WO2010089103A1

WO2010089103A1 - Method and furnace for making a metal workpiece with regions of different ductility

Info

Publication number: WO2010089103A1
Application number: PCT/EP2010/000664
Authority: WO
Inventors: Franz Divjak; Berthold Scheuring
Original assignee: Magna IHV Gesellschaft fur Innenhochdruck Verfahren mbH
Current assignee: Magna IHV Gesellschaft fur Innenhochdruck Verfahren mbH
Priority date: 2009-02-03
Filing date: 2010-02-03
Publication date: 2010-08-12
Anticipated expiration: 2011-08-03

Abstract

The invention relates to a method of heating at least two blanks for making a metal workpiece having regions of greater and lesser hardness comprising the steps of moving a first blank or blank batch and a second blank or blank batch offset from said first blank or blank batch in a furnace on a first transport mechanism, heating said at least first and second blank or blank batches in the continuous furnace. The invention also relates to a furnace for heating at least two blanks or blank batches comprising a blank feed, a furnace portion comprising a first transport mechanism for transporting the at least two blanks or blank batches offset from one another, and a furnace outlet for receiving the at least two blanks or blank batches from said furnace portion and for transporting the at least two blanks. The object of the present invention is to provide an improved method and furnace that is less complex than the prior art. It is suggested to provide a method that is characterized by moving said at least first and second blanks or blank batches independently from each other to a furnace outlet on at least a second and third transport mechanism, as well as by a furnace that is characterized in that said furnace outlet comprises at least a second and third transport mechanism.

Description

METHOD AND FXJRNACE FOR MAKING A METAL WORKPIECE WITH REGIONS OF DIFFERENT DUCTILITY

[0001] The present invention generally relates to the field of steel parts and a method of making the same. More particularly, the present invention relates to a steei part with regions of different ductility and/or hardness as well as a method and an oven for making the same.

[0002] The present invention pertains to a method of heating at least two blanks for making a metal workpiece having regions of greater and lesser hardness, the invention having the features of the preamble of claim 1. The invention also pertains to a furnace for heating at least two blanks or blank batches having the features of the preamble of claim 11.

[0003] It is known to produce tool-hardened shaped structural parts for motor vehicle components, for example drive components like steering rods or cross bars, or structural components like door impact beams, B-columns, struts or shock absorbers which have material properties which are distributed uniformly over the shaped bodies. This is done by completely hardening the shaped body which can be done in conjunction with an annealing or optionally a tempering process. These parts should have, on the one hand, a high strength so that they remain stable in, for example, a crash. On the other hand these parts should also be deformable in a crash so that the crash energy can be absorbed as deformation energy. In various applications in motor vehicle technology, shaped components should have a high strength over certain regions and other regions with a high ductility. For example, in the case of a B-column, the column foot would advantageously be relatively ductile, while high strength properties would be advantageously established on the upper part of the column.

[0004] Aside from reinforcement with additional plates or by joining together parts of different strength, it is already known to treat a structural component by heat treatment so that it has local regions of higher strength or higher ductility.

[0005] Thus, DE 197 43 802 C2 describes a process for producing a shaped article for motor vehicle components with regions of different ductility and with a starting billet, before or after pressing is only partially heated or starting from a prior homogeneous heating is subsequently heated in a targeted manner in the regions at which higher ductility is desired. The subsequent heating for producing ductile regions has however the drawback that the shaped body can distort.

[0006] DE 197 23 655 Al describes a process for the partial hardening of a shaped body whereby a starting billet is homogeneously heated in a furnace and then hardened in a cooled pair of tools, whereby partial regions of the workpiece have hardening inhibited by slower cooling in that at these regions in the tool, recesses or thermally insulating inserts are disposed or in these regions in the tool the induction heating is applied. The purpose of this process is to provide non- hardened regions in the shaped body at which, additional machining, for example, drilling can be carried out. The method of DE 197 23 655 Al is problematical in the context of a hot-forming process since at the locations of the recesses in the tool, shaping cannot occur and with larger ductile regions, thermally insulating inserts are provided in the tool which limit the hardening and interfere with the shaping process so that breakage is possible. The inductive hardening is possible only with finish-shaped parts and requires certain intrinsic operating steps. As a consequence the subsequent inductive hardening is expensive and has the danger of resulting in distortion.

[0007] European Patent EP 0 816 520 Bl describes a shaped article and a method for providing desired strength and hardening patterns over its length, whereby the shaped body, after its shaping, is inductively heated and then quenched to produce the hardened regions.

[0008] DE 200 14 361 Ul describes a B-column which also has regions of different strengths. The formation of the B-column is effected in a hot-forming process whereby starting from a blank or a preformed longitudinal profile, the workpiece is austenitized in a furnace and then shaped and hardened in a cooled tool. In the furnace large-area regions of the workpiece are insulated against the effect of the temperature whereby in these regions the austenitization temperature is not reached and as a consequence during the hardening no martensitic structure arises in the hardening and shaping tool.

[0009] Alternatively, DE 200 14 361 Ul proposes to initially completely austenitize a pre-formed blank or longitudinal section and, during the transport to the hardening tool, to limit the cooling rate in a targeted manner in a region so that it is not excessively rapid, for example by blowing to bring it to a temperature clearly below the austenitization temperature. In the hardening tool there may thus no purely martensitic structure formation but rather the formation of a mixed structure with clear ferrite/bainite components which has ductile properties.

[0010] DE 10 2008 055 980 Al and the corresponding EP 2 1 10 448 A2, both being published later than the priority of this application, that is U.S. Patent Application No. 61/149.533 filed on February 3, 2009, disclose a method and a continuous furnace for heating workpieces. The workpieces are heated in said furnace. When a workpiece arrives at an outlet end of the furnace in a manner that the workpiece partially projects from the oven outlet to have contact with ambient air, a motion of a transport device for transporting the workpiece through the furnace is interrupted for a predetermined amount of time so that the portion of the workpiece that projects from the oven outlet is allowed to slowly cool by the contact to ambient air while the workpiece portion that remains within the furnace is kept at or further heated to the austenitization temperature of the hardenable workpiece material. Said documents disclose that the transport mechanism's motion can be interrupted either by decoupling the workpiece from a roll mechanism of the transport device by means of push rods lifting the workpiece from the roll, or by providing a separate transport mechanism extending throughout the furnace for each workpiece, which transport mechanism can be controlled to stop separately and transports the workpiece from one end of the furnace to the other end.

[0011] The manufacture of workpieces as described above often required that forming tools were fed with workpieces within a matter of fractions of a minute, for instance, one-half to one-quarter of a minute, in order to technically and economically optimize the output of a production line. The subject matter of the above-identified patent family of DE 10 2008 055 980 Al tries to address this problem by moving several workpieces in parallel through the furnace at the same time and to buffer said workpieces at the furnace end for desired removal of the workpieces from the furnace end. Both solutions rendered by said patent family, however, make the respective furnace design technically complicated, complex and expensive, as either a push rod mechanism has to be installed or a number of separate transport mechanisms, such as roller mechanisms, have to be installed within the furnace in accordance with the number of workpieces to be simultaneously heated.

[0012] It is an object of the present invention to provide an improved method of heating and an improved furnace to inhibit the problems delineated hereinabove. [0013] It is suggested to provide a method of heating in accordance with claim 1. This simplifies the method of heating in comparison to the prior art and allows better control of the workpieces at the furnace outlet.

[0014] The first and/or the at least second and/or third transport mechanisms may be a roller mechanism. Roller mechanisms are an appropriate transport mechanism for transporting metal workpieces through a furnace in connection with the invention, as roller mechanisms are durable and reliable.

[0015] The furnace may be a continuous furnace through which the at least first and second blank or blank batches are moved. Continuous furnaces may be advantageous, for instance, when the inventive method is used and conducted in a line production.

[0016] The at least first and second blank or blank batches may be heated to austenitization temperature. In order to allow a microstructural transformation of at least parts of the workpiece, it may be advantageous to heat said workpieces to austenitization temperature, so that hardening effects in at least some portions of the workpiece may be achieved upon rapid cooling or quenching in subsequent process steps.

[0017] At least one of said at least first and second blank or blank batches may be centered in a centering position at said furnace outlet. This may allow controlling the blank position at the outlet and to thereby better control the position of the blank at the furnace outlet, which may influence the position of heated and cooled portions of the blank.

[0018] At least one of said first and second blank or blank batches may be moved at least partially out of a furnace portion to said furnace outlet and then partially back into a cooling position at said outlet in which cooling position a portion of a respective blank is allowed to cool. By performing such an operation, the position of the blank relative to the furnace outlet, and therefore, the location of heated, cooled and transition regions on the blank, may be better controlled.

[0019] Each respective blank may be moved further into the furnace outlet in the centering position than a cooling position. This may allow centering each blank to achieve a desired distribution between heated and cooled portions of the blank or to align its position before it is transferred to a process situation.

[0020] The at least second and/or third transport mechanisms may be operated while the first transport mechanism of the furnace may be stopped or slowed. This may allow flexible management of the logistics at the furnace outlet. For instance, it may allow handling said at least first and second blank or blank batches at the furnace outlet and may allow slowing or stopping a further supply of heated blanks to the furnace outlet until those blanks present at said furnace outlet are removed.

[0021] The first transport mechanism may be operated together with said at least second or third transport mechanism while the other of said third or second transport mechanism is stopped or accelerated. This may allow more flexible control of the output of the furnace and handling of heated workpieces within the furnace and at the furnace outlet. It would allow, for instance, holding one blank or blank batch at the second transport mechanism while the first transport mechanism and the third transport mechanism together transport another blank to the furnace outlet.

[0022] It may be possible to move a respective blank or blank batch faster on at least one of said second and third transport mechanism than on the first transport mechanism. This might allow more handling time at the furnace outlet and an increased distance to the subsequent blank.

[0023] The present invention may also provide a method of making a metal workpiece having regions of greater and lesser hardness comprising the steps of moving a first blank through a continuous furnace on a first roller mechanism, moving a second blank offset from the first blank through the continuous furnace on the first roller mechanism, heating the first and second blanks to austenitization temperature in the continuous furnace, and moving the first and second blanks independently from each other to a furnace outlet on at least a second and third roller mechanism.

[0024] It is further suggested to provide a furnace for heating at least two blanks or blank batches according to claim 1 1. Such a furnace is simplified and less complex than the furnaces known from the prior art because the furnace outlet now comprises at least a second and third transport mechanism for transporting the at least two blanks while there is one transport mechanism inside the furnace portion.

[0025] At least one of said transport mechanisms may be a roller mechanism. Roller mechanisms may be advantageous because they are durable and reliable.

[0026] The at least second and third transport mechanisms may operate independently from each other. This can simplify the handling of blanks at the furnace outlet and may allow operating the furnace outlet more flexibly. "Independently" shall mean that said transport mechanisms may be controlled separately so that blanks thereon may be moved differently from each other.

[0027] For instance, a centering device may be provided at said furnace outlet. Using a centering device may allow increasing the precision as the blanks may be centered prior to moving them back into the furnace for partial cooling, and/or prior to transferring the heated blanks to subsequent process stations such as forming tools.

[0028] For instance, said at least second and third transport mechanisms may be operated at different speeds than the first transport mechanism. This can allow increasing the flexibility in handling the blank at the furnace outlet because handling time may be saved due to moving the blanks faster on the at least second and third transport mechanisms.

[0029] For instance, the at least second and third transport mechanisms may be operated in different directions at the same time. This may allow moving blanks on said second and third transport mechanisms back and forth at the furnace outlet and may even allow at least partially reinserting blanks into the furnace.

[0030] For instance, said first transport mechanisms may be operated independently from each of said further transport mechanisms. In this context, "independently" shall mean that the first transport mechanism may be operated differently from each of said further transport mechanisms insofar as each further transport mechanism may be operated faster or slower than the first transport mechanism and may operate in different directions. However, the operation of all transport mechanisms may mainly be dependent on the loading pattern, for instance, the offset and number of tracks of the furnace. Operating said first transport mechanism independently/differently from each of said further transport mechanisms may allow more flexibly handling blanks in said furnace.

[0031] There may also be provided a continuous furnace for heating at least two blanks comprising a blank feed, a continuous furnace portion for transporting the at least two blanks offset from one another on at least 2 oven tracks, said at least two blanks being transported on a first roller mechanism, and a furnace outlet for receiving the at least two blanks from the continuous furnace and for transporting the at least two blanks on at least a second and third roller mechanism.

[0032] Exemplary embodiments of the invention will now be described in conjunction with the following drawings wherein like numerals represent like elements, and wherein:

[0033] Fig. 1 shows a schematic exemplary presentation of a process in accordance with the invention;

[0034] Fig. 2 shows a schematic example of an oven/furnace and an oven/furnace outlet in accordance with the present invention at a first point in time;

[0035] Fig. 3 shows a schematic example of an oven/furnace and an oven/furnace outlet in accordance with the present invention at a second point in time;

[0036] Fig. 4 shows a schematic example of an oven/furnace and an oven/furnace outlet in accordance with the present invention at a third point in time;

[0037] Fig. 5 shows a schematic example of an oven/furnace and an oven/furnace outlet in accordance with the present invention at a fourth point in time;

[0038] Fig. 6 shows an example of a formed workpiece having regions of different ductility in accordance with the invention. [0039] In accordance with the present invention, a workpiece may be provided having regions of different ductility. The present invention may also provide a method and an oven to produce such a workpiece.

[0040] Advantageously, the present invention provides an economic method and an apparatus for producing workpieces with regions of different ductility having reduced cycle times. The terms "furnace" and "oven" are used interchangeably in the entire disclosure.

[0041] Fig. 1 shows a schematic presentation of a process in relation with the invention in which a furnace 1 is provided to completely heat a blank 2, for instance, to austenitization temperature, as shown in a first phase indicated with reference numeral 3 in Fig. 1. In a second phase, indicated with reference numeral 4 in Fig. 1 , a portion of the blank 2 protrudes from a furnace outlet 5 so that the portion projecting from the furnace outlet 5, such as a lower part 6 of a blank, is allowed to slowly cool while an upper part, such as indicated by the reference numeral 7, stays within the furnace and is kept at a desired temperature, such as austenitization temperature. The lower part 6 of the blank 2 may correspond in a particular example to the toe area 8 of a B-pillar 9 for a motor vehicle while an upper part 7 may correspond to longitudinal pillar structure of the B-pillar 9. The lower part 6 thereby forms an area of cooling 11, for instance, cooling by air cooling. An exemplary workpiece is described in connection to Fig. 6 hereinafter.

[0042] Further referring to Fig. 1 , the blank 2 is then transferred to a process station, for instance, a hot stamping tool that forms and rapidly cools, i.e. quenches the blank in a die 14 in order to obtain a workpiece having regions of greater and lesser hardness.

[0043] The workpieces are made from hardenable steel. The basic process to produce such workpieces may accordingly comprise the steps of heating a blank 2 in an oven or furnace 1 to an austenitization temperature, then transport a predetermined portion of the blank 2 outside the oven or furnace 1 so that this portion of the blank 2, e.g. lower part 6, is allowed to air cool for a predetermined amount of time while the remaining portion of the blank 2, i.e. upper part 7, remains in the oven at austenitization temperature, which can be about 920 to 950° C but depends on the material of the blank 2. After a portion of the blank 2 was allowed to air cool, the blank 2 is transferred to a hot stamping tool 13 where the blank 2 is formed and allowed to cool. This results in a formed workpiece with regions of different ductility. Examining the microstructure of the formed workpiece after cooling shows that the portion of the workpiece that remained in the oven or furnace 1 , e.g. upper part 7, at austenitization temperature results in a martensitic structure, whereas the portion of the workpiece that was allowed to air cool, e.g. lower part 6, shows a different microstructure, for example martensite can be mixed with varying amounts of bainite and/or ferrite, depending on the amount of time that it was allowed to air cool. It may be advantageous to allow the workpiece to remain at a temperature over about 250° C upon cooling so that annealing effects may be obtained.

[0044] Fig. 2 shows a schematic example of an oven/furnace and an oven/furnace outlet 5 in accordance with the present invention at a first point in time. As can be seen, one or more blanks 2 are placed offset from each other on at least two oven tracks 15 in the furnace portion 16 and are then heated to austenitization temperature in a furnace portion 16 (dark grey area). The furnace 1 may be a continuous furnace, such as a roller hearth furnace for example. The blanks 2 are transported through the oven by means of one transport mechanism, such as one set of rollers. It is possible to use other transport mechanisms such as conveyors instead of rollers.

[0045] Fig. 2 also shows that the furnace 1 comprises a blank feed 17 where the blanks 2 are inserted. Fig. 2 through Fig. 5 also show that not only single blanks but also blank batches 19 may be inserted into the blank feed and heated in the furnace 1. An arrow denoted with the numeral 18 indicates the process flow direction from the blank feed 17 through the furnace portion 16 to the furnace outlet 5.

[0046] The blanks 2 or blank batches 19 are numbered by the white digits shown inside each of the black boxes indicating the blanks 2.

[0047] Having regard to Fig. 3, the blanks 2 of batch No. 1 are subsequently moved to the oven/furnace outlet 5 in this example on a divided roller system 20, forming the second and third transport mechanisms, and centered in a centering device 21. At the oven/furnace outlet 5, there is a divided roller system 20, for example two or more different roller tracks are provided, so that the blanks 2 arriving from the at least two different oven tracks 15 can be separately moved in and out of the furnace 1 for the purpose of centering the workpieces for partial air cooling and subsequently for hot stamping. The furnace outlet 5 and the transport mechanisms at the furnace outlet comprise respective drives and controls to move the blanks 2 back and forth. [0048] Having regard now to Fig. 4, a portion of the blanks 2 or blank batches 19 is subsequently moved back into the furnace 1 from the oven/furnace outlet 5 so that the portion of the blank 2 that is moved back into the furnace 1 is kept at austenitization temperature and the portion of the blank 2 that is protruding into the oven/furnace outlet 5 is allowed to air cool for a predetermined amount of time. It is noted that the blanks 2 are now on the divided roller tracks 29 of the oven outlet.

[0049] Turning now to Fig. 5, blanks 1 are completely moved out of the furnace again on the divided roller system 20 at the oven outlet where they are centered and subsequently will be hot stamped in the hot stamping tool 13 comprising the die 14. The die 14 is cooled so that the blank 2 or workpiece, respectively, can be quenched, i.e. rapidly cooled for hardening during the forming operation of the hot forming tool 13.

[0050] In Figs. 3 and 5, it is also shown that the blanks 2 or blank batch 19 of blank batch No. 1 are in the centering device 21 where they are in a centering position. In said centering position, both blanks 2 of the blank batch 19 are moved further out from or into the furnace outlet 5 than in the cooling position, which is assumed and indicated by the blanks 2 or blank batch 19 of the group No. 2 shown in Figs. 3 and 5.

[0051] In order to transfer said blank batches 19 or blanks 2 into the respective centering or cooling positions, the transport mechanisms of the furnace outlet 5 can be driven in opposite directions and independently from each other. That means, once the heated blank batch 19 can be handled by the transport mechanism of the furnace outlet shown on the right in Fig. 2 through Fig. 5 may move them out of the furnace independently of the operation of the transport mechanism of the furnace outlet shown on the left in said figures. That means that the transport mechanisms may operate differently in different directions and/or at different speeds. The supply of additional heated blanks, for instance, blank 2, group No. 3, may be considered when controlling the second and third transport mechanism at the furnace outlet.

[0052] It is possible to stop or slow down the first transport mechanism in the furnace portion 16 in order to allow for sufficient cooling time of the blanks 2 of the group Nos. 1 and 2, as shown in Fig. 4. Also, it may be possible to operate the first transport mechanism together with the second transport mechanism on the left in Figs. 2 to 5 in order to move the blank batch 19, blank group Nos. 2 and 3, while blank group No. 1 is being centered in the centering device 21 of the right track 22 of the furnace outlet 5, as shown in Fig. 5.

[0053] Further, the second and third transport mechanisms at the furnace outlet 5 may operate faster than the first transport mechanism inside furnace portion 16 so that handling of blanks 2 at the furnace outlet 5 may be carried out quickly and in a time saving manner while the first transport mechanism may run slower so that the furnace portion 16 may be designed to be shorter.

[0054] In accordance with the instant invention, the oven or furnace outlet 5 is provided with a divided roller mechanism to receive the blanks from the different oven tracks 15. While passing through the oven or furnace 1 , the blanks 2 are offset from each other and are moved on separate tracks 15 but on the same set of rollers or roller mechanism. The divided roller transport mechanism at the oven or furnace outlet 5 allows reducing the cycle time and hence is more economical.

[0055] As a result of the slowed cooling process in a portion of the workpiece, the microstructure is allowed to develop that has an advantageous crash load, i.e. it can achieve better strain values. Thus a workpiece can be formed with at least two different material properties at different locations of the workpiece.

[0056] Having regard to Fig. 6, an example of a workpiece with tailored properties in accordance with the present invention is presented. Thus, the toe area 8 of a B-pillar 9 was partially air cooled before the hot stamping process was performed, resulting in a toe area 8 with decreased hardness but increased ductility in comparison to the remaining portion, i.e. longitudinal B-pillar structure 10, of the B-pillar that was maintained at austenitization temperature.

[0057] It should be appreciated that the foregoing description is illustrative in nature and that the present invention includes modifications, changes, and equivalents thereof, without departure from the scope of the invention, which is defined in the claims.

Claims

ClaimsWhat is claimed is:

1. Method of heating at least two blanks (2) for making a metal workpiece having regions of greater and lesser hardness comprising the steps of: moving at least a first blank (2) or first blank batch (19) and a second blank (2) or second blank batch (19) offset from said first blank (2) or blank batch (19) in a furnace (1) on a first transport mechanism; heating said at least first and second blanks (2) or blank batches (19) in the furnace (1); characterized by moving said at least first and second blank (2) or blank batches (19) independently from each other to a furnace outlet (5) on at least a second and third transport mechanism.

2. Method according to claim 1 , characterized in that the first and/or the at least second and/or third transport mechanisms is/are a roller mechanism.

3. Method according to one of the preceding claims, characterized in that the furnace (1) is a continuous furnace through which the at least first and second blank (2) or blank batches (19) are moved.

4. Method according to one of the preceding claims, characterized in that said at least first and second blank (2) or blank batches (19) are heated to austenitization temperature.

5. Method according to one of the preceding claims, characterized by the step of centering at least one of said at least first and second blank (2) or blank batches (19) in a centering position at said furnace outlet (5).

6. Method according to one of the preceding claims, characterized by moving at least one of said first and second blank (2) or blank batches (19) at least partially out of a furnace portion (16) to said furnace outlet (5) and then partially back into a cooling position at said outlet (5) in which cooling position a portion of the respective blank (2) is allowed to cool.

7. Method according to one of the preceding claims, characterized in that each respective blank (2) is moved further into the furnace outlet (5) in a centering position than in a cooling position.

8. Method according to one of the preceding claims, characterized by operating said at least second and/or third transport mechanisms while said first transport mechanism is stopped or slowed.

9. Method according to one of the preceding claims, characterized by operating said first transport mechanism together with said at least second or third transport mechanism while the other of said third or second transport mechanism is stopped or accelerated.

10. Method according to one of the preceding claims, characterized by moving said respective blank (2) or blank batch (19) faster on at least one of said second or third transport mechanisms than on the first transport mechanism.

1 1. Furnace (1) for heating at least two blanks (2) or blank batches (19) comprising: a blank feed (17); a furnace portion (16) comprising a first transport mechanism for transporting the at least two blanks (2) or blank batches (19) offset from one another; and a furnace outlet (5) for receiving the at least two blanks (2) or blank batches (19) from said furnace portion (16) and for transporting the at least two blanks (2) or blank batches (19), characterized in that said furnace outlet (5) comprises at least a second and third transport mechanism.

12. Furnace (1) according to one of the preceding claims, characterized in that at least one of said transport mechanisms is a roller mechanism.

13. Furnace ( 1 ) according to one of the preceding claims, characterized in that said at least second and third transport mechanisms can operate independently from each other.

14. Furnace (1 ) according to one of the preceding claims, characterized in that a centering device (21) is provided at said furnace outlet (5).

15. Furnace (1) according to one of the preceding claims, characterized in that said at least second and third transport mechanisms can be operated at different speeds than said first transport mechanism.

16. Furnace (1) according to one of the preceding claims, characterized in that said at least second and third transport mechanisms can be operated in different directions at the same time.

17. Furnace ( 1 ) according to one of the preceding claims, characterized in that said first transport mechanism can be operated independently from each of said further transport mechanisms.