JP4337565B2 - Steel slab continuous casting method - Google Patents

Description

  The present invention relates to a steel slab continuous casting method, and more particularly to a continuous casting method capable of preventing vertical cracking and streak regardless of casting speed and steel type.

  In continuous casting of steel, so-called high speed casting is directed to improve productivity. Although the maximum casting speed varies depending on the slab thickness due to various technological developments in recent years, 3.5 m / min is achieved at a slab thickness of about 250 mm, and 8 m / min is achieved at a slab thickness of about 90 mm.

  However, along with the increase in casting speed, especially as the casting speed becomes extremely high at 2 m / min or more, the molten metal surface fluctuation in the mold becomes severe, and as a result, the mold flux is caught in the molten steel, resulting in an increase in product defects. In addition, an increase in the amount of heat extracted from the mold and surface cracks due to non-uniform solidification due to the increase in heat removal from the mold are likely to occur. On the other hand, steel that undergoes peritectic solidification transformation and steel that undergoes δ-γ solidification, such as medium carbon steel (C: 0.08 to 0.15 mass%), is prone to surface vertical cracks due to non-uniform solidification. There is. In severe cases, streak-like cracks accompanied by linear depressions (streaks) and cracks occur over the entire length of the slab of 7 to 10 m.

  In order to solve these problems, generally, (1) the viscosity of the mold flux is set to an appropriate high viscosity, or (2) the crystallization temperature of the mold flux is increased. As the viscosity increases and the crystallization temperature increases, the frequency of constraining breakouts due to poor lubrication increases, making stable operation difficult, and conversely, streaks tend to occur. Obstruction and slab care are forced. With respect to such a problem, Patent Document 1 discloses that the relationship between the inclination angle of the long side surface of the mold and the mold vibration stroke and the relationship between the maximum rising speed of the mold and the casting speed are within a predetermined range. Means have been proposed, and it is said that the mold lubrication is stable and a slab free from surface cracks can be produced stably.

Japanese Patent Laid-Open No. 2003-04155

  However, in the means described in Patent Document 1, although the effect is recognized when the steel type to be cast is a low carbon steel, the steel type to be cast is, for example, an extremely low carbon steel, a medium carbon steel, or a high carbon steel, and There is a problem that stable operation cannot always be performed in a high-speed casting region where the casting speed exceeds 2 m / min. In addition, medium carbon steel with remarkable non-uniform solidification (regardless of whether it is plain steel or high-tensile steel containing various alloy components) or stainless steel, the casting speed is relatively low at 2 m / min or less. No measures have been disclosed so far for countermeasures against surface cracks such as vertical cracks, streak-like cracks, and streaks.

  The present invention relates to product defects and surface cracks that occur during continuous casting of low carbon steel at high speed, and vertical cracks and streak-like cracks that occur regardless of casting speed in steels that undergo peritectic solidification transformation or steel that undergoes δ-γ solidification. An object of the present invention is to propose a slab continuous casting method capable of effectively preventing the occurrence of streaks, and to prevent the occurrence of these defects in continuous casting regardless of the steel type and casting speed.

In the continuous casting method according to the present invention, when casting is performed using a steel slab continuous casting assembled mold composed of four-sided mold plates, the setting conditions of the assembled mold are set to a short side taper β _n and a long side. The ratio β _n / β _w of the side taper β _w is in the range of 1.5 or more and 6 or less. Here, the long side taper β _w and the short side taper β _n are values (% / m) calculated by the following equations, respectively, and the range is β _w : 0.2% / m or more and 0.9% / M or less, β _n : 0.8% / m or more and 1.3% / m or less.
β _w = {(T _u −T _d ) / T _u / L} × 100 (%) (1)
β _n = {(W _u −W _d ) / W _u } / L} × 100 (%) (2)
put it here,
T _u , T _d : upper end on the short side of the mold, lower end width (mm),
W _u , W _d : upper end on the long side of the mold, lower end width (mm),
L: Mold length (m)
It is.

In the above-described invention, it is desirable in terms of operation to adjust the short side taper β _n after setting the long side taper β _w to a predetermined value.

As described above, the present invention performs continuous casting by determining the parameters β _n and β _w of the continuous casting mold and the ratio β _n / β _w thereof as described above. The following is a brief description.

Proper setting of the short side taper β _n and the long side taper β _w is a basic condition for obtaining a sound slab while preventing bulging and breakout. It has been practiced to operate with a side taper β _n of 0.7 to 1.3% / m and a long side taper β _w of 0 to 1.0% / m (iron and steel, Vol. 67, 1981). , P. 93, Japanese Patent Laid-Open No. 15-94155, etc.).

Conventionally, these short side taper and long side taper are basically adjusted independently within the above-mentioned range, and values that are considered to be optimal for each casting condition determined by the steel type and the like have been selected. However, since the long side shell and the short side shell are combined with each other to form a slab as a continuous body, in the continuous casting mold, if the mass balance of the steel material including molten steel and solidified shell is not considered, the short side side Even if the taper β _n and the long side taper β _w are individually within the proper range, problems such as slab surface cracks, slab shape defects and increased fluctuations in the molten metal surface may occur. The ratio of the side taper β _n and the long side taper β _w is related.

Specifically, it is as follows. If the long side taper β _w is too large relative to the short side taper β _n , the short side shell tends to protrude in the width direction in order to keep the mass balance of the steel material in the mold. When the shell is sufficiently thick and does not buckle, a compressive force is generated in the long-side shell as a reaction force, resulting in the long-side shell buckling in the width direction. For this reason, medium carbon steel (regardless of ordinary steel or high-tensile steel) or stainless steel with remarkable non-uniform solidification causes streak-like cracks and streaks even under relatively low casting speeds of 2 m / min or less. It will be. Such defects are characterized by occurring at almost equal intervals in the width direction, or at specific locations such as along the center line in the mold width direction.

This problem is that if the short side taper is gentle enough to absorb the shell protruding to the short side, there will be a space where the short shell swells to the mold side, so the reaction force to the long side shell will not work, Alternatively, as a result, unnecessary compression force does not act on the short side and long side shells, and the buckling of the shell is prevented and thereby solved. A similar phenomenon occurs even when the short side taper is too large. Thus, by setting β _w relatively small when β _n is large, while β _n is relatively small when β _w is large, the initial object of the present invention can be achieved. . Figures possible range of the short side taper beta _n and the long side taper beta _w ratio β _{n /} β _w and the short side taper beta _n and the long side taper beta _w is the present invention based on the above considerations It specifically defines the range in which the objective can be achieved.

  According to the present invention, the relationship between the long side taper and the short side taper of the mold is set to an optimum value, so that the molten metal surface fluctuation during continuous casting is reduced, and the occurrence of surface cracks is suppressed. As a result, even when low-carbon steel is cast at a high speed of over 2.0 m / min, product defects and surface cracks that occur when continuously casting low-carbon steel at a high speed can be prevented. It is possible to effectively prevent the occurrence of vertical cracks, streak-like cracks, and streaks that occur regardless of the casting speed in steel that undergoes peritectic solidification transformation or steel that undergoes δ-γ solidification. As a result, it is possible to stably obtain a slab having no defects over a wide range of steel types, that is, requiring no maintenance, thereby achieving stable operation and improved productivity.

  The present invention is directed to a continuous casting method using a steel slab continuous casting assembled mold composed of four-sided mold plates. As shown in FIG. 1, a continuous casting mold for steel is generally obtained by combining a pair of long-side mold plates 2A and 2B with a pair of short-side mold plates 1A and 1B constituted by water-cooled copper plates. Composed. The steel slab continuous casting assembled mold composed of these four-sided mold plates (1A, 1B, 2A, 2B) compensates for the amount of thermal shrinkage of the solidified shell during continuous casting of the steel, and the solidified shell becomes the mold. In order to prevent the mold from being greatly separated from the mold, a taper is provided so that the mold cross-sectional area does not become smaller as the long side mold plate and the short side mold plate are moved downward.

According to FIG. 1, the long side taper β _w is determined by the downward taper taper of the short side mold plate 1, while the short side taper β _n is uniquely determined by the tilt angle of the short side mold plate 1. It is determined. Among these, as shown in FIG. 2, the short side taper is the distance between the mold upper end 5 and the mold lower end 6 being L, and the length of the portion where the short side mold plate 1 (1A, 1B) is in contact with the long side mold plate 2 When the upper side dimension of the side mold is W _u and the lower side dimension of the mold is W _d ,
β _n = {(W _u −W _d ) / W _u } / L} × 100 (% / m) (2)
Represented by Similarly, referring to FIG. 3, the short side taper is β _w = {(T _u −T _d ) / T _u / L} × 100 (% / m) (1)
It is expressed. In the present invention, the following relationship is set for β _n and β _w .

Ratio of short side taper β _n and long side taper β _w , β _n / β _w : 1-6
As the casting speed increases, the thickness of the shell formed in the casting becomes thinner. Especially when the casting speed is 2 m / min or more, the bulging tendency in the mold increases in the long side shell, resulting in poor lubrication. Therefore, it is necessary to make the amount of mold flux flowing between the mold and the shell appropriate. For that purpose, it is necessary to maintain the relationship of β _n / β _w ≧ 1, in other words, β _n ≧ β _w , between the short side taper β _n and the long side taper β _w .

In addition, if the ratio β _n / β _{w of the} short side taper β _n and the long side taper β _w is too large, the heat extraction amount generated by the short side mold plate and the heat extraction amount generated by the long side mold plate are not uniform. The cross-sectional shape of the slab may become non-uniform during secondary cooling performed after leaving the mold. There is also a tendency for corner vertical cracks to occur. Therefore, the ratio β _n / β _w of the short side taper β _n and the long side taper β _w needs to be limited to 6 or less.

Note that the ratio β _n / β _w of the short side taper β _n and the long side taper β _w is less than 1 (when β _w is too large) and is more than 6 (when β _n is too large). However, depending on the casting conditions, it may promote fluctuations in the molten metal surface and corner cracks. For this reason, from the viewpoint of stable operation, the ratio β _n / βw must be 1 or more and 6 or less.

Short edge taper β _n : 0.8 to 1.3% / m
In general, the short side taper β _n is set to a contraction rate in the slab width direction of a slab shell (hereinafter referred to as “long side shell”) formed in contact with the long side mold plate. The shrinkage ratio of the long side shell varies depending on the casting speed and steel type, and fluctuates during the operation, so that it is very difficult to set the optimum value. However, the short side taper beta _n is too small, the slab shell (hereinafter referred to as "short side shell") bulging and corners vertical cracks even more likely to occur, which is formed in contact with the mold plate for a shorter side, and further the short The amount of mold flux that flows between the side mold plate and the short side shell becomes too large, the amount of heat removed to the mold is reduced and the growth of the short side shell is hindered, and the short side shell is jetted from the immersion nozzle. There is a risk of breakout due to redissolution. Therefore, the short side taper β _n is desirably 0.8% / m or more.

However, it becomes extremely thin long side shell, particularly when performing high-speed casting of more casting speed 2m / min, when the short side taper beta _n is too large, causing waviness based on buckling in a long side shell This tends to promote the generation of surface cracks (longitudinal cracks) and slag streaks on the long side surface of the slab. Such a phenomenon also occurs in medium carbon steel (regardless of whether it is plain steel or high-tensile steel) or stainless steel, which is markedly unevenly solidified even when the casting speed is low. Further, when the short side taper beta _n is large, cause the inflow of mold flux into between short side mold plate and the short side shell is inhibited to deteriorate lubrication between the mold plate and the shell, restricted There is a risk of breakout. In addition, as the short side taper β _n increases, the distance that the mold pushes the short side shell into the molten steel increases in the process of increasing mold vibration. Becomes larger, causing mold flux entrainment and poor lubrication. Considering such a problem, the short side taper β _n is limited to 1.3% / m or less.

Scope of long side taper β _w: 0.2~0.9% / m
When the long side taper beta _w are too small, there is a possibility that the tendency to elicit a bulging of the long side shell is large. In addition, the gap between the long side shell and the long side mold plate is increased near the corner of the slab corner, and the amount of mold flux inflow is excessive at that portion, so that heat removal from the mold is reduced and the shell grows. There is a risk of breakout due to hindrance and re-melting of the shell by the jet from the immersion nozzle. Therefore, the long side taper beta _w is more desirably 0.2% / m.

However, when the long side taper beta _w is too large, it becomes thinner is the short side shell when the casting speed is high and 2m / min or higher, slab short side central portion and the corner short side short side shell buckles A concave streak running in the casting direction may occur in the vicinity of ¼ to の of the width, which may promote surface cracks (longitudinal cracks) and breakout. Further, an excessively large fence the long side taper beta _w, causes the mold flux flowing into between the long side mold plate and the long side shell is inhibited to deteriorate lubrication between the mold plate and the shell, restrain Creates a risk of sex breakout. Furthermore, with the long side taper beta _w increases, the distance which the mold at elevated course of mold vibration pushes the long side shell Lok steel side becomes greater, as a result, the resonance melt surface is a mold oscillating water surface Fluctuations increase, causing mold flux entrainment and poor lubrication. Such consideration of the short side taper beta _w problems should be limited to 0.9% / m.

  By continuously casting the mold conditions as described above, the present invention reduces the fluctuation of the molten metal surface during casting, keeps the flow rate of the mold flux between the mold and the shell properly, and thereby makes it possible to While preventing breakout, it is possible to obtain a sound continuous cast slab free from slab surface cracks, particularly vertical cracks.

In carrying out the present invention, the long side taper β _w and the short side taper β _n and the ratio β _n / β _w of the assembled mold for continuous casting of steel slab composed of four-sided mold plates are predetermined. Must be in range. As the means, such as particular, as shown in FIG. 1, for example, long side taper beta _w is 0.4% / m become as short side mold plate 1A, select 1B, this long side mold plate 2A, combining 2B, include further short side molds plate 1A in this state, 1B and by applying means known mold width changes to adjust the short side taper beta _n appropriately.

In this case, since the long side taper beta _w is fixed by short side mold plate 1, which it is important to have selected an appropriate value depending on the steel type and the casting conditions, as a template width changing means Since the short side taper β _n and β _n / β _w can be set to appropriate values by using mold width changing means provided in many continuous casting facilities, there are many operational advantages. Of course, Figure 1 and a different form, for example, first the short side taper beta _n becomes a predetermined value so long side side mold plate 2A, select 2B, combined short side mold plate 1A, and 1B to this, further that The long side mold plates 2A and 2B can be adjusted in the state.

  Hereinafter, the embodiment of the present invention is made more specific by listing examples and comparative examples.

A slab having a thickness of 220, 235, 275 mm and a width of 750 to 1600 mm was cast under the conditions shown in Tables 1 to 4 using a continuous casting machine of the type shown in FIG. The mold height was 900 mm, and the immersion nozzle used had a discharge port diameter of 80 mm and a discharge angle of 20 ° downward (constant). In the case of carbon steel, the mold flux has a solidification temperature of 1000 ° C., a viscosity of 0.05 Pa · s (1300 ° C.) and a basicity (CaO / SiO ₂ ) of 1.0, and in the case of stainless steel, A solidification temperature of 1100 ° C., a viscosity of 0.02 Pa · s (1300 ° C.), and a basicity (CaO / SiO 2) of 1.1 were used. The degree of superheated molten steel in tundish was 10-40 ° C. As target steel types, very low carbon steel (steel type A), low carbon steel (steel type B), medium carbon steel (steel type C), and stainless steel (steel type D) were selected. The composition of these steels (both mass%) is as follows.

  Steel type A: C: 0.0005-0.0090%, Si <0.05%, Mn <0.50%, P <0.035%, S <0.020%, Al: 0.005-0. 060%, Ti <0.080%, Nb <0.050%, B <0.0003%, and the balance is Fe except for inevitable impurities.

  Steel type B: C: 0.03-0.06%, Si <0.3%, Mn <0.50%, P <0.035%, S <0.020%, Al: 0.005-0. 060%, the balance is Fe except for inevitable impurities.

  Steel type C: C: 0.08-0.16%, Si <0.3%, Mn <1.0%, P <0.035%, S <0.020%, Al: 0.005-0. 060%, the balance is Fe except for inevitable impurities.

  Steel type D: C: 0.10 to 0.20%, Si <0.3%, Mn <0.45%, P <0.020%, S <0.0010%, Al <0.002%, Cr : 8.5 to 9.0%, the balance is Fe except for inevitable impurities.

  In casting these steels, when the slab thickness is 220 mm, a static magnetic field application (EMBR) is applied to the entire width of the mold near the lower end of the mold (described in JP-A-2-284750), and when the slab thickness is 235 mm. Was subjected to static magnetic field application (EMLS) on the outlet side of the submerged nozzle discharge hole (described in JP-A-57-17356). Steel type D had a slab thickness of 275 mm, and molten steel flow control in the mold was not performed.

  The presence or absence of breakout during the casting was investigated, and the molten steel surface in the mold was measured with an overflow type level sensor at the center in the thickness direction 360 mm from the mold short side. Further, the obtained slab (length 7 to 10 m in length) was examined for the presence of vertical surface cracks and vertical corner cracks. The results are shown in Tables 1 to 4 together with the operating conditions. The investigation was conducted in units of 10 to 300 charges. FIG. 4 shows a summary of the relationship between the short side, long side taper, surface cracking, molten metal surface fluctuation, and breakout occurrence.

  As is apparent from Tables 1 to 4 and FIG. 4, when cast according to the present invention, the molten metal surface fluctuation width can be suppressed to 10 mm or less even in high-speed casting in which the casting speed exceeds 2.0 m / min. As a result, slabs without surface cracks could be manufactured under stable operation without breakout. In addition, steel type D, which tends to solidify unevenly even at a low casting speed, can be operated stably without the occurrence of streak-like vertical cracks on the slab surface or constraining breakout.

It is a schematic diagram which shows the typical assembly structure of the typical continuous casting mold to which this invention is applied. It is explanatory drawing of short side taper (beta) _n . It is explanatory drawing of long side taper (beta) _w . It is explanatory drawing which showed the surface crack, the molten metal surface fluctuation | variation, and the occurrence condition of a breakout in the relationship between a short side taper and a long side taper.

Explanation of symbols

1: Mold plate for short side 2: Mold plate for long side 5: Mold upper end 6: Mold lower end

Claims (2)

When casting using a steel slab continuous casting assembled mold composed of four mold plates, the setting condition of the assembled mold is set to a ratio β _n / short side taper β _n to long side taper β _w continuous casting method characterized by the beta _w in the range of 1.5 to 6.
The long side taper β _w and the short side taper β _n are values (% / m) calculated by the following equations, respectively, and the range thereof is β _w : 0.2% / m or more and 0.9% / m or less. , Β _n : 0.8% / m or more and 1.3% / m or less.
β _w = {(T _u −T _d ) / T _u / L} × 100 (%) (1)
β _n = {(W _u −W _d ) / W _u } / L} × 100 (%) (2)
here,
T _u , T _d : upper end on the short side of the mold, lower end width (mm),
W _u , W _d : upper end on the long side of the mold, lower end width (mm),
L: Mold length (m) After setting the long side taper beta _w to a predetermined value, the continuous casting method according to claim 1, wherein the adjusting the short side taper beta _n.

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