Nodular cast iron for lightweight sintering pallet and preparation method of nodular cast iron

Abstract

The invention relates to the technical field of metallurgical sintering, in particular to nodular cast iron for a lightweight sintering pallet and a preparation method of the nodular cast iron. The content of Si is 3.5%-4.3%; mn < = 0.2%, P < = 0.04%; s is less than or equal to 0.02%; cu < = 0.1%; less than or equal to 0.03% of Cr; ti is less than or equal to 0.03%; compared with the prior art, the content of silicon is increased, the silicon can promote formation of ferrite and reduce generation of cementite, the content of stable carbide and pearlite forming elements in chemical components can be properly widened, from the point of view of adding of smelting raw materials, the adding amount of waste steel reaches 50%, the use amount of pig iron is reduced, and the production cost is reduced. The smelting production cost is greatly reduced; compared with the prior art, the prepared nodular cast iron has higher tensile strength and yield strength; the service life of the sintering pallet is prolonged, and the light weight of the sintering pallet is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgical sintering, and in particular to ductile iron for a lightweight sintering trolley and a preparation method thereof. Background Art

[0002] The sintering car is the main mechanical component of the sintering machine. Its assembly weight accounts for more than 50% of the total weight of the sintering machine. It is a large number of easily worn mechanical parts consumed in the metallurgical industry. It is also the core process mechanical part in sintering production. The entire sintering process that constitutes the sintering principle is completed in the sintering car.

[0003] Steel companies generally require a large number of sintering trolleys, and the cost of a sintering trolley accounts for about 50% of the sintering machine. Therefore, research and application on reducing the weight of sintering trolleys, saving energy consumption, and reducing costs are particularly important for the sintering industry, and are the development direction of green production for steel companies.

[0004] At present, the sintering trolley body material of the existing technology is all made of ductile iron with the grade of QT500-7. The main chemical components of the existing QT500-7 ductile iron are C: 3.2%-3.8%; Si: 2.2%-2.8%; Mn: 0.4%-0.7%; S: ≤0.03%; P: ≤0.06%; Mg: 0.03%-0.06%; Re: 0.02%-0.05%.

[0005] The main mechanical properties of the existing QT500-7 ductile iron are: yield strength ≥320MPa, tensile strength ≥500MPa.

[0006] The charge ratio of QT500-7 ductile iron in the prior art is 86% of Q10 pig iron and 14% of scrap steel.

[0007] The production cost of sintering carts made of QT500-7 ductile iron is high and cannot adapt to market competition. Therefore, there is an urgent need for a sintering cart material that can improve performance, ensure life and reduce weight. Summary of the invention

[0008] The purpose of the present invention is to provide a ductile iron for a lightweight sintering trolley and a preparation method thereof, thereby reducing the production cost of the sintering trolley and achieving energy saving, consumption reduction and cost reduction in the sintering industry.

[0009] The chemical composition contents of the ductile iron for a lightweight sintering trolley of the present invention are calculated as follows according to weight percentage: C is 3.0%-3.5%; Si is 3.5%-4.3%; Mn≤0.2%, P≤0.04%; S≤0.02%; Cu≤0.1%; Cr≤0.03%; and Ti≤0.03%.

[0010] A method for preparing ductile iron for lightweight sintering trolley, characterized in that it comprises the following steps: S1. Select the charge, which includes Q10 pig iron: 30%; scrap steel: 15%; recycled charge: 55%; spheroidizer: 1.4%; 3mm-8mm inoculant: 0.9%; 0.2mm-0.7mm inoculant: 0.1%; S2. Add materials to the smelting furnace. The material adding hopper is required to add Q10 pig iron for melting. The melting temperature is controlled to be above 1200℃. Then, scrap steel is added and the temperature is raised to continue melting. When the melting temperature reaches 1300℃, recycled materials are added. When the melting temperature reaches above 1400℃, sampling begins. The overheating temperature is 1500℃. S3. Add spheroidizing agent into the smelting furnace, pour the spheroidizing agent into the reaction chamber on one side of the ladle and evenly spread and tamp it, evenly cover 50% of 3mm-8mm inoculant and iron filings on the spheroidizing agent and tamp it, heat the molten iron to the tapping temperature, the tapping temperature is 1420℃-1470℃, aim the furnace nozzle at the side where the spheroidizing agent, 50% inoculant and iron filings are placed, and start the spheroidizing reaction when the tapping amount in the ladle reaches more than 50%; S4. Add the remaining inoculant, first add the remaining 3mm-8mm inoculant, select the inoculation method of the ladle into the taphole inoculation, to ensure that the inoculated molten iron is uniform and sufficient, after slagging, add the remaining 0.2mm-0.7mm inoculant for instantaneous inoculation and stir evenly; S5. The shutdown temperature is 1438℃, the pouring temperature is 1347℃, and the time from the completion of spheroidization to the start of pouring is 15 minutes. After the molten iron is melted, the recarburizer is added to the bag according to the recovery rate of 70%.

[0011] Furthermore, the scrap steel is carbon steel; the recycled materials include one or two of calcium carbide pot and cooling wall; the spheroidizing agent is yttrium; and the inoculant is silicon-barium-calcium inoculant.

[0012] Furthermore, the step S2 of adding materials to the smelting furnace requires to ensure continuous addition of materials to avoid interruption of materials during the melting process; it requires to observe the melting conditions in the furnace at all times to avoid material shedding, and once material shedding occurs, it requires to promptly tilt the furnace or pry with iron bars; after all the materials are added, observe the melting conditions to avoid shedding.

[0013] The beneficial effects of the present invention are: Compared with the prior art, the silicon content in the present invention is increased, and silicon can promote the formation of ferrite and reduce the generation of cementite. The content of stable carbide and pearlite forming elements in the chemical composition can be appropriately relaxed. From the perspective of adding smelting raw materials, the amount of scrap steel added reaches 50%, which reduces the use of pig iron and greatly reduces the smelting production cost.

[0014] The ductile iron prepared by the present invention has higher tensile strength and yield strength than the prior art; the service life of the sintering trolley is improved, and the lightweight of the sintering trolley is achieved; more than 95% of the ductile iron matrix prepared by the present invention is ferrite, and the cutting performance during the processing is good, which can effectively extend the life of the tool and greatly reduce the cost of mechanical processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A top view of the sintering trolley in the present invention; Figure 2 It is a side view of the sintering trolley in the present invention; Figure 3 For the present invention Figure 1 AA section view. DETAILED DESCRIPTION

[0016] The chemical composition contents of the ductile iron for a lightweight sintering trolley of the present invention are calculated as follows according to weight percentage: C is 3.0%-3.5%; Si is 3.5%-4.3%; Mn≤0.2%, P≤0.04%; S≤0.02%; Cu≤0.1%; Cr≤0.03%; and Ti≤0.03%.

[0017] A method for preparing ductile iron for lightweight sintering trolley, characterized in that it comprises the following steps: S1. Select the charge, which includes Q10 pig iron: 30%; scrap steel: 15%; recycled charge: 55%; spheroidizer: 1.4%; 3mm-8mm inoculant: 0.9%; 0.2mm-0.7mm inoculant: 0.1%; the scrap steel is carbon steel; the recycled charge includes one or two of calcium carbide pot and cooling wall; the spheroidizer is dragon yttrium; the inoculant is silicon-barium-calcium inoculant; S2. Add materials to the smelting furnace. It is required to use a feeding hopper to add materials. The feeding process is to first add Q10 pig iron for melting, control the melting temperature to above 1200℃, then add scrap steel, continue to heat and melt, add recycled materials when the melting temperature reaches 1300℃, start sampling when the melting temperature reaches above 1400℃, and the overheating temperature is 1500℃; Add materials to the smelting furnace to ensure continuous addition of materials to avoid interruption of materials during the melting process; Always observe the melting condition in the furnace to avoid material shed. Once material shed occurs, promptly tilt the furnace or pry with iron bars; After all materials are added, observe the melting condition to avoid shed; After all the added materials are melted, monitor the temperature changes at any time, focusing on controlling the melting temperature, sampling temperature, and overheating temperature; S3. Add spheroidizing agent into the smelting furnace, pour the spheroidizing agent into the reaction chamber on one side of the ladle and evenly spread and tamp it, evenly cover 50% of 3mm-8mm inoculant and iron filings on the spheroidizing agent and tamp it, heat the molten iron to the tapping temperature, the tapping temperature is 1420℃-1470℃, aim the furnace nozzle at the side where the spheroidizing agent, 50% inoculant and iron filings are placed, and start the spheroidizing reaction when the tapping amount in the ladle reaches more than 50%; S4. Add the remaining inoculant, first add the remaining 3mm-8mm inoculant, select the inoculation method of the ladle into the taphole inoculation, to ensure that the inoculated molten iron is uniform and sufficient, after slagging, add the remaining 0.2mm-0.7mm inoculant for instantaneous inoculation and stir evenly; S5. The shutdown temperature is 1438℃, the pouring temperature is 1347℃, and the time from the completion of spheroidization to the start of pouring is 15 minutes. After the molten iron is melted, the recarburizer is added to the bag according to the recovery rate of 70%.

[0018] The Si content in the present invention is 3.5%-4.3%. Compared with the Si content of QT500-7 ductile iron of 2.2%-2.8%, the silicon content in the present invention is increased, Si significantly increases graphitization, and also promotes the formation of ferrite and effectively reduces the generation of cementite; silicon element is dissolved in molten iron to form a solid solution, and the solute atoms in the solid solution cause lattice distortion, thereby increasing the resistance to dislocation movement, hindering dislocation movement, and then increasing the strength and hardness of the solid solution; the ferrite strength is increased by high Si solid solution strengthening, and at the same time, high Si prevents the generation of pearlite during the eutectoid transformation of ductile iron, realizes the full ferrite matrix organization, and thus reduces the hardness fluctuation. Studies have shown that high Si content can make ductile iron obtain better comprehensive mechanical properties.

[0019] The carbon content in the present invention is 3.0%-3.5%. Compared with the carbon content of QT500-7 ductile iron of 3.2%-3.8%, the carbon content in the present invention is reduced, and the C content is mainly determined based on the mechanical properties and Si content.

[0020] Since the Mn element can improve the stability of austenite, reduce the transformation temperature of austenite to pearlite, and promote the formation of pearlite matrix, it is also easy to form segregation on the boundary of eutectic cells. The ductile iron of the present invention is to improve material properties and reduce costs, so the Si content is increased, so the main matrix is ​​ferrite, and the Mn content is reduced.

[0021] P comes from the metal charge. P will not affect the spheroidization of cast iron, but it is a harmful element. It will dissolve in the molten iron to reduce the C content in the Fe-C alloy eutectic group, and has a weak anti-graphitization effect. Since the solidification mode of ductile iron is paste solidification, P is very easy to segregate, thereby forming phosphorus eutectic, and this segregation will increase with the increase of wall thickness, especially in the wall thickness hot zone. Since the phosphorus eutectic is relatively low, they will precipitate and distribute at the boundary of the eutectic group, which will greatly reduce the mechanical properties of ductile iron. Therefore, for ductile iron, the lower the P content, the better. However, due to the restriction of raw material quality, the P content in the present invention is <0.04%.

[0022] Compared with the QT500-7 ductile iron in the prior art, the charge of the present invention is mainly different in that the amount of pig iron added is reduced from 86% to 30%, and the amount of scrap steel added is increased, thereby saving costs; the present invention obtains full ferrite, and the solid solution strengthening effect of Si is formed after the Si content is increased, so the amount of pig iron added is reduced and the amount of scrap steel is increased; calculated according to the current market price, the price of pig iron per ton is about 3,300 yuan, and the price of scrap steel per ton is about 1,900 yuan, so the cost of the present invention is saved by about 100 yuan per ton.

[0023] In step S2 of the present invention, since the scrap steel has different shapes and sizes and poor density, it is necessary to continue to heat up and melt. When the temperature reaches above 1300°C, the recycled materials are added. Since the recycled materials are dense and have been smelted, it can avoid furnace spraying and save energy. Therefore, the proportion of recycled materials added is increased. Sampling begins when the temperature reaches above 1400°C, and the overheating temperature is 1500°C. The main function of overheating is to make the gas slag float out after sedation.

[0024] The tapping temperature in step S3 of the present invention needs to be determined according to the casting temperature and the weather and ladle conditions. Generally, the weather temperature is low in winter, so the tapping temperature is high, and vice versa in summer. If the ladle is a new ladle, the tapping temperature is high, and if the ladle is an old ladle, the tapping temperature is low, because the new ladle has not formed a dense insulation shell inside, so the heat dissipation is fast, and the old ladle has been used many times, and a protective shell of a certain thickness has been formed inside, so the heat dissipation is slow, and the tapping temperature is low.

[0025] The main function of covering the spheroidizing agent with iron filings in step S3 of the present invention is to cover the spheroidizing agent to slow down its reaction, and no specific ratio of iron filings is set.

[0026] The method of the present invention is used to prepare ductile iron, and the obtained ductile iron is subjected to elemental inspection. The analysis and detection structure of the main elements is: C is 3.03%, Si is 4.05%, Mn is 0.16%, P is 0.024%, and S is 0.016%.

[0027] Using infrared temperature measurement and other technical means, the temperature data of the middle main beam, side beam, rib plate and end plate of the sintering trolley in use in the prior art is collected, and 500℃ is finally determined as the working temperature, and 500℃ is used for simulation analysis. The QT500-7 ductile iron of the prior art and the ductile iron prepared by the present invention are subjected to high-temperature tensile, high-temperature oxidation, hardness, thermal conductivity, thermal expansion coefficient and high-temperature creep tests at a working temperature of 500℃ to obtain test data. A model is established based on the geometric structure dimensions of the sintering trolley in the prior art, and the test data of the QT500-7 ductile iron in the prior art, the safety factor of 2.2, the mechanical load, the thermal load and other values ​​are assigned to the current trolley body, and the maximum deformation of the trolley beam after 5 years of operation is simulated as i=8.227mm, which is used as the judgment standard. Abaqus is integrated with Isight software, and a gradient-based optimization algorithm is adopted. With mass minimization as the objective function and the variable size of the trolley beam as the design variable, the material test data of ductile iron prepared by the method of the present invention, the safety factor of 2.2, mechanical load, thermal load, and the maximum deformation of the trolley beam i=8.227mm, which are the same as those of the current trolley body, are assigned to the model to obtain the specific values ​​of the variable size of the sintering trolley.

[0028] like Figure 1-Figure 3 As shown, the variable sizes of the sintering trolley include: The width of the web of the main beam is indicated as M_W1 in the figure; The lower width of the web of the main beam, represented as M_W2 in the figure; The width of the lower flange of the main beam, represented as M_W3 in the figure; The height of the lower flange of the main beam, represented as M_H1 in the figure; The width of the web of the secondary beam is indicated as S_W1 in the figure; The lower width of the web of the secondary beam is represented by S_W2 in the figure; The width of the lower flange of the secondary beam is 1, which is represented by S_W3 in the figure; The width of the lower flange of the secondary beam is 2, which is represented by S_W4 in the figure; Height of the lower flange of the secondary beam, indicated as S_H1 in the figure; The thickness of the guardrail mounting plate is indicated as B_H1 in the figure; The thickness of the end cross plate, indicated as B_H2 in the figure; The thickness of the side vertical plate, represented as B_W1 in the figure; The thickness of the transverse rib plate is indicated as T1 in the figure; The comparison of the variable dimensions of the sintering trolley obtained by using the QT500-7 ductile iron of the prior art and the ductile iron prepared by the present invention is shown in Table 1: Table 1: Comparison of variable sizes of sintering carts.

[0029] Through simulation calculation and the results of Table 1, the mass of the sintering trolley of QT500-7 ductile iron using the prior art is 3645 Kg, and the mass of the sintering trolley of the ductile iron prepared by the present invention is 3261 Kg. The mass of the sintering trolley of the ductile iron prepared by the present invention is reduced by 384 Kg, and the weight reduction ratio is 10.5%.

[0030] The mechanical properties of the ductile iron prepared by the present invention at 500° C. are tensile strength of 610 MPa and yield strength of 533 MPa; the yield strength is increased by 22.5% over the prior art; the tensile strength is increased by 11% over the prior art, and the service life of the sintering trolley is effectively increased while reducing the weight of the sintering trolley.

Claims

1. A ductile iron for lightweight sintering trolley, characterized in that: The chemical composition content calculated by weight percentage is: C is 3.0%-3.5%; Si is 3.5%-4.3%; Mn≤0.2%, P≤0.04%; S≤0.02%; Cu≤0.1%; Cr≤0.03%; Ti≤0.03%.

2. The method for preparing ductile iron for lightweight sintering trolley according to claim 1, characterized in that: The steps include: S1. Select the charge, which includes Q10 pig iron: 30%; scrap steel: 15%; recycled charge: 55%; spheroidizer: 1.4%; 3mm-8mm inoculant: 0.9%; 0.2mm-0.7mm inoculant: 0.1%; S2. Add materials to the smelting furnace. The material adding hopper is required to add Q10 pig iron for melting. The melting temperature is controlled to be above 1200℃. Then, scrap steel is added and the temperature is raised to continue melting. When the melting temperature reaches 1300℃, recycled materials are added. When the melting temperature reaches above 1400℃, sampling begins. The overheating temperature is 1500℃. S3. Add spheroidizing agent to the smelting furnace, pour the spheroidizing agent into the reaction chamber on one side of the ladle and evenly spread and tamp, evenly cover 50% of the 3mm-8mm inoculant and iron filings on the spheroidizing agent and tamp, heat the molten iron to the tapping temperature, the tapping temperature is 1420℃-1470℃, and align the furnace nozzle with the side where the spheroidizing agent, 50% inoculant and iron filings are placed. When the tapping amount in the ladle reaches more than 50%, the spheroidizing reaction begins; S4. Add the remaining inoculant, first add the remaining 3mm-8mm inoculant, select the inoculation method of the ladle into the taphole inoculation, to ensure that the inoculated molten iron is uniform and sufficient, after slagging, add the remaining 0.2mm-0.7mm inoculant for instantaneous inoculation and stir evenly; S5. The shutdown temperature is 1438℃, the pouring temperature is 1347℃, and the time from the completion of spheroidization to the start of pouring is 15 minutes. After the molten iron is melted, the recarburizer is added to the bag according to the recovery rate of 70%.

3. The method for preparing ductile iron for lightweight sintering trolley according to claim 2, characterized in that: The scrap steel is carbon steel; the recycled materials include one or two of calcium carbide pot and cooling wall; the spheroidizing agent is yttrium; and the inoculant is silicon-barium-calcium inoculant.

4. The method for preparing ductile iron for lightweight sintering trolley according to claim 2, characterized in that: The step S2 is to add material into the smelting furnace to ensure continuous addition of material to avoid material interruption during the melting process; It is required to observe the melting condition in the furnace at all times to avoid material shedding. Once material shedding occurs, it is required to promptly tilt the furnace or pry it out with an iron rod; After all the materials have been added, observe the melting conditions and avoid building a scaffold.

Images