JPH05263121A - Production of high carbon and high purity molten steel - Google Patents

Abstract

PURPOSE:To provide high carbon and high purity molten steel low in elongated inclusion. CONSTITUTION:After the molten steel containing 0.51-1.50% C is melted in an atmospheric refining furnace and tapped off in the undeoxidizing or weakly deoxidizing condition, the dissolved oxygen is made to be <=30ppm by a vacuum treatment at <=1Torr the vacuum degree, and successively, Al in the range of 0.005-O.015% and Si in the range of 0.1-0.8%, are added. Thereafter, by adding Mn, number of secondary deoxidizing produced materials as the crystallized nuclei of MnS crystallized in the last solidified part is reduced, and MnO concn. in the oxides is reduced and the crystallization of MnS is restrained.

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-carbon, high-cleanliness molten steel containing few elongated inclusions.

[0002]

2. Description of the Related Art Stretched inclusions in steel materials have been a material problem in various fields. For example, in rails, it is known that the presence of elongated inclusions is a problem as a starting point of shelling damage at the corners, and reduction thereof is desired. As the elongated inclusions, in addition to oxide-based inclusions such as those in which alumina as a main component is arranged in a row in a group and those in which a silicate is mainly stretched in a linear form, similarly linear It is also known that MnS exhibiting an elongated morphology causes a problem.

Of these, MnS is concentrated and segregated between dendrites as the solidification progresses. As a result, MnS precipitates in the final solidification portion between the dendrites in excess of the solubility product on the solid phase side. In particular, in the case of a slab containing 0.51% or more of carbon, its precipitation was unavoidable. That is, in this case, the solidification segregation of S is extremely large due to austenite solidification (γ solidification), and the solubility product of MnS in austenite is small, so that carbon content is 0.5
This is due to the disadvantage that precipitation of MnS is extremely likely to occur as compared with the case of ferrite solidification (δ solidification) at 0% or less.

On the other hand, it is difficult to deal with the problem with the conventional technique, and there are no cases in which it is implemented, but the following method is generally used. A method of thoroughly reducing the Al concentration to 0.001% or less. A method of suppressing the precipitation of MnS by reducing the S concentration. Japanese Patent Application Laid-Open No. 63-227748 discloses a method for manufacturing a high-strength spring steel wire, and Japanese Patent Application Laid-Open No. 52-2979.
A method of adding Ca in the same manner as the method disclosed in Japanese Patent No. 6 or "Iron and Steel" Vol.

However, although the alumina-based inclusions can be reduced only by the method of lowering Al to the limit and strengthening Si deoxidation like the method of 1, there arises a problem that the silicate-based elongated inclusions increase. In order to reduce MnS only by the method of, the S concentration should be 20 ppm.
Since it is necessary to make it below, there has been a problem that it causes a significant decrease in economic efficiency and productivity. Furthermore, alumina type, Mn
In order to reduce all of S, it is necessary to perform both and at the same time, so it is necessary to perform extremely low sulfurization with Si-killed steel whose oxygen potential cannot be lowered sufficiently. It will be accompanied by

On the other hand, when the above method is applied, CaS-based grouped inclusions may be generated due to coarse precipitation of CaS in molten steel, and the effect was only extremely unstable. .. Here, the conditions of the method disclosed in Japanese Patent Application Laid-Open No. 52-29796 and "Iron and Steel", Vol. 66, page 354 and thereafter, which are proposed for aluminum-killed steel, are as follows. In addition to its high absorption capacity, this oxide is stable and difficult to decompose even at the end of coagulation, so it is consumed in the formation of calcium aluminate by combining with oxygen or oxide in the added Ca based on the substance balance. The amount of addition is determined by deducting the remaining amount and regarding the rest as effective Ca. However, when CaS is coarsely precipitated around the oxide when added to the cast strip for rails based on this condition, CaS-based grouped inclusions are particularly prone to be generated, and in addition, calcium is added. Since the aluminate has a low melting point, there is a problem that it tends to be an elongated inclusion, and it is difficult to apply.

[0007]

DISCLOSURE OF THE INVENTION The present invention is to reduce the economical efficiency and productivity due to the increase of silicate-based elongated inclusions and the desulfurization of Si-killed steel in the method of thoroughly reducing the Al concentration and the S concentration. Problems and JP-A-63-2277
48, Japanese Unexamined Patent Publication No. 52-29796, and "Iron and Steel" Vol. 66, pages 354 et seq., The generation of CaS-based grouped inclusions and extended calcium aluminate-based inclusions. The present invention provides a method for producing high carbon silicon killed high clean molten steel that solves the problem of formation.

[0008]

The gist of the present invention is that high carbon molten steel containing 0.51 to 1.50% of C is melted in an atmospheric refining furnace such as a converter or an electric furnace. After tapping in undeoxidized state or weakly deoxidized state, vacuum degree 1
Performed vacuum treatment at less than Torr to reduce dissolved oxygen concentration to 30
ppm or less, then 0.005 to 0.015 Al
%, Si is added in the range of 0.10 to 0.80%, and Mn is further added after that. As a result, it is possible to realize the production of a high carbon silicon killed and highly clean molten steel with a small amount of extension inclusions, and to provide a rail with excellent shelling damage resistance, for example. Here, it is preferable to add Al, Si, and Mn in this order.

[0009]

The present invention is based on the discovery that the presence of secondary deoxidation products having a high MnO concentration plays an extremely large role in the precipitation of MnS. That is, MnS is
As the solidification progresses, Mn and S are densely segregated between the dendrites. As a result, in the final solidification part between the dendrites, the precipitates exceed the solubility product on the solid phase side. M
In order for nS to be easily precipitated, it is necessary to have a substance serving as a crystallization nucleus. On the other hand, in some of the SiO ₂ and MnO-based oxides, Mn, Si, and O are densely segregated between the dendrites as the solidification progresses, similar to MnS.
It precipitates in the final coagulation part beyond its solubility product and is called a secondary deoxidation product. Since the solubility product of such an oxide is smaller than that of MnS, it precipitates before MnS, and since the place of precipitation is the final solidified portion,
It acts as a precipitation nucleus for MnS that is subsequently precipitated. Therefore, when the amount of the secondary deoxidation product is large, MnS is easily precipitated in the final solidification portion, while when the amount of the secondary deoxidation product is small, the solubility product is reduced in the final solidification portion. Even if MnS is exceeded, MnS cannot be precipitated and can be solidified in a supersaturated solid solution.

Further, MnO is used as a secondary deoxidation product.
It was also found that an oxide having a high concentration easily acts as a precipitation nucleus of MnS. The first item of the MnS precipitation suppressing method based on such a technical viewpoint is as follows.
In order to reduce the secondary deoxidation product, the primary deoxidation power is strengthened to reduce the dissolved oxygen before solidification. As the second item, the composition of the secondary deoxidation product is MnO 2. The concentration should be low.

Regarding the reduction of the secondary deoxidation product, which is the first item, it has been clarified by many experiments that it is necessary to reduce the dissolved oxygen before coagulation to 5 ppm or less. However, in order to achieve this, when a strong deoxidizing element such as Al is used in the undeoxidized state, Al ₂
It is known that a large amount of O ₃ -based inclusions are produced and become harmful elongated inclusions in a row. On the other hand, high-carbon steel contains 0.51% or more of carbon due to its material characteristics, and therefore has the property of undergoing the reaction of formula (1), so-called carbon deoxidation, when subjected to vacuum treatment. There is.

[C] + [O] = CO (1) As a result of the study by the present inventors, the oxygen concentration cannot be reduced to a level equivalent to that of aluminum-killed steel only by deoxidizing carbon, but the degree of vacuum is 1 Torr or less. After the oxygen concentration is reduced to 30 ppm or less by performing vacuum treatment at
If 1 is added in the range of 0.005 to 0.015%, the formation of a large amount of Al ₂ O ₃ -based grouped elongated inclusions can be prevented and oxygen before solidification can be reduced to 5 ppm or less. The knowledge that it can be obtained was obtained.

When C is less than 0.51%,
If it does not sufficiently act on carbon deoxidation according to the formula (1), and if it exceeds 1.50%, the material characteristics as a rail cannot be obtained, so the C content is set to 0.51 to 1.50%. did.
When Al is less than 0.005%, oxygen before solidification cannot be reduced to 5 ppm or less,
If it exceeds 5%, a large amount of Al ₂ O ₃ -based grouped elongated inclusions are generated, so Al is 0.005 to 0.015%.
And the range. FIG. 1 shows the influence of Al on the MnS length in the rail and the length of the alumina group row inclusions, and it can be seen that there is an appropriate value in the above concentration range.

On the other hand, the timing of Mn addition is an important factor for the second item, the composition of the secondary deoxidation product. Since normal deoxidation is added in the order of weak deoxidizing power, in the case of a rail material, the order is Mn, Si, and Al. However, Mn
Thermodynamically has a weaker affinity for oxygen than Si or Al, so Mn is temporarily added by first adding Mn.
It was thought that although O was generated, it was reduced by Si and Al and eventually MnO did not exist. However, as a result of detailed investigation of the fine oxide composition, the present inventors have found that MnO remains in the oxide when the alloys are added in this order. This is because when molten spherical MnO produced before Si or Al is added and thereafter reacts with Si or Al inside the molten steel, a solid phase of SiO ₂ or Al ₂ O ₃ is formed on the outer periphery of the oxide. The reason is that the MnO reduction rate after that is extremely small because of the precipitation. Externally generated Si generated in this way
Oxides having a high concentration of O ₂ and Al ₂ O ₃ and having a region with a high MnO concentration inside have a high sulfide capacity in the internal region, so S easily diffuses from the surrounding molten steel to the periphery of the oxide and accumulates easily. Therefore, MnS is likely to precipitate in the final solidified portion.

In order to avoid this, it is important not to generate MnO first, and as in the present invention, Mn is added after all elements having strong deoxidizing power such as Si and Al have been added. Is required. Here, since Al is set to a range of 0.005 to 0.015% for the above reason, Si is set to a range of 0.10 to 0.80%. That is, when Si is less than 0.10%, Mn is added in a state where the concentration of dissolved oxygen is not sufficiently lowered, so that there is a problem that MnO is contained in the oxide, and Si is 0.80. If it exceeds%, the SiO ₂ concentration in the oxide becomes high, which causes a problem that harmful elongated silicate is produced. FIG. 2 shows the influence of Si on the MnS length on the rail and the length of the silicate-based elongated inclusions, and it can be seen that there is an appropriate value in the above concentration range.

[0016]

EXAMPLES Examples are results of melting in the following steps. Three
Carbon concentration of 0.70 due to 50 ton scale top-bottom blowing converter
After refining molten steel having a sulfur content of up to 0.90% and a sulfur concentration of 0.010% or less, vacuum treatment was performed using DH without adding a deoxidizer, and then Si, Al, and Mn were added.

The molten steel thus melted was cast into pieces (about 250 to 400 mm square) by a continuous casting machine, and then rail-rolled. The inclusion survey on the rail is 1 from the rail head surface.
The maximum length of the inclusions was evaluated by observing the entire 15 mm square cross section obtained by cutting out the inner surface of 10 to 25 mm from the side surface at the lower part of 0 mm with a 100 times optical microscope. Here, in the case of alumina-based inclusions, those connected in a group are
The lengths were evaluated by considering them as one continuous piece rather than as individual inclusion lengths. Each inclusion is Mn
The characteristics of the shape and color tone are summarized based on the results of EPMA analysis of typical ones, and in most cases, the characteristics of the shape and color tone with an optical microscope are selected. It was judged by. The longest inclusion length in the sample is 500 μm or more.
499 to 251 μm was marked with ◯, and 250 μm or less was marked with ⊚.
In addition, the evaluation section shows the anti-shelling damage characteristics on the rail. The results are shown in Tables 1 and 2.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to reduce any type of inclusions such as MnS-based elongated inclusions, alumina-based grouped inclusions, and silicate-based elongated inclusions.
The absence of harmful inclusions of 0 μm or more made it possible to produce high-carbon, high-cleanliness molten steel, and, for example, it was possible to provide a high-cleanness rail with excellent shelling damage resistance.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of Al on the MnS length of a rail and the length of alumina group row inclusions.

FIG. 2 is a diagram showing the influence of Si on the MnS length of rails and the length of silicate-based elongated inclusions.

Claims (1)

[Claims] 1. A high carbon molten steel containing 0.51 to 1.50% of C is melted in an atmospheric refining furnace such as a converter or an electric furnace, and is tapped in an undeoxidized state or a weakly deoxidized state. Vacuum treatment at a vacuum degree of 1 Torr or less is performed so that the dissolved oxygen concentration is 30 ppm or less, and then Al is 0.005 to 0.05.
A method for producing a high-carbon high-cleanliness molten steel, comprising adding Si in an amount of 0.10 to 0.80% in a range of 15%, and further adding Mn thereafter.