METHOD FOR PRODUCE A WELDED COMPONENT FROM A FORGED HIGH-STRENGTH STEEL

Description

[0001] The invention relates to a method for producing a welded component, in which a hot-rolled steel product made of high-strength steel with a material thickness of at least 1.5 mm is formed into a component and welded joints are then produced on the component by means of fusion welding.

[0002] The highly competitive automotive market forces manufacturers, among other things, to constantly search for solutions to reduce vehicle fuel consumption while maintaining maximum comfort and occupant protection. On the one hand, weight savings of all vehicle components play a crucial role, but on the other hand, the best possible behavior of individual body and chassis components under high static and dynamic loads during operation, as well as in the event of a crash, is also important. Suppliers are attempting to meet this need by providing high-strength and ultra-high-strength steels, which can reduce wall thicknesses while simultaneously improving component behavior during production and operation.Such steels must therefore meet comparatively high requirements with regard to strength, ductility, toughness, energy absorption, fatigue strength as well as operational strength and processability, for example by cold forming, welding and / or surface treatment.

[0003] Due to the high corrosion requirements, the surfaces of these steels often have to be coated with appropriate corrosion protection layers, such as zinc, whereby both conventional hot-dip galvanizing and high-temperature galvanizing are used.

[0004] In the past, conventional steels with relatively large sheet thickness, water-quenched and tempered high-strength fine-grain steels or multi-phase steels were mostly used for this application.

[0005] The use of conventional steels suffers from the disadvantage of heavy components. Alternative high-strength multiphase steels have disadvantages such as poor weldability and poor formability due to their high base hardness. Water-quenched and tempered steels are expensive to produce and therefore often uneconomical.

[0006] If these steels are subjected to subsequent thermal treatment, for example, by welding, resulting in temperatures in the weld area exceeding Ac3 (approximately 900 °C), these steels lose their original strength. With multiple heat treatments, such as intersecting welds in the heat-treated area, this phenomenon repeats itself, causing the steel to steadily lose strength.

[0007] In high-strength multi-phase steels, this loss of strength is even more pronounced after appropriate heat treatment because the original martensitic phase portion is lost when heated above the transformation temperature Ac3 if the cooling is not controlled and intensified.

[0008] This is particularly disadvantageous for dynamically stressed, welded chassis components or parts, as the required fatigue and operational strengths according to the specifications can no longer be achieved. These components can be made of steel strips or steel sheets, or of welded tubes, for example, high-frequency induction (HFI) welded tubes, or even seamless tubes.

[0009] In the following, steel strips, steel sheets, welded or seamless pipes are therefore summarized as steel products for the purposes of the invention.

[0010] For these reasons, air-hardening steel materials have been developed as an alternative to conventional steels. These steels overcome the disadvantages of conventional steels by achieving the required material properties simply by cooling the steel in air, for example, after heat treatment of the component. These steels combine good cold formability (e.g., Rp0.2: < 420 MPa for A80: >= 25%) in the annealed state with high strength and hardness of the formed component in the air-hardened state (e.g., Rm: > 800 MPa). Heat treatment can be, for example, furnace heating or high-temperature galvanizing.

[0011] In a steel strip made of air-hardening steel, the steel cools, at least in sections, in air after hot rolling, especially in still air, so quickly that the air-hardening effect sets in. The current state of the art has required achieving cold formability for the production of formed components through a subsequent soft annealing process, for example, in a hood annealing furnace or through homogenization annealing. Alternatively, cold formability can also be maintained after hot rolling if a correspondingly tightly wound coil is allowed to cool slowly, possibly in a special heat-insulated hood.

[0012] The same applies to state-of-the-art seamless hot-rolled tubes which, after air cooling and hardening, are subjected to a soft annealing process in order to restore cold formability.

[0013] The published patent application DE 102 21 487 A1 discloses the use of an air-hardening steel material for molded components in lightweight vehicle construction, with the main elements C (0.09 - 0.13 weight %), Si (0.15 - 0.30 weight %), Mn (1.10 - 1.60 weight %), Cr (1.0 - 1.6 weight %), Mo (0.30 - 0.60 weight %) and V (0.12 - 0.25 weight %), the remainder being iron including usual accompanying elements.

[0014] While this Cr-Mo-V-based alloy concept achieves the mechanical material properties required for the specified application in the air-hardened state, as well as good tempering resistance and galvanizability, the relatively high Cr content of 1.0–1.6 wt.% is a disadvantage. This can cause undesirable chromium carbide precipitation in the weld seam, particularly during HFI welding, which is frequently used in pipe production. These precipitations can lead to cracking in the weld seam during further processing of the welded pipe through forming or severe mechanical stress on the welded component during operation, thus leading to premature failure of the component. The relatively high chromium content also increases costs.

[0015] The patent specificationEP 0 576 107 B1 discloses an air-hardening steel with a reduced Cr content, which is used for the production of seamless, ungalvanized structural tubes, for example, as door reinforcement tubes in automotive construction. The Mn-Si-Ti-B-based alloy concept has as its main elements C (0.15-0.30 wt%), Mn (2.05-3.35 wt%), Si (0.50-0.80 wt%), Cr (0.5-1.0 wt%), Mo (max. 0.6 wt%), Ti (0.01-0.05 wt%), B (0.0015-0.0035 wt%), and N (0.002-0.015 wt%), the remainder being iron and common accompanying elements.

[0016] The disadvantage of this steel, which is known for seamless pipe production, is that with this alloy concept the general weldability of the steel is limited by the relatively high C and Mn contents and the galvanizability by hot-dip or high-temperature galvanizing is very limited by the also relatively high Si content of up to 0.8% by weight.

[0017] Furthermore, investigations have shown that, particularly due to the lack of vanadium, the extensive tempering resistance of this well-known steel is not guaranteed, so that at higher temperatures, for example ≥ 550 °C, as occur during high-temperature galvanizing, the strength drops significantly below the required values ​​for an air-hardened steel.

[0018] As is well known, sufficient tempering resistance requires, in particular, the formation of a sufficient amount of Cr, Mo, and / or V carbides or carbonitrides, which prevent slip dislocations at elevated temperatures by precipitating at the grain boundaries. This process is also called secondary hardening.

[0019] The use of an air-hardening steel for structural hollow profiles made of seamless hot-finished tubes, for example for door reinforcement elements, is known from the published patent application DE 44 46 709 A1. A steel alloy is used for this purpose with the following main elements: C (0.17 - 0.28 wt%), Mn (1.30 - 2.50 wt%), Si (0.30 - 0.49 wt%), Cr (≤ 0.49 wt%), Mo (0.20 - 0.40 wt%), Ni (0.05 - 0.19 wt%), Ti (0.02 - 0.07 wt%), B (0.0015 - 0.0050 wt%), Nb (0.01 - 0.10 wt%), V (0.01 - 0.10 wt%) and N (≤ 0.015 wt%), the remainder being iron and usual accompanying elements. In addition, the total content of V+Nb+Ti must not exceed 0.15 wt%.

[0020] This alloy concept with additions of Nb and Ni is expensive for the requirements of an air-hardening steel and problematic for welding due to its relatively high carbon content. Furthermore, this steel has a silicon content of 0.30 to 0.49 wt.%, which is critical for galvanizing.

[0021] The published patent application WO 99 / 05336 A1 discloses a process for producing an ultra-high-strength, boron-containing steel flat product, wherein a steel slab is first heated to a suitable temperature and rolled into a steel strip through one or more rolling steps at a first temperature at which austenite recrystallizes. Rolling is then continued through one or more rolling steps at a second temperature, below the first temperature and above a temperature at which austenite begins to transform into ferrite upon cooling. The steel strip is then quenched to a suitable quench-stop temperature. After stopping quenching at the quench-stop temperature, cooling is continued in air to ambient temperature (see abstract). Accordingly, WO 99 / 05336 A1 does not provide for pure air hardening by cooling in air alone, as proposed by the present invention.

[0022] The published patent application CN 109 402 499 A does not disclose air hardening. On the contrary, a so-called "slow cooling treatment" is carried out over 72 hours using a cooling hood. This is not the same as air hardening, i.e., cooling in air.

[0023] Publication US 2007 / 267110 A1 discloses a process for producing high-strength steel pipes. The process comprises producing a steel slab, heating the slab to temperatures above 2000 °F (~1093 °C), rolling the slab into a strip of the desired thickness at a temperature above the Ac3 starting temperature, cooling the strip to a temperature of 850 °F to 950 °F (454 °C to 510 °C) to obtain a predominantly bainitic structure, coiling the steel strip, and forming it into a pipe. Furthermore, it is disclosed that after rolling, the steel strip is cooled by water to the desired temperature (850 °F to 950 °F). Thus, US 2007 / 267110 A1 does not disclose air-hardened steel products or a process that includes an air-hardening step.

[0024] With known alloying concepts, the cold forming of air-hardenable steels into a component usually takes place in the soft-annealed state of the steel product in order to ensure sufficient cold formability and to be able to produce components with complex geometries.

[0025] According to the state of the art, cold forming refers to the following process variants: a) The direct production of corresponding components from soft hot-rolled strip by deep drawing or similar, with subsequent possible quenching and tempering to increase component strength. Quenching and tempering of air-hardening steel would consist of reheating the component after forming above the Ac3 temperature, cooling in air (air hardening), and annealing or tempering to an elevated temperature below the Ac3 temperature. b) Further processing into tubes using appropriate drawing and annealing processes. The tubes themselves are then formed into components, for example, by bending, hydroforming (IHU), or similar, and then quenched and tempered. c) Further processing of the hot-rolled strip into cold-rolled strip with an integrated (hood) annealing process. The cold-rolled strip is then processed by deep drawing or similar as under a).

[0026] All air-hardening steels known, for example, from published patent applications DE 10 2004 053 620 A1, DE 100 23 488 A1, DE 44 46 709 A1, or WO 2001 / 000351 A1 also have in common that, after forming into a component, the higher-strength air-hardening state must then be restored through subsequent heat treatment, for example, high-temperature galvanizing. The forming itself can take place at room temperature or elevated temperature, for example, up to the Ac1 temperature (semi-hot) or up to the Ac3 transformation temperature or above, if technically necessary.

[0027] However, the production of welded components, such as chassis parts or chassis components, from a soft-annealed steel strip made of the well-known air-hardening steels is cost-intensive, since the steel strip as hot-rolled strip (hot-rolled condition) or as cold-rolled strip (cold-rolled condition) must be subjected to an additional post-heat treatment after hot or cold rolling, as described above, to produce sufficient cold formability for component production.

[0028] Additional heat treatment can lead to significant, unwanted dimensional distortion of the component, particularly in the case of flat components.

[0029] On the other hand, for example, in the case of welded chassis components made of soft-annealed steel strip made of air-hardening steel, the achievable fatigue strengths are not yet sufficient due to the "softening" in the weld seam area described above, resulting in a loss of hardness and strength.

[0030] The invention is therefore based on the object of specifying a method for producing a welded component, in particular a dynamically stressed chassis component, from a high-strength steel, with which the aforementioned disadvantages are overcome, in particular with which comparable mechanical properties, in particular fatigue strengths, are achieved in the weld seam area as in the unaffected base material, preferably even with a higher strength than in the base material.

[0031] According to the teaching of the invention, this object is achieved by a process in which a hot-rolled steel product made of high-strength air-hardening steel with a material thickness of at least 1.5 mm is first produced, wherein the hot-rolled steel product has the following chemical composition in mass %: C: 0.03 to 0.4, preferably 0.06 to 0.12, particularly preferably 0.08 to 0.10 Mn: 1.0 to 4.0, preferably 1.80 to 2.20, particularly preferably 1.80 to 2.00 Si: 0.09 to 2.0, preferably 0.22 to 0.34, particularly preferably 0.25 to 0.30 Al: 0.02 to 2.0, preferably 0.02 to 0.06, particularly preferably 0.02 to 0.05 P: <=0.1, preferably ≤ 0.020 S: <=0.1, preferably ≤ 0.010 N: 0.001 to 0.5, preferably 0.0030 to 0.0125, particularly preferably 0.0030 to 0.0080 Ti: 0.01 to 0.2, preferably 0.010 to 0.050, particularly preferably 0.020 to 0.030 Cr: 0.05 to 2.0, preferably 0.60 to 1.0, particularly preferably 0.70 to 0.80 B: 0.001 to 0.1, preferably 0.0015 to 0.0060, particularly preferably 0.0025 to 0.0035 Mo: 0.01 to 1.0, preferably 0.10 to 0.40, particularly preferably 0.15 to 0.30 V: 0.01 to 0.2, preferably 0.05 to 0.09, particularly preferably 0.05 to 0.08 optional: Ni: 0.02 to 1.0 Nb: 0.01 to 0.1 remainder iron including usual steel-accompanying elements.

[0032] The resulting hot-rolled steel product is then air-hardened by cooling in air alone, after which the hot-rolled, air-hardened steel product has a minimum yield strength Rp0.2 of 450 MPa, a minimum tensile strength Rm of 700 MPa, and / or an elongation at break A5 of at least 6% and a complex phase microstructure with a bainite content of more than 50%. It is then directly formed into the component in the air-hardened state without the otherwise conventionally required soft annealing. The final step involves the creation of welded joints on the component by fusion welding. A quenching and tempering treatment of the component, consisting of heating the entire component to temperatures above Ac3, cooling, and tempering, does not take place.

[0033] This allows a welded component to be obtained, in particular by avoiding soft annealing before cold forming and by avoiding a final tempering treatment.

[0034] In this context, it should be noted that the final creation of welded joints on the component is not considered a heat treatment. While temperatures above the Ac3 temperature are reached locally around the weld seam during welding, these welds are not intended to increase the strength of the entire component material in the sense of a heat treatment.

[0035] In connection with range specifications in mass%, such as C: 0.03 to 0.4 mass%, the initial value and also the final value of the range are considered to be included.

[0036] The forming into a component can be carried out using the usual cold sheet forming processes, for example by deep drawing, folding, bending, roll forming or flanging.

[0037] The forming is preferably carried out as cold forming in the range from -5 °C to 40 °C, preferably at room temperature in the range from 15 °C to 25 °C.

[0038] However, if technically necessary, it can also be carried out at elevated temperatures up to the Ac1 temperature, up to the Ac3 temperature or beyond the Ac3 temperature up to 1000 °C.

[0039] According to the invention, the hot-rolled steel product, prior to cold forming, has a minimum yield strength Rp0.2 of more than 450 MPa or even more than 600 MPa and / or a minimum tensile strength Rm of 700 MPa or even more than 800 MPa. For the purposes of the invention, this is considered the definition of the high-strength property. The hot-rolled steel product, even in the air-hardened state, has an elongation at break A5 of at least 6%, preferably of at least 13%, and a complex-phase microstructure with a bainite content of more than 50%.

[0040] A welded component manufactured by cold forming from a hot-rolled and air-hardened steel product made of air-hardened high-strength steel with a material thickness of at least 1.5 mm is also provided, the steel product having the following chemical composition in mass %: C: 0.03 to 0.4, preferably 0.06 to 0.12, particularly preferably 0.08 to 0.10 Mn: 1.0 to 4.0, preferably 1.80 to 2.20, particularly preferably 1.80 to 2.00 Si: 0.09 to 2.0, preferably 0.22 to 0.34, particularly preferably 0.25 to 0.30 Al: 0.02 to 2.0, preferably 0.02 to 0.06, particularly preferably 0.05 P: <=0.1, preferably ≤ 0.020 S: <=0.1, preferably ≤ 0.010 N: 0.001 to 0.5, preferably 0.0030 to ≤ 0.0125, particularly preferably 0.0030 to 0.0080 Ti: 0.01 to 0.2, preferably 0.010 to ≤ 0.050, particularly preferably 0.020 to ≤ 0.030 Cr: 0.05 to 2.0, preferably 0.60 to ≤ 1.0, particularly preferably 0.70 to ≤ 0.80 B: 0.001 to 0.1, preferably 0.0015 to ≤ 0.0060, particularly preferably 0.0025 to ≤ 0.0035 Mo: 0.01 to 1.0, preferably 0.10 to ≤ 0.40, particularly preferably 0.15 to ≤ 0.30 V: 0.01 to 0.2, preferably 0.05 to ≤ 0.09, particularly preferably 0.05 to ≤ 0.08 optional: Ni: 0.02 to 1.0 Nb: 0.01 to 0.1 where the hot-rolled and air-hardened steel product in the air-hardened condition,before cold forming into a component, has a minimum yield strength Rp0.2 of 450 MPa or preferably more than 600 MPa, a minimum tensile strength Rm of 700 MPa or preferably more than 800 MPa and / or an elongation at break A5 of at least 6%, preferably 13%, and wherein the hot-rolled and air-hardened steel product (2, 3) by cooling only in air has a complex phase structure with a bainite content of more than 50%, preferably more than 80% and particularly preferably more than 90%, and wherein the hot-rolled, air-hardened and formed steel product (2, 3) in the region of a heat-affected zone of a welded joint (4) has a complex phase structure with a bainite content of more than 50%, preferably more than 80% and particularly preferably more than 90%.

[0041] Typical sheet thicknesses for the hot-rolled steel products mentioned are between 1.5 mm and 25 mm, preferably up to 15 mm.

[0042] All processes commonly used in industrial production are used as fusion welding processes, such as beam welding, in particular laser beam welding, or metal arc welding processes, such as gas-shielded arc welding.

[0043] In an advantageous development of the invention, a hot-rolled steel strip or steel sheet for cold forming is used to manufacture the component, which exhibits an air-hardened state after hot rolling and cooling. Since the cold formability of the hot-rolled and air-hardened steel strip or steel sheet is sufficiently high due to the alloying concept, soft annealing to achieve sufficient cold formability is no longer necessary, thus significantly reducing the manufacturing costs of the component.

[0044] Surprisingly, investigations have shown that the air-hardening steel exhibits sufficiently high cold formability in the air-hardened state. Tests determined A5 elongations of 13% and more for sheet thicknesses up to 25.0 mm. The hot-rolled steel products can be available in material thicknesses from 1.5 mm to 25.0 mm, preferably up to 15 mm, thus covering the typical thickness range in the chassis sector.

[0045] This eliminates the need for soft annealing of the steel product after hot rolling. The same applies to cold forming of hot-rolled seamless tubes into components.

[0046] In addition, investigations into the mechanical properties of the welded component have shown that the heat introduced during welding and the subsequent air cooling no longer lead to softening of the weld seam area, as the air-hardening properties of the steel prevent a loss of hardness and strength in this area and, in fact, actually increase it. The mechanical properties and fatigue strength of welded chassis components are therefore at least at a comparable level in the weld seam area and in the adjacent, unaffected base material of the steel product. A comparable level is defined as deviations between the base material and the weld seam area in the range of + / - 20% based on their tensile strength or hardness. Instead of just a comparable level, a higher level can also be achieved, with values ​​of up to +50% or even up to +100%.This enables higher voltage transfer in the heat-affected zone.

[0047] Advantageous for achieving high fatigue strength or operational strength is not only the preferably increased hardness of the weld area, but also the predominantly fine-grained bainitic structure for the heat-affected zone as well as for the weld or for the weld metal, which also depends on the selected welding wire.

[0048] According to the invention, the hot-rolled steel product thus has a complex phase microstructure with a bainite content of more than 50%, preferably more than 80%, and particularly preferably more than 90%. According to the invention, this microstructure also applies to the heat-affected zone of the welded joint.

[0049] The invention is explained in more detail below using an experiment illustrated and evaluated in the figures. They show: Figure 1a section of a welded component 1 according to the invention in a side sectional view, Figure 2 a qualitative presentation of the results of a grid-like hardness test over the section according to Figure 1 , Figure 3 a microstructure image from the area of ​​the base material of the welded component 1, Figure 4 a micrograph of the heat-affected zone of the welded component 1, Figure 5 a micrograph of the fusion line area of ​​the welded component 1, Figure 6 a micrograph of the weld seam area of ​​the welded component 1.

[0050] The Figure 1shows a section of a welded component 1 obtained according to the invention in a side sectional view of a typical lap joint. The welded component 1 in the form of a chassis component essentially consists of a first lower steel product 2, in particular a flat steel product, on which a further second upper steel product 3, in particular a flat steel product, is placed in a partially overlapping manner. The steel products 2, 3 each have a material thickness of 3 mm. The upper steel product 3 is connected to a surface 2o of the lower steel product 2 in the region of a front edge 3v via a welded connection 4 in the manner of a lap joint.

[0051] In the Figure 2is a qualitative representation of the result of a grid-like hardness test over the entire section of the welded component 1 shown. Starting from the hardness of the lower steel product 2 in the basic state, which is shown as the first hardness range 5a with vertical hatching, a heat-affected zone in the steel products 2, 3 follows in the direction of the weld metal 4a or the melt in the usual way, which is shown as the second hardness range 5b without hatching or pattern. Based on the air-hardening properties of the steel, the second hardness range 5b is hardened compared to the first hardness range 5a. The weld metal 4a of the weld joint 4 has a third hardness range 5c, which is identified by a dot pattern. The hardness of this third hardness range 5c can be adjusted depending on the choice of welding wire and will usually lie within a hardness range of the steel product 2, 3 in the basic state.

[0052] The measured values ​​for the first hardness range 5a are in the range 280 to 320 HV 0.1, for the second hardness range 5b in the range 430 to 470 HV 0.1 and for the third hardness range 5c in the range 230 to 270 HV 0.1.

[0053] The Figures 3 to 6 each show a micrograph from different areas of the welded component 1. All micrographs were etched with Nital in connection with sample preparation and are shown at a magnification of 1:500.

[0054] The Figure 3 shows a micrograph of the base material of welded component 1. This micrograph shows a complex phase structure with 5% ferrite, 3% pearlite, 90% bainite, and 2% martensite. The average ferrite grain size is 13.5 µm.

[0055] In the Figure 4A micrograph of the heat-affected zone of welded component 1 is shown. It shows a microstructure consisting of bainite with some ferrite. The average ferrite grain size is 13.5 µm.

[0056] The Figure 5 shows a micrograph of the fusion line area of ​​welded component 1. The microstructure there is 100% bainite.

[0057] In the Figure 6 A micrograph of the weld seam 4a of the welded component 1 is shown. The microstructure shows bainite interspersed with a light network of ferrite and bainite.

[0058] As already explained above, the welded component 1 obtained according to the invention achieves high fatigue strength and operational strength due to the predominantly fine-grained bainitic microstructure in the heat-affected zone and in the weld seam 4a in the area of ​​the welded joint 4. The base material of the welded component already exists as a complex phase microstructure with a predominant proportion of bainite.

[0059] The high-strength, air-hardening steel for lightweight vehicle construction according to the invention is also characterized by the fact that this alloy concept achieves excellent weldability in conventional welding processes, such as gas-shielded arc welding, gas-shielded brazing, or laser welding, without the disadvantages of known air-hardening steels. High-frequency induction welding (HFI welding) is also unproblematic, with no undesirable chromium carbide precipitation in the weld seam.

[0060] The reduced C and Mn content compared to the well-known air-hardening steel for seamless pipes ensures excellent general weldability with simultaneously excellent forming properties.

[0061] At the same time, the reduced Si content ensures the galvanizability of the steel and the addition of V ensures its tempering resistance.

[0062] The investigations have shown that the Cr content, which is crucial for the air-hardening effect, can be reduced to a value that is uncritical for avoiding chromium carbide precipitation during HFI welding if, at the same time, the air-hardenability of the steel is improved again using a complex alloying concept based on Cr-Mo-Ti-B.

[0063] According to the invention, the alloying concept for the steel product is based on the finding that, in contrast to the known steel for seamless pipes, in which nitrogen must be completely bound by titanium in order to avoid boron nitride precipitation and thus ensure the effectiveness of the added boron, the nitrogen is also bound by other alloying elements such as Cr or Mo.

[0064] Specifying a superstoichiometric titanium addition relative to nitrogen is therefore no longer mandatory. The addition of vanadium triggers precipitation of vanadium carbonitrides of type V(C,N) at higher tempering temperatures, which counteract a loss in strength through secondary hardening.

[0065] A disadvantage of these Mn-Si-Ti-B-based alloy concepts, however, is the excessively high Si content, which, while necessary to achieve high strength values, makes batch galvanizing difficult. Furthermore, at temperatures above approximately 550 °C, the material's strength drops significantly below the required values, meaning that tempering resistance is not guaranteed.

[0066] Based on these findings, the inventive alloy concept described above was defined, with the following analytical range in mass % proving advantageous. The respective specifications for the analytical ranges can be met individually or in total: C 0.06 to 0.12 Mn 1.8 to 2.20 Si 0.22 to 0.34 Al 0.02 to 0.06 P ≤ 0,020 S ≤ 0,010 N 0.0030 to 0.0125 Ti 0.010 to 0.050 Cr 0.60 to 1.0 B 0.0015 to 0.0060 Mon 0.10 to 0.40 V 0.05 to 0.09

[0067] Particularly advantageous processing and component properties are achieved if the following analysis range in mass% is maintained: C 0.08 to 0.10 Al 0.02 to 0.05 Si 0.25 to 0.30 Mn 1.80 to 2.00 P ≤ 0,020 S ≤ 0,010 N 0.0030 to 0.0080 Ti 0.020 to 0.030 Cr 0.70 to 0.80 B 0.0025 to 0.0035 Mon 0.15 to 0.30 V 0.05 to 0.08

[0068] The results demonstrate the high tempering resistance of the steel up to temperatures of 700 °C.

[0069] As tests on welded components made from the hot-rolled and air-hardened steel product according to the invention have shown, this steel is not only advantageous for use in the automotive sector, but also in all applications requiring good cold formability combined with high steel strengths or fatigue and operational strengths under dynamic loading of welded components. Accordingly, such components can be used, for example, in the automotive industry, particularly for chassis components, the construction equipment industry, the household appliance industry, or chemical equipment manufacturing. In the automotive industry, use as chassis components, bumpers, or cross members is conceivable.

[0070] The advantages of this inventive air-hardening steel are listed below: Very good cold formability in the air-hardened state, very good weldability in the soft and air-hardened state, very good HFI weldability, easy to coat with the usual coating processes, such as cathodic dip painting (KTL), hot-dip galvanizing and high-temperature galvanizing, use for welded, statically and dynamically highly loaded components, especially in the chassis of vehicles, cost-effective alloy concept. List of reference symbols

[0071] 1 welded component 2 lower steel product 3 upper steel product 4 welded joint 4a weld seam 5a first hardening zone 5b second hardening zone 5c third hardening zone

Claims

1. Method for manufacturing a welded component (1), the method comprising, in particular exclusively consisting of, the following steps: - producing a hot-rolled steel product (2, 3) from a high-strength air-hardenable steel having a material thickness of at least 1.5 mm, wherein the hot-rolled steel product (2, 3) has the following chemical composition in % by mass: C: 0.03 to 0.4, preferably 0.06 to 0.12, particularly preferably 0.08 to 0.10 Mn: 1.0 to 4.0, preferably 1.80 to 2.20, particularly preferably 1.80 to 2.00 Si: 0.09 to 2.0, preferably 0.22 to 0.34, particularly preferably 0.25 to 0.30 Al: 0.02 to 2.0, preferably 0.02 to 0.06, particularly preferably 0.02 to 0.05 P: <=0.1, preferably ≤ 0.020 S: <=0.1, preferably ≤ 0.010 N: 0.001 to 0.5, preferably 0.0030 to 0.0125, particularly preferably 0.0030 to 0.0080 Ti: 0.01 to 0.2, preferably 0.010 to 0.050, particularly preferably 0.020 to 0.030 Cr: 0.05 to 2.0, preferably 0.60 to 1.0, particularly preferably 0.70 to 0.80 B: 0.001 to 0.1, preferably 0.0015 to 0.0060, particularly preferably 0.0025 to 0.0035 Mo: 0.01 to 1.0, preferably 0.10 to ≤ 0.40, particularly preferably 0.15 to 0.30 V: 0.01 to 0.2, preferably 0.05 to ≤ 0.09, particularly preferably 0.05 to 0.08 optionally: Ni: 0.02 to 1.0 Nb: 0.01 to 0.1 remainder iron including conventional steel-accompanying elements, - subsequently air hardening solely by cooling the produced hot-rolled steel product (2, 3) in air, after which the hot-rolled, air-hardened steel product (2, 3) has a minimum yield strength Rp0.2 of 450 MPa, a minimum tensile strength Rm of 700 MPa, and / or a breaking elongation A5 of at least 6%, and a complex phase structure with a bainite content of more than 50%, and - then forming the hot-rolled steel product (2, 3) in the air-hardened state to form a component, - producing welded joints by fusion welding on the component.

2. Method according to claim 1, characterised in that the forming is a conventional cold sheet-metal-forming process, in particular deep drawing, folding, roll forming, bending or flanging.

3. Method according to claim 1 or 2, characterised in that the forming takes place at a temperature in the range of -5°C to 40°C, preferably at ambient temperature in the range of 15°C to 25°C.

4. Method according to at least one of claims 1 to 3, characterised in that metal arc welding or beam welding, in particular laser beam welding, are implemented as fusion welding processes.

5. Method according to claim 4, characterised in that shielded-arc welding is implemented as metal arc welding.