METHOD FOR PRODUCING STEEL STRIP

Abstract

A method for producing steel strip, in particular hot strip in the form of coiled coils or in the form of folded individual sheets, in which a steel melt is first produced, this is then formed into a strand in a continuous casting system, the strand is then fed into a heating unit and the heated strand is then rolled into hot strip in a subsequent rolling mill. The casting of the strand, the passage through the heating unit, and the rolling take place in a continuous process. To be able to produce hot-rolled steel strips in the most energy-efficient way possible and to make these strips available for further processing into high-quality cold-rolled and, if necessary, coated strips, the invention provides that, first of all, a steel melt is produced.

Claims

1-18. (canceled)

19. A method for producing steel strip, in particular hot strip, in the form of wound coils or in the form of folded individual sheets, in which first a steel melt is produced, then this steel melt is formed into a strand in a continuous casting system, then the strand is fed, either undivided or divided into individual slabs, into a heating unit, and then the heated strand or the heated slabs are rolled into strip in a downstream rolling mill, wherein, first, a steel melt is produced which has the following chemical composition: maximum 0.02 wt. % carbon, preferably less than 0.01 wt. % carbon, 0.01 to 3.5 wt. % silicon, preferably less than 0.1 wt. % silicon, maximum 2.5 wt. % manganese, preferably less than 1.0 wt. % manganese, 0.01 to 0.20 wt. % copper, preferably less than 0.15 wt. % copper, maximum 0.40 wt. % chromium and nickel, preferably less than 0.20 wt. % chromium and nickel, niobium, titanium, vanadium, and boron, each at less than 0.10 wt. %, preferably titanium, vanadium, and boron at less than 0.05 wt. %, maximum 70 ppm nitrogen, preferably less than 50 ppm nitrogen, optionally other elements without iron in a proportion of less than 1.0 wt. %, which are specifically added or which enter the melt as an unavoidable admixture via the input materials, and residual iron content, wherein the preparation of the steel melt comprises the steps of: a) melting of solid, ferrous starting material in a smelting unit; b) continuously feeding solid starting materials containing iron and carbon as well as air, oxygen and/or natural gas into the smelting unit to achieve a continuously strong boiling reaction in the flat bath phase over a period of between 2 and 30 min, preferably between 10 and 20 min; c) feeding the melt into a vacuum system and decarburizing the melt in the vacuum system at a maximum decarburization rate of 180 ppm/min carbon; wherein this is followed by the steps: d) feeding the melt thus pretreated into the continuous casting system; e) casting the melt in the continuously operating continuous casting system; f) feeding the strand or the slabs produced therefrom into the heating unit and setting the required rolling temperature, wherein the strand or the slab enters the heating unit directly at a temperature greater than A.sub.3-20 K, such that the volume fraction of ferrite in the near-surface regions of the strand or the slab is less than 5 vol % down to a depth of at least 5 mm, preferably down to a depth of 10 mm; g) feeding the strand or slabs into the rolling mill and rolling out the strand or slabs into the strip.

20. The method according to claim 19, wherein step a) is carried out such that the proportion of solid starting materials corresponds to 10 to 70% of the total charge weight.

21. The method according to claim 19, wherein step a) is carried out in such a way that the solid starting materials are at least partially replaced by liquid input materials.

22. The method according to claim 19, wherein step b) is carried out in such a way that an addition of at least 20 kg of carbon per minute, preferably between 30 kg and 150 kg of carbon per minute, into the melt results.

23. The method according to claim 19, wherein the material continuously fed according to step b) has an average carbon content of at least 0.5 wt. %, preferably between 1.0 and 3.5 wt. %.

24. The method according to claim 19, wherein step b) is carried out such that the melt has a nitrogen content of 5 to 60 ppm, preferably less than 30 ppm, prior to tapping from the smelting unit.

25. Method according to claim 19, characterized in that step c) is carried out to achieve an average decarburization rate between 30 ppm/min and 60 ppm/min, preferably between 40 ppm/min and 50 ppm/min.

26. The method according to claim 19, wherein the melt has a carbon content of 0.0005 to 0.01 wt. %, preferably below 0.0040 wt. %, before carrying out step d).

27. The method according to claim 19, wherein the strand downstream of the last segment of the continuous casting system has a surface temperature (T.sub.1) of at least A.sub.3-20 K, preferably above 800° C., when step e) is carried out.

28. The method according to claim 19, wherein the slabs are discharged from the production line for finishing before carrying out step f), in particular for carrying out inspection work, repairing surface defects, and for dividing.

29. The method according to claim 28, wherein the discharged slabs are fed to the heating unit after finishing and heated to the required rolling temperature.

30. The method according to claim 19, wherein the casting of the strand, the passage through the heating unit, and the rolling take place in a continuous process.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0075] The drawing shows an exemplary embodiment of the invention. The single FIGURE schematically shows the preparation of steel melt, a downstream continuous casting system with a downstream heating unit and rolling mill.

DETAILED DESCRIPTION

[0076] The FIGURE schematically shows a production line that can be used to manufacture hot-rolled strip 1.

[0077] First, starting material is melted in an electric arc furnace 6. The melt is then fed to a vacuum system 7, where it undergoes secondary metallurgical treatment. The ready-to-cast melt then enters a continuous casting system 2, in which a strand 3 (slab) is cast in a known manner. Strand 3 has the temperature T.sub.1 immediately downstream of the continuous casting system 2 (namely downstream of its last segment).

[0078] The slab then enters a heating unit 4, in which the slab is heated to a temperature T2 at which it then enters the rolling mill 5 and is rolled into the finished hot strip 1.

[0079] The method described can be used to produce highest-quality steels (for example for outer skin grades in the automotive industry) by means of electric arc furnaces by selecting the input materials, optimizing the process control, preventing non-metallic inclusions, and synchronizing with downstream process steps (in particular in the form of vacuum decarburization, i.e., secondary metallurgical treatment) by continuous casting into a strand 3 having a thickness of 90 to 310 mm.

[0080] The crude steel preferably has a maximum carbon content of 0.020 wt. % when tapped from the smelting unit and, as mentioned, is produced in the electric arc furnace 6. Preferably, solid metallic input materials are used, wherein the process control enables the production of a crude steel with low contents of undesirable accompanying elements (Cu, Cr, Ni) and the lowest contents of gases (nitrogen, hydrogen). The crude steel produced is decarburized in the vacuum system 7 and subsequently formed into strand 3 on the continuously producing casting machine 2.

[0081] In particular, scrap, pig iron, and sponge iron (DRI and/or HBI) are used as metallic input materials, which lead to a low sulfur input.

[0082] The metallic input materials also have a low content of undesirable accompanying elements.

[0083] New input material can be added with respect to the undesired steel companion and adapted to the brand of steel to be produced.

[0084] The metallic input materials are selected in such a way that a total carbon input of at least 1 wt. % is possible.

[0085] The metallic input materials are preferably fed in such a way that a strong boiling reaction is present throughout the flat bath phase, which is ensured by the addition of at least 65 kg carbon/min.

[0086] Preferably, melting and slagging are carried out in such a way that a nitrogen content below 30 ppm is achieved in the liquid steel before tapping.

[0087] Decarburization of the crude steel melt takes place in vacuum system 7 at a maximum decarburization rate of 120 ppm/min carbon down to carbon contents below 0.010 wt. % before delivery to the casting machine.

[0088] The decarburization of the crude steel melt in the vacuum system 7 is further preferably carried out in such a way that the system is operated at an average decarburization rate of 40 to 50 ppm/min carbon during the entire decarburization phase.

[0089] The secondary metallurgical treatment can also be provided for deoxidizing the decarburized steel melt in the vacuum system 7 and for adjusting the target composition and temperature homogeneity in the vacuum system or, if necessary, in a downstream atmospheric treatment plant.

[0090] The melt is poured on the continuously operating casting machine 2, at an outlet temperature from the last segment (temperature T.sub.1) at the surface preferably of at least 800° C.

[0091] The production time of the continuous casting system 2 preferably comprises at least four melts cast continuously in succession.

[0092] Furthermore, the slab 3 produced in this way is fed directly into the downstream heating unit 4 to set an average discharge temperature (temperature T2) between 1,050° C. and 1,280° C.

[0093] An automatic surface inspection of the slab 3 can be performed between the outlet of the slab 3 from the last segment of the continuous casting system 2 and its entry into the downstream heating unit 4.

[0094] Slabs 3 with surface defects can be automatically discharged from the production line and repaired after cooling. Repaired slabs can be returned to the production process.

[0095] Thus, the basic concept of the proposed method is aimed at arranging the processes of steel melting in the electric arc furnace 6, vacuum treatment in the vacuum unit 7, and continuous casting of the slabs 3, preferably with a thickness greater than 110 mm, in such a way that the slabs 3 leaving the continuous casting system 2 have sufficiently high temperatures so that they can be inserted into the heating unit 4 (preferably a walking beam furnace) without risk of surface defects.

[0096] To ensure the main requirement of high slab temperatures at the entry to the heating unit 4, the entire process upstream is optimized for high throughput rates.

[0097] Accordingly, a high slab temperature results in a high casting speed and from this in turn a rapid provision of the melt from the vacuum system 7, which in turn leads to short treatment times in the electric arc furnace 6.

[0098] The short treatment times in the electric arc furnace 6 while limiting the nitrogen content in the steel require a high boiling reaction in the bath and a constant decarburization rate during the melting phase, as described above. Continuous pumping of DRI and/or other ferrous and carbonaceous input materials promotes this.

[0099] Rapid treatment in a vacuum paired with simultaneous reduction of the carbon content to minimum values is favored by the required minimum decarburization rate.

[0100] Thus, the proposed concept is based on a coupled process with several units arranged one after the other, whose processes are logistically linked in such a way that at the end the slabs 3 can be fed directly into the heating unit 4 without subsequently forming surface defects.

[0101] The method according to the invention from steel production to steel strip can be controlled and/or regulated by means of a higher-level process control system.

LIST OF REFERENCE SYMBOLS

[0102] 1 hot strip [0103] 2 continuous casting system [0104] 3 strand (slab) [0105] 4 heating unit (reheating unit) [0106] rolling mill [0107] 6 smelting unit (electric arc furnace) [0108] 7 vacuum system [0109] T.sub.1 Temperature of the strand downstream of the last segment of the continuous casting system [0110] T.sub.2 Temperature of the strand at the outlet from the heating unit