Ferritic stainless steel sheet

Abstract

To provide a ferritic stainless steel sheet having superior high-temperature strength and excellent weld solidification crack resistance.SOLUTION: A ferritic stainless steel sheet comprises, in mass%, C: 0.001-0.030%, Si: 0.010-2.000%, Mn: 0.010-2.000%, P: 0.100% or less, S: 0.0100% or less, Cr: 16.0-20.0%, Cu: 1.00-2.00%, Mo: 0.01-1.00%, Ti: 0.01-0.30%, Nb: 0.050-0.300%, Al: 0.003-0.500%, N: 0.001-0.020%, B: 0.0001-0.0050%, optional elements, and the balance: Fe and impurities, wherein the following condition is satisfied: [X≤300+(-34P-38S-4.9Mn-19Ti-9.5Nb-273)].SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Among automotive parts, exhaust parts are the parts that serve as the pathway for exhaust gases, and refer to exhaust manifolds, front pipes, center pipes, etc. Exhaust parts, which stably ventilate high-temperature exhaust gases, use materials with excellent heat resistance and corrosion resistance, including high-temperature strength, oxidation resistance, high-temperature fatigue properties, and thermal fatigue properties.

[0003] Traditionally, cast iron has been the norm for automotive exhaust parts, but stainless steel exhaust manifolds have begun to be used in light of stricter exhaust gas regulations, improved engine performance, and lighter vehicle weight.

[0004] Among stainless steels, ferritic stainless steels have excellent thermal fatigue properties and scale spalling resistance due to their small thermal expansion coefficient. Furthermore, because they do not contain Ni, their material costs are low and they are widely used. However, because ferritic stainless steels generally have low high-temperature strength and high-temperature fatigue strength, alloying elements such as Nb and Cu have been used to improve their strength.

[0005] For example, Patent Documents 1 to 3 disclose the development of ferritic stainless steels containing Cu to improve high-temperature strength. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2015 / 174079 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-240143 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-248620 Summary of the Invention [Problem to be solved by the invention]

[0007] Most exhaust parts are cylindrical or elliptical cylindrical in shape and have a welded structure, so the ferritic stainless steel used for exhaust parts is required to have not only high-temperature strength but also weldability, specifically resistance to weld solidification cracking, because weld solidification cracking can easily cause defects in the exhaust part, which can easily cause exhaust gas to leak outside the passage, resulting in a decrease in part performance.

[0008] However, the inclusion of elements such as Nb and Cu that improve high-temperature strength makes solidification cracking more likely to occur during welding, and so it is difficult to achieve both high-temperature strength and resistance to weld solidification cracking in ferritic stainless steel. However, the ferritic stainless steels disclosed in Patent Documents 1 to 3 do not consider weld solidification cracking, and there is room for further improvement in weld solidification cracking resistance.

[0009] An object of the present invention is to provide a ferritic stainless steel sheet that is excellent in high-temperature strength and resistance to weld solidification cracking. [Means for solving the problem]

[0010] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following ferritic stainless steel sheet.

[0011] (1) Chemical composition, in mass%, C: 0.001 to 0.030%, Si: 0.010 to 2.000%, Mn: 0.010 to 2.000%, P: 0.100% or less, S: 0.0100% or less, Cr: 16.0~20.0%, Cu: 1.00-2.00%, Mo: 0.01 to 1.00%, Ti: 0.01 to 0.30%, Nb: 0.050~0.300%, Al: 0.003 to 0.500%, N: 0.001 to 0.020%, B: 0.0001~0.0050%, Mg: 0 to 0.0010%, Ca: 0 to 0.0050%, Ni: 0-2.00% V: 0~0.50%, W: 0~3.00%, Zr: 0 to 0.10% REM: 0~0.100%, Sn: 0 to 0.500% Co: 0 to 0.30% Sb: 0 to 0.500% Ga: 0 to 0.3000%, Ta: 0 to 1.000%, Hf: 0 to 1.000%, Bi: 0 to 0.020% The balance is Fe and impurities. A ferritic stainless steel sheet that satisfies the following formula (i): X≦300+(-34P-38S-4.9Mn-19Ti-9.5Nb-273) (i) In the above formula, each element symbol represents the content (mass%) of each element contained in the ferritic stainless steel, and if the element is not contained, it is set to zero. X in the above formula represents the solid-liquid coexistence temperature range (°C).

[0012] (2) The chemical composition is in mass%: Mg: 0.0001 to 0.0010%, Ca: 0.0001 to 0.0050%, Ni: 0.01 to 2.00% V: 0.01 to 0.50%, W: 0.10~3.00%, Zr: 0.01 to 0.10%, REM: 0.005~0.100%, Sn: 0.005 to 0.500%, Co: 0.01 to 0.30%, Sb: 0.005 to 0.500%, Ga: 0.0002 to 0.3000%, Ta: 0.001 to 1.000% Hf: 0.001 to 1.000%, and Bi: 0.001 to 0.020%, Contains one or more selected from The ferritic stainless steel sheet according to (1) above.

[0013] (3) The ferritic stainless steel plate is containing Ti-containing precipitates, The ferritic stainless steel sheet according to (1) above, wherein the average value of the ratio of the Nb content to the Ti content, [Nb] / [Ti], of the precipitates is 0.05 or less. However, the above [Nb] indicates the Nb content (at %) of the precipitate, and if no Nb is contained, it is set to zero, and [Ti] indicates the Ti content (at %) of the precipitate.

[0014] (4) The ferritic stainless steel plate is containing Ti-containing precipitates, The ferritic stainless steel sheet according to (2) above, wherein the average value of the ratio of the Nb content to the Ti content, [Nb] / [Ti], of the precipitates is 0.05 or less. However, the above [Nb] indicates the Nb content (at %) of the precipitate, and if no Nb is contained, it is set to zero, and [Ti] indicates the Ti content (at %) of the precipitate.

[0015] (5) An exhaust part comprising the ferritic stainless steel sheet according to any one of (1) to (4) above. [Effects of the Invention]

[0016] According to the present invention, a ferritic stainless steel sheet having excellent high-temperature strength and resistance to weld solidification cracking can be obtained. [Brief explanation of the drawings]

[0017] [Figure 1]FIG. 1 is a phase diagram showing the solid-liquid coexistence temperature range. [Figure 2] FIG. 2 is a diagram showing the relationship between formula (i) and weld solidification cracking. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present inventors have conducted research into achieving both high-temperature strength and weld solidification cracking resistance, and have obtained the following findings.

[0019] (a) Weld solidification cracking is also affected by the temperature range in which the liquid and solid phases coexist, i.e., the solid-liquid coexistence temperature range. In other words, the larger the solid-liquid coexistence temperature range, the wider the temperature range in which the solid and liquid phases coexist. As a result, during welding, as the molten metal solidifies and becomes solid, the remaining liquid phase cannot withstand the stress changes that accompany the solidification shrinkage of the solid phase, and cracks occur.

[0020] Although this solid-liquid coexistence temperature range is a value calculated thermodynamically, it is also affected by the content of elements such as P, S, Mn, Ti, and Nb. Therefore, it is effective to set the relationship between the content of these elements and the solid-liquid coexistence temperature range within an appropriate range.

[0021] (b) To improve high-temperature strength, it is effective to add a certain amount of Nb and form precipitates in a high-temperature environment. That is, it is effective to dissolve Nb in the matrix before use and form Nb precipitates during use. It is also effective to add a certain amount of Ti and cause Ti precipitates to precipitate during manufacturing, thereby strengthening the precipitation. Generally, Ti carbonitrides are precipitated as precipitates. These Ti carbonitrides may contain not only Ti but also Nb.

[0022] On the other hand, if Ti carbonitride contains Nb, the amount of Nb dissolved in the matrix is ​​reduced, and the strength-enhancing effect of Nb is not fully achieved. For this reason, it is desirable to reduce the Nb content relative to the Ti content, i.e., [Nb] / [Ti], in Ti-containing precipitates. This ensures the amount of dissolved Nb. This is because dissolved Nb precipitates finely as a Laves phase (mainly Fe2Nb) in the operating environment, improving high-temperature strength through precipitation strengthening.

[0023] An embodiment of the present invention has been made based on the above findings. Each requirement of the ferritic stainless steel sheet of this embodiment will be described in detail below.

[0024] 1.Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0025] C: 0.001 to 0.030% Carbon (C) forms carbides, which reduces corrosion resistance and high-temperature strength. Therefore, the lower the content, the better, and the C content is 0.030% or less. The C content is preferably 0.005% or less. However, excessive reduction of the C content leads to increased refining costs. For this reason, the C content is 0.001% or more.

[0026] Si: 0.010 to 2.000% In addition to acting as a deoxidizing element, Si (silicon) is an important element for improving high-temperature properties and oxidation resistance. Therefore, the Si content is 0.010% or more. The Si content is preferably 0.020% or more, more preferably 0.030% or more, and even more preferably 0.040% or more. However, if the Si content exceeds 2.000%, the steel sheet becomes excessively hard, resulting in reduced part workability and manufacturability. Therefore, the Si content is 2.000% or less. The Si content is preferably 1.500% or less, and more preferably 0.500% or less. From the viewpoints of room-temperature ductility and part workability, it is even more preferable that the Si content be 0.3000% or less.

[0027] Mn: 0.010 to 2.000% Manganese (Mn) is used as a deoxidizing element and also improves strength at medium temperatures of around 750°C. It also forms Mn-based oxides on the surface during long-term use, improving scale adhesion. Therefore, the Mn content is 0.010% or more. The Mn content is preferably 0.020% or more, more preferably 0.030% or more, and even more preferably 0.040% or more.

[0028] However, if the Mn content exceeds 2.000%, it not only reduces the uniform elongation at room temperature but also forms MnS, reducing corrosion resistance and oxidation resistance. It also raises the solid-liquid coexistence temperature range, promoting weld solidification cracking. For this reason, the Mn content is set to 2.000% or less. Considering high-temperature ductility and suppression of abnormal oxidation, the Mn content is preferably 1.500% or less, more preferably 1.000% or less, and even more preferably 0.300% or less.

[0029] P:0.100% or less P (phosphorus) is an element that deteriorates hot workability and promotes solidification cracking during manufacturing. Therefore, the P content is 0.100% or less. The P content is preferably 0.060% or less, and more preferably 0.050% or less. It is preferable to reduce the P content as much as possible, but excessive reduction of P increases refining costs. Therefore, the P content is preferably 0.010% or more. Furthermore, from the viewpoint of reducing manufacturing costs, the P content is preferably 0.020% or more, and more preferably 0.030% or more.

[0030] S: 0.0100% or less S (sulfur) is an element that reduces hot workability and corrosion resistance. Furthermore, when coarse sulfides (MnS) are formed, the inclusion cleanliness of the steel sheet is significantly reduced. In addition, it widens the solid-liquid coexistence temperature range, promoting weld solidification cracking. Therefore, the S content is 0.0100% or less. From the viewpoint of oxidation resistance, the S content is preferably 0.0050% or less. However, excessive reduction of S leads to increased refining costs. Therefore, the S content is preferably 0.0001% or more, and more preferably 0.0005% or more.

[0031] Cr: 16.0~20.0% Cr (chromium) is an element that improves corrosion resistance and oxidation resistance. From the viewpoint of suppressing abnormal oxidation in an exhaust part environment, the Cr content is 16.0% or more. The Cr content is preferably 16.5% or more, and more preferably 17.0% or more. However, if Cr is contained in excess, the material becomes hard and the manufacturing cost increases. For this reason, the Cr content is 20.0% or less. Furthermore, from the viewpoints of workability, manufacturability, and manufacturing cost, the Cr content is preferably 19.0% or less, and more preferably 18.5% or less.

[0032] Cu: 1.00-2.00% Copper (Cu) is an element effective in improving high-temperature strength in the mid-temperature range around 750°C. This is a precipitation hardening effect caused by the precipitation of ε-Cu, and is exhibited when the Cu content is 1.00% or more. Therefore, the Cu content is 1.00% or more. The Cu content is preferably 1.10% or more, and more preferably 1.15% or more.

[0033] However, excessive Cu content hardens the material, reducing uniform elongation and elongation at break and increasing room-temperature yield strength, resulting in reduced press formability. Furthermore, an austenite phase is formed in the high-temperature range, causing abnormal oxidation on the surface. Therefore, the Cu content is set to 2.00% or less. Considering manufacturability and scale adhesion, the Cu content is preferably 1.80% or less, and more preferably 1.50% or less.

[0034] Mo: 0.01 to 1.00% Mo (molybdenum) is an element that improves corrosion resistance and also improves high-temperature strength by inhibiting high-temperature oxidation and solid-solution strengthening. Therefore, the Mo content is 0.01% or more. The Mo content is preferably 0.11% or more, and more preferably 0.13% or more. However, Mo is expensive and reduces uniform elongation at room temperature. Therefore, the Mo content is set to 1.00% or less. From the viewpoints of inclusion cleanliness, manufacturability, and cost, the Mo content is preferably 0.90% or less, and more preferably 0.80% or less.

[0035] Ti: 0.01 to 0.30% Titanium (Ti) is an element that combines with carbon, nitrogen, and sulfur to improve corrosion resistance and intergranular corrosion resistance. When added in combination with niobium, it improves high-temperature strength and ductility, and improves high-temperature fatigue and thermal fatigue properties. Therefore, the Ti content is 0.01% or more. The Ti content is preferably 0.05% or more, and more preferably 0.10% or more. However, excessive Ti content increases the likelihood of nozzle clogging during the casting stage, significantly reducing manufacturability. It also widens the solid-liquid coexistence temperature range, promoting weld solidification cracking. Therefore, the Ti content is 0.30% or less. From the perspective of manufacturing costs, the Ti content is preferably 0.25% or less, and more preferably 0.19% or less.

[0036] Nb: 0.050 to 0.300% Nb (niobium) is an element that improves high-temperature strength and high-temperature fatigue strength through solid solution strengthening and precipitation strengthening by precipitating as FeNb (Laves phase). In addition, like Ti, it combines with C and N to improve corrosion resistance and intergranular corrosion resistance. Therefore, the Nb content is 0.050% or more. The Nb content is preferably 0.060% or more, more preferably 0.100% or more. However, if the Nb content exceeds 0.300%, hot workability is significantly reduced. Furthermore, the Nb content widens the solid-liquid coexistence temperature range, promoting weld solidification cracking. Furthermore, the manufacturing cost increases. Therefore, the Nb content is 0.300% or less. From the viewpoint of manufacturing cost, the Nb content is preferably 0.250% or less, more preferably 0.200% or less.

[0037] Al: 0.003 to 0.500% Al (aluminum) acts as a deoxidizing element and improves inclusion cleanliness. It also improves hot workability. For this reason, the Al content is 0.003% or more. The Al content is preferably 0.010% or more, and more preferably 0.030% or more. However, excessive Al content reduces pickling properties, increases surface roughness, and increases the amount of inclusions. As a result, the inclusions become the starting points for fatigue cracks, reducing fatigue strength. For this reason, the Al content is 0.500% or less. From the viewpoints of refining costs and surface quality, it is preferably 0.300% or less. It is more preferably 0.150% or less.

[0038] N: 0.001 to 0.020% Like C, N (nitrogen) reduces corrosion resistance and high-temperature strength. For this reason, the N content is 0.020% or less. The N content is preferably 0.010% or less, and more preferably 0.007% or less. It is preferable to reduce N as much as possible, but excessive reduction increases refining costs. For this reason, the N content is 0.001% or more. The N content is preferably 0.003% or more, and more preferably 0.004% or more.

[0039] B: 0.0001 to 0.0050% B (boron) is an element that not only improves hot workability but also suppresses work hardening and secondary work cracking at room temperature. Therefore, the B content is 0.0001% or more. From the viewpoint of ductility, the B content is preferably 0.0002% or more, and more preferably 0.0003% or more. However, excessive B content causes the formation of boron carbides, which reduces the inclusion cleanliness and intergranular corrosion resistance of the steel sheet. Therefore, the B content is 0.0050% or less. From the viewpoint of refining costs, the B content is preferably 0.0030% or less, and more preferably 0.0010% or less.

[0040] In addition to the above elements, one or more elements selected from Mg, Ca, Ni, V, W, Zr, REM, Sn, Co, Sb, Ga, Ta, Hf, and Bi may be contained within the ranges shown below. In other words, the lower limit of the above elements is 0%. The reasons for limiting each element will be explained below.

[0041] Mg: 0 to 0.0010% Magnesium (Mg) acts as a deoxidizing element, improving inclusion cleanliness and improving corrosion resistance and high-temperature fatigue properties. Therefore, it may be added as needed. However, excessive Mg content reduces weldability and corrosion resistance. Furthermore, the formation of coarse Mg inclusions can serve as the starting point for fatigue fracture, reducing fatigue strength. Therefore, the Mg content is 0.0010% or less. The Mg content is preferably 0.0008% or less, and more preferably 0.0005% or less. To achieve the above effects, the Mg content is preferably 0.0001% or more. From the viewpoints of weldability and corrosion resistance, the Mg content is more preferably 0.0002% or more.

[0042] Ca: 0 to 0.0050% Ca (calcium) acts on desulfurization and has the effect of improving inclusion cleanliness. Therefore, it may be added as needed. However, if the Ca content exceeds 0.0050%, CaS, a water-soluble inclusion, is formed. As a result, the inclusion cleanliness and corrosion resistance of the steel sheet are significantly reduced. For this reason, the Ca content is 0.0050% or less. The Ca content is preferably 0.0020% or less. On the other hand, to obtain the above effects, the Ca content is preferably 0.0001% or more, and more preferably 0.0003% or more. From the viewpoints of manufacturability and surface properties, the Ca content is preferably 0.0005% or more, and more preferably 0.0007% or more.

[0043] Ni: 0 to 2.00% Ni (nickel) improves the toughness of ferritic stainless steel and has the effect of suppressing brittle cracking during forming. Therefore, it may be added as needed. However, if Ni is added in excess, abnormal oxidation is induced by the precipitation of the austenite phase, and thermal fatigue properties are reduced. For this reason, the Ni content is 2.00% or less. The Ni content is preferably 0.50% or less, and more preferably less than 0.10%. On the other hand, to obtain the above effects, the Ni content is preferably 0.01% or more.

[0044] V: 0 to 0.50% V (vanadium) has the effect of improving corrosion resistance. It also forms carbides and nitrides, which has the effect of improving high-temperature strength and high-temperature fatigue properties. Therefore, it may be added as needed. However, excessive V content increases manufacturing costs. It also lowers the abnormal oxidation limit temperature. For this reason, the V content is 0.50% or less. The V content is preferably 0.20% or less, and more preferably 0.15% or less. On the other hand, to obtain the above effects, the V content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0045] W:0~3.00% W (tungsten) has the same effect as Mo, improving high-temperature strength, high-temperature fatigue properties, and corrosion resistance. Therefore, it may be added as needed. However, excessive W content leads to hardening of the material, reduced toughness during manufacturing, and increased manufacturing costs. Therefore, the W content is 3.00% or less. The W content is preferably 2.00% or less, and more preferably 1.50% or less. On the other hand, to obtain the above effects, the W content is preferably 0.10% or more. From the viewpoints of manufacturability and refining costs, it is more preferably 0.15% or more.

[0046] Zr: 0 to 0.10% Like Ti and Nb, Zr (zirconium) combines with C or N to form carbides or nitrides, which improves high-temperature strength, high-temperature fatigue properties, intergranular corrosion resistance of welds, and oxidation resistance. Therefore, it may be added as needed. However, excessive Zr content significantly reduces manufacturability and increases manufacturing costs. Therefore, the Zr content is 0.10% or less. From the viewpoints of manufacturability and refining costs, the Zr content is preferably 0.08% or less. On the other hand, to obtain the above effects, the Zr content is preferably 0.01% or more, and more preferably 0.03% or more.

[0047] REM: 0 to 0.100% REM (rare earth elements) are effective in improving oxidation resistance. Therefore, they may be added as needed. However, excessive REM content can form REM-containing oxides in the nozzle through which molten steel flows during casting, making the nozzle more susceptible to clogging. Furthermore, REM-containing sulfides can form, reducing corrosion resistance. Therefore, the REM content is 0.100% or less. The REM content is preferably 0.070% or less. To achieve the above effects, the REM content is preferably 0.005% or more, and more preferably 0.010% or more.

[0048] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0049] Sn: 0 to 0.500% Sn (tin) has the effect of improving corrosion resistance, high-temperature strength, and high-temperature fatigue properties. Therefore, it may be contained as needed. However, if Sn is contained in excess, slab cracking may occur during production. Therefore, the Sn content is 0.500% or less. Furthermore, from the viewpoint of manufacturability and refining costs, the Sn content is preferably 0.300% or less. On the other hand, in order to obtain the above effects, the Sn content is preferably 0.005% or more, and more preferably 0.030% or more.

[0050] Co: 0 to 0.30% Co (cobalt) has the effect of improving high-temperature strength and high-temperature fatigue properties. Therefore, it may be added as needed. However, if Co is added in excess, the steel sheet becomes hard and its toughness during manufacturing decreases. Furthermore, manufacturing costs increase. Therefore, the Co content is 0.30% or less. In consideration of manufacturability and refining costs, the Co content is preferably 0.10% or less. On the other hand, to obtain the above effects, the Co content is preferably 0.01% or more, and more preferably 0.03% or more.

[0051] Sb: 0 to 0.500% Sb (antimony) segregates at grain boundaries and has the effect of improving high-temperature strength and high-temperature fatigue strength. Therefore, it may be added as needed. However, excessive Sb content causes segregation and cracks during welding. Therefore, the Sb content is 0.500% or less. Considering toughness and manufacturing costs, the Sb content is preferably 0.300% or less. On the other hand, to obtain the above effect, the Sb content is preferably 0.005% or more, and more preferably 0.030% or more.

[0052] Ga: 0 to 0.3000% Ga (gallium) has the effect of improving corrosion resistance and suppressing hydrogen embrittlement. Therefore, it may be contained as needed. However, if Ga is contained in excess, coarse sulfides are formed, which reduces part workability. Therefore, the Ga content is 0.3000% or less. From the viewpoint of manufacturability and manufacturing costs, the Ga content is preferably 0.1000% or less. On the other hand, in order to obtain the above effects, the Ga content is preferably 0.0002% or more, and more preferably 0.0020% or more.

[0053] Ta: 0 to 1.000% Hf: 0 to 1.000% Bi: 0 to 0.020% Ta (tantalum) has the effect of improving high-temperature strength and high-temperature fatigue strength. Therefore, it may be contained as needed. However, if Ta is contained in excess, the manufacturing cost increases. For this reason, the Ta content is 1.000% or less. The Ta content is preferably 0.200% or less. On the other hand, in order to obtain the above effect, the Ta content is preferably 0.001% or more, and more preferably 0.010% or more.

[0054] Hf (hafnium) may also be contained as needed. For the same reason, the Hf content is also 1.000% or less. The Hf content is preferably 0.200% or less. For the same reason, the Hf content is preferably 0.001% or more, and more preferably 0.010% or more.

[0055] For the same reason, Bi (bismuth) may be added as needed. However, excessive Bi content increases production costs. Therefore, the Bi content is 0.020% or less. The Bi content is preferably 0.015% or less. On the other hand, to obtain the above effects, the Bi content is preferably 0.001% or more, and more preferably 0.005% or more.

[0056] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect this embodiment. It is desirable to reduce common harmful elements such as As and Pb, as well as impurity elements, as much as possible.

[0057] 2. Equation (i) The ferritic stainless steel sheet of this embodiment needs to satisfy the following formula (i) in order to suppress weld solidification cracking. X≦300+(-34P-38S-4.9Mn-19Ti-9.5Nb-273) (i) In the above formula, each element symbol represents the content (mass%) of each element contained in the ferritic stainless steel, and if the element is not contained, it is set to zero. X in the above formula represents the solid-liquid coexistence temperature range (°C).

[0058] Here, the solid-liquid coexistence temperature range X (°C), which is the left-hand side of equation (i), is a value calculated using the thermodynamic calculation system Thermo-Calc (manufactured by Thermo-Calc Software AB, database TCFE10), and is the temperature range in which the liquid phase and ferrite phase coexist in the calculated phase diagram.

[0059] Figure 1 shows a calculated phase diagram that indicates the solid-liquid coexistence temperature range. In Figure 1, the temperature range enclosed by the solidus and liquidus lines is the solid-liquid coexistence temperature range, which is the region where the liquid phase and the solid ferrite phase coexist. If the solid-liquid coexistence temperature range is large, when the molten metal solidifies and becomes solid during cooling after welding, the remaining liquid phase cannot withstand the stress changes that accompany the solidification and shrinkage of the solid phase, making cracks more likely to occur.

[0060] This solid-liquid coexistence temperature range is also affected by the content of elements such as P, S, Mn, Ti, and Nb. When formula (i) is satisfied, weld solidification cracking resistance is improved. A trans-Varestraint test was conducted to evaluate weld solidification cracking resistance using a ferritic stainless steel containing 14-18% Cr, 0.02-1.2% Cu, 0.1-0.4% Mn, 0.02-0.03% P, 0.0003-0.011% S, 0.05-0.6% Nb, 0.01-0.3% Ti, 0.004-0.011% C, and 0.009-0.012% N. The results, organized by the total crack length, as shown in Figure 2, indicate good weld solidification cracking resistance when the total crack length was 2.5 mm or less. However, when formula (i) was not satisfied, the total crack length exceeded 2.5 mm, resulting in poor weld solidification cracking resistance.

[0061] The above test was performed using a 90mm x 90mm x 2mm thick test piece, with welding conditions of current 90A, voltage 12.5V, welding speed 40cm / min, arc length 3mm, and a bending strain of 4% load strain. The length of the cracks that occurred in the weld metal was observed and measured using a microscope, and the sum of all the observed crack lengths was taken as the total crack length.

[0062] 3. Composition of precipitates In the ferritic stainless steel sheet of this embodiment, carbonitrides containing Ti (hereinafter also referred to as "Ti carbonitrides") and Nb dissolved in the matrix (hereinafter simply referred to as "solute Nb") mainly improve high-temperature strength. Ti carbonitrides are compounds that precipitate during manufacturing, and improve high-temperature strength through precipitation strengthening. Furthermore, solute Nb not only improves high-temperature strength through solid-solution strengthening, but also improves high-temperature strength by precipitating as a compound in a high-temperature usage environment.

[0063] Therefore, it is desirable that Nb be dissolved as much as possible in the matrix before use in a high-temperature environment. On the other hand, Nb may form Ti carbonitrides together with Ti during casting, hot rolling, and cold-rolled sheet annealing. In this case, since the amount of dissolved Nb is reduced, it is preferable to minimize the formation of Nb-containing Ti carbonitrides. Therefore, the ferritic stainless steel sheet of this embodiment contains precipitates containing Ti, and the average value of the ratio of the Nb content to the Ti content, [Nb] / [Ti] (hereinafter simply referred to as the "Nb / Ti ratio"), of the precipitates is preferably 0.05 or less. However, the above [Nb] indicates the Nb content (at %) of the precipitate, and if no Nb is contained, it is set to zero, and [Ti] indicates the Ti content (at %) of the precipitate.

[0064] If the Nb / Ti ratio is 0.05 or less, the amount of dissolved Nb can be sufficiently secured. Therefore, the Nb / Ti ratio is preferably 0.05 or less, and more preferably 0.03 or less. Note that, since it is preferable to reduce the Nb / Ti ratio as much as possible, the lower limit of the Nb / Ti ratio is preferably 0.

[0065] The Nb / Ti ratio is measured using the following procedure. A cross section parallel to the rolling direction and thickness direction is used as the observation surface, embedded in thermosetting resin, and mechanically polished to a mirror finish. The precipitates are then observed using a scanning electron microscope (hereinafter referred to as "SEM") equipped with an energy dispersive X-ray spectrometer (hereinafter referred to as "EDS"). The measurement range is the region from the center of the thickness to one-quarter of the thickness, and precipitates with an equivalent diameter of 3 μm or more are observed in 10 fields of view at 1000x magnification. All observed precipitates are subjected to EDS analysis, and precipitates containing 50.0 at% or more of Ti are extracted. For each extracted precipitate (each precipitate), the composition ratio [Nb] / [Ti] is calculated from the Ti and Nb amounts (at%), and the average value is calculated.

[0066] 4.Applications The ferritic stainless steel sheet of this embodiment is suitable for automobile exhaust parts, specifically exhaust manifolds, center pipes, front pipes, converter peripheral parts, mufflers, exhaust gas purification parts such as urea SCR, and turbocharger housings and internal parts.

[0067] 5. Target characteristics In this application, if the total crack length of the weld metal portion after the trans-Varestraint test is 2.5 mm or less and the maximum crack length of the weld metal portion is 0.45 mm or less, it is determined that the weld solidification cracking resistance is good.

[0068] Furthermore, if the 0.2% yield strength at 600°C is 120 MPa or more and the 0.2% yield strength at 850°C is 13 MPa or more, it is judged to have good high-temperature strength. This target value for high-temperature strength is equivalent to or better than that of high-Nb-containing steels that have traditionally been used for automobile exhaust parts such as exhaust manifolds that are exposed to environments of approximately 600 to 900°C.

[0069] 6. Manufacturing method The ferritic stainless steel sheet of this embodiment can be stably produced, for example, by the following production method: The production process for the ferritic stainless steel sheet of this embodiment comprises, for example, steelmaking-hot rolling-coiling-annealing and pickling-cold rolling-annealing and pickling or steelmaking-hot rolling-coiling-pickling-cold rolling-annealing and pickling.

[0070] 6-1.Steelmaking process A suitable method is to melt steel having the above chemical composition in an electric furnace or converter, followed by secondary refining. The melted steel is formed into slabs by a known casting method (such as continuous casting). To avoid excessive inclusions being mixed in during continuous casting and to reduce the number of nucleation sites for TiN crystallization, a settling time of at least 1 minute, preferably at least 5 minutes, is provided before continuous casting.

[0071] 6-2.Hot rolling process The slab is heated to 1000 to 1300°C and hot-rolled to a predetermined thickness by continuous rolling to form a hot-rolled sheet. For example, the heating temperature of the slab is preferably in the range of 1020 to 1200°C. The thickness of the slab and the reduction rate in hot rolling may be selected appropriately. In addition, the cooling rate after hot rolling is preferably 30°C / s or more, and more preferably 50°C / s or more.

[0072] After hot rolling, the hot-rolled sheet is coiled and cooled. It is preferable to suppress the precipitation of Nb-containing precipitates, such as Nb(C,N) and Laves phase (mainly FeNb), during the coiling and post-coiling cooling, and it is therefore preferable to lower the coiling temperature and increase the cooling rate. This is to keep the Nb / Ti ratio at 0.05 or less.

[0073] For this reason, the coiling temperature after hot rolling is preferably 600°C or lower. Considering manufacturability, the coiling temperature is preferably 500°C or lower. Note that annealing may be performed as needed to promote recrystallization after hot rolling. If annealing is performed, it is performed at 900 to 1200°C for 10 to 200 seconds. The cooling rate after annealing may be 30°C / s or higher. After hot rolling (or after annealing if annealing is performed), the steel sheet is usually subjected to pickling.

[0074] 6-3.Cold rolling process Next, the sheet is cold-rolled to a predetermined thickness. The reduction ratio of the cold rolling may be selected as appropriate. After cold rolling, the cold-rolled sheet is first annealed and pickled. The ferritic stainless steel sheet of this embodiment has a small amount of Nb in order to lower the recrystallization temperature. For this reason, the annealing temperature is preferably in the range of 850 to 950°C, and the annealing time is preferably in the range of 1 to 150 seconds.

[0075] Annealing of cold-rolled sheets may be performed between passes of cold rolling, and may be batch annealing or continuous annealing. During cooling in the final annealing, precipitates that affect the high-temperature strength of Cu particles and Laves phases (mainly FeNb) are precipitated. These precipitates can be effectively improved by suppressing the amount of precipitation in the steel sheet as much as possible and allowing them to precipitate in the usage environment. Therefore, with regard to cooling after the final annealing of cold-rolled sheets, it is preferable to increase the cooling rate in the temperature range where the above precipitates are likely to precipitate.

[0076] Specifically, the cooling rate from 800 to 500°C is more preferably 5°C / s or more. Also, the cooling rate from 500 to 300°C is preferably 1°C / s or more. These cooling rate conditions are preferable in order to keep the Nb / Ti ratio at 0.05 or less. If the cooling rate from 800 to 500°C is less than 5°C / s and the cooling rate from 500 to 300°C is less than 1°C / s, a large amount of precipitates will precipitate, which will harden the material and reduce workability, and there is a possibility that the desired high-temperature strength will not be obtained.

[0077] The cold-rolled sheet is annealed and then cooled to produce a ferritic stainless steel sheet. After cooling, it is preferable to perform pickling. Pickling can remove scale formed on the steel surface during annealing. The pickling method may be any chemical descaling method, such as sulfuric acid, nitric hydrofluoric acid, or nitric acid electrolysis, and immersion in a molten alkali salt may be performed as a pretreatment. The temperature and time conditions for immersion in a molten alkali salt can be appropriately changed depending on the desired properties of the steel sheet. The cold-rolled, annealed, and pickled steel sheet may be subjected to temper rolling and polishing processes.

[0078] The ferritic stainless steel sheet according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0079] Steel having the chemical composition shown in Table 1 was cast into a flat 17 kg mold, and then the following steps were carried out to obtain steel sheets as shown in Table 2. Specifically, the steel was hot-rolled at 1,050°C to obtain a 5 mm thick hot-rolled sheet. To simulate coiling temperatures of 500°C and 890°C, simulated heat treatments were performed at 500°C and 890°C for 1 hour, followed by cooling at a cooling rate of 10 to 50°C / s and pickling. Note that the temperature of the simulated heat treatment is referred to as the coiling temperature after hot rolling in the table. The steel was then cold-rolled to a thickness of 2 mm, annealed at 880°C for 120 seconds, cooled at a cooling rate of 2.3 to 8.4°C / s from 800 to 500°C and a cooling rate of 0.3 to 9.2°C / s from 500 to 300°C, and then pickled to obtain a steel sheet. (Hereinafter, this will also be referred to as the "product sheet.") The solid-liquid coexistence temperature range X (°C) in Table 1 was calculated using the thermodynamic calculation system Thermo-Calc (manufactured by Thermo-Calc Software AB, database TCFE10).

[0080] [Table 1]

[0081] [Table 2] The resulting product sheets were subjected to structural observation and measurement of the Nb / Ti ratio. In addition, the weld solidification cracking resistance, high-temperature strength, room-temperature ductility, and salt corrosion resistance were evaluated using the following procedures.

[0082] Nb / Ti ratio The specimens were embedded in thermosetting resin and mechanically polished to a mirror finish using a cross section parallel to the rolling and thickness directions. The specimens were then observed using an SEM equipped with an EDS device. The measurement area was a region extending from the center of the specimen to one-quarter of the thickness. Precipitates with a circle-equivalent diameter of 3 μm or greater were observed in 10 fields of view at 1000x magnification. All observed precipitates were analyzed using EDS, and precipitates containing 50.0 at% or greater of Ti were extracted. The composition ratio [Nb] / [Ti] was calculated from the Ti and Nb contents (at%) for each extracted precipitate (each precipitate), and the average value was calculated. In the table, Nb / Ti ratios of 0.03 or less were marked with ◎, 0.05 or less with 〇, 0.10 or less with △, and greater than 0.10 with ×.

[0083] (Weld solidification crack resistance) Weld solidification cracking resistance was evaluated using a trans-Varestraint test. Test pieces cut from the product plates measuring 90 mm x 90 mm x 2 mm were used. The welding conditions were a current of 90 A, a voltage of 12.5 V, a welding speed of 40 cm / min, and an arc length of 3 mm, with a bending strain of 4% applied. The length of any cracks in the weld metal was observed and measured using a microscope. The sum of all observed crack lengths was calculated as the total crack length, and the longest observed crack length was calculated as the maximum crack length. To ensure weld solidification cracking resistance suitable for exhaust components, the table indicates a total crack length of 2.5 mm or less as "good," and one greater than 2.5 mm as "bad." Similarly, maximum crack lengths of 0.45 mm or less were calculated as "good," and one greater than 0.45 mm as "bad."

[0084] (High temperature strength) High-temperature strength was evaluated by tensile tests at 600°C and 850°C. For this test, flanged test specimens with a 35 mm inter-mark distance and a 10 mm inter-mark width were taken from the resulting product sheets, with the rolling direction parallel to the longitudinal direction. Tensile strength was measured in accordance with JIS G 0567. A Shimadzu AG-100kNX autograph was used for this test. In the table, a 0.2% yield strength at 600°C of 200 MPa or greater was indicated as ◯, a 0.2% yield strength of 120 MPa or greater but less than 200 MPa was indicated as △, and a 0.2% yield strength of less than 120 MPa was indicated as ×. Similarly, a 0.2% yield strength at 850°C of 20 MPa or greater was indicated as ◎, a 0.2% yield strength of 15 MPa or greater but less than 20 MPa was indicated as ◯, a 0.2% yield strength of 13 MPa or greater but less than 15 MPa was indicated as △, and a 0.2% yield strength of less than 13 MPa was indicated as ×.

[0085] (room temperature ductility) Room temperature ductility was evaluated by a tensile test at room temperature. JIS No. 13B test pieces were prepared from the product sheets so that the longitudinal direction was parallel to the rolling direction, and a tensile test was conducted in accordance with JIS Z 2241 to measure the breaking elongation (total elongation). In the table, a room temperature total elongation of 27% or more was considered good and marked with a circle, and anything else was marked with an x.

[0086] (Salt corrosion resistance) Condensed water corrosion resistance was evaluated in a salt corrosion test. Test specimens measuring 150 mm (parallel to the longitudinal direction) x 70 mm (parallel to the transverse direction) were cut from the product plate and finished with a #600 wet polishing finish on all sides. These test specimens were pre-heat treated in air at 400°C for 8 hours, and then subjected to a salt corrosion test (180 cycles) in accordance with JASO M 609-91.

[0087] After the salt corrosion test, the test pieces were descaled and the corrosion depth was measured using a microscope focal depth method. The corrosion depth was measured at 10 points on each test piece, starting from the deepest, and evaluated as the average value of n = 2. In the table, to ensure salt corrosion resistance applicable to exhaust part materials, maximum corrosion depths of 160 μm or less are marked with a ◯, and those exceeding 160 μm are marked with an ×. The results are summarized in Table 3.

[0088] [Table 3]

[0089] Nos. 1 to 12, which satisfy the requirements of this embodiment, exhibit good resistance to weld solidification cracking, while Nos. 13 to 20, which do not satisfy the requirements of this embodiment, are inferior in at least one of resistance to weld solidification cracking and high-temperature strength. [Industrial Applicability]

[0090] The ferritic stainless steel of this embodiment is suitable for exhaust parts used in environments ranging from medium to high temperatures, and is particularly suitable for parts that comply with automobile exhaust gas regulations, weight reduction, resource conservation, and improved fuel economy. Furthermore, it is not limited to exhaust parts for automobiles and motorcycles, and can also be used for parts with welded structures, such as various boilers and fuel cell systems, that are durable in high-temperature environments and are used in environments exposed to salt corrosion.

Claims

1. The chemical composition, in mass%, is C: 0.001-0.030%, Si: 0.010-2.000%, Mn: 0.010-2.000%, P: 0.100% or less, S: 0.0100% or less, Cr: 16.0-20.0%, Cu: 1.00-2.00%, Mo: 0.01-1.00%, Ti: 0.01 to 0.30%, Nb: 0.050-0.300%, Al: 0.003-0.500%, N: 0.001-0.020%, B: 0.0001 to 0.0050%, Mg: 0 to 0.0010%, Ca: 0-0.0050%, Ni: 0-2.00%, V: 0-0.50%, W: 0-3.00%, Zr: 0 to 0.10%, REM: 0-0.100%, Sn: 0-0.500%, Co: 0 to 0.30%, Sb: 0 to 0.500%, Ga: 0-0.3000%, Ta: 0-1.000%, Hf: 0-1.000%, Bi: 0 to 0.020%, The balance is Fe and impurities. A ferritic stainless steel sheet that satisfies the following formula (i): X≦300+(-34P-38S-4.9Mn-19Ti-9.5Nb-273)...(i) In the above formula, each element symbol represents the content (mass%) of each element contained in the ferritic stainless steel, and if no element is contained, it is set to zero, and X in the above formula represents the solid-liquid coexistence temperature range (°C).

2. The chemical composition is, in mass %, Mg: 0.0001 to 0.0010%, Ca: 0.0001-0.0050%, Ni: 0.01-2.00%, V: 0.01-0.50%, W: 0.10-3.00%, Zr: 0.01 to 0.10%, REM: 0.005-0.100%, Sn: 0.005-0.500%, Co: 0.01 to 0.30%, Sb: 0.005-0.500%, Ga: 0.0002-0.3000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, and Bi: 0.001 to 0.020%, Contains one or more selected from The ferritic stainless steel sheet according to claim 1.

3. The ferritic stainless steel plate is containing Ti-containing precipitates, 2. The ferritic stainless steel sheet according to claim 1, wherein the average value of [Nb] / [Ti], which is the ratio of the Nb content to the Ti content of the precipitates, is 0.05 or less. Here, the above [Nb] indicates the Nb content (at %) of the precipitate, and if no Nb is contained, it is set to zero, and [Ti] indicates the Ti content (at %) of the precipitate.

4. The ferritic stainless steel plate is containing Ti-containing precipitates, 3. The ferritic stainless steel sheet according to claim 2, wherein the average value of [Nb] / [Ti], which is the ratio of the Nb content to the Ti content of the precipitates, is 0.05 or less. Here, the above [Nb] indicates the Nb content (at %) of the precipitate, and if no Nb is contained, it is set to zero, and [Ti] indicates the Ti content (at %) of the precipitate.

5. An exhaust part comprising the ferritic stainless steel sheet according to any one of claims 1 to 4.

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