JPH0729192B2 - Continuous casting injection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous casting method, and more particularly to an injection method for high-speed continuous casting, in which molten steel temperature, nozzle shape and casting speed are optimized under specific conditions. It is about the method.

[Prior Art] In recent years, a high-speed continuous casting method for slabs that has been cast at a speed of 4 m / min or more and the heat removal amount in the casting solidification process is about 2 × 10 ⁶ kcal / hr · m ² has been attempted, and it is newly generated in the mold. It is emphasized that vertical cracks have a great influence on productivity and production cost, and studies are being made on ways to prevent them.

In the above high-speed casting, shell discharge occurs due to the discharge flow of molten steel injected into the mold, which causes uneven shell thickness and vertical cracking.

This vertical crack remains as a surface flaw of the slab even after the subsequent processing steps, and not only decreases the yield as a product flaw, but also
The probability of this technology for preventing vertical cracking is strongly desired because it causes breakout during continuous casting.

As is well known, continuous casting of slabs, which is usually performed, uses a water-cooled copper mold and uses a flux as a lubricant to form a casting with a thickness of 200 to 300 mm and a width of 1000 to 2000 mm.
Casting at a speed of 2 m / min, the heat removal amount in the casting solidification process is 7 × 10
A continuous casting method of about ⁵ kcal / hr · m ² is used.

In this continuous casting method, vertical cracks are recognized as a typical surface flaw, and in producing defect-free cast pieces, it has been attempted to elucidate the formation process and prevent the occurrence of vertical cracks. .

In general, the cause of vertical cracking is the occurrence of non-uniform shell thickness in the mold width direction.

Stress generated during the solidification process of casting. It is known to be in.

The present invention is directed to the vertical cracks caused by the above-described generation of the non-uniform shell thickness in the mold width direction, and the improvement thereof.

There are various reports and proposals regarding vertical cracking due to non-uniform shell thickness in the mold width direction.

For example, on pages 1773 to 1781 of Iron and Steel Vol.68, No.13, there is the following description about the effect of local solidification delay on vertical cracking of a cast piece.

The non-uniform shell thickness caused by the temperature deviation in the mold width direction, which occurs near the meniscus, cannot be eliminated by the movement of the slab and causes vertical cracking.

Vertical cracking occurs when the nonuniformity of the solidified shell thickness exceeds 10% in medium carbon steel.

Slabs with carbon content of 0.12% to 0.16% have significant vertical cracking.

Etc. are described.

Next, when vertical cracking is not observed, there is almost no temperature difference in the width direction of the mold, and a stable temperature distribution is also observed in the vertical direction of the mold. On the contrary, when looking at the occurrence of vertical cracks, the temperature of the mold flaw is low, it is directed to the lower end of the mold, this tendency becomes stronger gradually, the slab surface temperature of the vertical cracks,
It says that it is higher than the health department.

However, in the method for high-speed casting of slabs at a casting speed of 4 m / min or more, the mold is composed of an endless steel strip belt, and the long side that moves in synchronization with the solidification and drawing process of the injected molten steel is connected endlessly. There is a method of casting using a continuous casting machine that is composed of a block and is composed of a short side that is sandwiched by the long side on a meniscus and moves integrally with the long side. It is considered that there is a cause of the occurrence of vertical cracks observed in No. 1, but the mechanism of occurrence of the cause seems to be greatly different.

[Problems to be Solved by the Invention] The present invention clarifies the cause of vertical cracking in high-speed continuous casting in which the continuous casting speed exceeds 4 m / min.
It is necessary to establish an injection method for high-speed continuous casting that does not substantially cause vertical cracks in high-speed continuous casting that exceeds / min, specifically, to optimize the molten steel temperature, nozzle shape, and casting speed under specific conditions. This is an issue.

[Means for Solving the Problem] In order to achieve the above object, the present invention provides a ratio of a casting speed V, a mold cross-sectional area, and a discharge port area of an injection nozzle so that a shell washing ratio φ described below is 0.1 or less. The means is to adjust at least one of S and inject the molten steel into the mold.

Shell wash rate φ = (δ _max −δ _min ) / δ _max φ = f (ΔT, V, S) or φ = A ・ ΔT ・ V ^0.473・ S ^0.8 However, δ _max : Maximum shell in the same width section Thickness δ _min : Minimum shell thickness within the same width A: 5.6 × 10 ^-4 (constant) V: Casting speed (m / hr) S: Cross-sectional area ratio of mold to nozzle outlet ΔT: Heating temperature of molten steel (Deg) [Operation] The inventors of the present invention have conducted various experiments and studies to achieve the above-mentioned object. In the high-speed continuous casting method with a casting speed of 4 m / min or more, when the casting speed is increased, the immersion nozzle The discharge velocity of molten steel of No. 2 increased, and the shell was washed with this increased discharge velocity, which promoted uneven shell thickness and caused vertical cracking.

At this time, a study was repeated with reference to the knowledge that was confirmed by the above-mentioned conventional continuous casting method "vertical cracking occurs when the nonuniformity of the solidified shell thickness exceeds 10% in medium carbon steel." In continuous casting performed at a casting speed of 4 m / min or more, it was similarly found that vertical cracking occurs when the non-uniformity of solidified shell thickness exceeds 10% in medium carbon steel.

The conventional knowledge is the knowledge obtained from an experiment in which a mold has a flaw. The temperature of the artificial flaw portion of the mold becomes lower, and the temperature of the corresponding slab portion becomes higher, which is the nonuniformity of the thickness of the shell caused thereby.

On the other hand, the findings of the experiment of high-speed continuous casting, which is the subject of the present invention, are for the nonuniformity of the thickness of the shell produced by the shell washing caused by the molten steel discharge flow from the immersion nozzle.

Incidentally, non-uniformity of the shell thickness by the shell washing (Shell washing rate), [delta] _max, is determined by the [delta] _min, the [delta] _max, [delta] _min is a tracer such as sulfur injected with molten steel during casting, casting It can be determined by measuring the concentration distribution of the tracer in a certain cross section of the subsequent slab to measure the shell thickness of that cross section.

However, in both cases, the non-uniformity of the solidified shell thickness is 10%.
It was found that, even if the mechanism of occurrence is different, in the end, the essential factor that forms vertical cracks is the non-uniformity of shell thickness, even if the mechanism of occurrence is different. .

Therefore, the present inventors continued experiments and studies in order to prevent shell washing due to molten steel discharge flow, which is the cause of uneven shell thickness in high-speed continuous casting, and summarized the results to obtain the following formula. It was

Shell wash rate φ = (δ _max −δ _min ) / δ _max φ = A ・ ΔT ・ V ^0.473・ S ^0.8 δ _max : Maximum shell thickness within the same width section δ _min : Minimum shell thickness within the same width section A: 5.6 × 10 ^-4 (constant) V: Casting speed (m / hr) S: Cross-sectional area ratio of mold to nozzle outlet ΔT: Superheated temperature of molten steel (deg) This formula shows that the shell washing rate is ΔT. It is a function of V and S. Calculated value and measured value by this formula
It is shown in the figure, indicating that they are in good agreement.

The present invention is based on the above findings.

That is, prior to continuous casting, S ₁ is calculated from the discharge port area of the nozzle to be used, and the target superheat temperature ΔT _{1 of the} molten steel is calculated.
And casting speed V ₁ are obtained, and the shell washing rate φ
₁ is calculated, and if φ ₁ is in the appropriate region, that is, 0.1 or less, the condition is determined to be appropriate. If φ ₁ is larger than 0.1, the nozzle shape is changed to correct S ₁ and / or the casting speed is adjusted so that φ ₁ is a value of 0.1 or less. Start the injection process.

In addition, after continuous casting is started, the nozzle outlet area
Although S ₁ is constant, ΔT ₁ can be measured, so the measured ΔT ₁
Then, when φ ₁ becomes larger than 0.1, the casting speed V is adjusted so that φ ₁ is in an appropriate range, that is, 0.1 or less,
It is desirable that φ ₁ can always be secured in an appropriate area during continuous casting. In general, ΔT ₁ decreases as the continuous casting time elapses, so that the casting speed V can be increased.

[Example] (1) Continuous conditions Two types of mold cross-sectional dimensions (mm x mm): 600 x 50, 1200 x 50 Nozzle discharge port dimensions (mm x mm): 200 x 10,300 x 10,400 x
3 types of 10 Casting speed (m / min): 2 types of 10 and 20 Shell washing rate: Shown in Fig. 2.

(2) Adjustment method The above items ,,, and were cases in which the shell washing ratio did not reach 0.1, and therefore do not require adjustment. On the other hand, A, A, A, A, and A are the cases where the shell washing rate was 0.1 or more, respectively. Therefore, after performing the following adjustments, B, B, and
Shown in B, B and B. The specific adjustment method is
In the case of, 2 nozzles of A are used in parallel in B, and in the case of, the discharge port width of the nozzle is changed from 300 mm of A to 600 mm,
Similarly, in the case of and, the nozzle outlet width was replaced with a doubled one, and in case of A, the superheat of molten steel was expected to be 10 ° C, but in case of A, it was 20 ° C, so φ> 0.1. , B are examples in which the casting speed is greatly reduced to 8.5 m / min.

In the case of A, φ ≦ 0.1 was achieved, but in B, the degree of superheat of the molten steel was lowered to 8 ° C. Therefore, this is an example in which the casting speed was increased.

EFFECTS OF THE INVENTION The present invention adjusts at least one of the casting rate and the single area ratio of the mold nozzle based on a characteristic formula to determine the shell washing rate that causes vertical cracking in high-speed continuous casting. Since it is controlled to 0.1 or less where vertical cracking does not occur, the occurrence of vertical cracks in high-speed continuous casting can be very easily prevented, and breakouts due to vertical cracks can be eliminated altogether. The effect of reducing production costs is great.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the calculated value and the actually measured value according to the present invention. FIG. 2 is a diagram showing an adjustment situation in the embodiment of the present invention.

Claims (2)

[Claims] 1. Molten steel is used as a mold by adjusting at least one of a casting speed V and a ratio S between a mold cross-sectional area and a discharge port area of an injection nozzle so that a shell washing ratio φ described below is 0.1 or less. A casting method for continuous casting, characterized by pouring. φ = f (ΔT, V, S) However, shell washing rate φ = (δ _max −δ _min ) / δ _max δ _max : maximum shell thickness within the same width section δ _min : minimum shell within the same width section Thickness V: Casting speed (m / hr) S: Cross-sectional area ratio of mold to nozzle outlet ΔT: Overheating temperature of molten steel (deg) 2. The casting method for continuous casting according to claim 1, wherein the shell washing ratio φ is represented by the following equation. φ = A ・ ΔT ・ V ^0.473・ S ^0.8 where A: constant term