JP2000054023A - Production of cast member made of shape memory alloy and cast member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cast member made of a shape memory alloy free from the generation of cracking caused by cold working after casting and its producing method and to provide a cast member made of a shape memory alloy by casting high in yield strength and its producing method. SOLUTION: This is a method for executing heat treatment at 1060 to 1200 deg.C after casting. The cracking results from the problem that an intermetallic compd. composed mainly of Mn and Si is produced at the time of casting and the heat treatment is the means of eliminating such problem. The heat treatment is effective even after cooling the compd. to a room temp. after casting and is also effective even if the heat treatment is executed at a stage in which the temp. does not reduce yet just after casting and the generation of cracking at the time of cold working can be suppressed. Then, hot working is executed after being cast into a cylindrical shape. The hot working is executed at >=600 deg.C finishing temp. after heating at 1060 deg. to 1200 deg.C or in the case of <600 deg.C finishing temp., the heat treatment is executed at 600 to 1050 deg.C after the hot working. In this way, the yield strength of the cast member made of a shape memory alloy improves and its reliability after joining increases.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a cast member made of a shape memory alloy which is used as a fastening member such as a pipe joint, and a cast member obtained by the method.

[0002]

2. Description of the Related Art As a shape memory alloy, a copper alloy such as Ti-Ni alloy, Cu-Zn-Al or Cu-Al-Ni, and an iron alloy such as Fe-Mn-Si-Cr are known. (For example, "Metal", March 1983, p. 38). In producing members utilizing these shape memory alloys,
Conventionally, forged and ingot materials are generally hot forged,
Rolling, extruding, drawing, etc. to make an intermediate processed product in the form of a plate or a rod (line), or by further processing such as cold rolling, drawing, etc. into an intermediate processed product, In addition, products are usually processed into products through finishing processes such as cutting, bending, cutting, and welding. Originally, the various processing steps performed before obtaining an intermediate processed product, especially for general shape memory alloy materials with poor workability, are factors that greatly increase the product price of these alloys. In addition, the high cost has caused a great limitation on the use of these alloys.

Recently, as a method of greatly simplifying the processing steps leading to the intermediate processed product, an epoch-making manufacturing method in which a molten shape memory alloy is directly cast into a mold having a shape close to the final product. Have been found (Japanese Patent Application No. 9-08
No. 3655). According to this method, for example, when manufacturing a pipe having an inner diameter of 50 mm or less, a shape memory alloy melted at a temperature of 1300 ° C. or more is poured into a mold having a predetermined shape close to a finished shape, and after cooling, 600 to 1000 ° C
After performing the heat treatment described above, the desired product can be produced by applying a cold deformation. Further, when manufacturing a relatively large cylindrical pipe joint having an inner diameter of 100 mm or more, 1300 ° C.
The shape memory alloy melted at the above temperature is poured into a steel drum rotating at a high speed and solidified.
A target product can be produced by performing a heat treatment at about 000 ° C. and deforming it cold.

According to such a method, not only hot and cold steps such as forging, rolling, extrusion and drawing, but also a part of finishing steps such as bending and welding can be omitted, and the manufacturing cost can be reduced. Is expected to be significantly reduced, and the range of use of the shape memory alloy is expected to be greatly expanded.

[0005]

However, when a shape memory alloy cast member is manufactured by the above-mentioned casting method, the member is frequently applied to the member at a certain frequency during cold deformation performed to impart shape recovery performance. It has become clear that cracking occurs. That is, of course, there are casting defects called "shrinkage cavities" which are generally formed during solidification due to differences in the cooling rate, etc., but there are no such casting defects, and members considered to be sound as cast products. It has become clear that cracking of the member at a certain frequency cannot be avoided. Therefore, the present inventors have investigated cracks during cold deformation that occur in a sound member as a cast product, and found that the microstructure of the member in which these cracks occur is within a few hundred μm of normal crystal grains. It was confirmed that an intermetallic compound phase of about several tens μm was present. Since this phase was harder and less ductile than the normal phase, it was found that it worked as a starting point for cracking during cold deformation.

[0006] If the amount of deformation given in the cold is about 4%, cracking may not occur, but if deformation of 7% or more is given, most members will crack starting from such a small hard phase. Had occurred. Further, there is also a problem that the fastening member manufactured by the above casting method has a low yield strength and a low tensile strength, and the one that is once fastened by utilizing shrinkage due to the shape memory effect is easily released due to vibration in a subsequent use environment. Had occurred.

Therefore, an object of the present invention is to achieve the following two solutions with respect to a shape memory alloy cast member manufactured by casting and a method of manufacturing the same. (1) To provide a cast member made of a shape memory alloy which does not crack by cold working after casting and a method of manufacturing the same. (2) To provide a cast member made of a shape memory alloy by casting and having a high yield strength and a high tensile strength and a method of manufacturing the same.

[0008]

Means for Solving the Problems and Embodiments of the Invention
The manufacturing process of the shape memory alloy cast member adopted by the present invention to solve the above two problems basically takes the following four embodiments. Melting → Casting → Cooling → Heat treatment from 1060 ° C to 1200 ° C →
Cooling → Cold deformation (cold expansion) Melting → Casting → Heat treatment from 1060 ° C to 1200 ° C → Cooling →
Cold deformation (cold expansion) Melting → Casting into cylindrical shape → Hot working up to 600 ° C after heating at 1060 to 1200 ° C → Cooling → Cold deformation (cold expanding) Melting → Casting into cylindrical shape → Hot working → 600 to 1050
℃ heat treatment → cooling → cold deformation (cold expansion)

First, the first invention will be described in detail. After melting the shape memory alloy comprising the desired component, it is cast into a fastening member having a shape close to the finished shape. The casting method may be any of sand casting, lost wax, shell molding, centrifugal casting and the like. In the present invention, the cooling rate after casting does not particularly matter. This is because it is difficult to avoid the presence of intermetallic compounds in the cooling process after casting regardless of slow cooling or rapid cooling, although the amount is small. Furthermore, it is considered that this intermetallic compound is mainly composed of Mn and Si, and is formed at a relatively early stage of the process of solidifying from a molten state. Although it tends to dissolve again and disappear, it will remain without completely disappearing in a general cooling process.

When the shape memory alloy is previously deformed in a cold state, an operation in a direction opposite to the deformation when heated next time can be obtained as shape recovery. Therefore, cold deformation is essential. Cracks at the time of cold deformation originated from this intermetallic compound. The inventors have
In order to make such intermetallic compounds disappear,
It has been found that a heat treatment at 1200 ° C. is effective. This is the first invention. That is, this method is a method in which an intermetallic compound is eliminated by performing a heat treatment at 1060 to 1200 ° C. for 10 minutes or more and 3 hours or less on a shape memory alloy that has been cast into an element member having a finished shape after melting and cooled to room temperature. .

The reason why the heat treatment temperature is set to 1060 ° C. or more is as follows.
If the temperature is lower than 1060 ° C., the above-mentioned intermetallic compound is present in a small amount.
The reason why the temperature is set to 0 ° C. is that if the temperature exceeds 1200 ° C., the yield strength of the material decreases, and sufficient strength as a fastening member cannot be maintained. The reason why the processing time is set to 10 minutes or more is 10 minutes.
If the heating time is less than 3 minutes, the entire material is not sufficiently heated, and there is a risk that the intermetallic compound may remain. The reason why the heating time is 3 hours or less is that if the heating time exceeds 3 hours, the crystal grains become coarse and the material yields. This is because the strength is reduced.

As described above, the first invention is a method of once cooling to room temperature after casting and re-dissolving the intermetallic compound by heat treatment.

Next, even if the cooling to room temperature is omitted after the casting, and the heat treatment is performed during the cooling, the dissolution of the intermetallic compound can be performed without any trouble. Since the sensible heat possessed by the cast shaped material can be used effectively, it can be said that this has a greater practical advantage. However, in cases where the casting size is significantly larger than the final product size (especially with respect to wall thickness), such as in the case of centrifugal casting, rough processing is performed from the finished shape before heat treatment, and it is close to the finished product size. In some cases, it is better to perform the heat treatment after forming the shape. In this case, it is rarely effective to perform the heat treatment after performing the roughing according to the first invention in which the casting is once cooled to room temperature. Not.

The third invention will be described in detail below.
First, after melting the shape memory alloy consisting of the desired components,
This is cast into a cylindrical fastening member close to the finished shape. The casting method may be any of sand casting, lost wax, shell mold, centrifugal casting and the like, but centrifugal casting is most suitable for casting a cylindrical member having no anisotropy. After casting, it is heated at a temperature of 1060 to 1200 ° C for 10 minutes or more and 3 hours or less, and then hot-worked. Hot working refers to, for example, ring rolling or ring forging. In the ring rolling, as shown in FIG. 1, rolling rolls 2 are inserted into the inside and outside of a cylinder 1 to perform rolling. The reason for setting the temperature to 1060 to 1200 ° C. and the heating time to 1
The reason for 0 minute to 3 hours is exactly the same as the reason explained in the first invention. The reasons for performing hot working are the following two.

In general, centrifugal casting is manufactured to be about 1.5 mm thicker on the outer diameter side and about 3 mm thicker on the inner diameter side with respect to the thickness of the product.
It is common to cut the inside and outside at any stage of the post-process to finish the product. These portions to be removed by cutting have irregularities and micro-segregation, and are usually considered to be undesirable as products. However, it has become clear that these parts can be reformed to a comparable state as a product by performing hot rolling and appropriate heat treatment. This is because ring rolling is most suitable for hot working a cylindrical material such as a centrifugally cast material, and this step can significantly improve the material yield. This is the first reason.

The second reason is that the cast structure of the material produced by the centrifugal casting method is completely destroyed by hot working, and the crystal grains recrystallized by heating are stretched and finely divided, thereby increasing the yield strength and tensile strength. To get the material. When the finished shape is a cylinder, ring rolling is suitable for rolling the cylinder. The reason for finishing at a temperature of 600 ° C. or more is to memorize the shape of the cylinder in the shape memory alloy cast into a cylindrical shape. If the temperature is lower than this, the memory of the shape is insufficient and the shrinkage ratio (when the diameter is expanded) at the time of recovery becomes small.

It is clear that the present invention includes a cylindrical shape having a different diameter and a diameter varying continuously along the way, and a tapered shape is also included in the present invention. It is also clear that a hollow cylinder having a non-circular shape such as a polygonal cross section can be easily imagined from the present invention.

A fourth invention is a manufacturing method according to the third invention, wherein the finishing temperature conditions for hot working are not specified. Even if the heating conditions before processing and the finishing temperature are lower than 600 ° C., if the heat treatment is performed at a temperature of 600 ° C. or more and 1050 ° C. or less as a process for storing the shape of the cylinder (shape memory process), the member as a fastening member It will be practical. 6
Heating at a temperature of 00 ° C. or more causes the shape memory alloy cast into a cylindrical shape to memorize the shape of the cylinder. 600
The reason why the temperature is set to not less than ° C. is that at a temperature lower than this, the memory of the shape is insufficient and the shrinkage rate at the time of recovery (when the diameter is expanded) becomes small. The reason why the temperature is set to 1050 ° C. or less is that if the temperature exceeds this, the yield strength of the material is significantly reduced, and the fastening portion is likely to come off due to vibrations in the use environment after fastening.
The reason why the heating time is set to 10 minutes to 3 hours is that if the heating time is less than 10 minutes, the entire material is not sufficiently heated and the shape memory is insufficient. This is because heat treatment for a long time has no particular effect compared with heat treatment for 3 hours or less and is unnecessary.

As the shape memory alloy in the first to fourth inventions, an Fe-Mn-Si based shape memory alloy is preferable.
The fifth invention is directed to any one of the first to fourth inventions, wherein Fe-Mn is used.
This is a manufacturing method using a Si-based shape memory alloy. Fe
—Mn—Si based shape memory alloys are Mn: 10 to 40% by weight, Si: 4 to 7% by weight, Cr: 0 to 15% by weight, N
i: an alloy containing 0 to 15% by weight and the balance being Fe as a main component, preferably, for example, Fe-32Mn-6Si
Alloy, Fe-28Mn-6Si-5Cr alloy, Fe-2
0Mn-5Si-8Cr-5Ni alloy, Fe-16Mn
-5Si-12Cr-7.5Cr alloy etc.
), And an alloy obtained by adding Cr or Ni for improving corrosion resistance to an alloy based on Fe-Mn-Si. The reason for using the Fe-Mn-Si based shape memory alloy is that when manufacturing a large cylindrical fastening member with a diameter of 100 mm or more, TiNi, Cu-Zn-Al, Cu-Al-N
Because the material cost is lower than other shape memory alloys such as i,
This is because it is particularly suitable for mass production.

A sixth aspect of the present invention is an inexpensive shape memory alloy cast member having high fastening reliability and manufactured by using the manufacturing method according to any one of the first to fifth aspects.

A seventh invention is a shape memory alloy cast member manufactured according to the first or second invention, which is a cylindrical shape memory alloy cast member. Fe-Mn
In an alloy having a face-centered cubic structure such as a Si-based shape memory alloy, the <100> direction has the maximum crystal growth rate, and the main trunk (first arm) develops in this direction, and then the secondary arm and the tertiary arm The arm and the branch progress and solidify. The solidification structure of the material of the present invention is characterized in that a dendritic solidification structure 3 is formed from the outside to the inside of the cylinder (arrow 4) as shown in the cross-sectional view of the cylinder in FIG. Is homogeneous, and a substance having another composition different from the matrix such as an intermetallic compound is not mixed. This cast member has the advantage that it does not crack during cold working.

[0022]

(Example 1) Fe-28Mn-6Si-5C
After melting r (weight%) shape memory alloy in a 100 kg atmosphere furnace, cylindrical fastening members of various sizes are produced by four methods: lost wax method, shell mold method, centrifugal casting method, and sand mold method. Manufactured. Table 1 shows the casting method, dimensions of the produced cylinder, heating temperature after casting, deformation rate of cold deformation after cooling (expansion rate of inner diameter), presence or absence of cracks during deformation, shrinkage rate at shape recovery, Yield strength as determined by specimens taken from a part of the material of the cylinder is indicated. The remarks column indicates which invention each embodiment corresponds to from the viewpoint of the method. The member obtained by the method of the present invention, compared with the comparative example,
It was found that there was no occurrence of cracks in each case and a good finish was exhibited. Further, it was found that the yield strength was improved in the member subjected to hot rolling (ring rolling) after casting.

[0023]

[Table 1]

[0024]

According to the method of manufacturing a shape memory alloy member according to the present invention, the following effects can be obtained. (1) After casting, a shape memory alloy member is obtained which does not crack even in strong working with a standard deformation of 7% or more in cold working as a shape memory alloy, and which can obtain sufficient shape recovery. Can be (2) It is possible to obtain a shape memory alloy member having high yield strength and tensile strength of the material and high reliability after fastening. (3) Large-sized cylindrical shape memory alloy members having an inner diameter of 100 mm or more can be mass-produced at low cost.

[Brief description of the drawings]

FIG. 1 is a schematic view of ring rolling according to third and fourth inventions of the present invention.

FIGS. 2A and 2B are a cross-sectional view showing a crystal structure and a direction of solidification of a cylindrical shape memory alloy casting member according to a seventh embodiment of the present invention, and FIG. 2B is a partially enlarged view of the crystal structure.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Circular cylinder of shape memory alloy 2 ... Rolling roll 3 ... Dendritic crystal structure 4 ... Direction of solidification

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tanaki withstand 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Tadakatsu Maruyama 2-3-3 Kanda Ogawamachi, Chiyoda-ku, Tokyo 13M & C Building 4F Awaji Sangyo Co., Ltd. (72) Inventor Mitsuki Tomaru 2-3-13 Kanda Ogawamachi, Chiyoda-ku, Tokyo M & C Building 4F Awaji Sangyo Co., Ltd.

Claims (7)

[Claims] 1. Casting a molten shape memory alloy into an elementary member having a shape close to the finished shape, cooling to room temperature, heating at a temperature of 1060 ° C. to 1200 ° C. for 10 minutes to 3 hours, and then to room temperature A method for producing a cast member made of a shape memory alloy, wherein a deformation is imparted in a cold state after cooling. 2. After the molten shape memory alloy is cast into an element member having a shape close to the finished shape, heating is performed at a temperature of 1060 ° C. or more and 1200 ° C. or less for 10 minutes or more and 3 hours or less during cooling from the casting temperature to room temperature. A method for manufacturing a cast member made of a shape memory alloy, wherein the member is cooled to room temperature and then deformed in a cold state. 3. After the molten shape memory alloy is cast into a cylindrical shape, the temperature is raised to a temperature of 1060 ° C. or more and 1200 ° C. or less.
Production of a shape memory alloy cast member, characterized in that hot working is performed immediately after heating for at least 600 minutes or more immediately after heating for not less than minutes and not more than 3 hours, and then deformation is imparted cold after cooling to room temperature. Method. 4. After the molten shape memory alloy is cast into a cylindrical shape, it is heated at a temperature of not less than 1060 ° C. and not more than 1200 ° C.
Immediately after heating for not less than min and not more than 3 hours, then hot working at a temperature of not less than 600 ° C and not more than 1050 ° C for not less than 10 minutes and not more than 3 hours, and then cool to room temperature and then apply deformation in the cold A method for producing a shape memory alloy cast member. 5. The method for manufacturing a shape memory alloy cast member according to claim 1, wherein the shape memory alloy is an Fe—Mn—Si based shape memory alloy. 6. A shape memory alloy cast member manufactured by the manufacturing method according to claim 1. Description: 7. A cylindrical cast member in which a dendritic solidification structure is formed from the outside to the inside of the cylinder, and a phase composed of an intermetallic compound is not mixed in the crystal structure. A cast member made of a shape memory alloy.