EP1510272B1 - Verfahren zur Herstellung von Stahlbrammen mit ultra-geringem Kohlenstoffgehalt - Google Patents

Verfahren zur Herstellung von Stahlbrammen mit ultra-geringem Kohlenstoffgehalt Download PDF

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Publication number
EP1510272B1
EP1510272B1 EP04020281A EP04020281A EP1510272B1 EP 1510272 B1 EP1510272 B1 EP 1510272B1 EP 04020281 A EP04020281 A EP 04020281A EP 04020281 A EP04020281 A EP 04020281A EP 1510272 B1 EP1510272 B1 EP 1510272B1
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Prior art keywords
mold
magnetic field
molten steel
application device
slab
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French (fr)
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EP1510272A1 (de
Inventor
Seiji JFE Steel Corporation Itoyama
Toshio JFE Steel Corporation Fujimura
Makoto JFE Steel Corporation Suzuki
Hirohide JFE Steel Corporation Uehara
Takeshi JFE Steel Corporation Matsuzaki
Chikashi JFE Steel Corporation Tada
Yuji JFE Steel Corporation Miki
Akira JFE Steel Corporation Yamauchi
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JFE Steel Corp
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JFE Steel Corp
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Priority claimed from JP2003307108A external-priority patent/JP4259232B2/ja
Priority claimed from JP2003395818A external-priority patent/JP4411945B2/ja
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/16—Controlling or regulating processes or operations
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/14—Plants for continuous casting
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10—Supplying or treating molten metal
    • B22D11/11—Treating the molten metal
    • B22D11/114—Treating the molten metal by using agitating or vibrating means
    • B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D41/00—Casting melt-holding vessels, e.g. ladles, tundishes, cups or the like
    • B22D41/50—Pouring-nozzles
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%

Definitions

  • This invention relates to methods for producing a steel slab having ultra low carbon by continuous casting and, more particularly, relates to a method for producing a steel slab suitably used for forming outer plates of automobiles and the like with superior surface qualities.
  • Ultra-low-carbon steel steel sheets used, for example, for forming outer plates of automobiles, which are to be processed by deep drawing and/or which are to be formed into complicated shapes by deformation, should have superior formability. Hence, so-called “ultra-low-carbon steel” has been used, the carbon content of which is decreased as low as possible. Ultra-low carbon steel generally contains a C content of 0.01 mass percent or less. Among ultra-low-carbon steel sheets as described above, a cold-rolled steel sheet for forming outer plates of automobiles is particularly helpful for superior appearance in addition to superior paintability.
  • a step of removing carbon in molten steel is carried out in a refining process by oxidation using oxygen when ultra-low-carbon steel is produced. Accordingly, a deoxidizing step for removing oxygen dissolved in molten steel in this oxidation removing step is further carried out using a deoxidizing agent such as aluminum, magnesium and titanium. In this deoxidizing step, the oxygen dissolved in molten steel is allowed to react with the deoxidizing agent to form reaction products such as alumina, magnesia and titania, and the reaction products thus formed remain in the molten steel as non-metallic inclusions.
  • Defects such as slivers and/or blisters are unfavorably generated on a surface of the steel sheet in forming the slab into a thin steel sheet by hot rolling and/or cold rolling when the non-metallic inclusions as described above are present in the vicinity of a slab surface.
  • Argon gas is supplied and mold powder is added to a molten steel surface in the mold in continuous casting to prevent an immersion nozzle from being clogged which is used to supply molten steel from a tundish into the mold.
  • the argon gas thus supplied may simply remain in the molten steel in the form of bubbles or may combine with the reaction products (hereinafter referred to as "deoxidation reaction products") formed by deoxidation described above to form bubbles which remain in the molten steel.
  • deoxidation reaction products reaction products formed by deoxidation described above to form bubbles which remain in the molten steel.
  • Surface defects are generated in both cases described above.
  • surface defects similar to those of the deoxidation reaction products are also generated when the mold powder thus added remains in the molten steel.
  • hot-rolling is performed in the case of an ordinary slab prepared by continuous casting for forming a cold-rolled steel sheet, without performing surface treatment of the slab.
  • a surface portion of the slab having a thickness of approximately 1 to 4 mm is removed, for example, by scarfing so as to remove inclusions of deoxidation reaction products, bubbles, mold flux, and the like which may cause surface defects of a steel sheet formed after hot-rolling and, subsequently, hot rolling and cold rolling are performed.
  • Slab finishing treatment as described above decreases the yield of the slab used as a starting material and, in addition, disadvantageously causes delays in the process.
  • attempts have been made to prevent generation of slab surface defects which cause the above-described surface defects of steel sheets.
  • a technique has been disclosed in Japanese Unexamined Patent Application Publication No. 5-76993 in which when casting of molten steel containing less than 0.10 percent by weight of carbon is performed using a continuous casting apparatus having a vertical portion 20 m or more long at a casting speed of 1.0 m/min or more and 4 ton/min or more to form a slab having a thickness of more than 200 mm and a width of more than 900 mm, while the powder viscosity is set to 1.0 poise or more, and an inert gas flow rate from an immersion nozzle is set to 1 liter/min or more, electromagnetic stirring is performed for molten steel present in the region from the meniscus to a depth of 1.5 m at a flow speed of 15 to 40 cm/sec in the horizontal direction.
  • This technique is primarily based on the above paragraphs (1), (2), (3), (4), and (6).
  • molten steel having a high flow speed unevenly collides against a part of the solidification shell along the short side and, as a result, the growth of the part of the solidification shell described above is delayed.
  • the variation in flow speed of molten steel in the slab thickness direction is also partly responsible for variation in flow speed of molten steel in the vicinity of the meniscus described above.
  • the respective teaching does not consider a combination of parameters for producing a carbon steel slab as defined by the present invention.
  • This invention provides a method for producing an ultra-low carbon steel slab comprising: introducing molten steel into a mold having a casting space with a short side length D of 150 to 240 mm through an immersion nozzle having at least one discharge spout with a lateral width d, in which a ratio D/d is in the range of from 1.5 to 3.0; casting the molten steel at a casting speed of more than 2.0 m/min with the continuous casting apparatus, applying a brake using an electromagnetic force to the flow of molten steel by applying static magnetic fields to the mold in a direction intersecting the mold thickness with an upper magnetic field application device and a lower magnetic field application device, wherein the upper magnetic field application device is provided at an upper portion of the mold including a surface level of the molten steel in the mold and the lower magnetic field application device is provided at a lower side of the upper magnetic field application device, wherein the immersion nozzle is disposed between the upper magnetic field application device, and wherein the lower magnetic application devices and has an immersion depth of 200 to 350 mm, and
  • the slab continuous casting method includes oscillating the mold at a frequency of 185 cycles/min or less.
  • the probability of occurrence of an abnormal phenomenon is suppressed in that the molten steel surface level is suddenly and largely varied.
  • the number of defects caused by flux can be decreased since the rate of occurrence of the resonance between the oscillation of a molten steel surface and that of the mold decreases when the mold oscillation cycle is about 185 cycles/min or less.
  • the casting speed is preferably about 2.4 m/min or more.
  • the nail depth becomes about 0.7 mm or less, that is, the thickness for trapping foreign materials becomes not more than the nail depth when the casting speed is about 2.4 m/min or more.
  • the casting speed is preferably set to about 2.4 m/min or more.
  • a cylindrical nozzle (so-called “straight nozzle") or a two-spout nozzle in which the front end is closed and two approximately circular discharge spouts are provided toward the two short sides of the mold are generally used.
  • the ratio D/d of the short side length D to the lateral width d of the discharge spout of the immersion nozzle is preferably about 2.1 to about 2.9 when the slab thickness, immersion nozzle durability and the desired flow rate are taken into consideration in addition to the product quality.
  • the ultra-low carbon steel slab described above is preferably a starting material for a cold-rolled steel sheet used for forming outer plates of automobiles.
  • the slab continuous casting method described above preferably further includes applying a brake using an electromagnetic force to the flow of the molten steel in the casting space of the mold.
  • the following paragraphs (A) to (C) may be mentioned as preferred methods for applying a brake using an electromagnetic force:
  • slabs having ultra-low carbon content can be advantageously produced by appropriately controlling the casting speed, the short side length D of the casting space of a continuous casting mold, and the ratio D/d of the short side length D to a lateral width d of a discharge spout of an immersion nozzle in addition to, whenever necessary, appropriate control of oscillation frequency of the mold, or effective use of an electromagnetic brake on a molten steel flow.
  • a type of steel in accordance with aspects of the invention is so-called "ultra-low-carbon steel" having a carbon content of about 0.01 mass percent or less.
  • Components other than C are not particularly limited.
  • a type of steel which can be suitably processed by deep drawing for forming outer plates of automobiles or the like is preferred.
  • An advantage of the invention is that, for steel used in applications with substantially no defects caused by inclusions, inclusions are substantially not allowed to be present in the region from the surface of a slab to a certain depth therefrom, which region is not to be scaled off in a subsequent step.
  • Ultra-low-carbon steel may receive most advantages of the invention since, in the ultra-low carbon steel, non-metallic inclusions such as alumina are liable to be generated as deoxidation reaction products in the refining process.
  • a typical composition (not including component C) of ultra-low-carbon steel the following may be mentioned by way of example: about 0.01 to about 0.04 mass percent of Si, about 0.08 to about 0.20 mass percent of Mn, about 0.008 to about 0.020 mass percent of P, about 0.003 to about 0.008 mass percent of S, about 0.015 to about 0.060 mass percent of Al, about 0.03 to about 0.080 mass percent of Ti, about 0.002 to about 0.017 mass percent of Nb, and 0 to about 0.0007 mass percent of B.
  • the continuous casting apparatus used in accordance with the invention is a continuous casting apparatus for forming a steel slab and may be optionally selected from a vertical continuous casting apparatus, a vertical bending continuous casting apparatus and a curved continuous casting apparatus.
  • a vertical bending continuous casting apparatus is particularly advantageous in consideration of productivity and product quality.
  • the mold is a so-called "slab continuous casting mold," and the short side length thereof is about 150 to about 240 mm.
  • the long side length of the mold is not particularly limited and is preferably substantially equivalent to the length of an ordinary cold-rolled steel sheet (in particular, a cold-rolled steel sheet for automobiles), such as approximately 900 to 2,200 mm.
  • the short side length corresponds to the slab thickness when the slab is formed and the long side length corresponds to a slab width.
  • the height of the mold in the vertical direction is not particularly limited. However, since a solidification shell is formed having a certain thickness so that a cast steel sheet passing through the mold does not bulge even when casting is performed at a casting speed of more than 2.0 m/min, the height is preferably set to approximately 800 to approximately 1,000 mm.
  • An immersion muzzle is used as the nozzle for supplying molten steel into the casting space of the mold from a tundish.
  • the material for the immersion nozzle may be a commonly used material such as alumina-graphite. However, the material is not only limited thereto.
  • the shape of the immersion nozzle there may be generally mentioned a cylindrical nozzle (so-called “straight nozzle") or a two-spout nozzle in which the front end is closed and two approximately circular discharge spouts are provided toward the two short sides of the mold.
  • the cross-sectional shape of the discharge spout may be circular, square, or rectangular (longer in a lateral direction, or longer in a longitudinal direction) and is not particularly limited, and any type of shape may be used as long as the maximum width d of the discharge spout satisfies the conditions of the invention.
  • the casting speed is set to more than about 2.0 m/min for the reasons described later.
  • the casting speed is more preferably set to about 2.4 m/min or more.
  • Formation of an initial solidification shell at the meniscus portion which is a so-called “nail”, can be significantly suppressed when the casting speed Vc is set to more than about 2.0 m/min or preferably set to about 2.4 m/min or more.
  • the reason for this is that since the thickness of a solidification shell formed at an optional constant depth from a molten steel surface level is decreased as the casting speed Vc is increased, due to the influence of a static pressure of molten steel, a force applied toward the mold side becomes larger than a force of the nail leaning toward the molten steel side caused by thermal contraction of the solidification shell which depends on the thickness thereof.
  • the absolute value of the amount of shell contraction in the thickness direction represented by "slab thickness ⁇ temperature difference ⁇ coefficient of thermal expansion" is decreased, the leaning of the shell toward the molten steel side is further suppressed and, as a result, the effect of suppressing the leaning of the nail becomes more significant.
  • Fig. 1 the influence of the casting speed on the nail depth is shown.
  • the nail depth becomes 1 mm or less when the casting speed is more than about 2.0 m/min and the short side length (slab thickness) of the casting space of the mold is about 240 mm or less.
  • the nail depth becomes about 0.7 mm or less when the casting speed is about 2.4 m/min or more.
  • the force of adsorbing and trapping foreign materials on the interface of the solidification shell is suppressed by increasing the casting speed Vc. That is, when the casting speed Vc is high, such as more than about 2.0 m/min, since the solidification amount at the meniscus portion is decreased, the segregation amount is also decreased. Hence, the gradient of surface tension, which functions as a force of attracting foreign materials, is also decreased. As a result, the amount of foreign materials adsorbed and trapped at the solidification shell side is also reduced.
  • Fig. 2 shows the relationship in a surface portion of the slab between a trapping depth h from the slab surface at which foreign materials are trapped and the number of trapped foreign materials.
  • Fig. 3 shows the relationship between the number of trapped foreign materials and a distance L from the meniscus (the surface of molten steel) which is obtained by converting the trapping depth h from the slab surface.
  • Vc indicates the casting speed, and a solidification constant k is 20 mm ⁇ min -1/2
  • the trapping depth is decreased as the casting speed is increased, and at a casting speed Vc of more than 2.0 m/min, the trapping depth h from the slab surface is 1 mm or less.
  • the trapping depth h is 1 mm or less, although foreign materials are trapped by the shell, in a subsequent process forming products through a hot rolling step and a cold rolling step, the foreign materials are scraped off and removed together with oxide scales formed on the surface of a cast steel sheet. Accordingly, a defect-free product can be obtained without performing slab conditioning.
  • the nail depth becomes 0.7 mm or less, that is, the trapping thickness h also becomes not more than the nail depth when the casting speed is about 2.4 m/min or more.
  • the casting speed is more preferably set to about 2.4 m/min or more.
  • the residence time of the solidification shell in the region from the molten steel surface to a depth of 20 mm in which foreign materials are likely to be trapped by the solidification shell decreases as the casting speed increases. Accordingly, the probability of trapping foreign materials by the solidification shell decreases even when the same amount of foreign materials is present floating in molten steel. For example, when Vc is 3.0 m/min, the trapping probability decreases to one half of that when Vc is 1.5 m/min.
  • the bulging phenomenon is a phenomenon in which the solidification shell is pushed toward the mold side by the influence of the static pressure of molten steel. In this bulging phenomenon, when the temperature of the shell is high, and when a type of steel is a ultra-low carbon steel or the like having a small shell strength as compared to that of other types of steel, the bulging (being pushed to the mold) speed becomes higher than the oscillation speed of the mold.
  • an abnormal phenomenon may occur in rare cases in which the molten steel surface level suddenly and largely varies. It has been difficult to detect this phenomenon using an ordinary eddy-current type level sensor for molten steel surface since this abnormal phenomenon occurs at the edge portion of the mold.
  • this phenomenon by investigation of the distortion of an oscillation mark of a cast steel slab with time.
  • the casting speed is more than about 2.0 m/min and the oscillation frequency of the mold is high, such as more than about 185 cycles/min
  • this abnormal phenomenon described above is likely to be observed.
  • mold flux may be engulfed in the molten steel and may be trapped in the solidification shell, thereby causing defects in the surface portion of the cast steel sheet. Accordingly, in the case of casting at a casting speed of more than about 2.0 m/min, the number of surface defects in the product caused by the mold flux is suddenly increased. As a result, it has been difficult to decrease the surface defects.
  • the lower limit of the oscillation frequency of the mold may be set in view of reduction in area of trapping foreign materials so as not to increase the nail depth and also in view of prevention of restraint breakout caused by the decrease in lubricant properties (consumption amount of mold flux) in the mold.
  • a negative strip time is about 0.02 seconds or more and that a negative strip length is about 0.1 mm or more.
  • the negative strip time is one characteristic value for defining the mold oscillation conditions and indicates a period of time in which the descending speed of the mold is higher than that of the cast steel slab.
  • the negative strip length indicates the maximum distance between the mold and the cast steel slab within the negative strip time, the mold passing by the cast steel slab which is being drawn.
  • ⁇ Sf/Vc>1 is satisfied when the oscillation waveform of the mold is assumed to have a sine waveform wherein S indicates the oscillation stroke of the mold, f indicates the mold frequency, and Vc indicates the casting speed.
  • Vc is 2.0 m/min and S is 9 mm
  • the lower limit of the mold frequency f is 71 cpm (cycles/minute)
  • S is 5 mm
  • the lower limit is 127 cpm.
  • the lower limit of the frequency and the waveform may be appropriately determined.
  • the short side length that is, the slab thickness
  • the fluctuation of the molten steel surface level is facilitated by an inversion flow and a secondary flow of the jet flow of the molten steel, which flows are from the short sides of the solidification shell, engulfment and trapping of mold flux are liable to occur.
  • stagnation of molten steel at the meniscus portion, particularly, in the vicinity of the immersion nozzle is liable to occur.
  • the number of slab surface defects and that of the product defects increases.
  • the short side length (slab thickness) of the casting space of the mold is less than about 150 mm, for the following reasons.
  • the above effect (1) cannot be obtained in view of controllability of molten steel surface level when the cross-sectional area of the slab excessively decreases.
  • the reason for this is that when the casting amount is changed, the fluctuation in molten steel surface level increases as compared to the case in which a slab having a large cross-sectional area is formed. Also, due to the formation of molten steel ripples thereby, the rate of generation of nails having a depth of 1 mm or more is increased. In addition, engulfment and trapping of mold flux are liable to occur (see Fig. 5 ) due to the fluctuation in molten steel surface level.
  • the outer diameter of an ordinary immersion nozzle is determined by the sum of the wall thickness (about 20 mm or more) determined in consideration of durability and the inside diameter (about 70 to about 130 mm) determined to ensure a throughput of from 5.4 ton/min (150 mm thick, 2,200 mm wide, and Vc of 2.1 m/min or more) to 14.5 ton/min (240 mm thick, 2,200 mm wide, and Vc of 3.5 m/min or more).
  • the short side length (slab thickness) D is excessively small, the distance between the outer wall of the immersion nozzle and the long side of the solidification shell becomes too small (less than 20 mm), the flow therebetween becomes non-uniform, thereby resulting in generation of longitudinal cracks.
  • the solidification shell is brought into contact with the nozzle and is bonded thereto, resulting in breakout generation.
  • the short side length (slab thickness) D is set to not less than about 150 mm (inside diameter of 70 mm + total outer wall thickness of 40 mm (20 ⁇ 2) + distance between the outer wall of the immersion nozzle and the long side of the solidification shell of 40 mm (20 ⁇ 2)).
  • the long side length (slab width) of the casting space of the mold is not particularly limited and may be equivalent to the width of an ordinary cold-rolled steel sheet (in particular, cold-rolled steel sheet for automobiles). A length of approximately 900 to 2,200 mm is preferred.
  • the height in the vertical direction of the mold is not particularly limited. However, the height is preferably set to approximately 800 to 1,000 mm since a solidification shell must be formed having a certain thickness so that a cast steel slab passing through the mold is not bulged even when casting is performed at a casting speed of more than about 2.0 m/min.
  • the molten steel jetted out of the discharge spout of the immersion nozzle extends its width until it collides against the short side shell.
  • the degree of deceleration and distribution of the jet flow speed of the molten steel which collides against the short side shell depend on the slab width W, the casting speed Vc, and the D/d ratio.
  • the molten steel collides against the short side of the solidification shell, ascends, and then flows along the molten steel surface at the long side when the ratio D/d is out of the optimum range due to the variation in flow speed of the molten steel in the slab thickness direction, the variation of the flow speed in the vicinity of the meniscus may be partly influenced thereby, and the amount of engulfed mold flux increases.
  • the ratio D/d is preferably in the range of from 1.5 to 3.0. However, the ratio is more preferably in the range of from about 2.1 to about 2.9 when the optimum slab thickness, the durability of the immersion nozzle and the required flow rate are also taken into consideration. TABLE 1 No.
  • Fig. 7A shows magnetic application devices 1 disposed at an upper portion of the mold including the molten steel surface level and at a predetermined distance thereunder for applying static magnetic fields in two stages.
  • Fig. 7B shows a magnetic application device 2 is disposed only at an upper portion of the mold including the molten steel surface level for superimposingly applying a static magnetic field and an AC magnetic field.
  • Fig. 7C shows the magnetic application device 2 is disposed at an upper portion of the mold including the molten steel surface level for superimposingly applying a static magnetic field and an AC magnetic field and the magnetic application device 1 is disposed at a predetermined distance under the magnetic field application device 2 for applying a static magnetic field.
  • the magnitude (magnetic flux density) of a DC magnetic field is preferably set to approximately 1,000 to approximately 7,000 gausses when the magnetic field application device for applying a static magnetic field is used.
  • the value mentioned above may be applied in both cases in which two devices are provided at the upper and the lower positions and in which only one device is provided at the lower position.
  • the AC magnetic field there are two types, that is, an AC oscillating magnetic field and an AC travelling magnetic field, and in the invention, both of them are preferably used.
  • Fig. 8 shows the AC oscillating magnetic field is a magnetic field in which AC currents having phases practically opposite to each other are applied to coils adjacent to each other or a magnetic field in which AC currents having the same phase are applied to coils having coiling directions opposite to each other so as to practically invert a magnetic field generated in the adjacent coils.
  • a local flow can be induced in molten steel in the mold when this AC oscillating magnetic field is superimposed on the DC magnetic field.
  • reference numeral 3 indicates a DC coil
  • reference numeral 4 indicates an AC coil
  • reference numeral 5 indicates a mold
  • reference numeral 6 indicates molten steel (portion shown by oblique lines is a slow flow region).
  • the AC travelling magnetic field is a magnetic field obtained when AC currents having phases shifted by 360°/N are applied to N pieces of adjacent optional coils.
  • N 3 (a phase difference of 120°) is used since a high efficiency can be obtained.
  • a local flow can be induced in molten steel in the mold when this AC travelling magnetic field is superimposed on the DC magnetic field.
  • the magnetic flux density of the AC magnetic field is preferably set to approximately 100 to approximately 1,000 gausses when the magnetic field application device for applying an AC magnetic field as described above is used, and the frequency of the oscillating magnetic field is preferably set to approximately 1 to approximately 10 Hz.
  • the magnitude of a DC magnetic field is preferably set to approximately 1,000 to approximately 7,000 gausses
  • the magnetic flux density of an AC magnetic field is preferably set to approximately 100 to approximately 1,000 gausses when the magnetic field application device for superimposingly applying a static magnetic field and an AC magnetic field is used.
  • the state of a circulation flow of molten steel in the mold is varied in accordance with the change in nozzle immersion depth.
  • the immersion depth is optimized since the flow speed from the immersion nozzle is high when the casting speed is high. That is, the flow speed of molten steel at the surface thereof becomes too high when the immersion depth is too small. Engulfment of flux is facilitated as a result.
  • the depth is too large, since the flow speed of molten steel at the surface thereof is decreased too much, the effect of washing the interface of the solidification shell decreases. Trapping of bubbles and inclusions is facilitated as a result.
  • the nozzle immersion depth is set in the range of from about 200 mm to about 350 mm.
  • a material for the immersion nozzle as described above for example, ordinary alumina-graphite is preferably used. However, the material is not limited thereto.
  • a cylindrical nozzle or a two-spout nozzle in which the front end is closed and two approximately circular discharge spouts are provided toward the two short sides of the mold may be generally used.
  • the cross-sectional shape of the discharge spout may be circular, square, or rectangular (longer in a lateral direction, or longer in a longitudinal direction) and is not particularly limited, and any type of shape may be used as long as the maximum width d satisfies the conditions of the invention described later.
  • the heights of the molds were 900 mm (by the production continuous casting apparatus) and 700 mm (by the test continuous casting apparatus), and the immersion nozzle was a two-spout nozzle made of alumina-graphite having a wall thickness of 25 mm, the shape of the discharge spout being square (when the slab thickness is 220 mm or less) or circular (when the slab thickness is more than 220 mm), the downward discharge angle being at a constant value of 20°, and the nozzle immersion depth (the distance from the molten steel surface to the upper end of the discharge spout) being set to 200 to 250 mm.
  • a material As mold flux, a material was used having a solidification temperature of 1,000°C, a viscosity of 0.05 to 0.2 Pa ⁇ s (0.5 to 2.0 poise) at 1,300°C, and a basicity (CaO/SiO 2 ) of 1.0. In addition, the degree of superheat for molten steel in a tundish was set to 10 to 30°C.
  • components of molten steel which had a ultra-low carbon steel composition, were 0.0005 to 0.0090 mass percent of C, less than 0.05 mass percent of Si, less than 0.50 mass percent of Mn, less than 0.035 mass percent of P, less than 0.020 mass percent of S, 0.005 to 0.060 mass percent of Al, less than 0.080 mass percent of Ti, less than 0.050 mass percent of Nb, and less than 0.0030 mass percent of B.
  • the mold oscillation waveform was a sine waveform.
  • the maximum short-side bulging amount, the maximum nail depth, the maximum number of slab surface defects and generation of breakout were measured for the various types of slabs thus formed. The results thereof are shown in Table 3.
  • the maximum short-side bulging amount is preferably 10 mm or less, more preferably 5 mm or less.
  • the maximum nail depth is preferably 1 mm or less, more preferably 0.7 mm or less.
  • Table 3 the results of measurement of rate of surface defects of a cold-rolled steel sheet (sheet thickness of 0.8 mm) are also shown, the cold-rolled steel sheet being obtained by the steps of heating each of the above slabs at a temperature of 1,100 to 1,200°C for 2 to 2.5 hours, followed by hot rolling, cold rolling, and finish annealing in accordance with an ordinary process.
  • the maximum number of slab surface defects was the number (pieces/m 2 ) of bubbles (a diameter of 0.2 mm or more), alumina clusters (a diameter of 500 ⁇ m or more), and slag (including mold flux, a diameter of 0.5 mm or more) per unit area observed after the following sequential steps of milling the slab surface by 1 mm, performing polishing using emery paper #1000, and performing etching using a mixed solution of hydrochloric acid and hydrogen peroxide.
  • the rate of surface defects of a cold-rolled steel sheet was the ratio, on a percent basis, of the number of defects, such as scratches and spills, caused by casting with respect to the total defects, the number of defects being measured on the front and the rear surfaces per 1,000 m of a cold-rolled steel sheet.
  • the generation of breakout was defined as "Yes” when even at least one breakout occurred in casting under each of the individual conditions.
  • Type 1 described as an electromagnetic brake indicates static magnetic field application (EMBR) performed for the entire mold at the vicinity of the bottom end of the mold
  • Type 2 described as an electromagnetic brake indicates static magnetic field application (EMLS) performed for the entire mold at the discharge spout of the immersion nozzle
  • EMLS static magnetic field application
  • the negative strip time tn is one characteristic value for defining the mold oscillation conditions and indicates a period of time in which the descending speed of the mold is higher than that of a cast steel sheet.
  • Table 3 and Fig. 6 when a slab is formed by casting in accordance with the invention, even when the casting speed is high, such as more than about 2.0 m/min, the degree of surface defects of the slab thus formed was slight, and surface defects of a cold-rolled steel sheet formed therefrom were not substantially detected, or even when the defects are present, the number thereof was very small.
  • the operation conditions are preferably optimized so that the following states can achieved:
  • Molten steel (approximately 300 tons), which was obtained by melting in a converter followed by RH treatment, was formed into a slab by continuous casting using a continuous casting apparatus provided with one of the magnetic field application devices shown in Figs. 7A to 7C , the molten steel having a composition containing 0.0015 mass percent of C, 0.02 mass percent of Si, 0.08 mass percent of Mn, 0.015 mass percent of P, 0.004 mass percent of S, 0.04 mass percent of Al, 0.04 mass percent of Ti, and the balance being Fe and inevitable impurities.
  • the manufacturing conditions in this example are shown in Table 2.
  • As the immersion nozzle a two-spout immersion nozzle was used having rectangular discharge spouts each provided with a downward discharge angle of 15°.

Claims (9)

  1. Verfahren zum Herstellen einer kohlenstoffarmen Stahlbramme, umfassend:
    Einbringen von geschmolzenem Stahl in eine Form mit einem Gussbereich mit einer kurzen Seitenlänge D von 150 bis 240 mm durch ein Tauchrohr mit mindestens einer Ablaufmündung mit einer lateralen Breite d, wobei ein Verhältnis D/d im Bereich von 1,5 bis 3,0 ist;
    Gießen des geschmolzenen Stahls mit einer Gießgeschwindigkeit von mehr als 2,0 m/min mit der kontinuierlichen Gießvorrichtung,
    Anwenden einer Bremse unter Anwendung einer elektromagnetischen Kraft auf den Fluss von geschmolzenem Stahl durch Anwenden eines von statischen magnetischen Feldern auf die Form in einer Richtung, die die Formdicke schneidet, mit einer oberen magnetischen Feldanwendungseinrichtung und einer unteren magnetischen Feldanwendungseinrichtung, wobei die untere magnetische Feldanwendungseinrichtung an einem unteren Teil der Form, die eine Oberflächenschicht des geschmolzenen Stahls in der Form beinhaltet, vorgesehen ist und die untere magnetische Feldanwendungseinrichtung an einer unteren Seite der oberen magnetischen Feldanwendungseinrichtung vorgesehen ist, wobei die Ablaufmündung zwischen der oberen magnetischen Feldanwendungseinrichtung und den unteren magnetischen Anwendungseinrichtungen gelegen ist und eine Eintauchtiefe von 200 bis 350 mm hat, und
    Oszillieren der Form bei einer Frequenz von 185 Zyklen/min oder weniger und so, dass ΠSf/Vc > 1 erfüllt wird, wobei S den Oszillationshub der Form anzeigt, F die Oszillationsfrequenz anzeigt und Vc die Gussgeschwindigkeit anzeigt,
    um eine kohlenstoffarme Stahlbramme mit einem Kohlenstoffgehalt von 0,01 Massenprozent oder weniger herzustellen.
  2. Verfahren nach Anspruch 1, wobei die Gussgeschwindigkeit 2,4 m/min oder mehr ist.
  3. Verfahren nach Anspruch 1, wobei das Tauchrohr ein Zwei-Tauchrohr ist.
  4. Verfahren nach Anspruch 1, wobei das Verhältnis D/d 2,1 bis 2,9 ist.
  5. Verfahren nach Anspruch 1, wobei die kohlenstoffarme Stahlbramme ein Ausgangsmaterial für ein kalt gewalztes Stahlblech zum Bilden äußerer Bleche von Automobilen ist.
  6. Verfahren nach Anspruch 1, wobei Anwenden der Bremse durch Anwendung der elektromagnetischen Kraft auf den Fluss von geschmolzenem Stahl durch überlagerndes Anwenden eines statischen magnetischen Felds und eines magnetischen Wechselstromfelds auf die Form in einer Richtung, die die Formdicke schneidet, durchgeführt wird mit einer magnetischen Feldanwendungseinrichtung, die an einem unteren Bereich der Form, eine Oberflächenschicht des geschmolzenen Stahls in der Form beinhaltet, vorgesehen ist, und
    das Tauchrohr an einer unteren Seite der magnetischen Feldanwendungseinrichtung gelegen ist und eine Tauchtiefe von 200 bis 350 mm aufweist.
  7. Verfahren nach Anspruch 1, wobei Anwenden der Bremse unter Anwendung der elektromagnetischen Kraft auf den Fluss von geschmolzenem Stahl durch überlagerndes Anwenden eines statischen magnetischen Felds und eines magnetischen Wechselstromfelds auf die ganze Form in der Richtung, die die Formdicke schneidet, durchgeführt wird unter Verwendung einer oberen magnetischen Feldanwendungseinrichtung und durch Anwenden eines statischen magnetischen Felds auf die Form in einer Richtung, die die Formdicke schneidet unter Verwendung einer unteren magnetischen Feldanwendungseinrichtung,
    wobei die obere magnetische Feldanwendungseinrichtung an einem oberen Bereich der Form, die eine Oberflächenschicht des geschmolzenen Stahls in der Form beinhaltet, vorgesehen ist, und die untere magnetische Feldanwendungseinrichtung an einer unteren Seite der oberen magnetischen Feldanwendungseinrichtung vorgesehen ist, und
    das Tauchrohr zwischen der oberen und der unteren magnetischen Feldanwendungseinrichtungen vorgesehen ist und eine Eintauchtiefe von 200 bis 350 mm aufweist.
  8. Verfahren nach Anspruch 1, wobei der geschmolzene Stahl 0,01 Massenprozent oder weniger von C; 0,01 bis 0,04 Massenprozent von Si, 0,08 bis 0,20 Massenprozent von Mn, 0,008 bis 0,020 Massenprozent von P, 0,003 bis 0,008 Massenprozent von S, 0,015 bis 0,060 Massenprozent von A1, 0,03 bis 0,080 Massenprozent von Ti, 0,002 bis 0,017 Massenprozent von Nb und 0 bis 0,0007 Massenprozent von B; und den Rest Fe und unvermeidbare Verunreinigungen umfasst.
  9. Verfahren nach Anspruch 8, wobei der geschmolzene Stahl 0,0005 bis 0,0090 Massenprozent von C umfasst.
EP04020281A 2003-08-29 2004-08-26 Verfahren zur Herstellung von Stahlbrammen mit ultra-geringem Kohlenstoffgehalt Active EP1510272B1 (de)

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JP2003395818A JP4411945B2 (ja) 2003-11-26 2003-11-26 極低炭素鋼のスラブ連続鋳造方法
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DE602004026253D1 (de) 2010-05-12
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KR100654738B1 (ko) 2006-12-08
EP1510272A1 (de) 2005-03-02
KR20050021961A (ko) 2005-03-07
CN1597182A (zh) 2005-03-23
TWI268820B (en) 2006-12-21
US20060102316A1 (en) 2006-05-18

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