JP2701737B2 - Manufacturing method of alloyed hot-dip galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvannealed steel sheet using a high-P steel sheet as a base metal.

[0002]

2. Description of the Related Art In recent years, in order to control exhaust gas and improve fuel efficiency,
As the body weight of automobiles is being reduced, the use of high-tensile steel sheets is increasing.

[0003] High-strength steel sheets are based on ultra-low carbon Ti-added steel or Ti-added steel, which is sufficiently decarburized in the steelmaking stage to reduce the carbon content to 0.01% or less, and then to Ti. Typically, the strength is increased by adding P, Si, Mn, and Cr. In particular, since P is lower in cost than other strengthening elements, a high-tensile steel sheet to which P is added is often used as a plating base material.

However, in a steel sheet having a large amount of P in steel, a non-uniform distribution of P is already recognized on the surface of the steel sheet often at the full hard stage. The causes of the non-uniform distribution of P include:
Non-uniform distribution in steel making, poor descaling during hot rolling, etc. are considered. Regardless of the cause, when a steel sheet having such a non-uniform distribution of P on the surface is Zn-plated and alloyed under normal conditions, alloying unevenness occurs.

This is because, as shown in FIG. 1, since P is an alloying retarding element, the plating film becomes a concave defect in the segregated portion of P, and conversely, the plated film becomes a convex defect in the negative segregated portion of P. It is a phenomenon that becomes. The detailed reason is that the growth of the Fe-Zn alloy layer is slower in the segregated portion of P than in the surroundings, and the liquid Zn in the upper layer of the segregated portion is consumed for the growth of the surrounding Fe-Zn alloy layer. It can be considered that the opposite phenomenon occurs in the negatively segregated portion of P.

[0006] The unevenness of the coating appears on the surface as remarkable alloying unevenness after temper rolling or press forming, and this is a particularly serious problem in the case of an outer panel of an automobile.

[0007] As far as the applicant knows, there is no direct measure for preventing alloying unevenness due to non-uniform distribution of P concentration on the steel sheet surface. However, as described above, since the alloying unevenness is caused by alloying delay due to P, a general alloying promoting method for a high-P steel sheet can be expected as a measure for preventing the alloying unevenness. For example, the following three methods are well known as such alloying promoting methods.

[0008] A method of pre-plating the surface of a steel sheet with Cu, Ni, Fe or the like (Japanese Patent Laid-Open No. 55-12286). A method in which the surface of a steel sheet is rapidly oxidized and then reduced to obtain a porous steel sheet surface (JP-A-55-12286 and JP-A-4-202631). A method of strongly pickling the surface of a steel sheet (JP-A-3-24375)
No. 1 and JP-A-3-243752).

[0009]

Among these alloying promoting methods, the method of pre-plating the surface of the steel sheet is also effective in preventing the alloying unevenness caused by the non-uniform P concentration on the steel sheet surface. However, since electroplating equipment is required separately,
This leads to an increase in equipment costs and manufacturing costs.

On the other hand, in the method of rapidly oxidizing a steel sheet surface,
The rapid oxidation prevents the concentration of P on the surface layer of the steel sheet,
Although the alloying in the alloying process is promoted, the non-uniform distribution of the P concentration existing on the steel sheet surface before the oxidation is not eliminated, so that the non-uniform alloying caused by the non-uniform distribution is not prevented.

[0011] Similarly, in the method of strongly pickling the steel sheet surface,
The removal of P concentrated at the crystal grain boundaries promotes overall alloying, but does not prevent alloying unevenness caused by uneven P concentration on the steel sheet surface.

An object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet which can economically prevent uneven alloying due to non-uniform P concentration on the surface of the steel sheet, which is peculiar to high-P steel sheets. It is in.

[0013]

In order to prevent non-uniform alloying caused by non-uniform P concentration on the surface of the steel sheet, which is peculiar to the high-P steel sheet, it is necessary to eliminate the non-uniform P concentration on the surface of the steel sheet. In addition, it is necessary to prevent the plating film from becoming uneven due to the non-uniform P concentration in the alloying treatment. From the latter viewpoint, the present inventors focused on the liquid phase existence time of Zn in the alloying treatment.

That is, as described above, in the alloy unevenness caused by the non-uniform P concentration on the steel sheet surface, the liquid Zn of the surface layer is consumed in the growth of the surrounding Fe—Zn alloy layer at the P segregation portion, and the plating film is formed. This is considered to be because the surrounding liquid Zn is consumed in the growth of the Fe-Zn alloy layer in the negatively segregated portion of P, and the plating film becomes convex. Therefore, if the liquid phase existence time of Zn is short, the unbalanced consumption of liquid Zn due to the non-uniform P concentration on the surface of the steel sheet is suppressed. It is expected that surface irregularities will be avoided.

Based on this idea, the present inventors conducted various experiments. As a result, the temperature rise process of the alloying treatment, in particular, from 420 ° C. in which Zn becomes a liquid phase to a relatively high temperature range, 20 ° C./s.
If the above rapid heating is performed, the liquid phase existence time of Zn is shortened without hindering the original alloying, and as a result, a remarkable P concentration non-uniformity peculiar to the high P steel exists on the steel sheet surface. It was also found that the unbalanced consumption of liquid Zn caused by the non-uniformity was suppressed, and the unevenness of the plating film surface was prevented.

The present invention has been made on the basis of the above-mentioned findings. When a high-P steel sheet having a P content of 0.025 wt% or more in steel is hot-dip Zn-plated and then subjected to alloying treatment, the material temperature is reduced to 4%.
Rapid heating from 20 ° C. to the highest temperature at a rate of 20 ° C./s or more is performed, and the highest temperature is 52
A method for producing an alloyed hot-dip galvanized steel sheet at a temperature of 0 to 600 ° C. is provided.

The high-speed heating itself in the alloying treatment is known from Japanese Patent Application Laid-Open No. Sho 50-13229. However, in the alloying treatment, the composition of the base material is unknown, the purpose of high-speed heating is not clear, and the rate of temperature rise cannot be specified. Therefore, in order to prevent non-uniform alloying due to non-uniform P concentration when a high-P steel sheet is used as a plating base material, it is necessary to use 20% in the Zn liquid phase region.
The idea that rapid heating above ℃ is effective is not shown here.

[0018]

The production conditions in the method of the present invention will be described below in detail.

The method of the present invention uses a high-P steel sheet as a plating base material. The reason why the amount of P in the steel is limited to 0.025% or more is that if the P content is less than 0.025%, the non-uniformity of the P concentration on the steel sheet surface is mild, so that even if the non-uniform distribution exists, the alloy unevenness is reduced. This is because it does not occur. Since the alloy unevenness becomes more remarkable as the amount of P in steel increases, the method of the present invention is particularly effective for steel sheets having P of 0.05% or more.

The components other than P are not particularly limited as long as they are in the category of high-tensile steel. C, Mn, S, SolAl, Ti and the like may be contained in a range generally used. The base material may be either a hot-rolled material or a cold-rolled material.

After the pretreatment, the plating base material is heated to, for example, 600 to 900 ° C. in a reducing atmosphere, subjected to annealing at the same time as reduction, and further subjected to hot-dip Zn plating after cooling.

The concentration of Al in the Zn plating bath may be in a generally employed range, and is usually 0.05 to 0.5%. A
l, Pb, Sh, Si, Fe, Sn, Mg, Mn,
1 of Ni, Cr, Ca, Li, Ti, misch metal, etc.
A seed or two or more kinds may be contained in a small amount in the plating bath.

After the plating, the amount of plating is adjusted by a wiping process, and an alloying process is performed.
Then, in the alloying treatment, the material is heated at high speed at a rate of 20 ° C./s or more from 420 ° C. to the highest temperature (520 to 600 ° C.).

With respect to the starting temperature of the high-speed heating, Zn contributes to the formation of unevenness of the film only after Zn is in a molten state.
420 ° C., the melting point of Zn, was used as the starting temperature.

The reason why the heating rate is set to 20 ° C./s or more is that 2
If the temperature is less than 0 ° C./s, the time during which Zn in the plating surface layer exists in the liquid phase becomes longer, and during this time, in the segregation portion of P, the liquid Zn is consumed for the growth of the surrounding Fe—Zn alloy layer, and the negative segregation portion of P This is because the surrounding liquid Zn is consumed for the growth of the Fe—Zn alloy layer, so that the surface of the plating film becomes uneven. A particularly desirable heating rate is 30 ° C. or higher.

The upper limit of the heating rate is not limited from the viewpoint of plating quality, but it is preferably 60 ° C./s or less because extremely high-speed heating requires a large increase in the capacity of the alloying furnace. , 40 ° C./s or less is more desirable.

The maximum temperature was limited to 520 to 600 ° C. for the following reason. If the temperature is lower than 520 ° C., the alloying is not completed even if the material reaches the maximum temperature, and a sufficient alloying rate cannot be expected in the temperature holding period after the temperature is raised or in the subsequent cooling process. The plating surface layer is kept in a molten state for a long time, causing unevenness. On the other hand, 600 ° C
If it exceeds 300, the alloying proceeds excessively in the process of raising the temperature, and the powdering resistance is significantly reduced. Particularly desirable maximum attainment temperature is 530 ° C. or higher for the lower limit and 550 ° C. or lower for the upper limit.

In the alloying treatment in the method of the present invention, it is sufficient that the initial heating process satisfies the above condition, and the subsequent heat pattern may be a normal condition, and the condition is not particularly limited.

As the heating means, any of combustion gas direct heating, combustion gas radiation heating, direct current heating, induction heating and the like may be employed.

[0030]

EXAMPLES Examples of the method of the present invention will be described below. Steels having the respective compositions shown in Table 1 were melted and hot-rolled to obtain 3.2 mm steel sheets. The winding temperature was 500 to 650 ° C. Next, the hot-rolled steel sheet was descaled with a pickling solution of 15% HCl, washed with water, and then cold-rolled to obtain a 0.8 mm thin sheet. Further, the cold-rolled steel sheet was plated after pickling and washing with water.

[0031]

[Table 1]

In the plating, after performing solvent degreasing, water washing and drying as pretreatments, a pre-heat treatment is performed at 550 ° C. × 30 s in N ₂ 100 ppm O ₂ using a hot-dip plating simulator, and further, 25% H ₂ − After performing reduction annealing at 760 ° C. × 60 s in an atmosphere with a dew point of −30 ° C., the Z at 460 ° C.
Immersion plating was performed in a Zn plating bath of n-0.15% Al. Then, after plating, the adhesion amount is 6 by wiping.
It was adjusted to 0 g / m ² and subjected to a draw alloying treatment.

In the alloying treatment, a plating sample obtained by spot welding a thermocouple is placed in an induction heating furnace (50 kW, 100 kHz).
The plating sample was heated under various conditions while measuring the temperature. When alloying was completed at the time when the temperature increase was completed, air cooling was performed as it was. When alloying was not completed, the temperature was maintained at the maximum temperature, and after completion of alloying was confirmed, air cooling was performed.

The cold-rolled steel sheet, which is the base metal for plating, has a heating rate of 10 ° C./s and an ultimate temperature of 480 ° C. after plating.
The alloyed hot-dip galvanized steel sheet was examined for surface unevenness, and those having surface unevenness were selected and used.

The surfaces of the various alloyed hot-dip galvanized steel sheets produced were polished with a grindstone, and the developed unevenness was visually observed.
It was rated on a scale. The results are shown in Tables 2 to 4. ◎ indicates no surface unevenness, は indicates some slight surface unevenness, △ indicates some surface unevenness, and × indicates the entire surface unevenness.

A disc specimen having a diameter of 60 mm was punched from each of the manufactured alloyed hot-dip galvanized steel sheets, and the specimen was subjected to a cylindrical drawing with a punch diameter of 30 mm and a die shoulder radius of 3R. A tape peeling test was performed on the portion. Tables 2 to 4 also show the results of examining the powdering resistance by visually evaluating the degree of the peeling in four stages. ◎ shows almost no peeling. In the case of ○, slight peeling was observed. Δ indicates that slight peeling was observed on the entire surface. × indicates that significant peeling was observed on the entire surface.

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

As can be seen from Tables 2 to 4, when the heating rate is less than 20 ° C./s, the alloying unevenness becomes remarkable even if other conditions are appropriate. Further, even when the heating rate is 20 ° C./s or more, when the maximum attainment rate is less than 520 ° C., the alloying unevenness becomes remarkable. When the maximum attainment temperature exceeds 600 ° C., the alloying unevenness is prevented. Powdering resistance decreases.

[0041]

As described above, the method for producing an alloyed hot-dip galvanized steel sheet according to the present invention performs a proper high-speed heating in the process of increasing the temperature of the alloying treatment, so that the surface of the steel sheet has a characteristic characteristic of high-P steel. Even if there is non-uniform P concentration, uneven alloying due to the non-uniformity can be prevented. Therefore, a high-strength, high-quality alloyed hot-dip galvanized steel sheet can be economically manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the mechanism of occurrence of alloying unevenness.

Claims (1)

(57) [Claims] 1. A high-P steel sheet having a P content of 0.025 wt% or more in steel is hot-dip Zn-plated and then subjected to an alloying treatment.
A method for producing an alloyed hot-dip galvanized steel sheet, wherein the steel sheet is rapidly heated at a heating rate of at least ℃ / s, and the maximum temperature thereof is set to 520 to 600 ° C.