EP2782689B1 - Method and forming tool for hot forming and press hardening work pieces made of sheet steel, in particular galvanized sheet steel - Google Patents

Description

The invention relates to a method for hot forming and press hardening of plate-shaped or preformed workpieces made of sheet steel, in particular galvanized workpieces made of sheet steel, in which the workpiece is heated to a temperature above the Austenitisierungstemperatur and then in a cooled forming tool having at least one punch and at least one die , reshaped and quenched. Furthermore, the invention relates to a forming tool for hot forming and press hardening of plate-shaped or preformed workpieces made of sheet steel, in particular galvanized workpieces made of sheet steel, with at least one punch and a die associated with the die, wherein the punch and / or die have cooling channels for passing a cooling fluid. Such a method and such a forming tool are eg in the JP-A-2007075834 disclosed.

With regard to the hot forming and press-hardening of workpieces made of sheet steel devices are known which have at least two mold halves, which are partially formed such that they have different thermal conduction properties, which are used to adjust locally different strength properties in the manufactured component can. The method carried out with these devices will be familiar to those skilled in the art termed "tailored tempering". A corresponding device is for example from the DE 10 2009 018 798 A1 known.

Furthermore, it is known to increase the dimensional and fitting accuracy of formed components by virtue of the fact that the tool halves used for production have positive radii in the region of curves of the workpiece and form air gaps in the opposite regions, with projections being formed adjacent to the air gaps. that a distortion-free terminals is possible. As a result, different degrees of hardness in the component can be adjusted. A corresponding device for producing hardened components made of sheet steel is in the DE 10 2004 038 626 B3 described.

Investigations have shown that during hot forming of galvanized steel blanks in conventional forming tools, cracks sometimes occur in the zinc coating. The cracks can continue into the board, which can lead to premature component failure.

The present invention has for its object to provide a method or a forming tool of the type mentioned, which improves the flow properties of steel materials during hot forming, so that the risk of occurrence of cracks in the hot forming of workpieces made of sheet steel, especially galvanized steel blanks considerably is reduced.

This object is achieved by a method having the features of claim 1 or by a forming tool having the features of claim 8.

Preferred and advantageous embodiments of the invention are specified in the subclaims.

According to the invention, the die used for hot forming and press hardening is coated in its defined by a positive drawing radius Ziehkantenbereich with material and / or provided with at least one insert which has a thermal conductivity which is lower by at least 10 W / (m * K) than the thermal conductivity the portion of the die adjacent the drawing edge region which contacts the workpiece during hot working and press hardening of the workpiece. The applied in the drawing edge region material or arranged there insert part, which according to the invention has a relatively low thermal conductivity, is formed so that its surface facing the workpiece has a extending over the drawing edge transverse dimension which is in the range of 1.6 times to 10 times, preferably in the range of 1.6 times to 6 times the positive drawing radius of the die is. The transverse extent (transverse dimension) of the relatively low thermal conductivity having arranged in the drawing edge region material or insert part is thus limited and relatively small.

In particular, in the drawing edge region of the die defined by a positive drawing radius, the coated steel sheet (workpiece) to be formed is subjected to high plastic deformation. By the action of the tool punch In this area, the workpiece initially experiences a compressive stress, which changes into tensile stress during the continued closing movement of the forming tool. Due to the high temperature difference between the workpiece and the forming tool, the local flow properties of the workpiece in a conventional forming tool, in particular in the drawing radius of the die of a conventional forming tool are adversely affected, which often leads to cracks in the coating, such as zinc coating. With increasing sheet thickness and depending on the complexity of the geometry of the component to be manufactured, different crack depths can form, which can extend into the sheet of the coated component.

According to the invention, the material applied in the drawing edge region, for example by coating, having a relatively low thermal conductivity or arranged there, a relatively low thermal conductivity insert is such that it is ensured that the component produced by hot forming and press hardening has a substantially completely martensitic structure , The part of the press-hardened component influenced by the drawing edge of the die, ie the material or insert part having a relatively low thermal conductivity, may have a lower hardness than another part or the remaining part of the component, but this so influenced part of the press-hardened component according to the invention always has a hardness above a required minimum hardness, which corresponds to a martensitic structure. In this way, cracks in the coating, for example in a Zinc layer, as well as in the corresponding coated sheet avoided or significantly reduced at least crack depths in the coating or the coated sheet metal.

The hot working of the coating, e.g. galvanized workpiece (steel sheet) occurring stresses and strains and the solidification occurring in the forming process are reduced by the reduced heat or temperature loss compared to that in a conventional tempered forming. This also reduces or prevents any local material failure.

The present invention thus improves the flow characteristics of steel sheet workpieces during hot forming, and thus significantly reduces the risk of cracking during hot forming of steel sheet workpieces, preferably galvanized steel blanks. In particular, by the present invention, the feasibility of components having a complex three-dimensional shape and of coated, e.g. galvanized steel sheet are improved.

A preferred embodiment of the solution according to the invention provides that the thermal conductivity of the insertion part or material applied in the drawing edge region is less than 40 W / (m * K), preferably less than 30 W / (m * K), particularly preferably less than 20 W / (m * K) is. As a result, the heat or temperature loss during the hot working of the workpiece is advantageously reduced and the forming behavior of the workpiece is improved accordingly.

A further advantageous embodiment of the solution according to the invention is characterized in that a heat insulating layer is or is arranged between the insert part and the die. In this way, the heat or temperature loss during the hot working of the workpiece can be further reduced. In particular, this embodiment makes it possible to use an insert which is made of a particularly wear-resistant material, but which has a relatively high thermal conductivity, wherein the heat insulating, which exposed to the Ziehkantenbereich insert part not exposed to high mechanical stress, in particular no high Reibbeanspruchung is, from a heat insulating material, for example, plastic or wood material, with low wear resistance can exist.

According to a further advantageous embodiment of the solution according to the invention, it is provided that the insert part has a projection protruding from the inner circumference of the die and / or opposite to the edge of the cavity adjacent to the cavity of the die. By this projection representing a local increase, the heat dissipation from the workpiece to be reshaped can be more effectively reduced before the drawing radius.

A further advantageous embodiment of the solution according to the invention is characterized in that the material applied in the drawing edge region of the die is applied to the die by build-up welding, preferably laser-deposition welding. In this way, it is relatively easy to reliably reduce the thermal conductivity in the drawing edge region of the die. The one relatively small Thermal conductivity having material application can be replaced inexpensively by build-up welding, preferably laser deposition welding, if this is required due to a wear-related wear (removal).

A further advantageous embodiment of the solution according to the invention is characterized in that the drawing edge region of the die is locally selectively heated by means of a heat source integrated in the matrix or a channel carrying a heating fluid. As a result, the heat dissipation from the workpiece to be formed can be significantly reduced and thus the flow properties of the steel material can be improved during hot forming.

It is also within the scope of the present invention to combine several of the aforementioned or all the aforementioned embodiments of the solution according to the invention.

Fig. 1

einen Abschnitt eines erfindungsgemäßen Umformwerkzeuges in Schnittansicht;

Fig. 2

einen Abschnitt eines weiteren erfindungsgemäßen Umformwerkzeuges in Schnittansicht;

Fig. 3

eine Schnittansicht eines eine Ziehkante umfassenden Abschnitts eines erfindungsgemäßen Umformwerkzeuges mit einer im Ziehkantenbereich angeordneten, eine relativ geringe Wärmeleitfähigkeit aufweisenden Beschichtung;

Figuren 4 und 7

jeweils eine Schnittansicht eines eine Ziehkante umfassenden Abschnitts eines erfindungsgemäßen Umformwerkzeuges mit im Ziehkantenbereich durch Auftragsschweißen aufgebrachtem Material, das jeweils eine relativ geringe Wärmeleitfähigkeit aufweist; und

Figuren 5, 6 und 8

jeweils eine Schnittansicht eines eine Ziehkante umfassenden Abschnitts eines erfindungsgemäßen Umformwerkzeuges mit einem im Ziehkantenbereich angeordneten Einsatzteil, das eine relativ geringe Wärmeleitfähigkeit aufweist.

The invention will be explained in more detail with reference to a drawing illustrating several embodiments. They show schematically:

Fig. 1

a section of a forming tool according to the invention in sectional view;

Fig. 2

a section of another forming tool according to the invention in sectional view;

Fig. 3

a sectional view of a pulling edge comprehensive portion of a forming tool according to the invention with a in the drawing edge region arranged, having a relatively low thermal conductivity coating;

FIGS. 4 and 7

each a sectional view of a drawing edge comprehensive portion of a forming tool according to the invention with applied in the drawing edge region by build-up welding material, each having a relatively low thermal conductivity; and

FIGS. 5, 6 and 8

in each case a sectional view of a pulling edge comprehensive portion of a forming tool according to the invention with a arranged in the drawing edge region insert part, which has a relatively low thermal conductivity.

In the Figures 1 and 2 are each sections of cooled forming tools for hot forming and press hardening of a plate-shaped or preformed workpiece 1 made of sheet steel, in particular a galvanized workpiece made of sheet steel. The reference numeral 2 is a stamp and the reference numeral 3 is a die (die) of the respective forming tool designated. In addition, that can be done in Fig. 1 and or Fig. 2 Shaping tool shown optionally have a holding-down device (blank holder), which presses the workpiece 1 against the die 3 during the forming process. Preferably, however, the forming tool according to the invention is designed as a blank holder (low-holder) forming tool.

Die (die) 3 contains a cavity 4, in which the punch 2 during forming or deep drawing of the workpiece. 1 penetrates. In the Figures 1 and 2 the respective forming tool is shown in each case in the closed state with the workpiece 1 formed therein.

In the die 2 and / or the die 3, cooling channels (not shown) for passing a cooling fluid are provided near the forming die surface. Before it is placed in the open forming tool, the workpiece 1 to be formed is first heated to a target temperature, preferably to a temperature above the austenitizing temperature, and then shaped and quenched in the cooled forming tool.

The temperature of the heated plate-shaped or preformed workpiece 1 is preferably kept as high as possible prior to the forming in order to achieve an improvement in the flow properties of the workpiece 1 or a reduction of the stresses and / or strains during the forming process. This may be, for example, via the selected level of heating temperature and / or by short transfer times, i. short handling times between the heating device (not shown), such as a continuous furnace, and the start of the forming, are influenced.

The forming tool according to the invention is characterized by an optimized heat transfer coefficient. As a result, too rapid local cooling of the heated workpiece 1 (for example, the galvanized steel plate) is prevented after positioning and during the forming in the tool. According to the invention, at least the die 3 is optimized with regard to the heat transfer coefficient. For this purpose, the die 3 is in its defined by a positive drawing radius Ziehkantenbereich cohesively coated with material and / or there provided with at least one insert part 5, which has a thermal conductivity which is lower by at least 10 W / (m * K) than the thermal conductivity of the drawing edge region adjacent section 3.1 of the die 3, during hot forming and Press hardening of the workpiece with the same comes into contact. The means with comparatively low thermal conductivity are dimensioned such that it is furthermore ensured that a completely martensitic microstructure is established in the formed component (workpiece) 1 after completion of the quenching process (press hardening), whereas the workpiece region influenced by the draw edge region designed according to the invention reduces the Hardness, however, must be within the required minimum hardness, may have, whereby cracks in the workpiece 1 can be avoided or crack depths can be reduced. The workpiece 1 facing surface of the applied in the drawing edge region material 6 (see. Fig. 3 ) or arranged insert 5 therefore has according to the invention a extending over the drawing edge 7 transverse dimension, which is in the range of 1.6 times to 10 times the positive drawing radius of the die 3.

In the in the Figures 1 and 2 illustrated embodiments, the respective die 3 in its positive drawing radius defined pulling edge region at least one insert part 5, whose thermal conductivity is preferably less than 40 W / (m * K), more preferably less than 30 W / (m * K). The at least one insert part 5 is annular or strip-shaped and inserted into a formed in the drawing edge region of the die 3 recess 3.2.

In the FIGS. 3 to 8 Further embodiments of a forming tool according to the invention, preferably a die 3 are shown schematically.

At the in Fig. 3 In the exemplary embodiment shown, a die 3 serving for hot forming and press-hardening is coated, in its drawing edge region defined by a positive drawing radius, with material 6 which has a relatively low thermal conductivity. The material (coating) 6 is preferably ceramic, for example alumina or zirconia. The selective coating of the drawing edge region can be effected, for example, by flame spraying, in particular powder flame spraying or wire flame spraying, or by arc spraying or plasma spraying.

The transverse dimension of the coating 6 extending over the drawing edge 7 is, for example, in the range of 1.6 times to 4 times, preferably in the range of 1.6 times to 2 times, the positive drawing radius of the die 3. The coating 6 stands or may protrude slightly from the adjacent surface 3.1 of the die 3, for example by about 0.25 mm to 0.5 mm or even more.

At the in Fig. 4 In the embodiment shown, the die 3 serving for hot forming and press hardening in the drawing edge region is provided with a material application 6 'produced by build-up welding, which has a relatively low thermal conductivity. Before the build-up welding, a depression 3.3 extending transversely across the drawing edge is produced in the drawing edge region of the die 3, for example by machining. In this recess (recess) 3.3 is then by deposition welding material 6 'with arranged relatively low thermal conductivity. This application material 6 'may be, for example, chromium steel, titanium or high alloy steel, such as X5CrNi18-10, which has a thermal conductivity of about 30 W / (m * K) or less than 30 W / (m * K ) exhibit. The material 6 'applied to the drawing edge region by build-up welding is applied to the extent or subsequently removed by milling or grinding in such a way that it terminates substantially flush in the surface 3.1 of the die 3 or protrudes slightly from the surface 3.1 of the die 3.

In the Fig. 5 in a section illustrated die 3 substantially corresponds to the in Fig. 1 shown embodiment. Again, a strip-shaped insert 5 is arranged with relatively low thermal conductivity in the drawing edge region of the die 3. The insert 5 consists for example of ceramic, preferably of aluminum oxide (Al ₂ O ₃ ) or zirconium oxide. The outer edge of the insert part 5 forming the pulling edge region terminates substantially flush with the surface 3.1 of the die 3.

In Fig. 5 In addition, another option or alternative for reducing the heat loss of the heated workpiece is shown. This alternative or additional option is that in the die 3, a heat source or a heating fluid leading channel 8 is integrated, by means of which the drawing edge region of the die 3 is locally selectively heated. According to a further preferred embodiment, it is provided that the heat source, for example in the form of one or more electrical heating wires, or the channel 8 carrying a heating fluid is integrated in the insertion part 5 forming the drawing edge region.

This in Fig. 6 illustrated embodiment differs from those in the FIGS. 1, 2 and 5 shown embodiments in that between the insert part 5 and the die 3, a heat insulating layer 9 is arranged. The heat insulating layer 9 is single-layered or multi-layered and consists for example of plastic and / or mineral wool.

At the in Fig. 7 In the exemplary embodiment shown, the die 3 used for hot forming and press hardening in the drawing edge region is again provided with a material application 6 'produced by build-up welding, which has a relatively low thermal conductivity. In contrast to the embodiment according to Fig. 4 However, the material application 6 'is designed so that it has a relation to the inner periphery of the die 3 or opposite to the cavity 4 of the die 3 adjacent edge peripheral surface projecting projection 6.1. By this local increase or local projection 6.1 from a relatively low thermal conductivity material having the heat dissipation from the heated workpiece 1 is reduced. In addition to this material application 6 ', a heat source or a heating fluid-conducting channel 8 can again be integrated in the matrix, by means of which the drawing edge region of the die 3 can be locally selectively heated.

This in Fig. 8 illustrated embodiment differs from the embodiment according to Fig. 6 in that the insert part 5 has a projection 5.1 protruding from the inner periphery of the cavity 4 of the die 3 or opposite the edge peripheral surface adjacent to the cavity 4 having. The reference numeral 9 again denotes a disposed between the insert part 5 and the die 3 heat insulating layer.

The embodiment of the present invention is not limited to the embodiments described above and / or illustrated in the drawings. On the contrary, numerous variants or modifications are conceivable which make use of the invention specified in the appended claims, even if the design deviates from the exemplary embodiments. Thus, for example, in addition, the stamp and / or optionally also the sheet holder (hold-down) with means 5, 5.1, 6, 6 ', 6.1 and / or 9 be provided with low thermal conductivity for optimizing the heat transfer coefficient.

Claims (14)

1. Method for hot forming and press hardening plate-shaped or preformed workpieces (1) of sheet steel, in particular galvanised workpieces (1) of sheet steel, in which the workpiece is heated to a temperature above the austenitisation temperature and is then formed and quenched in a cooled forming tool having at least one punch (2) and at least one female mold (3), characterised in that the female mold (3) used for hot forming and press hardening is coated in its drawing edge region, defined by a positive die radius, with material (6, 6') in a material-uniting manner and/or is provided there with at least one insert part (5) having a thermal conductivity which is at least 10 W/ (m * K) lower than the thermal conductivity of the portion (3.1) of the female mold (3), which portion is adjacent to the drawing edge region and comes into contact with the workpiece (1) when said workpiece is being hot formed and press hardened, the surface, facing the workpiece (1), of the material (6, 6') applied in the drawing edge region, or of the insert part (5) arranged there, having a transverse dimension which extends over the drawing edge (7) and is within the range of 1.6 times to 10 times the positive radius of the female mold (3).
2. Method according to claim 1, characterised in that the thermal conductivity of the insert part (5) arranged in the drawing edge region or of the applied material (6, 6') is less than 40 W/ (m * K), preferably less than 30 W/ (m * K).
3. Method according to claim 1 or claim 2, characterised in that the at least one insert part (5) is configured in the form of a strip and is inserted into a recess (3.2) formed in the corner region of the female mold (3).
4. Method according to any one of claims 1 to 3, characterised in that a heat insulating layer (9) is arranged between the insert part (5) and the female mold (3).
5. Method according to any one of claims 1 to 4, characterised in that the insert part (5) has a projection (5.1) which protrudes with respect to the inner periphery of the female mold (3) and/or with respect to the peripheral surface (3.1) adjoining the cavity (4) of the female mold (3).
6. Method according to any one of claims 1 to 5, characterised in that the material (6') applied in the drawing edge region of the female mold (3) is applied to the female mold (3) by build-up welding.
7. Method according to any one of claims 1 to 6, characterised in that the drawing edge region of the female mold (3) is heated in a locally selective manner by a heat source integrated into the female mold or by a duct (8) conducting a heating fluid.
8. Forming tool for hot forming and press hardening plate-shaped or preformed workpieces (1) of sheet steel, in particular galvanised workpieces (1) of sheet steel, having at least one punch (2) and a female mold (3) associated with said punch, the punch (2) and/or the female mold (3) having cooling ducts for conducting a cooling fluid, characterised in that the female mold (3) is coated in its drawing edge region, defined by a positive die radius, with material (6, 6') in a material-uniting manner and/or is provided there with at least one insert part (5) having a thermal conductivity which is at least 10 W/ (m * K) lower than the thermal conductivity of the portion (3.1) of the female mold (3), which portion is adjacent to the drawing edge region and comes into contact with the workpiece (1) when said workpiece is being hot formed and press hardened, the surface, facing the workpiece (1), of the material (6, 6') applied in the drawing edge region or of the insert part (5) arranged there having a transverse dimension which extends over the drawing edge (7) and is within the range of 1.6 times to 10 times the positive radius of the female mold (3).
9. Forming tool according to claim 8, characterised in that the thermal conductivity of the insert part (5) arranged in the drawing edge region or of the applied material (6, 6') is less than 40 W/ (m * K), preferably less than 30 W/ (m * K).
10. Forming tool according to claim 8 or claim 9, characterised in that the at least one insert part (5) is configured in the form of a strip and is inserted into a recess (3.2) formed in the drawing edge region of the female mold (3).
11. Forming tool according to any one of claims 8 to 10, characterised in that a heat insulating layer (9) is arranged between the insert part (5) and the female mold (3).
12. Forming tool according to any one of claims 8 to 11, characterised in that the insert part (5) has a projection (5.1) which protrudes with respect to the inner periphery of the female mold (3) and/or with respect to the peripheral surface (3.1) adjoining the cavity (4) of the female mold (3).
13. Forming tool according to any one of claims 8 to 12, characterised in that the material (6') applied in the drawing edge region of the female mold (3) is applied to the female mold (3) by build-up welding.
14. Forming tool according to any one of claims 8 to 13, characterised in that integrated into the female mold (3) is a heat source or a duct (8) conducting heating fluid, by which the drawing edge region of the female mold (3) can be heated in a locally selective manner.

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