JPS6111759B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention forms a circumferential notch on the cut surface of a steel bar or steel pipe (hereinafter collectively referred to as steel bar),
A method of cutting a steel bar by rapidly heating and cooling the vicinity of the notch to generate a crack from the bottom of the notch toward the center of the steel bar, and then applying a mechanical load to the notch, especially the notch in this cutting method. This invention relates to a forming means, and aims to form a notch suitable for generating a crack with good workability, and to use the crack to reliably break a steel bar with a small mechanical load. The forged material is cut to a certain size from a steel bar prior to the forging operation. Conventionally, shearing or sawing has been commonly used as this cutting means. Of these, shearing is the most widely used,
In particular, in the case of high-carbon hard steels such as bearing steel and tool steel, softening heat treatment is required, which increases costs and makes it impossible to obtain a flat cut surface. Furthermore, since a large shearing force is required for cutting, the cutting machine is large and expensive, and also produces large vibrations and noise during cutting, resulting in a poor working environment. Cutting with a saw has the advantage of producing a good cut surface and being free from vibration and noise, but the work is inefficient. In addition, when cutting a steel bar, attempts have been made to make a notch in the circumferential direction in advance and then break it mechanically, but if only a mechanical load is applied to the notch, the fracture surface will match the bottom of the notch. There is a large variation in weight, the cut surface is not smooth, and the upsetting process during forging causes collapse and inclination, resulting in a decrease in efficiency and yield. Therefore, the inventors first provided a notch in the circumferential direction at the cutting position of the steel bar, heated and cooled the area around the notch to generate warpage from the bottom of the notch toward the center of the bar, and then removed the notch. Developed a method for cutting steel bars by applying mechanical loads to the parts (patent no.
970416, Special Publication No. 54-2425). According to this method, there is no vibration noise during cutting, cutting can be performed with a small mechanical load, and the cut surface is extremely smooth. In this method, two methods can be used for the heating and cooling treatment to generate cracks at the bottom of the notch. That is, the first method is to rapidly heat and cool the area around the notch so that there is a large temperature difference between the surface and the inside, and at a temperature that does not reach the _A3 transformation point of the steel material at the bottom of the notch. (hereinafter referred to as the one-time method). Although this method is easy to operate, it may be difficult to produce the deep cracks necessary for cutting relatively thin steel bars. The second method is to harden only the surface area by rapidly heating and cooling the area around the notch, then at a temperature lower than the previous heat treatment and at a temperature at which the bottom of the notch does not reach the _A3 transformation point of the steel material. This method involves rapid heating and cooling (hereinafter referred to as the two-step method), and can generate the cracks necessary for cutting regardless of the diameter of the steel bar. High frequency heating or the like is used as the rapid heating means, and water cooling or the like is used as the rapid cooling means. We are generated during the cooling process. By the way, in the above-mentioned cutting method developed by the inventors, cutting or plastic working may be used as a means of notching the steel bar, but it goes without saying that the notching method makes it easy to determine the cutting position correctly. It is required that the subsequent heat treatment reliably causes cracks to occur at the bottom of the notch, and that the processing work is short, easy, and low cost. As a result of various experimental studies, the inventors found that by heating the cut position of the steel bar at a temperature of 400°C or higher and under conditions that do not cause recrystallization of the steel bar, the cut position of the steel bar is rolled. It has been confirmed that the above requirements can be met by forming a notch. That is, the present invention includes a notch forming step in which a notch is formed in the circumferential direction on the surface of a steel bar, a warp forming step in which the periphery of the notch is heated and cooled to generate a warp directed from the bottom of the notch toward the center of the steel bar, In a steel cutting method that includes a cutting process in which a mechanical load is applied to a notch portion having cracks to cut the steel bar, in the notch forming process, first, the portion of the steel bar surface where the notch is to be formed is heated to a temperature of 400°C or above. , and is characterized by heating the steel material to a temperature range that does not cause recrystallization, and then forming circumferential notches in the heated surface portion by rolling. The present invention will be explained below based on experiments conducted by the inventors. First, we experimented with notch formation using conventional cutting. Bearing steel with a diameter of 38 to 70 mm (SUJ2 material, as rolled) with an opening angle of 60° and a notch bottom radius.
Turning an annular notch of 0.25 mm and depth of 1.5 mm (hereinafter referred to as
(referred to as a cutting notch) and then rapidly heating and cooling the area around the cutting notch. In this case, by appropriately selecting the heat treatment conditions, the incidence of cracking required for cutting was 100%, regardless of the shaft diameter of the steel bar. However, when notches are formed by cutting, in the case of high carbon hard steel, commercially available cutting tools are prone to wear and chipping and have a short tool life. In addition, when the tip of the cutting tool wears and the radius of the notch bottom increases, the stress concentration factor decreases, reducing the incidence of cracking, and when the tip of the cutting tool becomes chipped, multiple grooves are formed at the bottom of the notch. Therefore, the fracture surface consists of multiple surfaces, making it unsuitable for subsequent forging. Furthermore, in order to increase the cutting speed, it is necessary to rotate the steel bar or tool at high speed, which makes the device complicated. As described above, notch formation by cutting has a limit in reducing the time required for cutting, and there are serious problems in practical terms, such as high cost for notch formation including tool costs. In this respect, forming the notch by rolling has an advantage. Normally, rolling for notch formation is performed by pressing the notch forming part of a disc-shaped rolling roll against the steel bar surface, but since the process is not performed with a sharp tool unlike cutting, the rolling roll Although it depends on the diameter of the roll and the rolling method, there is little wear or damage at the tip of the roll. Further, according to rolling processing, the rotation speed of the steel bar is low, and the processing is completed at a small number of rotations, and the equipment is simple and the processing time is short for steel bars of various diameters. However, forming notches by rolling has another drawback that is not found in cutting notches. The table below shows steel bars with notches formed in advance by cold rolling at room temperature (20℃).
This is an example of the results of an experiment in which the rate of occurrence of cracks was determined by applying heating and cooling treatment, which is considered to be optimal for the occurrence of cracks.

[Table] In this experiment, two types of rolling rolls with annular protrusions with an apex angle of 60° and a tip radius of 0.15 mm and 0.25 mm were used. An annular notch with a diameter of 1.5 mm (including the bulge during rolling) was cold rolled. Then, cracks were generated by the two-step heat treatment described above. In addition,
In the table, heating time t ₁ is the first heating time in the two-step method, and t ₂ is the second heating time. However, in the case of this experiment, the occurrence rate of cracking was approximately 80 for both notch shapes of the two types of rolling rolls.
It remained at %. The following is thought to be the reason why the occurrence rate of cracking is lower when notches are formed by cold rolling than when notches are cut. The first is the effect of compressive residual stress generated at the notch bottom due to cold rolling. That is, during notch rolling, the roll compresses the notched portion of the steel bar, and in the axial direction of the steel bar, the area in contact with the roll is expanded relative to the center of the steel bar. Therefore, after rolling is completed, compressive residual stress occurs in the axial direction of the steel bar to a considerable depth near the bottom of the notch. The reason why cracks occur at the notch bottom is that tensile residual stress is generated in the axial direction of the steel bar due to heating and cooling treatment, which causes the cracks, but the compressive residual stress near the notch bottom due to cold rolling is Due to its size and the depth of the affected layer, it has the effect of reducing the magnitude of tensile residual stress that occurs after heating and cooling, and this is recognized to be one of the causes of the reduction in the crack occurrence rate. The second cause is the flow and refinement of crystals near the bottom of the notch due to rolling. That is,
When the tip of the roll bites into the steel material during cold rolling, the material is pressed against the center of the steel bar, and the material flows from the tip of the roll in the axial direction of the steel bar. At that time, the crystals are crushed and become finer, so even if the same amount of tensile residual stress is applied by heating and cooling, it is recognized that the occurrence of cracks is restricted. Based on the above considerations, we conducted experimental research with the aim of forming a notch by rolling with good workability, and then ensuring that cracks occur at the bottom of the notch through heating and cooling operations. As a result, it was confirmed that if the notch is formed by rolling while the notch forming part of the steel bar is heated at an appropriate temperature and time range, cracks will definitely occur due to subsequent heating and cooling. To give an example of an experiment, the surface of a 38 mm diameter bearing steel bar (SUJ2 material) was heated by high frequency heating, and then immediately rolled to form an annular V-shaped notch (opening angle 60°, bottom radius 0.25 mm, depth 1.5mm). After cooling to room temperature, heating and cooling (two-step method) similar to those in the above experiment were performed to generate cracks. The results are shown in the figure. As shown in the figure, cracking occurs when the maximum temperature of the part where the notch is formed by high-frequency heating is 400℃ (heating time 4 seconds), 490℃ (heating time 6 seconds), and 600℃ (heating time 8 seconds). The rate was 100%. On the other hand, at temperatures above 750°C, the crack occurrence rate was less than 22%. Note that the number of experiments for each experiment was 50. The reason why we were able to achieve a cracking rate of 100% under temperature conditions of 400 to 600°C is that heating to 400°C or higher softens the steel material, making it easier for the material to flow. This is recognized to be due to the fact that when rolling was carried out, no compressive residual stress or microstructural refinement occurred as in cold rolling. The incidence of cracks increases as the heating temperature approaches 400℃, but for example, there are cases where cracks do not occur even when heated to 300℃, and the heating temperature that will stably and reliably cause cracks is 400℃. Or more. On the other hand, microscopic observation of the structure near the notch bottom of the steel bar heated at 750°C revealed that recrystallization occurred and the crystals became finer than in the as-rolled state, which is thought to be the reason for the low crack occurrence rate. Is recognized. From the above, in the notch forming process of the present invention in which a notch is formed by rolling, the heat treatment prior to rolling is performed at a temperature of 400°C or higher and within a temperature and time range that does not cause recrystallization in the structure. preferable. The temperature at which recrystallization does not occur in the structure usually refers to the recrystallization temperature specific to each metal. However, even if the temperature reaches a temperature higher than the recrystallization temperature seen in a normal phase diagram, recrystallization does not occur in the structure if the heating is performed for a short period of time, from several seconds to tens of seconds, as in the case of high-frequency heating. Therefore, the upper limit of the heating temperature in the present invention is not limited to the so-called recrystallization temperature. However, in the case of long-time heating lasting several tens of minutes to several hours, such as heating in an electric furnace, recrystallization tends to occur in the structure, so the upper limit of the heating temperature in the present invention is limited to the recrystallization temperature. The heating means for heating the steel bar prior to rolling is not particularly limited, but it is advantageous to use a means for rapidly heating only the surface of the steel bar, such as high-frequency heating, in order to increase the crack occurrence rate. This is because when a notch is rolled by heating the surface of the steel bar using high-frequency heating, etc., only the surface is heated, and the dimensions of the surface layer where the notch is formed and the inside of the bar reach an equilibrium state, and then the steel bar is cooled. When the high-temperature surface part of a steel bar begins to contract, the surface part becomes pulled by the inside, and as cooling progresses and the deformation resistance of the surface part increases, tensile residual stress is generated in the surface part. In this state,
If heating and cooling are performed in the warp forming step following the notch forming step, tensile residual stress will be further promoted, and as a result, warps will be more likely to occur. Note that cooling, slow cooling in a furnace, etc. may be performed after rolling in the notch forming step, or the step may be shifted to the crack forming step immediately after rolling. The present invention, which has been achieved through the above experiments and considerations, is achieved by heating a steel bar at a temperature of 400°C or higher and under conditions that do not cause recrystallization in the steel material, and rolling notches in the circumferential direction of the steel bar. By forming the notch portion by heating and rapidly cooling the notch portion, it is possible to reliably generate cracks from the bottom of the notch toward the center of the steel bar. If a mechanical load is applied to this cracking portion, the steel bar can be easily cut with a low load. In addition, it is more effective to heat only the surface of the steel bar, such as by high-frequency heating, for the above-mentioned heating performed prior to rolling the notch.
Heating only the surface makes it more certain that cracks will occur later. Particularly, the present invention uses rolling, which is economical as the notch forming means has little vibration and noise, can quickly form the notch, and has little wear and tear on the tool, and also ensures the occurrence of the above-mentioned cracks. This method has the greatest significance, and together with the above-mentioned steel cutting method that the inventors developed earlier, it greatly contributes to improving the workability and quality of cutting forged materials.

[Brief explanation of the drawing]

The figure is a diagram showing experimental results regarding the present invention.

Claims (1)

[Claims] 1. A notch forming process in which a notch is formed in the circumferential direction on the surface of the steel bar or steel pipe, and a crack formation process in which heating and cooling treatment is applied to the periphery of the notch to generate a warp from the bottom of the notch toward the center of the steel bar or steel pipe. In this method, the notch forming step first involves cutting the notch on the surface of the steel bar or steel pipe. It is characterized by heating the area to be formed to 400°C or higher and within a temperature range in which recrystallization does not occur in the steel material, and then forming circumferential notches by rolling on the surface of the steel bar or steel pipe at the heated area. How to cut steel.