JPS637849B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a continuous descaling method for steel strip. [Technical background of the invention and its problems] Since scale is attached to a hot-rolled steel strip, it is necessary to remove the scale when cold rolling is performed thereafter. This descaling method can be broadly classified into a pickling method and a mechanical descaling method. However, in the pickling method,
Disadvantages: It requires a long pickling tank and large-scale waste acid treatment equipment, which not only occupies a large space, but also increases equipment costs.Also, it is not economical because a large amount of acid is used, and the work environment is bad. There is. On the other hand, the mechanical descaling method attempts to remove scale using shot blasting, wire brushing, etc., and has the disadvantage that descaling cannot be done as uniformly as the pickling method.
Therefore, although there are the above problems at present,
The pickling method is the mainstream, and the mechanical descaling method is only used as a complement. Furthermore, in the pickling method, simply passing the plate through 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 and remove the acid. Make it easier for scale to dissolve during washing,
Attempts have also been made to adjust the layer composition of the scale layer in consideration of descaling properties. Brush descaling during pickling has also been developed.
However, even with these improvements, the above-mentioned problem of long line lengths associated with pickling, that is, immersion in a pickling tank, still remains unsolved. On the other hand, various mechanical descaling methods have been proposed from the viewpoint of how to combine them with the pickling method or on their own, and some of them have been implemented. However, this mechanical descaling method alone cannot perform uniform descaling, so it is used only as a pretreatment for the pickling method. For example, prior to pickling, skin pass mill,
Various proposals have been made to pass the material through a leveler or a rolling mill, but at present no one has been found that provides sufficient descaling properties. Especially when using skin palmyl, as in Japanese Patent Application Laid-open No. 55-30363,
Using a work roll of 5 to 35μHmax, rolling reduction rate of 0.5
Some were reduced by ~2%. When we conducted a scale breaking rolling experiment on this material, we found that
It is true that the coarser the grain, the greater the effect, but on the other hand, increasing the rolling reduction rate causes scale intrusion defects, which in turn leads to poor descaling, making it invincible to high-speed pickling. I understand. There have also been attempts to inject high-pressure slurry prior to pickling, but the results are similar. On the other hand, in order to shorten the time required for pickling and to effectively follow the scale condition and speed of the threaded material in place of the conventional method of passing the plate through the pickling tank, a method was developed in Japanese Patent Application Laid-Open No. 54 A spray pickling method disclosed in JP-A-81126 and the like is known. In addition, instead of performing complete pickling, there is a method in which cracks are placed in advance, acid is allowed to penetrate into the cracks, and then the final removal is performed by mechanical descaling such as brushing. - Proposed as No. 11713. Furthermore, methods to avoid using such a pickling tank include using a leveler as a method other than rolling to create cracks, and using a leveler →
There is also a method that involves processes such as acid application → brushing → pickling (using a tank). It is true that even if a spray pickling method or a pickling tank is used instead of the conventional method of passing the plate through a pickling tank, the method of pickling that only allows the acid to penetrate into the cracks is difficult because of the space occupied by the equipment. This is effective in reducing the size of the image. One of the present applicants also published a technical idea with a similar purpose in JP-A-58-84983.
It was previously disclosed in Publications Nos. 107211, 107212, and 110116. However, as the inventors conducted further intensive research, they discovered that, firstly, cracking conditions affect the descaling performance and have a particularly large effect on surface quality, and secondly, spray pickling It was found that there is a limit to descaling performance even when combined with a rolling process. For example, scale breaking methods by rolling, including the skin pass light reduction method, have limited effectiveness and cause scale indentation during rolling, reducing quality, and acid coating methods such as spray pickling have limitations. Therefore, when brushing is performed afterwards, brush marks are left on the surface of the material if the brush is pressed too hard to improve descaling performance, and sufficient descaling performance cannot be obtained if the brush is applied weakly. In addition, in the brushing method, if the steel strip has a shape or warp, the way the brush hits the steel strip will be uneven in the width direction, making it impossible to avoid brush marks and tending to cause over-pickling. [Object of the invention] The object of the present invention is to solve the above-mentioned conventional problems,
The object of the present invention is to provide a method for descaling a steel strip, which results in a product with excellent surface properties, a significantly short line length, and which can be processed at high speed. [Summary of the Invention] To achieve this object, the present invention involves rolling down a target steel strip using a dull work roll with a roll diameter of 50 to 400 mm and a roughness of 0.5 to less than 3.5 μRa to crack the scale layer. The method is characterized in that the temperature of the steel strip is increased, and then acid is injected onto the heated steel sheet to perform pickling. Note that the surface roughness in the present invention is based on JIS B0601
Specified by the center line average roughness (Ra) specified in . The main points of the invention are as follows. (1) For scale breaking, a work roll with a small diameter dull is used. From the viewpoint of rolling load, it is preferable to use a bright roll with a smooth roll surface, but the scale breaking effect is poor. Moreover, if scale breaking is performed under strong pressure using a bright roll, which is less effective, indentation flaws will occur on the surface of the material. Therefore, in the present invention, a work roll with a small diameter is used, and a synergistic effect with the rolling reduction ratio improves the scale breaking effect.
It is an attempt to achieve a goal. (2) The scale breaking steel strip is heated as a pre-process for acid application methods such as spray pickling. 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. However, although it is superior to the immersion pickling (tank) method, there are limits to descaling performance. Of course, it would be possible to increase the line length by increasing the number of rows of spray nozzles, but this would be contrary to the purpose of the present invention. On the other hand, the higher the temperature, the faster the acid reaction proceeds. However, since a preferred example of the present invention uses a spray pickling method, even if the temperature of the acid sprayed from the spray nozzle is increased, the effect will be low if the material temperature is low. In addition, although it is effective to raise the acid temperature if the acid immersion method is used, the present invention does not depend on the acid immersion method as described above. Therefore, in the present invention, the temperature of the material is raised before pickling, and the pickling is performed in a high temperature state where the temperature drop is suppressed as much as possible. At first glance, it seems that the line length becomes longer, which is contrary to the purpose of the present invention, but it becomes shorter than increasing the number of rows of spray nozzles, and it also serves the purpose of high-speed pickling. [Specific Examples of the Invention] The present invention will be explained in more detail below. FIG. 1 shows an outline of the continuous descaling equipment according to the present invention, in which the scaled hot rolled steel strip 1 that has been wound up is fed out from a payoff reel 2, and first equipped with dull work rolls 3a, 3a. For example 6
Scale breaking is performed by a stage cluster mill 3. Next, the temperature is increased by passing through a heating tank 4 filled with hot water, etc., and then passing through a spray pickling tank 5.
It is pickled in the process and finally wound onto a take-up reel 6. Next, this process will be explained in order while showing a preferred example. Scale breaking is performed on the steel strip 1. Since the conventionally commonly used bright roll has a low scale breaking effect as mentioned above, a dull work roll is used.
The surface roughness of the work roll is preferably from 0.5 μRa to less than 3.5 μRa, particularly from 1.5 μRa to 3.0 μRa. Below 0.5 μRa, the scale breaking effect is small;
If it exceeds 3.5 μRa, too many scale cracks occur, causing scale to fall off, resulting in scale intrusion flaws in the steel strip. The rolling reduction ratio is desirably 5 to 15%; if it is less than 5%, the scale breaking effect will be small, and if it exceeds 15%, the rolling reduction will be too strong and cause scale intrusion flaws. The work roll diameter is preferably 50 to 400 mmφ.
The smaller the diameter of the work roll, the higher the scale breaking effect, but in view of the strength of the roll itself, it should be avoided to be less than 50 mmφ, and on the other hand, if it exceeds 400 mmφ, no scale breaking effect can be expected. By the way, although FIG. 1 shows an example of rolling in one pass, in general, multiple passes have a greater descaling effect than rolling with a reduction ratio of 5 to 15% in one pass. One pass is practically sufficient since the equipment cost and the subsequent steps of the present invention are borne by the cost. Further, during rolling, it is desirable to perform rolling using water or a lubricant, and to remove scale powder adhering to the work roll surface using a high-pressure water spray nozzle or the like. The steel strip that has been subjected to scale breaking is heated at least at its surface in a heating tank. Water is usually sufficient as the temperature raising liquid, but an acid may be used if necessary. It is desirable that the temperature of at least the surface portion of the steel sheet be 40 to 80°C by increasing the temperature. In order to speed up the pickling reaction in the next process and to keep the tank length as short as possible,
The higher the temperature, the greater the effect, but it is not preferable to exceed 80°C from the standpoint of heating energy costs and practical considerations. As a means for raising the temperature, it is possible to use a heat transfer coil or the like in addition to a method of immersion in a liquid, but care must be taken not to generate new scale. It is preferable that the heated steel strip is passed through a spray pickling tank as soon as possible to be pickled. Spray pickling has a higher pickling efficiency than the acid dipping method. As shown in Figures 1 and 2,
The direction of acid injection from the spray nozzles 5a, 5a... is preferably in the opposite direction to the flow of the steel plate on both the upper and lower surfaces, and the direction is at an angle of 30° to 50° with respect to the vertical line. preferable. 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 it is parallel to the flow of the plate, 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,
Normally there should be at least three lines per row. Examples of the acid solution used include hydrochloric acid and sulfuric acid, but hydrochloric acid is more effective. If necessary, the acid solution may be jetted out at an elevated temperature of 40-85°C. In addition, the jetting pressure of acid liquid from the nozzle is 3.0
It is desired that it is Kg/cm ² or more. After the above-mentioned pickling process, a normal water washing process, that is, rinsing such as water spraying is performed. The thus descaled steel sheet can be wound as is, but if necessary, it may be passed through a short acid soaking bath or subjected to a light brushing process. However, in practical terms, it is preferable not to include such a step. [Example] Next, an example will be shown. Hot-rolled steel strips 3.0 mm thick and 1200 mm wide were wound at 680°C and descaled under various conditions. The evaluation of the descaling of the results is as follows: ◎: complete descaling;
○: Descaling of 90% or more, Δ: Descaling of 30 to 90%, ×: Descaling of 30% or less. Example 1 For the steel strip, the work roll diameter is 250mmφ
The scale breaking effect was investigated by changing the type of work roll, roughness, and rolling reduction ratio. In addition, in all processes after rolling, the steel strip is heated to 50℃.
The temperature is raised to , and the spray pickling tank is as shown in the drawing.
Arrange 5 nozzles in the width direction and 5 rows on the top and bottom surfaces, with the nozzles at both ends facing the center, and apply 5 kg/cm ² of hydrochloric acid.
This is an example of pressure injection. The results are shown in Table 1.

[Table] Example 2 Roll roughness 2.0μRa, which was suitable in Example 1,
The conditions of rolling reduction ratio of 7% were fixed, and the temperature raising conditions and pickling conditions were changed. The results are shown in Table 2.

【table】

[Table] As a comparative example, the results of the acid immersion method are also shown. Example 3 When an actual descaling test was conducted under the following conditions, complete descaling could be performed. Treated material: 3.0mm thick x 1200mm wide steel plate with scale Rolling work roll surface roughness: 2μRa Reduction ratio: 7% Rolling work roll diameter: 250mm (6-stage cluster mill) Steel plate temperature when heated: 75℃ Number of width direction spray nozzles : 5 pieces x 6 rows Spray direction: 2 pieces at both ends should be directed towards the center of the steel strip, and 1 piece in the middle should be sprayed towards the center of the plate Spray pressure: 5Kg/cm ² Also, the line length is about 100m and line speed
400mpm, 20Ton coil (approx. 700m length)
Processed in 1.75 minutes. Spray pickling time is 8 seconds. On the other hand, considering that the conventional method required a line length of about 300 m, a line speed of 200 mpm, and a processing time of 3.5 minutes, the effectiveness of the present invention became clear. [Effects of the Invention] As described above, according to the present invention, a synergistic effect between the roughness of the dull work roll and the rolling reduction ratio, as well as a synergistic effect between the heating tank and the spray nozzle can be expected, so that product quality can be improved, It is possible to reliably achieve faster and shorter lines.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a continuous 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... Work roll with dull, 4... Temperature raising tank, 5... Spray pickling tank.

Claims (1)

[Claims] 1. In the descaling method of hot rolled steel strip, a multi-stage rolling mill having a dull work roll with a roll diameter of 50 to 400 mm and a roughness of less than 0.5 to 3.5 μRa from the upstream side of the steel strip transfer path, and a steel strip a heating tank to raise the temperature of the
Pickling tanks with spray nozzles are sequentially installed, and the steel strip is lightly rolled at a rolling reduction rate of 5 to 15% in the multi-stage rolling mill, and then heated to 40 to 80°C in a heating tank. A continuous descaling method for steel strips characterized by spraying an acid solution and performing pickling.