JPS6252007B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a steel material for cold plastic working which has high tensile strength and extremely high workability such as elongation and reduction. Conventional plastic working methods are broadly classified into cold working methods and hot working methods. The cold working method is mainly used for manufacturing small diameter or thin-walled products, and the hot working method is mainly used for manufacturing large diameter or thick-walled products. This is because the larger the diameter or wall thickness of the material, the higher the strength, the more difficult it is to cold-work the material. To explain the situation in more detail using coil springs as an example, conventionally large-diameter coil springs of 10 mm or more are made by hot-forming the material into a coil spring shape, then quenching and tempering to give the required high strength. It is manufactured by the method of According to the hot forming method, it is easy to form a coil, but on the other hand, the decarbonization of the spring surface that occurs during heating,
The quality of the finished product may deteriorate due to the fact that the strength of the material decreases due to hot treatment, which tends to cause surface scratches, and because the heat treatment is performed on coiled springs, the strength tends to vary, and the surface roughness and deformation occur during heat treatment. Specifically, defects are more likely to occur compared to cold-formed products. According to the cold forming method, which manufactures coil springs by cold forming wire rods such as oil-tempered wires with a predetermined strength and a relatively small diameter, the strength of the wire rods remains the same because the wire rods are not heated during forming. In this respect, it is superior to the hot forming method because it retains its properties and does not cause surface roughness.However, if the material is a high-strength wire rod, there is a problem that forming becomes difficult as the wire diameter increases. The present invention aims to provide a method for manufacturing steel materials for cold plastic working that can remove the defects of the above-mentioned hot working method and cold working method. The objects of the present invention include coil springs, leaf springs, torsion bars, stabilizers, and anything else that requires high strength and that requires a plastic working process in the manufacturing process. The present invention will be explained in detail below with reference to FIGS. 1 to 3. The present invention consists of the following configuration. Hardenable steel materials are rapidly heated by high-frequency induction heating, etc., and then quenched, and then rapidly heated in the same manner in a temperature range of 300° to 600°C to bring the steel material to a desired temperature within the above heating temperature range. Immediately quench when reached. After rapidly heating a steel material made of carbon steel by high-frequency induction heating, etc., the steel material is kept in the austenite region at 1000℃ or less for a minimum time that can be set depending on the chemical composition of the steel material, and then water quenched to reduce the crystallization of the steel material. The grains become extremely fine with ASTM no.9 to 12,
Another feature of rapid heating quenching, such as high-frequency induction heating, is that by creating a microstructure with a non-uniform carbon concentration, it is possible to produce steel materials with higher strength and toughness than normal heat treatment during the quenching stage. Although it is already known, the present inventor has determined that the carbon content is 0.30.
The high-strength steel material obtained by subjecting the above-mentioned quenching to the steel material with a tempering temperature of 300% or more is comparatively higher than the conventional tempering temperature.
As soon as the desired temperature within the range of ~600°C is reached, it can be cooled immediately, without heating and holding, to have a tensile strength of 150 kgf/mm ² or more, and high toughness. It was discovered that a steel material with extremely high workability can be obtained. As mentioned above, this is achieved by departing from the conventional concept of tempering and heating rapidly at a relatively high temperature.
When heating gradually decomposes martensite containing supersaturated interstitial atoms such as carbon and precipitates carbides until the desired temperature within the heating temperature range determined by the chemical composition of the steel material is reached. Due to tempering, which occurs rapidly compared to the previous temperature and then rapidly cooled at that point, the distribution and shape of carbides are affected due to insufficient thermal energy supply to reach the generally considered sufficient tempering state at that temperature. It is presumed that this is due to the fact that the reaction is temporarily stopped in a state where the tempering is insufficient. In other words, rapid heating and short-time holding quenching transforms the steel into a martensitic structure with fine grains, thereby providing the steel with the base necessary for high strength and high ductility. 150Kgf/ by timeless tempering
mm ² or more, and the dislocation density is generally high, but the yield point is kept relatively low, and the ratio of dynamic dislocations required to obtain sufficient elongation after yielding is high, that is, the characteristics obtained. As such, it has high ductility and, specifically, high workability. Of course, at this stage, the tempering is insufficient, so it can be said that the resistance to settling is not favorable. The present invention, as disclosed in Japanese Patent Application No. 75446/1983, shows that a steel material with the above-mentioned tensile strength can be obtained by heating at 300° to 600°C, holding it for a short period of time, and then rapidly cooling it. This has been confirmed by the results of various tests and studies conducted by the present inventor, but the inventor's subsequent results show that even if the material is immediately quenched after tempering at 300° to 600°C without a short holding time, It has been found that similar effects can be obtained. Some of the experimental results to prove this are shown below. Experimental example 1 (1) Specimen material SAE 1552 Chemical composition C=0.51, Mn=1.56 P=0.017, S=0.006 Diameter 12mmφ Quenching hardness Hv=800 (2) Relationship between tempering conditions and mechanical properties

[Table] (3) Experimental results According to the above experimental results, after rapid heating and short holding time, quenching was performed, and then the temperature was 300° to 600°.
℃, and when it reaches the desired temperature within the above heating temperature range determined by the chemical composition of the steel material, it is immediately rapidly cooled and tempered (the holding time in normal tempering treatment is 30 to 60 minutes). ) It turns out that it is possible to obtain a steel material with a strength of 150 kgf/mm ² or more and good workability. Note that FIG. 1 shows the relationship between the tempering time and temperature of the specimen in this example, and FIG. 2 shows the hardness and toughness of the steel material at a tempering temperature of 525°C. In Figure 1, the vertical axis is the tempering temperature, and the horizontal axis is the tempering time (logarithm); in Figure 2, the horizontal axis is the tempering time (logarithm);
b and c indicate changes in tensile strength, area of area and elongation, respectively. As can be seen from the above experimental examples, the tempering treatment according to the present invention reduces the carbon content.
If 0.51% carbon steel is used as material, 150Kg
A steel material with high strength of f/mm ² or more and excellent workability can be obtained. Furthermore, according to other experimental examples of the present inventor, carbon steel with a carbon content of 0.30% or more or hardenable carbon steel as a base, containing one or more of Mn, Cr, Si, Mo, and other components. For example, if steel materials with known standards such as those whose chemical compositions are shown in Tables 1 to 8 are used as raw materials, it will be 150 Kgf/mm ² or more as above, or about 220 Kgf/mm ² depending on the steel type or composition system. It has been found that steel materials with high strength and excellent workability can be obtained.

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[Table] Note that Mo in Table 3 may be replaced with V or Nb as is known. When the above-mentioned steel material is subjected to cold plastic working, the steel material obtained by the above-mentioned method has high strength and high workability. The diameter is 5 to
A large diameter coil spring that is 60% thicker can be easily cold-formed. That is, it is possible to easily cold-form a high-strength, large-diameter coil spring using a known molding machine. The same applies to leaf springs and the like. If it is desired to increase the elastic limit of the cold-worked product obtained by the present invention, the cold-worked product may be further tempered in a second stage of reheating at 300° to 500°C for 30 to 60 minutes. It turns out that this is possible. In other words, after the steel material according to the present invention is cold-formed, the second stage of so-called low-temperature tempering, in which the steel material is heated to 300° to 500°C, imparts excellent creep resistance to the workpiece, resulting in a product with very little settling. It has been confirmed that it can be manufactured. In other words, the steel material that maintains an extremely high dislocation density through the tempering treatment of the present invention, which is characterized by rapid heating and rapid cooling without holding time in (1) above, undergoes plastic working by cold forming. , the upper limit of the tempering temperature of the present invention is 300° to 300°, which further increases the dislocations and does not reduce the high strength obtained by the tempering of the present invention.
When tempering is performed in the second stage in an electric furnace, which is generally carried out at a temperature range of 500℃ for a fixed period of 30 to 60 minutes, a sticking phenomenon occurs between dislocations and solute atoms or carbides, making the dislocations immobile. , a kind of strain aging effect is obtained by forming so-called passive dislocations, which improves the elastic limit, yield point and relaxation properties, and also improves marten during the heat treatment stage before cold forming of the wire. As mentioned above, the insufficient tempering phenomena such as site decomposition, carbide precipitation, distribution, and shape are stabilized, and the final result is a workpiece with high creep resistance while maintaining high strength. It is estimated that it can be manufactured. It goes without saying that the second stage of low-temperature tempering adds the general effect of removing residual stress due to cold working. The present inventor also conducted an experiment to confirm the effect of the second stage of stabilizing tempering. Figure 3 shows part of the experimental results. Experimental Example 2 (1) Experimental Conditions (1) Specimen Same as in Experimental Example 1 (2) Heat Treatment Conditions Heat treatment was performed under the same conditions as in Experimental Example 1. (3) One part of the heat-treated specimen was subjected to plastic deformation due to torsion (processed material), and the other part was not subjected to plastic deformation due to torsion (unprocessed material), and both were subjected to stabilizing tempering treatment under the same conditions (described above). provided. (2) Experimental results As shown in Figure 3, the vertical axis is the tensile load (P)
, the horizontal axis indicates strain (ε), m indicates the tensile load-strain curve of the treated material, and n indicates the tensile load-strain curve of the untreated material. From FIG. 3, it is clear that the processed material has a higher elastic limit than the unprocessed material, and has higher creep resistance. In order to compare the specific mechanical properties of processed products subjected to stabilization tempering with those of products obtained by conventional hot forming, the present inventor conducted the following research on coil springs. I also conducted an experiment. Experimental example 3 (1) Specimen Raw material Wire Diameter 14mmφ Material A: The one in which the present invention was implemented
SAE1552 B: Conventional method implemented
SUP6 manufacturing process

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Claims (1)

[Claims] 1. Hardenable steel is rapidly heated to a quenching temperature determined by the type of steel by high-frequency induction heating, etc. at a heating rate of 50°C/sec or higher, and then quenched. 300 at a heating rate of .
Rapidly heat in the same manner to a temperature range of ゜~600℃,
When the steel material reaches the desired temperature within the above heating temperature range, without any holding time,
A method for producing a steel material for cold plastic working which has high tensile strength and extremely high workability such as elongation and drawing, characterized by immediately quenching to achieve a strength of 150 kgf/mm ² or more.