JP2014077202A - Ferritic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel excellent in corrosion resistance at a weld zone.SOLUTION: A ferritic stainless steel contains, by mass%, C:0.003% to 0.014%, N:0.005% to 0.016%, C%+N%:0.023% or less, Si:0.01% to 0.90%, Mn:0.01% to 0.50%, P:0.020% to 0.040%, S:0.008% or less, Al:0.001% to 0.090%, Cr:14.5% to 23.0%, Ni:0.10% to 0.60%, V:0.010% to 0.040%, Mo:0.01% to 1.65%, further at least either of Ti and Nb in a range satisfying specific conditions and the balance Fe with inevitable impurities.

Description

  The present invention relates to a ferritic stainless steel, and more particularly to a ferritic stainless steel excellent in corrosion resistance of a welded portion.

  Stainless steel is roughly classified into ferritic stainless steel represented by SUS430 and austenitic stainless steel represented by SUS304. Ferritic stainless steel can be manufactured at low cost because the amount of Ni, which is an expensive element, is small compared to austenitic stainless steel. In addition, ferritic stainless steel has excellent characteristics such as the advantage of low deformation during welding due to its low thermal expansion coefficient and high thermal conductivity, excellent corrosion resistance in outdoor environments, and resistance to stress corrosion cracking. Yes. Therefore, ferritic stainless steel has been widely applied to various building materials, automobile parts, kitchen equipment, home appliances, water heaters, and the like, and the needs thereof have been further increased in recent years.

Ferritic stainless steel is often used by being welded with ferritic stainless steels or with austenitic stainless steel (for example, SUS304, etc.), and good corrosion resistance is required in the welded portion as well as the base material portion. However, when austenitic stainless steel such as SUS304 and ferritic stainless steel having higher C and N contents than ferritic steel types are welded, the corrosion resistance of the weld may be lower than that of the base metal due to a phenomenon called sensitization. is there. Sensitization means that C and N in steel are combined with Cr due to the thermal history of the welded portion, and precipitated as grain carbide as Cr carbide (eg Cr ₂₃ C ₆ ) or Cr nitride (Cr ₂ N). This is a phenomenon in which the Cr concentration in the boundary and the vicinity thereof is lower than that of the base material, and the corrosion resistance at the grain boundary is lowered. In recent years, as the structure of the welded part has become complicated, sufficient gas shielding cannot be performed during welding, and welding under incomplete conditions in which nitrogen in the air enters the molten pool has increased. Yes. Nitrogen that has entered the molten pool promotes sensitization of the welded portion by the same mechanism as described above, and causes a decrease in corrosion resistance. Therefore, ferritic stainless steel applied for such applications is required to be able to sufficiently secure the corrosion resistance of the welded portion even when the gas shield is insufficient during welding.

  In order to solve such problems, as disclosed in Patent Documents 1 and 2, a method of adding Ti and Nb to fix C and N in steel as carbides or nitrides and making them harmless has been proposed. However, when the gas shield is insufficient, sensitization may occur in the welded portion, and the corrosion resistance of the welded portion is not sufficient.

JP 51-88413 A JP 2007-270290 A

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a ferritic stainless steel excellent in corrosion resistance of a welded portion.

  The fixing mechanism of C and N by addition of Ti and Nb is that after Ti and Nb are dissolved in the base material during welding, they are precipitated again as Ti (C, N) or Nb (C, N) during cooling. C and N are fixed, and it is empirically found that it is effective to add Ti and Nb to 8 or more at Ti% / (C% + N%) or Nb% / (C% + N%). Are known.

  However, even if Ti and Nb are added in a range where Ti% / (C% + N%) or Nb% / (C% + N%) satisfies 8 or more, a sufficient sensitization suppressing effect can be obtained. The inventors found that there might not be, and the inventors investigated the cause. As a result, in conventional Ti and Nb-added ferritic stainless steels, the solution temperature and precipitation peak temperature of Ti (C, N) or Nb (C, N) (hereinafter referred to as Ti, Nb carbonitride) are low. Since both were high, they could not be sufficiently precipitated during cooling after welding, and it was found that solute C and N may remain and sensitization may occur due to precipitation of Cr carbonitride. Therefore, in order to improve the corrosion resistance of the welded portion, a technique for fixing solute C and N more than before is required in the cooling process after welding.

  Therefore, the present inventors promoted the precipitation of Ti and Nb carbonitride during cooling after welding by lowering the precipitation peak temperature of these Ti and Nb carbonitrides, so that C, N We examined a method to fix the stencil sufficiently. As a result, when Ti and Nb-based carbonitrides contain an appropriate amount of V, these precipitates are composite carbonitriding such as (Ti, V) (C, N) or (Nb, V) (C, N), respectively. The precipitation temperature is lower than that of conventional Ti and Nb-based carbonitrides, and these composite carbonitrides including V fix C and N more than conventional Ti and Nb-based carbonitrides. It was found that the corrosion resistance of the welded portion is greatly improved.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  (1) By mass%, C: 0.003% to 0.014%, N: 0.005% to 0.016%, C% + N%: 0.023% or less, Si: 0.01% 0.90% or less, Mn: 0.01% or more and 0.50% or less, P: 0.020% or more and 0.040% or less, S: 0.008% or less, Al: 0.001% or more. 090% or less, Cr: 14.5% to 23.0%, Ni: 0.10% to 0.60%, V: 0.010% to 0.040%, and Ti: 0.15% or more and 0.34% or less and Ti% + Nb% ≦ 0.70 and V% / (Ti% + 0.5 × Nb%): Ti is contained within a range satisfying 0.05 to 0.20. Or when Ti and Nb are contained, or Nb: 0.35% or more and 0.60% or less and Ti% + Nb% ≦ 0. 0 and V% / (Ti% + 0.5 × Nb%): Nb is contained in a range satisfying 0.05 to 0.20 or at least one of cases where Nb and Ti are contained, with the balance being Fe and Ferritic stainless steel characterized by inevitable impurities. The C%, the N%, the Ti%, the Nb%, and the V% represent the contents (mass%) of Ti, Nb, and V, respectively.

  (2) It is characterized by containing one or two of Cu: 0.01% to 0.80% and Mo: 0.01% to 1.65% (1% by mass) (1 ) Ferritic stainless steel.

  (3) By mass%, Zr: 0.01% to 0.20%, REM: 0.001% to 0.100%, Co: 0.01% to 0.20%, B: 0.0002% or more and 0.0009% or less, Mg: 0.0002% or more and 0.0010% or less, Ca: 0.0005% or more and 0.0020% or less The ferritic stainless steel according to (1) or (2), wherein

  According to the present invention, a ferritic stainless steel having excellent corrosion resistance of a welded portion can be obtained. The ferritic stainless steel of the present invention has excellent corrosion resistance without causing sensitization even under welding conditions in which carbon and nitrogen enter from a welding partner material or welding conditions in which nitrogen enters from air. Therefore, it can be suitably used for applications in which structures are produced by welding, for example, automobile exhaust system materials such as mufflers, building materials such as fittings, ventilation openings, and ducts, electrical equipment, and kitchen products.

  The reason why the composition of the steel of the present invention is specified will be described below. In addition, all the component% means the mass% unless there is particular notice.

C: 0.003% or more and 0.014% or less When C is contained in excess of 0.014%, the workability and the corrosion resistance of the welded part are significantly lowered. A lower C content is preferable from the viewpoint of corrosion resistance and workability, but refining takes time to make the C content less than 0.003%, which is not preferable in production. Therefore, the C content is in the range of 0.003% to 0.014%. Preferably it is 0.004% or more and 0.011% or less of range.

N: 0.005% or more and 0.016% or less When N is contained in excess of 0.016%, the workability and the corrosion resistance of the welded part are significantly reduced. From the viewpoint of corrosion resistance, the lower the N content, the better. However, in order to reduce the N content to less than 0.005%, it is necessary to lengthen the refining time, which increases the manufacturing cost and decreases the productivity. Absent. Therefore, the N content is set in a range of 0.005% to 0.016%. Preferably it is 0.005% or more and 0.011% or less of range.

C% + N%: 0.023% or less C and N cause a decrease in workability and a decrease in corrosion resistance of the weld. The effect has a synergistic effect. When the total amount of C and N (C% + N%) exceeds 0.023%, the workability and the corrosion resistance of the welded portion become remarkable. Therefore, the range of (C% + N%) is set to 0.023% or less. Preferably it is less than 0.020%.

Si: 0.01% or more and 0.90% or less Si is an element that is effective as a deoxidizing element in the steelmaking process as well as being effective in improving the corrosion resistance of the welded part by concentrating on the oxide film formed during welding. . These effects are obtained by containing 0.01% or more of Si, and the effect increases as the Si content increases. However, if the Si content exceeds 0.90%, the rolling load increases in the hot rolling process and a significant scale is generated. In the annealing process, the pickling property decreases due to the formation of the Si concentrated layer on the steel sheet surface layer. Respectively, which causes an increase in surface defects and an increase in manufacturing cost. Therefore, the Si amount is set to 0.01% or more and 0.90% or less. Preferably it is 0.05 to 0.60% of range. More preferably, it is 0.05 to 0.15% of range. In particular, when Ti is contained in an amount of 0.25% or more, the pickling property is significantly reduced by Si. Therefore, the Si content is preferably in the range of 0.05% to 0.20%.

Mn: 0.01% or more and 0.50% or less Mn has the effect of increasing the strength of the steel and also acts as a deoxidizer. In order to acquire the effect, it is necessary to contain 0.01% or more. However, if the amount of Mn exceeds 0.50%, precipitation of MnS, which is a starting point of corrosion, is promoted, and the corrosion resistance is lowered. Therefore, the range of the amount of Mn is 0.01% or more and 0.50% or less. Preferably it is 0.05 to 0.40% of range. More preferably, it is 0.10% or more and 0.30% or less of range.

P: 0.020% or more and 0.040% or less P is an element inevitably contained in steel, but it is an element harmful to corrosion resistance and workability, so the content should be reduced as much as possible. Is preferred. In particular, when it exceeds 0.040%, the workability is remarkably lowered due to solid solution strengthening. However, in order to make it less than 0.020%, it takes time for refining, which is not preferable in production. Therefore, the P content is 0.020% or more and 0.040% or less. Preferably, it is 0.025% or more and 0.030% or less.

S: 0.008% or less S is an element inevitably contained in steel like P. However, since it is an element harmful to corrosion resistance and workability, its content should be reduced as much as possible. preferable. Particularly when it exceeds 0.008%, the corrosion resistance is remarkably lowered. Therefore, the S amount is 0.008% or less. Preferably it is 0.006% or less. More preferably, it is 0.003% or less.

Al: 0.001% to 0.090% Al is an effective deoxidizer. Furthermore, since Al has a stronger affinity for nitrogen than Cr, when nitrogen penetrates into the weld zone, it has the effect of precipitating nitrogen by precipitating nitrogen as Al nitride instead of Cr nitride. These effects can be obtained by containing 0.001% or more of Al. However, if Al exceeds 0.090%, the penetration at the time of welding is lowered and the welding workability is lowered, which is not preferable. Therefore, the Al content is set to be in the range of 0.001% to 0.090%. Preferably it is 0.001% or more and 0.060% or less of range. More preferably, it is 0.001% or more and 0.040% or less of range.

Cr: 14.5% to 23.0% Cr is the most important element for securing the corrosion resistance of stainless steel. If the content is less than 14.5%, sufficient corrosion resistance cannot be obtained in the welded portion with the austenitic stainless steel. On the other hand, if the content exceeds 23.0%, the toughness of the hot-rolled sheet decreases due to the formation of the σ (sigma) phase, and continuous annealing of the hot-rolled sheet becomes difficult. Therefore, the Cr content is set in the range of 14.5% or more and 23.0% or less. Preferably it is 14.5% or more and 22.0% or less of range. More preferably, it is 16.0% or more and 21.5% or less of range.

Ni: 0.10% to 0.60% 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 in which 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. This effect is obtained by containing 0.10% or more of Ni, and increases as the Ni content increases. However, when the content exceeds 0.60%, workability is lowered and stress corrosion cracking is likely to occur. Furthermore, since Ni is an expensive element, an increase in the content of Ni causes an increase in manufacturing cost, which is not preferable. Therefore, the Ni content is set to 0.10% or more and 0.60% or less. Preferably it is 0.10% or more and 0.50% or less of range. More preferably, it is 0.10% or more and 0.40% or less of range.

V: 0.010% to 0.040% V is an extremely important element in the present invention. V forms a composite carbonitride with Ti and Nb. This composite carbonitride precipitates with a higher precipitation peak temperature than conventional Ti and Nb carbonitrides and contains more C and N during the cooling process after welding, thereby suppressing sensitization of the weld zone. To do. This effect is acquired by containing V 0.010% or more. However, if the content exceeds 0.040%, workability is remarkably lowered, which is not preferable. Therefore, the V amount is in the range of 0.010% to 0.040%. Preferably it is 0.010% or more and 0.030% or less of range.

Ti: 0.15% or more and 0.34% or less and Ti% + Nb% ≦ 0.70 and V% / (Ti% + 0.5 × Nb%): within a range satisfying 0.05 to 0.20, Ti Or Ti and Nb, or Nb: 0.35% to 0.60% and Ti% + Nb% ≦ 0.70 and V% / (Ti% + 0.5 × Nb%): 0 When Nb is contained in a range satisfying 0.05 to 0.20 or Nb and Ti are contained Ti and Nb are preferentially bonded to C and N, and are attributed to sensitization by precipitation of Cr carbonitride. It is an element that suppresses the decrease in corrosion resistance. In order to obtain this effect, one or two of Ti and Nb are contained at 0.15% or more of Ti or 0.35% or more of Nb. Preferably, it contains Ti: 0.20% or more, or Nb: 0.40% or more. More preferably, it contains Ti: 0.25% or more, or Nb: 0.45% or more. On the other hand, if Ti is contained in excess of 0.34%, coarse Ti carbonitrides are produced in the casting process and cause surface defects, which is not preferable for production. Therefore, the Ti amount is set to 0.34% or less. Preferably, it is 0.30% or less. Nb is also an element that raises the recrystallization temperature. If it contains more than 0.60%, the annealing temperature required for recrystallization increases, resulting in an increase in annealing cost and an uneven metal structure. The resulting ductility is reduced. Furthermore, since Nb increases the hot rolling load, if it is added excessively, it becomes difficult to manufacture hot rolled sheets. Therefore, the Nb amount is set to 0.60% or less. Preferably it is 0.55% or less. Moreover, when Ti or Nb is contained, the metal structure at the time of recrystallization becomes non-uniform and ductility is lowered. Therefore, Ti% + Nb% is set to 0.70% or less. Preferably it is 0.65 or less. As described above, all of the Ti amount, the Nb amount, and Ti% + Nb% must be equal to or lower than the upper limit value.

  The occurrence of sensitization cannot be completely prevented only when Ti and Nb are in the above range. Furthermore, it is necessary to contain an appropriate amount of V and satisfy an appropriate ratio of V, Ti, and Nb in order to suppress sensitization. V forms a composite carbonitride with Ti and Nb, suppresses sensitization and improves the corrosion resistance of the weld. This composite carbonitride is produced when one or two of Ti and Nb and V are contained so that V% / (Ti% + 0.5 × Nb%) is 0.05 or more. When V% / (Ti% + 0.5 × Nb%) is less than 0.05, V required for forming the composite carbonitride is insufficient, and the amount of precipitation of the composite carbonitride is reduced. For this reason, the solid solution C and N of a welding part cannot fully be fixed, but the predetermined | prescribed corrosion resistance improvement effect is not acquired. On the other hand, when V% / (Ti% + 0.5 × Nb%) exceeds 0.20, V becomes excessive with respect to Ti and Nb, and the N concentration in the composite carbonitride increases. As a result, the solid solution C in the welded portion cannot be sufficiently fixed as a precipitate, and a sufficient sensitization suppressing effect cannot be obtained. Therefore, V% / (Ti% + 0.5 × Nb%): 0.05 to 0.20. Preferably it is the range of 0.10-0.15. The Ti%, the Nb%, and the V% represent the contents (mass%) of Ti, Nb, and V, respectively.

  The present invention is a ferritic stainless steel characterized in that it contains the above essential components, and the balance consists of Fe and inevitable impurities. Furthermore, if necessary, one or more selected from Cu and Mo, or one or more selected from Zr, REM, W, Co, B, Mg, Ca, It can contain in the range of.

Cu: 0.01% or more and 0.80% or less 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 a weakly acidic water droplet adheres. It is. Moreover, Cu is a strong austenite-forming element like Ni, and has the effect of suppressing ferrite formation at the weld and suppressing sensitization due to precipitation of Cr carbonitride. These effects are obtained by containing 0.01% or more, and the effect becomes higher as the Cu content increases. However, if Cu is contained in excess of 0.80%, the hot workability is lowered and surface defects are induced, which is not preferable. Furthermore, since descaling after annealing becomes difficult, it is not preferable in production. Therefore, when contained, the Cu content is in the range of 0.01% to 0.80%. Preferably, it is 0.10% or more and 0.60% or less of range. More preferably, it is 0.30% or more and 0.45% or less of range.

Mo: 0.01% to 1.65% Mo is an element that significantly improves the corrosion resistance of stainless steel. This effect is obtained when the content is 0.01% or more, and the effect improves as the content increases. However, if the Mo content exceeds 1.65%, the rolling load at the time of hot rolling becomes large, the productivity is lowered, and the steel sheet strength is excessively increased. Moreover, since Mo is an expensive element, the addition of a large amount increases the manufacturing cost. Therefore, when contained, the Mo content is 0.01% or more and 1.65% or less. Preferably it is 0.10% or more and 1.40% or less of range. In particular, in Ti-containing steels whose hot-rolled sheet toughness is reduced, the addition of Mo further reduces the toughness and makes hot-rolled sheet annealing difficult. It is preferably 30% or more and 1.40% or less. More preferably, it is the range of 0.4% or more and 1.00% or less.

Zr: 0.01% or more and 0.20% or less Zr combines with C and N and has an effect of suppressing sensitization. This effect is obtained when the content is 0.01% or more. On the other hand, if the content exceeds 0.20%, workability is remarkably lowered, which is not preferable. Therefore, when it contains, Zr content shall be 0.01% or more and 0.20% or less of range. Preferably, the range is 0.01% or more and 0.10% or less.

REM: 0.001% or more and 0.100% or less REM has the effect of improving oxidation resistance, and suppresses the formation of a Cr-deficient region directly under the oxide film by suppressing the formation of an oxide film (weld temper color) in the weld zone. To do. In order to acquire this effect, it is necessary to contain REM 0.001% or more. On the other hand, if the content exceeds 0.100%, productivity such as pickling at the time of cold rolling annealing is lowered, which is not preferable. Therefore, when it contains, REM amount shall be 0.001% or more and 0.100% or less of range. Preferably, it is set as 0.001% or more and 0.050% or less of range.

Co: 0.01% or more and 0.20% or less Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, if the content exceeds 0.20%, the workability decreases. Therefore, when it contains, Co amount shall be 0.01% or more and 0.20% or less.

B: 0.0002% or more and 0.0009% or less B is an element effective for improving secondary work embrittlement resistance after deep drawing. This effect is obtained by making the B content 0.0002% or more. On the other hand, if the B content exceeds 0.0009%, workability and toughness deteriorate, which is not preferable. Therefore, when it contains, B amount shall be 0.0002% or more and 0.0009% or less of range. Preferably it is 0.0003% or more and 0.0006% or less of range.

Mg: 0.0002% or more and 0.0010% or less Mg is an element that improves the equiaxed crystal ratio of the slab and is effective for improving workability and toughness. Furthermore, in the steel containing Ti as in the present invention, when Ti carbonitride is coarsened, the toughness is lowered, but Mg also has an effect of suppressing the coarsening of Ti carbonitride. These effects appear by containing 0.0002% or more of Mg. On the other hand, when the amount of Mg exceeds 0.0010%, the surface properties of steel are deteriorated. Therefore, when it contains, Mg amount shall be 0.0002% or more and 0.0010% or less of range. Preferably it is 0.0002% or more and 0.0004% or less of range.

Ca: 0.0005% or more and 0.0020% or less Ca is an effective component for preventing nozzle clogging due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. The effect is acquired by containing 0.0005% or more of Ca. However, when it contains exceeding 0.0020%, corrosion resistance falls by the production | generation of CaS. Therefore, when contained, the Ca content is in the range of 0.0005% to 0.0020%. Preferably it is 0.0005% or more and 0.0015% or less of range. More preferably, it is 0.0005% or more and 0.0010% or less of range.

  Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.

  The ferritic stainless steel of the present invention is obtained by melting a molten steel having the above composition by a known method such as a converter, an electric furnace, a vacuum melting furnace, and the like, and a steel material (slab slab by a continuous casting method or an ingot-bundling method. ). This slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-rolled as cast without heating to obtain a hot-rolled sheet.

  Usually, the hot-rolled sheet is subjected to continuous annealing at 800 to 1100 ° C. or batch annealing at 600 to 900 ° C., but 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 annealed and pickled to obtain a product.

  Cold rolling is preferably performed at a rolling reduction of 50% or more from the viewpoints of extensibility, bendability, press formability, and shape correction.

  In general, the recrystallization annealing of a cold-rolled sheet is 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 1100 ° C. from the viewpoint of obtaining good mechanical properties and pickling. Further, BA annealing (bright annealing) may be performed in order to obtain more gloss.

  In addition, in order to further improve the surface properties after cold rolling and after processing, grinding or polishing may be performed.

  Hereinafter, the present invention will be described in more detail based on examples.

  50 kg of stainless steel having the chemical composition shown in Table 1 was melted in a small vacuum melting furnace. These steel ingots were heated to 1150 ° C. and then hot-rolled to obtain 3.5 mm thick hot rolled sheets. A test piece (JIS B 7722 V notch) was sampled with the rolling direction as the longitudinal direction, and a Charpy impact test was performed on the obtained hot-rolled sheet. Next, the hot-rolled sheet obtained as described above was annealed at 900 to 1100 ° C. for 10 minutes and then pickled, and cold-rolled to a cold-rolled sheet having a thickness of 0.8 mm by cold rolling. The obtained cold-rolled sheet was subjected to finish annealing at 850 to 1100 ° C. in an air atmosphere, and then pickled with a mixed acid of hydrofluoric acid and nitric acid.

  The surface of the cold-rolled annealed pickling plate obtained as described above was determined by visual observation, a tensile test and a pitting potential measurement. In the tensile test, a JIS No. 13B tensile test piece was taken in parallel with the rolling direction, and the elongation (El) (breaking ductility) was measured based on the tensile test according to JIS Z2201. For pitting corrosion potential measurement, a 20 mm × 20 mm test piece was sampled and the surface was polished with No. 600 polishing paper, and then covered with a sealing material leaving a 10 mm × 10 mm measurement surface, and 3.5% by mass at 30 ° C. Pitting potential was measured in NaCl solution. The test piece was not passivated, but other measurement methods were based on JIS G 0577 (2005).

  In addition, the cold-rolled annealed pickled steel plate of each steel type prepared above and SUS304 (C: 0.07 mass%, N: 0.05 mass%, Japanese Industrial Standard, JIS G 4305) with a thickness of 0.8 mm TIG welded. The welding conditions are welding speed: 600 mm / min, welding voltage: 10 to 12 V, welding current: 70 to 120 A. Although the front side was sealed by flowing 15 L / min of argon gas, the back side was not gas shielded in order to make nitrogen enter the molten pool due to insufficient gas shielding.

  Next, a test piece of 60 mm × 90 mm is taken so that the weld bead passes on the longitudinal center line, the surface is polished with No. 600 abrasive paper, the end surface is sealed with waterproof tape, and the corrosion resistance test by the salt spray cycle test is performed. went. In the salt spray cycle test, salt spray (5% NaCl, 35 ° C., spray 2 h) → dry (60 ° C., 4 h, relative humidity 40%) → wet (50 ° C., 2 h, relative humidity ≧ 95%) is one cycle. Five cycles were performed.

  The above evaluation was performed and each evaluation result was determined as follows.

Hot-rolled sheet Charpy test The Charpy impact value at 25 ° C of the hot-rolled sheet was determined to be 50 J / cm ² or more, and less than 50 J / cm ² was determined to be unacceptable.

Pitting corrosion potential The pitting corrosion potential of the base material was determined to be 120 mV or higher, and less than 120 mV was determined to be unacceptable.

Salt spray test The area where rust occurred was 20% or less, and 20% or more was judged to be unacceptable.

Breaking ductility The elongation at break in the tensile test was determined to be 25% or more, and less than 25% was determined to be unacceptable.

Surface determination: The surface of a cold-rolled annealed pickling plate of 20 cm × 40 cm is visually observed, and if the length or width is 5 mm or more and there are 3 or less surface defects (linear wrinkles, white streaks, etc.), 4 The case where there were more than one was judged as unacceptable.

  The results obtained as described above are shown in Table 2.

  From Table 2, A1 to A14 satisfying the scope of the present invention showed a pitting corrosion potential of 120 mV or more, and there was no sensitization and rusting of the welded part, and a predetermined corrosion resistance was obtained for both the base material and the welded part. 25% or more of fracture ductility was obtained, and no surface defects were observed.

  Since B1 containing the Cr amount exceeding the range of the present invention could not obtain a predetermined Charpy impact value in the hot-rolled sheet, the subsequent steps and tests were not performed. Moreover, in B2 which is less than the range of the present invention with the Cr amount of 13.8%, in addition to the pitting corrosion potential being as low as 108 mV, corrosion occurs from the welded part in the salt spray cycle test, and a predetermined welded part corrosion resistance is obtained. I could not. In B3 in which the amount of Nb exceeds the range of the present invention, a predetermined fracture ductility could not be obtained as a result of an uneven metal structure including non-recrystallized grains after annealing. In B4 where the amount of Ti exceeds the range of the present invention, surface defects (streak-like defects) due to coarse Ti carbonitrides occurred.

  On the other hand, in B5 and B6 where either Ti or Nb is below the range of the present invention, a predetermined amount of V is contained but Ti or Nb is insufficient, so (Ti, V) (C, N) and (Nb V) The precipitation amount of (C, N) became insufficient, and the predetermined welded portion corrosion resistance could not be obtained. In B7 where the amount of V is below the range of the present invention, (Ti, V) (C, N) and (Nb, V) (C, N) are hardly precipitated due to the lack of V, and solid solutions C and N are fixed. Sensitization occurred without complete conversion, and the predetermined corrosion resistance of the weld could not be obtained.

  Similarly, B9 to B10 in which V% / (Ti% + 0.5 × Nb%) is outside the range of the present invention can be obtained with (Ti, V) (C, N) although good base metal corrosion resistance is obtained. And (Nb, V) (C, N) precipitation amount is insufficient, or V concentration in the composite carbonitride is excessively high. As a result, solid solution C and N cannot be sufficiently fixed as precipitates and are sensitive. As a result, the predetermined weld corrosion resistance could not be obtained.

  From the above results, in order to obtain the predetermined weld corrosion resistance provided by the present invention while having excellent mechanical properties and surface aesthetics, the content of each element, V% / (Ti% + 0.5 It was confirmed that (Nb%) must be appropriately adjusted within the scope of the present invention.

  Ferritic stainless steel obtained in the present invention is used for the production of structures by welding, for example, automobile exhaust materials such as mufflers, building materials such as fittings, ventilation openings, ducts, electrical equipment, kitchen products, etc. It is suitable for application to.

Claims (2)

C: 0.003% or more and 0.014% or less, N: 0.005% or more and 0.016% or less, C% + N%: 0.023% or less, Si: 0.01% or more. 90% or less, Mn: 0.01% or more and 0.50% or less, P: 0.020% or more and 0.040% or less, S: 0.008% or less, Al: 0.001% or more and 0.090% or less Cr: 14.5% to 23.0%, Ni: 0.10% to 0.60%, V: 0.010% to 0.040%, Mo: 0.01% to 1.65 % Or less,
Further, within a range satisfying Ti: 0.15% or more and 0.34% or less, Ti% + Nb% ≦ 0.70, and V% / (Ti% + 0.5 × Nb%): 0.05 to 0.20. In the case of containing Ti or containing Ti and Nb, and Nb: 0.35% or more and 0.60% or less, Ti% + Nb% ≦ 0.70 and V% / (Ti% + 0.5 × Nb%): Nb is contained in a range satisfying 0.05 to 0.20 or at least one of cases containing Nb and Ti is satisfied,
A ferritic stainless steel characterized in that the balance consists of Fe and inevitable impurities. The C%, the N%, the Ti%, the Nb%, and the V% represent the contents (mass%) of Ti, Nb, and V, respectively.   Further, Zr: 0.01% to 0.20%, REM: 0.001% to 0.100%, Co: 0.01% to 0.20%, Mg: 0.0002 2. The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel contains at least one selected from the group consisting of% or more and 0.0010% or less and Ca: 0.0005% or more and 0.0020% or less.