JPH02122082A - Production of alloyed hot dip galvanized steel sheet having superior workability and coatability - Google Patents

Abstract

PURPOSE:To produce an alloyed hot dip galvanized steel sheet having superior workability and coatability by successively subjecting a pretreated steel sheet to hot dip galvanizing, alloying, Mn-Zn alloy plating and reheating under specified conditions. CONSTITUTION:A steel sheet pretreated as usual is hot dip galvanized by dipping in a molten Zn bath contg. 0.05-0.3wt.% Al and <=0.2wt.% Pb to form a Zn film by 30-90g/m<2>. The steel sheet is introduced into an alloying furnace and alloyed to such a degree that the Zn film has 3-8wt.% Fe content and contains part of Zn in the unalloyed state. One side or both sides of the steel sheet are further plated with an Mn-Zn alloy contg. >=35wt.% Mn to form a Mn-Zn alloy film by 0.5-10g/m<2> and the steel sheet is reheated in the temp. range of 250 deg.C to the m.p. of Zn for 20sec to 15hr in a furnace filled with a nonoxidizing or reducing atmosphere. A plated steel sheet having superior resistance to corrosion, powdering and cratering is obtd.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a zinc-based alloy coated steel sheet used for automobiles, home appliances, building materials, etc.

[Prior art] Galvanized steel sheets are widely used as materials that are inexpensive and have excellent corrosion resistance and strength.In particular, galvanized steel sheets are used in large quantities for the interior and exterior panels of automobiles, considering their workability and paintability in addition to their corrosion resistance. It is being said. Generally speaking, there are two methods for mass production of galvanized steel sheets: electroplating and hot-dip plating.Since electroplating is processed at low temperatures, there is no phase change due to heat effects, and it is easy to control the composition of the plating film. In order to increase the amount of plating deposited, it is necessary to increase the processing time.On the other hand, with the hot-dip plating method, the amount of plating can be easily increased without increasing the processing time, and it is possible to easily increase the amount of plating by applying heat treatment after plating. Fe-Zn alloy can be made from However, the composition of the plating film and the control of the generated phases require some ingenuity. In recent years, steel sheets for automobiles have been required to have a higher degree of corrosion resistance in order to deal with salt damage, etc. In response to this, we have developed plating compositions, mainly hot-dip galvanizing, which can easily secure a coating amount and is economical. Efforts are being focused on the development of toughened steel sheets that have excellent corrosion resistance and processability and paintability through skillful phase control.

The most important problem in processability is powdering resistance.
An issue with paintability is cratering resistance. Powdering is a phenomenon in which a plating film becomes powdery and falls off during press molding, and cratering is a phenomenon in which the plating film becomes powdery and falls off. Cratering is a phenomenon in which the plating film becomes visually visible in the electrocoating process that is performed after chemical conversion treatment. (crater) is a phenomenon that occurs. The former has r-phase (Fe) with high iron content in the plating film.
3 Zn1O+ Fe2O~28wt%) is generated,
This occurs because the plating film is hard and brittle, and the latter is caused by uneven surface shape of the plating film, oxide film, plating film phase structure, etc.)
Occurs due to.

Conventionally, alloyed hot-dip galvanized steel sheets used for automobiles have an average iron content of 1 after hot-dipping.
Alloying treatment is performed until the Fe content reaches around 0 wt%, and Fe is diffused to the plating surface to improve corrosion resistance, especially post-painting corrosion resistance.

That is, after pre-treating the steel plate (including heat treatment) continuously to adjust the material, it is plated by immersing it in a plating bath containing molten zinc, and then this plated steel plate is heated for 50 minutes in an alloying furnace.
Rapidly raise the temperature from 0℃ to 700"C for a short period of time (1
0 to 30 seconds) to reduce the iron content of the plating film to 10%.
The front and back are alloyed. However, since the alloyed hot-dip galvanized steel sheets produced in this way are heated to high temperatures due to rapid temperature rise, the iron content in the plating film tends to vary depending on the location, and the iron content in the plating film tends to vary in the surface direction and depth direction. In addition to this, the iron concentration gradient within the plating film increases, and in order to secure the iron content in the surface layer, the iron content at the interface with the steel base increases and the r-phase is formed. Furthermore, high temperature treatment and rapid cooling generate thermal stress in the plating film.

On the other hand, a method has been proposed in which the alloying process is divided into two steps, primary and secondary.
In No. 1, in the secondary heating, rapid heating and high temperature heating is performed to remelt the Zn plating film in order to obtain smoothness of the plating film. In this heating, the iron content is reduced to 2.2 to 5.5 wt.
Therefore, depending on the result of this secondary heating, secondary heating is performed at a low temperature below the melting point of zinc over a period of time to keep the iron content within a range of 6 to 13 wt%. . It is also disclosed that by this method, it is possible to obtain an alloyed hot-dip galvanized film with a smooth surface, excellent appearance, and no peeling or powdering during processing.

On the other hand, some proposals have been made that improve cratering resistance by increasing the iron content only in the surface layer of the plating film.
The proposal of Japanese Patent Publication No. 58-15554 is a plated steel sheet with a corrosion-resistant metal layer as an inner layer and a Fe--Zn alloy coating layer with a high iron content attached thereon to improve cationic electrodeposition coating properties. This proposal discloses an alloyed hot-dip galvanized layer that is formed into an Fe-Zn alloy by heat treatment after hot-dip galvanizing as the corrosion-resistant metal layer that is the inner layer.

[Problem to be solved by the invention] However, in the above-mentioned Japanese Patent Publication No. 59-14541,
Regarding the cratering resistance, which does not satisfy the cratering resistance, the iron content on the surface is insufficient. In addition, regarding powdering resistance, since the alloying process is performed by rapid temperature rise and high temperature heating after hot-dip galvanizing, it is inevitable that the alloying reaction progresses unevenly, resulting in the growth of an F layer with poor workability. Resulting in. Furthermore, in some cases,
Unalloyed portions and highly alloyed portions coexist, resulting in a so-called uneven burning phenomenon.

As described above, secondary heating tends to be non-uniform, so secondary heating conditions based on the results of secondary heating become extremely complicated, and implementation thereof is very difficult in actual operation.

In Japanese Patent Publication No. 58-15554, the iron concentration on the plating surface was dramatically increased, improving the cratering resistance, but since alloying was completed by heat treatment after hot-dip galvanizing, Similar to No. 14541, there is the problem of non-uniform alloying, and in addition, the iron concentration gradient within the plating film becomes large, and at the interface with the steel base where the iron concentration becomes high, "phases grow". In addition, thermal strain stress caused by rapid heating and cooling is also unfavorable for powdering resistance.Thus, efforts have been made to satisfy powdering resistance and cratering resistance, but molten zinc that satisfies both properties has not yet been developed. No plated steel sheets were obtained.

In order to solve this problem, the present invention was made, and an object of the present invention is to provide a method for manufacturing a plated steel sheet that satisfies not only corrosion resistance but also powdering resistance and cratering resistance.

[Means and effects for solving the problem] The means for achieving this objective are: (1) Manufacturing of an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, which includes the following steps: (a) A steel plate subjected to normal pretreatment is coated with Al0.05wt%.
30 g/m” or more by immersion in a hot-dip galvanizing bath containing Pb of 0.3 wt% or less and Pb of 0.2 wt% or less
0g/m" or less plating step; (b) Continuing with the plating step, introducing a galvanized steel sheet into an alloying treatment furnace to increase the iron content in the plating film from 3 wt% to 8 wt%. A step of performing alloying treatment in the following range and in a state where part of the zinc in the coating remains unalloyed, (c) 0.5 g/m on one or both sides of the hot-dip galvanized steel sheet subjected to the alloying treatment. ”Mn of 10g/m2 or less
A step of applying Mn-Zn alloy plating of 35 wt% or more, (d) The steel plate plated in the above step is kept in a furnace maintained in a non-oxidizing or reducing atmosphere at a temperature range of 250 ° C or more and below the melting point of zinc. A step of reheating for 20 seconds or more and 15 hours or less.

(2) A method for producing an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, which includes the following steps.

(a) All 0.05wt steel plate subjected to normal pretreatment
% or more and 0.3 wt% or less, and Pb0.2. 30g/m by immersion in a hot-dip galvanizing bath containing up to wt%
'The process of applying plating of at least 90 g/m2, (b)
Continuing with the plating process, a galvanized steel sheet is introduced into the alloying furnace to reduce the iron content in the plating film to 3.
A step of alloying the zinc in the range of wt% or more and 8 wt% or less and in a state where a part of the zinc in the coating remains unalloyed. A step of spraying Mn-Zn alloy powder containing 35 wt% or more of Mn to form an upper layer plating of 0.5 g/m" or more and 10 g/m" or less; (d) After the plating film has solidified, skin pass treatment is performed on the surface of the hot-dip galvanized film. (e) The steel plate with the toughening film smoothed in the above step is heated for 250' in a furnace maintained in a non-oxidizing or reducing atmosphere.
A step of reheating at a temperature range of C or more and below the melting point of zinc for 20 seconds or more and 15 hours or less.

The above means will be described in detail below, including their effects.

First, the steel plate for plating may be a cold-rolled steel plate or a hot-rolled steel plate, and may be subjected to surface conditioning and annealing treatment as a normal pretreatment.

A hot-dip galvanizing bath usually contains around 0.2% AfI for suppressing the Fe--Zn alloy reaction and smoothing the mating surface, and also contains Pb for spangle adjustment. Among these, Al has an effect of suppressing alloying, so 0.05
Fe-Z after adding wt% or more and immersing in hot-dip galvanizing bath
It is important to prevent Fe-Zn alloy from being formed unevenly in this process, and once it becomes uneven, it must be corrected in a later process. Cannot be done, the amount of Al added is too large, 0.3wt
If it exceeds %, the effect of suppressing alloying becomes excessive, and the subsequent alloying treatment takes too much time, making it unsuitable for industrial use. P
Although b does not directly participate in the alloying reaction, a large amount of pb reduces powdering resistance, so it must be limited to 0.2 wt% or less.

When the lower plating layer formed here becomes an integral structure with the thin upper plating layer that will be formed later, it forms the inner layer that occupies most of the plating film, but this layer provides most of the corrosion resistance and powdering resistance. Determined by The coating weight of the lower plating layer needs to be 30 g/m or more in order to exhibit high corrosion resistance. However, if it is deposited in excess of 90 g/m, it will not only result in excessive quality, but also cause the subsequent The reheating process performed at a low temperature in the process takes a long time and reduces productivity. Furthermore, if the plating film becomes thicker than -m, the film may break or peel off during processing, and in the case of alloyed hot-dip galvanized steel sheets, powdering is likely to occur.

In the case of alloyed hot-dip galvanized steel sheets, the corrosion resistance, particularly the post-painting corrosion resistance, is largely determined by the amount of coating deposited and the iron content in the coating. For this reason, alloying treatment is performed following the plating process. This treatment involves passing the plated steel sheet through an alloying furnace to raise the temperature of the steel sheet and diffusing Fe from the steel base into the galvanized layer. However, at this time, the degree of alloying is such that the iron content ranges from 3 wt% to 8 wt%.
% range and leave the alloying in an incomplete state. That is, in this invention, the alloying of the hot-dip galvanized layer is completed by the reheating treatment performed in a later step, but in order to shorten the reheating time as much as possible and not to deteriorate the powdering resistance, 3wt% or more of the hot-dip galvanized layer is added. Iron content is necessary. On the other hand, by keeping the amount below 8 wt%, the η phase (pure Z
If this η phase is not left, Mn will not easily diffuse from the upper layer to the lower layer during the reheating treatment performed in the later process, and as a result, the adhesion between the two layers will decrease. is not sufficiently improved, and part of the outer layer of the plating film inevitably falls off during pressing. In addition, although the alloying process in continuous hot-dip galvanizing equipment takes a short time (from a few seconds to several tens of seconds), the temperature is higher than the melting point of zinc (419.5°C), so it cannot be used as an Fe-Zn alloy. , r phase, δ1
Generation and growth of phase, ζ phase, etc. are considered. Of these, δ1
The phase and ζ phase improve corrosion resistance after coating of the plating film, but
``The phase is hard and brittle and deteriorates powdering resistance, which is undesirable. However, under the above alloying conditions, this r phase hardly grows and does not have a negative effect on powdering resistance.

Prior to alloying treatment in this continuous hot-dip galvanizing equipment, if the spangles are made finer by mist spray or powder spray, the macroscopic inhomogeneity of the zinc crystal orientation will be reduced.
This can be done as needed, as this will improve the coverage of the upper layer plating performed in the later process.Also, by performing a skin pass after the alloying treatment to smooth the surface of the plating film, it will improve the coverage of the upper layer plating film. It is possible to efficiently improve the image quality and sharpness after painting.

An upper layer of Mn-Zn alloy is applied to the hot-dip galvanized steel sheet that has been alloyed in this way. This is to diffuse Mn. From the viewpoint of cratering resistance, the Mn content of this plating layer is 3.
5W% or more, and the adhesion amount is 0.5g/m” to 10g
/m2. For automobiles, alloyed hot-dip galvanized steel sheets are subjected to chemical conversion treatment (phosphate treatment) and then cationic electrodeposition coating. Coarse, needle-shaped phosphate crystals called hopite [Z n3 (PO4)2.4 H20F are formed in the crystals.

However, when Mn coexists, a part of Zn in the crystal is replaced with Mn, and the phosphate crystal becomes dense. One of the causes of crater generation is thought to be local concentration of current in defective areas of the chemical conversion coating, but coatings made of dense crystals have fewer defects. If the Mn content in the upper layer plating is 35W% or more, even if there is diffusion of components between it and the lower layer due to reheating treatment in a later step, the Mn content in the outer layer should be 30wt% or more. According to research conducted by the inventors, when the Mn content on the plating surface becomes high and approaches 30 wt %, the occurrence of craters rapidly decreases. Further, corrosion resistance is further improved by alloying Zn with Mn, and the corrosion resistance is improved not only in the outer layer portion with a high Mn content but also in the inner layer portion where Mn diffuses by subsequent reheating.

If the adhesion amount is less than 0.5 g/m2, Mn cannot be sufficiently supplied to the entire plating surface during chemical conversion treatment, and if the adhesion amount exceeds 10 g/m2, the effect will be saturated and the cost will increase. It will only put you at a disadvantage.

The above-mentioned upper layer plating treatment may be any method such as electroplating, vapor deposition plating, thermal spraying, etc. as long as the treatment is performed at a temperature lower than the melting point of zinc.When performing this upper layer plating treatment by alloy powder spraying, This is done while the previous hot-dip galvanizing layer is in a molten state, and can also serve as a finer spangle. However, in this case, the surface cannot be expected to be smooth after the upper layer plating, so it is necessary to smooth the surface by skin pass treatment. This skin pass processing has an elongation rate of 0.
．． If the elongation is carried out at 3% or more, the attached surface will be smooth, but if the elongation is too large and exceeds 5%, there is a risk that the workability of ordinary thin steel sheets will be affected.

Furthermore, depending on the application, the appearance of one side may not be a problem, and in such cases, one side may not have this upper plating film, or may be provided with another plating film.

In the final step, the plated steel plate is heated again. In other words, the plating layer applied twice is carefully heated at a low temperature to complete alloying while preventing the formation of phases, and at the same time, the composition is made continuous by diffusion of ingredients between both layers, creating a monolithic plating film. The conditions for this reheating treatment are heating for 20 seconds to 15 hours at a temperature range of 250°C or higher and lower than the melting point of zinc.If it is lower than 250°C, the effect of promoting diffusion of Fe atoms in the plating layer is low; It takes too much time to obtain a degree of alloying sufficient to ensure corrosion resistance after painting, which is not industrially practical.On the other hand, if the temperature is higher than the melting point of zinc (419.5℃), the diffusion of Fe atoms is partially promoted. This may lead to the appearance of areas where alloying progresses rapidly, which may even promote non-uniformity and thermal distortion, and the formation of phases cannot be ignored.
The figure shows the conditions under which both powdering and cratering do not occur in the above temperature range. The horizontal axis is the heating time, and the vertical axis is the heating temperature. Points a, b, and c are shown in the figure.

The range surrounded by the line connecting d is the preferred range for actual operation to avoid powdering and cratering, and the heating time is from the time coordinate of point a to the time coordinate of point 0, that is, 20 seconds or more and 15 hours. When reheating is performed under the following heating conditions of 0 or more, Mn diffuses from the upper plating side, so the amount of Fe that diffuses from the steel cable side to the lower plating layer is small, and the amount of Fe diffuses from the steel cable side to the inner layer. Appropriate alloying is achieved without creating locally high Fe concentration areas.

At this time, since some η phase remains in the lower layer near the upper layer, diffusion between the upper and lower glabella tends to proceed. Therefore, the remaining η phase disappears, and "no phase is substantially generated." , a plating film consisting only of the Mn-Zn alloy with good corrosion resistance and the δ1 phase and ζ phase that gives good results in corrosion resistance after painting is obtained.At the boundary layer between the plating film and the steel base, the phase has a thickness of 0.5 μm or more. Although it is difficult to detect if the phase does not grow, the r-phase was not detected in the plating film treated under these conditions.
Only the δ1 phase and the ζ phase are detected. Since this plating film avoids rapid high-temperature heating, the iron content is uniform along the surface, and any part of the plated steel sheet exhibits the specified corrosion resistance and workability, resulting in a product with extremely stable quality. becomes. Further, the iron content falls within the range of 5 wt% to 20 wt%. However, considering the variations in conditions that tend to occur during actual operation, it is particularly preferable to set the heating temperature to 260°C.
to 400° C., and the heating time is from 10 minutes to 10 hours. In this case, the iron content of the plating film falls within the range of 5 wt% to 10 wt%. Further, by this heat treatment, the upper layer and the lower layer become an integral structure due to thermal diffusion of Mn-Zn, forming an outer layer portion and an inner layer portion of the plating film, respectively, and also removing thermal distortion. As a result, the mechanical properties and electrochemical properties of the plating film do not differ significantly between adjacent parts, so that the adhesion between the upper layer and the inner layer is perfect, and at the same time, it has excellent workability and corrosion resistance. This heat treatment is performed in a furnace maintained in a non-oxidizing or reducing atmosphere, but doing so in a non-oxidizing or reducing atmosphere prevents oxidation of the surface and causes uneven chemical conversion coating crystals during the chemical conversion treatment before painting. This is to avoid this, and if the process is to be carried out in a short period of time, a continuous furnace is used, and if the process is to be carried out over a long period of time, it is recommended to use a batch type annealing furnace.

[Example] Regarding the alloyed hot-dip galvanized steel sheets of 17 examples (Examples) in which two types of steel sheets were used and the hot-dip galvanizing conditions, upper layer plating conditions, and alloying treatment conditions were changed, the alloy in the plating film was The component content (Fe, Mn) was investigated and evaluated by performing a powdering test and a cratering test. For comparison, 7 samples were treated under conditions outside the scope of this invention.
The details of the 0 condition similarly investigated for the example (comparative example) and three examples (conventional example) according to the prior art are as follows. The steel plates used were cold-rolled steel plates with a thickness of 0.8 mm, including low carbon AJ quilt (Material A) for thin plates, which is commonly used, and ultra-low carbon titanium, which is said to easily cause powdering for high processing purposes. Table 1 shows the respective components of steel (Material B).

The hot-dip galvanizing of the lower layer is carried out in a continuous plating facility equipped with a non-oxidizing furnace and a reduction heating furnace, and after adjusting the coating amount with a gas throttle device installed after the surface of the plating bath, alloying treatment is carried out continuously. Ta. The plating layer has an elongation rate of 1.5% after cooling.
A skin pass is performed to smooth the surface, and Mn-Z is applied on top of this.
An upper plating layer of n alloy was formed. For the reheating treatment, a continuous furnace was used in Example No. 16 and Comparative Example NIL7, and a batch furnace was used in the other examples. For upper layer plating, electroplating,
Although plasma spraying or powder spraying methods were used,
Each of these was processed under the following conditions.

(1) Electroplating Zn2 SO4・7H2 MnSO4・H2O N a5 c, H507.

Bath temperature cathode current density (2 Plasma spraying Plasma gas spraying Heat input thermal spraying distance Average powder particle size Powder supply rate r 0KW 00am Approx. 5μm 5 g/mts -d m” 0 50-150g/ρ 50-150g #1 282 0 180g /41 50℃ 30-50 A / d m" (Mn70%) (3) Powder spray average powder particle size (Mn70%) Approx. 5 μm Powder supply rate 3 g/mia -d m" Plating film outer layer and plating film The iron and manganese contents in the inner layer were investigated by Auger electron spectroscopy and Grim glow discharge emission spectroscopy.

Powdering resistance is determined by bending the material 90 degrees with a radius of curvature of 2, attaching adhesive tape to the inside of the bend, and peeling it off.
The adhesion of the powder to this adhesive tape was visually observed and evaluated by giving a score. The rating criteria is on a five-point scale: 1: no adhesion at all, 2: very little adhesion, 3: slightly adhesion, 4: a little adhesion, and 5: considerable adhesion.

Cratering resistance was evaluated by applying a chemical conversion treatment to the plated surface, followed by electrodeposition coating, and evaluating the number of craters generated at this time. A commercially available dipping type phosphate treatment agent was used for the chemical conversion treatment. For the electrodeposition coating, a commercially available cationic electrodeposition paint was used, and after the paint was prepared, it was stirred for one week, and electrodeposition was carried out by instantaneously applying an electrodeposition voltage of 300 cm with a distance between electrodes of 4C1l.

The processing conditions and investigation results for each of these examples are shown in Table 2.

In the examples, material B was not inferior in powdering resistance, and although very slight powdering was observed at No. 6, which is the upper limit coating amount, there was no problem in practical use. Example N1L11 where the Mn content of is the lower limit and Example N where the upper layer plating amount is the lower limit
113, one or two small lubrication plates were found, but this also poses no practical problem.In this way, in the examples, all alloyed hot-dip galvanized steel sheets had good powdering resistance and cratering resistance. It has both gender. Further, the iron content of the inner layer is within the range of 6wL% to 10wt%, which ensures sufficient corrosion resistance after painting.

On the other hand, in a comparative example processed under conditions outside the scope of the invention, Nα1 did not contain AII during hot-dip galvanizing, Na2 was heated for an excessive amount of time, and iron content did not exceed 8 wt% in alloying after lower layer plating. 3. No. 4 with a lot of PB in the bath.
Too much N [L5] attached, too high reheating temperature
A, 7, etc. have problems with powdering resistance, and N[L6, which is not coated with upper layer plating, has poor cratering resistance.

In the conventional examples, N[L2 is alloyed by rapid heating at high temperature and then alloyed at low temperature, and has poor cratering resistance. , 3 is alloyed by rapid heating and high temperature heating,
A Fe--Zn alloy plating layer with an iron content of 8% wt.

In this way, both characteristics are not satisfied at the same time.

Next, the iron content distribution in the inner layer of the plating film according to the present invention was investigated.

Alloyed hot-dip galvanized coil of Example Na 1 (width 18
The iron content of the inner layer of the plating film was examined at intervals of 200 m+* in the width direction of 00 mm) and compared with conventional example N[L 2. The results are shown in Figure 2. In Figure 2, the horizontal axis is the distance from the left end of the coil, the vertical axis is the iron content, the O mark is plotted for Example N+L1, and the * mark is plotted for Conventional Example Na2. As is clear from Figure 0, the iron content of Example 1 N[Ll was 6.8 wt% on average, and all measurement points ranged from 6.6 wt% to 7.0 wt%. It was distributed between OwL%. On the other hand, the iron content of conventional example N[L 2 is 8.3 wt% on average, and 7.9 wt% at all measurement points.
However, it was distributed between 8.9 wt% and there was a large variation.

In addition, in order to investigate whether or not the F phase exists at the boundary between the plating film and the steel base, N[Li
As a result of removing about two-thirds of the upper layer of the plating film and performing X-ray diffraction on the samples up to 2, it was found that r
No phase was detected.

[Effects of the Invention] According to the present invention, there is substantially no r-phase in the plating film, and the outer layer with a high iron content and the inner layer with an appropriate iron content have an integral structure. A hot-dip galvanized steel sheet with a film that has a uniform distribution of iron content in the surface direction, that is, it has sufficient corrosion resistance, excellent powdering resistance and cratering resistance, and extremely stable quality. A hot-dip galvanized steel sheet is produced. The industrial effects of the present invention, which allow such excellent products to be easily manufactured through simple steps, are significant.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between heat treatment conditions and appropriate characteristics for detailed explanation of the present invention, and FIG. 2 is a diagram showing the distribution of iron content in one embodiment of the present invention.

Claims (2)

[Claims] (1) A method for producing an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, which includes the following steps. (b) Steel plate subjected to normal pretreatment with Al0.05wt%
A step of applying plating of 30 g/m^2 to 90 g/m^2 by immersion in a hot-dip galvanizing bath containing Pb of 0.3 wt% or less and Pb of 0.2 wt% or less; (b) A step of applying the plating. Continuously introducing a galvanized steel sheet into an alloying treatment furnace, the iron content in the plating film is in the range of 3 wt% or more and 8 wt% or less, and a part of the zinc in the film remains unalloyed. (c) Mn-Zn alloy plating of 35 wt% or more of Mn of 0.5 g/m^2 or more and 10 g/m^2 or less on one or both sides of the hot-dip galvanized steel sheet subjected to the alloying treatment. (d) Reheating the steel plate plated in the above step in a furnace maintained in a non-oxidizing or reducing atmosphere at a temperature range of 250°C or higher and below the melting point of zinc for 20 seconds or more and 15 hours or less. Process. (2) A method for producing an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, the method comprising the following steps. (b) Steel plate subjected to normal pretreatment with Al0.05wt%
30g/m^2 or more by immersion in a hot-dip galvanizing bath containing 0.3wt% or less and 0.2wt% or less of Pb9
A step of applying plating of 0g/m^2 or less, (b) Continuing to the above-mentioned plating step, introducing a galvanized steel sheet into an alloying treatment furnace to increase the iron content in the plating film to 3wt% or more 8wt % or less and a part of the zinc in the film remains unalloyed. Spraying Mn-Zn alloy powder to apply upper layer plating of 0.5 g/m^2 to 10 g/m^2; (d) After the plating film has solidified, skin pass treatment is performed to smooth the surface of the hot-dip galvanized film. (e) Re-processing the steel plate with the smoothed plating film in the above step in a furnace maintained in a non-oxidizing or reducing atmosphere at a temperature range of 250°C or higher and below the melting point of zinc for 20 seconds or more and 15 hours or less. The process of heating.