DE102020214427A1 - Method for producing a hot strip by means of a casting and rolling plant - Google Patents

Abstract

The invention relates to a method for producing a hot strip by means of a casting and rolling plant. Hot strips that are wound into a coil at a coiling temperature that is too low develop strain hardening. This hinders the subsequent cold rolling process. The work hardening is reduced or avoided by the method according to the invention. The method includes temperature control and a targeted setting of structural components during the forming in the different manufacturing steps.

Description

Area:

The invention relates to a method for producing a hot strip by means of a casting and rolling plant.

State of the art:

For the production of hot strips with properties that approximate the properties of cold strips, in particular dimensions and / or mechanical properties, casting and rolling plants are used, for example. In a casting and rolling process of this type, a slab is first cast in the continuous caster and this is then rolled out to form a hot strip directly after the continuous casting. The mechanical properties of the hot strip result from the structure or mixed structure present in the finished hot strip. The development of the structure or mixed structure is set by the temperature control starting from the continuous casting in connection with the forming in the rolling train.

To avoid an unwanted austenite / ferrite mixed structure, the final rolling temperature during hot forming of the hot strip in the rolling train is usually below the austenite-ferrite transformation temperature. As a result, the austenite was converted to ferrite before the last deformation. Following the last forming step, the hot strip is reeled into a coil at a comparatively low temperature. Due to the low temperature of the hot strip in the coil, a strongly work-hardened ferritic structure is formed, which no longer has sufficient cold forming properties for optimal further processing in a cold rolling mill. This work hardening occurs particularly frequently in the case of hot strips with a thickness of> 2.5 mm, since from this thickness the compression or stretching in the edge areas of the strip exceeds the material limits during reeling.

Hot strips produced in this way require additional work steps in a cold rolling mill to achieve the desired end properties, in particular the thickness, of the cold strip. An additional intermediate annealing to reduce the work hardening is mostly necessary here.

An exemplary structure of a casting and rolling plant can be found in the publication DE 196 13 718 C1 can be removed. A method and apparatus for hot rolling hot strip is disclosed in US Pat WO 2011/138 159 A1 disclosed. the KR 2001 004 857 describes a process in which the coil has to be transferred to another plant after the casting and rolling plant. In this ferritic rolling then takes place with a slight cooling to the coiling temperature.

Possible final rolling temperatures or final rolling temperatures for hot rolling are in the CN 107 597 844 A described. The high final rolling temperature in a range of 730 ° C to 850 ° C in connection with a coiling temperature of 580 ° C to 710 ° C ensures that the ferrite grains recrystallize. Subsequent processing is not provided here. The publication describes analogously CN 108 994 081 A a similar process in which a coiling temperature of 650 ° C to 700 ° C also leads to a recrystallization of the ferrite structure.

The disadvantage of the known methods is the continued work hardening of the hot strip by reeling the hot strip, which can only be reduced by at least one additional processing step during cold rolling of the hot strip to form a cold strip.

Object of the invention:

The object of the invention is therefore to further develop a known method for producing cold strips from hot strips in such a way that the hot strip produced has less work hardening after coiling and thus the cold rolling process is simplified.

Invention:

The object of the invention is achieved by a method having the features of claim 1. A casting and rolling plant has at least the following units: a continuous casting plant, a first temperature control element, in particular a furnace, a roughing train, preferably having 2 to 4 roll stands, a second temperature control element for heating and / or cooling, preferably a furnace, an inductive heater and / or a Transfer bar cooling, preferably transfer bar cooling, downstream of the roughing train, a finishing train, preferably having 5 to 8 finishing stands, a cooling section, preferably with cooling that starts directly after the last removal pass and a reel for winding the hot strip into a coil. The method according to the invention, in which at least the following process steps take place, is carried out by means of the casting and rolling plant: A slab is cast in a strand by means of the continuous casting plant. After completion of the casting process and before the start of the first acceptance pass, the slab is kept at or heated to the rolling temperature. Depending on the casting conditions, the slab can also be cooled with temperature equalization.

The rough rolling of the slab to a roughing strip in the roughing train or with the roughing stands of the roughing train takes place in an austenitic temperature range of the slab material. In the sense of the invention, the structure of the slab has more than 70%, preferably more than 95% austenite. The local proportion of austenite in the structure can fluctuate very strongly due to the segregation during solidification. Therefore, when determining the structural composition, the edge, the transition area and the core of the slab must be taken into account.

The temperature of the pre-strip is set before the first pass in a finishing stand of the finishing train and after the last acceptance pass in a pre-rolling stand. A temperature is set that triggers a conversion of austenite to ferrite. In the sense of the invention, this conversion does not have to be completed before the first pass in the finishing train. It is sufficient if the proportion of ferrite is greater than the proportion of austenite in the structure. The temperature setting, in particular the temperature control between the last roughing stand and the first finishing stand, can also include additional heating or holding steps for setting the desired structure.

The roughing strip, which at least partially consists of a ferritic structure, is rolled into a hot strip by means of the finishing rolling train or finishing stands of the finishing rolling train. The hot strip preferably has a temperature below 800 ° C. in the region of the finishing rolling stands. Following the last acceptance pass in the finishing stand, the hot strip is cooled directly to a temperature below the recrystallization temperature of the material. Directly in the sense of the invention means that the time course of the cooling takes place in such a way that undesired recrystallization and / or recovery processes in the structure are at least reduced, preferably completely avoided. Time courses can be taken from material-specific diagrams or determined using simulation programs. Cooling media are, for example, air, gas, water and / or corresponding mixtures that are brought into contact with the hot strip on one or both sides. The hot strip is then annealed in a recrystallizing manner.

Further preferred embodiments of the method are given in the dependent claims 2 to 18 shown. A temperature of the pre-strip is preferably set at which the conversion of austenite to ferrite takes place within the finishing train, preferably after the 2nd stand of the finishing train. As a result, ferrite is also formed within the finishing train. This has a positive effect on the microstructure, in particular on the grain size.

The hot strip ideally has more than 50%, preferably more than 65%, even more preferably more than 80%, ferrite in its structure during hot forming in the finishing rolling train. With increasing ferrite content in the finishing train, the properties of the hot strip improve.

The slab is preferably cast with a casting thickness of 100 mm - 150 mm, more preferably with a casting thickness of 120 mm - 140 mm. From these slabs, hot strip can be rolled into hot strip without exceeding the maximum degree of deformation of the material used, which can be rolled into cold strip without intermediate steps.

The pre-strip is preferably rolled to a thickness of 25 mm to 100 mm. This means that a large part of the forming work, i.e. the reduction in thickness, is carried out on an austenitic structure. Austenitic structures have a lower resistance to deformation. This reduces the effort required to achieve the final thickness in the finishing train.

The finished hot strip preferably has a thickness of 0.5 mm to 3.2 mm. Cold strips are used in this size range. This means that hot strips with cold strip properties can be used immediately.

The final rolling temperature in the finishing train is preferably 650 ° C to 800 ° C. This temperature range has proven to be ideal for the desired structure formation.

Preferably at least 30% to 60% of the deformation in the finishing train takes place in the range below 750 ° C. This is to ensure that the material no longer completely recrystallizes during rolling in the finishing train.

After the last acceptance pass, the hot strip is cooled from the final rolling temperature to a temperature of ≤ 450 ° C, ideally within 2 seconds.

The slab material preferably has the chemical analysis (in% by weight) C <0.08%, Si <0.1%, Mn <1%, Nb <0.1%, Ti <0.1%, C, Cr, Ni each < 0.5%, V <0.1%, remainder iron, preferably optionally other elements, as well as unavoidable impurities in the range of the usual contents, such as they occur in steel production. This group of materials can be used directly as hot strip with cold rolling properties.

In the production of the hot strip, a casting and rolling plant is used which additionally has at least one of the units interframe cooling, shears and / or scale washer. The use of these units in the casting and rolling plant serves to ensure plant and process safety. The temperature control can be better controlled by cooling between the stands. A scale washer can be used to remove scale from the surface of the slab or hot strip. A hot strip can be divided using the scissors.

The hot strip is pickled, preferably before and / or after hot forming. In this way, surface defects can be reduced or preferably avoided entirely.

The slab temperature is equalized before the first piercing in a furnace after continuous casting. Equalizing the slab temperature between the core and the edge of the slab reduces the property differences between the edge and core of the slab. This has a positive effect on the forming properties of the slab or hot strip or the structure development in the course of production. Depending on the cast structure and the deformation properties, the temperature compensation does not take place completely. In this way, for example, greater deformation in the core of the slab can be achieved or reduced.

A process model connected to a control or regulation of the casting and rolling plant or one of the units of the casting and rolling plant determines the temperature control and transfers this to the control or regulation of the casting and rolling plant or the units of the casting and rolling plant. Connected or cross-unit control and regulation concepts improve process stability and enable manufacturing tolerances to be reduced.

A temperature of the pre-strip is set at which the conversion of austenite to ferrite takes place within the finishing train, preferably after the 2nd stand of the finishing train. This enables a sufficiently high degree of deformation in the area of the ferrite, which has a positive effect on the structure to be achieved later.

Furthermore, the object of the invention is achieved by a method having the features of claim 17. After the hot forming, the hot strip according to the invention is cold rolled and the decrease in thickness during cold rolling is 70%. Before a recrystallizing annealing of the cold strip, a further additional cold rolling pass, preferably a skin pass, with a thickness reduction of 5% can optionally take place. A hot strip produced according to the invention is further processed in an optimal manner by means of these method steps.

• 1: Beispiel einer Gießwalzanlage
• 2: Entwicklung der Gefügeanteile im Warmband im Prozessverlauf
• 3: Diagramm Gefüge-/ Temperaturverlauf in der Fertigwalzstraße

Three figures are attached to the description of the invention:

• 1 : Example of a casting and rolling plant
• 2 : Development of the structural components in the hot strip in the course of the process
• 3 : Diagram of structure / temperature profile in the finishing train

The invention is described in detail below with reference to the figures. The same technical elements are provided with the same reference symbols in all figures.

1 shows the schematic structure of a casting and rolling plant for the production of hot strip. A continuous caster 1 creates a slab or thin slab from a liquid melt. By means of a first temperature actuator 2 is the temperature of the slab before the first pass in the roughing mill 3 influenced. The temperature actuator 2 can increase or cool the temperature of the slab and / or change the temperature difference between the core and the outer layer of the slab. The temperature actuator can do this 2 have gas-fired ovens, inductive ovens, isolated areas and / or cooling devices.

After the first temperature actuator 2 a scale washer is arranged. The scale washer cleans the surface of the slab and thus reduces surface defects on the finished hot strip. The roughing mill 3 consists here of two roll stands R1, R2. A roughing strip is produced from the slab by the two roughing stands R1, R2. Using the subsequent scissors, the pre-strip is divided and thus adapted to the planned coil weight.

The temperature of the sliver is controlled by a second temperature control element 4th influenced. The temperature actuator 4th can increase or cool the temperature of the sliver and / or change the temperature difference between the core and the edge layer of the sliver. The second temperature control element can do this 4th have gas-fired ovens, inductive ovens, isolated areas and / or cooling devices.

The finishing mill 5 consists of seven finishing stands F1 to F7 without intermediate stand cooling. The pre-strip is formed into a hot strip by the seven finishing stands F1 to F7. Following the finishing stands F1 to F7, the hot strip runs through a cooling section 6th . The cooling section 6th has different cooling units, which can represent different cooling curves by means of different cooling media and cooling intensities. After cooling from the final rolling temperature to the coiling temperature, the hot strip is wound into a coil by means of the coiler. The coil can be deposited in air or with temperature control.

The casting and rolling plant is a superordinate regulation 8th assigned. This monitors, regulates and / or influences the process control of all units or at least the temperature control in the casting and rolling plant. Process models in various forms are used for this purpose. In particular, structural models and temperature models are used here. The overriding regulation 8th directly or indirectly influences the control or regulation of the individual units. Furthermore, material tracking is integrated in connection with external production planning.

2 shows the schematic temperature curve starting with the first temperature control element up to the final coiling temperature. A slab leaves the casting plant at a temperature of approx. 900 ° C and is then heated to the roughing temperature of 1080 ° C. During pre-rolling, the temperature of the pre-strip is also reduced by approx. 900 ° C. The second temperature control element cools and evens the temperature of the pre-strip to the piercing temperature in the finishing stand of 850 ° C.

As the pre-strip is finished rolling into a hot strip, the temperature of the hot strip drops again to approx. 780 ° C. The subsequent cooling section first lowers the temperature of the hot strip quickly to 500 ° C and then at a slower cooling rate to room temperature. In an adjoining furnace, the hot strip is annealed to recrystallize at a temperature of 700 ° C.

3 shows an example of the change in the structural proportions in the area of the finishing stands of a rolling train. The slab is rolled into a pre-strip in a temperature range with an almost complete austenitic structure. In a combined equalizing and cooling unit, the temperature of the pre-strip is lowered before the first pass in the finishing train so that the transformation of the austenite to ferrite begins before the first roll stand F1. The proportion of ferrite increases accordingly.

The temperature of the hot strip drops in the course of the finishing train from the roll stand F1 to the roll stand F6, so that the conversion of austenite to ferrite is accelerated. From the roll stand F4 onwards, a small amount of pearlite is also formed.

List of reference symbols

1

Continuous caster

2

First temperature control element, furnace with partial inductive heating and thermal insulation device

3

Pre-rolling mill with mill stands R1, R2

4th

Second temperature control element, furnace with partial inductive heating and thermal insulation device

5

Finishing mill with rolling stands F1 to F7

6th

Cooling section

7th

reel

8th

Process control, including material tracking and temperature setting

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 19613718 C1 [0005]
• WO 2011/138159 A1 [0005]
• KR 2001004857 [0005]
• CN 107597844 A [0006]
• CN 108994081 A [0006]

Claims (18)

Method for producing a hot strip by means of a casting and rolling plant, having at least the following units: - a continuous caster (1), - a first temperature control element (2), in particular an oven, - A roughing train (3), preferably having 2-4 roughing stands, - A second temperature control element (4) for heating and / or cooling, preferably an oven, inductive heating and / or transferable cooling, preferably downstream of the pre-strip mill - a finishing rolling train (5), preferably having 5 - 8 finishing rolling stands, - a cooling section (6), preferably with a cooling system that starts directly after the last rolling pass, - A reel (7) for winding the hot strip into a coil and in which at least the following process steps take place: - Continuous casting of a slab by means of the continuous caster (1), - Heating up or keeping the slab at rolling temperature - Pre-rolling of the slab into a pre-strip in an austenitic temperature range of the slab material in the pre-rolling train (3), - Setting a temperature of the pre-strip by means of the second temperature control element (3) in the area between the roughing and finishing train (3,5), which triggers a conversion of austenite to ferrite, - Rolling of the pre-strip, which at least partially consists of a ferritic structure, in the finishing rolling train (5) to form a hot strip, preferably below a temperature of the pre-strip of 800 ° C, - direct cooling of the hot strip to a temperature below the recrystallization temperature of the material, - Crystallizing annealing of the hot strip. Procedure according to Claim 1 , characterized in that a temperature of the pre-strip is set at which the conversion of austenite to ferrite takes place within the finishing train (5), preferably after the 2nd stand of the finishing train. Method according to one of the preceding claims, characterized in that the hot strip has more than 50%, preferably more than 65%, even more preferably more than 80%, ferrite in the structure during hot forming in the finishing rolling train (5). Method according to one of the preceding claims, characterized in that the slab is cast with a casting thickness of 100 mm to 150 mm, preferably 120 mm 140 mm. Method according to one of the preceding claims, characterized in that the pre-strip is rolled to a thickness of 25 mm to 100 mm. Method according to one of the preceding claims, characterized in that the finished hot strip has a thickness of 0.5 mm to 3.2 mm. Method according to one of the preceding claims, characterized in that the final rolling temperature is 650 ° C to 800 ° C in the finishing train (5). Method according to one of the preceding claims, characterized in that at least 30% to 60% of the deformation, in particular the reduction in thickness, takes place in the finishing rolling train (5) in the range below 750 ° C. Method according to one of the preceding claims, characterized in that the hot strip is cooled after the last acceptance pass from the final rolling temperature to a temperature ≤ 450 ° C within 2 s. Method according to one of the preceding claims, characterized in that the coiling temperature of the hot strip is 200 ° C to 450 ° C, preferably 200 ° C to 400 ° C. Method according to one of the preceding claims, characterized in that the slab material is the chemical analysis in% by weight: C <0.08%, Si <0.1%, Mn <1%, Nb <0.1%, Ti <0.1% , C, Cr, Ni each <0.5%, V <0.1%, remainder iron, preferably possibly other elements, as well as unavoidable impurities in the range of the usual contents, as they occur in steel production. Method according to one of the preceding claims, characterized in that the casting and rolling plant additionally has at least one of the units - interframe cooling - shears - scale washer. Method according to one of the preceding claims, characterized in that the slab temperature is equalized before the first piercing in a furnace after the continuous casting. Method according to one of the preceding claims, characterized in that the hot strip is pickled, preferably before and / or after hot forming. Method according to one of the preceding claims, characterized in that a process model connected to a control or regulation (8) of the casting and rolling plant or one of the units of the casting and rolling plant determines the temperature control and transfers it to the control or regulation of the casting and rolling plant or the units of the casting and rolling plant. Method according to one of the preceding claims, characterized in that a temperature of the pre-strip is set at which the conversion of austenite to ferrite takes place within the finishing train (5), preferably after the second stand of the finishing train (5). A method for producing a cold strip from a hot strip which is produced by a method according to FIG Claims 1 until 16 was produced, characterized in that - after the hot forming, the hot strip is cold rolled; and - the decrease in thickness during cold rolling is ≤ 70%. Procedure according to Claim 17 , characterized in that a further cold rolling pass, preferably a skin pass with a thickness reduction of ≤ 5%, takes place before a recrystallizing annealing.

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