Ferritic stainless steel welded structure

Abstract

To provide a ferritic stainless steel welding structure that is excellent in high-temperature oxidation resistance, corrosion resistance and dimensional accuracy.SOLUTION: A ferritic stainless steel welding structure contains a base material and a weld metal part. The base material contains, in mass, C: 0.050% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.00 to 24.00%, N: 0.050% or less, Cu: 1.00% or less, Mo: 1.00% or less, Si: 3.00% or less, Al: 0.80 to 5.00%, Nb: 0.50% or less, Ti: 0.50% or less, with the total content of Nb and Ti being 6(C+N) or more (C and N denoting the content of C and N respectively), and with the balance being Fe and impurities. The base material has an average crystal orientation difference of 0.15° or more. In the weld metal part, the grain boundary C level is 10 mass% or more. The ferritic stainless steel welding structure has an oxide film containing Al of 30 mass% or more on its surface.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel welded structure.

Background Art

[0002] In recent years, in response to the global efforts to suppress CO2 emissions, efforts to effectively utilize exhaust heat have been expanding. For example, as a technology for extracting thermal energy from exhaust gas, many heat exchangers are used in automotive exhaust system components, plants, household energy equipment, etc., and its use is expected to expand in the future.

[0003] In a heat exchanger, heat exchange is performed between high-temperature exhaust gas and a refrigerant such as low-temperature water, but the environments on the exhaust gas side and the refrigerant side are very different. In particular, the exhaust gas side is exposed to an oxidation environment due to high-temperature (about 400°C to about 750°C) exhaust gas, and in addition, to a corrosion environment due to condensed water generated by cooling the exhaust gas in the heat exchanger. On the other hand, the refrigerant side is exposed to a corrosion environment due to the refrigerant, but the temperature is lower than that on the exhaust gas side, and for a refrigerant such as tap water, the concentration of corrosion factors such as chloride ions is also low and regulated, so its concentration is unlikely to occur. Therefore, since the heat exchanger is particularly required to have resistance to the environment on the exhaust gas side (high-temperature oxidation resistance and corrosion resistance), stainless steel materials are used as the material.

[0004] In addition, the heat exchanger is also exposed to a temperature difference from low temperature (room temperature to about 90°C) to high temperature (about 400°C to about 750°C). Among stainless steel materials, austenitic stainless steel materials and duplex stainless steel materials containing an austenite phase are easily deformed by this temperature difference, so these stainless steel materials are not suitable as materials for heat exchangers. Therefore, ferritic stainless steel materials are often used as materials for heat exchangers.

[0005] Examples of ferritic stainless steel materials with excellent acid resistance include, for example, in Patent Document 1, a ferritic stainless steel material having a composition containing Cr: 11 to 22% by mass, C: 0.03% by mass or less, N: 0.03% by mass or less, Mn: 1.5% by mass or less, S: 0.008% by mass or less, Si: 2% by mass or less, Al: 1.0 to 6.0% by mass, with the balance being Fe and unavoidable impurities, has been proposed. Also, in Patent Document 2, in mass%, C: 0.03% or less, Si: 3% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Ni: 0.5% or less, Cr: 11 to 21%, Al: 6% or less, Cu: 0.01 to 0.5%, Mo: 0.01 to 0.5%, Nb: 0.1% or less, Ti: 0.005 to 0.50%, Sn: 0.001 to 0.1%, N: 0.03% or less, O: 0.002% or less, H: 0.00005% or less, Pb: 0.01% or less, and the balance being Fe and unavoidable impurities, a ferritic stainless steel has been proposed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The ferritic stainless steel materials described in Patent Documents 1 and 2 contain Al, and an Al oxide (Al2O3) film is formed on the surface, so they have better oxidation resistance than general ferritic stainless steels. However, in the temperature range of 400 to 600 °C, even for ferritic stainless steel materials containing Al, Fe is preferentially oxidized. The Fe oxide film has a rough structure and cannot sufficiently shield oxygen, so oxidation proceeds continuously. Therefore, it cannot be said that the ferritic stainless steel materials described in Patent Documents 1 and 2 have sufficient oxidation resistance in a high-temperature environment (hereinafter referred to as "high-temperature oxidation resistance"). Here, in this specification, "high-temperature environment" mainly means a temperature environment of 400 to 750 °C.

[0008] On the other hand, regarding the corrosion resistance of ferritic stainless steel materials, the amounts of C and N dissolved in the ferritic stainless steel materials are important. The dissolved C and N combine with Cr to form carbides and nitrides of Cr (hereinafter referred to as "carbonitrides"), which preferentially precipitate at grain boundaries. The area where Cr carbonitrides precipitate becomes a state called sensitization where Cr is lacking. When exposed to an environment where corrosion factors such as chloride ions exist, corrosion progresses significantly. Therefore, it is effective to reduce the contents of C and N in the ferritic stainless steel material as much as possible and add elements such as Ti and Nb that preferentially combine with C and N to form carbonitrides to reduce the solid solution amounts of C and N.

[0009] In addition, various products such as heat exchangers are manufactured by subjecting ferritic stainless steel materials to processing treatments such as welding. When welding is performed, in the welded metal part, carbides of Ti and Nb dissolve and the solid solution amounts of C and N increase. In ferritic stainless steel materials with the C and N contents controlled to extremely low levels as described above, since carbides of Ti and Nb are reformed by natural cooling after welding, the solid solution amounts of C and N remain at low levels, and a decrease in corrosion resistance due to sensitization can be suppressed. However, in the ferritic stainless steel materials described in Patent Documents 1 and 2 containing Al, since the diffusion of Ti and Nb is slow, it is difficult for carbides of Ti and Nb to be reformed by natural cooling after welding, and the solid solution amounts of C and N increase. Since this is greatly affected by the diffusion rate, it cannot be solved by excessive addition of Ti or Nb. On the contrary, if Ti or Nb is excessively added, a decrease in surface quality and toughness will be caused due to an increase in inclusions such as TiO2. Thus, Patent Documents 1 and 2 do not recognize at all the problem of a decrease in the corrosion resistance of the welded metal part in a ferritic stainless steel welded structure obtained by welding a ferritic stainless steel material.

[0010] Furthermore, in the manufacture of various products such as heat exchangers, generally, after processing a ferritic stainless steel material into a predetermined shape, heat treatment is performed under high temperature (for example, 900 °C or higher) conditions to remove strain. However, when heat treatment is performed under high temperature conditions, the shape is likely to collapse, resulting in a decrease in dimensional accuracy.

[0011] The present invention has been made to solve the above problems, and an object thereof is to provide a ferritic stainless steel welded structure excellent in high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy.

Means for Solving the Problems

[0012] As a result of intensive research on a ferritic stainless steel welded structure including a base material and a weld metal part, the inventors of the present invention have found that the above problems can be solved by controlling the average crystal orientation difference, the composition of the base material, the grain boundary Cr concentration of the weld metal part, and the Al concentration in the surface oxide film, and have completed the present invention.

[0013] That is, the present invention is A ferritic stainless steel welded structure used in applications that require high-temperature acid resistance, corrosion resistance, and dimensional accuracy, a ferritic stainless steel welded structure including a base material and a weld metal part see , wherein the base material contains, on a mass basis, C: 0.050% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.00 to 24.00%, N: 0.050% or less, Cu: 1.00% or less, Mo: 1.00% or less, Si: 3.00% or less, Al: 0.80 to 5.00%, Nb: 0.50% or less, Ti: 0.50% or less, the total content of Nb and Ti is 6(C + N) or more (where C and N represent the contents of C and N, respectively), and the balance consists of Fe and impurities, the base material has an average crystal orientation difference of 0.15° or more, the weld metal part has a grain boundary Cr concentration of 10% by mass or more, and the ferritic stainless steel welded structure has an oxide film containing 30% by mass or more of Al on the surface.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a ferritic stainless steel welded structure excellent in high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy.

Brief Description of the Drawings

[0015]

Figure 1

Modes for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention completed based on the above viewpoints will be specifically described. The present invention is not limited to the following embodiments, and it should be understood that within the scope not departing from the gist of the present invention, modifications, improvements, etc. may be appropriately added to the following embodiments based on the ordinary knowledge of those skilled in the art and still fall within the scope of the present invention. In addition, in this specification, the “%” display regarding components means “mass %” unless otherwise specified.

[0017] The ferritic stainless steel welded structure according to an embodiment of the present invention includes a base material and a weld metal part. This ferritic stainless steel welded structure is manufactured by welding ferritic stainless steel materials. Here, in this specification, “ferritic” means that the metal structure is mainly a ferrite phase at normal temperature. Therefore, “ferritic” includes those that slightly contain phases other than the ferrite phase (for example, austenite phase, martensite phase, etc.). Further, “stainless steel material” means a material formed from stainless steel, and its material form is not particularly limited. Examples of the material form include plate shape (including strip shape), rod shape, tubular shape, etc. Further, the material may be various shaped steels such as T-shaped and I-shaped cross-sectional shapes.

[0018] Figure 1 shows a schematic partial enlarged cross-sectional view of a ferritic stainless steel welded structure. As shown in Figure 1, the ferritic stainless steel welded structure (100) includes a base material (10) and a weld metal part (30). Further, the ferritic stainless steel welded structure (100) further includes a heat affected zone (20) between the base material (10) and the weld metal part (30). Here, “base material” means a part not affected by welding. Further, “heat affected zone” means a part (also referred to as HAZ) that is not melted but affected by the heat of welding. Further, “weld metal part” means a part that is melted and re-solidified by the influence of welding.

[0019] Since the base material is not affected by welding, it has the same composition and metal structure as the ferritic stainless steel material, which is the material of the ferritic stainless steel welded structure. The base material (ferritic stainless steel material) contains C: 0.050% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.00 - 24.00%, N: 0.050% or less, Cu: 1.00% or less, Mo: 1.00% or less, Si: 3.00% or less, Al: 0.80 - 5.00%, Nb: 0.50% or less, Ti: 0.50% or less, and the total content of Nb and Ti is 6(C + N) or more (where C and N represent the contents of C and N respectively), and the balance consists of Fe and impurities. Here, in this specification, "impurities" means components that are mixed in due to raw materials such as ores and scraps and various factors in the manufacturing process when industrially manufacturing stainless steel materials, and are those that are allowed within a range that does not adversely affect the present invention. For example, impurities include inevitable impurities.

[0020] In addition, the base material (ferritic stainless steel material) can further contain at least one selected from Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, and W: 1.00% or less, if necessary. In addition, the base material (ferritic stainless steel material) can further contain at least one selected from REM: 0.10% or less and Ca: 0.10% or less, if necessary. Furthermore, the base material (ferritic stainless steel material) can further contain at least one selected from Sn: 0.10% or less and B: 0.0100% or less, if necessary. In the description of each element below, the term "base material" includes not only the base material of the ferritic stainless steel welded structure but also the ferritic stainless steel material used in the manufacture of the ferritic stainless steel welded structure.

[0021] (C: 0.050% or less) C is an element that affects properties such as the intergranular corrosion resistance (sensitization suppression effect) of the base material and the workability of ferritic stainless steel. If the C content is too high, the intergranular corrosion resistance of the base material and the workability of ferritic stainless steel will decrease. Therefore, the upper limit value of the C content is 0.050%, preferably 0.040%, more preferably 0.030%. On the other hand, the lower limit value of the C content is not particularly limited, but reducing the C content will lead to an increase in refining costs. Therefore, the lower limit value of the C content is preferably 0.0005%, more preferably 0.001%.

[0022] (Mn: 1.00% or less) Mn is an element useful as a deoxidizing element. If the Mn content is too high, it is likely to generate MnS, which serves as a corrosion initiation point, and destabilize the ferrite phase. Therefore, the upper limit value of the Mn content is 1.00%, preferably 0.90%, more preferably 0.80%. On the other hand, the lower limit value of the Mn content is not particularly limited, but is preferably 0.01%, more preferably 0.05%.

[0023] (Ni: 1.00% or less) Ni is an element effective in improving the corrosion resistance of the base material and the toughness of the weld metal part. If the Ni content is too high, the ferrite phase will be destabilized and the manufacturing cost will also increase. Therefore, the upper limit value of the Ni content is 1.00%, preferably 0.80%, more preferably 0.60%. On the other hand, the lower limit value of the Ni content is not particularly limited, but from the perspective of obtaining the above effects, it is preferably 0.01%, more preferably 0.05%.

[0024] (P: 0.100% or less) P is an element that affects properties such as the weldability and workability of ferritic stainless steel materials. If the P content is too high, the above properties may deteriorate. Therefore, the upper limit value of the P content is 0.100%, preferably 0.080%, more preferably 0.050%. On the other hand, the lower limit value of the P content is not particularly limited, but reducing the P content will lead to an increase in refining costs. Therefore, the lower limit value of the P content is preferably 0.001%, more preferably 0.010%.

[0025] (S: 0.050% or less) S is an element that generates MnS, which serves as a corrosion initiation point, and affects the toughness of the weld metal part. If the S content is too high, the toughness of the weld metal part may deteriorate. Therefore, the upper limit value of the S content is 0.050%, preferably 0.040%, more preferably 0.030%. On the other hand, the lower limit value of the S content is not particularly limited, but reducing the S content will lead to an increase in refining costs. Therefore, the lower limit value of the S content is preferably 0.0001%, more preferably 0.0005%.

[0026] (Cr: 10.00 - 24.00%) Cr is an element effective in improving the corrosion resistance and oxidation resistance of the base material. If the Cr content is too high, the toughness of the base material will decrease and the manufacturing cost will increase. Therefore, the upper limit value of the Cr content is 24.00%, preferably 23.50%, more preferably 23.00%. On the other hand, if the Cr content is too low, the above effects may not be fully obtained. Therefore, the lower limit value of the Cr content is 10.00%, preferably 10.50%, more preferably 10.90%.

[0027] (N: 0.050% or less) N is an element that affects properties such as the intergranular corrosion resistance (sensitization inhibition effect) of the base material and the workability of ferritic stainless steel. If the N content is too high, the intergranular corrosion resistance of the base material and the workability of ferritic stainless steel will decrease. Therefore, the upper limit value of the N content is 0.050%, preferably 0.030%, more preferably 0.020%. On the other hand, the lower limit value of the N content is not particularly limited, but reducing the N content will lead to an increase in refining costs. Therefore, the lower limit value of the N content is preferably 0.0005%, more preferably 0.001%.

[0028] (Cu: 1.00% or less) Cu is an element effective in improving the corrosion resistance of the base material. If the Cu content is too high, the ferrite phase will become unstable and the manufacturing cost will also increase. Therefore, the upper limit value of the Cu content is 1.00%, preferably 0.70%, more preferably 0.30%. On the other hand, the lower limit value of the Cu content is not particularly limited, but preferably 0.001%, more preferably 0.01%.

[0029] (Mo: 1.00% or less) Mo is an element effective in improving the corrosion resistance and oxidation resistance of the base material. If the Mo content is too high, the workability of ferritic stainless steel will decrease and the manufacturing cost will increase. Therefore, the upper limit value of the Mo content is 1.00%, preferably 0.80%, more preferably 0.50%. On the other hand, the lower limit value of the Mo content is not particularly limited, but preferably 0.001%, more preferably 0.005%.

[0030] (Si: 3.00% or less) Si is an element effective in improving the corrosion resistance of the base material. If the Si content is too high, the workability of ferritic stainless steel materials and the toughness of the weld metal part will decrease. Therefore, the upper limit value of the Si content is 3.00%, preferably 2.50%, more preferably 2.00%. On the other hand, the lower limit value of Si is not particularly limited, but from the viewpoint of obtaining the above effects, it is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%.

[0031] (Al: 0.80 - 5.00%) Al, like Si, is an element effective in improving the corrosion resistance of the base material. If the Al content is too high, the toughness of the base material will decrease. Therefore, the upper limit value of the Al content is 5.00%, preferably 4.50%, more preferably 4.00%. On the other hand, the lower limit value of the Al content is 0.80%, preferably 1.00%, more preferably 1.20% from the viewpoint of obtaining the above effects.

[0032] (Nb: 0.50% or less, Ti: 0.50% or less, total content of Nb and Ti: 6(C + N) or more) Nb and Ti are elements that affect properties such as the intergranular corrosion resistance (sensitization suppression effect) of the base material. If the Nb content is too high, the workability of ferritic stainless steel materials and the toughness of the base material will decrease. Therefore, the upper limit value of the Nb content is 0.50%, preferably 0.48%, more preferably 0.45%. Also, if the Ti content is too high, the workability and surface quality of ferritic stainless steel materials will decrease. Therefore, the upper limit value of the Ti content is 0.50%, preferably 0.48%, more preferably 0.45%. On the other hand, the lower limit value of the total content of Nb and Ti is controlled in relation to the contents of C and N that reduce the intergranular corrosion resistance. Specifically, the lower limit value of the total content of Nb and Ti is 6(C + N), preferably 7(C + N). Here, C and N represent the contents of C and N respectively.

[0033] (Zr: less than 1.00%, Co: less than 1.00%, V: less than 1.00%, W: less than 1.00%) Zr, Co, V, and W are elements effective in improving the oxidation resistance of the base material. If the contents of Zr, Co, V, and W are too high, the workability of the ferritic stainless steel material and the toughness of the base material will decrease, and the manufacturing cost will increase. Therefore, the upper limit value of the content of each of Zr, Co, V, and W is 1.00%, preferably 0.80%, more preferably 0.60%. On the other hand, the lower limit value of the content of each of Zr, Co, V, and W is not particularly limited, but is preferably 0.001%, more preferably 0.01%.

[0034] (REM: less than 0.10%, Ca: less than 0.10%) REM (rare earth elements) and Ca are elements effective in improving the oxidation resistance of the base material. If the contents of REM and Ca are too high, it will lead to an increase in the manufacturing cost of the ferritic stainless steel material. Therefore, the upper limit value of the content of each of REM and Ca is 0.100%, preferably 0.08%, more preferably 0.05%. On the other hand, the lower limit value of the content of each of REM and Ca is not particularly limited, but is preferably 0.0001%, more preferably 0.003%. Note that REM refers to the general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These may be used alone or as a mixture.

[0035] (Sn: less than 0.10%) Sn is an element effective in improving the corrosion resistance of the base material. If the content of Sn is too high, Sn will segregate and the manufacturability will decrease. Therefore, the upper limit value of the content of Sn is 0.10%, preferably 0.08%, more preferably 0.05%. On the other hand, the lower limit value of the content of Sn is not particularly limited, but is preferably 0.001%, more preferably 0.005%.

[0036] (B: less than 0.0100%) B is an element effective in improving the secondary workability of ferritic stainless steel materials. If the content of B is too high, the fatigue strength of ferritic stainless steel materials will decrease. Therefore, the upper limit value of the content of B is 0.0100%, preferably 0.0080%, more preferably 0.0050%. On the other hand, the lower limit value of the content of B is not particularly limited, but is preferably 0.0001%, more preferably 0.0005%.

[0037] The base material preferably has an average crystal grain size of 100 μm or less, more preferably 95 μm or less, still more preferably 90 μm or less. By controlling the average crystal grain size within such a range, it is possible to suppress the decrease in toughness due to coarsening of the crystal grain size. Note that the lower limit value of the average crystal grain size is not particularly limited, but is preferably 1 μm, more preferably 5 μm, still more preferably 10 μm. Here, in this specification, the average crystal grain size means the one measured by the method described in the examples described later.

[0038] The base material has an average crystal orientation difference of 0.15° or more. The average crystal orientation difference is an index indicating the state of retention of strain introduced during processing. By controlling such an average crystal orientation difference, the strain is sufficiently retained and the shape is less likely to collapse, so that the dimensional accuracy can be improved. From the viewpoint of stably retaining the strain, the average crystal orientation difference is preferably 0.18° or more, more preferably 0.20° or more. On the other hand, the upper limit value of the average crystal orientation difference is not particularly limited, but is, for example, 2.00°, preferably 1.80°, more preferably 1.50°. Here, in this specification, the average crystal orientation difference means the one measured by the method described in the examples described later.

[0039] The base material preferably has an absorbed energy in the Charpy impact test (hereinafter referred to as "Charpy impact value") of 100 J / cm 2 or more, more preferably 120 J / cm 2The above is the case. By setting the Charpy impact value within such a range, the desired toughness can be ensured. On the other hand, the upper limit value of the Charpy impact value is not particularly limited, but generally it is 300 J / cm 2 , preferably 280 J / cm 2 . Here, in this specification, the Charpy impact value means the value measured by the method described in the examples described later.

[0040] The weld metal part has a grain boundary Cr concentration of 10% or more. The grain boundary Cr concentration of the weld metal part is an index representing the amount of Cr-deficient regions around the Cr carbonitrides precipitated during welding. By controlling such a grain boundary Cr concentration, the Cr-deficient regions are reduced and sensitization is less likely to occur, so that the corrosion resistance of the weld metal part can be improved. From the viewpoint of stably reducing the Cr-deficient regions, the grain boundary Cr concentration of the weld metal part is preferably 11% or more, more preferably 12% or more. On the other hand, the upper limit value of the grain boundary Cr concentration of the weld metal part is not particularly limited, but for example, it is 30%, preferably 28%, more preferably 23%. Here, in this specification, the grain boundary Cr concentration of the weld metal part means the value measured by the method described in the examples described later.

[0041] The ferritic stainless steel welded structure according to an embodiment of the present invention has an oxide film containing 30% or more of Al on the surface (the surfaces of the base material, the heat-affected zone, and the weld metal part). By providing such an oxide film on the surface, the high-temperature oxidation resistance can be improved. From the viewpoint of stably enhancing the high-temperature oxidation resistance, the Al concentration in the oxide film is preferably 31% or more, more preferably 32% or more. On the other hand, the upper limit value of the Al concentration in the oxide film is not particularly limited, but for example, it is 90%, preferably 80%. Here, in this specification, the Al concentration in the oxide film means the value measured by the method described in the examples described later.

[0042] The ferritic stainless steel welded structure according to an embodiment of the present invention can be manufactured according to a method known in the art using a ferritic stainless steel material having the above composition. A typical manufacturing method of the ferritic stainless steel welded structure according to an embodiment of the present invention will be described below.

[0043] The ferritic stainless steel welded structure according to an embodiment of the present invention includes a welding step of welding a ferritic stainless steel material having the above composition to obtain a welded structure, and a heat treatment step of heating the welded structure under an oxygen partial pressure of 2×10 -5 Pa or more, in a temperature range of 600 to 800 °C and under conditions satisfying formula (1). 0.011×T A +logT B ≧8.8 …(1) In the formula, T A represents the heating temperature (°C), and T B represents the heating time (minutes).

[0044] The ferritic stainless steel material having the above composition can be manufactured by a conventional method. Specifically, first, a ferritic stainless steel having the above composition is melted and forged or cast, and then hot-rolled to obtain a hot-rolled material. Next, annealing, pickling, and cold rolling are sequentially performed on the hot-rolled material to obtain a cold-rolled material. Next, annealing and pickling are sequentially performed on the cold-rolled material to obtain a cold-rolled annealed material. Note that the conditions in each step may be appropriately adjusted according to the composition of the stainless steel and the like, and are not particularly limited. A hot-rolled material, a cold-rolled material, or a cold-rolled annealed material produced by such a method can be used as the ferritic stainless steel material. Among these, the ferritic stainless steel material is preferably a cold-rolled annealed material. Further, the hot-rolled material, the cold-rolled material, or the cold-rolled annealed material may be subjected to a forming process into a predetermined member shape. Examples of the forming process include various pressing processes using a die, and machining such as bending. Note that the conditions in each step may be appropriately adjusted according to the composition of the ferritic stainless steel material, and are not particularly limited.

[0045] The welding process is carried out using a ferritic stainless steel material having the above composition. The welding of the ferritic stainless steel material may be performed by welding a plurality of ferritic stainless steel materials, or by welding the ferritic stainless steel material to a metal material of another material. The welding method is not particularly limited, and known methods in the technical field such as arc welding (such as TIG welding), electron beam welding, laser welding, plasma arc welding, spot welding, etc. can be used. Also, a filler material may or may not be used for welding. Note that the welding conditions may be appropriately adjusted according to the type of welding and the composition of the ferritic stainless steel material, and are not particularly limited.

[0046] The heat treatment process is carried out at an oxygen partial pressure of 2×10 -5 Pa or more. By performing the heat treatment process under such an oxygen partial pressure, an oxide film containing 30% or more of Al can be formed on the surface. If the oxygen partial pressure is less than 2×10 -5 Pa, the Al concentration in the oxide film becomes low, so the high-temperature oxidation resistance decreases. From the viewpoint of stably forming an oxide film containing 30% or more of Al, the oxygen partial pressure is preferably 3×10 -5 Pa or more, more preferably 5×10 -5 Pa or more. On the other hand, the upper limit value of the oxygen partial pressure is not particularly limited, but is, for example, 1 Pa. Note that the gas other than oxygen in the heat treatment atmosphere is not particularly limited, and hydrogen, argon, etc. can be used.

[0047] In addition, the heat treatment step is performed by heating the welded structure in a temperature range of 600 to 800 °C and under conditions that satisfy formula (1). By controlling the heating temperature within the above range, it is possible to achieve both high-temperature oxidation resistance and dimensional accuracy. If the heating temperature is less than 600 °C, the formation of Cr carbonitrides and the diffusion of elements in the base material are slow, and long-time heating is required. In addition, since it is a temperature range in which Fe oxides are preferentially generated, it becomes difficult to form an oxide film containing 30% or more of Al. On the other hand, if the heating temperature exceeds 800 °C, the strain is removed and deformation is likely to occur, resulting in a decrease in dimensional accuracy. Also, by setting the conditions to satisfy formula (1), the Cr-deficient region around the Cr carbonitrides is reduced, and sensitization is less likely to occur, thus improving the corrosion resistance. The heat treatment step can be performed, for example, using a heating furnace. The form of the heating furnace may be batch type or continuous type.

[0048] The ferritic stainless steel welded structure according to the embodiment of the present invention is excellent in high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy, and thus can be used in various applications that require these characteristics. Examples of applications include heat exchangers used in automotive exhaust system parts, plants, and household energy equipment.

Examples

[0049] Hereinafter, the content of the present invention will be described in detail with reference to examples, but the present invention is not construed as being limited thereto.

[0050] (Examples 1 to 6 and Comparative Examples 1 to 7) As a ferritic stainless steel material, a ferritic stainless steel plate was produced according to the following procedure. A ferritic stainless steel having the composition shown in Table 1 was melted, hot-rolled to obtain a hot-rolled sheet with a thickness of 3.0 mm, and then the hot-rolled sheet was annealed at 1050 °C and pickled to obtain a hot-rolled annealed sheet. Next, the hot-rolled annealed sheet was cold-rolled to obtain a cold-rolled sheet with a thickness of 1.0 mm, and then the cold-rolled sheet was finish-annealed at 1000 °C and pickled to obtain a cold-rolled annealed sheet. Next, a test piece with a width of 150 mm × a rolling direction of 300 mm was cut out from the cold-rolled annealed sheet by cutting.

[0051]

Table 1

[0052] Welding treatment was performed on the above test pieces. For the welding treatment, mock welding (where melting is performed in the same manner as TIG dressing welding at the center in the width direction of the test piece, but welding is not actually carried out) was performed to obtain a welded test piece. The welding conditions were a welding current of 100 A, a welding speed of 60 cm / min, and a welding electrode diameter of 1.6 mm. Next, each of the above test pieces without welding (hereinafter referred to as "non-welded test pieces") and welded test pieces was placed in a vacuum furnace, heat-treated under the conditions shown in Table 2, and then cooled with an average cooling rate of 35 °C / min. The oxygen partial pressure was controlled by changing the pressure in the vacuum furnace. The oxygen partial pressure shown in Table 2 is a value obtained by doubling the pressure in the vacuum furnace. Also, the cooling rate was controlled by the introduction amount of nitrogen gas for cooling into the vacuum furnace. In Comparative Example 1, heat treatment and cooling were not performed on the above non-welded test pieces and welded test pieces.

Table 2

[0053] The following evaluations were performed on the non-welded test pieces and welded test pieces after heat treatment obtained above (however, in the case of Comparative Example 1, since heat treatment was not performed, non-welded test pieces and welded test pieces without heat treatment in Comparative Example 1 were used). For the average crystal orientation difference and dimensional accuracy, measurement test pieces were newly prepared using the above ferritic stainless steel sheet for evaluation.

[0054] (Al concentration in the oxide film) A 50 mm square test piece for measurement was cut out from the unwelded test piece after heat treatment by cutting, and its surface was degreased with acetone. Next, in accordance with JIS K0144:2001, analysis of the component concentration in the depth direction was performed using glow discharge optical emission spectrometry (GD-OES). In the component concentration profile in the depth direction obtained by this analysis, the Al, Fe, and Cr concentrations at the position where the O (oxygen) concentration becomes 3 / 4 of the maximum value were determined, and the Al concentration in the oxide film was determined by the following formula. Al concentration in the oxide film [mass%] = Al concentration / (Fe concentration + Cr concentration + Al concentration) × 100

[0055] (Average crystal grain size of the base metal) After cutting out a 10 mm square test piece for measurement from the unwelded test piece after heat treatment by cutting, resin embedding was performed so that the surface parallel to the rolling direction of the plate thickness and perpendicular to the width direction became the observation surface. Next, after mirror finishing the resin-embedded test piece for measurement by wet polishing, the metal structure revealed by etching with nitric hydrofluoric acid was observed with an optical microscope. Observation with an optical microscope was carried out in accordance with JIS G0551:2013. A straight line was drawn at an arbitrary position on the optical microscope image, the number of intersections between the straight line and the crystal grain boundaries was measured, and the average intercept length was taken as the crystal grain size. Measurement of the crystal grain size was performed by drawing 20 or more straight lines in a plurality of fields of view and measuring them, and the average value thereof was taken as the average crystal grain size.

[0056] (Average crystal orientation difference) A test piece for measurement with a width of 10 mm × a length of 75 mm in the rolling direction was cut out from the above-mentioned ferritic stainless steel sheet (cold-rolled annealed sheet) by cutting. Next, using a universal testing machine (UH-300kNI manufactured by Shimadzu Corporation), a punch with an inner radius of 8 mm was used to perform U-bending so that both legs were parallel. Next, each test piece for measurement was heat-treated under the same conditions as above. Next, the test piece for measurement after heat treatment was resin-embedded so that the plate thickness cross-section at the top of the U-bending would be the observation surface. The test piece for measurement subjected to resin embedding was wet-polished using SiC abrasive paper and diamond paste, and then polished using colloidal silica abrasive. Thereafter, the crystal orientation difference was measured using a FE-SEM equipped with an OIM (Orientation Imaging Microscopy) system (EBSD method). In this measurement, the evaluation area was set to 100 μm square. At this time, the evaluation range was set to include grain boundaries. The measurement results were subjected to crystal orientation difference analysis using a KAM map (Kernel Average Misorientation Map). After excluding points with a crystal orientation difference of 5° or more, which are grain boundaries, the average value of the crystal orientation differences within the measurement range was calculated as the average crystal orientation difference.

[0057] (Grain boundary Cr concentration in the weld metal part) From the welded test piece after heat treatment, a test piece for measurement was cut out by focused ion beam (FIB) processing from the weld metal part. The observation surface was a plate thickness cross-section perpendicular to the welding direction and was set to a range including at least one grain boundary. Next, the test piece for measurement was observed using a field emission transmission electron microscope, and energy dispersive X-ray spectroscopy (EDS analysis) was performed on the precipitates at the grain boundaries to identify Cr carbides. After that, EDS analysis was performed at a position 10 nm away from the interface between the Cr carbide and the grain in the direction of the grain interior (analysis diameter 1 nm). In this analysis, the Cr, Fe, and Al concentrations were determined, and the grain boundary Cr concentration in the weld metal part was determined by the following formula. Grain boundary Cr concentration in the weld metal part [mass%] = Cr concentration / (Fe concentration + Cr concentration + Al concentration) × 100

[0058] (Toughness of the base material) From the unwelded test piece after the above heat treatment, a test piece for measurement with dimensions of 50 mm (length in the plate width direction) × 10 mm (length in the rolling direction) × 1 mm (length in the plate thickness direction) was cut out by machining. Next, a V-notch (notch angle 45°, notch depth 2 mm, notch bottom radius 0.25 mm) was applied by cutting in the central part of the test piece for measurement in the plate width direction toward the rolling direction. Using this test piece for measurement, a Charpy impact test was conducted at a test temperature of 25°C in accordance with JIS Z2242:2018. The measurement was performed on three test pieces for measurement, and the average value was taken as the measurement result. In this evaluation, when the absorbed energy of the Charpy impact per unit area (Charpy impact value) is 100 J / cm 2 the above cases were judged to have good toughness.

[0059] (Corrosion resistance of the weld metal part) Taking into account the actual use environment of the ferritic stainless steel welded structure, for the welded test piece after the above heat treatment, a simulated heat treatment of the use environment was carried out using an electric resistance furnace by heating at 500°C for 100 hours in an air atmosphere. Next, a 50 mm square test piece for measurement was cut out from the welded test piece that had undergone the simulated heat treatment of the use environment so that the weld metal part was located at the center, and then the entire surface of the test piece for measurement was subjected to #600 wet polishing. For this test piece for measurement, in accordance with the method for corrosion test of stainless steel in sulfuric acid - copper sulfate specified in JIS G0575:2012, after laying copper grains on the bottom surface of the flask, 400 mL of a 15.7% sulfuric acid / 5.5% copper sulfate aqueous solution and the test piece for measurement were placed and heated on a hot plate. After maintaining the boiling state for 20 hours, the test piece for measurement was taken out, washed with water, and dried. Next, using a universal testing machine (UH-300kNI manufactured by Shimadzu Corporation), a 1 t bend was applied to the test piece for measurement in a direction perpendicular to the welding direction, and the bend apex was observed with an optical microscope. As a result of this observation, those with cracks occurring along the grain boundaries were evaluated as × (poor corrosion resistance), and those without crack occurrence were evaluated as ○ (good corrosion resistance).

[0060] (Oxidation resistance of the base metal) For the unwelded test piece after the above heat treatment, a simulated heat treatment of the use environment was carried out under the same conditions as above. Next, a 50 mm square test piece for measurement was cut out by cutting from an unwelded test piece that had been subjected to simulated heat treatment of the use environment, and its surface was degreased with acetone. Next, in accordance with JIS K0144:2001, the component concentration in the depth direction was analyzed using glow discharge optical emission spectrometry (GD-OES) in the same manner as above. In the component concentration profile in the depth direction obtained by this analysis, the Al, Fe, and Cr concentrations at the position where the O (oxygen) concentration becomes three-quarters of the maximum value were determined, and the Fe concentration in the oxide film was determined by the following formula. Fe concentration in oxide film [mass%] = Fe concentration / (Fe concentration + Cr concentration + Al concentration) × 100 In this evaluation, when the Fe concentration was 10 mass% or less, it was judged that the oxidation resistance was good.

[0061] (Dimensional accuracy) A test piece for measurement with a width of 10 mm in the width direction and 75 mm in the rolling direction was cut out by cutting from the above-mentioned ferritic stainless steel sheet (cold-rolled annealed sheet). Next, using a universal testing machine (UH-300kNI manufactured by Shimadzu Corporation), a punch with an inner radius of 8 mm was used, and U-bending was performed so that both legs were parallel, and the distance between both legs was measured at both ends. Next, after performing heat treatment on each test piece for measurement under the same conditions as above, the distance between both legs was measured again at both ends. In this evaluation, when the average distance between both legs was 7 mm or less, it was judged that the dimensional accuracy was good. The above evaluation results are shown in Table 3.

[0062]

Table 3

[0063] As shown in Table 3, in Examples 1 to 6 where the average crystal orientation difference, the composition of the base metal, the grain boundary Cr concentration of the weld metal part, and the Al concentration in the oxide film satisfy a predetermined range, the corrosion resistance of the weld metal part, the oxidation resistance of the base metal, and the dimensional accuracy were good. On the other hand, in Comparative Examples 1 and 3, since the Al concentration in the oxide film and the grain boundary Cr concentration of the weld metal part were too low, the corrosion resistance of the weld metal part and the oxidation resistance of the base metal were not sufficient. In Comparative Example 2, since the grain boundary Cr concentration in the weld metal part was too low, the corrosion resistance of the weld metal part was insufficient. In Comparative Examples 4 and 5, since the average crystal orientation difference and the grain boundary Cr concentration were too low, the corrosion resistance and dimensional accuracy of the weld metal part were insufficient. Further, in Comparative Example 5, the average crystal grain size of the base material was also large and the toughness of the base material was low. In Comparative Example 6, since the Al concentration in the oxide film was too low, the oxidation resistance of the base material was insufficient. In Comparative Example 7, since the Al content of the base material was too low, the Al concentration in the oxide film became low and the oxidation resistance of the base material was insufficient.

[0064] As can be seen from the above results, according to the present invention, it is possible to provide a ferritic stainless steel welded structure excellent in high temperature oxidation resistance, corrosion resistance, and dimensional accuracy.

Claims

A ferritic stainless steel welded structure used in applications that require high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy, comprising: a base material and a weld metal part; the base material contains, on a mass basis, C: 0.050% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.00 to 24.00%, N: 0.050% or less, Cu: 1.00% or less, Mo: 1.00% or less, Si: 3.00% or less, Al: 0.80 to 5.00%, Nb: 0.50% or less, Ti: 0.50% or less, and the total content of Nb and Ti is 6(C + N) or more (where C and N represent the contents of C and N respectively), and the balance consists of Fe and impurities, having a composition; the base material has an average crystal orientation difference of 0.15° or more; the weld metal part has a grain boundary Cr concentration of 10% by mass or more; the ferritic stainless steel welded structure is provided with an oxide film containing 30% by mass or more of Al on the surface. A ferritic stainless steel welded structure.

2. The ferritic stainless steel welded structure according to claim 1, wherein the base material further contains at least one selected from the group consisting of, on a mass basis, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, and W: 1.00% or less.

3. The ferritic stainless steel welded structure according to claim 1 or 2, wherein the base material further contains at least one selected from the group consisting of, on a mass basis, REM: 0.10% or less and Ca: 0.10% or less.

4. The ferritic stainless steel welded structure according to any one of claims 1 to 3, wherein the base material further contains at least one selected from the group consisting of, on a mass basis, Sn: 0.10% or less and B: 0.0100% or less.

5. The ferritic stainless steel welded structure according to any one of claims 1 to 4, wherein the base material has an average crystal grain size of 100 μm or less.

6. The base material has a Charpy impact value of 100 J / cm 2 The ferritic stainless steel welded structure according to any one of claims 1 to 5, wherein the value is 100 J / cm or more.

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