Ferritic stainless steel sheet, its manufacturing method and parts

Abstract

To provide a ferritic stainless steel sheet that resists cracking during its production and also has excellent toughness, and a part made using the same.SOLUTION: A steel contains, in mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, P: 0.010-0.050%, S: 0.0002-0.0100%, Cr: 10.0-20.0%, N: 0.001-0.030%, Nb: 0.10-0.40%, B: 0.0002-0.0030%, Al: 0.005-0.100%, with the balance being Fe and inevitable impurities. The amount of solute Nb is less than or equal to a smaller value of 0.20% and (Nb content-0.08) mass%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet, a method for producing the same, and a part using the stainless steel sheet. [Background technology]

[0002] An automobile's exhaust gas path is composed of various components, such as the exhaust manifold, muffler, catalyst, flexible tube, center pipe, and front pipe. Fasteners called flanges and brackets are often used to connect these components. Flange joints are widely used for automotive exhaust system components because they require minimal processing time and require a small working space. Furthermore, thick flanges (5 mm or thicker) are often used to reduce noise caused by vibration and ensure rigidity. Flanges are manufactured by press forming and punching processes, and traditionally, ordinary steel plate has been used as the material. However, due to its poor corrosion resistance, ordinary steel can develop initial rust after vehicle assembly, which can detract from the vehicle's aesthetics. For this reason, stainless steel plate is being actively replaced by ordinary steel plate as the flange material.

[0003] Ferritic stainless steel sheets have a lower Ni content and are less expensive than austenitic stainless steel sheets, but they are known to have poor toughness. Low toughness can lead to sheet breakage during threading and coil unrolling during the steel sheet manufacturing process. They can also crack during cutting, punching, and other processes in part manufacturing. Furthermore, impacts in low-temperature winter environments can cause parts to crack, resulting in damage to automobile exhaust pipes. In particular, thick ferritic stainless steel sheets (5 mm or thicker) can have low toughness, which can lead to poor reliability when used in fasteners.

[0004] Several methods have been devised to ensure the toughness of ferritic stainless steel sheets. For example, Patent Documents 1 and 2 disclose manufacturing conditions for mass-producing ferritic stainless steel hot-rolled coils or hot-rolled and annealed coils with a thickness of 5 to 12 mm. Patent Document 1 is directed to Ti-containing ferritic stainless steel, and discloses a method in which the coiling temperature is set to 570°C or higher and the coil is immersed in water in order to adjust the hardness and Charpy impact value. Patent Document 2 is directed to Nb-containing ferritic stainless steel, and discloses a method of adjusting the hardness and Charpy impact value by setting the hot rolling finishing temperature to 890°C or higher, coiling at 400°C or lower, and immersing the coil in water. Patent Document 3 discloses a ferritic stainless steel with excellent cold crack resistance, in which the length of subgrain boundaries, where the crystal orientation difference of the ferrite phase is small, is set to a certain level or more. This is obtained by a method in which the hot rolling finishing temperature is set to 800 to 1000°C, the coiling temperature is set to above 650°C to 800°C, and the steel is immersed in a water tank after coiling. Patent Document 4 discloses a ferritic stainless steel sheet having excellent toughness in which the proportion of precipitates at grain boundaries is specified. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-140687 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-140688 [Patent Document 3] International Publication No. 2013 / 085005 [Patent Document 4] Japanese Patent Application Laid-Open No. 2009-263714 Summary of the Invention [Problem to be solved by the invention]

[0006] The ferritic stainless steels described in Patent Documents 1 and 2 specify hot rolling conditions to improve the toughness of the hot-rolled sheet or hot-rolled annealed sheet, but it is difficult to control the entire coil length to meet the above conditions, and the metallurgical factors that govern toughness improvement are unclear. The ferritic stainless steels described in Patent Documents 3 and 4 attempt to improve toughness by controlling the grain boundary characteristics (for example, the magnitude of misorientation of the grain boundaries) and precipitates on the grain boundaries, but do not necessarily achieve a toughness level satisfactory for flange applications. Therefore, an object of the present invention is to solve the above-mentioned problems of the known techniques and to provide a ferritic stainless steel sheet having excellent toughness. [Means for solving the problem]

[0007] To solve the above problems, the present inventors investigated the toughness of ferritic stainless steel sheets from the viewpoint of controlling the chemical composition and structure during the manufacturing process. In particular, they conducted detailed research from the viewpoint of the amount of solute Nb contained in the steel as a measure against solid solution strengthening and intergranular corrosion of welds. As a result, they discovered that controlling the amount of solute Nb is extremely effective in improving the toughness of ferritic stainless steel sheets, particularly hot-rolled steel sheets or hot-rolled and annealed steel sheets.

[0008] The gist of the present invention to solve the above problems is: (1) In mass%, C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.010~0.050%, S: 0.0002~0.0100%, Cr: 10.0 to 20.0%, N: 0.001 to 0.030%, Nb: 0.10 to 0.40%, B: 0 to 0.0030%, Al: 0 to 0.100%, the balance being Fe and unavoidable impurities; A ferritic stainless steel sheet characterized in that the amount of solute Nb is equal to or less than the smaller of 0.20 mass % and (Nb content - 0.08) mass %. (2) Furthermore, in mass%, Ti: 0 to 0.20% Ni: 0 to 1.00% Mo: 0-2.0% Cu: 0-3.0% V: 0~1.00%, Mg: 0 to 0.0030%, Sn: 0~0.30% Sb: 0 to 0.30% Zr: 0 to 0.10% Ta: 0 to 0.10%, Hf: 0 to 0.10% W: 0~2.00%, Co: 0 to 0.20% Ca: 0 to 0.0030%, REM: 0~0.050%, Ga: 0 to 0.10% The ferritic stainless steel sheet according to (1), characterized in that it contains one or more of the following: (3) When the thickness of the stainless steel plate is t, the number density of Nb-containing precipitates having a particle size of 1 μm or more is 0.005 particles / μm in the depth range of t / 2 to t / 4 from the surface of the cross section in the plate thickness direction. 2 A ferritic stainless steel sheet according to (1) or (2) below. (4) The ferritic stainless steel sheet according to any one of (1) to (3), wherein the grain size of the Nb-containing precipitates is 2 μm or less. (5) A method for producing a ferritic stainless steel sheet according to (1) or (2), characterized in that a slab having the composition according to (1) or (2) is heated to 1230°C or higher, hot-rolled so that the ratio of the finish rolling start temperature (°C) to the finish rolling end temperature (°C) (finish rolling start temperature (°C) / finish rolling end temperature (°C)) is 1.10 or higher, and then coiled at 500°C or lower. (6) (5) A method for producing a ferritic stainless steel sheet according to (5), wherein the hot-rolled steel sheet is cooled at a cooling rate of 20°C / sec or more and then coiled. (7) The method for producing a ferritic stainless steel according to (5) or (6), wherein the coiled steel sheet is heated to 900 to 1050°C and then held for 60 seconds or more for annealing. (8) A part at least partly using the ferritic stainless steel sheet according to any one of (1) to (4). (9) The component according to (8), wherein the ferritic stainless steel plate is joined to another component by brazing or arc welding. [Effects of the Invention]

[0009] According to the present invention, a ferritic stainless steel sheet having excellent toughness can be efficiently produced without the need for new equipment. Furthermore, by using the ferritic stainless steel sheet according to the present invention for parts, parts having excellent toughness can be obtained by utilizing existing equipment. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the relationship between the Nb content, the amount of dissolved Nb, and the toughness. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described below. Unless otherwise specified, "%" for components indicates mass% in the steel. Unless a lower limit is particularly specified, it may include cases where no component is contained (0%).

[0012] It has been known for some time that refinement of grain size, refinement of precipitates, and crystal orientation contribute to improving toughness. However, it is difficult to ensure sufficient toughness for fastener applications using hot-rolled or hot-rolled annealed ferritic stainless steel sheets containing many elements based on conventional knowledge alone. Ensuring toughness is particularly difficult for thick ferritic stainless steel sheets with a thickness of 5 mm or more.

[0013] The inventors conducted a detailed investigation into the relationship between the amount of solute Nb dissolved in the ferrite matrix and toughness, and found that toughness can be improved by setting the amount of solute Nb to the smaller of 0.2 mass% or (Nb content - 0.08) mass% or less, thereby finely dispersing Nb precipitates. Figure 1 shows the relationship between the Nb content (mass % of the total) and the amount of solute Nb (mass % of the total) and toughness of a ferritic stainless steel (plate thickness 8 mm) with a composition of 17.2%Cr-0.005%C-0.2%Si-0.1%Mn-0.02%P-0.0003%S-0.0006%B-0.06%Al-0.012%N. The amount of dissolved Nb was determined by measuring the amount of precipitated Nb by extraction residue analysis, and the difference between this and the Nb content was used to determine the amount of dissolved Nb. For the extraction residue analysis, precipitates in the steel were extracted using a tetramethylammonium chloride solution and a 0.2 μm filter, and then analyzed by ICP.

[0014] The toughness was evaluated by a Charpy impact test. V-notch test pieces (V-notches in the width direction) were taken from the hot-rolled annealed sheets, and the impact value was measured at room temperature in accordance with JIS Z 2242. The impact value was 20 J / cm 2 Passed (○), 20J / cm 2 A strength of less than 20 J / cm (hereinafter sometimes referred to as low toughness in this specification) was rated as unacceptable (×). 2 If the above conditions are met, the hot rolled steel sheet can be produced without brittle fracture in the production process.

[0015] The present inventors have found from FIG. 1 that there is a relationship between the amount of dissolved Nb (or, from another perspective, the amount of precipitated Nb) and toughness, and have further investigated how to improve toughness, resulting in the following findings. In the Nb content range of 0.10 to 0.40%, it was found that when the amount of solute Nb was greater than the smaller of 0.2% and (Nb content - 0.08) mass%, the toughness was low, but when the amount of solute Nb was equal to or less than the smaller of 0.20% and (Nb content - 0.08) mass%, the toughness was improved. If the Nb content is less than 0.10%, Cr carbides and nitrides are formed, which act as the initiation points for brittle fracture, presumably resulting in reduced toughness. Furthermore, if the Nb content exceeds 0.40%, coarse Nb(C,N) (Nb carbonitrides) are formed during hot rolling, particularly during the slab heating stage, presumably resulting in reduced toughness. On the other hand, even if the Nb content is 0.40% or less, if the amount of solute Nb is greater than the smaller of 0.20% and (Nb content - 0.08) mass%, the solute Nb in the ferrite matrix significantly hardens the steel, presumably resulting in reduced toughness.

[0016] Although there is no previous knowledge about the effect of hardening due to solute Nb on toughness, it is thought that when brittle fracture (cleavage fracture) occurs originating from inclusions or oxides and the crack propagates along the cleavage plane of the ferrite phase, propagation is rapid if the ferrite phase is hard, but if the amount of solute Nb is made relatively soft by setting the value below the smaller of 0.2% or (Nb content - 0.08) mass%, the propagation speed of the cleavage plane slows down and toughness improves.

[0017] Furthermore, when the amount of solute Nb is set to the smaller of 0.2% or (Nb content - 0.08) mass%, the toughness value is further improved by distributing fine Nb-containing precipitates (Nb carbonitrides and composite precipitates containing them). That is, the number density of large Nb-containing precipitates with a grain size of 1 μm or more is set to 0.005 particles / μm at the center of the steel sheet. 2 It is preferable to set the density to 0.004 particles / μm or less, and more preferably 0.004 particles / μm 2 Below, 0.003 pieces / μm 2 Below, 0.002 pieces / μm 2 Less than or equal to 0.001 particles / μm 2 It would be better if it was below.

[0018] Furthermore, since the presence of coarse Nb-containing precipitates with a grain size exceeding 2 μm easily becomes the starting point of fracture and deteriorates toughness, it is desirable to minimize their presence. Therefore, it is preferable that the grain size of the Nb-containing precipitates be 2 μm or less.

[0019] Here, the density of Nb-containing inclusions is determined by observing a cross section in the thickness direction of the steel plate at a depth from the surface in the range of t / 2 to t / 4, where t is the thickness of the steel plate. The particle size of the Nb-containing inclusions is determined as the diameter of a circle having the same area.

[0020] Next, the range of the steel composition will be explained. Carbon (C) reduces toughness due to hardening caused by solute C and carbide precipitation, so the lower the content, the better. If the C content exceeds 0.030%, the formation of Cr carbides reduces toughness and the intergranular corrosion resistance of welds deteriorates. Therefore, the upper limit should be set to 0.030%, and preferably 0.010%. On the other hand, excessive reduction leads to increased refining costs, so the lower limit should be set to 0.001%, and preferably 0.002%.

[0021] Si may be contained as a deoxidizing element and also improves oxidation resistance, but since it is a solid solution strengthening element, the less Si the better from the viewpoint of toughness, with the upper limit being 1.00%, preferably 0.50%. On the other hand, to ensure oxidation resistance, the lower limit should be 0.01%, preferably 0.05%.

[0022] Like Si, Mn is a solid-solution strengthening element, so the lower the content, the better. Furthermore, if the Mn content exceeds 1.00%, precipitates such as MnS are formed, which become the starting point for brittle fracture. Therefore, the upper limit should be set to 1.00%, preferably 0.50%. On the other hand, excessive reduction leads to increased refining costs, and trace amounts of Mn improve scale spallability. Therefore, the lower limit should be set to 0.01%, preferably 0.05%.

[0023] Like Mn and Si, P is a solid-solution strengthening element that hardens materials, so from the perspective of toughness, the lower its content, the better. Furthermore, if its content exceeds 0.050%, phosphides that become the starting point for brittle fracture are formed, so the upper limit should be set to 0.050%, preferably 0.030%. However, excessive reductions lead to increased raw material costs, so the lower limit should be set to 0.010%, preferably 0.015%.

[0024] Since S deteriorates corrosion resistance, the lower its content, the better. Furthermore, if S exceeds 0.0100%, precipitates such as MnS and Ti4C2S2, which act as starting points for brittle fracture, are formed, so the upper limit should be set at 0.0100%, preferably 0.0060%. On the other hand, S combines with Mn and Ti to improve punchability in part molding, and this effect is only apparent from 0.0002%. Therefore, the lower limit should be set at 0.0002%, preferably 0.0004%.

[0025] Cr is an element that improves corrosion resistance and oxidation resistance. Considering the salt damage resistance required for fasteners, the lower limit should be 10%, preferably 11%. On the other hand, excessive Cr content hardens the material, degrading formability and toughness, which may lead to plate breakage during manufacturing. For example, if the content exceeds 20%, the toughness decreases due to the formation of coarse Cr carbides and nitrides, so the upper limit should be 20%, preferably 18%.

[0026] Like C, N deteriorates toughness and corrosion resistance, so the lower the content, the better. Furthermore, if the N content exceeds 0.030%, nitrides that become the starting point for brittle fracture are formed, so the upper limit should be set to 0.030%, preferably 0.020%. On the other hand, an excessive decrease in N content leads to increased refining costs, so the lower limit should be set to 0.001%, preferably 0.005%.

[0027] Nb is included to improve high-temperature strength and to bond with C and N to improve corrosion resistance, intergranular corrosion resistance, and toughness. To ensure this effect, the lower limit should be set to 0.10%, preferably 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, or 0.25%. On the other hand, excessive Nb content not only hardens the steel and reduces formability, but also significantly reduces toughness due to the precipitation of coarse Nb(C,N) and, depending on the thermal history, (Fe,Nb)6C and Fe2Nb. Therefore, the upper limit should be set to 0.40%, preferably 0.38%, 0.35%, 0.33%, or 0.30%.

[0028] B is an element that segregates at grain boundaries to improve the secondary workability of products, and may be added as needed to improve punchability in fastener processing. Therefore, there is no need to specifically limit the lower limit of the content, and it can be 0%, but to ensure this effect, the lower limit should be set to 0.0002%, preferably 0.0003%. On the other hand, excessive B content causes precipitation of borides, which reduces toughness, so the upper limit should be set to 0.0030%, preferably 0.0010%.

[0029] Al may be contained as a deoxidizing element and may be contained as needed. Therefore, there is no need to particularly limit the lower limit of the content, and it may be 0%, but to ensure that this effect is exerted, the lower limit should be set to 0.005%, preferably 0.010%. On the other hand, excessive content generates inclusions such as Al2O3, which not only harden the steel but also act as fracture initiation sites, resulting in reduced toughness, weldability (especially brazability), and surface quality. Therefore, the upper limit should be set to 0.100%, preferably 0.080%.

[0030] The balance of the above steel components is Fe and impurities. Here, the term "impurities" refers to components that are mixed in during industrial steel production due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

[0031] Furthermore, the ferritic stainless steel of this embodiment may further contain one or more of Ti, Ni, Mo, Cu, V, Mg, Sn, Sb, Zr, Ta, Hf, W, Co, Ca, REM, and Ga instead of Fe. These elements do not necessarily have to be contained, but by including them, further effects can be obtained. These elements will be explained below.

[0032] Ti is an element that is optionally contained to combine with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and toughness. The lower limit of the content does not need to be particularly limited and may be 0%, but to ensure the C and N fixing effect, the lower limit should be set to 0.01%, preferably 0.02%. On the other hand, a content of more than 0.20% causes the precipitation of coarse Ti (C, N), which not only significantly deteriorates toughness but also deteriorates weldability (particularly brazability), so the upper limit should be set to 0.20%, preferably 0.05%.

[0033] Ni is contained as needed to inhibit crevice corrosion and promote repassivation, thereby improving initial rust resistance. The lower limit of the content does not need to be particularly limited and may be 0%, but to ensure this effect, the lower limit should be set to 0.10%, preferably 0.20%. On the other hand, excessive Ni content hardens the steel, reducing toughness and making it more susceptible to stress corrosion cracking, so the upper limit should be set to 1.00%, preferably 0.30%.

[0034] Mo is an element that improves corrosion resistance and high-temperature strength, and is an element that suppresses crevice corrosion, especially in the case of a crevice structure. The lower limit of the content does not need to be particularly limited and can be 0%, but to ensure this effect, the lower limit should be set to 0.1%, preferably 0.5%. On the other hand, if the content exceeds 2.0%, formability will be significantly deteriorated and toughness will be deteriorated due to hardening, so the upper limit should be set to 2.0%, preferably 1.2%. Furthermore, considering manufacturing costs and punching ease during part processing, the upper limit is preferably 0.9%, and considering application to parts that require greater corrosion resistance and high-temperature strength, the lower limit should be 0.5%.

[0035] Cu is added as needed to improve high-temperature strength, inhibit crevice corrosion, and promote repassivation. The lower limit of the Cu content does not need to be particularly limited and can be 0%, but to ensure this effect, the lower limit should be set to 0.1%, preferably 0.2%. On the other hand, excessive Cu content causes hardening due to ε-Cu precipitation, deteriorating formability and toughness, so the upper limit should be set to 3.0%, preferably 1.2%.

[0036] V suppresses crevice corrosion and contributes to improving toughness when contained in trace amounts, so it is added as needed. The lower limit of the content does not need to be particularly limited and can be 0%, but to ensure this effect is exerted, it is recommended to set the lower limit at 0.05%. On the other hand, excessive content hardens the steel and deteriorates formability, and coarse V(C,N) precipitates, leading to a deterioration in toughness, so the upper limit should be set at 1.00%, preferably 0.20%.

[0037] Mg may be added as a deoxidizing element, and also contributes to refining the slab structure and improving formability. Furthermore, Mg oxides serve as precipitation sites for carbonitrides such as Ti(C,N) and Nb(C,N), resulting in the finely dispersed precipitation of these compounds. The lower limit of the Mg content does not need to be particularly limited and may be 0%, but to ensure this effect and contribute to improving toughness, the lower limit should be set to 0.0002%, preferably 0.0003%. On the other hand, excessive Mg content leads to deterioration of weldability and corrosion resistance, so the upper limit should be set to 0.0030%, preferably 0.0010%.

[0038] Sn and Sb contribute to improving corrosion resistance and high-temperature strength. There is no particular need to limit the lower limit of the content, and it may be 0%, but if necessary, it may be contained in an amount of 0.01% or more, preferably 0.02% or more. On the other hand, a content of more than 0.30% may cause slab cracking during steel sheet production, so the upper limit should be set to 0.30%, preferably 0.10%.

[0039] Zr, Ta, and Hf combine with C and N to contribute to improving toughness. There is no need to particularly limit the lower limit of the content, and it may be 0%, but if necessary, it may be contained in an amount of 0.01% or more, preferably 0.02% or more. On the other hand, a content of more than 0.10% increases costs and significantly deteriorates manufacturability, so the upper limit should be set at 0.10%, preferably 0.07%.

[0040] W contributes to improving corrosion resistance and high-temperature strength. The lower limit of the W content does not need to be particularly limited and may be 0%, but if necessary, it may be contained in an amount of 0.01% or more, preferably 0.05% or more. On the other hand, a content of more than 2.00% leads to a deterioration in toughness during steel plate production and an increase in costs, so the upper limit should be set to 2.00%, preferably 1.80%.

[0041] Co contributes to improving high-temperature strength. There is no particular need to set a lower limit for the Co content, and it may be 0%, but if necessary, it may be contained in an amount of 0.01% or more, preferably 0.02% or more. On the other hand, a content of more than 0.20% leads to a deterioration in toughness during steel plate production and an increase in costs, so the upper limit should be set to 0.20%, preferably 0.09%.

[0042] Ca may be added for desulfurization. The lower limit of the Ca content does not need to be particularly limited and may be 0%, but to ensure this effect, the lower limit should be set to 0.0001%, preferably 0.0002%. On the other hand, a Ca content of more than 0.0030% generates coarse CaS, which deteriorates toughness and corrosion resistance, so the upper limit should be set to 0.0030%, preferably 0.0020%.

[0043] REM may be added to improve toughness and oxidation resistance by refining various precipitates. The lower limit of the REM content does not need to be particularly limited and may be 0%, but it may be added as needed. To ensure this effect, the lower limit should be set to 0.001%, preferably 0.008%. On the other hand, a content of more than 0.050% significantly deteriorates castability, so the upper limit should be set to 0.050%, preferably 0.040%. REM (rare earth elements) is generally defined as the collective term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) ranging from lanthanum (La) to lutetium (Lu). These REM elements may be added alone or in combination. When multiple REM elements are added, the total amount should be within the above-mentioned range.

[0044] Ga improves corrosion resistance and suppresses hydrogen embrittlement. The lower limit of the Ga content does not need to be particularly limited and may be 0%, but from the viewpoint of sulfide and hydride formation, the lower limit should be set to 0.0002%, preferably 0.0010%. On the other hand, from the viewpoint of manufacturability and cost, the upper limit should be set to 0.1000%, preferably 0.0040%.

[0045] The present invention does not particularly specify other components, but in this embodiment, Bi and the like may be contained in an amount of 0.001 to 0.100% as needed.

[0046] Next, the manufacturing method will be described. A steel sheet according to an embodiment of the present invention is manufactured through the steps of steelmaking-hot rolling, steelmaking-hot rolling-pickling, or steelmaking-hot rolling-annealing-pickling. In steelmaking, a method is preferred in which steel containing the above-described components is melted in a converter or electric furnace, followed by secondary refining. The molten steel with the adjusted components is formed into a slab by a known casting method (e.g., continuous casting). The slab is heated to a predetermined temperature and hot rolled to a predetermined thickness.

[0047] Normally, to reduce the amount of solute Nb, Nb-based precipitates such as Nb(C,N), MC, and Laves phases are precipitated, but typical precipitation treatments result in a decrease in toughness due to the aggregation and coarsening of the precipitates. To solve this problem, in this embodiment, fine precipitation during the hot rolling process is utilized to reduce the amount of solute Nb and refine the precipitates.

[0048] In this embodiment, the heating temperature immediately before hot rolling (hot rolling heating temperature), the ratio between the finish rolling start temperature and the finish rolling end temperature, and the coiling temperature are specified, particularly to control the amount of solute Nb and achieve softening. The hot rolling heating temperature is set to 1230°C or higher from the viewpoints of reducing fracture initiation points due to fine precipitation of Nb(C,N) (Nb carbonitride) and controlling the amount of solute Nb. If the heating temperature is lower than 1230°C, Nb(C,N) precipitates and coarsens during slab heating, becoming fracture initiation points and reducing toughness, so the lower limit is set to 1230°C. On the other hand, if the temperature exceeds 1300°C, the slab will deform severely during heating, deteriorating plate thickness accuracy and surface quality, so a temperature of 1300°C or lower is desirable.

[0049] After the slab is heated, hot-rolled steel sheets are produced by rough rolling and finish rolling. Rough rolling after slab heating involves 5 to 9 passes with a reduction rate of 30% or less per pass, with the finishing temperature generally reaching around 1100°C. Between passes of this rough rolling, fine precipitates of Nb(C,N) and Laves phases occur, reducing the amount of solute Nb. After this rough rolling, finish rolling involves 5 to 9 passes of high-speed continuous rolling.

[0050] In this embodiment, the ratio of the finish rolling start temperature (°C) to the finish rolling end temperature (°C) (finish rolling start temperature (°C) / finish rolling end temperature (°C)) is preferably 1.10 or higher. After finish rolling, the obtained steel sheet is cooled and coiled at 500°C or lower. By controlling the temperature in this embodiment, the solute Nb at the hot-rolled steel sheet stage can be controlled to the smaller value of 0.20% or (Nb content - 0.08) mass% or lower, thereby ensuring toughness.

[0051] Although temperature fluctuations occur in the longitudinal direction of the steel strip after rough rolling depending on the slab heating temperature and rough rolling conditions, the start temperature of finish rolling is preferably 950 to 1100°C and the end temperature is preferably 800 to 900°C. In this case, if the difference between the start temperature and the end temperature of finish rolling is small, Nb-based precipitates such as Nb(C,N) will not precipitate during finish rolling, and the amount of solute Nb cannot be reduced. In this embodiment, by setting the ratio of the start temperature to the end temperature of finish rolling (= finish rolling start temperature (°C) / finish rolling end temperature (°C)) to 1.10 or more, it is possible to achieve both fine precipitation of Nb(C,N) between the finish stands and a reduction in the amount of solute Nb. The ratio of the start temperature to the end temperature is preferably 1.15 or more, 1.20 or more, 1.22 or more, 1.24 or more, or 1.25 or more. The start and end temperatures of the finish rolling are not particularly specified here, but from the viewpoint of surface quality, it is desirable that the start temperature be 1000°C or higher and the end temperature be 850°C or higher.

[0052] Following finish rolling, the resulting steel sheet is cooled and coiled at 500°C or below, but this cooling after finish rolling should be as rapid as possible. Rapid cooling can prevent coarsening of Nb(C,N) and Laves phases that have precipitated before finish rolling. The lower limit of the cooling rate is preferably 20°C / sec, 30°C / sec, 40°C / sec, 50°C / sec, or 60°C / sec. There are no particular restrictions on the upper limit of the cooling rate, but it is recommended to set it to 100°C / sec or less in consideration of the capacity of ordinary cooling equipment. The coiling temperature is set to 500°C or lower in order to prevent Nb(C,N) and the Laves phase from coarsening after coiling. From the viewpoints of toughness, surface defects, and pickling properties, the temperature is preferably set to 450°C or lower, 400°C or lower, 370°C or lower, or 350°C or lower. There is no particular lower limit for the coiling temperature, but at lower temperatures, the strength of the steel sheet increases, making coiling difficult, and therefore the coiling temperature is preferably set to 300°C or higher.

[0053] The hot-rolled steel sheet (hot-rolled sheet) thus obtained may be annealed (hot-rolled sheet annealing) as needed. When annealing is performed, it is recommended to heat the sheet to a temperature range of 900 to 1050°C to obtain a recrystallized structure. If the heating temperature during annealing exceeds 1050°C, Nb-based precipitates begin to dissolve, resulting in a rapid increase in the amount of dissolved Nb and an increase in grain size, resulting in a deterioration in toughness. Therefore, the heating temperature during annealing should be 1050°C or lower, preferably 1100°C or lower. To minimize the strain during hot rolling and obtain a uniformly grained recrystallized structure, a holding time of 60 seconds or longer within this temperature range is recommended. Holding times of less than 60 seconds result in the formation of residual hot-rolled processed structures, resulting in a decrease in toughness due to coarse grains. Therefore, a holding time of 60 seconds or longer is recommended. On the other hand, excessively long holding times not only significantly reduce productivity but also deteriorate pickling properties, so a holding time of 90 seconds or shorter is desirable.

[0054] Other conditions in the manufacturing process may be selected as appropriate. For example, the slab thickness, hot-rolled sheet thickness, etc. may be set as appropriate. After coiling, the hot-rolled sheet may be immersed in a water-cooled pool. There are no particular restrictions on the pickling process after hot rolling or hot-rolled annealing, and mechanical descaling methods such as shot blasting, bending, and brushing may be selected as appropriate. There are also no particular restrictions on the pickling solution, and existing conditions such as sulfuric acid, nitric hydrofluoric acid, etc. may be used. Furthermore, coil surface grinding may be performed thereafter.

[0055] The ferritic stainless steel sheet produced in this manner can be processed into parts by conventional processing methods such as punching and grinding. In particular, the ferritic stainless steel according to the present invention has both excellent toughness and corrosion resistance, making it suitable for use in parts used in corrosive environments. It can be used in at least some of the exhaust system parts of automobiles and motorcycles. For example, it can be used for some exhaust system parts (exhaust manifolds, mufflers, etc.) and fastening parts such as flanges and brackets for fastening these parts together.

[0056] Although parts can be manufactured using only this ferritic stainless steel, parts made of this ferritic stainless steel can also be joined with other parts to form a single part. The joining method is not particularly limited. For example, brazing or arc welding (TIG, MIG, MAG, or plasma welding) can be applied. The welding conditions can be selected appropriately depending on the shape of the fastening parts and the shapes of other exhaust parts. [Example]

[0057] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0058] Steel having the chemical composition shown in Table 1 (Table 1-1 and Table 1-2 are collectively referred to as Table 1) was melted and cast into a slab, which was then hot-rolled to a thickness of 5 mm or more to produce a hot-rolled coil (hot-rolled sheet). The slab heating temperature was 1230-1280°C, the finish rolling start temperature was 950-1100°C, the finish rolling end temperature was 800-900°C, the cooling rate was 25°C / sec-50°C / sec, and the coiling temperature was 300-450°C. Subsequently, coils to be annealed were also produced, with the annealing temperature being 900-1050°C and the holding time being 60 seconds or longer.

[0059] Table 2 shows the results of Charpy impact tests on coils obtained by changing the hot rolling conditions and annealing conditions for steel having the composition of this embodiment. When the amount of solute Nb was within a predetermined range according to the manufacturing conditions of this embodiment, good toughness was obtained, but in the comparative example in which the amount of solute Nb was not within the predetermined range, the toughness was so low that coil threading was impossible.

[0060] For toughness evaluation, Charpy impact test specimens were taken from the hot-rolled sheets or hot-rolled annealed sheets shown in Tables 1 and 2, and the Charpy impact test was carried out by the above method. The results are shown in the "Impact Properties" column. 2 Passed (○), 20J / cm 2 Anything less than this was deemed a failure (×) and is noted in the "Impact Properties" column of Table 1. All of the examples of the present invention had a Charpy impact test value of 20 J / cm 2As shown above, it is clear that the steel has excellent toughness.

[0061] The results of evaluation of filler metal spreadability during brazing using the hot-rolled or hot-rolled annealed sheets listed in Tables 1 and 2 are shown in the "filler metal spreadability" column. For filler metal spreadability, 0.1 g of Ni-29%Cr-4%Si-6%P (Tokyo Brace Co., Ltd.) filler metal was placed on a flat plate and brazed at 1130°C for 10 minutes under a vacuum of 50 PaN. The filler metal spreadability was then visually observed. A sufficient filler metal spread was evaluated as "pass" (○), and an insufficient filler metal spread was evaluated as "fail" (×). Specifically, a sufficient filler metal spread was defined as a filler metal spread area after the filler metal spread test that was at least five times the filler metal applied area before the test.

[0062] Furthermore, the results of evaluation of the hot-rolled or hot-rolled annealed sheets listed in Tables 1 and 2 when they were arc-welded to other exhaust pipe components after flange processing are shown in the "Intergranular corrosion properties of arc welds" column. For arc-welded joints, the intergranular corrosion susceptibility of the weld cross section was investigated after TIG welding. A 10% oxalic acid electrolytic etch test conforming to JIS G0571 showed that grooved structures were observed, indicating a failure (×), and stepped structures were observed, indicating a pass (○). The TIG welding conditions were current: 200 A, voltage: 10 V, speed: 10 cm / min, torch gas: Ar 10 L / min, back gas: Ar 15 L / min, and after-gas: Ar 30 L / min. It can be seen from Tables 1 and 2 that flanges made from the steel of the present invention have excellent brazeability and intergranular corrosion resistance.

[0063] [Table 1-1]

[0064] [Table 1-2]

[0065] [Table 2] [Industrial Applicability]

[0066] The ferritic stainless steel sheet, its manufacturing method, and fasteners including the ferritic stainless steel sheet according to the present invention can be used in a variety of industries. In particular, their use in automobile and motorcycle parts ensures reliability and increases social contribution, making them extremely useful industrially.

Claims

1. In mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, P: 0.010-0.050%, S: 0.0002-0.0100%, Cr: 10.0-20.0%, N: 0.001-0.030%, Nb: 0.10-0.40%, B: 0 to 0.0030%, Al: 0 to 0.100%, the balance being Fe and unavoidable impurities; The amount of solute Nb is equal to or less than the smaller value of 0.20 mass% or (Nb content - 0.08) mass%, A ferritic stainless steel sheet characterized in that, when the sheet thickness is t, the number density of Nb-containing precipitates having a particle size of 1 μm or more is 0.005 particles / μm 2 or less in a depth range of t / 2 to t / 4 from the surface of a cross section in the sheet thickness direction.

2. Furthermore, in mass%, Ti: 0 to 0.20%, Ni: 0 to 1.00%, Mo: 0-2.0%, Cu: 0-3.0%, V: 0 to 1.00%, Mg: 0 to 0.0030%, Sn: 0 to 0.30%, Sb: 0 to 0.30%, Zr: 0 to 0.10%, Ta: 0-0.10%, Hf: 0-0.10%, W: 0-2.00%, Co: 0 to 0.20%, Ca: 0-0.0030%, REM: 0-0.050%, Ga: 0-0.10% 2. The ferritic stainless steel sheet according to claim 1, further comprising one or more of the following:

3. 3. The ferritic stainless steel sheet according to claim 1, wherein the grain size of the Nb-containing precipitates is 2 μm or less.

4. 4. A method for producing a ferritic stainless steel sheet according to claim 1, wherein a slab having the chemical composition according to claim 1 or 2 is heated to 1,230°C or higher, hot-rolled so that the ratio of the finish rolling start temperature (°C) to the finish rolling end temperature (°C) (finish rolling start temperature (°C) / finish rolling end temperature (°C)) is 1.10 or higher, and then coiled at 500°C or lower.

5. 5. The method for producing a ferritic stainless steel sheet according to claim 4, wherein the hot-rolled steel sheet is cooled at a cooling rate of 20°C / sec or more and then coiled.

6. 6. The method for producing a ferritic stainless steel sheet according to claim 4, wherein the coiled steel sheet is heated to 900 to 1050°C and then held for 60 seconds or more for annealing.

7. A part at least partially made of the ferritic stainless steel sheet according to any one of claims 1 to 3.

8. 8. The part according to claim 7, wherein the part using the ferritic stainless steel plate is joined to another part by brazing or arc welding.

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