High-strength nodular cast iron production process based on full waste steel

Abstract

The invention discloses a high-strength nodular cast iron production process based on full waste steel, and relates to the field of metal casting, casting is carried out through the modes of raw material preparation, smelting casting, spheroidizing, in-ladle inoculation treatment, pouring, unboxing, heat treatment and processing and surface treatment, and the casting raw materials comprise silicon iron, ferromanganese iron, a carburant, a spheroidizing agent, an inoculant and waste steel. And the carburant adopts a low-sulfur and low-nitrogen carburant. The production cost is reduced while the product quality is guaranteed by replacing pig iron with full scrap steel for casting, and the graphite structure can be obviously improved and the spheroidization rate can be increased by adopting a secondary inoculation mode, namely a ladle inoculation mode and a stream inoculation mode.

Description

Technical Field

[0001] The invention relates to the field of metal casting, and in particular to a production process of high-strength ductile iron based on all scrap steel. Background Art

[0002] Ductile iron is a high-strength cast iron material developed in the 1950s. Its comprehensive properties are close to those of steel. Based on its excellent properties, it has been successfully used to cast some parts with complex stresses and high requirements for strength, toughness and wear resistance. Ductile iron has rapidly developed into a widely used cast iron material second only to gray cast iron.

[0003] High-strength ductile iron has been increasingly widely used in mechanical parts due to its high tensile strength and high elongation. Usually, the casting production of ductile iron is made of 60% pig iron, 30% recycled materials, 10% scrap steel, and then carbonizer, ferrosilicon and ferromanganese are added to make the chemical composition of the original iron water in the electric furnace reach C: 3.75-3.90%, Si: 1.20-1.40, Mn: 0.25-0.35, and then spheroidized and cast to obtain the casting blank.

[0004] Since scrap steel is abundant in social resources and its price is relatively low, in order to reduce production costs while ensuring product quality, scrap steel is often used instead of pig iron in the prior art. All scrap steel is used in combination with carburizer, ferrosilicon and ferromanganese to produce high-strength ductile iron. However, since steel is obtained by decarburization and refining of pig iron, the carbon content of the steel is low. More carburizers must be added when producing ductile iron. Ordinary carburizers contain high sulfur and nitrogen. The increase in the amount of carburizer added will inevitably cause a sharp increase in the sulfur and nitrogen content of the blended molten iron. Excessive sulfur content can easily cause spheroidization recession; increased nitrogen content can easily cause nitrogen pores and cracks in castings; the steel obtained by refining has a very high purity and contains few impurity elements, which results in fewer nucleation points in the blended molten iron, and the graphite nodules obtained in the casting after spheroidization are large and small in number, which affects the mechanical properties. A solution is proposed below to address the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a high-strength ductile iron production process based on all scrap steel to solve the problems raised in the above background technology.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] A high-strength ductile iron production process based on all scrap steel, characterized by comprising the following steps:

[0008] (1) Raw material preparation: The raw materials include ferrosilicon with a mass ratio of 1%-2.5%, ferromanganese with a mass ratio of 0.5%-3.5%, carburizer with a mass ratio of 1%-3.5%, spheroidizer with a mass ratio of 0.1%-1%, inoculant with a mass ratio of 0.5%-1.5%, cast iron chips with a mass ratio of 0.2%-1%, slag remover with a mass ratio of 0.1%-1%, perlite covering agent with a mass ratio of 0.05%-0.1%, and the rest are all scrap steel;

[0009] (2) Melting and casting: Add all scrap steel, ferrosilicon and ferromanganese into a medium frequency melting furnace, heat it up and melt it into molten iron, add a carburizer during the heating and melting process, and perform tempering and desulfurization treatment by adding the carburizer to make the treated molten iron reach the target composition of the original molten iron;

[0010] (3) Spheroidization and inoculation in a ladle: Spheroidization is carried out by a dam-type flushing method, wherein a spheroidizing agent is placed in a ladle pit and compacted, and a thin layer of ferrosilicon and cast iron chips is covered on the spheroidizing agent. An inoculant accounting for 0.3% to 0.4% by mass of the molten iron is used for ladle processing. The dissolved molten iron is poured into the ladle pit, and 70% to 80% of the molten iron is first poured in for reaction, and the remaining molten iron is poured in after the reaction is completed. When all the molten iron is poured in, it is fully stirred to prevent the inoculant from not melting and to allow the inoculant to be fully absorbed. After the spheroidization reaction is completed, a slag remover is added, stirred and slag removed, and then an inoculant is added for reaction, and the inoculant accounts for 0.4% to 0.5% by mass of the molten iron;

[0011] (4) Casting: Pour the molten iron into a casting mold, and use an inoculant with a mass percentage of 0.05% to 0.1% of the molten iron to perform inoculant during casting;

[0012] (5) Unpacking: After pouring, take out the casting after it cools and solidifies;

[0013] (6) Heat treatment: heat treatment of castings;

[0014] (7) Processing and surface treatment: The castings are processed and surface treated to achieve the required shape and surface quality.

[0015] Preferably, in step (2) smelting and casting, the furnace temperature is first raised to 1450°C to melt all the scrap steel. After the scrap steel is completely melted, a recarburizer accounting for 1%-3.5% of the total mass of the raw materials is added and mixed. Then the furnace temperature is raised to 1500-1520°C, and ferromanganese accounting for 0.5%-3.5% of the total mass of the raw materials is added to the raw materials. After the material is completely melted, before the temperature is raised, ferrosilicon accounting for 1%-2.5% of the total mass of the raw materials is added. After the melting and heating are continued to form molten iron, the temperature is finally raised to 1530°C-1550°C, and the molten iron is allowed to stand for 1-3 minutes before being taken out of the furnace.

[0016] Preferably, in step (3) of spheroidization and inoculation treatment, after the inoculant is added, it can be covered with a perlite covering agent so that subsequent treatment can be carried out.

[0017] Preferably, in step (5), the casting is unpacked after the casting is cooled to below 650° C. in the mold.

[0018] Preferably, the recarburizer selected in step (1) is a low-sulfur and low-nitrogen recarburizer, wherein the sulfur content in the recarburizer does not exceed 0.04% of the total mass, and the nitrogen content does not exceed 300 ppm.

[0019] Beneficial effects: The present invention replaces pig iron with all scrap steel for casting, thereby reducing production costs while ensuring product quality. Since the carbon content of scrap steel is low, more carburizers will be added. The present application uses a low-sulfur and low-nitrogen carburizer to increase the carbon content while preventing excessive sulfur and nitrogen contents.

[0020] The conventional method of only performing inoculation treatment once is prone to inoculation decay, and the number of graphite nodules is small and the spheroidization rate is low. The present application adopts a secondary inoculation method, namely, in-package inoculation and in-flow inoculation, which can significantly improve the graphite structure and increase the spheroidization rate. DETAILED DESCRIPTION

[0021] The following is only a preferred embodiment of the present invention, and the protection scope is not limited to this embodiment. All technical solutions under the concept of the present invention should belong to the protection scope of the present invention. At the same time, it should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

[0022] A high-strength ductile iron production process based on all scrap steel, characterized in that it includes the following steps: raw material preparation: the raw materials include 2% ferrosilicon by mass, 3.5% ferromanganese by mass, 3.5% carburizer by mass, 1% spheroidizer by mass, 1% inoculant by mass, 1% cast iron chips, 1% slag remover, 0.1% perlite covering agent, and the rest are all scrap steel;

[0023] (2) Melting and casting: In the melting and casting, the furnace temperature is first raised to 1450°C, and all the scrap steel is added to melt. When the scrap steel is completely melted, a recarburizer accounting for 3.5% of the total weight of the raw materials is added to mix. The recarburizer uses a low-sulfur and low-nitrogen recarburizer. The sulfur content in the recarburizer does not exceed 0.04% of the total weight, and the nitrogen content does not exceed 300ppm. The recarburizer is added to temper and desulfurize the treated molten iron to achieve the target composition of the original molten iron. Then the furnace temperature is raised to 1500°C, and ferromanganese accounting for 3% of the total weight of the raw materials is added to the raw materials. After the material is completely melted, before the temperature is raised, ferrosilicon accounting for 2% of the total weight of the raw materials is added. After the material is melted, the temperature is continued to rise to form molten iron. Finally, the temperature is raised to 1550°C, and the molten iron is allowed to stand for 2 minutes before being taken out of the furnace.

[0024] Medium frequency induction furnace is used in smelting and casting. The medium frequency induction furnace is a kind of furnace that converts three-phase industrial frequency alternating current into direct current after rectification, and then converts the direct current into an adjustable current, which supplies alternating current flowing through the capacitor and induction coil, generates high-density magnetic lines in the induction coil, and cuts the metal material contained in the induction coil, generates a large eddy current in the metal material, and heats the metal material to melt;

[0025] (3) Spheroidizing and inoculation in a ladle: The spheroidizing process is carried out by a dam-type flushing method. The spheroidizing agent is placed in the ladle pit and compacted. A thin layer of ferrosilicon and cast iron chips are covered on the spheroidizing agent. In the spheroidizing and inoculation process, cast iron chips are added to the molten iron. The cast iron chips can serve as the core of graphite nucleation and promote the graphitization process, thereby improving the microstructure of the casting and improving its mechanical properties. The addition of cast iron chips can improve the fluidity of the molten iron, help the molten iron to better fill the casting mold, and reduce casting defects. The cast iron chips in the molten iron can increase the cooling rate of the casting, which helps to shorten the production cycle and improve production efficiency.

[0026] Use an inoculant with a mass percentage of 0.4% of the molten iron for inoculation in the ladle. Pour the dissolved molten iron into the ladle pit, first pour 80% of the molten iron for reaction, and then pour the remaining 20% ​​of the molten iron after the reaction is completed. When all the molten iron is poured in, stir it thoroughly to prevent the inoculant from being unmelted so that the inoculant can be fully absorbed;

[0027] Since there is a lot of thick slag after ductile iron is spheroidized, after the spheroidization reaction is completed, a slag remover is added, and the thick slag in the molten iron is removed by stirring and slag removal, and then a 0.5% inoculant is added to the molten iron by mass percentage for secondary reaction, and finally the perlite covering agent is used to cover it so that it can be processed later;

[0028] (4) Casting: Pour the molten iron into a casting mold. During the casting, use an inoculant with a mass percentage of 0.1% of the molten iron for inoculation. The conventional inoculation process is prone to inoculation decay, and the number of graphite balls is small and the spheroidization rate is low. The present application adopts a secondary inoculation method, i.e., in-package inoculation and in-stream inoculation, which can significantly improve the graphite structure and increase the spheroidization rate.

[0029] The tendency of crystallization to be overcooled can be eliminated by inoculating ductile iron: the inoculant is dispersed in the molten iron, and its main component element silicon forms an enriched area, which increases the activity of carbon in this micro-area, thereby directly weakening the bonding of cementite and hindering the formation of cementite. In addition, trace alloying elements (such as aluminum and calcium) in the inoculant react with nitrogen, oxygen, and sulfur that stabilize cementite and hinder graphitization after being added to the molten iron, forming compounds and removing them; 2) Promote graphite spheroidization: inoculation treatment can increase the graphite core, refine the spherical graphite, improve the relative stability of spherical graphite growth, and improve the roundness of graphite. 3) Reduce intergranular segregation; inoculation treatment refines the eutectic group, thereby reducing the degree of segregation between eutectic groups, and the inoculant improves the plasticity and toughness of cast iron;

[0030] (5) Unpacking: After pouring, the casting needs to have enough heat preservation and slow cooling time in the mold, so it is necessary to wait until the casting is cooled to below 650℃ in the mold before unpacking;

[0031] (6) Qualified testing: After the castings are out of the box, the finished castings need to be sampled, measured and tested;

[0032] (7) Heat treatment: heat treatment of castings;

[0033] (8) Processing and surface treatment: The casting is processed and surface treated to achieve the required shape and surface quality.

Claims

1. A high-strength ductile iron production process based on all scrap steel, characterized in that: The following steps are involved: (1) Raw material preparation: The raw materials include ferrosilicon with a mass ratio of 1%-2.5%, ferromanganese with a mass ratio of 0.5%-3.5%, carburizer with a mass ratio of 1%-3.5%, spheroidizer with a mass ratio of 0.1%-1%, inoculant with a mass ratio of 0.5%-1.5%, cast iron chips with a mass ratio of 0.2%-1%, slag remover with a mass ratio of 0.1%-1%, perlite covering agent with a mass ratio of 0.05%-0.1%, and the rest are all scrap steel; (2) Melting and casting: Add all scrap steel, ferrosilicon and ferromanganese into a medium frequency melting furnace, heat it up and melt it into molten iron, add a carburizer during the heating and melting process, and perform tempering and desulfurization treatment by adding the carburizer to make the treated molten iron reach the target composition of the original molten iron; (3) Spheroidization and inoculation in a ladle: Spheroidization is carried out by a dam-type flushing method, wherein a spheroidizing agent is placed in a ladle pit and compacted, and a thin layer of ferrosilicon and cast iron chips is covered on the spheroidizing agent. An inoculant accounting for 0.3% to 0.4% by mass of the molten iron is used for ladle processing. The dissolved molten iron is poured into the ladle pit, and 70% to 80% of the molten iron is first poured in for reaction, and the remaining molten iron is poured in after the reaction is completed. When all the molten iron is poured in, it is fully stirred to prevent the inoculant from not melting and to allow the inoculant to be fully absorbed. After the spheroidization reaction is completed, a slag remover is added, stirred and slag removed, and then an inoculant is added for reaction, and the inoculant accounts for 0.4% to 0.5% by mass of the molten iron; (4) Casting: Pour the molten iron into a casting mold, and use an inoculant with a mass percentage of 0.05% to 0.1% of the molten iron to perform inoculant during casting; (5) Unpacking: After pouring, take out the casting after it cools and solidifies; (6) Heat treatment: heat treatment of castings; (7) Processing and surface treatment: The castings are processed and surface treated to achieve the required shape and surface quality.

2. A high-strength ductile iron production process based on all scrap steel according to claim 1, characterized in that: In step (2) of smelting and casting, the furnace temperature is first raised to 1450°C to melt all the scrap steel. After the scrap steel is completely melted, a recarburizer accounting for 1% to 3.5% of the total mass of the raw materials is added and mixed. Then the furnace temperature is raised to 1500-1520°C, and ferromanganese accounting for 0.5% to 3.5% of the total mass of the raw materials is added to the raw materials. After the material is completely melted, before the temperature is raised, ferrosilicon accounting for 1% to 2.5% of the total mass of the raw materials is added. After the melting and heating are continued to form molten iron, the temperature is finally raised to 1530°C to 1550°C, and the molten iron is allowed to stand for 1 to 3 minutes before being taken out of the furnace.

3. A high-strength ductile iron production process based on all scrap steel according to claim 1, characterized in that: In step (3) of spheroidization and inoculation in a ladle, after the inoculant is added, it can be covered with a perlite covering agent so that subsequent treatment can be carried out.

4. A high-strength ductile iron production process based on all scrap steel according to claim 1, characterized in that: In step (5) unpacking, the casting is unpacked after it is cooled to below 650° C. in the mold.

5. The high-strength ductile iron production process based on all scrap steel according to claim 1, characterized in that: The recarburizer selected in step (1) is a low-sulfur and low-nitrogen recarburizer, wherein the sulfur content in the recarburizer does not exceed 0.04% of the total mass, and the nitrogen content does not exceed 300 ppm.