HEAT TREATMENT PROCEDURES AND THE USE OF A HEAT TREATMENT DEVICE

Description

[0001] The invention relates to a method and a use of a device for the targeted, component-zone-specific heat treatment of a steel component.

[0002] In In many applications in different industries, there is a desire for high-strength sheet metal parts with low weight.

[0003] For example, the automotive industry is striving to reduce fuel consumption and CO2 emissions, while at the same time increasing occupant safety. There is therefore a rapidly growing demand for body components with a favorable strength-to-weight ratio. These components include, in particular, A- and B-pillars, side impact protection members in doors, sills, frame components, bumper guards, cross members for the floor and roof, and front and rear side members. In modern motor vehicles, the bodyshell with a safety cage is usually made of hardened steel sheet with a strength of approximately 1,500 MPa. Al-Si coated steel sheets are often used for this purpose. The process known as press hardening was developed to produce components from hardened steel sheet.In this process, steel sheets are first heated to austenite temperature, then placed in a press tool, rapidly formed, and rapidly quenched by the water-cooled tool to below the martensite starting temperature. This creates a hard, strong martensite structure with a strength of approximately 1,500 MPa. However, steel sheets hardened in this way have only low elongation at break. The kinetic energy of an impact cannot therefore be sufficiently converted into deformation heat. For the automotive industry, it is therefore desirable to be able to produce body components that have several different strain and strength zones within the component, so that more rigid areas (hereinafter referred to as the first areas) and more ductile areas (hereinafter referred to as the second areas) are present in one component. On the one hand, components with high strength are generally desirable in order to obtain components that can withstand high mechanical loads and are lightweight.On the other hand, even high-strength components should be able to have partially soft areas. This provides the desired, partially increased deformability in the event of a crash. Only in this way can the kinetic energy of an impact be dissipated and thus the acceleration forces on the occupants and the rest of the vehicle minimized. Furthermore, modern joining processes require softened areas that allow the joining of similar or dissimilar materials. Often, for example, seam, crimp, or rivet connections are used, which require deformable areas in the component.

[0004] The general requirements for a production facility should still be observed: there should be no cycle time losses on the press hardening system, the entire system should be universally applicable without restrictions, and it should be quickly convertible to specific products. The process should be robust and economical, and the production facility should require minimal space. The shape and edge accuracy of the component should be high.

[0005] Various methods for heat treating components are known from the prior art. A general method is known from EP 2 548 975 A1. Furthermore, DE 10 2010 048 209 B3, DE 102 08 216 C1, and US 2016 / 0010171 A1 disclose methods in which areas of the component intended to acquire ductile properties are held isothermally at a certain temperature after cooling. However, the achievable ductile properties are insufficient for many applications.

[0006] DE102010049205 A1 discloses a hot forming line for hot forming sheet material and a method for hot forming sheet material which is formed as a sheet metal blank or as a sheet metal strip.

[0007] In all known processes, 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 production line.

[0008] The object of the invention is therefore to provide a method and a use of a device for the targeted, component-zone-specific heat treatment of a steel component, 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.

[0009] According to the invention, this object is achieved by a method having the features of independent claim 1. Advantageous developments of the method emerge from subclaims 2 to 4. The object is further achieved by using the device according to claim 5. Advantageous embodiments of the use of the device emerge from subclaims 6 to 10.

[0010] The steel component is first heated to above the austenitizing temperature AC3 so that the microstructure completely transforms into austenite. In a subsequent hardening process, for example, press hardening, it is then quenched so quickly that a martensitic microstructure develops predominantly and strengths of around 1,500 MPa are achieved. Quenching is advantageously carried out from the fully austenitized microstructure. To achieve this, cooling must take place at least at the lower critical cooling rate after the temperature falls below the microstructural transformation start temperature ϑ 1 , at which microstructural transformations can begin. For example, for the material 22MnB5, which is commonly used for press hardening, around 660°C should be considered the ϑ 1 limit.Although an at least partially martensitic structure can still develop if quenching starts at a lower temperature, a reduced strength of the component in this area is to be expected.

[0011] This temperature profile is common in the press hardening process, especially for fully hardened components.

[0012] A second area or several second areas are initially heated to above the austenitizing temperature AC3 so that the microstructure can completely transform into austenite. Then, as quickly as possible within a treatment time t B, cooling takes place to a cooling stop temperature ϑ 2 , which, for example, for the material 22MnB5 should be below 650°C. The martensite start temperature for 22MnB5, for example, is approximately 410°C. A slight oscillation into temperature ranges below the martensite start temperature is also possible. After that, cooling is not carried out any further, so that a predominantly bainitic microstructure forms. This microstructure transformation does not occur suddenly, but requires a treatment time. The transformation is exothermic. There is no active heating of the second area or areas in the treatment station during the treatment time t B.Any possible temperature increase in the second region(s) during this time occurs only through recalescence. By adjusting the cooling rate and / or the temperature to which the component is cooled, as well as the dwell time until the component is pressed, the desired strength and elongation values ​​can generally be achieved. These values ​​lie between the maximum achievable strength of the structure in the first region and the values ​​of the untreated component. Studies have shown that suppressing the temperature increase due to recalescence through further, forced cooling tends to be detrimental to the achievable elongation values. Therefore, isothermal holding at the cooling temperature does not appear to be advantageous.

[0013] In an embodiment outside the claimed invention, the second region or regions are additionally actively heated during this phase. This can be achieved, for example, by thermal radiation.

[0014] In one embodiment, the cooling stop temperature ϑ 2 is chosen above the martensite start temperature MS.

[0015] In an alternative embodiment, the cooling stop temperature ϑ 2 is chosen below the martensite start temperature MS.

[0016] The heat treatment of the first and second regions is fundamentally different, with the treatment of the second region or regions primarily depending on the treatment duration. According to the invention, second regions are partially cooled to the cooling stop temperature ϑ 2 within a treatment time t B of a few seconds in a furnace downstream of a treatment station to reach the austenitizing temperature. In This treatment station ensures, if necessary by applying heat, that the first area(s) do not fall below a temperature during the treatment period at which sufficient martensite formation would not be expected during the subsequent press hardening. Depending on the treatment duration, it is sufficient to minimize the radiation losses of the first area(s), for example, through thermal insulation or reflectors for thermal radiation.

[0017] Optionally, the treatment station can be at least partially heated for this purpose. For this purpose, heat can be introduced via convection or thermal radiation, for example. In an advantageous embodiment, heating via laser radiation can also be used additionally or exclusively.

[0018] According to the invention, the components remain in the treatment station for a short time, for example a few seconds, in order to allow the structural transformation of the second areas to take place.

[0019] If the dwell time required for sufficient microstructural transformation in the treatment station is so long that the required cycle time can no longer be achieved, it is recommended to provide two or more similar treatment stations that are fed consecutively. In an advantageous design, the chambers are arranged one above the other. It is irrelevant whether the treatment stations are moved vertically to overcome the height difference or whether the feeding system performs the necessary vertical movement.

[0020] The furnace can be a continuous furnace or a batch furnace, such as a chamber furnace. Continuous furnaces generally have a large capacity and are particularly well-suited for mass production because they can be loaded and operated without great effort.

[0021] In an advantageous embodiment, the component is only blown onto one side. This creates a clear separation between the conveying system, for example, located below the component, and the cooling system, for example, located above the component, which significantly simplifies the design of the treatment station(s).

[0022] According to the invention, the treatment station comprises a device for rapidly cooling one or more second regions of the steel component. In In a preferred embodiment, the device has a nozzle for blowing a gaseous fluid, for example air or a protective gas, such as nitrogen, onto the second region(s) of the steel component.

[0023] In a further advantageous embodiment of the method, the second region(s) are exposed to air by blowing a gaseous fluid, with water, for example in atomized form, being added to the gaseous fluid. For this purpose, in an advantageous embodiment, the device has one or more atomizing nozzles. Blowing the gaseous fluid mixed with water increases heat dissipation from the second region(s). Evaporation of the water on the steel component achieves significant heat dissipation and high energy transfer.

[0024] In a further embodiment, the second region(s) is / are cooled via thermal conduction, for example, by bringing it into contact with one or more dies that have a significantly lower temperature than the steel component. For this purpose, the die can be made of a material with good thermal conductivity and / or can be cooled directly or indirectly. A combination of these cooling methods is also conceivable.

[0025] With the method according to the invention and the use of a device according to the invention, steel components each with one or more first and / or second regions, which may also have complex shapes, can be economically imprinted with a corresponding temperature profile, since the different regions can be brought to the required process temperatures very quickly with sharp contours. Clearly contoured boundaries between the individual regions can be realized. Small variations in the temperature level of the component have a beneficial effect during further processing in the press.

[0026] According to the invention, with the method shown and with the inventive use of a device, it is possible to set almost any number of second regions, which can also have different strength and elongation values ​​within a steel component. The selected geometry of the sub-regions is also freely selectable. Point-shaped or linear regions can be represented, as can, for example, large-area regions. The position of the regions is also unimportant. The second regions can be completely enclosed by first regions or be located at the edge of the steel component. Even full-surface treatment is conceivable. The number of steel components that can be treated simultaneously is limited at most by the press hardening tool or the conveyor technology of the entire heat treatment device. The application of the method to already pre-formed steel components is also possible.The three-dimensionally formed surfaces of pre-formed steel components simply result in greater design effort for the representation of the counter surfaces.

[0027] Furthermore, it is advantageous that existing heat treatment systems can also be adapted to the process according to the invention. For this purpose, in a conventional heat treatment system with only one furnace, only the treatment station needs to be provided behind the furnace and the loading equipment needs to be adapted.

[0028] Further advantages, special features and expedient developments of the invention emerge from the subclaims and the following representation of preferred embodiments with reference to the figures.

[0029] From the illustrations shows: Fig. 1 a typical temperature curve during the heat treatment of a steel component with a first and a second region Fig. 2 a thermal heat treatment device usable according to the invention in a plan view as a schematic drawing Fig. 3 another thermal heat treatment device usable according to the invention in a plan view as a schematic drawing Fig. 4 another thermal heat treatment device usable according to the invention in a plan view as a schematic drawing

[0030] In the Fig. 1 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 furnace 110 according to the schematically drawn temperature curve ϑ 200,110 during the residence time in the 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 drawn curve ϑ 220,150. The cooling ends after the treatment time t B , 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.The second region 220 has now reached the cooling stop temperature ϑ 2 above the martensite start temperature MS. The temperature of the first region 210 of the steel component 200 can fall below the AC3 temperature, but this is not mandatory. In contrast, the temperature of the second region 220 of the steel component 200 can rise slightly again during the residence time t 150 due to recalescence, according to the temperature profile ϑ 220,130 shown, without reaching the AC3 temperature, before then slowly continuing to fall.

[0031] After the residence time t 150 of the steel component 200 in the treatment station has ended, it is transferred into a press hardening tool 160 during the transfer time t 131, where it is formed and hardened during the residence time t 160.

[0032] Fig. 2 shows a heat treatment device 100 usable 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 furnace 110. The heat treatment device 100 also has the treatment station 150. Further arranged in the main flow direction D behind the furnace 110 is a removal station 131, which is equipped with a positioning device (not shown). The main flow direction now bends by essentially 90° to allow a press-hardening tool 160 in a press (not shown) to follow, in which the steel component 200 is press-hardened. A container 161, into which rejected parts can be placed, is arranged in the axial direction of the furnace 110.

[0033] Fig. 3 shows a heat treatment device 100 usable according to the invention in a straight arrangement. The heat treatment device 100 has a loading station 101 through which steel components are fed to the furnace 110.

[0034] The heat treatment device 100 further comprises the treatment station 150. Further downstream of the furnace 110 in the main flow direction D is a removal station 131 equipped with a positioning device (not shown). Next, in the now straight main flow direction, is 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.

[0035] Fig 4 shows a further variant of a heat treatment device 100 which 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 furnace 110. The furnace 110 is preferably designed as a continuous furnace in this embodiment. Furthermore, the heat treatment device 100 has the treatment station 150. The removal device 131 can, for example, have a gripping device (not shown). The removal station 131 removes the steel components 200 from the furnace 110, for example by means of the gripping device. In contrast to the Fig. 2In the embodiment shown, the treatment station 150 is arranged on the furnace 110. This arrangement saves installation space. In this embodiment, the main flow direction changes plane in that the steel component 200 is lifted from the removal station after leaving the furnace 110 and placed into the treatment station 150. After the residence time t 150 of the steel component 200 in the treatment station 150 has elapsed, the removal station 131 removes the steel component 200 from the treatment station 150 again and places it in a press hardening tool 160 installed in a press. In the embodiment shown, the press is arranged in line with the furnace 110, while a container 161 for rejects is arranged at an angle to the furnace axis. The positions of the press with tool 160 and container 161 can also be interchanged.

[0036] The embodiments shown here are merely examples of the present invention and should therefore not be considered limiting. Alternative embodiments contemplated by those skilled in the art are equally encompassed within the scope of the present invention. List of reference symbols:

[0037] 100Heat treatment device 110Furnace 131Unloading station 150Treatment station 160Press hardening tool 161Container 200Steel component 210First area 220Second area DMain flow direction MS Martensite start temperature t B Treatment time t 110 Retention time in the furnace t 120 Transfer time of steel component in treatment station t 131 Transfer time of steel component in press hardening tool t 150 Retention time in treatment station t 160 Retention time in the press hardening tool ϑ 1 Structural transformation start temperature ϑ 2 Cooling stop temperature ϑ 3 Internal temperature of the furnace ϑ 200,110 Temperature profile of the steel component in the furnace ϑ 210,150 Temperature profile of the first area of ​​the steel component in the treatment station ϑ 220,150 Temperature profile of the second area of ​​the Steel component in the treatment station ϑ 200,160 Temperature profile of the steel component in the press hardening tool

Claims

1. Method for targeted component-zone-specific heat treatment of a steel component (200), wherein, in the steel component (200), a primarily austenitic microstructure, from which a predominantly martensitic microstructure is able to be formed by way of quenching, is able to be established in one or more first regions (210), and a predominantly bainitic microstructure is established in one or more second regions (220), characterized in that the steel component (200) is firstly heated in a furnace (110) to a temperature above the AC3 temperature, so that the microstructure of the steel component (200) is transformed completely into austenite, and the steel component (200) is then transferred to a treatment station (150) and, in the treatment station (150), the one or more second regions (220) of the steel component (200) are cooled during a treatment time tB to a cooling stop temperature ϑ2, wherein, during a dwell time t150 in the treatment station (150), no active heating of the second region(s) (220) takes place, and wherein an increase in the temperature of the second region(s) (220) during the dwell time t150 in the treatment station (150) as a result of recalescence is not prevented and, during the dwell time t150, the one or more first regions (210) of the steel component (200) do not fall below a temperature below which insufficient formation of martensite during a subsequent press-hardening process would be expected.

2. Method according to Claim 1, wherein the cooling stop temperature ϑ2 is selected to be above the martensite start temperature MS.

3. Method according to Claim 1, wherein the cooling stop temperature ϑ2 is selected to be below the martensite start temperature MS.

4. Method according to one of the preceding claims, wherein the second region(s) (220) is / are cooled by way of blowing on one side using a fluid.

5. Use of a heat-treatment apparatus (100) for carrying out a method according to one of Claims 1 to 3, wherein the heat-treatment apparatus (100) has the furnace (110) for heating the steel component (200) to a temperature above the AC3 temperature, and wherein the heat-treatment apparatus (100) furthermore has the treatment station (150), wherein the treatment station (150) has an apparatus for rapid cooling of the one or more second regions (220) of the steel component (200) and has means for keeping the temperature of the one or more first regions (210) above the microstructure-transformation temperature, wherein the heat-treatment apparatus (100) has means for transferring the steel component (200) from the furnace (110) to the treatment station (150).

6. Use according to Claim 5, wherein the apparatus for rapid cooling of the one or more second regions (220) of the steel component (200) has a nozzle for blowing a gaseous fluid onto the second region(s) (220) of the steel component (200).

7. Use according to either of Claims 5 and 6, wherein the apparatus for rapid cooling of the one or more second regions (220) of the steel component (200) has a nozzle for blowing a gaseous fluid admixed with water onto the second region(s) (220) of the steel component (200).

8. Use according to either of Claims 6 or 7, wherein the nozzle for blowing onto the second region(s) (220) of the steel component (200) is arranged only on one side of the treatment station (150), so that blowing onto the steel component (200) is possible on one side only.

9. Use according to one of Claims 5 to 8, wherein the treatment station (150) has heat reflectors.

10. Use according to one of Claims 5 to 9, wherein the treatment station (150) has thermally insulated walls.