RU2376389C1 - Method of structural steel production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes receiving of semiproduct in arc furnace, discharge of metal into lade without slag, treatment by main slag, alloying and deoxidisation of metal, vacuum-slag treatment with simultaneous blowing by argon. During the treatment process it is regulated formation of composition of nonmetallics, measuring oxidation of metal and keeping at specified level content of aluminium and silicon in metallic melt, herewith aluminium is introduced in amount, depending on amount of aluminium, introduced into melt, %, of specified content of aluminium in metal, %, activity of oxygen in metal, %. Content of aluminium in final metal must correspond to 0.004-0.05 % and silicon not to exceed 0.10%.

EFFECT: reduction of amount of nonmetallics, providing of its specified composition and increasing of mechanical and performance attributes of finished product.

3 cl, 3 ex, 3 tbl

Description

The invention relates to metallurgy and can be used in the smelting and out-of-furnace treatment of structural steels of various grades, including those intended for the manufacture of parts of new generation power plants.

A known method for the production of structural steel, including the production of intermediate in an arc furnace, the release of metal into a ladle without slag, processing with main slag, alloying and deoxidation of metal, vacuum slag treatment with simultaneous purge with argon [RF Patent No. 2095429 from 10.11.97].

The disadvantage of this method is the impossibility of guaranteed provision of a given composition of non-metallic inclusions. Meanwhile, for grades of structural chrome steel intended for the manufacture of power plants and steam pipelines operating under conditions of super supercritical steam parameters (temperature up to 620 ° C and pressure up to 30 MPa) and high loads, the presence of certain types of non-metallic inclusions can cause catastrophic destruction of the metal.

The disadvantage of the prototype method is that the number of deoxidizers introduced into the metal is determined not by the state of the metal melt, but by the norms of their consumption specified in the technological instruction.

The technical result of the proposed technical solution is to reduce the number of non-metallic inclusions, ensure their desired composition and increase the mechanical and operational properties of the finished product.

The technical result is achieved by the fact that when implementing the method of production of structural steel, which involves obtaining an intermediate in an arc furnace, releasing metal into a ladle without slag, treating with main slag, alloying and deoxidizing the metal, vacuum slag treatment with simultaneous purging with argon, additionally control the formation the composition of non-metallic inclusions by measuring the oxidation of the metal and maintaining at a predetermined level the content of aluminum and silicon in the metal melt, and aluminum added to the melt in an amount which is determined by the formula:

[Al] = (1.3 ... 1.6) · [Al] _ass - (1 ... 5) · 10 ^-6 · α _o ^-1 ,

where [Al] is the amount of aluminum introduced into the melt, expressed as a percentage; [Al] _back - the specified aluminum content in the metal, expressed as a percentage;

α _about - the activity of oxygen in the metal, expressed as a percentage,

and the aluminum content in the final metal must comply

0.004-0.05% and silicon does not exceed 0.10%.

The measurement of oxidation and the introduction of aluminum is carried out one or more times to obtain a given level of aluminum content.

Aluminum is introduced into the metal in the form of a rolled or cored wire using a tribamer.

The reduction in the amount of non-metallic inclusions is ensured by deep deoxidation in the early stages of out-of-furnace treatment. In this case, the earliest possible binding of oxygen to non-metallic inclusions gives them the opportunity to remove most of the volume of liquid metal into slag, therefore, primary deoxidation should be carried out immediately at the outlet. In addition, primary deoxidation at the outlet contributes to a decrease in the oxidizing potential of the ladle lining and to a decrease in its oxidizing effect at subsequent stages of the redistribution, which together allows reducing the number of inclusions present in the finished metal. Increasing the purity of the metal in the inclusions helps to improve the properties of metal products.

Improving the properties of the finished product is ensured by limiting the silicon content in the metal to 0.10%. This element enhances the segregation of sulfur and phosphorus (especially in large ingots), and an increase in its content in the metal more than 0.10% leads to a significant decrease in viscosity and an increase in the temperature of the brittle-viscous transition. In addition, the inclusion of silicates have dimensions, as a rule, lying in the critical range. The elastic modulus of quartz glass is much lower than steel; silica silica glasses can precipitate in the form of films at the boundary of growing crystals and significantly degrade, for example, the ductility of steel during hot deformation. The same inclusions can occur during the joint deoxidation of silicon and aluminum. The combination of these factors explains the lower level of properties of steel deoxidized by silicon, compared with steel deoxidized by aluminum.

Obtaining higher properties is also achieved by controlling the composition of non-metallic inclusions and ensuring their least harmful composition, which, in turn, is achieved by introducing a strictly defined amount of aluminum into the metal, calculated from the measured metal oxidation. In this case, the amount of aluminum introduced is defined as the difference between its predetermined content and the actual:

[Al] = (1.3 ... 1.6) · [Al] _ass - (1 ... 5) · 10 ^-6 · α _o ^-1 .

The latter value is calculated through the measured oxygen activity in the metal melt. The coefficient (1 ... 5) · 10 ^-6 is the specific (per one mol of aluminum) equilibrium constant for the reaction of oxidation of aluminum to Al ₂ O ₃ . Its value characterizes the oxidizing potential of the phases in contact with the metal - the lining and slag. With a low oxidizing ability of neighboring phases, the value of the power-law coefficient approaches 1 · 10 ^-6 , with a larger oxidizing potential, the coefficient increases. The coefficient value less than 1 · 10 ^-6 cannot be established due to the thermodynamics of the process, greater than 5 · 10 ^-6 values occur when the oxidation potential of the lining or slag is very high and is unacceptable in the production of the considered type of steel (metal will have a very high contamination by non-metallic inclusions).

The coefficient 1.3 ... 1.6 is set based on the characteristics of the casting method used. With poor protection of the metal from secondary oxidation, the coefficient value is increased, with the complete prevention of the interaction of metal and oxygen, the value of this coefficient is set to 1.3.

The value of [Al] _back should be set, guided by the following considerations. If it is necessary to provide the oxide phase in the finished metal products, which is mainly represented by chromium-manganese spinels, the specified aluminum content is 0.004 ... 0.005%; if the specified type of inclusions is chromium aluminates, the required value of the [Al] value of the _back is 0.009 ... 0.015%; to ensure corundum, as the dominant type of inclusions, a given aluminum content is taken equal to 0.02 ... 0.05%. The specific value of the aluminum content to ensure one type or another of inclusions depends on the composition of the steel.

The desired composition of the oxide phase should be selected based on the results of numerous studies of the influence of the composition of inclusions on the properties of structural steel deoxidized by aluminum, which showed that the highest level of properties, including heat-resistant, is observed when corundum is the main type of non-metallic inclusions. In the absence of large exogenous inclusions in the metal and a uniform distribution of the oxide phase precisely in the case when corundum is the dominant type of non-metallic particles, steel has the highest technological and heat-resistant properties. The least negative effect of corundum particles, for example, on technological plasticity is explained by the fact that alumina inclusions fall out in the form of dispersed particles of less critical size (comparable to grain size), their elastic modulus is much higher than that of steel, they cannot form film inclusions and have low adhesion at the metal border. Therefore, at the boundary a corundum particle is a metal or in areas adjacent to these particles, high stresses do not occur, which explains the increased level of properties of steel deoxidized by aluminum.

Particles of chromium-manganese spinel (which form the basis of the oxide phase of non-oxidized chromium steel) are most undesirable in the steel of this type. Basically, they are relatively large inclusions (significantly larger than the precipitation of corundum). In addition, particles of this composition have a high adhesion work to chromium steel, therefore very high stresses appear in the metal near their surface, which leads to easier nucleation and development of a crack.

Thus, the least harmful type of oxide inclusions in the steel is a chromium corundum, hence to obtain the smallest reduction properties caused by nonmetallic inclusions, must deoxidize aluminum metal and the value [Al] _ass should be set equal to 0.02 ... 0.05%. But, nevertheless, the proposed method allows to provide other types of non-metallic inclusions in the metal, so the aluminum content in the final metal should be in the range of 0.004-0.05%. If, after measuring the oxidation of the metal and the introduction of aluminum, the predetermined level of its content does not reach, then this operation is repeated one or more times until the specified level of aluminum content is obtained.

Examples of the method. Industrial testing of the developed recommendations was carried out in the conditions of LLC OM3-Special Steel.

Three melts of steel grade 25KHNZMFA were carried out. Metal samples were taken from experimental ingots for further research. At all heats, the intermediate was smelted in a 50-ton electric furnace using metallized pellets with a sulfur content of 0.005% and scrap with a sulfur content of 0.015%. The mass of liquid metal was 56 tons (melts were overloaded). Before being released from the furnace, the metal temperature was 1650 ... 1655 ° С, carbon content - 0.040 ... 0.055%, sulfur content - about 0.007%. The intermediate product was produced in a steel ladle with complete slag cut-off. After filling about 1/4 of the bucket, a solid slag mixture (357 kg) was then added, then manganese (229 kg) and aluminum (86 kg). At the end of production, an additional amount (29 kg) of aluminum section was added to deoxidize the slag and enhance its desulfurizing ability. After that, the metal was transferred to the ASEA-SKF after-furnace treatment plant, where another portion of slag-forming and alloying was planted and averaging was performed. Next, the oxygen activity was measured and the calculated amount of aluminum wire was introduced. The absorption coefficient of aluminum with this input method is 0.75.

In the first heat, the oxygen activity in the metal melt before deoxidation was 0.0015%. The specified type of non-metallic inclusions is corundum, i.e. [Al] _ass = 0.040%. The required aluminum concentration was 0.061% in the metal melt, and the wire consumption was 45.5 kg.

In the second heat, the oxygen activity in the metal melt before deoxidation was 0.0008%. The specified type of non-metallic inclusions is corundum, i.e. [Al] _ass = 0.040%. The required concentration of aluminum was 0.058% in the metal melt. The wire consumption was 43.3 kg.

In the third heat, the oxygen activity in the metal melt before deoxidation was 0.0007%. The specified type of non-metallic inclusions is corundum, i.e. [Al] _ass = 0.040%. The required concentration of aluminum was 0.057% in the metal melt, the wire consumption was 42.6 kg.

After the introduction of aluminum, the metal was evacuated. The exposure time under vacuum was 20 ... 25 min, while the pressure was maintained at about 0.8 mm Hg. The argon flow rate while holding the metal under vacuum was maintained at a level of 214 l / min. The slag composition during the evacuation period in the experimental melts was as follows, wt.%: CaO - 50 ... 53, Al ₂ O ₃ - 21 ... 27, FeO <0.7. The slag thickness is 140 ... 165 mm. At the end of the evacuation, the inclusions were kneaded by blowing metal through bottom porous plugs with argon for 15 min at a gas flow rate of 43 l / min without exposing the metal mirror. Steel was cast in a 225-ton vacuum chamber at a speed of 6.2 t / min. During the overflow of metal from the steel ladle into the intermediate ladle, the jet was protected by a special pipe. The junction of the protective pipe with the steel-bucket gate was sheathed and flushed with argon. Profits were insulated with the Nermat mixture.

Samples for the analysis of non-metallic inclusions were taken from the middle part of the ingot at a distance of 1/3 of the radius from the longitudinal axis (from the center).

The chemical composition of the metal smelted during these three melts and the composition of the oxide phase in it are given in Table 1 and Table 2, respectively.

Table 1. The chemical composition of the metal experimental swimming trunks FROM Si Mn S P Cr Ni Mo V Al Given 0.25-0.30 no more than 0,04 0.3-0.6 <0.010 <0.010 1.3-1.7 3.4-3.8 0.5-0.7 0.12-0.18 0.04 Melting 1 0.27 0,03 0.35 0.005 0.006 1,5 3.41 0.51 0.17 0,039 Melting 2 0.28 0.01 0.37 0.005 0.005 1,6 3,55 0.57 0.14 0,042 Melting 3 0.26 0.02 0.45 0.005 0.005 1.35 3.74 0.53 0.15 0,041

Table 2. The composition of non-metallic inclusions in the metal of experimental swimming trunks melting [O],% * OKNV,% ** The composition of the oxide precipitate, wt.% [O] nv,% *** SiO ₂ MnO Cr ₂ O ₃ FeO Al ₂ O ₃ Melting 1 0.0040 0.0080 5.7 0.8 0,0 1,6 80.0 0.0033 Melting 2 0.0045 0.0095 6.3 4.7 1.9 3.2 81.0 0.0042 Melting 3 0.0041 0.0080 2.0 3.8 1,0 1,6 91.6 0.0037 * oxygen content determined by vacuum melting ** total number of non-metallic inclusions *** oxygen content calculated according to the composition and number of non-metallic inclusions

As can be seen from the method, steel smelting in accordance with this method made it possible to obtain a metal with an oxide phase of a given composition — corundum (Al ₂ O ₃ ) was the main type of non-metallic inclusions. As shown by the results of metallographic studies, oxide particles were uniformly distributed in the metal volume, their maximum size did not exceed 10 microns.

According to the results of studies of metal samples taken from ingots, the steel obtained has a homogeneous structure and increased values of the characteristics of heat resistance, ductility and short-term mechanical properties, which leads to high reliability of products made from it. The level of long-term strength of the smelted steel at 600 ... 630 ° C was 73 ... 95 N / mm ² and exceeded the specified level of properties.

To assess the resistance to brittle fracture under creep conditions (in the zones of structural stress concentrators - grooves, grooves, transitions, etc.), the level of long-term ductility of the rotor metal was determined. The critical level of this characteristic is in the range of 4 ... 8%. The obtained values of long-term ductility are at least 17%; in the bulk, they are in the range of more than 20% and significantly exceed the critical level.

The studies made it possible to identify the relationship in the system of metal oxidation - deoxidizer content - non-metallic inclusions, with the help of which it is possible to reliably predict the composition of the formed oxide inclusions and to develop a method for the production of structural steel that allows controlling the composition of the oxide phase and ensuring its minimum amount in the finished metal at a given composition.

Table 3. Parameters and test results of the test metal for long-term strength. Test temperature, ° С Voltage, N / mm ² Time to destruction, h Final elongation, δ,% Note 600 180 116 16.8 axial zone 180 280 21.3 surface 160 1950 22.4 axial zone 160 1480 24.3 surface 140 2500 ^* 19.6 axial zone 140 6400 ^* 22.4 surface 120 2050 → Tests are ongoing axial zone 120 7525 ^* 27.3 axial zone 120 1600 → Tests are ongoing surface 120 13187 ^* 29.4 surface 650 160 12 24.1 axial zone 160 28 27.3 surface 140 39 19.6 axial zone 140 94 22.4 surface 120 110 27.3 axial zone 120 187 29.4 surface one hundred 520 → Tests are ongoing axial zone one hundred 540 → Tests are ongoing surface ^* recalculation from a temperature of 650 ° C.

Claims (3)

1. Method for the production of structural steel, including the production of intermediate in an arc furnace, the release of metal melt into a ladle without slag, processing with main slag, alloying and deoxidation of the metal melt, vacuum slag treatment with simultaneous argon purging, characterized in that in the vacuum slag process the processing of metal melt in a ladle controls the formation of the composition of non-metallic inclusions by measuring the oxidation of the metal melt and adjusting the aluminum content to a predetermined level of indium and silicon in a metal melt, and aluminum is introduced into the metal melt in an amount determined by the formula:
[Al] = (1.3 ... 1.6) · [Al] _ass - (1 ... 5) · 10 ^-6 · α _О ^-l , where
[Al] is the amount of aluminum introduced into the metal melt, wt.%;
[Al] _back - the specified aluminum content in the metal melt, wt.%;
α _about - the activity of oxygen in a metal melt,%,
with the content in the final metal, wt.%: aluminum 0,004-0,05 and silicon not more than 0.10. 2. The method according to claim 1, characterized in that the measurement of oxidation and the introduction of aluminum is carried out one or more times to obtain a given level of aluminum content in the metal melt. 3. The method according to claim 1 or 2, characterized in that the aluminum is introduced into the metal melt in the form of a rolled or cored wire using a tribamer.