Patent Publication Number: US-2012024434-A1

Title: Method for producing strips of metal, and production line for performing the method

Description:
The invention relates to a method of continuous and discontinuous production of hot-rolled strips made of cast metal, in particular steel, having the features of the introductory clause of claim  1 . 
     The invention further relates to a plant for carrying out the method according to the features of the introductory clause of claim  9 . 
     In a known method of horizontal strip casting, it is possible for melts of different types of steel to be cast with less than 20 mm strip thickness close to their final dimensions. Using this method, lightweight steels in particular with a high content of C, Mn, Al, and Si may be produced. 
     A production method of the production of hot-rolled thin flat products is known from EP 1 047 510 [U.S. Pat. No. 6,527,882]. According to this method, steel melts are cast close to their final dimensions in a range of 5 mm to 18 mm strip thickness and cooled in a controlled manner and in a protective gas atmosphere before reaching a single-stand roughing train. 
     The blank hot strip is cooled, heated, or its temperature is maintained in a controlled fashion in a unit provided downstream of the roughing train, and the edges of the hot strip are reheated. 
     The single-stand roughing train is followed by a multistand finishing train, a run-out roller table having a device for cooling the hot strip, and coilers upstream and downstream for winding the hot strip. 
     The rolling temperature upstream of the multistand finishing train may be controlledly set by the device for cooling, heating, or maintaining the temperature of the hot strip, in the austenite or ferrite range or in the transition range from austenite to ferrite. 
     In addition, WO 2006/066551 [US 20100059196] discloses a method of the production of hot strips of lightweight steel that has particularly good cold deep drawability and has Fe, Mn, Si, and Al as its primary elements. The rough strip produced by casting upstream passes through a device for homogenization in a protective gas with optional maintenance of the temperature, cooling, or heating. The rough strip is subsequently subjected to a hot-rolling step having at least one pass with a total degree of deformation of at least 50%. After the last pass, the hot strip is cooled and wound up. The hot-rolling step occurs inline or decoupled depending on the ratio of casting speed to rolling speed. 
     The homogenization zone is intended to equalize the temperature over the surface of the hot strip and reduce tension within the hot strip independently of whether the temperature level is maintained, increased, or decreased. 
     The question of whether the rolling step should occur inline or decoupled from the casting step essentially becomes a function of the different speeds in the casting and rolling step, with the recrystallization behavior of the workpiece having significance as well. 
     In the casting step, a direct connection exists between the material in the liquid phase in the pouring region of the melt and the later process steps of the hardened workpiece via the cast strip. 
     The cast strip is guided to further processing along a transport path. The subsequent method steps may be: straightening, rolling, cutting, and winding (reeling, coiling). These and other components of a casting system may lead to fluctuations in the tension and mass flow in the cast strip. If these disruptions continue in the direction of the liquid steel, casting disruptions may occur as well as negative influences on the cast strip such as, for example, fluctuations in thickness, overflows, edge constrictions, and tears in the strip or the flow. 
     The object of the invention is to refine a method of the production of hot strips by a strip caster and a strip caster having a coiler or stacker in such a way that the method steps is following the casting step do not have any negative influence on the quality and material properties of the cast strip being produced and that the cast strip, after the casting step, may be separated into pieces of predetermined size that can be transported and/or stored such as, for example, stacked sheets or coils. 
     This object is attained according to the invention by the features of the independent claims  1  and  8 . 
     Thus, according to the characterizing features of claim  1 , the rough strip produced by strip casting passes through a homogenization zone in a protective gas atmosphere in order to homogenize its structural composition in conjunction with maintaining the temperature of the rough strip, reducing the temperature, or increasing the temperature [, driver, looper, driver error]. Then the rough strip passes through a temperature controller in order to maintain the temperature of the rough strip, reduce the temperature, or increase the temperature of the rough strip. Thereafter, the rough strip is subjected to a reduction in thickness of less than 49% in a hot-rolling step having at least one pass. Finally, the rough strip passes through a downstream homogenization zone before it is separated as a hot strip from the following rough strip. 
     In the upstream homogenization zone, an inert-gas argon/CO 2 -nitrogen mixture is used. In the upstream homogenization zone, the temperature is maintained in the region of 900 to 1000° C., reduced by 200° C., and increased by 250° C. 
     In order for the workpiece to be able to be bent into a coil, it must have a suitable structural composition that allows deformation under tensile and compressive stress. 
     These structural properties may be attained by the one or more rolling steps with one pass up to a degree of deformation of 49%. 
     This deformation step triggers a recrystallization of the workpiece. In the subsequent downstream homogenization zone, the thus treated hot strip undergoes recrystallization at least at its edges while the temperature of the hot strip is maintained, if the temperature is reduced, or if the temperature is increased. 
     In the downstream homogenization zone downstream of the upstream roll stand, the workpiece is able to recrystallize and is therefore able to withstand tensile and compressive stress. This method of recrystallization is particularly suitable for lightweight steels that, among other things, may have a very wide solidification range, i.e. a larger or smaller “temperature window” from the beginning of the hardening of the melt to complete curing and have zero solidity and ductility temperatures that depend on the “window.” 
     After the rough strip has passed through the downstream homogenization zone, the hot strip is guided to a cutter with a working speed synchronized to the roller or transport belt speed, subdivided into pieces of rough strip of predetermined length, and wound into a coil in a winding system. 
     However, the hot strip may also be cut directly after leaving the downstream homogenization zone, the pieces of hot strip of a particular length then being conveyed to a stacking system by suitable conveyors and stacked there as plates. 
     In the embodiment of the method according to the invention, the casting step is decoupled from the rolling step. 
     This has advantage that, on the one hand, the casting speed may be selected as a function of the progression of the solidification step and, on the other hand, the hot-rolling step is able to occur at a defined hot-strip temperature until a predetermined degree of deformation is attained. 
     In another embodiment of the method according to the invention, after the upstream homogenization of the structural composition, the transport speed of the rough strip is varied by a mass-flow controller. This prevents disruptions from the following process steps within the production method from negatively influencing the casting step and the cast strip and prevents overflows, edge constrictions, and tears in the strip or the flow. 
     According to the features of claim  9 , the plant for carrying out the method according to the invention comprises a casting machine known per se and downstream of which an upstream homogenization zone is provided in a protective-gas atmosphere for influencing the structural composition of the cast rough strip and where the temperature of the rough strip is maintained, reduced, or increased. 
     An upstream temperature controller for maintaining, reducing, or increasing the temperature of the rough strip is provided downstream of the upstream homogenization zone. 
     Relative to the transport direction of the rough strip, a downstream temperature controller is provided downstream of the upstream temperature controller, with a mass-flow controller integrated between an upstream and a downstream rough strip conveyor; in the downstream temperature controller, the cast strip is brought to a suitable rolling temperature, by maintaining, reducing, or increasing the temperature of the cast strip. 
     An upstream roll stand is provided downstream of the downstream temperature controller to subject the rough strip to a reduction in thickness of less than 49% at the appropriate temperature in at least one pass. In a downstream homogenization zone provided downstream of the upstream roll stand, the rough strip, now hot, is recrystallized at a temperature of 700 to 900° C., particularly at the edges of the hot strip, such that the workpiece is able to absorb tensile and compressive forces as well as plastic deformations that occur during winding or stacking without damage to the structural composition. 
     Finally, downstream of the downstream homogenization zone is a cutter operated at a working speed that is synchronized with the transport speed of the hot-strip conveyor. In the cutter, the hot-rolled hot strip continually approaching the device is wound after being cutting and is stacked as plates. 
     In an additional embodiment of the plant according to the invention, a mass-flow controller is provided downstream of the upstream temperature controller and is embodied as a dancer roll or a looper. The mass-flow controller, viewed in the transport direction of the strip, is integrated between an upstream rough strip conveyor and a downstream rough strip conveyor. 
     Moreover, in the invention, upstream of the roll stand is a downstream temperature controller in which the rough strip is brought to a suitable rolling temperature so that the rough strip may be subjected to a hot-rolling step having at least one pass, and the rough strip, as a hot strip, then has a degree of deformation less than 49%. 
     Finally, according to the invention a coiler or stacker in which the hot strip is wound into a coil or deposited as individual plates is provided downstream of the cutter. 
    
    
     
       Additional features and advantages of the invention may be found in the following description and in the illustrated embodiment shown in the drawing, in which: 
         FIG. 1  is a schematic view of the plant for carrying out the method according to the invention for strip casting close to the final dimensions, having a winder and/or stacker. 
     
    
    
     The single  FIG. 1  is schematic view of a plant according to the invention for carrying out the method according to the invention. The plant has a casting machine  1  in the form of a horizontal strip caster for carrying out the casting method or casting step and having a conveyor in the form of an endless conveyor belt  2  and two deflection rollers  3 ,  3 ′. The casting machine  1  is moreover provided with a side wall  4  that prevents the poured melt  5  from flowing down to the right and left sides of the conveyor  2 . The melt  5  is fed to the casting machine  1  from a ladle  6  and flows through an outlet opening  7  ing the base of the ladle  6  into a feed tank  8 . This feed tank  8  is formed as an overflow tank. 
     Devices for intensive cooling of the bottom of the support reach of the conveyor  2  and the complete housing surrounding the casting machine  1  and containing the corresponding protective gas atmosphere are not shown in greater detail. 
     After the melt  5  has been poured onto the moving conveyor  2 , intensive cooling results in solidification and the formation of a cast or rough strip  9  that is completely solidified to a large extent by the time it reaches the downstream end of the conveyor  2 . 
     In order to equalize temperature and reduce tension, a homogenizing zone  10  follows the casting machine  1 . The homogenizing zone is formed by a heat-insulated enclosure  11  and a roller conveyor. This upstream homogenizing zone may be used to maintain the temperature of the cast rough strip  9 , further cool it, or, alternately, to heat it somewhat. In particular, provision is made in this upstream homogenization zone  10  for the cast or rough strip  9  to be subjected to an upstream step for homogenizing its structure, by maintaining a temperature at approximately 900 to 1000° C., by reducing the temperature by approximately 200° C., or, alternately, by increasing the temperature by approximately 250° C., in a protective gas atmosphere composed of an inert-gas mixture of argon-CO 2 -nitrogen. 
     Subsequently, the cast rough strip  9  passes through an upstream temperature controller  12  and is then conducted to a downstream temperature controller  15  by an upstream rough-strip conveyor  14  and a downstream rough-strip conveyor  14 ′ by a mass-flow controller  13  provided between the upstream rough strip conveyor  14  and the downstream rough strip conveyor  14 ′ and comprised of a dancer roll or a looper. In this downstream temperature controller  15 , the cast rough strip  9  is brought to the rolling temperature before it is then subjected to a hot-rolling step in a following upstream roll stand  16 , with at least one pass being conducted until the cast rough strip  9  has a degree of deformation of less than 49% of the total deformation. In this upstream temperature controller  15 , the cast rough strip  9  is maintained at a temperature of approximately 880 to 940° C., its temperature is reduced by approximately 50° C., or, alternately, its temperature is increased by approximately 50° C. After passing through the upstream roll stand  16 , the cast rough strip  9  is subsequently conveyed to a downstream homogenization zone  17  in which the cast rough strip  9  is subjected to a downstream homogenization step, in particular for recrystallization of the structural composition. This occurs in that the temperature of the cast rough strip  9  in the downstream homogenization zone  17  is maintained at a temperature of approximately 700 to 900° C., its temperature is reduced by approximately 100° C., or its temperature is increased by approximately 50° C. Downstream of the downstream homogenization zone  17 , the cast rough strip  9  then passes through a subdividing unit  18  in the form of a cutter that is synchronized with the transport or belt speed, and is then conveyed as pieces of fully rolled hot strip that have been separated from the cast rough strip  9  to a coiler  19  or a stacker  20 .