WO2012133681A1 - 溶接部の耐食性及び強度に優れるフェライト系ステンレス鋼およびtig溶接構造物 - Google Patents
溶接部の耐食性及び強度に優れるフェライト系ステンレス鋼およびtig溶接構造物 Download PDFInfo
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- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- B23K9/00—Arc welding or cutting
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- C21D2211/00—Microstructure comprising significant phases
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Definitions
- the present invention relates to a ferritic stainless steel excellent in corrosion resistance and strength of a welded portion and a TIG welded structure thereof.
- Ferritic stainless steel is not only excellent in corrosion resistance, but also has a smaller thermal expansion coefficient and stress corrosion cracking resistance than austenitic stainless steel.
- stainless steel hot water storage cans are generally used in electric water heaters and household natural refrigerant (CO 2 ) heat pump water heaters.
- the hot water storage can has a capsule-like structure in which a mirror plate and a body plate are welded so as to withstand water pressure.
- hot water storage cans have been required to have higher strength.
- As the can body material it is necessary to increase the strength of the material itself, but it is more important to improve the strength of the weld metal portion where the stress of the can body tends to concentrate.
- the residual chlorine concentration or chloride ion concentration becomes high due to water quality problems, the environment becomes susceptible to corrosion.
- sterilization with an oxidizing agent such as addition of chlorine is necessary, and there is an increasing number of cases where the environment is severely corroded.
- Patent Document 1 describes a ferritic stainless steel containing 22 to 26% of Cr, “4 mm or more clearance depth and maximum clearance depth of 30 ⁇ m or less and no argon back shield”, and welding within 2 mm from the bond end.
- Patent Document 2 by defining the component range of ferritic stainless steel as an example, a value: Cr% + Mo% + 1.5Si% + 0.5Nb% + 0.9Mn% + 1.5Ni% ⁇ 23, A technique that can improve the corrosion resistance of a welded part even if the gas shield is omitted is disclosed.
- Patent Document 3 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.
- Patent Document 1 guarantees the corrosion resistance by forming a Cr oxide film even if the shield is omitted.
- a severe corrosive environment such as a place where the clearance is very narrow as in the hot water storage can, Even if the film is thick, crevice corrosion is very likely to occur.
- no filler material is used in Patent Document 1, in steels with a Cr content exceeding 22%, Cr easily absorbs oxygen and nitrogen, so that Cr-deficient portions are easily formed at grain boundaries, and sensitization occurs. Is very likely.
- Patent Document 2 shows that a large amount of expensive Ni is added to a high purity ferrite system. Since Si, Nb, and Cu, in addition to Ni, are elements that promote solid solution strengthening, the strength and hardness of the material are excessively increased and workability is deteriorated.
- the prescribed Ca concentration level is a general range if it is a normal high-purity ferritic stainless steel. Furthermore, since the production of CaS, which is considered as a problem in corrosion resistance in this document, is greatly affected by the manufacturing conditions of the steel sheet and other additive components, it is not determined only by the Ca concentration.
- An object of the present invention is to provide a ferritic stainless steel capable of suppressing a decrease in corrosion resistance of a welded portion even if an Ar gas shield is omitted in TIG welding. It is another object of the present invention to provide a ferritic stainless steel welded structure that has excellent corrosion resistance and excellent weld strength even in severer corrosive environments such as crevice corrosion.
- the present inventors have obtained the following knowledge.
- the corrosion-sensitive portion of the TIG welding back surface portion (on the side to be welded facing away from the side facing the welding torch) is 1 mm from the weld metal portion.
- a portion with strong corrosion susceptibility occurred in a HAZ portion that is several mm away from the weld metal portion.
- the generated weld scale composition is mainly composed of oxides mainly composed of Cr and Fe and not mixed with oxides of Ti, Al and Si. I found out. Therefore, it was found that the material composition must be controlled as shown in the following (formula 1). (Si + 2.5Al + 3Ti) ⁇ 1.50 (1 formula) However, the element symbol in a formula means the mass% of the said element.
- crevice corrosion may occur in a TIG welded part in which an Ar gas shield is omitted simply by satisfying (1).
- the ratio of the width Wb of the portion where the weld metal part is exposed on the back surface, that is, the bead exposed on the back surface (back bead), to the plate thickness t is 1.5. It has been found that it is important to make the width narrower below (that is, the bead width exposed on the back surface is not more than 3 times the plate thickness t). The reason is considered as follows. That is, the heat input of welding can be reduced by narrowing the back bead.
- the oxidation in the gap can be suppressed, and even if the shielding gas is omitted, the highly corrosion-sensitive part can be eliminated, or the highly corrosion-sensitive part is inside the part that is difficult to corrode in the gap. It is because it can be made.
- Ingredient is mass%, C: 0.002 to 0.020%, Si: 0.05-0.30%, Mn: 0.01 to 0.50%, Cr: 16.0-25.0%, Mo: 0.30 to 3.00%, Al: 0.005 to 0.150%, Ti: 0.05 to 0.30%, Nb: 0.10 to 0.50%, N: 0.002 to 0.025%, P: 0.035% or less, S: not more than 0.0100%, the balance consisting of Fe and inevitable impurities, And ferritic stainless steel characterized by the content of Si, Al and Ti satisfying one formula. Si + 2.5Al + 3Ti ⁇ 1.50 (1 formula) However, the element symbol in a formula means the mass% of the said element.
- the ingredients are by weight, Cu: 0.10 to 0.50%, Ni: 0.05 to 2.00%, V: 0.05-1.00% Sn: 0.05 to 1.00%, Sb: 0.05 to 1.00% Zr: 0.03 to 0.20%, B: 0.0001 to 0.0050%
- the ferritic stainless steel according to (1) characterized in that it contains one or more of the above.
- the element symbol of the three formulas means the content mass% of the element.
- the composition of the ferritic stainless steel sheet is mass%, C: 0.002 to 0.020%, Si: 0.05-0.30%, Mn: 0.01 to 0.50%, Cr: 16.0-25.0%, Mo: 0.30 to 3.00%, Al: 0.005 to 0.150%, Ti: 0.05 to 0.30%, Nb: 0.10 to 0.50% N: 0.002 to 0.025% P: 0.035% or less, S: not more than 0.0100%, the balance consisting of Fe and inevitable impurities, And the content of Si, Al, Ti satisfies the formula 1,
- a welded structure of ferritic stainless steel, wherein the bead width Wb of the weld metal exposed on the back surface of the ferritic stainless steel plate at the TIG welded portion and the thickness t of the ferritic stainless steel plate satisfy the following two formulas: object.
- formula means the content mass% of the said element.
- the components of the ferritic stainless steel sheet are Furthermore, in mass%, Cu: 0.10 to 0.50%, Ni: 0.05 to 2.00%, V: 0.05-1.00% Sn: 0.05 to 1.00%, Sb: 0.05 to 1.00% Zr: 0.03 to 0.20%, B: 0.0001 to 0.0050%
- the element symbol of the three formulas means the content mass% of the element.
- the present invention it is possible to provide a ferritic stainless steel having excellent corrosion resistance of the welded portion even if the Ar gas shield is omitted. Further, it is possible to provide a welded structure that is excellent in crevice corrosion resistance and strength of the welded portion.
- FIG. 2A is a schematic diagram of the TIG welding clearance structure used in the present invention.
- FIG.2 (b) is an enlarged view of the welding part of Fig.2 (a). In the present application, when it is simply referred to as FIG. 2, both FIG. 2 (a) and FIG. 2 (b) are shown. It is a figure which shows the relationship between (Formula 2) in a severe corrosive environment, and crevice corrosion depth at the time of omitting Ar gas shielding.
- FIG. 5A shows an example of overlap fillet TIG welding
- FIG. 5B shows an example of fillet TIG welding.
- % of component content means mass%.
- the corrosion-sensitive portion on the back side of the TIG weld is a HAZ part that occurs at a distance of several millimeters from the weld metal part known at the time of conventional Ar gas shield implementation.
- the generated weld scale composition is mainly composed of oxides mainly composed of Cr and Fe and not mixed with oxides of Ti, Al and Si. I found it. Therefore, it is necessary to control the material composition as shown in the following formula (1).
- the Ar gas shield may be omitted. Can satisfy crevice corrosion. (Si + 2.5Al + 3Ti) ⁇ 1.50 (1)
- the element symbol in a formula means the mass% of the said element.
- Nb ⁇ V ⁇ 0.01
- the element symbol in a formula means the mass% of the said element.
- one of the two steel plates 2 is brought into contact with the surface of the other steel plate 1 obliquely (that is, neither vertical nor horizontal) and contact is made.
- the test piece was manufactured by fillet TIG welding the part.
- this TIG welding of the steel plates crossed diagonally is referred to as “oblique fillet TIG welding” in the present application.
- a TIG welding torch is applied from the upper side of the steel plate 1 and is welded to the other steel plate 2.
- the welding torch side of each steel plate is referred to as “front side”, and the side opposite to the welding torch is referred to as “back side”. That is, the back side (anti-torch side) corresponds to the space surrounded by the lower side of the steel plate 1 and the upper surface of the steel plate 1 and the lower surface of the steel plate 2 in FIG.
- 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.
- the oblique fillet TIG welding shown in FIG. 2A will be described, but it is not necessary to be limited to this aspect.
- the same effect can be obtained in the lap fillet TIG welding shown in FIG. 5A and the fillet TIG welding shown in FIG.
- the effects of the present invention can be obtained as long as they have the technical features of the present invention other than these embodiments.
- the oblique fillet welded joint shown in FIG. 2 will be described as an example.
- a ferritic stainless steel sheet as a test material was manufactured as follows. Ferritic stainless steel based on Cr: 16.0 to 25.0% and Mo: 0.30 to 3.00% melted in a laboratory vacuum melting furnace, with various changes in Al, Ti, Si, Nb, and V A cold-rolled sheet was manufactured by rolling and heat-treating the material.
- the components of the test materials are shown in Table 1 (Table 1 is divided into 6 parts, and Table 1-1 to Table 1-6 are hereinafter collectively referred to as Table 1).
- the plate thickness of this test material is 0.8 mm. After wet-polishing the surface of the test material with # 600 emery paper, it was cut into 40 mm width ⁇ 200 Lmm and 55 mm width ⁇ 200 Lmm to prepare test piece 1 and test piece 2, respectively. In order to provide an opening angle of the clearance, the test piece 2 was bent at 15 mm from one end of the short side (width 55 mm) at 15 ° from the horizontal. As shown in FIG. 2, the end of the bent material was TIG welded in combination with the steel plate 1 (test piece 1) and the steel plate 2 (bend processed test piece 2).
- the welding conditions were such that the feed rate was 50 cm / min, the current value was constant at 70 A, and the backside bead was exposed.
- the back bead width was about 1.2 mm.
- Ar gas was used for the gas shield of the weld.
- the gas shield on the torch side 7 was fixed at a flow rate of 15 L (liter, hereinafter the same) / min, and the after gas was fixed at 20 L / min.
- the anti-torch side 6 was conducted under two conditions, with and without a gas shield.
- gas shielding was performed at a flow rate of 5 L / min for both the weld gap portion 5 and the back surface weld portion 4 in the vicinity of the back bead portion 8, and the flow rate was zero when the gas shield was stopped.
- 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. The cut end face was subjected to # 600 emery wet polishing. In order to expose only the anti-torch side 6 to the test environment, the weld metal part 3 on the torch side 7 was coated with a heat-resistant silicone resin.
- Cu 2+ is added to adjust the oxidizing property of the environment, and the concentration of 2 ppm is a concentration simulating a general hot water environment.
- the 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 4 and the crevice corrosion depth of the welded gap part 5 after the test were measured by using the depth of focus method.
- the corrosion measurement in the gap was performed by cutting the weld metal portion 3 and separating the steel plate 1 and the steel plate 2.
- the clearance depth indicates the maximum value of the pitting corrosion depth due to corrosion.
- the surface film of the welded part was subjected to surface element surface analysis at an observation magnification of 5000 times using AES manufactured by JEOL Ltd.
- FIG. 1 shows the relationship between the crevice corrosion depth after the corrosion test and Si + 2.5Al + 3Ti in the weld crevice where the Ar gas shield is omitted. As shown in FIG. 1, when the Ar gas shield was omitted, it was confirmed that the crevice corrosion depth would be 50 ⁇ m or less if the corrosion depth was Si + 2.5Al + 3Ti of 1.50 or less.
- the corrosion position when the Ar gas shield is carried out is 5 to 10 mm from the boundary between the weld metal and the material to be welded (in this case, the steel plate) (also referred to as a fusion line; hereinafter simply referred to as “weld metal boundary”). It was made in the remote base metal HAZ part. However, unlike that, the corrosion position when the shield was omitted occurred within 1 mm from the boundary of the weld metal.
- the corrosion resistance of ferritic stainless steel in such a corrosive environment is more excellent as the amount of Cr and Mo added is higher.
- the surface of the weld bead part and the HAZ part is not a passive film having a self-repair function, but is transformed into an oxide film of Cr and Fe generated, and its oxide
- a Cr-deficient portion is formed immediately below the film, and further, the base material is corroded by corrosion of the oxide film itself. For this reason, it is thought that corrosion resistance falls rather than the grade estimated from the amount of added Cr and Mo.
- the corrosion was suppressed if the Si + 2.5Al + 3Ti was 1.50 or less even if the Ar gas shield was omitted in the gap where the corrosion resistance was previously inferior. As described above, it is also presumed that the portion susceptible to corrosion moved to the inner side of the gap due to the omission of the Ar gas shield, and thus the portion was not easily exposed to the corrosive environment.
- the value of Si + 2.5Al + 3Ti is preferably small, preferably 1.0 or less, and more preferably 0.8 or less.
- Si, Al, and Ti are important as a deoxidation element at the time of refining. For this reason, if both are excessively reduced, it becomes difficult to produce high purity ferritic stainless steel, and the production cost is significantly increased. Therefore, each of these elements has a lower limit value.
- the value derived from the lower limit value of the content of each element is the lower limit.
- it is desirable that the lower limit of Si + 2.5Al + 3Ti is 0.30, and the lower limit is set to 0.45 in consideration of manufacturing margin. More desirable.
- the corrosion resistance of the welded portion improves not only in the back surface welded portion 4 but also in the welded gap portion 5 of the gap structure.
- the anti-torch side (weld gap 5 and back weld 4) was not subjected to Ar gas shielding.
- the welding current value was changed between 50 A and 120 A for the purpose of controlling the back bead width of the weld.
- the state of the back bead at this time is expressed as 0.13 to 1.88 (back bead width 0.2 to 3) as indicated by an index: Wb / 2t indicated by the back bead width Wb and the plate thickness t shown in FIG. 0.0 mm). Since the back bead width is not uniform over the entire length, five or more points were measured along the bead in the longitudinal direction, and the average value was taken as the value of Wb. In particular, when the welding current value is small, the back bead may not extend over the entire length. In this case, Wb was zero.
- the corrosion resistance evaluation test piece was manufactured under the same conditions as the above [Manufacture of test material] and [Welding conditions].
- the corrosion test solution had a composition simulating a highly corrosive environment by increasing only the above-mentioned 600 ppm Cl ⁇ +20 ppm Cu 2+ and Cu 2+ .
- Other test conditions and evaluation methods were as described in [Corrosion Resistance Evaluation Test (Part 1)].
- the gap angle is set to 15 ° in the above test, but this angle is not important and is not particularly limited.
- the back bead width tends to be larger than when the Ar gas shield is used. This is because the cooling effect by the gas is lost, and the amount of metal melt increases even with the same heat input. Furthermore, in this invention, since Al, Ti, and Si which reduce the penetration property of welding are reduced, the tendency is larger. Therefore, in order to control the back bead width in TIG welding with the material of the present invention, it is necessary to further reduce the amount of heat input from welding than when there is a gas shield.
- the value of Wb / 2t is more desirably 1.40 or less. More desirably, it is 1.25 or less. Further, as shown in FIG. 3, when the value of Wb / 2t is 1.00 or less, the crevice corrosion depth is constant at about 20 ⁇ m, and is most preferably 1.00 or less.
- each steel shown in Table 1 described later was used as the test material.
- the welding conditions were the same as those shown in the above [Corrosion Resistance Evaluation Test (Part 2)].
- Part 2 Corrosion Resistance Evaluation Test
- the strength of the welded portion was determined by measuring the hardness of the central portion of the weld metal portion using a Vickers hardness tester under the condition of 0.5 kgf using the sample for cross-sectional observation, and obtaining the average value of the Vickers hardness Hv.
- Hv is 180 or more.
- the average value of the measurable numbers was used.
- Material strength improvement mechanisms include refinement of the structure, solid solution strengthening with trace elements, and precipitation strengthening with precipitates.
- crystal grain coarsening of the weld metal portion is likely to occur, but when Nb is added, crystal grain refinement and precipitation strengthening are achieved by the carbonitride.
- Nb is also effective as a solid solution strengthening element.
- the strength is dramatically improved by adding Nb and V together. The cause is presumed that the V precipitates synergistically improved the strength due to the difference in size from the Nb precipitates. Therefore, in order to show the effect of addition of Nb and V notably, the product of the contents of Nb and V was used as an index.
- Nb ⁇ V The value of Nb ⁇ V is preferably 0.01 or more. More desirably, 0.02 or more is added. More desirably, it is 0.03 or more, and most desirably 0.04 or more. However, excessive addition causes a decrease in workability, so is 0.5 or less.
- the lower limit value of the Al content is set to 0.005% or more and the upper limit value is set to 0.150% or less. More desirably, the lower limit is 0.007% and the upper limit is 0.100%. More desirably, the lower limit is 0.010% and the upper limit is 0.050%.
- Ti is a very important element for fixing C and N and suppressing intergranular corrosion of the welded portion to improve workability. This effect cannot be obtained when the Ti content is less than 0.05%.
- the lower limit of the Ti content is set to 0.05% and the upper limit is set to 0.30%. More desirably, the lower limit is 0.06% and the upper limit is 0.25%. More desirably, the lower limit is 0.07% and the upper limit is 0.20%.
- Si is also an important element as a deoxidizing element and is effective in improving corrosion resistance and oxidation resistance. This effect cannot be obtained when the Si content is less than 0.05%.
- the upper limit value of the Si content is set to 0.30%. Since Si is an essential element for deoxidation as described above, the lower limit was set to 0.05%. The Si content is more desirably 0.06% at the lower limit and 0.25% at the upper limit. More desirably, the lower limit is 0.07% and the upper limit is 0.20%.
- 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 welded portion by the combined addition with V. This effect cannot be obtained when the Nb content is less than 0.10%. On the other hand, when Nb is contained in excess of 0.50%, there are cases where excessive addition results in a problem that the strength is excessively improved and the workability is lowered, and the cost is also increased.
- the lower limit of the Nb content is 0.10 and the upper limit is 0.50%. Desirably, the lower limit is 0.12% and the upper limit is 0.45%. More preferably, the lower limit is 0.15% and the upper limit is 0.36%.
- the upper limit of the C content is set to 0.020% or less.
- the lower limit is 0.005% and the upper limit is 0.015%. More desirably, the lower limit is 0.008% and the upper limit is 0.013%.
- the N content is preferably 0.002% or more.
- the upper limit value of N is preferably 0.015%, more preferably 0.010%.
- the lower limit value of N is preferably 0.003% from the viewpoint of refining costs, and more preferably 0.005%.
- Mn is an important element as a deoxidizing element. This effect cannot be obtained unless 0.01% or more is added. On the other hand, excessive addition over 0.50% makes it easy to produce MnS as a starting point of corrosion and destabilizes the ferrite structure. Therefore, the Mn content is set to 0.01% to 0.50% or less. More desirably, the lower limit is 0.05% and the upper limit is less than 0.40%. More desirably, the lower limit is 0.08% and the upper limit is less than 0.33%. From the viewpoint of suppressing MnS generation, it is more certain that the upper limit is 0.30% or less.
- P P is an unavoidable impurity, but not only lowers weldability and workability but also tends to cause intergranular corrosion. Therefore, the P content is limited to 0.035% or less. More desirably, the content is 0.001% to 0.020%.
- S is also an unavoidable impurity, but it needs to be reduced in order to generate water-soluble inclusions such as CaS and MnS that will start corrosion. Therefore, the S content is limited to 0.0100% or less. It is more desirable if it is 0.0050% or less. However, excessive reduction causes cost deterioration. For this reason, the content of S is desirably 0.0001% or more.
- Cr is the most important element for ensuring the corrosion resistance of stainless steel, and has the effect of stabilizing the ferrite structure.
- the Cr content needs to be 16.0% or more. Therefore, the Cr content in the present invention is set to 16.0% or more.
- the upper limit was made 25.0% or less.
- the lower limit of the Cr content is desirably 17.0%.
- the upper limit is preferably 24.0%, more preferably the lower limit is 18.0% and the upper limit is 23.0%.
- Mo is effective for repairing passive films and is an extremely effective element for 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, when Mo is increased, the workability is remarkably lowered and the cost is increased. For this reason, it is preferable to make the upper limit of Mo content 3.00% or less. Moreover, in order to improve said characteristic by containing Mo, it is preferable to contain 0.30% or more of Mo. The lower limit of the Mo content is preferably 0.50% and the upper limit is preferably 2.50%, more preferably 0.90% and the upper limit is 2.00%.
- Cu generally has an effect of not only reducing the active dissolution rate but also promoting repassivation, so it is preferable to add 0.10% or more as necessary.
- the upper limit of Cu is set to 0.50% or less. Desirably, the lower limit is 0.12% and the upper limit is 0.20%. More preferably, the lower limit is 0.13% and the upper limit is 0.18%.
- Ni has the effect of suppressing the active dissolution rate and has excellent repassivation characteristics due to the small hydrogen overvoltage, so it can be added if necessary.
- the Ni content is preferably 0.05% to 2.00%. More preferably, the lower limit is 0.10% and the upper limit is 1.50%. More desirably, the lower limit is 0.20% and the upper limit is 1.20%.
- V has the effect of improving weather resistance and crevice corrosion resistance as well as the effect of the combined addition with Nb described above.
- excessive addition of V lowers workability and saturates the effect of improving corrosion resistance.
- the addition amount is preferably 0.05 to 1.00%. More desirably, the lower limit is 0.10% and the upper limit is 0.70%. More preferably, the lower limit is 0.20% and the upper limit is 0.60%.
- the content is preferably 0.05 to 1.00%. More preferably, the lower limit is 0.08% and the upper limit is 0.70%. More desirably, the lower limit is 0.10% and the upper limit is 0.50%.
- the upper limit of the content when Zr is contained is preferably 0.20% or less. Moreover, in order to improve said characteristic by containing Zr, it is preferable to contain Zr 0.03% or more. More preferably, the lower limit is 0.05% and the upper limit is 0.10%. More preferably, the lower limit is 0.06% and the upper limit is 0.09%.
- the lower limit is preferably 0.0001% or less and the upper limit is preferably 0.0050% or less. More preferably, the lower limit is 0.0002% and the upper limit is 0.0020%. More desirably, the lower limit is 0.0003% and the upper limit is 0.0010%.
- the technical features of the present invention are the composition of the ferritic stainless steel used and the structure of the weld. Therefore, in this specification, the hot water storage can body is taken as an example, but the present invention is not limited to this, and is applicable to all ferritic stainless steels subjected to TIG welding. Further, the thickness t of the material to be welded is not particularly limited, and there is no problem as long as it is in a practical range of 0.1 to 10 mm. The thickness can be appropriately determined within the range of generally used thin plate material of 0.4 to 3 mm.
- test material a test piece made of ferritic stainless steel having the chemical composition (composition) shown in Table 1 was used, and a plate having a thickness of 0.8 mm was manufactured by the method shown in [Production of test material] described above. These were combined to produce a TIG welded test piece with a clearance angle of 15 °.
- the welding conditions were such that the Ar gas shield was not carried out only on the side opposite to the torch (weld gap and back weld).
- the welding current value was changed between 50 A and 120 A for the purpose of controlling the back bead width of the weld.
- the corrosion resistance evaluation method was evaluated by the method shown in the above-mentioned [Corrosion Resistance Evaluation Test (Part 2)]. That test solution, 600ppmCl -, and 2ppmCu 2+, 600ppmCl -, a 2 composition with 20ppmCu 2+, 80 °C, was 2 consecutive weeks immersion in oxygen blowing. As described above, 2 ppm Cu 2+ simulates a general hot water environment, and 20 ppm Cu 2+ simulates a severe environment. The crevice corrosion depth was measured using the depth of focus method. For the clearance structure material, the weld metal part was cut and the clearance part was opened, and the corrosion in the gap was measured.
- the degree of exposure of the weld metal part was evaluated by embedding a cross section of the weld metal part with resin and observing the structure. Specifically, the cross-sectional structure as shown in FIG. 2 was observed, the thickness t and the back bead width Wb of the material to be welded were measured, and the evaluation was made using (Formula 2): Wb / 2t.
- the ferritic stainless steel of the present invention is a member that requires excellent corrosion resistance in a structure having a TIG welded portion, such as various water storage and hot water storage tanks that store water and hot water in addition to the aforementioned hot water storage can body. It can be used suitably. Furthermore, even when a more severe corrosive environment is assumed, such as when there is a clearance in the welded joint structure, it is possible to provide the optimum material.
- TIG welded structure that has a clearance structure and requires welded portion strength
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Abstract
Description
(1)TIG溶接時に裏面のArガスシールドを省略した場合、TIG溶接裏面部(溶接される鋼板において、溶接トーチの面する側と反対側)における腐食感受性の強い部分は、溶接金属部から1mm以内に生じることを知見した。従来のArガスシールドをしたTIG溶接では、腐食感受性の強い部分は溶接金属部から数mm程度離れたHAZ部分に生じていた。
そして、この溶接金属部から1mm以内の部位の耐食性向上には、生成した溶接スケール組成をCr、Fe主体の酸化物を主体とし、Ti、Al、Siの酸化物を混入させないことが重要であることを見出した。そのため、材料組成を下記(1式)のように制御する必要があることがわかった。
(Si+2.5Al+3Ti)≦1.50 …(1式)
但し、式中の元素記号は、当該元素の含有質量%を意味する。
(Nb×V)≧0.01 …(3式)
但し、式中の元素記号は、当該元素の含有質量%を意味する。
本発明はこれら知見に基づき成されたものである。本発明の要旨は以下のとおりである。
成分が質量%で,
C:0.002~0.020%,
Si:0.05~0.30%,
Mn:0.01~0.50%,
Cr:16.0~25.0%,
Mo:0.30~3.00%、
Al:0.005~0.150%,
Ti:0.05~0.30%,
Nb:0.10~0.50%,
N:0.002~0.025%,
P:0.035%以下,
S:0.0100%以下
を含有し,残部がFeおよび不可避的不純物からなり、
かつSi、Al、Tiの含有量が1式を満足することを特徴とするフェライト系ステンレス鋼。
Si+2.5Al+3Ti≦1.50 …(1式)
但し、式中の元素記号は、当該元素の含有質量%を意味する。
(2)
更に、成分が質量%で、
Cu:0.10~0.50%、
Ni:0.05~2.00%、
V:0.05~1.00%
Sn:0.05~1.00%、
Sb:0.05~1.00%
Zr:0.03~0.20%、
B:0.0001~0.0050%
の1種又は2種以上を含有することを特徴とする(1)に記載のフェライト系ステンレス鋼。
(3)
前記フェライト系ステンレス鋼の成分におけるNbとVが3式を満たすことを特徴とする(2)に記載のフェライト系ステンレス鋼。
Nb×V≧0.01 …(3式)
但し、3式の元素記号は、当該元素の含有質量%を意味する。
(4)
フェライト系ステンレス鋼板どうしをTIG溶接した溶接構造物において、
前記フェライト系ステンレス鋼板の成分が、質量%で,
C:0.002~0.020%,
Si:0.05~0.30%,
Mn:0.01~0.50%,
Cr:16.0~25.0%,
Mo:0.30~3.00%、
Al:0.005~0.150%,
Ti:0.05~0.30%,
Nb:0.10~0.50%
N:0.002~0.025%
P:0.035%以下,
S:0.0100%以下
を含有し,残部がFeおよび不可避的不純物からなり、
かつSi、Al、Tiの含有量が1式を満足し、
前記TIG溶接した部分の前記フェライト系ステンレス鋼板の裏面に露出した溶接金属のビード幅Wbと前記フェライト系ステンレス鋼板の板厚tが、2式を満たすことを特徴とするフェライト系ステンレス鋼の溶接構造物。
Si+2.5Al+3Ti≦1.50 …(1式)
(5)
前記フェライト系ステンレス鋼板の成分が
更に、質量%で、
Cu:0.10~0.50%、
Ni:0.05~2.00%、
V:0.05~1.00%
Sn:0.05~1.00%、
Sb:0.05~1.00%
Zr:0.03~0.20%、
B:0.0001~0.0050%
の1種又は2種以上を含有することを特徴とする(4)に記載のフェライト系ステンレス鋼の溶接構造物。
(6)
前記フェライト系ステンレス鋼板の成分におけるNbとVが3式を満たすことを特徴とする(5)に記載のフェライト系ステンレス鋼の溶接構造物。
Nb×V≧0.01 …(3式)
但し、3式の元素記号は、当該元素の含有質量%を意味する。
(Si+2.5Al+3Ti)≦1.50 …(1)
但し、式中の元素記号は、当該元素の含有質量%を意味する。
0<Wb/2t≦1.50 …(2)
(t:被溶接材の板厚、Wb:裏面に露出したビード幅の長さ)
(Nb×V)≧0.01 …(3)
但し、式中の元素記号は、当該元素の含有質量%を意味する。
供試材となるフェライト系ステンレス鋼板は以下のように製造した。
ラボ真空溶解炉により溶解したCr:16.0~25.0%、Mo:0.30~3.00%を基準としたフェライト系ステンレス鋼で、Al,Ti,Si,Nb,Vを種々変化させた材料を、圧延・熱処理により冷延板を製造した。供試材の成分を表1(表1は6分割して表示している。以下、表1-1~表1-6をあわせて表1とよぶ。)に示す。
溶接条件は、送り速度50cm/min、電流値は70A一定とし、裏面のビードが露出する条件とした。裏ビード幅は約1.2mmであった。溶接部のガスシールドにはArガスを用いた。トーチ側7のガスシールドは流量15L(リットル、以下同じ)/min、アフターガスは20L/minで固定した。反トーチ側6についてはガスシールド有りとなしの2条件で実施した。ガスシールド実施時は、溶接すきま部5と裏ビード部8近傍の裏面溶接部4の両方について流量5L/minでガスシールドを行い、ガスシールド停止時は流量ゼロとした。
耐食性評価試験片は、本材料から溶接金属長さを20mm、長さ40mmの大きさに切断した。切断端面は#600のエメリー湿式研磨処理を施した。反トーチ側6のみを試験環境に晒すために、トーチ側7の溶接金属部3は耐熱シリコン樹脂でコーティングした。試験液としては、NaClの他に酸化剤としてCuCl2試薬を用いて、Cl-は600ppmと、Cu2+は2ppmCu2+となるように調整した試験液を用いた。なおCu2+は環境の酸化性を調整するために添加しており、2ppmの濃度は一般の温水環境を模擬した濃度である。浸漬条件は80℃、酸素吹き込みとし、2週間連続浸漬とした。なお試験液は、1週間毎に交換した。
(すきま部)
Arガスシールドを実施した場合は、Si+2.5Al+3Tiが高い場合には一部腐食が見られたが、その深さは臨界深さ(それ以上腐食が深くなると、穴あき貫通まで腐食が成長してしまうという腐食深さ)である50μmを超えることはなかった。
一方Arガスシールドを省略した溶接すきま部における腐食試験後のすきま腐食深さとSi+2.5Al+3Tiの関係を図1に示す。図1に示すように、Arガスシールドを省略した場合には、腐食深さはSi+2.5Al+3Tiの値が1.50以下であれば、すきま腐食深さが50μm以下となることを確認した。
Arガスシールドを実施した場合は、Si+2.5Al+3Tiが高い場合には一部腐食が見られたが、その深さは臨界深さとされる20μmを超えることはなかった。なお裏面溶接部の孔食発生位置は、裏面溶接金属境界から2~3mmはなれた部分、いわゆるHAZ部において観察された。
一方Arガスシールドを省略した場合には、Si+2.5Al+3Tiの値が1.50を超えると、腐食深さ20μmを超える腐食を生じていた。Arガスシールド省略時の腐食位置は、Arガスシールドを実施した場合と異なり、溶接金属境界部およびその境界から1mm以内に発生していた。
(Si+2.5Al+3Ti)≦1.50 …(1)
*式中の記号は、鋼中の各成分の含有量[質量%]を表す。
つづいて、[耐食性評価試験(その1)]で用いた条件よりも厳しい腐食環境下で耐食性を評価した。供試材は、表1のNo.5の鋼板を用いた。これは、(1式)を満たす成分系である。この供試材を前述の[供試材の製造][溶接条件]に記載したのと同様に、TIG溶接にてすきま溶接サンプルを作製した。
近年の給湯器の高圧給湯の要望にたいし、貯湯缶体の強度を上げるには溶接部の強度を向上させる必要がある。本発明において耐食性を優先させて溶接裏ビード幅を狭くすることは、溶接部の強度を低下させる傾向となる。この改善のため、溶接金属部の強度向上について検討した。
一方、Nbを0.50%超含有すると、過剰な添加となり過度に強度を向上させ加工性を低下させる問題が生じる場合もあり、またコストも上昇する問題がある。加工性を低下させるため,Nbの含有量は下限を0.10、上限を0.50%とする。望ましくは下限が0.12%、上限が0.45%である。さらに望ましくは、下限0.15%、上限0.36%にするとよい。
2 鋼板
3 溶接金属部
4 裏面溶接部
5 溶接すきま部
6 反トーチ側
7 トーチ側
8 裏ビード部
Claims (6)
- 成分が質量%で,
C:0.002~0.020%,
Si:0.05~0.30%,
Mn:0.01~0.50%,
Cr:16.0~25.0%,
Mo:0.30~3.00%、
Al:0.005~0.150%,
Ti:0.05~0.30%,
Nb:0.10~0.50%,
N:0.002~0.025%,
P:0.035%以下,
S:0.0100%以下
を含有し,残部がFeおよび不可避的不純物からなり、
かつSi、Al、Tiの含有量が1式を満足することを特徴とするフェライト系ステンレス鋼。
Si+2.5Al+3Ti≦1.50 …(1式)
但し、式中の元素記号は、当該元素の含有質量%を意味する。
- 更に、成分が質量%で、
Cu:0.10~0.50%、
Ni:0.05~2.00%、
V:0.05~1.00%
Sn:0.05~1.00%、
Sb:0.05~1.00%
Zr:0.03~0.20%、
B:0.0001~0.0050%
の1種又は2種以上を含有することを特徴とする請求項1に記載のフェライト系ステンレス鋼。
- 前記フェライト系ステンレス鋼の成分におけるNbとVが3式を満たすことを特徴とする請求項2に記載のフェライト系ステンレス鋼。
Nb×V≧0.01 …(3式)
但し、3式の元素記号は、当該元素の含有質量%を意味する。
- フェライト系ステンレス鋼板どうしをTIG溶接した溶接構造物において、
前記フェライト系ステンレス鋼板の成分が、質量%で,
C:0.002~0.020%,
Si:0.05~0.30%,
Mn:0.01~0.50%,
Cr:16.0~25.0%,
Mo:0.30~3.00%、
Al:0.005~0.150%,
Ti:0.05~0.30%,
Nb:0.10~0.50%、
N:0.002~0.025%,
P:0.035%以下,
S:0.0100%以下
を含有し,残部がFeおよび不可避的不純物からなり、
かつSi、Al、Tiの含有量が1式を満足し、
前記TIG溶接した部分の前記フェライト系ステンレス鋼板の裏面に露出した溶接金属のビード幅Wbと前記フェライト系ステンレス鋼板の板厚tが、2式を満たすことを特徴とするフェライト系ステンレス鋼の溶接構造物。
Si+2.5Al+3Ti≦1.50 …(1式)
0<Wb/2t≦1.50 …(2式)
但し、1式中の元素記号は、当該元素の含有質量%を意味する。
- 前記フェライト系ステンレス鋼板の成分が
更に、質量%で、
Cu:0.10~0.50%,
Ni:0.05~2.00%,
V:0.05~1.00%,
Sn:0.05~1.00%,
Sb:0.05~1.00%,
Zr:0.03~0.20%,
B:0.0001~0.0050%
の1種又は2種以上を含有することを特徴とする請求項4に記載のフェライト系ステンレス鋼の溶接構造物。
- 前記フェライト系ステンレス鋼板の成分におけるNbとVが3式を満たすことを特徴とする請求項5に記載のフェライト系ステンレス鋼の溶接構造物。
Nb×V≧0.01 …(3式)
但し、3式の元素記号は、当該元素の含有質量%を意味する。
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NZ615961A NZ615961B2 (en) | 2011-03-29 | 2012-03-29 | Ferritic stainless steel excellent in corrosion resistance and strength of weld zone and tig welded structure |
CN201280015893.6A CN103459641B (zh) | 2011-03-29 | 2012-03-29 | 焊接部的耐腐蚀性及强度优异的铁素体系不锈钢及tig焊接结构物 |
JP2013507737A JP5931053B2 (ja) | 2011-03-29 | 2012-03-29 | 溶接部の耐食性及び強度に優れるフェライト系ステンレス鋼およびtig溶接構造物 |
AU2012233388A AU2012233388B2 (en) | 2011-03-29 | 2012-03-29 | Ferrite stainless steel exhibiting excellent corrosion resistance and strength in weld zones, and TIG-welded structure |
US14/007,016 US20140023432A1 (en) | 2011-03-29 | 2012-03-29 | Ferritic stainless steel excellent in corrosion resistance and strength of weld zone and tig welded structure |
KR1020137025672A KR101573511B1 (ko) | 2011-03-29 | 2012-03-29 | 용접부의 내식성 및 강도가 우수한 페라이트계 스테인리스강 및 tig 용접 구조물 |
EP12765772.4A EP2692891B1 (en) | 2011-03-29 | 2012-03-29 | Welded structure obtained by tig welding ferrite stainless steel exhibiting excellent corrosion resistance and strength in weld zones |
KR1020157011765A KR101803050B1 (ko) | 2011-03-29 | 2012-03-29 | 용접부의 내식성 및 강도가 우수한 페라이트계 스테인리스강 및 tig 용접 구조물 |
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JP2011-072269 | 2011-03-29 | ||
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US (1) | US20140023432A1 (ja) |
EP (1) | EP2692891B1 (ja) |
JP (1) | JP5931053B2 (ja) |
KR (2) | KR101803050B1 (ja) |
CN (1) | CN103459641B (ja) |
AU (1) | AU2012233388B2 (ja) |
TW (1) | TWI450983B (ja) |
WO (1) | WO2012133681A1 (ja) |
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WO2014033372A1 (fr) * | 2012-09-03 | 2014-03-06 | Aperam Stainless France | Tôle d'acier inoxydable ferritique, son procédé de fabrication, et son utilisation, notamment dans des lignes d'échappement |
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JP2015124419A (ja) * | 2013-12-27 | 2015-07-06 | Jfeスチール株式会社 | フェライト系ステンレス鋼 |
CN105132812A (zh) * | 2015-09-01 | 2015-12-09 | 启东市荣盛铜业有限公司 | 一种铁素体易切削不锈钢 |
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JP2021085056A (ja) * | 2019-11-26 | 2021-06-03 | 日鉄ステンレス株式会社 | 水利施設用フェライト系ステンレス鋼製部材 |
CN115335174A (zh) * | 2020-03-25 | 2022-11-11 | 日铁不锈钢株式会社 | 焊接结构、不锈钢制焊接结构物、不锈钢制焊接容器以及不锈钢 |
CN115335174B (zh) * | 2020-03-25 | 2023-10-20 | 日铁不锈钢株式会社 | 焊接结构、不锈钢制焊接结构物、不锈钢制焊接容器以及不锈钢 |
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KR20150055112A (ko) | 2015-05-20 |
JP5931053B2 (ja) | 2016-06-08 |
CN103459641A (zh) | 2013-12-18 |
JPWO2012133681A1 (ja) | 2014-07-28 |
AU2012233388A1 (en) | 2013-10-17 |
TWI450983B (zh) | 2014-09-01 |
KR20130123463A (ko) | 2013-11-12 |
NZ615961A (en) | 2014-10-31 |
KR101803050B1 (ko) | 2017-11-29 |
EP2692891B1 (en) | 2021-05-05 |
US20140023432A1 (en) | 2014-01-23 |
EP2692891A1 (en) | 2014-02-05 |
EP2692891A4 (en) | 2015-11-11 |
CN103459641B (zh) | 2015-09-09 |
AU2012233388B2 (en) | 2015-05-07 |
KR101573511B1 (ko) | 2015-12-01 |
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