JPS6133898B2 - - Google Patents

Description

[Detailed description of the invention]

In the present invention, after applying plastic working strain at room temperature, As
The present invention relates to a method of utilizing steel that has a partial shape memory effect when heated above the Ms point and cooled to room temperature below the Ms point. In each field of industry, there are many locations where structures or mechanical parts must be fixed, tightened, or joined, and many of these locations are extremely important. Conventionally, various methods have been used for these parts, from screws and bolts to welding. Although various improvements have been made to each of these technologies and have contributed greatly to the development of industry, there is still much room for improvement. For example, when using screws and bolts, it is necessary to process not only the screws themselves but also the parts and structures themselves. It is difficult to completely prevent loosening of the tightening force of screws and bolts caused by vibration unless special means are used. In addition, even in the use of welding, which has been particularly widely used in recent years, it has the characteristic of heating a part of the material above its melting point and solidifying it again. Materials that have been given properties through various heat treatments will lose those properties at the welded part.
Problems such as welding cracks and deterioration of toughness occur. Current industrial technology employs a number of methods to improve these points, and some of them have achieved their objectives to a considerable extent;
The current situation is that no method of improvement based on a fundamentally new principle has yet been recognized. In view of these circumstances, the present inventor conducted various studies from a completely new perspective, and found that if the shape memory effect, which has been mainly recognized in certain non-ferrous alloy materials to date, is to be effectively utilized, it is necessary to use screws. In the case of bolts, no loosening is observed even under high mechanical vibrations, making it possible to obtain joints with sufficient tightening force, and to tighten parts with complex shapes that are difficult to reach. In this case, we have found that sufficient tightening can be achieved by using a simple heating method such as a gas burner. The present invention was made based on the discovery that there is a steel material with adjusted Mn as a material that meets this purpose. That is, the present invention provides a steel material having a partial shape memory effect in which the Mn content is in the range of 15.9 to 30.0% by weight and the balance is Fe and unavoidable impurities, and is subjected to plastic deformation strain of 20% or less below the Ms point. It has a partial shape memory effect characterized by adapting the shape to the fixed part and fixing it, then heating the material above the A point and cooling it again below the Ms point to generate stress in the fixed part. This is how steel is used. The present invention will be explained in detail below. The partial shape memory effect referred to in the present invention will be defined here as follows. First, the so-called shape memory effect in a certain material refers to the relationship between temperature and deformation, or in this case, temperature and elongation, as shown in A in Figure 1. When cooled to
This refers to the phenomenon in which the temperature history curve of elongation returns to point O. On the other hand, the partial shape memory effect referred to in the present invention refers to the Ms
When the material is cooled below the temperature point, the length of the material does not completely return to the length before heating, that is, the end point of the elongation temperature history curve does not return to point O, but returns to point F. In such a phenomenon, the material ends up in a stretched state. On the other hand, when tension is applied as strain before heating, the final length is not at point O, but is in a contracted state, being to the left of point O in B in FIG. 1, contrary to the case of compression. That is, a phenomenon in which the dimensions of the final material differ from the strained state before heating and cooling is defined as a partial shape memory effect. In particular, in the present invention, as described later, materials exhibiting martensitic transformation are
As after applying plastic deformation strain of 20% or less below the Ms point
This is specified as a phenomenon in which when a material is heated above the Ms point and then cooled again below the Ms point, it does not completely return to the shape it had when plastic deformation was applied before heating and cooling below the Ms point. In order to effectively utilize the partial shape memory effect defined above, it is necessary that the material's transformation point, As point, is not very high temperature, and that its Ms point is above room temperature.
First of all, it is necessary that the temperature is not too high. If the As and Ms points are too high or, conversely, below room temperature, there will be major restrictions in terms of workability or economy, so these transformation points must be temperatures that are extremely easy to reach in actual work. The steel used in the present invention is a steel having a composition range such that the Ms point is at room temperature or higher and the As point is as low as possible to meet these conditions. Next, the reason for limiting the range of components of the steel used in the present invention as described above will be described. When Mn is added alone to Fe and its content is less than 15.9%, reversible martensitic transformation to exhibit a partial shape memory effect does not occur, resulting in plastic deformation strain at temperatures below the Ms point. Even if the material is heated to a temperature above the As point and then cooled, the shape of the material at a temperature below the Ms point completely restores the shape before the heating and cooling heat treatment, and no change in shape is observed before and after the heat treatment. No partially attached shape memory effect is observed. In addition, when the amount of Mn exceeds 30.0%, the transformation point of this steel material is below room temperature, and steel with this composition exhibits a partial shape memory effect no matter how it is heat treated at least above room temperature. No reversible changes in martensite occur. Therefore, no matter what kind of heating and cooling heat treatments are applied, the shape of this material will always completely return to the shape before the heating and cooling heat treatments. However, when the Mn content is increased to 15.9% to 30.0%, the Ms point and the As point do not rise above room temperature or at a very high temperature, and the martensitic transformation exhibits reversibility, causing plastic working strain at room temperature. After
When heated and cooled, the shape of the material does not completely return to the shape before the heat treatment of heating and cooling, and the shape does not match. In other words, before heating and cooling, a certain amount of plastic deformation is applied to adapt the shape to the fixing part, and then heat treatment is performed to take advantage of the fact that the shape does not completely return to its original shape to generate stress in that part and fix it. It is possible to do this. This knowledge was obtained based on the following experimental results. Table 1 lists the Mn content from 12.1 to
The chemical composition of the steel samples varied between 36.8% and 36.8% is shown. All of these materials are melted in a laboratory-scale high-frequency melting furnace in an atmospheric atmosphere, and are injected into regular molds and ingot-formed, then hot-rolled to form a 20 mm plate. It has been tempered. From this board, 10×15×15
mm-shaped specimens were collected by machining. The test method was as follows: First, a test piece with this shape was cold-rolled to reduce the thickness of the 10 mm portion to 9 mm, and then a test jig such as the one shown in Figure 2 A was used to roll the test piece to a thickness of 9 mm. Insert the piece s into the gap between the beams a and b of this jig so that it takes the shape shown in Figure 2B. After that, move within a certain range around the test piece s.
Heat to 375℃, hold for 20 minutes, then cool to room temperature. Beam a of the jig before heating is fixed to column c by welding, but beam b is not fixed in any way. Column c and foundation material d are firmly fixed by welding. After cooling to room temperature, check whether the specimen s is sufficiently fixed. If it is fixed, a strain gauge is attached to the top surface of the beam b to measure the residual stress and confirm the partial shape memory effect. I investigated how much. The results are also listed in Table 1. As can be seen from these test results, it can be seen that steel with a Mn content in the range of 15.9 to 30.0%, with the balance consisting of Fe and unavoidable impurities, exhibits a partial shape memory effect.

【table】

[Table] By using steel that has a partial shape memory effect as described above, the above-mentioned fixing, tightening, and joining effects can be sufficiently achieved. Furthermore, it has been found that steel with such a composition exhibits more than twice the amount of reversible expansion and contraction when repeatedly cooled and heated to temperatures below the Ms point and above the As point, compared to conventional steels. are also obtained. In addition, within the above component range, if the amount of plastic deformation applied at room temperature exceeds 20%, cracks are likely to occur and sound machining becomes difficult, but if the strain is less than 20%, tensile, compressive or Since the desired partial shape memory effect can be sufficiently exhibited even with other strains, in the present invention, the amount of plastic deformation exhibiting the partial shape memory effect is defined as 20% or less. The steel used in the present invention can be melted in the air, in a vacuum, or in a special gas atmosphere, and may be melted by any melting method such as a converter, open hearth, or electric furnace, and after melting, continuous casting can be used. Alternatively, it can be made into a slab or steel ingot by a steel ingot casting method, or it can be made into a cast steel product as it is. After making a slab or steel ingot, it is rolled into a plate,
It can be manufactured into any shape such as a bar, wire, or shaped steel, and can also be subjected to the next stage of secondary processing. Further, as for heat treatment, ordinary quenching, tempering, normalizing, stress relief annealing, etc. can be performed without any problem. Next, the effects of the present invention will be illustrated with examples. Example 1 The test steels were shown in Table 2, and the comparative steels were heat-treated before producing test pieces so that the strength was almost the same as that of the steel used in the present invention. M10 x 50L bolts were fabricated from these materials by cold forging, and after tightening with M10 iron polished JIS class 2 nuts at a tightening torque of 212 kg cm, the comparative materials were used as they were, and the steel used in the present invention was heated at 375°C. A high-speed loosening test was conducted on the sample that was heated to 100 mL, held for 20 minutes, and then cooled to room temperature. The loosening test was conducted using an NS-type high-speed screw loosening tester manufactured by Nihon Giken Co., Ltd. at a vibration frequency of 1800 times/min and an oscillation width of 10 mm. To check the degree of loosening,
Nut loosens after 9000 vibrations.

[Table] Measured by loosening torque. This result is the third
Shown in the table. From these results, the comparative steel loosens due to mechanical vibration, but the steel used in the present invention shows no loosening at all even due to mechanical vibration due to heat treatment, indicating a partial shape memory effect. An improvement in bolt tightening force was observed.

[Table] ○ marks are comparative steel examples 2. The test steels are Steel 1, Steel 1,
In Examples 4 and 6, the heat treatment of the comparison steel before making the test specimens was adjusted so that the strength of this material was approximately the same as the steel used in the present invention. The test method was to cut out a round bar from these materials, then use an M10 x 50L bolt made from this round bar by cold forging to create a friction welding test specimen as shown in Figure 3. The bolt tightening force was determined by examining the sliding load using a tensile tester. In the same figure, e is a bolt, f is a nut, g is a washer, and h is a thickness of 5.
mm, width 70mm, length 110mm, i is thickness 15mm, width 70
mm and length of 30 mm, respectively. Ten friction bonding test specimens were manufactured for each test bolt material, and the sliding load was investigated 10 times. The tightening torque at this time was 212 kg-cm. Next, the bolt joint made of the steel of the present invention was heated at 375°C.
The mixture was heated to room temperature, held for 20 minutes, and then cooled to room temperature. The bolted joint of the comparative steel is not heated. Table 4 shows the sliding load test results for each specimen. From these results, the sliding load of the comparative steel shows large variations, but in the case of the inventive steel,
It is clearly seen that the heat treatment makes the bolt tightening force uniform due to the partial shape memory effect.

[Table] As explained above, the steel used in the present invention has a partial shape memory effect that is not observed in conventional steels. The use of this steel is extremely widespread;
The partial shape memory effect of this steel can be applied to all structures, fastening parts or joints of mechanical parts. The steel used in the present invention having such properties can be used extremely effectively in a wide variety of industries.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between temperature and elongation, where A shows the case where compressive strain is applied to a material with a shape memory effect, and B shows the case where compressive strain is applied to a material with a partial shape memory effect. , FIG. 2 is a diagram showing the structure of a jig for demonstrating the partial shape memory effect, and FIG. 3 is a configuration diagram of a friction bonding test specimen. a: beam, b: unfixed beam, c: column,
d: base material, s: test piece, e: bolt, f: nut, g: washer, h, i: mild steel plate.

Claims (1)

[Claims] 1 A steel material having a partial shape memory effect with a Mn content in the range of 15.9 to 30.0% by weight and the remainder consisting of Fe and unavoidable impurities is given a plastic deformation strain of 20% or less below the Ms point to shape the fixed part. A method of utilizing steel with a partial shape memory effect, characterized in that it is adapted and fixed, and then the material is heated above the As point and cooled again below the Ms point to generate stress in the fixed part.