Exhaust system component and producing method thereof as well as ferritic stainless steel material

Abstract

To provide an exhaust system component excellent in toughness of a base material and a welded portion.SOLUTION: An exhaust system component 1 comprises a first component 2, which is a forged component, and a second component 3 welded around the first component, the exhaust system component including a base material of the first component and a weld heat-affected zone of the first component, wherein the first component is made of a ferritic stainless steel having a predetermined chemical composition, the hardness of the base material of the first component is 250 HV or less, the difference in hardness between the weld heat-affected zone 5 of the first component and the base material of the first component is 30 HV or less, and the average grain size of the weld heat-affected zone of the first component is 500 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust system component, a manufacturing method thereof, and a ferritic stainless steel material. [Background technology]

[0002] Automotive exhaust system components such as exhaust manifolds, front pipes, and center pipes pass high-temperature exhaust gases emitted from the engine, so the materials that make up these components must have a variety of properties, including oxidation resistance, high-temperature strength, and thermal fatigue properties.

[0003] To satisfy these characteristics, ferritic stainless steel is used for exhaust system parts. For example, Patent Document 1 discloses a ferritic stainless steel intended for use in exhaust system parts. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-112020 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to manufacture exhaust system components, materials made of ferritic stainless steel need to be welded, but welding can easily reduce the toughness of the base material and the welded joint. When manufacturing exhaust system components using the ferritic stainless steel disclosed in Patent Document 1, there is still room for improvement in the toughness of the base material and the welded joint.

[0006] In view of the above, an object of the present invention is to provide an exhaust system component having excellent toughness in the base material and welded portion. [Means for solving the problem]

[0007] The present invention has been made to solve the above-mentioned problems, and is summarized as the following exhaust system component, a manufacturing method thereof, and a ferritic stainless steel material.

[0008] (1) An exhaust system component including a first component that is a forged component and a second component that is circumferentially welded to the first component, the exhaust system component includes a base material of the first component and a weld heat affected zone of the first component; The first part has a chemical composition, in mass %, of: C: 0.001 to 0.080%, Si: 0.01 to 2.00%, Mn: 0.01 to 2.00%, P: 0.20% or less, S: 0.050% or less, Ni: 0.01 to 4.00% Cr: 10.0 to 25.0%, Nb: 0.2 to 2.0% Ti: 0.001 to 2.00%, V: 0.001 to 2.00%, N: 0.001 to 0.100%, Mo: 0-4.0% Cu: 0-3.0% B: 0~0.10%, Al: 0-4.0% W: 0-2.5%, Ga: 0 to 0.05%, Co: 0-2.0% Sn: 0 to 2.0% Ta: 0 to 2.0%, Ca: 0-0.05%, Mg: 0 to 0.012% Zr: 0 to 0.012%, REM: 0~0.05%, The balance is Fe and impurities. The steel is made of a ferritic stainless steel material that satisfies the following formula (i): the hardness of the base material of the first component is 250 HV or less; a difference in hardness between the weld heat affected zone of the first component and the base metal of the first component is 30 HV or less; An exhaust system component, wherein the average crystal grain size of the weld heat affected zone of the first component is 500 μm or less. (C+N)(Ti+2Nb+3V)≦0.20 (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel material, and if the element is not contained, it is set to zero.

[0009] (2) A method for manufacturing an exhaust system part according to (1) above, hot forging the ferritic stainless steel material into a part shape; a step of cooling the ferritic stainless steel material shaped into a part at a cooling rate of 50 to 400 ° C / min in a temperature range of 900 to 500 ° C to obtain a first part which is a forged part; a step of circumferentially welding the first component and the second component by MIG welding; A method for manufacturing an exhaust system part.

[0010] (3) The ferritic stainless steel material according to (1), The chemical composition is, in mass%, C: 0.001 to 0.080%, Si: 0.01 to 2.00%, Mn: 0.01 to 2.00%, P: 0.20% or less, S: 0.050% or less, Ni: 0.01 to 4.00% Cr: 10.0 to 25.0%, Nb: 0.2 to 2.0% Ti: 0.001 to 2.00%, V: 0.001 to 2.00%, N: 0.001 to 0.100%, Mo: 0-4.0% Cu: 0-3.0% B: 0~0.10%, Al: 0-4.0% W: 0-2.5%, Ga: 0 to 0.05%, Co: 0-2.0% Sn: 0 to 2.0% Ta: 0 to 2.0%, Ca: 0-0.05%, Mg: 0 to 0.012% Zr: 0 to 0.012%, REM: 0~0.05%, The balance is Fe and impurities. A ferritic stainless steel material that satisfies the following formula (i): (C+N)(Ti+2Nb+3V)≦0.20 (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel material, and if the element is not contained, it is set to zero. [Effects of the Invention]

[0011] According to the present invention, an exhaust system component having excellent toughness in the base material and the welded portion can be obtained. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1(a) is a diagram schematically illustrating an example of a forged part (side view), which is a first part before welding, FIG. 1(b) is a diagram schematically illustrating a second part (side view) before welding, and FIG. 1(c) is a diagram schematically illustrating an example of an exhaust system part (side view) in which the first part and the second part are welded together. [Figure 2] FIG. 2 is a diagram (side view) that schematically shows an exhaust system part manufactured in the example. [Figure 3] Figure 3(a) is a side view of a test specimen taken for evaluating toughness, viewed from a direction perpendicular to the longitudinal direction, and Figure 3(b) is a cross-sectional view of the test specimen viewed from the axial direction. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the present invention has been made based on the above findings. The requirements for the exhaust system component and ferritic stainless steel material of this embodiment will be described in detail below.

[0014] 1. Exhaust system parts 1-1. Overall structure The exhaust system component of this embodiment includes a first component, which is a forged component, and a second component that is circumferentially welded to the first component. The exhaust system component is typically an exhaust manifold, a front pipe, a flexible pipe, a catalytic converter, a center pipe, a main muffler, or a tail end pipe, and is a component that releases exhaust gas generated by an engine to the outside. The exhaust component may be, for example, a component that is formed by welding individual components together.

[0015] In the exhaust component of this embodiment, the second component is circumferentially welded to the first component. The second component may be made of any material that can be welded to the first component, but is preferably made of ferritic stainless steel. For example, a metallic material is preferred, and the composition of the weld metal is preferably one that satisfies the weld metal composition described below. Circumferential welding is a welding method in which the entire outer periphery of the components is welded. While there are no particular limitations on which portions of the outer periphery of the components are welded, the outer periphery of the ends of the components is typically welded.

[0016] FIG. 1(a) is a diagram schematically illustrating an example of a forged part (side view) that is a first part before welding, FIG. 1(b) is a diagram schematically illustrating a second part (side view) before welding, and FIG. 1(c) is a diagram schematically illustrating an example of an exhaust system part (side view) in which the first part and the second part are welded together. In FIG. 1(c), 2 is the first part and 3 is the second part. In FIG. 1, the first part 1 is flange-shaped and the second part 2 is tubular-shaped, and these two parts are joined by circumferential welding to form the exhaust system part 1. The first part 2 and the second part 3 are joined by weld metal 4.

[0017] 1-2. First part 1-2-1. Configuration of the first part As described above, the first part is a forged part. Here, a forged part refers to a part formed by forging. The type of forging is not particularly limited. Any of hot forging, cold forging, and warm forging may be used, but for example, a part formed by hot forging is preferable. In the exhaust system part of this embodiment, the forged part is preferably a part formed by forging a bar-shaped steel material (hereinafter simply referred to as "bar material"). In this case, a linear structure called metal flow that follows the product shape is formed in the metal structure. This structure can be observed using a microscope or the like.

[0018] The first component includes a base material and a heat-affected zone. The base material refers to a region where the shape, performance, and characteristics of the material are not changed by welding. The weld heat-affected zone refers to a region where changes in the structure and mechanical properties occur due to welding heat, excluding the weld metal portion.

[0019] That is, the exhaust system component of this embodiment includes a base material of the first component and a weld heat-affected zone of the first component. Here, as shown in FIG. 1(c), the portion of the first component 2 close to the region of the weld metal side 4 is the weld heat-affected zone 5, and the region of the first component 2 excluding this weld heat-affected zone 5 is the base material. The base material and the weld heat-affected zone can be distinguished by observing the metal structure. The second component 3 similarly includes a base material and a weld heat-affected zone.

[0020] The chemical composition of the weld metal is preferably the same as that of the first component described below, i.e., C: 0.001 to 0.080%, Si: 0.01 to 2.00%, Mn: 0.01 to 2.00%, P: 0.20% or less, S: 0.050% or less, Ni: 0.01 to 4.00%, Cr: 10.0 to 25.0%, Nb: 0.2 to 2.0%, Ti: 0.001 to 2.00%, V: 0.001 to 2.00%, N: 0.001 to 0.10%. 0%, Mo: 0-4.0%, Cu: 0-3.0%, B: 0-0.10%, Al: 0-4.0%, W: 0-2.5%, Ga: 0-0.05%, Co: 0-2.0%, Sn: 0-2.0%, Ta: 0-2.0%, Ca: 0-0.05%, Mg: 0-0.012%, Zr: 0-0.012%, REM: 0-0.05%, balance: Fe and impurities. It is also preferable that the formula (i) described below is satisfied.

[0021] 1-2-2. Material of the first part The first component is made of a ferritic stainless steel material. Here, ferritic stainless steel material refers to a steel material having a ferrite phase as the main phase, and is determined to be a ferritic stainless steel if it satisfies the chemical composition described below. The chemical composition of this ferritic stainless steel material is as shown below. That is, the base metal and weld heat-affected zone of the first component satisfy the chemical composition below. The detailed reasons for limiting each element are the same as those described in the section on steel material.

[0022] The chemical composition is, in mass%, C: 0.001 to 0.080%, Si: 0.01 to 2.00%, Mn: 0.01 to 2.00%, P: 0.20% or less, S: 0.050% or less, Ni: 0.01 to 4.00%, Cr: 10.0 to 25.0%, Nb: 0.2 to 2.0%, Ti: 0.001 to 2.00%, V: 0.001 to 2.00%, N: 0.001 to 0.100%, Mo : 0-4.0%, Cu: 0-3.0%, B: 0-0.10%, Al: 0-4.0%, W: 0-2.5%, Ga: 0-0.05%, Co: 0-2.0%, Sn: 0-2.0%, Ta: 0-2.0%, Ca: 0-0.05%, Mg: 0-0.012%, Zr: 0-0.012%, REM: 0-0.05%, balance: Fe and impurities, and satisfying the following formula (i):

[0023] (C+N)(Ti+2Nb+3V)≦0.20 (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel material, and if the element is not contained, it is set to zero.

[0024] 1-2-3.Hardness of the base material of the first part The hardness of the base material of the first component is 250 HV or less. When the hardness of the base material of the first component is 250 HV or less, the toughness of the base material is easily ensured. Therefore, the hardness of the base material of the first component is preferably 220 HV or less, and more preferably 200 HV or less. Note that the lower limit of the hardness of the base material is not particularly limited, but considering that the first component is hot forged, the lower limit of the hardness of the base material is preferably 130 HV.

[0025] 1-2-4.Difference in hardness between the weld heat-affected zone of the first component and the base material of the first component The difference in hardness between the weld heat-affected zone of the first component and the base material of the first component is 30 HV or less. This is because, if the difference in hardness between the weld heat-affected zone of the first component and the base material of the first component is 30 HV or less, the toughness of the base material and the weld can be ensured. It is preferable that the difference in hardness between the weld heat-affected zone of the first component and the base material of the first component be as close to 0 as possible. Note that the difference in hardness between the weld heat-affected zone of the first component and the base material of the first component is the value obtained by subtracting the smaller of the harder ones of the weld heat-affected zone of the first component and the base material of the first component from the larger one of the hardnesses. For example, it can also be said to be the absolute value of the value obtained by subtracting the hardness of the base material of the first component from the hardness of the weld heat-affected zone of the first component.

[0026] The hardness of each area described in 1-2-3 and 1-2-4 is measured using the following procedure. The hardness of the base material is measured at a location sufficiently far from the weld heat-affected zone of the base material, at a position 1 / 2t, where t is the wall thickness of the part. The hardness test uses a micro-Vickers hardness tester, with a test force of 0.1 kgf, and hardness measurements are taken at five points, spaced at intervals of three or more indentations. The average hardness value is taken as the hardness of the base material.

[0027] The hardness of the weld heat affected zone is measured at the center of its width. Under the same conditions as for measuring the hardness of the base metal, the hardness is measured at five points at the center of its width, and the average value is taken as the hardness of the weld heat affected zone. Hardness tests of the base metal and weld heat affected zone are conducted in accordance with JIS Z 2244-1:2020 (Vickers hardness test).

[0028] 1-2-5. Average grain size in the heat-affected zone The average grain size of the weld heat affected zone of the first component is 500 μm or less. By having the average grain size of 500 μm or less, the toughness of the weld can be improved. The average grain size of the weld heat affected zone is preferably 400 μm or less, and more preferably 300 μm or less. There is no particular restriction on the lower limit of the average grain size of the weld heat affected zone, but it is usually preferably 10 μm, taking into account the average grain size of the base material.

[0029] As described above, the first component is preferably a forged component, particularly a hot forged component. In the case of a hot forged component, the average crystal grain size of the base material of the first component is preferably 10 to 500 μm.

[0030] The average grain size is measured using the following procedure. Test pieces are taken from an area including the base material, weld heat-affected zone, and weld metal, and the taken test pieces are polished. The polished test pieces are analyzed using EBSP-OIM, specifying an area from the side of the weld heat-affected zone adjacent to the weld metal to the entire side adjacent to the base material. Specifically, in orientation measurements taken at regular measurement steps, the position where the orientation difference between adjacent measurement points exceeds 15° is considered to be a grain boundary. 15° is the threshold for high-angle grain boundaries and is generally recognized as a grain boundary. The area surrounded by grain boundaries is considered to be a grain, and the average grain size of the grains within the measurement area is calculated. Note that measurements are made using more than 100 grains.

[0031] 2. Ferritic stainless steel 2-1.Chemical composition The steel material used in the exhaust system component of this embodiment and serving as the raw material for the first component is preferably a ferritic stainless steel material having the following chemical composition: In the following description, "%" in relation to the content means "% by mass."

[0032] C: 0.001 to 0.080% C (carbon) has the effect of increasing strength and toughness. For this reason, the C content is preferably 0.001% or more. The C content is more preferably 0.002% or more, and even more preferably 0.003% or more. However, if C is contained in excess, the toughness of the base metal and weld of the first component decreases. For this reason, the C content is preferably 0.080% or less. The C content is more preferably 0.060% or less, and even more preferably 0.040% or less.

[0033] Si: 0.01 to 2.00% Silicon (Si) has a deoxidizing effect and improves the high-temperature oxidation properties of exhaust system components. It also has the effect of increasing toughness. For this reason, the Si content is preferably 0.01% or more. The Si content is more preferably 0.10% or more, and even more preferably 0.30% or more. However, if Si is contained in excess, the toughness of the base metal and weld of the first component tends to decrease. For this reason, the Si content is preferably 2.00% or less. The Si content is more preferably 1.50% or less, and even more preferably 1.00% or less.

[0034] Mn: 0.01 to 2.00% Mn (manganese) has the effect of improving strength and toughness. For this reason, the Mn content is preferably 0.01% or more. The Mn content is more preferably 0.10% or more, and even more preferably 0.20% or more. However, if Mn is contained in excess, the toughness of the base metal and weld of the first component decreases. For this reason, the Mn content is preferably 2.00% or less. The Mn content is more preferably 1.50% or less, even more preferably 1.00% or less, and even more preferably 0.80% or less.

[0035] P:0.20% or less P (phosphorus) is an element contained as an impurity in steel and reduces strength and toughness. For this reason, the P content is preferably 0.20% or less. It is preferable to reduce P as much as possible, but excessive reduction of P increases refining costs. For this reason, the P content is preferably 0.010% or more.

[0036] S: 0.050% or less S (sulfur) is an element contained as an impurity in steel and reduces mechanical properties such as corrosion resistance. For this reason, the S content is preferably 0.050% or less. It is preferable to reduce S as much as possible, but excessive reduction of S increases refining costs. For this reason, the S content is preferably 0.0001% or more.

[0037] Ni: 0.01 to 4.00% Ni (nickel) has the effect of improving strength and toughness. For this reason, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more, and even more preferably 0.10% or more. However, if Ni is contained in excess, the toughness of the base metal and weld of the first component decreases. For this reason, the Ni content is preferably 4.00% or less. The Ni content is more preferably 2.00% or less, and even more preferably 1.00% or less.

[0038] Cr: 10.0 to 25.0% Cr (chromium) has the effect of improving corrosion resistance and toughness. Therefore, the Cr content is preferably 10.0% or more. The Cr content is more preferably 12.0% or more, and even more preferably 15.0% or more. However, if Cr is contained in excess, the toughness of the base metal and weld of the first component decreases. Therefore, the Cr content is preferably 25.0% or less. The Cr content is more preferably 22.0% or less, and even more preferably 20.0% or less.

[0039] Nb: 0.2 to 2.0% Nb (niobium) forms carbonitrides and has the effect of improving strength and corrosion resistance. Therefore, the Nb content is preferably 0.2% or more. The Nb content is more preferably 0.3% or more, and even more preferably 0.4% or more. However, if Nb is contained in excess, the toughness of the base metal and weld of the first component decreases. Therefore, the Nb content is preferably 2.0% or less. The Nb content is more preferably 1.0% or less, and even more preferably 0.8% or less.

[0040] Ti: 0.001 to 2.00% Ti (titanium) has the effect of increasing strength and corrosion resistance. For this reason, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.003% or more, and even more preferably 0.005% or more. However, if Ti is contained in excess, the toughness of the base metal and weld of the first component decreases. For this reason, the Ti content is preferably 2.00% or less. The Ti content is more preferably 1.00% or less, and even more preferably 0.50% or less.

[0041] V: 0.001 to 2.00% V (vanadium) has the effect of improving corrosion resistance. For this reason, the V content is preferably 0.001% or more. The V content is more preferably 0.003% or more, and even more preferably 0.005% or more. However, if V is contained in excess, the toughness of the base metal and weld of the first component decreases. For this reason, the V content is preferably 2.00% or less. The V content is more preferably 1.00% or less, and even more preferably 0.50% or less.

[0042] N: 0.001 to 0.100% N (nitrogen) has the effect of improving strength. For this reason, the N content is preferably 0.001% or more. The N content is more preferably 0.002% or more, and even more preferably 0.003% or more. However, if N is contained in excess, the toughness of the base metal and weld of the first component decreases. For this reason, the N content is preferably 0.100% or less. The N content is more preferably 0.060% or less, and even more preferably 0.040% or less.

[0043] In addition to the above elements, one or more elements selected from Mo, Cu, B, Al, W, Ga, Co, Sn, Ta, Ca, Mg, Zr, and REM 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.

[0044] Mo: 0-4.0% Mo (molybdenum) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if excessive Mo is contained, the toughness of the base metal and weld of the first component decreases. Therefore, the Mo content is preferably 4.0% or less. The Mo content is more preferably 2.0% or less, and even more preferably 1.5% or less. On the other hand, in order to obtain the above effect, the Mo content is preferably 0.01% or more.

[0045] Cu: 0 to 3.0% Cu (copper) has the effect of improving the thermal fatigue properties of exhaust system components. Therefore, it may be contained as needed. However, if Cu is contained in excess, the toughness of the base material and weld of the first component decreases. Therefore, the Cu content is preferably 3.0% or less. The Cu content is more preferably 2.0% or less, and even more preferably 1.5% or less. On the other hand, to obtain the above effect, the Cu content is preferably 0.01% or more.

[0046] B: 0 to 0.10% B (boron) has the effect of improving hot workability and corrosion resistance. However, if excessive B is contained, the toughness of the base metal and weld of the first component decreases. For this reason, the B content is preferably 0.10% or less. The B content is more preferably 0.02% or less, and even more preferably 0.01% or less. On the other hand, to obtain the above effects, the B content is preferably 0.0001% or more.

[0047] Al: 0 to 4.0% Al (aluminum) has a deoxidizing effect. Therefore, it may be contained as needed. However, if an excessive amount of Al is contained, the toughness of the base metal and weld of the first component decreases. Therefore, the Al content is preferably 4.0% or less. The Al content is more preferably 1.0% or less, and even more preferably 0.5% or less. On the other hand, to obtain the above effect, the Al content is preferably 0.01% or more.

[0048] W: 0 to 2.5% W (tungsten) has the effect of improving the high-temperature strength of exhaust system components. Therefore, it may be contained as needed. However, if excessive W is contained, the toughness of the base material and weld of the first component decreases. Therefore, the W content is preferably 2.5% or less. The W content is more preferably 1.0% or less, and even more preferably 0.5% or less. On the other hand, in order to obtain the above effect, the W content is preferably 0.01% or more.

[0049] Ga: 0 to 0.05% Ga (gallium) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if Ga is contained in excess, hot workability decreases. Therefore, the Ga content is preferably 0.05% or less. The Ga content is more preferably 0.02% or less, and even more preferably 0.01% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably 0.001% or more.

[0050] Co: 0-2.0% Co (cobalt) has the effect of improving strength and high-temperature properties. Therefore, it may be contained as needed. However, if excessive Co is contained, the toughness of the base metal and weld of the first component decreases. Therefore, the Co content is preferably 2.0% or less. The Co content is more preferably 1.0% or less, and even more preferably 0.5% or less. On the other hand, to obtain the above effect, the Co content is preferably 0.01% or more.

[0051] Sn: 0 to 2.0% Sn (tin) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if excessive Sn is contained, the toughness of the base material and weld of the first component decreases. The Sn content is preferably 2.0% or less. The Sn content is more preferably 1.0% or less, and even more preferably 0.5% or less. On the other hand, to obtain the above effect, the Sn content is preferably 0.01% or more.

[0052] Ta: 0 to 2.0% Ta (tantalum) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if excessive Ta is contained, the toughness of the base material and weld of the first component decreases. Therefore, the Ta content is preferably 2.0% or less. The Ta content is more preferably 1.0% or less, and even more preferably 0.5% or less. On the other hand, to obtain the above effect, the Ta content is preferably 0.01% or more.

[0053] Ca: 0 to 0.05% Ca (calcium) has a deoxidizing effect. Therefore, it may be contained as needed. However, if an excessive amount of Ca is contained, the toughness of the base metal and weld of the first component decreases. Therefore, the Ca content is preferably 0.05% or less. The Ca content is more preferably 0.02% or less, and even more preferably 0.01% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.001% or more.

[0054] Mg: 0 to 0.012% Mg (magnesium) has the effect of enhancing corrosion resistance. Therefore, it may be added as needed. However, if an excessive amount of Mg is added, the toughness of the base metal and weld of the first component decreases. Therefore, the Mg content is preferably 0.012% or less. The Mg content is more preferably 0.008% or less, and even more preferably 0.004% or less. On the other hand, to obtain the above effect, the Mg content is preferably 0.0001% or more.

[0055] Zr: 0 to 0.012% Zr (zirconium) has the effect of improving the high-temperature properties of exhaust system components. Therefore, it may be contained as needed. However, if Zr is contained in excess, the toughness of the base material and weld of the first component decreases. Therefore, the Zr content is preferably 0.012% or less. The Zr content is more preferably 0.008% or less, and even more preferably 0.004% or less. On the other hand, to obtain the above effect, the Zr content is preferably 0.0001% or more.

[0056] REM: 0 to 0.05% REM (rare earth elements) have the effect of improving the high-temperature properties of exhaust system components. Therefore, they may be added as needed. However, if REM is added in excess, the toughness of the base material and weld of the first component decreases. Therefore, the REM content is preferably 0.05% or less. The REM content is more preferably 0.02% or less, and even more preferably 0.01% or less. On the other hand, to obtain the above effect, the REM content is preferably 0.001% or more.

[0057] 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.

[0058] In the chemical composition of the steel material of this embodiment, the balance is preferably Fe and impurities. Here, "impurities" refer to components that are mixed in due to various factors in raw materials such as ore and scrap, or in the manufacturing process, when industrially manufacturing steel material, and are acceptable within a range that does not adversely affect this embodiment.

[0059] 2-2.(i) The ferritic stainless steel material of this embodiment preferably satisfies the following formula (i). (C+N)(Ti+2Nb+3V)≦0.20 (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel material, and if the element is not contained, it is set to zero.

[0060] Here, the elements in formula (i) are solid-solution strengthening elements. The higher the content of these elements, the more the toughness of the base metal and weld of the first component is reduced due to solid-solution strengthening. Furthermore, when the relationship between C, N, Ti, Nb, and V, which constitute the left-hand side of formula (i), is 0.20 or less, good toughness of the base metal and weld can be ensured. Therefore, the left-hand side of formula (i), (C + N)(Ti + 2Nb + 3V), is 0.20 or less, and preferably 0.18 or less. The lower limit of the left-hand side of formula (i) is not particularly limited, but is preferably 0.0008, for example.

[0061] 3. Shape of steel material The ferritic stainless steel material of this embodiment is preferably a bar-shaped steel material. Here, the bar-shaped steel material refers to a bar-shaped steel material having a circular, rectangular, or other cross section and a longitudinal direction.

[0062] 4. Manufacturing method The exhaust system component of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0063] 4-1. Manufacturing method of ferritic stainless steel material A steel having the above chemical composition is melted and a slab is produced. The resulting slab is heated and hot-rolled. Other hot-rolling conditions are not particularly limited and may be conventional. After hot-rolling, the slab is cooled to produce a ferritic stainless steel material. Heat treatment, pickling, etc. may be performed as necessary. The ferritic stainless steel material is preferably a bar-shaped steel material.

[0064] 4-2. Manufacturing of the first part (forged part) The ferritic stainless steel material is forged into a desired part shape to produce a forged part. The forging method is preferably hot forging. Hot forging is a processing method in which the ferritic stainless steel material is heated to a temperature equal to or higher than the recrystallization temperature and forged.

[0065] The ferritic stainless steel material formed into a part shape is cooled at a cooling rate of 50 to 400°C / min in a temperature range of 900 to 500°C to produce a forged part. If the cooling rate in this temperature range exceeds 400°C, the amount of solid solution strengthening elements precipitated during the cooling process is small, and the hardness of the base material of the first part increases due to solid solution strengthening. Furthermore, the effect of precipitates in suppressing grain coarsening is not obtained, and the average grain size of the weld heat-affected zone becomes excessively large. For this reason, the cooling rate in this temperature range is 400°C / min or less, preferably 300°C / min or less. On the other hand, if the cooling rate in this temperature range is less than 50°C / min, the precipitates aggregate and coarsen, the effect of suppressing grain coarsening is not obtained, and the average grain size of the weld heat-affected zone of the first part becomes excessively large. In addition, the increase in the amount of solid solution strengthening in the weld heat-affected zone results in an excessively large difference in hardness between the weld heat-affected zone and the base material. Therefore, the cooling rate in the above temperature range is 50°C / min or more, and preferably 80°C / min or more.

[0066] The first and second components, which are the obtained forged components, are circumferentially welded by MIG welding. That is, the end of the first component is welded to the end of the second component. Possible welding techniques include, for example, butt welding and fillet welding. Furthermore, as long as the chemical compositions of the base metal and weld metal of the first component are within the above-described ranges, welding may be performed using, for example, a welding material. The welding material is not particularly limited, but may be selected and used so that the chemical composition of the weld metal falls within the above-described range. Note that, for example, a ferritic stainless steel welding wire for MIG welding may be used as the welding material.

[0067] Here, the conditions for MIG welding are, for example, current: 100 to 200 A, voltage: 16 to 24 V, welding speed: 30 to 100 cm / min, and it is preferable to use a mixed gas of Ar and 2 to 5 mass % O2 as the shielding gas.

[0068] MIG welding tends to produce wider weld beads than other welding methods because the cooling rate of the weld is slower. MIG welding also reduces the hardness of the heat-affected zone.

[0069] EXAMPLES The exhaust system parts 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]

[0070] A ferritic stainless steel bar having the chemical composition shown in Tables 1 and 2 was prepared. This ferritic stainless steel bar had a diameter of 60 mm and a length of 50 mm and was manufactured by hot rolling. This ferritic stainless steel bar was heated to 1000°C, hot forged, and cooled at a cooling rate of 100°C / min in the temperature range of 900 to 500°C to produce a hot-forged part, which was then machined to form a first part. The first part had a cylindrical cavity and a wall thickness of 3 mm.

[0071] A 3mm thick ferritic stainless steel pipe was prepared as the second part. The first and second parts were circumferentially welded together using MIG welding, ensuring no misalignment in wall thickness. Butt welding was performed using Ar + 2 mass% O2 shielding gas, a current of 150 A, a voltage of 20 V, and a welding speed of 50 cm / min to produce an exhaust system part with the shape shown in Figure 2. The welding material used for welding was WEL MIG 430NbL (manufactured by Nippon Welding Rod Co., Ltd.), a MIG welding wire. The chemical compositions of the base metal and the weld heat-affected zone of the first part were the same as those of the steel bar used.

[0072] Samples were cut from the exhaust system components obtained, and the hardness of each portion and the average grain size of the weld heat-affected zone were measured using the following methods. Note that, with regard to the inventive examples, those not listed in the table satisfied the preferred ranges and manufacturing conditions described in the specification. For example, the chemical composition of the weld metal was within the preferred range described in the specification.

[0073] [Table 1]

[0074] [Table 2]

[0075] (Hardness) The hardness of the base material was measured at a location far enough away from the weld heat-affected zone of the base material, at a position 1.5 mm from the weld, assuming a wall thickness of 3 mm. The hardness test was performed using a micro-Vickers hardness tester, with a test force of 0.1 kgf, and measurements were taken at five points, spaced at intervals of at least three indentations. The average hardness was taken as the hardness of the base material.

[0076] The hardness of the weld heat affected zone was measured at the center of its width. Under the same conditions as for measuring the hardness of the base metal, the hardness was measured at five points at the center of its width, and the average value was taken as the hardness of the weld heat affected zone. The hardness tests for the base metal and the weld heat affected zone were conducted in accordance with JIS Z 2244-1:2020 (Vickers hardness test). For simplicity, the weld heat affected zone is referred to as the HAZ in the table.

[0077] (Average grain size in the heat-affected zone) Test specimens were taken from an area including the base metal, weld heat-affected zone, and weld metal, and then polished. The polished test specimens were analyzed using EBSP-OIM, specifying the entire area of ​​the weld heat-affected zone from the side adjacent to the weld metal to the side adjacent to the base metal. Specifically, in orientation measurements at fixed measurement steps, the location where the orientation difference between adjacent measurement points exceeded 15° was defined as a grain boundary. 15° is the threshold for high-angle grain boundaries and is generally recognized as a grain boundary. The area surrounded by grain boundaries was defined as a grain, and the average grain size of the grains within the measurement area was calculated. Measurements were performed using more than 100 grains.

[0078] (Toughness evaluation) The toughness of the resulting exhaust system components was evaluated. Evaluation was based on the bending test method for butt-welded joints in JIS Z 3122:2013. A 50 mm long, 8 mm wide, and 3 mm thick test piece was taken from the center of the weld of the exhaust system component, as shown in Figures 3(a) and (b). The excess reinforcement was removed from the test piece. The resulting test piece was subjected to a roller bending test at a test temperature of 23°C, with a 12 mm diameter push jig tip, a 20 mm radius support roller, and a 21 mm roller spacing to evaluate toughness. After the roller bending test, samples with crack lengths of less than 3 mm were marked with an ◯, and samples with crack lengths of 3 mm or greater were marked with an × in the table.

[0079] [Table 3]

[0080] [Table 4]

[0081] Inventive examples Nos. 1 to 35, which satisfied the chemical composition requirements of this embodiment, exhibited good toughness, while comparative examples Nos. 1 to 21, which did not satisfy the requirements of this embodiment, exhibited poor toughness. [Example]

[0082] Exhaust system components were manufactured under the conditions shown in Table 5 using a ferritic stainless steel material of the steel type No. B of Example 1. Other manufacturing conditions were the same as in Example 1. Furthermore, the obtained exhaust system components were subjected to measurement of various values ​​and characteristic evaluations using the same methods as in Example 1. The results are summarized below in Table 5.

[0083] [Table 5]

[0084] Inventive examples Nos. 36 to 42, which satisfied the requirements of the manufacturing method of this embodiment, exhibited good toughness, while comparative examples Nos. 22 to 25, which did not satisfy the requirements of the manufacturing method of this embodiment, exhibited poor toughness. [Explanation of symbols]

[0085] 1. Exhaust system parts 2. First part 3. Second part 4. Weld metal 5. Weld heat affected zone 6. Test specimen

Claims

1. An exhaust system component comprising a first component which is a forged component, and a second component which is circumferentially welded to the first component, the exhaust system component includes a base material of the first component and a weld heat affected zone of the first component; The first component has a chemical composition, in mass %, of: C: 0.001-0.080%, Si: 0.01-2.00%, Mn: 0.01 to 2.00%, P: 0.20% or less, S: 0.050% or less, Ni: 0.01 to 4.00%, Cr: 10.0-25.0%, Nb: 0.2-2.0%, Ti: 0.001 to 2.00%, V: 0.001-2.00%, N: 0.001-0.100%, Mo: 0-4.0%, Cu: 0-3.0%, B: 0 to 0.10%, Al: 0-4.0%, W: 0 to 2.5%, Ga: 0-0.05%, Co: 0-2.0%, Sn: 0-2.0%, Ta: 0-2.0%, Ca: 0-0.05%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0-0.05%, The balance is Fe and impurities. The steel is made of a ferritic stainless steel material that satisfies the following formula (i): the hardness of the base material of the first component is 250 HV or less; a difference in hardness between a weld heat affected zone of the first component and a base metal of the first component is 30 HV or less; an average crystal grain size in the weld heat affected zone of the first component is 500 μm or less; (C+N)(Ti+2Nb+3V)≦0.20...(i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel material, and if the element is not contained, it is set to zero.

2. 2. A method for manufacturing an exhaust system component according to claim 1, comprising: hot forging the ferritic stainless steel material into a part shape; a step of cooling the ferritic stainless steel material shaped into a part at a cooling rate of 50 to 400 ° C / min in a temperature range of 900 to 500 ° C to obtain a first part which is a forged part; a step of circumferentially welding the first component and the second component by MIG welding; A method for manufacturing an exhaust system part.

3. The ferritic stainless steel material according to claim 1, The chemical composition, in mass%, is C: 0.001-0.080%, Si: 0.01-2.00%, Mn: 0.01 to 2.00%, P: 0.20% or less, S: 0.050% or less, Ni: 0.01 to 4.00%, Cr: 10.0-25.0%, Nb: 0.2-2.0%, Ti: 0.001 to 2.00%, V: 0.001-2.00%, N: 0.001-0.100%, Mo: 0-4.0%, Cu: 0-3.0%, B: 0 to 0.10%, Al: 0-4.0%, W: 0 to 2.5%, Ga: 0-0.05%, Co: 0-2.0%, Sn: 0-2.0%, Ta: 0-2.0%, Ca: 0-0.05%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0-0.05%, The balance is Fe and impurities. A ferritic stainless steel material that satisfies the following formula (i): (C+N)(Ti+2Nb+3V)≦0.20...(i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel material, and if the element is not contained, it is set to zero.

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