EP3332041B1 - Method for heat treatment of a sheet steel component and heat treatment apparatus therefor - Google Patents

Description

The invention relates to a method for targeted, component-zone-specific heat treatment of sheet metal components and to a heat treatment device for carrying out the method.

In technology, there is a desire for high-strength sheet metal parts with low part weight in many applications in different industries. For example, the automotive industry is striving to reduce the fuel consumption of motor vehicles and lower CO2 emissions, while at the same time increasing passenger safety. There is therefore a rapidly increasing demand for body components with a favorable strength-to-weight ratio. These components include in particular A and B pillars, side impact protection beams in doors, sills, frame parts, bumper guards, cross members for the floor and roof, front and rear longitudinal members. In modern motor vehicles, the body shell with a safety cage usually consists of a hardened steel sheet with a strength of approx. 1,500 MPa. AlSi-coated steel sheets, i.e. steel sheets coated with aluminum-silicon, are often used. The process of so-called press hardening was developed to produce a component from hardened steel sheet. Steel sheets are first heated to an austenite temperature of between 850°C and 950°C, then placed in a press tool, quickly formed and rapidly quenched to a martensite temperature of around 250°C using the water-cooled tool. This creates a hard, solid martensite structure with a strength of around 1,500 MPa. However, a steel sheet hardened in this way has only a low elongation at break, which is disadvantageous in certain areas in the event of a vehicle collision. The kinetic energy cannot be converted into deformation heat. In this case, the component will break brittlely and also risk injuring the occupants.

For the automotive industry, it is therefore desirable to obtain body components that have several different expansion and strength zones in the component, so that very strong areas on the one hand and very elastic areas on the other are present in one component. The general requirements for a production plant should still be taken into account: there should be no loss of cycle time on the mold hardening plant, the entire plant should be able to be used without restrictions and quickly converted to customer-specific requirements. The process should be robust and economical and the production plant should only require minimal space. The shape and edge accuracy of the component should be so high that hard trimming can be largely eliminated in order to save material and labor.

To produce a component with areas of different hardness and ductility, different steels can be welded together so that non-hardenable steel is in the soft zones and hardenable steel is in the hard zones. In a subsequent hardening process, the desired hardness profile can be achieved over the component. The disadvantages of this process are the occasionally unsafe weld seam on an Al-Si-coated sheet of approximately 0.8-1.5 mm thick, which is usually used for body parts, the abrupt hardness transition there, and the increased cost of the sheet due to the additional manufacturing step of welding. In tests, failures due to breakage near the weld seam occasionally occurred, so the process cannot be described as robust. In addition, the process has limitations when it comes to complex geometries.

From the German patent specification EN 10 2007 057 855 B3 A process is known in which a molded component in the form of a plate cut from a strip material with an AlSi coating of high-strength boron steel is first heated completely homogeneously to such a temperature and is kept at this temperature level for a certain time that a diffusion layer forms as a corrosion or scale protection layer, whereby material from the coating and the base material diffuse together. The heating temperature is about 830°C to 950°C. This homogeneous Heating takes place in a first zone of a continuous furnace having multiple temperature zones. Following this process step, an area of the first type of the blank is cooled in a second zone of the furnace to a temperature at which austenite decomposes. This takes place at around 550°C to 700°C. This reduced temperature level is maintained for a certain time so that the decomposition of austenite proceeds smoothly. At the same time as the area of the first type of the blank is being locally cooled, the temperature in at least one area of the second type is kept just high enough in a third zone of the furnace so that sufficient martensite can still be produced during the subsequent hot forming in a corresponding press. This temperature is between 830°C and 950°C. When the area of the first type is cooling, this area of the blank can be brought into contact with cooling jaws for a short time.

However, this process only allows relatively simple and large-area geometries, usually with only two different areas, to be subjected to different heat treatments. Complex geometries, such as ductile spot weld edges of almost any shape in space on a B-pillar that is otherwise very hard, cannot be heat treated accordingly using this process. In addition, the temperatures of the individual zones of the furnace must be regulated very precisely. On the other hand, continuous furnaces are usually heated with gas burners for economic reasons, but this does not allow the temperatures of the individual zones to be regulated with the required accuracy in a simple and inexpensive way.

From the European Disclosure Document EP 2 497 840 A1 A furnace system and a method for targeted, zone-specific heat treatment of sheet metal components is known. The furnace system has a conventional, universal production furnace for heating the sheet steel parts to a temperature close to, but below, the AC3 temperature, i.e. the temperature at which the transformation of the ferrite into austenite ends, wherein the furnace system also has a profiling furnace with at least one level. The at least one level has an upper and a lower part, as well as a product-specific intermediate flange inserted into a corresponding holder, wherein the product-specific intermediate flange is designed to impose a predetermined temperature profile on the component with temperatures above the AC3 temperature for areas to be hardened and below the AC3 temperature for softer areas. The temperature profile is imposed by means of heat radiation. Since the process provides for heating the components in the production furnace only to a temperature below the AC3 temperature and the heat for heating defined areas to a temperature above the AC3 temperature is introduced in a later process step in the profiling furnace, very precise temperature control in the production furnace is not necessary, so that the disadvantage of the poorer controllability of gas burners compared to electric heaters can be accepted in favor of the cost-effectiveness of the cheaper energy source gas. The disadvantage of this process is that the areas with different temperatures cannot be separated exactly. In addition, the heat exchange via radiation takes place relatively slowly, so that several profiling furnaces have to be operated in parallel in order to be able to utilize the possible capacity of the continuous furnace.

From the German disclosure document EN 10 2012 102 194 A1 a furnace system and a method for operating a furnace system are known, wherein a radiant heat source is arranged within the furnace system and a metallic component can be thermally treated within the furnace system with two different temperature ranges. Furthermore, an air flow is circulated in a second area of the furnace system, with which a second temperature range is thermally treated due to forced convection. The first area of the metallic component is heated to at least AC3 by means of radiant heat and/or its temperature is kept at at least AC3 and the second area is cooled by convection from a temperature of at least AC3 to a temperature below AC3 or the second area is heated by convection to a temperature below AC3, the different temperature zones created thereby being thermally separated from one another by a separating device. It is difficult to thermally separate the temperature ranges in the furnace from one another. The separating device must be adapted to the contour of the metallic component in order to enable effective temperature separation. As a result, the furnace can only be used for other applications after conversion. Component geometries can be used, although furnace conversion is complex due to the size of the furnace, especially the size of a roller hearth furnace.

In addition, it is desirable for an AlSi layer to form on the component as corrosion protection during heat treatment and to be firmly bonded to the component. To do this, the AlSi can be diffused into the surface of the component. This usually occurs at temperatures of more than 930°C.

From the WO 2014/118723 A2 A process is known in which steel is heated differently in different areas exclusively by radiation. Furthermore, EN 20 2014 010318 U1 A device is known with which steel can be heated differently in certain areas.

All known devices require a relatively large amount of space. In addition, it is difficult with all known devices and methods to introduce the heat energy into the component in a targeted manner. All known types of heat introduction have the disadvantage that the energy cannot be introduced into specific areas of the component in a selective manner, but rather neighboring areas are also exposed to heat energy, so that a selective temperature generation above the AC3 temperature directly next to areas with temperatures below the AC3 temperature is only possible to a limited extent. In particular, measures must be taken, for example in the form of partitions, in order to maintain hard and ductile component areas directly next to one another after press hardening.

From the scientific journal Laser Technik and in particular from the article " High Power VCSEL Systems: A tool for digital thermal processing" by Holger Mönch Vol. 11, No. 2, 1 April 2014 (2014-04-01), Pages 43-47, XP055828652, ISSN: 1613-7728, DOI: 10. 1002/latj.201400024 A VSCEL is known for surface treatment of steel.

The object of the invention is to provide a method for the targeted heat treatment of sheet metal components, whereby a demarcation with minimized transition zones can be created between component areas with temperatures above the AC3 temperature and component areas with temperatures below the AC3 temperature. A further object of the invention is to provide a heat treatment device for the targeted, component-zone-specific heat treatment of sheet metal components, which takes up relatively little space and with which it is possible to achieve a demarcation between component areas with temperatures above the AC3 temperature and component areas with temperatures below the AC3 temperature without any sealing measures, whereby the transition zones between the areas are minimized.

According to the invention, this object is achieved by a method having the features of independent claim 1. Advantageous further developments of the method arise from the subclaims 2 to 8. The object is further achieved by a heat treatment device according to claim 9. Advantageous further developments of the heat treatment device arise from the subclaims 10 to 14. With the inventive method for impressing a temperature profile on a sheet steel component, a temperature below the AC3 temperature can be impressed on the sheet steel component in one or more first areas and a temperature above the AC3 temperature in one or more second areas. The AC3 temperature, like the recrystallization temperature, is alloy-dependent. For the materials usually used for vehicle body components, the AC3 temperature is around 870°C, while the recrystallization temperature at which the ferrite-pearlite structure is established is around 800°C. The method is characterized in that the sheet steel component is first preheated in a gas-heated production furnace, the sheet steel component is then transferred to a thermal post-treatment station, wherein in the thermal post-treatment station a radiant heat source is brought over the component, with which the first region or the plurality of first regions of the sheet steel component are optionally held at a temperature below the AC3 temperature or are further cooled and the second region or the plurality of second regions of the sheet steel component are optionally heated to or held at a temperature above the AC3 temperature. During preheating, the component can be brought to a temperature below the AC3 temperature or above the AC3 temperature. Depending on the temperature it has when it is introduced into the post-treatment station, the one or more first areas of the sheet steel component are kept at a temperature below the AC3 temperature or are cooled further in the post-treatment station and the one or more second areas of the sheet steel component are heated to a temperature above the AC3 temperature if they have a lower temperature when they are introduced into the post-treatment station, or kept at a temperature above the AC3 temperature if they already have this temperature when they are introduced into the post-treatment station. Natural convection, for example, can be used for cooling. Forced convection by blowing on the corresponding areas of the component is also possible. The blowing can be provided from above, i.e. the side of the component facing the radiant heat source, or from below, i.e. the side of the sheet steel component facing away from the radiant heat source. It is also conceivable to provide contact cooling from the underside of the component, i.e. the side of the sheet steel component facing away from the radiant heat source.

In the inventive method, the gas-heated production furnace does not have to be adapted to the geometry of the sheet steel component to be treated, in particular no part geometry-dependent separating device is provided in the furnace. On the contrary, a standard furnace can be used that does not need to be converted when production changes. In particular, a standard roller hearth furnace or a batch furnace can be used. Continuous furnaces generally have a large capacity and are particularly suitable for mass production because they can be loaded and operated without great effort. According to the invention, the production furnace is gas-fired.

In most cases, gas firing is the most economical way of heating a production furnace. Controlling the furnace temperature does not place any increased quality requirements, as the entire sheet steel component is heated to an essentially uniform temperature.

The radiant heat source can be moved over the component. In one embodiment, the radiant heat source is pivotable, for example pivotable essentially horizontally, arranged in the post-treatment station and pivotable over the component and also pivotable away again. This means that after heat treatment, the component can easily be picked up by a handling device, for example an industrial robot, and transported further without the radiant heat source interfering with the movement.

It has proven to be advantageous if the post-treatment station is directly connected to the production furnace. The production furnace can be a roller hearth furnace, for example. In a roller hearth furnace, the components are transported through the furnace using rollers. The post-treatment station can be directly connected to the furnace by extending the roller conveyor accordingly. One possible effect of this arrangement is, for example, that the component cools down as little as possible in the ambient air. It is also possible to have several post-treatment stations connected to the furnace in order to minimize the cycle time.

The production furnace is gas-fired and can be heated with gas burners, for example.

In the thermal post-treatment station, the radiant heat source is a field with surface emitters, so-called VCSELs (Vertical Cavity Surface Emitting Lasers), which emit radiation in the infrared range. Such a field consists of a large number, typically several thousand, very small lasers (microlasers) with diameters in the µm range, which are arranged in the field with a typical distance of approx. 40 µm between the individual lasers. Such VCSELs deliver radiation with a very narrow line width compared to infrared LEDs and an extremely forward-directed radiation characteristic. This makes it possible to imprint different temperatures on a substrate with great edge accuracy. Furthermore, very high power densities of over 100 W/cm2 are achieved on the irradiated area with this microlaser technology.

In an advantageous embodiment, the surface emitters emit radiation in the near infrared range between 780 nm and 3 µm, for example radiation of 808 nm or 980 nm wavelength.

It has also proven to be advantageous if the surface emitters can be controlled in groups. Alternatively, the surface emitters can also be controlled individually. Mixed forms are also possible, whereby individual surface emitters can be controlled individually and other surface emitters can be controlled in groups.

By controlling individual emitters or groups of surface emitters, it is possible to generate different radiation intensities and thus impose a temperature profile on a substrate. For example, the surface emitters located above the first areas of the component can be controlled so that they radiate with less power than the Surface emitters that are located above the second areas of the component. It is also possible to adapt the radiation power to a three-dimensional component profile in which, for example, the areas of the component that are closer to the surface emitters are irradiated with lower power than the component areas that are further away from the surface emitters due to the three-dimensional geometry of the component. If the surface emitters are pulsed lasers, the control can, for example, relate to the pulse lengths and/or the frequency. The control can also be based on which temperature is to be reached in the individual areas. The corresponding temperature, for example the AC3 temperature, depends on the alloy. Another parameter for the control can be the thermal conductivity of the substrate, which can also be alloy-dependent.

In a particularly advantageous embodiment, the production furnace has several zones with different temperatures, whereby the sheet steel component is heated in a first zone or one of the first zones to a temperature above approximately 900°C, and whereby it is cooled in the zones following in the direction of flow to such an extent that it has a temperature of less than approximately 900°C, for example approximately 600°C, when transferred to the post-treatment station. In the first zone or zones, an AlSi coating can diffuse into the component and the component can then cool down to such an extent that a pearlitic-ferritic structure is established. In the post-treatment station, the second areas of the component can be heated up very quickly to temperatures above the AC3 temperature again using the surface emitter field, so that an austenitic structure is formed in these areas.

(Continue on page 10 of the original description.)

A heat treatment device according to the invention comprises a gas-heated production furnace for preheating a sheet steel component and a thermal post-treatment station for imparting a temperature profile to the sheet steel component, and is characterized in that the post-treatment station comprises a radiant heat source, wherein the radiant heat source comprises an array of surface emitters (VCSELs) which emit radiation in the infrared range.

With the method according to the invention and the heat treatment device according to the invention, a corresponding temperature profile can be economically imposed on sheet steel components with several first and/or second regions, which can also have a complex shape, since the surface emitters used here result in a more selective treatment of the first and second regions of the sheet steel component in the post-treatment station than is possible in the production furnace.

Further advantages, special features and expedient developments of the invention emerge from the subclaims and the following presentation of preferred embodiments with reference to the figures.

• Fig. 1 eine erfindungsgemäße Wärmebehandlungsvorrichtung in einer Draufsicht
• Fig. 2 ein Stahlblechbauteil mit ersten und zweiten Bereichen in einer Draufsicht
• Fig.3 ein Beispiel eines anderen Stahlblechbauteils in Draufsicht nach Ausführung des erfindungsgemäßen Verfahrens

From the pictures shows:

• Fig.1 a heat treatment device according to the invention in a plan view
• Fig.2 a sheet steel component with first and second regions in a plan view
• Fig.3 an example of another sheet steel component in plan view after implementation of the method according to the invention

Fig.1 shows a heat treatment device 100 according to the invention in a plan view. A sheet steel component 200 is placed on an infeed table 120 of the heat treatment device 100 by a first handling device 130. From the infeed table 120, sheet steel components 200 enter the continuous furnace designed production furnace 110 and pass through it in the direction of the arrow, whereby their temperature increases to a temperature, for example, above the AC3 temperature. Viewed in the direction of flow behind the production furnace 110 there is an outlet table 121 designed as a post-treatment station 150, onto which the heated sheet steel components 200 arrive after passing through the production furnace 110. The post-treatment station 150 has a radiant heat source 151 in the form of a surface radiator with an array of surface emitters. The radiant heat source 151 is designed to be pivotable. The figure shows the situation in which the temperature profile has already been impressed on the sheet steel component 200. For this purpose, the radiant heat source 151 was pivoted over the sheet steel component 200 so that the infrared radiation could hit the sheet steel component. After applying the temperature profile, the radiant heat source is now pivoted away from the sheet steel component 200 so that a second handling device 131 can grip the sheet steel component 200 and transport it further without the radiant heat source 151 disturbing the movement.

More thermal post-treatment stations 150 can also be provided. The number of thermal post-treatment stations 150 that can advantageously be provided depends on the ratio of the cycle times of the production furnace 110 and the thermal post-treatment station 150, wherein the cycle times depend on the temperatures to be reached and thus depend, among other things, on the material being processed as well as the geometry and material thickness of the sheet steel component 200.

Fig.2 shows a sheet steel component 200 with first areas 210 and second areas 220 in a top view. The first areas 210 should have a high degree of ductility in the later finished part. If the sheet steel component 200 is a vehicle body part, these first areas 210 can be, for example, the areas where the later finished part is connected to the rest of the vehicle body. The second areas 220 of the sheet steel component 200, on the other hand, should have a high degree of hardness in the later finished part.

Fig.3 shows an example of another sheet steel component 200, here a B-pillar 200 for vehicles in plan view after carrying out the method according to the invention. The B-pillar is the connection between the vehicle floor and the vehicle roof in the middle of the passenger compartment. In the event of an accident in which the vehicle rolls over, the pillars in the vehicle, and therefore also the B-pillar, have the life-saving task of stabilizing the passenger compartment against vertical deformation. Much more important is the absorption of forces in the event of a side impact so that the vehicle occupants remain unharmed. In order to be able to guarantee this task, the B-pillar 200 has first areas 210 with high ductility and second areas 220 with high hardness. The B-pillar 200 was provided with the first areas 210 and second areas 220 shown here by means of the method according to the invention in the heat treatment device according to the invention, with the second areas 220 additionally being tempered.

The embodiments shown here are only examples of the present invention and should therefore not be understood as limiting.

List of reference symbols:

100

Heat treatment device

110

Production furnace

120

Infeed table

121

Discharge table

130

first handling device

131

second handling device

150

thermal post-treatment station

151

Radiation heat source

200

Sheet steel component

210

first area

220

second area

300

Handling device

Claims (14)

1. Method for forming a temperature profile in a sheet-steel component (200), wherein the sheet-steel component (200) can be brought to a temperature below the AC3 temperature in one or more first regions (210) and to a temperature above the AC3 temperature in one or more second regions (220),
   characterized in that
   the sheet-steel component (200) is firstly preheated in a gas-heated production furnace (110), and the sheet-steel component (200) is then transferred to a thermal post-treatment station (150), wherein a radiant-heat source (151) is moved over the component in the thermal post-treatment station (150), with which radiant-heat source the one or more first regions (210) of the sheet-steel component (200) can selectively be kept at a temperature below the AC3 temperature or cooled further and the one or more second regions (220) of the sheet-steel component (200) are selectively heated to or kept at a temperature above the AC3 temperature, wherein the radiant-heat source (151) is a field with vertical-cavity surface-emitting lasers (VCSELs), which emit radiation in the infrared range.
2. Method according to Claim 1, characterized in that the surface emitters emit radiation in the near-infrared range between 780 nm and 3 µm.
3. Method according to Claim 1 or 2, characterized in that
   the surface emitters can be actuated in groups.
4. Method according to Claim 1 or 2, characterized in that
   the surface emitters can be actuated individually.
5. Method according to one of the preceding claims,
   characterized in that
   the sheet-steel component (200) is heated to a temperature below the AC3 temperature in the production furnace (110).
6. Method according to one of Claims 1 to 4,
   characterized in that
   the sheet-steel component (200) is heated to a temperature above the AC3 temperature in the production furnace (110).
7. Method according to one of Claims 1 to 4,
   characterized in that
   the production furnace (110) has multiple zones of different temperatures, wherein the sheet-steel component (200) is heated to a temperature above approximately 900°C in a first zone or in multiple first zones, wherein it is cooled in the zones which follow said one or more first zones in the direction of flow to the extent that it has a temperature of less than approximately 900°C when transferred to the post-treatment station.
8. Method according to Claim 7, characterized in that the sheet-steel component (200) is cooled in the zones which follow the first zone or the first zones in the direction of flow to the extent that it has a temperature of approximately 600°C when transferred to the post-treatment station.
9. Heat treatment device (100), comprising a gas-heated production furnace (110) for pre-heating a sheet-steel component (200) and a thermal post-treatment station (150) for forming a temperature profile in the sheet-steel component (200),
   characterized in that
   the post-treatment station (150) comprises a radiant-heat source (151), wherein the radiant-heat source (151) comprises a field with vertical-cavity surface-emitting lasers (VCSELs), which emit radiation in the infrared range.
10. Heat treatment device (100) according to Claim 8,
    characterized in that
    radiation in the near-infrared range can be emitted by the surface emitters.
11. Heat treatment device (100) according to either of Claims 8 and 9, characterized in that
    the surface emitters can be actuated in groups.
12. Heat treatment device (100) according to either of Claims 8 and 9, characterized in that
    the surface emitters can be actuated individually.
13. Heat treatment device (100) according to one of Claims 8 to 11, characterized in that
    the post-treatment station (150) directly adjoins the production furnace (110).
14. Heat treatment device (100) according to one of Claims 8 to 12, characterized in that
    the radiant-heat source (151) is arranged pivotably in the post-treatment station (150).

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