JPH07164001A - Method and device for rolling steel sheet - Google Patents

Abstract

PURPOSE:To obtain a steel sheet which is prevented from surface flaw without imposing the restrictions on the component of a material to be rolled and inducing the increase in a special process load by making an average draft in specified sections from both ends in the width direction of a slab larger than the average draft in the section on the middle side of those sections. CONSTITUTION:In the case that the relationship between the draft in edge parts and the draft in the middle part of width must be changed in accordance with a rolling-down schedule, it is effective to enable arbitrary and independent setting of the draft in the edge parts and the draft in the middle part. For example, a rolling roll is taken as a divided rolling roll which is divided at the position in the axial direction of roll equivalent to the position of one-sixth slab width from both ends of the slab to be rolled and, with rolling-down screws and/or hydraulic cylinders which are provided at both ends of each divided roll, screw-down position is made so as to be adjustable so that the roll gap between upper and lower rolls in the edge part is smaller than that in the middle part. Then, the surface flaws are remarkably reduced and the yield of product is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet having few surface flaws generated during hot rolling, that is, for surface flaws generated during hot rolling of a steel sheet by changing the widthwise distribution of the rolling reduction. By doing so, the present invention relates to a rolling method and a rolling apparatus for reducing the rolling amount.

[0002]

2. Description of the Related Art Generally, surface defects are apt to occur at the edges of hot-rolled steel sheets, and surface defects called "scarring defects" caused by microcracks during hot-rolling are particularly likely to occur after cracking. If the produced scale bites into the inside by rolling and is subjected to the cold rolling step without being removed in the pickling step, it becomes a linear defect that is long in the rolling direction, and the yield reduction is particularly large.

Therefore, various techniques have been devised from the past in order to reduce surface defects during hot rolling. As a method of defining the component system of the material to be rolled, for example, Japanese Patent Laid-Open No. 57-16153 relating to austenitic stainless steel.
Japanese Patent Publication No. 3-294 related to steel containing Mn and S
Although the publication No. 001 and the like are disclosed, these limit the degree of freedom for the component system and are not general.

Further, Japanese Patent Publication No. 55-50723 and Japanese Patent Application Laid-Open No. 2-15806 disclose a technique for removing surface defects (pinholes) on a slab by maintenance so as to eliminate the occurrence of bald spots. However, in this technique, a step of care removal is indispensable, and if it is not sufficient, a slab surface defect that becomes a starting point of cracking will remain, and minute cracks will occur after hot rolling.

As a technique for reducing the occurrence of flaws in the shape of a slab, Japanese Patent Laid-Open No. 58-138502 and Japanese Patent Laid-Open No. 3-138502.
No. 207551 is cited. Both of them disclose a technique of reducing the edge seam flaw of stainless steel by recessing the central part of the short side of the slab and suppressing the width spread (metal flow in the width direction) in the vicinity (nearest) of the short piece. However, the effect of changing the shape of the cross section of the slab short piece C (cross section orthogonal to the casting direction) appears only near the edge, and when micro-cracks on the surface during hot rolling occur in a relatively wide range. Cannot handle.

[0006]

DISCLOSURE OF THE INVENTION The present invention prevents the occurrence of surface flaws in preventing microcracks that occur during hot rolling without restricting the composition of the material to be rolled or increasing the process load. It is an object of the present invention to provide a rolling method and an apparatus therefor capable of obtaining a rolled steel sheet.

[0007]

In order to achieve the above object, the present invention defines a rolling direction stress generated inside a slab during rolling by defining a distribution in the width direction of a rolling reduction during hot rolling of the slab. It is characterized in that fine cracks are prevented by adjusting the state and suppressing the generation of tension near the edges, which is the cause of cracking. As shown in FIG. 1, when the slab is hot-rolled, the gist thereof is 1/6 of the slab width from the edges of the slab, that is, both ends in the width direction.
The average rolling reduction r _e of the section is made larger than the average rolling reduction r _c of the width center portion, that is, the section closer to the width center than the edge portion, and further, the average rolling reduction r _e of the edge portion of the slab. Is 1.05 compared with the average rolling reduction r _c in the width center part.
The rolling is performed so that it becomes 1.4 times, or alternatively, the average rolling reduction r _e at the edge portion of the slab is rolled so that the relationship with the average rolling reduction r _c at the width center portion becomes the following formula (1). That is. _{(R e -r c) / (} 1-r e) = β × r c k (1) where, beta is a positive constant, k is a constant of 1.5 to 2.0.

Further, as shown in FIG. 2 (a), with respect to each of the edge portion and the width center portion of the slab, of the sections in the axial direction of the rolling roll in which both sections are reduced in thickness, sections in which the edge portion is reduced in thickness Rolling apparatus having a roll larger than the roll radius in the section where the roll radius in the width central part is reduced,
Alternatively, as shown in FIG. 3B, rolling rolls divided from both ends in the width direction of the slab at roll axial direction positions corresponding to 1/6 position of the slab width and installed on both ends of each of the divided rolling rolls. Among the divided rolling rolls, a divided rolling roll having a screw position and / or a hydraulic cylinder for adjusting a rolling position and having a reduced thickness at the edge portion of the slab has a reduction ratio lower than that of a divided roll having a reduced thickness at the width center portion. It is a rolling mill located on the side to be enlarged.

Here, the rolling reduction is the amount of reduction in thickness at any position in the width direction divided by the thickness on the rolling mill entrance side at that position, and the average rolling reduction is the rolling reduction within a predetermined section. It is an average value. The average reduction rate may be approximated by dividing the average thickness reduction amount within a predetermined section by the average thickness on the rolling mill entrance side.

[0010]

The present invention will be described in detail below. The present inventors have completed the present invention by thoroughly examining the relationship between the morphology / frequency of flaws generated during hot rolling and the widthwise distribution of the reduction ratio.

First, the inventors of the present invention have scrutinized the occurrence of hot rolling defects in slabs. As a result, as shown in FIG. 3, it was found that all the flaws that were generated were cracks in the C direction (cracks in the direction perpendicular to the rolling direction), and the frequency of occurrence was higher as the edge was closer to the slab edge. In addition, the size of the cracks is 0.1 to 2 mm in the C direction and 0.1 mm in depth, and the cracks are concentrated and appear after the first pass of rough hot rolling, and the subsequent rolling process such as hot rolling or cold rolling. It was confirmed that the resulting product was stretched in the L direction (rolling direction), resulting in a bald flaw or the like that fatally deteriorates the surface quality of the final product plate.

The present inventors have paid attention to the above experimental facts, particularly that all microcracks are cracked in the C direction, and the tension in the rolling direction generated inside the slab during rolling is the main cause of cracking, that is, surface flaws of the product. Presumed to be. The generation of the tension in the rolling direction can be roughly explained by the difference in the metal flow in the rolling direction between the material near the center of the slab width and the material near the edge. That is, the width easily expands near the slab edge during rolling, the amount of metal flow in the rolling direction becomes smaller than that near the width center, and the material near the edge is relatively stretched by the material near the width center. , Tension in the rolling direction is generated near the edge.

However, with such a qualitative interpretation, detailed examination of the flaw generation mechanism and proposal of the flaw occurrence prevention measure as in the present invention cannot be made. Therefore, the present inventors analyzed the stress in the rolling direction by the three-dimensional rigid-plastic finite element method and obtained the following findings.

The stress in the rolling direction (positive tension) generated inside the slab during rolling is distributed in a substantially parabolic shape in the width direction as shown in FIG. 4, and changes from compression to tension from the width center to the edge. To do. The area where tension is generated is about 1 from the edge.
It is a / 6 width portion, and becomes maximum near the edge. Also,
If the so-called slab size (plate thickness / plate width ≧ about 0.1), this tendency does not change. Of these findings, the width of the tension generation region is understood as follows. In single-stand rolling of a slab, the total stress in the rolling direction in the width direction is zero, that is, the average value of the stress in the rolling direction in the width direction is zero.
Here, if the distribution of stress in the rolling direction is a parabola (quadratic curve), the position in the width direction that gives the average value is 1 / (2
× 3 ^1/2 ) width, that is, (3-3 ^1/2 ) / 6 = from the edge
1.268 / 6 wide point, about 1 from the above edge
The result that tension is generated in the / 6 width portion is understood.

This stress distribution in the rolling direction corresponds well to the situation of flaw generation in FIG. 3, and indicates that flaws (cracks) are concentrated and occur at the portions where high tension is generated in the rolling direction. This means that if the tension in the rolling direction inside the slab that occurs near the edge during rolling can be reduced, the occurrence of flaws can be essentially suppressed.

As described above, the distribution of the stress in the rolling direction in the width direction can be understood macroscopically from the difference in the drawing in the rolling direction between the vicinity of the width center and the edge along with the widthwise metal flow (hereinafter referred to as the drawing difference). In other words, it is considered that the tension generated in the vicinity of the edge can be reduced by providing a widthwise distribution of the reduction ratio that increases the reduction ratio near the edge. However, it is not clear what kind of rolling reduction distribution can effectively suppress the generation of tension.

Therefore, the present inventors examined this rolling reduction distribution by the above-mentioned three-dimensional finite element method. As a result, as shown by the broken line in FIG. 5, when the range for increasing the rolling reduction is unnecessarily widened, the tension in the vicinity of the edge decreases, but the reaction, that is, the sum of the rolling direction stresses in the width direction as described above. As much as possible, the tension decrease near the edge and the tension increase near the width center occur as much as possible, and the tension increase near the width center becomes remarkable and cracks may occur in the width center. The range of increasing
As shown by the alternate long and short dash line in the figure, it was first found that the section from the edge to 1/6 of the slab width, that is, the edge portion defined in the present invention is suitable.

Further, as shown in FIG. 6, the effect of reducing the tension depends to a large extent on the pattern of increasing the draft of the edge portion (in FIG. 6, the one-dot chain line is straight line, the broken line is saturated type, and the dotted line is acceleration type). However, the average reduction rate of the edge portion (in FIG. 6, the average reduction rate r _e of the edge portion is 0.07 in any of the line types in FIG. 6) is substantially determined.
Further, the ratio α (= r _e of the average rolling reduction r _e of the edge portion and the average rolling reduction r _c of the section closer to the width center, that is, the width rolling center portion.
/ R _c , hereinafter abbreviated as a ratio of reduction ratio) and the distribution of tension generated inside the slab, as can be seen from FIG. 7, the average reduction ratio of the edge portion is about 1 compared to the average reduction ratio of the width center portion. ．
It was found that the tension at the edge portion can be reduced without causing a remarkable tension increase in the width center portion within the range of 05 to 1.4 times. Regarding the setting range of the average rolling reduction ratio of the edge part, if the average rolling reduction ratio of the width center part is less than 1.05 times, the effect of preventing micro-cracking due to the tension reduction of the edge part is not sufficient, and if it exceeds 1.4 times the width. The tension in the central portion increases, and the occurrence of fine cracks in the central portion of the width becomes a problem, so the above range is preferable.

As described above, the rolling reduction ratio α exceeds 1.0,
By preferably setting the range of 1.05 to 1.4,
The generation of minute cracks can be suppressed. However, if the normal often separately formed product of different thickness from a slab of the same thickness, reduction schedule, which particularly changes rolling reduction (mean reduction ratio width central portion average reduction rate r _c of the entire width) larger In order to respond, it is necessary to appropriately select the reduction ratio α according to the change. 8, 9, and 1
0, showed rolling direction tension distribution in the case of changing the rolling reduction ratio α for various width central portion average reduction rate r _c in FIG. 11, does not cause tension in substantially the entire slab width, i.e. optimum It is understood that it is necessary to select the reduction ratio α according to the level of the average reduction ratio r _{c in} the width center portion in order to obtain a wide tension distribution. As described above, the distribution of stress in the rolling direction in the width direction is essentially the stretching in the rolling direction of the edge portion along with the width direction metal flow (hereinafter abbreviated as stretching) λ _e (= 1 / (1-r _e )).
And the difference between the stretching λ _c (= 1 / (1-r _c )) in the width center portion (hereinafter, abbreviated as stretching difference). Therefore,
The stretching difference ratio γ shown in the equation (2) is defined as an index representing the degree of the stretching difference, and from FIGS. 8 to 11, the stretching difference ratio γ and the width center required to prevent the tension from being generated over almost the entire slab width. The relationship with the partial average rolling reduction r _c was obtained, and FIG. 12 was obtained. _{γ = (λ e -λ c)} / λ c = {(1-r e) / (1-r e)} - 1 = (r e -r c) / (1-r e) (2)

From the figure, the following difference (3) is established between the drawing difference ratio γ and the average width reduction ratio _{c in} the width center portion, and the average reduction ratio r _c in the width center portion is calculated based on the formula (3). By setting the average edge portion reduction ratio r _e according to the rolling and rolling, regardless of the rolling schedule, tensile force is not generated over almost the entire area of the slab width, that is, the generation of microcracks that are the starting point of product defects is suppressed to the minimum. It was found that γ = (r _e −r _c ) / (1−r _e ) = β × r _c ^k (3) β in the above formula is a rolling condition other than the reduction ratio, for example, a rolling roll diameter, a slab thickness before rolling, It is a positive constant determined by the slab width, steel type, etc. The exponentiation factor k is a constant of 1.5 to 2.0 (in FIG. 12, the solid line is k = 2.0 and the broken line is k = 1.5.
Of the rolling reduction ratio r _c , a constant value may be used within the changing range of the rolling reduction ratio r _c , or the rolling reduction ratio changing range is divided into sections.
The numerical value may be changed for each section.

As a rolling apparatus for carrying out the present invention, that is, as a rolling apparatus in which the rolling reduction distribution during hot rolling of a slab is set to be large at the edge portion and small at the width center portion, the following can be considered.

First, as shown in FIG. 2 (a), the rolling apparatus uses a roll having a roll radius larger than the roll radius in the section where the slab edge portion is reduced in thickness and the width center portion is reduced in thickness. The roll radius is set, for example, as the radius that is normally used for the section where the width center part is reduced,
For the section in which the thickness of the edge portion is reduced, a value obtained by subtracting the average rolling reduction at the width center portion from the desired rolling reduction distribution (distribution) multiplied by the slab thickness / 2 may be used as the roll radius distribution. Further, when the relationship between the edge reduction ratio and the width center reduction ratio must be changed according to the reduction schedule, the edge reduction ratio and the width center reduction ratio can be set arbitrarily and independently. . For example, as shown in FIG. 2B, rolling rolls are divided from both ends of the slab to be rolled at a roll axial position corresponding to 1/6 position of the slab width, and the rolling rolls are installed at both ends of each divided roll. The rolling device is capable of adjusting the rolling position by using the rolling screw and / or the hydraulic cylinder so that the gap between the upper and lower rolls of the edge portion is smaller than that of the width center portion. It goes without saying that the reduction position adjustment amount of the divided rolls is determined according to the desired reduction ratio distribution and the slab thickness.

That is, according to the rolling method and the rolling apparatus of the present invention, it is possible to effectively reduce the stress (tension) in the rolling direction which is directly related to the occurrence of surface cracks in the slab during rolling, which is the starting point of product defects. Recognize. In the present invention, the shape of the rolling reduction distribution from the central portion of the slab width to the slab most edge is not particularly specified, but when the rolling reduction distribution is discontinuous, additional shear deformation is concentrated near the discontinuous point. However, since there is a concern that defects due to this may occur, it is desirable to have a smooth rolling reduction distribution.

As another means for setting the rolling reduction distribution during hot rolling of the slab to be large at the edge portion and small at the width center portion, before the horizontal rolling (thickness rolling) pass using a normal roll, There is a method of obtaining a thickness distribution required to realize a predetermined rolling reduction distribution and forming this slab thickness distribution by edging rolling using a vertical roll. Hereinafter, detailed description will be given with reference to examples.

[0025]

EXAMPLE Stainless steel having the components shown in Table 1 was melted in a converter to cast a slab having a thickness of 165 mm and a width of 1250 mm. At the time of rolling, in order to give two kinds of reduction ratio distributions (uniform distribution and distribution according to the present invention) shown in FIG. 13, a flat roll having a radius of 600 mm and a roll radius between the center of the roll cylinder width and ± 410 mm are used. 600 mm,
Two types of rolls were used, a large edge diameter roll having a parabolic radius increase on the body end side and a radius of 602.5 mm at a position of ± 625 mm from the center of the body width.

FIG. 14 shows the widthwise distribution of the occurrence frequency of slab surface cracks after rolling at a rolling temperature of 1100 ° C. As is clear from the figure, when a large-edged roll to which the method of the present invention is applied is used, the frequency of cracks in the vicinity of the edge is drastically reduced, and the yield is significantly improved.

[0027]

[Table 1]

[0028]

As described in detail above, according to the present invention, when a steel sheet is manufactured by hot rolling, the width direction distribution of an appropriate reduction ratio is set and rolling is performed to prevent surface defects of the product. It is a great place to benefit industrially because it can be significantly reduced and the product yield can be improved.

[Brief description of drawings]

FIG. 1 is a graph showing an example of a reduction ratio distribution according to claims 1 and 2 of the present invention.

2A and 2B are schematic views showing one embodiment of a rolling apparatus (claims 3 and 4) for realizing the rolling reduction distribution according to claims 1 and 2;

FIG. 3 is a schematic diagram showing a distribution state of cracks on the surface of a slab after rolling by a conventional method.

FIG. 4 is a diagram showing an example of an analysis result by a three-dimensional rigid-plastic finite element method regarding a widthwise distribution of rolling direction stress generated inside a slab during rolling by a conventional method.

FIG. 5 is a diagram showing an example of results of an analysis performed to obtain knowledge about the width of an edge vicinity section for increasing the rolling reduction, a pattern of increase in rolling reduction near the edge, and the influence of the average rolling reduction when the present invention is carried out. It is a thing.

FIG. 6 is a diagram showing an example of the results of an analysis performed to obtain knowledge about the width of an edge-neighboring section that increases the rolling reduction, the pattern of increasing the rolling reduction near the edge, and the effect of the average rolling reduction when the present invention is implemented. It is a thing.

FIG. 7 is a diagram showing an example of results of an analysis performed to obtain knowledge about the width of an edge vicinity section for increasing the rolling reduction, the pattern of rolling reduction increase near the edge, and the influence of the average rolling reduction when the present invention is carried out. It is a thing.

FIG. 8 is a diagram showing an example of results of an analysis performed to obtain knowledge about an area of an edge vicinity section for increasing a rolling reduction, a pattern of rolling reduction increase near the edge, and an influence of an average rolling reduction when the present invention is carried out. It is a thing.

9 shows an example of analysis results similar to FIG.

FIG. 10 shows an example of analysis results similar to FIG.

FIG. 11 shows an example of analysis results similar to FIG.

FIG. 12 is a diagram showing an example of results of an analysis performed to obtain knowledge about the width of an edge vicinity section for increasing the rolling reduction, the pattern of increase in rolling reduction near the edge, and the influence of the average rolling reduction when the present invention is carried out. It is a thing.

FIG. 13 is a diagram showing a rolling reduction ratio distribution set in an example.

FIG. 14 is a comparison between the conventional method and the method of the present invention regarding the occurrence frequency of slab surface cracks after rolling in the examples.

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Claims (5)

[Claims] 1. When manufacturing a steel sheet by reducing the thickness of a slab by hot rolling, the average rolling reduction ratio r in the width direction of the slab from both ends to 1/6 of the slab width (hereinafter referred to as edge portion). _A method of rolling a steel sheet, wherein _e is set to be larger than an average reduction ratio r _{c in} a section closer to the width center (hereinafter, referred to as width center portion). 2. A slab is thinned by hot rolling to produce a steel sheet, and an average rolling reduction ratio r _e of an edge portion of the slab is manufactured.
Is 1.05 to 1.4 as compared with the average rolling reduction r _c in the width center part.
A method for rolling a steel sheet, comprising rolling in a doubled manner. 3. A slab is thinned by hot rolling to produce a steel sheet, and an average rolling reduction ratio r _e of an edge portion of the slab is produced.
Is rolled such that the relationship with the average rolling reduction r _c in the width center part is expressed by the following formula. _{(R e -r c) / (} 1-r e) = β × r c k Here, beta is a positive constant, k is a constant of 1.5 to 2.0. 4. A roll radius in a section in which the edge portion is thinned is reduced in the width center portion in each of the rolling roll axial direction sections in which both sections are thinned for each of the edge portion and the width center portion of the slab. A rolling device for steel sheet, which has a roll larger than the roll radius in the section. 5. A rolling roll divided from both ends in the width direction of the slab at a roll axial position corresponding to 1/6 position of the slab width, and screws and / or hydraulic pressures installed at both ends of each divided rolling roll. It has a rolling position adjusting mechanism by a cylinder, and among the divided rolling rolls, the divided rolls that reduce the thickness of the edge portion of the slab are located on the side where the reduction ratio is larger than that of the divided rolling rolls that reduce the width center portion. A steel plate rolling apparatus characterized by being.