JPH0581662B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet. Specifically, it is a method for producing an alloyed hot-dip galvanized steel sheet with excellent powdering resistance and flaking resistance. (Prior Art) Alloyed hot-dip galvanized steel sheets are produced by heating hot-dip galvanized steel sheets after plating to diffuse iron in the base steel sheet into the plating layer to form an iron-zinc alloy phase. It is widely used in the fields of automobiles and home appliances and construction materials because it has superior coating corrosion resistance and weldability compared to general galvanized steel sheets (Japanese Patent Laid-Open Nos. 62-124266 and 62-130268, etc.). In recent years, demands for corrosion resistance have increased more and more, and alloyed hot-dip galvanized steel sheets are desired to replace conventional cold-rolled steel sheets and pure zinc-based galvanized steel sheets. Moreover, as the number of areas in which it is used has expanded, it has come to be applied to areas that undergo more severe processing, and there has been a strong desire for improved workability such as pressability. (Problem to be solved by the invention) However, since alloyed hot-dip galvanized steel sheets are manufactured by thermal diffusion treatment as described above, Fe-Zn alloy phases with high iron concentrations (Γ phase and Γ _{A single-} phase intermetallic compound (generally referred to here as the Γ phase) is produced. Since the Γ phase thickness increases as the degree of alloying increases, plating adhesion deteriorates. For example, during press processing when the Γ phase becomes thicker, the plating layer tends to peel off in a powdered form, causing scratches and the like on the product, resulting in actual damage such as a decrease in yield or an increase in the frequency of mold cleaning. A control technology that suppresses the amount of Γ phase produced as much as possible is desired. This tendency becomes particularly noticeable as the basis weight increases. On the other hand, when the degree of alloying is reduced to reduce the Γ phase, the surface of the plating layer has a Fe-Zn alloy phase (ζ phase) with a relatively low Fe concentration and a zinc phase (η phase) in which Fe remains in solid solution. becomes more likely to remain. As the degree of alloying decreases, its thickness also increases. This tendency becomes more remarkable as the basis weight increases. If a large amount of such η phase or ζ phase remains on the surface of the plating layer, these alloy layers are relatively soft and are likely to cause mold galling during press working, resulting in so-called flaking that accumulates near the mold bead. , or fall into the mold die, which also reduces yield and workability in the pressing process. Considering the problems mentioned above during press processing,
The ideal plating layer structure of an alloyed hot-dip galvanized steel sheet is desirably composed of a uniform δ ₁ phase with no Fe concentration gradient from the iron substrate interface to the plating layer surface. It is extremely difficult to do so. As the basis weight decreases, the amount of Γ phase and ζ phase produced decreases, so the above-mentioned problem is relatively alleviated. However, as the areas of application have expanded in recent years, there has been a strong demand for alloyed hot-dip galvanized steel sheets, which have much better workability than before, because they are subjected to more severe processing. Workability can be considerably improved by reducing the basis weight to 30 g/m ^{2 or} less, but recently it is also required to improve rust prevention, so ^the The basis weight is required. In actual use, it is a prerequisite to ensure corrosion resistance at the target level.
It is not acceptable to simply reduce the weight. As mentioned above, alloyed hot-dip galvanized steel sheets are becoming more widely used due to their excellent characteristics, and because they are applied to areas with even more severe processing conditions, they have excellent powdering resistance and flaking resistance. There is a strong demand for a method for stably producing alloyed hot-dip galvanized steel sheets with excellent properties. In order to advantageously and reliably solve the above-mentioned problems, the present invention provides a method for producing a δ _1- based alloyed hot-dip galvanized steel sheet that has excellent powdering resistance and flaking resistance. be. (Means for Solving the Problems) The present invention applies plating in a zinc plating bath consisting of Al: 0.04 to 0.12 wt%, residual Zn, and unavoidable impurities, then controls the area weight, and then When performing alloying treatment by heating in an alloying furnace, after completion of area weight control, rapid heating is performed in the temperature rising process in the alloying furnace in which the time required to reach a plate temperature of 470℃ or higher is 2.0 seconds or less, and alloying is performed. After completion, the plate temperature will increase during the cooling process.
This is a method for producing an alloyed hot-dip galvanized steel sheet, which is characterized by rapid cooling to a temperature range of 420°C or less in 2 seconds or less. The immersion time in the plating bath should be 2.5 seconds or less, the immersion time in the plating bath should be 2.5 seconds or less, and the maximum alloying temperature should be in the range of 470 to 510℃, and the basis weight quantity 45-90
g/m ² , and it is preferable that the upper layer of the alloyed galvanized steel sheet be electroplated mainly consisting of iron and the balance being zinc. (Function) The reason for setting the Al content in the galvanizing bath to 0.04 to 0.12wt% (expressed as the effective Al concentration in the bath; the same applies hereinafter) is as follows.
If the Al concentration is lower than this value, the formation reaction of the alloy layer increases in the plating bath, and ζ
A thick Fe-Zn alloy layer mainly consisting of phase is formed, and this alloy layer remains on the surface of the plating layer immediately after alloying is completed, deteriorating flaking resistance. In addition, if further heating is performed immediately after alloying is completed, or if the cooling rate after alloying is slow, this ζ phase undergoes a solid phase diffusion reaction relatively easily, so the overalloying reaction tends to proceed, resulting in This transforms into a Γ phase with a high iron concentration, which deteriorates powdering resistance. On the other hand, when Al exceeds 0.12wt%, a ternary alloy layer with high Al concentration is formed in the plating bath, and Fe
- This will suppress the Zn alloying reaction. For this reason, alloying becomes difficult during superheating treatment in an alloying furnace, resulting in a metallic shiny plating appearance called half-baked, or if the heating alloying temperature is raised to prevent half-baked, the overalloying reaction progresses. Γ during cooling after completion of alloying.
This is disadvantageous because it transforms into a phase and deteriorates powdering resistance. In this case, the required alloying heating plate temperature is 540℃ or higher, so in order to increase the degree of cooling after alloying is completed and suppress the overalloying reaction,
It is necessary to increase the number of cooling facilities and also to increase the heating capacity of the alloying furnace. When plating is performed in the above bath AL concentration range, a barrier layer with a high Al concentration is generated due to the selective preferential reaction of Al in the bath, suppressing the Fe-Zn alloying reaction, but the suppression time is relatively short. . Therefore, the Fe--Zn alloying reaction starts during the temperature raising process after leaving the plating bath, controlling the basis weight, and then superheating in the alloying furnace. The temperature increase process from the time the area weight control is completed immediately after exiting the plating bath until the maximum plate temperature in the alloying furnace is reached is 420℃.
The temperature is relatively low, ranging from above ℃ to below 470℃,
Therefore, the main alloy layer that is formed is the ζ phase, which has the same problems as the ζ phase that is formed in the bath described above.
Deteriorates flaking resistance. In order to suppress the reaction during the heating process,
It is effective to increase the Al concentration in the bath to 0.12wt% or more, but as mentioned above, it not only impairs quality such as plating appearance and powdering resistance, but also economically reduces operating and equipment costs. increase As a result of various studies to solve this time advantageously, we found that the heating time from leaving the plating bath to reaching 470℃ in the temperature raising process in the alloying furnace was 2.0℃.
By ensuring that the temperature is within seconds, even in the bath Al concentration range mentioned above, the reaction to form an alloy layer mainly composed of the ζ phase during the heating process is substantially suppressed, so that the formation reaction in the plating bath as described above is suppressed. It has been found that, as well as the effect of suppressing the ζ phase, this can be achieved advantageously without deterioration of flaking resistance or powdering resistance.
If the heating time to 470℃ is longer than 2 seconds,
In this temperature range, Fe~Zn mainly consists of ζ phase.
Since an alloying reaction occurs and the reaction rate is relatively high, the above-mentioned problem of deterioration of plating quality due to the ζ phase occurs. In other words, if the ζ phase remains on the plated surface layer, flaking resistance deteriorates, and on the other hand, if the heating temperature is increased or the heating time is prolonged to eliminate this ζ layer, the overalloying reaction causes the formation of the ζ phase. This can lead to deterioration of powdering resistance. Furthermore, after completion of alloying, it is necessary to cool as rapidly as possible to suppress the overalloying reaction. As a result of examining the cooling conditions, it was found that after completion of alloying, it is important to rapidly cool the plate to a temperature of 420°C within 2 seconds. When plating is carried out under the bath Al concentration and heating rate conditions described above, there is considerably less ζ phase on the plating surface, resulting in an alloy layer consisting mainly of δ ₁ phase, so there is no problem with flaking resistance. However, a small amount of ζ remains on the plated surface to the extent that it does not deteriorate flaking resistance.
Since the phase transforms into the Γ phase at a fast reaction rate even at a relatively low temperature during the cooling process, there is a problem of deterioration of powdering resistance. On the other hand, although the reaction rate is relatively slow, δ ₁
Since the phase itself also transforms into the Γ phase during this cooling process, it is important to cool it as rapidly as possible and to suppress the reaction that produces the Γ phase. The formation reaction of the Γ phase during the cooling process can be suppressed by rapidly cooling the alloy to a temperature range of 420° C. or lower in 2 seconds or less after completion of alloying. The formation reaction of Γ phase is 420
Although it does not substantially occur at temperatures below ℃, the Γ phase is generated at a fairly rapid rate at temperatures above this temperature.
When heated in a temperature range of 420° C. or higher for a period longer than 2 seconds, the Γ phase thickness increases, resulting in a problem of deterioration of powdering resistance. Note that the plate temperature at the time of completion of alloying is generally approximately the same as the time when the highest plate temperature is reached in the alloying furnace. However, this is not necessarily necessary; for example, if alloying is completed in a conditioning cooling furnace or during the relatively slow cooling after being heated to the highest temperature, the alloy 420 from the temperature range at the time of completion of conversion
The cooling time to the plate temperature of °C may be set to 2 seconds or less. In this way, if the alloying is completed during cooling, 80% or more of the entire plating layer should be heated to 470 to 510℃.
It is desirable that the alloying reaction proceed in a relatively high temperature range. The reason for this is that if more than 20% of the plated layer remains unalloyed and alloying is completed by heating at a temperature below 470℃, a large amount of ζ phase tends to remain on the plated surface as described above, making it difficult to This is because deterioration of flaking properties occurs. Next, we will discuss the effect of immersion time. Although the alloying reaction in the bath can be sufficiently suppressed within the above-mentioned Al concentration range, it is more desirable to set the immersion time in the bath to 2.5 seconds or less. The immersion time here refers to the time from when the strip is immersed in the plating bath to when it leaves the plating bath to when the basis weight control is completed by a gas wiping method or the like. Soaking time
If the time is longer than 2.5 seconds, a sufficient Al barrier layer will not be formed because the reactivity of Al in the bath varies depending on the steel type, heat treatment conditions in the hot-melting line, and changes in the atmosphere composition within the Al concentration range mentioned above. Otherwise, the Fe-Zn alloying reaction proceeds in the bath, making it impossible to effectively suppress the formation of the ζ phase. Therefore, considering these variations in actual operation, it is desirable that the immersion time be 2.5 seconds or less. The immersion time can be set by adjusting the length from when the steel strip enters the plating bath via the snout to the area weight control position, or by adjusting the threading speed of the steel strip. On the other hand, when superheating is performed in an alloying heat treatment furnace, it is desirable to perform alloying at a maximum plate temperature of 470°C or higher and 510°C or lower. When the temperature is lower than 470℃, the reaction rate of Fe eluted from the substrate is
Since the diffusion rate is relatively high, an alloy layer with a low Fe concentration is formed, which increases the thickness of the ζ phase and remains even after alloying is completed, which is undesirable because it deteriorates flaking resistance. . on the other hand,
When the temperature of the alloyed plate is higher than 510℃, the elution rate of the base iron increases, making it easier to form the δ ₁ phase, so the above problem does not occur. Since the transformation reaction of not only the small amount of ζ phase remaining on the surface but also the δ ₁ phase to the Γ phase progresses at a relatively fast rate, there is a problem that the powdering resistance is significantly deteriorated. In particular, this tendency becomes more pronounced as the basis weight increases, making it difficult to ensure good quality. A possible solution to this problem is to increase the cooling rate, but simple methods such as water cooling are ineffective, so special equipment is required to ensure the cooling rate.
This is not preferable since it has the disadvantage of increasing equipment costs. As mentioned above, as an alloying heat treatment furnace,
Rapid heating during the temperature raising process after plating, rapid cooling after completion of alloying, and control of the optimal temperature range of the plate temperature are important. As a heat source for this alloying heat treatment furnace, methods such as gas combustion heating, electric resistance heating, infrared heating, and high frequency heating can be adopted. Also,
When the threading speed changes, by arranging heat sources in multiple stages in the threading direction, they can be turned on and off appropriately, and the effective length of the superheating zone and cooling zone can be adjusted according to the threading speed. desirable. The present invention is applicable regardless of the basis weight. Even if the basis weight is small (usually 45 g/ ^m2 or less), by applying the present invention, a plated layer structure consisting mainly of δ ₁ phase with less Γ and ζ phases can be created, making it suitable for applications and parts that undergo more severe molding processing. Even when this alloyed hot-dip galvanized steel sheet is applied, there is an advantage that flaking resistance and powdering resistance can be reliably prevented. However, the application of the present invention is most advantageous in the case of thick alloyed hot-dip galvanized steel sheets with a basis weight of 45 g/m ² or more, particularly 50 g/m ² or more. When alloying is performed by the heating diffusion method, the concentration distribution of Fe gradually decreases from the substrate to the plated surface, so that the Γ phase tends to grow thickly on the substrate side, and the ζ phase grows thickly on the plated surface side. Therefore, controlling the alloy phase to be mainly _δ1 phase by applying the present invention has the advantage of reliably preventing powdering resistance and flaking resistance. Although the present invention is applied to alloyed hot-dip galvanized steel sheets, it can be applied to either double-sided plating or single-sided plating. Further, there is no particular limitation as long as it has an alloyed hot-dip galvanized layer on at least one side. For example, a steel plate with different thickness on the front and back (for example, one side has a basis weight of 30g/m ² and the other side has a
60g/ ^m2 alloyed hot-dip galvanized steel sheet) or single-sided alloyed hot-dip galvanized steel sheet (one side has a basis weight of 30g/m2 ⁾
It can be advantageously applied to an alloyed galvanized layer (with the other side being 100 g/m ² of a normal hot-dip galvanized layer that is not alloyed). Furthermore, in order to improve the cationic electrodeposition coating properties, the upper layer of such an alloyed hot-dip galvanized steel sheet may be made of 60% or more iron and the balance zinc, or the alloy may contain Ni, Co, Cr, Mn:Si, The alloy layer containing one or more of P, Cu, B, etc. has a basis weight of 2.
By applying it by electroplating in the range of 5 g/m ² , it is possible to obtain a tanned steel sheet with excellent powdering resistance and flaking resistance, and excellent cationic electrodeposition properties. (Example) Next, Examples of the present invention are shown in Table 1 together with Comparative Examples.

【table】

[Table] (Effects of the invention) By applying the present invention, the powdering resistance and flaking resistance of alloyed hot-dip galvanized steel sheets are reliably improved, and they are used in the fields of automobiles, home appliances, and construction materials to undergo harsh processes such as press processing. This makes it suitable for receiving a lot of molding. Furthermore, by applying the present invention, high-quality, high-performance alloyed hot-dip galvanized steel sheets can be produced relatively easily and inexpensively, and excellent effects can be advantageously and reliably produced. As described above, the performance improvement effect of the alloyed hot-dip galvanized steel sheet produced according to the present invention is remarkable, and the industrial value is extremely large because it can accurately meet the needs that will further expand in the future.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a test method for evaluating flaking resistance. 1...Punch, 2...Dice, 3...Test piece.

Claims (1)

[Claims] 1. Plating is performed in a galvanizing bath consisting of Al: 0.04 to 0.12 wt%, residual Zn and unavoidable impurities, and after controlling the basis weight, heating is performed in an alloying furnace. When performing alloying treatment, after completion of area weight control, rapid heating is performed in the temperature raising process in the alloying furnace to reach a plate temperature of 470℃ or higher in less than 2.0 seconds, and after completion of alloying, in the cooling process. A method for producing an alloyed hot-dip galvanized steel sheet, characterized by rapid cooling in 2 seconds or less to a temperature range of 420°C or less. 2. The method according to claim 1, wherein the immersion time in the plating bath is 2.5 seconds or less. 3. The method according to claim 1, wherein the immersion time in the plating bath is 2.5 seconds or less, and the alloying treatment is carried out by heating at a maximum alloying temperature in the range of 470 to 510°C. 4. The method according to any one of claims 1 to 3, wherein the basis weight is 45 to 90 g/ ^m2 . 5 Claim 1, in which the upper layer of the alloyed galvanized steel sheet is electroplated mainly consisting of iron and the balance being zinc.
3. The method according to any one of items 3 to 3.