BRIEF DESCRIPTION OF THE DRAWING
The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:
FIGS. 1, 2, 3, and 4 are partly diagrammatic views illustrating steps of a shaping/hardening operation;
FIG. 5 is a diagram illustrating principles of this invention; and
FIG. 6 is another diagrammatic view illustrating another step of the method of this invention.
SPECIFIC DESCRIPTION
As seen in FIGS. 1-4 and 6, a flat steel plate constituting a workpiece blank 6 is formed into a cup 60.
FIG. 1 schematically illustrates parts of a deep-drawing die, specifically, an upper half or tool 1, a lower half or tool 2 having a cavity 3, a hold-down plate or element 4, and spacers 5. The workpiece blank 6 is a steel-alloy plate. All parts 1 through 6 are situated in a press (not illustrated in greater detail) that applies the shaping and locking forces. The heat distribution in the workpiece blank 6 is well described with reference to FIG. 1. Before being inserted into the deep-drawing tool, the blank 6 has been homogeneously heated to a temperature above the AC3 point of the alloy. After the blank 6 is inserted into the deep-drawing tool, the blank 6 initially rests above the cavity 3, at the edge regions 7 and 8 on the lower tool 2. Immediately upon contact with the edge regions 7 and 8 the blank 6 rapidly cools, so that these regions 7 and 8 harden somewhat. The press is still open.
When the press is closed according to FIG. 2 the hold-down element 4 also contacts the blank 6 at the edge regions 7 and 8, which consequently become even colder and harder. The hold-down element 4 then rests on the spacers 5. In the illustrated embodiment of FIGS. 1 through 4, the upper tool 1 makes initial contact with the blank 6 in the center 9. Cooling also begins in this center 9 of the blank 6 as the result of contact with the upper tool 1. In the process stage shown in FIG. 2, the edge regions 7 and 8 and the center 9 of the blank 6 are then relatively cold and strong. In contrast, specific regions 10 and 11 of the blank that are not yet shaped have a relatively high temperature and are still relatively soft. Without of temperature compensation by cooling according to the invention, further closing of the upper tool 1 in the transition regions 12 and 13 would cause the blank 6 to gradually become thinner from the cold zone toward the hot zone, and also possibly tear.
According to the invention, this problem is counteracted by subjecting the blank 6 to localized cooling, either before being placed on the lower tool 2 or in the shaping tool in regions 10 and 11. As schematically illustrated in FIG. 6, cooling in the tool may be achieved, for example, by use of nozzles 17 and 18 that are integrated into the press. The cooling may also be performed by blowing or otherwise cooling outside the tool. However, the cooling must be limited so that hardening does not occur. The object is to achieve temperature balancing between the relatively cold regions 7, 8, and 9 and the regions 10 and 11 during the step FIG. 2 so that regions 10 and 11 can better transmit forces that are necessary for deep-drawing and that occur during the shaping process to the edge regions 7 and 8 of the blank 6, and so that the transition regions 12 and 13 from the already relatively strong zone to the still relatively weak zone inside the blank 6 are relieved of stress.
In FIG. 3 the upper tool 1 travels further down into the direction of the cavity 3. Deep-drawing forces are increasingly absorbed by the transition regions 12 and 13.
In FIG. 4 the press is completely closed and the cup 60 has been shaped from the blank 6. As a result of the complete tool contact the cup 60 is completely cooled and hardened within a holding time that depends on the sheet thickness and other factors. Contact with the tool as well as the holding time should be designed such that the workpiece is not only hardened, but also warping is minimized so that the workpiece satisfies tolerance requirements.
FIG. 5 shows flow curves 14, 15, 16 for the same steel material at different temperatures. Curve 14 shows the flow curve at a low steel temperature, curve 15 shows the flow curve at a moderate steel temperature, and curve 16 shows the flow curve at a high steel temperature. The curves are theoretical curves; the degree of plastic elongation that the material can actually withstand before failure is not illustrated. However, curves 14, 15, and 16 show that the colder the material (curve 14), the more tension must be applied to achieve the same elongation values as at higher temperatures (curve 16). This is because the flow resistance at high temperatures is lower than at low temperatures. For hot deep-drawing or thermoforming, these differing flow resistances mean that for temperature gradients in the workpiece blank the entire deep-drawing force applied as tension always results in a localized elongation in the hottest region of the workpiece blank, since the flow resistance is lowest at this localized point. The locally concentrated tension frequently results in the prior-art system with drastic temperature gradients in failure of the material during shaping. The invention counteracts this problem by equalizing the differing temperature levels, and therefore also equalizes the differing flow resistances by ensuring that the regions that must transmit the deep-drawing forces during the shaping process are cooled in advance.
Patent Application
20070235113
