JPH111755A - Galvanized steel sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plated steel sheet, capable of properly producing, with ease, a hot dip galvanized steel sheet and a galvannealed steel sheet, both having superior adhesion, by the use of the same plating bath of identical composition and also capable of greatly contributing, at the time of production, to the improvement of productivity and reduction of manufacturing costs, and its production. SOLUTION: The galvanized steel sheet excellent in adhesion can be produced by forming, as a first layer, a surface-reduced scale layer 4 having cracks on a steel sheet to be a base material 1, further forming, as a second layer, an Fe-Al alloy layer or Fe-Al-Zn alloy layer 6 on the above layer, and further forming, as a third layer, a galvanizing layer 7 or Zn-Fe alloy plating layer. The galvanized steel sheet excellent in adhesion can be produced by forming, after hot rolling of a steel sheet to be a base material 1, cracks in the scale resultant from hot rolling by means of thermal shock, shot blasting, etc., then reducing the scale surface by a reduction furnace, etc., and further passing the steel sheet through a hot dipping bath.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet which is inexpensive and has good adhesion, and a method for producing the same.

[0002]

2. Description of the Related Art At present, a raw sheet of a continuous hot-dip galvanizing line is a pickled steel sheet obtained by completely removing a scale generated in a hot rolling step in a pickling line of a hot-rolled steel sheet, or A cold rolled steel sheet obtained by subjecting this pickled steel sheet to a cold rolling treatment is used. However, since it is necessary to completely remove the hot-rolled scale from all of these steel sheets, the processing cost for descaling is high, and since there are many steps to pass through the plating line, the plated product The construction period is long. Further, in order to enhance the plating adhesion of these steel sheets, a smooth alloy stable layer is formed between the base material and the galvanized layer. The type of the alloy layer is mainly Fe-Al alloy layer Fe
-Al-Zn based alloy layer.

Further, as a plated steel sheet having excellent weldability, such as an automobile body assembled by welding, an alloyed hot-dip galvanized steel sheet (hereinafter referred to as GA and GA) in which iron is diffused in a plating layer in order to improve weldability. There are two types of ordinary hot-dip galvanized steel sheets (hereinafter referred to as GI) for household appliances and building materials. To make these two types separately, it is conventionally necessary to change the Al concentration of the plating bath. There is. For this purpose, means such as performing line deceleration or line stop, adjusting the Al concentration of the bath, or switching to a plating pot having a different plating bath component is used.

[0004]

As described above, the original plate of the plated steel sheet needs to be descaled, and has a problem that the production cost is high and the construction period is long, and it is inexpensive and can be produced in a short construction period. A steel sheet having good adhesion is desired. Further, there is a problem that the product type switching between the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet greatly hinders productivity, and a method of separately producing two types of galvanized steel sheets that do not impair productivity is also desired. .

[0005]

As a method for obtaining a galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which are inexpensive and have good plating adhesion, a number of cracks are put on a hot-rolled scale. It has been found that it is effective to use a hot-rolled steel sheet as a base sheet for plating. Furthermore, they found a method of easily producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet in the same plating bath component by using this original sheet.

Hereinafter, the present invention will be described in detail.
The boundary between the base material and the hot-dip plating layer, which affects the plating adhesion, is formed of a uniform layer of an Fe-Al-based or Fe-Al-Zn-based alloy. In order to further improve the plating adhesion, the inventors of the present invention have conducted studies and found that it is desirable that this adhesion layer be joined with a highly irregular and random structure and a highly ductile metal or the like. did. This is because a metal having a high ductility acts as a cushioning material when working from various angles such as bending and impact, and has an effect of increasing the adhesion area and increasing the adhesion further by increasing the unevenness.

The inventors have invented that a method for obtaining a layer structure having excellent adhesion can be obtained by the following process.
First, as shown in FIG. 1, a hot-rolled sheet in which a number of cracks 3 are formed in a hot-rolled scale 2 of a base material 1 is used as a plating base sheet. As a method for forming the cracks 3 in the hot-rolled scale 2, rolling, bending by a leveler, shot blasting, thermal shock by quenching or the like, light pickling, or the like can be used.

Next, as shown in FIG. 2, in the reduction furnace of the plating line, the scale 4 surface layer of the base material 1 and the scale 4
Of the three cracks in the above to secure the reactivity with the plating bath. Note that reference numeral 5 is a reduction unit. Thereafter, when continuously immersed in the hot-dip galvanizing bath, the plating bath invades the cracks of the scale 4 on the surface of the steel sheet and reacts. Further, as shown in FIG. 3, a reaction with the plating bath also occurs on the surface layer of the scale 4 so that Fe-Al or Fe-Al
The Al-Zn based alloy layer 6 is formed. The plated steel sheet thus obtained has irregularities at the boundary between the plating layer and the base material,
In addition, the crack portion of the scale has an intermediate layer filled with highly ductile zinc, and thus has excellent adhesion. Reference numeral 7 denotes a Zn plating layer.

Another feature of this steel sheet is the Al reaction behavior during immersion in a plating bath. When the plating bath solution passes through the crack portion of the scale, the Al in the plating bath component and the reduced scale react preferentially to Zn, so that the Al in the plating bath reaching the vicinity of the base metal. The concentration is low. Therefore, the Fe-Al-based or Fe-Al-Zn-based alloy formed at the boundary between the plating bath and the base material becomes thin. When the intermediate layer becomes thinner, re-heating after plating or keeping the plating bath temperature at a high temperature facilitates the diffusion of Fe into the plating layer, and the Fe-Zn alloying reaction occurs relatively easily. This is equivalent to a substantial effect of lowering the Al concentration in the plating bath.

Therefore, according to the method of the present invention, the same Al
By changing the plating bath temperature, a hot-dip galvanized steel sheet (hereinafter, referred to as GI) and an alloyed hot-dip galvanized steel sheet (hereinafter, referred to as GA) can be separately formed by changing the plating bath temperature. Conversely, by maintaining the plating bath temperature at a relatively high temperature, GI and GA can be obtained even in a plating bath having the same Al concentration regardless of the presence or absence of scale cracks on the steel sheet surface. That is, Al for GI
For steel plates with scale cracks even in a plating bath with a high concentration,
The adhesion layer becomes thin, and the production of an alloyed hot-dip galvanized steel sheet is possible.

[0011]

Example 1 The apparatus of FIG. 4 is a schematic diagram of an experimental apparatus simulating a continuous hot-dip galvanizing facility, in which a test material 8 was heated in a non-oxidizing furnace 9 and nitrogen gas and hydrogen gas were used. Then, the scale surface layer and the scale crack surface layer are reduced in the reduction furnace 10 filled with the above. Subsequently, in a gas-sealed state, the zinc plating bath 11 is immersion-plated, and a gas wiping device 12 controls the amount of zinc deposition. 1.6mm thick low carbon hot rolled steel sheet (with hot rolled scale)
Was used. In order to crack the scale of the hot-rolled steel sheet, cold rolling or light pickling was performed in advance to produce a test material. Passing conditions are set to 5
50-850 ° C., the hydrogen concentration in the reduction furnace 10 is 2-15%,
In addition, the residence time of the reduction furnace 10 was 20 sec, the plating bath temperature was 450 to 470 ° C., and the Al concentration in the plating bath was 0.12 to 0.12.
0.28 wt%. The experimental results are shown in Table 1. It was confirmed that the plating adhesion was good at any of the levels 2 to 7 in which cracks were formed in the hot-rolled scale under roll pressure or pickling.

[0012]

[Table 1]

Example 2 A low-carbon hot-rolled steel sheet having a thickness of 1.6 mm (with a hot-rolled scale) was used as a test material.
The test material was obtained by cracking the test piece. Further, as a comparative material, GI and GA were produced using the apparatus shown in FIG. 4 using an acid-washed plate from which the scale of the test material was completely removed by pickling. As test conditions, a common condition is a steel plate heating temperature of 700 ° C., a hydrogen concentration of 5% in the reduction furnace 10, a reduction time of 20 sec, and a plating bath Al concentration of 0.18 wt%. Change
An experiment was performed. Table 2 shows the results of this test. Level 1
The plating bath temperature and the plating bath intrusion plate temperature of No. 4 are the production conditions of a normal hot-dip galvanizing line.
Concentrations are at the GI production level.

[0014]

[Table 2]

Here, level 1 is the same as the conventional GI manufacturing condition. Level 2 applies the application of the alloying furnace to the conditions of level 1, but since the plating adhesion layer is thick,
Even if an alloying furnace is applied, only GI can be manufactured. Level 3
Is obtained by applying the original plate having a scale crack of the present application to this, and a GA can be manufactured by applying an alloying furnace. That is, even under the same bath conditions, GI and G
A can be made separately. However, in this case, even if the original plate has scale cracks,
Not A but GI.

Further, levels 5 to 7 are conditions when the plating bath temperature is raised. In the case of level 5, when the plating bath temperature is increased, GA can be produced without using an alloy or a furnace by using an original plate having scale cracks. However, only a GI can be manufactured with a normal master plate as in the case of level 6. That is, by increasing the bath temperature, the GI can be maintained at the same bath condition even without the
And GA can be made separately. However, if the plating bath temperature is further increased, it is possible to manufacture GA using a normal original plate. In this case, however, the line must be stopped because the bath temperature must be increased in order to change the type of plated steel sheet. . On the other hand, the temperature of the plate 8 or 9 is raised, and the GA and GI can be separately formed depending on the presence or absence of the scale crack.

[0017]

As described above, according to the present invention, it is possible to manufacture a steel sheet having good adhesion which can be manufactured at a low cost and in a short period of time. It is easy to produce galvanized steel sheets, and when producing galvanized steel sheets with good adhesion,
This greatly contributes to improving productivity and reducing manufacturing costs.

[Brief description of the drawings]

FIG. 1 is a view showing a state of a base material in which cracks are added to a hot-rolled scale according to the present invention;

FIG. 2 is a diagram showing a state of a base material after reduction according to the present invention;

FIG. 3 is an enlarged cross-sectional view of a plated steel sheet according to the present invention;

FIG. 4 is a schematic diagram of an experimental apparatus used in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Base material 2 Hot rolled scale 3 Crack 4 Scale 5 Reduction part 6 Fe-Al type or Fe-Al-Zn type alloy layer 7 Zn plating layer 8 Test material 9 Non-oxidizing furnace 10 Reduction furnace 11 Plating bath 12 Wiping device 13 Alloying furnace

Claims (3)

[Claims] 1. A scale layer having cracks and a reduced surface is formed as a first layer on a steel sheet as a base material, and a Fe-Al-based alloy layer is formed thereon as a second layer. Alternatively, a galvanized steel sheet having excellent adhesion, wherein a Fe—Al—Zn-based alloy layer is formed and a Zn plating layer or a Zn—Fe alloy plating layer is formed as a third layer. 2. After hot rolling a steel sheet as a base material, cracks are formed on the scale generated during the hot rolling by thermal shock or shot blasting, and then the scale surface is reduced by a reducing furnace or the like. 2. The method for producing a galvanized steel sheet having excellent adhesion according to claim 1, wherein the steel sheet is reduced and then passed through a hot-dip plating bath. 3. An alloyed hot-dip galvanized steel sheet by using a steel sheet having scale cracks and a normal steel sheet having no scale as a base sheet to be provided to a plating bath.
A method for producing a galvanized steel sheet, which comprises separately producing a normal hot-dip galvanized steel sheet.