JPH0464773B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a continuous casting method for steel, etc. (hereinafter, continuous casting will be abbreviated as CC). Conventional technology In conventional steel horizontal CC, a ceramic break ring B is fitted to the tip of a water-cooled copper mold as shown in Fig. 4, and the mold is stationary and the slab is withdrawn intermittently as shown in Fig. 6. The slab is pulled out. In FIG. 6, denotes the end point of the pull-out, denotes the middle point of the pushback, denotes the start point of the pull-out, and denotes the middle point of the pull-out. In this case, as shown in Figure 4,
On the surface of the slab in the early stage of solidification in contact with the copper mold, there is cold shrapnel C due to solidification on the break ring surface.
And a hot spot H, which is the bonding mark between the initially solidified shell X and the drawn shell Y, is generated. Since the shell growth process progresses in the order of - in Figure 4, the cold spots and hot spots remain as surface defects on the slab without cracks in most cases and become a problem. On the other hand, if a conduit-mold made of non-metallic material such as graphite, ceramics, or cermets and having a continuous inner surface and the same cross-sectional dimensions as shown in Fig. 5 is used, the shell growth process will be as shown in Fig. 5. ~
The hot spots remain, but the cold spots disappear. However, hot spots can sometimes be more than 0.2 mm deep, accompanied by cracks, and become a problem as surface defects in slabs. As the conduit-mold shown in Fig. 5, there is a cast iron horizontal CC using a graphite conduit-mold.
It has been disclosed that steel can be produced with corrosion-resistant cermet conduit-molds. However, in either case, hot spots remain. Furthermore, Mitsubishi Steel Technical Report Vol.19, No.1, 2 (1985)
On page 28, it is stated that when intermittent drawing is performed with a BN mold, there is no occurrence of cold shatter cracks or hot tears (those with tension in hot spots), and further on page 30. It is stated that pull-out marks called cold shots are observed in BN molds, but hot tear patterns are not observed. It is said that the pull-out mark called a cold shot is generated at the interface between the new and old solidified shells, and is therefore the hot spot H shown in FIG. 4 of the present invention. As described above, even if the continuous conduit-mold made of non-metal and having the same cross-sectional dimensions as shown in FIG. 5 is used, the generation of hot spots is inevitable. Problems to be Solved by the Invention The present invention eliminates surface marks such as oscillation marks, cold spots, and hot spots, which are non-smooth defects that occur periodically on the surface of a cast slab, thereby producing a cast slab with excellent surface quality. This provides a continuous casting method that can produce pieces. Means for Solving the Problems The present invention provides a method for continuously casting molten steel using a continuous conduit-mold made of a non-metallic material, by increasing the conduit-mold transmission resistance R (cm ² ·s · °C/cal). On the other hand, the intermittent drawing cycle of slab or mold oscillation cycle f (1/min) is 1.52−0.25logR≦logf≦
This continuous casting method is characterized by operating by adjusting f so that it is 2.73-0.32 logR, and has excellent slab surface properties. Even if a continuous conduit-mold made of non-metallic material is used, hot spots as shown in FIG. 5 will normally occur as described above. However, as a result of casting using various nonmetallic conduit-molds and under various casting conditions, it has been found that the above-mentioned conditions exist under which the hot spots disappear or become slight. Generally, the heat transfer resistance R of a mold is expressed as t/k, where t (cm) is the thickness of the conduit-mold material, and k (cal/cm ² ·s ·° C.) is the thermal conductivity. For the thermal conductivity k, the value at the average temperature of the mold during casting should be adopted, but in the present invention, the value at 500° C. was used for simplicity. If R is small, the mold surface temperature tends to drop because the amount of heat removed from the mold increases, and when molten steel solidifies on the low-temperature mold surface, solidification occurs before the fusion of the initial solidification shell and the drawn shell in Fig. 5 has sufficiently progressed. It is thought that because the bond progresses, hot spots of bonding marks remain on the surface. On the other hand, when R is large, the amount of heat removed from the mold is small and the mold surface temperature is kept high, and the initial solidification shell and the drawn shell solidify after fusion progresses, so the bond marks become smooth and hot spots are presumed to disappear or become slight. The surface temperature of the mold repeatedly decreases and returns with each cycle of intermittent drawing of the slab, and the degree of decrease depends on the heat transfer resistance R of the mold as well as the time of one cycle, that is, the number of cycles f. Therefore, in order to prevent the mold surface temperature from decreasing, it is necessary to adjust f to be large when R is small, and to be small when R is large. The conditions for this are 1.52−0.25logR≦logf≦2.73−0.32logR shown in Figure 1,
When casting under conditions within this range, the surface marks on the slab obtained disappear or are only slight with a depth of less than 0.1 mm. When the casting conditions are logf<1.52−0.25logR, the surface mark becomes deep and noticeable by 0.1mm or more,
If logf>2.73−0.32logR, solidification progresses insufficiently during one drawing cycle and breakout occurs. The case of the stationary mold-casting intermittent drawing method shown in FIG. 6 has been explained above with reference to FIG. 5, but the mold oscillation-mold drawing method shown in FIG. It is the same as the intermittent pull-out method, and can be considered to have added smoothness to the operation. That is, in Fig. 7, ~
The points correspond to the points .about. in FIG. 6, and the solidification process is the same as in FIG. The method of the present invention can also be applied to such a system. Also, the horizontal position shown in Figure 2, where the tundish pipe and mold are arranged horizontally.
It goes without saying that, like the CC, this can also be applied to the vertical CC shown in FIG. 3, which is arranged vertically. The non-metal used in the conduit-mold of the present invention includes:
Nitrides such as BN, Si ₃ N ₄ etc., Al ₂ O ₃ ,
Oxides such as ZrO ₂ or composites thereof are suitable, but other substances with a melting point of 1800°C or higher, such as ZrB ₂ , may also be used alone or with the nitrides,
Suitable as a complex with oxides. Embodiments Embodiment 1 First, Embodiment 1 of the present invention will be explained in detail with reference to FIG. Molten steel 21 is injected from a ladle 1 through a flow rate adjustment device (sliding nozzle) 2 and from an immersion nozzle 3 into a tundish 5 equipped with a heating coil 4.
flows from the lower side wall of the tundish 5 through a horizontally installed refractory conduit 6 into a nonmetallic conduit-mold 7. The conduit-mold 7 has its rear half (mold) cooled by a water-cooled jacket 8, in which the molten steel solidifies to form a shell 22. The shell 22 is intermittently pulled out by the pinch rolls 12 as shown in FIG. It is cooled and completely solidified at step 11, and cut into molds 23 of required length by cutter 13. Stainless steel SUS304 was cast using such a horizontal CC. Boron nitride (hereinafter BN) is used as the conduit-mold 6
), silicon nitride (hereinafter referred to as Si ₃ N ₄ ), and zirconia (hereinafter referred to as ZrO ₂ ) tubes were used. The tube has an inner diameter of 20 mm, a length of 100 mm, and a wall thickness of t from 1 to 3.
mm, and the heat transfer resistance R was set using a value of 500°C for the thermal conductivity k. In the drawing operation, the drawing stroke per cycle was 10 to 50 mm, and the number of drawing cycles f was varied in the range of 10 to 500 cpm. The casting temperature is 1470℃ and the casting amount is 300Kg. Example 2 Next, Example 2 of the present invention will be explained in detail with reference to FIG. Molten steel 21 is injected from a ladle 1 through a flow rate adjustment device (sliding nozzle) 2 and from an immersion nozzle 3 into a tundish 5 equipped with a heating coil 4.
A non-metallic conduit is passed from the bottom of the tundish 5 through a refractory conduit 6 arranged vertically.
It flows into the mold 7. The lower half of the conduit mold 7 and the trailing mold (made of graphite) 9 are cooled by a water cooling jacket 8. The tundish 5 to the succeeding mold 9 are mounted on an integral frame 14, and this frame 14 is oscillated vertically by an oscillation device 15 in a waveform as shown in FIG. 7 at a constant number of cycles. Molten steel 21 in the mold section under the oscillation
solidifies to form a shell 22, which is continuously drawn out by pinch rolls 12 to form a subsequent mold 9.
After exiting, it is completely solidified by being cooled by a water spray group 11 while being supported by a roll group 10, and then cut by a cutter 13.
Then, the slab 23 is cut into the required length. Stainless steel SUS304 was cast with such a vertical CC. As the conduit-mold 7, BN, Si ₃ N ₄ and a composite Si ₃ N ₄ ·BN pipe were used. The tube had an inner diameter of 100 mm and a length of 150 mm, the wall thickness t was varied from 3 to 10 mm, and the heat transfer resistance R was set using a value of 500° C. for the thermal conductivity k. The drawing speed is 1 to 3 m/min, and the oscillation cycle number f is 10 to 500 cpm.
It was varied within the range of. The casting temperature is 1470℃, and the casting amount is 10 tons. The casting conditions and results of Examples 1 and 2 are summarized in Table 1. Comparative example No. 8, 10, 12, 20, 22,
24 had a breakout within a few minutes after starting casting, but the others were cast smoothly. The obtained slabs tested according to the present invention had surface marks that disappeared or were only slight (depth
(less than 0.1 mm) and did not require surface maintenance, whereas those in the comparative example test had noticeable hot spots (depth of 0.1 mm or more) and required surface maintenance. In addition, in the casting conditions in Table 1, ○ indicates complete casting, and × indicates breakout. Also, on the slab surface, ○ indicates that the surface mark has disappeared or is slight, × indicates that the surface mark is noticeable, and - indicates that no measurement was made.

【table】

[Table] Effects of the Invention As described above, when performing continuous casting using a conduit-mold made of a non-metallic material, the drawing cycle or oscillation cycle is set in a specific relationship with respect to the heat transfer resistance value of the conduit-mold. By adjusting the surface of the slab, surface marks on the slab can be eliminated or made slight, which eliminates the need for slab surface maintenance work, improves product retention, and reduces slab production costs. can be reduced.

[Brief explanation of the drawing]

Fig. 1 shows the relationship between the conduit-mold heat transfer resistance R, the drawing or oscillation cycle f, and the slab surface mark, and Fig. 2 shows the horizontal
Fig. 3 is an explanatory diagram of an embodiment of the CC device, Fig. 3 is an explanatory diagram of an embodiment of the vertical CC device to which the present invention is applied, and Fig. 4 is the solidification process in the horizontal CC of the conventional break ring-copper mold method and based on this. An explanatory diagram of the surface mark generation mechanism. Figure 5 is an explanatory diagram of the solidification process in the horizontal CC of the non-metallic conduit-mold method and the surface mark generation mechanism based on this. Figure 6 is an explanatory diagram of the stationary mold - intermittent slab. A diagram showing changes in the drawing speed of slabs in the drawing method, Figure 7 shows the mold oscillation.
It is a figure which shows the change of the drawing speed of a slab in a continuous slab drawing method. 1...Ladle, 2...Sliding nozzle, 3
... Immersion nozzle, 4 ... Heating coil, 5 ... Tundish, 6 ... Refractory conduit, 7 ... Nonmetallic conduit-mold, 8 ... Water cooling jacket, 9 ... Graphite subsequent mold, 10 ... ... Roll group, 11 ... Water spray group, 12 ... Pinch roll, 13 ... Cutter, 14 ... Frame, 15 ... Oscillation device, 21 ... Molten steel, 22 ... Shell, 23 ... Slab, B ...Break ring, W...Cooling water,...
...Copper mold, ...Nonmetallic conduit-mold, ...Copper water cooling jacket, L ...Mold center line,
X...Initial solidification shell, Y...Drawn shell, H...
...Hot spot, C...Cold shot.

Claims (1)

[Claims] 1. When continuously casting molten steel using a continuous conduit-mold made of non-metallic material, the heat transfer resistance R ( ^cm2・s・℃/cal) of the conduit-mold is determined by the intermittent drawing cycle or Mold oscillation cycle f (1/min) is 1.52−0.25logR≦logf≦2.73
A continuous casting method with excellent slab surface properties characterized by operating by adjusting f so that it is -0.32logR.