SIDE RAIL AND METHOD FOR PRODUCING A HOT-FORMED AND PRESS-HARDENED SIDE RAIL

Abstract

A side rail and to a method for producing a side rail are disclosed. The side rail has a region of a first type and a region of a second type which have mutually different strengths. A transition region having a width of less than 50 mm is formed between the two regions. The side rail has in the region of the first type a bainitic structure and in the region of the second type a martensitic structure.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application, Serial No. 10 2010 048 209.9, filed Oct. 15, 2010, and European Patent Application Serial No. 11 155 717.9, filed Feb. 23, 2011, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein.

This is one of two applications both filed on the same day. Both applications deal with related inventions. They are commonly owned and have the same inventive entity. Both applications are unique, but incorporate the other by reference. Accordingly, the following U.S. patent application is hereby expressly incorporated by reference: “AUTOMOBILE COLUMN AND METHOD FOR PRODUCING A HOT-FORMED AND PRESS-HARDENED AUTOMOBILE COLUMN”.

BACKGROUND OF THE INVENTION

The present invention relates to a side rail, produced by hot-forming and press hardening. The present invention also relates to a method for producing a side rail by hot-forming and press hardening.

The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.

The requirements profile for vehicle safety increases in the automotive industry due to regulatory and manufacturer-specific guidelines. At the same time, the automobile manufacturers strive to reduce the weight of the automobile bodies in order to minimize fuel consumption and CO₂ emission. This creates a divergence between low weight and high bending and torsion strength and high crash safety.

According to one approach, for example light-metal materials, in particular aluminum alloys, or bodies in hybrid construction, for example made of metallic alloys and fiber composite material or plastics, can be used. However, the aforementioned approaches are both associated with high material costs, which in turn increases the vehicle production costs of models produced in large quantities.

However, a metallic alloy, in particular steel, still remains the preferred material for constructing the body, in particular the raw body. Due to consequent improvements, steel is still viewed as a high-tech material which due to different processing approaches represents a good compromise between favorable manufacturability, excellent crash safety and long service life.

Heat-treatment is according to the state-of-the-art typically performed in a temperature range between 320° C. and 400° C. and hardly changes the material properties and the strength values adjusted in the hot-forming and trans-hardening process. At the same time, however, the ductility of the material is increased so as to allow superior fold formation in a crash.

However, the additional heat-posttreatment once more increases the production costs due to significantly higher tooling costs up to the start of the series production.

It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an side rail and a method for its manufacture, which has lower manufacturing costs compared to the state-of-the-art, while simultaneously allowing precise adjustment of material properties within the side rail.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a side rail with at least two regions of different strength is produced by hot-forming and press-hardening, wherein a region of a first type has after press-hardening a substantially bainitic structure and a region of a second type has after press-hardening a substantially martensitic structure, and a transition region between the region of the first type and the region of second type being smaller than 80 mm.

According to one advantageous feature of the present invention, the transition region is smaller than 50 mm, preferably smaller than 30 mm and still more preferably smaller than 20 mm. Because the transition region is very small, the component can within the context of the invention be specifically adjusted in a single production step, namely the production process itself, so that the required crash properties can be reliably implemented with the current manufacturing tolerances, while simultaneously having improved manufacturability.

According to another advantageous feature of the present invention, a side rail with advantageous material properties can be attained in previously defined regions of the first and second type by a reliable process and a specific design. During the hot-forming and press-hardening of a metal plate or of a preform or semi finished product made of high-strength hardenable steel, regions of the first type are intermediately cooled, so that regions of the first type and regions of the second type with different strengths, hardness and ductility properties can be intentionally adjusted. A material structure having a more ductile tendency is produced in the regions of the first type, as compared to the regions of the second type. The transition region between both regions tends to have clearly defined edges. This significantly relaxes or even entirely eliminates production tolerances. Regions with ductile material properties are produced by the substantially bainitic structure in the region of the first type.

An intentional deformation is favored in the regions of the first type, so that folds or a compressions can be formed in a crash, without causing cracks or detachment. The energy absorption capacity of the side rail according to the invention is hereby increased while retaining its high stiffness. A high degree of energy is then absorbed in an automobile equipped with the side rail according to the invention by converting kinetic energy from the impact into deformation energy, while retaining the high stiffness of the rest of the automobile body.

Moreover, the side rail may be used as engine support or may be employed in the area of the luggage compartment, where a higher energy absorption capacity may be required than in the region of the passenger compartment itself. A sequence of regions of the first type and regions of the second type may be produced along the length of the side rail according to the invention in the direction of the vehicle by specifically creating regions of the first type and regions of the second type. The side rail can the advantageously collapse in a crash like an accordion.

The side rail according to the invention may be arranged, for example, in an automobile body transversely at the front and/or rear side to intentionally absorb an impacting object or another automobile. The side rail should absorb the body colliding with the automobile or the stationary body hit by the automobile such that the side rail is minimally deformed for dissipating the energy and a deliberate intrusion of a body into the automobile itself is prevented. With the side rail according to the invention, the overall energy absorption capability is increased, which in turn increases the overall energy absorption capacity of the automobile body, while at the same time providing a high stiffness. In addition, material savings can be achieved in the regions of higher ductility because of, for example, thinner wall thicknesses, which in turn reduce the overall weight of the automobile body. A high degree of energy is then absorbed in an automobile equipped with the side rail according to the invention in that kinetic energy of the impact is converted into deformation energy, while the stiffness of the passenger compartment and hence of the body remains high or is even increased.

According to another advantageous feature of the present invention, a side rail according to the invention may also prevent unintentional buckling in regions which are intentionally formed as regions of the second type after hard-forming and press-hardening according to the invention. The high hardness of the regions of the second type therefore prevents an undesirable deformation in certain regions.

Weakening of the material caused by vibrations under high permanent load and/or vibrations in conjunction with a high stiffness is also prevented. The remaining components of the side rail, i.e. the regions of the second type, have a substantially martensitic structure with particularly high strength values, so that the minimally required strength and crash characteristic of the component is adequately attained.

The side rail produced according to the invention can thus be produced more cost-effectively compared to components produced with conventional production methods, because only a single reforming and press-hardening process is required for adjusting the most important required properties of the components. The adjustment by way of a substantially martensitic structure and an intermediate structure which is substantially defined by a bainitic structure, allows a particularly error-free, specific adjustment of the required material properties in clearly defined regions of the component.

According to another advantageous feature of the present invention, the region of the second type, which has a martensitic structure as the major component of the structure, includes other structures in concentrations of more than 50%, in particular more of than 80%, preferably of more than 90%, and particularly preferred situations of more than 95%.

The high torsional stiffness and bending stiffness due to the martensitic structure guarantees the elevated hardness of the side rail according to the invention, which preserves the integrity of the body and of passenger compartment as much as possible and thus protects the vehicle occupants.

According to another advantageous feature of the present invention, bainite may be present as an additional structure component in the region of the second type.

According to another advantageous feature of the present invention, the region of the first type may have as the primary structure component bainite, wherein additional structure components with less than 50%, preferably less than 30%, and in particular less than 15% may be present. For example, a mixed structure of bainite, with ferrite and/or perlite may be present. Optionally, within the context of the invention, martensite may also be present as a component of the structure in the region of the first type.

According to another advantageous feature of the present invention, the region of the first type may be at least partially enclosed by the region of the second type; preferably, the region of the first type may be completely enclosed by the region of the second type. The region of the first type is preferably completely enclosed by the region of the second type in the region of the attachment points for, for example, crash boxes. Due to the particularly small transition region according to the invention, the stiffness in the direction of the component remains unchanged, so that essentially no weakened location, for example in form of an undesired rated breakpoint, is produced. The region of the first type is also ductile so as to prevent crack formation as much as possible. The ductility of the region of the first type also largely prevents components attached to the side rail or other couples components from being torn off, for example in an offset crash.

According to another advantageous feature of the present invention, the region of the first type may be spot-shaped, preferably with a diameter of less than 40 mm, in particular less of than 20 mm and particularly preferred of less than 10 mm.

According to another advantageous feature of the present invention, a passage may be produced in the region of the first type. This means the passage may be formed simultaneously during the reforming process and/or press-hardening process; in one embodiment, the passage can also be created after the end of the press-hardening process. Due to the higher ductility, tool wear of a punching or stamping tool is reduced, or the passage can only be produced by this process without crack formation.

According to another advantageous feature of the present invention, marginal regions, in particular recesses and flanges, may be formed as regions of the first type, wherein cracks originating from the edge can be effectively prevented. Also regions subjected to mechanical processing after press-hardening, such as re-orientations, can advantageously be implemented as regions of the first type.

According to another advantageous feature of the present invention, the region of the first type may also be provided as a region for producing cutting edges. This provides an initial material characteristic which is gentle on the cutting or separation tool to advantageously allow cold cutting after hot-forming and press hardening, for example with simple cutting and/or separation methods. Further machining of the component, for example by cutting, is here particularly gentle, precise and cost-effective while maintaining the required tight tolerances. In particular, the need for an expensive laser cutting of the otherwise hard edge of the component can be eliminated. To this end, a circumferential, narrow region of the second type can advantageously be formed proximate to the edge contour. The risk of a delayed formation of cracks, caused by local stress in the hard structure, is at the same time significantly reduced.

According to another advantageous feature of the present invention, the region of the first type may have a stretchability A50 between 10 and 30%, preferably between 14 and 20%. This ensures sufficiently high strength, with simultaneously adequate ductility, thereby preventing the formation of cracks and hence individual structural automobile components to be torn off in a crash.

According to another advantageous feature of the present invention, the region of the first type may have a tensile strength between 500 and 1000 N/mm², preferably between 550 and 800 N/mm². The region of the first type may have an elongation limit between 200 and 800 N/mm², preferably between 250 and 600 N/mm², particularly preferred between 250 and 500 N/mm², and even more preferred between 300 and 500 N/mm².

Between the region of the second type and the region of the first type, the elongation limit and/or the tensile strength may be formed with a decreasing or increasing gradient of more than 100 N/mm², preferably more than 200 N/mm², and in particular more than 400 N/mm² per 10 mm. This means that the elongation limit and/or the tensile strength in the region of the first type increase by more than 100 N/mm² per 10 mm in the direction of the region of the second type.

According to another advantageous feature of the present invention, the region of the second type may have a strength of more than 1000 N/mm², in particular of more than 1200 N/mm², and preferably of more than 1400 N/mm².

According to another aspect of the invention, a method for producing a hot-formed and press-hardened side rail, wherein the side rail has at least two regions of different strength, includes the following method steps:

• • providing a hardenable metal plate or semi-finished product and heating the hardenable metal plate or semi-finished product to at least an austenizing temperature,
  • intermediately cooling a region of a first type of the metal plate or semi-finished product with a cooldown speed selected to be greater than a lower critical cooldown speed of a material of the metal plate or semi-finished product, and
  • hot-forming and press-hardening the metal plate or semi-finished product in a press-hardening tool to form the side rail.

With the method according to the invention, an intermediate stage structure may be adjusted under time control and/or temperature control. The intermediate stage structure may be adjusted, in particular, in the region of the first type of the metal plate by intermediate cooling. The cooldown speed of the intermediate cooling may be selected within the context of the invention so as to be above the lower critical cooldown speed of the bainite formation of the material of the metal plate. The cooldown speed may also be greater than the lower critical cooldown speed of the bainite formation. In particular, those regions are cooled which are designed to be soft after press-hardening, i.e., they have a higher ductility.

According to one advantageous feature of the present invention, the component may also be preformed to a semi-finished product while cold. The component is then at least partially preformed from a hardenable metal plate. Preferably, preforming matches at least 80% of the final shape of the component. Following the cold preforming process, which can be carried out, for example, at room temperature, a heating step to at least the austenizing temperature, i.e. to above the AC3 temperature, is performed. Thereafter, a region of the first type is at least partially intermediately cooled, followed by additional steps of the method according to the invention.

The cooldown process of the intermediate cooling is performed after the hardenable metal plate is heated to the austenizing temperature, but may also be performed within the context of the invention before or during the hot-forming and press-hardening process. In particular, if the cooldown process of the intermediate cooling is performed during press-hardening, suitable means are provided in the pressing tool capable of performing a corresponding cooldown as well as corresponding cooldown speeds.

If the intermediate cooling takes place before hot-forming and press-hardening, then this may be associated with a production line with corresponding intermediate transfers of the metal plate that was heated above the austenizing temperature.

The cooldown itself may be performed, for example, by free or forced convection, with cooling rollers, two-sided or one-sided annealing plates with an insulated abutment or by applying cooling media, such as water, or with other suitable cooling devices. The cooldown can hereby be performed in a fixedly installed intermediate station as well as in a cooling unit which moves commensurate with the production cycle. Preferably, a cooldown speed for the intermediate cooling is between 200 Kelvin per second and 5 Kelvin per second. Particularly preferred is a cooldown speed of 50 Kelvin per second. The cooldown is hereby preferably performed immediately after removal from the furnace. In this way, strength values between 550 and 900 MPa are adjusted in the first regions. Preferably, strength values of substantially 700 MPa are adjusted.

According to another advantageous feature of the present invention, a region of the second type may be held above the austenizing temperature, wherein the region of the second type may be any region of the metal plate that is not taken up by the region of the first type. This means that after the metal plate is heated to at least in the austenizing temperature, a corresponding temperature above the austenizing temperature is maintained. This may be done actively by using external heat sources, or passively by employing suitable insulation. A temperature above the temperature AC1 may also be maintained. Although a certain loss in strength may occur compared to forming from AC3, this is noncritical in most situations.

When employing external heat sources, the temperature may be held in the region of the second type, in particular with infrared lamps, heating coils, pore burners, insulation plates or similar external heat sources. Within the context of the invention, a temperature significantly above the austenizing temperature may be selected, wherein the time after the heat-up to above the austenizing temperature has ended to the start of the press-hardening process and the accompanying cooldown are matched to one another such that the region of the second type is at the start of the press-hardening process still at a temperature which is at least above the austenizing temperature.

According to one advantageous feature of the present invention, the cooldown speed during intermediate cooling of the region of the first type may be selected so that a bainitic structure is obtained; preferably, the material is cooled down to a temperature between 700 and 400° C., preferably 650 to 450° C., and in particular 650 to 500° C. With cooldown speeds that are greater than the lower critical cooldown speed of the respective employed material, but stop above the martensitic start temperature, the so-called bainite formation occurs during isothermal holding of the cooldown temperature, also known as intermediate structure or as intermediate stage.

Unlike with conventional methods, where perlite or ferrite is formed, with perlite being formed mainly directly from the austenite by diffusion, the diffusion of carbon in the austenite is significantly hindered in the intermediate stage of the bainite as a result of the more rapid cooldown. Small austenite regions, mostly originating at grain boundaries, are transformed during bainite formation into a distorted alpha lattice. Because the diffusion velocity in the alpha lattice is significantly greater than in the gamma lattice, small cementite grains precipitate in these alpha mixed crystals which are oversaturated with carbon, which become finer with faster cooldown. This produces a substantially needle-like bainitic structure. This also produces a grainy structure of the carbides caused by the increasing hardness which increases with the grain fineness. A further difference is made in the bainite structure between an upper intermediate stage, in which the carbides are combined for increased incursion, and a lower intermediate stage, in which the carbides are very finely distributed.

According to one advantageous feature of the method of the present invention, the region of the first type may be maintained at the cooldown temperature of the intermediate cooling for a predetermined time; preferably, the temperature may be held substantially isothermal. With this embodiment, the respective required or desired strength values of the bainitic intermediate structure can be adjusted exactly as a function of time. The intermediate cooling in this embodiment takes place substantially to a temperature where the material structure in the region of the first type has been transformed into austenite, or occurs directly into the intermediate structure. From this cooling temperature, the material structure is further transformed by isothermal holding for a specified time. The material is then transformed from an austenitic structure to a bainitic structure. If the material is cooled directly into the intermediate stage by selecting the cooldown speed, then a mixed structure between austenite and bainite are already adjusted. By holding at the cooldown temperature, holding is performed for a predetermined time in a purely bainitic structural transformation range. The longer the region of the first type is held at the temperature, the greater becomes the bainitic component of the structure.

According to another advantageous feature of the present invention, the intermediate structure range cooled to the cooldown temperature may be further quenched from the bainitic structural transformation stage in the press-hardening tool itself, so that a mixed structure of martensite and bainite is adjusted in the region of the first type. By quenching the region of the first type, where the structure has an intermediate stage, the residual austenite fractions are transformed to martensite fractions during press-hardening. As a result, a martensite-bainite mixed structure is produced in the regions of the first type. The fractions of the bainite in relation to martensite depend again from the duration during which the first region is held in the intermediate stage, before the press-hardening process begins.

According to another advantageous feature of the present invention, the region of the first type may be held isothermally during a certain time interval so as to transform the region of the first type is completely into bainite. This produces a material structure with a higher strength compared to a ferritic-perlitic structure. In particular, a perlitic structure is hereby intentionally avoided, which would reduce the ductility.

According to another advantageous feature of the present invention, the cooldown speed during intermediate cooling may be selected to be above a critical cooldown speeds of the employed material. In this way, an austenitic region can be selectively adjusted which is thereafter held, preferably isothermally, during a predetermined time at a temperature level, so that the structural transformation is specifically adjusted to be bainitic during the holding time. Depending on the employed holding time, a partially bainitic-austenitic structure or an exclusively bainitic structure can be adjusted. If a bainitic-austenitic structure is adjusted, this structure is transformed to a bainitic-martensitic structure in the subsequent press-hardening process.

Within the context of the invention, holding is to be understood as maintaining a substantially identical temperature below the ferrite and perlite temperature, but above a martensite start temperature, i.e. substantially below 700° C., in particular below 600° C., particularly preferred below 550° C. For example, when isothermally holding for a longer time, the temperature may decrease from 500 to 400° C., which however is still considered within the context of the invention to be substantially isothermal. Particularly preferred, the region of the first type is held isothermally during a time interval from 1 second to 80 seconds. Particularly preferred, the holding time is 15 seconds. However, these values are to be selected depending on the employed material alloy.

According to another advantageous feature of the method of the present invention, the intermediate cooling of the region of the first type may be performed in the press-hardening tool, for example with cooling plates arranged in the press-hardening tool. This reduces the cycle times and also the production costs. In particular, an automobile component having a region of different strength is produced with only two tool steps. Initially, heat-up is performed in a furnace system, followed by a combination of intermediate cooling and hot-forming and press-hardening using only a single tool.

A cooldown speed of at least 25 Kelvin per second may be selected as the cooldown speed in the actual press-hardening process. In another embodiment, the cooldown speed may be selected to be higher than 27 Kelvin per second. However, higher cooldown speeds may be selected for the actual press-hardening process. In particular, the press-hardening process may then be performed both in the region of the first type and in the region of the second type at the same cooldown speed depending on the local temperature gradient between press-hardening tool and the workpiece. Due to the different temperatures at the start of the press-hardly process in both regions, the cooldown speed may slightly diverge from the region of the first type to the region of the second type.

In one embodiment, a hardenable steel categorized as micro-alloyed heat-treated steel is used with the method according to the invention. This steel includes in particular the following alloy element in mass weight percent fractions:

	carbon	(C)	0.19 to 0.25
	silicon	(Si)	0.15 to 0.30
	manganese	(Mn)	1.10 to 1.40
	phosphorus	(P)	0 to 0.025
	sulfur	(S)	0 to 0.015
	chromium	(Cr)	0 to 0.35
	molybdenum	(Mo)	0 to 0.35
	titanium	(Ti)	0.020 to 0.050
	boron	(B)	0.002 to 0.005
	aluminum	(Al)	0.02 to 0.06.

According to one advantageous feature of the present invention, the intermediate cooling of the regions of the first type may be performed with a tool having integrated cooling plates. The cooling plates may here have an intrinsic temperature of up to 600° C., which is still less below the AC3 temperature of more than 900° C. The region of the first type can be cooled down with these cooling plates and then, if desired, held isothermally for a certain time. For example, such cooling plates can be brought to the respective required temperature with electrical heater cartridges or by backside burner heating or with thermal oils.

According to one advantageous feature of the present invention, the intermediate cooling may also be performed with substantially cold cooling plates. The cooling plates then have a temperature significantly below 400° C., preferably between −100° C. and +100° C., particularly preferred between −10° C. and +25° C. However, an isothermal holding time can only be performed with cold cooling plates a limited way. In one embodiment, both versions of cooling plates may be integrated, for example, in a hot-forming tool and pressing tool, so that the entire process following the actual furnace heating is performed in only a single tool. Within the context of the invention, the cooling plates for performing the intermediate cooling may also be housed in a separate tool, so that the process takes place from a heat-up furnace via intermediate cooling to the actual hot-forming in press-hardening tool. This embodiment has the advantage that the separate tool can be designed substantially as a flat tool with substantially flat heating and/or cooling plates.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

FIG. 1 shows a detail area of a side rail according to the invention with a region of a first type, a transition region and a region of a second type;

FIG. 2 shows a side rail according to the invention;

FIG. 3 shows a time-temperature diagram for carrying out a process according to the invention; and

FIG. 4 shows a side rail assembly according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.

Turning now to the drawing, and in particular to FIG. 1, there is shown a detail of a side rail 1. As can be seen, a region of the second type 3 according to the invention is formed in a region of the first type 2. A transition region 4 is arranged between the region of the first type 2 and the region of the second type 3. A material structure having a tendency to be ductile is produced in the region of the first type 2, whereas a hard material structure is produced in the region of the second type 3. Within the context of the present invention, the transition region 4 has essentially a width a which is quite small compared to the region of the first type 2.

FIG. 2 shows a side rail 1. The side rail 1 has beads 5, openings 6 and recesses 7. The side rail 1 according to the invention also has joining flanges 8 disposed in its marginal regions. The beads 5, openings 6, recesses 7 and joining flanges 8 are each implemented as regions of the first type, depending on the requirements, whereas the remaining region of the side rail 1 is implemented as a region of the second type.

FIG. 3 shows a time-temperature diagram of an exemplary steel, without limiting the field of the present invention. Several structures are indicated which are obtained in the material at various cooldown speeds as a function of temperature. The lower part of the FIG. shows the martensite formation. Above, in the center region of the FIG., the bainite formation is shown, and there above the perlite and/or ferrite formation.

In the illustrated exemplary embodiment, three different curves for the different cooldown processes are shown. Curve K1 shows the course of the temperature for a first region according to the invention, wherein this region is first heated to a temperature above the AC3 temperature. From this temperature, the material is cooled down to an intermediate temperature of about 520° C. with a cooldown speed which in this case is greater than the upper critical cooldown speed oK for the bainite formation of the illustrated material. When the cooldown temperature of the intermediate cooling of about 520° C. is reached, the first region is held substantially isothermally at a temperature for the time ti. The temperature thereby decreases from about 520° C. to about 480° C. due to heat loss in form of, for example, heat radiation, convection or heat conduction. An austenitic structure is produced at the time Z1 of the intermediate cooling, and a bainitic-austenitic mixed structure is produced at the time P1, corresponding to the start of press-hardening in the first embodiment.

In the first embodiment, quenching thereafter occurs in the press-hardening process from the time P1, such that the bainitic-austenitic mixed structure in the first region is transformed to a bainitic-martensitic mixed structure. In parallel, the second region according to the invention is quenched from a temperature above AC3 by press-hardening, producing a martensitic structure directly from an austenitic structure; however, this is not illustrated in detail for sake of clarity.

The second embodiment of the method according to the invention is illustrated with the cooldown sequence according to curve 2 of the first region. The cooldown sequence of the curve 2 is similar to the cooldown sequence of the curve K1, wherein the cooldown temperature is held for a longer time from a time Z2 (equal to Z1), so that the press-hardening process starts at a time P2. The time interval t2 is therefore greater than t1. The structure in the first region is completely transformed to bainite at the time P2 and therefore does not undergo any further structural transformation after the time P2 due to the cooldown speed.

In a third embodiment according to the present invention, a cooldown speed from a temperature above the AC3 temperature according to curve 3 is selected, so that a transformation occurs directly into the bainitic intermediate structure during the cooldown process of the intermediate cooling. In the first region, an austenitic-bainitic intermediate structure was adjusted, so that when the press-hardening process starts at the time P3, this bainitic-austenitic mixed structure in the first region is transformed to a bainitic-martensitic mixed structure. In the embodiments according to curves 2 and 3, the second region which was held above the AC3 temperature during the intermediate cooling, is in both cases transformed from the austenitic region directly to martensite by the cooldown during the press-hardening process. In the embodiment according to curve 3, the temperature is selected according to the invention to be always greater than the lower critical cooldown speeds uK of the corresponding employed material.

FIG. 4 shows a side rail assembly 9 formed of a side rail 1 and a heat-treated component 10. The side rail 1 is here formed in the center region as a region of the second type and in an outer region as the region of the first type. The side rail 1 and the component are coupled with one another at their corresponding lateral regions by joining flanges 8. The joining flanges 8 themselves are here formed as regions of the first type with a rather ductile material characteristic. In the event of a deformation, for example in a crash, a basic stiffness is provided by the side rail 1 itself. Detachment is prevented by the rather ductile material characteristic. Both components are connected with each other at the coupling locations 11.

While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

Claims

1. A side rail comprising:

at least two regions of different strength produced by hot-forming and press-hardening, wherein a region of a first type has after press-hardening a substantially bainitic structure and a region of a second type has after press-hardening a substantially martensitic structure, and

a transition region between the region of the first type and the region of second type being smaller than 80 mm.

2. The side rail of claim 1, wherein the transition region is smaller than 50 mm.

3. The side rail of claim 1, wherein the transition region is smaller than 30 mm.

4. The side rail of claim 1, wherein the transition region is smaller than 20 mm.

5. The side rail of claim 1, wherein the substantially martensitic structure of the region of the second type comprises additional structure components in a concentration of less than 50%.

6. The side rail of claim 1, wherein the substantially martensitic structure of the region of the second type comprises additional structure components in a concentration of less than 30%.

7. The side rail of claim 1, wherein the substantially martensitic structure of the region of the second type comprises additional structure components in a concentration of less than 15%.

8. The side rail of claim 5, wherein the additional structure component comprises bainite.

9. The side rail of claim 1, wherein the substantially bainitic structure of the region of the first type comprises additional structure components in a concentration of less than 50%.

10. The side rail of claim 1, wherein the substantially bainitic structure of the region of the first type comprises additional structure components in a concentration of less than 30%.

11. The side rail of claim 1, wherein the substantially bainitic structure of the region of the first type comprises additional structure components in a concentration of less than 15%.

12. The side rail of claim 1, wherein the region of the first type is at least partially enclosed by the region of the second type.

13. The side rail of claim 12, wherein the region of the first type is completely enclosed by the region of the second type.

14. The side rail of claim 1, wherein the region of the first type is spot-shaped with a diameter of less than 40 mm.

15. The side rail of claim 1, wherein the region of the first type is spot-shaped with a diameter of less than 20 mm.

16. The side rail of claim 1, wherein the region of the first type is spot-shaped with a diameter of less than 10 mm.

17. The side rail of claim 1, wherein the region of the first type is constructed as a coupling location for coupling additional components to the side rail.

18. The side rail of claim 1, wherein the region of the first type is formed in regions of the side rail which are subject to strong deformations in a crash or which are configured to dissipate crash energy through deformations.

19. The side rail of claim 1, wherein the region of the first type has an increased wall thickness in relation to the region of the second type.

20. The side rail of claim 1, further comprising a passage or an edge, or both, in the region of the first type after hot-forming.

21. The side rail of claim 1, wherein the region of the first type has a stretchability A50 between 10% and 30%.

22. The side rail of claim 1, wherein the region of the first type has a stretchability A50 between 12% and 20%.

23. The side rail of claim 1, wherein the region of the first type has a stretchability A50 between 12% and 16%.

24. The side rail of claim 1, wherein the region of the first type has a stretchability A50 between 14% and 16%.

25. The side rail of claim 1, wherein the region of the first type has a tensile strength between 500 and 1000 N/mm2.

26. The side rail of claim 1, wherein the region of the first type has a tensile strength between 550 and 800 N/mm2.

27. The side rail of claim 1, wherein a yield strength or a tensile strength decreases or increases in the transition region with a gradient of more than 100 N/mm2 per 10 mm.

28. The side rail of claim 1, wherein a yield strength or a tensile strength decreases or increases in the transition region with a gradient of more than 200 N/mm2 per 10 mm.

29. The side rail of claim 1, wherein a yield strength or a tensile strength decreases or increases in the transition region with a gradient of more than 400 N/mm2 per 10 mm.

30. The side rail of claim 1, wherein the region of the second type has a strength of more than 1000 N/mm2.

31. The side rail of claim 1, wherein the region of the second type has a strength of more than 1200 N/mm2.

32. The side rail of claim 1, wherein the region of the second type has a strength of more than 1400 N/mm2.

33. The side rail of claim 1, wherein region of the first type has a yield strength between 200 and 800 N/mm2.

34. The side rail of claim 1, wherein region of the first type has a yield strength between 250 and 600 N/mm2.

35. The side rail of claim 1, wherein region of the first type has a yield strength between 250 and 500 N/mm2.

36. The side rail of claim 1, wherein region of the first type has a yield strength between 300 and 500 N/mm2.

37. The side rail of claim 1, wherein the side rail is manufactured from a Tailor Welded Blank or a Tailor Rolled Blank.

38. A method for producing a hot-formed and press-hardened side rail having at least two regions of different hardness, the method comprising the steps of:

providing a hardenable metal plate or semi-finished product and heating the hardenable metal plate or semi-finished product to at least an austenizing temperature,

intermediately cooling a region of a first type of the metal plate or semi-finished product with a cooldown speed selected to be greater than a lower critical cooldown speed of a material of the metal plate or semi-finished product, and

hot-forming and press-hardening the metal plate or semi-finished product in a press-hardening tool to form the side rail.

39. The method of claim 38, wherein a region of a second type is held above the austenizing temperature until the region of a second type is transported into the press-hardening tool.

40. The method of claim 38, wherein the cooldown speed during intermediate cooling of the region of the first type is selected such that a bainitic structure is obtained.

41. The method of claim 40, wherein the region of the first type is cooled to a cooling temperature between 600 and 400° C.

42. The method of claim 41, wherein the region of the first type is cooled to a cooling temperature of about 500° C.

43. The method of claim 40, wherein the region of the first type is held at the cooling temperature for a predetermined time.

44. The method of claim 40, wherein the region of the first type is held at the cooling temperature isothermally.

45. The method of claim 38, further comprising the step of quenching the region of the first type in the press-hardening tool from a bainitic structure transformation stage, whereby a mixed structure of martensite and bainite, or a mixed structure of martensite, bainite and at least one of ferrite and perlite, is adjusted in the region of the first type.

46. The method of claim 38, further comprising the step of holding the region of the first type isothermally so as to form a substantially pure bainitic structure by press-hardening.

47. The method of claim 38, wherein the cooldown speed of the intermediate cooling is selected to be greater than an upper critical cooling-down speed.

48. The method of claim 38, wherein the intermediate cooling of the region of the first type is performed in the press-hardening tool.

49. The method of claim 48, wherein the intermediate cooling of the region of the first type is performed by using cooling plates arranged in the press-hardening tool.

50. The method of claim 38, wherein the metal plate is pre-formed into a semi-finished product while cold before being heated to at least the austenizing temperature.