Abstract
Disclosed are implementations for heat treatment of steel components. In one or more first regions of a steel component, a predominantly austenitic structure can be adjusted, from which, by way of quenching, a mainly martensitic structure is educible. In one or more second regions of the steel component, there is a mainly bainitic structure, wherein the metal component is initially heated in a first furnace to a temperature above the Ac3 temperature. Subsequently, the steel component is transferred into a treatment station, wherein the steel component can cool down during the transfer. In the treatment station, the one or more second regions of the steel component are cooled down to a cooling stop temperatures ϑ.sub.2 during a treatment period. Subsequently, said metal component is transferred to a second furnace, wherein the temperature of the one or more second regions increases again to a temperature below the Ac3 temperature.
Claims
1. A method comprising heat-treating individual zones of a steel component, which is usable for a vehicle component, to form a primarily austenitic structure in one or more first regions of the steel component, from which structure a predominantly martensitic structure can be produced by means of quenching, and to form a predominantly bainitic structure in one or more second regions, wherein heat-treating the zones comprises: first heating the steel component to a temperature above the Ac3 temperature in a first furnace, transferring the entire steel component to a treatment station wherein said component is cooled down during the transfer, cooling the one or more second regions of the steel component to a cooling stop temperature Θ.sub.2 in the treatment station during a treatment time t.sub.B, and transferring the entire steel component to a second furnace, the temperature of the one or more second regions increasing to a temperature below the Ac3 temperature again, wherein a temperature of the second furnace is set between 660 and 850° C.
2. The method according to claim 1, wherein the cooling stop temperature Θ.sub.2 is selected to be above the martensite start temperature M.sub.S.
3. The method according to claim 1, wherein the cooling stop temperature Θ.sub.2 is selected to be below the martensite start temperature M.sub.S.
4. The method according to claim 1, wherein a heat supply is used to increase the temperature at the second region or second regions of the steel component in the second furnace.
5. The method according to claim 1, wherein the internal temperature of the second furnace Θ.sub.4 is greater than the cooling stop temperature Θ.sub.2.
Description
(1) Additional advantages, features and advantageous developments of the invention can be found in the sub-claims and the following description of preferred embodiments on the basis of the figures, in which:
(2) FIG. 1 shows a typical temperature curve when heat-treating a steel component having a first and a second region,
(3) FIG. 2 is a schematic plan view of a thermal heat-treatment device according to the invention,
(4) FIG. 3 is a schematic plan view of another thermal heat-treatment device according to the invention,
(5) FIG. 4 is a schematic plan view of another thermal heat-treatment device according to the invention,
(6) FIG. 5 is a schematic plan view of another thermal heat-treatment device according to the invention,
(7) FIG. 6 is a schematic plan view of another thermal heat-treatment device according to the invention, and
(8) FIG. 7 is a schematic plan view of another thermal heat-treatment device according to the invention.
(9) FIG. 1 shows a typical temperature curve when heat-treating a steel component 200 comprising a first region 210 and a second region 220 according to the method of the invention. According to the schematically drawn temperature profile ϑ.sub.200,110, the steel component 200 is heated in the first furnace 110 during the dwell time thereof in the first furnace t.sub.110 to a temperature that is above the Ac3 temperature. The steel component 200 is then transferred to the treatment station 150 for a transfer time t.sub.120. In this case, the steel component loses heat. In the treatment station, a second region 220 of the steel component 200 is quickly cooled, the second region 220 losing heat quickly according to the profile ϑ.sub.220,150 drawn. Cooling ends once the treatment time t.sub.B, which only lasts for a few seconds depending on the thickness of the steel component 200, the desired material properties and the size of the second region 220, has elapsed. In a first approximation, the treatment time t.sub.B equals the dwell time t.sub.150 in the treatment station 150 in this case. The second region 220 has then reached the cooling stop temperature ϑ.sub.2 that is above the martensite start temperature M.sub.S. At the same time, the temperature of the first region 210 in the treatment station 150 has also decreased according to the temperature profile ϑ.sub.210,150, whereby the first region 210 is not in the region of the cooling apparatus. Once the treatment time t.sub.B has elapsed, the steel component 200 is transferred to the second furnace 130 during the transfer time t.sub.121, whereby it loses more heat if its temperature is greater than the internal temperature ϑ.sub.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 schematically drawn temperature profile ϑ.sub.210,130 during the dwell time t.sub.130, i.e. the temperature of the first region 210 of the steel component 200 slowly continues to decrease. In this case, the temperature of the first region 210 of the steel component 200 can fall below the Ac3 temperature, but does not have to. On the contrary, the temperature of the second region 220 of the steel component 200 once again increases during the dwell time t.sub.130 according to the temperature profile ϑ.sub.220,130 drawn, without reaching the Ac3 temperature. The second furnace 130 does not comprise any special devices for treating the different regions 210, 220 differently. Only one furnace temperature ϑ.sub.4, i.e. a substantially homogeneous temperature in the entire interior of the second furnace 130, is set and is between the austenitizing temperature Ac3 and the cooling stop temperature ϑ.sub.2, for example between 660° C. and 850° C. The different regions 210, 220 therefore approach the internal temperature ϑ.sub.4 of the second furnace 130. Provided that the drop in temperature in the first region 210 during the dwell time t.sub.150 in the treatment station 150 are small enough for the second region 220 that the temperature does not fall below the temperature ϑ.sub.4 of the second furnace 130, the temperature profile ϑ.sub.210,130 of the first region approaches the temperature ϑ.sub.4 of the second furnace 130 from above. In this embodiment, the cooling stop temperature ϑ.sub.2 is lower than the temperature ϑ.sub.4 selected for the second furnace 130. The temperature profile ϑ.sub.220,130 of the second region approaches the temperature ϑ.sub.4 of the second furnace 130 from below. The temperature of the region 210 does not fall below the structure transformation start temperature ϑ.sub.1. As a result of the small temperature difference between the two regions 210, 220, clearly contoured boundaries of the individual regions 210, 220 can be formed and the warpage of the steel component 200 is minimized. Small expansions in the temperature of the steel component 200 have an advantageous effect when further processing said component in the press-hardening die 160. The dwell time t.sub.130 required for the second region 220 can be established on the basis of the length of the steel component by setting the conveying speed and the dimensions of the length of the second furnace 130. The cycle time of the heat-treatment device 100 is thereby minimally affected, or even not at all. The first region 220 of the steel component 200 dissipates heat in the second furnace 130. The second region 220 of the steel component 200 absorbs heat in the second furnace 130, with the heat absorption being restricted by the heat released in the second region 220 of the steel component 200 during the recalescence of the structure. Overall, this only requires a relatively small amount of heating power in the second furnace 130. Additional heating of the second furnace 130 can optionally be omitted altogether. This treatment step is therefore particularly energy-efficient.
(10) Once the dwell time t.sub.130 of the steel component 200 in the second furnace 130 has finished, said component is transferred to a press-hardening die 160 during the transfer time t.sub.131, where it is reshaped and hardened during the dwell time t.sub.160.
(11) FIG. 2 shows a heat-treatment device 100 according to the invention in a 90° arrangement. The heat-treatment device 100 comprises a loading station 101, by means of which steel components are fed to the first furnace 110. Furthermore, the heat-treatment device 100 comprises the treatment station 150 and, arranged therebehind in the main direction of flow D, the second furnace 130. Arranged further downstream thereof in the main direction of flow D is a removal station 131, which is equipped with a positioning device (not shown). The main direction of flow then deviates by substantially 90°, in order to allow a press-hardening die 160 in a press (not shown) to follow, in which die the steel component 200 is press-hardened. A container 161, in which rejects can be placed, is arranged in the axial direction of the first furnace 110 and of the second furnace 130. In this arrangement, the first furnace 110 and the second furnace 120 are preferably formed as continuous furnaces, for example roller hearth furnaces.
(12) FIG. 3 shows a heat-treatment device 100 according to the invention in a linear arrangement. The heat-treatment device 100 comprises a loading station 101, by means of which steel components are fed to the first furnace 110. The heat-treatment device 100 also comprises the treatment station 150 and, arranged downstream thereof in the main direction of flow D, the second furnace 130. Arranged further downstream thereof in the main direction of flow D is a removal station 131, which is equipped with a positioning device (not shown). A press-hardening die 160 in a press (not shown), in which the steel component 200 is press-hardened, then follows in the main direction of flow that now continues straight. Arranged at substantially 90° to the removal station 131 is a container 161, in which rejects can be placed. In this arrangement, the first furnace 110 and the second furnace 120 are also preferably formed as continuous furnaces, for example roller hearth furnaces.
(13) FIG. 4 shows another variant of a heat-treatment device 100 according to the invention. The heat-treatment device 100 again comprises a loading station 101, by means of which steel components are fed to the first furnace 110. In this embodiment, the first furnace 110 is again preferably formed as a continuous furnace. Furthermore, the heat-treatment device 100 comprises the treatment station 150, which is combined with a removal station 131 in this embodiment. The removal device 131 can comprise a gripping device (not shown), for example. The removal station 131 removes the steel components 200 from the first furnace 110 by means of the gripping device, for example. The second region or second regions 200 is/are heat-treated and cooled and the steel component or the steel components 200 are loaded in a second furnace 130 that is arranged at substantially 90° to the axis of the first furnace 110. In this embodiment, this second furnace 130 is preferably provided as a chamber furnace, for example comprising a plurality of chambers. Once the dwell time t.sub.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 die 160 that is installed in a press (not shown). For this purpose, the removal station 131 can comprise a positioning apparatus (not shown). A container 161 is arranged downstream of the removal station 131 in the axial direction of the first furnace 110, in which container rejects can be placed. In this embodiment, the main direction of flow D describes a deflection of substantially 90°. In this embodiment, a second positioning system for the treatment station 150 is not required. Furthermore, this embodiment is advantageous when there is not enough space available in the axial direction of the first furnace 110, for example in a production hall. In this embodiment, the second regions 220 of the steel component 200 can also be cooled between the removal station 131 and the second furnace 130 so that a stationary treatment station 150 is not required. For example, a cooling device, for example a blowing nozzle, can be integrated in the gripping device. The removal device 131 ensures that the steel component 200 is transferred from the first furnace 110 to the second furnace 130 and to the press-hardening die 160 or to the container 161.
(14) In this embodiment, too, the press-hardening die 160 and the container 161 can switch positions, as can be seen in FIG. 5. In this embodiment, the main direction of flow D describes two deflections of substantially 90°.
(15) If the space in which the heat-treatment device is to be placed is restricted, a heat-treatment device according to FIG. 6 is advantageous: in comparison with the embodiment shown in FIG. 4, the second furnace 130 is moved to a second plane above the first furnace 110. In this embodiment, too, the second regions 220 of the steel component 200 can likewise be cooled between the removal station 131 and the second furnace 130, so that a stationary treatment station 150 is not required. Once again it is advantageous for the first furnace 110 to be formed as a continuous furnace and for the second furnace 120 to be formed as a chamber furnace, possibly comprising a plurality of chambers.
(16) Lastly, FIG. 7 is a schematic view of a final embodiment of the heat-treatment device according to the invention. In comparison with the embodiment shown in FIG. 6, the press-hardening die 160 and the container 161 have switched positions.
(17) The embodiments shown here only represent examples of the present invention and should therefore not be understood to be limiting. Alternative embodiments that a person skilled in the art would take into consideration are likewise covered by the scope of protection of the present invention.
LIST OF REFERENCE SIGNS
(18) 100 heat-treatment device 110 first furnace 130 second furnace 131 removal station 150 treatment station 160 press-hardening die 161 container 200 steel component 210 first region 220 second region D main direction of flow M.sub.S martensite start temperature t.sub.B treatment time t.sub.110 dwell time in the first furnace t.sub.120 transfer time of the steel component to the treatment station t.sub.121 transfer time of the steel component to the second furnace t.sub.130 dwell time in the second furnace t.sub.131 transfer time of the steel component to the press-hardening die t.sub.150 dwell time in the treatment station t.sub.160 dwell time in the press-hardening die ϑ.sub.1 structure transformation start temperature ϑ.sub.2 cooling stop temperature ϑ.sub.3 internal temperature of the first furnace ϑ.sub.4 internal temperature of the second furnace ϑ.sub.200,110 temperature profile of the steel component in the first furnace ϑ.sub.210,150 temperature profile of the first region of the steel component in the treatment station ϑ.sub.220,150 temperature profile of the second region of the steel component in the treatment station ϑ.sub.210, ϑ.sub.130 temperature profile of the first region of the steel component in the second furnace ϑ.sub.220,130 temperature profile of the second region of the steel component in the second furnace ϑ.sub.200,160 temperature profile of the steel component in the press-hardening die
