EP1951451B1 - Method for producing a hot-rolled steel strip and combined casting and rolling installation for carrying out the method - Google Patents

Abstract

A method for the production of a hot-rolled steel strip from a steel melt in a continuous manufacturing process: continuously casting a steel strand in a continuous casting mold of a continuous casting plant, roll-forming the case steel strand in a first group of roll stands into a pre-rolled hot strip, roll-forming the pre-rolled hot strip in a second group of roll stands into a hot-rolled steel strip, setting the hot strip to a rolling temperature between the first and second groups, and winding or dividing the hot-rolled steel strip; further: descaling the hot strip before temperature setting, holding the hot strip in a protective gas atmosphere during temperature setting. A combined casting and rolling plant for the method is disclosed.

Description

The invention relates to a method for producing a hot-rolled steel strip in coils or in sheets from a molten steel in a continuous production process with continuous passage through a combined casting and rolling plant and a combined casting and rolling plant for performing the method.

A process of this type comprises the following process steps: A steel strand is formed from liquid steel by continuous casting in a continuous casting mold of a continuous casting plant. In a subsequent first group of rolling mills, this cast steel strand is roll formed into a pre-rolled hot strip, and in a second set of rolling mills, this rough rolled hot strip is finish rolled into a hot rolled steel strip having the final desired dimensions and material properties. Between the first group of rolling stands and the second group of rolling stands, the pre-rolled hot strip was set to rolling temperature in a temperature adjuster to achieve favorable conditions for finish rolling. After finish rolling, the hot rolled steel strip passes through a cooling section and is wound into coils or cut into sheets.

The prior art discloses, in many variants, methods and arrangements of production equipment for producing a hot-rolled steel strip starting from the liquid phase, which can be assigned to a few basic types of process.

In a batch process for producing a hot-rolled steel strip, liquid steel is cast on a continuous casting machine into a continuous steel strand and cut into slabs with a casting thickness of more than 120 mm or into thin slabs with a casting thickness between 40 mm and 120 mm. Subsequently or with a production-related time interruption these precursors become rolled after tempera- ture equalization or basic reheating to rolling temperature in rolling mills to steel strip of specified target thickness. Usually this is a single- or multi-frame roughing and a multi-stand finishing train used.

From the WO 98/00248 already a combined casting and rolling plant for the production of a steel strip in thermoforming quality is known in which in the continuous casting mold of a continuous casting a steel strand is cast with a casting thickness of less than 100 mm. This cast steel strand is rolled after carrying out a descaling in a multi-stand Vorstraße at least to a winding tape thickness and after passing through an inductively heated furnace in which a non-oxidizing inert gas atmosphere maintained and in which the pre-rolled hot strip is heated to a temperature in the austenite Bound wound and stored in a cache. After rewinding the bundle, the pre-rolled hot strip is fed to a finishing train and rolled in the ferritic microstructure to a final product. Regardless of the specific requirements of the plant layout resulting from the specific steel quality of this prior art, the rolling speed in the finishing train can be product-specifically adjusted by decoupling from the caster. However, the winding and unwinding of the pre-rolled steel strip into bundles and their intermediate storage result in considerably greater investment costs than in a continuous strip pass. In a multi-strand casting plant, such a decoupled production process in which at least the rolling speed in the finishing train can be set independently of the casting speed in the continuous casting plant is absolutely necessary. In single-strand casting this decoupling can be made equally and leads to the disadvantages described.

From the WO 89/11363 a continuous process of the generic type for producing a hot-rolled steel strip is already known, in which the casting speed at the outlet from the continuous casting mold determines the rolling speed in the downstream respective deformation stages as a function of the degree of deformation in the respective deformation stage. Only after leaving the last mill stand, the continuously cast and hot rolled steel strip is divided according to the predetermined coil weight and wound into coils. Before entering the pre-rolled in a roughing mill hot strip in the finishing mill this is brought to a uniform belt temperature, which is above the rolling temperature and then descaled immediately before entering the finishing train. When descaling with water jets there is a loss of temperature, which must be compensated by the previous heating to a temperature above the rolling temperature.

The object of the present invention is therefore to avoid the disadvantages of the known prior art and a method for producing a hot-rolled steel strip and a combined casting and rolling plant for carrying out this method with a continuous and preferably uninterrupted strip pass from the continuous casting mold to the passage through To propose the last deformation stage of the finishing train, wherein by optimizing the sequence of process steps and the sequence of plant components, a minimization of the additional energy input is achieved in the hot strip.

Furthermore, the object of the invention is to minimize the investment costs for the combined casting and rolling plant according to the invention and to reduce the operating costs for the production of the rolled steel strip, in particular the energy costs for the additional strip heating.

Another object of the present invention is to increase the flexibility of the proposed production process and the combined caster and mill in terms of the potential production of hot rolled steel strip in a wide range of steel grades.

The object underlying the invention is achieved by a method of the type described above in that the pre-rolled hot strip is descaled immediately before entering the Tempertureinstelleinrichtung, the pre-rolled hot strip is kept in the temperature setting in a protective gas atmosphere and the pre-rolled hot strip after passing through the temperature adjustment is immediately roll formed in the second group of rolling mills.

Since the hot strip is kept in a protective gas atmosphere after descaling within the temperature adjustment, a further scaling process is largely avoided during heating of the hot strip to rolling temperature, but at least kept in a range that causes no loss of quality on the hot strip surface in the subsequent rolling process, so that an additional Descaling immediately before entry into the rolling stand is not necessary. By the The sequence of the method steps according to the invention ensures that the hot-rolled strip temperature set in the temperature setting device is essentially maintained until entry into the first deformation stage of the second group of rolling stands, and heating of the hot strip to a temperature above the rolling temperature is no longer necessary. Thus, the hot strip temperature can be kept lower by up to 80 ° C than in conventional methods with a belt descaling immediately before the roll stand.

During the heating of the rough-rolled hot strip to rolling temperature and the temperature compensation in the hot strip this is maintained in the temperature adjuster in an inert inert gas atmosphere having an oxygen content of less than 10.0 vol .-%. Preferably, the oxygen content is less than 2.0% by volume. Predominantly the protective gas atmosphere consists of nitrogen.

According to a further embodiment, the pre-rolled hot strip may be maintained in the temperature adjuster during heating to rolling temperature in a reducing inert gas atmosphere. As a result, the oxygen flowing into the protective gas chamber surrounding the temperature adjusting device is occasionally bound by the inlet or outlet openings and scaling at the surface of the strip is avoided.

Preferably, the pre-rolled hot strip is set in the temperature adjuster to a predetermined roll entry temperature. Optimal conditions for the rolling operation in the second group of rolling mills are given when the pre-rolled hot strip is set in the temperature adjuster depending on the current casting speed to a roll inlet temperature, which allows a final rolling temperature in the last stage of deformation of the second group of rolling mills, which in the austenitic Microstructure of the hot strip is. In addition to the actual casting speed, the degree of reduction of the strip thickness in the first group of rolling stands is expediently taken into account.

The pre-rolled hot strip is expediently descaled with water jets at a jet pressure between 200 bar and 450 bar immediately before it enters the temperature setting device. Preferably, rotating, obliquely directed against the surface of the belt water jets are used from a Rotorentzunderungseinrichtung.

The described method can be used particularly advantageously if a steel strand having a steel strand thickness between 50 and 150 mm is cast in the continuous casting mold, then the cast steel strand in a first group of rolling stands to a pre-rolled hot strip having a hot strip thickness between 6.0 and 30 mm The hot rolled strip is then thermoformed in a second group of rolling stands to a hot rolled steel strip having a final steel strip thickness between 0.6 and 5.0 mm.

The casting thickness at the outlet from the continuous casting mold essentially determines the number of downstream rolling stands. Depending on the casting thickness, the roll forming of this cast steel strand is carried out in the first group of rolling stands by at least one rolling stand, preferably by three successive rolling stands. Furthermore, the hot-forming of the pre-rolled hot strip in the second group of rolling stands is carried out by at least two, preferably by three to five successive rolling stands.

According to a possible embodiment of the invention, the pre-rolled hot strip is transversely divided between the first group of rolling stands and the descaling device. Thus, the hot working of the cast steel strand in the first group of rolling stands is coupled to the casting process and the further hot working in the second group of rolling stands is decoupled from the casting process and thus can be carried out uninfluenced by this. This opens up the possibility, preferably for thicker strips, of finish-rolling them in the conventional tapping process.

A combined casting and rolling plant for carrying out the process according to the invention comprises a continuous casting plant with a continuous casting mold for producing a cast steel strand, a first group of rolling stands for roll forming the cast steel strand into a pre-rolled hot strip, a second group of rolling stands for roll forming the rough rolled hot strip into one hot rolled steel strip, a temperature adjuster between the first group of rolling stands and the second group of rolling stands, and a belt coiler for winding the hot rolled steel strip into coils or a dicing device for cutting the hot rolled steel strip into sheets. Preferably, a Bogenstranggießanlage is used with an oscillating continuous casting mold.

To achieve the object, the combined casting and rolling plant is characterized in that the temperature adjusting is arranged in a closed protective gas chamber, which is equipped with inlet and outlet openings for the pre-rolled hot strip and supply lines for a protective gas that descaling the protective gas chamber immediately is upstream and that the second group of rolling stands directly adjoins the protective gas chamber. This arrangement ensures that no relevant scaling of the steel strip and on the transport of the steel strip from the exit from the protective gas chamber to the entrance to the first downstream rolling stand on the transport of the steel strip from entry into the protective gas chamber to the entrance to the first downstream mill relevant temperature loss occurs.

The temperature setting device is preferably designed as an induction heating device, since this allows for increased strip edge heating and possibly a zone-dependent different heating of the steel strip within its short cycle time. Furthermore, such an inductive heating device allows a very compact design of the temperature adjusting device, whereby the construction and operation of the protective gas chamber is cost-effective.

According to a preferred embodiment, the descaling device is formed by at least one rotor descaling device. Such a Rotorentzunderungseinrichtung is for example from the EP 640 413 already known for the descaling of a rolling stock immediately before a roll stand. Suitably, a plurality of rotor descaling devices are arranged parallel to the inlet opening of the protective gas chamber immediately in front of this.

To largely avoid renewed scaling of the steel strip prior to entry into the first rolling stand of the second group of rolling stands, the outlet opening of the protective gas chamber comprises an outlet channel which is at most 5.0 m, preferably at most 3.0 m, in front of the nip of the first rolling stand of the second Group of rolling stands ends. Experience has shown that on this short path, although there is a renewed build-up of a scale layer with a scale layer thickness of not more than 8 .mu.m, the strip speeds occurring there do not cause any problems for the surface quality of the rolling stock.

According to a possible variant of the combined casting and rolling plant, a transverse dividing shear, preferably designed as a pendulum shear, for cross-dividing the pre-rolled hot strip is arranged between the first group of rolling stands and the descaling device. The separated hot strip sections can be finished by the conventional tapping process.

Fig. 1

eine schematische Darstellung einer kombinierten Gieß- und Walzanlage zur Herstellung eines warmgewalzten Stahlbandes,

Fig. 2

den Temperaturverlauf am Stahlband nach dem Austritt aus einer Induktionsheizeinrichtung gemäß dem Stand der Technik mit einer Entzunderung vor der zweiten Gruppe von Walzgerüsten,

Fig. 3

den Temperaturverlauf am Stahlband nach dem Austritt aus einer Induktionsheizeinrichtung gemäß dem erfindungsgemäßen Verfahren mit einer Entzunderung vor der Induktionsheizeinrichtung.

Further advantages and features of the present invention will become apparent from the following description of a non-limiting embodiment, reference being made to accompanying figures which show:

Fig. 1

a schematic representation of a combined casting and rolling plant for producing a hot-rolled steel strip,

Fig. 2

the temperature profile on the steel strip after emerging from an induction heating device according to the prior art with a descaling before the second group of rolling stands,

Fig. 3

the temperature profile on the steel strip after emerging from an induction heater according to the inventive method with a descaling in front of the induction heater.

A combined casting and rolling plant for producing a hot-rolled steel strip from liquid steel in a continuous in-line production process comprises a Bogenstranggießanlage 1 of conventional design, which in FIG. 1 is shown schematically only by a continuous casting mold 2 and some of the following strand guide rollers 3 in the direction indicated by the arcuate steel strand course strand guide 4. In the cooled continuous casting mold 1, liquid steel is formed into a steel strand 5 with a slab or thin slab cross section. Typical steel strand thicknesses are between 40 and 150 mm. The casting speed for these systems is between 4.0 and 8.0 m / min and is very much dependent on steel products.

The cast steel strand 5 deflected in the strand guide 4 of the continuous casting plant in a horizontal belt transport direction R further passes through a first group of rolling stands 6 consisting of three rolling stands 6a, 6b, 6c, which form a roughing group and in which the cast steel strip 5 is pre-rolled Hot strip 7 is formed with a hot strip thickness between 6 and 30 mm. Here is the cast Steel strip in each of the rolling stands with a reduction of up to 60% thickness reduced.

In this plant area usually other plant components are positioned, which are not shown here in detail, such as a straightening unit at the end of the continuous casting for straightening the cast steel strand in the horizontal tape transport direction, an emergency cutting before or after the first group of rolling stands, the In addition to severing the Anfahrstrangkopfes is used, a dividing shear between the first group of rolling stands and the descaling or the protective gas chamber for occasionally shredding scrap boards and a descaling before the first roll stand group for removing the surface scale of the cast steel strand. In addition, one or more deformation units 8 formed by drive rollers may be arranged in the strand guide for a reduction in the thickness of the steel strand while the core is still liquid core (liquid core soft-reduction).

After the first rolling deformation in the first group of rolling stands 6, the pre-rolled hot strip 7 is descaled on both sides in a descaling device 9. This descaling device comprises a number of Rotorentzunderungseinrichtungen 10, which are arranged in at least one row transverse to the strip direction R immediately before a protective gas chamber 11. With the Rotorentzunderungseinrichtungen 10 rotating water jets are directed at a jet pressure of 200 to 450 bar obliquely against the surface of the pre-rolled hot strip and achieved a nearly complete descaling of the strip surface. Other descaling devices may be used.

Immediately to the descaling 9 includes a protective gas chamber 11, which is equipped with an inlet opening 12 and an outlet opening 13 for the passage of the pre-rolled hot strip and the temperature adjusting means 14 for reheating and for temperature compensation of the hot strip. These temperature adjusting devices are designed as induction heating devices 15, whereby a rapid introduction of heat into the hot strip moved past the heating devices is ensured as needed. A zone-related heating of the hot strip is thereby possible, in particular an increased heating in the band edge area. The hot strip is brought in the protective gas chamber to a necessary for the subsequent finish rolling roller inlet temperature, at least depend on the number of subsequent rolling stands and the desired target material properties of the finish rolled steel strip.

The protective gas chamber 11 is equipped with supply lines 16, 17 and with corresponding control devices for the supply and removal of a largely inert or reducing protective gas to maintain a predetermined protective gas atmosphere.

The outlet opening 13 of the protective gas chamber 11 comprises an outlet channel 18, which encloses the pre-rolled hot strip, shields the room atmosphere and leads to the second group of rolling stands 19. The free distance A between the outlet opening 13 of the outlet channel 18 to the nip 20 of the first stand 19a is not more than 5.0 m in order to avoid a relevant rescaling of the hot strip. The four rolling stands 19a to 19d form a finishing train in which the pre-rolled hot strip 7 is finished to a hot rolled steel strip 21 having the intended target thickness of between 0.6 and 5.0 mm and finished with the predetermined material properties.

Finally, the steel strip 21 is transversely divided with a transverse dividing shear 22 and wound into coils in a band reeling device 23. Similarly, the steel strip can be cut with the dividing shears into sheets, which are then stacked in a stacking system to sheet packages.

The dividing shear 24 shown by dashed lines between the first group of rolling stands 6 and the descaling 9 allows decoupling of the combined casting and rolling process at this point, so that the pre-rolled hot strip 7, especially in larger hot strip thicknesses in the conventional tapping in the second subgroup of Rolling stands 19 can occur.

The invention is by no means limited to a second group of rolling stands of the type described. It is quite possible that intermediate stand heaters are arranged between individual rolling mills of the second roll stand group, with which the strip temperature is raised, if there are metallurgical or rolling technical requirements to do so. Furthermore, further individual rolling stands or groups of rolling stands can be arranged before or after the transverse dividing shear.

In the FIGS. 2 and 3 the temperature profile on the pre-rolled hot strip (pre-strip) from the exit from the temperature setting or the Protective gas chamber set up to reach the final roll thickness in the nip of the last rolling mill of a set of five rolling stands second groups of rolling stands (finishing scale) for two procedures compared. In FIG. 2 FIG. 3 shows the temperature profile of the pre-strip in a system configuration which corresponds to the prior art, in which the descaling device is arranged between the temperature setting device and the finishing scale. In FIG. 3 In contrast, the temperature profile of Vorvorreifens is shown in a system configuration according to the invention, in which the descaling before the protective gas chamber and the protective gas chamber is located as close to the finishing scale.

Both temperature gradients are based on the production of a hot-rolled steel strip of the grade DD12 with a final finish thickness of 1.0 mm. The maximum temperature when leaving the temperature setting device or the protective gas chamber is typically about 1250 ° C. for this steel quality. At higher temperatures undesirable local melting of the tape can occur. In the case of the arrangement of a descaling device before the finishing scale, the descaling results in an average temperature loss of about 70 ° C. before the pre-strip enters the finishing scale.

The minimum casting speed to reach a final rolling temperature in the austenitic microstructure region of 850 ° C in the last stand is 6.3 m / min in the illustrated case with a steel strand thickness of 70 mm at the outlet from the continuous casting mold. The pre-strip thickness after exit from the first group of three rolling stands is 14 mm. The temperature loss at the descaling device is in this case ( FIG. 2 ) about 95 ° C, the inlet temperature in the first rolling stand of the finishing scale is about 1110 ° C.

It follows that a final rolling temperature above 850 ° C can not be achieved for casting grades below 6.3 m / min and the boundary conditions described above for this steel grade. Thus, the required quality standards, which result from the microstructure during rolling, can not be achieved.

FIG. 3 shows the temperature profile at a pre-strip, in which the descaling was done before entering the protective gas chamber and is used after exiting the protective gas chamber directly into the finishing scale. Here is the Protective gas chamber 3.0 m away from the nip of the first rolling stand. All other boundary conditions (steel quality, final thickness, starting temperature, etc.) correspond to the boundary conditions of the comparative example.

Starting at 1250 ° C at the exit from the protective gas chamber, the inlet temperature in the first rolling stand of the finishing scale is now at approx. 1185 ° C. A further increase in the inlet temperature of 20 to 30 ° C can be achieved by a corresponding thermal insulation of the protective gas chamber and the outlet channel to close to the first rolling stand of the finishing scale. In this case, the minimum casting speed for achieving a target final rolling temperature of 850 ° C. under these boundary conditions is 5.8 m / min, ie 0.5 m / min less than in the system configuration according to the prior art.

Since not all steel grades can be produced with equally high casting speeds, the field of application and the flexibility of the combined casting and rolling plant according to the invention are significantly expanded by the proposed method.

Claims (18)

1. Method for the production of a hot-rolled steel strip in bundles or in sheets from a steel melt in a continuous manufacturing process, with an uninterrupted strip runthrough, having the following method steps:

   - continuous casting of a steel strand (5) in a continuous casting mold (2) of a continuous casting plant (1),

   - roll-forming of the cast steel strand in a first group of roll stands (6) into a pre-rolled hot strip (7),

   - roll-forming of the pre-rolled hot strip in a second group of roll stands (19) into a hot-rolled steel strip (21),

   - setting of the pre-rolled hot strip to the rolling temperature between the first group of roll stands and the second group of roll stands in a temperature setting device (14),

   - winding of the hot-rolled steel strip into bundles or division of the hot-rolled steel strip into sheets,

   characterized in that

   - the pre-rolled hot strip (7) is descaled immediately before entry into the temperature setting device (14),

   - the pre-rolled hot strip (7) is held in a protective gas atmosphere in the temperature setting device (14), and

   - the pre-rolled hot strip (7) , after running through the temperature setting device (14), is roll-formed immediately thereafter in the second group of roll stands (19).
2. Method according to Claim 1, characterized in that the pre-rolled hot strip is held in the temperature setting device in an inert protective gas atmosphere having an oxygen content of less than 10%, preferably an oxygen content of less than 2%.
3. Method according to Claim 1, characterized in that the pre-rolled hot strip is held in a reducing protective gas atmosphere in the temperature setting device.
4. Method according to one of the preceding claims, characterized in that the pre-rolled hot strip is set to a roll entry temperature in the temperature setting device.
5. Method according to Claim 4, characterized in that the pre-rolled hot strip is set in the temperature setting device, as a function of the current casting speed, to a roll entry temperature which makes it possible to have, in the last forming stage of the second group of roll stands, a final rolling temperature which lies in the austenitic structural range of the hot strip.
6. Method according to one of the preceding claims, characterized in that the pre-rolled hot strip, before entry into the temperature setting device, is descaled by means of water jets at a nozzle admission pressure of between 200 bar and 450 bar.
7. Method according to one of the preceding claims, characterized in that roll-forming in the first group of roll stands takes place by means of at least one roll stand, preferably by means of three successive roll stands.
8. Method according to one of the preceding claims, characterized in that the roll-forming in the second group of roll stands takes place by means of at least two, preferably three to five successive roll stands.
9. Method according to one of the preceding claims,
   characterized in that

   - a steel strand having a steel strand thickness of between 50 and 150 mm is cast in the continuous casing mold,

   - the cast steel strand is roll-formed in a first group of roll stands into a pre-rolled hot strip having a hot strip thickness of between 6.0 and 30 mm,

   - the pre-rolled hot strip is hot-formed in a second group of roll stands into a hot-rolled steel strip having a final steel strip thickness of between 0.6 and 5.0 mm.
10. Method according to one of the preceding claims, characterized in that the pre-rolled hot strip is divided transversely between the first group of roll stands and the descaling device.
11. Combined casting and rolling plant for carrying out the method according to one of Claims 1 to 10,

    - with a continuous casting mold (2) in a continuous casting plant (1) for the production of a cast steel strand (5),

    - with a first group of roll stands (6) for the roll-forming of the cast steel strand into a pre-rolled hot strip (7),

    - with a second group of roll stands (19) for the roll-forming of the pre-rolled hot strip into a hot-rolled steel strip (21),

    - with a temperature setting device (14) between the first group of roll stands (6) and the second group of roll stands (19), and

    - with a strip coiling device (23) for winding the hot-rolled steel strip into bundles or with a dividing device for dividing the hot-rolled steel strip into sheets,

    characterized in that

    - the temperature setting device (14) is arranged in a closed protective gas chamber (11) which is equipped with entry and exit orifices (12, 13) for the pre-rolled hot strip and with supply lines (16, 17) for a protective gas,

    - a descaling device (9) directly precedes the protective gas chamber (11),

    - the second group of roll stands (19) directly follows the protective gas chamber (11).
12. Combined casting and rolling plant according to Claim 11, characterized in that the temperature setting device (14) is formed by an induction heating device (15).
13. Combined casting and rolling plant according to Claim 11 or 12, characterized in that the descaling device (9) is formed by at least one rotor descaling device (10).
14. Combined casting and rolling plant according to Claim 13, characterized in that a plurality of rotor descaling devices (10) are arranged directly upstream of the protective gas chamber (11), parallel to the entry orifice (12) of the latter.
15. Combined casting and rolling plant according to one of Claims 11 to 14, characterized in that the first group of roll stands (6) is formed by at least one roll stand, preferably by three successive roll stands (6a, 6b, 6c).
16. Combined casting and rolling plant according to one of Claims 11 to 15, characterized in that the second group of roll stands (19) is formed by at least two roll stands, preferably by three to five roll stands (19a, 19b, 19c, 19d).
17. Combined casting and rolling plant according to one of Claims 11 to 16, characterized in that the exit orifice (13) of the protective gas chamber (11) comprises an exit duct (18) which terminates at most 5.0 m, preferably at most 3.0 m, upstream of the roll nip (20) of the first roll stand (19a) of the second group of roll stands (19).
18. Combined casting and rolling plant according to one of Claims 11 to 17, characterized in that cross-dividing shears (24) are arranged between the first group of roll stands (6) and the descaling device (9) for the transverse division of the pre-rolled hot strip.

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