JPS61103617A - Method for descaling steel strip - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for descaling steel strip.

(Prior Art) Since scale is attached to a hot-rolled steel strip, it is necessary to remove the scale when performing cold rolling thereafter. This descaling method can be roughly divided into:
There are two methods: pickling method and mechanical descaling method. However, the pickling method not only requires a long pickling tank and large-scale waste acid treatment equipment, but also takes up a large amount of space.
The disadvantages are that the equipment costs are high, and since a large amount of acid is used, it is not economical, and the working environment is poor. For this,
The mechanical descaling method attempts to remove scale using shot blasting, wire brushing, etc., but has the drawback that it cannot descale as uniformly as the pickling method. At present, although there are the above problems, the pickling method is the mainstream, and the mechanical descaling method is only used as a supplement.

Furthermore, in the pickling method, simply passing the plate through the acid solution in the pickling tank has poor descaling properties, so a skin pass, scale breaker, and rapid cooling of the coil are performed as pretreatment to crack the scale layer. Various attempts have been made to improve descaling performance, such as adding water to make it easier to dissolve scale during pickling. Brush descaling during pickling has also been developed. However, even with these improvements, the problem of long line length, which is caused by the method of pickling, that is, immersion in a pickling tank, still remains unsolved.

In contrast, various mechanical descaling methods have been proposed and some have been implemented in terms of how to combine them with the pickling method, or on their own. However, mechanical descaling cannot achieve uniform descaling when used alone, so it is only used as a pretreatment for pickling.

For example, there have been various proposals to pass the material through a skin pass mill, a repeller, or a rolling mill prior to pickling, but at present none of them has been found to have sufficient descaling properties. Also,
Some attempts have been made to inject high-pressure slurry prior to pickling, but the results are the same.On the other hand, instead of the conventional method of passing the plate through the pickling tank, it is possible to shorten the pickling time and improve the In order to effectively descale depending on the state of scale and its speed, a spray pickling method is known, which is disclosed in Japanese Patent Laid-Open No. 54-81126. In addition, instead of performing complete pickling, there is a method in which cracks are created in advance, acid is penetrated into the cracks after NFC, and then the final removal is performed by mechanical descaling such as brushing.
It has been proposed as No.-11713. put crack n
In addition to rolling, there is also a descaling method that involves the use of a leveler, followed by steps such as leveler → acid coating → brushing → pickling (using a tank).

It is true that if a spray pickling method or a spray pickling tank is used instead of the conventional method of passing the plate through a pickling tank, the pickling process only requires the acid to penetrate into the cracks, which reduces the space occupied by the equipment. It is effective in that it can be made smaller. One of the applicants of the present application also proposed a technical idea with the purpose of
It was previously disclosed in 2.110116 and the like.

(Problem to be solved by the invention) However, as the inventors further conducted their research, they discovered that
Firstly, the descaling properties are influenced by the conditions for cracking, and the surface quality in particular is greatly affected.Secondly, even if spray pickling is combined with the rolling process,
It was found that there is a limit to descaling performance.

For example, the scale-plaking method by rolling, including the skin-pass light rolling method, has a limited effect and also causes scale intrusion during rolling, resulting in a decrease in quality. In addition, because there are limitations to acid application methods such as spray pickling, if the brush is pressed too hard to improve descaling properties during subsequent brushing, brush marks will be left on the surface of the material, and the brush will be weakly pressed. If it is only applied, sufficient descaling properties will not be obtained. ′
In addition, in the brushing method, if the steel strip has a shape or warp, the way the brush hits the steel strip is uneven in the width direction, and brush marks are unavoidable and over-pickling tends to occur.

Therefore, the purpose of the present invention is to solve the above-mentioned conventional problems,
It is an object of the present invention to provide a method for descaling a steel strip, in which the surface quality of the resulting product is excellent, the line length is significantly shortened, and high-speed processing is possible.

(Means for Solving the Problems) In order to achieve this object, the present invention processes a hot-rolled steel strip at a reduction rate of 5 to 25 using a multi-stage rolling mill consisting of small-diameter work rolls whose surfaces are brightly finished. After rolling with
The steel strip is heated to a temperature of 0 to 200°C and descaled by spraying an acid solution onto the heated steel strip.Furthermore, the small-diameter work roll is chromium-plated and/or ion-implanted. It is characterized by its use.

The main points of the present invention are as follows.

(1) In scale breaking, rolling is performed at a rolling reduction ratio of 5 to 25% using a small-diameter work roll whose surface is brushed or chrome-plated and/or ion-implanted.

When rolling a scaled steel strip, roll wear near both ends of the rolling plate becomes a problem, and the amount of wear increases in proportion to the rolling reduction ratio. Therefore, for the purpose of suppressing roll wear, we conducted various experiments on how to effectively perform scale breaking under light rolling.We found that the scale breaking effect is effective when the rolling reduction is within the range of 5 to 25 inches. It was found that the larger the value, the better. That is, if the rolling reduction ratio is less than 5 inches, the scale breaking effect will be small, and if it exceeds 25%, a scale crimping phenomenon will occur and the descaling performance will deteriorate.

In addition, if the rolls used for rolling are dull rolls, the scale intrusion phenomenon is already noticeable at a rolling reduction of about 15 inches, so bright finish rolls are preferred in order to further enhance the scale breaking effect. used n
In order to more effectively prevent roll wear, the roll surface is chromium plated and/or ion implanted.
This is because the surface treatment increases the hardness of the roll surface and lowers the coefficient of friction. If a lubricant is used during rolling, the coefficient of friction can be further reduced.

The roll diameter is preferably a small diameter of about 50 to 400 mφ. The smaller the diameter of the work roll, the higher the scale pre-king effect, but it is not preferable if it is less than 50 mm due to the strength of the roll itself, while if it exceeds 400 mm, the scale pre-king effect will be weakened.

(2) As a pre-process for acid application methods such as spray pickling, the temperature of the steel strip after scale plaiking is raised, and then acid is sprayed. Spray pickling is more efficient than immersion pickling because fresh acid is constantly in contact with the target steel strip, so the acid reaction is accelerated. That said, there are limits to the descaling effect. Of course, it is not impossible to obtain a sufficient descaling effect by increasing the number of rows of spray nozzles to lengthen the line length, but this goes against the purpose of the present invention, which is to shorten the line length. On the other hand, the higher the temperature, the faster the acid reaction proceeds. However, when using the spray pickling method as in the present invention, even if the temperature of the acid sprayed from the spray nozzle is increased, it is not very effective if the material temperature is low. Therefore, in the present invention, a pickling bath is used in which the temperature of the material itself is raised before pickling, and the temperature drop of the acid solution due to contact with the material is suppressed as much as possible. Heating temperature is 50-20
0°C is preferred. If the temperature is below 50°C, the descaling promoting effect is poor, and if it exceeds 200°C, energy cost for heating up is required and it is uneconomical.

FIG. 1 shows an outline of the continuous descaling equipment according to the present invention, in which a scaled hot-rolled steel strip 1 is wound up and fed out from a payoff reel 2. Scale breaking is carried out by, for example, a six-stage cluster mill 3 equipped with bright finishing small-diameter work rolls 3a+3a. Next, it is passed through a temperature raising tank 4 filled with hot water or the like, and after being heated, it is pickled in a spray pickling tank 5, and finally wound onto a take-up reel 6.

Incidentally, although FIG. 1 shows an example in which one pass of rolling is performed, in general, multiple passes have a greater descaling effect than rolling with a rolling reduction of 5 to 25 inches in one pass. However, in view of the equipment cost and the fact that the cost will be borne in subsequent steps of the present invention, one pass is practically sufficient.

Further, during rolling, as mentioned above, it is desirable to use water or a lubricant.

For NFC steel strips subjected to scale play kinging, at least the surface portion thereof is heated in a heating device. The temperature raising liquid is usually water, but it is also effective to use a pickling bath to raise the temperature.

In addition, the method using induction heating is also an effective means of easy temperature rise control, and it is desirable to bring the temperature of at least the surface of the steel sheet to 50 to 200°C. In order to proceed quickly and to make the tank length as short as possible, the temperature must be 50°C or higher.The effect is greater at high temperatures, but due to heating energy costs and practical considerations, It is preferable that the temperature exceeds 0', and the heated steel strip is preferably passed through a spray pickling tank as directly as possible for spray pickling. As mentioned above, spray pickling has higher pickling efficiency than the acid immersion method. As shown in FIGS. 1 and 2, it is preferable that the direction of acid injection from the spray nozzles 5a, 5a, etc. is countercurrent to the flow of the steel plate on both the upper and lower surfaces; 30° to 50° to the vertical line
° is preferred. Further, it is preferable that the direction of the acid solution from each nozzle is directed toward the center of the plate as shown in FIG. 2 when viewed from above. If the flow is parallel to the plate flow, the acid solution will concentrate on both sides of the steel strip in the width direction, making it impossible to achieve uniform descaling in the width direction. The number of nozzles may be 2 or more, and usually should be 3 or more per row. Examples of the acid solution and hydrochloric acid solution used include sulfuric acid, but hydrochloric acid is more effective. Also, the jetting pressure of acid liquid from the nozzle is 3.0'Y! I hope it's more than that.

After the above-mentioned pickling process, a normal water washing process, that is, rinsing such as water spraying is performed.

(Example) Next, examples will be shown.

3.0m1×1200mm wide hot rolled steel strip at 680℃
Descale the rolled material under various conditions.

When we conducted an actual machine test for descaling under conditions of shortage, we found that
Complete descaling was possible.

■Processed material: 3.0mm thick x 1200mm width steel plate with scale ■Rolling work roll surface: Φ120 bright finish Chrome plating followed by nitrogen injection treatment ■Rolling ratio = 12% ■Rolling work roll diameter: 25011W (5-stage cluster mill) ■Steel plate temperature when heating up: 85℃ ■Number of spray nozzles in the width direction = 5 nozzles x 6 rows ■Spray in one direction Two nozzles at both ends are directed toward the center of the steel strip, and one in the center is directed toward the center of the sheet. Spray 0 ■Spray pressure crab 5 ~ Please note that the line length is approximately 75 m and the line speed is 400 mpm.
A 20-coil (approximately 700 m long) was processed in 1.3 minutes. Spray pickling time is 6 seconds.

On the other hand, considering that in the conventional method, the line length was about 300 m, the line speed was 2007 ipms, and the processing time was 3.5 minutes, the effectiveness of the present invention was clearly demonstrated. As can be seen, according to the present invention, it is possible to reliably improve product quality, speed up and shorten the line.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a descaling facility according to the present invention, and FIG. 2 is a plan view of an example of the arrangement of spray nozzles. 1. Steel plate 3. Mill 3a. Nisso ion implantation treatment work roll after chromium plating on small diameter bright surface 4. Induction heating temperature raising device 5. Spray pickling tank Patent applicant Sumitomo Metal Industries, Ltd. Sumitomo Heavy Industries Co., Ltd. Industrial closed company

Claims (2)

[Claims] (1) Hot-rolled steel strip is rolled at a reduction rate of 5 to 25 using a multi-stage rolling mill consisting of small-diameter work rolls with a bright surface finish.
%, the steel strip is heated to a temperature of 50 to 200° C., and the heated steel strip is descaled by spraying an acid solution. (2) The method for descaling a steel strip according to claim 1, characterized in that the small-diameter work roll is chromium-plated and/or ion-implanted.