SAND CASTING EQUIPMENT.

The spheroidization level of ordinary nodular cast iron castings is required to reach level 4 or above, (that is, the spheroidization rate is 70%), the spheroidization rate reached by the general foundry is about 85%. In recent years, with the development of ductile 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%. Xinyuanzhu Group 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 for spheroidization is 1.3%~1.5% RE3Mg8SiFe alloy; the inoculant used for 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 inoculant, and then add the rest of the molten iron. Because the traditional method is used for spheroidization and inoculation, the spheroidization rate measured with a single cast wedge test block of 25 mm thickness is generally about 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:

2.1 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 was increased from 1.3% to 1.4% to 1.7%, but the spheroidizing rate did not meet the requirements.

2.2 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.

2.3 Continuing the analysis, it is believed that there are more inclusions in molten iron and high spheroidization interference elements may be the reason for 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℃, but the ball The conversion rate has not exceeded 90%.

2.4 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 only reaches 86%.

According to the test results of the above four schemes, the structure 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 reclaimed materials; the raw molten iron is desulfurized by adding soda ash (Na2CO3); Foseco 390 pretreatment agent is used for pre-deoxidation in the bag; Foseco nodulizer Spheroidizing treatment; combined inoculation with silicon carbide and ferrosilicon.

New technology of the original iron liquid composition control: omega (C) = (3.70% ~ 3.90%, omega (Si) = 0.80% ~ 1.20% [of the casting (Si) = 2.60% ~ 3.00%] omega, omega (Mn) of 0.30% or less, omega (P) of 0.05% or less, omega (S) of 0.02% or less. When the original iron liquid (S) more than 0.02%, the use of industrial soda desulfurization treatment before the furnace, because desulfurization reaction is endothermic reaction, desulphurization temperature control is required at 1500℃, the amount of soda added according to the furnace melting in the (S) amount of high and low control in 1.5 % ~2.5 %. At the same time, the spheroidizing treatment package adopts the ordinary dyke treatment package. Firstly, 1.7% of a certain brand of spheroidizing agent is added to one side of the dyke at the bottom of the package, then the spheroidizing agent is flattened and compacted. A layer is successively covered with 0.2% powder silicon carbide and 0.3% small 75SiFe, after tamping, it is covered with pressing iron, and 0.3% of Fusco 390 insemination agent is added to the other side of the molten iron package. At the time of iron release, 55%~60% of the total iron liquid was first injected. After the spheroidization reaction was completed, 1.2% 75siFE-C inoculant was added and the remaining iron liquid was poured into the slag and poured.

3.2 Test results
The composition of the original molten iron before and after desulfurization, the mechanical properties and metallographic structure corresponding to the 25mm single cast wedge-shaped test block, and the spheroidization rate in the metallographic structure are automatically detected by the metallographic image analysis system.

4. Analysis of results

4.1 Influence of main elements on spheroidization rate
C, Si: C can promote graphitization, reduce the tendency of white mouth, but (C) high amount will make CE too high and easy to produce graphite floating, generally controlled at 3.7%~3.9%. Si can strengthen graphitization ability and eliminate cementite. When Si is added as an inoculant, the supercooling capacity of molten iron can be greatly reduced. In order to improve the inoculation effect, the amount of (Si) in the original iron solution decreased from 1.3% ~1.5% to 0.8%~1.2%, and the amount of (Si) was controlled at 2.60%~3.00%. 

Mn: During crystallization, Mn increases the supercooling tendency of cast iron and promotes the formation of carbides (FeMn)&3C. During the eutectoid transition, Mn reduces the eutectoid transition temperature, stabilizes and refine the pearlite. Mn has no significant effect on spheroidization rate. Due to the influence of raw materials, general control is (Mn). 0.30%.

P: When (P) < It is difficult to form phosphorus eutectic when it is solidly dissolved in Fe at 0.05%, which has little influence on the spheroidization rate of nodular iron.

S: S is an anti-spheroidizing element. In the spheroidizing reaction, S consumes Mg and RE in the spheroidizing agent, hinders graphitization and reduces the spheroidizing rate. Sulfide slag will return sulfur before the solidification of molten iron, consume spheroidized elements again, accelerate the decline of spheroidization, and further affect the spheroidization rate. In order to achieve a high spheroidization rate, the amount of (S) of the original iron solution should be reduced to less than 0.02%.

4.2 Desulfurization treatment
When the burden melting, sampling analysis of chemical composition, when the amount of (S) is higher than 0.02% to be desulfurized treatment.

The principle of soda desulfurization is as follows: put a certain amount of soda in the casting ladle and stir it with the iron liquid flow. The soda is decomposed at high temperature. The reaction formula is Na2CO3=Na2O+CO2↑ : the generated Na2O is combined with the sulfurization in the iron liquid to produce Na2S, (Na2O) +[FeS]= (Na2S) + (FeO). The decomposition of CO2 from Na2CO3 resulted in violent agitation of iron solution, which promoted the desulfurization process. The pure alkali slag is easy to flow and float up quickly. The desulfurization reaction time is very short. After desulfurization, the slag should be scraped in time, otherwise the sulfur will return.


A 390 pretreatment agent plays a role of pre-deoxidization in the package, and increases the graphite nucleation core and the number of graphite spheres per unit area, which can also improve the absorption rate of Mg, greatly improve the anti-decay ability and increase the spheroidization rate. Fusco inoculant contained (Si) =60%~70%, (Ca) =0.4%~2.0%, (Ba) =7%~11%, and Ba could prolong the effective inoculation time.


Choose a nodularizer, the omega (Si) = 40% ~ 50%, omega (Mg) = 7.0% ~ 8.0%, omega (RE) = 0.3% ~ 1.0%, omega (Ca) = 1.5% ~ 2.5%, omega (Al) & lt; 1.0%. Because the iron liquid after desulfurization and pre-deoxidization treatment, iron liquid consumption of spheroidizing agent element greatly reduced, so the selection of (RE) low amount of spheroidizing agent, to reduce RE to the deterioration of the shape of spherical graphite; The spheroidizing elements are mainly Mg; Ca and Al can play a role in enhancing the inoculation.

By using silicon carbide and ferrosilicon combined inoculation treatment, the melting point of silicon carbide is around 1600℃, and graphite crystal nucleus is added during solidification, and large dose of ferrosilicon inoculation can prevent spheroidization decline.

5, conclusion

When the production of ferrite nodular cast iron requires the spheroidization rate to be above 90%, the following measures may be adopted:

5.1 select high quality charge to reduce anti-spheroidizing elements in charge.

5.2 Choose low (RE) amount of spheroidizing agent to reduce the deterioration effect of RE on the shape of spherical graphite.

5.3 The amount of (S) of the original iron solution shall be less than 0.020%, so as to reduce the consumption of spheroidizing agent, especially the consumption of spheroidizing elements consumed by sulfur secondary back to sulfide slag.

5.4 Predeoxidize the iron solution, increase the number of graphite balls per unit area, improve the spheroidization rate, greatly improve the anti-decay ability, and extend the effective incubation time.

5.5 Reduce the amount of (Si) in the original iron solution, increase the amount of spheroidizing agent, inoculant and various pretreatment agents, strengthen inoculant.