JPS6358225B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a steel sheet that has been subjected to hot-dip galvanizing followed by alloying treatment (hereinafter simply referred to as alloyed steel sheet).

Alloyed steel sheets are far superior to general galvanized steel sheets in terms of paint adhesion, paint corrosion resistance, weldability, etc., and the demand for them has been steadily increasing in recent years. There is.

In general, the production of alloyed steel sheets is often carried out on the same line between the production of regular galvanized steel sheets that are not subjected to alloying treatment. It is necessary to add 0.1 to 0.2% Al to the zinc melt in order to suppress the formation of aluminum as much as possible and improve the processing adhesion of the film. On the other hand, in the case of manufacturing alloyed steel sheets, it is better to have as little Al as possible in the zinc melt. In other words, the film on the alloyed steel sheet is mainly δ ₁ −FeZn ₇ or ζ
-It is composed of an intermetallic compound of FeZn ₁₃ , but if Al is added to the zinc melt, immediately before the steel plate is sent to the alloying furnace on the line,
The composition of the film is mostly η-Zn or its melt, and an ultra-thin Al-Bearing δ ₁ is directly on the steel plate.
(Al-rich δ ₁ −FeZn ₇ ) or Al such as Fe ₂ Al ₅
An enriched layer is formed. This Al-enriched layer has the function of suppressing mutual diffusion between Fe and Zn, and acts to delay alloying in the alloying process, so the amount of Al in the melt is small in order to promote alloying. This means that it is more advantageous. Therefore, when a line is used to manufacture both alloyed steel sheets and normal galvanized steel sheets that are not alloyed, it is ideal to keep the Al concentration in the zinc melt to a level that is sufficient for the production of normal galvanized steel sheets. It is desirable that the concentration be high enough to ensure good processing adhesion, and that it be as low as possible when producing alloyed steel sheets. However, in practice, changing the Al concentration in the zinc melt is not reliable due to complicated problems related to creating process plans and feeding and replenishing the Al-Zn master alloy. This is something I would like to avoid if possible. Rimmed steel is the most common base material for galvanized steel sheets, but when rimmed steel is used as the base material, conventionally, for the reasons mentioned above, the Al concentration in the zinc melt has been adjusted to We set a compromise that allows the film to maintain good processing adhesion and also maintains the alloying rate at a relatively high level during the alloying process, that is, around 0.1%, and alloyed it with the galvanized iron plate without changing this. The company manufactured both steel plates. In other words, in the past, the alloying speed had to be sacrificed to some extent in order to ensure the adhesion of the galvanized iron plate coating, and this alloying speed was never sufficient for continuous operation. Ta.

On the other hand, recently, killed steel produced by continuous casting has been increasingly used as the base material for galvanizing. The reason for this is that it can be provided at a lower price than rimmed steel, but this killed steel tends to have poorer adhesion of the hot-dip galvanized coating than general rimmed steel, so killed steel is used as the base material. In this case, it becomes necessary to make the Al concentration in the zinc melt even higher than in the case of the rimmed steel base material, specifically, 0.14% or more. However, in the case of alloying treatment, the Al concentration must be suppressed to at least the upper limit of 0.11% in actual operation. Therefore, when applying killed steel to the base material,
As with the rimmed steel base material mentioned above, the Al concentration in the zinc melt is kept constant throughout the production of both galvanized iron sheets and alloyed steel sheets.
It is not possible to take a method that is not necessarily advantageous, and it is necessary to change the Al concentration depending on the presence or absence of alloying treatment, which is even more problematic. Moreover,
Failure to smoothly adjust and transition the Al concentration when changing the Al concentration will result in defects that result in product losses, such as so-called raw discoloration of the alloyed steel sheet and poor adhesion of the coating on the galvanized iron sheet.

As mentioned above, in the past, not only killed steel base metal but also rimmed steel base metal has been
There is a problem that Al slows down the alloying process speed, but if we can obtain a plating film with good processing adhesion using a zinc melt containing almost no Al, or conversely, use a high Al bath. It is not hard to imagine that if there was a way to speed up the alloying rate under conditions using , it would be of great help in solving the above problems.

The purpose of the present invention is to provide a method for manufacturing an alloyed steel sheet that can ensure an extremely high alloying rate equivalent to or higher than that in the case of a low Al concentration even when using a high Al bath with an Al concentration of 0.14% or more. It is something to do.

That is, the gist of the present invention is to provide an alloying treatment characterized in that, in the production of an alloyed steel sheet, the surface of the steel sheet after softening annealing is machined to give a new surface at a stage before completion of passing through a zinc bath. There is a manufacturing method for steel plates.

As a result of extensive experiments and studies by the inventors,
If the surface of a steel sheet is mechanically processed in a zinc melt or before being immersed in a zinc bath to give it a new surface, the alloying speed of the film in the subsequent alloying process will be dramatically improved. A new fact has come to light. That is, such a phenomenon
This can be expected in the range of Al concentration in the bath from 0.05 to 0.3%, but this is because the formation of an Al-enriched layer on the steel sheet is suppressed by the addition of the above-mentioned new surface, and as a result, the alloying treatment It is considered that the latent time of the so-called Fe-Zn interdiffusion in the aluminum alloy is eliminated, and alloying is achieved in almost the same amount of time as in the case of a low Al concentration. The alloying rate in this case does not substantially depend on the Al concentration of the zinc melt, and therefore the time required for alloying treatment hardly varies depending on the Al concentration.

In addition, even when alloying treatment is performed at the same time as C precipitation treatment during post-annealing by box annealing after plating outside the line, as above, the alloying treatment can be performed at a constant rate regardless of the Al concentration in the plating bath, and the annealing process can be performed at a constant rate. The time can be greatly reduced.

The machining process to provide the new surface may be performed by any method such as polishing or grinding using a wire brush roll or an abrasive belt, shot blasting, or the like.

This machining step is basically performed either during the plating bath or after annealing and before entering the plating bath. In other words, the Sendzimir method (or non-oxidation furnace method) in which the steel plate is annealed in the Metsuki line.
In this case, a steel plate 1 is supplied from a payoff reel 2 as shown in FIG.
It passes through the recrystallization reduction zone 4, passes through the low temperature holding zone 5, enters the plating bath 6, and then passes through the alloying treatment furnace 7. In the case of such a plating process, the above-mentioned machining The implementation stage is as follows:
Recrystallization zone 4 as shown by wire brush roll 8 in the figure.
It is necessary to set the stage at any stage from the time of completion of passage to the time of completion of passage of plating bath 6. In machining before annealing,
This is because the effect is lost during the annealing process. On the other hand, in the case of the flux method, a similar plating method that requires annealing outside the plating line, or a plating method that does not undergo an annealing process within the line, the steel plate is supplied from the payoff reel 2 as shown in FIG. 1 enters the plating bath 6 after passing through a degreasing tank 9, a pickling tank 10, a flux tank 11, and a preheating drying oven 12 in order. As illustrated in 8, immediately after leaving the payoff reel 2, that is, before undergoing substantially in-line processing, pre-treatment steps 9, 10, 11, 12 are performed.
Exactly the same effect can be expected whether machining is performed at any stage, during the plating bath 6 or during the plating bath 6. Even if machining is performed before pretreatment, pickling process 1
There is no concern that the effect will be lost if the value is 0.

Needless to say, it is preferable to perform machining as uniformly as possible.

Next, the effects of implementing the present invention will be explained using specific examples.

Example 1 SPCC Steel plate based on JIS-G3141 (50mm width x 100mm
A new surface is given to the surface (length x 0.8 mm thick) by blasting or grinding in advance, and this steel plate is immersed in a plating bath at 460°C for 5 seconds and then rolled to 70g using the normal flux method. Hot-dip galvanizing was carried out to give an area weight of /m ² . Al in bath
Two concentrations were used: 0.1% and 0.2%. After plating, alloying treatment was carried out at 500°C in a heat treatment furnace, and at this time changes in the amount of Fe in the film over time were investigated. The results are shown in Figure 2 A and B. In the same figure, A is 0.1%, and B is 0.2%.
In the figure, △ indicates the blasting process, ● indicates the grinding process, respectively.
○ indicates that the corresponding processing was not performed.

In the same figure, the alloying speed when a new surface is provided in advance based on the present invention is at least 8 times that of the conventional general method (indicated by ○), as shown by △ or ●. It is clear that this difference reaches a value that reaches even more than 100%, and such a difference tends to further expand as the Al concentration in the plating bath increases.

Example 2 A test piece whose surface was polished with emery paper and a test piece which was not polished for comparison with the surface of an SPCC cold-rolled steel plate with a width of 50 mm, a length of 100 mm, and a thickness of 0.8 mm were prepared, and molten zinc was applied using the usual flux method. Plated, immersed for 5 seconds, rolled to a weight of 70g/ ^m2 , and post-annealed in a heat treatment furnace at 280℃, 330℃, 370℃.
Alloying treatment was carried out under three conditions at .degree. C. and with various holding times. The Al concentration in the plating bath was 0.17%. After alloying treatment, it diffused into the apparent epithelial membrane.
The amount of Fe (Fe content in the film) was measured by atomic absorption spectrometry using chemical dissolution.

The results are shown in Figure 3. In the figure, A is a polished test piece, B is an unpolished test piece, and subscripts 1, 2, and 3 indicate alloying treatment temperatures, 370°C, 330°C, and 330°C, respectively.
Indicates 280℃. In addition, the × mark in the figure is a polished test piece plated with an Al concentration of 0.10% in a galvanizing bath at 370℃.
These are the test results when alloying treatment was performed.

It is clear from FIG. 3 that the alloying behavior is hardly influenced by the Al concentration. Also, the comparative example
It can be seen that the alloying processing speed is much faster than that of the plated material of SPCC (unpolished specimen B).

As is clear from the above explanation, the present invention dramatically improves the alloying speed of zinc coatings when using a plating bath containing a high concentration of Al, and therefore improves the alloying rate of zinc coatings when using a plating bath containing a high concentration of Al. This eliminates the need to change the Al concentration in the plating bath or reduce the line speed to carry out the alloying process, which was previously necessary when the line was used for both types of steel sheet production.

[Brief explanation of the drawing]

FIG. 1 is a process diagram of a hot-dip galvanizing line suitable for carrying out the present invention, and A shows the case of the Sendzimir method and the case of the LoFlux method, respectively. Figure 2 shows the relationship between the alloying treatment time and the amount of Fe in the plating film obtained through experiments.
Each figure represents the case of 0.2%. Figure 3 shows
The relationship between the post-annealing time after hot-dip galvanizing and the amount of Fe in the galvanized film obtained through experiments is shown.
A represents the case where the surface was polished prior to plating, and B represents the case where the surface was not polished. In the figure, 1: steel plate, 2: payoff reel, 3: oxidation furnace (or non-oxidation furnace), 4: recrystallization ring zone,
5: Low temperature holding zone, 6: Plating bath, 7: Alloying furnace, 8: Wire brush roll, 9: Degreasing tank, 1
0: pickling tank, 11: flux tank, 12: preheating drying zone, 13: belt polishing machine.

Claims (1)

[Claims] 1. Manufacturing of a hot-dip galvanized alloyed steel sheet, characterized in that the surface of the steel sheet after softening annealing is machined to give a new surface at a stage before completion of passing through a zinc bath. Law.