JP2008133519A - Method for reforming steel material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a reforming layer to a desired depth from the surface of a steel material and to simply perform this formation at a low cost. <P>SOLUTION: A rotating tool T rotating at high speed is brought into contact with the surface of the steel material W and then, the contact part of the steel material W with the rotating tool T is softened with frictional heat and this softened part is stirred with a probe 2 to reform the steel material W. Since this rotating tool T is composed of a heat-resistant cemented carbide mainly composed of a tungsten carbide, wear hardly occurs, even in the temperature zone of ≥1,000°C where the steel material W is softened. The depth of the reforming layer 3 can be made a desired depth by changing the length of the probe 2 projectingly arranged on the rotating tool T. Further, when performing this treatment in a state the rotating tool T is inclined to the back part in the advancing direction, since a fluid f2 vertically stirring the softened steel material, is generated with the inclined probe 2, stirring with the fluid is more effectively performed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In this invention, the surface of a steel material such as high manganese steel is rubbed with a probe having a high hardness, and the steel material is softened by the frictional heat to cause plastic flow, and the region where the plastic flow is generated is determined. The present invention relates to a method for reforming.

  As steel materials for sliding parts such as machine tools, wear-resistant steel such as high manganese steel is widely used. As the initial characteristics of this high manganese steel, the material itself is rather low in hardness compared to other steel types and easily wears, so the hardness near the surface layer of the sliding part prior to assembly of the machine tool etc. A reforming process for improving the temperature is performed.

  As this modification treatment, a shot peening method in which hard small particles collide with the surface of a steel material is widely used. When this shot peening method is performed, crystal defects are generated at a high density in the vicinity of the surface layer of the steel material, and the crystal defects act as a so-called work-hardened layer and improve the hardness in the vicinity of the surface layer. Although the thickness of the work hardened layer depends on the collision speed of the hard small particles, it is extremely thin at most about 1 mm, and the work hardened layer is often insufficient in hardness to exhibit wear resistance. There is a problem.

Therefore, in order to increase the thickness of the work-hardened layer and increase its hardness, a method shown in Patent Document 1 has been proposed. This method includes a first step of applying a high hydrostatic pressure to the surface of the high manganese steel and a second step of causing hard particles to collide with the surface by a shot peening method. A work-affected layer is formed up to a deeper region, and the hardness of the work-affected layer is higher than when the shot peening method is carried out alone.
Japanese Patent Laid-Open No. 10-44043

Further, as shown in Patent Document 2, a shoulder that is brought into contact with a low-melting-point metal material such as aluminum while being rotated at a high speed, and is provided so as to protrude from the front end of the shoulder, while rotating around the rotation axis of the metal material. There is a reforming method using a rotating tool that is in contact with a surface and inserted into a metal material softened by frictional heat at that time and a probe for stirring the metal material. By this modification, the softened region is modified by refinement of precipitates and crystal structure generated by the stirring, so that the hardness of the surface of the metal material can be improved.
JP 2003-48060 A

  This rotary tool is usually made of stainless steel or the like, and the metal material can be modified to a desired depth corresponding to the length of the probe of the rotary tool. This modification is performed by pressing the probe into a softened metal material and inserting the probe, and moving the probe.

  In the method shown in Patent Document 1, the work-hardened layer is formed by two steps of the hydrostatic pressure load and the shot peening method, and an expensive dedicated device is required for each step. There is a problem that becomes expensive.

  In addition, as described above, the method disclosed in Patent Document 2 is applied only to a low-melting-point material such as aluminum and is not currently applied to a steel material. This is because the softening of the low melting point material occurs at a low temperature of about 300 to 400 ° C., whereas the steel material does not soften unless it reaches a high temperature of 1000 ° C. or more, and at that temperature, the strength of the rotary tool itself decreases. This is because the damage during processing is significant.

  Therefore, an object of the present invention is to form a modified layer from a surface to a desired depth in a steel material such as high manganese steel, and to form the modified layer inexpensively and easily.

  In order to solve the above-described problems, the present invention provides a probe that protrudes from the tip of a rotating tool made of a heat-resistant cemented carbide, contacts the steel material with the probe, and causes the friction of the steel material with the contact. It was softened by heat, and the softened part was stirred with the probe to modify the steel material.

  By performing the modification according to the present invention, the structure of the steel material is refined. Moreover, with this refinement | miniaturization, a part of additive elements, such as chromium in a steel material, manganese, and carbon couple | bond together and a carbide | carbonized_material is formed. The refinement of the structure and the generation of carbides improve the strength and surface hardness of the material itself.

  As a result of this modification, a modified layer is formed to a depth corresponding to the length of the probe. By changing the length of the probe, the depth of the modified layer can be changed to a desired depth. it can.

  The amount of carbide generated varies in accordance with the temperature rise and the cooling rate of the steel material surface accompanying the moving speed of the probe.

  Specifically, when the moving speed is increased, the rotary tool that is a heat supply source moves away quickly, so that the temperature rise is small and the cooling speed is large. Conversely, if the moving speed is small, the temperature rise is large and the cooling speed is large. Becomes smaller. It has been experimentally confirmed that when the temperature rise is reduced and the cooling rate is increased, the amount of carbide generated is increased, and when the temperature rise is increased and the cooling rate is reduced, the amount of carbide generated is decreased.

  It has also been experimentally confirmed that the crystal grains on the surface of the steel material are refined by increasing the moving speed of the probe to decrease the temperature rise and increase the cooling speed.

  In the method for reforming a steel material according to the present invention, a probe rotating at high speed is brought into contact with the surface of the steel material, the contact portion of the steel material with the probe is softened by frictional heat, and the softened portion is this A configuration in which the steel material is modified by stirring with a probe can be employed.

  The surface modification of the steel material W was performed with the configuration shown in FIG. 2 using the rotary tool T shown in FIG. The rotary tool T is provided with a shoulder 1 that contacts the steel material W, and is projected from the tip of the shoulder 1, and abuts against the surface of the steel material W while rotating around the rotation axis. A probe 2 is provided which is inserted into the steel material W softened by frictional heat and stirs the softened steel material W.

  The rotary tool T needs to have a mechanical strength that can stir the softened steel material W even at a high temperature of 1000 ° C. or higher, and has a tungsten carbide heat resistance as a material having such characteristics. Cemented carbide can be used.

  More specifically, the rotary tool T is formed by mixing a fine powder of tungsten carbide and cobalt powder acting as a binder, filling the mold into a mold of the shape of the rotary tool T, and further forming the powder. It is created by baking and solidifying this. In addition to this, by adding carbides such as chromium carbide, tantalum carbide, vanadium carbide, it is possible to improve the oxidation resistance when the rotary tool T is exposed to a high temperature.

  In this surface modification, the probe of the rotary tool T that rotates at high speed is pressed against the surface of the steel material W, and the pressed steel material W is softened by frictional heat. When the probe 2 is inserted into the softened steel material W, the shoulder 1 is in direct contact with the steel material W, and the contact area between the rotary tool T and the steel material W increases, so the amount of frictional heat generated increases. In addition, the softening is performed more quickly.

  When the rotary tool T is moved at a predetermined speed v in a state where the probe 2 is inserted into the steel material W, the softened steel material W is stirred by the probe 2 and a modified layer in which a refined structure or carbide is introduced. 3 is formed.

  The depth of the modified layer 3 formed by this process depends on the length of the probe 2 of the rotary tool T, and when the length of the probe 2 is increased, the modified layer 3 reaches a depth corresponding to the length. Is formed.

  When the rotary tool T is used in a state where it stands upright on the surface of the steel material W, the flow generated by the probe 2 of the rotary tool T is centered on the probe 2 as shown in FIG. The rotational flow f1 in the horizontal plane becomes the main flow, and the flow of stirring the softened steel material W in the vertical direction does not occur much.

  On the other hand, when the rotary tool T is used in a state where it is inclined with respect to the vertical axis by an inclination angle φ rearward in the advancing direction, as shown in FIG. The rotational flow f1 in the plane inclined by the angle φ becomes the mainstream. Since this rotational flow f1 occurs in a plane inclined from the horizontal plane, not only a horizontal flow but also a flow f2 that stirs the softened steel material W in the vertical direction is generated, whereby stirring by this flow is more efficient. Made.

  Further, since the rotary tool T is pressed against the steel material W with a very high pressure of about 4 tons, the shoulder 1 of the rotary tool T is recessed into the steel material W. This indentation becomes resistance when the rotary tool T is moved, and the shoulder 1 of the rotary tool T is easily damaged by this load. Therefore, when the rotary tool T is inclined rearward in the traveling direction, the shoulder 1 is not sunk into the steel material W in the front in the traveling direction, so that the rotary tool T moves smoothly and damage to the shoulder 1 is prevented.

  In order to eliminate the stagnation in the forward direction, it is preferable that the rotary tool T is inclined backward in the forward direction by at least 0.5 degrees. Further, when the inclination angle is increased, the sag in the front in the traveling direction is further eliminated, but the pressing pressure of the shoulder 1 against the steel material W is increased in the rear in the traveling direction, and the amount of generated frictional heat is increased. When the generation amount of frictional heat increases, the steel material W becomes locally high in temperature, and as described above, the amount of carbide generated decreases and the surface hardness decreases. Therefore, it is preferable to make the inclination angle of the rotary tool T smaller than 5 degrees.

  For example, when the rotary tool T is processed by inclining 3 degrees backward in the traveling direction, as shown in FIG. 4, the entire surface of the steel material W is softened from the surface layer portion to the bottom layer portion. It was confirmed that a homogeneous modified layer 3 having a black contrast was formed.

  When this modified layer 3 was observed in detail with a scanning electron microscope, as shown in FIG. 5, for example, crystal grains 4 of the steel material refined to a size of about 1 μm and carbides 5 of the same size of about 1 μm. When the hardness in this region was measured with a Vickers hardness tester, it was confirmed that the hardness was greatly improved. This is considered due to the precipitation of the fine crystal grains 4 and the carbides 5.

  Table 1 shows the presence or absence of the occurrence of groove defects, the formation of carbides, Vickers hardness, and the results of comprehensive evaluation taking these into consideration when the inclination angle φ, the moving speed and the rotational speed of the rotary tool T are changed. Summarized. Here, the inclination angle φ means the backward inclination of the rotary tool T in the traveling direction, as described above.

  It has been clarified that a groove defect may occur depending on the moving speed and the number of rotations when processing is performed at any inclination angle φ. This groove defect means a groove-like cavity defect that occurs continuously in the direction of movement of the probe 2 when the probe 2 is moved, and the cause of its occurrence has not been fully clarified. It is presumed that the temperature of steel materials is related. If this groove defect occurs, the homogeneity of the modified layer 3 is impaired, and it is necessary to suppress this occurrence.

  When the inclination angle φ was 3 degrees or 5 degrees, a groove defect occurred under the condition that the moving speed was 50 mm / min. Further, when the inclination angle φ is 0.5 degree, a groove defect occurs when the rotational speed is 130 to 150 rpm under the condition of a moving speed of 25 mm / min. However, when the rotational speed is reduced to 100 to 120 rpm, the groove The defect disappeared.

  In addition, regarding the generation status of carbide, carbide is uniformly generated in the entire reformed portion where softening has occurred under the condition that the inclination angle φ is 0.5 degrees and the moving speed is 10 to 25 mm / min. Was confirmed.

  Furthermore, regarding the Vickers hardness, it was confirmed that the carbide had a high hardness in the case where the carbide was confirmed uniformly in the entire modified portion.

  Taking into account the above-mentioned presence or absence of groove defects, the formation of carbides, and the results of Vickers hardness, the inclination angle φ is 0.5 degrees, the moving speed is 25 mm / min, and the rotational speed is 100 to 120 rpm. It was possible to conclude that the product treated under the above conditions was the best in terms of wear resistance.

  This optimum processing condition is not always the above processing condition because the temperature rise and cooling rate of the steel material during processing change when the thickness of the steel material, the length of the probe 2 of the rotary tool T, and the like change. Absent.

  On the other hand, it can be concluded that those not subjected to surface modification have low Vickers hardness because there is no carbide near the surface and are inferior in terms of wear resistance.

The perspective view of the rotary tool in one Example Action diagram showing surface modification using a rotating tool in one embodiment In one Example, it is an effect | action figure which shows the flow of the steel material by rotation of a rotary tool, (a) when a rotary tool is stood | rightened vertically, (b) when a rotary tool is inclined In the optical microscope photograph of the cross section of the high manganese steel which performed the surface modification using the rotary tool in one Example, (a) is a surface layer part, (b) is a middle layer part, (c) is a bottom layer part, d) is a diagram showing the shooting position of each photo Scanning electron micrograph of a cross section of a high manganese steel subjected to surface modification using a rotary tool in one example

Explanation of symbols

2 Probe T Rotary tool W Steel material

Claims (4)

  A probe (2) protruding from the tip of a rotating tool (T) that rotates at high speed is brought into contact with the surface of the steel material (W), so that the contact portion of the steel material (W) with the probe (2) A method for modifying a steel material (W), which is softened by frictional heat, and the probe (2) is inserted into the softened portion and stirred to modify the steel material (W).   The method for modifying a steel material (W) according to claim 1, wherein a heat-resistant cemented carbide containing tungsten carbide and cobalt is used as a material of the rotary tool (T).   The steel material (W) is reformed in a state where the rotating shaft of the rotating tool (T) is inclined 0.5 to 5 degrees from the vertical axis to the rear in the traveling direction of the rotating tool (T). Alternatively, the method for reforming a steel material (W) according to 2.   A steel material modified by the reforming method according to any one of claims 1 to 3.