EP3878993A1

EP3878993A1 - Ferritic stainless steel sheet

Info

Publication number: EP3878993A1
Application number: EP18939476.0A
Authority: EP
Inventors: Ken Kimura; Shinichi Tamura; Chikara Itoh; Atsushi Taguchi; Eiichiro Ishimaru; Yuji Kaga; Keiichi Oomura; Akihito Yamagishi
Original assignee: Nippon Steel Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2018-11-09
Filing date: 2018-11-09
Publication date: 2021-09-15
Also published as: KR102515016B1; BR112021000940B1; KR20210034054A; CN112513303A; CN112513303B; WO2020095437A1; BR112021000940A2; EP3878993A4; WO2020095437A8

Abstract

This ferritic stainless steel sheet contains Cr: 11.0% or more and 25.0% or less, C: 0.001 % or more and 0.010% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01 % or more and 1.0% or less, P: 0.10% or less, S: 0.01% or less, and N: 0.002% or more and 0.020% or less, and either one or both of Ti: 1.0% or less and Nb: 1.0% or less, with a remainder of Fe and impurities. The ferritic stainless steel sheet has a ferrite single-phase microstructure in which the grain size number is larger than 9.0, and the random intensity ratios of crystal orientations in planes parallel to a rolled surface at a sheet thickness 1/2 position and a sheet thickness 1/10 position are I_{554}<225> ≥ 7.0, I_{411}<148> ≥ 0.9, and I_{211}<011> ≥ 1.0.

Description

TECHNICAL FIELD

The present invention relates to a ferritic stainless steel sheet, and particularly the present invention relates to a ferritic stainless steel sheet which is excellent in terms of formability when formed and a surface characteristic after formation.

BACKGROUND ART

Austenitic stainless steel including SUS304 (18Cr-8Ni), which is a representative kind of steel, is excellent in terms of corrosion resistance, workability, beautiful appearance, and the like; and therefore, the austenitic stainless steel is widely used in home electric appliances, kitchenwares, building materials, and the like. However, the austenitic stainless steel contains a large amount of Ni, which is expensive and greatly fluctuates in price, and thus the prices of steel sheets are high, and there is a desire for less expensive steel sheets from the viewpoint of economic efficiency.
On the other hand, ferritic stainless steel contains no Ni or an extremely small amount of Ni, and thus, in recent years, the demand for ferritic stainless steel has been increasing as a material having an excellent cost performance. However, in a case where ferritic stainless steel is used for forming, there are problems of the forming limit and the deterioration of the surface characteristic attributed to the formation of surface irregularities after formation.
First, when the forming limits are compared, austenitic stainless steel has excellent stretchability, whereas ferritic stainless steel has poor stretchability and cannot be greatly deformed. However, the deep drawability can be controlled by adjusting the crystal orientation (texture) in the steel; and therefore, in a case where ferritic stainless steel is used for forming, a forming method mainly involving deep drawing is often used.
Next, the surface characteristic after formation (surface irregularities) will be described. In this regard, the term "surface irregularities" refer to fine irregularities (surface roughening) generated on the surface of a steel sheet after working or forming. These fine irregularities correspond to crystal grains, and, as the crystal grain size is larger, surface irregularities become more prominent.
In the case of austenitic stainless steel, the work hardening property is excellent, and a fine-grained microstructure is relatively easy to generate. Therefore, steel sheets having a grain size number of approximately 10 are manufactured. For this reason, surface irregularities (surface roughening) after formation are small, which rarely causes any problem. On the other hand, the crystal grain size of ferritic stainless steel is approximately nine in SUS430 and approximately seven in SUS430LX, which are small compared with the crystal grain size of austenitic stainless steel. A small grain size number indicates a large crystal grain size.
A reason for ferritic stainless steel to be likely to have coarse grains is that, in ferritic stainless steel, the recrystallized grain size is likely to be large, and, particularly, in high-purity ferritic stainless steel having workability and formability that are improved by reducing the amount of C and N such as SUS430LX, crystal grains are likely to grow; and the crystal grain size tends to be large compared with the crystal grain size of austenitic stainless steel.
In a case where relatively strict formability is required as in the housings or vessels of home electric appliance products, high-purity ferritic stainless steel such as SUS430LX is often used as ferritic stainless steel. In addition, typically, the sheet thicknesses of stainless steel sheets used to ensure strength after formation are 0.6 mm or more in the majority of cases. However, since the crystal grain size is large as described above, surface roughening after formation is significant, and it is common to remove surface irregularities by polishing after formation.
In view of the above-described background, methods for mitigating the surface roughening of high-purity ferritic stainless steel have been disclosed.
Patent Document 1 discloses ferritic stainless steel in which the sizes of precipitated particles and the crystal grain sizes are controlled using high-purity ferritic stainless steel to reduce working surface roughening and to improve formability and a manufacturing method thereof. However, in the method described in Patent Document 1, although a steel sheet having small crystal grain sizes is obtained, there is a problem in that the deep drawability during forming is not sufficient and surface roughening after formation is likely to occur regardless of the fact that the crystal grain sizes are small.
Patent Document 2 discloses a technique for manufacturing stainless steel having excellent resistance to surface roughening during forming in which ferritic stainless steel containing Ti and Nb is subjected to hot rolling at a low temperature and a high cold rolling ratio is selected to refine grains. With the above-described technique, the stainless steel sheet described in Patent Document 2 obtains a fine-grained microstructure having a grain size number of 9.5, but the resistance to surface roughening after cup drawing forming is not always sufficient.
Patent Document 3 discloses ferritic stainless steel in which deep drawability, a ridging property, and resistance to surface roughening are improved by controlling the crystal grain size of steel having a composition containing Nb and/or Ti before final cold rolling. However, the crystal grain size of the final product is 15 µm (the grain size number is 8.8), and the resistance to surface roughening is not sufficient.
In addition, in the related art, in order to reduce the surface irregularities of high-purity ferritic stainless steel, a method for reducing surface irregularities was studied in which surface irregularities was reduced by increasing the number of times of cold rolling at the time of manufacturing ferritic stainless steel sheet to decrease the crystal grain size. However, there is a case where surface irregularities are generated on a product sheet in reality, the cause therefor is not clear, and there is a desire for a technique capable of stably maintaining the high quality of the steel sheet surface.
As described above, in the case of considering the forming of ferritic stainless steel, the current status is that it is extremely difficult to form ferritic stainless steel into a predetermined shape and to satisfy the surface characteristic after formation. Therefore, at the moment, in order for ferritic stainless steel to ensure formability, it is necessary to perform a polishing step to remove surface irregularities generated after formation. However, polishing takes time, increases the manufacturing cost, and furthermore, generates a large amount of dust, which also creates an environmental issue.

PRIOR ART DOCUMENTS

Patent Document

Patent Document 1: Japanese Patent No. 4749888
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H7-292417
Patent Document 3: Japanese Patent No. 3788311

Non Patent Document

Non Patent Document 1: R. K. Ray, J. J. Jonas, and R. E. Hook: International Materials Reviews. vol. 39, No. 4 (1994), p 131
Non Patent Document 2: Hotaka Homma, Shuichi Nakamura, Naoki Yoshinaga: Iron and Steel, vol. 90, No. 7 (2004), p510 to 517
Non Patent Document 3: "Texture", written and edited by Shin-ichi Nagashima, Maruzen (1984), p23

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

The present invention has been made in view of the above-described problem and the present invention provides a ferritic stainless steel sheet which is excellent in terms of forming workability and the surface characteristic after formation.

Solutions for Solving the Problems

The features of one aspect of the present invention are as follows.

[1] A ferritic stainless steel sheet contains, by mass%, Cr: 11.0% or more and 25.0% or less, C: 0.001% or more and 0.010% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.10% or less, S: 0.01% or less, N: 0.002% or more and 0.020% or less, and either one or both of Ti: 1.0% or less and Nb: 1.0% or less, with a remainder of Fe and impurities, in which the ferritic stainless steel sheet has a ferrite single-phase microstructure in which a grain size number is larger than 9.0, and random intensity ratios of crystal orientations in planes parallel to a rolled surface at a sheet thickness 1/2 position and a sheet thickness 1/10 position are I_{554}<225> ≥ 7.0, I_{411}<148> ≥ 0.9, and I_{211}<011> ≥ 1.0.
I_{hkl}<uvw> indicates a random intensity ratio of an {hkl}<uvw> orientation.
[2] The ferritic stainless steel sheet according to [1] further contains, by mass%, one or more of B: 0.0001% or more and 0.0025% or less, Sn: 0.005% or more and 0.50% or less, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 2.0% or less, Al: 1.0% or less, W: 1.0% or less, Co: 0.50% or less, V: 0.50% or less, Zr: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, and Sb: 0.50% or less.

Effects of Invention

According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet which is excellent in terms of forming workability and the surface characteristic after formation.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of a ferritic stainless steel sheet of the present invention will be described.
The ferritic stainless steel sheet according to the present embodiment contains, by mass%, Cr: 11.0% or more and 25.0% or less, C: 0.001 % or more and 0.010% or less, Si: 0.01 % or more and 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.10% or less, S: 0.01% or less, N: 0.002% or more and 0.020% or less, and either one or both of Ti: 1.0% or less and Nb: 1.0% or less, with a remainder of Fe and impurities. The ferritic stainless steel sheet has a ferrite single-phase microstructure in which the grain size number is larger than 9.0, and the random intensity ratios of crystal orientations in planes parallel to a rolled surface at a sheet thickness 1/2 position (a position of 1/2 thickness) and a sheet thickness 1/10 position (a position of 1/10 thickness) are I_{554}<225> ≥ 7.0, I_{411}<148> ≥ 0.9, and I_{211}<011> ≥ 1.0.
Hereinafter, each feature will be described in detail.
First, the reason for limiting components will be described below. It should be noted that "%" for the amount of each element indicates "mass%".
Cr is an element that improves corrosion resistance, which is a basic characteristic of stainless steel. In a case where the amount of Cr is smaller than 11.0%, sufficient corrosion resistance cannot be obtained, and thus the lower limit is set to 11.0% or more. On the other hand, addition of an excessive amount of Cr promotes the generation of an intermetallic compound corresponding to a σ phase (intermetallic compound of Fe and Cr) to aggravate cracking during manufacturing, and thus the upper limit is set to 25.0% or less. From the viewpoint of stable manufacturability (yields, roll marks, and the like), the amount of Cr is desirably 14.0% or more and 22.0% or less. The amount of Cr is more desirably 16.0% or more and 20.0% or less.
C is an element that degrades formability (r value), which is important in the present embodiment. Therefore, the amount of C is preferably as small as possible, and the upper limit is set to 0.010% or less. However, excessive reduction leads to an increase in the refining cost, and thus the lower limit is set to 0.001% or more. In consideration of both refining cost and formability, the amount of C is preferably 0.002% or more and 0.008% or less and more preferably 0.002% or more and 0.006% or less.
Si is an element that improves oxidation resistance, but addition of an excessive amount of Si degrades formability. Therefore, the upper limit is set to 1.0% or less. From the viewpoint of formability, the amount of Si is preferably as small as possible. However, excessive reduction leads to an increase in the raw material cost, and thus the lower limit is set to 0.01% or more. From the viewpoint of manufacturability, the amount of Si is desirably 0.05% or more and 0.60% or less, and more desirably 0.05% or more and 0.30% or less.
Similar to Si, addition of a large amount of Mn also degrades formability, and thus the upper limit is set to 1.0% or less.
From the viewpoint of formability, the amount of Mn is preferably as small as possible. However, excessive reduction leads to an increase in the raw material cost, and thus the lower limit is set to 0.01% or more. From the viewpoint of manufacturability, the amount of Mn is desirably 0.05% or more and 0.40% or less, and more desirably 0.05% or more and 0.30% or less.
P is an element that degrades formability (r value and product elongation), and thus the amount of P is preferably as small as possible, and the upper limit is limited to 0.10% or less. However, excessive reduction leads to an increase in the raw material cost, and thus the lower limit is preferably set to 0.005% or more. In consideration of both formability and the manufacturing cost, the amount of P is preferably 0.007% or more and 0.030% or less, and more desirably 0.010% or more and 0.025% or less.
S is an unavoidable impurity element and aggravates cracking during manufacturing. Therefore, the amount of Si is preferably as small as possible, and the upper limit is limited to 0.01% or less. The amount of S is preferably as small as possible and desirably 0.0030% or less. On the other hand, excessive reduction leads to an increase in the refining cost, and thus the lower limit is desirably set to 0.0003% or more. From the viewpoints of manufacturability and cost, a preferable range is 0.0004% or more 0.002% or less.
Similar to C, N is an element that degrades formability (r value), and the upper limit is set to 0.020% or less. However, excessive reduction leads to an increase in the refining cost, and thus the lower limit is set to 0.002% or more. From the viewpoint of formability and manufacturability, a preferable range is 0.005% or more and 0.015% or less.
Either one or both of Ti and Nb are contained as follows.
Ti bonds to C and N and fixes C and N as precipitates such as TiC and TiN (that is, increases the purity); and thereby, formability (r value) and product elongation are improved. In order to obtain these effects, the lower limit is preferably set to 0.01% or more. On the other hand, addition of an excessive amount of Ti leads to an increase in the alloying cost and the degradation of manufacturability accompanying an increase in the recrystallization temperature, and thus the upper limit is set to 1.0% or less. From the viewpoint of formability and manufacturability, a preferable range is 0.05% or more and 0.50% or less. Furthermore, a preferable range where the above-described effects of Ti are positively utilized is 0.10% or more and 0.30% or less.
Similar to Ti, Nb is also a stabilizing element that fixes C and N and leads to improvement in formability (r value) and product elongation by increasing the purity of steel through the above-described action. In order to obtain these effects, the lower limit is preferably set to 0.01% or more. On the other hand, addition of an excessive amount of Nb leads to an increase in the alloying cost and the degradation of manufacturability accompanying an increase in the recrystallization temperature, and thus the upper limit is set to 1.0% or less. From the viewpoint of the alloying cost and manufacturability, a preferable range is 0.02% or more and 0.30% or less. Furthermore, a preferably range where the above-mentioned effects of Nb are positively utilized is 0.04% or more and 0.15% or less. The range is more desirably 0.06% or more and 0.10% or less.
The ferritic stainless steel sheet of the present embodiment may selectively contain, in addition to the above-described basic composition, one or more selected from the following element group.
B is an element that improves secondary workability. In order to exhibit the effect, the amount of B needs to be 0.0001% or more, and thus this amount is set as the lower limit. On the other hand, addition of an excessive amount of B leads to the deterioration of manufacturability, particularly, castability, and thus the upper limit is set to 0.0025% or less. A preferable range is 0.0002% to 0.0020%, and more preferably 0.0003% to 0.0012%.
Sn is an element having an effect of improving corrosion resistance, and thus Sn may be added depending on corrosive environment at room temperature. The effect is exhibited at an amount of Sn of 0.005% or more, and thus this amount is set as the lower limit. On the other hand, addition of a large amount of Sn leads to the deterioration of manufacturability, and thus the upper limit is set to 0.50% or less. In consideration of manufacturability, a preferable range is 0.01% to 0.20%, and more preferably 0.02% to 0.10%.
Ni, Cu, Mo, Al, W, Co, V, and Zr are elements effective for enhancing corrosion resistance or oxidation resistance and are added as necessary. However, there is a concern that addition of excessive amounts of these elements may lead not only to the degradation of formability but also to an increase in the alloying cost or the inhibition of manufacturability. Therefore, the upper limits of Ni, Cu, Al, and W are set to 1.0% or less. Mo brings about the degradation of manufacturability, and thus the upper limit is set to 2.0% or less. The upper limits of Co, V, and Zr are set to 0.50% or less. For all of the elements, more preferable lower limits of the amounts are set to 0.10% or more.
Ca and Mg are elements that improve hot workability or secondary workability and are added as necessary. However, addition of excessive amounts of Ca and Mg leads to the inhibition of manufacturability, and thus the upper limits of Ca and Mg are set to 0.0050% or less. Preferable lower limits are set to 0.0001 % or more. In consideration of manufacturability and hot workability, preferable ranges are 0.0002% to 0.0020%, and more preferably ranges are 0.0002% to 0.0010% for both Ca and Mg.
Y, Hf, and REM are elements effective for improving hot workability or the cleanliness of steel and for improving oxidation resistance and may be added as necessary. In the case of adding these elements, the upper limits are set to 0.10% or less for each of Y and REM and the upper limit is set to 0.20% or less for Hf. Preferable lower limits are 0.001 % or more for all of Y, Hf, and REM. "REM" in the present embodiment refers to elements belonging to atomic numbers of 57 to 71 (lanthanoid) which are, for example, Ce, Pr, Nd, and the like.
Similar to Sn, Sb is an element having an effect of improving corrosion resistance and may be contained as necessary. However, addition of a large amount of Sb leads to the deterioration of manufacturability, and thus the upper limit is set to 0.50% or less. On the other hand, the effect of improving the corrosion resistance is exhibited at an amount of Sb of 0.005% or more, and thus this amount is set as the lower limit.
The ferritic stainless steel sheet of the present embodiment contains Fe and impurities (including the unavoidable impurities) in addition to the above-described elements, but may contain elements other than the respective elements described above as long as the effect of the present invention is not impaired. In the present embodiment, for example, Bi, Pb, Se, H, Ta, or the like may be contained; however, in that case, the amount thereof is preferably reduced as much as possible. Meanwhile, the amount ratios of these elements are controlled to be within a range where the problem of the present invention is solved, and one or more of Bi ≤ 100 ppm, Pb ≤ 100 ppm, Se ≤ 100 ppm, H ≤ 100 ppm, and Ta ≤ 500 ppm may be contained as necessary.
Next, the metallographic structure will be described.
The ferritic stainless steel sheet of the present embodiment has a ferrite single-phase microstructure in which the grain size number is larger than 9.0.
The grain size number is set to larger than 9.0. Since surface irregularities after formation are less likely to be generated as the grain size number increases, that is, as the crystal grain sizes of ferrite crystal grains decrease, and thus the grain size number of larger than 9.0 is set as the lower limit. In order to further suppress surface irregularities, the grain size number is preferably larger than 9.5 and more desirably larger than 10.0.
The grain size number can be obtained by the line segment method of JIS G 0551 (2013). It should be noted that the grain size number of 9 corresponds to the average line segment length per crystal grain, which traverses the inside of the crystal grain, of 14.1 µm, and the grain size number of 10 corresponds to the average line segment length per crystal grain, which traverses the inside of the crystal grain, of 10.0 µm. In the measurement of crystal grain sizes, the number of traversing crystal grains per sample is set to 500 or more in an optical microscope microstructure photograph of a cross section of a test piece. The etching liquid is preferably aqua regia or inverse aqua regia, but other solutions may also be used as long as crystal grain boundaries can be determined. In addition, depending on the orientation relationship between adjacent crystal grains, there is a case where a grain boundary is not clearly visible, and thus it is preferable to deeply etch the metallographic structure. In addition, in the measurement of a crystal grain boundary, the grain boundary between twin crystals is not measured.
Normally, it is known that crystal orientations have a favorable correlation with formability (r value); however, in the present embodiment, the texture is defined as follows based on a new finding that the present inventors obtained. That is, the texture is defined based on the new finding that crystal orientations have a great influence on surface irregularities after formation. At each of the sheet thickness 1/2 position and the sheet thickness 1/10 position, the random intensity ratios of the crystal orientations in a plane parallel to a rolled surface are set as follows.

I_{554}<225> ≥ 7.0
I_{411}<148> ≥ 0.9
I_{211}<011> ≥ 1.0
I_{hkl}<uvw> indicates the random intensity ratio of the {hkl}<uvw> orientation.

It is known that a {554}<225> orientation is formed as a recrystallization orientation of high-purity ferritic stainless steel and is an orientation favorable to formability (Non Patent Document 1). Therefore, it is necessary to increase the {554}<225> orientation at the time of performing forming mainly involving drawing.
On the other hand, a {411}<148> orientation is formed when the cold rolling ratio is increased (for example, Non Patent Document 2), but this orientation is not favorable to formability. In addition, a {211}<011> orientation is an orientation that is formed by rolling (Non Patent Document 3), but this orientation is encroached during recrystallization and thus rarely remains after the completion of recrystallization. Therefore, in the related art, it was considered that, in order to ensure formability, it was effective to increase the integration degree (random intensity ratio) of the {554}<225> orientation and to decrease the integration degree of the {411}<148> orientation or the {211}<011> orientation, and the orientations were controlled in this manner.
However, the present inventors found that surface irregularities (surface roughening) after formation can be stably suppressed by controlling the orientations in combination with the crystal grain size such that not only the integration degree of the {554}<225> orientation, which is an orientation favorable to formability, is increased but also the integration degrees of the {411}<148> orientation, which is not favorable to formability, and the {211}<011> orientation, which is less likely to remain after recrystallization are increased.
That is, in the present embodiment, the random intensity ratio of the {554} <225> orientation is set to 7.0 or more in consideration of forming the steel sheet in a variety of shapes. As described above, the random intensity ratio of the {554}<225> orientation is preferably as high as possible in order to increase the forming limit and is thus desirably 8.0 or more.
The {411}<148> orientation is an important orientation for suppressing surface irregularities, and the random intensity ratio is set to 0.9 or more. The random intensity ratio is preferably 1.0 or more. In the case of manufacturing ferritic stainless steel sheets by a normal method, it is common that the random intensity ratio becomes less than 0.7. Therefore, in the present embodiment, in order to increase the random intensity ratio of the {411}<148> orientation, it is necessary to control the manufacturing method as described below.
The integration degree of the {211}<011> orientation is set to 1.0 or more. As described above, the {211}<011> orientation is unlikely to remain after the completion of recrystallization, and it is common that the random intensity ratio of this orientation also becomes 0.8 or less in the case of manufacturing ferritic stainless steel sheets by a normal method. Therefore, it is necessary to devise manufacturing conditions in the same manner as the control of the {411}<148> orientation.
A method for measuring the random intensity ratios of the crystal orientations will be described.
X-ray diffraction is performed on planes parallel to the rolled surface of the steel sheet at the 1/2 position of the sheet thickness and the 1/10 position of the sheet thickness. The 1/2 position often exhibits the average texture of steel and can be an index of formability. In addition, since surface irregularities (surface roughening) after formation are generated on the surface, the crystal orientation distribution in the vicinity of the surface is important, and thus the random intensity ratio is also measured at the 1/10 position.
A three-dimensional orientation analysis is performed based on the obtained data. As an analysis method, it is possible to use the "Bunge" method which is widely known. From a crystal orientation distribution map, the random intensity ratio in the corresponding orientation is read. It is also possible to use a local orientation analysis by EBSD; however, in this case, it becomes necessary to investigate a region where the number of crystal grains is 1000 or more and to be careful so that the average information of the texture can be obtained.
The reason for both characteristics of formability and surface irregularities (surface roughening) after formation being improved by the above-described definition of the texture is under intensive investigation and, at the moment, is presumed to be as follows.
During the forming of steel, each crystal grain deforms according to its crystal orientation. The slip system that becomes active at that time is considered to be different for each crystal orientation. Normally, the slip system (direction) that becomes active for an orientation having a high r value is different from the slip system (direction) that becomes active for an orientation having a low r value. Therefore, it is considered that, in a case where a crystal grain in an orientation having a high r value and a crystal grain in an orientation having a low r value are adjacent to each other on the surface of steel, a surface change (recess or protrusion) caused by the slip of one of the crystal grains is offset by a different surface change (protrusion or recess) of the adjacent crystal grains; and as a result, surface irregularities are suppressed. However, since the combination of crystal grain orientations adjacent to each other on the surface of steel is enormous, additional studies are required for the description of this mechanism.
The metallographic structure of the ferritic stainless steel sheet of the present embodiment is a ferrite single-phase microstructure. This means that the ferritic stainless steel sheet does not include austenite or a martensite microstructure. In a case where austenite phase or a martensite microstructure is included, it is relatively easy to decease the crystal grain size, and additionally, austenite exhibits favorable formability due to the TRIP effect. However, the raw material cost increases, and additionally, the yield is likely to decrease due to edge cracking or the like which occurs during manufacturing. Therefore, a ferrite single-phase microstructure is provided as the metallographic structure. It should be noted that a precipitate such as carbonitride is present in steel, but the precipitate has no great influence on the effect of the present invention, and thus the microstructure of the primary phase has been described without considering the precipitate in the following description.
It should be noted that the sheet thickness of the ferritic stainless steel sheet of the present embodiment is not particularly limited, but is desirably 0.5 mm or more and preferably 0.6 mm or more from the viewpoint of ensuring the strength. This is because, when the sheet thickness is thin, there is a case where the strength becomes insufficient in a formed component. The sheet thickness needs to be designed in consideration of the size or shape, withstand load, and the like of a component that is a manufacturing target.
Next, a method for manufacturing the above-described ferritic stainless steel sheet of the present embodiment will be described. In the manufacturing method, hot rolling, cold rolling, and individual heat treatments (annealing) are combined together, and pickling is appropriately performed as necessary. That is, as an example of the manufacturing method, it is possible to employ, for example, a manufacturing method including individual steps of steelmaking, hot rolling, annealing of hot rolled sheet, cold rolling, and annealing of cold rolled sheet.
Points that need to be controlled to satisfy both the crystal grain size and the crystal orientation (texture) that are important in the present embodiment as described above are the heat treatment condition after hot rolling, the cold rolling ratio, and the heat treatment condition after cold rolling, and the steps and conditions other than these are not particularly limited.
In the heat treatment after hot rolling (annealing of hot rolled sheet), the recrystallization temperature T₁ (°C) of the hot-rolled sheet differs depending on the sheet thickness, the component, and the rolling reduction ratio of the hot rolling, but it is necessary to control the maximum reaching temperature (maximum reached temperature) to be in a range of T₁ to (T₁ + 35) (°C). This is because, in the case where the maximum reaching temperature of the annealing of hot rolled sheet is lower than T₁°C, nonrecrystallized grains remain, and the ridging characteristic and formability of products become poor. On the other hand, in the case where the maximum reaching temperature is higher than (T₁ + 35)°C, crystal grains become coarse due to grain growth, and the crystal grain sizes after the cold rolling and the annealing of cold rolled sheet become coarse or the above-described crystal orientations, which are important for the surface roughening property, cannot be obtained after the cold rolling and the annealing of a cold rolled sheet.
The cold rolling ratio is set to 93% or more. In normal methods, it is common that the cold rolling ratio is set to approximately 90% at most; however, in the present embodiment, it is necessary to increase the introduced strain amount in order to decrease the crystal grain sizes of the recrystallized grains after the cold rolling. Recrystallization begins from a portion where a lot of strains are introduced. That is, as the amount of a material worked increases (the rolling reduction ratio increases), the number of portions from which recrystallization begins (nuclei) increases, and thus the crystal grain sizes of the recrystallized grains decrease. In addition, it is also important to control the {554}<225> orientation, the {411}<148> orientation, and the {211}<011> orientation, which are important for the surface roughening property after recrystallization, within the above-described ranges, and, in order to increase the random intensity ratios of these orientations, it is necessary to increase the rolling reduction ratio. Based on these facts, in the present embodiment, it is important to set the rolling reduction ratio to 93% or more. It should be noted that the upper limit of the rolling reduction ratio is not particularly limited, but may be set to 97% or less from the viewpoint of the capacity of rolling machines.
In addition, the other rolling conditions of the cold rolling of the present embodiment may be appropriately selected and set.
When the recrystallization temperature of the cold-rolled sheet is represented by T₂ (°C), it is necessary to control the maximum reaching temperature in the heat treatment after the cold rolling (annealing of cold rolled sheet or final annealing) to be within a range of (T₂ - 10)°C to (T₂ + 30)°C. This is because, in the case where the maximum reaching temperature of the annealing of cold rolled sheet is lower than (T₂ - 10)°C, the material is hardened, forming cracking is likely to occur, and there is a concern that formability may deteriorate. On the other hand, in the case where the maximum reaching temperature is higher than (T₂ + 30)°C, the crystal grain sizes become large, the defined grain size number cannot be obtained or the predetermined crystal orientations cannot be obtained, and surface roughening occurs after formation.
In the present embodiment, intermediate annealing may be performed in the middle of the cold rolling. That is, the cold rolling of the present embodiment may be performed once or may be performed twice or more before and after the intermediate annealing. It should be noted that the intermediate annealing and the final annealing may be batch annealing or continuous annealing. In addition, each annealing may be bright annealing where annealing is performed in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, if necessary, or may be performed in the atmosphere (air).
The recrystallization temperature T₁ or T₂ can be determined from the observation of the metallographic structure of the hot-rolled sheet or the cold-rolled sheet that has been heat-treated at different temperatures.
The ferritic stainless steel sheet according to the present embodiment can be obtained by the manufacturing method described above.

EXAMPLES

Next, examples of the present invention will be described. Conditions in the examples are examples of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention is capable of adopting a variety of conditions as long as the present invention does not deviate from the features of the present invention and the object of the present invention is achieved.
It should be noted that underlined values in tables shown below indicate that the values are outside the scope of the present embodiment.
Stainless steels having compositions shown in Table 1 were melted and cast into slabs, and the slabs were rolled by hot rolling. After that, annealing of hot rolled sheet, cold rolling, and annealing of cold rolled sheet were performed to manufacture stainless steel sheets (product sheets) No. 1 to No. 28 having a thickness of 0.6 mm. Conditions of each step were changed as shown in Table 2. It should be noted that the annealing times (retention times) in the annealing of hot rolled sheet and the annealing of cold rolled sheet were set to be in a range of 1 to 60 seconds, respectively, and, in the present example, intermediate annealing was not performed.
Next, the grain size numbers (GSN) of the obtained stainless steel sheets No. 1 to No. 28 were measured according to JIS G 0551 (2013).
In addition, textures were measured by X-ray diffraction, which is the method described above, at the sheet thickness centers (1/2t positions) and 1/10t positions of the stainless steel sheets No. 1 to No. 28, respectively, and the random intensity ratios I_{554}<225>, I_{411}<148>, and I_{211}<011> of the {554}<225> orientation, the {411}<148> orientation, and the {211}<011> orientation were obtained.
Furthermore, samples having a diameter of ϕ100 mm were cut out from the stainless steel sheets No. 1 to No. 28, and a cup forming test with a limit drawing ratio of 2.0 was performed with a hydraulic forming tester. It is known that surface roughening after cup forming is greatly affected by the limit drawing ratio, but is rarely affected by the other forming conditions. The conditions for the cup forming test performed in examples were that a punch diameter was 50 mm, a punch shoulder R was 5 mm, a die diameter was 53 mm, a die shoulder R was 8 mm, and a blank holding force was 10 ton, and, as a lubricant between the samples and the punch, a rust preventive oil "DAPHNE OIL COAT Z3 (registered trademark)" manufactured by Idemitsu Kosan Co., Ltd. was applied. After that, lubricating sheets "NAFLON tape TOMBO9001 manufactured by NICHIAS Corporation" were attached to protect the surfaces of the formed steel sheets.
For the samples formed with a limit drawing ratio of 2.0, surface roughening after the cup forming was evaluated. Specifically, the surface roughness was measured using a two-dimensional contact-type surface roughness measuring instrument for a length of 5 mm parallel to the height direction at the height center portion of a standing wall portion of the sample after the cup forming. Based on the arithmetic average roughness Ra of 2.0 µm, which is described in JIS B 0031 (2003), in a case where Ra was 2.0 µm or less, the surface roughening evaluation was determined as favorable ("○"), and, in a case where Ra was more than 2.0 µm, the surface roughening evaluation was determined as poor ("x").
Table 3 shows the results of the above-described characteristic evaluations of the stainless steel sheets No. 1 to No. 28. It should be noted that all of the stainless steel sheets of the Invention Examples had a ferrite single phase (did not include an austenite phase or a martensite microstructure).
As shown in Table 3, according to the Invention Examples, the grain size numbers and the textures were controlled, and it was possible to obtain ferritic stainless steel sheets being excellent in terms of resistance to surface roughening and formability.

It should be noted that, in the case of the comparative examples where Ra was more than 2.0 µm, surface irregularities were significant, the irregularities were removed by polishing in the end, and thus the evaluations were poor in terms of the manufacturing cost.

Table 1

Steel	Components (mass%): remainder was Fe and impurities
Steel	C	Si	Mn	P	S	Cr	Ti	Nb	N	Other
A	0.003	0.06	0.07	0.022	0.0001	16.3	0.15	0.07	0.007	0.0007B, 0.04Sn
B	0.005	0.07	0.24	0.015	0.0012	14.5	0.14	-	0.015
C	0.008	0.38	0.38	0.028	0.0025	17.3	-	0.26	0.009	0.08Ni, 0.04Cu, 0.0015Mg, 0.0014Ca
D	0.009	0.81	0.91	0.035	0.0007	18.9	0.22	-	0.006	3.25Mo, 0.01Sn, 0.07Sb
E	0.002	0.16	0.24	0.039	0.0035	24.1	0.12	0.12	0.014	0.01REM, 0.03Hf, 0.44V, 0.003Ca, 0.12Co
F	0.004	0.25	0.35	0.091	0.0015	18.3	0.02	0.34	0.011	3.0021B, 0.62Ni, 0.54A1, 0.35W
G	0.001	0.39	0.02	0.055	0.0038	13.5	-	0.27	0.017	3.0002B, 0.0005Ca, 0.07V, 0.06Ni
H	0.001	0.72	0.09	0.025	0.0061	11.7	0.34	-	0.005	3.28Sb, 0.03V, 0.04Co, 1.1Mo
I	0.004	0.02	0.19	0.030	0.0011	16.4	0.78	0.77	0.011	0.004A1, 0.09Sn, 0.0003Mg
J	0.008	0.25	0.35	0.027	0.0024	16.2	0.18	-	0.014
K	0.025	0.34	0.08	0.015	0.0009	17.2	0.15	0.15	0.009	0.06Co, 0.22A1, 0.15Cu
L	0.005	0.60	0.25	0.069	0.0018	17.9	-	-	0.010	0.022Zr, 0.33W, 0.11Hf, 0.013Y

Table 2

No.	Steel	Recrystallization temperature of hot-rolled sheet T₁ (°C)	Annealing temperature of hot-rolled sheet Ta (°C)	Cold rolling ratio (%)	Recrystallization temperature of cold-rolled sheet T₂ (°C)	Annealing temperature of cold-rolled sheet Tb (°C)	Note
1	A	850	860	93	780	800	Invention Example
2	A	850	920	93	820	840	Comparative Example
3	A	850	860	91	820	840	Comparative Example
4	B	825	828	94	760	770	Invention Example
5	B	825	950	90	770	850	Comparative Example
6	C	900	910	84	880	900	Comparative Example
7	C	900	950	94	880	900	Comparative Example
8	C	900	925	93	840	860	Invention Example
9	D	920	948	93	845	845	Invention Example
10	D	920	940	94	845	800	Comparative Example
11	E	925	930	94	850	880	Invention Example
12	E	925	935	93	850	890	Comparative Example
13	E	925	880	93	850	880	Comparati ve Example
14	F	950	960	80	910	920	Comparative Example
15	F	950	960	94	880	900	Invention Example
16	G	970	980	93	910	930	Invention Example
17	G	970	1050	94	910	930	Comparative Example
18	H	910	920	94	850	860	Invention Example
19	H	910	930	90	850	860	Comparative Example
20	H	910	1000	94	880	900	Comparative Example
21	I	1040	1050	95	1000	1020	Invention Example
22	I	1040	1050	65	1030	1000	Comparative Example
23	J	910	935	94	900	920	Invention Example
24	J	910	940	93	900	950	Comparative Example
25	K	900	910	93	840	860	Comparative Example
26	K	900	920	94	860	890	Comparative Example
27	L	880	900	93	850	880	Comparative Example
28	L	880	910	93	850	860	Comparative Example

Table 3

No.	Steel	Grain size number GSN	Random intensity ratios of crystal orientation density (sheet thickness 1/2t)			Random intensity ratios of crystal orientation density (sheet thickness 1/10t)			Forming test		Note
No.	Steel	Grain size number GSN	I {554} <225>	I {411} <148>	I {211} <011>	I {554} <225>	I {411} <148>	I {211} <011>	Success or failure of forming	Surface roughening	Note
1	A	9.8	9.5	1.0	1.2	8.3	1.1	1.1	○	○	Invention Example
2	A	8.9	6.3	1.1	1.1	5.4	1.0	1.1	×	-	Comparative Example
3	A	8.9	7.4	1.1	1.1	6.5	1.2	1.0	×	-	Comparative Example
4	B	10.2	7.7	1.7	1.4	7.7	2.1	1.2	○	○	Invention Example
5	B	8.0	9.1	0.9	0.7	808.0	1.0	0.7	○	×	Comparative Example
6	C	8.1	10.2	0.4	0.8	10.2	0.5	0.9	○	×	Comparative Example
7	C	9.5	10.2	0.4	0.8	10.4	0.4	0.7	○	×	Comparative Example
8	C	10.0	8.5	1.3	1.2	8.6	1.4	1.4	○	○	Invention Example
9	D	10.4	9.2	1.2	1.3	9.0	1.2	1.1	○	○	Invention Example
10	D	Non recrystallization	3.4	4.5	4.5	4.1	4.9	4.8	×	-	Comparative Example
11	E	10.0	11.3	1.0	1.6	12.1	1.1	1.3	○	○	Invention Example
12	E	9.2	7.3	0.6	1.1	7.5	0.7	1.2	○	×	Comparative Example
13	E	9.1	5.1	2.4	1.3	5.9	2.0	1.0	×	-	Comparative Example
14	F	8.8	7.8	0.6	1.0	7.0	0.8	1.2	○	×	Comparative Example
15	F	9.7	8.2	2.1	1.1	8.6	1.9	1.2	○	○	Invention Example
16	G	10.7	9.9	1.5	1.0	9.3	1.2	1.0	○	○	Invention Example
17	G	9.6	9.4	1.1	0.6	8.4	1.0	0.7	○	×	Comparative Example
18	H	9.1	7.1	2.1	1.6	7.1	1.8	1.8	○	○	Invention Example
19	H	9.0	6.4	1.4	1.1	4.8	1.3	1.0	×	-	Comparative Example
20	H	8.7	6.6	0.6	0.8	6.2	0.5	0.6	×	-	Comparative Example
21	I	10.6	11.0	1.4	1.4	9.7	1.2	1.3	○	○	Invention Example
22	I	Non recrystallization	3.5	0.4	4.1	3.5	0.6	4.4	×	-	Comparative Example
23	J	9.7	8.5	0.9	1.7	9.0	1.0	1.6	○	○	Invention Example
24	J	8.4	9.7	1.2	1.0	9.1	1.6	1.0	○	×	Comparative Example
25	K	9.2	3.4	0.7	1.5	3.5	0.6	1.3	×	-	Comparative Example
26	K	9.1	2.8	0.8	1.4	3.4	0.7	1.3	×	-	Comparative Example
27	L	8.8	4.5	1.5	1.2	4.1	1.3	1.0	×	-	Comparative Example
28	L	8.8	4.5	1.7	1.0	4.0	1.2	1.1	×	-	Comparative Example

Industrial Applicability

According to the present embodiment, it is possible to provide a ferritic stainless steel sheet which is excellent in terms of forming workability and the surface characteristic after formation. Furthermore, the ferritic stainless steel sheet according to the present embodiment is excellent in terms of the surface characteristic after formation. In the related art, a polishing step was performed after formation for the purpose of removing surface irregularities; however, in the ferritic stainless steel sheet of the present embodiment, it is possible to omit this polishing step, and thus it is possible to sufficiently exploit the effect even in terms of the manufacturing cost. Therefore, the ferritic stainless steel sheet of the present embodiment is preferably applied to forming applications.

Claims

A ferritic stainless steel sheet comprising, by mass%,
Cr: 11.0% or more and 25.0% or less;

C: 0.001 % or more and 0.010% or less;

Si: 0.01% or more and 1.0% or less;

Mn: 0.01% or more and 1.0% or less;

P: 0.10% or less;

S: 0.01% or less;

N: 0.002% or more and 0.020% or less; and

either one or both of Ti: 1.0% or less, and Nb: 1.0% or less,

with a remainder of Fe and impurities,

wherein the ferritic stainless steel sheet has a ferrite single-phase microstructure in which a grain size number is larger than 9.0, and

random intensity ratios of crystal orientations in planes parallel to a rolled surface at a sheet thickness 1/2 position and a sheet thickness 1/10 position satisfy

I_{554}<225> ≥7.0,

I_{411}<148> ≥ 0.9, and

I_{211}<011> ≥ 1.0,

where, I_{hkl}<uvw> indicates a random intensity ratio of an {hkl}<uvw> orientation.
The ferritic stainless steel sheet according to claim 1, further comprising, by mass%: one or more of B: 0.0001% or more and 0.0025% or less, Sn: 0.005% or more and 0.50% or less, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 2.0% or less, Al: 1.0% or less, W: 1.0% or less, Co: 0.50% or less, V: 0.50% or less, Zr: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, and Sb: 0.50% or less.