Martensitic stainless steel welded joint, welded structure, and welding method

Abstract

To provide a weld joint of martensitic stainless steel having improved HAZ toughness, a weld structure having the weld joint, and a weld method, with an intention of improving toughness of HAZ in welding of martensitic stainless steel.SOLUTION: A weld joint made of martensitic stainless steel having a predetermined component composition and having a γmax of 75 or more as expressed by Formula 1, wherein when rewelding such that at least a part of the weld metal constituting the weld joint is made to satisfy Formula 2, HAZ toughness is improved. γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-52×Al+189: Equation 1, 0.01≤L / √Q≤0.11: Equation 2, L: a shortest distance between a melt boundary line with weld metal due to rewelding and the melting boundary line of weld joint (mm), Q: heat input (J / mm).SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a welded joint of martensitic stainless steel, a welded structure having the joint, and a method for welding martensitic stainless steel. [Background technology]

[0002] Martensitic stainless steel is a Cr-based stainless steel that has a hard martensite phase that is formed by rapid cooling from a high-temperature austenite phase. Taking advantage of its hardness, it is used for cutting tools, shafts, etc. SUS410 is a known martensitic stainless steel that contains low amounts of expensive elements such as Ni and Mo, and has a relatively low Cr content compared to other stainless steels, making it a relatively inexpensive stainless steel with reasonable corrosion resistance (e.g., Patent Document 1 and Patent Document 2).

[0003] The application of martensitic stainless steel to storage facilities such as the holds of general-purpose transport tankers and general-purpose storage tanks is being considered. In general-purpose storage facilities, stored goods are frequently changed, and the interior surfaces of the storage facilities must be cleaned each time the goods are changed. Currently, even for general-purpose use, a certain level of corrosion resistance is required, so painted steel plates are typically used. However, painted steel plates are subject to paint damage during cleaning, and repairs require a great deal of time and effort. Therefore, efforts are being made to replace painted steel plates with unpainted stainless steel for storage facilities, thereby eliminating damage during cleaning and improving the efficiency and cost of the cleaning process. The application of martensitic stainless steel, which is relatively inexpensive and has a reasonable level of corrosion resistance, is being considered as such a stainless steel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 49-53521 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-190521 [Patent Document 3] International Publication No. 2016 / 088364 [Patent Document 4] Japanese Patent Application Laid-Open No. 2016-78060 [Patent Document 5] Japanese Patent Application Laid-Open No. 2000-271742 [Patent Document 6] Japanese Patent Application Laid-Open No. 2012-115886 Summary of the Invention [Problem to be solved by the invention]

[0005] In general, martensitic stainless steels have poor weldability and are not used in welded structures. This is because they have a high susceptibility to so-called cold cracking, which requires preheating at 200 to 400°C, which is a bottleneck in practical applications. Furthermore, the toughness of the weld heat-affected zone (HAZ) is poor, so high-temperature heat treatment at around 700 to 800°C is required to improve toughness. For this reason, in practice, a heat treatment furnace capable of preheating and high-temperature heat treatment is required for welded structures, which is why martensitic stainless steels are not used in welded structures.

[0006] Patent Document 1 proposes a martensitic stainless steel that can be welded without preheating and has ductility and toughness even after welding. This is achieved by reducing the carbon and nitrogen content and adjusting the chemical composition to produce a massive martensite structure with good ductility and toughness in the HAZ. However, although cracking susceptibility is improved, the HAZ toughness after welding is not necessarily sufficient when welding thick materials with a large heat input that provides good workability.

[0007] Patent Document 2 proposes a martensitic stainless steel that combines weld toughness and corrosion resistance while suppressing the content of expensive Ni. This is a 16%Cr-2%Ni steel base that contains 2% or more Mn to improve weld toughness. This allows for a certain level of corrosion resistance while ensuring weld toughness without post-weld heat treatment. However, Mn is an element that reduces corrosion resistance. While Mn is added to ensure weld toughness, the Cr content is increased to compensate for the resulting reduced corrosion resistance, and the Ni content is also increased to ensure austenite content. This results in a high price, making it comparable in cost performance to ferritic and duplex stainless steels.

[0008] Patent Document 3 describes that in girth welding of low-carbon martensitic stainless steel pipes, the low-temperature toughness of the weld metal can be improved by performing the first pass using CMT (Cold Metal Transfer) welding, and that there are no practical problems even if post-weld heat treatment is not performed. However, it is the fracture toughness of the weld metal that is improved, not the HAZ toughness of the base metal. To improve the HAZ toughness of the base metal, post-weld heat treatment is required.

[0009] On the other hand, when martensitic stainless steel is used in storage facilities, the storage facilities themselves are large, so preheating or post-weld heat treatment is practically impossible. In particular, when martensitic stainless steel plates with a thickness of 10 mm or more are welded, the toughness of the HAZ decreases significantly, leading to a decrease in the functionality of the structure.

[0010] Therefore, an object of the present invention is to improve the toughness of the HAZ in a welded joint of martensitic stainless steel without causing cold cracking even without preheating or high-temperature heat treatment, and to provide a welded joint of martensitic stainless steel with improved HAZ toughness, a welded structure having such a welded joint, and a welding method. [Means for solving the problem]

[0011] In order to achieve the above object, the present inventors have conducted extensive research and have obtained the following findings. (a) This study investigated the mechanisms of HAZ toughness degradation and improvement during the welding of martensitic stainless steels. Thermal cycles (temperature changes over time) due to welding and post-weld heat treatment were simulated for various martensitic stainless steels, and the changes in HAZ structure were confirmed experimentally. As a result, it was found that the heat input during welding coarsens the martensite crystals in the HAZ, which is the cause of the deterioration of toughness. Furthermore, it was found that this crystal coarsening is more pronounced with increasing welding heat input, resulting in a further decrease in HAZ toughness. On the other hand, it has been confirmed that when the HAZ is reheated to approximately 1000°C, the coarse martensite crystals (hereinafter sometimes referred to as "coarse martensite") change to fine martensite crystals (hereinafter sometimes referred to as "fine martensite"), improving toughness. On the other hand, when heated to around 1300°C, fine martensite did not form and toughness did not improve.Furthermore, when heated to less than 800°C, fine martensite did not form and toughness did not improve.

[0012] (b) Furthermore, some compositions did not exhibit the fine martensite transformation described above. Figure 1 shows, as an example, the microstructures of 11.3%Cr-0.95%Ni steel (also referred to as 11.3%Cr steel) and 13.75%Cr-0.88%Ni steel (also referred to as 13.75%Cr steel) in thermal cycle tests. With a heat input simulating welding (initial heat input), the 11.3%Cr steel was entirely coarse martensite, while the 13.75%Cr steel had a structure consisting primarily of ferrite with some martensite. When reheated (reheat input), the 11.3%Cr steel was entirely fine martensite, but the 13.75Cr steel retained the coarse ferrite phase, resulting in poor toughness.

[0013] (c) Further investigations were conducted to clarify the mechanism of the formation of fine martensite. It has already been mentioned that the martensite grains in the HAZ coarsen after welding, resulting in a deterioration in toughness. However, it is well known that the martensite phase contains a high density of fine dislocations. When the martensite phase transforms into the austenite phase upon reheating, these fine dislocations act as nuclei for austenite, transforming into a fine-grained austenite phase (hereafter referred to as the fine austenite phase). This fine austenite phase is then rapidly cooled, resulting in the formation of fine martensite. This effect can only be achieved if the alloy is composed primarily of martensite at the initial heat input. Furthermore, it cannot be achieved if the reheating temperature is below 800°C, which is lower than the austenite transformation temperature, or above 1250°C, which is higher than the retransformation temperature at which the austenite phase retransforms into the coarse-grained ferrite phase. From these findings, it was discovered that by maintaining the temperature at which the components change to martensite phase due to the heat input during welding and the temperature at which the components change to austenite phase upon reheating after welding, refined martensite can be obtained in the HAZ, improving HAZ toughness.

[0014] (d) The inventors further investigated a reheating method for imparting such a temperature history (thermal cycle) to a welded structure without using a heat treatment furnace, and as a result, they recalled the reheating method for welded parts of duplex stainless steel as described in Patent Document 4, and came up with the idea of ​​once creating a welded joint by welding, rewelding part of the weld metal (strictly speaking, welding for heat input), and reheating the martensite part of the coarse crystals in the HAZ.

[0015] Patent Document 4 discloses a technique for improving corrosion resistance by rewelding a portion of the weld to bring the nitrides in the high-temperature heat-affected zone into solution, since welding duplex stainless steel causes nitrides to precipitate in the high-temperature heat-affected zone, degrading corrosion resistance. Duplex stainless steel and martensitic stainless steel have different compositions and structures, and their mechanisms for structure control are also completely different, so the technique in Patent Document 4 cannot be applied directly to the present invention. However, as a method for reheating the HAZ, we came up with the idea of ​​reheating the HAZ by additionally welding a portion of the weld, similar to Patent Document 4.

[0016] The present invention has been made by integrating these findings, and the gist of the present invention is as follows. [1] At least one of them is expressed by mass %, C: 0.030% or less Si: 1.00% or less Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.003% or less, Cr: 10.0~13.5%, Ni: 0.10-3.00% N: 0.0200% or less, Cu: 2.00% or less, Al: 0.050% or less, C + N: 0.040% or less, the balance being Fe and unavoidable impurities; A welded joint that is a martensitic stainless steel having a γmax of 75 or more as shown in formula 1, At least a part of the weld metal constituting the weld joint has a reweld portion, A martensitic stainless steel welded joint, characterized in that in the weld heat-affected zone of the martensitic stainless steel corresponding to the re-welded portion of the welded joint, martensite crystals having a grain size of 0.05 mm or less in equivalent circle diameter at an area ratio of 75% or more. γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-52×Al+189: Formula 1 However, the element symbols in the formula indicate the content (mass%) of each element, and 0 is substituted if the element is not contained. [2] The martensitic stainless steel further comprises, in mass %, Mo: 1.00% or less W: 0.50% or less V: 0.30% or less, Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less REM: 0.050% or less, Ti: 0.020% or less, Nb: 0.05% or less, B: 0.0050% or less, A welded joint of martensitic stainless steel according to [1], containing one or more of the following: [3] The martensitic stainless steel weld joint according to [1] or [2], which is a weld joint between martensitic stainless steels. [4] A method for manufacturing a welded joint having a martensitic stainless steel having at least the components described in [1] or [2] above, re-welding at least a portion of the weld metal constituting the weld joint; When the heat input of the rewelding is Q (J / mm), When the shortest distance between the fusion boundary line between the weld metal constituting the weld joint and the weld metal by rewelding and the fusion boundary line which is the boundary between the weld metal constituting the weld joint and the martensitic stainless steel is defined as L (mm), A method for manufacturing a welded joint of martensitic stainless steel, characterized by satisfying formula 2. 0.01≦L / √Q≦0.11: Formula 2 Q=I×V / v: Formula 3 however, I: Welding current (A) V: Welding voltage (V) v: welding speed (mm / sec) [5] The method for manufacturing a martensitic stainless steel welded joint according to [4], wherein the filler material used in the rewelding is selected from the filler material used in the welded joint, austenitic stainless steel filler material, or duplex stainless steel filler material. [6] A method for manufacturing a martensitic stainless steel welded joint as described in [4] or [5], wherein in the martensitic stainless steel constituting the welded joint, in the weld heat affected zone (HAZ) adjacent to the fusion boundary line corresponding to the shortest distance L, martensite phase having a grain size of 0.05 mm or less in equivalent area circle diameter is 75% or more in area ratio. [7] A welded structure characterized by having, at least in part, a welded joint made of the martensitic stainless steel according to any one of [1] to [3] above. [Effects of the Invention]

[0017] The method for welding martensitic stainless steel according to the present invention has the effect of improving HAZ toughness, and the welded joint of martensitic stainless steel according to the present invention provides a welded joint with improved HAZ toughness. Furthermore, the welded joint according to the present invention makes it possible to obtain a welded structure with improved toughness. [Brief explanation of the drawings]

[0018] [Figure 1] Figure 1 is a schematic diagram showing the microstructures of 11.3%Cr-0.95%Ni steel and 13.75%Cr-0.88%Ni steel after initial heat input and after reheat input in a thermal cycle test. [Figure 2] FIG. 2 is a cross-sectional view of a welded joint formed by butt welding, showing an example of a conventional embodiment. [Figure 3] FIG. 3 is a diagram showing a cross-sectional view of a welded joint formed by butt welding, showing one example of an embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram showing an example of a cross section of a welded joint according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] [Composition of martensitic stainless steel] An example of an embodiment of the present invention will be described below. Unless otherwise specified, "%" regarding the content of an element means % by mass. Furthermore, unless a lower limit is particularly specified, it may include the case where no element is contained (0%). Furthermore, when an element symbol is used in a formula, the element symbol indicates the content (% by mass) of each element, and if no element is contained, 0 (%) is substituted.

[0020] C: 0.030% or less C has the effect of improving strength, but also increases susceptibility to weld cracking. Therefore, in the steel according to the present invention, which does not require preheating during welding, C content should be reduced as much as possible, to 0.030% or less. The upper limit of the C content can preferably be 0.028%, 0.026%, 0.024%, 0.022%, 0.020%, or 0.018%. On the other hand, the lower limit of the C content is not particularly limited and may be 0% or less, but an excessive reduction in the C content leads to a significant increase in costs, so the lower limit may be set at 0.001%, and preferably at 0.002%, 0.003%, 0.004%, or 0.005%.

[0021] Si: 1.00% or less Silicon has a deoxidizing effect during refining and is also useful for suppressing oxide scale formation during heat treatment, but it narrows the temperature range for the austenite single phase and reduces toughness, so the content is set to 1.00% or less. From the viewpoint of ensuring the austenite single phase temperature range, the upper limit of the Si content can be 0.90%, 0.80%, 0.70%, 0.60%, 0.55%, 0.50%, 0.45%, or 0.40%. On the other hand, the lower limit of the Si content is not particularly limited and may be 0% or less. However, since Si is also a deoxidizing element in the refining process, the lower limit of the Si content may be set to 0.01%, and preferably may be 0.03%, 0.05%, 0.08%, 0.10%, 0.13%, 0.15%, 0.17%, or 0.18%.

[0022] Mn: 0.10 to 3.00% Mn has a deoxidizing effect during refining and also expands the austenite single-phase region. To achieve this effect, the Mn content should be 0.10% or more. To ensure the expansion effect of the austenite single-phase region, the lower limit of the Mn content can be 0.20%, 0.30%, 0.40%, 0.50%, or 0.60%. On the other hand, since excessive Mn reduces corrosion resistance and promotes the formation of oxide scale, the Mn content should be set to 3.00% or less. Taking into account the reduction in corrosion resistance caused by MnS and other particulates, the upper limit of the Mn content can be 2.80%, 2.60%, 2.50%, 2.40%, 2.30%, 2.20%, 2.10%, or 2.00%.

[0023] P:0.050% or less P is an element contained as an impurity in the main raw materials such as molten pig iron and ferrochromium. Since P is an element that is the main cause of a decrease in hot ductility, its content should be reduced as much as possible. From this perspective, the P content should be set to 0.050% or less. Preferably, the upper limit of the P content should be 0.045%, 0.040%, 0.035%, or 0.030%. However, an excessive reduction in the P content leads to a significant increase in costs, so the lower limit of the P content is preferably 0.001%, 0.005%, or 0.010%.

[0024] S: 0.0030% or less S is an element that forms sulfide-based inclusions and deteriorates the general corrosion resistance (general corrosion and pitting corrosion) of steel. It also reduces hot ductility and increases the susceptibility to edge cracking in hot-rolled steel sheets. Therefore, its content is preferably as low as possible. Furthermore, the coexistence of S and P can reduce hot ductility and increase cracking susceptibility, so S is specifically limited to a content of 0.0030% or less. From these perspectives, the S content should be as low as possible, and its upper limit is preferably 0.0020% or 0.0010%. On the other hand, the lower the S content, the better the hot workability and corrosion resistance. However, reducing the S content increases the desulfurization load and production costs. Therefore, the lower limit of the S content may be 0.0001%, and preferably 0.0003%.

[0025] Cr: 10.00~13.50% To ensure corrosion resistance in martensitic stainless steel, the Cr content should be 10.00% or more. From the viewpoint of ensuring corrosion resistance, the lower limit of the Cr content can preferably be 10.10%, 10.20%, 10.30%, 10.50%, 10.70%, 10.90%, 11.00%, 11.10%, or 11.20%. On the other hand, if the Cr content is too high, the HAZ will be mainly ferrite phase at the time of the initial welding heat input, and the grain refinement effect at the time of reheating will not be achieved, so the Cr content must be 13.50% or less. The upper limit of the Cr content can preferably be 13.40%, 13.30%, 13.20%, 13.10%, 13.00%, 12.90%, 12.80%, 12.70%, 12.60%, or 12.50%.

[0026] Ni: 0.10 to 3.00% Like Mn, Ni is an austenite stabilizing element that expands the austenite single-phase region, improves toughness, and also has the effect of suppressing the progression of pitting corrosion. From this perspective, the Ni content is set to 0.10% or more. To ensure these effects, the lower limit of the Ni content can be 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, or 0.50%. On the other hand, if Ni is contained in a large amount, a retained austenite phase is formed, which reduces hardness, and from the viewpoint of alloy cost, the Ni content is set to 3.00% or less. The upper limit of the Ni content can be 2.80%, 2.60%, 2.40%, 2.20%, 2.00%, 1.80%, 1.60%, 1.40%, 1.20%, 1.00%, 0.90%, or 0.80%.

[0027] N: 0.0200% or less N is an austenite-forming element, but like C, it increases weld crack susceptibility, so it is best to reduce it as much as possible, and the N content is set to 0.0200% or less. The upper limit of the N content can preferably be 0.0180%, 0.0160%, 0.0150%, 0.0140%, 0.0130%, 0.0120%, 0.0110%, or 0.0100%. On the other hand, the lower limit of the N content is not particularly limited and may be 0% or less, but an excessive reduction in the N content leads to a significant increase in costs, so the lower limit may be set at 0.0010%, and preferably 0.0030% or 0.0050%.

[0028] Cu:2.00% or less Cu may be added because it has an austenite stabilizing effect. However, excessive Cu content leads to a decrease in hot workability and an increase in raw material costs, so the Cu content is set to 2.00% or less. The upper limit of the Cu content can preferably be 1.80%, 1.60%, 1.40%, 1.20%, 1.00%, 0.90%, 0.80%, 0.70%, 0.60%, or 0.50%. Since Cu may not be contained, the lower limit of its content is 0%, but because removing Cu is costly, the lower limit of the Cu content may be set to 0.01%, preferably 0.10% or 0.20%.

[0029] Al: 0.050% or less Al is a deoxidizing element that improves oxidation resistance, so it may be contained. However, excessive Al content tends to form large oxide-based inclusions, impairing toughness. Therefore, the Al content is set to 0.050% or less. Preferably, it can be 0.040% or 0.030%. Since Al does not necessarily have to be contained, the lower limit of its content is 0%, but because removing Al is costly, the lower limit of the Al content may be set to 0.001%. Preferably, it can be 0.005%. Here, the Al content is the T.Al (total Al) content.

[0030] C+N: 0.040% or less Carbon (C) has the effect of increasing the strength of steel when combined with nitrogen (N), but it also increases susceptibility to weld cracking. To prevent weld cracking even without preheating, the total amount of C and N (C + N) is limited to 0.040% or less. The upper limit of the total amount of C and N (C + N) can preferably be 0.039%, 0.038%, 0.037%, 0.036%, 0.035%, 0.034%, 0.033%, 0.032%, 0.031%, 0.030%, 0.029%, 0.028%, 0.027%, 0.026%, or 0.025%.

[0031] In one embodiment of the present invention, the balance is Fe and impurities in addition to the above elements. Here, the impurities refer to elements that are inevitably and unintentionally mixed in raw materials such as ores and scraps during the industrial production of steel, and are acceptable within a range that does not adversely affect the present invention.

[0032] Furthermore, in addition to these elements, the stainless steel of this embodiment may contain one or more of Mo, V, Sn, Ca, and REM in place of a portion of Fe. These elements do not necessarily have to be contained, but by including them, further effects can be obtained. These elements will be explained below.

[0033] Mo: 1.000% or less Mo may be added because it is effective in improving the corrosion resistance of martensite structures containing δ-ferrite. However, Mo stabilizes the ferrite phase, and excessive addition, like Cr, increases the amount of ferrite in the HAZ. Furthermore, because Mo is an expensive element, the Mo content is limited to 1.000% or less. The upper limit of the Mo content is preferably 0.900%, 0.800%, 0.700%, 0.600%, 0.500%, 0.450%, 0.400%, 0.350%, 0.300%, 0.250%, 0.200%, 0.150%, or 0.100%. The lower limit of the Mo content is not particularly limited, but since removing Mo is costly, it may preferably be 0.005% or 0.010%.

[0034] W: 0.50% or less W, like Mo, is an element that improves the corrosion resistance of stainless steel and may be contained. However, because it is an expensive element, its content should be 0.50% or less, and preferably 0.40% or 0.30% or less. When W is contained, in order to obtain its effect more reliably, it should be contained at 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more.

[0035] V: 0.30% or less V may be contained because it forms fine carbonitrides and is effective in improving wear resistance as well as corrosion resistance. However, excessive V content may lead to coarsening of precipitates, resulting in a decrease in toughness. Therefore, the upper limit of the V content is set to 0.30%, preferably 0.20% or 0.10%. There is no particular lower limit for the V content, but in consideration of production costs and manufacturability, it is preferably 0.01%, 0.03%, 0.05%, or 0.07%.

[0036] Sn: 0.100% or less Sn is an element effective in improving corrosion resistance after quenching, but excessive addition promotes edge cracking during hot rolling. Therefore, the upper limit of the Sn content is 0.100%, preferably 0.090%, 0.080%, 0.070%, 0.060%, or 0.050%. The lower limit of the Sn content is not particularly limited, but to ensure the desired effect, it is preferably 0.001%, preferably 0.002%, 0.005%, 0.010%, 0.015%, or 0.020%.

[0037] Ca: 0.0050% or less Mg: 0.0050% or less Ca and Mg are sometimes added during the steelmaking process to adjust the composition, and may be added because they act as powerful deoxidizers, promoting deoxidation and improving hot ductility. However, because there is a concern that they may reduce corrosion resistance, the Ca and Mg contents are each set to 0.0050% or less. The upper limits of the Ca and Mg contents are preferably 0.0030% and 0.0020%, respectively. There are no particular lower limits for the Ca and Mg contents, but from the standpoint of ensuring the desired effect, the contents are preferably 0.0001%, more preferably 0.0002%, 0.0003%, 0.0004%, or 0.0005%, respectively.

[0038] REM: 0.050% or less Similar to Ca, the inclusion of an appropriate amount of REM significantly improves hot ductility. To obtain this effect, the lower limit of the REM content should be set to 0.001%, 0.002%, 0.003%, 0.004%, or 0.005%. On the other hand, excessive addition of REMs is undesirable because it can easily form large REM oxides, causing nozzle clogging during casting. Therefore, the upper limit of REM content should be set at 0.050% or 0.030%. REMs are typically added in the form of misch metals, but the addition of single elements such as La, Ce, Pr, and Nd has similar effects. Here, REM (rare earth elements) refers to the two elements scandium (Sc) and yttrium (Y) and the 15 elements (lanthanoids) ranging from lanthanum (La) to lutetium (Lu), as generally defined. These REM elements may be contained alone or in combination. When multiple REM elements are contained, it is recommended that their total amount be within the above-mentioned lower and upper limits.

[0039] Ti: 0.020% or less Nb: 0.05% or less B: 0.0050% or less Ti and Nb have the effect of improving corrosion resistance, and B has the effect of improving hot ductility, so the steel may contain up to 0.020% Ti, up to 0.05% Nb, and up to 0.0050% B. While Ti, Nb, and B do not necessarily need to be contained, there is no need to remove them excessively. Therefore, the lower limits of the contents of these elements are not particularly limited, but may be Ti: 0.001%, Nb: 0.01%, and B: 0.0001%.

[0040] It is also preferable to reduce the amount of Zn, Pb, Bi, Se, Sb, Ga, Ta, Mg, Zr, etc. as much as possible. On the other hand, as long as the object of the present invention is achieved, one or more of these elements may be contained as needed, including Zn: 50 ppm or less, Pb: 10 ppm or less, Bi: 30 ppm or less, Se: 100 ppm or less, Sb: 100 ppm or less, Ga: 50 ppm or less, Ta: 500 ppm or less, Mg: 100 ppm or less, and Zr: 120 ppm or less.

[0041] [γmax≧75] γmax is expressed by formula 1 and is an index for predicting the maximum value of the austenite phase fraction generated in the range of 900°C to 1000°C. Here, the element symbols in formula 1 indicate the content (mass%) of each element, and 0 is substituted if the element is not contained. γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr -11.5Si-52×Al+189: Formula 1

[0042] A larger γmax indicates a higher austenite fraction near 1000°C. Furthermore, a larger γmax indicates a wider temperature range in which austenite single phase forms with respect to temperature changes near 1000°C. Therefore, by increasing γmax, a nearly austenite single phase can be achieved by the initial welding, resulting in fine martensite during reheating. Experiments by the inventors have confirmed that a γmax of 75 or more ensures sufficient austenite phase and toughness after reheating. Since a larger γmax is preferable, its lower limit can preferably be 80, 85, or 90. There is no particular upper limit. The upper limit is automatically determined according to Equation 1 from the composition range of martensitic stainless steel.

[0043] [Manufacturing method of martensitic stainless steel] The manufacturing method of the martensitic stainless steel described above is not particularly limited. It can be manufactured using a conventional manufacturing method. Since it is a low C+N steel (steel with low carbon and low nitrogen content), it is not extremely hard and has relatively good toughness. Therefore, unlike general martensitic stainless steel, it does not require long-term annealing, and it can have sufficient properties by water-cooling or air-cooling the martensite phase after hot rolling and then subjecting it to a short-term tempering heat treatment at about 700°C to 800°C.

[0044] Welded joint The structure of the welded joint in the embodiments of the present invention is not particularly limited. For example, commonly used welded joint structures such as butt welding, fillet welding, and lap fillet welding can be employed. Furthermore, the welding method is also not particularly limited. For example, existing welding methods such as so-called arc welding, such as TIG welding, MIG welding, MAG welding, plasma welding, and submerged welding, and laser welding can be applied. In particular, for thick steel materials (thick plates) such as storage facilities, arc welding involving a filler metal is common. In this case, the filler metal (welding wire, etc.) is not particularly limited, but it is preferable to use a filler metal for austenitic stainless steel or duplex stainless steel, which is compatible with martensitic stainless steel.

[0045] The steel materials constituting the welded joint may be a combination of steel materials containing at least a martensitic stainless steel having the above-described chemical composition. In this case, the present invention is applied to improving the HAZ toughness of the martensitic stainless steel constituting the welded joint. Of course, it may also be a welded joint between two martensitic stainless steels. The present invention can be applied to improving the HAZ toughness of all welded joints between two martensitic stainless steels.

[0046] The shape of the steel material is not particularly limited. It may be a strip-shaped steel material (steel plate), a bar-shaped steel material (steel bar), a wire-shaped steel material (wire rod, wire, etc.), or a tubular steel material (steel pipe). Of course, it may also be a combination of these steel materials.

[0047] Plate Thickness There are no particular restrictions on the thickness of the martensitic stainless steel plate that constitutes the welded joint, but if it is too thin, it becomes difficult to reweld onto the initial weld metal, so the plate thickness is preferably 6 mm or more, 8 mm or more, 10 mm or more, 12 mm or more, 15 mm or more, 20 mm or more, 25 mm or more, or 30 mm or more.

[0048] [Improved toughness of high-temperature heat-affected zones] Next, referring to Figs. 2 and 3, a welded joint by butt welding will be described as an example showing one embodiment of the present invention.

[0049] When two pieces of martensitic stainless steel are butt-welded to form a joint, as shown in Figure 2, coarse ferrite grains are formed in the weld heat-affected zone (HAZ) formed in the martensitic stainless steel base material adjacent to the fusion line 3, particularly in the region where the maximum temperature is approximately 1250°C or higher (high-temperature HAZ (hatched region 4 in Figure 2)). This is because in the case of martensitic stainless steel, the austenite phase fraction (area fraction) is nearly 100% (single phase) at 1000°C, but above approximately 1250°C, almost the entire amount becomes ferrite. Furthermore, at this time, the ferrite grains grow and become coarse due to transformation in the high-temperature region.

[0050] Although this high-temperature heat-affected zone 4 varies depending on the amount of welding heat input, in normal welding it is a region within approximately 1 mm from the fusion line 3 toward the base metal. The fusion line 3 is the boundary between the weld metal 2 that constitutes the weld joint and the martensitic stainless steel that serves as the base metal. The "range of 1 mm from the fusion line" refers to a region within the stainless steel that serves as the base metal, within 1 mm from the fusion line in a cross section perpendicular to the weld line of the welded joint.

[0051] Immediately after welding, the high-temperature heat-affected zone 4 is composed of coarse-grained ferrite crystals, but cooling after welding causes the austenite phase to re-precipitate. Because the transformation occurs from the coarse ferrite grains at high temperatures, the re-precipitated austenite also becomes coarse grained. As cooling continues, the austenite transforms into martensite. However, because the transformation occurs from the coarse austenite grains, the martensite at room temperature also becomes coarse grained. In this way, welding causes the high-temperature heat-affected zone to become coarse martensite, which reduces toughness. At this time, fine dislocations are generated during the martensite transformation during the cooling process.

[0052] Next, the steel is reheated to a temperature at which the austenite fraction becomes nearly 100% (austenite single phase). This temperature range is approximately 800°C to 1250°C in the case of the steel according to the present invention. Therefore, the reheating temperature should be 800°C or higher, preferably 850°C or higher or 900°C or higher, and 1250°C or lower, preferably 1200°C or lower or 1150°C or lower. When the ferrite transforms to austenite by this reheating, fine dislocations in the ferrite act as nuclei for austenite formation, resulting in refinement of the austenite grains.

[0053] These fine austenite grains are rapidly cooled to obtain fine martensite grains. This refinement of martensite improves toughness. The martensite grain size in the reheated region should be such that the area ratio of crystals with an equivalent circle diameter of 0.05 mm or less is 75% or more, preferably 76% or more, 78% or more, 80% or more, 82% or more, 84% or more, 86% or more, 88% or more, 90% or more, 92% or more, 94% or more, 95% or more, 96% or more, 97% or more, or 98% or more.

[0054] On the other hand, if the steel is heated too much, when the austenite phase retransforms into the ferrite phase, the ferrite crystal grains become coarse, just as they do during welding, and therefore cooling from this state does not result in a fine austenite phase. As a result, the martensite does not become finer, and therefore toughness does not improve.

[0055] As described above, by reheating after welding to a temperature range where the austenite phase is almost completely single-phase and then cooling, fine martensite crystals (fine martensite) are generated in the HAZ, improving the HAZ toughness. In particular, when a bending load is applied to a joint, the area near the joint surface becomes the source of crack initiation, so improving the toughness near the joint surface contributes to improving the toughness of the entire joint. Therefore, development focused on improving the toughness near the joint surface (more specifically, near the HAZ surface of the stainless steel base material).

[0056] [Rewelded parts and rewelding methods] The reheating method is not particularly limited. However, it is not practical to prepare a heat treatment furnace capable of heat treating a welded structure as is. In particular, welded structures such as storage facilities and ship tanks are large structures, making it difficult to heat treat them as is. Furthermore, although a localized heat treatment method can be used, welded structures have various individual shapes, and therefore no general-purpose device capable of localized heating is available. Therefore, we investigated a method for locally and easily heating a welded joint even in a welded structure, and came up with the idea of ​​using welding heat as a heat source for reheating, as described in Patent Document 4.

[0057] On the other hand, a method for utilizing welding heat to modify the structure of a weld heat-affected zone (HAZ) is known as the temper bead method, which aims to improve the toughness of the hardened region of the HAZ (Patent Documents 5 and 6). However, the target area (hardened region of the HAZ) and heating temperature range for this temper bead method differ from the target condition range of the present invention, which is to heat the region within 1 mm of the fusion line to approximately 1000°C. In other words, the conventional temper bead method tempers the portion of the base material quenched and hardened by the first layer welding using the welding heat from the remaining layer welding, and does not take into account the control of the structure. Furthermore, the temperature range must be lower than 700°C, the temperature at which the base metal transforms into the austenite phase, known as the Ac1 point. Therefore, the temper bead methods described in Patent Documents 5 and 6 cannot be immediately applied to the problem that the present invention aims to solve.

[0058] Furthermore, methods for rewelding welds, such as fillet welding or decorative welding of the weld toe using the TIG method, are widely known to improve fatigue strength. However, these methods are intended to improve the shape of the weld and do not improve the HAZ toughness of martensitic stainless steel. This is because rewelding a martensitic stainless steel weld using fillet welding or decorative welding creates a new weld heat-affected zone, which reduces the HAZ toughness in that area (the area within 1 mm of the fusion line of the fillet welding or decorative welding).

[0059] Therefore, we came up with the idea of ​​using welding heat as a heat source for reheating. As shown in Figure 3, we performed rewelding on weld metal 6 (hereinafter referred to as rewelding) and investigated the structure and toughness of the high-temperature weld heat-affected zone resulting from welding when producing the original welded joint (hereinafter referred to as initial welding). Regarding the position of reweld metal (weld metal generated by rewelding) 8, various studies were conducted using the shortest distance L between the fusion line 10 of reweld metal 8 (the fusion line between the weld metal (initial weld metal) and the reweld metal that constitutes the initial welded joint; hereinafter referred to as the reweld fusion line) and the fusion line 9 of the initial welded joint (the fusion line between the weld metal that constitutes the initial welded joint and the martensitic Slentes steel that serves as the base material; hereinafter referred to as the initial fusion line).

[0060] As a result, when L is small, the high-temperature heat-affected zone (HHAZ) 7, which had reduced toughness in the initial welded joint, is reheated by rewelding to temperatures above 1250°C, where a ferrite single-phase region forms, resulting in the formation of coarse ferrite crystals. Conversely, when L is large, the HHAZ is not heated above 800°C by the welding heat from rewelding, but only below that temperature, resulting in no change in martensite grains and no improvement in toughness. In other words, it was revealed that there is an optimum range for the rewelding location to improve toughness by reheating the surface of the HHAZ 7 from the initial weld to approximately 800°C to 1250°C. Furthermore, the thermal history of rewelding also varies depending on the welding heat input, which also clearly affects the optimum range for the rewelding location.

[0061] Therefore, a heat conduction analysis was conducted to determine the optimum range for the shortest distance L (mm) between the initial fusion boundary 9 and the reweld fusion boundary 10, with the welding heat input for rewelding Q (J / mm) so that the surface of the high-temperature heat-affected zone would be between 800°C and 1250°C. As a result, it was found that fine martensite can be obtained and HAZ toughness improved when the distance L between the initial fusion boundary and the reweld fusion boundary satisfies Equation 2. Here, the welding heat input Q is defined by the following Equation 3. 0.01≦L / √Q≦0.11: Formula 2 Q=I×V / v: Formula 3 L: The shortest distance between the fusion line of the initial weld and the fusion line of the reweld metal (mm) Q: Welding heat input (J / mm) I: Welding current (A) V: Welding voltage (V) v: welding speed (mm / sec)

[0062] That is, if the distance L is smaller than 0.01√Q, the high-temperature heat-affected zone is heated again to a temperature range of 1250°C or higher where a single ferrite phase appears, resulting in coarse martensite crystals and no improvement in toughness.On the other hand, if the distance L is larger than 0.11√Q, the high-temperature heat-affected zone is not heated above 800°C, so ferrite does not transform to austenite and toughness does not improve.

[0063] Rewelding can be performed at a position in the weld metal corresponding to a portion of the welded joint where it is desired to improve the HAZ toughness. In other words, rewelding can be performed on at least a portion of the weld metal from the initial welding so that the rewelding is at an appropriate distance (within the range of L in Equation 2) from the fusion boundary (initial fusion boundary) adjacent to the portion where it is desired to improve the HAZ toughness.

[0064] Note that Figure 3 shows the case where the toughness of the weld heat-affected zone on the right side of the initial weld joint is improved. When improving the toughness of the weld heat-affected zone on the left side of the initial weld joint, the position of the reweld metal is set based on the fusion boundary line on the left side of the initial weld joint.

[0065] In the case of multi-pass welding (lap welding with two or more passes, such as a first pass and a second pass), the HAZ created by the lower weld is reheated by the upper overlap welding, so the reheating effect can be achieved. Therefore, rewelding can be performed on the weld metal created by the final or uppermost pass welding.

[0066] By rewelding that satisfies Equation 2, the high-temperature heat-affected zone of the base metal (martensitic stainless steel) corresponding to the reweld zone can be made to have an area fraction of martensite crystals (fine martensite) with a grain size of 0.05 mm or less in equivalent-area circle diameter, of 75% or more. This area fraction is preferably 76% or more, 78% or more, 80% or more, 82% or more, 84% or more, 86% or more, 88% or more, 90% or more, 92% or more, 95% or more, 95% or more, 96% or more, 97% or more, or 98% or more. By making the area fraction of fine martensite 90% or more, the toughness of the high-temperature heat-affected zone can be improved.

[0067] Here, the high-temperature heat-affected zone of the base metal (martensitic stainless steel) corresponding to the reweld is the high-temperature heat-affected zone of the base metal that is the shortest distance between the reweld fusion boundary and the initial fusion boundary, and refers to the range from the surface to a depth equivalent to half the depth of the reweld metal in the region where the structure differs from the base metal structure (the structure extending parallel to the plate surface) on the base metal side from the weld line in a cross section perpendicular to the weld line of the initial weld, as shown in an example in Figure 4. Therefore, the crystal grain size of the martensite in this portion can be measured.

[0068] The area ratio of martensite having a crystal grain size of 0.05 mm or less in diameter equivalent to an area-equivalent circle can be measured by embedding a cross section perpendicular to the welding direction in resin, mirror-polishing it, etching it with aqua regia or the like, observing it under a microscope, taking a photograph, comparing the photographed image with a circle equivalent to 0.05 mm, filling in the area of ​​the parts with grains smaller than that, and measuring the area ratio.

[0069] The rewelding method is not particularly limited, and TIG welding, MIG welding, MAG welding, plasma welding, submerged welding, laser welding, electron beam welding, etc. can be applied. The filler metal (welding material) used does not need to be particularly limited, but it is preferable to use the same filler metal as used in producing the original welded joint. This is because using the same filler metal can maintain the corrosion resistance of the weld metal part. Alternatively, filler metal for austenitic stainless steel or duplex stainless steel may be used.

[0070] In addition, welding can be performed without using welding materials in TIG welding, laser welding, and electron beam welding. By rewelding at the welding position specified by Equations 2 and 3, the structure of the HAZ (heat affected zone) in the initial welded joint can be improved, resulting in a martensitic stainless steel welded joint with excellent toughness.

[0071] [Welded structure] A structure constructed by welding, i.e., a structure having a welded joint, is called a welded structure. The welded structure according to the present invention is a welded structure having at least one welded joint as described above. By having the welded joint according to the present invention, the toughness of the welded joint can be improved. Moreover, by applying the rewelding method described above, it is possible to easily improve the toughness even after the welded structure has been formed. [Example]

[0072] Examples of the present invention will be described below, but the present invention is not limited to the conditions used in the following examples. Steel with the chemical composition shown in Table 1 was melted in a MgO crucible in a 50 kg vacuum induction furnace in the laboratory and cast into a flat steel ingot approximately 100 mm thick. The main body of the steel ingot was processed into a hot-rolling blank, which was heated to 1150-1250°C for 1-2 hours and then hot-rolled into a 15 mm-thick hot-rolled steel plate. The final solution heat treatment was performed at 700°C for 20 minutes, followed by water cooling. From the resulting hot-rolled plate, several strips (at least four) of steel test material, each 200 mm long and 100 mm wide, were prepared.

[0073] First, the following tests were performed to determine the pass / fail status of the base material. For hot workability, a specimen was deemed pass if no edge cracks occurred at the widthwise edge during hot rolling. For toughness, Charpy test specimens were taken from the base material and subjected to a Charpy impact test (-20°C absorbed energy) in accordance with JIS Z 2242:2018. A specimen was deemed pass if the average value of three specimens was 80 J or greater. For corrosion resistance, samples were taken from the steel surface and polished to a #600 polishing finish. A specimen was deemed pass if the average value of three specimens was 0 V vs. Ag / AgCl or greater. In Table 1, a pass for each evaluation is indicated by a "○" and a fail is indicated by an "×."

[0074] Next, for the evaluation of cold cracking, two randomly selected pieces of test material for each steel prepared were butted together under the same conditions and welded under the normal welding conditions in Table 2 in accordance with the Y-type restraint cracking test (JIS Z3158:2016) and evaluated. The results are shown in Table 2. Test materials in which no cracks were observed are marked with a "◯", and test materials in which cracks were observed are marked with an "X".

[0075] Furthermore, two randomly selected pieces of each prepared steel test material were butt-welded (initial welding) under the same conditions to create welded joints. A V-groove with a groove angle of 60° was created at the butt end of each test material, and the welding method and filler metal (welding material) shown in Table 2 were used. The welding was performed by adjusting the welding current, welding voltage, and welding speed to achieve the specified welding heat input shown in Table 2. This initial welding was performed using a total of three passes: one pass at the bottom of the groove and two passes at the top.

[0076] Next, rewelding was performed on the weld metal of each welded joint thus prepared, as shown in Figure 3. This rewelding was performed under the same conditions for each steel test material. The rewelding was performed over the entire length of the initial weld metal (i.e., the entire length along the weld line) so that the rewelding was parallel to the fusion boundary of the initial weld. The welding method, welding conditions, and welding position (the distance L between the initial fusion boundary and the reweld fusion boundary (same as L shown in Figure 3)) for rewelding are shown in the "Rewelding" column of Table 2. The welding materials and welding method shown in Table 2 were used for rewelding, and the welding current, welding voltage, and welding speed were adjusted to achieve the specified welding heat input shown in Table 2. Rewelding was performed in one pass.

[0077] In the welding methods shown in Table 2, "GMAW" stands for gas metal arc welding, "TIG" stands for tungsten inert gas welding, and "SAW" stands for submerged arc welding. In addition, for the filler metals (welding metals), "308" and "329J3L" correspond to YUS308 and YUS329J3L specified in JIS Z 3321:2013 (stainless steel filler rods for welding).

[0078] The thus obtained rewelded welded joint test pieces (length 200 mm, width 200 mm) were subjected to cross-sectional observation and toughness evaluation.

[0079] After rewelding, the welded joint test material was cut perpendicular to the initial weld line at any position in the longitudinal direction, and five 20 mm long evaluation test pieces were taken. That is, the evaluation test material had the initial weld metal located in the center, and was 20 mm long, 100 mm wide, and 15 mm thick. The five evaluation test pieces were etched using aqua regia, and the distance between the reweld fusion boundary and the initial fusion boundary was measured, and the arithmetic average was calculated.

[0080] Two of the five test pieces were then sampled and cross-sectional observations were performed using an optical microscope to measure the area ratio of the martensite phase and the area ratio of the fine martensite phase with a grain size of 0.05 mm or less. The observation area was measured within the area from the surface to a depth equivalent to half the depth of the reweld metal, from the weld line to the base metal side, where the structure differs from the base metal structure, as shown in Figure 4. 2 The martensite crystals were identified and measured in the region.

[0081] Furthermore, Charpy test specimens were taken from the remaining three evaluation test materials, with the notch position aligned with the high-temperature heat-affected zone, and a Charpy impact test (-20°C absorbed energy) was performed in accordance with JIS Z 2242:2018. The absorbed energy of the three evaluation test materials was calculated as the arithmetic average to obtain the absorbed energy value of the welded joint. An absorbed energy of 27 J or more in the Charpy impact test was considered to have passed. The test results are shown in Table 2.

[0082] As can be seen from Table 2, none of the welded joints of the examples of the present invention had low-temperature cracking, and the toughness of the high-temperature heat-affected zone of the base material was improved. On the other hand, the base material toughness of welded structures Nos. 63, 65, and 70 was insufficient. The base material corrosion resistance of welded structures Nos. 64 and 66 was insufficient. Welded structure No. 69 had poor hot ductility and cracks occurred during hot working.

[0083] [Table 1]

[0084] [Table 2] [Industrial Applicability]

[0085] The present invention can be used in welded structures that use martensitic stainless steel plates, and its applications are not limited to storage facilities, but can also be used in a wide range of industries, including general buildings, social infrastructure structures such as bridges, and mechanical equipment. [Explanation of symbols]

[0086] 1. Martensitic stainless steel 2. Weld metal 3 Melting Boundary 4 High-temperature heat-affected zone (weld heat-affected zone with a maximum temperature of 1100°C or higher) 5 Base material (martensitic stainless steel) 6 Weld metal of initial weld joint (initial weld metal) 7 High-temperature heat-affected zone (weld heat-affected zone with a maximum temperature of 1100°C or higher) 8 Rewelding weld metal (rewelding molten metal) 9 Initial fusion boundary of welded joint (initial fusion boundary) 10 Fusion boundary of reweld metal (reweld fusion boundary)

Claims

1. At least one of them is expressed in mass %: C: 0.030% or less Si: 1.00% or less Mn: 0.10-3.00%, P: 0.050% or less, S: 0.003% or less, Cr: 10.0-13.5%, Ni: 0.10-3.00%, N: 0.0200% or less, Cu: 2.00% or less, Al: 0.050% or less, C + N: 0.040% or less, the balance being Fe and unavoidable impurities; A welded joint that is a martensitic stainless steel having a γmax represented by Equation 1 of 75 or more, At least a part of the weld metal constituting the weld joint has a reweld portion, A martensitic stainless steel welded joint, characterized in that in the weld heat-affected zone of the martensitic stainless steel corresponding to the re-welded portion of the welded joint, martensite crystals having a grain size of 0.05 mm or less in equivalent circle diameter at an area ratio of 75% or more. γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-52×Al+189: Formula 1 In the formula, the element symbols indicate the content (mass%) of each element, and 0 is substituted when the element is not contained.

2. The martensitic stainless steel further comprises, in mass %, Mo: 1.00% or less, W: 0.50% or less, V: 0.30% or less, Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.050% or less, Ti: 0.020% or less, Nb: 0.05% or less, B: 0.0050% or less, The martensitic stainless steel weld joint according to claim 1, further comprising one or more of the following:

3. 3. The martensitic stainless steel weld joint according to claim 1, wherein the martensitic stainless steel weld joint is a weld joint between two martensitic stainless steels.

4. A method for manufacturing a welded joint having a martensitic stainless steel having at least the components according to claim 1 or 2, re-welding at least a portion of the weld metal constituting the weld joint; When the heat input of the rewelding is Q (J / mm), When the shortest distance between the boundary line between the weld metal constituting the weld joint and the weld metal by rewelding and the fusion boundary line which is the boundary between the weld metal constituting the weld joint and the martensitic slender steel is defined as L (mm), A method for manufacturing a welded joint of martensitic stainless steel, characterized in that formulas 2 and 3 are satisfied. 0.01≦L / √Q≦0.11: Formula 2 Q=I×V / v: Formula 3 Where, I: welding current (A) V: Welding voltage (V) v: welding speed (mm / sec)

5. 5. The method for producing a martensitic stainless steel welded joint according to claim 4, wherein the filler material used in the rewelding is selected from the filler material used in the welded joint, austenitic stainless steel filler material, or duplex stainless steel filler material.

6. 6. The method for manufacturing a welded joint of martensitic stainless steel according to claim 4 or 5, wherein in the martensitic stainless steel constituting the welded joint, in a weld heat affected zone (HAZ) adjacent to the fusion boundary line corresponding to the shortest distance L, martensite crystals having a grain size of 0.05 mm or less in terms of an equivalent circle diameter at an area ratio of 75% or more.

7. A welded structure, at least in part, having a welded joint made of the martensitic stainless steel according to any one of claims 1 to 3.

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