, sol. A
l: 0.003 to 0.08%, N ≦ 0.004%, T
i: 0.002 to 0.015%, Nb: 0.010 to
0.030%, the sum of the contents of Ti and Nb satisfies the following equation, the balance being Fe and unavoidable impurities, tensile strength of 340 MPa or more, and paint bake hardenability of 1
An alloyed hot-dip galvanized steel sheet having 0 MPa or more and excellent in powdering resistance and low-temperature chipping resistance.

[0025]

(Equation 2)

(2) C, P and sol. The galvannealed steel sheet according to claim 1, wherein the content of Al is in the following range by weight%.

C: 0.005 to 0.007%, P: 0.
017-0.035%, sol. Al: 0.003 or more
0.04% (3) The base material is hot-dip galvanized, heated to an alloying treatment temperature at a heating rate of 20 ° C./sec or more, and subjected to alloying treatment, and then cooled at a cooling rate of 10 ° C./sec or more. The method for producing a galvannealed steel sheet according to claim 1 or 2, wherein the steel sheet is cooled.

The concept for solving the problem of the present invention is as follows.

(A) In order to obtain a steel sheet having excellent formability as a steel sheet for an automobile body, an ultra-low carbon steel is used as a base material. In order to increase the tensile strength at low cost without impairing the formability, a solid solution strengthening alloy element such as Si or P is contained. When P is added to the base material of the galvannealed steel sheet, the low-temperature chipping resistance of the plating film is deteriorated. However, by increasing the Si content in accordance with the P content, it is possible to not only prevent the low-temperature chipping resistance from deteriorating, but also to further improve it. In particular, when Si is contained at least 2.5 times the P content, the effect of improving low-temperature chipping resistance becomes remarkable.

As the P content increases, the segregation of P at the crystal grain boundaries on the surface of the base material increases. Therefore, Zn hardly diffuses to the crystal grain boundary, and the wedge effect of Zn at the crystal grain boundary cannot be obtained. However, when an appropriate amount of Si is contained, local penetration of Zn into crystal grain boundaries and the like of the base material of the steel sheet is promoted.
As a result, the anchoring effect of the plating film is improved, and the low-temperature chipping resistance is improved.

(B) In the present invention, in order to ensure the strength of the steel sheet, a small amount of Nb is contained in addition to the addition of P and Si. This is because it is necessary to limit the contents of P and Si in order to secure the plating property, and the strength of the steel sheet is insufficient with only P and Si. Conventionally, it has been considered that the strength increasing effect of Nb addition is small in ultra-low carbon steel. However, the strength of steel can be increased by adding a small amount of C and Nb in addition to a small amount of solid solution C necessary for expressing the BH property. This is presumed to be due to the effect of refinement of crystal grains and precipitation of fine carbonitrides.
Even if a small amount of Nb is contained in the base material, there is no adverse effect on the performance of the plating film.

In order to ensure the adhesion of the plating film, S
Since it is necessary to limit the content of alloying elements such as i, Mn, and P, the strength of the base material is insufficient. The strength improving effect by the addition of Nb can be utilized as a means for compensating for the insufficient strength.

(C) If the heating rate during the alloying treatment is increased, the low-temperature chipping resistance is improved. This is considered to be because zinc is easily diffused and penetrates into crystal grain boundaries on the surface of the base material by rapid heating, and the anchoring effect on the plating film is increased. Diffusion of Zn to the crystal grain boundary is promoted when the molten Zn, not the solid-phase Fe-Zn alloy, is in contact with the base material surface when heated to the alloying treatment temperature. You. The melting point of the Fe—Zn alloy increases as the Fe content in Zn increases. Therefore, rapid heating is required to leave Zn (η phase) having a low melting point when heated to the alloying treatment temperature.

When the cooling rate after the alloying treatment is increased, the powdering resistance is improved and the BH property is easily obtained. Powdering resistance deteriorates when the Fe-Zn alloying reaction proceeds too much. By performing rapid cooling after the alloying treatment, it is possible to prevent an excessive alloying reaction from occurring during cooling.

The BH content of a steel sheet is greatly affected by the amount of C dissolved in steel at normal temperature. If the cooling rate from the alloying treatment temperature is reduced, solid solution C precipitates in the course of cooling, so that the BH amount of the steel sheet decreases. Therefore, in order to secure the BH content of the galvannealed steel sheet, it is necessary to increase the cooling rate from the alloying treatment temperature.

[0036]

Embodiments of the present invention will be described in detail below. The percentages shown below are% by weight.
Means

C: 0.004% or more is contained in order to secure a BH amount and a predetermined tensile strength. C
When the content is less than 0.004%, the BH amount becomes insufficient, and the effect of improving strength by combining with Nb described below cannot be obtained. When the C content exceeds 0.008%, the cold-rolled base material has poor drawability after annealing. In addition, the strain aging progresses, and the workability of the steel sheet deteriorates, and at the same time, the stretcher strain is easily generated at the time of working, so that it cannot be used as an exterior material of an automobile body. Therefore, the upper limit of the C content is set to 0.008% or less. In order to easily secure the strength and the BH amount, the C content is 0.005 to 0.005.
More preferably, it is 0.007%. Si: P is contained for the purpose of improving the low-temperature chipping resistance, which decreases as the content of Si: P increases, and for the purpose of increasing the tensile strength of the base steel. In order to maintain low-temperature chipping resistance in a favorable range, it is necessary to increase the Si content in accordance with the increase in the P content. Therefore, the lower limit of the Si content is P
It needs to be managed in relation to the content. When the Si content is less than 2.5 × P (%), the low-temperature chipping resistance is not sufficiently improved. On the other hand, Si
If it is contained in excess of (%), a strong oxide film is formed on the surface of the steel sheet during annealing before hot-dip galvanizing and the like, resulting in poor wettability with Zn and non-plating may occur during hot-dip galvanizing. is there. Therefore, the Si content is 2.5 × P
(%) To 0.20%.

As the Si content of the base material increases, the alloying speed decreases. It is presumed that an Si oxide film is formed on the surface of the base material, which inhibits the diffusion of Fe atoms into the plating film. If the alloying speed is too slow, the productivity is deteriorated, which is not preferable.

The alloying speed was determined by adjusting the Al content in the plating bath.
The rate can be increased by reducing the amount or increasing the heating rate of the steel sheet to the alloying treatment temperature. However, even without such a method, the Si content is reduced to ｛(−6 /
7) The alloying speed is not slowed down if it is limited to not more than × P (%) + 0.16｝ (%). Therefore, the Si content is
It is more preferable to be {(−6/7) × P (%) + 0.16} (%) or less.

FIG. 1 is a diagram showing the Si and P content ranges of the base metal of the galvannealed steel sheet according to the present invention. The lower Si region than the line segment AB in the figure is the more preferable range described above.

In order to suppress embrittlement during hot rolling by Mn: S, the content is at least 0.1% or more. Mn has the effect of increasing the strength of steel, so it is added as necessary.
The effect of increasing the strength is small compared to the same amount of Si. In addition, Mn is expensive and, if it is contained excessively, the amount of BH decreases, which hinders the improvement of low-temperature chipping resistance by Si. For this reason, the upper limit of the Mn content is 0.40%.

P: It has the effect of increasing the strength of the steel sheet. Even if the strength is increased, the degree of impairing the formability is small compared to other strengthening elements. Moreover, it is inexpensive. In the present invention, P is contained in an amount of 0.017% or more in order to exert the effect of improving strength. However, as P increases, the low-temperature chipping resistance significantly decreases. If P is contained excessively, even if Si is increased, the low-temperature chipping resistance cannot be improved, so the upper limit of P is made 0.045%. To further improve low-temperature chipping resistance, the P content is preferably set to 0.035% or less.

Sol. Al: Used as a deoxidizing agent when refining steel. When the content is less than 0.003%, the deoxidizing effect is insufficient. If it is contained excessively, the deoxidizing effect is saturated and the adhesion of the plating film deteriorates.
The upper limit is set to 0.08%. To further improve the adhesion of the plating film, the content is preferably 0.04% or less.

Ti: works to fix a part of N, S and C in the steel as precipitates and to improve deep drawability. Ti
Is less than 0.002%, these effects are insufficient, and if it exceeds 0.015%, the BH property may be reduced. Therefore, the range of the content of Ti is 0.002 to
0.015%.

Nb: NbC is formed by the presence of a small amount of C, and has the effect of improving the strength of steel. If the Nb content is less than 0.010%, the effect is not sufficient, and
If it exceeds 030%, the workability is deteriorated. Therefore, Nb
Is set to 0.010 to 0.030%.

By simultaneously containing Ti and Nb, the press formability of the base steel sheet can be further improved. However, since both Ti and Nb have an effect of fixing carbon and reducing solid solution C, the total content (Ti + Nb) is set to 0.012 to 0.035%. T
When i + Nb is less than 0.012%, the amount of solid solution C becomes too large, so that press formability is deteriorated and strain aging at room temperature becomes unfavorable. If Ti + Nb exceeds 0.035%, the amount of dissolved carbon becomes too small and the BH amount becomes insufficient.

Elements other than the above elements are Fe and inevitable impurities. It is desirable that all of the unavoidable impurities be as small as possible. However, when S and N are increased, precipitates and non-metallic inclusions are combined with Ti to increase the workability. Therefore, S as an inevitable impurity is 0.015% or less, and N is 0.1% or less.
004% or less.

The galvannealed steel sheet of the present invention is preferably manufactured by the following method. Molten steel with the chemical composition as described above is smelted in a conventional converter or electric furnace, refined by vacuum treatment or the like, slabed by continuous casting or ingot casting and slab rolling, and then hot-rolled. You. The slab may be rolled directly while hot, or may be rolled after charging in a heating furnace and reheating. The preferred finishing temperature during hot rolling is 880 to 980 ° C. The upper limit of this temperature is defined by the problem of surface flaws, and the lower limit is defined by the Ar3 transformation point. The winding temperature is preferably from 450 to 750 ° C. When importance is placed on draw formability, winding at a high temperature is preferred.
If the temperature exceeds ℃, surface flaws such as scale flaws are generated.

The hot-rolled steel sheet is cold-rolled after an oxide film on the surface is removed by means such as pickling. The rolling reduction of the cold rolling is preferably performed in the range of 60 to 90%. If the rolling reduction is too low, the deep drawability is not preferable, and if the rolling reduction is too high, the deep drawability is impaired and the rolling load becomes excessive, so that surface flaws are easily generated. More preferably, the rolling is performed at a cold reduction ratio of 75 to 85%. After the cold rolling, the sheet is hot-dip galvanized by a continuous hot-dip galvanizing apparatus and subsequently alloyed.

The hot-dip galvanizing is preheated according to a usual method, and then heated in a reducing atmosphere to reduce the surface and to perform recrystallization annealing. The annealing temperature is preferably from 700 to less than Ac3 point. The higher the annealing temperature, the better the deep drawability, but conversely, the crystal grains become coarser and the strength of the steel sheet decreases,
Increased risk of rough skin. If the annealing temperature is too low, the deep drawability becomes insufficient. For this reason, a more preferable range of the annealing temperature is 750 to 850 ° C. The annealed steel sheet is cooled to the temperature of the plating bath and hot-dip galvanized. If the temperature of the galvanizing bath is too high, the evaporation of Zn will be severe, causing operational problems. If the temperature of the galvanizing bath is too low, Z
n becomes easy to solidify, and it becomes difficult to control the amount of zinc deposition. Therefore, the temperature of the galvanizing bath is preferably in the range of 450 to 490 ° C. Al is 0.08 ~ in galvanizing bath
The content is preferably 0.15% by weight. The reason for this is to make it easier to control the amount of zinc deposition. If the amount of zinc adhered is too large, powdering is likely to occur. Therefore, the amount of zinc is preferably about ²⁰ to 70 g / m ² per one surface. The weight of the zinc deposit is the weight of the Fe—Zn alloy.

The galvanized steel sheet is subsequently heated to an alloying temperature and alloyed. The alloying treatment temperature is preferably in the range of 480 to 600 ° C. When the temperature of the alloying treatment is lower than 480 ° C., the alloying reaction rate is slow, so that the treatment time becomes longer. When the temperature exceeds 600 ° C., the alloying proceeds excessively. More preferred alloying treatment temperature is 500 to
540 ° C. The alloying temperature and the holding time at that temperature are preferably controlled so that the Fe content in the plating film is 8 to 15% by weight. If the Fe content in the plating film is less than 8%, the adhesion of the paint is not preferred, and the corrosion resistance after painting is insufficient. From the viewpoint of improving the low-temperature chipping resistance, the higher the Fe content, the better. However, the lower the Fe content, the better the powdering resistance. In order to obtain a steel sheet excellent in both performances, it is desirable to control the Fe content to 15% or less.

Further, in order to improve the low-temperature chipping resistance, the heating rate in the temperature range of 480 ° C. or less during the alloying treatment should be 20 ° C./s or more, and the cooling rate from the alloying treatment temperature should be 1
It is desirable that the temperature be 0 ° C./s or more. Heating rate 20 ° C /
When it is not less than s, the low-temperature chipping resistance is improved. The heating rate may be as high as possible. However, if the heating rate is excessively rapid, the attained temperature becomes unstable or the equipment becomes excessively large. Therefore, the upper limit is preferably set to 50 ° C./s. More preferably, it is 20 to 40 ° C / s. If the cooling rate is increased, excessive Fe-Zn alloying reaction is suppressed, the powdering resistance is improved, and solid solution C is likely to remain, so that a sufficient amount of BH can be obtained. Therefore, the cooling rate from the alloying temperature is preferably 10 ° C./s or more. The upper limit of the cooling rate is not limited. However, if the cooling rate is excessively high, the shape of the steel sheet deteriorates.

When the alloying process is completed, the alloy is cooled and subjected to processes such as temper rolling and leveling. These treatments may be performed according to a conventional method. Further, post-treatment such as chromate treatment may be performed as necessary.

In order to exhibit the effect of weight reduction as a steel sheet for automobiles, the tensile strength of the galvannealed steel sheet is 340M.
Pa or more is required. Although the upper limit is not specified, good formability is ensured, and if the addition amount of the alloy element specified in the present invention is in the range, it is substantially 400M.
The upper limit is around Pa.

The amount of BH must be 10 MPa or more in order to ensure dent resistance. In order to obtain better dent resistance, a BH amount of 20 MPa or more is desirable. BH
The larger the amount, the more preferable. However, if the amount is too large, aging at room temperature tends to progress, and the moldability is deteriorated and defects such as stretcher strains are liable to occur during molding. For this reason, the upper limit of the BH amount is desirably 70 MPa or less. A more desirable range of the BH amount is 30-6.
0 MPa.

The BH amount is measured by the method described in JIS-G-3135, Appendix, Test for baking hardening amount. The outline of this method is as follows. The load (F _WH ) when a 2% elongation strain was applied to the tensile test piece was recorded and the load was removed. Thereafter, a heat treatment was performed at 170 ° C. for 20 minutes, and after cooling to room temperature, a tensile test was performed. F _SA )
Ask for. The BH amount is a value obtained by dividing the difference between the two loads by the initial cross-sectional area (A ₀ ) of the parallel part of the test piece, and is represented by (F _SA −
F _WH ) ÷ A ₀ .

The plating film is made of an Fe—Zn alloy,
Its chemical composition contains 8 to 15% Fe, with the balance being substantially Zn.

The alloyed hot-dip galvanized steel sheet of the present invention is prepared by subjecting a slab made of steel having the above chemical composition to a required thickness by hot rolling and cold rolling by a usual method. It can be manufactured by galvanizing with a line and alloying by heating to the alloying treatment temperature.

[0059]

【Example】

(Example 1) Slabs of various chemical compositions obtained by melting in a converter, vacuum degassing, and continuous casting were charged into a heating furnace and heated to 1200 ° C, and the thickness was: 3.6mm, width 1
It was hot rolled into a 250 mm coil. Table 1 shows the ladle analysis values of these steels. The finishing temperature of these coils during hot rolling is 910-930 ° C., and the winding temperature is 6 ° C.
40-670 ° C. These hot-rolled coils were pickled and cold-rolled into 0.70 mm thick steel sheets. The cold reduction was 81%.

[0060]

[Table 1]

From these steel plates, a length of 200 mm and a width of 80 mm
mm cut plates were collected. These cut plates were hot-dip galvanized under the following conditions using a hot-dip galvanizing simulator. First, an oxygen concentration of 500 pp was applied to these cut plates.
The temperature was raised to 550 ° C. at a rate of 15 ° C./s in a nitrogen atmosphere of m or less, and preheating was performed for 3 seconds. Then dew point -3
The sample was heated to 800 ° C. at a heating rate of 15 ° C./s in an atmosphere consisting of 10% by volume of hydrogen at 0 ° C. and the remaining nitrogen, and subjected to reduction annealing for 60 seconds. Thereafter, it was allowed to cool to 460 ° C. and immersed in a hot-dip galvanizing bath to perform plating. The plating conditions were as follows: the Al concentration in the plating bath was 0.12%; the temperature of the plating bath was 460 ° C .; The amount of zinc applied was adjusted to 30 to 60 g / m ² per side by changing the amount of high-pressure air blown. The cut plate after plating was once cooled and then heated to 480 to 580 ° C. at a heating rate of 25 ° C./s using an induction heating device. The holding time at the alloying treatment temperature depends on the F in the plating film.
The content was changed within 20 seconds so that the e content was within a predetermined range. Thereafter, a mist composed of water and nitrogen gas was sprayed to cool to room temperature at a cooling rate of 10 ° C./s.

From the cut plate after the alloying treatment, JIS-Z-2
A No. 5 test piece specified in 201 was cut out and subjected to a tensile test. In addition, the BH content was measured according to the method described in the appendix of JIS-G-3135. Further, the plating film of a sample collected from the cut plate after the alloying treatment was dissolved using a 6% hydrochloric acid solution to which an inhibitor was added, and the zinc adhesion amount was measured and the Fe content in the plating film was analyzed.

The powdering resistance was evaluated by the following method. A blank having a diameter of 60 mm was punched from the cut plate after the alloying treatment, and the blank was press-formed using a die having a punch diameter of 30 mm and a die shoulder radius of 3 mm to form a cylindrical cup. The total weight of the plating film peeled off from the outer surface of the side wall of the cylindrical cup by the adhesive tape was measured. A case where the amount of peeling was less than 25 mg was determined to be acceptable.

The low-temperature chipping resistance was evaluated by the following method. From the alloyed plate, length 150mm, width 7
A test piece of 0 mm was cut out, degreased and washed, and then subjected to an immersion type phosphate base treatment (adhesion amount: 3 to 7 g / m ² ). Next, undercoating with a cationic electrodeposition paint (coating thickness 20μ)
m), a middle coat (coating thickness 35 to 40 μm) and a top coat (coating thickness 35 to 40 μm) were applied (total coating thickness about 100 μm) to prepare a coated steel sheet test piece. These coated steel plate specimens were cooled to −20 ° C., and 10 gravel stones having a diameter of 4 to 6 mm were crushed at a speed of 1 using a Gravelo testing machine.
Collision was performed at 00 to 150 km / h, and the maximum size of each peeled piece of the plating film was measured, and the average value was obtained. A case where the average peel diameter was less than 3.5 mm was regarded as acceptable.

Table 2 also shows the results of the above tests.

[0066]

[Table 2]

As shown in the results of Test Nos. 1 to 12 in Table 2, the tensile strength of the galvannealed steel sheet satisfying the conditions specified by the present invention is 340 MPa or more, has a suitable BH property and good resistance to heat. Has powdering properties and low-temperature chipping resistance.

On the other hand, in steel L and steel M having a low C content and a low P and Nb content, the adhesion of the plating film is good but the tensile strength is too low. Steel L also has a low BH content. Steel N has an unfavorable low-temperature chipping resistance because the P content exceeds the range specified by the present invention. Steel O,
Since P, Q, R, S, U and V all have insufficient Si content, low-temperature chipping resistance is not preferable. Further, the steels S, T and U, in which the total content of Ti and Nb exceeds the range specified by the present invention, have insufficient BH content.

Example 2 A slab having the chemical composition of the symbols A, B and C shown in Table 1 was hot-rolled, pickled, and cold-rolled under the same conditions as described in Example 1. 0.7
A 0 mm cold-rolled steel sheet was obtained. From these steel plates, cut plates having a length of 200 mm and a width of 80 mm were collected. These cut plates were subjected to hot dip galvanizing and alloying treatment using a hot dip plating simulator. The hot-dip galvanizing was preheated under the same conditions as described in Example 1 and 800
The reduction annealing was performed at 60 ° C. for 60 seconds, and the sample was immersed in a zinc plating bath at 460 ° C. The amount of zinc deposited was adjusted to 35 g / m ² per one side by changing the amount of high-pressure air blown. The plated plate is once cooled to room temperature and then heated to 530 ± 10 ° C using an induction heating device, and the holding time is changed within 20 seconds so that the Fe content in the plating film is within a predetermined range, and held. Then, an alloying treatment was performed. Thereafter, the cooling rate was changed by a means such as nitrogen gas spraying or water cooling to cool to room temperature.

The mechanical properties, powdering resistance, and low-temperature chipping resistance of the cut sheet subjected to the alloying treatment were evaluated in the same manner as in Example 1. Table 3 shows the evaluation results.

[0071]

[Table 3]

The steels A, B and C all satisfy the chemical composition specified in the present invention. As shown in Table 3,
Each of these steel sheets has a tensile strength of 340 MPa or more, an appropriate BH property, good powdering resistance and low-temperature chipping resistance.

In particular, the steel sheets subjected to alloying treatment by increasing the heating rate up to the alloying treatment temperature or the cooling rate to room temperature after the alloying treatment are particularly excellent in low-temperature chipping resistance. And BH properties.

[0074]

The galvannealed steel sheet of the present invention and the galvannealed steel sheet produced by the method of the present invention have high strength, sufficient paint bake hardenability, and powdering resistance. And excellent low temperature chipping resistance.
This steel sheet has all the characteristics required of a steel sheet used for a recent automobile outer panel, and has extremely useful effects in industry, such as improvement in the performance of an automobile and rationalization of its manufacture.

[Brief description of the drawings]

FIG. 1 is a view showing the ranges of the contents of Si and P in a base material of a galvannealed steel sheet according to the present invention.

Claims (3)

[Claims] 1. The chemical composition of a base material is% by weight and C: 0.00
4 to 0.008%, Si: 2.5 × P (%) to 0.20
%, Mn: 0.10 to 0.40%, P: 0.017 to
0.045%, S ≦ 0.015%, sol. Al: 0.
003 to 0.08%, N ≦ 0.004%, Ti: 0.0
02-0.015%, Nb: 0.010-0.030
%, And the sum of the contents of Ti and Nb satisfies the following formula, the balance being Fe and unavoidable impurities, having a tensile strength of 340 MPa or more, and a paint bake hardenability of 10 MPa or more. Alloyed hot-dip galvanized steel sheet with excellent low-temperature chipping resistance. (Equation 1) 2. C, P and sol. The galvannealed steel sheet according to claim 1, wherein the content of Al is in the following range by weight%. C: 0.005-0.007%, P: 0.017-0.
035%, sol. Al: 0.003 to 0.04% 3. The base material is hot-dip galvanized, heated to an alloying treatment temperature at a heating rate of 20 ° C./sec or more, subjected to alloying treatment, and then cooled at a cooling rate of 10 ° C./sec or more. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein: