US20180016655A1 - Ferritic stainless steel for exhaust system member having excellent corrosion resistance after heating - Google Patents

Ferritic stainless steel for exhaust system member having excellent corrosion resistance after heating Download PDF

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US20180016655A1
US20180016655A1 US15/544,476 US201615544476A US2018016655A1 US 20180016655 A1 US20180016655 A1 US 20180016655A1 US 201615544476 A US201615544476 A US 201615544476A US 2018016655 A1 US2018016655 A1 US 2018016655A1
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stainless steel
ferritic stainless
corrosion resistance
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Nobuhiko Hiraide
Hiroshi Urashima
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Nippon Steel Stainless Steel Corp
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Nippon Steel and Sumikin Stainless Steel Corp
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    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a ferritic stainless steel excellent in corrosion resistance after heating and suitable for use in an exhaust system component for an automobile, a motor cycle, a commercial vehicle and a construction machine, an exhaust system component and a manufacturing method thereof. Specifically, the present invention relates to a ferritic stainless steel adapted to be heated to 573 to 1073 K to be used with an oxide film being formed on a surface thereof.
  • Ferritic stainless steel is often used for exhaust system components of an automobile, a motor cycle, a commercial vehicle, a construction machine and the like.
  • downstream exhaust system components are often made of SUH409L steel (a steel containing C and N fixed by Ti and 11% of Cr), SUS430LX steel (a steel containing C and N fixed by Ti and 17% of Cr), and SUS436J1L and SUS436L further added with Mo, in terms of corrosion resistance, formability and weldability.
  • the corrosion resistance herein includes corrosion resistance against condensed water of exhaust gas on an interior surface and corrosion resistance against salt-induced corrosion on an exterior surface.
  • reduction in lifetime resulting in breakage due to local thickness reduction and generation of through pit(s) causing leakage of exhaust gas are of problem. Accordingly, pitting resistance bears a high importance in corrosion resistance.
  • degradation in appearance due to generation of rust has recently been seen as a problem.
  • Patent Literature 1 discloses a stainless steel sheet with improved crevice corrosion resistance, the stainless steel containing 0.015% or less of C, 0.02% or less of N, 1.0% or less of Si, more than 0.6% to 3.0% of Ni, 16.0 to 25.0% of Cr, optionally one or both of 3.0% or less of Mo and 2.0% or less of Cu as necessary, and one or more of 2.0% or less of Mn, 0.5% or less of Ti, 0.5% or less of Nb, 0.5% or less of Al and 0.01% or less of B, where a matrix with restricted amount of 0.04% or less of P and 0.02% or less of S exhibits a ferrite single-phase texture.
  • Patent Literature 2 discloses a ferritic stainless steel that is excellent in crevice corrosion resistance, the ferritic stainless steel containing 0.001 to 0.02% of C, 0.001 to 0.02% of N, 0.01 to 0.3% of Si, 0.05 to 1% of Mn, 0.04% or less of P, 0.15 to 2% of Ni, 11 to 22% of Cr, 0.01 to 0.5% of Ti, and one or more of 0.5 to 3.0% of Mo, 0.02 to 0.6% of Nb and 0.1 to 1.5% of Cu in an amount satisfying Cr+3Mo+6(Ni+Nb+Cu) ⁇ 22.
  • Both of Patent Literatures 1 and 2 relate to a stainless steel containing Ni to provide improved crevice corrosion resistance, where corrosion growth speed is restrained to enhance the pitting resistance. However, nothing is disclosed on the corrosion resistance when an oxide film is formed by heating.
  • Patent Literature 3 discloses a ferritic stainless steel containing 0.0010 to 0.30% of C, 0.0010 to 0.050% of N, 0.01 to 1.0% of Si, 0.01 to 1.0% of Mn, 0.04% or less of P, 0.010% or less of S, 1.0% or less of Ni, 10.0 to 30.0% of Cr, 0.010% or less of 0, 0.005 to 0.10% of one or both of Sn and Sb, and, optionally, 0.0050 to 0.5% of Ti and/or 0.01 to 1.0% of Nb as necessary.
  • the presence of one or both of Sn and Sb prevents grain boundary segregation of P to restrain surface flaws caused due to intergranular corrosion during sulfuric acid pickling.
  • Patent Literature 4 discloses a manufacturing method of a steel plate containing high-purity Cr that is excellent in pressing formability, the steel plate containing 0.02% or less of C, 0.02% or less of N, 3 to 30% of Cr, and one or both of Ti and Nb in an amount satisfying (Ti+Nb)/(C+N) ⁇ 8, where a ferrite particle diameter of a cast product and a winding temperature during a hot rolling step are defined in predetermined ranges. It is also disclosed that 0.5% or less of Sn content is effective in order to restrain intergranular corrosion caused by Cr carbonitride.
  • Patent Literature 5 discloses a ferritic stainless steel that is excellent in crevice corrosion resistance, the ferritic stainless steel containing 0.001 to 0.02% of C, 0.001 to 0.02% of N, 0.01 to 0.5% of Si, 0.05 to 1% of Mn, 0.04% or less of P, 0.01% or less of S, 12 to 25% of Cr, one or both of 0.02 to 0.5% of Ti and 0.02 to 1% of Nb, and one or both of 0.005 to 2% of Sn and 0.005 to 1% of Sb.
  • Patent Literature 5 relates to a stainless steel containing Sn and/or Sb to provide improved crevice corrosion resistance, where corrosion growth rate is inhibited to enhance the pitting resistance.
  • Patent Literatures 3 to 5 nothing is disclosed in Patent Literatures 3 to 5 on the corrosion resistance under the circumstances that an oxide film is formed by heating.
  • Patent Literature 6 discloses an alloy-saving ferritic stainless steel for an automobile exhaust system component that is excellent in corrosion resistance after heating, the ferritic stainless steel containing 0.015% or less of C, 0.015% or less of N, 0.10 to 0.50% of Si, 0.05 to 0.50% of Mn, 0.050% or less of P, 0.0100% or less of S, 10.5 to 16.5% of Cr, one or both of 0.03 to 0.30% of Ti and 0.03 to 0.30% of Nb, and one or both of 0.03 to 0.50% of Sn and 0.03 to 0.50% of Sb in an amount satisfying Cr+Si+0.5Mn+10A1+15(Sn+Sb) ⁇ 13.
  • Patent Literature 7 discloses an Mo-saving ferritic stainless steel for an automobile exhaust system component that is excellent in corrosion resistance after heating, the ferritic stainless steel containing 0.015% or less of C, 0.015% or less of N, 0.01 to 0.50% of Si, 0.01 to 0.50% of Mn, 0.050% or less of P, 0.010% or less of S, 16.5 to 22.5% of Cr, 0.01 to 0.100% of Al, one or both of 0.03 to 0.30% of Ti and 0.03 to 0.30% of Nb, and one or both of 0.03 to 1.00% of Sn and 0.05 to 1.00% of Sb.
  • Patent Literature 8 discloses a ferritic stainless steel for an automobile exhaust system component, the ferritic stainless steel containing 0.015% or less of C, 0.015% or less of N, 0.01 to 0.50% of Si, 0.01 to 0.50% of Mn, 0.050% or less of P, 0.010% or less of S, 0.5 to 2.0% of Ni, 16.5 to 22.5% of Cr, 0.010 to 0.100% of Al, 0.01 to 0.50% of Sn, and one or both of 0.03 to 0.30% of Ti and 0.03 to 0.30% of Nb. All of Patent Literatures 6 to 8 disclose corrosion resistance under the circumstances that an oxide film is formed by heating. However, the composition and formation conditions of the oxide film are not mentioned in Patent Literatures 6 to 8.
  • Thickness and weight reduction and increase in lifetime are demanded of exhaust system components for an automobile, a motor cycle, a commercial vehicle, a construction machine and the like. Improvement in corrosion resistance is further required of downstream exhaust system components.
  • An oxide film is locally formed on the components in practical use due to heat applied during welding for assembly and travelling.
  • the formed oxide film is inferior in corrosion resistance as compared with a material without the oxide film, and thus the pitting corrosion lifetime and rust resistance are greatly influenced by the presence of the oxide film. Accordingly, an improvement in corrosion resistance with the oxide film being formed is effective for reducing the thickness, increasing the lifetime and maintaining the good appearance of the component.
  • An object of the invention is to provide a ferritic stainless steel excellent in corrosion resistance after heating and suitably usable as a material for an exhaust system component, an exhaust system component, and a manufacturing method of the ferritic stainless steel and the exhaust system component.
  • T represents a temperature (K) and t represents a time (s).
  • the ferritic stainless steel of the above aspects of the invention is suitable for a material of exhaust system components of an automobile, a motor cycle, a commercial vehicle, a construction machine and the like. Since the ferritic stainless steel of the above aspects of the invention improves the corrosion resistance of a portion including a welded portion that is subjected to heating in use, the ferritic stainless steel contributes to an increase in lifetime of the exhaust system component and thickness and weight reduction of the exhaust system component.
  • FIG. 1 shows an influence of Sn content exerted on a maximum pit depth.
  • the inventors of the invention focused on an oxide film formed by the heating and conducted detailed studies. This is because the inventors suspected that the deterioration in corrosion resistance due to heating was primarily dependent on a formation status of the oxide film.
  • an oxide film having Fe-rich external layer and Cr-rich internal layer is formed on the surface of the ferritic stainless steel.
  • the oxide film is inferior to a passivation film of unheated stainless steel in terms of performance for shielding a material from a corrosive environment. Accordingly, with an identical chemical composition of the material, the heated material is inferior in corrosion resistance. Thus, it is believed that an improvement in the formation status of the oxide film would lead to an improvement in corrosion resistance after heating.
  • the ferritic stainless steel is mostly formed of Fe and Cr, it is inevitable that the oxide film is primarily formed of these two elements. Accordingly, a use of a third element other than Fe and Cr is attempted.
  • ferritic stainless steel sheets containing 0.004C-0.008N-0.1Si-0.1Mn-16.5Cr-0.2Nb-0.1Ti-0.03A1 system (the numbers representing contents of individual elements (mass %)) as a base component and Sn content ranging from 0 to 0.5 mass % were prepared as samples, and each of the samples was subjected to a heat treatment in the atmosphere at 673 K for 24 hours and, subsequently, was subjected to two cyclic corrosion tests. It should be noted that, when the steel sheets were cooled from a finish annealing temperature during the production of the steel sheet, the cooling rate of the steel sheets was 15 degrees C./s in a temperature range from 800 to 600 degrees C. Grain size on a Z-surface of the steel plates was 6.5.
  • the first one of the cyclic corrosion tests which is intended to assess pitting corrosion resistance, one cycle of: spraying 5% NaCl solution at 35 degrees C. for two hours; drying at 60 degrees C. for four hours; and wetting at 50 degrees for two hours in accordance with JASO M609-91 was repeated for 120 times (i.e. 120 cycles). After completion of the test, corrosion product was removed using di-ammonium hydrogen citrate aqueous solution. Subsequently, a maximum pit depth was measured using a microscope focal depth method.
  • the second one of the cyclic corrosion tests which is intended to assess rust resistance, one cycle of: spraying ten-fold diluted artificial seawater at 35 degrees C. for four hours; drying at 60 degrees C.
  • rust generation level was graded using a rating number (abbreviated as RN hereinafter) according to JIS G0595. It should be noted that a larger number of RN represents a more excellent rust resistance.
  • FIG. 1 shows an influence of the Sn content on the maximum pit depth measured in the first test.
  • the presence of 0.02 mass % or more of Sn clearly reduces the maximum pit depth and the maximum pit depth is reduced in accordance with an increase in the Sn content.
  • RN rated in the second test was 5 when the Sn content was 0.001%, RN was 6 or more when 0.02 mass % or more of Sn was contained (i.e. the rust generation level was improved).
  • the presence of rust is easily observable and degradation of appearance is clearly recognizable when RN is 5. Accordingly, the steel is judged inferior in quality when RN is 5 or less whereas the steel is judged excellent when RN is 6 or more.
  • the presence of 0.02 mass % or more of Sn can improve the rust resistance in addition to the pitting corrosion resistance.
  • a sample containing 0.021 mass % of Sn was subjected to the same heat treatment as that in the above corrosion test and was examined using the XPS. It was found that an approximately 40-nm-thick oxide film having an Fe-rich external layer and a Cr-rich internal layer was formed on a surface of the sample and 0.02 to 0.04 at % in cation fraction of Sn was present in a region of approximately 2 nm at and near the border between the oxide film and the base material. The Sn content at and near the border between the oxide film and the base material increases in accordance with an increase in the Sn content in the sample.
  • the thickness of the Sn-concentrated layer is 2 nm or more and the Sn concentration in the Sn-concentrated layer is twice or more of the Sn content in the base material, the effect for improving the corrosion resistance of the invention can be exhibited.
  • the Sn content and thickness of the Sn-concentrated layer increase in accordance with an increase in heating temperature and heating time.
  • excessive heating results in an uneven growth of the oxide film and, consequently, uneven thickness of the Sn-concentrated layer, and also results in saturation of corrosion resistance improving effect.
  • the thickness of the Sn-concentrated layer was approximately 15 nm.
  • T temperature (K)
  • t time (s)
  • a preferable value for the right side of the formula (I) is 8.6 ⁇ 10 ⁇ 15 .
  • the upper limit of the value represented by the left side of the formula (I) is preferably 4.5 ⁇ 10 ⁇ 9 .
  • a grain size number on the surface is 6 or more in order to reach the Sn concentration required in the invention. Grain boundary diffusion dominantly occurs in the temperature range of 573 to 1073 K. Accordingly, with a small grain size, the diffusion of Sn is promoted to progress the concentration of Sn.
  • the grain size number is 6.5 or more and, more preferably, 7 or more.
  • it is effective to form a processed layer on the surface by polishing and the like in order to concentrate Sn.
  • the content of C should be kept at a low level. Accordingly, the upper limit of the C content is set at 0.015%, preferably at 0.012%. However, when the C content is excessively low, a necessary strength cannot be obtained and refining cost increases. Accordingly, the lower limit of the C content is preferably set at 0.002%, more preferably at 0.003%.
  • the content of N should be kept at a low level. Accordingly, the upper limit of the N content is set at 0.02%, preferably at 0.018%. However, when the N content is excessively low, necessary strength cannot be obtained and refining cost increases. Accordingly, the lower limit of the N content is preferably set at 0.002%, more preferably at 0.003%.
  • Si is an element effective for improvement in oxidation resistance and adapted to improve the corrosion resistance after heating, it is necessary that Si content is 0.03% or more.
  • the lower limit of Si content is preferably 0.05%, more preferably 0.1%, further preferably 0.2%.
  • the upper limit of Si content is set at 1.0%.
  • the upper limit of Si content is preferably 0.8%, more preferably 0.6%, further preferably 0.5%.
  • the content of Mn has to be limited. Accordingly, the upper limit of the Mn content is set at 1.0%, preferably at 0.5%. However, extremely lowering the Mn content results in an increase in the production cost. Accordingly, the lower limit of the Mn content is preferably set at 0.03%, more preferably at 0.05%.
  • the upper limit of the P content is set at 0.04%.
  • the upper limit of the P content is preferably set at 0.02%.
  • the content of S has to be limited. Accordingly, the upper limit of the S content is set at 0.01%, preferably at 0.005%, more preferably at 0.002%.
  • the lower limit of Cr content has to be set at 10.5%.
  • the Cr content is 11.0% or more, more preferably 12.5% or more, further preferably 14.0% or more.
  • the corrosion resistance can be improved in accordance with increase in the Cr content, excessive addition of Cr leads to deterioration in formability and productivity.
  • the Cr content is 22.5% or less, preferably 20.5% or less, further preferably 19.5% or less, further more preferably 18.0% or less.
  • the lower limit of the Sn content is set at 0.02%, preferably at 0.05%, more preferably at 0.07% and further preferably at 0.1%.
  • the Sn content is 0.5% or less, preferably 0.4% or less, further preferably 0.3% or less, further more preferably 0.25% or less.
  • the Cr content when the required level of the corrosion resistance after heating is low, the Cr content is suitably defined to be 0.02% or more and less than 0.05%. When a normal level of the corrosion resistance after heating is required, the Cr content is suitably defined to be 0.07% or more and 0.3% or less. When the required level of the corrosion resistance after heating is high, the Cr content is suitably defined to be 0.3% or more and 0.5% or less. It is more preferable that the Cr content is 0.1% or less when the normal level of the corrosion resistance after heating is required.
  • Al is effective as a deoxidizing element and it is necessary that 0.003% or more of Al is contained.
  • Al content is preferably 0.005% or more, more preferably 0.01%.
  • the upper limit of Al content is set at 0.2%.
  • the upper limit of Al content is preferably 0.15%, more preferably 0.1%.
  • the stainless steel of the exemplary embodiment contains one or both of Ti and Nb in the following amount.
  • Ti is an element that is fixed with C and N to form a Ti carbonitride to inhibit intergranular corrosion. Further, Ti is also fixed with S to form a Ti sulfide or Ti carbon-sulfide to improve the corrosion resistance. Accordingly, the lower limit of the Ti content is set at 0.03%, preferably at 0.05%, more preferably at 0.07%. However, since the addition of excessive amount of Ti results in an adverse effect in terms of formability and productivity, the upper limit of Ti content is set at 0.35%. The upper limit of the Ti content is preferably 0.32%, more preferably 0.28%. It should be noted that the Ti content should be 4(C+N)+3S or more.
  • Nb is an element that is fixed with C and N to form an Nb carbonitride to inhibit the intergranular corrosion.
  • Nb acts to improve high-temperature strength.
  • the lower limit of the Nb content is set at 0.03%, preferably at 0.1%, more preferably at 0.2%.
  • the upper limit of Nb content is set at 0.6%.
  • the upper limit of Nb content is preferably 0.55%, more preferably 0.5%.
  • the stainless steel of the exemplary embodiment may optionally further contain, in mass %, one or more of 0.05 to 1.5% of Cu, 0.1 to 1.2% of Ni, 0.03 to 3% of Mo, 0.03 to 1% of W, 0.05 to 0.5% of V, and 0.01 to 0.5% of Sb.
  • Cu is an element that enhances corrosion resistance and strength. Accordingly, 0.05% or more of Cu may be added as necessary.
  • the Cu content is preferably 0.2% or more, more preferably 0.3% or more. However, since the addition of excessive amount of Cu results in decrease in formability, the upper limit of Cu content is preferably set at 1.5% or less.
  • the Cu content is more preferably 1.0% or less and further preferably 0.8% or less.
  • Ni is an element that enhances corrosion resistance. Accordingly, 0.1% or more of Ni may be added as necessary. Ni content is preferably 0.2% or more, more preferably 0.3% or more. However, excessive addition of Ni, which is expensive, results in deterioration in formability and in an increase in the production cost. Accordingly, the Ni content is preferably 1.2% or less, more preferably 0.9% or less and further preferably less than 0.5%.
  • Mo is an element that enhances corrosion resistance and strength. Accordingly, 0.03% or more of Mo may be added as necessary.
  • the Mo content is 0.1% or more, more preferably 0.3% or more, further preferably 0.7% or more.
  • the Mo content is preferably 3% or less, more preferably 2.2% or less and further preferably 1.8% or less.
  • W is an element that enhances corrosion resistance. Accordingly, 0.03% or more of W may be added as necessary.
  • the W content is preferably 0.2% or more, more preferably 0.5% or more.
  • the W content is preferably 1% or less, more preferably 0.8% or less.
  • V 0.05% or More and 0.5% or Less
  • V is an element that enhances corrosion resistance. Accordingly, 0.05% or more of V may be added as necessary.
  • the V content is further preferably 0.1% or more.
  • the V content is preferably 0.5% or less, more preferably 0.3% or less.
  • Sb is an element that enhances corrosion resistance. Accordingly, 0.01% or more of Sb may be added as necessary.
  • the Sb content is preferably 0.03% or more, more preferably 0.05% or more. However, excessive addition of Sb results in deterioration in formability and productivity. Accordingly, the Sb content is preferably 0.5% or less, more preferably 0.3% or less.
  • the stainless steel of the exemplary embodiment may optionally further contain, in mass %, one or more of 0.03 to 0.5% of Zr, 0.02 to 0.2% of Co, 0.0002 to 0.002% of Ca, 0.0002 to 0.002% of Mg, 0.0002 to 0.005% of B, 0.001 to 0.01% of REM, 0.0002 to 0.01% of Ga, and 0.01 to 0.5% of Ta.
  • Zr is an element that enhances corrosion resistance, especially intergranular corrosion resistance. Accordingly, 0.03% or more of Zr may be added as necessary.
  • the Zr content is preferably 0.05% or more, more preferably 0.1% or more.
  • the Zr content is preferably 0.5% or less, more preferably 0.3% or less.
  • Co is an element that enhances secondary formability and toughness. Accordingly, 0.02% or more of Co may be added as necessary.
  • the Co content is preferably 0.05% or more, more preferably 0.08% or more. However, excessive addition of Co results in an increase in the production cost. Accordingly, the Co content is preferably 0.2% or less, more preferably 0.18% or less.
  • Ca is an element that has deoxidization effect and the like and thus is useful in a refining process. Accordingly, 0.0002% or more of Ca may be added as necessary.
  • the Ca content is more preferably 0.0004% or more.
  • the Ca content is preferably 0.002% or less and more preferably 0.0015% or less.
  • Mg is an element that has deoxidization effect and the like and thus is useful in a refining process.
  • Mg miniaturizes the texture to improve formability and toughness. Accordingly, 0.0002% or more of Mg may be contained, and more preferably 0.0005% or more of Mg may be contained as necessary. However, since excessive addition of Mg deteriorates the corrosion resistance, the Mg content is preferably 0.002% or less and more preferably 0.0015% or less.
  • B is an element that enhances formability, especially secondary formability. Accordingly, 0.0002% or more of B may be added as necessary.
  • the B content is more preferably 0.0003% or more.
  • the B content is preferably 0.005% or less and more preferably 0.002% or less.
  • REM represents a group of elements including La, Y, Ce, Pr, Nd and the like belonging to atomic numbers of 57-71.
  • REM is a group of elements that have deoxidization effect and the like and thus is useful in a refining process. Accordingly, 0.001% or more of REM may be added as necessary. However, excessive addition of REM results in an increase in the production cost. Accordingly, the REM content is preferably 0.01% or less.
  • Ga 0.0002% or More and 0.01% o r Less
  • Ga is an element that forms a stable sulfide to improve corrosion resistance and hydrogen embrittlement resistance. Accordingly, 0.0002% or more of Ga may be added as necessary. However, excessive addition of Ga results in an increase in the production cost. Accordingly, the Ga content is preferably 0.01% or less.
  • Ta 0.01% or More and 0.5% or Less
  • Ta is an element that enhances the corrosion resistance. Accordingly, 0.01% or more of Ta may be added as necessary.
  • the Ta content is preferably 0.05% or more, more preferably 0.1% or more.
  • the Ta content is preferably 0.5% or less, more preferably 0.4% or less.
  • the stainless steel of the exemplary embodiment is basically manufactured according to a method typically employed in order to manufacture ferritic stainless steel.
  • molten steel having the above chemical composition may be produced in a converter furnace or an electric furnace, refined in an AOD furnace or a VOD furnace, and formed into a steel piece through a continuous casting process or ingot-making process.
  • the steel piece is then sequentially subjected to hot rolling, hot-rolled sheet annealing, pickling, cold rolling-finish annealing, and pickling.
  • the hot-rolled sheet annealing may be omitted and/or the sequence of cold rolling, finish annealing and pickling may be repeated as necessary.
  • a use of a small-diameter roller having a diameter of 150 mm or less in the cold rolling step is effective for concentrating Sn at and near the border between the oxide film and the base material.
  • the finish annealing temperature is preferably 800 degrees C. or more and, in order to restrain the grains from being coarsened, the finish annealing temperature is preferably 1030 degrees C. or less.
  • the cooling rate in a temperature range from 800 to 600 degrees C. during cooling from the finish annealing temperature is less than 20 degrees C./s on average. More preferably, the cooling rate is less than 15 degrees C./s on average, more preferably less than 5 degrees C./s on average.
  • the finish annealing temperature of the cold rolling is set at an appropriate temperature of 1030 degrees C. or less and the cooling rate in a temperature range from 800 to 600 degrees C. during cooling from the finish annealing temperature is less than 20 degrees C./s on average, so that the grain size number on the surface of the steel becomes 6 or more. Accordingly, when the ferritic stainless steel sheet is heated in the atmosphere under the condition satisfying the formula (I), 2 to 15 nm of the layer containing Sn at a concentration twice or more of the Sn content in the base material can be formed.
  • the finish annealing temperature of the cold rolling is set at an appropriate temperature of 1030 degrees C. or less, the cooling rate in a temperature range from 800 to 600 degrees C. during cooling from the finish annealing temperature is less than 20 degrees C./s on average and the steel sheet is heated in the atmosphere under the condition satisfying the formula (I), so that a ferritic stainless steel sheet whose grain size number on the surface is 6 or more and having 2 to 15 nm of a layer containing Sn at a concentration twice or more of the Sn content in the base material can be manufactured.
  • the heating in the atmosphere under the condition satisfying the formula (I) corresponds to the heating applied on the exhaust system component when a vehicle travels.
  • the heating in the atmosphere under the condition satisfying the formula (I) may be applied on a steel sheet before being assembled into the exhaust system component.
  • the exhaust system component excellent in corrosion resistance after heating according to the exemplary embodiment is manufactured using the steel plate as a material according to a typical manufacturing method of a stainless steel pipe for exhaust system components such as electric resistance welding, TIG welding and laser welding.
  • 1 mm-thick cold-rolled steel sheets were prepared by: melting stainless steel of the compositions shown in Table 1-1 in a 180 kg vacuum melting furnace; casting the stainless steel into steel ingots of 45 kg; and subjecting the steel ingots to a process including hot rolling, hot-rolled sheet annealing, shot blasting, cold rolling, and finish annealing.
  • Each of the hot-rolled sheets was prepared by rolling each of the ingots of 50 mm thickness to a thickness of 5 mm at a heating temperature of 1200 degrees C. and subsequently air-cooling the hot-rolled sheet.
  • the hot-rolled sheet annealing was applied by air-cooling for one minute in a temperature range from 850 to 1050 degrees C. Subsequently, scales were removed by the shot blasting.
  • each of the steel sheets was cold-rolled to obtain a 1-mm-thick steel sheet and was subjected to the finish annealing in which the steel sheets were held for one minute under the temperatures shown in Table 1-2. Thereafter, the steel plates were cooled under the conditions shown in Table 1-2.
  • a specimen of 70 mm in width and 150 mm in length was cut out from each of the cold-rolled steel sheets.
  • a test surface of the specimen was wet-polished up to #600 using Emery paper.
  • the specimen was subjected to a heat treatment at 673 K in the atmosphere for 24 hours.
  • the value represented by the left side of the formula (I) at this time is 1.2 ⁇ 10 ⁇ 10 .
  • Comparative Example 5 in Table 1-2 (steel 7) was subjected to a heat treatment at 523 K in the atmosphere for 15 minutes instead of the heat treatment at 673 K for 24 hours.
  • the value represented by the left side of the formula (I) at this time is 7.1 ⁇ 10 ⁇ 17 .
  • the distribution of the Sn content at and near the surface of the steel sheet after the heat treatment was measured using an XPS.
  • the XPS was manufactured by ULVAC-PHI, Inc. having an X-ray source of mono-Al K ⁇ ray, where elemental analysis in the depth direction was performed using Ar-ion sputtering.
  • the sputtering rate was 1.5 nm/min in terms of SiO 2 .
  • the thickness of the Sn-concentrated layer present at the border region between the oxide film and the base material was measured (shown in Table 1-2).
  • the thickness of the Sn-concentrated layer represents a thickness of the region in which detected Sn concentration was higher than the Sn content in the base material.
  • the lowest Sn concentration in the Sn-concentrated layer is shown in Table 1-2 in atom %.
  • a value obtained by dividing the Sn concentration in the Sn-concentrated layer by the Sn content in the base material is shown in Table 1-2 as “Concentration Degree.”
  • the corrosion resistance was evaluated using the two types of cyclic corrosion tests.
  • one cycle of: spraying 5% NaCl solution at 35 degrees C. for two hours; drying at 60 degrees C. for four hours; and wetting at 50 degrees for two hours in accordance with JASO M609-91 was repeated for 120 times (i.e. 120 cycles).
  • corrosion product was removed using di-ammonium hydrogen citrate aqueous solution.
  • a maximum pit depth was measured using a microscope focal depth method.
  • one cycle of: spraying ten-fold diluted artificial seawater at 35 degrees C. for four hours; drying at 60 degrees C. for two hours; and wetting at 50 degrees for two hours was repeated for three times (i.e. three cycles).
  • a rust generation level was graded using the rating number according to JIS G0595.
  • a specimen of 20 mm in width and 20 mm in length was cut out from the same cold-rolled steel sheet.
  • a surface of the specimen was mirror-polished and subsequently etched to expose microstructure.
  • a grain size on a Z-surface (a surface parallel to the surface) was measured in accordance with JIS G0551.
  • Test results are shown in Table 1-2.
  • the grain size number shows the measurements on the specimen cut out from the cold-rolled steel sheet.
  • the same results as those of the specimen of the cold-rolled steel sheet not subjected to the heat treatment were obtained. It should be noted that, since the Sn-concentrated layer was not formed in Comparative Examples 5 and 6, the Sn concentration at and near the border between the oxide film and the base material is described in Comparative Examples 5 and 6.
  • inventive Examples 1 to 18 show 400 ⁇ m or less of the maximum pit depth and 6 or more of RN and are thus excellent in corrosion resistance.
  • Comparative Example 1 whose Sn content does not satisfy the requirements of the invention, Comparative Example 2 whose Cr content does not satisfy the requirements of the invention, Comparative Example 3 whose Si content does not satisfy the requirements of the invention, Comparative Example 5 whose heating condition does not satisfy the formula (I) and Comparative Example 6 whose cooling rate in the temperature range from 800 to 600 degrees C. during the finish annealing step exceeds 20 degrees C./s all show the maximum pit depth of more than 500 ⁇ m and 5 or less of RN and thus are inferior in corrosion resistance.
  • the Sn-concentrated layer is formed in Comparative Example 4 whose grain size number is 4, the Sn concentration is not sufficient due to the influence of the grain size number. Consequently, though a certain degree of the pitting corrosion resistance is ensured as shown by the 400 to 500 ⁇ m of the maximum pit depth, Comparative Example 4 is inferior in rust resistance as shown by the RN of 5.
  • Ferritic stainless steel of the invention is suitable for exhaust system components of an automobile, a motor cycle, a commercial vehicle, a construction machine and the like that are subjected to heating in use.
  • suitable exhaust system components include a converter case, a front pipe, a center pipe and a muffler.

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