DE102012016075B4 - Method and device for producing a metal component - Google Patents

Abstract

A method for producing a metal component having at least two regions of different strength properties within the same component obtained by different cooling rates of the different regions is characterized according to the invention by the following process features: a metallic base body (1) having at least austenitizing temperature is provided, an area of the main body (1) is shielded during a subsequent application of heat to the main body (1) until the component temperature of this region has dropped to a predetermined temperature (T2) below the austenitizing temperature but still above the martensite starting temperature, wherein in this component region an average cooling rate below the critical rate of martensite formation prevails, while the unshielded area remains at least austenitizing temperature the shield (3) is removed, the graduated temperature profile base body (1) is subjected to a forming process by a tool, at least in the previously unshielded area a cooling rate above the martensite critical cooling rate. A device for carrying out the method has a shielding device (3) with one or more contoured cover elements (4) for shielding a predetermined region of a base body (1), preferably a circuit board, wherein the shielding device (3) for staying in the heating unit (2, 5) is designed during a temperature control of the base body (1) before its deformation in the forming tool.

Description

The invention relates to a method and a device for producing a metal component, which is to obtain different strength properties within the same component, according to the preambles of claims 1 and 6, respectively.

In many metal components, for example in the automotive industry, there is a requirement that the components on the one hand have a low weight, on the other hand have specifically adjustable strength ranges. So z. As a motor vehicle B-pillar have a hard center area to maintain high stability and on the other hand be ductile at the upper and lower attachment points to the body, so that in an impact introduced energy can be dissipated, if possible without causing tearing. To obtain such components, the different structure formation within a steel material is made use of when choosing different cooling rates. Thus, a high cooling rate leads to the predominant formation of hard martensite, while lower cooling rates favor other microstructures such as bainite or the preferred because of their toughness Ferit and perlite.

A base body, which is subsequently thermoformed in a preferably press-hardening process, can be pushed out of the oven, for example, after being heated to austenitizing temperature with one partial area, which then cools slowly in the air, while the other portion is kept in the oven at austenitizing temperature. The subsequent transformation leads to a hard area (which remained longer in the furnace and therefore still had Austenitisierungstemperatur) at a sufficient cooling rate and a softer area, which had already cooled a little further in the air and therefore in the further cooling by deformation in the microstructure of ferrit, perlite or bainite. A corresponding method with the additional use of a buffer furnace as a buffer is, for example, in the EP 2 365 100 A2 described. This method has the disadvantage that only a softer area can be created, a maximum of two, if the oven is very short, so that the component can protrude on both sides. In addition, the transition zone between soft and hard area in the finished component is often undesirably large. Complementary in the EP 2 365 100 A2 a buffer furnace shown having cooled contact surfaces on which the workpiece can be placed in order to extract heat in some areas. Here, however, the buffer oven must be specially tailored to the workpiece, which is very expensive.

In a further method, therefore, the base body is inserted into the forming tool at a uniform temperature, where it is shaped over differently cooled regions of the tool at different cooling rates. However, this often leads to warping of the components and thus to high reject rates.

Another alternative according to the EP 1 180 470 B1 Therefore, by partially covering the base body with a cover placed on it in the oven during the first heating before the forming process certain areas should not be heated up to the austenite. This can also be done by partial heat removal during the heating process according to DE 10 2006 018 406 B4 or WO 2010/109012 A1 done with absorption masses. The DE 10 2009 023 195 A1 also describes a method in which areas of a component are covered. However, this is done in the method described there on an already formed component. Accordingly, the shield is complicated. The shielding also leads here to the fact that the shielded areas are not heated to the austenite. However, these methods have disadvantages when, for example, coated materials are used. For example, in the case of an AlSi coating, thorough heating is required so that alloying and optimum bonding of the layers takes place. If this is not the case, the aluminum coating often adheres to the tool in the subsequent forming process, which leads to high tool wear.

From the DE 102 08 216 C1 For example, a method is known in which regions of an end component which are to have a higher ductility later are quenched to a cooling-stop temperature which is above the martensite start temperature. Then these areas are kept isothermal and later cooled together with the entire component in the hardening process. To quench the later ductile areas they must be subjected to a cooling medium and sealed off from the other areas. The process is therefore complicated. In addition, the result is not uniform, since the cooling medium itself is subject to a change in temperature. Homogeneity problems can therefore occur.

Finally, that describes DE 10 2010 048 209 B3 a method in which a board heated to austenitizing temperature is cooled in a sub-range, which receives a martensite and bainite mixed structure in the following forming process. The intermediate cooling, which is preferably carried out by integrated cooling plates in the forming tool itself, however, is expensive. In addition, that obtained in the subarea Mixture of martensite and bainite due to the high bainite content is not optimal for many applications.

The invention is therefore based on the object of specifying a method and a device which avoid the disadvantages of the previously known processes and in particular enable the production of components with more complicated contours between the regions of different strength properties. This object is achieved by a method having the features of claim 1 and a device having the features of claim 6. Claim 15 relates to a shielding as inventively essential component of the device.

In the method according to the invention, a metallic base body, in particular a sheet steel plate, which may also be coated, is heated to at least austenitizing temperature or made available at this temperature. In a subsequent process step, a region of the base body, which can also be composed of several subregions, is shielded against the action of heat by a shielding device accompanying the workpiece, while the base body continues to be subjected to heat. Thus, the unshielded area of the body is kept at Austenitisierungstemperatur while the shielded area controlled below the Austenitisierungstemperatur but above martensite starting temperature drops, wherein in this component area an average cooling rate below the martensite critical cooling rate prevails, so that the structure of this area after the subsequent forming consists mainly of ferrite and perlite, regardless of the subsequent cooling rate during forming. The cooling of the shielded areas should be done slower than a self-given cooling in air. After the removal of the shield, which may consist of several sub-elements, the base body, which then has a graded temperature profile obtained by the shield, subjected to a forming process by a tool, in particular a hot forming with thereby resulting press hardening. In this case, at least in the previously unshielded region of the base body, a cooling rate above the critical for martensite cooling rate is maintained, d. H. The cooling of the body is so fast that in the formerly unshielded area a structure of predominantly martensite arises. During the forming process, it is not necessary to set different cooling rates for the different areas. In particular, both areas of the base body can be cooled at a cooling rate greater than the critical cooling rate for martensite formation, since a martensite transformation can only occur in the previously unshielded area because of the partial cooling of the shielded area or shielded areas.

This method avoids all the disadvantages of those known in the art. Thus, both coated and uncoated materials can be used as the main body, since the starting temperature above Austenitisierungstemperatur a previous Durchlegierung of the entire body, whereby the forming tools are protected. Also, components with complicated contours for the different strength properties can be created, as they can be imaged by the design of the shield. Thus, it is possible, for example, to produce a component for a B-pillar with a hard center, a soft upper and lower connection region to the body and a circumferential soft flange, which considerably facilitates the subsequent shape cutting along the entire component in the flange region. In addition, complicated temperature control devices for the forming tool are unnecessary because the deformation can be done with a uniform cooling rate over the entire body, which also avoids the risk of warping of the components when removed from the mold.

Preferably, the shielding takes place contactlessly, at least over the predominant area of the area to be shielded, ie the base body should touch the shielding at as few points as possible. It is particularly advantageous if the main body is mounted on supports, kiln furniture, oven carriers or the like and the shield is completely contactless to the main body, since then the shield is less expensive and almost wear-free. However, the shielding device is workpiece accompanying and not furnace bonded. Preferably, the shielding takes place at least partially via radiation shielding, so that the heat radiation in the shielded areas can not penetrate to the base body. This is preferably even the predominant type of shielding. In order not to leave the temperature profile in the shielded areas of the main body to itself, the shield can be subject to an active temperature control, which is realized by a targeted temperature / cooling of the cover. This serves in particular to protect the shield itself, in order to avoid its excessive heating by the radiant heat of the body, the furnace and / or by the heat application of the rest of the body. Since the surface of the shield may preferably be made of aluminum, it is thus prevented that it melts. But the introduction of heat energy is possible if, for example, the shield for the desired temperature drop is too strong and even slower cooling is desired.

A device according to the invention is also claimed, the at least one heating unit, or a series main furnace z. B. a roller hearth furnace, a (multi-layer) chamber furnace or a reciprocating furnace, and a forming tool, in particular a press-hardening tool or a forming press. An essential part of the device and claimed separately in claim 15 in a special embodiment is a shielding device which has one or more contoured cover to shield the predetermined area of the body and both in terms of their material and in terms of their functional design is designed so that they are in the heating unit can be inserted and can stay there with the main body, while the main body is tempered before forming. If the production does not permit a direct adaptation of the shielding device in the series main furnace, an autonomous heating unit is provided according to the invention, into which the shielding device can be integrated. Thus, the heating to austenitizing temperature and the maintenance of the temperature in the unshielded areas of the body, while the shielded areas are already undergoing deliberate cooling, can be decoupled from the main heating unit, which makes sense even if the timing in the different areas takes different lengths.

The covering element (s) are preferably slidably or pivotally mounted on the shielding device, so that they can be moved over, under and / or around the area of the base body to be shielded, depending on the necessity and intended use. In this case, a shielding device does not have to allow different possibilities of movement of the cover elements, since a shielding device designed especially for its geometry is usually produced for each workpiece to be produced, as there is also a separate press hardening mold for each workpiece. Therefore, the furnace itself does not have to be adapted to the workpiece, but only the shielding device accompanying the workpiece. The cover or the elements are preferably held pivotably or displaceably on a guide element of the shielding. In particular, they can be arranged to be closed and reopened in the manner of a mold. This can be done for example via a rotary knob and a toothed rail, which translates the operation of the rotary wheel in a lateral and opposite movement and extension of the cover.

The device according to the invention and the shielding device can also be used advantageously in processes if the main body has not yet completely reached the austenitizing temperature before it is received in the shielding device, ie H. A thorough warming of the body is not yet complete. This can be, for example, at a temperature just below Austenitisierungstemperatur, z. B. austenitizing temperature minus 30 K. The shielded areas of the body then reach despite austenitizing temperature, while this sets in the unshielded areas despite continuous heating. As a result, reduced cycle times can be realized because the Temperierungsphasen be shortened. Without disadvantage this can be used with uncoated base materials and even with coated base materials whose coating does not require alloy, eg. B. Zinc-based coating systems.

Arranged by a cover element or cover elements of constant thickness and at a constant distance from the surface of the base body, a transition region in which the material properties pass from those of one area to that of the other area is formed on the separating contour of the cover element in the finished workpiece. The methods of the prior art resulted in a mostly relatively large transition region. By contrast, a very small transitional area of, for example, only 15 to 25 mm can be achieved via the shielding device according to the invention with a sharp-edged contour of the cover elements. However, if a wider transition range is desired, this can also be adjusted with the device according to the invention by varying the thickness of the cover element - and thus the shielding effect - in the direction of the transition zones. A thus targeted adjustment of temperature and strength gradients after final shaping and cooling in the forming tool can also be done by varying the distance of the cover to the main body in the shield. The variation of the distance and the thickness of the cover can also be used together.

A targeted influencing of the component strength of the component to be produced can in particular also take place in that the cover element or the cover elements have an active temperature control, in particular active cooling. In addition, the extraction temperature can be adjusted specifically. In particular, by such a controllable and controllable time-dependent temperature control in the shielding significantly higher insertion temperatures and the shielded and thus already partially cooled areas of the body when inserted into the forming tool can be selected, whereby both an abrasive and a reduced adhesive wear and the formability is significantly improved. Increased degrees of deformation are thus also possible in the areas which should have softened properties after the deformation.

In order to control and, if appropriate, purposefully influence the cooling process during the residence of the base body in the shielding device, it is advantageous if it has an integrated temperature sensor which can be integrated in particular in the cover elements. In addition, the current temperature of the recorded body is to be determined in these areas. If a coupling of the temperature sensor with the active temperature control of the cover, a time-dependent temperature control can be automatically controlled and regulated.

Further advantages and details emerge from the claims and the figures, the u. a. contain preferred embodiments of the invention and are described below; show it:

1 schematically a ZTU diagram of the cooling processes according to the invention,

2 a sample board before the forming process,

3 schematically different process stages,

4 a perspective view of a heating unit with shielding device shown in front,

5 the decoupling of process steps through the use of a self-sufficient heating unit,

6 the shielding device according to the invention in different views and partly different scales.

In the diagram 1 are the different processes of cooling and thus microstructure of the metal component over time, but offset from one another. The unshielded region of the component undergoes a temperature profile along the curve A, in that it is kept above the austenitizing temperature after being heated to at least austenitizing temperature and during the further heat application. The essential cooling of this area takes place only in the forming tool at a cooling rate above the critical for martensite cooling rate (27 to 30 K / s), whereby a targeted martensite takes place. Thus, strength characteristics result in this range of about Rm 1300 MPa-1650 MPa, Rp0.2 1000 MPa-1250 MPa, hardness 400-520 HV and ε> 15%. The further region of the main body or the other regions which are shielded during the application of heat experience a temperature profile along the curve B and are cooled on removal from the shielding to a temperature below the Austenitisierungstemperatur but still above the Martenitstarttemperatur and have this cooling in one Experience speed below the martensite critical cooling rate. As a result, in the subsequent rapid cooling in the forming tool no predominantly martensitic structure is achieved, but rather a structure with a high ferrite and / or pearlite content is formed. For example, after forming, these areas have strength properties of the order of magnitude Rm output properties - 1300 MPa, Rp0.2 output properties - 1000 MPa, hardness 100-400 HV and ε> 15%.

(Note to 1 : To illustrate the structure, curves A and B are shown in the same diagram. Of course, however, both areas of the base body and later metal component linger for the same length of time in the application of heat, only just shielded area B and not shielded area A. The time line for the curve A therefore begins practically further to the left in the region of the graph which is no longer shown.)

2 shows a main body 1 after removal from the shielding and before in a further not shown forming. The main body 1 here has the form of a board with locally graded temperature profiles, namely a temperature range T1 and a temperature range T2, where T1 is greater than T2, since the area T1 was not shielded. Based on this board 1 the method according to the invention or the device according to the invention is also illustrated in the following figures.

3 shows in the first position the homogeneous heating of the board 1 in a heating unit 2 , here a series furnace of the production line, where appropriate, a permeation of a coating takes place when it was applied. In front of the stove 2 there is a shielding device 3 in the open state, in this case with two cover elements 4 , At the second position is the shielding device 3 over the board 1 in the oven 2 driven to set the desired temperature in the covered board area. position 3 shows the removal of the shielding 3 including the board 1 from the oven 2 , In fourth position, the shield is again shown open and gives the board 1 with graded temperature ranges, as in 2 shown, free for the subsequent forming process.

4 shows the process stage 3 out 3 in perspective view while in 5 for decoupling the process to form different temperature ranges in the board a self-sufficient heating unit 5 separate from the oven 2 is provided. Schematically are in the self-sufficient heating unit 5 heaters 6 recognizable and arranged below shielding 3 , This is with the included board 1 in more detail in the different views 6 shown. In particular, there is the contoured shape of the cover 4 to see and their sliding movable arrangement on a guide element 7 , In particular section BB shows that the cover elements 4 the board 1 in the areas to be shielded above and below and thus reduce heat radiation from both sides.

Particularly advantageous in the device according to the invention is that the shielding 3 can be integrated into existing furnace systems or coupled to them. Can an integration in the series main furnace 2 do not effect, or this is not desirable, the process can still with a self-sufficient heating unit 5 be performed. This can also be designed as a heating-cooling unit, in particular when the cover elements 4 the shielding device 3 an active temperature, preferably to have an active cooling, the temperature grading of the component 1 before the forming and thus the strength properties of the finished metal component to influence more targeted.

Claims (15)

Method for producing a metal component having at least two regions of different strength properties within the same component obtained by different cooling rates of the different regions, characterized by the following process features: a metallic base body ( 1 ) having at least austenitizing temperature is provided, - an area of the basic body ( 1 ) is during a subsequent heat application of the body ( 1 ) until the component temperature of this region has dropped to a predetermined temperature (T2) below the austenitizing temperature but still above the martensite start temperature, in which component region an average cooling rate below the martensite critical cooling rate prevails, while the unshielded region continues to exist at least austenitizing temperature is maintained, - the shielding ( 3 ) is removed, - the main body ( 1 ) with graded temperature profile is subjected to a forming process by a tool, wherein at least in the previously unshielded region a cooling rate above the critical for martensite cooling rate prevails. Method according to Claim 1, characterized in that the drop in the temperature of the basic body ( 1 ) in the shielded area slower than the material-specific cooling characteristic of the body ( 1 ) in ambient temperature. Method according to one of the preceding claims, characterized in that the shield takes place without contact at least over the predominant surface of the region to be shielded. Method according to one of the preceding claims, characterized in that the shielding takes place at least partially, preferably even predominantly via radiation shielding. Method according to one of the preceding claims, characterized in that during the shielding in the covered area a preferably controlled active temperature control, in particular cooling takes place. Device in particular for carrying out the method according to one of claims 1 to 5, with a heating unit ( 2 . 5 ) and a forming tool and a shielding device ( 3 ) with one or more cover elements ( 4 ) for shielding a predetermined area of a base body ( 1 ), preferably a circuit board, wherein the shielding device ( 3 ) to stay in the heating unit ( 2 . 5 ) is designed during a temperature control of the base body ( 1 ) before its forming in the forming tool, characterized by a self-sufficient heating unit ( 5 ) into which the shielding device ( 3 ) is integrally formed. Device according to claim 6, characterized in that the cover element ( 4 ) on the shielding device ( 3 ) is slidably and / or pivotally mounted and under, over and / or around the area of the body to be shielded ( 1 ) is arranged movable. Device according to one of claims 6 or 7, characterized in that the shielding device ( 3 ) at least two at least one guide element ( 7 ) sliding and / or pivotally supported cover elements ( 4 ) having. Device according to claim 8, characterized in that the cover elements ( 4 ) about the Guide element ( 7 ) are arranged closable and openable in the manner of a mold Device according to one of claims 6 to 9, characterized in that the cover elements of the shielding device are formed contoured. Device according to one of claims 6 to 10, characterized in that the distance of the cover ( 4 ) to the main body ( 1 ) and / or the thickness of the cover element ( 4 ) to the areas of different tempering of the body ( 1 ) separating contour changes, in particular the thickness is reduced. Device according to one of claims 6 to 11, characterized in that the cover ( 4 ) has an active temperature control. Device according to one of claims 6 to 12, characterized in that the shielding device ( 3 ) has a particularly integrated temperature sensor for determining the current temperature of the recorded base body ( 1 ) in at least one area. Apparatus according to claim 12 and 13, characterized by a coupling of the temperature sensor with the active temperature control. Shielding device ( 3 ) according to any one of claims 12 to 14.

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