Steel material machining method, and rolling component manufacturing method

Abstract

[Problem] To manufacture a rolling component having an excellent rolling life. [Solution] This method for processing a steel material involves applying compressive stress to a base material made of a steel material including a non-metallic inclusion, to process the same to a shape different from the base material, characterized in that a reduction ratio when the compressive stress is applied to the steel material is determined using a strain ratio εx / εx' as a control index. Here, εx' is a strain in an extension direction at a contact point between the non-metallic inclusion and a matrix of the steel material when the contact point reaches an inversion position at which the direction of shear stress inverts. εx is the strain in the extension direction at a contact-point-corresponding-portion corresponding to the contact point, in the steel material without the non-metallic inclusion, when the contact-point-corresponding-portion reaches an inversion position at which the direction of shear stress inverts.

Description

Technical Field

[0001] The present invention relates to a method for processing a steel material including a non-metallic inclusion by applying compressive stress to the steel material, and the like.Background Art

[0002] In recent years, high performance of various types of machine equipment has led to a very severe environment in which machine components and devices are used for which a rolling contact fatigue life is desired. The lives and reliability of these machine components and devices are strongly desired to be improved.

[0003] Steel material components such as bearings are known to inevitably include a foreign matter referred to as a non-metallic inclusion and derived from manufacturing steps such as a refining step, a casting step, and a solidification step which are manufacturing steps for steel.

[0004] Additionally, in steel material components such as bearings manufactured via a rolling step and a forging step, a gap may be formed around the non-metallic inclusion. The gap is considered to be generated at an interface due to a difference in deformability between the non-metallic inclusion and steel, which is a matrix. For example, in a case where the steel material component is a bearing, the gap encourages cracking in a bearing component subjected to rolling contact fatigue during use and which may become the origin of cracking. A similar problem is posed by rolling components other than the bearing for which a good rolling contact fatigue life is desired.

[0005] Therefore, reducing the gap around the non-metallic inclusion is effective for improving the rolling contact fatigue life of rolling components such as a bearing.

[0006] Patent Literature 1 discloses a method for manufacturing a ring-like machine component including a rolling section that is excellent in rolling contact fatigue life, wherein, in manufacture of a machine component including a rolling section including a rolling component rolling along an inner diameter of a ring-like material, the rolling section in which the rolling component rolls is formed on an inner diameter surface of the ring-like material using forging processing in which hydrostatic stress of compression that is 1.5 times as high as yield stress of the material is applied to the inner diameter surface of the ring-like material on which the rolling section is to be formed, making plastic strain in a rolling direction of the rolling component into compressive strain of 0.10 or more. Thus, the void present at the interface between the non-metallic inclusion in the steel and the steel, which is a matrix, moves in a direction in which the void is closed off, allowing manufacture of a machine component that is excellent in rolling contact fatigue life.

[0007] Patent Literature 2 discloses a method for manufacturing a ring-like machine component, the method including the step of forming a rolling section on an inner diameter surface of a ring-like processing base material on which the rolling section is to be formed and which is to be subjected to cold forging, by causing a hydrostatic stress of at least 1000 MPa to act on the inner diameter surface using an upper punch and a lower punch each with a diameter smoothly reduced toward a tip of the punch, wherein the action of the hydrostatic stress moves the void present at the interface between the non-metallic inclusion in the steel and the steel, which is the matrix, in a direction in which the void is closed off.Citation ListPatent Literature

[0008] Patent Literature 1: Japanese Patent No. 5896713 Patent Literature 2: Japanese Patent No. 5669128 Patent Literature 3: Japanese Patent No. 2923095 Patent Literature 4: Japanese Patent Laid-Open No. 2014-55346 Non Patent Literature

[0009] Non Patent Literature 1: LIGHT METALS / Vol.42 No.2(1992) Finite Element Analysis in Metal Forming Non Patent Literature 2: KOBE STEEL ENGINEERING REPORTS / Vol.48 No.1(Apr.1998) FEATURE: Metal Working Technology, Effects of Entrance Zone Length on 3-dimensional steady State Analysis Thickness Distribution for strip Rolling Summary of InventionTechnical Problem

[0010] In Patent Literatures 1 and 2, hydrostatic compressive stress or compressive strain greater than or equal to a certain value is only applied to the occlusion of the gap already present inside a steel component or a forging component, and a deformation phenomenon of a non-metallic inclusion and a surrounding base material is not controlled, and thus evaluating the effect on the occlusion of the gap is difficult.

[0011] An object of the present invention is to provide a method for manufacturing a rolling component with an excellent rolling life by controlling a deformation phenomenon of a non-metallic inclusion included in a steel material and a matrix.Solution to Problem

[0012] A steel material processing method of applying compressive stress to a base material made of a steel material including a non-metallic inclusion to process the base material into a shape different from a shape of the base material in order to solve the above-described problem, the steel material processing method including using a strain ratio εx / εx' as a control index to determine a rolling reduction rate used when the compressive stress is applied to the steel material.

[0013] εx' is strain of a contact between the non-metallic inclusion and a matrix of the steel material which strain is generated in an extending direction of the contact when the contact reaches a reversal position where a direction of shear stress is reversed.

[0014] εx is strain of a contact corresponding portion of a steel material with no non-metallic inclusion corresponding to the contact which strain is generated in an extending direction of the contact corresponding portion when the contact corresponding portion reaches the reversal position where the direction of the shear stress is reversed.

[0015] (2) The steel material processing method according to (1) described above, wherein the compressive stress is applied to the steel material by a rolling roll or a forging roll.

[0016] (3) The steel material processing method according to (2) described above, wherein the rolling reduction rate of the rolling roll or the forging roll for keeping the strain ratio εx / εx' less than or equal to a predetermined value is predetermined by a rolling or forging analysis model, and the steel material is rolled or forged at the rolling reduction rate.

[0017] (4) A rolling component manufacturing method including processing, into a shape of a rolling component, a semi-finished product manufactured by the steel material processing method according to any one of (1) to (3) described above.Advantageous Effect of Invention

[0018] According to the present invention, a rolling component such as a bearing which is excellent in rolling life can be provided by controlling a deformation phenomenon of a non-metallic inclusion included in a steel material and a matrix.Brief Description of Drawings

[0019] Figure 1 is an explanatory diagram for describing a method for rolling analysis. Figure 2 is a shear stress distribution of a plate model 11 acquired at a certain timing during rolling. Figure 3 is an explanatory diagram of a forward area and a backward area. Figure 4 is a schematic diagram of a contact between a non-metallic inclusion and a matrix for describing strain εx'. Figure 5 is a schematic diagram of a ring rolling. Figure 6 is a graph in which a relationship between strain εx (εx') and a rolling reduction rate is organized (Example). Description of Embodiment

[0020] The inventors have discovered a steel material processing method of applying compressive stress to a base material made of a steel material including a non-metallic inclusion to process the base material into a shape different from the shape of the base material, the processing method being capable of providing a rolling component such as a bearing which is excellent in rolling life by controlling a deformation phenomenon of the non-metallic inclusion included in the steel material and a matrix.

[0021] Rolling components include components for which a good rolling contact fatigue life is desired. Such rolling components include, for example, a bearing, a gear, a hub unit, a continuously variable transmission, a constant-velocity joint, a crank pin, and a piston pin.

[0022] The inventors have clarified, by using a rolling analysis model (see Non Patent Literature 1) based on a well-known rigid-plastic finite element method, the method capable of controlling the deformation phenomenon of the non-metallic inclusion included in the steel material and the matrix when rolling the steel material. CAE analysis was used for the rolling analysis.

[0023] The CAE analysis is an abbreviation of initials taken from Computer Aided Engineering, and is an analysis method for evaluating (simulating) a design problem of a product reproduced on a computer in a pseudo manner. The CAE analysis was realized by a computer program.

[0024] Figure 1 is an explanatory diagram for describing a method for rolling analysis. A relationship is analyzed that is observed between generation behavior of a gap around a non-metallic inclusion L when a plate model 11 is rolled using a vertical pair of rolling rolls 12 and resulting strain. A condition for contact between the non-metallic inclusion L and a matrix is defined using a shear friction coefficient. Shear stress and strain history are acquired when the non-metallic inclusion L passes from an inlet side toward an outlet side of the rolling rolls 12. Here, the strain refers to plastic strain of a contact between the non-metallic inclusion L and the matrix in an extending direction (x-axis direction in Figure 1) of the plate model 11.

[0025] In addition to the condition (shear friction coefficient) for the contact between the non-metallic inclusion L and the matrix described above, typical elements provided to the rolling analysis model include a plate thickness, hot or cold rolling conditions (temperature and the like), the position and size of the non-metallic inclusion, the position and size of a gap present around the non-metallic inclusion, a rolling reduction rate, physical property data of a material (Young's modulus, Poisson ratio, and stress-strain curve), the roll diameter and peripheral velocity of the rolling rolls, the shear friction coefficient between the rolling roll and the plate model, and the like.

[0026] Note that the rolling reduction rate refers to the degree of processing of rolling expressed in percentage and is calculated using a formula for computation (h1 - h2) / h1 when the plate thicknesses of the material before and after rolling are respectively represented as h1 and h2.

[0027] These elements can be determined by pre-analyzing the properties and the like of a steel material used for a rolling component such as a bearing. If two or more non-metallic inclusions L are present, it is preferable to assume a rolling component after processing and to designate as the target the non-metallic inclusion L located closer to the surface of the rolling component.

[0028] This is because a harmful effect on the rolling component is assumed to increase with decreasing distance from the non-metallic inclusion L to the surface of the rolling component.

[0029] Figure 2 is a shear stress distribution of the plate model 11 acquired at a certain timing during rolling. In Figure 2, the magnitude of the shear stress and the positivity and negativity of the shear stress are color-coded. The shear stress distribution allows location of a position where the direction of the shear stress is reversed (hereinafter also referred to as a reversal position). That is, the shear stress distribution in Figure 2 allows location of the position where the shear stress changes between positivity and negativity, in other words, location of the reversal position. A dotted line indicates the reversal position of the non-metallic inclusion L.

[0030] The concept of the reversal position can be defined by a backward area and a pre-deformation area. That is, the reversal position is located at a boundary between the backward area and the pre-deformation area. The backward area will be described with reference to Figure 3. When the velocity of the plate model 11 on a roll inlet side (point A) is defined as V A and the velocity of the plate model 11 on a roll outlet side (point B) is defined as V B , the magnitude relationship between V A and V B is V A < V B . A neutral point refers to a point (point N) where the velocity V N of the plate model 11 is equal to a peripheral velocity V of the rolls. An area preceding the neutral point (on the roll outlet side) is the forward area, and an area following the neutral point (on the roll inlet side) is a backward area. Patent Literature 3 also describes such definitions of the forward and backward areas.

[0031] The pre-deformation area is an area on the inlet side of the rolling rolls 12 where the plate thickness starts to decrease due to a draw-in force from the rolling rolls 12 before contact between the rolling roll 12 and a rolling material. Deformation of the rolling material before contacting the rolls, in other words, pre-deformation is technically common knowledge (see, for example, Non Patent Literature 2), and thus detailed description is omitted.

[0032] The inventors focused on εx', which is strain of the plate model 11 generated in a longitudinal direction (x-axis direction) of the plate model 11 when the non-metallic inclusion L reaches the reversal position. More specifically, as illustrated in a schematic diagram of Figure 4, the contact is focused on that is formed between the non-metallic inclusion L and the matrix when the non-metallic inclusion L reaches the reversal position, and the strain εx', which is the strain of the contact in the x-axis direction, is determined from strain history.

[0033] Additionally, similar rolling analysis is performed on a uniform plate model without the non-metallic inclusion L to determine εx, which is strain of the plate model 11 generated in the longitudinal direction (x-axis direction) of the plate model 11 when a contact corresponding portion corresponding to the contact reaches the reversal position, and a strain ratio εx / εx' is calculated. That is, εx, which is the strain of the contact corresponding portion generated in the x-axis direction of the contact corresponding portion when the contact corresponding portion reaches the reversal position, is determined from the strain history, and the strain ratio εx / εx' is calculated.

[0034] The rolling processing increases both strain εx and strain εx'. However, the magnitude of the increase varies depending on the presence of the non-metallic inclusion, and thus the strain ratio εx / εx' enables gap generation behavior to be evaluated. The inventors have discovered that, by performing rolling processing using, as a control index, the strain ratio εx / εx', which enables the gap generation behavior to be evaluated, a rolling component can be manufactured in which a matrix and a non-metallic inclusion interface are in close contact with each other and which is excellent in fatigue life.

[0035] Specifically, by performing the above-described rolling analysis a plurality of times with the rolling reduction rate varied, the relationship between the rolling reduction rate, the strain ratio εx / εx', and a final gap area (hereinafter also referred to as correlation information) can be determined. Here, with an acceptability criterion for the final gap area defined in advance, the appropriate range of the strain ratio εx / εx' (corresponding to "less than or equal to a predetermined value" in the claims) can be determined. The acceptability criterion for the final gap area can be set as appropriate according to the application of the rolling material and the like. Specifically, as quantitative information for mitigating the harmful effect of the gap, information such as "the strain ratio εx / εx' is set less than or equal to the predetermined value". Note that, for the rolling component, a final gap area rate of 8 to 9% or less is generally recommended.

[0036] The final gap area rate (%) refers to a gap area rate calculated after the rolling processing is ended and can be calculated as (gap cross-sectional area / non-metallic inclusion cross-sectional area) × 100. The final gap area rate (%) can also be obtained as an output value from the rolling analysis. Of course, the gap cross-sectional area is "CROSS-SECTIONAL AREA OF GAP IN X-Z CROSS SECTION" in FIG. 4, the non-metallic inclusion cross-sectional area is "CROSS-SECTIONAL AREA OF NON-METALLIC INCLUSION IN X-Z CROSS SECTION". Note that the gap cross-sectional area is desirably a "gap cross-sectional area" obtained when the plate model 11 is cut at a central position of the non-metallic inclusion L in a plate width direction (normal direction of the sheet of the drawing in Figure 1).

[0037] Then, from the above-described correlation information, the rolling reduction rate can be determined which is used to set the strain ratio εx / εx' to less than or equal to a predetermined value. The predetermined value varies according to the elements provided to a rolling analysis model, and is thus not limited herein.

[0038] Therefore, by rolling the steel material based on the determined rolling reduction rate, a semi-finished product for the rolling component can be manufactured in which the matrix and the non-metallic inclusion interface are in close contact with each other and which is excellent in fatigue life. By processing the shape of the semi-finished product, a rolling component excellent in fatigue life can be manufactured.

[0039] In the above-described embodiment, the rolling processing of the plate-like steel material has been described. However, the present invention is not limited to this and can be applied to ring rolling. Here, the ring rolling refers to "forging processing of increasing the diameter of a ring by reducing the thickness of a ring-like steel material in a radial direction thereof using a plurality of rolls," and is classified as a roll forging operation among plastic processing operations.

[0040] Figure 5 is a schematic diagram of the ring rolling. With reference to Figure 5, a forging roll includes a drive roll 21 and a driven roll 22. When a torque from a drive source not illustrated is applied to the drive roll 21, the drive roll 21 performs a rotating operation. A ring-like steel material 20 is sandwiched between the drive roll 21 and the driven roll 22. When the driven roll 22 is pressed toward the ring-like steel material 20 in a Z-axis direction, the driven roll 22 rotates due to a frictional force and plastically deforms the inner diameter of the ring-like steel material 20 in a diameter increase direction.

[0041] Also in the ring rolling, correlation information corresponding to the above-described correlation information can be acquired by performing CAE analysis using a forging analysis model. Then, based on the correlation information, a rolling reduction rate can be determined that is required to set the strain ratio εx / εx' to less than or equal to a predetermined value.Example

[0042] With reference to an example, the present invention will be specifically described. The above-described rolling analysis model was used to perform CAE analysis on the relationship between the strain ratio εx / εx' and the rolling reduction rate, and the strain ratio εx / εx' and the rolling reduction rate were checked that are required to reduce the final gap area rate to 8% or less. As analysis software for the CAE analysis, DEFORM-3D manufactured by Scientific Forming Technologies was used.

[0043] The elements provided to the model include, in addition to the rolling reduction rate, the condition for the contact between the non-metallic inclusion L and the matrix (shear friction coefficient): 0.3, the plate thickness of the plate model 11: 50 mm, the Young's modulus of the plate: 206 GPa, the Poisson's ratio of the plate: 0.3, the stress-strain curve of the plate σ: 129 · ε 0.045< · ε̇ 0.039< , the position of the non-metallic inclusion: 1 mm from a superficial layer, ϕ of the non-metallic inclusion: 1 mm, a temperature condition for hot rolling: 1000°C, the inclusion and matrix: close contact (no gap), the roll diameter of the rolling roll: ϕ 250 mm, the peripheral velocity of the roll: 12.6 rad / s, the shear friction coefficient between the rolling roll and the plate model: 0.7, and the like. Note that the non-metallic inclusion was defined as a rigid body. In the present example, the acceptability criterion for the final gap area rate (%) was set less than or equal to 8%.

[0044] A graph in Figure 6 indicates the relationship between the strain εx (εx') and the rolling reduction rate. The strain εx is plotted as a blacked-out square, and the strain εx' is plotted as a blacked-out circle. In Figure 6, the horizontal axis indicates the rolling reduction rate (%), and the vertical axis indicates the strain εx (εx'). The relationship between the strain εx and the rolling reduction rate was fitted with a linear function to obtain a relation equation y = 0.0030x - 0.018 (coefficient of determination: 0.98). Similarly, the relationship between the strain εx' and the rolling reduction rate was fitted with a linear function to obtain a relation equation y = 0.0099x - 0.0035 (coefficient of determination: 0.98). The strain ratio εx / εx' obtained when the rolling reduction rate is 2(%), 5(%), 10(%), 20(%), or 40(%) was determined from the above-described relation equation, and organized in Table 1 together with the output final gap area rate (%). [Table 1]Rolling reduction rate (%)εx / εx'Final gap area rate (%)Evaluation2-0.740.7○5-0.0862.4○100.137○200.2213×400.2624×

[0045] The results indicate that the final gap area rate can be reduced to 8% or less by setting the strain ratio εx / εx' to less than or equal to 0.13. Therefore, in the present example, determining the rolling reduction rate required to set the strain ratio εx / εx' to less than or equal to 0.13 is found to enable manufacture of a material in which the matrix and the non-metallic inclusion interface are in close contact with each other and which is excellent in fatigue life. In the case of a change in any of the elements provided to the rolling analysis model change, the analysis processing may be performed again to acquire the appropriate strain ratio εx / εx' .Reference Signs List

[0046] 11Plate model 12Rolling roll LNon-metallic inclusion 20Ring-like steel material 21Drive roll 22Driven roll

Claims

1. A steel material processing method of applying compressive stress to a base material made of a steel material including a non-metallic inclusion to process the base material into a shape different from a shape of the base material, the steel material processing method comprising: using a strain ratio εx / εx' as a control index to determine a rolling reduction rate used when the compressive stress is applied to the steel material, wherein εx' is strain of a contact between the non-metallic inclusion and a matrix of the steel material which strain is generated in an extending direction of the contact when the contact reaches a reversal position where a direction of shear stress is reversed, and εx is strain of a contact corresponding portion of a steel material with no non-metallic inclusion corresponding to the contact which strain is generated in an extending direction of the contact corresponding portion when the contact corresponding portion reaches the reversal position where the direction of the shear stress is reversed.

2. The steel material processing method according to claim 1, wherein the compressive stress is applied to the steel material by a rolling roll or a forging roll.

3. The steel material processing method according to claim 2, wherein the rolling reduction rate of the rolling roll or the forging roll for keeping the strain ratio εx / εx' less than or equal to a predetermined value is predetermined by a rolling or forging analysis model, and the steel material is rolled or forged at the rolling reduction rate.

4. A rolling component manufacturing method comprising processing, into a shape of a rolling component, a semi-finished product manufactured by the steel material processing method according to any one of claims 1 to 3.

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