Method for accurately controlling spheroidizing temperature of high-purity molten iron

Abstract

The invention relates to a method for accurately controlling the spheroidizing temperature of high-purity molten iron, and belongs to the technical field of the smelting industry. According to the method, the temperature of rough smelting and refining of the molten iron is controlled through the technical means of large induction furnace coil induction heating, ladle furnace graphite electrode electric arc heating, refining slag protection refining, ladle bottom argon blowing stirring temperature equalization and homogenization and the like; by combining a smelting and transporting container baking system, process temperature drop empirical parameter revision and secondary refining tapping temperature compensation for molten iron cooling in the subsequent process, the optimal process temperature requirement required by spheroidizing treatment is finally met, and a high-purity and high-uniformity molten iron manufacturing process control technology is added; therefore, precise control over the optimal process temperature of base iron spheroidizing for spheroidizing the barrel of the hundred-ton nodular cast iron spent fuel storage and transportation device is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of smelting industry, and particularly relates to a method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron, which can be applied to the production of molten iron and the control of process temperature for products made of ductile iron with a capacity of hundreds of tons. Background Art

[0002] Spent fuel, that is, irradiated nuclear fuel, generally refers to the nuclear fuel in a nuclear power plant nuclear reactor that can no longer maintain a nuclear reaction due to the reduction of uranium content to a certain extent. Spent fuel contains a large amount of radioactive elements, which are extremely harmful to the environment and have a long half-life. Therefore, it must be properly treated, and its treatment process includes storage, transportation, reprocessing, deep geological disposal, etc. The spent fuel storage and transportation device is the storage and transportation container for this treatment process.

[0003] In the prior art, ductile iron castings are mainly produced by cupola furnaces or induction furnaces to produce the original molten iron. After spheroidizing and inoculation treatment, they are cast into shape, and then obtained after a series of processes such as knocking out the mold, cleaning, and heat treatment. The amount of molten iron that can be provided at one time is limited by the nominal capacity of the furnace body, mostly ranging from several tons to dozens of tons. Its composition is limited by raw materials and melting processes, with large fluctuations. The single induction heating method results in poor controllability of the process temperature, lacks process means for removing inclusions, and it is difficult to control the overall metallurgical quality of the original molten iron.

[0004] The existing methods cannot simultaneously precisely control the purity, composition, and temperature uniformity of molten iron, as well as the process temperature of molten iron spheroidizing treatment. The performance indicators of the finished products are difficult to meet the requirements of strict technical conditions, and there is also a problem that it is difficult to control the graphite morphology of the matrix microstructure. Summary of the Invention

[0005] In view of the above analysis, the embodiments of the present invention aim to provide a method for precisely controlling the process temperature of high-purity molten iron spheroidizing treatment, so as to solve at least one of the problems in the prior art such as the difficult precise control of the spheroidizing treatment temperature of high-purity molten iron, the difficult realization of high-standard performance indicators of finished products, and the difficult control of the graphite morphology of the matrix microstructure. This is particularly important for large-tonnage ductile iron castings with a capacity of hundreds of tons.

[0006] The object of the present invention is mainly achieved through the following technical solutions:

[0007] A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron, the method comprising the following steps:

[0008] (1) Roughly smelt the steel raw materials at a temperature ≤ 1550 °C to obtain the roughly smelted molten iron at the first station, and roughly smelt the foundry pig iron and graphite carbon powder at a temperature ≤ 1550 °C to obtain the roughly smelted molten iron at the second station;

[0009] (2) The first-stage crude iron molten steel is subjected to primary refining at a temperature ≤ 1550°C to obtain primary-refined iron molten steel. The primary-refined iron molten steel and the second-stage crude iron molten steel are combined and mixed, and then subjected to secondary refining at a temperature ≤ 1550°C to obtain secondary-refined iron molten steel;

[0010] (3) The secondary-refined iron molten steel is transported and divided for pouring. The temperature of the molten steel in the transfer ladle is controlled at 1440 - 1460°C, and nodulizing treatment is carried out at a temperature of 1380 - 1420°C.

[0011] Further, in step (1), both the first-stage crude iron molten steel and the second-stage crude iron molten steel are tapped after being kept at a temperature of 1500 - 1520°C for 5 - 20 minutes after all the furnace charges are melted.

[0012] Further, in step (2), the tapping temperature of the primary-refined iron molten steel is 1500 - 1520°C, and the tapping temperature of the secondary-refined iron molten steel is 1480 - 1520°C.

[0013] Further, in step (2), the whole process of primary refining and secondary refining adopts the stirring method of bottom blowing argon in the ladle.

[0014] Further, special slag-making treatment is carried out in both primary refining and secondary refining. Specifically, for the slag system components, the slag-making materials in primary refining are metallurgical lime and fluorite, and the mass ratio of metallurgical lime to fluorite is 4:1. The slag-making material in secondary refining is fluorite.

[0015] Further, in step (3), the time from the completion of transporting and dividing the secondary-refined iron molten steel to the start of nodulizing treatment is controlled within 30 - 60 minutes.

[0016] Further, in step (3), the molten steel before nodulizing treatment is evenly cooled by means of bottom blowing argon.

[0017] Further, if the nodulizing treatment temperature is lower than 1380 - 1420°C, the molten steel is returned to the refining station for graphite electrode arc heating.

[0018] Further, the single-eye argon gas flow rate is 30 - 50 NL / min, and the cooling rate of the molten steel is 1.5 - 2.0°C / min.

[0019] Further, before nodulizing treatment in step (3), the content of C in the molten steel is 3.50 - 4.00%, Si is 0.30 - 0.40%, P ≤ 0.020%, S is 0.004 - 0.009%, and Ni is 0.55 - 0.65%.

[0020] Further, step (3) also includes transporting the nodulized molten steel to the casting process for final inoculation, slag skimming and pouring operations.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] (1) The method of the present invention controls the temperature of molten iron roughing and refining, and the temperature of the secondary refining iron needs to compensate for the cooling of the molten iron in the subsequent transportation, distribution and waiting process to meet the optimal process temperature requirement of spheroidization treatment. The method of the present invention adopts roughing and secondary refining to achieve accurate control of key alloy components such as C, P, and S in the molten iron; the secondary refined molten iron is distributed by the distribution equipment and then transported to the spheroidization treatment equipment for spheroidization treatment to ensure the identity and homogeneity of the overall composition of the cylinder casting;

[0023] (2) In the present invention, except for steel materials, cast iron, and alloy nickel plate, no alloy is added in the process to adjust the target composition of molten iron, so as to avoid excessive addition of alloys causing an increase in residual elements, affecting the purity of molten iron, and reducing the final evaluation of the graphite morphology of the product metallographic structure. In particular, in the secondary refining process in molten iron manufacturing, a dedicated alloy composition control process must be strictly implemented;

[0024] (3) The method of the present invention uses argon blowing and strong stirring at the bottom of the ladle throughout the primary refining and secondary refining to highly homogenize the temperature and composition of the molten iron, greatly reducing the temperature gradient and concentration gradient of the molten iron in the container, and assisted by a special slag system slag making method to promote the floating of endogenous and exogenous inclusions in the molten iron so that they are adsorbed and removed by the metallurgical slag, thereby achieving high purity of molten iron that cannot be achieved by traditional molten iron manufacturing processes, high uniformity of composition and temperature, and precise control of the optimal process temperature for the final spheroidization treatment; since high-pressure argon gas has strong stirring kinetic energy, the mass transfer process from the bottom of the molten iron to the slag-iron interface is enhanced, and the air-permeable bricks at the bottom of the ladle generate fine high-purity argon bubbles equivalent to countless micro vacuum chambers. In the process of the argon bubbles floating up and the volume increasing as the static pressure of the molten iron decreases, hydrogen and nitrogen gases in the molten iron enter the argon bubbles, and various inclusions are also adsorbed to the slag-iron interface in the collision with the surface of the argon bubbles to be adsorbed and removed by the alkaline slag; in addition, the method of the present invention controls the S in the molten iron before spheroidization 0.004~0.009%, practice has proved that it can effectively improve the spheroidizing inoculation effect and improve the metallographic structure of castings;

[0025] (4) The spheroidization temperature in the present invention is based on the tapping temperature of the secondary refined molten iron. The temperature drop rate of the molten iron during transfer, distribution and waiting is determined by empirical parameters. The optimal process temperature range of the spheroidization treatment is accurately controlled by blowing argon gas at the bottom of the ladle. The general-purpose ladles and special ladles involved in the molten iron are baked using a special baking curve before use to achieve accurate control of the optimal process temperature of the molten iron spheroidization treatment.

[0026] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combined solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages will be obvious from the description or can be understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained from the content specifically pointed out in the description and the drawings. Description of the Drawings

[0027] The drawings are only for the purpose of showing specific embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference signs denote the same components.

[0028] Figure 1 Microstructure diagram (magnification 50 microns) of the spent fuel storage and transportation cylinder made of ductile iron prepared in Example 1 of the present invention after corrosion of the cylinder;

[0029] Figure 2 Microstructure diagram (magnification 200 microns) of the spent fuel storage and transportation cylinder made of ductile iron prepared in Example 1 of the present invention before corrosion of the cylinder;

[0030] Figure 3 On-site physical diagram of the workpiece after semi-finishing of the spent fuel storage and transportation cylinder made of ductile iron in Example 1 of the present invention. Detailed Description of the Invention

[0031] The following will specifically describe the preferred embodiments of the present invention with reference to the drawings. The drawings form a part of this application and are used together with the embodiments of the present invention to explain the principles of the present invention, rather than to limit the scope of the present invention.

[0032] A specific embodiment of the present invention is a method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron. The method includes the following steps:

[0033] (1) Crudely smelt the steel raw materials at a temperature ≤ 1550 °C to obtain the first-station crudely smelted molten iron, and crudely smelt the foundry pig iron and graphite carbon powder at a temperature ≤ 1550 °C to obtain the second-station crudely smelted molten iron;

[0034] (2) Refine the first-station crudely smelted molten iron at a temperature ≤ 1550 °C to obtain the first-refined molten iron, and mix the first-refined molten iron and the second-station crudely smelted molten iron and refine them at a temperature ≤ 1550 °C to obtain the second-refined molten iron;

[0035] (3) Transfer and divide the second-refined molten iron, control the temperature of the molten iron in the transfer ladle at 1440 - 1460 °C, and perform spheroidizing treatment at a temperature of 1380 - 1420 °C.

[0036] Specifically, in step (1), the steel raw materials are cold steel materials, foundry pig iron, alloy nickel plates, and graphite carbon powder.

[0037] It should be noted that the rough smelting in the present invention is carried out in an induction furnace, and the induction furnace is a two-station induction furnace. The cold steel materials, foundry pig iron, alloy nickel plates, and graphite carbon powder are roughly smelted at the first station to obtain the rough molten iron at the first station. The foundry pig iron and graphite carbon powder are roughly smelted in the second process to obtain the rough molten iron at the second station. The primary refining is carried out in the first refining furnace, and the secondary refining is carried out in the second refining furnace.

[0038] The temperature during the whole process of rough smelting and refining of molten iron shall not be greater than 1550 °C, and the tapping temperature of the secondary refining at the end of the smelting process needs to compensate for the temperature drop of the molten iron during subsequent transportation, dispensing, and waiting processes to meet the best process temperature requirements for nodulizing treatment.

[0039] Compared with the prior art, for a method for precisely controlling the nodulizing treatment temperature of high-purity molten iron in the present invention, the method of the present invention controls the temperature of rough smelting and refining of molten iron, formulates a special baking process to bake and keep warm the general steel ladle and special molten iron ladle, ensures the controllability of the temperature drop during the molten iron process, and the tapping temperature of the secondary refining needs to compensate for the temperature drop of the molten iron during subsequent transportation, dispensing, and waiting processes to meet the best process temperature requirements for nodulizing treatment. The method of the present invention adopts rough smelting and secondary refining to precisely control the key alloy components such as C, P, and S in the molten iron; the molten iron after secondary refining is dispensed by a dispensing device and then transported to a nodulizing treatment device for nodulizing treatment to ensure the identity and homogenization degree of the overall composition of the hundred-ton nodular cast iron cylinder casting.

[0040] In this embodiment, except for cold steel materials, foundry pig iron, and alloy nickel plates, no additional alloy is added in the process to adjust the target composition of the molten iron, so as to avoid the increase of residual elements caused by excessive addition of alloys, which affects the purity of the molten iron and reduces the final evaluation of the graphite morphology in the product metallographic structure. Especially in the secondary refining process of molten iron manufacturing, the alloy composition control process must be strictly implemented. Among them, the auxiliary materials are graphite carbon powder, metallurgical lime, and massive fluorite. Except that the graphite carbon powder can be used for carbon addition and the alloy nickel plate can be used to adjust nickel in the rough smelting process of the induction furnace, and the cold steel materials and foundry pig iron are used to adjust the amount and composition of the molten iron in the refining process, and necessary metallurgical lime and fluorite are used to create slag, there are no other additional raw and auxiliary materials.

[0041] It should be noted that during the primary refining and secondary refining processes of the ladle furnace, except for cold steel materials and foundry pig iron, the adjustment of the chemical composition C of the molten iron in this refining process shall not be added to the molten iron in the form of elemental carbon such as graphite carbon powder and ordinary carbon powder. The purpose is to avoid the loss of control of the final finished product C composition caused by the yield of elemental carbon and the error of detection and inspection instruments for the key component C in high-temperature molten iron.

[0042] In addition, taking a 160-ton secondary ladle refining furnace as an example for explanation, after the hot metal refined by the 160-ton ladle furnace is tapped, it needs to be transported to the dispensing station for hot metal dispensing and transfer treatment. The dispensing sequence is: first, 60 tons of molten metal are dispensed into the intermediate ladle, and then 80 tons of molten metal are dispensed into the intermediate ladle; the transfer sequence is: first, the 60-ton intermediate ladle is transported, and then the 80-ton intermediate ladle is transported. After dispensing, the temperature is measured. The temperature of the hot metal in the intermediate ladle is 1440°C to 1460°C. To avoid re-heating treatment caused by excessive temperature drop of the hot metal, the time interval from the transfer and dispensing of the hot metal to the spheroidizing treatment after tapping should be strictly controlled within 30 min to 60 min. When the intermediate ladle reaches the spheroidizing treatment station, a disposable temperature measuring probe is obliquely inserted at least 200 mm below the liquid surface of the hot metal. If the temperature is too low, it is returned to the refining station for heating. If the temperature is too high, the hot metal is cooled by means of bottom blowing argon.

[0043] Spheroidizing inoculation and pouring: In the casting process, 140 tons of hot metal are dispensed into 60 tons and 80 tons of hot metal. After spheroidizing treatment, slag skimming, and floating silicon inoculation, they are poured into the cavity through different pouring basins at intervals for a certain time. To ensure the floating speed of the liquid surface, the pouring of the two should end simultaneously. The process spheroidizing treatment temperature is 1380 - 1420°C, and the actual spheroidizing temperature is 1405°C.

[0044] It should be noted that in the raw materials for spheroidizing treatment of high-purity hot metal by the method of the present invention, the mass of cold-state steel materials accounts for 15 - 20% of the total mass of steel materials, pig iron, and alloy nickel plates.

[0045] The hot metal spheroidized by the present invention can not only be used to prepare the cylinder body of the spent fuel storage device, but also be other large castings made of ductile iron.

[0046] Exemplarily, the cold-state steel materials described in step (1) include, by mass percentage, C 0.25 - 0.45%, Si ≤ 0.01%, Mn ≤ 0.05%, P ≤ 0.005%, S ≤ 0.005%, Cr ≤ 0.05%, Mo ≤ 0.05%, and trace amounts of Sb, W, V, Pb, As, Sn, and Zr.

[0047] Specifically, the cold-state steel materials are made by rough smelting in an electric arc furnace, refining in a ladle furnace, and finally casting into billets by atmospheric casting.

[0048] It should be noted that in this embodiment, the cold-state steel materials are prepared by the above method, with a block weight of 250 - 6000 kg. The component ratios in the cold-state steel materials are controlled. Preferably, the main components analyzed are C 0.25%, Si ≤ 0.01%, P ≤ 0.001%, and S ≤ 0.002%. The purpose of such setting is to meet the technical restrictive requirements such as the quality and weighted components of the subsequent hot metal smelting.

[0049] Specifically, the cast pig iron in step (1) contains, by mass percentage, 4.50 - 4.70% C, 0.40 - 0.60% Si, ≤0.100% Mn, ≤0.030% P, ≤0.025% S, ≤0.010% Cr, ≤0.10% Ni, ≤0.010% Mo, and ≤0.050% Ti. The weight of the cast pig iron block is 5 kg.

[0050] Specifically, in step (1), the mass fraction of Ni in the alloy nickel plate > 99.5%. In this embodiment, the grade of the alloy nickel plate is Ni9950.

[0051] It should be noted that in this embodiment, the graphite carbon powder contains more than 99% carbon and has a particle size less than 1 mm.

[0052] Specifically, in step (1), both the first-stage crude iron melt and the second-stage crude iron melt are kept at 1500 - 1520 °C for 5 - 20 min after all the furnace charges are melted and then tapped.

[0053] Specifically, in step (2), the tapping temperature of the primary refined iron melt is 1500 - 1520 °C, and the tapping temperature of the secondary refined iron melt is 1480 - 1520 °C.

[0054] Specifically, in step (2), the whole process of primary refining and secondary refining adopts the ladle bottom blowing argon stirring method.

[0055] It should be noted that with the ladle bottom blowing argon stirring method, the temperature and composition of the iron melt are highly homogenized. Supplementary slag-making methods are used to promote the floating of endogenous and exogenous inclusions in the iron melt, which are then adsorbed and removed by the metallurgical slag, achieving a high purity of the iron melt, high uniformity of composition and temperature, and precise control of the optimum process temperature for spheroidizing treatment that cannot be achieved by traditional iron melt manufacturing processes.

[0056] Exemplarily, special slag system slag-making treatments are carried out in both primary refining and secondary refining. Specifically, the technical requirements for the slag system components are as follows: in primary refining, the slag-making materials are metallurgical lime and fluorite, and the mass ratio of metallurgical lime to fluorite is 4:1; in secondary refining, the slag-making material is fluorite.

[0057] It should be noted that in this embodiment, first-grade metallurgical lime is selected. By mass percentage, the main technical indicators are CaO ≥ 90%, MgO ≤ 5.0%, SiO 2 ≤ 2.0%, S ≤ 0.03%, burning alkali ≤ 4%, activity ≥ 320 (activity 4 mol / L, 40 °C ± 1 °C for 10 min), and the lump size is 20 - 100 mm; fluorite lump ore, grade FL-85, the main technical indicators are (CaF 2 ) ≥ 85%, (SiO 2) ≤ 14.3%, P ≤ 0.06%, S ≤ 0.10%, block size 5 - 100 mm. All raw and auxiliary materials used are required to be clean, dry, and clearly labeled. Large metal cutting materials need to be labeled with the specific single weight.

[0058] It should be noted that in this embodiment, steel materials and foundry pig iron are used to adjust the quality and weight components of hot metal, and metallurgical lime and fluorite are used to make slag. The slag system ratio is CaO:CaF 2 = 4:1. The graphite electrode is used for arc heating to raise the temperature and carry out refining for sulfur control; after the hot metal smelting in the second station is completed, it is added to the second ladle refining furnace; after the mixing is completed, fluorite is added to make slag in the second ladle refining furnace, and the graphite electrode is used for arc heating to raise the temperature. Argon gas is blown at the bottom for strong stirring to evenly mix the components and temperature. After sampling, when the components and temperature are appropriate, the hot metal can be tapped and transferred to the subsequent hot metal sub - mixing and spheroidizing inoculation treatment.

[0059] In this embodiment, except for cold - state steel materials, foundry pig iron, and alloy nickel plates, no additional alloys are added in the process to adjust the target components of hot metal, so as to avoid the increase of residual elements caused by excessive addition of alloys, which affects the purity of hot metal and reduces the final evaluation of the graphite shape and morphology of the product's metallographic structure. Especially in the secondary refining process of hot metal production, the alloy composition control process must be strictly implemented. Among them, the auxiliary materials are graphite carbon powder, metallurgical lime, and massive fluorite. Except that graphite carbon powder can be used for carbon addition and nickel plates can be used to adjust nickel in the induction furnace rough smelting process, and cold - state steel materials and foundry pig iron are used to adjust the amount and composition of hot metal in the refining process, supplemented by necessary metallurgical lime and fluorite for slag making, there are no other additional raw and auxiliary materials.

[0060] It should be noted that during the secondary refining process of the ladle furnace, except for cold - state steel materials and foundry pig iron, the adjustment of the chemical component C of the hot metal in this process shall not be added to the hot metal in the form of elemental carbon such as graphite carbon powder and carbon powder. The purpose is to avoid the out - of - control of the final product composition due to the yield of elemental carbon and the error of detection and inspection instruments for the key component C of high - temperature hot metal.

[0061] In this embodiment, both the first ladle refining furnace and the second ladle refining furnace are newly built barrel - shaped ladle furnaces. The working layer refractory bricks are magnesia - carbon. These refractory bricks are cold - mixed and pressed from sintered magnesia, flaky graphite, organic binder, and antioxidant. When used for the first time, it needs to be baked according to a special baking curve for no less than 24 h. A regenerative vertical covered ladle roaster is used, with a heating rate of 50 - 80 °C / h, a holding time of more than 12 h at 1000 °C. The energy medium is natural gas or industrial gas. The stop - fire temperature is greater than 1000 °C. After 30 min of stopping the fire, the inner bottom of the ladle is visually red - hot, the infrared temperature measurement at 1 / 2 of the inner wall is not lower than 750 °C, and the infrared temperature measurement at 1 / 2 of the outer wall is not lower than 150 °C, so as to ensure that the newly built ladle and hot metal ladle are baked through and nearly in a heat - saturated state, and there is no residual moisture or crystal water in the refractory bricks to ensure the safety of the smelting process, and at the same time ensure that the hot metal does not increase H due to environmental factors during the melting process.

[0062] It should be noted that ladle furnace refining: (1) When adding molten iron and foundry pig iron, at the same time, slag-making materials need to be added so that the graphite electrode can be electrically heated, the molten slag can desulfurize, and adsorb and remove various inclusions in the hot metal, both endogenous and exogenous. During the smelting process of traditional hot metal induction furnaces, the floating slag on the surface of the molten pool is mainly acidic SiO 2 -based, which comes from the primary gangue brought by the raw material foundry pig iron during the production of hot metal. Compared with the basic slag mainly composed of CaO, the basicity of this acidic slag is too low to desulfurize, and its inclusion adsorption ability is weak; (2) Since it is easy for high-carbon hot metal to increase carbon in the form of carbon powder, un-dissolved graphite particles will enter the casting cavity, and the carbon powder recovery rate fluctuates greatly due to various factors. Therefore, during the rough smelting stage of the induction furnace, the C weight of the hot metal composition is adjusted to the lower limit of the specification, and the refining process only uses foundry pig iron and molten iron to adjust the carbon content; (3) Stirring the hot metal by blowing argon gas at the bottom of the ladle can greatly shorten the process of equalizing the temperature and homogenizing the inclusions of the hot metal, improve the purity and uniformity of the hot metal, and has technical advantages that cannot be compared with traditional processes. High-pressure argon gas has strong stirring kinetic energy, which strengthens the mass transfer process from the bottom of the hot metal to the slag-metal interface. The breathable bricks at the bottom of the ladle generate fine high-purity argon bubbles, which are equivalent to countless micro vacuum chambers. During the process of the argon bubbles floating up and their volume increasing as the static pressure of the hot metal decreases, the H and N gases in the hot metal enter the argon bubbles, and various inclusions are also adsorbed during the collision with the surface of the argon bubbles and come to the slag-metal interface to be adsorbed and removed by the basic molten slag. Its process parameters are: argon blowing square bricks at the bottom of the ladle, double-eye breathable, at the 1 / 2 radius, the flow rate is 150 NL / min to 300 NL / min, the argon purity is greater than 99.99%, and in actual operation, it is appropriate that the bulge of the slag surface does not exceed 200 mm × 200 mm. A complete temperature equalization and homogenization cycle depends on the height of the hot metal liquid level in the ladle and is between 10 min and 15 min; (4) In addition to the composition of the hot metal in the refining process meeting the design requirements, its temperature should consider the temperature drop compensation during the subsequent transfer, distribution, and waiting of the hot metal, and on this basis, meet the final spheroidizing treatment temperature of the hot metal. In actual operation, the empirical formula for the temperature drop rate during transfer: transfer time t = 20 min to 30 min, temperature drop rate ΔT / t ≈ 1.5 °C / min; transfer time t = 30 min to 45 min, temperature drop rate ΔT / t ≈ 1.0 °C / min. In the design of the process plan, the tapping temperature T of secondary refining is 1480 °C to 1520 °C.

[0063] Specifically, the time from the transfer and distribution of the secondary refined molten iron described in step (3) to the start of the spheroidization treatment is controlled at 30 to 60 minutes. In order to avoid the re-heating treatment caused by excessive temperature drop of the molten iron, the time interval from the transfer and distribution of the molten iron to the spheroidization treatment after the iron is tapped should be strictly controlled at 30 to 60 minutes. When the transfer bag arrives at the spheroidization treatment station, a disposable temperature measuring head is obliquely inserted at least 200mm below the molten iron liquid surface. If the temperature is too low, the graphite electrode is returned to the refining station for arc heating. If the temperature is too high, the molten iron is cooled by bottom argon blowing. The use of refined graphite electrode arc heating and ladle bottom argon blowing technology greatly enhances the accuracy and flexibility of the molten iron spheroidization treatment process.

[0064] Specifically, in step (3), the molten iron before spheroidization is cooled by blowing argon from the bottom of the ladle for stirring.

[0065] Specifically, if the temperature is lower than 1380-1420°C, the molten iron is returned to the refining station for heating.

[0066] It should be noted that the process spheroidization temperature is 1380℃~1420℃, and the tapping temperature of the secondary refined molten iron is used as the reference temperature. The temperature drop rate of the molten iron transfer, distribution and waiting process is determined by empirical parameters, and the argon blowing operation at the bottom of the ladle is used to accurately control the optimal process temperature range of the spheroidization treatment.

[0067] Specifically, the single-eye argon flow rate is 30-50NL / min, and the molten iron cooling rate is 1.5-2.0℃ / min.

[0068] Specifically, in step (3), the molten iron before spheroidization treatment contains 3.50-4.00% C, 0.30-0.40% Si, ≤0.020% P, 0.004-0.009% S, and 0.55-0.65% Ni.

[0069] In the present invention, the C content of the molten iron before spheroidization treatment is 3.50-4.00%. During the processes of rough refining, tapping, pouring, mixing, tapping, separation and spheroidization in the induction furnace, the high-temperature and high-carbon molten iron has a certain amount of carbon burnout or carbon loss. In order to ensure the target C of the product, it is necessary to perform carbon increase operation with graphite carbon powder during the induction furnace smelting process, and the C component value in the tapping of the induction furnace should meet the weight value of the final carbon in the refining process.

[0070] It should be noted that the precise control of the C element in molten iron is the most critical technological difficulty in the entire molten iron manufacturing process. As the main element affecting the metallographic structure and mechanical properties of the tank body finished product, the starting point of most process designs in the technical route used in this invention is based on the precise controllability of the C element process. In the whole process of molten iron manufacturing, due to multiple carbon loss links and large fluctuation ranges, it is difficult to precisely control the C element of the final finished molten iron by the method of adding carbon with graphite carbon powder. After multiple tests, the inventor further optimized the process technical route, and controlled the factors affecting carbon fluctuation within a minimum range through precise raw material detection, batching design, and process empirical carbon loss, so as to achieve precise controllability of the carbon composition in the smelting process.

[0071] Secondly, inaccurate carbon recovery rate occurs when using graphite carbon powder to adjust carbon, which also leads to process defects such as graphite floating and slag inclusion in the metallographic structure of the cylinder castings after spheroidizing treatment, and there is a phenomenon of carbon loss in high-carbon molten iron throughout the production and manufacturing process. The specific carbon loss empirical values are: when the carbon addition C is 2.50% - 3.50%, the carbon loss ΔC is 0.15%; when the carbon addition C is 4.50% - 5.50%, the carbon loss ΔC is 0.20%.

[0072] In addition, starting from improving the spheroidizing inoculation effect and the graphite shape and morphology of the metallographic structure of the castings, it is required that the internal control S of the spheroidizing original molten iron is 0.004 - 0.009%, and the target S content is 0.006%. This process value is obtained based on the analysis of the test data from multiple previous pilot tests, and is specifically related to the process effect of molten iron spheroidizing inoculation and the graphite property evaluation of the metallographic structure of the castings. The main difficulty in controlling this element process is that during the refining process in the ladle refining furnace, as the smelting process progresses, the removal amount of the S element is in an uncontrollable state. Through process path design and selection of different component slag systems, the best process value of the S element in the spheroidizing treatment original molten iron is obtained.

[0073] Since molten iron with a high phosphorus content is prone to generate phosphide eutectic phenomenon at the crystal interface, resulting in the deterioration of the matrix structure and the reduction of mechanical properties of the finished castings, it is required that the P in the spheroidizing treatment original molten iron is ≤ 0.020%. The traditional molten iron smelting route (induction furnace melting + out-of-furnace treatment) cannot meet this technical requirement. The main reason is that the phosphorus content of high-quality foundry pig iron in China is basically above 0.025% at present, and only relying on induction furnace melting, the P element cannot be controlled within the target composition range. Therefore, the cold-state steel and iron materials used in the method of this invention are obtained by rough smelting in an electric arc furnace, refining in a ladle furnace, and finally casting into billets by atmospheric casting, which can control P below 0.001%, so as to achieve the weight P ≤ 0.020% and solve the process limitation that the molten iron smelted by the induction furnace cannot remove P.

[0074] Specifically, step (3) further includes transporting the spheroidized molten iron to the casting process for final inoculation, slag skimming, and pouring operations.

[0075] It should be noted that before use, the ladles and torpedo ladles involved in the molten iron in the present invention are baked using a dedicated baking curve to achieve precise control of the temperature for the spheroidizing treatment of molten iron.

[0076] Example 1

[0077] This example will be explained by taking the production of ductile iron material QT400 - 18AL with a total amount of 140 tons of molten iron for spheroidizing treatment as an example.

[0078] 1. Technical conditions:

[0079] The technical requirements for the evaluation of the cylinder body material (smelting and finished products) (wt.%) are shown in Table 1; the technical standards for special steel materials for ductile iron (wt.%) are shown in Table 2; the technical standards for special ultra - pure pig iron for ductile iron (wt.%) are shown in Table 3.

[0080] Table 1 Technical requirements for the evaluation of the cylinder body material (smelting and finished products) (wt.%)

[0081]

[0082] Table 2 Technical standards for special steel materials for ductile iron (wt.%)

[0083]

[0084]

[0085] Table 3 Technical standards for special ultra - pure pig iron for ductile iron (wt.%)

[0086]

[0087] 2. Main equipment:

[0088] One EBT (eccentric bottom tapping method, which can effectively control the slag volume during tapping compared with the tapping method through the tapping trough) electric arc furnace with a nominal capacity of 40 tons, used for the rough smelting of cold - state steel materials in this example; one double - station intermediate - frequency induction furnace with a nominal capacity of 60 tons, used for the preliminary melting of molten iron; 4 ladle - type steel ladles lined with magnesia - carbon bricks, with nominal capacities of 160 tons, 130 tons, 90 tons, and 40 tons respectively, serving as metallurgical containers for molten iron refining and transfer; one refining station each for 160 tons and 130 tons, which can achieve metallurgical functions such as power - on temperature rise, argon blowing and stirring, slag making and desulfurization, and heat preservation waiting; one 80 - ton and one 60 - ton ladle - type torpedo ladle, serving as metallurgical containers for the spheroidizing treatment and pouring of molten iron.

[0089] 3. Equipment inspection and status requirements:

[0090] (1) For the charging basket of the electric arc furnace, check the charging basket before loading. There should be no residual scrap steel materials from the previous furnace hanging on it to prevent material mixing;

[0091] (2) The electric arc furnace body requires that the molten steel slag in the forehearth be completely discharged, smelting is not allowed in the later stage of the furnace campaign, and the Mo content in the post-furnace composition of the steel grade in the previous heat should be less than 0.20%;

[0092] (3) The induction furnace body requires that the molten steel slag be completely discharged during the previous furnace baking, and there should be no visible continuous sticking of molten steel and slag on the tapping trough, furnace wall, and furnace bottom;

[0093] (4) The refining ladle is a newly-built ladle, which is in good thermal condition after baking. Check the permeability of the bottom porous brick by purging argon;

[0094] (5) The hot-tapping ladle is in good thermal condition after baking, with no remaining molten steel and slag at the bottom and the edge of the ladle. Moreover, the molten slag of the steel grade smelted in the previous heat is a refined ladle with a powder slag system mainly composed of CaO and SiO 2 ;

[0095] (6) The transfer ladle is a newly-built ladle. Check the state of the tapping side edge of the ladle after bricklaying; Implement the new ladle baking process with a baking time greater than 24 h. Measure the temperature before tapping iron, requiring the middle part of the ladle wall lining to be greater than 750 °C. After baking, suck air to remove the exfoliated refractory materials in the ladle;

[0096] (7) The hot metal ladle is baked at a temperature of 500 °C to 800 °C. The baking time for a new ladle is greater than 24 h, and the baking time for an old ladle is greater than 12 h. Appropriately extend the time in humid weather. Measure the temperature of the hot metal ladle before spheroidizing treatment, requiring the middle part of the inner lining to be at a temperature of 150 °C to 300 °C and the middle part of the ladle shell to be greater than 100 °C.

[0097] 4. Preparation of raw and auxiliary materials:

[0098] 32 t of steel materials, 125 t of foundry pig iron, 2000 kg of first-grade metallurgical lime, 1000 kg of fluorite, 1000 kg of alloy nickel plate, and 300 kg of graphite carbon powder. The above raw and auxiliary materials shall all be provided with alloy composition test reports and can only be used after being re-verified on-site before use. It is required that the weighing be accurate, the identification be clear, clean, dry, and not be mixed. The large steel materials also need to be marked with the specific mass.

[0099] The steel materials in this embodiment are all self-made. The specific method is to first refine in an electric arc furnace, then refine in a ladle furnace, and finally cast into billets by atmospheric casting. By mass percentage, the steel materials include C 0.25%, Si ≤ 0.01%, P ≤ 0.001%, S ≤ 0.002%;

[0100] The foundry pig iron includes C 4.50%, Si 0.40%, Mn ≤ 0.100%, P ≤ 0.030%, S ≤ 0.025%, Cr ≤ 0.010%, Ni ≤ 0.10%, Mo ≤ 0.010%, Ti ≤ 0.050%;

[0101] The Ni content in the alloy nickel plate is > 99.50%, and the grade is Ni9950;

[0102] In the described graphite carbon powder, C > 99%, and the particle size of the graphite carbon powder < 1 mm; primary metallurgical lime, the main technical indicators are CaO ≥ 90%, MgO ≤ 5.0%, SiO 2 ≤ 2.0%, S ≤ 0.03%, ignition loss ≤ 4%, activity ≥ 320 (activity 4 mol / L, 40°C ± 1°C for 10 min), lump size 20 mm - 100 mm;

[0103] Fluorite, grade FL - 85, the main technical indicators are CaF 2 ≥ 85%, SiO 2 ≤ 14.3%, P ≤ 0.06%, S ≤ 0.10%, lump size 5 mm - 100 mm.

[0104] A method for precisely controlling the spheroidizing treatment temperature of high - purity molten iron in this embodiment includes the following steps:

[0105] (1) Charge 25 t of steel materials, 35 t of foundry pig iron, 800 kg of alloy nickel plate, and 75 kg of graphite carbon powder, with a carbon addition of 2.75%, and conduct rough melting at the first station of the induction furnace. After melting is complete, take a sample for full analysis. During the melting process, the molten iron temperature shall not exceed 1550°C throughout. After all the furnace charges are melted, conduct the first heat preservation treatment at 1500 - 1520°C for 5 - 20 min, and then tap the iron to obtain the rough - melted molten iron at the first station. If the slag amount is abnormally large during the melting process (visually greater than 200 kg), it is necessary to strictly control the slag amount during the ladle - pouring process;

[0106] Charge 61 t of foundry pig iron and 125 kg of graphite carbon powder, with a carbon addition of 4.85%, into the second station of the induction furnace for rough melting. After melting is complete, take a sample for full analysis. During the melting process, the molten iron temperature shall not exceed 1550°C throughout. After all the furnace charges are melted, conduct the second heat preservation treatment at 1500 - 1520°C for 5 - 20 min, and then tap the iron to obtain the rough - melted molten iron at the second station;

[0107] (2) The first ladle refining furnace has a capacity of 130 tons. Key points of process control: The diameter of the ladle nozzle is Φ100mm for a single hole, the baking temperature is greater than 1000°C, the receiving iron temperature is greater than 900°C, the thermal state is good, and the temperature drop of receiving iron is less than 50°C; after pouring the rough-smelted hot metal at the first station into the first ladle refining furnace, sampling and slag-making are carried out. The slag-making materials are 1000 kg of metallurgical lime and 250 kg of fluorite. During the smelting process, an appropriate amount of fluorite is supplemented according to the fluidity of the molten slag. During the refining period, the temperature of the hot metal shall not be greater than 1550°C throughout the process. The medium-grade voltage is evenly powered to increase the temperature. When the temperature rises to 1500 - 1520°C, a heat preservation operation is carried out for 5 - 20 minutes; argon is blown from the bottom throughout the process. When implementing the heat preservation process, the argon flow rate is adjusted so that the molten iron surface does not expose the slag layer. When powering to increase the temperature and adding foundry pig iron and steel materials, the argon flow rate can be appropriately increased to strengthen the mass transfer and heat transfer process of the hot metal. The composition and the amount of hot metal are adjusted by adding steel materials and foundry pig iron respectively. The tapping temperature is 1500 - 1520°C, and in the hot metal, C is 3.05 - 3.15%, Si ≤ 0.40%, P ≤ 0.020%, S ≤ 0.002%, Ni is 1.00 - 1.05%. The amount of hot metal is controlled at 81 - 85 t. Carbon powder carburization is strictly prohibited, and thus the first-refined hot metal is obtained;

[0108] The second ladle refining furnace has a selected capacity of 160 tons. Key points of process control: The diameter of the ladle nozzle is Φ100mm for a single hole, the baking temperature is greater than 1000°C, the receiving iron temperature is greater than 900°C, the thermal state is good, and the temperature drop of receiving iron is less than 50°C; among them, the first-refined hot metal is poured into the second ladle refining furnace, and then the rough-smelted hot metal at the second station of the induction furnace is poured in; after pouring, sampling and slag-making are carried out. The slag-making material is 750 kg of the first batch of fluorite. After the chemical composition results are reported, an appropriate amount of metallurgical lime is supplemented according to the S content to continue desulfurization to meet the internal control specifications. The temperature of the metallurgical lime added to the hot metal is 1500 - 1520°C. During the refining period, the temperature of the hot metal shall not be greater than 1550°C throughout the process. The medium-grade voltage is evenly powered to increase the temperature. When the temperature rises to 1500 - 1520°C, a heat preservation operation is carried out for 5 - 20 minutes. Argon is blown from the bottom throughout the process. When implementing the heat preservation process, the argon flow rate is adjusted so that the molten iron surface does not expose the slag layer. When powering to increase the temperature, the argon flow rate can be appropriately increased to strengthen the mass transfer and heat transfer process of the hot metal; the smelting time is 60 min - 120 min, the tapping temperature T is 1480°C - 1520°C, the best composition range of the hot metal is C 3.70 - 3.80%, Si 0.30 - 0.40%, P ≤ 0.020%, S 0.004 - 0.009%, Ni 0.55 - 0.65%. The amount of hot metal is controlled at 140 - 145 t. Carbon powder carburization is strictly prohibited, and thus the second-refined hot metal is obtained;

[0109] (3)Key points for controlling the process of hot metal transfer and dispensing: Check mechanical equipment such as weighing equipment, overhead crane equipment, hydraulic system, and motor system in advance to ensure they are in normal working condition; measure the temperature in the middle of the lining of the transfer ladle before dispensing the secondary refined hot metal. It is required to be visually red-hot and the temperature is greater than 900 °C, which can be regarded as the approximate thermal saturation state of the transfer ladle, and the empirical formula for the temperature drop of hot metal during the transfer process is valid; the dispensed hot metal amounts are 80t and 60t in sequence. After dispensing, measure the temperature of the hot metal in the transfer ladle and take samples; the spheroidizing treatment temperature is 1395 - 1405 °C. Precise control of the hot metal spheroidizing temperature is carried out by blowing argon at the bottom of the ladle. The single-eye argon gas flow rate is 30 - 50 NL / min, and the hot metal temperature drop rate is 1.5 - 2.0 °C / min. It is strictly prohibited to operate the hot metal cooling with a large argon gas flow rate; from the end of dispensing to the start of spheroidizing treatment, the entire process time should be controlled within 30 - 45 min. The 140-ton hot metal for spheroidizing in this embodiment needs to be secondary refined in a 160-ton ladle furnace, adjust the composition and temperature, and then carry out the spheroidizing treatment after temperature equalization and homogenization. Each process operation on-site is commanded by a dedicated person, strictly implement the on-site safety operation procedures, and make safety precautions and accident plans in advance. After the spheroidizing treatment is completed, the hot metal ladle is transported to the casting process for the final inoculation, slag skimming, and pouring work.

[0110] (1) The main components of the hot metal smelted by the method of this embodiment after spheroidizing and inoculation treatment are specifically as follows:

[0111] C 3.77%, Si 1.53%, Mn 0.11%, P 0.015%, S 0.005%, Ni 0.61%, Cr 0.04%, Mo 0.03%, H 1.6 ppm, O 10 ppm, N 33 ppm. It can be seen that the purity of the hot metal smelted by the method of the present invention is higher than that of the hot metal produced by the traditional method.

[0112] (2) The spent fuel storage and transportation cylinder body is made of the ductile iron material QT400 - 18AL prepared in this embodiment. The metallographic structure diagram (magnification 50 microns) obtained after the sampled cylinder body is corroded is as Figure 1 shown. It can be seen from Figure 1 that the graphite tissue morphology of the casting is approximately circular, evenly distributed after precipitation along the grain boundaries, with a size of 30 - 50 microns. Subsequent performance tests also show that the metallurgical quality of this casting has a high degree of uniformity and purity that traditional castings do not possess. The metallographic structure diagram (magnification 200 microns) of the cylinder body before corrosion is as Figure 2 shown. It can be seen that the metallographic structure changes little before and after the spent fuel corrosion.

[0113] The on-site physical drawing of the workpiece after semi-finishing the spent fuel storage and transportation cylinder body made of the ductile iron material of this embodiment is as Figure 3 shown. Subsequent non-destructive testing results show that this casting fully meets the technical requirements specified in the design outline.

[0114] Example 2

[0115] This example adopts the method of precisely controlling the temperature of the high-purity molten iron spheroidizing treatment in Example 1. The difference is that in step (1), the highest temperature of the molten iron during the entire crude refining process is 1550 °C;

[0116] In step (2), the highest temperature of the molten iron during the entire primary refining and secondary refining processes is 1550 °C. The tapping temperature of the molten iron in the primary refining is 1510 - 1520 °C, and the tapping temperature of the molten iron in the secondary refining is 1490 - 1510 °C. In the secondary-refined molten iron, C is 3.67%, Si is 0.39%, P is 0.002%, S is 0.006%, and Ni is 0.64%;

[0117] In step (3), the time for transporting and blending the secondary-refined molten iron to the spheroidizing treatment is controlled within 30 - 40 min. The temperature of the spheroidizing treatment is 1380 - 1400 °C, the single-eye argon flow rate is 40 - 50 NL / min, and the cooling rate of the molten iron is 1.8 - 2.0 °C / min.

[0118] Example 3

[0119] This example adopts the method of precisely controlling the temperature of the high-purity molten iron spheroidizing treatment in Example 1. The difference is that in step (1), the highest temperature of the molten iron during the entire crude refining process is 1530 °C;

[0120] In step (2), the highest temperature of the molten iron during the entire primary refining and secondary refining processes is 1530 °C. The tapping temperature of the molten iron in the primary refining is 1500 - 1510 °C, and the tapping temperature of the molten iron in the secondary refining is 1500 - 1520 °C. In the secondary-refined molten iron, C is 3.83%, Si is 0.40%, P is 0.018%, S is 0.004%, and Ni is 0.64%;

[0121] In step (3), the time for transporting and blending the secondary-refined molten iron to the spheroidizing treatment is controlled within 35 - 45 min. The temperature of the spheroidizing treatment is 1385 - 1400 °C, the single-eye argon flow rate is 40 - 50 NL / min, and the cooling rate of the molten iron is 1.5 °C / min.

[0122] As mentioned above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron, characterized in that, the method comprises the following steps: (1) Crude smelting of iron and steel raw materials is carried out at a temperature ≤ 1550 °C to obtain the crude molten iron at the first station. Crude smelting of foundry pig iron and graphite carbon powder is carried out at a temperature ≤ 1550 °C to obtain the crude molten iron at the second station; (2) The crude molten iron at the first station is subjected to primary refining at a temperature ≤ 1550 °C to obtain the primary refined molten iron. After the primary refined molten iron and the crude molten iron at the second station are mixed and homogenized, secondary refining is carried out at a temperature ≤ 1550 °C to obtain the secondary refined molten iron; (3) The secondary refined molten iron is transported and divided for pouring. The temperature of the molten iron in the transfer ladle is controlled at 1440 - 1460 °C, and spheroidizing treatment is carried out at a temperature of 1380 - 1420 °C.

2. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 1, characterized in that, in step (1), both the crude molten iron at the first station and the crude molten iron at the second station are tapped after being kept warm at 1500 - 1520 °C for 5 - 20 min after all the furnace charges are melted.

3. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 1, characterized in that, in step (2), the tapping temperature of the primary refined molten iron is 1500 - 1520 °C, and the tapping temperature of the secondary refined molten iron is 1480 - 1520 °C.

4. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to any one of claims 1 - 3, characterized in that, in step (2), the whole process of primary refining and secondary refining adopts the stirring method of bottom blowing argon in the ladle.

5. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 4, characterized in that, in step (3), the time from the completion of the transfer and division of the secondary refined molten iron to the start of the spheroidizing treatment is controlled within 30 - 60 min.

6. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 1, characterized in that, in step (3), the molten iron before spheroidizing treatment is subjected to temperature equalization and cooling treatment by means of bottom blowing argon.

7. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 6, characterized in that, if the spheroidizing treatment temperature is lower than 1380 - 1420 °C, the molten iron is returned to the refining station for graphite electrode arc heating.

8. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 7 or 6, characterized in that, the single-eye argon gas flow rate is 30 - 50 NL / min, and the cooling rate of the molten iron is 1.5 - 2.0 °C / min.

9. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 1, characterized in that, before spheroidizing treatment in step (3), the C content in the molten iron is 3.50 - 4.00%, Si is 0.30 - 0.40%, P ≤ 0.020%, S is 0.004 - 0.009%, and Ni is 0.55 - 0.65%.

10. A method for precisely controlling the spheroidizing treatment temperature of high-purity molten iron according to claim 1, characterized in that, Step (3) also includes transporting the spheroidized molten iron to the casting process for final inoculation, slag skimming, and pouring operations.

Images