EP3160667B1 - Method and forming tool for heat-forming, and corresponding work piece - Google Patents

Description

The present invention relates to a method for hot forming and in particular partial press hardening of a workpiece, in particular a board, made of sheet steel, in which the workpiece is heated before forming, in which then the heated workpiece in a forming tool, the two tool components in the form of a die and a stamp, is thermoformed by contact with both tool components hot, and in which then the formed workpiece is at least partially directly exposed to a surrounding the forming tool gas and thereby cooled.

Furthermore, the invention relates to a forming tool for hot forming and in particular partial press hardening of a workpiece, in particular a board, made of sheet steel, in particular for carrying out the method defined above, with two tool components in the form of a die and a punch, between which the workpiece can be arranged and the relative are mutually movable, wherein the one of the tool components has a relative to the rest of the tool component movable tool component part, which is movable away in the compressed state of the two tool components of the other of the tool components.

Finally, the invention relates to a wheel rim for a vehicle rim, which / is produced by the inventive method.

A vehicle wheel is a safety component and must therefore be able to withstand high mechanical alternating stresses during operation. Conventional steel wheels consist of a wheel disc, which ensures the connection to the wheel hub, and a rim tape for tire receiving, which is circumferentially connected to the wheel rim. The wheeled dishes are nowadays manufactured on step presses with up to eleven steps. Almost exclusively microalloyed and dual-phase steels with a strength of 400 to 600 MPa are used. In addition to the fatigue strength of the steel wheels, the weight plays an essential role, which affects the material costs, unsprung masses, rotationally moving masses and fuel consumption.

Of course, the foregoing also applies to a large number of other components, in particular cyclically stressed components, preferably in the chassis area. The present invention therefore relates to the manufacture of any components. The production of a wheel dish is therefore only mentioned as an example in the following.

In order to achieve further weight advantages in the wheel disc nowadays, material with higher yield strength and fatigue strength must be used to ensure a sufficient fatigue strength of the wheels and on the other compensated by geometry adjustments, for example stronger embossing, the loss of stiffness at a lower sheet thickness. With increasing strength of the steel materials (carbon steel) but usually decreases the formability, which is almost exhausted in today's wheeled dishes. Therefore, the further lightweight potential of cold-formed solutions seems to be limited.

In addition to the so-called cold forming, among other things, the so-called hot forming is used in vehicle / body construction. Through the use of hot forming, the requirement for a high formability with high strength of the resulting components can be met. Forming processes which involve involving a prior heat treatment of the workpiece, for example in a separate oven, are well known in the art. In particular, the hot forming and press hardening is known here.

In the above-mentioned example of the production of a wheel, but as I said in many other cyclic components in particular, a complete press-hardening is not effective, since in some areas of the workpiece in addition to high strength and a sufficient toughness in the material is necessary, ie the property of a material to deform plastically under load before it fails to transfer the high local stresses, for example by biasing the wheel bolts or system on the hub operatively. It is therefore desirable if the wheel disc is designed to be load-bearing and therefore has specific properties in the relevant areas (zones) or a different microstructure in the component. To achieve this, there are in the prior art method of so-called partial press hardening, in which not the entire hardening Structure is converted into martensite. Known methods will be briefly described below.

The hot forming of plates (plate-shaped workpieces) of higher and higher strength steels for producing press-hardened components, for example, by bringing the workpiece to a temperature above the austenitizing temperature and the immediately subsequent forming is carried out in a forming tool, which in at least one area with a heater for local adjustment of a softer microstructure is equipped.

The local setting of a softer microstructure is called partial press hardening. Partial press-hardening is based on the principle that the workpiece previously heated to a temperature above the austenitizing temperature is strongly cooled in the forming tool with the exception of one or more individual regions, in particular using a cooling device in the form of cooling channels in the die and the punch of the forming tool. Due to the rapid cooling, the microstructure at these points is completely converted into martensite and thus fully press-hardened. In the other areas of the workpiece, the heater allows a much slower cooling of the microstructure, whereby the microstructure in these areas is not completely converted into martensite, that is not completely press-hardened. A corresponding method for hot forming and a corresponding forming tool is for example from DE 10 2006 019 395 A1 known.

A disadvantage, however, in the prior art described above, that at least one heating device must be provided, which causes operating costs and can negatively influence the life of the tool components (punch and die) by the permanent application of heat.

From the DE 10 2010 027 554 A1 Further, a method and a corresponding forming tool for partial press hardening of a workpiece is known, which is based on the principle after compression of the punch and die directly apart a movable die part and an opposite die part apart, so that for the cooling process, a corresponding region of the formed Workpiece is not in contact with the tool components (die, punch), but the ambient air is exposed. In this way, at this point, the formed workpiece is slowly cooled on both sides slowly over the air, whereas the adjacent portions, which are still in contact with the tool components in the compressed state, are rapidly cooled by a cooling device in the form of cooling channels in the die and die , The rapid cooling also leads here to a complete transformation of the microstructure into martensite. The slowly cooled areas of the workpiece are not fully converted into martensite.

A disadvantage of this prior art, however, is that the forming tool is constructed relatively complex, since this consists of many moving parts, which also have cooling channels.

It is an object of the present invention to provide a method and a forming tool for hot working a workpiece, with which a high fatigue strength at low weight and a reduced production cost is achieved.

The object is achieved in a method for hot forming and partial press hardening of a workpiece, in particular a blank, of sheet steel, in which the workpiece is heated prior to forming (until the autenitic state is reached), in which then the heated workpiece in a forming tool, the has two tool components in the form of a die and a punch, is hot formed by contact with both tool components, and in which then the formed workpiece at least partially a surrounding the forming tool gas, in particular air, preferably ambient air, is directly exposed and thereby cooled, characterized solved that after forming at least one tool component part of a tool component which is annularly formed about a central axis which is parallel to the direction of movement of the tool component with the movable tool component part, of the said tool component facing Sei te of the formed workpiece is moved away, while the side facing away from this tool component of the formed workpiece remains fully in contact with the other tool component.

"At least one tool component part" means that, on the one hand, one or more tool component parts of one tool component, for example the punch, is moved away from the formed workpiece while the other part (tool component parts) of this tool component are not moved away, ie in contact with remain the reshaped workpiece. On the other hand, however, it also includes the case that the entire tool component, for example the entire punch, is moved away from the formed workpiece. In both cases, however, the respective other tool component, for example, the die, in contact with the formed workpiece, in full contact, that is, the entire workpiece is to cool on one side completely with the other tool component, such as the die, in Contact.

In the manner according to the invention, a hot forming and partial (but not complete) press hardening can take place with a reduced manufacturing outlay, as a result of which zone-specific microstructural properties are created in the workpiece which bring about a high fatigue strength at low weight. According to the invention, at least one unilateral tool cooling takes place in the entire workpiece, wherein in sections a double-sided tool cooling is also conceivable. In the case of a one-sided mold cooling only, global full-hardening or complete martensite formation, ie over the entire workpiece, does not take place. In the case of only one-sided tool cooling, there is no complete press hardening or complete martensite formation locally in this section or in this area.

In order to enable a partial (partial) press-hardening of the workpiece by simple means, the invention proposes in one embodiment that the forming tool has a cooling device and at least a first portion of the formed workpiece remains in contact with both tool components during cooling of the formed workpiece and by means the cooling device is cooled and at the same time at least a second portion of the formed workpiece is directly exposed to the gas surrounding the forming tool and thereby cooled more slowly than the at least one first portion. Slower means that the heat transfer between the formed workpiece and the surrounding gas, such as the ambient air, is less than the heat transfer upon contact with the cooled tool component (s). In particular, for the sections different cooling is provided that the movable tool component part of the one tool component is moved away from the said tool component side facing the at least a second portion of the formed workpiece, while the side facing away from this tool component of at least a second portion of the formed workpiece further in contact with the other Tool component remains.

The tool component with the movable tool component part, ie with the tool component part displaceable with respect to the rest of the tool component, is preferably the stamp. The tool component defined as the other tool component in each case is the die. However, it is also conceivable in principle the reverse case, namely that the die is formed in several parts and has a movable tool component part, whereas then the stamp remains after forming for cooling on the corresponding side with the workpiece in contact. Also, in the case that the tool components are both not formed in several parts, that is not have as defined previously movable tool components, in particular conceivable that the tool component, which is completely moved away from the tool component side facing the formed workpiece, the punch is and the other tool component is the die. Of course, the reverse case is also conceivable here, namely that the die is the tool component part which is moved away from the workpiece after forming, while the punch is the tool component part which remains on the workpiece.

According to a further embodiment, it is provided that the deformed workpiece or the at least one second section on the side which is remote from the tool component with the movable tool component part, that is in contact with the respective other tool component, is not cooled by means of the cooling device. In the event that at least one second section on one side is cooled after being deformed by the surrounding gas, it is in principle also conceivable to use the other side of the second section, which is not exposed to the surrounding gas, since it is in contact with the corresponding tool component part, for example, the die, is still in contact, nevertheless to cool by means of the cooling device (tool cooling).

According to another embodiment, it is provided that the at least one first section is cooled on both sides by means of the cooling device, that is to say both on the side facing the tool component with the movable tool component part and on the side remote from this tool component.

According to yet another embodiment, it is provided that for cooling the at least one first section, a liquid coolant (for example oil, water, ice water, saline solution) is passed through cooling channels in the die and / or the stamp, which form the cooling device. The same applies optionally also for the case that the tool component, which remains in contact with the at least one second section of the workpiece after forming, should also have a cooling function in the tool component section adjacent to the second section. Preferably, this tool component portion, which is adjacent to the second portion of the workpiece, but is free of cooling channels.

According to yet another embodiment, it is provided that the workpiece before forming a steel sheet of a thickness of at least 1.5 mm, in particular of at least 2.0 mm, and preferably of at least 2.5 mm. These sheet thicknesses are used in the chassis area such as in the manufacture of a wheel rim for a vehicle rim. The relatively large thicknesses are particularly advantageous because the introduced heat, for example, the heat introduced in a furnace upstream of the forming tool can be stored for a particularly long time with thick sheets, so that a one-sided tool contact for heat dissipation and tempering (hardening) used can be. The workpiece made of sheet steel may have a uniform thickness and be monolithic. Also conceivable are workpieces made of tailored products (tailor-made sheet steel semi-finished products), such as tailored blanks, tailored strips, tailored rolled blanks, steel sandwich materials or combinations thereof.

According to another embodiment, it is provided that the workpiece is formed into a wheel rim for a vehicle rim, wherein the wheel flange of the wheel is cooled by the direct contact with the gas surrounding the forming (relatively slow), while the entire remaining part of the wheel by means of the Cooling device (relatively fast) is cooled. With the wheel flange is the range of Radschüssel meant, which is provided circumferentially with the bores for the wheel bolts. As already mentioned above, the production of the wheel disc is mentioned here only as an example. In principle, however, the method according to the invention can also be applied to the production of other components.

According to another embodiment, it is provided that the gas surrounding the forming tool has a temperature in a range of 20 to 30 ° C, in particular a temperature in a range of 22 to 26 ° C, and in particular is air. In principle, it is also conceivable to expose the formed workpiece to a gas at a higher temperature, as long as it is guaranteed that the sections (zones) of the workpiece which come into contact with the gas cool down so slowly that the structure is not completely converted into martensite becomes. Lower temperatures of the gas are conceivable. Furthermore, it can also be provided that the workpiece is heated to a temperature in a range from 750.degree. C. to 1000.degree. C. before forming and / or after the forming in the first section at a cooling rate in a range from 25.degree. s is cooled.

The object is further according to another teaching of the present invention in a forming tool for hot forming a workpiece, in particular a board made of sheet steel, in particular for carrying out the method defined above, with two tool components in the form of a die and a punch, between which the workpiece can be arranged is and which are movable relative to each other, wherein the one of the tool components has a movable relative to the rest of the tool component component component part, which is moved away in the compressed state of the two tool components from the other of the tool components, achieved in that the tool component part is formed annularly about a central axis which runs parallel to the direction of movement of the tool component with the movable tool component part.

Here, too, the tool component which has a tool component part that is movable relative to the rest of the tool component, in particular the stamp, wherein the respective other tool component is then the die. As I said, but in principle, the reverse case is conceivable, that is, the die has a relative to the rest of the die moving part.

When talking about a movable tool component part, it is not excluded that there are also several such movable tool component parts that can be moved independently of each other or together in the previously defined manner. The movable tool component parts can also have any shape. According to the invention, it is provided that the tool component part is designed in the form of a ring around a central axis which runs parallel to the direction of movement of the tool component with the movable tool component part. The direction of movement is in particular the direction in which the punch is moved towards the die (pressing direction). The center axis of the movable tool component part extends in this direction. The ring-shaped movable tool component part is particularly suitable for producing in the production of the aforementioned wheel by cooling zone-specific microstructural properties, namely a non-complete transformation into martensite, especially in the region of the wheel flange (the holes for the wheel bolts).

According to a further embodiment of the forming tool, it is provided that the forming tool further comprises a cooling device, in particular a cooling device formed by cooling channels in the die and / or in the punch, wherein the movable tool component part is not connected to the cooling device and in particular free of cooling channels.

Finally, the object is also achieved by a wheel rim for a vehicle rim, produced by the previously defined method.

Steel materials with high material strength with a tensile strength of, for example, more than 800 MPa or high matensite content can fail just under cyclic load at high stress peaks, which occur for example at holes, notches or machined edges early, so that the high material strength is not on the component can be transferred. For this reason, the workpiece or the wheel disc according to the invention produced by the method according to the invention or the device according to the invention is not completely press-hardened in the heavily loaded areas, such as the wheel flange, which rests on the wheel hub on the vehicle side. In this case, either the complete wheel dish can not be completely press-hardened and so that the microstructure of the entire component can not be homogeneously transformed into martensite or only partially press-hardened, as for example in the wheel flange. As a result, the fatigue strength of the overall construction is significantly improved.

Fig. 1a)

ein Umformwerkzeug mit einem zwischen Stempel und Matrize eingesetzten plattenförmigen Werkstück vor dem Umformvorgang,

Fig. 1b)

das Umformwerkzeug aus Fig. 1a) nach dem Umformen,

Fig. 1c)

das Umformwerkzeug aus Fig. 1a) beim Abkühlen und

Fig. 1d)

das Umformwerkzeug aus Fig. 1a) bei der Entnahme des fertig umgeformten Werkstücks.

There are now a variety of ways to design and further develop the method according to the invention, the forming tool according to the invention and the workpiece according to the invention. In this regard, reference is first made to the claims subordinate to claim 1, on the other hand to the description of an embodiment in conjunction with the drawing. In the drawing shows:

Fig. 1a)

a forming tool with a plate-shaped workpiece inserted between punch and die before the forming process,

Fig. 1b)

the forming tool Fig. 1a ) after forming,

Fig. 1c)

the forming tool Fig. 1a ) while cooling and

Fig. 1d)

the forming tool Fig. 1a ) when removing the finished formed workpiece.

The following is briefly based on the FIGS. 1a ) to d) a method for hot forming and partial press hardening of a workpiece 1 made of sheet steel using the example of a wheel rim to be produced for a vehicle rim described.

Fig. 1a ) shows in the open state, a forming tool 2 with a first tool component 3 in the form of a die 3 and a second tool component 4 in the form of a punch 4. The die 3 is stationary and the punch 4 can for the purpose of forming the workpiece 1 in its entirety be moved to the die 3 until the punch 4 fills the corresponding shape or depression in the die 3. The latter condition is in Fig. 1b ), whereby the heated up to an austenitic state workpiece 1 receives the shape of the wheel. This state is also referred to as a compressed state of the tool components 3 and 4.

In the illustrated embodiment, the punch 4 is formed substantially in three parts, namely with an outer circumferential tool component part 4.2, arranged therein, also circumferential tool component part 4.1, which is annular, and in turn arranged therein another tool component part 4.3. The annular tool component part 4.1, also referred to as a stamp part, relative to the other tool component parts 4.2 and 4.3, so compared to the rest of the stamp, movably mounted and can be moved independently of the rest of the punch against the direction of movement or pressing direction X, as in Fig. 1c ) is shown.

By in the process step in Fig. 1c ) after the forming of the workpiece 1, the annular tool component part 4.1 is moved away from the stamp 4 facing surface 1a of the workpiece 1, ambient air penetrates to the surface 1a of the workpiece 1, but only in the annular section 1.3, the later wheel flange or Forming area with the holes for the wheel bolts. The stamp parts 4.2 and 4.3, however, are initially not moved away from the compressed state, but these tool component parts remain in contact with the top 1a of the workpiece 1. Also, the entire die 3 remains at this time with the complete workpiece 1, namely with its bottom 1b in contact.

The previously in the heated state according to the FIGS. 1a) and 1b ) formed workpiece is according to Fig. 1c ) now cooled as follows. The section 1.3 of the workpiece 1 is no longer in contact with the punch 4 on the upper side, since the punch parts 4.1 have been retracted immediately after forming. In this section 1.3 is a relatively slow cooling of the workpiece 1 by a heat transfer to the ambient air to the top 1a out. Towards the bottom 1b, the workpiece 1 is still in contact with the die 3 and slowly releases heat to the die 3, which is not additionally cooled here.

The remaining areas 1.1 and 1.2 of the formed workpiece 1 are in contact both with the top 1a and the bottom 1b with the die 3 and the punch 4, so that no ambient air, at least not to any appreciable extent, reaches the sections 1.1 and 1.2 , Here, the cooling takes place via a heat transfer to the die 3 and the corresponding stamp parts 4.2 and 4.3, which are still for an increased heat transfer additionally of cooling channels 5, which form a cooling device 5, are interspersed. The cooling channels 5 are flushed through, for example, with oil or ice water, whereby the sections 1.1 and 1.2 of the workpiece 1 cool relatively quickly.

The rapid cooling of the sections 1.1 and 1.2 of the workpiece 1 leads to a complete transformation of the microstructure into martensite. By contrast, the cooling of the annular region 1.3 of the workpiece 1 via the ambient air is so slow that complete conversion of the microstructure into martensite can not take place here. This area is thus not completely press-hardened.

The tool component 4.1 is shown here as an example as part of the punch 4. In principle, however, it is also conceivable to form the die 3 with a movable part as an alternative, but in this case the stamp 4 would abut against the workpiece 1 during cooling over the entire surface 1a of the workpiece 1. The workpiece 1 is thus exposed during cooling always only one side of the ambient air or another heat-dissipating gas.

The movable tool component part 4.1 is designed here by way of example only around the central axis M annular. For other applications, other forms of moving tool component sections 4.1 may be conceivable. Also, by way of example only a single movable tool component section is shown here. In principle, however, it is also possible for a plurality of such tool component sections to be provided, which can be moved away from the workpiece 1 for cooling in relation to the remaining tool component 4. Also exemplified here are two stamp parts 4.2 and 4.3, which remain in contact with the workpiece 1 for the purpose of cooling. In principle, however, it is also conceivable that no part of the punch 4 at all (or exactly one tool component part or more than two tool component parts) remains in contact with the deformed workpiece 1 for cooling.

Claims (13)

1. Method for hot-forming a workpiece (1), in particular a blank, from sheet steel,

   - in which the workpiece (1) prior to forming is heated;

   - in which the heated workpiece (1) thereafter in a forming tool (2), which has two tool components (3, 4) in the form of a die (3) and of a punch (4), is hot-formed by contacting both tool components (3, 4); and

   - in which the formed workpiece (1) thereafter at least in portions is exposed directly to a gas surrounding the forming tool (2), and on account thereof is cooled;

   characterized in that
   post forming at least one tool-component part (4.1) of the one tool component (4), which is configured in an annular manner about a central axis (M) which runs parallel with the movement direction (X) of the tool component (4) having the movable tool-component part (4.1), is moved away from that side (1a) of the formed workpiece (1) that faces this tool component (4), while that side (1b) of the formed workpiece (1) that faces away from this tool component (4) continues to completely contact the respective other tool component (3).
2. Method according to Claim 1,
   characterized in that
   the forming tool (2) has a cooling installation (5), and during cooling of the formed workpiece (1) at least one first portion (1.1, 1.2) of the formed workpiece (1) remains in contact with both tool components (3, 4) and is cooled by means of the cooling installation (5), and at least one second portion (1.3) of the formed workpiece (1) is simultaneously exposed directly to the gas surrounding the forming tool (2) and, on account thereof, is cooled more slowly than the at least one first portion (1.1, 1.2).
3. Method according to Claim 2,
   characterized in that
   the movable tool-component part (4.1) of the one tool component (4) is moved away from that side (1a) of the at least one second portion (1.3) of the formed workpiece (1) that faces this tool component (4), while that side (1b) of the at least one second portion (1.3) of the formed workpiece (1) that faces away from this tool component (4) continues to contact the respective other tool component (3).
4. Method according to one of the preceding claims,
   characterized in that
   the tool component (4) from which at least the tool-component part (4.1) or that side (1a) of the formed workpiece (1) that completely from which faces this tool component (4) is moved away is the punch (4), and the other tool component (3) is the die (3).
5. Method according to one of the preceding claims,
   characterized in that
   the formed workpiece (1) or the at least one second portion (1.3) on that side (1b) that faces away from the tool component (4) having the movable tool-component part (4.1) is not cooled by means of the cooling installation (5).
6. Method according to one of Claims 2 to 5,
   characterized in that
   the at least one first portion (1.1, 1.2) is cooled on both sides by means of the cooling installation (5).
7. Method according to one of Claims 2 to 6,
   characterized in that
   for cooling the at least one first portion (1.1, 1.2) a liquid coolant is directed through cooling ducts (5) in the die (3) and/or in the punch (4), said cooling ducts (5) forming the cooling installation (5).
8. Method according to one of the preceding claims,
   characterized in that
   the workpiece (1) prior to forming is a steel sheet (1) of a thickness of at least 1.5 mm, in particular of at least 2.0 mm, preferably of at least 2.5 mm.
9. Method according to one of Claims 2 to 8,
   characterized in that
   the workpiece (1) is formed into a wheel disk (1) for a vehicle rim, wherein the wheel flange (1.3) of the wheel disk (1) is cooled by way of the direct contact with the gas surrounding the forming tool (2), while the entire remaining part (1.1, 1.2) of the wheel disk (1) is cooled by means of the cooling installation (5).
10. Method according to one of the preceding claims,
    characterized in that
    the gas surrounding the forming tool (2) has a temperature in a range from 20 to 30°C, in particular a temperature in a range from 22 to 26°C, and in particular is air.
11. Forming tool (2) for hot-forming a workpiece (1), in particular a blank (1), from sheet steel, for carrying out the method according to one of the preceding claims, having two tool components (3, 4) in the form of a die (3) and of a punch (4), between which the workpiece (1) is disposable and which are movable in relation to one another, wherein the one of the tool components (3, 4) has a tool-component part (4.1) that is movable in relation to the remaining tool component (4.2, 4.3) and that in the compressed state of the two tool components (3, 4) is movable away from the respective other of the tool components (3, 4),
    characterized in that
    the tool-component part (4.1) is configured in an annular manner about a central axis (M) which runs parallel with the movement direction (X) of the tool component (4) having the movable tool-component part (4.1).
12. Forming tool (2) according to Claim 11,
    characterized in that
    the tool component (4) having the movable tool-component part (4.1) is the punch (4), and the respective other tool component (3) is the die (3).
13. Forming tool (2) according to one of Claims 11 or 12,
    characterized in that
    the forming tool (2) furthermore has a cooling installation (5), in particular a cooling installation (5) that is formed by cooling ducts (5) in the die (3) and/or in the punch (4), wherein the movable tool-component part (4.1) is not connected to the cooling installation (5) and in particular is free of cooling ducts (5).

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