EP3420111B1 - Process for targeted heat treatment of individual component zones - Google Patents

Description

The invention relates to a method for targeted heat treatment of a steel component in individual component zones.

In technology, there is a desire for high-strength sheet metal parts with low weight for many applications in different industries.

For example, in the vehicle industry there is an effort to reduce the fuel consumption of motor vehicles and reduce CO ₂ emissions, while at the same time increasing passenger safety. There is therefore a rapidly increasing need 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 side members. In modern motor vehicles, the bodyshell with a safety cage usually consists of a hardened steel sheet with a strength of approx. 1,500 MPa. Al-Si-coated steel sheets are often used. The process of so-called press hardening was developed to produce a component from hardened sheet steel. Steel sheets are first heated to austenite temperature, then placed in a press tool, quickly formed and quickly quenched by the water-cooled tool to less than the martensite starting temperature. This creates a hard, solid martensite structure with a strength of approx. 1,500 MPa. However, a steel sheet hardened in this way only has a low elongation at break. The kinetic energy of an impact cannot therefore be sufficiently converted into heat of deformation.

It is therefore desirable for the automotive industry to be able to produce body components that have several different expansion and strength zones in the component, so that rather solid areas (hereinafter first areas) on the one hand, Maximum stretchable areas (hereinafter second areas) on the other hand and additionally adjustable stretchable areas (hereinafter third areas) are present in a component. On the one hand, components with high strength are fundamentally desirable in order to obtain components that can withstand high mechanical loads and are low in weight. On the other hand, high-strength components should also be able to have partially soft areas, which achieves the desired, partially increased deformability in the event of a crash. This is the only way to reduce the kinetic energy of an impact and thus minimize the acceleration forces on the occupants and the rest of the vehicle. In addition, modern joining processes require softened areas that enable the joining of similar or different materials. For example, folding, crimping or riveting connections often have to be used, which require deformable areas in the component.

Soft edge areas of the component also allow contour trimming in the tool and can therefore make complex laser trimming unnecessary.

The general requirements for a production system should still be taken into account: there should be no loss of cycle time on the press hardening system, the entire system should be able to be used for general purposes without restrictions and quickly converted to product-specific requirements. The process should be robust and economical and the production facility should only require minimal space. The shape and edge accuracy of the component should be high.

A method for producing a press-hardened molded component is, for example, from EP 2 679 692 A1 known. Before the press forming process, a board is homogeneously heated to a forming temperature of 450°C to 700°C. Individual areas of the board are then heated to a higher temperature of up to or more than 900°C over a period of a few seconds and then formed and hardened.

In Altena H. et al.: "Process technology and plant design for bainite hardening", European Conference on Heat Treatment 2015 and 22nd IFHTSE Congress Metallurgical requirements, process technologies, suitable materials, suitable equipment and applications for bainite hardening are described. This technology is also described in H. Altena et al.: "Process technology and plant design for bainite hardening", La Metallurgia Italiana -n. 3 2016. The publication "22nd IFHTSE Congress and European Conference on Heat Treatment 2015 Venice " is an overview of a congress on heat treatment.

In all known methods, the targeted heat treatment of the component takes place in a time-consuming treatment step, which has a significant influence on the cycle time of the entire heat treatment device.

The object of the invention is therefore to provide a method for the targeted heat treatment of a steel component in individual component zones, whereby areas of different hardness and ductility can be achieved, in which the influence on the cycle time of the entire heat treatment device is minimized.

According to the invention, this object is achieved by a method with the features of independent claim 1. Advantageous developments of the method result from subclaims 2 to 8.

The inventive method for the targeted heat treatment of a steel component in individual component zones is defined in claim 1.

A heat treatment device not covered by the claims has a first furnace for heating a steel component to a temperature below the AC3 temperature, a treatment station and a second furnace, the treatment station having a device for quickly heating the first and third areas and a device for rapid cooling of one or more third areas of the steel component and the second furnace has a device for introducing heat.

In an advantageous embodiment of the method, the heat supply in the second oven is achieved via thermal radiation.

A steel component is first heated in an oven to below the austenitization temperature. The different areas are then treated differently in a treatment station:
In the treatment station, the first area or areas are first brought to a temperature above AC3 within a few seconds, for example with the help of a high-power laser, so that the structure is transformed as completely as possible into austenite. In a preferred embodiment, the areas irradiated by the laser are precisely defined by channel walls that are arranged as vertically as possible to the component surface.

The first area or areas are then not subjected to any further special treatment in the treatment station, i.e. they are neither blown on nor heated or cooled using other special measures. The first area or areas cool slowly in the treatment station, for example via natural convection and radiation. It has proven to be advantageous if measures are taken in the treatment station to reduce the temperature losses of the first or first areas. Such measures can be, for example, the attachment of heat radiation reflectors and/or the insulation of surfaces of the treatment station in the area of the first or first areas.

The second area or the second areas are not subjected to any special treatment in the treatment station, ie they are neither blown on nor heated or cooled using other special measures. The second area or areas cool slowly in the treatment station, for example via natural convection and radiation. It has proven to be advantageous if measures are taken in the treatment station to reduce the temperature losses of the second or second areas. Such measures can be, for example, the attachment of heat radiation reflectors and/or the insulation of surfaces of the treatment station in the area of the second or second areas.

The second or second areas were not completely austenitized during the process and, even after pressing in a subsequent press hardening process, have low strength values similar to the original strengths of the untreated steel component.

In the treatment station, the third area or areas are first brought to a temperature above AC3 within a few seconds, for example with the help of a high-power laser, so that the structure is transformed as completely as possible into austenite. In a preferred embodiment, the areas irradiated by the laser are precisely defined by channel walls that are arranged as vertically as possible to the component surface.

Immediately afterwards, the third or third areas are cooled down as quickly as possible within a treatment time t ₁₅₂ . In a preferred embodiment of the method, the third region or regions are rapidly cooled by blowing with a gaseous fluid, for example air or a protective gas. For this purpose, in an advantageous embodiment, the treatment station has a device for blowing on the third area or areas. This device can, for example, have one or more nozzles. In an advantageous embodiment of the method, the third or third regions are blown by blowing with a gaseous fluid, with water, for example in nebulized form, being added to the gaseous fluid. For this purpose, in an advantageous embodiment, the device has one or more misting nozzles. By blowing with the gaseous fluid mixed with water, the heat is dissipated from the or increased from the third areas. After the treatment time t ₁₅₂ has elapsed, the third area or areas have reached a cooling stop temperature ϑ _S. The treatment time t ₁₅₂ is usually in the range of a few seconds.

According to the invention, after a few seconds in the treatment station, which can also have a positioning device to ensure the precise positioning of the different areas, the components are transported into a second oven, which preferably does not have any special devices for treating the different areas differently. Clearly contoured boundaries have already been implemented in the treatment station. In one embodiment, only an oven temperature ϑ ₄ , ie a substantially homogeneous temperature in the entire oven space, is set, which is below the austenitization temperature AC3. The temperatures of the individual areas approach each other and the small temperature difference between the areas minimizes the distortion of the components. The smallest possible spread in the temperature level of the component has an advantageous effect during further processing in the press.

In a further advantageous embodiment of the method, the internal temperature ϑ ₄ in the second oven is less than the AC3 temperature.

In one embodiment, a continuous oven is advantageously provided as the first oven. Continuous ovens usually have a large capacity and are particularly suitable for mass production because they can be loaded and operated without much effort. But a batch oven, for example a chamber oven, can also be used as the first oven.

Advantageously, in one embodiment, the second oven is a continuous oven.

If both the first and second furnaces are designed as continuous furnaces, the necessary residence times for the first and second areas can be in Dependence of the component length can be realized via the setting of the conveying speed and the design of the respective oven length. In this way, an influence on the cycle time of the entire production line with the heat treatment device and press for subsequent press hardening can be avoided.

In an alternative embodiment, the second oven is a batch oven, for example a chamber oven.

In a preferred embodiment, the treatment station has a device for quickly heating one or more third areas of the steel component. In an advantageous embodiment, the device has one or more high-power lasers for irradiating the third region or regions of the steel component. In a preferred embodiment, the areas are clearly demarcated by appropriately shaped channels.

In a preferred embodiment, the treatment station has a device for quickly cooling one or more third areas of the steel component. In an advantageous embodiment, the device has a nozzle for blowing a gaseous fluid, for example air or a protective gas such as nitrogen, into the third region or regions of the steel component. For this purpose, in an advantageous embodiment, the device has one or more misting nozzles. By blowing with the gaseous fluid mixed with water, the heat dissipation from the third area or areas is increased.

In a further embodiment, the third or third areas are cooled via heat conduction and contact cooling, for example by bringing them into contact with a stamp or several stamps, which has or have a lower temperature than the steel component. For this purpose, the stamp can be made of a material that conducts heat well and/or can be tempered directly or indirectly. A combination of cooling types is also conceivable.

With the method according to the invention, a corresponding temperature profile can be economically imposed on steel components, each with one or more first, second and / or third areas, which can also be complex in shape, since the different areas can be brought to the necessary process temperatures very quickly with sharp contours.

According to the invention, it is possible with the method shown to set almost any number of the three different areas, with different third areas being able to achieve different strength values among themselves, if necessary.

The selected geometry of the sub-areas can also be freely selected. Point or line-shaped areas as well as large areas can be displayed. The location of the areas is also irrelevant. The individual areas can be completely enclosed by other areas or located at the edge of the steel component. Even full-surface treatment is conceivable. A special orientation of the steel component in relation to the direction of travel is not required for the purpose of the method according to the invention for targeted heat treatment of a steel component in individual component zones. The number of steel components treated at the same time is limited at most by the press hardening tool or the conveyor technology of the entire heat treatment device. It is also possible to apply the process to pre-formed steel components. The three-dimensionally shaped surfaces of pre-formed steel components simply result in a higher design effort to represent the counter surfaces.

Furthermore, it is advantageous that existing heat treatment systems can also be adapted to the method according to the invention. For this purpose, in a conventional heat treatment device with only one furnace, only the treatment station and the second furnace need to be installed behind it. Je Depending on the design of the existing oven, it is also possible to divide it so that the original one oven becomes the first and second oven.

Further advantages, special features and useful developments of the invention result from the subclaims and the following presentation of preferred exemplary embodiments based on the illustrations.

• Fig. 1 eine typische Temperaturkurve bei der Wärmebehandlung eines Stahlbauteils mit einem ersten, zweiten und einem dritten Bereich
• Fig. 2 eine erfindungsgemäß einsetzbare thermische Wärmebehandlungsvorrichtung in einer Draufsicht als Schemazeichnung
• Fig. 3 eine weitere erfindungsgemäß einsetzbare thermische Wärmebehandlungsvorrichtung in einer Draufsicht als Schemazeichnung
• Fig. 4 eine weitere erfindungsgemäß einsetzbare thermische Wärmebehandlungsvorrichtung in einer Draufsicht als Schemazeichnung
• Fig. 5 eine weitere erfindungsgemäß einsetzbare thermische Wärmebehandlungsvorrichtung in einer Draufsicht als Schemazeichnung
• Fig. 6 eine weitere erfindungsgemäß einsetzbare thermische Wärmebehandlungsvorrichtung in einer Draufsicht als Schemazeichnung.
• Fig. 7 eine weitere erfindungsgemäß einsetzbare thermische Wärmebehandlungsvorrichtung in einer Draufsicht als Schemazeichnung

From the pictures shows:

• Fig. 1 a typical temperature curve during the heat treatment of a steel component with a first, second and a third area
• Fig. 2 A thermal heat treatment device that can be used according to the invention in a plan view as a schematic drawing
• Fig. 3 a further thermal heat treatment device that can be used according to the invention in a top view as a schematic drawing
• Fig. 4 a further thermal heat treatment device that can be used according to the invention in a top view as a schematic drawing
• Fig. 5 a further thermal heat treatment device that can be used according to the invention in a top view as a schematic drawing
• Fig. 6 a further thermal heat treatment device that can be used according to the invention in a top view as a schematic drawing.
• Fig. 7 a further thermal heat treatment device that can be used according to the invention in a top view as a schematic drawing

In the Fig. 1 is a typical temperature curve during the heat treatment of a steel component 200 with a first area 210, a second area 220 and a third area 230 according to the inventive method. The respective areas can be present multiple times, that is, there can be several first areas 210, several second areas 220 and several third areas 230, with any combination of the number of areas possible. The steel component 200 is heated in the first furnace 110 according to the schematically drawn temperature curve ϑ _200,110 during the residence time t ₁₁₀ to a temperature below the AC3 temperature. The steel component 200 is then transferred to the treatment station 150 with a transfer time t ₁₂₁ . The steel component loses heat. In the treatment station, a first area 210 and a third area 230 of the steel component 200 are quickly heated above the austenitization temperature AC3 using laser radiation, with the second area 220 losing heat according to the drawn curve ϑ _220.151 and ϑ _220.152 . This happens within a few seconds. Immediately afterwards, the third area 230 is quickly cooled down to the desired cooling stop temperature ϑ _S according to the drawn temperature curve ϑ _{230, 152} . The cooling stop temperature ϑ _S can be different between the individual partial areas of the third areas 230 if variable material properties of the third areas 230 are desired within a component. The third region 230 can be cooled quickly, for example, by blowing with a gaseous fluid.

The blowing ends after the cooling time t ₁₅₂ has elapsed, which, depending on the thickness of the steel component 200, is only a few seconds. The third area 230 has now reached the cooling stop temperature ϑ _S. At the same time, the temperature of the first area 210 and also the second area 220 in the treatment station 150 has fallen according to the drawn temperature profile ϑ _210.152 or ϑ _220.151 , ϑ _220.152 .

After the residence time t ₁₅₀ in the treatment station 150 has elapsed, the steel component 200 is transferred into the second furnace 130 during the transfer time t ₁₂₂ . In the second In the furnace 130, the temperature of the first region 210 of the steel component 200 changes according to the schematically drawn temperature curve ϑ _210,130 during the residence time t ₁₃₀ . The temperature of the second region 220 of the steel component 200 also behaves according to the drawn temperature curve ϑ _220,130 during the residence time t ₁₃₀ , although it does not reach the AC3 temperature. The temperature of the third region 230 of the steel component 200 also behaves according to the drawn temperature curve ϑ _230,130 during the residence time t ₁₃₀ without reaching the AC3 temperature.

The second furnace 130 has no special devices for different treatment of the different areas 210, 220, 230. Only an oven temperature ϑ ₄ , ie a substantially homogeneous temperature ϑ ₄ is set in the entire interior of the second furnace 130, which is below the austenitization temperature AC3 is located.

The steel component can then be transferred during a transfer time t ₁₄₀ into a press hardening tool 160, which is installed in a press, not shown.

Clearly contoured boundaries can be achieved between the areas 210, 220, 230 and the distortion of the steel component 200 is minimized due to the small temperature difference. Small differences in the temperature level of the steel component 200 have an advantageous effect during further processing in the press hardening tool 160. The necessary residence time t ₁₃₀ of the steel component 200 in the second furnace 130 can be realized depending on the length of the steel component 200 by adjusting the conveying speed and designing the length of the second furnace 130. An influence on the cycle time of the heat treatment device 100 is thus minimized and can even be avoided entirely.

Fig. 2 shows a heat treatment device 100 that can be used according to the invention in a 90 ° arrangement. The heat treatment device 100 has a Loading station 101, via which steel components are fed to the first furnace 110. Furthermore, the heat treatment device 100 has the treatment station 150 and the second oven 130 arranged behind it in the main flow direction D. Arranged further behind it in the main flow direction D is a removal station 140, which is equipped with a positioning device (not shown). The main flow direction now bends by essentially 90° in order to allow a press hardening tool 160 to follow in a press (not shown) in which the steel component 200 is press hardened. A container 161 into which reject parts can be placed is arranged in the axial direction of the first oven 110 and the second oven 130. In this arrangement, the first oven 110 and the second oven 130 are preferably designed as continuous ovens, for example roller hearth ovens.

Fig. 3 shows a heat treatment device 100 that can be used according to the invention in a straight arrangement. The heat treatment device 100 has a loading station 101 via which steel components are fed to the first furnace 110. Furthermore, the heat treatment device 100 has the treatment station 150 and the second oven 130 arranged behind it in the main flow direction D. Arranged further behind it in the main flow direction D is a removal station 140, which is equipped with a positioning device (not shown). Next, in the now straight main flow direction, there follows a press hardening tool 160 in a press (not shown), in which the steel component 200 is press hardened. A container 161 into which reject parts can be placed is arranged essentially at 90° to the removal station 131. In this arrangement, the first oven 110 and the second oven 130 are also preferably designed as continuous ovens, for example roller hearth ovens.

Fig. 4 shows a further variant of a heat treatment device 100 that can be used according to the invention. The heat treatment device 100 again has a loading station 101, via which steel components are fed to the first furnace 110. In this embodiment, the first oven 110 is again preferably designed as a continuous oven. Furthermore, the heat treatment device 100 has the treatment station 150, which in this embodiment is combined with a removal station 131. The removal station 140 can, for example, have a gripping device (not shown). In the removal station 140, the steel components 200 are removed from the first furnace 110, for example by means of the gripping device. The heat treatment of the second or second regions 220 and/or the third or third regions 230 is carried out and the steel component or steel components 200 are placed in a second oven 130 arranged essentially at 90° to the axis of the first furnace 110. In this embodiment, this second oven 130 is preferably provided as a chamber oven, for example with several chambers. After the residence time t ₁₃₀ of the steel components 200 in the second furnace 130 has elapsed, the steel components 200 are removed from the second furnace 130 via the removal station 140 and placed in an opposite press hardening tool 160 installed in a press (not shown). For this purpose, the removal station 140 can have a positioning device (not shown). In the axial direction of the first furnace 110, a container 161 is arranged behind the removal station 140 in the main flow direction D, into which reject parts can be placed. In this embodiment, the main flow direction D describes a deflection of essentially 90°. In this embodiment, a second positioning system for the treatment station 150 is not required. In addition, this embodiment is advantageous if there is not enough space available in the axial direction of the first oven 110, for example in a production hall. In this embodiment, the heat treatment of the first or first regions 210 and the third regions 230 of the steel component 200 can also take place between the removal station 140 and the second furnace 130, so that there is no need for a stationary treatment station 150. For example, the treatment station 150 can be integrated into the gripping device. The removal station 140 ensures the transfer of the steel component 200 from the first furnace 110 into the second furnace 130 and into the press hardening tool 160 or into the container 161.

In this embodiment too, the position of press hardening tool 160 and container 161 can be swapped, as in Fig. 5 to see. In this embodiment, the main flow direction D describes two deflections of essentially 90°.

If the space for setting up the heat treatment device is limited, a heat treatment device according to Fig. 6 an: Compared to the in Fig. 4 In the embodiment shown, the second oven 130 is moved to a second level above the first oven 110. In this embodiment too, the treatment of the first region or regions 210 and the third region or regions 230 of the steel component 200 can also take place between the removal station 140 and the second furnace 130, so that there is no need for a stationary treatment station 150. Once again, it is advantageous to design the first oven 110 as a continuous oven and the second oven 130 as a chamber oven, possibly with several chambers.

In Fig. 7 Finally, a final embodiment of the heat treatment device that can be used according to the invention is shown schematically. Compared to that in Fig. 6 In the embodiment shown, the positions of press hardening tool 160 and container 161 are swapped.

List of reference symbols:

100

Heat treatment device

101

Loading station

110

First oven

130

Second oven

140

removal station

150

Treatment station

151

High power laser

152

Cooling device

160

Press hardening tool

161

container

200

Steel component

210

First area

220

Second area

230

Third area

D

Main flow direction

t110

Residence time in the first oven

t121

Transfer time steel component in the treatment station

t122

Transfer time steel component into second furnace

t130

Residence time in the second oven

t140

Transfer time steel component into press hardening tool

t150

Length of stay in the treatment station

t151

Warm-up time in the treatment station

t152

Cooling time in the treatment station

t160

Dwell time in the press hardening tool

ϑS

Cooling stop temperature

ϑ3

Internal temperature of the first oven

ϑ4

Internal temperature of the second oven

ϑ200,110

Temperature profile of the steel component in the first furnace

ϑ210,151

Temperature profile of the first area of the steel component in the treatment station during heating

ϑ220,151

Temperature profile of the second area of the steel component in the treatment station

ϑ220,152

Temperature profile of the second area of the steel component in the treatment station

ϑ230,152

Temperature profile of the third area of the steel component in the treatment station during cooling

ϑ210,130

Temperature profile of the first area of the steel component in the second furnace

ϑ220,130

Temperature profile of the second area of the steel component in the second furnace

ϑ230,130

Temperature profile of the third area of the steel component in the second furnace

ϑ200,160

Temperature profile of the steel component in the press hardening tool

Claims (8)

1. Method for targetedly heat-treating individual zones of a steel component (200), wherein in one or more first regions (210) of the steel component (200) a primarily austenitic structure is established, from which a predominantly martensitic structure is producible by quenching, and in one or more second regions (220) a predominantly ferritic-pearlitic structure is established, wherein further in one or more third regions (230) of the steel component (200) a predominantly bainitic structure is established, wherein the steel component (200) is first heated in a first furnace (110) to a temperature below the AC3 temperature, the steel component (200) is then transferred to a treatment station (150), and is able to cool down during the transfer, and in the treatment station (150) the one or more first regions (210) and the one or more third regions (230) of the steel component (200) are heated to a temperature above the AC3 temperature within a dwell time t₁₅₁, wherein the third region or regions (230) of the steel component (200) are then cooled to the cooling stop temperature ϑ_s, and wherein the steel component (200) is then transferred to a second furnace (130) in which the steel component (200) remains at a temperature below the austenitizing temperature for a dwell time (t₁₃₀) until sufficient bainitic structure has been formed in the third region or regions (230).
2. Method according to Claim 1, wherein heat is supplied in the second furnace (130) via thermal radiation.
3. Method according to Claim 1 or 2, wherein, in the treatment station (150), the one or more first regions (210) of the steel component (200) are brought to a temperature above the austenitizing temperature within a dwell time t₁₅₁ by means of a high-power laser.
4. Method according to any of the preceding claims, wherein, in the treatment station (150), the third region or regions (230) of the steel component (200) are brought to a temperature above the austenitizing temperature within a dwell time t₁₅₁ by means of a high-power laser.
5. Method according to any of the preceding claims, wherein, in the treatment station (150), a gaseous fluid is blown against the third region or regions (230) of the steel component (200) within a dwell time t₁₅₂ for cooling.
6. Method according to Claim 5, wherein the gaseous fluid contains water.
7. Method according to any of the preceding claims, wherein, in the treatment station (150), the third region or regions (230) of the steel component (200) are brought into contact with a punch within a dwell time t₁₅₂ for cooling, the punch having a lower temperature than the third region or regions (230).
8. Method according to any of the preceding claims, wherein the internal temperature ϑ₄ in the second furnace (130) is lower than the AC3 temperature.

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