JP6342056B2 - Ferritic stainless steel sheet - Google Patents

Description

  The present invention relates to a ferritic stainless steel sheet that is excellent in ridging resistance and workability, and has low in-plane anisotropy.

  When a cold-rolled annealed steel sheet made of ferritic stainless steel is formed, a striped pattern called ridging may occur on the surface of the molded product, which is often a problem in appearance quality. Ridging is considered to be caused by a non-uniform structure caused by the cast structure remaining in the cold-rolled annealed steel sheet.

  In Patent Document 1, hot rolling under strong pressure is performed while maintaining the steel plate temperature at 850 ° C. or higher with a reversible rolling mill equipped with a heat-retaining furnace, and then batch annealing is performed at a temperature of 900 ° C. or higher for 4 hours or longer. A technique for improving ridging resistance of a ferritic stainless steel sheet by a technique to be performed is described. However, steel containing B has not been applied, and it is difficult to realize a steel plate that can be satisfactorily satisfied by strict rigging evaluation criteria that take into consideration both the height of the surface irregularities and the wavelength.

  In Patent Document 2, the r value and resistance of a ferritic stainless steel sheet are obtained by a technique in which hot rolling is performed in a mixed structure of ferrite and austenite, and hot rolling annealed steel sheet is subjected to cold rolling under strong pressure and then annealed. Techniques for improving ridging properties are described. However, steel containing B is not applied. Further, in hot rolling in a mixed structure temperature range in which an austenite phase exists stably, it is difficult to achieve both excellent ridging resistance and workability.

  In Patent Document 3, the slab is subjected to reduction at a high temperature and then reheated to obtain a hot-rolled sheet at a temperature of 850 ° C. or higher by a rough rolling mill and a continuous finish rolling mill. A technique for improving ridging resistance of a ferritic stainless steel sheet in a process of performing cold rolling with a work roll having a diameter is described. However, steel containing B is not applied. The hot rolling in this document is also performed in the α + γ two-phase region. It is difficult to achieve both excellent ridging resistance and processability by this method.

  Patent Document 4 discloses that ferrite energy is accumulated in a hot-rolled steel sheet by a method of terminating hot rolling at 700 to 850 ° C., and the strain energy is used as a driving force for recrystallization during annealing. A technique for improving the ridging resistance of a stainless steel sheet is described. However, steel containing B is not applied. With the technique of this document, it is difficult to realize excellent ridging resistance evaluated by both the height of the surface irregularities and the wavelength.

  In Patent Document 5, the first hot rolling is performed at a temperature higher than 1000 ° C. in order to destroy the columnar crystals at the time of slab solidification, and then the second hot rolling is performed at a high pressure at a low temperature for grain refinement. A technique for improving the ridging resistance of a ferritic stainless steel sheet by employing a process performed by under rolling is described. However, steel containing B is not applied. It is difficult to realize excellent ridging resistance according to strict evaluation criteria by the technique of this document.

  In Patent Documents 6 and 7, B is added to soften the γ phase, rough rolling is performed under a high pressure of 40% or more at a high temperature of 1100 ° C. or more, hot rolling and winding at 600 ° C. or less. A technique for obtaining a ferritic stainless steel sheet with reduced roping (slight undulation) through a process of cold rolling and final annealing without performing hot rolling and annealing is described. However, as taught in Paragraph 0005 of Patent Document 6 and Paragraph 0006 of Patent Document 7, it cannot be said that there is always a good correspondence between ridging during forming and roping during cold rolling. With the techniques disclosed in these documents, it is difficult to achieve both excellent ridging resistance according to strict evaluation criteria and excellent workability.

  Patent Document 8 describes a technique for obtaining a ferritic stainless steel sheet with reduced roping by a method in which rough rolling is performed at a temperature of 1000 ° C. or higher and a cumulative rolling of 80% or higher and winding is performed at 700 ° C. or higher after finish rolling. Although steel added with B is used for the purpose of fixing N, it is difficult to achieve both excellent ridging resistance and excellent workability according to strict evaluation criteria even with the technique disclosed in this document.

Patent Document 9 discloses a ferrite having excellent roping characteristics by accumulating strain by securing a rolling reduction of 30% or more in a pass after the hot rolling and rough rolling, and promoting recrystallization before finish rolling. A technique for obtaining a stainless steel sheet is disclosed. However, with this technology, it is difficult to proceed the decomposition reaction and recrystallization from the austenite phase to the ferrite phase and carbide at a temperature range below the Ar ₁ transformation point, and it has excellent ridging resistance according to strict evaluation criteria. Cannot be realized.

  Patent Document 10 discloses a technique for obtaining a ferritic stainless steel sheet having excellent shape freezing properties by performing intermediate annealing in a cold rolling process. However, a conventional general method is applied to the hot working process (forging, hot rolling), and it is difficult to achieve both excellent ridging resistance and excellent workability according to strict evaluation criteria.

On the other hand, a steel sheet material used for overhanging or drawing is required to have a small in-plane anisotropy of r value (plastic strain ratio).
Patent Document 11 discloses a technique for improving ridging resistance and in-plane anisotropy by optimizing a strain rate and a coefficient of friction with a roll when rough rolling is performed at 1000 to 1150 ° C. . However, with this technique, it is difficult to proceed the decomposition reaction and recrystallization into a ferrite phase and a carbide in a temperature range below the Ar ₁ transformation point, and it is impossible to realize excellent ridging resistance based on strict evaluation criteria.

In Patent Document 12, a method of subjecting a hot-rolled sheet to two heat treatments, a pre-annealing and a main annealing with an appropriate temperature rise / cooling rate, and a homogenized annealing after hot rolling at high temperature and high pressure reduction are performed. A technique for improving the in-plane anisotropy and ridging resistance by performing a cold rolling process having intermediate annealing by dividing colonies by a method to be applied is disclosed. However, with this technique, it is difficult to proceed the decomposition reaction and recrystallization into a ferrite phase and carbide at a temperature range below the Ar ₁ transformation point, and it is impossible to realize excellent ridging resistance according to strict evaluation criteria. .

  Patent Document 13 discloses a technique for producing a ferritic stainless steel sheet for clad steel having a small in-plane anisotropy and excellent deep drawability by performing a cold rolling process having intermediate annealing. . However, it is difficult to achieve both excellent ridging resistance and excellent workability according to strict evaluation standards, as hot rolling has been applied a conventional method (considered using a tandem continuous hot rolling machine). It is.

JP 2010-270399 A Japanese Patent Publication No. 61-50126 Japanese Examined Patent Publication No. 62-34803 Japanese Patent Publication No. 7-51727 JP-A-11-256230 Japanese Patent Laid-Open No. 10-176223 JP 10-36911 A Japanese Patent Laid-Open No. 10-60543 Japanese Patent Laid-Open No. 9-256064 JP 2002-322548 A JP 7-310122 A JP 2006-328524 A JP 2008-127671 A

  Ferritic stainless steel sheets are widely used as processing materials for various products. In recent years, in the field of daily necessities such as pots, for example, more and more design properties are required in terms of product shape and surface appearance. In order to meet the demand, the steel sheet is required to have both excellent workability and ridging resistance, compared with the conventional steel sheet. In particular, regarding ridging, it has been found that not only the height elements of the irregularities appearing in a streak shape on the processed surface but also the spacing elements are important in evaluating the beauty of the appearance. On the other hand, in order to obtain excellent ridging resistance, it is considered advantageous to perform hot rolling under strong pressure in a temperature range in which the austenite phase coexists with the ferrite phase. It is difficult to achieve both. Moreover, in order to apply to various uses, it is also desired that weldability is good.

  As described above, various techniques for reducing ridging and roping of ferritic stainless steel sheets have been studied so far, and predetermined effects have been obtained in the respective techniques. However, even according to these techniques, it is difficult to achieve both excellent ridging resistance and workability (breaking elongation, in-plane anisotropy of r value) capable of meeting the recent demands for strict design properties. An object of the present invention is to provide a ferritic stainless steel sheet having excellent workability and weldability, and having excellent ridging resistance evaluated based on a strict standard in consideration of both the height and wavelength of surface irregularities. To do.

  According to the inventors' investigation, in the cold rolled annealed steel sheet of ferritic stainless steel, the crystal orientation of the ferrite recrystallized grains is sufficiently randomized to achieve the above-mentioned severe ridging resistance. It turned out to be effective. In general, in order to improve the r value of a ferritic stainless steel sheet, it is advantageous to form a {111} texture in which the <111> direction of the ferrite crystal is oriented in the plate thickness direction. On the other hand, in order to improve the ridging resistance, it is well known that the disappearance of ferrite crystal colonies extending in the rolling direction is important. However, as a result of detailed studies, the inventors have further confirmed that the orientation of the crystal orientation is small in order to improve the rigging resistance of strict standards considering both the height of undulation and the wavelength. It was found that it is important that the orientation of individual ferrite crystal grains be randomized as much as possible. Specifically, it is extremely effective to adjust the crystal orientation ratio to satisfy the formula (2) described later.

In order to achieve both excellent ridging resistance and workability, (i) γmax value is strictly regulated so that an austenite phase is not excessively formed at a high temperature, and an appropriate amount of B is contained before hot rolling. (Ii) Hot rolling is carefully performed in the temperature range above the recrystallization temperature and below the Ar ₁ transformation point to change from austenite phase to ferrite phase and carbide. It was found that it is extremely effective to advance the decomposition reaction of ferrite and recrystallization of the ferrite phase all at once in the hot rolling process. The present invention has been completed based on such findings.

In order to achieve the above object, the present invention discloses the following invention.
[1] By mass%, C: 0.020 to 0.150%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.00%, Ni: 0.01 to 0.60% , Cr: 11.00 to 19.00%, N: 0.000 to 0.050%, B: 0.0025 to 0.0100%, Al: 0 to 0.200%, Ti: 0 to 0.180 %, Mo: 0 to 0.50%, Cu: 0 to 0.50%, of which Al is selected from the group of 0.050 to 0.200% and Ti is 0.050 to 0.180%. It is composed of recrystallized ferrite crystal grains that contain one or more types, consist of the remainder Fe and inevitable impurities, have a chemical composition with a γmax value of 20.0 to 44.0 determined by the following formula (1) having a metallic structure that carbide particles in the matrix is dispersed that, R _ND value determined by the following equation (2) has a crystal orientation ratio of 2.8 or less Ferrite stainless steel sheet.
Applying a steel having a chemical composition that satisfies the above-described content range of the component elements and further contains one or more of Nb: 0.10% or less, Co: 0.10% or less, and V: 0.20 or less. Also good.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-52Al + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1), and 0 (zero) is substituted for the element not contained.
R _ND = S ₁₁₁ / (S ₀₀₁ + S ₀₁₁ ) (2)
Here, in S ₀₀₁ , S ₀₁₁ and S ₁₁₁ , the crystal orientation in the plate thickness direction of the ferrite crystal measured by EBSD (electron beam backscattering diffraction method) on the cross section (ND plane) perpendicular to the plate thickness direction is <001, respectively. > Direction, <011> direction, and <111> direction are the crystal grain area ratios within 15 °.
[2] The ferritic stainless steel sheet according to the above [1], which further has a chemical composition in which an Ar value determined by the following formula (3) is 870 or more.
Ar = -250C + 73Si-66Mn-115Ni + 35Cr + 60Mo-18Cu + 620Ti + 750Al-280N + 310 (3)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (3), and 0 (zero) is substituted for the element not contained.
[3] The ferritic stainless steel sheet according to the above [1] or [2], wherein the elongation at break (JIS Z2241: 2011) in a rolling direction by a JIS No. 13B tensile test piece is 26.0% or more.
[4] the and the R _ND value is 1.8 or less, elongation at break in the rolling direction by JIS13B No. Tensile test pieces (JIS Z2241: 2011) is described in [1] or [2] is 30.0% or more Ferritic stainless steel sheet.
[5] The in-plane anisotropy Δr (JIS Z2254: 2008) of the following equation (4) measured using a JIS No. 13B tensile test piece under the conditions of a gauge distance of 25 mm and a strain amount of 15% is 0.70 or less. The ferritic stainless steel sheet according to [1] or [2].
Δr = (r _L −2r _D + r _T ) / 2 (4)
Here, r _L , r _D, and r _T are r values (plastic strain ratios) measured by collecting test pieces in parallel, 45 ° direction, and 90 ° direction with respect to the rolling direction of the plate surface.

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel plate which has the outstanding ridging resistance evaluated by both the height of a surface unevenness | corrugation and a wavelength was able to be provided industrially stably. This steel plate has good workability and weldability and can be widely applied to various products. It is expected that the design flexibility of processing can be expanded by utilizing the excellent ridging resistance, and it can contribute to the improvement of the design properties of processed products.

The figure which illustrated the crystal orientation map based on 001 reverse pole figure by EBSD measurement of the cross section (ND surface) perpendicular | vertical to a plate | board thickness direction about invention example A1 (steel No. 1). The figure which illustrated the crystal orientation map based on 001 reverse pole figure by EBSD measurement of the cross section (ND surface) perpendicular | vertical to a plate | board thickness direction about invention example A2 (steel No. 1). The figure which illustrated the crystal orientation map based on the 001 reverse pole figure by EBSD measurement of the cross section (ND surface) perpendicular | vertical to a plate | board thickness direction about comparative example B6 (steel No. 13). The figure which illustrated the crystal orientation map based on the 001 reverse pole figure by EBSD measurement of the cross section (ND surface) perpendicular | vertical to a plate | board thickness direction about comparative example B23 (steel No. 38).

[Chemical composition]
In this specification, “%” related to the chemical composition of steel means “% by mass” unless otherwise specified.

  C is an austenite-forming element and is effective for preventing ferrite crystal grain coarsening during hot rolling. As a result of the study, the present invention requires a C content of 0.020% or more. However, if the C content is too large, the workability is reduced. The C content is limited to 0.150% or less. More preferably, it is less than 0.100%.

  Si is an element having a deoxidizing action, but if contained in a large amount, workability and toughness are lowered. On the other hand, excessively low Si leads to an increase in refining costs. The Si content is 0.10 to 1.00%. You may manage in the range of 0.20 to 0.70%.

  Mn is an austenite-forming element, is effective for preventing ferrite crystal grain coarsening during hot rolling, and ensures a Mn content of 0.10% or more. More preferably, the content is 0.25% or more. A large amount of Mn content causes deterioration of workability and corrosion resistance. The Mn content is limited to 1.00% or less.

  Ni is an austenite-forming element and is effective for preventing ferrite crystal grain coarsening during hot rolling. It is also effective in improving toughness and corrosion resistance. In order to exert these effects, a Ni content of 0.01% or more is ensured. More preferably, it is 0.05% or more. However, since excessive Ni content leads to an increase in raw material cost, the Ni content is set to 0.60% or less. You may manage in the range of 0.30% or less.

  From the viewpoint of corrosion resistance, Cr needs to ensure a content of 11.00% or more. More preferably, it is 13.5% or more. However, containing a large amount of Cr causes a decrease in workability, a decrease in toughness, and an increase in cost. The Cr content is in the range of 19.00% or less.

  N is an austenite-forming element, is effective for preventing ferrite crystal grain coarsening during hot rolling, and ensures an N content of 0.010% or more. However, if the N content increases, the workability tends to be lowered. The N content is limited to 0.050% or less. More preferably, it is 0.035% or less.

  B has the effect of finely dispersing the austenite phase at a high temperature and accelerating the decomposition of the austenite phase. In the present invention, it is necessary to secure a B content of 0.0025% or more, and more preferably 0.0033% or more. Since a large amount of B causes welding hot cracking, the content is set to 0.0100% or less.

  Al has a large effect of reducing the austenite phase generation temperature region and increasing the temperature at which the austenite phase is stabilized. That is, the contribution to the increase in Ar value in formula (3) described later is large. By this action, when performing finish hot rolling in a temperature range where the austenite phase cannot exist stably, finish hot rolling at a higher temperature becomes possible, thereby promoting the decomposition reaction from the austenite phase to the ferrite phase and carbide. Sufficient progress of this decomposition reaction during hot rolling results in improved workability. Moreover, Al has the effect | action which fixes N, and contributes also to high purity. Since Ti described later exhibits the same action as Al, in the present invention, at least one of Al and Ti is contained. As a result of various studies, it is effective to secure an Al content of 0.050% or more in order to sufficiently obtain the above effect by adding Al. However, Al is a strong ferrite-forming element, and excessive addition causes the austenite phase generation amount at a high temperature to be reduced more than necessary, so the Al content is limited to 0.200% or less.

  Ti, like Al, has a large effect of increasing the temperature at which the austenite phase formation temperature range is reduced to stabilize the austenite phase. That is, the contribution to the increase in Ar value in formula (3) described later is large. As described above, this action functions advantageously in advancing the decomposition reaction from the austenite phase to the ferrite phase and carbide during hot rolling, and contributes to improvement of workability. Ti also fixes N and C and contributes to high purity. In the present invention, at least one of Al and Ti is contained as described above. As a result of various studies, it is effective to secure a Ti content of 0.050% or more in order to sufficiently obtain the above effect by adding Ti. However, excessive Ti addition causes a reduction in the austenite phase generation amount at a high temperature more than necessary, and also causes a reduction in workability, so the Ti content is limited to 0.180% or less.

  Mo is effective in improving the corrosion resistance of the Cr-containing steel, and can be added as necessary. It is more effective to secure a Mo content of 0.01% or more. Excessive Mo content causes a decrease in workability and an increase in cost. When adding Mo, it is desirable to carry out in the range of 0.50% or less, and you may manage in the range of 0.15% or less.

  Cu is an austenite-forming element and is effective for preventing ferrite crystal grain coarsening during hot rolling, and therefore can be added as necessary. It is more effective to secure a Cu content of 0.01% or more. Excessive Cu content causes a decrease in corrosion resistance and workability. When adding Cu, it is desirable to carry out in the range of 0.50% or less, and you may manage in the range of 0.15% or less.

  Nb and V are effective for fixing and increasing the purity of C in steel and increasing the Ar value, and Co is effective for improving corrosion resistance and toughness and increasing the purity by fixing C in steel. Depending on the, one or more of these elements can be added. However, if these elements are added excessively, the cost increases and the workability decreases due to hardening, so the Nb content is 0.10% or less, the V content is 0.20% or less, and the Co content is 0. It is desirable to set it in the range of 10% or less.

  About P and S mixed as an unavoidable impurity, there is no problem as long as the content range is P: 0.050% or less and S: 0.030% or less, as in the conventional general ferritic stainless steel.

The γmax value determined by the following formula (1) is an index for estimating the amount of austenite (% by volume) from the component composition when isothermally maintained at 1100 ° C. to reach an equilibrium state. When the γmax value is 100 or more, it is estimated that the maximum austenite amount of the steel is 100%.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-52Al + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1), and 0 (zero) is substituted for the element not contained.

  In the present invention, steel in which the content of each component element is adjusted so that the γmax value falls within the range of 20.0 to 44.0 is used. According to the studies by the inventors, the pulverization of ferrite colonies during hot rolling leads to a significant improvement in ridging resistance. In the case of steel with γmax less than 20.0, the above-mentioned colony crushing effect may not be fully enjoyed because the amount of austenite phase generated at high temperatures is small. On the other hand, if the γmax value exceeds 44.0, the amount of austenite phase generated increases, and the pulverization effect of the ferrite colonies increases. In some cases, the entire austenite phase cannot be decomposed. The austenite phase that remains without being decomposed transforms into a hard martensite phase and remains in the hot-rolled steel sheet, causing deterioration of workability, and improving ridging resistance and workability at a time by hot rolling. It becomes difficult.

The Ar value determined by the following formula (3) is an index for estimating the austenite-ferrite transformation point (Ar ₁ transformation point) (° C.) from the component composition when the temperature falls from the temperature range where the austenite phase is stably present.
Ar = -250C + 73Si-66Mn-115Ni + 35Cr + 60Mo-18Cu + 620Ti + 750Al-280N + 310 (3)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (3), and 0 (zero) is substituted for the element not contained.

In order to significantly improve ridging resistance and workability, it is very effective to perform finish hot rolling in a temperature range from the recrystallization temperature to the Ar ₁ transformation point. Since the temperature range below the Ar ₁ transformation point is a temperature range where the austenite phase does not exist stably, the decomposition reaction from the austenite phase to the ferrite phase and carbide is promoted. The higher the Ar ₁ transformation point is, the higher the finishing rate of the hot diffusion at a high diffusion rate becomes, which is advantageous not only in improving ridging resistance but also in productivity. It is preferable to apply steel having a chemical composition in which the Ar value in the formula (3) is adjusted to 870 or more. There is no need to adjust to an excessively high Ar value. For example, the Ar value may be adjusted within a range of 1100 or less or 1050 or less.

[Metal structure]
The ferritic stainless steel sheet of the present invention exhibits a metal structure in which carbide particles are dispersed in a matrix composed of recrystallized ferrite crystal grains. If the matrix is not sufficiently recrystallized, or if the martensite phase remains, the workability deteriorates. The plate thickness of this steel plate is, for example, 0.4 to 3.0 mm.

(Crystal orientation ratio)
The ferritic stainless steel sheet of the present invention exhibits excellent ridging resistance that is evaluated by a strict standard that takes into account both the height of surface irregularities (waviness curves) and the wavelength. In the steel sheet exhibiting such excellent ridging resistance, it is important that the orientation of each ferrite crystal grain is randomized in addition to the disappearance of the ferrite crystal colony as described above. As an index of randomization, the present invention defines an _RND value determined by the following equation (2).
R _ND = S ₁₁₁ / (S ₀₀₁ + S ₀₁₁ ) (2)

Here, S ₀₀₁ is the plate thickness direction the crystal orientation of the ferrite crystal measured by the cross-section perpendicular to the thickness direction (ND plane) EBSD (electron backscatter diffraction method), within 15 ° from the <001> direction It is the area ratio of a certain crystal grain in the ND plane.
Similarly, S ₀₁₁ is the area ratio of the crystal grains with the plate thickness direction crystal orientation of the ferrite crystal measured by EBSD on the ND plane within 15 ° from the <011> direction in the ND plane.
Similarly, S ₁₁₁ is an area ratio of crystal grains having a plate thickness direction crystal orientation of the ferrite crystal measured by EBSD on the ND plane within 15 ° from the <111> direction in the ND plane.
The measurement area of the ND surface by EBSD may be a total of 10 mm ² or more in one or a plurality of fields set at random.

According to the study by the inventors, in order to improve the ridging resistance evaluated by both the height of the undulation and the wavelength, the crystal orientation ratio in which the R _ND value of the above formula (2) is 2.8 or less, It is effective to do. Such a cold-rolled annealed steel sheet has a ratio of crystal grains in which the <111> direction is close to the plate thickness direction as compared with a ferritic stainless steel cold-rolled annealed steel sheet manufactured in a conventional general process. The ratio of crystal grains that are small and the <001> direction and the <011> direction are close to the plate thickness direction is large. In particular, the R _ND value is more preferably 2.0 or less. However, even if the _ND value is excessively low, the effect of improving the ridging resistance commensurate with it is considered to be small. The R _ND value may be adjusted within a range of, for example, 1.0 or more. In order to obtain a low _RND value, it is effective to follow the hot rolling conditions described later. Furthermore, if hot-rolled sheet annealing is further performed, it can be adjusted to a particularly low _RND value of 2.0 or less, or 1.8 or less.

[Processability]
As described above, the ferritic stainless steel sheet according to the present invention has a metal structure whose matrix is composed of ferrite recrystallized grains, and exhibits good workability. The elongation at break (JIS Z2241: 2011) in the rolling direction by a JIS No. 13B tensile test piece is, for example, 26.0% or more. In particular, in a steel sheet R _ND values described above is controlled in a very randomized tissue condition such that 1.8 or less, with remarkable anti-ridging property improvement effect, breaking elongation in the rolling direction (JIS Z2241: 2011 ) Can achieve excellent ductility of 30.0% or more.

The ferritic stainless steel sheet according to the present invention is well maintained in terms of in-plane anisotropy, and is compatible with excellent ridging resistance. Specifically, the in-plane anisotropy Δr (JIS Z2254: 2008) of the following formula (4) measured using a JIS No. 13B tensile test piece under the conditions of a gauge distance of 25 mm and a strain amount of 15% is 1.10. The following are cleared, and in many applications, a processed product having a good surface property can be obtained without causing a problem due to the in-plane anisotropy of the r value.
Δr = (r _L −2r _D + r _T ) / 2 (4)
Here, r _L , r _D, and r _T are r values (plastic strain ratios) measured by collecting test pieces in parallel, 45 ° direction, and 90 ° direction with respect to the rolling direction of the plate surface.

  In addition, a steel sheet having a small Δr of 0.70 or less can be obtained by adopting a process of omitting hot-rolled sheet annealing after hot rolling described later. Thus, a steel sheet having a small r-value in-plane anisotropy is suitable for application to a processed product having large deformation elements such as overhang and deep drawing.

(Ridging resistance)
The ferritic stainless steel sheet according to the present invention has excellent ridging resistance that is evaluated according to a strict standard in consideration of both the height of surface irregularities and the wavelength. As a specific evaluation index, a ridging index “Wa × WSm value” defined in (A) below can be adopted.
(A) Using a JIS No. 5 tensile test specimen collected so that the longitudinal direction is the rolling direction, the tensile strain was applied until the elongation at the parallel portion was 20%, and then the load was gradually reduced. Five measurement lines in the direction perpendicular to the rolling direction are defined on the parallel part of the test piece at intervals of 5 mm, a surface profile having a reference length of 20 mm is measured on each measurement line in accordance with JIS B0601: 2013, a cutoff value λf = 8.0 mm, An undulation curve having a wavelength component of 0.5 to 8.0 mm is defined with λc = 0.5 mm, and an arithmetic average undulation Wa (μm) and an average length WSm (μm) of the undulation curve element are calculated from the undulation curve for every five measurement lines. ) And the product of the Wa average addition value Wa _(AVE) and the WSm addition average value WSm _(AVE) in each measurement line, Wa _(AVE) × WSm _(AVE) is calculated. This test was performed on three test pieces, and the total of six (three test pieces × both sides) Wa _(AVE) × WSm _(AVE) averages was calculated as “Wa × WSm” which is the ridging index of the steel sheet. Value ”.

The steel sheet of the present invention in which the above the R _ND value of 2.8 or less, Wa × WSm value can be realized ridging resistance as the 12000 or less. Such a steel sheet can be evaluated as having excellent ridging resistance on a strict standard in consideration of both the height of the surface irregularities and the wavelength. Especially in steel with controlled R _ND value to 1.8 or less, Wa × WSm value can be realized stably very good ridging resistance of 8500 or less.

〔Manufacturing process〕
The ferritic stainless steel sheet according to the present invention can be manufactured, for example, by the following process.
Slab heating-> hot rolling-> (hot-rolled sheet annealing)->cold-rolling-> finish annealing Among the above, the "hot-rolled sheet annealing" process is selective especially when emphasizing the realization of excellent ridging resistance Can be adopted. In addition, although description of "pickling" is abbreviate | omitted in the said process, pickling is suitably implemented according to a conventional method.

  In order to obtain the above-described structure state (metal structure and crystal orientation ratio), it is necessary to devise slab heating and hot rolling processes. Moreover, hot-rolled sheet annealing can be performed as needed. For other steps, conventional general conditions may be applied. Hereinafter, slab heating, hot rolling, and hot rolled sheet annealing will be described.

[Casting heating]
The slab heating before hot rolling is desirably performed in a temperature range of 1050 to 1150 ° C. If the slab heating temperature is too high, the amount of austenite when extracted from the slab heating furnace decreases. Since the austenite phase is harder than the ferrite phase, the larger the amount of austenite phase during rolling, the easier the introduction of strain into the ferrite phase is facilitated, which is advantageous for the disappearance of ferrite crystal colonies. Therefore, it is advantageous for improving the ridging resistance that the slab heating temperature is not set too high. On the other hand, if the slab heating temperature is too low, the deformation resistance increases and the rolling load increases.

(Hot rolling)
Hot rolling can be performed using a roughing mill and a finish rolling mill. The finish rolling mill is a hot rolling mill that can be rolled down to the final target thickness of the hot rolled steel sheet. Hereinafter, hot rolling performed with a rough rolling mill is referred to as “rough rolling”, and hot rolling performed with a finishing rolling mill is referred to as “finish hot rolling”. Here, finishing hot rolling is performed in a temperature range equal to or higher than the recrystallization temperature and equal to or lower than the temperature represented by the Ar value (° C.) in the formula (3), thereby improving ridging resistance and workability. For that purpose, it is very effective to use a reverse hot rolling machine having a coiler furnace and to heat between the recrystallization temperature and the temperature represented by the Ar value (° C.) between the finishing hot rolling passes. . The temperature range below the Ar value corresponds to a temperature range where the austenite phase cannot exist stably in an equilibrium manner. By repeating the reduction and heating in this temperature range, the decomposition reaction from the austenite phase to the ferrite phase and carbide is promoted. Further, by finishing the finish hot rolling at the recrystallization temperature or higher, the matrix becomes a recrystallized ferrite crystal. By this method, a hot-rolled steel sheet having a microstructure in which carbides are precipitated in the recrystallized ferrite crystal can be obtained. In addition, the recrystallization temperature of the target steel type of the present invention is around 800 ° C. Therefore, for example, if the finish hot rolling is performed in a temperature range of 850 ° C. or higher, the requirement of “hot rolling at a recrystallization temperature or higher” can be sufficiently satisfied. Therefore, the rolling temperature and the heating temperature between passes in the finish hot rolling may be managed within a range of, for example, 850 ° C. or more and a temperature (° C.) or less corresponding to the Ar value in the equation (2). The total rolling rate in finish hot rolling can be 55 to 90%, for example, and the number of rolling passes in finish hot rolling can be 5 to 7 passes, for example. Moreover, it is preferable that the time required from the start of the first pass of finishing hot rolling to the end of the final pass is in the range of, for example, 5 to 15 minutes including the holding time of the heating between passes. What is necessary is just to adjust the plate | board thickness of a hot-rolled steel plate in the range of 2.0-8.0 mm, for example.

  The hot rolling method disclosed here causes the austenite phase to be directly decomposed into a ferrite phase and carbide during hot rolling. When using the hot-rolled steel sheet obtained in this way, the randomization of the crystal orientation in the cold-rolled annealed steel sheet is increased compared to the conventional general method for precipitation of carbides by annealing after hot rolling, and the waviness curve The excellent ridging resistance on a strict standard as evaluated by the product of the parameter relating to the height of the light and the parameter relating to the wavelength can be remarkably improved. In general finish hot rolling using a tandem mill, there is no time to sufficiently proceed the decomposition reaction from the austenite phase to the ferrite phase and carbide.

  Patent Document 1 described above discloses a method of performing finish hot rolling at 850 ° C. or higher while performing heating between passes. However, steel that does not contain Al or Ti is targeted, and the temperature range of 850 ° C. or higher is considered to correspond to the region in which the austenite phase stably exists for the steel disclosed in Patent Document 1. Therefore, the austenite phase cannot be directly decomposed into a ferrite phase and carbide during hot rolling. Moreover, in this literature, the Nb addition steel which does not contain B is made into object, and applicable steel types differ. With the technique of Patent Document 1, it is difficult to realize the excellent ridging resistance intended by the present invention.

[Hot rolled sheet annealing]
Hot-rolled sheet annealing can be performed as necessary. Above R _ND value is for example 2.0 or less, or even to obtain a cold-rolled annealed steel sheet, such as taking a particularly low value such as 1.8 or less, very good as Wa × WSm values described above for example 8500 or less resistant When aiming at ridging, it is desirable to perform hot-rolled sheet annealing. Long-term batch annealing may be performed, but a large effect can be obtained by short-time annealing in a continuous annealing pickling line. In the batch annealing, for example, a condition of holding for 9 to 24 hours at a temperature represented by 830 ° C. or higher and Ar value (° C.) can be applied. In the continuous annealing, for example, a condition of holding for 0 to 60 seconds at a predetermined temperature set in a range of 870 ° C. or more and a temperature represented by an Ar value (° C.) is applicable. The holding time of 0 seconds means a heat pattern that cools immediately after reaching a predetermined temperature.

On the other hand, when realizing excellent ridging resistance in which the above Wa × WSm value is 12000 or less, for example, and also focusing on the reduction of the in-plane anisotropy Δr of the r value (for example, Δr is controlled to a small value of 0.70 It was found that it is advantageous to omit the hot-rolled sheet annealing and proceed to the cold rolling process as it is. By applying the hot rolling method described above to the steel having the chemical composition defined in the present invention, the decomposition from the austenite phase to the ferrite phase and carbide proceeds sufficiently, and recrystallization is also promoted. However, the hot-rolled sheet at this stage (as hot) stores strain caused by hot rolling to some extent in comparison with a hot-rolled annealed steel sheet that has been subsequently subjected to hot-rolled sheet annealing. By positively using this strain, the in-plane anisotropy of the r value can be more effectively reduced. That is, when hot-rolled sheet annealing is omitted, the final finish annealing is performed in a state in which processing strain introduced by cold rolling is added to strain resulting from the above hot rolling. . It is presumed that this may contribute to the formation of crystal orientation (the above R _ND value exceeds 1.8 and 2.8 or less) advantageous for reducing the in-plane anisotropy Δr of the r value. .

  Steels shown in Tables 1A and 1B were melted to obtain continuous cast slabs (plate thickness of about 200 mm). After heating this with a slab heating furnace, it was taken out of the furnace, subjected to rough rolling and finish hot rolling, and wound to obtain a hot rolled steel sheet. The slab (continuous cast slab) was heated at the slab heating temperature shown in Tables 2 and 3 for a holding time of 120 to 300 minutes. After a rough rolled material having a plate thickness of 18 to 23 mm by rough rolling, finish hot rolling was performed. The finishing mill is a reverse type except for Comparative Example B23, and is provided with a furnace (coiler furnace) that winds and holds a steel plate between each rolling pass on both sides of the mill. The total number of passes for finishing hot rolling was 5 to 7 passes, and heat treatment was performed between each pass with a coiler furnace. The heating temperature is shown in Tables 2 and 3. The heating temperature between each pass was made uniform. In Comparative Example B23, finish hot rolling was performed by a tandem mill. In any example, after finishing the final pass of the finish hot rolling, the film was wound up without being subjected to heat treatment and allowed to cool in the air. Thus, a hot rolled steel sheet having a thickness of 2.0 to 8.0 mm was obtained.

  A sample was taken from the obtained hot-rolled steel sheet, and the structure observation and hardness measurement of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction were performed. It is checked whether the ferrite crystal grains are recrystallized by observing the metallographic structure with an optical microscope. The recrystallized one is ○ (recrystallized: accepted), and the others are x (recrystallized: rejected). evaluated. The hardness test was performed in accordance with JIS Z2244: 2009, and the hardness of the hot-rolled steel sheet was defined as an average value of 10 hardness measurement values at positions randomly determined in the L cross section. Even if the ferrite phase is recrystallized, the hardness increases when the martensite phase is present. The results are shown in Tables 2 and 3.

  Subsequently, in some examples, the hot-rolled steel sheet was subjected to hot-rolled sheet annealing using a batch annealing furnace or a continuous annealing furnace. The presence or absence of hot-rolled sheet annealing or hot-rolled sheet annealing conditions are shown in Tables 2 and 3. Next, pickling, cold rolling, finish annealing (870 ° C. × 5 seconds), and pickling were sequentially performed to obtain a cold-rolled annealed steel plate having a thickness of 0.7 mm. Samples were taken from cold-rolled annealed steel sheets and the following investigations were performed.

[Metal structure]
The L structure was observed for the metal structure with an optical microscope. As a result, it was confirmed that all of the cold-rolled annealed steel sheets of the present invention exhibited a metal structure in which carbide particles were dispersed in a matrix composed of recrystallized ferrite crystal grains.

(Crystal orientation ratio)
With respect to the cross section (ND plane) perpendicular to the plate thickness direction, the ferrite crystal orientation in the plate thickness direction was measured by EBSD (electron beam backscatter diffraction method) using a crystal orientation analysis system (PEGASUS2300) manufactured by TSL. The measurement surface was a smooth surface prepared by polishing the rolled surface with SiC paper (grain sizes P180, P320, P500 and P1000 defined in JIS R6010: 2000) by 0.2 mm and then buffing. Three measurement fields of 2 mm × 2 mm were selected at random, and each field was irradiated with an electron beam with a step size (measurement pitch) of 0.5 μm to create a BCC crystal orientation map based on the 001 reverse pole figure. In the crystal orientation map, regions within 15 ° from the <001>, <011>, and <111> directions of the ferrite crystal can be distinguished. The area of each of the above regions was calculated by image analysis software. From the calculation results in each field of view, the total area of the regions within 15 ° from each of the <001>, <011>, and <111> directions is obtained, and the R _ND value is calculated by substituting them into the equation (2) Determined. The results are shown in Tables 2 and 3.

(Ridging resistance)
A JIS No. 5 tensile test piece was collected from each cold-rolled annealed steel sheet so that the longitudinal direction was the rolling direction. A tensile strain was applied to the test piece until the elongation at the parallel portion became 20%, and then the load was gradually loaded to obtain a ridging measurement sample. Five measurement lines in the direction perpendicular to the rolling direction (that is, the direction perpendicular to the longitudinal direction of the test piece) were determined at intervals of 5 mm on the parallel portion of the ridging measurement sample. Using a contact type surface roughness meter (manufactured by Tokyo Seimitsu Co., Ltd .; SURFCOM 2900DX), a surface profile having a reference length of 20 mm is measured on each measurement line in accordance with JIS B0601: 2013, and a cutoff value λf = 8.0 mm, λc = Wave curve with wavelength component of 0.5 to 8.0 mm was defined as 0.5 mm. By setting λf = 8.0 mm, the swell component due to the plate warpage is removed. The arithmetic average waviness Wa (μm) and the average length WSm (μm) of the waviness curve elements were determined from the waviness curves for the five measurement lines. The product of the addition average value Wa _{(AVE) of} the five Wa obtained in each measurement line and the addition average value WSm _{(AVE) of} the five WSm obtained in the same measurement line, Wa _(AVE) × WSm _{( AVE)} was defined as a ridging index value on the measurement surface. The same operation was performed on the opposite surface (the back surface of the measurement surface) of the same ridging measurement sample, and the ridging index value on the opposite measurement surface was obtained. Three ridging samples were prepared for each cold-rolled annealed steel sheet, and the ridging index values were calculated for both surfaces of each sample. In this way, a total of 6 (3 test pieces × both sides) ridging index values were determined for each cold-rolled annealed steel sheet. Then, the ridging index value of the cold-rolled annealed steel sheet was determined by averaging the six ridging index values. Hereinafter, the ridging index value obtained in this way is referred to as “Wa × WSm value”. According to the study by the inventors, if the Wa × WSm value taking into account both the height of the waviness and the wavelength is 12000 or less, it is possible to obtain a smooth surface appearance with high design properties in various processed products. It can be evaluated that it exhibits ridging properties. Therefore, a Wa × WSm value of 12000 or less was determined to be acceptable. Especially, it can be evaluated that the thing of WaxWSm value is 8500 or less has very excellent ridging resistance. The results are shown in Tables 2 and 3. In Tables 2 and 3, ◎ is added to those having a Wa × WSm value of 8500 or less, ○ is added to those having a value of 8500 and 12000 or less, and × is added to the others.

(Weldability)
The steel sheet surface was subjected to TIG tanning welding under the conditions of an electrode of 1.6 mmφ, a welding current of 60 A, and a speed of 10 mm / second, and a color check (penetration flaw inspection) was performed on the welded portion. The case where no crack was observed was judged as ◯ (weldability: good), and the others were judged as x (weldability; poor). The results are shown in Tables 2 and 3.

[Processability]
(Elongation at break)
A JIS No. 13B tensile test piece was collected from each cold-rolled annealed steel sheet, subjected to a tensile test in the rolling direction in accordance with JIS Z2241: 2011, and the elongation at break (%) was measured from the test piece after breakage. Those having an elongation at break of 30% or more were judged as ◎ (ductility; excellent), those with 26% or more were judged as ○ (ductility; good), and others were judged as × (ductility; poor).

(In-plane anisotropy)
From each cold-rolled annealed steel sheet, JIS13B tensile test specimens whose longitudinal direction is parallel (L direction), 45 ° direction (D direction), and 90 ° direction (T direction) with respect to the rolling direction of the plate surface were collected. These specimens were subjected to a tensile test according to JIS Z2241: 2011 by applying a strain with a gauge distance of 25 mm and a strain amount of 15%, and the in-plane anisotropy Δr of the r value specified in JIS Z2254: 2008 was 4) Obtained by the equation. When Δr is 0.70 or less, ◎ (in-plane anisotropy; excellent), when it exceeds 0.70 and is 1.10 or less, ○ (in-plane anisotropy; good), and other than x ( In-plane anisotropy; poor).
These results are shown in Tables 2 and 3.

All of the examples of the present invention are austenite during hot rolling by performing rolling while performing interpass heating in a temperature range where the austenite phase does not exist stably (temperature range lower than the Ar value) in finish hot rolling. The decomposition reaction from the phase to the ferrite phase and carbide is sufficiently promoted. In cold-rolled annealed steel sheets derived from these hot-rolled steel sheets, the crystal orientation ratio _RND value represented by the formula (2) is low, and randomization of ferrite crystal orientations is progressing. As a result, excellent ridging resistance was obtained. All of these are good in ductility and in-plane anisotropy of the r value, and both ridging resistance and workability on a strict evaluation standard are realized. Also, R _ND value is example that a hot-rolled sheet annealing is very low, ridging resistance with it is more is more improved. In the example in which hot-rolled sheet annealing is omitted, the effect of improving the in-plane anisotropy of the r value is great.

  In Comparative Examples B1 and B5, the heating temperature between passes in the finish hot rolling is low, so that the decomposition reaction from the austenite phase to the ferrite phase and carbide and recrystallization cannot be sufficiently achieved during hot rolling, and lysine resistance The improvement in sex was insufficient. Further, the austenite phase that could not be decomposed remained as a martensite phase in the hot-rolled steel sheet, and the workability could not be sufficiently improved even by the subsequent cold rolling and annealing. Since B2 has a high slab heating temperature, the amount of austenite of the rough rolled material used for finish hot rolling is small, and in finish hot rolling, the effect of introducing strain into the ferrite phase due to the coexistence of the hard austenite phase was small. Therefore, the disappearance of the ferrite crystal colony was insufficient and the ridging resistance was poor. B3 is an example in which heating between passes in finish hot rolling is raised to the austenite phase stable temperature range (temperature higher than the Ar value). In this case, since the finish hot rolling temperature was high, the disappearance of the ferrite crystal colony and the recrystallization of the ferrite phase in the finish hot rolling proceeded, and the effect of improving ridging resistance was obtained. However, the austenite phase that could not be decomposed during finish hot rolling remained as a martensite phase in the hot-rolled steel sheet, and the workability could not be sufficiently improved even by subsequent cold rolling and annealing. Since B4 had a high slab heating temperature and a small amount of austenite phase during extraction, the pulverization of ferrite colonies was not promoted and ridging resistance was poor. Moreover, since the heating temperature between passes in the finish hot rolling was higher than the Ar value, the martensite phase remained in the hot rolled steel sheet, and the workability decreased.

B6 does not contain B, and B7 has a low B content. These become insufficient finely dispersed austenite phase when both slab heating, R _ND value orientation can not be sufficiently eliminated in the ferrite crystal is large in cold-rolled annealed steel sheet. As a result, the ridging resistance was poor. In B6, the in-plane anisotropy of the r value was also large. Since B8 had a high B content, it caused welding hot cracking. Since B9, B10, B11, B14, B16, B17, B19, B20, and B22 all have high γmax values, they are hot-rolled in a state where there is a large amount of austenite phase, and the effect of eliminating the ferrite crystal colony is large. Therefore, the effect of improving ridging resistance was recognized. However, the martensite phase remained in these, and compatibility with good workability was not achieved. Since all of B12, B13, B15, B18, and B21 have a low γmax value, the hot rolling was performed in a state where the coexistence amount of the austenite phase was small, and the ridging resistance was not sufficiently improved. Moreover, the improvement of the in-plane anisotropy of the r value was insufficient. Among these, since B15 had high Si content, workability was also bad. B23 employs a finish hot rolling process by a general tandem mill. In this case, it is impossible to heat and hold the steel plate above the recrystallization temperature in the finish hot rolling, and it is difficult to sufficiently proceed the recrystallization of the ferrite phase during rolling. A coarse band-like structure was exhibited without conversion. As a result, in the cold-rolled annealed steel sheet, the ferrite crystal orientation was not sufficiently randomized and the ridging resistance was poor. Further, the in-plane anisotropy of the r value was large.

For reference, FIGS. 1 to 4 show Example A1 (steel No. 1), Example A2 (steel No. 1), Comparative example B6 (steel No. 13), and Comparative example B23 (steel No. 1), respectively. .38) illustrates a crystal orientation map based on the 001 reverse pole figure by EBSD measurement of a cross section (ND plane) perpendicular to the plate thickness direction. In these crystal orientation maps, the region where the crystal orientation of the ferrite crystal is within 15 ° from the <111> direction is black, and the region where the crystal orientation of the ferrite crystal is within 15 ° from the <001> direction The area within 15 ° from the <011> direction is displayed in gray, and the other areas are displayed in white. The scale in the figure is 700 μm long. A value obtained by dividing the area of the black region by the area of the gray region corresponds to the _RND value of the above equation (2). In the figure, the values of S ₀₀₁ , S ₀₁₁ and S _{111 on} the right side of equation (2) are also shown. In the example of the present invention (FIGS. 1 and 2), compared to the comparative example (FIGS. 3 and 4), the area ratio of the gray area decreases and the area ratio of the black area increases. That is, it can be seen that the examples of the present invention randomize the orientation (orientation) of the ferrite crystal.

Claims (4)

In mass%, C: 0.020 to 0.150%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.00%, Ni: 0.01 to 0.60%, Cr: 11.00 to 19.00%, N: 0.000 to 0.050%, B: 0.0025 to 0.0100%, Al: 0 to 0.200%, Ti: 0 to 0.180%, Mo : 0 to 0.50%, Cu: 0 to 0.50%, Nb: 0 to 0.10%, Co: 0 to 0.10%, V: 0 to 0.20%, of which Al: It contains at least one selected from the group of 0.050 to 0.200%, Ti: 0.050 to 0.180%, and consists of the balance Fe and unavoidable impurities, and the γmax value determined by the following formula (1) is It has a chemical structure of 20.0 to 44.0, and has a metal structure in which carbide particles are dispersed in a matrix composed of recrystallized ferrite crystal grains. Have a more defined crystal orientation ratio R _ND value is 2.8 or less, elongation at break in the rolling direction by JIS13B No. Tensile test pieces (JIS Z2241: 2011) and a 26.0% or more, JIS13B No. Tensile test pieces A ferritic stainless steel sheet having an in-plane anisotropy Δr (JIS Z2254: 2008) of the following formula (4) measured under the conditions of a gauge distance of 25 mm and a strain amount of 15% using 1 .
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-52Al + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1), and 0 (zero) is substituted for the element not contained.
R _ND = S ₁₁₁ / (S ₀₀₁ + S ₀₁₁ ) (2)
Here, in S ₀₀₁ , S ₀₁₁ and S ₁₁₁ , the crystal orientation in the plate thickness direction of the ferrite crystal measured by EBSD (electron beam backscattering diffraction method) on the cross section (ND plane) perpendicular to the plate thickness direction is <001, respectively. > Direction, <011> direction, and <111> direction are the crystal grain area ratios within 15 °.
Δr = (r _L −2r _D + r _T ) / 2 (4)
Here, r _L , r _D, and r _T are r values (plastic strain ratios) measured by collecting test pieces in parallel, 45 ° direction, and 90 ° direction with respect to the rolling direction of the plate surface. Furthermore, the ferritic stainless steel plate of Claim 1 which has a chemical composition whose Ar value defined by following (3) Formula is 870 or more.
Ar = -250C + 73Si-66Mn-115Ni + 35Cr + 60Mo-18Cu + 620Ti + 750Al-280N + 310 (3)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (3), and 0 (zero) is substituted for the element not contained. Wherein R _ND value is 1.8 or less, elongation at break in the rolling direction by JIS13B No. Tensile test pieces (JIS Z2241: 2011) is ferritic stainless steel sheet according to claim 1 or 2 is 30.0% or more. The ferritic stainless steel sheet according to claim 1 or 2, wherein the in- plane anisotropy Δr (JIS Z2254: 2008) is 0.70 or less .

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