Ferritic stainless steel welded structure

Abstract

To provide a ferritic stainless steel welding structure that is excellent in high-temperature oxidation resistance and corrosion resistance.SOLUTION: A ferritic stainless steel welding structure contains a base material and a weld metal part. The base material contains, in mass, C: 0.100% 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: 0.100% 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. In the weld metal part, the total amount of solid-dissolved C and N is 0.015 mass% or less. 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 welded structure of a ferritic stainless steel.

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. However, 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 not likely to increase either. 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 likely to deform due to this temperature difference, so these stainless steel materials are not suitable as the material for the heat exchanger. Therefore, ferritic stainless steel materials are often used as the material for the heat exchanger.

[0005] As a ferritic stainless steel material excellent in acid resistance, for example, Patent Document 1 proposes 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, and the balance being Fe and inevitable impurities. Further, Patent Document 2 proposes a ferritic stainless steel having a composition containing, 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 inevitable impurities.

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 continues to progress. 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, the "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 Cr carbides and nitrides (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 materials 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 amount of C and N in solid solution.

[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 are dissolved, and the dissolved amounts of C and N increase. In ferritic stainless steel materials in which the C and N contents are controlled to extremely low levels as described above, since carbides of Ti and Nb are re-formed by natural cooling after welding, the dissolved 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 re-formed by natural cooling after welding, and the dissolved amounts of C and N increase. Since this is greatly affected by the diffusion rate, it cannot be solved by excessive addition of Ti and Nb. Conversely, when Ti and Nb are excessively added, a decrease in surface quality and toughness is 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] 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 and corrosion resistance.

Means for Solving the Problems

[0011] As a result of intensive research on a ferritic stainless steel welded structure including a base material and a welded metal part, the present inventors have found that the above problems can be solved by controlling the composition of the base material, the total dissolved amount of C and N in the welded metal part, and the Al concentration in the oxide film on the surface, and have completed the present invention.

[0012] That is, the present invention A ferritic stainless steel welded structure used in applications where high-temperature acid resistance and corrosion resistance are required, including a base material and a welded metal part mi , The base material contains, by mass basis, C: 0.100% 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: 0.100% 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 total amount of C and N dissolved in the weld metal part is 0.015% by mass or less, The ferritic stainless steel welded structure is a ferritic stainless steel welded structure having an oxide film containing 30% by mass or more of Al on the surface.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide a ferritic stainless steel welded structure excellent in high-temperature oxidation resistance and corrosion resistance.

Brief Description of the Drawings

[0014]

Figure 1

Modes for Carrying Out the Invention

[0015] 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 those obtained by appropriately making changes, improvements, etc. to the following embodiments based on ordinary knowledge of those skilled in the art without departing from the gist of the present invention also fall within the scope of the present invention. In addition, in this specification, the “%” display regarding components means “% by mass” unless otherwise specified.

[0016] 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), bar shape, tubular shape, etc. Further, the material may be various shaped steels such as T-shaped and I-shaped cross-sectional shapes.

[0017] FIG. 1 shows a schematic partial enlarged cross-sectional view of a ferritic stainless steel welded structure. As shown in FIG. 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.

[0018] Since the base material is not affected by welding, it has the same composition and metal structure as the ferritic stainless steel material that is the material of the ferritic stainless steel welded structure. The base material (ferritic stainless steel) contains C: 0.100% 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: 0.100% 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, and are allowed within a range that does not adversely affect the present invention. For example, impurities also include inevitable impurities.

[0019] In addition, the base material (ferritic stainless steel) 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) 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) 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 of the following elements, when referring to the "base material", it includes not only the base material of the ferritic stainless steel welded structure but also the ferritic stainless steel used in the manufacture of the ferritic stainless steel welded structure.

[0020] (C: 0.100% 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 0.100%, preferably 0.080%, more preferably 0.050%. 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%.

[0021] (Mn: 1.00% or less) Mn is an element useful as a deoxidizing element. If the Mn content is too high, it is easy to generate MnS which becomes a corrosion initiation point, and it also destabilizes 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 preferably 0.01%, more preferably 0.05%.

[0022] (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 viewpoint of obtaining the above effects, it is preferably 0.01%, more preferably 0.05%.

[0023] (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 leads to an increase in refining costs. Therefore, the lower limit value of the P content is preferably 0.001%, more preferably 0.010%.

[0024] (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 leads to an increase in refining costs. Therefore, the lower limit value of the S content is preferably 0.0001%, more preferably 0.0005%.

[0025] (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 decreases and the manufacturing cost increases. 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 11.00%.

[0026] (N: 0.100% or less) N 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 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 0.100%, preferably 0.050%, more preferably 0.030%. 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%.

[0027] (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 it is preferably 0.001%, more preferably 0.01%.

[0028] (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 it is preferably 0.001%, more preferably 0.005%.

[0029] (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%.

[0030] (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.

[0031] (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.

[0032] (Zr: below 1.00%, Co: below 1.00%, V: below 1.00%, W: below 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 it will lead to an increase in manufacturing cost. Therefore, the upper limit 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 of the content of each of Zr, Co, V, and W is not particularly limited, but preferably 0.001%, more preferably 0.01%.

[0033] (REM: below 0.10%, Ca: below 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 of the content of each of REM and Ca is 0.10%, preferably 0.08%, more preferably 0.05%. On the other hand, the lower limit of the content of each of REM and Ca is not particularly limited, but 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.

[0034] (Sn: below 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 of the content of Sn is 0.10%, preferably 0.08%, more preferably 0.05%. On the other hand, the lower limit of the content of Sn is not particularly limited, but preferably 0.001%, more preferably 0.005%.

[0035] (B: below 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 preferably 0.0001%, more preferably 0.0005%.

[0036] 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 the coarsening of the crystal grain size. The lower limit value of the average crystal grain size is not particularly limited, but 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 below.

[0037] The base material preferably has an absorbed energy in the Charpy impact test (hereinafter referred to as the "Charpy impact value") of 100 J / cm 2 or more, more preferably 120 J / cm 2 or more. 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 300 J / cm 2 , preferably 280 J / cm 2 . Here, in this specification, the Charpy impact value means the one measured by the method described in the examples below.

[0038] The total amount of C and N dissolved in the weld metal part is 0.015% or less, preferably 0.014% or less. By setting the total amount of C and N dissolved to 0.015% or less, it is possible to suppress the precipitation of Cr carbides and Cr nitrides due to the combination of dissolved C and N with Cr in a high-temperature environment. Therefore, it is possible to improve the toughness of the weld metal part in a high-temperature environment and suppress sensitization. The lower limit of the total amount of C and N dissolved is not particularly limited, but is typically 0.0001%, preferably 0.0005%, more preferably 0.001%. In this specification, the total amount of C and N dissolved means the amount calculated by the method described in the examples below. Also, since it is difficult to calculate the amount of C and N dissolved individually, it should be noted that in the present invention, the total amount of C and N dissolved is calculated for convenience.

[0039] 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, heat-affected zone, and 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 32% or more, more preferably 35% or more. On the other hand, the upper limit of the Al concentration in the oxide film is not particularly limited, but is, for example, 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 below.

[0040] 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.

[0041] 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 2×10-5 A heat treatment step of heating the welded structure for 1 minute or more in a temperature range of 1000 °C or higher under an oxygen partial pressure of Pa or higher, and a cooling step of cooling the heat-treated welded structure so as to be held in a temperature range of 900 to 750 °C for 5 minutes or more are included.

[0042] The ferritic stainless steel material having the above composition can be manufactured by a conventional method. Specifically, first, the 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. The hot-rolled material, cold-rolled material, or cold-rolled annealed material produced in this way 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, cold-rolled material, or cold-rolled annealed material may be subjected to a forming process into a predetermined member shape. Examples of the forming process include various press processes using a mold, 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.

[0043] The welding step is performed using the 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 (TIG welding, etc.), electron beam welding, laser welding, plasma arc welding, and spot welding can be used. Further, a filler metal 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.

[0044] 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. When the oxygen partial pressure is less than 2×10 -5 Pa, the Al concentration in the oxide film decreases, resulting in a decrease in high-temperature oxidation resistance. 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. In addition, the gas other than oxygen in the heat treatment atmosphere is not particularly limited, and hydrogen, argon, etc. can be used.

[0045] Also, the heat treatment process is carried out by heating the welded structure in a temperature range of 1000 °C or more for 1 minute or more. By controlling such heating temperature and heating time, the Cr carbonitride precipitated in the cooling process of welding can be redissolved. From the viewpoint of stably redissolving the Cr carbonitride, the heating temperature is preferably 1020 °C or more and the heating time is preferably 2 minutes or more. On the other hand, the upper limit values of the heating temperature and heating time are not particularly limited, but if heating is performed at a high temperature for a long time, the crystal grains may coarsen and the toughness may decrease. Therefore, from the viewpoint of suppressing the decrease in toughness, the upper limit value of the heating temperature is preferably 1100 °C and the upper limit value of the heating time is preferably 60 minutes. The heat treatment process can be carried out, for example, using a heating furnace. The form of the heating furnace may be batch type or continuous type.

[0046] The cooling process is carried out by cooling while maintaining the temperature in the range of 900 to 750 °C for 5 minutes or more. By cooling under such conditions, it is possible to preferentially precipitate carbonitrides of Ti and Nb, so that the amount of C and N in solid solution can be reduced. As a result, the corrosion resistance of the welded metal part is improved. If it is held at a temperature higher than this temperature range, the carbonitrides of Ti and Nb will not precipitate, and the corrosion resistance of the welded metal part will decrease. Also, if it is held at a temperature lower than this temperature range, carbides of Cr will precipitate, and the toughness will decrease. In order to hold for 5 minutes or more in the temperature range of 900 to 750 °C, for example, the cooling rate in the temperature range of 900 to 750 °C may be set to 30 °C / min or less. Also, after holding at an arbitrary constant temperature in the temperature range of 900 to 750 °C for 5 minutes or more, it may be cooled to a temperature range below 750 °C at a cooling rate exceeding 30 °C / min.

[0047] The ferritic stainless steel welded structure according to the embodiment of the present invention is excellent in high-temperature oxidation resistance and corrosion resistance, so it can be used in various applications where these properties are required. Examples of applications include heat exchangers used in automotive exhaust system parts, plants, and household energy equipment.

Examples

[0048] 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.

[0049] (Examples 1 to 7 and Comparative Examples 1 to 5) 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 plate with a thickness of 3.0 mm, and then the hot-rolled plate was annealed at 1050 °C and pickled to obtain a hot-rolled annealed plate. Next, the hot-rolled annealed plate was cold-rolled to obtain a cold-rolled plate with a thickness of 1.0 mm, and then the cold-rolled plate was finish-annealed at 1000 °C and pickled to obtain a cold-rolled annealed plate. Next, a test piece with a width of 150 mm × a rolling direction of 300 mm was cut out from the cold-rolled annealed plate by cutting.

[0050]

Table 1

[0051] Welding treatment was performed on the above test pieces. For the welding treatment, mock welding (without actual welding) was carried out by melting the central part in the width direction of the test piece in the same manner as TIG dressing welding to obtain welded test pieces. The welding conditions were set as follows: welding current 100 A, welding speed 60 cm / min, and diameter of the welding electrode 1.6 mm. Next, each of the above test pieces without welding (hereinafter referred to as "non-welded test pieces") and the welded test pieces was placed in a vacuum furnace, heat-treated under the conditions shown in Table 2, and then cooled under the conditions shown in Table 2. The oxygen partial pressure was controlled by changing the pressure inside the vacuum furnace. The oxygen partial pressure shown in Table 2 is a value obtained by doubling the pressure inside 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

[0052] 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).

[0053] (Al concentration in the oxide film) A 50 mm square test piece for measurement was cut out from the non-welded test piece after heat treatment by cutting, and its surface was degreased with acetone. Next, in accordance with JIS K0144:2001, glow discharge optical emission spectrometry (GD-OES) was used to analyze the component concentration in the depth direction. 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-fourths 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

[0054] (Average crystal grain size of the base metal) After cutting out a 10 mm square measurement test piece from the non-welded 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 measurement test piece by wet polishing, the metal structure revealed by etching with hydrofluoric nitric 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. The measurement of the crystal grain size was performed by drawing 20 or more straight lines in a plurality of fields and measuring them, and the average value of them was taken as the average crystal grain size.

[0055] (Total solid solution amount of C and N in the weld metal part) First, the production amounts of titanium and niobium carbonitrides were measured by the following procedure. After cutting out a 50 mm square measurement test piece from the welded test piece after heat treatment so that the weld metal part was located at the center, #600 wet polishing was performed on the entire surface of the measurement test piece. This measurement test piece was electrolytically etched using the SPEED method. Electrolytic etching was carried out by constant potential electrolysis in a 10% acetylacetone solution at 400 mV until the electric quantity reached 5000 coulombs. The solution after electrolysis was filtered through a filter with a grid diameter of 0.05 μm to collect the carbonitrides. Then, the dissolution amount and the collected carbonitride amount were determined by mass measurement. Next, X-ray diffraction (XRD) analysis was performed on the collected carbonitrides. In the XRD analysis, the sum of the peak intensities of Ti carbonitride and Nb carbonitride was obtained from the XRD profile. Hereinafter, the sum of the peak intensities of Ti carbonitride is referred to as "Ti carbonitride peak intensity", and the sum of the peak intensities of Nb carbonitride is referred to as "Nb carbonitride peak intensity". The peak positions (diffraction angle 2θ) of each carbonitride used in the calculation are as follows. Ti carbonitride - TiC: 48.838°, TiN: 49.895° Nb carbonitrides ~ NbC: 40.557°, NbN: 41.307° Next, the consumption amounts of C and N used in the formation of Ti carbonitrides and Nb carbonitrides (hereinafter referred to as "consumed C+N amount") were calculated by the following formula. Consumed C+N amount [mass%] = Amount of taken carbonitrides [g] × (0.21 × Ti carbonitride peak intensity + 0.12 × Nb carbonitride peak intensity) / (Ti carbonitride peak intensity + Nb carbonitride peak intensity) / Amount of dissolved [g] × 100 Next, the total solid solution amounts of C and N [mass%] were calculated by subtracting the consumed C+N amount [mass%] from the total content of C and N [mass%].

[0056] (Toughness of the base metal) A test piece for measurement 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 cutting from the unwelded test piece after the above heat treatment. Next, a V-notch (notch angle 45°, notch depth 2 mm, notch bottom radius 0.25 mm) was cut by cutting in the center 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 according to 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 case was judged to have good toughness.

[0057] (Corrosion resistance of the weld metal part) Considering 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 by heating at 500°C for 100 hours in an air atmosphere using an electric furnace. Next, a 50 mm square test piece for measurement was cut out from the welded test piece that had undergone simulated heat treatment of the use environment so that the welded metal part was located at the center. 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 spreading 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 - 300 kNI manufactured by Shimadzu Corporation), a 1 t bending was applied to the test piece for measurement in a direction perpendicular to the welding direction, and the bending 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 cracks were evaluated as ○ (good corrosion resistance).

[0058] (Oxidation resistance of the base metal) For the unwelded test piece after the above heat treatment, simulated heat treatment of the use environment was performed under the same conditions as above. Next, a 50 mm square test piece for measurement was cut out from the unwelded test piece that had undergone simulated heat treatment of the use environment by cutting, and its surface was degreased with acetone. Next, in accordance with JIS K0144:2001, using glow discharge optical emission spectrometry (GD - OES) in the same manner as above, analysis of the component concentrations in the depth direction was performed. In the component concentration profile in the depth direction obtained from 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 Fe concentration in the oxide film was determined by the following formula. Fe concentration in the 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. The above evaluation results are shown in Table 3.

[0059]

Table 3

[0060] As shown in Table 3, in Examples 1 to 7 where the composition of the base material, the total amount of C and N dissolved in the weld metal part, and the Al concentration in the oxide film satisfy a predetermined range, the corrosion resistance of the weld metal part and the oxidation resistance of the base material were good. On the other hand, in Comparative Example 1, since the Al concentration in the oxide film was too low and the total amount of C and N dissolved in the weld metal part was too high, both the corrosion resistance of the weld metal part and the oxidation resistance of the base material were insufficient. Also, in Comparative Examples 2 and 3, since the total amount of C and N dissolved in the weld metal part was too high, the corrosion resistance of the weld metal part was insufficient. Furthermore, in Comparative Example 4, since the Al concentration in the oxide film was too low, the oxidation resistance of the base material was insufficient. In Comparative Example 5, 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.

[0061] 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 and corrosion resistance.

Claims

1. A ferritic stainless steel welded structure used in applications requiring high-temperature oxidation resistance and corrosion resistance, including a base material and a weld metal part, wherein the base material, on a mass basis, contains C: 0.100% 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: 0.100% 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, wherein the total amount of C and N in solid solution in the weld metal part is 0.015% by mass or less, and the ferritic stainless steel welded structure has 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 Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, and W: 1.00% or less on a mass basis.

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

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 Sn: 0.10% or less and B: 0.0100% or less on a mass basis.

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 or more, and is the ferritic stainless steel welded structure according to any one of claims 1 to 5.

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