GB2029303A - Descaling hot-rolled steel strip - Google Patents

Abstract

In a method of processing steel strip the strip is hot rolled and thereafter is descaled by ejecting descaling agents against the strip. In the rolling step the strip passes in an S-shaped path around two rolls whose peripheral speeds are different and whose speed ratio is equal to the reduction ratio of the strip. The degree of reduction is from 2 to 8 percent. <IMAGE>

Description

SPECIFICATION Descaling hot-rolled steel strip The present invention relates to a method of descaling hot-rolled metal surfaces.

According to the present invention a method of descaling surfaces of hot-rolled steel strip comprises passing the strip through a rolling drawing mill stand between a pair of work rolls rotating in opposite directions, in which the ratio of the peripheral velocity of the upstream roll, which is equal to that of the strip entering the work rolls, to the peripheral velocity of the downstream roll, which is equal to that of the strip leaving the work rolls, is equal to the ratio of the thickness of the strip leaving the work rolls to the thickness of the strip entering the work rolls, giving a reduction ratio between 2% and 8% and thence passing the strip through descaling devices for ejecting descaling agents against their surfaces to remove scale.

The strip may be passed in an 'S' shaped path round and between the work rolls.

The invention may be put into practice in various ways, but one specific embodiment will be described by way of example with reference to the accompanying drawings, in which; FIGURE 1 is a view used for the explanation of the term "impinging angle fl" used in this specification; FIGURE 2 is a graph illustrating the relationship between the impinging angle on the one hand and the surface roughness of the descaled steel sheet and the quantity of abrasives adhered thereto on the other hand; FIGURE 3 is a diagrammatic view of a known descaling line; FIGURE 4 is a graph illustrating the relationship between the reduction ratio as a percentage and the descaling agent ejecting time;; FIGURE 5 is a graph illustrating the relationship between the reduction ratio as a percentage on the one hand and the surface hardness, the quantity of abrasives adhered to the descaled surfaces and the volume of steel removed by the descaling on the other hand; FIGURE 6 is a diagrammatic view of a known four-high rolling mill stand; FIGURE 7 is a graph illustrating the relationship between the reduction ratio as a percentage and the reduction force; FIGURE 8 is a diagrammatic view of a hotrolled steel strip descaling line implementing the descaling method in accordance with the present invention; and FIGURE 9 is a diagrammatic view used for the explanation of the underlying principle of the rolling drawings process used in accordance with the present invention.

Extensive studies and experiments have been made to attain efficient techniques of descaling or removing oxides or scales on a steel strip by impinging abrasives or descaling agents against the surfaces of the steel strip. It has been found that efficient descaling cannot be attained by merely impinging the descaling agents against the surfaces of the steel strip. Therefore, there has been proposed a method wherein descaling devices are arranged in tandem so that the steel strip may be subjected to descaling steps many times.Furthermore, as shown in FIGURE 1 , the axis of a nozzle 'b' may be inclined at an angle 0 to the surface of a steel strip 'a', the angle 0 being large so that descaling agents consisting of slurry 's' and water 'w' under high pressure may exert great force on the surface of the steel 'a' so as to effectively remove the scales.

The known descaling methods are disadvantageous in that the descaling line becomes larger and longer in size because of a number of descaling devices arranged in tandem, and accordingly the consumption of the descaling agents inevitably increases. Furthermore, with a large impinging angle 0, rough surfaces result and the abrasives adhere so strongly to the surfaces of the descaled steel strip that they cannot be removed.In order to overcome these problems, there has been proposed a descaling method wherein, in order to compensate for the roughened surfaces of the steel strip which have been subjected to descaling with the descaling agents impinging against the surfaces of the steel strip at an impinging angle 0 greater than 300 in the initial descaling stages, there is provided in the last stage a descaling device which ejects the descaling agents at an impinging angle 0 less than 300 so that the surface roughness of the descaled steel sheet may be within the hatched area in FIGURE 2.

Furthermore, there has been proposed a descaling method wherein as shown in FIGURE 3, the steel strip 'a' is rolled by a two-high rolling mill stand 'c' at the reduction ratio of about 1% and therafter is subjected to descaling by three descaling devices 'd', 'e' and 'f,. Even with the descaling line shown in FIGURE 3, efficient descaling cannot be attained so that the descaling devices must be increased in number.

Furthermore, the impinging angle must be increased (for instance to 450) at the descaling devices 'd', 'e' and 'f' but the impinging angle at the last descaling device 'g' must be decreased (for instance to 200). This descaling line is also disadvantageous in that the line becomes large in size and that large reduction forces are required to attain the reduction ratio of about 1% with the two-high rolling mill stand 'c'.

Accordingly, one of the objects of the present invention is to provide a method for descaling hotrolled steel sheet surfaces which may substantially overcome the above and other problems.

The Inventors made extensive experiments in order to obtain the clear understanding of the relationship between the reduction ratio and the descaling time, i.e. a time required for removing scale from the surfaces of the steel strip. The result is that when the reduction ratio is on the order of 8%, descaling may be accomplished with substantially the same, short, time intervals at any of the impinging angles of 200,300, and 450 as shown in FIGURE 4. Furthermore, it must be emphasised that the descaling time may be almost reduced to one half as compared with the descaling time with the impinging angle of 45 in the known descaling line. In other words, descaling efficiency may be almost doubled in terms of time according to the present invention.

Furthermore, as shown in FIGURE 5, the surface hardness is increased with the reduction ratio as indicated by the curve 'A' while the volume of steel removed by the descaling is reduced as indicated by the curve 'B'. This means that the yield of the steel strip may be improved. In addition, the experiments show that the adhesion of abrasive to the surfaces of the steel strip may be decreased as indicated by the curve 'C' because the hard surfaces of steel strip may avoid the adhesion of abrasives to the surfaces. Thus, according to the descaling method of the present invention, high quality products may be provided.

In order to attain a reduction ratio as high as 8% great force is needed. That is, the desired reduction forces may be obtained only with a fourhigh rolling mill stand comprising work rolls h and h' and backing rolls i and i' as shown in FIGURE 6.

Use of such a rolling mill stand will result in increase in size of the descaling line as well as in installation and operation costs.

Shown in FIGURE 7 is the relationship between the reduction ratio and the reduction forces. With a conventional two-high rolling mill stand, a maximum available reduction force is indicated by the straight line 'X' and a maximum reduction ratio of only from 1 to 2% is obtained as indicated by the curve 'Y'. The four-high rolling mill stand of the type shown in FIGURE 6 has a considerably higher maximum rolling force than the two-high rolling mill stand, and, as indicated by the curve 'Z' the reduction ratio is increased with the reduction forces. However, excessively high reduction forces result not only in cracking of scale but also in more adhesion or wedging scale to the surfaces of the steel strip being descaled. As a result, complete removal of remaining scale becomes extremely difficult.In addition, use of the four-high rolling mill stand is not economically advantageous as already described.

However, according to the present invention, a desired high reduction ratio may be attained while minimizing the adhesion or wedging of scales to the steel sheet surface, by means of a rolling drawing mill stand to be described in detail below, and diagrammatically shown in FIGURE 8.

The descaling line includes a rolling drawing mill stand 1 comprising a pair of work rolls 4a and 4b. As shown in detail in FIGURE 9, a hot-rolled steel strip 2 is partly wrapped round the work rolls 4a and 4b in the form of a letter 'S' and the work rolls 4a and 4b are rotated in opposite directions at different peripheral velocities V1 and V2 respectively so that the following relations may be satisfied.

V2 = h1 V, h2 V1 = v1 V2 = v2 V1 < v2 where V1 = the peripheral velocity of the upstream work roll 4a; V2 = the peripheral velocity of the downstream work roll 46; h1 = the thickness of the steel strip entering thg work rolls; h2 = the thickness of the steel strip leaving the work rolls; v1 = the velocity of the steel strip entering the work rolls; and v2 = the velocity of the steel strip leaving the work rolls.

The rolling drawing mill stand 1 reduces the thickness of the steel strip 2, which is dry, at a reduction ratio from 2 to 8%. The steel strip leaves the rolling drawing mill stand f and passes through first and second descaling stands 3 as shown in FIGURE 8.

When the steel strip 2 is partly wrapped about the work rolls 4a and 4b of the rolling drawing mill stand 1 in the form of a letter 'S', a desired reduction ratio may be obtained easily with less reduction forces as shown in FIGURE 7 by the curve 5. In addition, the adhesion or wedging of scale to the surfaces of the steel strip 2 may be minimised.

Referring back to FIGURE 9, in the reduction zone 'x' friction forces developed between the work rolls 4a and 4b and the steel strip 2 being rolled are opposite in direction along the arcsjl and mk of contact. As a result, the condition of the scale are so changed that the scale may be easily removed from the surfaces of the strip 2. In addition, the steel strip 2 is bent to pass in the curved path over the peripheral surfaces of the work rolls 4a and 4b so that the removal of scale may be further facilitated.

As described elsewhere, the descaling time of the steel strip which has been rolled at the reduction ratio of from 2 to 8% may be considerably reduced so that the number of descaling devices 3 may be reduced (for instance to two as shown in FIGURE 8). Since the conditions of the scale have been changed as described above, the impinging angle 0 may be made less than 30C so that the desired surface roughness and qualities of the descaled steel strip 2 may be maintained without any special treatment.

As described above, with the two-high rolling 'mill stand of the type shown in FIGURE 3, considerable rolling forces are needed to attain a reduction ratio of from 2 to 8%. As a result, a multi-high rolling mill stand such as the four-high rolling mill stand as shown if FIGURE 6 must be used so that the descaling line becomes large and long.Furthermore, even when the steel strip is rolled by the four-high rolling mill stand at a reduction ratio of from 2 to 8% the conditions of scale are not so improved as to facilitate easy descaling as compared with the method of the present invention, wherein the upper and lower surfaces of the steel strip 2 are subjected to friction forces in opposite directions and are wrapped about the work rolls 4a and 4b in the form of a letter 'S' so that the conditions of scale may be so remarkably improved as to facilitate the descaling.

Effects and advantages of the present invention may be summarised as follows; (1) Since the rolling drawing process is employed, the steel strip may be rolled at a desired reduction ratio with less reduction force prior to descaling. As a result, not only the conditions of the scale are so changed that the descaling may be much facilitated but also the adhesion or wedging of scale to the surfaces of the steel strip may be minimised.

(2) Because of (1 ) the number of descaling devices may be decreased and consequently the descaling line may be shortened in length.

(3) Because of (1), the impinging angle 0 may be reduced so that a desired roughness and surface qualities may be avoided.

(4) A smaller rolling mill may be employed.

Furthermore because of the pre-rolling by the rolling mill, rolling mill stands may be reduced in number in a succeeding continuous rolling mill.

(5) When the steel strip passes through the rolling drawing mill stand, friction forces are developed between the work rolls and the steel strip in opposition directions along the upper and lower surfaces of the steel strip and the steel strip is bent to follow an S-shaped path, so that the condition of scale on the steel strip may be so changed as to facilitate the descaling.

It is to be understood that the present invention is not limited to the preferred embodiment described above with reference to the accompanying drawings and that various modifications may be effected without departing from the spirit of the invention.

Claims (3)

1. A method of descaling surface of hot-rolled steel strip which comprises passing the strip through a rolling drawing mill stand between a pair of work rolls rotating in opposite directions, in which the ratio of the peripheral velocity of the upstream roll, which is equal to that of the strip entering the work rolls, to the peripheral velocity of the downstream roll, which is equal to that of the strip leaving the work rolls is equal to the ratio of the thickness of the strip leaving the work rolls to the thickness of the strip entering the work rolls, giving a reduction ratio between 2% and 8% and thence passing the strip through descaling devices for ejecting descaling agents against their surfaces to remove scale.

2. A method as claimed in Claiml in which the strip is passed in an 'S' shaped path round and between the work rolls.

3. A method of descaling the surfaces of hotrolled steel strip as specifically described with reference to the accompanying drawings.