JP5838929B2 - Ferritic stainless steel with excellent corrosion resistance at welds with austenitic stainless steel - Google Patents

Description

  The present invention relates to a ferritic stainless steel that is unlikely to deteriorate in corrosion resistance even under welding conditions in which oxygen or nitrogen from a shielding gas and nitrogen or carbon from a welding partner material enter a weld bead.

  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 water-related materials such as kitchens and water / hot water storage tanks.

  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. For example, such a structure is subjected to processing such as pressing to form stainless steel into a product shape, and after processing into parts such as cutting and polishing, the members are joined by welding. Often used. As such a stainless steel welding method, TIG welding is generally used. Even in that case, it is required to have good corrosion resistance like the base material portion.

However, when TIG welding is performed with a steel type such as SUS304, the corrosion resistance of the welded portion may be lower than that of the base metal due to a phenomenon called sensitization. Sensitization means that Cr in steel combines with C and N due to the thermal history during welding and precipitates at grain boundaries as Cr carbide (Cr ₂₃ C ₆ etc.) or Cr nitride (Cr ₂ N etc.). This is a phenomenon in which the corrosion resistance at the crystal grain boundary is lowered due to the formation of a Cr-deficient layer having a Cr concentration lower than that of the base material in the vicinity of the crystal grain boundary of the weld zone, and austenitic stainless steel such as SUS304 C This may occur when welding a ferritic stainless steel and a steel having a higher N content than that of the ferritic stainless steel.

  In recent years, as the shape of the welded member has become complicated, sufficient gas shielding cannot be performed at the time of welding, and oxygen or nitrogen in the air has entered the molten pool (the part where the metal has melted during welding). Welding in imperfect conditions that intrude is increasing. When oxygen penetrates, a thin oxide film called a temper collar is formed at the weld. Since this oxide film is mainly a Cr-based oxide, when the oxide film grows, the base material Cr concentration in the welded portion decreases and the corrosion resistance decreases. On the other hand, when nitrogen penetrates, sensitization of the weld becomes more likely to occur.

  Therefore, the conventional ferritic stainless steel disclosed in Patent Document 1 has a problem that it is difficult to ensure corrosion resistance.

  Examples of ferritic stainless steels having excellent corrosion resistance of welded parts include, for example, Patent Document 2 discloses ferritic stainless steels having excellent corrosion resistance of welded parts, and Patent Document 3 discloses ferritic stainless steels having excellent corrosion resistance of weld gaps. However, Patent Document 4 discloses a ferritic stainless steel excellent in corrosion resistance of a welded portion with austenitic stainless steel.

  However, even in these ferritic stainless steels, sufficient corrosion resistance is not always ensured in welding with austenitic stainless steels such as SUS304 or in welding conditions in which oxygen and nitrogen enter the molten pool from the shielding gas. .

  On the other hand, in recent years, the conventional no. In addition to glossy products such as 2B finish and BA finish, there is an increasing demand for so-called functional products that are used for members that do not place importance on the appearance (car muffler materials, exhaust system members, etc.). In order to reduce the manufacturing cost, the functional product is manufactured by using a high-speed pickling technique as disclosed in Patent Document 5, for example, after annealing at about 850 to 900 ° C. using a carbon steel line. Therefore, no. In order to produce not only 2B finish and BA finish but also functional products, steel components that have a recrystallization temperature that can be annealed in an annealing line of carbon steel and that can be pickled at high speed are required.

JP 51-88413 A JP 2007-270290 A JP 2009-161836 A JP 2010-202916 A Japanese Patent No. 2842787

  In order to solve this problem, it is conceivable to suppress the occurrence of sensitization by simply increasing Ti or Nb in accordance with the conventional technical idea, but then, a surface caused by a TiN inclusion called a Ti stringer This is not an optimal measure because defects increase or solid solution Nb precipitates as coarse Nb precipitates at the weld and causes a problem of weld cracking. Furthermore, excessive Nb addition remarkably raises the recrystallization temperature, and thus is not preferable for the production of functional products using an annealing line of carbon steel.

  Therefore, the present invention has excellent corrosion resistance even under welding conditions in which oxygen penetrates from the shielding gas and a temper collar is generated, and in welding conditions in which nitrogen and carbon penetrate into the weld bead from the welding counterpart material, The object is to provide ferritic stainless steel with good weldability.

  In order to solve the above-mentioned problems, the present inventors investigated the influence on the oxide film properties and sensitization behavior caused by the penetration of oxygen, nitrogen and carbon into the molten pool, and further earnestly ensured the corrosion resistance by the steel-containing elements. I did research.

  First, No. 1 in Table 1 was used. Using the ferritic stainless steel shown in Fig. 1, the bead-on-plate TIG welding is performed by changing the oxygen concentration of the Ar-based shield gas in the range of 0 to 2 vol%, and the thickness of the temper collar (oxide film) on the weld bead Was measured. The results are shown in FIG. Welding was performed under the conditions of a welding current of 90 A, a welding speed of 60 cm / min, a plate thickness of 0.8 mm, a front shield gas flow rate of 15 L / min, and a back shield gas flow rate of 10 L / min. The thickness of the oxide film was measured by observation with a high-resolution SEM (scanning electron microscope) after precision polishing of the cross section including the oxide film.

  The temper color thickness showed a substantially constant value after increasing in proportion to the increase in the oxygen concentration of the shielding gas. Moreover, although the gas flow rate was different between the front surface and the back surface, no difference was observed in the temper color thickness. This indicates that the temper collar does not occur when oxygen enters the molten pool, but occurs when oxygen contacts the molten pool surface. Also, the temper color thickness becomes constant at a certain oxygen concentration or higher because the rate-determining step of oxidation is due to the diffusion of oxygen to the weld surface, and various elements in the steel (for example, Cr) move outward in the oxide ( This is thought to be due to the transition to the passing speed from the ground iron side to the reaction surface. This temper color was identified as an oxide enriched in Al, Si, and Ti by analysis such as Auger electron spectroscopy.

  Next, in order to examine the sensitization that is the main cause of the above-mentioned deterioration of the corrosion resistance of the welded portion, the influence of the nitrogen concentration of the shielding gas on the nitrogen content of the weld bead was investigated. No. in Table 1 Using the ferritic stainless steel shown in Fig. 1, the nitrogen concentration of the Ar-based shield gas was varied in the range of 0 to 2 vol%, and TIG welding of the bead-on plate (welding current 90 A, welding speed 60 cm / min, plate thickness) 0.8 mm, front shield gas flow rate 15 L / min, back shield gas flow rate 10 L / min), and the nitrogen content of the weld bead was measured.

  The results are shown in FIG. In the case of a front shield gas with a large gas flow rate, the nitrogen content of the weld bead increased in proportion to the increase in the nitrogen concentration of the shield gas. Even when the nitrogen concentration increased, the nitrogen content of the weld bead hardly changed. This is because the front shield gas was blown from the nozzle toward the molten pool at a large flow rate, and nitrogen entered the molten pool, while the back shield gas flowed slowly into the weld pool because the flow rate was small. It is thought that it did not invade the inside of the molten pool only by contact. As a result of observing the metal structure of the cross section of the welded portion, the sensitization of the weld bead became conspicuous as the amount of intruding nitrogen increased.

  From these, it became clear that oxygen or nitrogen can easily enter the molten pool. Therefore, in order to ensure the corrosion resistance of the welded part, it is necessary not only to adjust the welding shielding gas conditions but also to improve the corrosion resistance by improving the material itself.

Therefore, the effect of various additive elements on the corrosion resistance of the temper color was evaluated by pitting potential measurement. The sample steel, C, containing 0.5 wt% of Nb as an element for immobilizing the N, Ti, Si and Al is referred to as Si + Al + Ti (hereinafter _{O X} value. Note that element symbol in the formula is each element 17% Cr ferritic stainless steel contained in a range of about 0.4 to 1.9 in terms of the content (mass%) of (a). The reason for containing a large amount of Nb is to evaluate only the effect of temper color formation on the corrosion resistance by bonding and fixing C and N with Nb to prevent sensitization of the weld. The test steel was butt TIG welded with SUS304 steel (C: 0.05 mass%, N: 0.07 mass%) with a thickness of 0.8 mm using Ar gas with 2 vol% oxygen concentration as the front shield gas. went. Thereafter, the pitting corrosion potential was measured in a 3.5 mass% NaCl solution at 30 ° C. without removing the temper collar formed on the front side (torch side) of the weld bead. Measurement methods other than the concentration of NaCl were in accordance with JIS G 0577 (2005). The measurement results are shown in FIG.

O _X value becomes the pitting potential in a range of 0.60 or more 0 mV (vs SCE) or more, and improved corrosion resistance. This is thought to be because Si, Al, and Ti are concentrated in the temper collar to form a dense oxide film having good protective properties, and the decrease in Cr concentration of the weld bead surface layer due to oxidation is suppressed. Note that pitting corrosion at this time occurred in the HAZ part slightly outside the bead of the welded part temper collar part. On the other hand, if _{O X} value exceeds 1.60, the corrosion resistance is deteriorated. This is presumably because the crystallinity of the oxide film was increased and the effect of suppressing permeation of metal ions and the like was reduced.

Further, the austenite phase (gamma-phase) to investigate the effect of promoting the formation element (substantially to the austenite formers below), _{O X} value in the range of 0.60 or more 1.60 or less, and Cu, Ni, Mn and Mo is Cu + Ni + Mn—Mo (hereinafter referred to as γ value. The element symbol in the formula represents the content (% by mass) of each element) of about 0.40 to 1.80, 17% Cr ferrite system Stainless steel was produced as a test steel. SUS304 steel (C: 0.05 mass%, N: 0.07 mass%) with a plate thickness of 0.8 mm using Ar gas with 2 vol% oxygen concentration and 8 vol% nitrogen concentration as the surface shield gas Butt TIG welding was performed. Thereafter, a 20 mm square test piece including a weld bead was collected, the weld bead portion was polished to remove the temper collar, and then covered with a sealing material, leaving a 10 mm square measurement surface, 30 ° C., 3.5 mass. The pitting potential was measured in% NaCl solution. Measurement methods other than the concentration of NaCl were in accordance with JIS G 0577 (2005). The measurement results are shown in FIG.

  The γ value was 0.60 or more, and the pitting corrosion potential was 0 mV (vs SCE) or more, indicating good corrosion resistance. This is because when the Cr content is less than 18%, the concentrations of Cr and Mo that are ferrite phase formation promoting elements (hereinafter abbreviated as ferrite forming elements) are low, and the concentrations of Cu, Ni, and Mn that are austenite forming elements are high. In addition, the molten pool becomes an austenite phase, and this austenite phase transforms into a martensite phase during solidification, which suppresses the precipitation of Cr-based carbonitrides at the grain boundaries, which is thought to suppress sensitization. It is done.

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

  [1] By mass%, C: 0.001 to 0.030%, Si: more than 0.30 to 0.55%, Mn: 0.05 to 0.50%, P: 0.05% or less, S : 0.01% or less, Cr: 16.5 to less than 18.0%, Ni: 0.05 to less than 0.50%, Mo: 0.01 to 0.50%, Al: 0.10 to 1. 25%, V: 0.01 to 0.50%, Nb: 0.002 to 0.050%, Ti: 0.05 to 0.50%, Cu: 0.30 to 0.60%, N: 0 0.001 to 0.030%, satisfying the following formulas (1) and (2), the balance being made of Fe and inevitable impurities, excellent corrosion resistance of the welded portion with austenitic stainless steel Ferritic stainless steel.

0.60 ≦ Si + Al + Ti ≦ 1.60 (1)
0.60 ≦ Cu + Ni + Mn—Mo (2)
In addition, the element symbol in a formula represents content (mass%) of each element.

  [2] Further, in ferrite mass, Sb: 0.05 to 0.30% or less, the ferrite system having excellent corrosion resistance of the welded portion with the austenitic stainless steel according to the above [1] Stainless steel.

  [3] Further, by mass%, Zr: 1.0% or less, W: 0.2% or less, REM: 0.1% or less, Co: 0.2% or less, B: 0.1% or less The ferritic stainless steel excellent in corrosion resistance of the welded portion with the austenitic stainless steel according to the above [1] or [2], comprising at least one selected from the group consisting of:

  According to the present invention, a ferritic stainless steel having excellent corrosion resistance can be obtained even under welding conditions in which oxygen or nitrogen from the shielding gas and nitrogen or carbon from the welding partner material enter the weld bead. Further, the ferritic stainless steel of the present invention is as good in welding work as the conventional steel.

It is a figure which shows the influence of the oxygen concentration of the shielding gas on the oxide film thickness of a weld bead. It is a figure which shows the influence of the nitrogen concentration of the shielding gas on the nitrogen content of a weld bead. It shows the effect of 17 wt% on the pitting potential of the weld bead and SUS304 in Cr steel Si + Al + Ti _{(O X} value). It is a figure which shows the influence of Cu + Ni + Mn-Mo ((gamma) value) which acts on the pitting corrosion potential of the weld bead with SUS304 in 17 mass% Cr steel.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.001 to 0.030%
When the amount of C is large, the strength is improved, and when it is small, workability is improved. In order to obtain sufficient strength, the content of 0.001% or more is necessary. However, if the content exceeds 0.030%, the workability deteriorates remarkably, and Cr carbide is precipitated to local Cr. It tends to cause a decrease in corrosion resistance due to deficiency. Therefore, the C content is in the range of 0.001 to 0.030%. The lower the amount of C, the better the corrosion resistance, but if it is too low, it takes a long time for refining, so the range is preferably 0.003 to 0.018%.

Si: more than 0.30 to 0.55%
Si is an important element in the present invention. It is an important element that concentrates together with Al and Ti on the temper collar formed by welding to improve the protective property of the oxide film and to improve the corrosion resistance of the welded portion. The effect is obtained when the content exceeds 0.30%. However, when the amount of Si exceeds 0.55%, the workability is significantly lowered, and the molding process becomes difficult. Therefore, the Si amount is in the range of more than 0.30% to 0.55%. Preferably, it is 0.40 to 0.50% of range. More preferably, it is 0.35 to 0.45% of range.

Mn: 0.05 to 0.50%
Mn is also an important element in the present invention. Mn is an austenite-forming element, and when the Cr content in the steel is 16.5% or more and less than 18.0%, it functions to make the bead of the dissimilar steel type weld with SUS304 into a martensite phase. In order to acquire the effect, containing 0.05% or more is required. However, if the content exceeds 0.50%, the precipitation of MnS, which is the starting point of corrosion, is promoted and the corrosion resistance is lowered, so the amount of Mn is made 0.05 to 0.50%. Preferably, it is 0.05 to 0.40% of range. More preferably, it is 0.05 to 0.35% of range.

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% or less S is an element inevitably contained in steel, but inclusion exceeding 0.01% lowers the corrosion resistance. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less.

Cr: 16.5% or more and less than 18.0% Cr is the most important element for ensuring the corrosion resistance of stainless steel. If it is less than 16.5%, sufficient corrosion resistance cannot be obtained in the weld bead where the Cr of the surface layer is reduced by oxidation due to welding and in the vicinity thereof. On the other hand, since Cr is a ferrite-forming element, the content of 18.0% or more includes SUS304 even if an austenite-generating element such as Ni, Mn, or Cu is contained in a range that does not affect other elongation and hardness. The weld bead part and the like do not become a martensite phase. For this reason, in the case of dissimilar steel welding with SUS304 or the like, sensitization is particularly likely to occur. Therefore, the Cr content is in the range of 16.5% or more and less than 18.0%. Preferably, it is 16.5 to 17.5% of range. More preferably, it is 16.5 to 17.3%.

Ni: 0.05% or more and less than 0.50% Ni is also an important element in the present invention. Ni is an austenite-forming element, and when the Cr content in the steel is 16.5% or more and less than 18.0%, it functions to make the bead portion of a dissimilar steel type weld with SUS304 or the like into a martensite phase. In order to acquire the effect, 0.05% or more of content is required. 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. The effect is acquired by 0.05% or more of containing. However, if the content is 0.50% or more, in addition to lowering the workability, the stress corrosion cracking sensitivity becomes strong. Furthermore, since Ni is an expensive element, it causes an increase in cost. Therefore, the Ni content is in the range of 0.05% or more and less than 0.50%. Preferably, it is 0.10 to 0.30% of range. More preferably, it is 0.15 to 0.25% of range.

Mo: 0.01 to 0.50%
Mo is an element that promotes the repassivation of the passive film, including the weld, and improves the corrosion resistance of the stainless steel. The effect is obtained when the Mo content is 0.01% or more. However, when the amount of Mo exceeds 0.50%, the strength increases and the rolling load during hot rolling increases, so that the productivity decreases, and Mo is also an expensive element, which increases the manufacturing cost. Furthermore, from the viewpoint of weld corrosion resistance, Mo is a ferrite-forming element. Therefore, when the addition amount is excessive, the weld bead portion is not in the martensite phase but in the ferritic phase when welded with a different steel type such as SUS304. Thus, sensitization occurs, and the corrosion resistance of the welded portion is lowered, which is not preferable. Therefore, the Mo amount is set to a range of 0.01 to 0.50%. Preferably, it is 0.01 to 0.30% of range. More preferably, it is 0.01 to 0.15%.

Al: 0.10 to 1.25%
Al, like Si, is an element that concentrates in a temper collar formed by welding together with Si and Ti, and improves the corrosion resistance of the welded portion. In addition, since Al has a stronger affinity for nitrogen than Cr, even when nitrogen is mixed in the weld pool, nitrogen precipitates as Al nitride instead of Cr nitride, and even an element that has the effect of suppressing sensitization. is there. Al is also an element useful for deoxidation in the steel making process. These effects cannot be obtained unless the content is 0.10% or more. However, if it exceeds 1.25%, the ferrite crystal grains increase, and workability and manufacturability deteriorate. Therefore, the Al amount is set to a range of 0.10 to 1.25%. Preferably, it is 0.12 to 0.80% of range. More preferably, it is 0.15 to 0.50% of range.

V: 0.01 to 0.50%
V is an element that improves corrosion resistance and workability. In the present invention, when nitrogen enters the weld bead from the shielding gas, V is an element that suppresses sensitization by combining with nitrogen to become VN. The effect is obtained when the V content is 0.01% or more. However, if the content exceeds 0.50%, the processability decreases. Therefore, the V amount is in the range of 0.01 to 0.50%. Preferably, it is 0.03 to 0.30% of range. More preferably, it is 0.08 to 0.20% of range.

Nb: 0.002 to 0.050%
Nb is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. Therefore, in this invention, it is an important element in order to suppress the sensitization by nitrogen penetration | invasion from shielding gas, The effect is acquired by 0.002% or more. However, on the other hand, the content of Nb has the effect of increasing the recrystallization temperature during annealing, so if it exceeds 0.050%, it is manufactured at low cost by annealing and pickling using a continuous annealing line of carbon steel. There is a problem that it is difficult to manufacture functional products. Therefore, the Nb content is in the range of 0.002 to 0.050%. Preferably, it is 0.003 to 0.010% of range. More preferably, it is 0.005 to 0.010% of range.

Ti: 0.05 to 0.50%
Ti, like Si and Al, is an element that concentrates in a temper color formed by welding and improves the protective properties of the oxide film. Ti is also an element that preferentially binds to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. These effects cannot be obtained when the content is less than 0.05%. On the other hand, if the content exceeds 0.50%, the workability deteriorates and the Ti carbonitride becomes coarse and causes surface defects. Therefore, the Ti amount is set to a range of 0.05 to 0.50%. Preferably, it is 0.10 to 0.35% of range. More preferably, it is 0.20 to 0.30% of range.

Cu: 0.30 to 0.60%
Cu is also an important element for the present invention. Cu is also an austenite-forming element like Mn and Ni. In particular, when the Cr concentration is less than 18.0%, a martensite phase is generated in a dissimilar steel type welded portion such as SUS304 and the corrosion resistance of the welded portion is deteriorated (sensitive) Has a function to prevent Cu is an element that enhances corrosion resistance, particularly corrosion resistance when an aqueous solution or weakly acidic water droplets adhere. This is because Cu dissolves at a certain electrochemical potential in an aqueous solution or weakly acidic water droplets, and Cu reattaches to the ground iron to suppress dissolution resistance. These effects are obtained at 0.30% or more. However, when Cu is contained in excess of 0.60%, hot workability is deteriorated, and Cu-like hydrated oxide called red scale is formed on the hot-rolled slab during hot rolling, which may cause surface defects. Become. Therefore, the Cu amount is set to a range of 0.30 to 0.60%. Preferably, it is 0.30 to 0.50% of range. More preferably, it is 0.35 to 0.45% of range.

N: 0.001 to 0.030%
N, like C, is an element inevitably contained in steel. When the N content is large, the strength is improved, and when 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 0.030%, the workability deteriorates significantly, and when Cr nitride is precipitated, In order to reduce the corrosion resistance, the N content is in the range of 0.001 to 0.030%. N is preferably as low as possible for corrosion resistance, but if it is too low, it takes time for refining. Therefore, it is preferably in the range of 0.003 to 0.030%. More preferably, it is 0.003 to 0.013% of range.

Si + Al + Ti (O _X value): 0.60 or more and 1.60 or less In addition, the element symbol in a formula represents content (mass%) of each element.

Since Si, Al, and Ti all have a strong affinity for oxygen, when stainless steel to which these elements are added is welded, a temper collar mainly composed of Si, Al, and Ti is formed on the steel plate surface. The Since this temper color is dense and highly protective, the oxidation of Cr is reduced, and the deterioration of corrosion resistance due to the decrease in Cr concentration in the base material is reduced. To obtain a sufficient corrosion resistance is O _X value is required to be 0.60 or more. On the other hand, if O _X value exceeds 1.60, increases the crystallinity of the temper color, corrosion resistance to lower the effect of suppressing the transmission of such metal ions is lowered again. Therefore, _{O X} value is set to 0.60 or more 1.60 or less.

Cu + Ni + Mn—Mo (γ value): 0.60 or more In addition, the element symbol in a formula represents content (mass%) of each element.

  If the material to be welded is austenitic stainless steel (for example, SUS304) with a high C and N content, a large amount of C and N are mixed in the molten pool from the material and shield gas, and the bead portion is in the ferrite phase. In this case, sensitization occurs and the corrosion resistance decreases. However, when the γ value: Cu + Ni + Mn—Mo is high and Cr is less than 18.0% and the counterpart material is SUS304 or the like, the bead portion becomes a martensite phase, and the sharpness resulting from the precipitation of Cr carbonitride. Is suppressed. This effect is obtained when the γ value is 0.60 or more. The higher the γ value, the better in terms of forming the austenite phase. However, if it is too high, red heat embrittlement due to Cu, stress corrosion cracking due to Ni, and the like occur, so 0.60 to 1.00 is preferable. More preferably, it is 0.60-0.90.

The above is the basic chemical component of the present invention, and the balance is composed of Fe and unavoidable impurities, and Ca: 0.0020% or less is acceptable as the unavoidable impurities.
Furthermore, you may contain Sb as a selection element in order to stabilize nitrogen.

Sb: 0.05-0.30%
Sb, like Al, has an effect of suppressing sensitization due to Cr nitride precipitation by combining with nitrogen that enters the molten pool from the atmosphere when the gas shield of TIG welding is insufficient. This effect is obtained when the content is 0.05% or more. However, when Sb is contained exceeding 0.30%, non-metallic inclusions are generated at the slab stage, and the surface properties are deteriorated. Moreover, the toughness of a hot-rolled sheet is also deteriorated. Therefore, when it contains Sb, it is preferable to set it as 0.05 to 0.30% of range. More preferably, it is 0.05 to 0.15% of range. More preferably, it is 0.05 to 0.10% of range.

  Furthermore, you may contain 1 or more types chosen from Zr, W, REM, Co, and B as a selection element for the purpose, such as sensitization suppression and an improvement in corrosion resistance.

Zr: 1.0% or less Zr combines with C and N to suppress sensitization, but the effect is obtained with a content of 0.01% or more. However, if the content exceeds 1.0%, the workability is lowered, and the cost is increased because it is a very high element. Therefore, when Zr is contained, the amount of Zr is preferably 1.0% or less. More preferably, it is 0.5% or less. More preferably, it is 0.30% or less.

W: 0.2% or less W has the effect of improving the corrosion resistance like Mo, but the effect is obtained with a content of 0.01% or more. However, the content exceeding 0.2% increases the strength and decreases the productivity. Therefore, when it contains W, it is preferable that W amount shall be 0.2% or less.

REM: 0.1% or less REM improves oxidation resistance, reduces the temper color generation rate, and suppresses the formation of a Cr-deficient region immediately below the temper color. The effect is acquired by 0.001% or more of containing. However, if the content exceeds 0.1%, the productivity such as pickling properties is lowered and the cost is increased. Therefore, when it contains REM, it is preferable that the amount of REM shall be 0.1% or less.

Co: 0.2% or less Co is an element that improves toughness, and the effect is obtained when the content is 0.001% or more. However, if the content exceeds 0.2%, the productivity decreases. Therefore, when Co is contained, the Co content is preferably 0.2% or less.

B: 0.1% or less B is an element that improves secondary work brittleness, and the effect is obtained with a content of 0.0001% or more. However, the content exceeding 0.1% causes a decrease in ductility due to solid solution strengthening. Therefore, when it contains B, it is preferable to make B amount into 0.1% or less. More preferably, it is 0.03% or less. More preferably, it is 0.01% or less.

2. Production Conditions Next, a preferred production method for 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, and the like, and is made into a steel material (slab) by a continuous casting method or an ingot-bundling method. The steel material is then heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-rolled without heating to form a hot-rolled sheet.

  The hot-rolled sheet is usually subjected to hot-rolled sheet annealing at 800 to 1100 ° C. for 1 to 10 minutes, but depending on the application, the hot-rolled sheet annealing may be omitted. Then, after pickling the hot-rolled sheet, it is cold-rolled by cold rolling, and then recrystallized and annealed to obtain a product.

  The rolling reduction of the cold rolling is desirably performed at a rolling reduction of 50% or more in view of stretchability, bendability, press formability, and shape correction. The recrystallization annealing of the cold rolled sheet is preferably performed at 800 to 950 ° C. from the viewpoint of obtaining good mechanical properties and pickling.

  However, in the case of a functional product, production by an inexpensive process using the high-speed pickling (refer to Patent Document 5) of the above-described carbon steel annealing pickling line using a carbon steel line is most preferable, and the annealing temperature at this time Is most preferably performed at 800 to 900 ° C. In addition, it is effective to perform BA annealing for finishing the member where the luster is desired. In addition, as described above, polishing or the like can be performed to further improve the surface properties after cold rolling and after processing.

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

  Stainless steel having the composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated to 1150 ° C. in a furnace purged with Ar gas and hot-rolled to obtain 3.5 mm thick hot rolled sheets. Then, after subjecting these hot-rolled sheets to hot-rolled sheet annealing at 950 ° C. × 1 minute, the surface was subjected to shot blasting treatment of glass beads, and then 120 wt. After dipping for 2 seconds, pickling was performed by dipping in a mixed acid composed of 15% by mass nitric acid and 3% by mass hydrofluoric acid at a temperature of 55 ° C. for 60 seconds, and descaling was performed.

Further, it is cold-rolled to a thickness of 0.8 mm, annealed at 900 ° C. for 1 minute in a weak reducing atmosphere (H ₂ : 5 vol%, N ₂ : 95 vol%, dew point −40 ° C.), and cold-rolled annealed plate Got. This cold-rolled annealed plate was subjected to high-speed descaling in which electrolysis (10 A / dm ² × 2 seconds) was performed twice in a solution comprising a temperature of 50 ° C., 15% by mass nitric acid and 0.5% by mass hydrochloric acid, and cold-rolled acid A washed and annealed plate was obtained.
In _{O X} values in Table 1 are Si + Al + Ti, in γ value is Cu + Ni + Mn-Mo, are defined respectively (Note, each element symbol in the formulas represents the content of each element (mass%)).
About this cold-rolled pickled and annealed plate, the pitting corrosion potential was measured in a 3.5% by mass NaCl solution at 30 ° C. (except for the NaCl concentration in accordance with JIS G 0577 (2005)). did.

  Butt TIG welding was performed using the produced cold rolled sheet and a commercially available cold rolled sheet of SUS304 (C: 0.07 mass%, N: 0.05 mass%). The welding current was 90 A and the welding speed was 60 cm / min. As the shielding gas, Ar gas containing 8 vol% nitrogen and 2 vol% oxygen was used at a front shielding gas flow rate of 15 L / min and a back shielding gas flow rate of 10 L / min. The width of the front side weld bead was approximately 3 mm. This was subjected to the test in a state where a temper color as welded was generated.

  A 20 mm square test piece including the prepared weld bead was collected, covered with a sealing material, leaving a 10 mm square measurement surface, and a hole was formed in a 3.5 mass% NaCl solution at 30 ° C. with a temper collar by welding. Eating potential was measured. The specimen was not polished or passivated. The other measurement methods conformed to JIS G 0577 (2005). This pitting corrosion potential was taken as the pitting corrosion potential of the weld bead.

  In addition, a 60 × 80 mm test piece including a weld bead was sampled, and a neutral salt spray cycle test of JIS H 8502 (1999) was performed using the front side as a test surface. Salt spray cycle test: 1 cycle of spraying 5% NaCl solution (35 ° C, 2h) → drying (60 ° C, 4h, relative humidity 20-30%) → wet (40 ° C, 2h, relative humidity 95% or more) The number of cycles was 15 cycles. After the test, the presence or absence of corrosion was judged visually.

  Mechanical properties were evaluated by a tensile test in accordance with JIS Z2201, using a JIS No. 13B tensile test piece.

  These evaluations were made, and the pitting corrosion potential of the base metal was 150 mV vs SCE (hereinafter abbreviated to vs SCE) or more, the pitting corrosion potential of the weld bead was 0 mV or more, no corrosion was generated by the neutral salt spray cycle test, and the fracture in the tensile test A case where the elongation was 25% or more and the surface property was good was judged as acceptable (◎), and a case where any of the conditions did not achieve the predetermined characteristics was regarded as unacceptable (x).

  The evaluation results are shown in Table 2. In all the inventive examples, the pitting corrosion potential of the base metal was 150 mV or more, and the pitting corrosion potential of the weld bead was 0 mV or more, indicating an excellent pitting corrosion potential. Also, no corrosion occurred in the neutral salt spray cycle test results. From these, it can be seen that the inventive example has excellent corrosion resistance in both the base material and the welded part. In addition, it was confirmed that all of the inventive examples exhibited good mechanical properties of 25% or more in breaking elongation, and had no surface defects and excellent surface properties.

However, the Nb amount is outside the scope of the present invention. No. 8 was rejected because the fracture structure of 25% or more could not be obtained because the work structure introduced by cold rolling remained after annealing due to an increase in the recrystallization temperature. In addition, No. in which the Si amount is outside the scope of the present invention. No. 10 was rejected because the scale remained after annealing and pickling and a predetermined elongation at break was not obtained. From these, if appropriate the content of elements involved in processability and surface properties such as Nb and Si, even if secure corrosion resistance by optimizing the O _X value and γ values, etc., is inexpensive manufacturing cost It can be seen that the predetermined mechanical properties and surface properties cannot be obtained in the production of carbon steel in a continuous annealing line (a method of annealing at less than 900 ° C. and thinning the scale in advance).

The Cr content is 16.0% and the Cu content is 0.25%, which is outside the scope of the present invention. In No. 9, the pitting corrosion potential of the base material was as low as 100 mV, and corrosion occurred in the neutral salt spray cycle test. The Mn content is 0.04% and the γ value is 0.34, which is outside the scope of the present invention. 11 and, Si content of 0.12%, _{O X} value is outside the range of the present invention at 0.34 No. No. 12, the pitting corrosion potential of the weld bead was as low as -165 mV and -33 mV, and corrosion occurred in the neutral salt spray cycle test.

Furthermore, although the content of each element is within the scope of the present invention, O _X value, gamma value is outside the range of the present invention No. In 13 to 16, the pitting corrosion potential of the base metal is 150 mV or more, and the predetermined base metal corrosion resistance is obtained, but the pitting corrosion potential of the weld bead is less than 0 mV, and from the welded portion by the neutral salt spray cycle test. Corrosion occurred, and the predetermined weld corrosion resistance could not be obtained. Therefore, in order to obtain a predetermined weld corrosion resistance, not only the content of each element, O _X value and γ values it can be seen that needs to be adjusted within the scope of the present invention at the same time.

  The ferritic stainless steel obtained by 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 (3)

By mass%, C: 0.001 to 0.030%, Si: more than 0.30 to 0.55%, Mn: 0.05 to 0.50%, P: 0.05% or less, S: 0.00. 01% or less, Cr: 16.5% or more and less than 18.0%, Ni: 0.05% or more and less than 0.50%, Mo: 0.01 to 0.50%, Al: 0.10 to 1.25 %, V: 0.01 to 0.50%, Nb: 0.002 to 0.050%, Ti: 0.05 to 0.50%, Cu: 0.30 to 0.60%, N: 0.00. Ferrite excellent in corrosion resistance of welded portion with austenitic stainless steel, characterized by containing 001 to 0.030%, satisfying the following formulas (1) and (2), and the balance consisting of Fe and inevitable impurities Stainless steel.
0.60 ≦ Si + Al + Ti ≦ 1.60 (1)
0.60 ≦ Cu + Ni + Mn—Mo (2)
In addition, the element symbol in a formula represents content (mass%) of each element.   Furthermore, ferritic stainless steel excellent in the corrosion resistance of the welded part with austenitic stainless steel according to claim 1, characterized by containing Sb: 0.05 to 0.30% in mass%.   Further, it is selected from mass%, Zr: 1.0% or less, W: 0.2% or less, REM: 0.1% or less, Co: 0.2% or less, B: 0.1% or less. The ferritic stainless steel excellent in corrosion resistance of the welded portion with the austenitic stainless steel according to claim 1, comprising at least one kind.

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