1. Melting process of alloy vermicular graphite cast iron for brake disc
(1) Raw materials and chemical composition
Pig iron, scrap steel, return charge, Si, Mn, Mo, Cu, Ni, Vermicular Agent and other ferroalloys are selected according to the chemical composition of the furnace charge. Q10 or Q15 pig iron shall be selected as pig iron. It is required that the harmful elements s ≤ 0.03%, P ≤ 0.05%, the total amount of trace elements (Cr, Mo, Sn, Sb, Pb, Bi, Te, as, B, Al) ≤ 1%, and the anti spheroidizing element Ti ≤ 0.05%.
Scrap steel shall be clean without serious corrosion and coating. The returned materials shall be stacked by classification, and various brands shall not be mixed. Shot blasting shall be carried out before use.
The composition control is shown in Table 1.
Reasonable carbon equivalent can increase vermicular graphite, reduce the shrinkage tendency of vermicular graphite cast iron, increase tensile strength, yield strength and hardness, and effectively improve the casting and processing properties of brake disc.
The alloy element Mn can refine pearlite and effectively increase the content of matrix pearlite, so as to improve the strength of vermicular graphite cast iron castings; Cu can improve the strength, hardness and wear resistance, and reduce the section sensitivity of vermicular graphite cast iron; Mo element can improve the thermal strength of brake disc and the yield ratio of casting; Ni element can improve the uniformity of matrix structure of castings; Mg and rare earth elements are the guarantee of creep rate of vermicular graphite cast iron.
The melting equipment is a 5T medium frequency furnace, and the pouring ladle is a 2T ladle. The metal charge and ferroalloy are added to the medium frequency furnace for melting, and the power is increased to raise the temperature until the charge is melted. The slag is removed with slag remover, and the spectral sample is taken for inspection. After the composition of the molten iron is qualified, the molten iron is washed into the vermicular graphite cast iron vermicular ladle. For secondary tapping, the amount of tapping shall be strictly controlled. The amount of cast iron is the key point affecting the creep rate. If the amount of cast iron is too large, flake graphite is easy to appear and form gray iron; The amount of cast iron is too small, and it is easy to produce a large number of graphite balls into nodular cast iron.
(3) Creep inoculation treatment
Using the process of flushing method, the ladle pit of molten iron ladle is put into the special Vermicular Agent, inoculant and covering agent for vermicular graphite cast iron, and the molten iron is flushed into the ladle for Vermicular and inoculation treatment. The process of creep punching method is shown in Figure 1.
(4) Pouring and insulation
After the vermicular inoculation treatment of vermicular graphite cast iron, the molten iron shall be stirred, slag raked and ladle turned. When the molten iron temperature reaches the pouring temperature, the casting shall be carried out, and the pouring process shall be controlled within 10 minutes. After pouring, it shall be cooled naturally to a temperature ≤ 300 ℃, and the sand shall be removed after unpacking.
2. Creep rate of alloy vermicular graphite cast iron for brake disc
The graphite of vermicular graphite cast iron is vermicular, and its shape is between flake graphite and spherical graphite, so the process range of vermicular treatment is very narrow. Stable mass production of vermicular cast iron is a major control problem. After repeated tests on vermicular cast iron production, a set of mature and reliable smelting process scheme, operation specification and control system has been formed to realize stable mass production of high-quality low-alloy vermicular cast iron.
After long-term process optimization, the creep rate of vermicular graphite cast iron castings is stably controlled at 70% ~ 90% and the pearlite content is 35% ~ 55%. Figure 2 shows two groups of metallographic photos of castings produced on site. The first group is shown in Figure 1. The creep rate is 70%, pearlite is 50%, ferrite is 50% (no cementite); As shown in Fig. 2, the second group has the as cast diagram of 80% creep rate, 35% pearlite + 65% ferrite (no cementite). The general rule is that the lower the creep rate, the higher the pearlite, the higher the strength and hardness, and vice versa. Cementite is easy to appear due to poor inoculation treatment and high manganese content. If the nickel content is too high, sorbite is easy to appear.
3. Production control difficulties
(1) Vermicular graphite cast iron vermicular rate control. The creep rate of casting body and test block must be stable above 70%.
(2) The body of vermicular graphite cast iron brake disc shall not have casting defects. It shall be processed on four sides without surface cold insulation, slag inclusion and internal shrinkage defects. Each piece shall be subject to ultrasonic flaw detection and magnetic particle flaw detection.
(3) Control the proportion of various elements, and ensure that the chemical composition, mechanical properties and metallographic structure meet the requirements through vermicular graphite cast iron vermicular and inoculation treatment.
(4) Accurately control the amount and temperature of. The error of tapping amount is 1 500 kg ± 5 kg, and the tapping temperature range is 1 500 ℃± 10 ℃