JP2684307B2 - Highly efficient method for preventing Al2O3 aggregation in molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-quality steel material in which oxide-based inclusions in molten steel are prevented from agglomerating and alumina in the steel material is finely dispersed.

[0002]

2. Description of the Related Art In recent years, the quality required for steel materials has become increasingly severe and diversified, and there is a strong demand for the development of technology for producing cleaner steel. Oxide-based inclusions in steel are no exception, and further reductions have been required to reduce the degree of adverse effects in steel. That is, oxide-based inclusions, especially alumina (Al ₂ O ₃ ) -based inclusions, are the causes of wire breakage of wire materials such as tire cords, the deterioration of rolling contact fatigue characteristics for bar steel such as bearing steel, and the production of thin steel sheets such as DI cans. It is known to cause cracking during canning, and as a countermeasure against this, various reduction techniques have been developed or studied centering on alumina-based inclusions.

The outline thereof is described in detail on pages 11 to 15 of the 126.127th Nishiyama Memorial Technical Course "High Clean Steel" issued by the Iron and Steel Institute of Japan in November 1988, and Table 4 on the 12th page. A technical summary is given in. According to this, the removal technology can be roughly classified into a technology for reducing alumina in molten steel that is a deoxidation product, a technology for preventing and preventing alumina produced by air oxidation, and a technology for reducing aluminum-based inclusions mixed in from refractory materials. In the actual industrial process, it is the current situation that the above-mentioned classified elemental technologies are combined in various ways to reduce the inclusions in the oxidation system. That is,
Focusing on the reduction of deoxidation products by the application of secondary refining equipment such as RH and powder blowing equipment, prevention of reoxidation by deaeration, slag reforming, etc., and reduction of mixed oxide inclusions by slag cutting. The combination and the reduction needs are addressed.

[0004]

However, it is a fact that the removal of the above-mentioned alumina-based inclusions has reached the technical limit. That is, T.I., which is a measure of the content of alumina inclusions in molten steel. It is difficult to set the O concentration to 5 ppm or less. On the other hand, T. Even if it contains 5 to 7 ppm of O, alumina-based inclusions often cause defects in the product stage. Therefore this problem encountered a technically significant barrier. For such a situation, the inventors of the present invention removed the dissolved oxygen in the molten steel in advance by immersing the C-quality refractory-made immersion pipe in the molten steel and reducing the pressure inside the immersion pipe. In addition, we have established a method for manufacturing steel products that has reduced the alumina content to a level that has never been seen before. In addition, a method has been established in which Mg alloys such as Fe-Si-Mg and Fe-Mn-Mg alloys are added to molten steel to prevent agglomeration of alumina in molten steel and to finely disperse oxides to render alumina harmless. did. However, the quality level required for steel materials is expected to become more and more severe, and it is necessary to take measures in manufacturing technology. In view of the above points, the present invention has improved the method of adding Mg such as Fe-Si-Mg and Fe-Mn-Mg alloy to molten steel, and established a more efficient method of detoxifying alumina. It is a thing.

[0005]

The gist of the present invention is as follows. For molten steel containing Si and Mn and having a C content of 0.2% by weight or more and 1.2% by weight or less, M
The present invention provides a highly efficient method for preventing aggregation of Al ₂ O _{3 in} molten steel, characterized by adding a Mg-Al alloy having a g content of more than 5% by weight and 70% by weight or less. Further Mg-A
This section presents the appropriate conditions for the addition rate, addition location, and addition method of the 1-alloy.

[0006]

The basic idea of the present invention is that A existing in molten steel
l ₂ O ₃ is prevented from coarsening due to aggregation and Al ₂ O ₃ is added to 2
It is to avoid the adverse effect of Al ₂ O _{3 on} the quality of the steel material by dispersing it in the steel material while maintaining the small size of μ or less. That is, as the size of the inclusions in the steel material is larger, stress is more likely to be concentrated in that portion, which is more likely to cause defects, which is not preferable. Therefore, if Al ₂ O ₃ is also not large, no adverse effect is exerted. Based on this idea, the present inventors have made various studies on prevention of aggregation and coarsening of Al ₂ O ₃ . As a result, Al ₂ O ₃ is modified and Al ₂ O ₃ is modified.
By using O ₃ .MgO, it became possible to completely prevent aggregation and coarsening of Al ₂ O ₃ .

As is well known, Al ₂ O ₃ aggregates in molten steel to form clusters and coarsens. This is molten steel and Al
_This is due to the wettability of ₂ O ₃ . On the contrary,
Wettability of the molten steel and Al ₂ O ₃ · MgO, unlike the case of _{Al 2 O 3, Al 2 O} 3 · MgO never quite clustering. This is because Al ₂ O ₃ .MgO has a smaller interfacial energy with molten steel than Al ₂ O ₃ . Note that no larger size when reforming the Al ₂ O ₃ to Al ₂ O ₃ · MgO. By the addition of Mg, the modification of Al ₂ O ₃ progresses and the oxide composition becomes Al ₂ O 3.
_{3. It} may change from MgO to MgO. MgO
However, since the interface energy is smaller than that of Al ₂ O ₃ , the size is fine and clustering is difficult, and the object of the present invention is achieved. The details of the present invention will be described below.

In the present invention, Al ₂ O ₃ is replaced with Al ₂ O
An Mg-Al alloy having a Mg content of more than 5% by weight and 70% by weight or less is used for modifying to ₃ · MgO. Mg-Al
The alloy has a low melting temperature and a low alloy production cost, and since Al and Mg are added at the same time, the reforming reaction can proceed rapidly and efficiently. As a result, the yield of Mg addition in the alloy is dramatically improved. Mg-Al
The reason for defining the Mg content of the alloy to 70% by weight or less is
This is to prevent molten steel from scattering when an alloy is added. That is, when the Mg content in the alloy exceeds 70% by weight, the reactivity becomes intense,
Molten steel is scattered when the alloy is added, which is not preferable. On the other hand, if the Mg content is 5% by weight or less, the modification to Al ₂ O ₃ .MgO does not proceed sufficiently, which is not preferable. The Mg-Al alloy may contain 10% by weight or less of necessary components for molten steel such as Mn and Si.

The reason why the molten steel composition before adding the Mg-Al alloy is specified to contain Si and Mn and the C content to be 0.2% by weight or more and 1.2% by weight or less is as follows. C
If the content is less than 0.2% by weight, the dissolved oxygen in the molten steel increases, and the added Mg reacts with the dissolved oxygen to form MgO, so that Mg cannot be used in the Al ₂ O ₃ reforming reaction. When the C content exceeds 1.2% by weight, the added Mg and C form a compound, and the Al ₂ O ₃ reforming efficiency of the added Mg decreases, which is not preferable. If Si and Mn are added after the addition of Mn, the time from the addition of Mg to the addition of Mg to the casting becomes long and the loss of Mg increases, which is not preferable. Therefore, it is better to add Mg to the molten steel containing a predetermined amount of Si and Mn. In addition, Al in the molten steel before Mg addition
The object of the present invention can be achieved even when a certain amount is contained.

Next, the addition rate of the Mg component in the Mg-Al alloy added to the molten steel is 0.002-0.100 kg / T-
The reason for controlling to molten steel / min will be described. Mg reduces Al ₂ O ₃ in molten steel and converts Al ₂ O ₃ into Al ₂ O ₃ · M
It is added for the purpose of making gO (spinel). At this time, Al in the molten steel increases with the reduction reaction. However, since Mg is very highly reactive, it is not preferable if it is added at a constant rate or more because severe smoke generation, corrosion of refractory materials, and the like occur. As a result of conducting a test on this point, it was found that by setting the addition rate of the Mg component in the Mg alloy to 0.100 kg / T-molten steel / min or less, severe smoke generation, refractory erosion, etc. can be prevented. It was also clarified that at an addition rate of 0.002 kg / T-molten steel / minute or less, the evaporation loss ratio of Mg was large and the reduction of Al ₂ O ₃ was insufficient. From the above, the addition rate of Mg in the Mg alloy was 0.002
It is proper to control to ~ 100 Kg / T-molten steel / min.

Next, the conditions for adding the Mg-Al alloy will be described. The reason for adding the Mg-Al alloy in the continuous casting tundish and / or the mold on the left is as follows. Mg is a high vapor pressure element, and its evaporation loss increases as it is added at higher temperatures. Therefore, it should be added at a temperature lower than that in the melting step, and specifically, addition in a continuous casting tundish and / or a mold on the left side is desirable. Of course, if these are not possible due to equipment restrictions, Mg-Al alloy may be added to the molten steel in the ladle. Addition to molten steel in a ladle is a generally adopted method.

The advantages of the method of blowing granular Mg alloy into the molten steel with an inert gas and the method of filling the iron wire into the molten steel and supplying it into the molten steel are as follows. Since Mg has a strong affinity with oxygen and is easily oxidized, if it is added to the surface of molten steel by natural dropping, a considerable portion is oxidized on the surface of molten steel, which is not preferable. On the other hand, in the method of blowing granular Mg alloy into the molten steel with an inert gas, and the method of filling the steel wire and supplying it into the molten steel, there is no air oxidation, and the yield of addition of Mg is greatly improved. To do. The method of blowing granular Mg alloy into the molten steel with an inert gas is preferably applied to the continuous casting tundish, and the method of filling the iron wire and supplying it into the molten steel is best applied to the continuous casting mold. Is. As mentioned above,
By optimizing the addition rate, addition method, and addition location of the Mg-Al alloy, it is possible to prevent the molten steel from scattering and the refractory reaction due to the addition of Mg, and further suppress Mg evaporation. As a result, the yield of addition of Mg is improved, and as a result, the modification of Al ₂ O ₃ is promoted and the material properties are dramatically improved. Less than,
An example of the present invention will be described to describe the effect of the present invention.

[0013]

【Example】

Example 1 Molten mother steel was melted in a 120 ton converter, and Si, Mn, and Cr were added when the steel was tapped into a ladle. Then, the molten steel in the ladle is subjected to RH treatment to degas and remove inclusions, and fine adjustment of Si, Mn, and Cr is performed, and then Mg alloy is added under the conditions shown in Table 2 and shown in Table 1. 115ton of molten steel corresponding to the bearing of the composition was obtained. A bearing steel material (size 65 mmφ) was manufactured from this molten steel by continuous casting and bar rolling. At this time, as a result of adding the Mg alloy under the conditions shown in Table 2, the rolling bearing fatigue test results of the obtained bearing steel material were extremely good as compared with the comparative example. Most of the inclusions in the steel material were Al ₂ O ₃ .MgO and MgO, and the size thereof was 3.8 μm or less.

Comparative Example 1 Similar to Example 1, mother molten steel was melted in a 120 ton converter, and Si, Mn, Cr, and Al were added when tapping into a ladle. Subsequently, the molten steel in the ladle was subjected to RH treatment to degas, remove inclusions, and finely adjust Si, Mn, Cr, and Al.
15 ton was obtained. At this time, Mg alloys other than the present invention were added or no Mg alloy was added under the conditions shown in Table 2. A bearing steel material (size 65 mmφ) was manufactured from this molten steel by continuous casting and bar rolling. As a result, the rolling fatigue test results of the obtained bearing steel material were unfavorable as compared with Example 1. The inclusions in the steel material were mainly Al ₂ O ₃ , and the size thereof was 13 μm or less.
Was much larger than

[0015]

[Table 1]

[0016]

[Table 2]

Example 2 In the same manner as in Example 1, 120 ton of molten steel corresponding to spring steel having the composition shown in Table 3 was obtained. At this time, Mg under the conditions shown in Table 4
-Al alloy was added. A spring steel wire rod (having a size of 10 mmφ) was manufactured from this molten steel by continuous casting and wire rod rolling. As a result, the rotating bending fatigue test result of the obtained spring steel wire rod was extremely good as compared with Comparative Example 2. Most of the inclusions in the wire are Al ₂ O ₃ · MgO and M.
It was gO, and its size was as fine as 6 μm or less.

Comparative Example 2 According to the method of Comparative Example 1, 118 ton of molten steel corresponding to the spring steel having the composition shown in Table 3 was melted, and subsequently, continuous casting and rolling of the wire were performed to manufacture a spring steel wire (size 10 mmφ). However, as shown in Table 4, the results of the rotary bending fatigue test of the wire rod were unfavorable as compared with Example 2. Most of the inclusions in the wire were Al ₂ O ₃ , and the size was 18 μm or less, which was considerably larger than that in Example 2.

[0019]

[Table 3]

[0020]

[Table 4]

[0021]

As described above in detail, according to the present invention, the agglomeration of Al ₂ O _{3 in} molten steel can be completely prevented. As a result, the size of Al ₂ O ₃ -based inclusions in steel materials can be reduced to the conventional level. It has become possible to make the size smaller than that of. This makes it possible to produce a high-quality steel material in which Al ₂ O ₃ -based inclusions are rendered harmless, which is extremely beneficial to the industrial world.

Claims (5)

(57) [Claims] 1. A material containing Si and Mn and having a C content of 0.2.
Highly efficient agglomeration prevention of Al ₂ O _{3 in} molten steel, characterized in that Mg-Al alloy with Mg content of more than 5% by weight and 70% by weight or less is added to molten steel of more than 1% by weight and less than 1.2% by weight. Method. 2. When adding a Mg—Al alloy to molten steel, the addition rate of Mg in the alloy is 0.002 to 0.10.
The method according to claim 1, wherein the rate is controlled to kg / T-molten steel / minute. 3. The Mg-Al alloy is added by a ladle, a continuous casting tundish and / or a mold on the same left .
Or the method of 2. 4. The method according to claim 1 , 2 or 3, wherein the granular Mg—Al alloy is blown into the molten steel with an inert gas. 5. The iron-based wire is filled with a granular Mg—Al alloy and supplied into molten steel , or
Is the method described in 3.