JPH0515779B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet, and more specifically, a method for manufacturing an alloyed hot-dip galvanized steel sheet, and more specifically, a method for manufacturing an alloyed hot-dip galvanized steel sheet, in particular, a method for manufacturing an alloyed hot-dip galvanized steel sheet (hereinafter referred to as " GA steel sheet). (Prior Art) GA steel sheets are generally manufactured by continuously heat-treating hot-dip galvanized steel sheets immediately after plating. Usually, due to interdiffusion of Fe-Zn during heat treatment, the plating layer contains 8.5 to 13% Fe,
It is composed of a layered structure of intermetallic compounds. The material obtained in this way has better paintability and weldability than general galvanized steel sheets, but since the plating layer has no plastic deformability at all, it is prone to peeling of the plating layer called powdering during press processing. . Various attempts have been made to overcome the powdering phenomenon of GA steel sheets. One example is the one described in Japanese Patent Publication No. 49-4134, where the surface is η+ζ
However, this type of product has poor paintability and weldability because the η phase remains. Generally, in order to satisfy weldability and paintability, most of the surface layer of the coating layer needs to be δ ₁ phase. Another idea for improving powdering resistance is to consider the heat pattern during alloying. For example, in Japanese Patent Publication No. 59-14541, alloying heat treatment is divided into primary heating and secondary heating;
It is proposed that the subsequent heating be performed offline.
However, although this offline method has the effect of improving powdering resistance to some extent, there is no need to mention its disadvantages again, except for the effect of post-annealing. Furthermore, in JP-A No. 59-173255, the temperature is 520℃ after plating.
It is proposed below to hold the alloy for 6 seconds or more, but as there is no regulation on the final temperature, etc., such processing conditions cannot be said to be sufficient conditions, and therefore, the powdering resistance is poor. It is conceivable that a considerable number of such problems will occur, and that this will only unnecessarily increase the length of the reactor. (Problems to be Solved by the Invention) Thus, it is an object of the present invention to provide a method that eliminates the drawbacks of these conventional methods. Another object of the present invention is to provide a method for manufacturing a GA steel sheet that suppresses powdering with a relatively short alloying treatment time. (Means for Solving the Problems) In order to achieve the above object, the present inventors conducted extensive studies and discovered the following basic facts. (i) The powdering phenomenon generally becomes noticeable as the amount of Fe diffused into the coating increases. Moreover,
This is generalized in cases where the δ ₁ phase remains. (ii) The temperature of the coated steel sheet during alloying (hereinafter simply referred to as "material temperature") has an effect, and powdering is alleviated when alloying is performed at a lower material temperature. In other words, the material temperature is low, that is, the ζ phase can be formed.
It is necessary to slowly alloy the alloy at a temperature of 520°C or lower without causing overtreatment. However, in order to achieve this, an unnecessarily long alloying furnace is required. In this case, the purpose of the present invention is not achieved. Therefore, the present inventors conducted further studies based on the above-mentioned findings. It is true that in order to shorten the length of the alloying furnace, it is basically necessary to maintain the material temperature during alloying treatment on the high temperature side, but alloying at temperatures above 550°C requires powdering resistance. deteriorates significantly. Therefore, the present inventors considered incorporating the alloying treatment condition of 550°C or higher into the alloying process as one step during alloying, and found that the 550°C or higher temperature Even when the material temperature was maintained at ℃ or higher, no deterioration of powdering properties occurred, and the effect of shortening the alloying treatment time was observed. Further investigation revealed that if alloying is completed to the surface during this initial high-temperature heating, it is quite difficult to improve powdering properties during subsequent heating. Therefore, taking all the above into account, alloying is started with initial heating above 550℃, and the steel plate is rapidly cooled before alloying is completed to the surface, and the final alloying treatment is carried out.
The present invention was completed based on the knowledge that the above-mentioned object of the present invention can be effectively achieved by carrying out the process at 450 to 530°C. Therefore, the gist of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet, which comprises plating a hot-dip galvanized steel sheet having Al in the Zn-plated layer and then heat-treating the steel sheet. After passing through a hot-dip galvanizing bath, it is rapidly heated to 550 to 700°C at a heating rate of 30°C/S or more, and from a state where a liquid phase remains on the surface of the galvanized layer to 530°C or less, it is heated at a heating rate of 30°C/S or more. This is a method for producing an alloyed hot-dip galvanized steel sheet, which is characterized by rapidly cooling the steel sheet at a temperature of 450 to 530° C. Said rapid heating to 550-700°C may be carried out by high frequency induction heating, and/or after passing through said hot dip galvanizing bath, and/or after passing through said hot dip galvanizing bath, after rapid heating. It is also possible to set the time until the start of rapid cooling to within 10 seconds. In particular, by appropriately selecting the application frequency of high-frequency induction heating, only the interface between the surface of the steel sheet and the plating layer can be rapidly and intensely heated, which is preferable for the purpose of the present invention. Further, the low-temperature alloying treatment performed by rapidly cooling to a temperature range of 530° C. or less and then holding at that temperature is preferably performed for 3 to 120 seconds. Note that "liquid phase remains on the surface of the plating layer" refers to a state in which an Fe-Zn alloy layer is formed by mutual diffusion inside the plating layer, but Fe has not yet diffused to the surface, and generally The surface of the plating layer still has a metallic luster. Under the conditions specified by the present invention, such a state is sufficiently ensured within 10 seconds from the start of heating. (Function) Next, the reason why the plating layer structure and alloying treatment conditions are limited as described above in the present invention will be explained. The concentration of Al in the Zn plating layer was determined by a conventional method. It is generally contained at 0.05% or more. Since the present invention can promote alloying, the upper limit is preferably 0.35 wt%. Initial heating in the present invention is performed as rapidly as possible. If the acceleration rate is less than 30° C./S, the time the material remains at high temperature until the maximum temperature is reached will be longer, resulting in deterioration of powdering resistance and at the same time making the furnace length redundant. The temperature is preferably 50° C./S or higher, and it is also desirable to use induction heating to achieve such rapid heating. The material temperature reached during such initial heating is 550°C or higher and 700°C or lower. If the temperature is lower than 550°C, the effect of promoting alloying will be small, and the object of the present invention will not be met. If the temperature is higher than 700°C, the oxidation of Zn in the plating layer will become significant and the surface properties will deteriorate. Preferably 580℃~
The temperature is 680℃. If necessary, Zn
A soaking time may be given while the surface of the plating layer remains in a liquid phase. However, typically the time is no more than 5 seconds. Generally, it is desirable that the time from passing through the hot-dip galvanizing bath to the end of initial heating be 10 seconds or less. If the time is longer than this, it becomes difficult to maintain the surface of the Zn plating layer in a liquid phase, and sufficient powdering resistance cannot be ensured. The average Fe concentration in the plating layer after this initial heating is 4 to 8% by weight. Next, this plated steel plate is heated to a temperature of 530℃ or less for 30 minutes.
Rapid cooling at a rate of ℃/S or higher. Cooling at a rate slower than 30° C./S increases the time of exposure to high temperatures and reduces powdering resistance. In the present invention, the final stage of alloying treatment is performed by low-temperature alloying treatment after this rapid cooling, and it is preferable that this alloying temperature is as low as possible to the extent that alloying can be performed in-line. However, if the alloying rate is lower than 450℃, the alloying rate is too slow, and if it exceeds 530℃, the powdering resistance deteriorates.
The temperature is maintained at 530°C, preferably 480-520°C.
If the temperature is low, an alloying furnace is required after passing through the turn rolls, which causes the problem of metal pick-up due to contact between the plated steel plate and the turn rolls. Therefore, it is desirable that this low-temperature alloying treatment by heat retention at 450 to 530°C be completed in 3 to 120 seconds, although it depends on the sheet passing speed. 3
If it is held for less than 120 seconds, sufficient alloying will not occur, and on the other hand, if it is held for more than 120 seconds, no further alloying will occur and the economical efficiency will also decrease. In addition, cooling after heating at this final stage is 400℃.
In order to suppress powdering, it is preferable to cool down as rapidly as possible. Hereinafter, the present invention will be explained in more detail based on actual cooling. In addition, in each of the following examples, a single-sided 86 mm film with a thickness of 0.6 mm was manufactured continuously using the Sendzimer method.
A galvanized steel sheet with a Zn coating amount of g/m ² was used as the test material. The steel plate composition at this time was C: 0.035%,
Si<0.01%, Mn: 0.22%, P: 0.01%, S:
0.009%, sol.Al: 0.024%, and the effective Al concentration in the Zn plating layer was 0.135%. Example 1 In this example, the above steel plate was subjected to the first
Product name manufactured by Toyo Dentsu Kogyo Co., Ltd. under each condition shown in the table.
Heat treatment was performed using an alkali metal molten salt of MS5A. The temperature increase rate of the steel plate was 150 to 250°C/S.
When the heat treatment temperature is only one condition, it is soaked for a predetermined time and then cooled with water, and when the two conditions are combined, the molten salt bath is maintained at the temperature of the next secondary heating condition from the first molten salt bath. quenched into the molten salt. The cooling rate at this time is 75 to 130℃/
It was S. Cooling from the second molten salt bath was water cooling. The sample that has been alloyed in this way is punched into a disk shape with a diameter of 60 mm, and a cylindrical drawing is performed under the conditions that the sample slides directly against the mold, with a flat bottom diameter of 25 mm and an overhang height of 34 mm. I went. After drawing the cylinder, the sliding part was taped and peeled off, and the amount of powdering was evaluated by the change in weight before and after the test. The results are summarized in graphs in FIGS. 1 and 2. In the figure, the numbers are the experiment numbers in Table 1. As shown in FIG. 1, according to the method of the present invention, the experiment
As shown in Nos. 4 and 5, the performance is comparable to that of isothermal treatment at approximately 500℃ (see Experiment No. 1), but in the case of Experiments No. 6 and 7, where the final process alloying temperature is high, , the powdering resistance is close to that of isothermal treatment at 550℃ and 600℃, and the performance is significantly inferior. Figure 2 is a graph showing the relationship between alloying time and powdering, and the performance of Experiment Nos. 6 and 7 in Table 1 is not significantly different from that of Experiments Nos. 2 and 3, respectively. Excluded. In this figure, the minimum alloying treatment time for each condition is the lower limit time at which alloying will not be completed within an alloying treatment time shorter than this. For example, as shown by the dotted line in the figure, the amount of powdering increases within 15 seconds.
GA steel sheets with a weight of less than 0.02g/piece are manufactured using the method of the present invention (experimental
(See Nos. 4 and 5).

[Table] Example 2 Using the same sample as in Example 1, test samples of GA steel plates with each heat pattern shown in FIG. 3 were prepared using an infrared programmable heater. Taking into consideration the practical length of the furnace, it is planned that after holding the temperature at 450°C for 1 second, the process from the start of heating to the final start of cooling will be completed in 16 seconds. Among the heat patterns shown in FIG. 3, [A] to [C] belong to the scope of the present invention, and [D] to [G] are
This was added as a comparative example. Table 2 summarizes the test materials obtained using these heat patterns. As shown in these results, in all cases of the present invention, the alloying process was completed in 16 seconds, and
The amount of powdering was also small, whereas in the comparative example, the amount of powdering was large, or the processing was insufficient under the 16 second condition. The alloying status was determined based on whether the η phase was detected or not detected by X-ray diffraction in the plating film.

【table】

[Table] (Effects of the invention) As described above, the present invention is excellent as a high-speed, short-time heat treatment method for GA steel sheets, and is particularly suitable for materials with a relatively large amount of adhesion or when the Al concentration in Zn is high. It shows excellent effects, and as it is today,
The present invention has great significance as a means of increasing productivity in a situation where alloyed hot-dip galvanizing with a high Al concentration is desired.

[Brief explanation of the drawing]

1 and 2 are graphs showing the test results of Example 1; and FIG. 3 is a diagram showing the heat patterns of various alloying heat treatments.

Claims (1)

[Claims] 1. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises plating and heat-treating a hot-dip galvanized steel sheet having Al in the Zn-plated layer, wherein the steel sheet is heated in a hot-dip galvanizing bath. After passing through, it is rapidly heated to 550 to 700℃ at a heating rate of 30℃/s or more, and the temperature is reduced to 530℃ or less with a liquid phase remaining on the surface of the plating layer.
A method for producing an alloyed hot-dip galvanized steel sheet, characterized by rapidly cooling at a temperature of ℃/S or higher and further maintaining the temperature in a temperature range of 450 to 530℃. 2. The method according to claim 1, wherein the rapid heating to 550 to 700°C is performed by high-frequency induction heating. 3. The method according to claim 1 or 2, wherein the time from passing through the hot-dip galvanizing bath to the start of rapid cooling after rapid heating is within 10 seconds. 4. The method according to any one of claims 1 to 3, wherein the holding time after rapid cooling to 530°C or less is 3 to 120 seconds.