JP6197467B2 - Steel for machine structure - Google Patents

Description

  The present invention relates to a steel for machine structure mainly composed of a ferrite / pearlite structure, and more particularly to a steel for machine structure having excellent workability and fatigue strength.

  Known JIS alloy steels for machine structures include chromium steel SCr420 and chromium molybdenum steel SCM420. In general, these alloy steels are subjected to surface treatment such as carburizing for the purpose of increasing fatigue strength as necessary after heat-treating the forged material and performing machining such as cutting. Here, in order to improve the machinability in cutting, it is effective to precipitate inclusions such as MnS. On the other hand, a decrease in fatigue strength due to inclusions is also pointed out. Accordingly, a steel for machine structure that can improve the machinability while suppressing the precipitation of inclusions has also been proposed.

  By the way, regarding machinability of steel for machine structural use, it is easy to discharge and keep cutting resistance low by dividing finely the chips at the time of cutting and not storing them between the cutting tool and the workpiece. It is important to provide a smooth cutting surface.

  For example, Patent Document 1 discloses steel for machine structural use that is particularly excellent in chip disposal among machinability. In order to make it easy to divide the chips, it is stated that it is preferable to make the structure of ferrite and spheroidized cementite constant and not including spheroidized cementite. Such steel is, in mass%, C: 0.15 to 0.25%, Si: 0.30 to 0.70%, Mn: 0.7 to 1.3%, S: 0.002 to 0.012 %, Cr: 1.20 to 1.70%, Al: 0.005 to 0.035%, and N: 0.010 to 0.025%, specifying the upper limit of P and O as impurities The amount of Si is given to a predetermined amount or more while suppressing the amount of S as much as possible. Here, for Si, when the yield ratio in the shear strength characteristics is suppressed, shear strain in chip formation is generated with low stress, and the hardness of the workpiece (machine structural steel) is equal or high Even so, it states that cutting resistance can be reduced.

  Furthermore, regarding machinability, it is related to cutting the chips at the time of cutting finely to give a stable and smooth cutting surface, and sudden chipping (chipping) occurs in the cutting tool. The smoothness of the cut surface of the cut article is abruptly lost.

  For example, in Patent Document 2, chipping of a cutting tool can be suppressed by reducing the fluctuation range of the main component force of cutting resistance during the cutting process. It is disclosed that it is effective to reduce the yield ratio (0.2% proof stress / tensile strength) in the tensile properties with a pearlite structure and a constant proportion of ferrite in the structure. Such steel is, in mass%, C: 0.15 to 0.25%, Si: 0.15 to 0.50%, Mn: 0.70 to 1.30%, S: 0.015% or less, Cr : 1.25 to 1.80, Al: 0.005 to 0.035%, and N: 0.010 to 0.025%, and the combined amount of Si and Cr and of P and O as impurities While defining the upper limit and increasing the amount of Si that is a ferrite strengthening element, the yield ratio can be lowered by selecting the heat treatment conditions.

JP 2012-126953 A JP 2011-89189 A

  As described above, in the steel for mechanical structure that suppresses the decrease in fatigue strength without precipitating inclusions such as MnS, by controlling mainly the amount of Si and Cr and the structure of the component composition, Performance, especially chipping of cutting tools can be suppressed. On the other hand, in controlling the component composition, it was considered that the fatigue strength was lowered.

  The present invention has been made in view of the situation as described above, and its object is to have high fatigue strength and to suppress machinability, in particular, chipping of a cutting tool, so that it is stable. It is to provide a steel for machine structure that can provide a smooth cutting surface.

  The steel for machine structural use according to the present invention is in mass%, C: 0.15 to 0.25%, Si: 0.04 to 0.15%, Mn: 0.6 to 1.0%, P: ≦ 0. 0.030%, S: 0.015-0.030%, Cu: ≤0.30%, Ni: ≤1.00%, Cr: 1.0-2.0%, Mo: ≤1.00%, s-Al: 0.010 to 0.050% and N: 0.005 to 0.030%, consisting of the balance Fe and impurities, and the mass% of the element M is [M] is 10.0. X [Si] + [Mn] + [Cr] ≦ 3.90, 5.44 × [Si] + 34.6 × [S] ≧ 1.06, [Ni] + [Mo] ≦ 1.00, It is mainly composed of a ferrite / pearlite structure.

  According to this invention, while having high fatigue strength, the chipping of the cutting tool used for cutting this can be suppressed, and as a result, a stable and smooth cutting surface can be obtained.

It is a figure of the component composition of an Example and a comparative example. (A) Perspective view of a turning test piece and (b) Front view of a fatigue test piece. It is a figure which shows the test result of an Example and a comparative example. FIG. 3 is a diagram showing the relationship of fatigue limit stress ratio to Equation 1. FIG. 6 is a diagram illustrating a relationship of a flank defect life ratio with respect to Formula 2.

  The present inventor changed the composition of this component based on chromium steel or chromium molybdenum steel specified in JIS G4053, and after normalization (after normalization), the steel for mechanical structure mainly composed of ferrite and pearlite structure. Repeated evaluation. As a result, (1) the contents of Si, Mn and Cr affect the grain boundary oxidation in the vicinity of the surface layer subjected to surface hardening heat treatment such as carburizing treatment, and greatly change the bending fatigue strength, (2) Si and S It has been found that the content of has an effect on the life of the cutting tool used when cutting it, in particular the chipping life. The results of this evaluation test are shown below.

  In order to evaluate the machinability of machine structural steel from the rotating bending fatigue test to evaluate the fatigue strength and the chipping life of the cutting tool used when cutting this as an evaluation test for machine structural steel A turning test was conducted. First, the test piece used for these tests is demonstrated.

First, 150 kg steel ingots having the composition shown in Examples 1 to 3, 5 to 8 and Reference Example 4 and Comparative Examples 1 to 10 in FIG. 1 are ingoted by a vacuum induction furnace, and hot forging is performed. Then, for normalization, heat treatment was performed by holding at 910 ° C. for 1.5 hours and air cooling. Thereby, a normalized steel mainly composed of a ferrite / pearlite structure can be obtained. Here, a bainite structure having a cross-sectional area ratio of 10% or less may be included. Subsequently, a fatigue test piece used for the rotating bending fatigue test and a turning test piece used for the turning test are cut out by machining. As shown in FIG. 2 (a), the turning test piece 20 is a stepped round bar of φ60 × 200L, a test part 21 having a length of 186 mm, a chuck part 22 having a small diameter on one end side in the longitudinal direction, Consists of. Further, as shown in FIG. 2B, the fatigue test piece 10 is an Ono type rotating bending fatigue test piece having a smooth parallel portion of φ8. The fatigue test piece 10 is obtained by subjecting the test piece cut out as described above to carburizing treatment as a surface hardening heat treatment so that the target value of the carbon content in the vicinity of the surface layer is 0.8 mass%.

  Next, methods for the rotating bending fatigue test and the turning test will be described.

In the rotational bending fatigue test, the fatigue test piece 10 was attached to an Ono-type rotational bending fatigue tester, and fatigue was repeatedly given by full swing (stress ratio R = -1) at room temperature and in the atmosphere at a rotational speed of 3600 rpm. An SN curve was obtained from the plurality of fatigue test pieces 10, and the critical stress that did not break was measured with 10 ⁷ repetitions, and this was defined as the fatigue limit stress. Further, the fatigue limit stress ratio is expressed as a ratio where the fatigue limit stress of Comparative Example 1 using the conventional material SCM420 equivalent material (half Mo) is 1. The reference value of the fatigue limit stress ratio according to the present invention was 105% or more of the fatigue limit stress ratio in Comparative Example 1 serving as a reference, that is, 1.05 or more.

  In the turning test, first, the chuck portion 22 of the turning test piece 20 is chucked on a lathe, and the test portion 21 is pre-machined until φ58. In such pre-processing, a pre-processing tool is used as a cutting tool to reduce the eccentricity of the test portion 21 and to make the surface a predetermined machined skin. Subsequently, the cutting tool is replaced with a new chip (test bit), and a turning test process is started. In the turning test processing, the cutting speed is 250 m / min, the feed speed is 0.35 mm / rev, the cutting depth is 1.0 mm, and the processing is performed using the water-soluble cutting fluid until the test portion 21 becomes φ28. Until the cutting tool tip 20 is determined to have a life described later, the turning test piece 20 is replaced, and the pre-processing and the turning test are performed while the cutting tool is replaced with the pre-processing tool and the testing tool each time. Continue processing.

  The life of the test tool tip is assumed to be when a chip having a width of 250 μm is generated in the direction perpendicular to the tool cutting edge from the cutting edge on the side clearance surface of the test tool tip. The chip flank defect life was defined as the total time required for the turning test processing to reach the chip life. Further, the flank defect life ratio is expressed as a ratio when the life of Comparative Example 1 using a conventional material equivalent to SCM420 is 1. The reference value of the flank defect life ratio according to the present invention was set to be equal to or greater than the flank defect life ratio in Comparative Example 1 serving as a reference, that is, 1.00 or more.

Next, FIG. 3 summarizes the results of the rotating bending fatigue test and the turning test on the mechanical structural steels of Examples 1 to 3, 5 to 8 and Reference Example 4 and Comparative Examples 1 to 10 described above. These test results will be described.

Examples 1 to 3 and 5 to 8 are normalized steels in which the Si content is decreased and the S content is increased compared to Comparative Example 1 using the conventional SCM420 equivalent material. (See FIG. 1). In this example, the fatigue limit stress ratio was 1.05 to 1.13, which satisfied the above-described reference value. Further, the flank defect life ratio was 1.02 to 1.67, which satisfied the above-described reference value. That is, according to the machine structural steels of Examples 1 to 3 and 5 to 8, the cutting tool used has a high fatigue strength as compared with the conventional machine structural steel and is used for cutting the steel. The chipping can be suppressed to at least equal to or less than that of the conventional material, and the machinability is also excellent.

  On the other hand, in Comparative Example 2 (see FIG. 1) in which the Si content was reduced to 0.05% by mass with respect to Comparative Example 1, the fatigue limit stress ratio was 1.10, which satisfied the standard value, but the flank defect The life ratio was 0.14, which was significantly lower than the standard value.

  In Comparative Example 3 (see FIG. 1) in which the contents of Si and S are reduced compared to Comparative Example 1, the fatigue limit stress ratio is 0.99 and the flank defect life ratio is 0.37, both of which satisfy the standard values. There wasn't.

  In Comparative Example 4 (see FIG. 1) in which the Si content was decreased and the Mn content was increased compared to Comparative Example 1, the flank defect life ratio was 1.04, which met the standard value, but the fatigue limit. The stress ratio was 0.96, which did not satisfy the standard value.

  Compared to Comparative Example 1, in Comparative Example 5 (see FIG. 1) in which the Cr content was increased by decreasing the Si content, the fatigue limit stress ratio was 0.99, which was lower than Comparative Example 1 and met the reference value. Further, the flank defect life ratio was 0.39, which did not satisfy the standard value.

  In Comparative Example 6 in which the P content is increased compared to Comparative Example 1 and in Comparative Example 7 (see FIG. 1) in which the S content is increased, the flank defect life ratio is 1.14 and 1 respectively. The fatigue limit stress ratio was 0.98 and 0.91, which were lower than those of Comparative Example 1, respectively, but did not satisfy the standard value.

  In Comparative Example 8 (see FIG. 1) in which the Cr content is increased compared to Comparative Example 1, the flank defect life ratio is 1.18, which satisfies the standard value, but the fatigue limit stress ratio is 1.00. The value was the same as in Example 1 and did not satisfy the standard value.

  In Comparative Example 9 (see FIG. 1) in which the Si content was reduced to 0.05 mass% as in Comparative Example 2, compared with Comparative Example 1, the fatigue limit stress ratio was 1.10, which met the standard value. The flank defect life ratio was 0.10, significantly below the standard value.

  In Comparative Example 10 (see FIG. 1) in which the contents of s-Al and N are reduced compared to Comparative Example 1, the flank defect life ratio is 1.47, which satisfies the standard value, but the fatigue limit stress ratio is 1. Although it was larger than 01 and Comparative Example 1, the reference value was not satisfied.

Furthermore, the tendency of the fatigue limit stress ratio and the flank defect life ratio in the steels of Examples 1 to 3, 5 to 8 and Reference Example 4 and Comparative Examples 1 to 10 will be described with reference to FIGS.

  One of the causes that bending fatigue strength is reduced by carburizing treatment in steels containing elements having a strong binding force with oxygen, for example, elements such as Si, Mn, and Cr, is the oxidation of grain boundaries (intergranular oxidation). ) Oxygen in the carburizing atmosphere enters the steel through the grain boundaries and reacts with Si, Mn, Cr, etc. near the grain boundaries to generate oxides. On the other hand, in the vicinity of the crystal grain boundary, these Si, Mn, Cr and the like dissolved in the alloy are locally deficient. Therefore, the hardenability is lowered, the formation of martensite is inhibited, and the bending fatigue strength is lowered due to the incompletely hardened layer.

Therefore, the intergranular oxidation parameters N _go as a measure for the progression of such a grain boundary oxidation, Si series of mechanical structural steel containing Examples 1 to 8, Mn, and Cr content and fatigue limit stress ratio Was obtained by performing regression calculation. That is, when the mass% of the element M is [M], the grain boundary oxidation parameter N _go is
N _go = 10 × [Si] + [Mn] + [Cr]
It is.

As shown in FIG. 4, if the grain boundary oxidation parameter N _go is in the range of 3.90 or less, the fatigue limit stress ratio exceeds the reference value. That is, by adjusting the Si, Mn, and Cr components contained in the steel so that the grain boundary oxidation parameter N _go is in the range of 3.90 or less, the fatigue limit stress ratio is 1.05 or more which is a reference value. Can be controlled within the range. That is, it is a case where the following formula 1 is satisfied.
N _go = 10 × [Si] + [Mn] + [Cr] ≦ 3.90 (Formula 1)

In Comparative Example 6, Comparative Example 7, and Comparative Example 10, the grain boundary oxidation parameter N _go is 3.80, 3.75, and 3.60, respectively, and satisfies Equation 1. However, the fatigue limit stress ratios are 0.98, 0.91, and 1.01, respectively, which are below the standard value. It is considered that Comparative Example 6 and Comparative Example 7 had a high P or S content, produced a lot of inclusions, and reduced the strength of the crystal grain boundaries. In Comparative Example 10, the content of s-Al and N is small, and the precipitation of AlN that causes refinement of crystal grains is small, so that the crystal grains become rough.

  In addition, as described above, the lifetime due to the flank defect of the cutting tool that processes the work material is affected by the Si and S contents of the work material. That is, Si can reduce the chipping of the cutting tool by forming an oxide film on the surface of the cutting tool during the cutting process. Such an oxide film is presumed to improve lubricity and reduce cutting resistance. Further, S forms MnS as inclusions, makes it easy to finely cut off chips and reduce cutting resistance.

Therefore, with respect to the flank defect parameter N _vb as a measure for the flank defect life of such a cutting tool, the Si and S contents of a series of mechanical structural steels including Examples 1 to 3 and 5 to 8 The flank defect life ratio and the regression calculation were performed. That is, when the mass% of the element M is [M], the flank defect parameter N _vb is
N _vb = 5.44 × [Si] + 34.6 × [S] −0.71
It is.

As shown in FIG. 5, if the flank defect parameter _Nvb is within a range of 0.35 or more, the flank defect life ratio exceeds the reference value. That is, by adjusting the Si and S components contained in the steel so that the flank defect parameter _Nvb is in the range of 0.35 or more, the flank defect life ratio is 1.00 or more which is a reference value. Can be controlled within range. That is, it is a case where the following formula 2 is satisfied.
N _vb = 5.44 × [Si] + 34.6 × [S] −0.71 ≧ 0.35 (Formula 2)
That means
5.44 × [Si] + 34.6 × [S] ≧ 1.06 (Formula 3)
It is necessary to satisfy.

  Note that the flank defect life varies depending on the turning conditions. For example, it has been observed that the flank defect life tends to be longer than that of the conventional material in a region where the cutting speed is relatively high.

  As described above, with respect to the conventional material, the content of S is increased while reducing the content of Si, and further, the contents of Si, Mn and Cr are adjusted to satisfy (Equation 1). By suppressing grain boundary oxidation in surface hardening heat treatment such as carburizing treatment, higher bending fatigue strength than that of conventional materials can be ensured. In addition, by adjusting the Si and S contents so as to satisfy (Equation 2), a flank defect life equal to or greater than that of the conventional material can be obtained. That is, it is possible to provide a steel for machine structure that has high fatigue strength, can suppress chipping of a cutting tool used for cutting the steel, and can provide a stable and smooth cutting surface.

Based on these results, the ranges of the respective compositional components are set as follows within a range not impairing the characteristics as the steel for machine structural use obtained by the steels having the component compositions shown in Examples 1 to 3 and 5 to 8 described above. It was determined by such guidelines.

  C is an additive element indispensable for ensuring the mechanical strength, particularly the strength of the core, required for the steel for machine structure after the surface hardening heat treatment. That is, if the amount of C added is too small, the core strength cannot be secured. On the other hand, when there is too much addition amount of C, manufacturability, such as hot forgeability and machinability, will fall. Therefore, the C content is in mass% and is in the range of 0.15 to 0.25%.

  Si is a deoxidizer for molten steel and further enhances the hardenability of the steel. Further, an oxide film can be formed on the surface of the cutting tool during the cutting process to improve the life of the cutting tool. However, if the amount of Si added is too large, the grain boundary oxidation proceeds during the surface hardening heat treatment, and the fatigue strength required for the steel for machine structure cannot be ensured. Therefore, the Si content is in mass% and is in the range of 0.04 to 0.15%.

  Mn is necessary for enhancing the hardenability of steel and ensuring the mechanical strength required as steel for machine structural use. If the amount of Mn added is too small, quenching is insufficient, and the mechanical strength required for machine structural steel cannot be ensured. On the other hand, as described above, if the amount of Mn added is too large, the grain boundary oxidation proceeds during the surface hardening heat treatment, and the bending fatigue strength required for the machine structural steel cannot be ensured. Therefore, the content of Mn is mass% and is in the range of 0.6 to 1.0%.

  P can embrittle the crystal grain boundary to lower the mechanical strength and reduce the fatigue strength. However, if the content is below a certain level, this decrease in fatigue strength is slight. Moreover, since refining process will become long when content is reduced extremely, it may become a cause which increases cost. Therefore, the content of P is set to 0.030% or less by mass%.

  S is inevitably present in the steel, but combines with Mn to generate MnS inclusions, and makes it easy to break up the chips finely. That is, the chip fragility can be improved. When there is too little content of S, the crushability of a chip will fall and workability may be reduced. On the other hand, if the content of S is too large, the inclusions that are the starting points of stress concentration are increased, and it becomes difficult to ensure the fatigue strength required as steel for mechanical structures. Therefore, the S content is in the range of 0.015 to 0.030% by mass%.

  Cu can improve the hardenability of steel. However, the effect on the amount of addition gradually saturates and may increase the cost. Therefore, the Cu content is within a range of 0.30% or less in terms of mass%.

  Ni improves the hardenability of steel and can improve the wear resistance of steel. However, the effect on the amount of addition gradually saturates and may increase the cost. Therefore, the Ni content is within a range of 1.00% or less by mass.

  Mo improves the hardenability of the steel and can improve the wear resistance of the steel. However, the effect on the amount of addition gradually saturates and may increase the cost. Therefore, the Mo content is within a range of 1.00% or less by mass.

  In addition, since Ni and Mo can improve the wear resistance of steel almost equivalently, the total content of Ni and Mo is within a range of 1.00% or less by mass%, that is, the mass% of element M is reduced. Assuming [M], [Ni] + [Mo] ≦ 1.00.

  Cr can improve the hardenability of steel and improve the mechanical strength. That is, if the amount of Cr added is too small, the hardenability is insufficient and the bending fatigue strength can be reduced. On the other hand, if the amount of Cr added is too large, as described above, grain boundary oxidation proceeds during the surface hardening heat treatment, and the bending fatigue strength required for steel for machine structures cannot be ensured. Therefore, the Cr content is in mass% and is in the range of 1.0 to 2.0%.

  s-Al has a deoxidizing action of molten steel. Moreover, AlN which combines with N and refines crystal grains can be formed, and the bending fatigue strength required as mechanical structural steel can be improved. However, when there is too much content of s-Al, an inclusion will increase and the bending fatigue strength required as steel for machine structure cannot be ensured. Therefore, the content of s-Al is in the range of 0.010 to 0.050% by mass.

  N combines with Al, Nb, Ti and the like to form a nitride that refines the crystal grains, and can improve the bending fatigue strength required as a steel for mechanical structures. However, when there is too much content of N, the improvement ability of bending fatigue strength will be saturated. Therefore, the N content is in the range of 0.005 to 0.030%.

  As mentioned above, although the Example by this invention and the modification based on this were demonstrated, this invention is not necessarily limited to this, A person skilled in the art will deviate from the main point of this invention, or the attached claim. Various alternative embodiments and modifications could be found without doing so.

10 Fatigue test piece 20 Turning test piece 21 Test part 22 Chuck part

Claims (1)

By mass%, C: 0.15~0.25%, Si : 0.04~0.15%, Mn: 0.6~ 0.8%, P: ≦ 0.030%, S: 0.015 -0.030%, Cu: ≤0.30%, Ni: ≤1.00%, Cr: 1.0-2.0%, Mo: ≤1.00%, s-Al: 0.010-0 .050% and N: 0.005 to 0.030%, which consists of the balance Fe and impurities, and the mass% of the element M is [M].
10.0 × [Si] + [Mn] + [Cr] ≦ 3.90
5.44 × [Si] + 34.6 × [S] ≧ 1.06
[Ni] + [Mo] ≦ 1.00
Satisfying the requirements, and mainly composed of ferrite and pearlite structure.