JPH0342981B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for continuously casting high-Si steel with high cracking susceptibility at high speed.

[Conventional technology]

In the continuous casting method, molten steel from the tundish is
It is forced to harden through a mold and a secondary cooling zone. Next, after heating the slab in a heating furnace,
It is finished into a steel billet by hot rolling. Among such continuous casting methods, curved continuous casting machines in which the secondary cooling zone portion is curved are popular.

In this continuous casting method, as a way to reduce the unit consumption of heating fuel consumed in the subsequent hot rolling process, by producing high-temperature slabs, the heating time for slabs before hot rolling is reduced. It is possible to shorten the time. For this purpose, it is effective to increase the temperature of the slab delivered from the continuous casting machine by performing high-speed casting.

However, when the casting speed is increased, the solidified shell of the slab is not sufficiently developed, resulting in increased bulging distortion. The thickness d of this solidified shell is expressed by the following equation in relation to time t when the temperature of the solidified shell is constant.

d=k√ (where k is a constant) Therefore, as the casting speed υ _c increases, the moving time of the slab moving the same distance becomes shorter, so the thickness d of the solidified shell decreases, and bulging occurs accordingly. Distortion increases. In addition, the surface temperature of the solidified shell remains almost constant regardless of the casting speed υ _c at the same water density.

In this way, when the casting speed υ _c is simply increased, a thin, high-temperature solidified shell is generated, and the strength of the solidified shell becomes small. As a result, bulging strain increases and internal cracks occur frequently.

To solve this problem, in the secondary cooling zone of continuous casting, in order to prevent the slab from receiving tensile stress due to the thermal cycle from rapid cooling to reheating, the upper part of the secondary cooling zone, which is 25 to 35% of the total length of the secondary cooling zone, is The specific water content
Cool at 200-400/min/ ^m2 , then 100-400/min/m2
JP-A-53-4727 proposes to prevent subsurface cracking of slabs by cooling at 180/min.m ² and then at 50 to 130/min.m ² .

In addition, in Japanese Patent Application Laid-Open No. 53-26730, the amount of water injection is 1.0
/Kg- steel is distributed in the upper and lower regions of the secondary cooling zone at a ratio of 6 to 7:3 to 4 to form a solidified shell where thermal stress does not apply, thereby preventing the occurrence of internal cracks in steel materials such as stainless steel. It is prevented.

[Problem that the invention seeks to solve]

However, as shown in the graphs below, when continuously casting high-Si steel such as high-silicon electrical steel, which has low hot strength, aluminum-silicon killed steel (Al
-Si-K) is more sensitive to strain than common steels, so there is a problem that internal cracks are more likely to occur as continuous casting speeds up.

FIG. 8 shows the results of a melt bending test conducted on ordinary steel and electrical steel to investigate the critical strain for internal cracking. In the melt bending test, as shown in FIG. 10, the upper center of the slab test piece 11 is heated to partially melt it.
A force F was applied from the anti-melting side to exert a bending force on the solid-liquid interface to impart strain, and then the cast specimen 11 was cooled and the presence or absence of cracks C occurring in the molten cross section M of the slab test piece 11 was investigated. The electromagnetic steel test material was an electromagnetic steel slab with Si of 3.23% and 2.9%, and the ordinary steel test material had S of 0.006%.
and 0.01% ordinary steel slab. In the figure, ○ indicates no cracks, and ● indicates cracks. As is clear from this figure, electromagnetic steel has a lower internal crack limit strain than ordinary steel, and internal cracks occur with smaller strain.

Figure 9 is a graph showing the hot strength of ordinary steel and electrical steel. Tensile test pieces were made from pieces of ordinary steel and electrical steel, and hot tensile tests were conducted to determine the hot strength by varying the temperature. This is the result of a different investigation. The electromagnetic steel test pieces were made from electromagnetic steel slabs containing 2.9% Si, and the ordinary steel test pieces were made from aluminum-killed steel slabs. The hot strength in the same figure is 1% yield strength, that is, 1%
It is expressed as the stress required to cause the strain. As is clear from this graph, electromagnetic steel has lower hot strength than ordinary steel, so it is more susceptible to cracking and tends to have larger bulging due to static pressure of molten steel. In other words, it can be seen that electromagnetic steel is a steel material that is prone to internal cracks.

On the other hand, in order to enable direct conveyance from continuous casting to hot rolling, it is necessary to increase the casting speed and shorten the heating process for the slab before hot rolling. For example, we performed continuous casting at a casting speed of 1.2 m/min and shortened the heating time to just over 2 hours in the conventional process, which required heating the slab for about 6 hours before hot rolling. This requires increasing the casting speed to 1.3 m/min, preferably 1.4 m/min or higher. For the first time, this
Energy saving, which is unique to direct rolling, can be realized, and effects such as quality improvement and productivity improvement can be obtained. However, when the speed of continuous casting is increased in this way, the above-mentioned problem of internal cracking becomes more prominent.

The present invention was devised to solve the above-mentioned problems, and by devising a cooling pattern for steel types with high Si content, it is possible to perform continuous casting at high speed. By increasing the speed of casting, the temperature of the billet is increased, reducing the time required to heat the billet before hot rolling, thereby reducing fuel consumption and improving quality, yield, productivity, etc. The purpose is to

[Means for solving problems]

In order to achieve the purpose, the continuous casting method of high Si content steel of the present invention uses a curved continuous casting machine with a radius of curvature R of 6 m or more to cast steel containing [Si] 2.5% or more and 4.5% or less at a rate of 1.3 m/1.3 m/s. When casting at a casting speed υ _c equal to or higher than 6.5 m from the meniscus, Q = Q _w /S υ _c using the water injection amount Q _w (l/min) and the spray area S (m ² ) at the top of the curved part from the meniscus to 6.5 m. The water density Q, defined as
m ³ ) or more, the slab is strongly cooled, and only the central part in the width direction of the slab is cooled by water injection at the lower part of the curved part from 6.5 m to the point of unbending.

[Effect]

By cooling such a slab, the temperature of the solidified shell of the slab immediately after continuous casting molding is lowered, and the shell has a predetermined thickness. Therefore, the solidified shell produced has a strength that can withstand high-speed casting. Also,
By reheating after strongly cooling the edge portion of the slab, it is possible to raise the temperature of the slab while maintaining the required rigidity. Therefore, cracks do not occur and it is possible to maintain the slab at a high temperature suitable for direct rolling.

Hereinafter, the features of the present invention will be specifically explained with reference to the drawings.

FIG. 1 shows an example of a curved continuous casting machine to which the cooling method of the present invention is applied. Molten steel 2 in the tundish 1 is poured into a mold 3. A solidified shell 4 is formed on the surface of the molten steel 2 that contacts the mold 3, and the molten steel 2 becomes a slab 5 and is transported toward a secondary cooling zone 6. At this time, the inside of the slab 5 is still in a molten state.
Therefore, by spraying cooling water onto the slab 5 from the spray 8 while supporting the outside of the slab 5 with the guide rolls 7, the inside of the slab 5 is cooled and solidified while maintaining its outer shape.

In the curved continuous casting machine, the secondary cooling zone 6 is formed in a substantially 1/4 arc shape. The slab 5 is then driven by drive rolls provided in the guide roll band 7 and the pinch roll band 9 at the lower end of the secondary cooling zone 6.
While being pulled out, the bend is the bending point P
(See Figure 2). Next, the slab 5 is cut by a torch 10 and transported in the horizontal direction.

Here, in the present invention, the secondary cooling zone 6 is divided into an upper curved part 6a and a lower curved part 6b, and the slab 5 is divided into an upper curved part 6a and a lower curved part 6b.
Change the cooling pattern for each.

FIG. 2 illustrates a secondary cooling pattern in a curved continuous casting machine with a radius of curvature R. The secondary cooling zone 6 is divided into an upper curved part 6a and a lower curved part 6b, as shown in FIG. 2A. In the section corresponding to the upper curved portion 6a, that is, the upper curved section _lS , cooling water is injected over the entire width of the slab 5, as shown by the cooling water injection range 8a in FIG. 2B. However, in the section corresponding to the lower curved part 6b, that is, in the lower curved part _lE , water is injected only into the widthwise central part of the slab 5, and not into the non-cooled zone W at the edge part.

When the upper curved section l _S is 6.5 m, the lower curved section l _E is (πR/2-6.5) m.

The slab 5 that has come out of the mold 3 is strongly cooled in the upper part 6a of the curved part in the strong cooling region, with a large amount of water at the beginning and gradually decreasing it until the average water density Q is 220 l/m ³ or more. That is, the rigidity of the solidified shell 4 can be increased by lowering the solidified shell temperature in the upper curved section _lS . Therefore, bulging distortion is suppressed, and even when the casting speed is increased, internal cracking caused by bulging becomes slight, making it possible to cast at a casting speed of 1.4 m/min or higher.

Next, in the lower curved portion 6b, water is poured only to the center portion of the slab 5 in the width direction. In other words, if water injection cooling is performed over the entire width of the secondary cooling zone 6, the edges of the slab 5 will become supercooled in the next bending process, and cracks will occur at the edges. Prevent by.

During the entire length of the horizontal section _lH after the unbending point P, the slab 5 is not cooled. Therefore, the slab 5 is transported in a high temperature state due to the synergistic effect of high-speed casting and improved cooling. As a result, the heating process for the slab 5 before hot rolling can be shortened, and the amount of fuel consumed for heating can be reduced.

In such a curved continuous casting machine, the smaller the radius of curvature R of the secondary cooling zone 6, the more the height of the apparatus can be reduced. Suppression of this height also leads to a reduction in the static pressure of molten steel, increasing the effect of suppressing bulging distortion.

On the other hand, when the thickness of the slab 5 is _D1 and the thickness of the solidified shell 4 is _D2 , the correction strain _ΔD at the unbending point P is expressed by the following equation.

Δ _D = D ₁ −2D ₂ /R Therefore, if the radius of curvature R is made too small,
The correction strain at the unbending point P becomes large, which even encourages the occurrence of internal cracks. This trend is
This effect is compounded by the increased speed of continuous casting and becomes even more noticeable. For this reason, the radius of curvature R of at least 6 m
is necessary.

In addition, at the bottom of the curved part, water is not injected to the edges in the width direction of the steel slab, but only the central part is injected to cool the edges, and the edges are prevented from cracking by recuperating heat. However, if the radius of curvature R is 6 m or more, the meniscus After strong cooling to within 6.5 m from the center, the edges are heated again and no cracks occur when water is injected only at the center.

Figure 3 shows the radius of curvature R of the secondary cooling zone 6 as 6.0 m.
The above shows the distribution of the upper curved section L _S and the lower curved section L _E when the length required for strong cooling is 6.5 m. Radius of curvature R (6.0+x)
When m, the lower curved section _lE is expressed by the following equation.

l _E = π/2 (6.0 + x) - 6.5 However, x≧0 Figure 4 shows the water density Q in the upper section of the curve l _S , that is, the section from just below the meniscus to a position of 6.5 m, but the average It is necessary to secure at ^least 220l/m3. This water density Q is 220l/ ^m3
If it is smaller, the initial solidification shell formation will be insufficient and internal cracks due to bulging will occur frequently. Also,
The water density Q is expressed by the following formula.

Q=Q _W /S・υ _C (l/m ³ ) where Q _W : Water injection amount (l/min) S: Spray area (m ² ) υ _C : Casting speed (m/min) As can be seen from the figure, Slab 5 immediately after leaving casting 3
is rapidly cooled because it is injected with high water density Q. Therefore, the temperature of the solidified shell 4 of the slab 5 immediately after leaving the mold 3 decreases, and the thickness also increases. As a result, the required strength of the solidified shell is ensured.

Due to this cooling, the thickness D ₂ of the solidified shell increases as the distance M _L from the meniscus increases. As the thickness D ₂ increases, the amount of recuperated heat transferred from the unsolidified molten steel in the solidified shell to the solidified shell 4 becomes smaller. From this, in order to control the surface temperature of the solidified shell 4 to a low temperature necessary for suppressing bulging, a more favorable result can be obtained by decreasing the water density Q as the distance M _L from the meniscus increases.

Furthermore, in order to reduce the water volume density Q, although it is decreased stepwise in FIG. 4, it goes without saying that the water volume density Q may be decreased continuously.

In this way, in the upper curved section l _S ,
Water is injected over the entire width of the slab 5 at a water density Q shown by the dotted line to strongly cool it, and in the lower section _lE of the curved part, a portion of, for example, 200 mm from the widthwise edge of the slab 5 is not cooled. By doing so, electrical steel with a silicon content of 2.5% or more and 4.5% or less is
This becomes possible by casting at a casting speed of m/min or higher. The reason why the amount of Si is 2.5% or more is to lower the iron loss of the electromagnetic steel, and the reason why it is 4.5% or less is because if the amount increases, it becomes brittle and the workability deteriorates. When the range from this edge to 200 mm becomes large, bulging occurs due to the unsolidified portion that exists due to high speed, and internal cracking occurs accordingly. On the other hand, if it is smaller than this, the supercooling of the edge part or the utilization of internal solidification recuperation will be inhibited.
With cracking at the correction point.

〔Example〕

Next, the effects of the present invention will be specifically explained with reference to Examples.

Figure 5 shows the water density Q in an example in which electromagnetic steel with a silicon content of 3.3% is cast at a casting speed of 1.6 m/min using a continuous casting machine with a radius of curvature R of 10.5 m.
shows. In this example, the upper curved section _lS and the lower curved section _lE are 6.5 m and 10.0 m, respectively. That is, the distance M ₁ from the meniscus is 6.5
Cooling water was injected over the entire width of the slab 5 in the section up to m, and water was not injected to the width direction edge portion of the slab 5 in the range of distance M ₁ from 6.5 m to 16.5 m.

In the example shown in FIG. 5, the width of the water injection range 8a in the lower curved section _lE is divided into two stages. In other words, the distance M ₁ from the meniscus is 6.5 m.
In the range _from
Water injection cooling was performed with a width of mm. In the horizontal section _lH after the unbending point P, the slab 5 was not cooled. This made continuous casting suitable for high-temperature extrusion possible.

FIG. 6 shows changes in the water density Q of the cooling water in the embodiment shown in FIG. In the example shown in FIG. 6, the slab 5 immediately after leaving the mold 3 is cooled by water injection at a high water density Q. The water density Q is decreased as the distance M ₁ from the meniscus increases. The average water density Q in this section l _S is
280l/ ^m3 is secured. Also, if the distance M ₁ from the meniscus is in the range of 6.5 m to 16.5 m, that is,
Even in the section where the widthwise edge portion of the slab 5 is not cooled, the water density Q is gradually decreased.

FIG. 7 is a graph showing the influence of such a cooling pattern on internal cracks in slabs. In the figure, the number of internal cracks N indicates the number of internal cracks that occurred per 500 mm of slab length. These internal cracks include mild internal cracks, moderate internal cracks, which are harmless to the product, and severe internal cracks, which are harmful to the product.The number of internal cracks is displayed using a bar graph that distinguishes each type with white, dots, and diagonal lines. ing. As is clear from this figure, it can be seen that moderate and severe internal cracks are completely suppressed when strong upper cooling according to the present invention is employed.

〔Effect of the invention〕

As explained above, in the continuous casting method of the present invention, the strength of the solidified shell is maintained at the required value even during high-speed casting by strongly cooling the slab at the upper part of the curved part of the secondary cooling zone. be done. Therefore,
Bulging distortion is eliminated, and internal cracks can be prevented from occurring during bulging straightening of slabs. Further, since the edge portion in the width direction of the slab is not cooled in the lower part of the curved portion, the edge portion is maintained at the required temperature without being overcooled. For this reason,
No cracks will occur at the edges of the slab during bending back.

In this way, high-speed casting is possible even in steel grades that are particularly susceptible to internal cracking, such as high-silicon electrical steels, resulting in hot extrusions. Therefore,
By shortening the heating time before hot rolling, fuel consumption can be suppressed, and the characteristics of direct rolling can be fully exhibited. As described above, according to the present invention, yield, productivity, etc. can be improved, and the quality of the obtained products is also excellent.

[Brief explanation of drawings]

Fig. 1 is a schematic sectional view showing a continuous casting machine to which the cooling method of the present invention is applied, Fig. 2 is an explanatory drawing showing the cooling pattern in the secondary cooling zone, and Fig. 3 shows the division state of the secondary cooling zone. The illustrated explanatory diagrams, Figure 4 is a graph showing the relationship between the distance from the meniscus and the water volume density, Figure 5 is an explanatory diagram showing the relationship between the distance from the meniscus and the water injection range, and Figure 6 is a graph showing the relationship between the distance from the meniscus and the water injection range. A graph showing the relationship between the distance from the meniscus and the water density in the example shown, Figure 7 is a graph showing the influence of the cooling pattern and casting speed on the occurrence of internal cracks, and Figure 8 is the internal crack limit strain due to different steel types. FIG. 9 is a graph showing the relationship between temperature and proof stress, and FIG. 10 is a graph showing the melt bending test.

Claims (1)

[Claims] 1. When casting steel containing [Si] 2.5% or more and 4.5% or more with a curved continuous casting machine with a radius of curvature R of 6m or more at a casting speed υ _c of 1.3m/min or more, the meniscus Water injection amount Q _w (l/min) for the upper part of the curved part from to 6.5 m
and spray area S (m ² ), Q=Q _w /S・υ _c
The water density Q, defined as , is initially large and gradually reduced to ensure an average of 220 (l/m ³ ) or more, and the slab is strongly cooled, and the A continuous casting method for high Si-containing steel characterized by cooling only the widthwise center of the slab by water injection.