Ferritic stainless steel sheet and method for manufacturing the same

Abstract

To provide a ferritic stainless steel sheet capable of managing both improvement of moldability and reduction of processed rough skin.SOLUTION: A ferritic stainless steel sheet (1) contains P:0.005-0.05%. In a cross section (12) parallel to a rolling direction and vertical to a rolled surface, a crystal grain size is 15 μm or less, and an area ratio of ND / / <111>±10° azimuth grain is 40% or more, and an average major axis of deposit deposited at the temperature of 600°C or higher and below 900°C is 0.15 μm or less and larger than 0 μm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel sheet and a method for manufacturing the same.

Background Art

[0002] Generally, with respect to a ferritic stainless steel sheet (hereinafter sometimes abbreviated as "steel sheet"), in order to reduce irregularities (referred to as "processing surface roughness") generated on the surface of the steel sheet after deep drawing, it is effective to reduce the crystal grain size (refine) in the steel sheet. On the other hand, the steel sheet is more likely to have reduced workability (i.e., formability during deep drawing) as the crystal grain size is reduced.

[0003] Regarding steel sheets, development of technologies that achieve both reduction of processing surface roughness and improvement of formability during deep drawing has been carried out.

[0004] For example, Patent Document 1 discloses a technique for reducing processing surface roughness and improving formability by defining the components and microstructure of steel, and preferably controlling the crystal grain size within an appropriate range. In the technique described in Patent Document 1, in order to manufacture a steel sheet having a desired microstructure, hot rolling annealing of the hot rolled sheet is omitted, and a cold rolling process, intermediate annealing, final cold rolling, and final annealing are performed in this order, and the cold rolling rate and the temperature range of annealing are defined.

[0005] Also, in the technique described in Patent Document 2, while defining the components of steel, the heat treatment temperature and cooling rate in the hot rolling annealing process are controlled to define the crystal grain size and the maximum particle size of the precipitated particles. In the technique described in Patent Document 3, the crystal grain size, average r value, (tensile strength (MPa) × average r value) / (crystal grain size (μm)), and Nb / (C + N)+2(Ti / (C + N)) are defined. In the technique described in Patent Document 4, the amount of P present as phosphide and the crystal grain size number are defined.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2008-208412 [Patent Document 2] Japanese Patent Publication No. 2011-149101 [Patent Document 3] Japanese Patent Publication No. 2003-138349 [Patent Document 4] Patent No. 6906688 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the conventional technologies described above have the problem that they do not adequately achieve both improved formability and reduced surface roughness during processing. One aspect of the present invention aims to realize a ferritic stainless steel sheet that can achieve both improved formability and reduced surface roughness during processing. [Means for solving the problem]

[0008] To solve the above problems, a ferritic stainless steel sheet according to one aspect of the present invention has a composition in mass% of 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: 0.005~0.05%, and also contains at least one of Ti: 0.01~0.5% and Nb: 0.01~0.5%, with an S content of 0.01% or less, and the remainder being Fe and unavoidable impurities, wherein the grain size calculated by the cutting method in the cross-section when the ferritic stainless steel sheet is cut in a plane parallel to the rolling direction and perpendicular to the rolling surface is 15 μm or less, and the ND / / of the total sheet thickness measured by electron backscatter diffraction in the cross-section is 15 μm or less. <111> The area ratio of grains oriented ±10° is 40% or more, and in the cross-section, the average major axis of precipitates deposited in the central part of the plate thickness at 600°C to less than 900°C is greater than 0 μm and 0.15 μm or less. With the above configuration, it is possible to realize a ferritic stainless steel sheet that achieves both improved formability and reduced surface roughness during processing.

[0009] Furthermore, the ferritic stainless steel sheet according to one aspect of the present invention has the following composition: 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%, B: 0.0001~0.002% The composition may further contain at least one of the following: 5%, V: 0.05-0.50%, W: 0.05-1.00%, Sn: 0.005-0.50%, Sb: 0.005-0.50%, Zr: 0.05-0.50%, Y: 0.001-0.10%, Hf: 0.001-0.10%, and rare earth elements: 0.001-0.10%. According to the above configuration, it is possible to realize a ferritic stainless steel sheet with further improved formability and further reduced surface roughness during processing.

[0010] Furthermore, the ferritic stainless steel sheet according to one aspect of the present invention may have an average Rankford value of 1.6 or higher. According to the above configuration, a ferritic stainless steel sheet with further improved formability can be realized.

[0011] Furthermore, in one aspect of the present invention, a ferritic stainless steel sheet may be formed by cylindrical deep drawing under the conditions of punch diameter: Φ50 mm, punch shoulder radius: 5 mm, die diameter: 52 mm, die shoulder radius: 5 mm, wrinkle-holding force: 1 ton, and drawing ratio: 2.0, and the surface roughness expressed as a ten-point average roughness on the cylindrical side parallel to the rolling direction may be 8.0 μm or less. According to the above configuration, a ferritic stainless steel sheet with further reduced processing surface roughness can be realized.

[0012] Furthermore, a method for manufacturing a ferritic stainless steel sheet according to one aspect of the present invention contains, 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: 0.005~0.05%, and also contains at least one of Ti: 0.01~0.5% and Nb: 0.01~0.5%, with an S content of 0.01% or less. The method includes a first annealing step of heating a ferritic stainless steel having a composition in which the remainder is Fe and unavoidable impurities to a temperature range of 900 to 1000°C and holding it uniformly; a cold rolling step of cold rolling the ferritic stainless steel after the first annealing step; and a second annealing step of heating the ferritic stainless steel after the cold rolling step to a temperature range of 800 to 950°C at a heating rate of 50 to 1000°C / s, and then holding it uniformly for 5 to 60 seconds. According to the above method, a ferritic stainless steel sheet can be manufactured that achieves both improved formability and reduced surface roughness during processing.

[0013] Moreover, the method for manufacturing a ferritic stainless steel sheet according to one aspect of the present invention may have a composition in which the ferritic stainless steel further contains at least one of 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: 0.0001 to 0.0025%, V: 0.05 to 0.50%, W: 0.05 to 1.00%, Sn: 0.005 to 0.50%, Sb: 0.005 to 0.50%, Zr: 0.05 to 0.50%, Y: 0.001 to 0.10%, Hf: 0.001 to 0.10%, and rare earth elements: 0.001 to 0.10%. According to the above method, a ferritic stainless steel sheet with improved formability and reduced surface roughness can be manufactured.

Advantages of the Invention

[0014] According to one aspect of the present invention, it is possible to realize a ferritic stainless steel sheet that can achieve both improved formability and reduced surface roughness.

Brief Description of the Drawings

[0015] [Figure 1] It is a schematic diagram showing a cross-section of a steel sheet according to one aspect of the present invention. [Figure 2] It is a diagram showing a typical SEM photograph of a cross-section of a steel sheet according to one aspect of the present invention. [Figure 3] It is an enlarged view within the dotted line range in the SEM photograph shown in Figure 2. [Figure 4] It is a flowchart showing a method for manufacturing a steel sheet according to one aspect of the present invention.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, one embodiment of the present invention will be described in detail. The following description is for better understanding of the gist of the invention and does not limit the present invention unless otherwise specified.

[0017] In this application, unless otherwise specified, "%" as the unit of the content of each component element means "% by mass". Also, in this application, for numerical values X and Y (where X < Y), "X to Y" shall mean "not less than X and not more than Y". Further, in this application, "ferritic stainless steel" may be abbreviated as "stainless steel".

[0018] 〔Composition of Steel Sheet〕 The steel sheet according to one aspect of the present invention contains, 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 0.05%, and contains at least one of Ti: 0.01 to 0.5% and Nb: 0.01 to 0.5%, has a sulfur content of 0.01% or less, and the balance is Fe and inevitable impurities.

[0019] Also, the steel sheet according to one aspect of the present invention may have a composition that further contains, in mass %, one or more of 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: 0.0001 to 0.0025%, V: 0.05 to 0.50%, W: 0.05 to 1.00%, Sn: 0.005 to 0.50%, Sb: 0.005 to 0.50%, Zr: 0.05 to 0.50%, Y: 0.001 to 0.10%, Hf: 0.001 to 0.10%, and rare earth elements: 0.001 to 0.10%.

[0020] Hereinafter, the significance of the content of each element contained in the steel sheet according to one aspect of the present invention will be described. Note that the steel sheet may consist of iron (Fe) and a small amount of unavoidably mixed impurities (unavoidable impurities) other than the components shown below.

[0021] <C: Carbon> C is an element that forms carbides with Cr etc., generating interfaces that serve as dislocation generation sources when stainless steel is deformed. However, when C is added excessively, intergranular corrosion resistance and workability decrease, and the cost required for refining increases. Therefore, in one aspect of the present invention, the content rate of C is 0.001 - 0.030%.

[0022] <Si: Silicon> Si has the effect of being a deoxidizer in the melting stage. However, when Si is added excessively, the stainless steel hardens and the ductility decreases. Therefore, in one aspect of the present invention, the content rate of Si is 0.01 - 1.00%.

[0023] <Mn: Manganese> Mn has the effect of being a deoxidizer. However, when Mn is added excessively, the amount of MnS generated increases and the corrosion resistance of the stainless steel decreases. Therefore, in one aspect of the present invention, the content rate of Mn is 0.01 - 1.00%.

[0024] <Cr: Chromium> Cr is necessary to form a passive film on the surface of the cold-rolled steel sheet to enhance corrosion resistance. However, when Cr is added excessively, the ductility of the stainless steel decreases. Therefore, in one aspect of the present invention, the content rate of Cr is 10.5 - 30.0%.

[0025] <N: Nitrogen> N is an element that forms nitrides with Cr etc., generating interfaces that serve as dislocation generation sources when stainless steel is deformed. However, when N is added excessively, the ductility decreases due to solid solution strengthening. Therefore, in one aspect of the present invention, the content rate of N is 0.001 - 0.030%.

[0026] <P: Phosphorus> When P is contained excessively, weldability, the toughness of the welded part, and workability may deteriorate. Therefore, in one aspect of the present invention, the content rate of P is 0.005 - 0.05%.

[0027] <Ti and Nb: Titanium and Niobium> Ti and Nb combine with C or N to fix C and N as precipitates such as TiC, TiN, NbC or NbN, etc. Therefore, by purifying the stainless steel, the average Rankford value and product elongation can be improved. On the other hand, if Ti and Nb are contained excessively, the raw material cost increases and the manufacturability may decrease with the increase in the recrystallization temperature.

[0028] Therefore, in one aspect of the present invention, the content ratio of Ti and Nb is such that at least one of them is 0.01 - 0.5%. In one aspect of the present invention, the stainless steel plate may contain only one of Ti and Nb, or may contain both Ti and Nb.

[0029] <S: Sulfur> S is an element whose upper limit of content needs to be controlled in the steel plate according to one aspect of the present invention. If S is contained excessively, it may have an adverse effect on the formation of the Al2O3 film in stainless steel and deteriorate the oxidation resistance. Therefore, in one aspect of the present invention, the content ratio of S is 0.01% or less. The steel plate according to one aspect of the present invention may not contain S.

[0030] <<Other Components>> The steel plate according to one aspect of the present invention preferably further contains at least one element among Mo, Ni, Co, Cu, Al, Ca, Mg, B, V, W, Sn, Sb, Zr, Y, Hf and rare earth elements as elements other than those described above.

[0031] <Mo: Molybdenum> Mo is an element effective for improving corrosion resistance. However, if Mo is added excessively, the raw material cost of stainless steel increases. Therefore, in one aspect of the present invention, the content ratio of Mo is preferably 0.05 - 2.00%.

[0032] <Ni: Nickel> Ni is an element that improves the corrosion resistance of stainless steel. On the other hand, if Ni is contained excessively, the ferrite phase becomes unstable and the raw material cost of stainless steel increases. Therefore, in one aspect of the present invention, the content rate of Ni is preferably 0.01 to 1.00%.

[0033] <Co: Cobalt> Co is an element effective for improving corrosion resistance and heat resistance. However, if Co is added excessively, the raw material cost of stainless steel increases. Therefore, in one aspect of the present invention, the content rate of Co is preferably 0.05 to 0.50%.

[0034] <Cu: Copper> Cu is an element effective for improving corrosion resistance. Therefore, in one aspect of the present invention, the content rate of Cu is preferably 0.05 to 1.00%.

[0035] <Al: Aluminum> Al is an element effective for deoxidation and can reduce A2-based inclusions that have an adverse effect on press workability. However, if Al is added excessively, surface defects increase. Therefore, in one aspect of the present invention, the content rate of Al is preferably 0.01 to 1.00%.

[0036] <Ca: Calcium> Ca is an element effective for degassing. Therefore, in one aspect of the present invention, the content rate of Ca is preferably 0.0001 to 0.0050%.

[0037] <Mg: Magnesium> Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizer. On the other hand, if Mg is contained excessively, the toughness of stainless steel decreases and the productivity decreases. Therefore, in one aspect of the present invention, the content rate of Mg is preferably 0.0001 to 0.0050%.

[0038] <B: Boron> B is an element effective in improving toughness. However, if B is present in excess, its effect becomes saturated. Therefore, in one embodiment of the present invention, the B content is preferably 0.0001 to 0.0025%.

[0039] <V:バナジウム> V is an element effective in improving hardness and strength. However, if V is added in excess, the raw material cost of stainless steel increases. Therefore, in one aspect of the present invention, the V content is preferably 0.05 to 0.50%.

[0040] <W:タングステン> W is an effective element for improving high-temperature strength. However, if W is added in excess, the raw material cost of stainless steel increases. Therefore, in one aspect of the present invention, the W content is preferably 0.05 to 1.00%.

[0041] <Sn:スズ> Sn is an effective element for improving corrosion resistance. However, if Sn is added in excess, hot workability and toughness decrease. Therefore, in one aspect of the present invention, the Sn content is preferably 0.005 to 0.50%.

[0042] <Sb:アンチモン> Sb is effective in improving processability by promoting the formation of deformation strips during rolling. On the other hand, if Sb is present in excess, the effect saturates, and processability further decreases. Therefore, in one aspect of the present invention, the Sb content is preferably 0.005 to 0.50%.

[0043] <Zr:ジルコニウム> Zr is an effective element for denitrification, deoxidation, and desulfurization. However, if Zr is added in excess, the raw material cost of stainless steel increases. Therefore, in one aspect of the present invention, the Zr content is preferably 0.05 to 0.50%.

[0044] <Y:イットリウム> Y is an element effective in improving hot workability and oxidation resistance. However, these effects saturate when exceeding 0.20%. Therefore, in one aspect of the present invention, the content of Y is preferably 0.001 to 0.10%.

[0045] <Hf: Hafnium> Hf is an element that improves oxidation resistance. On the other hand, when Hf is contained excessively, the toughness of the steel sheet is decreased and the raw material cost of stainless steel is increased. Therefore, in one aspect of the present invention, the content of Hf is preferably 0.001 to 0.10%.

[0046] <REM: Rare earth elements> Rare earth elements (rare earth metals, hereinafter abbreviated as "REM") mean lanthanoid elements (elements with atomic numbers 57 to 71 such as La, Ce, Pr, Nd, Sm, etc.). REM is effective in improving hot workability and oxidation resistance. However, these effects saturate when exceeding 0.10%. Therefore, in one aspect of the present invention, the content of REM is preferably 0.001 to 0.10%.

[0047] 〔Highlights of the steel sheet〕 The steel sheet according to one aspect of the present invention has the above-described component composition, and generally has an internal structure that is refined while promoting recrystallization and has a crystal orientation effective in improving workability.

[0048] The assumed mechanism (i.e., the key points in the manufacturing method of the steel sheet) will be described in detail later. The steel sheet according to one aspect of the present invention is manufactured under conditions where P or Nb-based precipitates (low-temperature precipitates) that precipitate at a relatively lower temperature than TiC or the like are finely precipitated in the final annealing process. Further, the steel sheet according to one aspect of the present invention is manufactured by developing a crystal orientation effective in improving workability in the final annealing process.

[0049] In the final annealing process, the cold-rolled sheet is rapidly heated and then held (maintained uniform temperature) to suppress the growth of matrix grains while promoting matrix recrystallization, due to the influence of the finely precipitated low-temperature deposits.

[0050] Typically, the low-temperature precipitates precipitate in a stage prior to the final annealing process, or precipitate when the temperature is raised during the final annealing process. In this case, the recrystallization process during the final annealing process is inhibited, and the low-temperature precipitates also grow in grain size. In contrast, a steel sheet according to one aspect of the present invention has a small grain size and fine low-temperature precipitates.

[0051] A steel sheet according to one aspect of the present invention has an internal structure controlled to have properties that simultaneously improve formability and reduce surface roughness during processing. The features of the steel sheet according to one aspect of the present invention will be described in detail below.

[0052] [Grain size of steel plate] Figure 1 is a schematic diagram showing a cross-section 12 of a steel plate 1 according to one embodiment of the present invention. As shown in Figure 1, the cross-section 12 is the cross-section obtained when the steel plate 1 is cut by a plane parallel to the rolling direction of the steel plate 1 and perpendicular to the rolling surface 11.

[0053] In the cross-section 12 of the steel plate 1, the grain size calculated by the cutting method is 15 μm or less, and more preferably 12 μm or less. The smaller the grain size and the finer the grains, the less surface roughness can be reduced in the processed steel plate.

[0054] Here, the grain size calculated by the sectioning method can be measured according to the method specified in the JIS standard (JIS G 0552:1998). Specifically, first, a line segment with a total length L parallel to the rolling direction is drawn in section 12, and the number of grains n traversed by this line segment is measured. Grains whose ends are inside the line segment are counted as 1 / 2. Then, the average grain size d is calculated using the following formula.

[0055] d = L / n In this application, the total length L of the line segment was set to 185 μm.

[0056] Furthermore, when steel plates are used in products requiring a high level of aesthetic appeal, such as cooking utensils or home appliances, polishing may be performed before or after deep drawing. In such cases, since the steel plate 1 has a grain size of 15 μm or less, the polishing load can be reduced or the polishing time can be shortened. Therefore, the workability of polishing the steel plate 1 can be improved.

[0057] [ND / / <111> [Area ratio of grains with ±10° orientation] In the cross section 12, the ND / / of the entire thickness t measured by electron backscatter diffraction (EBSD) is as follows: <111> Area ratio of grains with an orientation of ±10° (hereinafter, <111> The area ratio (referred to as "area ratio") is 40% or more, and more preferably 43% or more.

[0058] In this application, "ND / / <111> "±10° orientation grains" means "relative to the direction normal to the rolling surface" <111> This refers to "crystal grains that are parallel or tilted within a range of -10° to +10°". <111> It is known that grain orientation of ±10° has a favorable effect on the formability of steel sheets during deep drawing. Therefore, <111> The higher the area ratio, the better the formability of the steel sheet can be improved.

[0059] As mentioned above, the steel sheet 1 has a grain size of 15 μm or less in the cross-section 12. It is known that the smaller the grain size of the steel sheet, the lower the formability tends to be. However, the steel sheet 1 according to one aspect of the present invention is <111> Since the area ratio is 40% or more, the formability of steel plate 1 can be improved.

[0060] [Average major diameter of precipitates] In the steel plate 1, the average major axis of precipitates (hereinafter referred to as "low-temperature precipitates") that precipitate in the central part of the plate thickness t at 600°C or more and less than 900°C in the cross section 12 is 0.15 μm or less, more preferably 0.12 μm or less. Examples of low-temperature precipitates include compounds containing at least one of Nb, Ti, and P, and Fe. More specifically, examples of low-temperature precipitates include (i) phosphides, such as FeTiP, FeNbP, and Fe(Ti,Nb)P, and (ii) intermetallic compounds that form a phase called the Laves phase, such as Fe2Nb.

[0061] Note that precipitates of compounds with a melting point of 900°C or higher (hereinafter referred to as "high-temperature precipitates") are not included in the "low-temperature precipitates" in this application because they precipitate at temperatures of 900°C or higher. Examples of high-temperature precipitates include (iii) metal carbides, such as TiC and NbC, and (iv) metal nitrides, such as TiN and NbN.

[0062] Figure 2 is a representative SEM image of a cross-section 12 of a steel plate 1 according to one embodiment of the present invention. As shown in Figure 2, low-temperature precipitates can be observed in the cross-section 12 as granular material dispersed within the material.

[0063] Figure 3 is an enlarged view of the area within the dotted line in the SEM image shown in Figure 2. Here, the major axis of the precipitate refers to the length of the longest line segment connecting two points at the edge of the particulate precipitate image. In this application, the average major axis of the low-temperature precipitate is defined as the average value of multiple measurements obtained by taking an image of the central part of the plate thickness t of cross section 12 at a magnification of 10,000x (imaging area 12 μm × 9 μm) using a field emission scanning electron microscope (FE-SEM), and then measuring the major axis of each low-temperature precipitate observed in the acquired SEM image.

[0064] As shown in Figure 2, high-temperature precipitates can also be observed as granular material dispersed within the material in cross-section 12. However, high-temperature precipitates can be distinguished from low-temperature precipitates because, in SEM images taken using the method described above, they are observed as coarse granular material with a major axis exceeding 0.5 μm, for example. Therefore, low-temperature precipitates can be rephrased as "fine precipitates among the precipitates with a major axis of 0.5 μm or less." High-temperature precipitates can be rephrased as "coarse precipitates among the precipitates with a major axis exceeding 0.5 μm."

[0065] [Average Rankford Value] In one aspect of the present invention, the steel sheet preferably has an average Rankford value (hereinafter referred to as "average r value") of 1.6 or higher. Here, the average r value can be calculated using the Rankford value (hereinafter referred to as "r value") measured at a plastic strain of 14.4% by the method specified in the JIS standard (JIS Z 2254:2021) using the following formula (1).

[0066] Average r-value = (r L +2r D +r C ) / 4 ···(1) In formula (1), r L , r D and r C These are the r values ​​of JIS No. 13B test specimens taken from directions parallel, 45°, and 90° to the rolling direction, respectively. A higher average r value indicates improved formability of the steel sheet.

[0067] [Surface roughness after deep drawing] In one aspect of the present invention, a steel sheet preferably has a surface roughness Rz of 8.0 μm or less, expressed as a ten-point average roughness on the cylindrical side parallel to the rolling direction, after cylindrical deep drawing. Here, cylindrical deep drawing can be performed under the following conditions: punch diameter: Φ50 mm, punch shoulder radius: 5 mm, die diameter: 52 mm, die shoulder radius: 5 mm, wrinkle-holding force: 1 ton, and drawing ratio: 2.0. Furthermore, the surface roughness Rz can be measured by the method specified in the JIS standard (JIS B 0601:2013).

[0068] The smaller the surface roughness Rz, the less surface roughness the steel sheet will have during processing. Furthermore, when the steel sheet is polished, a smaller surface roughness Rz reduces the polishing load or shortens the polishing time. Therefore, the workability of polishing the steel sheet can be improved.

[0069] [Manufacturing method] Figure 4 is a flowchart showing an example of a steel sheet manufacturing method according to one aspect of the present invention. As shown in Figure 4, the steel sheet manufacturing method includes a first annealing step S1, a cold rolling step S2, and a second annealing step S3. The steel sheet manufacturing method may also include a casting step and a hot rolling step in that order before the first annealing step S1. Each of the above steps will be described below.

[0070] <Casting Process> The casting process is a process for producing ferritic stainless steel slabs (hereinafter abbreviated as "slabs") by pouring molten steel having a desired composition into a mold and cooling it. After cooling, the slabs are cut to the desired length and used in the subsequent hot rolling process. The molten steel can be produced, for example, by using pig iron or iron scrap as raw materials, removing impurities, adding various components, and heating it.

[0071] <Hot rolling process> The hot rolling process is a process for producing a ferritic stainless steel strip of a predetermined thickness (hereinafter abbreviated as "steel strip") by rolling (hot rolling) the slab produced in the casting process at a high temperature. The hot rolling process can be carried out using known equipment and methods. The steel strip produced by the hot rolling process can be used in the first annealing process S1 as stainless steel having the composition described in [Component composition of steel sheet] above.

[0072] <First annealing process S1> The first annealing step S1 is a step of heating stainless steel having the composition described above [Composition of steel sheet] to a temperature range of 900 to 1000°C and maintaining uniform heat. The uniform heat temperature in the first annealing step S1 is preferably 910 to 980°C. The uniform heat time in the first annealing step S1 is preferably 30 to 90 seconds. The stainless steel used in the first annealing step S1 is not particularly limited, and for example, steel strip after the hot rolling step described above can be used. Furthermore, the annealing equipment used in the first annealing step S1 is not particularly limited, and for example, known equipment such as a continuous annealing furnace or a batch furnace can be used.

[0073] After the first annealing step S1 and before the cold rolling step S2, a pickling step may be performed. The pickling step is a process of washing off scale adhering to the surface of stainless steel using a pickling solution such as sulfuric acid, hydrochloric acid, or a mixture of nitric acid and hydrofluoric acid.

[0074] <Cold rolling process S2> The cold rolling process S2 is a process of cold rolling the stainless steel (hot-rolled annealed material) after the first annealing process S1. The temperature of the stainless steel in the cold rolling process S2 is, for example, room temperature to 200°C throughout the cold rolling process S2. Furthermore, the rolling ratio in the cold rolling process S2 is preferably 75% or more, as a higher rolling ratio is effective in improving the average r value and formability. The rolling equipment used in the cold rolling process S2 is not particularly limited, and known equipment can be used.

[0075] In the cold rolling process S2, the cold rolling treatment may be performed only once or two or more times. If the cold rolling treatment is performed two or more times, the first annealing process S1 may be performed between the cold rolling treatments. Alternatively, any annealing treatment different from the first annealing process S1 may be performed between the cold rolling treatments. Furthermore, after this optional annealing treatment and before the next cold rolling treatment, an acid pickling treatment may be performed.

[0076] <Second annealing process S3> The second annealing step S3 is a process in which the stainless steel (cold-rolled material) after the cold rolling step S2 is heated to a temperature range of 800 to 950°C at a heating rate of 50 to 1000°C / s, and then held at a uniform temperature for 5 to 60 seconds. The annealing equipment used in the second annealing step S3 is not particularly limited, and known equipment such as a continuous annealing furnace or a batch furnace can be used.

[0077] The heating rate in the second annealing step S3 is 50 to 1000°C / s, preferably 100 to 700°C / s. The soaking temperature in the second annealing step S3 is 800 to 950°C, preferably 820 to 920°C. The soaking time in the second annealing step S3 is 5 to 60 seconds, preferably 15 to 45 seconds.

[0078] After the second annealing step S3, a pickling step may be performed as needed. The pickling step is a process of washing off scale adhering to the surface of the stainless steel using a pickling solution such as sulfuric acid, hydrochloric acid, or a mixture of nitric acid and hydrofluoric acid.

[0079] After the second annealing process S3, a finishing process may be performed as needed. The finishing process is a process for finishing the steel sheet after the second annealing process S3. Specifically, in the finishing process, for example, temper rolling and cutting into the desired shape may be performed.

[0080] <Possible mechanism> The mechanism by which steel sheets produced by this manufacturing method achieve both improved formability and reduced surface roughness is not limited, but at present, it is speculated to be as follows. First, in the first annealing step S1, by heating the stainless steel to a temperature range of 900-1000°C and maintaining uniform heat, elements that form low-temperature precipitates can be dissolved in the stainless steel.

[0081] Next, after the cold rolling process S2, the stainless steel is rapidly heated in the second annealing process S3, which reduces the precipitation of low-temperature precipitates during heating. This promotes the recrystallization of the stainless steel, allowing it to be recrystallized at a relatively low temperature of 800-950°C. As a result, the growth rate of the stainless steel grains is reduced, and the stainless steel grains can be refined. It is known that when the stainless steel grains are refined, the surface roughness of the steel sheet is reduced. Therefore, the steel sheet produced by this manufacturing method can have a reduced surface roughness.

[0082] Furthermore, in the second annealing step S3, by rapidly heating the stainless steel and then maintaining uniform heat for 5 to 60 seconds, fine low-temperature precipitates can be precipitated. These fine low-temperature precipitates have an effect called "pinning," which can reduce the growth rate of the stainless steel crystal grains. This allows for further refinement of the stainless steel crystal grains. Therefore, the surface roughness of the steel sheets produced by this manufacturing method can be further reduced.

[0083] Furthermore, in the second annealing step S3, by precipitating fine low-temperature precipitates during soaking, ND / / <111> This can promote the growth of grains with an orientation of ±10°. Therefore, the formability of the steel sheet 1 can be improved.

[0084] Furthermore, since the high-temperature precipitates have a high melting point, they do not change at the soaking temperature (900-1000°C) in the first annealing step S1 and the soaking temperature (800-950°C) in the second annealing step S3. Therefore, it is presumed that the steel sheet and the method for manufacturing the steel sheet according to one aspect of the present invention have little effect on formability and surface roughness.

[0085] Furthermore, under typical annealing conditions where heating rate is less than 50°C / s, the recrystallization of stainless steel is inhibited by the precipitation of low-temperature precipitates during heating, requiring recrystallization at temperatures exceeding 950°C. At such high temperatures, the growth rate of stainless steel grains is high, which may cause the stainless steel grains to coarseen. Moreover, the low-temperature precipitates that precipitate during heating continue to grow during subsequent heating and soaking, reducing the pinning effect and further coarsening the stainless steel grains. Therefore, it is presumed that the surface roughness of the steel sheet produced will be significant under these typical annealing conditions.

[0086] The present invention is not limited to the embodiments described above, 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.

[0087] [Examples] An embodiment of the present invention will be described below. Note that the method for manufacturing steel sheets described in this embodiment is merely an example and does not limit the method for manufacturing steel sheets according to one aspect of the present invention.

[0088] <Slab manufacturing> To evaluate the physical properties of a steel sheet according to one aspect of the present invention, a 30 kg slab was first produced by vacuum melting steel having the component composition shown in Table 1 below. In Table 1, steel grades A to N are stainless steels produced within the scope of the present invention, serving as examples of the present invention. In addition, steel grades O to R in Table 1 are stainless steels produced under conditions outside the scope of the present invention, serving as comparative examples.

[0089] [Table 1]

[0090] Table 1 shows the composition of the components contained in each steel grade in mass percent. The remainder other than the components shown in Table 1 is Fe or small amounts of unavoidable impurities. Furthermore, the underlined text in Table 1 indicates that the composition of the components contained in each steel grade in the comparative examples falls outside the scope of the present invention.

[0091] <Manufacturing of hot-rolled steel sheets> Next, the slab was heated at 1200°C for 2 hours, and then hot-rolled to produce a hot-rolled steel sheet with a thickness of 3 mm.

[0092] <Manufacturing and property evaluation of steel plates> Next, steel sheets No. 1 to 27 with a thickness of 0.6 mm were produced by carrying out the first annealing process, the cold rolling process and the second annealing process under the manufacturing conditions shown in Table 2. In this embodiment, cold rolling was performed only once in the cold rolling process. The results of evaluating various physical properties for each steel sheet are also shown in Table 2. Note that underlines in Table 2 indicate that the manufacturing conditions and various physical properties of the steel sheets are outside the scope of the present invention or outside the preferred scope of the present invention.

[0093] [Table 2]

[0094] In the moldability assessment in Table 2, "○ (Good)" indicates an average r value of 1.6 or higher, while "× (Poor)" indicates an average r value of less than 1.6. Furthermore, in the assessment of surface roughness, "○ (Good)" indicates an Rz value of 8.0 μm or less, while "× (Poor)" indicates an Rz value exceeding 8.0 μm.

[0095] In Table 2, a "-" in the "Average major diameter of low-temperature precipitates (μm)" column indicates that no low-temperature precipitates were observed. Furthermore, the "Unformable" indication in the "Rz (μm)" and "Processing surface roughness" columns for steel sheet No. 26 indicates that steel sheet No. 26 was too hard and could not be deep drawn due to the high P content of 0.097% in steel grade Q.

[0096] As shown in Table 2, steel plates No. 1, 4, 7, 9, 10, 12-14, 16, 17, 19-21 and 23 (hereinafter referred to as "Steel Plates of the Invention") are steel plates corresponding to the Examples of the Invention, manufactured by the method for manufacturing steel plates according to one aspect of the Invention. Furthermore, the Steel Plates of the Invention have a component composition, average major axis of low-temperature precipitates, grain size, and <111> The area ratio standards are met. All of these example steel sheets of the present invention have a "○ (good)" rating for formability and surface roughness, demonstrating excellent formability and reduced surface roughness.

[0097] In contrast, steel plates No. 2, 3, 5, 6, 8, 11, 15, 18, 22 and 24-27 (hereinafter referred to as "comparative example steel plates") are steel plates that fall under comparative examples manufactured under conditions outside the scope of the steel plate manufacturing method according to one aspect of the present invention. Furthermore, comparative example steel plates have a component composition, average major axis of low-temperature precipitates, grain size, and <111> At least one of the area ratios did not meet the standard. In all of these comparative steel sheets, at least one of the formability and surface roughness was rated "× (good)," demonstrating that it is not possible to achieve both improved formability and reduced surface roughness. [Industrial applicability]

[0098] The present invention can be used, for example, in the processing of home appliances and other objects. [Explanation of Symbols]

[0099] 11 Rolling surface 12 Cross-section S1 First annealing process S2 cold rolling process S3 Second annealing process t Plate thickness W Rolling width

Claims

1. A ferritic stainless steel sheet having a composition in mass%, containing 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: 0.005-0.05%, and also containing at least one of Ti: 0.01-0.5% and Nb: 0.01-0.5%, with a total Ti and Nb content of 0.13% or more, an S content of 0.01% or less, and the remainder being Fe and unavoidable impurities, When the ferritic stainless steel sheet is cut in a plane parallel to the rolling direction and perpendicular to the rolling surface, the grain size calculated by the cutting method is 15 μm or less in the cross-section. Crystal grains whose <111> direction is within ±10° of the ND direction are defined as ND / / <111>±10° oriented grains, and in the cross-section, the area ratio of the ND / / <111>±10° oriented grains in the total plate thickness measured by electron backscatter diffraction is 40% or more. Regarding the particulate precipitates observed in a SEM image of the aforementioned cross-section taken with a field emission scanning electron microscope at a magnification of 10,000x, the length of the longest line segment connecting two points at the edge of the precipitate image is defined as the major axis, and the granular material having a major axis of 0.5 μm or less is defined as a fine precipitate. In the SEM image of the cross-section, the major axis of each micro-precipitate is measured, and the average major axis of the micro-precipitates, defined as the average of the multiple measured values ​​obtained, is 0.15 μm or less. The average Rankford value measured in accordance with JIS Z 2254:2021 is 1.6 or higher, A ferritic stainless steel sheet having a surface roughness Rz of 8.0 μm or less, expressed as a ten-point mean roughness measured in accordance with JIS B 0601:2013.

2. The ferritic stainless steel sheet according to claim 1, further comprising a composition containing at least one of the following: Mo: 0.05-2.00%, Ni: 0.01-1.00%, Co: 0.05-0.50%, Cu: 0.05-1.00%, Al: 0.01-0.05%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, B: 0.0001-0.0025%, V: 0.05-0.10%, W: 0.05-0.22%, Sn: 0.005-0.050%, Sb: 0.0050% or less, and rare earth elements: 0.001-0.010%.

3. A ferritic stainless steel sheet according to claim 1 or 2, wherein, after cylindrical deep drawing is performed under the conditions of punch diameter: Φ50 mm, punch shoulder radius: 5 mm, die diameter: 52 mm, die shoulder radius: 5 mm, wrinkle holding force: 1 ton, and drawing ratio of 2.0, the surface roughness expressed as a ten-point average roughness on the cylindrical side parallel to the rolling direction is 8.0 μm or less.

4. A method for manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 3, A first annealing step in which a ferritic stainless steel having the above composition is heated to a temperature range of 900 to 1000°C and maintained at a uniform temperature, A cold rolling step in which the ferritic stainless steel after the first annealing step is cold-rolled, A method for manufacturing a ferritic stainless steel sheet, comprising: a second annealing step in which the ferritic stainless steel after the cold rolling step is heated to a temperature range of 800 to 950°C at a heating rate of 50 to 1000°C / s, and then held at a uniform temperature for 5 to 60 seconds.

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