Ferritic stainless steel sheet

Abstract

Provided are: a ferritic stainless steel sheet having a high r value, reduced in-plane anisotropy, and low yield strength and wrinkle resistance; and a method for producing the ferritic stainless steel sheet. The ferritic stainless steel sheet (1) has a crystal grain size of 15 [mu] m or more and 40 [mu] m or less, and satisfies the relationships I111 > = 10.0 and 0.2 < = (Ia / Ib) < = 4.0, where I111 is the larger one of Ia, which is {111} < 110 > crystal orientation strength, and Ib, which is {111} < 112 > crystal orientation strength, in a cross-section (13) parallel to the rolling surface in the center of the sheet thickness.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel plate. Background Art

[0002] When deep drawing ferritic stainless steel sheets, the in-plane anisotropy of the material structure can cause irregularities on the surface of the formed product. Furthermore, ridge-like undulations can occur along the rolling direction of the ferritic stainless steel sheet. This surface defect is generally referred to as wrinkling.

[0003] Conventionally, technologies for improving the formability of ferritic stainless steel sheets have been studied (for example, see Patent Documents 1 to 5).

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent No. 5505555

[0007] Patent Document 2: Japanese Patent No. 4083669

[0008] Patent Document 3: Japanese Patent No. 2772237

[0009] Patent Document 4: Japanese Patent Application Laid-Open No. 8-311542

[0010] Patent Document 5: Japanese Patent Publication No. 51-35369 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] When ferritic stainless steel has a high yield strength, the forming load increases during forming, and the dimensional accuracy after forming may deteriorate (springback). While grain coarsening is effective in reducing yield strength, the development of a texture with a specific orientation during grain growth increases in-plane anisotropy. Furthermore, reducing in-plane anisotropy tends to reduce the r-value and worsen wrinkling.

[0013] An object of one embodiment of the present invention is to provide a ferritic stainless steel sheet having a high r-value, reduced in-plane anisotropy, low yield strength, and high ridging resistance.

[0014] Means for solving problems

[0015] In order to solve the above-mentioned problems, a ferritic stainless steel sheet according to one embodiment of the present invention has the following chemical composition: containing, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030%, and P: 0.005-0.050%, and at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, and the S content is 0.0 100% or less, the remainder being Fe and inevitable impurities, in a cross section of the ferritic stainless steel plate parallel to the rolling direction and perpendicular to the rolling surface, the crystal grain size calculated by a cutting method is 15 μm or more and 40 μm or less, in a cross section of the ferritic stainless steel plate at the center of the plate thickness and parallel to the rolling surface, the {111}<110> crystal orientation intensity is denoted as Ia, and the {111}<112> crystal orientation intensity is denoted as Ib, and the larger value of Ia or Ib is denoted as I 111 , satisfy I 111 The relationship is ≥10.0 and 0.2≤(Ia / Ib)≤4.0.

[0016] Effects of the Invention

[0017] According to one embodiment of the present invention, it is possible to provide a ferritic stainless steel sheet having a high r-value, reduced in-plane anisotropy, low yield strength, and high ridging resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram showing a cross section of a stainless steel plate according to one embodiment of the present invention.

[0019] Figure 2 This is a schematic diagram showing a cross section of a stainless steel plate according to one embodiment of the present invention.

[0020] Figure 3 This is a flowchart showing an example of a method for producing a ferritic stainless steel sheet according to one embodiment of the present invention. DETAILED DESCRIPTION

[0021] The following describes the embodiments of the present invention. It should be noted that the following description is for better understanding of the subject matter of the invention and does not limit the present invention unless otherwise specified. In addition, in this application, "A to B" means A or more and B or less.

[0022] It should be noted that, in this specification, the term "ferritic stainless steel" does not limit the specific form of steel strip, steel plate, etc., but is used to describe the properties of the raw material itself. Furthermore, "steel plate" can be considered a portion of "steel strip," and thus "ferritic stainless steel plate" includes "ferritic stainless steel strip."

[0023] The ferritic stainless steel sheet according to one embodiment of the present invention has a chemical composition that is a single-phase ferrite up to the melting point, has low carbon and nitrogen contents, and contains carbide-stabilizing elements (such as Ti and Nb). Ferritic stainless steel having such a chemical composition is sometimes referred to as high-purity ferritic stainless steel.

[0024] <About the Summary of the Invention>

[0025] Generally, the formability (deep drawability) of ferritic stainless steel sheets can be evaluated using the r-value (Lankford value, plastic working strain ratio). Typically, deep drawability can be evaluated using the average r-value obtained by averaging multiple r-values ​​measured in multiple different in-plane directions with the rolling direction as a reference. In-plane anisotropy can also be evaluated using the in-plane anisotropy index Δr calculated from these multiple r-values.

[0026] Ferritic stainless steel sheets (high-purity ferritic stainless steel sheets) with a chemical composition that does not undergo austenite transformation upon heating have a texture in the metal structure and are prone to the presence of colonies (hereinafter referred to as colonies). These colonies are formed by the aggregation of grains with similar crystal orientations. The crystal orientation of the colonies differs from the crystal orientation of the texture surrounding the colonies.

[0027] To reduce in-plane anisotropy Δr, a process is known in which hot-rolled sheet annealing is omitted. However, this method results in insufficient colony segmentation, exacerbating wrinkling. On the other hand, hot-rolled sheet annealing, while colony segmentation improves wrinkling, increases in-plane anisotropy because only certain orientations develop preferentially. Therefore, achieving both low Δr and improved wrinkling resistance is difficult.

[0028] The present inventors conducted extensive research on technologies for achieving high r-values, reduced in-plane anisotropy, low yield strength, and high ridging resistance, leading to the present invention. The ferritic stainless steel sheet according to one embodiment of the present invention is achieved through a manufacturing process that differs from conventional methods. The various properties and manufacturing method of the ferritic stainless steel sheet according to one embodiment of the present invention will be described in detail below.

[0029] (Regarding the prior art)

[0030] In order to facilitate understanding of the method for producing a ferritic stainless steel sheet according to one embodiment of the present invention, the techniques described in Patent Documents 1 to 5 will be briefly described below.

[0031] Generally, the manufacturing process of stainless steel plates includes steelmaking process, hot rolling process, hot rolled plate annealing process, intermediate processes such as cold rolling and final annealing process.

[0032] The technology described in Patent Document 1 improves formability by grain refinement, without considering the reduction in yield strength. The technologies described in Patent Documents 2 and 4 perform texture control during the hot rolling process, omitting the hot-rolled sheet annealing process. Patent Document 2 describes increasing the integration of both the {111}<112> and {111}<011> orientations to achieve a high r-value while reducing in-plane anisotropy, but this does not aim to achieve both low yield strength and wrinkling resistance. Patent Document 4 describes effectively controlling texture during the hot rolling process by significantly reducing the carbon content to below 0.005% and the nitrogen content to below 0.012%, achieving wrinkling resistance and a high r-value while reducing in-plane anisotropy. However, in the technologies described in Patent Documents 2 and 4, since the hot-rolled sheet annealing process is omitted, wrinkling may worsen. In the technology described in Patent Document 4, wrinkle resistance is taken into consideration, but improvement of wrinkle resistance is limited, and greatly reducing the carbon and nitrogen contents leads to problems in production costs.

[0033] Patent Document 3 describes that by controlling the conditions of the hot rolling process, the in-plane anisotropy is reduced while having wrinkling resistance and a high r value. However, the combination of low yield strength and low Δr value is not the purpose. If the annealing temperature of the cold-rolled sheet is lowered, the grains become finer, thereby increasing the yield strength. On the other hand, if the annealing temperature of the cold-rolled sheet is increased, the yield strength decreases, and on the other hand, Δr also increases. In addition, in the technology described in Patent Document 5, the carbon and nitrogen contents are reduced, and then Ti is added to fix carbon and nitrogen, thereby improving the workability of ferritic stainless steel. However, wrinkling resistance is unclear, and recrystallization can be suppressed by Ti-based precipitates. Therefore, there is a limit to increasing the r value.

[0034] <Steel Plate Composition>

[0035] First, the component composition (chemical composition) of the ferritic stainless steel sheet in one embodiment of the present invention is described below. In the following description, the ferritic stainless steel sheet in one embodiment of the present invention may be simply referred to as "this stainless steel sheet."

[0036] The present stainless steel plate may have a chemical composition containing, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030% and P: 0.005-0.050%, and at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, with a S content of 0.0100% or less.

[0037] The present stainless steel plate may have a chemical composition in which the remainder is iron (Fe) and inevitable impurities.

[0038] (C: Carbon)

[0039] C is an element that forms carbides with Cr and other elements, forming interfaces that become sources of dislocations during deformation of ferritic stainless steel. However, excessive addition of C reduces intergranular corrosion resistance and workability, and increases refining costs. Therefore, the C content can be 0.001 to 0.030 mass%, 0.001 to 0.020 mass%, or 0.002 to 0.010 mass%.

[0040] (Si: Silicon)

[0041] Si acts as a deoxidizer during the melting stage. However, excessive Si addition can harden ferritic stainless steel and reduce ductility. Therefore, the Si content can be 0.01-1.00 mass%, 0.02-0.70 mass%, or 0.03-0.30 mass%.

[0042] (Mn: manganese)

[0043] Mn has the effect of a deoxidizer. However, if excessive Mn is added, the amount of MnS generated increases, reducing the corrosion resistance of ferritic stainless steel. Therefore, the Mn content can be 0.01-1.00 mass%, 0.02-0.70 mass%, or 0.03-0.30 mass%.

[0044] (Cr: chromium)

[0045] Cr is necessary to form a passive film on the surface of cold-rolled steel sheets, improving corrosion resistance. However, excessive addition of Cr reduces the ductility of ferritic stainless steel. Therefore, the Cr content can be 10.5-30.0% by mass, or 12.0-25.0% by mass.

[0046] (N: Nitrogen)

[0047] Nitrogen (N) is an element that forms nitrides with Cr and other elements, forming interfaces that serve as sources of dislocations during deformation in ferritic stainless steel. However, excessive addition of N reduces ductility due to solid solution strengthening. Therefore, the N content can be 0.001 to 0.030 mass%, or 0.005 to 0.025 mass%. Furthermore, in this stainless steel plate, the total C and N content, i.e., the C+N content, can be 0.050 mass% or less, or 0.045 mass% or less. Excessive C+N content can lead to excessive carbonitride precipitation. On the other hand, excessively reducing the C+N content increases refining costs, so the C+N content can be 0.010 mass% or more, or 0.015 mass% or more. The C+N content, expressed in mass%, can be within the range of 0.010 ≤ C+N ≤ 0.050, or 0.015 ≤ C+N ≤ 0.050.

[0048] (P: Phosphorus)

[0049] Excessive P content can deteriorate weldability, weld toughness, and workability. Furthermore, P is associated with precipitates (described later) in the material structure. Therefore, the P content can be 0.005-0.050 mass%, 0.005-0.040 mass%, or 0.010-0.030 mass%.

[0050] (Ti and Nb: titanium and niobium)

[0051] Ti and Nb combine with C or N, fixing C and N in the form of precipitates such as TiC, TiN, NbC, or NbN. Therefore, by increasing the purity of ferritic stainless steel, the average r-value and product elongation can be improved. On the other hand, excessive inclusion of Ti and Nb increases raw material costs and may reduce manufacturability due to an increase in recrystallization temperature.

[0052] Therefore, in one embodiment of the present invention, the Ti content may be 0.01 to 0.50 mass%, 0.02 to 0.40 mass%, or 0.10 to 0.30 mass%. Furthermore, the Nb content may be 0.01 to 0.50 mass%, 0.02 to 0.40 mass%, or 0.10 to 0.30 mass%. Ferritic stainless steel may contain only one of Ti and Nb, or both.

[0053] (S: Sulfur)

[0054] S is an impurity atom that adversely affects hot workability, corrosion resistance, and oxidation resistance. Therefore, the S content may be 0.0100% by mass or less. Ferritic stainless steel may not contain S, and the lower limit of the S content is not particularly limited. The S content may be 0 (including no addition) to 0.0100% by mass. Regarding the S content of "0 (including no addition)", S is allowed to be contained as an unavoidable impurity.

[0055] (Other ingredients)

[0056] The present stainless steel plate may have a chemical composition further containing, in mass %, one or more selected from Mo, Ni, Co, Cu, Al, Ca, Mg, B, V, W, Sn, Sb, Zr, Y, Hf and rare earth elements.

[0057] (Mo: molybdenum)

[0058] Mo is an element that is effective in improving corrosion resistance. However, if Mo is added excessively, the raw material cost of stainless steel increases. Therefore, when Mo is included in the chemical composition, the Mo content can be 0.05-2.00 mass%.

[0059] (Ni: nickel)

[0060] Ni is an element effective in improving corrosion resistance. On the other hand, if Ni is excessively contained, the ferrite phase becomes unstable and the raw material cost of ferritic stainless steel increases. Therefore, when Ni is included in the chemical composition, the Ni content can be 0.01 to 1.00 mass%. The Ni content can be 0.01 to 0.10 mass%. In this stainless steel, the Ni content can be 0.40 mass% or less, 0.10 mass% or less, or 0 (including no addition) to 0.10 mass%. "No addition" means that Ni is not artificially added during steelmaking. Regarding the Ni content of "0 (including no addition)", Ni is allowed to be contained as an unavoidable impurity.

[0061] (Co: cobalt)

[0062] Co is an element effective in improving corrosion resistance and heat resistance. However, excessive addition of Co increases the raw material cost of ferritic stainless steel. Therefore, when Co is included in the chemical composition, the Co content can be 0.005 to 0.500 mass%.

[0063] (Cu: copper)

[0064] Cu is an element effective in improving corrosion resistance. Therefore, when Cu is included in the chemical composition, the Cu content may be 0.05 to 1.00% by mass.

[0065] (Al: aluminum)

[0066] Al is an effective deoxidizing element and can reduce A2-type inclusions that negatively affect stamping workability. However, excessive Al addition increases surface defects. Therefore, when Al is included in the chemical composition, the Al content can be 0.01 to 1.00 mass%.

[0067] (Ca: calcium)

[0068] Ca is an element effective for degassing. Therefore, when Ca is included in the chemical composition, the content of Ca may be 0.0001 to 0.0050 mass %.

[0069] (Mg: magnesium)

[0070] Mg forms Mg oxides with Al in molten steel, acting as a deoxidizer. However, excessive Mg content reduces the toughness of ferritic stainless steel and reduces manufacturability. Therefore, when Mg is included in the chemical composition, the Mg content can be 0.0001 to 0.0050 mass%.

[0071] (B: Boron)

[0072] B is an element effective in improving toughness. However, if B is present in excessive amounts, its effect becomes saturated. Therefore, when B is included in the chemical composition, the B content can be 0.0001 to 0.0025 mass%.

[0073] (V: Vanadium)

[0074] V is an element that is effective in increasing hardness and strength. However, excessive addition of V increases the raw material cost of ferritic stainless steel. Therefore, when V is included in the chemical composition, the V content can be 0.05-0.50 mass%.

[0075] (W: tungsten)

[0076] W is an element that is effective in improving high-temperature strength. However, excessive W addition increases the raw material cost of ferritic stainless steel. Therefore, when W is included in the chemical composition, the W content can be 0.05-1.00 mass%.

[0077] (Sn: Tin)

[0078] Sn is an element that is effective in improving corrosion resistance. However, excessive Sn addition can reduce hot workability and toughness. Therefore, when Sn is included in the chemical composition, the Sn content can be 0.005 to 0.500 mass%.

[0079] (Sb: Antimony)

[0080] Sb is effective in improving workability by promoting the formation of deformation bands during rolling. However, excessive Sb content saturates this effect, leading to a decrease in workability. Therefore, when Sb is included in the chemical composition, the Sb content can be 0.005 to 0.500 mass%.

[0081] (Zr: Zirconium)

[0082] Zr is an element effective for denitrification, deoxidation, and desulfurization. However, excessive Zr addition increases the raw material cost of stainless steel. Therefore, when Zr is included in the chemical composition, the Zr content can be 0.050-0.500 mass%.

[0083] (Y: yttrium)

[0084] Y is an element effective in improving hot workability and oxidation resistance. However, these effects are saturated when the content exceeds 0.20%. When Y is included in the chemical composition, the Y content can be 0.001 to 0.100 mass%.

[0085] (Hf: Hafnium)

[0086] Hf is an element that improves oxidation resistance. However, excessive Hf content reduces the toughness of the steel sheet and increases the raw material cost of the stainless steel. Therefore, when Hf is included in the chemical composition, the Hf content can be 0.001 to 0.100 mass%.

[0087] (REM: Rare Earth Elements)

[0088] Rare earth metals (REMs) refer to lanthanides (elements with atomic numbers 57 to 71, such as La, Ce, Pr, Nd, and Sm). REMs are effective in improving hot workability and oxidation resistance. However, these effects saturate when the content exceeds 0.100%. Therefore, when REMs are included in the chemical composition, the REM content can be 0.001 to 0.100 mass%.

[0089] <Characteristics of Steel Plate>

[0090] The present stainless steel plate has the chemical composition as described above, and a material structure (internal structure) having the following characteristics is formed by controlling the manufacturing conditions. In general, the colonies are divided in the hot-rolled plate annealing process, while the metal structure is recrystallized and low-temperature precipitates are dissolved. The low-temperature precipitates have the effect of inhibiting grain growth (the so-called "pinning" effect). Moreover, in the final annealing process, by heating to a temperature range where no precipitates are generated, recrystallization nuclei of grains of various crystal orientations are generated and grain growth is carried out, thereby promoting the randomization of crystal orientation. More details will be described later together with the description of the manufacturing method of the present stainless steel plate.

[0091] (Grain size of steel plate)

[0092] Figure 1 1 is a schematic diagram showing a cross section 12 of a stainless steel plate 1 in one embodiment of the present invention. Figure 1 As shown, the cross section 12 is parallel to the rolling direction of the stainless steel plate 1 and perpendicular to the rolling surface 11. The thickness of the stainless steel plate 1 is t, and the width is w.

[0093] The crystal grain size (hereinafter referred to as average grain size d) calculated by a cutting method in the cross section 12 of the stainless steel plate 1 is 15 μm or more and 40 μm or less, preferably 18 μm or more and 35 μm or less.

[0094] The grain size calculated by the cutting method can be measured using the method specified in the JIS standard (JIS G 0551:2020). Specifically, first, draw a line segment of length L parallel to the rolling direction in section 12, and count the number n of grains that the line segment intersects. Furthermore, grains with the end of the line segment located within the line segment are counted as 1 / 2. The average grain size d can be calculated using the formula d = L / n.

[0095] (Crystal Orientation)

[0096] Figure 2 1 is a schematic diagram showing a cross section 13 of a stainless steel plate 1 in one embodiment of the present invention. Figure 2 As shown, cross section 13 is a cross section of the steel plate 1 at the center of the thickness, parallel to the rolling surface 11. The rolling direction is abbreviated as RD (Rolling Direction), the normal direction to the rolling surface is abbreviated as ND (Normal Direction), and the direction perpendicular to the rolling surface is abbreviated as TD (Transverse Direction). Cross section 13 is the so-called ND plane.

[0097] By performing XRD measurement on the cross section 13, the extreme point diagram data can be obtained. The grain orientation distribution function (ODF) obtained by the extreme point diagram data is analyzed to determine the strength of each crystal orientation (crystal orientation strength). For example, by performing extreme point measurement using SmartLab manufactured by Rigaku, the extreme point diagram data can be obtained. As ODF analysis software, SmartLab Studio II can be used. Since a known analysis method can be used in this way, a detailed description is omitted, but if it is described in general terms, it is as follows.

[0098] When the (hkl) plane is parallel to the ND plane and the [uvw] direction is in the rolling direction, the crystal orientation of a certain grain is expressed as (hkl)[uvw]. Furthermore, an equivalent orientation group is expressed as {hkl}<uvw>.

[0099] The ODF is a function of three variables (φ1, Φ, and φ2) that uniquely specifies the crystal orientation of a grain relative to the material coordinate system. φ1, Φ, and φ2 are Euler angles defined using the Bunge method. The material coordinate system uses the x, y, and z axes as RD, TD, and ND, respectively. φ1 is the counterclockwise rotation angle around the z-axis, Φ is the counterclockwise rotation angle around the x'-axis after φ1 is rotated, and φ2 is the counterclockwise rotation angle around the z'-axis after Φ is rotated.

[0100] In the cross section of Euler space at φ2=45°, the position at φ=55° and φ1=30° is set to be the {111}<112> orientation, and the position at φ=55° and φ1=0° is set to be the {111}<110> orientation.

[0101] The stainless steel plate 1 is subjected to XRD measurement to obtain (200), (110), and (211) positive pole diagrams. The obtained positive pole diagram is used for ODF analysis. The {111}<110> crystal orientation strength obtained by ODF analysis is set as Ia, and the {111}<112> crystal orientation strength obtained by ODF analysis is set as Ib. The values ​​of Ia and Ib can be obtained, for example, by outputting the contour data (ODF diagram) obtained using SmartLab Studio II as numerical values. In addition, the calculation method in the analysis of ODF can use the WIMV method of Matthies and Vinel, which does not use continuous functions in the analysis. The larger value of Ia and Ib is called I 111 For example, a SmartLab can be used as an X-ray diffraction device, and a Mo-ray source can be used as the X-ray source. In this case, the intensity of the pole diagram data used for ODF analysis is standardized through correction processes such as background correction and randomization during processing in SmartLab. This standardization process is performed under certain conditions without individually setting conditions by selecting the standardization check box displayed on the display in the user interface of the X-ray diffraction device. The above-mentioned crystal orientation intensity can also be expressed as an orientation density or an X-ray random intensity ratio.

[0102] Stainless steel plate 1 I 111 is 10.0 or more and satisfies the relationship 0.2≤(Ia / Ib)≤4.0. 111 The higher the value, the larger the r value can be. By making Ia / Ib satisfy the above relationship, the in-plane anisotropy can be reduced.

[0103] Stainless steel plate 1 I 111 It may be 10.0 or more and 30.0 or less, or 15.0 or more and 30.0 or less, and may satisfy the relationship of 1.0≤(Ia / Ib)≤3.8. 111 When the ratio exceeds 30.0, it becomes difficult to satisfy the relationship 0.2 ≤ (Ia / Ib) ≤ 4.0. When manufacturing through the steps of hot rolling, hot-rolled sheet annealing, cold rolling, and final annealing (i.e., without intermediate annealing or additional cold rolling), Ia ≥ Ib is often the case. Therefore, (Ia / Ib) can be greater than 1. (Ia / Ib) can be less than 3.8, thereby further reducing in-plane anisotropy.

[0104] (Average r value and Δr)

[0105] The average r value (average Lankford value) of the present stainless steel plate can be 1.4 or more. The average r value can be calculated by the following formula (1) using the r value measured at a plastic strain of 14.4% by the method specified in the JIS standard (JIS Z 2254: 2021):

[0106] Average r value = (r L +2r D +r C ) / 4…(1).

[0107] In formula (1), r L 、r D and r C The r-values ​​are measured for JIS 13 B test pieces taken at 0° (parallel), 45°, and 90° relative to the rolling direction. The average r-value of the stainless steel plate may be 1.4 to 2.1, or 1.9 to 2.1.

[0108] In addition, the Δr of the present stainless steel plate can be greater than -0.50 and less than 0.50. The range of Δr is preferably greater than 0 and less than 0.50, and more preferably greater than 0 and less than 0.48. Δr can be calculated using the above r value by the following formula (2):

[0109] Δr=(r L -2r D +r C ) / 2…(2).

[0110] (Wrinkle height)

[0111] The wrinkle height of this stainless steel sheet can be 15 μm or less. The wrinkle height is obtained by taking a JIS No. 5 tensile test piece perpendicular to the rolling direction, applying a 16% strain to the test piece using the tensile testing method specified in the JIS standard (JIS Z 2241:2011), and measuring the height of the undulations in the width direction (orthogonal to the rolling direction of the stainless steel sheet) at the center between the evaluation points. The undulation height is the average height of the undulation curve elements measured using surface property measurements specified in JIS B 0601:2001 and other standards. The average height of the undulation curve elements measured in this manner is the undulation height (i.e., the wrinkle height) of this stainless steel sheet.

[0112] (Yield Strength)

[0113] The yield strength of this stainless steel plate can be 320 MPa or less. The yield strength is the value obtained by measuring the 0.2% yield strength of a JIS No. 13 B test piece using the tensile test method specified in the JIS standard (JIS Z2241:2011). The yield strength of this stainless steel plate can be 310 MPa or less, or 300 MPa or less.

[0114] <Method for Manufacturing Steel Sheet>

[0115] Figure 3 This is a flow chart showing an example of a method for producing a ferritic stainless steel sheet in one embodiment of the present invention. Figure 3 As shown, the present method for producing a stainless steel sheet (hereinafter sometimes referred to as simply the present method) includes a hot rolling step, a hot-rolled sheet annealing step, an intermediate step, and a final annealing step in this order.

[0116] The method for producing slabs for the hot rolling process is not particularly limited. For example, a steelmaking process may be included before the hot rolling process. In the steelmaking process, molten steel having a desired composition is poured into a mold and cooled, thereby producing a ferritic stainless steel slab. The slab is then cut into desired lengths for the hot rolling process. Furthermore, the present production method may include subsequent processes as appropriate after the final annealing process.

[0117] (Hot rolling process)

[0118] The hot rolling process is a process for producing hot-rolled sheets of a specified thickness by rolling (hot rolling) a slab at high temperature. The hot rolling process can be performed using known equipment and methods. In this production method, general production conditions can be adopted in the hot rolling process. For example, the heating temperature (rolling temperature) can be set to 1150-1250°C, and the total reduction ratio can be set to 95-99%. The coiling temperature after hot rolling can be set to 200-500°C.

[0119] (Hot-rolled plate annealing process)

[0120] In the hot-rolled sheet annealing step, the hot-rolled sheet is heated to a temperature range of 900-1000°C and held at a soaking temperature. The soaking temperature in the hot-rolled sheet annealing step is preferably 910-980°C. The soaking time in the hot-rolled sheet annealing step is 20-120 seconds.

[0121] In this manufacturing method, by performing the hot rolled sheet annealing step as described above, precipitates can be dissolved in the ferritic stainless steel. In this specification, the precipitates are divided into low-temperature precipitates and high-temperature precipitates.

[0122] Low-temperature precipitates refer to precipitates that precipitate in ferritic stainless steel in a temperature range of 600°C or higher and lower than 900°C. Examples of low-temperature precipitates include compounds containing Fe and at least one of Nb, Ti, and P. More specifically, examples of low-temperature precipitates include (i) phosphides such as FeTiP, FeNbP, and Fe(Ti,Nb)P, and (ii) intermetallic compounds that form a phase known as a Laves phase, such as Fe2Nb.

[0123] Furthermore, precipitates of compounds having a melting point of 900° C. or higher are referred to as high-temperature precipitates. Examples of high-temperature precipitates include metal carbides (such as TiC and NbC) and metal nitrides (such as TiN and NbN).

[0124] During the hot-rolled sheet annealing process, low-temperature and high-temperature precipitates are dissolved in the ferritic stainless steel, and recrystallization separates the hot-rolled structure, which causes wrinkling. Therefore, the lower limit of the heating temperature is set at 900°C, and the lower limit of the soaking time is set at 20 seconds. On the other hand, if the recrystallized structure of the hot-rolled sheet becomes coarse, wrinkling worsens. Therefore, the upper limit of the heating temperature is set at 1000°C, and the upper limit of the soaking time is set at 120 seconds. This separates the colonies in the metal structure.

[0125] The cooling rate after soaking in the hot-rolled sheet annealing process can be set to 5°C / second or higher within the temperature range from the soaking temperature to 500°C. This can suppress the formation of low-temperature precipitates during cooling. The cooling rate in the temperature range below 500°C is not particularly limited. The semi-finished product after cooling is referred to as hot-rolled annealed sheet.

[0126] The annealing equipment used in the hot-rolled sheet annealing step is not particularly limited, and for example, known equipment such as a continuous annealing furnace and a batch furnace can be used.

[0127] (Intermediate process)

[0128] Typically, at least one cold rolling step is performed between the hot-rolled sheet annealing step and the final annealing step. The hot-rolled annealed sheet may be subjected to a pickling step. The pickling step is a step in which scale adhering to the surface of the hot-rolled annealed sheet is removed using a pickling solution such as sulfuric acid, hydrochloric acid, or a mixture of nitric acid and hydrofluoric acid.

[0129] The cold rolling step is a step of cold rolling the hot-rolled annealed sheet (e.g., at room temperature to 200°C) to obtain a cold-rolled sheet of a predetermined thickness. Regarding the rolling reduction in the cold rolling step, a higher rolling reduction is more effective in improving the average r-value and formability, and therefore a rolling reduction of 75% or more is preferred. The rolling reduction in the cold rolling step can be 90% or less.

[0130] The rolling equipment used in the cold rolling step is not particularly limited, and known equipment can be used.

[0131] In the intermediate step, cold rolling may be performed only once or twice or more. When cold rolling is performed twice or more, an intermediate annealing step may be performed between the cold rolling steps. The heating temperature in the intermediate annealing step may be, for example, 900 to 1000°C.

[0132] <Final Annealing Process>

[0133] The final annealing step involves heating the cold-rolled sheet to a temperature range of 900-1000°C and then holding the temperature for 0-120 seconds. The heating rate during the final annealing step is 100°C / s or higher. The heating rate can be 100-2000°C / s, and preferably 100-1500°C / s.

[0134] In the final annealing process, by heating the cold-rolled sheet under the above conditions, low-temperature precipitates are less likely to precipitate during the heating process, and recrystallization occurs in random orientations while accelerating the growth rate of grains. By setting the heating rate to 100°C / s or higher, the amount of low-temperature precipitates formed during the heating process can be effectively reduced. Furthermore, if the heating time is too short, the heat required for recrystallization may be insufficient. Therefore, the heating rate can be set to 2000°C / s or lower, or even 1500°C / s or lower.

[0135] The temperature increase under the above conditions in the final annealing step suppresses the precipitation of low-temperature precipitates and promotes the growth of grains in the metal structure. By dissolving the low-temperature precipitates in the hot-rolled sheet annealing step and then performing the temperature increase under the above conditions in the final annealing step, the growth of grains with various crystal orientations can be promoted.

[0136] The lower limit of the heating temperature (soaking temperature) in the final annealing step is set to 900°C, at which low-temperature precipitates are not generated and recrystallization is complete. On the other hand, if the grains grow too large, the in-plane anisotropy becomes larger. Therefore, the upper limit of the heating temperature is set to 1000°C, and the upper limit of the soaking time is set to 120 seconds. The soaking temperature in the final annealing step is preferably 910-980°C, and the soaking time is preferably 5-100 seconds.

[0137] The final annealing process in the present manufacturing method is described in more detail as follows. First, as a comparison, the case of annealing at a general heating rate is described. When the temperature is increased at a general heating rate, recrystallization occurs in the order of {111} grains, {211} grains, {311} grains, and {100} grains as the temperature rises. That is, during the heating, recrystallization nuclei of {111} grains are generated at a temperature of about 800°C, for example. Then, with the subsequent heating, recrystallization nuclei of grains with other orientations are generated. When recrystallization is performed by annealing, grains with a specific direction (for example, {111}<112> orientation) in the {111} grains that were first nucleated grow preferentially. Therefore, when annealing is performed at a general heating rate, the anisotropy of the ferritic stainless steel sheet becomes larger. In contrast, in the final annealing step of the present manufacturing method, by increasing the temperature under the above-mentioned conditions, not only {111} grains but also recrystallization nuclei in other orientations are generated at approximately the same time, thereby suppressing the preferential growth of only a specific orientation.

[0138] The annealing equipment used in the final annealing step is not particularly limited, and for example, known equipment such as a continuous annealing furnace and a batch furnace can be used.

[0139] After the final annealing step, a pickling step may be performed as needed. In addition, subsequent steps may be performed as needed. In the subsequent steps, for example, temper rolling and cutting into a desired shape may be performed.

[0140] Summarize

[0141] The ferritic stainless steel sheet of the first embodiment of the present invention has a chemical composition containing, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030%, and P: 0.005-0.050%, and contains at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, and the S content is 0.0100% or less. , the remainder being Fe and inevitable impurities, wherein, in a cross section of the ferritic stainless steel plate parallel to the rolling direction and perpendicular to the rolling surface, the crystal grain size calculated by a cutting method is 15 μm or more and 40 μm or less, and in a cross section of the plate thickness center portion of the ferritic stainless steel plate parallel to the rolling surface, the {111}<110> crystal orientation strength is denoted as Ia, and the {111}<112> crystal orientation strength is denoted as Ib, and the larger value of Ia and Ib is referred to as I 111 , satisfy I 111 The relationship is ≥10.0 and 0.2≤(Ia / Ib)≤4.0.

[0142] The ferritic stainless steel sheet of the embodiment 2 of the present invention is characterized in that, in the embodiment 1, the yield strength is 320 MPa or less, the average r value is 1.4 or more, and the in-plane anisotropy index Δr of the r value is -0.50 or more and 0.50 or less. L +2r D +r C ) / 4, Δr=(r L -2r D +r C ) / 2、r L : Lankford value at 0° relative to the rolling direction, r D : Lankford value at 45° relative to the rolling direction, r C : Lankford value at 90° relative to the rolling direction.

[0143] The ferritic stainless steel sheet according to aspect 3 of the present invention is characterized in that, in aspect 1 or 2 above, a JIS No. 5 tensile test piece is obtained from the ferritic stainless steel sheet perpendicular to the rolling direction, a 16% strain is applied to the JIS No. 5 tensile test piece, and the wrinkle height obtained by measuring the height of the undulation in the width direction at the center between the evaluation points is 15 μm or less.

[0144] The ferritic stainless steel sheet of the fourth embodiment of the present invention is any one of the embodiments 1 to 3, further comprising, in mass%, Mo: 0.05-2.00%, Ni: 0.01-1.00%, Co: 0.005-0.500%, Cu: 0.05-1.00%, Al: 0.01-1.00%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, One or more of B: 0.0001-0.0025%, V: 0.05-0.50%, W: 0.05-1.00%, Sn: 0.005-0.500%, Sb: 0.005-0.500%, Zr: 0.050-0.500%, Y: 0.001-0.100%, Hf: 0.001-0.100% and rare earth elements: 0.001-0.100%.

[0145] The method for producing a ferritic stainless steel sheet according to the fifth embodiment of the present invention includes the following steps: a hot-rolled sheet annealing step of heating the hot-rolled sheet to a temperature range of 900 to 1000° C. and holding the hot-rolled sheet at this temperature for 20 to 120 seconds, wherein the hot-rolled sheet has the following chemical composition: C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, Cr: 10.5 to 30.0%, N: 0.001 to 0.030%, and P: 0.005 to 1.00%. The present invention relates to a method for preparing a hot-rolled sheet having a steel content of 0.050%, at least one of 0.01% to 0.50% Ti and 0.01% to 0.50% Nb, a S content of 0.01% or less, and the remainder being Fe and inevitable impurities; and a final annealing step of heating the hot-rolled annealed sheet obtained in the hot-rolled sheet annealing step by cold rolling the cold-rolled sheet obtained by the hot-rolled sheet annealing step to a temperature range of 900 to 1000°C at a heating rate of 100°C / s or more, and then soaking and holding for 120 seconds or less or cooling the sheet without soaking and holding.

[0146] [Additional matters]

[0147] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0148] Example

[0149] An embodiment of the present invention will be described below. It should be noted that the method for producing a ferritic stainless steel sheet described in this embodiment is just an example.

[0150] (Manufacturing of slabs)

[0151] Steels having the chemical compositions shown in Table 1 below were vacuum melted to produce 30 kg slabs. In Table 1, steel grades A to N have chemical compositions within the scope of the present invention. Furthermore, in Table 1, steel grades P, R, and S have chemical compositions outside the scope of the present invention. Table 1 shows the composition of the components contained in each steel grade in mass %. The remainder other than the components shown in Table 1 is iron or unavoidable impurities. Underlined elements in Table 1 indicate that the composition of the components contained in each steel grade of the comparative example is outside the scope of the present invention.

[0152]

[0153] The slab was heated at 1200° C. for 2 hours and then hot-rolled to produce a hot-rolled plate having a thickness of 3 mm.

[0154] Next, the hot rolled plate annealing process and the final annealing process were carried out under the manufacturing conditions shown in Table 2 to manufacture steel plates No. 1 to 38 with a plate thickness of 0.6 mm. In addition, in this embodiment, only one cold rolling was performed as an intermediate process. The rolling reduction of the cold rolling was set to 80%. In addition, the results of evaluating various physical properties of each steel plate are also shown in Table 2. In addition, the underline in Table 2 indicates that the manufacturing conditions and various physical properties of the steel plate are outside the scope of the present invention, or outside the preferred range of the present invention. Crystal grain size, crystal orientation strength (I 111 , Ia, Ib), yield strength, average r value, Δr, and wrinkle height were measured or calculated using the same methods as in the aforementioned embodiments. The hatched column describing the hot-rolled sheet annealing conditions for Steel Plate No. 3 indicates that the hot-rolled sheet annealing process was not performed (omitted).

[0155]

[0156] As shown in Table 2, the steel sheets of the examples of the present invention produced by the method for producing a ferritic stainless steel sheet according to one embodiment of the present invention all have the characteristics of grain size, crystal orientation strength, yield strength, average r value, Δr, and wrinkle height.

[0157] In contrast, at least one of the above-mentioned properties of the steel sheets of the comparative examples does not meet the benchmark. The yield strength, average r value, and Δr of the steel sheet No. 3 of the comparative example are within the range of the present invention, but due to the omission of the hot-rolled plate annealing process, the division of the colonies is insufficient, and the wrinkling resistance is insufficient. For the steel sheet No. 6 of the comparative example, the division of the colonies is also insufficient. In the steel sheet No. 8 of the comparative example, the heating in the hot-rolled plate annealing process is insufficient, resulting in a small grain size, an increase in yield strength, and insufficient wrinkling resistance. In the steel sheets No. 10 and 24 of the comparative examples, the recrystallized structure becomes coarse, thereby deteriorating the wrinkling resistance.

[0158] In Comparative Example Steel Sheet No. 13, the heating rate during the final annealing step was outside the range of the present invention, resulting in precipitation of low-temperature precipitates and preferential grain growth in a specific orientation (e.g., {111}<112> orientation), which increased in-plane anisotropy. In Comparative Example Steel Sheets Nos. 16 and 28, insufficient heating during the final annealing step resulted in small grain sizes and increased yield strength. On the other hand, in Comparative Example Steel Sheets Nos. 19, 21, 30, and 33, excessive grain growth due to heating during the final annealing step resulted in increased in-plane anisotropy.

[0159] Description of Reference Numerals

[0160] 1 stainless steel plate

[0161] 11 Rolled surface

[0162] 12 Sections

[0163] 13 Section

Claims

1. A ferritic stainless steel sheet having the following chemical composition: containing, by mass%, 0.001-0.030% C, 0.01-1.00% Si, 0.01-1.00% Mn, 10.5-30.0% Cr, 0.001-0.030% N, and 0.005-0.050% P; and at least one of 0.01-0.50% Ti and 0.01-0.50% Nb; wherein the S content is 0.0100% or less, and the remainder is Fe and unavoidable impurities. In the cross section of the ferritic stainless steel sheet parallel to the rolling direction and perpendicular to the rolling surface, the grain size calculated by a cutting method is 15 μm or more and 40 μm or less, In a cross section parallel to the rolling surface at the center of the plate thickness of the ferritic stainless steel plate, the {111}<110> crystal orientation intensity is denoted as Ia, the {111}<112> crystal orientation intensity is denoted as Ib, and the larger value of Ia and Ib is denoted as Ib. 111 , satisfy I 111 The relationship is ≥10.0 and 0.2≤(Ia / Ib)≤4.

0.

2. The ferritic stainless steel sheet according to claim 1, wherein The yield strength is 320 MPa or less, the average r value is 1.4 or more, and the in-plane anisotropy index Δr of the r value is -0.50 or more and 0.50 or less, Here, the average r value = (r L +2r D +r C ) / 4 Δr=(r L -2r D +r C ) / 2 r L : Lankford value at 0° relative to the rolling direction r D : Lankford value at 45° relative to the rolling direction r C : Lankford value at 90° relative to the rolling direction.

3. The ferritic stainless steel sheet according to claim 1 or 2, wherein: A JIS No. 5 tensile test piece was obtained from the ferritic stainless steel sheet perpendicular to the rolling direction. The JIS No. 5 tensile test piece was strained 16% and the wrinkle height was measured at the center between evaluation points in the width direction. The wrinkle height was 15 μm or less.

4. The ferritic stainless steel sheet according to any one of claims 1 to 3, wherein In terms of mass%, it further contains Mo: 0.05-2.00%, Ni: 0.01-1.00%, Co: 0.005-0.500%, Cu: 0.05-1.00%, Al: 0.01-1.00%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, B: 0.0001-0.0025% %, V: 0.05~0.50%, W: 0.05~1.00%, Sn: 0.005~0.500%, Sb: 0.005~0.500%, Zr: 0.050~0.500%, Y: 0.001~0.100%, Hf: 0.001~0.100% and rare earth elements: 0.001~0.100% or more.

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