EP2976441B1 - Iron-based shape memory alloy - Google Patents

Description

The invention relates to an iron-based shape memory alloy.

Shape memory alloys are known from different material systems. Iron-based shape memory alloys, ie consisting of substantial elements of the element iron, are also known from the prior art. For example, the European patent application discloses EP 2 194 154 A1 a shape memory alloy which has a two-way effect and consists of a low-cost, iron-based alloy. The said European patent application proposes to consider a certain combination of the elements of the group Mn, Si, Cr and Ni in addition to Fe in the alloy.

From the Japanese patent application JP-A 2003/268502 In addition, an iron-based shape memory alloy with Ni contents is known which is intended to form precipitates during the manufacturing process by adding titanium in the alloy. However, this has the disadvantage that the Ni-Ti precipitates, the properties of the nickel with respect to the shape memory effect due to its binding to titanium can not be used. Another iron-based shape memory alloy based on a Fe-Mn-Si alloy system to which elements Nb and C have been additionally added is disclosed in the publication EP 1 348 772 A1 known. The alloying components niobium and carbon alloyed in these alloy systems promote the formation of precipitates, which serve as nucleation sites for gamma-epsilon phase transformations, which then form the basis for the shape memory effect. Also the European patent application EP 1574 587 A1 describes an alloy system that uses NbC precipitates to provide the shape memory effect. Similar to the two alloying elements niobium and carbon, it is possible to form two groups of elements which have similar functions. The one group contains vanadium, niobium, titanium, tungsten and zirconium, the other group consists from carbon, nitrogen and boron. The targeted use of different types of precipitation are described in the following documents, for example, vanadium-carbon precipitates in the CN 1280444 C , Ti-Nb-VN carbides / nitrides and Ni3Ti as precipitation types are mentioned in the Japanese documents JP 2004/115864 and the Japanese patent JP 3970645 B2 disclosed. The European patent application EP 2 141 251 A1 discloses vanadium nitrogen and vanadium carbon precipitates used for shape memory alloying effect. The Chinese patent application CN 101215678 B relates to a so-called Ti-Nb-CN system. An Nb-Ti-VC system for forming precipitates is known from CN 100523263 C known. KAJIWARA S ET AL. disclosed in "Remarkable improvement of shape memory effect in Fe-Mn-Si based shape memory alloys by producing NbC precipitates" (SCRIPTA MATERIALIA, ELSEVIER, AMSTERDAM, NL, Vol. 44, No. 12, 2001 ) a shape memory alloy of the type Fe-28Mn-6Si-5Cr, to which 0.47 wt% Nb and 0.06 wt% C are alloyed, so as to improve the shape memory effect and to be able to do without the time-consuming "training". Finally, the Russian patent publication RU 2270267 C1 a V-Nb-WC excretion system for the representation of the shape memory effect. However, the iron-based shape memory alloy systems known from the prior art are in need of improvement in view of reducing the manufacturing costs, increasing the activation or switching temperature and their corrosion and forming properties.

On this basis, the present invention has for its object to provide an iron-based shape memory alloy which provides a disposable shape memory effect, which requires reduced manufacturing costs, an increased activation temperature compared to copper and nickel-based alloys and their corrosion and Forming properties are improved compared to nickel-free and chromium-containing concepts.

• N ≤ 0,1%,
• C ≤ 0,1%,
• B ≤ 0,1%

besteht und für die Summe der Legierungsbestandteile der Gruppe 1 gilt: $${\displaystyle \sum _{\mathit{Gruppe}1}N,C\mathrm{,10}\cdot B\ge \mathrm{0,005}\%}$$

und mindestens ein Element einer Gruppe 2 von Elementen vorhanden ist, die Gruppe 2 aus den Elementen Ti, Nb, W, V, Zr mit folgenden Gehalten

• Ti ≤ 1,5%,
• Nb ≤ 1,5%,
• W ≤ 1,5%,
• V ≤ 1,5%,
• Zr ≤ 1,5% besteht und für die Summe der Legierungsbestandteile der Gruppe 2 gilt: $${\displaystyle \sum _{\mathit{Gruppe}2}\mathit{Ti},\mathit{Nb},V,W,\mathit{Zr}\ge \mathrm{0,1}\%}$$
  
  und für das Verhältnis der Summe der Legierungsbestandteile der Gruppe 1 und der Gruppe 2 jeweils in Atom-% gilt: $$\mathrm{0,5}\le \frac{{\displaystyle \sum _{\mathit{Gruppe}2}}}{{\displaystyle \sum _{\mathit{Gruppe}1}}}\le \mathrm{2,0},$$
  
  mit Rest Eisen und unvermeidbaren Verunreinigungen.

The above object is achieved according to a first teaching of the present invention by a shape memory alloy consisting of an alloy with the following alloy components in weight percent: 25.0% ≤ Mn ≤ 32.0%, 3.0% ≤ Si ≤ 8.0%, 3.0% ≤ Cr ≤ 10.0%, 0.1% ≤ Ni ≤ 4.0%, P ≤ 0.1%, S ≤ 0.1%, Mo ≤ 0.5%, Cu ≤ 0.5%, al ≤ 5.0%, mg ≤ 5.0%, O ≤ 0.1%, Ca ≤ 0.1%, Co ≤ 0.5%, wherein at least one element of a group 1 of elements is present, wherein the group 1 of the elements N, C, B having the following contents

• N ≤ 0.1%,
• C ≤ 0.1%,
• B ≤ 0.1%

and for the sum of the alloying components of group 1: $${\displaystyle \underset{\mathit{group}1}{\Sigma }N.C10\cdot B\ge 0.005\%}$$

and at least one element of a group 2 of elements is present, the group 2 of the elements Ti, Nb, W, V, Zr with the following contents

• Ti ≤ 1.5%,
• Nb ≤ 1.5%,
• W ≤ 1.5%,
• V ≤ 1.5%,
• Zr ≤ 1.5% and the sum of the alloying components of group 2 is: $${\displaystyle \underset{\mathit{group}2}{\Sigma }\mathit{Ti}.\mathit{Nb}.V.W.\mathit{Zr}\ge 0.1\%}$$
  
  and for the ratio of the sum of the alloy constituents of Group 1 and Group 2, both in atomic%, the following applies: $$0.5\le \frac{{\displaystyle \underset{\mathit{group}2}{\Sigma }}}{{\displaystyle \underset{\mathit{group}1}{\Sigma }}}\le 2.0.$$
  
  with residual iron and unavoidable impurities.

In addition to the mandatory components of the shape memory alloy, the alloy components Mn, Si, Cr, Ni and one of the elements of group 1 (N, C, B) and one of the elements of group 2 (Ti, Nb, W, V, Zr), the shape memory alloy In addition, optionally contain the elements P, S, Mo, Cu, Al, Mg, O, Ca or Co, which can develop beneficial effects up to the specified values. The effects on the shape memory effect affecting precipitations, the formation of which is influenced by the relationship of the two element groups, group 1 and group 2, show a significant, positive influence on the shape memory effect, provided that the sum of the constituents of the group 2 elements in atomic% of Alloy in proportion to the sum of the alloy components of Group 1 in atomic% in the range of 0.5 to 2.0. As a result, a targeted stoichiometric ratio of the elements of group 1 to the elements of group 2 is established. It has been found that it is precisely at this ratio of the alloy constituents of group 2 to group 1 that precipitation formation is particularly favorable and supports the shape memory effect. For example, if the ratio given is less than 0.5, the precipitating elements in the form of N, C and / or B can not be set and the shape memory effect is reduced since the group 1 elements are present in dissolved form in the microstructure. in the In addition, a negative effect on the reversibility of the phase transformation, that is, the return transformation of martensite into austenite, is observed. If the ratio of the sums of the alloying components formed in this way is greater than 2.0, unwanted solidifications due to the elements of the group 2 occur, which become lodged as free atoms in the microstructure and thus hinder the shape memory effect.

The manganese content of 25 wt .-% to 32 wt .-% serves to stabilize the austenite in the structure and has particular influence on the switching temperature of the shape memory material. Below an Mn content of 25.0 wt .-% ferrite is increasingly formed, which adversely affects the shape memory effect. Increasing the Mn content above 32 wt .-%, the desired switching temperature is reduced too much, so that the switching temperature and the possible operating temperatures of a corresponding component to approach too much.

Silicon serves to ensure the reversibility of the phase transformation of martensite into austenite. Below 3.0% by weight, Si causes a reduction in the shape memory effect. Above 8% by weight, embrittlement of the material can be observed. In addition, at Si contents above 8% by weight, the increased formation of the unfavorable ferritic constituents takes place.

To ensure sufficient corrosion resistance, the shape memory alloy contains at least 3.0 wt% Cr. An increase in the Cr content to above 10.0 wt .-% in turn promotes ferrite formation, which, as already stated, has a negative effect on the shape memory effect.

Finally, nickel serves to stabilize the austenitic structure and also improves the formability of the material. However, a Ni content below 0.1% by weight has no significant influence on the properties of the material. However, Ni contents of more than 4.0% by weight lead to slight improvements only in connection with an increased Cr content Cost savings of Ni content is limited to a maximum of 4.0 wt .-%. In order to ensure that the desired precipitations take place without having a negative effect on further properties of the shape memory alloy, the upper limit for all elements of group 1, ie N, C and B, is at most 0.1% by weight. The elements of group 2 (Ti, Nb, W, V, Zr) are present at a minimum level of 0.1% by weight, which applies to at least one element of this group. With a proportion by weight of at least 0.1% by weight of Ti, Nb, W, V and / or Zr, the shape memory effect is positively influenced. In particular, the reversibility of the phase transformation can be ensured by a corresponding content of one of the group 2 elements. Preferably, each individual element of group 2 does not exceed the maximum content of 1.5% by weight, more preferably the maximum content of each individual element is 1.2% by weight or 1.0% by weight, respectively This results in a reduction of unwanted solidifications.

According to a first embodiment of the shape memory alloy according to the invention, the Cr content in weight percent is 3.0% ≤ Cr ≤ 8.0%, so that a good compromise between ferrite formation and corrosion resistance of the shape memory alloy is achieved. The ferrite formation counteracts the shape memory effect, since ferrite does not undergo the desired phase transformation.

According to another aspect of the shape memory alloy, the difference of Cr content and Ni content is 0% ≦ Cr-Ni ≦ 6.0%. The maximum difference in the contents of Cr and Ni is limited to 6%. It has been found that an increase in the difference of the chromium and nickel content to more than 6 wt .-% leads to no appreciable improvements in the mechanical properties, but rather to the embrittlement of the material. On the other hand, a drop in the difference to below 0%, ie the nickel content is greater than the chromium content, can have a negative effect on the switching temperature, in which the latter is lowered and approaches the starting temperature of the material.

so dass einerseits der Formgedächtniseffekt vollständig gewährleistet werden kann und andererseits Verfestigungen aufgrund von freien Atomen der Gruppe 2 im Gefüge deutlich reduziert werden können.According to a further embodiment of the shape memory alloy, the ratio of the sum of the alloy constituents of group 1 and group 2 in atomic% applies in each case: $$0.8\le \frac{{\displaystyle \underset{\mathrm{group}2}{\Sigma }}}{{\displaystyle \underset{\mathrm{group}1}{\Sigma }}}\le 1.5.$$

so that on the one hand the shape memory effect can be completely guaranteed and on the other hand solidifications due to free atoms of group 2 in the microstructure can be significantly reduced.

• 0,005% ≤ N ≤ 0,1%, und/oder
• 0,005% ≤ C ≤ 0,1% und/oder
• 0,0001% ≤ B ≤ 0,1%

auf. Enthält die Formgedächtnislegierung die Elemente N und/oder C in Gehalten von mindestens 0,005 Gew.-% und/oder B in einem Gehalt von mindestens 0,0001 Gew.-%, kann durch die Mindestgehalte die Bildung der Ausscheidungen verbessert werden. Durch die Obergrenze von 0,1 Gew.-%, vorzugsweise von 0,05 Gew.-%, besonders bevorzugt 0,01 Gew.-% von B, wird gewährleistet, dass die Oxidationsbeständigkeit der Formgedächtnislegierung nicht zu stark herabsinkt. Gleichzeitig wird der Gehalt von N und/oder C auf maximal 0,1 Gew.-%, vorzugsweise maximal 0,07 Gew.-% beschränkt, so dass die Ausscheidungen nicht zu groß werden und diese sich negativ auf mechanischen Eigenschaften der Legierung auswirken können.A further embodiment of the shape memory alloy comprises N, C and B in the following amount in percent by weight:

• 0.005% ≤ N ≤ 0.1%, and / or
• 0.005% ≤ C ≤ 0.1% and / or
• 0.0001% ≤ B ≤ 0.1%

on. If the shape memory alloy contains the elements N and / or C in amounts of at least 0.005% by weight and / or B in a content of at least 0.0001% by weight, the minimum contents can improve the formation of the precipitates. The upper limit of 0.1% by weight, preferably 0.05% by weight, particularly preferably 0.01% by weight of B, ensures that the oxidation resistance of the shape memory alloy does not drop too much. At the same time, the content of N and / or C is limited to a maximum of 0.1% by weight, preferably a maximum of 0.07% by weight, so that the precipitates do not become too large and can adversely affect the mechanical properties of the alloy ,

• Ti ≤ 1,2 Gew.-%,
• Nb ≤ 1,2 Gew.-%,
• W ≤ 1,2 Gew.-%,
• V ≤ 1,2 Gew.-%,
• Zr ≤ 1,2 Gew.-%,

wobei bevorzugt die Obergrenze auf 1,0 Gew.-% für jedes einzelne Element der Gruppe 2 herabgesenkt wird. Die Entstehung von Verfestigungen wird hierdurch weiter verringert, so dass die Formgedächtnislegierung ein gutes Umformverhalten aufweist.In a further embodiment of the alloy, the alloy contents of the alloying elements of the group 2 elements are limited. According to this embodiment, the alloying constituents of the elements of group 2

• Ti ≦ 1.2 wt.%,
• Nb ≤ 1.2 wt%,
• W ≦ 1.2% by weight,
• V ≤ 1.2% by weight,
• Zr ≦ 1.2 wt.%,

preferably, the upper limit is lowered to 1.0% by weight for each individual Group 2 element. The formation of solidifications is thereby further reduced, so that the shape memory alloy has a good forming behavior.

Finally, according to a further embodiment of the shape memory alloy, sulfur, phosphorus and oxygen should be limited to contents of not more than 0.1% by weight, preferably not more than 0.05% by weight and more preferably not more than 0.03% by weight. to reduce their negative impact, for example on corrosion resistance. Molybdenum, copper and cobalt can be alloyed individually or in different combinations to improve the strength properties. A corresponding influence is limited in each case to contents of not more than 0.5% by weight. Aluminum and magnesium can contribute individually or in combination to improve the corrosion resistance and also cause a reduction in the density of the molten steel. Their content is limited to a maximum of 5 wt .-%, preferably to a maximum of 2.0 wt .-%, more preferably to a maximum of 1.0 wt .-%.

In another embodiment, calcium may be added to set sulfur present to avoid undesirable sulfur-manganese to MnS bonding. In order not to reduce the corrosion resistance and too large To avoid contamination by Ca, the content of Ca is limited to a maximum of 0.015 wt .-%, preferably to a maximum of 0.01 wt .-%.

Table 1 now shows various embodiments of the invention and comparative examples. In embodiments Nos. 1 to 20 of the invention could be detected at sufficiently high switching temperature sufficient shape memory effect on samples. On the one hand, the comparative examples had problems with solidification, so that the shape memory effect was reduced (Comparative Example 1). The other two Comparative Examples 2 and 3 showed a significantly weaker shape memory effect than the embodiments of the invention. The iron-based shape memory alloy according to the invention can be produced as a flat product, for example in the form of a strip and / or sheet as a long product, for example in the form of a wire, as a slab, billet or the like.

Claims (7)

1. Shape memory alloy consisting of an alloy having the following alloy constituents in % by weight:

   25.0% ≤ Mn ≤ 32.0%,

   3.0% ≤ Si ≤ 8.0%,

   3.0% ≤ Cr ≤ 10.0%,

   0.1% ≤ Ni ≤ 4.0%,

   P ≤ 0.1%,

   S ≤ 0.1%,

   Mo ≤ 0.5%,

   Cu ≤ 0.5%,

   Al ≤ 5.0%,

   Mg ≤ 5.0%,

   O ≤ 0.1%,

   Ca ≤ 0.1%,

   Co ≤ 0.5%,

   where at least one element of a group 1 of elements is present, where group 1 consists of the elements N, C, B with the following contents:

   N ≤ 0.1%,

   C ≤ 0.1%,

   B ≤ 0.1%, and, for the sum total of the alloy constituents of group 1: $${\displaystyle \sum _{\mathit{Group}1}N,C\mathrm{,10}\cdot B\ge 0.005\%}$$

   

   and at least one element of a group 2 of elements is present, group 2 consists of the elements Ti, Nb, W, V, Zr with the following contents:

   Ti ≤ 1.5%,

   Nb ≤ 1.5%,

   W ≤ 1.5%,

   V ≤ 1.5%,

   Zr ≤ 1.5%, and, for the sum total of the alloy constituents of group 2: $${\displaystyle \sum _{\mathit{Group}2}\mathit{Ti},\mathit{Nb},W,V,\mathit{Zr}\ge 0.1\%}$$

   

   and, for the ratio of the sum total of the alloy constituents of group 1 and group 2, each in atom% $$0.5\le \frac{{\displaystyle {\sum }_{\mathit{Group}2}}}{{\displaystyle {\sum }_{\mathit{Group}1}}}\le 2.0,$$

   

   the balance being iron and unavoidable impurities.
2. Alloy according to Claim 1, characterized in that the Cr content in % by weight is 3.0% ≤ Cr ≤ 8.0%.
3. Alloy according to Claim 1 or 2, characterized in that the difference between the Cr content and the Ni content is:
   0% ≤ Cr-Ni ≤ 6.0%.
4. Alloy according to any of Claims 1 to 3, characterized in that, for the ratio of the sum total of the alloy constituents of group 1 and group 2, each in atom: $$0.8\le \frac{{\displaystyle {\sum }_{\mathit{Group}2}}}{{\displaystyle {\sum }_{\mathit{Group}1}}}\le \mathrm{1.5.}$$

   
5. Alloy according to any of Claims 1 to 4, characterized in that the alloy additionally contains N, C and/or B in the following minimum amount in per cent by weight:

   0.005% ≤ N ≤ 0.1%, and/or

   0.005% ≤ C ≤ 0.1%, and/or

   0.0001% ≤ B ≤ 0.1%.
6. Alloy according to any of Claims 1 to 5, characterized in that the alloy constituents of the elements of group 2 are:

   Ti ≤ 1.2%,

   Nb ≤ 1.2%,

   W ≤ 1.2%,

   V ≤ 1.2%,

   Zr ≤ 1.2%.
7. Alloy according to any of Claims 1 to 6, characterized in that the alloy has a Ca content in % by weight of not more than 0.015%.