EP3445885B1 - Reinforcement for a breaker strip for a thermal bridge for building construction, and breaker strip for a thermal bridge comprising same - Google Patents

Description

The present invention relates to metal products for the building, and more specifically the use of certain stainless steels as reinforcing elements and connecting parts between elements forming a building.

The construction of buildings with low energy consumption requires the implementation of devices that limit heat loss integrated into the structure of the building. We can thus cite the French thermal regulation RT2012 which defines the limits of primary energy consumption of buildings. It recommends dealing with thermal bridges, i.e. areas where heat can easily pass from one element of the building to another, by using insulating connectors called "thermal bridge breakers" at these bridges. . These thermal bridge breakers make it possible to divide by three, or even more, the flow of heat loss between the elements they connect. A typical case of use of such switches is the connection between a floor and a wall which is normally covered by a layer of insulation. At the level of the wall-floor junction, this layer is interrupted, and the heat can pass through this junction without being hindered other than by the wall/floor interface from one element to another, resulting in a significant loss of heat inside the heated building, or conversely a heat input coming from outside in a building initially at moderate indoor temperature.

It is therefore known to use thermal bridge breakers which introduce a layer of insulation into a zone which is normally devoid of it, and which comprise a metal reinforcement passing through this layer of insulation to take up the mechanical forces at the level of the connection. in question.

• Des propriétés mécaniques élevées, notamment la limite d'élasticité, de façon à assurer la cohésion de la structure du bâtiment, et la ductilité de façon à pouvoir faire résister le bâtiment à des sollicitations exceptionnelles comme un tremblement de terre ;
• Une bonne tenue à la corrosion en milieu atmosphérique, en particulier à la corrosion sous contrainte ; en effet, la partie de l'armature qui traverse l'isolant n'est pas noyée dans le béton, et est donc susceptible d'être soumise à la corrosion atmosphérique ;
• Une conductivité thermique la plus basse possible, pour limiter les déperditions énergétiques à travers l'armature ;
• Un coût matière raisonnable.

These thermal bridge breaker frames must have the following properties:

• High mechanical properties, in particular the elastic limit, so as to ensure the cohesion of the structure of the building, and the ductility so as to be able to make the building withstand exceptional stresses such as an earthquake;
• Good resistance to corrosion in an atmospheric environment, in particular stress corrosion; indeed, the part of the reinforcement which passes through the insulation is not embedded in the concrete, and is therefore liable to be subjected to atmospheric corrosion;
• The lowest possible thermal conductivity, to limit energy losses through the reinforcement;
• A reasonable material cost.

Obviously, stainless steels would be good candidates to fulfill the requirements related to corrosion resistance.

The document, " Thermal Bridging Solutions: Minimizing Structural Steel's Impact on Building Envelope Energy Transfer", Jeralee Anderson et al, SEI / AISC Thermal Steel Bridging Task Committee Members, A Supplement to Modern Steel Construction, March 2012 , thus proposes to produce a frame for a thermal bridge breaker in stainless steel.

Conventionally, stainless steels have a thermal conductivity λ of the order of 15 W/(m.K), which is very advantageous compared to coated carbon steels, whose thermal conductivity is of the order of 45 W/(m.K). ). The AISC (American Institute of Steel Construction) refers, for example, to the classic austenitic stainless steels 304 and 316 in its document “Thermal Bridging Solution” (March 2012).

It remains to be seen which stainless steels would be specifically capable of fulfilling all of the aforementioned requirements in the best possible way.

From the point of view of material cost, so-called "duplex" stainless steels with low levels of alloying elements such as Ni and Mo (so-called "lean duplex" grades) are a priori more economical solutions and less subject to variations in the cost of raw materials than austenitic stainless steels, and these grades are increasingly used.

The most widely used classic stainless steel grades are the austenitic grades 1.4301 (AISI 304), 1.4597 (UGI ^® 204Cu) and the duplex grades 1.4362 and 1.4062.

We also know of documents WO 2015/1074802 A1 , WO 02/27056 A1 and WO 2014/055010 A1 austenitic-ferritic stainless steels, in particular for building materials or for electric cables.

However, the thermal regulations governing the construction of buildings impose increasingly low linear coefficients ψ at the junctions. Thus, the French regulation RT 2012 imposes a ψ of less than 0.60 W/(mK), and the future regulation RT 2020 will probably lower ψ to less than 0.22 W/(mK). Thus, manufacturers must and will have to strive to make their thermal bridge breakers even more insulating, which could be done by reducing the section of the reinforcements and/or using materials that are even weaker heat conductors than those just mentioned, and which would have a thermal conductivity λ of less than 15 W/(mK).

But these solutions present two difficulties, if we stick to the most obvious state of the art. On the one hand, a reduction in the section of the reinforcements makes them less efficient mechanically, and there is a risk that this solution is not usable so that the switch can fulfill all its functions related to the structure of the building. On the other hand, the grades known to have a thermal conductivity of less than 14 W/(m.K) are highly alloyed austenitic grades, therefore very expensive, and would be prohibitively expensive for this type of use.

• Une conductivité thermique λ de au plus 13,5 W/(m.K), de préférence au plus de 13,0 W/(m.K), mieux au plus de 12,5 W/(m.K) ;
• Des propriétés mécaniques correspondant à celles requises pour cette application, à savoir notamment une limite élastique Rp_0,2 d'au moins 600 Mpa, mieux d'au moins 700 MPa, et un allongement total plastique sous charge maximale Agt d'au moins 5% après une mise en forme à froid ; on rappelle que Agt, auquel il est courant de se référer dans l'industrie du bâtiment, est la somme de l'allongement élastique et de l'allongement plastique à la charge d'essai maximale ;
• Une bonne aptitude à la mise en forme à chaud et à froid ;
• Un coût matière comparable à celui d'une nuance « lean duplex » 1.4362, voire inférieur.

The object of the invention is to provide thermal bridge breakers in which the material constituting the metal reinforcement best meets the various requirements which have been cited. The frame material should have:

• A thermal conductivity λ of at most 13.5 W/(mK), preferably at most 13.0 W/(mK), better still at most 12.5 W/(mK);
• Mechanical properties corresponding to those required for this application, namely in particular an elastic limit Rp _0.2 of at least 600 MPa, better still of at least 700 MPa, and a total plastic elongation under maximum load Agt of at least 5 % after cold forming; it is recalled that Agt, to which it is common to refer in the construction industry, is the sum of the elastic elongation and the plastic elongation at the maximum test load;
• Good aptitude for hot and cold forming;
• A material cost comparable to that of a “lean duplex” 1.4362 grade, or even lower.

• traces ≤ C ≤ 0,08% ; de préférence 0,01 ≤ C ≤ 0,04% ;
• 1,5% ≤ Si ≤ 4,0% ; de préférence 2,0% ≤ Si ≤ 3,0% ;
• 4,0% ≤ Mn ≤ 10,0% ;
• traces ≤ Ni ≤ 7,0% ; de préférence traces ≤ Ni ≤ 5,0% ;
• 16,0% ≤ Cr ≤ 23,0% ;
• traces ≤ Mo ≤ 2,0% ;
• traces ≤ W ≤ 1,0% ;
• traces ≤ Mo + W/2 ≤ 2,0% ;
• traces ≤ Co ≤ 2,0%: de préférence traces ≤ Co ≤ 0,8%
• traces ≤ Cu ≤ 3,0% ;
• 0,10% ≤ N ≤ 0,25% ;

le reste étant du fer, des éléments d'alliage autres que ceux précédemment mentionnés et des impuretés résultant de l'élaboration, le total de ces autres éléments d'alliages et des impuretés ne dépassant pas 1,0%, et aucun de ces autres éléments d'alliage n'étant présent individuellement à une teneur supérieure à 0,5%, et en ce que l'indice de conductivité thermique IC calculé selon :
IC = 22,2 + 2,11 (1 - IF/100) - 0,89 Si - 0,77 Ni - 0,44 Mn - 0,17 Cr - 0,16 Cu avec IF = 6,7 Cr + 5,7 Mo + 10,7 Si - 8,6 Ni - 2,4 Mn - 0,5 Cu - 110 C - 150 N - 42,7 est ≤ 13,5, de préférence ≤ 13,0, mieux ≤ 12,5.To this end, the subject of the invention is a thermal bridge breaker frame for the construction of buildings, characterized in that it is made of an austenitic or austenitic-ferritic stainless steel whose composition, in% by weight, consists of :

• traces ≤ C ≤ 0.08%; preferably 0.01≤C≤0.04%;
• 1.5% ≤ If ≤ 4.0%; preferably 2.0% ≤ Si ≤ 3.0%;
• 4.0% ≤ Mn ≤ 10.0%;
• traces ≤ Ni ≤ 7.0%; preferably traces ≤ Ni ≤ 5.0%;
• 16.0%≤Cr≤23.0%;
• traces ≤ MB ≤ 2.0%;
• traces ≤ W ≤ 1.0%;
• traces ≤ Mo + W/2 ≤ 2.0%;
• traces ≤ Co ≤ 2.0%: preferably traces ≤ Co ≤ 0.8%
• trace ≤ Cu ≤ 3.0%;
• 0.10%≤N≤0.25%;

the balance being iron, alloying elements other than those previously mentioned and impurities resulting from working, the total of these other alloying elements and impurities not exceeding 1.0%, and none of these other alloying elements not being present individually at a content greater than 0.5%, and in that the thermal conductivity index IC calculated according to:
IC = 22.2 + 2.11 (1 - IF/100) - 0.89 Si - 0.77 Ni - 0.44 Mn - 0.17 Cr - 0.16 Cu with IF = 6.7 Cr + 5 .7 Mo + 10.7 Si - 8.6 Ni - 2.4 Mn - 0.5 Cu - 110 C - 150 N - 42.7 is ≤ 13.5, preferably ≤ 13.0, better ≤ 12, 5.

The Cr content of the steel can be between 16.0% and 20.0%.

The Cr content of steel can be between 20.0% and 23.0%

The Ni content of the steel may be between 1.0% and 7.0%, preferably between 2.0% and 5.0%.

The ferritic index IF of the steel calculated according to:
IF = 6.7 Cr + 5.7 Mo + 10.7 Si - 8.6 Ni - 2.4 Mn - 0.5 Cu - 110 C - 150 N - 42.7 can be ≤ 20.

The ferritic index IF of the steel calculated according to:
IF = 6.7 Cr + 5.7 Mo + 10.7 Si - 8.6 Ni - 2.4 Mn - 0.5 Cu - 110 C - 150 N - 42.7 can be ≥ 40 and ≤ 70.

Among said other alloying elements may be at least one of Al, Ti, Nb, V, Ca and B, Al, Ti, Nb and V may each be present at a rate of at most 0.5% and Ca and B may each be present at most 0.05%.

The yield strength Rp0.2 can be greater than or equal to 600 MPa with a total elongation under maximum load Agt greater than or equal to 5%.

The thermal bridge breaker reinforcement can be obtained from a bar, a wire or a sheet

The invention also relates to a thermal bridge breaker for the construction of buildings, comprising an armature and a layer of insulation through which said armature passes, characterized in that said armature is made as stated previously.

As will have been understood, the invention is based on the use, to manufacture a metal frame for a thermal bridge breaker between two elements of a building (wall and floor, for example), of a grade of stainless steel of austenitic or austenitic-ferritic structure whose chemical composition is not strictly speaking new, in that steels which could sometimes conform to it had already been used in the past (see documents US-A-4,814,140 and WO94/04714 for example), but whose suitability for this use, in the precise range of compositions of the invention, had never been recognized.

• La figure 1 qui montre schématiquement en coupe longitudinale une zone de raccordement entre une façade et un plancher d'un bâtiment, dans laquelle, classiquement, on n'a pas placé de dispositif de rupture du pont thermique ;
• La figure 2, qui montre schématiquement en coupe longitudinale une zone de raccordement entre une façade et un plancher d'un bâtiment, dans laquelle on a placé un dispositif de rupture du pont thermique qui peut être réalisé selon l'invention ;
• La figure 3 qui montre la corrélation entre l'indice IF calculé traduisant la fraction de ferrite présente à 1100°C de l'invention et la fraction de ferrite effectivement mesurée par sigmamétrie ;
• La figure 4 qui montre la corrélation entre l'indice IC calculé traduisant la conductivité thermique de l'acier et la conductivité thermique λ effectivement mesurée à température ambiante par la méthode dite « hot disk » (disque chaud) qui utilise la technique de la source plane transitoire ;
• Les figures 5 et 6 qui montrent, sur des coulées de laboratoire, la ductilité à chaud à 1200°C (figure 5) et 1100°C (figure 6) des aciers testés, traduite par leur striction en %, en fonction de leur indice ferritique IF.

The invention will be better understood on reading the following description, given with reference to the following appended figures:

• The figure 1 which schematically shows in longitudinal section a connection zone between a facade and a floor of a building, in which, conventionally, no device for breaking the thermal bridge has been placed;
• The picture 2 , which schematically shows in longitudinal section a connection zone between a facade and a floor of a building, in which a device for breaking the thermal bridge which can be produced according to the invention has been placed;
• The picture 3 which shows the correlation between the calculated IF index reflecting the fraction of ferrite present at 1100° C. of the invention and the fraction of ferrite actually measured by sigmametry;
• The figure 4 which shows the correlation between the calculated IC index reflecting the thermal conductivity of the steel and the thermal conductivity λ actually measured at room temperature by the so-called “hot disk” method which uses the transient planar source technique;
• The figures 5 and 6 which show, on laboratory castings, the hot ductility at 1200°C ( figure 5 ) and 1100°C ( figure 6 ) of the steels tested, translated by their necking in %, according to their ferritic index IF.

The figures 1 and 2 recall, schematically, what is the problem that the invention intends to solve.

The figure 1 represents a connection zone between a facade 1 and a floor 2 of a building of conventional design, for which no attempt has been made to optimize the performance in terms of thermal insulation between the external environment 3 and the interior of the building 4 The interior side of the facade 1 is, of course, provided with an insulating coating 5. But this is interrupted at the level of the junction between the facade 1 and the floor 2, so that these two elements are in contact. direct and that the heat can pass from the interior to the exterior of the building (or vice versa) by crossing this contact zone (which is illustrated by the arrows in the figure 1 ). Conventional building materials impose a linear loss coefficient ψ at this junction which is of the order of 1 W/(mK).

The figure 2 schematically represents the same building equipped with a thermal bridge breaker at the facade 1-floor 2 junction. This breaker comprises, in known manner, an insulating layer 6 between the facade and the floor which replaces the usual direct contact between these two parts, and a metal reinforcement 7 which connects the facade 1 and the floor 2 by crossing the insulating layer 6. In this way, the coefficient ψ is lowered, and the absorption of the forces by the reinforcement 7 ensures the mechanical functions that the insulator 6 alone could not fulfill. It is this reinforcement 7 that the invention aims to improve with respect to known devices, by giving it particularly favorable mechanical and, above all, thermal properties, without it being necessary to modify the configuration of the reinforcement. This optimization is achieved by the choice of a particular grade of stainless steel which, at first glance, did not indicate that it would have been suitable for this purpose.

The limits set for the contents of the various elements present, on a mandatory, optional or imposed basis, are justified as follows. All contents are given in % by weight.

The C content is between traces and 0.08%, better still between 0.01% and 0.04%. A higher content would increase the risks of sensitizing the alloy to intergranular corrosion. A C content of less than 0.01 is difficult and costly to obtain industrially.

The Si content is between 1.5 and 4.0%, preferably between 2.0 and 3.0%. Si is an alphagenic element (promoting the stability of ferrite), and is acceptable as long as it is not present in too great a quantity to upset the desired balance between austenite and ferrite. Adding more than 4.0% would degrade the toughness of the steel too much, and it is preferable, from this point of view, not to exceed a content of 3.0%.

It is for the adjustment of thermal conductivity that Si is of particular interest. The tests which will be presented below show that an Si content in the prescribed range, and more particularly between 2.0 and 3.0%, makes it possible to lower the thermal conductivity of the steel of the invention up to 12 to 13.5 W/(m.K) approximately, whereas the steels usually used to make the reinforcements of the junctions of thermal bridges have thermal conductivities greater than 14 W/(m.K), often of the order of 15 W/( m.K) or more. Above 3.0% Si, however, a reduction in the toughness of the steel begins to be observed, which becomes inadequate above 4.0% Si.

The Mn content is between 4.0 and 10.0%. A large proportion of this cheap element is added which stabilizes the austenite and can, advantageously from the financial point of view, partially or totally replace Ni for this function. In addition, Mn increases the solubility of N in liquid steel, and as it will be seen that relatively large quantities of N are required in the invention, the production of steel is facilitated by the large presence of Mn.

The Ni content is between traces and 7.0%, preferably between traces and 5.0%. Ni is the gammagenic element typically used in the manufacture of austenitic stainless steels, and its content makes it possible to adjust the balance of the austenitic and ferritic phases to obtain the desired mechanical properties. However, Ni is an expensive element anyway, and whose price is likely to fluctuate in large proportions. To obtain a steel at a limited and relatively predictable cost price, which is one of the objectives of the invention, it is therefore necessary not to exceed the aforementioned values for the Ni content. In fact, Ni may even be present only in the form of traces, that is to say at a low or very low content which only results from the melting of the raw materials and not from a voluntary addition. Its usual gammagenic role is then assumed entirely by manganese, carbon, nitrogen and possibly copper.

However, as will be seen, Ni is an element which strongly tends to reduce the thermal conductivity of steel. From this point of view, an important advantage can be found in adding a significant quantity of it and, therefore, in not replacing it completely with Mn. However, it is difficult to set an optimum quantity of Ni in the grade used according to the invention, as this optimum will depend in particular on financial factors, which are liable to vary greatly according to the price of Ni. A balance will have to be found by those skilled in the art at the time of the manufacture of the steel, between purely technical considerations and financial considerations. It is generally considered that from a metallurgical and thermal point of view the Ni content is preferably at least 1.0%, more preferably at least 2.0%. Accordingly, particularly preferred ranges of the Ni content are 1.0 to 7%, more preferably 2.0 to 5.0%.

The Cr content is between 16.0 and 23.0%. As is well known, it gives steel its stainless character from 11%. Cr also has the advantage of slightly lowering the thermal conductivity of the steel, and a minimum content of 16.0% is required according to the invention to properly combine these two effects. A content less than or equal to 20.0% makes it possible to maintain the desired phase balance without proceeding to an excessive addition of Ni, Mn and other gammagenic elements. A content of 20.0% to 23.0% makes it possible to significantly increase the corrosion resistance and can be imposed, possibly by compensating for the effect of the increase in the Cr content on the mechanical properties by adjusting the contents of Mn, Ni and N that routine experiments make it possible to achieve. A Cr content higher than 23.0% unnecessarily increases the cost of the steel and would risk degrading certain mechanical properties too much.

In other words, above 20.0% preference is given to the corrosion resistance of the grade. Below 20.0%, the economic nature of the grade is preferred.

The Mo content is between traces resulting from the elaboration and 2.0%. This element is not essential, but it helps to improve resistance to corrosion. Its possible drawbacks are its alphagenic nature which risks preventing the achievement of the desired austenite-ferrite balance, in particular on the austenitic-ferritic grades, and the fact that it promotes the appearance of embrittling intermetallic phases. Moreover, its cost is high, which runs counter to one of the aims of the invention.

For the improvement of the corrosion resistance, the Mo can be partially or completely substituted by W. A substitution ratio of W/Mo of 2 is generally acceptable. Consequently, it is also considered that on the one hand, the content of W must not exceed 1.0%, and on the other hand that the sum Mo+W/2 must not exceed 2.0%. A Mo content of 2.0% would correspond to a case where W was not added voluntarily and where the possible presence of traces of W would only result from the melting of the raw materials. A W content of 1.0% would correspond to a case where Mo was not added voluntarily and where the possible presence of traces of Mo would only result from the melting of the raw materials.

The Cu content is between traces resulting from the mere melting of the raw materials and 3.0%. Adding Cu in the proportions mentioned has the advantages of slightly reducing the thermal conductivity and improving the ductility. But an addition of 3.0% should not be exceeded, because beyond that, the embrittling effect of the Cu would cause problems during hot forming, and moreover would unnecessarily increase the cost of the steel.

The Co content is between traces resulting from the sole fusion of very pure raw materials and 2.0%. Depending on the purity of the raw materials, in particular ferronickel, the residual Co content can reach 0.8%. It is preferred not to add Co voluntarily, as this costly element has no marked metallurgical effect in stainless steels below 2%, therefore for contents which would considerably increase the cost of the steel. 0.8% is therefore the maximum preferential content of Co.

The N content is between 0.10% (1000 ppm) and 0.30% (3000 ppm). This element is important to ensure the corrosion resistance necessary in the application targeted by the invention, and if its content which would simply result from the absorption of atmospheric nitrogen during production is not high enough, it must be added, for example by blowing nitrogen gas into the liquid metal or by using significantly nitrided ferroalloys (in particular nitrided ferromanganese which contains several % of N). N stabilizes the austenitic phase and makes it possible to adjust the balance of the various phases present. It also has an interesting hardening effect for achieving the desired high mechanical properties. But beyond 0.30%, it can cause problems during production, casting and hot rolling (formation of nitrides in the presence of alloying elements such as Al and especially Ti, and blowholes during solidification).

Other additional alloying elements may be present following a voluntary addition, among which may be mentioned, in a non-exhaustive manner: Ti, Nb and V to improve weldability, Al and Ca as deoxidizers and/or temperature control elements. number and composition of non-metallic inclusions, as well as B which improves forgeability. But the individual contents of these additional alloying elements must not exceed 0.5%, in particular for Al, Ti, Nb and V, and more particularly must not exceed 0.05% for Ca and B. And the sum of the content of alloying elements other than C, Si, Mn, Cr, Ni, Mo, W, Cu, Co, N and the content of impurities resulting from the production (for example S, P, etc.) does not exceed 1.0%. These limits aim to avoid the risk of disturbing the balances that the contents of the main alloying elements, obligatorily or optionally present within well-defined limits, make it possible to achieve.

Other conditions linking the contents of alloying elements are to be observed, according to the invention.

One of the objectives of the invention, as has been said, is to obtain a thermal bridge breaker armature element having low thermal conductivity. This depends on the chemical analysis of the steel, and the crystallographic structure of the matrix.

The crystallographic structure of the steel is also an important factor in the ability of the steel to be hot shaped, by forging or otherwise. Since the armatures of thermal breaks can have relatively complex shapes for relatively small dimensions, this ability to be hot-shaped is a criterion which is often to be considered for the steels used in the invention.

Depending on the balance of the main alloying elements which have been defined above, the steel has an austenitic or austenitic-ferritic microstructure. The ferritic index IF makes it possible to estimate the percentage of ferrite at 1100°C in the steel, therefore in the temperature zone most frequently encountered during hot forming, from the composition of the steel. It is obtained by the formula, where the contents of the various elements are expressed in %: $$\mathrm{IF}=6.7\mathrm{CR}+5.7\mathrm{Mo}+10.7\mathrm{Whether}-8.6\mathrm{Neither}-2.4\min -0.5\mathrm{Cu}-110\mathrm{VS}-150\mathrm{NOT}-42.7$$

For austenitic grades, IF is, according to the invention, preferably ≤ 20 if good hot formability is desired.

For austenitic-ferritic grades, IF is, according to the invention, preferably ≥ 40 if it is desired to obtain good hot formability.

It will be seen later that the FI zone comprised between 20 and 40 is rather to be avoided if it is desired to obtain high hot formability, as the invention may require.

Furthermore, to obtain satisfactory stress corrosion resistance, it is preferable to choose, rather than an austenitic grade, an austenitic-ferritic grade which is characterized by an IF of 40 to 70 at most. Beyond this limit, the steel would fall within the domain of ferritic steels, which is not desired from the point of view of the mechanical characteristics.

The picture 3 shows the correlation between the ferrite fraction at 1100°C measured by a magnetic method (known as sigmametry and as described in standard IEC 60404-14) and the ferritic index IF calculated by the previous formula, for the eight laboratory samples A to H and the industrial sample I of table 3. It can be seen that this correlation is very satisfactory.

The conditions on IF which were quoted above are, however, not absolutely imperative, in particular in the cases where the hot transformations are not very constraining. They are nevertheless recommended for most of the contact breaker armature configurations that one may wish to obtain.

It is interesting to note that the microstructures of the steels used in the invention are relatively little dependent on the heat treatment and cooling conditions of the metal during its transformations. This therefore leaves a lot of freedom to the metallurgists to design the precise method of manufacturing the reinforcements of the invention.

Concerning the thermal conductivity λ of steel, as we said it depends on the chemical composition and the crystallographic structure of the matrix.

The inventors have been able to determine a formula giving a conductivity index IC depending on the composition of the steel, and also on its microstructure since it involves the ferritic index IF defined above. This formula is (the contents of the different elements are expressed in %): $$\mathrm{CI}=22.2+2.11\left(1-\mathrm{IF}/100\right)-0.89\mathrm{Whether}-0.77\mathrm{Neither}-0.44\min -0.17\mathrm{CR}-0.16\mathrm{}\mathrm{Cu}$$

The figure 4 shows the good correlation obtained between IC calculated by the formula above and the thermal conductivity λ actually measured at 20°C by the so-called “hot disk” method which uses the transient plane source technique, on the thirteen samples in tables 1 and 2. This figure, and the tables on which it is based, also show that the thermal conductivity decreases when the quantity of alloying elements increases, and that Si in the first place and Ni in the second place are the most influential elements from this point of view. This is reflected in the above formula for calculating IC. According to the invention, the IC index of the steel used must be ≤ 13.5, preferably ≤ 13.0, better still ≤ 12.5.

The mechanical properties of the steels used in the invention prove to be sufficient for the application envisaged, in particular because of the high N content and the percentage of austenite which is always at least 40%. The N content and the percentage of austenite according to the invention provide the desired ductility both for the ease of hot transformations and for the capacity of the reinforcement to deform during exceptional stresses such as an earthquake. . The best ductilities are obtained for austenitic grades.

By way of example, laboratory castings according to the invention, referenced A to H, and reference castings, the compositions of which appear in Tables 1 to 2 which follow, in the form of ingots of 25 kg cross-section were developed. initial 100mm x 100mm and which have been hot transformed by forging to a thickness of 18 mm from 1250°C, then hot rolling to a thickness of 6 mm from 1250°C. A solution treatment was carried out at 1050°C, then milling to adapt the thickness, before cold transformation to a thickness of 3 mm. Microstructure, forgeability, thermal conductivity and other mechanical properties were characterized on all samples.

An industrial casting I according to the invention of 40 t was also developed by melting in an electric furnace, decarburization by the AOD process, continuous casting in blooms of 205 mm side and hot rolling in round bars with a diameter of 115 mm, then in wire rod with a diameter of approximately 10.5 mm. The wire rod was cold transformed into notched wire of 10 mm in diameter, at a reduction rate of 10 to 15%.

In the table grouping together the compositions of the different samples, the elements not mentioned are only present in trace amounts. Austenitic structures are designated by A, austenitic-ferritic structures are designated by AF.

Sample G is a sample in accordance with the invention. Indeed, its composition means that its thermal conductivity λ meets the broadest requirements set by the inventors: measured λ is 13.3 W/(m.K), which is very well correlated with the calculated IC which is 13.4 (for a maximum of 13.5 according to the invention, which already constitutes a significant progress compared to the most current prior art to ensure, in an economic way, the respect of the energy standards present and probably to come). This sample is poor in Cu and contains relatively little Ni and Si, hence its higher thermal conductivity than what the optimal variants of the invention make it possible to obtain, even if the individual contents of each of its elements are entirely in accordance with the corresponding requirements of the invention. It confirms that the composition of the steel to be used to implement the invention must imperatively be considered as a whole, as a coherent whole.

Furthermore, samples A to I, in accordance with the invention, have mechanical properties which are not inferior to those of the reference steel ^UGI® 204Cu, except for the degree of elongation Agt. But this one remains at values acceptable for the intended application, and several of the samples even have tensile strengths Rm and yield strengths Rp0.2 significantly higher than those of the reference steel. It will also be noted that sample B has an Agt of 6%, therefore slightly higher than the 5% that the inventors consider to be the minimum value to be obtained. But on the other hand, this sample B has a very high Rm and Rp0.2 and an IC which is the lowest of those calculated. This steel can therefore constitute a very satisfactory solution to the problems posed, at least for manufacturing frames of thermal breakers whose shapes are not too complex.

Furthermore, the figures 5 and 6 show the results of forgeability tests, therefore representative of hot ductility, carried out at 1200°C ( figure 5 ) and at 1100°C ( figure 6 ) on the aforementioned laboratory samples A to G. Their rate of necking was measured as a function of their ferritic index IF.

It emerges from these figures that these samples according to the invention have a ductility which is not very favorable when IF is between 20 and 40%, therefore corresponds to an austenitic-ferritic steel whose ferritic character is not yet very Mark. This "ductility dip" therefore justifies that, according to preferred variants of the invention, it is advisable either to use a frankly austenitic steel (with less than 20% ferrite, this percentage being calculated by the IF index of which one has since it took into account the real percentage of ferrite reasonably well, at least for the steels used), or to use an austenitic-ferritic steel containing between 40 and 70% ferrite according to the IF index.

As we have seen, the invention makes it possible to substantially improve the thermal insulation performance of stainless steel thermal bridge breakers, and this without having to sacrifice the mechanical properties of the usual stainless steel breakers, on the contrary. Certain variants of the invention have a particularly high hot-formability, which gives access to forms of armatures for thermal bridge breakers which were not easily conceivable hitherto. The constructors of buildings with low energy consumption therefore have, thanks to the invention, the possibility of exploiting new designs of thermal bridge breakers, which could be advantageous.

Claims (10)

1. -Thermal break reinforcement (7) for building construction, characterised in that it is made of an austenitic or austeno-ferritic stainless steel whose composition, in % by weight, consists of:

   - traces ≤ C ≤ 0.08%; preferably 0.01 ≤ C ≤ 0.04%;

   - 1.5% ≤ Si ≤ 4.0%; preferably 2.0% ≤ Si ≤ 3.0%;

   - 4.0% ≤ Mn ≤ 10.0%;

   - traces ≤ Ni ≤ 7.0%; preferably traces < Ni ≤ 5.0%;

   - 16.0% ≤ Cr ≤ 23.0%;

   - traces ≤ Mo ≤ 2.0%;

   - traces ≤ W ≤ 1.0%;

   - traces ≤ Mo + W/2 ≤ 2.0%;

   - traces ≤ Co ≤ 2.0%; preferably traces < Co ≤ 0.8%;

   - traces ≤ Cu ≤ 3.0%;

   - 0,10% ≤ N ≤ 0,25%;

   the balance being iron, alloying elements other than those mentioned above and impurities resulting from the smelting process, the total of these other alloying elements and impurities not exceeding 1.0%, and none of these other alloying elements being individually present in a content exceeding 0.5%, and in that the thermal conductivity index IC calculated according to:
   IC = 22.2 + 2.11 (1 - IF/100) - 0.89 Si - 0.77 Ni - 0.44 Mn - 0.17 Cr - 0.16 Cu with IF = 6.7 Cr + 5.7 Mo + 10.7 Si - 8.6 Ni - 2.4 Mn - 0.5 Cu - 110 C - 150 N - 42.7 is ≤ 13.5, preferably ≤ 13.0, better ≤ 12.5.
2. - The thermal break reinforcement (7) according to claim 1, characterised in that the Cr content is between 16.0% and 20.0%.
3. - The thermal break reinforcement (7) according to claim 1, characterised in that the Cr content is between 20.0% and 23.0%.
4. - The thermal break reinforcement (7) according to one of the claims 1 to 3, characterised in that the Ni content of the steel is between 1.0% and 7.0%, preferably between 2.0% and 5.0%.
5. - The thermal break reinforcement (7) according to one of the claims 1 to 4, characterised in that the ferritic index IF of the steel is calculated according to:
   IF = 6.7 Cr + 5.7 Mo + 10.7 Si-8.6 Ni -2.4 Mn - 0.5 Cu - 110 C - 150 N - 42.7 is ≤ 20.
6. - The thermal break reinforcement (7) according to one of the claims 1 to 4, characterised in that the ferritic index IF of the steel is calculated according to:
   IF = 6.7 Cr + 5.7 Mo + 10.7 Si-8.6 Ni - 2.4 Mn - 0.5 Cu - 110 C - 150 N - 42.7 is ≥ 40 and ≤ 70.
7. - The thermal break reinforcement (7) according to any of claims 1 to 6, characterised in that said other alloying elements include at least one of Al, Ti, Nb, V, Ca and B, in that Al, Ti, Nb and V may each be present in an amount of at most 0.5% and in that Ca and B may each be present in an amount of at most 0.05%.
8. - The thermal break reinforcement (7) according to one of claims 1 to 7, characterised in that the yield strength Rp0.2 is greater than or equal to 600 MPa or better than or equal to 700 MPa with a total elongation under maximum load Agt greater than or equal to 5%.
9. - The thermal break reinforcement (7) according to one of claims 1 to 8, characterised in that it is obtained from a bar, a wire or a sheet
10. - A thermal break for building construction, comprising a reinforcement (7) and a layer of insulation (6) through which said reinforcement (7) passes, characterised in that said reinforcement (7) is made according to one of claims 1 to 9.

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