RU2295421C2 - Steel ingot producing method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method comprises steps of filling ingot mold with metal and soaking it for time period τ = (0.05 - 0.06)τ_c where τ_c - time period of ingot crystallization at natural condition; then for time period consisting (0.03 -0.04)τ_c heating up head portion of ingot by means of hydrocarbon-containing plasma jet at mass relation of oxygen to reducing agent in range 0.8 - 0.9; soaking metal for time period (0.05 - 0.06)τ_c ; again heating head portion of ingot by means of plasma for time period (0.06 - 0.08)τ_c; setting axial density of heat flux according to its dependence upon mean density of heat flux in cut of plasmatron nozzle and length of plasma jet. Mean density of heat flux in cut of nozzle is determined at taking into account flow rate of plasma generating gas, mean-mass enthalpy of plasma jet and surface area of outlet nozzle of plasmatron.

EFFECT: increased rate of treating ingot, enhanced quality of ingot, improved labor conditions, lowered electric power consumption.

2 dwg, 1 ex

Description

The invention relates to the field of metallurgy and can be used in the casting of ingots obtained by casting metal into a mold.

A known method of producing ingots, including siphon casting of liquid metal into a stationary mold, its additional heating, mixing and subsequent solidification, characterized in that the additional heating is carried out by a jet of low-temperature plasma at a constant specific heat flow and dynamic pressure on the liquid metal mirror, starting from the moment of filling liquid metal half the volume of the mold (AS USSR No. 1387269, application No. 4064030 / 23-02 from 04.29.86, CL 4 B 22 D 7/00, 7/10).

However, this method does not provide an effective integrated effect on the oxidation of the metal with increasing its level in the mold and achieving optimal oxidation values for the formation of a favorable structure of the surface layer of the ingot.

The closest in technical essence and the achieved result (prototype) adopted a method of producing steel ingots, including casting metal into a mold and its subsequent heating with a jet of low-temperature plasma, which begins after a period of time equal to 0.3-0.35 of the solidification time of the ingot, starting since the end of the casting process, the heating is carried out by pulses, pauses between which are equal to the duration of the removal of overheating of the molten metal volumes to the liquidus temperature, and the duration τ each pulse is determined by the formula:

where m is the mass of the metal located in the two-phase zone, kg;

L is the specific heat of fusion, kJ / kg;

φ is the fraction of solidified metal in the two-phase zone;

N - net power, kW;

K is a coefficient taking into account convective mixing and heat and mass transfer (A.S. USSR No. 1611555, class B 22 D 7/00, decl. 06.06.88. Publ. Bull. No. 45, 1990).

The disadvantage of this method is its low efficiency and technological complexity due to the increased processing time of the head of the ingot, the increased content of oxygen and oxide inclusions in the metal, as well as the violation of its structural homogeneity.

The basis of the invention is the task of creating a method for producing a steel ingot, in which, due to the rational duration of plasma heating with the reducing gas of the head part, the rate of formation of a shrink shell decreases, its depth decreases, and due to this, the structural homogeneity of the metal increases, the head trim and rejection of metal on surface defects.

The problem is solved in that in the method of producing a steel ingot, which includes casting metal into a mold, periodically heating the head of the ingot with a low-temperature plasma jet after a predetermined time interval from the moment the mold is filled, according to the invention, the heating is started with a hydrocarbon-containing plasma jet with a mass ratio of α oxygen to reducing agent α = 0.8-0.9 after time, component τ = 0.05-0.06 τ _{to the} length of time τ _to crystallization of the ingot in vivo, and for a long time the duration of the plasma heating action is τ = 0.03-0.04τ _k , then the heating is stopped, pause with a time interval of τ = 0.05-0.06 τ _k and the head of the ingot is reheated by plasma for τ = 0, 06-0,08τ _k , while the axial density of the heat flux q _O set according to

where 0.375 is an empirical coefficient that takes into account heat and mass transfer when exposed to a plasma jet;

q _s is the average density of the heat flux at the nozzle exit of the plasma torch, W / m ² ;

- приведенная длина плазменной струи,

- the reduced length of the plasma jet,

and the average heat flux density q _s at the nozzle exit of the plasma torch is determined by the formula

q _s = Gh _s / F _CA (W / m ² ),

where G is the plasma gas flow rate, kg / s;

h _s is the mass-average enthalpy of the plasma jet, J / kg;

F _CA - the area of the output nozzle of the plasma torch, m ² .

These distinctive features provide an equivalent effect of the plasma jet on the head of the ingots when casting them with different weights. In this case, the time from the end of the casting of the metal to the inclusion of the plasma torch should be τ = 0.05-0.06 τ _{to the} duration of the crystallization time. An earlier start of the heating process is energetically and technically impractical, since, firstly, the metal is still in the molten state, and secondly, the removal of the plasma torch to the operating mode requires a certain amount of time. A later start of the inclusion of the plasmatron does not significantly affect the decrease in the depth of the shrink shell. The indicated heating time ranges are optimal from the point of view of increasing the processing speed, and the impact on the metal with hydrocarbon-containing jets in the above ratios intensifies the carbon deoxidation reaction, stabilizes the chemical composition of the upper part of the ingot, and reduces the rate of formation of a shrink shell.

The invention is illustrated by drawings,

where figure 1 shows a graph for two modes of processing the head of the ingot using a plasma jet and without plasma;

figure 2 is a graph of the dependence of the depth of the shrink shell on the duration of the plasma treatment of the head of the ingot at various values of the thermal power of the plasma torch.

The method is as follows.

The mold is mounted on a pallet to which a siphon wiring is connected, connecting the mold cavity with the center. A heat-resistant lid with a plasma torch is installed on top of the mold, to which an electric and water supply system is connected. The liquid metal is poured from the casting ladle into the center, from where it enters the mold cavity through the siphon wiring. After the working volume of the mold cavity is completely filled, a plasma-forming gas is supplied to the plasmatron, power supply, and after a time equal to τ = 0.05-0.066 _{to the} time duration τ _to ingot crystallization (in natural conditions), they initiate an arc discharge and act on the head jet with a plasma jet part of the ingot. As a plasma-forming gas, a mixture of natural gas and air is used with a mass ratio of α = 0.8-0.9, at a pressure of P = 3-5 atm. The duration of exposure to the plasma heat flux is τ = 0,03-0,04τ _k. After a predetermined heating time interval has elapsed, the plasmatron is turned off, a pause is maintained with a time interval of τ = 0.05-0.06 τ _to and metal is reheated by a plasma reduction stream for τ = 0.06-0.08 τ _to . Then the plasmatron is turned off and the natural crystallization of the ingot occurs.

The constant heat flux transferred to the head of the ingot was accepted to be normally distributed along the radius, and the control of the specific heat flux values and maintaining it constant during operation in the specified mode were ensured using a multi-parameter measuring complex for the diagnosis of high-temperature gas flows. Statistical processing of experimental materials showed that the axial density of the heat flux q _{O is} generalized by the following dependence

where 0.375 is an empirical coefficient that takes into account heat and mass transfer when exposed to a plasma jet;

q _s is the average density of the heat flux at the nozzle exit of the plasma torch, W / m ² ;

- приведенная длина плазменной струи.

- the reduced length of the plasma jet.

The average heat flux density at the nozzle exit of the plasma torch, calculated from the energy balance of the plasma torch operating mode, is represented by the dependence

q _s = Gh _s / F _CA (W / m ² ),

where G is the plasma gas flow rate, kg / s;

h _s is the mass-average enthalpy of the plasma jet, J / kg;

F _CA - the area of the output nozzle of the plasma torch, m ² .

An example of a specific implementation.

The ingots weighing 7.5 tons were cast from steel 30. The metal was poured into a square cast iron mold using a siphon method. A pre-prepared cover with a plasmatron mounted on it was mounted on the mold using a lifting and transport device. A mixture of natural gas and air at a pressure of P = 3–5 atm was used as a plasma-forming gas. The pouring temperature of steel is 1600 ° C. After the completion of pouring the metal into the mold and moving the steel casting ladle to fill the subsequent molds, after 10 minutes an arc discharge of 80 kW was excited in the plasmatron and the plasma jet was directed to the head of the ingot. The time of plasma exposure to the ingot was 5 min with a mass ratio of α oxygen to reducing agent α = 0.8. 10 minutes after the first heating, reheating was carried out for 10 minutes. The average heat flux density at the nozzle exit of the plasma torch (q _s ) was calculated from the energy balance of the plasma torch operating mode q _s = Gh _s / F _CA ,

where is the plasma-forming gas flow rate, G = 0.005 kg / s;

- mass average enthalpy of the plasma jet, h _s = 5000 kJ / kg;

- the area of the output nozzle of the plasma torch, F _CA = 0,000314 m ² .

The axial density of the heat flux q _{o was} generalized according to the dependence

where - the average heat flux density at the nozzle exit of the plasma torch was q _s = 79617 kW / m ² ;

принята 1,0, тогда- reduced plasma jet length

accepted 1.0 then

q _O = 29856 kW / m ² .

A significant increase in the shrinkage shell was observed at the beginning of the cooling process for 10 min, which is due to the dynamics of the formation of the hardened volume of the metal. After turning on the plasma torch, a gradual decrease in the rate of formation of a shrink shell occurs, continuing until the solidification of the main part of the ingot is completed. An increase in the duration of solidification of the metal by 15 min was observed during plasma heating due to the increased heat content of the profitable part of the ingot. However, the curing time of the head part with the traditional technology of warming the profitable part and with plasma heating is almost the same. A mixture of natural gas and an oxidizing agent with α = 0.8 ensures the creation of a reducing atmosphere, preventing the formation of oxides and the stability of the chemical composition of the upper part of the ingot.

Dependencies are experimentally determined that characterize the change in the maximum depth of the shrink shell in the conventional technology of warming the profitable part of the ingot and using the inventive method for plasma heating of the head part of the ingot. Figure 1 shows the curves for two modes of processing the head of the ingot using a plasma jet 1 and without exposure to plasma - 2. As can be seen from the graph, the use of plasma insulation of the head of the ingot during its solidification in the mold during the first operation mode allows to reduce the shrinkage depth sinks compared with conventional technology from 0.325 to 0.175 m.

At the same time, the control ingot was poured in accordance with the claims of the prototype. The test results showed that the metal consumption for trimming by the known method amounted to 6%, according to the proposed - 4%, and, taking the energy consumption of the prototype method for 100%, according to the proposed method, they amounted to 80%.

In order to choose a rational regime of plasma heating of the head part of ingots weighing 7.4 tons, a series of calculations of metal solidification processes was carried out for various values of the thermal power of the plasma torch and its duration.

Figure 2 presents graphs of the dependence of the depth of the shrink shell on the duration of the plasma treatment of the head of the ingot at various values of the thermal power of the plasma torch. The graphs make it possible to choose a rational regime of plasma processing of the ingot in order to reduce the shrinkage of the shell.

The application of the proposed process technology will increase the processing speed and quality of the ingot, improve working conditions and reduce energy consumption.

Currently developed terms of reference for the design of a pilot plant using the proposed method.

Claims (1)

A method of producing a steel ingot, including casting metal into a mold, periodically heating the head of the ingot with a low-temperature plasma jet after a predetermined time interval from the moment the mold is filled, characterized in that the heating is started with a hydrocarbon-containing plasma jet with a mass ratio of oxygen to reducing agent α = 0.8-0 9 after the time τ, constituent (0,05-0,06) τ _k, where τ _k - duration of crystallization time of the ingot in vivo, and the duration time of action of the plasma beneath roar is τ = (0,03-0,04) τ _k, then heating stopped, kept paused with a time interval τ = (0,05-0,06) τ lead _to and re-heating the ingot head portion plasma for τ = (0.06-0.08) τ _to , while the axial density of the heat flux q _o set taking into account the dependence

where 0.375 is an empirical coefficient that takes into account heat and mass transfer when exposed to a plasma jet; q _s is the average density of the heat flux at the nozzle exit of the plasma torch, W / m ² ;

- the reduced length of the plasma jet, and the average heat flux density q _s at the nozzle exit of the plasma torch is determined by the formula q _s = Gh _s / F _CA (W / m ² ), where G is the plasma gas flow rate, kg / s; h _s is the mass-average enthalpy of the plasma jet, J / kg; F _CA - the area of the output nozzle of the plasma torch, m ² .

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