JP7679269B2 - Ferritic stainless steel laser welded structure, manufacturing method thereof, and exhaust heat recovery device - Google Patents

Description

The present invention relates to a ferritic stainless steel laser welded structure, a manufacturing method thereof, and an exhaust heat recovery device.

In response to recent global efforts to reduce _CO2 emissions, efforts to effectively utilize waste heat are becoming more widespread. For example, heat exchangers are used in many automobile exhaust system components, plants, and home energy equipment as a technology for recovering thermal energy from exhaust gas, and their use is expected to continue to expand in the future.

The exhaust gas used in heat exchangers is, for example, the combustion exhaust gas of gasoline or LPG (liquefied petroleum gas), which is high temperature (about 400°C to about 750°C) and is an oxidizing gas containing a lot of water vapor. Therefore, heat exchangers are required to have resistance to such environments (heat resistance and corrosion resistance), and stainless steel materials are used as the material.
Heat exchangers are also exposed to temperature differences from low temperatures (room temperature to about 90°C) to high temperatures (about 400°C to about 750°C). In particular, when heat exchangers become large, the thermal stress caused by the temperature difference increases, making them more likely to deform. For this reason, ferritic stainless steel, which has low thermal expansion, is often used as the material for heat exchangers.

Stainless steel materials have excellent corrosion resistance due to the formation of a Cr oxide film on the surface. However, in a highly oxidizing environment such as high temperature and high water vapor, Cr oxide grows quickly and Cr vaporizes, causing the Cr oxide to turn into hexavalent Cr and scatter.
Therefore, for stainless steel materials used in highly oxidizing environments such as high temperatures and high water vapor content, ferritic stainless steel materials capable of forming aluminum oxide (Al ₂ O ₃ ) on the surface are used.

For example, Patent Document 1 proposes a ferritic stainless steel material having a composition containing 11 to 22 mass% Cr, 0.03 mass% or less C, 0.03 mass% or less N, 1.5 mass% or less Mn, 0.008 mass% or less S, 2 mass% or less Si, 1.0 to 6.0 mass% Al, with the balance being Fe and unavoidable impurities.
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 unavoidable impurities.
Furthermore, Patent Document 3 proposes a ferritic stainless steel material having a composition containing C: 0.03 mass% or less, Si: 0.5 mass% or less, Mn: 1.0 mass% or less, P: 0.04 mass% or less, S: 0.01 mass% or less, Ni: 0.6 mass% or less, Cr: 15 to 20 mass%, N: 0.03 mass% or less, Ti: 0.5 mass% or less, B: 0.0005 to 0.003 mass%, Al: 1.5 mass% or more and less than 4.0 mass%, with the balance being Fe and unavoidable impurities.

JP 2009-167443 A JP 2012-012674 A JP 2011-179088 A

The corrosion resistance of ferritic stainless steel is related to the amount of C and N dissolved in the ferritic stainless steel. The dissolved C and N combine with Cr to form Cr carbides and nitrides (hereinafter referred to as "carbonitrides"), which precipitate preferentially at grain boundaries. The area around the precipitated Cr carbonitrides is in a state called sensitization, where Cr is deficient. In the Cr-deficient area, Cr oxides and Al oxides are not formed, and Fe is continuously oxidized, so heat resistance cannot be guaranteed. Therefore, it is effective to reduce the amount of C and N in the ferritic stainless steel as much as possible, and to add elements such as Ti and Nb, which preferentially combine with C and N, to form carbonitrides and thereby reduce the amount of C and N dissolved in the ferritic stainless steel.

In addition, exhaust heat recovery devices including heat exchangers are manufactured by subjecting ferritic stainless steel materials to processing such as welding. When welding is performed, carbonitrides of Ti and Nb are dissolved in the weld metal parts, and the amount of dissolved C and N increases. In the ferritic stainless steel materials in which the C and N contents are controlled to an extremely low level as described above, carbonitrides of Ti and Nb are formed again by natural cooling after welding, so the amount of dissolved C and N remains at a low level, and the 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, the diffusion of Ti and Nb is slow, so that carbonitrides of Ti and Nb are unlikely to be formed again by natural cooling after welding, and the amount of dissolved C and N increases. This is largely affected by the diffusion rate, so it cannot be solved by excessive addition of Ti and Nb. Conversely, excessive addition of Ti and Nb leads to a decrease in toughness due to an increase in inclusions such as TiO ₂ . Thus, Patent Documents 1 to 3 do not recognize at all the problem of reduced heat resistance and toughness of the weld metal in a ferritic stainless steel welded structure obtained by welding ferritic stainless steel materials.

The present invention has been made to solve the above problems, and aims to provide a ferritic stainless steel welded (particularly laser welded) structure with excellent heat resistance and toughness (particularly the heat resistance and toughness of the welded metal part), a manufacturing method thereof, and an exhaust heat recovery device.

The inventors conducted extensive research into ferritic stainless steel laser welded structures including a base material and a weld metal part, and discovered that the above problems could be solved by controlling the composition of the base material and the number density of inclusions of a given 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, leading to the completion of the present invention. The inventors also discovered that the above ferritic stainless steel laser welded structure could be manufactured by using a ferritic stainless steel material having a specific composition as the base material and performing laser welding with a given irradiation energy, leading to the completion of the present invention.

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, 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/ ^mm2 or more, a number density of inclusions having a diameter exceeding 10 μm of 1.0 pieces/ ^mm2 or less, and a total solid solution amount of C and N of 0.015 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,
The weld metal part is a ferritic stainless steel laser-welded structure having an average crystal grain size of 200 μm or less.

The present invention also provides 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 has a composition comprising 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 being 6(C+N) or more (C and N represent the contents of C and N, respectively), with the balance being 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/mm2 ^or more, a number density of inclusions having a diameter exceeding 10 μm of 1.0 pieces/mm2 ^or less, and a total solid solution amount of C and N of 0.015 mass% or less,
The laser welding is a manufacturing method carried out under conditions in which the irradiation energy relative to the thickness of the ferritic stainless steel material is 10.0 to 40.0 kJ/ ^mm3 .

Furthermore, the present invention is a waste heat recovery device equipped with the above-mentioned ferritic stainless steel laser welded structure.

The present invention provides a ferritic stainless steel welded (particularly laser welded) structure with excellent heat resistance and toughness (particularly the heat resistance and toughness of the welded metal part), a manufacturing method thereof, and an exhaust heat recovery device.

FIG. 2 is a schematic enlarged partial cross-sectional view of a ferritic stainless steel laser-welded structure.

The following is a detailed description of the embodiments of the present invention that have been completed based on the above-mentioned viewpoints. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention.
In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

A 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 a base material and performing laser welding.
Here, in this specification, "ferritic" means that the metal structure is mainly ferritic phase at room temperature. Therefore, "ferritic" also includes those that contain small amounts of phases other than ferritic phase (e.g., austenite phase, martensite phase, etc.). Furthermore, "stainless steel material" means a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the material shape include plate (including strip), rod, tube, etc. Furthermore, the material may be various types of shaped steel with a cross-sectional shape such as T-shaped or I-shaped.

FIG. 1 shows a schematic enlarged partial cross-sectional view of a ferritic stainless steel laser-welded structure.
As shown in Figure 1, the ferritic stainless steel laser welded structure (100) includes a base material (10) and a weld metal portion (30). The ferritic stainless steel laser welded structure (100) further includes a heat affected zone (20) between the base material (10) and the weld metal portion (30).
Here, "base material" refers to the portion that is not affected by welding. "Heat-affected zone" refers to the portion that is affected by heat but does not melt due to welding (also called HAZ). "Weld metal zone" refers to the portion that melts and resolidifies due to welding.

The base material is not affected by welding and therefore has the same composition and metal structure as the ferritic stainless steel material that is the raw material for 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 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, Mg and/or Ca: 0.0100% or less, with the total content of Nb and Ti being 6(C+N) or more (C and N represent the contents of C and N, respectively), and the balance being Fe and impurities.
In this specification, the term "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes during industrial production of stainless steel materials, and are acceptable within a range that does not adversely affect the present invention. For example, impurities include unavoidable impurities. An example of an impurity is O. The O content is preferably 0.0020% or less.

In addition, the base material (ferritic stainless steel material) may 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 necessary.
In addition, the base material (ferritic stainless steel material) may further contain at least one selected from REM: 0.10% or less and Ca: 0.10% or less, as necessary.
Furthermore, the base material (ferritic stainless steel material) may further contain at least one selected from Sn: 0.10% or less and B: 0.0100% or less, as necessary.
In the following description of each element, the term "base material" includes not only the base material of a ferritic stainless steel laser-welded structure, but also the ferritic stainless steel material used in the manufacture of a ferritic stainless steel laser-welded structure.

(C: 0.100% or less)
C is an element that affects the intergranular corrosion resistance (sensitization suppression effect) of the base material and the workability of the ferritic stainless steel material. If the C content 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 of the C content is 0.100%, preferably 0.080%, and more preferably 0.050%. On the other hand, the lower limit of the C content is not particularly limited, but reducing the C content leads to an increase in refining costs. Therefore, the lower limit of the C content is preferably 0.0005%, and more preferably 0.001%.

(Mn: 1.00% or less)
Mn is a useful element as a deoxidizing element. If the content of Mn is too high, MnS, which is the starting point of corrosion, is easily generated and the ferrite phase is destabilized. Therefore, the upper limit of the content of Mn is 1.00%, preferably 0.90%, and more preferably 0.80%. On the other hand, the lower limit of the content of Mn is not particularly limited, but is preferably 0.01%, and more preferably 0.05%.

(Ni: 1.00% or less)
Ni is an element effective for improving the corrosion resistance of the base material and the toughness of the weld metal. If the Ni content is too high, the ferrite phase becomes unstable and the manufacturing cost increases. Therefore, the upper limit of the Ni content is 1.00%, preferably 0.80%, and more preferably 0.60%. On the other hand, the lower limit of the Ni content is not particularly limited, but is preferably 0.01%, and more preferably 0.05%, from the viewpoint of obtaining the above-mentioned effects.

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

(S: 0.050% or less)
S is an element that generates MnS, which becomes the starting point of corrosion, and affects the toughness of the weld metal part. If the S content is too high, the toughness of the weld metal part may decrease. Therefore, the upper limit of the S content is 0.050%, preferably 0.040%, and more preferably 0.030%. On the other hand, the lower limit of the S content is not particularly limited, but reducing the S content leads to an increase in refining costs. Therefore, the lower limit of the S content is preferably 0.0001%, and more preferably 0.0005%.

(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 of the Cr content is 24.00%, preferably 23.50%, and more preferably 23.00%. On the other hand, if the Cr content is too low, the above effects may not be sufficiently obtained. Therefore, the lower limit of the Cr content is 10.00%, preferably 10.50%, and more preferably 11.00%.

(N: 0.100% or less)
N is an element that affects the intergranular corrosion resistance (sensitization suppression effect) of the base material and the workability of the ferritic stainless steel material. If the N content 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 of the N content is 0.100%, preferably 0.050%, and more preferably 0.030%. On the other hand, the lower limit of the N content is not particularly limited, but reducing the N content leads to an increase in refining costs. Therefore, the lower limit of the N content is preferably 0.0005%, and more preferably 0.001%.

(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 becomes unstable and the manufacturing cost increases. Therefore, the upper limit of the Cu content is 1.00%, preferably 0.70%, and more preferably 0.30%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.001%, and more preferably 0.01%.

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

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

(Al: 0.80-5.00%)
Like Si, Al is an element effective in improving the corrosion resistance of the base material. In addition, the use of Al can reduce the content of O, making it possible to finely disperse inclusions. If the content of Al is too high, the toughness of the base material decreases. Therefore, the upper limit of the content of Al is 5.00%, preferably 4.50%, and more preferably 4.00%. On the other hand, the lower limit of the content of Al is 0.80%, preferably 1.00%, and more preferably 1.20%, from the viewpoint of obtaining the above-mentioned effects.

(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 the properties of the base material, such as intergranular corrosion resistance (sensitization suppression effect).
If the Nb content is too high, the workability of the ferritic stainless steel material and the toughness of the base metal are reduced, so the upper limit of the Nb content is 0.50%, preferably 0.48%, and more preferably 0.45%.
Moreover, if the Ti content is too high, the workability and surface quality of the ferritic stainless steel material deteriorates, so the upper limit of the Ti content is 0.50%, preferably 0.48%, and more preferably 0.45%.
On the other hand, the lower limit of the total content of Nb and Ti is controlled based on the relationship with the contents of C and N, which reduce intergranular corrosion resistance. Specifically, the lower limit of the total content of Nb and Ti is 6 (C+N), preferably 7 (C+N), where C and N represent the contents of C and N, respectively.

(Mg and / or Ca: 0.0100% or less)
Mg and Ca are elements that are easily oxidized, similar to Al. In addition, the oxides of Mg and Ca aggregate with the oxide of Al (Al ₂ O ₃ ), causing the inclusions to become coarse, thereby reducing the toughness. Therefore, the upper limit of the content of Mg and/or Ca (the total content of Mg and Ca) is 0.0100%, preferably 0.0090%, and more preferably 0.0080%. On the other hand, the lower limit of the content of Mg and/or Ca (the total content of Mg and Ca) is not particularly limited because the lower the better, but is, for example, 0.0001%.
The Mg content is not particularly limited as long as the total content of Mg and Ca satisfies the above range, and may be, for example, 0.0030% or less, 0.0025% or less, 0.0020% or less, or 0.0015% or less. The Ca content is also not particularly limited as long as the total content of Mg and Ca satisfies the above range, and may be, for example, 0.0070% or less, 0.0065% or less, 0.0060% or less, or 0.0055% or less.

(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 for improving the oxidation resistance of the base material. If the content of Zr, Co, V and W is too high, the workability of the ferritic stainless steel material and the toughness of the base material decrease, and the manufacturing cost increases. Therefore, the upper limit of the content 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 Zr, Co, V and W is not particularly limited, but is preferably 0.001%, more preferably 0.01%.

(REM: 0.10% or less)
REM (rare earth elements ) are effective elements for improving the oxidation resistance of the base material. If the content of REM is too high, it leads to an increase in the manufacturing cost of the ferritic stainless steel material. Therefore, the upper limit of the content of REM is 0.10%, preferably 0.08%, and more preferably 0.05%. On the other hand, the lower limit of the content of REM is not particularly limited , but is preferably 0.0001%, and more preferably 0.003%.
REM is a collective 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.

(Sn: 0.10% or less)
Sn is an element effective for improving the corrosion resistance of the base material. If the Sn content is too high, Sn segregates and the manufacturability decreases. Therefore, the upper limit of the Sn content is 0.10%, preferably 0.08%, and more preferably 0.05%. On the other hand, the lower limit of the Sn content is not particularly limited, but is preferably 0.001%, and more preferably 0.005%.

(B: 0.0100% or less)
B is an element effective for improving the secondary workability of ferritic stainless steel materials. If the B content is too high, the fatigue strength of the ferritic stainless steel material decreases. Therefore, the upper limit of the B content is 0.0100%, preferably 0.0080%, and more preferably 0.0050%. On the other hand, the lower limit of the B content is not particularly limited, but is preferably 0.0001%, and more preferably 0.0005%.

The base metal has a number density of inclusions having a diameter of 1 to 10 μm of 10.0 pieces/ ^mm2 or more, preferably 15.0 pieces/ ^mm2 or more, and more preferably 20.0 pieces/ ^mm2 or more. By controlling the number density of inclusions of such a size, these inclusions become crystal formation nuclei during the solidification process during welding, contributing to the refinement of crystal grains in the weld metal part. Therefore, the toughness of the weld metal part can be improved. On the other hand, the upper limit of the number density of these inclusions is not particularly limited, but is preferably 100.0 pieces/ ^mm2 , more preferably 80.0 pieces/ ^mm2 , and even more preferably 50.0 pieces/ ^mm2 .
In this specification, the term "inclusions" refers mainly to oxides of elements such as Al, Ca, Ti, etc. Furthermore, in this specification, the number density of inclusions refers to that measured by the method described in the examples described later.

The base metal has a number density of inclusions having a diameter exceeding 10 μm of 1.0 pieces/ ^mm2 or less, preferably 0.9 pieces/mm2 or ^less , and more preferably 0.8 pieces/ ^mm2 or less. By controlling the number density of inclusions of such a size, it is possible to suppress the formation of coarse inclusions in the weld metal part. Since coarse inclusions promote the propagation of fracture and reduce toughness, it is possible to improve the toughness of the weld metal part by controlling the number density of the inclusions.

The total amount of solute C and N in the base metal is 0.015% or less, preferably 0.014% or less, and more preferably 0.012% or less. By controlling the total amount of solute C and N within the above range, it is possible to suppress the precipitation of Cr carbonitrides by the solid solution of C and N bonding 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. On the other hand, the lower limit of the total amount of solute C and N is not particularly limited, but is typically 0.0001%, preferably 0.0005%, and more preferably 0.001%.
In this specification, the total amount of dissolved C and N means the amount calculated by the method described in the Examples below. It should be noted that since it is difficult to calculate the amounts of dissolved C and N individually, the total amount of dissolved C and N is calculated for convenience in the present invention.

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

The weld metal part has an average crystal grain size of 200 μm or less, preferably 180 μm or less, and 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 one measured by the method described in the Examples section below.

In general, in order to suppress sensitization in the weld metal part of a ferritic stainless steel laser-welded structure manufactured using a ferritic stainless steel material containing Al, it is necessary to perform heat treatment after welding to precipitate carbonitrides of Ti and Nb and reduce the amount of solid solution of C and N. However, the heat treatment requires detailed control of the furnace temperature and cooling rate, which increases the manufacturing cost. In addition, deformation may occur during the heat treatment, and it is difficult to perform the heat treatment itself when the size is large.
In contrast, in the ferritic stainless steel laser-welded structure according to the embodiment of the present invention, sensitization in the weld metal portion can be suppressed even without heat treatment after welding.
A ferritic stainless steel laser-welded structure according to an embodiment of the present invention, which is not subjected to heat treatment after welding, has an oxide film containing less than 30% Al on its surface (the surface of the base material, heat-affected zone, and welded metal part). The Al content in the oxide film is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. The lower limit of the Al content is not particularly limited, but is, for example, 0.1%.
In this specification, the Al content in the oxide film means that which is measured by the method described in the examples described later.

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 present invention, which is heat treated after welding, has an oxide film containing 30% or more Al on the surface. By providing such an oxide film on the surface, high-temperature oxidation resistance can be improved. From the viewpoint of stably increasing high-temperature oxidation resistance, the Al content in the oxide film is preferably 32% or more, more preferably 35% or more. On the other hand, the upper limit of the Al content is not particularly limited, but is, for example, 90%, preferably 80%.

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

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, the hot-rolled material is annealed, pickled, and cold-rolled in sequence to obtain a cold-rolled material. Next, the cold-rolled material is annealed and pickled in sequence to obtain a cold-rolled annealed material. The conditions in each step are not particularly limited and may be appropriately adjusted according to the composition of the stainless steel. The hot-rolled material, cold-rolled material, or cold-rolled annealed material manufactured by such a method can be used as the ferritic stainless steel material. Among these, the ferritic stainless steel material is preferably a cold-rolled annealed material. In addition, the hot-rolled material, cold-rolled material, or cold-rolled annealed material may be formed into a predetermined member shape. Examples of the forming process include various press processes using a mold, mechanical processes such as bending, and the like.
The conditions in each step may be appropriately adjusted depending on the composition of the ferritic stainless steel material, and are not particularly limited.
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 even more preferably 0.5 to 3.0 mm.

Ferritic stainless steel has a 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 are dissolved during welding, increasing the amounts of C and N dissolved, and the heat resistance is reduced due to sensitization.
In addition, as a method for suppressing the coarsening of crystal grains, a method is known in which _TiO2 is added to the flux of arc welding to refine the crystal grains in the weld metal part in order to disperse oxides that become the nuclei of crystal grains during solidification after welding. However, _TiO2 often has a diameter of 10 μm or more, and it is known that when it is contained as an inclusion in a ferritic stainless steel material, it causes surface defects. Therefore, it is not preferable to intentionally precipitate _TiO2 . 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 toughness.
Therefore, in order to avoid the above problems, in the manufacture of a ferritic stainless steel laser-welded structure according to an embodiment of the present invention, welding is performed using laser welding, which makes it easy to control the amount of heat input.

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/mm ³ , preferably 10.0 to 38.0 kJ/mm ³ , and more preferably 10.0 to 35.0 kJ/mm ³ . By performing laser welding with irradiation energy in this range, it is possible to suppress an increase in the amount of solid solution of C and N due to solid solution of Ti and Nb, and coarsening of inclusions. As a result, it is possible to improve both the heat resistance and toughness of a ferritic stainless steel laser-welded structure.

In laser welding, the irradiation energy relative to the thickness of the ferritic stainless steel material can be controlled by, for example, controlling the laser output [kW], the focal diameter [mm], and the scanning speed [m/min]. Specifically, the irradiation energy relative to the thickness of the ferritic stainless steel material [kJ/ ^mm3 ] can be calculated by the following formula (1).
Irradiation energy relative to thickness of ferritic stainless steel material [kJ/mm ³ ]=irradiation energy [kJ/mm ² ]/thickness of ferritic stainless steel material [mm] (1)
In formula (1), the irradiation energy [kJ/mm ² ] can be calculated by the following formula (2).
Irradiation energy [kJ/mm ² ]=power density [kW/mm ² ]×irradiation time [seconds] (2)
In formula (2), the power density [kW/mm ² ] can be calculated 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 calculated by the following formula (4).
Irradiation time [sec] = focal diameter [mm] / (scanning speed [m/min] × 1000/60) ... (4)

When laser welding ferritic stainless steel materials of different thicknesses, the irradiation energy is controlled based on the thinner one. Laser welding may be used to weld multiple ferritic stainless steel materials together, or to weld ferritic stainless steel materials to metal materials of other materials.
The laser used for laser welding is not particularly limited, and known lasers such as _CO2 laser, YAG laser, and semiconductor laser can be used.

In the ferritic stainless steel laser-welded structure according to the embodiment of the present invention, sensitization in the weld metal part can be suppressed even without performing heat treatment after laser welding. Therefore, it is not necessary to perform heat treatment after laser welding.
However, in order to improve the heat resistance of the ferritic stainless steel laser-welded structure according to the embodiment of the present invention, a heat treatment may be performed after the laser welding.
The conditions for the heat treatment are not particularly limited, but it is preferable to heat the ferritic stainless steel laser-welded structure for 1 minute or more at an oxygen partial pressure of 2×10 ⁻⁵ Pa or more and at a temperature range of 1000° C. or more. The gas other than oxygen in the heat treatment atmosphere is not particularly limited, and hydrogen, argon, etc. can be used. The heat treatment can be performed using, for example, a heating furnace. The type of the heating furnace may be a batch type or a continuous type.
After the heat treatment, it is preferable to cool the material by holding the material in the temperature range of 900 to 750° C. for 5 minutes or more. In order to hold the material in the temperature range of 900 to 750° C. for 5 minutes or more, for example, the cooling rate in the temperature range of 900 to 750° C. may be set to 30° C./min or less. Alternatively, the material may be held at any constant temperature in the temperature range of 900 to 750° C. for 5 minutes or more, and then cooled to a temperature range of less than 750° C. at a cooling rate exceeding 30° C./min.

The ferritic stainless steel laser welded structure according to the embodiment of the present invention has excellent heat resistance and toughness (particularly the heat resistance and toughness of the weld metal part), and can be used in various applications where these properties are required. Examples of applications include exhaust heat recovery devices including heat exchangers used in automobile exhaust system parts, plants, household energy equipment, etc.

The exhaust heat recovery device according to the embodiment of the present invention is equipped with the above-mentioned ferritic stainless steel laser welded structure. The exhaust heat recovery device according to the embodiment of the present invention, which has such characteristics, is equipped with a ferritic stainless steel laser welded structure that has excellent heat resistance and toughness (particularly the heat resistance and toughness of the welded metal part), and therefore its performance can be improved.

The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these.

<Preparation 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 and hot rolled to obtain a hot rolled sheet having a thickness of 3.0 mm, which was then annealed at 1050°C and pickled to obtain a hot rolled annealed sheet. Next, the hot rolled annealed sheet was cold rolled to obtain a cold rolled sheet having a thickness of 0.6 to 2.0 mm, which was then finish annealed at 1000°C and pickled to obtain a cold rolled annealed sheet. Next, a test piece (base metal test piece) having a width direction of 50 mm and a rolling direction of 500 mm was cut out from the cold rolled annealed sheet by cutting.

<Fabrication of Ferritic Stainless Steel Welded Structure>
Two of the above-mentioned base metal test pieces were used, and the two were butted together and laser welding or TIG welding was performed. A _CO2 laser was used for laser welding under the conditions shown in Table 2. For TIG welding, the welding width was 3.2 mm, the output was 90 A, and the welding speed was 0.5 m/min.
In some examples, heat treatment was performed after laser welding. The heat treatment was performed by placing the welded structure in a vacuum furnace, heating it at 1025°C for 5 minutes, and setting the oxygen partial pressure at ^5x10-4 Pa, and then cooling it 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. The cooling rate was controlled by the amount of cooling nitrogen gas introduced into the vacuum furnace.

The ferritic stainless steel welded structure obtained as described above was evaluated as follows.

<Number density of inclusions in base material>
The surface of the above-mentioned base metal test piece was mirror-polished so that the observation position was a depth of 15 μm (any depth within 100 μm) from the surface, and etching was performed with fluoronitric acid to reveal the grain boundaries and inclusions. Next, an FE-SEM (SU-5000 manufactured by Hitachi High-Tech Corporation) was used to photograph an area of 15 mm × 15 mm, and image analysis was performed to determine the number of inclusions having a diameter of 1 to 10 μm and inclusions having a diameter of more than 10 μm. The diameter of the inclusions was set to ^1/2 (long side × short side). The number of each inclusion was then divided by the observation field to calculate the number density of each inclusion.

<Total amount of C and N dissolved in base material>
First, the amounts of Ti and Nb carbonitrides formed were measured by the following procedure.
A 50 mm x 50 mm test specimen was cut out from the above base material test specimen, and the entire surface of the test specimen was subjected to #600 wet polishing. The test specimen was electrolytically etched using the SPEED method. The electrolytic etching was performed in a 10% acetylacetone solution at 400 mV with constant potential electrolysis until the amount of electricity reached 5000 coulombs, and the solution after electrolysis was filtered through a filter with a lattice diameter of 0.05 μm to collect carbonitrides. The amount of dissolved carbonitrides and the amount of collected carbonitrides were then determined by mass measurement.
Next, the collected carbonitrides were subjected to X-ray diffraction (XRD) analysis. In the XRD analysis, the sum of the peak intensities of the Ti carbonitrides and the Nb carbonitrides was calculated from the XRD profile. Hereinafter, the sum of the peak intensities of the Ti carbonitrides is referred to as the "Ti carbonitride peak intensity", and the sum of the peak intensities of the Nb carbonitrides is referred to as the "Nb carbonitride peak intensity". The peak positions (diffraction angles 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 amounts of C and N consumed in forming Ti carbonitrides and Nb carbonitrides (hereinafter referred to as "consumed C+N amount") were calculated by the following formula.
Amount of C+N consumed [mass %]=amount of collected carbonitride [g]×(0.21×peak intensity of Ti carbonitride+0.12×peak intensity of Nb carbonitride)/(peak intensity of Ti carbonitride+peak intensity of Nb carbonitride)/amount dissolved [g]×100
Next, the total amount of C and N in solid solution [mass %] was calculated by subtracting the consumed C+N amount [mass %] from the total content of C and N [mass %].

<Average grain size of weld metal>
From the above ferritic stainless steel welded structure, a sample of 10 mm in the rolling direction × 20 mm in the width direction was cut out by cutting so that the welded part was in the width direction center. Next, resin filling was performed so that the direction perpendicular to the welding direction was the observation surface. Next, the resin-filled measurement test piece was mirror-finished by wet polishing, and then etched with fluoronitric acid to reveal the metal structure, which was observed with an optical microscope. The observation with an optical microscope was performed in accordance with JIS G0551:2013, by drawing a straight line at an arbitrary position on a 50x optical microscope image, measuring the number of intersections between the straight line and the grain boundary, and the average intercept length was taken as the grain size. The grain size was measured by drawing and measuring 20 or more straight lines in multiple fields of view. This measurement was performed in any 5 fields of view, and the average value of these was taken as the average grain size.

<Difference between Vickers hardness of base metal and Vickers hardness of weld metal>
A sample of 25 mm in the rolling direction x 25 mm in the width direction was cut out from the above ferritic stainless steel welded structure by cutting so that the weld was in the center of the width direction. Next, resin filling was performed so that the thickness direction cross section was the observation surface. Next, the resin-filled test piece was mirror-finished by wet polishing using SiC polishing paper and diamond paste. Thereafter, the Vickers hardness of the base material and the weld metal part was measured using a micro Vickers hardness tester. In this measurement, the load was 0.01 kg, and measurements were performed at any five points, and the average value was taken as the result. The Vickers hardness of the base material was measured at a position 5 mm or more away from the position where the Vickers hardness of the weld metal part was measured. The difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal part was calculated from the obtained Vickers hardness of the base material and the weld metal part.

<Al concentration in oxide film>
A test piece of 30 mm square was cut out from the ferritic stainless steel welded structure, and the surface was degreased with acetone. Next, the component concentration of the surface was analyzed using Auger electron spectroscopy (AES) in accordance with JIS K0167:2011. 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 by this analysis, the Al, Fe, and Cr concentrations were obtained at the position where the O (oxygen) concentration was three-quarters of the maximum value, and the Al concentration in the oxide film was calculated by the following formula.
Al concentration in oxide film [mass %] = Al concentration / (Fe concentration + Cr concentration + Al concentration) × 100

<Toughness of weld metal>
Considering the actual usage environment of the ferritic stainless steel welded structure, the ferritic stainless steel welded structure was subjected to a heat treatment simulating the usage environment in which it was heated at 600°C for 100 hours in an air atmosphere with a moisture concentration of 25% by volume using an EREMA electric furnace.
Next, three test specimens of 50 mm x 10 mm x 1 mm were cut out from the ferritic stainless steel welded structure after the heat treatment simulating the usage environment. Next, a V-notch (notch angle 45°, notch depth 2 mm, notch bottom radius 0.25 mm) was cut in the center of the width direction of the test specimen toward the rolling direction. In each test specimen, the V-notch was made to be the weld metal part. Using this test specimen, a Charpy impact test was performed at a test temperature of 25°C in accordance with JIS Z2242:2018. The measurement was performed on three test specimens, and the average value was taken as the measurement result. In this evaluation, it was determined that the toughness was good when the Charpy impact absorption energy per unit area (Charpy impact value) was 20 J/ ^cm2 or more.

<Heat resistance of welded metal parts>
The ferritic stainless steel welded structure was subjected to a heat treatment simulating a usage environment in the same manner as described above.
Next, a test piece of 30 mm x 30 mm for measurement was cut out from the ferritic stainless steel welded structure that had been subjected to the heat treatment simulating the usage environment, and the surface was degreased with acetone. Next, in accordance with JIS K0167:2011, the component concentration of 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 by this analysis, the Al, Fe, and Cr concentrations were obtained at the position where the O (oxygen) concentration was three-quarters of the maximum value, and the Fe concentration in the oxide film was calculated by the following formula. In this evaluation, it was determined that the heat resistance was good when the Fe concentration in the oxide film was 30 mass% or less.
Fe concentration in oxide film [mass %] = Fe concentration / (Fe concentration + Cr concentration + Al concentration) x 100
The results of the above evaluations are shown in Table 3.

As shown in Table 3, in Examples 1 to 8 in which the composition of the base metal and the number density of inclusions of a specified size, the difference between the Vickers hardness of the base metal and the Vickers hardness of the weld metal part, and the average crystal grain size of the weld metal part satisfied the specified conditions, the toughness and heat resistance of the weld metal part were good.
In contrast, in Comparative Example 1, the irradiation energy relative to the thickness of the ferritic stainless steel material was too large, which resulted in a large difference in Vickers hardness between the base material and the weld metal, and an increase in the amount of solid solution of C and N in the weld metal, resulting in a decrease in the toughness and heat resistance of the weld metal.
In Comparative Examples 2 and 3, the irradiation energy relative to the thickness of the ferritic stainless steel material was even greater than in Comparative Example 1, so that the difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal increased, the amount of solid solution of C and N in the weld metal increased, and the average crystal grain size of the weld metal increased. As a result, the toughness and heat resistance of the weld metal decreased.
In Comparative Example 4, TIG welding was used as the welding method, so the difference between the Vickers hardness of the base material and the Vickers hardness of the weld metal was large, the amount of solid solution of C and N in the weld metal was large, and the average crystal grain size of the weld metal was also large. As a result, the toughness and heat resistance of the weld metal were reduced.

In Comparative Example 5, the Al content of the base metal was too low, so the O (impurity) content was high, resulting in a decrease in the heat resistance of the weld metal portion.
Comparative Example 6 did not contain Ti and Nb, and therefore the amounts of C and N dissolved in the base metal and the weld metal increased, resulting in a decrease in the heat resistance of the weld metal.
In Comparative Examples 7 and 8, the total content of Mg and Ca was too high, so that the number density of inclusions exceeding 10 μm in size in the base metal increased, and the toughness of the weld metal joint decreased.

As can be seen from the above results, the present invention can provide a ferritic stainless steel laser welded structure with excellent heat resistance and toughness (particularly the heat resistance and toughness of the weld metal part), a manufacturing method thereof, and an exhaust heat recovery device.

Claims (14)

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/ ^mm2 or more, a number density of inclusions having a diameter exceeding 10 μm of 1.0 pieces/ ^mm2 or less, and a total solid solution amount of C and N of 0.015 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. The ferritic stainless steel laser 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, by mass. 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 . The ferritic stainless steel laser 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 by mass. 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 the surface containing less than 30 mass% Al. 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 that contains 30 mass% or more of Al. A ferritic stainless steel laser welded structure according to any one of claims 1 to 6, used in an exhaust heat recovery device. 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 has a composition comprising 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 being 6(C+N) or more (C and N represent the contents of C and N, respectively), with the balance being 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/mm2 ^or more, a number density of inclusions having a diameter exceeding 10 μm of 1.0 pieces/mm2 ^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 . The manufacturing method according to claim 8, wherein the ferritic stainless steel 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, by mass. 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 . 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, by mass. The manufacturing method according to any one of claims 8 to 11, in which no heat treatment is performed after the laser welding. The manufacturing method according to any one of claims 8 to 11, in which heat treatment is performed after the laser welding. A waste heat recovery device comprising a ferritic stainless steel laser welded structure according to any one of claims 1 to 7.