METHOD AND DEVICE FOR PRODUCING HARDENED STEEL COMPONENTS

Abstract

The invention relates to a method for press hardening sheet steel components in which a blank is detached from a sheet steel band composed of a hardenable steel alloy and the blank is then austenitized, in that it is heated to a temperature greater than Ac.sub.3 and is then inserted into a forming tool and formed in the forming tool, and during the forming, is cooled at a speed greater than the critical hardening speed, characterized in that in order to inhibit microcracks of the second type from being produced during the forming and hardening process in the sheet metal blanks that are to be formed, oxygen is supplied adjacent to the positive radii and/or drawing edges; the invention also relates to a device for performing this method.

Claims

1. A method for press hardening sheet steel components comprising: cutting a blank out from a sheet steel band composed of a hardenable steel alloys; austenitizing the blank, by heating it to a temperature greater than Ac.sub.3, inserting it into a forming tool, and forming in the forming tool, and during the forming, cooling it at a speed greater than the critical hardening speed; characterized in that in order to avoid microcracks of second type from forming in the sheet metal blank to be formed during the forming and hardening process, oxygen is supplied in and/or adjacent to the positive radii and/or drawing edges and/or in contact regions.

2. The method according to claim 1, characterized in that the entry of oxygen by means of inserts (1) made of oxygen-storing materials are provided in the forming tool adjacent to or in the region of the drawing edges and/or positive radii, which are dimensioned so that deep drawing is not negatively affected and the inserts (1) form a reservoir for oxygen.

3. The method according to claim 2, characterized in that inserts (1) made of sintered metals, porous ceramics, or impervious ceramics are used.

4. The method according to claim 2, characterized in that the inserts (1) are supplied with oxygen or oxygen-containing fluids from the forming tool side, or the inserts (1) or mold cavity are flooded with oxygen or an oxygen-containing fluid between two forming procedures.

5. A device for the press hardening or hot forming and hardening of sheet steel components, having two forming tool halves; the two forming tool halves cooperate in order to deep-draw a blank and are embodied so that they can move toward and away from each other; depending on a desired forming contour, at least one positive radius or one drawing edge region is provided with a drawing edge (2), a ceramic insert being positioned in lieu of a metallic drawing edge (2), wherein the it is inserted into the respective forming tool half in a form-fitting way.

6. The device according to claim 5, characterized in that in the ceramic insert, a recess (7) is provided, which is dimensioned so that the remaining thickness of the drawing edge (2) between a surface, which adjoins the drawing edge (2), and the recess (5) corresponds approximately to its radius.

7. The device according to claim 6, characterized in that the recess (5) between the drawing edge (2) and a forming tool surface (4) has a height, which corresponds to approximately 25 to 35 mm at a depth of 5 to 9 mm or is embodied as a groove (8), which has a height between the surface (4) and the drawing edge (2), which totals approximately 8 to 12 mm, with a depth of 5 to 9 mm or in the region of the wall (4) adjacent to the drawing edge (2), a plurality of recesses in the form of slots (9) extending in the drawing direction; and the slots (9) have a slot width of 4 to 8 mm and a slot spacing of 7 to 11 mm so that remaining bridge pieces have a width of 1 to 5 mm.

8. The device according to claim 7, characterized in that the recess (7), the groove (8), or the slots (9) are supplied from the rear, i.e. from the tool side, with an oxygen-containing fluid by means of supply openings and correspondingly drilled lines.

9. The method according to claim 3, characterized in that the inserts (1) are supplied with oxygen or oxygen-containing fluids from the forming tool side, or the inserts (1) or mold cavity are flooded with oxygen or an oxygen-containing fluid between two forming procedures.

Description

[0040] The invention will be explained by way of example based on the drawings. In the drawings:

[0041] FIG. 1 shows an example of a tool insert in a massive embodiment;

[0042] FIG. 2 shows a tool insert with a recess;

[0043] FIG. 3 shows another tool insert with a recess;

[0044] FIG. 4 is a sectional side view of a slotted tool insert;

[0045] FIG. 5 shows the slotted tool insert in a view from the forming surface.

[0046] For example, an insert 1 is made of a ceramic and in particular, of an oxide ceramic. The ceramic insert extends along drawing edges 2 and is used in the tool in lieu of the metallic drawing edge 2; it has a back side 3 and an underside 4 with which it is inserted in a form-fitting way into a recess in the metallic tool. In addition, the ceramic insert 1 has a top side 6 and mold-front side 5, the mold-front side 5 and top side 6 preferably being flush with the corresponding surfaces of the tool.

[0047] This ceramic insert can be embodied as massive or impervious and hard or porous and hard.

[0048] In the region of the surfaces 3 or 4, leading from the metallic forming tool and corresponding to the latter, a gas connection (not shown) can be provided, if the ceramic is embodied as oxygen-conducting or porous, which brings a sufficient concentration of oxygen through the insert 1 to the region of the surfaces 5 and the drawing edge 2.

[0049] In another advantageous embodiment (FIG. 2), a recess 7 is produced in the region of the surface 5 adjacent to the drawing edge 2. For example, the recess 7 has a depth of 5 to 10 mm, whereas the insert as a whole has a height between the surfaces 4 and 6 of 35 to 50 mm and a width between the surfaces 3 and 5 of 15 to 30 mm, for example.

[0050] Preferably, the drawing edge 2 in this case is embodied so that the thickness of the drawing edge in front of the recess 7 corresponds approximately to its radius.

[0051] In another advantageous embodiment, in lieu of a recess 7 adjacent to the drawing edge 2 (FIG. 3), there is only a groove 8 extending parallel to the surface 6 that has a depth, for example, of 5 to 8 mm, with the height of the groove 8 between the drawing edge 2 and the surface 5 being 8 to 12 mm.

[0052] According to the invention, it has turned out that such a groove 8 with these dimensions stores enough oxygen in the form of a gas after the demolding of a component and the insertion of a new blank to ensure the sufficient oxygen supply during the forming.

[0053] In another advantageous embodiment (FIGS. 4, 5), the surface 5 is embodied with slots 9, which extend from a surface 4 in the direction of the drawing edge 2, but the drawing edge 2 still has a thickness that corresponds to its radius.

[0054] The slot width in this case is 4 to 8 mm, with a slot spacing of 7 to 11 mm so that a bridge piece width of 2 to 5 mm is achieved with a slot depth of 5 to 9 mm. Here, too, it has turned out that the bridge piece width does not negatively influence the oxygen supply.

[0055] In another advantageous embodiment (not shown), the recesses 7 or the groove 8 or the slots 9 are filled with a porous ceramic material or a porous sintered metal material; on the back side 3 of the insert, supply openings for oxygen-containing fluids can be provided and/or the sintered metal inserts or ceramic inserts are charged with oxygen between the forming procedures, for example by flooding the mold cavity with water vapor, or the ceramic and/or the sintered metal has a high enough oxygen affinity that during the forming procedures, oxygen is absorbed, which during the drawing procedure, is imparted to released zinc iron or zinc phases.

[0056] The invention has the advantage that relatively simple measures can be used to effectively prevent the formation of second-order microcracks; also, existing forming tools can be retrofitted by milling out the positive radius regions and/or the drawing edges inserting correspondingly shaped inserts.

[0057] In order to keep the oxygen content at a high level in the recesses 5, grooves 6, and slots 7 during continuous processing, the mold cavity can also be flushed with an oxygen-containing fluid so that at all times, there is a sufficient oxygen reservoir in the recesses 5, grooves 6, and slots 7.

[0058] Primarily in the direct press hardening process, 20MnB8, 22MnB8, and other manganese/boron steels are also used in addition to 22MnB5.

[0059] Consequently, steels of the following alloy composition are suitable for the invention (all indications in mass %):

TABLE-US-00001 C Si Mn P S Al Cr Ti B N [%] [%] [%] [%] [%] [%] [%] [%] [%] [%] 0.20 0.18 2.01 0.0062 0.001 0.054 0.03 0.032 0.0030 0.0041
and the rest made up of iron and smelting-induced impurities; in such steels, particularly the alloy elements boron, manganese, carbon, and optionally chromium and molybdenum, are used as transformation-delaying agents.

[0060] Steels of the following general alloy composition are also suitable for the invention (all indications in mass %):

TABLE-US-00002 Carbon (C) 0.08-0.6 Manganese (Mn)  0.8-3.0 Aluminum (Al)  0.01-0.07 Silicon (Si) 0.01-0.8 Chromium (Cr) 0.02-0.6 Titanium (Ti)  0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1  
and the rest made up of iron and smelting-induced impurities.

[0061] The following steel configurations have turned out to be particularly suitable (all indications in mass %):

TABLE-US-00003 Carbon (C) 0.08-0.35 Manganese (Mn) 1.00-3.00 Aluminum (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chromium (Cr) 0.02-0.3  Titanium (Ti) 0.03-0.04 Nitrogen (N)  <0.007 Boron (B) 0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1  
and the rest made up of iron and smelting-induced impurities.