CN115029508B - Method for improving magnesium modification effect of IF steel - Google Patents
Method for improving magnesium modification effect of IF steel Download PDFInfo
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Abstract
The invention discloses a method for improving the magnesium modification effect of IF steel, which regulates and controls refining slag components to a low melting point region in the RH refining process, wherein the slag alkalinity is 5-8, and CaO/Al 2 O 3 Between 1.5 and 2.0, mannesmann index MI is 0.25 to 0.35, key influencing factors of Mg modification are controlled before magnesium treatment at the end stage of RH refining, a magnesium-containing cored wire is fed into molten steel by a sectional type precise wire feeding method, and Al in IF steel is fed into the molten steel 2 O 3 Modification of inclusions into finely dispersed MgO-Al 2 O 3 Inclusions of MgO-Al 2 O 3 Wherein the mass percentage of MgO is 22-100%, and the oxide inclusion type is mainly MgO.Al 2 O 3 And MgO, modified MgO-Al 2 O 3 Is wrapped by TiN to form composite inclusions, so that the TiN is also thinned. The method can ensure stable wire feeding process, has no obvious nodulation phenomenon on the inner wall of the water gap, and can smoothly realize multi-furnace continuous casting. The product has good surface quality, low defect rate and excellent performance.
Description
Technical Field
The invention relates to the technical field of IF steel production, in particular to a method for improving the magnesium modification effect of IF steel, which is applied to the technical field of ferrous metallurgy.
Background
The IF steel is extremely excellent deep drawing steel, has the characteristics of low yield strength, high elongation, high vertical plasticity anisotropic strain ratio (r), high strain hardening index (n), no timeliness and the like, is the steel plate with the highest current stamping grade, and is widely used in the industries of automobile manufacture, household appliances, daily necessities, buildings and the like. However, inclusions in steel, especially large-size Al 2 O 3 Inclusions are important factors affecting the deep drawability and surface quality of steel.
In the production of IF steel, RH vacuum decarburization treatment is needed in the production process in order to ensure the purity of the IF steel and the low content of C, N, O and other elements. In the RH vacuum treatment process, in order to obtain lower C, O content and proper temperature, oxygen blowing decarburization and heating and aluminum adding deoxidation process are adopted, and Ti and Nb are added for alloying after the deoxidation is finished. A large amount of Al is formed due to the reaction of aluminum oxide in the RH refining process 2 O 3 And (3) mixing, wherein one part floats in the ladle and the tundish, and the other part gathers and grows on the inner wall of the water gap to form a nodulation substance. In the IF steel casting process, the phenomenon of nodulation often occurs in the tundish upper nozzle and the immersed nozzle, and the whole molten steel channel is blocked when serious. The nodulation substance enters a crystallizer under the scouring action of molten steel to form large-size inclusion. In addition to Al 2 O 3 The self-adhesive agent is extremely easy to polymerize with each other to form clusters to form inclusions, and the inclusions are captured by the solidified shell in the casting process. In the subsequent solidification and cooling process, tiN is separated out from molten steel and is prepared by Al 2 O 3 For heterogeneous nuclear point, form Al 2 O 3 -TiN complex inclusions, which further increase the size of the inclusions. Al in steel 2 O 3 Inclusions, especially large-size Al 2 O 3 The inclusion has a harmful effect on plasticity, toughness, fatigue performance and the like of the steel. There are studies showing that the composition is composed of Al 2 O 3 Product quality defects caused by inclusion can account for 29% in cold-rolled sheets and can reach 62% in galvanized sheets. Al in IF steel 2 O 3 The inclusion seriously deteriorates the punching performance, surface quality and plating quality of the IF steel, and becomes an important cause of the quality defect of the IF steel.
The magnesium treatment technology is to regulate Al at present 2 O 3 One of effective measures for inclusion of Al 2 O 3 The inclusions are modified into fine dispersion-distributed inclusions, and large-size Al is eliminated 2 O 3 Is a hazard of (3). The fine dispersion-distributed inclusions can be used as heterogeneous nucleation points of TiN and a steel matrix in the solidification process, promote the refinement of TiN and crystal grains, and in addition, the fine inclusions can be pinned with austenite grain boundaries to inhibitThe growth of the prepared crystal grains can induce the formation of acicular ferrite in the crystal grains, thereby further refining the structure and improving the strength and the toughness. However, in the Mg treatment process, there are problems of severe reaction, low element yield, unstable product quality, etc., and in addition, if the addition amount is not reasonable or the process control is not proper in the Mg treatment process, there is a risk of nozzle nodulation, so that the application of the magnesium treatment technology is limited.
The Chinese patent with the publication number of CN106399633B discloses a process for treating molten steel and magnesium of ship plate steel, wherein an Mg-Al-Fe alloy cored wire is added after alloy fine adjustment in an LF procedure or vacuum treatment in an RH procedure, and the cored wire comprises the following components in percentage by mass: 5-15%, al: 40-65%, and the balance of Fe and unavoidable impurities, feeding the steel into the position with a radius of 1/3-1/2 from the center of the steel ladle according to a wire feeding rate of 2.5-4.0 m/s under a certain cored wire structure, and then performing certain soft blowing to achieve the purpose of controlling the cleanliness of ship plate steel and the inclusion components, quantity, granularity and distribution of the steel, thereby improving the mechanical property and stability of the ship plate steel. The technology is a conventional wire feeding technology, the wire feeding target is not accurately controlled, the influence of slag system on the magnesium adding effect is not considered, and risks or problems of severe reaction, unstable product quality, easy water gap nodulation and the like still exist.
The Chinese patent with publication number of CN 109536840A discloses a method for improving continuous casting high-quality die steel by micro-magnesium treatment and a preparation method thereof, designs a magnesium-containing die steel component and provides a corresponding production process, and after vacuum degassing treatment of a VD furnace, a magnesium alloy wire is added, wherein the alloy wire component Al: 30-50%, 10-25% of Mg, and the balance of Fe, wherein the weight of the wire core is 180-220 g/m. Is added into molten steel at a certain linear feeding speed and feeding quantity to play the roles of deoxidizing, purifying and modifying inclusions, thereby improving the purity and toughness of the steel. The technology is a conventional wire feeding technology, the wire feeding target is not accurately controlled, the influence of slag system on the magnesium adding effect is not considered, and risks or problems of severe reaction, unstable product quality, easy water gap nodulation and the like still exist.
The chinese patent with publication No. CN110117694B discloses a "magnesium adding process method for free-cutting steel containing magnesium", which comprises the steps of, after adjusting other components except magnesium at the end of refining, adding a cored wire containing magnesium:
1) Controlling production conditions before wire feeding, and controlling slag layer, temperature and oxygen activity of molten steel;
2) Feeding a magnesium-containing cored wire;
3) And (5) carrying out treatment after wire feeding.
The process is used for improving the morphology of manganese sulfide inclusions in steel and reducing the deformation in the rolling process, so that the cutting performance of the steel is further improved, the process is suitable for the production of free-cutting stainless steel, and the sulfur content in the steel is 0.15-0.35%. The technology focuses on the magnesium adding process method, the condition control in the magnesium adding process is thinned, but the feeding quantity of the inclusion control target is not accurate enough, the element interrelation control which is easy to cause the water gap nodulation is not clear enough, the importance of the slag system on the magnesium treatment effect is not important enough, and the product quality stability cannot be ensured.
The Chinese patent with publication No. CN 113278763A discloses a product and a method for magnesium or magnesium-calcium treatment of molten steel, wherein the temperature of the molten steel is utilized to promote a composite reducing agent encapsulated in a sealed shell to reduce magnesium oxide and combine the magnesium oxide with a diluting agent to form a mutually dissolved alloy melt, so that metal magnesium is stabilized, diluted and dispersed, rapid evaporation of magnesium in the process of modifying and denaturing inclusions in the steel can be effectively inhibited, vaporization and oxidation loss of magnesium are reduced, the yield of magnesium element in the steel is improved and stabilized, and the purity of the molten steel after magnesium treatment is ensured. The technology generates a magnesium source by virtue of the reaction of magnesium oxide and a composite reducing agent, and combines a stable diluent to achieve the purposes of reducing the reaction intensity and reducing the gasification and oxidation loss of magnesium, but the introduced magnesium oxide causes oxygen enrichment of molten steel, is unfavorable for the cleanliness of the molten steel, is not accurate in wire feeding quantity control, is not clear enough in target inclusion, does not consider the influence of slag system on the magnesium adding effect, and still has risks or problems of unstable product quality, easy water gap nodulation and the like.
The invention discloses a method for refining grain size of sulfur-containing non-quenched and tempered steel, which is disclosed in Chinese patent publication No. CN112195305A, wherein after other components are regulated in VD or RH refining procedure, the activity oxygen in the steel is ensured to be 5-15 ppm, a certain amount of magnesium-silicon cored wire is fed, so that the magnesium content in the steel is 0.001-0.003%, mg element is utilized to modify MnS inclusion, so that fine and dispersed manganese sulfide and magnesium-aluminum oxide composite inclusion are formed, the composite inclusion is used as ferrite core, equiaxial ferrite is induced to form, and grains are refined. The technique has the advantages of inaccurate wire feeding quantity control, insufficient definition of target inclusion, no consideration of the influence of slag system on the magnesium adding effect, and risks or problems of unstable product quality, easy water gap nodulation and the like.
Therefore, the magnesium treatment technology improves the cleanliness of molten steel and modifies Al in the steel 2 O 3 The method has remarkable effects on aspects such as MnS inclusion, grain refinement and the like, however, the prior art only focuses on the beneficial effects of Mg, ignores adverse factors, leads to unstable product quality, and is still greatly limited in industrial production, wherein two main problems are nozzle nodulation and magnesium addition effect stability, and the two aspects are not explicitly related in the prior art, so that the method is also a technical problem which is urgently needed to be solved.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a method for improving the magnesium modification effect of IF steel. Before Mg treatment, al, S and Ca elements in steel are controlled to prevent nozzle nodulation, and the optimal Mg addition amount is determined by controlling the liquid oxygen activity of the steel and providing a relation between Mg and Al content, so that the control type of inclusions and the mass proportion of components are defined, and a good Mg treatment effect is ensured. In addition, by combining with optimization of refining slag system, excessive large-size impurities are removed by adsorption, the cleanliness of molten steel is improved, and the Mg treatment process and the product quality are stabilized.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a method for improving the modifying effect of magnesium in IF steel features that in the production of IF steel, the low-smelting-point high-Al alloy for magnesium treatment is regulated and controlled 2 O 3 RH refining slag system with low absorption rate and Mg consumption rate; controlling the content and superheat degree of Al, S, ca, O key influencing factors for magnesium modification before magnesium treatment at RH refining end stageDetermining the quantitative addition range of Mg according to the Al content in the steel, feeding the cored wire containing magnesium into the molten steel by a sectional type precise wire feeding method, and treating the Al in the IF steel by magnesium 2 O 3 Modification of inclusions into finely dispersed MgO-Al 2 O 3 To make modified MgO-Al 2 O 3 Is wrapped by TiN to form composite inclusions and single MgO-Al 2 O 3 Mixing TiN; mgO-Al 2 O 3 The size of the composite inclusion is mainly 0.5-3 mu m, and the size of the integral composite inclusion is mainly 0.5-6 mu m; the average size of all inclusions is 2-4 mu m, the inclusion ratio of the inclusions smaller than 2 mu m is more than 40%, and the inclusion ratio of the inclusions larger than 10 mu m is less than 0.5%.
Preferably, the IF steel is produced by the following steps:
converter smelting, RH vacuum refining, mg treatment, slab continuous casting, casting blank high-temperature heating, rough rolling, finish rolling, coiling, cold rolling, annealing and hot galvanizing; the IF steel finished product comprises the following components in percentage by mass: less than or equal to 0.004 percent of C, less than or equal to 0.03 percent of Si, 0.09 to 0.18 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.009 percent of S, 0.025 to 0.050 percent of Al, 0.05 to 0.07 percent of Ti, less than or equal to 0.0035 percent of N, 0.0005 to 0.0050 percent of Mg, and the balance of iron and unavoidable impurities.
Preferably, the low melting point and high Al which are beneficial to magnesium treatment are regulated 2 O 3 The method of RH refining slag system with absorption rate and low Mg consumption rate comprises the following steps:
lime and a top slag modifier are added in the converter tapping process, and slag making materials are added in the RH refining process, so that the refining slag components in percentage by mass are as follows:
CaO:38～48％，SiO 2 :4～9％，MgO:5～8％，Al 2 O 3 :25～35％，FeO:5～8％，MnO≤3％，CaF 2 less than or equal to 6 percent; and the slag alkalinity is 5-8, caO/Al 2 O 3 Between 1.5 and 2.0, the Mannesmann index MI is 0.25 to 0.35. The slag has lower melting point, better desulfurization capability and good fluidity, is favorable for removing inclusion adsorption and can avoid secondary oxidation of molten steel caused by slag oxidability.
Preferably, the content and the superheat degree of the key influencing factors Al, S, ca, O for controlling the modification of the magnesium before the magnesium treatment meet the following requirements:
at the end of RH refining, before feeding the magnesium-containing cored wire, ensuring that the mass percentages of Al, S and Ca elements in the molten steel meet w S ≤0.008％，w Al +w S ≤0.055％，w Ca Less than or equal to 4ppm, and the activity oxygen is required to meet the requirement of a O Less than or equal to 5ppm, and the superheat degree is 40-65 ℃.
Preferably, mgO-Al 2 O 3 Is pure MgO.Al 2 O 3 Phase, or solid solution Al 2 O 3 Or MgO.Al of MgO 2 O 3 Or MgO.Al 2 O 3 Mixed phase with MgO; making MgO-Al 2 O 3 The mass percentage of MgO in the alloy is 22-100%; the pure MgO.Al 2 O 3 The phases are MgO and Al 2 O 3 Magnesia alumina spinel with stoichiometric ratio of 1:1.
Preferably, the quantitative addition range of Mg is determined according to the Al content in steel:
magnesium content w obtained after feeding magnesium-containing cored wire into molten steel Mg Satisfy 0.0031w Al +5.3×10 -6 To 0.0066w Al +2.6×10 -5 Between, inclusion MgO-Al formed in steel 2 O 3 Wherein the mass percentage of MgO is 22-100%, and the oxide inclusion type is mainly MgO.Al 2 O 3 And MgO.
Further preferably, the magnesium content w Mg In the range of 0.0031w Al +5.3×10 -6 To 0.003w Al +1.2×10 -5 Between, inclusion MgO-Al formed in steel 2 O 3 Wherein the mass percentage of MgO is 22-35%, and the oxide inclusion type is mainly MgO.Al 2 O 3 。
Preferably, the sectional type accurate wire feeding method is to feed the magnesium-containing cored wire into molten steel, if the feeding amount of the cored wire is smaller than 200m, the magnesium-containing cored wire is fed for 2 times, and the twice feeding amounts respectively account for 0.382 and 0.618 of the total wire feeding length according to the golden section proportion; if the feeding amount of the cored wire is more than 400m, the magnesium-containing cored wire can be fed for 3 times, and the three feeding amounts respectively account for 1/4, 1/3 and 5/12 of the total feeding length; the feeding amount of the cored wire is 200-400 m, and the cored wire can be fed for 2 times or 3 times; the feeding speed is 50-200 m/min when feeding the magnesium-containing cored wire, and the time interval between every two feeding is 10-20 s, so that the wire feeding process is ensured to be stable; after the wire feeding is finished, soft blowing is carried out for 5-15 min, and molten steel is subjected to continuous casting.
Further preferably, the method of determining the feeding speed is as follows:
according to the depth h m of a ladle molten pool, the diameter D mm of a cored wire and the thickness delta mm of a cored wire iron sheet, determining the wire feeding speed v= (h-0.15) & delta -1 ·(1-δ/D) -1 ·D -0.5 。
The principle of the invention is as follows:
al generated by Al deoxidation during refining of IF steel 2 O 3 The wetting angle with molten steel is larger, the aggregation length is easy to be large, and large-size inclusions are formed. In addition, during continuous casting, al 2 O 3 The water gap is easy to adhere to the inner wall of the water gap to generate nodulation phenomenon, and the water gap is blocked. The nodulation substance enters a crystallizer under the scouring action of molten steel to form inclusion with larger size, and the inclusion is captured by a solidified blank shell to become a defect in steel. Al (Al) 2 O 3 High melting point and high hardness of inclusions, and has adverse effect on the quality and performance of steel, and how to regulate and control Al 2 O 3 Inclusion is critical in reducing its hazard.
Mg to Al 2 O 3 The inclusion modification process is generally a process of transition from the outer surface of the inclusion to the inside, and the larger the size of the inclusion is, the more Mg element is to Al 2 O 3 The greater the internal diffusion resistance, the less likely the modification is. To sufficiently modify Al in steel 2 O 3 Inclusion, before Mg is added, al in steel is made as much as possible 2 O 3 The inclusions are medium-small sized inclusions, and the large sized inclusions are removed. According to Stokes formula, the floating speed of large-size inclusions is faster, and the inclusions are easier to float to slag steel interfaces. Through regulating and controlling the slag system, the adsorption removal of large-size impurities can be promoted. In the RH refining process, a certain amount of slag making materials are properly added, so that the refining slag components in percentage by mass are as follows: 38-48% of CaO and SiO 2 :4～9％，MgO:5～8％，Al 2 O 3 :25～35％，FeO:5～8％，MnO≤3％，CaF 2 Less than or equal to 6 percent. The composition range can make slag implementHas lower melting point and good fluidity, and the slag layer uniformly covers the surface of molten steel, on one hand, the slag layer is used for absorbing Al 2 O 3 The inclusion provides good dynamic conditions, and meanwhile, in the subsequent cored wire feeding process, the volatilization path of Mg vapor can be reduced, the burning loss of Mg is reduced, and the yield of Mg is improved.
The slag contains 5-8% of MgO, so that on one hand, the MgO in the slag is saturated as much as possible, the corrosion to the magnesia carbon bricks of the furnace lining is reduced, on the other hand, the activity of MgO is higher, the reaction of alloy element Mg among slag steels in the wire feeding process is reduced, the loss of dissolved Mg in the steel is reduced, and the yield of Mg is improved. Too high MgO content will raise the melting point of the slag, while too low MgO content tends to attack the lining refractory and increase the reaction loss of Mg between slag steels. When FeO is contained in the slag by 5-8%, decarburization of molten steel can be promoted, but too high FeO content easily causes oxygen transfer from the slag to the molten steel, so that secondary oxidation of the molten steel is caused. On the premise of ensuring the fluidity of the slag, the alkalinity of the slag is controlled to be 5-8, so that better desulfurization can be achieved, the excessive high alkalinity of the slag can raise the melting point of the slag and deteriorate the fluidity, and the too low alkalinity of the slag is unfavorable for desulfurization. In addition, caO/Al in the slag is controlled 2 O 3 Between 1.5 and 2.0, the Mannesmann index MI is 0.25 to 0.35, so that the slag has proper viscosity and stronger Al 2 O 3 Adsorption capacity, and can effectively remove large-size Al 2 O 3 The cleanliness is improved, which is beneficial to subsequent Mg to Al 2 O 3 And 4, fully modifying the inclusions.
When Al is 2 O 3 Modified to MgO-Al 2 O 3 After that, if MgO-Al 2 O 3 The MgO content in the alloy is 28.2% and is MgO and Al 2 O 3 Pure magnesia-alumina spinel MgO.Al with stoichiometric ratio of 1:1 2 O 3 And (3) phase (C). When MgO-Al 2 O 3 When the MgO content in the magnesium aluminate spinel is 22-35%, the magnesium aluminate spinel is MgO-Al 2 O 3 Can be dissolved into a certain amount of MgO or Al 2 O 3 . When MgO-Al 2 O 3 When the MgO content is less than 22%, al in a non-solid solution state will be present 2 O 3 And (3) generating. When MgO-Al 2 O 3 When the MgO content is higher than 35%, mgO.Al will be present 2 O 3 And MgO.
MgO·Al 2 O 3 Or critical nucleation radius ratio of MgO to Al 2 O 3 Small, high nucleation rate, and is distributed in a state of finer dispersion in molten steel. MgO/Al 2 O 3 Or interface energy of MgO and molten steel is higher than that of Al 2 O 3 The interface energy with molten steel is small, the wetting angle is also small, and the attraction force between particles is also smaller than Al 2 O 3 The method is small, is not easy to gather and grow, avoids the formation of clustered inclusions, and therefore, the clustered inclusions are distributed in a finer dispersion mode in a final product, quality defects caused by large-particle inclusions are reduced, and small-size inclusions generally have no adverse effect on the performance of steel. In addition, mgO.Al 2 O 3 Or MgO is not easy to accumulate in the water gap, so that the risk of blocking the water gap is reduced, and continuous casting is ensured. In the solidification process of molten steel, due to MgO and Al 2 O 3 The degree of mismatching with delta-Fe is 1.2% or that of mismatching with MgO and delta-Fe is 3.8% compared with Al 2 O 3 Less than 16.1% mismatch with delta-Fe, mgO.Al 2 O 3 Or MgO is easier to induce the formation of intragranular acicular ferrite, thereby refining grains, improving tissues and improving the deep drawing performance of steel. When MgO-Al 2 O 3 When the MgO content in the alloy is less than 22%, the alloy contains Al 2 O 3 Is the presence of Al 2 O 3 Possibly with MgO.Al 2 O 3 Sintering increases the size of inclusions and adheres to the nozzle, which adversely affects the performance and quality of the steel. Thus MgO-Al 2 O 3 The MgO content in the alloy is 22-100%.
In ideal conditions, the Mg element can lead Al to be mixed with other elements 2 O 3 Better modified into MgO-Al with tiny dispersion distribution 2 O 3 However, in the actual production process, a certain amount of Ca and S are inevitably present in the steel. Part of Ca exists in the form of CaO and is equal to Al 2 O 3 Combined to form high-melting CaO-Al 2 O 3 Or with MgO and Al 2 O 3 Ternary CaO-MgO-Al combined to form complex stoichiometric ratio 2 O 3 Inclusion of CaO-Al 2 O 3 Or ternary inclusion of CaO-MgO-Al 2 O 3 All lose MgO and Al 2 O 3 Or MgO surface tension, wetting angle, critical nucleation radius and other characteristics, so that CaO-Al 2 O 3 Or CaO-MgO-Al 2 O 3 Is easy to gather and grow up. On the other hand, caS formed by combining Ca and S is easy to combine with MgO-Al 2 O 3 And the inner walls of the water gap are mutually adhered and sintered to cause water gap nodulation. Therefore, at the end of RH refining and during Mg treatment, the content of Ca and S in steel should be strictly limited to ensure that the mass percentage of S, ca elements in molten steel meets w S ≤0.008％，w Ca Less than or equal to 4ppm. In addition, when the S and Al contents are too high, caO-Al with high melting point can be generated more easily 2 O 3 、CaS、CaO-MgO-Al 2 O 3 And the inclusion is equal, so that the total amount of S and Al is required to satisfy w Al +w S Less than or equal to 0.055 percent. Mg has a strong chemical activity and has a strong binding force with oxygen. When the activity oxygen in the steel is higher, the added Mg can generate a severe reaction, so that a large amount of Mg is used for deoxidization and cannot play a role in modifying inclusions, and therefore, the activity oxygen in the molten steel needs to meet the requirement of a O ≤5ppm。
When the magnesium-containing cored wire is fed, a proper quantitative adding range of Mg is determined according to the Al content in steel, and the magnesium content w obtained by molten steel after wire feeding Mg Satisfy 0.0031w Al +5.3×10 -6 To 0.0066w Al +2.6×10 -5 Between, inclusion MgO-Al formed in steel 2 O 3 Wherein the mass percentage of MgO is 22-100%, and the oxide inclusion type is mainly MgO.Al 2 O 3 And MgO. Preferred magnesium content w Mg In the range of 0.0031w Al +5.3×10 -6 To 0.003w Al +1.2×10 -5 Between, inclusion MgO-Al formed in steel 2 O 3 Wherein the mass percentage of MgO is 22-35%, and the oxide inclusion type is mainly MgO.Al 2 O 3 . If w Mg The content is less than 0.0031w Al +5.3×10 -6 Will have Al 2 O 3 Inclusions are formed, resulting in under-modification if w Mg The content is higher than 0.0066w Al +2.6×10 -5 MgO alone is formed, and Mg is added in excess toLess than dissolution and diffusion are easy to volatilize, so that the yield of Mg is obviously reduced, and the excessive addition amount inevitably increases the intensity of the wire feeding reaction. Neither the under-modification nor the over-modification is detrimental to the Mg treatment effect.
In the wire feeding process, if the feeding length of the cored wire is less than 200m, the cored wire can be fed for 2 times, the length of the 2 times of feeding wires is controlled to be 0.382 and 0.618 according to the golden section proportion, and the time interval between every two times of feeding wires is 10-20 s. When the molten steel is fed for the first time, the active oxygen in the steel is higher, the fed cored wire mainly plays a role in deoxidization, so that the small amount of feeding is used for avoiding excessively severe reaction, and meanwhile, a certain local Mg concentration is ensured, so that the molten steel has a preliminary modifying effect, and is gradually diffused to other parts of the molten steel during the interval period before the next feeding. In the second feeding, the feeding quantity is increased, and the inclusions are fully modified in the process of stably adding Mg. If the feeding length of the cored wire is more than 400m, the cored wire can be fed for 3 times, and the 3 times of feeding amount respectively accounts for 1/4, 1/3 and 5/12 of the total feeding length. The time interval between every two feeding is 10-20 s, the first feeding amount is less, and the third feeding amount is more. When the steel is fed for the first time, the active oxygen in the steel is higher, the fed cored wire mainly plays a role in deoxidizing, so that the reaction is avoided being too violent when the feeding is carried out for a small amount, when the feeding is carried out for the second time, the feeding quantity is properly increased, the effect of modifying the inclusions is gradually achieved when the steel is further deoxidized, meanwhile, a certain local Mg concentration is ensured, the Mg concentration is gradually diffused to other parts of molten steel during the interval between the Mg concentration and the feeding of the steel for the next time, the feeding quantity is further increased for the third time, and the inclusions are fully modified in the process of stably adding the Mg. If the feeding amount of the cored wire is 200-400 m, the cored wire can be fed for 2 times or 3 times.
In order to ensure stable wire feeding process and Mg yield, wire feeding parameters are required to be reasonably set. The speed of feeding the magnesium-containing cored wire is generally 50-200 m/min. If the wire feeding speed is too low, the cored wire is melted at the upper part of the molten pool, and the alloy elements volatilize and burn out before being dissolved, so that the reaction is severe, and the yield is low. If the wire feeding speed is too high, the cored wire can scratch refractory materials at the bottom of the molten pool. The more accurate wire feeding speed is related to the depth h (unit m) of a ladle molten pool and the melting time t (unit s) of the cored wire iron sheet, and the most optimal melting position of the end part of the cored wire is positioned at a position 100-200 mm away from the bottom of the ladle, and 150mm is taken. The optimum feeding speed v (unit m/s) is shown in formula 1. The melting time t of the cored wire iron sheet is related to the diameter D (unit mm) of the cored wire, and the thickness delta (unit mm) of the cored wire iron sheet is shown in a corresponding relation as shown in a formula 2. From this, it can be determined that the preferable feeding speed is as shown in formula 3, ensuring that the feeding process is smoothly performed.
v＝(h-0.15)/t (1)
t＝δ·(1-δ/D)·D 0.5 (2)
v＝(h-0.15)·δ -1 ·(1-δ/D) -1 ·D -0.5 (3)
In the subsequent continuous casting process and cooling process after solidification, tiN is precipitated from the steel and forms composite inclusions by taking oxide inclusions in the steel as heterogeneous nuclei. When Al is added to steel 2 O 3 Regulated to be MgO-Al 2 O 3 After that, mgO-Al is formed 2 O 3 Composite inclusion with TiN and a certain amount of MgO-Al alone 2 O 3 TiN inclusions, additionally small amounts of Al 2 O 3 、TiO 2 And the like. Due to MgO-Al 2 O 3 Specific Al 2 O 3 The more fine dispersion distribution can provide more TiN heterogeneous nucleation points, so that the proportion of composite inclusions is improved, the sizes of the integral inclusions in the steel are reduced, and the large-size proportion is obviously reduced. By adopting the technology, the harm of inclusions in the IF steel can be obviously reduced, and the product quality is improved. In addition, the fine inclusion can be used as a nucleation core in the molten steel solidification process, so that the refinement of crystal grains is promoted, and the fine inclusion can be pinned with an austenite grain boundary to inhibit the growth of the crystal grains, so that the structure is finer and more uniform, and the mechanical properties of the steel are improved.
Compared with the prior art, the invention has the following obvious prominent substantive features and obvious advantages:
1. the method can ensure stable wire feeding process, does not generate severe reaction, has high Mg element yield, stable stopper curve in the continuous casting process, has no obvious nodulation phenomenon on the inner wall of the water gap, and can smoothly realize multi-furnace continuous casting;
2. the method of the invention can accurately regulate and control the generation type of the inclusions, and the inclusions are mostly distributed in a way of fine dispersion MgO-Al 2 O 3 Presence of complex inclusions with TiN, mgO-Al alone 2 O 3 TiN inclusions, etc.; mgO-Al 2 O 3 The size of the composite inclusion is 0.5-3 mu m, and the size of the composite inclusion is 0.5-6 mu m; in addition, the average size of all inclusions is 2-4 mu m, the inclusion proportion is less than 0.5% when the average size is more than 10 mu m, the area ratio of the inclusions is low, and the cleanliness is high;
3. the IF steel produced by the method has good surface quality, low defect rate and excellent performance.
Drawings
FIG. 1 shows the control of the refining slag system components in the CaO-SiO composition containing 5% MgO in comparative example one and examples one to four 2 -Al 2 O 3 Component points in the ternary phase diagram of the slag system.
FIG. 2 is a comparative example of a scanning electron micrograph of a typical inclusion in IF steel under a conventional Mg treatment process.
FIG. 3 is a scanning electron microscope image of typical inclusions in IF steel in a process according to an embodiment of the present invention.
Detailed Description
The foregoing embodiments are further described in conjunction with specific examples, which are intended to be exemplary only and not limiting, wherein the following examples are illustrative of the present invention and are not intended to be limiting, and wherein the first comparative example is a Mg treatment process that is not a technique of the present invention, and the first to fourth examples are Mg treatment processes that are a technique of the present invention, and the preferred embodiments of the present invention are described in detail below:
the integral production process flow of the IF steel comprises the following steps: converter smelting, RH vacuum refining, mg treatment, slab continuous casting, casting blank high-temperature heating, rough rolling, finish rolling, coiling, cold rolling, annealing and hot galvanizing; the IF steel finished product comprises the following components in percentage by mass: less than or equal to 0.004 percent of C, less than or equal to 0.03 percent of Si, 0.09 to 0.18 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.009 percent of S, 0.025 to 0.050 percent of Al, 0.05 to 0.07 percent of Ti, less than or equal to 0.0035 percent of N, and the balance of iron and unavoidable impurities.
Lime and top slag are added in the tapping process of the converterThe modifier is added with other slag-making materials properly in the RH refining process, the components of the refining slag are shown in table 1 when RH comes out, the components are in the content range of the invention, the slag alkalinity is between 5 and 8, and the CaO/Al ratio is between 5 and 8 2 O 3 Between 1.5 and 2.0, the Mannesmann index MI is between 0.25 and 0.35. Comparative example one and examples one to four refining slag compositions in CaO-SiO containing 5% MgO 2 -Al 2 O 3 The positions in the ternary phase diagram are shown in figure 1. Slag compositions under 5 conditions are near the liquidus line with low melting point of about 1500 ℃ and have low melting point, but the Mannesmann index MI of comparative example is low, the fluidity of the slag and the adsorption of Al 2 O 3 Poor inclusion capacity, higher FeO content in slag, easy secondary oxidation of molten steel and adverse effect on the cleanliness of steel.
TABLE 1RH outbound refining slag composition (%)
At the end of RH refining, the content of Al, S and Ca elements, the activity oxygen value and the superheat degree before feeding the magnesium-containing cored wire are shown in table 2, and each component and temperature meet the wire feeding condition. According to the content of Al, the weight of the alloy is 0.0031w Al +5.3×10 -6 To 0.003w Al +1.2×10 -5 The range feeding of the magnesium-containing cored wire, the thickness of the cored wire iron sheet is 0.3mm, the diameter of the cored wire is 9mm, the depth of a molten pool is about 2.2m, the feeding speed, the feeding times and the feeding quantity of the cored wire are shown in the table 3, the feeding time interval of each two times is 15s, the feeding process is stable, soft blowing is carried out for 5-15 min after the feeding is finished, continuous casting is carried out on molten steel, and the Mg content in final steel is shown in the table 2.
TABLE 2 content of key elements and degree of superheat before wire feeding and magnesium content in final Steel
TABLE 3 key production process parameters
By adopting the technical scheme of the invention, the wire feeding process is stable, no severe reaction is generated, the stopper curve in the continuous casting process is stable, no obvious nodulation phenomenon is generated on the inner wall of the water gap, the continuous casting of 6 furnaces is smoothly realized, and the Mg element yield is obviously improved compared with that of the conventional magnesium treatment process, as shown in Table 3. The inclusions are distributed in MgO and Al in a fine dispersion manner 2 O 3 Exists in composite inclusion with TiN and a certain amount of MgO-Al alone 2 O 3 TiN inclusions, and the like. MgO/Al 2 O 3 The size of the composite inclusions is 0.5-3 μm, and the size of the composite inclusions is 0.5-6 μm. Comparative example a typical sem result of inclusions in steel under a conventional magnesium treatment process is shown in fig. 2, and a typical sem result of inclusions in steel under one to four processes according to the examples of the present invention is shown in fig. 3. In addition, the average size of all inclusions is 2-4 μm, the inclusion proportion is below 0.5% when the inclusion proportion is more than 10 μm, the area ratio of the inclusions is low, and the cleanliness is high. The IF steel produced by the technology has good surface quality, low defect rate and excellent performance.
Table 4 results of inclusion detection in steel
Each component and index of the first to fourth examples are in accordance with the set range of the invention, and the catalyst has lower melting point, good fluidity and better desulfurization and Al adsorption 2 O 3 Capability of inclusion.
In summary, the above embodiment of the invention discloses a method for improving the magnesium modification effect of IF steel, which regulates and controls the components of refining slag system to a low melting point region in the RH refining process, wherein the slag alkalinity is 5-8, and the CaO/Al ratio is 2 O 3 Between 1.5 and 2.0, mannesmann index MI is 0.25 to 0.35, and key influencing factors for controlling Mg modification before magnesium treatment at the end of RH refining meet w S ≤0.008％，w Al +w S ≤0.055％，w Ca ≤4ppm, the active oxygen satisfies a O Less than or equal to 5ppm and according to the content of Al in the steel, the added magnesium content w Mg Satisfy 0.0031w Al +5.3×10 -6 To 0.0066w Al +2.6×10 -5 Feeding the magnesium-containing cored wire into molten steel by a sectional type precise wire feeding method, and feeding Al in the IF steel 2 O 3 Modification of inclusions into finely dispersed MgO-Al 2 O 3 Inclusions of MgO-Al 2 O 3 Wherein the mass percentage of MgO is 22-100%, and the oxide inclusion type is mainly MgO.Al 2 O 3 And MgO, modified MgO-Al 2 O 3 Is wrapped by TiN to form composite inclusions, so that the TiN is also thinned. By adopting the method of the embodiment of the invention, the stable wire feeding process can be ensured, the phenomenon of obvious nodulation on the inner wall of the water gap is avoided, and the multi-furnace continuous casting can be smoothly realized. The product has good surface quality, low defect rate and excellent performance.
The embodiments of the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to the embodiments described above, and various changes, modifications, substitutions, combinations or simplifications made under the spirit and principles of the technical solution of the present invention can be made according to the purpose of the present invention, and all the changes, modifications, substitutions, combinations or simplifications should be equivalent to the substitution, so long as the purpose of the present invention is met, and all the changes are within the scope of the present invention without departing from the technical principles and the inventive concept of the present invention.
Claims (7)
1. A method for improving the modifying effect of IF steel magnesium is characterized by comprising the following steps: in the IF steel production process, the low melting point and high Al which are beneficial to magnesium treatment are regulated and controlled 2 O 3 RH refining slag system with low absorption rate and Mg consumption rate;
controlling the content and superheat degree of Al, S, ca, O key influencing factors of magnesium modification before magnesium treatment at the end stage of RH refining, determining the quantitative addition range of Mg according to the Al content in steel, feeding a magnesium-containing cored wire into molten steel by a sectional type precise wire feeding method, and treating the Al in IF steel by magnesium 2 O 3 Modification of inclusions into finely dispersed MgO-Al 2 O 3 To make modified MgO-Al 2 O 3 Is wrapped by TiN to form composite inclusionsMgO-Al alone 2 O 3 Mixing TiN;
MgO-Al 2 O 3 the size of the composite inclusion is mainly 0.5-3 mu m, and the size of the integral composite inclusion is mainly 0.5-6 mu m; the average size of all inclusions is 2-4 mu m, the inclusion proportion of the inclusions with the size smaller than 2 mu m is more than 40%, and the inclusion proportion of the inclusions with the size larger than 10 mu m is less than 0.5%;
the IF steel finished product comprises the following components in percentage by mass: less than or equal to 0.004 percent of C, less than or equal to 0.03 percent of Si, 0.09 to 0.18 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.009 percent of S, 0.025 to 0.050 percent of Al, 0.05 to 0.07 percent of Ti, less than or equal to 0.0035 percent of N, 0.0005 to 0.0050 percent of Mg, and the balance of iron and unavoidable impurities;
the content and the superheat degree of the Al, S, ca, O key influencing factors for controlling the modification of the magnesium before the magnesium treatment meet the following requirements:
at the end of RH refining, before feeding the magnesium-containing cored wire, ensuring that the mass percentages of Al, S and Ca elements in the molten steel meet w S ≤0.008％，w Al +w S ≤0.055％，w Ca Less than or equal to 4ppm, and the activity oxygen is required to meet the requirement of a O Less than or equal to 5ppm, and the superheat degree is 40-65 ℃;
the quantitative addition range of Mg is determined according to the Al content in steel:
magnesium content w obtained after feeding magnesium-containing cored wire into molten steel Mg Satisfy 0.0031w Al +5.3×10 -6 To 0.0066w Al +2.6×10 -5 Between, inclusion MgO-Al formed in steel 2 O 3 Wherein the mass percentage of MgO is 22-100%, and the oxide inclusion type is mainly MgO.Al 2 O 3 And MgO.
2. The method for improving the magnesium modification effect of the IF steel according to claim 1, wherein the method comprises the following steps: the IF steel production process comprises the following steps: converter smelting, RH vacuum refining, mg treatment, slab continuous casting, casting blank high-temperature heating, rough rolling, finish rolling, coiling, cold rolling, annealing and hot galvanizing.
3. The method for improving the magnesium modification effect of the IF steel according to claim 1, wherein the method comprises the following steps: low melting point and high Al for magnesium treatment 2 O 3 The method of RH refining slag system with absorption rate and low Mg consumption rate comprises the following steps:
lime and a top slag modifier are added in the converter tapping process, and slag making materials are added in the RH refining process, so that the refining slag components in percentage by mass are as follows: 38-48% of CaO and SiO 2 :4～9％，MgO:5～8％，Al 2 O 3 :25～35％，FeO:5～8％，MnO≤3％，CaF 2 Less than or equal to 6 percent; and the slag alkalinity is 5-8, caO/Al 2 O 3 Between 1.5 and 2.0, the Mannesmann index MI is 0.25 to 0.35.
4. The method for improving the magnesium modification effect of the IF steel according to claim 1, wherein the method comprises the following steps: mgO-Al 2 O 3 Is pure MgO.Al 2 O 3 Phase, or solid solution Al 2 O 3 Or MgO.Al of MgO 2 O 3 Or MgO.Al 2 O 3 Mixed phase with MgO; making MgO-Al 2 O 3 The mass percentage of MgO in the alloy is 22-100%; the pure MgO.Al 2 O 3 The phases are MgO and Al 2 O 3 Magnesia alumina spinel with stoichiometric ratio of 1:1.
5. The method for improving the magnesium modification effect of the IF steel according to claim 1, wherein the method comprises the following steps: magnesium content w Mg In the range of 0.0031w Al +5.3×10 -6 To 0.003w Al +1.2×10 -5 Between, inclusion MgO-Al formed in steel 2 O 3 Wherein the mass percentage of MgO is 22-35%, and the oxide inclusion type is mainly MgO.Al 2 O 3 。
6. The method for improving the magnesium modification effect of the IF steel according to claim 1, wherein the method comprises the following steps: the sectional type accurate wire feeding method is that a magnesium-containing cored wire is fed into molten steel, if the feeding amount of the cored wire is smaller than 200m, the magnesium-containing cored wire is fed for 2 times, and the twice feeding amounts respectively account for 0.382 and 0.618 of the total wire feeding length according to the golden section proportion; if the feeding amount of the cored wire is more than 400m, the magnesium-containing cored wire can be fed for 3 times, and the three feeding amounts respectively account for 1/4, 1/3 and 5/12 of the total feeding length; the feeding amount of the cored wire is 200-400 m, and the cored wire can be fed for 2 times or 3 times; the feeding speed is 50-200 m/min when feeding the magnesium-containing cored wire, and the time interval between every two feeding is 10-20 s, so that the wire feeding process is ensured to be stable; after the wire feeding is finished, soft blowing is carried out for 5-15 min, and molten steel is subjected to continuous casting.
7. The method for improving the magnesium modification effect of the IF steel according to claim 6, wherein the method comprises the following steps: the method for determining the feeding speed comprises the following steps:
according to the depth h m of a ladle molten pool, the diameter D mm of a cored wire and the thickness delta mm of a cored wire iron sheet, determining the wire feeding speed v= (h-0.15) & delta -1 ·(1-δ/D) -1 ·D -0.5 。
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