JP7716177B2 - Ferritic stainless steel material and method for manufacturing vibration-damping member - Google Patents

Description

The present invention relates to ferritic stainless steel materials and vibration-damping members.

As automobiles become more electric, engine noise and vibrations are eliminated, improving the quietness of the vehicle interior. As a result, noises that were previously buried in engine noise and high-frequency sounds specific to electric vehicles are now more easily perceived by passengers as abnormal sounds, and the level of vibration-damping required for materials used in automobiles is increasing.
Furthermore, in electronic devices such as hard disks, vibrations can cause malfunctions and breakdowns, so materials used in electronic device components are also required to have vibration-damping properties.

Resins are a typical example of materials with vibration-damping properties, but they are often difficult to use due to their strength, environmental resistance (corrosion resistance and heat resistance), heat dissipation, etc. Therefore, there is a demand for metallic materials with vibration-damping properties.
Metallic materials with vibration-damping properties are broadly classified into composite, ferromagnetic, dislocation, and twin crystal types based on the vibration energy damping mechanism. Among these, ferritic stainless steel is a ferromagnetic material and has a ferromagnetic damping mechanism. In ferromagnetic materials, 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 strain generated at this time absorbs vibration energy and damps the vibration.

Ferritic stainless steel materials with excellent vibration damping properties include: C: 0.001-0.03 mass%, Si: 0.1-1.0 mass%, Mn: 0.1-2.0 mass%, Ni: 0.01-0.6 mass%, Cr: 10.5-24.0 mass%, N: 0.001-0.03 mass%, Nb: 0-0.8 mass%, Ti: 0-0.5 mass%, Cu: 0-2.0 mass%, Mo: 0-2.5 mass%, V: 0-1.0 mass%, Al: 0-0. A vibration-damping ferritic stainless steel material has been proposed that has a chemical composition of 0.3 mass% of Ni, 0-0.3 mass% of Zr, 0-0.6 mass% of Co, 0-0.1 mass% of REM, 0-0.1 mass% of Ca, and the balance being Fe and unavoidable impurities, a metal structure in which the matrix is a single ferrite phase and the average grain size of the ferrite crystal grains is 0.3-3.0 mm, and a residual magnetic flux density of 45 mT or less (Patent Document 1).

JP 2017-39955 A

However, the ferritic stainless steel material described in Patent Document 1 requires long-term high-temperature recrystallization treatment (final annealing at 1200°C for 120 minutes or at 950°C for 120 minutes) to improve vibration damping, which results in the problem of being prone to deformation.

The present invention has been made to solve the above-mentioned problems, and aims to provide a ferritic stainless steel material that can suppress thermal deformation while improving vibration damping properties through recrystallization treatment.
Another object of the present invention is to provide a vibration-damping member that is less susceptible to thermal deformation and has excellent vibration-damping properties.

As a result of extensive research aimed at solving the above-mentioned problems, the inventors discovered that by controlling the composition and average grain boundary strain rate of ferritic stainless steel material within a specified range, it is possible to shorten the recrystallization process, thereby achieving both improved vibration damping and suppressed thermal deformation, and thus completed the present invention.

That is, the present invention provides a ferritic stainless steel material containing C: 0.05 mass% or less, Mn: 1.0 mass% or less, Ni: 0.60 mass% or less, P: 0.05 mass% or less, S: 0.03 mass% or less, Cr: 10.5 to 24.0 mass%, N: 0.03 mass% or less, Cu: 0.60 mass% or less, Mo: 2.5 mass% or less, Si: 3.0 mass% or less, Al: 5.0 mass% or less, the balance being Fe and unavoidable impurities, and having an average grain boundary strain rate of 1% or more but less than 15%.

The present invention also provides a method for producing a vibration-damping member, which comprises subjecting the above-mentioned ferritic stainless steel material to recrystallization treatment at 900 to 1100° C. for 5 to 30 minutes .

The present invention provides a ferritic stainless steel material and a vibration-damping component that can enhance vibration-damping properties through recrystallization treatment while suppressing thermal deformation.

Embodiments of the present invention are described in detail below. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, as appropriate and based on the common knowledge of those skilled in the art, fall within the scope of the present invention, provided that they do not deviate from the spirit of the present invention.

A ferritic stainless steel material according to an embodiment of the present invention contains C: 0.05% by mass or less, Mn: 1.0% by mass or less, Ni: 0.60% by mass or less, P: 0.05% by mass or less, S: 0.03% by mass or less, Cr: 10.5 to 24.0% by mass, N: 0.03% by mass or less, Cu: 0.60% by mass or less, Mo: 2.5% by mass or less, Si: 3.0% by mass or less, Al: 5.0% by mass or less, with the balance being Fe and unavoidable impurities.
In this specification, the term "unavoidable impurities" refers to components that are difficult to remove, such as O. Such components are inevitably mixed in during the process of melting raw materials.
Furthermore, the ferritic stainless steel material according to the embodiment of the present invention may further contain at least one selected from Nb: 0.50 mass% or less, Ti: 0.50 mass% or less, Zr: 1.0 mass% or less, Co: 1.0 mass% or less, V: 1.0 mass% or less, W: 1.0 mass% or less, REM: 0.10 mass% or less, Ca: 0.10 mass% or less, Sn: 0.10 mass% or less, and B: 0.01 mass% or less.

C is an element that affects properties such as intergranular corrosion resistance (sensitization suppression) and workability of ferritic stainless steel materials. If the C content is too high, the workability and intergranular corrosion resistance of the ferritic stainless steel material will decrease. Therefore, the upper limit of the C content is controlled to 0.05 mass%, preferably 0.045 mass%, and more preferably 0.04 mass%. On the other hand, the lower limit of the C content is not particularly limited, but reducing the C content leads to increased refining costs. Therefore, the lower limit of the C content is preferably 0.0005 mass%, and preferably 0.001 mass%.

Mn is useful as a deoxidizing element. If the Mn content is too high, it will easily form MnS, which acts as a corrosion starting point, and will destabilize the ferrite phase. Therefore, the upper limit of the Mn content is controlled to 1.0 mass%, preferably 0.9 mass%, and more preferably 0.8 mass%. On the other hand, the lower limit of the Mn content is not particularly limited, but is preferably 0.01 mass%, and more preferably 0.05 mass%.

Ni is an element that is effective in improving the corrosion resistance and weld toughness of ferritic stainless steel materials. If the Ni content is too high, the ferrite phase becomes unstable and production costs increase. Therefore, the upper limit of the Ni content is controlled to 0.60 mass%, preferably 0.58 mass%, and more preferably 0.55 mass%. On the other hand, the lower limit of the Ni content is not particularly limited, but from the perspective of achieving the above effects, it is preferably 0.01 mass%, and more preferably 0.05 mass%.

P is an element that affects the properties of ferritic stainless steel, such as the weldability and workability. If the P content is too high, the above properties may be reduced. Therefore, the upper limit of the P content is controlled to 0.05 mass%, preferably 0.045 mass%, and more preferably 0.04 mass%. On the other hand, the lower limit of the P content is not particularly limited, but reducing the P content leads to increased refining costs. Therefore, the lower limit of the P content is preferably 0.001 mass%, and more preferably 0.01 mass%.

S is an element that generates MnS, which acts as a corrosion initiation site, and affects the toughness and other properties of welds in ferritic stainless steel materials. If the S content is too high, the above properties may be reduced. Therefore, the upper limit of the S content is controlled to 0.03 mass%, preferably 0.025 mass%, and more preferably 0.02 mass%. On the other hand, the lower limit of the S content is not particularly limited, but reducing the S content leads to increased refining costs. Therefore, the lower limit of the S content is preferably 0.0001 mass% or more, and more preferably 0.0005 mass% or more.

Cr is an element that is effective in improving the corrosion resistance and oxidation resistance of ferritic stainless steel materials. If the Cr content is too high, the toughness of the ferritic stainless steel material will decrease and manufacturing costs will increase. Therefore, the upper limit of the Cr content is 24.0 mass%, preferably 23.5 mass%, and more preferably 23.0 mass%. On the other hand, if the Cr content is too low, the above effects may not be fully achieved. Therefore, the lower limit of the Cr content is 10.5 mass%, preferably 11.0 mass%.

N is an element that affects properties such as intergranular corrosion resistance (sensitization suppression) and workability. If the N content is too high, the workability and intergranular corrosion resistance of the ferritic stainless steel material will decrease. Therefore, the upper limit of the N content is controlled to 0.03 mass%, preferably 0.028 mass%, and more preferably 0.025 mass%. On the other hand, the lower limit of the N content is not particularly limited, but reducing the N content leads to increased refining costs. Therefore, the lower limit of the N content is preferably 0.0005 mass%, and preferably 0.001 mass%.

Cu is an element that is effective in improving the corrosion resistance of ferritic stainless steel materials. If the Cu content is too high, the ferrite phase becomes unstable and production costs increase. Therefore, the upper limit of the Cu content is controlled to 0.60 mass%, preferably 0.55 mass%, and more preferably 0.50 mass%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.001 mass%, and more preferably 0.01 mass%.

Molybdenum is an element that is effective in improving the corrosion resistance and oxidation resistance of ferritic stainless steel materials. If the Mo content is too high, the workability of the ferritic stainless steel material decreases and manufacturing costs increase. Therefore, the upper limit of the Mo content is controlled to 2.5 mass%, preferably 2.3 mass%, and more preferably 2.0 mass%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.001 mass%, and more preferably 0.01 mass%.

Si and Al are elements effective in improving the vibration-damping properties of ferritic stainless steel materials. If the Si content is too high, the workability of the ferritic stainless steel material and the toughness of welds will decrease. Therefore, the upper limit of the Si content is controlled to 3.0 mass%, preferably 2.8 mass%, and more preferably 2.5 mass%. Furthermore, if the Al content is too high, the toughness of the ferritic stainless steel material will decrease. Therefore, the upper limit of the Al content is controlled to 5.0 mass%, preferably 4.5 mass%, and more preferably 4.0 mass%. The Si and Al contents are not particularly limited, but from the perspective of consistently improving the vibration-damping properties of ferritic stainless steel materials, the total content of Al and Si is preferably 1.0 mass% or more, more preferably 1.2 mass% or more, and even more preferably 1.5 mass% or more.

Nb and Ti are elements that affect properties such as intergranular corrosion resistance (sensitization suppression). If the Nb content is too high, the workability and toughness of the ferritic stainless steel material will decrease. Therefore, the upper limit of the Nb content is controlled to 0.50 mass%, preferably 0.48 mass%, and more preferably 0.45 mass%. Furthermore, if the Ti content is too high, the workability and surface quality of the ferritic stainless steel material will decrease. Therefore, the upper limit of the Ti content is controlled to 0.50 mass%, preferably 0.48 mass%, and more preferably 0.45 mass%. Meanwhile, the lower limits of the Nb and Ti contents are controlled in relation to the C and N contents, which decrease intergranular corrosion resistance. Specifically, the lower limit of the total Nb and Ti content is controlled to 6 (C + N), preferably 7 (C + N). Here, C and N represent the C and N contents, respectively.

Zr, Co, V, and W are elements effective in improving the oxidation resistance of ferritic stainless steel materials. If the content of Zr, Co, V, and W is too high, the workability and toughness of the ferritic stainless steel material will decrease and manufacturing costs will increase. Therefore, the upper limits of the contents of Zr, Co, V, and W are controlled to 1.0 mass%, preferably 0.8 mass%, and more preferably 0.5 mass%, respectively. On the other hand, the lower limits of the contents of Zr, Co, V, and W are not particularly limited, but are preferably 0.001 mass%, and more preferably 0.01 mass%, respectively.

REM and Ca are elements effective in improving the oxidation resistance of ferritic stainless steel materials. If the REM and Ca contents are too high, the manufacturing costs of ferritic stainless steel materials will increase. Therefore, the upper limits of the REM and Ca contents are controlled to 0.10 mass%, preferably 0.08 mass%, and more preferably 0.05 mass%. On the other hand, the lower limits of REM and Ca are not particularly limited, but are preferably 0.0001 mass%, and more preferably 0.003 mass%.

Sn is an element that is effective in improving the oxidation resistance of ferritic stainless steel materials. If the Sn content is too high, Sn segregates, reducing manufacturability. Therefore, the upper limit of the Sn content is controlled to 0.10 mass%, preferably 0.08 mass%, and more preferably 0.05 mass%. On the other hand, the lower limit of the Sn content is not particularly limited, but is preferably 0.001 mass%, and more preferably 0.005 mass%.

B is an element that is effective in 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 will decrease. Therefore, the upper limit of the B content is controlled to 0.01 mass%, preferably 0.008 mass%, and more preferably 0.005 mass%. On the other hand, the lower limit of the B content is not particularly limited, but is preferably 0.0001 mass%, and more preferably 0.0005 mass%.

The ferritic stainless steel material according to the embodiment of the present invention has an average grain boundary strain rate of 1% or more but less than 15%, preferably 3 to 10%. By controlling the average grain boundary strain rate within this range, coarse crystal grains can be grown with a short heat treatment (recrystallization treatment). Since the movement of magnetic domains, which is effective in exhibiting vibration-damping properties, is more hindered the more grain boundaries there are, the vibration-damping properties can be improved by coarsening the crystal grains and reducing the number of grain boundaries. The average grain boundary strain rate can be determined by the method described in the Examples below.
It is generally known that strain acts as a driving force for recrystallization. However, when the average grain boundary strain rate becomes high, fine crystal grains grow and then absorb surrounding crystal grains while growing, requiring a long heat treatment time.

The ferritic stainless steel material according to the embodiment of the present invention can be manufactured by melting stainless steel having the above-described composition, forming it into a steel sheet by a conventional method, and then imparting strain to the steel sheet. Specifically, first, the stainless steel having the above-described 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 sequence to obtain a cold-rolled sheet. Next, the cold-rolled sheet is annealed and pickled in sequence to obtain a cold-rolled annealed sheet. Next, strain is introduced into the cold-rolled annealed sheet using a light reduction imparting means such as a tension leveler or skin pass so as to achieve a predetermined average grain boundary strain rate. Note that strain can also be introduced by cold rolling, but the average grain boundary strain rate increases, making it difficult to control the average grain boundary strain rate to the desired level.
The conditions for each step may be adjusted appropriately depending on the composition of the stainless steel, and are not particularly limited.

The ferritic stainless steel material according to the embodiment of the present invention preferably has an average crystal grain size of 150 μm or more after recrystallization treatment. By setting the average crystal grain size within this range, it is possible to reduce the number of crystal grain boundaries, thereby improving vibration damping properties.
Here, the conditions for the recrystallization treatment are not particularly limited, but it is preferable that the heat treatment is performed for 5 to 30 minutes at 900 to 1100° C. The heat treatment atmosphere may be either an air atmosphere or a non-oxidizing atmosphere.

The ferritic stainless steel material according to the embodiment of the present invention preferably has a loss factor η of 5 × 10 ⁻⁴ or more, more preferably 1 × 10 ⁻³ or more after recrystallization treatment. By setting the loss factor η in this range, vibration damping properties can be improved.

The ferritic stainless steel material according to an embodiment of the present invention has a composition and average grain boundary strain rate controlled within a specified range, which allows for enhanced vibration-damping properties through recrystallization treatment while suppressing thermal deformation. Therefore, this ferritic stainless steel material is suitable for use in vibration-damping components. There are no particular limitations on the vibration-damping components that can be used, and it can be used in a variety of components that require vibration-damping properties, such as in automobiles and electronic devices.

The vibration-damping member according to an embodiment of the present invention includes the above-mentioned recrystallized ferritic stainless steel material. Because this vibration-damping member uses the above-mentioned ferritic stainless steel material, it exhibits little thermal deformation and excellent vibration-damping properties.

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

(Examples 1 to 11 and Comparative Examples 1 to 9)
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 3.0 mm thick hot-rolled sheet. The hot-rolled sheet 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 1.0 mm thick cold-rolled sheet. The cold-rolled sheet was then finish-annealed at 950 to 1050 ° C and pickled to obtain a cold-rolled annealed sheet. Next, for Examples 1 to 11 and Comparative Examples 3, 4, 7, and 8, tensile test specimens measuring 300 mm in the rolling direction (L direction) and 40 mm in the width direction (C direction) were cut by cutting, and strain was imparted using a tensile tester with the tensile force shown in Table 2. The tensile direction was the rolling direction. Note that strain was not imparted to Comparative Examples 1, 2, 5, 6, and 9.

The ferritic stainless steel material obtained above was evaluated as follows:

(Average grain boundary strain rate)
A 10 mm x 10 mm test piece was cut from the ferritic stainless steel material obtained above, and then resin-embedded so that the cross section in the thickness direction parallel to the rolling direction served as the observation surface. Next, the resin-embedded test piece was mirror-finished by wet polishing using SiC polishing paper and diamond paste, and then polished with a colloidal silica abrasive. The crystal orientation of the thus-treated test piece was measured using the EBSD method. An FE-SEM equipped with an OIM (Orientation Imaging Microscopy) system was used for the crystal orientation measurement. The evaluation area was a field of view of 100 μm square or more, and the strain area ratio was calculated from the measurement results using a Kernel Average Misorientation Map (KAM map). The strain area ratio was calculated for five arbitrary fields of view, and the average value was used as the average grain boundary strain ratio.

(Heat treatment: recrystallization treatment)
Test pieces measuring 20 mm in width direction × 270 mm in rolling direction were cut out from the ferritic stainless steel material obtained above, and then heat treated (recrystallization treatment) in an air atmosphere under the conditions shown in Table 2. After the heat treatment, the test pieces were cooled by air cooling.

(Thermal deformation amount)
The heat-treated test piece was placed on a flat table, and a range of 30 mm from one end (one end in the rolling direction) of the test piece was brought into contact with the table and fixed with a clamp, and the height (warpage height) of the other end (the other end in the rolling direction) floating from the table was measured. The warpage height was measured at both ends of the test piece in the rolling direction, and the average value of the measurements was taken as the warpage height. In this evaluation, a warpage height of 5 mm or less was evaluated as ◯ (small amount of thermal deformation), and a warpage height of more than 5 mm was evaluated as × (large amount of thermal deformation).

(Average grain size after heat treatment (recrystallization treatment))
After heat treatment, 10 mm x 10 mm test pieces were cut out from the test pieces by cutting, and then resin-embedded so that the cross section in the thickness direction parallel to the rolling direction was the observation surface. Next, the resin-embedded test pieces were subjected to a mirror finish by wet polishing, and then etched with fluoronitric acid to reveal the metal structure, which was then observed under an optical microscope. Observation using an optical microscope was performed in accordance with JIS G0551:2013. Lines were drawn at arbitrary positions on the optical microscope image, the number of intersections between the lines and the grain boundaries was counted, 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 was taken as the average grain size. In this evaluation, when the average grain size was 150 μm or more, it was rated as ◯ (sufficient coarsening of the grains), and when the average grain size was less than 150 μm, it was rated as × (insufficient coarsening of the grains).

(Loss factor η after heat treatment (recrystallization treatment))
Test pieces measuring 10 mm in width and 250 mm in rolling direction were cut out from the heat-treated test pieces by cutting. Using these test pieces, the loss factor η was measured in accordance with the "central vibration method" specified in JIS K7391:2008. Specifically, the test piece with its center fixed was vibrated using an impedance head, and the mechanical impedance was derived from the output force signal and acceleration vibration. The loss factor η was then derived based on the antiresonance frequency at which the mechanical impedance peaked and the frequency at which the amplitude dropped 3 dB from the peak. In this evaluation, a loss factor η of 5 × 10 ^-4 or more was evaluated as ⊚ (excellent vibration damping), a loss factor η of 1 × 10 ^-3 or more but less than 5 × 10 ^-4 was evaluated as ◯ (good vibration damping), and a loss factor η of less than 1 × 10 ^-3 was evaluated as × (insufficient vibration damping).
The results of the above evaluations are shown in Table 2.

As shown in Table 2, the ferritic stainless steel materials of Examples 1 to 11, which had an average grain boundary strain rate of 1% or more and less than 15%, all had good results for the amount of thermal deformation, average grain size, and loss factor η.
In contrast, the ferritic stainless steel materials of Comparative Examples 1 to 7 had average grain boundary strain rates outside the above ranges, and therefore, the results for one or more of the thermal distortion, average grain size, and loss factor η were insufficient. The ferritic stainless steel material of Comparative Example 8 had an insufficient result for the thermal distortion because the Cr content was too low. Furthermore, the ferritic stainless steel material of Comparative Example 9 had an insufficient result for all of the thermal distortion, average grain size, and loss factor η because the Cr content was too low and the average grain boundary strain rate was outside the above ranges.

As can be seen from the above results, the present invention can provide a ferritic stainless steel material and vibration-damping component that can enhance vibration-damping properties through recrystallization treatment while suppressing thermal deformation.

Claims (10)

A ferritic stainless steel material containing C: 0.05 mass% or less, Mn: 1.0 mass% or less, Ni: 0.60 mass% or less, P: 0.05 mass% or less, S: 0.03 mass% or less, Cr: 10.5 to 24.0 mass%, N: 0.03 mass% or less, Cu: 0.60 mass% or less, Mo: 2.5 mass% or less, Si: 3.0 mass% or less, Al: 5.0 mass% or less, the balance being Fe and unavoidable impurities, and having an average grain boundary strain rate of 1% or more but less than 15%. The ferritic stainless steel material according to claim 1, further comprising at least one selected from Nb: 0.50 mass% or less and Ti: 0.50 mass% or less, and the total content of Nb and Ti is 6(C+N) or more (C and N represent the contents of C and N, respectively). The ferritic stainless steel material according to claim 1 or 2, wherein the total content of Al and Si is 1.0 mass% or more. The ferritic stainless steel material according to any one of claims 1 to 3, further comprising at least one selected from Zr: 1.0 mass% or less, Co: 1.0 mass% or less, V: 1.0 mass% or less, and W: 1.0 mass% or less. The ferritic stainless steel material according to any one of claims 1 to 4, further comprising at least one selected from REM: 0.10 mass% or less and Ca: 0.10 mass% or less. The ferritic stainless steel material according to any one of claims 1 to 5, further comprising at least one selected from Sn: 0.10 mass% or less and B: 0.01 mass% or less. The ferritic stainless steel material according to any one of claims 1 to 6, wherein the average crystal grain size after recrystallization treatment at 900 to 1100°C for 5 to 30 minutes is 150 µm or more. The ferritic stainless steel material according to any one of claims 1 to 7, wherein the loss factor η after recrystallization treatment at 900 to 1100°C for 5 to 30 minutes is 5 x ^10-4 or more. The ferritic stainless steel material according to any one of claims 1 to 8 , which is used for a vibration-damping member. A method for producing a vibration-damping member, comprising recrystallizing the ferritic stainless steel material according to any one of claims 1 to 9 at 900 to 1100°C for 5 to 30 minutes .