Steel processing method and rolling part manufacturing method

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 nonmetallic inclusions to perform processing. [Background technology]

[0002] In recent years, with the increasing performance of various mechanical devices, mechanical parts and equipment that require a long rolling fatigue life have become increasingly important. The environments in which these machines are used are becoming increasingly severe, and there is a strong demand for improvements in the lifespan and reliability of these machine parts and devices. It is known that steel parts such as bearings inevitably contain foreign matter (non-metallic inclusions) that originate from the steel manufacturing processes, such as the refining process, casting process, and solidification process.

[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. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6665737 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-55346 [Non-patent literature]

[0007] [Non-Patent Document 1] Light Metals / Vol.42 No.2(1992) Current status of finite element analysis in plastic processing Summary of the Invention [Problem to be solved by the invention]

[0008] Since Patent Document 1 does not involve 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. [Means for solving the problem]

[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 product of the reduction ratios of each rolling process multiplied by 10 kwhere 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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is an explanatory diagram for explaining a rolling analysis method. [Figure 2] FIG. 2 is a schematic diagram of a nonmetallic inclusion L and gaps formed around it after the rolling process is completed. [Figure 3] 10 is a graph schematically showing relationship information. [Figure 4] 4 is a graph corresponding to FIG. 3, clearly showing the upper limit of the target gap area ratio. [Figure 5] FIG. 1 is a schematic diagram of ring rolling. [Figure 6] 10 is a graph of relational information (linear function) (embodiment); [Figure 7] 10 is a graph of relational information (logarithmic function) (embodiment); DETAILED DESCRIPTION OF THE INVENTION

[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 a steel material according to one embodiment of the present invention is a method for processing a steel material by compressing a base material made of steel containing non-metallic inclusions to form a steel material 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 step in this embodiment is a rolling step, and the rolling step is performed in multiple stages to achieve a target reduction ratio. Each step will be described in detail below.

[0019] (Regarding acquisition steps) 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 evaluating the reduction ratio. {(rolling rate n1)×(rolling rate n2)×(rolling rate n k )}×10 k ...Equation (1) Equation (1) is the product of the reduction rate of each rolling process and the multiplication value of 10 k "k" corresponds to the number of times the rolling process is performed and is an integer of 2 or more. For example, if the rolling process is performed in three separate steps, formula (1) is obtained by multiplying the reduction rate n1 (first reduction rate), the reduction rate n2 (second reduction rate), and the reduction rate n3 (third reduction rate) by 10. 3It goes without saying that the sum of the reduction ratios n1, n2, and n3 is the target reduction ratio.

[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 the 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 has been 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 given to the rolling analysis model include the contact conditions (shear friction coefficient) between the nonmetallic inclusions L and the parent phase, plate thickness, hot or cold conditions (temperature, etc.), the position and size of the nonmetallic inclusions L, the position and size of the gaps around the nonmetallic inclusions L, the number of rolling treatments, the reduction rate of each rolling treatment, 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, and the shear friction coefficient between the rolling rolls and the plate model. The rolling reduction is the degree of rolling processing 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 material used in rolling parts such as bearings. When two or more nonmetallic inclusions L are present, it is desirable to assume the 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 relative harmfulness it poses to 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 the gaps formed around them after the rolling process is completed, with the 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 paper surface in FIG. 1).

[0027] The parameters 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 rolling 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 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 rolling reduction ratio evaluation formula and the gap area ratio." Note that the target rolling reduction 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 nth-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, and 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 treatments, the reduction ratio of each rolling treatment, etc.) and increasing the number of analysis processes. Also, a relational expression satisfying the predetermined coefficient of determination may be searched for by changing the functional expression to be fitted (for example, by changing from a linear function to a logarithmic function). Note that, if two or more functional expressions satisfying the predetermined coefficient of determination can be constructed, it is preferable to select the functional expression with the higher coefficient of determination. In FIG. 3, as an example, equation (1) and the relational expression for 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 the central 50 mm of the cut surface was cut. 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 observation specimen was used as a plate model, and the above-mentioned CAE analysis was performed to obtain an estimated gap area ratio (2.6%). The error of the estimated value compared to the actual measured value was only about 1.1%. These experimental results also demonstrate that the gap area ratio can be accurately estimated using CAE analysis.

[0031] (Parameter determination step) The parameter determination step determines parameters included in the rolling reduction ratio evaluation formula (1) based on the relationship information acquired in the acquisition step so as to obtain a target gap area ratio, which is a target value of the gap area ratio. Figure 4 is a graph corresponding to Figure 3, clearly showing the upper limit (8%) of the target gap area ratio. From the relational expression (linear expression), 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). {(rolling rate n1)×(rolling rate n2)×(rolling rate n k )}×10 k ≦1...Equation (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 for the clearance area ratio is an estimated value, so it 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 by about 1 to 2% from 8%.

[0033] (Processing step) The steel is rolled 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 is rolled three times according to these rolling conditions. Note that a small reduction ratio per process is desirable. When the reduction ratio is small during rolling, deformation is limited to the steel surface. However, since the elongation deformation in the rolling direction is constrained to the undeformed portion inside the steel, it is known that compressive stress in the elongation direction is likely to be effective on the steel surface. Therefore, when focusing on the surface layer of the steel, 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 process not to 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 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 radial thickness of the ring-shaped steel material 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 roll is 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) Through CAE analysis, the solution to the rolling reduction evaluation formula (1) and the relationship between the gap area ratio were obtained. DEFORM-3D, manufactured by Scientific Forming Technologies, was used as the analysis software for the CAE analysis. The parameters given to the model are the contact condition (shear friction coefficient) of the nonmetallic inclusion L and the parent phase: 0.3, the 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, JPEG0007734261000001.jpg6170 Inclusion diameter: 1 mm, hot rolling temperature: 1000°C, inclusion and parent phase: 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. Analysis processing was performed while varying the number of rolling processes and the reduction ratio of each rolling process. This analysis processing was performed three times, changing the roll diameter of the rolling roll to φ250, φ175, and φ400. Non-metallic 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 fitted to a linear function, resulting in the relational equation y = 2.8x + 4.2 (see Figure 6). The coefficient of determination was 0.81. Furthermore, by fitting the above data to a logarithmic function, the relational equation y = 2.7In(x) + 8.9 was obtained (see Figure 7). The coefficient of determination was 0.86. Therefore, it was possible to construct a highly accurate relational expression for both linear and logarithmic functions.

[0040] By fitting the analytical data for a roll diameter of φ175 mm to a logarithmic function, the relational expression y = 3.0In(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 φ400 mm to a logarithmic function, the relational expression 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 equation could be constructed.

[0041] From the results in Figure 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 rolling processes k and the reduction ratio of each rolling process should be determined so as to satisfy the following equation (3). {(rolling rate n1)×(rolling rate n2)×(rolling rate n k )}×10 k ≦1.4...Equation (3) From the results in Figure 7, 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 rolling processes k and the reduction ratio of each rolling process may be determined so as to satisfy the following equation (4). {(rolling rate n1)×(rolling rate n2)×(rolling rate n k )}×10 k ≦0.72...Equation (4)

[0042] From the results in FIG. 7, it was found that when rolling is performed using rolls with a roll diameter of φ400 mm, in order to set the target gap area ratio to 8% or less, the number of rolling processes k and the reduction ratio of each rolling process should be determined so as to satisfy the following formula (5). {(rolling rate n1)×(rolling rate n2)×(rolling rate n k )}×10 k ≦1.14...Equation (5) From the results in FIG. 7, it was found that when rolling is performed using rolls with a roll diameter of φ175 mm, in order to set the target gap area ratio to 8% or less, the number of rolling processes k and the reduction ratio of each rolling process should be determined so as to satisfy the following formula (6). {(rolling rate n1)×(rolling rate n2)×(rolling rate n k )}×10 k ≦2.72...Equation (6) [Explanation of symbols]

[0043] 11 Board Model 12 Rolling mill L Nonmetallic inclusions 20 Ring-shaped steel material 21 Drive Roll 22 Follower roll

Claims

1. A method for processing a steel material, in which a base material made of a steel material containing nonmetallic inclusions is subjected to a compression treatment and processed 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 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 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; a processing step of performing a compression process k times in accordance with the predetermined rolling reduction evaluation formula determined in the parameter determination step; and The predetermined rolling reduction evaluation formula is a multiplication 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 rolling reductions of the respective rolling treatments included in the predetermined rolling reduction evaluation formula and k are parameters determined in the parameter determination step, The solution in the acquisition step is a solution when k, which is the number of rolling processes, and the reduction rate of each rolling process are substituted into the predetermined reduction rate evaluation formula. A method for processing steel material, characterized by:

2. A method for processing steel material as described in claim 1, characterized in that the compressive stress generated in the base material by the compression treatment is imparted to the steel material by rolling or forging rolls.

3. 3. The steel processing method according to claim 1, wherein the acquisition step acquires the relevant information by performing a CAE analysis using an analytical model.

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 claim 1 or 2 into a shape of a rolling part.

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