Method for welding steel plates 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 for welding a steel sheet containing TiB2 precipitates to at least one second steel sheet. [Background technology]

[0002] Steel sheets containing TiB2 precipitates (hereinafter referred to as TiB2 steel sheets) are attracting attention due to their excellent high modulus of elasticity, low density, and high tensile strength. However, welding such sheets can be difficult, and in particular, the stress during the cooling of the weld beads can lead to the propagation of intergranular cracks along the grain boundaries in the molten zone. [Overview of the Initiative] [Problems that the invention aims to solve]

[0003] Therefore, an object of the present invention is to provide a method for welding a TiB2 steel sheet to at least one second sheet without forming cracks in the molten zone. [Means for solving the problem]

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

[0005] Another objective is achieved by providing the welded joint described in claim 6.

[0006] Here, with reference to the attached drawings, the present invention will be described and illustrated in detail by examples without introducing any limitations. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 shows a schematic diagram of lap welding. [Figure 2] Figure 2 shows a cross-section of an lap weld from trial 11, which does not conform to the present invention. [Figure 3] Figure 3 shows a micrograph of the points used to calculate the average content Ti* of the melting zone and free titanium. [Figure 4] Figure 4 shows a micrograph of Trial 2 according to the present invention. [Figure 5] Figure 5 shows a micrograph obtained by SEM of Trial 10 not according to the present invention. [Figure 6] Figure 6 shows a micrograph obtained by confocal microscopy of Trial 10 not according to the present invention.

Mode for Carrying Out the Invention

[0008] Although not intended 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 the formation of cracks in the melting zone.

[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 composition in which the remainder is Fe and inevitable impurities resulting from refining, the following TiB2 steel plate; - providing a second steel plate; - A process of welding a TiB2 steel plate and a second steel plate using a filler wire having a chemical composition containing 0.8-2% titanium (Ti) expressed as a weight percentage. This includes methods.

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

[0011] According to the present invention, the titanium content of the filler wire is 0.8% to 2%, and the average free titanium content in the molten region is 0.8% to 2%. * Ensure that the amount is 0.60% or more. Free titanium Ti * This refers to titanium that is not captured as precipitates or inclusions.

[0012] During welding of TiB2 plate and second steel plate, the average content of free titanium in the molten zone is typically as follows: - Chemical composition of the board, - Chemical composition of filler wire, - atmosphere, It depends on.

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

[0014] The 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 weld plate and filler wire reacts with titanium in the molten zone to form TiC precipitates and / or Ti(C) y N 1-y ) can be formed.

[0015] The same applies to nitrogen present in the plate and filler wire, which can form TiN. However, most of the TiN formed originates from the reaction between titanium and the atmosphere due to poor gas protection during welding. Nitrogen has very low solubility in liquid metal, and contact between this liquid metal and the atmosphere generates TiN.

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

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

[0018] By having a titanium content of 0.8 wt.% or more, the filler wire is dimensionally sized to offset the above effects when welding is performed under a shielding gas, for example, argon containing up to 20 volume% CO2 in some cases, preferably up to 18 volume% CO2. Adding more than 2 wt% titanium 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 under a protective gas consisting of 82 volume% argon and 18 volume% CO2, ensuring no dilution effect between the metal plates and the metal wire, and by measuring the titanium content in the deposited metal.

[0020] The composition of the filler wire residues depends primarily 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% including

[0021] The remainder of the composition of the wire is iron and impurities. In this regard, P, S and N are considered residual elements which are at least inevitable impurities. Their contents 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, 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 %Ti in the melting range 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 contents in the melting range, 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 method, such as MAG welding, TIG welding, MIG welding, plasma welding, or laser welding with shielding gas.

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

[0028] Process welding methods that do not use shielding gas, such as submerged arc welding, which carry the risk of titanium oxidation, may be considered.

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

[0030] The present invention is illustrated by the following examples, but these are by no means limiting. [Examples]

[0031] A TiB2 steel sheet is joined to a second steel sheet (FB or DP) using overlap welding technology. As shown in Figure 1, the TiB2 steel sheet is the bottom plate, and the second steel sheet is the top plate. Two TiB2 sheets of different thicknesses are used. The chemical composition (expressed as weight percent wt.%) and their thicknesses (expressed in mm) of these sheets are shown in Table 1.

[0032] [Table 1]

[0033] Steel plates are welded together using a 1 mm diameter filler wire. The metal wire is deposited in seven layers between the two plates under a protective gas mixture of 82 vol% argon and 18 vol% CO2, ensuring no dilution effect between the plates and the metal wire. The chemical composition of these wires is determined before welding the two steel plates by measuring the elemental content in the deposited metal. These chemical compositions are shown in Table 2.

[0034] [Table 2]

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

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

[0037] [Table 3]

[0038] The molten zone obtained after welding two steel plates 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 molten zone, the specimen is cut around the molten zone, polished, and etched with a well-known reagent, such as Nital reagent. This cross-section can then be observed using a scanning electron microscope (SEM) and combined with energy-dispersive X-ray analysis (SEM-EDX).

[0039] In the resulting micrograph of the molten region, 21 markers are selected to encompass all cross-sections of the molten region corresponding to a magnification of ×2.5, as indicated by the black circles in Figure 2.

[0040] Next, free titanium is measured by EDX at four points around these markers, as shown by x1, x2, x3, and x4 in Figure 3 (magnification x50), making it possible to obtain 21*4=84 measurements of free titanium in the molten zone.

[0041] The average free titanium content Ti is calculated by averaging these 84 measurements. * The minimum value of free titanium content measured in the molten zone is Ti * min and the maximum value of the measured free titanium content Ti * max All of these are expressed in weight percentage (wt%) and are 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, the examples according to the present invention, namely Examples 1-8, have an average free titanium content Ti * The study showed that no cracks appeared in the molten zone when the concentration was 0.60 wt.% or higher. Figure 4 shows a micrograph obtained by SEM from Trial 2, in which there were no cracks in the molten zone and no Fe2B precipitates.

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

[0046] In Trial 8, the same plate is welded with wire of the same composition, but under a shielding gas containing CO2, compared to Trial 2 where the plate is welded under a shielding gas containing CO2. Subsequently, the amount of free titanium is reduced compared to Trial 2 because the titanium reacts with the present oxygen.

[0047] Subsequently, this lower average free titanium content allows for 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 wire is used to weld the same plates, but unlike Trial 1, where the welding is performed under a shielding gas consisting only of argon, two plates are welded under a shielding gas consisting of 82 volume% argon and 18 volume% CO2.

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

[0050] Subsequently, this low average free titanium content allows for the formation of Fe2B precipitates and is insufficient to avoid crack formation and propagation in the molten zone.

[0051] The plates in trials 10 and 11 were welded with a filler wire containing 0.01 wt.% titanium, and the plate in trial 12 was welded with a filler wire that did not contain titanium. This is because the average free titanium content in the molten zone was less than 0.60 wt% Ti * This leads to the formation of brittle Fe2B precipitates, as well as the formation and propagation of cracks.

[0052] Figure 6 shows a longitudinal cross-section of the molten region of trial 10 obtained by confocal microscopy, where cracks are observed. Fe2B precipitates can be seen in the micrograph obtained by SEM shown in Figure 5.

[0053] The plate in Trial 11 was welded with a filler wire containing too little titanium under argon shielding gas coupled to a trailing shield. Despite the 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 welding method for at least two steel plates, comprising the following sequential steps: - The following elements expressed as 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% It includes, and is optional, Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% Includes TiB ２ A step of providing at least a first steel sheet having a composition in which precipitates of and the remainder is Fe and unavoidable impurities resulting from refining, - The process of providing the second steel plate, - A process of welding a first steel plate and a second steel plate together with a filler wire and a shielding gas, wherein the filler wire is composed of the following elements expressed in weight percent, namely, Ti: 0.8-2% C: 0.02% to 0.25% Mn: 0.5% to 3.5% Si: 0.2-2.0% Al ≤ 0.5% P ≤ 0.020% S ≤ 0.020% N ≤ 0.050% It contains iron and unavoidable impurities resulting from refining, and the titanium content of such filler wire is 0.60 wt.% or more, with an average free titanium content of Ti ＊ A process selected to obtain a molten zone having Methods that include...

2. The welding method according to claim 1, wherein the steel of the second steel plate is duplex (DP) steel, composite (CP) steel, TRIP steel, ferritic-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 as weight percentages: 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% It includes, and is optional, Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% Includes TiB ２ The welding method according to claim 1, wherein the steel sheet has a composition in which precipitates are included and the remainder is Fe and unavoidable impurities resulting from refining.

4. The welding method according to claim 1, wherein the welding is performed by one of the following arc welding methods: MAG welding, TIG welding, MIG welding, plasma welding, or laser welding using a shielding gas.

5. The aforementioned shielding gas contains up to 20% by volume of CO ２ The welding method according to claim 4, which is made with argon that may contain [a specific substance].

6. A welded joint comprising at least two steel plates welded together, The at least two steel plates include a first steel plate and a second steel plate, and the first steel plate and the second steel plate are welded together. The first steel plate is, - The following elements expressed as 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% It includes, and is optional, Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% It has a composition that includes TiB₂ precipitates, with the remainder being Fe and unavoidable impurities resulting from refining. A welded joint that includes a molten zone containing an average of 0.60 wt% or more of free titanium.

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