KR101673218B1 - Ferritic stainless steel - Google Patents

Abstract

A ferritic stainless steel excellent in corrosion resistance of a welded portion is provided.
N: not less than 0.005% and not more than 0.016%, C: not more than 0.023%, Si: not less than 0.01% and not more than 0.90%, Mn: not less than 0.01% and not more than 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% Ti in the range of not less than 0.15% and not more than 0.34% and Ti in the range of + Nb%? 0.70 and V% / (Ti% + 0.5 x Nb% Nb is contained in a range satisfying Nb: 0.35% to 0.60% and Ti% + Nb%? 0.70 and V% / (Ti% + 0.5 x Nb%): 0.05 to 0.20. Nb and Ti, and the balance of Fe and inevitable impurities.

Description

Ferritic stainless steel {FERRITIC STAINLESS STEEL}

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

Stainless steels are classified into ferritic stainless steels typified by SUS430 and austenitic stainless steels typified by SUS304. Ferritic stainless steels can be manufactured at a low cost because the amount of Ni, which is an expensive element, is smaller than that of austenitic stainless steels. In addition, ferritic stainless steels have excellent properties such as low thermal expansion coefficient and high heat conductivity, excellent in corrosion resistance in outdoor environment, and excellent resistance to stress corrosion cracking. For this reason, ferritic stainless steels have been widely applied to various architectural materials, automobile parts, kitchen appliances, household appliances, water heaters, and the necessity thereof has been recently increasing.

Ferritic stainless steels are often used by welding with ferritic stainless steels or with austenitic stainless steels (e.g., SUS304, etc.), and good corrosion resistance is required in the welded parts as in the base steel parts. However, when an austenitic stainless steel such as SUS304 which is higher in C and N content than a ferritic steel is welded to a ferritic stainless steel, the corrosion resistance of the welded part may be lowered than that of the base metal due to a phenomenon called sensitization . The sensitization is caused by the thermal history of the welded portion, in which C and N in the steel are combined with Cr to precipitate in the grain boundary as Cr carbide (for example, Cr ₂₃ C ₆ ) or Cr nitride (Cr ₂ N) so that the Cr concentration in and around the grain boundary becomes lower than that of the base material and the corrosion resistance at the grain boundary is lowered. Recently, as the structure of the welded portion is complicated, sufficient gas shielding can not be performed at the time of welding, and welding in an incomplete condition such as the infiltration of nitrogen in the air into the molten pool . Nitrogen infiltrated into the melting paper promotes sensitization of the welded portion by the same mechanism as described above, resulting in deterioration of corrosion resistance. Therefore, ferritic stainless steels applicable to such applications are required to be able to sufficiently secure the corrosion resistance of the welded portion even when the gas shield is insufficient at the time of welding.

To solve this problem, as disclosed in Patent Documents 1 and 2, there has been proposed a method in which Ti and Nb are added to fix C and N in a steel as carbides or nitrides to harmless them. However, if the gas shield is insufficient, the welded portion may be susceptible to corrosion, and the corrosion resistance of the welded portion is not sufficient.

Japanese Laid-Open Patent Publication No. 51-88413 Japanese Patent Application Laid-Open No. 2007-270290

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

The mechanisms of fixing of C and N by addition of Ti and Nb are such that Ti and Nb dissolve in the base material during welding and then Ti (C, N) or Nb (C, N ), And C and N in the steel are fixed. It is empirically proved that it is effective to add Ti and Nb at 8 or more in Ti% / (C% + N%) or Nb% / (C% + N% It is known.

However, even when Ti and Nb are added in the range where Ti / (C% + N%) or Nb% / (C% + N%) satisfies 8 or more, sufficient amelioration inhibition effect may not be obtained The inventors investigated the cause. As a result, in the conventional ferritic stainless steels containing Ti and Nb, the solid solution temperature and the precipitation peak temperature of Ti (C, N) or Nb (C, N) It is impossible to sufficiently precipitate them at the time of cooling after the welding, and it is found that there is a case where the solid solution C and the N remain and the solidification due to the precipitation of the Cr carbonitride occurs. Therefore, in order to improve the corrosion resistance of the welded portion, it is necessary to immobilize the solid solution C and N in the cooling process after welding.

Therefore, the inventors of the present invention have found that, by lowering the precipitation peak temperature of these Ti and Nb-based carbonitrides, precipitation of Ti and Nb-based carbonitrides during cooling after welding is promoted and C and N are sufficiently fixed Review. As a result, when an appropriate amount of V is contained in the Ti and Nb based carbonitrides, these precipitates become composite carbonitrides such as (Ti, V) (C, N) or (Nb, V) , The precipitation temperature is lowered than that of the conventional Ti and Nb based carbonitrides and these composite carbonitrides containing V can fix C and N more than conventional Ti and Nb based carbonitrides, Which is significantly improved.

The present invention has been made on the basis of the above-described recognition, and its main points are as follows.

N: 0.005% or more and 0.016% or less, C% + N%: 0.023% or less, Si: 0.01% or more and 0.90% or less, Mn: 0.01% or more and 0.50% or less, 0.001% or more and 0.090% or less of Al, 14.5% or more and 23.0% or less of Cr, 0.10% or more and 0.60% or less of Ni, 0.010% or more and 0.040% or less of V, Ti in the range of not less than 0.15% and not more than 0.34%, and Ti% + Nb%? 0.70 and V% / (Ti% + 0.5 x Nb%): 0.05 to 0.20, Or Ti and Nb, or 0.35 to 0.60% Nb, and Ti% + Nb%? 0.70 and V% / (Ti% + 0.5 x Nb% Wherein at least one of Nb and Nb and Ti is contained, and the balance of Fe and inevitable impurities is contained in the ferritic stainless steel. The above C%, N%, Ti%, Nb% and V% represent contents (mass%) of C, N, Ti, Nb and V, respectively.

(2) The ferritic stainless steel according to (1), wherein the ferritic stainless steel according to (1) further contains one or two of Cu: 0.01 to 0.80% and Mo: 0.01 to 1.65%.

0.001 to 0.100%, Co: 0.01 to 0.20%, B: 0.0002 to 0.0009%, Mg: 0.0002% or less, Or more and not more than 0.0010%, and Ca: not less than 0.0005% and not more than 0.0020%, based on the total weight of the ferritic stainless steels (1) and (2).

According to the present invention, a ferritic stainless steel excellent in corrosion resistance of a welded portion is obtained. The ferritic stainless steel of the present invention is excellent in corrosion resistance even in a welding condition in which carbon or nitrogen enters from a mated steel or in a welding condition in which nitrogen enters from air, Respectively. Therefore, the present invention can be applied to applications in which the structure is manufactured by welding, for example, automotive exhaust system materials such as mufflers, building materials such as fittings or ventilation ducts, ducts, And can be suitably used.

(Mode for carrying out the invention)

The reason why the composition of the steel of the present invention is defined will be described below. The term "%" means "% by mass" unless otherwise specified.

C: not less than 0.003% and not more than 0.014%

When C is contained in excess of 0.014%, the workability is lowered and the corrosion resistance of the welded portion is remarkably lowered. Lower C content is preferable from the viewpoint of corrosion resistance and processability, but refining takes a long time in order to make the C content less than 0.003%, which is not preferable from the viewpoint of production. Therefore, the amount of C is set in the range of 0.003% to 0.014%. And preferably not less than 0.004% and not more than 0.011%.

N: not less than 0.005% and not more than 0.016%

When N exceeds about 0.016%, the workability is lowered and the corrosion resistance of the welded portion is remarkably reduced. From the viewpoint of corrosion resistance, it is preferable that the content of N is as low as possible. However, in order to reduce the N content to less than 0.005%, it is necessary to elongate the refining time, which leads to an increase in production cost and a decrease in productivity. Accordingly, the amount of N is set in the range of 0.005% or more and 0.016% or less. And preferably 0.005% or more and 0.011% or less.

C% + N%: 0.023% or less

C and N cause deterioration of workability and corrosion resistance of a welded portion. If the total (C% + N%) of the C content and the N content exceeds 0.023%, there is a synergistic effect in the effect, and the lowering of 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 less than 0.020%.

Si: not less than 0.01% and not more than 0.90%

Si is concentrated on an oxide film formed at the time of welding to improve the corrosion resistance of a welded portion and is also an element useful as a deoxidizing element in a steelmaking process. These effects are obtained by containing Si in an amount of 0.01% or more, and the larger the Si content is, the larger the effect is. However, when Si is contained in an amount exceeding 0.90%, the rolling load in the hot rolling step is increased and the scale is remarkably increased. In the annealing step, an acid The deterioration of the pickling property occurs, which is undesirable because it entails an increase in surface defects and an increase in manufacturing cost. Therefore, the amount of Si is set to 0.01% or more and 0.90% or less. , Preferably not less than 0.05% and not more than 0.60%. , More preferably not less than 0.05% and not more than 0.15%. In particular, when Ti is contained in an amount of not less than 0.25%, deterioration of acid-cleasing property by Si becomes remarkable, so that the content of Si is preferably in the range of not less than 0.05% and not more than 0.20%.

Mn: 0.01% or more and 0.50% or less

Mn has an effect of increasing the strength of the steel, and also has an effect as a deoxidizer. In order to obtain the effect, it is necessary to contain 0.01% or more. However, when the amount of Mn exceeds 0.50%, precipitation of MnS as a starting point of corrosion is promoted and corrosion resistance is lowered. Therefore, the range of the amount of Mn is set to 0.01% or more and 0.50% or less. , Preferably not less than 0.05% and not more than 0.40%. More preferably not less than 0.10% and not more than 0.30%.

P: 0.020% or more and 0.040% or less

P is an element inevitably contained in steel, but it is preferable to reduce the content of P as much as possible because it is an element harmful to corrosion resistance and workability. In particular, if it exceeds 0.040%, the workability is remarkably lowered due to solid-solution hardening. However, in order to make the content less than 0.020%, refining takes time, which is not preferable in view of 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: not more than 0.008%

S is an element which is inevitably included in steel in the same manner as P, but is a harmful element to corrosion resistance and workability. Therefore, it is preferable to reduce the content as much as possible. In particular, if it exceeds 0.008%, the corrosion resistance remarkably decreases. Therefore, the amount of S is 0.008% or less. It is preferably not more than 0.006%. More preferably, it is 0.003% or less.

Al: 0.001% or more and 0.090% or less

Al is an effective deoxidizer. Further, since Al has strong affinity with nitrogen than Cr, there is an effect of suppressing sensitization by precipitating nitrogen as Al nitride instead of Cr nitride when nitrogen enters the welded portion. These effects are obtained by containing at least 0.001% of Al. However, if Al is contained in an amount exceeding 0.090%, penetration at the time of welding is lowered, and welding workability is lowered, which is not preferable. Therefore, the Al content is set in the range of 0.001% or more and 0.090% or less. And preferably not less than 0.001% and not more than 0.060%. And more preferably 0.001% or more and 0.040% or less.

Cr: 14.5% or more and 23.0% or less

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 can not be obtained at the welded portion with the austenitic stainless steel. On the other hand, if it exceeds 23.0%, the toughness of the hot-rolled sheet is lowered due to the generation of the sigma phase (sigma phase), and continuous annealing of the hot-rolled sheet becomes difficult. Therefore, the amount of Cr is set in the range of 14.5% or more and 23.0% or less. , Preferably not less than 14.5% and not more than 22.0%. , More preferably not less than 16.0% and not more than 21.5%.

Ni: 0.10% or more and 0.60% or less

Ni is an element that improves the corrosion resistance of stainless steel and is an element that inhibits the progress of corrosion in a corrosive environment where active dissolution occurs because a passivation film can not be formed. Further, Ni is a strong austenite generating element and has the effect of suppressing the ferrite formation in the welded portion and suppressing the sensitization due to precipitation of Cr carbonitride. This effect is obtained by containing 0.10% or more of Ni, and the higher the content of Ni is, the higher the effect is. However, when the content exceeds 0.60%, in addition to the deterioration of workability, stress corrosion cracking tends to occur. Further, since Ni is an expensive element, an increase in the content of Ni is not preferable because it causes an increase in manufacturing cost. Therefore, the amount of Ni is set to 0.10% or more and 0.60% or less. And preferably not less than 0.10% and not more than 0.50%. More preferably, it is in the range of 0.10% or more and 0.40% or less.

V: 0.010% or more and 0.040% or less

V is a very important element in the present invention. V forms composite carbonitride with Ti and Nb. This composite carbonitride precipitates more C and N at a precipitation peak temperature lower than that of the conventional Ti and Nb based carbonitrides in the cooling process after welding, thereby suppressing the sensitization of the welded portion. This effect is obtained by containing V in an amount of 0.010% or more. However, if it exceeds 0.040%, the workability is significantly lowered, which is not preferable. Therefore, the V amount is set in a range of 0.010% or more and 0.040% or less. And preferably 0.010% or more and 0.030% or less.

Ti in the range satisfying Ti: not less than 0.15% and not more than 0.34%, and Ti% + Nb%? 0.70 and V% / (Ti% + 0.5 x Nb%): 0.05 to 0.20 Or Nb in the range satisfying Nb: 0.35% or more and 0.60% or less and Ti% + Nb%? 0.70 and V% / (Ti% + 0.5 x Nb% When

Ti and Nb are elements that bind preferentially to C and N and suppress the deterioration of corrosion resistance due to the sensitization due to precipitation of Cr carbonitride. In order to obtain this effect, at least one of Ti and Nb is contained at 0.15% or more of Ti or 0.35% or more of Nb. Preferably, it contains 0.20% or more of Ti or 0.40% or more of Nb. More preferably, it contains 0.25% or more of Ti or 0.45% or more of Nb. On the other hand, if Ti is contained in an amount exceeding 0.34%, coarse Ti carbonitride is produced in the casting step, which causes surface defects, which is not desirable for production. Therefore, the amount of Ti is set to 0.34% or less. Preferably, it is 0.30% or less. Nb is also an element for raising the recrystallization temperature. If it is contained in excess of 0.60%, the annealing temperature required for recrystallization is increased, resulting in an increase in the annealing cost and a decrease in ductility due to uneven metal structure Lt; / RTI > Further, since Nb increases the hot rolling load, if it is excessively added, it becomes difficult to manufacture the hot-rolled sheet. Therefore, the amount of Nb is 0.60% or less. And preferably 0.55% or less. Further, when Ti or Nb is contained, the metal structure at the time of recrystallization becomes uneven and the ductility is lowered. Therefore, Ti% + Nb% is set to 0.70% or less. It is preferably 0.65% or less. As described above, all of the amount of Ti, the amount of Nb, and the content of Ti% + Nb% must be equal to or less than the upper limit value.

Ti and Nb within the above range can not completely prevent the occurrence of sensitization. It is also necessary to contain an appropriate amount of V and satisfy the appropriate ratio of V, Ti and Nb to suppress sensitization. V forms composite carbonitride with Ti and Nb, and inhibits sensitization and improves the corrosion resistance of the welded portion. This composite carbonitride is produced when one or two kinds of Ti and Nb, V, are contained so that V% / (Ti% + 0.5 x Nb%) is 0.05 or more. When V% / (Ti% + 0.5 x Nb%) is less than 0.05, the amount of V necessary for forming the composite carbonitride is insufficient, and the precipitation amount of the composite carbonitride decreases. Therefore, the solid solutions C and N of the welded portion can not be sufficiently fixed, and the predetermined corrosion resistance improving effect can not be obtained. On the other hand, if V% / (Ti% + 0.5 x Nb%) exceeds 0.20, V becomes excessive with respect to Ti or Nb, and the concentration of N in the composite carbonitride becomes high. As a result, the solid solution C in the welded portion can not be sufficiently fixed as a precipitate, and sufficient amelioration suppression effect can not be obtained. Therefore, it is set in the range of V% / (Ti% + 0.5 × Nb%): 0.05 to 0.20. And preferably in the range of 0.10 to 0.15. The Ti%, Nb% and V% represent the content (% by mass) of Ti, Nb and V, respectively.

The present invention is a ferritic stainless steel characterized by containing the above-mentioned essential components and the balance of Fe and inevitable impurities. If necessary, one or two or more selected from the group consisting of Cu and Mo, or Zr, REM, W, Co, B, Mg, ≪ / RTI >

Cu: not less than 0.01% and not more than 0.80%

Cu is an element that improves corrosion resistance, and is an element particularly effective for improving the corrosion resistance of a base material and a weld portion in a case where a droplet is adhered to an aqueous solution or a weak acid. Further, Cu is an austenite-generating element strong as Ni, and has the effect of suppressing the ferrite formation in the welded portion and suppressing the 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 is larger. However, if Cu is contained in excess of 0.80%, the hot workability is deteriorated and surface defects are attracted, which is not preferable. And furthermore, the de-scale after annealing becomes difficult. Therefore, when contained, the amount of Cu is set in the range of 0.01% or more and 0.80% or less. , Preferably not less than 0.10% and not more than 0.60%. More preferably, it is in the range of 0.30% or more and 0.45% or less.

Mo: 0.01% or more and 1.65% or less

Mo is an element which remarkably improves the corrosion resistance of stainless steel. This effect is obtained by the content of 0.01% or more, and the effect is improved as the content is larger. However, when the Mo content exceeds 1.65%, the rolling load at the time of hot rolling becomes large, so that the composition is decreased and the steel sheet strength is excessively increased. In addition, since Mo is an expensive element, the addition of a large amount increases the manufacturing cost. Therefore, when contained, the amount of Mo is set to 0.01% or more and 1.65% or less. And preferably not less than 0.10% and not more than 1.40%. In particular, in the case of the Ti-containing steel in which the hot-rolled sheet toughness is lowered, the toughness is lowered by the addition of Mo and the hot-rolled sheet annealing becomes difficult, and when the Ti content is 0.15% or more, the Mo content is 0.30% or more and 1.40% . More preferably, it is in a range of 0.4% or more and 1.00% or less.

Zr: 0.01% or more and 0.20% or less

Zr has an effect of inhibiting sensitization by binding with C and N. This effect is obtained by the content of 0.01% or more. On the other hand, if it exceeds 0.20%, the workability is significantly lowered, which is not preferable. Therefore, when contained, the amount of Zr is in the range of 0.01% or more and 0.20% or less. Preferably, it is in the range of 0.01% or more and 0.10% or less.

REM: 0.001% or more and 0.100% or less

REM has an effect of improving oxidation resistance and inhibits the formation of an oxide film (weld temper color) of a weld portion and suppresses the formation of a Cr-depleted region immediately below the oxide film. In order to obtain this effect, it is necessary to contain 0.001% or more of REM. On the other hand, if it is contained in an amount exceeding 0.100%, it is undesirable to lower the productivity such as pickling qualities at the time of cold annealing. Therefore, when contained, the amount of REM is set in a range of 0.001% or more and 0.100% or less. It is preferably in the range of 0.001% to 0.050%.

Co: 0.01% or more and 0.20% or less

Co is an element for improving toughness. This effect is obtained by the content of 0.01% or more. On the other hand, if the content exceeds 0.20%, the workability is lowered. Therefore, when contained, the amount of Co is set in a range of 0.01% or more and 0.20% or less.

B: 0.0002% or more and 0.0009% or less

B is an effective element to improve resistance to secondary working embrittlement after deep drawing molding. This effect is obtained by setting the content of B to 0.0002% or more. On the other hand, if B is contained in excess of 0.0009%, the workability and toughness are lowered, which is not preferable. Therefore, when contained, the amount of B is in the range of 0.0002% to 0.0009%. And preferably 0.0003% or more and 0.0006% or less.

Mg: not less than 0.0002% and not more than 0.0010%

Mg improves the equiaxed crystal ratio of a slab and is an effective element for improving workability and toughness. Further, in the steel containing Ti as in the present invention, when the Ti carbonitride is coarsened, the toughness is lowered, but Mg also has an effect of suppressing the coarsening of the Ti carbonitride. These effects are shown by containing 0.0002% or more of Mg. On the other hand, if the amount of Mg exceeds 0.0010%, the surface properties of the steel are deteriorated. Therefore, when contained, the amount of Mg is set in the range of 0.0002% to 0.0010%. And preferably 0.0002% or more and 0.0004% or less.

Ca: 0.0005% or more and 0.0020% or less

Ca is an effective component for preventing the occlusion of the nozzle by crystallization of Ti inclusions likely to occur during continuous casting. The effect is obtained by containing 0.0005% or more Ca. However, if it exceeds 0.0020%, the corrosion resistance is lowered by the formation of CaS. Therefore, when contained, the amount of Ca is set in the range of 0.0005% to 0.0020%. And preferably in the range of 0.0005% to 0.0015%. And more preferably in the range of 0.0005% to 0.0010%.

Next, a method of producing the ferritic stainless steel of the present invention will be described.

The ferritic stainless steel of the present invention is produced by melting molten steel having the composition described above by a known method such as a converter, an electric furnace, or a vacuum melting furnace, a continuous casting process ) Or a steel material (slab) by the ingot making and blooming process. The slab is directly hot-rolled at a temperature of 1100 to 1250 占 폚 for 1 to 24 hours, or is cast without heating, thereby forming a hot-rolled sheet.

Normally, the hot-rolled sheet is subjected to continuous annealing at 800 to 1100 占 폚 or hot-rolled sheet annealing at batch annealing at 600 to 900 占 폚, but depending on the use, hot-rolled sheet annealing may be omitted. Subsequently, hot-rolled sheet is cleaned, cold-rolled to obtain a cold-rolled sheet, and then subjected to annealing and pickling to obtain a product.

The cold rolling is preferably carried out at a rolling reduction of 50% or more from the viewpoints of extensibility, bendability, press formability and shape correction.

The recrystallization annealing of the cold-rolled sheet is preferably carried out at 800 to 1100 占 폚 in terms of surface finish of JIS G 0203 in general, good mechanical properties in the case of No. 2B finished product, and pickling qualities. Further, BA annealing (bright annealing) may be performed to obtain further luster.

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

Example

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 dissolved in a small vacuum melting furnace. These ingots were heated to 1150 占 폚 and hot-rolled to obtain a hot-rolled sheet having a thickness of 3.5 mm. A test piece (JIS B 7722 V notch) was taken from the obtained hot rolled steel sheet in the rolling direction as a length, and a Charpy impact test was conducted. Then, the hot-rolled sheet obtained above was annealed at 900 to 1100 占 폚 for 10 minutes, pickled, and cold-rolled to obtain a cold-rolled sheet having a thickness of 0.8 mm. The obtained cold-rolled sheet was subjected to finish annealing at 850 to 1100 占 폚 in an air atmosphere, and then pickled with a mixed acid of hydrofluoric acid and nitric acid.

The cold-annealing pickling top plate thus obtained was subjected to surface determination, tensile test and formal potential measurement by visual observation. In the tensile test, tensile test specimens of JIS 13B were taken parallel to the rolling direction, and elongation (El) (fracture ductility) was measured according to JIS Z2201 in the tensile test. The pitting potential measurement was carried out by taking a specimen of 20 mm x 20 mm, polishing the surface with 600 polishing papers, covering the measuring surface of 10 mm x 10 mm with a sealing material, The pitting potential was measured in a 3.5 mass% NaCl solution at 30 占 폚. Passivation treatment of the test specimens was not carried out, but other measurement methods were in accordance with JIS G 0577 (2005).

Further, cold-annealing pickling top plates of the respective steel types produced by the above were subjected to TIG welding with 0.8 mm thick SUS304 (C: 0.07 mass%, N: 0.05 mass%, Japanese Industrial Standards, JIS G 4305). The welding conditions are a welding speed of 600 mm / min, a welding voltage of 10 to 12 V, and a welding current of 70 to 120 A. The front side (front side) was sealed with 15 L / min of argon gas, but the rear side was not subjected to gas shielding in order to allow nitrogen to enter the melting paper by an insufficient gas shield.

Subsequently, a specimen of 60 mm x 90 mm was taken so that the weld bead passed over the centerline of the length, the surface was polished with 600 abrasive paper, the end surface was sealed with a waterproof tape, and a salt spray cycle test spray cycle test). The saline spraying cycle test was carried out by using one cycle of spraying (5% NaCl, 35 ° C, spraying 2h) → drying (60 ° C, 4h, relative humidity 40%) → wet (50 ° C, 2h, relative humidity ≥95% , And 5 cycles.

The above evaluations were carried out, and the respective evaluation results were determined as follows.

Hot-rolled plate Charpy test

It was judged that the Charpy impact value of the hot-rolled sheet at 25 캜 was at least 50 J / cm 2, and that less than 50 J / cm 2 was rejected.

Official potential

It is judged that the official potential of the base material is more than 120 합, and less than 120 불 is unsatisfactory.

Salt spray test

It was judged that the area where the rust occurred was less than 20%, and that more than 20% was not.

Fracture ductility

It was judged that at least 25% of the elongation at break in the tensile test was acceptable and less than 25% of the elongation was rejected.

Surface determination

The surface of a cold-rolled annealing pickling top plate of 20 cm x 40 cm was visually inspected to see if there were three or less surface defects (linear scratches, white stripe patterns, etc.) having a length or width of 5 mm or more, It was judged to have failed.

Table 2 shows the results obtained by the above.

From Table 2, A1 to A14 that satisfy the range of the present invention exhibit a formal dislocation of 120 ㎷ or more, and have no susceptibility to rusting and rusting of the welded portion, , A fracture ductility of 25% or more was obtained, and surface defects were not recognized.

The B1 containing the amount of Cr exceeding the range of the present invention was not subjected to the subsequent processes and tests because a predetermined Charpy impact value could not be obtained in the hot rolled steel sheet. In addition, in the case of B2 in which the amount of Cr was 13.8% and below the range of the present invention, in addition to the low formaldehyde level of 108 kPa, corrosion occurred from the welded part in the salt water spray cycle test and the predetermined welded part corrosion resistance could not be obtained. In the case of B3 in which the amount of Nb exceeded the range of the present invention, uneven metal structure including unrecrystallized grain was formed after annealing, so that a predetermined fracture ductility was not obtained. In B4 where the amount of Ti exceeded the range of the present invention, surface defects (striped scratches) due to coarse Ti carbonitride occurred.

On the other hand, in B5 and B6 in which any one of Ti and Nb is less than the range of the present invention, (Ti, V) (C, N) and (Nb, V) (C, N) was insufficient, and the predetermined weld portion corrosion resistance was not obtained. (C, N) and (Nb, V) (C, N) are hardly precipitated by the shortage of V in the B7 in which the amount of V is less than the range of the present invention, It was not immobilized, so that sensitization occurred, and the predetermined weld portion corrosion resistance was not obtained.

Similarly, B9 to B10 having V% / (Ti% + 0.5 x Nb%) out of the range of the present invention have (Ti, V) (C, N) and (Nb, V (C, N), or the V concentration in the composite carbonitrile becomes excessively high, the solubility C and N can not be sufficiently fixed as precipitates, resulting in sensitization, and the predetermined welded portion corrosion resistance is not obtained .

From the above results, it is found that the content of each element, V% / (Ti% + 0.5 x Nb%), in order to obtain the predetermined weld portion corrosion resistance provided by the present invention with excellent mechanical properties and surface smoothness, It has been confirmed that it needs to be appropriately adjusted within the range.

The ferritic stainless steel obtained in the present invention can be used for applications in which a structural body is manufactured by welding such as automobile exhaust system materials such as mufflers, building materials such as dryers and vents, ducts, electric appliances, It is suitable for application.

Claims (4)

N: not less than 0.005% and not more than 0.016%, C: not more than 0.023%, Si: not less than 0.01% and not more than 0.90%, Mn: not less than 0.01% and not more than 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% ≪ / RTI &
Further, it is preferable that Ti is contained in a range satisfying 0.15% to 0.34% Ti and 0.05% to 0.20% V / Ti%
And the balance of Fe and inevitable impurities. The C%, N%, Ti% and V% represent the contents (% by mass) of C, N, Ti and V, respectively. The method according to claim 1,
Co: 0.01% or more and 0.20% or less; B: 0.0002% or more and 0.0009% or less; Mg: 0.0002% or more and 0.0010% or less; By mass or less, and Ca: not less than 0.0005% and not more than 0.0020% by mass of the ferrite based stainless steel. N: not less than 0.005% and not more than 0.016%, C: not more than 0.023%, Si: not less than 0.01% and not more than 0.90%, Mn: not less than 0.01% and not more than 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% Mo: not less than 0.30% and not more than 1.65%
Further, it is preferable that Ti is contained or Ti and Nb are contained in an amount satisfying 0.15% to 0.34%, Ti% + Nb% 0.70 and V% / (Ti% + 0.5 x Nb% , Nb is contained in the range satisfying Nb: 0.35% or more and 0.60% or less, Ti% + Nb%? 0.70 and V% / (Ti% + 0.5 x Nb% Is contained, and at least one of the cases in which
And the balance of Fe and inevitable impurities. The above C%, N%, Ti%, Nb% and V% represent contents (mass%) of C, N, Ti, Nb and V, respectively. The method of claim 3,
Co: 0.01% or more and 0.20% or less; Mg: 0.0002% or more and 0.0010% or less; Ca: 0.0005% or more and 0.0020% or less; Based ferritic stainless steels. The ferritic stainless steel according to claim 1 or 2, wherein the ferritic stainless steel contains at least one selected from the group consisting of the following.