CN100535153C - Method for processing ultra-pure ferrite stainless steel by using ladle refining furnace to control carbon and nitrogen content - Google Patents

Abstract

The invention discloses a carbon and nitrogen content control method of processing super ferrite stainless steel with ladle refining furnace, comprising the procedures that, 1) during smelting ferrite stainless steel, arc furnace, induction furnace or converter are used for tapping, and argon oxygen decarburization furnace or vacuum oxygen decarburization furnace is used to reach the demand of steel liquid components of super ferrite stainless steel. 2) Ladle refinement: during power-on process, by blowing the mixture of coke oven gas and argon gas through the central hole of graphite electrodes, hydrogen containing plasma arc is formed at the lower end of graphite electrodes to achieve decarburization and denitrogen of liquid steel and guarantee the cleanness of liquid steel; among which the mixture proportion of coke oven gas and argon gas is between 95:5 and 5:95, with the pressure range between 0.02 and 10 MPa, the flow range of mixture gases between 0.01 and 50 Nm3/h; 3) During power-off process: gases blowing to the central hole of graphite electrodes are suspended; ladles are hung away from ladle refining furnace after components and temperature of steel liquid meet the requirement, and sent to the next procedure for casting.

Description

Method of Controlling Carbon and Nitrogen Content in Ladle Refining Furnace Treating Ultra-pure Ferritic Stainless Steel

technical field

The invention relates to the technical field of metallurgy, in particular to a method for controlling the carbon and nitrogen content of ultra-pure ferritic stainless steel in a ladle refining furnace.

Background technique

Ferritic stainless steel has much better resistance to stress corrosion such as chloride and caustic alkali than austenitic stainless steel, good local corrosion performance in seawater and high temperature oxidation resistance, and low price, so it is widely used in various sectors of the national economy. Fields, such as household appliances, kitchen utensils, hardware products, automotive exhaust systems, construction, and petrochemicals. The disadvantage of ordinary ferritic stainless steel is that it is sensitive to intergranular corrosion, its plasticity and toughness are very low, its ductile-brittle transition temperature is above room temperature, and its welding performance is poor. Relevant studies have shown that the above-mentioned defects of ferritic stainless steel are caused by interstitial elements C and N. Ultra-pure ferritic stainless steel with C+N≤150ppm can completely overcome the above shortcomings.

Ultra-pure ferritic stainless steel is currently mainly produced by vacuum refining equipment (such as VOD, SS-VOD). In recent years, in order to achieve multi-furnace continuous casting in stainless steel plants, improve the cleanliness of molten steel and precisely control the composition, usually before continuous casting, molten steel needs to be treated in a ladle furnace (Ladle Furnace, referred to as LF, see Figure 1). It can be seen from Figure 1 that during the production of ultra-pure ferritic stainless steel using the conventional LF process, since the conventional ladle refining furnace (LF furnace) uses the free arc generated by the graphite electrode and the molten steel to heat the molten steel, the heating process will generate an increase. In addition, since the furnace gas around the electrode is mainly air in the conventional LF operation process, the arc will ionize the nitrogen in the air around it during the heating process, which will cause the problem of nitrogen increase, and the longer the heating time, the increase The problem of carbon and nitrogen increase is more serious. Taking a 300t ladle refining furnace as an example, usually under the condition of energizing for 30 minutes, the amount of carbon increase is 0.005% to 0.01%, and the amount of nitrogen increase is greater than 0.002%. This makes the production of ultra-low carbon and nitrogen ferritic stainless steel with conventional ladle refining furnace (LF) often cause the problem of changing the steel grade due to the unqualified steel grade composition due to excessive carbon and nitrogen, and the product refining rate is reduced. Since the problem of carbon and nitrogen increase in ladle refining furnace refining ferritic stainless steel has not been solved, it is impossible to use conventional ladle refining furnace (LF) to refine ultra-low carbon and nitrogen ultra-pure ferritic stainless steel.

Contents of the invention

The object of the present invention is to provide a method for controlling the carbon and nitrogen content of ultra-pure ferritic stainless steel in a ladle refining furnace, so as to realize the refining of ultra-low carbon and low-nitrogen ferritic stainless steel in the ladle refining furnace.

To achieve the above object, the technical solution of the present invention is,

A method for processing ultra-pure ferritic stainless steel in a ladle refining furnace to control carbon and nitrogen content, comprising the steps of:

1) When ferritic stainless steel is smelted, the initial furnace such as electric arc furnace, induction furnace or converter is tapped, and the composition of molten steel is basically ultra-pure through argon oxygen decarburization furnace (AOD) or vacuum oxygen blowing decarburization furnace (VOD) Requirements for ferritic stainless steel;

2) Transport the ladle to the ladle refining station and start ladle refining. During the electrification process, the mixed gas of coke oven gas and argon is injected through the center hole of the graphite electrode of the ladle refining furnace to form hydrogen-containing plasma at the lower end of the electrode of the ladle refining furnace Electric arc, to achieve decarburization and denitrification of molten steel, while ensuring the cleanliness of molten steel; among them, the length and power of the plasma arc are adjusted by adjusting the secondary voltage of the transformer, the mixing ratio of coke oven gas and argon, the pressure of the mixed gas, and the flow rate of the mixed gas And the electrode rise and fall to control; Among them, the voltage range of the secondary voltage of the transformer is 0 ~ 380V, the mixing ratio of coke oven gas and argon gas is 95:5 ~ 5:95, the pressure range of the mixed gas is 0.02 ~ 10Mpa, the mixed gas The flow range is 0.01～50Nm ³ /h;

3) When the power is off, stop blowing gas to the center hole of the graphite electrode, lift the ladle away from the ladle refining furnace after the molten steel composition and temperature are qualified, and send it to the next step for pouring.

In addition, the volume percentage of the coke oven gas is: H ₂ 50-60%, CH ₄ 20-30%, C _n H _m excluding CH ₄ 2-5%, CO ₂ 1-5%, CO 3-8%, O ₂ 0-1.0%, N 2-6%.

Using the hollow graphite electrode of the ladle refining furnace (LF furnace) to inject coke oven gas and argon mixed gas (because the coke oven gas contains a large amount of hydrogen, a hydrogen-containing plasma arc is formed) to realize the refining of ultra-pure ferrite in the LF furnace Body stainless steel.

Since the center of the graphite electrode of the ladle refining LF furnace is hollow, the above-mentioned mixed gas is blown into the center hole to form a plasma arc at the lower end of the graphite electrode. 95:5～5:95), pressure (the pressure range of the mixed gas is 0.02～10MPa), flow rate (the flow rate range of the mixed gas is 0.01～50Nm ³ /h) and other process parameters to prevent the carburization of molten steel and realize the steel Liquid denitrification to achieve the purpose of refining ultra-low carbon and low nitrogen ferritic stainless steel in the ladle refining furnace.

After the coke oven gas is injected, a hydrogen-containing plasma arc is formed at the end of the electrode. When the arc exists, the following chemical reactions that occur on the surface of the molten steel are the basic principles for effectively removing nitrogen, carbon and oxygen from the molten steel:

[N]+3H＝NH ₃

[C]+4H= _CH4

[O]+2H＝H ₂ O

[N]+H+C=HCN

The proportion of the injected mixed gas can be adjusted according to the composition and purity requirements of the smelted steel. In order to ensure that the hydrogen content in the steel after refining does not exceed the standard, the gas fed into the arc zone is switched to argon gas in the later stage of refining, or the hydrogen is easily removed to a lower level by bottom blowing argon stirring for a certain period of time after refining. Therefore, the present invention overcomes the problem of increasing carbon and nitrogen in molten steel in the current LF furnace, and can greatly expand the scope of steel treatment.

The processing of ultra-pure ferritic stainless steel by conventional ladle refining furnaces is prone to increase in carbon and nitrogen. The longer the energization time, the more serious the problems of carbon and nitrogen increase. This greatly limits the application of LF furnace in the smelting of ultra-pure ferritic stainless steel. application.

The present invention adopts the central drilling of graphite electrodes to blow air, and does not require major modifications to conventional LF furnace equipment. The method is simple and flexible to implement and saves investment. The new method of the invention is for smelting, which is flexible and convenient to start and stop, and does not affect the front and back production procedures and normal production process. The equipment is easy to use and maintain, and the maintenance cost is low.

Compared with ordinary ladle refining furnace (LF), the hydrogen-containing plasma arc formed by injecting coke oven gas can greatly increase the arc voltage and correspondingly greatly increase the arc power under the same arc length and current conditions. Thus forming a plasma arc with high energy density. This is because the enthalpy of the hydrogen plasma arc is about 16 times that of the argon plasma arc. For example, when 10% coke oven gas is added to argon, the arc voltage can be increased by 1.8 times. Therefore, on the premise of not increasing the arc current and arc length, the energy of the arc can be significantly increased by greatly increasing the secondary voltage of the transformer on the basis of the traditional ladle furnace, and a high-power and ultra-high-power ladle refining furnace can be realized. In this case, since the current does not increase, the cross-sectional area of the secondary high-current conductor can remain unchanged, and the corresponding electrode diameter will also remain unchanged, and the absolute amount of power loss in the secondary power supply circuit will basically remain unchanged. In fact, the electrical efficiency has improved. . While the arc length is constant, the absolute amount of heat loss of the arc heat transfer is basically unchanged. In fact, after the arc energy density is increased, the arc heat transfer efficiency is greatly improved. The final result is that the heating rate of molten steel is greatly increased, the power consumption and electrode consumption are significantly reduced, the refining efficiency is improved, the refining cycle is shortened, and the buffer and adjustment of the ladle furnace between the primary smelting furnace and the continuous casting are greatly improved. Function.

The beneficial effects of the present invention are:

By injecting the mixed gas of coke oven gas and argon gas into the graphite electrode hole of the ladle refining furnace to form a plasma arc at the lower end of the electrode, the heating efficiency of the molten pool is improved (because the enthalpy of the hydrogen plasma arc is high); at the same time, the LF furnace refining process is solved The problem of carburization and nitrogenation of molten steel caused by the process (because after the coke oven gas is injected, a hydrogen-containing plasma arc is formed at the end of the electrode, and when the arc exists, decarburization and denitrification chemical reactions occur on the surface of the molten steel), thus Realize the refining of ultra-low carbon and low nitrogen ferritic stainless steel on the LF furnace, and realize the multi-furnace continuous casting of continuous casting, improve the cleanliness of molten steel and precisely control the composition; the purpose of using coke oven gas is to Gas is a common hydrogen-containing gas in iron and steel complexes. Most steelmaking workshops have coke oven gas supply pipelines. The gas source is easy, and the price is low and easy to implement.

Description of drawings

Fig. 1 is the schematic diagram of existing conventional ladle refining furnace;

Fig. 2 is a schematic structural view of a ladle refining furnace according to an embodiment of the present invention;

Explanation of the symbols in the figure,

1 AC power supply 2 Secondary large current conductor 3 Electrode holder

4 Hollow graphite electrode 5 Hydrogen plasma arc 6 Ladle cover

7 ladle 8 slag 9 molten steel

10 Bottom gas pipeline 11 Ladle car 12 Bottom blowing vent plug

13 Hydrogen-containing gas source (a gas 1; b gas 2; gas 3)

14 Top blowing gas valve station (A pressure reducing valve; B stop valve; C flow meter; D flow regulating valve; E quick shut-off valve)

15 Gas mixing bag 16 Flow control valve 17 Top blowing gas pipeline and connection device

18 alloy feeding device

Detailed ways

The present invention will be further described below through the examples, but the present invention is not limited to the following examples.

Referring to Fig. 2, it is a schematic diagram of the structure of a hydrogen-containing plasma three-phase AC ladle refining furnace of the present invention. As shown in the figure, the AC power supply 1 is connected to the secondary high-current conductor 2 and then connected to the electrode holder 3, and the hollow graphite electrode 4 is The electrode holder 3 holds the hollow graphite electrode 4 up and down and conducts electricity to it. The hollow graphite electrode 4 passes through the electrode hole opened on the ladle cover 6 and enters the ladle 7, and the hollow graphite electrode 4 is raised and lowered. Contact and separation with slag 8 and molten steel 9.

An alloy feeding device 18 is also provided on the ladle cover 6, and the ladle 7 containing the slag 8 and molten steel 9 sits on the ladle car 11, and the bottom blowing pipeline 10 is connected with the bottom blowing vent plug 12 installed at the bottom of the ladle to discharge the bottom blowing gas. Blow into the molten steel 9 in the ladle 7. The hydrogen-containing gas source 13, the top blowing gas valve station 14, the gas mixing bag 15 and the flow control valve 16 are sequentially connected by the gas pipeline, and the flow control valve 16 is connected to the hollow graphite electrode 4 through the top blowing gas pipeline and the connecting device 17 A hydrogen-containing gas is blown into the hydrogen-containing plasma arc 5 formed at the end of the hollow graphite electrode.

When working, sit on the ladle car 11 when the ladle 7 that is filled with molten steel, and move the ladle car to make the ladle be in the heating position just below. Connect the AC power supply 1, lower the hollow graphite electrode 4 and short-circuit the slag 8 and molten steel 9 to strike the arc, and pass the gas from the hydrogen source 13 through the top blowing gas valve station 14, gas mixing bag 15, flow control valve 16, top The blowing gas pipeline and connecting device 17 quickly pass through the hollow graphite electrode 4 into the arc area, thereby generating a hydrogen-containing plasma arc 5 . According to the needs of the process, the ratio of the components of the mixed gas, the gas flow and the pressure can be adjusted through the valve station. The length and power (arc voltage, arc current) of the plasma arc can be controlled by adjusting the secondary voltage of the transformer, gas component ratio, pressure, flow rate and electrode lift. In order to control the furnace atmosphere between the ladle cover and the ladle, the electrode hole and the electrode can be well sealed. Other operations, such as argon blowing at the bottom of the ladle, slagging, alloying, etc., are basically the same as the current LF operation.

Example 1

When using the 30t EAF-AOD-LF-CC (electric arc furnace-argon oxygen decarburization furnace-ladle refining furnace-continuous casting) process to produce 409L ferritic stainless steel, when the electric arc furnace is smelted and passed through the argon oxygen decarburization furnace ( After AOD) refining, hoist the ladle to the LF station of the three-phase AC ladle refining furnace, and seat it on the ladle car, connect the bottom blowing pipeline and open the bottom blowing gas valve to test the gas. The ventilation brick has good air permeability, and the ladle car is driven Go to the heating station of the LF furnace and lower the ladle cover to keep a good seal between the ladle cover and the upper edge of the ladle. Then take a sample from the opening on the ladle cover, and take a sample from the temperature measurement port (T=1575°C), and send the sample to the laboratory for inspection and analysis. Add the weighed slag into the furnace from the alloy feeding device on the ladle cover. Pass the mixed gas containing 10% coke oven gas and 90% argon into the hollow graphite electrode hole according to 67m ³ /h·t steel, and energize at the same time, so that a hydrogen-containing plasma arc will be formed at the end of the electrode, and the arc passes through Radiation, conduction, convection and other methods transfer heat to the molten pool in the ladle to realize the heating of slag and molten steel. After powering on for 8 minutes, lift the hollow graphite electrode, turn off the power, and turn off the top blowing gas at the same time, take samples and measure the temperature (T=1583°C), take a slag sample and observe that it has become white slag. According to the results of the molten steel composition after the first sampling analysis and the steel composition target, the corresponding alloy is weighed and added to the molten steel in the furnace. Adjust the bottom blowing argon flow rate to 226 l/min for strong stirring for 3 min, then reduce the argon flow rate to 120 l/min. Turn on the top-blowing gas, lower the hollow graphite electrode, use the lower gear current and voltage to transmit power for 12 minutes, lift the hollow graphite electrode, cut off the power, turn off the top-blowing gas, take samples, measure temperature (T=1612°C), and lower the bottom blowing The argon flow rate was 45 l/min, and the argon gas was stirred weakly for 17 minutes, sampling and temperature measurement (T=1596°C), and the composition and temperature were both qualified, and the argon gas was blown at the bottom. Lift the ladle cover, drive the ladle car out of the heating station, lift the ladle to the next processing station, and the refining is over.

Spray 10% coke oven gas + 90% Ar mixed gas into the hollow electrode, and the arc power is increased by 1.8 times. After 10 minutes of refining, the nitrogen content of molten steel is reduced from 150ppm to below 95ppm. After 20 minutes, the nitrogen content drops to 42ppm. The carbon content dropped from 0.02% to 0.004%. After the heating is stopped, the hydrogen content in the molten steel can be reduced to below 3ppm by blowing and stirring at the bottom of the ladle for 17 minutes in the later stage. However, when 409L ferritic stainless steel is smelted by the conventional method, after the same treatment time, the carbon content and nitrogen content in the steel increase by 0.006% and 23ppm respectively, resulting in an out-of-standard steel composition that cannot meet the composition requirements of 409 finished products.

Example 2

The process flow of Example 1 is also adopted, except that the gas ratio in the blowing process is changed to a mixed gas containing 20% coke oven gas and 80% argon, the gas flow rate is 78m ³ /h·t steel, and other operating processes are basically the same. Change.

Spray 20% coke oven gas + 80% Ar mixed gas into the hollow electrode. Due to the increase in the proportion of coke oven gas, the blowing flow rate increases, and the arc power increases by 2.2 times. After refining for 9 minutes, the carbon content in the steel decreases from 0.02%. To 0.0079%, the nitrogen content was reduced from 164ppm to 93ppm. Continue the gas blowing treatment until the end of refining, and the nitrogen content continues to decline during the gas blowing process. At the end of the LF treatment, the nitrogen content drops to 35ppm, and the carbon content in the steel also continues to drop from 0.0079% to 0.0025%.

Example 3

The smelting process flow of embodiment 3 adopts 30t EAF-AOD-VOD-LF-CC (electric arc furnace-argon oxygen decarburization furnace-vacuum oxygen blowing decarburization furnace-ladle refining furnace-continuous casting) flow process, and the production steel type is 409L iron Body stainless steel. After the molten steel initially smelted in the electric furnace is refined by AOD, the steel is poured into the ladle and sent to VOD for refining. At the end of VOD treatment, the carbon content in the steel reaches 29ppm and the nitrogen content is 56ppm. The molten steel after VOD refining is hoisted into the LF station for heating and composition fine-tuning.

In the LF refining process of the three-phase AC ladle refining furnace, the basic operation of this furnace is similar to the operation steps of Example 1. During the refining and electrification process of this furnace, 5% coke oven gas + 95% coke oven gas is injected from the graphite electrode hole The mixed gas of argon, the gas flow rate is controlled at 53m ³ /h·t steel, and the soft argon blowing time at the end of refining is 7min.

In Example 3, the carbon and nitrogen contents in the steel after VOD treatment are both low, being [C]=29ppm and [N]=56ppm respectively. In the process of entering the LF furnace, the arc power is increased by 1.1 times, the carbon content is [C]=29ppm, 20ppm and 16ppm at the beginning of refining, 13min of refining and the end of refining (22min), and the nitrogen content at the corresponding time is [N]= 56ppm, 45ppm and 34ppm.

Claims (2)

1. ladle refining furnace is handled the method for super-purity ferrite stainless steel control carbon nitrogen content, and it comprises the steps:

1) during the ferrite stainless smelting steel,, and make molten steel composition reach the requirement of super-purity ferrite stainless steel substantially by argon oxygen decarburizing furnace or vacuum-oxygen decarbonizing stove when electric arc furnace, induction furnace or converter tapping;

2) ladle is transported to the ladle refining station, the beginning ladle refining, in galvanization, Graphite Electrodes centre hole winding-up coke-oven gas and argon gas mixed gas by ladle refining furnace, form hydrogeneous plasma arc in ladle refining furnace electrode lower end, realization guarantees the molten steel cleanliness factor simultaneously to molten steel decarburization, denitrogenation; Wherein, the length of plasma arc, power are controlled by regulating transformer secondary voltage, coke-oven gas and argon gas blending ratio, the pressure of mixed gas, the flow and the rise fall of electrodes of mixed gas; Wherein, the voltage range of transformer secondary voltage is 0～380V, and coke-oven gas and argon gas blending ratio are 95: 5～5: 95, and the pressure range of mixed gas is 0.02～10Mpa, and the flow range of mixed gas is 0.01～50Nm ³/ h;

When 3) having a power failure, stop, after molten steel composition, temperature are qualified, ladle is hung ladle refining furnace, send down to go on foot operation and pour into a mould to Graphite Electrodes centre hole blowing gas.

2. ladle refining furnace as claimed in claim 1 is handled the method for super-purity ferrite stainless steel control carbon nitrogen content, it is characterized in that the composition volume percent of described coke-oven gas is: H ₂50～60%, CH ₄20～30%, do not comprise CH ₄C _nH _m2～5%, CO ₂1～5%, CO 3～8%, O ₂0～1.0%, N 2～6%.

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