JPS58110116A - Method for continous descaling of steel sheet - Google Patents

Abstract

PURPOSE:To reduce the length of treating line and to perform an economical continuous-descaling with the least consumption of said to be used, by cold rolling a hot-rolled steel sheet in a specified draft rate after coiling up the steel sheet at a low temperature, and then, pickling the sheet after mechanically descaling it. CONSTITUTION:A hot-rolled steel sheet 1 is coiled up at <=500 deg.C low temperature. The sheet 1 is uncoiled from an uncoiler 2 and is fed to a rolling mill 3 to be cold-rolled or cold-worked in a 5-20% draft rate. Next, the sheet 1 is led to a mechanical descaler 6 consisting of several pairs of brushing rolls 4 and supporting rolls 5, etc. to be descaled, and then, the sheet 1 is coiled up by a coiler 10 after passing through a finish pickling bath 7 and a washing bath 8. By this method, occupation space and installation cost are reduced. Further, acid consumption is also reduced, and a waste water treating plant is made smaller.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous descaling method for steel plates.

Since scale is attached to the hot-rolled steel sheet, it is necessary to remove the scale when performing cold rolling afterwards.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.
It has the drawbacks of high equipment costs, uneconomical use of a large amount of acid, and poor working environment. Against this,
The mechanical descaling method attempts to remove scale by means of force blasting or wire brush pressure, and has the disadvantage that it cannot descale uniformly like the pickling method. Therefore, at present, although the above-mentioned problems exist, the pickling method is the mainstream, and the mechanical descaling method is only used as a supplement.

In addition, 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 a pretreatment to crack the scale layer. No attempt has been made to make it easier for scale to dissolve during pickling or to adjust the layer composition of the scale layer in consideration of descaling properties. It has also been proposed to descale by placing a brush in the pickling solution. However, even with these improvements, the method of pickling, that is, immersion in a pickling tank, still remains.
The problem of long line lengths mentioned above still remains unsolved.In contrast, the mechanical descaling method has various problems in terms of how to combine it with the pickling method, or by itself. However, this mechanical descaling method alone cannot perform uniform descaling, so it is mainly used only as a pretreatment for the pickling method. The inventors of the present invention conducted intensive research while keeping in mind the problems of 0 or more, and found that the major factors in descaling performance are the coiling temperature during hot rolling and the cooling temperature prior to mechanical descaling. It was found that the rolling reduction ratio during inter-rolling and/or cold working.

Therefore, the present invention has been proposed for NFC, and its purpose is to provide an economical continuous descaling method that can significantly shorten the line length and use a minimum amount of acid. The basic idea is to minimize the formation of scale during hot rolling by making the target steel plate a low-temperature rolled material, and to reduce the reduction rate during cold rolling and/or cold rolling by 5 to 20 inches. By making the scale relatively large, intensive descaling is performed by mechanical descaling without pickling, and it is a complementary finishing method for some scales that may not be removed even with this method. It performs pickling.

That is, the present invention involves cold rolling and/or cold working a hot-rolled steel sheet that has been cold-rolled at a temperature of 500°C or less at a rolling reduction of 5 to 20 inches, then mechanically descaling, and then finishing with acid. It is characterized by washing.

The hot-rolled steel sheet to which the present invention is directed is a low-temperature rolled steel sheet rolled at a temperature of 500°C or lower, particularly preferably 450°C or lower. In general, hot-rolled steel sheets come in two types: high-temperature rolled material of about 68 (J℃) and normal rolled material of about 560°C. High-temperature rolled steel is continuously annealed after cold rolling to align crystal grains and improve formability. However, because the winding temperature is high, the side edges of the steel plate are oxidized by the air and a thick layer of n-scale is formed.Usually, after winding, the air A thick scale is formed due to oxidation, and descaling cannot be carried out smoothly in the subsequent process.The same thing can be said about normal rolled material at about 560°C, although the scale thickness and composition are different. The scale is usually around 520℃ and the boundary layer with the base steel is Fe0O.
Since transformation to Fe*Fe504 occurs, it is desirable to wind it at a temperature lower than this temperature, leaving the boundary with the base iron & CFe0, and smooth descaling in the subsequent process.

The hot-rolled steel sheet thus wound at a low temperature is passed through an unwinding machine to a descaling line. The temperature of the steel plate coil in the unwinding machine is not limited to a separate installation, but may range from room temperature to 20oC,
Normally, the temperature of the unwound steel plate is about 120°C.A flattener is used to flatten the curved coil, and the off-gauge part at the front and rear ends of the steel plate is cut to make the end face square. The cold rolling and/or cold working process is carried out through a shear, a 7 lash unit, a towelder, a priddle roll, a loop car, etc. for welding the first steel plate and the next steel plate together and making them continuous.

As an example of cold rolling, a normal quadruple rolling mill A as shown in FIG. 3 can be used. Further, for cold working, a leveling mill B1 or a roll bender as shown in FIG. 4 can be used. It is also possible to combine a roll bender and a double rolling mill as shown in FIG. Furthermore, since the purpose of this process is to generate cracks in the steel plate, even if cold rolling and cold working are performed alone,
A combination of the two may be used, and when the two are combined, the order may be appropriately selected. Cold rolling and/or cold working gives the steel plate elongation and/or bending, creates cracks and cracks in the steel plate, and creates conditions that facilitate the removal of scale in the subsequent mechanical descaling process.

It is important that the rolling reduction ratio in this case is 5 to 2 degrees. In the present invention, the steel sheet is directly led to a mechanical descaling process, where intensive descaling is performed, and then finishing pickling is performed, but this process is only complementary and gives bending and elongation to the steel plate. It's nothing
Pickling is not carried out using large equipment as in the past. Therefore, it is necessary to sufficiently increase the rolling reduction rate to generate large and numerous cracks in the scale layer, and to create conditions that allow easy descaling in the next mechanical descaling step. In this respect, the rolling reduction ratio needs to be 5 inches or more. In addition, it is often thought that when the rolling reduction rate is increased, a large force acts on the scale, causing the scale to break into smaller pieces and making it easier to peel off the scale or peel off the scale through acid treatment, but in reality, the scale is compressed to the base steel. I tried it and it didn't have the effect I expected. Furthermore, if the rolling reduction rate is large, some of the scale will become trapped in the substrate, resulting in so-called trapped scale, which will not be removed in subsequent processing and may cause scale flaws in the steel plate during cold rolling. However, if the reduction is too strong, the surface properties will deteriorate, especially mechanical properties such as hardness and elongation. Therefore, it is preferable that the rolling reduction ratio does not exceed 20 inches. On the other hand, in this process, it is best to install priddle rolls before and after the steel plate and apply appropriate tension.The steel plate is then led to a mechanical descaling process that does not involve bending and/or elongation. The material is rolled at a low temperature of 500℃ or less, which reduces the amount of scale generated, and has a large rolling reduction during subsequent cold rolling and/or cold working.
~20%. Therefore, descaling can be easily performed in the subsequent mechanical descaling step. Further, in a step before this mechanical descaling step, variations in descaling performance in the width direction are prevented.

As mechanical descaling means, in addition to relatively soft nylon brushes and wire brushes containing abrasive grains, grinding, shot blasting, liquid honing, high-pressure fluid jetting, and iron oxide slurry projection can be used, but brushes are usually used. It is preferable to use 0. The steel plate is then led to a finishing pickling bath for the purpose of finishing (n
Because the purpose of the pickling tank is for finishing purposes only, unlike the long pickling tank commonly used in the past, it is usually 25cm long.
The plate is passed through a tank at about 30° C. for about 5 to 6 seconds. This finishing pickling completely removes even a small amount of unremoved scale and makes the surface beautiful.

The final pickling is preferably carried out using sulfuric acid or preferably hydrochloric acid from the viewpoint of high-speed processing, at a liquid temperature of 50 to 95°C, especially 65 to 90°C, and at a concentration of about 5 to 15%. After that, the acid adhering to the steel plate is removed in a water washing or rinsing tank. Next, the dryer, exit loop car,
It is wound up by a winding machine after passing through a side trimmer, an oiler for applying anti-rust oil, and a shear.

Next, an example of the outline of a processing line for carrying out the method of the present invention will be explained with reference to FIG. 1, which shows only the main parts. , the sheet is passed through the rolling mill 3.

Thereafter, the plowsinger roll 4 and the support roll 5 are guided to a mechanical descaler 6 which is a pair, then passed through a finish pickling tank 7 and a water washing tank 8, and further passed through a dryer 9 and wound up by a winder 10. FIG. 2 shows an example in which the rolling mill 3 is the leveling mill shown in FIG. 4, and the pickling tank 7 is changed from an immersion type to a spray pickling type. Ordinary hot rolled steel plate of Om
℃, 560℃, 500℃, and 450℃, and then cold-rolled at various rolling reductions of 3qb, 5%, 7 inches, 10 inches, and 12 inches, brushed, and then After passing through a 25 m finishing pickling tank with 10% hydrochloric acid II solution at 70°C, washing with water and drying with a dryer, and winding up on a winding machine,
The descaling status of the steel plate surface was measured at each position in the width direction of the steel plate, at both edges of the steel plate, at a quarter position of the plate width from both edges (referred to as 1/4 edge), and at each position in the center. (This was investigated along with the relationship with the rolling reduction ratio.) As a result, the rolling reduction ratio was found to be good for 5 to 20 inches. Table 1 shows the descaling status results for rolling reductions of 5% to 20 inches. 0 Table 1 In addition, ◎Complete descaling Descaling of 090 cm or more Descaling Δ 30 to 90 cm Descaling of xaos or less As can be seen from the above table, the coiling temperature of the hot rolled steel sheet is 56
The descaling rate of the edge portion of the 0°C sample is 30% or less, and the descaling rate of the edge portion and 1/4 edge portion of the 650°C sample is 30% or less, which is poor. Compared to that, when the winding temperature is 500°C, the descaling rate at the edges and quarter edges is 90% or more, and complete descaling occurs in the center. , 450°C, complete descaling was confirmed at all positions on the steel plate.

As described above, the present invention skillfully combines the coiling temperature during hot rolling, the rolling reduction during cold rolling and/or cold working, mechanical descaling, and finish pickling, and therefore The line length can be reduced to about 1/2 to 115 times that of the pickling method, which significantly reduces not only the space occupied but also the equipment cost. Further, in the present invention, since the acid used is only for the final pickling, there are significant effects in terms of reducing the amount of acid used, downsizing the waste acid equipment, and improving the working environment. Furthermore, the threading speed is 6
It is possible to increase the speed by more than twice the conventional speed up to about 00 m/mil+. Although these effects mainly involve descaling by mechanical descaling, Due to the fact that the winding temperature and rolling reduction are specified nfc% 0

[Brief explanation of the drawing]

FIGS. 1 and 2 are schematic diagrams showing an example of a processing line for carrying out the method of the present invention, and FIGS. 3 to 5 are schematic diagrams of examples of cold rolling and/or cold working.

Claims (1)

[Claims] (1) A hot-rolled steel sheet that is rolled at a low temperature of 500℃ or less,
A method for continuous descaling of a steel plate, which comprises cold rolling and/or cold working at a rolling reduction of 5 to 20 inches, followed by mechanical descaling and further pickling.