JP6354772B2 - Ferritic stainless steel - Google Patents

Description

  The present invention relates to a ferritic stainless steel used for an application in which welding with an austenitic stainless steel is performed, and thereafter, an overhang forming process is performed in a state including a weld bead. If the ferritic stainless steel of this invention is used, the weld bead of the welding part formed between austenitic stainless steel will be excellent in the surface property and corrosion resistance after a process.

  Compared with austenitic stainless steel, ferritic stainless steel has various excellent characteristics such as high cost performance with respect to corrosion resistance, good thermal conductivity, low thermal expansion coefficient, and resistance to stress corrosion cracking. For this reason, ferritic stainless steel is 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.

  The production of these structures often includes a step of joining stainless steel plates by welding. However, since ferritic stainless steel has a small solid solubility limit of C and N, Cr carbonitride is generated at the grain boundary of the welded part due to melting and solidification by welding, and a Cr-deficient layer is formed at the grain boundary of the welded part. May be formed. When a Cr-deficient layer is formed at the grain boundary, the corrosion resistance is lowered (corrosion resistance reduction phenomenon called sensitization).

  Therefore, conventionally, the content of C and N is kept low to make it difficult to precipitate carbonitride, and the addition of Ti or Nb, which has a higher affinity for carbon nitrogen than Cr, produces Cr carbonitride. The method of suppressing is taken.

  For example, Patent Document 1 discloses a steel in which intergranular corrosion resistance (corrosion resistance) of ferritic stainless steel is improved by adding Ti and Nb in combination.

  Patent Document 2 discloses a ferritic stainless steel for improving the corrosion resistance of a welded portion with an austenitic stainless steel.

JP 51-88413 A JP 2010-202916 A

  However, in the case of the steel described in Patent Document 1, it is effective when welding the same ferritic stainless steel, but when welding with austenitic stainless steel, the formation of Cr carbonitride is completely suppressed. It may not be possible and corrosion resistance may be reduced. This is because austenitic stainless steel contains a larger amount of C and N than ferritic stainless steel.

  Moreover, in the case of the steel of patent document 2, since ferritic stainless steel has suppressed the production | generation of the martensite in a fusion | melting part, when the welding part is processed, the surface unevenness | corrugation of a weld bead becomes remarkable. There is a problem of end.

  In view of the above-mentioned problems of the prior art, the present invention has a surface property after processing in a weld bead in an application in which an overhang forming process including a weld bead is performed after welding with austenitic stainless steel. An object of the present invention is to provide a ferritic stainless steel having excellent corrosion resistance with a weld bead excellent.

  In the present invention, in order to solve the above-mentioned problems, the influence of various additive elements and the influence of the structure of the melted part have been intensively studied regarding the corrosion resistance and workability of the welded part of austenitic stainless steel and ferritic stainless steel.

  First, the effect of additive elements on the corrosion resistance of welds was investigated.

  Austenitic stainless steel has a larger amount of C and N in solid solution than ferritic stainless steel and contains a large amount of C and N. When austenitic stainless steel and ferritic stainless steel are welded, most of the structure of the melted portion formed during the welding often becomes a ferrite phase. Since the ferrite phase has a small amount of solid solution of C and N, the entire amount of C and N contained in the austenitic stainless steel cannot be dissolved, and C and N that cannot be dissolved melt as carbonitride. It precipitates at the grain boundary of the part. At this time, when Cr carbonitride is formed, the Cr content of the substrate is locally reduced in the vicinity thereof, and the corrosion resistance is lowered. As a means for preventing this deterioration in corrosion resistance, there is generally a method of adding elements that are more easily bonded to C and N than Cr, such as Ti and Nb, to suppress the formation of Cr carbonitride. In the present invention, V and Al are further added in addition to Nb and Ti, and the amount of each added is defined by the formula (1) described later, thereby suppressing the formation of Cr carbonitride and the corrosion resistance of the molten part. The decline is prevented. Furthermore, as a means for suppressing the formation of Cr carbonitride, in the present invention, a martensite phase is generated at the crystal grain boundary in the molten part. The formation of the martensite phase can be controlled by adjusting the component composition of the steel. The martensite phase has a larger solid solution amount of C and N than the ferrite phase, and the presence of this phase increases the solid solution amount of C and N at the grain boundary where Cr carbonitride is likely to precipitate, Generation of Cr carbonitride is suppressed. By these measures, in the present invention, a ferritic stainless steel is obtained in which corrosion resistance is not easily lowered by the precipitation of Cr carbonitride even in welding with austenitic stainless steel.

  An oxide film called a temper collar is formed on the weld bead due to the heat effect during welding. This temper collar is also one of the causes of a decrease in the corrosion resistance of the weld. Therefore, in order to obtain appropriate corrosion resistance, the temper collar formed by welding is often removed and used. As a method for removing the temper color, dissolution with an acid may be employed because it can be easily and mass-processed and can cope with a complicated shape. However, the temper color cannot be completely removed by dissolution with an acid, and elements that are easily oxidized such as Si and Al concentrated in the temper color may remain on the surface of the substrate. Therefore, the basic components are 0.01% by mass of C, 0.2% by mass of Mn, 0.02 to 0.03% by mass of P, 0.001 to 0.003% by mass of S, and 20% by mass of Cr. 1.5 mass% Mo, 0.01-0.04 mass% Cu, 0.1 mass% Ni, 0.01 mass% N, and Si and Al, 0.15 mass% Si And 0.03% by mass of Al (0.15Si-0.03Al group), 0.20% by mass of Si and 0.01% by mass of Al (0.20Si-0.01Al group) As a street, the corrosion resistance after removing the temper collar of the welded part was evaluated on a 0.8 mm cold-rolled annealed pickled steel plate of ferritic stainless steel in which Nb, Ti, V, and Al were variously changed. These 0.8 mm cold-rolled annealed pickling plates were produced in the same manner as in the examples described later. The evaluation was made by shielding with 100% Ar gas and SUS316L cold-rolled annealed plate having a thickness of 0.8 mm by TIG welding (0.015 mass% C, 0.70 mass% Si, 1.0 mass% Mn, 17 Butt welding with .6 mass% Cr, 12.1 mass% Ni, 2.1 mass% Mo, 0.020 mass% N), 2 mass% hydrofluoric acid and 10 mass% nitric acid at 50 ° C. After removing the temper color by immersing in the mixed solution for 1 minute, the pitting corrosion potential was measured in a 3.5 mass% NaCl solution at 30 ° C. The results are shown in FIG. 1 with the value of equation (1) as the horizontal axis and the pitting corrosion potential as the vertical axis (the black circle relates to the 0.15Si-0.03Al group, and the white circle relates to the 0.20Si-0.01Al group). In the 0.15Si-0.03Al group, those satisfying the formula (1) exhibited good corrosion resistance at a pitting potential of 250 mV or more. However, in the 0.20Si-0.01Al group, the pitting corrosion potential was 200 mV or less including those satisfying the formula (1), and the corrosion resistance was inferior. When the surface of these weld beads was subjected to component investigation using Auger electron spectroscopy (AES), Si remained on the substrate surface in 0.20Si-0.01Al. For this reason, it is considered that the corrosion resistance was not improved even if the formula (1) was satisfied. Furthermore, as will be described later, the corrosion resistance after removing the temper collar of the weld with an acid using steel with a changed ratio of Si and Al is examined, and each component is within an appropriate range. When the amount satisfies Si ≦ 10Al, a ferritic stainless steel having excellent corrosion resistance was obtained after removing the temper color of the welded portion with an acid.

  Next, the surface property after processing of the weld bead of the weld formed by welding austenitic stainless steel and ferritic stainless steel was examined.

  Regarding the surface properties after processing of the welded portion, there is a problem that the surface unevenness is large because the crystal grain size of the melted portion is coarse. With regard to this, it has been clarified in completing the present invention that the presence of a hard martensite phase at the crystal grain boundary makes it difficult to form orange peel-like surface irregularities. However, since the martensite phase is hard and does not stretch easily, it is necessary to moderately suppress the formation of the martensite phase in the melted part in order to ensure the workability of the welded part. Therefore, in the present invention, the austenitic stainless steel is adjusted by adjusting the addition amount of Ni and Cu as austenite stabilizing elements for promoting the formation of martensite and the addition amount of Al as a ferrite stabilizing element for suppressing the formation of martensite. An appropriate martensite phase is generated in the molten part of the steel. The preferred martensite phase content in the melted part is 0.1 to 10%.

  Furthermore, when the influence of the grain size of the ferrite phase in the melt zone and the length of the grain boundary and the grain size of the martensite phase on the grain boundary on the surface properties after the extension processing was examined in detail, the ferrite phase crystals It became clear that the smaller the grain size and the longer the grain boundary length, the more uniform and fine crystal grains in the martensite phase of the grain boundary, and the smoother the surface properties after overhanging the melted part. . The refinement of the crystal grain size of the ferrite phase in the melted part is due to the inclusion of 0.21% or more of Ti, and the grain growth of the ferrite phase precipitated from the molten state as a component in which TiN is finely precipitated from the temperature range of the molten state This can be achieved by suppressing.

  The present invention is configured based on the above results. That is, the present invention is summarized as follows.

[1] By mass%, C: 0.001 to 0.030%, Si: 0.01 to 0.25%, Mn: 0.01 to 0.50%, P: 0.05% or less, S: 0.01% or less, Cr: 18.0 to 24.0%, Ni: 0.01 to less than 0.20%, Mo: 0.1 to 3.0%, Al: 0.01 to 1.2% V: 0.01-0.50%, Cu: less than 0.10%, Nb: 0.01-0.50%, Ti: 0.01-0.50%, N: 0.001-0. A ferritic stainless steel containing 030%, the balance being Fe and inevitable impurities and satisfying the following formulas (1) and (2).
Nb + 1.3Ti + 0.9V + 0.2Al> 0.50 (1)
Si ≦ 10Al (2)
In addition, the element symbol in a formula represents content (mass%) of each element.

  [2] Further, any of mass%, Zr: 0.5% or less, W: 1.0% or less, REM: 0.1% or less, Co: 0.3% or less, B: 0.01% or less 1 type or 2 types or more are contained, Ferritic stainless steel as described in [1] characterized by the above-mentioned.

  [3] The ferritic stainless steel according to [1] or [2], wherein the content of Ti is 0.21 to 0.50% by mass.

  According to the ferritic stainless steel of the present invention, the weld bead of the weld formed by welding with the austenitic stainless steel has excellent corrosion resistance, and the surface properties of the weld bead after processing are also excellent.

It is a figure which shows the influence of the additive element on the pitting corrosion potential of a weld bead.

  Embodiments of the present invention will be described below. In addition, this invention is not limited to the following embodiment.

  The ferritic stainless steel of the present invention is, in mass%, C: 0.001 to 0.030%, Si: 0.01 to 0.25%, Mn: 0.01 to 0.50%, P: 0.00. 05% or less, S: 0.01% or less, Cr: 18.0 to 24.0%, Ni: 0.01 to less than 0.20%, Mo: 0.1 to 3.0%, Al: 0.0. 01 to 1.2%, V: 0.01 to 0.50%, Cu: less than 0.10%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.50%, N: It has a component composition containing 0.001 to 0.030%. First, each element contained in the component composition will be described. In addition, unless otherwise indicated,% which shows content of each element shall be mass%.

C: 0.001 to 0.030%
When the C content is large, the strength is improved, and when it is low, the workability is improved. In order to obtain sufficient strength, the C content needs to be 0.001% or more. When the C content exceeds 0.030%, the workability is significantly reduced. On the other hand, when the C content exceeds 0.030%, Cr carbide is likely to precipitate, and the corrosion resistance is likely to be lowered due to local Cr deficiency. Therefore, the C content is set to 0.001 to 0.030%. More preferably, it is 0.002 to 0.018%.

Si: 0.01 to 0.25%
Si is an element useful for deoxidation. This effect is obtained when the Si content is 0.01% or more. However, upon completion of the present invention, it has been found that when the temper collar formed by welding is removed with an acid, Si remains on the surface of the substrate and the corrosion resistance decreases. Therefore, the Si content is preferably as low as possible in the present invention. Si is a ferrite stabilizing element and has an action of suppressing the formation of a martensite phase at the crystal grain boundary in the molten part formed when welding ferritic stainless steel and austenitic stainless steel. When the Si content exceeds 0.25%, it becomes difficult to generate a martensite phase at the crystal grain boundary in the molten part. Therefore, the Si content is set to 0.01 to 0.25%. More preferably, it is 0.05% to 0.18%.

Mn: 0.01 to 0.50%
Mn has an effect of accelerating the formation of a martensite phase at the crystal grain boundary in the molten part formed when welding ferritic stainless steel and austenitic stainless steel. Thereby, the surface property after processing of the weld bead can be stabilized. The effect is acquired by making Mn content 0.01% or more. On the other hand, the excessive Mn content promotes the precipitation of MnS, which is the starting point of corrosion, and reduces the corrosion resistance. Therefore, the Mn content is 0.50% or less. From the above, the Mn content was set to 0.01 to 0.50%. More preferably, it is 0.08% to 0.40%.

P: 0.05% or less P is an element inevitably contained in steel, and the excessive P content reduces weldability and easily causes intergranular corrosion. This tendency becomes prominent when the P content exceeds 0.05%. Therefore, the P content is set to 0.05% or less. More preferably, it is 0.04% or less. More preferably, it is 0.03% or less.

S: 0.01% or less S is an element inevitably contained in steel. If the S content exceeds 0.01%, the production of MnS, which is the starting point of corrosion, is promoted, resulting in a decrease in corrosion resistance. Therefore, the S content is set to 0.01% or less. More preferably, it is 0.005% or less.

Cr: 18.0 to 24.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 18.0%, sufficient corrosion resistance cannot be obtained in the weld bead in which the surface layer Cr decreases due to oxidation by welding or in the vicinity thereof. On the other hand, if the Cr content exceeds 24.0%, the formation of martensite phase at the crystal grain boundary in the melted part is inhibited, and the surface irregularities after processing become remarkable. Therefore, the content of Cr is set to 18.0 to 24.0%. More preferably, it is 19.0-22.0%, More preferably, it is 19.0-21.0%.

Ni: 0.01 to less than 0.20% Ni is a remarkable austenite stabilizing element. By adding Ni, it is possible to promote the formation of the martensite phase in the melted portion formed when welding the ferritic stainless steel and the austenitic stainless steel. In the present invention, it is important that both the workability of the weld bead and the surface properties after processing are compatible by keeping the martensite phase at the crystal grain boundary in the melted portion within an appropriate range. When the Ni content is less than 0.01%, it is difficult to generate a martensite phase. When the Ni content is 0.20% or more, the formation of the martensite phase is promoted too much. Therefore, the content of Ni is set to 0.01 to less than 0.20%. More preferably, it is 0.02% to 0.16%.

Mo: 0.1-3.0%
Mo is an element that promotes repassivation of the passive film and improves the corrosion resistance of stainless steel. The effect becomes more remarkable by containing Mo together with Cr. The effect of improving the corrosion resistance by Mo is obtained with a content of 0.1% or more. However, if the content exceeds 3.0%, it becomes difficult to generate a martensite phase in the molten portion formed during welding of ferritic stainless steel and austenitic stainless steel. Therefore, the Mo content is set to 0.1 to 3.0%. More preferably, it is 0.4 to 1.8%.

Al: 0.01-1.2%
Al is an element that binds to N supplied from austenitic stainless steel and suppresses the formation of Cr carbonitride in the molten part formed during welding of ferritic stainless steel and austenitic stainless steel. . Al is also an element that suppresses the formation of a martensite phase in the melted part. When the Al content is 0.01% or more, the effect of suppressing the formation of Cr carbonitride is obtained. On the other hand, when the Al content exceeds 1.2%, it becomes difficult to generate a martensite phase in the molten part. Therefore, the Al content is set to 0.01 to 1.2%. More preferably, it is more than 0.08% to 1.0%, and more preferably more than 0.15% to 0.8%. In addition, in this specification, carbonitride includes carbide and nitride in addition to carbonitride.

V: 0.01 to 0.50%
V is combined with C and N derived from austenitic stainless steel in the molten part formed during welding of ferritic stainless steel and austenitic stainless steel, and suppresses a decrease in corrosion resistance due to Cr carbonitride. Element. The effect is obtained when the V content is 0.01% or more. However, if the V content exceeds 0.50%, the workability decreases. Therefore, the V content is set to 0.01 to 0.50%. More preferably, it is 0.02 to 0.40%.

Cu: Less than 0.10% Cu is a ferritic stainless steel of the present invention that satisfies the above Cr content and Mo content, and increases the passive maintenance current to make the passive film unstable and reduce the corrosion resistance. There is an effect. Therefore, the Cu content is restricted to less than 0.10%.

Nb: 0.01 to 0.50%
Since Nb is preferentially combined with C and N to become Nb carbonitride, the deterioration of corrosion resistance due to precipitation of Cr carbonitride is suppressed. Therefore, Nb is an important element for suppressing a decrease in corrosion resistance due to welding between ferritic stainless steel and austenitic stainless steel. The effect is acquired by making Nb content 0.01% or more. However, if the Nb content exceeds 0.50%, the hot strength increases, the hot rolling load increases, and the productivity decreases. On the other hand, when the Nb content exceeds 0.50%, Nb carbonitride precipitates at the crystal grain boundary of the welded portion and easily causes weld cracking. Therefore, the Nb content is set to 0.01 to 0.50%. More preferably, it is 0.07 to 0.38%.

Ti: 0.01 to 0.50%
Ti preferentially bonds with C and N to form Ti carbonitride, and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. In the present invention, Ti is an important element for suppressing a decrease in corrosion resistance due to welding with austenitic stainless steel. The effect is obtained when the Ti content is 0.01% or more. Further, when the Ti content is 0.21% or more, the length of the crystal grain boundary in the molten part is increased by precipitation of TiN, and the grain size of the martensite phase formed in the crystal grain boundary in the molten part is made uniform. There is also an effect of improving the surface properties after the overhang processing of the melted part. However, when the Ti content exceeds 0.50%, the workability deteriorates, and the Ti carbonitride becomes coarse and causes surface defects. Therefore, the Ti content is set to 0.01 to 0.50%. Preferably, it is 0.07 to 0.38%. More preferably, it is 0.21 to 0.35%. Most preferably, it is 0.23 to 0.33%.

N: 0.001 to 0.030%
N is an element that is inevitably contained in steel like C, and has the effect of increasing the strength of the steel by solid solution strengthening. The effect is obtained when the N content is 0.001% or more. However, when Cr nitride precipitates, the corrosion resistance decreases. For this reason, the N content is suitably 0.030% or less. Therefore, the N content is set to 0.001 to 0.030%. Preferably, it is 0.002 to 0.018%.

In the present invention, the above essential components must satisfy the following formulas (1) and (2).
Nb + 1.3Ti + 0.9V + 0.2Al> 0.50 (1)
Si ≦ 10Al (2)
In addition, the element symbol in a formula represents content (mass%) of each element.

Nb + 1.3Ti + 0.9V + 0.2Al> 0.50
Ti and Nb are C and N stabilizing elements. In completing the present invention, it has been found that Al and V are also effective in suppressing the formation of Cr carbonitride contained in the weld formed by welding ferritic stainless steel and austenitic stainless steel. . In order to improve the corrosion resistance of the weld bead of the weld formed by welding the ferritic stainless steel and the austenitic stainless steel, Nb + 1.3Ti + 0.9V + 0.2Al needs to be more than 0.50. From the above, Nb + 1.3Ti + 0.9V + 0.2Al was set to more than 0.50.

Si ≦ 10Al
Si and Al are both elements concentrated in a temper color. In completing the present invention, the relationship between the element concentration on the surface after removal of the temper color with acid and the corrosion resistance was investigated, and when the concentration of Si was significant, the corrosion resistance of the weld bead decreased. Revealed. When the Si content contained in the steel exceeds 10 times the Al content, the Si concentration on the surface after temper color removal becomes significant. Therefore, Si ≦ 10Al. Preferably, Si ≦ Al.

  In addition to the above essential components, the ferritic stainless steel of the present invention may contain the following elements as necessary.

Zr: 0.5% or less Zr combines with C and N and has an effect of suppressing sensitization. The effect is acquired by making Zr content 0.01% or more. However, excessive inclusion of Zr reduces workability. In addition, an increase in the Zr content causes an increase in manufacturing cost. Therefore, the Zr content is set to 0.5% or less.

W: 1.0% or less W, like Mo, has an effect of improving corrosion resistance. The effect is acquired by making W content 0.01% or more. However, excessive inclusion of W increases strength and decreases manufacturability (workability). Therefore, the W content is set to 1.0% or less.

REM: 0.1% or less REM improves oxidation resistance, suppresses the formation of oxide scale, and suppresses the formation of a Cr-deficient region immediately below the temper collar of the weld. The effect is obtained when the REM content is 0.0001% or more. However, the inclusion of excess REM reduces the productivity such as pickling. Further, an increase in the REM content causes an increase in manufacturing cost. Therefore, the content of REM is set to 0.1% or less.

Co: 0.3% or less Co is an element that improves toughness. The effect is obtained when the Co content is 0.001% or more. However, excessive Co content reduces the manufacturability. Therefore, the Co content is set to 0.3% or less.

B: 0.01% or less B is an element that improves secondary work brittleness. In order to obtain the effect, it is appropriate that the B content is 0.0001% or more. However, addition of excess B causes a decrease in ductility due to solid solution strengthening. Therefore, the B content is set to 0.01% or less.

  When ferritic stainless steel and austenitic stainless steel are welded, a martensite phase may be formed in the molten part. In the present invention, the martensite phase is appropriately generated at the grain boundaries, and the surface properties after processing of the weld bead in the welded portion are improved. If the martensite phase in the melted part is less than 0.1% in area ratio, the effect of reducing surface irregularities after processing due to the martensite phase at the crystal grain boundary may not be obtained. If the area ratio of the martensite phase in the melted portion exceeds 10%, the martensite phase may increase so that the workability of the weld bead may deteriorate. Therefore, the area ratio of the martensite phase in the molten part is preferably 0.1 to 10%. More preferably, it is 1 to 8%. In addition, the area ratio of the martensite phase formed in the fusion | melting part can be evaluated by cross-sectional observation of the weld bead shown in an Example.

  Although any manufacturing method may be used for the stainless steel of the present invention, an example of a preferable manufacturing method is shown below.

  After heating the stainless steel having the above composition to 1100 ° C. to 1300 ° C., the finishing temperature is 700 ° C. to 1000 ° C., the winding temperature is 500 ° C. to 850 ° C., and the plate thickness is 2.0 mm to 5.0 mm. Roll. The hot-rolled steel strip thus produced is annealed at a temperature of 800 ° C. to 1200 ° C. and pickled, and then cold-rolled and cold-rolled sheet annealed at a temperature of 700 ° C. to 1100 ° C. After cold-rolled sheet annealing, pickling is performed to remove scale. Skin pass rolling may be performed after removing the scale.

  In order to ensure the corrosion resistance of the weld bead, as described above, it is an essential requirement to suppress the Cu content to less than 0.10%. In a normal melting method, the content of Cu mixed as an unavoidable impurity may be 0.10% or more, so a melting method that severely limits the mixing of Cu must be taken. Specifically, when scrap is not used or when scrap is used, the Cu content of the scrap is analyzed to control the total amount of Cu in the scrap. Furthermore, it is necessary to adopt a method such as not melting the molten steel immediately after melting the steel type containing Cu.

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

  Stainless steel shown in Table 1 was melted in vacuum and heated to 1200 ° C., then hot rolled to a plate thickness of 4 mm at a finishing temperature of 700 ° C. to 1000 ° C. and a winding temperature of 500 ° C. to 850 ° C., 850 to 1050 Annealing was performed in the range of ° C., and the scale was removed by pickling. Furthermore, it cold-rolled to plate thickness 0.8mm, annealed in the range of 800 degreeC-1000 degreeC, pickled, and it was set as the test material.

  In order to evaluate the corrosion resistance of the weld bead of the weld part from which the scale has been removed by acid, a SUS316L cold-rolled annealed plate having a thickness of 0.8 mm (0.015 mass% C, 0.70 mass) was used. % Si, 1.0% by mass Mn, 17.6% by mass Cr, 12.1% by mass Ni, 2.1% by mass Mo, 0.020% by mass N) It was. The electrode position was adjusted such that the welding current was 100 A, the welding speed was 60 cm / min, and the penetration ratio of SUS316L was 50%. As the shielding gas, 100% Ar gas was used at a flow rate of 15 L / min. The width of the front side weld bead was approximately 4 mm. The cross section of the weld bead was etched with aqua regia, and the ratio of the martensite phase of the weld bead was determined by area ratio. The results are shown in Table 2.

  After welding, it was immersed in a mixed solution of 2% by mass hydrofluoric acid and 10% by mass nitric acid at 50 ° C. for 1 minute to remove the temper collar at the welded part. A 20 mm square test piece including a weld bead was taken from the obtained test material, covered with a sealing material leaving a 10 mm square measurement surface, and pitting potential was measured in a 3.5 mass% NaCl solution at 30 ° C. It was measured. The specimen was not polished or passivated. A saturated calomel electrode was used as the reference electrode. The other measuring methods were based on JIS G 0577. The average of the three measurements is shown in Table 2 as the pitting potential Vc′100. In all examples of the present invention, the pitting potential V'c100 was 250 mV or more, and good corrosion resistance was exhibited in the weld bead between SUS316L. No. which is a comparative example. 35-No. 37, no. 39, no. 41-No. 45, no. 47, no. No. 48, the component or any of the formulas (1) and (2) is out of the scope of the present invention, so that the pitting corrosion potential of the weld bead is less than 250 mV and the corrosion resistance is poor.

  In order to evaluate the surface properties after processing of the welded portion, overhang processing was performed at a position including the weld bead. A specimen of 100 mm square centered on the weld bead was sampled from the test material TIG welded to SUS316L by the same method as the corrosion resistance evaluation, and after removing the temper collar with acid, the overhang height was 8 mm. The overhanging process was performed with the weld bead at the top. The punch diameter was 20 mm and the clamping pressure was 10 kN. Graphite grease was applied to the surface in contact with the punch and die. The presence or absence of cracks on the surface and the shape were judged by visual inspection. The case where cracks, wrinkles and orange peel were not confirmed was evaluated as ◎, the case where slight wrinkles and orange peel were generated was evaluated as ○, and the case where cracks and remarkable wrinkles were confirmed was evaluated as ×. The results are shown in Table 2. In the example of the present invention, cracks, remarkable wrinkles, orange peel and the like could not be confirmed, and in the example of the present invention in which Ti was 0.21 or more, better surface properties were exhibited. No. which is a comparative example. 34, no. 35, no. 38, no. 40, no. 44, no. Since No. 46 was excluded from the components of the present invention, remarkable wrinkles and orange peel could be confirmed on the surface of the weld bead after the overhanging process, and the surface properties were poor.

  In particular, No. containing 0.21% or more of Ti. In 1-7, 9-11, 13-18, 21-23, and 26-33, the surface property after the extending process of a fusion | melting part was excellent.

According to the present invention, a ferritic stainless steel having excellent corrosion resistance and excellent surface properties after processing in a weld bead is used in applications where welding with austenitic stainless steel is performed and an overhang forming process including a weld bead is performed. can get.
Ferritic stainless steel obtained in the present invention is used for the production of structures by welding with austenitic stainless steel, for example, for canned materials for hot water storage of electric water heaters, fittings, ventilation openings, ducts, etc. Suitable for application to materials and the like.

Claims (3)

In mass%, C: 0.001 to 0.030%, Si: 0.01 to 0.18% , Mn: 0.01 to 0.28% , P: 0.05% or less, S: 0.01 % Or less, Cr: 18.0 to 24.0%, Ni: 0.01 to less than 0.20%, Mo: 0.1 to 3.0%, Al: 0.01 to 1.2%, V: 0.01 to 0.50%, Cu: less than 0.10%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.50%, N: 0.001 to 0.030% Containing, the balance consisting of Fe and inevitable impurities,
Ferritic stainless steel characterized by satisfying the following formulas (1) and (2).
Nb + 1.3Ti + 0.9V + 0.2Al> 0.50 (1)
Si ≦ 10Al (2)
In addition, the element symbol in a formula represents content (mass%) of each element.   Furthermore, any one of mass%, Zr: 0.5% or less, W: 1.0% or less, REM: 0.1% or less, Co: 0.3% or less, B: 0.01% or less Or the ferritic stainless steel of Claim 1 containing 2 or more types.   The ferritic stainless steel according to claim 1 or 2, wherein the Ti content is 0.21 to 0.50% by mass.

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