Method For Producing Hardened Steel Sheet Components

Abstract

The invention relates to a method for producing a hardened sheet steel component, wherein sheet steel sheet bars are cut from a coil made of a hardenable steel alloy or steel strip, formed into sheet steel component blanks in a cold forming process, and the blanks are heated in a continuous furnace to a temperature above the austenitization temperature required for the hardening and pressed and quench hardened in a form hardening tool. The blanks have point-shaped or linear beads, raised bumps, or flanges whose free ends or partial lengths are bent relative to the contact element so that the blanks rest on the contact element using only the point-shaped or linear beads, raised bumps, or free ends or partial lengths of the flanges. During form hardening, the beads, bumps or bends are pressed or deformed into the desired geometry of the finished component.

Description

FIELD OF THE INVENTION

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

BACKGROUND OF THE INVENTION

When producing sheet steel components, particularly in automobile manufacturing, there are currently two common methods for producing hardened sheet steel components that are particularly intended for performing weight-bearing functions such as longitudinal members and pillars and are therefore light in weight while simultaneously having a high strength and rigidity in the event of a crash while also offering corrosion protection.

The first common method is so-called press hardening, also known as the direct method. In press hardening, a sheet steel component is produced in that a flat sheet bar made of a hardenable steel is heated to a temperature above the austenitization temperature so that at least in some regions, the structure of the steel takes the form of a high-temperature modification, namely austenite. This flat sheet bar is then formed in a forming tool, preferably in a single forming stroke, into a desired form and through the contact with the forming tool halves, the heat is removed from the steel material so quickly that a martensitic hardening occurs in which the austenite is essentially transformed into martensite. To accomplish this, the speed of the heat removal must be greater than the so-called critical hardening speed, which is usually above 20 Kelvin per second.

This press hardening method has been known for a long time and is well-established and is particularly used 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, which is the so-called form hardening method, also referred to as the indirect method, and is known from EP1651789B1. The form hardening method makes it possible to produce much more complex shapes.

For this purpose, a sheet steel sheet bar is cold formed in an intrinsically known way. This cold forming is carried out in the generally conventional way in a forming line, for example with five forming presses, which form a flat sheet steel sheet bar step by step into a complex component.

Then the component that has been cold formed in this way is heated in a continuous furnace to a temperature above the austenitization temperature in order to likewise achieve a largely or fully austenitic state of the steel structure.

This already formed and heated steel component is then transferred to a so-called form hardening tool and in the form hardening tool, by means of having the form hardening tools placed against it, is cooled at a speed above the critical hardening speed and is thus hardened. In this case, the cold forming is carried out so that a dimensional change due to thermal expansion during the austenitization is taken into account through a correspondingly smaller final shaping in all three spatial directions.

As already explained above, the advantage of the form hardening process is that the usual complete cold forming is able to achieve a significantly more complex component shape than is possible with the direct method.

In addition, the hardened sheet steel components can provide a corrosion protection if the sheet bar or the strip from which it has been produced is provided with a metallic corrosion protection layer, in particular a metallic corrosion protection layer based on zinc, aluminum, or the alloys thereof.

In so-called form hardening, a component that has undergone final forming in all three spatial directions, which is optionally reduced by the amount of the expansion in all three spatial directions, is heated to the austenitization temperature that is required for the hardening and is then inserted into the form hardening tool in which it is pressed on all sides, possibly with slight corrections, particularly also corrections of heating-induced distortion, and is quench hardened by being contacted on all sides by the forming tools.

This therefore means that the component whose forming is completely finished must be conveyed through a heating furnace.

In order to convey such components through a furnace, so-called goods carriers are known; the goods carriers pick up the component at predetermined locations and convey it through the furnace. Normally, such goods carriers are made of metal and are conveyed along with them out of the furnace, conveyed back to the furnace entrance, reloaded, and conveyed through the furnace again.

For energy reasons, however, this is disadvantageous since this means that energy in the form of heat is conveyed out of the furnace and the cooled goods carriers have to be reheated again in the furnace. In addition, the goods carriers are costly to manufacture. Since they have to be manufactured specifically for each component, when a switch to different components is made, different goods carriers must be provided and in addition, the furnace must be correspondingly reconfigured. For smaller production series, the goods carriers are thus hardly worth the expense.

It is also known instead of goods carriers to use chain conveyors, for example, onto which the preformed parts are placed and then transported through the furnace. This does make the process more flexible, but it has been determined that the contact points of the components with the chain conveyors often cause blisters in the metallic corrosion protection material, in particular zinc, covering the components.

In addition to so-called chain conveyor furnaces it is known, particularly from press hardening, that roller hearth furnaces and chamber furnaces or multi-layer chamber furnaces or related furnace designs are used in which the above-described blisters occur to a comparable degree. In general, any contacts in the furnace are to be avoided. Contact elements can be conveying means of any kind. In continuous furnaces, contact elements can be chains, but it is also conceivable for them to be plates, walking beams, rollers, or the like. In chamber furnaces and multi-layer chamber furnaces, support strips are provided. For purposes of the invention, the term contact element includes the above-mentioned conveying means as well as these support strips in chamber furnaces.

SUMMARY OF THE INVENTION

The object of the invention is to create a method for manufacturing hardened sheet steel components with which the components have a better quality and which can also be adapted to different components in a way that is flexible in terms of the process and can be used in an inexpensive and energy-efficient way for all furnace designs that are used for form hardening and press hardening.

The object is attained with a method having the features described and claimed herein.

Advantageous modifications are also described and claimed herein.

According to the invention, it has been discovered that zinc blisters on the surface that rests on the contact element, in particular the chain conveyor, can be reduced by correspondingly reducing the surface area in that formed shapes that usually rest on the chain conveyor, for example incompletely formed flanges or finally formed support struts, are produced in the cold forming process, which are undone again in the form hardening step.

Thus according to the invention, in the cold forming process, a bending or deforming is omitted or performed only partially or an additional bending or deforming of the component is performed so that the support surface on the contact element is reduced and in particular, is kept as small as possible. This can in particular be a deformation only in some regions. In particular, the additional bending or deforming is performed so that less than 5%, preferably less than 3%, and even more preferably less than 1% of the component surface rests on the contact element.

In this respect, according to the invention a flat support is changed to a point-shaped and/or linear support.

In the overwhelming number of cases, the components rest on the contact element with their flanges, which are required for the subsequent additional processing and in particular for the production of welded connections with other components.

According to the invention, it is possible, for example, to provide a point-shaped and/or linear support in such a way that the flanges are bent out of their usual position by about 2-10°, preferably 3 to 8°, in particular 7° in the direction toward the contact element so that for example the flange edge is bent by a few millimeters relative to its desired position, for example by 2.5 mm, in the direction toward the contact element. This deformation over the length of the flange edge or other support regions can also occur over only subregions or partial lengths. This achieves a linear contact on the respective points of the contact element, which largely or completely avoids damage to the galvanized.

The respective width of the support region can advantageously be adapted to the width of the contact element in particular the chain conveyor. For example, with a chain spacing of 30 cm relative to the walking beams, the width of the support region can be selected to be greater than 30 cm in order to reliably ensure a linear support on a chain or walking beam and to avoid a flat support.

Advantageously, between the subregions, a transition region can be provided, which has an advantageously continuous curvature of the bending relative to the region without deformation. For example, a region of 40 mm to 150 mm can be present between the region with 2 to 10° deformation and the regions without deformation. This can offer other advantages with regard to the sturdiness relative to microcracks or similar defects.

It has surprisingly turned out that such a pre-bending or final shaping of the flange during the cold forming can be corrected into the desired shape during the form hardening without the occurrence of microcracks or other surface defects and also, zinc blisters are no longer detectable after the form hardening since the support surfaces are too small to cause these changes. In addition, this shifts the support into the edge region that is non-critical anyway, which plays a subordinate role for the production of welded connections.

Instead of a bending-out of the corresponding flange, it is naturally also conceivable to embody the flat flange with corresponding point-shaped beads or linear beads, which likewise rest on the contact element only in small regions.

In components that rest on the contact element with a main surface and not with a flange, these main surfaces can also be provided with inward or outward bends or deformations, which are correspondingly reformed back into their original later during the subsequent form hardening.

The invention therefore has the advantage that for the component, a flexible use of chain conveyors or other contact elements is possible without negatively affecting the components.

The invention therefore relates to a method for producing a hardened sheet steel component, wherein sheet steel sheet bars are cut from a coil made of a hardenable steel alloy or steel strip and the sheet steel sheet bars are then formed into a sheet steel component blank in a cold forming process and then the blanks are heated in a continuous furnace to a temperature above the austenitization temperature that is required for the hardening and are then pressed in a form hardening tool and thus quench hardened, wherein the blank is embodied with point-shaped or linear beads or else raised bumps or else flanges resting on the contact element are bent with their free end or partial lengths thereof relative to the contact element so that they rest on it with the edge or with a partial length of the edge and then in the form hardening, the beads or outward bends or bends functioning as the support surface on the flange or flanges are pressed or deformed into the desired geometry of the finished component.

According to one embodiment, in order to produce the linear support surfaces, the flanges of the component or the edge regions of the flange are bent relative to a desired position of the flange by 2-10°, in particular 3-8° in the direction toward the contact element, in particular the chain conveyor. This means that with a desired position of 1°, a bending by 2-10° then corresponds to a resulting total bending of 3-11° away from the plane. For purposes of the invention, bent can also include tilted, folded, or similar deformations; this means that the bent surface can be intrinsically planar or also curved.

According to one embodiment, the flange edges are deflected or bent by 2-7, in particular 3-6 mm, in the direction toward the contact element, in particular the chain conveyor.

According to one embodiment, stamped point-shaped or linear beads for being supported against a contact element, in particular the chain conveyor, protrude from a surface of the component adjacent to a conveyor.

According to one embodiment, the point-shaped or linear beads protrude by 2-7 mm relative to the chain conveyor or other contact elements.

According to one embodiment, the sheet steel sheet bar that is used is a sheet bar, which is embodied with a metallic corrosion protection layer, in particular a metallic corrosion protection layer that is based on zinc or based on aluminum or is made of zinc or aluminum or alloys thereof. A coating based on zinc can advantageously be used in order to ensure a cathodic corrosion protection of the steel component.

An advantageous embodiment of the invention provides for the use of transformation-delayed steel grades in which a hot rolled or cold rolled steel strip is used, which has a concentration range of the following alloying elements within the following limits, which are expressed in percentage 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,
  • aluminum up to 0.2, preferably 0.02 to 0.07
  • boron up to 0.01, preferably 0.0005 to 0.005
  • sulfur max. 0.01, preferably max. 0.008
  • phosphorus max. 0.025, preferably max. 0.01
  • and the rest iron and impurities.

According to one embodiment, an alloy-galvanized steel strip made of a hardenable steel alloy, in particular a boron-manganese steel such as a 22MnB5 or 20MnB8, is used as the steel strip.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained by way of example based on the drawings. In the drawings:

FIG. 1: is a very schematic depiction of the method sequence in form hardening;

FIG. 2: schematically depicts the support regions of a preformed component on a chain conveyor without the bent regions according to the invention;

FIG. 3: shows a very schematic depiction of the standing behavior of a preformed component on a chain conveyor with flanges that are bent according to the invention;

FIG. 4: shows an example of a component according to the invention on a chain conveyor;

FIG. 5: schematically depicts a top view of a component according to the invention with bent flange regions for producing a linear support during a passage through a lifting step conveyor furnace;

FIG. 6: schematically depicts a top view of a component according to the invention with bent flange regions for producing a linear support during a passage through a roller hearth furnace;

FIG. 7: schematically depicts a top view of a component according to the invention with bent flange regions for producing a linear support in a multi-layer chamber furnace.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a very schematic depiction of the form hardening process in which sheet bars are first cut from a steel strip or steel coil, the sheet bars are then cold formed, and the cold formed blanks are then austenitized in a furnace. The austenitized blanks are then placed into the form hardening mold and are hardened therein, then removed from it, and, as a finished blank, undergo a possibly necessary surface conditioning.

FIG. 2 shows the support surfaces for such components on a chain conveyor, wherein the preformed component in this case is supported with the whole area of its flanges.

FIG. 3 is a very schematic depiction of an embodiment according to the invention in which the flanges are formed so that the component rests on the chain conveyor with only the edge regions of the flanges so that a whole-area contact and thus the blisters on the entire surface are avoided. Blisters possibly occurring anyway are not critical at this location since the edge regions are not relevant at least with regard to a welded connection.

FIG. 4 shows a component in which the flanges are deformed over a partial length in order to achieve a linear standing surface along the edge wherein partial lengths have the desired shape, e.g. flat and with transition regions between the bent and flat regions.

FIG. 5 shows a top view of a component according to the invention with a linear support in the passage through with a lifting step conveyor. The component shown in FIG. 4 corresponds essentially to the diagram in FIG. 5. In this case, the individual conveyor chains and the walking beams between them are visible. In this connection, the bent flange regions in this example have at least a length of CW, i.e. the distance between a chain and a walking beam, which is approximately 10 cm in this example. In this way, the support surface can be reliably reduced and instead of a flat support, it is possible to ensure an only linear support of the component—and this also only in the bent flange regions. The region B depicts the length of the bend flange region (only indicated on the left in FIG. 5). In this case, a linear support at the outer edges of the flanges is provided on both sides, i.e. in four regions in this example of a component. Naturally, depending on the component geometry, a support in fewer linear regions (for example three lines) or also more linear regions would also be possible.

FIG. 6 shows a top view of a component according to the invention with a linear support in the passage through a roller hearth furnace. As before, regions with bent flanges are shown again here, once again indicated with the letter B (only on the left). This region can advantageously exceed at least the length from one furnace roller to the next furnace roller (length FR).

FIG. 7 shows a top view of a component according to the invention with a linear support in a multi-layer chamber furnace. By contrast with FIGS. 5 and 6, in this case the schematic depiction does not show a continuous furnace, but rather a multi-layer chamber furnace (or single-chamber furnace). In this case, the region of the bent flanges can be selected to be smaller because of the stationary support. In particular, the region for the linear support can be selected to be any size down to the minimum width of the support strip (SS). In this example, 6 support lines are shown since it can possibly be advantageous, depending on the component geometry to also provide other support lines in the middle regions since deflections can occur, for example. This is independent of the furnace type, though, i.e. it can also be advantageous in continuous furnaces.

Naturally, point-shaped supports of this kind can also be achieved by bending only the edge region of the flange in the direction toward the chain conveyor or by producing beads, regardless of whether these are point-shaped beads or linear beads. According to the invention, all of these slight deformations can, while in the hot state, be bent into the desired state in the form hardening tool without the risk of microcrack formation.

Claims

1-9. (canceled)

10. A method for producing a hardened steel sheet component, comprising the steps of: cutting steel sheet bars from a steel strip, or from a coil made of a hardenble steel alloy;cold forming the steel sheet bars into steel component blanks;conveying the blanks through a furnace on a contact element;heating the blanks in the furnace to a temperature above an austenization temperature; andpressing and quenching the blanks in a form hardening tool;wherein the blanks comprise at least one of point-shaped beads, linear beads, raised bumps, and flanges having free ends regions that are bent relative to the contact element; andthe blanks rest on the contact element using only the point-shaped beads, linear beads, raised bumps, and/or free end regions of the flanges.

11. The method of claim 10, wherein the blanks comprise flanges having free end regions that are bent by about 2-10 degrees relative to a surface of the contact element.

12. The method of claim 10, wherein the blanks comprise flanges having free end regions that are bent by about 2-7 mm relative to a surface of the contact element.

13. The method of claim 10, wherein the contact elements comprise at least one of walking beams, chains, plates, and rollers.

14. The method of claim 10, wherein the contact elements comprise support strips.

15. The method of claim 10, wherein the blanks comprise point-shaped beads, linear beads or raised bumps that rest on the contact element.

16. The method of claim 15, wherein the point-shaped beads, linear beads or raised bumps protrude by about 2-7 mm toward the contact element.

17. The method of claim 10, wherein the steel strip or coil further comprises a metallic corrosion protection layer based on zinc, aluminum, or an alloy thereof.

18. The method of claim 10, wherein the steel strip or coil comprises a boron-manganese steel.

19. A method for producing a hardened steel sheet component, comprising the steps of: cutting steel sheet bars from a steel strip, or from a coil made of a hardenble steel alloy;cold forming the steel sheet bars into steel component blanks;forming deformations in the steel sheet blanks during the cold forming process conveying the blanks through a furnace on a contact element;heating the blanks in the furnace to a temperature above an austenization temperature; andpressing and quenching the blanks in a form hardening tool;wherein the blanks rest on the contact element using only the deformations, and the deformations are corrected in the form hardening tool.

20. The method of claim 19, wherein the deformations comprise at least one of point-shaped beads, linear beads, raised bumps, and flanges having free ends regions that are bent relative to the contact element.

21. The method of claim 19, wherein the deformations are formed such that less than about 5% of a surface area of the blanks rests on the contact element.

22. The method of claim 19, wherein the deformations comprise flanges in the blanks, the flanges having free end regions that are bent by about 2-10 degrees relative to a surface of the contact element.

23. The method of claim 19, wherein the deformations comprise point-shaped beads, linear beads or raised bumps in the blanks that rest on the contact element.

24. The method of claim 23, wherein the point-shaped beads, linear beads or raised bumps protrude by about 2-7 mm toward the contact element.

25. The method of claim 19, wherein the steel strip or coil comprises a boron-manganese steel.

26. The method of claim 25, wherein the steel strip or coil further comprises a metallic corrosion protection layer based on zinc, aluminum, or an alloy thereof.

27. A method for producing a hardened steel sheet component, comprising the steps of: cutting steel sheet bars from a steel strip, or from a coil made of a hardenble steel alloy;cold forming the steel sheet bars into steel component blanks;forming deformations in the steel sheet blanks during the cold forming process conveying the blanks through a furnace on a contact element;heating the blanks in the furnace to a temperature above an austenization temperature; andpressing and quenching the blanks in a form hardening tool;wherein the blanks rest on the contact element using only the deformations, and the deformations are formed such that less than about 5% of a surface area of the blanks rests on the contact element.

28. The method of claim 27, wherein the deformations are formed such that less than about 3% of a surface area of the blanks rests on the contact element.

29. The method of claim 27, wherein the deformations are formed such that less than about 1% of a surface area of the blanks rests on the contact element.