JPH06292907A - Manufacture of stainless steel sheet having minimized surface defect - Google Patents

Abstract

PURPOSE:To provide a method for reducing surface defect in a manufacturing method of a stainless steel sheet. CONSTITUTION:At the time of manufacturing the stainless steel sheet by hot rolling, the surface defect of the stainless steel sheet is reduced by executing the initial rolling with horizontal rolls at >=1000 deg.C: and taking the draft of the rolling as at least deg.15%. And, by applying at least one pass or more of the rolling with vertical rolls and taking the total draft with the vertical rolls as at least >=5% before executing the initial rolling with the horizontal rolls in the temp. range of of >=1000 deg.C, the surface defect of the stainless steel sheet is reduced. Where, the total draft (r) with the vertical rolls is computed according to r=1-w'/w from the slab width (w) before rolling and the slab width (w') after rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention reduces a surface flaw that occurs during hot rolling of a stainless steel sheet having a small number of surface defects, and particularly hot rolling of an austenitic stainless steel sheet, by defining the hot rolling conditions. It is about the method.

[0002]

2. Description of the Related Art Generally, surface defects are likely to occur on the edges of hot-rolled stainless steel sheets, and especially scales are generated inside the surface defects caused by cracks and surface irregularities on the steel sheet, and pickling in the subsequent process. At times, it is not sufficiently removed and becomes a serious surface defect after cold rolling, resulting in a decrease in product yield. In particular,
Surface defects called bald spots caused by microcracks during hot rolling are used in the cold rolling process without being removed in the pickling process because the scale generated after cracking penetrates into the inside by rolling. Then, it becomes a linear defect that is long in the rolling direction, and the yield reduction rate is particularly large. Further, a steel type in which the yield reduction due to the surface defects caused by the microcracks is a particular problem is an austenitic stainless steel in which the beauty of the product surface is likely to become a problem and microcracks are likely to occur in hot rolling.

Therefore, various techniques for reducing surface defects during hot rolling have hitherto been devised mainly for stainless steel. For example, JP-A-57-1615
No. 3 discloses a technique in which the components of an austenitic stainless steel are specified to secure hot workability and reduce the earing and bald spots of the steel. Japanese Unexamined Patent Publication No. 2-15806 discloses a technique of removing surface defects (pinholes) of a stainless steel slab by caring for it to eliminate the occurrence of bald spots. However, this technique cannot prevent minute cracks generated during hot rolling. Further, as a technique for reducing the occurrence of hot rolling flaws in a slab shape, Japanese Patent Laid-Open No. 58-138.
502 and JP-A-3-207551.
Both of them disclose a technique in which the central portion of the short side of the slab is recessed to reduce edge seam flaws in ferritic stainless steel. However, this technique cannot prevent the above-mentioned microcracking during hot rolling.

[0004]

DISCLOSURE OF THE INVENTION The present invention aims to improve surface defects generated during hot rolling of stainless steel.
It is an object of the present invention to provide a method for manufacturing a stainless steel sheet with improved surface defects without increasing the process load.

[0005]

Means for Solving the Problems The present invention specifies rolling conditions for hot rolling in order to solve the above problems, and the essence thereof is, firstly, in producing a stainless steel sheet by hot rolling, 100 rolls of the first pass with horizontal rolls
It is applied in a temperature range of 0 ° C. or higher, and the reduction rate of the first pass is at least 15% or higher. Secondly, before carrying out the first pass rolling with the horizontal rolls in the temperature range of 1000 ° C. or more, at least one pass rolling with the vertical rolls and the total rolling reduction with the vertical rolls should be at least 5% or more. It is to be. Here, the total rolling reduction r by the vertical roll rolling is obtained as r = 1-w '/ w from the slab width w before the rolling and the width w'after the rolling.

[0006]

The present invention will be described in detail below. The present inventors have completed the present invention by scrutinizing the relationship between defects generated during hot rolling and various conditions related to hot rolling.

First, the inventors of the present invention have conducted a close examination of the relationship between the occurrence of hot rolling flaws and the slab shape, and found that the recessed slab of FIG. 2 produces more hot defects than the rectangular slab of FIG. It was The rectangular slab is a slab obtained when bulging does not occur during casting, as shown in the cross section of FIG. 1, and h and w in the figure mean the slab thickness and the slab width, respectively. The recessed slab is a slab in which recesses are formed on the long side surface of the slab by bulging or the like during casting, as shown in the cross section of FIG. The position where the recess is generated is usually formed within the range of 10 mm to 300 mm from the slab edge. Δh in the figure is the maximum recess depth (depth from the horizontal plane of the slab center), and Δw is the maximum recess depth from the slab edge. The distance to the position and h _c indicate the average thickness of the central portion of the slab. In a normal continuous casting machine, it is generally difficult to completely eliminate the above-mentioned recess and form a rectangular slab cross section. In particular, recently, there has been a tendency to increase the casting speed during continuous casting in order to improve productivity, and as a result, bulging is likely to occur, so that it is extremely difficult to obtain a perfect rectangular slab.

Further, it has been found that the position of occurrence of the hot-rolling defect is higher in the rectangular slab as it is closer to the slab edge, and in the concave-portion slab the hot-rolling defect is generated in the concave portion at a frequency higher than that of the rectangular slab. Further, it is recognized that the frequency of hot-rolling defects in the recessed slab depends on the maximum recess depth Δh. The larger Δh, the higher the frequency of defects.
That is, when the recessed slab is used, thermal defects are likely to occur inside the edge, and the yield of the final product is significantly reduced.

Further, the present inventors investigated the hot rolling defects after the completion of the 1-pass rough hot rolling (horizontal roll rolling) in order to clarify the pattern of the defects. As a result, it was found that all the flaws generated in the slab edge and the slab recess were C-direction cracks (cracks in the direction perpendicular to the rolling direction). The size is a minute crack of 0.1 to 0.2 mm in the C direction and a depth of about 0.1 mm, and it is stretched in the L direction (rolling direction) by the subsequent rolling process such as hot rolling or cold rolling. It was confirmed that the product plate will cause bald defects and the like that will seriously deteriorate the surface quality.

In view of the above facts, the inventors of the present invention have developed and devised a hot rolling condition for reducing the hot rolling defect when a slab for forming recesses is used. That is, firstly, when a stainless steel sheet is manufactured by hot rolling, the rolling of the first pass by a horizontal roll is applied in a temperature range of 1000 ° C. or higher so that the rolling reduction of the first pass is at least 15% or higher. is there. Secondly, before carrying out the first pass rolling with the horizontal rolls in the temperature range of 1000 ° C. or more, at least one pass rolling with the vertical rolls and the total rolling reduction with the vertical rolls should be at least 5% or more. It is to be. Here, r means the total reduction ratio by the vertical roll rolling, and is obtained as r = 1−w ′ / w from the slab width w before the rolling and the width w ′ after the rolling.

The reasons for limitation in the claims will be described below. First, the invention described in item 1 will be described. Roll the first pass with a horizontal roll at 1000 ℃ or higher and
The reason for setting the rolling reduction to be not less than% is to reduce the occurrence of hot rolling defects to the same extent as when using a completely rectangular slab.
In particular, the flaw generation position in the coil width direction can be set to the same edge portion as when using the perfect rectangular slab. Where 1
The reason for limiting the rolling temperature of the pass to 1000 ° C or higher is
This is because if a stainless steel slab having a normal solidification structure is rolled at a temperature lower than that temperature by one-pass rolling with a rolling reduction of 15% or more, microcracks are likely to occur. Although the upper limit of the rolling temperature in the first pass is not particularly specified, the limit is about 1350 ° C. in the ordinary hot rolling of stainless steel. However, in the case of rolling with a rolling mill or a pinch roll incorporated in a casting machine, the rolling temperature can be up to about 1500 ° C.

Further, the reason why the reduction ratio of the first rolling in one pass is limited to 15% or more is that a flaw is generated in the recess area when the recess-forming slab is used at a reduction ratio of less than that. As mentioned above, the recessed area is about 300 from the slab edge.
It extends to the inside of mm, and if a flaw occurs in that area, the product yield will be greatly reduced. The upper limit of the first pass rolling reduction is not particularly specified, but if the rolling reduction is too large, scale flaws, roll seizure flaws, and the like will occur. The rolling reduction Re when rolling the recessed slab according to the present invention with a horizontal roll is
Re = 1-h '/ h depending on the average slab thickness h _c (see FIG. 2) at the center of the slab before rolling and the plate thickness h'after rolling.
_{It is calculated} as _c , and when displaying in%, Re may be multiplied by 100.

Next, the invention described in item 2 will be described.
The reason why the total rolling reduction is 5% or more in one pass or more before the horizontal roll rolling is performed in the temperature range of 1000 ° C or more is that the vertical roll is rolled before the horizontal roll hot rolling using the recessed slab. This is because the recesses are eliminated by rolling, and the occurrence of hot-rolled defects can be reduced to the same level as or more than that when using a completely rectangular slab. 1000 ° C here
The reason for limiting the above is that when the slab is rolled in a temperature range lower than this range, a flaw easily occurs. The upper limit temperature is not particularly specified, but it is up to about 1500 ° C. even when using a rolling mill or the like incorporated in a casting machine.

The reason why the total reduction ratio by the vertical rolls is limited to 5% or more is that the reduction ratio below that does not show the effect of eliminating the slab recesses by the vertical roll rolling, and microcracks and the like occur during the subsequent horizontal roll rolling. Because it does. The upper limit of the total rolling reduction is not particularly specified, but if it is extremely increased, it causes new cracks and flaws, and is up to about 40% at most. Moreover, the number of vertical roll rolling passes does not affect the effect of the present invention with any number of passes, but since increasing the number of passes causes obstacles such as a temperature decrease of the slab, the number of vertical roll passes in normal hot rolling is small. Is up to about 5 passes, and up to about 10 passes even in a vertical roll rolling machine incorporated in a casting machine.

The vertical roll rolling in the present invention means rolling the short side surface of the slab cross section with a roll,
Horizontal roll rolling means rolling the long side surface of the slab cross section. In addition, after performing rolling with a total rolling reduction of 5% or more by a vertical roll rolling machine or the like incorporated in the casting machine, the slab is once cooled to room temperature and heated again to 1000 ° C. or more to perform horizontal roll rolling. Also, it should be added that the effects of the present invention are not inferior to the effects of the present invention. The vertical roll rolling reduction ratio r in the present invention is calculated as r = 1-w ′ / w from the slab width w before vertical roll rolling and the slab width w ′ after rolling, and when r is expressed as 100, r is multiplied by 100. Good.

By the way, the reason why the hot-rolling defects can be improved or eliminated by the techniques described in the first and second aspects of the present invention is not always clear at present, but it is considered as follows. As described above, all the micro-cracks generated during the conventional rough hot rolling are cracked in the C direction, which indicates that tension in the rolling direction acts during rolling. It is considered that this rolling direction tension is due to the difference in metal flow between the slab center part and the edge part in the rolling direction. That is, the width of the edge portion is widened during rolling, and the amount of metal flow in the rolling direction is smaller than that in the central portion. As a result, the metal at the edge portion is dragged by the flow of the metal at the central portion, and tension in the rolling direction is generated at the edge portion. The above is considered to be the reason why even the conventional rectangular slab has microcracks in the edge portion. In the recessed slab shown in FIG. 2, it is considered that in the recessed area where the slab thickness is thin and the metal flow in the rolling direction is small, the tension in the rolling direction becomes high during the initial pass of the horizontal roll rolling, and defects are concentrated.

That is, the reason why the hot-rolling flaw is reduced even when the slab in which the recess is formed is hot-rolled by the technique described in the first aspect of the present invention is that the metal flow amount of the slab-edge recess is set by reducing the rolling reduction to 15% or more. By relatively reducing the difference from the metal flow amount in the central part, the tension in the rolling direction is made almost equal to that of a perfect rectangular slab,
It is considered that the occurrence of hot-rolled defects was reduced to the level of a complete rectangular slab. Further, the reason why the hot rolling flaw is reduced even if the slab in which the recess is formed is hot-rolled by the technique described in paragraph 2 is that the total rolling reduction of the vertical roll rolling before the initial pass of the horizontal roll rolling in which rolling direction tension is generated is 5% or more. By doing so, it is considered that the concave part of the slab edge disappears and the metal flow becomes equal to that of a perfect rectangular slab. Further, it is considered that the vertical roll rolling made the slab edge concave portion a convex portion, the amount of metal flow at the edge portion increased, the tension in the rolling direction disappeared, and the hot rolling defect was reduced more than when the perfect rectangular slab was used.

In order to verify the above hypothesis, the stress distribution generated in the rolling direction in the case where the slab edge has a recess was calculated using the three-dimensional rigid-plastic finite element method. The slab shape used is shown in Table 1, and the calculation results are shown in FIGS. 3 and 4. Table 1
Where w is the slab width, h _c is the thickness of the central part of the slab, Δ
h is the maximum depth of the recess of the recess generating slab (see FIG. 2), Δ
w indicates the distance from the slab edge to the position where the maximum depth of the recess occurs (see FIG. 2). FIG. 3 shows the stress distribution in the rolling direction when the rolling reduction of 1 pass of crude hot rolling (horizontal roll rolling) is set to 5% of the conventional method. In the figure, the horizontal axis represents the distance from the center of the slab width, and the vertical axis represents the stress value in the rolling direction.

[0019]

[Table 1]

From the figure, it can be seen that a higher tension is generated at the slab edge in the recessed slab (solid line and broken line with circle in the figure) than in the complete rectangular slab (solid line in the figure). Moreover, the high-tension generation area is on the inside of the recess-forming slab, which is in good agreement with the state of hot-roll defect generation. On the other hand, FIG. 4 shows the stress distribution in the rolling direction when the reduction ratio is 20% which is the technique described in the first aspect of the present invention. In the figure, the horizontal axis represents the distance from the center of the slab width, and the vertical axis represents the stress value in the rolling direction. From the figure, it can be seen that there is no great difference in the stress generation situation between the perfect rectangular slab (solid line in the figure) and the recessed slab (broken line in the figure).

[0021]

EXAMPLE Stainless steels having the components shown in Table 2 were melted according to a common melting method, and a slab having a slab center portion thickness of 165 mm and a slab width of 1250 mm was cast. A part of the obtained slab was subjected to hot rolling in the same shape, and a part of the slab was maintained and subjected to hot rolling while changing its shape. All hot-rolled coils were made into a 1.0 mm-thick cold-rolled coil through normal pickling and cold-rolling steps. Rewind the cold rolled coil to roll direction 1
The number of defects generated per m was determined and used as the frequency of defects. The above process conditions, slab shape, and defect occurrence frequency are summarized in Table 3. In the table, w is the slab width, h is the thickness of the central portion of the slab, Δh is the maximum depth of the concave portion when there is a concave portion in the slab (depth from the horizontal plane of the central portion of the slab), and Δw is the slab outermost edge ( It means the distance from the slab end face) to the maximum recess depth generation position.

[0022]

[Table 2]

As is clear from Table 3, it is recognized that the occurrence of surface flaws is less when the hot rolling condition according to the present invention is used as compared with the hot rolling condition according to the comparative method. In particular, the effect of the present invention is clear by comparing the defect occurrence frequency in the recessed slab (reference numeral: 7 and 8) that is usually obtained with the defect occurrence frequency when manufactured by the method of the present invention. Comparing the case of the reference numerals 1 and 2 according to the technique described in the first aspect of the present invention with the comparison methods 7 and 8 using the same degree of recess generation slab, a remarkable defect improvement effect is recognized, and a substantially perfect rectangle is obtained. It can be seen that the defect occurrence frequency is the same as that of the case considered to be a slab (reference numeral 9). Further, when manufactured by the present invention as set forth in claim 2 (reference numeral: 3, 4, 5, 6), the defect occurrence frequency is extremely low and the yield improving effect is remarkably large.

[0024]

[Table 3]

[0025]

As described in detail above, the effect of the present invention is that when a steel sheet is manufactured by hot rolling, hot rolling is performed under predetermined rolling conditions, so that surface defects of products can be reduced and product yield is improved. It is a great place to benefit industrially, such as being able to do it.

[Brief description of drawings]

FIG. 1 shows a cross-sectional shape of a rectangular slab obtained when bulging does not occur during casting.

FIG. 2 shows a cross-sectional shape of a slab when a recess is formed on the long side surface of the slab by bulging or the like during casting.

FIG. 3 shows a stress distribution in the rolling direction calculated by the three-dimensional rigid-plastic finite element method when the slab having the shape shown in Table 1 is rolled down by 5% with a horizontal roll.

FIG. 4 shows a stress distribution in the rolling direction calculated by the three-dimensional rigid-plastic finite element method when a slab having the shape shown in Table 1 is rolled down by 20% with a horizontal roll.

[Explanation of symbols]

w Length of opposing long side (that is, slab width) h Length of opposing short side (slab thickness) Δh Maximum recess depth (depth from center horizontal plane of slab) Δw Maximum recess depth from slab edge (end face) Distance to generation location h _c Average thickness of slab center

Claims (2)

[Claims] 1. When manufacturing a stainless steel sheet by hot rolling, the first pass rolling by a horizontal roll is performed for 1000 times.
A method for producing a stainless steel sheet having few surface defects, which has a reduction rate of at least 15% or more in a temperature range of ℃ or more. 2. When manufacturing a stainless steel sheet by hot rolling, at least one or more passes of vertical rolls are performed before the first pass of horizontal rolls in a temperature range of 1000 ° C. or higher, and A method for producing a stainless steel sheet with few surface defects, characterized in that the total rolling reduction by a vertical roll is at least 5% or more.