CN114058767B - Method for refining rare earth inclusions in super stainless steel - Google Patents
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Abstract
The invention provides a method for refining rare earth inclusions in super stainless steel, and belongs to the technical field of ferrous metallurgy. According to the invention, the primary refining and refining operation are carried out on the super stainless steel molten steel, so that the size of rare earth inclusions in the super stainless steel can be effectively refined, and the total oxygen content is obviously reduced. Wherein, during primary refining, the contents of oxygen, sulfur and aluminum in the tapped molten steel can be effectively controlled within a proper range; during refining, slag regulation, diffusion deoxidation, precipitation deoxidation, calcium treatment by feeding calcium silicate wire and rare earth treatment by feeding rare earth core-spun wire are sequentially carried out, so that the total oxygen content of molten steel can be further reduced, and the size of rare earth inclusions can be refined. The calcium treatment process is added before the rare earth treatment, and the feed amount formula of the calcium-silicon wire is designed, so that the calcium aluminate inclusion is in a semi-liquid state, the collision growth of the calcium aluminate inclusion is inhibited, and the overlarge size of the rare earth inclusion caused by the genetic effect is avoided.
Description
Technical Field
The invention relates to the technical field of ferrous metallurgy, in particular to a method for refining rare earth inclusions in super stainless steel.
Background
The super stainless steel mainly comprises super duplex stainless steel (such as S32760 and S32707) and super austenitic stainless steel (such as 254SMO), the alloy content of the super duplex stainless steel is greatly higher than that of common stainless steel, so the toughness and the corrosion resistance of the super duplex stainless steel are obviously superior to those of common stainless steel, and the super duplex stainless steel is widely applied to the fields of petrochemical industry, chemical ship manufacturing, nuclear power, paper pulp and papermaking, seawater desalination, desulfurization absorption devices and the like with severe corrosion environments. High alloy content causes great cold/hot processing difficulty of super stainless steel, and large-size nonmetallic inclusions in the steel can cause stress concentration during processing, so that cracks are generated, and the yield is seriously reduced. In addition, large-sized non-metallic inclusions seriously deteriorate mechanical properties and corrosion resistance of the super stainless steel.
Because rare earth has strong affinity with harmful elements such as oxygen, sulfur, phosphorus and the like, the rare earth is usually used for treating and realizing deep deoxidation, deep desulfurization, modification of MnS inclusions, refinement of steel solidification structure, purification of grain boundaries and the like in steel production, so that the thermoplasticity of super stainless steel is remarkably improved, and the generation of hot rolling cracks is reduced. However, rare earth oxygen-sulfur inclusions generated by rare earth treatment generally have three serious problems as follows: (1) the rare earth inclusions have high density, such as common Ce 2 O 3 Inclusions with a density of 6.2g/cm 3 . According to the Stokes law, the floating speed of inclusions with high density is low, so that rare earth inclusions generated by rare earth treatment are extremely difficult to float upwards and remove, and the cleanliness of steel is very unfavorable; (2) the rare earth inclusions are poor in wettability with molten steel, so that the rare earth inclusions are easy to agglomerate, large-size rare earth inclusions with irregular shapes are formed, and various performances of steel are seriously deteriorated; (3) al when aluminum deoxidized steel is directly treated with rare earth 2 O 3 The inclusions can be converted into rare earth inclusions in situ, and Al can be inherited to a great extent 2 O 3 The size of the inclusion is large, so that the large-size rare earth inclusion is formed.
Theoretically, for the aluminum deoxidized super stainless steel, calcium treatment is firstly carried out to denature alumina inclusions into calcium aluminate inclusions, and then rare earth treatment is carried out, so that rare earth inclusions are favorably refined. Compared with full-liquid calcium aluminate inclusion, the semi-liquid calcium aluminate inclusion is not easy to collide and grow, has smaller size and is more beneficial to refining rare earth inclusion. However, the conventional calcium treatment is usually to completely denature alumina inclusions into liquid calcium aluminate inclusions, which is very unfavorable for refining rare earth inclusions. For example, chinese patent (patent application No. 201810042651.5) provides a "purification method of low-carbon high-alloy steel", in which calcium treatment and rare earth treatment are sequentially performed at the VD/RH refining stage, the amount of added calcium-silicon wires is 3 to 5 m/ton steel, the amount of added rare earth is 30 to 100 μ g/g, the state of calcium aluminate inclusions after calcium treatment is unknown, and the amount of added rare earth is too low to be suitable for super stainless steel; chinese patent (patent application No. 202010832440.9) discloses a low-cost composite rare earth structural steel and a production method thereof, calcium treatment is carried out in a refining stage, and rare earth treatment is carried out in an RH vacuum treatment stage, but the patent only provides the content of calcium and rare earth in end-point molten steel, has no specific addition amount of calcium and rare earth, and cannot specifically guide actual operation; chinese patent (patent application No. 201511019835.2) discloses a high-strength steel plate with improved low-temperature impact toughness and a manufacturing method thereof, wherein an yttrium-based rare earth silicon calcium alloy cored wire is fed into an LF furnace at the refining stage by 300-500 m. The calcium and the rare earth are fed simultaneously, so that the beneficial effect of refining the rare earth inclusions by calcium pretreatment is lost.
Therefore, it is urgently needed to develop a method for refining rare earth inclusions in super stainless steel, so that the formation of large-size rare earth inclusions is effectively inhibited, and the beneficial effects of rare earth treatment on the hot workability and various comprehensive properties of the super stainless steel are fully exerted.
Disclosure of Invention
The invention aims to provide a method for refining rare earth inclusions in super stainless steel, which can effectively inhibit the formation of large-size rare earth inclusions during rare earth treatment after super stainless steel aluminum deoxidation.
In order to achieve the above object, the present invention provides the following technical solutions:
the invention provides a method for refining rare earth inclusions in super stainless steel, which comprises the following steps:
(1) carrying out primary refining on the super stainless steel to be treated to obtain molten steel with slag;
(2) refining the molten steel with slag obtained in the step (1) to obtain refined molten steel; the refining comprises the steps of temperature adjustment, slag adjustment, diffusion deoxidation, precipitation deoxidation and inclusion refining treatment which are sequentially carried out; the inclusion refining treatment comprises the steps of feeding calcium silicate wires and rare earth core-spun wires in sequence;
the feeding amount of the calcium silicate wire is calculated according to a formula I:
M t ×[T.O×(4.24-2.26×10 -3 T)-A]/w Ca (0.2-0.4) formula I;
in the formula I, M t Kg of the mass of the super stainless steel to be treated; T.O is the mass fraction percent of total oxygen in molten steel before feeding the calcium silicate wire; t is the temperature of molten steel before feeding the calcium silicate wire, and the temperature is DEG C; a is the mass fraction of calcium in molten steel before feeding the calcium-silicon wire; w is a Ca Is the mass fraction of calcium in the calcium silicate wire percent;
the feeding amount of the rare earth core-spun yarn is calculated according to a formula II:
4.38 XMtX (T.O + S) X (1-2) formula II;
in said formula II, M t The mass of the super stainless steel to be treated is kg; T.O is the mass fraction percent of total oxygen in molten steel before feeding the calcium silicate wire; s is the mass fraction percent of sulfur in the molten steel before feeding the calcium silicate wire;
(3) and (3) casting the refined molten steel obtained in the step (2) to obtain the super stainless steel with refined rare earth inclusions.
Preferably, the slag-carrying amount of the molten steel with slag in the step (1) is 70-100% of the total mass of the primary refined slag formed in the primary refining.
Preferably, in the step (1), the total oxygen content of the molten steel with slag is less than 0.008 percent, the sulfur content is less than 0.0015 percent, and the aluminum content is less than 0.01 percent by mass.
Preferably, the slag mixing in the step (2) comprises adding lime and fluorite into the molten steel with slag; the adding amount of the lime is 0-5 kg per ton of steel; the addition amount of the fluorite is 0-2.5 kg per ton of steel.
Preferably, the refining slag obtained by slag blending in the step (2) comprises the following components in percentage by mass: 40 to 60 percent of CaO and Al 2 O 3 10％～35％，CaF 2 0～12％，SiO 2 4% -8%, MgO 5% -10%, FeO + MnO 0-0.5% and less than 1% of impurities; and M in the refining slag CaO /M SiO2 The mass ratio of (A) is not less than 5, and M SiO2 ％+M Al2O3 ％+2M CaF2 % of the total mass fraction is 36-45%.
Preferably, the diffusion deoxidation in the step (2) comprises adding calcium silicate powder on the surface of the refined slag obtained by slag mixing; the addition amount of the calcium silicate powder is 0.8-1.5 kg per ton of steel.
Preferably, the precipitation deoxidation in the step (2) comprises feeding an aluminum wire into the molten steel obtained by diffusion deoxidation; the feeding amount of the aluminum wire is 0.1-0.3 kg per ton of steel.
Preferably, the temperature of the molten steel before the silicon-calcium wire is fed in the step (2) is 1550-1580 ℃.
Preferably, soft blowing is carried out before the operation of diffusion deoxidation in the step (2) and after the operations of precipitation deoxidation, calcium silicate wire feeding and rare earth core-spun wire feeding; the soft blowing time is independently 5-15 min.
Preferably, the mass fraction of Ce in the rare earth core-spun yarn in the step (2) is more than 50%, and the total mass fraction of Ce and La is more than or equal to 99.5%; the diameter of the rare earth core-spun yarn is 5-13 mm, and the thickness of an iron sheet layer coated on the outer layer of the rare earth core-spun yarn is 0.3-0.6 mm.
The invention provides a method for refining rare earth inclusions in super stainless steel, which can obtain refining slag and slag-carrying molten steel with high alkalinity and high fluidity after primary refining, preliminarily realize deep deoxidation and deep desulfurization of the super stainless steel, greatly reduce the content of the rare earth inclusions, and is more favorable for inhibiting the collision and growth of the rare earth inclusions. Refining is carried out again, so that the size of inclusions in the super stainless steel molten steel can be further refined; wherein, the refining comprises slag regulation, diffusion deoxidation, precipitation deoxidation and inclusion refining treatment which are sequentially carried out, and the components of the refining slag can be more favorable for adsorbing the inclusions in the molten steel and reducing the content of the inclusions after the slag regulation; the content of oxygen-containing inclusions in the molten steel can be effectively reduced through diffusion deoxidation and precipitation deoxidation; the refining treatment of the inclusions comprises feeding the calcium silicate wire and the rare earth core-spun wire in sequence, and the calcium silicate wire is fed before the rare earth core-spun wire is fed, so that the rare earth in-situ reduction can be avoidedAl 2 O 3 Forming rare earth oxygen-sulfur inclusion to avoid genetic Al 2 O 3 Coarse rare earth inclusions are obtained by the original size of the inclusions; meanwhile, a semi-liquid calcium aluminate inclusion with small size is formed after feeding calcium silicate wires, and then rare earth cored wires are fed, rare earth can reduce the semi-liquid calcium aluminate inclusion in situ, fine and dispersed rare earth oxygen-sulfur inclusions can be formed by utilizing size genetic effect, the fine rare earth oxygen-sulfur inclusions are weak in attraction, and the agglomeration growth tendency is very low, so that the size of the inclusions in the super stainless steel is further refined. The invention respectively sets the calculation methods of the fed calcium silicate wire and the rare earth core-spun wire, so that the yield of calcium and rare earth is higher during refining, the effect of refining impurities is more obvious, and compared with the traditional calcium treatment process, the feeding amount of the calcium silicate wire and the rare earth core-spun wire is obviously reduced, and the production cost of super stainless steel can be reduced.
The method for refining the rare earth inclusions in the super stainless steel has the following advantages and beneficial effects:
(1) compared with calcium-free treatment, the calcium treatment process of feeding calcium-silicon wires is added, so that the oxygen content in steel can be further reduced, and the comprehensive performance of the super stainless steel is facilitated;
(2) the invention enables the rare earth inclusion to be obtained by the in-situ transformation of the fine and dispersed semi-liquid calcium aluminate inclusion, and avoids the rare earth inclusion from large-size Al 2 O 3 The inclusions are directly converted and obtained, and meanwhile, the rare earth inclusions are prevented from being directly converted and obtained from large-size full-liquid calcium aluminate inclusions, so that compared with calcium-free treatment and traditional calcium treatment, the rare earth inclusions obtained by the method are smaller in size;
(3) compared with the traditional calcium treatment process, the feeding amount of the calcium silicate wire is obviously reduced, and the production cost of the super stainless steel can be reduced;
(4) according to the invention, rare earth is added in a rare earth core-spun yarn mode, so that the rare earth yield is higher, and the rare earth treatment cost is lower;
(5) the invention has simple and safe operation;
(6) the design idea of the invention has universality, so the invention is not limited to super stainless steel and can be popularized to other steel types.
The results of the embodiment also show that the invention not only obviously reduces the oxygen content of the super stainless steel, but also greatly improves the proportion of the small rare earth inclusions (1-5 mu m), reduces the proportion of the large-size rare earth inclusions (more than 10 mu m), and has very obvious effect of refining the size of the rare earth inclusions.
Drawings
FIG. 1 is an SEM image of an S32760 super duplex stainless steel ingot prepared in example 1 of the present invention;
fig. 2 is an SEM image of a calcium-free treated ingot of S32760 super duplex stainless steel prepared by omitting the feeding of calcium-silicon wire in comparative example 1 of the present invention with respect to the method of example 1.
Detailed Description
The invention provides a method for refining rare earth inclusions in super stainless steel, which comprises the following steps:
(1) carrying out primary refining on super stainless steel to be treated to obtain molten steel with slag;
(2) refining the molten steel with slag obtained in the step (1) to obtain refined molten steel; the refining comprises the steps of temperature adjustment, slag adjustment, diffusion deoxidation, precipitation deoxidation and inclusion refining treatment which are sequentially carried out; the inclusion refining treatment comprises the steps of feeding calcium silicate wires and rare earth core-spun wires in sequence;
the feeding amount of the calcium silicate wire is calculated according to a formula I:
M t ×[T.O×(4.24-2.26×10 -3 T)-A]/w Ca (0.2-0.4) formula I;
in the formula I, M t The mass of the super stainless steel to be treated is kg; T.O is the mass fraction of total oxygen in the molten steel before feeding the calcium silicate wire; t is the temperature of the molten steel before feeding the calcium-silicon wire, and is the temperature; a is the mass fraction of calcium in molten steel before feeding the silicon-calcium wire,%; w is a Ca Is the mass fraction of calcium in the calcium silicate wire percent;
the feeding amount of the rare earth core-spun yarn is calculated according to a formula II:
4.38 XMtX (T.O + S) X (1-2) formula II;
in said formula II, M t The mass of the super stainless steel to be treated is kg; T.O is the mass fraction of total oxygen in the molten steel before feeding the calcium silicate wire; s is the mass fraction percent of sulfur in molten steel before feeding the calcium silicate wire;
(3) and (3) casting the refined molten steel obtained in the step (2) to obtain the super stainless steel with refined rare earth inclusions.
The super stainless steel to be treated is primarily refined to obtain molten steel with slag.
The method provided by the invention is suitable for refining the rare earth inclusions in various grades of super stainless steel well known to those skilled in the art. In the present invention, the super stainless steel to be treated is preferably S32760 super duplex stainless steel or 254SMO super austenitic stainless steel.
In the present invention, the primary refining preferably includes decarburization, chromium reduction and desulfurization which are performed sequentially. The present invention is not particularly limited to the specific procedures for the decarburization, the chromium reduction and the desulfurization treatment, and those procedures for the decarburization, the chromium reduction and the desulfurization treatment which are well known to those skilled in the art may be employed.
In the present invention, the total oxygen content in the molten steel with slag is preferably less than 0.008% by mass; the sulfur content in the molten steel with slag is less than 0.0015 percent; the aluminum content in the molten steel with slag is less than 0.01 percent. According to the invention, by controlling the total oxygen content, the sulfur content and the aluminum content in the molten steel with slag, part of oxygen-containing inclusions and sulfur-containing inclusions can be eliminated, so that the size of the inclusions can be effectively refined when refining is carried out again in the follow-up process.
In the invention, the amount of slag in the molten steel with slag is 70-100% of the total mass of the primary refining slag formed by the primary refining. In the invention, if deep desulfurization is required, the slagging-off amount of the sulfur-containing refining slag can be increased, so that the slag carrying amount of molten steel is reduced, and the sulfur content in the refining slag is lower after the slag charge is supplemented subsequently.
And after obtaining the molten steel with the slag, refining the molten steel with the slag to obtain refined molten steel.
In the invention, the refining comprises the processes of temperature regulation, slag regulation, diffusion deoxidation, precipitation deoxidation and inclusion refinement which are sequentially carried out.
In the invention, the temperature of the molten steel after the temperature adjustment is preferably 1590-1610 ℃, and more preferably 1600 ℃. The temperature of the molten steel during slag adjustment is controlled within the range through temperature adjustment operation, so that the refining slag formed during slag adjustment has proper fluidity on the surface of the molten steel, the inclusion in the molten steel is better absorbed, the content of the inclusion is reduced, and the refining of the inclusion is more favorably realized.
In the present invention, the slag mixing preferably comprises adding lime and fluorite to the molten steel with slag; the adding amount of the lime is preferably 0-5 kg per ton of steel; the addition amount of the fluorite is preferably 0-2.5 kg/ton steel. According to the invention, the refining slag can be formed on the basis that the surface of the molten steel contains the primary refining slag by adding the lime and the fluorite, the alkalinity and the fluidity of the refining slag are regulated and controlled, and meanwhile, the slag amount is increased, so that the desulfurization and the impurity absorption are facilitated.
In the present invention, the mass fraction of CaO in the lime is preferably > 95%; CaF in said fluorite 2 Is preferably > 80%. The invention controls the CaO mass fraction in the lime and the CaF in the fluorite 2 The mass fraction of the slag is in the range, so that the reintroduction of more other impurities during the addition of lime and fluorite can be avoided, and meanwhile, the formed refining slag has more proper alkalinity and fluidity so as to better desulfurize and adsorb the impurities in the molten steel.
In the invention, the refining slag obtained by slag regulation preferably comprises the following components in percentage by mass: 40 to 60 percent of CaO and Al 2 O 3 10％～35％，CaF 2 0～12％，SiO 2 4% -8%, MgO 5% -10%, FeO + MnO 0-0.5% and less than 1% of impurities; more preferably, it comprises: 45 to 55 percent of CaO and Al 2 O 3 20％～30％，CaF 2 2～10％，SiO 2 5-7%, MgO 6-8%, FeO + MnO0.1-0.4% and less than 1% of impurities. The invention controls the components of the refining slagUnder the condition of the mixture ratio, the refining slag has proper alkalinity and fluidity, and is more favorable for desulfurization and the absorption of impurities in molten steel.
In the present invention, when the composition of the refining slag obtained by the slag adjustment does not satisfy the above conditions, it is preferable to adjust the composition by using a slag former known to those skilled in the art.
In the present invention, M in the refining slag CaO /M SiO2 The mass ratio of (A) is preferably not less than 5, and M SiO2 ％+M Al2O3 ％+2M CaF2 % of the total mass is preferably 36% to 45%. The invention controls CaO and SiO in the refining slag 2 Mass ratio of (A) and SiO 2 、Al 2 O 3 And CaF 2 The content of the refining slag is in the range, so that the refining slag has proper alkalinity and fluidity, and the refining slag is favorable for desulfurizing and adsorbing impurities in molten steel and reducing CaF in the refining slag 2 The content is low, which is beneficial to reducing the corrosion of the furnace lining and the environmental pollution.
In the present invention, the diffusion deoxidation preferably comprises adding a calcium silicate powder to the surface of the refined slag obtained by slag blending. In the invention, the addition amount of the calcium silicate powder is preferably 0.8-1.5 kg/ton of steel, and more preferably 1-1.2 kg/ton of steel. According to the invention, the silicon calcium powder is added for diffusion deoxidation and the addition amount of the silicon calcium powder is controlled within the range, so that the silicon calcium powder can be used as a deoxidizer to react with FeO in the refining slag, the FeO content in the refining slag is reduced, deep desulfurization is facilitated, the oxygen concentration balance between the refining slag and molten steel is destroyed, and oxygen in the molten steel is diffused into the refining slag, thereby achieving the effect of indirect deoxidation. In addition, the addition amount of the calcium silicate powder ensures that the alkalinity of the refining slag is only slightly reduced, and the desulfurization effect of the refining slag is protected.
In the invention, the mass fraction of silicon in the calcium silicon powder is preferably more than 60%; the mass fraction of calcium in the calcium silicate powder is preferably more than 30%. According to the invention, the gasification of calcium is inhibited by controlling the mass fractions of silicon and calcium in the calcium-silicon powder within the above ranges, so that the effect of diffusion deoxidation is improved.
In the present invention, the precipitation deoxidation preferably comprises feeding an aluminum wire to the molten steel obtained by the diffusion deoxidation. In the invention, the feeding amount of the aluminum wire is preferably 0.1-0.3 kg/ton steel, and more preferably 0.2 kg/ton steel. The invention adopts the precipitation deoxidation mode of feeding the aluminum wire into the molten steel and controls the content of the fed aluminum wire within the range, so that the oxygen in the molten steel and the aluminum in the aluminum wire can react to generate a product which is insoluble in the molten steel, and the product floats to the surface of the molten steel and is absorbed by the refining slag to achieve the effect of reducing the oxygen content in the molten steel. In addition, the feeding amount of the aluminum wire ensures that the content of aluminum in the molten steel is in a lower range on the premise of ensuring the deoxidation effect of the molten steel, and the formation of aluminum nitride harmful phases is avoided.
In the present invention, the mass fraction of aluminum in the aluminum wire is preferably > 99%. According to the invention, by controlling the mass fraction of the aluminum in the aluminum wire within the range, the amount of impurities mixed in the fed aluminum wire can be reduced, and the oxygen content in the molten steel can be effectively reduced.
In the invention, the inclusion refining treatment comprises the sequential feeding of calcium silicate wire and rare earth core-spun wire.
In the invention, the feeding amount of the calcium silicate wire is calculated according to a formula I:
M t ×[T.O×(4.24-2.26×10 -3 T)-A]/w Ca (0.2-0.4) formula I;
in the formula I, M t The mass of the super stainless steel to be treated is kg; T.O is the mass fraction of total oxygen in the molten steel before feeding the calcium silicate wire; t is the temperature of molten steel before feeding the calcium silicate wire, and the temperature is DEG C; a is the mass fraction of calcium in molten steel before feeding the silicon-calcium wire,%; w is a Ca Is the mass fraction of calcium in the calcium silicate wire percent;
the invention provides a method for calculating the feeding amount of a calcium-silicon wire shown in formula I, wherein M t T.O, A and w Ca Is a value actually detected (4.24-2.26X 10) -3 T) is actually the theoretical mass ratio of total calcium to total oxygen in the molten steel after the calcium silicate wire is fed, namely T.Ca/T.O, and (0.2-0.4) is actually the theoretical yield of calcium in the molten steel after the calcium silicate wire is fed. The invention adopts a calculation method according to the formula IThe method for determining the feeding amount of the calcium-silicon wire can ensure that the yield of calcium is higher during refining, is more favorable for fully forming calcium aluminate inclusion with inclusion in molten steel, and simultaneously can precipitate solid CaO-Al under certain conditions 2 O 3 And the structure is not beneficial to collision and fusion of the calcium aluminate inclusion, so that the calcium aluminate inclusion can be effectively inhibited from growing rapidly, the calcium aluminate inclusion is stably kept in a small size, and the effect of refining the inclusion is achieved. Compared with the traditional calcium treatment process, the feeding amount of the calcium silicate wire is obviously reduced, and the production cost of the super stainless steel can be reduced.
In the present invention, the mass fraction of silicon in the calcium silicon wire is preferably > 60%; the mass fraction of calcium in the silico-calcium wire is preferably > 30%. According to the invention, the mass fractions of silicon and calcium in the calcium-silicon wire are controlled within the above ranges, so that the gasification of calcium is inhibited, the yield of calcium is improved, and the effect of refining the size of inclusions is improved.
In the invention, the value of T in the formula I, namely the temperature of the molten steel after precipitation and deoxidation, is preferably 1550-1580 ℃, and more preferably 1560-1570 ℃. The present invention can form semi-liquid calcium aluminate inclusion more easily by controlling the temperature of molten steel after precipitation deoxidation within the above range. In addition, the temperature range can ensure that the molten steel does not need to be heated again before tapping, thereby avoiding steel slag mixing caused in the heating process and being more beneficial to refining the size of inclusions.
In the invention, the feeding speed of the calcium silicate wire is preferably 3-6 m/s, and more preferably 4-5 m/s. The invention can feed the calcium silicate wire deeply enough by controlling the feeding speed of the calcium silicate wire within the range, inhibit the gasification of calcium, improve the yield of the calcium and reduce the production cost.
In the invention, the feeding amount of the rare earth core-spun yarn is calculated according to a formula II:
4.38 XMt (T.O + S) x (1-2) formula II;
in said formula II, M t The mass of the super stainless steel to be treated is kg; T.O is the mass fraction of total oxygen in the molten steel before feeding the calcium silicate wire; s is the mass fraction percent of sulfur in molten steel before feeding the calcium silicate wire;
the invention provides a method for calculating the feeding amount of a rare earth core-spun yarn shown in formula II, wherein M t And T.O and S are actual detection numerical values, and (1-2) are empirical values and are related to the target content of the rare earth and the yield of the rare earth. The invention determines the feeding amount of the rare earth core-spun yarn according to the calculation method shown in the formula II, can obtain higher rare earth yield, reduces the rare earth consumption and reduces the production cost of the super stainless steel. In addition, the rare earth feeding amount determined by the calculation method shown in the formula II is lower, and the generation amount of rare earth inclusions can be reduced, so that the collision growth of the rare earth inclusions is effectively inhibited, and the size of the rare earth inclusions is more favorably refined.
In the invention, the mass fraction of Ce in the rare earth cored wire is preferably more than 50%, and the total mass fraction of Ce and La is preferably more than or equal to 99.5%. According to the invention, the mass relation of Ce, Ce and La in the rare earth core-spun yarn is controlled within the above range, so that the size of inclusions in molten steel can be refined by rare earth treatment.
In the invention, the feeding speed of the rare earth core-spun yarn is preferably 2-8 m/s, more preferably 3-7 m/s, and most preferably 4-6 m/s. The invention can ensure that the rare earth has higher yield by controlling the speed of feeding the rare earth core-spun yarn within the range.
In the invention, the diameter of the rare earth core-spun yarn is preferably 5-13 mm, and more preferably 8-10 mm; the thickness of the iron sheath layer coated on the outer layer of the rare earth cored wire is preferably 0.3-0.6 mm, and more preferably 0.4-0.5 mm.
In the present invention, soft blowing is preferably performed before the operation of the diffusion deoxidation and after each of the operations of the precipitation deoxidation, the feeding of the calcium silicate wire and the feeding of the rare earth cored wire. The soft blowing time is preferably 5-15 min, more preferably 8-12 min, and most preferably 10 min. In the present invention, the soft-blown gas is preferably argon. In the present invention, the flow rate of the argon gas is preferably 3 to 8 NL/(min.t), more preferably 4 to 7 NL/(min.t), and most preferably 5 to 6 NL/(min.t). The invention can make the refining slag micro-move on the surface of the molten steel by soft argon blowing and controlling the flow and time of the soft argon blowing within the range, so that the molten steel is not exposed, and simultaneously, the floating of the inclusion is better promoted and the inclusion is better absorbed by the refining slag, thereby being very beneficial to reducing the oxygen content and refining the inclusion.
After the refined molten steel is obtained, the invention casts the refined molten steel to obtain the super stainless steel with refined rare earth inclusions.
In the present invention, the casting is preferably die casting or continuous casting.
In the invention, the casting temperature is preferably 1500-1600 ℃, and more preferably 1530-1560 ℃. According to the invention, by controlling the casting temperature, a good solidification structure can be obtained, the solidification structure is prevented from coarsening, and the defects of shrinkage porosity, shrinkage cavity and the like are reduced.
In the present invention, the casting is preferably performed under an argon protective atmosphere.
The invention provides a method for refining rare earth inclusions in super stainless steel, which has the following design principle: in actual production of super stainless steel, aluminum is usually used for primary deoxidation, and then rare earth is used for deep deoxidation and deep desulfurization. Al produced by deoxidation of aluminium 2 O 3 The inclusions are easy to agglomerate and grow rapidly in the soft blowing process of molten steel to form large-size Al 2 O 3 Inclusions, with largest dimensions up to several hundred microns. After the aluminum is deoxidized, if the rare earth treatment is directly carried out, the rare earth has stronger binding capacity with oxygen, and Al can be reduced in situ 2 O 3 Inclusion, formation of rare earth oxygen-sulfur inclusion, inheritance of Al to a large extent 2 O 3 Coarse rare earth inclusions are obtained according to the original size of the inclusions. The coarse rare earth inclusions not only easily cause the water gap nodulation of the continuous casting tundish, but also seriously deteriorate various performances of the super stainless steel. After the aluminum is deoxidized, if the calcium treatment is carried out firstly, the binding capacity of calcium and oxygen is stronger than that of aluminum, and Al can be partially reduced 2 O 3 And (4) forming calcium aluminate inclusions. By accurately controlling the calcium/oxygen ratio of molten steel after calcium treatmentThe calcium aluminate inclusion component is located at 7Al 2 O 3 12CaO and CaO. Al 2 O 3 At the calcium treatment temperature, the calcium carbonate is just fully liquid or nearly fully liquid. In the subsequent soft blowing process, solid CaO and Al are precipitated from the liquid calcium aluminate inclusion along with the reduction of the temperature of the molten steel 2 O 3 And the calcium aluminate inclusion is in a semi-liquid state. CaO & Al in solid state according to Gibbs free energy minimum principle 2 O 3 The phase is located in the core part of the semi-liquid calcium aluminate inclusion, the residual liquid phase is located at the periphery, and the structure is not beneficial to collision and fusion of the calcium aluminate inclusion, so that the calcium aluminate inclusion can be effectively inhibited from growing rapidly, and the calcium aluminate inclusion can stably keep a fine size in the soft blowing process. And performing rare earth treatment after calcium treatment, and reducing calcium aluminate inclusion in situ by rare earth to form fine and dispersed rare earth oxygen-sulfur inclusion due to size genetic effect. The attraction among the fine rare earth oxygen-sulfur inclusions is weak, and the agglomeration and growth tendency in the subsequent soft blowing process is very low. In addition, by adopting aluminum deoxidation and calcium treatment and simultaneously producing refining slag with high alkalinity and high fluidity, the deep deoxidation and the deep desulfurization of the super stainless steel can be realized, the content of rare earth inclusions is greatly reduced, and the method is very favorable for inhibiting the collision and the growth of the rare earth inclusions.
The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
The method for refining the rare earth inclusions in the super stainless steel provided by the embodiment comprises the following steps of:
(1) carrying out primary refining on the super stainless steel to be treated to obtain molten steel with slag;
wherein the super stainless steel to be treated is selected from S32760 super duplex stainless steel, and the components of the super stainless steel are shown in Table 1; the mass of the super stainless steel to be treated is 45 t; the primary refining is decarburization, chromium reduction and desulfurization which are sequentially carried out according to conventional operation; the primary refining slag in the molten steel with slag is 70 percent of the total amount of the initially formed refining slag (namely, 30 percent of slag skimming amount); the total oxygen content in the molten steel with the slag is 0.0072%, the sulfur content is 0.0011%, and the aluminum content is 0.008%;
table 1 example 1 composition (mass%,%) of S32760 super duplex stainless steel used in step (1) of example 1
(2) Refining the molten steel with slag obtained in the step (1) to obtain refined molten steel;
the refining comprises the steps of temperature adjustment, slag adjustment, diffusion deoxidation, precipitation deoxidation and inclusion refining treatment which are sequentially carried out; the inclusion refining treatment comprises the steps of sequentially feeding calcium silicon wire (the mass fraction of silicon is 62 percent and the mass fraction of calcium is 35 percent) and rare earth core-spun wire (the mass fraction of Ce is 56.5 percent and the total mass fraction of Ce and La is 99.75 percent); more specifically: the temperature of the molten steel after the temperature adjustment is finished is 1597 ℃, and 3.5 kg/ton of steel lime (CaO content is 96.5 percent) and 2 kg/ton of steel fluorite (CaF) are added during slag adjustment 2 The mass fraction of (b) is 84.75%), and the refining slag components after completion of slag adjustment are shown in Table 2. After slag adjustment, soft blowing argon gas with the gas flow of 4 NL/(min.t), then adding 0.9 kg/ton of steel calcium silicon powder (the mass fraction of silicon is 62 percent, and the mass fraction of calcium is 35 percent) on the surface of the refined slag for diffusion deoxidation, then feeding 0.15 kg/ton of steel aluminum wire (the mass fraction of aluminum is 99.4 percent) into molten steel for precipitation deoxidation, and then soft blowing argon gas with the flow of 4 NL/(min.t) for 11 min; measuring the components of the molten steel to obtain the molten steel with the total oxygen content of 0.0031%, the sulfur content of 0.0006% and the calcium content of 0.0009%; calculating the feeding amount of the calcium silicate wire according to a formula I, wherein T in the formula I is 1554 ℃, the feeding amount of the calcium silicate wire is 7.0kg through calculation, feeding molten steel at the speed of 5m/s, and then continuously blowing argon with the flow of 4 NL/(min.t) for 7min in a soft blowing mode; calculating the feeding amount of the rare earth core-spun yarn according to formula II to be 10.2kg, and feeding the rare earth core-spun yarn at a speed of 3m/s to have a diameter of 13And (3) the rare earth core-spun yarn with the thickness of mm and the iron sheet of 0.4mm is subjected to soft blowing of argon with the flow of 4 NL/(min.t) for 8min again.
Table 2 example 1 composition of refining slag after slag adjustment in step (2) (mass%)
| CaO | Al 2 O 3 | CaF 2 | SiO 2 | MgO | FeO+MnO | Impurities |
| 56.5 | 16.9 | 10.6 | 6.2 | 9.3 | 0.1 | 0.4 |
M in the above refining slag CaO /M SiO2 Has a mass ratio of 9.11, and M SiO2 ％+M Al2O3 ％+2M CaF2 % of the total mass fraction was 44.3%.
(3) Casting the refined molten steel obtained in the step (2) to obtain super stainless steel with refined rare earth inclusions;
wherein the casting temperature is 1537 ℃, the casting mode is die casting, the bottom casting method is adopted for casting, the size of the die casting is 14 inches, and the argon protection measure is adopted at the joint of a ladle nozzle and a middle pouring pipe to prevent the secondary oxidation of the molten steel. The metallographic structure of the obtained S32760 super duplex stainless steel ingot was observed by a scanning electron microscope, as shown in fig. 1. Sampling and analyzing the R/2 position of the tail of the ingot, detecting the total oxygen content of the ingot and counting the size of the rare earth inclusions, wherein the result is shown in Table 3.
Comparative example 1
In order to reveal the effect of increasing the calcium pretreatment for refining rare earth inclusions, the same method as that of example 1 was adopted, and the operation of feeding calcium silicon wire in the inclusion refining treatment in step (2) was omitted, so as to compare the effect of refining the inclusions without calcium treatment before the rare earth treatment, and obtain one piece of S32760 super duplex stainless steel without calcium treatment. The metallographic structure of the obtained calcium-free treated S32760 super duplex stainless steel ingot was observed by a scanning electron microscope, as shown in fig. 2. Sampling and analyzing the tail R/2 position of the S32760 super duplex stainless steel ingot without calcium treatment, detecting the total oxygen content and counting the size of the rare earth inclusions, and the result is shown in Table 3.
And detecting the total oxygen content of the two steel ingots and counting the size of the rare earth inclusions. The results are shown in Table 3:
table 3 statistics of total oxygen content and rare earth inclusion size in two S32760 super stainless steels in example 1 and comparative example 1
The results in table 3 show that example 1 using calcium pretreatment prior to rare earth treatment can reduce the oxygen content of S32760 super duplex stainless steel from 0.0022% to 0.0018%. In addition, the quantity proportion of the fine rare earth inclusions (1-5 mu m) is increased from 68.4% to 81.1%, and the quantity proportion of the large-size rare earth inclusions (larger than 10 mu m) is reduced from 9.7% to 3.4%.
FIG. 1 is an SEM image of an S32760 super duplex stainless steel prepared by the method for refining inclusions in the super stainless steel provided by the invention in example 1. As can be seen from fig. 1, the S32760 super duplex stainless steel prepared in example 1 had a small inclusion content and a small size, and the inclusion refining effect was significant, at a scale of 2 μm.
FIG. 2 is an SEM photograph of a calcium-free treated S32760 super duplex stainless steel prepared without feeding a calcium silicate wire in comparative example 1. As can be seen from fig. 2, in the super duplex stainless steel without calcium treatment, which is obtained by not feeding calcium silicate wire before rare earth treatment, the inclusions in the super duplex stainless steel without calcium treatment are more and larger in size at a scale of 20 μm, i.e., the inclusion refining effect is not obvious.
Example 2
The method for refining the rare earth inclusions in the super stainless steel provided by the embodiment comprises the following steps:
(1) carrying out primary refining on the super stainless steel to be treated to obtain molten steel with slag;
wherein the super stainless steel to be treated is selected from 254SMO super austenitic stainless steel, and the components of the super stainless steel are shown in Table 4; the mass of the super stainless steel to be treated is 35 t; the primary refining is decarburization, chromium reduction and desulfurization which are sequentially carried out according to conventional operation; the primary refining slag in the molten steel with slag is 100 percent of the total amount of the initially formed refining slag; the total oxygen content in the molten steel with the slag is 0.0076%, the sulfur content is 0.0012%, and the aluminum content is 0.0069%;
table 4 composition (mass%,%) of 254SMO super austenitic stainless steel used in example 2, step (1)
(2) Refining the molten steel with slag obtained in the step (1) to obtain refined molten steel;
the refining comprises the steps of temperature adjustment, slag adjustment, diffusion deoxidation, precipitation deoxidation and inclusion refining treatment which are sequentially carried out; the refining treatment of the inclusion comprises the steps of sequentially feeding calcium silicon wire (the mass fraction of silicon is 62 percent and the mass fraction of calcium is 35 percent) and rare earth core-spun wire (the mass fraction of Ce is 56.5 percent, and the total mass fraction of Ce and La is 99.75 percent)) (ii) a More specifically: the temperature of the molten steel after the temperature adjustment is finished is 1604 ℃, and 2 kg/ton of steel lime (the mass fraction of CaO is 96.5%) and 1.2 kg/ton of steel fluorite (CaF) are added during slag adjustment 2 The mass fraction of (b) was 87.25%), and the refining slag components after completion of slag adjustment are shown in Table 5. After slag adjustment, soft argon blowing is carried out, the gas flow is 6 NL/(min.t), then, 1.3kg of silicon calcium powder (the mass fraction of silicon is 62 percent, and the mass fraction of calcium is 35 percent) per ton of steel is added to the surface of refined slag for diffusion deoxidation, then, 0.25kg of aluminum wire (the mass fraction of aluminum is 99.4 percent) per ton of steel is fed into molten steel for precipitation deoxidation, and then, argon with the flow of 6 NL/(min.t) is soft blown for 14 min; measuring the components of the molten steel to obtain the molten steel with the total oxygen content of 0.0027%, the sulfur content of 0.0009% and the calcium content of 0.0011%; calculating the feeding amount of the calcium silicate wire according to a formula I, wherein T in the formula I is 1571 ℃, the feeding amount of the calcium silicate wire is 3.1kg through calculation, feeding molten steel at the speed of 4m/s, and then continuously blowing argon gas with the flow of 6 NL/(min.t) for 9min in a soft blowing mode; and calculating the feeding amount of the rare earth core-spun yarn to be 8.9kg according to a formula II, feeding the rare earth core-spun yarn with the diameter of 7mm and the sheet thickness of 0.6mm at the speed of 7m/s, and soft blowing argon with the flow of 4 NL/(min.t) for 6min again.
Table 5 example 2 composition of refining slag after slag adjustment in step (2) (mass%)
| CaO | Al 2 O 3 | CaF 2 | SiO 2 | MgO | FeO+MnO | Impurities in the product |
| 54.8 | 23.6 | 4.9 | 7.4 | 8.7 | 0.1 | 0.5 |
M in the above refining slag CaO /M SiO2 Is 7.41, and M SiO2 ％+M Al2O3 ％+2M CaF2 % of the total mass fraction is 40.8%.
(3) Casting the refined molten steel obtained in the step (2) to obtain super stainless steel with refined rare earth inclusions;
wherein the casting temperature is 1543 ℃, the casting mode is continuous casting, the section size of a continuous casting machine is 200mm multiplied by 200mm, and the argon gas sealing protection measure is adopted at the joint of the tundish and the long nozzle to prevent the secondary oxidation of the molten steel.
Sampling and analyzing the tail center position of the continuous casting billet, detecting the total oxygen content of the continuous casting billet, and counting the size of the rare earth inclusions, wherein the result is shown in table 6.
Comparative example 2
In order to reveal the effect of increasing the calcium pretreatment for refining rare earth inclusions, the same method as that of example 1 was adopted, and the operation of feeding calcium silicon wire in the inclusion refining treatment in step (2) was omitted, so as to compare the inclusion refining effect of the calcium-free treatment before the rare earth treatment, and obtain one piece of the 254SMO super duplex stainless steel without the calcium treatment.
Sampling and analyzing the tail R/2 position of the 254SMO super duplex stainless steel without calcium treatment, detecting the total oxygen content and counting the size of the rare earth inclusions. The results are shown in Table 6:
TABLE 6 statistics of total oxygen content and rare earth inclusion size in two 254SMO superaustenitic stainless steels in example 2 and comparative example 2
The results in Table 6 show that example 2, which was fed to a calcium silicate wire prior to rare earth treatment, can reduce the oxygen content of the 254SMO superaustenitic stainless steel from 0.0018% to 0.0015%. In addition, the quantity proportion of the fine rare earth inclusions (1-5 mu m) is increased from 75.4% to 84.6%, and the quantity proportion of the large-size rare earth inclusions (larger than 10 mu m) is reduced from 6.9% to 2.8%.
The embodiment shows that the method for refining the rare earth inclusions in the super stainless steel is simple to operate, can greatly reduce the total oxygen content in the steel, and can obviously refine the size of the rare earth inclusions.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (9)
1. A method for refining rare earth inclusions in super stainless steel comprises the following steps:
(1) carrying out primary refining on the super stainless steel to be treated to obtain molten steel with slag;
(2) refining the molten steel with slag obtained in the step (1) to obtain refined molten steel; the refining comprises the steps of temperature adjustment, slag adjustment, diffusion deoxidation, precipitation deoxidation and inclusion refining treatment which are sequentially carried out; the inclusion refining treatment comprises the steps of feeding calcium silicate wires and rare earth core-spun wires in sequence;
the feeding amount of the calcium silicate wire is calculated according to a formula I:
M t ×[T.O×(4.24-2.26×10 -3 T)-A]/w Ca (0.2-0.4) formula I;
in the formula I, M t Is the material of the super stainless steel to be treatedAmount, kg; T.O is the mass fraction percent of total oxygen in molten steel before feeding the calcium silicate wire; t is the temperature of the molten steel before feeding the calcium silicate wire, and is DEG C; a is the mass fraction of calcium in molten steel before feeding the calcium-silicon wire; w is a Ca Is the mass fraction of calcium in the calcium silicate wire percent;
the feeding amount of the rare earth core-spun yarn is calculated according to a formula II:
4.38 XMtX (T.O + S) X (1-2) formula II;
in said formula II, M t The mass of the super stainless steel to be treated is kg; T.O is the mass fraction percent of total oxygen in molten steel before feeding the calcium silicate wire; s is the mass fraction percent of sulfur in the molten steel before feeding the calcium silicate wire;
(3) casting the refined molten steel obtained in the step (2) to obtain super stainless steel with refined rare earth inclusions;
the slag adjustment in the step (2) is to add lime and fluorite into the molten steel with slag; the adding amount of the lime is 0-5 kg per ton of steel; the addition amount of the fluorite is 0-2.5 kg per ton of steel.
2. The method according to claim 1, wherein the amount of slag in the molten steel with slag in the step (1) is 70 to 100% of the total mass of the primary refining slag formed by the primary refining.
3. The method of claim 1, wherein the total oxygen content of the slag-bearing molten steel in step (1) is less than 0.008%, the sulfur content is less than 0.0015%, and the aluminum content is less than 0.01% by mass.
4. The method according to claim 1, wherein the refining slag obtained by slag mixing in the step (2) comprises the following components in percentage by mass: 40 to 60 percent of CaO and Al 2 O 3 10％～35％，CaF 2 0～12％，SiO 2 4-8 percent of MgO, 5-10 percent of MgO, 0-0.5 percent of FeO + MnO and less than 1 percent of impurities; and M in the refining slag CaO /M SiO2 Has a mass ratio ofNot less than 5, and M SiO2 ％+M Al2O3 ％+2M CaF2 % of the total mass fraction is 36-45%.
5. The method according to claim 1, wherein the diffusion deoxidation in the step (2) comprises adding calcium silico-calcium powder on the surface of the refined slag obtained by slag mixing; the addition amount of the calcium silicate powder is 0.8-1.5 kg per ton of steel.
6. The method according to claim 1, wherein the precipitation deoxidation in the step (2) comprises feeding an aluminum wire to the molten steel obtained by the diffusion deoxidation; the feeding amount of the aluminum wire is 0.1-0.3 kg per ton of steel.
7. The method according to claim 1, wherein the temperature of the molten steel before the Si-Ca wire is fed in the step (2) is 1550-1580 ℃.
8. The method according to claim 1, wherein soft blowing is performed before the operation of diffusion deoxidation in the step (2) and after each of the operations of precipitation deoxidation, calcium silicate wire feeding and rare earth core-spun wire feeding; the soft blowing time is independently 5-15 min.
9. The method according to claim 1, wherein the mass fraction of Ce in the rare earth core-spun yarn in the step (2) is more than 50%, and the total mass fraction of Ce and La is more than or equal to 99.5%; the diameter of the rare earth core-spun yarn is 5-13 mm, and the thickness of an iron sheet layer coated on the outer layer of the rare earth core-spun yarn is 0.3-0.6 mm.
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