JP2009248089A - Continuous casting method for extralow carbon steel or low carbon steel using grooved immersion nozzle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique, when an extralow carbon steel or a low carbon steel is subjected to continuous casting, which can produce a high quality slab having reduced hole defects and powder defects at high productivity. <P>SOLUTION: The inner bottom face 3 of a molten steel discharge hole 2 is engraved with a molten steel discharge groove 5; the following formulae (1) to (3) are satisfied; casting velocity Vc [m/min] is controlled to 1.6 to 2.4; the superheat degree ΔT[°C] of molten steel is controlled to 20 to 45; a/A=0.1 to 0.9 (1), b/B=0.4 to 1.0 (2) and Δθ[deg.]=15 to 45 (3); wherein a denotes the engraving width of the molten steel discharge groove 5; A denotes the width of the opening edge 9 on the outer circumferential side of the molten steel discharge hole 2; b denotes the separation distance between an intersection line 10 between the inner bottom face 3 of the molten steel discharge hole 2 and the inner bottom face of the molten steel discharge groove 5, and the outer circumferential face of the immersion nozzle 1 in the radial direction; and B denotes the thickness of the circumferential wall of the immersion nozzle 1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a continuous casting method of ultra-low carbon steel or low-carbon steel having a carbon content C [wt%] of 0.06 or less.

  Patent Document 1 (Japanese Patent Application Laid-Open No. 2007-33003) is based on the applicant of the present application and has an extremely low carbon steel or a low carbon steel (hereinafter simply referred to as a carbon content C [wt%]) of 0.06 or less. It refers to the continuous casting method of low carbon steel etc.). First, this low carbon steel and the like will be introduced from various viewpoints as compared with a medium and high carbon steel (hereinafter, simply referred to as a medium and high carbon steel) having a carbon content C [wt%] of 0.07 or more.

  In general, low carbon steel and the like are often used for thin steel sheets used for press molding and the like because they are excellent in workability compared to medium and high carbon steels. From the viewpoint of continuous casting, this low-carbon steel has a high so-called liquidus temperature at which solidification starts and a solid-liquid coexistence temperature range is narrow, so that it is difficult to cause a significant solidification delay. However, it is said that continuous casting can be performed stably. However, the above-mentioned first and second problems are well known for the above-mentioned low carbon steel.

<First problem>
Low-carbon steel has a high liquidus temperature and a narrow solid-liquid coexistence temperature range, so that the solidus temperature at which solidification is completed is high. For this reason, there exists a characteristic which is easy to ensure the stable solidification shell thickness, and this point is a big merit in raising productivity as mentioned above. However, on the other hand, solidification proceeds just by a slight drop in temperature even immediately under the molten steel meniscus in the mold where solidification is not desirable (that is, the interface between the molten steel and the mold flux floating above it). There is a problem that it is easy. When a phenomenon called so-called `` skinning '' where a solidified shell is formed on the meniscus and a phenomenon called so-called `` nail '' where the upper end of solidification that grows between the mold copper plate and melting is bent to the meniscus side, The bubbles floating in the molten steel in the mold are blocked from passing through the mold flux and into the air, and the bubbles are easily trapped in the “skinned” and “claw” portions. As a result, defects such as blow holes (holes connected to the slab surface) and pin holes (holes not connected to the slab surface) (hereinafter referred to as hole defects) are likely to occur on the slab surface layer.

  The “inclusions” mainly refer to alumina and are inevitably generated in a normal operation of deoxidizing with aluminum or the like. The “bubbles” refer to Ar bubbles, and are unavoidable in a normal operation in which Ar gas is blown into the immersion nozzle so that the alumina does not adhere to the inner wall of the immersion nozzle. In addition, the hole defect is mainly caused by the trapping of Ar bubbles at the solid-liquid interface, and this hole defect may be accompanied by inclusions or may not be accompanied by inclusions.

<Second problem>
In addition, we want to set the casting speed to a high level in order to take advantage of the features of low carbon steel and the like that do not easily cause significant solidification delay. However, in such a case, since the flow rate of the molten steel discharge flow is large, the meniscus undulates and powder entrainment occurs, and a powdery defect (hereinafter referred to as a powdery defect) tends to occur on the slab surface layer.

<Summary of the first and second problems>
The hole defects and powder defects described above become surface defects (hereinafter referred to as sliver defects) commonly referred to as sliver at the stage where the slab is rolled into a thin steel plate product, and the quality of the thin steel plate product is improved. It will be significantly reduced.

  An object of the present invention is to provide a technique for producing a high-quality slab with few hole defects and powder defects with high productivity when continuously casting an extremely low carbon steel or a low carbon steel.

Means and effects for solving the problems

  The problems to be solved by the present invention are as described above, and the inventors of the present invention, after intensive research, can effectively diverge the discharge flow from the discharge holes in order to solve the above problems. I found out. Reference is now made to FIGS. FIG. 5 is an image of a molten steel discharge flow formed by a conventional immersion nozzle. FIG. 6 is an image of molten steel discharge flow achieved by the immersion nozzle of the present invention. That is, the present invention aims to solve the above-mentioned problem by branching a molten steel discharge flow having a large unit as shown in FIG. 5 into a plurality of branches as shown in FIG. In short, the above-mentioned Hall defect and powder defect, which are in a trade-off relationship, are to be solved simultaneously with the idea of “divergence of molten steel discharge flow”.

  Patent Document 2 (Japanese Patent Laid-Open No. 5-185192) discloses an immersion nozzle in which a notch extending to the bottom surface is provided at the bottom of the discharge hole. It is described that this notch opening further assists the original downward flow of the discharge hole, forms a so-called large downward flow, reduces the flow velocity of the molten steel surface flow at the meniscus, and suppresses surface defects caused by powder. Has been. Moreover, since it is hard to receive the influence of the deposit around a discharge hole, it describes that said effect lasts for a long time. Furthermore, the above-described notch is not limited to the notch formed by notching the entire width of the discharge hole, and for example, a desired effect can be obtained even if only a part is notched. However, the description that “a desired effect can be obtained even if only a part is cut out” described in Patent Document 2, page 3, column 4, lines 4 to 5 is “branch”, which is the focus of the present invention. It is not certain whether it matches. Moreover, the notch is standing vertically, and from this state, this notch is not intended to “branch”, but only to “help the original downward flow” of the discharge hole. I can ask.

  Next, the outline of the invention completed based on the above idea will be described together with its effects.

According to the aspect of the present invention, an extremely low carbon steel or a low carbon steel having a carbon content C [wt%] of 0.001 to 0.06, a mold width W [mm] of 800 to 2100, and a mold thickness D A bottomed cylindrical immersion nozzle used for pouring molten metal held in a tundish into the mold, with a mold having [mm] of 200 to 320, from the inner bottom surface of the immersion nozzle A pair of opposed molten steel discharge holes are drilled in the peripheral wall of the immersion nozzle at a position away by a predetermined distance, and the flow path cross-sectional area Asn [mm ² ] of the molten steel discharge hole is set to 70 to 120 in diameter [mm]. The area of the circle is [mm ² ], and the downward discharge angle θ1 [deg. ] Of 15 to 55, and a continuous low-carbon steel with a distance H [mm] between the lower end of the inner peripheral opening edge of the molten steel discharge hole and the inner bottom surface of 10 to 50. Alternatively, continuous casting of low carbon steel is performed by the following method. That is, a molten steel discharge groove extending in parallel with the drilling direction of the molten steel discharge hole in the bottom view of the immersion nozzle is formed on the inner bottom surface of the molten steel discharge hole, and the following expressions (1) to (3) are satisfied. The casting speed Vc [m / min] is set to 1.6 to 2.4, and the molten steel superheat degree ΔT [° C.] is set to 20 to 45.
a / A = 0.1 to 0.9 (1)
b / B = 0.4 to 1.0 (2)
Δθ [deg. ] = 15 to 45 (3)
However,
a is the engraving width A of the molten steel discharge groove, the width b of the outer peripheral opening edge of the molten steel discharge hole is the intersection line between the inner bottom surface of the molten steel discharge hole and the inner bottom surface of the molten steel discharge groove, and The radial separation distance B between the outer circumferential surface of the immersion nozzle is the radial separation distance Δθ between the inner circumferential surface of the immersion nozzle and the outer circumferential surface of the immersion nozzle is the downward discharge angle of the molten steel discharge hole. The difference between θ1 and the angle θ2 that the inner bottom surface of the molten steel discharge groove forms with the horizontal

  According to the above method, a high-quality slab with few hole defects and powder defects can be produced with high productivity. For example, when this slab is rolled into a thin steel sheet coil, the occurrence of sliver defects can be suppressed.

  Continuous casting of the above-mentioned ultra-low carbon steel or low-carbon steel is further performed by the following method. That is, the in-mold electromagnetic stirring strength MEMS [gauss] is set to 500 to 1000. According to the above method, a higher quality slab can be manufactured. For example, when this slab is rolled into a thin steel sheet coil, the care process for the slab can be omitted, so that productivity is improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view of a continuous casting machine. First, based on this figure, the structure and operation | movement of the continuous casting machine 100 used for the continuous casting of the ultra-low carbon steel or low carbon steel which concern on this embodiment are demonstrated easily as an example.

  The continuous casting machine 100 smoothly pours molten steel to be poured into the mold 10 at a predetermined flow rate, and a mold 10 for cooling the molten steel to form a solidified shell having a predetermined shape. And a pair of rolls 11 arranged in a plurality along the casting path Q from directly below the mold 10. In the present embodiment, the casting path Q includes a vertical path portion extending in the vertical direction, an arc path portion connected to the downstream side of the vertical path portion and extending in an arc shape, and further provided on the downstream side thereof in the horizontal direction. And a correction path part for smoothly connecting the arc path part and the horizontal path part.

  Moreover, each roll pair 11 is comprised from a pair of roll 11a * 11a which clamps the slab as a casting object with both wide surfaces. The roll gap [mm] as the shortest distance between the roll surfaces of the pair of rolls 11a and 11a is configured to be adjustable by appropriate means.

  Further, upstream of the casting path Q, there is appropriately provided a cooling spray 12 for spraying cooling water at a predetermined flow rate on the solidified shell formed in the mold 10 and pulled out from the mold 10. Generally, the mold 10 is referred to as a primary cooling zone, and in this sense, a path portion where the cooling spray 12 is disposed is referred to as a secondary cooling zone.

  The solidified shell pulled out from the mold 10 and conveyed along the casting path Q is further cooled and contracted by natural heat dissipation, the cooling spray 12 or the like. Therefore, the roll gap [mm] of the roll pair 11 is generally set so as to be gradually narrowed as it proceeds to the downstream side of the casting path Q.

  With the above configuration, when starting continuous casting of ultra-low carbon steel or low-carbon steel, a dummy bar (not shown) is inserted in advance into the casting path Q before pouring molten steel into the mold 10, The molten steel is started to be poured into the mold 10 through the immersion nozzle 1 at a predetermined flow rate, and the dummy bar is drawn downstream at a predetermined speed. The dummy bar is collected by an appropriate means when a predetermined meniscus distance is reached. Thus, ultra-low carbon steel or low-carbon steel is continuously cast.

Next, general operating conditions of the continuous casting machine 100 will be briefly introduced.
-Mold width W [mm] shall be 800-2100.
-Mold thickness D [mm] shall be 200-320.
-Mold height H [mm] shall be 800-1000.
-Casting speed Vc [m / min] shall be 1.0-2.5.
-Molten steel superheat degree (DELTA) T [degreeC] shall be 10-45.
The specific water amount Wt [L / kg Steel] is 1 to 3.
-Electromagnetic stirring intensity MEMS [gauss] in a mold shall be 0-1000.
-Molten steel composition is based on an agreement between the parties. Typical components are C, Si, and Mn. To this, Cr, Ti, Ni, Al, Cu or the like is appropriately added. Usually, P and S are adjusted to be as small as possible. In some cases, about 150 ppm of P and S are added because of machinability and other requirements. Contains other inevitable impurities.

Here, each term is briefly explained.
The mold width W [mm] and the mold thickness D [mm] are considered at the upper end of the mold 10.
The casting speed Vc [m / min] is a drawing speed of the slab, and is thought of as the peripheral speed of the roll pair 11 arranged in the most upstream of the plurality of roll pairs 11.
The molten steel superheat degree ΔT [° C.] is an index of the temperature of the molten steel poured into the mold 10. Details are described at the end of this specification.
The meniscus distance M [m] means a distance [m] that starts with the molten steel surface (meniscus) in the mold 10 and is considered along the casting path Q.
The specific water amount Wt [L / kg Steel] means the volume of cooling water used for 1 kg of steel. This cooling water is sprayed / sprayed on the slab in the above-described secondary cooling zone which is conceived with a meniscus distance [m] of 0.8 to 37.
In-mold electromagnetic stirring strength MEMS [gauss] is an index of the strength of the magnetic field applied to stir the molten steel in the mold 10. Details are described at the end of this specification.

  Next, specific operating conditions of the continuous casting machine 100 according to the present embodiment will be described. In the continuous casting according to the present embodiment, the operation is performed under the following conditions.

-Target steel type: Extremely low carbon steel or low carbon steel with carbon content C [wt%] of 0.001 to 0.06-Mold width W [mm]: 800-2100
Mold thickness D [mm]: 200 to 320
Casting speed Vc [m / min]: 1.6 to 2.4
-Molten steel superheat degree ΔT [° C.]: 20 to 45
In-mold electromagnetic stirring strength MEMS [gauss]: 0, preferably 500 to 1000
-Immersion nozzle: Suppose that the immersion nozzle 1 mentioned later is employ | adopted. The immersion nozzle 1 is disclosed in a previous application (Japanese Patent Application No. 2007-038467) of the applicant of the present application.

  Hereinafter, the shape and the like of the immersion nozzle 1 employed in the present embodiment will be described in detail with reference to the drawings. FIG. 2 is an elevation view of the immersion nozzle according to the embodiment of the present invention. 3 is a cross-sectional view taken along line 3-3 in FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. The immersion nozzle 1 shown in FIG. 2 has a bottomed cylindrical shape that is used to smoothly pour molten steel held in the tundish into the mold 10. Is done. As shown in this figure and FIG. 3, the immersion nozzle 1 is constituted by a hollow cylindrical refractory, and its lower end is closed. A pair of opposed molten steel discharge holes 2 are drilled in the peripheral wall of the immersion nozzle 1 at a position away from the inner bottom surface P of the immersion nozzle 1 by a predetermined distance. Further, on the inner bottom surface 3 of the molten steel discharge hole 2, a molten steel discharge groove 5 extending in parallel with the piercing direction 4 of the molten steel discharge hole 2 in the bottom view of the immersion nozzle 1 (see FIG. 4) is formed. Specifically, it is as follows.

  The thickness of the refractory on the peripheral wall of the immersion nozzle 1 (indicated by symbol B in FIG. 3), that is, the radial distance between the inner peripheral surface 6 of the immersion nozzle 1 and the outer peripheral surface 7 of the immersion nozzle 1. .) Is also slightly thinner than the thickness of the refractory on the bottom wall. The molten steel discharge hole 2 is a hole for appropriately discharging molten steel flowing from the tundish into the immersion nozzle 1 into the mold 10, and one end is connected to the inner peripheral surface 6 of the immersion nozzle 1. The end is connected to the outer peripheral surface 7 of the immersion nozzle 1, and the molten steel discharge hole 2 is formed so that the drilling direction is slightly downward in the vertical sectional view shown in FIG. 3. Specifically, an angle θ1 [deg.] Formed by the inner bottom surface 3 of the molten steel discharge hole 2 with the horizontal. ] (Downward discharge angle θ1 [deg.], Where downward is positive) is set to 15 to 55. The inner peripheral side opening edge 8 of the molten steel discharge hole 2 has a rectangular shape with a slightly rounded corner in the vertical sectional view shown in FIG. 2, and the same applies to the outer peripheral side opening edge 9 of the molten steel discharge hole 2. In addition, the outer peripheral side opening edge 9 of the molten steel discharge hole 3 is formed wider than the inner peripheral side opening edge 8, so that the molten steel discharge hole 2 has an outer periphery from the inner peripheral side opening edge 8 in the bottom view shown in FIG. It forms so that it may expand gradually as it goes to the side opening edge 9. As shown in FIG. The width of the outer peripheral opening edge 9 is also indicated by the symbol A.

In FIG. 3, when the distance between the lower end 8 a of the inner peripheral opening edge 8 of the molten steel discharge hole 2 and the inner bottom surface P is considered as a symbol H, this distance H [mm] is set to 10-50. The molten steel discharge hole 2 is conceived by the inner peripheral side opening edge 8 (a part of the inner peripheral side opening edge 8 is clearly shown using a hidden line in FIG. 2) that can appear in a plan view in an elevational view in FIG. The channel cross-sectional area Asn [mm ² ] is defined as an area [mm ² ] of a circle having a diameter [mm] of 70 to 120.

The shape of the molten steel discharge groove 5 will be described in detail below. As described above, the molten steel discharge groove 5 extends in parallel with the drilling direction 4 of the molten steel discharge hole 2 in the bottom view shown in FIG. 4, and the inner peripheral side end 10 is formed on the inner bottom surface 3 of the molten steel discharge hole 2. The outer peripheral side end 11 (see FIGS. 2 and 3) is connected to the outer peripheral surface 7 of the immersion nozzle 1. The molten steel discharge groove 5 is formed at substantially the center of the molten steel discharge hole 2 when attention is paid to the direction perpendicular to the piercing direction 4 of the molten steel discharge hole 2 in FIG. When the engraved width of the molten steel discharge groove 5 is considered by the symbol a, the following formula (1) is satisfied. Further, an intersection line between the inner bottom surface 3 of the molten steel discharge hole 2 and the inner bottom surface 12 of the molten steel discharge groove 5 (in this embodiment, the inner peripheral side end 10 of the molten steel discharge groove 5 corresponds) and the immersion nozzle 1. When the separation distance in the radial direction between the outer peripheral surface 7 and the outer peripheral surface 7 is considered by the symbol b, the following expression (2) is satisfied. Further, in the vertical sectional view of the immersion nozzle 1 shown in FIG. 3, the angle formed by the inner bottom surface 12 of the molten steel discharge groove 5 with the horizontal (however, the downward direction is positive) is considered as θ2, and the molten steel discharge hole 2 When the difference between the downward discharge angle θ1 and the angle θ2 formed by the inner bottom surface 12 of the molten steel discharge groove 5 to be horizontal is considered by the sign Δθ, the following expression (3) is satisfied.
a / A = 0.1 to 0.9 (1)
b / B = 0.4 to 1.0 (2)
Δθ [deg. ] = 15 to 45 (3)

  In addition, as shown in FIGS. 2-4, although the molten steel discharge groove | channel 5 is comprised so that the inner bottom face 12 and a side surface may be orthogonally crossed in this embodiment, in fact, the convenience on manufacture and the problem on intensity | strength In consideration of the above, it is advisable to appropriately add R to the portion where the inner bottom surface 12 and the side surface are orthogonal to each other. Similarly, an appropriate R is preferably attached to a portion where the surfaces intersect at an acute angle or a portion where the surfaces intersect at an obtuse angle. In addition, it cannot be overemphasized that the said crossing site | part before R is attached can be easily considered if a pair of surface which pinches | interposes this crossing site is extended virtually.

  By the way, the upper end of the immersion nozzle 1 is connected to the bottom of the tundish via a molten steel flow rate adjusting unit for adjusting the discharge flow rate of the molten steel discharged from the immersion nozzle 1 into the mold 10. And the immersion nozzle 1 comprised as mentioned above is a tundish (substantially) so that the drilling direction 4 (refer FIG. 4) of the molten steel discharge hole 2 drilled on the surrounding wall may correspond with the width direction of the casting_mold | template 10. FIG. Is attached to the molten steel flow rate adjusting unit). The meniscus is set so as to be positioned approximately 150 to 250 [mm] above the lower end of the outer peripheral opening edge 9 of the molten steel discharge hole 2 of the immersion nozzle 1.

[Test 1]
Hereinafter, the test for confirming the technical effect of the ultra-low carbon steel or the low carbon steel continuous casting method according to the present embodiment will be described. Each numerical range mentioned above is reasonably supported by the following test.

[Test 1.1] Index First, an index used for evaluation of each test will be described.

[Test 1.1.1] Slab Quality This index “slab quality” mainly focuses on hole defects or powder defects that may occur on the surface of a slab cast. The evaluation method is as follows. That is, first, the anti-reference surface (the anti-reference surface means the upper surface in the horizontal path portion) of the third slab slab (approximately 12.5 [m]) from the bottom side is deep for inspection. After about 1 mm of scarf (also referred to as a check scarf), it is visually observed to check for the presence of hole or powdery surface defects. The third slab slab from the bottom side is subject to inspection. The slab slab from the bottom side may have been inspected in the past, and the third slab slab from the bottom side may be representative in terms of surface quality. Because I understand. The reason for scarfing about 1 mm deep for inspection is to peel off the black skin (iron oxide) so that the presence or absence of hole defects or powder defects can be confirmed. The “hole surface defect (hole defect)” means “a defect in which a spherical recess having a diameter of 1 mm or more is recognized as a trace of Ar gas bubbles blown into the mold”. In many cases, inclusions such as alumina adhere to the curved surface of the depression. The “powder surface defect (powder defect)” means “a defect having a circumscribed circle diameter of 1 mm or more in which a mold powder is bitten in a slab”.

  ◆ If any defect is not visually recognized in the above visual observation, it can be rolled as it is without any care, so the test in this case should be evaluated as “◎ (very good)” in terms of slab quality. To do. On the other hand, surface defects were visually recognized in the visual observation, but when the anti-reference surface was ground by about 1.5 [mm] with a hot scarf or grinder and again visually observed, any surface defects were visually recognized in the visual observation. If not, the test in this case shall be evaluated as “◯ (good)” in terms of slab quality in the sense that the product can be collected by taking care of the slab surface. However, if a surface defect is also visually observed after the grinding, the product cannot be collected. Therefore, the test in this case is evaluated as “x (defect)” regarding the slab quality. In the above, the amount of cutting with a hot scarf or the like is set to 1.5 [mm], and it is empirically that the product quality after rolling is good when there is no wrinkle of a depth of 1.5 [mm] or less. It is because it understands.

[Test 1.1.2] Sliver This index “sliver” focuses on surface defects of a thin steel sheet coil having a thickness of about 1.0 to 2.0 mm obtained by appropriately rolling a slab. The evaluation method is as follows. That is, in the above slab quality item, the slab evaluated as “◎ (very good)” or “○ (good)” is then subjected to “heating → hot rolling → pickling → cold rolling”. Through a process of “→ continuous annealing”, a thin steel sheet coil having a thickness of about 1.0 to 2.0 mm is obtained. Then, the surface of the thin steel sheet coil is visually observed about 3000 m in the rolling direction, and surface defects discovered through visual observation are considered. Specifically, the surface defects that can be visually recognized on the surface of the thin steel sheet coil are the above-mentioned sliver defects (due to hole defects and powder defects, and stripes of approximately 20 mm or more appearing in the rolling direction after rolling). For example, the number of sliver defects, which are the former, can be confirmed per 100 m in the rolling direction. The number of sliver defects per 100 m in the rolling direction is defined as a sliver index.

  ◆ Thin steel sheet coils with a sliver index of 0.10 or less can be shipped as a normal grade without any problems. Therefore, the test in this case is evaluated as “good (good)”. ◆ Surface quality strict materials for outer plates are required to have a sliver index of 0.07 or less. Therefore, a test related to a thin steel sheet coil that satisfies this strict condition is evaluated as “◎ (very good)”. To do. ◆ On the other hand, since the steel sheet coil having the sliver index of 0.11 or more cannot be shipped as a product, the test in this case is evaluated as “x (defect)”.

[Test 1.2] Common Test Method Next, a test method common to each test will be described. Please also refer to Tables 1-2 below. Hereinafter, in Tables 1-2, test No. 1 will be described (following the above-described continuous casting operation unless otherwise specified). Test No. The test indicated by 1 corresponds to a continuous casting of a certain charge (one charge 250 [ton]) in a one-to-one relationship.

  First, test no. The components of the molten steel accommodated in the ladle corresponding to 1 are checked by sampling before pouring into the tundish, and the components of the molten steel are listed in the table. Next, test no. The contents relating to the mold 10 and the immersion nozzle 1 used when casting one charge are described in the table. The set values of the casting speed Vc [m / min] and the electromagnetic stirring strength MEMS [gauss] in the mold are described in the table, and the molten steel superheat degree ΔT [° C.] is measured and recorded in the table. Furthermore, slab quality is investigated in the cast primary cutting slab, and the results are listed in the table. The object of the investigation regarding the slab quality is a slab slab corresponding to the solidified shell existing in the mold 10 when the molten steel superheat degree ΔT [° C.] is measured.

[Test 1.3] Common Test Conditions Next, test conditions common to each test will be described. The mold height H [mm] is 900 and the specific water amount Wt [L / kg Steel] is Wt = Vc + 0.75.

[Test 1.4] Individual Test Conditions and Test Results Next, individual test conditions and test results for each test are shown in Tables 1 and 2 below. In the following Tables 1 and 2, the column title “(A) mm” is “diameter [mm] of the circle when the flow passage cross-sectional area Asn [mm ² ] of the molten steel discharge hole is converted into the area [mm ² ] of the circle. "Means. In the column title “slab quality”, specific surface defects are described in the form added to the above-described evaluation of slab quality. In the column title “Evaluation”, “◎ (very good)”, “◯ (good)”, “× (poor)” was entered based on the sliver quality of the product.

[Test 1.5] Discussion [Test 1.5.1] Carbon content C [wt%]
When the carbon content C [wt%] is less than 0.001, mass production with a normal RH degasser becomes difficult. On the other hand, if the carbon content C [wt%] exceeds 0.06, solidification delay due to the peritectic reaction may occur during solidification, making high-speed casting difficult. For this reason, in the above embodiment, it is assumed that the carbon content C [wt%] is 0.001 to 0.06.

[Test 1.5.2] Casting speed Vc [m / min]
According to Tables 1 and 2, it can be seen that when the casting speed Vc [m / min] is less than 1.6, hole-like surface defects have occurred. This is presumably because the kinetic energy of the molten steel injected into the mold is reduced because the casting speed Vc [m / min] is low, and as a result, the bubble cleaning effect on the inner surface of the solidified shell is reduced. On the other hand, when the casting speed Vc [m / min] exceeds 2.4, it can be seen that powdery defects occur. This is thought to be because the flow rate of the molten steel discharged from the immersion nozzle 1 is high, and as a result, the reverse flow of the meniscus is also strong, the interface between the molten steel and the powder is strongly disturbed, and the powder is entrained.

[Test 1.5.3] Molten steel superheat degree ΔT [° C.]
According to past knowledge, if the molten steel superheat degree ΔT [° C.] is less than 20, the immersion nozzle may be clogged. On the other hand, if the molten steel superheat degree ΔT [° C.] exceeds 45, troubles such as breakout may occur. Therefore, it is assumed that the molten steel superheat degree ΔT [° C.] is 20 to 45.

[Test 1.5.4] In-mold electromagnetic stirring strength MEMS [gauss]
According to Table 1, it can be seen that even when the in-mold electromagnetic stirring intensity MEMS [gauss] is set to 0, a certain slab quality can be obtained. And according to Table 2, when the electromagnetic stirring intensity | strength MEMS [gauss] in a mold shall be 500-1000, it turns out that favorable slab quality is obtained. And test no. According to 152, when the in-mold electromagnetic stirring strength MEMS [gauss] was less than 500, a certain slab quality was obtained, but the sliver index was not very good. Therefore, from the viewpoint of the bubble cleaning effect by the electromagnetic stirring in the mold, it can be said that it is more preferable to set the electromagnetic stirring intensity MEMS [gauss] in the mold a little stronger. On the other hand, test no. According to No. 156, when the in-mold electromagnetic stirring strength MEMS [gauss] exceeds 1000, it can be seen that a powdery surface flaw occurs. This is presumably because the flow velocity of the molten steel in the vicinity of the meniscus increases and the meniscus undulates, resulting in so-called powder entrainment.

[Test 1.5.5] a / A, b / B, Δθ
According to Tables 1 and 2, if a / A, b / B, and Δθ are not within a predetermined range, the function of branching the molten steel discharge flow carried by the molten steel discharge groove 5 is not fully exhibited, and a hole defect It can be seen that good results cannot be obtained with respect to powder defects.

In other words, the mold width W [mm] is set to 800 to 2100, the mold thickness D [mm] is set to 200 to 320, and the downward discharge angle θ1 [deg. ] Was used as a 15 to 55, the flow path cross-sectional area of the molten steel discharge hole 2 Asn [mm ^2] and the diameter [area of a circle mm] of the 70 to 120 [mm ^2], the distance H [mm] 10 to 50 These operating conditions are generally adopted. When the distance H [mm] was less than 10, the hot water pool portion disappeared and splash (spatter of molten steel at the start of molten steel pouring) was intense. On the other hand, when the distance H [mm] exceeds 50, the hot water reservoir is too deep, and a large amount of deposits are deposited on the hot water reservoir (immersion nozzle bottom).

(Claim 1)
As described above, in the above-described embodiment, continuous casting of ultra-low carbon steel or low-carbon steel is performed by the following method. That is, on the inner bottom surface 3 of the molten steel discharge hole 2, a molten steel discharge groove 5 extending in parallel with the drilling direction 4 of the molten steel discharge hole 2 in the bottom view of the immersion nozzle 1 is engraved, and the following formula (1) -(3) shall be satisfied. The casting speed Vc [m / min] is set to 1.6 to 2.4. Molten steel superheat degree (DELTA) T [degreeC] shall be 20-45.
a / A = 0.1 to 0.9 (1)
b / B = 0.4 to 1.0 (2)
Δθ [deg. ] = 15 to 45 (3)
However,
a is the engraved width A of the molten steel discharge groove 5, and b is the width b of the outer peripheral opening edge 9 of the molten steel discharge hole 2. The inner bottom surface 3 of the molten steel discharge hole 2 and the inner bottom surface 12 of the molten steel discharge groove 5. The distance B in the radial direction between the crossing line and the outer peripheral surface 7 of the immersion nozzle 1 is the radial distance between the inner peripheral surface 6 of the immersion nozzle 1 and the outer peripheral surface 7 of the immersion nozzle 1. The separation distance Δθ is the difference between the downward discharge angle θ1 of the molten steel discharge hole 2 and the angle θ2 formed by the inner bottom surface 3 of the molten steel discharge groove 5 to be horizontal.

  According to the above method, as shown in the said Table 1-2, a high quality slab with few hall | hole-like defects and powdery defects can be manufactured with high productivity (high Vc). For example, when this slab is rolled into a thin steel sheet coil, the occurrence of sliver defects can be suppressed.

  Note that the above method is not applied only to the so-called vertical sequential bending type continuous casting machine shown in FIG. 1, for example, a vertical continuous casting machine (all casting paths are formed along the direction of gravity). And a curved continuous caster (a vertical sequential bending type continuous caster in which the vertical path portion is excluded) can be applied without problems. Whether to perform the electromagnetic stirring in the mold is arbitrary as can be seen by comparing Table 1 and Table 2.

  Moreover, continuous casting of said ultra-low carbon steel or low-carbon steel is further performed as follows. That is, the in-mold electromagnetic stirring strength MEMS [gauss] is set to 500 to 1000. According to the above method, as shown in Table 2 above, a higher quality slab can be manufactured. For example, when this slab is rolled into a thin steel sheet coil, the care process for the slab can be omitted, so that productivity is improved.

  In addition, according to the said Table 1, even if it does not implement in-mold electromagnetic stirring, it turns out that a favorable result is obtained about slab quality. In this sense, the implementation of electromagnetic stirring in the mold is arbitrary, and there is no restriction on the implementation of the electromagnetic brake EMBr. In other words, the technical idea disclosed in the present specification can be realized without any problem whether it is an operation in which only one of the electromagnetic stirring in the mold and the electromagnetic brake is performed or an operation in which both are performed in combination. can do.

  The following are reference materials.

<Molten steel superheat degree ΔT [° C]>
Definition: A measure of the temperature of the molten steel poured into the mold.
(1) “Measurement time” is “Start casting using a tundish heated sufficiently in advance, the casting speed is constant with the same mold width, and the volume of molten steel in the tundish is constant. That is, the time when the pouring amount rate from the ladle to the tundish (ton / min) and the pouring amount rate from the tundish to the mold (ton / min) substantially coincide with each other and reach a steady state.
(2) “Measurement points” are as follows. That is, the “horizontal position” is the axis of the immersion nozzle provided on the bottom surface of the tundish, and the “vertical position” is 100 mm in depth based on the molten steel surface held in the tundish.
(3) The “measuring instrument” uses a consumable thermocouple. As described above, since the consumable thermocouple is immersed at a point having a depth of 100 mm, a configuration in which a consumable thermocouple is attached to the tip of a suitably prepared rod is suitable.
(4) The temperature of the molten steel measured according to the above “measurement time”, “measurement point”, and “measuring instrument” is compared with the liquidus temperature that is uniquely determined by the molten steel component of the molten steel. And the above-mentioned molten steel superheat degree (DELTA) T [degreeC] shall be calculated | required as the remainder which pulled the latter from the former.

<In-mold electromagnetic stirring strength MEMS [gauss]>
Definition: A measure of the strength of a magnetic field applied to stir molten steel in a mold.
(1) The “measurement time” is arbitrary.
(2) “Measurement points” are as follows. That is, the “horizontal position” is (i) the center in the mold width direction, (ii) 15 mm from the mold inner wall surface toward the center in the mold thickness direction, and (iii) in the mold height direction. Align with the coil center of the electromagnetic coil embedded in the mold.
(3) Use an appropriate Gauss meter as the “measuring instrument”.
(4) Measure multiple times according to the above “Measurement time”, “Measurement point”, and “Measurement instrument”. The above-described in-mold electromagnetic stirring intensity MEMS [gauss] is obtained by averaging the plurality of measured values.
(5) From various points of view, the above-mentioned in-mold electromagnetic stirring strength MEMS [gauss] is preferably 0 to 1000, and the frequency [Hz] ("magnetic field frequency") of the magnetic field applied to the molten steel in the mold. Means the number of times the current conducted to the electromagnetic coil changes direction per second.) Is preferably 1 to 5, and generally 2 is adopted as the frequency [Hz] of this magnetic field.

Schematic diagram of continuous casting machine Elevated view of an immersion nozzle according to an embodiment of the present invention Sectional view taken along line 3-3 in FIG. Sectional view taken along line 4-4 in FIG. Image of molten steel discharge flow formed by a conventional immersion nozzle Image of molten steel discharge flow achieved by the immersion nozzle of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Submerged nozzle 2 Molten steel discharge hole 3 Inner bottom face of molten steel discharge hole 5 Molten steel discharge groove 9 Outer peripheral side opening edge of molten steel discharge hole

Claims (2)

An extremely low carbon steel or a low carbon steel having a carbon content C [wt%] of 0.001 to 0.06,
A mold having a mold width W [mm] of 800 to 2100 and a mold thickness D [mm] of 200 to 320;
A bottomed cylindrical immersion nozzle provided for pouring molten steel held in a tundish into the mold, wherein the immersion nozzle is positioned at a position away from the inner bottom surface of the immersion nozzle by a predetermined distance. A pair of opposed molten steel discharge holes are perforated on the peripheral wall, the flow passage cross sectional area Asn [mm ² ] of the molten steel discharge hole is set to an area [mm ² ] of a circle having a diameter [mm] of 70 to 120, and the molten steel discharge Downward discharge angle θ1 [deg. ] Is 15 to 55, and the distance H [mm] between the lower end of the inner circumferential side opening edge of the molten steel discharge hole and the inner bottom surface is 10 to 50,
In the continuous casting method of ultra low carbon steel or low carbon steel,
On the inner bottom surface of the molten steel discharge hole, a molten steel discharge groove extending in parallel with the drilling direction of the molten steel discharge hole in the bottom view of the immersion nozzle is engraved, and the following formulas (1) to (3) are satisfied age,
The casting speed Vc [m / min] is set to 1.6 to 2.4,
The molten steel superheat degree ΔT [° C.] is set to 20 to 45,
A continuous casting method for ultra-low carbon steel or low-carbon steel.
a / A = 0.1 to 0.9 (1)
b / B = 0.4 to 1.0 (2)
Δθ [deg. ] = 15 to 45 (3)
However,
a is the engraving width A of the molten steel discharge groove, the width b of the outer peripheral opening edge of the molten steel discharge hole is the intersection line between the inner bottom surface of the molten steel discharge hole and the inner bottom surface of the molten steel discharge groove, and The radial separation distance B between the outer circumferential surface of the immersion nozzle is the radial separation distance Δθ between the inner circumferential surface of the immersion nozzle and the outer circumferential surface of the immersion nozzle is the downward discharge angle of the molten steel discharge hole. The difference between θ1 and the angle θ2 that the inner bottom surface of the molten steel discharge groove forms with the horizontal In the continuous casting method of the ultra-low carbon steel or the low-carbon steel according to claim 1,
In-mold electromagnetic stirring strength MEMS [gauss] is set to 500 to 1000,
A method for continuous casting of ultra-low carbon steel or low-carbon steel, characterized in that

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