Method for processing steel material and method for manufacturing rolling member

Abstract

In order to manufacture a rolling member having an excellent curve rolling life, the present invention provides a method for processing a steel material by applying a compressive stress to a base material made of a steel material containing non-metallic inclusions and processing the steel material into a shape different from that of the base material, characterized in that: in the method for processing the steel material, the compressive stress is applied to the base material; and determining the reduction rate when the compression stress is applied to the steel by taking the strain ratio epsilon x / epsilon x'as a control index. Wherein epsilon x'is the strain in the extension direction of the contact point when the contact point of the non-metallic inclusion and the steel parent phase reaches the reversal position where the shear stress direction is reversed. [epsilon] x is the strain in the extension direction of a contact point corresponding portion of the steel material not containing the non-metallic inclusion when the contact point corresponding portion corresponding to the contact point reaches a reversal position in which the shear stress direction is reversed.

Description

Technical Field

[0001] The present invention relates to a processing method for processing steel containing non-metallic inclusions by applying compressive stress, etc. Background Art

[0002] In recent years, with the high-performance of various mechanical devices, the operating environments of mechanical components and devices that require rolling fatigue life have become extremely harsh, and the requirements for improving the life and reliability of these mechanical components and devices are very strong. Steel components such as bearings are derived from manufacturing processes such as refining processes, casting processes, and solidification processes, all of which are steel manufacturing processes. As is well known, these processes inevitably contain foreign substances called non-metallic inclusions.

[0003] In addition, in steel components such as bearings manufactured by rolling processes and forging processes, gaps may form around non-metallic inclusions. It is considered that this gap is generated at the interface due to the difference in deformation ability between non-metallic inclusions and the matrix steel. For example, when the steel component is a bearing, the above gap may cause cracks to occur in the bearing component subjected to rolling fatigue during use and may become the starting point of cracks. In addition to bearings, the same problem exists in rolling components that require good rolling fatigue life.

[0004] Therefore, in order to improve the rolling fatigue life of rolling components such as bearings, it is effective to reduce the gaps around non-metallic inclusions.

[0005] Patent Document 1 discloses a method for manufacturing an annular mechanical component having a rolling part with excellent rolling fatigue life. The mechanical component has a rolling part on the inner diameter of an annular material where a rolling component rolls. In the manufacturing of this mechanical component, the rolling part for the rolling component to roll is formed on the inner diameter surface of the annular material by a forging process. In this forging process, a compressive hydrostatic stress of 1.5 times or more of the material yield stress is applied to the inner diameter surface of the annular material where the rolling part is to be formed, and a compressive strain of 0.10 or more is generated in the plastic strain in the rolling direction of the rolling component. Thus, the direction in which the voids existing at the interface between non-metallic inclusions and the matrix steel in the steel tend to close, and thus a mechanical component with excellent rolling fatigue life can be manufactured.

[0006] Patent Document 2 discloses a method for manufacturing an annular mechanical component, which includes: using upper and lower punches with a smoothly reduced diameter towards the front end, applying a hydrostatic stress of at least 1000 MPa to the inner diameter surface of an annular processed base material to be cold-forged to form a rolling part, and forming a rolling part by a cold-forging process. By the action of the hydrostatic stress, the direction in which the voids existing at the interface between non-metallic inclusions and the matrix steel in the steel tend to close. Prior Art Documents Patent Documents

[0007] Patent Document 1: Japanese Patent No. 5896713 Patent Document 2: Japanese Patent No. 5669128 Patent Document 3: Japanese Patent No. 2923095 Patent Document 4: Japanese Unexamined Patent Application Publication No. 2014-55346 Non-Patent Document

[0008] Non-Patent Document 1: Light Metals, Vol. 42, No. 2 (1992), Current Status of Finite Element Analysis in Plastic Working Non-Patent Document 2: Kobe Steel Technical Report, Vol. 48, No. 1 (April 1998), Special Issue on Plastic Working, Influence of Inlet-Side Calculation Region Length on Sheet Thickness Distribution in Analysis of Stable Sheet Rolling Summary of the Invention Problems to be Solved by the Invention

[0009] In the above Patent Documents 1 and 2, for the gaps already existing inside the closed steel components or castings and forgings, only a hydrostatic compressive stress or compressive strain of a certain level or more is applied to the steel components or castings and forgings, and the deformation phenomena of non-metallic inclusions and the surrounding base metal are not controlled. Therefore, it is difficult to evaluate the effect of the closed gaps.

[0010] An object of the present invention is to provide a method for manufacturing a rolling component having excellent rolling life by controlling the deformation phenomena of non-metallic inclusions and matrix phases contained in steel. Means for Solving the Problems

[0011] In order to solve the above problems, the present invention provides a processing method for steel, which applies a compressive stress to a base material made of steel containing non-metallic inclusions and processes it into a shape different from the base material, and is characterized in that the rolling reduction when applying the compressive stress to the steel is determined using the strain ratio εx / εx' as a control index. Here, εx' is the strain in the extending direction of the contact point when the contact point between the non-metallic inclusion and the matrix phase of the steel reaches the inversion position where the shear stress direction is reversed. εx is the strain in the extending direction of the contact point corresponding portion of the steel without non-metallic inclusions when the contact point corresponding portion corresponding to the contact point reaches the inversion position where the shear stress direction is reversed.

[0012] (2) The processing method for steel according to the above (1), characterized in that the compressive stress is applied to the steel by a rolling mill roll or a forging roll.

[0013] (3) The processing method of steel according to (2) above is characterized in that the reduction rate of the rolling roll or forging roll for suppressing the strain ratio εx / εx' below a specified value is obtained in advance by a rolling analysis model or a forging analysis model, and the steel is rolled or forged at this reduction rate.

[0014] (4) A method for manufacturing a rolling element, characterized in that a semi-finished product manufactured by the processing method of steel according to any one of (1) to (3) above is processed into the shape of a rolling element. Effects of the Invention

[0015] According to the present invention, by controlling the non-metallic inclusions contained in the steel and the deformation phenomenon of the matrix phase, it is possible to provide rolling elements such as bearings with excellent rolling life. Description of the Drawings

[0016] Figure 1 It is an explanatory diagram for explaining the rolling analysis method. Figure 2 It is the shear stress distribution of the sheet model 11 obtained at a certain moment during the rolling process. Figure 3 It is an explanatory diagram of the forward movement area and the backward movement area. Figure 4 It is a schematic diagram for explaining the contact point between the non-metallic inclusion and the matrix phase of the strain ε'. Figure 5 It is a schematic diagram of ring rolling. Figure 6 It is a graph (Example) for arranging the relationship between the strain εx (εx') and the reduction rate. Detailed Description of the Invention

[0017] The inventors have found a processing method of steel in which a compressive stress is applied to a base material made of steel containing non-metallic inclusions and processed into a shape different from the base material, and by controlling the non-metallic inclusions contained in the steel and the deformation phenomenon of the matrix phase, it is possible to provide rolling elements such as bearings with excellent rolling life.

[0018] Rolling elements include components that require good rolling fatigue life. Examples of such rolling elements include bearings, gears, wheel hub units, continuously variable transmissions, constant velocity joints, crank pins, piston pins, etc.

[0019] The inventors have analyzed by using a rolling analysis model (refer to Non-Patent Document 1) based on the well-known rigid-plastic finite element method, and clarified a method for controlling the deformation phenomenon of non-metallic inclusions and matrix phase contained in steel during rolling. Rolling analysis is performed by CAE analysis.

[0020] CAE analysis is an abbreviation of Computer Aided Engineering, which is an analytical method for evaluating (simulating) design problems of suspected reproduced products on a computer. CAE analysis is implemented through computer programs.

[0021] Figure 1 It is an explanatory diagram for illustrating the rolling analysis method. When analyzing the rolling of a sheet model 11 using a pair of upper and lower rollers 12, the relationship between the gap generation behavior around the non-metallic inclusion L and the strain is analyzed. The contact condition between the non-metallic inclusion L and the matrix phase is defined by the shear friction coefficient, and the shear stress and strain history when the non-metallic inclusion L passes from the entry side of the roller 12 to the exit side of the roller 12 are obtained. Among them, the strain refers to the plastic strain at the contact point between the non-metallic inclusion L and the matrix phase in the extension direction ( Figure 1 the X-axis direction in

[0022] The parameters given to the rolling analysis model, in addition to the above-mentioned contact condition (shear friction coefficient) between the non-metallic inclusion L and the matrix phase, representative parameters include plate thickness, hot and cold conditions (temperature, etc.), the position and size of the non-metallic inclusion, the position and size of the gap existing around the non-metallic inclusion, reduction ratio, physical property data of the material (Young's modulus, Poisson's ratio, stress-strain curve), the roll diameter and circumferential speed of the roller, the shear friction coefficient between the roller and the sheet model, etc. In addition, the reduction ratio is the degree of rolling processing expressed as a percentage. When the material plate thickness before and after rolling is h1 and h2 respectively, it is calculated according to the formula (h1 - h2) / h1. These parameters can be determined by pre-analyzing the properties of the steel used for rolling components such as bearings. When there are two or more non-metallic inclusions L, it is best to assume a machined rolling component and target the non-metallic inclusion L located near the surface of the rolling component. This is because it is considered that the shorter the distance of the non-metallic inclusion L from the surface of the rolling component, the relatively greater the harmful effect on the rolling component.

[0023] Figure 2 It is the shear stress distribution of the sheet model 11 obtained at a certain moment during the rolling process. The shear stress is distinguished by color according to its magnitude and positive / negative. Through this shear stress distribution, the position where the shear stress direction reverses (hereinafter also referred to as the reversal position) can be determined. That is, it is possible to Figure 2 grasp the position where the positive and negative of the shear stress change, that is, the reversal position, from the

[0024] The concept of the reversal position can be defined by the backward movement area and the pre-deformation area. That is, the reversal position is located at the boundary between the backward movement area and the pre-deformation area. Regarding the backward movement area, reference will be made toFigure 3 is described. When the speed of the sheet material model 11 at the entry side of the roll (point A) is defined as V A , and the speed of the sheet material model 11 at the exit side of the roll (point B) is defined as V B , the relationship between the magnitudes of V A and V B is V A < V B . The point (point N) where the speed V N of the sheet material model 11 is equal to the circumferential speed V of the roll is called the neutral point. The front (roll exit side) of this neutral point is the forward movement area, and the rear (roll entry side) is the backward movement area. The definitions of the forward movement area and the backward movement area are also described in Patent Document 3.

[0025] The pre-deformation area is the area on the entry side of the roll 12 where the sheet thickness starts to decrease due to the pulling force from the roll 12 before the roll 12 contacts the rolling material. It is common technical knowledge that the rolling material deforms, i.e., pre-deforms, before contacting the roll (for example, see Non-Patent Document 2), so detailed description is omitted.

[0026] The present inventors then focused on εx', which is the strain in the length direction (x-axis direction) of the sheet material model 11 when the non-metallic inclusion L reaches the inversion position. More specifically, as shown in the schematic diagram of Figure 4 , by focusing on the contact point between the non-metallic inclusion L and the matrix when the non-metallic inclusion L reaches the inversion position, the strain εx' in the x-axis direction of this contact point is obtained based on the strain history.

[0027] In addition, the same rolling analysis is also performed on a homogeneous sheet material model without non-metallic inclusions L, and the strain εx in the length direction (x-axis direction) of the sheet material model 11 when the contact point corresponding part corresponding to the above contact point reaches the inversion position is obtained, and the strain ratio εx / εx' is calculated. That is, the strain εx in the x-axis direction of the contact point corresponding part reaching the inversion position is obtained based on the strain history, and the strain ratio εx / εx' is calculated.

[0028] Both the strain εx and the strain εx' increase due to the rolling process, but the increase amplitude varies depending on the presence or absence of non-metallic inclusions. Therefore, the gap generation behavior can be evaluated by the strain ratio εx / εx'. The present inventors found that by performing the rolling process with the strain ratio ε / ε' that can evaluate the gap generation behavior as the control index, rolling components with high interfacial adhesion between the matrix and non-metallic inclusions and excellent fatigue life can be manufactured.

[0029] Specifically, by changing the reduction ratio and performing the above rolling analysis multiple times, the relationship among the reduction ratio, the strain ratio εx / εx', and the final clearance area (hereinafter also referred to as relevant information) can be grasped. Among them, if the acceptance criteria for the final clearance area are determined in advance, the appropriate range of the strain ratio εx / εx' (corresponding to "below the specified value" described in the technical solution) can be grasped from the said relevant information. The acceptance criteria for the final clearance area can be appropriately set according to the use of the rolled material, etc. Specifically, as quantitative information for reducing the harmfulness of the clearance, information such as "setting the strain ratio εx / εx' below the specified value" can be obtained. For rolling components, it is generally recommended that the final clearance area ratio be 8-9% or less.

[0030] The final clearance area ratio (%) is the clearance area ratio calculated after the rolling process and can be calculated by (clearance cross-sectional area / non-metallic inclusion cross-sectional area) × 100. The final clearance area ratio (%) is also obtained as an output value of the rolling analysis. Needless to say, the clearance cross-sectional area is Figure 4 the "cross-sectional area of the clearance on the X-Z section" in Figure 4 , and the non-metallic inclusion cross-sectional area is the "cross-sectional area of the non-metallic inclusion on the X-Z section" in Figure 1 . The clearance cross-sectional area is preferably the "clearance cross-sectional area" when cutting the sheet metal model 11 at the center position of the non-metallic inclusion L in the paper surface normal direction of the plate width direction ( Figure 1 ). According to the said relevant information, the reduction ratio for setting the strain ratio εx / εx' below the specified value can be grasped. The specified value varies according to the parameters given to the rolling analysis model and is not limited in this specification. Therefore, by rolling the steel based on the grasped reduction ratio, a semi-finished product for rolling components with high interfacial adhesion between the matrix phase and non-metallic inclusions and excellent fatigue life can be manufactured. By processing the shape of this semi-finished product, a rolling component with excellent fatigue life can be manufactured.

[0031] In the above embodiment, the rolling process of the plate-shaped steel is described, but the present invention is not limited thereto and can also be applied to ring rolling. The ring rolling mentioned here refers to "a forging process that uses multiple rollers to reduce the radial thickness of the ring-shaped steel, thereby expanding the diameter of the ring" and is classified as a kind of rotary forging in plastic processing.

[0032] Figure 5 is a schematic diagram of ring rolling. Referring to this figure, the forging roll consists of a driving roll 21 and a driven roll 22. The driving roll 21 rotates by applying a rotational force from a driving source (not shown). The ring-shaped steel 20 is sandwiched between the driving roll 21 and the driven roll 22. By pressing the driven roll 22 toward the ring-shaped steel 20 in the Z-axis direction, while the driven roll 22 rotates due to friction, the inner diameter of the ring-shaped steel 20 can be plastically deformed in the diameter-expanding direction.

[0033] In this ring rolling, by performing CAE analysis using a forging analysis model, relevant information corresponding to the above-mentioned relevant information can also be obtained. Then, based on such relevant information, the reduction ratio for setting the strain ratio εx / εx' to a value below a specified value can be grasped.

[0034] (Example) The present invention will be specifically described with reference to the examples. Using the above rolling analysis model, the relationship between the strain ratio εx / εx' and the reduction ratio was analyzed by CAE, and the strain ratio εx / εx' and the reduction ratio for reducing the final clearance area ratio to 8% or less were studied. The analysis software used in the CAE analysis was DEFORM-3D produced by Scientific Forming Technologies.

[0035] The parameters assigned to the model, except for the reduction ratio, are as follows: the contact condition (shear friction coefficient) between the non-metallic inclusions L and the matrix phase: 0.3, the thickness of the sheet model 11: 50 mm, the Young's modulus of the sheet: 206 GPa, the Poisson's ratio of the sheet: 0.3, The flow stress curve σ of the sheet: The position of the non-metallic inclusions: 1 mm from the surface layer, the diameter φ of the non-metallic inclusions: 1 mm, the hot rolling temperature condition: 1000 °C, the inclusions and the matrix phase: closely combined (no gap), the roll diameter of the roll: φ250 mm, the circumferential speed of the roll: 12.6 rad / s, the shear friction coefficient between the roll and the sheet model: 0.7. It should be noted that the non-metallic inclusions are defined as rigid bodies. In this example, the acceptance criterion for the final clearance area ratio (%) is set to 8% or less.

[0036] The relationship between the strain εx (εx') and the reduction ratio is as shown in the Figure 6 chart. The strain εx is plotted as solid squares, and the strain εx' is plotted as solid circles. In the figure, the horizontal axis is the reduction ratio (%), and the vertical axis is the strain εx (εx'). The relationship between the strain εx and the reduction ratio was fitted to a linear function to obtain the relationship expression y = 0.0030x - 0.018 (coefficient of determination: 0.98). Similarly, the relationship between the strain εx′ and the reduction ratio was fitted to a linear function to obtain the relationship expression y = 0.0099x - 0.0035 (coefficient of determination: 0.98). The strain ratios εx / εx' at reduction ratios of 2 (%), 5 (%), 10 (%), 20 (%), and 40 (%) were calculated using the above relationship expressions and summarized in Table 1 together with the output final clearance area ratio (%). Table 1 Reduction ratio (%) εx / εx′ Final clearance area ratio (%) Evaluation 2 -0.74 0.7 ○ 5 -0.086 2.4 ○ 10 0.13 7 〇 20 0.22 13 × 40 0.26 24 ×

[0037] From these results, it can be seen that by setting the strain ratio εx / εx' to 0.13 or less, the final clearance area ratio can be reduced to 8% or less. Therefore, in this embodiment, it was found that by determining the reduction ratio and making the strain ratio εx / εx' 0.13 or less, a material having high adhesion and excellent fatigue life at the interface between the matrix phase and the non-metallic inclusions can be manufactured. When the parameters given to the rolling analysis model are changed, the analysis process can be re-performed to obtain an appropriate strain ratio εx / εx'.

Description of reference numerals

[0038] 11 Sheet metal model; 12 Roll; L Non-metallic inclusion; 20 Ring-shaped steel; 21 Driving roll; 22 Driven roll.

Claims

1. A processing method for steel, which applies a compressive stress to a base material made of steel containing non-metallic inclusions and processes it into a shape different from that of the base material, characterized in that the reduction ratio when applying the compressive stress to the steel is determined with the strain ratio εx / εx' as a control index, wherein εx' is the strain in the extending direction of the contact point when the contact point between the non-metallic inclusion and the steel matrix reaches the inversion position where the shear stress direction is reversed, and εx is the strain in the extending direction of the corresponding contact part of the steel without non-metallic inclusions when the corresponding contact part of the contact point reaches the inversion position where the shear stress direction is reversed.

2. The processing method for steel according to claim 1, characterized in that the compressive stress is applied to the steel by a rolling roll or a forging roll.

3. The processing method for steel according to claim 2, characterized in that the reduction ratio of the rolling roll or the forging roll for suppressing the strain ratio εx / εx' below a specified value is obtained in advance by a rolling analysis model or a forging analysis model, and the steel is rolled or forged at this reduction ratio.

4. A manufacturing method for a rolling element, characterized in that a semi-finished product manufactured by the processing method for steel according to any one of claims 1 to 3 is processed into the shape of a rolling element.

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