RU2783829C1 - Method for production of welded steel billet and corresponding welded steel billet - Google Patents

Abstract

FIELD: welding.

SUBSTANCE: group of inventions relates to a method for the production of steel billet (1), a method for the production of a steel part welded and then molded by hot pressing and cooled, welded steel billet (1), a steel part welded, molded by hot pressing, and cooled, and the use of a steel part welded, molded by hot pressing, and cooled. For the production of welded steel billet (1), two pre-coated sheets (2) are provided. Each sheet (2) includes steel substrate (3) and has preliminary coating (5) on each of main surfaces (4). Each sheet (2), on an edge of welded joint (14), includes zone (18) of removal of preliminary coating (5). Butt welding of sheets (2) is performed using filler wire (20) to create welded joint (22) having an average aluminum content Al_WJ of 0.1-1.2 wt.%. A composition of wire (20) and a proportion of added wire (20) are such that welded joint (22) has: (a) a tempering factor FT_WJ such that

(b) a nickel content Ni_WJ of ≤ 14-3,4xAl_WJ, and a chromium content of Cr_WJ ≤ 5-2xAl_WJ, where Al_WJ is an average aluminum content in welded joint (22).

EFFECT: prevention of the probability of damage to a part obtained from a steel billet, manufactured of two welded pre-coated sheets, after hot pressing and cooling in a zone directly adjacent to a welded joint of the specified welded steel billet due to elimination of brittle fracture in the welded joint, and obtainment of a part having satisfying characteristics of safety in collision, even with relatively high aluminum content in the welded joint.

47 cl, 5 dwg, 6 tbl

Description

The present invention relates to a method for producing a welded steel billet, to a welded steel billet thus obtained, to a method for producing a welded, hot-pressed and cooled steel piece from a welded steel billet, and to a welded, hot-pressed and cooled steel piece thus obtained.

Methods for producing welded parts from steel sheets of various compositions and/or thicknesses that are butt welded to each other are known in the art. More specifically, the welded billets are usually heated to a temperature that austenizes the steel, and then subjected to hot pressing and cooling in a hot pressing apparatus. The composition of the steel can be chosen to allow subsequent heating and pressing operations, as well as to give the welded steel part high mechanical strength, high toughness and good corrosion resistance.

Steel parts of this type are used, in particular, in the automotive industry, and more specifically for the manufacture of parts against unauthorized access, structural parts or parts that ensure the safety of vehicles.

In order to prevent corrosion, the steel sheets are pre-coated with an aluminum-based primer, which is applied by dipping into the melt of a bath containing aluminum. If steel sheets are welded without any pre-treatment, the aluminum based pre-coating will be diluted with the steel substrate in the molten metal during the welding operation. Within the aluminum content range of the precoat, two phenomena can then occur.

If the local content of aluminum in the molten metal is high, then intermetallic compounds are formed in the weld, which is the result of dilution of part of the pre-coat in the molten metal and fusion, which occurs during the subsequent heating of the weld to the hot pressing stage. These intermetallic compounds are the centers where crack initiation is most likely to occur.

In addition, aluminum stimulates an increase in the austenitization temperature (Ac3) in the weld, and this modification of the austenitic region will become increasingly important as the aluminum content in the weld increases. In some cases, this can prevent complete austenitization of the weld, which necessarily occurs during heating prior to pressing and is the first step required for hot stamping and obtaining a martensitic structure in the weld after hot pressing and cooling.

Moreover, aluminum also has a negative effect on the hardenability of the weld, since aluminum increases the critical cooling rate required to obtain a martensitic or bainitic structure in the weld during cooling.

Therefore, it is no longer possible to obtain martensite or bainite during cooling after hot pressing, and the weld thus obtained will contain ferrite. Then the weld will have less hardness and mechanical strength than two adjacent sheets of steel, and therefore represents the weakest part of the part.

The published patent EP2007545 describes a solution to the problem of removing the surface layer of a metal alloy at the weld edge of pre-coated steel sheets, which is expected to be embedded, at least partially, in the metal weld zone. This removal can be done by brushing or using a laser beam. The intermetallic alloy layer is retained to provide corrosion resistance and to prevent decarburization and oxidation phenomena during the heat treatment prior to the pressing operation. In this case, the influence of aluminum is reduced due to the local removal of the surface layer of the coating.

However, the inventors of the present patent application have found that even if the surface layer of the metal alloy at the weld edge of the pre-coated steel sheets is removed, the weld may still have inadequate mechanical characteristics. Of course, the concentration of aluminum in the weld can still be quite high, due to the presence of raised parts of the coating on the side surface of the steel sheet, on the edge of the weld, as a result of the removal operation, and/or for thin steel sheets, for example, having a thickness less than or equal to 1 .0 mm.

Documents EP 2 737971, US 2016/0144456 and WO 2014075824 describe an attempt to develop a method in which pre-coated sheets are welded without prior removal of the pre-coat, using a filler wire containing austenite stabilizing elements such as carbon, manganese or nickel, with in order to obtain a fully martensitic structure in the weld, after hot pressing and cooling, despite the presence of aluminum in the weld, as a result of precoat melting.

However, these methods are not entirely satisfactory, as they address only one of the problems associated with the presence of aluminum in the weld pool: austenitization temperature compensation (Ac3) and, in some cases, the use of high carbon filler wire can cause segregation in the weld. Indeed, the inventors of the present invention have found that the methods described in the above documents do not provide satisfactory mechanical properties for parts obtained after hot pressing and cooling, especially when the aluminum content is greater than or equal to 0.7% by weight in the weld. In particular, for such parts, there is a high probability of damage in the weld during a tensile test in the direction opposite to the weld.

The method disclosed in documents WO 2015/086781 and EP 2942143 also addresses this problem, and methods are described in which pre-coated steel sheets are welded using specific welding methods with specific filler materials.

More specifically, document WO 2015/086781 proposes to use two-point laser welding by supplying a filler material in the form of metal powder, which has the following composition, in percent by weight: C: 0-0.03 wt.%, Mo: 2.0- 3.0 wt.%, Ni: 10-14 wt.%, Mn: 1.0-2.0 wt.%, Cr: 16-18 wt.% and Si: 0.0-1.0 wt.% , with the rest being iron.

EP 2 942143 proposes the use of hybrid laser/arc welding using arc welding with a torch located in front of the laser beam, supplying a filler material in the form of a filler wire, which has the following composition C: 0-0.3 wt.%, Mo: 0 -0.4 wt.%, Ni: 6-20 wt.%, Mn: 0.5-7 wt.%, Cr: 5-22 wt.% and Si: 0-1.3 wt.%, Nb: 0-0.7 wt.%, and the rest is iron.

These methods are also unsatisfactory. Indeed, the inventors of the present invention have found that the use of the filler wire described in these documents leads to a high probability of damage to the part after hot pressing and cooling, in the area immediately adjacent to the weld.

In addition, the use of hybrid laser/arc welding is undesirable because hybrid laser/arc welding cannot achieve the same welding speeds as laser welding, and therefore reduces the overall productivity of the process.

Moreover, the addition of powder is usually more difficult to accomplish in a large scale industrial plant than filler wire.

In all of the filler material addition methods mentioned above, filler material chemistry ranges are only set, and since welding parameters and conditions affect the filler material consumption rate, a single filler wire can lead to very different weld chemistry. Therefore, it turns out that the description of the composition of a single filler wire is insufficient to solve the problems mentioned above.

Therefore, the object of the invention is to develop a process for producing a welded steel billet from said two pre-coated sheets, which provides, after hot pressing and cooling, a part having satisfactory crash safety characteristics, even with a relatively high aluminum content in the weld.

For this purpose, in particular, it is desirable to completely eliminate brittle fracture in the weld.

For this purpose, the invention proposes a method for producing a welded steel billet, which includes the following steps:

- providing two pre-coated sheets, each pre-coated sheet containing a steel substrate (3) having a pre-coating on each of the two main surfaces, a pre-coating comprising a layer of an intermetallic alloy containing at least iron and aluminum, and optionally, a layer of metallic alloy extending over the intermetallic alloy layer, wherein the metallic alloy layer is an aluminum layer, an aluminum alloy layer or an aluminum-based alloy layer,

each pre-coated sheet, comprising on each of the two main surfaces, at the edge of the weld, intended to be included, at least partially, inside the weld, a removal zone in which the pre-coat is removed over the part to be removed, comprising between 30% and 100% on the thickness of the preliminary coating;

butt-welding the pre-coated sheets using a filler wire to create a weld at the junction of the pre-coated sheets, the weld having an average aluminum content Al _WJ in the range between 0.1 wt.% and 1.2 wt.%,

where

- the composition of the filler wire and the proportion of filler wire added to the weld pool are chosen so that the weld thus obtained has the characteristics:

(критерий C1), (a) the quenching factor FT _WJ of the weld is such that

(criterion C1),

where:

- FT _BM is the hardening factor of the least hardenable steel substrate among the steel substrates of the two pre-coated sheets, and

- hardening factors FT _WJ and FT _BM are determined using the following formula: FT=128 + 1553xC + 55xMn + 267xSi + 49xNi + 5xCr - 79xAl - 2xNi² - 1532xC² - 5xMn² - 127xSi² - 40xCxNi - 4xNixMn, where Al, Cr, Ni, C , Mn and Si mean, respectively, the average content of aluminum, chromium, nickel, carbon, manganese and silicon, expressed as a percentage by mass, of the area for which the hardening factor should be determined, and the specified area is the weld in the case of FT _WJ and the least hardenable substrate in the case of FT _BM ,

(b) the average nickel content Ni _WJ in the weld satisfies the following relationship: Ni _WJ ≤ 14-3.4xAl _WJ , where Al _WJ is the average aluminum content in the weld (criterion C2); and

(c) the average chromium content Cr _WJ in the weld satisfies the following relationship: Cr _WJ ≤ 5-2xAl _WJ , where Al _WJ is the average aluminum content in the weld (criterion C3).

In specific embodiments, the method may include one or more of the following features, taken alone or in any technically feasible combination:

- the steel of the substrate of at least one of the pre-coated sheets, and, for example, each pre-coated sheet contains, by weight:

0.10% ≤ C ≤ 0.5%

0.5% ≤ Mn ≤ 4.5%

0.1% ≤ Si ≤ 1%

0.01% ≤ Cr ≤ 1%

Ti ≤ 0.2%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.010%,

the rest is iron and impurities that appeared during production.

- the substrate of each pre-coated sheet is made of steel, hardened by pressing;

- the average aluminum content in the weld is greater than or equal to 0.15 wt.%;

- the average aluminum content in the weld is less than or equal to 0.8 wt.%;

- the average nickel content in the weld is between 0.1 wt.% and 13.6 wt.%, and more specifically between 0.2 wt.% and 12.0 wt.%;

- the welded steel billet is such that, after hot pressing and cooling, the Charpy impact strength of the weld at 20°C is greater than or equal to 25 J/cm², and the tensile strength of the hot-pressed and cooled welded steel billet is greater than or equal to the limit the tensile strength of the weakest substrate among the pre-coated sheet substrates, the weakest substrate being the substrate for which the product of thickness and tensile strength after hot pressing and cooling is minimal;

- filler wire has a carbon content between 0.01 wt.% and 0.45 wt.%;

- for at least one pre-coated sheet, and for example for both pre-coated sheets, the part to be removed is strictly less than 100% of the thickness of the pre-coating;

- for at least one pre-coated sheet, the pre-coating includes a metal alloy layer extending over the intermetallic alloy layer, where the metal alloy layer is an aluminum layer, an aluminum alloy layer or an aluminum-based alloy layer, and for at least one pre- coated sheet, and, for example, for both pre-coated sheets, the metal alloy layer is removed throughout the thickness, while the intermetallic alloy layer remains intact in the removal zone on each main surface of the pre-coated sheet;

- for at least one pre-coated sheet provided in the provisioning step and, for example, for both pre-coated sheets, in the removal zone on each main surface of the pre-coated sheet, the part to be removed is 100%, so that the pre-coating is removed over the entire sheet thickness;

- in addition, the method includes, up to the provisioning stage, the step of obtaining two pre-coated sheets from the respective original pre-coated sheets, this stage includes the sub-step of obtaining a removal zone on each main surface of each pre-coated sheet by removing the pre-coating on the removal part enclosed between 30 % and 100% precoat thickness by laser removal at the weld edge of the precoated sheet;

- the stage of obtaining two pre-coated sheets includes:

- provision of two original pre-coated sheets,

- placing these two original pre-coated sheets side by side, leaving a given gap between them; and

- simultaneous removal, by laser removal, of the pre-coating on two adjacent original pre-coated sheets in order to simultaneously create a removal zone on the adjacent surfaces of said two original pre-coated sheets, and the laser beam overlaps the two adjacent original pre-coated sheets during the removal stage, and optionally, during the welding step, the thus prepared two adjacent pre-coated sheets are welded with a laser beam, the spot of which is overlapped by the two adjacent pre-coated sheets, the time between the end of the laser removal and the start of the welding being preferably less than or equal to 10 seconds;

- in addition, the method includes, prior to butt welding, the preparation of the weld edge of at least one of the pre-coated sheets, using at least one of the following process steps: brushing, machining, countersinking and/or bevelling;

- the welding stage is carried out using a laser beam;

the two pre-coated sheets have the same thickness;

the two pre-coated sheets have different thicknesses;

- for at least one pre-coated sheet, and, for example, for both pre-coated sheets, the steel substrate contains, by weight:

0.15% ≤ C ≤ 0.25%

0.8% ≤ Mn ≤ 1.8%

0.1% ≤ Si ≤ 0.35%

0.01% ≤ Cr ≤ 0.5%

Ti ≤ 0.1%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.005%,

the rest is iron and impurities that appeared during production.

- for one pre-coated sheet, the substrate steel contains, by weight:

0.040% ≤ C ≤ 0.100%

0.80% ≤ Mn ≤ 2.00%

Si ≤ 0.30%

S ≤ 0.005%

P ≤ 0.030%

0.010% ≤Al ≤0.070%

0.015% ≤ Nb ≤ 0.100%

Ti ≤ 0.080%

N ≤ 0.009%

Cu ≤ 0.100%

Ni ≤ 0.100%

Cr ≤ 0.100%

Mo ≤ 0.100%

Ca ≤ 0.006%,

the rest is iron and impurities that appeared during production.

- for one pre-coated sheet, the substrate steel contains, by weight:

0.24% ≤ C ≤ 0.38%

0.40% ≤ Mn ≤ 3%

0.10% ≤ Si ≤ 0.70%

0.015% ≤ Al ≤ 0.070%

0% ≤ Cr ≤ 2%

0.25% ≤ Ni ≤ 2%

0.015% ≤ Ti ≤ 0.10%

0% ≤ Nb ≤ 0.060%

0.0005% ≤ B ≤ 0.0040%

0.003% ≤ N ≤ 0.010%

0.0001% ≤ S ≤ 0.005%

0.0001% ≤ P ≤ 0.025%,

where the content of titanium and nitrogen satisfies the following relationship:

Ti/N > 3.42,

and the content of carbon, manganese, chromium and silicon satisfies the following ratio:

,

,

steel optionally includes one or more of the following:

0.05% ≤ Mo ≤ 0.65%

0.001% ≤ W ≤ 0.30%

0.0005% ≤ Ca ≤ 0.005%,

the rest is iron and impurities that inevitably appear during production.

- welding is carried out using a shielding gas, in particular helium and/or argon.

In addition, the invention relates to a method for producing a welded, hot-pressed and cooled steel part, which includes the successive steps:

- the implementation of the method as defined above, in order to obtain a welded steel billet;

- heating the welded steel billet in order to obtain a fully austenitic structure in the substrates of the pre-coated sheet;

- hot pressing of a welded steel billet in a pressing apparatus in order to obtain a steel part; and

- cooling of the steel part in the pressing apparatus.

According to a particular embodiment of said process for producing a welded, hot-formed, and cooled steel part, during the cooling step, the cooling rate is greater than or equal to the cooling rate of bainite or martensite for the most hardenable precoated sheet among the substrates.

In addition, the invention relates to a welded steel billet comprising two pre-coated sheets, where each pre-coated sheet includes a steel substrate having a pre-coating on each of the main surfaces, where the pre-coating includes an intermetallic alloy layer,

comprising at least iron and aluminum, and optionally, a metal alloy layer extending over the intermetallic alloy layer, wherein the metal alloy layer is an aluminum layer, an aluminum alloy layer, or an aluminum-based alloy layer,

pre-coated sheets joined by a weld, wherein the weld has an average aluminum content between 0.1 wt% and 1.2 wt%, and the weld is further characterized by:

(критерий C1), где:(a) a weld hardening factor FT _WJ such that

(criterion C1), where:

- FT _BM is the hardening factor of the least hardenable steel substrate among the steel substrates of the two pre-coated sheets, and

- hardening factors FT _WJ and FT _BM are determined using the following formula: FT= 128 + 1553xC + 55xMn + 267xSi + 49xNi + 5xCr - 79xAl - 2xNi² - 1532xC² - 5xMn² - 127xSi² - 40xCxNi - 4xNixMn, where Al, Cr, Ni, C , Mn and Si respectively mean the average content of aluminum, chromium, nickel, carbon, manganese and silicon, expressed as a percentage by mass, in the area for which the hardening factor is to be determined, this area being the weld, in the case of FT _WJ , and the least hardenable substrate in the case of FT _BM , and

(b) the average nickel content Ni _WJ in the weld satisfies the following relationship: Ni _WJ ≤ 14-3.4xAl _WJ , where Al _WJ is the average aluminum content in the weld (criterion C2); and

(c) the average chromium content Cr _WJ in the weld satisfies the following relationship: Cr _WJ ≤ 5-2xAl _WJ , where Al _WJ is the average aluminum content in the weld (criterion C3), and

each pre-coated sheet comprising, on each of the two main surfaces adjacent to the weld, an intermediate zone in which the pre-coat is removed over the part to be removed, comprised between 30% and 100% of the thickness of the pre-coat.

According to specific embodiments of the welded steel billet, the welded steel billet includes one or more of the following features, taken alone or in any technically feasible combination:

- the steel of the substrate of at least one of the pre-coated sheets, and, for example, two pre-coated sheets contains, by weight:

0.10% ≤ C ≤ 0.5%

0.5% ≤ Mn ≤ 4.5%

0.1% ≤ Si ≤ 1%

0.01% ≤ Cr ≤ 1%

Ti ≤ 0.2%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.010%,

the rest is iron and impurities that appeared during production,

- the substrate of each pre-coated sheet is made of steel, hardened by pressing;

- for the pre-coated sheet, the width of the intermediate zone is between 5 µm and 2000 µm from the edge of the weld;

- for at least one pre-coated sheet, and, for example, for both pre-coated sheets, the part to be removed is equal to 100% of the thickness of the pre-coating;

- for at least one pre-coated sheet, and for example for both pre-coated sheets, the part to be removed is strictly less than 100% of the thickness of the pre-coating;

- for at least one pre-coated sheet, and for example for both pre-coated sheets, the pre-coating comprises a metal alloy layer extending over the intermetallic alloy layer, the metal alloy layer being an aluminum layer, an aluminum alloy layer or an aluminum-based alloy layer, and where for at least one pre-coated sheet, and for example both pre-coated sheets, the metal alloy layer is removed throughout the thickness, while the intermetallic alloy layer remains intact in the removal zone on each major surface of the pre-coated sheet;

- the nickel content in the weld is between 0.1 wt.% and 13.6 wt.%, and more specifically between 0.2 wt.% and 12.0 wt.%;

— the welded steel billet is such that, after hot pressing and cooling, the Charpy impact strength of the weld at 20°C is greater than or equal to 25 J/cm²; and the tensile strength of the welded steel billet, formed by hot pressing and cooled, is greater than or equal to the tensile strength of the weakest substrate among the substrates of the pre-coated sheet, and the weakest substrate means the substrate for which the product of the thickness and the tensile strength after hot pressing and cooling is minimal;

- the weld is such that after hot pressing and cooling, the maximum change in hardness ΔHV(WJ) across the weld is less than or equal to 20% of the average hardness HV _mean (WJ) of the weld;

- each intermediate zone includes a striation of solidification, and the striation of solidification on adjacent main surfaces of the two pre-coated sheets is symmetrical about the vertical median plane between the two pre-coated sheets;

- each intermediate zone includes an inner edge located on the weld and an outer edge located away from the weld, and where the distance between the outer edges of adjacent intermediate zones of the two pre-coated sheets is constant along the longitudinal direction of the weld;

- for at least one pre-coated sheet, and, for example, for both pre-coated sheets, the substrate steel contains, by weight:

0.15% ≤ C ≤ 0.25%

0.8% ≤ Mn ≤ 1.8%

0.1% ≤ Si ≤ 0.35%

0.01% ≤ Cr ≤ 0.5%

Ti ≤ 0.1%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.005%,

the rest is iron and impurities that appeared during production;

- for one pre-coated sheet, the substrate steel contains, by weight:

0.040% ≤ C ≤ 0.100%

0.80% ≤ Mn ≤ 2.00%

Si ≤ 0.30%

S ≤ 0.005%

P ≤ 0.030%

0.010% ≤Al ≤0.070%

0.015% ≤ Nb ≤ 0.100%

Ti ≤ 0.080%

N ≤ 0.009%

Cu ≤ 0.100%

Ni ≤ 0.100%

Cr ≤ 0.100%

Mo ≤ 0.100%

Ca ≤ 0.006%,

and the rest is iron and impurities that appeared during production;

- for one pre-coated sheet, the substrate steel contains, by weight:

0.24% ≤ C ≤ 0.38%

0.40% ≤ Mn ≤ 3%

0.10% ≤ Si ≤ 0.70%

0.015% ≤ Al ≤ 0.070%

0% ≤ Cr ≤ 2%

0.25% ≤ Ni ≤ 2%

0.015% ≤ Ti ≤ 0.10%

0% ≤ Nb ≤ 0.060%

0.0005% ≤ B ≤ 0.0040%

0.003% ≤ N ≤ 0.010%

0.0001% ≤ S ≤ 0.005%

0.0001% ≤ P ≤ 0.025%,

moreover, the content of titanium and nitrogen satisfies the following relationship:

Ti/N > 3.42

and the content of carbon, manganese, chromium and silicon satisfies the following ratio:

,

,

steel optionally includes one or more of the following:

0.05% ≤ Mo ≤ 0.65%

0.001% ≤ W ≤ 0.30%

0.0005% ≤ Ca ≤ 0.005%,

and the rest is iron and impurities that inevitably appear during production.

In addition, the invention relates to a welded, hot-pressed and cooled steel part, containing the first part of the coated steel part and the second part of the coated steel part, each part of the coated steel part, includes a steel substrate having a coating on at least one of the main surfaces , including at least iron and aluminum,

moreover, the first and second coated parts of the steel part are connected by a weld, the weld has an average aluminum content between 0.1 wt.% and 1.2 wt.%, and the weld is further characterized in that:

, где:(a) the quenching factor FT _WJ of the weld meets criterion C1

, where:

- FT _BM means the hardening factor of the least hardenable steel substrate among the steel substrates of the two pre-coated sheets, and

- hardening factors FT _WJ and FT _BM are determined using the following formula: FT = 128 + 1553xC + 55xMn + 267xSi + 49xNi + 5xCr - 79xAl - 2xNi² - 1532xC² - 5xMn² - 127xSi² - 40xCxNi - 4xNixMn, where Al, Cr, Ni, C , Mn and Si mean, respectively, the average content of aluminum, chromium, nickel, carbon, manganese and silicon, expressed as a percentage by mass, for the area in which the hardening factor is to be determined, this area being the weld in the case of FT _WJ and the least hardenable substrate in case of FT _BM ,

(b) the average nickel content Ni _WJ in the weld satisfies the following relationship: Ni ≤ 14 – 3.4xAl _WJ , where Al _WJ is the average aluminum content in the weld (criterion C2); and

(c) the average chromium content Cr _WJ in the weld satisfies the following relationship: Cr ≤ 5 - 2xAl _WJ , where Al _WJ is the average aluminum content in the weld (criterion C3), and

each part of the coated steel part, including on each of the two main surfaces adjacent to the weld, an intermediate zone in which the thickness of the coating is strictly less than in the adjacent zones of the part of the coated steel part located at a greater distance from the weld than the intermediate zone, or where coverage is not available.

According to particular embodiments of the welded, hot-formed, and chilled steel part, the welded, hot-formed, and chilled steel piece may include one or more of the following features, taken alone or in any technically feasible combination:

- the steel substrate of at least one of the first and second parts of the steel part, and, for example, the first and second parts of the steel part, contains, by weight:

0.10% ≤ C ≤ 0.5%

0.5% ≤ Mn ≤ 4.5%

0.1% ≤ Si ≤ 1%

0.01% ≤ Cr ≤ 1%

Ti ≤ 0.2%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.010%,

the rest is iron and impurities that appeared during production,

- the substrate of each of the first and second parts of the steel part is made of steel, hardened by pressing;

- each intermediate zone includes a solidification striation, and the solidification striation on adjacent main surfaces of the two coated parts of the steel part is symmetrical about the vertical median plane between the two coated parts of the steel part;

- each intermediate zone contains an inner edge located on the weld and an outer edge located away from the weld, and where the distance between the outer edges of adjacent intermediate zones of the two coated parts of the steel part is constant along the longitudinal direction of the weld;

- the average Vickers hardness HV _mean (WJ) in the weld is less than or equal to 700 HV;

- the average nickel content in the weld is between 0.1 wt.% and 13.6 wt.%, and more specifically between 0.2 wt.% and 12.0 wt.%;

- the Charpy impact strength of the weld at 20°C is greater than or equal to 25 J/cm² and the tensile strength of the welded, hot-pressed and cooled steel part is greater than or equal to the tensile strength of the weakest substrate among the substrates of the coated parts of the steel part , and the weakest is the substrate for which the product of thickness and tensile strength is minimal;

- the maximum change in hardness ΔHV(WJ) across the weld is less than or equal to 20% of the average hardness HV _mean (WJ) of the weld;

- the steel substrate of at least one among the first and second parts of the coated steel part, and, for example, the first and second parts of the coated steel part contains, by weight:

0.15% ≤ C ≤ 0.25%

0.8% ≤ Mn ≤ 1.8%

0.1% ≤ Si ≤ 0.35%

0.01% ≤ Cr ≤ 0.5%

Ti ≤ 0.1%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.005%,

the rest is iron and impurities that appeared during production;

- the steel substrate of one among the first and second parts of the coated steel part contains, by weight:

0.040% ≤ C ≤ 0.100%

0.80% ≤ Mn ≤ 2.00%

Si ≤ 0.30%

S ≤ 0.005%

P ≤ 0.030%

0.010% ≤Al ≤0.070%

0.015% ≤ Nb ≤ 0.100%

Ti ≤ 0.080%

N ≤ 0.009%

Cu ≤ 0.100%

Ni ≤ 0.100%

Cr ≤ 0.100%

Mo ≤ 0.100%

Ca ≤ 0.006%,

the rest is iron and impurities that appeared during production;

- the steel substrate of one among the first and second parts of the coated steel part contains, by weight:

0.24% ≤ C ≤ 0.38%

0.40% ≤ Mn ≤ 3%

0.10% ≤ Si ≤ 0.70%

0.015% ≤ Al ≤ 0.070%

0% ≤ Cr ≤ 2%

0.25% ≤ Ni ≤ 2%

0.015% ≤ Ti ≤ 0.10%

0% ≤ Nb ≤ 0.060%

0.0005% ≤ B ≤ 0.0040%

0.003% ≤ N ≤ 0.010%

0.0001% ≤ S ≤ 0.005%

0.0001% ≤ P ≤ 0.025%,

where the content of titanium and nitrogen satisfies the following relationship:

Ti/N > 3.42

and the content of carbon, manganese, chromium and silicon satisfies the following ratio:

,

,

steel optionally includes one or more of the following:

0.05% ≤ Mo ≤ 0.65%

0.001% ≤ W ≤ 0.30%

0.0005% ≤ Ca ≤ 0.005%,

and the rest is iron and impurities that inevitably appear during production.

In addition, the invention relates to the use of a welded, hot-formed and chilled steel part as described above for the production of anti-tamper parts or energy absorbers for an automobile.

The invention can be better understood by reading the following description, given by way of example only, and with reference to the accompanying drawings, in which:

fig. 1 is a perspective view of a precoated sheet including a removal zone in the precoat at the periphery of the sheet;

fig. 2 is a perspective view of the original pre-coated sheet;

in fig. 3 shows a schematic sectional view of the initial stage of welding according to the method according to the invention,

in fig. 4 shows a schematic sectional view of the final stage of welding according to the method according to the invention, and

in fig. 5 shows a schematic sectional view of a welded steel billet according to the invention.

Throughout the patent application, the content of the elements is expressed as a percentage by weight (wt%).

The invention relates to a method for producing a welded steel billet 1.

The method includes the first stage of obtaining two pre-coated sheets 2.

As shown in FIG. 1, each pre-coated sheet 2 includes two main surfaces 4 and at least one side surface 13 extending between the two main surfaces 4, from one main surface 4 to the other. In the shown in FIG. 1, the pre-coated sheet 2 has four side surfaces 13. For example, the side surfaces 13 form an angle with one main surface 4 between 60° and 90°.

Each pre-coated sheet 2 includes a metal substrate 3 having, on each of the major surfaces, a pre-coat 5. The pre-coat 5 is applied to and contacts the substrate 3.

More specifically, the metal substrate 3 is a steel substrate.

More specifically, the steel substrate 3 is a steel having a ferrite-pearlite microstructure.

Preferably, the substrate 3 is made of a heat-treatable steel, more specifically a pressure-hardening steel and, for example, a boron-manganese steel such as, for example, 22MnB5 type steel.

According to one embodiment, the steel substrate 3 includes and specifically consists of (% by weight):

0.10% ≤ C ≤ 0.5%

0.5% ≤ Mn ≤ 3%

0.1% ≤ Si ≤ 1%

0.01% ≤ Cr ≤ 1%

Ti ≤ 0.2%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.010%,

the rest is iron and impurities that appeared during production.

More specifically, the steel substrate 3 includes and specifically consists of (% by mass):

0.15% ≤ C ≤ 0.25%

0.8% ≤ Mn ≤ 1.8%

0.1% ≤ Si ≤ 0.35%

0.01% ≤ Cr ≤ 0.5%

Ti ≤ 0.1%

Al ≤ 0.1%

S ≤ 0.05%

P ≤ 0.1%

B ≤ 0.005%,

the rest is iron and impurities that appeared during production.

Alternatively, the steel substrate 3 comprises and specifically consists of (% by weight):

0.040% ≤ C ≤ 0.100%

0.80% ≤ Mn ≤ 2.00%

Si ≤ 0.30%

S ≤ 0.005%

P ≤ 0.030%

0.010% ≤Al ≤0.070%

0.015% ≤ Nb ≤ 0.100%

Ti ≤ 0.080%

N ≤ 0.009%

Cu ≤ 0.100%

Ni ≤ 0.100%

Cr ≤ 0.100%

Mo ≤ 0.100%

Ca ≤ 0.006%,

the rest is iron and impurities that appeared during production.

Alternatively, the steel substrate 3 comprises and specifically consists of (% by weight):

0.24% ≤ C ≤ 0.38%

0.40% ≤ Mn ≤ 3%

0.10% ≤ Si ≤ 0.70%

0.015% ≤ Al ≤ 0.070%

0% ≤ Cr ≤ 2%

0.25% ≤ Ni ≤ 2%

0.015% ≤ Ti ≤ 0.10%

0% ≤ Nb ≤ 0.060%

0.0005% ≤ B ≤ 0.0040%

0.003% ≤ N ≤ 0.010%

0.0001% ≤ S ≤ 0.005%

0.0001% ≤ P ≤ 0.025%,

where the content of titanium and nitrogen satisfies the following relationship:

Ti/N > 3.42,

and the content of carbon, manganese, chromium and silicon satisfies the following ratio:

,

,

steel optionally includes one or more of the following:

0.05% ≤ Mo ≤ 0.65%

0.001% ≤ W ≤ 0.30%

0.0005% ≤ Ca ≤ 0.005%,

the rest is iron and impurities that inevitably appear during production.

According to one example, the substrates 3 of the two pre-coated sheets 2 have the same composition.

According to another example, the substrates 3 of the two pre-coated sheets 2 have a different composition. In particular, the two substrates 3 have a different composition, each being selected from the four compositions mentioned above. For example, the steel substrate 3 of one pre-coated sheet 2 has the first composition listed above, while the steel substrate 3 of the other pre-coated sheet 2 has a composition selected from the second, third, or fourth compositions listed above.

The substrate 3 may be obtained, depending on the desired thickness, by rolling and/or cold rolling followed by annealing, or by any other suitable method.

The substrate 3 preferably has a thickness between 0.8 mm and 5 mm, and more specifically, it is between 1.0 mm and 3.0 mm. The two pre-coated sheets 2 may have the same thickness or different thicknesses.

The precoat 5 is obtained by hot dip coating, that is to say by immersing the substrate 3 in a bath of molten metal.

The precoat 5 comprises at least an intermetallic alloy layer 9 in contact with the substrate 3. The intermetallic alloy layer 9 contains at least iron and aluminum. In particular, the intermetallic alloy layer 9 is formed by the interaction between the substrate 3 and the molten bath metal. More specifically, the intermetallic alloy layer 9 contains intermetallic compounds of the Fe _x -Al _y type, and more specifically Fe ₂ Al ₅ .

In the example shown in FIG. 1, the precoat 5 further includes a metal alloy layer 11 extending over the intermetallic alloy layer 9. The metal alloy layer 11 has a composition that is close to that of the molten metal in the bath. The layer is formed by molten metal entrained in the sheet as it travels within the molten metal bath during the hot dip coating. The metal alloy layer 11 is an aluminum layer, or an aluminum alloy layer, or an aluminum-based alloy layer.

In this context, the term aluminum alloy refers to an alloy containing more than 50% by weight of aluminum. The term aluminum-based alloy refers to an alloy in which aluminum is the main element, by weight.

For example, the metal alloy layer 11 is an aluminum alloy layer further including silicon. More specifically, the metal alloy layer 11 contains, by weight:

- 8% ≤ Si ≤ 11%,

- 2% ≤ Fe ≤ 4%,

the rest is aluminum and possible impurities.

The metal alloy layer 11 has, for example, a thickness between 19 µm and 33 µm or between 10 µm and 20 µm.

In the example shown in FIG. 1, where the precoat 5 includes a metal alloy layer 11, the thickness of the intermetallic alloy layer 9 is typically on the order of a few micrometers. In particular, the average layer thickness is usually between 2 and 8 micrometers.

The specific structure of the precoat 5 comprising an intermetallic alloy layer 9 and a metal alloy layer 11 obtained by hot dip coating is specifically disclosed in EP 2007545.

According to another embodiment, the precoat 5 comprises only the intermetallic alloy layer 9 as described above. In this case, the thickness of the intermetallic alloy layer 9 is, for example, between 10 µm and 40 µm. Said precoat 5 composed of the intermetallic alloy 9 can be obtained, for example, by pre-melting the precoat 5 including the intermetallic alloy layer 9 and the metal alloy layer 11 as described above. Said pre-fusion treatment is carried out at the selected temperature and holding time so that the pre-coat 5 fuses to the substrate 3 for at least a portion of the thickness of the pre-coat 5. More specifically, the pre-fusion treatment may include the following steps: between 700°C and 900°C and holding the pre-fused sheet at this temperature for a time between 2 minutes and 200 hours. In this case, the intermetallic alloy layer 9 may be composed of various intermetallic sublayers such as Fe ₂ Al ₅ , FeAl ₃ , FeAl, Fe ₆ Al ₁₂ Si ₅ and FeAl ₃ sublayers.

Advantageously, as shown in FIG. 1, the substrate 3 has a precoat 5 as described above on both main surfaces 4.

In addition, as shown in FIG. 1, for each pre-coated sheet 2, the pre-coat 5 is removed at the edge of the weld 14 of the pre-coated sheet 2, on each major surface 4 of the pre-coated sheet 2, to create a removal zone 18 at the edge of the weld 14. More specifically, the pre-coat 5 is removed above the part F to be eliminated, which is between 30% and 100% (including borders) of the thickness of the precoat 5.

The edge of the weld 14 includes a peripheral part of the pre-coated sheet 2, which is designed to be at least partially included inside the weld 22 during butt welding. More specifically, the edge of the weld 14 includes a side surface 13 of the pre-coated sheet 2 and a part of the pre-coated sheet 2 extending from this side surface 13 and including a part of the pre-coat 5 and a part of the substrate 3.

The removal of the removable portion F of the precoat 5 at the edge of the weld 14 is preferably carried out using a laser beam, that is, by laser removal.

The removal zone 18 may extend in a width between 0.5 mm and 3 mm from the side surface 13 of the sheet 2.

Preferably, the part F to be removed is strictly less than 100%, which means that only part of the precoat 5 is removed in the removal zone 18, while part of the coating remains.

For example, in the embodiment shown in FIG. 1, in the removal zone 18, the metal alloy layer 11 is removed while the intermetallic alloy layer 9 remains for at least part of the layer thickness. In this case, the remaining layer of intermetallic alloy 9 protects the regions of the welded billet 1 immediately adjacent to the weld 22 from oxidation and decarburization during subsequent hot pressing steps and from corrosion during operation.

According to an embodiment, during the removal step, the intermetallic alloy layer 9 remains intact or retains a portion of the original thickness strictly less than 100%, such as only 60%, 80% or 90% of the original thickness, for example.

According to an alternative embodiment (not shown), during the stripping step, the precoat 5 is removed throughout the thickness in the stripping zone 18. In this embodiment, the part to be removed corresponds to 100% of the thickness of the precoat 5. In this embodiment, the precoat 5 is absent in the stripping zone eighteen.

More specifically, the method includes, prior to the provisioning step, the step of obtaining two pre-coated sheets 2 as shown in FIG. 1 from respective original pre-coated sheets 2' as shown in FIG. 2.

The original pre-coated sheets 2' have substantially the same geometry and composition as the pre-coated sheets 2, the only difference being the absence of removal zones 18. In other words, the pre-coat 5 of the original pre-coated sheets 2' remains intact on both main surfaces 4 of the original pre-coated sheets 2'. The coating remains completely on the two main surfaces of the original pre-coated sheets 2'.

This step includes the sub-step of obtaining a removal zone 18 on each main surface 4 of each pre-coated sheet 2 by removing the pre-coat 5 over the removal portion F at the edge of the weld 14 using laser removal.

Optionally, the method further includes the step of preparing the edge of the weld 14 on at least one of the pre-coated sheets 2, and for example both pre-coated sheets 2.

The preparation of the edge of the weld 14 may include at least one of the following process steps:

- brushing the edge of the weld 14,

- machining of the edge of the weld 14,

- countersinking of the edge of the weld 14, and/or

- beveling the edge of the weld 14.

The brushing step provides for partial removal of traces of the precoat 5 on the edge of the weld 14, and more particularly on the side surface 13 resulting from the mechanical cutting process and/or removal of the precoat 5 on the edge of the weld 14.

Countersinking or beveling the edge of the weld 14 allows you to increase the amount of added filler material without forming an excessive thickness of the weld 22.

Machining of the edge of the weld 14 is carried out in the case where the shape of the edge of the weld 14 before processing is not sufficiently correct for laser welding.

In addition, the method includes the step of butt-welding the pre-coated sheets 2, after optional preparation of the edge of the weld 14, using filler wire 20 to obtain a welded steel billet 1.

Fig. 3 and 4 illustrate two stages of welding in order to obtain a welded steel billet 1.

In the shown in FIG. 3 and 4, the two pre-coated sheets 2 are the pre-coated sheets shown in FIG. 1 which include a removal zone 18 at the respective edges of the weld 14 where the metal alloy layer 11 is removed through its entire thickness while the intermetallic alloy layer 9 remains intact.

The welding process leads to the formation of a zone of molten metal at the junction between two sheets 2, which subsequently solidifies, forming a weld 22.

The welding step is specifically a laser welding step where the laser beam 24 is directed towards the joint between two sheets 2. Said laser beam 24 is shaped to melt the filler wire 20 at the point of impact 26 of the laser beam 24.

The laser welding step is carried out, for example, using a CO ₂ laser or a solid state laser.

Preferably the laser source is a high energy laser source. For example, the source may be selected from a CO ₂ laser with a wavelength of approximately 10 micrometers, a solid state laser with a wavelength of approximately 1 micrometer, or a semiconductor laser source such as a diode laser with a wavelength between approximately 0.8 and 1 micrometer.

The power of the laser source is chosen depending on the thickness of the sheets 2. In particular, the power is chosen in such a way as to ensure the melting of the filler wire 20 and the weld edge 14 of the sheets 2, as well as effective mixing in the weld 22. For example, for a CO ₂ laser, the power is between 3 kW and 12 kW. For example, for a solid state laser or a semiconductor laser, the power is between 2 kW and 8 kW.

The diameter of the laser beam 24 at the point of impact 26 on the sheets 2 can be approximately 600 µm for both types of laser sources.

During the welding stage, the process is carried out, for example, in a protective atmosphere. This protective atmosphere, in particular, prevents oxidation and decarburization of the area where welding is carried out, as well as the formation of boron nitride in the weld 22 and possible cold cracking due to absorption of hydrogen.

An example of a protective atmosphere is an inert gas or a mixture of inert gases. The inert gas may be helium or argon or a mixture of these gases.

During said welding step, the distance between the front side surfaces 13 of the two sheets 1 is, for example, less than or equal to 0.3 mm, and more particularly less than or equal to 0.1 mm. Providing said gap between the front side surfaces 13 of the two sheets 1 promotes the deposition of filler metal during the welding process and prevents the formation of excess thickness of the weld 22. The deposition of filler metal and the prevention of excess thickness is also improved when, during the preparation stage, 14 sheets 2 has a small groove or beveled edge.

In particular, the average content of aluminum Al _WJ in the weld 22 is between 0.1 wt.% and 1.2 wt.%. More specifically, the average content of aluminum Al _WJ in the weld 22 is greater than or equal to 0.15 mass%. For example, the average content of aluminum Al _WJ in the weld 22 is less than or equal to 0.8 mass%.

The indicated average aluminum content Al _WJ is the result of the fact that part of the precoat 5 may remain in the removal zone 18 after removal of the removal part F, as well as traces of aluminum located on the side surface (surfaces) 13, at the edge of the weld (seams ) 14 as a result of the removal and/or cutting processes. In weld 22 aluminum is mixed with steel substrate 3 and filler wire 20.

During the welding step, the proportion of filler wire 20 added to the weld pool is between 10% and 50%, for example, and more specifically between 10% and 40%.

According to the invention, the composition of the filler wire 20 and the proportion of the filler wire 20 added to the weld pool are chosen so that the weld 22 thus obtained has the following characteristics:

, (a) hardening factor FT _WJ of weld 22 such that criterion C1 is met

,

where:

- FT _BM means the hardening factor of the least hardenable steel substrate 3 among the steel substrates 3 of the two pre-coated sheets 2, and

- hardening factors FT_WJ and FT_BMdetermined using the following formula: FT= 128 + 1553xC + 55xMn + 267xSi + 49xNi+ 5xCr- 79xAl - 2xNi² - 1532xC² - 5xMn² - 127xSi² - 40xCxNi - 4xNixMn, where Al, Cr, Ni, C, Mn and Si mean respectively the average aluminum content , chromium, nickel, carbon, manganese and silicon, expressed as a percentage by mass, in the area for which the hardening factor should be determined, the specified area is the weld 22 in the case of FT_WJand least hardened substrate 3 in case of FT_BM;

(b) the average nickel content Ni _WJ in the weld 22 satisfies the following relationship: Ni _WJ ≤ 14-3.4xAl _WJ , where Al _WJ is the average aluminum content in the weld (criterion C2); and

(c) the average chromium content Cr _WJ in weld 22 satisfies the following relationship: Cr _WJ ≤ 5-2xAl _WJ , where Al _WJ is the average aluminum content in weld 22 (criterion C3).

The least reinforced substrate 3 among the substrates 3 of the pre-coated sheets 2 is the substrate 3 having the lowest carbon content.

Indeed, the inventors of the present invention unexpectedly found that, in the case of the cumulative fulfillment of the criteria C1, C2 and C3, the part obtained from the specified welded steel billet 1 after heat treatment, including the stage of astenization (hot pressing and cooling in the pressing apparatus), shows an increased Charpy impact strength. (greater than or equal to 25J/cm² at 20°C) in the weld 22, wherein the tensile strength is greater than or equal to the tensile strength of the weakest substrate among the substrates 3 of the pre-coated sheets 2.

The weakest substrate 3 is the substrate for which the product of thickness and tensile strength after hot pressing and cooling is minimal.

In particular, the Charpy impact strength of the weld 22 at 20°C is greater than or equal to 25 J/cm², which makes it possible to completely eliminate brittle fracture in the weld.

Therefore, in the case of the cumulative fulfillment of criteria C1, C2 and C3, the presence of the weld 22 does not impair the performance of the welded steel part obtained by hot pressing and cooling from the welded billet, compared with the performance, after hot pressing and cooling, of the weakest substrate 3 among the substrates 3 pre-coated sheets 2, even if the weld 22 includes a relatively high aluminum content.

Thus, using the method according to the invention, it is possible to obtain parts having a satisfactory crash safety, despite the possibly relatively high aluminum content in the weld 22.

Preferably, the composition of the filler wire 20 and the proportion of the filler wire 20 added to the weld pool are further selected such that the average nickel content Ni _WJ in the weld 22 is between 0.1 wt% and 13.7 wt%, and more specifically between 0.2 wt.% and 12.0 wt.%.

For example, the composition of the filler wire 20 and the proportion of the filler wire 20 added to the weld pool are further selected such that the average chromium content Cr _WJ in the weld 22 is greater than or equal to 0.05 mass%. Said chromium content in the weld is advantageous as it improves the corrosion resistance and hardenability of the weld 22.

Preferably, the composition of the weld 22 is such that it has a predominantly martensitic microstructure after hot pressing and cooling. The expression "mainly" means that the weld contains at least 95% martensite, and more specifically 100% martensite.

The filler wire 20 in particular has a carbon content between 0.01 wt% and 0.45 wt%. According to the example, the carbon content of the filler wire 20 is greater than or equal to the carbon content of the least hardenable substrate 3 among the substrates 3 of the two pre-coated sheets 2.

Indeed, the present inventors have surprisingly found that, in order to reduce the risk of carbon segregation and hence hardness peaks in the weld 22 after hot pressing and cooling in the press apparatus, especially in the presence of a significant amount of aluminum in the weld 22, the carbon content of the filler wire should be enclosed between 0.01 wt.% and 0.45 wt.%. Therefore, using said filler wire 20 reduces the risk of brittleness of the weld 22 and partially eliminates damage to the weld 22 for a part obtained after hot pressing and cooling in a tensile press in a perpendicular direction to the weld 22.

, где ΔHV(WJ) означает разность между максимальной и минимальной твердостью, измеренной в сварном шве 22 и HV_mean(WJ) представляет собой среднюю твердость, измеренную в сварном шве 22.In particular, the inventors of the present invention have found that the carbon content of the filler wire must be between 0.01 wt.% and 0.45 wt.% in order to be able to obtain a weld 22 in which the maximum change in hardness ΔHV(WJ ) across weld 22 is less than or equal to 20% of the average hardness HV _mean (WJ) of weld 22, in other words,

where ΔHV(WJ) is the difference between the maximum and minimum hardness measured in weld 22 and HV _mean (WJ) is the average hardness measured in weld 22.

Preferably, the filler wire 20 has a manganese content strictly less than the manganese content of the substrates 3 of the pre-coated sheets 2.

For example, filler wire 20 has the following composition, by weight:

0.01% ≤ C ≤ 0.45%, and for example 0.02% ≤ C ≤ 0.45%

0.001% ≤ Mn ≤ 0.45%, and, for example, 0.05% ≤ Mn ≤ 0.45%, more specifically

0.05% ≤ Mn ≤ 0.20%,

0.001% ≤ Si ≤ 1%

0.02% ≤ Ni ≤ 56%, and for example between 0.2% ≤ Ni ≤ 10.0%,

0.001% ≤ Cr ≤ 30%

0.001% ≤ Mo ≤ 5%

0.001% ≤ Al ≤ 0.30%

0.001% ≤ Cu ≤ 1.80%

0.001% ≤ Nb ≤ 1.50%

0.001% ≤ Ti ≤ 0.30%

0.001% ≤ N ≤ 10%

0.001% ≤ V ≤ 0.1%

0.001% ≤ Co ≤ 0.20%,

the rest is iron and inevitable impurities.

In the filler wire composition example above, the Mn, Si, Cr, Mo, Al, Cu, Nb, Ti, N, V, and Co contents of approximately 0.001% correspond to traces of the above elements at the level of impurities arising from the melting of raw materials and from design or is the result of the accuracy of measuring devices at a very low content of elements, which in fact can mean the complete absence of the element in the analyzed steel, which is measured as the presence in a very small amount, or that the element is present in a very small amount, which is measured as the absence in the steel .

For example, the filler wire 20 consists of the above elements.

For example, filler wire 20 is a hard wire or flux-cored wire.

The invention also relates to a welded steel billet 1, which can be obtained using the above method.

An example of such a welded steel billet is shown in FIG. 5.

The welded steel billet 1 comprises two pre-coated sheets 2, each pre-coated sheet 2 including a steel substrate 3 having a pre-coat 5 on each of the major surfaces 4, a pre-coat 5 including a layer of an intermetallic alloy 9 containing at least iron and aluminum and, optionally, a metal alloy layer 11 extending over the intermetallic alloy layer 9, wherein the metal alloy layer 11 is an aluminum layer, an aluminum alloy layer, or an aluminum-based alloy layer, where the pre-coated sheets 2 are connected by a weld 22.

The welded steel billet 1 comprises on each side of the weld 22 an intermediate zone 28 in which the precoat 5 has been removed above the removal zone F as described above.

Moreover, as can be seen in FIG. 5, each intermediate zone 28 includes an inner edge 30 located on the weld 22 and an outer edge 32 located away from the weld 22.

The width W of each intermediate zone 28, measured from the edge of the weld 22, that is, the distance between the inner edge 30 and the outer edge 32, is between 5 µm and 2000 µm, and more specifically between 5 µm and 1500 µm.

Preferably, in the intermediate zone 28, the precoat 5 is removed above the part F to be removed, which is strictly less than 100%. In particular, the metal alloy layer 11 is removed, but the intermetallic alloy layer 9 remains intact.

According to an alternative, in the intermediate zone 28, the precoat 5 is removed above the part F to be removed equal to 100%, that is, over the entire thickness.

Therefore, in the intermediate zone 28, the thickness of the pre-coat 5 is strictly less than in the zones of the pre-coated sheets 2 further away from the weld 22, or even absent.

The intermediate zone 28 follows from the removal zone 18 on the respective pre-coated sheets 2.

The pre-coated sheets 2 and the weld 22 have the features described above in connection with the production method of the welded steel billet 1.

Therefore, the weld 22 meets the criteria C1, C2 and C3 defined above.

In addition, the average content of aluminum Al _WJ in the weld 22 is between 0.1 wt.% and 1.2 wt.%. More specifically, the average content of aluminum Al _WJ in the weld 22 is greater than or equal to 0.15 mass%. The average content of aluminum Al _WJ in the weld 22 is, for example, less than or equal to 0.8 mass%.

The weld 22 is, for example, such that after hot pressing and cooling in the pressing apparatus, the Charpy impact strength of the weld 22 at 20° C. is greater than or equal to 25 J/cm².

In addition, after hot pressing and cooling, the tensile strength of the hot-pressed and cooled welded steel billet is greater than or equal to the tensile strength of the weakest substrate among the substrates 3 of the pre-coated sheets 2. In this context, the weakest substrate is determined as described above.

For example, the average nickel content Ni _WJ in the weld 22 is between 0.1 wt.% and 13.6 wt.%, and more specifically between 0.2 wt.% and 12.0 wt.%.

For example, the average content of chromium Cr _WJ in the weld 22 is greater than or equal to 0.05 mass%.

. The weld 22 is, for example, such that after hot pressing and cooling in the press apparatus, the maximum change in hardness ΔHV(WJ) across the weld 22 is less than or equal to 20% of the average hardness HV _mean (WJ) of the weld 22. In other words ,

.

The weld 22 is, for example, such that the average hardness HV _mean (WJ) of the weld 22, after hot pressing and cooling in the pressing apparatus, is less than or equal to 700 HV.

Preferably, the composition of the weld 22 is such that it has a predominantly martensitic microstructure after hot pressing and cooling. The expression "mainly" means that the structure contains at least 95% martensite, and more specifically 100% martensite.

In addition, the invention relates to a method for producing a welded, hot-pressed and cooled steel part, which includes:

- obtaining a welded steel billet 1 using the method described above;

- heating the welded steel billet 1 in order to obtain a fully austenitic structure in the substrates 3 of the pre-coated sheets 2 constituting the welded billet 1;

- hot pressing of the welded steel billet 1 in the pressing apparatus in order to obtain a steel part; and

- cooling of the steel part in the pressing apparatus.

More specifically, during the heating step, the welded steel billet 1 is heated to an austenitizing temperature. Then, the workpiece is held at the austenitization temperature for a holding time depending on the thickness of the sheets 2 forming the welded steel workpiece 1. The holding time is selected depending on the austenitization temperature so that the welded workpiece 1 undergoes austenitization and in such a way that an intermetallic layer of a given thickness is formed. by fusing between the substrates 3 and the precoat 5. For example, the holding time is approximately 5 minutes.

Prior to hot pressing, the welded steel billet 1 thus heated is moved into the hot pressing apparatus. Preferably, the travel time is between 5 and 10 sec. The travel time is chosen to be as short as possible in order to avoid metallurgical transformations in the welded steel billet 1 prior to hot pressing.

During the cooling step, the cooling rate is greater than or equal to the critical cooling rate of martensite or bainite for at least one of the substrates 3 of the two steel sheets 2, and, for example, from the most hardenable steel sheet 1, that is, the steel sheet having the least critical cooling rate.

After cooling, the weld 22 has a predominantly martensitic microstructure. The expression "mainly" means that the structure contains at least 95% martensite, and more specifically 100% martensite.

In addition, the invention relates to a welded, hot-pressed and cooled steel part obtained using the method described above.

More specifically, said steel part includes the first part of the coated steel part and the second part of the coated steel part, respectively, resulting from hot pressing and cooling in the pressing apparatus of two pre-coated steel sheets 2.

More specifically, each part of the coated steel part includes a steel substrate having, on each of the main surfaces, a coating including iron and aluminum, the first and second parts of the steel part being connected by a weld 22 as described above.

In particular, the coating of the first and second parts of the steel part is the result of at least partial fusion of the precoat 5 during hot pressing.

The substrates of the first and second parts of the steel part have the composition described above for the pre-coated sheets 2. They are formed by hot pressing and cooling the substrates 3 of the pre-coated sheets 2.

Each part of the steel part includes an intermediate zone adjacent to the weld 22 and on each surface of the part of the steel part. Said intermediate zone is formed from the intermediate zone 28 described in connection with the welded billet 1. In the intermediate zone, the thickness of the coating is strictly less than in the rest of the steel part, or even no coating.

The weld 22 meets the criteria C1, C2 and C3 defined above.

In addition, the weld 22 has an average aluminum content Al _WJ between 0.1 wt.% and 1.2 wt.%. For example, the average content of aluminum in the weld 22 is less than or equal to 0.15 mass%. For example, the average content of aluminum in the weld 22 is less than or equal to 0.8 mass%.

The Charpy impact strength of the weld 22 at 20°C is greater than or equal to 25 J/cm² and the tensile strength of the part is greater than or equal to the tensile strength of the weakest substrate among the substrates 3 of the coated steel part.

For example, weld 22 has an average nickel content Ni _WJ between 0.1 wt.% and 13.6 wt.%, and more specifically between 0.2 wt.% and 12.0 wt.%.

For example, weld 22 has an average chromium content Cr _WJ greater than or equal to 0.05 wt%.

.The weld is, for example, such that the maximum change in hardness ΔHV(WJ) across the weld is less than or equal to 20% of the average hardness HV _mean (WJ) of the weld 22. In other words,

.

The average hardness HV _mean (WJ) in the weld 22, for example, is less than or equal to 700 HV.

Preferably, the weld 22 has a predominantly martensitic microstructure. The expression "mainly" means that the structure contains at least 95% martensite, and more specifically 100% martensite.

The present inventors conducted experiments in which welded steel blanks 1 were obtained by laser butt welding together with two pre-coated sheets A and B using filler wire W.

Table 1 below lists the experimental conditions for each experiment performed from E1 to E22.

The original pre-coated sheets A and B provided were pre-coated 5 on both main surfaces 4 with a thickness of approximately 25 micrometers.

For all pre-coated sheets A and B tested, pre-coating 5 was obtained by hot dip coating in a bath of molten metal and included a metal alloy layer 11 and an intermetallic alloy layer 9.

The metal alloy layer 11 of the preliminary coating 5 includes, by weight:

Si: 9%

Fe: 3%

with the rest consisting of aluminum and possible impurities resulting from development.

The metal alloy layer 11 has an average total thickness of 20 µm.

The intermetallic alloy layer 9 contains intermetallic compounds of the Fe _x -Al _y type, and mainly Fe ₂ Al _3. Fe ₂ Al ₅ and Fe _x Al _y Si _z . The layer has an average thickness of 5 µm.

For all pre-coated sheets A and B tested, a removal zone 18 was created on both major surfaces by removing the metal alloy layer 11 while the intermetallic alloy layer remained intact. The removal was carried out by laser removal using the method disclosed in the previous application WO 2007/118939.

Table 1. List of experimental conditions

* no data.

In Table 1, experiments are underlined that are not in accordance with the invention.

In the above table, “0” in the column “Proportion of filler wire added to the weld pool,” means that no filler wire was added.

The steel substrates used in the various experiments shown in Table 1 have the compositions shown in Table 2 below, where the content is expressed in wt. %.

Table 2. Substrate compositions

%C %Mn %Si %Al %Cr %Ni %Ti %B %S %P S1 0.22 1.17 0.28 0.04 0.19 - 0.04 0.0031 0.002 0.014 S2 0.22 1.18 0.26 0.05 0.19 - 0.032 0.0032 0.002 0.016 S3 0.23 1.19 0.25 0.03 0.17 - 0.004 0.0026 0.0006 0.01 S4 0.24 1.24 0.27 0.04 0.17 - - - - -

For all substrates, the rest of the composition is iron, possible impurities and unavoidable elements that appear during production.

In Table 2 above, the “-” sign means that the substrate contains the maximum traces of the element in question.

Filler wires W used in various experiments and listed in table 1 have the composition shown below in table 3, where the content is expressed in mass. %.

Table 3. Filler wire composition W

Filler wire W %C %Mn %Si %Cr %Ni %Mo %P %S Other elements
cops (in %) W1 0.29 0.94 0.13 0.04 0.05 0.008 0.01 0.004 W2 0.36 0.38 0.29 1.75 3.79 0.27 0.009 <0.0015 W3 0.27 0.34 0.40 - 12.37 - <0.001 <0.001 W4 0.02 0.30 0.30 0.05 53.24 <0.021 <0.001 0.002 Al 0.058 W5 0.03 1.80 0.50 20.50 25.00 4.70 <0.001 <0.001

For all filler wires, the rest of the composition is iron, possible impurities and unavoidable elements that appear during production.

Unless otherwise stated, these filler wires may contain Al, Cu, Nb, Ti, N, V and Co in an amount of approximately 0.001%, which corresponds to traces of these elements.

The inventors then determined, for each experiment from E1 to E21, the composition of the resulting weld 22 using conventional measurement methods.

The average content of manganese, aluminum, nickel, chromium and silicon in the weld 22 is determined by averaging over the entire surface of the analyzed weld using an energy dispersion spectral analyzer built into a scanning electron microscope. The average carbon content is determined using an electron Castaing microprobe, in the cross section of the sample, in the direction perpendicular to the weld 22. The results of these measurements are shown in Table 4 below.

Table 4. Measured content of elements in the weld

In addition, the inventors subjected the welded steel blanks 1 thus obtained to a heat treatment including austenitization followed by rapid cooling to obtain heat-treated parts. These heat-treated parts had the same properties as hot-pressed and cooled parts.

The inventors then carried out measurements to determine the mechanical characteristics of said parts (tensile strength of hot-pressed and cooled parts and Charpy impact strength of the weld).

In addition, the authors compared the measured tensile strength of hot-pressed and cooled parts (UTS _parts ) with the tensile strength of the weakest substrate after heat treatment (UTS _{weakest substrate} ).

The characteristics found in this way are shown in Table 5 below.

Table 5. Mechanical characteristics after heat treatment

In this table, “n.i.” means "not measured".

The tensile test was carried out at room temperature (about 20°C) using the procedure described in the following standards: NF EN ISO 4136 and NF ISO 6892-1 on a transverse welded tensile test specimen type EN 12.5 × 50 (240 × 30 mm ) extracted perpendicular to the direction of the laser weld. Five tensile tests were performed for each experiment (E1 to E21).

Charpy impact strength was measured using a standard Charpy impact test using a sample having a V-shaped notch in the weld 22; this V-notch had a depth of 2 mm and an overall width of 8 mm, and the accuracy of the position of the notch in the weld was better than or equal to 0.2 mm, where the width means the size of the sample parallel to the depth of the notch. The tests were carried out at 20°C.

Based on the measured composition of welds 22, the authors determined for each experiment from E1 to E21, whether the criteria C1 - C3, defined above, were met.

The results of these measurements are summarized below in Table 6.

Table 6. Definition of criteria for welds

Values that do not correspond to the invention are underlined.

As can be seen from Table 6, the experiments labeled E1, E2, E5 - E14, E17 and E18 are examples according to the invention: in these experiments criteria C1 - C3 are met.

On the contrary, the experiments labeled E3, E4, E15, E16 and E19 - E22 are not in accordance with the invention: in these experiments, at least one criterion among the criteria C1 - C3 is not fulfilled.

As can be seen from Table 5 above, in experiments E1, E2, E5 - E14, E17 and E18, in which the criteria C1 - C3 are met, the hot-pressed and cooled parts obtained from welded billets 1 have satisfactory mechanical properties, in in particular, the tensile strength is greater than or equal to the strength of the weakest substrate among the two substrates of the weld 1 after heat treatment, and the Charpy impact strength of the weld 22 at 20°C is greater than or equal to 25 J/cm².

Therefore, for the parts obtained from the blanks 1 according to the invention, the presence of the weld 22 does not impair the performance of the welded steel parts, compared to the properties of the weakest substrate 3 after hot pressing and cooling. Thus, said parts will have satisfactory crash safety regardless of the aluminum content in the weld.

On the contrary, in experiments E3, E4, E14 - E16 and E19 - E22, which do not correspond to the invention, since at least one among the criteria C1 - C3 is not fulfilled, at least one property - tensile strength of hot-pressed and cooled parts, or the Charpy impact strength of the weld are too low and therefore unsatisfactory. Therefore, for such parts, there is a risk that the part will fail in the weld, for example, in a collision situation.

Therefore, the method according to the invention is particularly advantageous as it provides a part which, after hot pressing and cooling in the pressing apparatus, has excellent mechanical characteristics, also in the weld 22, despite the aluminum content in the weld.

Therefore, the method is particularly well suited for the production of anti-tamper parts, structural parts or energy absorbers, which contributes to the safety of motor vehicles.

According to a specific embodiment of the method for producing a welded blank 1 according to the invention, the step of obtaining two pre-coated sheets 2 includes:

- providing two original pre-coated sheets 2',

- placing said two original pre-coated sheets 2' adjacent to each other, although a given gap remains between them; and

- simultaneous laser removal of the pre-coat 5 on two adjacent original pre-coated sheets 2' above the part F to be removed so as to simultaneously create a removal zone 18 on the adjacent surfaces of said two original pre-coated sheets 2', the laser beam overlapping the two adjacent original pre-coated sheet 2' during the removal step.

During the welding step, the two adjacent pre-coated sheets 2 thus obtained are welded with a laser beam whose spot overlaps the two original pre-coated sheets 2. Preferably, the time between the end of laser removal and the start of welding is less than or equal to 10 seconds.

Each intermediate zone 28 of the thus obtained welded billet 1 includes a solidification striation which is the result of laser removal.

By simultaneously removing the pre-coat 5 over the part F to be removed using a single laser beam that overlaps both original pre-coated sheets 2, the curing striation on the adjacent base surfaces 4 of the two pre-coated sheets 2 is symmetrical with respect to the vertical median plane M between the two pre-coated sheets 2 .

In addition, as noted earlier with respect to FIG. 5, each intermediate zone 28 includes an inner edge 30 located on the weld 22 and an outer edge 32 located away from the weld 22.

In this particular embodiment, due to the simultaneous removal method, the distance between the outer edges of the adjacent intermediate zones of the two pre-coated sheets is substantially constant along the longitudinal direction of the weld 22. The expression "substantially constant" means that the distance between the outer edges 32 of the adjacent intermediate zones of the two of coated sheets 2 changes by a maximum of 5% along the weld 22, that is, in the longitudinal direction of the weld 22.

The invention also relates to a part obtained by hot pressing and cooling of a welded blank 1 obtained using the method according to a particular embodiment of the invention.

Said hot-pressed and cooled part has the same features as described above.

In addition, in this part, each intermediate zone includes a solidification striation, and the solidification striation on the adjacent main surfaces 4 of the two coated parts of the steel part is symmetrical about the vertical median plane between the two coated parts of the steel part.

Preferably, each intermediate zone includes an inner edge located on the weld 22 and an outer edge located away from the weld 22, and the distance between the outer edges of the adjacent intermediate zones of the two coated parts of the steel part is substantially constant along the longitudinal direction of the weld 22. Expression "substantially constant" means that the distance between the outer edges of adjacent intermediate zones of two coated parts of the steel part varies by a maximum of 5% along the weld 22, that is, in the longitudinal direction of the weld 22.

Claims (247)

1. A method for producing a welded steel billet (1), including successive stages: - providing two pre-coated sheets (2), each pre-coated sheet (2) containing a steel substrate (3) has a pre-coat (5) on each of the two main surfaces (4), the pre-coat (5) contains an intermetallic alloy layer (9) containing at least iron and aluminum and optionally a metal alloy layer (11) extending over the intermetallic alloy layer (9), wherein the metal alloy layer (11) is an aluminum layer, an aluminum alloy layer, or a base alloy layer aluminum, each pre-coated sheet (2), containing on each main surface (4) of the sheet at the edge of the weld (14), intended to be included at least partially inside the weld (22), a removal zone (18) in which the pre-coating is removed (5) over a part to be removed (F) between 30% and 100% of the thickness of the precoat (5); butt-welding the pre-coated sheets (2) using a filler wire (20) to create a weld (22) at the junction between the pre-coated sheets (2), the weld (22) having an average aluminum content Al _WJ in the range 0, 1 wt.% - 1.2 wt.%, characterized in that: - the composition of the filler wire (20) and the proportion of the filler wire (20) added to the weld pool are selected from the condition of obtaining a weld (22) with the following characteristics: (a) hardening factor FT _WJ of the weld (22) such that FT _WJ - 0.96FT _BM ≥ 0 (criterion C1), where: - FT _BM is the hardening factor of the least hardenable steel substrate (3) among the steel substrates (3) of the two pre-coated sheets (2), and - hardening factors FT _WJ and FT _BM are determined using the following formula: FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi ² -1532xC ² -5xMn ² -127xSi ² -40xCxNi-4xNixMn, where Al, Cr , Ni, C, Mn and Si mean, respectively, the average content of aluminum, chromium, nickel, carbon, manganese and silicon, expressed as a percentage by weight of the area for which the hardening factor should be determined, and the specified area is the weld (22) in the case FT _WJ and the least hardened substrate in the case of FT _BM , (b) the average nickel content (Ni _WJ ) in the weld (22) satisfies the following relationship: Ni _WJ ≤ 14-3.4xAl _WJ , where Al _WJ is the average aluminum content in the weld (22) (criterion C2); and (c) the average chromium content (Cr _WJ ) in the weld (22) satisfies the following relationship: Cr _WJ ≤ 5-2xAl _WJ , where Al _WJ is the average aluminum content in the weld (22) (criterion C3). 2. The method according to p. 1, in which the steel substrate (3) of at least one of the pre-coated sheets (2) contains, wt.%: 0.10 ≤ С ≤ 0.5 0.5 ≤ Mn ≤ 4.5 0.1 ≤ Si ≤ 1 0.01≤Cr≤1 Ti ≤ 0.2 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.010, the rest is iron and inevitable impurities. 3. Method according to claim 1 or 2, wherein the average aluminum content (Al _WJ ) in the weld (22) is greater than or equal to 0.15% by weight. 4. The method according to any one of paragraphs. 1-3, in which the average content of aluminum (Al _WJ ) in the weld (22) is less than or equal to 0.8 wt.%. 5. The method according to any one of paragraphs. 1-4, in which the average nickel content (Ni _WJ ) in the weld (22) is 0.1 wt.% - 13.6 wt.%, and preferably 0.2 wt.% - 12.0 wt.%. 6. The method according to any one of paragraphs. 1-5, in which the welded steel billet after hot pressing and cooling has a Charpy impact strength of the weld (22) at 20°C greater than or equal to 25 J/cm ² and a tensile strength greater than or equal to the tensile strength of the weakest substrates among the substrates (3) of the pre-coated sheets (2), the weakest substrate (3) being the substrate (3) for which the product of thickness and tensile strength after hot pressing and cooling is minimal. 7. The method according to any one of paragraphs. 1-6, in which the specified filler wire (20) has a carbon content of 0.01 wt.% - 0.45 wt.%. 8. The method according to any one of paragraphs. 1-7, in which for at least one pre-coated sheet (2) the removed portion (F) is strictly less than 100% of the thickness of the pre-coat (5). 9. Method according to claim 8, wherein for at least one pre-coated sheet (2), the pre-coat (5) comprises a metal alloy layer (11) extending over the intermetallic alloy layer (9), wherein the metal alloy layer (11 ) is an aluminum layer, an aluminum alloy layer, or an aluminum-based alloy layer, wherein for at least one pre-coated sheet (2), the metal alloy layer (11) is removed throughout its thickness, while the intermetallic alloy layer (9) remains intact in a removal zone (18) on each major surface (4) of the pre-coated sheet (2). 10. The method according to any one of paragraphs. 1-7, in which at least one pre-coated sheet (2) provided at the provisioning stage, in the removal zone (18) on each main surface (4) of the pre-coated sheet (2), the part to be removed is up to 100%, so that the precoat (5) is removed throughout the thickness. 11. The method according to any one of paragraphs. 1-10, which before the stage of providing, including the stage of obtaining two pre-coated sheets (2) from the respective original pre-coated sheets (2'), includes an additional step of obtaining a removal zone (18) on each main surface (4) of each pre-coated sheet (2) by removing the precoat (5) over the part to be removed (F) in the range of 30%-100% of the thickness of the precoat (5) using laser removal at the edge of the weld (14) of the precoated sheet (2). 12. The method according to p. 11, in which the stage of obtaining two pre-coated sheets (2) includes: - providing two original pre-coated sheets (2'), - placing said two original pre-coated sheets (2') next to each other with a given gap; and - simultaneous removal by laser removal of the pre-coat (5) on two adjacent original pre-coated sheets (2') in order to simultaneously create a removal zone (18) on the adjacent surfaces of these two original pre-coated sheets (2'), and the laser beam overlaps two adjacent original pre-coated sheets during the removal step. 13. The method according to any one of paragraphs. 1-12, which further includes, prior to butt welding, preparing the edge of the weld (14) of at least one of the pre-coated sheets (2) using one of the following process steps: brushing, machining, countersinking and/or beveling. 14. The method according to any one of paragraphs. 1-13, in which the welding step is carried out using a laser beam. 15. The method according to any one of paragraphs. 1-14, in which at least one pre-coated sheet (2), the steel substrate (3) contains, wt.%: 0.15 ≤ С ≤ 0.25 0.8 ≤ Mn ≤ 1.8 0.1 ≤ Si ≤ 0.35 0.01 ≤ Cr ≤ 0.5 Ti ≤ 0.1 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.005, the rest is iron and inevitable impurities. 16. The method according to any one of paragraphs. 1, 3-14, in which for one pre-coated sheet (2) steel substrate (3) contains, wt.%: 0.040 ≤ С ≤ 0.100 0.80 ≤ Mn ≤ 2.00 Si ≤ 0.30 S ≤ 0.005 P ≤ 0.030 0.010 ≤ Al ≤ 0.070 0.015 ≤ Nb ≤ 0.100 Ti ≤ 0.080 N ≤ 0.009 Cu ≤ 0.100 Ni ≤ 0.100 Cr ≤ 0.100 Mo ≤ 0.100 Ca ≤ 0.006 the rest is iron and inevitable impurities. 17. The method according to p. 16, in which for another pre-coated sheet (2) steel substrate (3) contains, wt.%: 0.10 ≤ С ≤ 0.5 0.5 ≤ Mn ≤ 4.5 0.1 ≤ Si ≤ 1 0.01≤Cr≤1 Ti ≤ 0.2 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.010, the rest is iron and inevitable impurities. 18. The method according to any one of paragraphs. 1, 3-14, in which for one pre-coated sheet (2), the steel substrate (3) contains, wt.%: 0.24 ≤ С ≤ 0.38 0.40 ≤ Mn ≤3 0.10 ≤ Si ≤ 0.70 0.015 ≤ Al ≤ 0.070 0 ≤Cr ≤2 0.25 ≤ Ni ≤2 0.015 ≤ Ti ≤ 0.10 0 ≤ Nb ≤ 0.060 0.0005 ≤ V ≤ 0.0040 0.003 ≤ N ≤ 0.010 0.0001 ≤ S ≤ 0.005 0.0001 ≤ Р ≤ 0.025, while the content of titanium and nitrogen satisfies the following relationship: Ti/N > 3.42, and the content of carbon, manganese, chromium and silicon satisfies the following ratio:

wherein the steel optionally includes one or more of the following, wt %: 0.05 ≤ Mo ≤ 0.65 0.001 ≤ W ≤ 0.30 0.0005 ≤ Ca ≤ 0.005, the rest is iron and inevitable impurities. 19. The method according to p. 18, in which for another pre-coated sheet (2) steel substrate (3) contains, wt.%: 0.10 ≤ С ≤ 0.5 0.5 ≤ Mn ≤ 4.5 0.1 ≤ Si ≤1 0.01≤Cr≤1 Ti ≤ 0.2 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.010, the rest is iron and inevitable impurities. 20. The method according to any one of paragraphs. 1-19, in which welding is carried out using a shielding gas. 21. A method for producing a welded and then hot-pressed and cooled steel part, including successive stages: - implementation of the method according to any one of paragraphs. 1-20 to obtain a welded steel billet (1); - heating the welded steel blank (1) to obtain a fully austenitic structure in the substrate (3) of the pre-coated sheet (2); - hot pressing of the welded steel billet (1) in the pressing apparatus in order to obtain a steel part; and - cooling of the steel part in the pressing apparatus. 22. The method according to claim 21, wherein during the cooling step the cooling rate is greater than or equal to the cooling rate of bainite or martensite for the most hardenable of the substrates (3) of the precoated sheet (2). 23. A welded steel billet (1) containing two pre-coated sheets (2), where each pre-coated sheet (2), including a steel base (3), has a pre-coat (5) on each of the main surfaces (4), and the precoat (5) comprises an intermetallic alloy layer (9) containing at least iron and aluminum, and optionally a metal alloy layer (11) extending over the intermetallic alloy layer (9), wherein the metal alloy layer (11) is an aluminum layer , aluminum alloy layer or aluminum alloy layer, wherein the pre-coated sheets (2) are connected by a weld (22) which has an average aluminum content (Al _WJ ) of 0.1 wt.% - 1.2 wt.%, while the weld (22) is additionally characterized in that : (a) a hardening factor FT _WJ of the weld (22) such that FT _WJ - 0.96FT _BM ≥ 0 (criterion C1), where: - FT _BM is the hardening factor of the least hardenable steel substrate (3) among the steel substrates (3) of the two pre-coated sheets (2), and - hardening factors FT _WJ and FT _BM are determined using the following formula: FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi ² -1532xC ² -5xMn ² -127xSi ² -40xCxNi-4xNixMn, where Al, Cr , Ni, C, Mn and Si, respectively, mean the average content of aluminum, chromium, nickel, carbon, manganese and silicon, expressed as a percentage by weight, in the area for which the hardening factor should be determined, and the specified area is the weld (22) in in the case of FT _WJ and the least hardened substrate in the case of FT _BM , (b) the average nickel content (Ni _WJ ) in the weld (22) satisfies the following relationship: Ni _WJ ≤ 14 - 3.4xAl _WJ , where Al _WJ is the average aluminum content in the weld (22) (criterion C2); and (c) the average chromium content (Cr _WJ ) in the weld (22) satisfies the following relationship: Cr _WJ ≤ 5-2xAl _WJ , where Al _WJ is the average aluminum content in the weld (22) (criterion C3), and each pre-coated sheet (2) contains on each main surface (4), adjacent to the weld (22), an intermediate zone (28), in which the pre-coating (5) is removed over the removed part (F), comprising 30% -100% precoat thickness (5). 24. Welded blank (1) according to claim 23, in which the steel substrate (3) of at least one of the pre-coated sheets (2) contains, wt.%: 0.10 ≤ С ≤ 0.5 0.5 ≤ Mn ≤ 4.5 0.1 ≤ Si ≤ 1 0.01≤Cr≤1 Ti ≤ 0.2 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.010, the rest is iron and inevitable impurities. 25. Welded blank (1) according to claim 23 or 24, in which the width of the intermediate zone (28) of each pre-coated sheet (2) is 5 µm - 2000 µm from the edge of the weld (22). 26. Welded workpiece (1) according to any one of paragraphs. 23-25, in which the removed portion (F) for at least one pre-coated sheet (2) is up to 100% of the thickness of the pre-coat (5). 27. Welded workpiece (1) according to any one of paragraphs. 23-25, in which the removed portion (F) for at least one pre-coated sheet (2) is strictly less than 100% of the thickness of the pre-coat (5). 28. Welded blank (1) according to claim 27, in which the pre-coating (5) for at least one pre-coated sheet (2) contains a metal alloy layer (11) extending over the intermetallic alloy layer (9), while the layer metal alloy layer (11) is an aluminum layer, an aluminum alloy layer or an aluminum-based alloy layer, wherein for at least one pre-coated sheet (2) the metal alloy layer (11) is removed throughout its thickness, while the intermetallic alloy layer (9) remains intact in the removal zone (18) on each major surface (4) of the pre-coated sheet (2). 29. Welded workpiece (1) according to any one of paragraphs. 23-28, in which the nickel content (Ni _WJ ) in the weld (22) is 0.1 wt.% - 13.6 wt.%, and preferably 0.2 wt. % - 12.0 wt.%. 30. Welded workpiece (1) according to any one of paragraphs. 23-29, in which the welded steel billet (1) is such that after hot pressing and cooling: the Charpy impact strength of the weld (22) at 20° C. is greater than or equal to 25 J/cm ² ; and - the tensile strength of the hot-pressed and cooled welded steel billet is greater than or equal to the tensile strength of the weakest substrate among the substrates (3) of the pre-coated sheet (2), the weakest substrate (3) being the substrate for which the product The thickness of the tensile strength after hot pressing and cooling is minimal. 31. Welded workpiece (1) according to any one of paragraphs. 23-30, in which the weld (22) is such that after hot pressing and cooling, the maximum change in hardness ΔHV(WJ) across the weld (22) is less than or equal to 20% of the average hardness HV _mean (WJ) of the weld ( 22). 32. Welded workpiece (1) according to any one of paragraphs. 23-31, in which each intermediate zone (28) includes a striation of solidification, and the striation of solidification on adjacent main surfaces (4) of two pre-coated sheets (2) is symmetrical about the vertical median plane (M) between the two pre-coated sheets ( 2). 33. Welded workpiece (1) according to any one of paragraphs. 23-32, in which each intermediate zone (28) includes an inner edge (30) located on the weld (22) and an outer edge (32) located away from the weld (22), while the distance between the outer edges (32 ) of adjacent intermediate zones (28) of two pre-coated sheets (2) is constant along the longitudinal direction of the weld (22). 34. Welded, hot-pressed and cooled steel part, containing the first part of the coated steel part and the second part of the coated steel part, each part of the coated steel part containing the steel substrate (3) has a coating on at least one of the main surfaces, containing at least iron and aluminum, the first and second parts of the coated steel part are connected by a weld (22), wherein the weld (22) has an average aluminum content (Al _WJ ) of 0.1 wt.% - 1.2 wt.%, and the weld (22) is further characterized in that: (a) a hardening factor FT _WJ of the weld (22) such that FT _WJ - 0.96 FT _BM ≥ 0 (criterion C1), where: - FT _BM means the hardening factor of the least hardenable steel substrate (3) among the steel substrates (3) of the two parts of the coated steel part, and - hardening factors FT _WJ and FT _BM are determined using the following formula: FT=128+1553xC+55xMn+267xSi+49xNi+5xCr-79xAl-2xNi ² -1532xC ² -5xMn ² -127xSi ² -40xCxNi-4xNixMn, where Al, Cr , Ni, C, Mn and Si, respectively, mean the average content of aluminum, chromium, nickel, carbon, manganese and silicon, expressed as a percentage by weight in the area for which the hardening factor is to be determined, and this area is the weld (22) in in the case of FT _WJ and the least hardened substrate in the case of FT _BM , (b) the average nickel content (Ni _WJ ) in the weld (22) satisfies the following relationship: Ni _WJ ≤ 14-3.4xAl _WJ , where Al _WJ is the average aluminum content in the weld (22) (criterion C2); and (c) the average chromium content (Cr _WJ ) in the weld (22) satisfies the following relationship: Cr _WJ ≤ 5-2xAl _WJ , where Al _WJ is the average aluminum content in the weld (22) (criterion C3), and each part of the coated steel part contains, on each of the two main surfaces adjacent to the weld (22), an intermediate zone (28), in which the thickness of the coating is strictly less than in the adjacent zones of the part of the coated steel part located at a greater distance from the weld (22) than the intermediate zone, or where there is no coating. 35. The steel part according to claim 34, in which the steel substrate (3) of at least one of the first and second parts of the steel part contains, wt%: 0.10 ≤ С ≤ 0.5 0.5 ≤ Mn ≤ 4.5 0.1 ≤ Si ≤ 1 0.01 ≤ Cr ≤ 1 Ti ≤ 0.2 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.010, the rest is iron and inevitable impurities. 36. The steel part according to claim 34 or 35, in which each intermediate zone includes a solidification striation, and the solidification striation on adjacent main surfaces (4) of the two coated parts of the steel part is symmetrical about the vertical median plane between the two coated parts of the steel part. 37. Steel detail according to any one of paragraphs. 34-36, in which each intermediate zone includes an inner edge located on the weld (22) and an outer edge located away from the weld (22), and where the distance between the outer edges of adjacent intermediate zones of the two coated parts of the steel part is constant along the longitudinal direction of the weld (22). 38. Steel part according to any one of paragraphs. 34-37, in which the average hardness HV _mcan (WJ) in the weld (22) is less than or equal to 700 HV. 39. Steel detail according to any one of paragraphs. 34-38, in which the average nickel content (Ni _WJ ) in the weld (22) is 0.1 wt.% - 13.6 wt.%, and preferably 0.2 wt.% - 12.0 wt.%. 40. Steel detail according to any one of paragraphs. 34-39, in which: the Charpy impact strength of the weld (22) at 20° C. is greater than or equal to 25 J/cm ² ; and - the tensile strength of the welded, hot-pressed and cooled steel part is greater than or equal to the tensile strength of the weakest substrate among the substrates (3) of the coated parts of the steel part, and the weakest substrate (3) is the substrate for which the product of the thickness and the limit tensile strength is minimal. 41. Steel detail according to any one of paragraphs. 34-40, wherein the maximum change in hardness ΔHV(WJ) across the weld (22) is less than or equal to 20% of the average hardness HV _mean (WJ) of the weld (22). 42. Steel detail according to any one of paragraphs. 34-41, in which the steel substrate (3) of at least one among the first and second parts of the coated steel part contains, wt.%: 0.15 ≤ С ≤ 0.25 0.8 ≤ Mn ≤ 1.8 0.1 ≤ Si ≤ 0.35 0.01 ≤ Cr ≤ 0.5 Ti ≤ 0.1 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.005, the rest is iron and inevitable impurities. 43. Steel detail according to any one of paragraphs. 34, 36-41, in which the steel substrate (3) one among the first and second parts of the coated steel part contains, wt.%: 0.040 ≤ С ≤ 0.100 0.80 ≤ Mn ≤ 2.00 Si ≤ 0.30 S ≤ 0.005 P ≤ 0.030 0.010 ≤ Al ≤ 0.070 0.015 ≤ Nb ≤ 0.100 Ti ≤ 0.080 N ≤ 0.009 Cu ≤ 0.100 Ni ≤ 0.100 Cr ≤ 0.100 Mo ≤ 0.100 Ca ≤ 0.006 the rest is iron and inevitable impurities. 44. The steel part according to claim 43, in which the steel substrate (3) is different among the first and second parts of the coated steel part and contains, wt.%: 0.10 ≤ С ≤ 0.5 0.5 ≤ Mn ≤ 4.5 0.1 ≤ Si ≤ 1 0.01≤Cr≤1 Ti ≤ 0.2 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.010, the rest is iron and inevitable impurities. 45. Steel detail according to any one of paragraphs. 34, 36-41, in which the steel substrate (3) one among the first and second parts of the coated steel part contains, wt.%: 0.24 ≤ С ≤ 0.38 0.40 ≤ Mn ≤ 3 0.10 ≤ Si ≤ 0.70 0.015 ≤ Al ≤ 0.070 0 ≤Cr ≤2 0.25 ≤ Ni ≤ 2 0.015 ≤ Ti ≤ 0.10 0 ≤ Nb ≤ 0.060 0.0005 ≤ V ≤ 0.0040 0.003 ≤ N ≤ 0.010 0.0001 ≤ S ≤ 0.005 0.0001 ≤ Р ≤ 0.025, where the content of titanium and nitrogen satisfies the following relationship: Ti/N > 3.42, and the content of carbon, manganese, chromium and silicon satisfies the following ratio:

while the steel optionally includes one or more of the following elements, wt.%: 0.05 ≤ Mo ≤ 0.65 0.001 ≤ W ≤ 0.30 0.0005 ≤ Ca ≤ 0.005, the rest is iron and inevitable impurities. 46. The steel part according to claim 45, in which the steel substrate (3) is different among the first and second parts of the coated steel part, contains, wt.%: 0.10 ≤ С ≤ 0.5 0.5 ≤ Mn ≤ 4.5 0.1 ≤ Si ≤ 1 0.01≤Cr≤1 Ti ≤ 0.2 Al ≤ 0.1 S ≤ 0.05 P ≤ 0.1 B ≤ 0.010, the rest is iron and inevitable impurities. 47. The use of a welded, hot-pressed and cooled steel part according to any one of paragraphs. 34-46 for the production of anti-tamper parts or energy absorbers for the car.

Images