METHOD AND APPARATUS FOR PRODUCING A STEEL STRIP

Abstract

The invention relates to a method and to an apparatus for producing a steel strip, in particular a steel strip having a bainitic microstructure, such as for example a spring steel strip or a punching tool, wherein the steel strip is made to pass continuously through the following treatment steps: austenitization of the steel strip at a first temperature above the austenitization temperature; quenching of the steel strip, by means of a gaseous quenchant, to a lower, second temperature selected in accordance with a desired steel microstructure. According to the invention, the gaseous quenchant is conducted onto the steel strip in such a manner that uniform cooling is achieved over the width of the steel strip.

Claims

1-18. (canceled)

19: A method for producing a steel strip having a bainitic microstructure, comprising: passing carbon-containing steel strip continuously through following treatments: austenitization of the steel strip at a first temperature above the austenitization temperature; quenching of the steel strip, by a quenchant, to a lower, second temperature lying in the bainitization range of the steel strip; holding the steel strip at a temperature in the bainitization range for the quasi-isothermal formation of a bainite microstructure in the steel strip; wherein use is made of a gaseous quenchant, which is conducted onto the steel strip such that uniform cooling at a predefined cooling rate is achieved over the width of the steel strip, wherein the gaseous quenchant is guided in a temperature-controlled circuit and the flow rate of the gaseous quenchant is varied over the width of the steel strip.

20: The method as claimed in claim 19, wherein the gaseous quenchant is conducted onto the steel strip by two or more independently controllable gas streams.

21: The method as claimed in claim 19, wherein a hydrogen-containing gas mixture is used as the quenchant.

22: The method as claimed in claim 21, wherein the hydrogen proportion of the gas mixture used as the quenchant is between 50% by volume and 100% by volume.

23: The method as claimed in claim 19, wherein the surface of the steel strip is decarburized in a humid, hydrogen-containing nitrogen atmosphere before or during the austenitization.

24: The method as claimed in claim 23, wherein the humid, hydrogen-containing nitrogen atmosphere is guided in countercurrent to the direction of transport of the steel strip.

25: The method as claimed in claim 19, wherein the steel strip is tempered to final strength at a relatively high temperature in a hydrogen-containing nitrogen atmosphere after formation of the microstructure.

26: The method as claimed in claim 25, wherein the hydrogen proportion in the nitrogen atmosphere is between 1 and 10% by volume.

27: The method as claimed in claim 19, wherein the steel strip consists of a steel having a carbon content of between 0.3 and 0.8% by weight.

28: An apparatus for producing a steel strip having a bainitic microstructure, comprising: an austenitization unit heating a steel strip passing through to a first temperature above the austenitization temperature; a quenching unit quenching the steel strip passing through to a lower, second temperature lying in the bainitization range of the steel strip, wherein the quenching unit comprises a feed device for feeding a temperature-controlled gaseous quenchant onto the steel strip passing through; and a holding unit for holding the steel strip at a temperature in the bainitization range for the quasi-isothermal formation of a bainite microstructure in the steel strip; wherein the feed device is configured such that uniform cooling at a predefined cooling rate is achieved over the width of the steel strip, wherein the feed device comprises a plurality of nozzles, which are arranged above and below the steel strip passing through and are configured to produce a flow rate of the gaseous quenchant which varies over the width of the steel strip.

29: The apparatus as claimed in claim 28, wherein the nozzles are in a form of slotted nozzles, wherein at least some of the nozzles are arranged obliquely with respect to the steel strip passing through.

30: The apparatus as claimed in claim 28, wherein the nozzles are in a form of slotted nozzles, openings of which have adjustable apertures.

31: A steel strip having a bainitic microstructure, obtained with the method of claim 19.

32: A steel srip as claimed in claim 31, wherein the steel strip is a spring steel strip, a punching tool, or a cutting line.

Description

[0037] The invention will be explained in more detail hereinbelow with reference to an exemplary embodiment illustrated schematically in the accompanying drawing, in which:

[0038] FIG. 1 shows a schematic illustration of an apparatus according to the invention for carrying out the method according to the invention;

[0039] FIG. 2 shows a slotted nozzle arrangement according to the prior art, in which a noticeable edge effect arises;

[0040] FIG. 3 shows a variant according to the invention of the slotted nozzle arrangement with in some cases obliquely placed slotted nozzles; and

[0041] FIG. 4 shows a further arrangement according to the invention of the slotted nozzles, in which the openings of the slotted nozzles have adjustable apertures.

[0042] FIG. 1 shows a steel strip 10, which is guided via a gap 11 into a furnace 12 for the austenitization and optionally also for the surface decarburization of the steel strip. The direction of transport of the steel strip is denoted by the arrows 13 and 14. In the furnace 12, the steel strip 10 is heated to a temperature of approximately 900 C. The steel strip 10 leaves the austenitization furnace again via a lock 15. A dry or humid atmosphere which, in addition to nitrogen, can optionally also contain hydrogen is present in the austenitization/surface decarburization furnace. The atmosphere is blown into the furnace via an inlet opening 16 located in the proximity of the lock 15, and can leave the furnace 12 again via an outlet opening 17, which is located in the proximity of the entry gap 11. As denoted by the arrows 18, the atmosphere is thereby guided in countercurrent to the strip 10 passing through, such that cracked contamination can be discharged with the gas stream. The austenitization furnace 12 is adjoined by a quenching unit 19, which is separated from the austenitization furnace by the lock 15. In the quenching unit 19, a gaseous quenchant (for example a hydrogen/nitrogen gas mixture) is guided in a temperature-controlled circuit 20. To this end, the circuit 20 comprises a cooling device 21, in order to keep the circulating gas at a constant temperature, this ensuring that the steel strip 10 entering into the quenching unit 19 is cooled in a range of seconds to a temperature in the bainitization range of the steel strip 10. To this end, the quenching unit 19 has a plurality of nozzles 22, 23, which are arranged above and below the steel strip and blow the gaseous quenchant onto the surface of the steel strip passing through. A feed 24 can be used to feed fresh gas to the circuit 20, in order to compensate for losses in the circuit, primarily losses via the lock 15 and further via the outlet opening 17. The quenching unit 19 is adjoined by a holding unit 25, in which the steel strip passing through is held at a temperature in the bainitization range, for example at a temperature of 400 C., such that a bainite microstructure can form in the steel strip. By way of example, the atmosphere in the holding furnace 25 consists of a hydrogen/nitrogen mixture, which is introduced via an inlet opening 28. The holding furnace 25 also has suitable temperature-control means (not shown in FIG. 1), which, on account of the convection prevailing in the furnace (represented schematically by the arrows 26), ensure that the formation of the bainite microstructure can be effected in a quasi-isothermal manner. The steel strip with the bainite microstructure formed therein leaves the apparatus according to the invention at the exit 27. Subsequently, further devices can provide for the post-treatment which is known per se, for example an annealing furnace and/or cutting devices for separating the steel strip into a plurality of strips.

[0043] FIG. 2 shows a plan view of a steel strip 10 in the region of a quenching unit 19 according to the prior art. The direction of transport of the steel strip 10 (strip running direction) is again symbolized by an arrow 13. According to the prior art, a plurality of slotted nozzles 22 are arranged transversely to the strip running direction for cooling the steel strip 10. The cooling gas flows out of these slotted nozzles 22 onto the steel strip 10. The dashed lines 30a-30g symbolize the temperature profile of the steel strip 10 on the basis of isotherms having a temperature which decreases from 30a-30g. The profile of the isotherms shows the edge effect which is associated with the prior art, with lower temperatures being reached significantly earlier at the edge than in the center of the steel strip owing to the greater cooling of the edges of the steel strip 10.

[0044] In order to compensate for this edge effect, it is proposed according to the invention to vary the flow rate of the gaseous quenchant over the width of the steel strip.

[0045] According to the variant proposed in FIG. 3, use is made of slotted nozzles 22a, 22b, 22c, 22d having a width which increases in the strip running direction 13, such that firstly only the central region of the steel strip 10 is cooled and it is only toward the end of the quenching unit 19 that the edge regions are also cooled. In order to further homogenize the temperature distribution, provision may be made of slotted nozzles 22f, 22g which are arranged obliquely with respect to the strip running direction 13.

[0046] According to the variant of the quenching unit according to the invention as shown in FIG. 4, provision is made, as in the prior art, of slotted nozzles 22 arranged transversely to the strip running direction 13, but according to the invention these are provided with apertures 31 which can be adjusted in such a way that firstly in turn only the central region of the steel strip 10 is cooled, while the edge regions are cooled only at the end of the quenching unit 19. As symbolized by the arrows 32, the apertures are preferably formed in a movable manner, such that the respective opening can be adapted to different steel grades, strip dimensions or cooling profiles.

[0047] Isotherms of decreasing temperature are shown in turn in FIGS. 3 and 4 by way of the reference signs 30a-30g. The special arrangement or screening of the slotted nozzles achieves a temperature which is constant over the width of the steel strip 10 during the cooling operation.