Method for welding steel sheet containing TiB2 precipitates

Abstract

A method for welding at least two steel sheets, comprising the sequential steps of: providing at least one steel sheet having a composition comprising the following elements in percentage by weight: 0.01% < = C < = 0.2%, 2.5% < = Ti < = 10%, (0.45 * Ti)-1.35% < = B < = (0.45 * Ti) + 0.70%, S < = 0.03%, P < = 0.04%, N < = 0.05%, O < = 0.05%, and comprising a TiB2 precipitate, the at least one steel sheet having a composition comprising, in percentage by weight, 0.01% < = C < = 0.2%, 2.5% < = Ti < = 10%, (0.45 * Ti)-1.35% < = B < = (0.45 * Ti) + 0.70%, S < = 0.03%, P < = 0.04%, N < = 0.05%, O < = 0.05%, the first steel sheet is provided, the remainder being Fe and unavoidable impurities resulting from processing, a second steel sheet is provided, and the first steel sheet and the second steel sheet are welded by using a filler wire having a composition containing 0.8 wt% to 2 wt% Ti so as to obtain a molten zone having an average content of free titanium Ti * higher than or equal to 0.60 wt%.

Description

Technical Field

[0001] The invention relates to a method for welding a steel sheet comprising TiB2 precipitates to at least one second steel sheet. Background Art

[0002] Steel sheets containing TiB2 precipitates (hereinafter TiB2 steel sheets) have attracted much attention due to their excellent high elastic modulus, low density and high tensile strength. However, such sheets may be difficult to weld, and in particular, stress during the cooling of the weld bead may cause intergranular cracks to propagate along grain boundaries in the molten zone. Summary of the invention

[0003] It is therefore an object of the present invention to provide a method for welding a TiB2 steel sheet to at least a second steel sheet without forming cracks in the molten zone.

[0004] The object of the present invention is achieved by providing a welding method according to claim 1. The welding method according to the present invention may also have the optional features listed in claims 2 to 5 considered alone or in combination.

[0005] Another object is achieved by providing a welded joint according to claim 6 . BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention will now be described and illustrated in detail by way of example without introducing limitations, with reference to the accompanying drawings:

[0007] - Figure 1 shows a schematic diagram of lap welding,

[0008] - Figure 2 represents a cross section of a lap weld not according to the invention in test 11,

[0009] - Figure 3 Micrograph showing the molten zone and the points used to calculate the average content of free titanium Ti*,

[0010] - Figure 4 Represents a micrograph of Test 2 according to the invention,

[0011] - Figure 5 Represents a micrograph obtained by SEM of test 10 not according to the invention,

[0012] - Figure 6 Micrographs obtained by confocal microscopy represent test 10 not according to the invention. DETAILED DESCRIPTION

[0013] Without wishing to be bound by any theory, it appears that the welding method according to the invention of a TiB2 steel sheet with a second steel sheet allows avoiding the formation of cracks in the molten zone.

[0014] The present invention relates to a method for welding at least two steel plates, comprising the following sequential steps:

[0015] - providing at least a first steel sheet having a composition comprising the following elements expressed in weight percentage:

[0016] 0.01%≤C≤0.2%

[0017] 2.5%≤Ti≤10%

[0018] (0.45xTi)-1.35%≤B≤(0.45xTi)+0.70%

[0019] S≤0.03%

[0020] P≤0.04%

[0021] N≤0.05%

[0022] O≤005%

[0023] and optionally comprising:

[0024] Si≤1.5%

[0025] Mn≤3%

[0026] Al≤1.5%

[0027] Ni≤1%

[0028] Mo≤1%

[0029] Cr≤3%

[0030] Cu≤1%

[0031] Nb≤0.1%

[0032] V≤0.5%

[0033] And contains TiB2 precipitates, the balance is Fe and inevitable impurities produced by processing, hereinafter referred to as TiB2 steel sheet,

[0034] - provide a second steel plate,

[0035] - welding the TiB2 steel sheet and the second steel sheet by using a filler wire having a chemical composition, expressed in weight percent, of a titanium content of 0.8% to 2% Ti.

[0036] The composition of the filler wire according to the present invention will now be described, with the contents thereof being expressed in weight percent (wt %).

[0037] According to the invention, the titanium content of the filler wire is 0.8% to 2% to ensure that the average free titanium Ti* in the molten zone is higher than or equal to 0.60%. Free titanium Ti* is titanium that is not captured as precipitates or inclusions.

[0038] During welding of the TiB2 plate to the second steel plate, the average free titanium content in the molten zone generally depends on:

[0039] - chemical composition of the board,

[0040] - chemical composition of the filler wire,

[0041] -atmosphere.

[0042] The first effect of the chemical composition of the plates and the filler wire on the amount of free titanium in the molten zone is dilution. Dilution is a phenomenon that occurs between the two welded steel plates and the filler wire. Considering only the dilution phenomenon, the titanium content of the molten zone will depend on the contribution of each plate to titanium in the molten zone, as well as the contribution of the welding wire to titanium.

[0043] A second effect of the chemical composition of the plate and filler wire on the amount of free titanium in the molten zone is the formation of precipitates or inclusions. Carbon present in the welding plate and filler wire can react with titanium to form TiC precipitates and / or Ti(C) in the molten zone. y N 1-y ).

[0044] The same is true for nitrogen present in the plate and filler wire, which can form TiN. However, due to poor gas shielding during welding, most of the TiN formed comes from the reaction of titanium with the atmosphere. The solubility of nitrogen in liquid metal is very low, and this liquid metal in contact with the atmosphere produces TiN.

[0045] Titanium also reacts with oxygen from the atmosphere and / or protective gas to form TiO x (TiO, TiO2, Ti2O3).

[0046] All these phenomena reduce the amount of free titanium in the molten zone, which limits the boron protection and thus promotes the precipitation of the intergranular Fe-TiB2-Fe2B eutectic phase (hereinafter Fe2B). These Fe2B precipitates are brittle and can generate the formation of intergranular cracks during the cooling of the weld bead.

[0047] With a titanium content higher than or equal to 0.8% by weight, the filler wire is dimensioned in such a way as to counteract the above mentioned effects if welding is carried out under a shielding gas such as argon (which may contain up to 20% by volume of CO2, preferably up to 18% by volume of CO2). Adding more than 2% by weight of titanium would be expensive and ineffective in view of the required properties. The titanium content of the filler wire can be determined by depositing the metal wire in several layers, for example seven layers, between two metal sheets under a shielding gas (which may consist of 82% by volume of argon and 18% by volume of CO2), so that there is no dilution effect between the metal sheets and the metal wire, and measuring the titanium content in the deposited metal.

[0048] The remainder of the composition of the filler wire depends primarily on the desired mechanical properties of the molten zone. In addition to titanium, the filler wire also contains (in weight percent by weight%)

[0049] C 0.02% to 0.25%

[0050] Mn: 0.5% to 3.5%

[0051] Si 0.2% to 2%

[0052] Al≤0.5%

[0053] The remainder of the composition of the welding wire is iron and impurities. In this regard, P, S and N are at least considered as residual elements as unavoidable impurities. The S content is lower than or equal to 0.020%, the P content is lower than or equal to 0.020% and the N content is lower than or equal to 0.050%.

[0054] In a preferred embodiment of the present invention, the second steel plate may be a multiphase steel such as dual-phase (DP) steel, complex phase (CP) steel, ferritic-bainitic (FB) steel, TRIP steel, martensitic steel, TRIPLEX steel, TWIP steel, IF steel, high strength low alloy (HSLA) steel or aluminum killed steel.

[0055] In another preferred embodiment of the present invention, the second steel sheet is a TiB2 steel sheet.

[0056] Preferably, the total titanium content (%) Ti in the molten zone produced by the mixture of the TiB2 steel sheet, the second steel sheet and the welding wire is 总量 Satisfy the following formula:

[0057] %Ti 总量 ≥%B 总量*(47.867 / 2*10.811)+(47.867 / 14)*%N 总量 +(47.867 / 16*x)*%O 总量 +(47.867 / (12*y))*%C 总量

[0058] Where %B 总量 、%N 总量 、%O 总量 、%C 总量 are the total boron content, total nitrogen content, total oxygen content and total carbon content in the molten zone, and x and y are the TiO x and Ti(C y N 1-y ) of the precipitation stoichiometric coefficient, x is equal to 1, 2 or 3 / 2, and y varies between 0 and 1.

[0059] Such total titanium content includes both free titanium and titanium trapped in various precipitates (oxides, carbides, nitrides, carbonitrides).

[0060] The welding method used to weld the TiB2 steel plate to the second steel plate may be any arc welding such as MAG welding using a shielding gas, TIG welding, MIG welding, plasma welding or laser welding.

[0061] Preferably, the shielding gas is argon. The shielding gas may contain CO2.

[0062] In the case of a risk of oxidation of the titanium, welding processes without shielding gas, such as submerged arc welding, are conceivable.

[0063] According to the present invention, the welded joint between the TiB2 steel sheet and at least one steel sheet comprises a molten zone with an average free titanium content higher than or equal to 0.60% to obtain a crack-free molten zone.

[0064] The invention will now be illustrated by the following examples, which are not limiting in any way.

[0065] Example:

[0066] The TiB2 steel plate is joined to the second steel plate (FB or DP) by overlapping welding technology. The TiB2 steel plate is the bottom plate and the second steel plate is the top plate. Figure 1 Two thicknesses of TiB2 sheets were used. The chemical composition of the sheets (expressed in weight %) and their thickness (expressed in mm) are given in Table 1.

[0067] Table 1: Chemical composition of steel plates

[0068]

[0069] The steel plates were welded together using filler wires with a diameter of 1 mm. The chemical composition of these wires was determined before welding the two steel plates by depositing the metal wire in seven layers between the two plates under a shielding gas consisting of 82% by volume of argon and 18% by volume of CO2, so that there was no dilution effect between the plates and the metal wire, and measuring the content of the elements in the deposited metal. These chemical compositions are given in Table 2.

[0070] Table 2: Chemical composition of welding wire

[0071]

[0072] Underlined values: do not correspond to the present invention

[0073] Welding wire compositions A to D are according to the present invention, and welding wire compositions E and F are reference examples.

[0074] The top sheet and the lower sheet were welded together by MAG or MIG method under shielding gas at a welding speed of 500 mm / min. The welding parameters are given in Table 3.

[0075] Table 3: Welding process parameters

[0076]

[0077] Underlined values: do not correspond to the present invention

[0078] The molten zone obtained after welding the two steel plates is observed in the direction of the top plate, as shown in Figure 1 Schematically shown in FIG. The sample is cut around the molten zone to obtain a cross section including both the steel plate and the molten zone, polished and etched with a reagent known per se, such as Nital reagent. The cross section is then observed by scanning electron microscopy (SEM) and can be combined with energy dispersive X-ray analysis (SEM-EDX).

[0079] In the obtained micrograph of the melting zone, 21 marks were selected to cover all cross sections of the melting zone, e.g. Figure 2 (corresponding to a magnification of 2.5 times) indicated by black dots.

[0080] Then at the 4 points around these markers (such as Figure 3 The free titanium was measured by EDX at locations (indicated by ×1, ×2, ×3 and ×4 in the figure) (magnification 50 times), which made it possible to obtain 21*4=84 measurements of the free titanium in the molten zone.

[0081] The average free titanium content Ti* calculated from the average of these 84 measured values, the minimum value Ti* of the free titanium content measured in the molten zone 最小 and the maximum value of the measured free titanium content Ti* 最大 (all expressed in weight %) are given in Table 4.

[0082] Furthermore, the presence of Fe2B precipitates is given in Table 4.

[0083] Table 4: Free titanium content and cracks

[0084]

[0085] Underlined values: do not correspond to the present invention

[0086] nd: undetermined value

[0087] Due to the specific composition of the filler wire used and the process parameters, the examples according to the invention, namely Examples 1 to 8, show that no cracks occur in the molten zone when the average free titanium content Ti* is higher than or equal to 0.60 wt. %. Figure 4 The micrograph obtained by SEM of Test 2 is shown, wherein there are no cracks in the molten zone and no Fe2B precipitates are present.

[0088] In Test 8, a small amount of Fe2B precipitates were observed in the weld bead.

[0089] Compared to Test 2, in which the plates were welded under a shielding gas consisting only of argon, in Test 8 the same plates were welded with a welding wire of the same composition, but under a shielding gas also comprising CO 2. Compared to Test 2, the free titanium is reduced, since the titanium reacts with the oxygen present.

[0090] This lower average free titanium content then allows some Fe2B precipitates to form, but is still sufficient to avoid the formation and propagation of cracks in the molten zone.

[0091] Compared to Test 1, in which the same plates were welded together with the same welding wire but under a shielding gas consisting only of argon, in Test 9 the two plates were welded under a shielding gas consisting of 82% by volume argon and 18% by volume CO2.

[0092] Compared to Test 1, free titanium is reduced because titanium reacts with the oxygen present to produce titanium oxide.

[0093] However, this low value of the average free titanium content causes the formation of Fe2B precipitates, which is insufficient to avoid the formation and propagation of cracks in the molten zone.

[0094] The plates of tests 10 and 11 were welded with a filler wire containing 0.01 wt. % titanium, while the plates of test 12 were welded with a filler wire containing no titanium. This resulted in an average free titanium content Ti* in the molten zone of less than 0.60 wt. %, which favors the formation of brittle Fe2B precipitates and the formation and propagation of cracks.

[0095] Figure 6 A longitudinal cross-section obtained by confocal microscopy of the melt zone of test 10 is shown, on which cracks were observed. Figure 5 In the micrograph obtained by SEM shown in FIG. , Fe2B precipitates are visible.

[0096] The plates of test 11 were welded with a filler wire having too low a titanium content under a shielding gas of argon connected to the wake shield. Although the arrangement provided high quality gas coverage, the average free titanium content in the molten zone was still too low to avoid the formation of Fe2B precipitates and cracks in the molten zone.

Claims

1. A method for welding at least two steel plates, comprising the following steps in sequence: - providing at least a first steel sheet having a composition comprising the following elements expressed in weight percentages: 0.01％≤C≤0.2％ 2.5%≤Ti≤10% (0.45xTi)-1.35%≤B≤(0.45xTi)+0.70% S≤0.03％ P≤0.04％ N≤0.05％ O≤0.05％ and optionally comprising: Si≤1.5% Mn≤3% Al≤1.5％ Ni≤1% Mo≤1% Cr≤3％ Cu≤1% Nb≤0.1% V≤0.5％ It contains TiB2 precipitates, the balance being Fe and unavoidable impurities generated by processing. - provide a second steel plate, - Welding the first steel plate and the second steel plate using a filler wire under shielding gas, the filler wire containing the following elements expressed in weight percentage: Ti: 0.8% to 2% C: 0.02% to 0.25% Mn: 0.5% to 3.5% Si: 0.2% to 2.0% Al≤0.5％ P≤0.020％ S≤0.020％ N≤0.050％ The titanium content of such a filler wire, the balance being Fe and unavoidable impurities resulting from processing, is chosen so as to obtain a molten zone having an average content of free titanium Ti* greater than or equal to 0.60% by weight.

2. The welding method according to any one of claims 1 and 2, wherein the second steel is a dual phase (DP) steel, a composite phase (CP) steel, a TRIP steel, a ferrite-bainite (FB) steel, a martensitic steel, a TRIPLEX steel, a TWIP steel, an IF steel, a high strength low alloy steel, or an aluminum killed steel.

3. The welding method according to any one of claims 1 and 2, wherein the second steel plate is a steel plate having a composition containing the following elements expressed in weight percentage: 0.01％≤C≤0.2％ 2.5%≤Ti≤10% (0.45xTi)-1.35%≤B≤(0.45xTi)+0.70% S≤0.03％ P≤0.04％ N≤0.05％ O≤0.05％ and optionally comprising: Si≤1.5% Mn≤3% Al≤1.5％ Ni≤1% Mo≤1% Cr≤3％ Cu≤1% Nb≤0.1% V≤0.5％ It also contains TiB2 precipitates, with the balance being Fe and unavoidable impurities resulting from processing.

4. The welding method according to any one of claims 1 to 3, wherein welding is performed by any arc welding method, such as MAG welding using a shielding gas, TIG welding, MIG welding, plasma welding or laser welding.

5. The welding method according to claim 4, wherein the shielding gas consists of argon, optionally containing up to 20% by volume of CO2.

6. A welded joint of at least two steel plates obtained by the method according to any one of claims 1 to 5, wherein the welded joint comprises a molten zone comprising an average free titanium content greater than or equal to 0.60% by weight.

7. Use of a welded joint according to claim 6 or produced according to any one of claims 1 to 5 for the manufacture of motor vehicles.

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