JP5391842B2 - Melting method of high clean steel - Google Patents

Description

  The present invention relates to a smelting method for high clean steel, and more particularly to a smelting method for high clean steel in which residual flux particles introduced for desulfurization in a secondary refining process are suppressed.

In high-tensile steel with good workability, it is desired to reduce the S concentration as an impurity of the steel as much as possible. For this reason, molten steel is desulfurized in a secondary refining process (for example, RH equipment) during steel production. Regarding this desulfurization method, for example, Patent Document 1 describes the use of a flux containing CaO and having a solid-liquid two phase at the molten steel temperature. As a method for obtaining a solid-liquid two phase, for example, addition of CaF ₂ to CaO is also described.

However, in this method, the flux particles containing CaO and CaF ₂ remain in the molten steel even after desulfurization, and the high CaF ₂ region of the particles extends before and after the particles during the rolling of the steel, and both needles extend on both sides. There is a risk of inclusion. If there are a large amount of these inclusions, the hole expandability in a hole expansion test, which is a kind of workability evaluation method for good workability high-tensile steel, deteriorates. Such a phenomenon may occur if it is a CaO-based flux containing a low melting point oxide even if it is not CaF ₂ .

As a method for producing a hot-rolled steel sheet having excellent hole expansibility, Patent Document 2 and Patent Document 3 describe a production method using a desulfurized flux containing CaF ₂ , but the flux particles remaining in the molten steel after desulfurization are There is no explanation from the viewpoint of adversely affecting the hole expandability.

As a means of floating the flux particles remaining in the molten steel, there is a method in which the molten steel is refluxed under reduced pressure after the desulfurization flux blowing is completed, and the particles are floated. A long-time reflux is required, which is inappropriate from the viewpoint of improving productivity.

JP-A-6-240340 JP-A-8-176661 JP-A-8-291318

In high-tensile steel with good workability, it is desired to reduce the S concentration as an impurity of steel as much as possible, and molten steel is desulfurized in the secondary refining process (for example, RH equipment) at the time of steel production. However, a CaO-based flux containing a low melting point oxide such as CaF ₂ blown into the molten steel as a desulfurization material at that time remains in the molten steel and extends during rolling, and becomes a starting point of cracking during processing. May end up.

  Therefore, the present invention has been devised in view of the above-described problems, and provides a highly clean steel melting method for preventing a low melting point oxide-containing flux from remaining in molten steel as much as possible.

The present invention has been made to solve the above-mentioned problems, and the means thereof is as follows.
The method for melting highly clean steel according to the first invention is a secondary refining facility, after desulfurizing molten steel with a desulfurization flux containing at least one oxide or fluoride having a main component of CaO and a melting point of 1500 ° C. or less. Al is added to the molten steel in an amount of 0.02 to 0.04 mass%, and thereafter oxygen gas is blown into or blown into the molten steel in an amount of 0.1 to 0.2 Nm ³ per ton of molten steel.
The method for melting high-clean steel according to the second invention is characterized in that, in the first invention, the high-refining steel is refluxed for 2 minutes or more after completion of blowing of oxygen gas or after completion of blowing.

According to the present invention, without worrying about the problem of the desulfurization flux suspended in the molten steel, it has high desulfurization ability while ensuring high productivity, and it is mainly composed of CaO and having an melting point of 1500 ° C. or less. It becomes possible to produce a high-tensile steel having a very low S concentration and good workability using a desulfurization flux containing one or more compounds.

Diagram showing the relationship between the floating rate of inclusions and the amount of Al added-oxygen supply

The inventors examined a method for levitating the CaF ₂ -containing desulfurization flux remaining as inclusions in the molten steel. As a method for floating inclusions in molten steel, generally, a method in which molten steel is stirred with a gas to promote the floating of inclusions is performed. From this viewpoint, deoxidation is completed or desulfurization flux blowing is completed. It is effective to lengthen the stirring time. However, increasing the stirring time has a problem of causing a decrease in productivity.

Therefore, as a method of floating the desulfurization flux suspended in the molten steel in a short time, the idea of adding a large amount of Al into the molten steel and blowing oxygen gas there was conceived. The method itself of adding Al into molten steel and blowing oxygen gas is known as an Al heating method, and is used for the purpose of raising the temperature when the molten steel temperature is lowered. However, in that case, since Al addition and oxygen gas blowing are simultaneously performed, alumina (Al ₂ O ₃ ) is immediately formed, and the solubility product relating to deoxidation, that is, (mass% Al) ² × (mass% O ) The value of ³ does not increase. As a result, it is difficult to generate a large amount of inclusions, so that the effect of accelerating the floating cannot be expected because the generated inclusions described later are aggregated and coalesced with the desulfurization flux suspended in the molten steel.

In the present invention, when alumina is produced by reacting with oxygen in which Al in the molten steel is blown, desulfurized flux particles containing a low-melting point oxide such as CaF ₂ already suspended in the molten steel are used as nuclei. The aim is for the produced alumina to agglomerate and coalesce with each other or directly nucleated alumina to increase the particle size and float. In this case, in order to increase the solubility product (mass% Al) ² × (mass% O) ³ , it is important to increase the Al concentration (mass% Al) by adding Al prior to the oxygen gas blowing. It is.

Therefore, an experiment on a laboratory scale was conducted to determine what kind of Al concentration and oxygen gas blowing amount the suspended inclusions in the molten steel easily float. The experimental steel components (mass%) are shown in Table 1, and the experimental conditions are shown in Table 2. The definition of the inclusion floating rate is as follows. First, the molten steel is adjusted to the prescribed components, the flux is injected and the molten steel is desulfurized. Dividing the amount of inclusions of the ingot at the level where the concentration and the amount of oxygen sprayed were changed by the number of bases, the fraction was made 100 minutes. The oxygen blowing time was 2 minutes, and the elapsed time after desulfurization was set to 2 minutes, the same as the base level. Regarding the Al concentration, first, since deoxidation is necessary before performing molten steel desulfurization, a predetermined amount of Al was added before desulfurization. Al was further added after desulfurization, but the amount of Al added at this time was changed. The experimental results are shown in FIG. 1, where the Al added after desulfurization is in the range of 0.02 to 0.04 mass% and the amount of oxygen blown is in the range of 0.1 to 0.2 Nm ³ per ton of molten steel. Furthermore, it was found that the floating rate of inclusions suspended in the molten steel was as high as 80% or more.

The operation of the present invention will be described below. First, molten steel deflowing in secondary refining is based on the premise that molten steel oxygen is lowered as much as possible before desulfurization, and deoxidation is generally performed by adding Al. Therefore, at this time, the oxygen concentration in the molten steel is very low. Next, in order to perform desulfurization, a CaO-based desulfurization flux is blown into the molten steel. The desulfurization flux at this time is a desulfurization flux containing CaO as a main component and one or more oxides or fluorides having a melting point of 1500 ° C. or lower in order to obtain a good desulfurization ability. Such a desulfurization flux has a very good desulfurization ability, but if it remains suspended in molten steel, it is stretched by rolling, so that a low value is likely to appear in a steel sheet hole expansion test. During the molten steel desulfurization process, the molten steel is simultaneously stirred. When the desulfurization is completed, the desulfurization flux that has not been floated in the molten steel is suspended.

This is the important point of the present invention. First, Al is added in the range of 0.02 to 0.04 mass% while stirring the molten steel is continued. More preferably, it adds in 0.03-0.04 mass%. Al is already added before desulfurization for deoxidation. In the present invention, Al is newly added after desulfurization. After the addition of Al is completed, oxygen gas is blown into the molten steel in the range of 0.1 to 0.2 Nm ³ per ton of molten steel. More preferably, the molten steel is blown into the molten steel in the range of 0.15 to 0.2 Nm ³ per ton of molten steel. This operation generates a large amount of alumina.

There are two ways of producing this alumina, one of which is produced around the desulfurization flux that is suspended as a core, and becomes an integral particle with the desulfurization flux particles. The other is homogeneous nucleation in the form of single particles directly in molten steel. If the product of the Al concentration and the oxygen concentration is equal to or higher than a predetermined value, both particles aggregate and coalesce and float in the molten steel as large size particles.

After desulfurization, the amount of Al added before oxygen blowing or before blowing was defined as a concentration range of 0.02 to 0.04 mass% based on the Al concentration that was effective for inclusion flotation as a result of laboratory experiments. Were determined. When the addition amount is lower than 0.02, the amount of alumina inclusions produced is small, so that a large amount of desulfurization flux particles suspended in the molten steel cannot be levitated. In addition, the upper limit of the addition amount is set to 0.04 mass%. If the Al concentration is too high, a large amount of alumina is generated, so that the alumina that has not been lifted this time adversely affects the product characteristics. Because.

The reason why the amount of oxygen blown was defined to be in the range of 0.1 to 0.2 Nm ³ per 1 ton of molten steel was determined based on the amount of oxygen that had an effect on the floating of inclusions as a result of laboratory experiments. When the amount of oxygen is lower than 0.1 per 1 ton of molten steel, a large amount of desulfurized flux particles suspended in the molten steel cannot be levitated because the amount of alumina inclusions produced is small. Also, the upper limit of the oxygen amount is set to 0.2 Nm ³ , as in the case of Al, because a large amount of alumina is produced, the alumina that has not been lifted this time adversely affects the product characteristics. Because.

  In this way, the suspended desulfurized flux particles float up and are discharged out of the molten steel. Furthermore, in order to ensure the floating of the flux particles, it is desirable to secure a reflux time of 2 minutes or more after the completion of the oxygen gas blowing. When the present invention is carried out, the molten steel temperature rises due to the oxidation heat of Al. However, in general, since there are many cases where there is a concern about a temperature drop during molten steel desulfurization, the present invention works in a good direction in the desulfurization treatment.

The molten steel melted using a 400 t capacity converter was desulfurized using an RH vacuum degasser. After desulfurization, conditions were changed, Al addition and oxygen gas supply were performed, and then continuous casting was performed to perform hot rolling, and inclusions in the hot rolled sheet were investigated. In addition, it was evaluated by a hole expansion test. Table 3 shows the steel composition (mass%), Table 4 shows the desulfurization flux composition (mass%) used, and Table 5 shows the test conditions. Melting point of oxide / fluoride: CaO: about 2570 ° C., CaF ₂ : 1423 ° C., Na ₂ O: 1133 ° C.

The results are shown in Table 6. Levels 1 to 5 satisfy the conditions of the present invention, and the number of inclusions in the hot-rolled sheet decreased, and in particular, the elongated inclusions decreased remarkably. Moreover, the hole expansibility was also good.

  On the other hand, in any of the comparative examples of levels 6 to 11, the number of inclusions in the hot-rolled sheet, particularly the number of inclusions extended, was large, and the hole expandability was poor. That is, since the Al concentration of the additional addition is low at level 6, the injected oxygen gas amount is low at levels 7 and 8, and the Al concentration of the additional addition is too high at levels 9 and 10. The conditions of the present invention could not be satisfied, and the evaluation results were poor. Level 11 is the same as Level 3 that satisfies the conditions of the present invention except for oxygen gas blowing, but was ineffective because Al addition and oxygen gas blowing were simultaneous.

In particular, to further improve the workability of high-strength steel with good workability, which is a high-strength steel used for automobile undercarriage parts, and requires high formability and workability for the manufacture of parts with complex shapes. It is used as a highly clean steel melting technology.

Claims (2)

Secondary refining equipment, the main component is CaO, melting point is 1500 ℃ or less Desulfurization flux containing one or more oxides or fluorides after desulfurization of the molten steel, 0.02 to 0.04 mass% of Al in the molten steel added, followed by the oxygen gas into the molten steel, a method of melting the high cleanliness steel wherein the spraying 0.1 to 0.2 nm ³ blown or per molten steel 1t.   The method for melting high-clean steel according to claim 1, wherein the reflux is performed for 2 minutes or more after the end of blowing oxygen gas or after the end of blowing.

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