KR20180112820A - METHOD AND DEVICE FOR MANUFACTURING CURED STEEL PARTS - Google Patents

Abstract

The present invention After separating the blank from a plate-shaped steel band consisting of a hardenable steel alloy, the blank is heated to a temperature above Ac ₃ , then inserted into a forming tool and molded in the forming tool, In order to prevent 2-type micro cracks from being formed in the plate-like metal blank to be formed in the molding and curing process, the positive radius and / or the oxygen-containing side adjacent to the drawing edge The present invention relates to a press-hardening method of a plate-shaped steel part and an apparatus for carrying out the method.

Description

METHOD AND DEVICE FOR MANUFACTURING CURED STEEL PARTS

The present invention relates to a method and apparatus for manufacturing hardened steel parts.

The hardened steel parts, particularly the steel parts used in the car body structure of automobiles, are particularly stable because of their excellent mechanical properties, without the use of a larger amount of parts at normal strength (consequently being implemented as heavier) passenger compartment can be constituted.

To produce this kind of hardened steel part, a type of steel that can be cured by quenching hardening is used. This type of steel includes, for example, boron-alloy manganese carbon steel, and the most widely used steel is 22MnB5. However, other boron-alloyed manganese-carbon steels are also used for this purpose.

To produce these types of steel hardened parts, the steel material must be heated to the austenitizing temperature (> Ac ₃ ) and the steel material must be austenitized. Depending on the desired hardness, partial or complete austenitization can be obtained.

After austenitization, when such steel is cooled at a rate that exceeds the critical cure rate, the austenite structure is converted to a martensitic, very hard structure. In this way, it is possible to achieve a tensile strength R _m of more than up to 1500 MPa.

Currently two different procedural approaches are commonly used in the production of steel rebar parts.

In so-called form hardening, a plate-shaped steel blank is separated from the steel band (for example, by cutting or stamping out) and then, using conventional methods such as a five-step deep drawing process, To produce final parts. This final part has somewhat smaller dimensions to compensate for subsequent thermal expansion during austenitization.

The parts produced in this way are austenitized and then inserted into a molding hardening tool that is pressed but not molded or only slightly molded and the heat is transferred from the component to the press tool at a rate that is faster than the critical hardening speed, do.

Another procedural approach is to separate the blank from the flat steel band (e.g., by cutting or stamping out), then austenitize the blank and heat the hot blank to a temperature of less than 782 DEG C Called press hardening, which is molded at the same time and cools at a rate faster than the critical curing rate.

In both cases, for example, a blank with a metal corrosion-resistant coating with zinc or a zinc-based alloy can be used. Form hardening is also called indirect process, and press hardening is called direct process. An advantage of the indirect process is that more complex tool shapes can be obtained.

The advantages of the direct process can be achieved with higher material utilization, but the complexity of the parts that can be obtained is reduced, especially in the one-stage molding process.

However, there is a disadvantage that in the press hardening, micro-cracks are formed on the surface, particularly when a zinc-plated plate-type steel blank is used.

In this regard, the primary micro crack and the secondary micro crack are distinguished.

The primary microcracks are due to so-called liquid metal embrittlement. The theory is that when forming, that is when tensile stress is applied to the material, the liquid phase remains interacting with the still existing austenite phase, creating microcracks in the material with depths of up to several hundred micrometers.

Applicants have been successful in suppressing these primary micro cracks by actively or passively cooling the material to a temperature at which the liquid phase is no longer present until it is taken out of the furnace and commencing a hot forming process did. This means that hot forming takes place at a temperature of less than about 750 ° C.

Until now, secondary micro cracks have not been able to be controlled in hot forming despite the preliminary cooling, and they occur even under hot forming of less than 600 캜. In this case, the depth of the crack reaches several tens of micrometers.

The primary microcracks or secondary microcracks are not acceptable to the user because they cause potential damage.

With previous methods, it is still impossible to guarantee the production of parts without secondary micro cracks.

DE 10 2011 055 643 A1 discloses a molding tool and method of hot press cured parts made of galvanized workpieces made of plate steel, especially plate steel. In this case, the female die used for hot forming and press hardening is either liquid coated with a material in the drawing edge area defined by the positive drawing radius, or the insert piece must be provided, And has a thermal conductivity that is at least 10 W / (mxK) lower than the thermal conductivity of the female mold that is in contact with the workpiece when the drawing edge region is hotformed and press cured. The material applied to the surface of the drawing edge region facing the insert piece disposed in the workpiece or in position is a transverse dimension extending across the drawing edge in the range of 1.6 to 10 times the positive drawing radius of the female mold. . This should improve the flow characteristics of the work piece made of plate steel during hot forming and thus significantly reduce the risk of cracking during hot forming of the work piece made of plate steel, preferably galvanized steel blank. However, these tools can not avoid type 2 micro cracks.

DE 10 2011 052 773 A1 discloses a tool for press hardening in which the mold surface of the tool is micro-structured in some areas by two micro-cavities introduced into the mold surface. This step is intended to limit the effective contact area (area) for forming the blank between the mold surface having the blank and the surface portion arranged in the cavity to four. This is to reduce friction.

DE 10 2004 038 626 B3 performs the required final trimming of the molded part and any required punching process or the creation of the hole pattern before or after molding the molded part and then the molded part is then moved Heating to a temperature to allow austenitization of the material; The component is then transferred to a mold hardening tool and mold hardening is performed in the mold hardening tool where the component is cooled and hardened in at least some areas by contact and pressing of the component; Preferably, the part is fixed by two clamps in the region of the trim edge, and in the area where the part is not clamped (not clamped) (Half of the mold). With this method, the part can be fixed without twisting, and another curing gradient can be set by another curing speed.

An object of the present invention is to avoid 2-type micro cracks in direct hot-formed, i.e., press-hardened parts.

This object is achieved by a method having the features of claim 1. Advantageous changes are specified in the dependent claims.

It is still another object of the present invention to provide a device capable of hot-forming and curing a plate-shaped steel blank in a press-hardening process and avoiding micro-cracks.

This object is achieved with an apparatus having the features of claim 6. Advantageous modifications are specified in the dependent claims citing the claims.

The present inventors have found that, in the region subjected to tensile strain, a type 2 microcrack is generated at the so-called vapor metal embrittlement (VME) when the resulting zinc vapor reaches steel at sufficient concentration. Zinc vapor is produced by the tearing of the zinc / iron layer that occurs during stretching during the forming process. In particular, the area in which the sheet metal directly contacts the tool is dominant (wider) or sufficient concentration occurs in the area where the sheet metal is at a very short distance from the tool. The very short distance defined in the present invention is less than 0.5 mm.

In accordance with the present invention, secondary micro cracks must be avoided while maintaining the largest possible work window for material and temperature and providing an inexpensive implementation. At least the same residence time should not increase cycle time or reduce throughput during parts production.

According to the present invention, in the region under tensile strain (elongated edge fibers), the zinc vapor that is generated is moved away (convected), more precisely by thinning, or sufficiently diluted by the gas flow. Alternatively or additionally, through the inflow of fluid, zinc is rapidly converted into stable compounds such as zinc oxide or ZnI ₂ . In addition, protecting the steel from secondary micro cracks can also be accomplished by creating a protective layer, such as an oxide layer, by supplying fluid. All of the above methods have been shown to significantly reduce microcracks.

Gas-containing oxygen-containing fluids such as air or oxygen are particularly preferred because they can not excessively contaminate the tool and the undesirable massive cooling action, for example caused by water, Since it can be adjusted more easily by tempering.

In this case, the avoidance of the secondary microcracks may occur after a different contact area located outside the positive radius / drawing edge in the drawing radius or drawing direction in the positive radius area, i.e., the drawing edge area of the female mold and / On the one hand, the deep draw is not adversely affected, or the blank or workpiece is constructed (dimensioned) to be in a wave-like shape and, on the other hand, (Dimensions) that are not negatively influenced by the size of the product.

However, the recesses constitute a reservoir of oxygen to allow a sufficient amount of oxygen to reach the blank to be drawn and the material to be drawn, in order to supply oxygen for oxidation of the discharged zinc or zinc / iron phase.

This recess, in particular, functions as a fluid reservoir for oxygen, but the reservoir may contain other fluids such as water or nitrogen. When these reservoirs are filled with an inert gas or continuously flushed with an inert gas, they function by diluting or transporting the resulting zinc vapor rather than functioning by oxidation.

If necessary, oxygen-containing fluid may be continuously supplied to the concave portion, for example, from the tool side through an appropriate inlet opening to form a fluid cushion, advantageously. Further, after removing the workpiece from the mold and before inserting the other blank, the mold cavity can be flushed with a fluid, particularly a fluid containing oxygen, which is then present in the recess. Examples of oxygen-containing fluids include not only air but also water, that is, they can be supplied in liquid and gaseous form.

These recesses effectively prevent the formation of secondary microcracks through oxidation of zinc or zinc / iron, even if their dimensions are relatively small.

The present invention will be described by way of example based on the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a tool area adjacent a drawing edge with a recess of the present invention.
Fig. 2 shows a drawing edge region of a tool having a concave portion according to another embodiment of the present invention.
Figure 3 is a partial cutaway side view of a drawing edge region of a tool with a slot arrangement according to the present invention.
Figure 4 is a top view of the arrangement of Figure 3;

The drawing edge region 1 or the positive radius region 1 has two surfaces 3 and 4 which are arranged on the forming tool and which face the workpiece to be encountered in the region of the drawing edge or positive radius 2.

The recess 5 according to the invention is provided on the surface 4 located behind (behind) the drawing edge 2 in the drawing direction. The concave portion 5 in this case is formed so that the residual thickness of the drawing edge 2 between the surface 3 and the concave portion 5 substantially coincides with the radius thereof in order to provide sufficient supporting action for the material to be drawn .

Of course, another recess may be provided which is located in the area where the sheet metal contacts the tool; These contact areas are defined as a maximum distance of about 0.5 mm between the sheet metal and the tool.

Between the drawing edge 2 and the surface 4, the recess 5 has a height of about 25 to 35 mm and a depth of 5 to 9 mm.

In another advantageous embodiment (Fig. 2), instead of providing a large area of recess 5 adjacent the drawing edge 2 and leaving the abovementioned thickness, a groove 6 . In this case, the groove 6 has a total height of about 8 to 12 mm between the surface 4 and the drawing edge 2, and the depth is 5 to 9 mm.

In a further embodiment, instead of the continuous recesses 5 in the region of the wall 4 adjacent to the drawing edge 2, a plurality of grooves 7 are provided which extend in the drawing direction; For example, the groove 7 or slot 7 has a slot width of 4 to 8 mm and a slot spacing of 7 to 11 mm, so that the remaining bridge pieces have a width of 1 to 5 mm. In this case the groove 7 or slot 7 likewise has a depth of 5 to 9 mm.

Surprisingly, due to the shape described above, even a relatively small amount of fluid within the recesses 5, 6, 7, despite the presence (or the presence possibility) of the bridge pieces 4, Type microcracks formed on the surface of the substrate.

In the advantageous embodiment (not shown), the recess 5, the groove 6 and the slot 7 are provided with a plurality of drilled lines correspondingly pierced from the rear, i.e. from the tool side, The oxygen-containing fluid is supplied by the oxygen-containing gas, and the oxygen partial pressure in the recessed portion 5, the groove 6 and the slot 7 region can be further increased as required.

The mold cavity is flushed with the oxygen-containing fluid so as to form the concave portion 5, the groove 6, and the groove 7 in order to maintain the oxygen content in the concave portion 5, the groove 6, And the slot 7 always have sufficient oxygen reservoir.

In direct press hardening processes, besides 22MnB5, 20MnB8, 22MnB8 and other manganese / boron steel are mainly used.

As a result, steels having the following alloy composition are suitable for the present invention (all expressed in mass%).

And the balance of iron and smelting impurities. In such steels, alloying elements such as boron, manganese, carbon and optionally chromium and molybdenum are used as transformation-delaying agents.

Steel with the following general alloy composition is also suitable for the present invention (all expressed in mass%).

And the balance of iron and smelting impurities.

It has been found that steels having the following alloy composition are particularly suitable for the present invention (all expressed in mass%).

And the balance of iron and smelting impurities.

2: Drawing edge
3, 4: Surface
5:
6: Home
7: a plurality of grooves (slots)

Claims (10)

After separating the blank from a plate-shaped steel band consisting of a hardenable steel alloy, the blank is heated to a temperature above Ac ₃ , then inserted into a forming tool and molded in the forming tool, Wherein the oxygen-containing fluid reservoir is cooled at a rate exceeding the critical curing rate at the time of forming and the positive radius and / or the edge of the drawing is cooled in order to prevent the formation of a 2-type microcrack in the plate- And / or in another contact area outside the positive radius and / or the drawing edge. ≪ Desc / Clms Page number 20 > The method according to claim 1,
Oxygen is introduced by the recesses provided in the forming tool adjacent to the drawing edge and / or the positive radius, the recesses being configured so as not to adversely affect the deep drawing, the recesses forming a reservoir of oxygen- Or another oxygen-containing fluid can be supplied through the recess. 3. The method according to claim 1 or 2,
Wherein the introduction of the oxygen-containing fluid is ensured by the air present in the recess. 4. The method according to any one of claims 1 to 3,
Wherein a fluid, oxygen, or oxygen-containing fluid is supplied to the recess from the molding tool side, or a fluid, particularly an oxygen or oxygen-containing fluid, is supplied to the recess or the mold cavity between the two molding steps , Way. 5. The method according to any one of claims 1 to 4,
Wherein a vacuum is applied to the recess. The method according to claim 1,
Wherein the oxygen-containing fluid is continuously supplied. 7. An apparatus for carrying out the method of any one of claims 1 to 6,
Has two half-forming tools (half of the forming tool); The two half-formed tools are designed to cooperate to deep draw the blank and to be close to and away from each other; Depending on the desired shaping shape, a drawing edge is provided in at least one positive radius or drawing edge area; Wherein a recess is provided in a surface located behind the positive radius or drawing edge in the drawing direction and / or in another contact area outside the positive radius and / or the drawing edge. 8. The method of claim 7,
Wherein the recess is configured such that the remaining thickness of the drawing edge between the surface adjacent to the drawing edge and the recess coincides approximately with its radius. 9. The method according to claim 7 or 8,
Wherein the recess between the drawing edge and the forming tool surface has a height of about 25 to 35 mm at a depth of 5 to 9 mm or a height of about 8 to 12 mm in total between the surface and the drawing edge, And a plurality of grooves extending in the drawing direction in a wall region adjacent to the drawing edge, the plurality of grooves or slots having a slot width of 4 to 8 mm and a slot width of 7 to 11 mm mm so that the remaining bridge pieces have a width of between 1 and 5 mm. 10. The method according to any one of claims 7 to 9,
Wherein the recess, the groove, or the slot is supplied with an oxygen-containing fluid from behind, i.e. from the tool side, by a feed opening and a corresponding perforated line.

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