Steel material processing method and rolling component manufacturing method

Abstract

[Problem] To provide a rolling component, such as a bearing, that has excellent rolling life. [Solution] Provided is a steel material processing method for performing a compression treatment on a base material that is made of a steel material which contains a non-metal inclusion and processing the base material into a shape differing from that of the base material, said method being characterized by comprising: an acquisition step for acquiring, in advance, relationship information pertaining to a relationship between a solution to a rolling reduction rate evaluation formula and a gap area ratio; a parameter determination step for determining, on the basis of the relationship information, parameters that are included in the prescribed rolling reduction rate evaluation formula so as to obtain a target gap area ratio; and a processing step for performing the compression treatment k times in accordance with the prescribed rolling reduction rate evaluation formula that has been determined in the parameter determination step, wherein the prescribed rolling reduction rate evaluation formula is a formula in which a multiplication value obtained by multiplying the rolling reduction rate of each rolling treatment is multiplied by 10k, k is an integer of 2 or more, and k and the rolling reduction rate of each rolling treatment that are included in the prescribed rolling reduction rate evaluation formula are the parameters which are determined in the parameter determination step.

Description

Steel processing method and rolling part manufacturing method

[0001] The present invention relates to a processing method for applying compressive stress to a steel material containing nonmetallic inclusions to perform processing.

[0002] In recent years, with the advancement of performance in various types of machinery and devices, the environments in which machinery parts and devices that require a long rolling contact fatigue life have become extremely severe, and there is a strong demand for improvements in the life and reliability of these machinery parts and devices.It is known that steel parts such as bearings inevitably contain foreign matter (non-metallic inclusions) that originate from manufacturing processes such as the refining process, casting process, and solidification process of steel manufacturing.

[0003] Furthermore, in steel components such as bearings manufactured through rolling and forging processes, gaps can form around nonmetallic inclusions. These gaps are thought to arise at the interface due to differences in the deformability between the nonmetallic inclusions and the parent steel. For example, if the steel component is a rolling bearing, repeated contact loads are applied to the raceway surface on which the rolling elements roll during use. If the aforementioned gaps exist in this affected area, they can promote crack initiation within the bearing component and become the starting point for the cracks. If these cracks reach the raceway surface, they can lead to spalling and subsequent bearing failure. Similar issues exist in rolling components other than bearings, which require a long rolling contact fatigue life.

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

[0005] Patent Document 1 discloses a method of crushing voids that exist between inclusions and the matrix interface by utilizing hydrostatic stress during hot forging.

[0006] Japanese Patent No. 6665737 Japanese Patent Application Laid-Open No. 2014-55346

[0007] Light Metals / Vol.42 No.2(1992) Current status of finite element analysis in plastic processing

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

[0009] An object of the present invention is to provide a method for manufacturing a rolling part having an excellent rolling life by controlling the deformation phenomenon of non-metallic inclusions and parent phase contained in a steel material.

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

[0011] (2) The method for processing a steel material according to (1) above, characterized in that the compressive stress is imparted to the steel material by a rolling or forging roll.

[0012] (3) A method for processing steel material according to (1) or (2) above, characterized in that in the acquisition step, the relevant information is acquired by performing CAE analysis using an analytical model.

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

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

[0015] FIG. 1 is an explanatory diagram for explaining a method of rolling analysis. FIG. 2 is a schematic diagram of non-metallic inclusions L and gaps formed around them after the rolling process is completed. FIG. 3 is a graph schematically showing related information. FIG. 4 is a graph corresponding to FIG. 3 , clearly showing the upper limit of the target gap area ratio. FIG. 5 is a schematic diagram of ring rolling. FIG. 6 is a graph of related information (linear function) (Example). FIG. 7 is a graph of related information (logarithmic function) (Example).

[0016] The present inventors have discovered a processing method for steel material in which compressive stress is applied to a base material made of steel material containing non-metallic inclusions and processed into a shape different from that of the base material, and by controlling the deformation phenomenon of the non-metallic inclusions and base phase contained in the steel material, it is possible to provide rolling parts such as bearings with excellent rolling life.

[0017] Rolling parts include components that require a good rolling fatigue life, such as bearings, gears, hub units, continuously variable transmissions, constant velocity joints, crank pins, and piston pins.

[0018] A method for processing steel according to one embodiment of the present invention is a method for processing steel by compressing a base material made of steel containing non-metallic inclusions to form a shape different from that of the base material, and includes an acquisition step, a parameter determination step, and a processing step. The compression process in this embodiment is a rolling process, and the target reduction rate is achieved by performing the rolling process multiple times. Each step will be described in detail below.

[0019] (Regarding the Acquisition Step) In the acquisition step, relationship information between a solution of a predetermined rolling reduction evaluation formula and the gap area ratio is acquired. Equation (1) is a general formula for the rolling reduction evaluation formula. {(rolling reduction n １ )×(rolling reduction rate n ２ )×・・・(Rolling reduction rate n ｋ )} x 10 ｋ ...Equation (1) Equation (1) is the product of multiplying the reduction ratio of each rolling process by 10 ｋ"k" corresponds to the number of times of rolling treatment and is an integer of 2 or more. For example, when the rolling treatment is carried out in three separate times, the formula (1) is expressed as １ (First reduction rate), reduction rate n ２ (Second reduction rate) and reduction rate n ３ (3rd rolling reduction rate) multiplied by 10 ３ The formula is obtained by multiplying the reduction ratio n １ , rolling reduction rate n ２ and rolling reduction rate n ３ It goes without saying that the added value is the target rolling reduction.

[0020] The gap area ratio is the area ratio of gaps formed around inclusions, 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 rolling reduction evaluation formula and the gap area ratio can be obtained by a rolling analysis model according to a known rigid-plastic finite element method (see Non-Patent Document 1). CAE analysis can be used for the rolling analysis.

[0022] CAE analysis is an abbreviation of Computer Aided Engineering, and is an analytical method for evaluating (simulating) design problems in a product that is simulated on a computer. CAE analysis is performed using a computer program.

[0023] For example, DEFORM can be used for CAE analysis. DEFORM is CAE software that simulates all machining phenomena on a computer, including metal material flow, load conditions on tools, deformation due to heat treatment, and chip prediction during cutting. Therefore, CAE analysis can be used to determine the gap area ratio with high accuracy.

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

[0025] Typical parameters to be provided to the rolling analysis model include the contact conditions between the nonmetallic inclusions L and the parent phase (shear friction coefficient), plate thickness, hot or cold conditions (temperature, etc.), the location and size of the nonmetallic inclusions L, the location and size of gaps around the nonmetallic inclusions L, the number of rolling treatments, the reduction ratio of each rolling treatment, material property data (Young's modulus, Poisson's ratio, stress-strain curve), the roll diameter and peripheral speed of the rolling rolls, and the shear friction coefficient between the rolling rolls and the plate model. The reduction ratio is the degree of rolling reduction expressed as a percentage, and is calculated by the formula (h1-h2) / h1, where h1 and h2 are the thicknesses of the material before and after rolling, respectively. These parameters can be determined by prior analysis of the properties of the steel used in rolling parts such as bearings. When two or more nonmetallic inclusions L are present, it is desirable to assume a rolling part after machining and target the nonmetallic inclusion L that is located closer to the surface of the rolling part. This is because the shorter the distance from the nonmetallic inclusion L to the surface of the rolling part, the greater the harmful effect it has on the rolling part.

[0026] The rolling analysis can obtain the gap area ratio after the rolling process is completed. The gap area ratio can be calculated by "(gap cross-sectional area / non-metallic inclusion cross-sectional area) × 100." FIG. 2 schematically shows the non-metallic inclusions L and gaps formed around them after the rolling process is completed, with hatching indicating the gaps. Referring to the figure, the area of ​​the gap in the X-Z cross section is the "gap cross-sectional area," and the area of ​​the non-metallic inclusion L in the X-Z cross section is the "non-metallic inclusion cross-sectional area." The rolling analysis obtains the "gap cross-sectional area" and "non-metallic inclusion cross-sectional area" after the rolling process is completed, and the "gap area ratio" is calculated based on this obtained information. Note that the gap cross-sectional area is preferably the "gap cross-sectional area" when the plate model 11 is cut at the center of the non-metallic inclusion L in the plate width direction (normal direction to the plane of the paper in FIG. 1).

[0027] The specifications given to the model include the "number of rolling processes" and the "reduction ratio of each rolling process," so it is possible to obtain one-to-one information on the "solution of the reduction ratio evaluation formula in Equation (1)" and the "gap area ratio." Then, by changing the "number of rolling processes" and the "reduction ratio of each rolling process" given to the model in various ways and performing the same analysis process, it is possible to obtain the above-mentioned "one-to-one information" for the number of analysis processes. In this way, it is possible to obtain "information on the relationship between the solution of the reduction ratio evaluation formula and the gap area ratio." Note that the target reduction ratio in each analysis process does not necessarily have to be the same.

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

[0029] Here, if a relational expression satisfying the predetermined coefficient of determination cannot be obtained, a relational expression satisfying the predetermined coefficient of determination can be constructed by changing the specifications given to the model (for example, by increasing the variations in the number of rolling processes, the reduction ratio of each rolling process, etc.) and increasing the number of analysis processes. Furthermore, a relational expression satisfying the predetermined coefficient of determination may be searched for by changing the function equation to be fitted (for example, by changing from a linear function to a logarithmic function). Note that, if two or more function equations satisfying the predetermined coefficient of determination can be constructed, it is preferable to select the function equation with the higher coefficient of determination. In FIG. 3 , as an example, Equation (1) and the relational expression of the gap area ratio are constructed by fitting to a linear function.

[0030] A mixture of SUJ2 powder and alumina particles in a mass ratio of 500:1 was sintered to obtain a base material test piece. This base material test piece was rolled from 50 mm to 40 mm thick using a roll with a diameter of φ870 mm to obtain a single test piece for observation in which many alumina particles were dispersed. The roll peripheral speed was set to 715 mm / s. The test piece for observation was cut at the center position in the width direction, and a 50 mm diameter cut was made at the center of the cut surface. ２ Ten alumina particles present within the specimen were observed, and the gap area ratios around the alumina particles were measured. The average value of these gap area ratios (3.7%) was then calculated as the actual measured value of the gap area ratio. The above-mentioned CAE analysis was performed using the above-mentioned observation test piece as a plate model to obtain an estimated gap area ratio (2.6%). The error of the estimated value relative to the actual measured value was only about 1.1%. These experimental results also demonstrate that the gap area ratio can be accurately estimated by CAE analysis.

[0031] (Parameter Determination Step) In the parameter determination step, parameters included in the reduction evaluation formula of formula (1) are determined based on the relational information acquired in the acquisition step so that the target gap area ratio, which is the target value of the gap area ratio, can be obtained. FIG. 4 is a graph corresponding to FIG. 3, which clearly shows the upper limit (8%) of the target gap area ratio. From the relational formula (linear equation), it can be seen that the target gap area ratio (8% or less) can be achieved by setting formula (1) to 1 or less. In other words, in the parameter determination step, it is sufficient to determine the number of rolling processes k and the reduction ratio of each rolling process that satisfy the following formula (2): {(reduction ratio n １ )×(rolling reduction rate n ２ )×・・・(Rolling reduction rate n ｋ )} x 10 ｋ ≦1...Formula (2)

[0032] For rolling components, a clearance area ratio of 8 to 9% or less is generally recommended. Therefore, the target clearance area ratio (8% or less) in Figure 4 is an example. As mentioned above, the analytical value of the clearance area ratio is an estimated value, and therefore may deviate from the actual measured value by about 1 to 2%. Since the deviation is small, it can be ignored, but taking the deviation into consideration, the upper limit of the target clearance area ratio may be lowered from 8% by about 1 to 2%.

[0033] (Processing Step) The steel material is subjected to rolling according to the number of rolling processes and the reduction ratio of each rolling process determined in the parameter determination step. For example, if the number of rolling processes is three, and 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 steel material is rolled three times according to these rolling conditions. Note that a small reduction ratio per rolling process is desirable. When the reduction ratio is small during rolling, deformation is limited to the steel material surface. However, since the elongation deformation in the rolling direction is constrained to the undeformed portion inside the steel material, it is known that compressive stress in the elongation direction is likely to be effective on the steel material surface. Therefore, when focusing on the surface layer of the steel material, a small 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 rolling process to not exceed 0.5X%.

[0034] By carrying out the rolling treatment according to the processing steps, it is possible to manufacture a semi-finished product for a rolling part having high adhesion between the matrix and the non-metallic inclusions and an excellent fatigue life. By processing the shape of this semi-finished product, it is possible to manufacture a rolling part having an excellent fatigue life.

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

[0036] 5 is a schematic diagram of ring rolling. Referring to the figure, the forging rolls are composed of a drive roll 21 and a driven roll 22. The drive roll 21 rotates when a rotational force is applied from a drive source (not shown). A ring-shaped steel material 20 is sandwiched between the drive 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] In this ring rolling, too, by carrying out the above-mentioned acquisition step and parameter determination step, it is possible to search for processing conditions for manufacturing rolling parts with excellent fatigue life. Then, by carrying out processing in accordance with these processing conditions, it is possible to manufacture rolling parts with excellent fatigue life.

[0038] (Example) A solution to the rolling reduction evaluation formula (1) and information on the relationship between the gap area ratio were obtained by CAE analysis. DEFORM-3D manufactured by Scientific Forming Technologies was used as the analysis software for the CAE analysis. The parameters given to the model were: contact condition (shear friction coefficient) between non-metallic inclusion L and the parent phase: 0.3; thickness of plate model 11: 50 mm; Young's modulus of plate: 206 GPa; Poisson's ratio of plate: 0.3; The following conditions were set: location of nonmetallic inclusions: 1 mm from the surface; diameter of nonmetallic inclusions: 1 mm; hot rolling temperature conditions: 1000°C; inclusions and parent phase: intimate 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. The analysis process was performed while varying the number of rolling processes and the reduction ratio of each rolling process. This analysis process was performed three times, changing the roll diameter of the rolling roll to φ250, φ175, and φ400. The nonmetallic inclusions were defined as rigid bodies. The gap 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) (where the roll diameter was φ250 mm), and fitting to a linear function yielded the relational equation y = 2.8x + 4.2 (see Figure 6). The coefficient of determination was 0.81. Furthermore, fitting the above data to a logarithmic function yielded the relational equation y = 2.7In(x) + 8.9 (see Figure 7). The coefficient of determination was 0.86. Therefore, a highly accurate relational equation could be constructed for both linear and logarithmic functions.

[0040] By fitting the analytical data for a roll diameter of 175 mm to a logarithmic function, the relational equation y = 3.0 In (x) + 5.0 was obtained (see Figure 7). The coefficient of determination was 0.90. By fitting the analytical data for a roll diameter of 400 mm to a logarithmic function, the relational equation y = 3.8 In (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 equation could be constructed.

[0041] From the results of FIG. 6, it was found that when rolling is performed using rolls with a roll diameter of φ250 mm, in order to set the target gap area ratio to 8% or less, the number of times of rolling, k, and the reduction ratio of each rolling process should be determined so as to satisfy the following formula (3): {(reduction ratio n １ )×(rolling reduction rate n ２ )×・・・(Rolling reduction rate n ｋ )} x 10 ｋ 7, when rolling is performed using rolls with a roll diameter of φ250 mm, it was found that in order to set the target gap area ratio to 8% or less, the number of times k of rolling and the reduction ratio of each rolling process may be determined so as to satisfy the following formula (4): {(reduction ratio n １ )×(rolling reduction rate n ２ )×・・・(Rolling reduction rate n ｋ )} x 10 ｋ ≦0.72...Formula (4)

[0042] From the results of FIG. 7, it was found that when rolling is performed using rolls with a roll diameter of φ400 mm, the number of times of rolling, k, and the reduction ratio of each rolling process should be determined so as to satisfy the following formula (5) in order to set the target gap area ratio to 8% or less. １ )×(rolling reduction rate n ２ )×・・・(Rolling reduction rate n ｋ )} x 10 ｋ7, when rolling is performed using rolls with a roll diameter of φ175 mm, it was found that in order to set the target gap area ratio to 8% or less, the number of times k of rolling and the reduction ratio of each rolling process should be determined so as to satisfy the following formula (6): {(reduction ratio n １ )×(rolling reduction rate n ２ )×・・・(Rolling reduction rate n ｋ )} x 10 ｋ ≦2.72...Formula (6)

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

Claims

1. In a method for processing a steel material that 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 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, where the predetermined reduction rate evaluation formula is a formula obtained by multiplying a multiplication value obtained by multiplying the reduction rates of respective rolling treatments by 10 k and multiplying it, k is an integer of 2 or more, and the reduction rate of each rolling treatment 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 element, characterized by processing a semi-finished product manufactured by the method for processing a steel material according to any one of claims 1 to 3 into the shape of a rolling element.

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