JPH0297653A - Alloying hot dip galvanized steel sheet excellent in workability and its production - Google Patents

Abstract

PURPOSE:To obtain a galvanized steel sheet excellent in workability by subjecting a steel sheet plated with a hot dip galvanizing bath having a bath composition in which Al concentration and a difference between Fe concentration and the above are specified, respectively, applying alloying treatment and regulating Fe concentration in the plating layer to the prescribed value. CONSTITUTION:A hot dip galvanizing bath having a composition in which the overall concentration of Al and a difference between Fe concentration and the above are regulated to 0.13-0.2% and 0.05-0.17%, respectively, is prepared. Galvanizing is applied to a steel sheet by using the above plating bath, and then, the above steel sheet is heated to undergo alloying treatment, by which the plating layer is formed into a Zn-Fe alloy layer in which Fe concentration is regulated to 8-17%, and further, Zn is diffused and infiltrated into grain boundaries in the surface of the steel sheet as base material.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an alloyed hot-dip galvanized steel sheet with excellent workability and a method for manufacturing the same.

[Conventional technology]

Alloyed hot-dip galvanized steel sheets are widely used as components of automobiles, light electrical equipment, and the like. Alloyed hot-dip galvanized steel sheets are manufactured by passing the steel sheet through a hot-dip galvanizing bath to form a galvanized layer with a predetermined amount of coating. ) is manufactured by performing an alloying treatment to improve the

The hot-dip galvanizing has a total Al concentration (T.

taI! , A42% (hereinafter rT, A1%) about 0.1
A plating bath adjusted to 4% or less is used. This is because when the An concentration in the plating bath increases, a large amount of Fe-Al! , an alloy layer is formed, and the diffusion of Fe from the steel sheet base to the galvanized skin in the subsequent alloying process is hindered by the Fe-A1 alloy layer,
This is because it causes unevenness in the alloying reaction (uneven firing), and if the alloying reaction is strengthened by increasing the alloying temperature to reduce the uneven baking, so-called powdering occurs in which the plating layer turns into powder. This is because phenomena occur.

Alloying treatment after hot-dip galvanizing improves the adhesion of the plating layer,
From the standpoint of improving the coating properties and the like on the surface of the plating layer, the zinc plating layer is a Zn-Fe alloy layer having a Fefi degree of about 9 to 15%, so that the alloying reaction occurs in just the right amount. The treatment temperature, time, etc. are controlled to ensure that.

The alloyed hot-dip galvanized steel sheet is subjected to an appropriate surface treatment (for example, chromate treatment, etc.), and then subjected to a coating process such as a roll coater, curtain coat, or powder coating. In recent years, from the perspective of improving productivity, there has been a trend toward so-called pre-coating, where coating is applied before forming, instead of the conventional process of applying a coating after forming a plated steel sheet. There is also an increasing number of requests for the formation of Also,
2. Description of the Related Art The use of rust-proof steel sheets for automobile bodies and the like has progressed, and alloyed hot-dip galvanized steel sheets are now being used for deep-drawn parts and the like.

[Problem to be solved by the invention]

However, by reversing the order of forming and painting a hot-dip galvanized steel sheet from the conventional process, and performing forming after applying a film as described above, the cut edge of the processed steel sheet Moreover, as shown in FIG. 4, peeling (G) is likely to occur between the steel plate (1) and the galvanized layer (2). This peeling is an interfacial peeling that occurs due to stress concentration at the part where the adhesion performance between the layers is lowest due to the difference in deformability of each layer of the base plate plating layer and the paint film during the deformation process accompanied by shear stress, and is commonly known as enamel hair. has been done. The thicker the coating, the more conspicuous the peeling tends to occur.

In view of the above, the present invention does not cause peeling of the plating layer or coating film, not only when deep drawing a plated product but also when forming after forming a thick coating film. The object of the present invention is to provide an alloyed hot-dip galvanized steel sheet with excellent adhesive workability and a method for manufacturing the same.

[Means and effects for solving the problems] The alloyed hot-dip galvanized steel sheet of the present invention has a galvanized layer with an Fe concentration of 8
~17%, and is characterized by having a structure in which zinc diffuses into the grain boundaries on the surface of the steel sheet base.

The alloyed hot-dip galvanized steel sheet of the present invention is hot-dip galvanized to a total A14 degree (T, A42%): 0.1.3
~0.2%, and the difference between T, An% and Fe4 degrees (Fe%) is 70.05-0.17%, and then in the alloying treatment,
While heating, the plating layer was coated with Zn with an Fe concentration of 28 to 17%.
-It is manufactured by forming a Fe alloy layer.

The present invention will be explained in detail below.

The hot dip galvanizing bath used in the present invention is T.

Affi% is 0.13 to 0.20%, and the difference between T, A42% and Fe% (T, Af-Fe) is 0.05 to 0.17
The bath composition is adjusted to be %. That T,, An! % and F
The difference from e% corresponds to the effective concentration of A/2 in the bath, that is, the concentration of Af that reacts with the steel sheet base. The reason why we decided to set the hot-dip galvanizing bath composition to a high A421 degree is to generate a slightly larger Fe-Aff alloy layer at the interface between the steel sheet base and the galvanized layer, and to prevent the formation of the subsequent plating layer. In the alloying process (as described below, the processing conditions are on the overbaking side compared to the alloying process in the production of conventional hot-dip galvanized steel sheets), suppressing the excessive progress of the alloying reaction and producing an alloy with just the right amount. This is to accomplish the conversion process.

Fe in the galvanized layer formed using the above plating bath
% is approximately the same as that in the plating bath, 0.03 to 0.08
It is about %.

In the alloying treatment after hot-dip galvanizing, the galvanized layer is changed to a Zn--Fe alloy layer with just the right degree of alloying to improve adhesion and paintability, that is, with an Fe concentration of 8 to 17%. This alloying process is accomplished under more overbaked side process conditions than conventional ones.

The specific processing conditions are individually set depending on the capacity of the alloying furnace used, but an example is shown below in comparison with conventional processing conditions using the same alloying furnace. . In both the invention example and the conventional example, the Fe concentration in the galvanized layer before alloying treatment is about 0.04%, and the Fe concentration after alloying treatment is about 12%. The temperature inside the furnace was set at approximately 850°C, and the fuel was butane.

Invention example Conventional example Fuel supply MCN I/H) 140 110 Processing time (sec) 18 14 Above high A
Zinc plating performed using a plating bath with an ffi concentration;
Hot-dip galvanized steel sheets obtained through alloying treatment under conditions that can be called overbaking according to conventional wisdom have high adhesion between the plating layer and the steel sheet base, and high adhesion of the paint film to the plating layer. As shown in the Examples below, even in the molding process after applying a thick film such as powder coating, peeling of the paint film such as enamel hair does not occur. In addition, in the present invention, a slightly large number of Fe-A/! Since an alloy layer is generated,
A proper alloying reaction is achieved in the alloying treatment,
There is no possibility of overbaking and resulting powdering.

The alloyed hot-dip galvanized steel sheet of the present invention has a very characteristic microstructure. That is, on the surface of the steel sheet substrate exposed by dissolving and removing the alloyed galvanized layer with a corrosion reagent, a clear mesh pattern of α-Fe grain boundaries is observed, unlike conventional materials. FIG. 1 (1) shows the microscopic structure (x 3000) of the surface of the steel plate base material after the plating layer has been dissolved and removed (sample material: invention material in the section by implementation below). In contrast to this, Fig. 2 (1) shows the microscopic structure (x 3000
) (sample material: comparative material in the section by implementation below), and no α-Fe crystal grain boundaries can be seen on the surface of the steel sheet base.

As mentioned above, in the alloyed hot-dip galvanized steel sheet of the present invention,
The reason why grain boundaries, which are not seen in conventional materials, appear on the surface of the steel sheet base from which the plating layer has been melted and removed is due to the alloying reaction of the plating layer (from the tail plate to the plating layer) during the alloying process as described above. In parallel with the diffusion and solid solution of Fe), the selective diffusion and solid solution of Zn from the coating layer to the grain boundaries of the steel sheet base is promoted, and Zn is enriched in a wedge shape along the grain boundaries. by. In addition, the reason why the galvanized steel sheet of the present invention has excellent adhesion and workability and does not easily cause enamel hair damage is due to the wedge effect caused by the penetration of Zn into the grain boundaries of the base steel sheet. This is thought to be due to the strong bonding force imparted to the interface. In addition, the first
Figure [■] and Figure 2 (n) are the microscopic structure of the plating layer of the above-mentioned test plated steel sheet < x 36oo), and Figure 1 [■]
] (invention example) has a granular structure, and FIG. 2 [■] (conventional material) exhibits an acicular structure. However, this difference is not essential to distinguish the material of the present invention from the conventional material (the structure of the plating layer of the material of the present invention may have an angular structure in addition to a granular structure, Similar acicular structures may also be present.

〔Example〕

Hot-dip galvanizing and alloying treatment were performed in a continuous hot-dip galvanizing line, and then a coating film was applied to the plated surface to obtain test materials A (invention material) and B (conventional material).

(r) Manufacturing conditions for test material A (invention example) (1) Plating original plate: 525C carbon steel strip (thickness = 0.5 mm) (2) Hot-dip galvanizing plating bath composition: T, Al O, 15% , Fe0.03
%, (T, Al - Fe) 0.11% Bath temperature: 4
65°C±5°C Plating deposition amount: 45g/I (both sides) (3) Alloying furnace temperature: 850°C Fuel and supply amount Nibutane gas, 14ONrd/Hr Processing time: 18 seconds (4) Coating film coating powder A thermosetting resin coating with a thickness of 40 to 50 μm is formed using a coating method.

(ff) Manufacturing conditions (1) Plating original plate: Same as sample material A (2) Hot-dip galvanizing plating bath composition: T, A1 0.12%, F e O, 0
4%, Bath temperature: Same as sample material A. Plating coverage: Same as sample material A. ) Same as paint film coating sample A (powder coating, film thickness 40 to 50 μm) 1st
Figure (1) shows the steel plate base surface after the plating layer of test material A (invention example) before painting was dissolved and removed with an aqueous hydrochloric acid solution (1/4), and the same figure (II) shows a scan of each of the plating layers. The microstructure obtained using a type electron microscope is shown in Figure 2 (1) (I
N indicates the microstructure and plating layer on the surface of the steel sheet substrate observed under the same conditions as above for sample B (conventional example) (all magnifications are 3000).

In addition, Fig. 3 shows the cross section of the cut end of sample material A (invention example) after the shearing test, and Fig. 4 shows the cross section of the cut end of test material B (conventional example) after the same test. Each is shown enlarged approximately 200 times. In each figure, (1) is a steel plate, (2) is a plating layer, and (3) is a coating film. In FIG. 4, (G) shows peeling that occurred between the steel sheet base and the plating layer. As is clear from the comparison between the two figures, ``Specimen B prepared by the conventional method''
In contrast, the coating film peeled off at the interface between the plating layer and the steel sheet base, whereas the sample material A according to the present invention had no such abnormality and had a healthy adhesion state.

〔Effect of the invention〕

The hot-dip galvanized steel sheet manufactured by the present invention has excellent workability, and does not peel off like conventional materials, and does not suffer from defects such as powdering, not only after plating but also during forming after thick coating. Not at all. Therefore, automatic army,
Pre-coating and thick coating are possible when used as components of light electrical equipment, and its good peeling resistance allows it to be used not only for plated products but also for painted products, such as when processing strong folds such as picture frames. It is also possible to apply it to bending processing and even ultra-deep drawing processing.

[Brief explanation of the drawing]

Fig. 1 (1) and Fig. 2 (1) are micrographs substituted for drawings showing the metallic Mi weave on the surface of the steel sheet substrate from which the alloyed hot-dip galvanized layer has been dissolved and removed, Fig. 1 (II) and Fig. 2 (II)
) is a photomicrograph substituted for a drawing showing the metal composition of the alloyed hot-dip galvanized layer, and Figures 3 and 4 are enlarged cross-sectional views showing the cut end surface of the coated alloyed hot-dip galvanized steel sheet after processing. . Steel plate, 2: plating layer, 3: coating film. #! 1s L side' made

Claims (1)

[Claims] 1. In an alloyed hot-dip galvanized steel sheet, the galvanized layer is a Zn-Fe alloy layer with an Fe concentration of 8 to 17%, and the steel sheet base has zinc at the grain boundaries on its surface. An alloyed hot-dip galvanized steel sheet with excellent workability, characterized by having a structure in which is diffused and penetrated. 2. Hot-dip galvanizing with total Al concentration: 0.13-0.
20%, and the difference between total Al concentration and Fe concentration: 0.05
It is carried out using a hot-dip galvanizing bath having a bath composition of ~0.17%, and then in the alloying treatment, the plating layer is heated to become a Zn-Fe alloy layer with an Fe concentration of 8 to 17%. The method for producing an alloyed hot-dip galvanized steel sheet with excellent workability according to claim 1.