JPH027363B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for dephosphorizing chromium-containing steel, particularly various low alloy steels containing 3.0% or more of Cr, heat-resistant steels, stainless steels, etc. [Prior Art] Phosphorus (P) in steel is generally a harmful element to the quality of steel. In particular, in austenitic stainless steel, it is desirable to keep the P content as low as possible because it increases the susceptibility to stress corrosion cracking, hot cracking, etc. Removal of P during steel refining is carried out during oxidation refining.
This is generally carried out by an oxidative dephosphorization method in which P is oxidized to P ₂ O ₅ and this is fixed in slag as 4CaO P ₂ O ₅ , but chromium-containing steel, especially containing 3.0% or more Cr In the case of steel, when the oxidative dephosphorization method is applied, preferential oxidation of Cr occurs, so it is practically impossible to reduce P in the steel to a predetermined level. As a countermeasure to this problem, methods have been taken so far to carefully select raw materials with low P content and to melt and refine steel using low-phosphorus refractories for the furnace wall lining. [Problem to be solved] However, raw materials with low phosphorus levels have been selected and used as chromium sources, and due to repeated use as rotating materials, low phosphorus level raw materials have been used. Procuring raw materials has become extremely difficult. It should be noted that ordinary steel scraps cannot be blended in such a large amount because they are accompanied by significant segregation of phosphorus. Recently, instead of the oxidative dephosphorization method described above, research has been progressing on reductive dephosphorization using the reaction shown by 3Ca ⁺⁺ + 2P ^--- →Ca ₃ P ₂ , _and
A method has been proposed in which a flux containing calcium fluoride or CaF ₂ (calcium fluoride) is used as a dephosphorizing agent. However, there are problems with reaction efficiency, etc., and sufficient results have not yet been achieved in actual production. In view of the above, the present invention aims to improve the reaction efficiency in reductive dephosphorization of Cr-containing steel and to guarantee the quality of the steel with a low phosphorus level. [Technical means and action] The method for dephosphorizing chromium-containing steel according to the present invention
Chromium-containing molten steel containing 3.0% or more of CaC ₂ (calcium carbide) with a C concentration of 0.5 to 3.0% and a Si concentration of 0.5% or less ), 0.2 to 3.0% CaO (lime), and 0.03 to 0.3% NaF (soda fluoride),
The method is characterized in that reductive dephosphorization is performed by administering a flux consisting of 0.05 to 0.5% Na ₃ AlF ₆ (cryolite). In the method of the present invention, CaC ₂ directly participates in the dephosphorization reaction as shown by the following formula: 3CaC ₂ +2P→Ca ₃ P ₂ +6C. To perform sufficient dephosphorization, at least
Requires an amount of 0.5%. The reason why the upper limit is set to 5.0% is that if the content is more than 5.0%, the smooth progress of the reaction will be hindered as the steel bath temperature decreases. In order to promote the above dephosphorization reaction, it is necessary to impart ease of slagging to the flux and form a highly fluid slag. For this purpose, in the present invention, CaO is used as a sludge forming agent, and CaO is used as a slagging accelerator.
Combined use of NaF and Na ₃ AlF ₆ . The amount of these substances necessary to carry out the reaction efficiently is 0.2 to 3% of CaO and 0.2 to 3% of NaF based on the amount of molten steel to be treated.
0.03-0.3% _{, Na3AlF6} 0.05-0.5%. That is, CaO needs to be at least 0.2% to form a suitable amount of slag with suitable basicity, and NaF needs to be at least 0.03% to form a slag with rapid slag and sufficient fluidity. ,and
Na ₃ AlF ₆ is required at 0.05% or more. On the other hand, setting the upper limit of CaO to 3% and the upper limits of NaF and Na ₃ AlF ₆ to 0.3% and 0.5%, respectively, not only does not require addition of amounts exceeding these, but also increases the This is because the lowering of the steel bath temperature and the associated lowering of the fluidity of the slag will actually hinder the smooth progress of the dephosphorization reaction. The flux used in the present invention is CaC ₂ :
The ratio of CaO: _NaF : _Na3AlF6 is 20-40:5-15:
1-5:1-5, preferably 25-35:5-10:1
It is prepared by blending at a ratio of ~3:2 to 5. Although CaF ₂ is generally used as a desaturation accelerator in conventional fluxes of this type, the present invention eliminates the use of CaF ₂ and uses NaF as described above. This avoids significant erosion of the furnace wall refractories as is the case when _CaF2 is used. In addition, as a slag accelerator, NaF
Although it is possible to form a slag using only one of Na ₃ AlF ₆ and Na 3 AlF 6 , the present invention particularly requires the combined use of both as described above. This results in the formation of a more reactive slag than when used alone. In particular, in conventional actual operations, flux powder tends to aggregate and form clumps in the furnace, making it difficult to form a sufficient slag.
In the present invention, in which a certain amount of Na ₃ AlF ₆ is used in combination,
There is no such problem, and even if some flux powder remains, it takes the form of dispersion as small particles.
Therefore, compared to the case where agglomeration and agglomeration occur, the contact area of the molten steel-slag interface becomes significantly larger, and a correspondingly higher dephosphorization reaction rate can be obtained. In the present invention, the molten steel to be treated containing 3.0% or more of Cr preferably has a C concentration of 0.5 to 3.0% and a Si concentration of
Each is adjusted to 0.5% or less. C concentration 0.5%
The reason for this is that if it is lower than that, it will be difficult to ensure sufficient fluidity of the steel bath due to the temperature drop that accompanies the administration of flux due to the relationship with the melting point of molten steel. In addition, in the Fe-C-P system, the C concentration is 0.5% or more (0.5 to 3.0%) for the progress of the dephosphorization reaction.
%). On the other hand, setting the upper limit of Si concentration to 0.5% means that if it exceeds
Flux due to Si oxidation products (SiO ₂ )
This is because the consumption of CaC ₂ and the resulting decrease in dephosphorization efficiency become significant. The dephosphorization treatment of molten steel according to the present invention is preferably carried out under strong stirring action such as gas bubbling stirring or impeller stirring. For example, in an argon-oxygen decarburization blowing furnace (AOD furnace), flux is administered and Ar
By carrying out stirring by gas blowing, extremely good dephosphorization treatment can be achieved, with a dephosphorization rate of 75% or more, reaching around 80%. It goes without saying that after the dephosphorization process is completed, the slag should be sufficiently removed to prevent phosphorus from returning to the steel bath before proceeding to the next refining process. [Example] Table 1 shows the results of a dephosphorization test conducted in the refining of 18-8 stainless steel in an AOD furnace. The processing conditions are as follows. (1) Amount of molten steel to be treated: 5.8 to 11.0 tons/charge (2) Molten steel before treatment: C: 0.5 to 3.0%, Si: 0.5% or less, Steel melting temperature:
1500-1570℃ (3) Flux: CaC ₂ 1.5-5%, CaO 0.5-3 relative to the amount of molten steel
%, NaF 0.05-0.2%, _{Na3AlF6 0.05-0.37} %. (4) Dephosphorization treatment: Performed for about 5 to 8 minutes under bubbling stirring at 25 to 40 Nm ³ /min of Ar. (5) After dephosphorization treatment: After almost complete removal of slag, decarburization and prescribed refining are carried out using conventional methods. As a comparative example, instead of the above flux,
Flux consisting of CaC ₂ , CaO and NaF (based on the amount of molten steel, CaC ₂ 0.5-5.0%, CaO 0.2-3.0%,
Dephosphorization was carried out under the same conditions as above except that NaF (0.1-1.0%) was used. The result is the first
Also listed in the table. As shown in Table 1, the dephosphorization rate in the comparative example is
In contrast, the dephosphorization rate in the examples of the present invention was as high as 75% or more and around 80%, which shows that the dephosphorization effect of the present invention is excellent.

【table】

〔Effect of the invention〕

According to the method of the present invention, even if the phosphorus content of the raw material is high, steel with a sufficiently low phosphorus level can be produced due to the excellent dephosphorization effect. Therefore, it is possible to deal with the raw material situation, especially the increasing trend of phosphorus content in rotating materials, and it can also be used to greatly improve the quality of materials for applications that require high quality, such as nuclear power piping materials and other various equipment and equipment materials. Contribute.

Claims (1)

[Claims] 1 Contains 3.0% or more Cr, C concentration is 0.5-3.0%,
Chromium-containing molten steel with Si concentration adjusted to 0.5% or less is added with 0.5 to 5.0% of CaC ₂ and 0.2 to 3.0% of the amount of molten steel.
CaO, 0.03-0.3% NaF, 0.05-0.5%
A method for dephosphorizing chromium-containing steel, which comprises administering a flux consisting of Na ₃ AlF ₆ to cause a reductive dephosphorization reaction.