High-performance ductile iron castings supplied as cast

High-performance castings supplied as cast in China adopt strict melting, spheroidizing and inoculation processes. Strict raw material control, using high-purity pig iron to prevent the interference of anti-spheroidizing elements in pig iron trace elements on spheroidization; With high-quality scrap steel, the content of Mn, P, S and Ti is lower. The nodular cast iron castings for the chassis of commercial vehicles are mainly QT800-5, with density of 7.3 kg/m3, tensile strength of 800 MPa, yield strength of 480 MPa, elongation of 5%, and material hardness of 245-335 HBW10/3000. Compared with QT800-2 in the national standard, QT800-5 has less practical batch application due to its low plasticity and toughness.

Dongfeng Precision Casting adopts investment casting process to achieve as-cast performance QT800-5. The main technical measures are as follows:

A. Use alloy elements such as Mn and Cu to strengthen the matrix structure;

B. Through the use of low magnesium and low rare earth nodularizing agent FeSiMg6Re2, 75SiFe is used for in-mold inoculation, 4-8 mm barium ferrosilicon inoculant is used for in-mold inoculation, and 2-4 mm barium ferrosilicon inoculant is used for in-mold inoculation;

C. The chemical composition of the original molten iron of ductile iron is controlled as follows: C mass fraction is 3.3% ~ 3.5%, Si mass fraction is 2.3% ~ 2.5%, Mn mass fraction is 0.8% ~ 1.0%, and Cu mass fraction is 0.7% ~ 0.9%. The pearlite content of the as-cast matrix structure of nodular cast iron is ≥ 85%, and the spheroidization grade is ≤ 2. The mechanical properties can meet the requirements.

GF (George Fischer Metal Forming Technology) nodular cast iron castings in as-cast supply shall be subject to subcontracting pure magnesium spheroidizing technology, that is, before spheroidizing treatment, the spheroidizing ladle shall be placed horizontally, the quantitative molten iron shall be injected, and then the magnesium block shall be put into the spheroidizing chamber from the special entrance. During spheroidizing treatment, turn the spheroidizing bag to make it stand upright, the molten iron enters the spheroidizing chamber, the magnesium is heated and gasified, and the magnesium steam floats up and reacts fully with the molten iron. The spheroidizing effect of pure magnesium is superior to the traditional spheroidizing methods such as “wire feeding method” and “flushing method”, and has obvious economic advantages. It can greatly reduce the amount of spheroidizing agent, reduce non-metallic inclusions produced in the spheroidizing process, improve the purity of molten iron, and greatly reduce the spheroidizing cost. The sulfur content of the original molten iron can be as high as 0.2%, which does not affect the nodularization effect. Desulfurization and nodularization can be carried out at the same time without pre-desulfurization. The treatment process is safe and stable. The temperature of the molten iron is reduced by about 40 ℃, and the absorption rate of magnesium is about 60%. Because the pure magnesium nodularization will not bring in silicon, the silicon content can be easily controlled.

The difference of GF pouring method lies in the difference of insulation pouring furnace equipment. The function of the insulation pouring furnace is similar to that of the ladle in the traditional pouring method. Compared with the traditional ladle, its advantages are embodied in four aspects: the upper part is protected by nitrogen to isolate oxygen in the air and maintain the composition of molten iron; There is electromagnetic heating at the bottom to keep the set temperature of molten iron stable; Accurately control the outflow of molten iron required for each casting; Continuous casting can replenish molten iron into the holding furnace at any time as required.

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