JPS5937355B2 - Manufacturing method of alloyed galvanized steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an alloyed galvanized steel sheet that has excellent workability, weldability, coating durability, and cosmetic properties during press working, such as for automobile bodies.

Traditionally, large amounts of snow-melting agents mainly composed of chlorides such as calcium chloride have been sprayed on the snow surface of roads in order to secure the road network in winter, mainly in North America and Northern Europe. Corrosion has become a social problem. For this reason, various anti-corrosion measures have been adopted for automobile bodies, and one of the effective measures is to use steel plates that are resistant to corrosion.

For this reason, the development of anti-corrosion steel plates for car bodies is being vigorously pursued, but they do not satisfy various performance requirements such as high workability, high-speed weldability, paint film durability, and cosmetic appearance, just like car bodies. Currently, steel plates have not yet been obtained. Currently available steel sheets for corrosion protection include hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, electrolytic galvanized steel sheets, and single-sided coated steel sheets, each of which has its advantages and disadvantages, and each of them has its own merits and demerits, and all of them satisfy various performances as steel sheets for car body corrosion protection. It's not a thing.

Hot-dip galvanized steel sheets generally have poor press workability and cannot withstand the advanced processing required for automobile bodies.

In addition, the galvanized layer on the surface stains the electrode during spot welding, and the welding strength changes greatly during continuous welding, making the reliability of the welding strength extremely low.

Alloyed hot-dip galvanized steel sheets also generally have poor press workability, and although spot weldability has been slightly improved, it is industrially difficult to reduce the amount of zinc on the side that contacts the electrode tip to 309/rr■ or less. Weldability is poor. In the electrogalvanizing method, there is no need for processes that require high temperatures during the process, and it is possible to obtain steel sheets with the same high level of press formability as cold-rolled steel sheets. It is relatively easy to obtain, and by using the bare surface (that is, the surface not plated with zinc in a single-sided plated steel sheet) as the exterior surface, it is possible to satisfy the durability of the coating film and the cosmetic properties.

However, the problem with the electrogalvanizing method is weldability, and even if a single-sided galvanized steel sheet is obtained, if the zinc surface is on the electrode tip side, the tip will be heavily contaminated by zinc and the reliability of welding strength will be compromised. becomes extremely low.

At this time, if the bare surface is on the electrode chip side, weldability will improve slightly, but if the zinc coating is thicker than 509/M2, the required welding current value will be extremely large, and the appropriate current The range of values is narrow, making it difficult to handle, and this has the disadvantage of resulting in unstable welding in which peeling and splintering occur alternately.

In addition, there are steel plates coated on one side with paint, but the coating peels off when subjected to severe processing that involves mold galling, making them unusable. Further, this type of organic coating film generally has low electrical conductivity and is a material that is difficult to weld. As described above, a steel plate for use in automobile bodies that combines high workability, high-speed weldability, paint film durability, and cosmetic properties has not yet been obtained. In view of the current situation, the present inventors have discovered, as a result of their research, a new surface-treated steel sheet that does not have these drawbacks and has various characteristics essential for a steel sheet for corrosion protection of automobile bodies, and hereby they provide it.

The present invention electrically galvanizes annealed strips obtained through normal steel manufacturing, hot rolling, and cold rolling processes in a galvanizing solution, and the amount of galvanizing on the front and back sides is different.
Thick plating side is Zn5O9/B or more, thin plating side is Z59
/If' or less, heat treatment is performed after NL to diffuse iron (Fe) into the galvanized layer, and alloy the Fe content in the galvanized layer on the thick zinc plating side by 7 to 12% by weight. This is a method for producing an alloyed galvanized steel sheet with excellent workability.

The method of the present invention exhibits its effects after combining the above conditions.

In the present invention, there is a large amount of zinc deposited on the zinc-thick plating side (1), which has excellent corrosion resistance, and has good weldability despite the two-thickness plating (3) Good paintability on both the exterior and interior surfaces.
It has excellent properties that were not previously known, such as excellent press formability of 4 steel sheets.

Therefore, according to the present invention, it is possible to stably obtain a steel plate having various characteristics essential to a corrosion-protective steel plate for automobile bodies.

The reason why we chose the electroplating method in this invention is that 1. It is easy to plate one side, 2. A uniform plating thickness can be obtained, the surface after pressing is smooth, and 3. There is no need for high temperature heating in the plating process. This is because it does not reduce the press formability of the steel plate, but the other biggest reason is that F caused by post-heating after plating
This is based on the inventor's knowledge that the 8-Zn alloy layer is formed much faster and at a lower temperature than the hot-dip plating method.

Conventional alloyed galvanized steel sheets have been galvanized exclusively by hot-dip galvanizing, and then alloyed by post-heat treatment.

However, according to the findings of the present inventor, as shown in Figure 1, it can also be seen from the relationship between time and temperature between the electroplating method @) and the melt plating method (b) until the same amount of alloying is reached. As shown, electroplating is much faster than F8-Z.
n alloy is reached. The reason for this is that in the case of the melt plating method,
This is because A, is added to the plating zinc bath in order to suppress the formation and development of F8Zn alloy, and therefore, this A1 suppresses formation of F8-Zn alloy by heating after plating.

Attempts have been made to reduce the addition of A in this hot-dip galvanizing, but in that case a brittle F8-Zn alloy layer develops and the bending performance of the F8-Zn layer specified in the JIS standard cannot be satisfied. Therefore, if an alloyed steel sheet and a galvanized steel sheet are to be manufactured on the same line, it is necessary to change the concentration of A in the zinc bath each time, resulting in extremely inefficient operation.

If the A1 concentration in the zinc bath is reduced, the corrosion loss of steel materials used in sink rolls, snap rolls, etc. will increase significantly, so from this point of view, there is a limit to reducing the A1 concentration in the zinc bath. be done. For these reasons, the electroplating method was adopted, but the electroplating method has the following two additional advantages. In other words, when performing Fe-Zn alloying treatment in one line (in this case, the temperature is preferably maintained at 400 to 550°C), the electroplating method increases the line speed by four times as much as the hot-dip plating method. Or the length of the F8-Zn alloying furnace can be shortened to 1/4, and if the F8-Zn alloying process is performed outside the 2nd line,
Alloying 7 to 12% F8 at a low temperature of about 350°C is extremely superior in terms of thermoeconomics and efficiency, but on the other hand, these advantages cannot be achieved with the hot-dip galvanizing method, even as seen in Figure 1. This is because it is clear that according to the present invention, the amount of zinc deposited on the thick plating side is set to 509/M.
The reason why the number is 2 or more is to ensure corrosion resistance.

There is still no standardized standard for the required performance of steel plates for corrosion protection on automobile bodies, but when a salt spray test is conducted after applying the primer coating, the peeling width at the cross cut portion is 3 m after 240 hours of spraying.
The general standard is less than m. The results of the test under these conditions are shown in FIG. Zinc plating amount and 240
As can be seen from the diagram showing the relationship between cross-cut coating peeling width after the time salt spray test, the amount of galvanizing was 50
This is because if it is 9/M2 or more, the above performance can be satisfied. Even if there is no alloying treatment up to a zinc plating amount of 409/In2, there will be a slight decrease in productivity (welding efficiency),
It is possible to score more than 1000 consecutive points, but the score is 509/
For M2 or higher, it is difficult to exceed 1000 points unless alloying treatment is performed, and the electrode chip requires maintenance, resulting in a decrease in efficiency. The reason why the amount of zinc plating on the thin plating side is set to 59/Nl2 or less in the present invention is as follows.

That is, one of the objects of the present invention is to obtain a steel plate that has paintability as an exterior material for an automobile body and has excellent continuous welding properties in spot welding. It is better not to have galvanization.
However, in the post-heat treatment of the F8-Zn alloy after galvanizing, an oxide film (so-called temper color) is formed when heated in a furnace in air, resulting in problems with chemical conversion treatment as a pre-painting treatment, uniformity of the paint film, and actual appearance. reduce sex.

For this reason, if full plating is not applied, it is necessary to heat the steel plate in a non-oxidizing atmosphere such as nitrogen gas.In order to supply steel sheets at a lower cost, it is necessary to perform alloying treatment in air. Desirably, an effective means for this purpose is to cover the surface that should originally be bare with a plating surface.

In this case, it is desirable to suppress the amount of plating deposited to the necessary minimum, and as a result of various studies, the upper limit was set at 5f1/d. Next, an example will be explained. Example 1 An annealed cold-rolled coil (0.8 mT1L thickness x 914 mm width) obtained through a normal steelmaking, hot-rolling and cold-rolling process was electroplated in a continuous electrogalvanizing line using a zinc sulfate bath.

In this case, the amount of zinc plating is 609/d on the thick plating side,
The other thinly plated side was made to have a ratio of 39/n]2, and alloyed by holding it in an atmospheric furnace maintained at 400° C. for 8 seconds in the line.

The results, as shown in Table 1, were sufficiently satisfactory as steel materials for corrosion protection of automobile bodies. Example 2 Ordinary steelmaking,
The amount of zinc deposited on the thick plating side was measured on a continuous galvanizing line using a zinc chloride bath using a killed steel annealed cold rolled coil (1.1 m thick x 1000 mm width) obtained through hot rolling and cold rolling processes. 909/m', electrically galvanized so that the amount of zinc deposited on the thinly plated side was 09/1n2, and was wound into a coil.

Alloying treatment was performed by holding this coil in a nitrogen gas atmosphere furnace maintained at 290° C. for 10 hours. The properties of the obtained steel sheet are shown in Table 1, and it was found to be sufficiently satisfactory as a steel sheet for corrosion protection of automobile bodies. FIG. 3 is a diagram showing ``the relationship between the number of consecutive dots and the weld diameter, which indicates the superiority of the alloyed material'' when the Zn content on the thin plating side is 0 in the electroplating method.

In the figure, (1) is 0/909/M2 alloyed, (6) is 0/40f1/ml without alloying, and 0/609/InJ is alloyed. Indicate what is missing. As can be seen from Fig. 3, method (1) is the most superior.As can be seen from Fig. 3, the present invention provides a method that has never been seen before in terms of weldability, processability, paintability, etc. for automobile body steel sheets. It exhibits excellent properties and can be used not only for automobile bodies but also for motorcycles, cruisers, etc. to demonstrate its excellence and cosmetic properties, and greatly contributes to the improvement of industrial material technology.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between time and temperature for electroplating and hot-dip plating until the same amount of alloying is reached.

Claims (1)

[Claims] 1 Annealed strips obtained through normal steelmaking, hot rolling, and cold rolling processes are electrically galvanized in a pressure-lead plating solution, and the amount of galvanization on the front and back sides is different, with the thicker plated side having a different plating thickness. 50g/m^2 or more, and the plating thickness on the thinly plated side is 5g/m^2 or less, and then heat treatment is performed to diffuse iron into the galvanized layer, and the iron is diffused into the galvanized layer on the thickly galvanized side. A method for producing alloyed galvanized steel with excellent workability, characterized in that the iron content is 7 to 12% by weight.