JP3167902B2 - Decarburization refining method for Cr-containing molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decarburizing and refining chromium-containing molten steel. More specifically, the present invention relates to chromium-containing molten steel contained in a smelting vessel such as a converter, an AOD furnace, a ladle or the like. And a technique of spraying or blowing a refining gas such as a mixed gas thereof from above and / or from the bottom of the molten steel.

[0002]

2. Description of the Related Art Generally, in decarburization refining of chromium-containing molten steel,
It is important to carry out the decarburization reaction efficiently by minimizing the oxidation loss of Cr in steel. The reason for this oxidation loss is that the Cr present in the steel greatly reduces the activity of C, so that Cr is oxidized with the progress of decarburization. And this tendency becomes more remarkable as it approaches the end of the decarburization refining, so it is an important technical issue how to reduce the oxidation loss of Cr in the low carbon concentration region. Therefore, in the decarburization refining of chromium-containing molten steel, a method of reducing the supply amount of oxygen gas by diluting the oxygen gas with an inert gas in accordance with the decrease in the carbon concentration in the steel has been adopted.

For example, Japanese Patent Application Laid-Open No. 4-56083 discloses an AO
When oxygen gas and inert gas are blown into the molten stainless steel held in a D furnace, converter, etc. under atmospheric pressure to carry out decarburization blowing, a double-flow lance is used for the upper blowing gas lance. And a gas mixture of inert gas and oxygen gas from the side holes around the center hole. The ratio of oxygen gas amount / inert gas amount in a certain range according to the carbon content [C] in the molten steel. A method of increasing the acid supply rate while suppressing the oxidation loss of Cr by sequentially decreasing the oxidation rate is disclosed.

Japanese Patent Application Laid-Open No. 59-104421 discloses that a protective gas is blown from the outer tube of a double tube tuyere, and a mixed gas of a diluent gas and oxygen gas is blown into molten steel from an inner tube. In the decarburization refining method, only the oxygen gas is blown from the inner tube of the double tube tuyere to the region where the C content in the molten steel is up to 0.7%, and the region where the C content is from 0.7% to 0.25 to 0.05% The ratio of the diluent gas and oxygen gas blown from the inner tube is continuously changed and blown according to the C amount, and the C content is changed from 0.25 to 0.05 to the final [C].
In the region up to the value, a method for decarburizing Cr-containing molten steel, which comprises blowing only a dilution gas from an inner tube, is disclosed.

Further, as an example of blowing an inert gas onto a bath surface from a top blowing lance in a low carbon region, see Japanese Patent Application Laid-Open No.
No. 329818 discloses that a decarburization reaction is carried out by blowing an inert gas and oxygen into a bath of a chromium-containing molten steel housed in an AOD furnace to sufficiently reduce the carbon content in the molten steel. Thereafter, a method of further decarburizing the molten steel by blowing inert gas from above toward the bath surface of the molten steel while continuously blowing inert gas and oxygen or an inert gas is disclosed. I have. In this method, the maximum value of the partial pressure of CO on the bath surface in the conventional AOD method was 6 × 10 ⁻² , but it was halved to 3 × 10 ^−2. Is once decarburized to about 0.01% by weight, and then so-called finish decarburization to a lower carbon concentration.

In addition, as disclosed in JP-A-59-21367, the height of the upper blowing lance is increased in the initial stage of the decarburization refining, and the acid supply from the upper blowing lance is soft blown. , CO gas generated as a decarburization reaction product on the steel bath to accelerate the oxidation reaction is converted to CO ₂ gas, the heating energy generated at that time, of the steel bath produced by the oxygen and inert gas is blown under the bath surface There is also a method in which the temperature of the steel bath is raised by a so-called secondary combustion promoting effect, which is propagated into the steel bath through vigorous stirring, and the decarburization reaction proceeds advantageously. At that time, since the molten steel is exposed to a high-temperature gas atmosphere, the decarburization reaction between carbon and a metal oxide such as a Cr oxide is further promoted.

[0007]

However, any of the above-mentioned prior arts is a method of diluting the supplied oxygen gas with an inert gas at the end of the decarburization refining. To reduce the amount of oxidation loss of Cr by reducing
Blowing is not performed from the viewpoint of promoting agitation of slag-metal from the initial stage of decarburization refining.

Further, as in the technique disclosed in Japanese Patent Application Laid-Open No. 59-21367, it is thermodynamically possible to raise the temperature of molten steel early from the initial stage of decarburization blowing to reduce the initial chromium oxidation loss. However, as a result of raising the temperature in the initial stage, the oxidation of chromium becomes remarkable with the decrease of [C] at the end of decarburization, so that the temperature of the molten steel rises sharply and the wear of refractories becomes remarkable. In addition, there are problems such as the ingots and oxides adhering to the inside of the furnace melting and contaminating molten steel. Further, there was a problem that the pickup of C and S became remarkable at the end of the decarburization which further increased. As described above, in the current decarburization refining method for chromium-containing molten steel, it is a fact that it is impossible to simultaneously suppress the oxidation loss of Cr and adjust the molten steel temperature at the end of refining.

The present invention has been developed in view of the above circumstances. Even if the temperature is raised early from the early stage of decarburization, it is possible to suppress the rise in the temperature of molten steel at the end of decarburization and, consequently, the refractory melting. An object of the present invention is to propose a method for decarburizing and refining chromium-containing molten steel that can effectively suppress the oxidation loss of Cr without causing any risk of loss or contamination of molten steel.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventor has carried out a thorough review of the decarburization refining reaction, and as a result, the chromium oxide transferred to the slag or the floating chromium oxide is converted into carbon in steel. Therefore, it has been found that since the reduction reaction is a large endothermic reaction, it is possible to suppress the increase in the temperature of molten steel at the end of decarburization while suppressing chromium oxidation. The present invention is based on the above findings, and in the last stage of decarburization blowing of chromium-containing molten steel, slag-thamel was actively stirred by blowing an inert gas from a top blowing lance, and oxidation loss was caused in the slag. The intended purpose has been achieved by effectively reducing Cr ₂ O ₃ with carbon in steel.

That is, according to the present invention, when decarburizing and refining molten Cr-containing steel in a vessel having a refining gas top-bottom blowing function, the molten steel having a carbon concentration in the range of 1.0 to 0.1% by weight on a bath surface of the molten steel. In a decarburization refining method for molten Cr-containing steel, the inert gas is blown from an upper blowing lance so that the recess depth of the center of the bath surface satisfies the following equation for at least a part of the period. is there. L / L ₀ ≧ 0.08 --- (1) where, L ₀ : steel bath depth (mm) L = L _h · exp (−0.78 _h / L _h ) (mm) L _h = 63.0 (F / nd) ^2/3 and L when h = 0
Value F: Gas flow rate (Nm ³ / h) h: Lance height (mm) n: Number of nozzle holes d: Nozzle hole diameter (mm)

Further, the present invention is a method for decarburizing and refining chromium-containing molten steel, which comprises performing vacuum decarburizing refining following the above-described decarburizing refining.

In the present invention, the carbon concentration in the steel is 1.0 to 0.1.
Up to 1% by weight, decarburization may be performed by any conventionally known method. The present invention is particularly effective when applied to molten steel in which the range of the carbon concentration in steel is in the range of 0.7 to 0.1% by weight.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, when decarburizing and refining molten Cr-containing steel in a vessel having a refining gas top and bottom blowing function, a bath of the molten steel having a carbon concentration in a range of 1.0 to 0.1% by weight is used. on the surface, at least part of the period, as depth of the depression in the bath surface center portion satisfies a supra equation. Thus blowing inert gas from the top lance, slag in smelting vessel - Since the metal is sufficiently stirred and the generated oxide or slag is violently beaten into the molten steel, Cr ₂ O ₃ in the slag is effectively reduced by the carbon in the molten steel, and as a result, The oxidation loss of Cr from the molten steel is suppressed, and the temperature rise of the molten steel is also suppressed. in this case,
The bottom blown gas may be blown in any one of three patterns of oxygen gas alone, inert gas alone, and mixed gas of oxygen gas and inert gas.

Further, in the present invention, the chromium-containing molten steel decarburized and refined by the above-mentioned method can be subjected to vacuum decarburization and refining using a vacuum degassing apparatus such as VOD, RH or DH.
Here, when nitrogen gas is used as the upper blowing inert gas in the decarburization refining in the preceding step, the dissolved nitrogen concentration when starting vacuum decarburization is higher than in the past , but Nishikawa et al. Kawatetsu Technical Report 25 (1993) 2, p. 79-) shows that RH (with top-blowing acid transfer function) shows an example of vacuum decarburization when dissolved nitrogen is high. Forming of the molten steel is caused by the denitrification reaction, thereby promoting the decarburization reaction. In the present invention, the reaction of Cr in the slag from the molten steel in the slag or the chromium floating after oxidation and the carbon in the steel is represented by the following formula (2). In order to promote this so-called slag-metal reaction, Because it is advantageous to increase the area of the slag-metal reaction interface, the stirring is strengthened with an inert gas blown upward. (Cr ₂ O ₃ ) + 3C = 2Cr + 3CO --- (2)

The contents of the present invention will be supplemented with reference to the drawings, and the reasons for limiting the carbon concentration in steel and the depth of the recess at the center of the bath surface will be described below. FIG. 1 (a) schematically shows a decarburization refining state of a Cr-containing molten steel according to the present invention. In the figure, number 1 is a top blow lance, 2 is slag, 3 is molten steel (metal), 4 is a refining vessel, 5 is a tuyere for bottom blow gas, 6 is top blow gas (inert gas), 7 is slag and This is the flow of molten steel. In the drawing, the bath surface of the molten steel 3 becomes concave due to the blowing of the inert gas 6 from the upper blowing lance 1, and the flow 7 of the slag 2 and the metal 3 is directed downward in the vicinity thereof. The inventors have confirmed by experiments that the purpose of the present invention cannot be achieved unless the stirring energy is at least a certain level in order to drive the generated oxide or slag into the molten steel, and the bath surface shown in FIG. The dent depth L is defined.

By the way, the inventors have found that [% C] = 0.1
In the range of ~ 1.0, when nitrogen gas was used as the top blowing gas, the flow rate was kept constant, and the lance height from the bath surface was changed in various ways, the fluctuation of the Cr oxidation loss during that time occurred. Was found. At this time, since the gas flow rate to be supplied is set to be constant, changing only the lance height hardly changes P _CO (CO partial pressure). However, the fact that the oxidation loss of Cr is reduced by lowering the time lance height, the inventors decarburization effect by the top-blown gas is not due to reduction in P _CO, stirring energy of top-blown gas Was determined to come from.

Further, when the temperature change of the molten steel before and after the blowing of the top-blown inert gas was examined, a temperature drop was observed unlike the conventional case. Therefore, when the heat balance in that section was examined, it was found that the influence of the sensible heat of nitrogen on the temperature drop was small and was mainly due to the decrease in the amount of oxidized Cr.

Here, the stirring energy by the top-blown gas is the same parameter as the recess depth of the steel bath by the top-blown gas. As a result, as shown in FIG. 2, the amount of oxidation loss of Cr could be arranged by the depth L / L ₀ of the depth of the steel bath due to the top blow gas. Figure 2 is shows the relationship between the oxidation loss of recess depth and Cr of the steel bath by the top-blown gas, the oxidation loss of rapidly Cr in L / L ₀ ≧ 0.08 is reduced I understand. At this time, the stirring power density due to the top blowing gas

[Ex.1] is the formula derived by Kato et al. (Iron and Steel 66 (1980)
9, S1307), it is expressed by the following equation (3).

(Equation 1) n: each nozzle hole Q _n: n nitrogen gas flow rate of the nozzle _{^{(Nm 3 / min) d n}} : n nozzle diameter of the nozzle of the lance (m) u _n: gas n nozzles initial velocity (m / s) θ _n: Nozzle inclination W: Weight of molten steel (t) x: Lance height from bath surface (m)

Therefore, the stirring power density [outside 1] is 60
0 W / t-steel or more
According to the equation (1), L / L₀≧ 0.08
Therefore, in the present invention, it is set as the limited condition of the upper blowing gas.
is there. A more preferred range is L / L ₀≧ 0.2. Steel charcoal
The reason for limiting the elemental concentration to 1.0-0.1% by weight is that 1.0%
%, The supplied O_TwoOr C in equation (2)
r_Two O_ThreeBecause there is enough [C] for
Rather, raise the bath temperature and oxidize C preferentially in equilibrium.
By doing so, it is only necessary to reduce Cr oxidation.
is there. On the other hand, in the region of less than 0.1% by weight, the reaction of equation (3)
Because there is not enough [C] necessary to cause
You.

On the other hand, as for the temperature change of the molten steel, the temperature change during the period in which the nitrogen gas was blown upward was measured. As shown in FIG. 3, when the nitrogen gas was blown upward, the bath temperature was lowered. The decline was seen. Examination of the heat balance in this region showed that
It was found that the sensible heat of the N ₂ gas had a small effect and was mainly due to a reduction in the oxidation amount of Cr. In FIG. 3, the horizontal axis is proportional to the upper blowing time, and the numerical value 0 on the vertical axis is based on no temperature change during that time.

As described above, it has been known that raising the temperature early in the early stage of decarburization is advantageous for reducing Cr oxidation. The temperature further rises and becomes high enough to adversely affect the refractory. Therefore, conventionally, the initial temperature could not be increased. In this regard, in the present invention, as described above, Cr oxide is reduced at the end of decarburization, and at the same time, an increase in the temperature of molten steel can be suppressed by endotherm accompanying this reduction reaction. Control becomes possible, and as a result, a more effective reduction of the Cr oxidation loss is realized.

[0023]

【Example】

Example 1 Using a 5t test converter having a top and bottom blowing function of refining gas,
A method for decarburizing and refining Cr-containing molten steel according to the conventional method and the method of the present invention was carried out in the manner shown in FIG. First, the carbon concentration of the hot metal was reduced to 1.0% by weight by ordinary top and bottom blown oxygen refining, and in the case of Examples, the method was switched to the method of the present invention from that point.
The operating conditions are as shown in Table 1.

[0024]

[Table 1]

In the embodiment, the carbon concentration is 1.0 to 0.1% by weight.
Part of the period, ie, [% C] = 0.3 to
Only in the region of 0.1, the dent depth (L /
While changing L ₀ ) variously, nitrogen gas was sprayed on the bath surface as a top blowing gas. FIGS. 4 (a) and 4 (b) show bottom blowing acid transfer patterns in the embodiment and the conventional example. Note that the conventional decarburization refining method is a case in which the top-blown nitrogen gas is not blown in this carbon concentration region. Table 2 and FIG. 5 show the amount of Cr oxidation loss during the above-mentioned decarburization refining. As is clear from the table and FIG. 5, according to the method of the present invention, the amount of Cr oxidation loss was smaller than that of the conventional method. On average 4.95
kg / t could be reduced.

[0026]

[Table 2]

As shown in FIG. 3, the temperature of the molten steel drops due to the blowing of nitrogen gas, and the amount of the drop is almost proportional to the blowing time. If the time is determined, decarburization refining can be performed while adjusting to the target temperature, and C
The oxidation loss of r can be reduced. Note that the final carbon concentration in the molten steel in this example was 0.1% by weight.

In the above embodiment, the case where nitrogen gas is used as the inert gas has been mainly described. However, in some cases, the same effect can be obtained by using other inert gas such as argon gas. Needless to say. Further, in the above embodiment, the refining for reducing the carbon concentration in steel to 1.0% by weight was performed in the same test converter, but the present invention is not limited to this, and the refining is performed in an electric furnace, an AOD furnace, or the like. Is also good.

Example 2 Molten steel that had been decarburized and refined in the same manner as in Example 1 was subsequently subjected to vacuum feeding and decarburization refining using a 5t test VOD facility. Table 3 shows the experimental conditions. Decarburization was performed by blowing oxygen gas from a top blowing lance. Before refining [%
C] ≒ 0.1, and the end time [% C] 終了 0.05. In Table 4,
This shows the refining results.

[0030]

[Table 3]

[0031]

[Table 4]

As shown in Table 4, during the decarburization refining in the converter, the charge was blown with N ₂ gas as an inert gas, and the [N] at the start of vacuum acid decarburization was high. Immediately thereafter, the de-N-reaction becomes active, so that particularly good decarboxylation efficiency is achieved.

[0033]

As described above, according to the present invention, in the decarburization refining of chromium-containing molten steel, the oxidation loss of Cr has been conventionally increased and the temperature has been sharply increased [% C] = 1.0 to 0.1.
In the region of the above, the inert gas is sprayed from the top blowing lance onto the steel bath surface under the condition that the bath surface recess depth (L / L ₀ ) is 0.08 or more, thereby enhancing the slag-metal stirring. The chromium oxide that has already been generated, or the chromium oxide that is already floating, and slag are beaten into the molten steel, and the reduction reaction of Cr ₂ O _{3 in} the slag by the carbon in the molten steel is promoted, thereby effectively reducing the Cr oxidation loss in this region. Can be reduced.

Further, since the above-mentioned reduction reaction is an endothermic reaction, it is possible to suppress a rise in temperature during the reduction reaction, so that it is possible to reduce the erosion of the refractory and to raise the heat quickly from the initial stage of blowing. Become.

[Brief description of the drawings]

FIG. 1 (a) is a schematic diagram showing a decarburization refining situation of a Cr-containing molten steel according to the present invention. (b) is explanatory drawing of the dent depth of a bath surface.

FIG. 2 is a diagram showing a relationship between a stirring power density of an upper blow of an inert gas and an oxidation loss amount of Cr.

FIG. 3 is a diagram showing a change amount of a molten steel temperature in an inert gas top blowing.

FIG. 4 is a diagram showing a bottom-blown acid feeding pattern in Examples and Conventional Examples.

FIG. 5 is a diagram showing the relationship between the bottom blowing acid unit consumption of inert gas top blowing and the amount of oxidation loss of Cr.

[Explanation of symbols]

 Reference Signs List 1 lance 2 slag 3 molten steel (metal) 4 smelting vessel (5t test converter) 5 tuyere for bottom-blown gas 6 top-blown gas (inert gas) 7 flow

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-276009 (JP, A) JP-A-4-329818 (JP, A) JP-A-5-43931 (JP, A) 158142 (JP, A) JP-A-60-52505 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21C 5/28-5/35 C21C 7/00, 7/068, 7/072

Claims (2)

(57) [Claims] When decarburizing and refining molten Cr-containing steel in a vessel having a refining gas top and bottom blowing function, at least one molten steel having a carbon concentration in a range of 1.0 to 0.1% by weight is placed on a bath surface of the molten steel. A method of decarburizing and refining molten steel containing Cr, characterized in that an inert gas is blown from an upper blowing lance so that the depth of the recess at the center of the bath surface satisfies the following formula during the period of the part. L / L ₀ ≧ 0.08 --- (1) where, L ₀ : steel bath depth (mm) L = L _h · exp (−0.78 _h / L _h ) (mm) L _h = 63.0 (F / nd) ^2/3 and L when h = 0
Value F: Gas flow rate (Nm ³ / h) h: Lance height (mm) n: Number of nozzle holes d: Nozzle hole diameter (mm) 2. A method for decarburizing and refining chromium-containing molten steel, comprising performing vacuum decarburization refining after the treatment according to claim 1.