EP3414027B1 - Method and device for producing hardened steel components - Google Patents

Description

The invention relates to a method and a device for producing hardened steel components.

Hardened steel components have the advantage, particularly in the body shop of motor vehicles, that their outstanding mechanical properties make it possible to create a particularly stable passenger cell without having to use components that are much more massive and therefore heavier under normal strength.

Steel grades which can be hardened by quench hardening are used to produce hardened steel components of this type. Such types of steel are, for example, boron-alloyed manganese carbon steels, the most widely used steel being the 22MnB5. Other boron-alloyed manganese carbon steels are also used for this.

In order to produce hardened components from these types of steel, the steel material must be heated to the austenitizing temperature (> Ac ₃ ) and waited until the steel material is austenitized. Depending on the desired degree of hardness, partial or full austenitization can be achieved here.

If such a steel material is cooled after austenitization at a speed above the critical hardening rate, the austenitic structure converts to a martensitic, very hard structure. In this way tensile strengths R _m up to over 1500 MPa can be achieved.

Two process methods are currently used to produce the steel components.

In so-called form hardening, a sheet steel blank is cut out of a steel strip and then deep-drawn to the finished component in a conventional, for example five-step, deep-drawing process. This finished component is dimensioned somewhat smaller in order to compensate for subsequent thermal expansion during austenitizing.

The component produced in this way is then austenitized and then placed in a form hardening tool, in which it is pressed, but not or only very slightly reshaped, and the heat flows from the component into the pressing tool due to the pressing, with the pressure exceeding the critical hardening speed Speed.

The further process is the so-called press hardening, in which a blank is cut out of a sheet steel strip, the blank is then austenitized and the hot blank is formed in a one-step step and at the same time is cooled at a speed above the critical hardening rate.

In both cases, metallic corrosion protection layers, e.g. Zinc-coated boards can be used. Mold hardening is also referred to as an indirect process and press hardening as a direct process. The advantage of the indirect process is that more complex workpiece geometries can be realized.

The advantage of the direct process is that a higher degree of material utilization can be achieved with less component complexity.

In press hardening, however, it is disadvantageous that, particularly in the case of galvanized sheet steel blanks, microcracks are formed in the surface.

A distinction is made between first order microcracks and second order microcracks.

First order microcracks are attributed to the so-called Liquid Metal Embrittlement. It is believed that liquid zinc phases during forming, i.e. while tensile stresses are applied to the material, they interact with existing austenite phases, creating microcracks with depths down to a few 100 µm in the material.

The applicant has succeeded in preventing this by cooling the material between removal from the heating furnace and before insertion into the forming tool to temperatures at which there are no longer any liquid zinc phases. This means that hot forming takes place at temperatures below around 750 ° C.

The second order microcracks have so far not been manageable in hot forming despite pre-cooling and also occur at hot forming temperatures below 600 ° C. The crack depths are some 10 pm.

Neither first order microcracks nor second order microcracks are accepted by users as this is a potential source of damage.

With the previous methods, however, production of components without second order microcracks cannot be reliably represented.

From the DE 10 2011 055 643 A1 A method and forming tool for hot press hardening workpieces made of sheet steel is known, and in particular made of galvanized workpieces made of sheet steel. In this case, the die used for hot forming and press hardening should be liquid-coated in its drawing edge area defined by a positive drawing radius or be provided with an insert that has a thermal conductivity that is at least 10 W / (mx K) lower than the thermal conductivity of the section of the die which is adjacent to the drawing edge region and which comes into contact with the workpiece during hot forming and press hardening. The surface of the material applied in the region of the drawing edge or of the insert part facing the workpiece should have a transverse dimension which extends over the drawing edge and is in the range from 1.6 times to 10 times the positive drawing radius of the die. This is intended to improve the flow properties of workpieces made of sheet steel during hot forming and thus significantly reduce the risk of cracks occurring during the hot forming of workpieces made of sheet steel, preferably galvanized steel blanks. With such a tool, however, second type microcracks cannot be avoided.

From the DE 10 2011 052 773 A1 A tool for a press hardening tool is known, the shape-giving surface of the tool being microstructured in regions by two micro-depressions made in the mold surface. This measure is intended to limit the effective contact area between the mold surface with a blank for forming a blank to the area parts four located between the depressions. This is intended to reduce friction.

From the DE 10 2004 038 626 B3 A method for producing hardened components from sheet steel is known, wherein before, during or after the molding of the molded part, a necessary final trimming of the molded part and possibly necessary punching out or the production of a hole pattern is carried out and the molded part is then heated to a temperature at least in some areas , which enables the steel material to be austenitized, and wherein the component is subsequently transferred to a mold hardening tool and a mold hardening is carried out in the mold hardening tool, in which the component is cooled and thereby hardened by the at least partial application and pressing of the component by the mold hardening tools, the Component is supported by the form hardening tool in the area of the positive radii, in some areas at least and preferably in the area of the trimmed edges by two clamps and in areas in which the component is not clamped, the construction part is at least spaced from a mold half with a gap. This measure serves to be able to clamp the component without distortion and to set different hardness gradients through different hardness speeds.

The object of the invention is to avoid microcracks of the second type in directly hot-formed, that is to say press-hardened, components.

The object is achieved with a method having the features of claim 1.

Advantageous further developments are characterized in the subclaims.

It is also an object to provide a device with which sheet steel blanks are hot formed using the press hardening process and can be hardened and avoid micro-cracks.

The object is achieved with a device having the features of claim 3. Advantageous further developments are characterized in dependent claims dependent thereon.

The inventors have recognized that microcracks of the second type occur when the zinc vapor occurring in tensile areas reaches the steel in sufficient concentration, so-called Vapor Metal Embrittlement (VME). Zinc vapor is created by tearing open the zinc iron layer during stretching during the forming process. Adequate concentration occurs particularly in those areas in which there is direct contact of the sheet with the tool or in which there is a very small distance between the sheet and the tool. A very small distance in the sense of the invention is less than 0.5 mm.

According to the invention, second order microcracks are to be avoided, the largest possible working window with regard to material and temperature being retained and the implementation being cost-effective. With at least the same throughput time, there should be no increase in cycle time or throughput reduction in component production.

According to the invention, the zinc vapor or free zinc occurring in the tensile stressed areas (stretching edge fiber) is quickly converted into a stable compound such as zinc oxide or ZnO2 by the entry of fluids containing oxygen. Furthermore, the protection of the steel against second-order microcracks can also be achieved by producing a protective layer, such as an oxide layer, by supplying a fluid. The measures described have shown that microcracks are significantly reduced.

Gaseous oxygen-containing fluids such as air or oxygen are particularly preferred because they cannot contaminate the tool unduly or also have an undesirable massive cooling effect such as that caused by e.g. Water can be regulated more easily by tempering the fluid.

According to the invention, inserts are used in the tool, preferably in the region of the positive radii or adjacent to the positive radii, which allow oxygen to enter when the sheet metal blank is deformed, that is to say when the blank material flows. Furthermore, inserts can also be provided at narrow points or contact areas of the sheet metal blank with the tool, these contact areas being defined as those areas in which the sheet metal is at a maximum 0.5 mm from the tool.

To do this, the corresponding material must of course be supported in the area of the positive radii, because these are the edges that cause the deformation and initiate the material flow.

Adjacent to these edges and spaced from them so that the inserts are not damaged, the inserts have means that allow oxygen to enter. These agents are sintered metal inserts or porous ceramic inserts, in which, after the workpiece has been moved apart and hardened and before a new circuit board has been inserted, so much oxygen is stored that it can be released to released zinc or released zinc phases.

In addition, the inserts can have exposed areas so that the material is spaced apart from the insert after flowing past the edge.

In an advantageous embodiment, this exemption area is slit, so that a minimum support of the material is possible, but the access of oxygen is guaranteed.

In all of these cases, there may also be fluid connection lines which open into the clearances or into the areas which are filled with sintered metal or porous ceramic, so that sufficient oxygen is supplied. In the simplest case, this can be air or, for example, water vapor.

Materials that inherently have a high oxygen diffusion capacity, for example certain ceramics, can also be solid and are either exposed to fluids containing oxygen while the press is open or from the back and store it until it is released Zinc iron phases or released zinc can be released.

These inserts can be formed on both the female and male.

Oxygen can also be loaded by flooding the mold cavity, for example with water vapor or the media already mentioned.

• Figur 1 beispielhaft einen Werkzeugeinsatz in massiver Ausgestaltung;
• Figur 2 einen Werkzeugeinsatz mit Freistellung;
• Figur 3 einen weiteren Werkzeugeinsatz mit Freistellung;
• Figur 4 einen geschlitzten Werkzeugeinsatz in einer seitlichen Schnittansicht;
• Figur 5 den geschlitzten Werkzeugeinsatz in einer formflächenseitigen Ansicht.

The invention is explained by way of example with reference to a drawing. It shows:

• Figure 1 exemplary a tool insert in a massive design;
• Figure 2 a tool insert with exemption;
• Figure 3 another tool insert with exemption;
• Figure 4 a slotted tool insert in a side sectional view;
• Figure 5 the slotted tool insert in a mold side view.

An insert 1 according to the invention is formed, for example, from a ceramic and in particular an oxide ceramic. The ceramic insert runs along drawing edges 2 and is used in the tool instead of the metallic drawing edge 2, it having a rear side 3 and an underside 4 with which it is inserted in a form-fitting manner in a recess in the metallic tool. In addition, the ceramic insert 1 has an upper side 6 and a mold front side 5, the mold front side 5 and the upper side 6 preferably being aligned with the corresponding surfaces of the tool.

This ceramic insert can be solid or dense and hard or porous hard.

In the area of the surfaces 3 or 4, a gas connection (not shown) can accordingly be provided from the metallic mold, insofar as the ceramic is oxygen-conducting or porous, which through the insert 1 has sufficient concentration of oxygen in the area of the surfaces 5 and the pulling edge 2 brings.

A cutout 7 is made in the area of the area 5 adjacent to the drawing edge 2 ( Figure 2 ). The clearance 7 has, for example Depth of 5 to 10 mm, while the entire insert, for example, has a height between surfaces 4 and 6 of 35 to 50 mm and a width between surfaces 3 and 5 of 15 to 30 mm.

The drawing edge 2 is designed such that the thickness of the drawing edge in front of the recess 7 corresponds approximately to its radius.

In a further advantageous embodiment, instead of a recess 7 adjacent to the drawing edge 2 ( Figure 3 ) there is only a groove 8 running parallel to the surface 6, which has a depth of 5 to 8 mm, for example, the height of the groove 8 between the drawing edge 2 and the surface 5 being 8 to 12 mm.

According to the invention, it has been found that such a groove 8 already stores so much oxygen after shaping a component and inserting a new circuit board as a gas in this dimensioning that the sufficient oxygen supply is ensured during the shaping.

In a further advantageous embodiment ( Figures 4 , 5 ) the surface 5 is formed with slots 9 which run from a surface 4 in the direction of the drawing edge 2, but the drawing edge 2 still has a thickness which corresponds to its radius.

The slot width is 4 to 8 mm with a slot spacing of 7 to 11 mm, so that a web width of 2 to 5 mm is realized with a slot depth of 5 to 9 mm. Here, too, it has been shown that the web width has no negative influence on the oxygen supply.

In a further advantageous embodiment (not shown), the cutouts 7 or the groove 8 or the slots 9 are filled with a porous ceramic material or a porous sintered metal material, supply openings for fluids containing oxygen may be present on the rear side 3 of the insert and / or the sintered metal or ceramic inserts are loaded with oxygen between the shaping processes, for example by flooding the mold cavity with water vapor, or the ceramic and / or the sintered metal has such a high affinity for oxygen that during the shaping processes oxygen is absorbed, which is present during the drawing process released zinc iron or zinc phases is released.

The invention has the advantage that the formation of second-order microcracks can be effectively prevented by relatively simple measures, and existing forming tools can also be retrofitted to insert appropriately shaped inserts by milling out the positive radius areas or drawing edges.

In addition to the 22MnB5, the 20MnB8, 22MnB8 and other manganese-boron steels are also used, especially in the direct press hardening process.

Steels of this alloy composition are therefore suitable for the invention (all figures in% by mass): 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

Balance iron and melting-related impurities, in particular as a retarding agent in such steels the alloying elements boron, manganese, carbon and optionally chrome and molybdenum are used.

Steels of the general alloy composition are also suitable for the invention (all figures in% by mass): Carbon (C) 0.08-0.6 Manganese (Mn) 0.8-3.0 Aluminum (Al) 0.01-0.07 Silicon (Si) 0.01-0.8 Chrome (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

Remainder iron and melting impurities.

Steel arrangements have proven to be particularly suitable as follows (all figures in% by mass): Carbon (C) 0.08-0.35 Manganese (Mn) 1.00-3.00 Aluminum (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chrome (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

Remainder iron and melting impurities.

Claims (5)

1. A method for press-hardening galvanized sheet steel components, wherein a blank is cut out of a sheet steel strip made of a hardenable steel alloy and the blank is then austenitized by heating it to a temperature greater than Ac₃ and then placing it in a forming tool and forming it in the forming tool and cooling it during forming at a speed above the critical hardening speed, and that, in order to avoid microcracks of the second type on the sheet metal blank to be formed during the forming and hardening process, oxygen is supplied

   - at and/or adjacent to positive radii and/or drawing edges and/or

   - at contact areas,

   the oxygen being admitted by inserts (1) of oxygen-storing materials which are provided in the forming tool adjacent to or in the region of the drawing edges and/or positive radii, which inserts are dimensioned in such a way that deep-drawing is not impaired, and the inserts (1) form a reservoir for oxygen, wherein the inserts (1) are made of sintered metals, porous ceramics or dense ceramics, wherein a recess (7) is arranged in the ceramic inserts, which is dimensioned such that the remaining thickness of the drawing edge (2) between a surface bounding the drawing edge (2) and the recess (5) approximately corresponds to its radius.
2. A method according to any of the preceding claims, characterized in that the inserts (1) are fed with oxygen or oxygen-containing fluids from the forming tool side, or the inserts (1) or the mold cavity are/is flooded with oxygen or an oxygen-containing fluid between two forming operations.
3. A device for carrying out the method according to any of the preceding claims for press-hardening or hot-forming and hardening steel sheet blanks with two forming tool halves, wherein the two forming tool halves cooperate with a blank in a deep-drawing manner and are designed so as to be able to be moved towards each other and away from each other, wherein, corresponding to a desired forming contour, at least one positive radius or one drawing edge region comprising a drawing edge (2) is present, the ceramic insert is set in place of a metallic drawing edge (2), wherein it is inserted in a form-fitting manner in the respective mold half, wherein a recess (7) is arranged in the ceramic insert, which recess is dimensioned such that the remaining thickness of the drawing edge (2) between a surface bounding the drawing edge (2) and the recess (5) corresponds approximately to its radius.
4. The device according to claim 3, characterized in that the recess (5) between the drawing edge (2) and a forming tool surface (4) has a height which is about 25 to 35 mm at a depth of 5 to 9 mm or is formed as a groove (8) which has a height between the surface (4) and the drawing edge (2) which is approximately 8 to 12 mm at a depth of 5 to 9 mm, or in the region of the wall (4) adjacent to the drawing edge (2) a plurality of slots (9) extending in the drawing direction are present as recesses, the slots (9) having a slot width of 4 to 8 mm and a slot spacing of 7 to 11 mm, so that the remaining webs have a width of 1 to 5 mm.
5. The device according to claim 3 or 4, characterized in that the recesses (7), the grooves (8) or the slots (9) can be supplied with an oxygen-containing fluid at the rear, i.e. from the side of the tool, by means of feeding systems and correspondingly drilled lines.

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