JPH1029048A - Method for drawing out cast slab in high velocity at initial stage of continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To quickly bring the temp. of a bottom slab close to a stationary part at the time of starting draw-out of the cast slab, to improve the quality of the bottom slab and to reduce a bottom crop, at the initial stage of continuous casting. SOLUTION: When the cast slab is drawn out from the starting of molten steel pouring into a mold at the starting time of casting, to the reaching into a stationary casting velocity Vc at the point T0 of starting time to draw-out of the cast slab when the molten metal surface in the mold reaches a control level after passing the fixed time since the starting of molten metal pouring, the drawing velocity of the cast slab is accelerated from the initial velocity zero to a fixed acceleration (a1 ) and once raised to casting velocity V1 having higher than the stationary casting velocity V0 . Successively, this casting velocity V1 is decreased to the stationary casting velocity Vc with the deceleration (a2 ) to shift the casting to the stationary casting, and the raise of molten steel temp. in the mold at the initial stage of casting is contrived, and the internal quality is improved by promoting the float-up of inclusion to prevent the deterioration of the surface quality caused by super-cooling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-speed slab drawing method in the early stage of casting from the start of drawing of slab after the start of molten steel injection to the time when the drawing speed reaches a steady casting speed in continuous casting.

[0002]

2. Description of the Related Art In continuous casting of steel, molten steel conveyed by a ladle or the like is temporarily stored in a tundish, then cast into a mold through an immersion nozzle, primary cooling by the mold, and a subsequent roll supporting and guiding device (roller). The molten steel is solidified by the secondary cooling in the apron, and the steady speed is controlled while controlling the molten steel flow rate by a flow control device such as a sliding nozzle provided in the tundish so that the crater end (unsolidified tip) maintains a predetermined position. The slab is pulled out.

[0003] At the start of the continuous casting, a dummy bar head is inserted into the lower portion of the bottomless mold to form a temporary bottom, and molten steel is cast into such a mold from an immersion nozzle, and a molten metal surface in the mold is formed. Reaches a preset control level, when a solidified shell of a predetermined thickness is formed in the cast molten steel and there is no fear of breakout, the dummy bar starts to be pulled out, and thereafter, the drawing speed (casting speed) is reduced. To a steady casting state.

As a method of controlling the slab drawing speed at the start of casting, a conventional method has been disclosed in, for example, Japanese Patent Publication No. 60-39.
There is a control method described in Japanese Patent Publication No. 00. In this control method, as shown in FIG. 6 (a), when a certain time has elapsed from the start of the molten metal injection and the slab drawing start time at which the molten metal level in the mold reaches the control level, the slab drawing speed is increased stepwise. As a result, a steady casting speed is reached (conventional method I).

However, in the control method according to the conventional method I, the molten steel level in the mold fluctuates up and down due to the stepwise change in the drawing speed, and the coated powder on the molten metal level is caught. However, since the drawing speed is low, the throughput of the molten steel is small, the flow of the molten steel is suppressed, the temperature of the molten steel in the mold is low, and the floating of inclusions is hindered.

Also, Japanese Patent Publication No. 60-3900 proposes a casting speed control method for suppressing the above-mentioned vertical fluctuation of the molten metal level in the mold to achieve a stable operation and improve the quality of the slab. As shown in FIG. 6 (b), the first acceleration α ₁ at the time of the start of molten metal injection (the minimum speed at which the initial nozzle clogging does not occur when a predetermined time has elapsed after the start of slab drawing after the start of molten metal injection). raising the initial velocity 0 acceleration) Hayashi, then the temperature in the cast strip withdrawing speed reaches V _a second acceleration alpha ₂ (acceleration as at the time of observing the minimum mold residence time reaches a steady cast strip withdrawing speed) Hayashi and apex of the dummy bar is tighten lead to steady casting speed V _c to reach the mold bottom (conventional method II).

[0007] The casting speed control method according to the conventional method II is a method in which the speed change at the start of casting is started in a state as smooth as possible to eliminate the boundary in cooling, but the upper surface of the dummy bar head, that is, the lower end of the slab (bottom). ) To reach the steady pouring speed when passing through the lower end of the mold,
There are problems such as leakage of the seal between the dummy bar head and the inner wall surface of the mold and breakout.

In order to solve the above-mentioned problems (1) to (5), the present applicant has already filed an application for "a method for drawing a slab in the early stage of continuous casting and a dummy bar head thereof" (Japanese Patent Laid-Open No. 7-185768). Gazette, Conventional Method III). As shown in FIG. 7, this drawing method uses a slab drawing speed from the initial speed of 0 to the first speed at a slab drawing start time T ₀ at which the molten metal surface in the mold reaches a control level after a certain time has elapsed after the start of the injection of the molten metal. increased to casting speed V _b Hayashi, then, after holding the first casting speed V _b predetermined time, linearly rising around the time T ₁₂ at a constant acceleration α of the dummy bar head is passed through the mold bottom Hayashi , those occupying lead to steady casting speed V _c.
Further, if necessary, a heat generation powder is used as the powder for covering the molten metal surface in the mold from the start of the molten metal injection until the steady pouring speed is reached, thereby preventing the temperature of the entire molten steel in the mold from lowering. In addition, the dummy bar head has a concave groove into which the molten metal penetrates into an arcuate hook shape that cuts in the direction of the center of curvature of the casting line, so that the dummy bar head can be easily attached and detached and the slab and the head can be firmly connected.

According to the drawing method according to the conventional method III, (a) the first casting speed _Vb is maintained until just before the dummy bar head passes the lower end of the mold, so that the solidified shell having a sufficient thickness is obtained. Is formed, sealing leakage between the dummy bar head and the mold wall surface is prevented, the temperature of the molten metal in the mold is prevented from lowering, and breakout or the like due to uneven solidification is prevented. (b) After the V _b holds a certain time, it is increased at a constant acceleration α to V _c,
Also, by using the heat generating powder, the temperature of the molten steel in the mold rises, thereby suppressing the deformation of the slab shape due to the overcooling of the slab bottom, and preventing the surface defects of the slab bottom due to the overcooling. (c) As the throughput of molten steel at the immersion nozzle increases at a constant acceleration α, the temperature of molten steel in the mold rises continuously, promoting the floating of inclusions and promoting the formation of powder slag in the meniscus. Quality can be improved. (d) By completely cooling the tip of the slab at the connection between the slab and the dummy bar head,
The tip of the slab cooled by the concave groove of the dummy bar head is prevented from extending due to the pulling force of the dummy bar.

[0010]

However, in the drawing method according to the conventional method III, the first casting speed V is low.
_{Since b} is retained longer, subcooling portion can not be reduced in the casting direction, it becomes insufficient supply of the molten steel heat, and when it reaches the steady casting speed V _c, since the supplied heat is reduced once However, it causes a temporary decrease in the molten steel temperature, and the quality deterioration is not sufficiently improved, and the quality deterioration is not reduced, that is, the tri-bottom crop is not reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems. An object of the present invention is to make it possible to quickly bring the temperature of a bottom slab close to a steady portion at the start of drawing a slab, and to improve the bottom slab quality. It is an object of the present invention to provide a high-speed slab-drawing method in the early stage of casting, which can improve bottoming and reduce bottom crop.

[0012]

SUMMARY OF THE INVENTION The present invention solves the above problems of the conventional methods I and II, and
The present invention has been made to solve the problems of the conventional method III, and the high-speed slab-drawing method in the early stage of casting according to the present invention is, as shown in FIG. At the time of slab withdrawal from the start of pouring until reaching the steady pouring speed V _C , the slab withdrawing is started at a point T ₀ at which slab withdrawal starts when a certain time has elapsed after the start of pouring and the molten metal surface in the mold has reached the control level. raising the speed from the initial speed zero at a constant acceleration a ₁ Hayashi, once raising speed to a steady casting speed V _C above the casting speed V _1. Followed by reduction of the casting speed V ₁ to the deceleration a ₂ at a steady casting speed V _c, it is a method to shift to the steady casting.

Here, the acceleration a ₁ is V ₁ / (T ₁ -T
₀ ) (T ₁ : the point at which the casting speed V ₁ is reached, which is higher than the steady casting speed V _C ), and the deceleration a ₂ when decelerating from V ₁ to V _C is (V _C −V ₁ ) / (T ₂ −T
Represented by ₁₎ (T _2: the time is reduced from the casting speed V ₁ higher than the steady casting speed V _C, to reach a steady casting speed V _C). Moreover, the casting upper limit speed V ₁ was considering final temperature drop is expressed by _{(a 2 / (T 2 -T} 1) 2) + V C.

(1) Improvement of bottom slab quality / reduction of bottom crop / In the conventional method III, since the low first casting speed _Vb is maintained for a long time, the supercooling due to the heat removal from the mold and the heat Is not sufficient (the throughput of molten steel is reduced), the temperature of the molten steel in the mold is lowered, the floating effect of inclusions is hindered, the internal quality deteriorates, and the surface quality deteriorates due to overcooling. Further, even when the steady casting speed V _C is reached, the supplied heat is reduced temporarily due to the low temperature of the molten steel in the mold, causing a temporary decrease in the molten steel temperature, and the deterioration of the internal quality and surface quality is prolonged. Become.

[0015] In contrast, in the present invention method, in order to stretch speed increasing until V ₁ exceeds the V _C from the pull-out started at a constant acceleration a _1, without molten steel is supercooled, and the molten steel heat also sufficiently As the molten steel is supplied (increased throughput of molten steel), the temperature of the molten steel in the mold can be raised in the initial stage of casting, the internal quality is improved by promoting the floating of inclusions, and the supercooling is eliminated to eliminate surface cooling. Quality is also improved and bottom crops can be reduced. Furthermore, V exceeding V _C
After speed-up phase to _1, in order to again V _C, it can keep sufficiently enhanced in-mold molten steel temperature before entering the steady casting, mold molten steel temperature after reaching V _C temporarily decreases Therefore, the portion where internal quality and surface quality are deteriorated can be shortened, and the bottom crop can be greatly reduced.

In the conventional method II, two-stage acceleration α ₁
・ Since α ₂ is moderate and is kept constant after reaching V _C , it has the same drawbacks as in the above-mentioned conventional method III. preventing, and it is impossible to prevent temporary lowering of the mold in the molten steel temperature after reaching V _C.

(2) Seal leakage and breakout between the dummy bar head and the inner wall surface of the mold In the method of the present invention, unlike the conventional method III, a low acceleration speed is not maintained for a certain period of time, and a constant acceleration from the start of drawing is achieved. a ₁
At a stretch to the casting maximum speed V _1, but since a solidified shell of sufficient thickness is formed from the start of pouring to the start of casting slab withdrawal T ₀ , the dummy bar head and the mold inner wall surface The seal leakage during is prevented. Further, by strengthening the seal of the dummy bar head before casting, the seal leakage can be reliably prevented.

Further, according to the method of the present invention, as described above, the temperature of molten steel in the mold at the initial stage of casting can be increased, so that breakout due to uneven solidification due to a decrease in temperature of molten steel can be prevented. .

Further, by using a dummy bar head having a concave groove in the shape of an arc hook, the contact area between the dummy bar head and the cast piece is increased, and the connecting portion has an angle orthogonal to the drawing direction, so that the dummy bar is firmly fixed. Concatenated,
Even if the dummy bar is pulled out at a relatively large acceleration as in the method of the present invention, the separation of the dummy bar and the slab in the roller apron can be reliably prevented.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. FIG. 5 shows a general curved-type continuous casting equipment, in which molten steel is cast from a tundish 2 into a mold 1 through a sliding nozzle device 3 and an immersion nozzle 4, and is primarily cooled by the mold 1. After that, the slab is secondarily cooled by a cooling water spray or the like by a roller apron 5 as a roll supporting and guiding device, and a completely solidified slab is pulled out by a pinch roll 6 and cut to a predetermined length by a traveling cutting device (not shown). . The surface of the molten metal in the mold is covered with powder 7 to prevent quenching of the surface of the slab and trap inclusions.

Dummy bar 10 used at the start of casting
Is a flexible member in which unit blocks are generally connected by pin bonding, and is an upper part of the mold 1 (top loading type).
Alternatively, the dummy bar head 1 is loaded from the back of the pinch roll 5 (bottom loading method) or the like and located at the upper end thereof.
1 is disposed in the lower part of the mold 1. After the start of casting, it is taken out from the middle of the roller apron 5 or from the latter stage of the pinch roll 6.

In the above configuration, in the present invention,
At the start of casting, a slab drawing speed as shown in FIG. 1 is controlled, and a dummy bar head 11 as shown in FIG. 4 is used. As shown in FIG. Take out upwards.

As shown in FIGS. 4 and 5, the dummy bar head 11 has an inclined surface 12 and an arc-shaped hook-shaped concave groove 13 at the upper part.
To form The inclined surface 12 is formed on the upper surface of the connection with the slab S with a downward slope toward the outside of the center direction A of the curvature of the line (the slab thickness direction) in a state where the inclined surface 12 is located in the mold 1. Reference numeral 13 is formed so as to extend in the direction A from the outside in the direction A of the inclined surface 12 and penetrate to the center.

The concave groove 13 is formed such that the radius Ri of the inner surface in the direction A is t + α and the radius Ro of the outer surface is t + β,
Also, the center Oi of the inner surface radius Ri is near the upper end inside the curve,
The center Oo of the outer surface radius Ro is shifted to the outside. By making the radii Ri and Ro larger than the thickness t of the slab S, the contact area with the slab is increased, and the angle is made closer to the angle perpendicular to the drawing direction so that the slab and the dummy bar do not separate even with a large drawing force. I do. Note that if this radius is too large, the distance becomes far from the angle perpendicular to the drawing direction, so that β ≦ 1.5t. Further, by setting α ≦ β, it can be easily separated after the drawing.

The outer surface of the concave groove 13 in the direction A is opened to form an opening 13a, and the upper part of the inner surface is formed as an inclined surface 13b inclined toward the upper part so that the groove 13 can be easily separated after being pulled out. In the slab width direction, the distance from the center is r,
The strength of the dummy bar head 11 is ensured. The upper end of the dummy bar 11 having such a structure is provided with a sealing member 14.
To prevent the molten steel from leaking.

Next, regarding the loading method of the dummy bar 6,
In the dummy bar top charging type CC, a machine mainly equipped with a drive roll after the second of the roller apron 4 is mainly used. However, since the number of drive rolls is about two to three, a large acceleration like the pull-out control of the present invention is required. Is required, the force for pulling out the slab and the dummy bar is insufficient, so that the slab cannot be pulled out. Therefore, in the present invention, a bottom loading method using a pinch roll drive is adopted.

As shown in FIG. 5, the dummy bar table 15 is placed on the pinch roll horizontal zone 16 at the subsequent stage of the pinch roll 6, and its rear end is pivotally mounted. From the position located at the exit of the vehicle to the horizontal position. Further, the dummy bar 10 on the dummy bar table 15 is loaded and moved by a driving device including a winch and a winch wire (not shown).

At the time of loading, the dummy bar table 15
Is tilted from the horizontal state, the dummy bar 10 is fed into the pinch roll 6, and the pinch roll 6 is reversely driven by the reverse rotation drive. At this time, a position sensor 17 of a photoelectric tube type or the like installed at a position where the dummy bar is detached from the pinch roll 6 exit.
, The tip of the dummy bar is detected, the moving distance is detected by the number of rotations of the pinch roll 6, and the dummy bar insertion length with the position of the sensor 17 as the origin is measured.

The dummy bar insertion length is determined in advance by the C
When it matches the numerical value stored in the PU, it is determined that the charging is completed. The dummy bar head 11 is located at a predetermined position in the mold 1, and in this state, the pinch roll 6 is driven to rotate forward to start pulling. The dummy bar 11 is transferred onto the dummy bar table 15 through the inside of the roller apron 5.

At this time, the amount of movement of the dummy bar is measured by detecting the number of rotations of the driving pinch roll 6, and when the amount of movement of the dummy bar coincides with the amount of movement of the pinch roll 6 in reverse rotation,
The dummy bar table 15 is raised, and the dummy bar 10 is pulled obliquely upward. The slab is continuously pulled out by the pinch roll 6 as it is.

[Embodiment] Under the casting conditions shown in Table 1, as shown in FIG. 1, a predetermined time elapses after the start of the injection of the molten metal, and the time T at which the level of the molten metal in the mold 1 has reached the control level has started.
_0, to speed-up the cast strip withdrawing speed at a constant acceleration a ₁ from the initial velocity zero to casting upper limit speed V ₁ (> steady casting speed V _c). Then a steady casting speed V _c is decelerated at the deceleration a _2.
As a comparative example, a drawing method according to the conventional method III was performed under the casting conditions shown in Table 2.

[0032]

[Table 1]

[0033]

[Table 2]

FIG. 1 (b) shows the transition of the molten steel temperature in the mold, and FIG. 2 shows the number of inclusions in the slab from the slab bottom to the steady portion. From FIG. 1 (b) and FIG. 2, the molten steel temperature in the mold was significantly higher than in the conventional method III,
It is also clear that there is no temporary temperature drop after the transition to steady casting as in the conventional method III, and the inclusions in the slab can be significantly reduced as compared with the conventional method III.

FIG. 3 shows a slab surface flaw at the tip of the slab. There is also a clear difference between the conventional method and the method of the present invention in surface flaws, and the effect of improving surface quality is apparent.

Further, by using the dummy bar head 11 shown in FIG. 4, even if the drawing speed is increased at an acceleration of 1.0 m / min ² , the tip end of the slab connection does not extend, and the detachment in the roller apron can be prevented. There was no operation trouble, and detachment from the slab tip at a fixed position was easy. Note that α ≦
By setting β, β ≦ 1.5t, the accidental detachment of the dummy bar in the roller apron was reduced, and the dummy bar was easily detached from the slab at the exit of the pinch roll.

[0037]

As described above, according to the present invention, the slab withdrawal speed is increased from the initial speed of 0 to a constant acceleration at the point of time when the slab withdrawal is started when the molten metal level in the mold reaches the control level after a certain time has elapsed after the start of pouring. Since the casting speed is increased to the casting speed equal to or higher than the steady casting speed, and then the casting speed is reduced to the steady casting speed at a deceleration, the following effects are obtained.

(1) Since the speed is increased at a stretch from the start of drawing to a casting speed exceeding a steady casting speed at a constant acceleration, the molten steel is not supercooled and the molten steel heat is sufficiently supplied. The temperature of molten steel in the mold can be raised in the early stage of casting, the internal quality is improved by promoting the floating of inclusions, and the surface quality is improved by eliminating supercooling. Since there is no subsequent temporary decrease in the temperature of the molten steel in the mold, a portion where internal quality and surface quality are deteriorated can be shortened, and the bottom crop can be significantly reduced.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a method for extracting a high-speed slab at an early stage of casting according to the present invention, and FIG.
(B) is a graph showing a change in the casting speed and the temperature in the mold with respect to the casting time.

FIG. 2 shows the number of inclusions in a slab according to the method of the present invention and the conventional method III.
6 is a graph showing an example in which the comparison is made between FIG.

FIG. 3 is a graph showing an example in which the surface defect code of a slab is compared between the method of the present invention and the conventional method III.

FIG. 4 is a dummy bar head used in the present invention,
(A) is a longitudinal sectional view of a dummy bar head in a mold, (b) is a longitudinal sectional view of a beam bar head at a pinch roll outlet, and (c) is a plan view of the dummy bar head.

FIG. 5 is a schematic side view showing a continuous casting facility used in the present invention.

FIG. 6 is a graph showing a change in a slab drawing speed with respect to time in the conventional method I and the conventional method II.

FIG. 7 is a graph showing a change in a slab drawing speed with respect to time in a conventional method III.

[Explanation of symbols]

T ₀ … Start time of drawing slab T ₁ … Time when slab drawing speed reaches upper casting speed V ₁ T ₂ … Time when slab drawing speed reaches steady casting speed V _c V ₁ … Higher casting limit velocity V _c ... steady casting speed 1 ... mold 2 ... tundish 3 ... sliding nozzle device 4 ... submerged nozzle 5 ... roller apron 6 ... pinch rolls 7 ... molten metal surface coating powder 10 ... dummy bar 11 ... dummy bar head 12 ... inclined surface 13 ... concave groove 14 ... sealing material 15 ... dummy bar table 16 ... pinch roll horizontal zone 17 ... position sensor

Claims (1)

[Claims] In a continuous casting, a slab withdrawal speed is increased from an initial speed of 0 at a constant acceleration during a slab withdrawal from the start of slab withdrawal at the start of casting to a steady pouring speed. After speeding up to a speed higher than the
A high speed slab drawing method in an early stage of continuous casting, wherein the slab drawing speed is reduced to reach a steady casting speed.