Method for processing steel material and method for manufacturing rolling component

Abstract

To provide a rolling component such as a bearing excellent in rolling life.SOLUTION: A method for processing a steel material that performs a compression process on a base material made of a steel material including non-metallic inclusions to transform the steel material into a shape different from that of the base material. The method includes: an obtaining step of obtaining relationship information between a solution of a rolling reduction evaluation formula and a gap area ratio in advance; a parameter determining step of determining parameters included in the predetermined rolling reduction evaluation formula such that a target gap area ratio can be obtained on the basis of the relationship information; and a processing step of performing the compression process k times according to the predetermined rolling reduction evaluation formula determined in the parameter determining step. The predetermined rolling reduction evaluation formula is a formula obtained by multiplying a multiplication value by 10k, the multiplication value being obtained by multiplying a rolling reduction of each rolling process, k being an integer of 2 or more, and the rolling reduction of each rolling process included in the predetermined rolling reduction evaluation formula and k being parameters determined in the parameter determining step.SELECTED DRAWING: Figure 6

Description

Technical Field

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

Background Art

[0002] In recent years, with the improvement of the performance of various mechanical devices, the operating environments of mechanical parts and devices for which rolling fatigue life is required have become extremely severe, and there is a strong demand for improving the life and reliability of these mechanical parts and devices. It is known that steel parts such as bearings inevitably contain foreign substances (non-metallic inclusions) derived from manufacturing processes such as the refining process, casting process, and solidification process, which are the manufacturing processes of steel.

[0003] Also, in steel parts such as bearings manufactured through a rolling process or forging process, gaps may be formed around non-metallic inclusions. This gap is considered to be generated at the interface due to the difference in deformability between the non-metallic inclusion and the steel, which is the matrix phase. For example, when the steel part is a rolling bearing, a repeated contact load is applied to the raceway surface where the rolling elements roll during bearing use. If the above-mentioned gap exists in the affected area, it may promote crack generation within the bearing part and may become the origin of cracks. When this crack reaches the raceway surface, it may lead to peeling and may result in the breakage of the bearing. The same problem exists in rolling parts other than bearings that require good rolling fatigue life.

[0004] Therefore, in order to improve the rolling fatigue life of rolling parts such as bearings, it is particularly effective to reduce the gap around non-metallic inclusions on the surface layer of the raceway surface.

[0005] Patent Document 1 discloses a method of using the hydrostatic pressure during hot forging to crush the voids existing between the inclusion and the matrix interface.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Non-Patent Document

[0007]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] Since Patent Document 1 is not a method for controlling the deformation phenomena of non-metallic inclusions and the surrounding base material, it is difficult to evaluate the effect regarding the closing of gaps. Further, Patent Document 1 is originally an invention related to die forging, and the technical field is different from that of the present invention which targets rolling and ring rolling.

[0009] 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 the matrix phase contained in a steel material.

Means for Solving the Problems

[0010] In order to solve the above problems, the present invention provides a method for processing a steel material, which comprises: (1) a steel material made of a base material containing non-metallic inclusions is subjected to a compression treatment and processed into a shape different from that of the base material, and an acquisition step of previously acquiring relationship information between a solution of a predetermined reduction rate evaluation formula and a gap area ratio which is an area ratio of gaps formed around the inclusions; a parameter determination step of determining a parameter included in the predetermined reduction rate evaluation formula so that a target gap area ratio which is a target value of the gap area ratio is obtained based on the relationship information; and a processing step of performing the compression treatment k times according to the predetermined reduction rate evaluation formula determined in the parameter determination step, wherein the predetermined reduction rate evaluation formula is a multiplication value obtained by multiplying the reduction rate of each rolling process by 10 kIt is an expression multiplied by, where k is an integer of 2 or more, and the reduction ratio of each rolling process and k included in the predetermined reduction ratio evaluation formula are parameters determined in the parameter determination step.

[0011] (2) The compression stress is applied to the steel material by a rolling or forging roll. The method for processing a steel material according to (1) above.

[0012] (3) In the acquisition step, the relationship information is acquired by performing CAE analysis using an analysis model. The method for processing a steel material according to (1) or (2) above.

[0013] (4) A method for manufacturing a rolling element, characterized in that a semi-finished product manufactured by the method for processing a steel material according to any one of (1) to (3) above is processed into the shape of a rolling element.

Effect of the Invention

[0014] According to the present invention, by controlling the deformation phenomena of non-metallic inclusions and the matrix phase contained in the steel material, it is possible to provide rolling elements such as bearings having excellent rolling life.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiment for Carrying Out the Invention

[0016] The inventors of the present invention have discovered a processing method for steel materials that applies compressive stress to a base material made of a steel material containing non-metallic inclusions and processes it into a shape different from that of the base material. By controlling the deformation phenomena of the non-metallic inclusions and the matrix phase contained in the steel material, a processing method capable of providing rolling parts such as bearings with excellent rolling life can be obtained.

[0017] Rolling parts include parts that require good rolling fatigue life. This type of rolling parts includes, for example, bearings, gears, hub units, continuously variable transmissions, constant velocity joints, crank pins, piston pins, and the like.

[0018] The processing method of the steel material according to an embodiment of the present invention is a processing method for a steel material that performs a compression treatment on a base material made of a steel material containing non-metallic inclusions and processes it into a shape different from that of the base material, and includes an acquisition step, a parameter determination step, and a processing step. The compression treatment in this embodiment is a rolling treatment, and the target reduction ratio is achieved by performing the rolling treatment in multiple stages. Hereinafter, each step will be described in detail by itemizing.

[0019] (Regarding the acquisition step) In the acquisition step, relationship information between the solution of a predetermined reduction ratio evaluation formula and the gap area ratio is acquired. Equation (1) is a general formula of the reduction ratio evaluation formula. {(Reduction ratio n1) × (Reduction ratio n2) × ···· (Reduction ratio n k )} × 10 k ···· Equation (1) Equation (1) is obtained by multiplying the product value of the reduction ratios of each rolling treatment by 10 k "k" corresponds to the number of rolling treatments and is an integer of 2 or more. For example, when the rolling treatment is carried out in three stages, Equation (1) is the product value of the reduction ratio n1 (the reduction ratio of the first rolling), the reduction ratio n2 (the reduction ratio of the second rolling), and the reduction ratio n3 (the reduction ratio of the third rolling) multiplied by 10 3It becomes an expression multiplied by. Needless to say, the sum value of the reduction ratio n1, the reduction ratio n2, and the reduction ratio n3 is the target reduction ratio.

[0020] The gap area ratio is the area ratio of the gap formed around the inclusion and is calculated after the rolling process is completed. The method for calculating the gap area ratio will be described later.

[0021] The relationship information between the solution of the reduction ratio evaluation formula and the gap area ratio can be obtained by a rolling analysis model (see Non-Patent Document 1) according to the known rigid-plastic finite element method. CAE analysis can be used for the rolling analysis.

[0022] CAE analysis is an abbreviation taking the initial letters of Computer Aided Engineering and is an analysis method for evaluating (simulating) the design problems of products pseudo-reproduced on a computer. CAE analysis is realized by a computer program.

[0023] For CAE analysis, for example, DEFORM can be used. DEFORM is a CAE software that simulates all processing phenomena such as the material flow of metal, the load state on the tool, the deformation due to heat treatment, and the prediction of chips during cutting on a computer. Therefore, a highly accurate gap area ratio can be obtained by CAE analysis.

[0024] Figure 1 is an explanatory diagram for explaining the method of rolling analysis. The X-axis corresponds to the rolling direction (drawing direction) of the plate model 11, and the Z-axis corresponds to the plate thickness direction of the plate model 11. The behavior of gap generation around the non-metallic inclusion L when the plate model 11 is rolled using a pair of upper and lower rolling rolls 12 can be analyzed.

[0025] Typical parameters for the rolling analysis model include the contact conditions (shearing friction coefficient) between the non-metallic inclusion L and the matrix phase, plate thickness, hot or cold conditions (temperature, etc.), the position and size of the non-metallic inclusion L, the position and size of the gaps existing around the non-metallic inclusion L, the number of rolling passes, the reduction ratio of each rolling pass, the physical property data of the material (Young's modulus, Poisson's ratio, stress-strain curve), the roll diameter and peripheral speed of the rolling rolls, the shearing friction coefficient between the rolling rolls and the plate model, etc. The reduction ratio represents the degree of rolling work as a percentage. When the plate thicknesses of the material before and after rolling are h1 and h2 respectively, it is calculated by the formula (h1 - h2) / h1. These parameters can be grasped by pre-analyzing the properties of the steel material used for rolling parts such as bearings. When there are two or more non-metallic inclusions L, it is desirable to assume the rolling parts after machining and target the non-metallic inclusion L located closer to the surface of the rolling parts. This is because the shorter the distance from the non-metallic inclusion L to the surface of the rolling parts, the relatively greater the harmfulness to the rolling parts.

[0026] Through rolling analysis, the gap area ratio after the rolling process is completed can be obtained. The gap area ratio can be calculated from "(gap cross-sectional area / non-metallic inclusion cross-sectional area) × 100". Fig. 2 schematically shows the non-metallic inclusion L and the gaps formed around it after the rolling process is completed, with the hatching indicating the gaps. Referring to this figure, the area in the X-Z cross-section of the gap is the "gap cross-sectional area", and the area in the X-Z cross-section of the non-metallic inclusion L is the "non-metallic inclusion cross-sectional area". Through rolling analysis, the "gap cross-sectional area" and the "non-metallic inclusion cross-sectional area" after the rolling process is completed are obtained, and the "gap area ratio" is calculated based on the obtained information. It is desirable that the gap cross-sectional area be the "gap cross-sectional area" when cutting at the central position of the non-metallic inclusion L in the plate width direction (the direction normal to the paper surface in Fig. 1) of the plate model 11.

[0027] The specifications given to the model include the "number of rolling processes" and the "rolling reduction rate of each rolling process", so one-to-one information on "the solution of the rolling reduction rate evaluation formula of Equation (1)" and the "gap area ratio" can be obtained. Then, by variously changing the "number of rolling processes" and the "rolling reduction rate of each rolling process" given to the model and performing the same analysis process, the above-mentioned "one-to-one information" can be obtained as many times as the number of analysis processes. Thereby, "relationship information between the solution of the rolling reduction rate evaluation formula and the gap area ratio" can be acquired. Note that the target rolling reduction rate in each analysis process does not necessarily have to be the same.

[0028] Figure 3 is a schematic diagram of the relationship information, where the horizontal axis is "the solution of the rolling reduction rate evaluation formula of Equation (1)" and the vertical axis is the "gap area ratio". Referring to the figure, the plotted data is fitted to a predetermined function formula to construct a relational formula between Equation (1) and the gap area ratio. Here, the function formula only needs to satisfy a predetermined coefficient of determination, and its formula form may be a linear function, an nth-order function (n is an integer of 2 or more), a logarithmic function, etc. The predetermined coefficient of determination is preferably 0.70 or more, more preferably 0.80 or more.

[0029] Here, when a relational formula that satisfies the predetermined coefficient of determination cannot be obtained, the specifications given to the model can be changed (for example, increasing the variations such as the number of rolling processes and the rolling reduction rate of each rolling process), and by increasing the number of analysis processes, a relational formula that satisfies the predetermined coefficient of determination can be constructed. Also, by changing the function formula to be fitted (for example, changing from a linear function to a logarithmic function), a relational formula that satisfies the predetermined coefficient of determination may be searched for. Note that when two or more function formulas that satisfy the predetermined coefficient of determination can be constructed, it is preferable to select the function formula with a higher coefficient of determination. In Figure 3, as an example, a relational formula between Equation (1) and the gap area ratio is constructed by fitting it to a linear function.

[0030] A base material test piece was obtained by sintering a mixture of SUJ2 powder and alumina particles at a mass ratio of 500:1. By rolling such a base material test piece from a thickness of 50 mm to 40 mm using a roll with a roll diameter of φ870 mm, a single observation test piece in which a large number of alumina particles were dispersed was obtained. The roll peripheral speed was set at 715 mm / s. The observation test piece was cut at the center position in the width direction, and 10 alumina particles existing within 50 mm at the center of the cut surface were observed. After measuring the gap area ratio around the alumina particles, the average value (3.7%) of these gap area ratios was calculated as the actually measured value of the gap area ratio. By performing the above-described CAE analysis using the above-described observation test piece as a plate model, an estimated value (2.6%) of the gap area ratio was obtained. The error of the estimated value with respect to the actually measured value was only about 1.1%. From such experimental results, it can be seen that the gap area ratio can be accurately estimated by CAE analysis. 2 Ten alumina particles existing within 2 were observed, and after measuring the gap area ratio around the alumina particles, the average value (3.7%) of these gap area ratios was calculated as the actually measured value of the gap area ratio. By performing the above-described CAE analysis using the above-described observation test piece as a plate model, an estimated value (2.6%) of the gap area ratio was obtained. The error of the estimated value with respect to the actually measured value was only about 1.1%. From such experimental results, it can be seen that the gap area ratio can be accurately estimated by CAE analysis.

[0031] (Parameter determination step) Based on the relationship information obtained in the acquisition step, the parameter determination step determines the parameters included in the reduction ratio evaluation formula of Equation (1) so that a target gap area ratio, which is the target value of the gap area ratio, can be obtained. Figure 4 is a graph corresponding to Figure 3 that shows the upper limit (8%) of the target gap area ratio. From the relational expression (linear expression), it can be seen that if Equation (1) is set to 1 or less, the target gap area ratio (8% or less) can be achieved. That is, as the parameter determination step, the number of rolling processes k and the reduction ratio of each rolling process that satisfy the following Equation (2) may be determined. {(Reduction ratio n1) × (Reduction ratio n2) × ··· (Reduction ratio n k )} × 10 k ≦ 1 ··· Equation (2)

[0032] In rolling elements, generally, a gap area ratio of 8 to 9% or less is recommended. Therefore, the target gap area ratio (8% or less) in Figure 4 is an example. As described above, since the analytical value of the gap area ratio is an estimated value, it may deviate from the actually measured value by about 1 to 2%. Although it may be ignored because the deviation amount is small, it may also be operated by lowering the upper limit of the target gap area ratio from 8% by about 1 to 2% in consideration of the deviation amount.

[0033] (Processing Step) According to the number of rolling processes determined in the parameter determination step and the reduction ratio of each rolling process, a rolling process is performed on the steel material. For example, when the number of rolling processes is 3, the reduction ratio of the first rolling process is 20%, the reduction ratio of the second rolling process is 5%, and the reduction ratio of the third rolling process is 5%, the rolling process of the steel material may be performed 3 times according to such rolling conditions. Note that it is desirable that the reduction ratio per time be smaller. When the reduction ratio is small in rolling, the deformation is limited to the surface of the steel material. However, since the stretching deformation in the rolling direction is restricted by the undeformed portion inside the steel material, it is known that the compressive stress in the stretching direction easily acts on the surface of the steel material. Therefore, focusing on the surface layer of the steel material, reducing the reduction ratio is more effective in suppressing gaps. Specifically, when the target reduction ratio is X%, it is desirable to set the reduction ratio per time so as not to exceed 0.5X%.

[0034] By performing the rolling process according to the processing step, it is possible to manufacture a semi-finished product for a rolling part having high adhesion at the matrix phase and non-metallic inclusion interface and excellent fatigue life. By processing the shape of this semi-finished product, it is possible to manufacture a rolling part having excellent fatigue life.

[0035] In the above-described embodiment, the rolling process of the plate-shaped steel material has been described. However, the present invention is not limited to this and can also be applied to ring rolling. Here, ring rolling means "a forging process that expands the diameter of a ring by reducing the thickness in the radial direction of a ring-shaped steel material using several rolls", and is classified as one of the rotary forging in plastic processing.

[0036] FIG. 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 receiving a rotational force from a driving source (not shown). The ring-shaped steel material 20 is sandwiched between the driving roll 21 and the driven roll 22. By pressing the driven roll 22 toward the ring-shaped steel material 20 in the Z-axis direction, the driven roll 22 rotates due to frictional force, and the inner diameter of the ring-shaped steel material 20 can be plastically deformed in the diameter-expanding direction.

[0037] Also in this ring rolling, by implementing the above-described acquisition step and parameter determination step, it is possible to search for processing conditions for manufacturing rolling components with excellent fatigue life. Then, by performing a processing treatment according to such processing conditions, it is possible to manufacture rolling components with excellent fatigue life.

[0038] (Example) By CAE analysis, the relationship information between the solution of the reduction rate evaluation formula of Equation (1) and the clearance area ratio was obtained. As the analysis software for CAE analysis, DEFORM-3D manufactured by Scientific Forming Technologies was used. The specifications given to the model are as follows: contact condition (shear friction coefficient) of non-metallic inclusions L and matrix phase: 0.3, plate thickness of plate model 11: 50 mm, Young's modulus of the plate: 206 GPa, Poisson's ratio of the plate: 0.3, φ of inclusions in JPEG2025104280000002.jpg6170: 1 mm, temperature condition of hot rolling: 1000 °C, inclusions and matrix phase: in close contact (no gap), roll diameter of rolling roll: φ250 mm, roll peripheral speed: 12.6 rad / s, shear friction coefficient between rolling roll and plate model: 0.7, etc. The number of rolling processes and the reduction rate of each rolling process were variously changed, and the analysis process was implemented. Such an analysis process was carried out 3 times while changing the roll diameter of the rolling roll to φ250, φ175, and φ400. The non-metallic inclusions were defined as rigid bodies. The clearance area ratio was calculated according to the method described in the embodiment.

[0039] The data was plotted on a graph with the solution of Equation (1) on the horizontal axis (x-axis) and the gap area ratio (%) on the vertical axis (y-axis) (however, roll diameter: φ250 mm), and by fitting it to a linear function, a relational expression of y = 2.8x + 4.2 was obtained (see Figure 6). The coefficient of determination was 0.81. Also, by fitting the above-mentioned data to a logarithmic function, a relational expression of y = 2.7In(x) + 8.9 was obtained (see Figure 7). The coefficient of determination was 0.86. Therefore, regardless of whether it was a linear function or a logarithmic function, a highly accurate relational expression could be constructed.

[0040] By fitting the analysis data with a roll diameter of φ175 mm to a logarithmic function, a relational expression of y = 3.0In(x) + 5.0 was obtained (see Figure 7). The coefficient of determination was 0.90. By fitting the analysis data with a roll diameter of φ400 mm to a logarithmic function, a relational expression of y = 3.8In(x) + 7.5 was obtained (see Figure 7). The coefficient of determination was 0.96. Even when the roll diameter was changed, a highly accurate relational expression could be constructed.

[0041] From the results of Figure 6, it was found that when performing rolling using a roll with a roll diameter of φ250 mm, in order to set the target gap area ratio to 8% or less, the number of rolling passes k and the reduction ratio of each rolling pass should be determined so as to satisfy the following Equation (3). {(Reduction ratio n1) × (Reduction ratio n2) × ··· (Reduction ratio n k )} × 10 k ≦ 1.4 ··· Equation (3) From the results of Figure 7, it was found that when performing rolling using a roll with a roll diameter of φ250 mm, in order to set the target gap area ratio to 8% or less, the number of rolling passes k and the reduction ratio of each rolling pass may be determined so as to satisfy the following Equation (4). {(Reduction ratio n1) × (Reduction ratio n2) × ··· (Reduction ratio n k )} × 10 k ≦ 0.72 ··· Equation (4)

[0042] From the results of Fig. 7, when performing rolling using a roll with a roll diameter of φ400 mm, in order to set the target gap area ratio to 8% or less, it was found that the number of rolling passes k and the reduction ratio of each rolling pass should be determined so as to satisfy the following formula (5). {(Reduction ratio n1) × (Reduction ratio n2) × ··· (Reduction ratio n k )} × 10 k ≤ 1.14 ··· Formula (5) From the results of Fig. 7, when performing rolling using a roll with a roll diameter of φ175 mm, in order to set the target gap area ratio to 8% or less, it was found that the number of rolling passes k and the reduction ratio of each rolling pass should be determined so as to satisfy the following formula (6). {(Reduction ratio n1) × (Reduction ratio n2) × ··· (Reduction ratio n k )} × 10 k ≤ 2.72 ··· Formula (6)

Explanation of symbols

[0043] 11 Plate model 12 Rolling roll L Non-metallic inclusions 20 Ring-shaped steel 21 Driving roll 22 Driven roll

Claims

1. In a method for processing a steel material, which comprises subjecting a base material made of a steel material containing non-metallic inclusions to a compression treatment and processing it into a shape different from that of the base material, an acquisition step of acquiring in advance relationship information between a solution of a predetermined reduction rate evaluation formula and a gap area ratio which is the area ratio of gaps formed around the inclusions; a parameter determination step of determining parameters included in the predetermined reduction rate evaluation formula so that a target gap area ratio which is a target value of the gap area ratio is obtained based on the relationship information; a processing step of performing the compression treatment k times according to the predetermined reduction rate evaluation formula determined in the parameter determination step; and The predetermined reduction rate evaluation formula is an expression obtained by multiplying a multiplication value obtained by multiplying the reduction rates of respective rolling processes by 10 k and k is an integer of 2 or more, and the reduction rate of each rolling process and k included in the predetermined reduction rate evaluation formula are parameters determined in the parameter determination step a method for processing a steel material, characterized by the above.

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

3. The method for processing a steel material according to Claim 1 or 2, wherein in the acquisition step, the relationship information is acquired by performing CAE analysis using an analysis model.

4. A method for manufacturing a rolling component, characterized by processing a semi-finished product manufactured by the method for processing a steel material according to Claim 1 or 2 into the shape of a rolling component.

Images