Ferritic stainless steel sheet, method for producing the same, and component

Abstract

To provide a ferritic stainless steel sheet that prevents cracking during steel sheet production and exhibits superior toughness, and a component made using the same.SOLUTION: A ferritic stainless steel sheet comprises, in mass%, C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.010 to 0.050%, S: 0.0002 to 0.0100%, Cr: 13.0 to 20.0%, N: 0.001 to 0.030%, Nb: 0.10 to 0.40%, B: 0 to 0.0030%, and Al: 0 to 0.100%. The remainder consists of Fe and unavoidable impurities. The grain boundary segregation level of Nb is 6% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] The exhaust gas path of an automobile is composed of various components such as an exhaust manifold, a muffler, a catalyst, a flexible tube, a center pipe, and a front pipe. When connecting these components, fastening components called flanges and brackets are often used. In automotive exhaust system components, flange joining is actively adopted because the number of processing steps is small and the working space can be narrow. Also, from the viewpoints of noise due to vibration and ensuring rigidity, thick flanges with a thickness of 5 mm or more are often used. Flanges are manufactured by processing such as press forming and punching, and conventional carbon steel sheets have been used as materials. However, since carbon steel is inferior in corrosion resistance, rust called initial rusting occurs after automobile manufacturing, which may impair the appearance. For this reason, the use of stainless steel sheets instead of carbon steel sheets as flange materials is being actively promoted.

[0003] Ferritic stainless steel sheets are known to have a lower Ni content and lower cost than austenitic stainless steel sheets, but are inferior in toughness. When the toughness is low, plate breakage may occur during sheet passing and coil unwinding in the steel sheet manufacturing process. Also, cracks may occur during processing such as cutting and punching in component processing. Furthermore, the components may crack when an impact is applied in a low-temperature environment. In particular, when the plate thickness increases (for example, a thick steel sheet with a thickness of 5 mm or more), the toughness further decreases, and brittle fracture is likely to occur in the manufacturing and processing of ferritic stainless steel sheets at low temperatures, which may become a problem.

[0004] Some devices have been made to ensure the toughness of ferritic stainless steel plates. For example, Patent Documents 1 and 2 disclose the manufacturing conditions of ferritic stainless steel hot-rolled coils or hot-rolled annealed coils with a plate thickness of 5 to 12 mm.

[0005] Patent Document 1 targets Ti-containing ferritic stainless steel, and in order to adjust the hardness and Charpy impact value, a method is shown in which the coiling temperature is set to 570 °C or higher and the coil is immersed in water.

[0006] Patent Document 2 targets Nb-containing ferritic stainless steel, and in order to adjust the hardness and Charpy impact value, a method is shown in which the hot-rolled finishing temperature is set to 890 °C or higher and coiling is performed at 400 °C or lower, and the coil is immersed in water.

[0007] Patent Document 3 discloses a ferritic stainless steel excellent in cold cracking resistance in which the length of sub-grain boundaries with a small crystal orientation difference in the ferrite phase is made equal to or greater than a certain value. This is obtained by a method in which the hot-rolled finishing temperature is 800 to 1000 °C, the coiling temperature is above 650 °C to 800 °C, and the coil is immersed in a water tank after coiling.

[0008] Patent Document 4 discloses a ferritic stainless steel plate excellent in toughness in which the ratio of precipitates at grain boundaries is defined.

[0009] Patent Document 5 discloses a ferritic stainless steel plate in which the number density of solid solution Nb and Nb-containing precipitates is defined.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0011] The ferritic stainless steels described in Patent Documents 1 and 2 define hot rolling conditions from the viewpoint of improving the toughness of hot rolled sheets or hot rolled and annealed sheets, but it is substantially difficult to control the entire length of the coil under the above conditions.

[0012] Regarding the ferritic stainless steels described in Patent Documents 3 and 4, improvement in toughness by controlling grain boundary character (for example, the magnitude of the orientation difference at grain boundaries) and precipitates on grain boundaries has been proposed, but they have not necessarily reached a satisfactory toughness level for flange applications.

[0013] Also, as defined in the manufacturing methods of Patent Documents 1 to 3, the method of immersing the coil in a water tank after hot rolling has poor productivity, and the toughness may vary greatly in the longitudinal and width directions of the coil due to the length of time until immersion in the water tank and uneven cooling. Patent Document 5 discloses a technique for improving toughness by controlling the amount of solid solution Nb and the number density and particle size of Nb-based precipitates. As a means, it is shown that the cooling rate after hot rolling is increased to 20°C / s or more and coiling is performed at 500°C or lower. However, solid solution Nb may segregate, and toughness may not be improved simply by the amount of solid solution Nb, and it was insufficient to simply increase the cooling rate after hot rolling and coil at a low temperature.

[0014] Also, in conventional knowledge (for example, Patent Document 5), an impact value at room temperature of 20 J / cm 2 or more is considered qualified, but in winter in cold regions, the temperature may drop to near 0°C, and in the technologies of Patent Documents 1 to 5, the impact value at 0°C may not be ensured to be 20 J / cm 2 or more in some cases.

[0015] As described above, in the conventional ferritic stainless steel sheet, further improvement in toughness, particularly toughness at low temperature (0 °C), is required. Therefore, an object of the present invention is to improve the toughness at further low temperature (0 °C) in a ferritic stainless steel sheet, particularly a Nb-added ferritic stainless steel sheet.

Means for Solving the Problems

[0016] In order to solve the above problems, the present inventors studied the toughness of ferritic stainless steel sheets from the viewpoints of components and microstructure control in the manufacturing process. In particular, a detailed study was conducted focusing on the structure of grain boundaries, which are the starting points of brittle fracture, and the size of precipitates. As a result, it was first found that Nb is segregated at the grain boundaries in the Nb-added ferritic stainless steel sheet. It was found that reducing the grain boundary segregation concentration of Nb (Nb grain boundary segregation concentration), which is the concentration (content) of Nb at the grain boundaries, and controlling the maximum size of precipitates are extremely effective for improving the toughness of ferritic stainless steel sheets, particularly hot-rolled steel sheets or hot-rolled annealed steel sheets annealed after hot rolling.

[0017] The present invention has been made based on the above findings, and the gist thereof is as follows. (1) In mass%, C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.010 to 0.050%, S: 0.0002 to 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%, Ti: 0 to 0.20%, Ni: 0 to 1.00%, Mo: 0 to 2.00%, Cu: 0 to 3.00%, 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 to 0.10%, Hf: 0 to 0.10%, W: 0 to 2.00%, Co: 0 to 0.20%, Ca: 0 to 0.0030%, REM: 0 to 0.050%, Ga: 0 to 0.1000%, and contains the balance consists of Fe and unavoidable impurities, a ferritic stainless steel sheet characterized in that the grain boundary segregation concentration of Nb is 6% or less and the maximum particle size (equivalent diameter of the area circle) of the precipitate is 3.0 μm or less. (2) The ferritic stainless steel sheet according to (1) above, having a thickness of 5.0 mm or more. (3) A method for manufacturing the ferritic stainless steel sheet according to (1) or (2) above, wherein a slab having the components according to (1) above is heated to 1250 °C or higher, and then the starting temperature of finish rolling in hot rolling is 1000 °C or higher and the finishing temperature is 800 °C or higher, and water cooling treatment is started within 5 seconds after the end of rolling and coiled at 100 to 350 °C. A method for manufacturing a ferritic stainless steel sheet, characterized by this. (4) A component using at least a part of the ferritic stainless steel sheet according to (1) or (2) above.

Effect of the Invention

[0018] According to the present invention, a ferritic stainless steel sheet excellent in toughness can be efficiently manufactured without requiring new equipment. Furthermore, by using the ferritic stainless steel sheet according to the present invention for a component, a component excellent in toughness can be obtained by utilizing existing equipment.

Brief Description of the Drawings

[0019]

Figure 1

DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as the present invention) will be described. Unless otherwise specified, “%” regarding components indicates mass % in steel. When no lower limit is specified or the lower limit is 0%, it includes the case of not containing (0%).

[0021] It has been conventionally known that grain refinement, reduction of precipitates, and crystal orientation contribute to improving toughness. The present inventors have advanced the development aiming at further improving the toughness for hot-rolled plates or hot-rolled annealed plates of ferritic stainless steels containing many elements. In particular, thick ferritic stainless steel plates with a thickness of 5 mm or more still require improvement in toughness.

[0022] As a result of extremely detailed investigation of factors governing toughness, such as solid-solution Nb dissolved in the ferrite phase as the parent phase, Nb carbonitrides (Nb(C,N)) and Laves phases that become precipitates, etc., the present inventors found that the toughness is improved by setting the Nb concentration segregated at the grain boundaries to 6% or less and the equivalent circle diameter of the area of the largest precipitate (the diameter of the area equivalent circle. Hereinafter simply referred to as the particle size) to 3.0 μm or less. FIG. 1 shows the result of examining the distribution of the amount of Nb in the vicinity of the grain boundaries of a ferritic stainless steel (plate thickness 8.5 mm) 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. Although having the same composition, the Charpy impact test value at 0°C is 40 J / cm 2 for the steel plate and 10 J / cm 2These are the results of measurements using the steel plate. Here, the Nb content near the grain boundaries was determined by observing and identifying with a field emission transmission electron microscope (FE-TEM) capable of analyzing fine regions and performing EDS analysis with a spherical aberration corrected transmission electron microscope (Cs-TEM) capable of analyzing extremely fine regions after preparing a thin film sample for TEM observation using the electrolytic polishing method. At this time, the sample was cut to a thickness of 0.5 mm using a precision cutting machine, then cut and polished from both sides to a thickness of several tens of μm using emery paper with grit sizes of P320 to 1200, punched out into a 3 mmφ sample, and subjected to double-sided jet electrolytic polishing until a hole opened in the center to obtain a sample for TEM observation. After searching for grain boundaries with a transmission electron microscope, EDS analysis was performed at a pitch of 0.5 nm at a distance of 10 nm centered on the grain boundaries to examine the concentration distribution of Nb.

[0023] From Figure 1, it is clear that Nb segregation occurs at the grain boundaries, and it was found that the lower the Nb grain boundary segregation concentration, the better the toughness. Figure 1 shows steel plates with the same composition. When about 11% of Nb is segregated at the grain boundaries, the Charpy impact value at 0 °C is 10 J / cm 2 However, when the Nb grain boundary segregation concentration is about 5%, the Charpy impact value at 0 °C is 40 J / cm 2 It can be seen that. When Nb segregates at the grain boundaries, the grain boundaries and the vicinity of the grain boundaries are hardened, and it is presumed that brittle fracture is likely to occur starting from the vicinity of the grain boundaries. It is considered that by reducing the Nb grain boundary segregation concentration, hardening in the vicinity of the grain boundaries is suppressed, and the probability of becoming a fracture initiation point decreases. To make the toughness more stable, it is preferable to reduce the Nb grain boundary segregation concentration.

[0024] The above-mentioned grain boundary segregation of Nb occurs during the hot rolling process. In addition, precipitates such as Nb(C, N) (niobium carbonitride) and Laves phase precipitate and coarsen. Coarse precipitates tend to be the starting points of brittle fracture. Therefore, by setting the maximum particle size of the precipitates to 3.0 μm or less, a decrease in toughness can be suppressed. Here, although the types of precipitates are not limited, typical target precipitates include Nb(C, N), Laves phase, carbonitrides such as Ti and Cr, and AlN. Here, the size of the precipitates is determined by observing a cross-section at a depth from the surface in the range of t / 2 to t / 4 in the cross-section in the thickness direction of the steel sheet (a cross-section perpendicular to the steel sheet surface. Hereinafter, it may be simply referred to as a cross-section). Observe 10 fields at a magnification of 500 times, measure the area of the largest precipitate in each field, obtain the equivalent circle diameter, and take it as the size of the largest precipitate.

[0025] Toughness was evaluated by the Charpy impact test. A V-notch test piece (with a V-notch applied in the width direction) was sampled, and the impact value was measured at 0 °C in accordance with JIS Z2242. Data are shown in the examples. The steel sheet according to the present invention has predetermined components, suppresses the grain boundary segregation of Nb to 6% or less, and sets the maximum particle size of the inclusions to 3.0 μm, thereby confirming that the impact value at 0 °C is 20 J / cm2 or more. That is, the impact value at room temperature in the conventional knowledge is 20 J / cm 2 Since it has an impact value equivalent to the above at a low temperature (0 °C), it can be evaluated as having excellent low-temperature (0 °C) toughness.

[0026] As described above, when the grain boundary segregation concentration of Nb decreases, the toughness improves (the Charpy impact value increases). Therefore, the grain boundary segregation concentration of Nb is preferably 6% or less, preferably 5% or less, or 4% or less. The lower limit of the grain boundary segregation concentration of Nb is not particularly limited, and it is desirable to be 0% (no segregation). However, eliminating the grain boundary segregation of Nb in Nb-added steel significantly deteriorates the manufacturability. Therefore, the lower limit of the grain boundary segregation concentration of Nb may be set to 1%. Also, since toughness improves (the Charpy impact value increases) when the maximum particle size of the inclusions is smaller, the maximum particle size of the inclusions is preferably 3.0 μm or less, more preferably 2.8 μm or less, 2.6 μm or less, 2.4 μm or less, 2.2 μm or less, or 2.0 μm or less.

[0027] Next, the component range of the steel will be described. C (carbon) deteriorates toughness due to hardening by solid-solution C and carbide precipitation, so the lower its content, the better. When contained excessively, toughness decreases due to the formation and coarsening of Cr carbides, and the intergranular corrosion resistance of the welded part deteriorates. Therefore, the upper limit is preferably 0.030%, more preferably 0.020% or 0.010%. On the other hand, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.001%, more preferably 0.002%.

[0028] Si may be contained as a deoxidizing element and also improves oxidation resistance. Since it is a solid-solution strengthening element, from the perspective of toughness, the lower its content, the better. The upper limit is 1.00%, preferably 0.50%. On the other hand, to ensure oxidation resistance, the lower limit is 0.01%, preferably 0.05%.

[0029] Mn, like Si, is a solid-solution strengthening element, so the lower its content, the better. When contained excessively, coarse precipitates such as MnS are generated, which become the starting points of brittle fracture. Therefore, the upper limit is 1.00%, preferably 0.70% or 0.50%. On the other hand, excessive reduction leads to an increase in refining costs, and a small amount of Mn content improves scale peelability. Therefore, the lower limit is 0.01%, preferably 0.05%.

[0030] Like Mn and Si, P is a solid-solution strengthening element that hardens the material. From the perspective of toughness, it is better to have a lower content. However, adding a small amount of P can cause grain boundary segregation and has the effect of suppressing the grain boundary segregation of Nb. Since this effect appears from 0.010%, the lower limit is set at 0.010%, preferably 0.015%. Also, when the content is excessive, the formation of phosphides that are the starting points of brittle fracture occurs, so the upper limit is set at 0.050%, preferably 0.030%.

[0031] Since S is an element that deteriorates corrosion resistance, the lower its content, the better. On the other hand, adding a small amount of S can cause grain boundary segregation and has the effect of suppressing the grain boundary segregation of Nb. Since this effect appears from 0.0002%, the lower limit is set at 0.0002%, preferably 0.0004%. Also, when the content is excessive, the formation of precipitates such as MnS and Ti4C2S2 that are the starting points of brittle fracture occurs, so the upper limit is set at 0.0100%, preferably 0.0060%.

[0032] The grain boundary segregation of Cr has the effect of suppressing the grain boundary segregation of Nb. Furthermore, since Cr is an element that improves corrosion resistance and oxidation resistance, the lower limit is preferably set at 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 15.0%, or 16.0%. On the other hand, excessive content causes a decrease in toughness due to the formation of precipitates such as coarse Cr carbides and nitrides, so the upper limit is preferably set at 20.0%, preferably 18.0%.

[0033] Like C, N deteriorates toughness and corrosion resistance, so the lower its content, the better. Also, when the content is excessive, the formation of nitrides that are the starting points of brittle fracture occurs, so the upper limit is set at 0.030%, preferably 0.020%. On the other hand, excessive decrease leads to an increase in refining costs, so the lower limit is set at 0.001%, preferably 0.005%.

[0034] Nb is contained not only to improve high-temperature strength but also to combine with C and N to improve corrosion resistance and intergranular corrosion resistance. To ensure the manifestation of this effect, the lower limit may be set at 0.10%. On the other hand, excessive content not only causes hardening and deteriorates formability but also, depending on the precipitation of coarse Nb(C, N) and thermal history, precipitates (Fe, Nb)6C and Fe2Nb, significantly deteriorating toughness. Also, since the Nb grain boundary segregation concentration increases with the increase in the addition amount, the upper limit may be set at 0.40%, preferably 0.30%.

[0035] B may not be particularly contained, but it is an element that improves the secondary workability of products by segregating at grain boundaries. In addition to improving the punching property in the processing of fastening parts, it is also an element that suppresses the segregation of Nb, so it may be contained as necessary. Therefore, the lower limit of the content does not particularly need to be limited and may be 0%, but to ensure the manifestation of this effect, it may be contained at 0.0002% or more, preferably 0.0003% or more. On the other hand, excessive content causes the precipitation of borides and deteriorates toughness, so the upper limit may be set at 0.0030%, preferably 0.0020%, or 0.0010%.

[0036] Al may not be particularly contained, but it may be contained as a deoxidizing element and may be contained as necessary. Therefore, the lower limit of the content does not particularly need to be limited and may be 0%, but to ensure the manifestation of its effect, it may be contained at 0.005% or more, preferably 0.010% or more. On the other hand, excessive content generates inclusions such as Al2O3, which not only causes hardening but also serves as a fracture initiation point, resulting in a decrease in toughness, deterioration of weldability (especially brazing property), and surface quality, so the upper limit may be set at 0.100%, preferably 0.080%.

[0037] Furthermore, it may contain one or more of Ti, Ni, Mo, Cu, V, Mg, Sn, Sb, Zr, Ta, Hf, W, Co, Ca, REM, Ga. These elements may not be contained, but further effects can be obtained by containing them. Hereinafter, these elements will be described.

[0038] Ti may be contained as necessary to combine with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and toughness. In particular, the lower limit may not be set, but in order to surely exhibit the C and N fixing effect, it may be contained at 0.01% or more, preferably 0.02% or more. On the other hand, excessive content not only causes precipitation of coarse Ti(C, N) and significantly deteriorates toughness, but also deteriorates weldability (especially brazing property), so the upper limit should be 0.20%, preferably 0.15%, 0.10%, 0.07%, or 0.05%.

[0039] Ni may be contained as necessary to improve the initial rust resistance by suppressing crevice corrosion and promoting re-passivation. In particular, the lower limit may not be set, but in order to surely exhibit this effect, it may be contained at 0.10% or more, preferably 0.20% or more. On the other hand, excessive content not only hardens and deteriorates toughness, but also makes stress corrosion cracking more likely to occur, so the upper limit should be 1.00%, 0.50%, or 0.30%.

[0040] Mo may be contained as necessary to improve corrosion resistance and high-temperature strength, and in particular, to suppress crevice corrosion when having a crevice structure. In particular, the lower limit may not be set, but in order to surely exhibit this effect, it may be contained at 0.10% or more, preferably 0.50% or more. Also, Mo is an element that segregates at grain boundaries like Nb, and the segregation of Mo suppresses the segregation of Nb, resulting in an effect of improving toughness. On the other hand, excessive content not only significantly deteriorates formability, but also causes toughness deterioration due to hardening, so the upper limit should be 2.00%, preferably 1.50%, or 1.20%. Furthermore, considering the manufacturing cost and the punching property during component processing, the upper limit is preferably 0.90%.

[0041] Cu may be contained as needed to improve high-temperature strength, suppress crevice corrosion, and promote re-passivation. In particular, the lower limit may not be set, but to reliably exhibit this effect, it may be contained at 0.10% or more, preferably 0.20% or more. On the other hand, excessive containment may cause hardening due to ε-Cu precipitation, deteriorating formability and toughness, so the upper limit may be set to 3.00%, 2.50%, 2.00%, 1.50%, or 1.20%.

[0042] V may be contained as needed to suppress crevice corrosion and contribute to toughness improvement by containing a small amount. In particular, the lower limit may not be set, but to reliably exhibit this effect, it may be contained at 0.05% or more. On the other hand, excessive containment may cause hardening and deteriorate formability, and coarse vanadium carbonitrides (V(C, N)) may precipitate, leading to toughness deterioration, so the upper limit may be set to 1.00%, preferably 0.50%, or 0.20%.

[0043] Mg may be contained as a deoxidizing element, and it is also an element that refines the structure of the slab and contributes to formability improvement. In addition, Mg oxide serves as a precipitation site for carbides such as titanium carbonitride (Ti(C, N)) and Nb(C, N), and has the effect of finely dispersing and precipitating these, so it may be contained as needed. In particular, the lower limit may not be set, but to reliably exhibit this effect and contribute to toughness improvement, it may be contained at 0.0002% or more, preferably 0.0003% or more. On the other hand, excessive containment may lead to deterioration of weldability and corrosion resistance, so the upper limit may be set to 0.0030%, preferably 0.0010%.

[0044] Sn and Sb contribute to improving corrosion resistance and high-temperature strength. They also have the effect of suppressing Nb segregation due to grain boundary segregation, so they may be contained as needed. In particular, the lower limit may not be set, but as needed, they may be contained at 0.01% or more, preferably 0.02% or more. On the other hand, excessive containment may cause slab cracking during steel sheet production, so the upper limit may be set to 0.30%, preferably 0.20%, or 0.10%.

[0045] Zr, Ta, and Hf may be contained as needed because they combine with C and N to contribute to the improvement of toughness. In particular, the lower limit may not be set, but it may be contained at 0.01% or more, preferably 0.02% or more as needed. On the other hand, excessive containment will increase costs and significantly deteriorate manufacturability, so the upper limit should be 0.10%, preferably 0.07%.

[0046] W may be contained as needed because it contributes to the improvement of corrosion resistance and high-temperature strength. In particular, the lower limit may not be set, but it may be contained at 0.01% or more, preferably 0.05% or more as needed. On the other hand, excessive containment will lead to deterioration of toughness and cost increase during steel plate manufacturing, so the upper limit should be 2.00%, preferably 1.80%.

[0047] Co may be contained as needed because it contributes to the improvement of high-temperature strength. In particular, the lower limit may not be set, but it may be contained at 0.01% or more, preferably 0.02% or more as needed. On the other hand, excessive containment will lead to deterioration of toughness and cost increase during steel plate manufacturing, so the upper limit should be 0.20%, preferably 0.14% or 0.09%.

[0048] Ca may be contained as needed because it may be contained for desulfurization. In particular, the lower limit may not be set, but from the perspective of reliably expressing this effect, it may be contained at 0.0001% or more, preferably 0.0002% or more. On the other hand, excessive containment will generate coarse CaS and deteriorate toughness and corrosion resistance, so the upper limit should be 0.0030%, preferably 0.0020%.

[0049] REM may be contained as necessary from the viewpoints of improving toughness and oxidation resistance by refining various precipitates. In particular, the lower limit may not be set, but it may be contained in some cases. From the viewpoint of surely exhibiting this effect, it may be contained in an amount of 0.001% or more, preferably 0.008% or more. On the other hand, since excessive content significantly deteriorates castability, the upper limit is set to 0.050%, preferably 0.040%. REM (rare earth element) refers to the general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu) according to the general definition. These REM elements may be contained alone or a plurality of REM elements may be contained. When a plurality of REM elements are contained, the total amount thereof is taken as the content of REM.

[0050] Ga may be contained as necessary to improve corrosion resistance and suppress hydrogen embrittlement. In particular, the lower limit may not be set, but it may be contained in an amount of 0.0002% or more, preferably 0.0010% or more from the viewpoints of sulfide and hydride formation. On the other hand, from the viewpoints of manufacturability and cost, the upper limit is set to 0.1000%, preferably 0.0070%, or 0.0040%.

[0051] In the present invention, other components are not particularly specified and may not be contained. However, for example, Bi or the like may be contained in an amount of 0.100% or less as necessary.

[0052] The balance of the above steel components is Fe and impurities. Here, the impurities mean components that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention.

[0053] Next, the manufacturing method will be described. The steel sheet according to an embodiment of the present invention is manufactured through the processes of steelmaking - hot rolling, steelmaking - hot rolling - pickling, or steelmaking - hot rolling - annealing - pickling. In steelmaking, a method of melting steel containing the above components in a converter or an electric furnace and then performing secondary refining is suitable. The molten steel with adjusted components is made into a slab according to a known casting method (for example, the continuous casting method). The slab is heated to a predetermined temperature and hot-rolled to be processed into a hot-rolled steel sheet with a predetermined thickness.

[0054] In this embodiment, in order to particularly suppress the grain boundary segregation of Nb, in addition to the slab heating temperature before hot rolling, the finishing rolling start temperature, and the finishing temperature, the water cooling treatment start time and the coiling temperature in the water cooling zone from the end of finishing rolling to coiling are defined. Regarding the heating temperature of hot rolling, in order to diffuse Nb segregated at the grain boundaries at the slab stage and from the viewpoint of solid solution of Nb(C, N), it is set to 1250°C or higher. On the other hand, when the slab temperature is heated above 1300°C, the deformation of the slab during heating is intense, and the sheet thickness accuracy and surface quality deteriorate. Therefore, the slab heating temperature is desirably 1300°C or lower.

[0055] After the slab is heated, a hot-rolled steel sheet is produced by hot rolling (rough rolling and finish rolling). In this embodiment, by setting the finish rolling start temperature to 1000°C or higher and the finish rolling end temperature to 800°C or higher, Nb segregation is suppressed. Also, until water cooling is started on the run-out table until it is coiled after the finish rolling, usually it takes more than 5 seconds after the rolling is finished. By adjusting the sheet passing speed, the arrangement of the water cooling device, etc., water cooling treatment (the cooling rate is preferably 50°C / sec or higher, and if the cooling rate can be ensured, the cooling method is not limited to water cooling treatment.) is started within 5 seconds after the rolling is finished. Preferably, it is within 4 seconds. This prevents Nb from diffusing and segregating onto the dislocations between the end of rolling and the start of water cooling. This is one of the new findings in the present invention. If the time from the end of rolling to the start of water cooling treatment is made too short, the sheet shape deteriorates, so it is preferably started after 1 second has passed. The hot-rolled steel sheet is coiled after being water-cooled, and the coiling temperature at that time is preferably 100 to 350°C. This is because if coiling is performed at an excessively high temperature, Nb segregation occurs after coiling, so it is 350°C or lower, preferably 300°C or lower. On the other hand, if it is made too low, the sheet shape becomes defective, so it is 100°C or higher, preferably 150°C or higher.

[0056] The hot-rolled steel sheet (hot-rolled plate) thus obtained may be annealed (hot-rolled plate annealing) as required. When annealing is performed, it is preferably heated in the temperature range of 900 to 1050°C to obtain a recrystallized structure. When the heating temperature during annealing exceeds 1050°C, not only do Nb-based precipitates start to dissolve and the amount of dissolved Nb increases rapidly, but also the crystal grain size increases, resulting in deterioration of toughness. Therefore, the heating temperature during annealing is 1050°C or lower, preferably 1100°C or lower. In order to reduce the processing strain of hot rolling as much as possible and obtain a grain-refined recrystallized structure, the holding time in this temperature range is preferably 60 seconds or more. If the holding time is less than 60 seconds, the processing structure of hot rolling remains, resulting in a decrease in toughness due to coarse grains, so the holding time is 60 seconds or more. On the other hand, if the holding time is made too long, not only does the productivity decrease significantly, but also the pickling property deteriorates, so the holding time is desirably 90 seconds or less.

[0057] In addition, other conditions in the manufacturing process may be appropriately selected. For example, the slab thickness, hot-rolled plate thickness, etc. may be set as appropriate. Also, it may be immersed in a water-cooling pool after coiling the hot-rolled plate. The pickling process after hot rolling or hot-rolled annealing is not particularly limited, and mechanical descaling methods such as shot blasting, bending, and brushing may also be appropriately selected. Since the pickling solution is not particularly limited, existing conditions such as sulfuric acid and nitric hydrofluoric acid may be used. Furthermore, coil grinding may be applied to the surface thereafter.

[0058] The plate thickness of the steel plate according to the present invention is not particularly limited. Since the product of the present invention is advantageous against cracks caused by processing due to high toughness, it is possible to suppress cracks caused by processing that are likely to occur as the plate thickness increases. For example, if the plate thickness is 5.0 mm or more, the effect of suppressing cracks caused by processing becomes remarkable. Therefore, it is advisable to apply it to a hot-rolled steel plate with a plate thickness of 5.0 mm or more, preferably 8.0 mm or more, which is used for fastening members such as automobile exhaust pipe flanges that require high toughness.

[0059] The ferritic stainless steel plate manufactured in this way is processed into parts by existing processing methods such as punching and grinding. In particular, since the ferritic stainless steel according to the present invention has excellent toughness and corrosion resistance, it can be applied to parts used in a corrosive environment. In particular, it can be used for at least a part of exhaust system parts of automobiles and motorcycles. For example, it can be applied to a part (such as an exhaust manifold and a muffler) of the exhaust system parts. Also, although the toughness decreases as the plate thickness increases and cracks caused by processing are likely to occur, the steel plate according to the present invention is advantageous against cracks caused by processing due to high toughness. Therefore, it is preferably applied to fastening parts such as flanges and brackets for fastening parts to each other and mechanical structure parts.

Examples

[0060] Hereinafter, the present invention will be described more specifically by way of examples. The present invention is not limited to these examples.

[0061] Steel with the component composition shown in Table 1 was melted and cast into slabs, and the slabs were hot-rolled to a thickness of 5 mm or more to obtain hot-rolled coils (hot-rolled sheets). At this time, the slab heating temperature was controlled to 1250 - 1300 °C, the finishing rolling start temperature was 1000 - 1100 °C, the finishing rolling end temperature was 800 - 900 °C, the time from the end of rolling to the start of cooling was 4 seconds, and the coiling temperature was controlled to 100 - 350 °C. Then, when annealing (hot-rolled annealing) was performed, the annealing temperature was maintained at 900 - 1050 °C for 30 seconds or more.

[0062] Table 2 shows the results of the Charpy impact test of the steel sheet when the hot-rolling conditions and annealing conditions were changed.

[0063] For the Nb concentration distribution near the grain boundary, after preparing a TEM observation thin film sample by the electrolytic polishing method, observation and identification analysis were carried out by a field emission type transmission electron microscope (FE-TEM) capable of analyzing fine regions, and EDS analysis was carried out by a spherical aberration corrected transmission electron microscope (Cs-TEM) capable of analyzing extremely fine regions. At this time, the sample was cut to a thickness of 0.5 mm with a precision cutting machine, then cut and polished from both sides to a thickness of 50 μm with emery paper of P320 - 1200, a 3 mmφ sample was punched out, double-sided jet electrolytic polishing was performed, and electrolytic polishing was continued until a hole opened in the center to obtain a TEM observation sample. After searching for grain boundaries with a transmission electron microscope, EDS analysis was performed at a pitch of 0.5 nm at a distance of 10 nm centered on the grain boundary to examine the Nb concentration.

[0064] For the maximum particle size of the precipitates, in the cross-section in the plate thickness direction, a cross-section in the range where the depth from the surface is 1 / 2 - 1 / 4 of the plate thickness was observed at a magnification of 500 times for 10 fields of view. The area of the largest precipitate in each field of view was measured to obtain the equivalent circle diameter, and the particle size of the largest precipitate among them was taken as the particle size.

[0065] Toughness was evaluated by the Charpy impact test. A V-notch test piece (with a V-notch applied in the width direction) was taken, and the impact value was measured at 0 °C in accordance with JIS Z2242, and 20 J / cm 2 or more was considered qualified (○), and less than 20 J / cm 2 (hereinafter, may be referred to as low toughness in this specification) was considered unqualified (×).

[0066] The evaluation results are shown in Table 1 and Table 2. As can be seen from Table 1 and Table 2, the steel sheet according to the present invention has good toughness. On the other hand, in the comparative example where the Nb grain boundary segregation concentration at the grain boundary is not within the predetermined range, there are cases where the toughness becomes so low that coiling through the sheet becomes impossible.

[0067]

Table 1

[0068]

Table 2

Industrial Applicability

[0069] The ferritic stainless steel sheet according to the present invention, its manufacturing method, and the fastening parts including the ferritic stainless steel sheet can be used in all industries. Particularly, since it has excellent toughness even in a high-temperature corrosion environment, for example, by using it as parts for automobiles and motorcycles, reliability can be ensured, the social contribution can be increased, and it is extremely beneficial industrially.

Claims

1. By mass percentage, C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.010 to 0.050%, S: 0.0002 to 0.0100%, Cr: 10.0 to 20.0%, N: 0.001 to 0.030%, Nb: 0.10 to 0.50%, B: 0 to 0.0030%, Al: 0 to 0.100%, Ti: 0 to 0.20%, Ni: 0 to 1.00%, Mo: 0 to 2.00%, Cu: 0 to 3.00%, 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 to 0.10%, Hf: 0 to 0.10%, W: 0 to 2.00%, Co: 0 to 0.20%, Ca: 0 to 0.0030%, REM: 0 to 0.050%, Ga: 0 to 0.1000% contain, the balance consisting of Fe and inevitable impurities, a ferritic stainless steel sheet characterized in that the grain boundary segregation concentration of Nb is 6% or less and the maximum particle size (equivalent diameter of area circle) of the precipitate is 3.0 μm or less.

2. The ferritic stainless steel sheet according to Claim 1, having a thickness of 5.0 mm or more.

3. A method for manufacturing a ferritic stainless steel sheet according to Claim 1 or 2, comprising heating a slab having the components according to Claim 1 to 1250°C or higher, then setting the finishing rolling start temperature in hot rolling to 1000°C or higher and the finishing temperature to 800°C or higher, and starting water cooling treatment within 5 sec after the completion of rolling and winding it in a coil shape at 100 to 350°C. A method for manufacturing a ferritic stainless steel sheet, characterized by the above.

4. A component using the ferritic stainless steel sheet according to Claim 1 or 2 at least in part.

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