JP5937867B2 - Ferritic stainless steel with excellent corrosion resistance of welds - Google Patents

Description

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

  Ferritic stainless steel generally has not only excellent corrosion resistance, but also has features such as a smaller thermal expansion coefficient and superior stress corrosion cracking resistance than austenitic stainless steel. Because of its characteristics, it is widely used not only for general kitchen equipment, but also for roofing materials, water storage and hot water storage cans.

  When such stainless steel is used as a structure, welding is indispensable. A general-purpose ferritic stainless steel such as SUS430 has a problem that the C and N solid solubility limit is small, so that the welded portion is sensitized and the corrosion resistance is lowered. In order to solve this problem, high-purity ferritic steel has been developed that suppresses the sensitization of the weld metal by reducing the amount of C and N and fixing C and N by adding stabilizing elements such as Ti and Nb. Has been widely used.

  Also, in high purity ferritic steel, it is known that the corrosion resistance of the scale portion generated by the heat input of welding deteriorates. In TIG welding used as a general stainless steel welding method, the tendency is large due to large heat input. As a preventive measure, Non-Patent Document 1 and the like indicate that it is important to sufficiently suppress the generation of a welding scale by sufficiently performing a shield using an inert gas such as Ar during welding.

  In Patent Document 1, in order to improve the corrosion resistance of the weld heat-affected zone that occurs even when Ar gas shielding is performed, as a method of forming an oxide film of Al and Ti on the surface layer of steel during welding, the formula P1 = A technique is disclosed in which Ti and Al are added so as to satisfy 5Ti + 20 (Al-0.01) ≧ 1.5 (Ti and Al in the formula indicate respective contents in steel).

  Patent Document 2 discloses a technique for improving the crevice corrosion resistance of a welded part by adding a certain amount or more of Si in addition to the combined addition of Al and Ti.

  On the other hand, in Patent Document 3, 4Al + Ti ≦ 0.32 (Ti and Al in the formulas indicate respective contents in the steel) is satisfied, thereby reducing the heat input during welding and the scale of the welded portion. A technique for suppressing generation and improving corrosion resistance of a welded portion is disclosed.

  The above-mentioned conventional techniques are all techniques in which Ar gas shielding is performed, but Patent Documents 1 and 2 are techniques that control the oxide film in the vicinity of the weld by positively adding Al, Ti, or Si. In addition, Patent Document 3 is characterized in that, by conversely suppressing Al and Ti, the weldability of the weld metal part is improved and the heat input during welding is reduced, both of which are Ar gas during welding. It is a technology that requires shielding, and there is no mention of its omission. Moreover, it aims at improving the corrosion resistance of a welding part and a welding heat affected zone, and is not the technique which referred to the intensity | strength and structure of a weld metal part.

  Incidentally, in recent years, a method has been disclosed in which corrosion resistance can be ensured even if back gas shielding by Ar is omitted. Patent Document 4 describes a ferritic stainless steel containing 22 to 26% Cr, “a gap depth of 4 mm or more and a maximum gap depth of 30 μm or less and no argon back shield”, and welding within 2 mm from the bond end. A ferritic stainless steel excellent in the corrosion resistance of the weld gap oxide film, characterized in that the average Cr ratio of the oxide scale in the gap portion is 20% by mass or more of all metal elements, is disclosed.

  In Patent Document 5, the component range of ferritic stainless steel is defined as an example such that a value: Cr% + Mo% + 1.5Si% + 0.5Nb% + 0.9Mn% + 1.5Ni% ≧ 23 And the technique which can improve the corrosion resistance of a welding part even if a gas shield is abbreviate | omitted is disclosed.

  Further, Patent Document 6 stipulates that when Ar gas shielding is omitted, Ca is controlled to 0.0010% or less, assuming that a weld scale is not sufficiently formed if CaS is present on the material surface.

  However, Patent Document 4 says that corrosion resistance can be ensured by forming a Cr oxide film even if the shield is omitted. However, since the clearance is very narrow, the Cr film is used in a relatively severe environment such as in a hot water tank. Even if it is thick, crevice corrosion is very likely to occur. Further, in Patent Document 4, since a filler metal is not used, especially in steels exceeding 22Cr, since Cr easily absorbs oxygen and nitrogen in a molten weld metal, a Cr-deficient part is formed in the grain boundary part as high Cr steel. It is very easy to form and is very likely to cause sensitization.

  Further, in Patent Document 5, the composition of the dissimilar weld metal with the austenitic stainless steel can be determined by determining the components of the ferritic stainless steel, but the actual composition of the weld metal part is the mutual welded portion. Therefore, it is very difficult to simply control the structure with only one material component on one side. Further, Patent Document 5 shows that a large amount of more expensive Ni is added to a high purity ferrite system. Since Si, Nb, and Cu are elements that promote solid solution strengthening in addition to Ni, when the a value is increased, the strength and hardness of the material is excessively increased, making it difficult to draw the end plate. The shape may become unstable. Although the a value increases with the addition of Mn, in the text of Patent Document 5, Mn lowers the Cr concentration in the passive film and causes a decrease in corrosion resistance. The target corrosion resistance is obtained by the effect of all the elements involved in the value a, not by the active improvement of corrosion resistance only by Mn.

  The Ca concentration level defined in Patent Document 6 is a general range if it is a normal high purity ferritic stainless steel. Furthermore, the generation of CaS, which is regarded as a problem in corrosion resistance in this document, is greatly determined by the manufacturing conditions of the steel sheet and other additive components, and thus is not determined only by the Ca concentration.

Examples of high purity ferritic stainless steel welded structures include stainless steel hot water storage cans for electric water heaters and household natural refrigerant (CO ₂ ) heat pump water heaters such as Ecocute (registered trademark). Is generally used. Most hot water storage cans are supplied with hot water pressure, and a capsule-like structure in which the end plate and the body plate are welded is used to withstand the pressure. This welded portion exhibits a clearance structure in the welded portion where the end plate and the body plate are overlapped. When the Ar gas shield is omitted at this part, it is an important technical problem how to suppress the corrosion resistance deterioration in the weld gap.

  In TIG welding of ferritic stainless steel, shielding with an inert gas such as Ar is performed in order to suppress corrosion resistance deterioration in the weld metal portion and its heat-affected zone (HAZ portion). However, in recent years, TIG welding construction in which the Ar gas shield is omitted has been desired due to demands for omitting the process and reducing the Ar gas cost. However, even if the Ar shielding gas is simply omitted with the conventional materials and welding conditions, the corrosion resistance of the weld as described above is reduced.

  Against this background, there is a demand for the development of ferritic stainless steel that is excellent in corrosion resistance of TIG welds even if the Ar gas shield is omitted.

JP-A-5-70899 JP 2006-241564 A JP 2007-270290 A JP 2009-185382 A JP 2010-202916 A JP 2011-202254 A

Stainless steel handbook third edition

  The present invention provides a ferritic stainless steel that can suppress a decrease in corrosion resistance of a welded portion under conventional welding conditions even when an Ar gas shield is omitted during welding, particularly TIG welding. is there.

  The present inventors have found that, even if the Ar gas shield is omitted, in order to develop a ferritic stainless steel having excellent corrosion resistance, as a result of intensive research, the above problem can be solved by adopting the following configuration. .

The gist of the present invention is as follows.
(1) By mass%, C: 0.020% or less, N: 0.025% or less, Si: 0.05 to 0.25% , Mn: 0.46 to 1.5% , P: 0.035 % Or less, S: 0.01% or less, Cr: 18.0 to 23.5%, Mo: 0.3 to 2.5%, Al: 0.005 to 0.15%, Ti: 0.05 to A ferritic stainless steel containing 0.25%, Nb: 0.10 to 0.5%, the balance being Fe and inevitable impurities, and the composition of the component satisfying the formula (A).
(Si + 2.5Al + 3Ti) /Mn≦2.0 (A)
However, the element symbol in a formula means the mass% of the said element.
(2) Further, by mass%, Cu: 0.5% or less, Ni: 0.05-2.0%, V: 0.05-1.0%, or one or more selected from The ferritic stainless steel as described in (1) above.
(3) Further described in (1) or (2) above, characterized by containing one or two kinds selected from Sn: 0.05 to 1% and Sb: 0.05 to 1% by mass%. Ferritic stainless steel.
(4) In addition, any one of the above (1) to (3), further comprising one or two kinds selected from Zr: 0.2% or less and B: 0.005% or less in mass% Ferritic stainless steel described in 1.

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel which is excellent in the corrosion resistance of a welding part can be provided. In particular, even if the Ar gas shield in TIG welding is omitted, it is possible to suppress a decrease in corrosion resistance.

It is a figure showing the test piece for a corrosion resistance test. It is a figure which shows the relationship between Cu2 ⁺ density ^| concentration, the Ar gas shield presence at the time of welding, and the left side of A type | formula, and crevice corrosion depth. It is a figure which shows the relationship between A type left side and crevice corrosion depth.

  Hereinafter, the present invention will be described in detail. Unless otherwise specified,% means mass%.

As a result of various investigations on the possibility of providing ferritic stainless steel having excellent corrosion resistance even if the Ar gas shield is omitted in the corrosion resistance of TIG welds of ferritic stainless steel, the present inventors have obtained the following knowledge. Obtained.
(1) If the shield at the time of TIG welding is omitted, the corrosion occurrence site in the vicinity of the weld changes as compared with the case where the shield is implemented. That is, when the shield is omitted, it is not a so-called HAZ portion, but a portion separated by about 1 mm from the boundary between the weld back bead and the base material.
(2) As a result of investigating the weld scale structure without a gas shield at this part, if the corrosion resistance is low, Ti, Al, Si oxides are locally scattered on the surface of the Cr, Fe film. It was found that this was the starting point for corrosion. Therefore, it is important to reduce Ti, Al, and Si as much as necessary.
(3) In TIG welding in which the gas shield is omitted, when the heat input during welding is large, the corrosion resistance may not be ensured only by the above (2) because the scale of the welded portion develops. Therefore, (2) When Mn is added in addition to the conditions, the Mn-based oxide is formed on the surface layer of the Cr-based oxide, and the corrosion resistance of the weld scale can be improved regardless of the heat input during welding. I found out. For this purpose, it is necessary to add 0.33% or more of Mn and satisfy the following formula (A).
(Si + 2.5Al + 3Ti) /Mn≦2.0 (A)

  In order to obtain this knowledge, a specimen as shown in FIG. 1 was manufactured, and the following experiment was performed. In addition, in the TIG welding structure shown in FIG. 1, what is located on the anti-torch side 6 corresponds to the lower half of FIG. That is, it is a clearance space including the plane where the back bead exists and the weld clearance 5. Further, among these portions on the side opposite to the torch 6, the corrosion resistance becomes a problem in the vicinity of the back bead portion 8 and the weld gap portion 5 in the enlarged view on the right side of FIG. 1. Therefore, in this invention, corrosion resistance evaluation is performed about the above-mentioned back bead part 8 vicinity and the welding clearance gap part 5. FIG. Among these, the back bead part 8 vicinity is described as the back surface weld part 4. FIG.

[Manufacture of test materials]
The test material was manufactured as follows. Ferritic stainless steels with different steel compositions and comparative steel compositions with various changes in Al, Ti, Si, and Mn shown in Table 1 melted in a vacuum melting furnace, which are cold rolled to a thickness of 0.8 mm by rolling and heat treatment A board was produced.

  This weld test material was produced as follows. The surface of the test material was wet-polished with # 600 emery paper and then cut into 40 mm width × 200 Lmm and 55 mm width × 200 Lmm. Of these, a 55 mm wide material was bent 15 mm from the end to 15 ° from the horizontal, and combined with a 40 mm wide flat plate as shown in FIG. 1, the end of the bent material was TIG welded.

[Welding conditions]
The welding conditions were a feed rate of 50 cm / min, and the current value was changed between 50 and 100 A based on 70 A in order to change the amount of penetration. When performing gas shielding, Ar gas is used, the torch side is fixed at a flow rate of 15 L / min, the after gas is fixed at 20 L / min, and the gas on the anti-torch side is flowed at a flow rate of 5 L / min and when stopped. The flow rate was zero.

[Corrosion resistance evaluation test]
The corrosion resistance evaluation test piece was cut from this material into a weld metal length of 20 mm and a length of 40 mm. In order to expose only the anti-torch side to the test environment, the test surface of the weld metal part on the torch side was subjected to # 600 emery wet polishing treatment together with the cut end surface.

The test solution, with CuCl ₂ reagent as an oxidizing agent in addition to NaCl, 600ppmCl ^- and, using the adjusted test solution so as to 2ppm or 20ppmCu ^2+. Cu ²⁺ is added to adjust the oxidizing property of the environment, and the higher the concentration, the more severe the corrosive environment. The adjusted immersion conditions were 80 ° C. and oxygen blowing, and continuous immersion for 2 weeks. The test solution was changed every week.

  The pitting corrosion of the back surface welded part after the test and the crevice corrosion depth of the welded crevice part were measured using the depth of focus method. The measurement of corrosion in the gap was performed after cutting the weld metal and opening the weld gap.

  The surface film of the welded portion was subjected to surface element surface analysis at an observation magnification of 5000 times using AES manufactured by JEOL Ltd.

[Evaluation results]
First, the change in the corrosion sensitivity of the welding material when the Ar gas shield was omitted during welding was evaluated. The composition of No. 2 and 17 of Table 1 was used for the test material. Here, No. 2 satisfies the conditions of the present invention with A formula (Si + 2.5Al + 3Ti) /Mn=1.52, and No. 17 is 8.08, which is a comparative example. In both cases, Cr is about 21% and Mo is about 1%.

The TIG welding conditions were such that the current value was 95 A, and Ar gas shielding on the back surface was implemented and omitted. The original plate thickness was 0.8 mm, and the bead width on the back of the weld was about 1.2 mm. Corrosion test 600PpmCl ^- in was performed in two conditions plus 2ppm or 20ppm of Cu ^2+.

The relationship between the weld crevice corrosion depth and Cu concentration after this test is shown in FIG. First, when Ar gas shielding was carried out, both of the Cu ²⁺ = 2 and 20 ppm, both the invention example (symbol ◯) in which the A value is 1.52, and the comparative example (●) in which the A value is 8.08 are the corrosion depth. The thickness was 50 μm or less. In particular, the inventive examples showed a tendency for the amount of corrosion at Cu ²⁺ = 20 ppm to be smaller than that of the comparative examples. In addition, the corrosion position when this Ar gas shield is carried out is a position about 2 to 3 mm away from the welding gap, which is a so-called heat affected zone (HAZ) of welding, which is also a weld bead portion of the back surface welded portion. Similarly, pitting corrosion was observed at a position 2 to 3 mm apart. However, the pitting corrosion of this back surface welded part was shallower than the gap part.

On the other hand, when the Ar gas shield is omitted, the corrosion depth is 50 μm or less for both the inventive example and the comparative example at Cu ²⁺ = 2 ppm, but the inventive example (Δ) is 50 μm when Cu ²⁺ = 20 ppm. As shown below, the crevice corrosion depth was significantly increased to about 100 μm in the comparative example (▲).

The corrosion position when the Ar gas shield is omitted is different from the case where the Ar gas shield is applied for both the 2 and 20 ppm Cu ²⁺ conditions, and the clearance starts within 1 mm from the weld bead boundary of the back surface welded part and also in the clearance part. Corrosion occurred at a position within 1 mm from the point (that is, the boundary between the weld metal and the welded material in the weld gap).

  The reason why the crevice corrosion depth was increased when the Ar gas shield was omitted is because the site where the corrosion sensitivity is increased by omitting the Ar gas shield has changed to a position where the distance from the weld metal body and the weld metal part is close, Even with a shape with a relatively open weld gap, such as this test material, the corrosion sensitivity is increased at the narrowest gap, so the occurrence of corrosion is accelerated, and the highly corrosive gap internal liquid generated due to its structure. It is understood that it stays in the gap.

  Similarly, the reason why the corrosion depth of the back surface welded part was suppressed to 50 μm or less was also due to the high corrosion susceptibility, but corrosion occurred. It is judged that there was not.

  In order to investigate the difference in crevice corrosion behavior when the Ar gas shield was omitted between the inventive example and the comparative example, the scale portion near the weld gap was subjected to surface analysis by AES. As a result, although Mn and Cr-based oxides were uniformly observed on the surface in the inventive example, Ti, Al, and Si oxides were scattered between the Cr-based oxides on the surface in the comparative example. It was confirmed that Further, from the observation after the corrosion test, traces of the vicinity of the oxides of Ti and Al starting from the corrosion test were observed. As a result, when the material of the present invention is used, even if the Ar gas shield is omitted, there is no oxide of Ti, Al, or Si on the surface scale portion, so that a high oxide film of Cr is generated. This is presumably due to the presence of an oxide mainly composed of Mn on the outer layer side. As a result of measuring the element profile in the depth direction with respect to the weld scale portion by AES, when Mn is high, an oxide mainly composed of Mn is formed on the surface layer, and an oxide mainly composed of Cr is present thereunder. It was. Generally, as in the evaluation of oxidation resistance at high temperatures, the oxide structure when stainless steel is exposed to high temperatures for a long time, such as several hundred hours, has a Mn-based spinel-type oxide on the inner layer side. Is known to produce a corundum type oxide mainly composed of Cr, and it is known that scale peeling is suppressed when the spinel oxide contains a large amount of Mn. In this TIG welding, since it is an oxide generated by heating for a very short time of several seconds or less, the detailed structure is different from the above long-time oxidation test, but the general structure is estimated to be the same. Since the bead generated by welding and the surrounding scale are oxides that are generated in a shorter time than the above oxidation test, the Mn-based oxide in the outer layer has a dense structure with excellent adhesion. It is presumed that the corrosion resistance has improved. Even when the Mn content was high, the crevice corrosion depth increased in the above test when the content of Ti, Al, or Si was high. This is presumably because when the amount of Ti, Al, and Si is large, the scale on the inner layer side is not uniform, and the Mn oxide generated in the outer layer is not generated as stably as the scope of the present invention.

  From the above, the corrosion resistance of TIG welds decreases when the Ar gas shield is omitted, and even when the Ar gas shield is omitted, the value of the left side of the formula A (Si + 2.5Al + 3Ti) / Mn indicated by the chemical composition of the material is reduced. As a result, it was clarified that the deterioration of the corrosion resistance of the back surface welded part and the gap part can be suppressed even in a hot water environment where the corrosiveness is severe.

Therefore, the test was carried out under the same conditions as the test in [Corrosion resistance test] using materials with different Ti, Al, Si and Mn concentrations. Among the other components, Cr and Mo, which have a great influence on crevice corrosion, were 18 to 23.5% for Cr and 0.3 to 2.5% for Mo. The gap shape was 15 °, the current value was 85A, the bead width on the back surface was 1.2 mm, and the Ar gas shield on the non-torch side was omitted. The corrosion test was performed under the condition of 600 ppm Cl ⁻ +20 ppm Cu ²⁺ . As a result, as shown in FIG. 3, a good correlation is obtained between the left side of Formula A: (Si + 2.5Al + 3Ti) / Mn and the crevice corrosion depth, and the crevice corrosion depth when the Ar gas shield is omitted is set to 50 μm or less. It was found that the value on the left side of Formula A must be 2.0 or less. From the above, when TIG welding is performed with the Ar gas shield omitted, it is clear that the value of the left side of the formula A (Si + 2.5Al + 3Ti) / Mn is 2.0 or less, preferably 1.8 or less. did.

  In addition, the clearance shape is evaluated with a clearance angle of 15 ° in the above test, but the critical clearance shape causing clearance corrosion is the starting point of the clearance, that is, the innermost clearance. If the clearance opening at 2 mm from the position is 40 μm or more, it has been confirmed that crevice corrosion does not occur. Accordingly, the clearance shape that can provide the effect of the configuration of the present invention is not limited to the clearance angle of 15 °, but means that the clearance opening is 40 μm or more at a position 2 mm from the clearance generation portion.

  From the above findings, it was clarified that it is important to control the value of (Si + 2.5Al + 3Ti) / Mn to 2.0 or less in order to prevent crevice corrosion when the Ar gas shield is omitted. In addition, in this component system, even when Ar gas shielding is performed as usual, excellent welded portion corrosion resistance is exhibited. Even if the Ar gas shield is implemented, a scale having a slightly discolored surface is generated depending on conditions. Even in this scale, it is determined that the corrosion resistance of the welded portion is improved by the above-described action.

  Next, the component composition of the ferritic stainless steel of the present invention will be described in detail.

  As described above, Al is important as a deoxidizing element, and also has an effect of refining the structure by controlling the composition of nonmetallic inclusions. However, as described above, Al inhibits the uniformization of the oxide film at the weld. Moreover, excessive addition of Al leads to coarsening of non-metallic inclusions during metal refining, which may be a starting point for product defects. Therefore, the lower limit value of the Al content is set to 0.005% or more and the upper limit value is set to 0.15% or less. Since it is preferable to contain Al in an amount of 0.01% or more for deoxidation, it is more preferably 0.01% to 0.08%.

  Ti is an extremely important element for fixing C and N and suppressing intergranular corrosion of the welded portion to improve workability. Furthermore, it is also useful to improve the corrosion resistance by fixing S which can be a starting point of corrosion in the material. However, Ti, like Al, inhibits the uniformization of the oxide film at the weld. Moreover, excessive addition causes a surface flaw at the time of steel plate manufacture. For this reason, the range of Ti content was made 0.05% to 0.25%. More desirably, it is 0.07% to 0.20%.

Si is also an important element as a deoxidizing element, and is effective in improving corrosion resistance and oxidation resistance. However, as with Ti and Al, it inhibits the uniformization of the oxide film at the weld. Furthermore, excessive addition reduces workability and manufacturability. Therefore, the upper limit value of the Si content is set to 0.25% or less. Since Si is an essential element for deoxidation as described above, 0.05% or more is contained. More desirably, it is 0.05% to 0.21%.
It is clear that Mn is an important element as a deoxidizing element, and when Ar gas shielding is omitted during welding as in the present invention, it is formed as Mn oxide at the weld scale and improves crevice corrosion resistance. I made it. However, if added in excess, MnS that becomes the starting point of corrosion tends to be generated, and the ferrite structure is destabilized. Moreover, since it is also a solid solution strengthening element, excessive addition of Mn reduces workability. Therefore, the Mn content is set to 0.33% to 1.5% or less. More desirably, it is 0.40% to 1.0%, and further desirably more than 0.5%.

  Next, other elements constituting the ferritic stainless steel of the present invention will be described.

  Since C lowers intergranular corrosion resistance and workability, its content needs to be reduced. For this reason, the upper limit of the C content is set to 0.020% or less. However, if the C content is excessively reduced, the refining cost deteriorates, so 0.002% to 0.015% is more desirable.

  N, like C, reduces intergranular corrosion resistance and workability, so its content needs to be reduced. For this reason, the upper limit of the content of N is set to 0.025% or less. However, if the N content is excessively reduced, the refining cost is deteriorated, so 0.002% to 0.015% is more desirable.

  P not only deteriorates weldability and workability, but also tends to cause intergranular corrosion, so P needs to be kept low. Therefore, the content of P is set to 0.035% or less. More desirably, it is 0.001% to 0.02%.

  S needs to be reduced because it generates water-soluble inclusions that cause corrosion such as CaS and MnS. Therefore, the S content is 0.01% or less. However, excessive reduction causes cost deterioration. For this reason, the S content is more preferably 0.0001% to 0.005%.

  Cr is the most important element for securing the corrosion resistance of stainless steel, and needs to be contained in an amount of 18.0% or more in order to stabilize the ferrite structure and improve the corrosion resistance of the scale during welding. However, Cr lowers the workability and manufacturability, so the upper limit was made 23.5% or less. The content of Cr is desirably 18.5% to 23.0%, and more desirably 20.0% to 22.5%.

  Mo is an element that is effective in repairing a passive film and is very effective in improving corrosion resistance. Further, when Mo is contained together with Cr, there is an effect of improving pitting corrosion resistance. Further, when Mo is contained together with Ni, it has a repassivation characteristic and an effect of suppressing the dissolution rate when corrosion occurs. However, since Mo is a rare metal, excessive addition increases the cost, and the workability is significantly reduced. For this reason, it is preferable that the upper limit of Mo content be 2.5% or less. Moreover, in order to improve said characteristic by containing Mo, it is necessary to contain 0.30% or more of Mo. The Mo content is desirably 0.50% to 2.0%, and more desirably 0.70% to 1.8%.

  Nb is added as a stabilizing element of C and N in combination with Ti to suppress intergranular corrosion of the weld and improve workability. It is preferable to satisfy (Ti + Nb) / (C + N) ≧ 6 (Ti, Nb, C, and N in the formula are the contents [mass%] of each component in the steel).

  Furthermore, Nb can improve the strength of the weld. On the other hand, excessive addition may cause a problem that the strength is excessively increased and the workability is lowered, and the cost is also increased. In order to reduce the workability, the Nb content is 0.10 to 0.50%. Desirably, it is 0.15% to 0.40%.

  Since Cu generally has an effect of reducing the active dissolution rate, it is added in an amount of 0.10% or more as necessary. However, in a hot water environment, Cu is not only considered not to have an effective corrosion inhibiting effect, but depending on the corrosive environment, not only Cu present in the environment but also Cu eluted from the material may promote corrosion. is there. Therefore, the upper limit of Cu is 0.50% or less. Preferably, it is 0.20% or less.

  V has the same effect as Nb described above, and also has an effect of improving weather resistance and crevice corrosion resistance, so V is added as necessary. However, excessive addition of V lowers workability and also saturates the effect of improving corrosion resistance. Therefore, the addition amount is preferably 0.05 to 1.0%. More desirably, it is 0.10% to 0.70%.

  Ni has the effect of suppressing the active dissolution rate and has excellent repassivation characteristics due to a small hydrogen overvoltage, so it can be added as necessary. In order to improve the above characteristics, it is preferable to contain 0.05% or more of Ni. However, excessive addition of Ni degrades workability and makes the ferrite structure unstable. Moreover, the price of raw materials often fluctuates, and it is desirable that Ni be as small as possible. Therefore, the Ni content is desirably 0.05% to 2.0%, and more desirably 0.2% to 1.1%.

  Sn and Sb not only improve crevice corrosion resistance, but also have an effect of suppressing the dissolution rate when corrosion occurs, and therefore one or two of them are added as necessary. However, excessive addition reduces workability and also saturates the effect of improving corrosion resistance, so the content is preferably 0.05 to 1%. More desirably, it is 0.1% to 0.5%.

  Zr not only improves the crevice corrosion resistance like V, but also has an effect as a stabilizing element of C and N, so is added as necessary. However, excessive addition of Zr reduces workability and also saturates the effect of improving corrosion resistance. Therefore, the upper limit of the content when Zr is contained is preferably 0.2% or less. Moreover, in order to improve said characteristic by containing Zr, it is preferable to contain Zr 0.03% or more. More desirably, it is 0.05% to 0.1%.

  B is a grain boundary strengthening element effective for improving secondary work embrittlement, and is added as necessary. However, excessive addition causes solid solution strengthening of ferrite and causes a decrease in ductility. Therefore, when B is added, the lower limit is preferably 0.0001% or more and the upper limit is preferably 0.005% or less. % To 0.0020% is more desirable.

  Test pieces made of ferritic stainless steel having the chemical components (composition) shown in Table 1 were produced by the method shown below. First, a cast steel having chemical components shown in Table 1 was melted by vacuum melting to produce a 40 mm thick ingot, which was hot rolled to a thickness of 5 mm. Thereafter, after heat treatment at 980 ° C. for 1 minute, the oxide scale was ground and removed, and a steel plate having a thickness of 0.8 mm was manufactured by cold rolling. Then, heat processing for 1 minute was performed in the range of 950-1050 degreeC. The oxidized scale on the surface was removed by pickling to obtain a test material.

  In the chemical components shown in Table 1, the balance is iron and inevitable impurities. Moreover, since the element of a blank is not added intentionally, it means not measuring.

  The structure and conditions of the TIG welded test piece were based on the conditions described in [Production of Specimen]. That is, the TIG welded test material was cut into 40 mm width × 200 Lmm and 55 mm width × 200 Lmm after the surface of the test material was wet-polished with # 600 emery paper. In order to create an opening angle of the gap, the 55 mm width material (test piece 2) is bent 15 mm from the end at 30 ° from the horizontal, and combined with a 40 mm width flat plate (test piece 1) as shown in FIG. The end of the workpiece was TIG welded. The welding current value was changed between 100 and 85 A to control the bead thickness. In this test, in order to evaluate the characteristics without an Ar gas shield, the flow rate on the torch side 7 was set to 15 L / min, and the flow rate was set to zero otherwise. All the feed rates were 50 cm / min.

  Moreover, in order to measure the proof stress of a material, the No. 5 test piece specified by JIS was produced, and it was set as the tensile test piece. The higher the yield strength, the higher the resistance to deformation as a welded structural member such as a can body. However, if it is too high, the moldability of the material is impaired. In particular, the end plate of the hot water can body is often subjected to a hemispherical drawing process, and if the proof stress of the material is too high, the working mold may be damaged. Therefore, the proof stress value was evaluated as x when 400 MPa or less, and x when exceeding.

  After preparing a cross-sectional embedded sample for the structure of the weld metal part, 10% oxalic acid electrolytic etching indicated by JIS was performed to investigate the presence or absence of sensitization. For some materials, the structure of the material itself was determined by this method.

The corrosion test of this welding material was performed as follows. The basic conditions are the same as in [Corrosion Resistance Test]. The test piece was cut into a length of 20 mm and a length of 40 mm of the weld metal exposed from the material, and the cut end surface was subjected to # 600 emery wet polishing. Only the weld metal part on the torch side was coated with silicon resin so as not to be exposed to the corrosive environment. In addition, in order to stabilize the passive state film | membrane of a grinding | polishing part, it used for the corrosion test, after passing 2 days or more after grinding | polishing. As the corrosion test solution, a test solution adjusted to 600 ppm Cl ⁻ +20 ppm Cu ²⁺ using NaCl and CuCl ₂ reagent was used as a highly oxidizing condition. The immersion conditions were 80 ° C. and oxygen blowing, and continuous immersion for 2 weeks. The test solution was changed every week. The corrosion depth after the test was determined by measuring the depth of the corrosion hole generated in the back surface welded portion and the weld gap. A maximum of 10 points were measured from the depth of each corrosion depth, and the maximum value was taken as each corrosion depth. Here, the corrosion depth of 50 μm or less was regarded as acceptable, and when exceeding, it was regarded as unacceptable. In the table, “sensitization” represents the evaluation result of the corrosion depth of the back surface welded portion as sensitization of the bead portion, and the case of 50 μm or less was represented as “◯” and the case of more than 50 μm as “x”. In addition, the evaluation result of the crevice corrosion depth was shown as a crevice corrosion depth as a measured value.

  Next, the crevice corrosion depth after the corrosion test in the sample welded without the Ar gas shield satisfies the present invention component as shown in Table 1, and the value of the left side of the formula A (Si + 2.5Al + 3Ti) / Mn is In Nos. 1 to 15 that are 2.0 or less, the crevice corrosion depth is 50 μm or less. Further, in Nos. 1 to 15, it was confirmed that a ferrite single phase structure was exhibited and the proof stress satisfied a condition of 400 MPa or less.

  On the other hand, when the value of (Si + 2.5Al + 3Ti) / Mn (A value) exceeds 2.0, In each of 16-19, 21, and 23, the crevice corrosion depth exceeded 50 μm. In addition to the A value exceeding the specified value, No. No. 16 is outside the specified range for Mn, Cr and Nb, No. 17 is outside the specified range for Mn and Al, No. 16 No. 18 has Mn and Nb out of specified range, No. 19 has Si and Ti out of specified range, No. 18 In No. 21, Mn and Cr were outside the specified ranges. As for No21, Cr is a value higher than the specified range. Therefore, as a result of cross-sectional observation, sensitization was observed at the bead portion of the back surface welded portion where the Ar gas shield was omitted. It is presumed that this is because it is easy to absorb nitrogen in the atmosphere due to high Cr, and Cr nitride is precipitated at the grain boundary. Therefore, in the actual environment, there is a concern about corrosion not only from the weld gap but also from the bead. . No. In 23, only the A value was higher than the specified value.

  In Nos. 20 and 22, the crevice corrosion depth exceeded 50 μm despite the A value being 2.0 or less. This is presumed that, in No20, since Ti was less than the specified value and a large amount of CaS was generated in the material as a corrosion starting point, corrosion progressed even if the scale was controlled. In addition, in No. 20, Mn was higher than the specified value, and the strength was also higher than the specified value. This is judged from the fact that the structure of the material is a two-phase structure in which ferrite and martensite are mixed, and although it is high in strength, it is judged that drawing of the end plate or the like is very difficult. Moreover, No22 is judged that Mo was as low as below specification, and the corrosion resistance of the material was not sufficient.

  Moreover, in No18 and 20, since the stabilizing element Ti or Nb was not added, sensitization was confirmed in the welded portion.

  From the above results, in the ferritic stainless steel in which the components are within the specified range and the A value (Si + 2.5Al + 3Ti) / Mn is controlled to 2.0 or less, the oxidation is performed when the Ar gas shield at the time of welding is omitted. It was clarified that the corrosion is suppressed even in severe corrosive environment. Even when the Ar gas shield is not omitted, this steel has excellent welded portion corrosion resistance.

  The ferritic stainless steel of the present invention can be suitably used for a member that requires excellent weld strength in a structure having a TIG weld, such as a water storage / hot water storage tank. Furthermore, even if it has a welded joint structure and excellent corrosion resistance of the welded portion is required, it is possible to provide an optimum material.

  It also has suitable workability and weld strength, such as general appliances such as home appliances, bathtubs, kitchen equipment, drain water recovery devices for latent heat recovery gas water heaters and their heat exchangers, various welding pipes, etc. It is also suitable for various application members.

DESCRIPTION OF SYMBOLS 1 Test piece 2 Test piece 3 Weld metal part 4 Back surface weld part 5 Weld clearance part 6 Anti-torch side 7 Torch side 8 Back bead part

Claims (4)

% By mass, C: 0.020% or less, N: 0.025% or less, Si: 0.05 to 0.25% , Mn: 0.46 to 1.5% , P: 0.035% or less, S: 0.01% or less, Cr: 18.0 to 23.5%, Mo: 0.3 to 2.5%, Al: 0.005 to 0.15%, Ti: 0.05 to 0.25 %, Nb: 0.10 to 0.5%, the remainder is made of Fe and inevitable impurities, and the component composition satisfies the formula (A).
(Si + 2.5Al + 3Ti) /Mn≦2.0 (A)
However, the element symbol in a formula means the mass% of the said element.   Furthermore, it contains one or two or more of Cu: 0.5% or less, Ni: 0.05-2.0%, V: 0.05-1.0% in mass%. Item 2. The ferritic stainless steel according to Item 1.   The ferritic stainless steel according to claim 1 or 2, further comprising one or two kinds selected from Sn: 0.05 to 1% and Sb: 0.05 to 1% by mass%. steel.   The ferritic stainless steel according to any one of claims 1 to 3, further comprising one or two kinds of Zr: 0.2% or less and B: 0.005% or less in mass%. steel.

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