Method for manufacturing hot-forged materials

Abstract

To provide a manufacturing method of a hot forged material enabling prevention of variation in crystal grain sizes during hot forging time and after hardening of an alloy equivalent to M50NiL.SOLUTION: A manufacturing method of a hot forged material is provided, comprising: a preparing step of preparing a rod-shaped raw material for forging which comprises by mass%: 0.11 to 0.15% of C; 0.10 to 0.25% of Si; 0.15 to 0.35% of Mn; 4.00 to 4.25% of Cr; 3.20 to 3.60% of Ni; 4.00 to 4.50% of Mo; 1.13 to 1.33% of V; and the balance consisting of Fe and inevitable impurities; a blooming step of heating the material for forging to a temperature of 1070 to 1200°C and then performing hot forging on the heated material for forging, thereby acquiring raw material for finish forging; a forging temperature heating step of heating the raw material for finish forging to a forging temperature of 1100 to 1200°C; and a finish hot forging step of performing radial forging on the heated raw material for forging, thereby acquiring the hot forged material.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a hot forging material.

Background Art

[0002] The alloy defined by AMS6278 is called M50NiL, and its composition is, in mass %, C (carbon) of 0.11 to 0.15%, Si (silicon) of 0.10 to 0.25%, Mn (manganese) of 0.15 to 0.35%, Ni (nickel) of 3.20 to 3.60%, Cr (chromium) of 4.00 to 4.25%, Mo (molybdenum) of 4.00 to 4.50%, V (vanadium) of 1.13 to 1.33%, and the balance consists of Fe (iron) and impurities. This M50NiL is used as a bearing steel, for example, as aircraft engine parts. As a method for manufacturing this M50NiL, for example, as described in Patent Document 1, a bar-shaped workpiece is used as a raw material, cut to a predetermined length, and processed by forging, turning, etc., so as to be processed into a predetermined shape.

[0003] In addition, the applicant of the present application has proposed, in Patent Document 2, a method for manufacturing a hot forging material aimed at preventing ultrasonic flaw detection test defects in the hot forging process of an M50NiL equivalent alloy. This manufacturing method includes a forging temperature heating step of heating a forging raw material to a forging temperature of 1000 to 1100°C, and a hot forging step of repeating an operation of extending the entire length by pressing the heated forging raw material from four directions over the entire length while rotating the forging raw material in the circumferential direction by radial forging to obtain a hot forging material. In the hot forging step, the forging end temperature is 800°C or higher.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] 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, resulting in a mixed grain structure with a mixture of coarse and fine grains, there is a concern that cracks may occur due to the coarse grains. Therefore, 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. However, 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. [Means for solving the problem]

[0006] 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 block division step in which the material is hot forged afterwards to obtain a material for finish forging, and a finish hot forging step 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 repeatedly forming a hot forged material. [Effects of the Invention]

[0007] 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. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional photograph of a sample after a simulated compression test of the present invention. [Figure 2] This is a cross-sectional photograph of a sample after a simulated compression test of the present invention. [Figure 3] This is a cross-sectional photograph of a sample of the finished simulated compressed material of the present invention after annealing and quenching. [Figure 4] These are cross-sectional mirror photographs of hot-forged materials of the present invention and comparative examples. [Figure 5] These are the grain size measurement results for hot forged materials after annealing and quenching of the present invention example and comparative example. [Modes for carrying out the invention]

[0009] 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 kept between 0.15% and 0.35%.

[0010] 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, increases the amount of retained austenite, and leads to problems such as increased deformation after cold working. Therefore, the amount of Ni 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 (chromium) improves hardenability and resistance to tempering softening at high temperatures, so a minimum of 4.00% is necessary. On the other hand, if Cr exceeds 4.25%, carbide precipitation is promoted, making it difficult to control hardness during manufacturing. Therefore, the Cr content should be between 4.00% and 4.25%.

[0011] Mo: 4.00~4.50% Mo (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 Mo content exceeds 4.50%, carbide precipitation is promoted, making it difficult to control hardness during manufacturing. Therefore, the Mo content 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.

[0012] Next, the present invention will be described in the order of the manufacturing process. <Preparation process> First, an ingot of an M50NiL equivalent alloy having the above composition is manufactured. Since the M50NiL equivalent alloy is an alloy used for aircraft engine parts and the like as described above, it is preferable to manufacture a consumable electrode by vacuum melting and perform vacuum remelting to reduce non-metallic inclusions and component segregation. In the present invention, the ingot may be used as a forging material, and for example, it may be further machined into a round bar-shaped or square bar-shaped forging material. In addition, in order to further reduce component segregation, a homogenization heat treatment may be performed on the forging material.

[0013] <Chunking process> After heating the forging material to a forging temperature of 1070 to 1200 °C, a chunking process of hot forging at this forging temperature (forging start temperature) is performed. By performing the chunking process at the above-described temperature, it is possible to obtain a structure with fine and uniform crystal grains in combination with a finishing hot forging process described later. When the heating temperature is less than 1070 °C, there is a concern that the mechanical properties may deteriorate due to a mixed grain structure in which fine crystal grains that have not fully grown and coarse non-recrystallized grains remain. In addition, the "mixed grains" in the present embodiment means that there are those different from the grain size number having the maximum frequency within one visual field by 3 or more, and that occupies 20% or more in terms of area ratio. When the heating temperature exceeds 1200 °C, there is a concern that coarse crystal grains may be formed due to excessive growth of crystal grains. The "hot forging" in the chunking process of the present embodiment can be hot free forging in which the workpiece is placed on an anvil whose surface is flat or curved and the material is processed by a hammer whose surface is also flat or curved in order to facilitate the processing of a large ingot shape, but impression die forging using a die may also be used. Further, while rotating the forging material in the circumferential direction, an operation of extending the entire length by repeatedly pressing from four directions over the entire length to obtain a forged material may be combined with the above-described free forging or impression die forging. In addition, a heat treatment process or a polishing process may be introduced between the chunking process and the finishing hot forging process.

[0014] <Finishing hot forging process> Subsequently, the finishing forging material after the above-mentioned block dividing process is heated to a forging temperature of 1100 to 1200 °C, and a hot forging material (hereinafter also referred to as "forging material") is obtained by radial forging at this forging temperature (forging start temperature), and a finishing hot forging process is carried out. The reason for applying radial forging in the finishing hot forging process is to utilize a control mechanism that enables gradual forging into a polygon while precisely controlling the feed rate, the rotational speed of the material, and the reduction amount of the material, and finally machining into a round shape, so that the heat generated during processing can be controlled and the temperature of the product inside and on the surface 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 crystal grain size, and the reason for setting the upper limit of the heating temperature to 1200 °C is to suppress the coarsening of crystal grains. The preferable lower limit of the heating temperature is 1120 °C. In addition, for the elongation (forging) by radial forging, it is preferable that the reduction by pressing is in the range of 50 to 210 times / minute, the area reduction rate per pass is 20 to 28%, and the feed rate on the insertion side of the forged material is 2.7 to 6 m / minute. By forging under these conditions, uniform tissue control over the entire length can be achieved, and since the adjustment of the heat generated during processing is easy, the end temperature can be controlled. And the forging end temperature can be set to 800 °C or higher. In addition, in order to make the forging end temperature 800 °C or higher, it is preferable to forge under conditions where the forging time per pass is short. When the forging end temperature is less than 800 °C, the temperature of the material surface layer drops, and the recrystallization of austenite grains is not promoted, so partially coarse crystal grains remain. Therefore, the forging elongation end temperature should be 800 °C or higher. The preferable forging elongation end temperature is 820 °C or higher. Note that the forging end temperature is the surface temperature of the forging material. According to the method for manufacturing the hot forging material of the present invention described above, the variation in crystal grain size during hot forging and after quenching can be prevented.

Example

[0015] (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.

[0016] [Table 1]

[0017] First, an experiment was conducted to determine 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 compressed at temperatures 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 showed the presence of coarse unrecrystallized grains (Figure 1(e)A), as shown in Figure 1(e).

[0018] (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 microstructural images of the longitudinal section after the simulated finishing compression test (upper panel: 100x magnification, lower panel: 200x magnification). Figure 3 shows microstructural images of the longitudinal section after annealing and quenching at 1100°C for 15 min following the simulated finishing compression test (upper panel: 100x magnification, lower panel: 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, exhibited a fully 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, contained 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, which were compressed at temperatures of 1100°C or higher, the maximum grain size was smaller and the variation in grain size was reduced.

[0019] [Table 2]

[0020] (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 finish 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 finish forging material with a diameter of 140 mm and a total length of 3000 mm (finish 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 (finish 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 results of measuring the crystal grain size using an optical microscope according to ASTM standards are shown in Figure 5. 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 these results, it was confirmed that applying the hot forging process defined in this invention makes it possible to prevent variations in grain size of M50NiL equivalent alloys during hot forging and after quenching.

Claims

[Claim 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.

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