JPH08225822A - Reformation of aluminum inclusion in molten steel - Google Patents

Abstract

PURPOSE: To provide a method for modifying alumina inclusions in molten steel so as to have a composition of low melting point, and to suppress the generation of CaS in the molten steel intended for steel bar and wire. CONSTITUTION: In modifying aluminum inclusions in the molten steel containing, by weight, <=0.100% Al and <=0.150% S to the oxides of a low molting point, Ca is first added to the molten steel, and successively Mg is added thereto. The ratio of the added amount of Mg to that of Ca, the position of addition, the method of addition, and the kind of Ca source and Mg source are appropriate. Casting with smaller cross-section of the molten steel for steel bar or wire completely free from the nozzle stuffing becomes possible to greatly reduce the manufacturing cost. Defects due to the inclusions in a product are almost completely eliminated to enable the supply of steel of excellent quality.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to the continuous casting of steel containing Al and S, in order to obtain a bloom billet of good quality with less large non-metallic inclusions under the skin and in the center, The present invention relates to a method of modifying an alumina-based inclusion into a low-melting oxide.

[0002]

2. Description of the Related Art In bloom / billet continuous casting for rods and wires, because of its smaller cross-sectional size than continuous slab casting, there is a restriction on the installation space of the immersion nozzle in the mold, and the amount of molten steel injected. From the viewpoint of improving controllability, casting has been performed using a small diameter immersion nozzle. Recently, billet continuous casting employing a smaller cross-sectional size and smaller diameter dipping nozzle has been developed aiming at cost reduction by omitting the agglomeration step which is a post-step of continuous casting.

Molten steel for rods and wires generally needs to be deoxidized with Al. In this case, the deoxidized product Al ₂ O ₃
There exists inevitably to accretion to the immersion nozzle inner wall during casting (giant), it becomes the impediment caused strainer stable casting nozzle clogging was peeled from the immersion nozzle inner wall, huge Al ₂ O ₃ system Inclusions may be mixed in the slab and cause defects of inclusions in the product. This tendency is remarkable in small-section casting, and in order to establish the billet continuous casting technique, prevention of clogging of the small diameter immersion nozzle has become a very important issue.

As a measure for preventing nozzle clogging, Al in molten steel is used.
₂ O ₃ removal technology and nozzle adhesion reduction technology by modifying Al ₂ O ₃ are being studied. Of these, No. 126/12 issued by the Iron and Steel Institute of Japan in November 1988.
It is described in the 7th Nishiyama Memorial Technical Lecture "Highly Clean Steel" on pages 11 to 15, but at present, few technologies have a sufficient Al ₂ O ₃ removal function. This is because the behavior of Al ₂ O ₃ is complicated and has not been sufficiently clarified.

On the other hand, regarding "Calcium Steel" 81-83 issued by Maruzen Co., Ltd. in April 1981.
See page for details. According to it, the mainstream of the reforming technique is to add Ca to the molten steel, whereby Al ₂ O ₃ is reformed to 12CaO · 7Al ₂ O ₃ having a low melting point to prevent the adhesion to the inner wall of the nozzle. However, in the case of rods, this method is often not achieved as expected. The reason is that the molten steel for rod and wire contains 0.01% by weight or more of S. Therefore, CaS competes with the reforming of Al ₂ O ₃ to produce CaS, and CaS adheres to the inner wall of the nozzle, This is to induce clogging. As a countermeasure against this, JP-A-6
No. 3-7322 has been proposed but is not sufficient. Further, the CaS composite inclusions separated from the inner wall of the nozzle often result in product inclusion defects.

The present inventors have made various studies in order to deal with such a problem, and disclosed in Japanese Patent Laid-Open No. 3-165952,
In addition, in JP-A-3-183721, a method for efficiently suppressing CaS production was proposed. The method of JP-A-3-165952 appropriately controls the ratio of total Ca concentration / total Al concentration in molten steel according to the total O concentration. Further, JP-A-3-183721 discloses a method of appropriately controlling the Ca supply rate according to the C concentration in the molten steel in order to suppress CaS generation in the reaction process.

[0007]

[0008] The present invention was to further develop and improve the method of the prior application, Al ₂ O ₃
Is a composite oxide having a melting point lower than that of 12CaO · 7Al ₂ O ₃ , and at the same time, an epoch-making method that hardly produces CaS is presented. This makes it possible to completely prevent nozzle clogging and significantly reduce defects in product inclusions.

[0008]

The gist of the present invention is as follows. Al: 0.100% by weight or less,
S: When the alumina-based inclusions in the molten steel containing 0.150% by weight or less are modified into low-melting oxides, Ca is first added to the molten steel, and then Mg is added. A method for reforming a temporary alumina-based inclusion is presented. Further, the present invention proposes a proper addition ratio of Mg and Ca, a proper addition place and a proper addition method, and preferable types of Ca source and Mg source.

[0009]

The function of the present invention will be described in detail below. First, C
The superiority of adding Mg to modify after a will be described. Ca
Followed purpose of adding Mg and has an oxide composition after reforming CaO-Al ₂ O _{3 2-component} from CaO-Al ₂ O ₃ -M
By using a gO ternary system, the melting point is further lowered. 12CaO · In other words CaO-Al ₂ O _{3 2-component}
7Al ₂ O ₃ has the lowest melting point (about 1400 ° C),
CaO-Al In ₂ O ₃ -MgO3 ternary, for example Ca
When O: 46%, Al ₂ O ₃ : 47%, and MgO: 7%, the melting point drops to about 1340 ° C. When the melting point of the oxide in the molten steel is lowered from 1400 ° C. to 1340 ° C., the oxide is not attached to the nozzle at a normal casting temperature (1470 to 1520 ° C.). More preferably, since the activity of CaO in the oxide is reduced, the production of CaS is significantly suppressed, and the oxide compounded with CaS disappears.
From the above two points, while completely preventing nozzle clogging,
It is possible to significantly reduce defects in product inclusions.

Next, the reasons for defining the molten steel composition in the present invention will be described. Al is necessary for adjusting the grain size of steel,
Even if added in excess of 0.100% by weight, the effect of adjusting the grain size is saturated, so the upper limit was made 0.100% by weight. S is necessary from the aspect of imparting machinability to steel bar wire rod products used for machine structural steel or cold forging steel, etc.
If the content exceeds 50% by weight, the mechanical properties rather deteriorate, so the upper limit is defined as 0.150% by weight.

Next, the reason for controlling the ratio of Mg addition amount W _Mg and Ca addition amount W _Ca to the molten steel and R (W _Mg / W _Ca ) in the range of 0.05 to 0.70 is as follows. . According to the experiments conducted by the present inventors, it was found that R = W _Mg / W _Ca is closely related to the composition of the produced oxide. That is, R is 0.
When it is less than 05, the MgO concentration in the oxide composition does not increase and the melting point of the oxide is not sufficiently lowered, and R is 0.
It was clarified that the effect of lowering the melting point of the oxide cannot be expected even if it is 70% by weight or more. Therefore, the preferred R
The range is from 0.05 to 0.70.

On the other hand, the reasons for adding Ca and Mg and the reasons for adding them are as follows. Since Ca and Mg are high vapor pressure elements and are prone to evaporation loss, it is desirable to add them immediately before solidification as much as possible so as to contribute to the reforming of the alumina-based oxide as soon as possible. For that purpose, considering the workability of the steelmaking process, it is desirable to add in the order of Ca → Mg in a molten steel ladle, a continuous casting tundish or a continuous casting mold. Ca may be added in a molten steel ladle and Mg may be added in a continuous casting tundish. As a source of Ca and Mg, it is efficient to supply the granular material into the molten steel. For that purpose, a method of blowing the granular material together with an inert gas,
It is preferable to fill the iron wire with the granular material and supply the iron wire into the molten steel.

Next, the types of Ca and Mg sources will be described. Considering the reactivity and reaction efficiency at the time of adding molten steel, it is possible to use metallic Ca, Ca-Si alloy, Fe-Ca alloy as the Ca source, and one or more of them may be used depending on the steel type. The Mg source has the same concept as the Ca source, but considering the reactivity and reaction efficiency when adding molten steel, metallic Mg, Mg-Si alloy, Mg-Coke, F
An e-Mg-Si alloy and an Fe-Mg-Si-Mn alloy can be used, and it is also possible to use one or more of these.
Furthermore, iron powder, Fe-Si alloy powder, Fe together with granular Mg source
-Mn alloy powder, Fe-Cr alloy powder, Fe-Mo alloy powder,
By mixing and adding one or more kinds of Fe-V alloy powder, the addition cost of these alloys can be reduced, and further, the molten steel scattering can be suppressed when the Mg source is added, and more preferable effects can be obtained. In this case, the Mg concentration in the mixture is 0.5 to 30.
Weight% is the proper range. If it is less than 0.5% by weight, the intended action of Mg is not exhibited, which is not preferable. On the other hand, if it exceeds 30% by weight, the molten steel is greatly scattered when the Mg source is added, which is not preferable.

[0014]

EXAMPLES The effects of the present invention will be described below by extending the examples of the present invention. When using the converter and RH degassing equipment to melt 120t / heat molten steel for rod and wire, the converter slag was removed at the tapping stage from the converter to the ladle,
Subsequently, a flux was added to the molten steel in the ladle to form a non-oxidizing slag, and the components were adjusted and dehydrogenated using an RH degassing facility. Then, a Ca source and a Mg source were added to the molten steel in the ladle or the continuous casting tundish according to the method of the present invention. The final molten steel after adding Mg has a slab size of 162
A billet for rod and wire rod was manufactured by using a continuous billet of mm × 162 mm. Table 1 shows the chemical composition and manufacturing conditions of the obtained billet.

[0015]

[Table 1]

On the other hand, as a comparative example, a slab size 1 was obtained from the mother molten steel melted regardless of the present invention, using the same continuous casting machine.
A billet for rod and wire rod having a size of 62 mm × 162 mm was manufactured. Table 1 also shows the chemical composition and manufacturing conditions of the billet of the comparative example. The billets obtained from the heat of each of the Examples and Comparative Examples were 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 failure rate due to large inclusions. The results are also shown in Table 1. While the average defective rate of the comparative example is 0.25%, the average defective rate of the example of the present invention is 0.06%, which is a very preferable level. Further, regarding the degree of nozzle clogging during continuous casting work, the nozzles of the present invention did not cause nozzle clogging at all, and there was no sign of this. In the comparative example, the nozzles tended to be clogged in most of the heats, and troubles such as operation interruption occurred in some of the heats.

[0017]

As described in detail above, according to the method of the present invention,
An efficient reforming method of alumina-based inclusions for molten steel for rods and rods containing Al and S has been established. That is, A
l a ₂ O ₃ and further lower melting point of CaO-Al ₂ O ₃ -MgO based composite oxide than 12CaO · 7Al ₂ O _3, innovative reforming methods have been developed that can be almost completely suppressed CaS produced simultaneously . As a result, it is possible to cast molten steel for rod and wire rods with a small cross-section, in which nozzle clogging does not occur at all. Further, the defects caused by inclusions in the product obtained by the method of the present invention are almost completely eliminated, and it becomes possible to supply good quality steel.

Claims (8)

[Claims] 1. Al: 0.100% by weight or less, S: 0.
When reforming an alumina-based inclusion in molten steel containing 150 wt% or less into a low melting point oxide, first, Ca is added to the molten steel, and then Mg is added, which is characterized by adding Mg. How to modify things. 2. The amount of Mg added W _Mg and the amount of Ca added W to molten steel
The method according to claim 1, wherein the ratio R of _Ca (= W _Mg / W _Ca ) is controlled in the range of 0.05 to 0.70. 3. The method according to claim 1, wherein Ca and Mg are added in a molten steel ladle, a continuous casting tundish or a continuous casting mold. 4. A granular Ca source and a granular Mg source are blown into the molten steel by an inert gas.
The described method. 5. The method according to claim 1, wherein the granular Ca source or the granular Mg source is filled in an iron wire and supplied into molten steel. 6. Metal Ca, Ca-Si as a source of granular Ca
The method according to any one of claims 1 to 5, wherein one or more kinds of alloys and Fe-Ca alloys are used. 7. Metal Mg, Mg—Si as a source of granular Mg
Alloy, Mg-Coke, Fe-Mg-Si alloy, Fe-
The method according to any one of claims 1 to 5, wherein one or more kinds of Mg-Si-Mn alloys are used. 8. An iron powder, a Fe—Si alloy powder, a Fe—Mn alloy powder, a Fe—Cr alloy powder, and a Fe—Mo together with a granular Mg source.
The method according to claim 1, wherein one or more kinds of alloy powder and Fe-V alloy powder are used.