WO2019235615A1 - 鋼の薄スラブ鋳造に用いる連続鋳造用設備および連続鋳造方法 - Google Patents
鋼の薄スラブ鋳造に用いる連続鋳造用設備および連続鋳造方法 Download PDFInfo
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- WO2019235615A1 WO2019235615A1 PCT/JP2019/022730 JP2019022730W WO2019235615A1 WO 2019235615 A1 WO2019235615 A1 WO 2019235615A1 JP 2019022730 W JP2019022730 W JP 2019022730W WO 2019235615 A1 WO2019235615 A1 WO 2019235615A1
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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
- B22D11/12—Accessories for subsequent treating or working cast stock in situ
- B22D11/122—Accessories for subsequent treating or working cast stock in situ using magnetic fields
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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
- B22D11/10—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
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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
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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
- B22D41/00—Casting melt-holding vessels, e.g. ladles, tundishes, cups or the like
- B22D41/50—Pouring-nozzles
Definitions
- the present invention relates to an equipment for continuous casting used for thin slab casting of steel and a continuous casting method.
- This application claims priority on June 7, 2018 based on Japanese Patent Application No. 2018-109469 for which it applied to Japan, and uses the content for it here.
- a thin slab casting method for casting a thin slab (thin cast piece) having a slab thickness of 40 to 150 mm, and further 40 to 100 mm is known.
- the cast thin slab is heated and then rolled by a small-scale rolling mill of about 4 to 7 stages.
- a continuous casting mold used for thin slab casting a method using a funnel mold (funnel mold) and a method using a rectangular parallel mold are employed.
- the casting thickness is generally as thin as 150 mm or less, and further 100 mm or less, while the casting width is about 1.5 m and the aspect ratio is high. Since the casting speed is 5 m / min and high speed casting, the throughput is also high.
- a funnel mold is often used, and the flow in the mold becomes more complicated. Therefore, in order to brake the nozzle discharge flow, a method (electromagnetic brake) in which an electromagnet is arranged on the long side of the mold to brake the flow has been proposed (see Patent Document 1).
- Patent Document 2 discloses a method of installing an immersion nozzle discharge hole at a position where the magnetic flux density in the immersion nozzle discharge hole is 50% or less of the maximum magnetic flux density of the electromagnetic stirring device.
- an electromagnetic brake is generally used to brake the nozzle discharge flow and stabilize the molten metal surface level.
- the gap between the immersion nozzle and the mold long side becomes narrow, and thus the flow of molten steel tends to stagnate in this narrow gap.
- an electromagnetic stirrer (hereinafter sometimes referred to as EMS) is installed on the back side of the long side wall of the mold, and the opposing long side walls are placed.
- EMS electromagnetic stirrer
- the pressure rises at the portion where the stirring flow collides, and the molten metal surface rises.
- the thickness central part In the central part in the thickness direction (hereinafter also referred to as the thickness central part), a phenomenon occurs in which the molten metal surface is recessed. Specifically, as shown in FIG. 2A, the molten steel surface 7 rises at the corner portion by applying a stirring flow so as to swirl within the horizontal section by EMS, and the thickness on the short side wall side is increased. Raises in the center. A powder layer 18 is present on the molten steel surface 7.
- a solidified shell 19 is first formed at the corner portion as shown in FIG. In the central part, solidification starts later than the corner part due to unevenness at the level of the hot water surface. Therefore, as shown in FIG. 2C, the solidification is most delayed at the center of the thickness, and the solidification delay portion 20 is formed further down in the mold.
- the immersion nozzle 2 is provided with a discharge hole 3 directed in the long side direction of the mold 12, and when a discharge flow of molten steel (hereinafter also referred to as a nozzle discharge flow 4) is formed from the discharge hole 3, the thickness of the slab In the direction, the flow velocity is the fastest at the center of the thickness.
- the nozzle discharge flow 4 collides with the short side solidified shell. The solidification delay due to the collision of the nozzle discharge flow with the short-side solidified shell is most noticeable in the thickness center in the thickness direction of the slab.
- the present invention has been made in view of such circumstances, and an object of the present invention is to provide a steel continuous casting equipment and a continuous casting method capable of preventing a vertical crack at the center of the long side of a slab in thin slab casting.
- a first aspect of the present invention is a continuous casting facility used for casting a thin slab of steel having a cast slab thickness of 150 mm or less and a casting width of 2 m or less in the mold, each of which is composed of a copper plate.
- the mold for molten steel casting provided with a pair of long side walls and a pair of short side walls, an immersion nozzle for supplying molten steel into the mold, and the back side of the pair of long side walls, An electromagnetic stirrer arranged along the long side wall and capable of applying a swirling flow on the surface of the molten steel in the mold, and satisfies the following formulas (1) -a and (1) -b
- the thickness T (mm) of the slab casted in step (2) may satisfy the following formula (2). 0.01 ⁇ ⁇ / T ⁇ 0.1 (2)
- a second aspect of the present invention is a steel continuous casting method using the steel continuous casting equipment described in (1) or (2) above, wherein the following (1) -a formula, ( 1) The thickness D Cu (mm) of the copper plate, the thickness T (mm) of the slab, the frequency f (Hz) of the electromagnetic stirrer, and the electric conductivity ⁇ ( S / m) and a continuous casting method of steel for adjusting the electrical conductivity ⁇ Cu (S / m) of the copper plate.
- ⁇ 2 ⁇ f: angular velocity (rad / sec)
- ⁇ vacuum permeability (N / A 2 ).
- the equipment for continuous casting and the continuous casting method used for thin slab casting of steel according to the present invention includes an electromagnetic stirrer installed in a mold in thin slab casting, and further optimizes the frequency of the alternating current applied to the electromagnetic stirrer. As a result, a swirl flow is formed in the vicinity of the molten metal surface level even in thin slab casting with a slab thickness of 150 mm or less. Thereby, it is possible to make solidification uniform on the long side surface, and it is possible to prevent vertical cracks at the center of the long side of the slab.
- the flat cross-sectional shape of the inner surface of the short side wall is a curved shape and the formation range is defined, solidification on the short side wall side can be made uniform, and the shape of the solidified part on the short side wall side is made rectangular (flat) Shape).
- FIG. 1 It is a perspective conceptual diagram explaining the molten steel flow in the casting_mold
- a facility for continuous casting of a thin slab cast having a cast slab thickness of 150 mm or less according to an embodiment of the present invention (hereinafter referred to as a continuous casting facility according to the present embodiment) will be described.
- the slab thickness may be greater than 100 mm.
- the equipment for continuous casting includes a mold 12 for casting a molten steel having a pair of long side walls and a pair of short side walls, each of which is made of a copper plate and arranged opposite to each other, and a molten steel 6 in the mold.
- the immersion nozzle 2 to be supplied and the rear surface of the pair of long side walls are arranged along the long side wall, and a swirl flow 9 is given to the molten steel in the vicinity of the molten steel surface 7 (hereinafter also referred to as a molten metal surface) in the mold.
- FIG. 1 the schematic diagram of the molten steel flow in a casting_mold
- the long side wall and the short side wall of the mold 12 are not shown, and the casting space 5 surrounded by the long side wall and the short side wall is shown. Since the molten steel surface 7 in the mold is normally cast around 100 mm from the upper end of the mold, the position below 100 mm from the upper end of the mold is referred to as a meniscus position P1 in the following description.
- the continuous casting equipment has the following configuration (a).
- the continuous casting facility preferably further has the following configuration (b) and configuration (c).
- the cross-sectional shape of the inner surface of the short side wall 10 (hereinafter also referred to as the inner surface shape) is a curved shape projecting outside the mold near the meniscus position P1, as shown in FIG.
- the amount of protrusion of the curved shape is sequentially reduced (narrowed) downward, and the lower portion (other than the curved shape) is flattened.
- the part which protruded in the curved shape turns into a recessed part seeing from the casting_mold
- template 12 it is also called the recessed part 14.
- the formation range of the curved shape is equal to or lower than the lower end 16 (lower end position of the core (iron core)) of the electromagnetic stirrer from the meniscus position P1, and the immersion depth 17 of the immersion nozzle. It is set as the range to position P2 above.
- the immersion depth 17 of the immersion nozzle is the depth of the lower end position of the discharge hole 3 (for example, about 200 to 350 mm), and the lower end position of the discharge hole 3 of the immersion nozzle is from the lower end 16 of the electromagnetic stirrer. Located below.
- the solidification on the short side wall side can be made uniform, and the shape of the solidified part on the short side wall side can be made rectangular (flat shape). This eliminates subepidermal cracking at the center of the long side width and at the center of the short side thickness, and further eliminates breakout due to solidification delay near the center of the short side thickness.
- the present inventors examined conditions for forming a stirring flow at the surface of the molten steel in the mold in thin slab casting with a slab thickness of 150 mm or less. For that purpose, first, it is important that the skin depth of the AC magnetic field formed by the electromagnetic stirring device 1 is larger than the copper plate thickness D Cu of the long side wall 15 of the mold. This condition is defined by the following equation (1) -a. That is, the skin depth of the electromagnetic field in the conductor needs to be larger than the copper plate thickness D Cu . D Cu ⁇ (2 / ⁇ Cu ⁇ ) (1) -a
- This equation shows the relationship between the skin depth of electromagnetic force and the slab thickness, and the skin depth of electromagnetic force is defined by 1/2 of the skin depth of the electromagnetic field in the conductor. This is because the electromagnetic force is current density ⁇ magnetic flux density, but the penetration of the current density and magnetic field into the conductor is described by ⁇ (2 / ⁇ ), and therefore the skin depth of the electromagnetic force of the product is 1 / 2 ⁇ ⁇ (2 / ⁇ ), which is described by ⁇ (1 / 2 ⁇ ).
- ⁇ 2 ⁇ f: angular velocity (rad / sec), ⁇ : permeability of vacuum (N / A 2 ), D Cu : mold copper plate thickness (mm), T: slab thickness (mm), f: frequency (Hz), ⁇ : electric conductivity (S / m) of molten steel, ⁇ Cu : copper plate electric conductivity (S / m).
- FIG. 1 An example of the influence of the electromagnetic stirring frequency on the mold skin depth and the molten steel electromagnetic force skin depth is shown in FIG.
- the expression (1) -a can be satisfied if the electromagnetic stirring frequency f is less than 20 Hz.
- the expression (1) -b can be satisfied if the electromagnetic stirring frequency f is greater than 10 Hz.
- an electromagnetic stirrer is installed in the mold in the thin slab casting, and the frequency of the alternating current applied to the electromagnetic stirrer is optimized so that the surface of the slab is cast even in a thin slab cast with a slab thickness of 150 mm or less.
- a swirling flow is formed near the level.
- the present inventors have studied a method for homogenizing solidification in the vicinity of the short side wall under the flow of molten steel obtained by applying EMS.
- C steel subperitectic steel
- a slab having a width of 1200 mm and a thickness of 150 mm was cast at a casting speed of 5 m / min.
- the molten steel surface position in the mold was set to 100 mm from the upper end of the mold.
- the casting was performed using a continuous casting facility in which the electromagnetic stirrer 1 (EMS) is mounted on the back side of the long side wall 15 for the purpose of forming a swirl flow in the horizontal cross section near the meniscus. .
- EMS electromagnetic stirrer 1
- the EMS was placed so that the upper end of the EMS core coincided with the meniscus position P1 in the mold (100 mm from the upper end of the mold).
- the core thickness of the EMS is 200 mm
- the lower end 16 of the electromagnetic stirrer is 200 mm from the meniscus position.
- the immersion depth 17 of the immersion nozzle was 250 mm from the meniscus position P1. Further, casting was performed without using an electromagnetic stirring device under the same conditions.
- a linear negative segregation line called a white band 21 showing a solidified shell front at a certain moment is observed on the slab cross section. This occurs because the molten steel flow hits the solidified shell and the concentrated molten steel in front of the solidified shell is washed away. Therefore, the thickness from the surface 25 of the slab 22 to the white band 21 represents the thickness of the solidified shell at the position where the molten steel flow collides.
- the flow rate of the stirring flow at the molten metal surface is ensured to be 20 cm / sec by adjusting the thrust 8 of electromagnetic stirring. I also found it possible.
- the formation range of the curved shape is a range from the meniscus position P1 (position 100 mm from the upper end of the mold) to the position P2 shown in FIG.
- the curved shape is continuously formed from the meniscus position P1 to the upper end of the mold as shown in FIG.
- the molten metal level in the mold is adjusted so that the meniscus position P1 is at the molten metal level (molten steel surface 7).
- the electromagnetic stirring conditions were such that the above formulas (1) -a and (1) -b were satisfied, and the thrust of the electromagnetic stirring was adjusted so that the flow rate of the stirring flow on the molten metal surface was 30 cm / second. .
- the lower end position P2 of the curved shape forming range was set to 200 mm in the casting direction from the surface level (meniscus position P1).
- the lower end position P2 is equal to the lower end 16 of the electromagnetic stirrer and is located above the immersion depth 17 of the immersion nozzle.
- the amount of overhang ⁇ at the meniscus position P1 was changed to 0 to 15 mm, and the influence on the solidification uniformity of the slab was evaluated by setting B / A in FIG. 5 as the solidification uniformity.
- the flat cross-sectional shape can be selected from an arc shape, an elliptical shape, a sine curve, and other arbitrary curved shapes.
- an arc shape adopted, the inner side shape of the short side wall is made into a gently curved shape so as to protrude outside the mold near the meniscus, based on the schematic diagram shown in FIG.
- ⁇ / T at the meniscus position P1 is expressed by the curvature radius R (mm) of the curved shape and the thickness T (mm) of the slab, the relationship of the following expression (3) is obtained.
- ⁇ / T R / T ⁇ ( ⁇ (4R 2 ⁇ T 2 )) / (2T) (3)
- FIG. 8 shows the results (relationship between the radius of curvature R and the overhang amount ⁇ ) obtained by using the above equation (3) with the thickness T of the slab being 150 mm. It was found that within the range shown, the above formula (2) was satisfied and high coagulation uniformity was obtained.
- FIG. 9 shows the result.
- the overhang range on the horizontal axis is the distance from the meniscus position P1 to the lower end position P2 of the curved shape.
- the upper end of the core of the EMS is at the meniscus position P1, and the thickness in the height direction of the core (hereinafter also referred to as core thickness) is 200 mm. Therefore, the lower end 16 of the electromagnetic stirrer is 200 mm from the meniscus position P1. is there. If the lower end position P2 of the region (formation range) provided with the overhang is equal to or lower than the lower end 16 of the electromagnetic stirring device, the improvement effect by providing the overhang was obtained. However, when the formation range of the overhang is 100 mm, which is shorter than the core thickness of EMS, the improvement in solidification uniformity was insufficient.
- the overhang formation range was longer than the core thickness of EMS and longer than 250 mm, which is the immersion depth 17 of the immersion nozzle, the effect was small. Therefore, the above-described configuration (c) is also included in the preferable configuration of the short side wall of the mold.
- the test was performed by changing the EMS current value and swinging the molten steel flow velocity at the meniscus to 1 m / sec.
- the molten steel flow velocity was calculated from the dendrite inclination angle of the slab cross section.
- the effect of improving the solidification uniformity was obtained under the above conditions until the molten steel flow velocity at the meniscus was 60 cm / second or less, including the condition where EMS was not applied. Uniform solidification could not be achieved only by changing the inner surface shape.
- solidification and homogenization can be achieved by applying a molten steel flow velocity of 20 cm / second or more, more preferably by applying a molten steel flow velocity of about 30 cm / second.
- the application range of the steel continuous casting equipment of the present invention is that the meniscus flow rate is 60 cm / second or less (particularly, the lower limit is 10 cm / second), and the raised height on the short side wall side is 30 mm or less. I can say that.
- the upper end width and the lower end width of the mold may be set by changing the setting angle of the short side wall according to the taper rate selected in each casting condition on the basis of the corner portion when no overhang is formed.
- the overhang formation range may be set so as to be in a range from the meniscus position P1 to the position P2 which is equal to or greater than the core thickness of the EMS and above the immersion depth of the immersion nozzle.
- the ratio ⁇ / T between the overhang amount ⁇ (mm) at the position P1 and the thickness T (mm) of the slab can be adjusted to 0.01 or more and 0.1 or less (that is, the above equation (2)). preferable.
- the lower end position of the short side protrusion is preferably set to a position from the core lower end position of the EMS to a maximum of 150 mm from the lower end of the core. .
- the size of the mold can be variously changed according to the size of the cast slab (slab) to be cast.
- the thickness (the interval between the opposing long side walls) is about 100 to 150 mm and the width (the opposing short sides).
- the size of the slab is about 1000 to 2000 mm.
- the continuous casting equipment according to this embodiment has a casting speed of 3 m / min. It is preferable to apply to the above casting.
- the upper limit value is not defined, but the upper limit value that is currently available is, for example, about 6 m / min.
- the continuous casting according to the present embodiment is performed even when the stirring flow is applied so as to form a swirling flow near the molten metal surface, that is, the molten metal surface is raised at the corner and recessed at the center of the thickness.
- solidification delay in the central part of the short side can be prevented, and solidification progresses uniformly.
- the solidification can be made uniform by narrowing down uniformly in the thickness direction by a normal taper.
- the shape of the short side wall can be a straight line, and the solidification delay at the short side thickness central portion can be eliminated.
- the effect of relieving the pressure when the swirling flow collides with the corner can be obtained. Therefore, it also has an effect of reducing the unevenness of the hot water surface shape on the short side wall side.
- the mold skin depth is larger than the mold copper plate thickness (satisfaction (1) -a is satisfied), and the skin depth of electromagnetic force is larger than the slab thickness. It was found that by setting the frequency to be small (satisfaction of (1) -b formula), the molten steel flow velocity was 20 cm / second or more, and the swirling flow was efficiently formed at the level of the molten metal.
- the taper of the short side wall was 1.4% / m.
- the taper of the short side wall is the inner surface (slab contact surface) of the short side wall on both sides (when there is a recess, the deepest part of the recess when the short side wall is viewed in plan view) )
- Is a value expressed in% by dividing the difference between the distance A at the upper end of the mold and the distance B at the lower end of the mold by the length L of the short side wall in the vertical direction (casting direction). That is, taper (%) (AB) / L ⁇ 100.
- the solidification structure of C cross section of slab was investigated. Similar to FIG. 6, the white band 21 (see FIG. 5) in which the solidified structure appears and is observed by etching is observed from the surface in the region from the corner portion 26 toward the center of the width on the long side 23 side of the slab.
- solidification uniformity 0.7 or more was evaluated as favorable. Furthermore, it was investigated whether or not subepidermal cracking was observed in the coagulation delay part. The method for evaluating subepidermal cracking is as described above.
- the mold resistance was also examined.
- the oscillation current is measured, and the value smaller than the oscillation current value when the sticking breakout occurs is “small”.
- the oscillation current value when the sticking breakout occurs The above cases were evaluated as “large”.
- Table 2 shows the test conditions and results.
- Invention Examples 2 to 4 shown in Table 2, the lower end of the curved shape of the short side wall is unified to the meniscus position P1 to 200 mm ( the same position as the lower end of the electromagnetic stirrer), and ⁇ / T Shows the results when 0.012, 0.05, and 0.093 within the preferable range (0.01 to 0.1), but the solidification uniformity is not increased without increasing the mold resistance. A value of 0.7 or more was obtained, which was greatly improved. Moreover, since the coagulation uniformity was improved, no coagulation delay part was observed, and no epidermal crack was observed. On the other hand, Invention Example 1 was a condition in which no overhang was provided, but the solidification uniformity was lower than that of Invention Examples 2 to 4.
- Invention Example 5 is a condition in which ⁇ / T is set to 0.12, which exceeds the upper limit value of the preferred range, although an overhang is provided. In this case, although the solidification uniformity was relatively good, the resistance value was locally increased, and the surface property was partially constrained.
- Invention Example 6 is a condition in which ⁇ / T is set to 0.007, which is less than the lower limit of the preferred range, although an overhang is provided. In this case, the solidification uniformity was 0.66, which was better than that of Invention Example 1 without bending, but small epidermal cracks were scattered.
- Invention Example 7 although the overhang was provided and ⁇ / T was set to 0.03 within a preferable range, the formation range of the overhang was shorter than the core thickness of the EMS, so that the solidification uniformity was invented. Compared to Examples 2 to 4, the value was low.
- Invention Example 8 provided an overhang, ⁇ / T was set to 0.03 within a preferable range, and the formation range of the overhang was set to 0.4 m that is equal to or greater than the core thickness of the EMS and equal to or greater than the immersion depth of the immersion nozzle. It is a result. In this case, the effect of improving the solidification uniformity was small as compared with Invention Examples 2-4. In addition, subepidermal cracking due to the delayed solidification part was also observed.
- Invention Example 9 provided an overhang and ⁇ / T was set to 0.04 within the preferred range, but the overhang formation range was set to 0.5 m, which is equal to or greater than the immersion depth of the immersion nozzle. Was smaller than Invention Examples 2 to 4.
- Inventive Example 10 provided an overhang and ⁇ / T was set to 0.013 within the preferred range, but the overhang formation range was set to 0.4 m, which is equal to or greater than the immersion depth of the immersion nozzle. Compared to Invention Examples 2 to 4, it was small.
- subepidermal cracking due to the delayed solidification part was also observed. In any of Invention Examples 7 to 10, no vertical crack was observed at the center of the long side surface of the slab.
- Comparative Example 1 does not perform electromagnetic stirring in the mold and does not have a curved shape of the short side wall.
- the solidification uniformity was only 0.2, which was a level at which there was a risk of interruption of casting (breakout).
- breakout since no swirl flow was formed, a large vertical crack occurred in the center of the width of the long side of the slab.
- the maximum value of the overhang amount ⁇ is set to be the central portion of the thickness of the short side wall, but, for example, it is shifted from the central portion of the thickness to the corner side according to the size and configuration of the mold. You can also.
- the curved protrusion is formed in a range from the upper end of the short side wall to the position P2 below the EMS lower end and above the immersion depth of the immersion nozzle, but at least from the meniscus position P1. If it forms in the direction, it will not specifically limit.
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Abstract
Description
本願は、2018年6月7日に、日本に出願された特願2018-109469号に基づき優先権を主張し、その内容をここに援用する。
(1)本発明の第一の態様は、鋳型内の鋳片厚みが150mm以下、鋳造幅が2m以下の鋼の薄スラブ鋳造に用いる連続鋳造用設備であって、それぞれ銅板から構成されると共に対向配置された、一対の長辺壁と一対の短辺壁とを備えた溶鋼鋳造用の鋳型と、前記鋳型内に溶鋼を供給する浸漬ノズルと、前記一対の長辺壁の裏面側に前記長辺壁に沿って配置され、前記鋳型内の溶鋼表面で旋回流を付与することのできる電磁攪拌装置と、を有し、下記(1)-a式、(1)-b式を満足するように、前記長辺壁の前記銅板の厚みDCu(mm)、前記鋳片の厚みT(mm)、前記電磁攪拌装置の周波数f(Hz)、前記溶鋼の電気伝導度σ(S/m)、及び、前記長辺壁の前記銅板の電気伝導度σCu(S/m)が調整される鋼の連続鋳造用設備である。
DCu<√(2/σCuωμ) (1)-a
√(1/2σωμ)<T (1)-b
ここで、ω=2πf:角速度(rad/sec)、μ=4π×10-7:真空の透磁率(N/A2)である。
(2)上記(1)に記載の鋼の連続鋳造用設備では、前記短辺壁の内面の平断面形状が、前記鋳型の上端から100mm下方の位置であるメニスカス位置で前記鋳型の外側に張り出す湾曲形状であり、前記湾曲形状の張り出し量が鋳造方向の下方に向けて順次減少し、前記鋳型内の下部で平坦形状であり、前記湾曲形状の形成範囲が、前記メニスカス位置から、前記電磁攪拌装置の下端と同等またはそれよりも下方であって前記浸漬ノズルの浸漬深さよりも上方の位置までの範囲であり、前記湾曲形状の前記メニスカス位置での張り出し量δ(mm)と、前記鋳型で鋳造する前記鋳片の厚みT(mm)とが、下記(2)式の関係を満足してもよい。
0.01≦δ/T≦0.1 (2)
DCu<√(2/σCuωμ) (1)-a
√(1/2σωμ)<T (1)-b
ここで、ω=2πf:角速度(rad/sec)、μ:真空の透磁率(N/A2)である。
さらに、短辺壁の内面の平断面形状を湾曲形状とし、その形成範囲を規定する場合、短辺壁側における凝固の均一化が図れ、短辺壁側の凝固部分の形状を矩形化(平坦形状)することができる。これにより、長辺幅中央部や短辺厚み中央での表皮下割れがなくなり、更には、短辺厚み中央近傍での凝固遅れによるブレークアウトがなくなる。
その結果、鋳型内の湯面近傍で旋回流を付与しつつ凝固の均一化が図れ、鋳造速度の高速化も可能となり好適である。
構成(a)を有することで、鋳型内の鋳片厚みが150mm以下の薄スラブ鋳造においてもメニスカス部で攪拌流を形成することができる。
構成(b):短辺壁10の内面の平断面形状(以下、内面形状ともいう)を、図3に示すように、メニスカス位置P1の近傍で鋳型の外側に張り出した湾曲形状とし、鋳造方向の下方に向けて、湾曲形状の張り出し量を順次減少させ(絞り込む)、下部(湾曲形状以外)で平坦形状とする。なお、湾曲形状に張り出した部分は、鋳型12から見て凹んだ部分となるため、凹部14ともいう。
構成(c):湾曲形状の形成範囲を、メニスカス位置P1から、電磁攪拌装置の下端16(コア(鉄芯)の下端位置)と同等またはそれよりも下方であって浸漬ノズルの浸漬深さ17よりも上方の位置P2までの範囲とする。なお、浸漬ノズルの浸漬深さ17とは、吐出孔3の下端位置の深さ(例えば、200~350mm程度)であり、浸漬ノズルの吐出孔3の下端位置は、電磁攪拌装置の下端16より下方に位置している。
本発明者らは、150mm以下の鋳片厚みの薄スラブ鋳造において、鋳型内溶鋼表面部で攪拌流を形成するための条件について検討した。
そのためには、まず、電磁攪拌装置1によって形成される交流磁場の表皮深さが鋳型長辺壁15の銅板厚みDCuよりも大きくすることが重要である。この条件は下記(1)-a式で規定される。すなわち、導体中での電磁場の表皮深さが銅板厚みDCuよりも大となる必要がある。
DCu<√(2/σCuωμ) (1)-a
√(1/2σωμ)<T (1)-b
上記(1)-a式、(1)-b式において、ω=2πf:角速度(rad/sec)、μ:真空の透磁率(N/A2)、DCu:鋳型銅板厚み(mm)、T:鋳片厚み(mm)、f:周波数(Hz)、σ:溶鋼の電気伝導度(S/m)、σCu:銅板電気伝導度(S/m)である。
本発明者らは、EMSを印加することによって得られる溶鋼の流動下で、短辺壁近傍の凝固を均一化する方法について検討した。
1)長辺壁と短辺壁の各方向への凝固収縮を補償できること
2)コーナー部近傍の形状変化に対し、鋳型自体の構成で追随できること
3)攪拌流の衝突によるコーナー部での圧力上昇を緩和できること
の3点が可能となるのではないかと考えた。
そこで、短辺壁10の内面形状が異なる鋳型を作製し、その鋳型を用いて鋳造を行い、短辺壁10の内部形状が鋳片の形状に及ぼす影響を調査した。
ここで、鋳造は、メニスカス近傍で水平断面内に旋回流を形成することを目的として、長辺壁15の背面側に電磁攪拌装置1(EMS)を搭載した連続鋳造用設備を用いて行った。なお、EMSの設置は、EMSコアの上端が鋳型内のメニスカスの位置P1(鋳型上端から100mm)と一致するように行った。EMSのコア厚は200mmであり、電磁攪拌装置の下端16はメニスカス位置から200mmである。浸漬ノズルの浸漬深さ17はメニスカス位置P1から250mmであった。また、同一条件ながら、電磁攪拌装置を用いない鋳造も行った。
また、鋳型抵抗は、測定したオシレーション電流値と、スティッキング性ブレークアウトが生じた際のオシレーション電流値とを比較することで、大小を評価した。
まず、鋳型銅板の材質、厚みが異なる鋳型を幾つか製作するとともに、電磁攪拌装置1に印加する交流電流の周波数fが異なる条件で鋳造を行った。鋳造した鋳片の幅中央部について、凝固組織を調査し鋳片表面から内部に向けて成長しているデンドライトの傾き角、すなわち、長辺表面の垂線に対する角度を測定するとともに、その傾き方向について調査した。デンドライトの傾き角と傾き方向から、非特許文献2に基づき、当該部位における溶鋼の流速と流れ方向の評価を行った。その結果、電磁攪拌装置1に通電する交流電流の周波数fと鋳型銅板の電気伝導度σCu(S/m)、銅板厚みDCu(S/m)、及び鋳片の厚みT(mm)との間で以下の関係を満足する条件であれば、メニスカス部で好ましい旋回流が形成されていることを見出した。
DCu<√(2/σCuωμ) (1)-a
√(1/2σωμ)<T (1)-b
ここで、ω=2πf:角速度(rad/sec)、μ:真空の透磁率(N/A2)、σ:溶鋼の電気伝導度(S/m)である。
0.01≦δ/T≦0.1 (2)
δ/T=R/T-(√(4R2-T2))/(2T) (3)
1)短辺壁の内面を湾曲形状とすることにより、平断面視した短辺壁の内面長さが実質的に変わる(増大する)ことになるため、メニスカス近傍で長辺壁にテーパーを付与したのと同じ効果が得られる。
2)コーナー部の形状についても、メニスカスでは90度よりも鈍角となるため、コーナー部の圧力上昇が緩和され、盛り上がり量そのものが小さくなる。
3)鋳型は、鋳片に対して鋳造方向に、短辺全体を絞り込むように短辺形状をR状からフラットに変化させる。そのため、EMSによる溶鋼の盛り上がりが生じて短辺厚み中央部で盛り下がることで、凝固遅れが生じやすい、短辺厚み中央部の凝固均一化に有効である。
従って、鋳型の短辺壁の好ましい構成に、上記した構成(c)も含めた。
ここでは、EMSの電流値を変化させ、メニスカスでの溶鋼流速を1m/秒まで振って試験を行った。溶鋼流速は、前述のように、鋳片断面のデンドライト傾角から算出した。その結果、EMSを印加しない条件を含めて、メニスカスでの溶鋼流速が60cm/秒以下までは、上記した条件で凝固均一化の改善効果が得られたが、60cm/秒を超えると、鋳型の内面形状の変更のみでは、凝固の均一化が図れなかった。
短辺壁は、一段のテーパーを前提としている。そのため、張り出しを形成しない場合のコーナー部を基準にして、それぞれの鋳造条件において選択されるテーパー率に従い、短辺壁の設定角度を変え、鋳型の上端幅と下端幅を設定すればよい。その際、メニスカスの位置P1から、EMSのコア厚以上であって浸漬ノズルの浸漬深さよりも上方の位置P2までの範囲となるように、張り出しの形成範囲を設定すればよく、更には、メニスカスの位置P1での張り出し量δ(mm)と鋳片の厚みT(mm)との比δ/Tを、0.01以上0.1以下(即ち、前記した(2)式で調整することが好ましい。
転炉での精錬と還流式真空脱ガス装置での処理、並びに合金添加により、0.1%C鋼(亜包晶鋼)を溶製した。そして、この溶鋼を、幅1800mm、厚み150mmのスラブに鋳造した。
前記した図6と同様、凝固組織をエッチングにて現出し観察されるホワイトバンド21(図5参照)について、鋳片の長辺23側でコーナー部26から幅中央に向かった領域において、表面からホワイトバンドまでの厚みが、略一定となった部位の厚みAと、短辺厚み中央の最も薄い部位の厚みBとの比、即ちB/Aを、凝固均一度とした。なお、凝固均一度については、0.7以上を良好として、評価した。
更に、凝固遅れ部に表皮下割れが見られるか否かを調査した。表皮下割れの評価方法は前述のとおりである。
そして、発明例7については、張り出しを設けて、δ/Tを好適範囲内の0.03としたものの、張り出しの形成範囲が、EMSのコア厚と比較して短かったため、凝固均一度が発明例2~4に対比して低値であった。発明例8は、張り出しを設けて、δ/Tを好適範囲内の0.03とし、張り出しの形成範囲を、EMSのコア厚以上、かつ、浸漬ノズルの浸漬深さ以上の0.4mとした結果である。この場合、凝固均一度の改善効果が発明例2~4に対比して小さかった。また、凝固遅れ部による表皮下割れも観察された。発明例9は、張り出しを設けて、δ/Tを好適範囲内の0.04としたものの、張り出しの形成範囲を浸漬ノズルの浸漬深さ以上の0.5mとしたため、凝固均一度の改善効果が発明例2~4に対比して小さかった。また、凝固遅れ部による表皮下割れも観察された。発明例10は、張り出しを設け、δ/Tを好適範囲内の0.013としたものの、張り出しの形成範囲を浸漬ノズルの浸漬深さ以上の0.4mとしたため、凝固均一度の改善効果が発明例2~4に対比して小さかった。また、凝固遅れ部による表皮下割れも観察された。発明例7~10のいずれも鋳片の長辺面中央には縦割れ発生が見られなかった。
2 浸漬ノズル
3 吐出孔
4 ノズル吐出流
5 鋳造空間
6 溶鋼
7 溶鋼表面
8 推力
9 旋回流
10、11 短辺壁
12 鋳型
14 凹部
15 長辺壁
16 電磁攪拌装置の下端
17 浸漬ノズルの浸漬深さ
18 パウダー層
19 凝固シェル
20 凝固遅れ部
21 ホワイトバンド
22 鋳片
23 長辺
24 短辺
25 表面
26 コーナー部
27 厚み中央
P1 メニスカス位置
P2 湾曲形状下端位置
δ 張り出し量
T 鋳型内の鋳片厚み
Claims (3)
- 鋳型内の鋳片厚みが150mm以下、鋳造幅が2m以下の鋼の薄スラブ鋳造に用いる連続鋳造用設備であって、
それぞれ銅板から構成されると共に対向配置された、一対の長辺壁と一対の短辺壁とを備えた溶鋼鋳造用の鋳型と、
前記鋳型内に溶鋼を供給する浸漬ノズルと、
前記一対の長辺壁の裏面側に前記長辺壁に沿って配置され、前記鋳型内の溶鋼表面で旋回流を付与することのできる電磁攪拌装置と、
を有し、
下記(1)-a式、(1)-b式を満足するように、前記長辺壁の前記銅板の厚みDCu(mm)、前記鋳片の厚みT(mm)、前記電磁攪拌装置の周波数f(Hz)、前記溶鋼の電気伝導度σ(S/m)、及び、前記長辺壁の前記銅板の電気伝導度σCu(S/m)が調整されることを特徴とする鋼の連続鋳造用設備。
DCu<√(2/σCuωμ) (1)-a
√(1/2σωμ)<T (1)-b
ここで、ω=2πf:角速度(rad/sec)、μ=4π×10-7:真空の透磁率(N/A2)である。 - 前記短辺壁の内面の平断面形状が、前記鋳型の上端から100mm下方の位置であるメニスカス位置で前記鋳型の外側に張り出す湾曲形状であり、前記湾曲形状の張り出し量が鋳造方向の下方に向けて順次減少し、前記鋳型内の下部で平坦形状であり、
前記湾曲形状の形成範囲が、前記メニスカス位置から、前記電磁攪拌装置の下端と同等またはそれよりも下方であって前記浸漬ノズルの浸漬深さよりも上方の位置までの範囲であり、
前記湾曲形状の前記メニスカス位置での張り出し量δ(mm)と、前記鋳型で鋳造する前記鋳片の厚みT(mm)とが、下記(2)式の関係を満足する
ことを特徴とする請求項1に記載の鋼の連続鋳造用設備。
0.01≦δ/T≦0.1 (2) - 請求項1又は2に記載の鋼の連続鋳造用設備を用いた鋼の連続鋳造方法であって、
下記(1)-a式、(1)-b式を満足するように、前記銅板の厚みDCu(mm)、前記鋳片の厚みT(mm)、前記電磁攪拌装置の周波数f(Hz)、前記溶鋼の電気伝導度σ(S/m)、及び、前記銅板の電気伝導度σCu(S/m)を調整する
ことを特徴とする鋼の連続鋳造方法。
DCu<√(2/σCuωμ) (1)-a
√(1/2σωμ)<T (1)-b
ここで、ω=2πf:角速度(rad/sec)、μ:真空の透磁率(N/A2)である。
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