JPH0645816B2 - Calcium treatment method for molten steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a calcium treatment method for molten steel containing Al and S, by which nonmetallic inclusions present in the molten steel are rendered harmless and continuous casting is performed. In this case, it is possible to manufacture good quality bloom billets with few inclusion defects.

(Prior art) In bloom / billet continuous casting for rods and wires, because of its smaller cross-sectional size than slab continuous casting, there is a restriction on the installation space of the immersion nozzle in the mold and control of the amount of molten steel injected. From the viewpoint of improving the property, casting has been performed using a dipping nozzle having a small diameter. Recently, billet continuous casting that employs a dipping nozzle with a small cross-sectional size has been developed aiming at cost reduction by omitting the agglomeration step, which is a post-step of continuous casting.

Generally, molten steel for rods and rods needs to be deoxidized with Al, but in this case, the deoxidation product Al ₂ O ₃ is unavoidably present, and during the casting, it adheres and grows on the inner wall of the immersion nozzle (giant Of the Al ₂ O ₃ particles (Al ₂ O ₃ particles that have become detached from the inner surface of the immersion nozzle, causing nozzle clogging and inhibiting stable casting.
Was a cause of generation of inclusion defects in the slab. Such a tendency is particularly remarkable in the case of small-section casting, that is, casting using a small diameter immersion nozzle,
Preventing Al ₂ O _{3 from} adhering to the inner surface of a small diameter immersion nozzle was an important technical issue in establishing a small cross-section casting technology.

As a measure for preventing Al ₂ O _{3 from} adhering to the inner surface of the immersion nozzle, a technique for removing Al ₂ O _{3 in} molten steel and a technique for modifying Al ₂ O ₃ into inclusions that are difficult to adhere to the inner surface of the nozzle are being studied. These are described in detail in November 1988, the 126th and 127th Nishiyama Memorial Technical Lecture "High Clean Steel", published by the Iron and Steel Institute of Japan, No. 11 to No. 15, and various in the actual machine process. A method that combines elemental technologies is adopted.
However, few of these methods have a sufficient Al ₂ O ₃ removal function. This means Al ₂ O ₃
Is based on a complex physicochemical phenomenon,
Furthermore, since Al ₂ O ₃ is very fine, it is difficult to separate it from the molten steel.

On the other hand, the details are described in "Calcium Steel" Nos. 81 to 83 issued by Maruzen Co., Ltd. in April 1981. According to it, the mainstream of the modification technology is a method of adding Ca into molten steel to modify the high melting point Al ₂ O ₃ into a low melting point 12CaO · 7Al ₂ O ₃ , whereby the Al on the inner surface of the immersion nozzle is changed.
_It prevents the adhesion of ₂ O ₃ . However, even with this method, in the case of molten steel for rods and rods, modification is often not sufficiently achieved. The reason is that the molten steel for wire rods contains 0.01% by weight or more of S. Therefore, CaS is generated simultaneously with the modification of Al ₂ O ₃ by the addition of Ca, and CaS adheres to the inner surface of the immersion nozzle to induce nozzle clogging. This is because there is a proposal by JP-A-63-7322, but it is not sufficient. Furthermore, the CaS composite inclusions separated from the inner surface of the nozzle cause defects in product inclusions.

The present inventors have made various investigations in order to deal with such a problem, and have made molten steel as described in “Continuous casting method of bloom and billet by calcium treatment” applied on November 22, 1989 (hereinafter referred to as “prior application”). CaS by controlling the ratio of total Ca concentration / total Al concentration in molten steel according to the total O concentration in molten steel
Invented a method of suppressing the generation of

(Problems to be Solved by the Invention) The present invention is a further development of the above-mentioned method of the prior application, focusing on the production of CaS in the reaction process, and presenting a method of stably suppressing CaS production to a low level. To do. As a result, defects of product inclusions (for example, ultrasonic flaw detection) are significantly improved, and complete prevention of nozzle clogging is achieved.

(Means for Solving the Problems) The gist of the present invention is Al; 0.100% by weight or less,
S; When adding calcium to molten steel containing 0.150% by weight or less, the addition rate of calcium is controlled according to the following formula in accordance with the C content in the molten steel.
This is an instruction for a calcium treatment method for molten steel for rod and wire, which can control the generation of S as much as possible.

V _Ca ≦ −25 [% C] +35 In the formula [% C]; C content in molten steel (% by weight) V _Ca ; Calcium addition rate (g / min / ton · steel) (function) Details will be described.

The present inventors further developed the method described in the prior application and pursued a method for suppressing the production of CaS at a low level. As a result, in the vicinity of the Ca addition position in the molten steel, that is, when Ca is added in the iron-coated Ca wire, the wire immersed in the molten steel is melted, and in the region where Ca is supplied into the molten steel, other It was clarified that this is due to the fact that the Ca concentration is extremely high compared to the molten steel part. in this way
When a region with a high Ca concentration is formed, excess Ca more than the amount used for reforming Al ₂ O ₃ exists, and excess Ca becomes Ca.
It will contribute to the generation of S. The result is that Ca in molten steel
It was obtained by analysis of inclusions in molten steel taken from the vicinity of the addition position.

From the above findings, high Ca near the Ca addition position in molten steel
The method of controlling the addition rate of Ca was conceived as a means to prevent the formation of the concentration region, and the appropriate range was determined by the iron-coated Ca wire addition experiment on a 120 ton scale and the inclusion analysis of molten steel taken from the vicinity of the Ca addition position in molten steel Clarified.

Figure 1 shows the results, and the molten steel C content and Ca
The proper range was shown in relation to the addition rate. In the figure, the straight line A corresponds to V _Ca = −25 [% C] +35, but V _Ca > −25
When it becomes [% C] +35, the production of CaS becomes remarkable, which is not preferable.
That is, when V _Ca ≦ −25 [% C] +35, the ratio of inclusions that combine CaS is 5% or less, whereas when V _Ca > −25 [% C] +35, the ratio is 30% or more. Increase to. The inclusion ratio of CaS was determined by randomly extracting 100 inclusions in the steel and analyzing the composition with an X-ray microanalyzer. The reason why the straight line A can increase the V _Ca as the lower [% C] is, is that the lower the [% C] is, the higher the O content in the molten steel is, and the condition in which CaS is hard to be generated is set.

On the other hand, in the range of V _Ca ≦ −25 [% C] +35, if V _Ca, that is, the Ca addition rate is too slow, the treatment time becomes long,
It is not preferable because the temperature drops to the molten steel temperature at which continuous casting is impossible. As a guideline for actual operation, it is necessary to set V _Ca ≧ −15 [% C] +15 in consideration of the fact that the higher [% C] is, the lower the solidification temperature is.

From the above, it should be controlled within the range of V _Ca ≦ −25 [% C] +35, and more preferable range is −15 [% C] + 15 ≦
V _Ca ≦ −25 [% C] +35.

The Ca source added in the present invention is not particularly limited, and V _Ca may be set in an appropriate range according to the Ca content of each additive. As the Ca source, iron-coated Ca wire, Ca-Si alloy or the like can be used. Further, the method of supplying Ca is not specified, and it is applicable to a wire addition method, a blowing method using an immersion lance, and the like. The molten steel [% C] to which the present invention can be applied is 0.01 to 1.00%. If [% C] is 1% or more, the amount of Al ₂ O _{3 in the} molten steel decreases, and the need for Ca treatment becomes weak.

In the present invention, the ranges of Cl and S in steel are specified for the following reasons.

Although Al is necessary for adjusting the grain size of steel, the upper limit is set to 0.100% because the grain size adjusting effect is saturated even if added in excess of 0.100%.

S is necessary from the aspect of imparting machinability to steel bar wire rod products used for machine structural steel or cold forging steel,
If the content exceeds 0.150%, the mechanical properties deteriorate, so the upper limit is set to 0.150%.

When the present invention is combined with the method of the prior application, the CaS production suppression effect becomes more remarkable.

(Example) The Example of this invention is shown below.

When smelting 120ton / Heat molten steel for rod and wire using a converter and degassing equipment, the converter slag was removed at the stage of tapping from the converter to the ladle, and then the molten steel in the ladle was not removed. After forming an oxidizing slag, adjusting the components and performing dehydrogenation, an iron-coated Ca wire having a wire outer diameter of 13 mmφ (Ca content in the filler: 92% by weight) was added to the deep inside of the molten steel in the ladle. In the examples, a total of 10 heats were carried out, and the Ca addition condition was V _Ca satisfying the present invention according to the C content of each heat as shown in the example column of Table 1. The molten steel after the Ca treatment was subjected to a curved billet continuous casting machine having a slab cross-sectional size of 162 mm × 162 mm to manufacture a billet for rod and wire rods.

On the other hand, as a comparative example, as shown in Table 1, V _Ca was cast using the same continuous casting machine for 5 heats adjusted independently of the present invention.

The billet obtained from each heat was rolled into a steel bar having a diameter of 40 mm, and then an ultrasonic flaw detection test was performed on the steel bar product to determine the ultrasonic flaw detection rate due to large hard inclusions. The results are also shown in Table 1. In the comparative example, the average defective rate = 0.31%
On the other hand, in the example of the present invention, the defect rate average was 0.11%, which was an extremely low level, and inclusion defects were significantly improved.
Further, in the examples of the present invention, nozzle clogging did not occur at all, and the amount of nozzle deposits was small.

(Effect of the Invention) As described in detail above, even in the molten steel for a rod and wire containing Al and S by the method of the present invention, the formation of CaS during Ca treatment is suppressed to a low level, and only Al ₂ O ₃ has a low melting point. The technology for reforming into 12CaO ・ Al ₂ O ₃ was established. As a result, it has become possible to cast molten steel with a small cross-section for rod and wire without nozzle clogging. Further, the small-section cast slab obtained by the present invention is of good quality with few inclusion defects, which is extremely beneficial to the steel industry.

[Brief description of drawings]

Figure 1 shows the relationship between the C content of molten steel and the Ca addition rate.
5 is a drawing showing a preferable range in which generation of S can be prevented.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshimasa Kusano 12 Nakamachi, Muroran-shi, Hokkaido Inside Nippon Steel Co., Ltd. Muroran Works

Claims (1)

[Claims] 1. When adding calcium to molten steel containing Al; 0.100% by weight or less and S; 0.150% by weight or less, the addition rate of calcium is controlled according to the following formula according to the C content in the molten steel. Characteristic method for calcium treatment of molten steel V _Ca ≦ −25 [% C] +35 [% C] in the formula; C content (% by weight) in molten steel V _Ca ； Calcium addition rate (g / min / ton ・ steel) .