JP5142610B2 - Manufacturing method of high cleanliness steel - Google Patents

Description

  The present invention relates to a method for producing a high cleanliness steel with a reduced oxygen content.

  In general, in the steel manufacturing process, molten steel melted in a converter or electric furnace is taken out into a ladle, and deoxidizers such as Al, Si, and Ca-Si alloys are added to the oxygen in the molten steel. Is removed as an oxide and the components are adjusted, and then the molten steel is cast into a mold and solidified to obtain a steel ingot. In this case, when deoxidizing the molten steel in the refining process, a method is adopted in which the molten steel is agitated using gas agitation in the ladle after the deoxidizer is added, and inclusions are aggregated and coalesced to promote floating separation. However, it is difficult to float and separate all the inclusions, and inclusions inevitably remain in the steel ingot after solidification.

  These inclusions have different shapes and sizes depending on the chemical components to be constructed. For example, in the case of alumina inclusions, large inclusions are easily formed by agglomeration and coalescence in molten steel. When trapped in a lump, a member for a generator that requires mechanical strength in a high-temperature environment may cause a decrease in ductility or a starting point of breakage. Therefore, it is extremely important to prevent the formation of large inclusions in such products.

  In addition, mold steel used as a material for plastic molding molds, etc. requires a very small surface roughness on the design surface that requires beauty, and if large inclusions of several hundred μm or more remain, Since the unevenness of the mold surface is transferred to the product and the surface properties are deteriorated, it becomes a big problem. Therefore, in such products, it is important to reduce the inclusions themselves to the limit as well as alumina inclusions.

Conventionally, as a steel processing method for preventing the formation of alumina inclusions, for example, in Patent Document 1, Ca is added so that the Ca concentration in molten steel is 0.001% by mass or more, and Al and And a method of changing the form of the generated oxide to a low melting point oxide such as Al ₂ O ₃ —CaO or TiO ₂ —CaO has been proposed.

Further, in Patent Document 2, an alloy of two or more selected from Ca, Mg and REM and Al is added as a deoxidizer in molten steel, and Al ₂ O ₃ in the generated inclusions is added in 30 to 30%. There has been proposed a method of obtaining Al killed steel free from alumina inclusions by adjusting within the range of 85 wt%.

In Patent Documents 3, 4 and 5, a Mn-Si alloy is introduced into the molten steel as a deoxidizing material, followed by vacuum degassing treatment to bring the O content in the molten steel to 100 to 600 mass ppm, and then to the deoxidizing material. A method is proposed in which Al, Mg, Zr and rare earth elements are added to the molten steel to perform composite deoxidation, and the resulting inclusions are refined and dispersed.
Japanese Patent Publication No. 63-41671 JP-A-9-263820 Japanese Patent Laid-Open No. 11-323426 JP 2002-105527 A JP 2003-13132 A

However, the above-described deoxidation method disclosed in Patent Document 1 has a problem in that the yield is low because the vapor pressure of Ca is high, and the Ca concentration in the molten steel is not stable.
Moreover, in the deoxidation method shown by patent document 2, when manufacturing the large steel ingot whose oxygen concentration in molten steel is 10 ppm or less, it is difficult to prevent the production | generation of an alumina type inclusion.
Furthermore, in the methods shown in Patent Documents 3 to 5 , although large inclusions having a diameter of 100 μm or more can be reduced, small inclusions of about several tens μm and fine elements of about several μm generated by trace elements remaining in molten steel. There is a problem that the secondary deoxidation product accumulates in the residual liquid phase part as solidification proceeds in the steel ingot and remains as an alumina inclusion in the central part. Therefore, in order to manufacture a more highly clean product, it is necessary to apply an additive that does not generate alumina inclusions and to find a new treatment method that can reduce the inclusions to the limit.

  The present invention has been made against the background of the above circumstances, and an object of the present invention is to solve the above-described problems, to suppress the formation of alumina inclusions, and to provide a method for producing steel with high cleanliness.

The present invention has been newly discovered as a result of studies to solve the above-described problems of the prior art.
That is, in the manufacturing method of the high cleanliness steel of the present invention, the invention according to claim 1 is composed out a rare earth element and (REM) and mullite _{(3Al 2 O 3 · 2SiO 2} ) in the molten steel during refining A highly clean molten steel is obtained by adding a composite treatment material.

The invention of the method for producing a high cleanliness steel according to claim 2 is the invention according to claim 1, wherein the composite additive is a mixture of a rare earth element (REM) and mullite (3Al ₂ O ₃ · 2SiO ₂ ). It is characterized by that.

The invention of the method for producing a high cleanliness steel according to claim 3 is the invention according to claim 2, wherein the composite additive is 0.05 to 1.5% by mass, preferably 0.1 to 1% of the molten steel, respectively. It is characterized by being composed of rare earth elements (REM) and mullite (3Al ₂ O ₃ .2SiO ₂ ) in the range of 0.0 mass%.

  The invention of the method for producing a high cleanliness steel according to claim 4 is characterized in that, in the invention according to any one of claims 1 to 3, deoxidation is performed with a primary additive prior to the addition of the composite additive. And

  Invention of the manufacturing method of the high cleanliness steel of Claim 5 is set in the invention in any one of Claims 1-4, The said primary additive is Al, Si, Ca, Ca-Si alloy, Ti, Mg It consists of 1 type or 2 types or more of these.

  The invention of the method for producing a high cleanliness steel according to claim 6 is the invention according to any one of claims 1 to 5, wherein the mixed steel after addition of the composite additive or the primary additive or both is vigorously stirred. By doing so, the floating separation of inclusions is promoted, and a highly clean molten steel is obtained.

  In the present invention, it is preferable to use a high cleanliness steel that does not contain large-sized alumina inclusions having a diameter of 100 μm or more, and preferably has a reduced oxygen concentration in the steel of 0.001% by mass or less. It shall be called steel. However, these numerical values do not limit the scope of the present invention.

  Usually, the size and form of inclusions generated in molten steel vary depending on the elements added. For example, Al tends to form huge alumina inclusions in molten steel, so it floats and separates in the refining process. Cheap. However, if the Al concentration in the molten steel exceeds 0.005% by mass, the Al concentration in the residual liquid phase rises due to solid-liquid distribution during the solidification process of the steel ingot, and therefore the alumina intervenes in the residual liquid phase. Things are generated. Since these alumina inclusions are formed in the final stage of solidification of the steel ingot, levitation separation is almost impossible, and they remain in the steel ingot after solidification and become defects.

  There is also a deoxidation method using Si that does not contain Al. However, alumina is inevitably contained in the out-of-furnace refining pan and refining slag. Is reduced, so that alumina inclusions are produced at the end of solidification. The formation of these alumina inclusions is greatly influenced by the Al and oxygen concentrations contained in the molten steel, and is easily generated even in a low oxygen region where the oxygen concentration in the molten steel is 0.001% by mass or less. Therefore, it is very difficult to stably control the Al and oxygen concentrations that do not generate alumina inclusions in the production process of the steel ingot.

For this reason, in order to prevent the formation of alumina inclusions in the solidification process of the steel ingot, it is necessary to lower the melting point of the generated inclusions and to melt the inclusions in the liquid phase.
Therefore, in the present invention, rare earth elements (REM) having a lower free energy for oxide formation than Al are mixed with mullite (3Al ₂ O ₃ .2SiO ₂ ) or with mullite (3Al ₂ O ₃ .2SiO ₂ ) in molten steel. Found a method to promote the deoxidation reaction and to lower the melting point by reacting the generated inclusions with mullite (3Al ₂ O ₃ · 2SiO ₂ ) and to promote the floating separation and to remove it. It is. The rare earth element may be composed of one kind of element, or may be composed of a plurality of kinds of elements. In addition, it is desirable that the rare earth element contains Ce that makes the above effect remarkable. The rare earth element and the oxide may be added to the molten steel at the same time, or may be added to the molten steel at different times.

Generally, Al, Si, Ca, Ca-Si alloy, etc. are usually used as additive materials for deoxidation of molten steel. However, deoxidizers are elements that combine with oxygen contained in molten steel to form oxides. For example, Ti, Mg, etc. may be added. In the present invention, the above elements may be used as a primary additive for the purpose of deoxidation. After the addition of the primary additive, rare earth elements (REM) and mullite (3Al ₂ O ₃ .2SiO ₂ ) are used as secondary additives. A composite treatment in which the composite additive material is added is effective. The reason for performing the composite treatment is that it is industrially effective to effectively reduce the oxygen concentration with the smallest possible addition amount because the rare earth elements used in the secondary additive are relatively expensive. For this reason, it is desirable to use the above-described Al, Si, or the like as the primary additive.

When only the rare earth element (REM) is used as the secondary additive, if the initial oxygen concentration in the molten steel is high, the oxygen concentration cannot be sufficiently lowered if the amount of rare earth element (REM) added is small. When the amount of (REM) added exceeds 1.5% by mass of the molten steel, a large amount of dendritic inclusions are formed in the steel ingot after solidification and the mechanical properties are remarkably lowered. In addition, the generated inclusions are heavier than the molten steel, so that floating separation becomes difficult. Therefore, the addition amount is 0.05% by mass or more and 1.5% by mass or less, preferably 0.1% by mass or more and 1.0% by mass or less of the molten steel.
On the other hand, when the present invention is added to mullite _{(3Al 2 O 3 · 2SiO 2} ) with a rare earth element (REM) as a secondary additive, it was found to be very effective in inclusion removal.
The mullite _{(3Al 2 O 3 · 2SiO 2} ) the addition amount more than 0.05 wt% and 1.5 wt% or less with respect to the molten steel, is preferably 0.1 mass% or more, the range of 1.0 wt% or less desirable. The reason for this is that if the amount of mullite (3Al ₂ O ₃ .2SiO ₂ ) added is less than 0.05% by mass, the ability to modify and adsorb oxides is lowered, and at the same time, sufficient floating separation is not possible. The cleanliness of the molten steel by excessive mullite exceeds 1.5 _{mass% (3Al 2 O 3 · 2SiO} 2) is deteriorated. For the same reason, the addition amount is preferably 0.1% by mass or more and 1.0% by mass or less.

By using the composite treatment method according to the present invention, rare earth element (REM) and mullite (3Al ₂ O ₃ .2SiO ₂ ) are added at the same time, so that mullite (3Al ₂ O _3. 2SiO ₂₎ is reacted, creating clusters of atoms population with the active surface, it is possible to effectively flotation while adsorbing the microscopic inclusions contained in the molten steel to lower the melting point.

As described above, the rare earth element in the molten steel during refining and (REM) mullite _{(3Al 2 O 3 · 2SiO 2} ) and by adding the formed composite additive out, the production of alumina-based inclusions It is possible to effectively remove the microscopic inclusions in the molten steel while suppressing, and to obtain a steel having a high cleanliness.
In addition, by performing the treatment with the primary additive prior to the addition of the composite additive, it is possible to reduce the amount of REM added and to obtain steel with high cleanliness effectively at an advantageous cost.

The method of the present invention can be effectively applied when refining steel melted in a converter or an electric furnace. In addition, the melting method of steel is not specifically limited as this invention, Various melting methods can be selected. Further, the components of steel to be cleaned are not particularly limited, and steels having various compositions can be targeted.
In refining steel, various refining methods such as out-of-furnace refining can be selected, and the refining method is not limited.

In refining steel, as described above, it is desirable to perform primary treatment by adding Al, Si, Ca, Ca—Si alloy, Ti, Mg or the like. In the present invention, this primary processing may not be performed.
REM and mullite _{(3Al 2 O 3 · 2SiO 2} ) , compared molten steel respectively, addition is made in an amount of preferably within the range of each 0.1 to 1.0 mass%. Inclusions produced by the REM reaction react with mullite (3Al ₂ O ₃ .2SiO ₂ ) added at the same time to lower the melting point and easily migrate into the slag.
As the molten steel solidifies, inclusions and the like remain in the slag, and a purified steel is obtained.

Examples of the present invention will be described below.
Using a round electric furnace, C: 0.2 mass%, Si: 0.1 mass%, Mn: 0.3 mass%, Ni: 3.8 mass%, Cr: 1.8 mass%, Mo: 300 g of molten steel containing 0.5% by mass and V: 0.1% by mass with the balance being Fe and inevitable impurities is melted in a dense MgO crucible having a diameter of 30 mm and a height of 100 mm, and iron oxide (Fe ₂ O ₃ ) is added so that the oxygen concentration in the molten steel is 100 ppm, and then, as a primary additive, metal Al or 44 mass% Ca-56 mass% Si alloy is added by 0.1 mass% of the molten steel and stirred. It was.
Further, after the lapse of 60 minutes, a rare earth element and (REM) and mullite (56 _mass% Al ₂ O 3 -40 wt% SiO ₂₎ was ground to 1mm~2mm adjust the particle size as a secondary additive, in the molten steel respectively In contrast, 0.1 wt% or 1.0 wt% weighed and mixed is added to the molten steel and continuously stirred while flowing 300 cc of Ar gas from an MgO tube with a diameter of 4 mm immersed in the molten steel per minute. went.

FIG. 1 shows the oxygen concentration in the molten steel before and after the addition of the additive of Example 1 above. The oxygen concentration in the molten steel gradually decreased after the addition of the additive. The oxygen concentration after solidification was 22 ppm when Al was used as the primary additive, and 37 ppm when the Ca-Si alloy was used. As the next additive, rare earth element (REM) and mullite (56 mass% Al ₂ O ₃ -40 mass% SiO ₂ ) were each added to 0.1 mass% of molten steel and stirred to reduce to 5 ppm. (REM) and to the molten steel mullite (56 _mass% Al ₂ O 3 -40 wt% SiO _2), was reduced to 3ppm when added respectively 1.0% by weight is stirred.

Moreover, the typical inclusion recognized in the steel ingot after solidification is shown in the electron micrograph (magnification 1000 times) of FIG. Alumina inclusions were observed in the inclusions in the steel ingot that was solidified by adding Al as a primary additive. From these results, alumina inclusions were observed in the case where Al was added as a primary additive, and those in which Al was added as a primary additive and those in which Ca-Si alloy was added as a primary additive were oxygen concentrations. Could not be reduced to 10 ppm or less.
On the other hand, the presence of alumina inclusions was not observed in the steel ingot to which the present invention was applied, and the oxygen concentration could be reduced to 10 ppm or less.

Next, Example 2 will be described.
Using a vacuum induction melting furnace, C: 0.3% by mass, Si: 0.2% by mass, Mn: 0.4% by mass, Ni: 4.0% by mass, Cr: 1.8% by mass, Mo: As a primary additive, melt 15 kg of molten steel containing 0.5% by mass and V: 0.1% by mass with the balance being Fe and inevitable impurities in a porous MgO crucible having a diameter of 130 mm and a height of 240 mm. 44 mass% Ca-56 mass% Si alloy was added 0.1 mass% of the molten steel, and the primary treatment was performed.

Further, 10 minutes after the addition of the primary additive, the secondary additive was pulverized with rare earth elements (REM) and mullite (56 mass% Al ₂ O ₃ -40 mass% SiO ₂ ) to 1 mm to ₂ mm to adjust the particle size. Were mixed and weighed and mixed with 1.0% by mass of each of the molten steel. In addition, the example which performed only the said primary process (Ca-Si alloy addition) was prepared for the comparison.

FIG. 2 shows the oxygen concentration in the molten steel before and after the addition of the additive of Example 2. The oxygen concentration in the molten steel has risen to about 0.03% by mass due to the reduction reaction from the MgO crucible after it has melted down, but it drops sharply after the addition of the additive, and the oxygen concentration after solidification is primary. When only the Ca—Si alloy was used as the additive, it was 18 ppm. On the other hand, it was reduced to 7ppm the addition of rare earth elements (REM) and mullite (56 _mass% Al ₂ O 3 -40 wt% SiO ₂₎ as a composite process. In addition, the oxygen concentration decreased earlier when the composite treatment was performed than when only the Ca—Si alloy was used, and the effect of the composite treatment was recognized. By using the method according to the present invention, it has become possible to obtain molten steel with higher cleanliness in a shorter time than before, and the cost reduction effect by shortening the refining time was obtained.

It is a figure which shows the oxygen concentration change before and behind the addition of the additive in Example 1 of this invention. Similarly, it is a figure which shows the oxygen concentration change before and after addition of the additive in Example 2. FIG. Similarly, it is the drawing substitute photograph which shows the typical inclusion after the solidification in the metal structure of Example 1.

Claims (6)

Process for producing a high cleanliness steel to obtain a highly clean molten steel by adding a composite treating agent composed out to the molten steel a rare earth element (REM) and mullite _{(3Al 2 O 3 · 2SiO 2} ) during refining. Process for producing a high cleanliness steel according to claim 1, wherein said composite process material is a mixture of rare earth elements (REM) and mullite _{(3Al 2 O 3 · 2SiO 2} ). _3. The high of claim 2, wherein the composite treatment material is composed of rare earth elements (REM) and mullite (3Al ₂ O ₃ .2SiO ₂ ) in a range of 0.05 to 1.5 mass% of the molten steel, respectively. Manufacturing method of cleanliness steel.   The method for producing a high cleanliness steel according to any one of claims 1 to 3, wherein deoxidation is performed with a primary additive prior to the addition of the composite treatment material.   The said primary additive consists of 1 type, or 2 or more types of Al, Si, Ca, Ca-Si alloy, Ti, Mg, The manufacture of the high cleanliness steel in any one of Claims 1-4 characterized by the above-mentioned. Method.   The molten steel after the addition of the composite treatment material or the primary additive or both is vigorously stirred to promote the floating separation of inclusions to obtain a highly clean molten steel. A method for producing a high cleanliness steel as described in 1.