Ferritic stainless steel sheet and method of manufacturing the same

Abstract

To provide a ferritic stainless steel plate that achieves enhanced wear resistance without impairing the workability, corrosion resistance, or high-temperature strength of the steel plate and a method for producing the same.SOLUTION: A ferritic stainless steel plate comprises, in mass%, C: 0.01% or less, Si: 0.05-1.2%, Mn: 0.05-1.5%, P: 0.035% or less, S: 0.01% or less, Cr: 10.5-20.0%, Ni: 0.01-0.60%, Mo: 0.01-2.0%, Cu: 0.01-1.6%, Al: 0.001-0.10%, N: 0.001-0.02%, Nb: 0.20-0.60%, an optional element, and the balance being Fe and impurities. In the area spanning 10 μm deep from the surface in the thickness direction of the plate, the average nitrogen level is 0.05-0.10%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet for use in components that require corrosion resistance and high-temperature strength, such as exhaust system components, suspension components, and structural members of automobiles, and a method for producing the same, and further to a ferritic stainless steel sheet with excellent wear resistance and a method for producing the same. [Background technology]

[0002] As a measure to improve fuel efficiency in response to global environmental issues, efforts are being made to reduce the weight of automobile bodies. To this end, efforts are being made to increase the strength of the steel plates used in automobiles and reduce their thickness, as well as to replace them with aluminum or resin materials to reduce weight. The need for weight reduction is not limited to automobile bodies, but extends to various other parts as well. One example is the exhaust system parts that treat engine exhaust gases.

[0003] Exhaust system parts are exposed to high-temperature engine exhaust gases, so they require high-temperature strength, oxidation resistance, and corrosion resistance.For this reason, heat-resistant stainless steels such as SUH409, SUS436, SUS429, and SUS441 are used, and exhaust system parts for one vehicle weigh 20 to 30 kg.

[0004] The exhaust manifold, which is directly connected to the engine, is exposed to high-temperature engine exhaust gases from the most upstream location. For this reason, exhaust manifolds, which were previously made of cast metal, have been replaced by stainless steel pipes welded together or plates welded after press forming, resulting in weight reduction. While mufflers and center pipes are exposed to lower exhaust gas temperatures than exhaust manifolds, their thicknesses have been limited to prevent internal corrosion caused by condensation from cooled exhaust gases and external corrosion caused by deicing salt on the road. However, even in these applications, the use of highly corrosion-resistant stainless steels has led to the reduction in thickness.

[0005] As stainless steels for exhaust system components become more corrosion-resistant and thinner and lighter, factors other than corrosion have begun to affect the lifespan of exhaust system components. These include wear from contact with the road surface and chipping from road debris. While using austenitic stainless steel, which is relatively common, is one way to address this issue, austenitic stainless steel is prone to stress corrosion cracking in the environments to which automotive exhaust system components are exposed, and the alloy costs are high, making this undesirable.

[0006] Patent Document 1 discloses a ferritic stainless steel material with excellent corrosion resistance and high surface hardness achieved by increasing the amount of nitrides in the surface layer of the ferritic stainless steel. This ferritic stainless steel material contains 14-18% Cr, 0.02-0.05% N, 0-0.2% Al, and 0-0.05% Ti, with a nitrogen concentration of 0.05% or more in the surface layer. For this reason, the material is manufactured by bright annealing at 750-950°C in an atmospheric gas containing 20% ​​or more N2 by volume and 50% or more H2 by volume.

[0007] Patent Document 2 discloses a chromium-based stainless steel foil with excellent spring properties, which is produced by heating a chromium stainless steel in a nitrogen-containing atmosphere, allowing it to absorb nitrogen, and then cooling it to produce a metal structure consisting of a two-phase structure containing a martensite phase and a retained austenite phase, or a three-phase structure containing a martensite phase, a retained austenite phase, and 75 volume% or less of a ferrite phase. The composition of this stainless steel foil is, by mass%, 0.01-0.2% C, 10-20% Cr, 0.05-0.5% N, 2% or less Si, 0.05% or less Al, 2% or less Ni, 2% or less Mn, and 2% or less Cu, and further contains one or more of: 0.001-0.02% Ti, 0.01-0.1% Nb, 0.1-2% Mo, and 0.05-0.3% V. In order to absorb nitrogen into the steel, it is preferable that the atmosphere be 50 to 80% by volume of hydrogen and 20 to 50% by volume of nitrogen, with a dew point of -30°C or lower.

[0008] Furthermore, Patent Document 3 discloses a chromium-based stainless steel material for use in color cathode ray tube mask frames, which contains, by mass, 0.05-0.20% C and 10.5-18% Cr, and is characterized by a metal structure consisting of a ferrite phase and a martensite phase, with Cr-based carbides less than 1 μm in size dispersed therein. This stainless steel material may also contain an austenite phase in addition to the ferrite and martensite phases, or may contain one or more of 0.003-0.03% Ti, 0.005-0.10% Nb, 0.02-0.5% V, and 0.1-1.0% Mo.

[0009] To obtain the above-mentioned structure, Patent Document 3 discloses that the material is heated to a temperature within a range of 900 to 1020°C at an average heating rate of 5°C / sec or more in an atmosphere containing 10% by volume or more of nitrogen gas and hydrogen so as to absorb nitrogen from the annealing atmosphere. The material is held within this temperature range for 15 seconds or less, and then cooled to a temperature below 500°C at a rate of 1°C / sec or more.

[0010] These inventions are based on bright annealing, which is specific to stainless steel annealing, and are intended to be manufactured using general-purpose continuous annealing equipment for stainless steel (optical annealing furnaces), which are generally called BA finishes. On the other hand, ordinary steel strips are generally annealed using continuous annealing equipment called CAL (Continuous annealing Line), etc., and the atmosphere is generally 95% nitrogen and 5% hydrogen. Both BA annealing and CAL annealing use radiant heating using radiant tubes.

[0011] In the CAL annealing of ordinary steel, the use of a combined current heating or induction heating has been considered to improve productivity. For example, Patent Document 4 discloses a highly efficient method for producing cold-rolled steel sheets, which is characterized by installing an induction or current heating device in a part of a continuous annealing furnace that is heated by radiation heat, and partially increasing the annealing temperature at a temperature increase rate of 100°C / s or more, thereby changing the annealing temperature to a predetermined annealing temperature in a short time.

[0012] Furthermore, when applying such rapid heating, Patent Document 5 discloses a technique for obtaining a uniform heating rate in the longitudinal direction by combining a first heating device in the first heating zone that heats the steel strip to 500°C or higher and less than the Curie point Tc (°C) - 50°C, a solenoid coil type high-frequency induction heating device in the second heating zone that heats the steel strip heated in the first heating zone to a temperature range of the Curie point Tc - 30°C to the Curie point Tc - 5°C, and a third heating device in the third heating zone that heats the steel strip heated in the second heating zone to a processing target temperature above the Curie point. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 11-350088 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-194504 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-244691 [Patent Document 4] Japanese Patent Application Publication No. 7-97635 [Patent Document 5] Japanese Patent Application Laid-Open No. 2009-221578 Summary of the Invention [Problem to be solved by the invention]

[0014] Thus, even if the weight of stainless steel sheets for automobile exhaust system parts is reduced using the technologies disclosed so far, there is still room for further improvement in properties such as strength and wear resistance. On the other hand, if the stainless steel composition is adjusted to contain 0.02 to 0.05% N or 0.01 to 0.2% C in order to improve these properties, and a technique is applied in which nitrogen is absorbed during BA atmosphere annealing to form a martensite phase in the surface layer or to precipitate Cr carbides, sufficient corrosion resistance cannot be ensured in the environment to which automobile exhaust system parts are exposed.

[0015] In addition, BA annealing is unsuitable for annealing automotive exhaust system components due to its relatively low productivity and high cost. Furthermore, CAL, which is used to anneal ordinary steel strips, cannot prevent oxidation of stainless steel, making it impossible to absorb nitrogen into the surface layer. This issue is also true when CAL is combined with rapid heating equipment such as induction heating and resistance heating. Furthermore, induction heating up to near the Curie point is insufficient for annealing stainless steel because the temperature is too low.

[0016] An object of the present invention is to provide a ferritic stainless steel sheet having improved wear resistance without impairing the workability, corrosion resistance and high-temperature strength of the steel sheet, and a method for producing the same. [Means for solving the problem]

[0017] The present invention has been made to solve the above-mentioned problems, and is summarized as the following ferritic stainless steel sheet and method for producing the same.

[0018] (1) Chemical composition, in mass%, C: 0.01% or less, Si: 0.05 to 1.2%, Mn: 0.05 to 1.5% P: 0.035% or less, S: 0.01% or less, Cr: 10.5 to 20.0%, Ni: 0.01 to 0.60% Mo: 0.01 to 2.0%, Cu: 0.01 to 1.6% Al: 0.001 to 0.10%, N: 0.001 to 0.02%, Nb: 0.20 to 0.60%, Sn: 0 to 0.20% Co: 0 to 0.10% Ti: 0 to 0.05% V: 0~0.20%, Zr: 0 to 0.10% B: 0~0.0030%, The balance is Fe and impurities. A ferritic stainless steel sheet having an average nitrogen concentration of 0.05 to 0.10% in a region extending from the surface to 10 μm in the sheet thickness direction.

[0019] (2) The chemical composition is in mass%: Sn: 0.001 to 0.20%, Co: 0.001 to 0.10%, Ti: 0.005 to 0.05%, V: 0.005~0.20%, Zr: 0.005 to 0.10%, and B: 0.0005~0.0030%, The ferritic stainless steel sheet according to (1) above, containing one or more selected from the following:

[0020] (3) A cold-rolled sheet of ferritic stainless steel sheet having the chemical composition described in (1) or (2) is annealed in an atmosphere containing 2 to 10% or more of hydrogen and 80% or more of nitrogen, and in a first step, the sheet is heated from room temperature to a temperature range of 800 to 950°C at a heating rate of 20°C / s or less, in a second step, the sheet is heated from the temperature reached at the heating rate in the temperature range of 800 to 950°C to a temperature range of 1000 to 1100°C at a heating rate of 50°C / s or more, and in a third step, the sheet is held in the temperature range of 1000 to 1100°C for 4 to 60 seconds. A method for producing a ferritic stainless steel sheet, in which the average nitrogen concentration in the region from the surface to 10 μm in the sheet thickness direction is 0.05 to 0.10%. [Effects of the Invention]

[0021] The present invention makes it possible to obtain a ferritic stainless steel sheet having improved wear resistance without impairing the workability, corrosion resistance, and high-temperature strength of the steel sheet. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a graph showing the relationship between the average nitrogen concentration in the surface layer and the friction coefficient in a Bowden friction and wear test. [Figure 2]FIG. 2 is a graph showing the relationship between the rapid heating start temperature and the surface layer nitrogen concentration. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present inventors have investigated ways to increase the strength of ferritic stainless steel sheet without compromising its corrosion resistance, formability, and cost competitiveness, with the aim of reducing the weight and improving the productivity of automotive exhaust system parts and the like, and have obtained the following findings.

[0024] In the prior art, when the steel composition after annealing the cold-rolled sheet is particularly high in C and N content and low in Ti and / or Nb content, the workability and corrosion resistance of the steel sheet are reduced. Therefore, it is unsuitable for press forming or pipe-making. Furthermore, because the surface layer near the surface becomes austenite during annealing, it is less likely to become sensitized during cooling after annealing, and corrosion resistance is not impaired even after the austenite phase transforms to martensite.

[0025] On the other hand, because the inner layer is a ferrite phase, it is easily sensitized during annealing and cooling. If scratches are made on the steel sheet surface and the inner layer is exposed, the corrosion resistance of the cut edge of the steel sheet decreases. Therefore, it cannot be said to have sufficient properties as a material for automobile exhaust system parts. Therefore, even when increasing strength by forming a nitrogen-enriched layer on the surface, the material must have a reduced C and N content and contain stainless steel that contains the stabilizing elements Ti and Nb.

[0026] In addition, Ti and Nb are generally considered to have similar additive effects as stabilizing elements. When concentrating nitrogen in the surface layer of steel sheet, both Ti and Nb form nitrides, which improve wear resistance. However, Ti nitrides tend to adhere to the Ti carbonitride coating layer applied to press dies to improve their wear resistance, increasing the friction coefficient and degrading formability. Therefore, when concentrating nitrogen in the surface layer of steel sheet to harden the surface, Nb should be the main stabilizing element, and when Ti is added, the upper limit should be kept to 0.05%.

[0027] The inventors have conducted various studies to increase the surface hardness of various stainless steels for automobile exhaust systems by concentrating nitrogen by absorbing it into the surface layer of the steel sheet during CAL annealing. In order to maintain corrosion resistance compared to current materials, it is necessary to reduce the C and N contents in the steel composition, and this is premised on a chemical composition in which the stabilizing elements Ti, Nb, Zr, or a combination thereof is 8 × (C% + N%) to 0.80%.

[0028] Furthermore, the present inventors have found that although Ti, Nb, and Zr are all considered to have the same corrosion resistance, the effect of improving wear resistance due to nitride formation when nitrogen is concentrated in the surface layer of a steel sheet is not uniform, and that Ti and Zr in particular have almost no effect. This is due to the solid solution strengthening and precipitation strengthening abilities of Ti, Nb, and Zr, and Nb has the highest strengthening ability when nitrogen is concentrated in the surface layer of a steel sheet.

[0029] Furthermore, methods for concentrating nitrogen by absorbing it into the surface layer of steel sheets are difficult to achieve using conventional CAL annealing, and when using equipment that combines induction heating, the oxide film that forms on the surface of stainless steel sheets prevents sufficient absorption and concentration of nitrogen. Therefore, a new heat pattern was devised: (A) to suppress the formation of Cr oxide, which inhibits nitrogen absorption and concentration, the heating rate in the temperature range where an iron oxide film forms is reduced to form iron oxide, (B) the temperature range where Cr oxide forms is rapidly heated, and (C) by maintaining the temperature in the nitrogen absorption and concentration range, the iron oxide is reduced and nitrogen is absorbed and concentrated in the surface layer of the steel sheet.

[0030] The Curie point of stainless steel is around 700°C, but in order to form an iron oxide film, it is ideal to heat the steel from room temperature to a temperature range of 800-950°C at a heating rate of 20°C / s or less.From the temperature reached at this heating rate, the steel is rapidly heated to a temperature range of 1000-1100°C at a heating rate of 50°C / s or more and 200°C / s or less using induction heating or resistance heating, and then held at a temperature range of 1000-1100°C for 4-60 seconds, which allows nitrogen to be absorbed into the surface layer of the steel sheet and concentrate the nitrogen to the required nitrogen concentration.

[0031] To improve the surface wear resistance and maintain corrosion resistance and press workability, the nitrogen concentration in the surface 10 μm of the steel sheet must be 0.10% or less, and the average nitrogen content across the entire sheet thickness must be 0.020% or less. Furthermore, elements that significantly affect the composition of the surface oxide film, such as Cr, Si, Al, and Ti, must also be controlled within appropriate composition ranges.

[0032] When annealing cold-rolled steel, controlling the oxide film that forms on the steel surface is important for ensuring stable nitrogen absorption and enrichment in the steel surface. Bright annealing, a common method for annealing stainless steel, uses a high hydrogen content and low dew point in the atmosphere, resulting in the formation of a silicon oxide film on the steel surface, which inhibits nitrogen absorption. To prevent this, the annealing atmosphere must be primarily nitrogen, with a hydrogen content of 2-10%.

[0033] The atmosphere used in CAL annealing of ordinary steel is primarily nitrogen and contains approximately 5% hydrogen, making it suitable for nitrogen absorption in the steel sheet surface. However, this atmosphere promotes Cr oxidation and is therefore unsuitable for nitrogen absorption in the steel sheet surface. Rapid heating is therefore required to suppress Cr oxide film formation. However, rapid heating below the Curie point does not prevent Cr oxide film formation. Therefore, rapid heating from the temperature reached at a heating rate of 20°C / s or less to a temperature range of 1000 to 1050°C is required. However, although this rapid heating significantly suppresses Cr oxide film formation, nitrogen absorption in the steel sheet surface may be hindered due to the state of the passive film in the cold-rolled steel sheet and the influence of deoxidizing elements such as Al and Si.

[0034] We therefore discovered a method in which an iron oxide film is first formed during heating and then reduced during rapid heating. Specifically, during annealing, an iron oxide film can be formed on the surface by heating from room temperature to a temperature range of 800–950°C at a heating rate of 20°C / s or less. Subsequently, rapid heating from the temperature reached by this heating rate to a temperature range of 1000–1050°C at a heating rate of 50°C / s or more reduces the Fe oxide film and suppresses the formation of oxide films of Si, Cr, and Al. This promotes absorption and concentration of nitrogen in the surface layer of the steel sheet during annealing at 1000–1100°C, resulting in the precipitation of Nb nitrides. This results in a stainless steel sheet with high surface hardness and excellent wear and corrosion resistance without compromising workability, making it suitable for automotive exhaust system components.

[0035] One embodiment of the present invention has been made based on the above findings, and has found an optimum chemical composition balance and a surface strengthening method for ferritic stainless steel for the intended use.

[0036] 1.Chemical composition The reasons for limiting the amount of each element are explained below. In the following explanation, "%" indicating the content of each element means "% by mass" unless otherwise specified.

[0037] C: 0.01% or less Solute C in the matrix forms Cr carbides, which causes sensitization and reduces corrosion resistance. Therefore, it is necessary to include stabilizing elements, namely Nb, Ti, V, and Zr, in amounts commensurate with the C content. Increasing the content of these elements increases alloy costs. Furthermore, after annealing in a nitrogen atmosphere, martensite forms on the surface of the steel, degrading workability. Therefore, the C content should be 0.01% or less. The C content is preferably 0.008% or less.

[0038] However, carbon is an element that is incorporated into molten steel during the smelting of iron ore. While most of the carbon can be removed by converter refining or the degassing process, this increases the refining time, impairing productivity, and also causes Cr to be oxidized, reducing alloy yield. For this reason, the carbon content is preferably 0.001% or more. Considering the balance of strength and ductility required for sheet products, the carbon content is preferably 0.002 to 0.006%.

[0039] Si: 0.05 to 1.2% Silicon is necessary for deoxidation during melting and refining, and is also effective in suppressing the formation of oxide scale during hot-rolling heating. Therefore, the Si content is set to 0.05% or more. To ensure the oxidation resistance required for automobile exhaust system parts, the Si content is preferably set to 0.1% or more.

[0040] However, excessive Si content reduces the ductility of the steel sheet due to solid solution strengthening. Therefore, the Si content is set to 1.2% or less. Furthermore, Si forms an Si oxide film during annealing of the cold-rolled sheet, which inhibits absorption of N into the steel sheet surface and reduces wear resistance. Therefore, the Si content is preferably set to 0.6% or less.

[0041] Mn: 0.05 to 1.5% Mn is an element contained as a deoxidizer and also contributes to increasing high-temperature strength in the medium temperature range. Furthermore, Mn-based oxides are formed on the surface during long-term use, contributing to the adhesion of scale (oxides) and the suppression of abnormal oxidation. Therefore, the Mn content is set to 0.05% or more.

[0042] However, excessive Mn content not only reduces the toughness of the hot-rolled sheet due to the precipitation of the γ phase (austenite phase), but also reduces corrosion resistance by forming MnS. Furthermore, oxidation resistance is reduced and a thick iron oxide film is formed during annealing, inhibiting nitrogen absorption and deteriorating wear resistance. Therefore, the Mn content is set to 1.5% or less. Considering high-temperature ductility, scale adhesion, and the prevention of abnormal oxidation, the Mn content is preferably 0.2 to 1.0%.

[0043] P:0.035% or less P is an element contained as an impurity in the raw materials, such as molten pig iron and alloys such as ferrochrome. Because it is an element that is harmful to hot workability and toughness, the P content is set to 0.035% or less. Because P also reduces workability, it is preferable to set it to 0.030% or less. Furthermore, excessive reduction of P requires the use of high-purity raw materials, which leads to increased costs, so the P content is preferably set to 0.010% or more.

[0044] S: 0.01% or less S is an element that has a small amount of solid solubility in the austenite phase and segregates at grain boundaries, promoting a decrease in hot workability. If the S content exceeds 0.01%, the effect becomes significant, so the S content is set to 0.01% or less. The lower the S content, the fewer sulfide-based inclusions there are and the better the corrosion resistance, but excessive reduction of S increases the desulfurization load and increases manufacturing costs. For this reason, the S content is preferably set to 0.001% or more. The S content is preferably set to a range of 0.001 to 0.008%.

[0045] Cr: 10.5 to 20.0% Cr is an essential element for ensuring oxidation resistance and corrosion resistance. Therefore, the Cr content is set to 10.5% or more. However, excessive Cr content leads to reduced workability and toughness. Furthermore, a Cr oxide film is formed during annealing heating, which makes it difficult for nitrogen to be absorbed on the steel surface and reduces wear resistance. For this reason, the Cr content is set to 20.0% or less. In addition, taking into consideration the crevice corrosion resistance and nitrogen absorption performance of the surface layer due to the structure of exhaust system parts, the Cr content is preferably set to 13.5 to 19.0%.

[0046] Ni: 0.01 to 0.60% Ni is an element that is mixed as an inevitable impurity in the alloy raw materials of ferritic stainless steel, but it is an element that is effective in suppressing the progression of pitting corrosion, and this effect is stably exhibited at a content of 0.01% or more. Therefore, the Ni content is set to 0.01% or more. However, excessive Ni content may lead to material hardening due to solid solution strengthening. Therefore, the Ni content is set to 0.60% or less. In consideration of alloy cost, the Ni content is preferably set to the range of 0.05 to 0.40%.

[0047] Mo: 0.01 to 2.0% Mo, like Cu, has the effect of suppressing active dissolution and inhibiting the progression of pitting corrosion, making it an effective element for improving corrosion resistance. Therefore, the Mo content is set to 0.01% or more. To obtain higher corrosion resistance, the Mo content is preferably set to 0.03% or more. However, excessive Mo content increases strength through solid solution strengthening, impairing press formability. Therefore, the Mo content is set to 2.0% or less. To achieve both corrosion resistance and workability, the Mo content is preferably set to 1.5% or less.

[0048] Cu: 0.01 to 1.6% Cu is often unavoidably contained, such as through contamination from scrap during melting. However, excessive reduction of the Cu content using high-purity reducing agents can promote active dissolution during pit growth, impairing corrosion resistance. Therefore, the Cu content is set to 0.01% or more. To further improve corrosion resistance, the Cu content is preferably set to 0.03% or more. Although Cu is sometimes intentionally added to improve high-temperature strength, excessive Cu content reduces hot workability and corrosion resistance. Furthermore, Cu-rich clusters are formed during the cooling process after annealing, degrading workability. For this reason, the Cu content is set to 1.6% or less. Considering the reduction in corrosion resistance due to Cu precipitation, the Cu content is preferably set to 1.2% or less.

[0049] Al: 0.001 to 0.10% Al is an effective element for deoxidation, and this effect is manifested when the Al content is 0.001% or more. Therefore, the Al content is set to 0.001% or more. To obtain the deoxidation effect of the combination of Si and Mn, the Al content is preferably set to 0.005% or more. However, Al increases the basicity of the slag, which may precipitate water-soluble inclusions (CaS) in the steel, reducing corrosion resistance. For this reason, the Al content is set to 0.10% or less. Furthermore, considering the reduction in polishability due to alumina-based non-metallic inclusions, the Al content is preferably set to 0.01% or less.

[0050] N: 0.001 to 0.02% N is an element that forms Cr nitrides at grain boundaries, causing sensitization and resulting in a decrease in corrosion resistance. While N content can be reduced by degassing in a vacuum, prolonged degassing is difficult due to the drop in molten steel temperature. Therefore, there is an industrial limit to how much N content can be reduced. Therefore, the N content is set to 0.001% or more. However, excessive N content reduces workability and corrosion resistance, and the content of stabilizing elements must be increased. Therefore, the N content is set to 0.02% or less. To ensure workability, corrosion resistance, and wear resistance, the N content is preferably set to a range of 0.008 to 0.015%.

[0051] Nb: 0.20 to 0.60% Nb is an element that improves high-temperature strength and thermal fatigue properties, and also improves wear resistance without impairing workability when nitrogen is absorbed in the surface layer. To obtain the high-temperature strength required for exhaust system components, the Nb content is set to 0.20% or more. From the viewpoints of wear resistance and corrosion resistance, the Nb content is preferably set to 0.30% or more. However, excessive Nb content generates Laves phases when used as exhaust system components, impairing the solid-solution strengthening ability at high temperatures. It also reduces workability. For this reason, the Nb content is set to 0.60% or less. To ensure surface hardness, corrosion resistance, and workability after nitrogen absorption, the Nb content is preferably set to 0.45% or less.

[0052] In addition to the above elements, one or more elements selected from groups A to C may be contained.

[0053] Group A elements Sn: 0 to 0.20% Like Mo and Cu, Sn has the effect of suppressing the progression of pitting corrosion and thereby enhancing corrosion resistance. Therefore, Sn may be added as needed. However, it is known that Sn concentrates below the oxide scale and causes hot rolling cracks and defects. Furthermore, long-term aging at 400 to 700°C can reduce the toughness of the steel. For this reason, the Sn content is set to 0.20% or less. On the other hand, to obtain the above effect, the Sn content is preferably set to 0.001% or more.

[0054] Co: 0 to 0.10% Co, like Nb, Mo, and Cu, has the effect of increasing high-temperature strength. Therefore, it may be added as needed. However, Co is a relatively expensive element and also delays the precipitation of Nb nitrides, which improve wear resistance, in the nitrogen-enriched layer on the steel sheet surface. Therefore, the Co content is preferably 0.10% or less. On the other hand, to obtain the above effect, the Co content is preferably 0.001% or more.

[0055] Group B elements Ti: 0 to 0.05% Ti, together with Nb, bonds with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room-temperature ductility, and deep drawability. Therefore, Ti may be added as needed. The Ti content is determined based on the economically achievable reduction amounts of C, N, and S and the Nb content. However, assuming nitrogen absorption in the steel sheet of this embodiment, excessive Ti impairs formability due to adhesion to the press die. Therefore, the Ti content is set to 0.05% or less. Since Nb is a more expensive element than Ti, adding Nb alone as a stabilizing element is preferable, and Ti is added as a supplement to Nb within a range that does not impair press formability. Therefore, the Ti content is preferably set to 0.005% or more. Furthermore, since Ti has a higher sulfide system performance than Nb and is an effective element for suppressing pitting corrosion, the Ti content is preferably set to 0.01% or more.

[0056] V: 0 to 0.20% V has the effect of acting as a stabilizing element for forming carbonitrides. Therefore, it may be contained as needed. However, excessive V content may promote the formation of coarse carbides due to solidification segregation, which may reduce ductility and toughness. For this reason, the V content is set to 0.20% or less. On the other hand, to obtain the above effect, the V content is preferably set to 0.005% or more.

[0057] Zr: 0 to 0.10% Zr has the effect of acting as a stabilizing element for forming carbonitrides. Therefore, it may be contained as needed. However, excessive Zr content may promote the formation of coarse carbides due to solidification segregation, which may reduce ductility and toughness. For this reason, the Zr content is set to 0.10% or less. On the other hand, to obtain the above effect, the Zr content is preferably set to 0.005% or more.

[0058] Group C elements B: 0 to 0.0030% B has the effect of increasing grain boundary strength through grain boundary segregation and improving secondary workability. Therefore, it may be added as needed. However, if B is added in excess, Cr2B, (Cr, Fe) 23 The precipitation of (C, B)6 impairs toughness and corrosion resistance. Therefore, the B content is set to 0.0030% or less. On the other hand, in order to obtain the above effects, the B content is preferably set to 0.0005% or more.

[0059] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial production of ferritic stainless steel sheet 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.

[0060] 2. Average nitrogen concentration in the surface layer In order to enhance wear resistance by absorbing and concentrating nitrogen in the surface layer of the steel sheet during the annealing process of the cold-rolled sheet, the average nitrogen concentration in the region from the steel sheet surface to 10 μm in the sheet thickness direction in the surface layer of the steel sheet after annealing and pickling (hereinafter referred to as "average surface layer nitrogen concentration") is set to 0.05% or more. When used at high temperatures, such as in exhaust manifolds, a decrease in the average nitrogen concentration due to nitrogen diffusion is expected, so the average surface layer nitrogen concentration is preferably 0.06% or more.

[0061] However, if the surface layer average nitrogen concentration is excessively high, sensitization may occur due to the formation of Cr nitrides. Therefore, the surface layer average nitrogen concentration is set to 0.10% or less. Since the critical nitrogen amount for sensitization varies depending on the Cr, Mo, or Ti content, the surface layer average nitrogen concentration is preferably set to 0.08% or less. The above-mentioned surface layer average nitrogen concentration is also expressed in mass%.

[0062] The average surface nitrogen concentration is determined by analyzing the N concentration in the depth direction of the plate using a glow discharge optical emission spectrometer (GDS) and calculating the average value.

[0063] 3. Manufacturing method The ferritic stainless steel sheet according to this embodiment can be stably produced, for example, by the following production method.

[0064] A stainless steel sheet having the above-described chemical composition can be obtained by hot rolling, pickling, cold rolling, annealing under known conditions in the atmosphere and heat pattern described below, and then going through the steps of temper rolling and pickling to obtain a stainless steel sheet having a thickness of 1.0 to 2.5 mm and an average surface layer nitrogen concentration of 0.05 to 0.10%. Temper rolling may be performed before pickling. The steel sheet produced under the preferred production conditions of this embodiment may be welded and made into a stainless steel pipe for use in exhaust system components. The annealing conditions for the cold-rolled sheet are specifically described below.

[0065] It is preferable to anneal the cold-rolled steel sheet obtained by cold rolling. The annealing step is a heat treatment step in which the steel sheet is held at a temperature range of 1000 to 1100°C for 4 to 60 seconds, and the annealing atmosphere is controlled as follows. In addition, for the steel sheet of this embodiment, the heating rate in a specific temperature range up to annealing is also controlled. This will be specifically explained below.

[0066] <Annealing atmosphere> To achieve the above-mentioned range of average nitrogen concentration in the surface layer, the atmosphere during annealing of the cold-rolled sheet is a nitrogen atmosphere containing mainly nitrogen and 2 to 10% hydrogen. If the hydrogen content is less than 2%, the oxide film thickness increases due to the additional oxygen or water vapor mixed in, inhibiting nitrogen absorption. Therefore, the amount of hydrogen in the atmosphere is set to 2% or more.

[0067] However, in an atmosphere containing more than 10% hydrogen, such as in BA annealing, a silicon oxide film forms on the surface of the steel sheet, inhibiting the absorption and concentration of nitrogen in the surface of the steel sheet. Therefore, the amount of hydrogen in the atmosphere is set to 10% or less. To stabilize the absorption and concentration of nitrogen in the surface of the steel sheet, the amount of hydrogen in the atmosphere is preferably set to a range of 5 to 7%. The gas other than hydrogen is primarily nitrogen, and specifically, it is sufficient for the nitrogen content to be 80% or more, preferably 90% or more. In addition to hydrogen and nitrogen, elements such as Ar, He, O2, and CO may also be contained. The % for each gas in the atmosphere indicates volume %.

[0068] <Heating and holding conditions> In the steel sheet of this embodiment, the heating and holding during annealing are controlled in the following three stages. Specifically, the heating rate in the temperature range from room temperature to 800 to 950°C is 20°C / s or less (first stage). Furthermore, the heating rate from the temperature reached at the heating rate of 20°C / s or less in the temperature range of 800 to 950°C to the temperature range of 1000 to 1100°C is 50°C / s or more (second stage). Then, the steel sheet is held in the temperature range of 1000 to 1100°C for 4 to 60 seconds (third stage).

[0069] In the first stage, i.e., in the temperature range from room temperature to 800 to 950°C, the heating rate is set to 20°C / s or less to raise the temperature. If the heating rate in the first stage exceeds 20°C / s, an iron oxide film is not formed on the surface, and even if the temperature is raised to 1000°C or higher, the necessary nitrogen absorption layer is not formed. In order to grow the surface nitrogen absorption layer to a sufficient thickness, it is preferable to set the heating rate in the first stage to 10°C / s or less.

[0070] Next, in the second stage, the material is rapidly heated from the temperature reached at a heating rate of 20°C / s or less in the temperature range of 800 to 950°C to a temperature range of 1000 to 1100°C at a heating rate of 50°C / s or more. This is because rapid heating in the second stage can suppress the formation of a Cr oxide film. A faster heating rate in the second stage is preferable from the viewpoint of suppressing the formation of a Cr oxide film, and a heating rate of 100°C / s or more is more preferable. On the other hand, if the heating rate is too fast, there is a risk of grain coarsening due to overheating and material variations due to temperature non-uniformity. Therefore, a heating rate of 200°C / s or less is preferable.

[0071] The subsequent third stage, i.e., in the temperature range of 1000 to 1100°C, is held for 4 to 60 seconds. This is because the third stage requires reducing the oxide film on the steel sheet surface and absorbing and concentrating nitrogen in the steel sheet surface. If the holding temperature is less than 1000°C, recrystallization is not completed, resulting in a sheet with high strength and low ductility. On the other hand, if the holding temperature exceeds 1100°C, coarse grains are formed, causing roughness of the processed surface, which is undesirable. Furthermore, if the holding time is less than 4 seconds, a uniform grain size cannot be obtained after recrystallization. On the other hand, holding for a long period of time, such as more than 60 seconds, causes roughness of the processed surface due to coarse grains.

[0072] Following the third stage of holding, the steel sheet is cooled to room temperature, but since no phase transformation occurs and the precipitation of precipitates during the cooling process is slow, rapid cooling is not necessary. For this reason, the cooling rate after holding is preferably in the range of 2 to 50°C / s.

[0073] The present inventors conducted the following preliminary experiments in order to examine the conditions for the first to third stages.

[0074] Cold-rolled ferritic stainless steel sheets with a thickness of 1.2 mm and the chemical composition shown in Table 1 were experimentally produced in the laboratory. These steel sheets were annealed and pickled under the conditions shown in Table 2 to prepare test materials. The annealing atmosphere was 95% nitrogen and 5% hydrogen by volume. In the first stage of annealing the cold-rolled sheets, the temperature was raised from 23°C to 800–950°C at a heating rate of 3–5°C / s. In the second stage, the rapid heating start temperature was 23–950°C, the rapid heating end temperature was 1000–1100°C, and the rapid heating rate was 100°C / s. In the third stage, the sheets were held between 1000–1100°C for 6–40 s, then cooled to 200°C at 20°C / s, and then allowed to cool naturally. The annealed sheets were pickled, and the nitrogen concentration in the surface layer was analyzed using a glow discharge optical emission spectroscopy (GDS). Bowden tests were performed to measure the dynamic friction coefficient.

[0075] [Table 1]

[0076] [Table 2]

[0077] The relationship between the average nitrogen concentration in the surface layer and the dynamic friction coefficient is shown in Figure 1. As the average nitrogen concentration in the surface layer increased, the dynamic friction coefficient decreased. The target wear resistance of the dynamic friction coefficient of 0.10 or less was achieved when the nitrogen concentration in the surface layer was 0.05% or more. However, with 0.15% Ti steel or 0.01% Ti-0.01% Nb steel, the friction coefficient exceeded 0.10 even when the average nitrogen concentration in the surface layer was 0.05% or more.

[0078] The effect of the rapid heating start temperature on the average nitrogen concentration in the surface layer of 0.40% Nb steel is shown in Figure 2. When rapid heating is performed from a temperature below 800°C, the average nitrogen concentration in the surface layer is less than 0.05%, but when the rapid heating start temperature is 800°C or higher, the average nitrogen concentration in the surface layer becomes 0.05% or higher.

[0079] The ferritic stainless steel sheet of this embodiment will be described in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0080] Steel sheets with a thickness of 1.2 mm and having the chemical composition shown in Table 3 were manufactured through ordinary hot rolling and cold rolling processes, and then annealed under the annealing conditions shown in Table 4, followed by pickling to obtain ferritic stainless steel sheets. The ferritic stainless steel sheets thus obtained were analyzed by GDS for the average nitrogen concentration in the surface layer in the thickness direction.

[0081] The resulting ferritic stainless steel sheets were also evaluated for their wear resistance by measuring the coefficient of dynamic friction using a Bowden friction and wear test (hereinafter simply referred to as the "Bowden test"). A dynamic friction coefficient of 0.10 or less was judged to have good wear resistance and passed the test. On the other hand, a dynamic friction coefficient of more than 0.10 was judged to have poor wear resistance and failed the test. The Bowden test involved applying G364 oil manufactured by Nippon Kogyo Oil to the surface of the steel sheet, and using a TiN-coated steel ball with a diameter of 10 mm, sliding the ball over a sliding distance of 100 mm at a load of 1 kgf 10 times to determine the average value of the dynamic friction coefficient.

[0082] The obtained ferritic stainless steel sheets were subjected to a tensile test in accordance with JIS Z 2241:2011 to measure elongation and evaluate workability. When the elongation was 20% or more, the workability was judged to be good and the sheet was rated as passing. On the other hand, when the elongation was less than 20%, the workability was judged to be poor and the sheet was rated as failing.

[0083] In addition, a high-temperature tensile test was conducted in accordance with JIS G 0567:2020 to measure the tensile strength at 700°C, and the high-temperature strength was evaluated. The tensile strength at 700°C was 140N / mm 2 On the other hand, if the tensile strength at 700°C is 140N / mm 2 If the value was below the above, the workability was judged to be poor and the sample was deemed to have failed.

[0084] A salt spray test was conducted in accordance with JIS Z 2371:2015 to check for the occurrence of rust and evaluate corrosion resistance. After 24 hours of testing, specimens with no rust were deemed to have the best corrosion resistance and were given a rating of A. After 24 hours of testing, specimens with spot rust were deemed to have relatively good corrosion resistance and were given a rating of B. Furthermore, after 24 hours of testing, specimens with flowing rust were deemed to have poor corrosion resistance and were given a rating of C. Here, the above ratings of A and B were deemed to be passing.

[0085] [Table 3]

[0086] [Table 4]

[0087] S1 to S30, which satisfy all the requirements of this embodiment, exhibited good wear resistance, processability, high-temperature strength and corrosion resistance.

[0088] On the other hand, R1 to R19, which do not satisfy the requirements of this embodiment, resulted in a decrease in at least one of the properties of wear resistance, workability, high-temperature strength, and corrosion resistance. In particular, R1 to R7 did not satisfy the manufacturing conditions for preferable annealing conditions, so the average nitrogen concentration in the surface layer was less than 0.05%, and the friction coefficient exceeded 0.10, resulting in poor wear resistance. R8 had a C content of less than 0.001% and an N content of less than 0.001%, so the tensile strength was 450 N / mm 2 The S content was less than 0.010%, resulting in a decrease in high-temperature strength. In addition, the corrosion resistance was poor because the S content exceeded 0.010%. In R9, the C content exceeded 0.01%, resulting in a decrease in corrosion resistance.

[0089] R10 had a Si content of less than 0.05%, which resulted in low oxidation resistance, resulting in the formation of a thick iron oxide film during annealing. This resulted in insufficient subsequent reduction, preventing nitrogen absorption and resulting in a high friction coefficient. Furthermore, the P content exceeded 0.035%, resulting in reduced elongation due to solid solution strengthening by P. R11 had a Si content of more than 1.2%, resulting in high oxidation resistance, resulting in the formation of a Si oxide film during annealing, preventing nitrogen absorption and resulting in a high friction coefficient.

[0090] R12 had a Mn content of less than 0.05%, which resulted in insufficient deoxidation and the presence of many oxide-based inclusions, resulting in a high friction coefficient and reduced high-temperature strength.

[0091] R13 had a manganese content of over 1.5%, which reduced oxidation resistance and caused a thick iron oxide film to form during annealing, inhibiting nitrogen absorption and resulting in a high friction coefficient. R14 had a chromium content of less than 10.5%, which resulted in poor corrosion resistance. R15 had a chromium content of over 20.0%, which prevented nitrogen absorption due to the protective effect of the chromium oxide film during annealing, resulting in a high friction coefficient.

[0092] In R16, the Ni content exceeded 0.6%, resulting in reduced elongation due to solid solution strengthening by Ni. Furthermore, the Nb content was less than 0.2%, resulting in reduced high-temperature strength. In R17, the Mo content was less than 0.01% and the Cu content was less than 0.01%, resulting in accelerated pitting corrosion and reduced corrosion resistance. In R18, the Cu content exceeded 1.60%, resulting in the formation of fine Cu-rich clusters during the cooling process after annealing the cold-rolled sheet, resulting in reduced elongation.

[0093] R19 had a Ni content of less than 0.01%, which led to accelerated pitting corrosion and reduced corrosion resistance. Furthermore, because the Nb content exceeded 0.60%, the precipitation of Nb-containing intermetallic compounds and solid solution strengthening by Nb reduced elongation. [Industrial Applicability]

[0094] According to the present invention, it is possible to efficiently produce stainless steel sheets that have high wear resistance and excellent workability and corrosion resistance. Therefore, the present invention contributes to reducing the weight and extending the life of automobile exhaust system parts, fuel system parts, structural members, etc. Furthermore, for automobile exhaust system parts, which could not be reduced in thickness due to the low hardness characteristic of ferritic stainless steel, it is now possible to reduce the thickness and weight, which has a significant economic effect.

Claims

1. The chemical composition, in mass%, is C: 0.01% or less, Si: 0.05-1.2%, Mn: 0.05-1.5%, P: 0.035% or less, S: 0.01% or less, Cr: 10.5-20.0%, Ni: 0.01 to 0.60%, Mo: 0.01-2.0%, Cu: 0.01-1.6%, Al: 0.001-0.10%, N: 0.001-0.02%, Nb: 0.20-0.60%, Sn: 0-0.20%, Co: 0 to 0.10%, Ti: 0 to 0.05%, V: 0-0.20%, Zr: 0 to 0.10%, B: 0 to 0.0030%, The balance is Fe and impurities. A ferritic stainless steel plate having an average nitrogen concentration of 0.05 to 0.10% in a region extending from the surface to 10 μm in the plate thickness direction.

2. The chemical composition is, in mass %, Sn: 0.001-0.20%, Co: 0.001 to 0.10%, Ti: 0.005 to 0.05%, V: 0.005-0.20%, Zr: 0.005 to 0.10%, and B: 0.0005-0.0030%, The ferritic stainless steel sheet according to claim 1, comprising one or more selected from the following:

3. A cold-rolled sheet of ferritic stainless steel sheet having the chemical composition according to claim 1 or 2 is annealed in an atmosphere containing 2 to 10% or more of hydrogen and 80% or more of nitrogen, and in a first step, the sheet is heated from room temperature to a temperature range of 800 to 950 ° C at a heating rate of 20 ° C / s or less, in a second step, the sheet is heated from the temperature reached at the heating rate in the temperature range of 800 to 950 ° C to a temperature range of 1000 to 1100 ° C at a heating rate of 50 ° C / s or more, and in a third step, the sheet is held in the temperature range of 1000 to 1100 ° C for 4 to 60 seconds. A method for producing a ferritic stainless steel sheet, in which the average nitrogen concentration in the region from the surface to 10 μm in the sheet thickness direction is 0.05 to 0.10%.

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