JP6744416B2 - Method and apparatus for producing hardened steel parts - Google Patents

Description

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

Hardened steel parts are specially used in car body manufacturing, especially in automobiles, because of their excellent mechanical properties, without the need to use parts that have to be embodied as heavy according to their much larger size at normal strength. It has an advantage that it is possible to realize a stable passenger compartment.

To produce hardened steel parts of this kind, steel types which can be hardened by quench hardening are used. Steel types of this type include, for example, boron alloyed manganese carbon steel, the most widely used of which is 22MnB5. However, other boron alloyed manganese carbon steels are also used for this purpose.

To produce the cured part from these types of steel, steel material has to be heated to the austenitizing temperature (> Ac _3), the steel material needs to wait until the austenite. Depending on the desired hardness, partial or complete austenitization can be achieved in this connection.

After austenitizing, when such a steel material is cooled at a rate above the critical hardening rate, the austenitic structure transforms into a very hard structure, martensite. In this way, it is possible to realize a tensile strength Rm exceeding 1500 MPa at the maximum.

Currently, two different procedural approaches are commonly used to manufacture steel parts.

In so-called form hardening, sheet steel blanks are cut (eg cut or punched from it) from the strip steel and then-using conventional, eg 5-step deep-drawing processes-deep-drawn to produce finished parts. .. In this case, the finished part is sized somewhat smaller to compensate for thermal expansion during subsequent austenitization.

The parts thus produced are austenitized and then inserted into a form-hardening tool and pressed therein, but not formed or formed to a very small extent, the pressing step Heat flows out of the part, specifically at a rate greater than the critical cure rate, and into the press tool.

The other procedural approach is that the blank is cut (e.g., cut or stamped from it) from the sheet steel and the blank is then austenitized, the hot blank preferably at a temperature below 782°C in a one-step process. This is so-called press hardening, in which at the same time as being molded, it is cooled at a rate higher than the critical hardening rate.

In either case, it is possible to use a blank provided with a metal anticorrosion coating (eg zinc or a zinc-based alloy). Mold hardening is also called indirect process and press hardening is called direct process. The advantage of the indirect process is that more complex tool geometries can be achieved.

The advantage of the direct method is that higher material utilization can be achieved. However, the achievable part complexity is low, especially in the one-step molding process.

Press hardening, however, has the disadvantage that microcracks form on the surface, especially in galvanized sheet steel blanks.

In this connection, a distinction is made between primary and secondary microcracks.

The primary microcracks are said to be due to so-called liquid metal embrittlement. The theory is that during molding, i.e. when a tensile stress is exerted on the material, the liquid zinc phase interacts with the austenite phase, which is still present, causing microcracks with a depth of up to several hundred μm to be created in the material. It will be.

The Applicant has determined that this primary temperature by cooling the material to a temperature at which the liquid zinc phase is no longer present-during the time between removal from the furnace and the start of the hot forming process-actively or passively. Succeeded in suppressing microcracks. This means that hot forming occurs at temperatures below about 750°C.

So far, it is not possible to control secondary microcracks in hot forming even with pre-cooling, and secondary microcracks also occur at hot forming temperatures below 600°C. The crack depth in this case amounts to tens of μm.

Both primary and secondary microcracks constitute a potential source of damage and are unacceptable to the user.

Until now, it has not yet been possible to guarantee the production of parts without secondary microcracks.

Patent Document 1 discloses a method and a forming tool for hot forming press hardening a part made of a thin steel plate, in particular, a galvanized work made of the thin steel plate. In this case, the female mold used for hot forming and press hardening is-in the drawing angle region of the female mold defined by the positive drawing radius-adjacent to the drawing angle region and the work is hot forming and pressing. It should be liquid coated or provided with an insert piece with a material having a thermal conductivity that is at least 10 W/(m×K) less than the thermal conductivity of the section of the female mold that contacts the work when cured. The material applied to the area of the drawing angle facing the workpiece or the surface of the insert piece placed in a suitable position should be in the range of 1.6 to 10 times the positive drawing radius of the female die. It should have a lateral dimension that extends to. This should improve the flow properties of the work made of sheet steel during hot forming, and thus the cracking in hot forming of work made of sheet steel, preferably made of galvanized steel blanks. It should significantly reduce the risk of occurrence. However, such tools do not make it possible to avoid the second type of microcracks.

Patent Document 2 discloses a tool for a press hardening tool in which the mold surface of the tool is microstructured in a partial region by two micro holes introduced into the mold surface. This step is intended to limit the effective contact area for the molding of the blank between the mold surface and the blank to the surface part located between the holes. This is intended to reduce friction.

Patent Document 3 discloses a method for manufacturing a hardened part from a thin steel plate. In this method, before or after forming the molded part, the required final trimming of the molded part and the production of any necessary perforation procedure or hole pattern are carried out, the molded part then in at least some areas being made of steel material. Heated to a temperature that enables austenitization; the part is then transferred to a mold hardening tool where mold hardening is carried out and the part is cooled, thus at least partly by contact and pressurization of the part. In the area of the positive radius, the part is supported by the forming and hardening tool in the area of the positive radius and is preferably held by two clamps in the area of the trim end, in the area where the part is not clamped It is separated from the mold half by at least a gap. This measure secures the part against distortion and makes it possible to set different hardness gradients with different hardening rates.

German patent application publication No. 10 2011 055 643 A1 German Patent Application Publication No. 10 2011 052 773 A1 German Patent No. 10 2004 038 626 B3

The object of the invention is to avoid microcracks of the second type in direct hot-formed, ie press-hardened parts.

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

Advantageous variant features are indicated in the dependent claims.

Another object of the present invention is to create a device that is capable of hot forming and hardening sheet steel blanks in a press hardening process and avoids microcracks.

This object is achieved by a device having the features of claim 6. The dependent claims, which are dependent on this claim, show advantageous variant features.

The present inventors have found that in the so-called vapor metal embrittlement (VME), in which the generated zinc vapor reaches the steel at a sufficient concentration in the region subjected to tensile strain, the second type of microscopic I recognized that a crack would be created. Zinc vapor is created by the splitting of the zinc/iron layer that occurs during stretching during the molding process. Sufficient concentrations occur especially in areas where direct contact between the thin metal plate and the tool is predominant or where the thin metal plate is at a very small distance from the tool. A very small distance as defined by the present invention is less than 0.5 mm.

According to the invention, secondary microcracks should be avoided while retaining the maximum possible working window in terms of material and temperature and ensuring cheap materialization. With at least the same residence time, there should be no increase in cycle time or reduction in throughput during part manufacture.

According to the invention, in the region under tensile strain (stretched end fibers), the zinc vapor generated is carried away by the gas flow (convection), or more precisely blown away or sufficiently Diluted. Alternatively or additionally, zinc can be rapidly converted to a stable compound such as zinc oxide or ZnI ₂ by the entry of a fluid. Furthermore, protection of the steel from secondary microcracks can also be achieved by supplying a fluid to create a protective layer such as an oxide layer. All of the above measures each show a marked reduction in microcracks.

Gaseous oxygen-containing fluids such as air or oxygen cannot over-contaminate the tool and, moreover, by adjusting the fluid more easily regulate possibly powerful undesired cooling effects of the kind that can occur, for example, with water. Therefore, it is particularly preferable.

In this case, avoidance of secondary microcracks is ensured by: In the area of positive radius (ie in the drawing angle area of the female and/or male mold), after a drawing angle or other contact area lying outside the positive radius/drawing angle in the drawing direction, a depression Are sized so that, on the one hand, the deep drawing is not adversely affected or the blank or workpiece is corrugated, and on the other hand, the heat outflow required for hardening is likewise of a considerable extent. It is dimensioned so that it is not adversely affected.

The dimples, however, do not allow the dimples to be oxygenated so that a sufficient amount of oxygen goes into the blank to be drawn and into the material to supply the released zinc or zinc/iron phase with oxygen for oxidation. Sized to form a reservoir for the.

The well functions as a fluid reservoir for oxygen in particular, but this reservoir can also contain other fluids such as water or nitrogen. Even when these reservoirs are filled with an inert gas or continuously flushed with an inert gas, they act by diluting or carrying away the zinc vapor that is generated, not by oxidation. ..

If necessary, a flow cushion can advantageously be formed, for example via a suitable inlet opening, preferably into the depression, an oxygen-containing fluid is continuously supplied from the tool side during molding. be able to. In addition, before removal of the workpiece from the mold and insertion of another blank, the mold cavity can be flushed with a fluid, which fluid in particular contains oxygen, which is then present in the depression. Examples of oxygen-containing fluids include air and water, that is, they can be supplied in both liquid and gas forms.

It has been found that these depressions, even when their dimensions are relatively small, effectively prevent the second type of microcracking through the oxidation of the zinc phase or the zinc/iron phase.

The invention is explained by way of example with reference to the drawings.

Figure 5 shows the tool area adjacent to the drawing angle in the depression according to the invention. 3 shows the drawing angle area of the tool in different embodiments of the depression according to the invention. FIG. 7 is a side view, partly cut away, of the drawing angle area of the tool in a slot arrangement according to the invention. FIG. 4 is a top view of the arrangement according to FIG. 3.

The drawing angle region 1 or the positive radius region 1 is located on the forming tool and has two surfaces 3, 4 towards the workpiece, the workpieces intersect at the drawing angle or the positive radius 2 region.

The indentation 5 according to the invention is provided in the surface 4 located after the aperture angle 2 in the aperture direction. In order to provide a sufficient supporting action for the drawing material, the depressions 5 are in this case dimensioned such that the thickness of the drawing angle 2 between the surface 3 and the depressions 5 corresponds approximately to its radius. To be done.

Of course, other depressions can be provided which are located in the area where the thin metal plate comes into contact with the tool. These contact areas are defined by a maximum distance of about 0.5 mm between the thin metal plate and the tool.

Between the squeezing angle 2 and the surface 4, the depression 5 has a depth of 5 to 9 mm and a height of about 25 to 35 mm.

In another advantageous embodiment (FIG. 2), instead of providing a wide depression 5 adjacent to the aperture angle 2, a groove 6 is introduced in the surface 4, leaving its above-mentioned thickness unchanged. In this case, the groove 6 has a depth between 5 and 9 mm, with a height between the surface 4 and the diaphragm angle 2 of a total of about 8 to 12 mm.

In a further embodiment, instead of a continuous depression 5 in the region of the wall 4 adjacent to the diaphragm angle 2, a plurality of grooves 7 extending in the diaphragm direction are provided. For example, the groove 7 or slot 7 has a slot width of 4-8 mm and a slot spacing of 7-11 mm, the remaining bridge pieces having a width of 1-5 mm. In this case, the groove 7 or slot 7 likewise has a depth of 5-9 mm.

In order to effectively prevent the formation of the second type of microcracks through the provision of oxygen, with the above-mentioned shape, with a relatively small amount of fluid in the depressions 5, 6, 7-in some cases also the presence of a bridge piece 4. Nevertheless-surprisingly, it turns out to be sufficient.

In an advantageous embodiment (not shown), if necessary, in order to further increase the partial pressure of oxygen in the region of the depression 5, the groove 6 and the slot 7, the depression 5, the groove 6 and the slot 7 are From the rear, i.e. from the tool side, the oxygen-containing fluid is supplied by the supply opening and the correspondingly perforated line.

In order to keep the oxygen content in the depression 5, groove 6 and slot 7 at a high level during continuous machining, there is always sufficient oxygen storage in the depression 5, groove 6 and slot 7, The holes can also be flushed with an oxygen containing fluid.

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

残りは鉄および製錬由来の不純物で占められる。そのような鋼において、特に、合金元素ホウ素、マンガン、炭素および任意選択としてクロムおよびモリブデンが、変換遅延剤として用いられる。 As a result, steels with the following alloy compositions are suitable for the present invention (all given in mass%):

The balance is made up of iron and impurities from smelting. In such steels, in particular the alloying elements boron, manganese, carbon and optionally chromium and molybdenum are used as conversion retarders.

Steels with the following general alloy compositions are also suitable for the present invention (all expressed in% by weight).
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
The balance is made up of iron and impurities from smelting.

The following steel configurations have been found to be particularly suitable (all given in 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
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
The balance is made up of iron and impurities from smelting.

Claims (10)

A blank is cut out from a steel strip made of hardenable alloy steel, said blank is then austenitized by heating it to a temperature above Ac ₃ and then inserted into a forming tool, said forming A method for press hardening a sheet steel part formed in a tool and cooled at a rate greater than the critical hardening rate during said forming, said thin sheet being coated with zinc or a zinc-based alloy during said forming and hardening process. In order to avoid the formation of a second type of microcracks in the sheet metal blank, the oxygen-containing fluid reservoir is
Adjacent to a positive radius and/or aperture angle, and/or to other contact areas outside the positive radius and/or aperture angle,
A method characterized by being present. The inflow of oxygen is caused by a depression provided in the forming tool adjacent to the drawing angle and/or a positive radius, which deep drawing is not adversely affected and the depression forms a reservoir for the oxygen-containing fluid. Or the oxygen-containing fluid is dimensioned such that it can be supplied through this recess. Method according to claim 1 or 2, characterized in that the inflow of oxygen-containing fluid is ensured by the air present in the depression. A fluid or oxygen or an oxygen-containing fluid is supplied to the recess from the forming tool side, or a fluid, in particular an oxygen or oxygen-containing fluid, is supplied to the recess or the mold cavity between two molding procedures. Method according to any one of claims 1 to 3, characterized in that Method according to any one of claims 1 to 4, characterized in that a vacuum is applied to the depressions. Method according to claim 1, characterized in that the oxygen-containing fluid is supplied continuously. Device for carrying out the method according to any one of claims 1 to 6, comprising two forming tool halves; two forming tool halves cooperating to deep draw the blank. However, they are embodied such that they can move towards and away from each other; depending on the desired shaping contour, at least one positive radius (1) or one throttle angle region (1). ) Is provided with an aperture angle (2) and the depression (5) is
A surface (4) located after the aperture angle (2) or a positive radius (1) in the aperture direction, and/or other contact areas outside the positive radius (1) and/or the aperture angle (2). To
A device provided. The recess (5) is dimensioned such that the remaining thickness of the stop angle (2) between the surface adjacent the stop angle (2) and the recess (5) corresponds approximately to its radius. Device according to claim 7, characterized in that it is determined. The depression (5) between the drawing angle (2) and the forming tool surface (4) has a height corresponding to about 25-35 mm at a depth of 5-9 mm, or a total of about 8-12 mm. Adjacent to the aperture angle (2) in the region of a groove (6) having a height between said surface (4) and said aperture angle (2) and a depth of 5-9 mm, or a wall (4). And is embodied as a plurality of depressions in the form of grooves (7) extending in the drawing direction; said grooves (7) or slots (7) having a slot width of 4-8 mm and a slot spacing of 7-11 mm. Device according to claim 7 or 8, characterized in that it has, so that the remaining bridge pieces have a width of 1-5 mm. It is provided that an oxygen-containing fluid is supplied to the recess (5), the groove (6) or the slot (7) from behind, ie from the tool side, by means of a supply opening and a correspondingly perforated line. Device according to any one of claims 7 to 9, characterized.

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