Ferritic stainless steel sheet

Abstract

Provided is a ferritic stainless steel sheet that has a high r-value while having a fine average crystalline particle diameter and that is excellent in deep drawing formability. A stainless steel sheet (1) has an average crystalline particle diameter of 15.0-30.0 μm, and satisfies, at a cross section (13) thereof that is parallel to a rolling surface and positioned at the midpoint of the sheet thickness, a relationship of Ia-Ib ≥ 20.0 and (Ia-Ib) / d ≥ 1.00, where Ia is the {111}<112> crystal orientation strength, Ib is the {311}<011> crystal orientation strength, and d is the average crystalline particle diameter.

Description

Ferritic stainless steel plate

[0001] The present invention relates to a ferritic stainless steel sheet.

[0002] The plastic working strain ratio (r-value) is one of the indicators of the deep drawability of ferritic stainless steel sheets. The r-value can usually be improved by growing crystal grains during the manufacturing process of ferritic stainless steel sheets. However, if the metal structure of a ferritic stainless steel sheet contains coarse crystal grains, surface irregularities may occur during deep drawing, and these surface irregularities can become the starting points for cracks.

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

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

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

[0006] In addition, it has also been generally studied to improve the r-value by controlling the texture in the metal structure of a ferritic stainless steel sheet. In the technology described in Patent Document 3, the conditions for hot-rolled sheet annealing and cold-rolled sheet annealing are specified so that the proportion of ferrite grains with an orientation misorientation from {111} / / ND of 10° or less in a cross section formed by the rolling direction and the sheet thickness direction is 20% or more.

[0007] Japanese Patent Publication No. 2002-180206 Japanese Patent Publication No. 2003-138349 Japanese Patent Publication No. 2009-299116

[0008] However, in the technology described in Patent Document 1, it is necessary to refine the average grain size dx of the intermediate product sheet to a certain extent in accordance with the range of the average grain size dz required for the final product sheet. If the manufacturing conditions in the hot rolling process or the like are specified or if cold rolling is performed multiple times in order to refine the average grain size dx, the manufacturing cost of the ferritic stainless steel sheet may increase.

[0009] In the technology described in Patent Document 2, the average crystal grain size of the ferritic stainless steel sheet is actually in the range of 35 μm to 50 μm, and surface irregularities may occur during deep drawing. When high requirements are placed on the surface quality of deep-drawn products, a ferritic stainless steel sheet having a finer average crystal grain size and excellent deep drawability is required.

[0010] Furthermore, as in the technology described in Patent Document 3, the r-value of a ferritic stainless steel sheet can be improved by increasing the proportion of crystal grains having {111} planes, but such a method alone may not be sufficient to improve the r-value.

[0011] An object of one aspect of the present invention is to provide a ferritic stainless steel sheet that has a fine average crystal grain size and a high r value, and that has excellent deep drawability.

[0012] In order to solve the above problems, a ferritic stainless steel sheet according to one embodiment of the present invention contains, by mass%, 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 0.050%, and also contains at least one of Ti: 0.01 to 0.50% and Nb: 0.01 to 0.50%, with an S content of 0.01% or less, and the balance being Fe and and unavoidable impurities, wherein in a cross section of the ferritic stainless steel sheet parallel to the rolling direction and perpendicular to the rolling surface, 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 the center of the sheet thickness of the ferritic stainless steel sheet, the {111}<112> crystal orientation intensity is Ia, the {311}<011> crystal orientation intensity is Ib, and the average crystal grain size is d, satisfying the relationships Ia-Ib≧20.0 and (Ia-Ib) / d≧1.00.

[0013] According to one aspect of the present invention, a ferritic stainless steel sheet having a fine average crystal grain size and a high r value, and excellent deep drawability, can be provided.

[0014] Fig. 1 is a schematic diagram showing a cross section of a stainless steel sheet according to one embodiment of the present invention. Fig. 2 is a schematic diagram showing a cross section of a stainless steel sheet according to one embodiment of the present invention. Fig. 3 is a flowchart showing an example of a method for producing a ferritic stainless steel sheet according to one embodiment of the present invention. Fig. 4 is a graph showing a schematic diagram of an example of a heat treatment pattern in a final annealing step. Fig. 5 is a graph showing a schematic diagram of another example of a heat treatment pattern in a final annealing step. Fig. 6 is a graph showing a schematic diagram of another example of a heat treatment pattern in a final annealing step. Fig. 7 is a flowchart showing an example of a method for producing a ferritic stainless steel sheet according to one embodiment of the present invention.

[0015] The following describes embodiments of the present invention. Note that the following description is intended to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. Furthermore, in this application, "A to B" indicates A or more and B or less.

[0016] In this specification, the term "ferritic stainless steel" is not limited to the specific form of steel strip, steel plate, etc., and is used to describe the properties of the material itself. Also, since a "steel plate" can be considered to be a part of a "steel strip," "ferritic stainless steel plate" includes a "ferritic stainless steel strip."

[0017] The ferritic stainless steel sheet according to one embodiment of the present invention has a chemical composition that allows it to remain in a ferrite single phase up to its melting point, and contains reduced carbon and nitrogen contents as well as carbide-stabilizing elements (Ti, Nb, etc.). Ferritic stainless steels having such a chemical composition are sometimes called high-purity ferritic stainless steels.

[0018] <Overview of the present invention> Generally, the formability (deep drawability) of a ferritic stainless steel sheet can be evaluated by the r-value (Lankford value, plastic working strain ratio). Typically, deep drawability can be evaluated by the average r-value obtained by averaging multiple r-values ​​obtained by measuring multiple different in-plane directions based on the rolling direction.

[0019] 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 their metal structure and are prone to the presence of colony structures (hereinafter referred to as colonies). These colonies are formed by the aggregation of crystal grains with similar crystal orientations. The crystal orientation of a colony differs from the crystal orientation of the texture around the colony.

[0020] Although techniques for increasing the r-value have been proposed, such as growing coarse grains or controlling the texture that is effective for increasing the r-value, annealing generally produces grains with various crystal orientations. This not only produces grains with orientations that are effective for increasing the r-value, but also grains with orientations that hinder the improvement of the r-value, limiting the improvement in deep drawability.

[0021] The present inventors have conducted extensive research into ferritic stainless steel sheets that have a high r-value while maintaining a small average grain size, and have arrived at the present invention. The ferritic stainless steel sheet of one aspect of the present invention is realized by a manufacturing approach that is different from conventional approaches. The various properties and manufacturing method of the ferritic stainless steel sheet of one aspect of the present invention will be described in detail below.

[0022] <Composition of Steel Sheet> First, the composition (chemical composition) of the ferritic stainless steel sheet according to one embodiment of the present invention will be described below. In the following description, the ferritic stainless steel sheet according to one embodiment of the present invention may be abbreviated as "the stainless steel sheet".

[0023] The stainless steel plate may have a chemical composition containing, in mass%, 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 0.050%, as well as at least one of Ti: 0.01 to 0.50% and Nb: 0.01 to 0.50%, with an S content of 0.0100% or less.

[0024] The stainless steel sheet may have a chemical composition with the balance being iron (Fe) and unavoidable impurities. Each of the elements mentioned above will be explained below.

[0025] (C: Carbon) Carbon is an element that forms carbides with Cr and other elements, thereby generating interfaces that become sources of dislocations when ferritic stainless steel is deformed. However, excessive addition of carbon reduces intergranular corrosion resistance and workability, and increases the cost of refining. Therefore, the carbon content may be 0.001 to 0.030 mass%, 0.001 to 0.020 mass%, or 0.002 to 0.010 mass%.

[0026] (Si: Silicon) Si acts as a deoxidizer during the melting stage. However, if excessive Si is added, the ferritic stainless steel becomes hard and its ductility decreases. Therefore, the Si content may be 0.01 to 1.00 mass%, 0.02 to 0.70 mass%, or 0.03 to 0.30 mass%.

[0027] (Mn: Manganese) Mn has the effect of acting as a deoxidizer. However, if excessive Mn is added, the amount of MnS produced increases, reducing the corrosion resistance of the ferritic stainless steel. Therefore, the Mn content may be 0.01 to 1.00 mass%, 0.02 to 0.70 mass%, or 0.03 to 0.30 mass%.

[0028] (Cr: Chromium) Cr is necessary to form a passive film on the surface of the cold-rolled steel sheet and improve corrosion resistance. However, if excessive Cr is added, the ductility of the ferritic stainless steel decreases. Therefore, the Cr content may be 10.5 to 30.0 mass%, or 12.0 to 25.0 mass%.

[0029] (N: Nitrogen) N forms nitrides with Cr and other elements, thereby generating interfaces that serve as sources of dislocations when ferritic stainless steel is deformed. However, excessive addition of N reduces ductility due to solid solution strengthening. Therefore, the N content may be 0.001 to 0.030 mass%, or 0.005 to 0.025 mass%. Furthermore, in this stainless steel sheet, the C+N content, which is the sum of the C content and the N content, may be 0.050 mass% or less, or 0.045 mass% or less. If the C+N content is too high, the amount of carbonitride precipitation may be excessive. On the other hand, because excessive reduction of the C+N content increases refining costs, the C+N content may be 0.008 mass% or more, or 0.015 mass% or more. The C+N content, in mass %, may be in the range of 0.008≦C+N≦0.050, or may be in the range of 0.015≦C+N≦0.050.

[0030] (P: Phosphorus) Excessive P content can deteriorate weldability, weld toughness, and workability. 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%.

[0031] (Ti and Nb: Titanium and Niobium) Ti and Nb bond with C or N and fix them as precipitates such as TiC, TiN, NbC, or NbN, so that purifying ferritic stainless steel can improve the average r-value and product elongation. On the other hand, excessive Ti and Nb content increases raw material costs and can reduce manufacturability due to an increase in recrystallization temperature.

[0032] 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%. The Nb content may be 0.01 to 0.50 mass%, 0.02 to 0.40 mass%, or 0.10 to 0.30 mass%. The ferritic stainless steel may contain only either Ti or Nb, or may contain both Ti and Nb.

[0033] (S: Sulfur) S is an impurity atom that adversely affects hot workability, corrosion resistance, and oxidation resistance. Therefore, the S content may be 0.0100 mass% or less. Ferritic stainless steels do not need to contain S, and there is no particular lower limit for the S content. The S content may be 0 (including no addition) to 0.0100 mass%. An S content of "0 (including no addition)" means that S is allowed to be contained as an unavoidable impurity.

[0034] (Other Components) The present stainless steel sheet may have a chemical composition further containing, in mass %, one or more elements selected from the group consisting of Mo, Ni, Co, Cu, Al, Ca, Mg, B, V, W, Sn, Sb, Zr, Y, Hf, and rare earth elements.

[0035] (Mo: Molybdenum) Mo is an element effective in improving corrosion resistance. However, if excessive Mo is added, the raw material cost of stainless steel increases. Therefore, when Mo is included in the chemical composition, the Mo content may be 0.05 to 2.00 mass%.

[0036] (Ni: Nickel) Ni is an element effective in improving corrosion resistance. On the other hand, excessive Ni content destabilizes the ferrite phase and increases the raw material cost of ferritic stainless steel. Therefore, when Ni is included in the chemical composition, the Ni content may be 0.01 to 1.00 mass%. The Ni content may be 0.01 to 0.10 mass%. In this stainless steel, the Ni content may 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. A Ni content of "0 (including no addition)" allows the inclusion of Ni as an unavoidable impurity.

[0037] (Co: Cobalt) Cobalt is an element that is effective in improving corrosion resistance and heat resistance. However, if excessive Co is added, the raw material cost of ferritic stainless steel increases. Therefore, when Co is included in the chemical composition, the Co content may be 0.005 to 0.500 mass%.

[0038] (Cu: Copper) 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.

[0039] (Al: Aluminum) Al is an element effective for deoxidation, but also has a negative effect on press workability. ２ It is possible to reduce system inclusions. However, if excessive Al is added, surface defects increase. Therefore, when Al is included in the chemical composition, the Al content may be 0.01 to 1.00 mass%.

[0040] (Ca: Calcium) Ca is an element effective for degassing. Therefore, when Ca is included in the chemical composition, the Ca content may be 0.0001 to 0.0050 mass %.

[0041] (Mg: Magnesium) Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizer. On the other hand, excessive Mg content reduces the toughness of the ferritic stainless steel, resulting in poor manufacturability. Therefore, when Mg is included in the chemical composition, the Mg content may be 0.0001 to 0.0050 mass%.

[0042] (B: Boron) B is an element effective in improving toughness. However, if an excessive amount of B is contained, the effect is saturated. Therefore, when B is contained in the chemical composition, the content of B may be 0.0001 to 0.0025 mass%.

[0043] (V: Vanadium) V is an element effective in improving hardness and strength. However, if excessive V is added, the raw material cost of ferritic stainless steel increases. Therefore, when V is included in the chemical composition, the V content may be 0.05 to 0.50 mass%.

[0044] (W: Tungsten) W is an element effective in improving high-temperature strength. However, if excessive W is added, the raw material cost of ferritic stainless steel increases. Therefore, when W is included in the chemical composition, the W content may be 0.03 to 1.00 mass%.

[0045] (Sn: Tin) Sn is an element effective in improving corrosion resistance. However, if excessive Sn is added, hot workability and toughness decrease. Therefore, when Sn is included in the chemical composition, the Sn content may be 0.005 to 0.500 mass%.

[0046] (Sb: Antimony) Sb is effective in improving workability by promoting the formation of deformation bands during rolling. However, if an excessive amount of Sb is contained, the effect saturates and workability further deteriorates. Therefore, when Sb is contained in the chemical composition, the Sb content may be 0.005 to 0.500 mass%.

[0047] (Zr: Zirconium) Zr is an element effective for denitrification, deoxidation, and desulfurization. However, if Zr is added in excess, the raw material cost of stainless steel increases. Therefore, when Zr is included in the chemical composition, the Zr content may be 0.050 to 0.500 mass%.

[0048] (Y: Yttrium) Yttrium 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 may be 0.001 to 0.100 mass%.

[0049] (Hf: Hafnium) Hf is an element that improves oxidation resistance. However, excessive Hf content reduces the toughness of the steel plate and increases the raw material cost of the stainless steel. Therefore, when Hf is included in the chemical composition, the Hf content may be 0.001 to 0.100 mass%.

[0050] (REM: Rare Earth Metals) Rare earth elements refer to lanthanoid elements (elements with atomic numbers 57 to 71, such as La, Ce, Pr, Nd, and Sm). REM is effective in improving hot workability and oxidation resistance. However, these effects saturate when the REM content exceeds 0.100%. Therefore, when REM is included in the chemical composition, the REM content may be 0.001 to 0.100 mass%.

[0051] <Steel Sheet Properties> The present stainless steel sheet has the chemical composition described above, and by controlling the manufacturing conditions, a material structure (internal structure) with the following properties is formed. Briefly, in the final annealing step, the temperature is raised to a range of 500 to 900°C and maintained within that temperature range for 5 to 120 seconds. This allows the preferential growth of {111}-oriented crystal grains, which contribute significantly to improving the r-value, while reducing the amount of {311} crystal grains that contribute less to improving the r-value. As a result, the average r-value can be increased by controlling the texture in the metal structure while maintaining a relatively small average grain size, thereby improving deep drawability. More details will be described later in conjunction with the explanation of the manufacturing method of the present stainless steel sheet.

[0052] (Grain size of steel sheet) Figure 1 is a schematic diagram showing a cross section 12 of a stainless steel sheet 1 according to one embodiment of the present invention. As shown in Figure 1, the cross section 12 is parallel to the rolling direction of the stainless steel sheet 1 and perpendicular to the rolled surface 11. The thickness and width of the stainless steel sheet 1 are denoted by t and w, respectively.

[0053] The stainless steel plate 1 has an average crystal grain size of 15.0 μm or more and 30.0 μm or less in cross section 12, calculated by a cutting method. The stainless steel plate 1 may have an average crystal grain size of 20.0 μm or more and 29.0 μm or less.

[0054] The average grain size calculated by the intercept method can be measured by the method specified in the JIS standard (JIS G 0551:2020). Specifically, first, a line segment with a total length L parallel to the rolling direction is drawn on the cross section 12, and the number n of grains crossed by this line segment is measured. Note that grains with the end of the line segment inside are counted as 1 / 2 grain. The average grain size d can be calculated by the formula d = L / n.

[0055] (Crystal orientation) Figure 2 is a schematic diagram showing a cross section 13 of a stainless steel sheet 1 according to one embodiment of the present invention. As shown in Figure 2, the cross section 13 is a cross section at the center of the sheet thickness, parallel to the rolled surface 11 of the steel sheet 1. The rolling direction is abbreviated as RD (Rolling Direction), the normal direction to the rolled surface is abbreviated as ND (Normal Direction), and the direction perpendicular to the rolling is abbreviated as TD (Transverse Direction). The cross section 13 is a so-called ND plane.

[0056] Pole figure data can be obtained by performing XRD measurement on the cross section 13. From the obtained pole figure data, the grain orientation distribution function (ODF) can be analyzed to measure the intensity (crystal orientation intensity) for each crystal orientation. For example, pole figure data can be obtained by performing pole measurements using SmartLab manufactured by Rigaku. SmartLab Studio II can be used as ODF analysis software. Since known analysis methods can be used in this way, detailed explanations will be omitted, but a brief explanation is as follows.

[0057] Regarding the crystal orientation of a certain crystal grain, if the (hkl) plane is located parallel to the ND plane and the [uvw] direction is the rolling direction, the crystal orientation of the crystal grain is expressed as (hkl)[uvw]. The equivalent orientation group is expressed as {hkl}<uvw>.

[0058] ODF is a function of three variables (φ1, Φ, φ2) that uniquely specify the crystal orientation of a grain relative to the material coordinate axis system. φ1, Φ, and φ2 are Euler angles defined by Bunge's method. The material coordinate axis system defines 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 the rotation of φ1, and φ2 is the counterclockwise rotation angle around the z' axis after the rotation of Φ.

[0059] In the cross section of Euler space at φ2=45°, the position at φ=55° and φ1=30° is the {111}<112> orientation, and the position at φ=25° and φ1=0° is the {311}<011> orientation. The {311}<011> orientation is equivalent to the {311}<110> orientation.

[0060] XRD measurement was performed on the stainless steel sheet 1 to obtain (200), (110), and (211) pole figures. ODF analysis was performed using the obtained pole figures. The {111}<112> crystal orientation intensity obtained by ODF analysis was designated Ia, and the {311}<011> crystal orientation intensity obtained by ODF analysis was designated 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. The calculation method for ODF analysis can be the WIMV method proposed by Matthies and Vinel, which does not use continuous functions in the analysis. For example, SmartLab is used as the X-ray diffractometer, and a Mo source can be used as the X-ray source. In this case, the pole figure data used in the ODF analysis is intensity-normalized by correction processes such as background correction and randomization in SmartLab processing. This normalization process is performed under fixed conditions without individually setting conditions by selecting a normalization check box displayed on the display of the user interface of the X-ray diffraction instrument. The above crystal orientation intensity can also be expressed as an orientation density or an X-ray random intensity ratio.

[0061] The stainless steel sheet 1 satisfies the relationship Ia - Ib ≥ 20.0 and (Ia - Ib) / d ≥ 1.00, where d is the average grain size. By satisfying the above relationship, the stainless steel sheet 1 can have a fine average grain size and a high r-value.

[0062] The stainless steel sheet 1 may have a value calculated by the Ia-Ib relationship of 20.0 or more and 30.0 or less, and a value calculated by the (Ia-Ib) / d relationship of 1.00 or more and 1.50 or less. If the value calculated by the Ia-Ib relationship exceeds 30, the in-plane anisotropy may deteriorate. Furthermore, a stainless steel sheet 1 having an average grain size d, a crystal orientation intensity Ia, or a crystal orientation intensity Ib such that the value calculated by the (Ia-Ib) / d relationship exceeds 1.5 may make it difficult to specify manufacturing conditions, which may result in increased production costs. The stainless steel sheet 1 may have a value calculated by the (Ia-Ib) / d relationship of 1.00 or more and 1.35 or less.

[0063] (Average r-value) The present stainless steel sheet can have an average r-value (average Lankford value) of 1.9 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% according to the method specified in the JIS standard (JIS Z 2254:2021): Average r-value = (r Ｌ +2r Ｄ +r Ｃ ) / 4... (1).

[0064] In formula (1), r Ｌ , r Ｄ and r Ｃ are the r-values ​​measured for JIS No. 13B test pieces taken from directions at 0° (parallel), 45°, and 90° to the rolling direction, respectively. The stainless steel sheet may have an average r-value of 1.9 or more and 2.3 or less.

[0065] (Drawing Limit Ratio) The present stainless steel sheet can have a drawing limit ratio (LDR) of 2.4 or more. Generally, the drawing limit ratio (forming limit drawing ratio) can be determined by a deep drawing test and is used as an index showing the deep drawability (deep draw formability) of a stainless steel sheet. The drawing limit ratio is a value obtained by dividing the maximum blank diameter at which deep drawing can be performed without the occurrence of cracks in the test piece (stainless steel sheet) by the diameter of the punch used in deep drawing. The drawing limit ratio of the present stainless steel sheet may be 2.4 or more and 2.6 or less.

[0066] <Method for Manufacturing Steel Sheet> Figure 3 is a flowchart showing an example of a method for manufacturing a ferritic stainless steel sheet according to one embodiment of the present invention. As shown in Figure 3, this method for manufacturing a stainless steel sheet (hereinafter sometimes abbreviated as this manufacturing method) includes a preparation step of preparing a cold-rolled steel sheet and a final annealing step of annealing the cold-rolled steel sheet. In this method for manufacturing a stainless steel sheet, a cold-rolled steel sheet manufactured by a general manufacturing method can be used, and the specific manufacturing method for the cold-rolled steel sheet is not necessarily limited. In the preparation step for preparing the cold-rolled steel sheet, the cold-rolled steel sheet may be manufactured by cold rolling a ferritic stainless steel material (steel material) having the above-mentioned chemical composition to be rolled, or a pre-manufactured cold-rolled steel sheet may be prepared. An example of a manufacturing process for a cold-rolled steel sheet will be described later. In addition, this manufacturing method may optionally include a post-process after the final annealing step.

[0067] (Final annealing step) The final annealing step in the production of this stainless steel sheet includes a first annealing process S1 in which the cold-rolled steel sheet is heated to a predetermined first ultimate 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 in which, successively from the first annealing process, annealing is performed with a holding time in the first temperature range of 5 to 120 seconds.

[0068] Hereinafter, with reference to the drawings, the heat treatment pattern in the final annealing step and its technical significance will be explained, categorizing them according to the conditions of the second annealing process.

[0069] (Heat Treatment Pattern 1) Fig. 4 is a graph schematically showing an example of a heat treatment pattern in the final annealing step. In Fig. 4 and other graphs, it is natural that the temperature change over time is not necessarily linear in practice. Fig. 4 shows an example (heat treatment pattern 1) in which the temperature is raised to a second ultimate temperature in the second annealing step S2 and then soaked at the second ultimate temperature exceeding 900°C.

[0070] 4, the first target temperature, which is the target temperature to which the temperature is raised in the first annealing process S1, is designated as T1, and the second target temperature, which is the target temperature to which the temperature is raised in the second annealing process S2, is designated as T2. Also, the heating time required to raise the temperature from the first target temperature T1 to the second target temperature T2 is designated as t1, the soaking time at the second target temperature T2 is designated as t2, and the cooling time required to cool from the second target temperature T2 to 500°C is designated as t3. These symbols will have the same meaning in the following description, and repeated explanations will be omitted.

[0071] In the example shown in Fig. 4, the second ultimate temperature T2 may be greater than 900°C and equal to or less than 1000°C. The first ultimate temperature T1 in the first annealing step S1, and the second ultimate temperature T2, heating time t1, soaking time t2, and cooling time t3 in the second annealing step S2 may be appropriately adjusted so that the average 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. If the second ultimate 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.

[0072] In the example shown in Figure 4, the second ultimate temperature T2 exceeds 900°C. In this case, the holding time t within the first temperature range of 500 to 900°C can be the time required for the temperature to rise from the first ultimate temperature T1 to 900°C. In this manufacturing method, by raising the temperature to the temperature range of 500 to 900°C in the first annealing step S1, 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. Next, in the second annealing step S2, during the holding time t, recrystallization of {111} crystal grains, which contribute greatly to improving the r-value, occurs preferentially, while recrystallization of {311} crystal grains, which contribute little to improving the r-value, is relatively unlikely to occur.

[0073] On the other hand, if the second annealing step S2 does not include the holding time t, for example, if the first annealing step S1 is heated to a temperature exceeding 900°C, recrystallization occurs in various orientations other than {111}. This will be explained in more detail below.

[0074] Generally, when a cold-rolled ferritic stainless steel sheet is annealed, recrystallization occurs in the following order as the temperature rises: {111} grains, {211} grains, {311} grains, and {100} grains. For example, during the temperature rise, recrystallization nuclei of {111} grains are formed at a temperature of about 800°C, and as the temperature rises further, recrystallization nuclei of grains of other orientations are formed. The present inventors have focused on this property and conceived the idea of ​​preferentially forming recrystallization nuclei of {111} grains in the metal structure in the early stages of the first annealing process S1 and the second annealing process S2, developing a {111} texture and reducing the proportion of {311} grains, followed by further recrystallization. This allows a ferritic stainless steel sheet with a fine average grain size, a high r-value, and excellent deep drawability to be produced by a relatively simple method.

[0075] In the example shown in Fig. 4, the {111} orientation can be preferentially developed during the holding time t in the second annealing step S2, and the temperature may then be raised to above 900°C. However, the various conditions of the second annealing step S2 are set so that the average grain size d of the ferritic stainless steel obtained by the final annealing step is 15.0 µm or more and 30 µm.0 or less.

[0076] For the reasons described above, the first ultimate temperature T1 is set to 900°C or lower to selectively develop the {111} orientation. On the other hand, the first ultimate temperature T1 is set to 500°C or higher to ensure a minimum amount of heat. The time during which the temperature is within the first temperature range of 500 to 900°C during cooling from the second ultimate temperature T2 to 500°C is not included in the holding time t in the second annealing process S2.

[0077] In the first annealing step S1 in the final annealing process, the temperature is increased from the heating start temperature to the first ultimate temperature T1 at a heating rate of 100 to 2000°C / s to promote the development of the {111} texture. This is because if the heating rate is too slow to exceed 500°C, the lower limit of the first temperature range, recrystallization nuclei are less likely to be generated in the second annealing step S2, which may result in insufficient development of the {111} orientation. On the other hand, increasing the heating rate more than necessary will result in excessive cost increases. Therefore, the upper limit of the heating rate is set to 2000°C / s.

[0078] In the example shown in FIG. 4, in the second annealing process S2, the material to be annealed is soaked at the second ultimate temperature T2 for a predetermined time (having a soaking time t2), but this is not limited thereto, and the soaking time at the second ultimate temperature T2 may be 0 seconds (t2 = 0 seconds), that is, cooling may be started immediately after the temperature of the material to be annealed reaches the second ultimate temperature T2.

[0079] The description made with reference to Figure 4 can be summarized as follows: In one embodiment of the method for producing a ferritic stainless steel sheet, the second annealing process S2 includes increasing the temperature from a first ultimate temperature T1 to a second ultimate temperature T2, (i) soaking at the second ultimate temperature T2 for 0 seconds, or (ii) soaking at the second ultimate temperature T2 for a predetermined soaking time t2, and cooling from the second ultimate temperature T2 to 500°C.

[0080] In one embodiment of the method for producing a ferritic stainless steel sheet, in the second annealing process S2, the second ultimate temperature T2 is a temperature exceeding 900°C, and the holding time t is the time during which the temperature is raised from the first ultimate temperature T1 to 900°C.

[0081] (Heat Treatment Patterns 2 and 3) Fig. 5 is a graph schematically showing another example of a heat treatment pattern in the final annealing step. Fig. 6 is a graph schematically showing another example of a heat treatment pattern in the final annealing step. Fig. 5 shows an example (heat treatment pattern 2) in which the temperature is raised to a second ultimate temperature in the second annealing step S2 and soaked at the second ultimate temperature T2 that is 900°C or less. Fig. 6 shows an example (heat treatment pattern 3) in which the temperature is raised to a second ultimate temperature in the second annealing step S2 and soaked at the second ultimate temperature T2 that is 900°C or less for 0 seconds.

[0082] In the examples shown in FIGS. 5 and 6 , the second ultimate temperature T2 is set within a temperature range greater than the first ultimate temperature T1 and equal to or less than 900°C. As described above, in the second annealing process S2, the temperature is maintained within the first temperature range of 500 to 900°C, thereby preferentially generating recrystallization nuclei of {111} crystal grains in the metal structure and developing the {111} texture. Therefore, in the examples shown in FIGS. 5 and 6 , the time point at which the first ultimate temperature T1 is reached, at which the second annealing process S2 is initiated, is defined as the first time point TP1, and the time point at which the temperature is increased from the first ultimate temperature T1 to the second ultimate temperature T2 and then cooled from the second ultimate temperature T2 to 500°C is defined as the second time point TP2. The holding time t can be the time between the first time point TP1 and the second time point TP2.

[0083] In the example shown in FIG. 5, the heat treatment pattern is such that the soaking is maintained at the second ultimate temperature T2 for a predetermined soaking time t2, and in the example shown in FIG. 6, the heat treatment pattern is such that the soaking is maintained at the second ultimate temperature T2 for 0 seconds.

[0084] (Heat Treatment Pattern 4) Fig. 7 is a graph schematically showing another example of a heat treatment pattern in the final annealing step, in which the temperature is not increased in the second annealing step S2, and soaking is performed at the first attained temperature T1 (heat treatment pattern 4).

[0085] In the example shown in Fig. 7, the second attainment temperature T2 and the first attainment temperature T1 are set to be the same temperature, that is, the temperature is not increased in the second annealing process S2, and the first attainment temperature T1 is maintained. Also in the example shown in Fig. 7, for the reasons described above, the holding time t can be the time between the first time point TP1 and the second time point TP2.

[0086] The annealing equipment used in the final annealing step is not particularly limited, and known equipment such as a continuous annealing furnace or a batch furnace can be used.

[0087] After the final annealing step, a pickling step may be performed as needed. Also, if necessary, post-processing may be performed as needed, such as temper rolling and cutting into a desired shape.

[0088] (Manufacturing Process of Cold-Rolled Steel Sheet) Figure 8 is a flowchart showing an example of a method for manufacturing a ferritic stainless steel sheet according to one aspect of the present invention. As shown in Figure 8, in an example of a preparation process in this manufacturing method, a cold-rolled steel sheet may be manufactured. In this case, this 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.

[0089] The method for producing slabs in the steelmaking process is not particularly limited, but 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 then cut to the desired length and used in the hot rolling process.

[0090] The hot rolling process is a process of producing a hot-rolled sheet (hot-rolled steel sheet) of a predetermined thickness by rolling (hot-rolling) a slab at a high temperature. The hot rolling process can be performed using known equipment and methods. In this production method, typical production conditions for hot rolling processes can be adopted. For example, the heating temperature (rolling temperature) can be 1150 to 1250°C, and the total reduction can be 95 to 99%. The coiling temperature after hot rolling can be 200 to 500°C.

[0091] The intermediate process may include a hot-rolled sheet annealing process for annealing the hot-rolled sheet and a pickling process. When the hot-rolled sheet annealing process is included, in the hot-rolled sheet annealing process, for example, the hot-rolled sheet is heated to a temperature range of 900 to 1000°C and soaked. The soaking temperature in the hot-rolled sheet annealing process is preferably 910 to 980°C. The soaking time in the hot-rolled sheet annealing process is 20 to 120 seconds (seconds).

[0092] When the intermediate process includes a hot-rolled sheet annealing step, the hot-rolled structure that causes ridging can be disrupted by recrystallization, thereby improving the ridging resistance of the ferritic stainless steel sheet.

[0093] In the hot-rolled sheet annealing process, the lower limit of the heating temperature is set to 900°C, and the lower limit of the soaking time is set to 20 seconds. On the other hand, if the recrystallized structure of the hot-rolled sheet becomes coarse, ridging will worsen. 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. This breaks up the colonies in the metal structure.

[0094] The annealing equipment used in the hot-rolled sheet annealing step is not particularly limited, and known equipment such as a continuous annealing furnace or a batch furnace can be used.

[0095] The hot-rolled annealed steel sheet may be subjected to a pickling process, in which scale adhering to the surface of the hot-rolled annealed steel sheet is removed using a pickling solution such as sulfuric acid, hydrochloric acid, or a mixture of nitric acid and hydrofluoric acid.

[0096] The intermediate step may include an intermediate cold rolling step and an intermediate annealing step, if necessary. The heating temperature in the intermediate annealing step may be, for example, 900 to 1000°C.

[0097] Following the intermediate step, a final cold rolling step is carried out. The cold rolling step is a step in which the material to be treated (steel material) is cold-rolled (for example, at room temperature to 200°C) to obtain a cold-rolled steel sheet of a predetermined thickness. The rolling ratio in the cold rolling step is preferably 75% or more, since a higher rolling ratio is more effective in improving the average r-value and formability. The rolling ratio in the cold rolling step may be 90% or less.

[0098] The rolling equipment used in the cold rolling step is not particularly limited, and known equipment can be used.

[0099] [Summary] The ferritic stainless steel sheet according to aspect 1 of the present invention is a ferrite sheet having a chemical composition containing, by mass%, 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 0.050%, and also containing at least one of Ti: 0.01 to 0.50% and Nb: 0.01 to 0.50%, with an S content of 0.01% or less, and the balance being Fe and unavoidable impurities. The present invention relates to a ferritic stainless steel plate, wherein in a cross section of the ferritic stainless steel plate parallel to the rolling direction and perpendicular to the rolling surface, 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 the center of the plate thickness of the ferritic stainless steel plate, the {111}<112> crystal orientation intensity is Ia, the {311}<011> crystal orientation intensity is Ib, and the average crystal grain size is d, the relationships Ia-Ib≧20.0 and (Ia-Ib) / d≧1.00 are satisfied.

[0100] The ferritic stainless steel sheet according to the second aspect of the present invention has an average r-value of 1.9 or more in the first aspect. Ｌ +2r Ｄ +r Ｃ ) / 4.

[0101] In a third aspect of the present invention, the ferritic stainless steel sheet of the first or second aspect has a limiting drawing ratio of 2.4 or more.

[0102] A ferritic stainless steel sheet according to Aspect 4 of the present invention is the same as any one of Aspects 1 to 3, and contains, by mass%, Mo: 0.05 to 2.00%, Ni: 0.01 to 1.00%, Co: 0.05 to 0.50%, Cu: 0.05 to 1.00%, Al: 0.01 to 1.00%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, B: 0.00 and further contains one or more elements selected from the group consisting of: 0.01 to 0.0025%, V: 0.05 to 0.50%, W: 0.05 to 1.00%, Sn: 0.005 to 0.500%, Sb: 0.005 to 0.500%, Zr: 0.05 to 0.50%, Y: 0.001 to 0.100%, Hf: 0.001 to 0.100%, and rare earth elements: 0.001 to 0.100%.

[0103] A method for producing a ferritic stainless steel sheet in aspect 5 of the present invention includes, by mass%, 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 0.050%, and also includes at least one of Ti: 0.01 to 0.50% and Nb: 0.01 to 0.50%, and the content of S is 0.01% or less, and the balance is Fe and unavoidable impurities. A method for producing a ferritic stainless steel sheet includes the steps of: cold rolling a steel material having a chemical composition containing, by mass%, 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 0.050%, and the steel material has a chemical composition containing, by mass%, at least one of Ti: 0.01 to 0.50% and Nb: 0.01 to 0.50%, and the content of S is 0.01% or less, and the balance is Fe and unavoidable impurities, and a method for producing a cold-rolled steel sheet by cold rolling the cold-rolled steel material. and a final annealing step of annealing the steel sheet, wherein the ferritic stainless steel sheet has an average grain size of 15.0 μm or more and 30.0 μm or less in a cross section parallel to the rolling direction and perpendicular to the rolling surface, as calculated by a cutting method, and the final annealing step includes a first annealing process in which the cold-rolled steel sheet is heated to a predetermined first ultimate 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 in which, consecutively from the first annealing process, annealing is performed with a holding time in the first temperature range of 5 to 120 seconds.

[0104] A sixth aspect of the present invention provides a method for producing a ferritic stainless steel sheet according to the fifth aspect, wherein the second annealing step comprises raising the temperature from the first ultimate temperature to a predetermined second ultimate temperature, (i) soaking at the second ultimate temperature for 0 seconds, or (ii) soaking at the second ultimate temperature for a predetermined soaking time, and cooling from the second ultimate temperature to 500°C.

[0105] A seventh aspect of the present invention provides a method for producing a ferritic stainless steel sheet according to the sixth aspect, wherein in the second annealing step, the second ultimate temperature is a temperature exceeding 900°C, and the holding time is the time required for the temperature to rise from the first ultimate temperature to 900°C.

[0106] Aspect 8 of the present invention is a method for producing a ferritic stainless steel sheet according to aspect 6, wherein in the second annealing step, the second ultimate temperature is a temperature higher than the first ultimate temperature and not higher than 900°C, and the holding time is the time from the first time point when the first ultimate temperature is reached at which the second annealing step is started to the second time point when the temperature is increased from the first ultimate temperature to the second ultimate temperature and then cooled from the second ultimate temperature to 500°C.

[0107] A ninth aspect of the present invention relates to the method for producing a ferritic stainless steel sheet according to the fifth aspect, wherein the second annealing process comprises soaking at the first ultimate temperature and cooling from the first ultimate temperature to 500°C, and the holding time is the time between the first time point, which is the time when the first ultimate temperature at which the second annealing process is started, and the second time point, which is the time when the sheet is cooled from the first ultimate temperature to 500°C.

[0108] [Additional Notes] 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 the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0109] An embodiment of the present invention will be described below. The method for manufacturing a ferritic stainless steel sheet described in this embodiment is merely an example.

[0110] (Production of Slabs) Steels having the chemical compositions shown in Table 1 below were vacuum melted to produce 30 kg slabs. In Table 1, steel types A to J have chemical compositions within the range of the present invention. Also in Table 1, steel types K, L, and M have chemical compositions outside the range of the present invention. Table 1 shows the composition of the elements contained in each steel type in mass %. The remainder other than the elements shown in Table 1 is Fe or unavoidable impurities. Also, the underlines in Table 1 indicate that the composition of the elements contained in each steel type according to the comparative example is outside the range of the present invention.

[0111]

[0112] The slab was heated at 1200°C for 2 hours and then hot-rolled to produce a hot-rolled sheet having a thickness of 3 mm. The hot-rolled sheet was then annealed at 950°C for 60 seconds, pickled, and then cold-rolled at a rolling ratio of 80% to obtain a cold-rolled sheet as a test material.

[0113] Next, a final annealing process was carried out under the manufacturing conditions shown in Table 2, and steel sheets No. 1 to 26 with a sheet thickness of 0.6 mm were manufactured. In this example, cold rolling was carried out only once. Table 2 also shows the results of evaluating various physical properties for each steel sheet. Note that underlines in Table 2 indicate that the manufacturing conditions and various physical properties of the steel sheet were outside the range of the present invention or outside the preferred range of the present invention. The average grain size d, the X-ray random intensity ratio (Ia, Ib) of the crystal orientation, and the average r-value were measured or calculated using the same methods as in the above-mentioned embodiment. Deep drawability was evaluated as follows.

[0114] Each test material was subjected to cylindrical deep drawing under the following conditions: punch diameter: Φ50 mm, punch shoulder R: 5 mm, die diameter: 52 mm, die shoulder R: 5 mm, blank holding force: 1 ton, and drawing ratio: 2.4. In Table 2, those that could be formed were marked as "good" and those that could not be formed were marked as "bad."

[0115]

[0116] As shown in Table 2, all of the steel sheets of the examples of the present invention, which were manufactured by the manufacturing method of a ferritic stainless steel sheet according to one embodiment of the present invention, had average grain size d and random intensity ratio of crystal orientation satisfying the predetermined relationship, and had both an average r value and deep drawability properties.

[0117] In contrast, the steel sheets of the comparative examples did not satisfy the criteria for at least one of the above properties. Comparative steel sheet No. 3 had a small average r-value and insufficient deep drawability due to insufficient heating in the second annealing step S2, resulting in an average grain size d of less than 15.0 μm. Comparative steel sheets No. 6 and 22 had a heating rate of less than 100° C. / s in the first annealing step S1, resulting in insufficient generation of recrystallized nuclei of {111}<112> orientation grains. Therefore, development of the {111} texture and reduction of {311}<011> orientation grains were insufficient in the second annealing step S2, resulting in a small average r-value. Comparative steel sheet No. In Steel Sheet No. 21, the heating rate in the first annealing process S1 was less than 100°C / s, while the average r-value was increased by grain growth of crystal grains of the {111}<112> orientation in the second annealing process S2. Although the value calculated using the Ia-Ib relationship satisfies the range of the present application, the value calculated using the (Ia-Ib) / d relationship was outside the range of the present application, and the deep drawability was insufficient.

[0118] Comparative steel sheets Nos. 8 and 19 were heated to a first ultimate temperature T1 greater than 900°C in the first annealing step S1, and therefore did not satisfy the conditions of the predetermined relational expression. Comparative steel sheets Nos. 10 and 12 had an excessive amount of heat in the second annealing step S2, and therefore had an average grain size d exceeding 30 µm and a relatively high average r-value, but had insufficient deep drawability (fracture occurred due to the occurrence of surface irregularities).

[0119] In the comparative steel sheet No. 15, the first attained temperature T1 in the first annealing step S1 was less than 500°C, and therefore the amount of heat in the annealing was insufficient, resulting in insufficient development of the {111} texture.

[0120] In the comparative steel sheet No. 17, the holding time t in the second annealing step S2 was short, and the {111} texture was not sufficiently developed, so that the condition of the predetermined relational expression was not satisfied.

[0121] 1 Stainless steel plate 11 Rolled surface 12 Cross section 13 Cross section

Claims

1. A ferritic stainless steel sheet having a chemical composition containing, by mass%, 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 0.050%, and also containing at least one of Ti: 0.01 to 0.50% and Nb: 0.01 to 0.50%, with an S content of 0.0100% or less, and the balance being Fe and unavoidable impurities, 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, In a cross section parallel to the rolled surface at the center of the thickness of the ferritic stainless steel plate, the {111}<112> crystal orientation intensity is Ia, the {311}<011> crystal orientation intensity is Ib, and the average crystal grain size is d, Ia-Ib≧20.0, and A ferritic stainless steel plate satisfying the relationship (Ia-Ib) / d≧1.

00.

2. The ferritic stainless steel sheet according to claim 1, having an average r-value of 1.9 or more. (where, Average r value = (r Ｌ +2r Ｄ +r Ｃ ) / 4 r Ｌ : Lankford value in the direction of 0° to the rolling direction r Ｄ : Lankford value in the direction at 45° to the rolling direction r Ｃ : Lankford value in the direction 90° to the rolling direction)

3. 3. The ferritic stainless steel sheet according to claim 1, wherein the limiting drawing ratio is 2.4 or more.

4. In mass%, Mo: 0.05 to 2.00%, Ni: 0.01 to 1.00%, Co: 0.005 to 0.500%, Cu: 0.05 to 1.00%, Al: 0.01 to 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% 3. The ferritic stainless steel sheet according to claim 1, further comprising one or more elements selected from the group consisting of Sn: 0.005 to 0.500%, Sb: 0.005 to 0.500%, Zr: 0.050 to 0.500%, Y: 0.001 to 0.100%, Hf: 0.001 to 0.100%, and rare earth elements: 0.001 to 0.100%.