JPH08253805A - Method for refining stainless steel using decarburizing slag having high productivity - Google Patents

Abstract

PURPOSE: To provide an efficiently refining method of a stainless steel, by which chromium refining rate is large without increasing the erosion of a refractory under condition of high productivity and expensive reducing agent is not used at all. CONSTITUTION: A top blowing lance, in which a slitting type oxygen supplying tube 7 having concentrically 3-16 angular polygonal or concentrically circular cross section is arranged and 2-10 shield plates 8 are arranged at a part of the tip opening surface of the tube 7 and the ratio B/h of the long side length B (mm) and the short side length (h) (mm) on the individual tip opening surface separated with the plates 8 is 10-225 and in the case of using R (mm) for lance diameter, B×h/R ratio is 0.4-4, is used. Oxygen supplying rate is made to be 125-250Nm<3> /Hr.ton without shifting the tip part of the lance including the lance center point (a) in the vertical direction to the lance body 6. Under condition of remaining decarburizing slag produced in the previous heat in a furnace, molten iron in the following heat is charged and blown, temp.-raised and refined and then, after reducing the chromium content in the decarburizing slag, the decarburizing slag is removed and successively, a ferro-chromium alloy is melted and decarburization-refined in the same converter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is directed to decarburization and refining in the same furnace after reducing the chromium oxide content in the decarburizing slag produced by the previous charge with the hot metal of the next charge, and then slagging the slag. It is an efficient method for refining stainless steel.

[0002]

As a raw material for refining molten steel containing chromium, blast furnace hot metal dephosphorized and desulfurized by out-of-pile refining, chromium-containing hot metal produced by smelting reduction of chromium ore, and stainless steel scrap are used. There is chromium-containing molten iron melted in an electric furnace. These raw materials are top-blown converters, bottom-blown converters, top-bottom-blended converters, and AOD blown decarburization refining, and in some cases, VOD and RH / OB vacuum refining furnaces. As a result, the carbon concentration is reduced to satisfy the material characteristics.

However, in these refining furnaces, the chromium oxide produced during the decarburization of propellic acid is removed by Fe-S at the end of propellant acid.
It is widely practiced to carry out reduction and recovery using an expensive reducing agent such as i, and the cost is high, which is a big problem.

On the other hand, JP-A-53-119210
In the gazette, the slag with high chromium oxide concentration generated during decarburization remains in the converter even after tapping, the hot metal of the next charge is received, and the chromium oxide is reduced and recovered by the carbon contained in the hot metal. The technology is disclosed. However, in this method, the amount of slag becomes extremely large during the decarburization with the second charge after the chromium oxide generated by the decarburization with the first charge is reduced and recovered, so that the oxidation of chromium during the decarburization becomes large. However, there is a very big problem that it takes a long time to reduce the third charge by the hot metal, and it is difficult to put it into practical use. Further, there is a problem that the reduction rate of chromium oxide by the carbon contained in the hot metal needs to be high, and it is slow even at high temperature, and it takes a long time to raise the temperature and reduce the productivity, resulting in a significant decrease in productivity.

In order to shorten the temperature rising and reducing times, the oxygen supply rate is increased. It weakens the contact between top-blown oxygen and molten iron and suppresses reoxidation of top-blown oxygen in molten iron. However, at the same time, it is essential to unnecessarily increase secondary combustion and prevent wear of the converter refractory. However, since a normal top-blowing lance is composed of a circular multi-hole nozzle, increasing the oxygen supply rate increases the jet strength (hard blow) and strengthens the contact with molten iron, resulting in severe reoxidation. If the lance gap is increased in order to avoid this, the secondary combustion becomes extremely high, and there is a problem that the converter refractory is significantly worn.

On the other hand, Japanese Patent Publication No. 47-4770 discloses that
A lance is disclosed in which a spindle having an actuating mechanism capable of moving up and down in a pipe line is provided between a throat portion and a tip outlet portion of a circular oxygen nozzle of an upper blowing lance. In this case, oxygen flows through the slit formed in the gap between the circular nozzle and the spindle, but the jet flow after passing through the gap merges immediately after the outlet and always becomes a hard blow, so the above problem cannot be solved.

[0007]

SUMMARY OF THE INVENTION The present invention is disclosed in Japanese Patent Laid-Open No. Sho 53-53.
In the technology disclosed in Japanese Patent Laid-Open No. 119210, there is a problem in that the amount of slag increases during decarburization, which increases the oxidation of chromium and requires a long time for reduction. And the problem in the technology disclosed in Japanese Patent Publication No. 47-4770 that the jets coalesce immediately after the outlet and always become a hard blow, and stainless steel using a decarburizing slag with high productivity is solved. A method for refining steel is provided.

[0008]

DISCLOSURE OF THE INVENTION The inventors of the present invention have noticed that when the nozzle shape of the lance is not a circle but a proper “elongate” shape, the jet flow after being discharged from the lance has a great attenuation. However, when this principle is applied during the reduction of the chromium oxide generated during pre-charge decarburization with carbon in the hot metal, compared with the circular nozzle, even if the oxygen supply rate is increased, it does not It is possible to weaken the contact with molten iron and suppress reoxidation by top-blown oxygen in chromium in molten iron, and the secondary combustion rate does not increase significantly, but even if it increases, the heat transfer efficiency defined by the following equation It has been found that it is possible to prevent the wear of converter refractory because it is extremely high. Chakunetsu efficiency = {1 - (sensible heat of the partial exhaust gas is heated above a bath temperature) / (the amount of heat generated by the combustion of CO to CO _2)}
× 100

The present invention is based on this finding. The main points are (1) Chromium 5 to 2
When decarburizing and refining molten iron containing 5% in a converter-type reaction vessel, the chromium oxide produced by precharging was added to 12 to 50%.
% With the slag contained in the furnace remaining in the furnace, the hot metal of the next charge was charged and the temperature of the molten oxide was refined by blowing acid to reduce the chromium oxide in the decarburizing slag to less than 8%, and then tilt the furnace. In the refining method of stainless steel for decarburizing and refining while slagging the slag and subsequently melting the ferrochrome alloy in the same furnace, one slit-shaped strip having concentric 3 to 16 polygonal or concentric cross sections is used. An oxygen supply pipe is provided, and 2 to 10 shielding plates are arranged on a part of a front end opening surface of the oxygen supply pipe.
The ratio B / h of the long side length B (mm) and the short side length h (mm) of each of the tip end opening surfaces separated by the shielding plate is 10 to 22.
5, (B × h) / R when the lance diameter is R (mm)
Of 0.4 to 4 is used, and oxygen is supplied at 125 to 250 Nm ³ / (Hr · ton) without moving the lance tip including the lance center point in the vertical direction with respect to the lance body. And a method for refining stainless steel using a decarburizing slag having a high productivity.

[0010]

The operation of the present invention is shown in FIG. Step I is a step of charging the dephosphorization desulfurized hot metal 2 into the converter 3, Step II is a temperature-reduction step, Step III is an intermediate slag step, Step IV is a decarburization and ferrochrome alloy melting step, Process V is a tapping process.
In this way, the slag 5 produced by the decarburization of the precharge
With the above remaining, the hot metal 2 for the next charge is charged, then oxygen is blown upward to raise the temperature and the chromium oxide in the slag is reduced. After that, after the reduction, the slag 4 is tilted in the converter 3 and discharged, and then the furnace is erected upright, oxygen is blown upward to decarburize, and at the same time ferrochrome alloy is added and melted to dissolve stainless steel crude molten steel or product. Is a process of manufacturing.

FIG. 2 shows an example of the top blowing lance described in the present invention. As in the nozzle shown in FIG. 2, when the nozzle shape of the lance is not a circle but a proper “elongate” shape, the jet after being discharged from the lance has a great attenuation. When decarburizing and refining molten iron containing chromium in a converter-type reaction vessel based on this principle, while leaving the slag containing chromium oxide generated in the precharge in the furnace,
After charging the hot metal of the next charge and reducing the chromium content in the decarburizing slag by refining the temperature with blowing acid, tilt the furnace to discharge the slag, and subsequently melt the ferrochrome alloy in the same furnace. When applied to the refining method of stainless steel for decarburizing and refining, it can easily slow down the jet slag surface arrival speed (so-called soft blow) compared to a circular nozzle,
Even if the supply rate of oxygen is increased, the contact between the top-blown oxygen and the molten iron can be weakened, reoxidation by the top-blown oxygen of chromium in the molten iron can be suppressed, and the secondary combustion rate does not increase significantly. However, it has been found that even if the heat resistance is increased, the heat generation efficiency defined by the following equation is extremely high, so that the wear of the converter refractory can be prevented.

In particular, in the case of a circular nozzle, when the operation is changed for soft blowing (for example, the diameter of the nozzle is increased, the diameter of the nozzle is increased, and the distance between the lance molten metal surfaces is increased), the CO inevitably becomes a space. In the case of the properly elongated nozzle described in the present invention, in the case of the properly elongated nozzle described in the present invention, while the exhaust gas temperature rises sharply and the converter refractory is significantly worn because the secondary combustion rate of gas combustion into CO ₂ rises. Although it is a blow, it has a great feature that the secondary combustion rate does not increase. This is because the jet flow emitted from the nozzle has a strong damping and the length of the jet core with a Mach number of 1 or more becomes extremely short, so that the jet flow has a soft blow effect in which the flow velocity at the surface of the jet becomes small.
The cross-sectional shape of the jet flow, which controls the entrainment of atmospheric gas in the space and the state of gas scattering from the jet flow to the atmospheric space and, as a result, controls the secondary combustion rate, is already elongated at a position relatively close to the nozzle outlet. This is because the shape is converted into a circle. As described above, in order to shorten the length of the jet core and obtain the state in which the cross-sectional shape of the jet flow is easily changed from the elongated shape to the circular shape, detailed examination and examination by the inventors of the nozzle shape has been performed. For example, the following points are important.

(1) As shown in FIG. 2, the ratio of the long side (B) to the short side (h) of the tip opening surface of each slit-shaped oxygen supply pipe 7 separated by the shielding plate 8 is large. Use so-called elongated ejection holes. This is because the circumferential length of the jet cross section is longer than that of the gas discharged from the circular hole, and it is greatly affected by the interaction with the gas outside the jet, and the jet has a large damping effect immediately after it leaves the nozzle (due to the shortening of the jet core). This is because a soft blow effect) can be obtained. This effect is obtained when B / h is 10 or more, and a high chromium reduction rate is obtained as shown in FIG. Further, if B / h is larger than 225, piping of lance cooling water becomes difficult, which is not realistic.

(2) The gas emitted from the elongated nozzle greatly attenuates immediately after being ejected, but after that,
It has a characteristic that it attenuates only at the 1/2 power of the distance from the nozzle tip. On the other hand, the gas emitted from the circular nozzle has a small attenuation immediately after being ejected, but thereafter, it attenuates at the first power of the distance from the nozzle tip. Therefore, it is necessary to convert the jet flow from the elongated shape to the circular cross-sectional shape after exiting from the nozzle in order to increase the subsequent attenuation while utilizing the characteristic of (1) that it is greatly attenuated immediately after ejection. This condition is (B when the lance diameter is R (mm).
Xh) / R is 4 or less, and as a result, the secondary combustion rate can be suppressed within an appropriate range as shown in FIG. When (B × h) / R is smaller than 0.4, it is difficult to maintain the machining accuracy of the nozzle, which is not realistic.

The cross section of the gas supply pipe is a concentric polygon or a slit surrounded by concentric circles, and the concentric polygon is in the range of 3 to 16 polygons shown in FIG. This is because a polygon does not have a digon, and if the number of angles is larger than that of a hexagon, machining becomes difficult. When the number of shields is less than 2, B is very large, and when it is more than 10, B is very small. Therefore, in both cases, B / h and (B × h) / R
However, the effect cannot be obtained because it does not enter the proper range.

Further, in the present invention, the lance body 6 and the tip of the lance including the lance center point a are fixed via the shield plate 8, and the center point a moves vertically with respect to the lance body 6. There is nothing to do. For this reason, it is not necessary to provide a complicated drive mechanism according to the technique of dividing the lance tip portion including the center point a in the prior art into a lance body as a core and moving only the core up and down. The structure has the great advantage that the lance can be manufactured.

Here, when the chromium concentration of the molten steel 2 produced in step IV to which the present invention is applied is lower than 5%, the effect of applying the present invention is that the amount of chromium oxide lost in oxidation in step IV is small. When the content is less than 25% and the content of chromium oxide is more than 25%, the amount of chromium oxide lost in oxidation in Step IV becomes extremely large, so that even if the present invention is applied, it cannot be sufficiently reduced in Step II and the present invention cannot be applied. When the chromium oxide of the slag 1 remaining in the furnace in the step I is lower than 12%, it is more economical to discard the slag without carrying out the present invention, and thus the effect of applying the present invention is less, When it is more than 50%, the present invention cannot be applied because the reduction cannot be sufficiently performed in the step II even if the present invention is applied. In step III, when the chromium oxide contained in the slag 4 to be discharged is higher than 8%, the fluidity of the slag is poor and the discharge becomes difficult.

Further, when oxygen is fed using the top-blown lance of the present invention, the oxygen supply rate is 125 to 250 Nm ^3.
It is necessary to supply with / (Hr ・ ton). 125Nm ³ / (Hr ・ to
If it is lower than n), the productivity is not improved because the temperature-reducing rate is slow, and if it is higher than 250 Nm ³ / (Hr · ton), even if the upper blowing lance of the present invention is used, soft blow is not possible. However, the chromium in the molten iron is reoxidized by the oxygen gas and the reduction rate of chromium is reduced.

[0019]

EXAMPLE The steps of the example are the same as those in FIG. First, 1
1 to 18% chromium steel was decarburized and refined, and a 175 ton top-bottom blow converter with slag containing 25 to 35% of chromium oxide left in the furnace was generated by pre-charge blowing. Charge hot metal that has been sufficiently dephosphorized and desulfurized. next,
Oxygen blowing is carried out on the hot metal and slag to raise the temperature and reduce the chromium oxide in the slag (heating reduction period). After that, the furnace is tilted to remove a part or most of the slag, and then the furnace is erected upright, and the ferrochrome alloy and the flux are added to feed oxygen to carry out decarburization refining. After the target carbon concentration is reduced, the furnace is tilted again and only the molten steel is tapped, and the slag containing chromium oxide is left in the furnace. The tapped molten steel moves to the finishing decarburization process, and the dephosphorization desulfurized hot metal of the next charge is received in the converter while leaving the slag. Here, depending on thermal conditions and slag conditions, carbonaceous material or flux may be used during the temperature-raising reduction period, or scrap may be used during the temperature-raising reducing period or the decarburization period.

The upper blowing lance is based on the shape shown in FIG. 2 (type a), and the hexagonal one shown in FIG. 5 (a) is also used (type b), and the number of nozzles (openings) , Changed the shape. The distance between the tip of the lance and the bath surface is 1.5 to 3 m
However, the tip of the lance including the lance center point did not move vertically with respect to the lance body.

Table 1 shows the present invention together with comparative examples. Here, the acid transfer rate is shown in the unit of Nm ³ / (Hr · ton), and the Cr reduction rate is shown in the unit of kg / (min · ton). Test Nos. 1 to 8 are examples of the present invention, and all have high reduction rates and high heat deposition efficiency. On the other hand, test numbers 9 to 16 show comparative examples, but the number of shielding plates and B of the opening are
If any of / h, (B × h) / R, and the acid transfer rate is out of the range of the present invention, there is a problem that the reduction rate is decreased or the secondary combustion rate is increased and the heat-adsorption efficiency is decreased. are doing. When a porous lance having four circular nozzles with a diameter of 12 mm having the same opening area as test number 1 was used, the secondary combustion rate was about 20%, but the chromium reduction rate was 1%. It was as small as 2 kg / (min · ton).

[0022]

[Table 1]

[0023]

EFFECTS OF THE INVENTION The present invention enables efficient refining of stainless steel in a state where the rate of chromium reduction is high and the productivity is high, without increasing the refractory melting loss and using no expensive reducing agent at all. It was

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an implementation process of the present invention. Step I is a step of charging dephosphorized desulfurized hot metal into a converter, step II is a temperature-reducing step, step III is an intermediate slag step, step IV is a decarburization and ferrochrome alloy melting step, and step V is a step. Indicates a tapping process.

FIG. 2 is an example of a top blowing lance described in the present invention (concentric type).

FIG. 3 is an experimental result showing the relationship between the chromium reduction rate and B / h.

FIG. 4 is an experimental result showing the relationship between the secondary combustion rate and (B × h) / R.

FIG. 5 is an example (concentric polygon type) of the top blowing lance described in the present invention, where (a) shows a hexagon, (b) shows a triangle, and (c) shows a hexagon type.

[Explanation of symbols]

 1 Slag generated by decarburization of pre-charge 2 Hot metal or molten steel 3 Converter 4 Slag after reduction 5 Slag generated by decarburization of the charge 6 Lance body 7 Slit-like oxygen supply pipe 8 Shielding plate a Lance center point h Short side Length B Long side length

Claims (1)

[Claims] 1. When the molten iron containing 5 to 25% of chromium is decarburized and refined in a converter reactor, a slag containing 12 to 50% of chromium oxide produced by precharging is left in the furnace. As it is, after charging the hot metal of the next charge and refining the temperature with blowing acid, the chromium oxide in the decarburizing slag is reduced to 8% or less, and then the furnace is tilted to remove the slag, and subsequently the same furnace is used. In a refining method for decarburizing and refining a ferrochrome alloy while melting a ferrochrome alloy, there is provided one slit-shaped oxygen supply pipe having a concentric 3 to 16 polygonal or concentric cross section, and the oxygen supply pipe 2 to 10 shielding plates are arranged on a part of the front end opening surface of each of the above, and the long side length B (mm) and the short side length h of each of the front end opening surfaces separated by the shield plate.
(Mm) ratio B / h is 10 to 225, and lance diameter is R (m
(B x h) / R is 0.4 to 4 when m) is used, and oxygen is supplied without moving the lance tip including the lance center point vertically with respect to the lance body. 12
A method for refining stainless steel using a decarburizing slag having high productivity, which is characterized by supplying at 5 to 250 Nm ³ / (Hr · ton).