DE102016124539A1 - Process for producing locally hardened sheet steel components - Google Patents

Abstract

The invention relates to a method for producing locally hardened sheet steel components, wherein steel sheet parts are passed through a heating furnace, wherein the heating furnace in two in the conveying direction 3 successively arranged furnace regions 7, 8 is divided, wherein at least in a first furnace region 7, the board to a temperature below Acerwärmt In this case, areas which are not to be hardened are heated less and in a second furnace area 8 the board is irradiated and / or heated in areas which are to be subjected to additional heating by means of electrically operated radiant heaters and / or burners in which no over-heating is desired, individual radiators and / or burners are switched off in 15 or patterns of radiators and / or burners 15 are turned off, and / or reflector stencils above non-heated areas between the radiator and / or burner field 11 and the Board 1 are pushed so that the board is shielded in nichtheizheizenden areas of additional heat input via radiation and remains below the AcTemperatur.

Description

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

Hardened sheet steel components have exerted a great influence in the bodywork of automobiles in recent years. A goal in the use of hardened sheet steel components in motor vehicles was to achieve a very high resistance to deformation at relatively low material thickness, especially in accidents of motor vehicles. The comparatively low weight of such hardened components in contrast to significantly thicker, more massive components has a compromise between low body weight and thus low fuel consumption and high stability, and thus provide passenger safety.

In order to produce such hardened steel components, two common methods have prevailed, the press hardening and the form hardening.

Since today's motor vehicle bodies are usually formed from steel sheets formed with a metallic protective layer, i.e., either galvanized steel sheets or aluminized steel sheets, both aluminized and galvanized sheets conditioned accordingly may be used for some time in both press hardening and tempering. Such components then blend harmoniously into the overall body of the corrosion protection system.

In both processes, both press hardening and mold hardening, the effect is exploited that steel is more or less complete at high temperatures, namely temperatures above the austenite start temperature (Ac ₁ ) and in particular above the austenitizing temperature (Ac ₃ ) into the high temperature phase, namely the austenite (gamma iron) converts. If this austenitic structure is cooled at a speed which is above the so-called critical hardening speed (eg around 22 Kelvin / second for the steel alloy 20MnB8), the austenitic structure results in a martensitic structure.

In press-hardening, flat sheet-steel blanks are usually austenitized, ie heated to a temperature greater than Ac ₃ and held at this temperature until a completely austenitic structure is present and then transferred to a pressing tool in which the flat sheet-steel plate enters the sheet-steel component with a single forming stroke is thermoformed and quenched by the high heat dissipation through the cold tool simultaneously at a rate above the critical cure speed. Thus, a steel sheet component is achieved with a single press stroke, which has the required high hardness or tensile strength R _m .

Form hardening has been developed for molding more complex components and provides for the conversion of a flat sheet steel blank into the corresponding sheet steel component in a conventional cold forming line, i.e. with five successive forming steps. This cold-formed sheet steel component is then heated to the austenitizing temperature and kept there for the desired time. Subsequently, this heated steel sheet component is transferred to a mold hardening tool, wherein the mold hardening tool has the shape of the finished sheet steel component (in the cold state) including a volume scaling, so that by inserting the hot pre-formed sheet steel component in the mold hardening tool, the mold hardening tool, which also consists of a die and a male, after closing all sides rests against the hot steel sheet component and the hot steel sheet member cools at a speed above the critical hardness rate. The shape-hardening tool ensures, among other things, that there is no delay due to the rapid cooling. Preferably, the mold-hardening tool has exactly the shape that the finished, hardened, cold component is to have, including a volume scaling, so that cold-working of the steel sheet prior to heating is carried out such that when heated in the oven and then cooled during transfer from the oven in the press unavoidable thermal expansion is taken into account so that the cold-formed component in all directions about 2%, but at least so much smaller that the thermal expansion is compensated and the inserted into the mold hardening tool fits exactly on this. In the case of mold hardening, due to the prevention of shrinkage of the sheet steel component during cooling, in addition to positive curing and geometry effects. In a further development of both methods has been found that often a uniformly hardened component does not fully meet the requirements, so that components with different hard or soft portions are desired. To achieve such so-called tailored-property parts (TPP), also several ways are feasible. In one possibility, a sheet steel component of steels of different quality and / or sheet thickness is welded together and then subjected to the press or mold hardening, the steels of different quality, for example, less or not curable, so that soft zones remain.

In addition, it is known not to heat certain areas of the board or the component so far that they austenitize before the press or form hardening, so that they in the subsequent Quenching also did not harden, as there was no austenitic structure.

It is also known to heat the component uniformly over the austenitizing temperature, but not to cool the portions which are to remain soft at a sufficiently high speed, but to leave the speed below the critical hardening speed, so that no martensitic structure is achieved that even in these areas no hardness or lower hardness is present.

From the DE 10 2008 030 279 D1 or the EP 214 808 B1 a method for producing a molded component having at least two structural regions of different ductility from a component blank is known, wherein the component blank consists of a hardenable steel, which is partially heated differently and then formed in a thermoforming and hardening tool and partially cured, wherein the component blank in a Heating device is heated to a homogeneous temperature less than the Ac ₃ point of the alloy and then brought by means of an infrared lamp array in the first type to a temperature above the Ac ₃ point of the alloy and the component blank in the thermoforming and hardening tool in the areas of the first Kind is hardened. A corresponding infrared lamp array for carrying out this method has different temperature fields, wherein the different temperature fields of the infrared lamp field are separated by a bulkhead.

From the EP 2 520 382 A2 a press hardening method is known in which a steel sheet is heated to a uniform first temperature and then at least partially heated to a higher temperature and then press-hardened, wherein the steel material is a zinc-coated steel material, to a first temperature, higher than 400 ° C and is heated lower than 600 ° C, and held at this temperature for up to 20 minutes.

The steel material may also be aluminum coated.

From the EP 2 322 672 A1 a method is known in which acts on a sheet steel plate with electromagnetic radiation, wherein the electromagnetic radiation is generated in respective generators and to produce a certain temperature pattern on the board absorber or reflector plates are present, which shield or absorb the emitted electromagnetic radiation and have a special pattern that can be placed between the generator and the material.

From the DE 10 2014 11 95 45 A1 there is known a method of making a hot formed and press hardened sheet steel component having partially different strength properties, wherein a hardenable steel alloy board is heated in a heating station, wherein a first portion is heated above austenitizing temperature and at least a second portion below austenitizing temperature; preferably smaller Ac _{1 is} heated and between the two areas a transition region is formed, wherein the thus heated board is transferred to a tempering station or a hot forming and pressing station. The temperature-control station or the hot-forming and press-hardening tool should be segmented and have at least one tempering segment, wherein the tempering segment is arranged in the region of the resulting transition region of the temperate blanks which are partially different from each other and the transition region is tempered before or during the press-hardening and the hot-forming and press-hardening of the steel component takes place with at least one hard area and a soft area and an intermediate transition zone, wherein the transition zone is smaller in area than the transition area.

From the DE 10 2014 101 891 A1 a system for warming up workpieces, in particular for a hot forming station is known, which has a warm-up device and at least one, to be transported by the warming device goods carrier, the goods carrier can be equipped with a tool, and at least one tempering component for conductive heating of the workpiece and the heating device comprises at least one movable electrode for electrically contacting the temperature control component.

From the DE 10 2011 053 698 B3 a method for producing in particular structure of the chassis components for a motor vehicle by hot or semi-warm forming is known, wherein a metal sheet is heated in a heating station at least in a first range from an initial temperature to a target temperature and then transferred the heat board in a cooled pressing tool and is formed therein and press-hardened, and the heating station comprises at least one burner zone with at least one burner in which the metal sheet is heated in at least a first range from the starting temperature to the target temperature, wherein at least one burner is operated with a fuel gas and an oxygen-containing gas, and the sheet metal board comes in direct contact with the burner flame, the heating to the target temperature in the heating station via the nozzle orientation and / or the distance of the burner nozzle of the sheet metal blanks is influenced.

Also from the EP 2 799 559 A1 Infrared heating of a steel sheet is known, in which shielding plates, in particular also of ceramic, are described.

In addition, from the EP 2 951 325 A2 and the DE 10 2007 057 855 B3 Method is known in which only is heated homogeneously and then cooled down in furnace zones.

From the AT 509596 B1 a method for heating a mold component for a subsequent press-hardening is known, wherein the mold component is first heated to a predetermined temperature and then heated by means of mutually independently controllable heating elements of a Heizelementfeldes to a higher temperature, wherein the mold component during its promotion by the Heizelementfeld with help the conveying direction in longitudinal and transverse rows arranged, at least in groups with different heating power controllable heating elements is heated.

The object of the invention is to provide a method by means of which sheet steel components with partially hardened regions can be created, wherein the method can be carried out in a simple manner and results in good reproducibility and selectivity of the individual regions on the one hand and high flexibility and a large process window on the other hand.

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

Advantageous developments are characterized in the dependent claims.

It is a further object to provide an apparatus for carrying out the method.

The object is achieved with a device having the features of claim 8.

Advantageous developments are specified in the dependent claims.

According to the invention, steel sheet blanks provided with a metallic coating are produced by separating off, for example, punching or chipping, from a correspondingly coated sheet steel strip. For example, these boards have an aluminum, aluminum-silicon, zinc or zinc-iron coating. In particular, the boards have, for example, a so-called ZF180 coating, which means that they have a zinc-iron coating which is applied to both sides of the sheet with a weight of 180 g / m ² . The sheet consists of a hardenable steel alloy, in particular a customary boron-manganese steel, in particular a 22 MnB5, a 20MnB8 or similar steels.

The boards produced in this way are heated in an oven. Preferably, the furnace is operated with a clocked driving, advantageous is the use of a Hubschritt conveyor furnace ..

This oven has a front oven area and a rear oven area, whereby two successively arranged separate ovens are conceivable.

In the front furnace area there is a furnace base temperature, which is dimensioned so that the boards are heated in a cycled manner to a temperature which is below the austenite start temperature (Ac ₁ ). This temperature should be below about 720 ° C for most steel alloys known for press-hardening or toughening.

In this front furnace area, the board is heated inhomogeneously according to hardening and non-hardening areas as well as transition areas. This means that the areas that should not be hardened and the transition areas remain colder than the areas that are to be hardened later.

Thus, only the areas are heated to a temperature slightly below the austenitizing start temperature Ac ₁ , which are later to be cured while the other areas are heated significantly lower and in particular at a temperature which is at a maximum of 620 ° C. The temperature difference between those areas heated by the furnace base temperature and the colder areas is therefore in particular at least 30 Kelvin, preferably 50 Kelvin and particularly preferably 100 Kelvin.

The colder areas are achieved, for example, by actively cooling the board in these areas, be it by contact and / or radiation heat exchange with cold bodies (absorption masses), and / or by blowing with cold media.

In the rear furnace area or in the second furnace or second furnace section, the furnace base temperature is still at a temperature which is such that the boards would not be heated above the Ac ₁ point of the corresponding alloy (ie, about 720 ° C.) due to the furnace base temperature , wherein the areas to be hardened, in this furnace area means electrically heated radiator or switchable burner to a temperature above Ac ₃ , so the Austenitisierungstemperatur the corresponding steel alloy (usually 840 to 930 ° C) are heated.

Both the circuit board and the radiator or burner field are hereby preferably flat, wherein the radiator or burner field is segmented in the direction of passage and transversely thereto in particular checkerboard-like or honeycomb-shaped or irregular. The corresponding radiators or burners are individually switched on and off, so that even a clocked supplied board a corresponding heating pattern by the individually switched on and off heated radiator or burner can be applied.

A suitably heated board can then be transferred to a pre-cooling station and then reshaped and cured. According to the invention, the individual radiator or burner temperatures can be regulated, so that it is also possible, if necessary, to discuss different sheet thicknesses or to discuss different absorption capacities of the sheets.

In accordance with the invention, reflector templates can also be provided which can be inserted between the radiator or burner and the circuit board in order to allow only locally radiation or flames to pass through and achieve locally higher separation intensities than would be possible with a radiator field alone. In this case, these reflector templates can also be actively cooled so as not to unnecessarily increase the basic oven temperature according to the invention and to further increase the selectivity.

In particular, the areas remaining colder in the first furnace area can on the one hand be correspondingly covered by reflector templates, on the other hand these areas can also remain in the areas in which the radiators are switched off or radiate with reduced power.

When heating in the second furnace section, the temperature difference between the colder and hotter areas of the board does not decrease, so that the low starting temperature of the board in the colder areas means that the board is not heated in these areas in the second furnace section to the extent that there Austenitization would take place. This effect and also the selectivity can be further increased by the fact that these areas are not irradiated or covered with respect to the radiation by the reflector templates.

In addition, a thermal separation can be provided between the front and rear furnace area, unless it is a separate furnace, which is designed, for example, as a bulkhead.

According to the invention, in order to achieve a higher selectivity between individual switched on and off radiator or burner segments, a thermal separation may be provided, so that, for example, ceramic plates and the like between the individual segments are present, which may be inserted depending on the component or from above in the direction of the board to be heated in the radiator or burner field can be inserted to increase the selectivity between adjacent segments.

The invention will be explained by way of example with reference to a drawing.

• 1 stark schematisiert einen Längsschnitt durch eine erfindungsgemäße Ofenanlage;
• 2 stark schematisiert ein Strahlerfeld innerhalb der Ofenanlage mit einer einschiebbaren Reflektorschablone.

It shows:

• 1 very schematically a longitudinal section through a furnace installation according to the invention;
• 2 very schematically a spotlight field within the furnace system with a retractable reflector template.

The inventive method provides for sheet metal blanks 1 through a furnace 2 along a conveying direction 3 through it.

The furnace plant 2 Here is, for example, a clocked working stroke conveyor furnace with a corresponding usually horizontally extending conveyor 4 , wherein the conveyor 4 is known per se and, for example, may be a clocked roller conveyor or the like.

According to the invention, the furnace plant 2 with a furnace entrance 5 and a furnace exit 6 formed, being from the furnace entrance 5 to the oven exit 6 two oven areas arranged one behind the other 7 . 8th are arranged, wherein a front furnace area 7 and a rear oven area 8th through passage direction 3 arranged one behind the other.

These oven areas 7 . 8th each form a furnace chamber in which prevail a predetermined furnace atmosphere and a predetermined furnace temperature. In particular, there is a furnace temperature that is such that the sheet metal blanks 1 can not be heated above AC ₁ by the furnace room temperature.

The two oven areas 7 . 8th can through a bulkhead 9 be separated from each other, which in particular of a furnace roof 10 and furnace side walls (not shown) is formed connecting that only the boards on the conveyor 4 between the oven areas 7 . 8th can happen.

In the area of the furnace roof 10 of the second furnace area 8th is a spotlight field 11 arranged, which consists of electrically operated radiators and / or burners, which are adjacent to each other like a checkerboard ( 2 ) are arranged.

The spotlight field 11 This can be done via checkerboard subdivisions 12 have, which both in the longitudinal and in the transverse direction, in particular by ceramic or other subdivisions ensures that the individual radiator of the radiator field are delimited.

The spotlight field 11 may be furnace side transformations 13 . 14 and a furnace exit 6 on the one hand and from a bulkhead 9 on the other hand be spaced.

The individual spotlights / burners 15 within the radiator field can each be individually controllable, ie be switched on and off and also be adjustable in temperature. To distinguish certain areas of the radiator field from other areas of the radiator field, in particular to the temperature influences of the radiator field and the individual radiator 15 on the board 1 To regulate, plug-in bulkheads 16 may be present, in particular from a furnace ceiling forth according to the checkerboard-like subdivision of the radiator field 11 can be plugged in and over the radiator field 11 can reach out into the oven room.

For special curing contours of the board 1 To bring about and to further increase the selectivity of the heating of the radiator field, can from the Ofenseitenwandungen 13 . 14 forth, reflector stencils 17 in the oven room and over the board 1 be inserted inserted, according to a direction of insertion 18 ,

In the simplest case, the reflector stencils may be metal sheets which are optionally heat-resistant and polished, moreover, the reflector stencils may also be formed from a ceramic and also have cooling in order to avoid overheating above or below the reflector stencil, if necessary.

In the first furnace area 7 areas 18 a circuit board 1 , which should not be cured, less to heat, z. B. absorption masses 19 on the board 1 be hung up and resumed. Here, for example, the absorption mass with a manipulation linkage 20 in front of a furnace side wall 14 in the room 7 be introduced and filed.

In the invention, it is advantageous that for the individual sheet metal blanks with a high accuracy and reproducibility, a corresponding hardening pattern can be produced over different temperatures in the sheet metal blank.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102008030279 D1 [0010]
• EP 214808 B1 [0010]
• EP 2520382 A2 [0011]
• EP 2322672 A1 [0013]
• DE 102014119545 A1 [0014]
• DE 102014101891 A1 [0015]
• DE 102011053698 B3 [0016]
• EP 2799559 A1 [0017]
• EP 2951325 A2
• DE 102007057855 B3 [0018]
• AT 509596 B1 [0019]

Claims (10)

Method for producing locally hardened sheet steel components, wherein steel sheet parts are passed through a heating furnace, wherein the heating furnace is divided into two furnace regions (7, 8) arranged one behind the other in the conveying direction (3), wherein the printed circuit board is at a temperature of at least in a first furnace region (7) is heated below Ac ₁ , wherein areas which are not to be hardened while less heated and irradiated in a second furnace area (8), the board in areas to be subjected to additional heating by means of electrically operated radiant heater and / or burner and / or is heated, wherein in areas where no above Ac ₁ heating is desired individual emitters and / or burners in (15) off or pattern of radiators and / or burners (15) are turned off, and / or reflector templates above nichtheizheizenden areas between the radiator and / or torch field (11) and the platinum e (1) are pushed so that the board is shielded in nichtheizheizenden areas of additional heat input via radiation and remains below the Ac ₁ temperature. Method according to Claim 1 , characterized in that the board (1) clocked by the furnace (2) is guided. Method according to one of the preceding claims, characterized in that the temperature in the furnace chambers (7, 8) is selected so that the board is not heated by the furnace chamber temperature Ac ₁ over. Method according to one of the preceding claims, characterized in that the radiators and / or burners (15) of the radiator and / or burner field (11) are thermally separated or decoupled from each other by thermal separations. Method according to one of the preceding claims, characterized in that for the shielding of the board (1) heat radiation by the radiator and / or burner field, in particular local shielding of heat from the radiator and / or burner field preformed reflector templates are slidable over the board. Method according to one of the preceding claims, characterized in that the reflector templates are actively cooled. Method according to one of the preceding claims, characterized in that only the areas heated to a temperature below the austenitizing start temperature Ac ₁ , which are later to be cured while the other areas are heated significantly lower and in particular at a temperature remain at is at most 620 ° C, wherein the temperature difference between those heated by the furnace base temperature areas and the colder areas in particular at least 30 Kelvin, preferably 50 Kelvin and more preferably 100 Kelvin. Apparatus for carrying out the method according to one of the preceding claims, characterized in that a furnace (2) is provided, the furnace having two in the conveying direction (3) subsequently arranged furnace chambers (7, 8), wherein the furnace via two in the conveying direction (3) subsequently arranged furnace spaces (7, 8), wherein the furnace chambers have a conveyor (4), wherein the conveyor is a roller conveyor and / or a Hubwolkenfördereinrichtung or the like means for clocked conveying of boards (1), wherein in the second furnace chamber (8) a radiant heater and / or burner field (11) with radiant heaters and / or burners (15) is present, wherein the radiant heater and / or burner (15) is present, wherein the radiant heater / burner (15) switched on and off and the intensity of the radiation / flame controllable emitters and / or burners are. Device after Claim 8 , characterized in that the individual radiator / burner (15) are arranged like a checkerboard or honeycomb or irregular in the radiator / burner field (11), wherein between the radiator (15) separations or foreclosures are arranged inserted. Device according to one of Claims 8 or 9 , characterized in that reflector templates (18) are provided, which are formed so that they reflect the radiation / flame of the radiator / burner (15) between the conveyor (4) and the radiator / burner field (11) or of the Conveying device (4) are arranged shielding.

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