WO2024165168A1 - Method for producing hardened steel components - Google Patents

Abstract

The invention relates to a method for producing hardened steel components, wherein a steel sheet blank made of a hardenable steel, which is provided with a metal anti-corrosion layer, in particular with a zinc-based metal anti-corrosion layer, is first cold-formed, and the resultant cold-preformed preliminary part is then fully or partially austenitised by being heated and is subsequently inserted into a press-hardening tool, wherein the insertion temperature is at least 500 °C, in particular above 600 °C, and is cooled in the press-hardening tool by contact with the press-hardening tool at a speed above the critical hardening speed and is thus quench-hardened and formed into the finished component, wherein the metal anti-corrosion layer has a layer thickness of 7 µm to 20 µm per side, wherein, based on the total surface area of the finished component, cold-finished regions make up 85-99% of the surface area and hot-finished regions make up 1-15% of the surface area.

Description

Process for producing hardened steel components

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

When producing sheet steel components, particularly in automotive engineering, there are currently two common processes for producing sheet steel components that are intended to perform load-bearing functions in particular, such as longitudinal beams and pillars, and therefore offer high strength and rigidity in the event of a crash while being lightweight, as well as corrosion protection.

The first common process is so-called press hardening, also known as the direct process. In press hardening, a sheet steel component is produced by heating a flat plate made of a hardenable steel to a temperature that is above the austenitizing temperature, so that the structure of the steel is at least partially in the high-temperature modification, namely austenite. This flat plate is then formed into a desired shape in a forming tool, preferably with a single forming stroke, and the contact with the forming tool halves removes the heat from the steel material so quickly that martensitic hardening occurs, in which the austenite is essentially converted into martensite. The speed of heat dissipation must be above the so-called critical hardening rate, which is usually above 20 Kelvin per second. This press hardening process has been known and introduced for a long time and is used in particular to produce components whose shape is not so complex that it cannot be produced with a single press stroke.

A second known method was developed by the applicant, it is the so-called form hardening, also called the indirect method and known from EP1651789B1. Form hardening allows the creation of significantly more complex shapes.

For this purpose, a sheet steel plate or a sheet steel strip is first cold formed into a prefabricated part in a conventional manner. This cold forming takes place as generally Usually, for example, in a press line with, for example, five forming presses and so-called step tools or in a transfer press with so-called transfer tools, where a flat sheet steel blank or a sheet steel strip is formed and trimmed step by step into a complex prefabricated part.

The cold-formed pre-component is then usually heated in a furnace, for example in a continuous furnace or a multi-layer chamber furnace, to a temperature above the austenitizing temperature in order to also achieve a largely or completely austenitic state of the steel structure.

This already formed and heated pre-component is then placed in a so-called form hardening tool and cooled and thus hardened in the form hardening tool by applying the form hardening tool surfaces at a speed above the critical hardening speed. The preceding cold forming is carried out in such a way that a dimensional change of the pre-component (caused by expansion in the furnace, shrinkage during phase transformation, further expansion in the furnace and shrinkage during transfer from the furnace to the form hardening tool) is taken into account by a correspondingly smaller forming in all three spatial directions, so that the size of the inserted heated pre-component matches the size of the form hardening tool.

This process is also known from W02006/015849. It is also known from this publication that if the cold preformed parts have dimensional deviations that lie outside the tolerance range, these can be corrected or re-stamped during the actual form hardening in the manner of hot calibration.

From DE102020200808A1, the so-called Smartform technology is known, which discloses upsetting with compressive stress in order to produce a cold-formed part that is as precise as possible.

EP2848715A1 discloses an electrolytic coating of a steel plate for the hot forming process, whereby this coating should be deposited as thinly as possible in order to enable a high flexibility of the processing properties.

EP2338618B1 describes a hybrid of indirect and direct process with trimming step after cold forming and after final forming with laser or water jet. cutting. The proportion of cold forming is defined as 50-80% of the drawing depth of the finished component.

As already mentioned, the advantage of the hot-forming process is that the usual cold multi-stage forming allows a significantly more complex component shape than the direct process.

At the same time, the sheet steel components can offer corrosion protection even in the hardened state if the plate or strip is provided with a metallic corrosion protection layer, in particular with a metallic corrosion protection layer based on zinc.

In the so-called form hardening, a cold-formed pre-component which is already fully formed and which is reduced in size in all three spatial directions in order to compensate for a later dimensional change is heated to the austenitizing temperature required for hardening and then placed in the form hardening tool in which it is pressed on all sides with minimal corrections, in particular corrections for heating-related distortion, and quench-hardened by the all-round contact of the forming tools.

The object of the invention is to provide a method for producing hardened sheet steel components which enables more flexible and economical production.

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

Advantageous further developments are identified in the dependent subclaims.

The inventors have recognized that the advantage of the direct process is that only a single hot forming tool is needed, but the disadvantage is that only simple geometries are possible and, in addition, the external trimming and the creation of a hole pattern must be carried out in the hardened state.

The advantage of the indirect process, however, is that no hard trimming or laser trimming is required after hardening, as the external surface and hole pattern are already created in the cold forming process before hardening. In addition, more complex geometries can be created due to the multi-stage cold forming. However, it has been proven that It has been found that the costs are comparatively high, especially for small series or smaller quantities, since in addition to the form hardening tool, a cold forming tool set is also required, i.e. several cold forming tools arranged one after the other, for example five cold forming tools and the associated presses.

A cold forming tool within the meaning of the invention is one of the tools of a cold forming tool set of, for example, step or transfer tools, in which a sheet steel blank or a sheet steel strip is successively formed into a component in a multi-stage forming and trimming process by a combination of mainly deep drawing, trimming and post-forming.

It has also been found that problems occur with the direct process, which are clearly related to the reaction of liquid or gaseous zinc with austenite during hardening. Apparently, these contacts of austenite grains with liquid zinc and gaseous zinc lead to two phenomena known as liquid metal embrittlement (due to liquid zinc) or vapor metal embrittlement (due to zinc vapor). These two phenomena lead to material embrittlement and thus to the formation of microcracks within the steel material during forming.

Liquid metal embrittlement (LME) can be counteracted by using steel grades with a delayed transformation, pre-cooling them below the liquidus point of the zinc alloy coating before forming and only then forming them. This is disclosed, for example, in DE102015113056. To counter vapor metal embrittlement (VME), it is possible to blow air or other gases, and in particular oxygen-containing gases, onto the component or into the mold before forming. This process is known from DE102016102344.

For the same reasons, no post-forming is permitted in the indirect process with zinc coatings, i.e. final forming must be carried out using the cold forming process, as well as cold forming scaling and the insertion temperature and geometry must be suitable for the form hardening process. Waviness caused by cold forming or other shape deviations that are leveled during form hardening are not permitted or only permitted to a very small extent, which means that necessary measures and corrections to avoid them entail a high level of tool technology and/or material use in cold forming. In addition, there is a risk that micro- cracks or other undesirable negative effects on the mechanical properties of the components.

According to the invention, it was found that there are various reasons for locally and precisely forming or finishing a component in the form hardening tool. According to the invention, this is surprisingly successful despite high zinc coatings and unavoidable high insertion temperatures, whereby the requirements regarding microcracks and mechanical properties of the end product can also be reliably met.

The insertion temperature within the meaning of the invention is the temperature of the pre-component shortly after its insertion into the tool before reaching the bottom dead center of the tool, i.e. at the beginning of the final forming process.

In contrast to the press hardening of flat blanks, during the form hardening of pre-assembled parts, due to the complex geometry of the pre-assembled parts, they cannot be pre-cooled either by contact or convection with sufficient uniformity and accuracy to low insertion temperatures, i.e. below the liquidus point of the zinc alloy coating, after austenitization and before insertion into the tool.

It was found that shifting forming processes from the cold forming tools to the form hardening tool and thus reducing cold forming processes can have a significant savings effect even for smaller component areas by eliminating entire cold forming tools.

In addition, correction grinding of cold forming, particularly for eliminating cold forming waviness, etc., can be saved and can be sensibly integrated into the form hardening tool as warm post-forming during form hardening. According to the invention, warm post-forming can also be referred to as warm final forming.

In addition, it was found that the effort for waviness-free, cold-formed parts can be saved, especially with regard to the use of materials and the use of draw beads or other cold-forming tool modifications and the like.

If required for manufacturing reasons, it may also be useful to design the components with certain overhangs, e.g. in the area of the flanges, which are used for example for handling from cold forming into the furnace, in the furnace and for handling from the furnace. to the form hardening tool as well as for the stable insert in the form hardening tool and to subsequently reshape these few protrusions in the form hardening tool.

Surprisingly, it was found that, contrary to all expectations, in the indirect process, i.e. in die hardening, austenitized cold-formed parts can be locally and definedly further formed or fully formed in the die hardening tool despite high zinc coatings and despite high insertion temperatures, even without pre-cooling, with good properties regarding microcracks, mechanical parameters, layer thicknesses, etc.

Even larger post-forming operations, i.e. defined forming processes such as bending, raising, leveling of corrugations and the like, can easily be carried out in the form hardening tool during form hardening, i.e. as hot post-forming.

In particular, microcracks caused by VME and/or LME can be prevented, if necessary, by targeted local injection of air. However, it has been found that smaller reshapings, i.e. undulations up to a height of a few millimeters, can be realized without microcracks even without injection of air.

In order to count as defined forming or final forming during the hot forming process, the geometric deviations between the component geometry after cold forming and the component geometry after the hot forming process must be at least 1 mm, particularly in the case of waviness.

According to the invention, a steel sheet plate or a steel strip is used which is formed with a metallic corrosion protection layer, in particular with a metallic corrosion protection layer based on zinc, which has a layer thickness of 7 pm to 20 pm per side. This enables good corrosion protection. In particular, the coating can be a Z120 or Z140 or Z180 according to DIN EN 10346.

Zinc-based corrosion protection layers can have a comparatively high zinc content of 85% to 98% by weight and, in addition to unavoidable impurities, also contain aluminum in the range of 0.2 to 2% by weight. They can also contain other oxygen-affine elements such as magnesium.

Particularly preferably, the metallic anti-corrosive layer based on zinc can be applied using a hot-dip process. This can be a simple and robust method of application. According to the invention, based on the total area of the finished component, 85-99% of the area is cold-formed (and thus only hardened during the mold hardening process) and 1-15% of the area is hot-formed and hardened during the mold hardening process.

The percentage of the component surfaces that are hot-formed and hardened is calculated in such a way that component areas with geometric deviations from the state before form hardening (i.e. after cold forming) are added to the state after form hardening and related to the entire surface of the component after form hardening (i.e. the finished part).

For the above-mentioned portion of the component surfaces that are hot-finished, the slightest deviation in the sense of hot calibration, i.e. deviations of less than 1 mm from the component geometry after form hardening and known springback effects of the material such as frame and flange projections from cold forming are not taken into account. This means that only deviations of greater than 1 mm, whether due to deliberately specified geometry adjustments such as shifting forming parts from cold forming tools to the form hardening tool and leveling of waviness, are used for the hot-finished surface parts. Advantageously, this can be used to correct any waviness from cold forming, which is usually difficult to avoid in cold forming or causes greater effort if avoided. Previously, for example, it was necessary to modify a cold forming tool in such a way that additional draw beads had to be introduced, which meant increased material expenditure because the board had to be enlarged.

The invention therefore relates in particular to a method for producing hardened steel components, in which a steel sheet blank made of a hardenable steel, which is formed with a metallic corrosion protection layer, in particular with a metallic corrosion protection layer based on zinc, is first cold formed, the cold preformed pre-component thus obtained is then completely or partially austenitized by heating and then inserted into a form hardening tool, wherein the insertion temperature is at least 500 °C, in particular greater than 600 °C, and is cooled in the form hardening tool by contact with the mold at a speed above the critical hardening speed and is thereby quench hardened and formed into the finished component, wherein the metallic corrosion protection layer has a layer thickness of 7 pm to 20 pm per side, wherein 85-99 area% are cold-finished formed areas and 1-15 area- % of the hot-finished areas during the form hardening process are present, based on the total surface area of the finished component.

This preferred surface distribution of cold and hot finished surfaces can optimally combine both the advantages of saving entire cold forming tools and the advantages of saving correction loops and material expenditure to eliminate cold forming waviness with process-reliable hot forming components during the die hardening.

One embodiment provides that 90-98% of the area are cold-finished areas and 2-10% of the area are hot-finished areas during the mold hardening process, based on the total area of the finished component.

One embodiment provides that the cold forming is carried out in three to ten cold forming tools and the form hardening and the final forming in the warm state are carried out in one tool, namely the form hardening tool.

One embodiment provides that deviations to be formed after cold forming from the geometry of the hardened, i.e. the finished, component, in particular in the case of waviness, have a deviation of at least 1 mm from the geometry in order to count as a surface that has been hot-formed during the form hardening.

One embodiment provides that before the mold hardening and during or after the heated preformed component is placed in the mold hardening tool, the mold hardening tool and/or the preformed component are blown or blown in or flushed with an oxygen-containing gas, in particular air. This can further reduce the possible risk of micro-cracking caused by LME and/or VME in the case of unfavorable geometric conditions.

One embodiment provides that before the mold hardening and during or after the insertion of the heated preformed component into the mold hardening tool, the preformed component is blown with an oxygen-containing gas, in particular air, exclusively in those areas which will experience a deformation of more than 5 mm in the mold hardening tool. One embodiment provides that the metallic, in particular alloy-galvanized, corrosion protection layer has a layer thickness of 10 pm to 15 pm per side. This can further increase the corrosion protection, in particular cathodic corrosion protection.

One embodiment provides that the metallic, in particular alloy-galvanized, corrosion protection layer was applied by means of a hot-dip galvanizing process. This can represent a particularly simple and robust coating process.

One embodiment provides that the steel strip is made of a hardenable steel alloy, in particular a boron-manganese steel. For example, this can be a 22MnB5 or 34MnB5 steel alloy.

One embodiment provides that a material is used as the steel material which is transformation-retarded with respect to the austenite-martensite transformation during cooling, wherein the strip is hot- or cold-rolled.

One embodiment provides that a steel strip with the following composition is used (all data in % by weight):

Carbon up to 0.4, preferably 0.15 to 0.3

Silicon up to 1.9, preferably 0.11 to 1.5

Manganese up to 3.0, preferably 0.8 to 2.5

Chromium up to 1.5, preferably 0.1 to 0.9

Molybdenum up to 0.9, preferably 0.1 to 0.5

Nickel up to 0.9,

Titanium up to 0.2 preferably 0.02 to 0.1

Vanadium up to 0.2

Tungsten up to 0.2,

Aluminium up to 0.2, preferably 0.02 to 0.07

Boron up to 0.01, preferably 0.0005 to 0.005

Sulphur max. 0.01, preferably max. 0.008

Phosphorus max. 0.025, preferably max. 0.01

Rest iron and impurities

The invention is explained by way of example using a drawing. It shows: Figure 1: the comparison of the result of cold forming according to the prior art in contrast to the result of cold forming according to the invention and the form hardening according to the invention in a highly schematic form;

Figure 2: exemplary permissible waviness in the cold-formed part according to the invention;

Figure 3: the drawing of an exemplary component in a perspective view before and after die hardening

Figure 4: the component according to Figure 3 with marked deformations during

form hardening;

Figure 5: an exemplary sequence of operations of a cold forming tool set (transfer tools) and the form hardening tool in the production of the component according to Figure 3 according to the prior art and according to the method according to the invention;

Figure 6: an exemplary sequence of operations of a cold forming tool set (step tools) and the form hardening tool in the production of a component according to the prior art and according to the method according to the invention;

Figure 7: Schematic of the form hardening process from the board to the finished component.

In Figure 1, cold forming according to the prior art as a preliminary stage to form hardening is compared in a highly schematic manner with cold forming according to the invention and form hardening according to the invention.

The basic requirement for conventional cold forming processes before hot stamping is that, as shown in the top row of the left column in Figure 1, there are no wavinesses and that any subsequent forming is not necessary as all shapes are already formed. This is referred to as KU SdT, i.e. cold forming according to the state of the art.

The cold forming according to the invention (Figure 1, left column, middle row - KU requirement) allows, as shown in Figure 2, certain wavinesses (greater than 1mm). In the subsequent form hardening step (Figure 1, left column, bottom row - FH requirement) the wavinesses are the die hardening tool surfaces are leveled so that the finished component has smoothed waviness.

The right-hand column shows that a subsequent forming in the flange area, in this case a raising process in the form hardening tool, takes place, i.e. it is hot formed with a raising jaw in the form hardening tool (Figure 1, right-hand column, bottom row - FH Erf.). This means that a cold forming step and thus possibly an entire cold forming tool can be saved. In the current state of the art, the entire component must be completely formed in the cold forming (Figure 1, right-hand column, top row - KU SdT).

In Figure 2 you can see photographs of existing wavinesses, which can still be present according to the invention and are caused by the cold forming process. These wavinesses can be eliminated in a simple and surprisingly good way in the hot state in the form hardening process and in particular can be eliminated considerably better and more efficiently than in a further cold forming correction step.

Figures 3 and 4 show corresponding components in which post-embossing or post-forming operations were carried out at the marked locations, which had no negative impact on the mechanical properties of the component and also did not produce any microcracks or similar, even though a relatively thick zinc layer was present. In this example component, the total component surface of the finished component was 1236 cm ² , although the proportion of component surface with deviations (greater than 1 mm) after cold forming and before form hardening from the geometry of the finished component was 38 cm ² . Therefore, the percentage of component surfaces after cold forming with a deviation from the component geometry of the finished component was 3.1%. The surface distribution measured on the finished component is therefore 96.9% cold-formed areas and 3.1% hot-formed areas during form hardening.

Figures 5 and 6 show corresponding comparisons of exemplary operation sequences in the manufacture of such an exemplary component according to the prior art to the invention. It can be seen that one operation step, i.e. one cold forming tool, could be saved in the cold forming. With the transfer tool set (Fig. 5), it was one operation step out of seven necessary steps in the process according to the prior art. The local post-forming, i.e. the raising of the flange, could be completely shifted to the form hardening process (i.e. hot forming). With the step tool set (Fig. 6), one operation step out of five could be saved. The manufacturing the final trimming edge (FBK), i.e. the final trimming could be integrated in operation step 4 and the post-forming and bending in the form hardening process (i.e. hot forming). Depending on the complexity of the component, three to ten cold forming steps may be necessary for the method according to the invention, but one step can be saved compared to the state of the art method. This means that in the

State of the art technology would have required four to eleven cold forming steps, depending on the complexity.

The advantage of the invention is that the overall manufacturing effort and thus also the costs can be reduced because the process is considerably more tolerant of inaccuracies that occur during the process and also enables other deliberate measures to simplify component handling and at the same time achieve a component with the desired and necessary properties.

Claims

Claims 1. A method for producing hardened steel components, wherein a steel sheet blank made of a hardenable steel, which is formed with a metallic corrosion protection layer, in particular with a metallic corrosion protection layer based on zinc, is first cold formed, the cold pre-formed pre-component thus obtained is then completely or partially austenitized by heating and then inserted into a form hardening tool, wherein the insertion temperature is at least 500 °C, in particular greater than 600 °C, and is cooled in the form hardening tool by contact with the mold at a rate above the critical hardening rate and is thereby quench-hardened and formed into the finished component, wherein the metallic corrosion protection layer has a layer thickness of 7 pm to 20 pm per side, characterized in that 85-99 area% are cold-finished formed areas and 1-15 area% are hot-finished formed areas during form hardening, based on the entire area of the finished component. 2. Process according to claim 1, characterized in that 90-98 area % are cold-finished formed areas and 2-10 area % are hot-finished formed areas during the mold hardening, based on the total area of the finished component. 3. Method according to claim 1 or 2, characterized in that the cold forming is carried out in three to ten cold forming tools and the form hardening and the final forming in the warm state are carried out in one tool, namely the form hardening tool. 4. Method according to one of the preceding claims, characterized in that deviations to be formed after cold forming from the geometry of the finished component, in particular in the case of undulations, have a deviation of at least 1 mm from the geometry in order to count as a surface that has been hot-formed during the form hardening. 5. Method according to one of the preceding claims, characterized in that before the mold hardening and during or after the insertion of the heated preformed component into the mold hardening tool, the mold hardening tool and/or the preformed component are blown or injected or flushed with an oxygen-containing gas, in particular air. 6. Method according to one of the preceding claims, characterized in that before the form hardening and during or after the insertion of the heated preformed component into the form hardening tool, the preformed component is blown with an oxygen-containing gas, in particular air, exclusively in those areas which will experience a deformation of more than 5 mm in the form hardening tool. 7. Method according to one of the preceding claims, characterized in that the metallic, in particular alloy-galvanized, corrosion protection layer has a layer thickness of 10 pm to 15 pm per side. 8. Method according to one of the preceding claims, characterized in that the metallic, in particular alloy-galvanized, corrosion protection layer was applied by means of a hot-dip galvanizing process. 9. Method according to one of the preceding claims, characterized in that the steel strip is formed from a hardenable steel alloy, in particular a boron-manganese steel. 10. Process according to claim 9, characterized in that the steel strip used is a strip with the following composition (all values in % by weight): Carbon up to 0.4, preferably 0.15 to 0.3 Silicon up to 1.9, preferably 0.11 to 1.5 Manganese up to 3.0, preferably 0.8 to 2.5 Chromium up to 1.5, preferably 0.1 to 0.9 Molybdenum up to 0.9, preferably 0.1 to 0.5 Nickel up to 0.9, Titanium up to 0.2, preferably 0.02 to 0.1 Vanadium up to 0.2 Tungsten up to 0.2, Aluminium up to 0.2, preferably 0.02 to 0.07 Boron up to 0.01, preferably 0.0005 to 0.005 Sulphur max. 0.01, preferably max. 0.008 Phosphorus max. 0.025, preferably max. 0.01 Rest iron and impurities