KR20000057923A - A method of continuously casting steel strip - Google Patents

Abstract

In a twin roll casting of steel strips, molten metal is introduced into the nip 16b between the parallel casting rolls 16 to make the casting pool 30 supported on the casting surface 16a of the roll and the roll is solidified strip ( The roll rotates to feed 20) downward from the nip. The casting surface 16A is shaped like a fiber by an irregular pattern of clear protrusions with sharp peaks, and the strip moves at a speed of 60 meters or more from the casting pool. To overcome chatter defects, the molten metal has a manganese of less than 0.6% by weight and a silicon content in the range of 0.1-1-35% by weight. Irregular fibrous shapes are produced by grit casting a casting surface on a substrate covered by a protective coating. Fiber shapes, on the other hand, are created by chemical or electrical electrodeposition of the coating on the substrate forming the casting surface.

Description

A method of continuously casting steel strip

The present invention relates to the casting of steel strips. Continuous casting of metal strips in two-win roll casting machines is well known. In this technique, the molten metal is introduced between a pair of counter-rotating horizontal casting rolls that are cooled so that the metal shell solidifies on the moving rolls and moves together to the nips between them to make a solidified strip product fed downward from the nip between the rolls. do. The term "nip" is used herein to mean the general area where the rolls come closest together. Molten metal is poured into a small vessel or series of vessels through which the metal flows through the metal feed nozzle located above the nip to direct the metal from the ladle into the nip between the rolls, thus casting the surface of the roll just above the nip. A cast pool of molten metal is formed that is supported on and extends along the length of the nip. This casting pool is generally defined between side plates or dams which remain in sliding contact with the cross section of the roll to prevent the two ends of the casting pool against outflow, even if the electromagnet barrier is used by other optional means.

Although two-win roll casting has been successfully applied to non-ferrous metals which solidify rapidly on cooling, there is a problem in applying this technique to casting of iron. One particular problem relates to achieving rapid and uniform cooling over the casting surface of the roll. In particular, it is difficult to obtain a sufficiently high cooling rate for solidification onto a casting roll with a smooth casting surface, so that in order to control the heat transfer rate, a finely-textured casting surface is formed by a regular pattern of protrusions and depressions. The use of a roll has been proposed to control the heat flux at the casting surface during solidification.

U. S. Patent No. 5,701, 948 discloses a casting roll shape formed by a series of parallel grooves and ridges. In particular, in a twin roll casting machine, the casting surface of the casting roll is basically shaped by installation of grooves and ridges extending in the circumferential direction of a constant depth and pitch. This feature is optimized for steel casting to achieve enhanced heat flux during metal solidification and to achieve both high heat flux and fine structure of the steel strip. Basically, when casting steel strips, for best results, the depth of the feature from the ridge peak to the groove base should be in the range of 5-60 microns, and the pitch of the feature should be within the range of 100-250 microns. For proper results, the shape should be in the range of 15-25 microns and pitch 150-200 microns.

Although rolls having a shape known from US Pat. No. 5,701,948 can achieve high solidification rates in the casting of ferrous metal strips, they are of two types known as "crocodile-skin" and "chatter" defects. It is shown that it is sensitive to casting conditions that must be precisely controlled to prevent common strip defects. In particular, it is necessary to control crocodile skin defects by controlled addition of sulfur to the metal and to prevent chatter defects by operating the casting machine within a narrow range of casting speeds. Crocodile skin defects include δ and ?? It occurs when the iron phase solidifies simultaneously in the shell on the casting surface of the roll of the twin roll casting machine under conditions in which there is a change in heat flux through the solidification shell. δ and ?? The iron phase has different thermal strength properties, and the heat flux deformation then results in local deformation of the solidification shell that comes together in the nip between the casting rolls and produces crocodile skin defects on the surface of the product strip.

Light oxidation deposits on the rolls having a melting temperature below the melting temperature of the metal being cast help to ensure controlled homogeneous heat flux during solidification of the metal on the casting roll face. When the roll surface enters the molten metal casting pool, the oxidized electrodeposits melt to form a thin liquid boundary layer between the casting surface and the molten metal of the casting pool to promote good heat flux. However, if too much oxide is present, the melting of the oxide results in a very high initial heat flux but then the oxide resolidifies and thus the heat flux decreases drastically. This problem is aggravated by efforts to continue the accumulation of oxides on cast rolls within the strict limits imposed by the roll cleaning apparatus. However, where the roll cleaning is not uniform, there is a variation in oxide buildup, which results in local deformation which causes variations in shell solidification and results in crocodile skin defects.

Chatter defects are initiated at the meniscus level of the casting pool where initial metal solidification takes place. One of the chatter defects, called "low speed chatter", occurs at low casting speeds because of the premature freezing of the metal on the casting rolls to create a weak shell that deforms as it is further drawn into the casting pool. Another one of the chatter defects called "fast chatter" occurs at high casting speeds when the shell begins to form further down the casting roll and there is liquid on the forming shell. This liquid supplying the meniscus region cannot follow the moving roll face, resulting in slippage between the liquid and the roll on top of the casting pool and causing high speed chatter defects as transverse strain bands across the strip.

Moreover, operation on a very narrow range of casting speeds is required to prevent low speed chatters on the one hand and high speed chatters on the other. In general, it is necessary to operate at casting speeds within a narrow range of 30-36 meters per minute. Certain speed ranges can vary from roll to roll, but generally the casting speed should be less than 40 meters per minute to prevent high speed chatter.

The present invention makes it possible to significantly reduce the tendency of chatter defects by careful selection of the chemical composition of the steel in relation to irregularly shaped cast surfaces. The present invention makes it possible to broaden the range of possible casting speeds very largely. In particular, it is possible to achieve very high casting speeds.

Irregularly shaped cast faces result in a dramatic increase in casting speed. At casting speeds in excess of 60 meters per minute, a new form of "high frequency chatter" arises, which can be reduced by the choice of the chemical composition of the metal in connection with the present invention.

1 is a front view of a continuous strip casting machine operable in accordance with the present invention;

2 is a side cross-sectional view of the strip casting machine shown in FIG. 1;

3 is a vertical cross-sectional view taken along line 3-3 of FIG.

4 is a vertical section along line 4-4 of FIG.

FIG. 5 is a vertical section along line 5-5 of FIG. 1;

Figure 6 shows a typical cast face shape used in the method of the present invention;

7 is a diagram showing the result of a test casting using steel whose composition is changed;

8 is a diagram showing the effect of the content of manganese in the generation of high speed chatter defects.

<Description of the symbols for the main parts of the drawings>

11: frame 13: carriage

16: casting roll 16A: casting surface

20: strip product 21, 22: coiler

25: emergency plug 32: wheel

35, 36: sliding member

According to the present invention, there is provided a method for continuous casting of steel strip comprising supporting a casting pool of molten metal on one or more chilled casting surfaces and moving the chilled casting surfaces to produce a solidifying strip moving from the casting pool. Provided, each casting surface is shaped by an irregular pattern of clear protrusions with sharp peaks and the strip moves the casting pool at a speed of 60 m / min or more, with molten steel of 0.6 wt% manganese and 0.1-0.35 wt% Silicon content.

The steel has a carbon content of less than 0.07% by weight.

Preferably, the sharp peaks have a surface distribution between 5-100 per mm 2 and have an average height of at least 10 microns.

Preferably, the average height of the apparent protrusions is at least 20 microns.

The strip is moved out of the casting pool at a speed of at least 75 meters per minute. In particular, the strip is moved out of the casting pool at a speed of 75-150 m per minute.

The manganese content of the steel is in the range of 0.6-0.9% by weight.

The process of the invention is carried out in a twin roll casting machine.

According to the present invention, molten metal is introduced into a nip between a pair of parallel casting rolls via a metal supply nozzle located above the casting surface nip to create a casting pool of molten steel supported on the casting surface of the roll just above the nip. The casting roll is rotated to feed the solidified steel strip downward from the nip, shaped by an irregular pattern of clear protrusions each with a sharp peak, and the strip moves the casting pool at a speed of 60 m or more per minute The molten steel has a manganese content of 0.6% by weight and a silicon content of 0.1-0.35% by weight.

The shaping of the cast face is made by grit casting a metal substrate protected by a surface coating to make each cast face or cast face. For example, each coating surface is produced by grit casting a copper substrate plated with a thin layer of chromium. Or the cast surface is formed of nickel with a nickel surface grit casted and no protective coating applied.

The desired shape of each cast surface is also obtained by electrodepositing the coating onto the substrate. In this case, the coating material is selected to control the heat flux during metal solidification. The material has a low affinity for steel oxide, so that wetting of the cast surface by such electrodeposits is poor. In particular, the cast surface is formed of nickel chromium and molybdenum or nickel chromium and cobalt alloys and the alloy is electrodeposited to make the desired shape.

The invention will be explained in more detail and the test results will be described with reference to the attached drawings.

The specification of US Pat. No. 5,701,948 discloses how steel strips are cast in a two-win roll casting machine in which the casting rolls have regular grooves with parallel grooves and ridge formations. The present invention can use a twin roll casting machine of the same type as known in the patent, but the casting roll has an irregularly shaped surface formed by grit casting. A preferred form of the device is shown in Figures 1-5 of the accompanying drawings.

The casting machine shown in FIGS. 1-5 includes a machine frame that stands up from the factory bottom 12. The frame 11 supports a casting roll carriage 13 which is movable in parallel between the assembly station 14 and the casting station 15. The carriage 13 is a pair of parallel casting rolls 16 in which molten material is fed from the ladle 17 to the feed nozzles 19 via the distributor 18 to make the casting pool 30 during the casting operation. Support it. The casting rolls 16 are water cooled so that the shells are solidified at the moving roll face 16A and brought together to the nip 16B therebetween to make the strip product 20 solidified at the roll exit. This product is supplied to the standard coiler 21 and simultaneously transported to the second coiler 22. The receptacle 23 is mounted on the machine frame adjacent to the casting station and the molten metal is on the distributor side via the overflow spout 24 on the dispenser or if there is a severe malformation of the product or other serious dysfunction during the casting operation. The operation of the emergency plug 25 allows it to branch into this receptacle.

The roll carriage 13 comprises a carriage frame 31 mounted by a wheel 32 on a rail 3 extending along a portion of the main machine frame 11 so that the roll carriage 13 is entirely rail 33. Mounted to move along. The carriage frame 31 carries a pair of roll cradles 34 mounted so that the rolls 16 rotate. The roll cradle 34 is mounted on the carriage frame 31 by the inner contacting auxiliary sliding members 35, 36 and thus the cradle to adjust the nip between the casting rolls 16 and as described in detail below. When it is necessary to form a transverse line of the weak part over the same strip, the rolls are allowed to move on the carriage under the influence of the hydraulic cylinder units 37 and 38 so that they can be quickly spaced apart from each other for a short period of time. The carriage connected between the drive bracket 40 on the roll carriage and the main machine frame is operable to move the roll carriage between the assembly station 14 and the casting station 15 and upside down a dual action hydraulic cylinder and cylinder unit. By operation of 39 the carriage is generally movable along rail 33.

The casting roll 16 reversely rotates through the electric motor mounted to the carriage frame 31 and the drive shaft 41 from the transmission part. The roll 16 is a longitudinally spaced and circumferentially spaced series in which cooling water is supplied through the roll end from the water supply duct of the roll drive shaft 41 connected to the water supply hose 42 via a rotating line 43. It has a peripheral wall made of a copper material having a cooling water passage of. The rolls generally have a diameter of about 500 mm and a length of 2000 mm to make a wide strip product of 2000 mm.

Ladles 17 are generally of general construction and are supported via yoke 45 at the top of the crane and brought to the position of the hot metal receiving step. The ladle is secured by a stopper rod 46 operable by the server cylinder to allow molten metal to flow from the ladle through the output nozzle 47 and the refractory shroud 48 into the distributor 18.

The distributor 18 is formed as a wide plate made of a refractory material, for example magnesium oxide (Mgo). One side of the distributor receives the molten metal in the ladle and has the overflow section 24 and emergency plug 25 described above. The other side of the dispenser has a series of longitudinally spaced metal outlet openings 52. The lower part of the dispenser bears a mounting bracket 53 for mounting the dispenser on the roll carriage frame 31 and has an opening for receiving the indexing peg 54 on the carriage frame to accurately position the dispenser.

The feed nozzle 19 is formed as an elongated body made of refractory material such as alumina graphite. Its lower portion is tapered to collect inwardly and downwardly, projecting into the nip between the casting rolls 16. A mounting bracket 60 is provided to support it on the roll carriage frame, the upper portion of which has an outer protruding side flange 55 positioned on the mounting bracket.

The nozzles 19 are horizontally spaced and generally vertical in order to achieve a moderately low viscosity discharge of the metal through the width of the roll and to supply molten metal into the nip between the rolls without direct penetration onto the roll surface where initial solidification occurs. It has a series of flow passages extending into. On the other hand, the nozzle has a single continuous slot outlet for feeding the low viscosity curtain of molten metal directly into the nip between the rolls and it can be impregnated into the molten metal pool.

Pull is a roll of roll when the roll carriage is at the casting station. It is defined on the end of the roll by a pair of side cover plates 56 supported against the end 57. The side cover plate 56 is made of a strong refractory material such as, for example, boron nitride and has a scalloped side edge 81 to match the curvature of the roll 57 of the roll. The side plates are actuated by the operation of a pair of hydraulic cylinders 83 to keep the side plates in contact with the stop of the casting roll to form an end cover for the molten pool of metal formed on the casting roll during the solidification process. It is mounted to the plate receiver 82 movable in the casting step.

During the casting operation, the ladle stopper rod 46 operates to flow molten metal from the ladle to the distributor through the metal supply nozzle so that the molten metal flows to the casting roll. The clean head end of the strip product 20 is guided to the jaws of the coiler 21 by the operation of an apron table 96. The apron table 96 is suspended from the pivot mount 97 on the main frame and is swingable toward the coiler by the operation of the hydraulic cylinder unit 98 after the clean head end is formed. The table 96 is operated with respect to the upper strip guide flap 99 which is operated by the piston and cylinder unit 101, and the strip product 20 is defined between a pair of vertical side rollers 102. After the head end is guided to the jaw of the coiler, the coiler rotates to coil the strip product and the apron table returns to the non-operational position, which is simply suspended from the machine frame without direct product going to the coiler 21. Can be. The resulting strip product 20 is then transported to the coiler 22 to become the final coil for remote transport from the casting machine.

A special form of a twin roll casting machine of the kind shown in FIGS. 1 to 5 is described in more detail in US Pat. No. 5,184,668, and International Patent Application No. PCT / AU93 / 00593.

Roll cast surfaces made with an irregular pattern of distinct protrusions are manufactured by grit or shot casting at casting speeds of up to 60 meters per minute, even if high frequency chatter occurs at high casting speeds. Has a sharp peak because of the occurrence of much less. Irregularity of the shape is very important to achieve a homogeneous and crack-resistant microstructure.

Appropriately irregular shapes can be dispensed on a metal substrate by grit casting with a hard particulate material such as alumina, silicon or silicon carbide having a particle size of 0.7-1.4 mm. For example, the copper roll face can be grit cast in such a way that it adds the appropriate shape and the shaped face is protected with a 50 micron thick thin chrome coating. Fig. 6 shows a typical cast surface produced in this way. On the other hand, it is possible to apply the shaped surface directly to the nickel keypad without additional protective coating.

Properly irregular shapes can be achieved by forming a coating by chemical electrodeposition or electrical electrodeposition. In this case, the coating material should be chosen to contribute to high thermal conductivity and increased heat flux during solidification. In addition, oxidation of the product in the steel shows poor wettability of the coating material and the molten steel itself has a great affinity for the coating material and thus can wet the coating with oxides. Two suitable materials are nickel, chromium and molybdenum alloys sold under the trade name "HASTALLOY C" and nickel, molybdenum and cobalt alloys sold under the trade name "T800". International Patent Application No. PCT / AU99 / 00641 describes test results using irregularly shaped cast surfaces formed by coatings of HASTALLOY C and T800, in which solidified shells electrodeposited on such surfaces were markedly of uniform thickness. It shows a homogeneous microstructure.

Casting is performed on a twin roll casting machine with a grit cast feature roll using low carbon steel with a range of manganese and silicon content configured to test both assumptions for the cause of high frequency chatter defects encountered when the casting speed is increased. .

One assumption for high frequency chatter is due to the lack of wettability of the casting surface as the casting speed increases. In this assumption, the problem is reduced by controlling the chemical composition of the steel to produce low temperature liquid inclusions which enhance the wettability of the oxidation product. This is achieved by controlling the manganese and silicon content of the steel.

A second assumption for the generation of high frequency chatter defects is that they are caused by the lack of cushioning effect of the mush region where the solidified shell is formed with the strip. This hypothesis was tested by varying the carbon content of the steel for the same manganese content to make a thicker mesh region. The results of these tests show that the effects of both hypotheses work together on high frequency chatter defects. These tests show that it is necessary to control the manganese content and silicon content of the steel to reduce high frequency chatter defects at high casting speeds.

FIG. 7 shows the effect of varying liquidus temperature changes with changes in manganese and silicon content, and FIG. 8 shows the effect on changes in manganese content in heavily chattered areas. These tests maintained a carbon content of less than 0.07 weight percent. These tests show that changes in manganese content are critical to controlling high frequency chatter defects. Changing the silicon content does not have the same effect, but the silicon content needs to be maintained in the range of 0.1-0.35%. If the silicon content is too high, castability problems occur, and the strip is fragile and there are solid phase inclusions. If the silicon content is too low, the volume of the oxide increases.

In Fig. 8, the content should be at least 0.6% to prevent the occurrence of chatter defects. As the casting speed increases, the manganese content of the steel must also increase to prevent high speed chatter. In general, the manganese content is in the range of 0.6-1.9% for a casting speed range of 75-150 m per minute.

Claims (20)

Supporting the molten metal casting pool 30 on one or more chilled casting surfaces, and moving the chilled casting surface 16A to make the solidification strip 20 moving from the casting pool 30. In the steel strip continuous casting method, Each casting surface 16A is shaped by an irregular pattern of clear protrusions with sharp peaks and the strip 20 moves the casting pool 30 at a speed of at least 60 meters per minute, with molten steel of 0.6 wt% manganese and 0.1 A method of continuous casting of steel strip, characterized by having a silicon content of -0.35% by weight. The method of claim 1, A steel strip continuous casting method, characterized in that the steel has a carbon content of less than 0.07% by weight. The method according to claim 1 or 2, Wherein said sharp peak has a surface distribution between 5-100 per mm 2 and has an average height of at least 10 microns. The method according to any one of claims 1 to 3, A method of continuous casting of steel strip, characterized in that the average height of the overhanging protrusion is at least 20 microns. The method according to any one of claims 1 to 4, Steel strip continuous casting method, characterized in that the strip 20 is moved from the casting pool 30 at a speed of at least 75 m per minute The method of claim 5, Steel strip continuous casting method characterized in that the strip (20) is moved from the casting pool (30) at a speed of 75-150m per minute. The method according to any one of claims 1 to 6, Process for continuous casting of steel strip, characterized in that the manganese content of the steel is in the range of 0.6-0.9% by weight. The method according to any one of claims 1 to 7, There is a pair of casting surfaces 16A constituted by the circumferential surfaces of a pair of parallel casting rolls 16 forming a nip 16B therebetween, and the molten steel rolls just above the nip 16B. Steel strip continuous casting method, characterized in that it is introduced into the nip (16B) between the casting rolls (16) to make a casting pool (30) supported on the casting surface (16A). The method of claim 8, Characterized in that the molten steel is fed to the nip (16B) between the casting rolls (16) via a metal supply nozzle (19) located above the nip. The method according to any one of claims 1 to 9, A steel strip continuous casting method characterized in that each casting surface (16A) is formed by a grit casting substrate covered by a protective coating. The method of claim 10, And the protective coating is an electroplated metal coating. The method of claim 11, And the substrate is copper and the electroplating coating is chromium. The method according to any one of claims 1 to 9, And each cast face (16A) is a grit cast face. The method of claim 13, And the grit casting surface is formed of nickel. The method according to any one of claims 1 to 9, Wherein each cast surface (16A) is formed by a coating electrodeposited onto the substrate to create irregular shapes of the substrate. The method of claim 15, And the coating is formed by chemical electrodeposition. The method of claim 16, And the coating is formed by electrical electrodeposition. The method according to any one of claims 15 to 17, The coating is formed of a material having a low affinity for the oxide in the molten metal such that the molten metal itself has a greater affinity for the coating material and therefore wets the coating better than the oxide. . The method according to any one of claims 15 to 18, A steel strip continuous casting method, wherein the coating is formed of an alloy of nickel, chromium and molybdenum. The method according to any one of claims 15 to 18, And the coating is formed of nickel, molybdenum and cobalt.