Abstract
A heat treatment furnace and a method for heat treatment of a steel sheet blank is disclosed having at least one furnace chamber and a transport system for conveying the steel sheet blanks through the furnace chamber. A preheating chamber, a metallurgical bonding path and a cooling chamber, wherein the steel sheet blank can be heated in the preheating chamber to a temperature of above 200 C. A method for the production of a hot-formed and press-quenched motor-vehicle part is also disclosed.
Claims
1. Method for heat treatment of a pre-coated steel sheet blank, in which a pre-coating is provided on a steel sheet blank, the method comprising: preheating the pre-coated steel sheet blank to a preheat temperature between 200 C. and 450 C., then heating the preheated, pre-coated steel sheet blank to a metallurgical bonding temperature above an austenizing temperature, such that the pre-coating is metallurgically bonded to the steel sheet blank to form a coating, then cooling the metallurgically bonded steel sheet blank to a cooling temperature of less than 450 C. at a cooling rate of greater than 30 s per millimeter of sheet thickness of the steel sheet blank, and then hot-forming and press-hardening the cooled, metallurgically bonded steel sheet blank, wherein the metallurgically bonded steel sheet blank has, in at least one region, a fraction of atomic hydrogen below 0.5 ppm.
2. Method according to claim 1, wherein the preheat temperature is between 250 C. to 450 C., and/or the cooling temperature is between 450 C. and 300 C.
3. Method according to claim 1, wherein the heating to the metallurgical bonding temperature is performed as rapid heating at a heating rate of less than 20 s per mm of sheet thickness of the steel sheet blank to be heated.
4. Method according to claim 1, wherein a thickness of the coating is between 0.6 m and 0.15 m and/or the fraction of atomic hydrogen is below 0.3 ppm.
5. Method according to claim 1, wherein the steel sheet blank is of a quenchable steel alloy, the pre-coating is of an AlSi alloy, and at least an intermetallic phase of FeAl is formed when the pre-coating is metallurgically bonded to the steel sheet blank.
6. Method according to claim 1, wherein heat radiation of metallurgically bonded steel sheet blanks that are guided through a cooling path during the cooling is used to preheat other, pre-coated steel sheet blanks that are guided through a preheating path during the preheating.
7. Method according to claim 1, wherein the heating to metallurgical bonding temperature is performed as rapid heating at a heating rate of less than 5 s per mm of sheet thickness of the steel sheet blank to be heated.
8. Method according to claim 1, wherein the cooling temperature is between 450 C. and 300 C., and the method further comprises a further cooling process to cool the metallurgically bonded steel sheet blank to a temperature of less than 300 C.
9. Method for producing a hot-formed and press-quenched motor vehicle part, the method comprising: preheating a pre-coated steel sheet blank, in which a pre-coating is provided on a steel sheet blank, to a preheat temperature between 200 C. and 450 C., then heating the preheated, pre-coated steel sheet blank to a metallurgical bonding temperature above an austenizing temperature, such that the pre-coating is metallurgically bonded to the steel sheet blank to form a coating, then cooling the metallurgically bonded steel sheet blank to a cooling temperature of less than 450 C. at a cooling rate of greater than 30s per millimeter of sheet thickness of the steel sheet blank, then reheating the cooled, metallurgically bonded steel sheet blank at least partially, in a time of less than 20 s, to a temperature greater than or equal to the austenizing temperature, and then hot-forming and press-quenching the reheated, metallurgically bonded steel sheet blank into the hot-formed and press-quenched motor vehicle part, wherein the metallurgically bonded steel sheet blank has, in at least one region, a fraction of atomic hydrogen below 0.5 ppm.
10. Method according to claim 9, wherein in the reheating, the cooled, metallurgically bonded steel sheet blank is reheated from the cooling temperature, or from room temperature.
11. Method according to claim 9, wherein the produced motor vehicle part has, in at least one region, a tensile strength Rm of greater than 1250 MPa, and/or the fraction of atomic hydrogen is below 0.3 ppm.
12. Method according to claim 11, wherein the tensile strength Rm is greater than 1450 MPa.
13. Method according to claim 1, wherein the heating to the metallurgical bonding temperature is performed as rapid heating at a heating rate of less than 10 s per mm of sheet thickness of the steel sheet blank to be heated.
Description
(1) Other advantages, features, properties and aspects of the present invention are dealt with in the following description. Preferred embodiment variants are presented in the schematic figures. These serve to make the invention easy to understand. In the figures:
(2) FIGS. 1a and 1b show a first variant, according to the invention, of a heat treatment furnace and its temperature profile,
(3) FIGS. 2a and 2b show a second variant, according to the invention, of a heat treatment furnace and its temperature profile,
(4) FIGS. 3a and 3b show a third variant, according to the invention, of a heat treatment furnace and its temperature profile,
(5) FIGS. 4a and 4b show a fourth variant, according to the invention, of a heat treatment furnace and its temperature profile,
(6) FIGS. 5a and 5b show a fifth variant, according to the invention, of a heat treatment furnace and its temperature profile, and
(7) FIG. 6 is a view of the method, carried out according to the invention, for producing a motor vehicle part.
(8) The figures use the same reference signs for identical or similar parts, even if a repeated description is omitted for reasons of simplicity.
(9) FIG. 1a shows a heat treatment furnace 1 according to the invention in the form of a continuous furnace. This furnace has, in relation to the plane of the image, a metallurgical bonding path 2 at the bottom, a cooling path 3 in the middle and a preheating path 4 at the top. In this regard, pre-coated steel sheet blanks 5 from a stack 6 are introduced into the preheating path 4 at one end 7 of the heat treatment furnace 1. The heat radiation of the steel sheet blanks 16 that are to be cooled and are transported through the cooling path 3 can simultaneously be used to preheat the steel sheet blanks that are to be transported through the preheating path 4. Also depicted is a distance A between the preheating path 4 and the cooling path 3, such that the transfer of heat {dot over (Q)} takes place in the form of heat radiation from the steel sheet blanks that are to be cooled to the steel sheet blanks that are to be preheated. This distance is preferably 20 to 300 mm.
(10) As transport means 9, rollers 8 can be arranged throughout the furnace. It is however also possible to use other transport means for transit. The pre-coated steel sheet blanks 5 are conveyed through the preheating path 4 in a transport direction of the preheating path 4.
(11) At the opposite end 10 of the heat treatment furnace 1 there is provided a vertical conveyor 11 which lowers the preheated steel sheet blanks 5 (with regard to the plane of the image) and transfers them to the metallurgical bonding path 2. Then, the preheated steel sheet blanks are conveyed through the metallurgical bonding path 2 in the transport direction 12. Heating means 13, for example burners or alternatively induction coils, are arranged in the metallurgical bonding path 2. The preheated steel sheet blanks transported through the metallurgical bonding path 2 are heated, at least at the end of the metallurgical bonding path 2, to a temperature above the Ac3temperature such that the pre-coating forms an intermetallic phase with the steel sheet blank and the steel sheet blanks 14 are metallurgically bonded.
(12) Also provided at the previously described end 7 is a vertical conveyor 11 which raises the metallurgically bonded steel sheet blanks 14 and introduces them into the cooling path 3. In the transport direction 15 through the cooling path 3, the metallurgically bonded steel sheet blanks 14 are cooled to a temperature and removed at the end of the cooling path 3, and the metallurgically bonded and cooled steel sheet blanks 16 are stored on a blank stack 17. These can undergo further processing (not shown in greater detail), in particular a subsequent hot-forming and press-quenching process.
(13) FIG. 1b shows an exemplary temperature profile that prevails in the individual paths 2, 3, 4. With regard to the plane of the image, the temperature within the metallurgical bonding path 2 increases from left to right from 750 C. to 930 C. The steel sheet blank conveyed through the metallurgical bonding path 2 thus heats up owing to the furnace temperature prevailing inside the metallurgical bonding path 2, or owing to the effect of heat on the steel sheet blank that is to be heated and metallurgically bonded. A relatively constant temperature of 350 C. prevails in the cooling path 3 and in the preheating path 4. By choosing the transport speed through the preheating path 4 or the cooling path 3, it is thus possible to influence the heating time and the preheating temperature or cooling temperature adopted at the end 7, 10 of the respective path 2, 3, 4. The preheating path 4 and the cooling path 3 have no heating means of their own. To that end, there is provided a separating layer 18 between the metallurgical bonding path 2 and the cooling path 3 and/or the preheating path 4. By prior selection, closed-and/or open-loop control of the separating layer, it is possible to influence the transfer of heat from the metallurgical bonding path 2 into the cooling path 3 and/or the preheating path 4.
(14) FIGS. 2a and b show an alternative embodiment variant to FIG. 1a and b. Here, too, the individual paths 2, 3, 4 are arranged stacked one atop the other with regard to the vertical direction V. However, in contrast to FIG. 1, the preheating path 4 is arranged in the middle, the cooling path 3 is arranged at the top and the metallurgical bonding path 2 is again arranged at the bottom, in each case with regard to the plane of the image or the vertical direction V. Thus, the pre-coated steel sheet blanks 5 are once again inserted into the preheating path 4 from a stack 6 at one end 7, pass through the preheating path 4 and are transferred to the metallurgical bonding path 2 by a vertical conveyor 11 arranged at the end of the preheating path 4. Then, the blanks pass through the metallurgical bonding path 2 in the transport direction 12 of the latter, and are once again transferred, at the starting end 7 and by a vertical conveyor 11, to the cooling path 3, in this example raised, and pass through the cooling path 3.
(15) At the end 10 of the cooling path 3, the cooled steel sheet blanks 16 are removed and brought to a blank stack 17. Here, too, heating means 13 are once again provided, both in the metallurgical bonding path 2 and in the thermal separating layer 18, such that heat energy is transferred from the metallurgical bonding path 2 to the preheating path 4 or to the cooling path 3.
(16) The temperature profile of the heat treatment furnace 1 shown in FIG. 2a can be seen in FIG. 2b.
(17) FIG. 2b also shows that, with regard to the plane of the image, the temperature profile of the metallurgical bonding path 2 increases from left to right. The thermal separating layer has the effect that the temperature profiles of the cooling path 3 and the preheating path 4 are less than that of the metallurgical bonding path 2. However, the left-to-right profile, in the plane of the image, also shows how the temperature increases within the path.
(18) FIGS. 3a and b show an alternative embodiment variant of the heat treatment furnace 1 according to the invention. In this case, the individual paths 2, 3, 4 are arranged lying next to one another in the horizontal direction H. The pre-coated steel sheet blanks 5 are once again inserted into a preheating path 4 from a stack 6 at one end 7 of the heat treatment furnace 1 and pass through the preheating path 4 in the transport direction 9 of the latter. At the end 10, the blanks are transferred, in the horizontal direction H by means of a horizontal conveyor 19, into a parallel metallurgical bonding path 2 and pass through the metallurgical bonding path 2 in the transport direction 12 of the latter, at the starting end 7 the metallurgically bonded steel sheet blanks 14 are transferred, in the horizontal direction H by means of another horizontal conveyor 19, into a cooling path 3 parallel to the metallurgical bonding path 2, and pass through the cooling path 3 in the transport direction 15 of the latter. At the end 10 of the cooling path 3, the cooled steel sheet blanks 16 are removed and are stored on a blank stack 17 such that they can be supplied for another use.
(19) FIG. 3b again shows a temperature profile of the parallel, mutually adjacent paths 2, 3, 4. It can be seen that, in the preheating path 4, use is initially made of excess temperature for more rapid preheating of the pre-coated steel sheet blanks 5, then in the metallurgical bonding path 2 the temperature increases from 750 C. to 930 C. internal temperature, and therefore so does that of the blanks passing through the furnace, such that metallurgical bonding takes place. Thereafter, a cooling path 3 is passed through from 400 C. to 300 C. such that controlled cooling of the metallurgically bonded steel sheet blanks 14 to approximately below 350 C. at the end of the cooling path 3 takes place. Both the cooling path 3 and the preheating path 4 are parallel and adjacent to the metallurgical bonding path 2 such that, in this embodiment variant, heating means (not shown) of the metallurgical bonding path 2 accordingly also control the temperature of the cooling path 3 and/or the preheating path 4.
(20) FIG. 4a shows a heat treatment furnace 1 with a separate preheating chamber 20, and a metallurgical bonding path 2 and cooling path 3 in the form of a stacked continuous furnace. First, the pre-coated steel sheet blanks 5 are transferred from a stack 6 into the preheating chamber 20. In that context, the preheating chamber 20 is optionally operated using exhaust air 21 from the actual heat treatment furnace 1. The pre-coated steel sheet blanks 5 are transported upwards, in the vertical direction V, in the transport direction 9 through the preheating chamber 20 and thence moved by means of a vertical conveyor 11 back down into the metallurgical bonding path 2. This is once again designed as a continuous furnace with heating means 13 such that the blanks are metallurgically bonded and the metallurgically bonded steel sheet blanks 14 are raised in the vertical direction V by a vertical conveyor 11 at one end 7 of the metallurgical bonding path 2 and transferred to the cooling path 3. The blanks pass through the cooling path 3 in the transport direction 15 of the latter, according to the contraflow principle relative to the metallurgical bonding path 2. Additional cooling means 22, for example cooling plates that can be placed on top, can be provided at the end of the cooling path 3. The metallurgically bonded and cooled steel sheet blanks 16 can then be supplied to further processing or storage.
(21) FIG. 4b once again shows a temperature profile of the cooling path 3 and the metallurgical bonding path 2, and of the preheating chamber 20 as shown in FIG. 4a.
(22) FIGS. 5a and b show a further alternative embodiment variant with a preheating path 4 and a metallurgical bonding path 2 arranged below this in the vertical direction V, and an exemplary temperature profile. Shown here are a preheating path 4 and a metallurgical bonding path 2. A cooling means 22 is provided at the end of the metallurgical bonding path 2. Alternatively or in addition to the cooling means 22, there is provided an insulated transport frame 23 into which the metallurgically bonded steel sheet blanks 14 are placed and then cooled in a targeted manner therein. The cooling rate can be influenced by the thickness of the insulation material of the insulated cooling frame.
(23) In FIG. 6, a pre-coated steel sheet blank 5 is first conveyed to a heat treatment furnace 1. After passing through the heat treatment furnace 1, this steel sheet blank 14 is metallurgically bonded and is conveyed to a tempering station 24 where rapid heating takes place. The metallurgically bonded steel sheet blank 14, which is tempered, at least in certain regions, to above Ac3 with the rapid heating, is then conveyed to a combined hot-forming and press-quenching tool 25 where it is hot-formed and quenched by rapid cooling. This produces a motor vehicle part 26 in accordance with the invention, which part has, owing to the heat treatment according to the invention, both an anti-corrosion layer and also reduced cracking tendency. The method can in particular be used for steel sheet blanks made of AlSi-precoated sheet metal strips with regionally reduced sheet thickness in the rolling direction of strips, also termed Tailor Rolled Blanks. In particular, the regions with a greater reduction in thickness and thinner sheet thickness are less susceptible to cracking and/or breakage owing to the low hydrogen content. Rolling is ideally performed as cold rolling. It is thus possible to produce coated parts with a load-appropriate sheet thickness profile without a tendency to crack. With the method, it is also possible to produce other steel parts with at least two regions of different thicknesses. The above-mentioned advantages apply accordingly.
