JPS60194009A - Method for refining stainless steel - Google Patents

Abstract

PURPOSE:To maintain the decarburization of molten crude stainless steel at a high speed and to shorten the refining time by blowing a gaseous mixture of oxygen with an inert gas on the surface of the bath from a top blowing lance and by blowing a gaseous mixture of a hydrocarbon fluid with an inert gas into the bath from the gap between the inner and outer tubes of a tuyere having a double-tubed structure. CONSTITUTION:In an oxidation stage for gradually removing carbon from molten crude steel in a refining vessel, a gaseous mixture of oxygen with an inert gas as a gas to be blown from the top is blown on the surface of the bath from a top blowing lance. At the same time, a tuyere having a double-tubed structure is used to blow a gas to be blown from the bottom, and a gaseous mixture of a hydrocarbon fluid for cooling the tuyere with an inert gas is blown as a tuyere protecting gas into the bath from the gap between the inner and outer tubes of the tuyere.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The technical content described in this specification in connection with the advantageous refining of stainless steel using a converter, particularly a converter having a double tube wall for bottom blowing, is as follows: The purpose of this paper is to propose development results that can advantageously realize appropriate promotion of decarburization in the oxidation process of the refining process, as well as reducing the consumption of the slag.

(Background Art) Stainless steel refining generally uses a refining vessel such as a so-called AOD furnace or, more recently, a bottom blowing converter. For example, the carbon in the crude molten steel is gradually oxidized by blowing a refining gas, such as a mixture of oxygen and an inert gas, into the bath through the tuyere.

That is, the main raw materials such as scrap and iron alloy are melted in advance in a melting furnace such as an electric furnace, and the resulting crude molten stainless steel is charged into a bottom-blowing converter or AOD furnace, where it is mixed with 02 gas or 02 gas. Oxidation refining is performed by so-called bottom blowing, in which a mixture with an active gas (Ar gas or N2 gas) is blown into a bath of crude molten steel as a refining gas, and carbon is mainly oxidized and decarburized.

However, in this oxidation refining process, not only carbon but also C
r becomes 6M, and the oxidation of Cr becomes more significant, especially as the carbonic acid becomes lower.

Therefore, in order to suppress the oxidation loss of Cr, the ratio of inert gas in the refining gas is gradually increased as the carbon concentration in the molten steel decreases, thereby lowering the 00 partial pressure in the furnace and lowering the equilibrium carbon concentration. , preferential decarbonization is being carried out.

However, even if we try to suppress the oxidation of C in this way, at the end of the oxidation process, about 2 to 3% of C in the molten steel remains.
The existence of oxidation loss is unavoidable, and therefore, following the oxidation process, the loss of Cr1l in the molten steel,
That is, it is usual to carry out a reduction step in which chromium oxide that has migrated into slag is reduced into molten steel.

The conventional stainless steel refining method described above requires the addition of a τ reduction refining process in addition to diluting the injected refining gas as a mixture for oxidation refining, so the refining time is longer than that of ordinary steel. As a result, it becomes difficult to synchronize with continuous casting in the subsequent process, especially adapting an efficient process such as continuous casting, and this poses a problem in terms of high production efficiency.

<Conventional development efforts> Regarding rapid refining of stainless steel,
8-130216, in addition to blowing a mixture of oxygen and nitrogen or oxygen and argon into the steel bath as a refining gas through the bottom blowing slats provided below the bath surface, 02 and A using the installed top blow lance
``Alternatively, it has been proposed to spray a mixture of 02 and N2 onto the surface of the steel bath for refining.

According to this method, the time required for oxidation refining of stainless steel can be shortened by the amount of oxygen supplied from the top blowing lance. However, in this case, even though the refining time may be shortened, there is little contribution to lowering the erosion rate of the bottom blown surface, which is important in stainless steel refining operations. . This is because the inert gas mixed into the bottom blowing gas does not sufficiently cool the tuyere against the oxidation exothermic reaction near the tuyere tip. Of course, if the inert gas is blown into a larger flow port than the oxygen, it is possible to achieve a noticeable cooling effect due to the inert gas, but in this case, the flow rate m of the oxygen gas to the inert gas is 1-1. , C becomes significantly smaller than the flow rate ratio of 5/1 to 1/4, which is suitable for ordinary steel refining, and cannot be put to practical use.

On the other hand, a converter for mass production of ordinary steel, a pure oxygen bottom-blown converter (Q-BOP), is known, which uses a so-called double-tube tuyere as a bottom-blown tuyere to ensure sufficient cooling of the bottom-blown tuyere. ing. This converter uses a concentric double-tube structure as a bottom blower, and pure oxygen and powders such as CaO are blown into the inner tube of the bottom blower, and carbonization such as propane is blown into the gap between the inner and outer tubes. A hydrogen-based fluid is supplied and the ends of the slats are cooled by an endothermic reaction caused by thermal decomposition of the hydrocarbons.

In this double pipe siding, the powder supplied from the inner tube serves as a slag-forming agent, and the sensible heat of the powder contributes to cooling the siding. However, the double pipe structure in Q-BOP cannot be directly applied to stainless steel refining. This is because when a large amount of hydrocarbon fluid is injected into molten steel, carbon content remains in the molten steel bath due to the thermal decomposition reaction, and the molten steel is carburized, which significantly impairs the decarburization efficiency in oxidation refining. This is because refining itself becomes impossible.

(Objective of the invention) Smelting of stainless steel that makes it possible to shorten the refining time while maintaining a high decarburization rate of crude molten stainless steel up to low carbon dioxide levels, as well as to reduce melting damage of the double tube tuyeres. It is an object of this invention to provide a method.

(Structure of the Invention) This invention uses a refining vessel that houses crude molten stainless steel and is equipped with a tuyere that is opened below the surface of the molten steel bath. During the oxidation process in which carbon in the crude molten steel is gradually removed when stainless steel is blown into the smelting vessel, oxygen and inert gas are blown into the smelting vessel through a top blowing lance attached to the smelting vessel. A top-blown gas consisting of a mixture of gases is blown onto the bath surface, and a concentric double-pipe structure is used to blow bottom-blown gas, and a hydrocarbon system for cooling the siding is used through the gap between the inner and outer tubes. This is a stainless steel melting method characterized by blowing a siding protective gas consisting of a mixture of a fluid and an inert gas.

In this invention, a mixture of 02 and Ar or/and N2 gas is also blown onto the surface of the molten steel bath from the top blowing lance.
A mixture of hydrocarbon fluid and inert gas is blown into the gap between the inner and outer tubes of the double-tube tuyere as a cooling gas for molten steel, without raising the partial pressure. In addition, appropriate refining is carried out while minimizing erosion of the double pipe lining and carburization of the molten steel.

First, the mixture ratio of the top-blown refining gas may be constant during the oxidation refining process, but in order to perform more efficient scouring, the mixture ratio of the bottom-blowing refining gas should be similarly adjusted. It is better to change it as the skill progresses.

For example, [%C]≧0.6, 02/N2 = 5/1~
4/1.0.6> (%Q) ≧0.25 Teha,
Oz/Nz'-2/1.0.25 > (%C)≧o,
Change it as follows: o4 Niattach, 02/N2Φ1/2.

When blowing the mixture onto the surface of molten steel, it is possible to make a wider range of changes than by blowing top-blown pure oxygen gas as in the conventional LD method. These include the timing of changes in the air ratio, the air mixture flow opening, and the lance height.

If these are appropriately controlled, the refining time can be significantly shortened compared to the conventional technology without increasing the oxidation loss of Cr at the final stage of oxidation refining.

Next, regarding the bottom blow stirring effect itself, it occurs in LD because (T, Fe!>111 degrees in the slag in Q-BOP is closer to the equilibrium between slag and metal.
e) This is clear from the well-known fact that ai is much lower than 1 degree.

In this invention, a mixture of oxygen and an inert gas is blown onto the surface of the steel bath as a top blower, and the inert gas behind it lowers the gas partial pressure of CO produced by the reaction between oxygen and carbon in molten steel. Oxidation refining, which suppresses the combustion of Cr and increases decarburization efficiency, becomes effective only when the molten steel is strongly stirred by blowing refining gas into the bath.

Regarding the inlet of the hydrocarbon fluid to be mixed with this scouring gas, the optimum amount exists within a range that can minimize the decarburization rate and minimize the tuyere melting rate. .

Figure 1 shows the melt volume of 5 to 5.5 tons in a converter, initial c.
[%C) = 0.07-0.0 when crude molten stainless steel with m degree 1.3-1.5% and Qr 11 degree 16-16.5% is decarburized to 0.04-0.05%. These are the results of investigating the relationship between the average decarburization rate, the tuyere melting rate, and the ratio of propane flow m to the blown O2 flow rate.

In this case, four tuyeres of a double-tube structure with an inner diameter of 8mm, an inner wall thickness of 2mm, and a gap of 0.8n between the inner and outer tubes were arranged at the bottom of the hearth. The bottom-blown total gas flow rate of the inner tube is 5Nyn3/1n and 1ONm'7mrn throughout the entire blowing, and the top-blown total gas flow rate is 11;iha5 N m3/ll1inF.
It was set as constant. The cooling gas supplied from the gap between the inner tube and the outer tube was a mixture of A and propane, and the total gas flow rate was 6% of the oxygen gas flow rate from the inner tube.

Here, the mixed gas ratio of oxygen and Ar, 02/Ar, is the same for both top and bottom blowing, and is 02/Ar -4/ 1 ((%C)
≧0.6), 2/1 (0,25≦(%C)≦0.6
), 1/2 ((%C)≦0.25).

It is obvious that it is better to suppress the propane ratio as much as possible in order to improve the decarburization rate, and to increase the propane ratio in order to reduce the tuyere erosion rate, but looking at Figure 1, , as shown by the arrow, the optimum propane ratio that does not significantly reduce the decarburization rate and suppresses the amount of tuyere erosion is 0.5.
It can be seen that it exists in the range of ~2%.

That is, when blowing a mixed gas of oxygen and an inert gas as its diluent gas using a top blowing lance and a double pipe siding as in this test example, the propane gas for cooling the siding has a flow rate ratio of 0. It has been found that it is particularly economical to set the content to 5 to 2%.

(Example) Examples of the present invention will be shown below in comparison with comparative examples.

5 tons of molten stainless steel crude molten steel was charged into a top and bottom blowing converter in advance in an electric furnace.

Four 2-inch pipes with an inner diameter of 12 m1φ are used as the bottom-blowing surface, and the bottom-blowing mixture (02+Ar>I) from the inner pipes is
ll is 8NmlB/Win, and from the gap between the outer tube and the inner tube, the total mixture of propane and Ar is 0.2Nn.
The flow rate was kept constant at ff/5in.

Further, as the top blowing lance, one with three holes and each nozzle having a throat diameter of 9 miφ was used.

The composition of crude molten steel is [%C) = 1.1~1.2° (%S
i) −0.5 to 0.55, (%Mn)=0.4
~0.5. (%P) = 0.020-0,025
．． (%5) -0.015 to 0.020, and the same composition was selected as much as possible.

The initial temperature was 1540-1560°C.

Massive CaO for basicity adjustment was introduced at the time of 02/Ar-571.

After oxidation refining, reduction refining was performed to adjust the slag composition after reduction to (%Ca0)/(%Si02)÷1.1. Moreover, the tapping temperature was also adjusted to 1650 to 1610°C.

Examples and comparative examples are shown in Table 1.

In Examples ■ to ■, the decrease in Cr concentration at the final stage of oxidation is almost the same or lower than that in Comparative Example ■ in which bottom blowing was performed alone.
There was almost no change in fl, and the refining time during the oxidation stage could be significantly shortened to half or less.

On the other hand, a comparative example using only pure oxygen gas for top blowing■,
In case (2), there is a lot of Or combustion in the final stage of oxidation, and as a result, the refining time during the oxidation stage cannot be shortened to the extent that the present invention can achieve, compared to Comparative Example 5, which was conducted during bottom blowing alone.

In Comparative Example (2), a mixed gas was used for top blowing and the blowing conditions were almost the same as in Example (2), but since the propane ratio was as high as 2.5%, carburization occurred in the steel bath. The coal efficiency decreases, the amount of 01203m increases, the amount of SiM used to reduce cCr203 also increases, and the refining time also becomes considerably longer than in Example (2).

Comparative Example ■, [Phase] is C when the propane ratio is 0%, but the refining time and amount of Si for reduction are the same as in the example, but the tuyere erosion rate is 2 to 3 times higher than in the example. .

Comparative example ■ is a bottom-blowing injection of CaO.
This is the case when 0 kg/t was carried out at the time of 02/Ar = 4/1 and 2/1, where the oxidation loss of chromium is large, and therefore the amount of reduced Si and the refining time also increase.

(Effects of the Invention) As can be seen from the above Examples and Comparative Examples, the method according to the present invention enables rapid stainless steel refining while advantageously avoiding the melting loss of the lining.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of the proportion of propane mixed in the protective gas in the siding on the decarburization rate and tuyere melting loss.

Claims (1)

[Claims] 1. Using a refining vessel containing crude molten stainless steel and having pores formed below the surface of the molten steel bath, a bottom-blown gas consisting of oxygen and an inert gas is blown from the tuyeres into the bath. When blowing stainless steel in this refining vessel, oxygen and nitrogen are blown into the refining vessel through a top blowing lance attached to the refining vessel during the oxidation process that gradually removes the bulk elements in the compatible steel. A top-blown gas consisting of a mixture of active gases is blown onto the bath surface, and a concentric double-pipe structure is used to blow bottom-blown gas into the tuyere cooling through the gap between the inner and outer tubes. A stainless steel melting method characterized by blowing a siding protective gas consisting of a mixture of a hydrocarbon fluid and an inert gas. 2. The ratio of the hydrocarbon fluid to the M element in the bottom blowing gas is 0. The method according to claim 1, wherein the content is adjusted to 5 to 2%.