JP5251360B2 - Manufacturing method of clean steel by ladle refining method - Google Patents

Description

  The present invention uses a ladle refining facility having a pressure reducing function such as VOD, VAD, tank degassing apparatus, etc., and does not use a flux containing fluorine as a refining agent. The present invention relates to a method for producing clean steel, which enables melting of O ≦ 0.0010% bearing steel and similar clean steel.

  When performing secondary refining for smelting of clean steel, as a refining material to be added to the ladle, a flux that mixes or synthesizes fluorine-containing fluorite with raw materials that do not contain fluorine, such as quick lime and alumina, is used. So-called slag refining, in which the mixture is added and stirred, has been performed.

  The purpose of adding fluorine-containing fluorite is to lower the melting temperature of the slag containing CaO, that is, to promote hatching. By containing fluorine, even in the case of slag having a high CaO concentration, so-called high basicity slag, hatching is improved, and slag-metal reaction can be promoted. As a result, molten steel desulfurization and oxygen concentration reduction are promoted, and it can be expected that processing time is shortened and cleanliness is improved.

  However, when the fluorine concentration of steelmaking slag increases, fluorine is eluted into the surrounding soil when this slag is recycled as roadbed material or civil engineering material. Fluorine is an important monitoring item for water pollution, and it is stipulated that the concentration of fluorine in the elution water is 0.8 mg / L or less in the water quality standard and soil environment standard of the Basic Environment Law. When used in the field, it is necessary not to cause contamination exceeding these standards. Moreover, when fluorine-containing slag is used, the problem which promotes the melting loss of a ladle refractory also arises. Therefore, there is an urgent need to reduce the fluorine-containing concentration of slag due to environmental and operational issues.

  In order to adhere strictly to the above standards, even if a very small amount of fluorine is contained, it may easily exceed the standard value depending on the slag composition, so it is necessary to add no fluorine-containing refining material such as fluorite. is there.

  When no fluorine-containing refining agent such as fluorite is added, (1) slag hatching is delayed, (2) slag-metal reaction (desulfurization S reaction efficiency) decreases, that is, desulfurization failure, oxygen concentration Does not decrease (S and O are difficult to decrease).

In order to promote hatching of slag containing CaO at a high concentration, it is common to add Al ₂ O ₃ (alumina) to adjust to a low melting point composition region, and as a typical flux, calcium aluminum Nate.

  In Patent Document 1, a low melting point composite steelmaking material is introduced into the steel from the electric furnace, ladle refining is started, and then the quick lime, calcium aluminate, and deoxidizing material are ensured to have slag fluidity. A fluorite-free additive treatment method in LF (Laddle Furnance) to be introduced is disclosed. However, it is clear that hatchability is deteriorated by adding no fluorite, and there is a lack of consideration for further hatching.

In addition, Patent Document 2 discloses the composition of the ladle refractory and the slag composition after refining in the method for producing aluminum killed steel. As a method for appropriately ensuring the fluidity of the slag in this document, It only defines the Al ₂ O ₃ concentration.

Patent Document 3 discloses the use of an alumina-based steelmaking material having a TiO ₂ content of 0.05% by mass or less. However, due to lack of consideration of conditions such as ladle contamination, considering the characteristics of molten steel refining with the generated slag, it is considered insufficient to control the Ti concentration in the steel.
JP 2004-204313 A JP 2007-84838 A JP-A-8-81710

  The present invention uses a ladle refining facility having a pressure reducing function such as VOD, VAD, tank degassing apparatus, etc., and does not use a flux containing fluorine as a refining agent. The object is to enable the production of bearing steel with O ≦ 0.0010% and similar clean steel.

As a result of studies by the present inventors to solve the above-described problems, in a ladle refining facility (for example, VOD, VAD) having a decompression function, Ti ≦ 0.0030%, T.P. When melting a bearing steel of O ≦ 0.0010% and similar clean steel, a slag composition after ladle refining CaO, SiO ₂ , Al ₂ is used with the use of a flux containing no fluorine as a refining agent. It was found that it is preferable to use TiO ₂ in order to use Na ₂ O and K ₂ O in addition to O ₃ and MgO and to control the Ti concentration in the steel at a low level. Here, the purpose of low-level control of the Ti concentration in the steel is from the viewpoint of quietness when using bearing steel products.

In obtaining this knowledge, the present inventor conducted the following examination.
In order to promote the hatching of CaO-based slag without containing fluorine, it was considered to increase the concentration of FeO and MnO, which are acidic oxides. These FeO and MnO are so-called lower oxides. It itself becomes an oxidizing source that supplies oxygen to the molten steel, and rather deteriorates cleanliness.

Therefore, as a new idea for promoting the hatching of CaO-based slag, Na ₂ O and K that can suppress the activity of amphoteric or acidic oxides such as Al ₂ O ₃ and SiO ₂ that coexist and can also be expected to lower the melting point. We focused on ₂ O. These compounds are industrially used as components such as mold powder in the continuous casting process. In addition, Na ₂ O and K ₂ O have the effect of lowering the activity of Al ₂ O ₃ (alumina) and SiO ₂ (silica) in addition to promoting initial hatching. It can be expected to suppress the adverse effect of SiO _{2 in} the slag, which is a factor.

However, these compounds themselves readily evaporate and could rather become an oxygen source during the evaporation process.
In consideration of this point, the present inventors have examined the conditions for containing Na ₂ O and / or K ₂ O that can ensure the hatchability and various slag properties that can provide a cleaning effect. In addition, as a supply source of Na ₂ O, sodium carbonate, sodium metasilicate, soda ash (main compound is Na ₂ CO ₃ ), and K ₂ O include potassium carbonate.

Further, in the melting of high clean steel such as bearing steel, which is the main purpose of the present invention, it is necessary to suppress the impurity elements that form precipitates to an extremely low concentration. In particular, the Ti concentration in the steel needs to be lowered because it forms TiN as a precipitate and impairs cleanliness. To that end, the TiO ₂ concentration in the slag must be controlled to be low. In the case of refining with fluorine-containing slag, the control of TiO ₂ concentration was gradual and good because it has the effect of chemically stabilizing the oxide. There is a need. However, the prior art has not made any specific disclosure regarding the control value and limit value of TiO ₂ in the slag composition after ladle refining in slag not containing fluorine.

Based on the above examination, the present invention provided to solve the above-described problems is specifically as follows.
(1) A method for producing clean steel by a ladle refining method in which molten steel and slag are stirred and refined under reduced pressure using Ar or N ₂ gas after steel is output from a converter or electric furnace. fluorine without using flux containing as agent, slag composition after ladle refining, in mass concentration, CaO: 30% ~60%, MgO: 3.0% ~15%, Al 2 O 3: 10% ~30%, (% CaO) / (% SiO 2): 2.0~10. 0, TiO ₂ : 1.0% or less, and the total content of Na ₂ O and K ₂ O is 0.05 to 1.00%, and the mass content of the elements contained in the product However, C: 0.15% to 1.10%, Si: 0.10% to 0.50%, Mn: 0.20% to 1.20%, P ≦ 0.025%, S ≦ 0.010 %, Cr: 1.30 to 1.60%, Mo ≦ 0.08%, sol.Al ≦ 0.045%, Ti ≦ 0.0030%, T.I. A manufacturing method of clean steel which is O ≦ 0.0010%, balance Fe and impurities.

  When melting clean steel using a ladle refining facility such as a ladle heating device or a tank degassing device, a flux containing fluorine is used as a refining agent by carrying out the production method according to the present invention. Even if not, the hatchability, fluidity and refining characteristics equivalent to those of conventional refining agents containing fluorine can be obtained.

Hereinafter, the present invention will be described in detail. In the following description, both% in the slag composition and% in the high composition mean mass%.
1. In a ladle refining facility with a slag decompression function, if calcium aluminate-based fluxes without fluorite are used, problems remain in hatchability and cleanliness. Therefore, by implementing the following measures, hatching is improved and it is possible to obtain refining characteristics equivalent to fluorite-added flux.

(A) For the purpose of improving hatchability, sodium silicate containing 10% Na ₂ O is added to the calcium aluminate flux.
(B) About sodium silicate, it mixes directly in the flux which does not contain fluorine, or is added separately from the flux which does not contain fluorine.
(C) In order to further promote the initial hatching, the stirring conditions are also strong stirring.
Below, a slag composition and stirring conditions are demonstrated in detail.

(1) Slag composition The CaO concentration in the slag needs to be 30% by mass or more and 60% or less. The reason is that if it is less than 30%, deoxidation of the molten steel becomes insufficient, and if it exceeds 60%, hatching is poor even if this technology is used. The CaO concentration in the slag is desirably 45% or more and 60% or less.

The SiO ₂ concentration in the slag is set so that (% CaO) / (SiO ₂ ) = 2.0 to 10.0 from the viewpoint of the cleanliness of the molten steel.
When the Al ₂ O ₃ concentration in the slag is less than 10% or more than 30%, the melting point of the slag increases and the hatchability deteriorates. Therefore, the Al ₂ O ₃ concentration in the slag is 10% or more and 30% or less.

The ladle refractory contains MgO, and 3-4% of MgO is added to the slag in order to prevent MgO from being damaged as much as possible. Still, MgO slightly melts from the refractories in the ladle slag line. Therefore, considering the MgO pickup, the MgO content of the slag is set to 3% or more and 15% or less.
The other slag component FeO and MnO concentrations are each desirably 1% or less. This is because although it has the effect of lowering the melting point, it itself becomes an oxygen supply source.

(2) Stirring conditions In the method according to the present invention, slag hatching is promoted by considering not only the adjustment of the flux composition but also the stirring conditions at the initial stage of the treatment. Moreover, it is preferable to consider the stirring conditions at the end of the treatment from the viewpoint of improving the cleanliness.

  Specifically, management is also performed based on the stirring power density ε (W / t) given by the following equation and the stirring time (s).

here,
ε: Stirring energy density (W / t)
Q: Blowing gas flow rate (Nl / min)
T: Molten steel temperature (K)
Wg: Molten steel weight (t)
Z: Molten steel depth (cm)
V: Degree of vacuum (torr)
It is.

  As a basic agitation treatment condition, strong agitation (ε ≧ 30 kW / t) is set for 10 minutes or more after the start of treatment to promote initial hatching. However, if excessive stirring is performed, molten steel and slag are scattered from the ladle and the operation becomes difficult, so ε ≦ 60 kW / t is appropriate. Moreover, this strong stirring time needs to be suppressed within about 15 minutes. This is because, if strong stirring longer than this is required for promoting the initial hatching, the total refining time will be extended, resulting in problems such as a decrease in productivity and a decrease in temperature.

  On the other hand, in order to float and remove slag droplets and generated inclusions mixed into the molten steel during the refining treatment, it is preferable to use weak agitation (ε ≦ 20 kW / t) for 3 minutes or more at the end of the treatment based on conventional knowledge. If the stirring force is too weak, the inclusion floating effect cannot be obtained, so ε ≧ 5 kW / t is appropriate. Even if this weak stirring time is too long, the effect is saturated, so about 5 minutes is usually sufficient.

  In addition, it is not necessary to control the stirring force in the middle period from the initial strong stirring to the final weak stirring. In addition, the total stirring time from the start of stirring to the end of stirring in the ladle refining process according to the present invention is typically 15 minutes to 30 minutes.

2. Molten steel The chemical composition of the molten steel targeted by the production method according to the present invention is as follows: C: 0.15% to 1.10%, Si: 0.10% to 0.50%, Mn: 0.20% to 1. 20%, P: 0.025% or less, S: 0.010% or less, Cr: 1.30 to 1.60%, Mo: 0.08% or less, sol. Al ≦ 0.045%, Ti: 0.0030% or less, T.I. O (total oxygen content in molten steel): 0.0010% or less, remaining Fe and impurities.

As for this molten steel component, all of C content, Si content, and Mn content are Ti ≦ 0.0030% and T.I. It is a component for obtaining O ≦ 0.0010% bearing steel and similar clean steel.
Moreover, the temperature of said molten steel when implementing the manufacturing method which concerns on this invention shall be 1480-1650 degreeC. If the molten steel temperature is lower than 1480 ° C, the slag will not hatch, and if it is higher than 1650 ° C, the refractory melts severely.

In addition, it is suitable for the slag line of the pot refractory to use to use the magnesia carbon-type brick in consideration of corrosion resistance.
FIG. 1 shows the influence of (Na ₂ O + K ₂ O) mass% and initial stirring force on the initial hatching time of the added refining agent under the above-described slag component conditions.

Here, as an additive refining agent, calcium aluminate-based flux (CaO: 57%, Al ₂ O ₃ : 35%) was added at 3.0 to 25.0 kg / t, and then sodium silicate (Na ₂ O: 24%). , SiO ₂ : 74%) was added in an amount of 0.03 to 0.25 kg / t.

  The quality of the initial hatching was determined by conducting the current heating with the electrode immediately after the refining agent was added, and the time required until a temperature increase rate of 2.5 ± 3.5 ° C./min or more was obtained. It was defined as “time required for crystallization”, and the case where the time was 10 minutes or less was determined as “initial hatching good”.

This “time required for initial hatching” is based on the idea that “the time during which the flux is melted and becomes slag and becomes fluid and uniform at the molten steel interface”.
It was confirmed that initial hatching is promoted when the total of Na ₂ O and K ₂ O is 0.05 mass% or more and ε ≧ 30 kW / t is set for 10 to 11 minutes after the start of treatment. . (Figure 1)

(1) Experimental conditions The molten steel weight in the ladle is about 70 t, the molten steel temperature is 1500-1600 ° C., the specifications are C: 0.50% -1.00%, Si: 0.15% -0.45%, Mn: 0 30% to 0.80%, P ≦ 0.025%, S: 0.010% or less, Cr: 1.30 to 1.60%, Mo: 0.08% or less, sol. Al: 0.045% or less, Ti: 0.0030%, T.I. O: 0.0010 or less, the remaining Fe and clean steel as impurities were treated in a ladle refining equipment (VAD). When a flux not containing fluorine is used, the hatchability deteriorates. Therefore, after adding 3.0 to 25.0 kg / t of calcium aluminate flux (CaO: 57%, Al ₂ O ₃ : 35%), silicic acid Soda (Na ₂ O: 24%, SiO ₂ : 74%) was added in an amount of 0.03 to 0.25 kg / t.
VAD ladle refining conditions were within the above-mentioned basic processing conditions.

(2) Experimental results Table 1 shows the slag composition after VAD treatment.

Here, regarding a comparative example, it is a slag composition at the time of using the flux containing a fluorine, Comprising: A fluorine concentration is 7.5%. Further, Na ₂ O or / and K ₂ O are not added.

  Table 2 shows the results of ladle refining.

  Here, the desulfurization rate (%) was calculated by ((treatment start [% S] concentration−treatment end [% S] concentration) / treatment start [% S] concentration × 100).

The cleanliness was measured by T. after casting with a bloom continuous caster and rolling into billets. Evaluation was carried out with O concentration.
Inclusions are non-metallic inclusion evaluations in products, and evaluation was performed by the ASTM-A method. Among them, the evaluation was carried out using a spherical or random distribution D system (granular oxide) Heavy which shortens the life.

The viewpoint of comprehensive evaluation is as follows.
○: Good result (F concentration in slag is low, and S, T.O and Ti contents of molten steel (product) after refining are below predetermined range)
×: Bad result

  Comparative Examples 1 to 6 are the results of refining by the conventional method, and since 7.5% of F is contained in the slag after treatment, the slag cannot satisfy the standards defined in the Basic Environment Law.

  On the other hand, in Examples 1 to 8 of the present invention, the flux containing fluorine was not used, so that the standard defined in the Environmental Basic Law can be satisfied. In addition, in the comparative example, S ≦ 0.010% in the product, T.I. Although satisfying the specifications of O ≦ 0.0010% and Ti ≦ 0.0030%, according to this example, S ≦ 0.007% in the product, T.I. It was confirmed that higher specifications of O ≦ 0.0007% and Ti ≦ 0.0020% could be satisfied.

The above results will be described in detail as follows.
When the concentration of Na ₂ O and K ₂ O added to promote hatching was less than 0.05%, hatching was worse than when 0.05% or more and 1.0% or less. . When the concentration of Na ₂ O and K ₂ O was 0.05% or more and 1.0%, hatching was as good as flux containing fluorine.

It has also been clarified that when the amount of Na ₂ CO ₃ and K ₂ CO ₃ added is too large, splash and smoke increase, and it is necessary to suppress the amount to 1% by weight or less.
Since the slag-metal reaction is promoted under the same stirring conditions that improve hatching, the de-S reaction is also good. O also became low.

Also, if often TiO ₂ concentration in the slag, slag TiO ₂ is reduced, tended to Ti value of a product is increased. Therefore, it was confirmed that the Ti content of the molten steel after refining can be managed in a low state by restricting the TiO ₂ concentration in the slag to 1% or less.

It is a graph showing the relationship between the total concentration and the agitation power density between in Na 2 _O and K _{2 O} slag.

Claims (1)

A method of producing clean steel by a ladle refining method in which molten steel and slag are stirred and refined using Ar or N ₂ gas under reduced pressure after steel from a converter or electric furnace,
Without using fluorine-containing flux as a refining agent,
Slag composition after ladle refining, in mass concentration, CaO: 30% ~60%, MgO: 3.0% ~15%, Al 2 O 3: 10% ~30%, (% CaO) / (% SiO ₂ ): 2.0-10. 0, TiO ₂ : 1.0% or less and 0.05 to 1.00% as a total of Na ₂ O and K ₂ O,
The mass content of the elements contained in the product is C: 0.15% to 1.10%, Si: 0.10% to 0.50%, Mn: 0.20% to 1.20%, P ≦ 0.025%, S ≦ 0.010%, Cr: 1.30 to 1.60%, Mo ≦ 0.08%, sol.Al ≦ 0.045%, Ti ≦ 0.0030%, T.I. A manufacturing method of clean steel which is O ≦ 0.0010%, balance Fe and impurities.

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