JP7844937B2 - Method for manufacturing hot-forged materials - Google Patents

Description

This invention relates to a method for manufacturing hot-forged materials.

The alloy specified in AMS6278 is called M50NiL, and its composition, by mass%, is 0.11–0.15% C (carbon), 0.10–0.25% Si (silicon), 0.15–0.35% Mn (manganese), 3.20–3.60% Ni (nickel), 4.00–4.25% Cr (chromium), 4.00–4.50% Mo (molybdenum), and 1.13–1.33% V (vanadium), with the remainder being Fe (iron) and impurities. This M50NiL is used as bearing steel, for example, in aircraft engine components.
As for the manufacturing method of M50NiL, as described in Patent Document 1, for example, a rod-shaped workpiece is used as the raw material, cut to a predetermined length, and then processed into a predetermined shape by forging, turning, and other processes.

Furthermore, in Patent Document 2, the applicant has proposed a method for manufacturing hot-forged materials aimed at preventing defects in ultrasonic testing during the hot forging process of an alloy equivalent to M50NiL. This manufacturing method includes a forging temperature heating step in which the forging material is heated to a forging temperature of 1000 to 1100°C, and a hot forging step in which the heated forging material is subjected to radial forging, repeatedly extending its length by rotating it circumferentially and pressing it from four directions along its entire length, thereby producing a hot-forged material. In the hot forging step, the forging completion temperature is set to 800°C or higher.

Japanese Patent Publication No. 2009-236232 Japanese Patent Publication No. 2018-144065

One of the factors that degrades the mechanical properties of M50NiL products as described above is grain coarsening. In particular, even if the average grain size is small, if the grains are partially coarse and a mixed grain structure with both coarse and fine grains is formed, there is a concern that cracks may occur due to the coarse grains, so it is important to also reduce the maximum grain size. The aforementioned patent documents have not considered how to reduce the maximum grain size in this way. In particular, in Patent Document 2, the size of coarse grains that cause ultrasonic testing defects (UT defects) is approximately 600 μm or more in terms of the major axis of the grain, but the size of coarse grains that cause deterioration of mechanical properties may be smaller than the coarse grains that cause UT defects, for example, around 200 to 400 μm in terms of the major axis, and may not be detected by ultrasonic testing.
Therefore, the object of the present invention is to provide a method for producing a hot-forged material that can suppress variations in grain size during hot forging and after quenching of an alloy equivalent to M50NiL.

This invention was made in view of the above-mentioned problems.
In other words, the present invention comprises a preparation step of preparing a forging material consisting of, by mass%, C: 0.11-0.15%, Si: 0.10-0.25%, Mn: 0.15-0.35%, Cr: 4.00-4.25%, Ni: 3.20-3.60%, Mo: 4.00-4.50%, V: 1.13-1.33%, with the remainder being Fe and unavoidable impurities, and a step of heating the forging material to a temperature of 1070-1200°C. This method for manufacturing a hot-forged material includes a step of dividing the material into blocks to obtain a material for finish forging by hot forging, and a step of finishing hot forging in which the material for finish forging is heated to a temperature of 1100 to 1200°C, and then the heated material for finish forging is subjected to radial forging, in which the material is rotated in the circumferential direction and pressed from four directions along its entire length, thereby extending the overall length and producing a hot-forged material.

According to the present invention, it is possible to suppress variations in grain size during hot forging and after quenching of an alloy equivalent to M50NiL.

This is a cross-sectional photograph of a sample after a simulated compression test of the present invention. This is a cross-sectional photograph of a sample after a simulated compression test of the present invention. This is a cross-sectional photograph of a sample of the finished simulated compressed material of the present invention after annealing and quenching. These are cross-sectional mirror photographs of hot-forged materials of the present invention and comparative examples. These are the grain size measurement results for hot forged materials after annealing and quenching of the present invention example and comparative example.

First, we will explain the reason for limiting the composition of the M50NiL equivalent alloy as defined in this invention.
C: 0.11-0.15%
Carbon (C) is an effective element for improving hardness, and a minimum of 0.11% is required. However, adding more than 0.15% of C reduces toughness, so the amount of C should be between 0.11% and 0.15%.
Si: 0.10~0.25%
Si is an effective element for strengthening the ferrite phase. If the Si content is less than 0.10%, the material becomes too ductile, hindering cold machinability. On the other hand, adding more than 0.25% Si reduces toughness, so the Si content should be between 0.10% and 0.25%.
Mn: 0.15-0.35%
Mn is an effective element for improving hardenability, and a minimum of 0.15% is required. On the other hand, if the amount of Mn exceeds 0.35% by mass, the hardness increases too much, causing problems such as poor machinability. Therefore, the amount of Mn should be between 0.15% and 0.35%.

Ni: 3.20-3.60%
Ni is an effective element for improving hardenability, and a minimum of 3.20% is required. However, excessive addition lowers the Ms point, leading to an increase in retained austenite and problems such as increased deformation after cold working. Therefore, the Ni content should be in the range of 3.20% to 3.60%.
Cr: 4.00-4.25%
Similar to Mo (which will be discussed later), Cr has the effect of improving hardenability and resistance to tempering softening at high temperatures, so a minimum of 4.00% is necessary for its addition. On the other hand, if the Cr content exceeds 4.25%, carbide precipitation is promoted, making it difficult to control the hardness during manufacturing. Therefore, the Cr content should be between 4.00% and 4.25%.

Mo: 4.00-4.50%
Mo is necessary at a minimum of 4.00% because it improves hardenability and resistance to tempering softening at high temperatures. On the other hand, if the amount of Mo exceeds 4.50%, carbide precipitation is promoted, making it difficult to control hardness during manufacturing. Therefore, the amount of Mo should be between 4.00% and 4.50%.
V: 1.13-1.33%
V has the effect of improving tempering softening resistance and refining the crystal grains. If V is less than 1.13%, VC precipitation is small and the crystal grains become coarser. On the other hand, if V exceeds 1.33%, carbide precipitation is promoted, making it difficult to control the hardness during manufacturing. Therefore, V should be set between 1.13% and 1.33%.
Besides the elements described above, the only other components are Fe and unavoidable impurities.

Next, the present invention will be described in the order of the manufacturing process.
<Preparation process>
First, a steel ingot of an M50NiL equivalent alloy having the above composition is manufactured. As mentioned above, since the M50NiL equivalent alloy is an alloy used in aircraft engine parts and the like, it is preferable to manufacture consumable electrodes by vacuum melting and then perform vacuum remelting to reduce nonmetallic inclusions and component segregation. In this invention, the steel ingot may also be used as a forging material, and for example, it may be further machined to form a round bar or square bar forging material. Furthermore, to further reduce component segregation, the forging material may be subjected to homogenization heat treatment.

<Bulking process>
After heating the forging material to a forging temperature of 1070 to 1200°C, a splitting process is performed in which hot forging is carried out at this forging temperature (forging start temperature). By performing the splitting process at the above temperature, a fine and uniform structure can be obtained in combination with the finishing hot forging process described later. If the heating temperature is below 1070°C, there is a concern that the mechanical properties will deteriorate due to the formation of a mixed-grain structure in which fine crystal grains that have not grown sufficiently and coarse, unrecrystallized grains remain. In this embodiment, "mixed grains" refers to grains in which there are grain size numbers that differ by 3 or more from the grain size number with the highest frequency within one field of view, and these grains account for 20% or more of the area. If the heating temperature exceeds 1200°C, there is a concern that coarse crystal grains will be formed due to excessive growth of the crystal grains. In this embodiment, the "hot forging" in the ingot-splitting process can be performed as free hot forging, in which the workpiece is placed on an anvil with a flat or curved surface and the material is processed with a hammer that also has a flat or curved surface, in order to facilitate the processing of large steel ingot shapes. However, die forging using a die may also be used. Furthermore, radial forging, in which the forging material is rotated circumferentially and pressed from four directions along its entire length to extend the overall length, may be performed in combination with the free forging or die forging described above. In addition, heat treatment and polishing processes can be introduced between the ingot-splitting process and the finish hot forging process.

<Finishing Hot Forging Process>
Next, the material for finish forging after the mass separation process is heated to a forging temperature of 1100 to 1200°C, and a hot forged material (hereinafter also referred to as "forged material") is obtained by radial forging at this forging temperature (forging start temperature) in a finish hot forging process. Radial forging is applied in the finish hot forging process because by utilizing a control mechanism that enables the gradual forging into a polygon while precisely controlling the feed rate, material rotation rate, and reduction rate onto the material, and ultimately processing into a round shape, processing heat generation can be controlled and the temperature inside and on the surface of the product can be controlled. In this embodiment, the reason for setting the lower limit of the heating temperature to 1100°C is to promote recrystallization and reduce the variation in grain size, and the reason for setting the upper limit of the heating temperature to 1200°C is to suppress grain coarsening. The preferred lower limit of the heating temperature is 1120°C.
For radial forging, it is preferable to reduce the material by pressing 50 to 210 times per minute, reduce the surface area per pass by 20 to 28%, and feed rate on the insertion side of the material to be forged in the range of 2.7 to 6 m/min. Forging under these conditions enables uniform microstructure control over the entire length and allows for easy adjustment of the amount of heat generated during processing, thus enabling control of the final temperature. The final forging temperature can be set to 800°C or higher. To achieve a final forging temperature of 800°C or higher, it is preferable to forge under conditions that shorten the forging time per pass. If the final forging temperature is below 800°C, the temperature of the material surface drops, and the recrystallization of austenite grains is not promoted, resulting in the retention of partially coarse grains. Therefore, the final forging temperature should be 800°C or higher. A preferred final forging temperature is 820°C or higher. The final forging temperature is the surface temperature of the forged material. According to the method for manufacturing hot-forged materials of the present invention described above, it is possible to prevent variations in grain size during hot forging and after quenching.

(Example 1)
A consumable electrode was prepared by vacuum melting, and then a steel ingot of an M50NiL equivalent alloy was obtained by vacuum remelting using the consumable electrode. Subsequently, hot working and machining were performed to obtain a rectangular bar-shaped forging material with a diameter of 320 mm and a total length of 2000 mm. From this forging material, a 15 mm long material for a simulated experiment was obtained. The composition is shown in Table 1.

First, an experiment was conducted to confirm the optimal temperature for the mass separation process. Compression test material was taken from the simulated experimental material described above, and a compression test simulating the mass separation process was carried out. The processing conditions were determined using plastic deformation simulation software, and the material was compressed to be equivalent to the compression conditions when a steel ingot is processed into a finish forging material with an area circle equivalent diameter of 240 mm in the actual process. Five processing temperature conditions were used: 1120°C, 1100°C, 1080°C, 1060°C, and 1040°C. The processed samples were observed with an optical microscope. Figure 1 shows microstructural images of the longitudinal section (section perpendicular to the compression plane) after the simulated mass separation compression test. In Figures 1(a) to (e), the observation magnification in the upper section is 100x, and the observation magnification in the lower section is 200x. Samples with a compression temperature of 1070°C or higher showed a uniform structure with recrystallized tissue throughout the observation field, as shown in Figures 1(a) to (c). On the other hand, the sample compressed at 1060°C contained unrecrystallized grains (Figure 1(d)A) and fine grains in the early stages of recrystallization (Figure 1(d)B), as shown in Figure 1(d), and was in a mixed state with grown crystal grains. The sample compressed at 1040°C, as shown in Figure 1(e), showed the presence of coarse unrecrystallized grains (Figure 1(e)A).

(Example 2)
Next, an experiment was conducted to confirm the optimal temperature for the finish hot forging process. Compression test material was taken from the same simulated experimental material as in Example 1, and a compression test simulating the finish hot forging process was carried out. The processing conditions were determined using plastic deformation simulation software, and the material was compressed to the same compression conditions as when a finish forging material with an area circle equivalent diameter of 240 mm is made into a forging material with an area circle equivalent diameter of 140 mm in the actual process. Six processing temperature conditions were used: 1150°C (Sample No. 1), 1130°C (Sample No. 2), 1100°C (Sample No. 3), 1070°C (Sample No. 4), 1050°C (Sample No. 5), and 1000°C (Sample No. 6). The processed samples were observed with an optical microscope. In addition, to confirm the effect on the grain size after the quenching treatment performed after forging, annealing and quenching treatments were performed on the samples after the compression test. The grain size after quenching was observed using an optical microscope at a magnification of 200x and a field of view of 30, and derived using image analysis software. Figure 2 shows microstructure images of the longitudinal section after the finish simulation compression test (upper row: 100x magnification, lower row: 200x magnification). Figure 3 shows microstructure images of the longitudinal section after annealing and quenching at 1100°C for 15 min following the finish simulation compression test (upper row: 100x magnification, lower row: 200x magnification), and Table 2 shows the grain size measurement results (average grain size and maximum grain size) measured by image analysis. From Figure 2, it can be seen that samples No. 1 to 5, compressed at temperatures of 1050°C or higher, had a completely recrystallized structure, confirming that the higher the compression temperature, the fewer fine grains there were and the more uniform the grain size tended to be. In contrast, sample No. 6, compressed at 1000°C, had a mixture of unrecrystallized grains and fine grains. Furthermore, as shown in Figure 3 and Table 2, while the average grain size of all samples is at a similar level, it was confirmed that in samples No. 1 to 3, where the compression temperature was 1100°C or higher, the maximum grain size was smaller and the variation in grain size was reduced.

(Example 3)
In Examples 1 and 2, the effect of processing temperature on grain size was confirmed. However, Examples 1 and 2 were simulated experiments, and the sample size was small, resulting in less influence from uneven heating and distortion compared to actual forged materials. Therefore, the effect of heating temperature in the finishing hot forging process was confirmed in an actual manufacturing process. A block processing was performed on a steel ingot of the same M50NiL equivalent alloy used in Example 1, using hot free forging at 1100°C. This yielded a round bar-shaped finishing forging material with a diameter of 140 mm and a total length of 3000 mm (the finishing forging material of the present invention example) and a square bar-shaped forging material with an area circle equivalent diameter of 240 mm and a total length of 3000 mm (the finishing forging material of the comparative example). The material for finishing forging in the present invention example was radially forged to a diameter of 80 mm while being heated to 1150°C to obtain the forged material of the present invention example, and the material for finishing forging in the comparative example was radially forged to a diameter of 140 mm while being heated to 1050°C to obtain the forged material of the comparative example. Here, the finishing hot forging was carried out in the present invention example and the comparative example so that the actual forging ratio was the same. Other hot forging conditions for both the present invention example and the comparative example were set to a reduction by pressing of 75 to 105 times/min, a reduction ratio per pass of 25 to 30%, and a feed rate on the insertion side of the material to be forged in the range of 4 to 5.5 m/min.
Figure 4 (upper panel: 200x magnification, lower panel: 500x magnification) shows the results of observing test specimens taken from the forged materials of the present invention example and comparative example using an optical microscope. Regarding the sampling locations, in the radial direction, samples were taken at a depth D/4 (D: diameter of the area circle equivalent diameter) from the surface of the forged material axially, and in the longitudinal direction, samples were taken at three locations: the top, center, and bottom of the columnar forged material. Figure 4 shows that in the comparative example, coarse, unrecrystallized grains elongated in the forging direction were observed. On the other hand, in the present invention, the crystal grain size was fine and uniform, resulting in an equiaxed crystal grain structure. Each material after forging underwent annealing and quenching treatments, and the crystal grain size was measured using an optical microscope according to ASTM standards. Figure 5 shows the results. The observation method was the same as in Example 2. Figure 5 shows that the average crystal grain size of the forged material of the present invention example was at the same level as the forged material of the comparative example, but the maximum crystal grain size was finer than that of the forged material of the comparative example. Based on the above results, it was confirmed that applying the hot forging process defined in this invention makes it possible to prevent variations in grain size during hot forging and after quenching of an alloy equivalent to M50NiL.

Claims (1)

A preparation step to prepare a forging material consisting of, by mass%, C: 0.11-0.15%, Si: 0.10-0.25%, Mn: 0.15-0.35%, Cr: 4.00-4.25%, Ni: 3.20-3.60%, Mo: 4.00-4.50%, V: 1.13-1.33%, with the remainder being Fe and unavoidable impurities,
The process involves heating the forging material to a temperature of 1070 to 1200°C and then hot forging it to obtain a material for finishing forging, and
A method for manufacturing a hot forged material, comprising: heating the aforementioned finishing forging material to a temperature of 1100 to 1200°C; and then, in a finishing hot forging step, repeatedly performing radial forging on the heated finishing forging material, in which the finishing forging material is rotated circumferentially and pressed from four directions along its entire length to extend its overall length, thereby producing a hot forged material.