1. Chemical composition design
When the diameter of roll body is more than 600 mm, the carbon equivalent is 3.8% – 3.9%. Under the condition of constant carbon equivalent, appropriately reducing the amount of W (SI) will help to reduce the tendency of graphite floating. The chemical composition control of the rollers of our company is shown in the table.
| Detection location | C | Si | Mn | P | S | Ni | Cr | Mo | V | Mg |
| Outer layer | 2.6-2.7 | 0.45-0.55 | 0.85-0,95 | ≤0.05 | ≤0.02 | 1.25-1.35 | 17.7-17.9 | 1.25-1.35 | 0.2-0.3 | — |
| Inner layer | 3.0-3.3 | 2.0-2.2 | 0.4-0.6 | ≤0.08 | ≤0.03 | 0,3-0.5 | ≤0.25 | — | — | ≥0.04 |
2. Optimization of burden ratio
2.1 Selection of raw materials
Select high-purity raw materials [w (SI) <1.0%, w (MN) <0.3%, w (s) <0.03%, w (P) <0.03%], and the burden ratio is 25%~35% high-quality pig iron +55%~65% reheating charge +5%~15% high-quality scrap.
2.2 Loading and melting
Slag should be formed in time in the early stage of smelting, and slag is covered under high temperature in the later stage. Give full play to the protective role of slag, lay chips at the bottom of the furnace, and use pig iron and scrap steel on it.
2.3 Utilization technology of alloys and trace elements
Only Ni, w (Ni) 0.3%~0.5% can improve the tensile strength by 30~50 MPa.
3. Spheroidization and inoculation treatment
Three inoculants are used for bottom inoculation: Nimg composite spheroidizing agent [w (Ni) ≥ 80%, w (mg) 14%~18%], with an addition of 5kg/t; RESiFe spheroidizing agent, with the addition amount of 10 kg / T; Sizr compound inoculant [w (SI) 60%~65%, w (AL) 0.75%~1.5%, w (CA) 1%~2%, w (Zr) 5%~7%], the addition amount is 3 kg / T. The stream inoculation adopts the sizr compound inoculant with particle size of 1~3 mm, and the addition amount is 1.5 kg / T.
4. Control technology of smelting process
4.1 Strictly control the outlet temperature of molten iron
Based on the advantages that the temperature of medium frequency furnace is easy to increase and can be smelted quickly, the process operation method of “fast melting and quick exit” is formulated to shorten the melting time as far as possible and shorten the holding time of molten iron in the furnace and ladle as far as possible. Because the molten iron reacts with sio2+2c=si+2co at 1510~1 530 ℃ to generate CO gas, the content of SiO2 slag in the molten iron continues to decrease, and the purity of the molten iron increases after standing for 5 minutes. Therefore, the tapping temperature of molten iron is controlled at (1480 ± 10) ℃.
4.2 Chemical composition control
Adjust the chemical composition in the order of manganese, carbon and silicon. At the later stage of smelting, in order to promote the dissolution of carburizing agent, the electric frequency is reduced to strengthen the stirring of molten pool. If the “Hump” is too high and the frequency modulation time is too long, the probability of contact between molten iron and the atmosphere will increase, and the dissociated oxygen ions will enter the molten iron.
4.3 Proper slagging and timely slagging
The main reason for the formation of one-time slag inclusion in the roll is that the amount of molten iron w (s) is high and the oxidation is serious; The main reason for the secondary slag inclusion is that the amount of W (mg residue) is too high, which increases the formation temperature of oxide film. Therefore, the measures taken are to reduce the amount of W (s) and w (o) in the raw molten iron, ensure the reduction of W (mg residual) during spheroidization, and add an appropriate amount of rare earth to reduce the film forming temperature. At the later stage of smelting, slag should be raked frequently and efficiently until the surface of molten iron is clean and there is no new scum.
4.4 Reduce the erosion of furnace lining materials
The furnace lining material with more than 98% w (SiO2) is selected, and the sintering temperature of the furnace lining is higher than 1550 ℃, so as to ensure the cristobalization of quartz in the sintering layer. The liquid level should be near the height of the furnace mouth, so that the internal surface of the furnace lining can be sintered up and down uniformly, so as to reduce the erosion of smelting on the furnace lining.
4.5 Avoid staying in the molten iron furnace for too long
Before spheroidizing treatment and inoculation treatment, it should be avoided to stay in the original molten iron furnace for too long. If the spheroidizing treatment and inoculation treatment are carried out after tapping for more than 15 minutes, it is easy to form carbides and produce shrinkage cavity and porosity defects, and the carbides are difficult to eliminate even if the methods of in ladle inoculation and flow inoculation are used.
4.6 Stable control of W (s) amount of raw molten iron
If the amount of W (s) in the raw molten iron is too high, the graphite will precipitate prematurely, with more graphite balls and uniform size, resulting in an increase in the tendency of shrinkage cavity and porosity. When the amount of W (s) of raw molten iron exceeds 0.02%, industrial soda ash is used for pre furnace desulfurization. Because the desulfurization reaction is endothermic, the desulfurization temperature is required to be controlled at about 1500 ℃. According to the amount of W (s) during pre furnace melting, the amount of soda ash is controlled at 1.5% – 2.5%.
5. Control the time of graphite precipitation and expansion
When the feeding molten iron cannot be provided by the pouring and riser at the later stage of solidification, the graphitization expansion will be too small, resulting in a large number of shrinkage cavity and shrinkage porosity defects. Therefore, from the beginning of solidification to the end of solidification, the precipitation of graphite should
Keep the same speed, especially at the initial stage of solidification, the precipitation of graphite should not be too fast. Within 0.5 hours after pouring, the insulation cover shall be timely buckled, and the insulation cover shall be removed after 8 hours. The insulation in the mold shall be ≥ 96 hours, and the mold shall be placed in a dry leeward place.
The pouring temperature is 1360 ~ 1380 ℃, otherwise the liquid shrinkage is too large. Take full advantage of the graphitization expansion to carry out the self feeding of nodular cast iron to ensure the internal compactness of the roll.