How to improve the spheroidization rate of ductile iron
The spheroidization level of domestic ordinary spheroidal graphite cast iron castings is required to reach level 4 or above, (that is, the spheroidization rate is 70%), the spheroidization rate achieved by the general foundry is about 85%. In recent years, with the development of nodular cast iron production, especially in industries with high requirements for wind power casting production and casting quality, the spheroidization level is required to reach level 2, that is, the spheroidization rate reaches more than 90%. The author's company analyzed and improved the spheroidization and inoculation process used in QT400-15, as well as the spheroidizing agent and inoculant, so that the spheroidization rate of nodular cast iron reached more than 90%.
1. The original production process
Original production process: The smelting equipment adopts 2.0T intermediate frequency furnace and 1.5T power frequency furnace; the composition of QT400-15 raw iron liquid 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 spheroidizing agent used in the spheroidizing treatment is 1.3%~1.5% RE3Mg8SiFe alloy; the inoculant used in the inoculation treatment is 0.7% ~0.9% 75SiFe-C alloy. The spheroidizing treatment adopts the two-shot iron rushing method: first tap 55% to 60% of the iron, perform spheroidizing treatment, then add the inoculant, and then add the rest of the molten iron. Due to the traditional method of spheroidization and inoculation, the spheroidization rate detected by a single cast wedge test block with a thickness of 25 mm is generally around 80%, that is, the spheroidization level is 3rd.
2. Test plan to improve the spheroidization rate
In order to increase the spheroidization rate, the original spheroidization and inoculation process has been improved. The main measures are: increasing the amount of spheroidizing agent and inoculant, purifying molten iron, and desulfurizing treatment. The spheroidization rate is still tested with a single cast wedge test block of 25 mm. The specific plan is as follows
(1) Analyze the reason for the low spheroidization rate of the original process. It was thought that the amount of spheroidizing agent was small, so the amount of spheroidizing agent added was increased from 1.3% to 1.4% to 1.7%, but the spheroidizing rate did not meet the requirements. .
(2) Another guess is that the low spheroidization rate may be caused by poor gestation or fertility decline. Therefore, 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 and believe that there are more inclusions in molten iron and high spheroidization interference elements may be the cause of the low spheroidization rate. Therefore, high-temperature purification of molten iron is carried out. The high-temperature purification temperature is generally controlled at 1500±10°C, but Its spheroidization rate has not exceeded 90%.
(4) The high amount of ω(S) seriously consumes the spheroidizing dose and accelerates the decline of spheroidization. Therefore, the desulfurization treatment is increased to reduce the original iron liquid ω(S) amount from 0.035% to less than 0.020%, but the spheroidization rate is also Only 86% meet the requirements.
3. The last improvement plan adopted
3.1 Specific improvement measures Raw materials are pig iron, rust-free or less rusty scrap and reclaimed materials; the original molten iron is desulfurized by adding soda ash (Na2CO3); Foseco 390 pretreatment agent is used for pre-deoxidation treatment in the package; Foseco The spheroidizing agent is spheroidized; silicon carbide and ferrosilicon are used for joint inoculation. The original molten iron composition control of the new process: ω (C) = (3.70% ~ 3.90%, ω (Si) = 0.80% ~ 1.20% [casting ω (Si final) = 2.60% ~ 3.00%], ω (Mn) ≤ 0.30%, ω(P)≤0.05%, ω(S)≤0.02%. When the original molten iron ω(S) exceeds 0.02%, industrial soda ash is used for desulfurization in front of the furnace, because the desulfurization reaction is an endothermic reaction Requires the desulfurization temperature to be controlled at about 1500℃, and the amount of soda ash added is controlled at 1.5% ~2.5% according to the amount of ω(S) during the melting in front of the furnace. At the same time, the spheroidization treatment package adopts an ordinary embankment type treatment package. Foseke NODALLOY7RE brand spheroidizing agent 1.7% is added to the side of the bottom of the dam, flattened and compacted, covered with 0.2% powdered silicon carbide and 0.3% small lumps of 75SiFe, and covered with iron after tamping , Add 0.3% Foseke 390 inoculant on the other side of the molten iron ladle. When tapping iron, first flush into 55%~60% of the total molten iron volume. After the spheroidization reaction is completed, add 1.2% 75SiFe-C After the inoculant is flushed into the remaining molten iron, the slag is removed and poured.
4. Result analysis
4.1 The influence of the main elements on the spheroidization rate C, Si: C can promote graphitization and reduce the tendency of white mouth, but a high amount of ω (C) will make CE too high and easily cause graphite to float, generally controlled at 3.7%~3.9% . Si can strengthen graphitization ability and eliminate cementite. When Si is added as an inoculant, it can greatly reduce the supercooling ability of molten iron. 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 (final Si) was controlled at 2.60% to 3.00%. Mn: During the crystallization process, Mn increases the tendency of cast iron to overcool and promotes the formation of carbides (FeMn) 3C. In the eutectoid transformation process, Mn reduces the eutectoid transformation temperature, stabilizes and refines pearlite. Mn does not have much influence on the spheroidization rate. 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, which has little effect on the spheroidization rate of ductile iron. S: S is a despheroidizing element. S consumes Mg and RE in the spheroidizing agent during the spheroidizing reaction, hindering graphitization and reducing the spheroidizing rate. Sulfide slag will also return to sulfur before the molten iron solidifies, again consuming spheroidizing elements, accelerating the decline of spheroidization, and further affecting the spheroidizing rate. In order to achieve a high spheroidization rate, the amount of ω(S) in the raw iron should be reduced to less than 0.02%.
4.2 Desulfurization treatment After the charge is melted, take samples and analyze the chemical composition. 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 the ladle, use molten iron flow to flush and stir, soda ash decomposes at high temperature, the reaction formula is Na2CO3=Na2O+CO2↑: the generated Na2O is in the molten iron again Sulfuration and formation of Na2S, (Na2O) + [FeS] = (Na2S) + (FeO). Na2CO3 separates and resolves CO2, causing violent agitation of molten iron, which promotes the desulfurization process. Soda ash slag is easy to flow and float quickly, and the desulfurization reaction time is very short. After desulfurization, the slag should be removed in time, otherwise it will return to sulfur.
4.3 Pre-deoxidation treatment, spheroidization treatment and inoculation treatment Foseke 390 pretreatment agent plays the role of pre-deoxidation treatment in the bag, at the same time increases the graphite nucleation core, increases the number of graphite spheres per unit area, and can also increase the absorption rate of Mg. Significantly improve the ability to resist recession and increase the spheroidization rate. Fochke inoculant contains ω (Si) = 60% ~ 70%, ω (Ca) = 0.4% ~ 2.0%, ω (Ba) = 7% ~ 11%, of which Ba can extend the effective incubation time. The NODALLOY7RE grade of Fozco Nodulizer is selected, and its 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 undergoes desulfurization and pre-deoxidation treatments, the elements that consume nodulizers in the molten iron are greatly reduced, so a nodulizer with a low amount of ω (RE) is selected to reduce the deterioration of the spheroidal graphite morphology by RE; The main element of action is Mg; Ca and Al can play a role in strengthening the gestation. Using silicon carbide and ferrosilicon combined inoculation treatment, the melting point of silicon carbide is about 1600°C, and graphite crystal nucleus is increased during solidification, and large doses of ferrosilicon are used for inoculation, which can prevent spheroidization from declining.
5. Conclusion
In the production of ferritic ductile iron, when the spheroidization rate is required to reach more than 90%, the following measures can be adopted: (1) Use high-quality charge to reduce the de-spheroidization elements in the charge.
(2) Choose a spheroidizing agent with a low amount of ω (RE) to reduce the deteriorating effect of RE on the morphology of spheroidal graphite.
(3) The ω (S) content of the original molten iron should be less than 0.020%, which can reduce the consumption of nodulizers, especially the nodulized elements consumed by the secondary sulfurization 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 recession, and extend the effective incubation time.
(5) Reduce the amount of ω (Si) in the original molten iron, increase the amount of spheroidizing agent, inoculant and various pretreatment agents, and strengthen the inoculation treatment.SHAPE* MERGEFORMAT
