EP3408417B1 - Heat treatment method - Google Patents

Description

Heat treatment of a steel component.

In technology, there is a desire for high-strength sheet metal parts with low part weight in many applications in different sectors. For example, in the vehicle industry, there is an effort to reduce the fuel consumption of motor vehicles and lower CO ₂ emissions, but at the same time to increase 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, rocker panels, frame parts, bumper catchers, cross members for the floor and roof, and front and rear side members. In modern motor vehicles, the bodyshell with a safety cage usually consists of hardened sheet steel with a strength of around 1,500 MPa. Al-Si-coated steel sheets are often used for this. The process of so-called press hardening was developed to produce a component from hardened sheet steel. Steel sheets are first heated to the austenite temperature, then placed in a press tool, quickly formed and quickly quenched to less than the martensite start temperature by the water-cooled tool. This creates a hard, solid martensite structure with a strength of approx. 1,500 MPa. However, a steel sheet hardened in this way has only a low elongation at break. Therefore, the kinetic energy of an impact cannot be sufficiently converted into heat of deformation.

For the automotive industry, it is therefore desirable to be able to produce body components that have several different expansion and strength zones in the component, so that more solid areas (hereinafter first areas) on the one hand and more stretchable areas (hereinafter second areas) on the other hand in one component present. On the one hand, high-strength components are fundamentally desirable in order to obtain low-weight components that can withstand high mechanical loads. On the other hand, high-strength components should also be able to have partially soft areas. This brings 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 the acceleration forces on the occupants and the rest vehicle are minimized. In addition, modern joining processes require softened areas that enable the joining of similar or different materials. Folded, crimped or riveted connections, for example, often have to be used, which require deformable areas in the component.

The general demands on a production plant should continue to be observed: there should be no loss of cycle time in the press hardening plant, the entire plant should be used in general without restrictions and be able to be quickly converted to suit the product. 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 high.

From the EP 2 548 975 A1 a method for producing a hardened metallic component with at least two areas of different ductility is known.

In all known methods, the specific 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 specify a method for the specific component zone-specific heat treatment of a steel component, areas of different hardness and ductility being achievable, 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 having the features of independent claim 1 . Advantageous developments of the method result from the dependent claims 2 to 5.

The steel component is first heated above the austenitization temperature AC3 so that the structure can be completely transformed into austenite. In a subsequent hardening process, for example the press hardening process, it is quenched so quickly that a primarily martensitic structure is formed and strengths of around 1,500 MPa are achieved. The quenching is advantageously carried out from the fully austenitized microstructure. For this purpose, cooling must take place at least at the lower critical cooling rate once the microstructure start temperature ϑ ₁ has been fallen below, at which microstructure transformations can start. For example around 660°C should be considered as the limit ϑ ₁ for the material 22MnB5 usually used for press hardening. Although an at least partially martensitic microstructure can still form if quenching starts at a lower temperature, a reduced strength of the component is to be expected in this area.

This temperature curve is common in the press hardening process, particularly for fully hardened components.

A second area or several second areas are first also heated to above the austenitization temperature AC3 so that the structure can be completely transformed into austenite. It is then cooled as quickly as possible within a treatment time t _B down to a cooling stop temperature θ ₂ . For example, the martensite start temperature for 22MnB5 is around 410 °C. A slight transient in temperature ranges below the martensite start temperature is also possible. There is no further rapid cooling afterwards, so that a predominantly bainitic structure is formed. This structural change does not take place suddenly, but requires a treatment period. The conversion is exothermic. If this conversion is allowed to take place in a heated environment with a temperature similar to the component temperature at the end of cooling, the cooling stop temperature ϑ ₂ , the temperature increase in the component caused by recalescence can be clearly seen. By adjusting the cooling rate and/or the temperature to which cooling takes place, as well as the dwell time before the component is pressed, the desired strength and elongation values can be set, which lie between the maximum achievable strength of the structure in the first area and the values of the untreated component. Investigations have shown that suppressing the temperature rise as a result of recalescence through further, forced cooling is rather disadvantageous for the achievable strain values. Holding isothermally at the cooling temperature therefore does not appear to be advantageous. Reheating, on the other hand, is beneficial.

In one embodiment, the second area or the second areas are additionally actively heated in this phase. This can be done, for example, by thermal radiation.

In one embodiment, the cooling stop temperature θ ₂ is selected above the martensite start temperature M _S .

In an alternative embodiment, the cooling stop temperature θ ₂ is selected below the martensite start temperature M _S .

The heat treatment of the first and second areas is fundamentally different, with the treatment of the second area or the second areas primarily being dependent on the treatment duration. According to the invention, second areas in a first furnace are partially cooled down to the cooling stop temperature ϑ ₂ within a treatment time t _B of a few seconds in order to reach the austenitization temperature. In this treatment station, the first area or areas are not treated in a special way.

Optionally, the treatment station can also be heated for this purpose. For this purpose, for example, the introduction of heat via convection or heat radiation can be used.

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 transferred to a second oven, which preferably does not have specific devices for treating the different areas differently. Only a furnace temperature θ ₄ , ie an essentially homogeneous temperature θ ₄ in the entire furnace space, is set, which generally lies between the austenitizing temperature AC3 and the minimum quenching temperature. An advantageous value is between 660°C and 850°C, for example. In this way, the different areas approach the temperature ϑ ₄ of the second furnace. If the temperature losses in the first zones during the stay in the treatment station for the second zones are so low that the temperature does not fall below the temperature ϑ ₄ of the second furnace, the temperature profile of the first zones kind approaches the temperature ϑ ₄ of the second oven from above. In an advantageous embodiment, the minimum cooling temperature, ie the cooling stop temperature θ ₂ in the areas of the second type, is lower than the selected temperature θ ₄ of the second furnace. In this respect, the temperature profile of the second areas approaches Temperature ϑ ₄ of the second oven from below. As a result of this procedure, the temperatures of the differently treated areas approach one another.

The first area or areas give off heat in the second oven when they enter the second oven at a higher temperature than the internal temperature θ ₄ of the second oven. The second or second areas absorb heat in the second furnace. Overall, this requires only a relatively small amount of heating power in the second furnace. If necessary, further heating can be dispensed with entirely during the production process. This treatment step is particularly energy-efficient. A continuous furnace or a batch furnace, such as a chamber furnace, for example, can be provided as the first furnace. Continuous furnaces usually have a large capacity and are particularly well suited for mass production because they can be loaded and operated without great effort.

The treatment station can have a device for rapid cooling of one or more second regions of the steel component. The device preferably has a nozzle for blowing a gaseous fluid, for example air or an inert gas such as nitrogen, onto the second area or areas of the steel component.

In a further advantageous embodiment of the method, the second or the second regions are blown on by blowing on them with a gaseous fluid, water being added to the gaseous fluid, for example in nebulized form. For this purpose, in an advantageous embodiment, the device has one or more atomizing nozzles. Blowing on with the gaseous fluid mixed with water increases the heat dissipation from the second region or regions. With the evaporation of the water on the steel component, a large heat dissipation and a high energy transport is achieved.

A continuous furnace or a batch furnace, for example a chamber furnace, can also be provided as the second furnace.

In a further preferred embodiment, the second or the second area(s) is/are cooled via heat conduction, for example by bringing it into contact with one or more stamps, one or more stamps being significantly lower Has or have temperature than the steel component. For this purpose, the stamp can be made of a material with good thermal conductivity and/or be cooled directly or indirectly. A combination of the types of cooling is also conceivable.

It has proven to be advantageous if measures are taken in the treatment station to reduce the temperature losses in the first area or areas. Such measures can be, for example, the attachment of a thermal radiation reflector and/or the insulation of surfaces of the treatment station in the area of the first area or the first areas.

With the method according to the invention and a corresponding

Heat treatment device, which is not claimed, steel components with one or more first and / or second areas, which can also be shaped complex, can be economically imprinted with a corresponding temperature profile, since the different areas can be brought to the necessary process temperatures very quickly with sharp contours. Clearly contoured delimitations of the individual areas can be realized between the two areas and the distortion of the components is minimized due to the small temperature difference. Small spreads in the temperature level of the component have an advantageous effect during further processing in the press. The necessary dwell times for the second area or the second areas can be implemented, for example, in a continuous furnace depending on the component length by adjusting the conveying speed and the design of the furnace length. An influence on the cycle time of the heat treatment device is minimized in this way, it can even be avoided entirely.

According to the invention, it is possible with the method shown and with the corresponding heat treatment device, which is not claimed, to set almost any number of second areas, which can also have different strength and elongation values within a steel component. The selected geometry of the partial areas can also be freely selected. Dotted or line-shaped areas can be displayed as well as, for example, large-area areas. The location of the areas is also irrelevant. The second areas can be completely enclosed by the first areas or can be located at the edge of the steel component. Even a full-surface treatment is conceivable. A special orientation of the steel component to the direction of passage is for the purpose of the method according to the invention for the targeted heat treatment of a steel component for individual component zones not mandatory. 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. The process can also be used on preformed steel components. The three-dimensionally shaped surfaces of already preformed steel components only result in a higher design effort for the representation of the counter surfaces.

Furthermore, it is advantageous that already existing heat treatment plants can 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 have to be installed behind it. Depending on the design of the existing oven, it is also possible to divide it so that the first and second ovens are created from the original one oven.

Further advantages, special features and expedient developments of the invention result from the subclaims and the following description of preferred exemplary embodiments with reference to the illustrations.

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

From the illustrations shows:

• 1 a typical temperature curve during the heat treatment of a steel component with a first and a second area;
• 2 a thermal heat treatment device in a plan view as a schematic drawing;
• 3 another thermal heat treatment device in a plan view as a schematic drawing;
• 4 another thermal heat treatment device in a plan view as a schematic drawing;
• figure 5 another thermal heat treatment device in a plan view as a schematic drawing;
• 6 another thermal heat treatment device in a plan view as a schematic drawing; and
• 7 another thermal heat treatment device in a plan view as a schematic drawing.

In the 1 12 is a typical temperature curve during the heat treatment of a steel component 200 having a first region 210 and a second region 220 according to the inventive method. The steel component 200 is heated in the first furnace 110 according to the temperature curve θ _200,110 shown schematically during the residence time in the first furnace t ₁₁₀ to a temperature above the AC3 temperature. The steel component 200 is then transferred to the treatment station 150 with a transfer time t ₁₂₀ . In the process, the steel component loses heat. In the treatment station, a second region 220 of the steel component 200 is rapidly cooled, with the second region 220 rapidly losing heat according to the curve θ _220,150 shown. The cooling ends after the treatment time t _B has elapsed, which, depending on the thickness of the steel component 200, the desired material properties and the size of the second region 220, is only a few seconds. In a first approximation, the treatment time t _B is equal to the residence time t ₁₅₀ in the treatment station 150. The second area 220 has now reached the cooling stop temperature θ ₂ above the martensite start temperature M _S . At the same time, the temperature of the first area 210 in the treatment station 150 has also fallen according to the temperature profile θ _{210,150 shown} , with the first area 210 not being located in the area of the cooling device. After the treatment time t _B has elapsed, the steel component 200 is transferred into the second furnace 130 during the transfer time t ₁₂₁ , during which it continues to lose heat if its temperature is greater than the internal temperature θ ₄ of the second furnace 130 . In the second furnace 130, the temperature of the first region 210 of the steel component 200 changes according to the temperature profile θ _210,130 shown schematically during the dwell time t ₁₃₀ , ie the temperature of the first region 210 of the steel component 200 slowly continues to decrease. The temperature of the first region 210 of the steel component 200 can fall below the AC3 temperature, but this does not necessarily have to happen. In contrast, the temperature of the second region 220 of the steel component 200 increases again according to the temperature profile θ _{220,130 shown} during the dwell time t ₁₃₀ without reaching the AC3 temperature. The second oven 130 does not have any specific devices for treating the different areas 210, 220 differently only a furnace temperature θ ₄ , ie an essentially homogeneous temperature in the entire interior of the second furnace 130, is set, which lies between the austenitization temperature AC3 and the cooling stop temperature θ ₂ , for example between 660° C. and 850° C. In this way, the various areas 210 , 220 approach the internal temperature θ ₄ of the second oven 130 . If the temperature losses in the first area 210 during the dwell time t ₁₅₀ in the treatment station 150 for the second area 220 are so low that the temperature does not fall below the temperature ϑ ₄ of the second oven 130, the temperature profile ϑ _{210,130 approaches} the first Range of temperature ϑ ₄ of the second oven 130 from above. In this embodiment, the cooling stop temperature θ ₂ is lower than the selected temperature θ ₄ of the second oven 130. The temperature profile θ _220,130 of the second region approaches the temperature 94 of the second oven 130 from below. The temperature of region 210 does not fall below the fabric transformation start temperature ϑ ₁ . Due to the small temperature difference between the two areas 210, 220, clearly contoured delimitations of the individual areas 210, 220 can be realized and the distortion of the steel component 200 is minimized. Small spreads in the temperature level of the steel component 200 have an advantageous effect during further processing in the press-hardening tool 160 . The necessary dwell time t ₁₃₀ for the second region 220 can be implemented as a function of the length of the steel component by adjusting the conveying speed and designing the length of the second furnace 130 . An influencing of the cycle time of the heat treatment device 100 is thus minimized, it can even be completely avoided. The first region 220 of the steel component 200 gives off heat in the second furnace 130 . The second region 220 of the steel component 200 absorbs heat in the second furnace 130, the heat absorption being limited by the heat released during the recalescence of the microstructure in the second region 220 of the steel component 200. Overall, this requires only a relatively small amount of heat output in the second oven 130. If necessary, additional heating of the second oven 130 can be dispensed with entirely. This treatment step is particularly energy-efficient.

After the end of the dwell time t ₁₃₀ of the steel component 200 in the second furnace 130 it is transferred during the transfer time t ₁₃₁ into a press hardening tool 160 where it is shaped and hardened during the dwell time t ₁₆₀ .

2 Figure 12 shows a heat treatment device 100, not claimed, in a 90° arrangement. the

Heat treatment device 100 has a loading station 101 via which the steel components are fed to the first furnace 110 . Furthermore, the heat treatment device 100 has the treatment station 150 and the second furnace 130 arranged behind it in the main flow direction D. Arranged further downstream in the main flow direction D is a removal station 131 which is equipped with a positioning device (not shown). The main flow direction now bends by essentially 90° in order to be followed by a press-hardening tool 160 in a press (not shown), in which the steel component 200 is press-hardened. In the axial direction of the first furnace 110 and the second furnace 130 there is a container 161 into which rejects can be placed. In this arrangement, the first furnace 110 and the second furnace 120 are preferably designed as continuous furnaces, for example roller hearth furnaces.

3 Figure 12 shows a heat treatment apparatus 100, not claimed, in a straight configuration. the

Heat treatment device 100 has a loading station 101 via which the steel components are fed to the first furnace 110 . Furthermore, the heat treatment device 100 has the treatment station 150 and the second furnace 130 arranged behind it in the main flow direction D. Arranged further downstream in the main flow direction D is a removal station 131 which is equipped with a positioning device (not shown). A press-hardening tool 160 in a press (not shown), in which the steel component 200 is press-hardened, follows in the main flow direction, which is now further straight. A container 161 into which rejects can be placed is arranged essentially at 90° to the removal station 131 . In this arrangement, the first furnace 110 and the second furnace 120 are also preferably designed as continuous furnaces, for example roller hearth furnaces.

4 FIG. 1 shows another variant of a heat treatment device 100 which is not claimed. The heat treatment device 100 again has a loading station 101 via which the steel components are fed to the first furnace 110 . The first oven 110 is with this one Execution again preferably designed as a continuous furnace. Furthermore, the heat treatment device 100 has the treatment station 150 which is combined with a removal station 131 in this example. The removal device 131 can have a gripping device (not shown), for example. The removal station 131 removes the steel components 200 from the first furnace 110, for example by means of the gripping device. The heat treatment with the cooling of the second or the second regions 220 is carried out and the steel component or the steel components 200 are in a substantially 90 ° to the axis of the first Furnace 110 arranged second furnace 130 inserts. In this example, this second furnace 130 is preferably provided as a chamber furnace, 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 131 and placed in an opposite press-hardening tool 160 built into a press (not shown). For this purpose, the removal station 131 can have a positioning device (not shown). In the axial direction of the first oven 110, a container 161 is arranged behind the removal station 131, into which rejects can be placed. In this example, the main flow direction D describes a deflection of essentially 90°. In this example, no second positioning system for the treatment station 150 is required. In addition, this example 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 example, the cooling of the second regions 220 of the steel component 200 can also take place between the removal station 131 and the second furnace 130, so that no stationary treatment station 150 is required. For example, a cooling device, for example a blowing nozzle, can be integrated into the gripping device. The removal device 131 ensures that the steel component 200 is transferred from the first furnace 110 into the second furnace 130 and into the press-hardening tool 160 or into the container 161.

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

If the space for installing the heat treatment device is limited, a heat treatment device, which is not claimed, according to FIG 6 an: Compared to the in 4 In the example shown, the second oven 130 is offset to a second level above the first oven 110 . In this example, too, the cooling of the second regions 220 of the steel component 200 can likewise take place between the removal station 131 and the second furnace 130, so that no stationary treatment station 150 is required. Again, it is advantageous to design the first furnace 110 as a continuous furnace and the second furnace 120 as a chamber furnace, possibly with multiple chambers.

In 7 finally a last example of a heat treatment device, which is not claimed, is shown schematically. Compared to the in 6 shown example, the positions of press-hardening tool 160 and container 161 are reversed.

Reference list:

100

heat treatment device

110

first oven

130

second oven

131

extraction station

150

treatment station

160

press hardening tool

161

container

200

steel component

210

first area

220

second area

D

main flow direction

MS

martensite start temperature

tB

treatment time

t110

Residence time in the first oven

t120

Transfer time steel component in treatment station

t121

Transfer time steel component in second furnace

t130

Residence time in the second oven

t131

Transfer time of steel component in press-hardening tool

t150

Residence time in treatment station

t160

Residence time in the press hardening tool

ϑ1

microstructural transformation start temperature

ϑ2

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

ϑ210,150

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

ϑ220,150

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

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

ϑ200,160

Temperature profile of the steel component in the press-hardening tool

Claims (5)

1. Method for heat treating a steel component (200) in a targeted manner in individual component zones, it being possible for a predominantly austenitic structure to be produced in the steel component (200) in one or more first regions (210), from which structure a predominantly martensitic structure can be produced by quenching, and a predominantly bainitic structure being used in one or more second regions (220),
   the steel component (200) first being heated in a first furnace (110) to a temperature above the AC3 temperature, the steel component (200) then being transferred to a treatment station (150), where it can cool down during the transfer, and in the treatment station (150) the one or more second regions (220) of the steel component (200) being cooled to a cooling stop temperature ϑ₂ during a treatment time t_B, characterized in that the steel component is then transferred to a second furnace, in the second furnace a furnace temperature being set so that the different regions (210, 220) of the steel component (200) approach the furnace temperature of the second furnace, in the second furnace the one or more first regions (210) cooling to a temperature above the structural transformation start temperature ϑ₁ of the steel component (220), from which temperature structural transformations from the austenitic structure can start, and the temperature of the one or more second regions (220) rising again to a temperature below the AC3 temperature.
2. Method according to claim 1,
   wherein the cooling stop temperature ϑ₂ is selected so as to be above the martensite start temperature M_s.
3. Method according to claim 1,
   wherein the cooling stop temperature ϑ₂ is selected so as to be below the martensite start temperature M_s.
4. Method according to any of the preceding claims,
   wherein the reheating of the second region or regions (220) in the second furnace is assisted by supply of heat.
5. Method according to any of the preceding claims,
   wherein the internal temperature in the second furnace ϑ₄ is greater than the cooling stop temperature ϑ₂.

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