JPH02301514A - Method for allowing shape memory stainless steel to memorize shape - Google Patents

Abstract

PURPOSE:To allow even a stainless steel having >10% Cr content and superior corrosion resistance to advantageously exhibit shape memory characteristics by rendering a specified compsn. based on the compsn. of an Fe-Cr steel and having property regulated Mn, Si and Co contents. CONSTITUTION:A steel consisting of, by weight, <=0.03% C, 3.0-6.0% Si, 6.0-25.0% Mn, <=7.0% Ni, 10.0%<Cr<=17.0%, 0.02-0.3% N, 2.0-10.0% Co, 0.05-0.8% one or more among Nb, V, Zr and Ti and the balance Fe or if necessary further contg. <=2.0% Mo and/or <=2.0% Cu and having >=-26.0 D defined by the equation is worked into a prescribed shape, annealed, deformed at ordinary temp. or below and heat-treated at 450-700 deg.C. After this deformation and heat treatment are repeated once or more, the temp. of the steel is returned to ordinary temp. The steel is allowed to memorize the final shape.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for improving the memory properties of shape memory stainless steel, which exhibits an excellent shape memory effect. This invention relates to a stainless steel with excellent corrosion resistance that takes advantage of the shape memory effect of steel.

[Conventional technology and problems]

Conventionally, nonferrous alloys such as Ni-Ti alloys and Cu alloys, Fe-Pd alloys, and Fe alloys have been used as alloys that exhibit shape memory effects.
Iron alloys such as -Ni series and Fe-Mn series are known. Among them, the Fe-Mn type is inexpensive.

Because of its great industrial value, 1, for example, JP-A-55-
Fe-(15.9-30.0%) in Publication No. 73846
Mn alloy, Fe-
Mn-(Si, Ni, Cr) alloy.

JP-A No. 61-76647 discloses that Fe-(20 to 40%
)Mn-(3.5~8%)Si alloy, JP-A-63-16
No. 946 reports Fe-(15-30)Mn-N alloy and the like. Furthermore, JP-A No. 62-112720 reports a method for improving the shape memory effect of Fe-Mn-Si alloy, which requires processing of 20% or less and heating at 400°C.
Apply the above heating one or more times. It uses the so-called training effect.

However, these iron-based shape memory alloys have a major drawback of poor corrosion resistance. Therefore, JP-A-61
-201761 publication describes Cr in Fe-Mn-Si alloy.
There are examples of improving corrosion resistance by adding C.
Since the r content is as low as 10.0% or less, it cannot be said that it has the corrosion resistance of a so-called stainless steel, and JP-A No. 63-216946 also states that it contains Cr to improve corrosion resistance. However, in actual examples, the Cr content is up to 10%, and it does not teach how to advantageously develop shape memory properties when C, which is a ferrite-forming element, is contained higher than this. Not yet.

On the other hand, in stainless steel, r 5Cripta M
eta-11urgicaJ, 1977, VOl, 51
1.196°C in 5US304 steel from 663 to 667
It has been reported that a shape memory effect can be exhibited by deforming the material at 9°C and then heating it to room temperature, but the shape recovery and recovery rate were small and it was far from practical use.

[Purpose of the invention]

The purpose of the present invention is to advantageously exhibit shape memory properties even in stainless steel containing a large amount of Cr, a ferrite-forming element, exceeding 10%. This paper aims to provide a shape memory method.

[Structure of the invention]

In the present invention, at 1% by weight, C: 0.03% or less, St
, 3.0-6.0%, Mn; 6.0-25.
0%, N1H7.0% or less, Cr; 10.0%a~17
．． 0%, N; 0.02-0.3%.

Co; 2.0-10.0%, Nb, V, Zr, Ti
One or more of: 0.05 to 0.8%, and in some cases further contains one or two of Mo or less than 2.0% Cu, with the remainder being Fe and unavoidable impurities. It is a steel made of.

D=Ni+0.30Mn+56.8C+19.0N
+0.73Co-+Cu 1.85[Cr+1.6
After processing steel with a D value of -26.0 or more, defined as Si + 1.5 (Nb + V + Zr + Ti) + Mo) into a predetermined shape and annealing, it is deformed at a temperature below room temperature and heated at 450°C. As described above, we provide a shape memory method for shape memory stainless steel, which is characterized by repeating heat treatment in a temperature range of 700°C or less once or more, and then returning to room temperature to memorize the final shape. .

[Details of the invention] Explain the shape memory phenomenon of stainless steel.

Deformation is applied to steel which has a substantially austenite I-(r) single phase structure in an annealed state without the presence of a 6-ferrite phase and a martensitic phase.

In order to prevent the generation of dislocations and permanent strain of the strain-induced martensitic (α'') phase, deformation proceeds only by the formation of strain-induced ε phase, and the temperature is higher than the temperature (As point) at which the ε phase starts to transform into the γ phase. The material is heated to cause ε→T transformation, thereby recovering the shape before deformation.

“Stainless Steel Handbook” (Nikkan Kogyo Shimbun) 11.18
7 to 5tlS304L# ε phase 5 after tensile deformation of q
The formation behavior of the α' phase has been reported, and it has been shown that the ε phase is formed in large quantities at low temperatures. However, since the α° phase that inhibits the shape memory effect is also generated at the same time, it is thought that the shape memory effect during low temperature deformation of the 5LIS304L steel described above is small.

The present inventors believe that the key to exhibiting excellent shape memory effects in γ-based stainless steel is to suppress the generation of permanent strains such as dislocations and α° phase during the deformation process, while promoting the generation of deformation-induced ε phase. After much thought and extensive research, we found that Fe-Cr steel contains appropriate amounts of Mn + SiI N and Co-, and also controls the content of alloying elements such as C and Ni, so that the T phase is free from dislocations and α If the structure is stable against the formation of the ° phase, the deformation-induced ε phase will be more likely to be generated below the Md point (the temperature at which the ε phase starts to form due to deformation) than when deformed at low temperatures. It became clear.

On the other hand, if the 1T phase is not sufficiently stable, α.

If a phase is formed or the deformation temperature is too low below the Ms point (the temperature at which the ε phase begins to form simply by cooling),
It was found that since the ε phase is already generated during cooling, the formation of the ε phase is suppressed even when deformation is applied, and the shape memory effect is reduced.

Based on the above knowledge, Mn based on Fe-Cr1il.

By appropriately controlling the contents of Sl, Co, N, C, Ni, etc., there is no δ ferrite phase or martensitic phase in the annealed state, and a T-phase single phase structure is created.
By bending and deforming in a low temperature range below °C, it is possible to suppress the generation of dislocations and α゛ phase, while significantly promoting the formation of deformation-induced ε phase, and as a result, heating above the As point after deformation is possible. It has been found that a more excellent shape memory effect can be achieved.

However, although the above-mentioned method shows an excellent shape memory effect when the amount of deformation is small, such as in bending deformation, the amount of deformation may reach about 8% when it is assumed to be used in practical applications such as pipe joints. When the amount of deformation is large, it cannot necessarily be said that a good shape memory effect is exhibited. It has been found that this is because when the amount of deformation is large, not only a large amount of deformation-induced ε phase but also dislocations and α゛ phase are generated, which inhibits the shape memory effect.

Therefore, in order to obtain an excellent shape memory effect even when the amount of deformation is large, we have conducted various studies and found that the temperature at which deformation and ε→T transformation are completed at temperatures below room temperature and the permanent strain is recovered, that is, 450 By repeating heat treatment in the temperature range above °C, the resistance to the generation of permanent strain increases, and the generation of permanent strain is suppressed in subsequent deformation, and deformation is substantially only due to the generation of the ε phase. It has been found that the shape memory effect is significantly improved by subsequent heat treatment. Furthermore, if the above-mentioned heating temperature is too high, it will reach the so-called sensitization temperature range, and if low-temperature deformation and heat treatment are repeated, C'carbide will be generated and a Cr-deficient layer will be formed, which will impair corrosion resistance. C and 5 further regulate the C content, and Nb.

It has been found that by containing appropriate amounts of one or more of V, Zr, and Ti, the formation of Cr carbides can be suppressed and excellent corrosion resistance can be maintained.

The reason why the content of each component of the steel is limited to the above range in the present invention is as follows.

C produces Cr carbide with repeated low-temperature deformation and heat treatment and deteriorates corrosion resistance, so the upper limit is set at 0.03.
%.

St prevents the generation of permanent strain during deformation and reduces deformation-induced ε
It is an essential element that promotes phase formation and exhibits an excellent shape memory effect, and must be contained in an amount of 3.0% or more. However, Si is a strong ferrite-forming element;
When contained in large amounts.

The upper limit of Si is set to 6.0% because a large amount of δ ferrite phase remains in the annealed state, reducing the shape memory effect and also deteriorating hot workability, making manufacturing difficult.

Mr+ is an austenite-forming element, and in the annealed state δ
Contributes to suppressing the formation of ferrite phase. Also Mn
is an effective element that prevents the generation of permanent strain during deformation, promotes the formation of deformation-induced ε phase, and enhances the shape memory effect, and must be contained in an amount of 6.0% or more. However, if it is contained in a large amount, it will conversely suppress the formation of the deformation-induced ε phase and reduce the shape memory effect, so the upper limit is set at 25.0%.

Ni is an austenite-forming element and is an effective element for preventing the formation of a δ ferrite phase in an annealed state. However, N1 induces the generation of permanent strain during deformation,
In order to reduce the shape memory effect, the upper limit is set to 7.0%.

Cr is an essential element for stainless steel, and to obtain excellent corrosion resistance, the content must exceed 10%.

Cr is also an element that suppresses the generation of permanent strain during deformation and improves the shape memory effect. However, Cr is a ferrite-forming element, and if it is contained in a large amount, the δ ferrite phase tends to remain in the annealed state, reducing the shape memory effect, so the upper limit is set at 17.0%.

N is an austenite-forming element and is effective in preventing the δ ferrite phase from remaining in the annealed state. Furthermore, N suppresses the generation of permanent strain during deformation and improves the shape memory effect. Furthermore, N has a very significant effect on increasing the strength of steel, 1 increasing the pull-out resistance (strength) required when it is used, for example, in pipe joints. For these reasons, N needs to be contained in an amount of 0.02% or more. However, if a large amount of N is contained, blowholes will be generated in the steel ingot, making it impossible to obtain a complete steel ingot, so the upper limit has been set at 0.3%.
shall be.

Co is an austenite-forming element and is effective in preventing the hexaferrite phase from remaining in the annealed state.

Co also suppresses the generation of permanent strain during deformation.

It is an effective element that promotes the formation of the deformation-induced ε phase and enhances the shape memory effect, and must be contained in an amount of 2.0% or more. However, even if a large amount of Co is contained, these effects are saturated, so the upper limit is set to 1010%.

Nb, V, Zr, and Ti are essential elements to suppress the formation of Cr carbide during repeated deformation at temperatures below room temperature and heat treatment at 450°C to 700"C, and to maintain excellent corrosion resistance. However, since these elements are ferrite-forming elements, if they are included in large amounts, the δ ferrite phase tends to remain in the annealed state, which may reduce the shape memory effect. Therefore, each upper limit is set to 0.8%.

Mo is an effective element for improving corrosion resistance.

However, Mo is a ferrite-forming element, and if it is contained in a large amount, the δ ferrite phase will remain in the annealed state, reducing the shape memory effect, so the upper limit is set at 2.0%.

Cu is an element that improves corrosion resistance. Further, Cu is an austonite-forming element and effectively acts to prevent the δ ferrite phase from remaining in the annealed state. These effects do not change even if the content exceeds 2.0%, so 2.0%
is the upper limit.

The D value regulates the residual form of the δ ferrite phase that adversely affects the shape memory effect, and the above-mentioned relational expression between the D value and the alloying elements is an empirical expression obtained experimentally. D value is -26,
If D41i is less than 0, a large amount of δ ferrite phase remains, reducing the shape memory effect, so D41i is set to -26.0 or more.

The stainless steel of the present invention, which has the above-mentioned composition and has excellent corrosion resistance, is hot or cold worked into a predetermined shape, then annealed, then deformed at a temperature of 450°C or higher and 70°C at a temperature below room temperature.
It has an excellent shape memory effect even if it is deformed by about 8% by repeating heat treatment in a temperature range of 0'C or lower.

Let me explain this shape memory method further.

First, the steel of the present invention is processed into a predetermined shape and then annealed. This processing can be performed at room temperature or at a warm temperature. ) There is no δ ferrite phase or martensitic phase, and a substantially austenite single phase is present, and by this processing, ε
phase, as well as dislocations and permanent strain of the α” phase.5
By annealing this workpiece, the ε phase and permanent strain are completely removed.

Next, deformation at a temperature below room temperature and 450°C to 700°C
Repeat the heat treatment in the temperature range of °C. The deformation produces a deformation-induced epsilon phase, but deformation at low temperatures increases the amount of epsilon phase produced. However, when a high deformation amount of about 8% is applied, it is not possible to completely prevent the generation of not only the ε phase but also permanent strain, and the shape memory effect is inhibited. Therefore, the heat treatment after deformation must be carried out at a temperature higher than the temperature at which the transformation of the ε phase to the T phase is completed and the permanent strain is recovered.For these reasons, the heating temperature is 45°C.
It is necessary to carry out the process at 0°C or higher. However, if the heating temperature is too high, Cr carbides are likely to be formed, and the corrosion resistance, which is a characteristic of the steel of the present invention, will deteriorate. Therefore, the upper limit of the heating temperature is set to 700'C. By repeating this low-temperature deformation and heat treatment one or more times, permanent strain is less likely to be generated in the subsequent deformation, and the deformation proceeds substantially by the formation of the ε phase, so the heat treatment after deformation is This shows an excellent shape memory effect.

〔Example〕

The alloys shown in Table 1 were produced by high frequency melting.

Steels A1 to AI4 are steels of the present invention. Bl to B4 steels are comparative steels, Bl steel and B2 steel have Mn and Si that are outside the range of the present invention, and B3 steel does not contain C and O, and the D value is also outside the range of the present invention. It is something. In 84m, C is outside the scope of the present invention, and Nb and V are included.

It does not contain Zr and Ti.

These steel ingots were forged and hot rolled to a thickness of 3 ladle.
Thereafter, annealing and cold rolling were repeated to obtain an annealed plate having a thickness of 0.511+1. A tensile test piece (0,
After cutting out 5mm' x 20-kasumi' x 200m-1).

Tensile strain and heat treatment at 20°C, -73°C or -196°C were repeated, and the shape recovery rate (R value) was measured. In addition, regarding corrosion resistance, 0.5mmLX 50
A test piece measuring s+I@"

The shape recovery rate (R value) was calculated as follows. Final deformation - Gauge length before deformation in heat treatment cycle 1
0=50+sm), then applying a tensile strain and measuring the strain fil).

Heat treatment was performed, and the amount of gauge length recovery (p2) was measured. The recovery rate (%) was calculated from these measured values using the linear % formula %().

Table 2 shows the 20'C, -73"C and -196 Al steels.
Tensile strain amount and heating temperature in °C (holding time: 15 minutes).

The evaluation results of the number of repetitions, shape recovery rate (R value), and partial corrosion resistance are shown. From these results, first, in the comparative example, process N[111, Ntx12. Looking at Nn15.

In the case of only 6% deformation and heat treatment at 600'C, the R value is high at low temperatures, but it is still only 41% at -196°C. In addition, if the amount of deformation in step k13 or step 16 is as large as 12%, or if the heating temperature is as low as 400°C as in step k14, the R value will be 17% to 20%.
%, on the other hand, the process Nll! When the heating temperature is as high as 750°C as in 7, the R value is 75% and has an excellent shape memory effect, but when the heating temperature is 600°C
The corrosion resistance is inferior to that of steps Na 10, No. 15, etc.

On the other hand, in the method of the present invention, as seen in steps Nctl, k3, and Sake 6, the deformation temperature is increased to 20'C by repeating 3% deformation and heat treatment at 600°C four times.
However, when the R value is as high as 76%, at -73°C and -196°C, the R value is 98% and 100%, and the shape is almost completely recovered.

Moreover, even when the amount of deformation is as high as 6 to 7%, step N114.
Na5 and N[L8. As seen in 9.

It deforms in the low temperature range of -73°C or -196°C,
It can be seen that the shape memory effect is significantly improved by repeating the heat treatment at 600°C.

Table 3 shows the R4M of the invention steel and comparative steel when heat treatment at 600°C and 6% strain at 20°C, -73°C or -196°C was repeated four times, and after the repeated treatments. This figure shows the evaluation of corrosion resistance in the CASS test.

From the results in Table 3, first of all, looking at the R value, among the 5 comparative steels, the R value of Bl steel is as low as 21% at 20'C, and most of them do not show any shape memory effect, and at -73'C and 1+96
Although the R value improves at °C, it is only 32% for each.

The shape memory effect of B2 steel and B3 steel is as low as 34%, and the R value at 20°C is 13% and 24%, respectively. Furthermore, although the R value at low temperature deformation increases considerably compared to the case at 20°C, it is still low at 40% or less and does not exhibit a sufficient shape memory effect.

On the other hand, all the steels of the present invention have an R value of 4 at 20°C.
If it is 5% or more and it is deformed at low temperature.

The R value is significantly increased to 70% or more.

On the other hand, regarding corrosion resistance, the present invention steel and B1-1B
3 steel has low C content and Nb, V, Zr and TI
It has excellent corrosion resistance because it contains appropriate amounts of one or two of the following. On the other hand, B4 steel has a high C content and does not contain Nb, V, Zr, and Ti, and therefore has poor corrosion resistance.

Table 3 0 Evaluation of corrosion resistance ○: Rust developed 1△: Slightly cracked, ×
As described above, according to the present invention, Cr is added to 10
% of Mn and Si to improve corrosion resistance, even when the amount of deformation strain is high.

In addition to properly controlling the content of alloying elements such as N + Co + N
By repeating heat treatment at ~700°C, excellent shape memory properties can be developed, and by regulating the C content and containing appropriate amounts of Nb, V, Zr, or Ti, corrosion resistance can be prevented from deteriorating. It could have been prevented,
It can be used to fix and tighten mechanical parts in fields that require corrosion resistance, and can be used as a suitable member for pipe joints, etc., and its industrial value is extremely large.

Claims (2)

[Claims] (1) In weight%, C: 0.03% or less, Si: 3.0
~6.0%, Mn; 6.0-25.0%, Ni; 7.0
% or less, Cr; more than 10.0% to 17.0%, N; 0.0
2-0.3%, Co; 2.0-10.0%, and Nb
, V, Zr, Ti or more: 0.05-0
．． 8%, with the balance consisting of Fe and unavoidable impurities, and D=Ni+0.30Mn+56
．． 8C+19.0N+0.73Co-1.85[Cr+
1.6Sl+1.5(Nb+V+Zr+Ti)] After processing the steel with a D value of -26.0 or more as defined by A shape-memory method for shape-memory stainless steel, which comprises repeating heat treatment in a temperature range of .degree. C. or higher and 700.degree. C. or lower one or more times, and then returning the temperature to room temperature to memorize the final shape. (2) In weight%, C: 0.03% or less, Si: 3.0
~6.0%, Mn; 6.0-25.0%, Ni; 7.0
% or less, Cr; more than 10.0% to 17.0%, N; 0.0
2-0.3%, Co; 2.0-10.0%, Nb, V,
One or more of Zr and Ti: 0.05 to 0.8%
, and 2.0% or less Mo or 2.0% or less C
A steel containing one or two types of u, the balance consisting of Fe and unavoidable impurities, and D=Ni+0.3
0Mn+56.8C+19.0N+0.73Co+Cu
-1.85 [Cr+1.6Si+1.5(Nb+V+Z
r+Ti)+Mo] Steel having a D value defined as -26.0 or more is processed into a predetermined shape, annealed, and then deformed at a temperature below room temperature and at a temperature of 450°C or more and 700°C or less. A shape memory method for shape memory stainless steel, which is characterized by repeating heat treatment in a temperature range one or more times and then returning the temperature to room temperature to memorize the final shape.