JPH0621287B2 - Method for dephosphorizing molten steel containing chromium - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for dephosphorizing molten steel containing chromium.

(Prior Art) Generally, phosphorus (hereinafter referred to as [P]) in a low alloy steel containing stainless steel or chromium is known to be a harmful impurity which adversely affects the brittleness and stress corrosion cracking resistance of the steel. . For this reason, in recent years, in the fields of materials for nuclear power generation, materials for reaction vessels, and the like, the number of steel types that strictly regulate [P] in steel is increasing.

However, it is well known that lowering [P] in the production of chrome steel is more difficult than lowering [P] in the production of ordinary steel. As a reason for this, the oxidation refining method used for dephosphorization of ordinary iron alloys,
In other words, a method of adding oxygen such as quicklime and CaO-FeO
In the method of adding a system flux, the affinity of chromium and oxygen is stronger than the affinity of phosphorus and oxygen, so chromium is preferentially oxidized and the generated Cr ₂ O ₃ migrates to the slag side and the slag hardens. As a result, the oxidation and fixation of phosphorus can be inhibited. In the production of low-phosphorus chromium steel, it has been generally accepted that after removing [P] as much as possible in a molten iron pretreatment or in a converter, an expensive low-phosphorus iron alloy is used.

In contrast to this method, a method of dephosphorization using Ca-CaF ₂ flux in the electroslag remelting method for the purpose of efficiently removing [P] in the molten iron alloy containing chromium (Japan Institute of Metals) Bulletin, 15 (1976), P.387),
A method of dephosphorization using a CaC ₂ -CaF ₂ system flux in a reaction vessel such as a ladle (Iron and Steel, 63 (1977), P.2287) has been attempted. These methods perform dephosphorization with Ca, and as the dephosphorization reaction, P represented by 3Ca + 2P → Ca ₂ P ₂ is reduced and removed, which is an advantage that chromium oxidation is suppressed. Have.

However, in the above reaction, Ca ₃ P ₂ exists in the slag after the treatment, and when it contacts the atmosphere, it reacts with H ₂ O, and the toxic phosphine (as shown by the formula Ca ₃ P ₂ + 3H ₂ O → 3CaO + 2PH ₃ Since PH ₃ ) is generated, an industrial problem remains in the slag treatment after dephosphorization.

In order to avoid the disadvantages caused by the dephosphorization method by reduction, as a method of oxidizing and removing phosphorus, a method of adding BaCO ₃ -BaCl ₂ based flux (iron and steel, 68 (1982), S971) and Li
₂ -CO ₃ added CaO-CaF ₂ -FeO system method of adding flux (Iron and Steel, 69 (1983), S878) and the like were also attempted. These have been tested on an industrial scale for hot metal with a carbon content of 1 to 4%, and they can be dephosphorized by reducing the oxidation of chromium, and also have a special dephosphorization method. It was confirmed that this is an excellent method that does not require slag treatment.

However, in these methods, the carbon concentration in the molten iron has a great influence on the dephosphorization efficiency, and if [C] is too high, the oxygen partial pressure of the system decreases, and if [C] is too low, the distribution ratio of chromium increases. Alternatively, it has been reported that dephosphorization is disadvantageous because the activity a _P of phosphorus in steel decreases. (Iron and Steel, 69 (1983), S
186) (Problems to be Solved by the Invention) The present invention is directed to dephosphorization of molten steel containing chromium in order to provide a chromium steel with low phosphorus in which the above-mentioned disadvantages of the conventional method are eliminated. According to the present invention, for example, after decarburizing to a target [C] concentration in a converter, an electric furnace or the like, or after refining in a converter, an electric furnace or the like, and after performing secondary refining such as vacuum degassing treatment, It is possible to remove only phosphorus without causing oxidation of chromium in molten steel containing 0.5 to 10% of [Cr].

(Means for Solving Problems) The object of the present invention is to provide C: 0.4% or less after refining, P: 0.
1% or less, more than 1500 containing Cr: 0.5-10%, 1700
This is achieved by a dephosphorization method for molten steel containing chromium, characterized in that a flux comprising CaO and a halide of an alkali metal or an alkaline earth metal and chromium oxide is added to the molten steel at a temperature of not higher than 0 ° C.

That is, the present inventors did not proceed with the oxidation reaction of chromium in molten steel containing chromium having a low [C] concentration, and in order to cause the oxidation reaction of phosphorus, the oxygen concentration determined equilibrium from the chromium concentration in molten iron was determined. It was confirmed that the oxidation of phosphorus occurs in the oxygen region where the oxidation reaction of chromium does not occur, that is, the oxidation reaction of chromium does not occur, and that chromium oxide in the flux is effective as such an oxidation source. It is desirable to use pure Cr ₂ O ₃ as the chromium oxide, but P or S
There is no problem even if a chromium ore with a low content of is used.

It was also confirmed that an inexpensive CaO system can sufficiently achieve the purpose without using the conventionally reported expensive BaO system or Li ₂ -CO ₃ system as a fixing agent for fixing the oxidized phosphorus. It was However CaO -Cr In ₂ O ₃ based flux is important to use a medium solvent for promoting slag formation for unexpected can more than enough exhibits its effect to a fixed phosphorus melting point of the slag is higher oxidized is there. As the medium solvent, CaF ₂ , CaCl ₂ , Ba
Examples include halides of alkali metals or alkaline earth metals such as Cl ₂ and NaF. Among them, CaF ₂ which is a compound of the same family as CaO and has a large effect of lowering the melting point of slag
Etc. are preferred. However, the present invention is not limited to this, and one or more compounds may be used in combination without any problem. These solvent media not only promote the above-mentioned slag slag formation, but also reduce the activity of P ₂ O _{5 in} the slag and also reduce the value of phosphorus that can be reached equilibrium.

The above components are blended to form a dephosphorization flux. As mentioned above, CaO contributes to the fixation of phosphorus, chromium oxide contributes to the oxidation of phosphorus, and the solvent solvent contributes to the promotion of slag slag. Therefore, as a weight ratio, 20-50% CaO-10-40%
Cr ₂ O ₃ -10 to 70% solvent solvent is suitable. These may be used simply in a mixed state, but it is preferable to sinter or melt them in advance to synthesize them, because slag formation will proceed rapidly.

The amount of flux added to molten steel is determined by the required amount of dephosphorization, that is, the difference between the target phosphorus concentration and the initial phosphorus concentration, but in practice it is used within 100 kg per ton of molten steel due to operational problems. Good to do.

Regarding the molten steel composition for dephosphorization, the carbon concentration is 0.4.
If it is more than%, the oxygen concentration in the steel is dominated by the carbon concentration, and the oxidation of carbon is prioritized over the oxidation of phosphorus.
The carbon concentration is 0.4% or less.

Also, regarding the temperature during dephosphorization treatment, considering the molten steel composition, at 1500 ° C or lower, the treatment cannot be performed due to the temperature drop due to slag formation of the added flux and heat dissipation during the treatment. Therefore, the lower limit of the dephosphorization treatment temperature is 1500 ° C or more. On the other hand, as one of the features of the present invention, since the oxygen concentration in the molten steel is determined equilibrium from the chromium concentration in the steel,
The higher the molten steel temperature with respect to the same chromium concentration, the higher the oxygen concentration, and the lower the phosphorus concentration that can be reached after the dephosphorization treatment. Considering that slag slagging property is also advantageous at high temperature, it does not cause work problems such as melting of refractory materials.
It is preferable to keep the temperature as high as possible in the range of 1700 ° C or lower.

FIG. 1 shows the component behavior during dephosphorization according to the present invention described above. In the figure, ○ and △ are phosphorus and chromium in steel at 1600 ℃, ● and ▲
Shows the concentrations of phosphorus and chromium in steel at 1700 ° C, respectively. As is clear from this figure, the concentration of chromium does not decrease at all with the passage of time, while the dephosphorization surely proceeds. It is clear. Further, the concentration of phosphorus reached at a higher temperature is lowered, that is, a higher dephosphorization rate is obtained.

From the viewpoint of increasing the dephosphorization reaction rate, it is, of course, preferable that the molten steel and the flux are agitated after the addition of the flux to increase the reaction interfacial area and promote the mass transfer that is the rate-determining reaction. .

(Example) Next, an example according to the present invention will be described.

Example 1 500 g of molten steel having the composition shown in Table 1 was melted in a MgO crucible using a high-frequency melting furnace and kept at 1600 ° C.
The _{0% CaO -20% Cr 2 O} 3 -20% CaF 2 -20% flux mixture 40g consisting of CaCl ₂ collectively added, was allowed to react for 20 minutes to obtain a such experimental results are also shown in Table 1.

From the results in the above table, it is clear that 90% dephosphorization proceeded without oxidation of chromium before and after the treatment.

Example 2 200 kg of molten steel having the composition shown in Table 2 was melted using a high-frequency melting furnace and kept at 1600 ° C., then 40% CaO-30
10 kg of the burning flux composed of% Cr ₂ O ₃ -30% CaF ₂ was added all at once, and the treatment was performed for 15 minutes while stirring by blowing Ar gas through a lance of Al ₂ O ₃ quality. It is also shown in the table.

As shown in the above table, 80% dephosphorization proceeded in a short time without change in chromium concentration before and after the treatment.

Example 3 500 g of molten steel having the composition shown in Table 3 was melted in a MgO crucible using a Tammann furnace, and the temperature was maintained at 1700 ° C. and then 50%.
The _{CaO -25% Cr 2 O 3 -25} % flux mixture 50g consisting of CaF ₂ was obtained such results are shown in same Table 3 was allowed to added to react for 20 minutes in bulk.

As shown in the table above, the flux
While Cr ₂ O ₃ was reduced and returned to the molten steel side, the chromium concentration increased and the dephosphorization of 83% proceeded.

Example 4 Example 4 will be described while showing the steps in FIG. After hot-melted hot metal which has been subjected to hot metal pretreatment and de-sulphurization and de-sulfurization is blown with oxygen in a converter 1, a chromium alloy having a high phosphorus content is added at the time of tapping to obtain the composition shown in Table 4. With respect to the molten steel 3 (150 t) delivered to the ladle 2, CaO 40% -Cr ₂ O ₃ 20% -CaF ₂ 30% -CaCl ₂ 1
Flux 4 (2000 kg) consisting of 0% was charged from above, Ar gas 5 was blown from the bottom of the ladle, and electrode heating 6 was performed from above, and treatment was performed at 1600 ° C for 30 minutes. The results shown in the table were obtained.

As shown in the above table, the chromium concentration in the steel increased due to the reduction of Cr ₂ O ₃ in the flux before and after the treatment, and 89% of dephosphorization proceeded. After that, the dephosphorization slag was completely discharged, and desulfurization and degassing steps were performed, whereby a low-phosphorus chromium steel could be stably produced.

Comparative Example 1 In the same process as in Example 4, for the molten steel 150t having the composition shown in Table 5, CaO4 used for conventional dephosphorization was used.
_{0% -CaF 2 20% -Fe 2} O consisting of ₃ to 40% flux 2
Add 000kg, and then perform the same dephosphorization treatment at 1600 ℃ for 30
When the test was conducted for a minute, the same results as shown in Table 5 were obtained.

As shown in the table above, when iron oxide Fe ₂ O ₃ is used as an oxidation source, chromium is oxidized preferentially over phosphorus, resulting in a 82% chromium loss, and the oxidized chromium migrates to the slag side, resulting in high melting. Only 38% of dephosphorization occurred because it became slag.

Even when Haikasu dephosphorization slag Thereafter, because of the high melting point slag above, slag will be solidified, it can not be carried out completely Haikasu this result after the desulfurization step, slag P ₂ O ₅
Phosphorus is reduced to molten steel, and a chromium alloy must be added to reach the target composition. Phosphorus also migrates to molten steel from this alloy, and the target phosphorus level cannot be achieved. Yield has dropped significantly.

(Effects of the Invention) As described above, according to the present invention, the molten steel containing chromium does not require complicated steps such as the treatment of slag after the dephosphorization treatment as in the conventional reduction dephosphorization method, and As is clear from comparison between Comparative Example 4 and Comparative Example 1, dephosphorization can be performed efficiently and reliably using an inexpensive flux without affecting the chromium content in the oxidative dephosphorization method. Combined with a significant improvement in the yield, an extremely low phosphorus chromium steel such as [P] 0.005% or less having excellent properties such as brittleness resistance can be obtained inexpensively and easily, and an industrially effective effect is brought about.

[Brief description of drawings]

FIG. 1 is a diagram showing the behavior of phosphorus and chromium during the dephosphorization treatment according to the present invention, and FIG. 2 is a diagram showing the steps of an example (Example 4) of the present invention. 1: Converter, 2: Ladle (refining vessel), 3: Molten steel, 4: Flux, 5: Ar gas for stirring, 6: Electrode for heating.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Hori 5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Co., Ltd. Inside the Nagoya Works (56) References JP-A-57-32319 (JP, A) JP-A-58-151416 (JP, A)

Claims (1)

[Claims] 1. C: 0.4% or less after refining, P: 0.1%
Below, more than 1500 ℃, containing Cr: 0.5-10%, 1700 ℃
A method for dephosphorizing molten steel containing chromium, which comprises adding a flux comprising CaO and a halide of an alkali metal or an alkaline earth metal and chromium oxide to the molten steel below.