JPH0230734A - Iron-base shape memory alloy having excellent shape memory characteristics and corrosion resistance - Google Patents

Abstract

PURPOSE:To develope the title alloy by containing specific amounts of Cr, Si and Mn and at least one kind among Ni, Co, Cu and N into Fe. CONSTITUTION:An alloy steel contg., by weight, 0.1 to 5.0% Cr, 2.0 to 8.0% Si and 1.0 to 14.8% Mn, contg. at least one kind among 0.1 to 20.0% Ni, 0.1 to 30.0% Co, 0.1 to 3.0% Cu and 0.001 to 0.400% N, having the quantitative relationship so as to satisfy the inequality I between the content of Ni, Mn, Co, Cu and N and the content of Cr and Si which are ferrite-formation elements, and the balance Fe is melted and cast into an ingot. The ingot is heated, e.g., to the temp. range of 1,000 to 1,250 deg.C and is thereafter subjected to hot rolling. The iron-base shape-memory alloy having excellent shape-memory characteristics and corrosion resistance can be manufactured at low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an iron-based shape memory alloy that has excellent properties.

[Prior Art] Shape memory alloys are formed by applying plastic deformation to an alloy at a predetermined temperature near the martensitic transformation point, and then heating the alloy to a predetermined temperature higher than the temperature at which it reversely transforms into its parent phase. An alloy that exhibits the property of recovering to its original shape before being subjected to plastic deformation. By applying plastic deformation to a shape memory alloy at a predetermined temperature, the crystal structure of the alloy transforms from its parent phase to martensite. When the alloy that has been plastically deformed in this way is then heated to a predetermined temperature higher than the temperature at which it reversely transforms into its parent phase, the martensite transforms back into its original parent phase, and thus the alloy has a shape Indicates memory properties. As a result, the plastically deformed alloy recovers to its original shape before being plastically deformed.

Many non-ferrous shape memory alloys have been known as alloys having such shape memory characteristics. (for example,
Shape Memory Alloys (ed., Hiroyasu Funakubo, 1984, Sangyo Tosho).

Among them, Ni-Ti-based and Cu-based shape memory alloys have already been put into practical use, and pipe fittings, clothing, medical equipment, actuators, etc. are made using these non-ferrous shape memory alloys. Manufactured. In recent years, the development of technology that applies shape memory alloys to various uses has been progressing.
It is actively practiced.

However, since non-ferrous shape memory alloys are expensive, they are economically limited. Under these circumstances, iron-based shape memory alloys that are cheaper than non-ferrous shape memory alloys are being developed. As described above, it is expected that the range of application of iron-based shape memory alloys will be expanded in place of non-ferrous shape memory alloys, which are economically limited.

From the viewpoint of the crystal structure of martensite, which transforms iron-based shape memory alloys from their parent phase by applying plastic deformation, iron-based shape memory alloys can be classified into fct (face-centered tetragonal) and bct (body-centered cubic) ) and hcp (close-packed hexagonal crystal).

Fe is an iron-based shape memory alloy that transforms from its matrix to fct martensite by applying plastic deformation.
-Pb system and Fe-Pt system are known.

(For example, Ryuichi Oshima et al., “Journal of the Japan Institute of Metals, Vol. 48.
Volume, No. 9, 1984. P881J). These iron-based shape memory alloys exhibit good shape memory properties.

As an iron-based shape memory alloy that transforms from its parent phase to bct martensite (hereinafter referred to as "α'fumarunsite") by applying plastic deformation, Fe-Pt type (for example, rActa
Metalurgica, Vol 26. Per
gaLlon Press 1978Printed
in Great Br1tainJ P1529) and Fe-Ni-Co-Ti system (JP-A-60-23
4950, Japanese Patent Application Laid-Open No. 61-106746) are known. α′ fumarunsite is a phase formed in alloys with high stacking fault energy;
As a result, the volume change during metamorphosis is large. Therefore,
During transformation, deformation tends to occur within α' martensite, and these iron-based shape memory alloys do not exhibit good shape memory characteristics in their original state.

However, by making the parent phase of these iron-based shape memory alloys into a parent phase that has the Invar effect (i.e., a phenomenon in which the coefficient of thermal expansion becomes extremely small in a certain temperature range), the α' fumarole of these alloys can be reduced. It is known that the sliding deformation of the fiber optic sites is suppressed and, as a result, these alloys exhibit good shape memory properties.

High manganese steel and 5US304 austenitic stainless steel specified in the JIS standard are used as iron-based shape memory alloys that transform from their parent phase to heρ martensite (hereinafter referred to as "ε martensite") by applying plastic deformation. Steel is known. ε martensite is a phase formed in alloys with low stacking fault energy, resulting in small volume changes during transformation. Therefore, during transformation, slip deformation is unlikely to occur in ε martensite, and these iron-based shape memory alloys
Shows good shape memory properties. (For example, written by Zenji Nishiyama, [
Martensitic Metamorphosis - Basics 1i4 December 1971 Maruzen).

The following alloys have been proposed as iron-based shape memory alloys whose parent phase transforms into ε-martensite by applying plastic deformation.

Iron-based shape memory alloy disclosed in Japanese Patent Publication No. 49-10409: ■ Ni: 10-IEht,%, Cr: 1
0 to 25 wt, %, if necessary, Co: 2
System containing 0 wt, % or less, ■ Cr: 10~
20tit, %Nioite, Ni: 17.5wt, Z
Below, 1Mn: 30.5wt% or less, Co: 15% or less 6% system, ■ Mn: 15-35wt 0% system,
■ Mn: 12.5~30tit,%, N
i: A system containing 15 wt.% or less (hereinafter referred to as "prior art 1").

In Prior Art 1, as seen in the examples as shape memory characteristics, when a tensile strain of about 4% is applied and then heated to a predetermined temperature, a recovery strain of about 1% can be obtained.

Iron-based shape memory alloy disclosed in Japanese Patent Publication No. 59-83744: ■ C: 0.1-0.35t1%, Si: 0.5υt
, 1 or less, Mn: 8.0-15. Owt, %, Son
AQ: 0.01-0.06tst, % system, ■ C: 0.1-0.35wt, %, Si: 0
．． 5wt, % or less, Mn=8.0-15. (ht,%,
So Q A Q: O, O1~0.06wt, %
In, Ni: 1. Owt, x or less, Cr:
1. , Ovt, % or less. (Hereinafter referred to as "prior art 2").

In Prior Art 2, as seen in the Examples, the shape memory property can be recovered by about 30% by applying a tensile strain of 10% or less and then heating to a predetermined temperature.

Advance nuclear t4! In the iron-based shape memory alloys disclosed in i1 and 2, the shape recovery is only partial, and the amount of one recovery is only about 30% of that of the additive type.

Iron-based shape memory alloy disclosed in Japanese Patent Publication No. 61-54859: Mn: 20-40wt, %, Si: 3.5-
8. (A system containing ht, %.

(Hereinafter referred to as "prior art 3").

Iron-based shape memory alloy disclosed in JP-A No. 61-201761: Mn: 20-40 wt, %, Si: 3.5-
g, 0wt0%, Ni: 10wt, x or less, Cr:
10tzt, % or less, Co: 1 (ht, % or less,
N.

= 2υt 1% or less, C: 1wt, x or less, AQ:
1wt, % or less.

Cu: ltgt, a system containing % or less (hereinafter,
(referred to as "Prior Art 4").

The iron-based shape memory alloys disclosed in Prior Art 3 and 4 have excellent shape memory properties. That is, prior art 3, 4
The shape memory properties obtained are as follows.

The iron-based shape memory alloys of Prior Art 3 and 4 are melted in a high-frequency heating atmospheric furnace, the melted alloy is then cast into an ingot, and the ingot thus cast is then
0.5 mm x 1.5 m by holding at a temperature in the range of 1,050-1250 °C for 1 hour and then hot rolling the thus heated ingot.
Test specimens with dimensions of m x 30 mn+ were prepared.

The specimen thus prepared was then plastically deformed - by bending it at an angle of 45'' at room temperature, and said specimen was heated to a predetermined temperature above the austenite transformation point. When the shape recovery rate of the alloy was investigated, it was found that the alloy had a recovery rate of 75 to 90.
% shape recovery rate.

[Problems to be Solved by the Invention] However, although the iron-based shape memory alloy disclosed in Prior Art 3 has excellent shape memory properties, it is insufficient in corrosion resistance.

The iron-based shape memory alloy disclosed in the preceding core #i4 contains Cr, Ni, Co and MO for the purpose of improving corrosion resistance.
At least one element of the above is added to the alloy. However, Prior Art 4 has the following problem. That is, as mentioned above, in order to improve the corrosion resistance of the alloy, at least one element among Cr, Ni, Co, and No is added, and in particular, manganese is added in a large amount of 20 to 4 (ht, %). Since it is added,
The effect of improving corrosion resistance is not necessarily sufficient, 15wt
, 1 or more, 20-40wt.

% Mn and further contains Cr, the alloy of Prior Art 4 tends to form very brittle intermetallic compounds (hereinafter referred to as "σ phase") due to the presence of Cr.

The formation and existence of this σ phase significantly deteriorates the shape memory properties, workability, and toughness of iron-based shape memory alloys; for this reason, it is necessary to develop iron-based shape memory alloys with excellent shape memory properties and corrosion resistance. Although highly desired, such iron-based shape memory alloys have not yet been proposed.

Therefore, an object of the present invention is to provide an iron-based shape memory alloy having excellent shape memory properties and corrosion resistance.

[Means for Solving the Problems] When plastic deformation is applied to a hep-type iron-like shape memory alloy at a predetermined temperature, the phase of the alloy changes to its parent phase.

That is, austenite transforms into ε-martensite. The alloy whose parent phase has been transformed into ε-martensite in this way is then heated to the austenite transformation point (hereinafter referred to as rAf).
When heated to a temperature above (referred to as "point") and near the Af point, ε martensite transforms back into its parent phase, i.e., austenite, and as a result, the plastically deformed alloy undergoes plastic deformation. Recovers to its original shape before adding.

In order to exhibit the shape memory properties of the above-mentioned HCP type iron shape memory alloy, it is necessary to satisfy the following conditions.

(1) Before plastic deformation is applied to the alloy at a predetermined temperature, the parent phase of the alloy must: - be composed mainly of austenite; The above-mentioned predetermined temperature is
This is the temperature at which the parent phase can transform into ε-martensite when plastic deformation is applied to the alloy at that temperature.

(2) The stacking fault energy of austenite must be low. Furthermore, by applying plastic deformation to the alloy, it is necessary that the parent phase transforms only into ε martensite, and not into α' fumarunsite.

(3) The yield strength of austenite must be high. Furthermore, when plastic deformation is applied to the alloy, no sliding deformation should occur in the crystal structure of the alloy.

The present invention solves the problems of the prior art described above while satisfying the above conditions.Cr: 0.1 to 5.
Owt, %, Si:2. O~8. Ovt1%, Mn: 1.0-14.8wt, %, at least one element selected from the group consisting of the following, Ni: 0.1-20, Oi+t, %, Co: 0
．． 1-30. Ovt, %, Cu: 0.1-3. Ow
t,%, N: 0.001-0.40kt,%.

However, Ni+ 0.5Mn+ 0.4Co+ 0.06
C: u+ 0.002N≧0.67 (Cr+ 1.2
Si) and the remainder: Fe and inevitable impurities.

Next, the chemical composition of the iron-based shape memory alloy of this invention is as follows:
The reason for limiting the range to the above-mentioned range will be described below.

(1) Cr (Chromium): Cr has the effect of lowering the stacking fault energy of austenite and improving the corrosion resistance and high temperature oxidation resistance of the alloy. Furthermore, Cr has the effect of increasing the yield strength of austenite.

However, if the Cr content is less than 0.1 tyt, 1, the desired effects described above cannot be obtained. On the other hand, C
The content of r is 5. (ht,% is not allowed for the following reason. That is, since Cr is a ferrite-forming element, when the Cr content increases, the formation of austenite is inhibited. Therefore, in this invention, ,
In order to form austenite, it is an austenite-forming element as described below. Mn, and also the austenite-forming elements Ni, Go, and C.
At least one element of u and N is added to the alloy. As the Cr content increases, it is also necessary to add larger amounts of the austenite-forming elements mentioned above. However, adding a large amount of austenite-forming elements is economically disadvantageous. Furthermore, as the Cr content increases, the σ phase tends to form in the alloy. For these reasons, the Cr content is 5. Owt,%, it is necessary to increase the content of austenite-forming elements, so
The economy is impaired, and furthermore, due to the formation of the σ phase, 1
The shape memory properties, processability and toughness of the alloy deteriorate. Therefore, the Cr content should be limited within the range of 0.1 to 5, (ht,%).

(2) Si (Silicon): Si has the effect of lowering the stacking fault energy of austenite. Furthermore, Si has the effect of increasing the yield strength of austenite. However, the Si content is 2. (If the Si content is less than 1, the desired effects described above cannot be obtained. On the other hand, if the Si content exceeds 8.Owt,%, the ductility of the alloy will decrease significantly, and the hot workability of the alloy will decrease.) and cold workability deteriorates significantly. Therefore, the Si content should be limited within the range of 2.0 to 8, (ht,%).

(3) Manganese: Manganese is a strong element that forms austenite, and manganese has the effect of making the parent phase of the alloy mainly austenite before plastic deformation is applied to the alloy. However, the content of manganese is 1, O
When wt is less than 3, the desired effects described above cannot be obtained. On the other hand, when the manganese content exceeds 14.8 wt.%, the corrosion resistance of the alloy deteriorates and σ phase is likely to be formed. Therefore, the manganese content is between 1.0 and 1.
It should be limited within the range of 4.8wt.%.

We investigated the influence of manganese, chromium and silicon content on corrosion resistance in iron-based shape memory alloys by the following tests: 2.0 to 8
Various specimens were prepared according to the method described in the ``Example'' section below, while varying the contents of chromium and manganese in alloy steel containing silicon of , 0 tit, %. Next, each of the specimens thus prepared was subjected to an atmospheric exposure test for 3 months, and the state of rust generation was evaluated by visual inspection for each specimen. The test results are shown in FIG.

In FIG. 1, the horizontal axis is the manganese content (tit).

%), and the vertical axis is the chromium content (wt.

%). In FIG. 1, the area surrounded by dotted lines indicates that the manganese content and chromium content are within the scope of this invention. In addition, in Fig. 1, the '01 mark indicates that no rust was observed, the 'O' mark indicates that some rust was observed, and the '×
The mark '' indicates that significant rust formation was observed. 1st
As is clear from the figure, from 1.0 to 14. kt, manganese content in the range of 0.1 to 5. Chromium content within the range of Owt,% and 2.0 to 8.0vt,%
Specimens with a silicon content within the range of 0.05 to 1.00% show excellent corrosion resistance.

In this invention, Cr, Si and Mn, which reduce the stacking fault energy of austenite, are added to the alloy, and N, which is an austenite forming element, is added to the alloy.
At least one element selected from i, Go, Cu, and N is added to the alloy, thereby making the parent phase of the alloy primarily austenite before plastic deformation is applied to the alloy.

(4) Ni (nickel): Ni is a strong element that forms austenite,
Further, Ni has the effect of making the parent phase of the alloy mainly austenite before plastic deformation is applied to the alloy.

However, when the Ni content is less than 0.1wt, 3,
The desired effect cannot be obtained from the above-mentioned action. On the other hand, the Ni content is 20. (ht,%), the transformation point of ε-martensite (hereinafter referred to as rMs point) shifts significantly to a lower temperature range, and the temperature at which plastic deformation is applied to the alloy becomes significantly lower. Therefore, the Ni content ,0.1~20.0t
it, should be limited within the range of the pair.

(5) Co (cobalt): CO is an austenite-forming element, and Co
has the effect of making the parent phase of the alloy mainly austenite before plastic deformation is applied to the alloy.

Furthermore, Mn, Ni, Cu, and N have the effect of lowering the Ms point, whereas Co has the effect of hardly lowering the Ms point. Therefore, co is an extremely effective element for controlling the 2Ms point within a desired temperature range. However, the Co content is 0.1
When wt is less than 1, the desired effects described above cannot be obtained. On the other hand, the Co content is 30. (ht,%, no particular improvement in the above-mentioned effect can be obtained. Therefore, C
The o content should be limited within the range of 0.1 to 30.0%.

(6) Cu (copper): Cu is an austenite-forming element, and Cu
has the effect of making the parent phase of the alloy mainly austenite before plastic deformation is applied to the alloy.

Furthermore, Cu has the effect of improving the corrosion resistance of the alloy. However, if the Cu content is less than 0.1 wt.%, the desired effects described above cannot be obtained.

On the other hand, the Cu content is 3. If it exceeds Owt, %, the formation of ε-martensite is inhibited. The reason is that Cu has the effect of increasing the stacking fault energy of austenite. Therefore, the Cu content is.

0.1-3. It should be limited within the range of Owt, %.

(7) N (nitrogen): N is an austenite-forming element, and.

N is the parent phase of the alloy before plastic deformation is applied to the alloy,
It mainly has the effect of converting it into austenite.

Furthermore, N does not have the desired effect on improving the corrosion resistance of the alloy. On the other hand, when the N content is 0.
If it exceeds 400st1%, nitrides of Cr and Si are likely to form, and the shape memory properties of the alloy deteriorate. Therefore, the N content is 0.001 to 0.400t
It should be limited within the range of +t,%.

(8) Ratio of the total content of austenite-forming elements to the total content of ferrite-forming elements: In this invention, as described above, before plastic deformation is applied to the alloy at a predetermined temperature, the parent phase of the alloy is: It is absolutely necessary that it consist primarily of austenite.

Therefore, in this invention, in addition to the above-mentioned limitations on the chemical composition of the alloy of this invention, it is necessary to satisfy the following formula.

Ni+ 0.5Mn+ 0.4Co+ 0.06Cu+
0.002N≧0.67 (Cr+1.2Si) The austenite-forming power of the austenite-forming elements contained in the alloy of this invention can be expressed as follows from the standpoint of Ni equivalent.

Ni equivalent = Ni+ 0.5Mn+ 0.4Co+0.0
6Cu+ 0.002NNi equivalent is an indicator of austenite forming ability.

The ferrite-forming power of the ferrite-forming elements contained in the alloy of this invention can be expressed as follows from the viewpoint of Cr equivalent.

Cr equivalent=Cr+1.2Si Cr equivalent is an index of ferrite forming ability.

By satisfying the above formula, the parent phase of the alloy before plastic deformation is applied to the alloy at a predetermined temperature can be made to be a parent phase mainly consisting of austenite.

(9) Impurities: The content of C1P and S, which are impurities, is less than ltgt,z for C, and 0.1wt,% for P.
The content of S is preferably 0.1 wt.% or less.

Next, the iron-based shape memory alloy of the present invention will be explained in more detail by way of examples, while comparing it with comparative alloys outside the scope of the present invention.

[Example] As shown in Table 1, the alloy steel of the present invention has a chemical composition within the scope of the present invention, and as also shown in Table 1, the alloy steel of the present invention has a chemical composition outside the scope of the present invention. Comparative alloy steels having the following properties were melted in a melting furnace under atmospheric pressure or vacuum and then cast into ingots. The obtained ingot was then heated to a temperature within the range of 1000 to 1250°C and then hot rolled to a thickness of 12 nn to obtain a specimen of the alloy steel of the present invention (hereinafter referred to as "the present invention"). (referred to as "specimen") Nα1 to 11. Comparative alloy steel specimens C (hereinafter referred to as "comparative specimens") Nα1 to Nα9 outside the scope of this invention were prepared.

Next, 1. Shape memory properties and corrosion resistance were investigated by the tests described below. The results of these tests are also shown in Table 1.

(1) Shape memory properties Shape memory properties were investigated by a tensile test consisting of the following. Specimen Nα1 of the present invention prepared as described above
~11 and comparative specimens &1~9, test pieces on round bars with a diameter of 6I and a gauge length of W130III11 were cut out, and each of the test pieces thus cut out had the following properties:
At the deformation temperature shown in Table 1, a 4% tensile strain is applied, and then each test piece is heated to a predetermined temperature above and near the Af point. Measure the gage distance of each specimen after
Then, the shape recovery rate is calculated based on the measurement results between the gauge points, and the shape memory characteristics of each specimen are evaluated.

The results of the above-mentioned tensile test are also shown in the "shape memory properties" column of Table 1.

The evaluation criteria for shape memory properties were as follows.

O: Shape recovery rate is 70% or more; O: Shape recovery rate is 30 to less than 70%; ×: Shape recovery rate is less than 30%.

The shape recovery rate was calculated according to the formula below.

Ll-L.

However, Lo: the initial gauge distance of the test piece, L: the gauge distance of the test piece after adding tensile strain.

L: Gauge length of the test piece after heating.

Since the Ms point differs for each specimen, the optimum temperature for applying plastic deformation was set for each specimen. This temperature is shown in Table 1, column "Deformation temperature".

(2) Corrosion resistance Inventive specimens Nα1 to 11 and comparative specimen Nα1
A two-year atmospheric exposure test was conducted for each of the items 9 to 9, and
Its corrosion resistance was investigated. After the above test was completed, each specimen was visually inspected to evaluate the occurrence of rust. The results of the above test are also shown in the "Corrosion Resistance" column of Table 1.

The evaluation criteria for the occurrence of rust were as follows.

O: No rust formation observed. 0: Some rust is observed, ×: Significant occurrence of rust is observed.

As is clear from Table 1, the comparative specimen Nal has poor shape memory properties because the silicon content is low and outside the range of the present invention.

Comparative specimen Na 2 has poor shape memory properties due to its high silicon content, which is outside the range of the present invention. Furthermore, in comparison specimen Nα2.

Occurrence of cracks is observed.

- Comparative specimen Nα3 has a low manganese content that is outside the range of the present invention, so it is inferior in shape memory properties.

Comparative specimen Nα4 has a high manganese content that is outside the range of the present invention, and therefore is inferior in corrosion resistance.

Comparative specimen Nα5 has a low chromium content that is outside the range of the present invention, and therefore is inferior in corrosion resistance.

Comparative specimen surface 6 has a high nickel content that is outside the range of the present invention, and therefore has poor shape memory properties.

Comparative specimen Nα7 has a high kappa content that is outside the range of the present invention, and therefore has poor shape memory properties.

Comparative specimen Na 8 has a high nitrogen content that is outside the range of the present invention, and therefore has poor shape memory properties.

Comparative specimen N (L 9 is the formula 'Ni+0.5Mn+0
．． 4Co+0.06Cu+ 0.002N≧0.67(
Cr+ 1.2SL)' is not satisfied,
It is inferior in shape memory property.

On the other hand, all of the present invention specimens Nα1 to Nα11 are excellent in shape memory properties and corrosion resistance.

[Effects of the Invention] As explained above, the iron-based shape memory alloy of the present invention has the following properties:
It has excellent shape memory properties and corrosion resistance, and is suitable for use as pipe fittings, various fastening devices, etc. materials, and biological materials, and reduces its manufacturing cost. 1, thus producing industrially useful effects.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of the contents of Cr, SL and Mn on corrosion resistance in an iron-based shape memory alloy. Figure 1

Claims (1)

[Claims] 1 Cr: 0.1 to 5.0wt. %, Si: 2.0-8.0wt. %, Mn: 1.0-14.8wt. %, at least one element selected from the group consisting of: Ni: 0.1-20.0wt. %, Co: 0.1-30.0wt. %, Cu: 0.1-3.0wt. %, N: 0.001-0.400wt. %, however, Ni+0.5Mn+0.4Co+0.06Cu+
An iron-based shape memory alloy with excellent shape memory properties and corrosion resistance, characterized by comprising 0.002N≧0.67 (Cr+1.2Si) and the remainder: Fe and inevitable impurities.