The present invention relates to the steel, and particularly welding processes by points of steel sheets.
Discloses stainless martensitic steel sheets processed hot, including adjusting the composition of the initial microstructure and the parameters of their heat treatments enabled to provide high mechanical characteristics and high ability to be formed into complex ways. Such plates are described in PCT / IB2017 / 051636 in the name of the Applicant, and are intended primarily for the automotive industry.

Their composition is as follows, in weight percent:

* 0,005%≤ C≤ 0,3% ;

* 0,2%≤ Mn≤ 2,0% ;

* traces≤ Si≤ 1 ,0% ;

* traces≤ S≤ 0,01 % ;

* traces≤ P≤ 0,04% ;

* 10.5% 17.0% ≤ Cr≤; preferably 10.5% Cr≤ ≤ 14.0%;

* traces≤ Ni≤ 4,0% ;

* traces≤ Mo≤ 2,0% ;

* Mo + 2 x W≤ 2,0%;

* Traces≤ Cu≤ 3%; preferably traces≤ Cu≤ 0.5%;

* traces≤ Ti≤ 0,5% ;

* traces≤ Al≤ 0,2% ;

* traces≤ O≤ 0,04% ;

* 0,05%≤ Nb≤ 1 ,0% ;

* 0,05%≤ Nb + Ta≤ 1 ,0% ;

* 0,25%≤ (Nb + Ta )/(C + N)≤ 8 ;

* traces≤ V≤ 0,3% ;

* traces≤ Co≤ 0,5% ;

* Traces≤ Co≤ Cu + Ni + 5.0%;

* traces≤ Sn≤ 0,05% ;

* traces≤ B≤ 0,1 % ;

* traces≤ Zr≤ 0,5% ;

* Ti + V + Zr≤ 0,5%;

* Traces≤ H≤ 5 ppm, preferably 1 ppm traces≤ H≤;

* traces≤ N≤ 0,2% ;

* (Mn + Ni) ≥ (Cr -10.3 - 80 x [(C + N) 2 ]);

* traces≤ Ca≤ 0,002% ;

* Traces≤ rare earths and / or Y≤ 0.06%;

* The balance being iron and impurities resulting from preparation;

- the starting temperature of martensitic transformation (Ms) of the sheet étant≥ 200 ° C;

- and the end temperature of martensitic transformation (Mf) of the sheet being ≥

-50°C.

The microstructure of the original sheet, which has been obtained by suitable means which may include hot transformation operations and / or cold, is composed of ferrite and / or martensite and from 0.5% to 5% by volume carbides, and the ferritic grain size is from 1 to 80 μηι, preferably from 5 to 40 μηι. This initial sheet has a thickness of 0.1 to 10 mm and more typically from 0.1 mm to 6 mm.

The treatment method is typically applied to them begins with an austenitization of the sheet, that is to say by increasing the temperature above the Ac1 temperature of the steel so as to form austenite in instead of ferrite and carbides constituting the starting microstructure, and under conditions that limit as much as possible decarburization and oxidation surface of the sheet. There typically exists as more than 20% of ferrite and residual 0.5% or less of carbides.

Then successively performs several steps of hot forming of the sheet (at least two) under conditions of temperature and time such that the structure in low levels of ferrite and carbides obtained after austenitization is maintained throughout the entire setting in shape. These hot layouts are held at a temperature higher than the Ms temperature of the beginning of the martensitic transformation. If necessary, one can carry out reheating or holding at temperature between heat-starts, or during the latter, by means of heated tools, so that the temperature of the sheet being set shaped and between layouts (for the transfer of the sheet from one tool to another, or if the sheet remains on the same tool for the configuration changes to the

It should be understood that the term "hot-shaping step," Including the operations of deformation or material removal as diverse as, inter alia, deep drawing, hot stampings, stampings, cutouts, holes, these steps can take place in any order chosen by the manufacturer.

After the hot forming, the part is cooled, without particular constraints on the cooling conditions.

During cooling, a step of cutting or ultimate setting hot forming can be performed between Ms and Mf. (End temperature of martensitic transformation), under conditions where the microstructure is constituted by at least 10% austenite, more than 20% ferrite, the remainder being martensite.

La tôle ainsi obtenue possède des propriétés mécaniques élevées à température ambiante, du fait notamment de sa teneur élevée en martensite. Typiquement, la résistance à la traction Rm est d'au moins 1000 MPa, la limite élastique Re est d'au moins 800 MPa, l'allongement à la rupture A mesuré selon la norme ISO 6892 est d'au moins 8%, et la capacité d'angle de pliage pour une épaisseur de 1 ,5 mm est d'au moins 60°, mesurée selon la norme VDA 238-100. Cela implique que la tôle finale obtenue présente une excellente formabilité et est utilisable notamment dans l'industrie automobile, ou pour constituer des pièces à fonction structurale dans l'aéronautique, le bâtiment ou le ferroviaire.

Finally, after cooling to room temperature following the last shaping, the microstructure of the sheet contains a maximum of 0.5% carbides volume fraction and at most 20% residual volume fraction of ferrite, remainder being martensite.

These sheets, whose thickness is typically from 0.10 to 6.0 mm, however, have a drawback, which is that weldability can sometimes be regarded as insufficient when the welding is performed by spot welding process in the most usual conditions among automakers. It turns out that in the welded zones, it is not easily obtained a tensile strength in cross that would be sufficient for a given thickness of the sheet (that is to say, typically at least 450 daN for thick plates 1, 2 mm): the material is too fragile in the weld.

The results could be improved by modifying the parameters of the welding, that is to say by adding to standard welding cycles postheating landing nets as normally used with the martensitic steels, but far optimizations n ' had failed to obtain a satisfactory quality of welding for welding time cycles less than 5 seconds. This time is much too high for car manufacturers to weld these plates within the constraints of productivity they face for application to mass production vehicles. A total of about welding cycle 1 s at most would be to aim for this purpose. A total duration of welding cycle of 1, 5 s, or even 2 s, might sometimes be acceptable.

The object of the invention is to provide a welding cycle Points specially adapted to the use of stainless steel sheet for hot stamping martensitic previously described, and which allows to carry out this welding under industrially suitable conditions for the automobile industry.

To this end, the invention relates to a method of welding two sheets of steel of thickness 0.10 to 6.0 mm and a composition, in percentages by weight:

* 0,005%≤ C≤ 0,3% ;

* 0,2%≤ Mn≤ 2,0% ;

* traces≤ Si≤ 1 ,0% ;

* traces≤ S≤ 0,01 % ;

* traces≤ P≤ 0,04% ;

* 10.5% 17.0% ≤ Cr≤; preferably 10.5% Cr≤ ≤ 14.0%;

* traces≤ Ni≤ 4,0% ;

* traces≤ Mo≤ 2,0% ;

* Mo + 2 x W≤2,0%;

* Traces≤ Cu≤ 3%; preferably traces≤ Cu≤ 0.5%;

* traces≤ Ti≤ 0,5% ;

* traces≤ Al≤ 0,2% ;

* traces≤ O≤ 0,04% ;

* 0,05%≤Nb≤ 1 ,0% ;

* 0,05%≤ Nb + Ta≤ 1 ,0% ;

* 0,25%≤ (Nb + Ta )/(C + N)≤ 8 ;

* traces≤ V≤ 0,3% ;

* traces≤ Co≤ 0,5% ;

* Traces≤ Co≤ Cu + Ni + 5.0%;

* traces≤ Sn≤ 0,05% ;

* traces≤ B≤ 0,1 % ;

* traces≤ Zr≤ 0,5% ;

* Ti + V + Zr≤ 0,5%;

* Traces≤ H≤ 5 ppm, preferably 1 ppm traces≤ H≤;

* traces≤ N≤ 0,2% ;

* (Mn + Ni) ≥ (Cr -10.3 - 80 x [(C + N) 2 ]);

* traces≤ Ca≤ 0,002% ;

* Traces≤ rare earths and / or Y≤ 0.06%;

* The balance being iron and impurities resulting from preparation;

- the starting temperature of martensitic transformation (Ms) of the sheet étant≥ 200 ° C;

- the end temperature of martensitic transformation (Mf) of the sheet étant≥

-50°C ;

- the microstructure of the sheet containing at most 0.5% by volume fraction carbides and at most 20% residual volume fraction of ferrite, the remainder being martensite;

characterized in that it comprises the following steps, (e) is the thickness of each of said sheets or the thinnest of them;

A first welding step, duration (t) ms:

* For the thicknesses (e) of 0.10 to 0.50 mm:

t = (40 x e + 36) ± 10%

* For the thicknesses (e) from 0.51 to 1, 50 mm:

t = (124 x e - 13) ± 10%

* For the thicknesses (e) of 1, 51 6.0 mm

t = (12 x e + 47) ± 10%

and clamping force (F) in daN:

* For the thicknesses (e) from 0.10 to 1, 50 mm:

F = (250 x e + 90) ± 10%

* For the thicknesses (e) of 1, 51 mm to 6.0 mm:

F= (180 x e + 150) ± 10%

during which is applied between the electrodes of a welding current having an intensity between 80 and 100% of the maximum allowable intensity corresponding to the expulsion of molten metal;

A second step during which fixed current intensity between zero and

1 kA;

And a third step, during which shows the current flow at an intensity of 3.5 kA and 4.5 kA, for a duration of at least 755 ms, to perform heat treatment of the weld zone.

Preferably, in the second step it interrupts the current flow in the weld zone.

Advantageously, the sum of the durations of said first, second and third stages is not more than 2 s, preferably at most 1, 5 s, more preferably at maximum 1 s.

Said sheets may be hot-rolled sheet.

As will have been understood, the invention consists in applying to the sheets to which the invention is primarily aimed, the composition of which has been specified above, a particular spot welding cycle in its choice of parameters and its sequence of operations.

Recall that the execution conditions of spot welding are sufficiently defined by:

- the pressure exerted by the welding electrodes on the parts to be welded which influences the contact resistance, in conjunction with the chemical composition and the roughness of the surface of the parts;

- the intensity of the current flowing through the zone to be welded, and which is subject to a control that controls the current supply of the installation, depending on the other not strictly controllable parameters;

the duration of welding or of its various stages.

Thus, the potential difference between the two plates varies depending on the contact resistance, and likewise, therefore, the power injected into the welding region. This potential difference and power do not directly represent themselves of the process parameters, but suffered because of controllable operating conditions that are controlled and the clamping force and the current intensity.

Welding starts with a first step, during which is passed an electric current of intensity regulated in the sheets to be welded, previously put in contact with one another by one effort. The force to be applied and the duration of the current are usually dictated by the standard referred to by the user (eg SEP1220 or IS018278-2). Once these two parameters selected and charged, the user varies the welding current until the expulsion of molten metal which is the maximum intensity value of the field weldability. The intensity of the inventive welding current is in a range between 80% and 100% of this maximum intensity. Typically, in the case of the invention, the welding current intensity is 5, 5 kA when the sheets to be welded have a thickness of 1 2 mm. Generally, the intensity of maximum tolerable corresponding to the expulsion of molten metal welding is obtained experimentally by a standardized method, see, e.g., September 1220 and ISO 18278-2 standards. His determination by the skilled person in each particular case it faces for the implementation of the invention is therefore to achieve during the implementation phase to the point of accurate welding process performed according to the invention. But this determination is not typical of the invention, such as the question of optimizing the intensity of the welding current can occur in any expulsion of molten metal is obtained experimentally by a standardized method, see, e.g., September 1220 and ISO 18278-2 standards. His determination by the skilled person in each particular case it faces for the implementation of the invention is therefore to achieve during the implementation phase to the point of accurate welding process performed according to the invention. But this determination is not typical of the invention, such as the question of optimizing the intensity of the welding current can occur in any expulsion of molten metal is obtained experimentally by a standardized method, see, e.g., September 1220 and ISO 18278-2 standards. His determination by the skilled person in each particular case it faces for the implementation of the invention is therefore to achieve during the implementation phase to the point of accurate welding process performed according to the invention. But this determination is not typical of the invention, such as the question of optimizing the intensity of the welding current can occur in any therefore be provided during the commissioning phase at the point of the specific welding process performed according to the invention. But this determination is not typical of the invention, such as the question of optimizing the intensity of the welding current can occur in any therefore be provided during the commissioning phase at the point of the specific welding process performed according to the invention. But this determination is not typical of the invention, such as the question of optimizing the intensity of the welding current can occur in any

performing a spot welding method, and is carried out in conventional manner as has been said.

The effort F in daN pour les épaisseurs and 0.1 to 1 mm, 50 mm exprime east by the equation:

F = 250 x e + 90

and for one thickness, 51 mm to 6 mm by the equation:

F= 180 x e + 150

e being the thickness of the two welded sheets, or the thinnest of them if they are of different thicknesses.

A variation of the force F of ± 10% around those expressed values ​​is allowed.

The welding time t in milliseconds is also expressed, for thicknesses of 0.10 to 0.50 mm, by the equation:

t = 40 x e + 36

for thicknesses of 0.51 1, 50 mm:

t = 124 x e - 13

and for thicknesses of 1, 51 6.0 mm

t = 12 x e + 47

A variation of ± 10% around the specified values ​​is permitted.

In a second step, the pressure is maintained of the electrodes and the current flow is eliminated or greatly reduced, and it imposes an intensity of 1 kA maximum, and ideally 0 kA, for a minimum time tf in ms by equation:

tf≥ 34 xe + 2

This leads to a sudden cooling of the sheets in the welding zone to a temperature between Ac1 and Ac5, temperature range leading to réausténitisation zone.

In a third step, the electric current is restored to an intensity value between 3.5 and 4.5 kA, so as to maintain the temperature between Ac1 and Ac5 and perform a heat treatment on the welded zone, which will modify its structural characteristics and impart the desired mechanical properties. This third step is to last at least 755 ms to be certainly effective, and there is no required maximum time. The longer it is the more the heat treatment is effective and provides the tensile certainly high cross resistance. But we better not excessively extend this third stage to not lengthen the welding cycle by giving it a period incompatible with the requirements of industrial production. Advantageously, as we said,

When executing this processing under the conditions that have been said, one can obtain a tensile strength in cross reaching suitable values ​​referred to the considered plate thicknesses, or even exceeding, and a welding cycle time of the order of 1 s or less, therefore compatible with industrial carmakers current requirements for large-series vehicles. so you can enjoy it in good economic conditions, on sheets welded points, advantages of the process described in PCT / IB2017 / 051636, relating to the ease of producing a complex shaped part in a martensitic stainless steel processed hot and with high mechanical properties, well-defined composition.

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

Figure 1 shows a micrograph of the welded zone in the case of welding of two plates according to a process not according to the invention;

Figure 2 which shows a detail of the weld area of ​​Figure 1;

Figure 3 shows a micrograph of the welded area after the second step of a method according to the invention thus in an intermediate state prior to the third stage of welding, and showing the disappearance of the residual ferrite at this stage;

Figure 4 which shows a complete micrograph of the welded area after the execution of a method according to the invention.

Figure 5 which shows a detail of the weld area of ​​Figure 4; Experiments conducted by the inventors on the composition of welding two metal sheets, in percentages by weight, Cr = January 1, 02%; Nb = 0.1% 1; Mn = 0.50%; C = 0.059%; N = 0.0107%; the remainder being iron and impurities resulting from the smelting, the austenitized and tempered condition press, thus falling within the invention disclosed in PCT / IB2016 / 052302, and 1, 2 mm thick, gave the following results.

In a first series of experiments, we applied a conventional weld cycle total duration of 560 ms, comprising applying between the electrodes a pressure of 4000 N, a current with an intensity of 5.5 kA for 280 ms, followed by a zero current period of 280 ms during which the pressure was maintained (parameters dictated by the ISO 18278-2 and typically used by vehicle manufacturers). We can see the results in Figures 1 and 2, which show micrographs of the weld zone. It is found in the center of Figure 1, the presence of the melted area 1 corresponding to the actual welding, and around it the heat affected zone the (ZAC, in English "Heat-Affected Zone," HAZ). The molten zone 1 is bordered by a crack 2 that propagates the within the ZAC 3 to high grain size within which we distinguish ferrite 4, white (clearly visible also in Figure 1). It is this four ferrite, brittle, which is responsible for the propagation of the crack 2, and therefore of the poor tensile strength crosswise. The proportion of ferrite in the HAZ 3 is 20 to 80% depending on the area, which is significantly higher than what could be expected by simple consultation of equilibrium diagrams. The tensile strength measured cross is 290 daN, very insufficient for the needs of car manufacturers, for example. which is responsible for the propagation of the crack 2, and therefore of the poor tensile strength crosswise. The proportion of ferrite in the HAZ 3 is 20 to 80% depending on the area, which is significantly higher than what could be expected by simple consultation of equilibrium diagrams. The tensile strength measured cross is 290 daN, very insufficient for the needs of car manufacturers, for example. which is responsible for the propagation of the crack 2, and therefore of the poor tensile strength crosswise. The proportion of ferrite in the HAZ 3 is 20 to 80% depending on the area, which is significantly higher than what could be expected by simple consultation of equilibrium diagrams. The tensile strength measured cross is 290 daN, very insufficient for the needs of car manufacturers, for example.

Reductions in the duration of application of current (280 ms to 140 ms) have been beneficial in that they have reduced the scale of the ZAC 3 to high grain size and decrease the rate of residual ferrite without significantly altering the molten zone 1. However, the ZAC 3 still contains an amount of ferrite brittle, and the tensile strength in the cross is not sufficiently improved.

In a second series of experiments, according to the invention, after a first step identical to previous experiments, it was interrupted current flow for 46 ms while maintaining the pressure of the electrodes. And a third step in the previous process is added in which was taken up the flow of current to a 4kA intensity for a duration of 814 ms, for performing heat treatment of the weld zone.

Total cycle therefore lasted in the case of the example according to the invention 140 + 46 + 814 = 1000 ms.

The objective is to achieve a weld of the two parts which does not represent a weak point in the assembly, meaning that the tensile strength in cross at the weld zone is sufficient for this purpose, and to realize this welding during a total cycle that ensures the facility satisfactory productivity in industrial conditions. Typically, a duration of the order of 1 s welding cycle as in the described example is such a result satisfactory for mass production of sheets welded to the automotive industry.

Figure 3 shows the appearance of the welding zone that the invention thus allows to obtain after the second step of the process according to the invention which lasted only 46 ms. Figures 4 and 5 show the welding zone after the execution of the entire process according to the invention. The coarse ferritic ZAC 1 of Figure 4 not only have disappeared, but the ZAC 3 and the molten zone 1 have a tenacity such that the crack 2 whose trace is shown in Figure 5 deviates in the base metal 5 .

the tensile strength is thus obtained in the upper cross 450 daN at the weld bead, which is the objective was to achieve in the example described, considering the thickness of sheets to be welded.

The inventors explain the advantages of the method according to the invention, compared to more conventional methods spot welding, by the sum of the following factors, it appears that it has a remarkable synergistic effect that was not expected.

The execution of a rapid first welding cycle reduces the time to stay above the point Ac5 and minimize the partition of gammagenic and alphagenic elements leading to the large grain ferrite formation in the HAZ. Thus, we see that the ferrite 4 in white in Figure 1 has completely disappeared HAZ 3 in Figure 3.

The interruption of the current flow during the second step (or at least the drastic reduction of the current flow) causes a cooling of the welding zone to a réausténitisation temperature of about 900 ° vs.

A third step where the power is restored with a relatively large intensity, albeit less than in the first step, allows to permanently destroy any residual ferrite coarse grained HAZ in the periphery of the overlap area and retrieve properties satisfactory mechanical (figures 4 and 5). There are also as cracking 2 of Figure 4 no longer follows the HAZ as shown in Figure 1, and it is carried out in the base metal 5 of Figure 4, leaving a large-diameter button on the one of the two sheets.

The sheet used for the implementation of the invention may have been hot-rolled or cold. What is important is, firstly, that their composition and microstructure are consistent with what has been said, and secondly that their thickness is among those that allow spot welding, so typically 0.10 6.0 mm.

CLAIMS

1 .- A method for welding two thick steel plate 0.10 to 6.0 mm and a composition, in percentages by weight:

* 0,005%≤ C≤ 0,3% ;

* 0,2%≤ Mn≤ 2,0% ;

* traces≤ Si≤ 1 ,0% ;

* traces≤ S≤ 0,01 % ;

* traces≤ P≤ 0,04% ;

* 10.5% 17.0% ≤ Cr≤; preferably 10.5% Cr≤ ≤ 14.0%;

* traces≤ Ni≤ 4,0% ;

* traces≤ Mo≤ 2,0% ;

* Mo + 2 x W≤ 2,0%;

* Traces≤ Cu≤ 3%; preferably traces≤ Cu≤ 0.5%;

* traces≤ Ti≤ 0,5% ;

* traces≤ Al≤ 0,2% ;

* traces≤ O≤ 0,04% ;

* 0,05%≤ Nb≤ 1 ,0% ;

* 0,05%≤ Nb + Ta≤ 1 ,0% ;

* 0,25%≤ (Nb + Ta )/(C + N)≤ 8 ;

* traces≤ V≤ 0,3% ;

* traces≤ Co≤ 0,5% ;

* Traces≤ Co≤ Cu + Ni + 5.0%;

* traces≤ Sn≤ 0,05% ;

* traces≤ B≤0,1 % ;

* traces≤ Zr≤ 0,5% ;

* Ti + V + Zr≤ 0,5%;

* Traces≤ H≤ 5 ppm, preferably 1 ppm traces≤ H≤;

* traces≤ N≤ 0,2% ;

* (Mn + Ni) ≥ (Cr -10.3 - 80 x [(C + N) 2 ]);

* traces≤ Ca≤ 0,002% ;

* Traces≤ rare earths and / or Y≤ 0.06%;

* The balance being iron and impurities resulting from preparation;

- the starting temperature of martensitic transformation (Ms) of the sheet étant≥ 200 ° C;

- the end temperature of martensitic transformation (Mf) of the sheet étant≥

-50°C ;

- the microstructure of the sheet containing at most 0.5% by volume fraction carbides and at most 20% residual volume fraction of ferrite, the remainder being martensite;

characterized in that it comprises the following steps, (e) is the thickness of each of said sheets or the thinnest of them;

A first welding step, duration (t) ms:

* For the thicknesses (e) of 0.10 to 0.50 mm:

t = (40 x e + 36) ± 10%

* For the thicknesses (e) from 0.51 to 1, 50 mm:

t = (124 x e - 13) ± 10%

* For the thicknesses (e) of 1, 51 6.0 mm

t = (12 x e + 47) ± 10%

and clamping force (F) in daN:

* For the thicknesses (e) from 0.10 to 1, 50 mm:

F = (250 x e + 90) ± 10%

* For the thicknesses (e) of 1, 51 mm to 6.0 mm:

F= (180 x e + 150) ± 10%

during which is applied between the electrodes of a welding current having an intensity between 80 and 100% of the maximum allowable intensity corresponding to the expulsion of molten metal;

A second step during which fixed current intensity between zero and

1 kA;

- and a third step, during which shows the current flow at an intensity of 3.5 kA and 4.5 kA, for a duration of at least 755 ms, to perform heat treatment of the weld zone.

2. A process according to claim 1, characterized in that during the second step stops the flow of current in the weld zone.

3. A process according to one of claims 1 to 3, characterized in that the sum of the durations of said first, second and third stages is not more than 2 s, preferably at most 1, 5 s, more preferably at maximum 1 s.

4. A process according to one of claims 1 to 3, characterized in that said sheets are hot-rolled sheet.