JP2574011B2 - Manufacturing method of galvannealed steel sheet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet used for automobiles, home electric appliances, building materials, etc., and particularly to a plated steel sheet having a plating film excellent in workability. Things.

[Prior art] Galvanized steel sheet is widely used as an inexpensive material with excellent corrosion resistance and strength. However, in order to improve durability in many parts of automobiles, galvanized steel sheets are more numerous than other surface-treated steel sheets. It is used. For these surface-treated steel sheets, various properties such as surface properties, coating performance, weldability, and workability are required. Among them, in recent years,
There is an increasing demand for corrosion resistance, especially for pitting corrosion resistance of the base steel sheet, and additionally for workability.

There are two methods of mass production of galvanized steel sheet: electroplating and hot-dip galvanizing.Electroplating is a low-temperature treatment, so there is no phase change due to heat and component control is easy. To do so, the processing time must be increased.

On the other hand, in the hot-dip plating method, the amount of adhesion can be easily increased without increasing the processing time, and by performing a heat treatment after plating, the Fe-Zn alloy plating can be easily formed. In order to cope with the pitting resistance described above, it is indispensable to increase the amount of galvanized coating, and from this viewpoint, a galvanized steel sheet that can easily secure the coated amount and is economical is promising, At present, there is another dissatisfaction with workability, and this solution is strongly desired.

When a hot-dip galvanized steel sheet is used for automobiles for the purpose of high corrosion resistance, a coating is applied on the plating film, and the corrosion resistance after coating is evaluated. The plating film made of an iron-zinc alloy has better adhesion to the coating film than zinc, and the corrosion resistance after painting is improved. The most problematic in the workability is the powdering resistance. This is a phenomenon in which the plating film becomes powdery and falls off during press forming, and a 含有 phase (Fe ₃ Zn ₁₀ , Fe ₂₀ to 28 wt%) having a high iron content is generated in the plating film, which is hardened. It is caused by brittleness. That is, alloy plating having a high iron content is required from the viewpoint of corrosion resistance after painting, and alloy plating having a low iron content is required from the viewpoint of powdering resistance.

Conventionally, alloyed hot-dip galvanized steel sheets used for automobiles are subjected to alloying treatment until the average iron content of all coating films reaches around 10 wt% after hot-dip coating in order to satisfy both characteristics from the above viewpoints. And improved corrosion resistance, especially after coating. That is, after the steel strip is continuously subjected to pretreatment (including heat treatment) to adjust the material, the steel strip is immersed in a plating bath in which zinc is melted, plated, and subsequently, the plated steel strip is placed in an alloying furnace. The alloy is formed by rapidly raising the temperature from 500 ° C. to 700 ° C. and holding it for a short time (10 to 30 seconds). The reason why the average iron content is set to about 10% is to secure a certain degree of uniformity of the iron content in the plane direction. For this reason, the alloying conditions are excessive with respect to the powdering resistance, which is disadvantageous. That is, since the alloyed hot-dip galvanized steel sheet made in this way is heated to a high temperature by a rapid temperature rise, the iron content in the plating film tends to vary from place to place, in order to secure the iron content of the surface layer. However, there is a problem that the iron content at the interface with the steel substrate is increased, the growth of the Γ phase is inevitable, and thermal stress is generated in the plating film by high-temperature treatment and rapid cooling.

On the other hand, a method has been proposed in which the alloying treatment is divided into primary and secondary processes. For example, Japanese Patent Publication No. 59-14541
In No. 1, in the temporary heating, rapid heating and high temperature heating to re-melt the Zn plating film is performed to obtain the smoothness of the plating film. In this heating, the iron content remains in the low range of 2.2 to 5.5 wt%, and in accordance with the result of the primary heating, the secondary heating is performed at a low temperature equal to or lower than the melting point of zinc, and the iron content is reduced to 6%. It should be within the range of ~ 13wt%. It discloses that this method provides an alloyed hot-dip galvanized film having a smooth surface, excellent appearance, and no peeling or powdering during processing.

[Problems to be Solved by the Invention] However, in Japanese Patent Publication No. 59-14541, in order to obtain the smoothness of the plating surface, primary heating is performed by rapid heating and high temperature heating to re-melt the galvanized film. The heating condition in this case is that iron is not diffused to the plating film surface, and a part of η phase (pure Zn) is mixed. In addition, since rapid heating and high-temperature heating are performed, the formation of a partial Fe-Zn alloy is inevitable, the growth of the alloy layer is non-uniform, and particularly when the coating weight is 90 g / m ² or less, The portion where the alloy layer has grown up to the surface and the portion where the η phase remains are mixed, resulting in a so-called uneven burning state. When alloying by secondary heating is performed in this state, the non-uniformity of the primary heating remains as a history, and, for example, in the η-phase remaining portion, the other portion is grayish even after the secondary heating, whereas the portion is whitish. Has become. In order to solve the non-uniformity, it is disclosed that a secondary heating alloying treatment in a batch refining furnace is performed in response to a difference in the degree of alloying due to the primary heating. The alloying process is a complicated process, and it is extremely difficult to control the uniformity over the entire width of the sheet.

In order to solve this problem, the present invention has been made, and an object of the present invention is to provide a method for reliably and easily manufacturing a plated steel sheet that satisfies powdering resistance in addition to corrosion resistance.

[Means and Actions for Solving the Problems] To achieve this object, a steel strip that has been subjected to a normal pretreatment is prepared by converting Al: 0.05 wt% to 0.3 wt% and Pb: 0.20 wt%.
% Or less and continuously immersed in the molten zinc plating bath containing other inevitable elements, after galvanized sided per 30 g / m ² or more 90 g / m ² or less of adhesion amount range, continuously Alloy Introduced into a chemical treatment furnace, alloyed until the iron diffused to the plating film surface, and then, in a non-oxidizing or reducing atmosphere, the heating time was plotted on the horizontal axis and the heating temperature was plotted on the vertical axis. A '(4 hours, 320 ° C), b (20 seconds, 3
Alloying hot-dip galvanizing characterized by performing reheating treatment within a range surrounded by a line connecting points of 20 ° C), c (2 hours, 250 ° C) and d (15 hours, 250 ° C) This is a method for producing a steel sheet, and as one of the methods for performing the reheating treatment, is a method for producing an alloyed hot-dip galvanized steel sheet using a batch type refining furnace.

The above means, including the operation thereof, will be described in detail below.

First, the steel strip for plating may be a cold-rolled steel strip or a hot-rolled steel strip, or may be subjected to an annealing treatment together with a surface adjustment as a normal pretreatment.

Usually, about 0.2 wt% of Al is added to the hot-dip galvanizing bath for suppressing the reaction of the Fe-Zn alloy and smoothing the plated surface, and contains Pb for adjusting the spangle. Of these, Al is related to the Fe-Zn alloying reaction. If it is less than 0.05 wt%, the Fe-Zn alloy phase is partially and non-uniformly formed immediately after immersion in the plating bath. However, if the content exceeds 0.3 wt%, the Fe-Zn alloying reaction is significantly suppressed macroscopically. Is partially destroyed, which results in the formation of a non-uniform alloy phase. In order to alloy to the surface of the plating film as described later without significantly changing the production conditions (line speed, plating bath temperature, alloying furnace temperature, etc.) in the current continuous hot-dip galvanizing equipment, the Al concentration Is 0.1 ~
0.2 wt% is most desirable. Although Pb does not directly participate in the alloying reaction, a large amount of Pb lowers the powdering resistance, so it must be limited to 0.2 wt% or less. Other unavoidable elements may be included because they have a small amount and do not affect alloying.

In the case of galvannealed steel sheets, most of the corrosion resistance is determined by the coating weight and the iron content in the film.
g / m ² or more is required, but attaching more than 90 g / m ² per side not only results in excessive quality, but also requires a long Will be reduced.

Subsequent to plating adhesion, rapid heating and high-temperature heating
-Zn alloying treatment is performed, but this treatment condition is a condition in which iron diffuses to the plating film surface and does not cause excessive alloying, and compared with the alloying condition of conventional alloyed hot-dip galvanizing.
Either the heating time is slightly shorter or the heating temperature is slightly lower. The alloying up to the surface of the plating film is carried out in the case of rapid heating and high-temperature heating, since the alloying reaction partially proceeds from the interface between the plating film and the steel base material toward the surface in the early stage of alloying. When cooled, a very uneven film is formed. When the alloying reaches the surface, the alloying reaction toward the surface stops here. Therefore, if the alloying is advanced until the diffusion of iron in the slowest part of the reaction reaches the surface, the alloying state becomes uniform. Further, if alloying is excessively promoted, the iron content in the plating film increases, and the Γ phase grows and impairs the powdering resistance. The iron content of the alloyed plating film obtained by this alloying treatment is preferably in the range of 7 to 15 wt%. If the content is less than 7 wt%, the subsequent secondary heating is not preferable because the non-uniformity of the primary heating remains as it is.
%, The workability of the plating layer is already inferior in the primary heating, and the improvement cannot be achieved by the secondary heating.

Next, reheating is carried out at a low temperature, and by this treatment, the iron content is made uniform and thermal distortion is removed. That is, in the alloying process, since the temperature is rapidly raised and heated, the iron content is easily distributed in the depth direction of the alloyed plating film. This is a distribution in which the iron content is low at the surface layer and increases toward the interface with the steel base. Slow heat treatment at a relatively low temperature promotes the diffusion of atoms in the plating film, while the diffusion of new Fe atoms from the steel substrate hardly occurs. Instead, the plating film becomes a homogeneous film composed of the ζ phase and the δ ₁ phase. In addition, the Fe content becomes uniform even in the surface direction of the film. Further, thermal strain in the alloy phase caused by rapid temperature rise and high temperature heating is removed, and not only powdering resistance but also overall workability is improved. When the processing conditions were examined, the results shown in FIG. 1 were obtained. In the figure, the horizontal axis represents the heating time, and the vertical axis represents the heating temperature. When the conditions range for good powdering resistance are shown, points a ′ (4 hours, 320 ° C.), b (20 seconds, 320 ° C.), This is a range surrounded by a line connecting c (2 hours, 250 ° C.) and d (15 hours, 250 ° C.). If the heating temperature is lower than 250 ° C., the effect of promoting the diffusion of atoms is extremely low and the retention time is too long, which is not industrially suitable.

On the other hand, if the temperature exceeds 320 ° C., diffusion of Fe atoms from the steel base is promoted, and on the contrary, unevenness of the alloy phase and thermal distortion are promoted. If the heating time is less than 20 seconds, diffusion of atoms in the plating film cannot be sufficiently performed. In principle, it can be said that it is better to reduce the temperature and take a long time to process, but spending more than 15 hours is not industrially suitable and the upper limit of time is set to 15 hours in relation to the lower limit temperature Things. Furthermore, considering the industrial properties, a safe and preferable range is a heating temperature of 280 ° C.
° C and a heating time of 1 minute to 5 hours. This reheating treatment may be performed using a batch annealing furnace or a continuous heating furnace. In such a reheating treatment, the iron content of the entire plating film hardly increases and is at most 1%.
Unlike the alloying treatment, the effect of the treatment is not limited to the above-described homogenization and thermal strain removal.

[Example] The degree of alloying was examined for eight alloyed hot-dip galvanized steel sheets that were processed using two types of steel sheets while changing the hot-dip galvanizing conditions, alloying treatment conditions, and reheating conditions, and a powdering test was performed. And evaluated. For comparison, eight cases (comparative example) and three cases (conventional example) according to the prior art which were processed under conditions outside the scope of the present invention were similarly examined. Details of the conditions are as follows.

The steel plate used is a cold-rolled steel plate with a thickness of 0.8 mm, a low-carbon Al-killed steel (material A) for thin sheets that is widely used, and an ultra-low-carbon titanium-containing steel that is said to be easily powdered for high-processing applications (Material B). Each component is shown in Table 1.

Hot-dip galvanizing is performed in a continuous plating facility equipped with a non-oxidizing furnace and a reduction heating furnace, and the amount of coating is adjusted by a gas throttle device provided immediately after the plating bath. Until alloyed. In Examples and Conventional Examples 2 and 3, the alloyed hot-dip galvanized steel strip produced in this manner was reheated in an open coil state.

 The iron content in the plating film was determined by chemical analysis.

Powdering resistance, after bending 90 degrees with a radius of curvature of 2 mm,
Paste the adhesive tape inside the bend, peel it off, visually observe the situation where the powder adhered to this adhesive tape,
The evaluation was given with a score. The evaluation criteria are as follows: 1; no adhesion, 2; extremely slight adhesion, 3; slight adhesion, 4; slight adhesion, 5; considerable adhesion.

Table 2 shows the processing conditions and results of each of these examples.

In the example, the powdering resistance was not inferior even in the material B, and extremely slight powdering was observed on the front side of the example 8 in which the lower limit adhesion amount was 30 g / m ^{2 in} the thickness difference plating.
There is no problem in practical use. The alloying degree is also 8wt% iron content.
To 13 wt% to ensure sufficient corrosion resistance after painting. In Examples, all alloyed hot-dip galvanized steel sheets have both corrosion resistance and powdering resistance.

On the other hand, in the comparative example in which the reheating treatment was not performed, even though the iron content was not high, powdering occurred, albeit slightly. This is considered to be due to the effect of removing the thermal strain by reheating. Further, Example 5 and Comparative Example 7
As is clear from the comparison with Example 6 and the comparison between Example 6 and Comparative Example 8, the reheating is performed in a reducing atmosphere so that the iron content in the plating film is easily increased as compared with the case where the reheating is performed in the air atmosphere. It has a special effect of being able to do things.

In the conventional example, alloying is not performed up to the surface of the plating film by continuous heating, but alloying is terminated by reheating. Among the three identical processing conditions, there are those in which powdering does not occur at all and those in which it occurs only slightly, indicating the difficulty of actual operation.

Next, the iron content distribution of the plating film according to the present invention was examined. Here, the iron content was examined at 200 mm intervals in the width direction of the galvannealed coil of Example 5 (width 800 mm). In this case, comparison was made with Conventional Example 2. This result is
Shown in the figure. In the figure, the abscissa represents the distance from the left end of the coil, the ordinate represents the iron content, ○ represents plotting of Conventional Example 5, and ● represents plotting of Conventional Example 2.

As is clear from the figure, the iron content of Example 5 averaged 9.7.
% And all measured points were distributed between 9.5 and 9.9%. On the other hand, the iron content of Conventional Example 5 was 9.7% on average, and all the measurement points were distributed between 9.0 and 10.2%, and the dispersion was large.

[Effects of the Invention] According to the present invention, alloying up to the surface of the plating film is performed by continuous heating after hot-dip galvanizing, and then reheated at a low temperature to homogenize the alloy and remove thermal strain. It is possible to reliably and easily manufacture an alloyed hot-dip galvanized steel sheet having good workability, especially powdering resistance, in addition to corrosion resistance after painting. The effect of the present invention in which such a surface-treated steel sheet can be manufactured without going through a complicated process is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between heat treatment conditions and proper characteristics for explaining the main part of the present invention, and FIG. 2 is a diagram showing the distribution of iron content in one embodiment of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akihiko Nakamura 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. Jury President Shunji Ogishima Judge Judge Mitsuo Karato Judge Yoshihisa Jahita (56) References Special 1983-34168 (JP, A)

Claims (2)

(57) [Claims] 1. An ordinary pretreated steel strip is made of Al: 0.05 wt%.
More than 0.3 wt% or less and Pb: 0.20 wt% or less and continuously immersed in a hot dip galvanizing bath containing other unavoidable elements,
After galvanizing with a coating amount of 30 g / m ² or more and 90 g / m ² or less per side, it is continuously introduced into an alloying furnace, alloyed until the iron diffuses to the surface of the plating film, and then non-alloyed In an oxidizing or reducing atmosphere, the heating time is plotted on the horizontal axis and the heating temperature is plotted on the vertical axis.
(4 hours, 320 ° C), b (20 seconds, 320 ° C), c (2 hours,
A method for producing an alloyed hot-dip galvanized steel sheet, wherein reheating is performed within a range surrounded by a line connecting points (250 ° C.) and d (15 hours, 250 ° C.). 2. The method for producing a galvannealed steel sheet according to claim 1, wherein the reheating treatment is performed using a batch annealing furnace.