JPH0762166B2 - Steel refining method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for refining steel, and more particularly to a method for producing a washed steel containing significantly less nitrogen as an impurity.

(Prior Art) In refining using an electric furnace such as an arc furnace, the molten steel melted in the furnace is subjected to reduction refining to remove [S] in the molten steel, and then degassing is performed exclusively for dehydrogenation. Is done. At this time, [N] in the molten steel is also removed to some extent, but its efficiency is generally low and cannot be removed sufficiently. For example, even when [N] before degassing is at a high level of 50 ppm or more, the denitrification rate is a low value of about 10 to 25%, and when the [N] level before treatment is 40 ppm or less, denitrification progresses almost. Absent. The [N] remaining in the molten steel is combined with Al, Ti, etc., and remains in the steel as non-metallic inclusions such as AlN, TiN, which causes the fatigue life of the steel to be shortened.

By the way, in recent years, the demand for the quality of steel has become more and more strict, and the demand for reduction of such non-metallic inclusions in the steel has become stronger. For example, in the bearing industry, the load on bearings is increasing with the increase in size and speed of equipment. Therefore, it is desired to improve the fatigue life of bearing steel, and to reduce such non-metallic inclusions. There is a strong demand.

(Problems to be Solved by the Invention) However, as described above, the denitrification rate by degassing is low, and in the past, the [N] level, which was slightly lowered in the oxidative refining of molten steel, cannot be raised. At best, it is important to keep in mind whether or not to carry out the subsequent processing, and the fact is that no other special means for nitrogen reduction can be found.

(Means for Solving Problems) Therefore, the present inventors have conducted earnest research on how to reduce the [N] level in molten steel, and have found the following facts. That is, when the [S] level in the molten steel is lowered in the above-mentioned reduction refining process, the denitrification rate in the degassing process is significantly improved depending on the degree of the reduction, and the [S] level in the molten steel and It was found that there was a proportional relationship with the nitrogen rate.

The present invention has been made on the basis of such knowledge, and the gist thereof is to reduce [S] in molten steel by subjecting the molten steel melted in a melting furnace to reduction refining Is once lowered to a level lower than the permissible amount in the finally obtained steel by degassing the molten steel to remove [N] in the molten steel, and then [S] in the molten steel is again removed. To increase to a desired level.

That is, in the present invention, first, the raw material is melted in a melting furnace, and then P in the molten steel is mainly removed by oxidative refining. Subsequently, reduction refining is performed to remove S and O in the molten steel. At this time, the level of [S] is lowered by a predetermined amount from the allowable amount in the steel finally obtained. The lower the level of [S], the higher the denitrification rate in the subsequent degassing step.

Conventionally, in this reduction smelting, [S] was lowered only to a level corresponding to the allowable amount of steel finally obtained, but in the present invention, regardless of this allowable amount,
The [S] level is determined according to the target amount of decrease in [N].

By the way, in this reduction refining, when desulfurization is performed prior to or concurrently with desulfurization by adding a deoxidizer such as Al, Ca, Si to molten steel, desulfurization by slag is promoted. Here, to perform deoxidation prior to or concurrently with desulfurization does not refer to the order of the treatment steps of desulfurization and deoxidation, but only to a phenomenological front and back, and desulfurization is in progress. When it is deoxidized to some extent with Al, when adding a deoxidizing agent such as Al and a slag generator that contributes to desulfurization at the same time, or when adding a deoxidizing agent such as Al after desulfurization has progressed to some extent Including either. However, the most preferable case is to add a deoxidizing agent such as Al to deoxidize the slag before the slag is formed on the upper surface of the steel bath, and accelerate the sulfuric acid in the slag.

In the present invention, this reduction refining is preferably carried out in a vessel separate from the above melting furnace. This is because if reduction refining is performed in a melting furnace, it takes time to reduce the oxide remaining on the furnace wall, but this does not occur if it is carried out in a separate container. Moreover, it is desirable to provide a heating device in a separate container. It takes a long time to reduce [S] in the molten steel to a very low level, but if a heating device is provided, desulfurization can be promoted by heating the molten steel, and the refining time can be shortened.

The molten steel thus reduced and refined is then degassed, where [N] in the molten steel is efficiently removed.
That is, [N] has a higher denitrification rate than conventionally achieved.
Is removed, which causes AlN and Ti remaining in the steel.
Non-metallic inclusions such as N are reduced, and the fatigue resistance of steel is improved.

It is desirable to perform this degassing process by the RH vacuum degassing method. In this method, even though the molten steel is refluxed, the molten steel and the slag do not mix with each other, there is no phenomenon that the components that have entered the slag return to the molten steel, and when it is desired to increase the [S] in the steel, this step Because you can easily do that with.

(Example) Next, in order to clarify the present invention, the example will be specifically described below.

First, as shown in Fig. 1 (a) and (b), the raw material is melted using a 70 ton arc furnace 10, then the molten steel is transferred to a ladle 12 and oxygen is blown from a lance 14 to oxidize and refine the molten steel. In
P, Mn, Si, etc. were excluded. Next, after removing the slag on the upper surface of the molten steel as shown in (c) of the figure, the slag generator is added again to heat the molten steel at the electrode 16 and the container (ladle 12) as shown in (d) of the figure. Reduction refining was performed while blowing Ar gas from the bottom. At this time, the amount, type, and
By changing conditions such as refining time, [S] in molten steel
The level was variously changed from about 150ppm of the conventional level to nearly 0ppm (the composition of molten steel at this time was 0.35%
C-0.8% Mn-Fe).

When the reduction and refining was completed, the RH vacuum degassing device 18 was attached to the ladle 12 for degassing as shown in FIG. As shown in FIG. 2, this device 18 has a vacuum chamber 24 with two legs, an ascending tube 20 and a descending tube 22, and
When the legs of the book are dipped in the molten steel in the ladle 12 to exhaust the inside of the tank 24, and when Ar gas is blown from the rising pipe 20 to drive, the molten steel enters the vacuum tank 24 from the rising pipe 20 and descends further. Reflux into the ladle 12 via the pipe 22.

In this degassing step, nitrogen in the molten steel is removed together with hydrogen and oxygen. Conventionally, the denitrification rate was very low in this degassing process, but by reducing the [S] level in the molten steel in the above-mentioned reduction refining to a level lower than that of the conventional method, the denitrification rate is significantly increased. Was confirmed. 3 and 4 show this relationship concretely. As shown in the figure, the [N] after degassing decreases with the decrease of [S] in the molten steel, especially when the [S] level is lowered to 10 ppm, the final [N] level is 20 to 30 ppm. It decreased (Fig. 4).

In general, the allowable amount of S in steel is not so low (for example, about 0.03%), and therefore, in the conventional reduction refining, it was at best possible to reduce the [S] level to 100 to 150 ppm. It is for this reason that nitrogen in molten steel has not been sufficiently removed in the past (this fact is of course known in the present invention), but according to this example, [S ]
Once the level is drastically reduced below the permissible amount, the denitrification rate also increases accordingly, and the nitrogen in the steel can be effectively reduced. Therefore, if it is desired to increase the amount of S finally, it suffices to add S to the molten steel after completion of the degassing. In this case, the RH vacuum degassing method of this example is very convenient. By adding S together with Si, Mn, Cr and other alloying elements from the alloy addition hole 26 of the equipment, S can be easily added to the molten steel, and at this time, the added components do not come into contact with the slag on the upper surface of the steel bath. This is because It should be noted that, especially in the case of steel that requires machinability, S
However, it is needless to say that it is not necessary to add S after degassing when the amount of S may be low.

Now, when the degassing process is completed in this way, the first
As shown in the figure (f), molten steel was continuously cast by a continuous casting device 28 to obtain an ingot.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific examples, and various modifications may be made without departing from the spirit of the present invention. Is.

(Effects of the Invention) As described above, according to the present invention, the N level in steel can be significantly reduced, and accordingly, non-metallic inclusions such as AlN and TiN can be significantly reduced. This has the effect of extending the fatigue life of the steel and increasing the reliability of the product.

[Brief description of drawings]

FIG. 1 shows the steps for carrying out a steel refining method which is an embodiment of the present invention, and FIG. 2 is a view showing the constitution of the RH vacuum degassing apparatus of FIG. FIG. 3 is a diagram showing the relationship between the [S] level in the reducing and refining process of FIG. 1 (d) and the [N] level after degassing of FIG. 1 (e), and FIG. 4 is the degassing time. It is a figure showing the relation between and [N] level. 10: Arc furnace 12: Ladle 16: Electrode 18: RH vacuum degasser

Claims (4)

[Claims] 1. When a molten steel melted in a melting furnace is subjected to reduction refining to reduce the [S] in the molten steel, the [S] is a predetermined amount more than an allowable amount in the finally obtained steel. A method for refining steel, which comprises once lowering to a low level, then degassing the molten steel to remove [N] in the molten steel, and then increasing [S] in the molten steel to a desired level again. . 2. The steel refining method according to claim 1, wherein in the reduction refining step, deoxidation is performed prior to or concurrently with desulfurization. 3. The method for refining steel according to claim 1 or 2, wherein the desulfurization is performed in a vessel equipped with a heating device, which is separate from the melting furnace. 4. The steel refining method according to claim 1, wherein the degassing is performed by an RH vacuum degassing method.