JPS6156270A - Method for hot dip galvanizing dead soft steel - Google Patents

Abstract

PURPOSE:To make the rate of alloying of Zn proper and to prevent deterioration in workability due to an excessive rate of alloying by increasing the Al content in a hot dip galvanizing bath, hot dip galvanizing a dead soft steel strip, and heat treating it under specified conditions to alloy Zn. CONSTITUTION:When a dead soft steel strip contg. <0.005wt% C is hot dip galvanized in a hot dip galvanizing bath, the concn. of Al in the bath is regulated to 0.14-0.17wt%. The amount of molten Zn on the surface of the steel strip passed through the bath is regulated to a prescribed value, and the steel strip is heated at 530-580 deg.C for 5-15sec to alloy a proper amount of Zn with iron in the steel strip. Deterioration in the workability of the resulting Zn layer due to an excessive amount of Zn alloyed is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for hot-dip galvanizing ultra-low carbon steel.

(Prior art) As a galvanizing method for low-carbon steel sheets, a method in which a steel strip is hot-dip galvanized and then alloyed is commonly used.
When this alloying hot-dip galvanizing method is applied to ultra-low carbon steel (steel with a C content of o, ooswt% or less, including steel stabilized with Ti, Nb, etc., the same applies hereinafter), the alloying rate is There was a problem in that the processability of the plating layer was likely to deteriorate.

Figure 2 shows the Cik in steel and the alloying rate ratio when the alloying amount of steel with a C content of 0.04 wt$ is set to 1.
It can be seen that the -Zn reaction rate varies greatly depending on the amount of C in the steel, and that the ultra-low carbon steel has a faster Fe-Zn reaction rate. This is because solid solute C that segregates at grain boundaries suppresses the Fe-Zn interdiffusion reaction, but in the case of low carbon steel, there is little solid solute C,
This is thought to be because the above-mentioned suppressing effect is weak.

f like this! 'e The low reactivity of Zn has a great influence on the quality of the plated layer after alloying treatment, and the influence of reactivity can be said to be fatal, especially for the workability of the plated layer. Figure 2 shows the alloying rate and workability of the plating layer (powdering)
This shows the relationship that the workability of the plating layer is greatly deteriorated compared to normal steel types.

From the above points, it is clear that when manufacturing an alloyed steel sheet using ultra-low carbon steel, it is important to control the alloying ratio within an appropriate range.

As a countermeasure against this problem, a method has conventionally been proposed in which the alloying temperature after hot-dip galvanizing is set to a low value to suppress the alloying rate of the plating layer.

(Problems to be Solved by the Present Invention) This method is effective to some extent for relatively light processing such as instant bending. However, when subjected to advanced press processing such as in automobile applications, the effect could not be expected.

That is, Fig. 4 shows 90 grade 0005wtg6 obtained by the conventional method.
This shows the results of examining the workability of the plated layer when the alloying temperature is lowered using a 90 degree curve test and a draw speed test. The “powdering fox” in the diagram is
In the test, the 90 degree curve was removed by removing the curve using adhesive tape, and the strength of Zr+ -K was determined by removing the tacking pieces that adhered to the boot using an image light X-boss tool. In addition, in the case of the drawbead test, after 3 non-destinations, remove the accumulating pieces that adhered to the dice with ljc/, and then use Z as described above.
The intensity was measured at n- to make it constant.

From FIG. 4, it can be seen that the method of lowering the alloying rate by lowering the alloying temperature cannot sufficiently improve the plating layer workability.

In addition, the method of alloying at a low temperature as described above is not suitable for alloying due to non-uniformity of the temperature distribution in the alloying furnace, non-uniformity of the coating thickness in the width direction or longitudinal direction, and even the shape of the steel strip. It was difficult to obtain a product with a uniform conversion rate, and it also had the disadvantage that it was easy to cause color unevenness due to alloying particles having a semi-alloyed state.

(11 = Steps for Solving Problems) The present invention Fi solves the above-mentioned conventional problems and attempts to provide a method for manufacturing an alloyed hot-dip galvanized steel sheet with excellent workability in coating layers. It is something to do.

For this purpose, the inventor focused on the concentration of A in the hot-dip galvanizing bath and set the concentration higher than that when plating general low carbon At-killed steel.
- The alloying reaction rate of Zn has been reduced to the same level as that of ordinary steel, and this has succeeded in improving the workability of plating and layering after alloying treatment. .00
A steel strip of ultra-low carbon steel of 5 wt% or less is heated to 448 degrees in a bath.
After plating in a hot-dip galvanizing bath of 14 to 0.17 wt%, immediately control the plating amount to a predetermined amount, and then heat the steel strip at a plate temperature of 530 to 580 ° C for 5 to 15 seconds. This is a characteristic feature.

Hereinafter, the present invention will be explained in detail based on the accompanying drawings.

Ultra-low carbon steel has high reactivity with zinc, so when it is alloyed, the alloying rate is not high, which tends to deteriorate the workability of plating and layering.

As a countermeasure for this, even if the alloying rate is controlled during the processing process, the workability of the plating layer cannot be sufficiently improved in the case of ultra-low carbon steel. This is because the layer growth process is definitely different from other steel types.

This means that if the alloy layer formation when passing through the plating bath can be made to the same level as normal steel, the alloying ratio of ultra-low carbon steel can be maintained at an appropriate level without changing the post-alloying process (galvanil temperature). I enjoy being able to control it.

Therefore, the present inventors repeatedly conducted numerous experiments and investigations regarding changes in alloy layer formation during passage through a hot-dip galvanizing bath. As a result, by controlling the concentration of A- in the hot-dip galvanizing bath, the rapid growth of the alloy layer can be suppressed, and the alloying rate can be controlled to an appropriate level in the subsequent alloying process. It was confirmed in the drawbead test that the workability is superior to that of ordinary steel.

Figure 1 shows the At
This shows the results of examining the relationship between concentration, difficulty of alloying, and plating layer workability.
After plating two types of ultra-low carbon steel strips of 0.05 wt% in a hot-dip galvanizing bath with an AtlA degree of 0.11 to 0.18 wt%, they were heated at 550°C (soaking temperature) for 0 to 30 seconds. However, the alloying ratio is discounted to 11 to 12%, and area A in the figure is the appropriate alloying area for low carbon steel.

As is clear from Fig. 1, the At concentration in the plating bath has a great effect on the workability of plating layers and alloying. However, it can be seen that a higher 1'd and fa degree than this is suitable for ultra-low carbon steel. However, if the At concentration is made too high, alloying becomes difficult.

Therefore, the present invention sets the A7 concentration of the plating bath within a predetermined range, specifically 0.14≦M≦0.17wt.
It is desirable to control within a range of %. When the αM concentration in the plating bath is less than 0.14 wt%, the effect of suppressing alloying is insufficient, and the improvement in workability of plating layers is unsatisfactory. In addition, when the At concentration exceeds 0.17wt,
Since the Fe-Zn alloying reaction is extremely suppressed, CG
It becomes difficult to combine within the L line. Furthermore, alloying under these conditions results in high-temperature and long-time heating conditions, which inevitably increases costs and reduces efficiency.

Next, the present invention controls the plating amount to a predetermined amount immediately after passing through the plating bath, and subsequently heats the plated steel strip to 530 to 580°C.
Alloying treatment is performed by heating to a temperature (plate temperature) for 5 to 15 seconds.

Here, the lower limit of the heating temperature is set to 530° C. because if the heating temperature is lower than this, the heating time becomes long, which is not efficient, and also tends to cause uneven alloying treatment. Upper limit is 580
℃ because above this temperature, the alloying ratio is likely to fluctuate due to operational changes such as changes in sheet threading speed and change in thickness, and as a result, the workability of the plating layer is not stable.

The reason why the lower limit of the heating time is set to 5 seconds is that heating for a shorter time than this tends to result in incomplete alloying. The upper limit of the heating time was set at 15 seconds because longer heating times would result in excessive alloying and deterioration of workability; This is because it can be considered as an upper limit.

(Example) An example of hot-dip galvanizing ultra-low carbon steel according to the invention will be compared with the hot-dip galvanizing method of low-carbon At-killed steel and the conventional method of lowering the alloying temperature. This is the front page. The amount of plating for each sample was 4597 m''.

As is clear from Table 1, according to the present invention, l”e
It can be seen that the workability of plating layers after alloying treatment is improved because the alloying reaction rate of -Zn is optimized.

(Effects of the Invention) According to the present invention as described above, the alloying ratio can be properly controlled in hot-dip galvanizing of ultra-low carbon steel including stabilized steel with a C content of 0.005 wt% or less. However, the processability of the plating layer is excellent, and an excellent effect can be obtained in that a high-quality alloyed hot-dip galvanized copper plate without uneven alloying can be produced.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the M concentration in the plating layer and workability and difficulty of alloying according to the method of the present invention, Figure 2 is a graph showing the relationship between the amount of C in steel and the alloying rate ratio, and Figure 3 is a graph showing the relationship between the M concentration in the plating layer and workability and difficulty of alloying. FIG. 4 is a graph showing the relationship between the alloying rate of a layer and workability. FIG. 4 is a graph showing the relationship between the decrease in alloying rate and powdering property according to the conventional method. Special m'lH Applicant 1" Honkoukan Co., Ltd. Invention
Person: Shigeru Kamihara
Kendo Fumiyama
Masaki Abe, Patent Attorney, Ryosho Yoshi, High School Sando
Tachibana Keiyo no F Joji Yoshi
Hiroko Hara No. 1 Figure Merisa; λ meter aA (ljira & (wt%) Figure 2 JP-A-6L-56270 (5) Figure 3 Figure 4

Claims (1)

[Claims] In producing an alloyed hot-dip galvanized steel sheet of ultra-low carbon steel in which the amount of C in the steel is 0.005 wt% or less, the steel strip with the above amount of C is mixed with an Al concentration in the bath of 0.005 wt% or less.
After plating with a 14-0.17 wt% hot-dip galvanizing bath, the amount of plating is immediately controlled to a predetermined amount, and then the steel strip is heated at a plate temperature of 530-580°C for 5-15 seconds. Hot-dip galvanizing method for ultra-low carbon steel.