Patent Grant
10287650
The invention relates to a furnace system and to a method for the controlled heat treatment of sheet metal parts.
In the technical realm, many applications in a wide array of sectors call for high-strength sheet metal parts that are lightweight. For instance, the automotive industry is striving to reduce the fuel consumption of motor vehicles so as to lower the CO2 emissions while, at the same time, improving passenger safety. For this reason, there is an ever-growing demand for autobody parts that have a favorable strength-to-weight ratio. These parts especially include A and B pillars, side-impact bars in doors, rocker panels, frame parts, bumpers, crossbeams for the floor and roof as well as front and rear longitudinal beams. In modern motor vehicles, the bodyshell with a safety cage is usually made of hardened sheet steel with a strength of about 1500 MPa. Al—Si-coated steel sheets are often used for this. The process of so-called press hardening has been developed for purposes of manufacturing parts made of hardened sheet steel. Here, steel sheets are first heated up to the austenitic temperature between 850° C. and 950° C. [1562° F. and 1742° F.], then placed into a pressing die, quickly formed and rapidly quenched by the water-cooled die to the martensitic temperature of approximately 250° C. [482° F.]. This gives rise to a hard, strong martensitic structure with a strength of about 1500 MPa. A steel sheet hardened in this manner, however, only has an elongation at break of about 6% to 8%, which is a drawback in certain areas if two vehicles collide, especially in the case of a side impact. The kinetic energy of the impacting vehicle cannot be converted into deformation heat. Rather, in this case, the part will undergo brittle fracture, additionally posing a risk of injury to the passengers.
For this reason, the automotive industry is striving to develop autobody parts that have several, different elongation and strength zones so that one single part can have very strong areas on the one hand and very extensible areas on the other hand. In this context, the general requirements made of a production installation should also be taken into account: for instance, the cycle time of the press hardening installation should not be detrimentally affected, it should be possible to use the entire installation universally without restrictions and to quickly retool it according to customer specifications. The process should be robust and cost-efficient, and the production installation should only take up a minimal amount of space. The shape and the edge precision of the part should be so high that the need for hard-trimming the hardened part is virtually eliminated, thus saving material and work.
The state of the art describes such methods and devices. In this context, these methods make use of partially heated dies, whereby one area of the part is cooled off above the martensite-forming quenching velocity. The rest of the part is cooled off abruptly as is normally done, thereby forming martensite. European publication EP 2 012 948, for example, describes a forming die for press-hardening and for the temperature-controlled forming of a blank consisting of high-strength and/or ultra-high strength steel grades; this die that has means for controlling the temperature of the forming die and this publication also describes a method for press-hardening and for temperature-controlled forming of a blank consisting of high-strength and/or ultra-high strength steel grades in which the blank is heated prior to the forming process and subsequently formed in a forming die while it is hot or warm, whereby the forming die has means for controlling the temperature. Here, several temperature-control means are provided in the forming die, as a result of which a plurality of temperature zones can be defined, whereby at least the contact surfaces of the die elements used for the forming process are associated with individual temperature zones.
German patent document DE 10 2005 032 113 discloses a device and a method for hot-working and partially hardening a part positioned between two die halves in a press. The die halves are each divided into at least two segments that are separated from each other by thermal insulation. The two segments can be heated or cooled by means of a temperature-control unit, so that different temperatures and thus different cooling curves can be established in different areas of the part. This makes it possible to manufacture a part with areas of different hardness and ductility.
International patent document WO 2009/113 938 describes a press-hardening process with which soft areas can be created in the finished product by reducing the cooling rate of these material sections. This diminishes the martensite fraction in these areas and consequently increases the elongation at break of these areas.
In this context, all of the methods that use a partially heated die entail the drawback that the part becomes warped since the part is removed from the die with partially different temperatures ranging from about 300° C. to 500° C. [572° F. to 932° F.] in the soft area, and of about 100° C. [212° F.] in the martensitic areas, after which it is further cooled away from the fixed shape of the die. Moreover, the cycle time of the process is lengthened since the fast cooling is slowed down in order to promote pearlite-ferrite formation, as a result of which the cost-effectiveness is likewise reduced. In addition, such dies are very complex and therefore expensive and malfunction-prone.
In another method known from the state of the art, for example, German patent documents DE 10 350 885, DE 10 240 675, DE 10 2005 051 403 or DE 10 2007 012 180, in a dual-zone furnace, the soft area of the part is heated up to a temperature below the material-dependent Ac3 temperature, whereas the area that is to be hardened, in contrast, is heated up to a temperature above the Ac3 temperature. In this process, an extensible soft pearlite-ferrite is formed in one area of the part and a hard martensite is formed in another area of the part. The disadvantage of this process is that the furnace can only be employed with certain limitations and can no longer serve as a universal furnace. This translates into a loss of cost-effectiveness for this method. Another disadvantage is that the separation of the areas usually cannot be accomplished with sufficient precision over the long run. Moreover, it is not feasible to implement more than two different zones. Furthermore, when Al—Si-coated parts are used, the temperature has to be kept at approximately 950° C. [1742° F.] for about 300 seconds so that the coating can diffuse into the base material. This process takes considerably longer at lower temperatures, thus reducing the cost-effectiveness of the entire installation.
Moreover, another method is known in actual practice in which the soft areas are partially cooled slowly. In this process, the entire part is heated up above the austenitic temperature beyond the diffusion time and diffusion temperature, and subsequently, either in a separate furnace or in the same furnace, it is cooled down again slowly to below the austenitic temperature in that it is partially exposed to air. When the press-hardening process is subsequently carried out in the die, the drawbacks in terms of the insufficient dimensional precision and the cost-effectiveness of the production furnace are eliminated. A disadvantage of this method is the slower cycle time caused by the additional work step. Yet another disadvantage is the undefined cooling rate that occasionally leads to martensite formation in parts that are less than 1.2 mm-thick. The cooling rate is undefined because the cooling takes place at an ambient temperature that cannot be precisely defined. For this reason, the process cannot be said to be robust. Moreover, this process can only be carried out with two zones of different hardness.
European Preliminary Published Application EP 2 143 808 A1 describes a method for the production of a shaped part having at least two structural areas of different ductility from a blank made of hardenable steel, different areas of which are heated differently and subsequently shaped in a heat-forming and hardening die and then hardened in certain areas, and also having an infrared lamp array. The blank made of hardenable steel is heated in a heating device to a homogeneous temperature that is lower than the Ac3 point of the alloy. Subsequently, the infrared lamp array is used to bring areas of the blank that are of the first type to a temperature above the Ac3 point of the alloy, and hardened in a heat-forming and hardening die in the areas of the first type. The result is a shaped part made of steel and having at least two structural areas of different ductility. The appertaining furnace system has a profiling furnace with one level, whereby the one level has an upper section and a lower section as well as a receptacle for a product-specific intermediate flange and the product-specific intermediate flange installed in it. Here, the product-specific intermediate flange is designed to impart a prescribed temperature profile to the part with temperatures above the Ac3 temperature for an area that is to be hardened and below Ac3 for a more ductile area.
Finally, German patent specification DE 10 2009 051822 B3 discloses a method for the production of shaped sheet metal parts made of high-strength steel and having partially differing strength properties in which a blank is heated up to a temperature that is higher than an Ac3 temperature, whereby the blank heated in this manner is subsequently fed into a forming die, where it is shaped and quenched, whereby it is preferably provided that partial zones of the shaped sheet part are merely annealed by controlling the temperature. In order to create a simpler and more efficient method, after being heated, the blank is partially cooled to a defined temperature, especially to a temperature below the Ac3 temperature, in an upstream conveying installation having upper and/or lower coolable conveying rollers.
Finally, it is also possible to weld different grades of steel together, so that unhardenable steel is present in the soft zones while hardenable steel is found in the hard zones. During the subsequent hardening process, the desired hardness profile is achieved over the entire part. The drawbacks of this process are the occasionally unreliable weld seam of the approximately 0.8 to 1.5 mm-thick Al—Si-coated sheet metal normally used for chassis parts, the abrupt hardness transition there as well as the increased costs of the sheet metal due to the additional production step of welding. During testing, failures occasionally occurred due to breakage in the vicinity of the weld seam, so that the process cannot be considered to be robust.
Before this backdrop, it is the objective of the invention to put forward a furnace system and a method for the controlled heat treatment of sheet metal parts which avoids the above-mentioned disadvantages.
The furnace system according to the invention lends itself for imparting a temperature profile to sheet steel parts, whereby a temperature above the Ac3 temperature is reached in a first area that is supposed to have an especially high hardness after the forming process, while a temperature below the Ac3 temperature is reached in a second area that is supposed to have a higher elongation at break than the first area after the forming process. In this context, it is accepted that the second area has a lower hardness than the first area.
The furnace system according to the invention has a production furnace for heating up the sheet steel parts as well as a profiling furnace in which the part can be imparted with a prescribed temperature profile at temperatures above the Ac3 for an area that is to be hardened and below Ac3 for a softer area. The profiling furnace comprises at least one level that has an upper section and a lower section as well as a receptacle for a product-specific intermediate flange and the product-specific intermediate flange installed therein. In this context, the product-specific intermediate flange is configured to impart the temperature profile to the part.
In a first embodiment, the furnace system has a conventional, universal production furnace for heating up the sheet steel parts to a temperature that is close to but below the Ac3 temperature. The profiling furnace has means to further heat up a selected area that is later going to be hardened at a temperature above the Ac3 temperature, while another area that remains less hard but that is supposed to have a higher elongation at break is kept below the Ac3 temperature.
In a second embodiment, the furnace system likewise has a conventional, universal production furnace for heating up the sheet steel parts, whereby this furnace lends itself for heating up the sheet steel parts to a temperature that is above the Ac3 temperature. For instance, the production furnace is configured to heat up the sheet steel parts at least to a diffusion temperature at which a coating diffuses deep enough into the steel matrix to later ensure corrosion resistance and good welding properties. The product-specific intermediate flange is configured to impart the part with a prescribed temperature profile at temperatures above the Ac3 temperature for areas that are to be hardened, and at temperatures below the Ac3 temperature for more ductile areas, that is to say, areas having a higher elongation at break. In this context, such more ductile areas normally have a lower hardness. For this purpose, the profiling furnace has means for maintaining a temperature above the Ac3 temperature in one selected area, while another area is brought to a temperature below the Ac3 temperature so slowly that the structure change that took place when the part was heated to the temperature above the Ac3 temperature is reversed. Here, the typical temperature gradient for the often-employed 22MnB5 steel is, for example, less than 25 K/s. With such a temperature gradient, the austenitic structure does not become a martensitic structure, but rather, it becomes a non-martensitic structure, for instance, a pearlite/ferrite structure that has a higher ductility at a lower hardness than a martensitic structure does.
In a preferred embodiment, the furnace system according to the first or second embodiment also has a positioning system on which the part can be placed in a defined position after it has been heated in the production furnace and/or in the profiling furnace. This ensures that the part is in a predefined position after it has been heated up in the production furnace or after it has been partially heated up in the profiling furnace. Then the part can be subsequently placed in a predefined position into the profiling furnace or into a press for the subsequent press-hardening process. The more precisely the placement position of the part can be adhered to, the less trimming work is needed for the finished, partially hard sheet metal part.
In an especially advantageous embodiment, the product-specific intermediate flange has means for actively cooling individual areas. In another advantageous embodiment, the cooling is effectuated by means of liquid cooling, for example, water or oil cooling.
In another particularly advantageous embodiment, the product-specific intermediate flange has means for heating an individual area or individual areas, whereby, in a special embodiment, these means are in the form of electric heaters. This makes it possible to systematically heat and/or cool individual, product-specific areas, so that the temperatures of these areas can be kept within narrow tolerance ranges. If individual areas are conveyed at a temperature above the Ac3 temperature to the subsequent press-hardening process, they become extremely hard. The other areas that undergo the press-hardening process systematically at a temperature below the Ac3 temperature will become considerably less hard and instead, they have a higher elongation at break. Electric heaters allow very precise temperature regulation.
It has been found to be advantageous for the production furnace according to the first or second embodiment to be heated by means of gas burners. This allows an especially economical heating of the parts. Since the method according to the invention in accordance with the first embodiment provides for the parts to be heated up in the production furnace only to a temperature below the Ac3 temperature and for the heat needed for heating defined areas to a temperature above the Ac3 temperature to be fed into the profiling furnace during a later process step, it is not necessary to have a very precise temperature regulation in the production furnace, so that the disadvantage of the less accurate regulation of gas burners in comparison to electric heaters is offset by the greater cost efficiency of gas as a cheaper energy carrier. This also applies to the furnace system according to the second embodiment. Here, the production furnace heats the part to a temperature above the diffusion temperature of a coating. Likewise here, there is no need for very narrow temperature regulation, provided that a temperature above the diffusion temperature is reached. More precise temperature regulation is only necessary in the next process step, in which a selected area of the part is partially cooled down to a temperature below the Ac3 temperature so slowly that the structure change that took place when the part was heated above the Ac3 temperature is reversed once again, while in another area, which is to become particularly hard later on, the temperature is kept at values above the Ac3 temperature.
In another advantageous embodiment, the furnace system according to the first or second embodiment has a production furnace which, as a continuous furnace, has a transport system to convey the parts through the production furnace. The cycle time for heating up the parts can thus be kept at the level of conventional heating furnaces used for the press-hardening process. If the subsequent process step of imparting the part with a temperature profile affects the cycle time so that the cycle time for the entire process is at risk of becoming prolonged, a profiling furnace with several levels can be employed in which the parts are partially further heated in parallel or partially in parallel. The parallel use of several profiling furnaces is also conceivable.
In order to keep the temperature tolerances on the part especially narrow during the controlled heating of individual areas, it has proven to be advantageous to regulate the temperature in a closed control circuit. For this purpose, in an advantageous embodiment, the profiling furnace has means for temperature regulation in a closed control circuit. Here, it is advantageously possible to also provide more than one control circuit.
It has proven to be particularly advantageous for the furnace system according to the first or second embodiment to also have a handling system for handling the parts. The handling system can place the parts quickly and systematically into the positioning system, can then remove them from the positioning system and can put them into the product-specific intermediate flange in the profiling furnace and take them out again. Moreover, the handling system can subsequently place the parts into a press die for the subsequent press-hardening. The use of a handling system minimizes the risk of injury to the operating personnel due to hot parts. A handling system executes the movements in defined and reproducible times, so that the parts can be placed with minimum temperature tolerances into the pressing die for the press-hardening, which has a positive effect on the quality of the part.
The method according to the invention is characterized by the following process steps:
In this process, in a first embodiment, the part is heated up in the production furnace to a temperature close to its Ac3 temperature and the temperature profile in the profiling furnace is achieved by the controlled further heating of the selected area to a temperature above the Ac3 temperature, while another area is kept at a temperature below the Ac3 temperature.
In a second embodiment, the part is heated up in the production furnace to a temperature above the diffusion temperature and thus also above the Ac3 temperature of a coating. At the end of the requisite temperature-dependent holding time, the heated part is positioned by means of a positioning system and the thus-positioned part is placed in a defined position into the profiling furnace, where a temperature profile is imparted to it. In this process, a selected area is kept at a temperature above the Ac3 temperature, while another area is cooled down to a temperature below the Ac3 temperature so slowly that the structure change that took place when the part was heated above the Ac3 temperature is reversed. Subsequently, the part that has been imparted with a temperature profile is removed from the profiling furnace.
It has proven to be advantageous for the part to be heated up in the production furnace by means of gas burners, whereby natural gas, for example, can be employed as the energy carrier.
In another advantageous embodiment, the positioned part is brought in a defined position into the profiling furnace by means of a handling system. The advantages of this are that the risk of injury to the operating personnel is minimized and that the process is rendered more robust due to the constant handling times. An advantage here is that such a system can be retrofitted into existent installations.
Advantageously, imparting a temperature profile to the part in the profiling furnace is regulated by means of a closed control circuit. This makes it possible to achieve very narrow temperature tolerances for the part, which has a positive impact on the quality of the press-hardened part. In this context, it has proven to be advantageous if, in order to impart the temperature profile, areas of the part that are to be hardened are systematically heated up via a product-specific intermediate flange to a temperature above the Ac3 temperature, while other areas that are supposed to display a higher extensibility in the finished part are kept at a temperature below the Ac3 temperature.
Other advantages, special features and practical refinements of the invention ensue from the subordinate claims as well as from the presentation below of preferred embodiments with reference to the figure.
The following is shown:
Instead of the above-mentioned robot, it is likewise possible to employ any other suitable handling system. In the embodiment shown in the figure, only a profiling furnace 40 with one level is described. However, it is likewise possible to have more than one level in the profiling furnace 40, whereby each level has an upper section and a lower section as well as a receptacle for a product-specific intermediate flange, so that several parts 5 can be imparted with a temperature profile in parallel or partially in parallel. By the same token, several profiling furnaces 40 can be provided in order to increase the capacity of the furnace system 1.
Although the invention has been described with a certain degree of particularity, it should be understood that those skilled in the art can make various changes to it without departing from the spirit or scope of the invention as hereinafter claimed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11157721 | Mar 2011 | EP | regional |
This application is a divisional application of U.S. Ser. No. 14/003,997, filed on Nov. 21, 2013 that was filed under 35 U.S.C. § 371 and which is a national phase application of International Application Serial Number PCT/EP2012/054139 filed Mar. 9, 2012, (copy and English translation, attached as Appendix 1), which claims priority to European Patent Application No. EP 11157721.9, filed on Mar. 10, 2011, the disclosures of all of the above applications being incorporated herein by reference, in their entireties for all purposes.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20090320968 | Boeke | Dec 2009 | A1 |
| 20110315281 | Charest | Dec 2011 | A1 |
| 20140083572 | Schwartz | Mar 2014 | A1 |
| Number | Date | Country |
|---|---|---|
| 10240675 | May 2004 | DE |
| 10350885 | Jun 2005 | DE |
| 102005032113 | Feb 2007 | DE |
| 102005051403 | Mar 2007 | DE |
| 102007012180 | Jun 2008 | DE |
| 102007057855 | Oct 2008 | DE |
| 102009016027 | Oct 2010 | DE |
| 102009051822 | Mar 2011 | DE |
| 2143808 | Jan 2010 | EP |
| 2009112938 | Sep 2009 | WO |
| Entry |
|---|
| PCT/EP2012/054139 International Search Report; completed Jun. 15, 2012. |
| PCT/EP2012/054139 International Preliminary Report on Patentability; date of completion Feb. 25, 2013. |
| Number | Date | Country | |
|---|---|---|---|
| 20170037489 A1 | Feb 2017 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14003997 | US | |
| Child | 15333352 | US |
