WO2015015735A1 - Ferritic stainless steel having excellent weld corrosion resistance - Google Patents

Ferritic stainless steel having excellent weld corrosion resistance Download PDF

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Publication number
WO2015015735A1
WO2015015735A1 PCT/JP2014/003767 JP2014003767W WO2015015735A1 WO 2015015735 A1 WO2015015735 A1 WO 2015015735A1 JP 2014003767 W JP2014003767 W JP 2014003767W WO 2015015735 A1 WO2015015735 A1 WO 2015015735A1
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corrosion resistance
range
content
stainless steel
ferritic stainless
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PCT/JP2014/003767
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French (fr)
Japanese (ja)
Inventor
正崇 吉野
太田　裕樹
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Ｊｆｅスチール株式会社
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Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to CN201480041182.5A priority Critical patent/CN105408511B/en
Priority to KR1020157036796A priority patent/KR101809812B1/en
Priority to JP2014553008A priority patent/JP5700182B1/en
Priority to US14/908,176 priority patent/US20160168673A1/en
Publication of WO2015015735A1 publication Critical patent/WO2015015735A1/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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

Definitions

  • the present invention relates to a ferritic stainless steel that is unlikely to deteriorate in corrosion resistance due to carbon and nitrogen entering a weld bead from a welding partner material or nitrogen entering from the atmosphere when welding is performed.
  • Ferritic stainless steel can ensure corrosion resistance with a smaller amount of Ni than austenitic stainless steel. Since Ni is an expensive element, ferritic stainless steel can be manufactured at a lower cost than austenitic stainless steel. Ferritic stainless steel has excellent properties such as higher thermal conductivity, lower thermal expansion coefficient, and less stress corrosion cracking than austenitic stainless steel. For this reason, ferritic stainless steel has been applied to a wide range of applications such as automobile exhaust system members, building materials such as roofs and fittings, and watering materials such as kitchen equipment, water storage tanks and hot water storage tanks.
  • austenitic stainless steel particularly SUS304 (18% Cr-8% Ni) (JIS G 4305), etc.
  • ferritic stainless steel are often used in combination.
  • TIG welding is generally used. Even in such a case, it is required that the welded portion has good corrosion resistance like the base material portion.
  • Patent Document 1 discloses a steel in which the intergranular corrosion resistance of ferritic stainless steel is improved by adding Ti and Nb in combination.
  • Patent Document 2 discloses steel having improved intergranular corrosion resistance by adding Nb.
  • an object of the present invention is to provide a ferritic stainless steel that does not require excessive addition of Ti or Nb and has excellent corrosion resistance of a welded portion.
  • the present inventors diligently studied a technique that can suppress a decrease in the corrosion resistance of the welded portion more than before.
  • the inventors systematically investigated the degree of sensitization allowed to obtain sufficient weld corrosion resistance using 16-20 wt% Cr steel.
  • the grain boundary coverage of the ferrite phase by Cr carbonitride for the grain boundary coverage, refer to the measurement method of the example. It has been found that it becomes apparent.
  • the inventors examined a method for reducing the grain boundary coverage of the ferrite phase with Cr carbonitride. As a result, it has been found that stabilizing the austenite phase by adding Mn and Cu, which are austenite-forming elements, is extremely effective in improving the corrosion resistance of welds.
  • the present invention has been made based on the above findings, and the gist thereof is as follows.
  • a ferritic stainless steel having excellent corrosion resistance can be obtained even under welding conditions in which C and N enter the base metal from the welding counterpart.
  • C 0.001 to 0.025%
  • C is an element inevitably included.
  • the content exceeds 0.025%, the workability is remarkably lowered, and the corrosion resistance is lowered (sensitization) due to local Cr deficiency due to precipitation of Cr carbide.
  • the C content is in the range of 0.001 to 0.025%.
  • the lower the amount of C the better from the viewpoint of corrosion resistance and workability, but extremely reducing the amount of C takes time for refining and is not preferable in production. Therefore, it is preferably in the range of 0.003 to 0.018%. More preferably, it is in the range of 0.005 to 0.012%.
  • Si 0.05-0.30%
  • Si is an element effective for improving the corrosion resistance of the weld. In order to acquire this effect, 0.05% or more of content is required, and the effect increases as the content increases. However, if the Si content exceeds 0.30%, the formability and toughness of the welded portion are not preferred. Therefore, the Si content is set in the range of 0.05 to 0.30%. Preferably it is 0.05 to 0.25% of range. More preferably, it is 0.08 to 0.20% of range.
  • Mn 0.35 to 2.0% Mn is a particularly important element in the present invention.
  • Mn is an element effective as a deoxidizer and has an effect of stabilizing the austenite phase.
  • C and N contained above the solid solubility limit of the ferrite phase have an austenite phase having a larger solid solubility limit than the ferrite phase.
  • stabilizing in a solid solution state the effect of preventing sensitization due to the formation of Cr carbonitride is exhibited, and the corrosion resistance of the welded portion is improved. In order to obtain these effects, it is necessary to contain 0.35% or more of Mn.
  • the amount of Mn is set in the range of 0.35 to 2.0%. Preferably, it is in the range of 0.50 to 1.5%. More preferably, it is in the range of 0.75 to 1.25%.
  • P 0.05% or less
  • the P content is 0.05% or less.
  • S 0.01% or less S, like P, is an element inevitably contained in steel. Corrosion resistance is reduced by the inclusion of over 0.01%. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less.
  • Al 0.05 to 0.80% Al, like Si, is an element that improves the corrosion resistance of the weld. Since Al has a stronger affinity for N than Cr, when N is mixed in the welded portion, N is precipitated as Al nitride instead of Cr nitride and has the effect of suppressing sensitization. Al is also an element useful for deoxidation in the steel making process. These effects can be obtained with a content of 0.05% or more. However, when Al is contained exceeding 0.80%, ferrite crystal grains become coarse, and workability and manufacturability deteriorate. Therefore, the Al content is set in the range of 0.05 to 0.80%. Preferably, it is 0.10 to 0.60% of range. More preferably, it is in the range of 0.15 to 0.50%.
  • N 0.001 to 0.025%
  • N is an element inevitably contained in steel.
  • the N content is high, the strength is improved, and as the N content is low, the workability is improved.
  • the content of 0.001% or more is appropriate.
  • the ductility is remarkably lowered and the corrosion resistance is reduced by promoting the precipitation of Cr nitride, which is not preferable. Therefore, the N content is set in the range of 0.001 to 0.025%.
  • N is preferably as low as possible.
  • the range of 0.003 to 0.025% is preferable. More preferably, it is in the range of 0.003 to 0.015%. More preferably, it is 0.003 to 0.010% of range.
  • Cr 16.0-20.0% Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 16.0%, sufficient corrosion resistance cannot be obtained in the weld bead where the surface layer Cr decreases due to oxidation by welding and in the vicinity thereof. Further, sensitization due to N mixed from the welding partner material or the atmosphere during welding is further promoted, which is not preferable. On the other hand, if the Cr content exceeds 20.0%, the toughness and the descalability after annealing decrease, which is not preferable. Therefore, the Cr content is set in the range of 16.0% to 20.0%. Preferably, it is in the range of 16.5% to 19.0%. More preferably, it is in the range of 17.0 to 18.5%.
  • Ti 0.12 to 0.50%
  • Ti is an element that preferentially binds to C and N and suppresses a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitride. This effect is obtained when the content is 0.12% or more. However, if the content exceeds 0.50%, coarse Ti carbonitrides are generated and cause surface defects, which is not preferable. Therefore, the Ti amount is set in the range of 0.12 to 0.50%. Preferably, it is in the range of 0.15 to 0.40%. More preferably, it is 0.20 to 0.35% of range.
  • Nb 0.002 to 0.050%
  • Nb is an element that preferentially binds to C and N and suppresses a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitride.
  • Nb also has the effect of reducing the crystal grain size of the weld and improving the toughness and bendability of the weld. These effects are obtained when the content is 0.002% or more.
  • Nb is also an element that raises the recrystallization temperature, and if it exceeds 0.050%, the annealing temperature necessary for recrystallization increases, so annealing in the annealing process using a high-speed cold-rolled sheet annealing line Is insufficient, and undesired grains and recrystallized grains coexist, resulting in a decrease in workability. Therefore, the Nb amount is set to a range of 0.002 to 0.050%. Preferably it is 0.010 to 0.045% of range. More preferably, it is in the range of 0.015 to 0.040%.
  • Cu 0.30 to 0.80%
  • Cu is an element that improves the corrosion resistance, and is an element that is particularly effective for improving the corrosion resistance of the base material and the welded part when an aqueous solution or weakly acidic water droplets adhere.
  • Cu is a strong austenite-forming element like Ni, and has the effect of suppressing the formation of ferrite phase in the weld and suppressing sensitization due to the precipitation of Cr carbonitride. These effects are obtained when the content is 0.30% or more.
  • the Cu content is set in the range of 0.30 to 0.80%.
  • it is 0.30 to 0.60% of range. More preferably, it is in the range of 0.35 to 0.50%.
  • Ni 0.05% or more and less than 0.50%
  • Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment where a passive film cannot be formed and active dissolution occurs.
  • Ni is a strong austenite generating element, and has the effect of suppressing ferrite formation at the weld and suppressing sensitization due to precipitation of Cr carbonitride. These effects are obtained when the content is 0.05% or more.
  • the amount of Ni is made 0.05% or less and less than 0.50%.
  • it is 0.10 to 0.30% of range. More preferably, it is in the range of 0.15 to 0.25%.
  • V 0.01 to 0.50%
  • V is an element that improves corrosion resistance and workability, and has the effect of suppressing the formation of Cr nitride and reducing the sensitization of the welded portion by combining with N when N is mixed in the welded portion. This effect is obtained when the content is 0.01% or more. However, if the content exceeds 0.50%, workability is lowered, which is not preferable. Therefore, the V amount is in the range of 0.01 to 0.50%. Preferably it is 0.05 to 0.30% of range. More preferably, it is 0.08 to 0.20% of range.
  • the element symbol in a formula means content (mass%) of each element.
  • the right side of the formula (1) is 0.50 or less, the austenite phase is not sufficiently stabilized, and the austenite phase that only effectively fixes the solid solution C and solid solution N provided by the present invention in the weld zone. Cannot be generated. Therefore, the right side of the formula (1) is over 0.5. Preferably it is 0.60 or more. More preferably, it is 0.70 or more.
  • the above are the basic chemical components of the present invention, and the balance consists of Fe and inevitable impurities.
  • the following elements may be contained for the purpose of suppressing the sensitization of the weld bead and improving the corrosion resistance.
  • Zr 0.01 to 0.50%
  • Zr has an effect of binding to C and N to suppress sensitization. This effect is obtained when the content is 0.01% or more. However, if it exceeds 0.50%, workability is lowered. Moreover, since Zr is an expensive element, excessive addition causes an increase in manufacturing cost, which is not preferable. Therefore, when Zr is contained, the content is preferably in the range of 0.01 to 0.50%. More preferably, it is in the range of 0.10 to 0.35%.
  • W 0.01-0.20% W, like Mo, has the effect of improving corrosion resistance. This effect is obtained when the content is 0.01% or more. However, if the content exceeds 0.20%, the strength increases, and the productivity decreases due to an increase in rolling load, etc., which is not preferable. Therefore, when it contains W, it is preferable to set it as 0.01 to 0.20% of range. More preferably, it is in the range of 0.05 to 0.15%.
  • REM 0.001 to 0.10% REM has an effect of improving oxidation resistance, and is effective in suppressing the formation of a Cr-deficient region directly under the weld temper collar by suppressing the formation of oxide scale. In order to acquire this effect, 0.001% or more needs to be contained. However, if the content exceeds 0.10%, productivity such as pickling properties is lowered. Moreover, since REM is an expensive element like Zr, excessive inclusion causes an increase in manufacturing cost, which is not preferable. Therefore, when it contains REM, it is preferable to set it as 0.001 to 0.10% of range. More preferably, it is in the range of 0.010 to 0.08%.
  • Co 0.01-0.20%
  • Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, when it contains exceeding 0.20%, manufacturability will be reduced. Therefore, when Co is contained, the content is preferably 0.01 to 0.20%. More preferably, it is in the range of 0.05 to 0.15%.
  • B 0.0002 to 0.010%
  • B is an element that improves secondary work brittleness, and the effect is obtained by the content of 0.0002% or more. However, when it exceeds 0.010%, it induces a decrease in ductility due to excessive solid solution strengthening. Therefore, when B is contained, the content is preferably in the range of 0.0002 to 0.010%. More preferably, it is in the range of 0.0010 to 0.0075%.
  • Sb 0.05 to 0.30% Sb, like Al, has an effect of capturing N mixed in from the atmosphere when the gas shield of TIG welding is insufficient, and is applied to a structure having a complicated shape that makes it difficult to perform sufficient gas shield. Is a particularly effective element. The effect is acquired by 0.05% or more of containing. However, when it contains exceeding 0.30%, a nonmetallic inclusion will produce
  • Molten steel having the above component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and a steel material (slab) is obtained by a continuous casting method or an ingot-bundling method.
  • the slab is hot-rolled after heating at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-cast as cast without heating to form a hot-rolled sheet.
  • hot-rolled sheets are annealed at 800 to 1100 ° C. for 1 to 10 minutes. Depending on the application, the hot-rolled sheet annealing may be omitted.
  • hot-rolled sheet pickling it is cold-rolled by cold rolling, and then subjected to recrystallization annealing and pickling to obtain a product.
  • Cold rolling is desirably performed at a rolling reduction of 50% or more from the viewpoint of stretchability, bendability, press formability, and shape.
  • the recrystallization annealing of cold-rolled sheets is generally performed according to JIS G 0203 surface finish, No.
  • BA annealing (bright annealing) for finishing the member where the luster is desired.
  • Stainless steel having the chemical composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated at 1150 ° C. for 1 hour and then hot rolled to form 3.5 mm thick hot rolled sheets. Next, these hot-rolled sheets were subjected to hot-rolled sheet annealing at 950 ° C. for 1 minute, and then subjected to shot blast treatment on the surface, and then immersed in a 20% by mass sulfuric acid solution at a temperature of 80 ° C. for 15 seconds. The pickling was performed by dipping in a mixed acid composed of mass% nitric acid and 3 mass% hydrofluoric acid at a temperature of 55 ° C. for 60 seconds, and descaling was performed.
  • the cold-rolled annealed sheet was descaled by electrolytic pickling in a mixed acid solution consisting of a temperature of 50 ° C., 15% by mass nitric acid and 0.5% by mass hydrochloric acid to obtain a cold-rolled pickled annealed sheet. .
  • Table 1-1 and Table 1-2 are a series of continuous tables.
  • Thickness 0 of manufactured cold rolled sheet and commercially available austenitic stainless steel SUS304 (C: 0.07 mass%, N: 0.05 mass%, Cr: 18.2 mass%, Ni: 8.2 mass%) .8mm cold rolled plate, Butt TIG welding was performed (cold rolled sheet of the present invention: base material, welding partner material: SUS304).
  • the welding current was 90 A, the welding speed was 60 cm / min, and Ar gas containing 8 vol% nitrogen and 2 vol% oxygen was used at 15 L / min as the shielding gas.
  • the width of the obtained front-side weld bead was about 3 mm.
  • test piece including the produced weld bead was collected and the following test was performed.
  • the pitting corrosion potential of the base material 150 mV or more, and the pitting corrosion potential of the welded portion: 0 mV or more was regarded as acceptable.
  • Neutral salt spray cycle test A 100 mm square test piece including a weld bead was taken from the welded test piece, and the surface was polished with # 600 emery paper, and then a test piece with a sealed end face was prepared. The sample was subjected to a neutral salt spray cycle test specified in 8502. Neutral salt spray cycle test: 5% by weight NaCl solution spray (35 ° C., 2 h) ⁇ dry (60 ° C., 4 h, relative humidity 20-30%) ⁇ moistness (40 ° C., 2 h, relative humidity 95% The above is one cycle. The case where there was no occurrence of corrosion from the base metal or the welded part after 15 cycles was regarded as acceptable.
  • the predetermined base metal and welded portion corrosion resistance were obtained, the ductility decreased due to the hardened steel sheet, and the predetermined mechanical properties could not be obtained.
  • the content of the austenite stabilizing element is less than the range of the formula (1).
  • predetermined base metal corrosion resistance and mechanical properties were obtained.
  • the pitting corrosion potential of the predetermined weld bead and the corrosion resistance of the weld were not obtained.
  • No. Examination of the cross-sectional structure of the welds 27 to 30 confirmed that an extremely large amount of Cr carbonitride was precipitated at the ferrite phase grain boundary with a grain boundary coverage of 50% or more.
  • the ferritic stainless steel obtained in the present invention is suitable for applications in which structures are produced by welding, for example, automotive exhaust materials such as mufflers, building materials such as fittings, ventilation openings, ducts, etc. is there.

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  • Engineering & Computer Science (AREA)
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Abstract

Provided is a ferritic stainless steel having excellent weld corrosion resistance. The ferritic stainless steel is characterized by containing, in terms of mass %, 0.001-0.025% of C, 0.05-0.30% of Si, 0.35-2.0% of Mn, 0.05% or less of P, 0.01% or less of S, 0.05-0.80% of Al, 0.001-0.025% of N, 16.0-20.0% of Cr, 0.12-0.50% of Ti, 0.002-0.050% of Nb, 0.30-0.80% of Cu, 0.05% to less than 0.50% of Ni, and 0.01-0.50% of V and satisfying formula (1), with the remainder consisting of Fe and unavoidable impurities. 0.50 < 25×C + 18×N + Ni + 0.11×Mn + 0.46×Cu　(1) Moreover, the chemical symbols in the formula denotes the content (mass %) of the elements in question.

Description

溶接部の耐食性に優れたフェライト系ステンレス鋼Ferritic stainless steel with excellent corrosion resistance of welds
　本発明は、溶接を行った際に溶接相手材から溶接ビードに侵入する炭素および窒素あるいは、大気から侵入する窒素に起因した耐食性の低下が生じにくいフェライト系ステンレス鋼に関する。 The present invention relates to a ferritic stainless steel that is unlikely to deteriorate in corrosion resistance due to carbon and nitrogen entering a weld bead from a welding partner material or nitrogen entering from the atmosphere when welding is performed.
　フェライト系ステンレス鋼は、オーステナイト系ステンレス鋼と比較して少ないＮｉ量で耐食性を確保できる。Ｎｉは高価な元素であることから、フェライト系ステンレス鋼はオーステナイト系ステンレス鋼に比べて低コストで製造することができる。また、フェライト系ステンレス鋼はオーステナイト系ステンレス鋼に比べて熱伝導率が高く、熱膨張率が小さい、さらには応力腐食割れが起こりにくい等の優れた特性がある。このことから、フェライト系ステンレス鋼は、自動車排気系部材、屋根や建具などの建材、厨房機器や貯水タンク、貯湯タンクなどの水まわり用材料など幅広い用途に適用されてきた。 Ferritic stainless steel can ensure corrosion resistance with a smaller amount of Ni than austenitic stainless steel. Since Ni is an expensive element, ferritic stainless steel can be manufactured at a lower cost than austenitic stainless steel. Ferritic stainless steel has excellent properties such as higher thermal conductivity, lower thermal expansion coefficient, and less stress corrosion cracking than austenitic stainless steel. For this reason, ferritic stainless steel has been applied to a wide range of applications such as automobile exhaust system members, building materials such as roofs and fittings, and watering materials such as kitchen equipment, water storage tanks and hot water storage tanks.
　これら構造物の作製にあたっては、オーステナイト系ステンレス鋼、特にＳＵＳ３０４（１８％Ｃｒ－８％Ｎｉ）（ＪＩＳ　Ｇ　４３０５）等とフェライト系ステンレス鋼を組み合わせて使用される場合が多い。このようなステンレス鋼の溶接方法としては、一般にＴＩＧ溶接が用いられる。その場合も、溶接部に対して母材部と同様に良好な耐食性を持つことが求められる。 In producing these structures, austenitic stainless steel, particularly SUS304 (18% Cr-8% Ni) (JIS G 4305), etc., and ferritic stainless steel are often used in combination. As such a stainless steel welding method, TIG welding is generally used. Even in such a case, it is required that the welded portion has good corrosion resistance like the base material portion.
　このような課題に対し、ＣｒよりもＣおよびＮとの親和力が大きいＴｉやＮｂを添加することで、ＣおよびＮをＴｉやＮｂの炭窒化物として固定し、Ｃｒ炭窒化物の生成をおさえて鋭敏化の発生を抑制し、良好な耐食性を確保する方法が提案されている。たとえば、特許文献１にはＴｉとＮｂを複合添加することでフェライト系ステンレス鋼の耐粒界耐食性を向上させた鋼が開示されている。また、特許文献２ではＮｂを添加することで耐粒界腐食性を向上させた鋼が開示されている。 In response to such problems, by adding Ti or Nb, which has a higher affinity for C and N than Cr, C and N are fixed as Ti or Nb carbonitrides, thereby suppressing the formation of Cr carbonitrides. Thus, a method for suppressing the occurrence of sensitization and ensuring good corrosion resistance has been proposed. For example, Patent Document 1 discloses a steel in which the intergranular corrosion resistance of ferritic stainless steel is improved by adding Ti and Nb in combination. Patent Document 2 discloses steel having improved intergranular corrosion resistance by adding Nb.
　しかし、いずれの発明も０．３質量％以上のＭｏ添加を必要とする。Ｍｏは母材の耐食性を向上させる元素ではある。しかしながら、強いフェライト生成元素であるため、０．３質量％ものＭｏが添加された場合、溶接部におけるフェライト相の生成が促進されて、溶接部の鋭敏化が助長されるため、十分な溶接部の耐食性を得ることができない。また、いずれも０．１質量％以上のＮｂ添加を必要としている。しかしながら、多量のＮｂを含む鋼を溶接した場合、固溶Ｎｂが溶接部で粗大Ｎｂ析出物として生成し、溶接割れなどの問題が発生する場合がある。溶接部の耐食性をＮｂやＴｉなどの炭窒化物生成元素の添加のみによって得ることは、製造性や実用性を考慮すると最適な方策とは言えない。 However, all inventions require addition of 0.3% by mass or more of Mo. Mo is an element that improves the corrosion resistance of the base material. However, since it is a strong ferrite-forming element, when 0.3% by mass of Mo is added, the formation of a ferrite phase in the welded portion is promoted and the sensitization of the welded portion is promoted. The corrosion resistance cannot be obtained. In addition, Nb addition of 0.1% by mass or more is required in all cases. However, when steel containing a large amount of Nb is welded, solid solution Nb is generated as coarse Nb precipitates at the weld, and problems such as weld cracking may occur. Obtaining the corrosion resistance of the welded part only by adding carbonitride-forming elements such as Nb and Ti is not an optimal measure in view of manufacturability and practicality.
特開２００７－２７０２９０号公報JP 2007-270290 A 特開２０１０－２０２９１６号公報JP 2010-202916 A
　そこで本発明では、ＴｉやＮｂの過度の添加を必要としない、溶接部の耐食性に優れたフェライト系ステンレス鋼を提供することを目的とする。 Therefore, an object of the present invention is to provide a ferritic stainless steel that does not require excessive addition of Ti or Nb and has excellent corrosion resistance of a welded portion.
　本発明者らは上記課題を解決するため、従来以上に溶接部の耐食性の低下を抑制できる技術について鋭意検討した。まず、発明者らは十分な溶接部の耐食性を得るために許容される鋭敏化の程度について１６～２０質量％Ｃｒ鋼を用いて系統的に調査した。その結果、鋭敏化による溶接部の耐食性の低下は、Ｃｒ炭窒化物によるフェライト相の粒界被覆率（粒界被覆率については、実施例の測定方法を参照。）が４０％を超えた場合に顕在化することを知見した。 In order to solve the above-mentioned problems, the present inventors diligently studied a technique that can suppress a decrease in the corrosion resistance of the welded portion more than before. First, the inventors systematically investigated the degree of sensitization allowed to obtain sufficient weld corrosion resistance using 16-20 wt% Cr steel. As a result, the deterioration of the corrosion resistance of the weld due to sensitization is caused when the grain boundary coverage of the ferrite phase by Cr carbonitride (for the grain boundary coverage, refer to the measurement method of the example) exceeds 40%. It has been found that it becomes apparent.
　次に発明者らはＣｒ炭窒化物によるフェライト相の粒界被覆率を低減する手法について検討した。その結果、オーステナイト生成元素であるＭｎおよびＣｕの添加によりオーステナイト相を安定化することが、溶接部の耐食性向上に極めて効果的であることを知見した。 Next, the inventors examined a method for reducing the grain boundary coverage of the ferrite phase with Cr carbonitride. As a result, it has been found that stabilizing the austenite phase by adding Mn and Cu, which are austenite-forming elements, is extremely effective in improving the corrosion resistance of welds.
　すなわち、ＭｎおよびＣｕの添加によりオーステナイト相を安定化した場合、溶接時のフェライト相の生成を抑制することができるとともに、フェライト相の固溶限以上に含まれたＣおよびＮをフェライト相よりも固溶限が圧倒的に大きいオーステナイト相中に固溶させた状態で安定化することにより、Ｃｒ炭窒化物の生成による鋭敏化を防止することができ、溶接部の耐食性が格段に向上することを知見した。 That is, when the austenite phase is stabilized by the addition of Mn and Cu, the formation of the ferrite phase during welding can be suppressed, and C and N contained above the solid solubility limit of the ferrite phase can be reduced more than the ferrite phase. By stabilizing in the austenite phase where the solid solubility limit is overwhelmingly large, it is possible to prevent sensitization due to the formation of Cr carbonitrides, and the corrosion resistance of the welded portion is greatly improved. I found out.
　さらに、ＣおよびＮをオーステナイト相中で固溶するため、従来のＴｉやＮｂ等のみによるＣおよびＮの固定化に比べて、より多いＣおよびＮ量においても鋭敏化を防止することができることも明らかとなった。発明者らは上記効果が得られる鋼成分について系統的に調査し、上記効果はオーステナイト相を安定化するＣ、Ｎ、Ｎｉ、ＭｎおよびＣｕの含有量が下式（１）を満たす場合に得られることを知見した。
０．５０＜２５×Ｃ＋１８×Ｎ＋Ｎｉ＋０．１１×Ｍｎ＋０．４６×Ｃｕ　　（１）
　　なお、式中の元素記号は各元素の含有量（質量％）を意味する。 Furthermore, since C and N are dissolved in the austenite phase, it is possible to prevent sensitization even with a larger amount of C and N compared to conventional immobilization of C and N only by Ti, Nb or the like. It became clear. The inventors systematically investigated the steel components from which the above effects are obtained, and the above effects are obtained when the contents of C, N, Ni, Mn, and Cu that stabilize the austenite phase satisfy the following formula (1). I found out that
0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
In addition, the element symbol in a formula means content (mass%) of each element.
　さらに、本効果は、従来技術であるＴｉやＮｂ添加によるＣおよびＮの固定化と併用することができるため、ＴｉやＮｂの過度な添加を必要とせずとも、従来鋼よりも格段に優れた溶接部の耐食性を得ることが可能となる。 Furthermore, since this effect can be used together with C and N fixation by adding Ti or Nb, which is a conventional technique, it is much better than conventional steel without requiring excessive addition of Ti or Nb. Corrosion resistance of the welded portion can be obtained.
　本発明は、上記の知見に基づきなされたもので、その要旨は以下の通りである。 The present invention has been made based on the above findings, and the gist thereof is as follows.
　［１］　質量％で、Ｃ：０．００１～０．０２５％、Ｓｉ：０．０５～０．３０％、Ｍｎ：０．３５～２．０％、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．０５～０．８０％、Ｎ：０．００１～０．０２５％、Ｃｒ：１６．０～２０．０％、Ｔｉ：０．１２～０．５０％、Ｎｂ：０．００２～０．０５０％、Ｃｕ：０．３０～０．８０％、Ｎｉ：０．０５％以上０．５０％未満、Ｖ：０．０１～０．５０％を含有し、かつ下記式（１）を満たし、残部がＦｅおよび不可避不純物からなることを満たすことを特徴とする溶接部の耐食性に優れたフェライト系ステンレス鋼。
０．５０＜２５×Ｃ＋１８×Ｎ＋Ｎｉ＋０．１１×Ｍｎ＋０．４６×Ｃｕ　　（１）
　　なお、式中の元素記号は各元素の含有量（質量％）を意味する。 [1] By mass%, C: 0.001 to 0.025%, Si: 0.05 to 0.30%, Mn: 0.35 to 2.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.05 to 0.80%, N: 0.001 to 0.025%, Cr: 16.0 to 20.0%, Ti: 0.12 to 0.50%, Nb: 0.002 to 0.050%, Cu: 0.30 to 0.80%, Ni: 0.05% or more and less than 0.50%, V: 0.01 to 0.50%, and A ferritic stainless steel excellent in corrosion resistance of a welded portion, characterized in that the following formula (1) is satisfied and the balance is composed of Fe and inevitable impurities.
0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
In addition, the element symbol in a formula means content (mass%) of each element.
　［２］　更に、質量％で、Ｚｒ：０．０１～０．５０％、Ｗ：０．０１～０．２０％、ＲＥＭ：０．００１～０．１０％、Ｃｏ：０．０１～０．２０％、Ｂ：０．０００２～０．０１０％、Ｓｂ：０．０５～０．３０％の中から選ばれる１種以上を含有することを特徴とする［１］に記載の溶接部の耐食性に優れたフェライト系ステンレス鋼。 [2] Further, in terms of mass%, Zr: 0.01 to 0.50%, W: 0.01 to 0.20%, REM: 0.001 to 0.10%, Co: 0.01 to 0. It contains at least one selected from 20%, B: 0.0002 to 0.010%, Sb: 0.05 to 0.30%, and the corrosion resistance of the welded portion according to [1] Excellent ferritic stainless steel.
　本発明によれば、溶接相手材から母材にＣおよびＮが侵入するような溶接条件においても優れた耐食性を有するフェライト系ステンレス鋼が得られる。 According to the present invention, a ferritic stainless steel having excellent corrosion resistance can be obtained even under welding conditions in which C and N enter the base metal from the welding counterpart.
　以下に本発明の各構成要件の限定理由について説明する。 Hereinafter, the reasons for limitation of each component of the present invention will be described.
　１．成分組成ならびに金属組織について
　はじめに、本発明の鋼の成分組成を規定した理由を説明する。なお、成分％はすべて質量％を意味する。 1. Component Composition and Metallographic Structure First, the reason for defining the component composition of the steel of the present invention will be described. In addition, all component% means the mass%.
　Ｃ：０．００１～０．０２５％
　Ｃは不可避的に含まれる元素である。Ｃ量が高いほど強度が向上し、少ないほど加工性が向上する。十分な強度を得るためには０．００１％以上の含有が必要である。しかしながら、０．０２５％を超えて含有すると加工性の低下が顕著となるうえ、Ｃｒ炭化物の析出による局所的なＣｒ欠乏に起因した耐食性の低下（鋭敏化）が生じやすくなる。そのため、Ｃ量は０．００１～０．０２５％の範囲とする。ただし、Ｃ量が低いほど耐食性および加工性の観点では好ましいものの、Ｃ量を極度に低下させることは精錬に時間がかかり、製造上好ましくない。このため、好ましくは０．００３～０．０１８％の範囲である。より好ましくは０．００５～０．０１２％の範囲である。 C: 0.001 to 0.025%
C is an element inevitably included. The higher the amount of C, the better the strength, and the lower the amount, the better the workability. In order to obtain sufficient strength, it is necessary to contain 0.001% or more. However, when the content exceeds 0.025%, the workability is remarkably lowered, and the corrosion resistance is lowered (sensitization) due to local Cr deficiency due to precipitation of Cr carbide. For this reason, the C content is in the range of 0.001 to 0.025%. However, the lower the amount of C, the better from the viewpoint of corrosion resistance and workability, but extremely reducing the amount of C takes time for refining and is not preferable in production. Therefore, it is preferably in the range of 0.003 to 0.018%. More preferably, it is in the range of 0.005 to 0.012%.
　Ｓｉ：０．０５～０．３０％
　Ｓｉは溶接部の耐食性の向上に有効な元素である。この効果を得るためには０．０５％以上の含有が必要であり、含有量が多いほどその効果は大きくなる。しかし、Ｓｉ含有量が０．３０％を超えると溶接部の成形性や靭性が低下するため好ましくない。そのため、Ｓｉ量は０．０５～０．３０％の範囲とする。好ましくは０．０５～０．２５％の範囲である。さらに好ましくは０．０８～０．２０％の範囲である。 Si: 0.05-0.30%
Si is an element effective for improving the corrosion resistance of the weld. In order to acquire this effect, 0.05% or more of content is required, and the effect increases as the content increases. However, if the Si content exceeds 0.30%, the formability and toughness of the welded portion are not preferred. Therefore, the Si content is set in the range of 0.05 to 0.30%. Preferably it is 0.05 to 0.25% of range. More preferably, it is 0.08 to 0.20% of range.
　Ｍｎ：０．３５～２．０％
　Ｍｎは本発明において特に重要な元素である。Ｍｎは脱酸剤として有効な元素であるとともに、オーステナイト相を安定化する効果を有する。所定量のＭｎを含有させることにより、溶接時のフェライト相の生成を抑制することができ、フェライト相の固溶限以上に含まれたＣおよびＮをフェライト相よりも固溶限が大きいオーステナイト相中に固溶させた状態で安定化することにより、Ｃｒ炭窒化物の生成による鋭敏化を防止する効果が発現し、溶接部の耐食性が向上する。これらの効果を得るためには０．３５％以上のＭｎを含有する必要がある。しかし、Ｍｎ含有量が２．０％を超えると、母材が過度に硬質化し延性が低下するとともに、溶接部においては、溶接部の硬質化による靭性の低下が生じるため好ましくない。そのため、Ｍｎ量は０．３５～２．０％の範囲とする。好ましくは０．５０～１．５％の範囲である。さらに好ましくは０．７５～１．２５％の範囲である。 Mn: 0.35 to 2.0%
Mn is a particularly important element in the present invention. Mn is an element effective as a deoxidizer and has an effect of stabilizing the austenite phase. By containing a predetermined amount of Mn, the formation of a ferrite phase during welding can be suppressed, and C and N contained above the solid solubility limit of the ferrite phase have an austenite phase having a larger solid solubility limit than the ferrite phase. By stabilizing in a solid solution state, the effect of preventing sensitization due to the formation of Cr carbonitride is exhibited, and the corrosion resistance of the welded portion is improved. In order to obtain these effects, it is necessary to contain 0.35% or more of Mn. However, if the Mn content exceeds 2.0%, the base material is excessively hardened and ductility is lowered, and in the welded portion, the toughness is lowered due to the hardened welded portion, which is not preferable. Therefore, the amount of Mn is set in the range of 0.35 to 2.0%. Preferably, it is in the range of 0.50 to 1.5%. More preferably, it is in the range of 0.75 to 1.25%.
　Ｐ：０．０５％以下
　Ｐは鋼に不可避的に含まれる元素であり、過剰な含有は溶接性を低下させ、粒界腐食を生じやすくさせる。この傾向は０．０５％超の含有で顕著となる。そのため、Ｐ量は０．０５％以下とする。好ましくは０．０３％以下である。 P: 0.05% or less P is an element inevitably contained in steel. Excessive content decreases weldability and easily causes intergranular corrosion. This tendency becomes remarkable when the content exceeds 0.05%. Therefore, the P content is 0.05% or less. Preferably it is 0.03% or less.
　Ｓ：０．０１％以下
　ＳもＰと同様に鋼に不可避的に含まれる元素であり、０．０１％超の含有によって耐食性が低下する。そのため、Ｓ量は０．０１％以下とする。好ましくは０．００８％以下である。 S: 0.01% or less S, like P, is an element inevitably contained in steel. Corrosion resistance is reduced by the inclusion of over 0.01%. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less.
　Ａｌ：０．０５～０．８０％
　ＡｌもＳｉと同様に溶接部の耐食性を向上させる元素である。ＡｌはＮとの親和力がＣｒよりも強いため、溶接部にＮが混入した場合に、ＮをＣｒ窒化物ではなくＡｌ窒化物として析出させ、鋭敏化を抑制する効果がある。また、Ａｌは製鋼工程における脱酸に有用な元素でもある。これらの効果は０．０５％以上の含有で得られる。しかし、０．８０％を超えてＡｌを含有すると、フェライト結晶粒が粗大化し、加工性や製造性が低下する。そのため、Ａｌ量は０．０５～０．８０％の範囲とする。好ましくは、０．１０～０．６０％の範囲である。さらに好ましくは０．１５～０．５０％の範囲である。 Al: 0.05 to 0.80%
Al, like Si, is an element that improves the corrosion resistance of the weld. Since Al has a stronger affinity for N than Cr, when N is mixed in the welded portion, N is precipitated as Al nitride instead of Cr nitride and has the effect of suppressing sensitization. Al is also an element useful for deoxidation in the steel making process. These effects can be obtained with a content of 0.05% or more. However, when Al is contained exceeding 0.80%, ferrite crystal grains become coarse, and workability and manufacturability deteriorate. Therefore, the Al content is set in the range of 0.05 to 0.80%. Preferably, it is 0.10 to 0.60% of range. More preferably, it is in the range of 0.15 to 0.50%.
　Ｎ：０．００１～０．０２５％
　ＮはＣと同様に鋼中に不可避的に含まれる元素である。Ｎ含有量が高いと強度が向上し、少ないほど加工性が向上する。十分な強度を得るためには０．００１％以上の含有が適当である。しかしながら、含有量が０．０２５％を超えると延性が顕著に低下する上、Ｃｒ窒化物の析出を助長することによる耐食性の低下が生じるため好ましくない。そのため、Ｎ量は０．００１～０．０２５％の範囲とする。耐食性の観点からＮは低いほど好ましい。しかしながら、Ｎ量を低減するには精錬時間を増加させる必要があり、製造性の低下を招く。このため、０．００３～０．０２５％の範囲とすることが好ましい。より好ましくは０．００３～０．０１５％の範囲である。さらに好ましくは０．００３～０．０１０％の範囲である。 N: 0.001 to 0.025%
N, like C, is an element inevitably contained in steel. When the N content is high, the strength is improved, and as the N content is low, the workability is improved. In order to obtain sufficient strength, the content of 0.001% or more is appropriate. However, if the content exceeds 0.025%, the ductility is remarkably lowered and the corrosion resistance is reduced by promoting the precipitation of Cr nitride, which is not preferable. Therefore, the N content is set in the range of 0.001 to 0.025%. From the viewpoint of corrosion resistance, N is preferably as low as possible. However, in order to reduce the amount of N, it is necessary to increase the refining time, resulting in a decrease in manufacturability. For this reason, the range of 0.003 to 0.025% is preferable. More preferably, it is in the range of 0.003 to 0.015%. More preferably, it is 0.003 to 0.010% of range.
　Ｃｒ：１６．０～２０．０％
　Ｃｒはステンレス鋼の耐食性を確保するために最も重要な元素である。Ｃｒ含有量が１６．０％未満では、溶接による酸化で表層のＣｒが減少する溶接ビードやその周辺において十分な耐食性が得られない。また、溶接の際に溶接相手材あるいは大気から混入するＮに起因する鋭敏化が一層助長されるため好ましくない。一方、Ｃｒ含有量が２０．０％を超えると、靭性の低下や焼鈍後の脱スケール性が低下するため好ましくない。そのため、Ｃｒ量は１６．０％～２０．０％の範囲とする。好ましくは、１６．５％～１９．０％の範囲である。さらに好ましくは１７．０～１８．５％の範囲である。 Cr: 16.0-20.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 16.0%, sufficient corrosion resistance cannot be obtained in the weld bead where the surface layer Cr decreases due to oxidation by welding and in the vicinity thereof. Further, sensitization due to N mixed from the welding partner material or the atmosphere during welding is further promoted, which is not preferable. On the other hand, if the Cr content exceeds 20.0%, the toughness and the descalability after annealing decrease, which is not preferable. Therefore, the Cr content is set in the range of 16.0% to 20.0%. Preferably, it is in the range of 16.5% to 19.0%. More preferably, it is in the range of 17.0 to 18.5%.
　Ｔｉ：０．１２～０．５０％
　ＴｉはＣおよびＮと優先的に結合してＣｒ炭窒化物の析出による鋭敏化に起因した耐食性の低下を抑制する元素である。この効果は、０．１２％以上の含有によって得られる。しかし、含有量が０．５０％を超えると、粗大なＴｉ炭窒化物が生成し、表面欠陥を引き起こすため好ましくない。そのため、Ｔｉ量は０．１２～０．５０％の範囲とする。好ましくは、０．１５～０．４０％の範囲である。さらに好ましくは０．２０～０．３５％の範囲である。 Ti: 0.12 to 0.50%
Ti is an element that preferentially binds to C and N and suppresses a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitride. This effect is obtained when the content is 0.12% or more. However, if the content exceeds 0.50%, coarse Ti carbonitrides are generated and cause surface defects, which is not preferable. Therefore, the Ti amount is set in the range of 0.12 to 0.50%. Preferably, it is in the range of 0.15 to 0.40%. More preferably, it is 0.20 to 0.35% of range.
　Ｎｂ：０．００２～０．０５０％
　ＮｂはＣおよびＮと優先的に結合してＣｒ炭窒化物の析出による鋭敏化に起因した耐食性の低下を抑制する元素である。また、Ｎｂは溶接部の結晶粒径を微細化させ、溶接部の靭性および曲げ性を向上させる効果もある。これらの効果は、０．００２％以上の含有で得られる。一方、Ｎｂは再結晶温度を上昇させる元素でもあり、０．０５０％を超えて含有すると、再結晶に必要な焼鈍温度が高温化するため、高速冷延板焼鈍ラインを用いた焼鈍工程において焼鈍が不十分となり、未再結晶粒と再結晶粒が混在することによる加工性の低下が生じるため好ましくない。そのため、Ｎｂ量は０．００２～０．０５０％の範囲とする。好ましくは０．０１０～０．０４５％の範囲である。さらに好ましくは０．０１５～０．０４０％の範囲である。 Nb: 0.002 to 0.050%
Nb is an element that preferentially binds to C and N and suppresses a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitride. Nb also has the effect of reducing the crystal grain size of the weld and improving the toughness and bendability of the weld. These effects are obtained when the content is 0.002% or more. On the other hand, Nb is also an element that raises the recrystallization temperature, and if it exceeds 0.050%, the annealing temperature necessary for recrystallization increases, so annealing in the annealing process using a high-speed cold-rolled sheet annealing line Is insufficient, and undesired grains and recrystallized grains coexist, resulting in a decrease in workability. Therefore, the Nb amount is set to a range of 0.002 to 0.050%. Preferably it is 0.010 to 0.045% of range. More preferably, it is in the range of 0.015 to 0.040%.
　Ｃｕ：０．３０～０．８０％
　Ｃｕは耐食性を向上させる元素であり、水溶液中や弱酸性の水滴が付着した場合の母材および溶接部の耐食性を向上させるのに特に有効な元素である。また、ＣｕはＮｉと同様に強いオーステナイト生成元素であり、溶接部でのフェライト相の生成を抑制し、Ｃｒ炭窒化物の析出による鋭敏化を抑制する効果がある。これらの効果は０．３０％以上の含有で得られる。一方、０．８０％を超えてＣｕを含有すると、熱間加工性が低下するため好ましくない。そのため、Ｃｕ量は０．３０～０．８０％の範囲とする。好ましくは０．３０～０．６０％の範囲である。さらに好ましくは０．３５～０．５０％の範囲である。 Cu: 0.30 to 0.80%
Cu is an element that improves the corrosion resistance, and is an element that is particularly effective for improving the corrosion resistance of the base material and the welded part when an aqueous solution or weakly acidic water droplets adhere. Further, Cu is a strong austenite-forming element like Ni, and has the effect of suppressing the formation of ferrite phase in the weld and suppressing sensitization due to the precipitation of Cr carbonitride. These effects are obtained when the content is 0.30% or more. On the other hand, if Cu exceeds 0.80%, the hot workability is lowered, which is not preferable. Therefore, the Cu content is set in the range of 0.30 to 0.80%. Preferably it is 0.30 to 0.60% of range. More preferably, it is in the range of 0.35 to 0.50%.
　Ｎｉ：０．０５％以上０．５０％未満
　Ｎｉはステンレス鋼の耐食性を向上させる元素であり、不動態皮膜が形成できず活性溶解が生じる腐食環境において腐食の進行を抑制する元素である。また、Ｎｉは強いオーステナイト生成元素であり、溶接部でのフェライト生成を抑制し、Ｃｒ炭窒化物の析出による鋭敏化を抑制する効果がある。これらの効果は０．０５％以上の含有によって得られる。しかし、Ｎｉを０．５０％以上含有すると、加工性が低下することに加えて、応力腐食割れ感受性が強くなる。さらには、Ｎｉは高価な元素であるため、製造コストの増大を招くため好ましくない。そのため、Ｎｉ量は０．０５％以下０．５０％未満の範囲とする。好ましくは０．１０～０．３０％の範囲である。さらに好ましくは０．１５～０．２５％の範囲である。 Ni: 0.05% or more and less than 0.50% Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment where a passive film cannot be formed and active dissolution occurs. Ni is a strong austenite generating element, and has the effect of suppressing ferrite formation at the weld and suppressing sensitization due to precipitation of Cr carbonitride. These effects are obtained when the content is 0.05% or more. However, when Ni is contained in an amount of 0.50% or more, workability is lowered and stress corrosion cracking sensitivity is increased. Furthermore, since Ni is an expensive element, it causes an increase in manufacturing cost, which is not preferable. Therefore, the amount of Ni is made 0.05% or less and less than 0.50%. Preferably it is 0.10 to 0.30% of range. More preferably, it is in the range of 0.15 to 0.25%.
　Ｖ：０．０１～０．５０％
　Ｖは耐食性や加工性を向上させる元素であり、溶接部にＮが混入した場合にＮと結合することによってＣｒ窒化物の生成を抑制し、溶接部の鋭敏化を低減する効果を有する。この効果は０．０１％以上の含有によって得られる。しかし、含有量が０．５０％を超えると加工性が低下するため好ましくない。そのため、Ｖ量は０．０１～０．５０％の範囲とする。好ましくは０．０５～０．３０％の範囲である。さらに好ましくは０．０８～０．２０％の範囲である。 V: 0.01 to 0.50%
V is an element that improves corrosion resistance and workability, and has the effect of suppressing the formation of Cr nitride and reducing the sensitization of the welded portion by combining with N when N is mixed in the welded portion. This effect is obtained when the content is 0.01% or more. However, if the content exceeds 0.50%, workability is lowered, which is not preferable. Therefore, the V amount is in the range of 0.01 to 0.50%. Preferably it is 0.05 to 0.30% of range. More preferably, it is 0.08 to 0.20% of range.
　０．５０＜２５×Ｃ＋１８×Ｎ＋Ｎｉ＋０．１１×Ｍｎ＋０．４６×Ｃｕ　　（１）
　　　なお、式中の元素記号は各元素の含有量（質量％）を意味する。 0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
In addition, the element symbol in a formula means content (mass%) of each element.
　溶接部のＣおよびＮをオーステナイト相中に固溶させて固定化するためには、溶接後の冷却組織中にオーステナイト相を生成させる必要がある。溶接後の冷却過程においてオーステナイト相を生成させるためには、上記式（１）を満たす必要がある。式（１）の右辺が０．５０以下の場合、オーステナイト相の安定化が不十分となり、溶接部において本発明が提供する固溶Ｃおよび固溶Ｎを効果的に固定化するだけのオーステナイト相を生成させることができない。そのため、式（１）の右辺は０．５超えとする。好ましくは０．６０以上である。より好ましくは、０．７０以上である。 In order to fix and fix C and N in the weld zone in the austenite phase, it is necessary to generate an austenite phase in the cooled structure after welding. In order to generate the austenite phase in the cooling process after welding, it is necessary to satisfy the above formula (1). When the right side of the formula (1) is 0.50 or less, the austenite phase is not sufficiently stabilized, and the austenite phase that only effectively fixes the solid solution C and solid solution N provided by the present invention in the weld zone. Cannot be generated. Therefore, the right side of the formula (1) is over 0.5. Preferably it is 0.60 or more. More preferably, it is 0.70 or more.
　以上が本発明の基本化学成分であり、残部はＦｅおよび不可避不純物からなる。なお、不可避不純物としては、Ｃａ：０．００２０％以下が許容できる。 The above are the basic chemical components of the present invention, and the balance consists of Fe and inevitable impurities. As an inevitable impurity, Ca: 0.0020% or less is acceptable.
　さらに、上記基本成分に加えて、溶接ビードの鋭敏化の抑制および耐食性の向上等の目的で以下の元素を含有してもよい。 Furthermore, in addition to the above basic components, the following elements may be contained for the purpose of suppressing the sensitization of the weld bead and improving the corrosion resistance.
　Ｚｒ：０．０１～０．５０％
　ＺｒはＣおよびＮと結合して鋭敏化を抑制する効果がある。この効果は０．０１％以上の含有により得られる。しかしながら、０．５０％を超えて含有すると加工性が低下する。また、Ｚｒは高価な元素であるため、過度な添加は製造コストの増加を招くため好ましくない。そのため、Ｚｒを含有する場合は０．０１～０．５０％の範囲とすることが好ましい。より好ましくは、０．１０～０．３５％の範囲である。 Zr: 0.01 to 0.50%
Zr has an effect of binding to C and N to suppress sensitization. This effect is obtained when the content is 0.01% or more. However, if it exceeds 0.50%, workability is lowered. Moreover, since Zr is an expensive element, excessive addition causes an increase in manufacturing cost, which is not preferable. Therefore, when Zr is contained, the content is preferably in the range of 0.01 to 0.50%. More preferably, it is in the range of 0.10 to 0.35%.
　Ｗ：０．０１～０．２０％
　ＷはＭｏと同様に耐食性を向上させる効果がある。この効果は０．０１％以上の含有により得られる。しかしながら、０．２０％を超えて含有すると強度が上昇し、圧延荷重の増大等による製造性の低下を招くため好ましくない。そのため、Ｗを含有する場合は０．０１～０．２０％の範囲とすることが好ましい。より好ましくは、０．０５～０．１５％の範囲である。 W: 0.01-0.20%
W, like Mo, has the effect of improving corrosion resistance. This effect is obtained when the content is 0.01% or more. However, if the content exceeds 0.20%, the strength increases, and the productivity decreases due to an increase in rolling load, etc., which is not preferable. Therefore, when it contains W, it is preferable to set it as 0.01 to 0.20% of range. More preferably, it is in the range of 0.05 to 0.15%.
　ＲＥＭ：０．００１～０．１０％
　ＲＥＭは耐酸化性を向上させる効果があり、酸化スケールの形成の抑制による、溶接テンパーカラー直下におけるＣｒ欠乏領域の形成の抑制に有効である。この効果を得るためには０．００１％以上の含有が必要である。しかしながら、０．１０％を超えて含有すると酸洗性などの製造性を低下させる。また、ＲＥＭはＺｒと同様に高価な元素であるため、過度な含有は製造コストの増加を招くため好ましくない。そのため、ＲＥＭを含有する場合は０．００１～０．１０％の範囲とすることが好ましい。より好ましくは、０．０１０～０．０８％の範囲である。 REM: 0.001 to 0.10%
REM has an effect of improving oxidation resistance, and is effective in suppressing the formation of a Cr-deficient region directly under the weld temper collar by suppressing the formation of oxide scale. In order to acquire this effect, 0.001% or more needs to be contained. However, if the content exceeds 0.10%, productivity such as pickling properties is lowered. Moreover, since REM is an expensive element like Zr, excessive inclusion causes an increase in manufacturing cost, which is not preferable. Therefore, when it contains REM, it is preferable to set it as 0.001 to 0.10% of range. More preferably, it is in the range of 0.010 to 0.08%.
　Ｃｏ：０．０１～０．２０％
　Ｃｏは靭性を向上させる元素である。この効果は０．０１％以上の含有によって得られる。一方、０．２０％を超えて含有すると製造性を低下させる。そのため、Ｃｏを含有する場合は０．０１～０．２０％の範囲とすることが好ましい。より好ましくは、０．０５～０．１５％の範囲である。 Co: 0.01-0.20%
Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, when it contains exceeding 0.20%, manufacturability will be reduced. Therefore, when Co is contained, the content is preferably 0.01 to 0.20%. More preferably, it is in the range of 0.05 to 0.15%.
　Ｂ：０．０００２～０．０１０％
　Ｂは二次加工脆性を改善する元素であり、その効果は０．０００２％以上の含有によって得られる。しかし、０．０１０％を超えて含有すると、過度な固溶強化による延性の低下を誘引する。そのため、Ｂを含有する場合は０．０００２～０．０１０％の範囲とすることが好ましい。より好ましくは、０．００１０～０．００７５％の範囲である。 B: 0.0002 to 0.010%
B is an element that improves secondary work brittleness, and the effect is obtained by the content of 0.0002% or more. However, when it exceeds 0.010%, it induces a decrease in ductility due to excessive solid solution strengthening. Therefore, when B is contained, the content is preferably in the range of 0.0002 to 0.010%. More preferably, it is in the range of 0.0010 to 0.0075%.
　Ｓｂ：０．０５～０．３０％
　ＳｂはＡｌと同じく、ＴＩＧ溶接のガスシールドが不十分な場合に、大気から混入するＮを捕らえる効果があり、十分なガスシールドを行うことが困難な複雑形状を有する構造体に適用する場合に、特に有効な元素である。その効果は０．０５％以上の含有により得られる。しかしながら、０．３０％を超えて含有すると、製鋼工程において非金属介在物が生成し、表面性状が悪化する。また、熱延板の靭性を悪化させる。そのため、Ｓｂを含有する場合は、０．０５～０．３０％の範囲とすることが好ましい。より好ましくは０．０５～０．１５％の範囲である。 Sb: 0.05 to 0.30%
Sb, like Al, has an effect of capturing N mixed in from the atmosphere when the gas shield of TIG welding is insufficient, and is applied to a structure having a complicated shape that makes it difficult to perform sufficient gas shield. Is a particularly effective element. The effect is acquired by 0.05% or more of containing. However, when it contains exceeding 0.30%, a nonmetallic inclusion will produce | generate in a steelmaking process and surface properties will deteriorate. Moreover, the toughness of a hot-rolled sheet is deteriorated. Therefore, when it contains Sb, it is preferable to set it as 0.05 to 0.30% of range. More preferably, it is in the range of 0.05 to 0.15%.
　２．製造条件について
　次に、本発明鋼の好適製造方法について説明する。上記した成分組成の溶鋼を、転炉、電気炉、真空溶解炉等の公知の方法で溶製し、連続鋳造法あるいは造塊－分塊法により鋼素材（スラブ）とする。このスラブを、１１００～１２５０℃で１～２４時間加熱後に熱間圧延するか、あるいは加熱することなく鋳造のまま直接、熱間圧延して熱延板とする。 2. Next, a preferred method for producing the steel of the present invention will be described. Molten steel having the above component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and a steel material (slab) is obtained by a continuous casting method or an ingot-bundling method. The slab is hot-rolled after heating at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-cast as cast without heating to form a hot-rolled sheet.
　通常、熱延板は８００～１１００℃で１～１０分の熱延板焼鈍が施される。なお、用途によっては熱延板焼鈍を省略しても良い。ついで、熱延板酸洗後、冷間圧延により冷延板とした後、再結晶焼鈍、酸洗を施して製品とする。 Usually, hot-rolled sheets are annealed at 800 to 1100 ° C. for 1 to 10 minutes. Depending on the application, the hot-rolled sheet annealing may be omitted. Next, after hot-rolled sheet pickling, it is cold-rolled by cold rolling, and then subjected to recrystallization annealing and pickling to obtain a product.
　冷間圧延は伸び性、曲げ性、プレス成形性および形状の観点から５０％以上の圧下率で行うことが望ましい。冷延板の再結晶焼鈍は、一般的にはＪＩＳ　Ｇ　０２０３の表面仕上げ、Ｎｏ．２Ｂ仕上げ品の場合、良好な機械的性質を得ること、および酸洗性の面から８００～９５０℃で行うことが好ましい。また、より光沢を求める箇所の部材には仕上げにＢＡ焼鈍（光輝焼鈍）を行うことが有効である。なお、冷間圧延後および加工後にさらに表面性状を向上させるために、研磨等を施しても良い。 Cold rolling is desirably performed at a rolling reduction of 50% or more from the viewpoint of stretchability, bendability, press formability, and shape. The recrystallization annealing of cold-rolled sheets is generally performed according to JIS G 0203 surface finish, No. In the case of a 2B finished product, it is preferable to carry out at 800 to 950 ° C. from the viewpoint of obtaining good mechanical properties and pickling properties. In addition, it is effective to perform BA annealing (bright annealing) for finishing the member where the luster is desired. In addition, you may give grinding | polishing etc. in order to improve a surface property further after cold rolling and after a process.
　以下、実施例に基づいて本発明をさらに詳しく説明する。 Hereinafter, the present invention will be described in more detail based on examples.
　表１に示す化学組成を有するステンレス鋼を５０ｋｇ小型真空溶解炉にて溶製した。これらの鋼塊を１１５０℃で１時間加熱後、熱間圧延を施して３．５ｍｍ厚の熱延板とした。ついで、これらの熱延板に９５０℃で１分間の熱延板焼鈍を施した後、表面にショットブラスト処理を行った後、温度８０℃、２０質量％硫酸溶液中に１２０秒浸漬後、１５質量％硝酸および３質量％弗酸よりなる温度５５℃の混合酸中に６０秒浸漬することにより酸洗を行い、脱スケールを行った。 Stainless steel having the chemical composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated at 1150 ° C. for 1 hour and then hot rolled to form 3.5 mm thick hot rolled sheets. Next, these hot-rolled sheets were subjected to hot-rolled sheet annealing at 950 ° C. for 1 minute, and then subjected to shot blast treatment on the surface, and then immersed in a 20% by mass sulfuric acid solution at a temperature of 80 ° C. for 15 seconds. The pickling was performed by dipping in a mixed acid composed of mass% nitric acid and 3 mass% hydrofluoric acid at a temperature of 55 ° C. for 60 seconds, and descaling was performed.
　さらに、板厚０．８ｍｍまで冷間圧延し、弱還元性雰囲気（水素：５ｖｏｌ％、窒素：９５ｖｏｌ％、露点：－４０℃）において９００℃で１分間の再結晶焼鈍を行い、冷延焼鈍板を得た。この冷延焼鈍板を、温度５０℃、１５質量％硝酸および０．５質量％塩酸からなる混合酸液中で電解酸洗することにより脱スケール処理を行い、冷延酸洗焼鈍板を得た。 Further, it was cold-rolled to a thickness of 0.8 mm and recrystallized and annealed at 900 ° C. for 1 minute in a weakly reducing atmosphere (hydrogen: 5 vol%, nitrogen: 95 vol%, dew point: −40 ° C.). I got a plate. The cold-rolled annealed sheet was descaled by electrolytic pickling in a mixed acid solution consisting of a temperature of 50 ° C., 15% by mass nitric acid and 0.5% by mass hydrochloric acid to obtain a cold-rolled pickled annealed sheet. .
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　なお、表１－１、表１－２は連続した一連の表である。 Table 1-1 and Table 1-2 are a series of continuous tables.
　作製した冷延板および市販のオーステナイト系ステンレス鋼ＳＵＳ３０４（Ｃ：０．０７質量％、Ｎ：０．０５質量％、Ｃｒ：１８．２質量％、Ｎｉ：８．２質量％）の板厚０．８ｍｍの冷延板を用いて、
突合せＴＩＧ溶接(butt TIG welding)を行った（本発明の冷延板：母材、溶接相手材：ＳＵＳ３０４）。溶接電流は９０Ａ、溶接速度は６０ｃｍ／ｍｉｎとし、シールドガスには８ｖｏｌ％の窒素、２ｖｏｌ％の酸素を含有するＡｒガスを１５Ｌ／ｍｉｎで使用した。得られた表側の溶接ビードの幅はおよそ３ｍｍであった。 Thickness 0 of manufactured cold rolled sheet and commercially available austenitic stainless steel SUS304 (C: 0.07 mass%, N: 0.05 mass%, Cr: 18.2 mass%, Ni: 8.2 mass%) .8mm cold rolled plate,
Butt TIG welding was performed (cold rolled sheet of the present invention: base material, welding partner material: SUS304). The welding current was 90 A, the welding speed was 60 cm / min, and Ar gas containing 8 vol% nitrogen and 2 vol% oxygen was used at 15 L / min as the shielding gas. The width of the obtained front-side weld bead was about 3 mm.
　作製した溶接ビードを含む試験片を採取して以下の試験を行った。 The test piece including the produced weld bead was collected and the following test was performed.
　１．母材および溶接部の孔食電位試験
　冷延焼鈍後の試験片、および溶接後の試験片から２０ｍｍ角の試験片を採取し、１０ｍｍ角の測定面を残してシール材で被覆した試験片を作製した。溶接後の試験片については溶接ビードを含むように試験片を採取し、溶接によるテンパーカラー（酸化皮膜）を残したままとした。これらの試験片について、３０℃の３．５質量％ＮａＣｌ溶液中で母材および溶接部の孔食電位を測定した。測定に際して、試験片の研磨や不動態化処理を行わなかったが、それ以外の測定方法はＪＩＳ　Ｇ　０５７７（２００５）に準拠した。 1. Pitting corrosion potential test of base metal and welded part A test piece of 20 mm square was taken from a test piece after cold rolling annealing and a test piece after welding, and a test piece covered with a sealing material was left leaving a 10 mm square measurement surface. Produced. About the test piece after welding, the test piece was extract | collected so that a weld bead was included, and the temper color (oxide film) by welding was left. About these test pieces, the pitting corrosion potential of the base material and the weld was measured in a 3.5 mass% NaCl solution at 30 ° C. At the time of measurement, the specimen was not polished or passivated, but other measurement methods were based on JIS G 0577 (2005).
　母材の孔食電位：１５０ｍＶ以上、溶接部の孔食電位：０ｍＶ以上を合格とした。 The pitting corrosion potential of the base material: 150 mV or more, and the pitting corrosion potential of the welded portion: 0 mV or more was regarded as acceptable.
　２．中性塩水噴霧サイクル試験
　溶接後の試験片から、溶接ビードを含む１００ｍｍ角の試験片を採取し、表面を＃６００エメリーペーパーにより研磨仕上げした後に端面部をシールした試験片を作製し、ＪＩＳ　Ｈ　８５０２に規定された中性塩水噴霧サイクル試験に供した。中性塩水噴霧サイクル試験は、５質量％ＮａＣｌ溶液噴霧（３５℃、２ｈ）→乾燥（６０℃、４ｈ、相対湿度２０～３０％）→湿潤(moistness)（４０℃、２ｈ、相対湿度９５％以上）が１サイクルである。これを１５サイクル実施後に母材あるいは溶接部からの腐食の発生がない場合を合格とした。 2. Neutral salt spray cycle test A 100 mm square test piece including a weld bead was taken from the welded test piece, and the surface was polished with # 600 emery paper, and then a test piece with a sealed end face was prepared. The sample was subjected to a neutral salt spray cycle test specified in 8502. Neutral salt spray cycle test: 5% by weight NaCl solution spray (35 ° C., 2 h) → dry (60 ° C., 4 h, relative humidity 20-30%) → moistness (40 ° C., 2 h, relative humidity 95% The above is one cycle. The case where there was no occurrence of corrosion from the base metal or the welded part after 15 cycles was regarded as acceptable.
　３．Ｃｒ炭窒化物によるフェライト相の粒界被覆率測定
　溶接した試験片の溶接ビードの直角方向に金属組織観察用試験片を採取し、鏡面研磨後、ピクリン酸塩酸水溶液によるエッチングにより金属組織および析出物を現出させ、走査型電子顕微鏡およびエネルギー分散型Ｘ線分光法を用いた組織観察および析出物の相同定を行い、溶接ビード部のフェライト相のＣｒ炭窒化物による粒界被覆率を測定した。 3. Measurement of grain boundary coverage of ferrite phase with Cr carbonitride Samples for metallographic observation were collected in the direction perpendicular to the weld bead of the welded test pieces, mirror-polished, and etched by picric acid hydrochloric acid aqueous solution. The structure was observed using a scanning electron microscope and energy dispersive X-ray spectroscopy, the phase of the precipitate was identified, and the grain boundary coverage of the ferrite phase of the weld bead by Cr carbonitride was measured. .
　粒界被覆率は、撮影した組織写真中の結晶粒の粒界長さを画像解析装置により計測するとともに、粒界上に析出したＣｒ炭窒化物の粒界に平行な方向の径を同じく画像解析装置により計測し、粒界被覆率（％）＝（粒界上のＣｒ炭窒化物の粒界に平行な方向の径の総計）÷（結晶粒の粒界長さの総計）×１００の式により算出した。 The grain boundary coverage is measured by measuring the grain boundary length of the crystal grains in the photographed structure photograph using an image analyzer, and the diameter in the direction parallel to the grain boundary of Cr carbonitride deposited on the grain boundaries is also measured. Measured with an analysis device, grain boundary coverage (%) = (total diameter in the direction parallel to the grain boundary of Cr carbonitride on the grain boundary) ÷ (total grain boundary length of crystal grains) × 100 Calculated by the formula.
　Ｃｒ炭窒化物によるフェライト相の粒界被覆率が４０％以下の場合を合格とした。 The case where the grain boundary coverage of the ferrite phase with Cr carbonitride was 40% or less was regarded as acceptable.
　４．機械特性評価
　作製した冷延焼鈍板から、圧延方向と平行にＪＩＳ　１３Ｂ号引張試験片を採取し、引張試験をＪＩＳ　Ｚ２２４１に準拠して行い、破断伸びを測定した。 4). Mechanical property evaluation A JIS No. 13B tensile test piece was taken in parallel with the rolling direction from the produced cold-rolled annealed plate, a tensile test was performed in accordance with JIS Z2241, and elongation at break was measured.
　破断伸びが２５％以上を合格とした。 An elongation at break of 25% or more was considered acceptable.
　５．表面品質評価
　冷延焼鈍後、脱スケールした鋼板表面を肉眼で観察し、脱スケール不良および線状疵等の表面欠陥がないか確認した。 5. Surface Quality Evaluation After cold-rolling annealing, the scaled steel sheet surface was observed with the naked eye, and it was confirmed that there were no surface defects such as poor descaling and linear wrinkles.
　スケール残りや表面欠陥がなく、良好な表面外観が得られていることを合格基準とした。 The acceptance criteria were that there was no scale residue or surface defects and a good surface appearance was obtained.
　試験結果を表２に示す。 Table 2 shows the test results.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　本発明の要件を満たす鋼Ｎｏ．１～２０はいずれも母材の孔食電位が１５０ｍＶ以上、溶接ビードの孔食電位が０ｍＶ以上であるとともに、中性塩水噴霧サイクル試験によっても腐食は発生せず、オーステナイト系ステンレス鋼と溶接を施した場合においても十分な耐食性が得られている。また、鋼Ｎｏ．１～２０はいずれも溶接後のＣｒ炭窒化物によるフェライト相の粒界被覆率が４０％以下であり、所定の鋭敏化防止効果が得られている。さらに、引張試験による破断伸びはいずれも２５％以上と良好な加工特性が得られているとともに、表面欠陥も認められなかった。 Steel No. satisfying the requirements of the present invention In all of Nos. 1 to 20, the pitting corrosion potential of the base metal is 150 mV or more, the pitting corrosion potential of the weld bead is 0 mV or more, and no corrosion occurs even in the neutral salt spray cycle test. Even when applied, sufficient corrosion resistance is obtained. Steel No. In all of Nos. 1 to 20, the grain boundary coverage of the ferrite phase by Cr carbonitride after welding is 40% or less, and a predetermined sensitization preventing effect is obtained. Furthermore, the elongation at break by the tensile test was 25% or more, and good processing characteristics were obtained, and no surface defects were observed.
　一方、Ｃｒ量が本発明の範囲を上回る鋼Ｎｏ．２１では熱間圧延後の鋼板の靭性が著しく低下し、以降の製造工程を実施することができず、特性を評価することができなかった。 On the other hand, steel No. with a Cr content exceeding the range of the present invention. In No. 21, the toughness of the steel sheet after hot rolling was remarkably lowered, the subsequent manufacturing process could not be carried out, and the characteristics could not be evaluated.
　Ｃｒ量が本発明の範囲を下回る鋼Ｎｏ．２２では、十分な孔食電位が得られず、また、中性塩水噴霧サイクル試験において母材および溶接部から腐食が生じ、所定の耐食性を得ることができなかった。 Steel No. whose Cr content is below the range of the present invention. In No. 22, a sufficient pitting corrosion potential was not obtained, and in the neutral salt spray cycle test, corrosion occurred from the base material and the welded portion, and the predetermined corrosion resistance could not be obtained.
　Ｍｎ量が本発明の範囲を下回る鋼Ｎｏ．２３では、母材に関しては十分な耐食性が得られたものの、溶接部に関して十分な耐食性が得られなかった。 Steel No. whose Mn content is below the range of the present invention. In No. 23, sufficient corrosion resistance was obtained for the base material, but sufficient corrosion resistance was not obtained for the welded portion.
　一方、Ｍｎ量またはＡｌ量が本発明の範囲を上回る鋼Ｎｏ．２４および２５では所定の母材ならびに溶接部耐食性は得られたものの、鋼板が硬質化したことにより延性が低下し、所定の機械特性を得ることができなかった。 On the other hand, the steel No. whose Mn amount or Al amount exceeds the range of the present invention. In 24 and 25, although the predetermined base metal and welded portion corrosion resistance were obtained, the ductility decreased due to the hardened steel sheet, and the predetermined mechanical properties could not be obtained.
　Ｔｉ量が本発明の範囲を上回る鋼Ｎｏ．２６では所定の耐食性および機械特性が得られたものの、粗大なＴｉ系介在物が多量に生成したことに起因する表面欠陥が発生し、所定の表面性状が得られなかった。 Steel No. with Ti amount exceeding the range of the present invention. In No. 26, although predetermined corrosion resistance and mechanical properties were obtained, surface defects were caused due to the generation of a large amount of coarse Ti-based inclusions, and predetermined surface properties could not be obtained.
　各元素の含有量は本発明の範囲を満たすものの、オーステナイト安定化元素の含有量が式（１）の範囲を下回る鋼Ｎｏ．２７～３０では、所定の母材耐食性ならびに機械特性は得られた。しかしながら、所定の溶接ビード部の孔食電位ならびに溶接部の耐食性が得られなかった。Ｎｏ．２７～３０の溶接部の断面組織を調査したところ、いずれもフェライト相粒界に粒界被覆率で５０％以上と極めて多量のＣｒ炭窒化物が析出していたことが確認された。鋼Ｎｏ．２７～３０ではオーステナイト安定化元素が不足したために、溶接後の冷却中にフェライト相が生成し、粒界上へのＣｒ炭窒化物の生成を抑制することができなかった結果、顕著な鋭敏化が生じ、所定の溶接部の耐食性が得られなかったと考えられる。 Although the content of each element satisfies the scope of the present invention, the content of the austenite stabilizing element is less than the range of the formula (1). In 27 to 30, predetermined base metal corrosion resistance and mechanical properties were obtained. However, the pitting corrosion potential of the predetermined weld bead and the corrosion resistance of the weld were not obtained. No. Examination of the cross-sectional structure of the welds 27 to 30 confirmed that an extremely large amount of Cr carbonitride was precipitated at the ferrite phase grain boundary with a grain boundary coverage of 50% or more. Steel No. In 27-30, since the austenite stabilizing element was insufficient, a ferrite phase was formed during cooling after welding, and the formation of Cr carbonitride on the grain boundary could not be suppressed. It is considered that the corrosion resistance of the predetermined weld was not obtained.
　以上の結果より、本発明によればＴｉやＮｂの過剰な添加を必要とせずに、優れた耐食性、機械特性ならびに表面性状を有するフェライト系ステンレス鋼が得られることが明らかとなった。 From the above results, it was found that the ferritic stainless steel having excellent corrosion resistance, mechanical properties and surface properties can be obtained according to the present invention without requiring excessive addition of Ti or Nb.
　本発明で得られるフェライト系ステンレス鋼は、溶接によって構造体の作製が行われる用途、例えば、マフラー等の自動車排気系材料、建具や換気口、ダクト等の建築用材料等への適用に好適である。 The ferritic stainless steel obtained in the present invention is suitable for applications in which structures are produced by welding, for example, automotive exhaust materials such as mufflers, building materials such as fittings, ventilation openings, ducts, etc. is there.

Claims (2)

  1. 　質量％で、Ｃ：０．００１～０．０２５％、Ｓｉ：０．０５～０．３０％、Ｍｎ：０．３５～２．０％、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．０５～０．８０％、Ｎ：０．００１～０．０２５％、Ｃｒ：１６．０～２０．０％、Ｔｉ：０．１２～０．５０％、Ｎｂ：０．００２～０．０５０％、Ｃｕ：０．３０～０．８０％、Ｎｉ：０．０５％以上０．５０％未満、Ｖ：０．０１～０．５０％を含有し、かつ下記式（１）を満たし、残部がＦｅおよび不可避不純物からなることを特徴とするフェライト系ステンレス鋼。
    　０．５０＜２５×Ｃ＋１８×Ｎ＋Ｎｉ＋０．１１×Ｍｎ＋０．４６×Ｃｕ　　（１）
    　　なお、式中の元素記号は各元素の含有量（質量％）を意味する。     In mass%, C: 0.001 to 0.025%, Si: 0.05 to 0.30%, Mn: 0.35 to 2.0%, P: 0.05% or less, S: 0.01 %: Al: 0.05 to 0.80%, N: 0.001 to 0.025%, Cr: 16.0 to 20.0%, Ti: 0.12 to 0.50%, Nb: 0 0.002 to 0.050%, Cu: 0.30 to 0.80%, Ni: 0.05% to less than 0.50%, V: 0.01 to 0.50%, and the following formula ( A ferritic stainless steel satisfying 1), the balance being Fe and inevitable impurities.
    0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
    In addition, the element symbol in a formula means content (mass%) of each element.
  2. 　更に、質量％で、Ｚｒ：０．０１～０．５０％、Ｗ：０．０１～０．２０％、ＲＥＭ：０．００１～０．１０％、Ｃｏ：０．０１～０．２０％、Ｂ：０．０００２～０．０１０％、Ｓｂ：０．０５～０．３０％の中から選ばれる１種以上を含有することを特徴とする請求項１に記載のフェライト系ステンレス鋼。 Further, by mass, Zr: 0.01 to 0.50%, W: 0.01 to 0.20%, REM: 0.001 to 0.10%, Co: 0.01 to 0.20%, 2. The ferritic stainless steel according to claim 1, comprising at least one selected from B: 0.0002 to 0.010% and Sb: 0.05 to 0.30%.
PCT/JP2014/003767 2013-07-29 2014-07-16 Ferritic stainless steel having excellent weld corrosion resistance WO2015015735A1 (en)

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CN201480041182.5A CN105408511B (en) 2013-07-29 2014-07-16 The ferrite-group stainless steel of the excellent corrosion resistance of weld part
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JP2014553008A JP5700182B1 (en) 2013-07-29 2014-07-16 Ferritic stainless steel with excellent corrosion resistance of welds
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KR102249965B1 (en) 2019-05-28 2021-05-11 주식회사 포스코 Austenitic stainless steel having excellent corrosion resistance of weld

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