JPH1058028A - Descaling method in hot rolling line - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent scale defects by executing a primary descaling by jetting high-pressure water or high-pressure water together with abrasives on the surfaces of a steel sheet, after the lapse of >=1sec, executing similar secondary descaling and leaving the first scale layer. SOLUTION: Spray headers 1a, 1b for jetting high-pressure water on the surface and back faces of the steel sheet 3 are provided on the inlet and outlet sides of a finish rolling rolls 2 and, by selectively using one of them in accordance with the advancing directions A, B of the steel sheet 3, descaling treatment is executed. While passing the steel sheet 3 forward in the direction A without applying reduction to the steel sheet, by jetting the high-pressure water from the spray header 1a, the primary descaling is executed. After the lapse of 1sec from the completion of this process, the steel sheet 3 is reversely passed in the direction B and, by jetting the high-pressure water from the spray header 1b while applying the reduction at a prescribed draft, the secondary descaling is executed. In this way, in the second scale layer which is not removed by the first descaling, crack is progressed by temp. difference between the scale and the steel sheet, the scale is removed by the secondary descaling and scale defects are not generated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a descaling method for a hot rolling line for a thick plate, a section steel or the like.

[0002]

2. Description of the Related Art With respect to a conventional descaling method in a hot rolling line, removal of primary scale formed during heating is performed by injection of high-pressure water by a hydraulic scale breaker arranged immediately before a rough rolling mill. ing. Further, the removal of the secondary scale formed at the time of rolling is similarly performed by injecting high-pressure water by a hydraulic scale breaker disposed immediately before each of a rough rolling mill and a finishing rolling mill.

[0003] However, these prior arts, in particular,
In the above-described secondary scale removing method, the scale may not be sufficiently removed due to interference or stagnation of high-pressure water during descaling. If the removal of the scale is insufficient as described above, the steel sheet is rolled down while the scale unevenness is formed on the surface of the steel sheet. As a result, problems such as scale flaws and variations in the strength of the steel sheet have occurred.

[0004]

As means for solving the above-mentioned problem, Japanese Patent Application Laid-Open No. Hei 7-48622 discloses a method for removing secondary scale by cooling a steel sheet from 700 to 800 ° C. immediately after the end of rolling. A method is disclosed (hereinafter, referred to as “prior art 1”). However, in order to carry out the method of Prior Art 1, it is necessary to use equipment for uniformly cooling the steel sheet within the above-mentioned temperature range, and therefore, enormous equipment costs are required. Due to such water cooling, there is a possibility that heat distortion occurs in the steel sheet.

As another means for solving the above-mentioned problem,
Japanese Patent Laying-Open No. 7-48623 discloses a method for removing secondary scale by limiting the rolling end temperature to 850 ° C. or lower (hereinafter, referred to as “prior art 2”). However, in the method of Prior Art 2, since the rolling end temperature is limited to 850 ° C. or less as described above,
This method cannot be applied to a steel sheet finish-rolled at a high temperature exceeding 850 ° C., resulting in low versatility.

Accordingly, an object of the present invention is to provide a hot rolling line capable of solving not only problems such as scale flaws and variations in the strength of steel sheets, but also problems involved in prior arts 1 and 2. To provide a de-scaling method in.

[0007]

Means for Solving the Problems The present inventors have developed a hot rolling line which can solve the problems of scale flaws and variations in the strength of steel sheets, and the problems of prior arts 1 and 2. We worked diligently to develop a descaling method.

As a result, the following findings were obtained. The scale unevenness includes a black-gray scale layer formed on the surface of the steel sheet (hereinafter, referred to as a “first scale layer”),
And a red scale layer formed on the first scale layer (hereinafter, referred to as “second scale layer”). The first scale layer is made of magnetite (Fe ₃ O ₄ ), has a uniform structure, has excellent adhesion to a steel plate, and has a thickness as thin as 10 μm or less. On the other hand, the second scale layer has a wustite (F
eO), hematite (Fe ₂ O ₃ ), and other fine powders, the structure is not uniform, and the thickness is as thick as 20 μm or more. The second scale layer is likely to partially remain on the first scale layer due to interference or stagnation of high-pressure water during descaling in the high-temperature rolling process of the steel sheet. The second scale layer partially remaining on the first scale layer is pulverized by rolling to increase its surface area, and grows thicker by being oxidized. When the steel sheet on which the grown second scale layer remains is further rolled, scale flaws occur due to the grown second scale layer. In the case of accelerated cooling of a steel sheet, the cooling capacity of the second scale layer having a large thickness is smaller than the cooling capacity of the first scale layer having a small thickness. Therefore, in the case of accelerated cooling of the steel sheet, if scale unevenness including the second scale is formed, a variation in the strength of the steel sheet occurs, and the range is not only the surface of the steel sheet, but also １ ／ of the sheet thickness. Range. In the early stages of the finish rolling pass schedule,
By removing the second scale layer and leaving only the first scale layer made of magnetite having a thin and uniform structure on the surface of the steel sheet, it is possible to solve the above-mentioned problems of scale flaws and variations in the strength of the steel sheet. it can. The steel sheet with only the first scale layer left on its surface is excellent in paintability and laser cutting properties, and can be used in various fields such as shipbuilding, construction, bridges, pressure vessels, line pipes, etc. Can be provided.

[0009] The present invention has been made based on the above-mentioned findings, and the invention described in claim 1 is to provide a steel plate with a steel plate at an early stage of a finish rolling pass schedule of a hot rolling line.
Performing a primary descaling step to remove scale on the surface of the steel sheet by spraying high-pressure water or high-pressure water and abrasive onto the surface of the steel sheet without applying substantial reduction; After one second or more has passed after the end of the primary descaling step, at least a secondary descaling step of removing high-pressure water or high-pressure water and abrasives onto the surface of the steel sheet to remove the scale is performed. Once, thus leaving a scale layer having a thin and uniform structure on the surface of the steel sheet, that is, a first scale layer.

[0010] The invention described in claim 2 is characterized in that the secondary descaling step is performed without substantially reducing the steel plate. The invention described in claim 3 is characterized in that the secondary descaling step is performed while reducing the steel sheet at a predetermined reduction rate.

According to a fourth aspect of the present invention, the secondary descaling step is performed a plurality of times, and the second de-scaling step is performed a plurality of times.
Performing at least one of the following descaling steps without substantially reducing the steel sheet, and performing at least one remaining of the secondary descaling steps performed a plurality of times on the steel sheet, It is characterized in that the reduction is performed while applying a predetermined reduction ratio.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In finish rolling of a hot rolling line, a steel sheet is subjected to a descaling process according to a plurality of kinds of descaling patterns using a hydraulic scale breaker.

That is, as shown in FIG. 1 to FIG. 5, a pair of upper and lower spray headers 1a and 1b for injecting high-pressure water toward the front and back surfaces of the steel sheet are provided on the entrance and exit sides of the pair of finishing rolls 2. 1b, and between the finish rolling rolls 2, while performing a forward pass and a reverse pass of the steel sheet 3, selectively using one of the spray headers 1a and 1b according to the traveling direction of the steel sheet 3. Then, high-pressure water was sprayed on the front and back surfaces of the steel sheet 3 passing between the finish rolling rolls 2, and thus the steel sheet was subjected to a descaling treatment. The descaling process described above is performed in the following 7
Made according to different descaling patterns.

Descaling pattern As shown in FIG. 1 (a), the steel sheet 3 is not subjected to substantial reduction and is transferred from the spray header 1a while being forward-passed between the finish rolling rolls 2 in the direction of arrow A. The primary descaling process was performed by injecting high-pressure water onto the front and back surfaces of the steel plate 3. One second after the end of this step, FIG.
As shown in (b), the steel sheet 3 is reverse-passed between the finish rolling rolls 2 in the direction of arrow B, and while the steel sheet 3 is subjected to reduction at a predetermined reduction rate, a high pressure is applied from the spray header 1b to the front and back surfaces of the steel sheet 3. Water was injected to perform a secondary descaling step.

Descaling pattern As shown in FIG. 1 (a), the steel sheet 3 is not subjected to substantial reduction and is transferred from the spray header 1a while passing forward between the finish rolling rolls 2 in the direction of arrow A. The primary descaling process was performed by injecting high-pressure water onto the front and back surfaces of the steel plate 3. 2 seconds after the end of this step, FIG.
As shown in (b), the steel sheet 3 is reverse-passed between the finish rolling rolls 2 in the direction of arrow B, and while the steel sheet 3 is subjected to reduction at a predetermined reduction rate, a high pressure is applied from the spray header 1b to the front and back surfaces of the steel sheet 3. Water was injected to perform a secondary descaling step. That is, the de-scaling pattern is the same as the de-scaling pattern except that after the primary descaling step is completed, a two-second wait is performed before the secondary descaling step is started.

Descaling pattern As shown in FIG. 2 (a), the steel sheet 3 is not subjected to substantial reduction and is transferred from the spray header 1a while being forward-passed between the finish rolling rolls 2 in the direction of arrow A. The primary descaling process was performed by injecting high-pressure water onto the front and back surfaces of the steel plate 3. One second after the end of this step, FIG.
Similarly, as shown in (b), the front and back surfaces of the steel plate 3 are sprayed from the spray header 1b while the steel plate 3 is subjected to a reverse pass in the direction of arrow B between the finish rolling rolls 2 without substantial reduction. To perform a secondary descaling step.

Descaling pattern As shown in FIG. 3 (a), the steel sheet 3 is not subjected to a substantial reduction and is transferred from the spray header 1a while passing forward between the finishing rolls 2 in the direction of arrow A. The primary descaling process was performed by injecting high-pressure water onto the front and back surfaces of the steel plate 3. One second after the end of this step, FIG.
Similarly, as shown in (b), the front and back surfaces of the steel plate 3 are sprayed from the spray header 1b while the steel plate 3 is subjected to a reverse pass in the direction of arrow B between the finish rolling rolls 2 without substantial reduction. To perform a first process as a secondary descaling process. After the end of this process 1
After a lapse of seconds, as shown in FIG. 3 (c), the steel sheet 3 is again forward-passed between the finish rolling rolls 2 in the direction of the arrow A, and while the steel sheet 3 is being reduced by a predetermined reduction ratio, the steel sheet 3 is removed from the spray header 1a. High-pressure water was sprayed on the front and back surfaces of the steel sheet 3 to perform a second step as a secondary descaling step.

Descaling pattern As shown in FIG. 4 (a), the steel sheet 3 is not subjected to substantial reduction and is transferred from the spray header 1a while passing forward between the finish rolling rolls 2 in the direction of arrow A. The primary descaling process was performed by injecting high-pressure water onto the front and back surfaces of the steel plate 3. One second after the end of this step, FIG.
As shown in (b), the steel sheet 3 is reverse-passed between the finish rolling rolls 2 in the direction of arrow B, and while the steel sheet 3 is subjected to reduction at a predetermined reduction rate, a high pressure is applied from the spray header 1b to the front and back surfaces of the steel sheet 3. Water was injected to perform a first step as a secondary descaling step. Immediately after the end of this step, as shown in FIG. 1
A high pressure water was injected from a to the front and back surfaces of the steel plate 3 to perform a second step as a secondary descaling step. One second after the end of this step, as shown in FIG.
The steel sheet 3 is reverse-passed again in the direction of arrow B between the finish rolling rolls 2 to apply high-pressure water to the front and back surfaces of the steel sheet 3 from the spray header 1b while applying a reduction to the steel sheet 3 at a predetermined reduction rate. A third step as a scaling step was performed.

Descaling pattern As shown in FIG. 5 (a), the steel sheet 3 is forward-passed between the finish rolling rolls 2 in the direction of the arrow A, and while the steel sheet 3 is subjected to the reduction by a predetermined reduction rate, the spray header 1a is formed. , High-pressure water was sprayed on the front and back surfaces of the steel plate 3 to perform a primary descaling step. One second after the end of this step,
As shown in FIG. 5 (b), the steel plate 3 is reverse-passed between the finish rolling rolls 2 in the direction of the arrow B, and the steel plate 3 is subjected to rolling at a predetermined rolling reduction while the front and rear surfaces of the steel plate 3 are sprayed from the spray header 1 b. To perform a secondary descaling step.

Descaling pattern As shown in FIG. 1 (a), the steel sheet 3 is not subjected to substantial reduction and is transferred from the spray header 1a while being forward-passed between the finish rolling rolls 2 in the direction of arrow A. The primary descaling process was performed by injecting high-pressure water onto the front and back surfaces of the steel plate 3. Immediately after the completion of this step, as shown in FIG. 1 (b), the steel sheet 3 is reverse-passed between the finish rolling rolls 2 in the direction of arrow B, and while the steel sheet 3 is subjected to a reduction by a predetermined reduction rate, a spray header is formed. High pressure water was injected from 1b to the front and back surfaces of the steel sheet 3 to perform a secondary descaling step. That is, the descaling pattern is the same as the descaling pattern except that the secondary descaling step is started immediately after the primary descaling step.

Table 1 shows the presence or absence of the reduction in each of the first and second descaling steps from the above-described descaling pattern, and the waiting time between the first and second descaling steps.

[0022]

[Table 1]

According to the descaling pattern described above, the steel sheet was subjected to a descaling process, and after the completion of the process, the presence or absence of the second scale layer on the surface of the steel plate was examined, and the area ratio was determined. FIG. 6 shows the area ratio of the second scale layer on the surface of the steel sheet thus determined.

As is apparent from FIG. 6, the second scale layer did not remain on the surface of the steel sheet in any of the descaling patterns. On the other hand, in the descaling pattern, the second scale layer remained on the surface of the steel sheet, and its area ratio was about 65%. Also, in the descaling pattern,
The second scale layer remained on the surface of the steel sheet, and its area ratio was about 35%.

The reason why the second scale layer did not remain on the surface of the steel sheet in any of the descaling patterns is as follows. (1) No substantial reduction is applied to the steel sheet in the primary descaling step. (2) After one second or more has elapsed after the end of the primary descaling step, the secondary descaling step is performed.

On the other hand, the reason why the second scale layer remained on the surface of the steel sheet in the descaling pattern is that the steel sheet was subjected to the reduction at a predetermined reduction rate in the primary descaling step. It is. On the other hand, in the descaling pattern, the second
The reason that the scale layer remained was that the secondary descaling step was performed immediately after the primary descaling step.

As in the case of the descaling pattern, when the primary descaling step is performed without applying substantial reduction to the steel sheet, the second scale that cannot be removed by the primary descaling step is removed. Even if the layer remains on the surface of the steel sheet, it is not reduced, so that the second scale layer remaining on the surface of the steel sheet is pulverized by rolling, and the second scale layer thus pulverized. Of the steel sheet on the surface of the steel sheet is prevented.

Therefore, in the first descaling step of the descaling method in the hot rolling line of the present invention, the steel sheet should not be subjected to substantial reduction. As in the case of the descaling pattern, if the standby time after the end of the primary descaling step until the start of the secondary descaling step is 1 second or more, it cannot be removed by the primary descaling step. In the second scale layer, the temperature difference between the steel sheet and the second scale layer promotes the generation of cracks, and as a result, in the second descaling step, it can be removed by the first descaling step. The second scale layer that could not be removed can be easily and completely removed.

Therefore, in the descaling method for the hot rolling line according to the present invention, the secondary descaling step should be performed at least one second after the end of the primary descaling step.

As is apparent from the above, the descaling method in the hot rolling line of the present invention is performed according to a pattern represented by a descaling pattern.

The descaling method in the hot rolling line of the present invention should be performed at the beginning of the finish rolling pass schedule. Even if the above descaling method is performed in the middle or late stage of the finish rolling pass schedule, the second scale layer cannot be completely removed. The reason is that the second scale layer mainly grows at the temperature before and after the start of the finish rolling, and the thick second scale layer pressed at the early stage of the finish rolling adheres firmly and deeply to the steel sheet surface. However, it is difficult to completely remove the second scale layer in the middle and late stages of finish rolling. On the other hand, if the above-mentioned descaling method is carried out in the early stage of the finish rolling, it is possible to relatively easily remove the thick second scale layer before press-bonding the second scale layer. Can be removed.

Incidentally, it is also possible to add an abrasive having a predetermined chemical component composition to the high-pressure water used in the descaling method of the present invention, if necessary.

[0033]

EXAMPLES Next, the descaling method in the hot rolling line according to the present invention will be described in more detail with reference to examples and comparative examples.

By weight%, C: 0.12%, Si: 0.3
6%, Mn: 1.36%, P: 0.008%, S: 0.
001%, Nb: 0.009%, Ti: 0.007%,
Al: 0.054%, N: 0.0031%, and a steel sheet composed of iron, inevitable impurities,
C. to remove the scale generated during the heating, the so-called primary scale. Next, the steel sheet from which the primary scale had been removed was subjected to finish rolling. The start temperature of finish rolling is 880 ° C. and the end temperature is 830 ° C.
Met. While performing the finish rolling process in this way,
At the beginning of the process, descaling according to any one of the descaling patterns was performed, and thus the methods A to E of the present invention were performed.

Table 2 shows the thickness of the steel sheet, the timing of descaling, and the descaling pattern for each of the methods A to E of the present invention.

[0036]

[Table 2]

The surface properties of the steel sheet after the above-mentioned finish rolling treatment were examined. That is, the type of the scale layer on the surface of the steel sheet was visually confirmed, and the thickness of the scale layer on the cross section of the steel sheet was measured with an optical microscope. The type of the scale layer thus confirmed and the thickness of the scale layer thus measured are also shown in Table 2.

Further, the adhesion of the scale on the surface of the steel sheet after completion of the above-mentioned finish rolling was examined. That is,
The steel sheets in the methods A, C and D of the present invention were reduced to 20 mm in thickness while leaving one surface, and cut into 100 mm width to prepare each sample. Regarding the steel sheet in the methods B and E of the present invention, the steel sheet was used as it was without reducing its thickness as described above,
Each sample was prepared by cutting to width. Each of the samples prepared in this manner was placed 180 degrees apart with a surface bend radius of twice its thickness, with its surface located outside.
° folded. The degree of peeling of the scale at the bent portion of each sample was observed. In the bent portion of the sample, when peeling of the flaky scale was recognized, it was evaluated as poor, and when the peeling was not recognized, it was evaluated as good. Table 2 also shows the above-mentioned evaluations regarding the scale adhesion for each sample.

As is apparent from Table 2, the methods A to E of the present invention in which the descaling according to the descaling pattern was performed at the beginning of the finish rolling pass schedule.
In, no residual second scale layer was observed,
4.5 to 7.3 μm observed on the surface of the steel plate
The first scale layer had a small thickness in the range of 1. and the adhesion was good.

For comparison, a steel sheet having the same composition as the above-mentioned steel sheet was similarly heated to 1200 ° C., and the scale generated during heating, that is, the so-called primary scale was removed. Next, the steel sheet from which the primary scale had been removed was subjected to finish rolling. The starting temperature of finish rolling is
As before, it is 880 ° C. and its end temperature is 830 ° C.
° C. While performing the finish rolling process in this way, in any one of the initial, middle and late stages of the process, the descaling pattern and any one of
, And thus performed the comparison methods F to I outside the scope of the present invention.

Table 2 also shows the thickness of the steel sheet, the timing of descaling, and the descaling pattern for each of the comparison methods F to I. For each of Comparative Methods F to I, the surface properties of the steel sheet after the finish rolling treatment were examined in the same manner as in Methods A to E of the present invention. Table 2 also shows the type of scale layer on the surface of the steel sheet and the thickness of the scale layer for each of Comparative Methods F to I.

Further, for each of the comparison methods F to I,
The scale adhesion on the surface of the steel sheet after the finish rolling treatment was examined by the same method as in methods A to E of the present invention. Table 2 also shows the evaluation of scale adhesion for each sample for each of Comparative Methods F to I.

As is clear from Table 2, the descaling was performed at the beginning of the finishing rolling pass schedule, but in the comparison method F or G performed according to the descaling pattern or according to the first scale layer only. The remaining second scale layer was also observed, and the thickness of the scale layer was as thick as 63.8 μm or 34.1 μm.
Moreover, the adhesion was poor. Also, although the descaling was performed according to the descaling pattern,
However, in the comparison method H or I performed in the middle or late stage of the finish rolling pass schedule, since descaling was not performed at the beginning of the finish rolling pass schedule, not only the first scale layer but also the second scale layer Was observed, and the thickness of the scale layer was 20.7 μm
Or, it was as thick as 25.2 μm, and the adhesion was poor.

[0044]

As described above, according to the present invention, (1) the primary descaling step is performed without applying substantial reduction to the steel sheet. Even if the second scale layer, which could not be removed, remains on the surface of the steel sheet, it is not reduced, so the second scale layer remaining on the surface of the steel sheet is not reduced.
The scale layer is pulverized by rolling down, and the pressure of the pulverized second scale layer on the surface of the steel sheet is prevented,
(2) Since the standby time of one second or more is provided after the end of the primary descaling step and before the secondary descaling step is started, it cannot be removed by the primary descaling step. In the second scale layer, the temperature difference between the steel sheet and the second scale layer promotes cracking, so that in the secondary descaling step, it can be removed by the primary descaling step. The second scale layer that did not exist can be easily and completely removed, and (3) the first scale layer remaining on the surface of the steel sheet has a uniform structure and excellent adhesion to the steel sheet; Furthermore, since its thickness is as thin as 10 μm or less, no scale flaws are generated, and as a result, problems such as scale flaws and variations in the strength of steel sheets can be solved. , The prior art 1 and 2 can be solved the problem of occluded, thus, results in a useful effect.

[Brief description of the drawings]

FIG. 1 is a schematic side view showing a descaling pattern of the method of the present invention.

FIG. 2 is a schematic side view showing another descaling pattern of the method of the present invention.

FIG. 3 is a schematic side view showing another descaling pattern of the method of the present invention.

FIG. 4 is a schematic side view showing another descaling pattern of the method of the present invention.

FIG. 5 is a schematic side view showing a descaling pattern outside the scope of the present invention.

FIG. 6 is a graph showing a relationship between a descaling pattern and an area ratio of a second scale layer on a surface of a steel sheet.

[Explanation of symbols]

 1a, 1b: Spray header 2: Finish rolling roll 3: Steel plate

Claims (4)

[Claims] At the beginning of the finishing rolling pass schedule of the hot rolling line, the steel sheet is not subjected to substantial reduction,
High pressure water or high pressure water and abrasive are sprayed on the surface of the steel sheet to perform a primary descaling step for removing scale on the surface of the steel sheet, and then, after the primary descaling step is completed. After 1 second or more, spraying high-pressure water or high-pressure water and an abrasive on the surface of the steel sheet, performing a secondary descaling step for removing the scale at least once, Descaling method in hot rolling line. 2. The method according to claim 1, wherein the secondary descaling step is performed without substantially reducing the steel sheet. 3. The method according to claim 1, wherein the secondary descaling step is performed while subjecting the steel sheet to reduction at a predetermined reduction rate. 4. The method according to claim 1, wherein the secondary descaling step is performed a plurality of times, and at least one of the secondary descaling steps performed a plurality of times is performed without substantially reducing the steel sheet. Performing at least one of the remaining secondary descaling steps performed on the steel sheet,
The method according to claim 1, wherein the reduction is performed while applying a reduction at a predetermined reduction rate.