JP7688985B2 - Ferritic stainless steel material and its manufacturing method, welded member and its manufacturing method, and vibration damping member - Google Patents

Description

The present invention relates to ferritic stainless steel materials and their manufacturing methods, welded components and their manufacturing methods, and vibration-damping components.

Electric vehicles are quieter because they do not have the noise and vibration caused by engines. Therefore, gasoline-powered vehicles are also required to be quieter to match the level of electric vehicles.
Noise and vibration in gasoline-powered vehicles are often amplified by resonance in exhaust system components such as the exhaust pipe, muffler, etc. Therefore, it is desirable for exhaust system components to be made of materials with vibration-damping properties in order to suppress the amplification of noise and vibration caused by resonance.

Typical examples of materials with vibration-damping properties include rubber and resin, but when rubber and resin are used in high-temperature environments (e.g., 400-750°C) such as exhaust system components, they often lack sufficient properties such as strength, corrosion resistance, and heat resistance. For this reason, there is a need for metal materials with vibration-damping properties that can be used in high-temperature environments.

Metal materials with vibration-damping properties are broadly classified into composite, ferromagnetic, dislocation, and twin crystal types based on the mechanism by which vibration energy is attenuated. Each type has its own advantages and disadvantages, but for use in high-temperature environments, the ferromagnetic type, which has good strength and heat resistance, is preferred. In the ferromagnetic type, the magnetic domains are rearranged in one direction when an external force such as vibration is applied, and when the load is removed, the magnetic domains are rearranged randomly. The residual distortion at this time absorbs vibration energy and can attenuate the vibration.

As a ferromagnetic metal material, for example, a vibration-damping ferritic stainless steel material has been proposed that contains, by mass%, C: 0.001-0.04%, Si: 0.1-2.0%, Mn: 0.1-1.0%, Ni: 0.01-0.6%, Cr: 10.5-20.0%, Al: 0.5-5.0%, N: 0.001-0.03%, Ti: 0.5% or less, Cu: 0.3% or less, Mo: 0.3% or less, with the balance being Fe and unavoidable impurities, has a metal structure in which the matrix is a single ferrite phase, the ferrite crystal grains have an average crystal grain size of 0.3-3.0 mm, and has a residual magnetic flux density of 45 mT or less (Patent Document 1).

Patent No. 6370276

The ferritic stainless steel material of Patent Document 1 exhibits vibration-damping properties by enlarging the ferrite crystal grains and controlling the average crystal grain size to 0.3 to 3.0 mm. However, in this ferritic stainless steel material, the amount of dissolved C and N is not controlled within an appropriate range, so there is a risk that C and N will combine with Cr in a high-temperature environment to precipitate Cr carbides and Cr nitrides. These precipitates not only induce sensitization that removes surrounding Cr and reduces corrosion resistance, but also significantly reduce toughness because Cr carbides in particular preferentially precipitate at grain boundaries.

The present invention has been made to solve the above problems, and aims to provide a ferritic stainless steel material and its manufacturing method, a welded member and its manufacturing method, and a vibration-damping member that are excellent in toughness and vibration-damping properties in high-temperature environments.

The inventors conducted extensive research to solve the above problems and discovered that by controlling the composition of the ferritic stainless steel material, the total amount of C and N in solid solution, and the average crystal grain size, it is possible to improve vibration damping while suppressing a decrease in toughness in high-temperature environments. The inventors also discovered that by adjusting the heat treatment (recrystallization treatment) and the subsequent cooling conditions, it is possible to control the total amount of C and N in solid solution, and the average crystal grain size. The present invention was completed based on these findings.

That is, the present invention provides a ferritic stainless steel material used for exhaust system members,
The composition includes, on a mass basis, C: 0.100% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.50 to 24.00%, N: 0.030% or less, Cu: 1.00% or less, Mo: 2.50% or less, Si: 3.00% or less, Al: 1.00 to 5.00%, Nb: 0.01 to 0.50%, and Ti: 0.01 to 0.50%, 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 0.20% or more, and the balance is Fe and impurities, and the total solid solution amount of C and N is 0.015% or less,
The ferritic stainless steel has an average crystal grain size of 100 μm or more.

The present invention also provides a welded member used in an exhaust system, in which metal materials are welded together, comprising:
At least one of the metal materials is a welded member that is the ferritic stainless steel material.

The present invention also provides a method for producing a ferritic stainless steel material used for exhaust system members, comprising the steps of:
Based on mass, C: 0.100% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.50 to 24.00%, N: 0.0 30% or less, Cu: 1.00% or less, Mo: 2.50% or less, Si: 3.00% or less, Al: 1.00 to 5.00%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.50%. The method for producing a ferritic stainless steel material comprises heating a ferritic stainless steel plate having a composition in which 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 0.20% or more, with the balance being Fe and impurities, at 900°C or more for 10 minutes or more, and then cooling the plate while maintaining the temperature in the 900 to 750°C range for 5 minutes or more.

The present invention also provides a method for manufacturing a welded component used in an exhaust system, in which metal materials are welded together, comprising the steps of:
At least one of the metal materials is
(i) the ferritic stainless steel material; or (ii) a ferritic stainless steel material having a composition comprising, on a mass basis, C: 0.100% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.50 to 24.00%, N: 0.030% or less, Cu: 1.00% or less, Mo: 2.50% or less, Si: 3.00% or less, Al: 1.00 to 5.00%, Nb: 0.01 to 0.50%, and Ti: 0.01 to 0.50%, 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 0.20% or more, with the balance being Fe and impurities;
This is a method for manufacturing a welded member, in which the metal materials are welded together, heated at 900° C. or higher for 5 minutes or more, and then cooled while being held in a temperature range of 900 to 750° C. for 5 minutes or more.

Furthermore, the present invention provides a vibration damping member for use in an exhaust system, comprising:
The vibration-damping member includes the ferritic stainless steel material or the welded member.

The present invention provides a ferritic stainless steel material and its manufacturing method, a welded member and its manufacturing method, and a vibration-damping member that are excellent in toughness and vibration-damping properties in high-temperature environments.

The present invention was completed based on the following viewpoints through research focusing on ferromagnetic ferritic stainless steel (hereinafter abbreviated as "stainless steel") material having good strength and heat resistance.
The vibration damping properties of ferromagnetic stainless steel materials are determined by the amount of deformation (magnetostriction) that occurs when magnetic domains move. Therefore, the amount of magnetostriction is increased by adding Al to the composition of the stainless steel material. Also, Si is added to improve the magnetic properties (i.e., to make magnetic domain movement easier).
In order for the stainless steel material to exhibit vibration-damping properties, it is necessary for the magnetic domains to be able to move freely. However, since the strain (dislocation), precipitates, and grain boundaries in the stainless steel material hinder the movement of the magnetic domains, heat treatment is required to reduce these. In the manufacturing process of stainless steel material, heat treatment (annealing treatment) is generally intended to remove strain, whereas heat treatment for exhibiting vibration-damping properties requires the growth of crystal grains to suppress the inhibition of magnetic domain movement by grain boundaries. Therefore, heat treatment for exhibiting vibration-damping properties needs to be performed at a higher temperature and for a longer period of time than annealing treatment. On the other hand, excessive growth of crystal grains reduces toughness, making it difficult to use in applications where stress loads due to vibration or impacts are applied. Therefore, it is necessary to select heating conditions that can exhibit vibration-damping properties while suppressing the decrease in toughness.

Furthermore, when stainless steel materials are used in high-temperature environments, if the stainless steel contains a large amount of dissolved C and N, they combine with Cr to precipitate Cr carbides and Cr nitrides. These precipitates not only cause sensitization, which reduces corrosion resistance by removing surrounding Cr, but also cause a significant decrease in toughness, as Cr carbides in particular preferentially precipitate at grain boundaries. In this specification, the term "high-temperature environment" primarily refers to a temperature environment of 400 to 750°C.
Therefore, in order to reduce the amount of dissolved C and N and suppress sensitization and a decrease in toughness in a high-temperature environment, the contents of C and N are reduced as much as possible, and Ti and Nb, which preferentially form compounds with C and N, are added in amounts several times that of C and N, so that carbides and nitrides of Ti and Nb (hereinafter sometimes abbreviated as "carbonitrides") are preferentially precipitated. That is, by precipitating Ti and Nb carbonitrides, the amount of dissolved C and N is reduced, making it difficult for Cr carbides to be generated in a high-temperature environment.

On the other hand, in stainless steel materials containing Al, the precipitation rate of carbonitrides of Ti and Nb is slow, and it is difficult to reduce the amount of solute C and N under normal cooling conditions after heat treatment. Therefore, it is necessary to select cooling conditions that allow sufficient precipitation of carbonitrides of Ti and Nb to reduce the amount of solute C and N.
Furthermore, when stainless steel materials are welded, the welded portion is likely to have a high solid solution amount of C and N. Therefore, in this case, it is necessary to perform heat treatment after welding and to cool under cooling conditions that allow sufficient precipitation of carbonitrides of Ti and Nb to reduce the solid solution amounts of C and N.

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.

(1) Stainless Steel Material The stainless steel material according to an embodiment of the present invention 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: 10.50 to 24.00%, N: 0.030% or less, Cu: 1.00% or less, Mo: 2.50% or less, Si: 3.00% or less, Al: 1.00 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 the 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. Furthermore, the term "stainless steel material" is a concept that includes various shapes such as stainless steel strips and stainless steel plates.

In addition, the stainless steel material according to the embodiment of the present invention 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.
Furthermore, the stainless steel material according to the embodiment of the present invention may further contain at least one selected from REM: 0.100% or less and Ca: 0.100% or less.
Furthermore, the stainless steel material according to the embodiment of the present invention may further contain at least one selected from Sn: 0.100% or less and B: 0.0100% or less.

(C: 0.100% or less)
C is an element that affects the properties of stainless steel materials, such as intergranular corrosion resistance (sensitization suppression effect) and workability. If the C content is too high, the workability and intergranular corrosion resistance of the 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 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 and toughness of stainless steel materials. 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 stainless steel materials, such as weldability and workability. If the P content is too high, the above properties may be deteriorated. 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 is a corrosion starting point, and affects the properties of stainless steel materials, such as toughness. If the S content is too high, the above properties may be reduced. 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: 10.50-24.00%)
Cr is an element effective in improving the corrosion resistance and oxidation resistance of stainless steel materials. If the Cr content is too high, the toughness of the stainless steel 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 10.50%, preferably 10.80%, and more preferably 11.00%.

(N: 0.030% or less)
N is an element that affects the properties of stainless steel materials, such as intergranular corrosion resistance (sensitization suppression effect) and workability. If the N content is too high, the workability and intergranular corrosion resistance of the stainless steel material will decrease. Therefore, the upper limit of the N content is 0.030%, preferably 0.028%, and more preferably 0.025%. 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 preferably 0.001%.

(Cu: 1.00% or less)
Cu is an element effective for improving the corrosion resistance of stainless steel materials. 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: 2.50% or less)
Mo is an element effective for improving the corrosion resistance and oxidation resistance of stainless steel materials. If the Mo content is too high, the workability of the stainless steel material decreases and the manufacturing cost increases. Therefore, the upper limit of the Mo content is 2.50%, preferably 2.00%, and more preferably 1.50%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.001%, and preferably 0.005%.

(Si: 3.00% or less)
Si is an element effective for improving the vibration-damping property and corrosion resistance of stainless steel materials. If the Si content is too high, the workability and toughness of the stainless steel material will decrease. 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 stably improving the vibration-damping property of the stainless steel material.

(Al: 1.00-5.00%)
Like Si, Al is an element effective in improving the vibration damping and corrosion resistance of stainless steel materials. If the Al content is too high, the toughness of the stainless steel material decreases. Therefore, the Al content is 5.00%, preferably 4.50%, and more preferably 4.00%. On the other hand, the lower limit of the Al content is 1.00%, preferably 1.20%, and more preferably 1.40%, from the viewpoint of obtaining the above-mentioned effects.
From the viewpoint of stably improving the vibration damping properties of the stainless steel material, the total content of Al and Si is preferably 1.00% or more, more preferably 1.20% or more, and even more preferably 1.50% or more.

(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 stainless steel materials, such as intergranular corrosion resistance (sensitization suppression effect).
If the Nb content is too high, the workability and toughness of the stainless steel material 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 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 stainless steel materials. If the content of Zr, Co, V and W is too high, the workability and toughness of the stainless steel material will decrease, and the manufacturing cost will increase. Therefore, the upper limit of the content of Zr, Co, V and W is 1.00%, preferably 0.80%, and 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%, and more preferably 0.01%.

(REM: 0.100% or less, Ca: 0.100% or less)
REM (rare earth elements) and Ca are elements effective for improving the oxidation resistance of stainless steel materials. If the content of REM and Ca is too high, it leads to an increase in the manufacturing cost of stainless steel materials. Therefore, the upper limit value of the content of REM and Ca is 0.100%, preferably 0.080%, and more preferably 0.050%. On the other hand, the lower limit value 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: 0.100% or less)
Sn is an element effective for improving the corrosion resistance of stainless steel materials. If the Sn content is too high, Sn segregates and manufacturability decreases. Therefore, the upper limit of the Sn content is 0.100%, preferably 0.080%, and more preferably 0.050%. 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 stainless steel materials. If the B content is too high, the fatigue strength of the 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%.

In the stainless steel material according to the embodiment of the present invention, the total amount of solute C and N is 0.015% or less, preferably 0.014% or less. By making the total amount of solute C and N 0.015% or less, it is possible to suppress the precipitation of Cr carbides and Cr nitrides due to 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 stainless steel material in a high-temperature environment and suppress sensitization.
The lower limit of the total amount of C and N in solid solution 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.

The stainless steel material according to the embodiment of the present invention has an average crystal grain size of 100 μm or more, preferably 120 μm or more, and more preferably 150 μm or more. By making the average crystal grain size 100 μm or more, the crystal grain boundaries that hinder the movement of magnetic domains that are effective in expressing vibration-damping properties are reduced, thereby improving vibration-damping properties.
The upper limit of the average crystal grain size is not particularly limited, but is preferably 500 μm, more preferably 450 μm, and even more preferably 400 μm. If the upper limit is set to such a value, the deterioration of toughness due to the extreme coarsening of the crystal grains can be stably suppressed.
In this specification, the average crystal grain size refers to one measured by the method described in the Examples section below.

The stainless steel material according to the embodiment of the present invention has a loss factor η of preferably 5.0×10 ⁻⁴ or more, more preferably 1.0×10 ⁻³ or more. By setting the loss factor η in such a range, it is possible to ensure the desired vibration damping properties.
The upper limit of the loss factor η is not particularly limited, but is generally 10.0×10 ⁻³ , and preferably 5.0×10 ⁻³ .
In this specification, the loss factor η means a value measured by the method described in the examples described later.

The stainless steel material according to the embodiment of the present invention has an absorbed energy in a Charpy impact test after heating at 600° C. for 100 hours in an air atmosphere (hereinafter referred to as "Charpy impact value") of 10 J/cm ² or more, preferably 12 J/cm ² or more. By setting the Charpy impact value in this range, the desired toughness can be ensured.
The upper limit of the Charpy impact value is not particularly limited, but is generally 300 J/cm ² , and preferably 280 J/cm ² .
In this specification, the Charpy impact value means a value measured by the method described in the examples described later.

The thickness of the stainless steel material according to the embodiment of the present invention is not particularly limited, but is preferably 3.0 mm or less, more preferably 2.8 mm or less, and even more preferably 2.5 mm or less. The thickness of this stainless steel material is preferably 0.2 mm or more, and more preferably 0.3 mm or more.

(2) Manufacturing method of stainless steel material The manufacturing method of the stainless steel material according to the embodiment of the present invention can be carried out in accordance with a method known in the art using a stainless steel sheet having the above-mentioned composition. A typical manufacturing method of the stainless steel material according to the embodiment of the present invention will be described below.

The manufacturing method of the stainless steel material according to the embodiment of the present invention is carried out by heating a stainless steel plate having the above composition at 900°C or higher for 10 minutes or more, and then cooling it by holding it in the temperature range of 900 to 750°C for 5 minutes or more.

The stainless steel sheet having the above composition can be manufactured by a conventional method. Specifically, first, the stainless steel having the above composition is melted and forged or cast, and then hot-rolled to obtain a hot-rolled sheet. Next, the hot-rolled sheet is annealed, pickled, and cold-rolled in order to obtain a cold-rolled sheet. Next, the cold-rolled sheet is annealed and pickled in order to obtain a cold-rolled annealed sheet. The conditions in each step can be appropriately adjusted according to the composition of the stainless steel, and are not particularly limited. The hot-rolled sheet, cold-rolled sheet, or cold-rolled annealed sheet manufactured by such a method can be used as the stainless steel sheet. Among them, the stainless steel sheet is preferably a cold-rolled annealed sheet.

The stainless steel sheet may be processed into a desired component before heat treatment (recrystallization treatment), or may be heat treated while still in plate or coil form. Processing methods include various press processes using dies, mechanical processes such as bending, etc.

The heat treatment (recrystallization treatment) of the stainless steel sheet is carried out by heating at 900°C or higher, preferably 1000°C or higher, for 10 minutes or more. By heating at 900°C or higher for 10 minutes or more, it is possible to grow the crystal grains so that the average crystal grain size is 100 μm or more. The upper limit of the heating temperature is not particularly limited, but since excessive growth of the crystal grains may reduce the toughness, it is preferably 1200°C, more preferably 1180°C. The upper limit of the heating time may be appropriately adjusted depending on the heating temperature, but is generally 120 minutes.
The heat treatment can be carried out, for example, by using a heating furnace, which may be of a batch type or a continuous type.
In addition, since this stainless steel sheet has oxidation resistance, the heat treatment atmosphere may be an air atmosphere or a non-oxidizing atmosphere (e.g., a vacuum or hydrogen gas atmosphere), etc. From the viewpoint of preventing the oxide film from growing excessively, it is preferable to perform the heat treatment in a non-oxidizing atmosphere.

Cooling after the heat treatment (recrystallization treatment) is performed by holding the temperature in the range of 900 to 750°C for 5 minutes or more. By cooling under such conditions, Ti and Nb carbonitrides can be preferentially precipitated, making it possible to reduce the amount of C and N dissolved in solid solution. If the material is held at a temperature higher than this temperature range, Ti and Nb carbonitrides will not precipitate. Also, if the material is held at a temperature lower than this temperature range, Cr carbide will precipitate, resulting in a decrease in toughness.
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.

(3) Welded member In the welded member according to the embodiment of the present invention, metal materials are welded together, and at least one of the metal materials is the above-mentioned stainless steel material. Since the above-mentioned stainless steel material has excellent toughness and vibration damping properties in a high-temperature environment, the welded member also has excellent toughness and vibration damping properties in a high-temperature environment.

The metal material may be a metal material different from the stainless steel material described above, but it is preferable that both of the metal materials be the stainless steel materials described above. By using the stainless steel materials as both of the metal materials to be welded, the toughness and vibration damping properties in a high-temperature environment can be further improved.
The method for welding the metal materials together is not particularly limited, and any method known in the art, such as arc welding (TIG welding, etc.), electron beam welding, laser welding, plasma arc welding, spot welding, etc., can be used. It is preferable that the welding does not use a filler metal.

(4) Manufacturing Method of Welded Member A manufacturing method of a welded member according to an embodiment of the present invention can be performed in accordance with a method known in the art. A typical manufacturing method of a welded member according to an embodiment of the present invention will be described below.

The manufacturing method of a welded member according to an embodiment of the present invention is performed by welding metal materials together, heating them at 900°C or higher for 5 minutes or more, and then cooling them by holding them in a temperature range of 900 to 750°C for 5 minutes or more.
At least one of the metal materials is the above-mentioned (i) stainless steel material or the above-mentioned (ii) stainless steel plate.

The other of the metal materials may be a metal material different from the (i) stainless steel material and (ii) stainless steel plate, but it is preferable that both are (i) stainless steel material or (ii) stainless steel plate. For example, both of the metal materials may be (i) stainless steel material, both of the metal materials may be (ii) stainless steel plate, or one of the metal materials may be (i) stainless steel material and the other of the metal materials may be (ii) stainless steel plate.
The method for welding the metal materials together is not particularly limited, and the above-mentioned methods can be used.

(ii) Stainless steel sheet can be manufactured by a conventional method. Specifically, first, stainless steel having the above composition is melted and forged or cast, and then hot-rolled to obtain a hot-rolled sheet. Next, the hot-rolled sheet is annealed, pickled, and cold-rolled in order to obtain a cold-rolled sheet. Next, the cold-rolled sheet is annealed and pickled in order to obtain a cold-rolled annealed sheet. The conditions in each step can be appropriately adjusted according to the composition of the stainless steel, and are not particularly limited. The hot-rolled sheet, cold-rolled sheet, or cold-rolled annealed sheet manufactured by such a method can be used as the stainless steel sheet. Among them, the stainless steel sheet is preferably a cold-rolled annealed sheet.

(ii) The stainless steel sheet may be processed into a specified component before heat treatment (recrystallization treatment), or may be heat treated while still in plate or coil form. Processing methods include various press processes using dies, mechanical processes such as bending, etc.

When (ii) stainless steel plate is used as the metal material, the crystal grains in the stainless steel plate are small, and therefore the vibration-damping properties are insufficient as is. Therefore, it is necessary to improve the vibration-damping properties by growing the crystal grains through a heat treatment (recrystallization treatment).
In addition, when metal materials are welded together, sufficient amounts of Ti and Nb carbonitrides do not precipitate during the cooling process of the weld, and the amount of solid solution of C and N increases, so that Cr carbides precipitate during use in a high-temperature environment, and the toughness is likely to decrease. Therefore, it is necessary to intentionally precipitate Ti and Nb carbonitrides and reduce the amount of solid solution of C and N by controlling the cooling conditions after heat treatment, thereby improving the toughness.

The heat treatment (recrystallization treatment) is performed by heating at 900° C. or higher, preferably 1000° C. or higher, for 5 minutes or more. By heating at 900° C. or higher for 5 minutes or more, crystal grains can be grown so that the average crystal grain size is 100 μm or more. The upper limit of the heating temperature is not particularly limited, but is preferably 1200° C., more preferably 1180° C., since excessive growth of crystal grains may reduce toughness.
In addition, when (i) stainless steel material is used as the metal material, since the crystal grains are in a grown state, it is only necessary to re-dissolve the Cr carbides precipitated in the welded portion. Therefore, the heating time may be shorter than the heating time described in (2) Stainless steel material manufacturing method. The upper limit of the heating time may be appropriately adjusted depending on the heating temperature, but is generally 120 minutes.
The heat treatment can be carried out, for example, by using a heating furnace, which may be of a batch type or a continuous type.
In addition, since this stainless steel sheet has oxidation resistance, the heat treatment atmosphere may be an air atmosphere or a non-oxidizing atmosphere (e.g., a vacuum or hydrogen gas atmosphere), etc. From the viewpoint of preventing the oxide film from growing excessively, it is preferable to perform the heat treatment in a non-oxidizing atmosphere.

Cooling after the heat treatment (recrystallization treatment) is performed by holding the temperature in the range of 900 to 750°C for 5 minutes or more. By cooling under such conditions, Ti and Nb carbonitrides can be preferentially precipitated, making it possible to reduce the amount of C and N dissolved in solid solution. If the material is held at a temperature higher than this temperature range, Ti and Nb carbonitrides will not precipitate. Also, if the material is held at a temperature lower than this temperature range, Cr carbide will precipitate, resulting in a decrease in toughness.
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.

(5) Vibration-damping member The vibration-damping member according to the embodiment of the present invention includes the above-mentioned stainless steel material and the above-mentioned welded member. Since the above-mentioned stainless steel material and the above-mentioned welded member have excellent toughness and vibration-damping properties in a high-temperature environment, the vibration-damping member also has excellent toughness and vibration-damping properties in a high-temperature environment.
Examples of the vibration-damping member are not particularly limited, but are preferably various members used in high-temperature environments, such as exhaust system members such as exhaust pipes and mufflers.

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 5)
A ferritic stainless steel material was prepared according to the following procedure.
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 950 to 1050°C and pickled to obtain a cold rolled annealed sheet. Next, a test piece having a width direction of 50 mm and a rolling direction of 300 mm was cut out from the cold rolled annealed sheet by cutting.

The above test pieces were placed in an EREMA electric furnace and heat-treated in air at 1100°C for a specified time. The heating time is shown in Table 2. After heat treatment, the test pieces were cooled by controlling the cooling rate. The cooling conditions (holding time and cooling rate) in the temperature range of 900 to 750°C are shown in Table 2. The test pieces were cooled by introducing nitrogen gas for cooling into the electric furnace. In Comparative Example 2, the test pieces were cooled by removing them from the electric furnace and cooling them with water.

Next, the test pieces of Example 2 and Comparative Examples 3 and 4 were welded. The test pieces were fused in the width direction center in the same manner as TIG spot welding (but no welding was performed) by pseudo welding. The welding conditions were a welding current of 90 A, a welding speed of 60 cm/min, and a welding electrode diameter of 1.6 mm.
Next, the welded test pieces of Example 2 and Comparative Example 4 were placed in an EREMA electric furnace and heat-treated in an air atmosphere at 1050°C for 5 minutes. After the heat treatment, the welded test pieces were cooled by controlling the cooling rate. The cooling conditions (holding time and cooling rate) in the temperature range of 900 to 750°C are shown in Table 2.
The test pieces and welded test pieces obtained above were subjected to the following evaluations.

(Average grain size)
The above test pieces and welded test pieces were cut into 10 mm x 10 mm test pieces by cutting, and then resin-filled 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 pieces were 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.

(Total amount of C and N in solid solution)
First, the amounts of Ti and Nb carbonitrides formed were measured by the following procedure.
The above test pieces and welded test pieces were electrolytically etched using the SPEED method. The above test pieces and welded test pieces were electrolyzed in a 10% acetylacetone solution until a constant potential electric field of 400 mV reached 5000 coulombs, and the solution after electrolysis was filtered through a filter with a lattice size 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 consumed C+N [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]
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 %].

(Vibration damping: loss factor η)
From the above test pieces and welded test pieces, test pieces for measurement, each having a width direction of 10 mm and a rolling direction of 250 mm, were cut out by cutting. Using these test pieces for measurement, the loss factor η was measured in accordance with the "central vibration method" specified in JIS K7391:2008. Specifically, the test pieces with the central part fixed were vibrated by an impedance head, and the mechanical impedance was derived from the output force signal and acceleration vibration. Then, the loss factor η was derived based on the anti-resonance frequency at which the mechanical impedance peaked and the frequency at which the amplitude was 3 dB lower than the peak. In this evaluation, it was determined that the vibration damping property was good when the loss factor η was 5×10 ⁻⁴ or more.

(Toughness)
Test pieces having a width of 50 mm and a length of 100 mm were cut out from the above test pieces and welded test pieces by cutting so that the longitudinal direction was perpendicular to the rolling direction.
Next, the test piece for measurement was heated in an EREMA electric furnace in an air atmosphere at 600° C. for 100 hours.
Next, a V-notch (notch angle 45°, notch depth 2 mm, notch bottom radius 0.25 mm) was cut in the longitudinal center of the heat-treated test specimen. For the welded test specimen, the V-notch was made to be the welded 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, the toughness was determined to be good when the Charpy impact absorption energy per unit area (Charpy impact value) was 10 J/cm2 or ^more .
The results of the above evaluations are shown in Table 2.

As shown in Table 2, Examples 1 to 6, in which the composition, the total amount of solid solution of C and N, and the average crystal grain size satisfy the predetermined ranges, have a Charpy impact value of 10 J/cm2 or ^more and a loss factor η of 5 × ^10-4 or more, and are confirmed to have excellent toughness and vibration damping properties in high-temperature environments.
In contrast, in Comparative Example 1, the heating time was too short, so the crystal grains did not grow sufficiently, resulting in a small average crystal grain size. As a result, the loss factor η was low and the vibration damping properties were insufficient. In addition, Comparative Example 1 also had a low Charpy impact value and insufficient toughness.
In Comparative Example 2, the holding time in the temperature range of 900 to 750°C was too short, so that carbonitrides of Ti and Nb could not be sufficiently precipitated, and the total amount of solid solution of C and N increased. As a result, the Charpy impact value was low and the toughness was insufficient.
In Comparative Example 3, since no heat treatment was performed after welding, Cr carbides were formed in the welded part, and the total amount of dissolved C and N was large. As a result, the Charpy impact value was low and the toughness was insufficient.
In Comparative Example 4, although heat treatment was performed after welding, the holding time in the temperature range of 900 to 750°C during subsequent cooling was too short, so that carbonitrides of Ti and Nb could not be sufficiently precipitated, resulting in a large total amount of solid solution of C and N. As a result, the Charpy impact value was low and the toughness was insufficient.
In Comparative Example 5, the Al content was too low, so the loss factor η was low and the vibration damping properties were insufficient.

As can be seen from the above results, the present invention can provide a ferritic stainless steel material and its manufacturing method, a welded member and its manufacturing method, and a vibration-damping member that are excellent in toughness and vibration-damping properties in high-temperature environments.

Claims (16)

A ferritic stainless steel material used for exhaust system members,
The composition includes, on a mass basis, C: 0.100% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.50 to 24.00%, N: 0.030% or less, Cu: 1.00% or less, Mo: 2.50% or less, Si: 3.00% or less, Al: 1.00 to 5.00%, Nb: 0.01 to 0.50%, and Ti: 0.01 to 0.50%, 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 0.20% or more, and the balance is Fe and impurities, and the total solid solution amount of C and N is 0.015% or less,
A ferritic stainless steel material having an average crystal grain size of 100 μm or more. The ferritic stainless steel material according to claim 1, further comprising 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 material according to claim 1 or 2, further comprising at least one selected from REM: 0.100% or less and Ca: 0.100% or less, by mass. The ferritic stainless steel material according to any one of claims 1 to 3, further comprising at least one selected from Sn: 0.100% or less and B: 0.0100% or less, by mass. A welded member used in an exhaust system, in which metal materials are welded together,
A welded member, wherein at least one of the metal materials is the ferritic stainless steel material according to any one of claims 1 to 4. A method for producing a ferritic stainless steel material used for exhaust system members, comprising the steps of:
By mass, C: 0.100% or less, Mn: 1.00% or less, Ni: 1.00% or less, P: 0.100% or less, S: 0.050% or less, Cr: 10.50 to 24.00%, N: 0.030% or less, Cu: 1.00% or less, Mo: 2.50% or less, Si: 3.00% or less, Al: 1.00 to 5.00%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.50% The present invention relates to a method for producing a ferritic stainless steel material, comprising heating a ferritic stainless steel plate having a composition in which the total content of Nb and Ti is 6(C+N) or more (C and N represent the content of C and N, respectively) and 0.20% or more, with the balance being Fe and impurities, at 900°C or more for 10 minutes or more, and then cooling the plate while maintaining the temperature in the 900-750°C range for 5 minutes or more. The method for producing ferritic stainless steel material according to claim 6, in which the cooling rate in the temperature range of 900 to 750°C is 30°C/min or less. The method for producing a ferritic stainless steel material according to claim 6 or 7, wherein the ferritic stainless steel sheet 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 method for producing a ferritic stainless steel material according to any one of claims 6 to 8, wherein the ferritic stainless steel sheet further contains at least one selected from REM: 0.100% or less and Ca: 0.100% or less, by mass. The method for producing a ferritic stainless steel material according to any one of claims 6 to 9, wherein the ferritic stainless steel sheet further contains at least one selected from Sn: 0.100% or less and B: 0.0100% or less, by mass. A method for manufacturing a welded component used in an exhaust system, in which metal materials are welded together, comprising the steps of:
At least one of the metal materials is
(i) A ferritic stainless steel material according to any one of claims 1 to 4; or (ii) a ferritic stainless steel plate having a composition, on a mass basis, containing 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: 10.50 to 24.00%, N: 0.030% or less, Cu: 1.00% or less, Mo: 2.50% or less, Si: 3.00% or less, Al: 1.00 to 5.00%, Nb: 0.01 to 0.50%, and Ti: 0.01 to 0.50%, 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 0.20% or more, with the balance being Fe and impurities;
A method for manufacturing a welded member, comprising: welding the metal materials together; heating the materials at 900°C or higher for 5 minutes or more; and then cooling the materials by holding the materials in a temperature range of 900 to 750°C for 5 minutes or more. The method for manufacturing a welded member according to claim 11, in which the cooling rate in the temperature range of 900 to 750°C is 30°C/min or less. The method for manufacturing a welded member according to claim 11 or 12, wherein the ferritic stainless steel plate 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 method for manufacturing a welded member according to any one of claims 11 to 13, wherein the ferritic stainless steel plate further contains at least one selected from REM: 0.100% or less and Ca: 0.100% or less, by mass. The method for manufacturing a welded member according to any one of claims 11 to 14, wherein the ferritic stainless steel plate further contains at least one selected from Sn: 0.100% or less and B: 0.0100% or less, by mass. A vibration-damping member for use in an exhaust system,
A vibration-damping member comprising the ferritic stainless steel material according to any one of claims 1 to 4 or the welded member according to claim 5.