RU2446215C2 - Nitrogen-alloyed steel making method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: nitrogen-alloyed steel making is performed in gaseous nitrogen atmosphere with pressure of more than 1 atm. As charge there used is thermit mixture of powders of iron, chrome and manganese oxides, nitrides or nitrided ferrous alloys of alloy elements and aluminium, which are taken in the ratio required for obtaining the steel of the specified composition. At that, thermit mixture is locally ignited and self-ignition of charge is provided without any external heat sources. Mass of the added aluminium powder is 0.94-1.04 of total mass of oxygen contained in oxides included in charge composition.

EFFECT: simpler melting technology, lower electric power consumption and shorter melting process.

2 cl, 2 tbl

Description

The invention relates to the field of ferrous metallurgy, and in particular to methods of smelting steel alloyed with nitrogen.

A known method of smelting nitrogen-containing steel in open (in atmospheric conditions) steelmaking units (induction or electric arc furnaces) [1]. Doping with nitrogen is carried out by blowing gas through a melt in a steelmaking ladle or by introducing nitrated ferroalloys into a liquid metal (in a furnace or ladle). This method does not make it possible to obtain high nitrogen concentrations in steel due to the rapid desorption (removal) of excess (super-equilibrium relative to a pressure of 1 atm) nitrogen from a liquid metal into the atmosphere. Therefore, even with a high content of nitride-forming elements (for example, chromium, manganese, vanadium and others) that increase the solubility of nitrogen, its content in the melt does not exceed several tenths of a percent [1].

Closest to the technical essence of the invention is a method of smelting high-nitrogen steel in an induction furnace in a specially created atmosphere of nitrogen having a pressure above 1 atm (almost 10-160 atm) [1-3]. Alloying the metal with nitrogen is carried out mainly from the gas phase by bubbling through the porous bottom of the furnace or a false stopper. High gas pressure contributes to the dissolution of nitrogen in the melt and allows you to get its super-equilibrium concentration in steel up to 1% and above. The duration of the smelting in this method is 40-180 minutes

The disadvantage of this method is the complexity of the melting technology, which includes the successive stages of melting the metal charge, heating the melt to the desired temperature, alloying, deoxidation, nitriding from the gas phase, which is accompanied by the expenditure of time, electricity and physical labor.

The present invention is aimed at simplifying the process by combining in time the above stages of melting, reducing energy consumption, the duration of melting, expanding the range of materials used for melting, simplifying the design of the melting plant.

The specified technical result is achieved by the fact that, as the initial charge for melting in a nitrogen atmosphere with a pressure above 1 atm, a thermite (aluminothermic) mixture of powders of iron oxides, oxides of alloying elements of steel and aluminum is used, the components of which are taken in the ratio necessary to obtain the specified steel composition and ensuring spontaneous combustion without external sources of heat. After local ignition, the mixture burns, resulting in a metal melt of the desired composition. To control the oxidation of the metal, the ratio of the mass of aluminum to the total mass of oxygen contained in the oxides of the mixture is maintained in the range of 0.94-1.04. To increase the nitrogen content in steel, powders of nitrogen-containing materials, for example, nitrides or nitrided ferroalloys of alloying elements, are additionally introduced into the mixture.

The aluminothermic process conducted in air at atmospheric pressure is used to obtain ferroalloys, alloys, for welding rails, reinforcing bars, surfacing worn parts, as well as for producing steel castings of complex configuration [4-6]. For the successful implementation of the aluminothermic process without external heating, it is necessary that the heat released during combustion is at least 2300 J per 1 kg of the mixture [7].

The termite mixture for the implementation of the method consists of two main groups of components, taken in powder form:

a) Oxidizing agents - iron oxides and oxides of alloying components, upon reduction of which the bulk of steel is formed. These components are the source of oxygen necessary for the aluminothermic reaction.

b) Reducer - aluminum, which takes oxygen from oxidizing agents and restores them to metal.

The composition of the resulting steel is determined by the ratio of the components of the mixture. To increase the nitrogen content in the steel, powders of nitrogen-containing materials, for example, nitrides or nitrided ferroalloys of alloying elements, are introduced into the thermite mixture.

The desired technical effect when using a termite mixture follows from the mechanism of the termite process. Upon local ignition of the thermite mixture, a combustion wave arises, which subsequently propagates throughout the charge volume. In this wave, parallel spontaneous exothermic reactions of oxide reduction develop, for example:

Fe ₂ O ₃ + 2Al = 2Fe + Al ₂ O ₃ ;

Cr ₂ O ₃ + 2Al = 2Cr + Al ₂ O ₃ ;

3 / 2MnO ₂ + 2Al = 3 / 2Mn + Al ₂ O ₃ , etc.

During the course of these reactions, a large amount of heat is released, causing melting of substances and heating of the reaction products to temperatures above 2000 ° C. At the same time, in the combustion zone, the mutual dissolution of the released metals and the formation of the steel composition proceed. The entry of manganese, chromium, and aluminum into the metal melt causes its alloying and deoxidation. Alloying the metal with nitrogen occurs both due to the dissolution of solid nitrogen-containing charge materials in steel, for example CrN = [Cr] + [N], and due to the dissolution of nitrogen gas: ½ N ₂ = [N], which is favored by the developed surface of the dispersed metal droplets initially standing out in front of the combustion wave. Doping with gaseous nitrogen also takes place later on when the melt jet is released from the melting crucible into the mold. In the tail of the combustion wave, coalescence of metal droplets separating from liquid slag occurs, with the formation of compact volumes of liquid steel. At the end of the process, a liquid metal bath forms at the bottom of the crucible, on the surface of which a layer of liquid slag floats. The atmosphere of high-pressure gaseous nitrogen helps maintain its high content in steel, and also increases combustion stability, preventing metal emissions from the melting crucible.

Thus, during the burning of a thermite mixture, the processes of metal melting, its heating, alloying, and deoxidation are combined in time.

In addition to simplifying the melting technology, the application of the described method leads to several more positive technical results:

1. There is the possibility of using cheap metallurgical waste as rolling stock instead of metal materials — mill and forge scale, as well as rich ore concentrates.

2. Smelting does not require the cost of heat and electric energy, since all the energy needed for melting is enclosed in a chemical form in the charge materials themselves.

3. The high linear burning rate of the charge (of the order of 1 cm / s or more) leads to a short melting time.

4. The design of the installation for implementing the method is greatly simplified, since there is no need to place an induction furnace in the high-pressure chamber, which is replaced by a simple melting crucible.

5. The simplicity of the proposed method allows to use it to obtain shaped castings in the conditions of machine-building plants.

6. The simplicity of the installation configuration allows the use of higher gas pressures than in existing devices, which will lead to an increase in the nitrogen content in steel.

Pilot melts were conducted to produce steel alloyed with chromium, manganese and nitrogen by the aluminothermic method. Chromium and manganese are the most important alloying elements of steel; the relatively low affinity for oxygen of these elements ensures their successful reduction from oxides during the aluminothermic process.

The laboratory installation for experimental melting includes a cylindrical melting crucible made of refractory material. There is a hole in the bottom of the crucible. Under the crucible is a mold for receiving molten steel. The crucible and the mold are placed in a cylindrical water-cooled reactor, designed for a maximum working pressure of 200 atm. The reactor is equipped with a pressure gauge, an electric valve inlet and communicates with a gas system that allows gaseous nitrogen to be injected to the desired pressure or to vent gas into the atmosphere.

The following powder materials were used as starting materials in the experiments:

- iron oxide with an oxygen content of 29.05% (FeO _x );

- chromium oxide grade OHM-1 (Cr ₂ O ₃ );

- primary aluminum grade PA-1 (Al);

- chromium nitride containing 20.2% nitrogen (CrN);

- pure manganese dioxide (MnO ₂ ).

Iron oxide FeO _x used in smelting is a mill scale from low-carbon steels, calcined in an oxidizing flame at a temperature of 880-900 ° C to approximate the composition of Fe ₂ O ₃ , the richest in oxygen. Chromium nitride CrN was obtained by treating technical chromium with high-pressure nitrogen in the mode of self-propagating high-temperature synthesis. The starting materials were dried at a temperature of 200 ° C for 1-1.5 h, weighed in the ratio necessary to obtain steel of a given composition, and mixed in a mixer. The finished mixture was loaded into a melting crucible with a small seal. After sealing, the reactor was filled with gaseous nitrogen to a pressure of 80 atm, the valve connecting the reactor to the gas system was closed, and the charge was ignited. During combustion, the pressure in the reactor monotonically increases, reaching 95-100 atm. Stabilization of gas pressure indicates the end of combustion. The liquid metal formed as a result of combustion, after a short exposure, is poured into the mold. The melting time in our experiments was 8–10 s, which corresponds to a linear charge burning rate of 1.5–2 cm / s. Following the metal, slag flowed out of the melting crucible, leaving the melting crucible free for the next melting. Smelted ingots had a mass of about 1.5 kg, satisfactory surface quality, a semi-open compact shrink shell located in the upper part of the ingot. Samples of smelted steel were analyzed for the content of the main components. The nitrogen content was determined by pulsed melting in a graphite crucible in an inert gas stream. The results of experimental swimming trunks, for which the ratio of the mass of aluminum to the mass of oxygen in the charge was taken Z = 0.96, are presented in table 1.

Table 1. The composition of the charge and the steel No. The composition of the charge,% Steel composition,% FeO _x Cr ₂ O ₃ MnO ₂ CrN Al Fe Cr Mn N Al FROM N ₍₁₎ one 64.7 13.3 - - 22.0 83.1 15.6 - 0.69 0,050 0,063 0.32 2 64.7 10.0 - 4.2 21.1 82.5 16.1 - 1.06 0,065 0,071 0.35 3 58.1 10,4 6.6 4.1 20.8 74.0 17.6 6.3 1.31 0,055 0,060 0.51

Notes to table 1:

1. All steels contain 0.06-0.07% carbon, which falls within the range 0.05-0.08% C specified by us. This element was not specially introduced into the mixture, since natural impurities of carbon in the charge materials are sufficient for alloying (for example, iron oxide contains 0.07-0.08% C).

2. N (i) is the ultimate solubility of nitrogen in the resulting steel at 1600 ° C and a nitrogen pressure of 1 atm.

The values of N _{(1) were} estimated from the thermodynamic data given in [8]. From table 1 it follows that the actual nitrogen content in steel significantly exceeds the value of its ultimate solubility at atmospheric pressure. Chromium nitride additives help increase the nitrogen content in steel.

Table 2 shows the experimental data of the dependence of the content of aluminum and oxygen in the metal on the ratio Z between the masses of aluminum and oxygen in the charge.

Table 2. The content of aluminum and oxygen in steel at various values of the ratio Z between the masses of aluminum and oxygen in the initial charge. Component% Z 0.90 0.94 0.96 1.00 1,04 1.06 [Al] 0.001 0.01 0.05 0,07 0.18 0.30 [ABOUT] 0.01 0.006 0.002 <0.001 <0.001 <0.001

From table 2 it follows that the optimal ratio between the masses of aluminum and oxygen in the charge provides Z = 0.94-1.04, since at lower values of Z the steel is not sufficiently deoxidized, and at Z> 1.04 an undesirably high aluminum content in the metal is observed.

Literature

1. Rashev C.V. Alloy steel production. M .: Metallurgy. - 1981, p. 162-181.

2. Rashev C.V. High nitrogen steels. Sofia: Publishing House of the Bulgarian Academy of Sciences. - 1995, p. 23-32, 35-38.

3. A.S. SU 908915 A, 02.28.1982.

4. Encyclopedia of inorganic materials, v.2 / ed. Fedorchenko I.M. Kiev: Ch. ed. Ukrainian owl Encyclopedias, 1977, p. 530-531.

5. Murach N.N. and other non-furnace metallothermy. M: Metallurgy - 1956.

6. Malkin B.V., Vorobev A.A. Termite welding. M .: Publishing House of the Ministry of Agriculture of the RSFSR. - 1963, p. 100-101.

7. Rostovtsev S.T. Theory of metallurgical processes. M .: Metallurgizdat. - 1956, p. 375.

8. Balachandran G. et al. Some theoretical aspects on designing nickel free high nitrogen austenitic stainless steels. - ISIJ International, 2001, V.41, No.9, p. 1018-1027.

Claims (2)

1. The method of smelting steel alloyed with nitrogen in an atmosphere of gaseous nitrogen with a pressure of more than 1 atm, characterized in that the thermite mixture is used as a mixture of powders of iron, chromium and manganese oxides, nitrides or nitrided ferroalloys of alloying elements and aluminum, taken in the ratio necessary to obtain steel of a given composition, while the thermite mixture is locally ignited to ensure spontaneous combustion of the mixture without external heat sources. 2. The method according to claim 1, characterized in that the aluminum powder is introduced with a mass of 0.94-1.04 of the total mass of oxygen contained in the oxides that make up the charge.