JPH0353054A - Production of alloying hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To produce an alloying hot dip galvanized steel sheet excellent in surface characteristics and coating suitability by plating the surface of a dead- soft carbon steel sheet with trace amounts of Ni and subjecting the above steel sheet to hot dip galvanizing and then to alloying treatment. CONSTITUTION:After the surface of a dead-soft carbon steel sheet containing <0.005wt.% C is plated with trace amounts of Ni by 20-70mg/m<2>, the steel sheet is immersed into a hot dip galvanizing bath to undergo hot dip galvanizing and subjected to alloying treatment to form a Zn-Fe alloyed plating layer, or, the steel sheet is subjected to Ni-Fe alloy plating, in which Ni content and coating weight are regulated, respectively, to 50-200mg/m<2> and 70-1000mg/m<2>, and further to hot dip galvanizing and is then alloyed by means of heating to form a Zn-Fe alloyed plating layer. By this method, the steel sheet suitable for use in automobile sheath and excellent in plane smoothness and coating suitability can be obtained.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides an alloyed hot-dip galvanized steel sheet with excellent surface smoothness, particularly suitable as a galvanized steel sheet for automobile exteriors.
This article relates to a method for manufacturing alloyed hot-dip galvanized steel sheets with excellent surface properties. (Prior art) Alloyed hot-dip galvanized steel sheet (hereinafter referred to as OA) is a plating process in which a steel sheet is hot-dip galvanized and then heat treated to cause mutual diffusion of iron and zinc to form an alloy layer. m
Fi, has relatively excellent weldability and corrosion resistance after painting,
It has been used in a wider range of fields than before. Particularly in recent years, it has been used as steel sheets for automobile bodies, especially as exterior steel sheets. The steel type used for automobile bodies must be a steel type with excellent deep drawing and formability, but when hot-dip galvanizing is performed, if ordinary low-carbon An-killed steel is used, the manufacturing process As the material undergoes rapid cooling, the supersaturated carbon remains in solid solution and hardens. Therefore, ultra-low carbon steels are generally used, particularly ultra-low carbon Ti steels and ultra-low carbon Ti-Nb steels in which carbon is fixed with Ti and/or Nb. Due to the high cleanliness of the ferrite grain boundaries of this type of ultra-low carbon type tl[Lt, the Fe-Zn alloying reaction occurs more easily at the grain boundaries than inside the ferrite grains, and the precipitate of the alloy layer is reduced during the alloying process. tends to become uneven. Plating amount is Znl0
If it is 0g/a+'' or more, the FeZn alloying reaction within the ferrite grains will also progress by the time the alloying reaction at the grain boundary is completed, so that eventually, as shown in Figure 3(a), A smooth plating layer 2 is obtained, but the amount of Zn deposited is 80 g/+
"In the following cases, the plating layer 2 tends to have many irregularities as shown in FIG.
/m'' or less, as shown in Figure 3(c), recesses (craters) are very likely to occur.If such defects exist, coating defects are likely to occur in electrodeposition coating, and Even with electrostatic powder coating, defects are likely to occur due to moisture adsorbed to some of the craters in the plating layer during baking.Therefore, galvanized steel sheets, especially those used for the exterior of automobile bodies, are susceptible to such irregularities. It is desirable to have a plating layer with good smoothness and less turbidity.

As a method of suppressing the non-uniformity of the alloying reaction inside the ferrite grains and at the grain boundaries, A
It is possible to set the concentration of 1 to 0.08% or less in terms of effective Al'4 degrees (analytical value of Al - analytical value of Fe), but
When the iron is reduced, the elution of Fe from the steel sheet in the plating bath increases significantly, and a large amount of bottom dross mainly composed of δ1 phase (FeZnt) is generated in the plating bath, which tends to float in the bath. A problem arises in that surface defects due to adhesion of particles increase. (Problems to be Solved by the Invention) As described above, in the conventional GA manufacturing method, there are many crater-like defects on the surface of the plating layer, which has a problem in that it adversely affects the finished state after painting. In particular, the above-mentioned defects are likely to occur in IP (Interstitial Free) steel, which is an ultra-low carbon steel that is often used for automobile bodies and contains Ti or Nb.

The present invention has been made to solve the above-mentioned problems, and in particular, it is an object of the present invention to provide a method for producing GA having a surface quality suitable for use as a plated steel sheet for automobile exteriors.

(Means for Solving the Problems) The present inventors plated a tempered steel sheet with a very small amount of Ni, and then applied hot-dip galvanizing to the surface of the plated layer after the above-mentioned alloying treatment. We discovered that craters can be significantly reduced. In the production of hot-dip galvanized steel sheets, a method of preliminary plating with Ni, Fe, Cu, Go, etc. before hot-dip galvanizing is already known, and it is difficult to improve the adhesion of the plating layer of non-alloyed material. It is applied for the purpose of improving the plating properties of plated steel materials and further promoting the alloying process. For example, when manufacturing hot-dip galvanized steel sheets,
6176 discloses a method of plating Ni and/or Co to a thickness of 0.01 to 1.0 μm before hot-dip galvanizing, and JP-A-58-120771 discloses a method of plating Ni and/or Co to a thickness of 0.01 to 1.0 μm before hot-dip galvanizing. A method for plating Co with a thickness of 0.05 μm or more is disclosed. However, as a result of studying the above method, we found that Fe and Zn during hot-dip galvanizing
It has been found that the Fe-Zn alloy layer grows significantly and is difficult to thin due to the accelerated reaction with the Fe-Zn alloy layer. The present inventors investigated from the viewpoint of imparting smoothness to the surface of the plating layer after alloying treatment, and found that the smoothness of the surface of the plating layer could be improved by applying a very small amount of Ni plating in advance, as described above. Much improved, and furthermore,
By applying Ni-Fe alloy plating and controlling the amount of Ni-Fe plating within a specified range, the smoothness of the GA plating surface can be ensured without significantly promoting the Pa-Zn reaction during hot-dip galvanizing. I confirmed that it can be done.

The present invention was made based on the above knowledge, and the following
The main points are the methods from to. ■ A method for manufacturing an alloyed hot-dip galvanized steel sheet, characterized in that the steel sheet is pre-plated with Ni at 20 to 70 mg/m'', and then heat-alloyed after hot-dip galvanizing (hereinafter referred to as the method described in ``Clause (1)''). invention). ■ The steel plate contains Ni equivalent to 50 to 200 mg/m" in the plating layer, and the plating coverage is 70 to 1000 m.
A method for manufacturing an alloyed hot-dip galvanized steel sheet (hereinafter referred to as "Second
). ■ The steel plate contains 1o. oos
% (hereinafter, % means weight %) of substantially solid t
The method for manufacturing the alloyed hot-dip galvanized steel sheet described in the above (1) or (2), which is a steel sheet in which 8C does not exist (hereinafter referred to as "No. (3)
). (Function) As described above, the method of the present invention is characterized in that the surface of the steel sheet is coated in advance with a very small amount of Ni plating or Ni-Fe alloy plating, and then hot-dip galvanizing and alloying treatment are carried out. The reasons for limiting the amount of plating deposited are explained below. In the invention of item (1), the amount of Ni plating deposited is 20
If it is less than 1 mg/1, the amount of plating is small, so the coverage is low, and no effect on the smoothness of the surface of the plating layer can be obtained. On the other hand, if the amount of Ni plating exceeds 70 mg/m'', the Pe-Zn reaction during hot-dip galvanizing becomes significant, making thin plating difficult.
The invention of item (2) is an invention made by further improving the invention of item (1) from the viewpoint of improving the coverage. If the amount of alloy plating deposited is less than 70s+g/m'', the coverage is insufficient and there is no effect superior to the invention of item (1). On the other hand, 1000
If the Ni-Fe alloy plating amount exceeds 70 to 1000 mg/m, the Fe-Zn reaction during hot-dip galvanizing will be intense, making thinning difficult and increasing the amount of dross generated during operation. ”. In addition, the above-mentioned Ni
- Even if the amount of Fe alloy plating is within the specified range, Ni
-Ni contained in the Fe alloy plating layer is 501mg/
If it is less than 200 mg/s, there is no effect, and if it exceeds 200 mg/s, the Fe-Zn reaction will be significant and thinning will be difficult, as in the case where the amount of Ni-Fe alloy plating exceeds the above upper limit. Therefore, the Ni contained in the Ni-Fe alloy plating layer was set at 50 to 200+wg/m''.In applying the invention of item (+) and the invention of item (2), regarding the strength of the steel plate, Not particularly limited.

However, the invention of item (1) or the invention of item (2) suppresses the C content to 0.005% or less, and furthermore
When applied to IF steel containing substantially no solid solution C, a great effect can be obtained (invention of item (3)).
This is because, as mentioned above, concave defects on the surface of the plating layer are likely to occur in IFw4. IF like this
To give an example of steel, C: 0.005% or less and the following formula Ti(X) ≧3.99C (X) +1.495 (
X) +2.42N (χ) It is a steel that contains nickel so as to satisfy (χ). In carrying out the method of the present invention, the steel plate is subjected to pretreatment such as alkaline cleaning. In order to improve the uniformity of the plating, it is desirable to perform an additional pickling treatment. When the Ni plating and Ni-Pe alloy plating described above are performed by electroplating, the amount of plating deposited can be easily controlled. A Ni-plated or Xi-Fe alloy-plated steel sheet is heated to the recrystallization temperature of the steel sheet in a reducing atmosphere containing hydrogen gas, subjected to reduction annealing, and then hot-dip galvanized. For this heating, a non-oxidizing furnace, etc. commonly used at present can be used, and Ni plating or Ni-Fe
The effects of the present invention are not impaired even if the alloy-plated fil plate is subjected to weak oxidation in a non-oxidizing furnace or the like. The method of the present invention is also effective when performing pretreatment using Franks, such as the Tuck-Nortman method, but in this case, even if the amount of Ni plating or Ni-Fe alloy plating is small, F
Since alloying of e-Zn is promoted, it is desirable to suppress the upper limit of the coating amount to about 60% of the above-mentioned upper limit. Pb, Mg, Mn, Sn, S in galvanizing bath
If the total content of b is 0.3% or less, there is no effect on the effect. Usually 0.05-0.2% A is present in the galvanizing bath.
ffi is added, but the Al concentration in the bath is 0.08
% or less, crater-like defects rarely occur on the surface of the plating layer. Therefore, the effective An! in the plating bath! The present invention is particularly effective when the il1 degree is 0.1 to 0.2%. The appropriate temperature for the material during alloying treatment is 470 to 600°C, which is conventionally practiced. Ni plating or Ni-Fe alloy plating does not necessarily need to be applied to both sides of the steel plate, and may be applied to only one side if necessary. for example,
The amount of plating deposited is 30g/+” on the exterior surface and 60g/+ on the inner surface.
GA60/30, which is designated as ``i'', is currently used in some areas, but this crater-like defect is particularly likely to occur because the amount of adhesion is low on this exterior surface.In such cases, only the exterior surface is It is also possible to apply the method of the present invention. (Example) C: 0.003%, St: tr., Mn: O.1
6%, P: 0.008%, S: Q. 008%, So
l. Ai: 0.23%, Ti: 0.038%, Nb
: 0.008%, and the remainder consists of Fe and unavoidable impurities.
A fully hardened steel plate (80mm) was used as the test material, and this test material was cut into 100 x 250+am, electrolytically degreased in an alkaline aqueous solution of sodium carbonate and sodium hydroxide, and then pickled with hydrochloric acid. N by the method shown in the table
i plating and Nt-Fe alloy plating were applied. In addition,
The plating bath &lf2 and current density for Ni-Fe alloy plating were adjusted as appropriate within the ranges shown in the table. This steel plate was set in a hot-dip plating simulator with a vertical atmosphere furnace, annealed at 850°C for 45 seconds in a 26% lh 1Nz atmosphere, cooled to 500'C, and then heated to 0.1
1% effective AI! ．． Zn plating bath (460'C) containing
2 seconds, and use a wiper to reduce the coating weight to 60%.
g/m''. Furthermore, alloying treatment was performed at 500°C for 15 seconds. Small pieces were taken from the obtained plated steel sheet, embedded in resin, and plated in the cross-sectional direction perpendicular to the rolling direction. The unevenness of the layer was observed using a scanning electron microscope. (Hereinafter in the margin) Table 1 The results are shown in Figures 1 and 2. Figure 1 shows the case where Ni plating was performed, and the horizontal axis shows the plating. The left vertical axis represents the amount of Ni deposited on the surface, the left vertical axis represents the smoothness of the plated surface, and the right vertical axis represents the reaction Re amount during hot-dip galvanizing. was evaluated on a five-point scale with ○ for normal, Δ for poor, × for extremely poor, and XX for extremely poor. In this case, good smoothness (marked with ○) was observed, and the cross section of the plating layer was equivalent to or similar to that shown in FIG. If it is within the range, during hot-dip galvanizing or reaction Fe
The amount also remains at a level of less than 1.5 g/m'', and Fe
- It was confirmed that the problem of abnormally accelerated Zn reaction did not occur. Figure 2 shows the case where Ni-Fe alloy plating is performed, and the horizontal axis represents the amount of Ni contained in the Ni-Fe alloy plating layer (Ni adhesion it), and the vertical axis represents the amount of Ni-Fe alloy plating deposited. ◎, ○, Δ, and × in the figure represent the smoothness of the plated surface, and are used according to the same evaluation criteria as in Figure 1 above. The area surrounded by the solid line in the figure shows the range determined by the method of the present invention, and it can be seen that within this range, the smoothness of the plated surface is extremely good (marked with ◎). The area surrounded by the broken line is the area where the reaction fail during hot-dip galvanizing is 1.5 g/m2 or more, and although the smoothness is extremely good, the growth of the Fe-Ni alloy layer is excessive and thinning is difficult. This is not desirable. From the comparison between Figure 1 and Figure 2, it is clear that Ni-Fe alloy plating is better for attaching Ni to the steel plate surface.
It can be seen that the smoothness of the plated surface after alloying treatment is good. Furthermore, using the same material as the test material used in this example, a plated steel plate was produced by a method based on the conventionally known Japanese Patent Application Laid-Open No. 57-76176 or Japanese Patent Application Laid-Open No. 58-120771. Although the plated surface after the alloying treatment had a color tone different from that of normal GA, the plated steel sheet produced by the method of the present invention showed the same appearance as normal material, and it was confirmed that there were no problems. (Effect of the invention) When manufacturing alloyed hot-dip galvanized mFi, by applying the method of the present invention in which a very small amount of N1 is pre-plated on a steel plate and then hot-dip galvanizing and alloying treatment are applied, the surface texture can be improved. It is possible to produce coated steel sheets with excellent properties and no coating defects. This method is particularly effective when IFjl containing Ti and Nb is used as the material to be plated, and is extremely effective as a method for producing GA with a surface quality suitable for plated steel sheets for automobile exteriors.

[Brief explanation of drawings]

Figure 1 shows the N
A graph showing the relationship between the adhesion amount, the smoothness of the plated surface, and the amount of reacted Fe during hot-dip galvanizing. FIG. 3 is a graph showing the relationship between the amount of plating deposited and the smoothness of the plating surface, and FIG. 3 is a schematic diagram showing a cross section of the GA plating layer.

Claims (3)

[Claims] (1) Apply 20 to 70 mg/m^2 of N to the steel plate in advance.
A method for producing an alloyed hot-dip galvanized steel sheet, which comprises applying i-plating, hot-dip galvanizing, and then heat-alloying treatment. (2) Ni- equivalent to 50 to 200 mg/m^2 of Ni is pre-contained in the plating layer of the steel plate, and the Ni-
A method for producing an alloyed hot-dip galvanized steel sheet, which comprises applying Fe alloy plating, hot-dip galvanizing, and then heat-alloying treatment. (3) Claim (1) or (
2) The method for producing an alloyed hot-dip galvanized steel sheet.