JPH0448061A - Production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To produce a galvannealed steel sheet excellent in die sliding characteristics and powdering resistance at the time of press forming by subjecting a hot-dip galvanized steel sheet to heating under specific conditions and controlling iron content in a plated layer. CONSTITUTION:A steel sheet is galvanized by immersion in molten zinc and the resulting galvanized steel sheet is heated, by which a galvannealed steel sheet is produced. At this time, the above galvanized steel sheet is heated up to 470-530 deg.C at >=30 deg.C/sec temp.-rise rate. Subsequently, this steel sheet is held at a temp. in the above temp. region, by which iron content in the plated layer is controlled so that zinc coating weight W(g/m<2>) and the above Fe content CFe(wt.%) in the plated layer satisfy the condition of 18-(W/10)>=CFe>=9. By this method, the galvannealed steel sheet excellent in die sliding characteristics and powdering resistance at the time of press working even under the condition of high zinc coating weight, improving productivity at the time of press working, and capable of producing a high quality press worked part can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet,
More specifically, the present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet that exhibits excellent powdering resistance and mold sliding properties during press forming.

[Prior art] Alloyed hot-dip galvanized steel sheets (hereinafter referred to as GA) are widely used as materials for automobile bodies because they have excellent corrosion resistance after painting, but powdering occurs when the plating layer peels off into powder during press processing. In this case, a decrease in mold sliding properties due to adhesion of the plating layer to the press mold often becomes a problem.

Powdering not only causes convex defects in pressed parts and significantly impairs the finished appearance (but also leads to a decrease in corrosion resistance), on the other hand, a decrease in mold sliding properties causes press cracks and poor press shapes.

Conventional knowledge suggests that increasing the iron (Fe) content in the plating layer improves mold sliding properties, but causes significant powdering, and conversely, keeping the Fe content low improves powdering resistance. However, the sliding properties of the mold are reduced.
It has been revealed that it exhibits antinomic properties.

The reason why the powdering resistance deteriorates when the Fe content is increased is said to be that one layer containing about 22% by weight of Fe is formed at the base steel sheet-plating layer interface.

On the other hand, in a plated layer with a low Fe content, the ζ layer containing about 7% by weight of Fe increases in the surface layer, resulting in a decrease in mold slidability.

Therefore, in order to obtain a GA with excellent powdering resistance and mold sliding properties, it is necessary to suppress the formation of the r phase and ζ phase and to make the plating layer so that the δ1 phase occupies most of the plated layer. It is necessary to control the composition.

Normally, when the Fe content in the plating layer exceeds 10% by weight, the ζ phase in the surface layer decreases and the problem of reduced mold sliding properties disappears. On the other hand, the Fe content range in which powdering does not practically become a problem becomes low as the zinc basis weight increases. It is necessary to suppress phase formation as much as possible.

For example, when the basis weight is 45g/rn'', the upper limit of the Fe content is 12% by weight, and when the basis weight is 60g/rn'', it is 11% by weight.
In order to obtain a GA with good powdering resistance, it is necessary to set the content to about 7% by weight at 90 g/rn''.

Therefore, as the basis weight increases, the Fe content range in which both powdering resistance and mold sliding properties are good becomes narrower, and it is difficult to industrially produce GA that is excellent in both properties with a high basis weight. becomes virtually impossible.

As a method for manufacturing GA with good powdering resistance, a method has been proposed in which GA is rapidly heated to a high temperature during alloying treatment and then rapidly cooled while leaving a liquid phase, as disclosed in JP-A No. 61-223174. However, although this method has good powdering resistance, the remaining liquid phase decreases to η after cooling.
It remains on the plating surface layer as a compatible phase and significantly deteriorates the sliding properties of the mold.

In addition, in order to manufacture a steel plate with excellent powdering resistance and mold sliding properties, as shown in Japanese Patent Publication No. 61-26600 and Japanese Patent Application Laid-Open No. 1-172578,
A method is used in which a film is formed on the plated surface with a low Fe content to improve sliding properties with the mold.

However, such a method not only loses the original surface properties of GA, which may adversely affect chemical conversion treatment properties, spot weldability, or coating adhesion, but also causes the formation of a thin film layer on the outermost layer. When the sliding properties of the mold are improved, if the pressing conditions are such that the pressure of the mold applied to the material is large, the outermost layer film may be destroyed and the improvement effect may not be achieved. Further, since post-treatment after plating is required, the manufacturing process becomes complicated and costs inevitably increase.

Problem to be Solved by the Invention 1 The present invention provides a method for producing GA that has excellent mold sliding properties and powdering resistance during press processing even with a high zinc coating weight, and overcomes the drawbacks of the above-mentioned prior art. This is what we are trying to solve.

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides a method for producing an alloyed hot-dip galvanized steel sheet by immersing a steel sheet in hot-dip zinc to galvanize it, and then heating the galvanized steel sheet. In the method of
”) and Fe content C iron (weight %) in the plating layer is 1g
-(W/l o)≧CFe≧9 A method for manufacturing an alloyed hot-dip galvanized steel sheet is provided, characterized in that the iron content is adjusted so as to satisfy the following condition.

[Effect 1 Generally, when manufacturing GA using continuous beach dip galvanizing equipment, the annealed material is immersed in molten zinc maintained at 440 to 490°C, and immediately after being pulled out, it is exposed to air, N2 gas, etc. The amount of zinc coating is controlled by blowing G
get an A.

In this process, the steel plate temperature is increased to 390~390 by gas blowing.
Although it is cooled to 420°C, it is necessary to heat it to a temperature range of 450°C to 550°C in order to effectively achieve alloying within a limited time.

The present inventors set the temperature range for heating alloying to 470 to 530.
℃ range, an alloy plating layer consisting mainly of δ1 phase with few phases and ζ phase can be obtained in a wide range of Fe content, and as a result, both powdering resistance and mold sliding properties are obtained. They discovered that excellent GA could be obtained and completed the present invention.

When alloyed in a temperature range below 470°C, F in the plating layer
The e content changes continuously, and even if the η phase (pure Zn) remains on the outermost surface, a layered phase is formed at the steel plate-plating layer interface.

In particular, in plating with a high basis weight, the phase formed at the interface becomes thick before alloying reaches the outermost surface, resulting in a significant deterioration in powdering resistance.

In order to alloy with a high basis weight and obtain GA with good powdering resistance, it is necessary to leave the ζ phase in the plating surface layer and soften the entire plating layer. However, when the zeta phase increases and the plating layer becomes soft, it tends to adhere to the mold tool during press working, and the sliding properties of the mold are significantly deteriorated.

Therefore, in order to suppress the layered growth of the phase and prevent deterioration of powdering resistance even with a small amount of ζ phase, it is necessary to
The progress of alloying in the temperature range below 70°C is suppressed as much as possible, and 4
Alloying must be carried out in a temperature range of 70°C or higher.

That is, when heating to the first alloying temperature after wiping, it is necessary to heat as rapidly as possible in order to shorten the time spent in the temperature range below 470°C. In order to prevent the η phase from forming a layer during heating and to substantially prevent deterioration of the powdering resistance of the plating layer, the required heating rate is 30
'C/sec or higher. 30℃/se
In order to obtain a heating rate of c or more, a heating burner with a larger capacity than the conventionally used alloying furnace is provided, but it is appropriate to use methods such as direct current heating and electromagnetic induction heating.

In the temperature range of 470 to 530°C, when the plating layer is alloyed, the growth rate of the initial alloy layer is high, so cracks are likely to occur in the alloy layer, and molten zinc infiltrates into the cracks, causing a breakdown between the steel sheet and the plating layer. Since zinc is supplied to the interface to suppress phase growth, a plated layer with good powdering resistance can be obtained.

In the temperature range above 530°C, the ζ phase does not exist and the δ1 phase crystallizes directly from the liquid phase, but as in the temperature range below 470°C, the Fe content in the solid phase of the plating layer changes continuously. death,"
Powdering resistance tends to deteriorate because the phase develops in a layered manner.

470℃ or higher at a heating rate of 30℃/sec or higher5
When performing alloying in a temperature range of 30°C or lower, the Fe content in the plating layer needs to be 9% by weight or more in order to prevent the mold sliding properties from decreasing due to the presence of a large amount of ζ phase. Also,
"In order to prevent powdering resistance from decreasing due to phase growth, F
It is necessary to keep the e content below (18-[W/101)% by weight.

In other words, as the zinc weight increases, the critical peeling amount at which the powdering resistance deteriorates significantly increases, so lower F.
It is necessary to set the content to e. When the heating rate is low or when the alloying temperature is less than 470°C, in order to ensure good powdering resistance (18-[W
Since it is necessary to keep the Fe content much lower than the weight percent (20g/l ol ), the Fe content range that maintains both good mold sliding properties and powdering resistance is extremely narrow (60g/l ol )wt%.
- It is virtually impossible to manufacture with a zinc area weight of more than -. However, according to the present invention, even if the basis weight is 70 g/ba, the Fe content can be controlled to 9 to 11% by weight.
It becomes possible to produce GA that is industrially stable and has excellent mold sliding properties and powdering resistance.

〔Example〕

The present invention will be explained below based on examples.

Example 1 Continuous hot-dip galvanizing equipment, zinc basis weight 30g/rn
', 45g/m', 60g/rn"175g/rn
”, we prototyped a GA of 90 g/d.

The temperature increase rate after wiping was 12℃/sec, 30
℃/sec, 45℃/sec, heating temperature 4
Alloying was carried out at 90° C. and the alloying time was varied so that the Fe content in the plating layer was 7 to 15% by weight.

The powdering resistance of each mIfit is determined by bending and unbending the steel plate 1 at right angles, peeling it off, and removing the zinc in the pinning layer adhering to the adhesive tape 2 using fluorescent X-ray.
Linear analysis and evaluation by measuring counts per second (CPS).

If this count number is 2000 CPS or less, press processing can be performed without any problem in an actual press.

In addition, using a tensile testing device 3 as shown in Figures 4 and 5, the load required to pull out a steel plate l held down at a constant pressure by a ball-headed die 4 between the dies was measured, and the pullout load and The ratio of the pressure that suppressed the die was used as a coefficient of friction and an index of mold slidability. When the friction coefficient of a cold rolled steel plate is determined using this method, it is approximately 0.2 to 0.25, and the friction coefficient is 0.2.
A mold sliding property of 5 or less was considered to be good.

Judging the quality of powdering property and mold sliding property when heated and alloyed at each temperature increase rate, and finding a GA with good both properties.
The range of Fe content and zinc basis weight that can be obtained is shown in Fig. 1.

Temperature increase rate: 30℃/sec, 45℃/sec
In the case of c, the powdering resistance is good and the friction coefficient is low (the range in which GA with good mold sliding property can be obtained is wide, and 90
It can also be produced with a zinc basis weight of g/m''.

On the other hand, when the heating rate is as low as 12°C/sec, GA with excellent both of the above properties has an Fe content in the narrow range of 9 to 10% by weight at a low basis weight of 45 g/m'' or less. Must be controlled.

Example 2 Continuous hot-dip galvanizing equipment with zinc coating weight of 30g/
m", 45/rr1', 60g/ba, 75g/rn',
A prototype GA of 90 g/m'' was manufactured.

The heating rate after wiping was 40°C/sec,
Alloying was carried out at heating temperatures of 450°C, 470°C, 500°C, and 540°C, and the Fe content in the plating layer was adjusted to 7 to 15% by weight by changing the alloying treatment time.

FIG. 2 shows the range of zinc area weight and Fe content that provide good powdering resistance and mold sliding properties for each heating temperature. In the figure, O1., mouth, and 1 mark are the same as in FIG.

When the alloying treatment temperature is 470°C or 500°C, GA with excellent powdering resistance and mold sliding properties is 30 to 90 g/
It is possible within the range of zinc coating weight of r+1'', but if the alloying temperature is as low as 450°C, GA with good powdering resistance and mold sliding properties cannot be obtained, but if the alloying temperature is as high as 540°C GA that satisfies both of these characteristics can only be produced with a zinc basis weight in the range of 30 g/rn''.

[Effect of the invention 1] The present invention improves the productivity of press work, and improves the productivity of press work since there are concerns about mold sliding properties and powdering resistance during press work even with high zinc coating weight. An alloyed hot-dip galvanized steel sheet, which is extremely effective in improving the appearance, quality, and corrosion resistance of products, can be manufactured without complicating the manufacturing process and increasing costs.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the zinc basis weight, the Fe content in the plating layer, and the heating speed, powdering resistance, and mold sliding properties. A diagram showing the relationship between content and alloying temperature, powdering resistance and mold sliding properties, Figure 3 is an explanatory diagram of the powdering resistance measuring method, Figure 4 is an explanatory diagram of the friction coefficient measuring device, FIG. 5 is a diagram showing the tip rA shape of the die in FIG. 4. ■... Alloyed hot-dip galvanized steel plate 2... Adhesive tape 3... Tension = test device 4... Dice 5... Load cell 6... Hydraulic cylinder

Claims (1)

[Scope of Claims] 1. A method for manufacturing an alloyed hot-dip galvanized steel sheet by immersing a steel sheet in hot-dip zinc and then heating the galvanized steel sheet, wherein the galvanized steel sheet is heated at 470°C.
After heating to a temperature range of 530°C or less at a temperature increase rate of 30°C/sec or more, the temperature is maintained within the above temperature range to determine the zinc basis weight W (g/m^2) and the iron content in the plating layer. Rate CF
A method for manufacturing an alloyed hot-dip galvanized steel sheet, characterized in that the iron content is adjusted so that e (wt%) satisfies the following condition: 18-(W/10)≧CFe≧9.