Patent Publication Number: US-2021189513-A1

Title: Method and device for producing hardened steel components

Description:
The invention relates to a method and device for producing hardened steel components. 
     Hardened steel components, particularly in vehicle body construction for motor vehicles, have the advantage that due to their outstanding mechanical properties, it is possible to achieve a particularly stable passenger compartment without having to use components that are much more massive at normal strengths and must therefore be embodied as much heavier. 
     To produce hardened steel components of this kind, steel types are used that can be hardened by means of a quench hardening. Steel types of this kind include, for example, boron-alloyed manganese carbon steels, the most widely-used steel being 22MnB5. But other boron-alloyed manganese carbon steels are also used for this purpose. 
     In order to produce hardened components from these types of steel, the steel material must be heated to the austenitization temperature (&gt;Ac 3 ) and it is necessary to wait until the steel material is austenitized. Depending on the desired degree of hardness, partial or complete austenitization can be achieved in this connection. 
     If after the austenitization, such a steel material is cooled at a speed that is above the critical hardening speed, then the austenitic structure converts into a martensitic, very hard structure. In this way, it is possible to achieve tensile strengths R m  of up to over 1500 MPa. 
     Currently, two different procedural approaches are commonly used for producing steel components. 
     In so-called form hardening, a sheet steel blank is cut out from a steel band and then—using a conventional, for example five-step, deep drawing process—is deep drawn to produce the finished component. This finished component in this case is dimensioned somewhat smaller in order to compensate for a subsequent thermal expansion during the austenitization. 
     The component produced in this way is austenitized and then inserted into a form hardening tool in which it is pressed, but is not formed or is only formed to a very slight extent and by means of the pressing, the heat flows out of the component and into the press tool, specifically at the speed greater than the critical hardening speed. 
     The other procedural approach is so-called press hardening in which a blank is cut out from a sheet steel band, then the blank is austenitized and the hot blank is formed in a one-stage step and at the same time, is cooled at a speed greater than the critical hardening speed. 
     In both cases, it is possible to use blanks provided with anticorrosion coatings such as zinc. Form hardening is also referred to as the indirect process and press hardening is referred to as the direct process. The advantage of the indirect process is that it is possible to achieve more complex tool geometries. 
     The advantage of the direct method is that a higher material utilization ratio can be achieved, but with a lower component complexity. 
     In press hardening, however, it is disadvantageous that microcracks form in the surface, particularly with galvanized sheet steel blanks. 
     In this connection, a distinction is drawn between first-order microcracks and second-order microcracks. 
     First-order microcracks are attributed to so-called liquid metal embrittlement. The theory is that during the forming, i.e. as tensile stresses are being exerted on the material, liquid zinc phases interact with still existing austenite phases, causing microcracks with depths of up to a few hundred μm to be produced in the material. 
     The applicant has succeeded in suppressing this by cooling the material—in the time between the removal from the heating furnace and the insertion into the forming tool—to temperatures at which liquid zinc phases are no longer present. This means that the hot-forming takes place at temperatures below approximately 750° C. 
     Up to this point, it has not been possible to control the second-order microcracks in hot-forming despite pre-cooling and they occur even at hot-forming temperatures below 600° C. The crack depths amount to a few tens of μm. 
     Neither first-order microcracks nor second-order microcracks are accepted by users since they constitute potential sources of damage. 
     With the previous methods, however, it has not been possible to ensure a production of components without second-order microcracks. 
     DE 10 2011 055 643 A1 has disclosed a method and a forming tool for hot-form press hardening components made of sheet steel, particularly made of galvanized workpieces composed of sheet steel. In this case, the female dies used for the hot-forming and press hardening—in their drawing edge region that is defined by a positive drawing radius—should be liquid-coated with a material or provided with an insert piece, which has a thermal conductivity that is at least 10 W/(m×K) less than the thermal conductivity of the section of the female die that is adjacent to the drawing edge region and that comes into contact with the workpiece as the latter is being hot-formed and press hardened. The material that is applied to the surface of the drawing edge region facing the workpiece or of the insert piece that has been put into position should have a transverse dimension extending across the drawing edge that is in a range of 1.6 to 10 times the positive drawing radius of the female die. This should improve the flow properties of workpieces made of sheet steel during the hot-forming and should thus significantly reduce the risk of the occurrence of cracks in the hot-forming of workpieces made of sheet steel, preferably made of galvanized steel blanks. Such a tool, however, does not make it possible to avoid microcracks of the second type. 
     DE 10 2011 052 773 A1 has disclosed a tool for a press hardening tool in which the mold surface of the tool is microstructured in some regions by two micro-cavities that are introduced into the mold surface. This step is intended to four restrict the effective contact area for the forming of a blank between the mold surface with a blank to the surface portions situated between the cavities. This is intended to reduce the friction. 
     DE 10 2004 038 626 B3 has disclosed a method for producing hardened components out of sheet steel in which before or after the forming of the formed part, a required final trimming of the formed part and any necessary punching procedures or the production of a hole pattern is carried out and the formed part is then heated at least in some areas to a temperature that enables an austenitization of the steel material; the component is then transferred to a form hardening tool and a form hardening is carried out in the form hardening tool in which the component is cooled and thus hardened at least in some areas by the contact and pressing of the component; and the component is supported by the form hardening tool, at least in some areas, in the region of the positive radii and is preferably held by two clamps in the region of the trim edges and in regions in which the component is not clamped, the component is at least spaced apart from a mold half by means of a gap. This measure makes it possible to clamp the component in a distortion-free manner and to set different hardness gradients by means of different hardening speeds. 
     The object of the invention is to avoid microcracks of the second type in directly hot-formed, i.e. press hardened, components. 
     The object is attained with a method having the features of claim  1 . 
     Advantageous modifications are characterized in the dependent claims. 
     Another object of the invention is to create a device with which sheet steel blanks can be hot-formed and hardened in the press hardening process and in which microcracks are avoided. 
     The object is attained with a device having the features of claim  5 . Advantageous modifications are characterized in the dependent claims that depend on this claim. 
     The inventors have realized that microcracks of the second type are produced when, in regions under tensile strain, the zinc vapor that occurs arrives at the steel in a sufficient concentration, so-called vapor metal embrittlement (VME). Zinc vapor is produced due to the tearing of the zinc/iron layer that occurs in the stretching during the forming process. A sufficient concentration particularly occurs in those regions in which direct contact of the sheet metal with the tool prevails or the sheet metal is a very small distance from the tool. A very small distance as defined by the invention is being less than 0.5 mm. 
     According to the invention, second-order microcracks should be avoided, while retaining the largest possible working window with regard to the material and temperature and ensuring an inexpensive implementation. With at least the same residence time, there should be no increase in cycle time or reduction in throughput during component production. 
     According to the invention, in the regions under tensile strain (elongation edge fiber), through an influx of oxygen-containing fluids, the zinc vapor or free zinc that occurs is quickly transformed into a stable compound such as zinc oxide or ZnI 2 . In addition, the protection of the steel from second-order microcracks can also be achieved by producing a protective layer such as an oxide layer by supplying a fluid. The measures described above have demonstrated that microcracks are significantly reduced. 
     Gaseous oxygen-containing fluids such as air or oxygen are particularly preferable because they cannot excessively contaminate the tool and in addition, a possibly unwanted massive cooling action of the kind that can occur, for example, by means of water can be more easily regulated by tempering the fluid. 
     According to the invention, in the tool—preferably in the region of the positive radii or adjacent to the positive radii—inserts are used, which permit an entry of oxygen when the sheet metal blank is being deformed, i.e. when the blank material is flowing. In addition, inserts can also be provided at narrow points or contact regions of the sheet metal blank with the tool, these contact regions being defined as the regions in which the distance of the sheet metal to the tool is at most 0.5 mm. 
     To this end, the corresponding material naturally has to be supported in the region of the positive radii because these are the edges that produce the deformation and initiate the flow of material. 
     Adjacent to these edges and spaced apart from them so that the inserts are not damaged, the inserts have means that enable an entry of oxygen. These means can, for example, be sintered metal inserts or porous ceramic inserts in which, after the move away from each other and the workpiece hardens and before a new blank is inserted, enough oxygen is stored that it can be imparted to released zinc or released zinc phases. 
     Furthermore, the inserts have surfaces that are left open so that the material, after it has flowed past the edge, is spaced apart from the insert. 
     In an advantageous embodiment, this left-open region is embodied as slotted so that a minimum support of the material is possible, but the entry of oxygen is ensured. 
     In all of these cases, there can also be fluid connection lines, which feed into the open regions or into the regions that that are filled with sintered metal or porous ceramic so that a sufficient amount of oxygen is supplied. In the simplest case, this can be air or also water vapor, for example. 
     Materials that inherently have a high oxygen diffusion capacity such as certain ceramics can also be embodied in a massive form and are acted on with oxygen-containing fluids either while the press is open or from the rear and store this oxygen until it can be imparted to released zinc iron phases or released zinc. 
     These inserts can be embodied on both the female die and the male die. 
     A charging with oxygen can also be carried out by flooding the mold cavity, for example with water vapor or with the mediums already mentioned above. 
    
    
     
       The invention will be explained by way of example based on the drawings. In the drawings: 
         FIG. 1  shows an example of a tool insert in a massive embodiment; 
         FIG. 2  shows a tool insert with a recess; 
         FIG. 3  shows another tool insert with a recess; 
         FIG. 4  is a sectional side view of a slotted tool insert; 
         FIG. 5  shows the slotted tool insert in a view from the forming surface. 
     
    
    
     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. 
     This ceramic insert can be embodied as massive or impervious and hard or porous and hard. 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 . 
     Primarily in the direct press hardening process, 20MnB8, 22MnB8, and other manganese/boron steels are also used in addition to 22MnB5. 
     Consequently, steels of the following alloy composition are suitable for the invention (all indications in 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                    
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.
 
     Steels of the following general alloy composition are also suitable for the invention (all indications in 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           Chromium (Cr)   0.02-0.6           Titanium (Ti)    0.01-0.08           Nitrogen (N)   &lt;0.02           Boron (B)   0.002-0.02           Phosphorus (P)   &lt;0.01           Sulfur (S)   &lt;0.01           Molybdenum (Mo)   &lt;1                          
and the rest made up of iron and smelting-induced impurities.
 
     The following steel configurations have turned out to be particularly suitable (all indications 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)    &lt;0.007           Boron (B)   0.002-0.006           Phosphorus (P)   &lt;0.01           Sulfur (S)   &lt;0.01           Molybdenum (Mo)   &lt;1                          
and the rest made up of iron and smelting-induced impurities.