JP3128487B2 - Method for producing ferritic stainless steel sheet with good ridging characteristics - Google Patents

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 ferritic stainless steel sheet having good ridging characteristics.

[0002]

2. Description of the Related Art As a method of improving the ridging characteristics of a ferritic stainless steel sheet by a hot rolling method, there are techniques disclosed in Japanese Patent Publication Nos. 49-17932 and 59-13026. The former 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, there is a problem that the yield stress of the product plate increases, and the ductility and deep drawability also deteriorate. In the latter, a technique is disclosed in which the ridging characteristics are improved by increasing the inter-pass time during rough hot rolling. However, if the inter-pass time is increased, the hot rolling time naturally increases,
There is a problem that productivity is reduced.

[0003]

SUMMARY OF THE INVENTION The present invention is directed to a manufacturing method for improving the ridging characteristics while overcoming the problems of the prior art, and in particular, for a product characteristic other than the ridging without lowering the productivity (mechanical properties). It is an object of the present invention to provide a manufacturing method for improving ridging characteristics without deteriorating properties and the like.

[0004]

In order to achieve the above object, the present invention specifies hot rolling conditions for ferritic stainless steel sheets, particularly finishing hot rolling conditions. In a method for producing a ferritic stainless steel sheet containing -30% by weight and a C content of 0.1% by weight or less, when the stainless steel is hot-rolled, the surface roughness of a rolling roll used for finish rolling is set to 1 0.5 μm or more. Further, when rolling with a rolling roll having the above roughness, at least one pass of different peripheral speed rolling with a different peripheral speed ratio of 15% or more is performed, or the finished hot rolling finished plate thickness is set to 2.5 mm or less. .

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
In general, it is considered that ridging becomes apparent due to the plastic anisotropy of crystal colonies existing in a steel sheet. A crystal colony is a group in which crystal grains having substantially the same crystal orientation are adjacent to each other, and the most prominent colony as a cause of ridging is a {100} colony having a {100} orientation parallel to a plate surface normal direction. . Therefore, in order to improve the ridging, it is sufficient to destroy the {100} colony, and the prior art aiming at the improvement of the ridging generally falls into this category. For example, according to the research by the present inventors, the low-temperature winding disclosed in the above-mentioned Japanese Patent Publication No. 49-17932 discloses a low-temperature transformation phase represented by martensite in a steel sheet by low-temperature winding during hot rolling. It is believed that this low-temperature transformation phase destroys the {100} colonies of the α matrix during the cold rolling after being introduced. 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 distortion deformation).
00 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 a deterioration in ductility and deep drawability.

The time between passes during rough hot rolling disclosed in the above-mentioned JP-A-59-13026 promotes recrystallization during rough hot rolling and causes the destruction of {100} colonies. However, this recrystallization takes time, resulting in a decrease in productivity. In view of this {100} colony destruction, the present inventors have searched for a method based on metallurgical phenomena other than the above-mentioned low-temperature transformation phase and recrystallization, and the change in strain distribution on the surface layer of the steel sheet during hot-rolling in the finish shows the ridging of the product sheet. The inventors have found that the characteristics are improved, and have completed the present invention.

The reasons for limiting the method of the present invention will be described below. First, the reason why the content of Cr is set to 10% or more is that if the content is less than that, the corrosion resistance as stainless steel is not satisfied, and the α / γ complete transformation occurs to greatly reduce the occurrence of ridging. . The reason why the upper limit of the Cr content is set to 30% is that further addition of Cr is not economical, the effect of improving corrosion resistance is reduced, and the tendency of embrittlement is increased, thereby impairing the formability. Further, the upper limit of the C content is set to 0.1% because not only addition of C further impairs corrosion resistance but also hardens and deteriorates moldability.

[0008] The finish roll roughness during hot rolling is 1.5
The reason for limiting the thickness to μm or more is that if the roll roughness is less than this, no change occurs in the strain distribution on the surface layer of the hot-rolled sheet, and the ridging characteristics of the product sheet are not improved. Although the upper limit is not particularly defined, if the roll roughness is too large, the surface properties of the steel sheet are deteriorated. The roll roughness in the present invention is represented by an average roughness Ra defined by JIS. Although the finish rolling condition by the roughness roll is not particularly specified, the temperature range is usually 110 ° C.
From 0 ° C. to 800 ° C., the rolling reduction is usually 60% or more, but it is considered that the lower the rolling temperature and the higher the rolling reduction, the greater the effect of improving the ridging characteristics. Therefore, when carrying out the present invention in a continuous hot rolling mill, it is more effective to use the roughness roll in the latter half of rolling. In this case, the total draft by the roughness roll is desirably 50% or more. Furthermore, it is naturally effective that the rolling reduction per pass of the roughness roll is large. However, the rolling reduction per pass in the finish rolling is usually up to 50%, and the rolling reduction more than that degrades the surface properties of the steel sheet.

In the present invention, the roll roughness may be basically the roll roughness before rolling. However, in a hot rolling mill used for industrial production, since a large amount of steel sheet is continuously rolled, the roll may be worn and the roll roughness may be reduced during rolling. In this case, it is desirable from the viewpoint of improving productivity that the roll is polished by a recently developed online roll polishing machine without interrupting the rolling and the roughness of the present invention is maintained.

Next, the reason why the different peripheral speed ratio is set to 15% or more when performing the hot rolling is that ridging does not improve at a different peripheral speed ratio lower than 15%. The number of rolling passes is 1
The pass works well. In the present invention, the upper limit of the different peripheral speed ratio and the upper limit of the number of different peripheral speed rolling passes are not particularly defined, but it is needless to say that the larger the both are, the greater the effect of improving the ridging is. However, different peripheral speed ratio of 50% or more is difficult at present,
The number of finishing hot rolling passes is usually up to about 8 passes. Here, the different peripheral speed ratio in the present invention is a value obtained by dividing a difference between the peripheral speeds of the upper and lower rolling rolls by the peripheral speed of the lower peripheral speed roll in percentage. In addition, since the different peripheral speed rolling of the present invention is performed in order to increase the above-described change in the surface layer strain distribution of the steel sheet, there is no difference in the ridging improvement effect regardless of which of the upper and lower roll peripheral speeds is large.

Further, the reason why the thickness of the finished hot-rolled sheet is limited to 2.5 mm or less is that if the sheet thickness is more than this, ridging is not improved. In addition, it goes without saying that as the plate thickness becomes smaller, the ratio of the above-mentioned strain distribution changing portion to the total plate thickness increases, and the effect of improving the ridging increases. However, the lower limit of the finished sheet thickness by ordinary hot rolling is about 1 mm, and even a continuous hot rolling mill that has been recently developed has a thickness of about 0.5 mm.

The reason why the ridging characteristics of the product sheet is remarkably improved by increasing the roughness of the hot-rolled roll, reducing the peripheral speed rolling, or reducing the thickness of the rolled sheet is not clear at present. It can be considered as follows. As described above, destroying the {100} colony improves the ridging characteristics of the product plate. According to our research,
The formation of {100} colonies is considered to occur as follows. First, when the crystal rotates during rolling deformation (plane deformation deformation) and the {100} direction becomes parallel to the normal direction of the sheet surface,
Even if recrystallized and refined in the subsequent annealing step, the orientation of each crystal grain is close to {100} even after recrystallization,
One group of the crystal grains is a {100} colony. Therefore, the formation of {100} orientation at the time of plane strain deformation is prevented or
Ridging characteristics are improved by reducing the size of the colonies by making the grains finer (and further randomizing) before the plane strain deformation. For example, ordinary steel in which α / γ complete transformation occurs before cold rolling (plane strain deformation) and α grains are refined and randomized has much better ridging characteristics than ferritic stainless steel. In ferritic stainless steel, since no complete transformation occurs, the hot rolling step can be regarded as a {100} plane deformation deformation step. At this time, according to the study of the present inventors, recrystallization is not usually sufficient in the rough hot rolling step but recrystallization occurs and the formation of {100} is small, and recrystallization does not occur in the finish hot rolling step and the {100} orientation is It is formed. That is, it can be said that the important {100} orientation forming steps in ferritic stainless steel are the cold rolling step and the finishing hot rolling step. Considering the above, the low-temperature winding technique disclosed in Japanese Patent Publication No. 49-17932 realizes the prevention of {100} formation in the cold rolling process, and is disclosed in Japanese Patent Application Laid-Open No. Sho 59-13026. It is considered that the obtained rough hot rolling inter-pass time effect realized grain refinement and randomization before finish hot rolling ({100} forming step).

Assuming that the above hypothesis is correct, the effects of roll roughness, different peripheral speed hot rolling and finish hot rolling in the present invention are as follows:
It can be considered that the formation of {100} in the finishing hot rolling step was inhibited, and the {100} colonies of the product plate were reduced to improve the ridging characteristics. That is, when the roll roughness is increased, the friction coefficient between the steel sheet and the roll increases, shear strain deformation occurs on the surface layer of the steel sheet, the strain distribution changes, and the plane strain condition is not maintained and {100} formation is inhibited. Conceivable.
Further, it can be considered that the different peripheral speed hot rolling promotes this shearing deformation, and that the thinner sheet thickness increases the ratio of the strain distribution changing portion to the total sheet thickness. However, it is thought that the {100} colonies normally present in the center of the thickness cause ridging, and it is currently unclear how the change in strain distribution on the surface of the steel sheet affects the colony formation in the center of the thickness. is not. Furthermore, if the surface shear deformation simply has an effect of improving ridging, ridging characteristics should be improved only by performing different peripheral speed hot rolling. There is no. That is, it is considered that the effect of improving the ridging is inherent in the change in the surface layer strain distribution when the roll roughness is increased.

On the other hand, it is considered that the effect of the roll roughness in the present invention is that recrystallization occurs due to surface layer shear deformation, and grain refinement and randomization have been achieved before 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 recrystallizes during hot rolling. Further, also in the hot-rolling winding step, since the partially transformed γ-phase precipitated during hot-rolling is decomposed into α-phase and carbide, recrystallization of the α-matrix is significantly delayed, and the
At a coiling temperature of 0 ° C. or less, recrystallization hardly occurs. That is, at present, it is unlikely that the surface strain distribution changes due to an increase in the roll roughness, and that recrystallization effective for improving ridging has occurred. However, when performing hot-rolled sheet annealing, it is considered that this recrystallization effect naturally contributes.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. Example 1 A 25 mm thick ferritic stainless steel sheet having the chemical components shown in Table 1 was heated to 1000 ° C., hot rolled in 6 passes, and subjected to a winding equivalent treatment at 650 ° C. for 1 hour after hot rolling. Table 2 shows the details of the hot rolling conditions at this time.

[0016]

[Table 1]

[0017]

[Table 2]

After the pickling, the obtained hot rolled sheet is subjected to a total reduction of 80%.
%, And after annealing at 875 ° C. for 30 seconds, the ridging characteristics were evaluated. The results are shown in Table 2 and FIGS.
Shown in In addition, the ridging characteristics were evaluated as follows. Ten JIS No. 5 tensile test pieces were placed in parallel with the rolling direction from the product plate.
This cutting was performed, and the maximum value of roughness (each test piece) when each test piece was subjected to a tensile strain of 17% in the rolling direction was determined, and the average of the maximum values (for 10 pieces) was defined as the ridging height. In this evaluation method, if the ridging height is about 20 μm or less, the ridging characteristics are good, and if it is 15 μm or less, the ridging characteristics are extremely good.

FIG. 1 is a view showing the effect of the roughness of a hot-rolled roll on ridging characteristics when the thickness of the hot-rolled sheet is kept constant.
Reference numeral 1 in Table 2 (comparison method: open circles in the figure) and reference numerals 7, 8, and 16
The results of (first term of the present invention: black circles in the figure) are shown. From FIG. 1, 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 plate becomes 20 μm or less, and the ridging characteristics are improved.

FIG. 2 shows that the roughness of the hot-rolled roll was 1 μm and 3 μm.
FIG. 4 is a diagram showing the effect of different peripheral speed rolling rates on ridging characteristics when the two levels are used, and reference numerals 1, 2, 3, 4, and 5 in Table 2 (comparison method: open circles in the figure) and reference numerals 8 and 9 ( The results of the first term of the present invention: black circles in the figure and the reference numerals 10, 11, and 12 (the second term of the present invention: black squares in the figure) are shown. As is clear from FIG. 2, when the roll roughness is large, the ridging height is reduced, and when the roll roughness is large, if the different peripheral speed rolling ratio is set to 15% or more, the ridging characteristics are remarkably improved, and the ridging height is improved. 15 μm
It is recognized that:

FIG. 3 shows that the roughness of the hot-rolled roll was 1 μm and 3 μm.
7 is a diagram showing ridging characteristics when the thickness of the hot-rolled sheet is changed as two levels of Nos. 1 and 6 (Comparative method: open circles in the figure) and 8 in Table 2 (Method 1 of the present invention: In the figure) Black circle) and symbol 1
The results of 3, 14, 15 (the method of the present invention, item 3: black triangle in the figure) are shown. As shown in FIG. 3, when the roll roughness is large, the ridging characteristics are good, and when the roll roughness is large, the ridging characteristics of the cold-rolled annealed plate are remarkably improved by reducing the thickness of the finished hot-rolled sheet, and the ridging height is improved. Is 15 μm or less. (Example 2) A ferritic stainless steel having the chemical components shown in Table 3 was melted in accordance with a normal melting method, and 250 mm
It was a thick continuous cast slab. The slab was heated to 1200 ° C., and after the completion of rough hot rolling, the slab was continuously heated by a 7-pass finishing hot rolling mill.
A hot-rolled coil having a thickness of mm was used. The obtained hot rolled coil was partially annealed (840 ° C. × 5 hours), pickled and cold rolled, and then 850
Annealing was performed at 30 ° C. for 30 seconds to obtain a product plate having a thickness of 0.4 mm.
The remaining hot-rolled coil was pickled and cold-rolled as it was, and annealed at 875 ° C. for 30 seconds to obtain a product sheet having a thickness of 0.4 mm. Table 4 shows the material properties of these product plates together with the manufacturing conditions.

[0022]

[Table 3]

[0023]

[Table 4]

Tables (1) to (5) show the results for steel type A in Table 3. (1) is a result of low-temperature winding in accordance with the conventional method, which shows good ridging characteristics but yield stress (YP).
And the deep drawing characteristics (r value) are low. So usually
As shown in (2), it is necessary to lower the YP and increase the r-value by performing hot-rolled sheet annealing. When high-temperature winding is performed as in (3), the ridging characteristics are extremely poor. Such (1)-
The method of the present invention was carried out with respect to the comparative method of (3) (4),
(5) In the material, the ridging characteristics can be improved without impairing other material characteristics. (4) The material was manufactured by performing hot-rolled sheet annealing, and it can be seen that the ridging characteristics are further improved as compared with the comparative method (2) material. (5)
The material has good ridging characteristics regardless of high temperature winding,
(3) The characteristics of the method of the present invention for improving the ridging characteristics without deteriorating the YP and r values as compared with the material are well shown. Also, as compared with the material (2), the material properties are almost the same. With such material properties, it is possible to omit hot rolled sheet annealing by the method of the present invention.

Tables (6) to (13) show the results for steel type B in Table 3. Since the Al content is higher than that of steel type A, the r value is generally increased at a low YP.
Comparing (8) with (9) to (11) of the method of the present invention, the same effect of the method of the present invention as the result of steel type A described above is observed. Further, the materials (12) and (13) show the effects of the second and third items of the method of the present invention, respectively. As shown in Table 4, when the method of the present invention is carried out, the ridging characteristics can be reliably reduced to 15 μm or less without lowering the YP or r-value characteristics.

[0026]

As described above in detail, according to the present invention, the ridging characteristics of a ferritic stainless steel sheet can be improved without deteriorating other material characteristics without lowering productivity.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a drawing of a stainless steel sheet having a thickness of 25 mm, which is hot-rolled to a thickness of 3 mm by changing the roll roughness, subjected to a winding equivalent treatment at 650 ° C. for one hour, and then reduced in total pressure after pickling. Rate 80
FIG. 3 is a diagram showing the relationship between the ridging characteristics and the hot roll roughness of a sheet subjected to cold rolling at 875 ° C. for 30 seconds.
(White circles in the figure: Comparative method, black circles: the production method of item 1 of the present invention method.
Roll roughness: Evaluated by the average roughness Ra defined by JIS. )

FIG. 2 shows a SUS430 steel plate having a thickness of 25 mm hot-rolled to a thickness of 3 mm and then cold-rolled to a thickness of 0.6 mm to 875 ° C.
FIG. 6 is a diagram showing ridging characteristics of a sheet annealed at 30 seconds for different peripheral speed rates during hot rolling. Roll roughness during hot rolling is 1
μm (white circle in the figure) and 3 μm (black mark in the figure)
Further, different peripheral speed hot rolling was performed while changing the peripheral speed of the upper roll and the lower roll. (White circles in the figure: Comparative method, black circles and black squares in the figure: Production methods of the first and second aspects of the present invention, respectively.)

FIG. 3 is a view showing ridging characteristics when the thickness of a hot-rolled sheet is changed in the same experiment as in FIG. 1; At this time, the roughness of the hot-rolled roll was set at two levels of 1 μm (open circles in the figure) and 3 μm (black marks in the figure). (White circles in the figure: Comparative method, black circles and black triangles in the figure: Production methods of the first and third aspects of the present invention, respectively.)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/18 C22C 38/18 (72) Inventor Takashi Oda 1-1 Nichihatacho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Inside the Yawata Works (72) Inventor Masayuki Abe 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation Technology Development Division (56) References JP-A-57-70234 (JP, A) JP-A-63 JP-B-177903 (JP, A) JP-A-7-116702 (JP, A) JP-B-57-61096 (JP, B2) JP-B-60-16281 (JP, B2) JP-B-59-33645 (JP, B2) Japanese Patent Publication No. 55-10648 (JP, B2) Japanese Patent Publication No. 61-19688 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 1/26, 3/02, 27 / 00 C21D 8/02 C22C 38 / 00,38 / 18

Claims (3)

(57) [Claims] (1) containing 10 to 30% by weight of Cr;
In a method for producing a ferritic stainless steel sheet having a content of 0.1% by weight or less, when hot rolling the stainless steel, the surface roughness of a rolling roll used for finish rolling is set to 1.5 μm or more. A method for producing a ferritic stainless steel sheet having good ridging characteristics. 2. The ferritic stainless steel sheet having good ridging characteristics according to claim 1, wherein when performing hot rolling, at least one pass of different peripheral speed rolling with a different peripheral speed ratio of 15% or more is performed. Manufacturing method. 3. The hot-rolled steel sheet according to claim 1, wherein the thickness of the finished hot-rolled sheet is 2.5 mm or less.
A method for producing a ferritic stainless steel sheet having good ridging characteristics as described above.