KR100495620B1 - Manufacturing method of galvannealed steel sheet for electric application and automotive - Google Patents

Abstract

The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet for automotive exterior having excellent surface appearance and cracking resistance.

In the present invention, a cold rolled steel sheet, which is a plating material, is rolled to an average roughness of 1.0 μm or less and a peak number (PPI) of 350 or more using a discharge-dall roll or an electron beam-dall roll, followed by alloyed hot dip galvanizing. Provided is a method for manufacturing a hot dip galvanized steel sheet.

According to the present invention, a GA material excellent in surface appearance and paintability can be easily produced. In particular, since there is no surface damage compared to the existing method of performing temper rolling under high pressure after alloying treatment, it has the effect of preventing the occurrence of powdering defects during the processing and the black spot of the plating layer turning black.

Description

Manufacturing method of galvannealed steel sheet for electric application and automotive}

The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet for automotive exterior having excellent surface appearance and anti-cratering resistance. More specifically, optimization of surface average roughness (Roughness average, Ra) and peak count (peak number, peak number / inch), which are roughness components of a cold rolled steel sheet, which is a plating material, causes plating surface cracking and local overalloy. The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet for home appliances and automobile exteriors, characterized by dramatically improving the smoothness and the cracking resistance of the surface appearance of the alloyed hot-dip galvanized steel sheet by preventing outburst reaction.

Galvanized galvanized steel sheet (hereinafter referred to as GA) is obtained by hot-dip galvanizing of cold rolled steel sheet, followed by continuous alloying treatment, whereby the hot-dip galvanized layer and the steel sheet are mutually diffused and reacted with η phase, ζ phase, δ phase, A Fe-Zn alloy plated steel sheet made of a Fe-Zn-based intermetallic compound such as Γ phase. GA material has excellent spot weldability, corrosion resistance and corrosion resistance after painting compared to hot-dip galvanized steel sheet, and its demand is increasing for building materials, home appliances, and automobiles. As surface size increases, excellent surface quality is required.

In particular, since GA materials for home appliances and automobile exterior plates are used as the bases of painted steel sheets, surface irregularities have a close relationship with paint selectivity, and thus, a beautiful surface appearance without such surface defects is required.

It is known that the surface unevenness of the GA material is mainly due to the alloying reaction being nucleated first at the grain boundaries of the plating material and the alloying proceeds. That is, in the early stage of alloying, flower-shaped outburst tissue is formed along the grain boundary of the plating material, and it grows faster by inhaling the molten zinc in the liquid phase in the adjacent area. It is known that the surface of the GA material exhibits fine concavities and convexities due to the generation of craters.

As such, when creering, which is the surface unevenness of the GA material, is caused, the portion of the cracker has a small amount of plating adhesion, which causes local corrosion resistance degradation.

Therefore, as a method for improving the cruttering and coating selectivity of the surface of the GA material, such as: 1) temper rolling after alloying heat treatment such as Japanese Patent Application Laid-Open No. 5-269505 and Japanese Patent Application Laid-open No. 4-285147 2) A method of controlling roughness of alloyed hot-dip galvanized steel sheet and 2) a method of adjusting plating conditions and alloying conditions such as steel sheet inlet temperature, Al concentration of plating bath, alloying temperature and alloying time as in Korean Patent Application No. 1999-57669. This occupies most of it.

However, in the method of 1), in order to remove the microscopic irregularities on the surface of the GA material, it is necessary to perform temper rolling with high rolling force, but during the high pressure temper rolling, cracks are generated from the Γ phase, which is the weakest among the alloy phases of the GA material, at high pressure. This dropping powdering occurs, which is undesirable.

In addition, in the above method 2), the alloying reaction occurs at the grain boundary of the thermodynamically unstable plating material, and thus, it is not possible to essentially eliminate the occurrence of the cracker, and also the alloying degree in consideration of the properties of the GA material such as powdering or flaking properties. Since it is limited to a constant value (9-12%), since the alloying degree cannot be arbitrarily adjusted, it is not preferable.

Therefore, the present invention, as a result of studying the method for removing the cutter, which causes the surface appearance of the GA material and inhibiting the coating sensitization, the surface average roughness and peak number (peak count, peak number) as the roughness component of the cold rolled steel sheet / Inch) by artificially adjusting, so that the outburst reaction that causes the cruttering occurs at the same time not only at the grain boundary of the plating material but also at the same time in the mouth. It is an object of the present invention to dramatically improve the surface smoothness and cracking resistance of an alloyed hot-dip galvanized steel sheet, characterized in that the alloying reaction occurs uniformly without.

The present invention for achieving the above object is to roll the cold rolled steel sheet of the plating material to the discharge number roll or electron beam roll roll so that the peak number (PPI) is 350 or more while the average roughness of the cold rolled steel sheet of the plating material to 1.0㎛ or less, alloying Provided is a method for producing an alloyed hot dip galvanized steel sheet for an automotive exterior, characterized by performing hot dip galvanizing.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The cruttering of alloyed hot-dip galvanized steel sheet grows faster as the outburst reaction, which is a non-uniform alloyed shape, occurs locally at the grain boundary of the plating material, and sucks and consumes the liquid zinc in the adjacent part, thereby depleting zinc. It was found to be the surface irregularities of the GA material, which appeared as crater-shaped craters formed there.

Therefore, the present inventors have analyzed in detail the effect of the surface state of the cold rolled steel plate plated material on the cruttering phenomenon, the outburst reaction that causes the cracking, as well as the grain boundaries, scratches, pits, indentations, etc. It was confirmed that the same occurred in the surface defects of the surface, and after artificially applying surface roughness by the method of roughening the final stand roll of cold rolling, it was investigated whether the occurrence of the cruttering during the alloying treatment. Among the various roughness components, the peak number (peak count, peak number / inch) and surface average roughness were found to be closely related.

That is, the peak number among the roughness components refers to the number of peaks and valleys out of a constant height (Slice level, 0.25㎛) and depth from the average line. In contrast, the roughness average (Ra) is an arithmetic mean value of the cross section separated by the center line within the average length, and means one short wavelength. Therefore, if the peak is a mountain, the average roughness is equivalent to a mountain range.

Therefore, it was confirmed that the peak actually acts as a nucleus site in contact with molten zinc, and when the peak number is small, the average roughness acts as a nucleus site. In other words, it can be seen that the cruttering of the alloyed hot-dip galvanized steel sheet can be prevented by adjusting the surface average roughness and the peak number, which are the surface roughness components of the cold rolled steel sheet.

In other words, as shown in FIG. 1 showing the change in the cruttering index according to the peak number of the plating material, when the peak number is 350 or more, the outburst reaction occurs at many nuclei sites, so that the local outburst reaction is eliminated and uniformly alloyed. It can be seen that since it is possible to prevent the occurrence of the cruttering. However, even if the peak number of the plating material is 350 or less, or the peak number is 350 or more, if the average roughness is 1.0 µm or more, the short roughness average roughness component acts as a single nucleus. Will occur.

Therefore, in the present invention, the surface average roughness and the number of peaks of the cold rolled steel sheet, which is a plating material, are respectively limited to 1.0 μm or less and 350 or more.

Since the roughness components, such as the average roughness and the number of peaks, of a cold rolled steel plate which are such plating materials change largely according to the roughness provision method of the final stand roll of cold rolling, it is necessary to control from the roll roughness provision stage.

As shown in Table 1, when cold-rolled steel sheet is used as a plating material by using a bright roll polished by a work roll of a cold rolled final stand roll, an average roughness of 0.3 μm or less is due to scratches. And because the peak number is 280 or less, the outburst reaction does not occur locally or as a whole, so the reaction rate is slow and is not completely alloyed under general alloying conditions.

In addition, the plated material, which is cold rolled with shot blast texturing Dull Roll, gives amorphous irregularities, so the average roughness increases more than the peak number due to the reduction in rolling force and roll roughness, resulting in local outburst. It was impossible to prevent lettering. In the case of Electric Discharge Texturing Dull roll, the indentation transferred to the surface of the plating material is constant and somewhat dense in the shape of crater, so that the peak number and average roughness increase proportionally by increasing the rolling force and roll roughness. Only under conditions, the outburst reaction could be suppressed locally.

In particular, when cold rolled with Electron Beam Texturing Dull roll, irregularities transferred to the surface are denser and have a uniform shape in spherical shape than discharge dall roll, so the peak number increase is much larger than the average roughness increase. As a result, the target number of peaks can be secured even at relatively low illuminance and reduction, thereby preventing local outburst reactions.

Therefore, the method of providing roughness of the cold rolled steel sheet, which is a plating material, is limited to the discharge and roll beams.

As described above, when the average roughness and the number of peaks of the cold rolled steel sheet, which is a plated material, are controlled by the discharge and roll beams, it is possible to improve the surface appearance and to prevent the cracking.

Hereinafter, embodiments of the present invention will be described.

< Example>

Steel plates with 0.04% C, 0.17% Mn, 0.01% P, 0.01% S, and 0.05% Al are applied to the finish rolls of the cold rolling process, such as bright roll, shot blast roll, discharge roll, and electron beam roll. Cold rolled to a thickness of 0.7mm to produce a cold rolled steel (full hard) having a surface roughness as shown in Table 1. Using this as a plating material, after annealing at 760 ° C. for 60 seconds, 0.135% Al, 0.01% Pb, and the balance were deposited in a plating bath component of Zn, and then excess zinc was adjusted with a gas wiping device to obtain a cross-sectional plating amount. This was 60 g / m 2, followed by an alloying treatment at 480 ° C. for 10 seconds.

In the cold rolling process, the surface average roughness and peak number were measured by the surface roughness meter. In the hot dip galvanizing process, the average roughness of the plating material after the hot dip plating was evaluated. In addition, the reactivity according to the surface roughness component of the plated material was evaluated by the iron content in the plated layer and the presence of outburst structure at the plated layer / ferrous iron interface.

In particular, in the alloyed hot dip galvanizing process, surface average roughness, alloying degree, and the like were evaluated. The surface roughness, paintability, powdering resistance, and corrosion resistance were evaluated, and the results were classified into four grades according to the following criteria. Shown in

-Grade 1 (Excellent): When the fraction of crease is less than 5%

-Grade 2 (good): In case of 5 ~ 10% fraction

-Grade 3 (Defective): When the percentage of occurrence of creter is 10 ~ 20%

-Grade 4 (very poor): When the fraction of cretter is more than 20%

As shown in Table 1, when using a cold-rolled steel sheet whose surface average roughness and peak number were adjusted by the cold rolled work roll roughness providing method proposed by the present invention as a plating material Nos. 10, 11, 13 to 17 The surface appearance was excellent with no local outburst reaction and no cracking.

On the contrary, the method of applying the cold rolled work roll roughness is outside the scope of the present invention (Comparative Examples No. 1 to 8), or the method of applying the work roll roughness satisfies the present invention, but the surface average roughness and the number of peaks are outside the scope of the present invention. In the case of (Comparative Examples No. 9 and 12), local outburst reaction and creering occur, which is not preferable.

According to the present invention for optimizing the work roll roughness providing method and roughness components of the cold-rolled steel sheet, which is a plating material in the alloying hot dip galvanizing process, it is possible to easily produce a GA material excellent in surface appearance and paint filmability. In particular, since there is no surface damage compared to the existing method of performing temper rolling under high pressure after alloying treatment, it has the effect of preventing the occurrence of powdering defects during the processing and the black spot of the plating layer turning black.

Figure 1 is a graph showing the change in the cruttering index according to the peak number of the coating material and the cold rolling roll roughness providing method.

Claims (2)

In the manufacturing process of the alloyed hot-dip galvanized steel sheet, the cold rolled steel sheet, which is a plating material, has an average roughness of 1.0 µm or less and is rolled so that the peak number (peak number / inch) is 350 or more, and the alloy is hot-dip galvanized. Method for producing alloyed hot-dip galvanized steel sheet for outer plate. The method for manufacturing an alloyed hot-dip galvanized steel sheet for an automotive exterior plate according to claim 1, wherein an electric discharge moon roll or an electron beam moon roll is applied as a final rolling work roll of the cold rolled steel sheet of the plating material.