Patent Application
20170183754
The present application claims priority to German Application Number 10 2015 122 827.0filed Dec. 23, 2015 and German Application Number 10 2016 100 648.3 filed Jan. 15, 2016, the disclosure of which is hereby incorporated by reference herein in its entirety.
The present invention relates to a heat treatment furnace according to the features in the preamble of Claim 1.
The invention also relates to a method for heat treatment of a coated steel blank according to the features in the preamble of Claim 7.
The invention also relates to a method for production of a motor vehicle part according to the features in the preamble of Claim 14.
Hot-forming and press-quenching technology is known from the prior art. In this context, a sheet metal blank made of a quenchable steel alloy is heated to a temperature above the Ac3 temperature, which generally corresponds to above 900° C. The steel sheet blank, which is at that temperature, is then placed in a hot-forming tool and is formed in this hot state. After forming, the formed steel sheet product is held in the hot-forming tool or is transferred to a separate press-quenching tool and rapidly cooled so as to harden the material structure.
This has the drawback that although the steel sheet product produced in this manner does have, at least in part, high hardness, it is susceptible to corrosion. Since these parts are used specifically in the body construction of motor vehicles, an appropriate corrosion protection measure must be adopted. The parts produced are provided with an anti-corrosion coating, for example in a CDP process.
It is however also known to provide pre-coated steel sheet blanks which for example have an aluminum silicon (Al—Si) coating. These are heat treated such that the pre-coating and the steel sheet blank are metallurgically bonded, thus creating an anti-corrosion coating on the produced part. At the same time, the pre-coating is also provided as scale protection.
This has the drawback that, specifically in the case of thin sheet metal blanks and/or partially rolled sheet metal blanks, hydrogen-induced tensile crack formation can arise during the heating of the pre-coated blank that precedes the hot-forming. This is also known as hydrogen embrittlement or delayed cracking. This can also lead to cracks appearing after hot-forming and press-quenching.
The present invention therefore has the object of indicating a possible way of avoiding, as far as possible, the hydrogen embrittlement that arises in pre-coated steel sheet blanks for a subsequent hot-forming and press-quenching process. It is possible to process constant-thickness steel sheet blanks but also steel sheet blanks of differing thickness.
The above-mentioned object is achieved according to the invention with a heat treatment furnace according to the features in Claim 1.
The method part of the object is moreover achieved with a method for heat treatment of a pre-coated steel sheet blank according to the features in Claim 7.
The further method part of the object is achieved with a method for production of a motor vehicle part according to the features in Claim 14.
Advantageous embodiment variants of the invention are described in the subclaims.
The present invention provides that a pre-coated steel sheet blank is first preheated, then metallurgically bonded with the pre-coating and then the blank that is metallurgically bonded with the coating is cooled in a targeted manner. Only after this does the actual heating in preparation for hot-forming take place. In that context, cooling is not performed too rapidly, such that the diffusible hydrogen atoms that are present in the blank can defuse out of the material. This establishes a hydrogen content of less than 0.5 ppm, preferably less than 0.3 ppm.
It is thus possible that the steel sheet blanks, heated and coated according to this inventive concept, have a markedly reduced hydrogen content and thus the risk of hydrogen-induced tensile cracking is almost completely avoided. Subsequent heating, in particular rapid heating, of the coated steel sheet blank together with austenization thus almost completely avoids the risk of hydrogen-induced tensile cracking.
To that end, the heat treatment furnace for coated steel sheet blanks has at least one furnace chamber and a transport system for guiding the steel sheet blanks through the furnace chamber. It is characterized, according to the invention, in that there are provided a preheating chamber, a metallurgical bonding path and a cooling chamber, wherein the pre-coated steel sheet blank can be heated in the preheating chamber to a temperature of above 200° C., can be heated in the metallurgical bonding path to a temperature above Ac3, and can be cooled in the cooling chamber to a temperature of below 450° C. in a controlled manner.
Preferably, to that end at least the metallurgical bonding path is designed as a continuous furnace with one furnace chamber. However, in another preferred embodiment variant, the preheating chamber and the cooling chamber are also respectively designed as a preheating path and a cooling path according to the principle of a continuous furnace.
In order that the heat treatment furnace can be operated economically with regard to the required installation space in an assembly hall and also with regard to energy considerations, it is provided that the preheating path, the metallurgical bonding path and the cooling path are arranged in a continuous furnace, in particular in parallel one above the other or in parallel next to one another.
The heat treatment furnace according to the invention is in particular characterized in that the metallurgical bonding path is primarily heated by heat sources or heating sources arranged therein. Heating is for example effected by means of radiative heaters, heating cartridges, induction, conduction, burner heating and/or in a similar manner. Thus, an air recirculator can be provided in the metallurgical bonding path. The parallel arrangement of the preheating path and the cooling path makes it possible for the heat energy, in particular excess heat energy, of the metallurgical bonding path to also be used in the preheating path and/or the cooling path. To that end, a temperature-permeable separating layer is provided. This can for example be a perforated plate or another, in particular physical, separating layer which permits temperature permeability that is pre-configured in a targeted manner, or can be subject to closed-and/or open-loop control. This allows part of the heat energy in the metallurgical bonding path to be transferred to the preheating path and/or to the cooling path. Furthermore, a reduced distance between the cooling path and the preheating path means that the heat radiation from the steel sheet blanks that are to be cooled can be used to heat the steel sheet blanks that are transported in the preheating path.
Manipulators are provided at the respective end of the heat treatment furnace such that the individual paths of the heat treatment furnace can be passed through in particular according to the contraflow or counter-current principle. In particular in the case of paths that are arranged one above the other with regard to the vertical direction, vertical conveyors are used, and in the case of paths that are arranged next to one another with regard to the vertical direction, horizontal conveyors are used.
Within the context of the invention, path is to be understood as the metallurgical bonding path, the cooling path and the preheating path.
However, other designs can also be used for both the cooling chamber and the preheating chamber. For example, for the preheating chamber use can be made of a multiple hearth furnace, a rotary furnace or a paternoster furnace, that is to say a vertical conveyor furnace.
The cooling chamber can also be of multiple hearth design. Also particularly preferred, when the preheating chamber and/or the cooling chamber are arranged separate from the metallurgical bonding path, is that the exhaust air from the metallurgical bonding path is routed into the preheating chamber and/or the cooling chamber.
The present invention is further characterized by a method for heat treatment of a coated steel sheet blank, wherein a pre-coated steel sheet blank is metallurgically bonded. The method is in particular carried out in an above-described heat treatment furnace. It is characterized by the following method steps:
The method according to the invention thus makes it possible, in particular, to decouple the metallurgical bonding of the steel sheet blank from the actual hot-forming and press-quenching process.
In particular, this preheating temperature is above 250° C., in particular at a temperature between 250° C. and 450° C.
Thereafter, the pre-coated and preheated steel sheet blank is heated from the preheat temperature to a temperature (metallurgical bonding temperature) above the Ac3 temperature and optionally held there in order that the pre-coating fully metallurgically bonds to the surface of the steel sheet blank. This involves a metallurgical bond with the pre-coating so as to form an intermetallic phase with the steel sheet blank.
After metallurgical bonding, the invention provides for targeted cooling to be carried out in a cooling chamber and/or a cooling path. This is brought about by targeted cooling to a cooling temperature below 450° C., in particular between 450° C. and 300° C. Also, and particularly preferably, two-stage cooling can be carried out. In particular, cooling is thus first carried out in a slow and controlled manner to a cooling temperature. This takes place slower than cooling in air at room temperature. Once the cooling temperature has been reached, further active rapid cooling can take place. The slow cooling thus allows the hydrogen to slowly diffuse out. The subsequent rapid cooling avoids warping of the blank.
Furthermore, the cooling path and the preheating path being arranged in parallel one above the other or next to one another, at least in certain sections, in the transport direction, means that the heat radiation from the steel sheet blanks that are to be cooled can be used to heat the steel sheet blanks that are transported through the preheating path.
In a further preferred manner, heating to metallurgical bonding temperature, that is to say heating for metallurgical bonding, from the preheat temperature for metallurgical bonding is carried out as rapid heating, in a time of less than 20 s/mm of sheet thickness of the pre-coated steel sheet blank. In particular at a time of less than 10 s per mm of sheet thickness, preferably less than 5 s per mm of sheet thickness. At the same time, heating from the preheat temperature to above the Ac3 temperature is carried out in the above-described time span. Sheet thicknesses deviating unevenly from a full millimeter can be interpolated appropriately.
Also, and particularly preferably, cooling from heating temperature to cooling temperature is carried out in a time of greater than 30 s per mm of sheet thickness of the sheet metal blank that is to be cooled.
In particular, it is thus possible to generate a coating layer thickness of less than 0.6 μm, the layer thickness being preferably greater than 0.15 μm. Particularly preferably, a layer thickness of between 10 μm and 35 μm is created.
Alternatively or in addition, the fraction of atomic hydrogen is less than 0.5 ppm, in particular less than 0.3 ppm. In particular, this indication relates to the hydrogen content in the steel material of the metallurgically bonded steel sheet blank. It is thus possible, with the method according to the invention, to heat treat a steel sheet bank made of a quenchable steel alloy and pre-coated with an aluminum-silicon alloy so as to form an intermetallic phase, in particular with an iron-aluminum fraction, between the steel sheet blank and the pre-coating. In particular, the method for heat treating the coated sheet metal blank is used for homogeneous heat treatment thereof.
The steel sheet blank homogeneously heat-treated in this manner can then be processed, in a subsequent hot-forming and press-quenching process, to give a quenched steel part, in particular a motor vehicle part. In particular, the secondary heating takes place as rapid heating. This is characterized in that the coated and metallurgically bonded steel sheet blank, which is to be reheated after heat treatment, is heated to the austenization temperature, that is to say the Ac3 temperature, in a time of less than 20 s, preferably less than 10 s, in particular less than 5 s. In particular, heating takes place in a time of less than 20 s per mm, preferably less than 10 s per mm, particularly preferably less than 5 s per mm of sheet thickness of the steel sheet blank that is to be heated. This avoids hydrogen diffusing back in. Thus, the hydrogen content is set at less than 0.5 ppm, preferably less than 0.3 ppm, even in the hot-formed and press-quenched part. The rapid heating can be effected in particular using contact plates or induction or also by direct resistance heating. Thereafter, the coated austenized steel sheet blank is hot-formed and press-quenched. Preferably, a part having a tensile strength Rm of greater than 1250 MPa, in particular greater than 1450 MPa is produced.
The previously mentioned part is in particular a sheet metal formed part, very particularly preferably a motor vehicle part. It is in particular produced such that the heat treated and metallurgically bonded steel sheet blank produced by means of the previously described method has an atomic hydrogen content of less than 0.5 ppm, in particular 0.3 ppm. The metallurgically bonded steel sheet blank is either supplied directly after heat treatment to a hot-forming and press-quenching process, or alternatively stored therebetween. Therefore, either the steel sheet blank is heated from the cooling temperature, for example in a range between 450° C. and 100° C., back above Ac3 for the subsequent hot-forming process, or is heated from room temperature to above Ac3 if the sheet metal blank is taken from the store.
Heating to above Ac3 takes place at least in certain regions and in particular entirely by means of a rapid heating process. This means that the steel sheet blank is heated from its current temperature to a temperature equal to or above the Ac3 temperature in a time of less than than 20 s, preferably less than 10 s, in particular less than 5 s. This can for example take place by means of contact heating, but also by means of induction or as direct resistance heating. The rapid heating in turn allows that no hydrogen in the surrounding air can penetrate into the coating, the intermetallic phase formed between the coating and the steel sheet blank, or the steel sheet blank itself. This avoids brittle breakages arising after hot-forming and press-quenching.
The motor vehicle part produced in this manner therefore has an atomic hydrogen fraction of less than 0.5 ppm, in particular less than 0.3 ppm. More preferably, it is thus possible to produce a high-strength, a super high-strength or preferably an ultra high-strength formed steel part. The indication expressed in ppm is preferably to be understood, within the context of this invention, as an indication in terms of mass relative to the entire motor vehicle part. In particular, the hydrogen content is also present in the quenched regions. The indication in ppm thus relates to the total mass of the produced motor vehicle part;
ppm=μg of hydrogen/g of motor vehicle part.
The motor vehicle part has, in certain regions and in particular throughout, a tensile strength Rm of greater than 1250 MPa, in particular greater than 1450 MPa. The tensile strength should be limited by the tensile strengths that can be technically achieved. In particular, the tensile strength is thus less than 3000 MPa, preferably less than 2000 MPa.
Other advantages, features, properties and aspects of the present invention are dealt with in the following description. Preferred embodiment variants are presented in the schematic figures. These serve to make the invention easy to understand. In the figures:
The figures use the same reference signs for identical or similar parts, even if a repeated description is omitted for reasons of simplicity.
As transport means 9, rollers 8 can be arranged throughout the furnace. It is however also possible to use other transport means for transit. The pre-coated steel sheet blanks 5 are conveyed through the preheating path 4 in a transport direction of the preheating path 4.
At the opposite end 10 of the heat treatment furnace 1 there is provided a vertical conveyor 11 which lowers the preheated steel sheet blanks 5 (with regard to the plane of the image) and transfers them to the metallurgical bonding path 2. Then, the preheated steel sheet blanks are conveyed through the metallurgical bonding path 2 in the transport direction 12. Heating means 13, for example burners or alternatively induction coils, are arranged in the metallurgical bonding path 2. The preheated steel sheet blanks transported through the metallurgical bonding path 2 are heated, at least at the end of the metallurgical bonding path 2, to a temperature above the Ac3temperature such that the pre-coating forms an intermetallic phase with the steel sheet blank and the steel sheet blanks 14 are metallurgically bonded.
Also provided at the previously described end 7 is a vertical conveyor 11 which raises the metallurgically bonded steel sheet blanks 14 and introduces them into the cooling path 3. In the transport direction 15 through the cooling path 3, the metallurgically bonded steel sheet blanks 14 are cooled to a temperature and removed at the end of the cooling path 3, and the metallurgically bonded and cooled steel sheet blanks 16 are stored on a blank stack 17. These can undergo further processing (not shown in greater detail), in particular a subsequent hot-forming and press-quenching process.
At the end 10 of the cooling path 3, the cooled steel sheet blanks 16 are removed and brought to a blank stack 17. Here, too, heating means 13 are once again provided, both in the metallurgical bonding path 2 and in the thermal separating layer 18, such that heat energy is transferred from the metallurgical bonding path 2 to the preheating path 4 or to the cooling path 3.
The temperature profile of the heat treatment furnace 1 shown in
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 122 827.0 | Dec 2015 | DE | national |
| 10 2016 100 648.3 | Jan 2016 | DE | national |
