JPH1046305A - Galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet excellent in powdering resistance, chipping resistance, and smoothness of plating surface by using an IF steel excellent in deep drawability as a base material. SOLUTION: At least one side of a base material, having a chemical composition which contains, by weight, <=0.006% C, 0.02-0.08% Si, 0.06-0.25% Mn, 0.007-0.014% P, 0.005-0.08% Al, 0-0.2% Cr, 0-0.003% B, and 0.03-0.30%, in total, of at least either of Ti and Nb and in which respective contents of Si and P satisfy the relations of inequalities Si+10×P<=0.18 and P>=0.005+0.1×Si, has a zinc alloy plating film containing 0.15-0.60% Al and 8.5-13.5% Fe. Further, the average crystalline grain size of the base-material surface from which the plating layer is removed is regulated to <=15μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet mainly used for automobiles and having excellent adhesion of a plating film and excellent smoothness of a plating surface.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are excellent in rust prevention function, after-painting performance and economical efficiency, and are used in large quantities in industrial fields such as automobile body materials, home appliances and building materials. . This steel sheet is prepared by heating a hot-dip galvanized steel sheet to 480-600 ° C to form a zinc coating on Zn-Fe.
Manufactured by forming an alloy film. The content of Fe in the plating film is often 7 to 12% by weight on the average of the cross section of the film.

The base material of the galvannealed steel sheet is a low-carbon Al killed steel, or a very low-carbon Interstitial Free steel (hereinafter referred to as I) in which solid solution C and N are fixed with Ti or Nb.
High strength steel reinforced by adding P or Mn is also widely used. In particular, IF steel is non-ageing and has excellent deep drawability, and is frequently used as a material for an automobile body.

[0004] Alloyed hot-dip galvanized steel sheets using IF steel as a base material have problems such as powdering resistance, low-temperature chipping resistance, and smoothness of the plating surface. Powdering is a phenomenon in which the plating film of the part that has undergone compression deformation is broken from the inside of the plating layer, falls into a powder state, and falls off. Become. Zn-Fe alloys become harder with increasing Fe content. In particular gamma ₁ phase referred to as Zn-Fe intermetallic compound is brittle hard, tends which the results powdering takes place. To improve this, Fe is necessary that properly manage the alloying degree so as not to excess the alloy, it is necessary to select and gamma _1-phase product difficult component or manufacturing conditions. In particular, in IF steel, since the grain boundaries are clean, the alloying reaction via the grain boundaries is fast. Therefore gamma _1-phase of ferrite grain boundaries is likely to grow, it is said to promote the vulnerability of the plating film.

[0005] When an impact such as a stone bomb is applied to a painted alloyed galvanized steel sheet of an automobile body on a road in a cold region, the plating film may peel off from the base material interface of the base material together with the coating film. This phenomenon is called low temperature chipping. This phenomenon is due to the weak adhesion at the plating interface, but it is also related to the deformation behavior of the steel sheet when subjected to impact.If the adhesion is the same, it is as soft as IF steel and used for car bodies. The problem is significant when the plate thickness is small as in the above.

[0006] Further, in a painted product, importance is attached to the beauty of the appearance after painting and the fact that an image reflected on the painted surface is clearly visible (hereinafter referred to as sharpness). The sharpness after coating becomes better as the surface of the steel plate to be coated is smoother. For this reason, in the alloyed hot-dip galvanized steel sheet used for the exterior material of the automobile, the smoothness of the plating film surface is emphasized. In order to make the plating surface smooth, it is necessary that the surface of the base material at the interface with the plating film is smooth and that the thickness of the plating film itself is uniform.

[0007] The rapid and local progress of the Zn-Fe alloying reaction at the crystal grain boundaries of the base material is called an outburst phenomenon. When this phenomenon occurs, the unevenness of the plating film surface becomes severe, and in some cases, a brittle layer such as a _first layer is locally formed to impair the powdering resistance of the film.
In IF steel with clean grain boundaries, Fe-Z
n The outburst phenomenon is likely to occur due to the fast alloying reaction. On the other hand, the alloying reaction rate also depends on the crystal orientation of the base material surface at the interface in contact with the plating film, and in particular, (111)
On a plane, the alloying reaction is delayed as compared with the plane having another crystal orientation. If the alloying reaction is slow in the plane, the portion where the alloying reaction is fast attracts Zn in the peripheral slow portion, so that the unevenness of the plating surface is promoted. To improve drawability (11
1) In the IF steel in which the surface is intentionally increased, the outburst phenomenon described above and the delay in alloying on the (111) surface are combined, so that the surface irregularities after the alloying treatment are enlarged and the surface roughness is increased. The sharpness after painting is easily impaired.

[0008] There is also a method of reducing the roughness by performing skin pass rolling after the alloying treatment. However, since only a slight reduction can be performed in order not to impair the mechanical properties, the center line average roughness R
With a, only an improvement effect of about 0.1 μm was obtained, and it was not a sufficient solution.

Among these three problems, the low-temperature chipping resistance can be improved by slightly increasing the Al content in the plating film. It is said that when Al is high, the reaction at the interface of the base material becomes non-uniform, and the unevenness is increased to improve the adhesion. In some cases, it is required to reduce the coating weight from the viewpoints of economy, improvement in powdering resistance, and the like. However, as the coating weight decreases, the low-temperature chipping resistance decreases. For this reason, the amount of adhesion is 40 g
In the case where it is less than / m ^2, as a countermeasure, it is usually produced by increasing the Al content of the film.

However, since Al tends to promote the outburst reaction at the grain boundary, increasing the Al content results in a roughened surface, which deteriorates the sharpness after coating. Also, Al
In the actual operation, it is necessary to perform the alloying at a high temperature because the alloying reaction is delayed. As a result, it is also a problem that a hard alloy layer such as a _single phase is formed and the risk of impairing the powdering resistance is increased.

The present inventors have proposed a method for improving the low-temperature chipping property of a galvannealed steel sheet using IF steel as a base material in Japanese Patent Application Laid-Open No. 6-41707. This is between 0.02 and
By subjecting an IF steel base material containing 0.1% by weight of Si and less than (0.005 + 0.1 × Si)% by weight of P to hot-dip galvanizing and alloying under specific conditions, the surface of the base material has a specific roughness. This method improves the adhesion between the plating and the surface of the base material. However,
In this method, the roughness of the plating surface may be increased at the same time that the surface of the base material is roughened, and the sharpness after coating may be insufficient.

[0012] Galvatech '95 Conference Procedure
gs p.753 suggests that when Si is added to IF steel, the adhesion of the galvannealed film is improved as in the case of JP-A-6-41707. However, since the balance to Si and P in this document is not taken into consideration, it is easy to grow gamma ₁ layers in shown as Experiment steel, terms of powdering resistance is lowered, the suggestion for preventing coarsening of the surface roughness I lack.

As described above, among alloyed hot-dip galvanized steel sheets using IF steel as a base material, which are frequently used in automobiles, steel sheets satisfying simultaneously low-temperature chipping resistance, powdering resistance and surface smoothness have been established. And its realization is strongly desired.

[0014]

SUMMARY OF THE INVENTION The present invention provides an alloyed hot-dip galvanized steel sheet having an IF steel excellent in deep drawability as a base material, and having excellent powdering resistance, chipping resistance and plating surface smoothness. The purpose is to provide.

[0015]

The gist of the present invention resides in the following galvannealed steel sheet.

When the chemical composition is C: 0.006% or less by weight%, S
i: 0.02 to 0.08%, Mn: 0.06 to 0.25%, P: 0.007 to
0.014%, Al: 0.005 to 0.08%, Cr: 0 to 0.2%,
B: 0 to 0.003%, at least one of Ti and Nb is contained in a total of 0.03 to 0.30%, and the contents of Si and P satisfy the following formulas and the relationship of the following formulas, with the balance being Fe and unavoidable impurities. Al: 0.15 to 0.60% by weight, Fe: 8.5 to 13.5%, balance: Zn and unavoidable impurities. At least one surface of the base material has an alloy plating film, and the base at the interface with the plating film. An alloyed hot-dip galvanized steel sheet having an average crystal grain size of 15 μm or less on the surface of the material.

Si + 10 × P ≦ 0.18—P ≧ 0.005 + 0.1 × Si The inventors of the present invention have investigated the powdering characteristics of the coating of the galvannealed steel sheet. As a result, it has been found that P may impair the powdering resistance of the plating film, but that the addition of an appropriate amount of Si improves the powdering resistance. Although the mechanism is not clear, it is considered that when Si exists, the interface between the Γ phase and the Γ ₁ phase has a complicated structure with severe irregularities, and has an effect of suppressing the growth of cracks generated in the Γ ₁ phase. And the effect of this Si is P
Is presumed to be effective even if some coexist.

Further, the present inventors have studied diligently the improvement of chipping resistance based on Japanese Patent Application Laid-Open No. 6-41707, which was previously filed. As a result, it has been found that the addition of Si improves the chipping resistance after coating. The reason for this is not clear, but when Si is contained, the diffusion of Zn into the ferrite grain boundaries of the base material is promoted, and Zn is concentrated and segregated in the surface layer portion of the base material to form ferrite grain boundaries. This part seems to be more brittle than a general ferrite grain boundary. When an impact is applied after painting, the propagation of cracks that normally propagate at the interface between the plating film and the base material is dispersed and attenuated by the brittle portions of the crystal grain boundaries on the surface layer of these base materials. It is presumed that the peeling of the resin does not proceed.

P generally suppresses the diffusion of Zn into the ferrite grain boundaries and delays the alloying reaction. As a result, the outburst reaction is suppressed and the plating surface after the alloying treatment is smoothed. On the other hand, according to the conventional general knowledge that P suppresses the diffusion of Zn into the ferrite grain boundary, P acts in a direction that impairs chipping resistance. However, according to the study of the present inventors, when a specific amount of Si and P coexist, there is a range of P which does not hinder the diffusion of Zn, and in this region, P does not impair chipping resistance and outgoes. It was found that it contributed only to suppression of the burst reaction. That is, in this region, the surface smoothness after the alloying treatment can be secured while maintaining the adhesion between the plating film and the base material interface.

In the present invention, in order to improve the adhesion of the plating film and the smoothness of the film surface, the crystal size on the surface of the base material is further restricted. The reason why the adhesion is improved by making the crystal grains fine is that the number of embrittled grain boundaries due to the penetration of Zn increases, and the number of paths through which cracks are dispersed when subjected to an impact increases. Also, the smoothness of the plating film is improved because the crystal grain boundaries are active points for alloying, and the increased active points make it easier for the crystal grains to be covered with the alloy layer, and the apparent uniformity of the alloy It is presumed that this is due to the progression of the formation and the difficulty in forming irregularities in the plane.

In order to make the crystal grains on the surface of the base material fine, it is necessary to contain an appropriate amount of P and Si in the steel. That is,
Si promotes the diffusion of Zn into the base material during the alloying process when a specific amount of P coexists, and the boundary where Zn is concentrated in the ferrite crystal grains on the base material surface (hereinafter referred to as pseudo grain). Are formed, and the crystal grain size is observed to be small.

In the present invention, the pseudo grain boundaries are also regarded as grain boundaries, and the crystal grain size on the surface of the base material is determined.

Since this phenomenon occurs at a later stage of alloying, it does not contribute much to the above-mentioned improvement in surface smoothness, but has a great effect in improving the interfacial adhesion of the plating film. In order to further improve the smoothness, the crystal grain size on the surface of the base material may be reduced at the beginning of the plating reaction. This can be attained by grinding the surface of the base material containing appropriate amounts of P and Si to provide a work layer on the surface, and then performing an annealing heat treatment. This promotes formation of pseudo grain boundaries. The present invention has been made based on these findings.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The constituent features of the present invention and the effects thereof will be described below. In addition,% of the chemical components of the base material and the plating film means% by weight.

(A) Chemical composition of the base material C: The cold workability of the product is impaired.
Further, if the precipitation of C at the crystal grain boundaries is reduced, the diffusion of Zn to the ferrite grain boundaries during the alloying process is promoted, the anchoring effect is increased, and the adhesion of the plating film is also improved. For this reason,
Solid solution C is fixed as a precipitate by adding Ti, Nb and the like. If C is increased, a large amount of Ti, N
Since the addition of b and the like becomes necessary and the economic efficiency decreases, the content of C is set to 0.006% or less. Desirably, it is 0.004% or less. The lower limit is not specified, but it is set at 0.
It is preferably at least 001%.

Si and P: In the reduction / annealing step when hot-dip galvanizing is applied, Si is easily concentrated as an oxide on the surface of the base material and causes non-plating. Therefore, it is generally preferred that Si be smaller. . However, Si has a certain range of P
And Z at the grain boundary of the base metal during alloying
Promotes the diffusion of n and improves the adhesion of the plating film.
At the same time, the crystal grains on the surface of the base material after the alloying treatment are refined. This is because Zn is locally concentrated in ferrite grains on the surface of the base material during the alloying reaction, and is corroded in the same manner as ordinary grain boundaries by an etching operation for detecting ferrite grain boundaries. This is considered to form pseudo grain boundaries. It is presumed that this pseudo grain boundary also contributes to the dispersion of the propagation of cracks generated by impacts such as stone splashes, and that the low-temperature chipping resistance is improved. In addition, since these grain boundaries also act as active points of the alloying reaction, such a fine grain structure can improve the low-temperature chipping resistance even if the Al content in the coating is low. Further, Si is uneven the interface structure between gamma phase and gamma _1-phase, for improving powdering resistance of the film. In order to exert such an effect, Si of 0.02% or more is required.

However, when Si is excessively contained, the alloying reaction becomes slow. Particularly in the case of a component range coexisting with P as in the present invention, the effect of suppressing the alloying reaction becomes remarkable,
As a result, an increase in the alloying temperature is required, whereby the powdering property is reduced, and alloying is locally delayed, so that unevenness in the alloying process is likely to occur on the surface. Therefore, the content of Si is
It is limited to a range of not more than 0.08% and satisfying the following formula.

P segregates at the grain boundary and suppresses the diffusion of Zn, thereby suppressing the outburst reaction at the ferrite grain boundary and smoothing the plating surface. To obtain this effect, the P content needs to be 0.007% or more. Also,
As the Si increases, the outburst reaction is accelerated.
Must also satisfy the following equation. When P is increased, the moldability is impaired, and the powdering property of the film is impaired. In addition, the effect of Si on promoting the diffusion of Zn is impaired, thereby impeding the improvement of low-temperature chipping resistance. For this reason, the upper limit of the P content is set to 0.014%.

Further, as described for the reason for limiting Si, if Si and P are excessively contained, alloying unevenness of the surface is likely to occur due to local alloying delay.

Si (%) + 10 × P (%) ≦ 0.18 ---- P (%) ≧ 0.005 + 0.1 × Si (%) ---- Mn: Hot embrittlement due to inevitable impurities S 0.06% or more is added in order to prevent the above. If added in a large amount, the moldability (r value) of the product will be reduced, so the content should be 0.25% or less. Since Mn has the effect of suppressing the occurrence of non-plating due to Si, Mn is contained when Si is contained at 0.05% or more.
It is desirable to add 0.13% or more.

Al: A deoxidizing agent for steel and N added as unavoidable impurities are added to fix as AlN. In this case, if the content is less than 0.005%, there is no effect, and if the content exceeds 0.08%, not only the effect is saturated, but also non-plating tends to occur. Therefore, the content range is 0.005
To 0.08%. Ti and Nb: Used for fixing solid solution C, S or N and improving workability of the base material. Further, since the cleanliness of the ferrite grain boundaries is enhanced, the diffusion of Zn at the ferrite grain boundaries is promoted, and there is also an effect of increasing the adhesion between the plating film and the base material. Ti has a somewhat greater effect on promoting the diffusion of Zn, but at the same time tends to promote the outburst reaction.
In order to ensure the formability of the steel sheet and the adhesion of the film, it is necessary to contain at least one of Ti and Nb in a total amount of 0.03% or more. If the sum of them exceeds 0.30%, the outburst reaction is promoted, the surface smoothness after the alloying treatment is reduced, and the economy is low.

Cr: It is not necessary to use Cr. However, Cr has an effect of improving the powdering resistance and is added as necessary. In that case, Cr may be contained in the range of 0.02 to 0.2%. If the Cr content is less than 0.02%, the effect of improving the powdering resistance is not sufficient, and even if it exceeds 0.2%, the effect is saturated.

B: It is not necessary to use B, but B
The effect of suppressing the in-plane anisotropy of the r value related to the deep drawability and strengthening the ferrite grain boundaries to avoid secondary working embrittlement can be expected by adding. For this, if necessary 0.00
You may add in the range of 1-0.003%. B content is 0.
If less than 001%, these effects are not enough,
Even if it exceeds 0.003%, the effect is saturated.

(B) Chemical Composition of Plating Film Next, in the galvannealed steel sheet of the present invention, Al: 0.15 to 0.60%, Fe: 8.5 to 15%
It has an alloy plating film consisting of Zn and unavoidable impurities.

When the Al content in the plating film is less than 0.15%, the powdering resistance of the plating film is reduced, and the plating film has an Al content of less than 0.15%.
If it exceeds 60%, the outburst reaction becomes remarkable and the smoothness of the plating surface is impaired. As described in connection with the prior art, if the amount of plating is reduced for reasons such as pursuing economic efficiency, the low-temperature chipping property is impaired. To compensate for this, Al is usually increased, but the outburst is severe and the sharpness has to be sacrificed.

According to the present invention, this problem can also be advantageously solved. In the steel sheet manufactured according to the limitation of the composition range of Si and P and the limitation of the crystal grain size on the surface of the base material specified in the present invention,
Low temperature chipping resistance can be ensured without increasing the Al content of the plating film. As a result, a steel sheet excellent in both low-temperature chipping resistance and surface smoothness can be obtained.

As a particularly effective embodiment of the present invention, when the amount of plating is in the range of 25 to 40 g / m ² , the Al content in the plating film is preferably 0.18 to 0.28%. According to the present invention, the chipping resistance can be ensured at a relatively low Al, and the smoothness is also improved. Al content 0.18
%, The powdering property of the film is reduced, and if it exceeds 0.28%, the surface smoothness is somewhat reduced.

When the Fe content in the plating film is less than 8.5%, the η (Zn) phase is likely to remain due to the low degree of alloying, and the weldability and the corrosion resistance after painting are reduced. General performance as a steel sheet is reduced. On the other hand, when the Fe content in the coating exceeds 13.5%, powdering becomes likely to occur .GAMMA ₁ phase is increased. For this reason, the Fe content is limited to the range of 8.5 to 13.5%.

In the plating film, other impurities such as Pb
, Cd, Sn and Sb impair the powdering resistance and corrosion resistance, so that the total content is preferably within 0.3%. In addition, Mg and Mn as elements for improving corrosion resistance
Even if it is contained within 0.5%, the effect of the present invention is not significantly reduced.

(C) Crystal Grain Size of Base Material Surface In the present invention, in order to ensure sufficient adhesion and smoothness of the plating film, the average crystal grain size of the base material surface at the boundary with the plating film is further determined. Is limited to 15 μm or less. This is because if the average crystal grain size exceeds this, the adhesiveness at the interface is reduced. In order to further improve the adhesion of the coating, the average crystal grain size on the surface of the base material is preferably 8 μm or less. The lower limit of the average crystal grain size is preferably 1 μm.
The effect of grain refinement on adhesion and smoothness saturates,
This is because further refinement is not economical.

(D) Manufacturing Method Next, an outline of a method for manufacturing an alloyed hot-dip coated steel sheet according to the present invention will be described. The following is an example, and the plated steel sheet according to the present invention is not limited to the following manufacturing method.

As the base material, a steel sheet having the composition described in (a) is used. Either a hot-rolled plate or a cold-rolled plate may be used, but in order to maximize the effects of the present invention, it is preferable to use a cold-rolled plate used as an automotive exterior steel plate. As the cold-rolled sheet, a sheet that has not been annealed (hereinafter referred to as an unannealed cold-rolled sheet) is generally used, but an annealed steel sheet may be used.

The base material is desirably immersed in an alkaline solution such as NaOH or degreased by electrolysis. At the same time as or before and after the degreasing step, the surface of the base material is ground with a rotary brush containing abrasive grains or the like at a rate of 0.2 to 8 g / ground surface.
It is desirable to m ² grinding. By this grinding, the refinement of the crystal grain size on the surface of the base material after the alloying treatment can be promoted. When the base material is a hot-rolled sheet or when the annealing temperature is high, it is particularly effective to use this grinding together. In particular, when the average crystal grain size of the surface of the base material after the alloying treatment is set to 8 μm or less, it is preferable to grind 2.0 g / m ² or more.

The base material thus treated is passed through a continuous furnace of a galvanizing facility. Continuous furnaces include a system in which a preheating furnace such as an oxidation-free furnace or a direct-fired furnace and a radiant tube-type reduction furnace are combined, and an all-radiant tube reduction furnace system in which the whole is a radiant tube system. Is not limited. The base material is a continuous furnace,
The material is heated to a predetermined temperature in accordance with the required material properties. However, when the base material is an unannealed cold rolled sheet, the material is heated to a temperature higher than the recrystallization temperature. In the case of the unannealed cold-rolled sheet of the composition of the present invention, 780
Anneal at ~ 870 ° C, preferably 780-830 ° C. The annealing atmosphere is an atmosphere of H ₂ : 2 to 50% by volume, residual N ₂ or an inert gas, and a dew point of −20 ° C. or less is desirable.

The heat-reduced base material is cooled to a temperature close to the temperature of the plating bath. Since the temperature of the plating bath is usually 460 to 480 ° C, the base material temperature before plating is 400 to 520 ° C.
If the temperature of the plating bath or the temperature of the base material is too high, alloying tends to be delayed. Therefore, it is desirable that the base material temperature does not exceed 480 ° C. Al content in plating bath is effective A
l (total Al concentration-total Fe concentration) is preferably set to 0.07 to 0.15%. If it is out of this range, it becomes difficult to control the content of Al in the plating film to the range described in (b) above. The immersion time in the plating bath is preferably 3 seconds or less. If the immersion time is longer than this, dross is likely to occur. Zinc is adjusted to a predetermined amount by a high-pressure gas knife.

The plated base material is subsequently alloyed in a heat treatment furnace. The temperature of the steel sheet during the alloying treatment is preferably 490 to 530 ° C, and it is preferable to heat the steel sheet up to 490 ° C at a rate of 20 ° C / sec or more. If the temperature of the steel sheet during the alloying treatment is lower than 490 ° C, a ζ phase will precipitate. This phase is F
Since the e-Zn alloy is relatively soft and has a large coefficient of friction, peeling (flaking) of the film may occur due to sliding with a mold during press molding. If the temperature of the steel sheet during the alloying treatment exceeds 530 ° C., the powdering resistance tends to decrease. The ζ phase tends to grow in the temperature range of 500 ° C or less, so if the heating rate of the steel sheet is less than 20 ° C / sec,
A large amount of phase may precipitate, which is not preferable.

On the alloyed hot-dip galvanized steel sheet of the present invention, a usual upper plating such as Fe-based plating or Zn-Ni is applied, or a lubricant is applied thereon. You can also.

[0048]

EXAMPLE An unannealed cold-rolled sheet having a thickness of 0.8 mm having the chemical composition shown in Table 1 was used as a base material, and 10% by weight of NaOH at 75 ° C.
After being degreased and washed with a solution, it is preheated to 650 ° C. by direct flame reduction heating of a continuous hot-dip galvanizing equipment, and H ₂ 20% by volume, N ₂
Anneal at 790 ° C. for 60 seconds in an atmosphere of 80% by volume and a dew point of −37 ° C., and further add 20% by volume of H ₂ , 80% by volume of N ₂ and a dew point of −47.
Heat treatment at 550 ° C for 60 seconds in an atmosphere at 480 ° C
And the effective Al (total Al concentration-total Fe concentration) is reduced to 0.11.
It was immersed in a galvanizing bath at 460 ° C. containing 0.15% by weight for 2 seconds to obtain a hot-dip galvanized steel sheet having an adhesion amount of 55 g / m ² per side. The coated steel sheet was heated to various temperatures at a rate of 40 ° C / sec using an induction heating type heat treatment apparatus, held for 18 seconds, cooled at a rate of 15 ° C / sec, and subjected to alloying treatment. Obtained. A part of the base material was ground with a rotating nylon brush containing abrasive grains before the degreasing and washing step, and then a sample of an alloyed hot-dip galvanized steel sheet was obtained by the same degreasing and subsequent manufacturing steps as described above.

[0049]

[Table 1]

The appearance after the alloying treatment was observed with the naked eye, and those in which unevenness in processing (streak-like appearance defect) was recognized were regarded as defective. The plated steel sheet sample subjected to the alloying treatment was analyzed by chemical analysis by dissolving the plated film with a 6% by weight hydrochloric acid solution containing 0.5% by weight of an inhibitor (Asahi Chemical Co., Ibit 710N).

The performance of the galvannealed steel sheet was evaluated by the following method.

Powdering resistance: A blank having a diameter of 60 mm was punched from each steel sheet sample, and this was formed into a cylindrical cup having a depth of 25 mm using a punch having a diameter of 33 mm. The wrinkle pressure during molding was 3.92 kN. The powdering resistance was evaluated by determining the weight of the zinc pieces peeled off from the outer side wall surface of the cylindrical sample with an adhesive tape.

Low temperature chipping resistance: length 150 mm, width 70 mm
After phosphate treatment with CF168 manufactured by Chemfil Co., Ltd., a cationic electrodeposition coating (using Uniprime manufactured by PPG, film thickness: 30 μm), and intermediate coating (using an epoxy ester-based coating manufactured by PPG) were used. 15μm thick) and top coat (P
An acrylic / enamel-based paint manufactured by PG Co., Ltd. was applied to a thickness of 45 μm). The steel plate was cooled and kept at -20 ° C, and 10 gravel stones with a diameter of 4 to 6 mm were hit with a gravel tester at a collision speed of 100.
After colliding under conditions of ~ 150 km / h, the average value of the plating film pieces having the largest diameter to 5 pieces among the pieces of the plating film peeled off with the adhesive tape was obtained as an index of the low-temperature chipping property of each steel sheet.

Surface roughness of plating surface: Using a stylus type surface roughness meter, using a stylus having a 55 ° conical shape and a tip diameter of 1 μm, surface irregularities were measured under the conditions of a scanning distance of 8 mm and a cutoff of 0.8 mm.

Crystal grain size on the surface of the base material at the boundary with the plating layer: The plating layer of the alloyed plated steel sheet is made to have a concentration of 6% containing 0.01% inhibitor (Ibit, manufactured by Asahi Chemical Co., Ltd.).
And dissolved in hydrochloric acid, and further corroded with a 3% nitric alcohol solution (Nital solution) for 2 minutes. The surface was photographed with an electron microscope at a magnification of 1000 for 10 fields of view, and the number of crystal grains cut by a straight line drawn at the center of the photograph was determined to calculate the average crystal grain size.

Table 2 shows the production conditions and performance evaluation results for each steel sheet. Here, each of the grinding amount and the adhesion amount of the plating indicates the amount per one side of the steel sheet, and the x mark in the column of the external appearance indicates that processing unevenness was observed.

[0057]

[Table 2]

As shown in Table 2, the steel sheet produced by the method of the present invention has no processing unevenness during alloying. Its performance is within 15mg of powdering, peeling diameter of plating film is less than 4mm in low temperature chipping test, and surface roughness of plating surface is less than 1.0μm. ing. In particular, Test Nos. 25, 28, 33, and 38 are extremely excellent in overall performance, and can secure low-temperature chipping resistance and powdering resistance even if the Al content of the film is low.

When the conditions of the present invention are not satisfied, one of the above three conditions is inferior as shown in the comparative example.
A steel sheet with excellent overall performance cannot be obtained.

[0060]

The alloyed hot-dip galvanized steel sheet of the present invention comprises:
In addition to the low temperature chipping resistance after painting and the powdering resistance of the plating film, the plating surface has excellent smoothness. Since this steel sheet can be manufactured rationally, it is economically rich and is also suitable for deep drawing, so it is particularly suitable as an exterior steel sheet for automobiles.

Claims (1)

[Claims] (1) The chemical composition is C: 0.006% or less by weight%,
i: 0.02 to 0.08%, Mn: 0.06 to 0.25%, P: 0.007 to
0.014%, Al: 0.005 to 0.08%, Cr: 0 to 0.2%,
B: 0 to 0.003%, at least one of Ti and Nb is contained in a total of 0.03 to 0.30%, and the contents of Si and P satisfy the following formulas and the relationship of the following formulas, with the balance being Fe and unavoidable impurities. Al: 0.15 to 0.60% by weight, Fe: 8.5 to 13.5%, balance: Zn and unavoidable impurities. At least one surface of the base material has an alloy plating film, and the base at the interface with the plating film. An alloyed hot-dip galvanized steel sheet having an average crystal grain size of 15 μm or less on the surface of the material. Si + 10 × P ≦ 0.18 ------ P ≧ 0.005 +0.1 × Si ----