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    • 专利摘要:
    spot welded joint, set of two steel sheets, method to produce a spot welded joint, structural part and vehicle. it is a joint welded to the point of at least two steel sheets, where at least one of the steel sheets has a yield limit greater than or equal to 600 mpa, a final tensile strength greater than or equal to 1,000 mpa, elongation uniform greater than or equal to 15%. the chemical composition of the base metal comprises 0.05? ç ? 0.21%, 4.0? mn? 7.0%, 0.5? al? 3.5%, si? 2.0%, you? 0.2%, v? 0.2%, nb? 0.2%, p? 0.025%, b? 0.0035%, and the spot welded joint contains a fused zone microstructure that contains more than 0.5% al and that contains a surface fraction of segregated areas less than 1%, and said segregated areas they are zones larger than 20 m² and contain more than the nominal steel phosphorus content.
    公开号:BR112015032684B1
    申请号:R112015032684-6
    申请日:2014-07-22
    公开日:2021-04-27
    发明作者:Astrid Perlade；Samuel Vignier；Frédéric KEGEL；Artem Arlazarov
    申请人:Arcelormittal Investigación Y Desarrollo S.L.；
    IPC主号:
    专利说明:

    FIELD OF THE INVENTION
    [001] The present invention relates to a joint welded to the point of at least two steel sheets, in which at least one of the steel sheets has a yield limit greater than or equal to 600 MPa, a final tensile strength greater than or equal at 1000 MPa, uniform elongation greater than or equal to 15%. BACKGROUND OF THE INVENTION
    [002] In the automotive industry in particular, there is a continuing need to make vehicles lighter and increase safety with use and by joining light steels or steels that have high tensile strength to compensate for less thickness. In this way, several steel families such as those mentioned below that offer various levels of strength have been proposed.
    [003] First, steels have been proposed that have alloy microforming elements whose hardening is achieved both by precipitation and by grain size refinement. The development of such steels was followed by those of greater strength called Advanced High Strength Steels that maintain satisfactory levels of strength together with satisfactory cold formability.
    [004] For the purpose of obtaining even higher levels of tensile strength, steels that exhibit TRIP (transformation-induced plasticity) behavior with highly advantageous combinations of properties (tensile strength / deformability) have been developed. These properties are associated with the structure of such steels, which consists of a ferritic matrix that contains bainite and residual austenite. Residual austenite is stabilized by the addition of silicon or aluminum, and these elements delay the precipitation of carbides in austenite and bainite. The presence of residual austenite improves the ductile behavior under the effect of subsequent deformation, for example, when uni- axially stressed, the residual austenite of a part made from TRIP steel is progressively transformed into martensite, resulting in substantial hardening and delaying the appearance of corrosion.
    [005] To achieve an even greater tensile strength, that is, a level greater than 800 to 1,000 MPa, multi-phase steels that have a predominantly bainitic structure have been developed. In the automotive industry or in industry in general, such steels are advantageously used for structural parts such as shock absorber cross members, abutments, various reinforcements and abrasion resistant wear parts. However, the formability of these parts simultaneously requires a sufficient elongation, greater than 10% and not a very high yield strength / tensile strength ratio in order to have a sufficient reserve of plasticity.
    [006] All of these steel sheets have sufficiently satisfactory balances of resistance and ductility, but new challenges appear when these sheets are assembled using, for example, conventional spot welding techniques. From that point on, new concepts that present high strength and high formability when being weldable with the use of existing welding techniques are necessary.
    [007] In order to reduce the body in white weight, Order No. EP1987904 aims to provide a joint product of a steel product and an aluminum material, and a spot welding method for the joint product, ensuring that the spot welding with high bond strength can be performed. In one embodiment, a steel product having a t1 sheet thickness of 0.3 to 3.0 mm and an aluminum material having a t2 sheet thickness of 0.5 to 4.0 mm are joined together by spot welding to form a joint product of a steel product and an aluminum product. In this joint product, the nugget area in the joint part is 20 x t20.5 to 100 x t20.5 mm2, the area of a portion where the thickness of the interface reaction layer is 0.5 to 3 μm is 10 x t20'5 mm2 or more, and the difference between the interface, reaction layer thickness at the center of the joint part and the interface reaction layer thickness at a point distant from the center of the joint part by one distance of a quarter of the joint diameter Dc is 5 μm or less. According to this construction, a joint product of dissimilar material with excellent bonding strength is provided, which can be formed by an existing spot welding apparatus at a low cost without using other materials such as roofing material. This is done without adding a separate step and spot welding method for the dissimilar material joint product. Such a method involves welding the steel sheet to an aluminum sheet, the resistance of the joint material will have a soft area on the aluminum side compared to the steel side.
    [008] Order No. US2012141829 discloses a spot welded joint that includes at least one thin steel plate with a tensile strength of 750 MPa to 1850 MPa and a carbon equivalent Ceq equal to or greater than 0.22% by weight at 0.55% by weight and in which a nugget is formed at an interface of the thin steel plates. In the area of the outer layer of nugget, a microstructure consists of a dendrite structure in which an average value of arm intervals is equal to or less than 12 μm, an average grain diameter of carbides contained in the microstructure is 5 nm to 100 nm, and a density of carbide number equal to or greater than 2 x 106 / mm2. Such application does not focus on third generation steels, but conventional ones only.
    [009] None of the above mentioned techniques have addressed or solved the challenge of welding steel with unconventional amounts of alloying elements in steel, which remains unresolved. DESCRIPTION OF THE INVENTION
    [010] The present invention relates to a joint welded to the point of at least two sheets of steel, in which at least one of the sheets of steel is a sheet of steel with aluminum alloy which has: - an upper yield limit or equal to 600 MPa - a tensile strength greater than or equal to 1,000 MPa - a uniform elongation greater than or equal to 15%.
    [011] Since the welded joint comprises: - a molten zone containing at least 0.5% by weight of Al and a fraction of the surface of coarse segregated areas less than 1%. Thick segregated areas are defined as zones larger than 20μm2 that contain at least the nominal base metal phosphorus content.
    [012] Optionally, a fused zone microstructure that contains a density of iron carbides greater than 50nm equal to or greater than 2x106 per mm2.
    [013] Optionally, a microstructure at the border between the molten zone and the steel according to the invention does not have martensite 18R within the ferritic grains.
    [014] Another focus of the invention is to provide a process for creating such a welded joint with a steel that can be easily cold rolled to its final thickness by being compatible with usual continuous annealing lines and that has a low sensitivity to process parameters.
    [015] The invention has, as a first objective, a joint welded to the point of at least two sheets of steel, with at least one of them being an aluminum alloy sheet, which comprises, in weight percent: 0, 05 <C <0.21% 4.0 <Mn <7.0% 0.5 <Al <3.5% Si <2.0% Ti <0.2% V <0.2% Nb <0, 2% P <0.025% B <0.0035%; S <0.004%
    [016] Since the rest of the composition is iron and unavoidable impurities resulting from the smelting, the said steel having a yield limit greater than or equal to 600 MPa, a final tensile strength greater than or equal to 1,000 MPa, and uniform elongation greater than or equal to 15%, the microstructure of said steel contains 20% to 50% of austenite, 40% to 80% of annealed ferrite, less than 25% of martensite and where the welded joint is defined by a microstructure fused zone containing more than 0.5% Al and containing a surface fraction of coarse segregated areas less than 1%. Thick segregated areas are defined as zones larger than 20μm2 that contain phosphorus in an amount greater than the content of phosphorus and steel.
    [017] In another preferred embodiment, said chemical composition of steel with aluminum alloy has an aluminum content so that, 1.0 <Al <3.0%, or even 1.0 <Al <2.5% .
    [018] Preferably, said chemical composition of steel with aluminum alloy has a silicon content so that, Si <1.5% or even Si <1.0%.
    [019] In a preferred embodiment, said microstructure of steel with aluminum alloy contains between 50% and 70% of annealed ferrite.
    [020] In a preferred embodiment, said steel with aluminum alloy has less than 20% of martensite.
    [021] Preferably, the density of iron carbides greater than 50nm is equal to or greater than 2x106 per mm2 in the joint welded to the molten zone.
    [022] Preferably, the microstructure at the border between the molten zone and the steel according to the invention does not have martensite 18R with a phase similar to the orthorhombic needle inside the ferritic grains.
    [023] The invention also has, as an objective, a set of two steel sheets that includes a spot welded joint according to the invention.
    [024] The second objective of the invention is a process to produce the welded joint to the point of at least two sheets of steel, with at least one of them being a sheet of steel with aluminum alloy, produced by: - shaping steel of aluminum alloys whose composition takes place according to the present invention in order to obtain a plate, - reheat the plate at a Treaquec temperature between 1,150 ° C and 1,300 ° C, - hot-laminate the reheated plate with a temperature between 800 ° C and 1,250 ° C to obtain a hot-rolled sheet, with the last hot-rolling pass occurring at a Tlp temperature greater than or equal to 800 ° C,
    [025] - cool hot rolled steel between 1 and 150 ° C / s to a coiling temperature Tbinding less than or equal to 650 ° C.
    [026] - Then, wind the hot rolled steel cooled in Tbobbing.
    [027] - Optionally, hot rolled steel is annealed in batches between 400 ° C and 600 ° C between 1 and 24 hours, or continuously annealed between 650 ° C and 750 ° C between 20 and 180s.
    [028] The invention also aims at a process to obtain steel directly using a molding machine in which the product is immediately laminated after molding. This process is called ‘thin plate molding’.
    [029] Then: - Descaling the hot rolled steel - Cold rolling the steel sheet with a cold rolling ratio between 30% and 70% in order to obtain a cold rolled steel sheet. - heat the steel sheet at a rate of heating Htaxa at least equal to 1 ° C / s to the Trecoz annealing temperature - anneal the steel to a Trecoz temperature between Tmin and Tmax defined by Tmin = 721-36 * C-20 * Mn + 37 * Al + 2 * Si (in ° C) Tmax = 690 + 145 * C-6.7 * Mn + 46 * Al + 9 * Si (in ° C) for a period between 30 and 700 seconds, - cool the steel sheet at a cooling rate preferably between 5 ° C / s and 70 ° C / s, Cut the cold rolled steel into sheets to obtain a cold rolled steel sheet - weld at least one of the cold rolled steel sheets cold in another metal with an effective intensity between 3kA and 15kA and a force applied to the electrodes between 150 and 850 daN, and the said active electrode face diameter is between 4 and 10 mm.
    [030] Optionally, the steel sheet is cooled in Cooling2 to a TOA temperature between 350 ° C and 550 ° C and kept in TOA for a period between 10 and 300 seconds in order to be coated by hot immersion.
    [031] - additionally cool the steel sheet at a cooling rate V Cooling3 preferably above 5 ° C / s and below 70 ° C / s below room temperature to obtain a cold-rolled and annealed steel sheet.
    [032] Optionally, the cold-rolled and annealed steel is tempered at a temperature of between 170 and 400 ° C for a period of time between 200 and 800 s.
    [033] In a preferred embodiment, the cold-rolled steel sheet according to the invention is, after annealing, coated with Zn or a Zn alloy.
    [034] In another embodiment, the cold-rolled steel sheet according to the invention is, after annealing, coated with Al or Al alloy.
    [035] Optionally, the spot welded joint according to the invention, after welding, undergoes a post-thermal treatment that is applied with an intensity between 60% and 90% of the welding intensity for a period between 0.1 and 2 seconds.
    [036] Steel sheets or a set of two steel sheets welded according to the invention can be used to produce blank structural parts for blank vehicle bodies in the automotive industry. BRIEF DESCRIPTION OF THE DRAWINGS
    [037] Other features and advantages of the invention will appear through the detailed description below. The attached Figures are given by way of example and should not be taken as limiting the scope of the present invention. The same are given so that: - Figure 1 illustrates the evolution of the hardness of hot-rolled materials B1, C1, E1 and F1, - Figure 2 illustrates the tensile properties of hot-rolled materials B1, C1, E1 and F1, - Figure 3 illustrates the tensile properties of cold rolled materials B1, C1, E1 and F1 before annealing, - Figure 4 A shows the tensile properties of cold rolled and annealed materials B1, C1, E1 and F1, - Figure 4 B shows the tensile properties of cold rolled and annealed materials G1, H1, H2, H3 and I2, - Figure 5 shows the scanning electron micrographs of the molten zone after Nital recording and image analysis that highlights the effect of aluminum content on cementite particles (in white) in the microstructure for assemblies A + A, B + B, C + C and E + E as detailed in Table 5, - Figure 6 shows the heterogeneous weld resistance defined by cross-traction specimen (A, B, C, E and F welded with J) - Figure 7 illustrates the coefficient of C TS as a function of Al content (A, B, C, E and F welded with J for heterogeneous), - Figure 8 shows the welding range for homogeneous welding (A, B, C, E and F), - Figure 9 shows the welding range for heterogeneous welding (A, B, C, E and F welded with J), - Figure 10 shows the results of heterogeneous tensile shear stress (A, B, C, E and F welded with J), - Figure 11 shows the micrographs for spot welded joints with an aluminum alloy sheet containing 2.9 & 3.9% Al (spot welds E + E and F + F as detailed in Table 5) and illustration of martensite 18R, - Figure 12 shows the microhardness affiliation for homogeneous spot welding (A, B, C, E and F), - Figure 13 shows the microhardness affiliation for heterogeneous spot welding using a sheet aluminum alloy and a typical 600 MPa Double Phase resistance. (A, B, C, E and F welded with J), - Figure 14 illustrates the effect of aluminum content in the molten zone on the hardness (A, B, C, E and F welded with J for heterogeneous), - A Figure 15 shows the failure modes as a function of the Al content of a 1 to 4% aluminum alloy sheet (left to right) for B, C, E and F, - Figure 16 A shows the plug ratios heterogeneous, for example, A, B, C, E and F welded with J, - Figure 16 B shows the homogeneous plug ratios, for example, G and H, - Figure 17 provides a schematic description of Tensile Shear tests and Cross Stress used to define the resistance of spot welding, - Figure 18 provides a non-limiting example of a plug ratio and a fused zone geometry between an aluminum alloy sheet according to the invention and a double phase 600 (DP). H is the height of MZ, PD is the diameter of Buffer, MZ-D is diameter of MZ, where MZ stands for molten zone, - Figure 19 shows the microbe probe images with a threshold at the nominal P content that shows the effect of Al on the segregation of P to A, B, C, E, - Figures 20 A and B illustrate the surface fraction of areas with more than the nominal P content as a function of their size, with the Figure 20A is, for example, A, B, C, E while Figure 20B is for G and H, - Figure 21 shows the evolution of the surface fraction of areas larger than 20μm2 with more than the nominal P content in the fused zone as a function of Al content for one, B, C, E, - Figures 22 A and B illustrate the CTS coefficient as a function of Al content with and without post-treatment: A, for example, A, B, C, E and F in homogeneous welding and B, for example, A, B, C, E and F welded with J. DESCRIPTION OF ACCOMPLISHMENTS OF THE INVENTION
    [038] The present invention relates to a joint welded to the point of two steel sheets in which at least one of the steel sheets, called an aluminum alloy sheet, has a yield limit greater than or equal to 600 MPa, a final tensile strength greater than or equal to 1,000 MPa, uniform elongation greater than or equal to 15%. The chemical composition of base metal which comprises more than 0.5% Al, which is easy to weld and cool to its desired final thickness. In doing so, the chemical composition is very important as well as the annealing parameters in order to achieve all the objectives. The following chemical composition elements are given in percentage by weight.
    [039] According to the invention, the carbon content is between 0.05 and 0.21%. Carbon is a gamma-forming element. It promotes, with the Mn content of the invention, the stabilization of austenite. Less than 0.05%, the tensile strength above 1,000 MPa is difficult to achieve. If the carbon content is greater than 0.21%, the cold rolling capacity is reduced and the weldability becomes weaker. Preferably, the carbon content is between 0.10 and 0.21%.
    [040] Manganese needs to be between 4.0% and 7.0%. This element, also an austenite stabilizer, is used to stabilize sufficient austenite in the microstructure. It also has a solid solution hardening and a refinement effect on the microstructure. For the Mn content less than 4.0%, the fraction of austenite retained in the microstructure is less than 20% and the combination of uniform elongation greater than 15% and the tensile strength greater than 1,000 is not achieved. Above 7.0%, weldability becomes weak, while segregations and inclusions deteriorate damage properties.
    [041] Regarding aluminum, its content needs to be between 0.5% and 3.5%. Above 0.5% by weight, aluminum additions are interesting in many ways to increase the stability of retained austenite through an increase in carbon in the retained austenite. Al allows to reduce the hardness of the heated strip, which can then be easily cold rolled to its final thickness as seen in Figures 1, 2 and 3. The strength is also improved during annealing with additions of Al. The addition of Al induces the minor variation in austenite fraction as a function of temperature and induces to improve the plug ratio as shown in Figures 15 and 16. In addition, Al is the most efficient element when it reopens a large window of viability for annealing temperature in continuous annealing as it favors the combination of advanced recrystallization at temperatures above the non-recrystallization temperature and austenite stabilization. Aluminum should be less than or equal to 3.5% to prevent the formation of coarse grains of primary ferrite formed during solidification and not transformed into austenite during additional cooling, inducing tensile strength of less than 1,000MPa. It should be understood that since Al is alfagene, although C and Mn are both gamma, the ideal content of Al to limit the formation of coarse grains of primary ferrite decreases when the levels of C and Mn decrease.
    [042] Aluminum is also harmful for continuous molding as the molding powder can react with the liquid metal, and the reaction kinetics is increased when Al content is increased. These coarse grains of primary ferrite reduce the tensile strength to less than 1,000 MPa. As a consequence, the Al content is preferably between 1.0 and 3.0% and even more preferably between 1.0 and 2.5%.
    [043] Silicon is also very efficient for increasing resistance through a solid solution. However, its content is limited to 2.0%, due to the fact that, in addition to this value, the rolling loads increase too much and the hot rolling process becomes difficult. The cold rolling capacity is also reduced. Preferably, to avoid edge breaks, the Si content is less than 1.5% or even less than 1.0%.
    [044] Microforming elements of alloy such as titanium, vanadium and niobium can be added in an amount less than 0.2% each, respectively, in order to obtain additional precipitation hardening. In particular, titanium and niobium are used to control grain size during solidification. A limitation, however, is necessary due to the fact that in addition, a saturation effect is obtained.
    [045] As for sulfur, greater than a 0.004% content, ductility is reduced due to the presence of excess sulfides such as MnS, in particular, bore expansion tests show lower values in the presence of such sulfides.
    [046] Phosphorus is an element that hardens in a solid solution, but that reduces spot weldability and hot ductility, particularly due to its tendency to segregate at the grain boundaries or cosegregation with manganese. For these reasons, its content needs to be limited to 0.025%, and preferably 0.020%, in order to obtain satisfactory spot weldability.
    [047] The maximum boron content allowed by the invention is 0.0035%. Above this limit, a saturation level is expected with respect to the hardening capacity.
    [048] The balance is made from iron and unavoidable impurities. The level of impurity means less than 0.04% of elements such as Ni, Cr, Cu, Mg, Ca.
    [049] The steel microstructure contains, as a surface fraction, 20% to 50% austenite, 40% to 80% annealed ferrite and less than 25% martensite. The sum of these microstructural phases is equal to more than 95%. The balance is realized from inevitable small precipitates such as carbides.
    [050] Austenite is a structure that provides ductility, its content needs to be greater than 20% so that the steel of the invention is ductile enough with uniform elongation greater than 15% and its content needs to be less than 50% due to the fact that, above that amount, the balance of mechanical properties deteriorates.
    [051] The ferrite in the invention is defined by a central cubic structure obtained from recovery and recrystallization by annealing either the anterior ferrite formed during solidification or the bainite or martensite of the hot-rolled steel sheet. Therefore, the term annealed ferrite implies that more than 70% of the ferrite is recrystallized. Recrystallized ferrite is defined by an average disorientation, as measured by SEM-EBSD, less than 3 ° inside the grains. Its content must be between 40 and 80% in order to have a minimum of 1000 MPa of tensile strength, with at least 600 MPa of yield strength and at least 15% of uniform elongation.
    [052] Martensite is the structure formed during cooling after immersion of the unstable austenite formed during annealing. Its content needs to be limited to 25% so that the uniform elongation remains above 15%. A specific type of martensite is the so-called 18R martensite structure, which is a phase similar to the orthorhombic needle with a specific crystallography that has been specified and documented by Cheng et al. [W.-C. Cheng, C.-F. liu, Y.-F. Lai, Scripta Mater., 48 (2003), pages 295 to 300].
    [053] The method for producing steel according to the invention involves shaping steel with the chemical composition of the invention.
    [054] Molded steel is reheated to between 1,150 ° C and 1,300 ° C. when the plate reheat temperature is below 1,150 ° C, the lamination loads increase too much and the hot lamination process becomes difficult. Above 1,300 ° C, oxidation is very intense, which induces loss of scale and surface degradation.
    [055] The hot lamination of the reheated plate is carried out at a temperature between 1,250 ° C and 800 ° C, with the last hot lamination pass occurring at a Tlp temperature greater than or equal to 800 ° C. if Tlp is less than 800 ° C, hot workability is reduced
    [056] After hot rolling, the steel is cooled at a cooling rate of V1 cooling between 1 ° C / s to 150 ° C / s, until the coiling temperature T coiling less than or equal to 650 ° C. Less than 1 ° C / s, a thick microstructure is formed and the final balance of mechanical properties deteriorates. Above 150 ° C / s, the cooling process is difficult to control.
    [057] The winding temperature Tbinding must be less than or equal to 650 ° C. If the winding temperature is above 650 ° C, a thick ferrite and bainite structure is formed inducing a more heterogeneous microstructure after cold rolling and annealing.
    [058] Optionally, the steel undergoes an intermediate annealing at this stage to produce its hardness and facilitate the subsequent cold rolling process and eventually to avoid breaks during cold rolling. The annealing temperature must be between 450 ° C and 600 ° C between 1 and 24 hours in the case of batch annealing, or between 650 ° C and 750 ° C between 20 and 180s in the case of continuous annealing.
    [059] The next step consists of descaling and cold rolling the steel with a cold rolling ratio between 30% and 70% in order to obtain a cold rolled steel with thickness generally between 0.6 and 3 mm. Less than 30%, recrystallization during subsequent annealing is not favored enough and uniform elongation above 15% is not achieved due to a lack of recrystallization. Above 70%, there is a risk of edge breakage during cold rolling.
    [060] Annealing can then be carried out by heating the steel at an Htaxa heating rate of at least 1 ° C / s to the Trecoz annealing temperature. Such Trecoz temperature has minimum and maximum values defined by the following equations: - Tmin = 721-36 * C-20 * Mn + 37 * Al + 2 * Si, in ° C - Tmax = 690 + 145 * C-6.7 * Mn + 46 * Al + 9 * Si, in ° C, where elements of chemical composition are given in percentage by weight.
    [061] The control of the annealing temperature is an important feature of the process since it allows controlling the fraction of austenite and its chemical composition as well as the recrystallization of the steel of the invention. Less than Tmin, the minimum austenite fraction is not formed, or its stability is very high, inducing a limited tensile strength of less than 1,000 MPa. Above Tmax, there is a risk of forming too much martensite, leading to a uniform limited elongation of less than 15%.
    [062] After annealing, the steel sheet is cooled at a cooling rate between 5 ° C / s and 70 ° C / s.
    [063] Optionally, the steel sheet is cooled to a TOA temperature between 350 ° C and 550 ° C and kept in TOA for a period between 10 and 300 seconds. It has also been shown that such a heat treatment that facilitates the coating of Zn by hot dipping process, for example, does not affect the final mechanical properties.
    [064] Optionally, the annealed and cold-rolled steel sheet is tempered at a Ttemper temperature between 170 and 400 ° C for a ttemper period between 200 and 800s. This treatment allows quenching of martensite, which can be formed during cooling after immersion of unstable austenite. The hardness of martensite is thereby decreased and the ductility of steel is improved. Less than 170 ° C, the quench treatment is not efficient enough. Above 400 ° C, the loss of resistance becomes high and the balance between resistance and ductility is no longer improved.
    [065] The cold-rolled and annealed steel sheet is then spot welded in order to obtain a welded joint with high strength.
    [066] To obtain a spot weld according to the invention, the welding parameters can be defined as follows. The effective intensity can be between 3kA and 15kA. As a non-limiting example, the spot welding intensity according to the invention is shown in Figures 8 and 9. The force applied to the electrodes is between 150 and 850 daN. The active electrode face diameter is between 4 and 10 mm. A suitable spot weld is defined by its characteristic dimension of the molten zone. Its fused zone height is between 0.5 and 6mm and diameter between 3 and 12mm as in Figure 18.
    [067] The spot welded joint according to the invention is defined by a fused zone microstructure that contains a surface fraction of coarse segregated areas less than 1%. Coarse segregated areas are defined as zones larger than 20μm2 that contain phosphorus in an amount greater than the nominal phosphorus content of the base metal. Higher than this value, the segregation is very high, which reduces the nugget hardness as in Figures 19, 20 and 21.
    [068] Additionally, the molten zone microstructure contains an iron carbide density greater than 50nm equal to or greater than 2x106 per mm2. Below this density, the martensite is not sufficiently tempered and the nugget microstructure does not have sufficient hardness as in Figures 5, 12, 13 and 14.
    [069] Preferably, on at least one side of the welded joint, the microstructure at the border between the molten zone and the steel according to the invention has no 18R martensite within the ferritic grains so that the coarse grained area maintains sufficient hardness as in Figure 11 for the 3% Al content.
    [070] Optionally, the spot welded joint according to the invention undergoes a thermal post-treatment to further improve the spot weld resistance as shown in Figures 22 A and B. Such post-treatment can be done in welding either homogeneous as heterogeneous. The post-treatment in the oven consists of an austenitization treatment above 1,000 ° C for at least 3 minutes followed by rapid cooling, that is, greater than 50 ° C / s for the welded joint.
    [071] Post-treatment in situ consists, after welding, of a treatment in two stages: • a first stage with no applied current of at least 0.2 seconds • a second stage that consists of applying a current to the welded joint between 60% and 90% of the average intensity applied during welding.
    [072] In order to season the martensite and improve the hardness of the nugget and the Heat Affected Zone. The total time of step 1 and step 2 is between 0.1 to 2 seconds.
    [073] The invention will be better understood with the following non-limiting examples. In fact, the spot welded steel of the invention can be obtained with any other steel, for example: interstitial free steels, double-phase steels, TRIP steels, BH steels, press-hardened steels, multiple-phase steels.
    [074] Semi-finished products were produced from steel molding. The chemical compositions of the semi-finished products, expressed as a percentage by weight, are shown in Table 1 below. The rest of the steel composition in Table 1 consists of iron and unavoidable impurities resulting from smelting.
    TABLE 1 CHEMICAL COMPOSITION (% BY WEIGHT).
    [075] The levels of Ti and V in steels A to J are less than 0.010%. The boron content is less than 35 ppm.
    [076] A to I steels were first reheated and hot rolled into 2.4 mm thick plates. J steel is a typical double-phase steel with 600 MPa of tensile strength, this type of steel is known for its technical know-how, the same is used as the steel to which steels A to I are welded for welding cases heterogeneous. The hot rolled steel plates A to I were then cold rolled and annealed. The process parameters submitted are shown in Table 2 with the following abbreviations: - Treaquec: is the reheating temperature - Tlp: is the finishing laminating temperature - Cooling1: cooling rate after the last hot rolling pass - Tiling: is the coiling temperature - IA T: is the temperature of the intermediate annealing performed on the heated strip - IA t: is the duration of the intermediate annealing performed on the heated strip - Rate: is the reduction rate of the cold rolling - Htaxa: is the heating rate - Trecoz: is the immersion temperature during annealing. - trecoz: is the duration of immersion during annealing. - Cooling2: is the cooling rate after annealing at room temperature.

    TABLE 2 CONDITIONS OF HOT LAMINATION AND COLD LAMINATION AND IRONING.
    [077] In Table 2, “empty” means that no intermediate annealing was performed and “*” means that the heating rate was 20 ° C / s to 600 ° C and then 1 ° C / s until the annealing temperature.
    [078] Table 3 presents the following characteristics: - Ferrite: “OK” refers to the presence of ferrite with a volume fraction between 40 and 80% in the microstructure of the annealed sheet. “KO” refers to comparative examples where the ferrite fraction is outside this range. - Austenite: “OK” refers to the presence of austenite with a volume fraction between 20 and 50% in the microstructure of the annealed sheet. “KO” refers to comparative examples where the austenite fraction is outside this range. - Martensite: “OK” refers to the presence or absence of martensite with a volume fraction less than 25% in the microstructure of the annealed sheet. “KO” refers to comparative examples in which the fraction of martensite is greater than 25%. - UTS (MPa) refers to the final tensile strength measured by a tensile test in the longitudinal direction in relation to the rolling direction. - YS (MPa) refers to the flow limit measured by tensile test in the longitudinal direction in relation to the rolling direction. - UEl (%) refers to the uniform elongation measured by tensile test in the longitudinal direction in relation to the rolling direction. - YS / TS refers to the relationship between yield strength and final tensile strength. - TEl refers to the total elongation measured in the ISO 12.5x50 specimen.

    TABLE 3: PROPERTIES OF COLD LAMINATED AND RECOILED SHEETS
    [079] A to I steels are then spot welded to a DP 600 GI as an example following the welding parameters presented in Table 4: The sheet thickness for material from A to I and DP600 GI is 1.2mm. The welding parameters are the same between degrees and differ only between homogeneous and heterogeneous welding.
    TABLE 4: STEEL WELDING PARAMETERS.
    [080] The different values are explained here below: - Welding current range: The welding current range (also called welding intensity) is expressed in kA. The minimum of the welding range is defined by the welding current required to develop a nugget where the diameter is 4.25Vt or more, where t is the thickness of the material in mm. The maximum of the welding current range is defined by the current in which the expulsion of the molten metal from the nugget occurs.
    [081] - The alpha value is the maximum load in the cross test divided by the weld diameter and thickness. It is a standardized load for resistant spot welding expressed in daN / mm2
    [082] - plug ratio: The plug ratio is equal to the plug diameter divided by the diameter of MZ. The smaller the plug ratio, the lower the hardness of the molten zone as shown in Figure 18.

    TABLE 5: WELDING RESULTS BY POINTS. CGHAZ MEANS COFFEE HEAT AFFECTED AREA.
    [083] All cold rolled and annealed steels produced with chemical compositions of B, C, D, E, H (except H2) and I are produced according to the invention, they showed YS above 600 MPa, resistance to traction greater than 1,000 MPa and uniform elongation 15% as shown in Figure 4 A for B1, C1, E1 and F1 (reference) and Figure 4B for G1, H1, H2, H3, and I2 where G1 and H2 are references. The chemical composition is within the target range as well as the microstructure; the process parameters of the invention were also followed. A1, F1, G1, and H2 are not in accordance with the invention. The resistance test of spot welds was carried out according to the test as shown in Figure 17. They are called tensile shear tests and cross tension tests. These tests are used to determine the weld strength. As shown in Figures 6, 7 and 10, the spot welding resistance increases with the Al content within the Al range of the invention.
    [084] In addition, an examination of macrogravure specimens can reveal the nugget diameters (Figure 11) as well as penetration and weld microstructures in the different zones.
    [085] When it returns to post-heat treatments, as can be seen from Figure 22, the cross-tensile strength coefficient is additionally improved with this said treatment for spot welded joints with at least one steel containing Al. it is due to the alpha effect of Al, which opens a tempering window lower than Ac1, allowing it not to be re-energized by welding the critical parts of the welded joint.
    [086] The steel sheet set according to the invention will be used beneficially for the manufacture of structural or safety parts in the automobile industry.
    权利要求:
    Claims (21)
    [0001]
    1. WELDED POINT JOINT, of at least two sheets of steel, characterized in that at least one sheet is made from an aluminum alloy sheet comprising, in weight percentage: 0.05 <C <0.21% 4.0 <Mn <7.0% 0.5 <Al <3.5% Si <2.0% Ti <0.2% V <0.2% Nb <0.2% P <0.025% B < 0.0035%; S <0.004% and the balance of the composition is iron and unavoidable impurities resulting from the foundry, and the steel sheet with aluminum alloy has a yield limit greater than or equal to 600 MPa, a final tensile strength greater than or equal at 1,000 MPa and uniform elongation greater than or equal to 15%, with the microstructure of the aluminum alloy steel sheet containing 20% to 50% austenite, 40% to 80% annealed ferrite, less than 25% martensite and where the spot welded joint has a fused zone microstructure that contains more than 0.5% Al and that contains a surface fraction of segregated areas less than 1%, with the segregated areas being larger areas than than 20 μm2 that contain more phosphorus than the nominal phosphorus content of steel with aluminum alloy.
    [0002]
    2. WELDED JOINT, according to claim 1, characterized by the chemical composition of steel with aluminum alloy having an aluminum content such that: 1.0 <Al <3.0%.
    [0003]
    3. WELDED POINT JOINT, according to claim 2, characterized by the chemical composition of steel with aluminum alloy having an aluminum content such that: 1.0 <Al <2.5%.
    [0004]
    4. POINT WELDED JOINT, according to any one of claims 1 to 3, characterized in that the chemical composition of steel with aluminum alloy has a silicon content so that Si <1.5%.
    [0005]
    5. POINT WELDED JOINT, according to claim 4, characterized by the chemical composition of steel with aluminum alloy having a silicon content so that Si <1.0%.
    [0006]
    6. SPOT WELDED JOINT, according to any one of claims 1 to 5, characterized in that the microstructure of steel with aluminum alloy contains between 50% and 70% of annealed ferrite.
    [0007]
    7. POINT WELDED JOINT, according to any one of claims 1 to 6, characterized in that the microstructure of steel with aluminum alloy contains less than 20% of martensite.
    [0008]
    8. SPOT WELDED JOINT, according to any one of claims 1 to 7, characterized in that the joint contains iron carbides greater than 50 nm whose density is equal to or greater than 2x106 per mm2 and the microstructure on the border between the molten zone and the steel with aluminum alloy is absent of martensite 18R with phase similar to the orthorhombic needle inside the ferritic grains.
    [0009]
    9. SET OF TWO STEEL SHEETS, which includes a spot welded joint, characterized in that it is as defined in any one of claims 1 to 8.
    [0010]
    10. METHOD TO PRODUCE A WELDED JOINT IN POINT, of at least two sheets of steel, characterized in that at least one sheet is made from a sheet of steel with aluminum alloy, as defined in any one of claims 1 to 8, comprising the following successive steps: - molding an aluminum alloy steel whose composition is as defined in any one of claims 1 to 5 in order to obtain a plate, - reheating the plate at a Treaquec temperature between 1,150 ° C and 1,300 ° C, - hot-laminate the reheated plate with a temperature between 800 ° C and 1,250 ° C to obtain a hot-rolled steel, with the last hot-rolling passage occurring at a Tlp temperature greater than or equal to 800 ° C, - cool hot-rolled steel between 1 and 150 ° C / s to a Tembobing coiling temperature less than or equal to 650 ° C, then - cooling hot-rolled steel cooled in Tembobing, - descaling, - cold-rolling, lamination ratio a cold between 30% and 70% in order to obtain a cold rolled steel sheet, - heat at a rate of heating Htaxa at least equal to 1 ° C / s until the annealing temperature Trecoz - anneal at a temperature Trecoz between Tmin and Tmax defined by Tmin = 721-36 * C-20 * Mn + 37 * Al + 2 * Si (in ° C) Tmax = 690 + 145 * C-6.7 * Mn + 46 * Al + 9 * Si (in ° C) for a period between 30 and 700 seconds, - cool to the desired temperature at a cooling rate that is between 5 ° C / s and 70 ° C / s, - cut cold-rolled steel into sheets to obtain steel sheets cold rolled, - weld at least one of the cold rolled steel sheets to another metal with an effective intensity between 3kA and 15kA and a force applied to the electrodes between 150 and 850 daN, with the active electrode face diameter being between 4 and 10 mm.
    [0011]
    11. METHOD, according to claim 10, characterized in that the hot-rolled steel sheet is annealed by batch between 400 ° C and 600 ° C between 1 and 24 hours.
    [0012]
    METHOD, according to claim 10, characterized in that the hot-rolled steel sheet is continuously annealed between 650 ° C and 750 ° C between 20 and 180s.
    [0013]
    13. METHOD according to any one of claims 10 to 11, characterized in that the molding of the steel is carried out using a thin plate molding machine to obtain the hot-rolled steel sheet.
    [0014]
    14. METHOD, according to claim 10, characterized in that the desired temperature is a TOA temperature between 350 ° C and 550 ° C and maintained in TOA for a period between 10 and 300 seconds.
    [0015]
    15. METHOD according to claim 14, characterized in that the steel sheet is additionally cooled to room temperature at a cooling rate Vcooling3 greater than 5 ° C / s and less than 70 ° C / s to obtain a laminated steel sheet cold and annealed.
    [0016]
    16. METHOD according to any one of claims 10 to 14, characterized in that the steel sheet is tempered at a temperature of between 170 and 400 ° C for a period between 200 and 800 seconds.
    [0017]
    17. METHOD according to any one of claims 10 to 15, characterized in that, after annealing, the cold-rolled steel sheet is additionally coated with Zn or a Zn alloy.
    [0018]
    Method according to any one of claims 10 to 15, characterized in that, after annealing, the cold-rolled steel sheet is additionally coated with Al or an Al alloy.
    [0019]
    19. METHOD, according to any one of claims 10 to 17, characterized in that a post-heat treatment is applied with an intensity between 60% and 90% of welding intensity for a period between 0.1 and 2 seconds.
    [0020]
    20. STRUCTURAL PART, characterized by containing a spot welded joint or a set of two steel sheets, as defined in any one of claims 1 to 9, or a produced spot welded joint, as defined in any of claims 10 to 19.
    [0021]
    21. VEHICLE, characterized in that it contains a spot welded joint, structural part or set of two steel sheets, as defined in any one of claims 1 to 9 or produced with a spot welded joint, as defined in any one of claims 10 to 20.
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    同族专利:
    公开号 | 公开日
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    CN105408513A|2016-03-16|
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    PL3052672T3|2018-02-28|
    UA114859C2|2017-08-10|
    HUE035451T2|2018-05-02|
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    MX2016001017A|2016-04-11|
    MA38696B1|2017-04-28|
    US20190193187A1|2019-06-27|
    EP3052672B1|2017-09-27|
    RU2016106169A|2017-08-30|
    CA2916632C|2020-10-27|
    KR101797408B1|2017-11-13|
    JP2016531200A|2016-10-06|
    WO2015011510A1|2015-01-29|
    CA2916632A1|2015-01-29|
    WO2015011547A3|2015-04-16|
    EP3052672A2|2016-08-10|
    CN105408513B|2018-09-04|
    RU2647425C2|2018-03-15|
    KR20160035015A|2016-03-30|
    MA38696A1|2016-09-30|
    ES2645731T3|2017-12-07|
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    法律状态:
    2018-11-06| B06F| Objections, documents and/or translations needed after an examination request according [chapter 6.6 patent gazette]|
    2019-10-08| B06U| Preliminary requirement: requests with searches performed by other patent offices: procedure suspended [chapter 6.21 patent gazette]|
    2020-09-08| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application [chapter 6.1 patent gazette]|
    2021-01-26| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-04-27| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 22/07/2014, OBSERVADAS AS CONDICOES LEGAIS. |
    2021-06-01| B09W| Decision of grant: rectification|Free format text: REFERENTE A RPI 2612 DE 26/01/2021. |
    优先权:
    申请号 | 申请日 | 专利标题
    IBPCT/IB2013/001609|2013-07-25|
    PCT/IB2013/001609|WO2015011510A1|2013-07-25|2013-07-25|Spot welded joint using high strength and high forming and its production method|
    PCT/IB2014/001366|WO2015011547A2|2013-07-25|2014-07-22|Spot welded joint using high strength and high forming and its production method|
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