JP7705015B2 - Ferritic stainless steel welded structure - Google Patents

Description

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

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

In a heat exchanger, heat is exchanged between high-temperature exhaust gas and a low-temperature refrigerant such as water, but the environments on the exhaust gas side and the refrigerant side are very different. In particular, the exhaust gas side is exposed to an oxidizing environment due to high-temperature (approximately 400°C to 750°C) exhaust gas, as well as a corrosive environment due to condensed water generated when the exhaust gas is cooled in the heat exchanger. On the other hand, the refrigerant side is exposed to a corrosive environment due to the refrigerant, but the temperature is lower than that of the exhaust gas side, and the concentration of corrosive factors such as chloride ions is specified to be low in refrigerants such as tap water, so they are less likely to concentrate. Therefore, heat exchangers are required to be resistant to the environment on the exhaust gas side in particular (high-temperature oxidation 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). Among stainless steel materials, austenitic stainless steel materials containing an austenite phase and duplex stainless steel materials are easily deformed by this temperature difference, so these stainless steel materials are not suitable as materials for heat exchangers. For this reason, ferritic stainless steel materials are often used as the material for heat exchangers.

As a ferritic stainless steel material having excellent oxidation resistance, 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 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: 11-21%, Al: 6% or less, Cu: 0.01-0.5%, Mo: 0.01-0.5%, Nb: 0.1% or less, Ti: 0.005-0.50%, Sn: 0.001-0.1%, 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.

JP 2009-167443 A JP 2012-012674 A

The ferritic stainless steel materials described in Patent Documents 1 and 2 contain Al, and an Al oxide (Al ₂ O ₃ ) film is formed on the surface, so that the oxidation resistance is better than that of general ferritic stainless steel. However, in the temperature range of 400 to 600°C, Fe is preferentially oxidized even in ferritic stainless steel materials containing Al. The Fe oxide film has a coarse structure and cannot adequately block oxygen, so oxidation continues to progress. Therefore, the ferritic stainless steel materials described in Patent Documents 1 and 2 cannot be said to have sufficient oxidation resistance in high-temperature environments (hereinafter referred to as "high-temperature oxidation resistance"). Here, in this specification, "high-temperature environment" mainly means a temperature environment of 400 to 750°C.

On the other hand, the amount of dissolved C and N in ferritic stainless steel is important for the corrosion resistance of 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 becomes sensitized, with a lack of Cr, and corrosion progresses significantly when exposed to an environment containing corrosion factors such as chloride ions. Therefore, it is effective to reduce the amount of dissolved C and N in 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 dissolved C and N.

In addition, various products such as heat exchangers are manufactured by subjecting ferritic stainless steel materials to processing such as welding. When welding is performed, carbides of Ti and Nb are dissolved in the weld metal part, and the amount of dissolved C and N increases. In the ferritic stainless steel material in which the C and N content is controlled to an extremely low level as described above, carbides 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 deterioration of corrosion resistance due to sensitization can be suppressed. However, in the ferritic stainless steel material described in Patent Documents 1 and 2 containing Al, the diffusion of Ti and Nb is slow, so that carbides 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 deterioration of surface quality and toughness due to an increase in inclusions such as TiO ₂ . Thus, Patent Documents 1 and 2 do not recognize at all the problem of reduced corrosion resistance of welded metal parts in ferritic stainless steel welded structures obtained by welding ferritic stainless steel materials.

Furthermore, in the manufacture of various products such as heat exchangers, ferritic stainless steel material is generally processed into a desired shape and then heat-treated under high-temperature conditions (e.g., 900°C or higher) to remove distortion. However, when heat-treated under high-temperature conditions, the shape is easily distorted, resulting in reduced dimensional accuracy.

The present invention has been made to solve the above problems, and aims to provide a ferritic stainless steel welded structure that has excellent high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy.

As a result of extensive research into ferritic stainless steel welded structures including base metal and weld metal, the inventors discovered that the above problems could be solved by controlling the average crystal orientation difference, the composition of the base metal, the grain boundary Cr concentration of the weld metal, and the Al concentration in the surface oxide film, which led to the completion of the present invention.

That is, the present invention is a ferritic stainless steel welded structure used in applications requiring high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy,
Includes base material and weld metal parts,
The base material contains, by mass, C: 0.050% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.00 to 24.00%, N: 0.050% or less, Cu: 1.00% or less, Mo: 1.00% or less, Si: 3.00% or less, Al: 0.80 to 5.00%, Nb: 0.50% or less, Ti: 0.50% or less, 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 an average crystal orientation difference of 0.15° or more,
The weld metal portion has a grain boundary Cr concentration of 10 mass% or more,
The ferritic stainless steel welded structure is a ferritic stainless steel welded structure having an oxide film on its surface that contains 30 mass % or more of Al.

The present invention provides a ferritic stainless steel welded structure that has excellent high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy.

FIG. 2 is a schematic partially enlarged cross-sectional view of a ferritic stainless steel 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 welded structure according to an embodiment of the present invention includes a base material and a weld metal part. This ferritic stainless steel welded structure is produced by welding ferritic stainless steel members.
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 welded structure.
As shown in Figure 1, the ferritic stainless steel welded structure (100) includes a base material (10) and a weld metal portion (30). The ferritic stainless steel 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 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 welded structure.
The base material (ferritic stainless steel material) contains C: 0.050% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.00 to 24.00%, N: 0.050% or less, Cu: 1.00% or less, Mo: 1.00% or less, Si: 3.00% or less, Al: 0.80 to 5.00%, Nb: 0.50% or less, Ti: 0.50% or less, with the total content of Nb and Ti being 6(C+N) or more (C and N 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 in the manufacturing process during industrial production of stainless steel materials, and are acceptable within a range that does not adversely affect the present invention. For example, impurities also include unavoidable impurities.

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 welded structure, but also the ferritic stainless steel material used in the manufacture of a ferritic stainless steel welded structure.

(C: 0.050% 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 0.050%, preferably 0.040%, and more preferably 0.030%. 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 10.90%.

(N: 0.050% 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 0.050%, preferably 0.030%, and more preferably 0.020%. 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 Si 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. 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 effect.

(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.

(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, Ca: 0.10% or less)
REM (rare earth elements) and Ca are effective elements for improving the oxidation resistance of the base material. If the content of REM and Ca 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 and Ca is 0.100%, preferably 0.08%, and more preferably 0.05%. On the other hand, the lower limit of REM and Ca 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 material has an average crystal grain size of preferably 100 μm or less, more preferably 95 μm or less, and further preferably 90 μm or less. By controlling the average crystal grain size within such a range, it is possible to suppress a decrease in toughness due to coarsening of the crystal grain size.
The lower limit of the average crystal grain size is not particularly limited, but is preferably 1 μm, more preferably 5 μm, and further preferably 10 μm.
In this specification, the average crystal grain size refers to one measured by the method described in the Examples section below.

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

The base material has an absorbed energy in a Charpy impact test (hereinafter referred to as "Charpy impact value") of preferably 100 J/ ^cm2 or more, more preferably 120 J/cm2 or ^more . By setting the Charpy impact value in such a range, it is possible to ensure the desired toughness. On the other hand, the upper limit of the Charpy impact value is not particularly limited, but is generally 300 J/ ^cm2 , preferably 280 J/ ^cm2 .
In this specification, the Charpy impact value means a value measured by the method described in the examples described later.

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

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

The ferritic stainless steel welded structure according to the embodiment of the present invention can be manufactured using a ferritic stainless steel material having the above composition in accordance with a method known in the art. A typical manufacturing method for the ferritic stainless steel welded structure according to the embodiment of the present invention is described below.

A ferritic stainless steel welded structure according to an embodiment of the present invention includes a welding process in which ferritic stainless steel materials having the above-mentioned composition are welded to obtain a welded structure, and a heat treatment process in which the welded structure is heated under an oxygen partial pressure of 2 x ^10-5 Pa or more in a temperature range of 600 to 800°C under conditions that satisfy formula (1).
0.011×T _A +logT _B ≧8.8…(1)
In the formula, T _A represents the heating time (° C.), and T _B represents the heating time (minutes).

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 welding step is performed using a ferritic stainless steel material having the above composition. The welding of the ferritic stainless steel material may involve welding a plurality of ferritic stainless steel materials together, or the ferritic stainless steel material may be welded to a metal material of another material.
The welding method is not particularly limited, and methods known in the art such as arc welding (TIG welding, etc.), electron beam welding, laser welding, plasma arc welding, spot welding, etc. In addition, the welding may or may not use a filler metal.
The welding conditions are not particularly limited and may be appropriately adjusted depending on the type of welding and the composition of the ferritic stainless steel material.

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

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

The ferritic stainless steel welded structure according to the embodiment of the present invention has excellent high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy, and can be used in a variety of applications where these properties are required. Examples of applications include heat exchangers used in automobile exhaust system parts, plants, and home energy equipment.

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.

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

The above test specimens were subjected to a welding process. The welding process was carried out by dissolving the widthwise center of the test specimen in the same manner as TIG spot welding (but without welding), to obtain a welded test specimen. The welding conditions were a welding current of 100 A, a welding speed of 60 cm/min, and a welding electrode diameter of 1.6 mm.
Next, the above test pieces that had not been welded (hereinafter referred to as "unwelded test pieces") and the welded test pieces were each placed in a vacuum furnace and heat-treated under the conditions shown in Table 2, and then cooled at an average cooling rate of 35°C/min. The oxygen partial pressure was controlled by changing the pressure in the vacuum furnace. The oxygen partial pressure shown in Table 2 is a value obtained by multiplying the pressure in the vacuum furnace by 0.2. The cooling rate was also controlled by the amount of cooling nitrogen gas introduced into the vacuum furnace.
In Comparative Example 1, the unwelded test piece and the welded test piece were not subjected to heat treatment and cooling.

The following evaluations were performed on the unwelded test pieces and welded test pieces obtained after the heat treatment described above (however, since no heat treatment was performed in Comparative Example 1, unwelded test pieces and welded test pieces that were not heat treated were used in the case of Comparative Example 1). Note that the average crystal orientation difference and dimensional accuracy were evaluated by preparing new test pieces for measurement using the above ferritic stainless steel plate.

(Al concentration in oxide film)
A 50 mm square test piece was cut out from the unwelded test piece after the heat treatment, and the surface was degreased with acetone. Next, the component concentration in the depth direction was analyzed using glow discharge optical emission spectroscopy (GD-OES) in accordance with JIS K0144:2001. In the component concentration profile in the depth direction 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

(average grain size of base material)
After cutting out a 10 mm square test piece from the unwelded test piece after heat treatment, the test piece was filled with resin so that the surface parallel to the rolling direction of the plate thickness and perpendicular to the width direction was the observation surface. Next, the resin-filled test piece was mirror-finished by wet polishing, and the metal structure revealed by etching with fluoronitric acid 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 the optical microscope image, counting 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, and the average value of these was taken as the average grain size.

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

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

(Toughness of base material)
From the unwelded test pieces after the above heat treatment, test pieces for measurement of 50 mm (length in the plate width direction) x 10 mm (length in the rolling direction) x 1 mm (length in the plate thickness direction) were cut out by cutting.
Next, a V-notch (notch angle 45°, notch depth 2 mm, notch bottom radius 0.25 mm) was cut in the center of the plate width direction of the test specimen toward the rolling direction. 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, the toughness was determined to be good when the Charpy impact absorption energy per unit area (Charpy impact value) was 100 J/cm2 or ^more .

(Corrosion resistance of welded metal parts)
Considering the actual usage environment of ferritic stainless steel welded structures, the welded test pieces after the above heat treatment were subjected to a heat treatment simulating the usage environment by heating them in an air atmosphere at 500° C. for 100 hours using an EREMA electric furnace.
Next, a 50 mm square test specimen was cut out from the welded test specimen that had been heat-treated to simulate the operating environment so that the weld metal part was located in the center, and then the entire surface of the test specimen was wet-polished with #600. This test specimen was placed in a flask with copper particles spread over the bottom surface in accordance with the sulfuric acid/copper sulfate corrosion test method for stainless steel specified in JIS G0575:2012, and then heated on a hot plate with 400 mL of 15.7% sulfuric acid/5.5% copper sulfate aqueous solution and the test specimen. The boiling state was maintained for 20 hours, and then the test specimen was taken out, washed with water, and dried.
Next, a universal testing machine (UH-300kNI manufactured by Shimadzu Corporation) was used to bend the test specimens by 1t in a direction perpendicular to the welding direction, and the top of the bend was observed with an optical microscope. As a result of this observation, specimens in which cracks occurred along the grain boundaries were rated as × (poor corrosion resistance), and specimens in which no cracks occurred were rated as ○ (good corrosion resistance).

(Oxidation resistance of base material)
The unwelded test pieces after the above heat treatment were subjected to a heat treatment simulating a usage environment under the same conditions as above.
Next, a 50 mm square test piece was cut out from the unwelded test piece that had been heat-treated to simulate the operating environment, and the surface was degreased with acetone. Next, in accordance with JIS K0144:2001, analysis of the component concentration in the depth direction was performed using glow discharge optical emission spectroscopy (GD-OES) in the same manner as above. In the component concentration profile in the depth direction obtained by this analysis, the Al, Fe, and Cr concentrations were determined 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 using the following formula.
Fe concentration in oxide film [mass %] = Fe concentration / (Fe concentration + Cr concentration + Al concentration) x 100
In this evaluation, the oxidation resistance was determined to be good when the Fe concentration was 10 mass % or less.

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

As shown in Table 3, in Examples 1 to 6 in which the average crystal orientation difference, the composition of the base material, the grain boundary Cr concentration of the weld metal part, and the Al concentration in the oxide film all fell within the specified ranges, the corrosion resistance of the weld metal part, the oxidation resistance of the base material, and the dimensional accuracy were good.
In contrast, in Comparative Examples 1 and 3, the Al concentration in the oxide film and the grain boundary Cr concentration in the weld metal were too low, so the corrosion resistance of the weld metal and the oxidation resistance of the base metal were insufficient.
In Comparative Example 2, the grain boundary Cr concentration in the weld metal part was too low, and therefore the corrosion resistance of the weld metal part was insufficient.
In Comparative Examples 4 and 5, the average crystal orientation difference and the grain boundary Cr concentration were too low, so the corrosion resistance and dimensional accuracy of the weld metal were insufficient. In Comparative Example 5, the average crystal grain size of the base metal was large, and the toughness of the base metal was also low.
In Comparative Example 6, the Al concentration in the oxide film was too low, and therefore the oxidation resistance of the base material was insufficient.In Comparative Example 7, the Al content in the base material was too low, and therefore the Al concentration in the oxide film was low, and therefore the oxidation resistance of the base material was insufficient.

As can be seen from the above results, the present invention can provide a ferritic stainless steel welded structure that has excellent high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy.

Claims (6)

A ferritic stainless steel welded structure used in applications requiring high-temperature oxidation resistance, corrosion resistance, and dimensional accuracy,
Includes base material and weld metal parts,
The base material contains, by mass, C: 0.050% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.00 to 24.00%, N: 0.050% or less, Cu: 1.00% or less, Mo: 1.00% or less, Si: 3.00% or less, Al: 0.80 to 5.00%, Nb: 0.50% or less, Ti: 0.50% or less, 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 an average crystal orientation difference of 0.15° or more,
The weld metal portion has a grain boundary Cr concentration of 10 mass% or more,
The ferritic stainless steel welded structure has an oxide film on its surface that contains 30 mass % or more of Al. The ferritic stainless steel welded structure according to claim 1, wherein the base material further contains at least one selected from Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, and W: 1.00% or less, by mass. The ferritic stainless steel welded structure according to claim 1 or 2, wherein the base material further contains at least one selected from REM: 0.10% or less and Ca: 0.10% or less, by mass. The ferritic stainless steel welded structure according to any one of claims 1 to 3, wherein the base material further contains at least one selected from Sn: 0.10% or less and B: 0.0100% or less by mass. A ferritic stainless steel welded structure according to any one of claims 1 to 4, wherein the base material has an average crystal grain size of 100 μm or less. The ferritic stainless steel welded structure according to any one of claims 1 to ⁵ , wherein the base material has a Charpy impact value of 100 J/cm2 or more.