RU2842922C1 - Complex additive for steel deoxidation and method of steel manufacturing - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, specifically to a complex additive for steel deoxidation, which is suitable for use in steels for offshore construction, main pipelines, industrial moulds, metal supports, production of rails and automobiles. Complex additive contains the following chemical components in wt.%: Fe 41–59%, Zr 5–11%, Ti 14–26%, Mg 11–19%, rare earth metals (RE) 4–10%, wherein the weight percentage content of the elements Zr, Ti, Mg and RE satisfies the following formula: (Ti+Mg+RE)/Zr=4–8. In the method, the temperature and composition of the melt are controlled to obtain liquid steel, which is discharged into a ladle, performing its preliminary reduction, Now, complex additive is used for final deoxidation. Liquid steel after final deoxidation is subjected to sequential refining and, then, used for continuous casting.

EFFECT: complex additive increasing steel ductility, viscosity, fatigue strength and resistance to localized corrosion is used for steel deoxidation.

8 cl, 7 dwg

Description

Field of technology

[0001] This invention relates to the technological field of metal smelting, namely to a complex additive that forms an impurity with a core-shell structure, a method for its manufacture and a smelting method.

State of the art

[0002] Steel materials are the basis of industrial production, the most important structural materials for use in industry, which play a decisive role in economic and social development. Steel purification technology is the basis for creating high-quality products with high characteristics. Since the 1980s, the purity of steel has been continuously improved. However, in the process of using these materials, deficiencies in fatigue resistance, resistance and localized corrosion and other performance characteristics are still found that do not meet the requirements of socio-economic development; the main reason for this is the lack of a deep understanding of the processes of impurity formation in steel, the functioning of micromechanisms and their control methods. Impurities (mainly oxide inclusions) in steel have a serious impact on the quality of steel material. The presence of impurities in steel destroys the continuity of the metal, reduces the mechanical, physical, chemical and technological properties of steel. Impurities exist in steel mainly in the form of non-metallic compounds such as oxides, sulfides and nitrides; their presence causes non-uniformity of the steel structure and adversely affects its physical, mechanical and chemical properties. The presence of impurities in the composition of steel reduces its ductility, toughness and fatigue life, worsens its workability and has a direct negative effect on the surface finish of the steel material, its weldability and resistance to localized corrosion. For example, the presence of impurities in the composition of steel may cause surface defects in thin sheet steel used in the production of automobiles or electrical goods, cracking of thin sheet steel used in the production of DI tanks, hydrogen cracking of pipeline steel, rupture in the process of manufacturing radial components of wheel tires and deterioration of the fatigue performance of bearing steel; in addition, non-metallic inclusions in steel have a negative effect on the rupture resistance, low-temperature impact toughness and marine corrosion resistance of steel.

[0003] Melting, pouring and crystallization of steel is a single complex physical and chemical process. Endogenous impurities are various types of compounds formed as a result of complex chemical reactions when steel is in a liquid state or in the process of solidification; when steel is in the process of solidification, such impurities do not have time to rise to the surface and remain in the steel; endogenous impurities can also be non-metallic substances dissolved in steel under high temperature conditions; when the temperature of molten steel decreases, the solubility of such metallic substances in steel worsens, resulting in their release from the steel, but they remain in the steel in the form of impurities.

[0004] In particular, the main type of impurities are oxide inclusions in the steel. Al ₂ O ₃ , MgO, CaO and other oxides, as well as complex oxides, calcium aluminate, magnesium aluminate and aluminum magnesium spinel are the main forms demonstrating the presence of oxides in steel. Such impurities easily settle on the inner walls of the immersion nozzle, which leads to clogging of the nozzle, can lead to the occurrence of defects on the surface of steel products and deterioration of the surface purity of the steel. Al ₂ O ₃ , among the frequently encountered oxide inclusions, has the greatest negative effect on the quality of steel; this compound belongs to brittle non-deformable impurities, its thermal deformability differs significantly from the deformability of the matrix; under the action of stress arising during heat treatment, large pieces of Al ₂ O ₃ and other brittle impurities are destroyed as a result of deformation, creating inclusions with sharp rhombic angles, which are distributed in the matrix in the form of chains; such hard inclusions of Al ₂ O ₃ of irregular shape can scratch the matrix; and a stress concentration field is formed around the impurities, which can form cracks and fissures on the boundary surfaces. For example, in medium-carbon and high-carbon steel, especially in bearing steel and steel for heavy rails, Al ₂ O ₃ inclusions under the action of cyclic stress can form fatigue fracture centers, which ultimately, as a result of the expansion of fatigue cracks, lead to the "chipping" of large pieces of metal up to the formation of ruptures in the steel.

[0005] For steel grades with a normal structure, the sulfides in the steel do not significantly affect its strength at room temperature; however, their volume and shape have a very large effect on toughness (including reduction in cross-sectional area and impact toughness), especially on toughness at low temperatures. For the vast majority of steel grades, an increase in the sulfur content leads to a decrease in the properties of the steel. At the temperature used in heat treatment, MnS inclusions are elongated into long strips, causing anisotropy in the toughness of the steel. Sulfide inclusions have a noticeable effect on the properties of the product; manganese sulfide impurities are easily deformed during rolling, while calcium sulfides have relatively high rigidity and are practically not deformed during rolling. Sulfides can provoke the occurrence of hydrogen cracks. Hydrogen is easily trapped by elongated MnS inclusions, and this reduces the resistance of steel to hydrogen cracking. Weldability, machinability and mechanical properties of steel also depend on the type and volume of sulphide inclusions it contains. Reticulated sulphides contained in steel can cause red brittleness.

[0006] The effect of impurities contained in steel on its properties depends on many factors. The properties of steel are affected by such factors as the amount of impurities contained in steel, their grain size, shape and distribution, the adhesion force of inclusions to the steel matrix, the ductility of impurities and the value of the elasticity coefficient, their coefficient of thermal expansion and hardness. The use of appropriate technological measures capable of effectively controlling the parameters of impurities in the composition of steel can not only eliminate their harmful effects, but also fully utilize their useful functions. Large and concentrated inclusions cause great harm to the properties of steel, and finely distributed, dispersed and fine-grained impurities not only do not cause any harm, but in some cases can improve the properties of steel. For example, the use of fine-grained and finely dispersed oxide inclusions in the composition of steel, which have excellent temperature resistance, can improve the weldability of steel, especially under conditions where it is necessary to withstand a large volume of incoming heat.

[0007] In the conventional smelting process, ferromanganese, ferrosilicon and aluminum are mainly used as deoxidizers. They react with oxygen to form oxides that cannot float to the surface and are not removed with the slag, that is, they form non-metallic inclusions in the steel. Nitrogen in steel is also generally considered a harmful element; it has extremely low solubility in steel, but at the same time has a strong affinity for titanium, vanadium and niobium; in addition, molten steel can absorb a large amount of nitrogen from the atmosphere, so nitride inclusions are easily formed in steel.

[0008] The existing deoxidation technology using Al is a frequently used deoxidation technology in which Al blocks, Al grains, Al wires or Fe-Al alloys are used as deoxidizers. The deoxidation product Al ₂ O ₃ among the frequently occurring oxide inclusions has the greatest negative effect on the quality of steel; this compound belongs to brittle non-deformable impurities, its thermal deformability is significantly different from the deformability of the matrix; under the action of stress arising in the process of heat treatment, large pieces of Al ₂ O ₃ and other brittle impurities are destroyed as a result of deformation, creating inclusions with sharp rhombic angles, which are distributed in the matrix in the form of chains; such hard inclusions of Al ₂ O ₃ of irregular shape can scratch the matrix; and a stress concentration field is formed around the impurities, which can form cracks and fissures on the boundary surfaces.

[0009] The existing Ca treatment technology is the most widely used smelting technology with inclusion shape improvement in the metallurgy at present. Ca is a very good purifier for molten steel; it can perform not only deep deoxidation but also deep desulfurization; after the deoxidation of molten steel, when the [O] content is already at a very low level, the direct deoxidation reaction with Ca turns into an auxiliary process; in this phase, it mainly reacts chemically with Al ₂ O ₃ in the steel to form calcium aluminate. Therefore, it not only solves the deoxidation problem but also reduces the impurity content of Al ₂ O ₃ in the steel; however, a part of the calcium aluminate that has not floated up and been removed with the slag still remains in the steel; For steel materials with special performance requirements, such as resistance to localized corrosion, fatigue resistance, and resistance to severe frost, this approach does not solve the problem in principle.

[00010] Control of non-metallic inclusion channels in steel, firstly, allows to reduce the formation of impurities during the melting process and during the process of casting operations, as well as to reduce the ingress of contaminants into liquid steel from the outside; secondly, it allows to remove impurities present in the liquid steel or to minimize their negative impact on the properties of steel. The main technological methods: (1) control of impurities during the melting process in a converter furnace, includes reducing additional blowdown and minimizing the amount of bottom slag to reduce the oxygen content at the end of the converter furnace; increasing the MgO content and alkalinity of the final converter slag to reduce the volume of bottom slag; application of slag removal during melt tapping, slag tapping, slag denaturation; slag washing in the process of tapping the smelt from the converter furnace for desulfurization, so as to reduce the sulfur content of the liquid steel and reduce the harmful effects of sulfide inclusions, etc. (2) Inclusion control in the refining process: including ladle blowing, air curtain formation at the retaining wall of the tundish, pressure increase/decrease method (NK-PERM), magnetohydrodynamic stirring of the casting ladle, centrifugation of the tundish, application of electromagnetic brake for the crystallizer, calcium processing technology, etc.

[00011] Several elements are distinguished that promote the spheroidization of oxides, sulfides and sulfide oxides: calcium, tellurium, magnesium, rare earth elements, etc. The four elements mentioned: calcium, tellurium, magnesium and rare earth elements, have extremely low solid solubility in the iron matrix. The reason that these elements stimulate the spheroidization of sulfides is that the sulfides of these elements have low wettability in the steel matrix, high interphase energy and a large wetting angle, which easily leads to the formation of spherical shapes.

[00012] However, the above-mentioned methods have various disadvantages: (1) Regarding steel treated with calcium: the methods for introducing calcium are highly complicated; the first method is to add calcium to liquid steel by spraying it with a spray gun into the casting ladle; the other method is to add calcium by using a steel wire with a protective sheath. (2) Regarding steel treated with tellurium and selenium: Both elements are expensive and not in demand for economic reasons; both elements are toxic; Tellurium can also cause red brittleness during rolling. (3) Regarding steel processed with magnesium: Because magnesium is very light, it is very difficult to incorporate it into liquid steel; it also has a low yield rate. (4) Regarding steel processed with rare earth elements: During the production and use of rare earth metals, radioactive dust is easily generated, and the resulting radioactive hazard cannot be ignored. When pouring steel treated with rare earth elements, burr is formed on the nozzle; the problem of burr formation on the nozzle also often occurs when using such strong deoxidizers as Al. Rare earth element impurities have a high specific gravity, usually in the range of 5.5-6.5, which explains the difficulty of their floating to the surface; this is especially pronounced when adding excessive amounts of rare earth elements, which can lead to an increase in harmful inclusions in the steel, including large-piece inclusions, the formation of brittle intermetallic compounds of rare earth elements with iron and deterioration of the steel properties.

[00013] From the above analysis, it can be understood that regardless of the type of control applied, whether it is impurity control during smelting in a converter furnace or impurity control during refining, both types are characterized by the complexity of technological processes, a large number of technological operations, a large amount of investment in equipment, and a high complexity of control. Based on this, the present invention provides a complex additive that forms impurities with a core-shell structure, a method for producing the same, and a smelting method.

Contents of the invention

[00014] In order to eliminate the existing technical disadvantages, the present invention provides a complex additive that forms impurities with a core-shell structure, a method for producing the same, and a method for melting. The aim is to provide a core-shell structure forming impurity that has a number of useful effects, including fine grain, spheroidization, and diffusion; and is capable of significantly increasing the plasticity, toughness, fatigue strength, localized corrosion resistance, weldability, and cold bendability of ferrous materials.

[00015] In order to eliminate the above-mentioned technical disadvantages, the present invention, on the one hand, represents a complex additive that forms impurities with a core-shell structure, its composition includes the following chemical components in a weight percentage content: Fe: 41-59%, Zr: 5-11%, Ti: 14-26%, Mg: 11-19%, RE: 4-10%; the weight percentage content of the elements Zr, Ti, Mg and RE satisfies the following formula: (Ti+Mg+RE)/Zr=4-8.

[00016] The above mentioned components perform the following functions as part of the complex supplement:

[00017] Zirconium: zirconium is a frequently used deoxidizer; however, due to the fact that it is often used for complex deoxidation in combination with titanium or titanium-aluminum alloy, this leads to the formation of fine-grained dispersed complex impurities; such complex impurities bind the movement of crystallite boundaries during the thermal cycle of welding, limiting the growth of austenite grains, and during the transformation of austenite, they stimulate the nucleation of acicular ferrite crystallization centers, which contributes to the effective refinement of crystal grains and an increase in viscosity in the coarse weld grain zone; at the same time, zirconium is a strong carbide-forming element, its functions in the composition of steel are similar to the functions of niobium, vanadium and titanium; carbides or nitrides are formed that are capable of effectively refining crystal grains, which contributes to an increase in the low-temperature properties of steel; However, excessive zirconium content leads to grain coarsening and loss of the function of limiting crystal grain coarsening, therefore the mass content of Zr according to this invention is 5-11%.

[00018] Titanium: titanium has an extremely strong affinity for oxygen, nitrogen and carbon, is an excellent deoxidizer and an element that effectively binds nitrogen and carbon; titanium oxides are considered to be the most effective impurities that promote the nucleation of crystallization centers in steel, which can effectively stimulate the nucleation of crystallization centers of acicular ferrite, and are widely used in oxide metallurgy; titanium forms fine dispersed TiN grains in the steel, which slowly dissolve only when the steel is heated to a temperature of 1400 °C; during the thermal cycle of welding, TiN particles effectively prevent the coarsening of austenite crystals and contribute to an increase in viscosity; TiN particles effectively stimulate the formation of acicular ferrite and effectively improve the weldability of steel; However, excessive Ti content has a negative effect on improving the properties of steel, creates a risk of forming large titanium carbonitrides, becomes a source of crack formation and can lead to a decrease in toughness, therefore, the mass percentage of Ti in this invention is 14-26%.

[00019] Magnesium: magnesium is a strong deoxidizing element that forms complex sulfide oxides; magnesium can reduce the amount and grain size of impurities, promotes their uniform distribution, and improves the shape of inclusions in steel; adding magnesium in microdoses can improve the particle size of carbides in stainless steel and their distribution, promotes more uniform fineness of carbide particles, and the forming impurity MgO plays the role of binding the boundaries of austenite crystallites and has a positive regulating effect on the grain size, so the mass content of Mg according to this invention is 11-19%.

[00020] Rare earth elements: in the composition of steel materials, they can be used as deoxidizers, especially lanthanum (La) and cerium (Ce); when added to molten steel, the properties of rare earth elements are sharply activated, which react with O and S in the steel to form extremely stable oxides, sulfides and sulfide oxides; the addition of rare earth elements obviously improves the mechanical properties of the steel material and its corrosion resistance; steel alloyed with rare earth elements has once been the subject of active research and has been used to develop steel materials meeting certain needs, so the mass content of RE in the present invention is 4-10%.

[00021] It should be explained that the symbols “Ti”, “Mg”, “RE” and “Zr” in the above formula (Ti+Mg+RE)/Zr mean the mass percentage of the corresponding element, and the value substituted into the above formula is the value before the percent sign, for example, if the mass percentage of Ti is 20%, the mass percentage of Mg is 15%, the mass percentage of RE is 7% and the mass percentage of Zr is 8%, then the following values will be substituted into the above formula: (Ti+Mg+RE)/Zr=(20+15+7)/8=5.2.

[00022] This invention has the following beneficial effects:

[00023] (1) The complex additive provided in the present invention has a number of beneficial effects including fine grain, spheroidization and diffusion; it is capable of significantly increasing the plasticity, toughness, fatigue strength, localized corrosion resistance, weldability of ferrous materials and their cold bending ability.

[00024] (2) The complex additive of the present invention is suitable for use in steels for marine construction, steels for pipelines and vessels, steels for cryogenic vessels, steels for bridge beams, steels for metal supports, steels for rails, bearing steels, toothed steels, steels for making knives for tunnel boring machines, steels for cords, spring steels, cold heading steels, steels for the production of automobiles, electrical steels and other types of steels that have high requirements for the shape of inclusions; the resulting impurities are complex impurities with a core-shell structure, the granularity, spheroidization, dispersion and bulk modulus of elasticity of which are similar to the corresponding indicators of the matrix phase of iron; they noticeably increase the corrosion resistance in a neutral aqueous environment, the resistance to marine corrosion, the fatigue resistance, and also increase the plastic viscosity and reduce the local stress concentration.

[00025] Based on the above-described technical solution, this invention also makes it possible to implement the improvements presented below.

[00026] Further, it contains the following chemical components in mass percentage: Fe: 46-57%, Zr: 6-11%, Ti: 15-16%, Mg: 12-19%, RE: 9-10%.

[00027] Further, it contains the following chemical components in mass percentage: Fe: 50%, Zr: 8%, Ti: 20%, Mg: 15%, RE: 7%.

[00028] Further, the mentioned RE element includes an La element and a Ce element, the mass ratio of which is (70-90):(10-30).

[00029] Further, said Zr element is spongy metallic zirconium and/or metallic zirconium; said Mg element is any one kind or a combination of at least two kinds from among metallic magnesium pieces, magnesium grains, magnesium-zirconium alloy pieces, and magnesium-zirconium alloy grains.

[00030] On the other hand, the present invention provides a method for producing the said complex additive forming an impurity with a core-shell structure, which includes the following sequence: first, Zr, Ti and RE elements are added to the Fe-Ti alloy, then smelting is carried out in a vacuum induction furnace, after which casting is carried out under vacuum conditions and the complex additive is obtained.

[00031] Further, the melting time in a vacuum induction furnace according to the mentioned preparation method is 4-8 hours.

[00032] On the third hand, the present invention provides a melting method which includes the following sequence:

[00033] (1) After processing liquid pig iron and/or steel scrap in a converter or electric arc furnace, the temperature and composition of the melt are adjusted to obtain liquid steel;

[00034] (2) Liquid steel is introduced into a ladle, its preliminary deoxidation is performed, and then, using the mentioned complex additive according to any of the claims of the invention, final deoxidation is performed and liquid steel is obtained after final deoxidation;

[00035] (3) The said liquid steel, after final deoxidation, is successively subjected to refining and then used for continuous casting.

[00036] Next, in step 1) of the sequence, the temperature of said molten steel is 1551-1690°C, the free oxygen content of said molten steel is 101-399 parts per million; step (2) specifically includes the following: liquid steel is put into a ladle, under the condition of microbubble stirring, said molten steel in the ladle is preliminarily deoxidized using a Fe-Si alloy or a Fe-Si-Mn alloy, and the free oxygen content of said molten steel is adjusted in the range of 11-99 parts per million; then, under the condition of microbubble stirring using a complex additive, final deoxidation is performed and molten steel after the final deoxidation is obtained; in step (3), first LF refining, VD refining or RH refining is performed on said molten steel after the final deoxidation, and then it is used for continuous casting.

[00037] Next, in step (2) of the sequence, the addition volume of said complex additive to each ton of said liquid steel is 0.51-4.9 kg. The complex additive is introduced into the liquid steel in the form of a lump ligature or wire with a protected core, the grain size of the complex additive is 3-19 mm.

Description of the attached drawings

[00038] Fig. 1 is an illustration of inclusions in a low alloy high strength steel obtained after electrolytic extraction in a non-aqueous solution according to the present invention;

[00039] Fig. 2 is an illustration of the distribution of inclusion elements in a low-alloy high-strength steel obtained after electrolytic extraction in a non-aqueous solution according to the present invention;

[00040] Fig. 3 is a SEM image after electrolytic extraction of impurities in the composition of a low-alloy high-strength steel of the present invention, deoxidized by a standardized procedure using Al, and an EDS analysis;

[00041] Fig. 4 is a curve of the precipitation of sulfide and nitride grains during the solidification process of steel according to the present invention;

[00042] Fig. 5 is a density of states curve of various oxides of the present invention, including the density of states of (a) Ti ₂ O ₃ , (b) ZrO ₂ , and (c) Al ₂ O ₃ ; the dashed lines indicate the Fermi surface;

[00043] Fig. 6 is a result of testing the KAM steel of the present invention after complex alloying using the rare earth element La; including (a) the IQ diagram, (b) the KAM diagram;

[00044] Fig. 7 is a diagram of the elastic tensile Young's modulus of the matrix phase of Fe and inclusions of Al ₂ O ₃ and AlLaO ₃ according to the present invention; including (a) the matrix phase of Fe, (b) Al ₂ O ₃ , (c) AlLaO ₃ .

Specific implementation options

[00045] The following is a description of the principles and features of the present invention; the embodiments provided are intended solely to describe the present invention and do not limit the scope of the present invention in any way. Techniques or conditions not described in the embodiments are governed by the technique or conditions described in the relevant literature or in the product instructions. Reagents and measuring equipment for which the manufacturer is not indicated are common products that can be purchased through official channels.

[00046] Implementation option 1

[00047] This embodiment relates to a complex additive that forms impurities with a core-shell structure; its composition includes the following chemical components in a weight percentage content: Fe is 50%, Zr is 8%, Ti is 20%, Mg is 15%, RE is 7%; the remaining composition is inevitable impurities. The weight percentage content of the elements Zr, Ti, Mg and RE satisfies the following formula: (Ti+Mg+RE)/Zr=5.2. In particular, the mentioned element RE includes the element La and the element Ce, the weight ratio of which is 80:20.

[00048] This embodiment relates to a method for producing the said complex additive forming an impurity with a core-shell structure, which includes the following sequence: using a Fe-Ti alloy as a base, adding Zr, Ti and RE; wherein RE includes lanthanum La and cerium Ce; La accounts for 80%, Ce accounts for 20%; the composition is melted in a vacuum induction furnace for 6 hours, after which casting is performed under vacuum conditions. The Zr element used in the complex additive is spongy metallic Zr and/or metallic Zr. The RE used in the complex additive is a mixture of rare earth elements, which includes lanthanum La and cerium Ce; La accounts for 80%, Ce accounts for 20%. The Mg element used in the complex additive is pieces of metallic magnesium, grains of magnesium or pieces or grains of an Mg-Zr alloy.

[00049] This embodiment relates to a melting method that includes the following sequence:

[00050] (1) After processing liquid pig iron or scrap steel or liquid pig iron from scrap steel in a converter furnace or electric arc furnace, the temperature and composition of the melt are adjusted so that the tapping temperature of the melt is 1620°C and the free oxygen content in the liquid steel is 250 parts per million;

[00051] (2) The liquid steel is put into the ladle and stirred by microbubble stirring for 6 minutes; then the liquid steel in the ladle is pre-deoxidized using a Fe-Si alloy or a Fe-Si-Mn alloy to adjust the free oxygen content of the liquid steel to 55 ppm; after the liquid steel is stirred by microbubble stirring for 4 minutes, a complex additive is added for final deoxidation; the complex additive is introduced into the liquid steel in the form of a lump master alloy or a wire with a protected core, the grain size of the complex additive is 11 mm; the addition amount of the complex additive is 2.7 kg for each ton of liquid steel; thereby obtaining liquid steel after final deoxidation;

[00052] (3) Then, according to a standardized technology, LF refining, VD refining or RH refining is carried out on said liquid steel after final deoxidation; finally, the liquid steel having undergone the refining stage is used for continuous casting according to a standardized technology.

[00053] Implementation option 2

[00054] This embodiment relates to a complex additive that forms impurities with a core-shell structure; its composition includes the following chemical components in a weight percentage content: Fe is 46%, Zr is 11%, Ti is 15%, Mg is 19%, RE is 10%; the remaining composition is inevitable impurities. The weight percentage content of the elements Zr, Ti, Mg and RE satisfies the following formula: (Ti+Mg+RE)/Zr=4. In particular, the mentioned element RE includes the element La and the element Ce, the weight ratio of which is 90:10.

[00055] This embodiment relates to a method for producing the said complex additive forming an impurity with a core-shell structure, which includes the following sequence: using a Fe-Ti alloy as a base, adding Zr, Ti and RE; wherein RE includes lanthanum La and cerium Ce; La accounts for 90%, Ce accounts for 10%; the composition is melted in a vacuum induction furnace for 4 hours, after which casting is performed under vacuum conditions. The Zr element used in the complex additive is spongy metallic Zr and/or metallic Zr. The RE used in the complex additive is a mixture of rare earth elements, which includes lanthanum La and cerium Ce; La accounts for 90%, Ce accounts for 10%. The Mg element used in the complex additive is pieces of metallic magnesium, grains of magnesium or pieces or grains of an Mg-Zr alloy.

[00056] This embodiment relates to a melting method that includes the following sequence:

[00057] (1) After processing liquid pig iron or scrap steel or liquid pig iron from scrap steel in a converter furnace or electric arc furnace, the temperature and composition of the melt are adjusted so that the tapping temperature of the melt is 1560°C and the free oxygen content in the liquid steel is 150 parts per million;

[00058] (2) The liquid steel is put into the ladle and stirred by microbubble stirring for 5 minutes; then the liquid steel in the ladle is preliminarily deoxidized using a Fe-Si alloy or a Fe-Si-Mn alloy to adjust the free oxygen content of the liquid steel to a value of 20 parts per million; after the liquid steel is stirred by microbubble stirring for 3 minutes, a complex additive is added for final deoxidation; the complex additive is introduced into the liquid steel in the form of a lump master alloy or a wire with a protected core, the grain size of the complex additive is 3-19 mm; the addition amount of the complex additive is 0.70 kg for each ton of liquid steel; thereby obtaining liquid steel after final deoxidation;

[00059] (3) Then, according to a standardized technology, LF refining, VD refining or RH refining is carried out on said liquid steel after final deoxidation; finally, the liquid steel having undergone the refining stage is used for continuous casting according to a standardized technology.

[00060] Implementation option 3

[00061] This embodiment relates to a complex additive that forms impurities with a core-shell structure; its composition includes the following chemical components in a weight percentage content: Fe is 57%, Zr is 6%, Ti is 16%, Mg is 12%, RE is 9%; the remaining composition is inevitable impurities; the weight percentage content of the elements Zr, Ti, Mg and RE satisfies the following formula: (Ti+Mg+RE)/Zr=6.2. In particular, the mentioned element RE includes the element La and the element Ce, the weight ratio of which is 70:30.

[00062] This embodiment relates to a method for producing the said complex additive forming an impurity with a core-shell structure, which includes the following sequence: using a Fe-Ti alloy as a base, adding Zr, Ti and RE; wherein RE includes lanthanum La and cerium Ce; La accounts for 70%, Ce accounts for 30%; the composition is melted in a vacuum induction furnace for 7 hours, after which casting is performed under vacuum conditions. The Zr element used in the complex additive is spongy metallic Zr and/or metallic Zr. The RE used in the complex additive is a mixture of rare earth elements, which includes lanthanum La and cerium Ce; La accounts for 70%, Ce accounts for 30%. The Mg element used in the complex additive is pieces of metallic magnesium, grains of magnesium or pieces or grains of an Mg-Zr alloy.

[00063] This embodiment relates to a melting method that includes the following sequence:

[00064] (1) After processing liquid pig iron or scrap steel or liquid pig iron from scrap steel in a converter furnace or electric arc furnace, the temperature and composition of the melt are adjusted so that the tapping temperature of the melt is 1670°C and the free oxygen content in the liquid steel is 350 parts per million;

[00065] (2) The liquid steel is put into the ladle and stirred by microbubble stirring for 8 minutes; then the liquid steel in the ladle is pre-deoxidized using a Fe-Si alloy or a Fe-Si-Mn alloy to adjust the free oxygen content of the liquid steel to 80 ppm; after the liquid steel is stirred by microbubble stirring for 5 minutes, a complex additive is added for final deoxidation; the complex additive is introduced into the liquid steel in the form of a lump master alloy or a wire with a protected core, the grain size of the complex additive is 16 mm; the addition amount of the complex additive is 3.9 kg for each ton of liquid steel; thereby obtaining liquid steel after final deoxidation;

[00066] (3) Then, according to a standardized technology, LF refining, VD refining or RH refining is carried out on said liquid steel after final deoxidation; finally, the liquid steel having undergone the refining step is used for continuous casting according to a standardized technology.

[00067] Test examples

[00068] The following are the results of visual inspection, analysis and testing of the characteristics of the low-alloy high-strength steel obtained according to embodiment 1 and its main impurities:

[00069] (1) Characteristics of the core-shell structure of the impurity

[00070] In particular, Fig. 1 and Fig. 2 show an image and a diagram of the distribution of inclusion elements in low-alloy high-strength steel obtained after electrolytic extraction in a non-aqueous solution. Fig. 1 and Fig. 2 show that the inclusions have a spherical shape. The type of impurity obtained as a result of electrolytic extraction in a non-aqueous solution is a complex sulfide oxide; the core of the impurity is complex oxides of Zr, Ti, Mg and RE, on the surface of the spherical particles is sulfide (MnS) and carbide (TiN).

[00071] Using an electron beam scanning microscope, a comparison was made of the types and sizes of grains of inclusions in the composition of a complex additive added in different amounts to low-alloy high-strength steel. In steel deoxidized using a standardized technology using Al, the oxide inclusions have a cluster appearance (literature source: Deng Z, Zhu M. Evolution Mechanism of Non-metallic Inclusions in Al-Killed Alloyed Steel during Secondary Refining Process[J]. [J]. Isij International, 2013, 53(3):450-458); the inclusions are mainly concentrated in particles of d2-5 μm; sulfides have a stripe shape; their length, as a rule, is 5-20 μm; after the complex deoxidation procedure, it is seen that the inclusion size is noticeably finely refined and is mainly concentrated in particles of 1-3 μm (as shown in Fig. 3).

[00072] (2) Thermodynamics of complex deoxidation

[00073] In the above study example (1), visual inspection and inclusion analysis are carried out; the inclusions are a core-shell structure with oxysulfide in the center and nitrides and sulfides attached to the periphery; this part presents a thermodynamic analysis of the formation and evolution of complex impurities. Considering the variety of alloying elements in the composition of steel, it should be understood that a number of physicochemical reactions may occur during the processing and solidification of molten steel, forming oxides, sulfides, nitrides and other non-metallic impurities. The change in Gibbs free energy is a standard that determines whether a reaction can proceed spontaneously at a constant temperature and pressure; ΔG<0 means that the reaction can proceed spontaneously. The chemical reaction equation of deoxidation and the thermodynamic formula used in adding alloying elements to molten steel are represented by formulas (1) and (2).

,

.

[00074] Where ΔG and ΔG ^θ respectively denote the Gibbs free energy of the reaction and the standard Gibbs free energy (J/mol); R is the gas constant (J/(mol⋅K)), T is the temperature (K); a _i is the activity of the element. Table 1 presents data on the Gibbs free energy corresponding to the formation of inclusions that can be part of low-alloy high-strength steels; the temperature of liquid steel is set as 1873 K. As shown in Table 1, the following sequence of formation of pure metal oxides is observed based on the values of Gibbs free energy: Al ₂ O ₃ (-734.33 kJ/mol), La ₂ O ₃ (-610.85 kJ/mol), Ti ₂ O ₃ (-273.72 kJ/mol), ZrO ₂ (-156.09 kJ/mol) and CaO (-96.36 kJ/mol). In addition, the Zr and RE elements completely or partially replace the Al element in the composition of the Al ₂ O ₃ oxide in molten steel; ZrO ₂ (-7298.12 kJ/mol) and LaAlO ₃ (-646.15 kJ/mol) are formed. The element Ca reacts with molten aluminum and oxygen to form calcium aluminate (-9458.40 kJ/mol). In the traditional deoxidation technology using aluminum in steel smelting, Al ₂ O ₃ oxide occupies a leading position among impurities. At the same time, refractory bricks easily react with Al ₂ O ₃ to form Al ₂ O ₃ MgO (similar to spinel).

[00075] Table 1. Gibbs free energy during the reaction of impurity formation at 1873 K

No. Chemical reaction ΔG ^θ (J/mol) Δ G (kJ/mol) 1 2[Al]+3[O]= _{( Al2O3} ) -1682927+323,240T -734.33 2 [Mg]+2[Al]+4[O]=(MgO⋅Al ₂ O ₃ ) -1848696+574,144T ΔG ^θ = -773.32 3 [Zr]+2[O]=( _ZrO2 ) -845532+266,100T -156.09 4 2[Ti]+3[O]= _{( Ti2O3} ) -1072872+346,000T -273.72 5 [Ca]+[O]=(CaO) -138227-63,000T -96.36 6 [Ca]+6[Al]+4[O]= [CaO⋅Al ₂ O ₃ ] -1023637+142,120T -9458,40 7 2[La]+3[O]=(La ₂ O ₃ ) -542531+124,150T -610.85 8 3[Zr]+2(Al ₂ O ₃ ) = 4[Al]+3(ZrO ₂ ) -8233279+464,510T -7298,12 9 3[Zr]+2(Ti ₂ O ₃ ) = 4[Ti]+3(ZrO ₂ ) -1073389+538,830T -11.99 10 [La]+3[O]+[Al]=( _LaAlO3 ) -801616+129,000T -646.15 11 [Ce]+(Al ₂ O ₃ ) = (CeAlO ₃ ) + [Al] -423900-247,300T -833.13 12 [Ca]+[S]=(CaS) -542531+124,150T -126.03 13 [Ti]+[N]=(TiN) -307620+113,400T 65.09

[00076] By performing complex deoxidation using Zr, Ti, Mg and RE (rare earth elements), complex sulfide oxides are easily formed in the molten steel. Below, a thermodynamic calculation is given by taking Zr, La and Ce as an example (formula _3-6 ). The results show that Zr directly reacts with _Ti2O3 and _{Al2O3 oxides} (formulas 3, 4); the complex addition of Zr and Ti elements for deoxidation leads to a uniform distribution of the complex oxides in the molten steel. This finding provides theoretical support for the fact that Zr can improve the impurities (e.g. _Al2O3 ) commonly found in steel. Formulas 5 and 6 show that, under low oxygen concentration _, the rare earth elements RE directly react with Al contained in the molten steel to form (La, Ce) _-AlO3 .

[00077] Including Δ G and Δ G ^θ (J/mol) - respectively, the Gibbs free energy and the standard Gibbs free energy; R is a constant (J/(mol K)); T is the temperature (K); a _i is the activity of element I,I; J ₁ , J ₂ , J ₃ , J ₄ - respectively , , , . For low-alloy high-strength steel according to the embodiment of the invention, the melting temperature is set at 1873 K, the corresponding Gibbs free energy is successively -7298.12 kJ/(mol⋅K), -11.99 kJ/(mol⋅K), -646.152 kJ/(mol⋅K), -833.13 kJ/(mol⋅K); all Gibbs free energy values are negative; this means that all reactions occur spontaneously.

[00078] (3) Mechanism of formation of complex sulfide oxides

[00079] Table 2 presents the formulas for the solid solubility product of carbides, nitrides and sulfides. Fig. 4 shows the curve of precipitation and sedimentation of impurities from molten steel during its solidification and cooling, calculated using the JMatPro thermodynamic software. It can be seen from Fig. 4 that at a temperature of about 1400°C, the process of sedimentation and elongation of the nitride shape begins; sedimentation and elongation of the sulfide shape begins at a temperature of about 1300°C. Sedimentation of carbonitride begins at a temperature of about 1100°C. In the late phase of solidification, CaS, TiN and other compounds mainly settle on the surface of pre-formed oxide inclusions. Therefore, after complex alloying using Zr, Ti, Mg and RE (rare earth elements) after the procedure of continuous casting of liquid steel or its pouring into molds during subsequent rolling, complex impurities with a core-shell structure are formed, shown in Fig. 1.

Table 2. Formulas for the solid solubility product of carbides, nitrides and sulfides

No. The release phase Product formula
solid solubility 1 NBC lg{[Nb]·[C]}= 2.26-6770/ T 2 NbN lg{[Nb]·[N]} = 2.80-8500/ T 3 TiC lg{[Ti]·[C]} = 2.75-7000/ T 4 TiN lg{[Ti]·[N]} = 0.32-8000/ T 5 MnS lg{[Mn]·[S]} = 5.02-11625/ T

[00080] MnS and ZrO ₂ have very similar lattice constants, the details are shown in Table 3. MnS and ZrO ₂ have good bonding based on the similarity of the crystal lattice, and this reduces the interfacial energy between them. The relatively low interfacial energy results in good adhesion properties between the crystal grains of different interfaces. This again proves the reason why sulfide bands or bundles are not formed in the test samples. The reason is that MnS tends to precipitate on the preformed ZrO ₂ particles, which leads to refinement, spheroidization and dispersion of sulfides. And this, in turn, contributes to the increase in ductility and toughness of ferritic stainless steels complexly deoxidized with Zr and Mg.

[00081] Table 3. Crystal lattice constants of MnS and ZrO ₂

View Crystal structure Crystallographic plane(hkl) Interplanar space β int h k l a b c _ZrO2 Monoclinic 002 2,621 99.23 20 0 0 2 5,145 5,207 5,311 022 1,847 14 0 2 2 113 1,509 4 1 1 3 MnS GCC 111 2,612 90 100 1 1 1 5,224 5,224 5,224 220 1,847 50 2 2 0 222 1,509 20 2 2 2

[00082] (4) Mechanisms of grinding and dispersion of complex impurities-deoxidizers

[00083] The density of states (DOS) is an applicable tool for the analysis of solid electronic structures. The DOS of oxides is shown in Fig. 5. For Ti ₂ O ₃ , there are no gaps on the Fermi surface, indicating its metallicity. For _{ZrO 2} and Al ₂ O ₃ , the gaps on the Fermi surface are 3.5 eV and 6.3 eV, respectively. The large gap value means that the material has good insulating property; at 1273 K, the specific conductivity of Ti ₂ O ₃ , ZrO ₂ , and Al ₂ O ₃ is 10 ² Ω ⁻¹ m ⁻¹ , 10 ^-1 Ω ⁻¹ m ^{−1 ,} and 10 ^-2 Ω ⁻¹ m ⁻¹ , respectively; In the DOS data and literary sources (Zhang X, Qin R. Controlled motion of electrically neutral microparticles by pulsed direct current [J]. 2015,5:10162) the data on specific electrical conductivity coincide.

[00084] The specific electrical conductivity of oxides is a key factor in the movements in molten steel. From the existing research results, it is clear that the driving force of the movement of Al ₂ O ₃ in molten steel is greater than that of ZrO ₂ . During the galvanizing and refining process, ZrO ₂ particles tend to repel each other and rarely aggregate together, while Al ₂ O ₃ particles easily collide and form large granules that float to the surface of the molten steel and are absorbed by the coating agent of the molten steel. Therefore, compared with the standardized deoxidation using Al, the complex deoxidation using Zr, Ti, Mg, and RE results in the formation of fine-grained, dispersed, and distributed complex oxides; this is also proven by the experimental results shown in Fig. 1 and Fig. 2.

[00085] According to the basic principles of metallurgical thermodynamics, the elements Zr, Ti, Mg and RE have strong oxide-forming abilities; complex deoxidation using Zr, Ti, Mg and RE helps to reduce the free oxygen content in molten steel. Table 4 shows the density of the main oxides in the steel. It can be seen from the content of Table 4 that the density of ZrO ₂ is 5.68 g/cm ³ ; it is higher than the density of Al ₂ O ₃ (3.97 g/cm ³ ) and closer to the density of liquid steel (7.15 g/cm ³ ). Rare earth oxides have an even higher density (6.87 g/cm ³ ) compared to ZrO ₂ , so the addition of rare earth elements helps the density of the complex oxides to approach the density of liquid steel. Therefore, immediately after stable oxides are formed under high temperature conditions, complex oxides mainly including _ZrO2 and rare earth oxides are uniformly distributed on the surface of molten steel, and _Al2O3 particles collide and concentrate on the surface of molten steel, _becoming a component of steel slag; part of _{the Al2O3 particles} that are unable to float to the surface remain in the steel as large cluster inclusions.

[00086] Table 4. Density of liquid steel and different types of impurities

Admixture _{Ce2O3 ​ Al2O3 ​ ZrO2 Ti2O3 ​ TiO2} CaO FeO MnO _MnO2 Density, (g/ ^cm3 ) 6.87 3.97 5.68 4.48 4.23 3.34 5.75 5.37 5.03 Admixture MgO _{SiO2 Fe2O3 ​} CaS MnS MgS FeS TiN AlN Density 3.58 2.65 5.24 2.59 4.89 2.80 4.85 5.22 3.26

[00087] (5) Physical mechanisms of approximation of bulk modulus of elasticity of impurities to that of the matrix phase of iron

[00088] Table 5 presents the crystal structures of the selected iron matrix phase and impurities; Table 6 presents the physical properties of the corresponding impurities determined on the basis of first-principle calculations. According to the calculation results, the bulk modulus of elasticity of LaAlO ₃ (192.61) and La ₂ O ₇ Zr ₂ (165.83) is lower than that of Al ₂ O ₃ (249.54) and ZrO ₂ (271.06), and the bulk modulus of elasticity of LaAlO ₃ (192.61) approaches the corresponding parameter of the iron matrix phase (194.76). Other impurities, such as CaO (114.11), MgO (165.84), TiN (175.02) and CaS (57.05), exhibit a lower bulk modulus of elasticity compared to the matrix.

[00089] Table 5. Parameters of the crystal structures of impurities and the BCC matrix

[00090] Table 6. Results of calculations of physical properties of impurities and matrix

[00091] (6) Signs of local stress of complex impurities

[00092] Electron back-scattering diffraction (EBSD) technology provides information on crystal orientation, phase distribution and deformations in the microstructure of materials. For EBSD testing in this test work, a voltage of 20 kV and a current of 13 nA are used; in order to determine the lattice distortion between impurities and the matrix, it is necessary to exclude the influence of external stresses on the impurities as much as possible; therefore, after polishing, the samples must be additionally ground using an ion-argon polishing machine (GATAN 685). The image quality (IQ) diagrams obtained from EBSD testing are mainly used to describe the quality of the EBSD patterns. Specifically, the change in pattern quality expresses the distribution of deformations in the microstructure. For example, when lattice distortions occur, the IQ pattern produces relatively low-quality dispersed diffraction patterns, and the corresponding locations in the pattern show an increase in gray intensity. And the Kernel Average Misorientation (KAM) pattern is used to characterize the degree of homogenization of local stress concentration or lattice distortions. As a rule, a relatively high KAM value indicates the presence of a relatively high degree of deformation/dislocation density in that area; the higher the value, the higher the stress concentration.

[00093] Fig. 6 shows the results of the study of the EBSD features of the regions with complex impurities in the composition of the steel sample. The results show that the IQ diagram has a clear image quality; the KAM diagram shows that the areas around the complex inclusions are mainly blue (low deformation), that is, clusters with only a small degree of deformation are located around the complex inclusions. This indicates that the complex impurities formed as a result of complex deoxidation using the rare earth element La create an extremely small stress in the matrix. The addition of rare earth elements has a positive effect on the formation of complex impurities of rare earth elements in the steel composition, such as LaAlO ₃ and La ₂ O ₇ Zr ₂ , and helps to reduce the stresses created during the formation of impurities.

[00094] (7) The mechanism of influence of the addition of rare earth elements on the physical properties of complex impurities

[00095] The addition of rare earth elements affects not only the grain size and microstructure, but also the type of impurities formed and their physical properties. Rare earth elements have a stronger affinity for oxygen and sulfur compared to other deoxidizers, including Zr, Ti, and are capable of forming oxides and sulfides of rare earth elements, and also contribute to the formation of complex impurities of rare earth elements in the composition with other deoxidizers, for example, LaAlO ₃ and La ₂ O ₇ Zr ₂ . Compared with the oxide Al ₂ O ₃ , the bulk modulus of elasticity of LaAlO ₃ is closer to the similar indicator of the Fe matrix; this indicates that the inclusion of LaAlO ₃ has an incompressibility similar to the similar parameter of the matrix. In addition, the elastic tensile Young's modulus of LaAlO ₃ is also close to the similar indicator of the matrix (Fig. 7); As is known, Young's modulus of elastic tension is directly proportional to the hardness of the material, that is, the hardness of a complex mixture of rare earth metals is closer to the similar indicator of the matrix.

[00096] To summarize the above, it should be noted that, in contrast to the traditional deoxidation technology using Al, complex mixtures of rare earth metals formed when using deoxidation technology with rare earth metals reduce the microgaps between the inclusions and the matrix, which leads to the formation of uniform deformation between them and contributes to the improvement of the mechanical properties of the steel material, improving its fatigue resistance and resistance to localized corrosion.

[00097] From the above results, it is obvious that this complex additive and smelting method are suitable for use in marine steel, pipeline and vessel steel, cryogenic vessel steel, bridge girder steel, metal support steel, rail steel, bearing steel, toothed steel, cord steel, spring steel, cold heading steel, automobile steel, electrical steel, bridge pier steel, bridge cable steel, stainless steel, I-beam steel and other steels that have high requirements for the shape of inclusions; the resulting impurities are complex impurities with a core-shell structure, the granularity, spheroidization, dispersion and bulk modulus of elasticity of which are similar to those of the matrix phase of iron; they markedly improve the corrosion resistance in a neutral aqueous medium, the resistance to marine corrosion, the fatigue resistance, and also increase the plastic viscosity and reduce the local stress concentration.

[00098] As used in this description, reference terms such as "one embodiment," "multiple embodiments," "example," "specific example," and "multiple examples" mean a combination of these embodiments or the exemplified specific features, structure, material, or characteristics contained in one or more embodiments of the present invention. The meaning conveyed by the use of the above terms in this description does not necessarily indicate a similar embodiment or example. However, the specific features, structure, material, or characteristics contained in the description may be combined in a suitable manner in one or more embodiments or examples. In addition, without mutual contradiction, the technical personnel in this field can combine or combine different embodiments or examples presented in this description, as well as their specific features.

[00099] The embodiments of the present invention shown and described above should be interpreted as illustrative examples of its use and cannot be interpreted as any limitation of the present invention, while ordinary technical personnel in this field can, within the scope of the present invention, make changes, amendments, replacements or modifications to the mentioned embodiments.

Claims (11)

1. A complex additive for steel deoxidation, characterized by the fact that it contains the following chemical components in mass percentage content: Fe 41-59%, Zr 5-11%, Ti 14-26%, Mg 11-19%, rare earth elements (RE) 4-10%; the mass percentage content of the elements Zr, Ti, Mg and RE satisfies the following formula: (Ti+Mg+RE)/Zr=4-8. 2. A complex additive according to item 1, characterized in that it contains the following chemical components in mass percentage content: Fe 46-57%, Zr 6-11%, Ti 15-16%, Mg 12-19%, RE 9-10%. 3. A complex additive according to item 1, characterized in that it contains the following chemical components in mass percentage content: Fe 50%, Zr 8%, Ti 20%, Mg 15%, RE 7%. 4. A complex additive according to any one of paragraphs 1-3, characterized in that the rare earth elements (RE) include the element La and the element Ce, the mass ratio of which is (70-90):(10-30). 5. A complex additive according to any one of paragraphs 1-3, characterized in that said element Zr is spongy metallic zirconium and/or metallic zirconium; said element Mg is any one type or a combination of at least two types from among pieces of metallic magnesium, grains of magnesium, pieces of magnesium-zirconium alloy and grains of magnesium-zirconium alloy. 6. A method of producing steel characterized by the fact that it includes the following sequence: (1) regulate the temperature and composition of the melt to produce liquid steel; (2) liquid steel is introduced into a ladle, its preliminary deoxidation is performed, and then, using a complex additive according to any of paragraphs 1-5, final deoxidation is performed and liquid steel is obtained after final deoxidation; (3) The liquid steel after final deoxidation is sequentially refined and then used for continuous casting. 7. The method according to claim 6, characterized in that in step (1) the temperature of said liquid steel is 1551-1690°C, the content of free oxygen in said liquid steel is 101-399 parts per million; step (2) - liquid steel is launched into a ladle, under bubble mixing conditions said liquid steel in the ladle is preliminarily deoxidized using an Fe-Si alloy or a Fe-Si-Mn alloy, wherein the content of free oxygen in said liquid steel is adjusted in the range of 11-99 parts per million; then, under bubble mixing conditions using a complex additive, final deoxidation is performed and liquid steel is obtained after final deoxidation; in step (3), first performing refining by means of a ladle furnace (LF), refining the liquid steel by means of recirculation vacuum degassing (VD) or vacuum arc refining (RH) of said liquid steel after final deoxidation, and then using it for continuous casting. 8. The method according to item 6, characterized in that in step (2) of the sequence the volume of addition of the said complex additive to each ton of the said liquid steel is 0.51-4.9 kg.