JPH0957327A - Scale removal method of steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent scale indentation at the time of hot straightening and surface defect at the product and further, to conduct uniform cooling at the time of forced cooling. SOLUTION: Descaling is carried out on at least one hand of just before and just behind of lest pass at the finishing rolling and scale is formed with a uniformed thickness at an extent of unscaling off during from termination of finishing rolling to hot straightening. Successively, hot straightening is carried out and after that, descaling of growing secondary scale is carried out and within 5 seconds from descaling, the forced cooling is started.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scale removing method for thick steel plates.

[0002]

2. Description of the Related Art In the hot rolling of thick steel plates, when hot straightening and forced cooling by a hot leveler are carried out after finish rolling, if secondary scale generated on the surface of the thick steel plates after finish rolling remains, In the hot leveler, peeling of scale and indentation of the scale cause defective surface quality of the product.

Therefore, it is extremely important to prevent the secondary scale from remaining during hot straightening with a hot leveler and remove the secondary scale.

Various disclosures have heretofore been made regarding methods for removing secondary scales and methods for suppressing the generation of secondary scales.

For example, in Japanese Unexamined Patent Publication No. 49-90256, a secondary scale flaw in which a steel plate temperature is adjusted to 650 ° C. to 800 ° C. immediately before a final finishing pass during hot rolling of a thick steel plate and then a final pass is performed. A method for preventing the generation of the steel sheet is disclosed, and in Japanese Patent Publication No. 59-13926, the S of the steel sheet is subjected to descaling during finish rolling of hot rolling.
A method is disclosed in which the content, the target thickness of the product, the surface grade of the product, the surface temperature, and the passing speed of the descaling device are input to the arithmetic processing device to perform the descaling control and to be reflected in the descaling control of the next material and thereafter.

Further, in Japanese Unexamined Patent Publication No. 59-218202, at the time of controlled rolling of a thick steel plate, a steel plate temperature and a temperature adjusting time following a temperature adjusting start temperature are scaled based on the scale and the scale peeling characteristic with respect to the time. A method for reducing the scale pattern is disclosed in which the peeling area is set and then rolling is restarted.
In Japanese Patent Laid-Open No. 44-44, after completion of primary descaling, 2
A scale removing method is disclosed in which the elapsed time until the next descaling or the elapsed time between arbitrary descaling during the secondary descaling is controlled so as to satisfy the relational expression between the steel plate surface temperature and the elapsed time.

[0007]

However, all of the above prior arts are intended to improve the descaling property or suppress the generation of secondary scale by controlling or adjusting the steel sheet temperature during finish rolling. ,
Even if these were completely performed and the secondary scale could be completely removed by the end of finish rolling, the steel plate temperature at that time was in the high temperature range of approximately 600 to 800 ° C. A large amount of secondary scale was generated, and indentation of the scale during hot straightening by a hot leveler and poor surface quality of the product were unavoidable.

On the other hand, in Japanese Unexamined Patent Publication No. 63-101018, in a hot leveler for hot straightening a thick steel plate, high-pressure water or high-pressure air is jetted to at least one place between upper leveling rolls and between lower leveling rolls. A descaling device with a descaling nozzle is disclosed.

However, when descaling of rolling of a forced coolant is carried out by using this apparatus, a sharp temperature drop occurs due to descaling in an unsuitable temperature range, resulting in a steel plate shape or a machine. The effect on the characteristics becomes large.

Further, in Japanese Patent Laid-Open No. 3-110017, a hot-rolling online forcedly cooled thick steel plate is
Disclosed is a method for producing a thick steel plate without secondary scale flaws, which is corrected so as to satisfy the relational expression between the cooling water amount of the leveler roll and the cooling stop temperature when hot-correcting with a hot leveler on the rear surface.

However, in this method, the line itself is originally different from the method for forcibly cooling after hot correction according to the present invention in that hot straightening is carried out by a hot leveler after cooling. Even if it is applied to the target line, since the temperature after forced cooling is about room temperature, it is not within the temperature range where the secondary scale can be peeled off, and therefore the invention cannot be applied.

As described above, the problems in the prior art are that the generation of secondary scale remains a problem in the descaling before the end of finish rolling, and the steel plate shape and the product required as a product in the descaling at the time of hot straightening remain. No mechanical properties can be obtained.

[0013] Therefore, a first object of the present invention is to prevent the peeling of the scale during hot straightening by the hot leveler due to the secondary scale and the surface quality defect of the product due to the pushing.

The second problem is to avoid unnecessary excessive temperature drop and to improve the scale property of the product by performing uniform cooling during forced cooling, and at the same time, excellent in steel plate shape and mechanical properties. To get a steel plate.

[0015]

A method of removing scales of a thick steel sheet according to the present invention, which has solved the above-mentioned problems, is a method of hot-rolling a thick steel sheet, in which descaling is performed at least immediately before or immediately after a final pass in finish rolling. Descaling of a secondary scale that is scaled to a uniform scale with a thickness that does not peel after the completion of finish rolling and until the hot straightening is performed, followed by hot straightening and subsequent growth Is performed, and forced cooling is started within 5 seconds from this descaling.

That is, the present invention is conceptually described as follows. Corresponding to different pass schedules of finish rolling depending on the target plate thickness and temperature, conventionally, descaling was performed at various pass stages suitable for the steel plate temperature and the scale occurrence situation.
The secondary scale grew after the descaling and before the hot straightening, and the scale peeling and the indentation during the hot straightening occurred in any case.

Therefore, in the present invention, in order to uniformly generate the scale formed after finishing rolling and before hot straightening with a thickness that does not cause peeling, the scale immediately before and after the final pass in finish rolling is selected. We decided to do descaling on at least one of the above. In this case, if the scale thickness at the time of hot straightening is 30 μm or less, it is a thickness that does not generally peel off. Therefore, it is preferable to perform descaling so as to achieve the scale thickness at the time of hot straightening. . This descaling may be done either just before or just after the final pass, or both just before and just after the final pass.

In this way, it is possible to prevent the scale from peeling from the thick steel plate and its indentation during hot straightening.

Even after the hot straightening and the forced cooling, the secondary scale grows thickly. It is relatively easy to pulverize and remove the grown secondary scale, and in the present invention, the secondary scale is pulverized and removed before forced cooling.

Moreover, by pulverizing and removing the secondary scale immediately before the forced cooling, that is, within 5 seconds before the forced cooling, the steel sheet surface and the internal temperature can be cooled all at once to a temperature range where the scale growth rate is slow. , Scale growth is extremely small, and it can be made tight, so uneven cooling during forced cooling (non-uniform cooling)
Can be obtained, and not only a thick steel plate excellent in mechanical properties and steel plate shape can be obtained, but also a product having excellent surface properties can be obtained, and the maintenance rate is reduced and the yield is improved.

The present invention is particularly effective for steel plates having a target thickness of 20 mm or more, a finishing temperature of 700 ° C. or more, and a temperature of 600 ° C. or more at the time of forced cooling disclosure. . Also, for higher strength,
Even a thick steel plate containing 0.2% or more of Si can achieve sufficient descaling.

[0022]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a process diagram showing a manufacturing process of a thick steel plate according to the present invention. After finishing rolling a plurality of times, hot compulsion is performed by a hot leveler and compulsory cooling is performed to obtain a product.

The pass schedule of the finish rolling process varies depending on the plate thickness, temperature range, etc., but generally, descaling is performed several times during the pass. This descaling is performed at various timings depending on the steel plate temperature and the scale generation situation, but the secondary scale grows during the descaling and after the final descaling.

In particular, when a thick steel plate added with Si for high strength is used, FeO--Fe ₂ SiO ₂ is heated at the time of heating.
_Although a eutectic compound of ₄ is formed, Fe ₂ SiO ₄ (firelite) greatly enhances the adhesion between FeO (wustite) and the base iron structure, which causes the descaling residue.

If any descaling remains, a new 2 is added between that part and the other descaled parts.
Since the growth rate of the next scale is different, an uneven secondary scale will be formed on the surface of the steel sheet before the next descaling. Then, in the next descaling, the secondary scale is removed in the direction of further promoting the unevenness. When such a secondary scale having irregularities is brought into the hot forcing step, partial peeling of the secondary scale is brought about.

Therefore, in the present invention, the finishing temperature is 700
In the finish rolling process at ℃ or higher, descaling is performed immediately before and / or immediately after the final pass in finish rolling.

Thus, since the descaling is performed in the final pass stage of finish rolling, even if the secondary scale grows thereafter, it is uniform and the thickness of the secondary scale is not so thick.

As a result, in the subsequent hot straightening step, since the secondary scale having a uniform thickness and not peeled off is formed, the scale peeling and the indentation thereof are prevented.

After the hot straightening, the forced cooling is started at a temperature of 600 ° C. or higher. In the present invention, however, the descaling is performed again between the hot straightening and the forced cooling, and the final pass of the finish rolling is performed. After the descaling at the stage, the secondary scale grown until just before the start of the forced cooling is removed, and the forced cooling is started within 5 seconds after the descaling.

Therefore, according to the present invention, immediately after the descaling, the steel sheet surface and the internal temperature can be cooled at once to a temperature range where the scale growth rate is slow (the forced cooling rate is preferably 5 ° C./sec or more). The growth of scale is extremely small, and since it can be made tight, it is possible to prevent uneven cooling during forced cooling, and to obtain a thick steel plate excellent in mechanical properties and steel plate shape.
As the descaling immediately before the forced cooling, for example, the mode of FIG. 2 can be used.

That is, before the thick steel plate M flowing in the arrow direction on the table rolls 1, 1, ... After being subjected to hot straightening enters the cooling device 2, several descaling nozzles are arranged along the width direction of the line. It is performed by injecting high-pressure water from No. 3. Descaling nozzle 3 is cooling header 4
Is integrally attached to the elevating cooling header 4 so that the elevating cooling header 4 can be elevated and lowered, and the distance from the plate surface to the descaling nozzle 3 can be adjusted by the thickness of the thick steel plate. . Here, in consideration of the descaling effect, it is desirable that the distance between the descaling and the steel sheet M is 350 mm or less. A pump 6 is connected to the descaling nozzle 3 via a pipe 5, and the discharge pressure of the descaling high-pressure water W ₁ injected from the descaling nozzle 3 can be adjusted by the pump 6, and the discharge The pressure is preferably 150 kgf / cm ² or more. If the discharge pressure is less than 150 kgf / cm ² , the descaling ability is poor.

Further, the tip of the descaling nozzle 3 is inclined 5 to 10 ° in the direction of the finishing mill (upstream side),
In addition to preventing interference with the cooling water for forced cooling, the descaling scale has good removability (removability).

The descaled steel sheet M is forcibly cooled by the cooling water W ₂ jetted from the cooling nozzles 7, 7, ... Attached to the cooling header 4.

[0034]

Embodiments of the present invention will be described below. (Basic Experiment Example) Using the target material having the composition shown in Table 1, the method for manufacturing a thick steel plate according to the present invention was executed in the steps shown in FIG. As the steel material used here, the most representative one having a plate thickness, a component system, and a strength grade is selected.

[0035]

[Table 1]

Since this steel type is sometimes used as a tank steel material such as LPG, its surface scale property or mechanical property is always important as a control item. As a result of the present implementation, the temperature process of the target material is as shown in FIG.

Incidentally, the effect of descaling between passes during finish rolling on the scale thickness is shown in FIG. It is known that scale growth follows the parabolic law, and if descaling is performed between all passes, the removal of the secondary scale is complete, but in reality the temperature drop becomes too severe, so for example during rolling. Descaling cannot be performed between all passes for a controlled rolling material that has temperature control (designation) for each pass.

Therefore, in all six passes of finish rolling,
As shown in Table 2, when the occurrence rate of indentation flaws at the time of hot straightening was examined due to the difference in descaling paths, the results shown in the same table were obtained. In any case, descaling before forced cooling was not performed.

[0039]

[Table 2]

As can be seen from Table 2, by performing descaling at least in the final pass stage among all 6 passes,
It can be seen that the product indentation rate can be reduced.

Further, as shown in FIG. 5, the scale thickness after that greatly changes depending on whether or not descaling is performed before the forced cooling. This is because, if the scale is removed immediately before the forced cooling, the new scale can be immediately cooled immediately to a temperature range where the new scale hardly grows.

Further, if there is cooling unevenness during forced cooling, it becomes necessary to perform re-correction after that. In each case,
Table 3 shows the results of using the re-forced implementation rate as a criterion for determining the quality of the steel sheet shape, depending on the presence or absence of descaling immediately before forced cooling after performing descaling just before the final finishing pass.

[0043]

[Table 3]

It can be seen from Table 3 that the flatness of the steel sheet shape can be secured when descaling is performed immediately before forced cooling. From this result, the steel plate shape is improved by the uniform cooling effect by descaling immediately before the forced cooling. Further, as shown in FIG. 6, when the descaling was performed immediately before the forced cooling, the tensile strength TS value could satisfy the specifications.

Next, FIG. 7 shows the effect of the descaling nozzle discharge pressure during descaling immediately before forced cooling. As can be seen from FIG. 7, when the area ratio of the scale remaining to the discharge pressure was taken, the effect was large at the discharge pressure of 100 to 150 kgf / cm ² . Furthermore, considering the versatility, 150
Descaling is preferably performed at a discharge pressure of kgf / cm ² or more.

FIG. 8 shows the effect of the distance between the steel plate surface and the descaling nozzle during descaling before forced cooling. As can be seen from Fig. 8, the distance is 300 mm.
At the boundary of ~ 350mm, the remaining scale interview rate is extremely worsening. Therefore, it is desirable that the distance between the steel plate surface and the descaling nozzle is 350 mm or less.

(Specific Example) Next, as an example of the present invention,
FIG. 9 shows an example of changes in the scale thickness in the first pass and the final sixth pass in finish rolling, and when descaling is performed immediately before the start of forced cooling. The temperature history is shown in Fig. 3.
And as shown in FIG.

On the other hand, as a conventional example, when descaling is performed only in the first pass in finish rolling,
10 to 12 show changes in the scale thickness when the descaling is performed in the first pass and the third pass and when the descaling is performed in the first pass and the sixth pass, respectively. In addition, as a comparative example, FIG. 13 shows changes in scale thickness when the first and sixth passes of finish rolling and when descaling is performed before hot straightening.

Looking at the above results, FIG. 10 and FIG.
In the conventional example shown in (1), the scale thickness at the start of hot straightening and forced cooling grows to 35 to 40 μm, which causes scale peeling during hot straightening and uneven cooling during forced cooling.

In the conventional example shown in FIG. 12, 2 is applied during hot straightening.
Although the scale is about 8 μm and peeling of the scale is small, the scale thickness is about 31 μm at the start of forced cooling.
This will cause non-uniform cooling during forced cooling.

On the other hand, in the comparative example, the scale thickness at the start of hot straightening and the start of forced cooling is greatly improved, but the scale thickness at the start of forced cooling is about 13 μm, which is also nonuniform. Cooling occurs.

In contrast to these examples, in the example of the present invention, FIG.
As shown in (4), the scale thickness at the start of forced cooling is as thin as about 5 μm, which is a tight property. Therefore, it was possible to secure the stable mechanical properties and the steel plate shape by uniform cooling.

[0053]

As is apparent from the above description, according to the present invention, it is possible to prevent peeling of the scale at the time of hot straightening by the hot leveler due to the secondary scale and prevention of surface quality defect of the product due to the pushing. it can.

Furthermore, while avoiding unnecessary excessive temperature drop and achieving uniform cooling during forced cooling, it is possible to improve the properties of the product scale and obtain a thick steel plate having excellent steel plate shape and mechanical properties. You can

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram of a steel sheet with secondary scale removal according to the present invention.

FIG. 2 is a schematic diagram of a descaling device and a forced cooling device according to the present invention.

FIG. 3 is a graph showing a temperature history of a steel sheet before forced cooling according to the present invention.

FIG. 4 is a graph showing the influence of descaling between passes during finish rolling.

FIG. 5 is a graph showing a change in scale thickness according to a descaling execution timing.

FIG. 6 is a diagram showing a difference in TS value depending on the presence or absence of descaling immediately before forced cooling.

FIG. 7 is a graph showing the relationship between the discharge pressure of the descaling nozzle and the scale remaining area ratio.

FIG. 8 is a graph showing the relationship between the steel plate surface-nozzle distance and the scale remaining area ratio.

FIG. 9 is a graph showing changes in scale thickness according to the present invention.

FIG. 10 is a graph showing changes in scale thickness in a conventional example.

FIG. 11 is a graph showing changes in scale thickness in another conventional example.

FIG. 12 is a graph showing changes in scale thickness in still another conventional example.

FIG. 13 is a graph showing changes in scale thickness in a comparative example.

FIG. 14 is a graph showing the temperature history of the steel sheet after forced cooling according to the present invention.

[Explanation of symbols]

1 ... table roll, 2 ... forced cooling apparatus, 3 ... descaling nozzle, 4 ... cooling header, 5 ... pipe, 6 ... pump, 7 ... cooling nozzle, M ... steel, W ₁ ... descaling for high-pressure water, W ₂ …Cooling water.

Claims (1)

[Claims] 1. When hot rolling a thick steel plate, descaling is performed at least immediately before or immediately after the final pass in finish rolling, and a scale generated between the end of finish rolling and hot straightening is It is characterized in that it is uniformly generated with a thickness that does not cause peeling, followed by hot straightening, descaling of the secondary scale that grows after that, and forced cooling is started within 5 seconds after this descaling. Descaling method for thick steel plate.