KR102672034B1 - Heat treatment method and heat treatment device - Google Patents

Abstract

The present invention relates to a method and apparatus for heat treatment of steel parts specifically directed to individual areas of the part. One or more first regions of the steel part may be set to a predominantly austenitic structure, from which a predominantly martensitic structure may be obtained through quenching, and one or more second regions of the steel part may have a predominantly ferrite-pearlitic structure. do. The steel part is first heated entirely in a first furnace to a temperature below the Ac3 temperature, and then the steel part is transferred to a processing station. Cooling may occur during transport of the steel part, wherein one or more second areas within the processing station are cooled to a cooling stop temperature (θ _S ) for a residence time (t ₁₅₀ ) and then transferred to a second heating furnace, where the steel part is subjected to cooling. Heat is transferred. The temperature of the one or more second regions is raised again to a temperature below the Ac3 temperature during the residence time t ₁₃₀ , while the temperature of the one or more first regions is heated to a temperature above the Ac3 temperature during the same residence time t ₁₃₀ .

Description

Heat treatment method and heat treatment device

The present invention relates to a method and apparatus for heat treatment of a steel component specifically targeting individual zones of the component.

The need for high strength sheet metal parts with technically low part weights exists in many applications in different fields. For example, in the automobile industry, efforts are being made to reduce vehicle fuel consumption and _CO2 emissions while simultaneously improving passenger safety. Therefore, the demand for vehicle body parts with excellent strength-to-weight ratio is rapidly increasing. These parts include, in particular, the A- and B-pillars, the side impact protection beams of the doors, the sills, frame parts, bumpers, floor and roof cross members. , including anterior and posterior lateral members. In modern vehicles, the body shell with the safety cage is typically made of hardened steel sheet with a strength of about 1,500 MPa. Al-Si coated steel sheet is used in many cases here. The so-called press hardening process has been developed to manufacture parts from hardened steel sheets. In this process, the steel sheet is first heated to the austenitic temperature, then placed in a pressing tool and rapidly molded within the water-cooled tool to initiate martensite. It is rapidly quenched to below the martensite start temperature. Here, a hard and strong martensite structure with a strength of approximately 1,500 MPa is produced. However, this type of hardened steel sheet has only low elongation at the break. Because of this, the kinetic energy of the impact cannot be properly converted into deformation heat.

Therefore, there are several different elongation and strength regions within the part, with solid regions on the one hand (hereinafter referred to as first regions) and ductile regions on the other hand (hereinafter referred to as second regions) in one part. It is desirable for the automotive industry to manufacture vehicle body parts that exist within. On the one hand, parts with high strength are, in principle, desirable to obtain parts with high mechanical load capacity and low weight. On the other hand, even high-strength parts must be able to have partially flexible areas. This advantageously provides partially enhanced deformability in case of collision. This is the only way to dissipate the kinetic energy of the impact, so acceleration forces on the passengers and the rest of the vehicle can be minimized in this way. Additionally, modern joining processes require softened points to join materials of the same type or different materials. For example, lock seam joints, crimped joints or riveted joints must be used, which require deformable areas in the part.

The general requirements for manufacturing equipment must also be observed here: no cycle time losses occur in the press hardening process and the overall plant can be used normally without restrictions and can be quickly modified in a product-oriented manner. The process is robust and economical, and the manufacturing facility requires minimal space. The shape and edge precision of the part must be high.

In all known methods, targeted heat treatment of the component takes place in time-intensive processing steps, which significantly affects the cycle time of the overall heat treatment device.

Therefore, the object of the present invention is to present a method and apparatus for heat treatment of steel parts, specifically aimed at individual regions of the part, such that different hardness and ductility regions can be achieved while minimizing the impact on the cycle time of the overall heat treatment apparatus.

According to the invention, this object is achieved by a method having the features of independent claim 1. Useful developments of this method are derived from dependent claims 2 to 5. This object is also achieved by the device according to claim 8. Useful embodiments of the device are derived from dependent claims 6 to 15.

Steel parts are first heated to below the austenitizing temperature (Ac3).

The steel parts are then transferred to a handling station. Here the second region or second regions are cooled as rapidly as possible within the handling time (t _B ). In a preferred embodiment of the heat treatment apparatus, the treatment station has a positioning device, with the help of which the correct positioning of the individual areas is ensured. The rapid cooling of the second zone or second zones is, in a preferred embodiment of the method, achieved by blowing of a gaseous fluid, for example air or an inert gas. In one preferred embodiment, the treatment station has for this purpose a device for spraying the second zone or second zones. The device may have, for example, one or more nozzles. In one preferred embodiment of this method, the injection into the second zone or second zones consists of a gaseous fluid with the addition of water, for example in nebulized form. For this purpose the device in one preferred embodiment has one or more atomizing nozzles. Heat dissipation from the second region or regions is increased by spraying a gaseous fluid with added water. When the processing time (t _B ) expires, the second region or second regions reach a cooling stop temperature (θ _S ). Processing time (t _B ) is generally in the range of several seconds. In this case, the second region or second regions can be cooled to a temperature significantly lower than the martensite start temperature ( _MS ). For example, the martensite onset temperature (M _S ) of 22MnB5, a commonly used vehicle body structural steel, is approximately 410 °C. The first zone or first zones do not undergo any special treatment at the treatment station, ie they are not sprayed, heated or cooled by any other special device. The first zone or first zones are cooled slowly in the processing station, for example through natural convection. It has been found useful to take measures to reduce heat loss in the first zone or first zones within the processing station. These means can be, for example, attachment of heat radiation reflectors and/or insulation of the surface of the processing station in the first zone or area of first zones.

Subsequently, that is, when the processing time (t _B ) expires, the steel part is transferred to the second heating furnace. In this second heating furnace the entire steel component is heated. Heating may take place, for example, by thermal radiation. The steel component remains in this second furnace for a residence time (t ₁₃₀ ), which is determined by the time it takes for the temperature of the first zone or first zones to rise above the Ac3 temperature. At the start of the residence time (t ₁₃₀ ), the second region or the second regions have a much lower temperature than the first region or the first regions due to the preceding method steps, so that the residence time (t ₁₃₀ ) in the second furnace is reduced. At the end they do not reach the Ac3 temperature. The steel part is then transferred to a press hardening tool, where the first zone or first zones are fully austenitized while the second zone or second zones are not austenitized, so that in subsequent press hardening The first region or first regions may form a martensite structure having a high strength value. Since the second region or secondary regions are not austenitized at any stage of the process, they have a ferritic-pearlitic structure with low strength values and high ductility after the press hardening step.

According to the invention, the parts are transferred after a few seconds at the processing station, preferably to ensure accurate positioning of the different zones into the second furnace, which does not have special devices for different processing of the different zones. The station may also have a positioning device. In one embodiment, only one furnace temperature (furnace temperature θ ₄ ) above the austenitization temperature (Ac3) is set, i.e. a temperature that is almost uniform throughout the entire furnace space. The low temperature difference between the two areas creates clearly defined boundaries between the individual areas and minimizes distortion of the part. A small spread in the temperature level of the part has a beneficial effect on subsequent processing in the press.

In one embodiment, a continuous furnace is preferably provided as the first heating furnace. Continuous furnaces are particularly suitable for mass production as they can be charged and operated without significant outlay and generally have a large throughput. However, a batch furnace such as a chamber furnace can also be used as the first heating furnace.

In one embodiment, the second furnace is preferably a continuous furnace.

If both the first and second heating furnaces are implemented as continuous furnaces, the residence time required for the first and second zone or zones can be implemented as a function of the part length through setting the feed rate and designing the length of each furnace. The influence on the cycle time of the entire production line, which has a heat treatment unit and a press for the subsequent press hardening process, can be avoided in this way.

In an alternative embodiment, the second furnace is a batch furnace, such as a chamber furnace.

In a preferred embodiment, the processing station has a device for rapid cooling of one or more second regions of the steel component. In one useful embodiment, the device has a nozzle for spraying a gaseous fluid, for example air or an inert gas, for example nitrogen, to the second region or regions of the steel component. For this purpose the device in one useful embodiment has one or more nebulization nozzles. Heat dissipation from the second region or regions is increased by spraying a gaseous fluid with added water.

In another embodiment, the second region or regions are cooled by heat conduction, for example by contacting it with one or several dies that have a much lower temperature than the steel part. The die may be made of a sufficiently thermally conductive material for this purpose and may be cooled directly or indirectly. Combinations of cooling methods may also be considered.

Using the method according to the invention and the heat treatment device according to the invention, the different zones can reach the required process temperature very quickly in a sharply contoured manner, each having one or more first and/or second zones and in a complex manner. The corresponding temperature profile can be economically implemented even in steel parts that can be composed of .

According to the invention, almost any number of second zones can be established using the described method and the heat treatment device according to the invention. The second zone or regions are not austenitized during the execution of the method and thus have low strength values similar to the original strength of the untreated steel component even after pressing. The selected geometry of the partial regions can also be freely selected. Punctiform or linear areas can also be calculated, as is possible for large areas, for example. Even the location of the areas doesn't matter. The second regions may be completely surrounded by the first regions or may be located at the corners of the steel part. Full-surface treatment can also be considered. A specific orientation of the steel part with respect to the throughput direction is not necessary for the purpose of the method according to the invention for heat treating the steel part by orienting measurements to individual areas of the part. The limit to the number of steel parts that can be processed simultaneously is, at best, set by the overall conveying technology of the press hardening tool or heat treatment unit. Application of the method (invention) to previously preformed steel parts is likewise possible. The three-dimensionally molded surface of a pre-formed steel part simply makes the manufacturing of the corresponding surface more expensive to design.

In addition, it is useful because even existing heat treatment equipment can be adapted to the method according to the present invention. To achieve this, in the case of a conventional heat treatment device having only one heater, a processing station and a second heater can be installed behind it. Depending on the structure of the existing heating furnace, it may be divided to form first and second heating furnaces from one original heating furnace.

Other advantages, special features and advantageous developments deriving from the dependent claims will become apparent from the description of the preferred exemplary embodiments following reference to the drawings. In the drawing,
Figure 1 is a typical temperature curve in the heat treatment of a steel part with first and second zones.
Figure 2 is a schematic plan view of a heat treatment device according to the present invention.
Figure 3 is a schematic plan view of another heat treatment apparatus according to the present invention.
Figure 4 is a schematic plan view of another heat treatment apparatus according to the present invention.
Figure 5 is a schematic plan view of another heat treatment apparatus according to the present invention.
Figure 6 is a schematic plan view of another heat treatment apparatus according to the present invention.
Figure 7 is a schematic plan view of another heat treatment apparatus according to the present invention.

Figure 1 shows a typical temperature curve in the heat treatment of a steel component 200 having a first region 210 and a second region 220 according to the invention. The steel component 200 is heated in the first furnace 100 along the schematically shown temperature curve θ _200,110 to below the Ac3 temperature during the residence time t ₁₁₀ in the first furnace 100 . The steel part 200 is then transferred to a handling station 150 during a transfer time t ₁₂₀ . Steel parts lose heat in this process. At the processing station, the second region 220 of the steel part 200 cools rapidly, with the second region 220 losing heat along the drawn-in curve θ _220,150 . Blowing ends with the expiration of the handling time (t _B ), which is only a few seconds depending on the thickness of the steel part 200 and the size of the second region 220. The processing time here (t _B ) is approximately equal to the residence time (t ₁₅₀ ) at the processing station 150. The second region 220 has now reached the cooling stop temperature ( _θS ). At the same time, the temperature of the first area 210 decreases along the temperature curve θ _210,150 shown at the processing station 150, where the first area 210 is not located within the area of the cooling device. When the processing time (t _B ) expires, the steel part 200 is transferred to the second heating furnace 130 during the transfer time (t ₁₂₁ ), and in this process, the steel part further loses heat. The temperature of the first region 210 of the steel component 200 in the second heating furnace 130 changes along the schematically shown temperature curve θ _210,130 during the residence time t ₁₃₀ , that is, up to a temperature above the Ac3 temperature. It is heated. The temperature of the second region 220 of the steel component 200 also rises along the shown temperature curve θ _220,130 during the residence time t ₁₃₀ but does not reach the Ac3 temperature. The second furnace does not have special devices to treat the different areas 210, 220 differently. Only the furnace temperature (θ ₄ ), that is, a temperature (θ ₄ ) that is almost uniform throughout the entire internal space of the second heating furnace 130, is set, which is above the austenitization temperature (Ac3). At the start of the residence time (t ₁₃₀ ), the second region or regions have a much lower temperature than the first region or regions, so when both regions are heated equally in the second heating furnace 130, they remain. At the end of time (t ₁₃₀ ), it has a different temperature as well. The residence time (t ₁₃₀ ) of the steel component 200 in the second heating furnace ₁₃₀ is at this point The temperature of the second region or second regions is set so that it does not reach the Ac3 temperature.

The steel part is then transferred to a press hardening tool 160 installed in a press, not shown, during the transfer time t ₁₃₁ . During the transfer time (t ₁₃₁ ), the steel component 200 may lose heat again and the temperature of the first region or regions may also fall below the Ac3 temperature. However, this region or regions are almost completely austenitized when leaving the second heating furnace 130, so that hard martensite is formed by quenching during the residence time (t ₁₆₀ ) in the press hardening tool 160. undergoes a structural transformation.

A clear delimitation of the outlines of the individual regions 210 and 220 is achieved between the two regions 210 and 220, and distortion of the steel component 200 can be minimized thanks to the small temperature difference. The small spread in temperature levels of the steel part 200 has a beneficial effect on subsequent processing in the press hardening tool 160. The required residence time (t ₁₃₀ ) of the steel part 200 in the second furnace 130 is determined as a function of the length of the steel part 200 by setting the transport speed and the design of the length of the second furnace 130. It can be implemented. The impact on the cycle time of the heat treatment apparatus 100 can thus be minimized or even completely avoided.

Figure 2 shows a heat treatment device 100 according to the invention in a 90° arrangement. The heat treatment apparatus 100 has a loading station 101 through which steel parts are supplied to the first heating furnace 110. The heat treatment apparatus 100 also has a processing station 150 and a second heating furnace 130 disposed behind its main throughput direction (D). Further rearward in the main processing direction, a removing station 131 is arranged, which is equipped with a positioning device (not shown). The main processing direction is here bent approximately 90° to a press hardening tool 160 in a press (not shown), where it is press hardened. A container 161 is disposed in the axial direction of the first heating furnace 110 and the second heating furnace 130, and rejected parts can pass through it. In this arrangement, the first heating furnace 110 and the second heating furnace 130 are preferably implemented as continuous furnaces, such as roller hearth furnaces.

Figure 3 shows the heat treatment apparatus 100 according to the invention in a linear arrangement. The heat treatment apparatus 100 has a loading station 101 through which steel parts are supplied to the first heating furnace 110. The heat treatment apparatus 100 also has a processing station 150 and a second heating furnace 130 is disposed behind it in the main processing direction D. Further rearward in the main processing direction D, a take-out station 131 is arranged, which is equipped with a positioning device (not shown). Following the linearly continuous main processing direction is located the press hardening tool 160 of a press (not shown), where the steel part 200 is press hardened. A container 161 is placed at an angle of approximately 90° at the extraction station 131, through which the discharged parts can pass. In this arrangement, the first heating furnace 110 and the second heating furnace 130 are also preferably implemented as continuous furnaces, for example roller hearth furnaces.

Figure 4 shows another modification of the heat treatment apparatus 100 according to the present invention. The heat treatment device 100 has a loading station 101 through which steel parts are supplied to the second heating furnace 110. In this embodiment the first heating furnace 110 is also preferably configured as a continuous furnace. The heat treatment apparatus 100 also has a processing station 150, which in this embodiment is combined with an extraction station 131. The take-out station 131 may have, for example, a gripper device (not shown). The take-out station 131 takes out the steel component 200 from the first heating furnace 110 by, for example, a holding device. Heat treatment is performed with cooling of the second region or second regions 9220 and the steel part or steel parts 200 are disposed at approximately 90° to the axis of the first heating furnace 110 in the second heating furnace 130. is located in In this embodiment the second heating furnace 130 is preferably provided as a chamber furnace, for example having several chambers. When the residence time (t ₁₃₀ ) of the steel part 200 in the second heating furnace 130 expires, the steel part 200 is taken out from the second heating furnace 130 through the extraction station 131, and It is located on a press hardening tool 160 integrated into a press (not shown) located at. The take-out station 131 may have a positioning device (not shown) for this purpose. A container 161 is disposed behind the extraction station 131 in the axial direction of the first heating furnace 110, and the extracted parts can pass through it. The main processing direction D in this embodiment exhibits a deflection of almost 90°. In this embodiment, there is no need for a second positioning system at processing station 150. This embodiment is also useful when there is not enough space in the axial direction of the first furnace 110, for example in a production hall. In this embodiment as well, cooling of the second region 220 of the steel component 200 occurs between the extraction station 131 and the second heating furnace 130, eliminating the need for a fixed processing station 150. For example, cooling devices, such as spray nozzles, can be integrated into the gripping device. The take-out station 131 handles the transfer of the steel part 200 from the first furnace 110 to the second furnace 130 and to the press hardening tool 160 or container 161.

In this embodiment, the positions of the press hardening tool 160 and the container 161 can also be changed as shown in FIG. 5. In this embodiment the main processing direction D exhibits two bends of approximately 90°.

If the space to install the heat treatment device is limited, the heat treatment device according to FIG. 6 may be proposed: Compared to the embodiment shown in FIG. 4, the second heating furnace 130 is located on the second floor above the first heating furnace 110 ( Go to the second level. In this embodiment too, cooling of the second region 200 of the steel component 200 can likewise take place between the extraction station 131 and the second heating furnace 130, eliminating the need for a fixed processing station 150. It is also desirable to implement the first heating furnace 110 as a continuous furnace and the second heating furnace 130 as a chamber furnace having as many chambers as possible.

Finally, a final embodiment of the inventive heat treatment apparatus is schematically shown in Figure 7. Compared to the embodiment of FIG. 6, the positions of the press hardening tool 160 and the container 161 were changed.

The embodiments shown above are merely representations of examples of the present invention and should not be construed in a limiting manner. Alternative embodiments that can be considered by those skilled in the art are likewise encompassed by the protection scope of the present invention.

100 Heat treatment device
110 First furnace
130 Second furnace
131 Removal station
135 Removal station
150 Handling station
152 Punctiform infrared radiator
153 Heating panel
160 Press hardening tool
161 Container
200 Steel component
210 First region
220 Second region
D Main throughput direction
M _S Martensite start temperature
t _B Handling time
t ₁₁₀ Residence time in first furnace
t ₁₂₀ Transfer time steel component to handling station
t ₁₂₁ Transfer time steel component to second furnace
t ₁₃₀ Residence time in second furnace
t ₁₃₁ Transfer time steel component to press hardening tool
t ₁₅₀ Residence time in handling station
t ₁₆₀ Residence time in press hardening tool
θ _S Cooling stop temperature
θ ₃ Internal temperature of first furnace
θ ₄ Internal temperature of second furnace
θ _200,110 Temperature curve of steel component in first furnace
θ _210,150 Temperature curve of first region of metal component in handling station
θ _220,150 Temperature curve of second region of steel component in handling station
θ _210,130 Temperature curve of first region of steel component in second furnace
θ _220,130 Temperature curve of second region of steel component in second furnace
θ _200,160 Temperature curve of steel component in press hardening tool

Claims (16)

A heat treatment method for a steel part 200 that specifically targets individual areas of the steel part 200,
The steel component 200 is first heated to a temperature below the Ac3 temperature in the first heating furnace 110,
The steel part 200 is then transported to the processing station 150 and cooled during transport,
Within the processing station (150), one or more second regions (220) of the steel component (200) are cooled to a cooling stop temperature (θ _S ) for a residence time (t ₁₅₀ ) by spraying a gaseous fluid;
Next, it is transferred to the second heating furnace 130, which is different from the first heating furnace 110, where the chamber furnace of the second heating furnace 130 is set to a temperature higher than the Ac3 temperature, and the second heating furnace 110 is transferred to the second heating furnace 130. At 130, heat is transferred to the steel part 200 by thermal radiation, thereby forming one or more first regions 210 of the steel part 200 and one or more second regions 220 of the steel part 200. ) is reduced, and the temperature of the one or more second regions 220 is raised again to a temperature below the Ac3 temperature within the second heating furnace 130 during the residence time (t ₁₃₀ ), while the The temperature of one or more first regions 210 is heated to a temperature equal to or higher than the Ac3 temperature during the same residence time (t ₁₃₀ ), and each of the first heating furnace 110 and the second heating furnace 130 is a continuous furnace or a batch furnace. What is composed of
Heat treatment method for steel parts characterized by: In claim 1,
The gaseous fluid includes water
Heat treatment method for steel parts characterized by: In claim 1 or 2,
The internal temperature (θ ₄ ) of the second heating furnace 130 is higher than the Ac3 temperature.
Heat treatment method for steel parts characterized by:



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