FI3445885T3 - Reinforcement for a breaker strip for a thermal bridge for building construction, and breaker strip for a thermal bridge comprising same - Google Patents

Description

Reinforcement for a breaker strip for a thermal bridge for building construction, and breaker strip for a thermal bridge comprising same

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

The construction of low-energy buildings requires the use of heat loss control devices integrated into the building structure. One example is the French thermal regulation RT2012, which sets limits on the primary energy consumption of buildings. It recommends that thermal bridges, i.e. areas where heat can easily pass from one part of the building to another, should be dealt with by using insulating connectors called “thermal bridge breaker strips” at these bridges. These thermal bridge breaker strips enable the flow of heat loss between the elements they connect to be divided by three or more. A typical case of use is the connection between a floor and a wall which is normally covered by a layer of insulation. At the wall/floor junction, this layer is interrupted, and heat can pass through this junction without being impeded other than by the wall/floor interface from one element to the other, resulting in significant heat loss inside the heated building, or conversely, heat gain from the outside in a building with an initially moderate indoor temperature.

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

These thermal break reinforcements must have the following properties: - High mechanical properties, including yield strength, so as to ensure the cohesion of the building structure, and ductility so as to be able to withstand exceptional loads such as an earthquake; - Good resistance to corrosion in an atmospheric environment, in particular to stress corrosion; in fact, the part of the reinforcement that passes through the insulator is not embedded in the concrete, and is therefore susceptible to atmospheric corrosion; - Lowest possible thermal conductivity, to limit energy loss through the reinforcement; - Reasonable material cost.

Clearly, stainless steels would be good candidates to meet the corrosion resistance requirements.

The paper, “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 Modem Steel Construction, March

2012, thus proposes to make a stainless steel reinforcement for a thermal bridge breaker strip.

Typically, stainless steels have a thermal conductivity À on 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 American Institute of Steel Construction (AISC), for example, refers to the classic austenitic stainless steels 304 and 316 in its document “Thermal Bridging Solution” (March 2012).

It remains to be seen which specific stainless steels would be best able to meet all the above reguirements.

From a material cost point of view, so-called “duplex” stainless steels with low content 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 raw material costs than austenitic stainless steels, and these grades are increasingly used.

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

Also known from WO 2015/1074802 A1, WO 02/27056 A1 and WO 2014/055010 A1 are austenitic-ferritic stainless steels, in particular for building materials or for electrical cables.

However, thermal regulations governing the construction of buildings impose increasingly low linear y coefficients at junctions. For example, the French RT 2012 regulation requires a w of less than 0.60 W/(m K), and the future RT 2020 regulation will probably lower w to less than 0.22 W/(m.K). Thus, manufacturers must and should strive to make their thermal breaker strips even more insulating, which could be done by reducing the cross-section of the reinforcement and/or by using materials that are even less thermally conductive than those just mentioned, and which would have a thermal conductivity A of less than 15 W/(m.K).

However, there are two difficulties with these solutions, based on the most obvious state of the art. On the one hand, a reduction in the cross-section of the reinforcement makes it less efficient mechanically, and there is a risk that this solution will not be usable for the breaker strip to fulfill all its functions related to the building structure. On the other hand, grades known to have a thermal conductivity of less than 14 W/(m.K) are highly alloyed austenitic grades, and therefore very expensive, and would be prohibitively expensive for this type of use.

The aim of the invention is to offer thermal breaker strips whose metal reinforcement material best meets the various requirements mentioned above. The reinforcement material should have:

- A thermal conductivity À of not more than 13.5 W/(m.K), preferably not more than 13.0 W/(m.K), more preferably not more than 12.5 W/(m.K); - Mechanical properties corresponding to those reguired for this application, namely a yield strength Rpo. of at least 600 MPa, preferably 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, which is commonly referred to in the construction industry, is the sum of the elastic elongation and the plastic elongation at the maximum test load; - Good hot and cold forming skills; - A material cost comparable to that of a lean duplex 1.4362 grade, or even — lower.

To that end, an object of the invention is a thermal break reinforcement for building construction, characterized 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%; - 1.590 < If < 4.0%; preferably 2.0% < If < 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%; - 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.

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

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

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

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 can be < 20.

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 can be > 40 and < 70.

Said other alloying elements may include at least one of Al, Ti, Nb, V, Ca and B, Al,

Ti, Nb and V may each be present in an amount of at most 0.5% and Ca and B may each be present in an amount of at most 0.05%.

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

The thermal break reinforcement can be obtained from a bar, wire, or sheet.

A further object of the invention is a thermal break for building construction, comprising a reinforcement and a layer of insulation through which said reinforcement passes, characterized in that said reinforcement is made as previously stated.

As will have been understood, the invention is based on the use, for the manufacture of a metal reinforcement for a thermal break between two elements of a building (wall and floor, for example), of a grade of stainless steel with an austenitic or austeno-ferritic structure, the chemical composition of which is not strictly speaking new, in that steels which could sometimes comply with 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 compositional range of the invention, had never been recognised.

The invention will be better understood on reading the following description, given with reference to the following attached figures: - Figure 1, which schematically shows in longitudinal section of a connection area between a façade and a floor of a building, in which, conventionally, no thermal break has been placed; - Figure 2, which schematically shows in longitudinal section a connection area between a facade and a floor of a building, in which a thermal break device that can be made according to the invention has been placed; - Figure 3 shows the correlation between the calculated IF index reflecting the ferrite fraction present at 1,100 *C of the invention and the ferrite fraction actually measured by sigmametry; - Figure 4 which shows the correlation between the calculated IC index reflecting the thermal conductivity of the steel and the thermal conductivity A actually measured at room temperature by the so-called “hot disk” method using the transient plane source technique; - Figures 5 and 6 show, on laboratory castings. the hot ductility at 1,200°C (Figure 5) and 1,100°C (Figure 6) of the tested steels, translated by their shrinkage into a %, expressed as a function of their ferritic index IF. 5 Figures 1 and 2 show, schematically, the problem that the invention intends to solve.

Figure 1 shows a connection area between a façade 1 and a floor 2 of a conventionally designed building, where no attempt has been made to optimize the thermal insulation performance between the external environment 3 and the interior of the building 4. The inner side of the façade 1 is provided with an insulating coating 5. However, this is interrupted at the junction between the façade 1 and the floor 2, so that these two elements are in direct contact and heat can pass from the inside to the outside of the building ( or vice versa) through this contact area (as illustrated by the arrows in Figure 1). Conventional building materials impose a linear heat loss coefficient w at this junction which is on the order of 1 W/(m.K).

Figure 2 shows the same building eguipped with a thermal break at the junction of facade 1 and floor 2. This breaker comprised, in a known way, 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 y is lowered, and the stresses taken up by the reinforcement 7 ensure the mechanical functions that the insulator 6 alone could not fulfill. It is this reinforcement 7 that the invention aims to improve compared 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 choosing a particular grade of stainless steel which at first sight was not suitable for this purpose.

The limits set for the proportions of the various elements present, whether mandatory, optional or suffered, are justified as follows All proportions are given in % by weight.

The C proportion is between traces and 0.08%, preferably between 0.01% and 0.04%.

A higher proportion would increase the risk of sensitization of the alloy to intergranular corrosion. A C proportion of less than 0.01 is difficult and expensive to obtain industrially.

The Si content is between 1.5 and 4.0%, preferably between 2.0 and 3.0%. If is an alphagenic element (promoting ferrite stability), and is acceptable as long as it is not present in such large quantities as to upset the desired balance between austenite and ferrite. 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 3.0%.

If is of particular interest for the adjustment of thermal conductivity. The tests that will be presented below show that a Si proportion 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 to approximately 12 to 13.5 W/(m.K) , whereas the steels usually used to make the reinforcements of thermal bridge junctions have thermal conductivities of more than 14

W/(m.K), often of the order of 15 W/(m.K) or more. Above 3.0% Si, however, a decrease in steel toughness begins to be observed, which becomes inadequate above 4.0% Si.

The Mn content is between 4.0 and 10.0%. A significant proportion of this cheap element is added, which stabilizes the austenite and can, from a financial point of view, partially or totally replace Ni for this function. In addition, Mn increases the solubility of N in the liquid steel, and as it will be seen that relatively large amounts of N are required in the invention, steelmaking 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 gamma element typically used in the manufacture of austenitic stainless steels, and its proportion is able to balance the austenitic and ferritic phases to be adjusted to obtain the desired mechanical properties. However, Ni is an expensive item anyway, and its price is — likely to fluctuate widely. In order to obtain a steel with a limited and relatively predictable cost price, which is one of the objectives of the invention, it is therefore necessary not to exceed the above-mentioned values for the Ni content. In fact, Ni may even be present only in trace amounts, i.e. at a low or very low level that results only from the melting of the raw materials and not from a deliberate 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 that strongly tends to reduce the thermal conductivity of steel. From this point of view, there is a significant advantage in adding a significant amount of Mn and therefore not replacing it entirely with Mn. However, it is difficult to fix an optimum guantity of Ni in the grade used according to the invention, as this optimum will depend in particular on financial factors, which are likely to vary greatly according to the price of Ni. A balance will have to be found by the skilled person at the time of steelmaking, between purely technical 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%, preferably at least 2.0%. Consequently, the particularly preferable ranges of Ni content are 1.0 to 7%, better 2.0 to 5.0%.

The CR content is between 16.0 and 23.0%. As is well known, it gives the steel its stainless character starting at 11%. Cr also has the advantage of lowering the thermal conductivity of the steel somewhat, and a minimum content of 16.0% is required according to the invention to combine these two effects well. A proportion of 20.0% or less allows the desired phase balance to be maintained without adding too much Ni, Mn and other gamma elements. A proportion of 20.0% to 23.0% significantly increases the corrosion resistance and can be imposed, possibly compensating for the effect of the increased Cr content on the mechanical properties by adjusting the Mn, Ni and N proportions, which can be done by routine experiments. À Cr proportion of more 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% the corrosion resistance of the grade is preferred. Below 20.0%, the focus is on the economic nature of the grade.

The MB content ranges from trace amounts resulting from production to 2.0%. This element is not essential, but it helps to improve corrosion resistance. Its possible disadvantages are its alphagenic nature, which may prevent the desired austenite-ferrite — balance from being achieved, particularly in austeno-ferritic grades, and the fact that it favors the appearance of brittle intermetallic phases. Furthermore, the cost is high, which defeats one of the purposes of the invention.

For improved corrosion resistance, Mo can be partially or fully substituted by W. A

W/Mo substitution ratio of 2 is generally acceptable. Consequently, it is also considered that the W content must not exceed 1.0% and that the sum of MB + W/2 must not exceed 2.0%.

A Mo content of 2.0% would correspond to a case where W is not deliberately added and the possible presence of trace amounts 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 is not deliberately added and the possible presence of trace amounts of Mo would only result from the melting of the raw materials.

The Cu content ranges from traces resulting from the melting of the raw materials alone to 3.0%. Adding Cu in the proportions mentioned has the advantage of slightly decreasing the thermal conductivity and improving the ductility. However, an addition of 3.0% should not be exceeded, as the embrittling effect of the Cu would cause problems during hot forming, and would also needlessly increase the cost of the steel.

The Co content ranges from traces, resulting from the melting of very pure raw materials alone, to 2.0%. Depending on the purity of the raw materials, especially ferronickel, the residual Co content can be as high as 0.8%. It is preferred not to add Co deliberately, as this expensive element has no marked metallurgical effect in stainless steels below 2%, i.e. for proportions that would considerably increase the cost of the steel. 0.8% is therefore the maximum preferred Co content.

The N content is between 0.10% (1,000 ppm) and 0.30% (3,000 ppm). This element is important to provide the corrosion resistance required in the application intended by the invention, and if its content, which would simply result from the absorption of atmospheric nitrogen during the manufacturing method, 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 % N). N stabilizes the austenitic phase and allows the balancing of the various phases to be adjusted. It also has an interesting hardening effect for achieving the desired high mechanical properties. However, above 0.30%, it can cause problems during production, casting and hot rolling (such as the 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 as a result of deliberate addition, including but not limited to: Ti, Nb and V to improve solderability, Al and Ca as deoxidisers and/or control elements for the number and composition of non-metallic inclusions, and B to improve forgeability. But the individual proportions 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. Additionally, the sum of the proportions of alloying elements other than

C, Si, Mn, Cr, Ni, Mo, W, Cu, Co, N and the proportions of impurities resulting from the smelting process (e.g. S, P...) shall not exceed 1.0%. These limits are intended to avoid the — risk of disturbing the equilibria achieved owing to the proportions of the main alloying elements, either mandatory or optional within well-defined limits.

Other conditions relating to the proportions of the alloying elements are to be adhered to, according to the invention.

One of the objectives of the invention, as mentioned above, is to obtain a thermal break reinforcement element with 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 suitability of the steel for hot forming, forging or otherwise. As thermal breaker reinforcements can have relatively complex shapes for relatively small dimensions, this ability to be hot-formed is a criterion that is often considered for the steels used in the invention.

Depending on the balance of the main alloying elements defined above, the steel has an austenitic or austeno-ferritic microstructure. The ferritic index IF is used to estimate the percentage of ferrite in the steel at 1,100 °C, i.e. in the temperature range most frequently encountered in hot forming, based on the composition of the steel. It is obtained by the formula, where the proportions of the different elements are expressed in %:

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

For austenitic grades, IF is, according to the invention, preferably < 20 if good hot formability is to be achieved.

For austeno-ferritic grades, IF is, according to the invention, preferably > 40 if good hot formability is to be achieved.

It will be seen later that the IF range between 20 and 40 should be avoided if high hot formability is to be achieved, as the invention may require.

Furthermore, to achieve satisfactory stress corrosion resistance, it is preferable to choose, rather than an austenitic grade, an austeno-ferritic grade which is characterized by an IF of 40 to 70 at most. Beyond this limit, the steel would fall into the domain of ferritic steels, which is undesirable from the point of view of mechanical properties.

Figure 3 shows the correlation between the ferrite fraction at 1.100 °C measured by a magnetic method (known as sigmametry and as described in 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 | in Table 3. We can see that this correlation is very satisfactory.

The above-mentioned conditions for IF are, however, not absolutely imperative, especially in cases where the hot transformations are not very restrictive. However, they are recommended for most thermal break reinforcement configurations that may be desired.

It is interesting to note that the microstructures of the steels used in the invention are relatively unaffected by the heat treatment and cooling conditions of the metal during its transformations. This leaves a lot of freedom for metallurgists to design the precise way of making the reinforcements of the invention.

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

The inventors were 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 proportions of the different elements are expressed in %):

CI = 22.2 + 2.11 (1 - 1F/100) - 0.89 Si - 0.77 Ni - 0.44 Mn - 0.17 Cr - 0.16 Cu

Figure 4 shows the good correlation obtained between IC calculated by the above formula and the thermal conductivity A actually measured at 20 °C by the "hot disk” method using 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 thermal conductivity decreases with increasing amounts of alloying elements, and that Si first and Ni second are the most influential elements in this respect. This is reflected in the above formula for calculating IC .

According to the invention, the IC index of the steel used should be < 13.5, preferably < 13.0, better < 12.5.

The mechanical properties of the steels used in the invention are sufficient for the intended application, in particular due to the high N content and the austenite percentage, which is always at least 40%. The N content and austenite percentage according to the invention provide the desired ductility both for ease of hot working and for the ability of the reinforcement to deform under exceptional stresses such as an earthguake. 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 are given in Tables 1 to 2 below, were produced in the form of 25 kg ingots with an initial cross-section of 100 mm x 100 mm and were hot-worked by forging to a thickness of 18 mm from 1,250 °C, then hot-rolled to a thickness of 6 mm from 1,250 °C. A solution treatment was carried out at 1,050 °C, followed by milling to adjust the thickness, before cold-forming to a thickness of 3 mm. The microstructure, forgeability, thermal conductivity, and other mechanical properties were characterized on all samples.

A 40 t industrial casting | according to the invention was also developed by melting in an electric furnace, decarburisation by the AOD method, continuous casting into blooms 205 mm on the side and hot rolling into round bars of 115 mm diameter, then into wire rod of about 10.5 mm diameter. The wire rod was cold processed into 10 mm diameter notched wire at a reduction rate of 10-15%.

In the table with the compositions of the different samples, the elements not mentioned are only present in traces. Austenitic structures are designated A, austenitic-ferritic structures are designated AF.

Grade C Si Mn Ni Cr | Mo | Cu N P Vv % % % % % % % % % % o | 14301 |0.040 18.15 0.079 | 9 | 0.025 2 | 14362 [0.020] 0.43 | 1.08 | 4.23 [22.21 0.122 0.025 5 | 14062 [0020] 0.50 | 1.92 | 272 [22.87 [0.12] 0.40 | 0.182 0.024 0.051 16.96 0.125 0.021 | 0.09

B [0.054 16.82 0.132 0.021 | 0.09 0.060 17.02 0.135| 21 | 0.021 | 0.09

S| D [0.063] 2.48 | 6.09 | 5.54 |16.46]0.31| 0.30 | 0.126 0.021 | 0.09 5 0.023 17.85 0.119 0.020 | 0.09 2 0.023 17.83 0.121 0.020 | 0.09 0.018 18.58 0.131 0.020 | 0.09 0.022 22.13 0.215 0.020 | 0.09 0022 18.68 0.133 0.023

Al Nb Ti Co Ca B W 5 | 14801 |<0002 0.01 |<0.002 0.076 con

S 8| 14362 | 0.02 | 0.02 |<0.002] 0.06 | 24 | 17 | 0.018

D

Y 1.4062 0.01 0.01 |<0.002| 0.05 | 25 7 0.007 AF ome] am [mana] [ [em]

A | 0.06 |<0.002|<0.002] 0.01 | 9 | 17 | <0003 | AF

B | 0.07 |[<0.002]<0.002] 0.01 | 6 | 18 | <0003 | AF

S| D | 0.03 |<0.002]<0.002] 0.02 | 17 | 16 | <0003 | At 2

H | 004 |<0.002]<0.002] 0.05 | 16 | 15 | <0004 | — AF1 [001 | 0.01 |<0.002| 0.05 | 21 | 17 | 0.004 | AF

Table 1: Compositions and structures of test samples (laboratory and industrial) o Jee TREE Mmk ue

Grade Structure measured at IF at 20 °C d IC 1.100 °C (%) (W/(m.K)) (E + isi a 5 5 e

A | AF | 25 |] 130 | 128

B | AF | 3 | 31 | 120 | 120

SL D | A | 5 | 8 | 123 | 121 5 | E | AF | 58 of SM | 125 | 130z

O | AF | 45 [uo | 123 | 124 5 Table 2: Ferrite fractions at 1,100 °C, IF, A measured at 20 °C and calculated IC of test samples (laboratory and industrial)

Rank | structure | Rm (MPa) |Rpoz (MPa) | Act (20)

B | AF | 96 | 848 | 6 c = | D | A | 970 | 724 | 19 5

E

H AF 934 854 10

AF | 85 | 792 | 16

Table 3: Tensile mechanical properties of different test castings made in the laboratory after cold-rolling

Sample G is a sample according to the invention. In fact, its composition means that its thermal conductivity À meets the broadest requirements set by the inventors: A measured is 13.3 W/(m.K), which correlates very well with the calculated IC, which is 13.4 (for a maximum of 13.5 according to the invention, which already constitutes significant progress compared with the most common prior art in order to economically ensure compliance with present and probably future energy standards). This sample is low in Cu and contains relatively little Ni and Si, resulting in a higher thermal conductivity than the optimum variants of the invention, even though the individual proportions of each of its elements are well within the corresponding requirements of the invention. It confirms that the composition of the steel to be used to implement the invention must be considered in its entirety, as a coherent whole.

Furthermore, samples A to |, in accordance with the invention, have mechanical properties that are not inferior to those of the reference steel UG*204Cu, except for the elongation rate Agt. However, this remains at acceptable values for the intended application, and several of the samples even have tensile strengths Rm and yield strengths Rp0.2 that are significantly higher than those of the reference steel. It should also be noted that sample

B has an Agt of 6%, thus 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 that is the lowest of those calculated. This steel can therefore provide a very satisfactory solution to the problems posed, at least for the manufacture of thermal breaker reinforcements whose shapes are not too complex.

Furthermore, Figures 5 and 6 show the results of forgeability tests, thus representative of hot ductility, carried out at 1,200 *C (Figure 5) and 1,100 *C (Figure 6) on the above-mentioned laboratory samples A to G. Their rate of shrinkage was measured as a function of their ferritic index IF.

It can be seen 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%, and therefore corresponds to an austeno-ferritic steel whose ferritic character is not yet very marked. This “ductility dip” therefore shows 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, which we have seen to reasonably well reflect the real percentage of ferrite, 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 significantly improve the thermal insulation performance of stainless steel thermal breakers, without having to sacrifice the mechanical properties of conventional stainless steel breakers.

Some variants of the invention have a particularly high degree of heat processability, which allows for forms of thermal break reinforcement that were not previously readily available.

The invention therefore offers builders of low-energy buildings the possibility of exploiting new and potentially advantageous designs of thermal breakers.

Claims (10)

Patent Claims 1. Reinforcement of the protective buffer (7) intended against cold bridging for the construction of buildings, characterized in that this reinforcement is made of austenitic or austenitic-ferritic stainless steel with the following composition in weight percentages: - 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%; - 0.110% < N < 0.25%; where the remainder is iron, alloy elements other than those mentioned above, and impurities arising in connection with manufacturing, in which case the total amount of these other alloy elements and impurities is not more than 1.0% and none of these other alloy elements alone is in a concentration of more than 0.5%, and of , that the thermal conductivity index IC calculated as follows: IC = 22.2 + 2.11(1 - 1F/100) - 0.89 Si - 0.77 Ni - 0.44 Mn - 0.17 Cr - 0.16 C and 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. Reinforcement (7) of the protective buffer intended against cold bridging according to claim 1, characterized in that the Cr content is between 16.0% and 20.0%. 3. Reinforcement (7) of the protective buffer intended against cold bridging according to claim 1, characterized in that the Cr content is between 20.0% and 23.0%. 4. Reinforcement (7) of the protective buffer intended against cold bridging according to one of claims 1-3, characterized in that the Ni content of the steel is between 1.0% and 7.0%, preferably between 2.0% and 5, 0%. 5. Reinforcement (7) of the protective buffer intended against cold bridging according to one of claims 1-4, characterized in that the steel's ferrite index IF calculated as follows: 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. Reinforcement (7) of the protective buffer intended against cold bridging according to one of claims 1-4, characterized in that the steel's ferrite index IF calculated as follows: 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. Reinforcement (7) of the protective buffer intended against cold bridges according to one of claims 1-6, characterized in that among the mentioned other alloying elements at least one of the following occurs: Al, Ti, Nb, V, Ca and B, and that Al , Ti, Nb and V can each be present at a concentration of up to 0.5%, and that Ca and B can each be present at a concentration of up to 0.05%. 8. Reinforcement (7) of the protective buffer intended against cold bridging according to one of claims 1-7, characterized in that the yield strength Rp0.2 is greater than or equal to 600 MPa or more preferably greater than or equal to 700 MPa and the total elongation at maximum load Agt is greater or equal to 5%. 9. Reinforcement (7) of the protective buffer intended against cold bridges according to one of claims 1-8, characterized in that it is obtained from a rod, wire or plate. 10. A protective buffer intended against cold bridging for the construction of buildings, which comprises a reinforcement (7) and an insulating layer (6), through which said reinforcement (7) passes, characterized in that said reinforcement (7) is — manufactured according to one of claims 1-9 in accordance with.