JPH02190448A - Iron-base shape memory alloy excellent in machinability, corrosion resistance, and high-temperature oxidation resistance - Google Patents

Abstract

PURPOSE:To manufacture an iron-base shape memory alloy excellent in machinability, corrosion resistance, and high-temp. oxidation resistance by incorporating specific percentages of Si, Cr, Mn, S, etc., to Fe. CONSTITUTION:An iron-base shape memory alloy having a composition consisting of, by weight, 2.0-8.0% Si, 5.0-20.0% Cr, at least one element selected from <15.0% Mn, <=20.0% Ni, <=30.0% Co, <=3.0% Cu, and <=0.4% N, further at least one element selected from <=0.4% S, <=0.4% Te, <=0.4% Se, <=0.4% Pb, <=0.4% Bi, and <=0.01% Ca, and the balance Fe with inevitable impurities is prepared. By this method, the iron-base shape memory alloy excellent in shape memory effect, corrosion resistance, and high-temp. oxidation resistance and having superior machinability can be obtained.

Description

[Detailed description of the invention] [Field of sedentary use] This invention relates to an iron-based shape memory alloy that has excellent machinability, corrosion resistance, and high temperature oxidation resistance. The object of the present invention is to provide an alloy which is preferable for use and which has significantly improved properties without impairing its shape memory properties or other properties.

(Prior Art) Shape memory alloys use martensite to undergo plastic deformation and recover their shape using reverse transformation from martensite to the parent phase.

A large number of alloys exhibiting such a shape memory effect have been reported, mainly non-ferrous alloys, and some of them have already been put into practical use. After that, several cheaper iron-based alloys were published in Japanese Patent Application Laid-Open No. 59-83744, Japanese Patent Publication No. 61-
It was reported in JP-A No. 54859 and JP-A No. 61-20176, etc., and in the non-ferrous metals, it was expected to expand its application to fields that were constrained by cost. However, these iron-based alloys have a problem of insufficient corrosion resistance.

On the other hand, recently, an iron-based shape memory alloy with excellent shape memory, corrosion resistance, and high temperature oxidation resistance was filed by the applicant in Japanese Patent Application No. 63-83494 on April 5, 1988. It has excellent properties such as hardness, corrosion resistance, and high-temperature oxidation resistance, and it also has great economic benefits, so the fields of application have expanded greatly.

[Problem to be solved by the invention]

As a result of subsequent research by the inventors, the above-mentioned patent application
The iron-based shape memory alloy filed in No. 3494 often requires cutting work during product processing (in this case,
It was found that machinability was not necessarily sufficient.

Therefore, this invention was made to solve the above problems, and provides an iron-based shape memory alloy that has excellent shape memory effect, corrosion resistance, high temperature oxidation resistance, and excellent machinability. The purpose is to

[Means to solve the problem]

This invention has the following characteristics: Si: 2. 0wt. %~B, 0wt. %, Cr
: 5. 0wt. %～2 0. 0wt. %,
At least one element selected from the group consisting of: Mn: l 5. Owt, less than 1.

Ni: 20. 0wt. %below.

Co: 30. 0wt. %below.

Cu: 3.0 wt, % or less.

N: 0.4wt, % or less, and At least one element selected from the group consisting of: S: 0.4wt, % or less.

To: 0.4 wt, % or less.

Se: 0.4 wL, % or less.

Pb: 0.4 wt, % or less.

Bi: 0.4 wt, X or less.

Ca: 0.01wt, % or less, and the remainder: Fe and inevitable impurities.

It is characterized by the following.

In this invention, it is necessary for the iron-based shape memory alloy to improve its machinability without impairing the originally required shape memory properties, corrosion resistance, and high temperature oxidation resistance. limited. The reason why this invention is limited to the above-mentioned chemical components will be described below.

Cr (Chromium): Cr has the effect of lowering the stacking fault energy of austenite and improving corrosion resistance and high temperature oxidation resistance. Furthermore, Cr also has the effect of increasing the strength of austenite. However, the content of C" is 5.Ow
If t is less than 1, the above-mentioned effects will be insufficient. Also, Cr
Since is a ferrite-forming element, when the content increases, it becomes necessary to also increase the content of an austenite-forming element, which will be described later.

However, when a large amount of Cr is contained, the σ phase tends to be generated. Cr content is 20. When Owt and χ are reduced, significant deterioration of properties occurs due to the inclusion of a large amount of austenite-forming elements and the formation of σ phase, which will be described later.

Therefore, the content of C' should be limited within the range of 5.0 to 20.Owt, χ.

Si (Silicon): Si is an important element in this invention because it lowers the stacking fault energy of austenite and increases the yield strength of austenite. That is, by lowering the stacking fault energy of austenite and increasing the yield strength, an alloy that utilizes ε-martensite and has an excellent shape memory effect can be obtained. However, the content of Sl is 2. When Owt is less than 1, the above-mentioned effects are insufficient. Moreover, the Si content is 8. Exceeding Owt, χ causes a significant decrease in ductility and significantly inhibits hot and cold workability.

Therefore, the content of St should be limited within the range of 2.0 to B, Owt, χ.

In this invention, in addition to containing ferrite-forming elements Cr and Si, at least one of austenite-forming elements Mn, Nt, Co, Cu, and N is contained to add plastic working strain. At a predetermined temperature, it is necessary that the microstructure before straining be mainly austenite or mainly austenite and partially containing ε-martensite.

Mn (manganese): Mn is a strong austenite-forming element and is an effective element for obtaining a predetermined microstructure.

However, the Mn content is 15. Owt, 1 or more deteriorates corrosion resistance and high temperature oxidation resistance, and furthermore, σ
It significantly facilitates the formation of phases and deteriorates processability.

Therefore, the Mn content is 15. It should be limited to less than Owt, χ.

Ni (nickel): Ni is a strong austenite-forming element and is an effective element for obtaining a predetermined microstructure.

However, the Ni content is 20. If Owt, χ is exceeded, the transformation point (Ms) of ε-martensite shifts to a significantly lower temperature range, and martensite becomes difficult to generate when strain is applied.

Therefore, the Ni content is 20.0wt. % or less.

Co (cobalt): Co is an austenite-forming element and is
It is an effective element for obtaining texture. Also, Ni, Cu
, N lowers the transformation point (Ms) when included, whereas Co hardly lowers the transformation point (H3). Therefore, CO adjusts the transformation point (Ms) within a predetermined temperature and reduces strain. It is an extremely effective element for generating ε-martensite during addition.

However, the Co content is 30. When Owt,X is exceeded, the above-mentioned effect is saturated.

Therefore, the content of CO should be limited to 30.0 wt% or less.

Cu (copper): Cu is an austenite-forming element. Since corrosion resistance is improved by containing a small amount of Cu, Cu is an effective element. However, since Cu has the effect of increasing the stacking fault energy of austenite, if the Cu content exceeds 3.0iit, χ, the formation of ε martensite is inhibited.

Therefore, the Cu content is 3.0wt. % or less.

N (nitrogen): N is an austenite forming element. N is an effective element because the corrosion resistance and the yield strength of austenite are increased by containing a small amount of N. However, when the N content exceeds Q, 4wt, χ, Cr, St
may facilitate the formation of nitrides and inhibit the shape memory effect.

Therefore, the N content should be limited to 0.4wt0% or less.

In this invention, in order to obtain an iron-based alloy with excellent machinability, in addition to the above-mentioned elements, S, TeSe, Pb
, Bi, and Ca.

S (sulfur)2S is a free-cutting element. S is normally treated as an impurity, but in this invention, by actively adding S, it is uniformly distributed in the alloy in the form of sulfide, and becomes a concentration source of strain during cutting, reducing machinability. Improve. However, when the S content exceeds Q, 4wt, and χ, this effect not only saturates, but also the corrosion resistance is significantly reduced and the shape memory properties are deteriorated.

Therefore, the S content should be limited to: o, 4 wt.

Te (Tellurium Te is a free-cutting element. Ta is in the form of a Te compound, uniformly distributed in the alloy, and becomes a source of strain concentration during cutting, improving machinability. However, the content of Te When the amount exceeds 0.4 wt.χ, not only this effect is saturated, but also corrosion resistance is significantly impaired and shape memory properties are deteriorated.

Therefore, the content of Te should be limited to 0.4 wt.% or less.

Se (Selenium): Se is a free-cutting element. Se is in the form of Se compound,
It is uniformly distributed in the alloy and becomes a source of strain concentration during cutting, improving machinability. However, when the Se content exceeds Q, 4wt, χ, this effect not only becomes saturated, but also the shape memory properties deteriorate.

Therefore, the Se content should be limited to 0.4 wt.% or less.

PB (Lead): PB is a free-cutting element. Pb is an element that improves machinability without impairing corrosion resistance. However, p
When the content of b exceeds Q, 4wt, and χ, not only the machinability improvement effect is saturated, but also the hot workability is significantly impaired.

Therefore, the content of pb should be limited to Q,4wt,% or less.

81 (Bismuth): Bi is a free-cutting element. Bi is an element that improves machinability without impairing corrosion resistance. However, if the original content increases to 0.4 wt, χ, not only the machinability improvement effect is saturated, but also the hot workability is significantly impaired.

Therefore, the Bi content should be limited to 0.4 wt.% or less.

Ca (calcium): Ca is a free cutting element. Ca is an element that improves machinability without impairing corrosion resistance by adding a small amount. However, when Ca exceeds 0.01 wt, χ, the effect is saturated.

Therefore, the Ca content should be limited to 0.01 wt.% or less.

Next, the present invention will be explained with reference to examples.

〔Example〕

Inventive steel having a chemical composition within the scope of this invention and comparative steel having a chemical composition outside the scope of this invention shown in Table 1 are melted using an air or vacuum melting furnace, and after ingot formation, , heated at 1000°C to 1250'C, with a thickness of 20
Hot rolling was carried out up to ■, and the specimens of the invention steel (hereinafter referred to as "the specimens of the invention") Nos. 1 to 10 and the comparison steel specimens (hereinafter referred to as the "comparison specimens") were prepared. )Nα11
~k15 was prepared.

A round bar tensile test piece (6Iφ, gauge pressure M30flIl11) was used for each of the prepared specimens, and 4%
After applying a tensile strain of
S, M, E, ) J are also shown in Table 1.

The evaluation criteria is that the shape memory effect is 70% or more: ◎, 3
0% to less than 70% was indicated as ○, and less than 30% was indicated as ×.

The method for calculating the shape memory effect is as follows.

S, M, E, (χ)-((11-1ri/(1,-1°)l Later gauge distance.

Each of the prepared specimens was subjected to a one-year atmospheric exposure test, and the state of rusting was visually evaluated.
It is also shown in the table.

The evaluation criteria are ◎ for those with no rust, O1 for those with some rust, and × for those with significant rust.
It was displayed as .

Each of the prepared specimens was heated to 600'C in an air atmosphere, and the oxidation state of the surface was visually observed to evaluate high temperature oxidation resistance, which is also shown in Table 1 as "oxidation resistance".

The evaluation criteria are: ◎ if no oxidation is observed, ○ if some oxidation is observed, and × if significant oxidation is observed.
And so.

The machinability was evaluated by the ratio of the life of the cemented carbide tool in face milling when comparison specimens Ntll and 12 were taken as 1.

As shown in Table 1, comparative specimens Nα11 and 12
It was found that the shape memory effect, corrosion resistance, and high-temperature oxidation resistance were equivalent to that of the specimen bt~10 of the present invention, but the machinability was inferior.

Comparative specimen stone 13, which has a high S content outside the range of this invention, and comparative specimen Sou 14, which has a high Te content outside the range of this invention, both have shape memory effects, corrosion resistance, and High temperature oxidation properties were poor.

Comparative specimen No. 15, which has a high Se content outside the range of this invention, has a poor shape memory effect and has Se added compared to specimen No. 3 of the present invention. Despite this, almost no improvement in machinability was observed.

On the other hand, the specimen N11l-10 of the present invention was excellent in shape memory effect, corrosion resistance, high temperature oxidation resistance, and machinability.

〔Effect of the invention〕

As explained above, the present invention brings about the industrially useful effect of obtaining an iron-based shape memory alloy having excellent machinability, corrosion resistance, and high-temperature oxidation resistance.

Claims (1)

[Claims] 1 Si: 2.0wt. %~8.0wt. %, Cr:5.0wt. %~20.0wt. %, at least one element selected from the group consisting of: Mn: 15.0wt. %, Ni: 20.0wt. % or less, Co: 30.0wt. % or less, Cu: 3.0wt. % or less, N: 0.4wt. % or less, and at least one element selected from the group consisting of: S: 0.4wt. % or less, Te: 0.4wt. % or less, Se: 0.4wt. % or less, Pb: 0.4wt. % or less, Bi: 0.4wt. % or less, Ca: 0.01wt. % or less, and the remainder: Fe and unavoidable impurities.