JPH10296399A - Mold for continuously casting molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent the longitudinal crack without lowering casting speed and to improve the productivity by forming grooves crossed with a meniscus at least on the surface at the center part in the width direction of long sides in a mold and having specific sizes of the width and the depth. SOLUTION: The concentration of stress is dispersed by infiltrating the air having thermal conductivity being 1/1000 of that of powder into cavity parts of the grooves 2 in the width direction of the long sides 1 in the mold and producing the thin part of the solidified layer thickness, so that the longitudinal crack on a cast slab is surely eliminated. For this purpose, the grooves 2 crossed wit the meniscus 8 at least on the surface at the center part in the width direction of the mold and having 0.25-0.9 mm width and >0.3-<0.5 mm depth, are formed under condition satisfying the equation Ws/Wi=0.2-2.0. Wherein, Ws is the groove width (mm) and Wi is interval between the grooves. Also, the groove 2 is formed so as to be crossed within the range from at least the upper part of the meniscus by 20 mm to the lower part of the meniscus by 50 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold for continuous casting of molten steel.

[0002]

2. Description of the Related Art In continuous casting of molten steel, in order to prevent a vertical crack of a slab, a ratio of 20% to a total area of a long side mold is required.
The formation of a large number of concave portions of 90% is disclosed in Japanese Patent Publication No. 57-11735. Also, the long side mold has a width of 250 in the casting direction.
Japanese Patent Application Laid-Open No. 61-92756 discloses forming a groove satisfying the conditions of 750750 μm, depth of 60-300 μm, and area ratio of 20-90%.

[0003]

When a groove is formed in the long side of the slab and the molten steel is cast as described above, a certain improvement effect can be obtained, but this does not mean that defect-free slab can be supplied stably. If the groove shape such as groove width, depth or groove spacing is inappropriate, it may rather cause vertical cracks, and even if these are formed properly, 1. It is evident from the results of a casting experiment conducted by the present inventors that casting using a powder having a relatively high viscosity of about 5 poise or more has almost no effect. There has been a strong demand for optimization of operating conditions in addition to optimization of the shape. The present invention has been made in order to advantageously solve such problems, and by optimizing the mold structure and operating conditions, the width direction of the cast slab is surely cooled almost uniformly, An object of the present invention is to provide a mold for continuous casting of molten steel capable of making the thickness of a slab solidified layer substantially uniform.

[0004]

A feature of the present invention is that a groove width and a groove which can be applied to a powder having relatively high viscosity at least in a direction intersecting a meniscus on the surface of the central portion in the width direction of the mold. A mold for continuous casting of molten steel, wherein a groove having a set depth is formed.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of various experiments conducted by the present inventors on the heat removal of molten steel by longitudinal grooves formed on the long side of a mold, the powder placed on the surface of molten steel injected into the mold during casting is as follows. (Slab), and as shown in FIG. 4a,
When a groove is not formed in the long side 1 of the slab, the form in which the molten steel 4 solidified layer 6 near the meniscus generally becomes non-uniform is as follows.
Non-uniform inflow of the powder 3 between the inner wall of the mold 1 and the mold 1, uneven meniscus temperature and influence of molten steel injection flow from the tundish are considered. For example, if the powder inflow becomes non-uniform, as shown in FIG.
The thickness of the solidified layer 6 is small and thick at a few places, and when the δ / γ transformation and the heat shrinkage stress are concentrated at the place where the solidified layer 6 is thin, a vertical crack occurs.

As shown in FIG. 4B, when a groove 2 is formed in the width direction of the long side 1 of the slab, the groove 2 of the mold 1 is filled with powder, and the powder 2 is interposed between the groove 2 and the molten steel 4. The thickness is increased, and the heat removal effect of the molten steel 4 from the mold 1 is reduced, and the interposed thickness of the powder 3 is reduced in the protrusions 5 other than the groove 2 portion, and the heat removal effect of the molten steel 4 from the mold 1 is increased. . However, since the thermal conductivity of the powder 3 at a high temperature is relatively large, about 1/10 of that of the copper material constituting the mold 1, it is not so effective as to reduce disturbance factors such as uneven inflow of the powder 3. . Therefore, it is difficult to reliably eliminate vertical cracks in the cast slab.

In view of the above, in the present invention, air having about 1/1000 of the thermal conductivity of the powder 3 is introduced into the gap 2 in the width direction of the mold 1 on the long side of the mold 1 and the thinned portion of the solidified layer 6 is removed. By generating, the stress concentration is dispersed, and the vertical crack of the cast slab is surely eliminated. That is, as shown in FIG. 4c, the powder 3 flows into the groove 2 to form the void 7, and the air having a small thermal conductivity of about 1/1000 of the powder flows into the void 7 accompanying the flow of the powder 3 along with the flow of the powder 3. , The void 7 (air) significantly reduces the heat removal from the molten steel 4 due to the cooling of the mold 1, so that the solidified layer 6 in the groove 2 is formed.
Becomes thinner. Accordingly, such air (7 voids) forms a large number of grooves 2 interposed between the powder 3 and the inner wall of the mold 1 in the width direction of the long side of the mold 1, thereby forming a thin portion of the solidified layer 6. It has been found that stress concentration due to δ / γ transformation and heat shrinkage of molten steel 4 (cast slab) within 1 can be dispersed and longitudinal cracks of the cast slab can be reliably prevented.

The formation of such a groove 2 crosses at least the central part (width direction 1/5 to full width) of the meniscus on the surface (the contact portion of the surface of the molten steel in the mold with the inner wall of the mold) in the width direction of the mold. To form. In other words, the powder placed on the surface of the molten steel flows out between the molten steel (cast slab) and the mold by drawing the slab to form a void. In order to surely exert such an effect, it is formed so as to cross over a range of at least 20 mm from the upper portion of the meniscus to 50 mm from the lower portion of the meniscus. If the upper part of the meniscus is less than 20 mm, the molten steel surface in the mold is positioned at the upper part of the groove due to fluctuations in the surface of the molten steel in the mold, and it may become impossible to entrain air due to the inflow of powder into the groove. The formation is not uniform, which is not preferable. Also, if the groove is shorter than 50 mm below the meniscus, the δ /
It is apt to be affected by γ transformation and heat shrinkage, and if a solidified layer is formed non-uniformly, longitudinal cracks occur, which is not preferable. Further, as described above, such a groove is formed in at least the central part, that is, the entire width from the long side mold width direction 1/5. If a groove having a distance of less than 1/5 in the width direction is formed in the center of the mold slab in the width direction, the stress cannot be sufficiently dispersed, so that vertical cracks in the slab width direction may not be reliably prevented.

Next, the shape of the groove to be formed is linear (vertical groove) in the direction perpendicular to the meniscus. Further, it can be formed in a lattice shape or the like. If such a groove does not set the groove width and the groove depth in accordance with the powder viscosity, as described above, the powder flowing into the groove forms a void, and air entrainment to the void may not be ensured. . However, in actual operation, changing the mold for each different powder is not practical because it is inefficient and increases the cost. Therefore, it is desirable to design the groove shape according to the powder having the most severe penetration conditions.
As a result of various experiments conducted by the present inventors on the intrusion of the powder into the groove, it was found that a viscous powder that is easily vitrified easily penetrates into a narrow groove than a low-viscosity powder that is easily crystallized. This is thought to be due to the increase in viscosity due to the formation of crystals when the powder penetrated into the grooves.Based on these findings, the groove shape was designed using a highly viscous powder that easily vitrified on the mold surface. Doing so is practically advantageous. That is, even when a low-viscosity powder that is easy to crystallize is used, it is possible to secure a gap between the groove bottom and the penetrating powder, and it is not necessary to frequently change the mold. Regarding the crystallization of this powder, the present inventors poured powders of various viscosities onto the surface of the copper mold and investigated the solidification form of the powder.
It turned out that it is easy to vitrify at a viscosity of about poise or more. Therefore, it is desirable to use a vitrified powder of 1.5 poise or more in designing the groove shape.

At various powder viscosities, a gap is formed in a groove, and an example of a groove width, a groove depth and a vertical cracking state of a slab which can reliably entrain air into the gap is as follows: 0.070 to 0.20% medium carbon steel, injection molten steel temperature
1520 ~ 1550 ℃, casting thickness 250mm, width 1500mm, casting speed 1.
At 0 to 2.5 m / min, the values are as shown in the table below.

[0011]

[Table 1]

[0012]

[Table 2]

[0013]

[Table 3]

[0014]

[Table 4]

[0015]

[Table 5]

As shown in the table, the results show that no matter which powder is used, if the groove width is not less than 0.25 mm and not more than 0.9 mm and the depth is not more than 0.3 mm and less than 0.6 mm, the uniformity of the solidified shell can be obtained. It was found that it was not possible to ensure the properties, and sometimes the formation of grooves would rather cause vertical cracks. In the case of casting using such a mold, it is desirable to apply to a steel type in which longitudinal cracks are likely to be generated, for example, C: 0.07 to 0.20%, Mn: 0.40%
~ 1.60%, Si: 0.08 ~ 0.50%, P: 0.002 ~ 0.025%,
S: 0.001 to 0.020%, one or two of trace amounts of Nb, Cu, Cr and Ti generally used in steels comprising the remaining Fe and impurities
It is advantageous to apply the present invention to medium carbon steel such as molten steel to which more than one kind of alloy is added as necessary.

[0017]

Next, examples of the mold of the present invention will be described. Example 1 In FIGS. 1 and 2, the long side molds 1 and 1 a and the short side mold 1
The width of the groove 2 and the depth of the groove 2 that can form a void in the groove 2 regardless of the powder viscosity are set by intersecting (in the orthogonal direction) the long side mold 1 and the meniscus 8 of the mold 1a composed of b and 1c. Groove 2
Is formed at least in the center in the width direction.

Embodiment 2 In FIG. 3, a groove 2 width and a groove 2 depth which can form a gap in the groove 2 regardless of the powder viscosity in a grid pattern intersecting the meniscus 8 of the long side molds 1 and 1a. Is formed at least at the center in the width direction.

Next, an example of a casting operation using the mold of the present invention will be described.

[Table 6]

[0020]

[Table 7]

Note 1: Groove type A is a vertical groove. B is a lattice (square). Note 2: The upper part of the groove position is the upper part of the meniscus. The lower part is the meniscus lower part. A at the position on the long side of the mold has a groove formed at a 420 mm width at the center in the long side of the mold. B forms a groove on the entire surface in the mold long side direction. For C, a groove was formed 210 mm wide on one side from the center of the mold in the long side direction. Note 3: The powder used is a commonly used powder mainly composed of SiO ₂ , CaO, etc. Powder viscosity is 1300 ℃
Viscosity. Note 4: For steel type A, C: 0.07 to 0.20%, Mn: 0.40 to 1.60
%, Si: 0.08 to 0.50%, P: 0.002 to 0.025%, S:
0.001 to 0.020%, remaining Fe and impurities. B is C: 0.025
~ 0.055%, Mn: 0.15 ~ 0.3%, Si: 0.01 ~ 0.03%,
P: 0.015 to 0.02%, S: 0.015 to 0.020%, remaining Fe and impurities. C is C: 0.005% or less, Mn: 0.15 to 0.25%,
Si: 0.40% or less, P: 0.02% or less, S: 0.018% or less, remaining Fe and impurities. Note 5: Continuous casting was performed by a vertical bending type slab casting facility.

[0022]

According to the mold of the present invention, a solidified layer of a slab can be uniformly formed, and vertical cracks can be reliably prevented without lowering the casting speed, so that productivity can be improved. . Further, since the occurrence of vertical cracks and breakouts can be prevented, quality yield can be improved, and a reduction in the operation rate of continuous casting equipment due to breakouts can be avoided, and further excellent effects such as further increasing productivity can be achieved. Is obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of the mold of the present invention.

FIG. 2 is a front view of a long side of the mold showing an example of the mold of the present invention.

FIG. 3 is a front view of a long side of the mold showing an embodiment of the mold of the present invention.

FIG. 4 is a plan view of a main part showing the action of a groove.

[Explanation of symbols]

 1 Mold long side 1a Mold long side 1b Mold short side 1c Mold short side 2 groove 8 Meniscus

Claims (5)

[Claims] 1. A width 0.25 or more to 0.9 mm or less and a depth of 0.
3. A casting mold for continuous casting, wherein a groove of more than 3 to less than 0.5 mm is provided under a condition satisfying the expression (1). W _s / W _i = 0.2 to 2.0 (1) where W _s : groove width (mm), W _i : groove interval (mm) 2. The mold for continuous casting of molten steel according to claim 1, wherein a groove is formed so as to intersect in a range from at least 20 mm above the meniscus to 50 mm below the meniscus. 3. The mold for continuous casting of molten steel according to claim 1, wherein a groove is formed so as to cross the meniscus in a direction orthogonal to the meniscus. 4. The mold for continuous casting of molten steel according to claim 1, wherein a groove is formed by intersecting the meniscus in a lattice pattern. 5. The mold for continuous casting of molten steel according to claim 1, 2 or 3, for C: 0.07 to 0.20 wt% for medium carbon steel casting.