JPH01246348A - Manufacture of alloying hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To easily manufacture a product hardly causing the occurrence of powdering at the time of working, such as press forming, by subjecting a steel strip plated by dipping into a molten Zn bath containing specific amounts of Al and Pb to heating in an open-coil state under specific conditions. CONSTITUTION:A steel sheet after pretreatment is dipped into a molten Zn bath containing, by weight, 0.05-0.3% Al and <=0.2% Pb to undergo plating by 30-90g/m<2> coating weight per side. Subsequently, this steel strip is rewound into an open coil state and then heated in a batch-type annealing furnace of nonoxidizing or reducing atmosphere at 320-419.5 deg.C (the melting point of Zn) for 1-50hr, by which the plating layer is alloyed. When Al content in the Zn bath exceeds 0.3%, the necessity of raising the heating temp. of the steel strip to a temp. beyond 419.5 deg.C arises and powdering resistance at the time of working is deteriorated. Further, when Pb content in the plating bath exceeds 0.2%, powdering resistance at the time of working is deteriorated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet in which the plating layer is difficult to peel off during processing.

[Conventional technology]

In order to improve the durability of automobile bodies, galvanized steel sheets,
Various surface-treated steel sheets such as alloy-plated steel sheets and composite coated steel sheets are used. For these steel plates, surface texture,
Various properties such as painting performance and weldability are required, but in recent years, among these properties, requirements for corrosion resistance, particularly pitting corrosion resistance of the base steel plate, have become stricter. From this point of view, there has been an increasing demand for alloyed hot-dip galvanized steel sheets.

Alloyed hot-dip galvanized steel sheets are usually manufactured as follows. That is, hot-rolled and cold-rolled steel strips are continuously pretreated, then the pretreated steel sheet is immersed in a hot-dip galvanizing bath for plating, and then the alloy is placed on the outlet side of the plating bath. The steel sheet is passed continuously through a curing furnace to rapidly raise the temperature to a temperature of 500 to 700'C, and is held at this temperature for a short time to form a plating layer of Fe--Zn alloy. Hereinafter, this method of manufacturing an alloyed hot-dip galvanized steel sheet will be referred to as Prior Art 1.

As another manufacturing method, Japanese Patent Publication No. 59-14541 discloses that a galvanized steel sheet is rapidly primary heated, thereby remelting the plating surface, smoothing the plating surface, and partially removing the plating surface. The method for producing Fe-Zn galvanized steel sheet is referred to as Prior Art 2.

[Problem to be solved by the invention]

According to the above-mentioned prior art 1, galvanizing and alloying can be completed continuously in a short time using only continuous hot-dip galvanizing equipment, which is extremely advantageous in terms of productivity. However, since the alloying process is a rapid heating-high temperature short time alloying process, the Fe--Zn alloying reaction tends to occur non-uniformly. Therefore, since the Fe content in the plating layer varies in the board width direction and the line direction, a homogeneous product cannot be stably obtained. Furthermore, the F phase (Fe3Zn, .), which has the highest hardness and is the most brittle among the Fe--Zn alloy phases, forms thickly at the interface between the plating layer and the steel substrate, and this F phase grows. For this reason, the plating layer may peel off during processing such as press molding.
So-called powdering occurs.

The above-mentioned prior art 2 has the following problems. That is, when only a part of the plating layer is alloyed by rapid primary heating in continuous hot-dip galvanizing equipment,
A Fe-Zn alloy phase is partially generated. For this reason, the alloy layer does not grow uniformly, and especially when the coating weight is less than 909/rrj per side, there is a mixture of areas where the alloy layer has grown to the steel plate surface and areas where the η phase (Zn) remains. However, uneven baking occurs. After this, secondary heating and alloying treatment is performed in a batch annealing furnace in response to the difference in the degree of alloying caused by the primary heating, which makes the process complicated and affects the workability of the plating layer over the entire width of the plate. It is not easy to control the process of obtaining a superior product.

Therefore, an object of the present invention is to provide a method for easily manufacturing an alloyed hot-dip galvanized steel sheet that is less likely to cause powdering during processing such as press forming.

[Failure to solve the problem]

In this invention, the pretreated copper strip has an AM value of 0.0.
5 to 0.3% by weight, Pb: 30 to 902% per side by immersion in a molten zinc bath containing 0.2% by weight or less
The plated steel strip A is then charged into a patch type annealing furnace, and the atmosphere in the furnace is maintained at a non-oxidizing atmosphere or a reducing atmosphere. The strip is heated in an open coil state to a temperature within the range of 320° C. to the melting point of Zn for 1 to 50 hours, thereby alloying the plating layer.

Next, this invention will be explained in more detail.

Hot-dip galvanizing is applied to copper strips that have undergone conventional pretreatment. The material to be plated may be either a hot-rolled steel strip or a cold-rolled steel strip. However, the copper strip may be subjected to an annealing treatment before being plated by the continuous hot-dip galvanizing apparatus.

The amount of plating deposited is limited to a range of 30 to 90 mm per side of the steel strip. The amount of plating deposited may be different on both sides. The hot-dip galvanizing bath in which the copper strip is immersed is u:0. os
0.3% by weight, and 0.2% by weight or less of Pb). Plating amount from 30 to 90? /m” because sufficient corrosion resistance cannot be obtained below 30 riβ.
On the other hand, 90 y/rr? This is because, as will be described later, if the value exceeds 1, it will be necessary to significantly change the alloying conditions. A8 content in hot dip galvanizing bath from 0.05 to 0.3% by weight
The reason for this is that if it is less than 0.05% by weight, the Fe-Zn alloy phase will be partially and non-uniformly formed immediately after immersion in the plating bath.
On the other hand, if it exceeds 0.3% by weight,
This is because the formation of the Fe-Zn alloy phase is significantly suppressed, and subsequent alloying conditions need to be significantly changed.

Although Pb does not significantly influence the Fe--Zn alloying reaction, if it exceeds 0.2% by weight, the powdering resistance of the plated layer during processing decreases.

Next, after winding the galvanized steel strip as described above into an open coil, heat treatment is performed to alloy the plating layer in a batch-type annealing furnace in a non-oxidizing or reducing atmosphere. . Here, the reason for using an open coil is to uniformly heat the copper strip, prevent uneven alloying, and prevent the steel strips from adhering to each other. That is, when a tight copper strip is heated in a batch type furnace, the temperature distribution becomes partially non-uniform, and the formation of the Fe--Zn alloy phase differs locally. For this purpose, Fe-Zn
The alloy layer formed by the growth of the alloy phase becomes non-uniform. Furthermore, from a macroscopic perspective, the Fe content in the plating layer is particularly non-uniform in the longitudinal direction of the copper strip, making it impossible to obtain an alloyed galvanized steel sheet with uniform performance. On the other hand, if the open coil is heated in the same way, the heat is easily transmitted to the inside of the coil, so an alloyed galvanized steel sheet with uniform performance can be obtained. Furthermore, when heat treatment is performed at a relatively low temperature using a batch annealing furnace, no rapid diffusion reaction occurs, so that the Fe content in the plating layer becomes uniform in the depth direction of the plating layer. As a result, it is possible to obtain an alloyed galvanized steel sheet with excellent powdering resistance of the plating layer during processing.

As shown in FIG. 1, the alloying treatment in the batch annealing furnace is performed by heating the copper strip from 320°C to the melting point of Zn, that is, 419°C.
This is done by heating to a temperature in the range of 5° C. for 1 to 50 hours. When the steel strip is alloyed in this way,
The Fe content in the plating layer is uniform throughout the depth of the plating layer, with the exception of a very thin F phase that exists at the interface between the plating layer and the steel base, and its value ranges from 7 to 20% by weight. Become. As a result, an alloyed galvanized steel sheet is obtained in which the plating layer is difficult to peel off during processing such as press forming.

Here, the reason why the heating temperature of the copper strip was limited to the range of 320 to 419.5°C is because below 320°C, even if the M content in the plating bath is as low as 0.05% by weight, M The effect of suppressing the Fe-Zn alloying reaction lasts for a long time,
This is because the heating and holding time of the alloying stage becomes extremely long, making it industrially meaningless. On the other hand, if the temperature exceeds 419.5°C, the diffusion rate of atoms is too fast, and the concentration of Fe in the plating layer decreases. The slope becomes steep, resulting in poor workability, and when the copper strips are made into open coils, there is a risk that the spacer inserted between the copper strips will adhere to the plated surface, resulting in spacer marks on the plated surface. This is because there is.

The M content in the plating bath is high;
-Zn diffusion reaction is suppressed, but if the M content exceeds 0.3% by weight, it is necessary to set the heating temperature of the copper strip to a temperature exceeding 419.5°C, in which case partial diffusion This tends to occur, and on the contrary, it leads to a decrease in workability. Moreover, it also leads to the formation of spacer marks on the plating surface.

In addition, as has been traditionally said, powdering
Fe in the plating layer after alloying treatment as well as the plating amount
It depends largely on the content. That is, it is desirable to keep the average Fe content as low as possible. For this reason, the M content in the plating bath is 0.15 to 0.30.
Within the range of weight percentages, the heating time and holding time in the annealing furnace are particularly preferably from 2 to 0 hours at 340 to 380° C., respectively. In this case, the Fe content in the plating layer is 7 to 13% by weight, and the Fe content in the alloy layer is also uniform both in the plate surface direction and in the depth direction, so powdering hardly occurs. .

〔Example〕

Next, the present invention will be explained in detail.

As the material for galvanizing, both the example and the comparative example
Two types of cold-rolled steel sheets A and B with a thickness of 0.8W and having the chemical composition shown in the table were used.

Zinc plating was performed using a continuous hot-dip galvanizing facility equipped with a non-oxidizing heating furnace and a reduction heating furnace, and the amount of plating deposited was adjusted using a gas throttling device installed immediately after immersion in the hot-dip galvanizing bath. In the example, the galvanized steel strip produced in this manner was continuously alloyed in an alloying furnace in a continuous hot-dip galvanizing facility. The powdering resistance of the plating layer was determined by bending it 90 degrees to a radius of 2 rrm, pasting cellophane tape on the inside of the bend, peeling it off, and visually observing the state of the adhesion to the cellophane tape, and using the evaluation criteria shown in Table 2. Judgment was made according to the following.

Table 2 Example 1 Material A, AR: 0.21% by weight, Pb: 0.1
A galvanized steel strip was immersed in a galvanizing bath containing 0% by weight and the coating weight was adjusted to 60y/- per side.
The coil was changed into an open coil and alloyed at a heating temperature of 360° C. for 4 hours. The alloyed galvanized steel sheet thus obtained had an Fe content of 8.6% by weight in the plating layer, and had extremely good powdering resistance.

Example 2 Material B was subjected to alloying treatment under the same conditions as in Example 1. The obtained alloyed galvanized steel sheet had an Fe content of 9.2% by weight in the plating layer, and had extremely good powdering resistance.

Example 3 Material A was treated with At: 0. os weight%, Pb: 0.10
% by weight, and the coating amount was 609/lr per side. The galvanized steel strip adjusted to
The alloying process was carried out for a period of time. The obtained alloyed galvanized steel sheet had an Fe content of 7.4% by weight in the plating layer, and had extremely good powdering resistance.

Example 4 A galvanized steel strip was prepared by immersing material B in a galvanizing bath containing /u: 0.30% by weight and Pb: 0.1o% by weight, and adjusting the coating amount to 8 og/m2 per side. ,
It was changed into an open coil and alloyed at a heating temperature of 415° C. for 1 hour. The obtained alloyed galvanized steel sheet had an Fe content of 12.8% by weight in the plating layer, and had good powdering resistance.

Example 5 Material A: At: 0.15% by weight, Pb: 0.10
% by weight, and the amount of plating coating was determined to be 30y/y/y on the surface and 80y/11y on the back. The galvanized steel strip adjusted to
Alloying treatment was carried out at 80° C. for 5 hours.

In the obtained alloyed galvanized steel sheet, the Fe content in the plating layer was 12.5% by weight on the front surface and 10.1% by weight on the back surface, and the powdering resistance was extremely good on both surfaces.

Example 6 Material B was subjected to alloying treatment under the same conditions as in Example 5. In the obtained alloyed galvanized steel sheet, the Fe content in the plating layer was 13.1% by weight on the surface and 10.4% by weight on the back side.
The powdering resistance was good on both sides.

Comparative Example 1 Material A was immersed in a galvanizing bath containing AA:O, 15% by weight and Pb: 0.10% by weight, and after adjusting the coating weight to 609/- per side, a copper strip was continuously applied. was introduced into an alloying furnace and alloyed at a heating temperature of 560' for 10 seconds.The resulting alloyed galvanized steel sheet had a Fe content of 10.2% by weight in the coating layer. However, the powdering resistance was poor.

Comparative Example 2 Material B was subjected to alloying treatment under the same conditions as Comparative Example 1. The obtained alloyed galvanized steel sheet had a Fe content of 11.0% by weight in the plating layer, but its powdering resistance was poor.

Comparative Example 3 Material A, AI! : 0.12% by weight, Pb: 0.1
It was immersed in a zinc plating bath containing 0% by weight, and the amount of plating deposited on one side was 601/n? After the adjustment, the copper strip was continuously introduced into an alloying furnace and alloyed at a heating temperature of 500° C. for 15 seconds. Although the obtained alloyed galvanized steel sheet had an Fe content of 9.6% by weight in the plating layer, its powdering resistance was not very good.

Comparative Example 4 Material B: Ae:O, 15% by weight, Pb: 0.10
After adjusting the coating amount to 30 f/m' on the front side and 8 og/m2 on the back side, the copper strip was continuously introduced into an alloying furnace and heated at a heating temperature of 520°C for 12 hours.
Alloying treatment was carried out for seconds. The obtained alloyed galvanized steel sheet had an Fe content of 12.2% by weight on the surface and 8.8% by weight on the back, but the powdering resistance was poor on both surfaces.

The results of Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Table 3.

As is clear from Table 3, Examples 1 to 6 are all excellent in powdering resistance, while Comparative Examples 1 to 4 are all poor in powdering resistance. This is Comparative Examples 1 to 4.
Do not heat-treat the copper strip in an open coil state,
Another reason is that the Fe content in the plating layer is not uniform in the depth direction of the plating layer because the alloying treatment is performed according to conditions other than the alloying conditions of the present invention.

〔Effect of the invention〕

As explained above, according to the present invention,

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between heating temperature and heating time. Fig. 1 Heating time (J Town procedural amendment (voluntary) 8711 1988 Director General of the Patent Office Yoshi 1) Tsuyoshi Moon 1, Indication of the case Patent application No. 1983-74166 2 Name of the invention Alloyed molten zinc Manufacturing method for galvanized steel sheets 3, relationship with the amended case Patent applicant address: 1-2 Marunouchi-chome, Chiyoda-ku, Tokyo AA (Ai
h+ Nippon Kokan Co., Ltd. Representative Akinari Yamaguchi 4, Agent address thIII for Kanagawa Prefecture + 6 Hiramatsu Kawasaki Building Sui, 2-1-4 Sunako, Yozaki-ku 〒210 Telephone +
0441222-730 (Representative) Self Add the following to the Detailed Description of the Invention column of the specification of the invention, page 6, line 19, next to "The powdering resistance of the layer decreases.""In addition, you may perform minimized processing if necessary."

Claims (1)

[Claims] 1 The pretreated steel strip is Al: 0.05 to 0.3
% by weight, Pb: 0.2% by weight or less is immersed in a molten zinc bath to provide plating of 30 to 90 g/m^2 per side, and then the above-mentioned steel plated in this manner. The strip is charged into a batch annealing furnace, the atmosphere in the furnace is maintained in a non-oxidizing atmosphere or a reducing atmosphere, and the steel strip is heated in an open coil state to a temperature within the range of 320° C. to the melting point of Zn. 1. A method for producing an alloyed hot-dip galvanized steel sheet, comprising heating for 1 to 50 hours, thus alloying the plating layer.