Alloy steel

Abstract

Provided is an alloy steel in which the maximum grain size of an alloy corresponding to M50 after quenching can be reduced and variation in grain size can be suppressed. This alloy steel contains, in mass%, 0.70%-0.95% of C, 0.05%-0.35% of Si, 0.05%-0.45% of Mn, 3.90%-4.35% of Cr, 3.90%-4.60% of Mo and / or W as defined by the relational expression (Mo + W / 2), 0.80%-1.20% of V, and the balance Fe and unavoidable impurities, and further contains Al and N so as to satisfy the following conditions (1) and (2). Condition (1) is Al: 0.02% or more or N: 0.005% or more in mass%, and condition (2) is 7.5 <-ln ([Al]% * [N]%) < 11.5.

Description

Technical Field

[0001] The invention relates to alloy steel. Background Art

[0002] The alloy specified in Aerospace Material Specification (AMS) 6491 is called M50, and its composition is steel containing 0.80% to 0.85% C (carbon), 0.15% to 0.35% Mn (manganese), 4.00% to 4.25% Cr (chromium), 4.00% to 4.50% Mo (molybdenum), 0.90% to 1.10% V (vanadium), and the remainder is Fe (iron) and impurities. The M50 is quenched and tempered, and used as bearing steel, for example, for aircraft engine parts.

[0003] For example, Patent Document 1 describes a method for manufacturing bearing steel, characterized in that steel containing, by weight, C: 0.60% to 1.50%, Si: 0.15% to 1.00%, Mn: 0.15% to 0.50%, Cr: 3.0% to 19.0%, Mo: 0.50% to 6.50%, V: 0.05% to 1.50%, Al: 0.020% to 1.50%, and the remainder Fe and inevitable impurities is nitrided at a temperature of 500° C. to 600° C. Patent Document 1 also discloses that adding V is effective in grain refinement, and setting the quenching temperature to 1200° C. or less is effective.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Laid-Open No. 5-179401 Summary of the invention

[0007] Problems to be solved by the invention

[0008] As one of the factors that reduce the mechanical properties of the M50 product described above, the coarsening of crystal grains can be cited. In particular, even if the average grain size is small, a part of the crystal grains is coarsened to form a mixed grain structure in which coarse particles and fine particles are mixed, and cracks caused by the coarse particles are feared to occur, so it is important to also reduce the maximum grain size. In the patent document, the reduction of such a maximum grain size has not been studied.

[0009] Therefore, an object of the present invention is to provide an alloy steel capable of reducing the maximum grain size of an alloy equivalent to M50 after quenching and suppressing the deviation of the grain size.

[0010] Technical means of solving problems

[0011] The present invention has been made in view of the above-mentioned problems.

[0012] That is, the present invention is an alloy steel, which contains, by mass%, C: 0.70% to 0.95%, Si: 0.05% to 0.35%, Mn: 0.05% to 0.45%, Cr: 3.90% to 4.35%, one or two of Mo and W defined by the relationship (Mo+W / 2): 3.90% to 4.60%, V: 0.80% to 1.20%, the remainder: Fe and inevitable impurities, and further contains Al and N in a manner satisfying the following conditions (1) and (2). Here, condition (1) is that by mass%, Al: 0.02% or more, or N: 0.005% or more, and condition (2) is 7.5<-ln([Al]%×[N]%)<11.5.

[0013] Preferably, the maximum grain size number according to American Society for Testing and Materials (ASTM)-E930 is 6.0 or more.

[0014] Preferably, the average grain size number according to ASTM-E112 is 8.0 or more.

[0015] Effects of the Invention

[0016] According to the present invention, the maximum grain size of an alloy equivalent to M50 after quenching can be reduced and the variation of the grain size can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] [ Figure 1 ] is a cross-sectional photograph of the samples after quenching of the examples of the present invention and the comparative examples. DETAILED DESCRIPTION

[0018] First, the reasons for limiting the composition of the alloy steel (alloy equivalent to M50) specified in the present invention will be described.

[0019] C: 0.70 mass % to 0.95 mass % (hereinafter also referred to as "%" for brevity),

[0020] C is an element effective in improving hardness, and the minimum content needs to be 0.70%. However, adding more than 0.95% of C will reduce toughness, so C is set to 0.70% to 0.95%. It is preferably 0.80% or more. In addition, it is preferably 0.85% or less.

[0021] Si: 0.05%~0.35%,

[0022] Si is used as a deoxidizer during steelmaking and is an effective element for improving machinability. When Si is less than 0.05%, the ductility of the material is too high, which will hinder the machinability during cold working. On the other hand, adding more than 0.35% Si will reduce the toughness, so Si is set to 0.05% to 0.35%. It is preferably 0.10% or more. In addition, it is preferably 0.25% or less.

[0023] Mn: 0.05%~0.45%,

[0024] Mn is used as a deoxidizer or desulfurizer during steelmaking. It is an effective element for improving hardenability. The minimum content is 0.05%. On the other hand, if Mn exceeds 0.45%, the hardness will increase excessively, which will cause problems such as poor workability. Therefore, Mn is set to 0.05% to 0.45%. It is preferably 0.15% or more. In addition, it is preferably 0.35% or less.

[0025] Cr: 3.90%~4.35%,

[0026] Cr, like Mo described later, has the effect of improving hardenability and tempering softening resistance at high temperatures, so a minimum of 3.90% needs to be added. On the other hand, if Cr exceeds 4.35%, the precipitation of carbides is promoted, and the control of hardness in manufacturing becomes difficult. Therefore, Cr is set to 3.90% to 4.35%. Preferably, it is 4.00% or more. In addition, it is preferably 4.25% or less.

[0027] One or both of Mo and W specified by the relationship (Mo+W / 2): 3.90% to 4.60%,

[0028] Mo and W have the same effect, which is specified by (Mo+W / 2) according to the relationship between atomic weights. Since it has the effect of improving hardenability and improving tempering softening resistance at high temperatures, a minimum of 3.90% needs to be added. On the other hand, if it exceeds 4.60%, the precipitation of carbides is promoted, and the control of hardness in manufacturing becomes difficult. Therefore, (Mo+W / 2) is set to 3.90% to 4.60%. It is preferably 4.00% or more. In addition, it is preferably 4.50% or less.

[0029] In addition, if the composition of the alloy equivalent to M50 is imitated, the entire content of W can be replaced by the entire content of Mo. In this case, it becomes Mo: 3.90% to 4.60%. Moreover, it is preferably 4.00% or more. In addition, it is preferably 4.50% or less. Moreover, W can be included as an impurity. For example, if the composition of the alloy equivalent to M50 is imitated, W can be included in an amount of 0.25% or less.

[0030] V: 0.80%~1.20%,

[0031] V has the effect of improving the temper softening resistance and the effect of making the grains finer. When V is less than 0.80%, the precipitation of MC carbides is small and the grains are coarsened. On the other hand, if V exceeds 1.20%, the precipitation of carbides is promoted, and the control of hardness in manufacturing becomes difficult. Therefore, V is set to 0.80% to 1.20%. Preferably, it is 0.90% or more. In addition, it is preferably 1.10% or less.

[0032] The alloy steel of the present invention is characterized in that: it further contains Al and N in a manner that satisfies the following conditions (1) and (2). Here, condition (1) is Al: 0.02% or more, or N: 0.005% or more in mass %, and condition (2) is 7.5<-ln([Al]%×[N]%)<11.5. Al is used as a deoxidizer, and N is an element that can remain as an impurity. Since Al and N exist in a solid solution state in the parent phase, Al acts as a nucleus during the precipitation of MC carbides that are effective for grain refinement, and N has the effect of dissolving in carbides to increase the amount of MC carbides and disperse MC carbides finely and uniformly. Therefore, the alloy steel of the present invention contains more than 0.02% Al or more than 0.005% N. Here, in the steel of the present invention, Al may be more than 0.02% and N may be more than 0.005%.

[0033] Moreover, in the present invention, in order to obtain the effect brought about by the solid solution of Al or N, it is effective to adjust the "solubility product" represented by the product of the content (mass %) of Al and N. That is, if -ln ([Al]% × [N]%) becomes less than 7.5, a large amount of AlN is generated, and the precipitation site of MC carbide is locally buried. Moreover, the amount of Al or N dissolved in the formation site of AlN is also reduced, so the distribution of the remaining MC carbide during quenching becomes uneven, resulting in coarsening of the grains. On the other hand, if -ln ([Al]% × [N]%) exceeds 11.5, although the formation of AlN can be suppressed, the amount of Al and N dissolved is also small, so the effect brought about by the solid solution of Al or N is not obvious. Therefore, it is set to 7.5<-ln ([Al]% × [N]%)<11.5. The preferred lower limit of -ln ([Al]% × [N]%) is 8.0, more preferably 8.5, and further preferably 9.0. The upper limit of -ln([Al]%×[N]%) is preferably 11.0, more preferably 10.5, and still more preferably 10.0.

[0034] The parts other than the above-described elements are Fe and inevitable impurities. As inevitable impurities, P, S, Ni, Co, Cu, and O can be listed. For example, if the composition of the alloy equivalent to M50 is imitated, the upper limits of each can be set to P≦0.015%, S≦0.008%, Ni≦0.15%, Co≦0.25%, Cu≦0.10%, and O≦0.0030% in mass %.

[0035] The alloy steel of the present invention with the alloy composition range described above has a small maximum grain size (large maximum grain size number) and a small deviation of grain size, so it is expected to stably obtain good mechanical properties. The preferred maximum grain size number of the present invention (according to ASTM-E930) is more than 6.0. In addition, the preferred average grain size number of the present invention (according to ASTM-E112) is more than 8.0. The lower limit of the preferred average grain size number is 8.5, and then the lower limit of the preferred average grain size number is 9.0. These grain size numbers can be measured in the state after quenching or after quenching-tempering. The reason is that the grain diameter does not change substantially after quenching and after quenching-tempering.

[0036] Example

[0037] 10 kg of an alloy equivalent to M50 having the composition shown in Table 1 was melted to obtain steels No. 1 to No. 5 as examples of the present invention and No. 6 to No. 8 as comparative examples. Then, hot working was performed to obtain experimental raw materials in the form of 30 mm square bars.

[0038] [Table 1]

[0039]

[0040] In order to confirm the grain size after quenching, the experimental raw materials of the present invention examples and comparative examples obtained in the above are annealed and then the test pieces are collected. The collection point of the test piece is set to the W / 4-T / 4 (W: length in the width direction, T: length in the thickness direction) position of the 30 mm square section and the L / 2 (L: length in the length direction) position in the length direction. Then, each raw material is quenched at 1115°C, and the results obtained by measuring the grain size using an optical microscope according to the ASTM standard (average grain size is ASTM-E112-13 (2021), maximum grain size is ASTM-E930-18) are shown in Table 2. In addition, No. 6 is a mixed grain and the average grain size cannot be measured, so it represents the distribution ratio of the grain size. According to Table 1, it can be confirmed that the examples of the present invention, which are Al: 0.020% or more, or N: 0.005% or more and satisfy 7.5<-ln([Al]%×[N]%)<11.5, are at the same level as the comparative examples of Sample No. 7 and Sample No. 8 in terms of average grain size, but the maximum grain size is 6.0 or more in terms of grain size No., which is finer than the comparative examples, and the deviation of grain size is small.

[0041] Regarding No.2, No.3, No.6, No.8, also Figure 1 An optical microscope photograph is shown in . The observation position is set to the longitudinal section of the test piece after quenching (TL section obtained by dividing it into two halves parallel to the longitudinal direction). In addition, Figure 1 The observation magnification of the upper section is 200 times, and the observation magnification of the lower section is 500 times. Figure 1 It can be confirmed that in the examples of the present invention (No. 2 and No. 3) that satisfy the conditions (1) and (2) of the present invention, the grain size is fine as a whole and the deviation is small. In the comparative examples (No. 6 and No. 8), coarse particles such as those exceeding 50 μm can be confirmed among the fine particles, and the deviation of the grain size is large. According to the above results, by controlling the Al amount and the N amount specified in the present invention, the deviation of the grain size of the alloy equivalent to M50 after quenching can be suppressed. In addition, by tempering the alloy equivalent to M50 after quenching, an alloy product equivalent to M50 with reduced mechanical properties can be provided.

[0042] [Table 2]

[0043]

Claims

1. An alloy steel comprising, by mass %,: C：0.70%～0.95%、 Si: 0.05%~0.35%, Mn: 0.05%~0.45%, Cr：3.90%～4.35%、 One or both of Mo and W specified by the relationship (Mo+W / 2): 3.90% to 4.60%, V：0.80%～1.20%、 The remainder: Fe and unavoidable impurities, Furthermore, Al and N are contained so as to satisfy the following conditions (1) and (2), Condition (1) is that, in terms of mass %, Al: 0.02% or more, or N: 0.005% or more, Condition (2) is 7.5<-ln([Al]%×[N]%)<11.

5.

2. The alloy steel according to claim 1, wherein: The maximum grain size number according to ASTM E930 is 6.0 or above.

3. The alloy steel according to claim 2, wherein: The average grain size number according to ASTM E112 is 8.0 or more.

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