According to the main technical requirements of cast steel rolls, the material composition of the rolls is determined.
The causes of casting defects such as roll slag, riser shrinkage and end face crack were analyzed, and the processes of modeling, smelting, pouring, electric heating riser, hole type pre-processing and heat treatment were controlled. The test and production results show that the electric heating riser process can effectively increase the process yield and greatly reduce the production cost. The developed ZU6OCrNiMoSL roller can fully meet the application requirements.
- Roller material determination
According to the performance requirements of the rolls, the selected roll material is ZU60CrNiMo.
- Rolls main defect analysis
1) Normal casting steel The crack defect of the roll blank is mainly the end crack of the roll surface. The reason for this crack is that the cold coating process of the roll is not satisfactory, the manufacturing process is unreasonable and the casting process is not properly controlled.
2) After the casting of the cast steel roll, various slag inclusions visible to the naked eye on the working surface are collectively referred to as large-surface slag inclusions. The defect is mainly caused by low paint strength and tight thickness control, as well as inclusions in the cavity and casting system and inclusions in the molten steel.
3) After the processing, various kinds of slag holes, slag inclusions, shrinkage, shrinkage holes, pores, etc. appearing on the upper and lower roll necks of the cast steel rolls,
This is due to the difference in cooling time and cooling rate after the completion of the pouring of molten steel, especially if the feeding of the riser end is insufficient or the control time is unreasonable, and such defects are more likely to occur.
- Roll production process control
3.1 Manufacturing Process
1) Add 30% zirconium sand in the sand, the sand hardness is controlled above 90, and the sand thickness is 35mm. Increasing the amount of bentonite in the molding sand can ensure the strength of the hanging sand, but the amount of the added sand is too large, which is easy to produce cracks, causing the hanging sand to fall off. If the amount of addition is too small, the strength of the molding sand is not enough, and it is easy to cause sand washing during pouring, causing slag inclusion.
2) After the modeling, the surface of the floating sand is cleaned, and the coating on the sand type should be evenly coated to ensure the thickness of the coating is 0.5 to 1.0 mm. When the box is combined, the debris in the cavity should be cleaned and cleaned, so as to completely eliminate the source of inclusions and reduce the chance of forming slag inclusions.
3.2 smelting process
The endogenous inclusions produced during the smelting process are mainly oxides, sulfides and composite inclusions of the two. These inclusions are generally small in size and evenly distributed in the steel. In addition, the low tapping temperature and short standing time are not conducive to the floating of inclusions and gases, and the rolls are prone to slag inclusion and porosity defects. According to the existing conditions of the heavy machine company, it is refined by 40ton electric furnace + 40ton LF ladle, which can desulfurize and deoxidize the molten steel and effectively improve the quality of molten steel.
- Before the furnace, W(C) is controlled as much as possible in the lower and middle limits of the demand, and w(Si) is controlled as much as possible in the middle limit. The hardness of the normal cast steel roll is mainly achieved by the heat treatment process, and the chemical composition must be strictly controlled.
- LF refining is carried out at the bottom of the package. The increase of the soft blowing time is beneficial to the floating and discharging of the inclusions. However, the injection flow rate of the inert gas must be controlled to prevent the molten steel from being secondary to oxidation.
- Before the tapping, ensure that the ladle is cleaned and the toast is in compliance with the requirements. The pouring temperature is controlled at 55 to 60 °C on the liquidus. Make sure that the steel has enough rest time in the bag to help the inclusions float. At the same time, the bottom of the ladle is used to blow Ar, and its main function is to remove oxygen and hydrogen inclusions and purify molten steel. The coating is added to protect the molten steel at the same time.
3.3 pouring process
In order to make the molten steel level rise smoothly, the surface of the roll body does not have slag inclusion defects, and the machining allowance of the diameter of the roll body is reduced, and the bottom injection casting process is adopted. The pouring temperature is controlled at 45~50 °C on the liquid phase line, and the pouring speed is controlled during the pouring process. When pouring, the control is within 8 min to ensure low temperature fast pouring. After pouring the fill filler, add 3n2oomm thick shrinkage insulation to cover the riser to prevent premature solidification, which is not conducive to feeding.
3.4 Electric heating riser process
After pouring, the riser is insulated with a heat retaining agent. In order to prevent the molten steel from boiling excessively and exposed to the air, the highest temperature is not used initially. After 1 hour, the temperature of the molten steel drops rapidly, and the maximum power rise must be used to ensure that the molten steel at the surface of the riser and the sand is melted. As the molten steel gradually solidifies from the outside and inside, the heat supplied by the electric heating is gradually reduced [z]; the total heating time is 5h.
Electric heating riser operation method: After the pouring is completed, insert q550mm×1000mm round steel into the pouring cup and connect with the output negative electrode, move the graphite electrode to the upper side of the roll riser, lower it to insert the molten steel, adjust the gear position, turn on the power. The heating agent should be added during the heating process so that the molten steel should not be exposed. In the initial stage of heating, the current and voltage are not stable due to the shrinkage of molten steel. The depth of the molten steel can be adjusted to stabilize the current and voltage by adjusting the depth of the graphite electrode. The roll size is 1080mm × 2800 mrn, the roll tool is 1168mm × 3110mm, the riser is 1002mm/964mmX2500mm, and the actual steel consumption is 44t, which is 10t less than the steel without electric heating riser. The results of the solidification simulation before and after the electric heating riser are shown in Figure 1. It can be seen from the simulation results that the unheated riser is lengthened more, the amount of molten steel is relatively large, and the long riser is cut off after the roll is opened, which not only wastes molten steel, but also uses a large amount of oxygen and acetylene gas, and the cost is high; The heating process can be carried out after the rolls are unpacked. The electric heating riser improves the feeding efficiency, and the casting process yield rate is increased by more than 10%, which eliminates the shrinkage of the roll neck end and greatly reduces the production cost.
3.5 hole type pre-processing
The hardness of the roll after cooling with the furnace is around 40HSD, and the processing performance is good. The cast steel roll has good hardenability and small hardness drop. The analysis considers that the normalized surface structure is finer than the core and the wear resistance is better. Before the normalizing, the hole type should be required according to the requirements. In order to prevent deformation, surface oxidation and decarburization of the rolls during heat treatment, the hole type should be left with a certain margin, and other parts, including the roll neck, should also leave a margin .
3.6 Heat treatment process
The heat treatment process for large cast steel rolls is: 880 ° C diffusion annealing (surface coating anti-oxidation coating when heating), heat preservation 12h; blow cooling 3 ~ 5h, 500 ~ 550 ° C tempering, holding time 30h; with furnace cooling to 80 °C. The cast steel roll is subjected to diffusion normalizing and tempering heat treatment to refine and strengthen the matrix structure of the roll surface material. The as-cast secondary reticulated cementite is eliminated and the mechanical properties are improved; the retained austenite amount is reduced to the desired To the extent that the residual stress of the rolls is effectively reduced. The tempered cast steel roll can obtain high hardness, good thermal shock resistance and anti-accident ability, and wear resistance is also good.
- Roll performance and use effect
Cast steel rolls are produced in the medium and thick plate steel wire of Tangshan Iron and Steel Co., Ltd., which have angle steel, U-shaped steel and steel plate piles, all of which have achieved good results. The data from the medium-thick plate steel wire shows that when the ZU60CrNiMo~L roller is used to roll the steel sheet pile, the per-hole per-rolling capacity is about 1000t, and each pair of rolls has a total of 3 holes, and the total amount of rolling rolls is I500t. At the same time, the ZU60CrNiMo roll production process was also successfully used in the production of ZU180CrNiMo semi-steel rolls.