Case-hardening steel having superior coarse grain prevention property during carburization

Abstract

To provide case-hardening steel enabling suppression of crystal grain coarsening during carburization.SOLUTION: The case-hardening steel comprises C: 0.17-0.23%, Si: 0.80-1.00%, Mn: 0.65-1.00%, P: 0.030% or less, S: 0.030% or less, Cu: 0.01-1.00%, Ni: 0.01-3.00%, Cr: 0.80-1.00%, Al: 0.030-0.047%, N: 0.015-0.030%, and Nb: 0.010-0.045%, with the balance being Fe and unavoidable impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a case-hardening steel material that is excellent in preventing coarse grains during carburization. [Background technology]

[0002] For example, in the case of gear parts used as power transmission parts in automobiles, case-hardened steels such as JIS steels SCr420H and SCM420H are carburized to ensure surface strength and internal toughness.

[0003] Carburizing is a high-temperature, long-term heat treatment, which can cause coarsening of austenite grains, resulting in a decrease in fatigue strength and impact properties. A widely used technique for preventing coarsening of austenite grains during carburizing is to disperse and precipitate particles such as NbC and AlN to pin the grains (more specifically, grain boundaries) (see, for example, Patent Documents 1 and 2 listed below). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-163168 [Patent Document 2] Japanese Patent Application Publication No. 2020-41199 Summary of the Invention [Problem to be solved by the invention]

[0005] However, even with the above-mentioned technique of pinning crystal grains with precipitate particles (pinning particles), it is difficult to fully prevent the phenomenon of abnormal grain growth in which crystal grains become locally coarse. SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a case-hardened steel material that has excellent coarse grain prevention properties and is capable of suppressing the coarsening of crystal grains during carburization. [Means for solving the problem]

[0006] As a result of extensive research into solving the above problems, the inventors discovered that if the structure before carburizing is fine, coarsening of grains (abnormal grain growth) is more likely to occur in the subsequent carburizing process, and that by optimizing the composition of the case-hardened steel, it is possible to avoid the structure before carburizing becoming fine, and to suppress coarsening of grains during carburizing.

[0007] The gist of the present invention is as follows.

[0008] [1] Case-hardening steel with excellent grain coarsening prevention properties during carburizing, containing, by mass%, C: 0.17-0.23%, Si: 0.80-1.00%, Mn: 0.65-1.00%, P: 0.030% or less, S: 0.030% or less, Cu: 0.01-1.00%, Ni: 0.01-3.00%, Cr: 0.80-1.00%, Al: 0.030-0.047%, N: 0.015-0.030%, Nb: 0.010-0.045%, with the remainder being Fe and unavoidable impurities.

[0009] [2] Case-hardened steel material according to [1], which has a ferrite-pearlite structure and has an average grain size number of ferrite of 10.6 or less as measured in accordance with JIS G 0552 in 5 fields of view at 100x magnification using an optical microscope.

[0010] With case-hardening steel specified in this way, most of the Al and Nb are dissolved during hot rolling, which reduces the amount of pinning particles that precipitate before forging heating and makes it possible to reduce the grain size number of the metal structure before carburizing (to make the grains larger).As a result, the driving force for grain growth during carburizing is reduced, and coarsening of grains (abnormal grain growth) during carburizing can be effectively suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is an explanatory diagram of hot forging of the case-hardened steel material of this embodiment and the subsequent rough heat treatment. [Figure 2](A) is a diagram showing the temperature patterns of hot forging and rough heat treatment when preparing the test material, and (B) is a diagram showing the temperature pattern of carburizing and quenching treatment when preparing the test material. DETAILED DESCRIPTION OF THE INVENTION

[0012] The case-hardened steel material according to this embodiment contains predetermined amounts of C, Si, Mn, P, S, Cu, Ni, Cr, Al, N, and Nb, with the remainder being Fe and unavoidable impurities.

[0013] The reasons for limiting each chemical component in the case-hardened steel material of this embodiment will be described in detail below. In the following description, "%" means "mass %" unless otherwise specified.

[0014] C: 0.17 to 0.23% C is an element necessary for ensuring strength, and is contained in an amount of 0.17% or more to ensure the internal hardness of the part. However, since a high content of C reduces machinability, the upper limit is set to 0.23%. The preferred range is 0.20 to 0.23%.

[0015] Si: 0.80 to 1.00% Silicon is an element that is effective in increasing the high-temperature temper hardness of the carburized and quenched layer. To achieve this effect, 0.80% or more must be added. However, if the content exceeds 1.00%, workability decreases, so the upper limit is set to 1.00%. The preferred range is 0.80 to 0.95%.

[0016] Mn: 0.65 to 1.00% Mn is added as a deoxidizer during melting. Mn is a useful component for ensuring hardenability, and for this purpose, the content is set to 0.65% or more. However, if the content is too high, there is a concern that machinability may decrease, so the upper limit is set to 1.00%. The preferred range is 0.80 to 0.95%.

[0017] P: 0.030% or less, S: 0.030% or less P and S are impurities. These elements are undesirable for the mechanical properties of parts, such as causing embrittlement, so the smaller the amount, the better. However, if the content is 0.030% or less, there is not much effect on the properties, so the upper limit is set at 0.30%.

[0018] Cu: 0.01 to 1.00% Cu, together with Ni and Cr, is an element that improves tensile strength, impact resistance, and fatigue strength. The lower limit of Cu is set at 0.01% because a content lower than this level reduces hardenability and strength. On the other hand, the upper limit of Cu is set at 1.00% because too much Cu deteriorates workability, particularly machinability. The preferred range is 0.10 to 0.20%.

[0019] Ni: 0.01 to 3.00% Ni, together with Cu and Cr, is an element that improves tensile strength, impact resistance, and fatigue strength. The lower limit of Ni is set at 0.01% because a content lower than this level reduces hardenability and strength. On the other hand, the upper limit of Ni is set at 3.00% because too much Ni deteriorates workability, particularly machinability. The preferred range is 0.05 to 0.50%.

[0020] Cr: 0.80~1.00% Cr is a useful component for improving hardenability and ensuring internal hardness. To achieve this effect, the Cr content is set to 0.80% or more. However, if the Cr content is too high, there is a concern that the machinability may decrease, so the upper limit is set to 1.00%. The Cr content is preferably 0.80 to 0.98%.

[0021] Al: 0.030 to 0.047% Al reacts with N in steel to form AlN, which acts to prevent coarsening of austenite grains during carburization, and to obtain this effect, it is necessary to add 0.030% or more. However, if added in excess, the effect of suppressing grain coarsening saturates, so the upper limit is set to 0.047%, and preferably 0.033 to 0.042%.

[0022] N: 0.015 to 0.030% N reacts with Al in the steel to form AlN, which prevents coarsening of austenite grains during carburization, and to obtain this effect, a content of 0.015% or more is required. However, if the N content is too high, the effect of suppressing coarsening of grains saturates and the amount of nitrides increases, causing a decrease in strength, so the upper limit is set to 0.030%. A preferred range is 0.018 to 0.027%.

[0023] Nb: 0.010 to 0.045% Nb forms carbides and acts to pin the austenite grain boundaries during carburization. To obtain this effect, a content of 0.010% or more is required. However, excessive addition of Nb results in insufficient solid solution of Nb during hot rolling and hot forging, and some of this forms carbides, causing the structure before carburization to become finer. Therefore, the upper limit is set to 0.045%. The preferred range of Nb content is 0.010 to 0.030%, and more preferably 0.015 to 0.025%.

[0024] The manufacturing process for producing steel parts using the case-hardened steel material 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 steel part is manufactured through the following processes: hot rolling → hot forging → rough heat treatment → machining → carburizing and quenching → tempering → finish machining.

[0025] The rough heat treatment performed after hot forging is a heat treatment to suppress the formation of bainite in the structure after forging, thereby ensuring machinability in subsequent machining. The rough heat treatment can be performed immediately after forging, as shown in Figure 1(A). In this case, for example, the workpiece is held at a temperature of 640 to 700°C for 30 minutes or more, and then cooled to approximately room temperature. The metal structure after rough heat treatment is mainly ferrite-pearlite. In this case, the ferrite-pearlite area ratio (the total area ratio of ferrite and pearlite) is 80% or more, and the remainder is bainite.

[0026] According to the case-hardened steel material of this embodiment, the contents of Al and Nb are specified so that they are sufficiently dissolved during hot rolling, the amount of pinning particles that precipitate before forging heating is suppressed, and the grain size number of the metal structure after hot forging and rough heat treatment (before carburizing) can be made small (the grains can be made large). Specifically, the average grain size number of ferrite in the metal structure after rough heat treatment (before carburizing) is preferably 10.6 or less.

[0027] As shown in Fig. 1(B), the rough heat treatment can be performed on a workpiece that has been cooled to approximately room temperature. In this case, the workpiece is held at a temperature of 890 to 950°C for 30 minutes or more, then held at a temperature of 640 to 700°C for 30 minutes or more, and then cooled to approximately room temperature.

[0028] The case-hardened steel material of this embodiment that has undergone rough heat treatment is then machined and then carburized and quenched. In carburizing and quenching, the workpiece is first heated to a predetermined carburizing temperature. During this temperature rise, the dissolved Nb precipitates as NbC and Al precipitates as AlN, and the effect of these pinning particles is to suppress the movement of austenite grain boundaries. In this case-hardened steel material of this embodiment, the metal structure before carburizing is somewhat enlarged, so the driving force for grain growth during carburizing is small, and abnormal grain growth is effectively suppressed. [Example]

[0029] 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 the average ferrite grain size of the structure before carburizing and the grain coarsening during carburizing were evaluated.

[0030] [Table 1]

[0031] <Preparation of test material> Steel with the chemical composition shown in Table 1 was melted and homogenized at 1300°C for at least 2.5 hours, after which it was hot rolled at 950°C or higher to produce steel bars with a diameter of 70 mm. Then, cylindrical specimens with a diameter of 8 mm and a length of 12 mm were machined, followed by hot forging and rough heat treatment as shown in Figure 2(A).

[0032] FIG. 2(A) is a diagram showing the temperature patterns of hot forging and rough heat treatment when preparing the test material. In the hot forging, the workpiece was heated to the forging temperature (1150°C) shown in Figure 2(A) and then worked to a rolling reduction of 50%. After the hot forging, the workpiece was held at a temperature of 640 to 700°C for 30 minutes or more, and then cooled to approximately room temperature as a rough heat treatment.

[0033] <Measurement of the average ferrite grain size number before carburizing> For the test material after rough heat treatment (before carburizing), the ferrite grains in five fields of view at 100x magnification were measured using an optical microscope in accordance with "JIS G 0552 Testing method for ferrite grain size of steel," and the average value of the grain size numbers was taken as the average ferrite grain size number. The results are shown in Table 1.

[0034] <Carburizing and quenching> After the rough heat treatment, the test specimens were machined to the specified dimensions and then carburized and quenched in a vacuum carburizing furnace, as shown in Figure 2(B). The test specimens were held at a carburizing temperature of 1020°C for 180 minutes, then at a temperature of 890°C for 15 minutes, and then quenched in oil at 80°C.

[0035] <Evaluation of grain coarsening> The test materials after carburizing (after carburizing and quenching) were evaluated for the presence or absence of grain coarsening. Specifically, the prior austenite grains in five fields of view at 100x magnification were measured for the carburized and quenched test materials in accordance with "JIS G 0551 Testing method for austenite grain size in steels," and the results were evaluated based on the following criteria. The results are shown in Table 1. When the average grain size number of the prior austenite grains was 6 or more and the area ratio of coarse grains (grain size 4.5 or less) in the observed region was less than 20%, it was judged as "good" with no coarsening. On the other hand, if the area ratio of coarse particles (particle size 4.5 or less) in the observed area was 20% or more, or if there were even a small number of coarse particles with a particle size of 3 or less in the observed area, coarsening was observed and the result was judged as "X".

[0036] The evaluation results in Tables 1 and 2 reveal the following: Comparative Example 1 is an example in which Nb was added in an amount exceeding the upper limit (0.045%) of the range defined in this embodiment. In Comparative Example 1, the ferrite average grain size number of the structure before carburizing was as large as 11.1 (the crystal grains were fine), and coarsening of the crystal grains during carburizing was observed. Comparative Example 2 is also an example in which Nb was added in excess of the upper limit of the range specified in this embodiment. As in Comparative Example 1, the ferrite average grain size number of the structure before carburizing was large at 11.1 (the crystal grains were fine), and coarsening of the crystal grains during carburizing was observed.

[0037] In contrast, in Examples 1 to 4, whose chemical compositions satisfy the conditions of this embodiment, the ferrite average grain size number of the structure before carburizing is smaller (grains are larger) than in Comparative Examples 1 and 2, and no coarsening of grains is observed during carburizing. As such, it is clear that case-hardened steel materials having the chemical compositions of each Example can effectively suppress coarsening of grains (abnormal grain growth) during carburizing.

Claims

1. By mass% C: 0.17-0.23% Si: 0.80-1.00% Mn: 0.65-1.00% P: 0.030% or less S: 0.030% or less Cu: 0.01-1.00% Ni: 0.01-3.00% Cr:0.80~1.00% Al: 0.030-0.047% N: 0.015-0.030% Nb: 0.010-0.045% and the balance being Fe and unavoidable impurities, and has excellent properties for preventing coarse grains during carburizing.

2. 2. The case-hardened steel material according to claim 1, having a ferrite-pearlite structure and an average grain size number of ferrite measured in accordance with JIS G 0552 at a 100x magnification and 5 visual fields of an optical microscope of 10.6 or less.

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