Alloy steel

Abstract

Provided is an alloy steel that is a M50NiL-equivalent alloy which has miniaturized crystal grains after quenching.　The alloy steel contains, in terms of mass%, 0.11-0.15% C, 0.10-0.25% Si, 0.15-0.35% Mn, 4.00-4.25% Cr, 3.20-3.60% Ni, 4.00-4.50% Mo, 1.13%-1.33% V, and 0.040-0.30% of one or more of Nb, Ti, Zr, Hf, and Ta, the remainder being Fe and unavoidable impurities. Preferably, the alloy steel further contains, in terms of mass%, 0.001-0.10% N.

Description

alloy steel

[0001] The present invention relates to alloy steels.

[0002] The alloy specified by AMS6278 is called M50NiL, and its composition, by mass%, is 0.11 to 0.15% C (carbon), 0.10 to 0.25% Si (silicon), 0.15 to 0.35% Mn (manganese), 3.20 to 3.60% Ni (nickel), 4.00 to 4.25% Cr (chromium), 4.00 to 4.50% Mo (molybdenum), 1.13 to 1.33% V (vanadium), and the balance is Fe (iron) and impurities. This M50NiL is quenched and tempered and used as bearing steel for, for example, engine and transmission parts for automobiles, industrial machinery, and aircraft.

[0003] For example, in Patent Document 1, the applicant of the present application has proposed a method for producing hot-forged material for the purpose of suppressing variations in the grain size after the hot forging process and quenching of an M50NiL-equivalent alloy. This production method includes a forging temperature heating process in which a forging material is heated to a forging temperature of 1100 to 1200°C, and a hot forging process in which the heated forging material is radially forged by rotating the forging material in the circumferential direction while pressing it from four directions over its entire length to elongate its entire length, repeatedly performing this process to produce a hot-forged material, with the forging completion temperature in the hot forging process being 800°C or higher.

[0004] JP 2018-144065 A

[0005] One of the factors that reduces the mechanical properties of products using M50NiL as described above is coarsening of crystal grains. Refining crystal grains is important because it increases strength and toughness at the same time. Meanwhile, aircraft engine parts are required to achieve higher power and efficiency, so alloy materials are also required to have higher strength and toughness. While the manufacturing method described in Patent Document 1 is an excellent invention that is effective in refining crystal grains, there is still room for improvement in order to stably control even finer crystal grains. Therefore, an object of the present invention is to provide an alloy steel that can refine the crystal grains of an M50NiL-equivalent alloy after quenching.

[0006] The present invention has been made in view of the above-mentioned problems. Specifically, the present invention relates to an alloy steel containing, by mass%, 0.11 to 0.15% C, 0.10 to 0.25% Si, 0.15 to 0.35% Mn, 4.00 to 4.25% Cr, 3.20 to 3.60% Ni, 4.00 to 4.50% Mo, 1.13 to 1.33% V, 0.040 to 0.30% of one or more of Nb, Ti, Zr, Hf, and Ta, with the balance being Fe and unavoidable impurities. Preferably, the alloy steel further contains, by mass%, 0.001 to 0.10% N.

[0007] According to the present invention, it is possible to refine the crystal grains of an alloy equivalent to M50NiL after quenching.

[0008] 1 is a photograph of a cross section of a sample of an example of the present invention and a comparative example after quenching.

[0009] First, the reasons for the compositional limitations of the M50NiL-equivalent alloy specified in this invention will be described. Note that the percentages in the chemical components are mass percent. C: 0.11-0.15% C is an element effective in improving hardness, and a minimum of 0.11% is required. However, adding more than 0.15% C reduces toughness, so the C content is set to 0.11-0.15%.

[0010] Si: 0.10 to 0.25% Si is an element effective in strengthening the ferrite phase. If the Si content is less than 0.10%, the material has too much ductility, which impairs cold machinability. On the other hand, adding more than 0.25% Si reduces toughness, so the Si content is set to 0.10 to 0.25%.

[0011] Mn: 0.15 to 0.35% Mn is an element effective in improving hardenability, and a minimum of 0.15% is required. On the other hand, if the Mn content exceeds 0.35 mass%, the hardness increases too much, causing problems such as poor workability. Therefore, the Mn content is set to 0.15 to 0.35%.

[0012] Ni: 3.20 to 3.60% Ni is an element effective in improving hardenability, and a minimum of 3.20% is required, but excessive addition lowers the Ms point, increases the amount of retained austenite, and causes problems such as increased deformation after cold working, so the Ni content is set to the range of 3.20 to 3.60%. To ensure the effects of Ni, the preferred lower limit is 3.25%, and the preferred upper limit is 3.50%.

[0013] Cr: 4.00 to 4.25% Like Mo, which will be described later, Cr has the effect of improving hardenability and temper softening resistance at high temperatures, so it is necessary to add at least 4.00%. On the other hand, if Cr exceeds 4.25%, precipitation of carbides is promoted, making it difficult to control hardness during manufacturing. Therefore, Cr is set to 4.00% to 4.25%. To ensure the effects of Cr, the preferred lower limit is 4.05%, and the preferred upper limit is 4.20%.

[0014] Mo: 4.00 to 4.50% Mo has the effect of improving hardenability and temper softening resistance at high temperatures, so it is necessary to add at least 4.00%. On the other hand, if Mo exceeds 4.50%, precipitation of carbides is promoted, making it difficult to control hardness in manufacturing. Therefore, Mo is set to 4.00% to 4.50%. To reliably obtain the effects of Mo, the preferred lower limit is 4.05%, and the preferred upper limit is 4.40%.

[0015] V: 1.13 to 1.33% V has the effect of improving temper softening resistance and refining crystal grains. If V is less than 1.13%, VC precipitation is low and crystal grains become coarse. On the other hand, if V exceeds 1.33%, carbide precipitation is promoted, making it difficult to control hardness during manufacturing. Therefore, V is set to 1.13 to 1.33%. To reliably obtain the effects of V, the preferred lower limit is 1.15%, and the preferred upper limit is 1.25%.

[0016] One or more of Nb, Ti, Zr, Hf, and Ta: 0.040 to 0.30% The alloy steel of the present invention is characterized by further containing one or more of Nb, Ti, Zr, Hf, and Ta so as to satisfy the following condition. The above-mentioned elements are among the elements that strongly form carbides. Preferably, at least Nb is contained, and more preferably, Nb is selected. The following will be described using an embodiment in which Nb is selected as an example. Inclusion of Nb within the above-mentioned range has the effect of forming fine Nb-based carbides, nitrides, or carbonitrides (also referred to as Nb(C,N)) to refine crystal grains. If the Nb content is less than 0.040%, the Nb(C,N) content is low and the effect is not apparent. On the other hand, if the Nb content increases, the sliding properties deteriorate due to an increase in the amount of Nb(C,N) and coarsening. Therefore, the content of one or more of Nb, Ti, Zr, Hf, and Ta is set to 0.040% to 0.30%. The lower limit of Nb is preferably 0.05%, 0.06%, or 0.065%, and the upper limit of Nb is preferably 0.25%, more preferably 0.20%, or even more preferably 0.15%.

[0017] The alloy steel of the present invention may contain a small amount of Al to obtain additional effects such as improved mechanical properties and manufacturing efficiency, as long as the effects of the present invention are not impaired. In this case, the preferred upper limit of Al is 0.05%. The alloy steel of the present invention may also contain N to form Nb nitrides or carbonitrides to more reliably obtain the grain refinement effect. On the other hand, since too much N tends to produce coarse nitrides that do not contribute to grain refinement, the N content is preferably in the range of 0.001 to 0.30%. A preferred lower limit of nitrogen is 0.003%, a preferred upper limit of nitrogen is 0.10%, and a more preferred upper limit of nitrogen is 0.05%. Furthermore, in the alloy steel of the present invention, a portion of the Ni can be substituted with Co. However, since Co has similar effects to Ni and is expensive, if substituted, the maximum amount should be 0.25% or less. Furthermore, a portion of the Mo can also be substituted with W. In this case, the maximum amount should be 0.5% or less, assuming 0.5W. In addition to the elements described above, the steel contains Fe and unavoidable impurities, such as P, S, Cu, and O. The upper limits of the impurities can be set, for example, in mass % as follows: P≦0.015%, S≦0.010%, Cu≦0.10%, and O≦0.0030%.

[0018] The alloy steel of the present invention having the above-mentioned alloy composition ranges is expected to have a small average grain size of prior austenite grains after quenching (a large average grain size number), and to stably obtain good mechanical properties. A preferred average grain size number (based on ASTM-E112) of the present invention is 6.5 or more. A more preferred lower limit of the average grain size number is 7.0. These grain size numbers can be measured in a state after quenching or after quenching and tempering. This is because the grain size does not substantially change between after quenching and after quenching and tempering.

[0019] Ten kg of steel ingots of M50NiL-equivalent alloys with the chemical compositions shown in Table 1 were melted to obtain steel ingots Nos. 1 to 4 (inventive examples) and Nos. 5 and 6 (comparative examples). Although not listed in Table 1, the nitrogen (N) content of all steel ingots was in the range of 0.004 to 0.025% by mass. These ingots were then hot-worked to obtain 30 mm square rods for testing. Chemical composition can be analyzed according to analytical methods specified by ASTM, and C can be analyzed by combustion-infrared absorption analysis according to ASTM-E1019. Other elements can be analyzed by X-ray fluorescence analysis according to ASTM-E1085. S, N, and O can also be analyzed by combustion-infrared absorption analysis or inert gas fusion-infrared absorption analysis according to ASTM-E1019.

[0020]

[0021] To confirm the grain size after quenching, the experimental materials of the present invention and comparative examples obtained above were annealed, and then test specimens were taken. The test specimens were taken at the W / 4-T / 4 (W: width, T: thickness) position and the L / 2 (L: length) position on a 30 mm square cross section. The longitudinal cross sections after quenching were then observed using an optical microscope at 200x magnification and 30 observation fields, and grain size measurements were performed using image analysis software. Optical microscope photographs are shown in Figure 1, and the average grain size measurements are shown in Table 2. Figure 1 confirms that the present invention examples (Nos. 1, 2, 3, and 4) had fewer coarse grains and more fine grains than the comparative examples (Nos. 5 and 6) with Nb content less than 0.040%. Table 2 also confirms that the present invention examples had an average grain size of 7.0 or more, indicating that the grains were finer than the comparative examples. From the above results, it is possible to refine the crystal grains of an alloy equivalent to M50NiL after quenching by controlling the Nb content to the range specified in the present invention.

[0022]

Claims

1. An alloy steel consisting of, by mass%, C: 0.11 to 0.15%, Si: 0.10 to 0.25%, Mn: 0.15 to 0.35%, Cr: 4.00 to 4.25%, Ni: 3.20 to 3.60%, Mo: 4.00 to 4.50%, V: 1.13 to 1.33%, one or more of Nb, Ti, Zr, Hf, and Ta: 0.040 to 0.30%, and the balance: Fe and unavoidable impurities.

2. The alloy steel according to claim 1, further containing, by mass%, N: 0.001 to 0.30%.

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