JPS61162256A - Improvement of surface characteristic of continuous casting steel ingot - Google Patents

Abstract

PURPOSE:To prevent the cracking of an ingot and the rupture of a shell in the stage of oscillating vertically a casting mold at a specified pitch and drawing the ingot by oscillating the mold by a nonsinusoidal waveform. CONSTITUTION:The casting mold is so oscillated vertically at the specified speed as to form the nonsinusoidal waveform in the stage of casting continuously a carbon steel, etc. having about 0.08-0.16wt.% carbon content. More specifically, the oscillating speed Vm of the mold is made higher than the drawing speed Vc of the mold to shorten the negative strip time by oscillating the mold in the nonsinusoidal waveform in which the mold is quickly lowered after rising. The relative speed difference (Vm-Vc) between the oscillating speed Vm of the mold and the drawing speed Vc of the mold is thereby decreased, by which the frictional resistance between the mold and the shell is decreased. The casting with high efficiency is thus made possible without the cracking of the ingot and the rupture of the shell and without dropping the casting speed.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] This invention provides carbon content of 0.08 to 0.16 wt,
% carbon steel or stainless steel (SUS 430) with a chromium content of 17 wt'%, surface defects such as transverse cracks on the slab or breakouts due to shell breakage in the mold occur. does not occur, and
The present invention relates to a method for improving the surface properties of continuously cast slabs that can be cast with high efficiency without reducing casting speed.

[Prior art and its problems]

This corresponds to continuous casting of carbon steel with a carbon content of 0.08 to 0.16 wt%, and 149% in the solidification process.
A peritectic reaction of L+δ→γ (L: molten metal, δ: ferrite, γ niostenite) is observed in the slab at a temperature around 0°C.

During the above reaction, the slab shrinks and its volume changes rapidly, so the shape of the solidified shell becomes uneven as shown in Figure 10, and a gap 3 is created between the mold 1 and the shell 2. . Since the heat removed from the shell 2 in the area where such voids 3 are formed is small, solidification is delayed. Therefore, as a result of the uneven thickness of the shell 2 in the mold 1, the frictional resistance generated between the mold 1 and the shell 2 causes cracks starting from the oscillation marks in the thin part of the shell 2. It becomes easier. FIG. 11 shows the shape of the crack that occurred in the slab 4 as described above.

In the figure, 5 is a hook-shaped crack that occurs in a part of the corner of the slab 4, 6 is a crack that occurs in a part of the corner of the slab 4 in the thickness direction, and 7 is a crack that occurs in a part of the corner of the slab in the width direction. This is a crack that occurs in

The following is known about the cracks that occur in the slab 4 due to the peritectic reaction as described above.

(1) There is no relationship between the occurrence of cracks and the conditions of secondary cooling, that is, the strength of cooling.

(2) The short sides of the mold 1 are inclined inward from the upper end to the lower end as shown in FIG.
When the dimension of is made small, the frictional resistance between the inner surface of the short side and the shell on the short side is reduced, and as a result, cracking is reduced.

(3) The cracks generated in the slab are found to be filled with powder added to the surface of the molten steel in the mold, so the cracks occur within the mold 1.

For this reason, the only way to prevent cracking of the slab is to draw the slab at a low speed of 0.9 V or less, which inevitably leads to a decrease in production efficiency.

In addition, stainless steel (SUS 430) with a chromium content of 17 wt or less has a low high temperature strength, reduction of area, and reduction value near the melting point compared to other austenitic stainless steels, so there is a The shell is likely to break due to the frictional resistance, resulting in a breakout. Figure 13 shows SUS 430 (melting point 1425-1510°C).
) is a graph showing the tensile strength and reduction of area of stainless steel (SUS 430) as described above (D To prevent shell rupture, draw the slab at 0.8rr
It is necessary to perform the casting at a low speed of Xfj or lower while ensuring the thickness of the minimum thickness, which causes the problem of an unavoidable decrease in production efficiency.

[Purpose of the invention]

Therefore, the object of this invention is to reduce the carbon content from 0.08 to 0.
．． 16 wt, % carbon steel or chromium content 1
During continuous casting of 7 wt. First, it is an object of the present invention to provide a method for improving the surface properties of continuously cast slabs, which allows for highly efficient casting without reducing the casting speed.

[Summary of the invention]

In this invention, when producing a continuously cast slab by pulling the slab from the mold at a predetermined speed while vibrating the mold vertically at a constant pitch, after the mold is slowly raised, The mold is vibrated by a non-sinusoidal waveform consisting of a rapid lowering and a shortening of the negative strip time as the vibration speed of the mold is faster than the withdrawal speed of the slab, thus changing the surface texture of the slab. The present invention is characterized in that it reduces horizontal cracks and flake-outs that occur in the slab.

[Structure of the invention]

Next, the present invention will be explained with reference to the drawings.

Fig. 1 is a graph showing the vibration waveform of a mold showing an embodiment of the method of the invention, Fig. 2 is a graph showing the vibration speed of the mold, and Fig. 3 is a graph showing the vibration waveform of a conventional mold. FIG. 4 is a graph similarly showing the vibration speed of the mold. Continuous casting of steel using a vertical continuous casting machine involves feeding molten steel contained in a tundish into a water-cooled vertical mold, forming a solidified shell in the mold, and in this way, the shell is formed around the tundish. The formed unsolidified slab is continuously pulled out from the mold, but in order to form a healthy shell within the mold, the mold is usually vibrated vertically at a constant pitch and the inside of the mold is Powder is added onto the surface of molten steel.

The vibration of the mold described above has conventionally been performed using a sine waveform as shown in FIG. Figure 4 is a graph showing the vibration velocity of the mold (Vni, )= in the case of a conventional sine waveform, where Vc is the blowing velocity of the slab, and Vm-Vc is the vibration velocity of the mold (when rising). It shows the relative speed difference between the maximum speed) and the slab drawing speed. This relative velocity difference represents the maximum tensile stress that the mold will exert on the shell if there is a frictional resistance between the mold and the shell of the slab.

In the method of the invention, the mold is vibrated vertically at a constant pitch so as to form a non-sinusoidal waveform as shown in FIG. That is, as shown in Fig. 2, the mold is vibrated with a non-sinusoidal waveform that slowly raises it and then quickly lowers it.
The vibration speed of the mold (,Vm,) is the drawing speed of the slab (Vc)
The faster time or negative strip time is reduced compared to the conventional sinusoidal waveform shown in FIG.

As a result, the relative speed difference (Vm - V c ) between the vibration speed of the mold and the drawing speed of the slab becomes small, and the frictional resistance between the mold and the shell can be reduced.

FIG. 5 is a graph showing the relationship between the distortion rate of the vibration waveform of the mold and the frictional force index. In Figure 5, the distortion factor of the vibration waveform (
α) indicates the amount of deviation B/A x 100 (%) of the non-sinusoidal waveform with respect to the sine waveform shown in the figure.

The frictional force index still shows (Vm-Vc)/Q. Here, Q is the amount of powder flowing in per m2 of slab (Yn・), and this amount of powder flowing in is:
It was determined from the relationship of q""atp, which is proportional to the positive strip time (the time during which the vibration speed of the mold is slower than the drawing speed of the slab). In the above equation, q is the amount of powder flowing in (weight of powder flowing in one cycle per slab ICrn, g/cycle m), tp is the positive strip time (see), the melting characteristics of the aid powder, the vibration conditions of the mold, and the slab. This is a coefficient determined by the drawing speed.

, FIG. 5 shows the vibration stroke of the mold, the loaf is 8 degrees, the vibration frequency is 120 cpm, and the slab drawing speed is 1. This is an example of the case of Q m mrn, and as is clear from the figure, the friction force index increases as the distortion rate of the vibration waveform becomes smaller compared to the case where the distortion rate of the vibration waveform is 0% (sine waveform). decreases, and the above distortion rate is 40
When it comes to friction, the frictional force index is 48.6% (,8,5×1.00) compared to 18.5-9.5 in the case of 0%.
Decrease.

The factor of 70 to 80 for such a decrease in the friction force index is the relative speed difference between the vibration speed of the mold and the drawing speed of the slab (Vm
c) This is due to becoming smaller, and part 2
0 to 30 is because the positive strip time (tp) is increased by making the vibration waveform of the mold a non-sinusoidal waveform, the amount of powder flowing in is increased, and the lubrication between the mold and the shell is improved. be.

[Embodiments of the invention] Next, the present invention will be explained with reference to examples.

The mold was subjected to a vibration stroke of 8 tran and a frequency of 10.
Carbon steel having a carbon content of 0.08 to 0.16 wt.% was continuously cast while vibrating in the vertical direction under the condition of 0 cpm to produce a slab with a thickness of 25.0+ nm and a width of 1,000 mm.

FIG. 6 shows the index of occurrence of transverse cracks in slabs when the above casting was carried out by the method of the present invention shown below and by the conventional method.

Invention method slab drawing speed (Vc): 1. Om 7 minutes Shape of mold Vibration waveform of slightly tapered mold: Non-sinusoidal wave (α-40 ratio) Conventional example 1 Slab drawing speed (Vc): 1. Shape of 7 minute mold Vibration waveform of double taper mold: Sine wave Conventional example 2 Slab drawing speed (vc): 0.8 m Shape of 7 minute mold Vibration waveform of double taper mold: Sine wave Conventional example 3 Slab Drawing speed (Vc): 0.8 m7 minutes Mold shape: weak taper Mold vibration waveform: sine wave In the above, strong taper refers to the taper from the upper end of mold 1 to the lower end as shown in Fig. 12 -BI This is the case where T(,-,l-×100×T)% is 0.8%/m, and the weak taper is the case where T is 0.65%/m.

As is clear from FIG. 6, in the case of the method of the present invention, even when the slab drawing speed is high at 1.0 m/min, the horizontal crack occurrence index of the slab is extremely low, and the Set speed to 0
．． This is almost the same as when using a slightly tapered mold at 8 m/min, and the production efficiency was reduced by about 2 compared to Conventional Example 3.
I was able to improve the 0 ratio.

FIG. 7 is a graph showing the relationship between the distortion rate of the vibration waveform of the mold and the frequency of occurrence of claws in the valleys of the oscillation marks of the slab. In general, the oscillation marks that occur in carbon steel slabs (Fig. 8(a), where claws are formed in the valleys of the oscillation marks and are accompanied by segregation lines, and those shown in Fig. 8(b)) As shown in Figure 2, there are cases where there are no segregation lines in which no claws are formed in the valleys of the una oscillation marks.Such external claws are formed during slab straightening when casting slabs using a curved continuous casting machine. Tensile stress is applied on the top side of the plate, causing horizontal cracks to occur along the claws or valleys.

As is clear from Figure 7, the frequency of occurrence of the above-mentioned nails is
40 when the distortion rate of the vibration waveform of the mold is 0% (sine waveform)
However, in the case of a non-sinusoidal waveform with a distortion factor of 401), it becomes about 20 factors, making it possible to significantly reduce transverse cracking of the slab caused by the claws.

Next, the mold was shaken with a vibration stroke of 8 strokes and a frequency of 100 c.
While vibrating in the vertical direction depending on the pm ° condition,
Stainless steel (SUS 43) with a content of 17wt,%
0) was continuously cast to produce a slab with a thickness of 250 mm and a width of 1.500 mm. FIG. 9 shows the breakout occurrence index that occurred in slabs when the above casting was performed by the method of the present invention shown below and by the conventional method.

Method of the present invention Slab drawing speed (Vc): 017 m/min Mold shape: Slight taper Mold vibration waveform: Non-sinusoidal wave (α = 40%) Conventional example 1 Slab drawing speed (Vc): 0.9 m 7 Shape of minute mold: Weak taper = 11- Vibration waveform of mold: Sine wave Conventional example 2 Slab drawing speed (Vc): 0.7 m Shape of 7 minute mold Vibration waveform of two-weak taper mold: Sine wave From Figure 9 As is clear, according to the method of the present invention, the breakout occurrence index was reduced to about one quarter of that in Conventional Example 2, and stable casting could be performed. This reduction in the breakout occurrence index is considered to be due to the above-mentioned frictional force index being reduced by a factor of 40 by making the vibration waveform of the mold a non-sinusoidal wave.

〔Effect of the invention〕

As mentioned above, according to the method of the present invention, carbon steel with a carbon content of 0.08 to 0.16 wt.% or stainless steel with a chromium content of 17 wt.% (5US4
30), the surface properties of the slab are improved, no cracks or shell breaks occur in the slab, and it is possible to cast at high efficiency without reducing the casting speed. Excellent effects are brought about.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the vibration waveform of a mold showing an embodiment of the method of the present invention, FIG. 2 is a graph showing the vibration speed of the mold, FIG. 3 is a graph showing the vibration waveform of a conventional mold, and FIG. Figure 4 is a graph showing the vibration speed of the mold, Figure 5 is a graph showing the relationship between the strain rate of the vibration waveform of the mold and the frictional force index, and Figure 6 is a graph showing the relationship between the strain rate of the vibration waveform of the mold and the frictional force index, and Figure 6 is a graph showing the relationship between the strain rate of the vibration waveform of the mold and the frictional force index. A graph showing the index of occurrence of transverse cracks that occur in slabs in comparison with the conventional method. Figure 7 shows the relationship between the strain rate of the mold vibration waveform and the frequency of occurrence of claws that occur in the valleys of the oscillation marks of slabs. FIG. 8 is a graph showing the shape of the oscillation mark. FIG. 9 is a graph showing the breakout occurrence index when stainless steel is continuously cast by the method of the present invention compared to the conventional method. Figure 10 is a diagram showing the gap that occurs between the mold and the shell during continuous casting of carbon steel, Figure 11 is a diagram showing the shape of cracks that occur in the slab, Figure 12 is a diagram showing the shape of the mold, and Figure 13 is a diagram showing the shape of the crack that occurs in the slab. The figure is a graph showing the relationship between the tensile strength and the aperture value of stainless steel. In the drawings, 1... fJa type, 2... shell, 3... void,
4. Slab, 5.6.7...Crack.

Claims (1)

[Claims] When producing continuous cast slabs by pulling the slab from the mold at a predetermined speed while vibrating the mold vertically at a constant pitch, the mold is slowly raised and then rapidly lowered. and vibrating the mold with a non-sinusoidal waveform consisting of shortening the negative strip time in which the vibration speed of the mold is faster than the drawing speed of the slab, thus improving the surface quality of the slab. A method for improving the surface properties of continuously cast slabs, the method comprising: reducing transverse cracks and breakouts occurring in the slabs.