One of our customers needs us to cast Mn14 mill liners for his ball mill. The detailed technical requirements as following：
- Material: C 1. 1～1. 5, Mn 11. 0～14. 0, Si 0.3-0.8, P＜ 0.05， S＜ 0.05，Mn/C ＞9.0
- Tensile strength σ B / MPa ≥637
- Elongation (%) ≥20
- Impact toughness / (J / m*m) ≥15
- Hardness ≤229 HB
- The alloy composition, shape and size, surface quality, and carbide grade of castings are required to be accepted, and no defects affecting strength and appearance are required. The hardness of castings after heat treatment is 197-228 Hb, while the mechanical properties and inclusions of castings (which can not meet the metallurgical quality assessment) are not subject to acceptance conditions.
Modeling process design
In order to meet the requirements of surface quality and dimensional accuracy, the molding sand should be compacted and more air holes should be punched. The following design principles should be adopted:
- Negative tolerance is adopted for the overall dimension of castings, and the machining allowance is generally 3-5 mm or no machining allowance is left. The maximum machining allowance is less than or equal to 10 mm.
- The slot hole of the casting adopts positive tolerance, and the core is made of magnesia refractory.
- The free linear shrinkage is 2.2% ～ 3.2%. The riser is designed according to the solidification shrinkage of 6%. The easy cutting pouring system is adopted.
- Forsterite powder basic coating is used.
Gating system design
An open gating system was selected, with ∑ f in ∶ f transverse ∶∑ f straight = 1 ∶ (1 ～ 1.1) ∶ (1 ～ 1.4); The diameter of the straight gate is 45 mm according to the size of the ladle gate. Then ∑ f = 15.9 cm²; 1 transverse gate: ∑ f = 15.9-17.5 cm²; 3 inner gates: ∑ f = 15.9 ～ 22.3 cm².
Melting and pouring
GW212500J medium frequency induction furnace is used for melting, and the covering and protection of molten steel are emphasized. When charging the furnace charge, a layer of lime (GB 1594-79) is first loaded at the bottom of the furnace. The quality of the lime is about 1% of the mass of the charging metal. With the melting process, the molten metal appears and rises, and the slag covers the surface of the molten steel all the time. The slag can protect the molten steel from gettering and oxidation, collect inclusions, keep heat, and save energy. With the melting going on, an appropriate amount of lime can be added, and the appropriate amount of slag should be able to cover the molten steel completely, and some fluorspar (gb826-87) should be added appropriately. The mass ratio of lime to fluorspar is about (4-5) ∶ 1, so as to reduce the melting point of slag, adjust the viscosity of slag, and remove slag easily.
High carbon ferromanganese femn75c7.5 (gb3795-87) is used for pre-deoxidation, and carbon oxidation is used to strengthen pre-deoxidation, so as to reduce the content of iron oxide in molten steel to a lower level, so as to reduce inclusions. During the treatment, the temperature of molten steel is 1 610 ~ 1 640 ℃, the addition amount is about 1% of the mass of liquid steel, and the yield is 90%. At the same time, ferromanganese with a block size of 50-100 mm is added after full preheating (above 750 ℃). After each batch is added, it shall be fully stirred,
“Prevent freezing” and precipitation, and add the next batch after each batch is basically melted. In this order, the yield of ferromanganese is 95%. If ferromanganese is added directly before pre-deoxidation, the yield is 90%, with a difference of 5%. The ratio of Mn / Mn to Mn C is more than 5.
After chemical composition adjustment, aluminum (Yb / Z4 – 75) is generally used for final deoxidation, and the amount is about 0.1% of the total mass of molten steel. Considering that the thoroughness of precipitation oxidation is less than that of diffusion deoxidation, the amount can be adjusted to 0.2% of the total mass of molten steel to increase the residual amount of aluminum (> 0.08%) in molten steel and to precipitate high melting point al2p compounds in the grains, Thus, the disadvantageous forms of phosphorus such as ternary phosphorus eutectic can be eliminated.
The molten steel should be kept after it is discharged from the furnace. The standing is beneficial to the floating of gas and inclusion in molten steel, improving metallurgical quality and adjusting pouring temperature. The liquidus temperature of high manganese steel zgmn1321 is 1400 ℃, the discharge temperature is 1 360 ～ 1 420 ℃, and the pouring temperature is 1 340 ～ 1 380 ℃.
According to this rule, the static isothermal time of molten steel at different temperatures is further determined.
The manganese mill liners with conventional water toughening treatment belong to thick and large parts (δ > 75 mm). During heat treatment, the heating rate must be controlled at 30-50 ℃ / h in the temperature range from normal temperature to 600 ℃. When the temperature is heated to above 600 ℃, the heating speed can be increased to 100-150 ℃ / h until the water quenching temperature is 1050-1080 ℃ and kept for 4 h, It is necessary to ensure that carbides in steel are fully dissolved in austenite and homogenized by diffusion, so as to reduce the possibility of carbide reprecipitation.
After heat preservation, the casting should be quickly pulled out of the furnace and put into water. The time from opening the furnace door to all the water entering the workpiece should be within 2-3 min, the shorter the better, so as to ensure that the casting temperature is not lower than 950 ℃. The water temperature should be controlled at 10 ~ 30 ℃, and the water temperature at the end of quenching should not be greater than 60 ℃. In the case of mass production, dry ice can be added to the water tank to cool down.
- The dimensional accuracy and surface quality of high manganese steel can be significantly improved by using metal plate molding and alkaline coating and strictly controlling other casting processes.
- Using lime and fluorite to protect the smelting furnace, the smelting process of pre-deoxidation with high carbon ferromanganese, followed by alloying ferromanganese and aluminum final deoxidization, can improve the yield of alloy ferromanganese and improve the metallurgical quality of high manganese steel.
- Both conventional and direct water toughening treatment schemes can be used for high manganese steel ball mill liner, and qualified castings can be obtained.