JP2001071097A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method by which the surface crack can be reduced even in the case of casting high carbon steel at a high speed. SOLUTION: In the continuous casting method in which molten steel is supplied into a mold from an immersion tube, the kind of steel contains 0.3-2.0 wt.% C, 0.15-3.5 wt.% Si and 0.0001-0.2 wt.% V, and casting speed is made to >=1.0 m/min. Further, the min. value or solidified shell thickness at a depth of 400 mm from the meniscus in the mold is made to >=10 mm, and the difference between the max. value and the min. value of the solidified shell thickness in the same horizontal cross section is made to <=5 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a continuous casting method for steel, and more particularly to a high-speed continuous casting method for high-C, high-Si steel.

[0002]

2. Description of the Related Art In continuous casting of steel, various proposals have been made in order to improve the surface quality of a slab.

(1) By controlling the viscosity and solidification temperature of the mold powder, the heat removal during the initial solidification can be slowed down,
Reduce surface defects of slab (CAMP-ISIJ 9 (1996) 204, C
AMP-ISIJ6 (1993) 1175).

(2) Surface defects of cast slabs are reduced by adjusting the taper on the short side of the mold (JP-A-56-538)
No. 49).

(3) A heater and a thermocouple are provided in a mold to control the heat flow rate at the meniscus portion to achieve uniform growth of the solidified shell (Japanese Patent Laid-Open No. 6-285606).

(4) By providing a slit in the mold to control the heat flow rate and strain in the meniscus portion, longitudinal cracks generated in the slab are reduced (Japanese Patent Laid-Open No. 7-284880,
JP-A-8-257695).

(5) An electromagnetic brake and an anemometer are installed in the mold to control the flow rate of molten steel near the meniscus to make the solidified shell growth during initial solidification uniform (Japanese Patent Laid-Open No. 11-123514).

(6) Slow cooling of the secondary cooling zone downstream of the mold (JP-A-63-115657, Iron and Steel 6
6 (1980) S743).

(7) In continuous casting of a slab, cooling on the long side of the mold is strengthened, and cooling on the short side is stopped immediately below the mold (iron and steel 67 (1981) 1345).

[0010]

However, each of the above techniques has the following problems.

(1) There is a limit to the adjustment of the surface eaves by the mold powder. It is possible to change the heat removal rate or the heat removal amount distribution by adjusting the viscosity or the solidification temperature of the mold powder. However, since mold powder performs four functions at the same time (heat retention of molten steel, lubrication of solidified shell and water-cooled copper plate, adjustment of cooling rate of solidified shell, and prevention of oxidation of molten steel), if one property is changed, The properties are adversely affected, leading to other defects such as breakouts or powder entrainment. Also, adjusting the powder to the slow cooling side
Not applicable to high speed casting.

(2) There is a limit in reducing surface cracks due to mold taper. It is possible to change the angle of the taper to make the heat transfer from the molten steel in the mold to the water-cooled steel plate uniform. However, the non-uniformity of the solidified shell thickness caused by the non-uniform flow of the molten steel cannot be avoided even by changing the Taber. Further, the appropriate taper varies depending on the steel type, and when performing casting of multiple steel types, it is impossible to find the optimum taper for all steel types.

(3) It is impossible to prevent surface defects in high-speed casting by a heater installed in a mold. By installing a heater in the mold, it is possible to achieve slow cooling and uniform growth of the solidified shell. However, in order to secure a shell thickness that does not cause a breakout in the mold in high-speed casting, it is necessary to cool the cooling to a rather high cooling side.

(4) It is impossible to prevent surface defects in high-speed casting by adjusting slits provided in the mold. Similar to the above-mentioned heater, the adjustment of the slit is mainly performed for the purpose of slow cooling. When a large heat removal is required as in high-speed casting, it is very difficult to obtain uniform solidified shell growth only by adjusting the slit.

(5) The use of the electromagnetic brake increases the equipment cost. Although the control of molten steel flow by an electromagnetic brake is possible, it is disadvantageous in terms of equipment cost, and the efficiency is very poor particularly when applied to only a limited type of steel.

(6) Slow cooling of the secondary cooling zone cannot reduce slab cracks depending on the steel wire. In high carbon steels and the like, longitudinal cracks occur starting from solute segregation regions between dendrides, and slab cracks can be said to be due to partial melting of grain boundaries. Considering this, the lower the temperature of the slab surface is, the more preferable it is, and the secondary cooling zone cannot be moderately cooled.

(7) Bloom surface flaws cannot be reduced by enhancing cooling on the long side of the mold and stopping cooling on the short side. Stopping the cooling of the short side in the bloom leads to an increase in bulging, and the slab method cannot be directly applied to the bloom or the like.

Accordingly, it is a main object of the present invention to provide a continuous casting method capable of reducing surface cracks even when high-carbon steel is cast at high speed.

[0019]

According to the present invention, there is provided a continuous casting method comprising:
In the continuous casting method in which molten steel is supplied from a dip tube into a mold, the steel type is C: 0.3 (particularly 0.8) to 2.0 wt%, Si: 0.15%
(Especially 0.8) to 3.5 wt%, V: 0.0001 to 0.2 wt%, the casting speed is 1.0 m / min or more, and the minimum value of the thickness of the solidified shell is 10 m at a depth of 400 mm from the meniscus in the mold.
m or more and the same horizontal cross section (the surface orthogonal to the longitudinal direction of the slab)
Wherein the difference between the maximum value and the minimum value of the thickness of the solidified shell is 5 mm or less. During solidification, there is usually a solid-liquid coexisting phase in which a solid phase and a liquid phase coexist, but the solidified shell in the present invention refers to a portion where the solid phase is 95% or more.

Until now, prevention of vertical cracks in slabs has been studied mainly for medium carbon steel (C: around 0.15 wt%). This is because the slab cracks of the medium carbon steel were due to the peritectic reaction and accompanied by large vertical cracks. In order to prevent longitudinal cracking of these medium carbon steels, countermeasures have been taken in the direction of making the solidified shell thickness uniform by slow cooling in the mold. Even if the carbon content increases to the high carbon steel region, the mechanism of the longitudinal cracking is the same because the non-uniformity of the solidified shell thickness is the major cause. In this mechanism, when the casting speed is increased, the slab surface cracks cannot be reduced by a slow cooling method. This is because if slow cooling is performed by high-speed casting, the solidified shell cannot be sufficiently grown in the mold, leading to operational trouble such as breakout. Therefore, in the present invention, high-speed casting without breakout is realized by limiting the minimum value of the chemical component and the solidified shell thickness and the variation thereof, and the surface crack of the slab is suppressed. In particular, the casting speed is 1.3m
The speed can be as high as / min or more. In this casting method, blooms or billets after slab rolling are particularly effective for suppressing surface cracks.

Here, as a specific means for realizing the uniformed thickness of the solidified shell, it is desirable to use one of the following constitutions alone or in combination. a: An electromagnetic stirrer is provided in the mold, and a straight nozzle is used as an immersion tube. Immersion pipes are roughly classified into two types, a straight type having a discharge hole at the tip end surface and a horizontal hole type (T type) having a discharge hole at a position facing the side wall. In the case of a horizontal bored dip tube, it produces an excessive drift of molten steel in the mold and causes non-uniformity of the solidified shell. On the other hand, the straight type does not generate the drift like the horizontal hole type, but will stagnate the molten steel flow near the meniscus,
There is a concern that other product quality will be adversely affected. In addition, even in the straight type, since the drift of the molten steel flow may be generated from the change during operation, it is necessary to supplementarily make the flow of the molten steel uniform. Therefore, in the present invention, mold electromagnetic stirring is used supplementarily. The application of electromagnetic stirring to the mold is one means for correcting the bias of the molten steel flow in the mold, and can thereby make the solidified shell uniform. Various known devices may be used as the electromagnetic stirring device.

B: The lowermost part of the dip tube is 130 m from the meniscus
Install at a depth of at least m. Even when the operation is performed using a straight dip tube, the flow of the molten steel flow becomes unstable and the uniformity of the solidified shell thickness decreases as the nozzle outlet approaches the meniscus. In particular, when performing high-speed casting, the discharge speed of the molten steel is very fast, and it is necessary to suppress the influence as much as possible. For this purpose, it is desirable that the lowermost position of the immersion pipe be 130 mm or more deeper than the meniscus.

C: The concentration of P contained in the molten steel is 0.01% by weight.
The following is assumed. Of the elements segregated between dendrites, only P is an impurity, and the other elements are additive elements. The segregation concentration of P greatly affects the solidus temperature of the segregated portion between dendrites, and surface cracks can be further reduced by setting the P concentration to 0.01% or less.

D: In the area of the secondary cooling zone installed at the lower part of the mold, the portion up to 20 mm from the surface of the slab is 1200
℃ or below. The mechanism of slab surface crack generation in high carbon steel is that segregation between dendrites formed by the action of bulging on the slab surface in the mold breaks with greater bulging in the secondary cooling zone region That is. Therefore, cracking during bulging can be prevented by cooling the surface to 1200 ° C. or less in the secondary cooling zone.

[0025]

Embodiments of the present invention will be described below, including processes leading to the present invention. (A) Elucidation of mechanism of surface flaw generation in high C and high Si steels In order to elucidate the mechanism of surface crack generation in high C and high Si steels, casting was performed using a continuous casting apparatus shown in FIG. In this apparatus, an immersion pipe 2 is arranged at an upper part in a mold 1 made of water-cooled copper, and molten steel 3 is supplied from a tip of the immersion pipe 2.
As the immersion pipe 2, a two-hole immersion pipe having a discharge hole for molten steel 3 at a position facing the side wall at the tip is used. A mold powder 5 is disposed on the meniscus 4 of the molten steel 3. The molten steel supplied into the mold 1 is cooled, forms a solidified shell 6 on the surface side, is drawn downward, and is further cooled in a secondary cooling zone (not shown) to form a slab. Specific casting conditions are as follows.

Mold cross section: 256 mm × 164 mm Casting speed: 1.6 m / min Immersion tube nozzle: 2-hole horizontal hole type immersion tube Steel type: See Table 1 (all units are wt%) Tundish temperature 1480-1500 ° C.

[0027]

[Table 1]

A) Observation of the actual condition of surface cracks Observation of the actual condition of surface cracks was carried out for each steel type A to E. As a result, as shown schematically in FIG. 2, a slab surface crack 11 was observed at the surface corner of the billet 10. In order to quantitatively evaluate the degree of these surface cracks, a surface crack score calculated based on the depth from the surface and the number of existing members was used. The evaluation criteria for the surface crack rating are as follows. For each specimen, billet (110 x 110 mm square / 1
The t) is inspected for 20 pieces, of which the largest crack depth is determined for each billet. The following points are given according to the depth. Then, a value obtained by adding these scores for 20 pieces is used as a surface crack rating. 1mm or less → more than 1mm 2mm or less → 0.3 more than 2mm, 3mm or less → 0.8 more than 3mm, 4mm or less → more than 24mm, 5mm or less → more than 45mm → 10 This is shown in the graph of FIG.
It can be inferred from this figure that, when A and E, A and B, and C and D are compared, differences in the concentrations of C, Si, and V respectively lead to differences in surface cracking. In other words, it can be said that the higher the concentration of C, Si, and V, the more easily cracks occur.

FIG. 4 is a graph showing the effect of casting speed on surface cracks in steel type E. Casting speed 0.7m / m
The surface cracking score tends to increase as it increases from in to 1.4 m / min and 1.6 m / min. One possible reason is that the increase in casting speed results in a reduced thickness of the solidified shell formed in the mold, and that a stress is applied to the thinned shell at a higher strain rate. Another reason is considered to be that the speed of the molten steel flowing from the dip tube into the mold was increased and the drift of the molten steel was increased as described below.

B) Observation of Surface Cracking Mechanism In order to elucidate the surface cracking mechanism, the shell being solidified in the mold was taken out (after breaking out), and its cross section was observed mainly on the form of surface cracking.
Using the steel type C shown in Table 1, the casting speed and the dip tube were under the same conditions as above. As a result, the formation of surface cracks in the mold under high-speed casting of high carbon steel was simulated as shown in FIG. 5, and the following was found.

Due to the uneven flow of the molten steel flow in the upper part of the mold 1, the thickness of the solidified shell becomes uneven. The speed of the molten steel flow discharged from the immersion tube 2 increases as the casting speed increases, and the molten steel 3 having a high temperature coming out of the immersion tube 2 easily hits the solidified shell 6.

When the thinnest corner portion goes down to the middle portion of the mold 1, bulging occurs, and tensile stress is intensively applied to the corner portion. The solidified shell 6 itself contracts due to its own solidification, but causes bulging due to the pressure received from the molten steel 3.

[0033] Cracks occur between the dendrites grown at the thinnest corners subjected to tensile stress. Since the solute elements segregate between dendrites, the solidification temperature is lowered, and cracks are likely to occur.

The molten steel having a high solute concentration near the interface between the solidified shell 6 and the molten steel 3 flows into the cracks between the dendrites,
It remains as larger segregation.

At the stage of cooling in the secondary cooling zone, stress is further applied to the segregated portion formed by bulging or large thermal stress, resulting in complete surface cracking.

As a result of a detailed investigation of the relationship between the non-uniformity of the solidified shell thickness and the cracks on the surface, it was found that the minimum value of the solidified shell thickness was less than 10 mm at 400 mm below the meniscus. It was found that the difference between the maximum value and the minimum value of the solidified shell thickness in the horizontal section exceeded 5 mm. Maximum value of solidified shell thickness max
An example of the minimum value min is as shown in FIG. As a result of the investigation, it was found that the greatest bulging due to the molten steel pressure occurred at a position 400 mm from the meniscus. As shown in FIG. 3, when steel type C was cast under the same conditions as described above (see 0026), the surface cracking score was 1.9. At this time, the minimum value of the solidified shell thickness was 9 mm, and the maximum value was 15 mm. On the other hand, when the steel type C was set to 1.0 mm / min only under the same conditions as described above (see 0026), the surface cracking score was 0.3
Was a point. At this time, the thickness of the solidified shell was uniform over the entire circumference and approximately 18 mm.

Regarding the so-called segregation of the surface portion formed in the initial solidification stage shown in FIG. 5 (in extreme cases, cracking already occurs in the mold), EPMA (Electron Probe Mic)
roAnalyzer) line analysis results, C, Si, C
It was found that r, P, and V were concentrated. It is considered that these solutes segregated in the above and the above processes, leading to surface cracking. That is, the liquidus temperature of the portion where these solutes are concentrated is much lower than that of the bulk portion, and it is considered that the portion is preferentially cracked when a stress is applied.

FIG. 7 shows the results of a grease test performed on steels A and E in Table 1 using test pieces taken out of billets.
Is shown in the graph. In the grease test, tensile stress was applied at each temperature at a strain rate of 0.42 / s, and the drawing at break was measured. A reduction of 0% is a so-called grain boundary rupture, in which the segregated grain boundary part of the solute partially melts and breaks. Also, as can be seen from this graph, steel types with higher C and Si concentrations crack at lower temperatures. This phenomenon is considered to be the same as the above-described phenomenon in which the segregated portion formed by the surface crack is cracked.

Considering the mechanism thus clarified, the following methods can be considered as means for preventing surface cracks. I (Countermeasures) Uniform growth of the solidified shell by eliminating the drift of the molten steel flow. II (Countermeasure) Reduce the concentration of elements segregated between dendrites so that bulging does not cause cracks between dendrites. III (Countermeasure) Even if molten steel with a high solute element concentration flows into the solidified shell, the solute element concentration is reduced to such an extent that it does not lead to cracking thereafter. IV (Countermeasures) When the segregated part with high solute element concentration leaves the mold and passes through the secondary cooling zone, take measures to prevent complete cracking even if stress is applied.

In these measures, II and III are treated in the same way because there is a difference in the process of taking the measures, but they are the same in that they can reduce the solute concentration of molten steel. .

(B) Initial solidification control test for high C and high Si steels An initial solidification control test was conducted to reduce surface cracking based on the above policy I. One way to eliminate the drift is to use a straight nozzle for the immersion tube, as shown from the above investigation results. However, the use of only the straight nozzle is considered to be disadvantageous for the floating of inclusions and the like, and disadvantageous for keeping the temperature near the meniscus. Therefore, it is necessary to additionally provide some means. In the present invention, the weak point of the straight nozzle is compensated by applying the electromagnetic stirring of the mold.

FIG. 8 shows a schematic diagram of the test equipment. This device uses a straight nozzle having a discharge hole for molten steel 3 at the tip as an immersion tube 2, and an electromagnetic coil 7 around the mold 1.
The apparatus is the same as the apparatus in FIG. 1 except that an electromagnetic stirring device is provided. In addition, in order to see the effect of the immersion depth of the immersion pipe on the formation of surface cracks, the test was performed by changing the immersion depth. The test content used a casting speed of 1.4 m / min for steel type E. The results are shown in the graph of FIG.

As can be seen from this, it is considered that the application of the straight nozzle and the electromagnetic stirring of the mold was effective in reducing the surface cracks. It is considered that the effect of the electromagnetic stirring helped to even out the slight drift of the molten steel flow caused by the distortion of the straight nozzle and the nozzle clogging that changes with time. Further, it can be seen that the immersion depth of the immersion tube also affects the surface crack. Even when using a straight dip tube, it is thought that the flow of molten steel flow becomes unstable and the uniformity of the solidified shell thickness decreases as the nozzle outlet approaches the meniscus, especially when high-speed casting is performed. The speed is very fast and its effects need to be kept as low as possible. To do so, position the bottom of the dip tube from the meniscus.
The position should be at least 130 mm deep.

(C) Strong cooling test of the secondary cooling zone After the slab has left the mold, it is assumed that bulging in the secondary cooling zone leads to a large crack in the surface segregated portion, and measures are taken based on the above policy IV. It was investigated. Since the cracks were caused largely by partial melting of the segregated portions, a test was repeated in which cooling was strengthened in the secondary cooling zone and the temperature of the slab surface was changed. In the test, steel type E was used, and the casting speed was 1.4m /
min, no electromagnetic stirrer, and a 2-hole horizontal hole type immersion nozzle.

As a result, it was found that the surface cracks were reduced by keeping the portion from the slab surface to 20 mm from the slab surface at 1200 ° C. or less in the entire secondary cooling zone. At 1200 ° C or lower, it is considered that cracking due to melting does not occur, as in the above-mentioned grease test results (FIG. 7).

(D) Influence of Operating Factors (Effect of Impurity Reduction) Based on the above policies II and III, it is considered to lower the liquidus temperature of the segregation part. Of the segregated elements of high carbon steel, only P can be handled as an impurity, not an additive element. Therefore, it was examined whether the concentration of P could be reduced to thereby reduce surface cracking. Casting conditions: casting speed 1.4m / min, steel type D and only P among steel type D
The content was 0.009 wt%, no electromagnetic stirrer was used, and the immersion nozzle was examined as a two-hole horizontal hole type.

The results are shown in the graph of FIG. As can be seen from this graph, the surface cracking level was reduced by about 20% by suppressing the P concentration from 0.012 wt% to 0.009 wt%.

It should be noted that the continuous casting method of the present invention is not limited to the above example, and various changes can be made without departing from the scope of the present invention.

[0049]

As described above, according to the continuous casting method of the present invention, high-carbon steel can be cast at high speed while suppressing surface cracks. Further, there is no need to use an electromagnetic brake, and the equipment can be simplified.

[Brief description of the drawings]

FIG. 1 is a schematic view of the vicinity of a mold in a continuous casting apparatus.

FIG. 2 is a schematic view showing a vertical crack of a billet.

FIG. 3 is a graph showing a relationship between a steel type and a surface crack rating.

FIG. 4 is a graph showing a relationship between a casting speed and a surface crack rating.

FIG. 5 is an explanatory diagram showing a mechanism of surface cracking in continuous casting.

FIG. 6 is an explanatory diagram showing a maximum value and a minimum value of a solidified shell thickness.

FIG. 7 is a graph showing the relationship between temperature and aperture.

FIG. 8 is a schematic view of a casting apparatus used in the method of the present invention.

FIG. 9 is a graph showing the relationship between the structure and immersion depth of the immersion tube and the surface crack rating.

FIG. 10 is a graph showing a relationship between a P concentration and a surface crack rating.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion pipe 3 Molten steel 4 Meniscus 5 Mold powder 6 Solidified shell 7 Electromagnetic coil

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Claims (6)

[Claims] 1. A continuous casting method for supplying molten steel from a dip tube into a mold, wherein the steel type is C: 0.3 to 2.0 wt%, Si: 0.15 to 3.5 wt%, V: 0.000
The casting speed is 1.0m / min or more, the minimum thickness of the solidified shell is 10mm or more at a depth of 400mm from the meniscus in the mold, and the thickness of the solidified shell is A continuous casting method characterized in that a difference between a maximum value and a minimum value is 5 mm or less. 2. The continuous casting method according to claim 1, wherein the casting speed is 1.3 m / min or more. 3. The continuous casting method according to claim 1, wherein an electromagnetic stirrer is provided in the mold, and a straight nozzle is used as the immersion tube. 4. The lowermost part of the dip tube is 130 mm from the meniscus.
The continuous casting method according to claim 3, wherein the continuous casting method is provided at a deep position. 5. The continuous casting method according to claim 1, wherein the concentration of P contained in the molten steel is set to 0.01% by weight or less. 6. The continuous casting method according to claim 1, wherein a portion from a surface portion of the slab to 20 mm from the surface of the slab is set to 1200 ° C. or less in a region of a secondary cooling zone provided at a lower portion of the mold.