Steel for machine structural use

Abstract

To suppress abnormal grain growth without making manufacturing conditions of steel for machine structural use complicated.SOLUTION: Steel for machine structural use contains C:0.10 to 0.55%, Si:0.03 to 2.0%, Mn:0.30 to 2.0%, P:≤0.030%, S:≤0.050%, Al:0.005 to 0.050%, Cu:≤0.5%, Ni:≤2.0%, Cr:≤2.0%, Mo:≤1.0%, N:≤0.01% and REM:0.03 to 1.0 in mass%, and the balance are Fe and inevitable impurities.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to structural steel used as a material for important parts of transportation equipment such as automobiles, construction equipment, industrial equipment, and the like. [Background technology]

[0002] Structural steel is often used after its strength has been improved through heat treatments such as quenching and carburizing. Carburizing refers to the process of heating and quenching above the A3 point in a carburizing atmosphere to form a high-carbon martensite structure at the surface. Furthermore, with this carburizing and quenching process, as well as with conventional quenching, abnormal grain growth of gamma grains can occur during heat treatment above the A3 point, resulting in significant enlargement. Abnormal grain growth (grain coarsening) is likely to occur during long-term carburizing and quenching at high temperatures or when cold-forged materials are directly heat-treated. Grain coarsening can result in reduced strength and toughness and increased heat treatment distortion, making it essential to prevent coarsening in structural steels.

[0003] Several techniques are known for preventing the coarsening of crystal grains. For example, a technique is known in which fine pinning particles such as AlN are reduced as much as possible, steady grain growth is promoted, and abnormal grain growth is suppressed to suppress abnormal coarse grains (see, for example, Patent Documents 1 to 3 listed below). Another technique is known in which a predetermined amount of pinning particles such as Nb or Ti is added to suppress grain growth itself (see, for example, Patent Documents 4 and 5 listed below). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-140482 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-140481 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-234552 [Patent Document 4] Japanese Patent Application Publication No. 9-324848 [Patent Document 5] Japanese Patent Application Publication No. 2019-183266 [Non-patent literature]

[0005] [Non-Patent Document 1] Kyohei Nakayama, Takahiro Miyazaki, Keisuke Inoue, "Development of Low-Alloy Case-Hardening Steel with Excellent Manufacturability and Abnormal Grain Growth Suppression," Electric Steel Manufacturing, Vol. 85, No. 2, pp. 133-138, Daido Steel Co., Ltd., Research and Development Division, December 2014 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the techniques described in Patent Documents 1 to 3 and Non-Patent Documents raise concerns that it may be difficult to establish manufacturing conditions, as it is necessary to precisely control the amounts of added elements such as Al, Ti, and N in order to reduce pinning particles. Also, the techniques described in Patent Documents 4 and 5 tend to add pinning particles such as Ti and Nb in excess, which raises concerns that this may worsen machinability or adversely affect manufacturing conditions due to precipitated particles.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to suppress abnormal grain growth without complicating the manufacturing conditions of steel for machine structural use.

[0008] As a result of extensive research into solving the above-mentioned problems, the present inventors have found that by adding a relatively small amount of REM (rare earth element), the number of pinning particles can be reduced, steady grain (regular coarse grain) growth can be easily maintained, and coarsening of crystal grains, including abnormal grain growth that results in #3 or less, can be suppressed during quenching. [Means for solving the problem]

[0009] Specific means for solving the above problems are as follows. First, a first invention is a steel for machine structural use containing, by mass%, C: 0.10 to 0.55%, Si: 0.03 to 2.0%, Mn: 0.30 to 2.0%, P: ≦0.030%, S: ≦0.050%, Al: 0.005 to 0.050%, Cu: ≦0.5%, Ni: ≦2.0%, Cr: ≦2.0%, Mo: ≦1.0%, N: ≦0.01%, REM: 0.03 to 1.0%, and the balance being Fe and unavoidable impurities.

[0010] Next, a second invention is characterized in that the REM comprises at least one of Ce, La, and Nd.

[0011] Next, the third invention is characterized in that the contents of La, Ce, Nd, S, and N, [La], [Ce], [Nd], [S], and [N], satisfy the following formula (1): (0.991×[La]+[Ce]+1.029×[Nd])-2.91×[S] -10.0×[N]>0...Equation (1) However, the amount in brackets [ ] is the content of each element expressed in mass %. [Effects of the Invention]

[0012] According to the steel for machine structural use specified in this way, abnormal grain growth can be suppressed without complicating the manufacturing conditions of the steel for machine structural use. [Brief explanation of the drawings]

[0013] [Figure 1] 10 is a photograph showing the state of a machine structural member according to a comparative example after carburizing and quenching. [Figure 2] 3 is a photograph showing the state of the machine structural member according to Example 1 after carburizing and quenching. [Figure 3] 10 is a photograph showing the state of a machine structural member according to Example 2 after carburizing and quenching. DETAILED DESCRIPTION OF THE INVENTION

[0014] The steel for machine structural use according to this embodiment contains predetermined amounts of C, Si, Mn, P, S, Al, Cu, Ni, Cr, Mo, N, and REM, with the balance being Fe and inevitable impurities. The reasons for limiting each chemical component in the steel for machine structural use according to this embodiment will be explained below. In the following explanation, "%" means "mass %" unless otherwise specified.

[0015] C: 0.10 to 0.55% C is contained in an amount of 0.10% or more to ensure hardness and strength. However, if the C content exceeds 0.50%, the workability decreases when cold forging or the like is performed on the steel material to form gears or other components. Therefore, the upper limit is set to 0.55%. The C content is preferably 0.30 to 0.40%.

[0016] Si: 0.03 to 2.0% The Si content must be 0.03% or more to ensure hardenability and strength. However, if the Si content exceeds 2.0%, the forgeability and machinability will be reduced, so the upper limit is set to 2.0%. The Si content is preferably 0.10 to 0.30%.

[0017] Mn: 0.30 to 2.0% Mn is contained in an amount of 0.30% or more to control the morphology of inclusions such as MnS and to ensure hardenability. Furthermore, if Mn is contained in an amount less than 0.30%, ferrite forms in the core, resulting in a decrease in strength. Therefore, in this sense, Mn is contained in an amount of 0.30% or more. However, if Mn is contained in an amount exceeding 2.0%, it promotes machinability and P segregation, resulting in a decrease in toughness, so the upper limit is set at 2.0%. The preferred range is 0.70 to 0.90%.

[0018] P:≦0.030% In the present invention, P is an impurity component that reduces strength, and is restricted to 0.030% or less.

[0019] S:≦0.050% In the present invention, S is an element that is undesirable for the mechanical properties of machine structural parts, such as causing embrittlement during hot working, and therefore is restricted to 0.050% or less.

[0020] Al: 0.005 to 0.050% Al is contained in steel as a deoxidizer, and in the present invention, its content is set to be within the range of 0.005% to 0.050%.

[0021] Cu:≦0.5% If Cu is contained in a large amount exceeding 0.5%, hot workability will be reduced, so the upper limit is set to 0.5%.

[0022] Ni:≦2.0% If Ni is contained in an amount exceeding 2.0%, the amount of residual γ increases during quenching, resulting in a decrease in strength, so the upper limit is set to 2.0%, preferably 1.0% or less, and more preferably 0.5% or less.

[0023] Cr:≦2.0% Cr is an element that is effective in improving hardenability and increasing strength, and therefore may be contained in a specified amount or more. However, if the content exceeds 2.0%, workability, particularly machinability, is reduced, so the upper limit is set at 2.0%.

[0024] Mo:≦1.0% Mo is an element that improves strength, and may be contained in a predetermined amount or more. However, if it is contained in a large amount exceeding 1.0%, it will deteriorate workability and increase costs, so the upper limit is set to 1.0%.

[0025] N:≦0.010% N increases nitrides, which reduces strength, and increases costs due to the increased amount of REM added, so the upper limit is set to 0.010%.

[0026] REM: 0.03 to 1.0% REM tends to maintain regular grain growth during quenching, suppressing grain coarsening. It has the function of reducing the hardness of the steel. To obtain this effect, the REM content must be 0.03% or more. Preferably, it is 0.05% or more, and more preferably, it is 0.5% or more. However, excessive addition of REM will combine with the C in the base material, reducing hardness. It will also form coarse carbides, reducing hot workability. Therefore, the upper limit is set to 1.0%. From the viewpoints of availability and handling, it is preferable that REM consist of one or more of Ce, La, and Nd.

[0027] The following formula (1), which shows the contents of La, Ce, Nd, S, and N as [La], [Ce], [Nd], [S], and [N], has the following meaning: La, Ce, and Nd, which are representative REM elements, bond preferentially with S, and then bond with N. Any excess La, Ce, or Nd that does not bond as sulfides or nitrides bonds with C to form carbides. Formula (1) defines the contents of La, Ce, and Nd that can bond with C based on the ratio of their atomic weights. (0.991×[La]+[Ce]+1.029×[Nd])-2.91×[S] -10.0×[N]>0...Equation (1) However, the amount in brackets [ ] is the content of each element expressed in mass %.

[0028] The manufacturing process for manufacturing machine structural parts using the steel for machine structural use of this embodiment can be exemplified as follows: First, an ingot or continuous cast material having a predetermined chemical composition is bloomed, and then the machine structural part is manufactured through the processes of hot rolling, hot forging, rough heat treatment, machining, carburizing and quenching or quenching, tempering, and finish machining.

[0029] According to the steel for machine structural use of this embodiment, the REM content is specified so that regular coarse grain growth is easily maintained during carburizing and quenching or normal quenching. Therefore, the structure of the steel for machine structural use of this embodiment after carburizing and quenching or quenching can be characterized as a structure in which the average crystal grain size of the prior austenite grains is 4 to 7. In the prior art, a predetermined amount of Ti or the like is added to maintain regular coarse grain growth (to form regular coarse grains). However, controlling the amount of Ti or the like added during manufacturing is difficult, and excessive addition leads to the formation of fine precipitates of TiC, which creates a pinning effect and hinders the formation of regular coarse grains. On the other hand, in the steel for machine structural use of this embodiment, when REM combines with C in the steel, it crystallizes not as fine precipitates but as carbides of appropriate size, and therefore the pinning effect is thought to be extremely small. Furthermore, the addition of the predetermined amount of REM results in almost no crystallization of AlN, which would cause a pinning effect. That is, according to the steel for machine structural use of this embodiment, abnormal grain growth can be suppressed without complicating the manufacturing conditions. [Example]

[0030] Next, examples of the present invention will be described below. Test materials were prepared based on the chemical compositions of the examples and comparative examples shown in Table 1 below, and various evaluations were carried out.

[0031] [Table 1]

[0032] Steels having the chemical compositions shown in Table 1 were melted by vacuum melting, homogenized at 1300°C for 2.5 hours or more, and then hot forged at 950°C or higher to form steel bars with a diameter of 32 mm, which were then cut to lengths of 30 mm.

[0033] <Quenching and tempering treatment> After the above treatment, the material was heated in a salt furnace at 900°C for 1 hour and then oil-quenched for quenching.

[0034] <Evaluation of crystal grains> The state of crystal grain formation was evaluated for the test materials after quenching. For grain observation, a test piece (which had been oil-quenched) was cut in half in the longitudinal cross section, and the cut surface was mirror-polished and then etched with supersaturated picric acid to reveal the prior austenite grain boundaries and measure the grain size. The measurement was carried out in the center of the longitudinal cross section using an optical microscope at a specified magnification in five fields of view in accordance with JIS G 0551, and the average value was calculated.

[0035] The following can be seen from Table 1, Figure 1, Figure 2, and Figure 3. The comparative example is an example in which no REM is added. As shown in Table 1, the comparative example has a large prior austenite grain size of 11 or more, and the grains are fine as shown in Figure 1. Furthermore, the comparative example is outside the range of the above-mentioned formula (1).

[0036] In contrast, Examples 1 to 15, whose chemical compositions satisfy the conditions of this embodiment, have prior austenite grain sizes of 4 to 7. Furthermore, all of Examples 1 to 15 fall within the range of the above-mentioned formula (1). Furthermore, as shown in Figures 2 and 3, in Examples 1 and 2, the crystal grains grow uniformly to a certain size, effectively preventing specific crystal grains from undergoing abnormal grain growth. In this way, when the steel for mechanical structural use has uniform coarse grains that are uniformly composed of a certain size, it is expected that mechanical structural parts made of that steel for mechanical structural use will have excellent low-cycle fatigue properties, which are determined by whether or not fatigue failure occurs after about 100 to 1,000 repetitions, as described in Non-Patent Document 1, when a large repeated load that causes plastic deformation is applied.

[0037] From the above, it is clear that the steel for machine structural use having the chemical composition of each example can suppress abnormal grain growth without complicating the manufacturing conditions. Furthermore, when a steel for machine structural use having the chemical composition of each example is carburized and quenched, if the steel is a machine structural part having a prior austenite grain size of 4 to 7, it is expected that the low cycle fatigue properties will be improved.

[0038] Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments and various modifications can be made.

Claims

1. By mass% C: 0.10-0.55% Si: 0.03-2.0% Mn: 0.30-2.0% P: ≦0.030% S: ≦0.050% Al: 0.005-0.050% Cu: ≦0.5% Ni: ≦2.0% Cr: ≦2.0% Mo: ≦1.0% N: ≦0.01% REM: 0.03~1.0% and the balance being Fe and unavoidable impurities.

2. 2. The steel for machine structural use according to claim 1, wherein the REM comprises at least one of Ce, La, and Nd.

3. The steel for mechanical structural use according to claim 2, characterized in that the contents of La, Ce, Nd, S, and N, [La], [Ce], [Nd], [S], and [N], satisfy the following formula (1): (0.991×[La]+[Ce]+1.029×[Nd])-2.91×[S] -10.0×[N]>0 ...Formula (1) However, the amount in brackets [ ] is the content of each element expressed in mass %.

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