Ferritic stainless steel laser welded structure, manufacturing method thereof, and exhaust heat recovery device

Abstract

To provide a ferritic stainless steel laser welded structure which is excellent in heat resistance and toughness, especially, heat resistance and toughness of a weld metal part.SOLUTION: A ferritic stainless steel laser welded structure includes a base material and a weld metal part. The base material has a composition which consists of, by mass, 0.100% or less C, 1.00% or less Mn, 1.00% or less Ni, 0.100% or less P, 0.050% or less S, 10.00-24.00% Cr, 0.100% or less N, 1.00% or less Cu, 1.00% or less Mo, 3.00% or less Si, 0.80-5.00% Al, 0.50% or less Nb, and 0.50% or less Ti, and contains 0.0100% or less Mg and / or Ca, wherein the total content of Nb and Ti is 6(C+N) or more (C and N each represent contents of C and N), with the balance consisting of Fe and impurities. The base material has the number density of an inclusion having a diameter of 1-10 μm of 10.0 pieces / mm2 or more, and the number density of an inclusion having a diameter of more than 10 μm of 1.0 piece / mm2 or less, and the total solid solution amount of C and N is 0.015 mass% or less. A difference between Vickers hardness of the base material and Vickers hardness of the weld metal part is 50 HV or less. The weld metal part has an average crystal grain size of 200 μm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel laser welded structure, a method for manufacturing the same, and a waste heat recovery device.

Background Art

[0002] In recent years, in response to global efforts to suppress CO 2 emissions, efforts to effectively utilize waste heat have been expanding. For example, as a technology for recovering 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] The exhaust gas used in heat exchangers is, for example, combustion exhaust gas of gasoline or LPG (liquefied petroleum gas), which is an oxidizing gas at a high temperature (about 400°C to about 750°C) and contains a large amount of water vapor. Therefore, since heat exchangers are required to have resistance (heat resistance and corrosion resistance) to such an environment, stainless steel materials are used as the material. In addition, heat exchangers are 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). In particular, when the heat exchanger becomes large, the thermal stress due to the temperature difference becomes large and it is likely to deform. Therefore, ferritic stainless steel materials with low thermal expansion are often used as the material for heat exchangers.

[0004] Stainless steel materials have excellent corrosion resistance by forming a Cr oxide film on the surface. However, in an oxidizing environment with high temperature and a large amount of water vapor, the growth of Cr oxide is fast, and Cr evaporation also occurs in which the Cr oxide becomes hexavalent Cr and scatters. Therefore, for stainless steel materials used in an oxidizing environment with high temperature and a large amount of water vapor, ferritic stainless steel materials capable of forming Al oxide (Al 2 O 3 ) on the surface are used.

[0005] 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, with the balance being Fe and inevitable impurities. Also, Patent Document 2 proposes a ferritic stainless steel material having a composition containing, by 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, with the balance being Fe and inevitable impurities. Furthermore, Patent Document 3 proposes a ferritic stainless steel material having a composition containing C: 0.03% by mass or less, Si: 0.5% by mass or less, Mn: 1.0% by mass or less, P: 0.04% by mass or less, S: 0.01% by mass or less, Ni: 0.6% by mass or less, Cr: 15 to 20% by mass, N: 0.03% by mass or less, Ti: 0.5% by mass or less, B: 0.0005 to 0.003% by mass, Al: 1.5% by mass or more and less than 4.0% by mass, with the balance being Fe and inevitable impurities.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] The corrosion resistance of ferritic stainless steel materials is related to the amounts of C and N dissolved in the ferritic stainless steel materials. 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 the carbonitrides of Cr precipitate becomes a state called sensitization where Cr is lacking. Since Cr oxides and Al oxides are not formed in the part lacking Cr and Fe is continuously oxidized, heat resistance cannot be ensured. Therefore, it is effective to reduce the contents of C and N in the ferritic stainless steel materials as much as possible and to add elements such as Ti and Nb that preferentially combine with C and N to form carbonitrides, thereby reducing the amount of C and N in solid solution.

[0008] In addition, an exhaust heat recovery device including a heat exchanger or the like is manufactured by performing processing such as welding on a ferritic stainless steel material. When welding is performed, in the weld metal part, carbonitrides of Ti and Nb are dissolved and the amount of C and N in solid solution increases. In a ferritic stainless steel material in which the contents of C and N are controlled to an extremely low level as described above, since carbonitrides of Ti and Nb are re-formed by natural cooling after welding, the amount of C and N in solid solution remains at a low level, and a decrease in heat resistance due to sensitization can be suppressed. However, in the ferritic stainless steel materials described in Patent Documents 1 to 3 containing Al, since the diffusion of Ti and Nb is slow, it is difficult for carbonitrides of Ti and Nb to be re-formed by natural cooling after welding, and the amount of C and N in solid solution increases. Since this is greatly affected by the diffusion rate, it cannot be solved by excessive addition of Ti and Nb. On the contrary, when Ti and Nb are excessively added, inclusions such as TiO 2 etc. increase, leading to a decrease in toughness. Thus, Patent Documents 1 to 3 do not recognize at all the problem that the heat resistance and toughness of the weld metal part decrease in a ferritic stainless steel welded structure obtained by welding a ferritic stainless steel material.

[0009] The present invention has been made to solve the above problems, and an object thereof is to provide a ferritic stainless steel welded (particularly, laser welded) structure excellent in heat resistance and toughness (particularly, heat resistance and toughness of a weld metal part), a method for manufacturing the same, and a waste heat recovery device.

Means for Solving the Problems

[0010] As a result of intensive studies on a ferritic stainless steel laser welded structure including a base material and a weld metal part, the present inventors have found that the above problems can be solved by controlling the composition of the base material, the number density of inclusions having a predetermined size, the difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal part, and the average crystal grain size of the weld metal part, and have completed the present invention. Further, the present inventors have found that the above ferritic stainless steel laser welded structure can be manufactured by using a ferritic stainless steel material having a specific composition as a base material and performing laser welding with a predetermined irradiation energy, and have completed the present invention.

[0011] That is, the present invention is a ferritic stainless steel laser welded structure including a base material and a weld metal part, the base material contains, on a mass basis, C: 0.001~0.050% , 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: 0.001~0.050% , 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, Mg and / or Ca: 0.0100% or less, the total content of Nb and Ti is 6(C+N) or more (C and N represent the contents of C and N, respectively), and the balance consists of Fe and impurities, the base material has a number density of inclusions having a diameter of 1 to 10 μm of 10.0 pieces / mm 2 or more, a number density of inclusions having a diameter exceeding 10 μm of 1.0 piece / mm 2 or less, and the total solid solution amount of C and N is 0.015 mass% or less, The difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal part is 50 HV or less, The weld metal part has an average crystal grain size of 200 μm or less, and is a ferritic stainless steel laser welding structure in the body there is.

[0012] In addition, the present invention is a method for manufacturing a ferritic stainless steel laser welding structure in which a ferritic stainless steel material is used as a base material and laser welding is performed, The ferritic stainless steel material has C: 0.001~0.050% , 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: 0.001~0.050% , 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, Mg and / or Ca: 0.0100% or less, and the total content of Nb and Ti is 6(C+N) or more (C and N represent the contents of C and N respectively), and the balance consists of Fe and impurities, The ferritic stainless steel material has a number density of inclusions having a diameter of 1 to 10 μm of 10.0 pieces / mm 2 or more, and a number density of inclusions having a diameter exceeding 10 μm of 1.0 piece / mm 2 or less, and the total amount of solid solution of C and N is 0.015 mass% or less, The laser welding is a manufacturing method performed under the condition that the irradiation energy with respect to the thickness of the ferritic stainless steel material is 10.0 to 40.0 kJ / mm 3 .

[0013] Furthermore, the present invention is a waste heat recovery device including the ferritic stainless steel laser welding structure.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a ferritic stainless steel welding (especially laser welding) structure excellent in heat resistance and toughness (particularly, the heat resistance and toughness of the weld metal part), a manufacturing method thereof, and a waste heat recovery device.

Brief Description of the Drawings

[0015]

Figure 1

Embodiments 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, those obtained by appropriately making changes, improvements, etc. to the following embodiments based on the ordinary knowledge of those skilled in the art also 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 laser welded structure according to an embodiment of the present invention includes a base material and a weld metal part. This ferritic stainless steel laser welded structure is manufactured by using a ferritic stainless steel material as the base material and performing laser welding. 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.). In addition, “stainless steel material” means a material formed from stainless steel, and its material shape is not particularly limited. Examples of the material shape 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 in cross-sectional shape.

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

[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 laser welded structure. The base material (ferritic stainless steel material) 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, Mg and / or Ca: 0.0100% 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" mean components that are mixed in due to raw materials such as ore and scrap, and various factors in the manufacturing process when industrially manufacturing stainless steel materials, and are allowed within the range that does not adversely affect the present invention. For example, impurities include inevitable impurities. Examples of impurities include O. The content of O is preferably 0.0020% or less.

[0020] Also, 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, as required. Also, 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, as required. 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 of the following elements, the term "base material" includes not only the base material of the ferritic stainless steel laser welded structure but also the ferritic stainless steel material used in the manufacture of the ferritic stainless steel laser welded structure.

[0021] (C: 0.100% or less) C is an element that affects the intergranular corrosion resistance (sensitization suppression effect) of the base material and properties such as the workability of the ferritic stainless steel material. If the content of C is too high, the intergranular corrosion resistance of the base material and the workability of the ferritic stainless steel material will decrease. Therefore, the upper limit value of the content of C is 0.100%, preferably 0.080%, more preferably 0.050%. On the other hand, the lower limit value of the content of C is not particularly limited, but reducing the content of C will lead to an increase in refining cost. Therefore, the lower limit value of the content of C 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 content of Mn is too high, it is easy to generate MnS which is a corrosion initiation point, and it destabilizes the ferrite phase. Therefore, the upper limit value of the content of Mn is 1.00%, preferably 0.90%, more preferably 0.80%. On the other hand, the lower limit value of the content of Mn is not particularly limited, but 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 becomes unstable and the manufacturing cost also increases. 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%.

[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, there is a risk that the above properties will 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 cost. 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 becomes a corrosion initiation point and affects the toughness of the weld metal part. If the S content is too high, there is a risk that the toughness of the weld metal part will 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 cost. 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 11.00%.

[0027] (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 materials. If the N content is too high, the intergranular corrosion resistance of the base material and the workability of ferritic stainless steel materials 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 cost. 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 be destabilized 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 content of Mo is too high, the workability of ferritic stainless steel materials will decrease and the manufacturing cost will increase. Therefore, the upper limit value of the content of Mo is 1.00%, preferably 0.80%, more preferably 0.50%. On the other hand, the lower limit value of the content of Mo 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 content of Si 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 content of Si is 3.00%, preferably 2.50%, more preferably 2.00%. On the other hand, the lower limit value of the content 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 is, like Si, an element effective in improving the corrosion resistance of the base material. Also, by using Al, the content of O can be reduced, so that inclusions can be finely dispersed. If the content of Al is too high, the toughness of the base material will decrease. Therefore, the upper limit value of the content of Al is 5.00%, preferably 4.50%, more preferably 4.00%. On the other hand, the lower limit value of the content of Al 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 content of Nb 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 content of Nb is 0.50%, preferably 0.48%, more preferably 0.45%. In addition, if the Ti content is too high, the workability and surface quality of the ferritic stainless steel material will deteriorate. 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 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] (Mg and / or Ca: 0.0100% or less) Mg and Ca are elements that are easily oxidized like Al. In addition, the oxides of Mg and Ca aggregate with the oxide of Al (Al 2 O 3 ), which causes the inclusions to coarsen and reduces toughness. Therefore, the upper limit value of the content of Mg and / or Ca (total content of Mg and Ca) is 0.0100%, preferably 0.0090%, more preferably 0.0080%. On the other hand, the lower limit value of the content of Mg and / or Ca (total content of Mg and Ca) is not particularly limited because the less the better, but for example, it is 0.0001%. Note that the content of Mg is not particularly limited as long as the total content of Mg and Ca satisfies the above range, but for example, it is 0.0030% or less, 0.0025% or less, 0.0020% or less, or 0.0015% or less. Also, the content of Ca is not particularly limited as long as the total content of Mg and Ca satisfies the above range, but for example, it is 0.0070% or less, 0.0065% or less, 0.0060% or less, or 0.0055% or less.

[0034] (Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less) 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, leading to an increase in manufacturing cost. Therefore, the upper limit values of the contents of Zr, Co, V, and W are all 1.00%, preferably 0.80%, more preferably 0.60%. On the other hand, the lower limit values of the contents of Zr, Co, V, and W are not particularly limited, but are preferably 0.001%, more preferably 0.01%.

[0035] (REM: 0.10% or less lower) REM (rare earth elements ) is , are elements effective in improving the oxidation resistance of the base material. RE of M If the content is too high, it will lead to an increase in the manufacturing cost of the ferritic stainless steel material. Therefore, RE of M The upper limit value of the content is is 0.10%, preferably 0.08%, more preferably 0.05%. On the other hand, RE of M The lower limit value of the content is is not particularly limited, but is preferably 0.0001%, more preferably 0.003%. Note that REM refers to the general term of 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.

[0036] (Sn: 0.10% or less) 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%.

[0037] (B: 0.0100% or less) B is an element effective for 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%.

[0038] The base metal has a number density of inclusions with a diameter of 1 to 10 μm of 10.0 pieces / mm 2 or more, preferably 15.0 pieces / mm 2 or more, more preferably 20.0 pieces / mm 2 or more. By controlling the number density of inclusions of such a size, in the solidification process during welding, these inclusions become crystal nucleation sites and contribute to the refinement of the crystal grains in the welded metal part. Therefore, the toughness of the welded metal part can be improved. On the other hand, the upper limit value of the number density of these inclusions is not particularly limited, but is preferably 100.0 pieces / mm 2 , more preferably 80.0 pieces / mm 2 , still more preferably 50.0 pieces / mm 2 . In addition, in this specification, the inclusions mainly refer to oxides of elements such as Al, Ca, and Ti. Also, in this specification, the number density of inclusions means that measured by the method described in the examples below.

[0039] The base metal has a number density of inclusions with a diameter exceeding 10 μm of 1.0 piece / mm 2 or less, preferably 0.9 piece / mm 2 or less, more preferably 0.8 piece / mm 2 or less. By controlling the number density of inclusions of such a size, it is possible to suppress the generation of coarse inclusions in the welded metal part. Coarse inclusions promote the propagation of fracture and reduce toughness. Therefore, by controlling the number density of such inclusions, the toughness of the welded metal part can be improved.

[0040] The base material has a total solid solution amount of C and N of 0.015% or less, preferably 0.014% or less, more preferably 0.012% or less. By controlling the total solid solution amount of C and N within the above range, it is possible to suppress the precipitation of Cr carbonitride due to the combination of dissolved C and N with Cr in a high-temperature environment. Therefore, the toughness of the welded metal part in a high-temperature environment can be improved, and sensitization can be suppressed. On the other hand, the lower limit of the total solid solution amount of C and N is not particularly limited, but is typically 0.0001%, preferably 0.0005%, more preferably 0.001%. In this specification, the total solid solution amount of C and N means the amount calculated by the method described in the examples below. Also, since it is difficult to calculate the solid solution amounts of C and N individually, it should be noted that in the present invention, the total solid solution amount of C and N is calculated for convenience.

[0041] It is desirable that the total solid solution amount of C and N in the welded metal part is approximately the same as that in the base material. However, when producing a ferritic stainless steel laser welded structure using laser welding, since the area of the welded metal part is very small, it is difficult to measure the total solid solution amount of C and N in the welded metal part. Therefore, Vickers hardness is used as an index representing the total solid solution amount of C and N in the welded metal part. In the ferritic stainless steel laser welded structure according to an embodiment of the present invention, the difference between the Vickers hardness of the base material and the Vickers hardness of the welded metal part is 50 HV or less, preferably 45 HV or less, more preferably 40 HV or less. If the difference in Vickers hardness is within such a range, it can be said that the total solid solution amount of C and N in the welded metal part is approximately the same as that in the base material, so the toughness of the welded metal part can be improved. In this specification, the Vickers hardness means the value measured by the method described in the examples below.

[0042] The weld metal part has an average crystal grain size of 200 μm or less, preferably 180 μm or less, more preferably 150 μm or less. By controlling the average crystal grain size within such a range, the toughness of the weld metal part can be improved. In this specification, the average crystal grain size means the one measured by the method described in the examples below.

[0043] Generally, for a ferritic stainless steel laser welded structure manufactured using a ferritic stainless steel material containing Al, in order to suppress sensitization in the weld metal part, it is necessary to perform heat treatment after welding to precipitate carbonitrides of Ti or Nb and reduce the solid solution amounts of C and N. However, heat treatment requires detailed control of the furnace temperature and cooling rate, so the manufacturing cost increases. Also, deformation may occur during heat treatment, and it is difficult to perform heat treatment itself when the size is large. In contrast, the ferritic stainless steel laser welded structure according to the embodiment of the present invention can suppress sensitization in the weld metal part without performing heat treatment after welding. The ferritic stainless steel laser welded structure according to the embodiment of the present invention that does not perform heat treatment after welding has an oxide film containing less than 30% Al on the surface (the surfaces of the base material, heat affected zone, and weld metal part). The Al content in the oxide film is preferably 20% or less, more preferably 10% or less, still more preferably 5% or less. The lower limit value of the Al content is not particularly limited, but is, for example, 0.1%. Here, in this specification, the Al content in the oxide film means the one measured by the method described in the examples below.

[0044] However, the ferritic stainless steel laser welded structure according to the embodiment of the invention may be heat treated after welding. The ferritic stainless steel laser welded structure according to the embodiment of the invention that is heat treated after welding has an oxide film containing 30% or more of Al on the surface. 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 content of Al in the oxide film is preferably 32% or more, more preferably 35% or more. On the other hand, the upper limit value of the Al content is not particularly limited, but is, for example, 90%, preferably 80%.

[0045] The ferritic stainless steel laser welded structure according to the embodiment of the invention can be manufactured by using a ferritic stainless steel material having the above composition as a base material and performing laser welding with a predetermined irradiation energy.

[0046] 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 such a manner 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 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. In addition, the thickness of the ferritic stainless steel material is not particularly limited, but is preferably 0.1 to 4.0 mm, more preferably 0.3 to 3.5 mm, and still more preferably 0.5 to 3.0 mm.

[0047] Ferritic stainless steel materials have high thermal conductivity, and when arc welding such as TIG welding is used, the heat input is likely to be excessive. As a result, carbonitrides of Ti and Nb dissolve during welding, increasing the solid solution amounts of C and N, and the heat resistance decreases due to sensitization. Conventionally, as a method for suppressing grain coarsening, in order to disperse oxides that serve as grain formation nuclei during solidification after welding in advance, TiO 2 is added to the flux for arc welding to refine the grains in the weld metal part. However, TiO 2 often has a diameter of 10 μm or more, and it is known that when it is included as an inclusion in ferritic stainless steel materials, it causes surface defects. Therefore, it is not preferable to intentionally precipitate TiO 2 . In particular, in the weld metal part of a ferritic stainless steel laser welded structure formed using a ferritic stainless steel material containing Al with low toughness, such coarse inclusions become the starting point of crack propagation and reduce the toughness. Therefore, in order to avoid the above problems, in the production of the ferritic stainless steel laser welded structure according to the embodiment of the present invention, welding is performed using laser welding that is easy to control the heat input.

[0048] For laser welding, the irradiation energy with respect to the thickness of the ferritic stainless steel material is 10.0 to 40.0 kJ / mm 3 , preferably 10.0 to 38.0 kJ / mm 3 , more preferably 10.0 to 35.0 kJ / mm 3It is carried out under the following conditions. By performing laser welding with irradiation energy in such a range, it is possible to suppress an increase in the solid solution amounts of C and N due to the solid solution of Ti and Nb, and coarsening of inclusions. As a result, it becomes possible to improve both the heat resistance and toughness of the ferritic stainless steel laser welded structure.

[0049] In laser welding, the irradiation energy with respect to the thickness of the ferritic stainless steel material can be controlled, for example, by controlling the output [kW] of the laser, the focal diameter [mm], and the scanning speed [m / min]. Specifically, the irradiation energy [kJ / mm 3 with respect to the thickness of the ferritic stainless steel material can be obtained by the following formula (1). The irradiation energy [kJ / mm 3 with respect to the thickness of the ferritic stainless steel material 2 = irradiation energy [kJ / mm / thickness of ferritic stainless steel material [mm] ···(1) 2 In formula (1), the irradiation energy [kJ / mm can be obtained by the following formula (2). 2 Irradiation energy [kJ / mm 2 = output density [kW / mm In formula (2), the output density [kW / mm 2 can be obtained by the following formula (3). Output density [kW / mm 2 = output [kW] / ((focal diameter [mm] / 2) 2 × 3.14) ···(3) In formula (2), the irradiation time [seconds] can be obtained by the following formula (4). Irradiation time [seconds] = focal diameter [mm] / (scanning speed [m / min] × 1000 / 60) ···(4)

[0050] When laser welding ferrite stainless steel materials with different thicknesses, the irradiation energy is controlled based on the one with the smaller thickness. Also, in laser welding, a plurality of ferrite stainless steel materials may be welded together, or a ferrite stainless steel material may be welded to a metal material of another material. The laser used for laser welding is not particularly limited, and known lasers such as CO 2 laser, YAG laser, semiconductor laser, etc. can be used.

[0051] The ferrite stainless steel laser welded structure according to the embodiment of the present invention can suppress sensitization in the welded metal part even without performing heat treatment after laser welding. Therefore, heat treatment may not be performed after laser welding. However, in order to improve the heat resistance of the ferrite stainless steel laser welded structure according to the embodiment of the present invention, heat treatment may be performed after laser welding. The conditions of the heat treatment are not particularly limited, but it is preferable to heat the ferrite stainless steel laser welded structure for 1 minute or more in a temperature range of 1000 °C or higher under an oxygen partial pressure of 2×10 -5 Pa or more. The gas other than oxygen in the heat treatment atmosphere is not particularly limited, and hydrogen, argon, etc. can be used. Also, the heat treatment can be performed, for example, using a heating furnace. The form of the heating furnace may be batch type or continuous type. Also, after the heat treatment, it is preferable to cool while maintaining for 5 minutes or more in a temperature range of 900 to 750 °C. In order to maintain 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 maintaining for 5 minutes or more at an arbitrary constant temperature in the temperature range of 900 to 750 °C, it may be cooled to a temperature range below 750 °C at a cooling rate exceeding 30 °C / min.

[0052] The ferritic stainless steel laser welded structure according to an embodiment of the present invention is excellent in heat resistance and toughness (particularly, the heat resistance and toughness of the welded metal part), and thus can be used in various applications that require these properties. Examples of applications include exhaust heat recovery devices including heat exchangers used in automotive exhaust system components, plants, and household energy equipment.

[0053] The exhaust heat recovery device according to an embodiment of the present invention includes the above-described ferritic stainless steel laser welded structure. The exhaust heat recovery device according to an embodiment of the present invention having such characteristics includes a ferritic stainless steel laser welded structure excellent in heat resistance and toughness (particularly, the heat resistance and toughness of the welded metal part), and thus its performance can be improved.

Example

[0054] 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.

[0055] <Fabrication of ferritic stainless steel plate (base material)> A ferritic stainless steel having the composition shown in Table 1 (the balance being Fe and impurities) 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 0.6 to 2.0 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 (base material test piece) with a width of 50 mm × a rolling direction of 500 mm was cut out from the cold-rolled annealed plate by cutting.

[0056]

Table 1

[0057] <Fabrication of ferritic stainless steel welded structure> Using two of the above base material test pieces, the two were butt-joined and laser welding or TIG welding was performed. In laser welding, CО 2A laser was used under the conditions shown in Table 2. In TIG welding, the welding width was 3.2 mm, the output was 90 A, and the welding speed was 0.5 m / min. For some examples, heat treatment was performed after laser welding. The heat treatment was carried out by putting the welded structure into a vacuum furnace, with a heating temperature of 1025 °C, a heating time of 5 minutes, and an oxygen partial pressure of 5×10 -4 Pa, and then cooling at a cooling rate of 5 °C / min in the temperature range of 900 - 750 °C. The oxygen partial pressure was controlled by changing 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.

[0058]

Table 2

[0059] The following evaluations were performed on the ferritic stainless steel welded structure obtained as described above.

[0060] <Number density of inclusions in the base material> The surface of the above-mentioned base material test piece was mirror-polished so that the position at a depth of 15 μm (any within 100 μm) from the surface was the observation position, and etching was carried out with hydrofluoric nitric acid to reveal the grain boundaries and inclusions. Next, an area of 15 mm × 15 mm was photographed using FE-SEM (SU-5000 manufactured by Hitachi High-Tech Corporation), and by performing image analysis, the number of inclusions having a diameter of 1 - 10 μm and the number of inclusions having a diameter exceeding 10 μm were respectively obtained. The diameter of the inclusions was (long side × short side) 1 / 2 And the number density of each inclusion was calculated by dividing the number of each inclusion by the observation field of view.

[0061] <Total amount of C and N in solid solution in the base material> First, the formation amounts of titanium and niobium carbonitrides were measured by the following procedure. After cutting out a 50 mm × 50 mm test piece for measurement from the above-mentioned base metal test piece, #600 wet polishing was performed on the entire surface of the test piece for measurement. This test piece for measurement was electrolytically etched using the SPEED method. The electrolytic etching was carried out by constant potential electrolysis in a 10% acetylacetone solution at 400 mV until the amount of electricity reached 5000 coulombs. The solution after electrolysis was filtered through a filter with a grid diameter of 0.05 μm to collect carbonitrides. Then, the dissolved 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 carbonitride - NbC: 40.557°, NbN: 41.307° Next, the consumption amounts of C and N used for the formation of Ti carbonitride and Nb carbonitride (hereinafter referred to as "consumed C+N amount") were calculated by the following formula. Consumed C+N amount [mass%] = Collected carbonitride amount [g] × (0.21 × Ti carbonitride peak intensity + 0.12 × Nb carbonitride peak intensity) / (Ti carbonitride peak intensity + Nb carbonitride peak intensity) / Dissolved amount [g] × 100 Next, the total solid solution amount of C and N [mass%] was calculated by subtracting the consumed C+N amount [mass%] from the total content of C and N [mass%].

[0062] <Average crystal grain size of the weld metal part> A sample with a size of 10 mm in the rolling direction and 20 mm in the width direction was cut out by machining from the above-described ferritic stainless steel welded structure such that the welded part was at the center in the width direction. Next, resin embedding was performed so that the direction perpendicular to the welding direction became the observation surface. Next, the test piece for measurement with resin embedding was mirror-finished by wet polishing, and then etched with hydrofluoric nitric acid, and the metallographic structure revealed was observed with an optical microscope. The observation with the optical microscope was carried out in accordance with JIS G0551:2013. A straight line was drawn at an arbitrary position on an optical microscope image at 50 times magnification, 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 of view and measuring them. This measurement was carried out in any 5 fields of view, and the average value thereof was taken as the average crystal grain size.

[0063] <Difference between the Vickers hardness of the base metal and the Vickers hardness of the weld metal part> A sample with a size of 25 mm in the rolling direction and 25 mm in the width direction was cut out by machining from the above-described ferritic stainless steel welded structure such that the welded part was at the center in the width direction. Next, resin embedding was performed so that the cross-section in the thickness direction became the observation surface. Next, the test piece for measurement with resin embedding was mirror-finished by wet polishing using SiC abrasive paper and diamond paste. Thereafter, the Vickers hardness of the base metal and the weld metal part was measured using a micro-Vickers hardness tester. In this measurement, the load was set to 0.01 kg, and the measurement was carried out at any 5 points, and the average value thereof was taken as the result. The Vickers hardness of the base metal was measured at a position more than 5 mm away from the position where the Vickers hardness of the weld metal part was measured. The difference between the Vickers hardness of the base metal and the Vickers hardness of the weld metal part was calculated from the obtained Vickers hardness of the base metal and the weld metal part.

[0064] <Al concentration in the oxide film> A 30 mm square test piece for measurement was cut out from the ferritic stainless steel welded structure, and its surface was degreased with acetone. Next, in accordance with JIS K0167:2011, Auger electron spectroscopy (AES) was used to analyze the component concentrations on the surface. In this analysis, the analysis diameter was set to 0.1 mm, and the analysis position was the weld metal part. In the component concentration profile obtained from 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 Al concentration in the oxide film was determined by the following formula. Al concentration in oxide film [mass%] = Al concentration / (Fe concentration + Cr concentration + Al concentration) × 100

[0065] <Toughness of the weld metal part> Considering the actual use environment of the ferritic stainless steel welded structure, for the ferritic stainless steel welded structure, a simulated heat treatment of the use environment was carried out using an electric arc furnace at 600 °C for 100 hours in an air atmosphere with a moisture concentration of 25% by volume. Next, three 50 mm × 10 mm × 1 mm test pieces for measurement were cut out by machining from the ferritic stainless steel welded structure after the simulated heat treatment of the use environment. Next, a V-notch (notch angle 45°, notch depth 2 mm, notch bottom radius 0.25 mm) was machined in the center of the width direction of the test piece for measurement toward the rolling direction. In each test piece for measurement, the V-notch was made to be the weld metal part. Using this test piece for measurement, a Charpy impact test was carried out in accordance with JIS Z2242:2018 at a test temperature of 25 °C. 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 20 J / cm 2 The above case was judged to have good toughness.

[0066] <Heat resistance of the weld metal part> For the ferritic stainless steel welded structure, a simulated heat treatment of the use environment was carried out in the same manner as above. Next, a 30 mm × 30 mm test piece for measurement was cut out from a ferritic stainless steel welded structure that had undergone simulated heat treatment in the usage environment, and then the surface was degreased with acetone. Next, in accordance with JIS K0167:2011, the component concentration on the surface was analyzed using Auger electron spectroscopy (AES). In this analysis, the analysis diameter was 0.1 mm, and the analysis position was the weld metal part. In the component concentration profile obtained from this analysis, the Al, Fe, and Cr concentrations at the position where the O (oxygen) concentration was 3 / 4 of the maximum value were determined, and the Fe concentration in the oxide film was determined by the following formula. In this evaluation, when the Fe concentration in the oxide film was 30 mass% or less, it was judged that the heat resistance was good. Fe concentration in oxide film [mass%] = Fe concentration / (Fe concentration + Cr concentration + Al concentration) × 100 The above evaluation results are shown in Table 3.

[0067]

Table 3

[0068] As shown in Table 3, in Examples 1 to 8 where the composition of the base material, the number density of inclusions of a predetermined size, the difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal part, and the average crystal grain size of the weld metal part satisfied the predetermined conditions, the toughness and heat resistance of the weld metal part were good. On the other hand, in Comparative Example 1, since the irradiation energy with respect to the thickness of the ferritic stainless steel material was too large, the difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal part became large, and the solid solution amounts of C and N in the weld metal part increased. As a result, the toughness and heat resistance of the weld metal part decreased. In Comparative Examples 2 and 3, since the irradiation energy with respect to the thickness of the ferritic stainless steel material was made even larger than that in Comparative Example 1, the difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal part became large, the solid solution amounts of C and N in the weld metal part increased, and the average crystal grain size of the weld metal part also became large. As a result, the toughness and heat resistance of the weld metal part decreased. In Comparative Example 4, since TIG welding was used as the welding method, the difference between the Vickers hardness of the base material and that of the weld metal part became large, the solid solution amounts of C and N in the weld metal part increased, and the average crystal grain size of the weld metal part also became large. As a result, the toughness and heat resistance of the weld metal part decreased.

[0069] In Comparative Example 5, since the Al content of the base material was too low, the O (impurity) content increased. As a result, the heat resistance of the weld metal part decreased. In Comparative Example 6, since it does not contain Ti and Nb, the solid solution amounts of C and N in the base material and the weld metal part increased. As a result, the heat resistance of the weld metal part decreased. In Comparative Examples 7 and 8, since the total content of Mg and Ca was too high, the number density of inclusions exceeding 10 μm in the base material increased, and the toughness of the weld metal part decreased.

[0070] As can be seen from the above results, according to the present invention, it is possible to provide a ferritic stainless steel laser welded structure excellent in heat resistance and toughness (particularly, the heat resistance and toughness of the weld metal part), a method for manufacturing the same, and a waste heat recovery device.

Claims

1. A ferritic stainless steel laser welded structure including a base material and a weld metal part, The base material contains, by mass, C: 0.001 to 0.050%, 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: 0.001 to 0.050%, 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, Mg and / or Ca: 0.0100% or less, the total content of Nb and Ti is 6(C+N) or more (C and N represent the contents of C and N, respectively), and the balance is Fe and impurities, The base material has a number density of inclusions having a diameter of 1 to 10 μm of 10.0 pieces / mm 2 The number density of inclusions having a diameter exceeding 10 μm is 1.0 pieces / mm 2 The total amount of C and N in solid solution is 0.015% by mass or less, The difference between the Vickers hardness of the base metal and the Vickers hardness of the weld metal portion is 50 HV or less, A ferritic stainless steel laser-welded structure, wherein the weld metal part has an average crystal grain size of 200 μm or less.

2. The ferritic stainless steel laser welded structure according to claim 1, wherein the base material further contains, by mass, 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.

3. The ferritic stainless steel laser-welded structure according to claim 1 or 2, wherein the base material further contains, on a mass basis, REM: 0.10% or less.

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

5. The ferritic stainless steel laser-welded structure according to any one of claims 1 to 4, wherein the ferritic stainless steel laser-welded structure has an oxide film on its surface containing less than 30 mass% Al.

6. The ferritic stainless steel laser-welded structure according to any one of claims 1 to 4, wherein the ferritic stainless steel laser-welded structure has an oxide film on its surface containing 30 mass% or more of Al.

7. The ferritic stainless steel laser-welded structure according to any one of claims 1 to 6, which is used in an exhaust heat recovery device.

8. A method for producing a ferritic stainless steel laser welded structure using a ferritic stainless steel material as a base material and performing laser welding, The ferritic stainless steel material contains C: 0.001 to 0.050%, 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: 0.001 to 0.050%, 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, Mg and / or Ca: 0.0100% or less, the total content of Nb and Ti is 6(C+N) or more (C and N represent the contents of C and N, respectively), and the balance is Fe and impurities, The ferritic stainless steel material has a number density of inclusions having a diameter of 1 to 10 μm of 10.0 pieces / mm 2 or more, a number density of inclusions having a diameter exceeding 10 μm of 1.0 pieces / mm 2 or less, and a total solid solution amount of C and N of 0.015 mass% or less, The laser welding is performed under conditions where the irradiation energy relative to the thickness of the ferritic stainless steel material is 10.0 to 40.0 kJ / mm3.

9. The manufacturing method according to claim 8, wherein the ferritic stainless steel material further contains, by mass, 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.

10. The manufacturing method according to claim 8 or 9, wherein the ferritic stainless steel material further contains, on a mass basis, REM: 0.10% or less.

11. The manufacturing method according to any one of claims 8 to 10, wherein the ferritic stainless steel material further contains at least one selected from Sn: 0.10% or less and B: 0.0100% or less, on a mass basis.

12. The method according to any one of claims 8 to 11, wherein no heat treatment is performed after the laser welding.

13. The method according to any one of claims 8 to 11, further comprising performing a heat treatment after the laser welding.

14. A waste heat recovery device comprising the ferritic stainless steel laser-welded structure according to any one of claims 1 to 7.

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