JPS5939225B2 - Continuous steel casting method - Google Patents

Description

[Detailed description of the invention] The present invention relates to the creation of a continuous casting method for steel, and an object of the present invention is to most efficiently improve the internal quality of slabs obtained by continuous casting, and to reduce non-metallic inclusions in the slabs. The aim is to establish a method that can

Continuous casting of steel has been carried out for a long time, and there are various methods such as curved type, vertical type, and vertical bending type. Internal defects such as center porosity, which is a fine shrinkage hole that occurs in the center, center pipe, which is a pipe-shaped shrinkage hole that is formed by the growth of this center porosity, and segregation, often occur. The molten steel static pressure t near the tip of the crater of the piece
This causes mechanical bulging, or by the suction action that occurs when the molten steel near the tip of the crater of the slab solidifies and shrinks, the unsolidified part is pulled into the solidified part and moved, and the pulled part moves. This is caused by the formation of pores due to insufficient supply of molten steel.

Therefore, as a method for eliminating the cause of such internal defects and preventing internal defects in slabs, for example, Japanese Patent Application Laid-Open No. 49-12
No. 1738 and JP-A No. 51-60633, at least two
A casting method in which pairs of reduction rolls are arranged and multiple reductions are applied to each pair of reduction rolls, or the inside of a frame is placed near the tip of a crater where the molten steel inside the slab has completely solidified, and is formed along the direction in which the slab is pulled out. 0.5 to 2% of the thickness of the slab
A method has been proposed in which the slab is narrowed down by a taper of m, and each of these methods is effective, but the injection temperature of the steel to be cast (usually the molten steel temperature in the tundish) has not been accurately elucidated. Therefore, the effect will be significantly different, and for this reason, it is not always possible to obtain the desired slab.

The present invention was created after repeated studies in view of the above-mentioned circumstances.

According to the detailed study results of the present inventors, in continuous casting of ordinary steel, the internal structure of the slab after casting changes depending on the temperature inside the tundish of the molten steel being cast, and the degree of this change is The degree of supermaturity of steel steel (
ΔT).

However, ΔT0: Degree of superheating of the molten steel in the tundish T ('C):
Tundish inner bath steel temperature Ts■: @ Solidification (liquidus line) temperature of steel The above-mentioned Ts is uniquely determined by the composition of the molten steel, and is expressed, for example, by the following equation H.

TS=1536-(780X [hole)+7.6 mowing%S i
]+4.9X [$n]+34.4 mowing%P]+38.
OX[%S'l→-4,9X[%Cu]+3.1x[%
N i']+1.3 x[%Cr)+3.6X[%A7
]) ・-・T1 [hole], [%Si], [to], [
%P], [%S], [hole U], [drawing guide], [hole r) and [hook V] are carbon, silicon, manganese, phosphorus, sulfur, copper, nickel, chromium, and aluminum in molten steel, respectively. This is the concentration by weight.

A typical example of the solidified structure inside the slab after casting is shown in Fig. 1, in which A indicates a columnar crystal structure, and the central part of the thickness is entirely composed of columnar crystals a (or branched columnar crystals b). ), which occurs when the degree of superheating of the molten steel is high, and B indicates a partially equiaxed crystal structure, with columnar crystals a or branched columnar crystals in the center of the thickness. Partial equiaxed crystals C are formed during crystallization, and this occurs when the degree of superheating of molten steel becomes relatively low.

Further, C is an equiaxed crystal structure in which the central part of the thickness is entirely covered with equiaxed crystals C, which occurs when the above-mentioned degree of superheating of the steel is lower.

Therefore, when we investigated the relationship between the solidification structure inside the slab and the degree of superheating of the steel inside the tundish, we found the relationship shown in Figure 2 below.

In the case of 40 kg class steel, if the molten steel superheat degree ΔT≦lθ℃, it will have the above-mentioned C equiaxed crystal structure, and if ΔT≧30℃, it will have a columnar crystal structure, and the intermediate conditions between these will be At the lower temperature, that is, at 10° C.<ΔT and 30° C., the above-mentioned partially equiaxed crystal structure is obtained.

In addition, 50-60 kg class steel has an equiaxed crystal structure at 21615°C.
When ΔT≧40°C, it becomes a columnar crystal structure, and when 15°C<ΔT<40°C, it becomes a partially equiaxed crystal structure.

Moreover, the above-mentioned 40 kg class steel and 50 to 60 kg class steel are shown in Table 1 below in terms of steel composition values.

In addition, the present inventors have practically investigated a vertical bending type continuous casting machine that has a vertical section of about 5 m and then bends at 5 points to form a constant circular arc section of 10.577 ZR. Approximately 4.2 m in the coagulation part (reduction roll XX illusion)
A casting test was conducted by changing the casting temperature using a device equipped with a slab unsolidified reduction device, and the casting conditions at this time are shown in Table 2 below.

Table 2 Casting conditions ■ Molten steel amount: 270 t/ch ■ Slab size: 230 thick cutting 600 o r 1900
run width ■Casting speed: 0.9~1.0 m/m
■ Grade of steel and degree of superheating of molten steel in tundish (ΔT) ■ Unsolidified slab: O, 0, 5, 0, 75, 1, 0° reduction (im) 2. Orran/m■Total number of charges = 40kg class steel, 50ch, 50~60kg class steel 20ch After casting, the slab is cut in the transverse direction of the casting, and after polishing, large inclusions with a diameter of 100 microns or more found on the polished surface are removed. After visually counting and measuring the amount of inclusions, S-printing was performed and the degree of center segregation observed on the S-print was determined by measuring di and ti from the center segregation area observed in the cross section as shown in Figure 3. From the relationship between the slab thickness D and the center segregation distribution range, it was determined and quantified as the center segregation area ratio using the following formula.

Furthermore, the measurement results of the center segregation area ratio obtained in this way are plotted in a diagram for the case of 40kg steel, with the unsolidified slab reduction on the horizontal axis and the degree of superheating of the steel in the tundish as a parameter, as shown in Figure 4. As shown, measurement point ○ is ΔT
-30~50℃, △ is ΔT=15~25℃, × is 316
This shows the case at 10°C.

In other words, as is clearly shown in FIG.
In the case of 0 kg class steel, the unsolidified slab is 0.5 to 2.
The degree of improvement in center segregation in the slab when the slab is reduced by 0 rran/m is greatly influenced by the degree of superheating of the molten steel in the tundish, and a significant improvement is observed only when casting at a high temperature of ΔT = 30 to 50°C.

However, the reason for this is thought to be due to the difference in the cast structure at the final stage of solidification of the slab, and the reduction of the unsolidified slab is effective only when the casting structure at the center of the thickness of the slab is a columnar crystal structure as shown in Figure 1A. It was confirmed that this material contributes to the improvement of center segregation.

The same thing applies to 50-60 kg class steel, and when ΔT is 40° C. or more as shown in FIG. 2, a significant improvement in center segregation can be obtained.

However, from the perspective of the casting structure, there is no upper limit for ΔT, but if the molten steel temperature in the tundish exceeds 1580°C, the solidified shell directly under the mold will not be fully developed, increasing the risk of breakout. For steel, ΔT is 60
℃ or less, it is desirable that ΔT be 70°C or less for 50 to 60 kg class steel.

Next, the quantitative values of large inclusions remaining in the slab are shown in Figure 5 for the case of 40 kg class steel. Since it has a vertical section of about 5 m, it tends to cause inclusions to float and separate, and the amount of inclusions is less than 1/15 of that of normal S type casting equipment. However, it is clear that in the case of high-temperature casting at ΔT-30 to 50°C, inclusions float and separate more easily than in the case of low-temperature casting at 31,610°C, resulting in a cleaner steel. This result was also observed in the case of 50 to 60 kg class steel, and it was known that high-temperature casting as described above is advantageous in obtaining clean steel.

When using the method of the present invention as explained above, the degree of superheating of the molten steel in the tundish is set to 30 to 70 during continuous casting.
℃, and by applying a light reduction of 0.5 to 2.0 mm/m near the crater end of this slab Q, the center segregation can be greatly improved, resulting in a slab with few internal defects. Moreover, it is possible to obtain clean steel by appropriately flotation and separation of inclusions, and this invention is industrially highly effective.

[Brief explanation of drawings]

The drawings show embodiments of the present invention, and FIG. 1 is an explanatory diagram of a typical example of the solidification structure of a continuously cast slab, and FIG. Figure 3 is an explanatory diagram of the method for measuring the center segregation area ratio of slabs, and Figure 4 shows the effect on improving slab center segregation when rolling unsolidified slabs of 40 kg class steel. Figure 5 is a chart showing the influence of casting temperature on the amount of large inclusions remaining in the slab for 40 kg class steel. In these drawings, a indicates a columnar crystal structure, b indicates a branched columnar crystal structure, and C indicates an equiaxed crystal structure.

Claims (1)

[Claims] 1. The cast slab shaped from the tundish through the mold is guided by guide rollers to continuously cast drawn steel. 1. A method for continuous casting of steel, characterized in that a cast slab is injected into a mold and a rolling reduction of 0.5 to 2.0 w/m is applied to the slab near the crater end of the slab pulled out from the mold.