Method for welding a steel plate containing TiB2 precipitates

Abstract

A welding method for at least two steel plates, comprising the following continuous steps, the following elements expressed in weight percent, namely, 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%, including precipitates of TiB2, and the balance being Fe and inevitable impurities resulting from refining, providing at least a first steel plate having a composition, providing a second steel plate, and welding the first steel plate and the second steel plate using a filler wire, wherein the filler wire has a chemical composition containing 0.8 to 2% Ti and an average content of free titanium Ti of 0.60 wt% or more * A method, including a step of obtaining a molten zone having the same.

Description

Technical Field

[0001] The present invention relates to a method of welding a steel sheet containing TiB2 precipitates to at least one second steel sheet.

Background Art

[0002] Steel sheets containing TiB2 precipitates (hereinafter referred to as TiB2 steel sheets) have attracted attention because they are excellent in high elastic modulus, low density, and high tensile strength. However, such sheets may be difficult to weld, and in particular, the stress during cooling of the weld bead can lead to the propagation of intergranular cracks along the grain boundaries in the molten region.

Summary of the Invention

Problems to be Solved by the Invention

[0003] Accordingly, an object of the present invention is to provide a method of welding a TiB2 steel sheet to at least one second sheet without forming cracks in the molten region.

Means for Solving the Problems

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

[0005] Another object is achieved by providing the welded joint according to claim 6.

[0006] Here, with reference to the accompanying drawings, the present invention will be described in detail and illustrated by way of examples without introducing limitations.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

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Figure 5

Figure 6

DETAILED DESCRIPTION OF THE INVENTION

[0008] Without wishing to be bound by any theory, the welding method according to the present invention performed on the TiB2 steel plate and the second steel plate makes it possible to avoid crack formation in the molten region.

[0009] The present invention is a method for welding at least two steel plates, comprising the following consecutive steps: - The following elements expressed in weight percent, namely: 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% including, optionally, Si ≦ 1.5% Mn ≦ 3% Al ≦ 1.5% Ni ≦ 1% Mo ≦ 1% Cr ≦ 3% Cu ≦ 1% Nb ≦ 0.1% V ≦ 0.5% providing at least a first steel plate having a composition containing precipitates of TiB2 and having a balance of Fe and inevitable impurities resulting from refining, the following TiB2 steel plate; - providing a second steel plate; - A step of welding a TiB2 steel plate and a second steel plate using a filler wire having a chemical composition containing titanium at a content of 0.8 to 2% represented by weight percentage, which relates to a method.

[0010] Next, the composition of the filler wire according to the present invention will be described, and its content will be expressed in weight percentage (wt.%).

[0011] According to the present invention, the titanium content of the filler wire is 0.8% to 2%, and the average free titanium T in the melting zone * is ensured to be 0.60% or more. Free titanium Ti * is titanium that is not trapped as a precipitate or inclusion.

[0012] During the welding of the TiB2 plate and the second steel plate, the average content of free titanium in the melting zone is usually as follows, - The chemical composition of the plate, - The chemical composition of the filler wire, - The atmosphere, depends on.

[0013] The first effect of the chemical composition of the plate and the filler wire on the amount of free titanium in the melting zone is dilution. Dilution is a phenomenon that occurs between the two welded plates and the filler wire. Considering only the dilution phenomenon, the titanium content in the melting zone will depend on the contribution of titanium by each of the plates in the melting zone and the contribution of titanium by the welding wire.

[0014] The second effect of the chemical composition of the plate and the filler wire on the amount of free titanium in the melting zone is the formation of precipitates or inclusions. Carbon present in the welded plate and the filler wire reacts with titanium in the melting zone to form TiC precipitates and / or Ti(C y N 1-y ).

[0015] The same applies to nitrogen present in the plate and the filler wire, and TiN can be formed. However, most of the formed TiN is due to the reaction of titanium with the atmosphere because of poor gas protection during welding. The solubility of nitrogen in the liquid metal is very low, and the contact of this liquid metal with the atmosphere generates TiN.

[0016] Titanium also reacts with oxygen from the atmosphere and / or the gas shield to form TiO x (TiO, TiO2, Ti2O3).

[0017] All of these phenomena reduce the amount of free titanium in the molten zone, limit boron protection, and thus promote the precipitation of the intergranular Fe-TiB2-Fe2B eutectic phase (hereinafter, Fe2B). These Fe2B precipitates are brittle and can cause the formation of intergranular cracks during the cooling of the bead.

[0018] By having a titanium content of 0.8 wt.% or more, the filler wire is sized to offset the above effects when welding is carried out under a shielding gas, for example, argon containing up to 20% by volume of CO2, preferably up to 18% by volume of CO2 in some cases. The addition of titanium exceeding 2 wt% is costly and ineffective considering the required properties.

[0019] The titanium content of the filler wire can be determined by depositing the wire metal in several layers, for example, seven layers, between two metal plates so that there is no dilution effect between the metal plate and the metal wire under a protective gas that can consist of 82% by volume of argon and 18% by volume of CO2, and by measuring the titanium content in the deposited metal.

[0020] The remaining composition of the filler wire mainly depends on the expected mechanical properties of the molten zone. In addition to titanium, the filler wire contains, in (weight percent wt.%), the following C 0.02% - 0.25% Mn 0.5% - 3.5% Si 0.2 - 2% Al ≦ 0.5% includes

[0021] The remainder of the wire composition is iron and impurities. In this regard, P, S, and N are considered to be residual elements that are at least inevitable impurities. Their content rates are 0.020% or less for S, 0.020% or less for P, and 0.050% or less for N.

[0022] In a preferred embodiment of the present invention, the second steel plate can be a multiphase steel such as dual-phase (DP) steel, complex-phase (CP) steel, ferrite (ferrito)-bainite (FB) steel, TRIP steel, martensite steel, TRIPLEX steel (stee), TWIP steel, interstitial-free (IF) steel, high-strength low-alloy (HSLA) steel, or aluminum-killed steel.

[0023] In another preferred embodiment of the present invention, the second steel plate is a TiB2 steel plate.

[0024] Preferably, the total titanium content rate %Ti in the melting region obtained from the mixture of the TiB2 steel plate, the second steel plate, and the wire total satisfies the following formula. %Ti total ≥ %B total *(47.867 / 2 * 10.811) + (47.867 / 14) * %N total + (47.867 / 16 * x) * %O total + (47.867 / (12 * y)) * %C total In the formula, %B total , %N total , %O total , %C total are the total boron, nitrogen, oxygen, and carbon content rates in the melting region, and x and y are the stoichiometric coefficients respectively related to the precipitation of TiO x and Ti(C y N 1-y ), and x is equal to 1, 2, or 3 / 2, and y varies between 0 and 1.

[0025] Such total titanium content includes both free titanium and titanium trapped in various precipitates (oxides, carbides, nitrides, carbonitrides).

[0026] The welding method used to weld the TiB2 steel plate to the second steel plate can be any arc welding such as MAG welding, TIG welding, MIG welding, plasma welding, or laser welding using a shielding gas.

[0027] Preferably, the shielding gas is argon. The shielding gas can contain CO2.

[0028] Process welding without using a shielding gas such as submerged arc welding, which involves a risk of titanium oxidation, can be envisaged.

[0029] According to the present invention, the weld joint between the TiB2 steel plate and at least one steel plate includes a molten zone having an average free titanium of 0.60% or more in order to obtain a crack-free molten zone.

[0030] The present invention is illustrated by the following examples, which are in no way limiting.

Examples

[0031] The TiB2 steel plate is joined to the second steel plate (FB or DP) by an overlap welding technique. As shown in FIG. 1, the TiB2 steel plate is the bottom plate and the second steel plate is the upper plate. Two TiB2 plates of different thicknesses are used. The chemical composition (expressed in weight percent wt.%) and their thicknesses (expressed in mm) of these plates are shown in Table 1.

[0032]

Table 1

[0033] Using a filler wire with a diameter of 1 mm, the steel plates are welded together. Under a shielding gas consisting of 82% by volume of argon and 18% by volume of CO2, the metal wire is deposited in 7 layers between the two plates to avoid dilution effects between the plates and the metal wire, and by measuring the elemental content in the deposited metal, the chemical composition of these wires is determined before welding the two steel plates. These chemical compositions are shown in Table 2.

[0034]

Table 2

[0035] Wire compositions A - D are according to the present invention, and wire compositions E and F are reference examples.

[0036] Under a shielding gas, the upper and lower plates are welded together at a welding speed of 500 mm / min by the MAG or MIG process. The welding parameters are shown in Table 3.

[0037]

Table 3

[0038] After welding the two steel plates, the fusion zone obtained is observed in the direction of the upper plate, as schematically shown in Figure 1. To obtain a cross-section including both the steel plate and the fusion zone, the specimen is cut around the fusion zone, polished, and etched with a reagent known per se, for example, Nital reagent. Then, this cross-section can be observed by a scanning electron microscope (SEM) and combined with energy-dispersive X-ray analysis (SEM-EDX).

[0039] In the obtained micrograph of the fusion zone, 21 markers are selected to encompass all cross-sections of the fusion zone corresponding to a magnification of ×2.5, as represented by the black circles in Figure 2.

[0040] Next, the free titanium is measured by EDX at four points around these markers as represented by x1, x2, x3, and x4 in Fig. 3 (magnification x50), enabling the acquisition of 21 * 4 = 84 measurement values of the free titanium in the melting zone.

[0041] The average free titanium content Ti calculated by the average of these 84 measurement values * , the minimum value Ti of the free titanium content measured in the melting zone * min and the maximum value Ti of the measured free titanium content * max are all expressed in weight percent wt% and shown in Table 4.

[0042] In addition, the presence of Fe2B precipitates is shown in Table 4.

[0043]

Table 4

[0044] Thanks to the specific composition of the filler wire and the process parameters used, Examples according to the present invention, namely Examples 1 to 8, showed that no cracks appeared in the melting zone when the average free titanium content Ti * was 0.60 wt.% or more. Fig. 4 represents a micrograph obtained by SEM of Trial 2 where there are no cracks in the melting zone and no Fe2B precipitates are present.

[0045] In Test 8, a small number of Fe2B precipitates were observed in the beads.

[0046] Compared with Trial 2 where the plate is welded under a shielding gas consisting only of argon, the same plate in Trial 8 is welded with a wire of the same composition but under a shielding gas containing CO2. Then, since titanium reacted with the existing oxygen, the free titanium is reduced compared to Trial 2.

[0047] Subsequently, this lower value of the average free titanium content enables the formation of some Fe2B precipitates but is still sufficient to avoid crack formation and propagation in the molten zone.

[0048] In Trial 9, the same plate is welded using the same wire, but compared to Trial 1 where it is welded under a shielding gas consisting of only argon, the two plates are welded under a shielding gas consisting of 82 vol% argon and 18 vol% CO2.

[0049] Subsequently, free titanium is reduced compared to Trial 1 because the titanium reacts with the oxygen present to produce titanium oxide.

[0050] Subsequently, this low value of the average free titanium content enables the formation of Fe2B precipitates but is not sufficient to avoid crack formation and propagation in the molten zone.

[0051] The plates of Trials 10 and 11 were welded with a filler wire containing 0.01 wt.% titanium, and the plate of Trial 12 was welded with a filler wire containing no titanium. This results 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. * and this favors the formation of brittle Fe2B precipitates and the formation and propagation of cracks.

[0052] Figure 6 shows a longitudinal section of the molten zone of Trial 10 obtained by confocal microscopy, on which cracks are observed. The Fe2B precipitates can be seen in the micrograph obtained by SEM shown in Figure 5.

[0053] The plate of Trial 11 was welded with a filler wire with too low a titanium content under a shielding gas of argon coupled with a trailing shield. Despite this device providing a high-quality gas coating, the average free titanium content in the molten zone remains 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 continuous steps: - The following elements expressed in weight percentages, namely: 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% including, optionally, Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% including TiB ２ providing at least a first steel sheet having a composition including precipitates of ２ and with the remainder being Fe and inevitable impurities resulting from refining - A step of providing a second steel plate; - A step of welding the first steel plate and the second steel plate using a filler wire together with a shielding gas, wherein the filler wire contains the following elements expressed in weight percentages, namely: T1: 0.8 - 2% C: 0.02% - 0.25% Mn: 0.5% - 3.5% Si: 0.2 - 2.0% Al ≤ 0.5% P ≤ 0.020% S ≤ 0.020% N ≤ 0.050% including, and the remainder being iron and inevitable impurities resulting from refining, and the titanium content of such filler wire is an average content Ti of free titanium of 0.60 wt.% or more ＊ a process, selected such that a molten region having A method.

2. The welding method according to claim 1 or 2, wherein the second steel is dual-phase (DP) steel, complex-phase (CP) steel, TRIP steel, ferrite-bainite (FB) steel, martensitic steel, TRIPLEX steel, TWIP steel, IF steel, high-strength low-alloy steel, or aluminum-killed steel.

3. The second steel plate contains the following elements expressed in weight percentages, namely: 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% including, optionally, Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% including TiB ２ The welding method according to claim 1 or 2, which is a steel sheet having a composition including precipitates of ２ and with the balance being Fe and inevitable impurities resulting from refining.

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

5. The shielding gas is argon which may contain up to 20% by volume of CO ２ The welding method according to claim 4, which is made of argon that may contain up to 20% by volume of CO

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 includes a molten region containing an average free titanium of 0.60 wt% or more.

7. Use of the welded joint according to claim 6 for the manufacture of an automobile or a welded joint manufactured by the method according to any one of claims 1 to 5.

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