JPH03114638A - Method for pouring molten metal in continuous casting - Google Patents

Abstract

PURPOSE:To prevent the development of longitudinal crack on a continuous cast slab by setting molten steel temp., shape of nozzle for pouring molten steel and casting velocity to the specific conditions and controlling shell washing ratio to the specific value or less at the time of continuously casting the steel slab at a high speed of the specific value or more. CONSTITUTION:At the time of continuously casting the molten steel, in the case of continuously casting at a high velocity, e.g. >=4m/min the casting velocity, the shell washing in the mold is developed with discharging flow of the molten steel poured into the mold and with this cause, uneven thickness of the shell in the width direction of the mold is developed and the longitudinal crack on the continuous cast slab is developed. This shell washing ratio phiat the time of continuous casting is shown in the equation I with functions of the casting velocity V, cross sectional film ratio S of the mold to discharging hole in the nozzle for pouring the molten steel and overheat temp. T and also since this is decided with the shell thickness of the mold as shown in the equation II, by adjusting either V or S in the equation I to make phi value <=0.1, the development of longitudinal crack to the continuous cast slab at the time of continuously casting at a high velocity is prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a continuous casting method, and in particular to a pouring 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, high-speed continuous casting methods have been attempted for slabs in which the casting speed is 4 m/min or more and the amount of heat removed during the casting solidification process is approximately 2 x 106 kcal/hr-m2. It is emphasized that this has a great impact on productivity and production costs, and methods to prevent it are being studied repeatedly.

In the above-mentioned high-speed casting, shell washing occurs due to the discharge flow of molten steel injected into the mold, which causes non-uniform shell thickness and causes vertical cracks.

These vertical cracks remain as surface flaws on the slab even after subsequent processing steps, and not only do they become product flaws and reduce yield.
Since breakouts occur during continuous casting, it is strongly desired to establish a technology to prevent vertical cracks.

As is well known, the conventional continuous casting of slabs uses a water-cooled copper mold and uses flux as a lubricant to cast slabs approximately 200 to 300 mm thick and 1000 to 200 mm wide.
A slab of around 0 mm is cast at a speed of 1 to 2 ml Tljn, and the amount of heat removed during the casting solidification process is 7 x 105 kcal/
A continuous casting method of about hr-m2 is used.

In this continuous casting method, vertical cracks are recognized as a typical surface flaw, and attempts are being made to elucidate the formation process and prevent the occurrence of vertical cracks in producing defect-free slabs. .

In general, the causes of vertical cracks are: ■ Occurrence of uneven shell thickness in the mold width direction.

■Stress generated during the casting solidification process.

It is known that there are

The present invention aims to improve the above-mentioned cause (1) of vertical cracking due to the occurrence of unrestricted shell thickness in the width direction of the mold.

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

For example, Tetsu to Hagane Vol. 1.68. No. 3, pages 1773 to 1781, there is the following description regarding the influence of local solidification delay, which does not extend to longitudinal cracking of slabs.

■Uneven shell thickness caused by temperature deviation in the width direction of the mold, which occurs near the meniscus, cannot be resolved by moving the slab and causes vertical cracks.

■If the degree of non-uniformity of the solidified shell thickness exceeds 1096 in medium carbon steel, vertical cracks will occur.

■The carbon content is O,]2C4~0.16! The occurrence of vertical cracking is noticeable in the slabs.

etc. are listed.

Next, when no vertical cracks occur, there is almost no temperature difference in the width direction of the mold, and a stable temperature distribution can also be seen in the lengthwise direction of the mold. On the other hand, when observing the occurrence of vertical cracks, the temperature at the mold eaves is low, and as it moves toward the bottom of the mold, this tendency gradually becomes stronger. It says something expensive.

However, in the method of high-speed casting of slabs at a casting speed of 4 m/min or more, the mold is composed of an endless steel belt, which is connected endlessly to the long side that moves in synchronization with the solidification and drawing progress of the injected molten steel. There is a method of casting using a continuous casting machine consisting of a short side that is sandwiched by the long side on a meniscus and moves integrally with the long side. Although it is possible that there is a cause for the occurrence of vertical cracks observed in casting, the mechanism of occurrence of this cause is thought to be very different.

[Problems to be Solved by the Invention] The present invention clarifies the causes of vertical cracking in high-speed continuous casting where the continuous casting speed exceeds 4 m/min, and solves the problem in high-speed continuous casting where the continuous casting speed exceeds 4 m/min. The objective of this project is to establish a high-speed continuous casting injection method that does not substantially generate vertical cracks, specifically, a method that optimizes the molten steel temperature, nozzle shape, and casting speed under specific conditions.

[Means for Solving the Problems] In order to achieve the above-mentioned problems, the present invention sets the casting speed ■, the cross-sectional area of the mold, and the discharge opening area of the injection nozzle so that the shell washing rate φ, which will be described later, becomes 0.1 or less. The method is to adjust at least one of the ratios S and inject the molten steel into the mold.

Shell washing rate φ=(δ, n, , X−δman)/δ. 8
8φ=f(ΔT, V, S) or φ=A・ΔT, Vo, 473・So, 8However, δmax: Maximum shell thickness within the same width cross section δwin
: Minimum shell thickness within the same width cross section A: 5.6
10-' (constant) ■ = casting speed (m/hr) S: cross-sectional area ratio of the mold to the nozzle discharge port ΔT: superheating temperature of molten steel (deg) [Function] The present inventors have achieved the above object. Therefore, various experiments and studies have been conducted to find out that in high-speed continuous casting methods where the casting speed is 4 m/min or more, as the casting speed increases, the discharge flow rate of molten steel from the immersion nozzle increases, and this sharp discharge flow rate It was discovered that the shell was washed with water, which promoted unevenness in the shell thickness and caused vertical cracks.

At this time, the knowledge that had been confirmed with the conventional continuous casting method mentioned above was that ``■ The degree of non-uniformity of the solidified shell thickness was 10 tons in medium carbon steel.
If it exceeds this, vertical cracks will occur. After repeated studies with reference to ``Continuous casting at a casting speed of 4 m/min or higher, the non-uniformity of the solidified shell thickness is 10% for medium carbon steel.''
It was found that vertical cracking occurs when the

The conventional knowledge was based on experiments in which artificial flaws were created in the mold. The temperature of the artificial flaw in the mold becomes low, and the temperature of the corresponding part of the slab becomes high, resulting in non-uniformity in the thickness of the shell.

On the other hand, the findings from experiments on high-speed continuous casting that are the object of the present invention are directed to non-uniformity in the thickness of shells produced by shell washing caused by the flow of molten steel discharged from a submerged nozzle.

Incidentally, the degree of non-uniformity of shell thickness due to shell washing (shell washing rate) is δmaX+δ. is determined by,
The δma

However, in both cases, the nonuniformity of the solidified shell thickness is 10
＊The fact that vertical cracks occur when the cracks occur is due to the fact that even though the mechanism of occurrence is different, the essential factor for the formation of vertical cracks is the non-uniformity of the shell thickness, and that both are caused by the same phenomenon. Ta.

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

Shell washing rate φ=(δ□ax−δ.1o)/δmayφ
=A・ΔT-V'・473・SO・8However, δ. Maximum shell thickness δ1 within the same width cross section of aX2. . :
Minimum shell thickness A within the same width cross section: 5.6
10-' (Constant) V: Casting speed (m/hr) S: Ratio of cross-sectional area of mold to nozzle discharge port T: Superheating temperature of molten steel (deg) This indicates that it is a function of . The calculated value using this formula and the actual measured value are
The results are shown in the figure, showing that the two are in good agreement.

The present invention is based on the above findings.

That is, before continuous casting, calculate Sl from the discharge opening area of the nozzle planned to be used, and determine the target superheating temperature Δ of the molten steel.
T1 and casting speed ■1 are determined, and shell cleaning rate φ1 is calculated from them. If φ1 is small and close to the appropriate range, that is, 0.1, the conditions are determined to be appropriate. If φ1
is larger than 0.1, the nozzle shape is changed to correct Sl and/or the casting speed is adjusted so that φ1 is 0.1. If φ1 is much smaller than 0.1, similarly modify the nozzle shape and/or casting speed to bring φ1 into the appropriate range. Then, pouring work for continuous casting will start.

In addition, after starting continuous casting, the discharge opening area S1 of the nozzle is constant, but ΔT1 can be measured, so if φ becomes larger than 0.1 with the actual measurement ΔT1, the casting speed ■ is changed to φ1. It is desirable to adjust so that φ1 falls within an appropriate range, so that φ1 can always be maintained within an appropriate range during continuous casting. In general, ΔT1 decreases as the continuous casting time increases, so the casting speed (2) can be increased.

[Example] (1) Continuous conditions ■ Mold cross-sectional dimensions (mm x mm): 600 x 50 ° 1200 x 50 ■ Nozzle discharge opening dimensions ( mm x mm): 200 x I
Q.

0 300X 10° 400X 10 3 types Casting speed (m/m1n): 10.20 2 types Shell cleaning rate: Shown in Figure 2.

(2) Adjustment method ■ No adjustment ■ Before A adjustment ■ After B adjustment ■ No adjustment ■ No A adjustment ■ After B adjustment ■ No adjustment ■ No adjustment ■ Before A adjustment After 0 adjustment ■ Before adjustment ■ After B adjustment ■ A adjustment flight 00 00 00 00 00 00 00 00 200 200 200 200 200 00 00 200+ 200 00 00 00 00 00 00 00 00 00 00 1 ■After B adjustment! 200 800 10 10
@ Before adjustment 1200 800 20.
20 [Phase] After B adjustment 1200 800 8.5
19 Items (1), (2), (2), (2), and (2) above are valid when the shell washing rate does not reach 0.1, and therefore do not require adjustment. On the other hand, ■A, ■8, ■8, ■8, and @lA are cases where the shell washing rate is 0.1 or more, so after the following adjustments are made, ■B, ■B, ■B, ■B and [phase] Not shown in B. The specific adjustment method is to use two nozzles of A in parallel in case of ■, and to adjust the nozzle discharge opening width to 300 m in case of ■.
600.m from mm, and in the case of ■ and ■, similarly replace the nozzle discharge opening width with one that is twice as wide, and [phase]
In case of , the degree of superheating of the molten steel was expected to be 10℃, but in case of A, it was as high as 20℃, so φ>0.1.In case of B, the casting speed was significantly lowered to 8.5n+/min. It is.

In the case of ■, A also achieved φ≦0.1, but B
In this example, the degree of superheating of the molten steel decreased to 8°C, so the casting speed was increased accordingly.

2 [Effects of the Invention] The present invention adjusts at least one of the casting speed and the cross-sectional area ratio of the mold and the nozzle based on a characteristic formula, and eliminates shell cleaning, which is a cause of vertical cracking in high-speed continuous casting. Since the ratio is controlled to 0.1 or less, which prevents vertical cracks from occurring, the occurrence of vertical cracks in high-speed continuous casting can be extremely easily prevented, and breakouts caused by vertical cracks can be completely eliminated, thereby improving production in this industry. The effects of improving performance and reducing production costs are significant.

[Brief explanation of drawings]

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

Claims (1)

[Claims] 1. Adjust at least one of the casting speed V and the ratio S of the mold cross-sectional area and the discharge port area of the injection nozzle so that the shell cleaning rate φ, which will be described later, is 0.1 or less. A continuous casting injection method characterized by injecting molten steel into a mold. φ=f(ΔT, V, S) However, shell washing rate φ=(δ_m_a_x−δ_m_i_n)
/δ_m_a_xδ_m_a_x: Maximum shell thickness within the same width cross section δ_m_i_n: Minimum shell thickness within the same width cross section V: Casting speed (m/hr) S: Cross-sectional area ratio of the mold to the nozzle discharge port ΔT: Superheating temperature of molten steel (deg) 2. The injection method for continuous casting according to claim 1, wherein the shell washing rate φ is expressed by the following formula. φ=A・ΔT・V＾0＾. ^4^7^3・S^0^. ^
8 However, A: constant term