JP2000273611A - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Abstract

(57) [Summary] The object of the present invention is to use a general low-carbon steel,
An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which is inexpensive and has excellent surface properties (glossiness) or more excellent workability and a method for producing the same. The invention has developed an alloyed hot-dip galvanized steel sheet having a 60-degree mirror glossiness of 20 or more. In a method for producing a galvannealed steel sheet in which a steel strip is subjected to a hot-dip galvanizing treatment and an alloying treatment, after the steel strip is hot-rolled as a low-carbon steel, and then annealed at a steel strip temperature of 850 ° C. or more, Hot-dip galvanizing and alloying are performed, and temper rolling is performed to reduce the sheet thickness by 1.5% or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet, and more particularly to a technique for making the steel sheet excellent in surface properties (glossiness) or workability.

[0002]

2. Description of the Related Art In recent years, the demand for alloyed hot-dip galvanized steel sheets has been rapidly expanding as a material for automobiles because of its good corrosion resistance and good paintability. Along with this, in order to be able to press-mold into a complicated shape, it is required to have good surface properties or to further improve workability in order to make the appearance beautiful. Regarding this workability, a defect (referred to as a stretcher strain) that causes the steel sheet to “wrinkle” during press forming must not be caused. Therefore, more research has been done than before, and related technologies have been published.

[0003] For example, Japanese Patent Application Laid-Open No. 4-72017 discloses that
A steel slab having a carbon content of 0.0050% by weight or less
Cold rolling of 0% or less, then dew point of -30 ° C or more +
Under an atmosphere of 15 ° C. or less, decarburizing annealing is performed for 40 seconds or more at a temperature of not less than the recrystallization temperature and 900 ° C.
10% by weight or more and less than 0.0030% by weight,
1. hot-dip galvanized, and elongation of 1.0% or more;
It discloses a technique for producing a hot-dip galvanized steel sheet having good workability and aging characteristics, which is characterized by performing temper rolling of 0% or less, and having appropriate bake hardenability. Japanese Patent Application Laid-Open No. Hei 6-158172 discloses that an ultra-low carbon steel to which Ti and Nb are added is subjected to hot finish rolling at an Ar temperature of ₃ points or less, and then to 30 ° C /
Cooling in more than seconds, winding up at 650 ° C or more, rolling reduction 60%
After the above cold rolling, continuous annealing at 800 ° C. or more and Ac ₃ points or less for 5 minutes or less, immersion in a hot-dip galvanizing bath at 420 to 530 ° C. for plating, and re-heating, followed by galvannealing for alloying A method for manufacturing a steel sheet is disclosed. Further, in Japanese Patent Application Laid-Open No. 2-118088, Fe-Zn is added to a steel sheet in which a plating film after galvanizing is in a molten state.
A technique for manufacturing an alloyed hot-dip galvanized steel sheet having excellent workability and coatability by spraying an alloy powder and then smoothing and heating the surface has also been generalized.

[0004]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Laid-Open No. 4-7 is disclosed.
The steel sheets manufactured by the methods described in JP-A-2017 and JP-A-6-158172 have a desired workability because the carbon is reduced to an extremely low region. However, the surface properties, particularly the gloss, are still insufficient. In addition, the cost of refining ultra-low carbon steel at the steelmaking stage increases, so that the cost of the manufactured hot-dip galvanized steel sheet increases.
On the other hand, in the method described in Japanese Patent Application Laid-Open No. 2-118088,
There is a problem in cost that an apparatus for spraying Fe—Zn alloy powder, a smoothing treatment, and a heat treatment apparatus must be separately provided after the alloying treatment apparatus. Further, there is also a problem that a change in the amount of powder sprayed and the heating temperature causes poor workability.

In view of such circumstances, an object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having excellent surface properties (glossiness) or more excellent workability using a general ordinary low-carbon steel at a low cost and its production. It is to provide a method.

[0006]

Means for Solving the Problems In the production of galvannealed steel sheets, the inventor of the present invention has conducted intensive studies on the possibility of improving the surface properties of the steel sheets, particularly the glossiness, compared with conventional steel sheets. It was found that the gloss was surely improved. And, in addition to the above knowledge, the present invention was completed in consideration of conditions that can maintain good workability even when low-carbon steel is used.

That is, the present invention is an alloyed hot-dip galvanized steel sheet having a 60-degree specular gloss of 20 or more.

The present invention also provides a method for producing an alloyed hot-dip galvanized steel sheet in which a steel strip is subjected to a hot dip galvanizing treatment and then to an alloying treatment. Alloying hot-dip galvanizing characterized by performing hot-dip galvanizing after annealing at a zone temperature of 850 ° C. or more, and then performing alloying treatment and temper rolling to reduce the sheet thickness by 1.5% or more. This is a method for manufacturing a steel sheet.

Further, the present invention is a method for producing an alloyed hot-dip galvanized steel sheet, wherein the thickness of the steel strip is reduced to 1.6 mm or less in the hot rolling.

In the present invention, the cold rolling is omitted to improve the glossiness, the annealing conditions and the temper rolling (also referred to as skin pass rolling) conditions are appropriately set, and the stretcher strain which is a defect in press working is prevented. Therefore, an alloyed hot-dip galvanized steel sheet having excellent surface properties and further excellent workability can be manufactured. In addition, since the steel strip to be plated is made of low-carbon steel, not only does cold rolling be omitted, but it is not necessary to add an Fe-Zn alloy powder spraying device, a smoothing and heat treatment device, etc. It will be significantly cheaper than that.

[0011]

FIG. 1 shows a production line for a general alloyed hot-dip galvanized steel sheet. First, a steel slab is hot and cold rolled to a predetermined thickness to obtain a steel strip as a material to be plated. Then, the steel strip is successively passed through an annealing furnace, a plating apparatus, an alloying furnace, and a temper rolling mill sequentially to obtain a product.

In the present invention, the steel strip of the material to be plated is made of low carbon steel. Its main chemical composition is: C: 0.03-0.
05% by weight, Si: 0.03% by weight or less, Mn: 0.1
8 to 0.30% by weight, P: 0.025% by weight or less, S:
0.020% by weight or less, Al: 0.035 to 0.075
% By weight, Mn / S: 12.0 or more, the balance being Fe and unavoidable impurities. Recently, materials for hot-dip galvanized steel sheets include
Very low carbon steel having a carbon concentration of 50 ppm or less is often used. However, in view of the fact that steel strips with a thickness of 1.6 mm or less, which could not previously be manufactured stably by hot rolling, can now be manufactured, and from the viewpoint of inexpensive manufacturing costs,
In the present invention, a low carbon steel is used.

First, in order to improve the glossiness of the product, the inventor repeated many plating experiments on the low carbon steel specimen.
I found that using a hot-rolled material as a material is superior in glossiness to a material obtained by plating and alloying a cold-rolled material. Therefore, it was conceived that plating was performed by omitting the cold rolling step in the above line.

However, a further problem was whether the workability could be improved with a steel strip having a relatively high carbon content as a material to be plated. Therefore, the inventor has studied the annealing conditions and the temper rolling conditions before the hot-rolled material is plated, and has found the following conditions.

In the present invention, "good workability" refers to
It means that the yield elongation after aging of the alloyed hot-dip galvanized steel sheet of the product is small. This is due to the age hardening of the alloyed hot-dip galvanized steel sheet before it is processed into steel products of various shapes, the yield point increases, the workability decreases, and the yield elongation occurs. Occasionally produces stretcher strain. In order to prevent this, it is desirable that the yield elongation is small.

The glossiness in the present invention is 20 or more as a 60-degree specular glossiness. If it is less than 20, it is equivalent to an alloyed hot-dip galvanized steel sheet using a conventional cold-rolled material as a raw material, and is inferior in glossiness. By setting the glossiness to 20 or more, there is also an effect that the appearance becomes beautiful and the coating adhesion after pressing is excellent. Preferably, it is 20 to 80, more preferably 20 to 60.

Annealing temperature: 850 ° C. or more When annealing a cold-rolled material, it is said that the annealing temperature should be higher than the recrystallization temperature of the steel strip from the viewpoint of workability. As a result of various studies on hot-rolled materials, 850 ° C.
It became clear that the workability was excellent by the above.
Therefore, in the present invention, the annealing temperature is set to 850 ° C. or higher.
On the other hand, if the temperature is too high, the crystal grains become coarse and the workability is rather deteriorated.

Preferred atmosphere for annealing: dew point of less than -30 ° C.
A gas of 5% by volume of hydrogen and the balance of nitrogen may be blown. In the hot-rolled material according to the present invention, the dew point is preferably suppressed to less than −30 ° C. The reason is that at -30 ° C. or higher, the glossiness after alloying decreases due to oxidation.
Note that the annealing time may be the same as that of the cold-rolled material.

The hot-rolled material annealed under the above conditions is immersed in a hot-dip galvanizing bath and plated. The plating conditions such as the composition of the plating bath and the bath temperature may be conventionally known conditions.
The same applies to the temperature of the alloying treatment performed after plating.

In the present invention, temper rolling is performed after the alloying treatment in the present invention, but the workability is further improved in addition to the glossiness. The conditions were examined from. The results are shown in FIG. 2 in terms of the relationship between the reduction rate of the sheet thickness during temper rolling (indicated by the symbol SK), the sheet temperature during annealing, and the yield elongation after aging. FIG. 2 shows that the steel strip temperature during annealing was 8
When the temperature is set to 50 ° C. or more and the rate of thickness reduction during temper rolling is set to 1.5% or more, it is suggested that the yield elongation after aging is reduced and the workability is improved. Therefore, in the present invention, the reduction rate of the sheet thickness in the temper rolling is set to 1.5% or more. Also, 2.
If it exceeds 5%, the workability may be reduced or the appearance may be degraded. Therefore, the content is preferably 2.5% or less.

[0021]

EXAMPLE In the hot dip galvanizing line shown in FIG. 1, a low carbon steel strip containing 0.035% by weight of carbon was plated and alloyed. The traveling speed of the steel strip is 70 m /
In min, plating and alloying were performed under conventional conditions. At that time, the characteristics of the steel sheets obtained by using the manufacturing method according to the present invention and the conventional method were compared. That is, in the practice of the present invention, the hot finish rolling shown in Table 1 was performed, and the plate thickness was 1.4.
The hot-rolled material having a thickness of 1 mm was annealed under the conditions shown in Table 1, galvanized to form an alloy, and temper-rolled. On the other hand, in the conventional method, the hot-rolled material having a thickness of 3.5 mm by the hot rolling is cold-rolled to a thickness of 1.4 mm, and then annealed under the conditions shown in Table 1, followed by galvanizing and alloying. Processing and temper rolling were performed. Yield elongation after aging was measured after heat treatment at 100 ° C. for 30 minutes.

The results are collectively shown in Table 1. From Table 1, it is clear that according to the invention, the glossiness or even the workability is better than the steel strip obtained by the conventional method. Further, the manufacturing cost was reduced to about 80% of the conventional one.

[0023]

[Table 1]

[0024]

As described above, according to the present invention, an alloyed hot-dip galvanized steel sheet which is a thin sheet obtained by hot rolling a low-carbon steel and which has excellent surface properties (glossiness) and further excellent workability compared to the prior art. Can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a view showing a production line for a continuous alloyed hot-dip galvanized steel sheet.

FIG. 2 is a graph showing a relationship between a reduction rate of a thickness of a tempered rolling of an alloyed hot-dip galvanized steel sheet, a steel strip temperature in annealing, and a yield elongation after aging.

Claims (3)

[Claims] 1. A galvannealed steel sheet having a 60-degree specular gloss of 20 or more. 2. A method for producing an alloyed hot-dip galvanized steel sheet in which a steel strip is subjected to a hot dip galvanizing treatment and then to an alloying treatment, wherein the steel strip is hot-rolled as a normal low carbon steel, and then the steel strip temperature is 850 ° C. A method for producing an alloyed hot-dip galvanized steel sheet, comprising performing hot dip galvanizing after performing the above-described annealing, then performing alloying treatment, and performing temper rolling to reduce the sheet thickness by 1.5% or more. . 3. A steel strip having a thickness of 1.6 m in the hot rolling.
3. The method for producing a galvannealed steel sheet according to claim 2, wherein the thickness is not more than m.