EP1983063A2 - Method for moulding and hardening a workpiece made from steel in a stamping machine - Google Patents

Abstract

Method for press-forming and hardening a steel workpiece in a stamping machine comprises partially moving the stamps (2, 3) away from each other after deforming and passing coolant (6) through the gap (10, 11) between the stamps and the workpiece. The heated workpiece is placed into the molding hollow space (9) above the coolant meniscus (8). The coolant meniscus is raised high enough for the coolant to flow into the hollow space.

Description

The invention relates to a method for compression molding and hardening of a workpiece made of steel in a stamping press with the measures in the preamble of patent claim 1.

It is for example through the DE 24 52 486 C2 known, workpieces made of steel, especially sheets, warm to transform in a press and to harden in the press. For this purpose, a blank is heated to a temperature in the specific austenitizing temperature range of the material, ie to a temperature above the transition temperature AC1, preferably greater than AC3, formed in the die press and cured in the closed pressing tool. The pressing tool is liquid cooled. The cooling can also be done during the closing of the die press.

In order to achieve the desired degree of hardness, the cooling rate of the workpiece must be greater than the critical material-specific cooling rate. The workpiece must also be cooled to a temperature below the martensite start temperature. In this way, a diffusion-free folding of the austenitic structure of the workpiece is achieved in a martensitic microstructure, wherein the martensitic microstructure is characterized by high hardness and high tensile strength. A certain proportion of retained austenite remains, but it is harmless in small quantities.

It is considered disadvantageous that in known press-hardening processes often only blanks of small thickness can be reliably processed. When building armored vehicles, for example, it is important to process thicker steel plates. These may have a thickness of several millimeters and even more than 10 mm. The known Presshärtverfahren come here often to its limits. There is therefore a desire, even thick-walled steel plates with complex three-dimensional geometries to reshape in order to adapt to the vehicle interior contour. So far, flat armor plates were welded together. However, the welds significantly change the mechanical properties of the steel and lead to a weakening of the entire component. Therefore, complex design measures had to be taken to ensure the required level of security of the armor components in the area of the welds.

In the DE 10 2004 006 093 B3 A method for producing a three-dimensionally shaped armor component for vehicle bodies is described, in which the cooling of the hot-formed component is part of the hardening treatment and takes place in the closed pressing tool, which is held in full-surface contact with the workpiece. Although the cooling of the deformed component in the closed pressing tool should be carried out at a cooling rate which at least corresponds to the material-specific critical cooling rate, but this will be very placed high demands on the cooling devices of the pressing tool in order to ensure the critical cooling speed, in particular for thick-walled armor components. Particularly in the case of thick-walled components, achieving the required critical cooling rate is particularly important so that the workpiece used for armoring has the desired hardness and tensile strength over its entire surface. The forming tools used are therefore relatively complex and due to the necessary cooling facilities extremely expensive to manufacture.

An improvement in cooling could theoretically be achieved by bringing the cooling liquid into direct contact with the workpiece. Such a method is used, for example, in DE 26 03 618 A described. There, a Abschreckpresse is described, which has a mold cavity which corresponds to the desired final shape of the workpiece. Inlets are provided in the mold cavity, through which cooling liquid flows into the mold cavity to rapidly cool and quench the workpiece fixed there between the mold halves. After quenching, the workpiece is removed again from the forming tool. The workpiece remains at least partially in contact with the workpiece during the quenching process, since it is considered another object of the quenching press to secure the article against deformation and to rigidly hold it.

Very elaborate forming tools are also used in this procedure, wherein a smaller area of the tool surface is in contact with the workpiece due to the multiplicity of grooves in the forming tools. But this also leads to premature tool wear, due to the forming forces, especially when it comes to very thick-walled components.

On this basis, the invention is based on the object to provide a method which is used for press forming and hardening of steel workpieces and in particular for the production of armor components for Vehicle bodies is suitable, with the method using simple and inexpensive design molds molded and hardened components of complex geometry and uniform hardness distribution can be made. At the same time, the method is intended to produce workpieces with reproducible precision and with significantly smaller dimensional tolerances than comparable weldments.

This object is achieved in a method having the features of patent claim 1.

Advantageous developments of the inventive concept are the subject of the dependent claims.

The invention is based on the essential finding that critical cooling rates can be achieved only with great effort by the tool contact in thick-walled components with wall thicknesses between 5 and 20 mm, especially when dealing with complex three-dimensional geometries. The resulting from the tool contact fluctuations in hardness and tensile strength due to falling below the critical cooling rate, can be prevented in the context of the invention that the cooling is not exclusively by heat dissipation via the tool or via the closed tool initiated coolant, but that the tool or the upper die and the lower die are partially moved apart after the hot working of the workpiece to allow the access of coolant in the gap formed by the moving apart between the dies and the workpiece.

It is essential that relatively large amounts of the coolant are introduced into the gap between the die and the workpiece or flow by itself, whereby it can be ensured that the entire workpiece is flowed around by coolant, resulting in a uniform cooling with a cooling rate above the critical cooling rate leads and thus to a uniform hardness and tensile strength distribution. The hardening is thus still within the forming tool, but without the forming tool is closed during curing. As a result, with regard to the tool cooling, considerably simpler upper and lower dies can be used, since the dies primarily have the task of reshaping the workpiece, while the separately supplied coolant causes the hardening of the workpiece.

It is considered to be particularly advantageous if the forming tool is closed again after the molds have been moved apart and after quenching by introducing coolant into the gap, so that the workpiece is pressed once more in the same forming tool. In this case, the workpiece is once again in contact with the upper die and the lower die. The workpiece is thereby directed, whereby a high dimensional accuracy of the workpieces produced in this process can be ensured. The moving apart of the dies for quenching and the subsequent pressing with intermediate cooling of the workpiece can be repeated several times before the reshaped and hardened workpiece is finally removed from the forming tool and is supplied for further use. The workpiece can be subjected to further heat treatments, for example.

The introduction of coolant into the coolant gap can be realized in different ways. For example, the heated workpiece above a coolant level can be inserted into the mold cavity. The lower die is to some extent in a filled with the liquid coolant pan. The coolant preferably flows simultaneously from all sides into the gap between the upper die and lower die that arises when moving apart. Instead of raising the coolant level, theoretically the lower die can also be lowered relative to the coolant level. In practice, however, it is easier to raise the coolant level. This can be done, for example, by displacement elements, which dip into the coolant when the upper die is lowered.

Of course, it is also possible to raise the coolant level by coolant supply.

It is considered to be particularly advantageous if the workpiece is lifted by ejector mandrels when the dies move apart relative to the lower die, so that a gap accessible to the coolant is formed on both sides of the workpiece and thus the contact area with the lower die is as low as possible, in order to allow coolant access on all sides and to ensure even cooling. The coolant is especially water, which is provided with a lubricant additive. A lubricant additive, such as graphite or a graphite-free, water-miscible die lubricant, supports the forming of the workpiece in the die press and helps to reduce tool wear.

In particular, three-dimensionally shaped armor components can be produced with the method according to the invention. The gradient of the cooling, ie the cooling rate of the workpiece, can be set very precisely via the properties of the coolant and in particular via the process of the tool opening and the holding times in the workpiece.

Complex tool geometries may require that drain holes be provided in the lower die to avoid liquid pockets that would interfere with repeated closing movements of the forming tool.

Figur 1

eine Schnittdarstellung durch ein Umformwerkzeug mit eingelegtem Werkstück in Form einer härtbaren Platine aus Stahl;

Figur 2

das Umformwerkzeug der Figur 2 in geschlossenem Zustand und

Figur 3

das Umformwerkzeug der Figur 1 im teilweise geöffneten Zustand bei geflutetem Formhohlraum.

The invention will be explained in more detail with reference to an embodiment shown in schematic drawings. Show it:

FIG. 1

a sectional view through a forming tool with a pickled workpiece in the form of a hardenable steel board;

FIG. 2

the forming tool the FIG. 2 in closed condition and

FIG. 3

the forming tool the FIG. 1 in the partially opened state with flooded mold cavity.

In FIG. 1 is greatly simplified to recognize a forming tool 1 a not shown pressing press. The forming tool 1 consists of an upper die 2 and a lower die 3. The upper die 2 may be referred to as an upper tool and the lower die as a lower tool. Between the dies 2, 3 is a workpiece 4 in the form of a board made of a hardenable steel. From this flat board three-dimensionally shaped armor components for motor vehicles are to be produced. The board or the workpiece 4 has a large wall thickness of several millimeters.

In FIG. 1 is the forming tool 1 in the open state. The lower die 3 is surrounded by a coolant trough 5, in which a coolant 6 is located. It is water with a lubricant additive. The coolant 6 is continuously cooled in a process, not shown. The upper die 2 is linearly displaceable with respect to the lower die 3. Furthermore, adjacent to the upper die 2 displacement bodies 7 can be seen, which can dip into the coolant 6.

Before inserting the workpiece 4 into the forming tool 1, the workpiece 4 has been heated to its austenitizing temperature range, ie to a temperature above the transformation point AC3. The hot workpiece is then reshaped by lowering the upper die 2 to a three-dimensional geometry ( FIG. 2 ). As a result, the workpiece 4 is briefly over the entire surface in contact with the upper die 2 and the lower die 3, wherein a certain amount of heat from the workpiece 4 on the dies 2, 3 is transmitted. As a result, however, the material-specific critical cooling rate of the workpiece 4 is not achieved. Rather, the forming tool 1 is opened a gap wide in the next step, so that the coolant 6 in the gap 10, 11 between the Dies 3, 4 flows in ( FIG. 3 ). Due to the significantly lower temperature of the coolant 6 relative to the temperature of the workpiece 4 is a uniform curing of the entire workpiece 4, which is enclosed on both sides of the incoming coolant 6.

In FIG. 2 can be seen that the coolant level 8 is still as low as in FIG. 1 because the displacement body 7 is not yet immersed in the coolant 6. As a result, the coolant can not yet flow into the mold cavity 9 of the forming tool 1. In FIG. 3 However, the displacer 7 are at least partially in the coolant 6, so that the coolant level 8 has risen so far that the coolant 6 in the gap 10 between the lower die 3 and the workpiece 4 and in the gap 11 between the upper die 2 and the Workpiece 4 can flow.

In a manner not shown in the FIGS. 2 and 3 repeats steps as needed until the desired cooling of the workpiece 4 has been achieved. Subsequently, the workpiece 4 is removed from the forming tool 1 and fed to further processing.

Reference numerals:

1 -

forming tool

2 -

upper die

3 -

lower die

4 -

workpiece

5 -

Coolant tray

6 -

coolant

7 -

displacement

8th -

Coolant level

9 -

mold cavity

10 -

gap

11 -

gap

Claims (9)

A method of press-forming and hardening a steel workpiece in a swaging press, wherein the workpiece (4) is heated to its austenitizing temperature range, then placed between an upper die (2) and a lower die (3) of a swaging press forming tool (1), hereinafter Workpiece (4) between the upper die (2) and the lower die (3) is thermoformed and then hardened by direct contact with a coolant (6) within the forming tool (1), characterized in that the dies (2, 3) After forming partially be moved apart again and that the coolant (6) through the gap (10, 11) between the dies (2, 3) and the workpiece (4) flows into the mold cavity (9). A method according to claim 1, characterized in that the forming tool (1) after moving apart of the dies (2, 3) and quenching again closed and the workpiece (4) in the forming tool (1) is pressed a further time. A method according to claim 2, characterized in that the moving apart of the dies (2, 3) for quenching and subsequent pressing is repeated several times before the reshaped and hardened workpiece (4) is removed from the forming tool (1). Method according to one of claims 1 to 3, characterized in that the heated workpiece (4) above a coolant level (8) in the mold cavity (9) is inserted and that the coolant level (8) is raised to harden so far that the coolant (6) when moving apart of the dies (2, 3) flows into the mold cavity (9). A method according to claim 4, characterized in that the coolant level (8) is lifted by displacement elements (7) which dip into the coolant (6). Method according to one of claims 1 to 4, characterized in that the coolant level (8) is lifted by coolant supply. Method according to one of claims 1 to 6, characterized in that the workpiece (4) when moving apart of the dies (2, 3) by Auswerferdorne against the lower die (3) is raised. Method according to one of claims 1 to 7, characterized in that the coolant (6) is water with a lubricant additive. Method according to one of claims 1 to 8 for producing a three-dimensionally shaped armor component from the workpiece (4).

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