JPS6133899B2 - - Google Patents

Description

[Detailed description of the invention]

In the present invention, after applying plastic working strain at room temperature, As
It relates to a 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, and there is room for further improvement in eliminating manufacturing steps. Loosening of the tightening force of screws and bolts caused by physical vibration is
It is difficult to completely prevent it unless special measures 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 lose their properties at the welded part, resulting in problems such as weld warping and deterioration of toughness. 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 has yet been found to improve the situation based on a fundamentally new principle. 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, can be effectively utilized, screws and bolts should be used. In such cases, no loosening is observed even under high mechanical vibrations, and it is possible to obtain a joint with sufficient tightening force.In addition, when tightening parts with complex shapes that are difficult to reach, It has been discovered that sufficient tightening can be achieved by using a heating method such as a simple gas burner. The material that meets this purpose is Si.
The present invention was made based on the discovery that there is a steel material with adjusted Mn. That is, the present invention allows high Mn steel to contain Si: 1.0 to 5.5%, and
It is a steel with a partial shape memory effect characterized by a total content of Mn of 17.7 to 21.5% and the remainder consisting of Fe and unavoidable impurities. 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 of all, 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. This refers to a phenomenon in which the final length completely returns to the length before heating when cooled, even if plastic deformation strain is applied before heating, and the temperature history curve returns to point 0. On the other hand, the partial shape memory effect referred to in the present invention refers to the effect of applying compressive plastic working strain to a certain material before heating it to a temperature above point A, as shown in B in Figure 1.
When cooled below the Ms 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 0, but returns to point F. In such a phenomenon, the material ends up in a stretched state. On the other hand, when applying tension as a strain applied before heating, the final length is not at point 0, but at point O in B in Figure 1, contrary to the case of compression.
It moves further to the left side and becomes contracted. 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 of the present invention has Ms
It is a steel that has a composition range such that the point is above room temperature and the As point is as low as possible. Next, the reason for limiting the range of components of the present invention as described above will be described. Combined with Si and Mn to increase the total content of Mn to 17.7
~21.5% and further limited Si to 1.0 to 5.5% because when Si is combined with Mn and the total content is less than 17.7%, reversible martensite exhibits a partial shape memory effect. Transformation no longer occurs, and As is produced by applying plastic deformation strain at temperatures below the Ms point.
Even if the material is heated to a temperature above the Ms point and then cooled, the shape of the material at a temperature below the Ms point completely recovers to the shape before the heating/cooling heat treatment, and no shape change is observed before or after the heat treatment, and a partial shape memory effect is observed. I can't. Similarly, when the total content exceeds 21.5%, the transformation point of this steel material will be below room temperature, and no matter how the steel with this composition is subjected to heat treatment at least above room temperature,
No reversible change of martensite occurs to exhibit a partial shape memory effect. Therefore, no matter what kind of heating and cooling heat treatment is applied, the shape of this material will always completely return to the shape before the heating and cooling treatment. In addition, even if the total content is 17.7 to 21.5%, the lower limit of the content of Si that is combined with Mn is set at 1.0% because martensite is reversible to exhibit a partial shape memory effect. This is because it is the minimum necessary content to indicate the property. In addition, the reason why we set the upper limit of Si content to 5.5% is that when it is contained in excess of this upper limit,
At this time, the transformation point of the steel material is below room temperature, and no matter how heat treatment above room temperature is applied, the reversible change of martensite to exhibit the partial shape memory effect will not occur. However, when Si: 1.0 to 5.5% is combined with Mn,
When the total amount with Mn is 17.7 to 21.5%,
The Ms and As points are each above room temperature and do not reach very high temperatures, and the martensitic transformation is reversible; heating and cooling after applying plastic working strain at room temperature will result in the final shape of the material. does not completely return to the shape before the heat treatment of heating and cooling, and the shapes do 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 something that can be done. This knowledge was obtained based on the following experimental results. In Table 1,
Si up to 6.0% and Mn content 12.6 to 20.1
The chemical composition of the test steel is shown in various percentages. All of these materials were melted in a laboratory-scale high-frequency melting furnace in an atmospheric atmosphere, poured into a regular mold, and hot-rolled into a 20 mm plate. It has been tempered. 10×15×15mm from this board
A shaped test piece was taken 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 as shown in Figure 2A 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. Thereafter, a certain area around the test piece S is heated to 375°C, held for 20 minutes, and then cooled to room temperature. Beam a of this jig before heating is fixed to column c by welding, but beam b is not fixed in any way. Pillar c
and the base material d are firmly fixed by welding. After cooling to room temperature, check whether the test piece S is sufficiently fixed. If it is fixed, measure the residual stress by pasting a strain gauge on the top surface of the beam b and confirm that the partial shape memory effect has been confirmed. I investigated how much. The results are also listed in Table 1. As can be seen from these test results, Si: 1.0
~5.5% is combined with Mn and the total amount with Mn is 17.7 ~
It can be seen that the steel, which is 21.5% and the balance is Fe and unavoidable impurities, exhibits a partial shape memory effect.

[Example 1]

The test steels were those shown in Table 2, and the comparative steels were heat-treated to have almost the same strength as the steel of the present invention before specimen production. From these materials, M10 x 50L bolts are made by cold forging, and after tightening with M10 iron polished JIS class 2 nuts to a tightening torque of 212Kg cm,
The comparative material was left as is, and the inventive steel was heated to 375°C.
A high-speed loosening test was performed on the sample that was 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,
The loosening of the nut after 9000 vibrations was measured using the nut loosening torque. The results are shown in Table 3. These results show that while the comparative steel loosens due to mechanical vibration, the steel of the present invention shows no loosening even due to mechanical vibration due to heat treatment, indicating that the bolt loosens due to the partial shape memory effect. Improvement in tightening force was observed.

[Table] ○ indicates comparative steel

[Table] ○ marks are comparative examples [Example 2] The test steels are Steels 1 and 4 among the test steels shown in Table 2.
The heat treatment of the comparative steel before making test pieces was adjusted so that the strength of this material was almost the same as that of the steel of the present invention. The test method was to cut out a round bar from these materials, and then cold forge it into an M10×
Using 50L bolts, we fabricated a friction bonding test specimen as shown in Figure 3, and determined the bolt tightening force by examining the sliding load using an ordinary tensile tester. In the same figure, e is a bolt, f is a nut,
g is washers, h is thickness 5mm, width 70mm, length
110mm, i indicates a mild steel plate with a thickness of 15mm, a width of 70mm, and a length of 300mm. 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 bolted joint made of the steel of the present invention was heated to 375°C, 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 of 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. In particular, we focused on the characteristic that the steel of the present invention exhibits expansion and contraction more than twice as much as ordinary steel, and we focused on the characteristics of the steel of the present invention, which expands and contracts more than twice as much as ordinary steel. It has the possibility of new effective use of thermal energy, such as by using heat to periodically cause large expansions and contractions to the steel of the present invention, converting this into rotational motion, and applying it to small generators. . The steel of the present invention having such properties can be used very 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. High Mn steel contains Si: 1.0 to 5.5%, and the total amount of this content and Mn is 17.7 to 21.5%,
A steel having a partial shape memory effect, characterized in that the remainder consists of Fe and unavoidable impurities.