Press-hardened laser welded steel part

Abstract

A press hardened laser welded steel part is provided, the part comprising first and second coated steel part portions and wherein at least one of the first and second coated steel part portions has on at least one of its main faces an aluminum containing coating comprising at least 30 wt.% of aluminum, the first and second coated steel part portions being joined by a weld joint (22), the weld joint having an aluminum content of 0.5 wt.% to 1.25 wt.% and a microstructure comprising martensite and / or bainite and a fraction of (a IC ) 15% to the maximum fraction of pro-eutectoid ferrite is defined using the following formula, wherein Ts1 and Ts2 are the ultimate tensile strengths of the weakest and strongest matrix after press hardening, C FW is the carbon content of the filler material, β is the fraction of the filler material, p is the ratio of the thickness of the weakest and strongest matrix, and the matrix of at least one of the first and the second coated steel part portions has a microstructure which is predominantly martensitic and / or bainitic.

Description

[0001] This application is a divisional application of the application patent application with the filing date of 26 February 2019, the application number 201980015662.7 (PCT / IB2019 / 051528), and the title “Method for producing press-hardened laser welded steel parts and press-hardened laser welded steel parts”.

[0002] The present invention relates to a method for producing press-hardened laser welded steel parts and to the press-hardened laser welded steel parts thus obtained.

[0003] Such type of steel parts are particularly used in the automotive industry and more particularly for manufacturing crash management parts, such as intrusion resistant or shock absorbing parts, structural parts or parts contributing to the safety of motor vehicles.

[0004] For such type of parts, motor vehicle manufacturers specify that the welded joint should not constitute the weakest area of the welded steel part.

[0005] In order to prevent corrosion, the steel sheet used to manufacture such welded steel parts is pre-coated with an aluminum-based pre-coating by hot dip coating in an aluminum containing bath. If the steel sheet is welded without any prior preparation, during the welding operation, the aluminum-based pre-coating will be diluted by the steel substrate within the molten metal. Aluminum tends to increase the full austenitization temperature of the molten metal and thus prevents full transformation into austenite during hot forming using conventional heat treatment temperatures. Therefore, it can no longer be possible to obtain a fully martensitic or bainitic microstructure in the welded joint during the press cooling which occurs during the hot forming process.

[0006] Moreover, the use of higher heat treatment temperatures allowing full austenitization of the welded joint is not possible because it would lead to excessive alloying of the coating and potential negative effects on the adhesion of the paint and / or on the spot weldability of the press hardened part.

[0007] Facing this situation, when manufacturing parts from such pre-coated steel sheets, in order to be able to obtain a fully martensitic structure in the welded joint after hot forming and quenching using conventional heat treatment temperatures, two types of solutions have been developed in the prior art.

[0008] In particular, EP2007545 describes a first solution which consists in removing a surface layer of the metal alloy at the welded edges of the pre-coated steel sheet to significantly reduce the total content of aluminum in the welded joint and thus to obtain a full austenitization temperature close to the full austenitization temperature of the pre-coated steel sheet substrate.

[0009] Furthermore, EP 2 737 971, US 2016 / 0144456 and WO 2014075824 describe a second solution which consists in welding the precoated steel sheets using a filler wire comprising an austenite stabilizing element, such as carbon, manganese or nickel, in order to compensate for the presence of aluminum in the welded joint and to lower its full austenitization temperature, so that a fully martensitic structure can be obtained in the welded joint after hot forming and quenching using conventional heat treatment temperatures.

[0010] However, these methods are not entirely satisfactory.

[0011] Indeed, the first method is relatively time consuming. Furthermore, the second method can require the addition of relatively large amounts of austenite-forming elements in order to be able to obtain a fully martensitic structure in the welded joint after heat treatment. This addition increases the production costs and can also lead to problems and risks of local retained austenite resulting from an unsatisfactory welded joint geometry or from an inhomogeneous mixing between the material from the precoated steel sheet and the material from the filler wire in the welded joint.

[0012] It is therefore an object of the present invention to provide a method for producing a welded steel blank from two such precoated sheets at relatively low cost, which allows obtaining a part having satisfactory crash performance characteristics after press hardening even for relatively high aluminum contents in the welded joint.

[0013] To this end, the present invention relates to a method for producing a press-hardened laser-welded steel part, said method comprising the following sequential steps:

[0014] - providing a first precoated steel sheet and a second precoated steel sheet, each comprising a steel matrix, at least one of the first and second precoated steel sheets having an aluminum-containing precoating comprising at least 50% by weight of aluminum on at least one of its main faces,

[0015] the first precoated steel sheet having a first thickness and the second precoated steel sheet having a second thickness,

[0016] the ultimate tensile strength of the matrix of the first precoated steel sheet after press hardening being strictly greater than the ultimate tensile strength of the matrix of the second precoated steel sheet after press hardening, and

[0017] the product of the first thickness by the ultimate tensile strength of the first precoated steel sheet after press hardening being strictly greater than the product of the second thickness by the ultimate tensile strength of the second precoated steel sheet after press hardening, then

[0018] - removing at least a portion of the thickness of the aluminum-containing pre-coating on at least one major face of the welding edge or of the edge to be welded of at least one of the first pre-coated steel sheet and of the second pre-coated steel sheet if the theoretical average aluminum content in the weld joint obtained by butt welding, possibly using a filler material comprising at most 0.05 wt.% of aluminum, of the first pre-coated steel sheet and of the second pre-coated steel sheet as provided at the providing step is strictly greater than 1.25 wt.%, so that the theoretical average aluminum content in the weld joint obtained by butt welding, possibly using a filler material comprising at most 0.05 wt.% of aluminum, of the first pre-coated steel sheet and of the second pre-coated steel sheet thus prepared is comprised between 0.5 wt.% and 1.25 wt.%,

[0019] - butt welding the first pre-coated steel sheet and the second pre-coated steel sheet using laser welding to obtain a weld joint between the first pre-coated steel sheet and the second pre-coated steel sheet, thereby obtaining a welded blank, the welding step possibly comprising the use of a filler material,

[0020] - heating the welded blank to a heat treatment temperature which is at least 10 °C lower than the full austenitization temperature of the weld joint and which is at least 15 °C higher than the minimum temperature T min wherein

[0021]

[0022] wherein

[0023] Ac3(WJ) is the full austenitization temperature of the weld joint in °C, Al is the aluminum content in the weld joint in wt.%

[0024] and is the maximum intercritical ferrite content of the weld joint calculated using the following formula

[0025]

[0026] wherein

[0027] Ts1 is the ultimate tensile strength of the strongest matrix after press hardening in MPa

[0028] Ts2 is the ultimate tensile strength of the weakest matrix after press hardening in MPa

[0029] C FW C is the carbon content of the filler material in wt.%

[0030] β is the proportion of filler material added to the weld pool, comprised between 0 and 1

[0031] p is the ratio of the thickness of the pre-coated steel sheet comprising the weakest matrix to the thickness of the pre-coated steel sheet comprising the strongest matrix (p = t2 / t1)

[0032] and holding the welded blank at this heat treatment temperature for a time of 2 minutes to 10 minutes;

[0033] - press-forming the welded blank into a steel part; and

[0034] - cooling the steel part thus formed at a cooling rate greater than or equal to the critical martensitic or bainitic cooling rate of the most hardenable matrix of the matrix of the first pre-coated steel sheet and of the second pre-coated steel sheet, to obtain a press-hardened welded steel part.

[0035] According to a particular embodiment of the method:

[0036] - a step of removing the aluminum-containing pre-coating is performed:

[0037] - if the theoretical average aluminum content in the welded joint obtained by butt welding, possibly using a filler material comprising at most 0.05% by weight of aluminum, of the first pre-coated steel sheet and of the second pre-coated steel sheet provided at the providing step is strictly greater than 1.25% by weight,

[0038] and optionally, if the theoretical average aluminum content in the welded joint obtained by butt welding, possibly using a filler material comprising at most 0.05% by weight of aluminum, of the first pre-coated steel sheet and of the second pre-coated steel sheet provided at the providing step is between 0.5% and 1.25% by weight, and more particularly strictly greater than 0.5% by weight,

[0039] then this step is performed so that the theoretical average aluminum content in the welded joint obtained by butt welding, possibly using a filler material comprising at most 0.05% by weight of aluminum, of the first pre-coated steel sheet and of the second pre-coated steel sheet thus prepared is between 0.5% and 1.25% by weight;

[0040] - the microstructure of the matrix of the first pre-coated steel sheet and of the second pre-coated steel sheet at the end of the heating step is fully austenitic; - the ratio of the ultimate tensile strength of the matrix of the first pre-coated steel sheet after press-hardening to the ultimate tensile strength of the matrix of the second pre-coated steel sheet after press-hardening is greater than or equal to 1.2;

[0041] - the carbon content of the matrix of the first pre-coated steel sheet is at least 0.05% by weight greater than the carbon content of the matrix of the second pre-coated steel sheet;

[0042] - the first pre-coated steel sheet and the second pre-coated steel sheet provided at the providing step each comprise on at least one of their main faces an aluminum-containing pre-coating comprising at least 50% by weight of aluminum;

[0043] - the first and second pre-coated steel sheets provided in the providing step comprise, on both of their main faces, an aluminum-containing pre-coating layer comprising at least 50% by weight of aluminum;

[0044] - the aluminum-containing pre-coating layer remains intact on at least one of the first and second pre-coated steel sheets, and for example on both main faces of each of the first and second pre-coated steel sheets, at the time of the butt welding;

[0045] - the method further comprises, prior to the butt welding, a step of preparing a weld edge of at least one of the first and second pre-coated steel sheets, which is intended to be at least partially incorporated into the welded joint, by removing the aluminum-containing pre-coating layer within at least one portion of the thickness of the aluminum-containing pre-coating layer on at least one main face of at least one of the first and second pre-coated steel sheets, even if the theoretical average aluminum content in the welded joint obtained by butt welding, possibly using a filler material comprising at most 0.05% by weight of aluminum, of the first and second pre-coated steel sheets provided in the providing step is between 0.5% and 1.25% by weight,

[0046] - the method further comprises, prior to the butt welding, a step of preparing a weld edge of at least one of the first and second pre-coated steel sheets, which is intended to be at least partially incorporated into the welded joint, by removing the aluminum-containing pre-coating layer within at least one portion of the thickness of the aluminum-containing pre-coating layer on at least one main face of the first and second pre-coated steel sheets, even if the theoretical average aluminum content in the welded joint obtained by butt welding, possibly using a filler material comprising at most 0.05% by weight of aluminum, of the first and second pre-coated steel sheets provided in the providing step is between 0.5% and 1.25% by weight, this removing step being performed in such a way that the theoretical average aluminum content in the welded joint obtained by butt welding, possibly using a filler material comprising at most 0.05% by weight of aluminum, of the first and second pre-coated steel sheets thus prepared remains between 0.5% and 1.25% by weight;

[0047] - for at least one of the first and second pre-coated steel sheets, the steel of the base body comprises, by weight:

[0048] 0.10% < C < 0.5%

[0049] 0.5% < Mn < 3%

[0050] 0.1% < Si < 1%

[0051] 0.01% < Cr < 1%

[0052] Ti < 0.2%

[0053] Al < 0.1%

[0054] S < 0.05%

[0055] P < 0.1%

[0056] B < 0.010%

[0057] the remainder being iron and impurities resulting from the manufacture;

[0058] - for at least one of the first and second pre-coated steel sheets, the steel of the base body comprises by weight:

[0059] 0.15% < C < 0.25%

[0060] 0.8% < Mn < 1.8%

[0061] 0.1% < Si < 0.35%

[0062] 0.01% < Cr < 0.5%

[0063] Ti < 0.1%

[0064] Al < 0.1%

[0065] S < 0.05%

[0066] P < 0.1%

[0067] B < 0.005%

[0068] the remainder being iron and impurities resulting from the manufacture;

[0069] - for at least one of the first and second pre-coated steel sheets, the steel of the base body comprises by weight:

[0070] 0.040% < C < 0.100%

[0071] 0.70% < Mn < 2.00%

[0072] Si < 0.50%

[0073] S < 0.009%

[0074] P < 0.030%

[0075] 0.010% < Al < 0.070%

[0076] 0.015% < Nb < 0.100%

[0077] Ti < 0.080%

[0078] N < 0.009%

[0079] Cu < 0.100%

[0080] Ni < 0.100%

[0081] Cr < 0.2%

[0082] Mo < 0.100%

[0083] Ca < 0.006%,

[0084] the remainder being iron and impurities resulting from the manufacture;

[0085] - for at least one of the first pre-coated steel sheet and the second pre-coated steel sheet, the steel of the substrate comprises by weight:

[0086] 0.06% < C < 0.100%

[0087] 1.4% < Mn < 1.9%

[0088] 0.2% < Si < 0.5%

[0089] 0.010% < Al < 0.070%

[0090] 0.04% < Nb < 0.06%

[0091] 3.4 x N < Ti < 8 x N

[0092] 0.02% < Cr < 0.1%

[0093] 0.0005% < B < 0.004%

[0094] 0.001% < S < 0.009%

[0095] the remainder being iron and impurities resulting from the manufacture;

[0096] - for at least one of the first pre-coated steel sheet and the second pre-coated steel sheet, the steel of the substrate comprises by weight:

[0097] 0.24% < C < 0.38%

[0098] 0.40% < Mn < 3%

[0099] 0.10% < Si < 0.70%

[0100] 0.015% < Al < 0.070%

[0101] 0% < Cr < 2%

[0102] 0.25% < Ni < 2%

[0103] 0.015% < Ti < 0.10%

[0104] 0% < Nb < 0.060%

[0105] 0.0005% < B < 0.0040%

[0106] 0.003% < N < 0.010%

[0107] 0.0001% < S < 0.005%

[0108] 0.0001% < P < 0.025%

[0109] wherein the contents of titanium and nitrogen satisfy the following relationship:

[0110] Ti / N > 3.42,

[0111] and the contents of carbon, manganese, chromium and silicon satisfy the following relationship:

[0112]

[0113] The steel optionally comprises one or more of the following elements:

[0114] 0.05% < Mo < 0.65%

[0115] 0.001% < W < 0.30%

[0116] 0.0005% < Ca < 0.005%

[0117] the remainder being iron and impurities resulting from the manufacture;

[0118] - the laser welding is performed using a shielding gas, in particular helium and / or argon; and

[0119] - the first pre-coated steel sheet and the second pre-coated steel sheet have different thicknesses.

[0120] The application also relates to a press-hardened laser-welded steel part comprising a first coated steel part portion and a second coated steel part portion,

[0121] each coated steel part portion comprising a steel substrate, at least one of the first coated steel part portion and the second coated steel part portion having, on at least one of its main faces, an aluminum-containing coating comprising at least 30% by weight of aluminum,

[0122] the first coated steel part portion having a first thickness and the second coated steel part portion having a second thickness, the ultimate tensile strength of the substrate of the first coated steel part portion being strictly greater than the ultimate tensile strength of the substrate of the second coated steel part portion, and the product of the first thickness by the ultimate tensile strength of the first coated steel part portion being strictly greater than the product of the second thickness by the ultimate tensile strength of the second coated steel part portion;

[0123] The first coated steel part portion and the second coated steel part portion are joined by a weld joint, the weld joint having an aluminum content comprised between 0.5 wt% and 1.25 wt%, and the microstructure of the weld joint comprising martensite and / or bainite, and the fraction of intercritical ferrite being comprised between 15% and the maximum intercritical ferrite fraction minus 5%, the maximum intercritical ferrite fraction being determined using the following formula:

[0124]

[0125] wherein

[0126] Ts1 is the ultimate tensile strength of the strongest matrix after press hardening, in MPa

[0127] Ts2 is the ultimate tensile strength of the weakest matrix after press hardening, in MPa

[0128] β is the proportion of filler material added to the weld pool, comprised between 0 and 1

[0129] C FW is the carbon content of the filler material, in wt%

[0130] p is the ratio of the thickness of the coated steel part portion comprising the weakest matrix to the thickness of the coated steel part portion comprising the strongest matrix (p = t2 / t1)

[0131] and

[0132] The matrix of at least one of the first coated steel part portion and the second coated steel part portion has a microstructure that is mainly martensite and / or bainite.

[0133] According to a particular embodiment of the steel part, the ratio of the ultimate tensile strength of the matrix of the first coated steel part portion to the ultimate tensile strength of the matrix of the second coated steel part portion is greater than or equal to 1.2;

[0134] - the carbon content of the matrix of the first coated steel part portion is at least 0.05 wt% higher than the carbon content of the matrix of the second coated steel part portion;

[0135] - for at least one of the first coated steel part portion and the second coated steel part portion, the steel of the matrix comprises, by weight:

[0136] 0.10% < C < 0.5%

[0137] 0.5% < Mn < 3%

[0138] 0.1% < Si < 1%

[0139] 0.01% < Cr < 1%

[0140] Ti < 0.2%

[0141] Al < 0.1 %

[0142] S < 0.05 %

[0143] P < 0.1 %

[0144] B < 0.010 %

[0145] the remainder being iron and impurities resulting from the manufacture;

[0146] - for at least one of the first coated steel part portion and the second coated steel part portion, the steel of the base body comprises by weight:

[0147] 0.15 % < C < 0.25 %

[0148] 0.8 % < Mn < 1.8 %

[0149] 0.1 % < Si < 0.35 %

[0150] 0.01 % < Cr < 0.5 %

[0151] Ti < 0.1 %

[0152] Al < 0.1 %

[0153] S < 0.05 %

[0154] P < 0.1 %

[0155] B < 0.005 %

[0156] the remainder being iron and impurities resulting from the manufacture;

[0157] - for at least one of the first coated steel part portion and the second coated steel part portion, the steel of the base body comprises by weight:

[0158] 0.040 % < C < 0.100 %

[0159] 0.70 % < Mn < 2.00 %

[0160] Si < 0.50 %

[0161] S < 0.005 %

[0162] P < 0.030 %

[0163] 0.010 % < Al < 0.070 %

[0164] 0.015 % < Nb < 0.100 %

[0165] Ti < 0.080 %

[0166] N < 0.009%

[0167] Cu < 0.100%

[0168] Ni < 0.100%

[0169] Cr < 0.2%

[0170] Mo < 0.100%

[0171] Ca < 0.006%,

[0172] the remainder being iron and impurities resulting from the manufacture;

[0173] - for at least one of the first coated steel part portion and the second coated steel part portion, the steel of the base body comprises, by weight:

[0174] 0.24% < C < 0.38%

[0175] 0.40% < Mn < 3%

[0176] 0.10% < Si < 0.70%

[0177] 0.015% < Al < 0.070%

[0178] 0% < Cr < 2%

[0179] 0.25% < Ni < 2%

[0180] 0.015% < Ti < 0.10%

[0181] 0% < Nb < 0.060%

[0182] 0.0005% < B < 0.0040%

[0183] 0.003% < N < 0.010%

[0184] 0.0001% < S < 0.005%

[0185] 0.0001% < P < 0.025%

[0186] wherein the content of titanium and nitrogen satisfies the following relationship:

[0187] Ti / N > 3.42,

[0188] and the content of carbon, manganese, chromium and silicon satisfies the following relationship:

[0189]

[0190] The steel optionally comprises one or more of the following elements:

[0191] 0.05% < Mo < 0.65%

[0192] 0.001% < W < 0.30%

[0193] 0.0005% < Ca < 0.005%

[0194] the remainder being iron and impurities resulting from the manufacture; and

[0195] - for at least one of the first coated steel part portion and the second coated steel part portion, the steel of the base body comprises by weight:

[0196] 0.06% < C < 0.100%

[0197] 1.4% < Mn < 1.9%

[0198] 0.2% < Si < 0.5%

[0199] 0.010% < Al < 0.070%

[0200] 0.04% < Nb < 0.06%

[0201] 3.4 x N < Ti < 8 x N

[0202] 0.02% < Cr < 0.1%

[0203] 0.0005% < B < 0.004%

[0204] 0.001% < S < 0.009%

[0205] the remainder being iron and impurities resulting from the manufacture.

[0206] The application will be better understood after reading the following detailed description, given only as an example and with reference to the attached drawings in which:

[0207] - Figure 1 is a schematic cross-sectional view of the start of the welding step according to the method of the application,

[0208] - Figure 2 is a schematic cross-sectional view of the welded blank obtained at the end of the welding step, and

[0209] - Figure 3 is a perspective view of the pre-coated steel sheet after the preparation step.

[0210] Throughout the patent application, the content of the elements is expressed in percentage by weight (wt%).

[0211] The application relates to a method for producing a press-hardened laser-welded steel part.

[0212] More particularly, the method comprises a first step of providing a first pre-coated steel sheet 1 and a second pre-coated steel sheet 2.

[0213] Each pre-coated steel sheet 1, 2 comprises two opposite main faces 5, 6, and at least one lateral face 13 extending between the two opposite main faces 5, 6 from one main face 5, 6 to the other. Figure 3 In the illustrated example, the pre-coated steel sheets 1, 2 comprise four lateral faces 13. For example, the lateral faces 13 form an angle of 60° to 90° with one of the main faces 5, 6.

[0214] As illustrated, Figure 1 Each pre-coated steel sheet 1, 2 comprises a metal substrate 3, 4 having an aluminum- containing pre-coating 7, 8 on at least one of its main faces. The pre-coating 7, 8 is superimposed on and in contact with the substrate 3, 4.

[0215] The metal substrate 3, 4 is more particularly a steel substrate.

[0216] The steel of the substrate 3, 4 is more particularly a steel having a ferritic-pearlitic microstructure.

[0217] Preferably, the substrate 3, 4 is made of a steel intended for heat treatment, more particularly a press-hardenable steel, for example a manganese-boron steel (for example a steel of the type 22MnB5).

[0218] According to one embodiment, the steel of the substrate 3, 4 comprises, by weight:

[0219] 0.10% < C < 0.5%

[0220] 0.5% < Mn < 3%

[0221] 0.1% < Si < 1%

[0222] 0.01% < Cr < 1%

[0223] Ti < 0.2%

[0224] Al < 0.1%

[0225] S < 0.05%

[0226] P < 0.1%

[0227] B < 0.010%

[0228] the remainder being iron and impurities resulting from the manufacture.

[0229] More particularly, the steel of the substrate 3, 4 comprises, by weight:

[0230] 0.15% < C < 0.25%

[0231] 0.8% < Mn < 1.8%

[0232] 0.1% < Si < 0.35%

[0233] 0.01% < Cr < 0.5%

[0234] Ti < 0.1%

[0235] Al < 0.1%

[0236] S < 0.05%

[0237] P < 0.1%

[0238] B < 0.005%

[0239] the remainder being iron and impurities resulting from the manufacture.

[0240] According to one alternative, the steel of the matrix 3, 4 comprises by weight:

[0241] 0.040% < C < 0.100%

[0242] 0.70% < Mn < 2.00%

[0243] Si < 0.50%, and more particularly, Si < 0.30%

[0244] S < 0.009%, and more particularly, S < 0.005%

[0245] P < 0.030%

[0246] 0.010% < Al < 0.070%

[0247] 0.015% < Nb < 0.100%

[0248] Ti < 0.080%

[0249] N < 0.009%

[0250] Cu < 0.100%

[0251] Ni < 0.100%

[0252] Cr < 0.2%

[0253] Mo < 0.100%

[0254] Ca < 0.006%,

[0255] the remainder being iron and impurities resulting from the manufacture.

[0256] According to one alternative, the steel of the matrix 3, 4 comprises by weight:

[0257] 0.24% < C < 0.38%

[0258] 0.40% < Mn < 3%

[0259] 0.10% < Si < 0.70%

[0260] 0.015% < Al < 0.070%

[0261] 0% < Cr < 2%

[0262] 0.25% < Ni < 2%

[0263] 0.015% < Ti < 0.10%

[0264] 0% < Nb < 0.060%

[0265] 0.0005% < B < 0.0040%

[0266] 0.003% < N < 0.010%

[0267] 0.0001% < S < 0.005%

[0268] 0.0001% < P < 0.025%

[0269] wherein the contents of titanium and nitrogen satisfy the following relationship:

[0270] Ti / N > 3.42,

[0271] and the contents of carbon, manganese, chromium and silicon satisfy the following relationship:

[0272]

[0273] The steel optionally comprises one or more of the following elements:

[0274] 0.05% < Mo < 0.65%

[0275] 0.001% < W < 0.30%

[0276] 0.0005% < Ca < 0.005%

[0277] the remainder being iron and impurities resulting from the manufacture.

[0278] According to one alternative, the steel of the matrix 3, 4 comprises, by weight:

[0279] 0.06% < C < 0.100%

[0280] 1.4% < Mn < 1.9%

[0281] 0.2% < Si < 0.5%

[0282] 0.010% < Al < 0.070%

[0283] 0.04% < Nb < 0.06%

[0284] 3.4 x N < Ti < 8 x N

[0285] 0.02% < Cr < 0.1%

[0286] 0.0005% < B < 0.004%

[0287] 0.001% < S < 0.009%

[0288] the remainder being iron and impurities resulting from the manufacture.

[0289] The substrate 3, 4 can be obtained by hot rolling and / or by cold rolling followed by annealing, or by any other appropriate method, according to the desired thickness of the substrate 3, 4.

[0290] The thickness of the substrate 3, 4 is advantageously comprised between 0.6 mm and 5 mm, more particularly between 0.8 mm and 5 mm, and even more particularly between 1.0 mm and 2.5 mm.

[0291] According to one example, the thickness of the substrate 3 of the first precoated steel sheet 1 is different from the thickness of the substrate 4 of the second precoated steel sheet 2.

[0292] According to one alternative, the substrates 3, 4 of the first precoated steel sheet 1 and of the second precoated steel sheet 2 have the same thickness.

[0293] According to the application, the ultimate tensile strength Ts1 of the substrate 3 of the first precoated steel sheet 1 after press hardening is strictly greater than the ultimate tensile strength Ts2 of the substrate 4 of the second precoated steel sheet 2 after press hardening.

[0294] In this context, by "after press hardening" is meant after heating to a temperature greater than or equal to the complete austenitization temperature Ac3 of the steel substrate considered, followed by hot press forming, and then cooling to obtain hardening compared to the initial state.

[0295] For example, the ultimate tensile strength Ts1 of the substrate 3 of the first precoated steel sheet 1 after press hardening is comprised between 1400 MPa and 1600 MPa or between 1700 MPa and 2000 MPa.

[0296] For example, the ultimate tensile strength Ts2 of the substrate 3 of the second precoated steel sheet 2 after press hardening is comprised between 500 MPa and 700 MPa or between 1000 MPa and 1200 MPa.

[0297] For example, the ratio of the ultimate tensile strength Ts1 of the matrix 3 of the first pre-coated steel sheet 1 after press hardening to the ultimate tensile strength Ts2 of the matrix 4 of the second pre-coated steel sheet 2 after press hardening greater than or equal to 1.2, more particularly greater than or equal to 1.4.

[0298] Furthermore, the first pre-coated steel sheet 1 has a first thickness t1. The second pre-coated steel sheet 1 has a second thickness t2.

[0299] The thicknesses t1, t2 are for example comprised between 0.6 mm and 5 mm, more particularly between 0.8 mm and 5 mm, and even more particularly between 1.0 mm and 2.5 mm.

[0300] According to one embodiment, the thicknesses t1 and t2 are identical. According to one alternative, the thicknesses t1 and t2 are different.

[0301] The product of the first thickness t1 by the ultimate tensile strength Ts1 of the first pre-coated steel sheet 1 is strictly greater than the product of the second thickness t2 by the ultimate tensile strength Ts2 of the second pre-coated steel sheet 1.

[0302] In particular, the composition of the matrices 3 and 4 of the first and second pre-coated steel sheets 1 and 2 is chosen from the compositions described above.

[0303] For example, the steel of the matrix 3 of the first pre-coated steel sheet 1 comprises by weight:

[0304] 0.15% < C < 0.25%

[0305] 0.8% < Mn < 1.8%

[0306] 0.1% < Si < 0.35%

[0307] 0.01% < Cr < 0.5%

[0308] Ti < 0.1%

[0309] Al < 0.1%

[0310] S < 0.05%

[0311] P < 0.1%

[0312] B < 0.005%

[0313] The remainder being iron and impurities resulting from the manufacture.

[0314] According to another example, the steel of the matrix 3 of the first pre-coated steel sheet 1 comprises by weight:

[0315] 0.24% < C < 0.38%

[0316] 0.40% < Mn < 3%

[0317] 0.10% < Si < 0.70%

[0318] 0.015% < Al < 0.070%

[0319] 0% < Cr < 2%

[0320] 0.25% < Ni < 2%

[0321] 0.015% < Ti < 0.10%

[0322] 0% < Nb < 0.060%

[0323] 0.0005% < B < 0.0040%

[0324] 0.003% < N < 0.010%

[0325] 0.0001% < S < 0.005%

[0326] 0.0001% < P < 0.025%

[0327] wherein the contents of titanium and nitrogen satisfy the following relationship:

[0328] Ti / N > 3.42,

[0329] and the contents of carbon, manganese, chromium and silicon satisfy the following relationship:

[0330]

[0331] The steel optionally comprises one or more of the following elements:

[0332] 0.05% < Mo < 0.65%

[0333] 0.001% < W < 0.30%

[0334] 0.0005% < Ca < 0.005%

[0335] the remainder being iron and impurities resulting from the manufacture.

[0336] For example, the steel of the base 4 of the second precoated steel sheet 2 comprises by weight:

[0337] 0.040% < C < 0.100%

[0338] 0.70% < Mn < 2.00%

[0339] Si < 0.50%, and more particularly Si < 0.30%

[0340] S < 0.009%, and more particularly S < 0.005%

[0341] P < 0.030%

[0342] 0.010% < Al < 0.070%

[0343] 0.015% < Nb < 0.100%

[0344] Ti < 0.080%

[0345] N < 0.009%

[0346] Cu < 0.100%

[0347] Ni < 0.100%

[0348] Cr < 0.2%

[0349] Mo < 0.100%

[0350] Ca < 0.006%,

[0351] the remainder being iron and impurities resulting from the manufacture.

[0352] According to another example, the steel of the base 4 of the second pre-coated steel sheet 2 comprises by weight:

[0353] 0.06% < C < 0.100%

[0354] 1.4% < Mn < 1.9%

[0355] 0.2% < Si < 0.5%

[0356] 0.010% < Al < 0.070%

[0357] 0.04% < Nb < 0.06%

[0358] 3.4 x N < Ti < 8 x N

[0359] 0.02% < Cr < 0.1%

[0360] 0.0005% < B < 0.004%

[0361] 0.001% < S < 0.009%

[0362] the remainder being iron and impurities resulting from the manufacture.

[0363] Preferably, the carbon content of the base 3 of the first pre-coated steel sheet 1 is at least 0.05% by weight greater than the carbon content of the base 4 of the second pre-coated steel sheet 2.

[0364] According to the application, the aluminum-containing pre-coating 7, 8 comprises at least 50 wt.% of aluminum for at least one of the first pre-coated steel sheet 1 and the second pre-coated steel sheet 2.

[0365] Preferably, the pre-coating 7, 8 is obtained by hot-dip coating, i.e. by dipping the substrate 3, 4 into a molten metal bath. In this case, as Figure 1 illustrated, the pre-coating 7, 8 comprises an intermetallic alloy layer 9, 10 in contact with at least the substrate 3, 4.

[0366] The intermetallic alloy layer 9, 10 comprises an intermetallic compound comprising at least iron and aluminum. In particular, the intermetallic alloy layer 9, 10 is formed by a reaction between the substrate 3, 4 and the molten metal bath. More particularly, the intermetallic alloy layer 9, 10 comprises an intermetallic compound of the type Fe x - Al y and more particularly of the type Fe2Al5.

[0367] In the example illustrated in Figure 1 , the pre-coating 7, 8 also comprises a metallic alloy layer 11, 12 extending on top of the intermetallic alloy layer 9, 10. The metallic alloy layer 11, 12 has a composition close to that of the molten metal in the bath. It is formed by molten metal that is carried away by the sheet as it travels through the molten metal bath during hot-dip coating.

[0368] The metallic alloy layer 11, 12 is for example a layer of aluminum, of an aluminum alloy or of an aluminum-based alloy.

[0369] In the present text, an aluminum alloy refers to an alloy comprising more than 50 wt.% of aluminum. An aluminum-based alloy is an alloy in which aluminum is the main element by weight.

[0370] For example, the metallic alloy layer 11, 12 is a layer of an aluminum alloy also comprising silicon. More particularly, the metallic alloy layer 11, 12 comprises by weight:

[0371] - 8% < Si < 11%,

[0372] - 2% < Fe < 4%,

[0373] the remainder being aluminum and possible impurities.

[0374] The thickness of the metallic alloy layer 11, 12 is for example between 19 pm and 33 pm or between 10 pm and 20 pm.

[0375] In the example illustrated in Figure 1 , in which the pre-coating 7, 8 comprises a metallic alloy layer 11, 12, the thickness of the intermetallic alloy layer 9, 10 is generally of the order of a few microns. In particular, its average thickness is generally between 2 pm and 7 pm.

[0376] The specific structure of the pre-coating layers 7 and 8, which are obtained by hot-dip coating and include intermetallic compound alloy layers 9 and 10 and metal alloy layers 11 and 12, is specifically disclosed in patent EP 2 007 545.

[0377] According to another embodiment, the aluminum-containing pre-coatings 7 and 8 comprise only the intermetallic compound alloy layers 9 and 10 as described above. In this case, the thickness of the intermetallic compound alloy layers 9 and 10 is, for example, 10 μm to 40 μm. Such pre-coatings 7 and 8 consisting of intermetallic compound alloy layers 9 and 10 can be obtained, for example, by subjecting the pre-coatings 7 and 8 comprising the intermetallic compound alloy layers 9 and 10 and the metal alloy layers 11 and 12 as described above to a pre-alloying treatment. Such pre-alloying treatment is performed at a temperature and holding time selected such that the pre-coatings 7 and 8 are alloyed with the substrates 3 and 4 at least a portion of the thickness of the pre-coatings 7 and 8.

[0378] More specifically, the pre-alloying process includes the following steps: heating the plate to a pre-alloying temperature of 620°C to 1000°C, and holding the pre-alloyed plate at that temperature for a period of several minutes to several hours, depending on the processing temperature used. In this case, the intermetallic compound alloy layers 9 and 10 themselves can be composed of different intermetallic sublayers such as Fe2Al5, FeAl3, FeAl, and Fe6Al. 12 It consists of Si5 and FeAl3 sublayers.

[0379] Advantageously, such as Figure 1 As shown, the substrates 3 and 4 have aluminum pre-coatings 7 and 8 as described above on both of their main surfaces.

[0380] The first pre-coated steel plate 1 and the second pre-coated steel plate 2 may have the same pre-coatings 7 and 8.

[0381] Alternatively, the pre-coatings 7 and 8 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2 may have different compositions.

[0382] Then, the theoretical average aluminum content in the weld joint 22 obtained by butt welding (possibly using filler material) between the first pre-coated steel plate 1 and the second pre-coated steel plate 2 is determined.

[0383] When a filler material is intended to be used, the filler material is preferably a steel-based filler material with an aluminum content of less than or equal to 0.05% by weight.

[0384] This determination shall be made in any manner known to a technician.

[0385] For example, the theoretical average aluminum content in welded joint 22 can be determined using the following formula:

[0386]

[0387] wherein

[0388] is the theoretical average content of aluminum in the weld joint 22, in wt.-%,

[0389] Al 涂层 is the average aluminum content in the aluminum-containing pre-coat 7, 8, in wt.-%,

[0390] M c is the weight per unit area of the aluminum-containing pre-coat 7, 8 on each of the two pre-coated steel sheets 1, 2, in g / m 2

[0391] β is the proportion of steel-based filler material optionally added to the weld pool, being 0 to 1, wherein β equals zero in case no filler material is added to the weld pool,

[0392] t1 is the thickness of the first pre-coated steel sheet 1, in mm, and

[0393] t2 is the thickness of the second pre-coated steel sheet 2, in mm.

[0394] The above formula can be used even in case a filler material is used, as long as the filler material comprises an aluminum content of less than or equal to 0.05 wt.-%.

[0395] The above formula can be used even in case the base body 3, 4 comprises aluminum, as long as the aluminum content of the base body 3, 4 is less than or equal to 0.05 wt.-%.

[0396] When the proportion is expressed in percent, the proportion β of steel-based filler material optionally added to the weld pool is for example 0 to 0.5, i.e. 0% to 50%.

[0397] theoretical average content of aluminum in the weld joint 22 In case the theoretical average content of aluminum in the weld joint 22 is strictly greater than 1.25 wt.-%, the method according to the present application further comprises the step of preparing the weld edge 14 of at least one of the pre-coated steel sheets 1, 2 in such a way that the theoretical average content of aluminum in the weld joint after preparation is

[0398] More particularly, the weld edge 14 of the pre-coated steel sheet 1, 2 under consideration is the edge of the pre-coated steel sheet 1, 2 which is intended to be welded to another pre-coated steel sheet 1, 2.

[0399] As Figure 3 ​​As shown more specifically, the weld edge 14 includes the peripheral portion of the pre-coated steel plates 1, 2 that is intended to be at least partially incorporated into the weld joint 22 during butt welding. More specifically, the weld edge 14 includes the side 13 of the pre-coated steel plates 1, 2 and portions of the pre-coated steel plates 1, 2 extending from the side 13 and including a portion of the pre-coated coatings 7, 8 and a portion of the substrates 3, 4.

[0400] More specifically, the step of preparing the weld edge 14 includes removing the aluminum-containing pre-coating 7, 8 from at least a portion of the thickness of at least one main surface 5, 6 of the first pre-coated steel plate 1 and the second pre-coated steel plate 2. The pre-coating 7, 8 is removed in a removal area 18 extending from the side surface 13 of the pre-coated steel plates 1, 2 at the weld edge 14. The removal area 18 may extend from the side surface 13 of the pre-coated steel plates 1, 2 in a width of 0.5 mm to 2 mm. An example of the pre-coated steel plate 1 thus prepared is shown in… Figure 3 middle.

[0401] Removal is preferably performed using a laser beam.

[0402] Advantageously, in the removal zone 18, the metal alloy layers 11 and 12 are removed, while the intermetallic compound alloy layers 9 and 10 are retained with at least a portion of their thickness.

[0403] More specifically, in removal zone 18, metal alloy layers 11 and 12 are removed, while intermetallic compound alloy layers 9 and 10 are completely retained.

[0404] The remaining intermetallic compound alloy layers 9 and 10 protect the area of ​​the weld blank immediately adjacent to the weld joint 22 from oxidation and decarburization during subsequent hot forming steps, and from corrosion during service.

[0405] exist Figure 3 In the example shown, the metal alloy layers 11 and 12 have been removed from the removal area 18 at the weld edge 14, leaving the intermetallic compound alloy layers 9 and 10 intact.

[0406] Specifically, the number of pre-coating portions 7 and 8 to be removed, as well as the number of main surfaces of the pre-coated steel plates 1 and 2 to be removed, results in a theoretical average aluminum content in the welded joint 22 after removal. It ranges from 0.5% to 1.25% by weight.

[0407] Specifically, at least a portion of the thickness of the pre-coatings 7 and 8 on the following main surfaces can be removed:

[0408] -Only one main surface 5, 6 of the first pre-coated steel plate 1 or the second pre-coated steel plate 2, or

[0409] - on a total of two main faces, for example on only one main face 5, 6 of each of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2, or on both main faces 5, 6 of only one of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2; or

[0410] - on a total of three main faces 5, 6, i.e. on both main faces 5, 6 of one of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2 and on only one main face 5, 6 of the other pre-coated steel sheet 1, 2; or

[0411] - on a total of four main faces 5, 6, i.e. on both main faces 5, 6 of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2.

[0412] the theoretical average aluminum content in the welded joint 22 obtained by butt welding between the first pre-coated steel sheet 1 and the second pre-coated steel sheet 2 provided in the providing step, through possibly using a filler material having an aluminum content of less than or equal to 0.05% by weight is 0.5% to 1.25% by weight, in particular the first pre-coated steel sheet 1 and the second pre-coated steel sheet 2 are welded without having previously removed the pre-coating 7, 8. More particularly, in this case, the welding is performed using the first pre-coated steel sheet 1 and the second pre-coated steel sheet 2 whose pre-coating 7, 8 is at least intact on the welding edge 14.

[0413] Optionally, even in the case where the theoretical average aluminum content in the welded joint 22 obtained by butt welding between the first pre-coated steel sheet 1 and the second pre-coated steel sheet 2 provided in the providing step, through possibly using a filler material having an aluminum content of less than or equal to 0.05% by weight is 0.5% to 1.25% by weight and more particularly strictly greater than 0.5% by weight, it is also possible to remove the pre-coating 7, 8 within at least one portion of the thickness of the pre-coating 7, 8 at the welding edge 14 on at least one main face 5, 6 of at least one of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2 (and for example on only one main face 5, 6 of at least one of the two pre-coated steel sheets 1, 2). For example, the pre-coating 7, 8 is removed within at least one portion of the thickness of the pre-coating 7, 8 at the welding edge 14 on only one main face 5, 6 of each of the two pre-coated steel sheets 1, 2. In such a way that the theoretical average aluminum content in the welded joint 22 obtained by welding the first pre-coated steel sheet 1 and the second pre-coated steel sheet 2 thus prepared, through possibly using a filler material containing at most 0.05% by weight of aluminum remains 0.5% to 1.25% by weight.

[0414] In particular, such a removal can be performed in order to even further reduce the heat treatment temperature T t, the heat treatment temperature T t As will be explained later. Indeed, the austenitization temperature Ac3 of the weld joint 22 (WJ) decreases as the aluminum content decreases. In particular, the heat treatment temperature T t This optional removal step is performed strictly above 950°C. Indeed, in order to maintain good sprayability and weldability, it is preferred to use a heat treatment temperature T t .

[0415] After determining the theoretical average aluminum content in the weld joint 22 After the preparation steps (as needed or desired), the method further comprises a step of butt welding the first precoated steel sheet 1 with the second precoated steel sheet 2 using laser welding to obtain a weld joint 22 between the first precoated steel sheet 1 and the second precoated steel sheet 2 and thereby obtain a welded steel blank 15.

[0416] The aluminum content of the weld joint 22 is between 0.5wt% and 1.25wt%.

[0417] According to one embodiment, the welding step comprises the use of a filler material.

[0418] The filler material is advantageously a steel-based filler material having an aluminum content less than or equal to 0.05wt%. The filler material has a low aluminum content in order to dilute the aluminum in the coating.

[0419] For example, the filler material further comprises an austenite-forming element in order to partially balance the ferrite-forming and / or intermetallics-forming effect of the aluminum from the precoating 7, 8.

[0420] The filler material is for example a filler wire or a powder.

[0421] The proportion of filler material added to the weld pool is for example between 0 and 0.5.

[0422] According to one example, the filler material has the following composition by weight:

[0423] 0.1% < C < 1.2%

[0424] 0.01% < Mn < 10%

[0425] 0.02% < Ni < 7%

[0426] 0.02% < Cr < 5%

[0427] 0.01% < Si < 2%

[0428] Optionally:

[0429] Trace < Mo < 1%

[0430] trace amounts < Ti < 0.1 %

[0431] trace amounts < V < 0.1 %

[0432] trace amounts < B < 0.01 %

[0433] trace amounts < Nb < 0.1 %

[0434] trace amounts < Al < 0.05 %

[0435] the remainder being iron and impurities inevitably produced by the manufacturing.

[0436] According to a particular example, the filler material can have one of the compositions W1, W2 or W3 described in Table 1 below.

[0437] ％C %Mn ％Al %Ni ％Cr %Si %Mo %Ti ％B W1 0.29 0.85 0.03 0.1 0.15 0.15 0.025 0.035 0.004 W2 0.70 2.00 0.03 - 1.0 0.40 - 0.2 - W3 0.10 3.61 0.03 1.84 0.36 0.68 0.45 - -

[0438] Table 1 : Composition of filler wire

[0439] In all these compositions, the contents are expressed in weight percent.

[0440] Furthermore, for each composition, the remainder of the composition is iron and inevitable impurities.

[0441] In the above Table 1,“-” means that the composition contains at most trace amounts of this element.

[0442] According to a variant, the welding step is a self-fusion welding step, which means that the welding is performed without using a filler material. In this case, the composition of the welded joint 22 depends only on the composition of the base 3, 4 of the first and second precoated steel sheets 1, 2 and on the amount of precoating 7, 8 incorporated in the welded joint 22.

[0443] The welding operation results in the formation of a welded joint 22 at the joint between the two sheets 1, 2.

[0444] The welding step is a laser welding step, in which a laser beam 24 is directed towards the joint between the two precoated steel sheets 1, 2.

[0445] The laser welding step is performed, for example, using a CO2 laser or a solid-state laser or a semiconductor laser.

[0446] The laser source is preferably a high-power laser source. It can be chosen, for example, from a CO2 laser with a wavelength of 10 microns, a solid-state laser source with a wavelength of 1 micron or a semiconductor laser source such as a diode laser with a wavelength of 0.8 to 1 micron.

[0447] The power of the laser is chosen according to the thickness of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2. In particular, the power is chosen to allow the melting of the welded edges 14 of the pre-coated steel sheets 1, 2 and a sufficient mixing in the welded joint 22. For a CO2 laser, the laser power is for example comprised between 3 kW and 12 kW. For a solid state laser or a semiconductor laser, the laser power is for example comprised between 2 kW and 8 kW.

[0448] For both types of laser sources, the diameter of the laser beam 24 on the pre-coated steel sheets 1, 2 at its impact point 26 can be equal to about 600 pm.

[0449] During the welding step, the welding is for example carried out under a protective atmosphere. Such a protective atmosphere in particular prevents the oxidation and the decarburization of the area in which the welding is carried out, prevents the formation of boron nitride in the welded joint 22 and possible cold cracks caused by hydrogen absorption.

[0450] The protective atmosphere is for example formed by an inert gas or a mixture of inert gases. The inert gas can be helium or argon or a mixture of these gases.

[0451] The welding can be carried out using the laser beam as the only heat source.

[0452] Optionally, the laser welding step comprises, in addition to the laser beam, an additional heat source, for example an electric arc or an induction heating. This additional heat source helps to melt the edges of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2 in order to form the welded joint 22.

[0453] Optionally, the welding step comprises the use of a filler wire 20, as shown by the dotted line in Figure 1 In this case, the laser beam 24 is additionally configured to melt the filler wire 20 at the impact point 26 of the laser beam 24.

[0454] During the welding step, the distance between the facing welded edges 14 of the two pre-coated steel sheets 1, 2 is for example less than or equal to 0.3 mm and more particularly less than or equal to 0.1 mm. Providing such a gap between the facing welded edges 14 of the two sheets 1, 2 facilitates the deposition of material from the possible filler wire 20 during the welding operation and prevents the formation of an excessive thickness at the welded joint 22.

[0455] At the end of the welding step, a welded steel blank 15 is obtained as shown in Figure 2 .

[0456] After the welding step, the method according to the application comprises a step of heating the welded steel blank 15 thus obtained in a heat treatment furnace.

[0457] More particularly, the heating step comprises heating the welded steel blank 15 to a heat treatment temperature T t .

[0458] According to the application, the heat treatment temperature T t at least 10 °C lower than the full austenitization temperature Ac3(WJ) of the welded joint 22.

[0459] The full austenitization temperature Ac3(WJ) of the welded joint 22 (in °C) is determined from the composition of the welded joint 22, for example using the following formula:

[0460] Ac3(WJ) = 102.2 x Al + 439 x C + 181.9 x Mn + 364.1 x Si + 148 x Al 2 - 425.2 x C 2 - 29.2 x Mn 2 - 497.8 x Si 2 - 400 x Al x C + 9.9 x Al x Mn - 50.5 x Al x Si - 208.9 x C x Mn + 570.3, wherein Al, C, Mn and Si refer to the content of aluminum, carbon, manganese and silicon, respectively, in the welded joint 22, in weight %.

[0461] The above formula for Ac3(WJ) can be used within the content ranges indicated in Table 2 below:

[0462] %Mn %Max C 0.05 0.35 Mn 0.1 5 Si 0.1 0.5 Al 0.03 1.5 Cr 0.01 2 Ni - 2 Ti 0.001 0.2 Nb 0.001 0.1 Mo - 0.1 Cu 0.001 0.1 B - 0.004 N - 0.01 Ca - 0.006 W - 0.30 S - 0.05 P - 0.1

[0463] Table 2: Content ranges for Ac3 formula

[0464] In Table 2 above:

[0465] - all contents are expressed in weight percent.

[0466] -“-” means that there is no lower limit.

[0467] According to the application, the heat treatment temperature T t is also at least 15 °C higher than the minimum temperature T min In this context, the minimum temperature T min is defined as follows:

[0468]

[0469] wherein

[0470] Ac3(WJ) is the full austenitization temperature of the welded joint 22, in °C,

[0471] Al is the aluminum content in the welded joint 22, in weight % and

[0472]

[0473] wherein

[0474] Ts1 is the ultimate tensile strength of the strongest matrix 3 after press hardening, in MPa

[0475] Ts2 is the ultimate tensile strength of the weakest matrix 4 after press hardening, in MPa

[0476] β is the proportion of filler material added to the weld pool, from 0 to 1

[0477] C FW C is the carbon content of the filler material, in wt.%

[0478] p is the ratio of the thickness of the pre-coated steel sheet 2 comprising the weakest matrix 4 to the thickness of the pre-coated steel sheet 1 comprising the strongest matrix 3 (p = t2 / t1).

[0479] In the present text, a matrix is weaker than the other if it has a lower ultimate tensile strength Ts after press hardening.

[0480] The minimum temperature T min can be calculated based on:

[0481] - the chemical composition of the welded joint 22,

[0482] - the properties of the matrices 3, 4 of the pre-coated steel sheets 1, 2,

[0483] - in the case of use of a filler material, the proportion and composition of the filler material.

[0484] The step of heating the welded blank 15 further comprises a step of holding the welded steel blank 15 at the heat treatment temperature T t for a time of 2 minutes to 10 minutes.

[0485] At the end of the heating step, since the welded steel blank 15 has been heated to a temperature at least 10°C lower than the full austenitization temperature Ac3(WJ) of the welded joint 22, the microstructure of the welded joint 22 is not fully austenitic. The fraction of the intercritical ferrite in the welded joint 22 depends on the temperature difference between the heat treatment temperature T t and the full austenitization temperature Ac3(WJ). In particular, at the end of the heating step, the fraction of the intercritical ferrite in the welded joint 22 a IC is greater than or equal to 15% and at least 5% less than the maximum intercritical ferrite fraction

[0486] the maximum intercritical ferrite fraction (in %) can be determined using the following formula:

[0487] ​

[0488] wherein

[0489] Ts1 is the ultimate tensile strength of the strongest matrix 3 after press hardening in MPa,

[0490] Ts2 is the ultimate tensile strength of the weakest matrix 4 after press hardening in MPa,

[0491] β is the proportion of filler material added to the weld pool, from 0 to 1

[0492] C FW is the carbon content of the filler material in wt.-%

[0493] p is the ratio of the thickness of the pre-coated steel sheet 2 comprising the weakest matrix 4 to the thickness of the pre-coated steel sheet 1 comprising the strongest matrix 3 (p = t2 / t1).

[0494] The sub- ferrite fraction can be measured, as known to the skilled person, for example by direct quenching of the welded blank 15 after heating to the heat treatment temperature T t After appropriate nitric acid ethanol etch (Nital) etching, the sub-ferrite appears as a light colored constituent on a light grey martensitic matrix.

[0495] The sub-temperature ferrite fraction in the weld joint 22 can also be determined by analysis of a manganese element map of a sample illustrating the manganese content distribution in the sample. Such a map can be obtained for example by analysis of the sample via electron probe microanalysis (EPMA). In this Mn map, the areas with the lowest Mn content coincide with the sub-temperature ferrite areas, while the areas with higher Mn content correspond to the phases resulting from the austenite transformation during the intercritical annealing. Thus, the surface fraction of sub-temperature ferrite in this map corresponds to the surface fraction of the areas with the lowest Mn content. This method is for example described in the following document: Hanlon, D, Rijkenberg, A, Leunis, E et al: Quantitative phase analysis of multi-phase steels, PHAST (2007), ISBN 92-79-02658-5, pages 77 to 79. Indeed, it is known that during the intercritical annealing, a partitioning of manganese occurs between the austenite and the ferrite, the manganese migrating from the ferrite to the austenite, so that at the end of the intercritical annealing, the Mn content of the sub-temperature ferrite is strictly lower than the Mn content of the austenite. The phases formed from the austenite during the subsequent cooling (for example martensite, transformed ferrite and / or bainite) inherit the Mn content of the austenite, while the sub-temperature ferrite retains its lower Mn content resulting from the partitioning. Thus, on the Mn element map, the sub-temperature ferrite can be distinguished from the other phases, and in particular from the other types of ferrite, and corresponds to the areas where the Mn content is the lowest.

[0496] In the context of the present patent application, all fractions relating to the microstructure are expressed in surface percentage.

[0497] At the end of the heating step, the microstructure of the base 3, 4 of the first 1 and second 2 pre-coated steel sheets is entirely austenitic. In particular, the full austenitization temperature Ac3 of the base 3, 4 is strictly lower than the full austenitization temperature Ac3(WJ) of the weld joint 22 due to the presence of aluminum from the pre-coating 5, 6 on the weld edge 14 of the pre-coated steel sheets 1, 2 at the time of welding.

[0498] At the end of the heating step, the welded steel blank 15 is hot formed under press in a press forming tool into a steel part. For example, the welded steel blank 15 is formed into a steel part by hot stamping using a suitable hot stamping tool.

[0499] Preferably, the transfer time between the heat treatment furnace and the press forming tool is less than or equal to 10 seconds. It is for example comprised between 5 and 10 seconds. The transfer time is chosen to be as short as possible to avoid metallurgical transformations in the welded blank 15 before the hot forming, in particular the formation of ferrite.

[0500] The steel part thus formed is then cooled at a cooling rate greater than or equal to the critical martensitic or bainitic cooling rate of the most hardenable matrix 3, 4 of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2.

[0501] Advantageously, the cooling step is performed in the press forming tool, for example by using a forming tool provided with a cooling system comprising, for example, cooling channels formed in the press forming tool.

[0502] According to the application, at the end of the cooling step, the welded joint 22 has a microstructure comprising martensite and / or bainite and a sub- austenite ferrite fraction a greater than or equal to 15% and less than the maximum sub-austenite ferrite fraction sub-austenite ferrite fraction a less than 5% IC maximum sub-austenite ferrite fraction It can be determined as described above.

[0503] At the end of the cooling step, at least one of the matrices 3, 4 has a predominantly martensitic and / or bainitic microstructure. The martensite and / or bainite results from the transformation of the austenite formed during the heating step during the cooling step.

[0504] According to one example, both matrices 3, 4 have a predominantly martensitic and / or bainitic structure.

[0505] Herein, "predominantly" means that the microstructure consists of martensite and / or bainite and at most 5% of ferrite.

[0506] The application also relates to a press-hardened laser-welded steel part obtained using the method described above.

[0507] The part is in particular a crash control part, for example an intrusion prevention part or a shock absorbing part, a structural part or a part contributing to the safety of a motor vehicle.

[0508] The press-hardened laser-welded steel part comprises a first coated steel part portion and a second coated steel part portion connected by the welded joint 22 as described above.

[0509] More particularly, the first coated steel part portion and the second coated steel part portion result respectively from the hot press forming and cooling of the first pre-coated steel sheet 1 and of the second pre-coated steel sheet 2 in the press forming tool.

[0510] More particularly, each coated steel part portion comprises a steel matrix having, on at least one of its main faces, an aluminum-containing coating comprising iron and at least 30% by weight of aluminum.

[0511] In particular, the aluminum-containing coating of the first steel part portion and of the second steel part portion results from the at least partial alloying of the pre-coatings 7, 8 during the hot press forming.

[0512] The substrates of the first and second coated steel part portions have the composition described above for the pre-coated steel sheets 1, 2. They result from hot press forming and cooling of the substrates 3, 4 of the pre-coated steel sheets 1, 2.

[0513] The ultimate tensile strength Ts1 of the substrate of the first coated steel part portion is strictly greater than the ultimate tensile strength Ts2 of the substrate of the second coated steel part portion.

[0514] For example, the first coated steel part portion has a first thickness and the second coated steel part portion has a second thickness, and the product of the first thickness of the first coated steel part portion by the ultimate tensile strength is strictly greater than the product of the second thickness of the second coated steel part portion by the ultimate tensile strength Ts2.

[0515] The aluminum content of the weld joint 22 is comprised between 0.5 and 1.25 wt%.

[0516] The weld joint 22 has a microstructure comprising martensite and / or bainite and a maximum sub-ambient ferrite fraction a sub-ambient ferrite fraction a that is at least 5% less IC

[0517] the maximum sub-ambient ferrite fraction may be determined as described above.

[0518] On a press-hardened laser welded steel part, the proportion β of filler material added to the weld pool during the welding operation can be determined by measuring the aluminum content Al 焊接 of the weld joint 22 via any appropriate method. The aluminum content Al 涂层 of the coating of the welded steel sheet is known and is considered negligible in view of the aluminum content of the filler material, based on the above formula The proportion β can be calculated using the following formula: The carbon content C FW of the filler material can then be determined based on the carbon content of the substrates 3, 4, the proportion β of filler material determined based on the aluminum content of the weld joint 22, and the measured carbon content in the weld joint 22.

[0519] After press hardening, the ultimate tensile strength of the weld joint 22 is greater than or equal to the ultimate tensile strength of the weakest substrate 4.

[0520] The steel on at least one side of the weld joint 22 (corresponding to the steel of at least the first substrate 3) has a predominantly martensitic and / or bainitic structure. For example, the steel on either side of the weld joint 22 (corresponding to the steel of the first substrate 3 and the second substrate 4) has a predominantly martensitic and / or bainitic structure.

[0521] The inventors of the present application surprisingly found that when subjecting the welded blank 15 to a heat treatment under the above conditions, the ultimate tensile strength of the welded joint 22 will be strictly greater than the ultimate tensile strength of the base 4 of the second pre-coated steel sheet 2, i.e. the base having the lowest ultimate tensile strength. Thus, the part obtained after the above heat treatment will not fail in the welded joint 22 when subjected to a tensile test in a direction perpendicular to the welded joint 22, even though the microstructure of the welded joint 22 after the heat treatment is not entirely martensitic or bainitic.

[0522] Thus, the method according to the present application is particularly advantageous as it allows to obtain satisfactory mechanical properties at reduced costs. Indeed, when welding together pre-coated steel sheets comprising an aluminum-containing pre-coating, there is no longer a need to adjust the composition of the welded joint in such a way that the full austenitization temperature of the welded joint is less than or equal to the full austenitization temperature of the base, for example by removing the pre-coating on both sides of the pre-coated steel sheet or by adding a large amount of austenite-forming elements to the weld by using a filler material such as a filler wire. In particular, avoiding the removal of the pre-coating on both faces of the steel sheet reduces the total processing time. Moreover, reducing the amount of austenite-forming elements that must be added by the filler material or even completely avoiding the use of a filler material reduces the production costs and prevents problems resulting from the addition of a high proportion of filler material, in particular related to the geometry of the welded joint and obtaining a homogeneous mixture between the material from the pre-coated steel sheet and the material from the filler material in the welded joint.

[0523] The inventors of the present application carried out experiments E1 to E36 in which welded steel blanks 15 were produced using pre-coated steel sheets 1, 2. Each pre-coated steel sheet 1, 2 had a base 3, 4 having the following composition (see Table 5) and on both main faces a pre-coating 7, 8 formed by hot dip coating comprising a metallic alloy layer 11, 12 containing 88 wt.% aluminum, 10 wt.% silicon and 2% iron.

[0524] The total weight per unit area of the pre-coating 7, 8 on both main faces of each pre-coated steel sheet 1, 2 was 150 g / m2before any removal step. 2 .

[0525] The total weight per unit area of the remaining pre-coating 7, 8 on each pre-coated steel sheet 1, 2 was 100 g / m2after removing the metallic alloy layer 11, 12 on only one of the main faces 5, 6 of the pre-coated steel sheet 1, 2, leaving the intermetallic compound alloy layer 9, 10 intact. 2 .

[0526] The composition of the bases used for the experiments is disclosed in Table 3 below. The composition of the filler wires used for the experiments is disclosed in Table 4 below.

[0527] ％C %Mn ％Al %Ni ％Cr %Si %Mo %Ti ％B %Nb ％S ％P S1 0.22 1.22 0.043 0.02 0.228 0.304 - 0.050 0.003 0.003 0.001 0.015 S2 0.07 1.60 0.038 0.01 0.071 0.443 - 0.022 0.004 0.054 0.001 0.007 S3 0.07 1.57 0.022 - 0.003 0.041- 0.081 - 0.046 0.002 0.014

[0528] Table 3: Composition of base material used for experiments

[0529] ％C %Mn ％Al %Ni ％Cr %Si %Mo %Ti ％B W1 0.29 0.85 0.03 0.1 0.15 0.15 0.025 0.035 0.004 W2 0.70 2.00 0.03 - 1.0 0.40 - 0.2 - W3 0.10 3.61 0.03 1.84 0.36 0.68 0.45 - -

[0530] Table 4: Composition of filler wire used for experiments

[0531] In Tables 3 and 4 above, the compositions are expressed in weight percentages.

[0532] Furthermore, for each composition in Tables 3 and 4, the remainder of the composition is iron and unavoidable impurities.

[0533] "-" means that the composition contains at most traces of the element.

[0534] The complete austenitization temperature Ac3 and the ultimate tensile strength Ts of the above matrices S1, S2 and S3 are as follows:

[0535] S1 : 834°C; Ts = 1500 MPa

[0536] S2: 858°C; Ts = 1050 MPa

[0537] S3: 806°C; Ts = 700 MPa

[0538] The pre-coated steel sheets 1, 2 were subjected to butt laser welding using a disc-shaped laser with a power of 5.6 kW or a YAG laser with a power of 4 kW.

[0539] In all examples, a protective atmosphere consisting of helium or argon was used to avoid oxidation and decarburization of the area in which the welding was performed, as well as the formation of boron nitride in the welded joint and possible cold cracking caused by hydrogen absorption. The flow rate of the gas was greater than or equal to 15 1 / min.

[0540] The welded blank 1 was then subjected to a heat treatment comprising heating to a heat treatment temperature T t of 920°C and maintaining at this temperature for six minutes, chosen so as to prevent the transfer time between the heating furnace and the hot forming tool, which prevents the formation of ferrite, the blank was transferred into a hot press forming tool, then cooled in the press forming tool with a cooling speed greater than or equal to 30°C / s for one minute to obtain a press hardened blank.

[0541] The experimental conditions used for experiments E1 to E36 are summarized in Tables 5 and 6 below.

[0542] Tensile specimens were then cut from the heat treated blank thus obtained in a direction perpendicular to the welded joint.

[0543] Tensile tests were performed on longitudinal tensile specimens of the EN 12,5 x 50 (240 x 30 mm) type extracted parallel to the rolling direction using the method disclosed in the standards NF EN ISO 4136 and NF ISO 6892-1 at ambient temperature (about 20°C). For each heat-treated welded blank, 5 tensile tests were performed.

[0544] The results of the tensile tests are shown in the column entitled "Failure location" in the following Table 6, which shows the location where the failure occurred during the tensile test.

[0545] In this column:

[0546] - "BM" means a failure in the base metal (i.e. in the substrate of one of the pre-coated sheets);

[0547] - "Weld" means a failure in the welded joint; and

[0548] - "Mix" means a situation where some of the tensile specimens failed in the welded joint while others failed in the base metal.

[0549]

[0550]

[0551]

[0552]

[0553]

[0554] In the above Tables 5 and 6, no line is added for the embodiments according to the application.

[0555] These results show that when the welded blank 15 is heated to a heat treatment temperature in the above-mentioned temperature range and the holding time at this heat treatment temperature is between 2 minutes and 10 minutes before press forming and cooling, the failure occurs in the weakest base metal of the assembly (the "substrate of the second pre-coated steel sheet" in the above Tables 5 and 6) and not in the welded joint 22 (experiments E1, E2, E5, E10, E12, E13, E16, E18, E22 and E29 to E32).

[0556] On the contrary, for a heat treatment temperature T min+ 15°C and for a holding time at this heat treatment temperature of 2 to 10 minutes, failure is observed or always occurs in the welded joint 22 (experiments E3, E4, E6 to E8, E14, E15, E17, E19, E21, E23, E27 to E27 and E33 to E36) or in at least some of the welded joints 22 in the tensile specimens for the considered experiments (experiments E9, E11, E20, E24 and E28, referred to as "hybrid" in the table).

[0557] The inventors also noted that, in all the experiments according to the application, the welded joint 22 had a sub-temperature ferrite fraction a IC of 15% to of the microstructure.

[0558] These results demonstrate that, when the welded blank 15 is heat treated using the heat treatment conditions according to the application, the ultimate tensile strength of the welded joint 22 is strictly greater than the ultimate tensile strength of the weakest base material corresponding to the base body 4 of the second pre-coated steel sheet 2. This base body 4 thus forms the weakest zone of the part, rather than the welded joint 22. Failure will thus occur in the base body 4 of the second pre-coated steel sheet 2, rather than in the welded joint 22 itself. These results are unexpected, since the welded joint 22 is not fully austenitized and thus does not have a predominantly martensitic and / or bainitic microstructure, even after the heat treatment, but these results are still obtained.

[0559] The method according to the application is thus particularly advantageous, since it allows the optimal process parameters to be determined (including the lowest heat treatment temperature and the amount of filler material to be added) in order to obtain a part having satisfactory properties while minimizing the production costs and time.

Claims

1. A pressure-hardened laser-welded steel component, the steel component comprising a first coated steel component portion and a second coated steel component portion, Each coated steel component portion includes a steel substrate, and at least one of the first coated steel component portion and the second coated steel component portion has an aluminum-containing coating comprising at least 30% by weight of aluminum on at least one of its main surfaces. The first coated steel component portion has a first thickness (t1) and the second coated steel component portion has a second thickness (t2). The ultimate tensile strength (Ts1) of the substrate of the first coated steel component portion is strictly greater than the ultimate tensile strength (Ts2) of the substrate (4) of the second coated steel component portion. Furthermore, the product of the first thickness (t1) and the ultimate tensile strength (Ts1) of the first coated steel component portion is strictly greater than the product of the second thickness (t2) and the ultimate tensile strength (Ts2) of the second coated steel component portion. said first and second coated steel part portions are joined by a weld joint (22) having an aluminium content comprised between 0.5 and 1.25 wt% and a microstructure comprising a fraction of martensite and / or bainite, and of intercritical ferrite (a IC ) comprised between 15% and the maximum intercritical ferrite fraction said maximum intercritical ferrite fraction is defined using the following formula: in Ts1 is the ultimate tensile strength of the strongest matrix (3) after compression hardening, expressed in MPa. Ts2 is the ultimate tensile strength of the weakest matrix (4) after compression hardening, expressed in MPa. β is the proportion of filler material added to the weld pool, ranging from 0 to 1. C FW for the carbon content of the filler material in wt.-% ρ is the ratio of the thickness of the coated steel component portion including the weakest substrate to the thickness of the coated steel component portion including the strongest substrate, where ρ = t2 / t1. as well as The substrate (3, 4) of at least one of the first coated steel component portion and the second coated steel component portion has a microstructure that is primarily martensitic and / or bainitic.

2. The press-hardened laser-welded steel component according to claim 1, wherein the ratio of the ultimate tensile strength (Ts1) of the substrate of the first coated steel component portion to the ultimate tensile strength (Ts2) of the substrate of the second coated steel component portion is greater than or equal to 1.

2.

3. The press-hardened laser-welded steel component according to claim 1 or 2, wherein the carbon content of the substrate of the first coated steel component portion is at least 0.05% higher by weight than the carbon content of the substrate of the second coated steel component portion.

4. Press-hardened laser welded steel part according to claim 1 or 2, wherein, For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the substrate comprises, by weight: 0.10％≤C≤0.5％ 0.5% ≤ Mn ≤ 3% 0.1% ≤ Si ≤ 1% 0.01％≤Cr≤1％ Ti≤0.2% Al≤0.1％ S≤0.05％ P≤0.1％ B≤0.010％ The remainder consists of iron and impurities produced during manufacturing.

5. Press-hardened laser welded steel part according to claim 3, wherein, For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the substrate comprises, by weight: 0.15％≤C≤0.25％ 0.8% ≤ Mn ≤ 1.8% 0.1% ≤ Si ≤ 0.35% 0.01％≤Cr≤0.5％ Ti≤0.1% Al≤0.1％ S≤0.05％ P≤0.1％ B≤0.005％ The remainder consists of iron and impurities produced during manufacturing.

6. Press-hardened laser welded steel part according to claim 1 or 2, wherein, For at least one of the first coated steel component portion and the second coated steel component portion, the steel of the substrate comprises, by weight: 0.040％≤C≤0.100％ 0.70% ≤ Mn ≤ 2.00% Si≤0.50% S≤0.005％ P≤0.030％ 0.010％≤Al≤0.070％ 0.015% ≤ Nb ≤ 0.100% Ti≤0.080% N≤0.009％ Cu≤0.100% Ni≤0.100% Cr≤0.2％ Mo≤0.100% Ca≤0.006%, The remainder is iron and impurities resulting from the manufacture.

7. Press-hardened laser welded steel part according to claim 1 or 2, wherein, For at least one of the first coated steel part portion and the second coated steel part portion, the steel of the base body comprises by weight: 0.24％≤C≤0.38％ 0.40% < Mn < 3% 0.10% < Si < 0.70% 0.015％≤Al≤0.070％ 0％≤Cr≤2％ 0.25% < Ni < 2% 0.015% < Ti < 0.10% 0% < Nb < 0.060% 0.0005％≤B≤0.0040％ 0.003％≤N≤0.010％ 0.0001％≤S≤0.005％ 0.0001％≤P≤0.025％ wherein the content of titanium and nitrogen satisfies the following relationship: Ti / N > 3.42, and the content of carbon, manganese, chromium and silicon satisfies the following relationship: The steel optionally comprises one or more of the following elements: 0.05% < Mo < 0.65% 0.001％≤W≤0.30％ 0.0005% < Ca < 0.005% The remainder is iron and impurities resulting from the manufacture.

8. Press-hardened laser welded steel part according to claim 1 or 2, wherein, For at least one of the first coated steel part portion and the second coated steel part portion, the steel of the base body comprises by weight: 0.06％≤C≤0.100％ 1.4% < Mn < 1.9% 0.2% < Si < 0.5% 0.010％≤Al≤0.070％ 0.04% < Nb < 0.06% 3.4 x N < Ti < 8 x N 0.02％≤Cr≤0.1％ 0.0005％≤B≤0.004％ 0.001％≤S≤0.009％ The remainder is iron and impurities resulting from the manufacture.

Images