Ferritic stainless steel sheet

Abstract

Provided is a ferritic stainless steel sheet which has a fine average crystal grain diameter, a high r value, and excellent deep drawing formability. The average crystal grain diameter of the stainless steel plate (1) is 15.0-30.0 [mu] m, and in a cross-section (13) parallel to the rolling surface in the center of the plate thickness, the relationships Ia-Ib > = 20.0 and (Ia-Ib) / d > = 1.00 are satisfied, where Ia is the crystal orientation strength {111} < 112 >, Ib is the crystal orientation strength {311} < 011 >, and d is the average crystal grain diameter.

Description

Technical Field

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

[0002] One indicator of the deep drawability of ferritic stainless steel sheets is the plastic working strain ratio (r-value). Generally, the r-value can be increased by increasing grain growth during the manufacturing process of ferritic stainless steel sheets. However, when the metallurgy of ferritic stainless steel sheets contains coarse grains, surface irregularities may occur during deep drawing, which can become the starting point of cracks.

[0003] Conventionally, various methods have been studied to improve the formability of ferritic stainless steel sheets (for example, see Patent Documents 1 to 3).

[0004] Patent Document 1 discloses a technique for improving the formability of a ferritic stainless steel sheet by setting the average grain size of the final product sheet to dz and the average grain size of the intermediate product sheet before final cold rolling to dx, and satisfying the relationship dx / dz≤3.

[0005] Patent Document 2 describes a technique for improving deep drawability and surface roughness resistance by defining conditions in each step of the production process of a ferritic stainless steel sheet.

[0006] Furthermore, research is generally underway to control the texture of the metallurgical structure of ferritic stainless steel sheets to increase the r-value. Patent Document 3 describes a technique for annealing hot-rolled and cold-rolled sheets, specifying conditions for achieving a ratio of ferrite grains with an orientation difference of within 10° from {111} / / ND of 20% or greater in a cross-section defined by the rolling and thickness directions.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-180206

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2003-138349

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 2009-299116 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] However, the technology described in Patent Document 1 requires that the average grain size dx of the intermediate product sheet be made fine to a certain extent, corresponding to the range of the average grain size dz required for the final product sheet. If the manufacturing conditions in the hot rolling process, etc., are specified or multiple cold rolling steps are performed to reduce the average grain size dx, the manufacturing cost of the ferritic stainless steel sheet may increase.

[0014] In the technology described in Patent Document 2, the average grain size of the ferritic stainless steel sheet is actually within the range of 35 μm to 50 μm, which may cause surface irregularities during deep drawing. In situations where high demands are placed on the surface quality of deep-drawn products, there is a demand for ferritic stainless steel sheet with a finer average grain size and excellent deep drawability.

[0015] Furthermore, as in the technique described in Patent Document 3, the r-value of a ferritic stainless steel sheet can be improved by increasing the ratio of grains having {111} planes. However, such a method alone may not sufficiently improve the r-value.

[0016] An object of one embodiment of the present invention is to provide a ferritic stainless steel sheet having a fine average crystal grain size, a high r-value, and excellent deep drawability.

[0017] Means for solving problems

[0018] 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%, with a S content of 0.01% or less, and the remainder being Fe and non-ferrous metals. Avoid impurities, wherein, in a cross section parallel to the rolling direction and perpendicular to the rolling surface of the ferritic stainless steel plate, the average crystal grain size calculated by a cutting method is 15.0 μm or more and 30.0 μm or less, and in a cross section parallel to the rolling surface at a center portion of the plate thickness of the ferritic stainless steel plate, the {111}<112> crystal orientation strength is denoted as Ia, the {311}<011> crystal orientation strength is denoted as Ib, and the average crystal grain size is denoted as d, and the relationship Ia-Ib≥20.0 and (Ia-Ib) / d≥1.00 is satisfied.

[0019] Effects of the Invention

[0020] According to one embodiment of the present invention, a ferritic stainless steel sheet having a fine average crystal grain size, a high r-value, and excellent deep drawability can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram showing a cross section of a stainless steel plate according to one embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram showing a cross section of a stainless steel plate according to one embodiment of the present invention.

[0023] 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.

[0024] Figure 4 This is a graph schematically showing an example of a heat treatment pattern in the final annealing step.

[0025] Figure 5 This is a graph schematically showing another example of the heat treatment pattern in the final annealing step.

[0026] Figure 6 This is a graph schematically showing another example of the heat treatment pattern in the final annealing step.

[0027] Figure 7 This is a graph schematically showing another example of the heat treatment pattern in the final annealing step.

[0028] Figure 8 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

[0029] 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.

[0030] 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."

[0031] 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.

[0032] <About the Summary of the Invention>

[0033] Generally, the formability (deep drawability) of ferritic stainless steel sheets can be evaluated by the r-value (Lankford value, plastic working strain ratio). Typically, deep drawability can be evaluated by averaging multiple r-values ​​measured in multiple different in-plane directions with the rolling direction as the reference.

[0034] 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.

[0035] Techniques for increasing the r-value by growing coarse grains or controlling the texture that is effective for increasing the r-value have been proposed. However, annealing generally produces grains with various crystal orientations. This growth not only of grains with orientations that are effective for increasing the r-value but also of grains with orientations that hinder the increase in the r-value limits the improvement in deep drawability.

[0036] The present inventors conducted extensive research on ferritic stainless steel sheets that maintain a small average grain size and exhibit a high r-value, leading to the present invention. The ferritic stainless steel sheet in 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 in one embodiment of the present invention will be described in detail below.

[0037] <Steel Plate Composition>

[0038] 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."

[0039] 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.

[0040] The present stainless steel plate may have a chemical composition in which the remainder is iron (Fe) and inevitable impurities.

[0041] (C: Carbon)

[0042] 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%.

[0043] (Si: Silicon)

[0044] 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%.

[0045] (Mn: manganese)

[0046] 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%.

[0047] (Cr: chromium)

[0048] 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.

[0049] (N: Nitrogen)

[0050] 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 0.008 mass% or more, or 0.015 mass% or more. The C+N content, expressed in mass%, can be within the range of 0.008 ≤ C+N ≤ 0.050, or 0.015 ≤ C+N ≤ 0.050.

[0051] (P: Phosphorus)

[0052] If P is excessively contained, weldability, toughness of welded portions, and workability may deteriorate. Therefore, the P content may be 0.005 to 0.050 mass%, 0.005 to 0.040 mass%, or 0.010 to 0.030 mass%.

[0053] (Ti and Nb: titanium and niobium)

[0054] 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.

[0055] 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.

[0056] (S: Sulfur)

[0057] 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.

[0058] (Other ingredients)

[0059] 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.

[0060] (Mo: molybdenum)

[0061] Mo is an element that is effective in improving corrosion resistance. However, if excessive amounts of Mo are added, the raw material cost of stainless steel increases. Therefore, when Mo is included in the chemical composition, the Mo content can be 0.05 to 2.00 mass%.

[0062] (Ni: nickel)

[0063] Ni is an element that is 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, or 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.

[0064] (Co: cobalt)

[0065] 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%.

[0066] (Cu: copper)

[0067] 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.

[0068] (Al: aluminum)

[0069] 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%.

[0070] (Ca: calcium)

[0071] 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 %.

[0072] (Mg: magnesium)

[0073] 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%.

[0074] (B: Boron)

[0075] 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%.

[0076] (V: Vanadium)

[0077] 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%.

[0078] (W: tungsten)

[0079] 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 0.03 to 1.00 mass%.

[0080] (Sn: Tin)

[0081] 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%.

[0082] (Sb: Antimony)

[0083] 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%.

[0084] (Zr: Zirconium)

[0085] 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%.

[0086] (Y: yttrium)

[0087] 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 content of Y can be 0.001 to 0.100 mass%.

[0088] (Hf: Hafnium)

[0089] 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%.

[0090] (REM: Rare Earth Elements)

[0091] 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%.

[0092] <Characteristics of Steel Plate>

[0093] This stainless steel sheet has the chemical composition described above, and by controlling the manufacturing conditions, a material structure (internal structure) having the following characteristics is formed. In brief, in the final annealing process, the temperature is raised to a temperature range of 500 to 900°C and maintained within this temperature range for 5 to 120 seconds. This allows {111} oriented grains, which contribute greatly to the improvement of the r value, to grow preferentially, while on the other hand, the amount of {311} grains, which contribute less to the improvement of the r value, to be generated. As a result, the average crystal grain size can be maintained small, and the average r value can be increased by controlling the texture in the metal structure, thereby improving deep drawability. More details will be described later together with the description of the manufacturing method of this stainless steel sheet.

[0094] (Grain size of steel plate)

[0095] 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.

[0096] The stainless steel plate 1 has an average grain size of 15.0 μm or more and 30.0 μm or less calculated by a cutting method in the cross section 12. The average grain size of the stainless steel plate 1 may be 20.0 μm or more and 29.0 μm or less.

[0097] The average 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 the cross 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.

[0098] (Crystal Orientation)

[0099] 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 center thickness of the steel plate 1, parallel to the rolling surface 11. The rolling direction is abbreviated as RD (Rolling Direction), the normal direction of the rolling surface is abbreviated as ND (Normal Direction), and the direction perpendicular to the rolling is abbreviated as TD (Transverse Direction). Cross section 13 is the so-called ND plane.

[0100] 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.

[0101] 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>.

[0102] The ODF is a function of three variables (φ1, Φ, and φ2) that uniquely specifies the crystallographic 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.

[0103] In the Euler space cross section with φ2 = 45°, the position with φ = 55° and φ1 = 30° is defined as the {111}<112> orientation, and the position with φ = 25° and φ1 = 0° is defined as the {311}<011> orientation. The {311}<011> orientation is equivalent to the {311}<110> orientation.

[0104] XRD measurement is performed on the stainless steel plate 1, thereby obtaining positive pole diagrams of (200), (110), and (211). ODF analysis is performed using the obtained positive pole diagram. The crystal orientation intensity of {111}<112> obtained by ODF analysis is set to Ia, and the crystal orientation intensity of {311}<011> obtained by ODF analysis is set to 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. It should be noted that the calculation method in the ODF analysis can use the WIMV method of Matthies and Vinel, which does not use continuous functions in the analysis. For example, SmartLab can be used as an X-ray diffraction device and a Mo radiation source can be used as an X-ray source. In this case, the pole diagram data used for ODF analysis is processed in SmartLab, and the intensity is standardized by correction processing such as background correction and randomization. This standardization processing is performed under certain conditions without setting conditions individually by selecting the standardization check box displayed on the display in the user interface of the X-ray diffraction device. The crystal orientation strength can also be expressed as an orientation density or an X-ray random intensity ratio.

[0105] The stainless steel plate 1 satisfies Ia-Ib≥20.0, and the average grain size is d, satisfying the relationship (Ia-Ib) / d≥1.00. Satisfying the above relationship allows the stainless steel plate 1 to have a fine average grain size and a high r value.

[0106] The value calculated by the relationship formula of Ia-Ib of the stainless steel plate 1 can be greater than 20.0 and less than 30.0, and the value calculated by the relationship formula of (Ia-Ib) / d can be greater than 1.00 and less than 1.50. When the value calculated by the relationship formula of Ia-Ib exceeds 30, the in-plane anisotropy may deteriorate. In addition, it is not easy to define the manufacturing conditions of the stainless steel plate 1 having an average crystal grain size d, crystal orientation strength Ia or crystal orientation strength Ib such that the value calculated by the relationship formula of (Ia-Ib) / d exceeds 1.5, which may lead to an increase in production costs. The value calculated by the relationship formula of (Ia-Ib) / d of the stainless steel plate 1 can be greater than 1.00 and less than 1.35.

[0107] (Average r value)

[0108] The average r-value (average Lankford value) of the present stainless steel plate can be 1.9 or more.

[0109] 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): Average r value = (r L +2rD +r C ) / 4…(1).

[0110] 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 present stainless steel plate can be 1.9 or more and 2.3 or less.

[0111] (Limiting drawing ratio)

[0112] The limited drawing ratio (LDR) of the present stainless steel plate may be greater than 2.4. Generally, the limited drawing ratio (forming limit drawing ratio) can be determined through a deep drawing test and is used as an indicator of the deep drawing workability (deep drawing formability) of the stainless steel plate. The limited drawing ratio is the value obtained by dividing the maximum billet diameter that can be deep drawn without cracking the test piece (stainless steel plate) by the diameter of the punch used in the deep drawing process. The limited drawing ratio of the present stainless steel plate may be greater than 2.4 and less than 2.6.

[0113] <Method for Manufacturing Steel Sheet>

[0114] 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 manufacturing method of the stainless steel sheet (hereinafter sometimes referred to as the manufacturing method) includes a preparatory step of preparing a cold-rolled steel sheet and a final annealing step of annealing the cold-rolled steel sheet. In the manufacturing method of the stainless steel sheet, the cold-rolled steel sheet can be a cold-rolled steel sheet manufactured by a general manufacturing method, and the specific manufacturing method of the cold-rolled steel sheet is not necessarily limited. In the preparatory step of preparing the cold-rolled steel sheet, the cold-rolled steel sheet can be manufactured by cold rolling the rolling object material (steel material) of the ferritic stainless steel having the above-mentioned chemical composition, or a pre-manufactured cold-rolled steel sheet can be prepared. An example of the manufacturing process of the cold-rolled steel sheet will be described later. In addition, the manufacturing method may appropriately include subsequent processes after the final annealing process.

[0115] (Final annealing process)

[0116] The final annealing process in the manufacture of the present stainless steel plate includes: a first annealing process S1, which heats the cold-rolled steel plate to a predetermined first reaching temperature in a first temperature range of 500 to 900°C at a heating rate of 100 to 2000°C / s; and a second annealing process S2, which continuously performs annealing from the first annealing process with a holding time in the first temperature range of 5 to 120 seconds.

[0117] Hereinafter, with reference to the accompanying drawings, the heat treatment pattern in the final annealing process and its technical significance will be described case by case according to the conditions of the second annealing process.

[0118] (Heat treatment mode 1)

[0119] Figure 4 This is a graph schematically showing an example of a heat treatment pattern in the final annealing step. Figure 4 In fact, the temperature change caused by the passage of time is of course not limited to a linear shape. Figure 4 , an example is shown in which the temperature is raised to the second ultimate temperature in the second annealing step S2 and soaking is performed at the second ultimate temperature exceeding 900° C. (heat treatment pattern 1).

[0120] like Figure 4 As shown, the target temperature for heating in the first annealing step S1, i.e., the first reaching temperature, is designated as T1, and the target temperature for heating in the second annealing step S2, i.e., the second reaching temperature, is designated as T2. Furthermore, the heating time required to heat the material from the first reaching temperature T1 to the second reaching temperature T2 is designated as t1, the soaking time at the second reaching temperature T2 is designated as t2, and the cooling time required to cool the material from the second reaching temperature T2 to 500°C is designated as t3. These symbols have the same meanings in the following description, and repeated descriptions are omitted.

[0121] exist Figure 4 In the example shown, the second peak temperature T2 can be greater than 900°C and less than 1000°C. The first peak temperature T1 in the first annealing step S1, the second peak temperature T2 in the second annealing step S2, the heating time t1, the soaking time t2, and the cooling time t3 can be appropriately adjusted so that the average grain size d of the ferritic stainless steel obtained by the final annealing step is greater than 15.0 μm and less than 30.0 μm. If the second peak temperature T2 exceeds 1000°C, the grain growth rate increases, making it difficult to stably control the average grain size d within the desired range.

[0122] exist Figure 4In the example shown, the second peak temperature T2 exceeds 900°C. In this case, the hold time t in the first temperature range of 500-900°C can be set to the time during which the temperature is raised from the first peak temperature T1 to 900°C. In this manufacturing method, by raising the temperature to the 500-900°C temperature range in the first annealing step S1, the strain introduced into the cold-rolled steel sheet, which serves as a driving force for recrystallization, can be effectively utilized in the second annealing step S2. Subsequently, in the second annealing step S2, during the hold time t, the {111} grains, which contribute significantly to the increase in r-value, preferentially recrystallize, while the {311} grains, which contribute less to the increase in r-value, are less likely to recrystallize.

[0123] On the other hand, when the second annealing step S2 does not have a holding time t, for example, if the temperature is heated to a temperature exceeding 900°C in the first annealing step S1, recrystallization occurs in various orientations other than {111}. This is described in more detail below.

[0124] Typically, when annealing cold-rolled ferritic stainless steel sheets, recrystallization occurs in the order of {111} grains, {211} grains, {311} grains, and {100} grains as the temperature rises. For example, during heating, recrystallization nuclei for {111} grains form at temperatures around 800°C, and recrystallization nuclei for grains with other orientations form as the temperature rises further. Focusing on this property, the present inventors devised a method to preferentially generate recrystallization nuclei for {111} grains in the metal structure during the initial stages of the first annealing step S1 and the second annealing step S2, thereby developing a {111} texture and reducing the proportion of {311} grains, allowing for subsequent recrystallization. This method enables the production of ferritic stainless steel sheets with excellent deep drawability, fine average grain size, and a high r-value, using a relatively simple method.

[0125] exist Figure 4 In the example shown, the {111} orientation can be preferentially developed during the holding time t in the second annealing step S2, and the temperature can be subsequently raised to a temperature exceeding 900°C. However, the various conditions of the second annealing step S2 are set so that the average crystal grain size d of the ferritic stainless steel obtained by the final annealing step is 15.0 μm or more and 30.0 μm or less.

[0126] For the reasons described above, the first peak temperature T1 is set to 900°C or lower in order to selectively develop the {111} orientation. On the other hand, the first peak temperature T1 is set to 500°C or higher to ensure a minimum amount of heat. Furthermore, the time spent within the first temperature range of 500°C to 900°C during cooling from the second peak temperature T2 to 500°C is not included in the aforementioned hold time t in the second annealing step S2.

[0127] Furthermore, in the first annealing step S1 of the final annealing process, the development of the {111} texture is promoted by increasing the temperature from the start temperature to the first target temperature T1 at a heating rate of 100-2000°C / s. This is because if the heating rate is too slow until the temperature exceeds 500°C, the lower limit of the first temperature range, it will be difficult to generate recrystallization nuclei in the second annealing step S2, and the development of the {111} orientation may be insufficient. On the other hand, increasing the heating rate more than necessary will lead to excessive cost increases. Therefore, the upper limit of the heating rate is set to 2000°C / s.

[0128] It should be noted that in Figure 4 In the example shown, in the second annealing process S2, the annealing object material is soaked at the second reaching temperature T2 for a predetermined time (having a soaking time t2), but this is not limited to this. It can also be soaked at the second reaching temperature T2 for 0 seconds (t2=0 seconds), that is, cooling can also be started immediately after the temperature of the annealing object material reaches the second reaching temperature T2.

[0129] Reference Figure 4 The following description can be summarized. In one embodiment of a method for manufacturing a ferritic stainless steel sheet, the second annealing step S2 includes: heating from a first reaching temperature T1 to a second reaching temperature T2; (i) soaking at the second reaching temperature T2 for 0 seconds or (ii) maintaining the second reaching temperature T2 for a predetermined soaking time t2; and cooling from the second reaching temperature T2 to 500°C.

[0130] In one embodiment of the method for producing a ferritic stainless steel sheet, in the second annealing step S2 , the second ultimate temperature T2 is a temperature exceeding 900° C., and the holding time t is a time period during which the temperature is raised from the first ultimate temperature T1 to 900° C.

[0131] (Heat treatment modes 2 and 3)

[0132] Figure 5 This is a graph schematically showing another example of the heat treatment pattern in the final annealing step. Figure 6 is a graph schematically showing another example of the heat treatment pattern in the final annealing step. Figure 5 In FIG. 2 , an example is shown in which the temperature is raised to the second reaching temperature in the second annealing process S2 and soaked at the second reaching temperature T2 below 900° C. (heat treatment mode 2). Figure 6 , an example is shown in which the temperature is raised to the second ultimate temperature in the second annealing step S2 and soaked at the second ultimate temperature T2 of 900° C. or lower for 0 seconds (heat treatment pattern 3).

[0133] exist Figure 5 as well as Figure 6 In the example shown, the second reaching temperature T2 is set within a temperature range higher than the first reaching temperature T1 and lower than 900°C. As described above, in the second annealing step S2, by maintaining the temperature within the first temperature range of 500 to 900°C, the recrystallization nuclei of {111} grains are preferentially generated in the metal structure, and the {111} texture is developed. Figure 5 and Figure 6 In the example shown, the moment when the first reaching temperature T1 is reached to start the second annealing process S2 is set as the first moment TP1, and the moment when the temperature is increased from the first reaching temperature T1 to the second reaching temperature T2 and then cooled from the second reaching temperature T2 to 500°C is set as the second moment TP2, and the holding time t can be set to the time between the first moment TP1 and the second moment TP2.

[0134] exist Figure 5 In the example shown, the heat treatment mode is to maintain the second reaching temperature T2 during the predetermined soaking time t2. Figure 6 In the example shown, a heat treatment mode is set to soak at the second reaching temperature T2 for 0 seconds.

[0135] (Heat treatment mode 4)

[0136] Figure 7 is a graph schematically showing another example of the heat treatment pattern in the final annealing step. Figure 7 , an example is shown in which the temperature is not increased in the second annealing step S2 but soaking is performed at the first reaching temperature T1 (heat treatment pattern 4).

[0137] exist Figure 7 In the example shown, the second reaching temperature T2 is set to be the same as the first reaching temperature T1, that is, the temperature is not increased in the second annealing step S2 but is maintained at the first reaching temperature T1. Figure 7 In the example shown, the holding time t can also be set to the time from the first time TP1 to the second time TP2 for the reasons described above.

[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] (Cold-rolled steel sheet manufacturing process)

[0141] Figure 8This 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 8 As shown, in one example of the preparatory process in the present manufacturing method, a cold-rolled steel sheet can be manufactured. In this case, the present manufacturing method may include a first manufacturing process S11 for manufacturing the cold-rolled steel sheet and a second manufacturing process S12 including a final annealing process. The first manufacturing process S11 may include, for example, a steelmaking process, a hot rolling process, an intermediate process, and a cold rolling process.

[0142] The method for producing slabs in the steelmaking process is not particularly limited. For example, ferritic stainless steel slabs can be produced by pouring molten steel having a desired composition into a mold and cooling it. The slabs are cut into desired lengths and used in the hot rolling process.

[0143] The hot rolling process is a process for producing hot-rolled plates (hot-rolled steel sheets) of a specified thickness by rolling (hot rolling) slabs at high temperatures. The hot rolling process can be carried out using known equipment and methods. In this manufacturing method, general manufacturing 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.

[0144] The intermediate steps may include a hot-rolled plate annealing step and a pickling step for annealing the hot-rolled plate. In the case of a hot-rolled plate annealing step, the hot-rolled plate is heated to a temperature range of 900-1000°C and then soaked. The soaking temperature in the hot-rolled plate annealing step is preferably 910-980°C. The soaking time in the hot-rolled plate annealing step is 20-120 seconds.

[0145] When the hot-rolled sheet annealing step is included in the intermediate steps, the hot-rolled structure that causes wrinkling can be divided by recrystallization, thereby improving the wrinkling resistance of the ferritic stainless steel sheet.

[0146] During the hot-rolled sheet annealing process, 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. However, as the recrystallized structure of the hot-rolled sheet becomes coarser, 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 helps to separate the colonies in the metal structure.

[0147] 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.

[0148] The hot-rolled annealed sheet may be subjected to a pickling step. The pickling step is a step of removing oxide scale attached to the surface of the hot-rolled annealed sheet using a pickling solution such as sulfuric acid, hydrochloric acid, or a mixture of nitric acid and hydrofluoric acid.

[0149] The intermediate steps may include an intermediate cold rolling step and an intermediate annealing step as needed. The heating temperature in the intermediate annealing step may be, for example, 900 to 1000°C.

[0150] After the intermediate steps, the final cold rolling step is performed. The cold rolling step involves cold rolling the material (steel) to produce a cold-rolled steel sheet of a specified thickness (e.g., at room temperature to 200°C). The reduction ratio in the cold rolling step is preferably 75% or higher, as a higher reduction ratio is more effective in improving the average r-value and formability. The cold rolling reduction can be 90% or lower.

[0151] The rolling equipment used in the cold rolling step is not particularly limited, and known equipment can be used.

[0152] Summarize

[0153] 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 at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, with a S content of 0.01% or less, and the remainder being Fe and unavoidable impurities. , wherein, in a cross section of the ferritic stainless steel plate parallel to the rolling direction and perpendicular to the rolling surface, the average grain size calculated by a cutting method is 15.0 μm or more and 30.0 μm or less, and in a cross section of a plate thickness center portion of the ferritic stainless steel plate parallel to the rolling surface, the {111}<112> crystal orientation strength is denoted by Ia, the {311}<011> crystal orientation strength is denoted by Ib, and the average grain size is denoted by d, and the relationship Ia-Ib≥20.0 and (Ia-Ib) / d≥1.00 is satisfied.

[0154] The ferritic stainless steel sheet of the embodiment 2 of the present invention is the one in which the average r value is 1.9 or more in the embodiment 1. Here, the average r value = (r L +2r D +r C ) / 4.

[0155] The ferritic stainless steel sheet according to aspect 3 of the present invention is characterized in that, in aspect 1 or 2, the limiting drawing ratio is 2.4 or more.

[0156] The ferritic stainless steel sheet of the fourth embodiment of the present invention is one of the embodiments 1 to 3, further comprising, in mass%, selected from the group consisting of Mo: 0.05-2.00%, Ni: 0.01-1.00%, Co: 0.05-0.50%, 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: 0.05-0.50%, Y: 0.001-0.100%, Hf: 0.001-0.100% and rare earth elements: 0.001-0.100%.

[0157] The method for producing a ferritic stainless steel sheet according to the fifth embodiment of the present invention comprises the step of preparing a cold-rolled steel sheet obtained by cold-rolling a steel material having a chemical composition comprising, in 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%, wherein the content of S is 0.01% or less, and the remainder is Fe and unavoidable impurities; and a final annealing step of annealing the cold-rolled steel sheet, wherein in a cross section of the ferritic stainless steel sheet parallel to the rolling direction and perpendicular to the rolling surface, the average grain size calculated by a cutting method is 15.0 μm or more and 30.0 μm or less, the final annealing step comprising: a first annealing step of heating the cold-rolled steel sheet at a heating rate of 100 to 2000°C / s to a predetermined first reaching temperature in a first temperature range of 500 to 900°C; and a second annealing step of continuously performing annealing in the first temperature range for a holding time of 5 to 120 seconds starting from the first annealing step.

[0158] In a sixth aspect of the present invention, the method for manufacturing a ferritic stainless steel sheet in the fifth aspect is characterized in that the second annealing step includes: heating from the first reaching temperature to a predetermined second reaching temperature; (i) soaking at the second reaching temperature for 0 seconds or (ii) soaking and holding at the second reaching temperature for a predetermined soaking time; and cooling from the second reaching temperature to 500°C.

[0159] According to a seventh aspect of the present invention, in the sixth aspect, the second ultimate temperature in the second annealing step is higher than 900°C, and the holding time is the time from the first ultimate temperature to 900°C.

[0160] In the method for manufacturing a ferritic stainless steel sheet according to aspect 8 of the present invention, in aspect 6, in the second annealing step, the second reaching temperature is a temperature higher than the first reaching temperature and not higher than 900° C., and the holding time is the time from the first moment when the first reaching temperature is reached at the start of the second annealing step, to the second moment when the temperature is raised from the first reaching temperature to the second reaching temperature and then cooled from the second reaching temperature to 500° C.

[0161] In the method for manufacturing a ferritic stainless steel sheet according to aspect 9 of the present invention, in aspect 5, the second annealing process includes: holding at the first reaching temperature; and cooling from the first reaching temperature to 500° C., wherein the holding time is the time between the first and second moments, with the time when the first reaching temperature is reached at the start of the second annealing process being the first moment and the time when the temperature is cooled from the first reaching temperature to 500° C. being the second moment.

[0162] [Additional matters]

[0163] 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.

[0164] Example

[0165] 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.

[0166] (Manufacturing of slabs)

[0167] 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 J have chemical compositions within the scope of the present invention. Furthermore, in Table 1, steel grades K, L, and M have chemical compositions outside the scope of the present invention. In Table 1, the composition of the components contained in each steel grade is expressed in mass %. The remainder other than the components shown in Table 1 is Fe or unavoidable impurities. Furthermore, 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.

[0168]

[0169] The slabs were heated at 1200°C for 2 hours and then hot rolled to produce 3 mm thick hot rolled sheets. The hot rolled sheets were then annealed at 950°C for 60 seconds, pickled, and cold rolled at a rolling reduction of 80% to obtain cold rolled sheets for testing.

[0170] Next, the final annealing process was carried out under the manufacturing conditions shown in Table 2 to manufacture steel plates No. 1 to 26 having a thickness of 0.6 mm. In addition, in this embodiment, only one cold rolling was performed. 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. The respective values ​​of the average crystal grain size d, the X-ray random intensity ratio of the crystal orientation (Ia, Ib) and the average r value were measured or calculated by the same method as in the above-mentioned embodiment. Deep drawing workability was evaluated as follows.

[0171] Each test piece was subjected to cylindrical deep drawing under the following conditions: punch diameter: 50 mm, punch shoulder R: 5 mm, die diameter: 52 mm, die shoulder R: 5 mm, wrinkle resistance: 1 ton, and drawing ratio: 2.4. In Table 2, those that could be formed are designated as suitable, and those that could not be formed are designated as unsuitable.

[0172]

[0173] 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 satisfy the prescribed relationship between the average grain size d and the random intensity ratio of the crystal orientation, and exhibit both an average r value and deep drawability.

[0174] In contrast, at least one of the aforementioned properties of the comparative example steel sheets failed to meet the benchmark. Comparative example steel sheet No. 3 exhibited insufficient heating during the second annealing step S2, resulting in an average grain size d below 15.0 μm, a low average r-value, and insufficient deep drawability. Comparative example steel sheets Nos. 6 and 22, due to a heating rate of less than 100°C / s during the first annealing step S1, insufficient recrystallization nuclei were generated in {111}<112>-oriented grains. Consequently, the development of the {111} texture and the reduction of {311}<011>-oriented grains were insufficient during the second annealing step S2, resulting in a low average r-value. Comparative example steel sheet No. 21, by maintaining a heating rate of less than 100°C / s during the first annealing step S1 and promoting grain growth in the {111}<112>-oriented grains during the second annealing step S2, improved the average r-value. In Steel Sheet No. 21, although the value calculated from the relational expression Ia-Ib satisfies the range of the present application, the value calculated from the relational expression (Ia-Ib) / d is outside the range of the present application, indicating insufficient deep drawability.

[0175] Comparative Example Steel Sheets No. 8 and 19 were heated to a first ultimate temperature T1 exceeding 900°C in the first annealing step S1, and therefore did not satisfy the conditions of the prescribed relationship. Comparative Example Steel Sheets No. 10 and 12 had an excessive amount of heat in the second annealing step S2, resulting in an average grain size d exceeding 30 μm and a relatively high average r value. However, deep drawability was insufficient (fractures occurred due to surface irregularities).

[0176] In the steel sheet No. 15 of the comparative example, the first reaching temperature T1 in the first annealing step S1 was lower than 500° C., and thus the heat amount during annealing was insufficient, resulting in insufficient development of the {111} texture.

[0177] In the case of the steel sheet No. 17 of the comparative example, the holding time t in the second annealing process S2 is short.

[0178] {111} The texture is not sufficiently developed and does not satisfy the conditions of the prescribed relationship.

[0179] Description of Reference Numerals

[0180] 1 stainless steel plate

[0181] 11 Rolled surface

[0182] 12 Sections

[0183] 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 average grain size calculated by a cutting method is 15.0 μm or more and 30.0 μm or less, In a cross section parallel to the rolled surface at the center of the plate thickness of the ferritic stainless steel plate, the {111}<112> crystal orientation strength is set to Ia, the {311}<011> crystal orientation strength is set to Ib, and the average crystal grain size is set to d, satisfying the relationship of Ia-Ib≥20.0 and (Ia-Ib) / d≥1.

00.

2. The ferritic stainless steel sheet according to claim 1, wherein The average r value is above 1.9, Here, the average r value = (r L +2r D +r C ) / 4 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: The limiting drawing ratio is 2.4 or more.

4. The ferritic stainless steel sheet according to any one of claims 1 to 3, wherein The present invention also contains, 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%, B: 0.0001-0.0025% , V: 0.05~0.50%, W: 0.03~1.00%, 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% one or more.

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