The spheroidization level of domestic common ductile iron castings is required to reach level 4 or higher (ie, the spheroidization rate is 70%). The spheroidization rate achieved by general foundries is about 85%. In recent years, with the development of ductile cast iron production, especially in wind power casting production and high quality industries, the spheroidization level is required to reach level 2, that is, the spheroidization rate is more than 90%. The author's company analyzed and improved the nodularization and inoculation treatment process and the nodulizer and inoculant used in QT400-15 to make the nodularity of nodular cast iron reach 90%. 1.Original production process
Original production process: The melting equipment uses 2.0T intermediate frequency furnace and 1.5T industrial frequency furnace; QT400-15 raw molten iron composition is ω (C) = 3.75% ~ 3.95%, ω (Si) = 1.4% ~ 1.7%, ω (Mn) ≤ 0.40%, ω (P) ≤ 0.07%, ω (S) ≤ 0.035%; the nodulizer used in the nodulizing treatment is 1.3% to 1.5% of the RE3Mg8SiFe alloy; the inoculant used in the inoculation treatment is 0.7% ~ 0.9% of 75SiFe-C alloy. The nodularizing process uses two iron tapping methods: firstly, iron 55% ~ 60%, then nodulizing, then add inoculant, and then add the rest of the molten iron. Due to the traditional method of nodularization and inoculation, the nodularization rate measured with a 25 mm thick single-cast wedge-shaped test block is generally about 80%, that is, nodularization level 3. 2. Test plan for improving nodularity
In order to improve the nodularization rate, the original nodularizing and inoculating treatment process was improved. The main measures were: increasing the amount of nodularizing agent and inoculant, purifying the molten iron, and desulfurizing treatment. The nodularization rate is still tested by using a 25 mm single-cast wedge test block. The specific scheme is as follows: (1) The reason for the low nodularization rate of the original process was analyzed. It was thought that the amount of nodulizer was small, so the nodulizer was added. The amount increased from 1.3% to 1.4% to 1.7%, but the spheroidization rate did not meet the requirements. (2) Another conjecture is that the low spheroidization rate may be caused by poor fertility or infertility, so the experiment increased the inoculation dose from 0.7% to 0.9% to 1.1%, and the spheroidization rate did not meet the requirements. (3) Continue to analyze that it is believed that more inclusions in the molten iron and higher spheroidizing interference elements may be the cause of the lower spheroidization rate. Therefore, the molten iron is subjected to high-temperature purification. The high-temperature purification temperature is generally controlled at 1 500 ± 10 ° C, but Its spheroidization rate has not yet exceeded 90%. (4) High amount of ω (S) consumes nodulizing dose seriously and accelerates spheroidizing decay. Therefore, the desulfurization treatment is increased to reduce the amount of ω (S) in the original molten iron from 0.035% to less than 0.020%. Only 86%. The test results of the above four schemes are shown in Table 1. The microstructure and mechanical properties of the wedge-shaped test block did not meet the requirements. 3. The last improvement plan adopted
3.1 Specific improvement measures The raw materials are pig iron, rust-free or less rusty scrap and regrind; raw iron liquid is desulfurized by adding soda ash (Na2CO3) to the furnace; pre-deoxidation treatment is carried out in the bag using Foshike 390 pretreatment agent; Fushike is used Spheroidizing agent is used for spheroidizing treatment; silicon carbide and ferrosilicon are used to conceive. Composition control of original molten iron in the new process: ω (C) = (3.70% ~ 3.90%, ω (Si) = 0.80% ~ 1.20% [casting ω (Si end) = 2.60% ~ 3.00%], ω (Mn) ≤ 0.30%, ω (P) ≤ 0.05%, ω (S) ≤ 0.02%. When the amount of ω (S) of the original molten iron exceeds 0.02%, industrial soda ash is used for desulfurization treatment before the furnace. The desulfurization reaction is an endothermic reaction. It is required to control the desulfurization temperature at about 1500 ° C, and the amount of soda ash to be controlled should be 1.5% to 2.5% according to the ω (S) amount when melting in front of the furnace. At the same time, the spheroidization treatment package uses ordinary dam-type treatment packages. Foseco NODALLOY7RE brand nodulizer 1.7% was added to the side of the bottom dam, flattened and compacted, covered with a layer of 0.2% powdered silicon carbide and 0.3% small lump 75SiFe, and then covered with a compacted iron Add 0.3% Foseco 390 inoculant on the other side of the iron bath. When iron is tapped, first charge 55% ~ 60% of the total molten iron. After the nodulizing reaction is completed, add 1.2% 75SiFe-C. After the inoculant is poured into the remaining molten iron, slag is poured and poured. 3.2 Test results The components before and after the desulfurization of the original molten iron are shown in Tables 2 and 3. The corresponding mechanical properties and metallographic structure of the 25mm single-cast wedge test block See Table 4. The method of evaluating the nodularity rate in the metallographic structure is automatically detected by the metallographic image analysis system. 4. Analysis of results
4.1 Influence of main elements on nodularity C, Si: C can promote graphitization and reduce white mouth tendency, but high ω (C) amount will make CE too high and easily cause graphite to float, generally controlled at 3.7% ~ 3.9% . Si can strengthen the graphitization ability and eliminate cementite. When Si is added as an inoculant, the supercooling ability of the molten iron can be greatly reduced. In order to improve the inoculation effect, the amount of ω (Si) in the original molten iron was reduced from 1.3% to 1.5% to 0.8% to 1.2%, and the amount of ω (Si) was controlled to 2.60% to 3.00%. Mn: During the crystallization process, Mn increases the undercooling tendency of cast iron and promotes the formation of carbides (FeMn) 3C. During the eutectoid transformation process, Mn lowers the eutectoid transition temperature, stabilizes and refines the pearlite. Mn does not have much effect on the spheroidization ratio. Due to the influence of raw materials, generally control ω (Mn) <0.30%. P: When ω (P) <0.05%, it is solid-soluble in Fe, and it is difficult to form a phosphorus eutectic, and the influence on the nodularity of ductile iron is not great. S: S is an anti-spheroidizing element. S consumes Mg and RE in the spheroidizing agent during the spheroidizing reaction, hinders graphitization, and reduces the spheroidizing rate. The sulphide inclusion slag will return to sulfur before the molten iron solidifies, consuming nodularizing elements again, accelerating the nodularizing decay and further affecting the nodularizing rate. In order to achieve a high spheroidization ratio, the amount of ω (S) of the original molten iron should be reduced to less than 0.02%. 4.2 Desulfurization treatment When the furnace charge is melted off, the chemical composition is sampled and analyzed. When the amount of ω (S) is higher than 0.02%, desulfurization treatment is required. The principle of soda ash desulfurization is: put a certain amount of soda ash in a ladle, stir it with a molten iron stream, the soda ash is decomposed at high temperature, and the reaction formula is Na2CO3 = Na2O + CO2 ↑: the generated Na2O is in the molten iron Sulfuration produces Na2S, (Na2O) + [FeS] = (Na2S) + (FeO). The separation of Na2CO3 into CO2 causes the molten iron to stir vigorously, which promotes the desulfurization process. Soda ash slag is very easy to flow and floats quickly. The desulfurization reaction time is very short. It should be picked up in time after desulfurization, otherwise it will return to sulfur. 4.3 Pre-deoxidation treatment, nodularization treatment and inoculation treatment Fushike 390 pretreatment agent plays the role of pre-deoxidation treatment in the package, at the same time increase the graphite nucleation core, increase the number of graphite spheres per unit area, and can also improve the absorption rate of Mg Significantly improve the ability to resist recession and increase the nodularity. Foster's inoculant contains ω (Si) = 60% ~ 70%, ω (Ca) = 0.4% ~ 2.0%, ω (Ba) = 7% ~ 11%, among which Ba can prolong the effective incubation time. Foseco nodularizer is selected as NODALLOY7RE, whose ω (Si) = 40% ~ 50%, ω (Mg) = 7.0% ~ 8.0%, ω (RE) = 0.3% ~ 1.0%, ω (Ca) = 1.5 % ~ 2.5%, ω (Al) <1.0%. Since the molten iron has undergone desulfurization and pre-deoxidation treatments, the elements that consume nodulizers in the molten iron have been greatly reduced. Therefore, a nodulizer with a low ω (RE) amount was selected to reduce the deterioration of spherical graphite by RE; The acting element is mainly Mg; Ca and Al can play a role in strengthening inoculation. Combined inoculation treatment with silicon carbide and ferrosilicon, the melting point of silicon carbide is around 1600 ° C, and graphite crystal nucleus is increased during solidification. Inoculation with large dose of ferrosilicon can prevent nodularity and decay. 5 Conclusion
For the production of ferritic ductile iron, when the spheroidizing rate is required to be above 90%, the following measures can be adopted: (1) Selecting high-quality charge materials to reduce the anti-spheroidizing elements in the charge materials. (2) Select nodulizer with low ω (RE) content to reduce the effect of RE on the deterioration of spherical graphite morphology. (3) The amount of ω (S) of the original molten iron should be less than 0.020%, so as to reduce the consumption of nodulizing agent, especially the nodularizing element consumed by the second sulfur return of the sulfide slag. (4) Pre-deoxidize the molten iron, increase the number of graphite spheres per unit area, increase the spheroidization rate, greatly improve the ability to resist decay, and extend the effective incubation time. (5) Reduce the amount of omega (Si) in the original molten iron, increase the amount of nodulizer, inoculant and various pretreatment agents, and strengthen the inoculation process.