Method for manufacturing a motor vehicle component from a high-strength steel alloy with ductile properties

Abstract

Method for manufacturing a motor vehicle component (2) with a tensile strength Rm greater than 1,800 MPa, produced by hot forming and press hardening, wherein a blank (3) made of a hardenable carbon-containing steel alloy with a carbon content greater than or equal to 0.3 mass percent is heated in a continuous furnace (4) to a temperature greater than or equal to AC3, subsequently removed from the continuous furnace (4) and hot formed and press hardened in a hot forming and press hardening tool (8), characterized in that the oxygen content in vol.% is measured in a furnace atmosphere in the continuous furnace (4) and nitrogen is supplied to the continuous furnace (4), wherein the amount of nitrogen is supplied in such a controlled manner that an oxygen content of 0.5 to 15 vol.% is established in the furnace atmosphere and the motor vehicle component has a bending angle greater than 50°.

Description

[0001] The present invention relates to a method for manufacturing a motor vehicle component produced by hot forming and press hardening according to the features in the preamble of claim 1.

[0002] It is known from the prior art to manufacture automotive components from a hardenable steel alloy. This involves hot forming and press hardening. In this process, a blank made of a hardenable steel alloy is heated to a temperature greater than or equal to the AC3 temperature. The AC3 temperature is also known as the austenitizing temperature and, depending on the steel alloy used, is greater than 800 °C.

[0003] If the circuit board is fully austenitized, it possesses a high degree of deformation. In a hot forming process, the circuit board is then used to manufacture the automotive component through forming.

[0004] During and after forming, the blank or formed component is cooled so rapidly that the austenitized material is transformed into a hardened microstructure. This is a martensitic microstructure. Ferrite, pearlite, or bainite may also be present in the hardened microstructure. The process described above is also called press hardening.

[0005] It has been known in recent years from the state of the art to manufacture motor vehicle components that have a tensile strength RM greater than 1000 MPa, in particular greater than 1200 and preferably also greater than 1500 MPa.

[0006] If components with even higher tensile strengths are manufactured, particularly above 1700 and preferably above 1800 MPa, these components exhibit only low ductility. Bending angles of approximately 30° result. In the event of an accident, brittle fractures and / or the detachment of other components, for example, those attached to this component by welding, can occur.

[0007] Metal components made of multi-layer steel are known from EP 3 296 104 A1 and DE 10 2026 114 062 B3, respectively. Furthermore, a motor vehicle component manufactured by hot forming and press hardening with a coating is known from DE 10 2016 108 836 A1.

[0008] The object of the present invention is therefore to produce a component with extremely strong material properties, which nevertheless also has high ductility, whereby the process costs for producing the component are low and, in particular, existing plant technology can be used.

[0009] The aforementioned problem is solved according to the invention by a method having the features in claim 1.

[0010] Advantageous embodiments of the present invention are the subject of the dependent claims.

[0011] The inventive method for manufacturing a motor vehicle component involves the use of a hot forming and press hardening process. For this purpose, a blank made of a hardenable carbon-containing steel alloy is heated, at least partially, and in particular completely, to a temperature above AC3. The carbon content is greater than or equal to 0.3% by mass. However, the carbon content should not exceed 1% by mass. The blank to be heated is heated in a continuous furnace. The heated blank is removed from the continuous furnace and transferred to a hot forming and press hardening tool. A hot forming operation and a press hardening process are carried out in the hot forming and press hardening tool.

[0012] This process uses a high-strength steel alloy, which makes it possible to provide a motor vehicle component after completion of the press hardening process that has a tensile strength Rm of at least 1800 MPa.

[0013] To ensure that this automotive component, also referred to as a component, exhibits high strength properties and a high degree of ductility, particularly a bending angle greater than 50°, preferably greater than 60°, a surface decarburization is performed during heating in a continuous furnace. The bending angle is determined, in particular, in a plate bending test according to VDA 238-100.

[0014] It is provided that an oven atmosphere is established within the continuous furnace by supplying ambient air and technically pure nitrogen. The oxygen content in the oven atmosphere is measured in volume percent. According to the invention, an oxygen content of 0.5 to 15 vol.%, preferably between 0.5 and 10 vol.%, particularly between 0.5 and 5 vol.%, and most preferably between 0.5 and 3 vol.%, is maintained in the oven atmosphere. The oxygen content is regulated by controlling the nitrogen volume flow rate into the continuous furnace.

[0015] According to the invention, it has been found that it is possible to carry out edge decarburization on the circuit board to be heated, such that the carbon atoms in a respective edge layer of the circuit board to be heated combine with oxygen. Due to the edge-decarburized layer, the component subsequently produced by hot forming and press hardening has higher ductility. Scaling of the surface is also largely avoided.

[0016] Ambient air can be supplied by drawing air from outside into the interior of the furnace. Preferably, however, the air or oxygen in the interior of the furnace originates from the immediate vicinity of the continuous furnace.

[0017] It has proven advantageous according to the invention that if a decarburized layer with a layer thickness of 10 to 70 µm or between 10 and 50 µm and preferably of 20 to 40 µm is applied to both sides of the component, a motor vehicle component with a tensile strength Rm greater than 1800 MPa and a bending angle greater than 50°, in particular greater than 60°, can be produced.

[0018] Furthermore, it is possible to carry out the inventive method on existing production facilities by retrofitting a nitrogen supply and a control method for adjusting the oxygen content in the furnace atmosphere to an existing continuous furnace.

[0019] A continuous furnace is typically operated with gas burners in radiant tubes. The combustion process itself takes place separately from the adjustment of the oxygen content of the furnace atmosphere. Alternatively, radiant tubes can be resistance-heated.

[0020] Furthermore, and particularly preferably, the nitrogen flow rate fed into the continuous furnace can have a value per hour, for example in m³. 3 This value is preferably between two and four times, preferably between 2.5 and 3.5 times, and most preferably corresponds to three times the furnace volume of the continuous furnace.

[0021] It has also proven advantageous to introduce the nitrogen into the continuous furnace above the circuit boards to be heated, relative to the direction of travel. This creates convection of the nitrogen within the furnace, eliminating the need for further mixing of the furnace's internal atmosphere.

[0022] The method according to the invention is preferably used to process tailored blanks. In particular, the tailored blanks are rolled blanks. However, tailored formed blanks or tailored welded blanks can also be processed. Likewise, blanks with a constant wall thickness can of course also be processed.

[0023] Furthermore, the inventive method also makes it possible to apply an additional coating to the manufactured component. This is particularly relevant for a subsequently applied anti-corrosion coating, for example cathodic dip coating or zinc diffusion coating.

[0024] It has widely proven advantageous for the circuit board to pass through the continuous oven in a time of 120 sec to 10 min, in particular 120 sec to 400 sec, especially preferably 160 sec to 200 sec and in particular about 180 sec.

[0025] Furthermore, it is provided that a temperature between 910 and 980 °C, in particular between 930 and 960 °C, prevails in the continuous furnace itself.

[0026] Alternatively or additionally, the temperature in the continuous furnace can be at least 5%, preferably 10%, particularly 11%, and preferably 12% above the AC3 temperature of the steel alloy used. However, the furnace internal temperature should not exceed the AC3 temperature of the steel material used by 30%, and particularly preferably not by 20%.

[0027] Within the scope of the invention, it has proven particularly advantageous to use a steel alloy which, in addition to iron and impurities resulting from the melting process, contains the following alloying elements, expressed as mass percent: C (carbon) 0,3-0,4 preferred 0,32-0,38 Si (silicon) 0,15-1 preferred 0,2-0,5 Mn (Manganese) 0,5-2 preferred 0,8-1,5 P (phosphorus) max. 0.05 preferred max. 0.02 S (sulfur) max. 0.01 preferred max. 0.005 N (Nitrogen) max. 0.01 preferred max. 0.005 Cr (Chromium) 0,05-1 preferred 0,1-0,5 Ni (Nickel) max. 0.3 preferred max. 0.1 Cu (copper) max. 0.1 preferred max. 0.05 Mo (molybdenum) max. 0.5 preferred max. 0.3 Al (Aluminum) max. 0.1 preferred max. 0.06 Nb (Niobium) 0,02-0,1 preferred 0,02-0,06 V (Vanadium) max. 0.06 preferred max. 0.05 Ti (Titanium) max. 0.1 preferred max. 0.01 B (Boron) 0,001-0,01 preferred 0,001-0,005

[0028] The carbon content determines the strength / hardness of the manufactured component. Silicon retards the transformation and improves tempering resistance. Manganese also retards the transformation by stabilizing the austenite. Chromium also retards the transformation and improves scale resistance. Boron also retards the transformation. Niobium results in a fine grain structure in the material.

[0029] Within the scope of the invention, the process can therefore preferably be carried out with the steel alloy specified in the table. However, the process can also be carried out with other carbon-containing steel alloys, particularly those with a carbon content greater than 0.3% by mass. The automotive component described below can also be made from the aforementioned steel alloy. However, the carbon content should not exceed 1% by mass.

[0030] The present invention further relates to a motor vehicle component manufactured from a blank by hot forming and press hardening. The blank itself is made of a hardenable steel alloy. In particular, the motor vehicle component is manufactured according to a previously described method according to the invention.

[0031] The automotive component is characterized according to the invention in that it has a tensile strength Rm greater than 1800 MPa, in particular greater than 1900 MPa, preferably greater than 2000 MPa. The tensile strength should in particular not exceed 2500 MPa.

[0032] Furthermore, the automotive component possesses high ductility. This ductility is characterized by a bending angle greater than 50°, and in particular greater than 60°, on the automotive component. The automotive component typically has a thickness between 0.7 mm and 3.5 mm. A decarburized layer is preferably formed on each surface of the automotive component, with the decarburized layer having a thickness of 10 to 70 µm, preferably 20 to 40 µm.

[0033] A layer boundary between a decarburized and a non-decarburized layer is characterized by the fact that the carbon content in the decarburized layer is a maximum of 50% relative to a core layer, i.e., a middle layer, of the manufactured automotive component. This means that, starting from the surface, the decarburized layer ends at the point where the carbon content exceeds 50% of the carbon content of a middle layer of the automotive component in the direction of its interior. These specifications also apply to the manufacturing process described above.

[0034] The aforementioned invention is further described by the following explanations and illustrated by the schematic figures, which are intended to facilitate the simple understanding of the invention.

[0035] They show: Fig. 1: a schematic process flow for the manufacture of a motor vehicle component, Fig. 2: a motor vehicle component manufactured according to the invention in the form of a B-pillar and Fig. 3 a cross-sectional view through a motor vehicle component according to the invention.

[0036] The figures use similes or reference symbols, even if a repeated description is omitted for the sake of simplicity.

[0037] Fig. Figure 1 shows a hot forming line according to the invention for the production of a motor vehicle component produced by hot forming and press hardening.

[0038] First, a circuit board 3 is placed in a continuous furnace 4. Ambient air U is supplied to the continuous furnace 4 to regulate the furnace atmosphere. Technically pure nitrogen N is also supplied to the continuous furnace 4. The amount of technically pure nitrogen N supplied is adjusted, in particular, depending on the measured volume percent oxygen content within the furnace atmosphere. For this purpose, several measuring points can be located within the continuous furnace 3 to measure the volume percent oxygen content. An average value can then be calculated from the measurements taken at these points. The circuit board 5, heated in this way, has a decarburized layer on each surface 6, 7. The heated circuit board 5 is then transferred to a hot forming and press hardening tool 8, where it is hot formed and press hardened.The manufactured motor vehicle component 2 is removed from the hot forming and press hardening tool 8 and fed to further processing.

[0039] Fig. Figure 2 shows a perspective view of a motor vehicle component 2. This could be, for example, a motor vehicle pillar, in particular a motor vehicle B-pillar. However, other motor vehicle components 2, in particular motor vehicle structural components, can be manufactured using the method according to the invention. These other motor vehicle components 2 are, for example, longitudinal members, cross members, sills, roof rails, rocker panels, or similar components of a motor vehicle body.

[0040] Fig. Figure 3 shows a cross-sectional view according to section line III-III. Fig.2 by the manufactured automotive component 2. The automotive component 2 has a wall thickness W. From each surface 6, 7 of the automotive component 2, a decarburized layer 10, 11 extends to a middle layer 9, also referred to as core layer or core layer. The decarburized layer 10, 11 has a layer thickness 12. The layer thickness 12 is particularly preferably 20 to 40 µm. A layer boundary 13 between the decarburized layer 10, 11 and the non-decarburized material is characterized in that the carbon content in the decarburized layer is 50% of the carbon content of the middle layer 9. Thus, if the carbon content starting from the surface 6, 7 of the manufactured automotive component exceeds 50%, it is no longer considered a decarburized layer within the scope of the invention.

[0041] Another advantage of the present invention is that the manufactured hot-formed and press-hardened automotive component or circuit board scales to a negligible extent during heating. Reference symbol: 1 Hot forming line 2 Motor vehicle component 3 circuit boards 4 continuous oven 5 heated circuit board 6 surface area to 5, 2 7 surface area to 5, 2 8 Hot forming and press hardening tools 9 Middle class 10 decarbonized layers 11 decarbonized layer 12 layer thickness to 10, 11 13 Layer boundary Ambient air N Nitrogen W wall thickness

Claims

[1] Method for manufacturing a motor vehicle component (2) with a tensile strength Rm greater than 1,800 MPa, produced by hot forming and press hardening, wherein a blank (3) made of a hardenable carbon-containing steel alloy with a carbon content greater than or equal to 0.3 mass percent is heated in a continuous furnace (4) to a temperature greater than or equal to AC3, then removed from the continuous furnace (4) and hot formed and press hardened in a hot forming and press hardening tool (8), characterized by , that in an oven atmosphere in the continuous oven (4) the oxygen content is measured in vol% and nitrogen is supplied to the continuous oven (4), wherein the amount of nitrogen is supplied in such a controlled manner that an oxygen content of 0.5 to 15 vol% is established in the oven atmosphere and the motor vehicle component has a bending angle greater than 50°. [2] Method according to claim 1, characterized bythat an oxygen content between 0.5 and 10 vol.%, preferably between 0.5 and 5 vol.% and particularly preferably between 0.5 and 3 vol.% is adjusted. [3] Method according to claim 1 or 2, characterized by that the nitrogen volume flow rate has a value per hour which is between two and four times, preferably 2.5 to 3.5 times and preferably three times the furnace volume of the continuous furnace (4). [4] Method according to claims 1 to 3, characterized by , that the nitrogen is introduced into the continuous furnace (4) above the circuit boards (3) to be heated, in relation to the spatial direction. [5] Method according to claims 1 to 4, characterized by that a tailored blank is processed and / or that the formed component is coated in a subsequent processing step. [6] Method according to claims 1 to 5, characterized by, that a circuit board (3) is passed through the continuous furnace (4) for a time of 120 s to 10 min, preferably 120 s to 360 s, particularly preferably 160 to 200 s and most preferably 180 s. [7] Method according to claims 1 to 6, characterized by , that in the continuous furnace (4) a temperature between 910 and 980 °C, preferably 930 to 950 °C prevails and / or that the temperature inside the continuous furnace (4) is preferably more than 5%, in particular more than 10% higher than the AC3 temperature of the steel material. [8] Method according to claims 1 to 7, characterized by that a steel alloy is used which, in addition to iron and impurities resulting from the melting process, contains the following alloying elements, expressed in mass percent: C (carbon) 0,3-0,4 preferred 0,32-0,38 Si (silicon) 0,15-1 preferred 0,2-0,5 Mn (Manganese) 0,5-2 preferred 0,8-1,5 P (Phosphorus) max. 0.05 preferred max. 0.02 S (sulfur) max. 0.01 preferred max. 0.005 N (Nitrogen) max. 0.01 preferred max. 0.005 Cr (Chromium) 0,05-1 preferred 0,1-0,5 Ni (Nickel) max. 0.3 preferred max. 0.1 Cu (copper) max. 0.1 preferred max. 0.05 Mo (molybdenum) max. 0.5 preferred max. 0.3 Al (Aluminum) max. 0.1 preferred max. 0.06 Nb (Niobium) 0,02-0,1 preferred 0,02-0,06 V (Vanadium) max. 0.06 preferred max. 0.05 Ti (Titanium) max. 0.1 preferred max. 0.01 B (Boron) 0,001-0,01 preferred 0,001-0,005

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