EP1778886A1

EP1778886A1 - Object comprising a steel part of a metal construction consisting of an area welded by a high power density beam and exhibiting an excellent toughness in a molten area, method for producing said object

Info

Publication number: EP1778886A1
Application number: EP05778661A
Authority: EP
Inventors: Dominique Kaplan
Original assignee: Arcelor France SA
Current assignee: ArcelorMittal France SA
Priority date: 2004-07-05
Filing date: 2005-06-21
Publication date: 2007-05-02
Also published as: FR2872442B1; BRPI0512996A; CN1989268A; CA2572869A1; US20080302450A1; WO2006013242A1; WO2006013242A9; FR2872442A1

Abstract

The invention relates to an object comprising at least one steel part whose composition consists of the following contents expressed by weight: 0.005-0.27 % carbon, 0.5-1.6 % manganese, 0.1-0.4 % silicon, less than 2.5 % chromium content, less than 1 % molybdenum content, optionally one or several elements selected form nickel, copper, aluminium, niobium, vanadium, titanium, boron, zirconium and nitrogen, the rest being iron and impurities of production, wherein said steel part comprises at least one area welded by a high power density beam and is characterised in that the microstructure of the welded area is constituted of 60-75 % self-tempered martensite, preferentially 60-70 % self-tempered martensite, and, in addition, of 40-30 % lower bainite.

Description

OBJECT COMPRISING A PART OF METALLIC CONSTRUCTION STEEL, THIS PART INCLUDING A WELDED AREA USING A HIGH-DENSITY BEAM AND HAVING EXCELLENT TENACITY IN THE MOLTEN AREA, - METHOD OF MANUFACTURING THIS OBJECT

The present invention relates to steel metallic constructions concerned by a high energy density beam, and more particularly

10 those where a minimum level of toughness is required in the molten zone in order to protect against the risk of sudden rupture.

The assembly by high energy density beam, such as LASER or the electron beam, of hot-rolled steel sheets and plates has particularly developed in the last twenty years due

15 of certain specific characteristics: mention will be made, for example, of very small deformations of the assemblies, the high accuracy of positioning of the beam and the possibility of melting only the quantity of material strictly necessary, the appearance of the beads not requiring completion, and the possibility of dispensing with relaxation treatments.

20 Among the fields of application of these processes, mention will be made in particular of shipbuilding, public works equipment, the automobile industry, tubes for transporting natural gas and crude oil. For certain applications, in particular those in which the thicknesses, the elastic limits involved or the service constraints are the most important, we require

25 toughness guarantees in order to protect against the risk of sudden break.

This eventuality is all the more to take into account that the assembly by beam with high energy density can generate defects such as microporosities or shrinkage likely to initiate a fragile rupture. The welded zones should therefore have the highest possible toughness

30 to guard against any risk.

Different methods have been proposed in order to obtain a high tenacity in the molten zone: Based on the observation that tenacious structures of needle-shaped ferrite are obtained by germination on non-metallic inclusions, we have sought to introduce this type of particles.

35 in the molten zone, for example by means of a prior deposit, as indicated in JP document No. 2000288754. However, this method has various drawbacks: the dispersion of the oxides within the molten zone may not be uniform, which leads to a dispersion of mechanical properties within this area. In addition, the increase in the inclusion fraction results in a drop in the ductile level.

For the same purpose, attempts have also been made to control the ratio between the aluminum and oxygen contents so as to promote the formation of inclusions favorable to the germination of acicular ferrite. Starting from aluminum calmed steel, this method however requires an increase in the oxygen content in the molten zone, which leads to the above drawbacks. In addition, in LASER welding, the kinetic conditions for the formation of these desired acicular ferrite structures are not necessarily compatible with the requirements of productivity and therefore of cooling speed after welding.

It has also been proposed to increase the toughness of the molten zones by adding nickel (gamma element lowering the transformation temperature γ-α), or of nickel alloy, so that the weight content of the molten zone in this element is between 0.5 and a few percent. The document US Pat. No. 4527040 describes for example the addition of a nickel alloy in the form of an insert 0.1 mm thick before assembly by LASER. However, this method increases the difficulties of positioning the beam relative to the joint plane and the risks of the appearance of defects, possibly corrosion.

There is therefore a need to have assemblies of steel welded with high energy density processes, which have every guarantee of toughness in the molten zone, without excessive dispersion of the mechanical characteristics, and to have a manufacturing method. economical of these assemblies without the drawbacks mentioned above.

The present invention aims to provide such welded assemblies and a method for obtaining such assemblies from structural steel.

To this end, a first object of the invention consists of an object comprising at least one steel part, the composition of which comprises, the contents being expressed by weight, carbon with a content of between 0.005 and 0.27%, manganese. between 0.5 and 1.6%, silicon between 0.1 and 0.4%, chromium in content less than 2.5%, Mo in content less than 1%, possibly one or more elements chosen from nickel, copper, aluminum, niobium, vanadium, titanium, boron, zirconium, nitrogen, the rest being iron and impurities resulting from the production. The steel part comprises at least one zone melted by high energy density beam with a microstructure consisting of 60 to 75% of self-returning martensite and, in addition, 40 to 25% of lower bainite, and preferably 60 to 70 % of self-returning martensite and, in addition, 40 to 30% of lower bainite.

Advantageously, the object is a steel tube comprising at least one section having a zone welded in the longitudinal or transverse direction. Advantageously also, the object consists of at least two hot-rolled or forged sheets of steel of identical or different composition, of identical or different thickness, welded together.

Preferably, the high energy density beam is a beam

LASER.

Also preferably, the high energy density beam is an electron beam.

The subject of the invention is also a method of manufacturing one of the preceding objects, comprising the steps consisting in:

- supply an object comprising at least one steel part, the composition of which comprises the contents being expressed by weight, carbon in content of between 0.005 and 0.27%, manganese between 0.5 and 1.6%, silicon between 0.1 and 0.4%, chromium in content less than 2.5%, Mo in content less than 1%, optionally one or more elements chosen from nickel, copper, aluminum, niobium, vanadium, titanium, boron, zirconium, nitrogen, the rest being iron and impurities resulting from the production,

- welding by a high energy density process the steel part with a steel part of identical or different composition, already or not part of the object,

- the welding power, the welding speed, the means of a possible pre or post-heating or cooling, being chosen so that a molten zone is obtained with a microstructure consisting of 60 to 75% of martensite self-returned and, in addition, 40 to 25% of lower bainite, preferably 60 to 70% of self-returned martensite and, in addition, 40 to 30% of lower bainite.

- According to a characteristic of the process, the nitrogen content of the molten zone is less than or equal to 0.020%, the welding power, the welding speed, the means of a possible pre or post-heating or cooling, are chosen so that the molten zone cools according to a parameter Δ / ₅ ⁸ ₀ ° ₀ ° such that:

Δt _B exp ° ^'75 ^Ln < ^{ΔtB / Δt M)} <(fa ™) ≤ Δt _B exp ^" °' ^{6 Ln} < ^{ΔtB / Δt M} >

and preferably: Δt _B exp- ° - ^{7 Ln (ΔtB / Δt M)} <(fa ™) <Δt _B exp ° - ^{6 Ln (ΔtB / Δt M)} Δt ₅ ⁸ oo expressed in seconds designating the time elapsing between the temperature of 800 ^° C. and the temperature of 500 ^° C. during the cooling after welding of said welded zone,

with: Δt _B = exp ⁽⁶ - ^{2 CE II + 0} ^'74) ,

Δt _M = exp ⁽¹⁰ - ^{6 CE |} - ⁴ - ⁸ >

CE, = C + Mn / 6 + Si / 24 + Mo / 4 + Ni / 12 + Cu / 15 + (Cr (1-0,16-v / Cr) / 8) + f (B)

CE ,, = C + Mn / 3,6 + Cu / 20 + Ni / 9 + Cr / 5 + Mo / 4,

With: f (B) = O, if B <0.0001%

f (B) = (0.03-1, 5N) if 0.0001% <B <0.00025%

f (B) = (0.06-3N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.5N) if B≥0.0004%,

C, Mn, Si, Mo, Ni, Cu, Cr, B and N respectively designating the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as a percentage by weight, of said molten zone.

According to another characteristic of the process, the welding is carried out by a LASER beam in a homogeneous and autogenous manner, the nitrogen content of the steel is less than or equal to 0.020%, and the welding power, the welding speed, the means of a possible pre or post-heating or cooling, are chosen so that the molten zone cools according to a parameter At ^ such that:

Δt _B exp- ° '75 ^{Ln (ΔtB / Δt M)} <(fa ™) ^{≤ Δt} B ^eχ P ^" °' ^{6 Ln (AtB / Δt M)}

and preferably: Δt _B exp ^" ° - ^{7 Ln (ΔtB / Δt M)} <(fa ™) <Δt _B exp ^" ° ' ^{6 Ln (ΔtB / Δt M)} At ™, expressed in seconds, designating the elapsed time between 800 and

500 ⁰ C during cooling after welding of the molten zone, with: Δt _B = exp < ^{6 2 CE} " ⁺ ° - ⁷⁴ >,

Δt _M = exp ⁽¹⁰ - ^{6 CE l} - ⁴ ' ⁸ >

CE, = C + Mn / 6 + Si / 24 + Mo / 4 + Ni / 12 + Cu / 15 + (Cr (1-0,16VCr) / 8) + f (B)

CE ₁ , = C + Mn / 3.6 + Cu / 20 + Ni / 9 + Cr / 5 + Mo / 4,

with: f (B) = O, if B <0.0001%

f (B) = (0.03-1, 5N) if 0.0001% <B <0.00025%

f (B) = (0.06-3N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.5N) if B> 0.0004%, C, Mn, Si, Mo, Ni, Cu, Cr, B, N respectively denoting the contents of carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen, expressed as a percentage by weight, of the welded steel .

According to another characteristic of the process, the welding is carried out by electron beam in an autogenous and homogeneous manner, the nitrogen content of the steel is less than or equal to 0.022%, the welding power, the welding speed, the means of a possible pre or post-heating or cooling, are chosen so that the zone melted by the electron beam cools according to a parameter At ^ such that:

Δt _B exp- ⁰ - ^{75 Ln (ΔfB / Δt M)} <(Δθ ^≤ Δt _B exp- ° - ^{6 Lπ} < ^{ΔtB / Λt M)}

and preferably: Δt _B exp ^{"0J Ln (ΔtB / Δt M} ><(fa ™) <Δt _B exp ° ' ^{6 Ln (ΔtB / Δt M} > Δt ₅ ⁸ ° o, expressed in seconds, designating the elapsed time between 800 and

500 ⁰ C during cooling after welding of said molten zone, with: Δt _B = exp < ⁶ - ^{2 CE} " ⁺ ° ^{> 74} \

Δt _M = exp ⁽¹⁰ ' ^{6 CE |} - ⁴ ' ⁸⁾

CE, = C + Mn / 6.67 + Si / 24 + Mo / 4 + Ni / 12 + Cu / 15 + (Cr (I-0, 16-VCr) / 8) + f (B)

CE ,, = C + Mn / 4 + Cu / 20 + Ni / 9 + Cr / 5 + Mo / 4,

With: f (B) = O, if B <0.0001%

f (B) = (0.03-1, 35N) if 0.0001% <B <0.00025%

f (B) = (0.06-2.7N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.05N) if B≥O.0004%,

C, Mn, Si, Mo, Ni, Cu, Cr, B, N respectively designating the contents of carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron, and nitrogen, expressed as a percentage by weight, of the welded steel .

According to a particular embodiment of the invention, the steel part is welded with a steel part of identical or different composition, of identical or different thickness, whether or not part of said object, using a filler product metallic

The invention will now be described in more detail, but without limitation, with reference to the appended figures in which:

- Figure 1 illustrates the comparison of the hardness of the Heat Affected Zone with that of the molten zone in LASER welding and in electron beam welding of steel structural steel.

- Figure 2 presents the comparison of the Charpy V transition temperature at the 28 Joule level (TK ₂ Sj) of the Heat Affected Zone with that of the molten zone in LASER and electron beam welding of structural steels metallic. - Figure 3 illustrates a typical change in the ductile-brittle transition temperature and the hardness in the Heat Affected Zone of a metallic structural steel, as a function of the cooling rate.

- Figures 4 and 5 illustrate the influence of the amount of self-returning martensite on the toughness in the molten zone in LASER welding and in electron beam welding respectively.

FIG. 6 shows the modification of the nitrogen content in the molten zone compared to that of the base metal during electron beam welding.

In the assemblies obtained by LASER welding or by electron beam, the welded part consists of two distinct zones:

- The molten zone, which corresponds to a zone which has passed through the liquid state during welding, ie that where the temperature has been higher than that of the liquidus of the welded material.

- The Zone Affected by Heat (or “ZAC”), which can include in the broad sense all the zones having undergone an allotropic transformation during welding. Subsequently, this term ZAC will be reserved here for the parts of the assembly remaining in the solid state brought to the highest temperatures during welding which are the seat of a greater magnification of the austenitic grain. These zones, very often the most critical from the point of view of toughness, correspond to maximum temperatures greater than 1200-1300 ^° C.

By placing it in the case of an autogenous (that is to say without filler material) and homogeneous welding (welding carried out between two parts having an identical chemical composition), it has been demonstrated for a wide range of compositions of structural steel, carbon content ranging from 0.005% C to 0.27% C by weight, manganese ranging from 0.5 to 1.6%, Si ranging from 0.1 to 0.4%, in Cr up to 2.5%, in Mo up to 1% that the mechanical properties of the molten zone and the ZAC are very similar: Thus, Figure 1 indicates that the hardness in LASER welding and in beam welding of electrons, are very similar in these two areas. This similarity also applies to the toughness properties, as shown in Figure 2, which compares the Charpy V transition temperature at the 28 Joule level of the Heat Affected Zone with that of the molten zone for the two types of welding using high energy density beams. The microstructures of these two zones are also very similar. In other words, provided that their composition is similar, the molten zone with high energy density can be assimilated to a ZAC of great width from the point of view of mechanical properties. This indicates that the means of improving the toughness in the LASER melted area can be based on the experience previously acquired in the area of ZACs. As such, Figure 3 presents a typical example of the evolution of the hardness and the ductile-brittle transition temperature of the ZAC of a metallic structural steel at 0.04% C, 1, 3% Mn in function of the cooling rate after welding. This speed is here characterized by A / °° _o , parameter which designates the time which elapses between the passage at the temperature of 800 ⁰ C and the temperature of 500 ⁰ C during cooling in welding. There is a cooling speed range (located for this steel composition around ^ ₅₀₀ ”1-2s), for which the toughness is optimal. For much faster cooling rates, there is the formation of non-returned martensite ("fresh martensite"), whose properties are inferior. Conversely, a decrease in the cooling rate results in the formation of superior bainite or coarse ferritic structures, also less stubborn. The microstructures corresponding to the optimum toughness consist partly of self-tempered martensite, the tempering being due to the welding cycle itself, and partly of lower bainite. The self-returning structure is characterized by the presence of fine carbides precipitated in the martensite slats. These optimal structures from the point of view of toughness are located towards the end of the martensitic onset range, that is to say correspond to the beginning of the reduction in hardness from a substantially horizontal "plateau" corresponding to the hardness martensite, when ^ ₅₀₀ increases.

According to the invention, it has been demonstrated, as shown in FIG. 4, that a proportion of self-healing martensite of between 60 and 75%, associated in addition to a proportion of lower bainite of between 40 and 25%, results in obtain excellent toughness in the LASER melt zone. When the proportion of martensite is more especially between 60 and 70% associated in addition to a proportion of lower bainite of between 40 and 30%, the transition temperature is less than - 100 ^° C., which translates a level of tenacity particularly Student.

A similar conclusion can be drawn from Figure 5, relating to electron beam welding tests on structural steel. whose carbon content is between 0.1 and 0.17%. A proportion of 60 to 75% of self-returning martensite and, in addition, 40 to 25% of lower bainite, is therefore particularly favorable for obtaining molten zones of excellent toughness in high energy density welding. With a given steel composition, among the various assembly variables in high energy density welding (welding power and speed, possible pre or post-heating or cooling means), we will choose those which lead to a proportion of 60 to 75% martensite in the molten zone, and preferably 60 to 70%, associated with an adequate supplement of lower bainite. The relationships between the cooling rate in welding and the martensite fraction will now be explained, taking into account the similarity between the ZAC and the molten zone in the high energy density assembly:

In the area of heat affected areas, it is known from the publication of "Metal Construction", April 1987, pp. 217-223, that the proportion of martensite can be evaluated by the following expressions:

log— f

Martensitic fraction f _M = ^

you _r

log- or, equivalently:

^M)

with:

At _f5 ⁸ 8 _Q 0 _O 0

f500 = Time elapsing between 800 and 500 ⁰ C during the cooling of the welded area after welding,

Δt _M = Critical cooling time leading to 100% martensite, Δt _B = Critical cooling time leading to 100% bainite.

log and Ln respectively designating the decimal and Neperian logarithms This expression applies when: Δtwi≤Δt ⁸ ™ <Δt _B

The critical cooling times are related to the chemical composition by the following expressions:

Δt _B = exp < ⁶ - ^{2 CE Il + 0} ^'74) ,

Δt _M = exp ⁽¹⁰ - ^{6 CE l} - ⁴ - ⁸⁾ .

with:

CE ,, = C + Mn / 3,6 + Cu / 20 + NÎ / 9 + Cr / 5 + Mo / 4,

With: f (B) = O, if B <0.0001% f (B) = (0.03-1, 5N) if 0.0001% <B <0.00025%

f (B) = (0.06-3N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.5N) if B> 0.0004%,

these expressions assuming that f (B)> 0, that is to say that N≤0.020%.

C, Mn, Si, Mo, Ni, Cu, Cr, B and N respectively denote the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed in weight percent, of the steel.

However, as shown above, the similarity of the ZAC and the molten zone in homogeneous and autogenous welding at high energy density indicates that the previous formulations valid for the ZAC are also applicable to the molten zone.

According to the invention, in the molten zone, a martensite content of between 60 and 75%, preferably between 60 and 70%, combined with a supplement in lower bainite results in excellent toughness. This is obtained if the cooling parameter obeys the following expression:

Δt _B exp ^" ° - ^{75 Ln (ΔtB / Δt M)} <(ΔC _o ) ^{≤ Δt} B ^eχ P ^" ° ' ^{6 Ln (ΔtB / MM)}

and preferably:

Δte exp- ^{α7 Ln (ΔtB / Δt M)} <( ) ^{≤ Δt} B ^eχ P ^" ° ^{'6 Ln (ΔtB / At M)}

Depending on the high energy density process used, two cases should be distinguished:

- In the case of homogeneous and autogenous LASER welding, the composition of the molten zone is practically identical to that of the base metal. The expressions mentioned above, relating to the elementary composition of the molten zone, also apply to the composition of the base metal, that is to say to the composition of the steel from which the assembly is carried out.

- In the case of homogeneous and autogenous electron beam welding, we have observed a change in the composition of the molten zone compared to the base metal: The nitrogen content is reduced on average by about 10%, as l 'indicates Figure 6, as a result of the low partial pressure above the liquid metal. On the other hand, there is also an average reduction of 10% in the initial content of manganese, an element with a high vapor pressure. From the initial contents of N and Mn in the base metal, the contents of

N and Mn in the molten zone are respectively equal to 0.9C and 0.9Mn. Under these conditions, the preceding expressions become - Δt _B = exp ⁽⁶ - ^{2 CE II + 0} Λ

Δt _M = exp ⁽¹⁰ - ^{6 CE l} - ⁴ ' ⁸⁾

CEi = C + Mn / 6.67 + Si / 24 + Mo / 4 + NÏ / 12 + Cu / 15 + (Cr (IO ₁ IeVO ⁷ ) / 8) + f (B)

CEn = C + Mn / 4 + Cu / 20 + Ni / 9 + Cr / 5 + Mo / 4,

With: f (B) = 0, if B≤O.0001%

f (B) = (0.03-1, 35N) if 0.0001% <B <0.00025%

f (B) = (0.06-2.7N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.05N) if B> 0.0004%,

these expressions assuming that f (B)> 0, that is to say that N <0.022%.

C, Mn, Si, Mo, Ni, Cu, Cr, B and N respectively denote the contents of carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen, expressed as a percentage by weight, of the welded steel .

Naturally, the invention can also be transposed in the case where a steel part is welded with another steel part of different composition, and this taking into account the relative participation of each element to form the molten zone, c 'ie the coefficient of dilution. The same remark also applies in the case of welding with metallic filler, the composition and dilution coefficient of which must be taken into account, in order to assess the composition of the molten area.

The present invention will now be illustrated from the following example, relating to LASER beam welding:

A 12 mm thick steel used for the manufacture of tubes with elastic limit greater than 400 MPa having the following composition: C = 0.1%, Mn = 1.45%, Si = 0.35%, AI = 0.030%, Nb = 0.040%, N = 0.004%, was welded in autogenous mode by LASER beam without filler metal with parameters chosen so that the cooling rate Δ ^ °° is equal to 1.7 s. Under these conditions, the fraction of self-returning martensite from the molten zone calculated from the above expression (case of homogeneous and autogenous welding) is equal to 68%, very close to that determined by metallographic observation, supplemented by 32% lower bainite. These conditions correspond to those of the invention, which are associated with optimum toughness of the molten zone: in fact, the transition temperature, determined from impact tensile tests on notched cylindrical specimens of 4mm in diameter, is of -120 ^° C., which translates an excellent toughness and a high resistance to brittle fracture of the tubes manufactured under these conditions by LASER welding. Thanks to the invention, the production of welded structures with high energy density is therefore carried out economically, without the need for expensive addition elements. The invention makes it possible to choose the assembly conditions so as to meet the security requirements with respect to the risk of sudden rupture.

Claims

1. Object comprising at least one steel part, the composition of which comprises, the contents being expressed by weight, carbon in content of between 0.005 and 0.27%, manganese between 0.5 and 1.6%, silicon between 0.1 and 0.4%, chromium in content less than 2.5%, Mo in content less than 1%, optionally one or more elements chosen from nickel, copper, aluminum, niobium, vanadium, titanium, boron, zirconium, nitrogen, the rest being iron and impurities resulting from production, said steel part comprising at least one zone melted by beam with high energy density, characterized in that said molten zone has a microstructure consisting of 60 to 75% of self-returned martensite and, in addition, 40 to 25% of lower bainite, and preferably 60 to 70% of self-returned martensite and, in addition, of 40 to 30% lower bainite.

2. Object according to claim 1, characterized in that it is a steel tube, comprising at least one section having a welded zone in the longitudinal or transverse direction.

3. Object according to claim 1 or 2, characterized in that it consists of at least two hot rolled or forged sheets of steel of identical or different composition, of identical or different thickness, welded together.

4. Object according to any one of claims 1 to 3, characterized in that said beam with high energy density is a LASER beam

5. Object according to any one of claims 1 to 3, characterized in that said beam with high energy density is an electron beam

6. A method of manufacturing an object according to any one of claims 1 to 3, characterized in that it comprises the steps consisting in:

- welding by a high energy density process said steel part with a steel part of identical or different composition, already or not part of said object,

- the welding power, the welding speed, the means of a possible pre or post-heating or cooling, being chosen so that a molten zone is obtained with a microstructure consisting of 60 to 75% of martensite self-returning and, in addition, from 40 to 25% of lower bainite, preferably 60 to

70% self-returning martensite and, in addition, 40 to 30% lower bainite.

7. The method of claim 6, further characterized in that the nitrogen content of said molten zone is less than or equal to 0.020%, that the welding power, the welding speed, the means of a possible pre or post -heating or cooling, are chosen so that said molten zone cools according to a parameter At ^ such that:

Δt _B exp ° - ^{75 Ln} < ^{ΔtB / Δt M} ><(ΔCo) ^{≤ Δt} B ^eχ P ^" ° ^{'6 Ln (ΔtB / Δt M)}

and preferably: Δt _B exp ° - ^{7 Ln (ΔtB / Δt M)} <(fa ™) <Δt _B exp ^" ° ' ^{6 Ln} < ^{ΔtB / Δt M)}

Δt | oo expressed in seconds designating the time elapsing between the temperature of 800 ^° C. and the temperature of 500 ^° C. during the cooling after welding of said welded zone,

with: Δt _B = exp ⁽⁶ - ^{2 CE II + 0} ^'74) ,

Δt _M = exp < ¹⁰ ' ^{6 CE |} - ⁴ ' ⁸⁾

CE ,, = C + Mn / 3,6 + Cu / 20 + Ni / 9 + Cr / 5 + Mo / 4,

With: f (B) = O, if B <0.0001%

f (B) = (0.03-1, 5N) if 0.0001% <B <0.00025%

f (B) = (0.06-3N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.5N) if B≥0.0004%,

8 Method according to claim 6, characterized in that said welding is carried out by LASER beam in a homogeneous and autogenous manner, that the nitrogen content of said steel is less than or equal to 0.020%, and that the welding power, the welding speed, the means of a possible pre or post-heating or cooling, are chosen so that said molten zone cools according to a parameter At ₅ ⁸ _O0 ⁰ such that:

Δt _B exp ° - ^{75 Ln (ΔtB / Δt M)} <(fa ™) ^{≤ Δt} B ^e > Φ ^~ ° ' ^{6 Ln (ΔtB / Δt M)}

and preferably: Δt _B exp- ^{0J Ln (ΔtB / Δt M)} ≤ (fa ™) ^{≤ Δt} B ^eχ P ^" ° ' ^{6 Ln (ΔtB / At M)} Δt ₅ ⁸ oo _" expressed in seconds, designating the time s' flowing between 800 and

500 ⁰ C during cooling after welding of said molten zone, with: Δt _B ≈ exp < ^{6 2 CE} " ⁺ ° Λ

Δt _M = exp ⁽¹⁰ ' ^{6 CE |} - ⁴ - ⁸⁾

CE _n = C + Mn / 3.6 + Cu / 20 + Ni / 9 + Cr / 5 + Mo / 4,

With: f (B) = O, if B <0.0001%

f (B) = (0.03-1, 5N) if 0.0001% <B <0.00025%

f (B) = (0.06-3N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.5N) if B> 0.0004%,

C, Mn, Si, Mo, Ni, Cu, Cr, B, N respectively denoting the contents of carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen, expressed as a percentage by weight, of the welded steel .

9. Method according to claim 6, characterized in that said welding is carried out by electron beam, in an autogenous and homogeneous manner, that the nitrogen content of said steel is less than or equal to 0.022%, that the welding power , the welding speed, the means of a possible pre or post-heating or cooling, are chosen such that said zone melted by the electron beam cools according to a parameter Δt ₅ ⁸ o ° such that:

Δt _B exp- ° - ^{75 Ln (ΔtB / Δt M} ><(fa ™) ^{≤ Δt} B ^eχ P ^" ° ' ^{6 Ln (ΔtB / Δt M)}

and preferably: Δt _B exp ° - ^{7 Ln (ΔtB / Δt M)} <(fa ™) ≤ Δt _B exp ^" ° - ^{6 Ln (ΔtB / Δt M)}

Δt ⁸ oo, expressed in seconds, designating the time elapsing between 800 and 500 ⁰ C during the cooling after welding of said molten zone, with: Δt _B = exp <" ^CE " ⁺ ° ^{> 74} \

Δt _M = exp ⁽¹⁰ ' ^{6 CE |} - ⁴ ' ⁸⁾

CE, = C + Mn / 6.67 + Si / 24 + Mo / 4 + Ni / 12 + Cu / 15 + (Cr (1-0,16Λ / Cτ) / 8) + f (B)

CE ,, = C + Mn / 4 + Cu / 20 + Ni / 9 + Cr / 5 + Mo / 4,

With: f (B) = O, if B <0.0001%

f (B) = (0.03-1, 35N) if 0.0001% <B <0.00025% f (B) = (0.06-2.7N) if 0.00025% <B <0.0004%

f (B) = (0.09-4.05N) if B≥O.0004%,

C, Mn, Si, Mo, Ni, Cu, Cr, B, N respectively designating the contents of carbon, manganese, silicon, molybdenum, nickel, copper chromium, boron and nitrogen, expressed as a percentage by weight, of the welded steel.

10. The manufacturing method according to claim 6 or 7, characterized in that said steel part is welded with a steel part of identical or different composition, of identical or different thickness, whether or not part of said object, using a metallic filler