Steel part, wire and method for manufacturing a steel part

Abstract

The invention relates to a steel part manufactured through additive manufacturing, said steel part being made of a steel having the following composition, in wt.%: 0.10% ≤ C ≤ 0.20% 0.3% ≤ Si ≤ 1.5% 1.5% ≤ Mn ≤ 2.5% 1.5% ≤ Ni ≤ 3.5% 0 ≤ V ≤ 0.25% 0 ≤ Nb ≤ 0.1% 0.15% ≤ V + 4 x Nb ≤ 0.50% 0.005% ≤ N ≤ 0.025% 0 ≤ Cr ≤ 1.5% 0 ≤ Mo ≤ 1.5% 0 ≤ Al ≤ 0.05% 0 ≤ Ti ≤ 0.1% 0 ≤ Cu ≤ 0.5% 0 ≤ B ≤ 0.005% 0 ≤ S ≤ 0.05% 0 ≤ P ≤ 0.05% with 0.7% ≤ Ceq ≤ 1.0%, Ceq being determined using the following formula (I), the rest being iron and unavoidable impurities resulting from the elaboration process.

Description

[0001] Steel part, wire and method for manufacturing a steel part

[0002] The present invention concerns a steel part, as well as a wire and a method for manufacturing this steel part through additive manufacturing.

[0003] In recent years, additive manufacturing has become increasingly important, since it allows manufacturing complex parts in a precise manner and at a relatively low cost.

[0004] Additive manufacturing is a method in which the part is built by adding the material layer by layer.

[0005] Additive manufacturing may be performed using a filler wire as a feedstock. This filler wire is melted using an energy source and the melted filler wire material is deposited, in the case of the initial layer, onto a substrate or, for each subsequent layer after the initial layer, onto a previously formed layer, in order to form each successive layer of the part. The energy source may be a laser beam (wire laser additive manufacturing), an electron beam (wire electron beam additive manufacturing) and / or an electric arc (wire arc additive manufacturing).

[0006] Due to the successive deposition of melted material layers during each deposition step, the previously deposited layers are subjected to reheating and cooling. Therefore, the parts manufactured by additive manufacturing are subjected to complex thermal cycles during manufacturing. These thermal cycles are in particular very different from the thermal cycles involved in the formation of a weld joint between two parts.

[0007] The inventors of the present invention have realized that the mechanical properties, in particular in terms of tensile strength, yield strength and elongation, obtained on parts manufactured by additive manufacturing using conventional filler wire materials are not satisfactory, and are in particular lower than those obtained in a conventional weld joint, i.e. when using the filler wire material for producing a weld joint between two parts.

[0008] Furthermore, the inventors of the present invention have observed that the mechanical properties of the as-welded material advertised in the datasheets issued by the manufacturers for conventional welding wire compositions are actually not obtained on parts produced by additive manufacturing using these wires.

[0009] For example, a part produced by wire arc additive manufacturing using a conventional ER120 type alloy, for which the datasheets indicate a tensile strength of up to 1050 MPa and a yield strength of about 890 MPa, has been observed to have a tensile strength of about 970 MPa and a yield strength of about 760 MPa. The ER120 grade is currently considered as the standard in the field of welding for welds with high mechanical properties. In particular, it is often used in high-load applications requiring high durability. Therefore, one purpose of the invention is to provide an alloy composition which results in good mechanical properties in a part manufactured through additive manufacturing, for example for high-load applications requiring high durability.

[0010] More particularly, the alloy composition should be adapted to the special thermal cycles associated with additive manufacturing.

[0011] Preferably, the part should have a tensile strength greater than or equal to 1200 MPa and a yield strength greater than or equal to 760 MPa. In such a case, the tensile strength and the yield strength of the part manufactured through additive manufacturing will be better than or at least equal to those advertised for commercial alloy ER120.

[0012] For this purpose, the invention relates to a steel part manufactured through additive manufacturing, said steel part being made of a steel having the following composition, in wt.%:

[0013] 0.10% < C s 0.20%

[0014] 0.3% < Si < 1.5%

[0015] 1.5% < Mn < 2.5%

[0016] 1.5% < Ni < 3.5%

[0017] 0 < V < 0.25%

[0018] 0 < Nb < 0.1%

[0019] 0.15% < V + 4 x Nb < 0.50%

[0020] 0.005% < N < 0.025%

[0021] 0 < Cr < 1 .5%

[0022] 0 < Mo < 1.5%

[0023] 0 < Al < 0.05%

[0024] 0 < Ti < 0.1 %

[0025] 0 < Cu < 0.5%

[0026] 0 < B < 0.005%

[0027] 0 < S < 0.05%

[0028] 0 < P < 0.05% with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula: the rest being iron and unavoidable impurities resulting from the elaboration process.

[0029] The steel part may further comprise one or more of the following features, taken alone or according to any technically possible combination:

[0030] - the steel part has a microstructure comprising a plurality of stacked layers. - the steel part has a microstructure comprising dendritic structures oriented in a direction of stacking of the layers.

[0031] - the carbon equivalent Ceq of the alloy is from 0.85% to 0.90%.

[0032] - the Mn content of the alloy is from 1.9% to 2.5%.

[0033] - the Si content of the alloy is from 0.80% to 1 .5%.

[0034] - the Nb content of the alloy is from 0.01 % to 0.1 % and / or the V content of the alloy is from 0.05% to 0.25%.

[0035] - the Mo content of the alloy is from 0.1% to 1.0%.

[0036] - the alloy composition is such that 0.15% < V + 4 x Nb < 0.35%.

[0037] - the C content of the alloy is from 0.12% to 0.20%.

[0038] - the steel part has a microstructure comprising a mixture of bainite and martensiteaustenite phases.

[0039] - the microstructure further includes niobium carbides and / or carbonitrides and vanadium carbonitrides.

[0040] - the hardness of the steel part is higher than or equal to 375 HV.

[0041] - the tensile strength of the steel part is higher than or equal to 1200 MPa.

[0042] - the yield strength of the steel part is higher than or equal to 760 MPa.

[0043] The invention also relates to a wire intended for use as filler wire for additive manufacturing, said wire having the following composition, in wt.%:

[0044] 0.10% < C s 0.20%

[0045] 0.3% < Si < 1.6%

[0046] 1.5% < Mn < 2.8%

[0047] 0 < Cr < 1 .5%

[0048] 0 < Mo < 1.5%

[0049] 1.5% < Ni < 3.5%

[0050] 0 < Al < 0.05%

[0051] 0.005% < N < 0.025%

[0052] 0 < Ti < 0.1 %

[0053] 0 < V < 0.25%

[0054] 0 < Nb < 0.1%

[0055] 0.15% < V + 4 x Nb < 0.50%

[0056] 0 < Cu < 0.5%

[0057] 0 < B < 0.005%

[0058] 0 < S < 0.05%

[0059] 0 < P < 0.05% with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula: the rest being iron and unavoidable impurities resulting from the elaboration process.

[0060] The wire may further comprise one or more of the following features, taken alone or according to any technically possible combination:

[0061] - the wire is a solid wire or a flux-cored wire.

[0062] - the wire is a solid wire having a diameter from 0.8 mm to 1.6 mm.

[0063] - the wire is coated with a copper-based coating.

[0064] The invention also relates to a method for manufacturing a steel part through additive manufacturing comprising the following successive steps:

[0065] - provision of a wire having the following composition, in wt.%:

[0066] 0.10% < C s 0.20%

[0067] 0.3% < Si < 1.6%

[0068] 1.5% < Mn < 2.8%

[0069] 0 < Cr < 1 .5%

[0070] 0 < Mo < 1.5%

[0071] 1.5% < Ni < 3.5%

[0072] 0 < Al < 0.05%

[0073] 0.005% < N < 0.025%

[0074] 0 < Ti < 0.1 %

[0075] 0 < V < 0.25%

[0076] 0 < Nb < 0.1%

[0077] 0.15% < V + 4 x Nb < 0.50%

[0078] 0 < Cu < 0.5%

[0079] 0 < B < 0.005%

[0080] 0 < S < 0.05%

[0081] 0 < P < 0.05% with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula: the rest being iron and unavoidable impurities resulting from the elaboration process; and

[0082] - manufacturing the steel part though additive manufacturing using this wire.

[0083] The method may further comprise one or more of the following features, taken alone or according to any technically possible combination: the wire is a solid wire or a flux-cored wire. - additive manufacturing is carried out through wire arc additive manufacturing, in particular through gas metal arc welding.

[0084] - the additive manufacturing comprises the deposition of at least ten superposed layers of material, and preferably at least forty superposed layers of material.

[0085] - the steel part is a steel part as described above.

[0086] The invention will be better understood upon reading the following description, provided only by way of example and with reference to the appended drawings, in which:

[0087] - Figure 1 is a schematic view of the sampling locations in the sample walls; and

[0088] - Figure 2 is a schematic view of the tensile samples.

[0089] The invention relates to a steel part manufactured by additive manufacturing, i.e. a part built by adding the material layer by layer.

[0090] The steel part is made of a steel having the following composition, in wt.%:

[0091] 0.10% < C s 0.20%

[0092] 0.3% < Si < 1.5%

[0093] 1.5% < Mn < 2.5%

[0094] 1.5% < Ni < 3.5%

[0095] 0 < V < 0.25%

[0096] 0 < Nb < 0.1%

[0097] 0.15% < V + 4 x Nb < 0.50%

[0098] 0.005% < N < 0.025%

[0099] 0 < Cr < 1 .5%

[0100] 0 < Mo < 1.5%

[0101] 0 < Al < 0.05%

[0102] 0 < Ti < 0.1 %

[0103] 0 < Cu < 0.5%

[0104] 0 < B < 0.005%

[0105] 0 < S < 0.05%

[0106] 0 < P < 0.05% with 0.7% < Ceq < 1.0%, Ceq being determined using the following formula: the rest being iron and unavoidable impurities resulting from the elaboration process.

[0107] The carbon content of the alloy is from 0.10% to 0.20%. Providing a carbon content within this range contributes to increasing the tensile strength and hardenability of the steel, without affecting the ductility of the bainite. Carbon contents below 0.10% result in an insufficient tensile strength of the part, while carbon contents above 0.20% are detrimental for the weldability and toughness of the part.

[0108] Preferably, 0.12% < C < 0.20%. Such carbon contents result in a further improvement of the mechanical properties of the part, in particular of the tensile strength thereof.

[0109] The silicon content of the alloy is from 0.3% to 1 .5%.

[0110] At these contents, the silicon contributes to solid solution hardening of the alloy, in particular of the bainite, and therefore improves the mechanical properties of the steel, more particularly in terms of microhardness and of tensile strength. In addition, the silicon reduces the risk of porosities in the part and results in an improved control of the shape of the deposited material, by making the melt more fluid during manufacturing of the part through additive manufacturing.

[0111] A silicon content below 0.3% results in insufficient mechanical properties of the part, as well as in possible defects in the shape of the part and / or increases the risk of the presence of undesired porosities in the part.

[0112] At silicon contents greater than 1.5%, there is an increased risk of formation of SiC>2 inclusions, which results in degraded ductility and impact resistance of the part.

[0113] Preferably, 0.80% < Si < 1.5%. Such silicon contents result in a further improvement of the mechanical properties of the part, in particular of the tensile strength thereof.

[0114] The manganese content of the alloy is from 1 .5% to 2.5%.

[0115] At these contents, manganese contributes to improving the tensile strength of the part. In addition, it contributes to lowering the bainite start temperature, thus resulting in more refined bainitic structures.

[0116] A manganese content lower than 1.5% results in decreased mechanical properties of the part.

[0117] At manganese contents greater than 2.5%, the drawability of the steel wire that may be used for manufacturing the part through additive manufacturing becomes more difficult.

[0118] Preferably 1.9% < Mn < 2.5%. Such manganese contents result in even further improved mechanical properties of the part, in particular in terms of tensile strength thereof.

[0119] The nickel content of the alloy is from 1.5% to 3.5%.

[0120] At these contents, the nickel improves the toughness of the steel. It further improves the tensile properties of the part.

[0121] A nickel content lower than 1 .5% results in decreased mechanical properties of the part, in particular in terms of toughness and tensile properties.

[0122] At nickel contents greater than 3.5%, the drawability of the steel wire that may be used for manufacturing the part through additive manufacturing becomes more difficult, and the cost of the alloy further increases. The alloy further contains niobium and / or vanadium, the niobium and vanadium contents fulfilling the following cumulative relationships:

[0123] 0.15% < V + 4 x Nb < 0.50% (1);

[0124] 0 < V < 0.25%; and

[0125] 0 < Nb < 0.1%.

[0126] The presence of niobium and / or vanadium at the above-mentioned contents is advantageous, since these elements improve the mechanical properties, in particular tensile strength, yield strength and toughness, of the part through precipitation hardening and grain refinement.

[0127] In particular, niobium precipitates in the form of niobium carbides and / or niobium carbonitrides, which result in precipitation hardening and austenite grain refinement, even at high temperatures and after multiple reheating cycles.

[0128] Vanadium has been observed to keep the steel in fine grain condition even after multiple reheating cycles. In addition, vanadium increases the tensile strength, yield strength and toughness through precipitation hardening via the precipitation of vanadium carbonitrides. Vanadium carbonitrides which are dissolved at high temperatures tend to reduce the austenitic grain sizes and to improve the ductile properties.

[0129] Furthermore, the provision of the above formula (1) allows adjusting the respective Nb and V contents depending on the relative cost thereof.

[0130] At contents of vanadium and / or niobium below the lower limit set in the above formula (1), the strengthening effect is not considered sufficient.

[0131] Contents of vanadium and niobium above the lower limit set in the above formula (1) are detrimental for the toughness of the part, due in particular to a coarser grain size.

[0132] Preferably 0.05% < V < 0.25% and / or 0.01% < Nb < 0.1 % and / or 0.15% < V + 4 x Nb < 0.35%. Vanadium and / or niobium contents within these preferred ranges result in further improved mechanical properties resulting from the precipitation amounts of niobium carbides or carbonitrides or of vanadium carbonitrides.

[0133] The alloy contains nitrogen at a content from 0.005% to 0.025%.

[0134] At these contents, the nitrogen precipitates with the niobium and the vanadium present in the alloy to form niobium carbonitride or vanadium carbonitride precipitates. As mentioned above, these precipitates result in a grain refinement, as well as in precipitation strengthening which improves the tensile strength of the part. The nitrogen content allows for a sufficient precipitation of carbonitrides.

[0135] A nitrogen content lower than 0.005% results in a decreased strength of the part, due in particular to a reduced precipitation of niobium or vanadium carbonitrides, resulting in a coarser grain size. At nitrogen contents greater than 0.025%, the manufacturing of the steel billets used for manufacturing the steel wire becomes more difficult. Furthermore, the risk of formation of pinholes in the billets increases, resulting in an increase of surface defects, such as porosities, on the steel wire. These surface defects deteriorate the quality of the part, and in particular result in the presence of porosities in the part.

[0136] Preferably, the nitrogen content is higher than or equal to 0.01 %. Such a nitrogen content allows maximizing the precipitation of niobium carbides, niobium carbonitrides and vanadium carbonitrides, and therefore further contributes to improving the mechanical properties of the part, in particular in terms of tensile strength.

[0137] Chromium, molybdenum, aluminum and titanium are optionally present in the steel composition.

[0138] The chromium content of the alloy is from 0 to 1.5%. When present, chromium improves the tensile properties of the part.

[0139] A chromium content greater than 1.5% results in an increased cost of the part without significant improvement of the mechanical properties of the part.

[0140] Preferably, 0.2% < Cr < 1.5%. Such chromium contents result in even further improved mechanical properties of the part, in particular in terms of tensile strength.

[0141] The molybdenum content of the alloy is from 0 to 1 .5%.

[0142] When present, molybdenum improves the tensile properties of the part. In addition, the molybdenum contributes to improving the toughness of the part.

[0143] At molybdenum contents greater than 1.5%, there is a risk of decrease of the toughness of the part. In addition, such contents are detrimental in terms of cost.

[0144] Preferably, 0.1% < Mo < 1.0%. Such molybdenum contents result in further improved mechanical properties, in particular in terms of tensile strength, at a reasonable cost.

[0145] The aluminum content of the alloy is from 0 to 0.05%.

[0146] When present, aluminum contributes to the deoxidation of the steel. It may further contribute to improving the strength of the part through grain refinement mechanisms.

[0147] An aluminum content greater than 0.05% results in the formation of aluminum oxides, which are detrimental for the toughness of the part.

[0148] The titanium content of the alloy is from 0 to 0.1%.

[0149] When present, titanium contributes to improving the strength of the part through grain refinement mechanisms, as well as through precipitation hardening through the precipitation of carbonitrides.

[0150] The alloy may further include copper at a content up to 0.5%. The copper present in the part may result from the elaboration process of the steel. For example, when elaborating the steel using an electric furnace, with a very high ratio of recycled scrap steel, copper may be included in the steel at a content of up to 0.25%.

[0151] In addition, the copper present in the part may additionally result from a copper coating present on the filler wire used for manufacturing the part. Indeed, filler wires used for additive manufacturing are often coated with a thin copper coating for the purpose of preventing corrosion of the wire during storage, as well as for improving the electrical contact between the filler wire and the contact tip of the end of the welding torch during additive manufacturing and for improving the sliding of the filler wire in the wire conduit during unwinding of the filler wire. At least some of this copper may enter into the composition of the part when the wire is melted during additive manufacturing of the part.

[0152] The alloy may additionally include, as impurities resulting from the elaboration process: up to 0.005% of boron; up to 0.05% of sulphur; and up to 0.05% of phosphorus.

[0153] The remainder of the composition is iron and impurities resulting from the elaboration process.

[0154] The level of impurities resulting from the elaboration process will depend on the production route used. For example, when using a Blast Furnace route with a low level of scrap, the level of impurities will remain very low. On the other hand, when elaborating the steel using an electric furnace, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. In this latter processing route, for example, Sn can go up to 0.05%, As can go up to 0.03%, Sb can go up to 0.03% and Pb can go up to 0.03%.

[0155] Furthermore, 0.7% < Ceq < 1.0%, Ceq being the carbon equivalent of the alloy, which is determined using the following formula:

[0156] A carbon equivalent within this range allows obtaining parts having a high tensile strength.

[0157] Preferably, 0.85% < Ceqs 0.90%.

[0158] The Ms temperature of the alloy is preferably lower than or equal to 360°C.

[0159] Ms corresponds to the martensite start temperature, at which martensite starts to form on cooling of the steel.

[0160] Ms may be calculated using the following formula (so-called Andrews formula):

[0161] Ms = 539 - 423 x C - 30.4 x Mn - 17.7 x Ni - 12.1 x Cr - 11 x Si - 7.5 x Mo. An Ms temperature within the above range contributes to obtaining good mechanical properties.

[0162] The part according to the invention has a hardness higher than or equal to 375 HV1 .

[0163] Preferably, the part further has the following mechanical properties: a tensile strength higher than or equal to 1200 MPa and a yield strength higher than or equal to 760 MPa.

[0164] Preferably, the part has an elongation at break A higher than or equal to 10%.

[0165] The microstructure of the part is preferably mainly bainitic. The part according to the invention has a microstructure comprising a mixture of bainite and martensite-austenite (MA) phases.

[0166] The part further comprises precipitates in the form of vanadium carbonitrides and niobium carbides and / or niobium carbonitrides.

[0167] The part is obtained through additive manufacturing, in particular through additive manufacturing using a wire as a feedstock, more particularly through wire arc additive manufacturing.

[0168] A part manufactured through additive manufacturing can be distinguished from a part obtained by other manufacturing methods, since it is possible to observe the stacking of layers resulting from the additive manufacturing process on the part itself.

[0169] The part according to the invention thus has a microstructure comprising a plurality of stacked layers.

[0170] This layered structure may be observed by the following method:

[0171] - provision of at least two cross-sectional samples of the part, taken along directions which are orthogonal to each other;

[0172] - observation of each cross-sectional sample, over an observation field of 10 mm x 10 mm, using an optical microscope with a x50 magnification, after mechanical polishing and etching with a suitable reagent.

[0173] The skilled person is able to determine the polishing and etching parameters based on its general knowledge.

[0174] Mechanical polishing is for example carried out down to a polishing grain size of 1 pm, such as by successively polishing with smaller polishing grain sizes starting from an initial grain size of 1200 pm.

[0175] Etching is for example carried out with a 4% Nital solution.

[0176] Using the above method, the layered structure is observed in at least one of the cross- sectional samples, and more particularly at least in the cross-sectional samples taken along one of the orthogonal directions.

[0177] In addition, parts manufactured by additive manufacturing have a microstructure comprising dendritic structures oriented in the direction in which the layers are stacked. These dendritic structures may be observed using the same method as described above, using a Bechet-Beaujard etchant in the etching step.

[0178] It is noted that, for parts which were not subjected to post-processing operations after solidification, the layered structure may also be visible to the naked eye on the as-solidified.

[0179] The invention also relates to a wire having the following composition, in wt.%:

[0180] 0.10% < C s 0.20%

[0181] 0.3% < Si < 1.6%

[0182] 1.5% < Mn < 2.8%

[0183] 0 < Cr < 1 .5%

[0184] 0 < Mo < 1.5%

[0185] 1.5% < Ni < 3.5%

[0186] 0 < Al < 0.05%

[0187] 0.005% < N < 0.025%

[0188] 0 < Ti < 0.1 %

[0189] 0 < V < 0.25%

[0190] 0 < Nb < 0.1%

[0191] 0.15% < V + 4 x Nb < 0.50%

[0192] 0 < Cu < 0.5%

[0193] 0 < B < 0.005%

[0194] 0 < S < 0.05%

[0195] 0 < P < 0.05% with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula: the rest being iron and unavoidable impurities resulting from the elaboration process.

[0196] The level of impurities resulting from the elaboration process will depend on the production route used.

[0197] For example, when using a Blast Furnace route with a low level of scrap, the level of impurities will remain very low. On the other hand, when elaborating the steel using an electric furnace, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. In this latter processing route, for example, Sn can go up to 0.05%, As can go up to 0.03%, Sb can go up to 0.03% and Pb can go up to 0.03%.

[0198] The wire is preferably a solid wire or a flux-cored wire.

[0199] A flux-cored wire is a wire comprising an envelope consisting of a low alloyed steel and an alloy powder contained in the envelope. The above-mentioned composition corresponds to the composition of the solid wire or, if a flux-cored wire is used, to the mean composition of the wire, taking into account the composition of the wire and of the envelope.

[0200] The solid wire for example has a diameter from 0.8 mm to 1.6 mm.

[0201] The wire may further be coated with a copper-based layer.

[0202] This wire is adapted to be used in a method for manufacturing the part as described above through additive manufacturing.

[0203] The invention also relates to a method for manufacturing a steel part through additive manufacturing comprising the following successive steps:

[0204] - provision of a wire having the following composition, in wt.%:

[0205] 0.10% < C s 0.20%

[0206] 0.3% < Si < 1.6%

[0207] 1.5% < Mn < 2.8%

[0208] 0 < Cr < 1 .5%

[0209] 0 < Mo < 1.5%

[0210] 1.5% < Ni < 3.5%

[0211] 0 < Al < 0.05%

[0212] 0.005% < N < 0.025%

[0213] 0 < Ti < 0.1 %

[0214] 0 < V < 0.25%

[0215] 0 < Nb < 0.1%

[0216] 0.15% < V + 4 x Nb < 0.50%

[0217] 0 < Cu < 0.5%

[0218] 0 < B < 0.005%

[0219] 0 < S < 0.05%

[0220] 0 < P < 0.05% with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula: the rest being iron and unavoidable impurities resulting from the elaboration process ; and

[0221] - manufacturing the steel part though an additive manufacturing process using this wire.

[0222] In the welding wire, the level of impurities resulting from the elaboration process will depend on the production route used. For example, when using a Blast Furnace route with a low level of scrap, the level of impurities will remain very low. On the other hand, when elaborating the steel using an electric furnace, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. In this latter processing route, for example, Sn can go up to 0.05%, As can go up to 0.03%, Sb can go up to 0.03% and Pb can go up to 0.03%.

[0223] The wire is preferably a solid wire or a flux-cored wire.

[0224] A flux-cored wire is a wire comprising an envelope consisting of a low alloyed steel and an alloy powder contained in the envelope.

[0225] The above-mentioned composition corresponds to the composition of the solid wire or, if a flux-cored wire is used, to the mean composition of the wire, taking into account the composition of the wire and of the envelope.

[0226] The solid wire for example has a diameter from 0.8 mm to 1.6 mm.

[0227] The wire may further be coated with a copper-based layer.

[0228] It is noted that the exact chemical composition of the part is different from the exact chemical composition of the filler wire, due to the loss of chemical elements during the manufacturing process, in particular through evaporation. Based on its general knowledge, the skilled person is able to determine which wire composition should be used for the purpose of obtaining a target steel part composition.

[0229] The part obtained by the method described above is a part as described above, in particular a steel part having the steel part composition described above.

[0230] The additive manufacturing process comprises:

[0231] - melting the wire using an energy source; and

[0232] - depositing the melted wire material onto a substrate in an initial deposition step and onto a previously formed layer in each subsequent deposition step.

[0233] The additive manufacturing process preferably comprises the deposition of at least 10 superposed layers of material, and preferably at least 40 superposed layers of material, to form the part.

[0234] These steps are repeated until the part is obtained.

[0235] Additive manufacturing methods using a wire as feedstock are advantageous, since they allow for higher deposition rates, and the feedstock is generally easier to handle and more cost effective than additive manufacturing methods using a powder as feedstock, for example.

[0236] The energy source may be a laser beam (wire laser additive manufacturing), an electron beam (wire electron beam additive manufacturing) and / or an electric arc (wire arc additive manufacturing).

[0237] The additive manufacturing is preferably carried out using an arc as energy source (wire arc additive manufacturing). The wire arc additive manufacturing method is for example metal inert gas welding (MIG), tungsten inert gas welding (TIG) or plasma arc welding (PAW).

[0238] Wire arc additive manufacturing is advantageous, since it allows for a relatively high flexibility in material composition, and a relatively low capital cost.

[0239] The following parameters may be used for wire arc additive manufacturing.

[0240] The average tension of the arc is from 10 V to 35 V.

[0241] The average intensity of the arc is from 40 A to 400 A.

[0242] The torch travel speed, corresponding to the speed at which the torch travels along the welding direction, is from 1 mm / s to 40 mm / s.

[0243] The wire feed speed is from 1 m / min to 20 m / min. The wire feed speed depends on the metal deposition requirements. It is chosen so as to avoid discontinuous deposition and dimensional inaccuracy.

[0244] The shielding gas is for example a mixture of argon and CO2.

[0245] The maximum interlayer temperature is equal to 500 °C.

[0246] The above exemplary parameters are provided only by way of example. The skilled person is able to choose the most adapted additive manufacturing parameters based on his general knowledge, in particular depending on the shape of the part that is to be manufactured and the composition and the nature, i.e. solid wire or cored wire, of the wire.

[0247] Optionally, the method according to the invention includes at least one postprocessing step carried out after additive manufacturing for finishing the part. The postprocessing step for example includes a machining step and / or a polishing step.

[0248] Experiments

[0249] The inventors of the present invention carried out the following experiments.

[0250] Sample walls were manufactured on a substrate through additive manufacturing using the Single Pass Multi Layer (SPML) method using wires having the compositions listed in Table 1 below. The values outside the range of the invention have been underlined.

[0251] Table 1 : Compositions of filler wires

[0252] In Table 1 , the contents are indicated in wt.%.

[0253] In each of the above compositions:

[0254] B < 0.005%

[0255] P < 0.05%

[0256] S < 0.05%.

[0257] In each of the above compositions, means that the element is present at most as an impurity resulting from the elaboration process of the steel.

[0258] In addition, for each composition, the rest of the composition is iron and impurities resulting from the elaboration process of the steel.

[0259] S1 and S2 are steels according to the invention, while C1 and ER120 are comparative steels. ER120 corresponds to a commercially available ER120 type steel.

[0260] The sample walls have the following dimensions:

[0261] Length: 250 mm

[0262] Height: 90 mm

[0263] Thickness: 5 mm.

[0264] Melting of the wire was performed using the gas metal arc welding (GMAW) process.

[0265] The additive manufacturing parameters used for producing the sample walls are listed in the below Table 2:

[0266] In the above table, “ZIG” means that, for each layer, the deposition starts and stops at the same location, taken along the direction of displacement of the welding torch. The thus obtained sample walls were then subjected to the following analyses.

[0267] First, the composition of the sample walls was measured using spark spectrometry. The measured compositions are listed in Table 3 below. The values outside the range of the invention have been underlined. Table 3: Measured compositions of the sample walls

[0268] Table 3 (continued) : Measured compositions of the sample walls

[0269] In Table 3, the contents are indicated in wt.%.

[0270] In each of the above compositions: 0 < B < 0.005% 0 < S < 0.05%

[0271] 0 < P < 0.05%.

[0272] In each of the above compositions, means that the element is present at most as an impurity resulting from the elaboration process of the steel.

[0273] In addition, for each composition, the rest of the composition is iron and impurities resulting from the elaboration process of the steel.

[0274] The following additional measurements were carried out on the sample walls.

[0275] Metallographic structures and hardness were assessed in one cross-sectional sample taken in the center C of the wall along the printing direction (see Figure 1), the cross-section being taken perpendicular to the printing direction.

[0276] This cross-sectional sample was mounted, polished up to 3 pm and subjected to LePera etching during a few seconds in order to reveal the microstructure of the steel. The sample was then observed with an optical microscope with the following magnifications: x100, x200, x500, x1000. Metallurgical phases were counted with the Metalia software, taking 15 frames per area with 49 targets per frame.

[0277] The sample was then polished again to remove the etching and Vickers measurements (HV1) were carried out automatically to map the samples with a measurement pitch of 1 mm in the vertical direction and of 250 pm in the horizontal direction. Hardness measurements were carried out according to standard NF EN ISO 6507-1 :2023. The hardness values provided in Table 5 below correspond to the mean of the measured hardness values.

[0278] For these tests, the hardness values were measured on samples extracted from the center of the wall, since the center is considered to be the most representative area of the wall, since the heat exchanges are not influenced by the substrate. It is also a zone with an important thermal re-affectation, which is characteristic of additive manufacturing thermal cycles.

[0279] Four flat tensile samples, as shown in Figure 2, were cut, respectively, in the area marked V in Figure 1 and in the area marked H in Figure 1. In the area marked V, the four samples were cut with their longitudinal axes extending horizontally and parallel to each other, from four vertically adjacent zones of the sample wall, while in the area marked H, the samples were cut with their longitudinal axes extending vertically and parallel to each other, from four horizontally adjacent zones of the sample wall.

[0280] The dimensions of the samples are listed in Table 4 below:

[0281] Table 4: Dimensions of tensile samples (in mm) The tensile strength, yield strength and elongation at break A were then measured in these tensile samples at room temperature, in accordance with standard NF EN ISO 6892- 1 : 2019.

[0282] The results of these measurements are listed in Table 5.

[0283] The experiments carried out show that sample walls E1 and E2, which are according to the invention, have a significantly higher tensile strength, as well as an improved yield strength, compared to the sample wall E4, produced using a conventional ER120 steel.

[0284] In addition, even though the elongation measured for sample walls E1 and E2 is lower than for sample wall E4, the results remain above the requirements set, i.e. A > 10%.

[0285] Therefore, the parts having a composition according to the invention have good mechanical properties, even though they were obtained through additive manufacturing.

[0286] In addition, these parts have a tensile strength greater than 1200 MPa and a yield strength greater than 760 MPa, and therefore have a tensile strength and a yield strength of the part manufactured through additive manufacturing which are better than those advertised for commercial alloy ER120.

[0287] Furthermore, the sample walls E1 and E2 are observed to have a high hardness HV1 , in particular a hardness greater than or equal to 375 HV, which makes the alloy of these sample walls particularly adapted for high strength applications. In particular, the hardness is greater than that observed for sample walls E3 and E4. It is further observed that samples E1 and E2 have a microstructure comprising a mixture of fine-grained bainitic phases and martensite-austenite (MA) phases. The MA phases are observed to correspond to the segregated zones of dendritic solidification.

[0288] Sample wall E3, which is made of an alloy having a higher carbon equivalent Ceq than sample walls E1 and E2 and does not include any niobium or vanadium, has a lower tensile strength and yield strength than sample walls E1 and E2, and further has a lower yield strength than sample wall E4, which makes it less satisfactory than sample walls E1 and E2, which are according to the invention.

Claims

1. CLAIMS1. Steel part manufactured through additive manufacturing, said steel part being made of a steel having the following composition, in wt.%:0.10% < C s 0.20%0.3% < Si < 1.5%1.5% < Mn < 2.5%1.5% < Ni < 3.5%0 < V < 0.25%0 < Nb < 0.1%0.15% < V + 4 x Nb < 0.50%0.005% < N < 0.025%0 < Cr < 1.5%0 < Mo < 1.5%0 < Al < 0.05%0 < Ti < 0.1%0 < Cu < 0.5%0 < B < 0.005%0 < S < 0.05%0 < P < 0.05% with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula:the rest being iron and unavoidable impurities resulting from the elaboration process.

2. Steel part according to claim 1, wherein the part has a microstructure comprising a plurality of stacked layers.

3. Steel part according to claim 2, wherein the part has a microstructure comprising dendritic structures oriented in a direction of stacking of the layers.

4. Steel part according to any one of claims 1 to 3, wherein 0.85% < Ceqs 0.90%.

5. Steel part according to any one of claims 1 to 4, wherein: 1.9% < Mn < 2.5%.

6. Steel part according to any one of claims 1 to 5, wherein: 0.80% < Si < 1.5%.

7. Steel part according to any one of claims 1 to 6, wherein 0.01 % < Nb < 0.1 % and / or 0.05% < V < 0.25%.

8. Steel part according to any one of claims 1 to 7, wherein: 0.1% < Mo < 1.0%.

9. Steel part according to any one of claims 1 to 8, wherein 0.15% < V + 4 x Nb < 0.35%.

10. Steel part according to any one of claims 1 to 9, wherein 0.12% < C < 0.20%.

11. Steel part according to any one of claims 1 to 10, wherein the steel part has a microstructure comprising a mixture of bainite and martensite-austenite (MA) phases.

12. Steel part according to claim 11 , wherein the microstructure further includes niobium carbides and / or niobium carbonitrides and vanadium carbonitrides.

13. Steel part according to any one of claims 1 to 12, wherein the hardness of the steel part is higher than or equal to 375 HV.

14. Steel part according to any one of claims 1 to 13, wherein the tensile strength of the steel part is higher than or equal to 1200 MPa.

15. Steel part according to any one of claims 1 to 14, wherein the yield strength of the steel part is higher than or equal to 760 MPa.

16. Wire intended for use as filler wire for additive manufacturing, said wire having the following composition, in wt.%:0.10% < C s 0.20% 0.3% < Si < 1.6%1.5% < Mn < 2.8%0 < Cr < 1 .5%0 < Mo < 1.5%1.5% < Ni < 3.5%0 < Al < 0.05%0.005% < N < 0.025%0 < Ti < 0.1 %0 < V < 0.25%0 < Nb < 0.1%0.15% < V + 4 x Nb < 0.50%0 < Cu < 0.5%0 < B < 0.005%0 < S < 0.05%0 < P < 0.05% with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula:the rest being iron and unavoidable impurities resulting from the elaboration process.

17. Wire according to claim 16, wherein the wire is a solid wire or a flux-cored wire.

18. Wire according to claim 17, wherein the wire is a solid wire having a diameter from 0.8 mm to 1.6 mm.

19. Wire according to any one of claims 16 to 18, wherein the wire is coated with a copper-based coating.

20. Method for manufacturing a steel part through additive manufacturing comprising the following successive steps:- provision of a wire having the following composition, in wt.%:0.10% < C s 0.20%0.3% < Si < 1.6%1.5% < Mn < 2.8%0 < Cr < 1 .5%0 < Mo < 1.5%1.5% < Ni < 3.5%0 < Al < 0.05%0.005% < N < 0.025%0 < Ti < 0.1 %0 < V < 0.25%0 < Nb < 0.1%0.15% < V + 4 x Nb < 0.50%0 < Cu < 0.5%0 < B < 0.005%0 < S < 0.05%0 < P < 0.05%with 0.7% < Ceq s 1.0%, Ceq being determined using the following formula:the rest being iron and unavoidable impurities resulting from the elaboration process; and- manufacturing the steel part though additive manufacturing using this wire.21 . Method according to claim 20, wherein the wire is a solid wire or a flux-cored wire.

22. Method according to claim 20 or claim 21 , wherein additive manufacturing is carried out through wire arc additive manufacturing, in particular through gas metal arc welding.

23. Method according to any one of claims 20 to 22, wherein the additive manufacturing comprises the deposition of at least ten superposed layers of material, and preferably at least forty superposed layers of material.

24. Method according to any one of claims 20 to 23, wherein the steel part is according to any one of claims 1 to 15.

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