Patent Application
20180370578
The disclosure is related to a body component or chassis component for a motor vehicle having improved crash performance, and to a method for producing a body component or chassis component having improved crash performance and protection against corrosion.
In the vehicle industry, use is made of components which are crash-relevant for the body, impart stability and rigidity to the vehicle and, in the event of an accident, absorb crash energy and ensure a survival space for the occupants. Similarly, use is increasingly made in the chassis of a motor vehicle of components which, in addition to rigidity and a dynamic load-bearing capacity, meet in particular exacting requirements imposed on defined deformation capability, but also on economic efficiency.
A customary method for this purpose is what is referred to as hot forming and die quenching of heat-treatable steel grades with the steps of heating for austenitization, hot forming and quench hardening in a press forming die. It is customary here to provide an aluminum-silicon coating on a heat-treatable steel sheet as temporary protection against corrosion and protection against scaling during the heating. This material from ArcelorMittal is known and widespread for heat forming under the tradename Usibor. For this purpose, WO 2009 090 555 A1 describes the corresponding material and the coating build-up before and after hot forming and die quenching. The heating strategy for economical production of body components is also discussed here. A disadvantage of coated hot forming steels, even more than in the case of uncoated hot forming steels, is an increased cracking tendency starting from the surface of the component in the die-quenched state, which may become noticeable during a crash by component failure. A reason for this is the cold forming which acts on the die-quenched component during the crash and by means of which microcracks which are frequently already present on the hard and brittle surface, in particular of the aluminum coating alloyed with iron atoms grow. The same problems occur to an even greater effect also in the case of steels which are coated with zinc alloy for the hot forming.
Taking this as the starting point, it is the object of the present invention to propose body or chassis components, which have an appropriate improvement in the crash properties, for a motor vehicle, and methods for producing same, said components nevertheless being able to be produced and processed cost-effectively and also being lightweight.
This object is achieved in terms of the subject matter by means of the features of claim 1. Claims 2 to 16 which are dependent thereon form advantageous developments of the invention.
In terms of method, the object is achieved by the features of claim 17. Dependent claims 18 to 21 are advantageous developments of the method.
A body component or chassis component for a motor vehicle with improved crash performance is proposed, comprising at least one surface section consisting of a multi-layer, in particular triple-layer, laminated metal sheet having a center layer and two external layers which bound the center layer on the outside. According to the invention, the external layers are connected over an extensive area and materially to the center layer. The characteristic here is that the external layers apre composed of a rust-resistant steel alloy with a microstructure selected from the group consisting of ferritic, austenitic or martensitic microstructures and the center layer is composed of a heat-treatable, in particular hardened, steel alloy, and the body component or chassis component has a bending angle greater than 80 degrees (°), determined in the plate bending test according to VDA 238-100:2010 with an Rp0.2 proof stress greater than 900 MPa. This allows a maximum of corrosion protection over the entire life of the vehicle, even taking into account harsh processing and operating conditions. Furthermore, the external layer, which is connected firmly to the harder center layer and is softer, has the effect that the tendency for cracking during an envisaged load in a crash, but also even during a joining or cold forming process following component shaping falls significantly. According to the invention, the external layers and the center layer are connected over an extensive area by material bonding in such a way that there are essentially no inclusions or impurities between the layers, wherein, in particular, a metallurgical joint is formed. According to the invention, the individual layers are preferably connected to one another materially and metallurgically over the full area. Here, the starting material used for the invention can, for example, be produced by hot rolling three connected slabs prefixed in advance mechanically and/or materially, or by hot rolling a slab cast in multiple stages, or by hot rolling a deposit-welded slab.
As a ferritic rust-resistant steel alloy, it has been found particularly advantageous to use an alloy which, apart from impurities caused by the melting process and iron, comprises the following alloy components in percent by weight:
Carbon (C): 0.08 to 0.16%
Silicon (Si): 0.5% to 1.8%
Manganese (Mn): 0.8% to 1.4%
Chromium (Cr): 13.0% to 22.0%
Aluminum (Al): 0.5% to 1.5%
Phosphor (P): 0.06% maximum
Sulfur (S): 0.02% maximum.
An advantage of the ferritically stainless steel alloy in conjunction with a hardenable, ferritically-perlitic steel alloy of the center layer is that the composition of the external layer undergoes a particularly homogenous and durable material bond with the center layer during the heat treatment. Even during the conversion of the microstructure of the center layer during the hot forming and die quenching, there is no risk of cracks, peeling or the like of the external layer.
While chromium ensures heat resistance and thus a scale-free surface during heating and hot forming, the heat-treatable steel alloy of the center layer ensures a maximum of tensile strength. As regards other ferritic rust-resistant steel alloys that can be used, attention may furthermore be drawn here to the content of EN 10088-1, with chromium contents of from 10.5 to 30%, depending on the grade. To ensure weldability, use is made of stabilizing additives of less than 0.5% of titanium, niobium or zirconium and of a carbon content limited to 0.16%.
As regards the austenitic rust-resistant steel alloys which can be used, attention is drawn here to grades EN 1.4310 and EN 1.4318. The ductility and elongation at break of rust-resistant austenitic steels is very high both in the low temperature range and during hot forming. The susceptibility to brittle fracture is extremely low. During cold forming and during a crash, its strength is increased by the conversion of metastable austenitic phases into martensite.
As regards martensitic rust-resistant steel alloys, attention may be drawn, by way of example, to the easily weldable grades EN 1.4313 and EN 1.4418 and to super martensitic steels of grade EN 1.4415. The latter are simultaneously of high strength and are very tough and, apart from impurities caused by the melting process and iron, have a chemical composition, expressed in percent by weight, comprising:
Carbon (C) 0.03% maximum
Chromium (Cr) 11-13%
Nickel (Ni) 2-6%
Molybdenum (Mo) 3% maximum
Nitrogen (N) >0.005%.
The bending angle of the body component or chassis component is preferably greater than 95°, and the Rp0.2 proof stress is greater than 950 megapascals (MPa).
As a particular preference, the bending angle is greater than 90°, in particular greater than 100°, preferably greater than 110°. In particular, the body component or chassis component has a product of the bending angle and the Rp0.2 proof stress of between 90 000° MPa (degrees megapascals) and 180 000° MPa, whereby optimum behavior of components in crashes is obtained without special process control measures during or after hot forming, without the risk of cracks or even component failure.
To make maximum use of the potential of the heat-treatable steel alloy for lightweight construction, according to the invention the center layer of the surface section preferably has an ultra-high-strength microstructure, with at least 80% martensite. In this case, the tensile strength Rm within the surface section having a triple-layer laminated metal sheet is greater than 1300 megapascals (MPa).
It is furthermore possible for the center layer of a surface section to have a microstructure selected from a group consisting of tempered martensite, which makes up at least 80 percent, or a hybrid microstructure comprising at least 70 percent ferrite and perlite, with the remainder being martensite, residual austenite and/or bainite.
The percentage figures for the constituents of the microstructure relate to area percentages that can easily be determined by metallographic methods.
The surface section comprising the triple-layer laminated metal sheet preferably has a total thickness and one of the external layers has a thickness, wherein the thickness of one of the external layers corresponds to at least 3 percent and at most 15 percent, preferably 4 percent to 10 percent, of the total thickness of this surface section. Here, total thickness should be taken to mean the sum of the thicknesses of the two external layers and of the center layer in the respective surface sections. The total thickness is preferably between 1 and 10 millimeters (mm), in particular between 1.7 and 3.5 mm A thicker external layer provides hardly any further advantages in terms of corrosion protection but significantly reduces the overall strength of the surface section. Currently, a thinner external layer can be produced only with difficulty in a reliable process by rolling and, given a conventional service life of a motor vehicle, can furthermore not be reduced in view of corrosion protection. Within the scope of the invention, the mutually opposite external layers preferably have the same thickness. However, it is also possible for external layers of different thickness to be formed in at least one surface section, if this is necessary, to ensure that, in the case of a hollow body component or chassis component for example, said component is particularly well adapted to different crash or corrosion requirements on the inside and the outside.
Particularly suitable as a heat-treatable steel alloy is a manganese-boron steel such as 16MnB5, but preferably 22MnB5 or, alternatively, 36MnB5. In particular, heat-treatable steel alloys with a carbon content greater than or equal to 0.27% by weight can be used, e.g. MBW 1900. These would be too brittle for direct hot forming and die quenching. By virtue of the external layers, it is possible to process them by means of hot forming and die quenching, even in a direct hot forming process, consequently without cold forming.
Diagram 1 shows the mechanical characteristics of tensile strength Rm, Rp0.2 proof stress and elongation at break A30, and diagram 2 shows the bending angle of a body component according to the invention comprising a center layer of steel of grade 22MnB5 and two external layers, each with a thickness of 5 percent of the total thickness and composed of a ferritic rust-resistant steel alloy. Here, X10CrAlSi18 was used as the ferritic steel alloy.
In comparison with this, diagrams 3 and 4 show the results for a component made of steel with the commercial name Usibor with an aluminum-silicon coating on both sides in accordance with the prior art. All the components had a thickness of 2 millimeters.
In an improved development of the invention, the body component or chassis component has a second surface section made of a triple-layer laminated metal sheet. Here, the first surface section has a first center layer having an ultra-high-strength microstructure containing at least 80 percent of martensite, while the second surface section has a second center layer with a microstructure selected from a group consisting at least 80 percent of tempered martensite or a hybrid microstructure containing at least 70 percent of ferrite and perlite and residual percentages of martensite and/or residual austenite and/or bainite. In this way, it is possible to produce components with softer and more ductile surface sections.
It is furthermore possible to envisage that the second surface section is a triple-layer laminated metal sheet, and the first center layer and the second center layer each have a thickness, and the thickness of the first center layer differs from the thickness of the second center layer. In this way, a particularly thick surface section can be arranged in zones of extremely high stress and load bearing capacity or where material reinforcement within a thinner surface section is required for joining by means of a rivet or screw, for example.
In this case, the first surface section can have a total thickness which differs from the total thickness of the second or further surface sections by at least 10 percent, in particular between 20 and 100 percent.
According to the invention, it is also possible to envisage that the body component or chassis component has a second surface section or further surface sections made of a ferritic or martensitic or austenitic rust-resistant steel alloy. In particular, the surface sections of the body component or chassis component can adjoin one another and can be butt welded.
However, it is also possible for the body component or chassis component to have a second surface section made of a ferritic steel alloy, in particular the surface sections once again being butt welded to one another.
It is also possible for the second surface section or further surface section to be selected from a low-alloyed steel alloy or a multiphase steel alloy or from a steel alloy with TWIP and/or TRIP properties. As a particular preference, the second surface section can be heated only to a temperature less than the AC 1 temperature during the process for the manufacture of the chassis component or body component in order to prevent scaling.
The body component or chassis component preferably has a rim, wherein, at least in some sections, the rim is surrounded at one end, in the surface section with the triple-layer laminated metal sheet, by the external layer, such that the end of the center layer is screened from the environment by the external layer. This further improves corrosion resistance.
Body components or chassis components of a motor vehicle according to the invention are particularly selected from the group consisting of a door pillar, especially in the form of an A pillar or center pillar, roof frame, sill board, bumper crossmember, longitudinal member, floor crossmember and transverse link, longitudinal link, transverse link, stabilizer, twist beam axles, axle carrier, crash box, door impact support, tunnel, for example transmission tunnel. The use as a battery box for a traction battery of an electric or hybrid vehicle is also possible.
In the case of a door pillar or a roof frame of the motor vehicle, it is preferred that the second surface section or a further surface section having low tensile strength is in each case arranged in the rim. In this case, the rim is more ductile and serves for simpler connectability of connection components, edge plates, local reinforcements or other mechanical processing steps.
The methodological part of the invention is achieved by a method for manufacturing a body component or chassis component of the kind described above, comprising the following steps:
In the method, a particular advantage is obtained if the hot forming and hardening of the sheet metal blank is carried out in or by means of a single press having a plurality of die stages. As an alternative or preferably at the same time, provision can be made for the heating and hot forming of the sheet metal blank to be carried out in a single press having a plurality of die stages. Thus, at least one sheet metal blank is in each case simultaneously heated, hot-formed and hardened in a single press cycle. It is self-evident that an extremely short cycle time and hence high throughput can be made possible when using servo-motor-operated or mechanical presses.
In a development of the method according to the invention, the heating is carried out within 30 seconds, preferably within 20 seconds, in particular within 10 seconds, allowing space-saving austenitization with little loss of heat. It is advantageous if heating is carried out sequentially in synchronism with the hot forming or the press cycle of a hot forming line. As a further preference, heating can comprise at least one holding phase. Heating can be carried out without a protective gas atmosphere since the external layers do not have any tendency for scaling. As an advantage, there is the fact that blast cleaning of the fully formed component before painting or cathodic dip coating can be omitted.
In the heating of the sheet metal blank, contact heating can be used to particular advantage since it is associated not only with high efficiency and low heat losses but also with the possibility of adjusting a first surface section of the sheet metal blank to more than the austenitization temperature and a second surface section to less than 700° C. before hot forming by means of contact plates adjusted to different temperatures. This also applies to a temperature adjustment stage after heating and ahead of the press forming die stage. In particular, there is the advantage over precoated steels that there is no need for full alloying beforehand.
In comparison with this,
According to the invention, provision can be made here for component trimming or cutting, in particular piercing of the component, to take place after hot forming and die quenching and consequently only on the hardened component. This can then be accomplished by combined rolling and cutting or pressure cutting, in particular in a press die stage following the press forming die stage but also outside in a separate operation. In this case, part of the external layer is displaced in the rim region into the face of the separation region or hole rim and, at the same time, a slug or trimmed edge is removed. According to the invention, in contrast to the aluminum-clad heat-treated steel, the center layer is protected from environmental influences, in particular the introduction of process-related molecular hydrogen, during heating by the external layers made of rust-resistant steel alloy, and the risk of embrittlement of the component caused by the introduction of hydrogen is prevented. The high-grade steel external layers are not susceptible to cracking or fracture, in contrast to coated components. It is therefore very readily possible to cut them in the cold and hard state after die quenching. Particularly in the case of pressure cutting, the lower layer does not have any significant proportion of cracks. The cut edges are substantially free from burrs owing to the increase in ductility of the external layers. Trimming can also be carried out in a separate cutting tool. In particular, trimming is carried out in full. This means to the final shape. In particular, a crack-free surface is provided. Moreover, the cut edge is provided very largely without cracks. There are preferably no cracks or microcracks larger than 10 μm at the surfaces. Thus, laser trimming or, alternatively, complex hot cutting can be eliminated.
Further aims, advantages, features and possibilities of using the present invention emerge from the description below of several embodiments with reference to schematic drawings. All of the features which are described and/or illustrated pictorially form the subject matter of the present invention by themselves or in any meaningful combination, also irrespective of the summary thereof in the claims or the reference back of the latter.
For an understanding of embodiments of the disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:
In the Figures, the same reference signs are used for identical or similar components, even if a repeated description is omitted for reasons of simplicity.
Some embodiments will be now described with reference to the Figures.
Both examples illustrate body components or chassis components 1 made from sheet metal and which have been formed three-dimensionally by means of die forming. Both the center pillar 20 and the transverse link 30 comprise at least one surface section 2 comprising a triple-layer laminated metal sheet 10 with, as FIG. 2 shows in more detail, a center layer 11 and two external layers 12, 13 bounding the center layer 11 on the outside, wherein the external layers 12, 13 are composed of a rust-resistant, in particular ferritic steel alloy and the center layer 11 is composed of a heat-treatable steel alloy. The tensile strength Rm within the surface section 2 with the triple-layer laminated metal sheet 10 is more than 1300 MPa.
A detail which is shown in enlarged form in
The construction of the laminated metal sheet according to
An alternative embodiment of the surface sections 2, 3 of the center pillar 20′ from
According to the invention, in addition to a B pillar, another body component or chassis component can have, next to a triple-layer laminated metal sheet in a first surface section, a more ductile, particularly corrosion-loaded second surface section composed of a stainless steel alloy.
The second surface section 3 has greater ductility and a lower tensile strength in its center layer 16, illustrated in
In contrast to
The detail which is considered in more detail in
A layer build-up of the alternative embodiments as per the center pillar 20″ and 20′″ from
A transition region 41 in the center layer between the first and the second center layers has a width B1 which is between 10 mm and 150 mm, but preferably below 50 mm since a state which is mechanically difficult to determine and is inhomogeneous is present in the transition region 41. Of course, said layer build-up of the described center pillar 20″, 20′″ is also transferrable to other body components and chassis components, as present in claim 15, wherein a targeted design of the component appropriate to the load is expedient.
A layer build-up of an alternative embodiment of the invention is apparent in
A first surface section 2 has the center layer 11 which is bounded upward and downward by two external layers 12 and 13. The first surface section 2 has a first center layer 11 of an ultra-high-strength microstructure, with at least 80 percent martensite, wherein the tensile strength within the first surface section 2 comprising a triple-layer laminated metal sheet 10 is greater than 1300 MPa. The body component or chassis component 1 has a second surface section 3 of the same triple-layer laminated metal sheet 15 in terms of material, wherein the second surface section 3 has a second center layer 16 with approximately the same metallic microstructure. With regard to the material, the surface sections correspond to one another in the individual layers, wherein the external layers 12, 13, 17, 18 are each composed of a ferritic rust-resistant steel alloy. It is also possible here for the jump in thickness to be formed only on one side, for example on the upper side, whereas the opposite lower side is flat. This facilitates subsequent heat forming since better die contact is produced. In addition, it results in a sheet-like welding plane.
A transition region 44 between the first and the second surface section 2, 3 has a width B2 which is between 50 millimeters (mm) and 250 mm, but preferably below 200 mm since the coupling to further components is made difficult in uneven sections. It can be seen that the total thickness D3 of the laminated metal sheet 15 in the second surface section 3 is greater than the total thickness D2 of the laminated metal sheet 10 in the first surface section 2, wherein the ratios of the thickness of the layers with respect to one another within a laminated metal sheet do not change.
Of course, it is possible for the body component or chassis component 1 to comprise further surface sections which adjoin the second surface section and permit a further increase in the overall thickness and therefore a strengthening of the design appropriate to the load. It should also be noted that the different thickness is preferably already present before the press forming into the three-dimensional component geometry.
Subsequently, the press-formed sheet metal blank is transferred into a subsequent cooling die stage 53 where the formed sheet metal blank is at least partially hardened. After a further transfer into a third die stage 54, final cooling to approximately ambient temperature takes place, but so does at least complete hardening of at least the first surface section.
Instead of three cooled die stages 52, 53, 54, as in
Trimming and piercing of the formed, but still unhardened component (not illustrated) can also take place by means of the press forming die 52 or the first cooling die stage 53. A decisive advantage of the methods according to the invention is that, by means of the external layers of ferritic or austenitic or martensitic rust-resistant steel alloy, scaling or oxidation during the heating and during hot forming are prevented and therefore a complicated coating, final cleaning of the surface, surface errors and a protective gas housing of the press or contact heating is avoided.
In
It can be seen in
As can be seen in
By contrast,
The foregoing description of some embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The specifically described embodiments explain the principles and practical applications to enable one ordinarily skilled in the art to utilize various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. Further, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as described by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 112 327.4 | Jul 2015 | DE | national |
The present application is a National Phase of International Application Number PCT/DE2016/100223 filed May 12, 2016 and is related to and claims priority benefits from German Application No. 10 2015 112 327.4 filed on Jul. 28, 2015.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/DE2016/100223 | 5/12/2016 | WO | 00 |
