Method for producing precipitation hardening austenitic alloy member and method for producing precipitation hardening austenitic alloy steel material

Abstract

The present invention provides a precipitation hardening austenitic alloy member capable of suppressing wear on a rake face of a tool even during cutting of a high-strength precipitation hardening austenitic alloy.　Provided is a method for producing a precipitation hardening austenitic alloy member wherein a precipitation hardening austenitic alloy steel material having a grain size number of not less than 6.0 is subjected to continuous cutting, so as to obtain a precipitation hardening austenitic alloy member. Also provided is a method for producing a precipitation hardening austenitic alloy steel material wherein a material having the composition of a precipitation hardening austenitic alloy is subjected to at least hot working, a solution treatment, and an aging treatment, so as to obtain a precipitation hardening austenitic alloy steel material having a grain size number of not less than 6.0.

Description

Method for manufacturing precipitation-hardened austenitic alloy member and method for manufacturing precipitation-hardened austenitic alloy steel

[0001] The present invention relates to a precipitation hardened austenitic alloy member and a method for producing a precipitation hardened austenitic alloy steel material.

[0002] Precipitation-hardened austenitic alloys such as SUH660 have good mechanical properties over a wide temperature range and are known as components suitable for use in hydrogen stations (hydrogen stands) because they have excellent resistance to hydrogen embrittlement in hydrogen environments. For example, Patent Document 1 describes forging A286 alloy (equivalent to SUH660) suitable for hydrogen energy facilities at a total forging ratio of 5:1 to obtain a forged product.

[0003] Although the above-mentioned precipitation-hardened austenitic alloys have excellent resistance to hydrogen embrittlement, in high-temperature environments or in high-pressure hydrogen gas, localized deformation occurs due to plastic deformation in the presence of hydrogen, and stacking faults are likely to form. Such defects are likely to cause cracks, and mechanical properties such as tensile strength tend to be lower than in air. Patent Document 2 discloses a precipitation-hardened austenitic alloy steel material that has improved mechanical properties compared to conventional materials and is expected to extend the life of parts in high-pressure hydrogen environments, and a method for manufacturing the same.

[0004] Chinese Patent Application Publication No. 11354982 International Patent Publication No. 2023 / 145423

[0005] Cutting is the mainstream method for obtaining various product shapes from the above-mentioned precipitation-hardened austenitic alloys, and improving processing efficiency is required to improve productivity. In this case, if the tool life is short, processing must be interrupted and the tool replaced, which may lead to a decrease in productivity. In particular, high-strength materials such as precipitation-hardened austenitic alloys tend to increase the cutting temperature, which makes it easy for adhesive wear and diffusion wear to progress on the rake face of the tool. Therefore, an object of the present invention is to provide a method for manufacturing a precipitation-hardened austenitic alloy member that can suppress rake face wear even when cutting high-strength precipitation-hardened austenitic alloys.

[0006] The present invention has been made in view of the above-mentioned problems. That is, one aspect of the present invention is a method for producing a precipitation hardened austenitic alloy member, which comprises performing continuous cutting on a precipitation hardened austenitic alloy steel material having a grain size number of 6.0 or more to obtain a precipitation hardened austenitic alloy member.

[0007] Another aspect of the present invention is a method for producing a precipitation hardened austenitic alloy steel material, which comprises subjecting a material having the composition of the precipitation hardened austenitic alloy described above to at least hot working, solution treatment, and aging treatment to have a grain size number of 6.0 or more.

[0008] According to the present invention, wear on the rake face of a cutting tool can be suppressed when producing a high-strength precipitation hardened austenitic alloy member.

[0009] 1A and 1B are SEM photographs of flank wear observed in an example of the present invention and a comparative example, optical microscope photographs of rake face wear observed in an example of the present invention and a comparative example, and optical microscope photographs of chip cross sections observed in an example of the present invention and a comparative example.

[0010] The present invention will be described in detail below. However, the present invention is not limited to the embodiments described herein, and appropriate combinations and improvements are possible without departing from the technical concept of the invention. The present invention is directed to precipitation hardened austenitic alloy steel. The precipitation hardened austenitic alloy steel refers to SUH660 and SUH661, which are described as austenitic in JIS-G-4311, and precipitation hardened austenitic stainless steels, which are improved versions of these. Specific compositions include, by mass, 0.2% or less of C, 1% or less of Si, 2% or less of Mn, 15 to 30% of Ni, 10 to 25% of Cr, and 0.5 to 4.5% of Mo, and may further include one or more of 23% or less of Co, 4% or less of W, 2% or less of Nb, 1% or less of V, 2.5% or less of Ti, 0.5% or less of Al, 0.02% or less of B, and 0.2% or less of N, with the balance being Fe and unavoidable impurities. Examples of unavoidably contained impurity elements include S, P, and O, and it is preferable to set the upper limit of each of these to 0.1%. The preferred lower limit of the Ni content is 20% by mass, and the preferred upper limit of Cr is 20% by mass. It is more preferable that Fe + Ni + Cr is 95% or more by mass. Of the precipitation hardenable austenitic stainless steels that are the subject of the present invention, it is particularly preferable to use SUH660 as described in JIS-G-4311, which is an alloy having a composition range, in mass %, of C 0.08% or less, Si 1.00% or less, Mn 2.00% or less, P 0.040% or less, S 0.030% or less, Ni 24.00 to 27.00%, Cr 13.50 to 16.00%, Mo 1.00 to 1.50%, Ti 1.90 to 2.35%, Al 0.35% or less, V 0.10 to 0.50%, B 0.001 to 0.010%, and the balance being Fe and unavoidable impurities.

[0011] In the manufacturing method of this embodiment, a material for hot working having a precipitation-hardened austenitic alloy composition is first prepared. This material for hot working is preferably a steel ingot that can be obtained by casting. Alternatively, the steel ingot may be subjected to hot plastic processing such as hot pressing or hot rolling, and then machined into a round or square bar shape to produce a billet, bloom, or other steel piece. This billet may then be subjected to homogenization heat treatment, and this steel piece may be used as the material for hot working. When producing the steel ingot, remelting may be performed to reduce elemental segregation and nonmetallic inclusions.

[0012] Next, the prepared hot-worked material is heated in a heating furnace and then subjected to at least one hot working to obtain a hot-worked material. Typical examples of "hot working" in this embodiment include hot forging and hot rolling, and a combination of hot forging and hot rolling is also possible. When hot forging is applied as the hot working method, solid forging alone may be used, or solid forging and upset forging may be combined. The temperature of the heating furnace during forging is preferably 800 to 1300°C, and the surface temperature of the material at the end of forging is preferably 600°C or higher. Furthermore, in terms of the degree of freedom in the shape to be processed, "hot forging" preferably includes hot free forging, in which the workpiece is placed on an anvil with a flat or curved surface and the material is processed by pressing or striking with an anvil or die also with a flat or curved surface. Of course, when processing into complex shapes, die forging using a mold may be performed, or radial forging may be performed in which the material is rotated in the circumferential direction and pressed from four directions over its entire length to obtain a forged material, or a combination of these processes may be performed.

[0013] In this embodiment, a hot-worked material obtained by hot working, such as a hot forging process or a hot rolling process, is subjected to solution treatment and aging treatment to obtain a precipitation-hardened austenitic alloy steel material (hereinafter simply referred to as alloy steel material). In the present invention, it is preferable to adjust the solution treatment temperature in this process to obtain an alloy steel material having a grain size number of 6.0 or higher. This tends to suppress the progression of rake face wear in the continuous cutting process described below. The preferred lower limit of the grain size number is 6.5. The preferred temperature for the above-mentioned solution treatment is 850 to 1050°C. A more preferred lower limit of the solution treatment temperature is 880°C, and a more preferred upper limit of the solution treatment temperature is 980°C. The solution treatment time can be determined appropriately depending on the weight of the hot-worked material, for example, approximately 1 to 100 hours. The preferred aging treatment temperature is 650 to 800°C, more preferably 700 to 760°C. The aging treatment time may be 1 to 20 hours, and most preferably 16 hours based on JIS-G-4311.

[0014] The alloy steel material subjected to the above-described solution treatment and aging treatment is then subjected to continuous cutting, such as turning, to obtain the precipitation-hardened austenitic alloy member of the present invention. Note that the above-described continuous cutting does not include intermittent cutting, such as with an end mill, in which a single cutting edge of the tool repeatedly cuts and misses. In cutting, tool wear progresses over time. Tool wear primarily progresses on the flank, which contacts the newly formed surface of the workpiece, and on the rake face, which contacts the chips. Since flank wear can be easily observed and measured from the appearance after cutting, it is often used to evaluate tool life. On the other hand, when chips adhere to the rake face, rake face wear is difficult to evaluate in the production field because it requires observing the worn area from the tool cross section, making it difficult to predict sudden damage caused by rake face wear. Therefore, suppressing rake face wear is important for stable continuous cutting. In the present invention, by adjusting the grain size number of the alloy steel to 6.0 or more using the above-mentioned process, it is possible to reduce the maximum thickness of the chips and suppress the rake face wear that occurs on the cutting tool. The grain size number can be measured, for example, by cutting a micro specimen (6 mm x 10 mm x 8 mm) for observation from the alloy steel so that a cross section or a longitudinal section can be observed, and measuring the grain size number on the mirror-finished micro specimen for observation using the comparative method in accordance with ASTM-E112.

[0015] The alloy steel material of this embodiment preferably has a chip breakability of 0.65 or less. This chip breakability is calculated by dividing the number of breakages of chips generated by 10 minutes of cutting using a cemented carbide insert (80° rhombic, negative, uncoated) under dry conditions of a cutting speed of 30 m / min, a feed rate of 0.2 mm / rev, and a depth of cut of 0.75 mm by the cutting time. In cutting tools, when chips break, adhered material carries away parts of the tool, causing adhesion damage. Therefore, chip breakability can be considered a factor that affects rake face wear. As described above, by setting the chip breakability to 0.65 or less, adhesion damage can be reduced, thereby further suppressing rake face wear. This chip breakability can be reduced to 0.65 or less by adjusting the grain size of the precipitation-hardened austenitic stainless steel to 6.0 or more.

[0016] Furthermore, according to the present invention, the effect of suppressing crater wear can be achieved even in harsh machining environments such as dry cutting and using uncoated tools. Here, the cutting conditions applied should be the recommended cutting conditions disclosed by each tool manufacturer or conditions close to the recommended cutting conditions (for example, -30% to 0% of the lower limit of the recommended conditions).

[0017] The precipitation-hardened austenitic alloy member obtained by the manufacturing method of the present invention described above is expected to exhibit good mechanical properties in high-temperature environments and hydrogen environments. In particular, the present invention is preferably used for manufacturing parts for use in hydrogen environments. Here, "for use in hydrogen environments" refers to parts exposed to high-pressure hydrogen gas, for example, for use in fuel cell vehicles and hydrogen stations, and examples of parts include piping, tanks, valves, dispensers, etc.

[0018] The present invention will be described in more detail in the following examples. (Example 1) A steel ingot (equivalent to SUH660) having the composition shown in Table 1 was prepared and used as a columnar forging material for the present invention examples and comparative examples. Each was hot forged. For the present invention examples, the forging material was hot forged multiple times to obtain a hot-forged material having a cross section perpendicular to the axis of the columnar material with an equivalent area circle diameter of 150 mm. Subsequently, multiple test pieces (φ43 mm × 100 mm) were taken axially from the surface of the obtained hot-forged material at a depth D / 8 (D: diameter of equivalent area circle) in the axial direction. The sampled test pieces were then subjected to solution treatment under various conditions (see Table 1), followed by aging treatment under the same conditions to obtain samples for the present invention examples and comparative examples. Each sample was then machined to obtain machinability test pieces measuring φ42 mm × 100 mm. Each machinability test piece was then subjected to a lathe-based machinability test. The machinability test was carried out by dry turning using a cemented carbide insert (80° diamond, negative, uncoated) manufactured by Mitsubishi Materials Corporation, at a cutting speed of 30 m / min, a feed rate of 0.2 mm / rev, and a depth of cut of 0.75 mm for approximately 10 minutes.

[0019] After the machinability test, the flanks of the recovered inserts were observed using a scanning electron microscope (SEM) to measure flank wear. The inserts were then embedded in cold resin and polished, and the cross sections were observed using an optical microscope to measure rake wear. The grain size number was determined by cutting an 8 mm thick sample from the end of the machinability test piece near the gripping portion. A micro-specimen (6 mm x 10 mm x 8 mm) was then cut from the sample so that the cross section or longitudinal section could be observed. The micro-specimen was mirror-finished and the grain size number was determined using the comparative method in accordance with ASTM-E112. Tensile properties were obtained by cutting a 10 mm diameter sample from the remaining cut portion, processing it into a tensile test piece with a parallel section of 16 mm and a parallel section diameter of 4 mm, and conducting a tensile test in accordance with ASTM-E8. The chips were embedded in cold resin and polished, and the cross section of the center of the width direction was observed using an optical microscope. The relationship between chip breakability and grain size was also evaluated. Chip breakability was calculated by counting the number of times chips broke from the cutting state photographed with a fixed camera in the machinability test and dividing the number of times chips broke by the cutting time. The obtained tensile properties, grain size number, rake face wear width, and chip breakability are shown in Table 2.

[0020] Table 2 confirms that the grain size numbers of the samples of the invention examples are 6.5 or greater, which is larger than that of the comparative examples, i.e., the samples are finer grained. It was also confirmed that the lower the solution treatment temperature, the finer the grain size, and that reducing the grain size (increasing the grain size number) tends to reduce chip breakability. Furthermore, the samples of the invention examples had higher 0.2% proof stress and tensile strength than any of the comparative examples, and had similar levels of elongation and reduction of area, confirming their excellent mechanical properties. Specifically, the tensile strength of the invention examples was 1000 MPa or greater, the 0.2% proof stress was 700 MPa or greater, the elongation was 20% or greater, and the reduction of area was 45% or greater.

[0021] Figure 1 shows an SEM photograph taken after the test to observe flank wear. Figure 2 shows a cross-sectional photograph of the tool to observe rake face wear. As shown in Figure 1, when comparing the inserts used to cut the inventive and comparative examples, the flank wear was roughly the same or slightly greater for the inventive example. According to JIS-B-4011 (1971), the standard tool life criterion is 300 μm for flank wear. Therefore, when evaluated based on flank wear, it is believed that neither the inventive example nor the comparative example reached the end of their tool life. Meanwhile, Figure 2 shows that rake face wear occurred on the comparative example insert after the machinability test, while no rake face wear occurred on the inventive example insert. This confirms that the steel material used in the inventive example successfully suppressed rake face wear on the insert. Furthermore, while the rake face wear life criterion according to JIS-B-4011 (1971) is 50 to 100 μm, Sample No. The rake face wear of No. 102 exceeded 50 μm, confirming the possibility that the tool life had expired. Figure 3 also shows a cross-sectional photograph of chips generated in the machinability test. Figure 3 confirms that the lower the solution treatment temperature, the smaller the maximum chip thickness tends to be, with the chip thickness of the inventive example being the smallest. From the above, it was found that the maximum chip thickness is correlated with the grain size, and that increasing the grain size number, i.e., decreasing the grain size, tends to reduce the maximum chip thickness and reduce chip separability, thereby suppressing rake face wear. Furthermore, the precipitation-hardened austenitic alloy steel of the present invention has a high rake face wear suppression effect, suggesting that it is possible to suppress sudden cutting tool damage caused by rake face wear when manufacturing precipitation-hardened austenitic alloy members by continuous cutting.

[0022]

[0023]

Claims

1. A method for manufacturing a precipitation hardened austenitic alloy member, comprising the steps of: subjecting a precipitation hardened austenitic alloy steel material having a grain size number of 6.0 or more to continuous cutting to obtain a precipitation hardened austenitic alloy member.

2. A method for producing a precipitation hardened austenitic alloy steel material according to claim 1, wherein a material having a precipitation hardened austenitic alloy composition is subjected to at least hot working, solution treatment and aging treatment to have a grain size number of 6.0 or more.

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