JPH09253712A - Method for producing ferritic stainless steel sheet with good workability - Google Patents

Abstract

(57) Abstract: The present invention provides a method for producing a ferritic stainless steel sheet having good ridging characteristics and deep drawing characteristics. The present invention specifies hot rolling conditions of a ferritic stainless steel sheet in particular, and the gist thereof is that the content of Cr is 10 to 30% by weight and the content of C is 0.1% by weight. In the following ferritic stainless steel hot rolling step, the rolling roll roughness used for finish rolling is 1.5 μm or more, and the rolling tension is 1 kg / mm ² .

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a ferritic stainless steel sheet having good workability.

[0002]

2. Description of the Related Art As a technique for improving the workability of a ferritic stainless steel sheet by a hot rolling method, Japanese Patent Publication No.
17932, JP-B-58-32217, and JP-A-59-13026. Japanese Patent Publication Sho 49-1
Japanese Patent No. 7932 discloses a technique for improving the ridging characteristics of a cold rolled product sheet by lowering the winding temperature during hot rolling. However, when the hot rolling coiling temperature is lowered, the yield stress of the product sheet becomes high, and the ductility and the deep drawability also deteriorate. On the contrary, Japanese Patent Publication Sho-58-322
In Japanese Patent No. 17, the winding temperature at the time of hot rolling is 850 to 95.
It is disclosed that, by raising the temperature to 0 ° C., the deep drawability is improved and becomes approximately the same as that of the conventional batch type hot rolled sheet annealing process material. However, if high-temperature winding at 850 ° C. or higher is performed, it is difficult to make the material uniform over the entire length of the rolled coil. Especially, the top portion and the bottom portion of the rolled coil cool faster than the middle portion, and the deep drawability deteriorates. Etc., and the yield is significantly reduced. Furthermore, this high-temperature winding method has a problem that the ridging property of the product plate deteriorates as described later. Also, JP-A-59-1302
Japanese Patent Publication No. 6 discloses a technique for improving workability by increasing the time between passes during rough hot rolling. However, if the time between the passes is lengthened, the hot rolling time is naturally lengthened and the productivity is lowered.

[0003]

SUMMARY OF THE INVENTION The present invention is a method for producing a ferritic stainless steel sheet which overcomes the problems of the prior art and improves the workability, that is, the ridging characteristics and deep drawing characteristics without lowering the productivity. It is an object of the present invention to provide a method for manufacturing a ferritic stainless steel sheet capable of improving both of the above.

[0004]

In order to achieve the above object, the present invention specifies hot rolling conditions, particularly finish hot rolling conditions, of a ferritic stainless steel sheet. In the method for producing a ferritic stainless steel sheet containing 30 to 30% by weight and a C content of 0.1% by weight or less, the surface roughness of a rolling roll used for finish rolling is 1 when hot rolling the stainless steel. 0.5 μm or more, and the tension acting on the steel sheet at the time of rolling is 1 kg / mm ² or more.

Hereinafter, the present invention will be described in detail.

[0006] Generally, it is considered that ridging is manifested by the plastic anisotropy of crystal colonies existing in a steel sheet. A crystal colony is a group of adjacent crystal grains whose crystal orientations are substantially the same, and the most prominent colony as a cause of ridging is a {100} colony whose {100} orientation is parallel to the plate surface normal direction. . Therefore, to improve ridging, destroy this {100} colony,
Conventional technologies aiming to improve ridging generally fall into this category.
For example, the low temperature winding of Japanese Patent Publication No. 49-17932 is
According to the research conducted by the present inventors, a low temperature transformation phase represented by martensite is introduced into a steel sheet by low temperature winding during hot rolling, and this low temperature transformation phase during the subsequent cold rolling is {10
It is thought to destroy the 0} colony. That is, since this low temperature transformation phase is harder than the α matrix, it does not deform as much as the α phase during cold rolling, and is promoted by cold rolling (plane strain deformation) {100}.
Prevents colonization and improves the ridging properties of the final product. However, at the same time, this low temperature transformation phase causes an increase in yield stress and deterioration of ductility and deep drawability. Therefore, conversely, if high-temperature winding is performed, the above-mentioned low-temperature transformation phase does not occur (γ phase decomposes into α phase and carbide), and deep drawability is improved, but {1
00} colonies remain, resulting in deterioration of ridging characteristics. That is, it is considered difficult to achieve both the ridging characteristic and the deep drawing characteristic as long as the workability of the product sheet is controlled by the low temperature transformation phase before cold rolling.

However, the pass-to-pass time in rough hot rolling disclosed in JP-A-59-13026 promotes recrystallization during rough hot rolling, resulting in destruction of {100} colonies. It is noted that the destruction of {100} colonies not only does not deteriorate the deep drawing property but also improves the ridging property. However, this recrystallization takes time (interpass time), resulting in deterioration of productivity.

[0008] Therefore, the inventors of the present invention sought a method based on metallurgical phenomena other than the above-mentioned low temperature transformation phase and recrystallization for the {100} colony destruction, and examined the hot rolling conditions and the ridging characteristics in the cold rolled / annealed sheet. Scrutinize the relationship between the deep drawing characteristics and
The present invention has been completed.

The reasons for limiting the method of the present invention will be described below.

First, the content of Cr is set to 10% or more, because if the content is less than that, the corrosion resistance as stainless steel is not satisfied, and α ← → γ complete transformation occurs, and the occurrence of ridging is significantly reduced. This is because The upper limit of the Cr content is set to 30% because the addition of more Cr is not economical, the effect of improving the corrosion resistance is reduced, and the embrittlement tendency is increased to deteriorate the formability. Moreover, the upper limit of the C content is set to 0.1%, because the addition of more C not only impairs the corrosion resistance, but also hardens and deteriorates the formability.

The roughness of the rolling roll during the finish hot rolling is 1.5
The reason why the thickness is limited to μm or more is that a roll roughness of less than this does not cause a change in strain distribution in the surface layer of the hot-rolled sheet and the ridging property of the product sheet is not improved. The upper limit thereof is not particularly specified, but if the roll roughness is too large, the surface quality of the steel sheet is deteriorated, and it is usually up to about 50 μm. The roll roughness in the present invention is indicated by the average roughness Ra defined by JIS. Further, the conditions for finish rolling with the roughness roll are not particularly specified, but the temperature range is usually from 1100 ° C to 800 ° C, and the rolling reduction is usually 60%.
That is all. Incidentally, the rolling temperature is low from the point of the present invention,
It is considered that the larger the rolling reduction is, the greater the effect of improving the ridging durability is. Therefore, when the present invention is carried out by a continuous hot rolling mill, it is more effective to use the roughness roll in the latter half of rolling. At that time, the total rolling reduction by the roughness roll is preferably 50% or more. Further, it is effective that the reduction rate per pass of the roughness roll is large as a matter of course. However, the rolling reduction in one pass in the normal finish rolling is up to 50%, and the rolling reduction higher than that deteriorates the surface properties of the steel sheet.

The roll roughness in the present invention may be basically the roll roughness before rolling. However, in a hot rolling mill used for industrial production, a large amount of steel sheets are continuously rolled, so that the roll may be worn and the roll roughness may be lowered during rolling. At this time, it is desirable from the viewpoint of productivity to polish the rolls without interruption of rolling by the recently developed online roll polishing machine to maintain the roughness of the present invention.

Next, the reason why the tension acting on the steel sheet during hot rolling is limited to 1 kg / mm ² or more is that the tension lower than this does not improve the deep drawing property and the ridging property of the product sheet. Although the upper limit is not particularly specified, it goes without saying that when the breaking stress of the steel sheet is reached, the sheet breaks and the productivity of hot rolling is significantly reduced. Further, the rupture stress of the steel sheet changes depending on the temperature and the strain rate, and the rupture stress becomes higher at lower temperatures and higher strain rates. Further, the tension specified in the present invention may be applied to either the entry side or the exit side of rolling, or the tension may be applied to both sides. The tension in the present invention is defined as the force acting in the rolling direction of the steel sheet divided by the cross-sectional area obtained by multiplying the width and thickness of the steel sheet.

The reason why the ridging property and deep drawing property of the product sheet are improved by increasing the roughness of the hot-rolled roll and the tension during rolling is not always clear at present, but it is considered as follows. .

As described above, if the {100} colonies are destroyed, the ridging property of the product plate is improved. According to the study by the present inventors, it is considered that the formation of {100} colonies occurs as follows. First, when the crystal is rotated during rolling deformation (plane strain deformation) and the {100} orientation becomes parallel to the plate surface normal direction, the orientation of each crystal grain remains the same even if it is recrystallized and refined in the subsequent annealing step. Even after recrystallization, the orientation is close to {100}, and a group of these crystal grains becomes {100} colonies. Therefore, if the formation of {100} orientation at the time of plane strain deformation is prevented or the colony size is reduced by making the grains finer (further randomization) before the plane strain deformation, the ridging characteristics are improved. For example, α ← → γ before cold rolling (plane strain deformation)
Compared with ferritic stainless steel, ordinary steel, which undergoes complete transformation to cause α grain refinement and randomization, has significantly better ridging characteristics. In ferritic stainless steel, since the complete transformation does not occur, the hot rolling process can be regarded as the {100} forming plane strain deformation process. At this time, according to the research conducted by the present inventors, recrystallization does not usually occur in the rough hot rolling process, but the formation of {100} is small, and recrystallization does not occur in the finish hot rolling process and the {100} orientation is It is formed. That is, the important {10
It can be said that the 0} orientation forming step is a cold rolling step and a finish hot rolling step. Considering this, the low-temperature winding technique disclosed in Japanese Patent Publication No. Sho 49-17932 realizes the prevention of {100} formation in the cold rolling process. It is considered that the inter-time effect realized the fine graining / randomization before the final hot rolling ({100} forming step).

If the above hypothesis is correct, the effects of roll roughness and tension in the present invention hinder {100} formation in the finish hot rolling process, reduce {100} colonies on the product sheet, and improve ridging characteristics. It can be thought of as a brutal thing. That is, when the roll roughness is increased, the friction coefficient between the steel plate and the roll is increased, shear strain deformation occurs in the steel plate surface layer, the strain distribution is changed, and the plane strain condition is not maintained, and {100} formation is obstructed. Conceivable. Further, it is considered that the change in strain distribution is promoted by applying the rolling tension. However, it usually exists in the center of the plate thickness {1
00} colonies are considered to cause ridging, and it is not clear at present how the change in strain distribution in the surface layer of the steel sheet affects the formation of colonies in the central portion of the sheet thickness. It is considered that the above-mentioned effect penetrated and acted even more in the central part of the layer thickness due to the increase in tension.

On the other hand, it is considered that the effect of the roll roughness in the present invention is that recrystallization is caused by the shear deformation of the surface layer, and the grain refinement / randomization is achieved before the cold rolling in the {100} forming step. However, as described above, recrystallization of ferritic stainless steel is slower than that of ordinary steel, and hardly recrystallized during finish hot rolling. Further, even in the hot rolling coiling step, the recrystallization of the α mother phase is significantly delayed because the partially transformed γ phase precipitated during hot rolling is decomposed into the α phase and carbides, and the value is about 80
Recrystallization is unlikely to occur at a winding temperature of 0 ° C or lower. That is, at present, it is unlikely that the surface strain distribution is changed by the increase of the roll roughness and the recrystallization effective for improving the ridging occurs. However, when the hot-rolled sheet annealing is carried out, it is considered that the recrystallization effect in the hot-rolled sheet annealing naturally contributes.

Next, the effect of improving the moldability will be described. As described later, when the roll roughness is increased, the ridging property is remarkably improved, but the deep drawability is not so improved. However, when the roll roughness is increased and the tension is applied, a remarkable effect of improving the deep drawability is recognized. The above results are not necessarily obvious at present, but they are interpreted as follows. First, due to the effect of roll roughness, {10
It is considered that the 0} colony is destroyed (that is, ridging is improved), and as a result, the crystal orientation formed after cold rolling / annealing is “an orientation that does not contribute so much to deep drawability (for example, {110} etc.)”. However, applying tension at the same time not only promotes the destruction of {100} colonies, but also changes the texture of the hot-rolled sheet, resulting in a crystal orientation of the cold-rolled / annealed sheet that is advantageous for deep drawing. It is considered that it has been changed to a different azimuth (eg, {111} etc.). Also,
When only tension is applied, neither the ridging property nor the deep drawing property is improved. Therefore, the effect of roll roughness and the effect of applying tension are combined, and the surface properties (ridging) and workability (deep drawing) of cold rolled and annealed sheets are combined. It is judged that this has improved the sex).

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

(Examples) Hereinafter, the present invention will be described in detail according to examples.

First, the experimental results that form the basis of the present invention will be described. A 25 mm thick ferritic stainless steel sheet having the chemical composition shown in Table 1 was heated to 1000 ° C., hot-rolled in 6 passes, and after hot rolling, a treatment equivalent to winding for 1 hour was performed at 650 ° C. Details of the hot rolling conditions at this time are shown in Table 2. The obtained hot-rolled sheet was pickled and then cold-rolled at a total reduction rate of 80% to obtain 87
The ridging characteristics were evaluated after annealing at 5 ° C for 30 seconds.
Table 2 shows the results. From Table 2, it is recognized that when the roll roughness during hot rolling is set to 1.5 μm or more, the ridging height of the cold rolled annealed sheet becomes 20 μm or less, and the ridging property becomes good. The ridging characteristics were evaluated as follows. Ten JIS No. 5 tensile test pieces were cut out from the product plate parallel to the rolling direction, and the maximum value of roughness (each test piece) was obtained when each test piece was subjected to a tensile strain of 17% in the rolling direction. The average value (10 pieces) was taken as the ridging height.
According to this evaluation method, it can be said that the ridging characteristic is good when the ridging height is about 20 μm or less, and it is extremely good when the ridging height is 15 μm or less.

Next, examples of the present invention will be described. Ferritic stainless steels having the chemical components shown in Table 3 were melted according to a common melting method to obtain a 250 mm thick continuously cast slab. After the slab is heated to 1200 degrees and the rough hot rolling is finished,
Subsequently, a hot rolling coil having a thickness of 3 mm was made by a 7-pass finishing hot rolling mill. At this time, finish hot rolling was performed by changing the roughness and tension of the hot rolling roll. The obtained hot-rolled coil was subjected to pickling / cold rolling after hot-rolling the negative part of the hot-rolled sheet (840 ° C. × 5 hr).
Annealing was performed for 30 seconds at 0 ° C. to obtain a 0.4 mm thick product plate. The remaining hot rolled coil is pickled and cold rolled as it is, 875
Annealing was performed at 30 ° C. for 30 seconds to obtain a 0.4 mm thick product plate. Table 4 shows the material characteristics of these product plates together with the manufacturing conditions.
Shown in

Tables 4a to 4d show the results for steel type A in Table 3. “A” is the result of low-temperature winding according to the conventional method, and the ridging property is good, but the yield stress (hereinafter YP) is high and the deep drawing property (r value) is low. Therefore, it is usually necessary to carry out hot-rolled sheet annealing to lower the YP and increase the r-value as shown in b. Moreover, when high-temperature winding is carried out as in the case of c, the ridging characteristic is extremely poor. In contrast to the comparative method of a to c, the material d subjected to the method of the present invention has extremely good ridging characteristics of 15 μm or less, although the hot rolled sheet annealing is not performed. R value is higher than any of the cases,
In addition, YP is also of a degree that can satisfy ordinary applications. Compared with the material b in particular, even if the hot-rolled sheet annealing is omitted according to the present invention,
It can be seen that the workability is improved.

Tables 4e through 4j show the results for steel type B in Table 3. Higher Al content than steel type A, so low YP overall
However, the same effects as the results of the steel type A described above can be recognized by comparing the materials e to g of the comparative method with the material h of the method of the present invention. Furthermore, the effect of the present invention becomes clear when compared with the materials i and j. Material i is the case where the hot rolling roll roughness is increased at the same winding temperature as material f of the comparative method. It can be seen that the ridging property is improved as compared with the f material, but the r value is not so changed. On the other hand, the material j has the same winding temperature as the material f and only the rolling tension is increased. It can be seen that the material characteristics have hardly changed as compared with the f material. Comparing the h material and the f material of the method of the present invention based on the results of the i and j materials, the extremely good ridging characteristics and high deep drawing characteristics of the h material are brought about by the synergistic effect of the roll roughness and the tension. It is thought that it was done.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

As described in detail above, according to the present invention, it is possible to improve the ridging characteristics and the deep drawing characteristics of the ferritic stainless steel sheet at the same time without lowering the productivity. Is large.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masayuki Abe 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (1)

[Claims] 1. A method for producing a ferritic stainless steel sheet containing 10 to 30% by weight of Cr and 0.1% by weight or less of C, which is used for finish rolling when hot rolling the stainless steel. A method for producing a ferritic stainless steel sheet having good workability, characterized in that the rolling roll has a surface roughness of 1.5 μm or more and a tension applied to the steel sheet at the time of rolling is 1 kg / mm ² or more.