With the advancement of technology and the improvement of rolling product requirements, the performance requirements for roll materials are getting higher and higher. The performance of a single metal often cannot meet the needs of production, and bimetallic composite materials have been increasingly widely used because they have the advantages of both metal materials. Many scholars have conducted research on new roll materials and their manufacturing processes.
Compared with traditional old-fashioned rolling mills that use bainitic ductile iron rolls or infinitely chilled cast iron rolls for the finishing rolling stands, high-speed steel rolls not only have a long service life and high rolling mill operation rates but also have improved dimensional accuracy and significantly improved surface quality of rolled materials. improve. High-speed steel rolls have broad market space and application prospects in bar and wire rod rolling mills. Improving the quality and service life of high-speed steel rolls is of great significance to both roll production companies and steel rolling companies. A domestic foundry company conducted research on the quality control of high-speed steel rolls in centrifugal composite casting.
1. Process flow
Focusing on the metallurgical bonding problem between the outer layer and the core of the roll, the manufacturing technology of high-speed steel rolls continues to develop, from the earliest centrifugal casting method to the subsequent continuous casting composite method, electro slag casting method, hot isostatic pressing method and injection moulding. Each casting process has different advantages and disadvantages. The centrifugal casting method has simple production equipment, a stable process, high efficiency, and low production cost. Through the reasonable design of alloy composition and process parameters, the rolls produced can still meet the needs of most rolling mills, so they will still be in a leading position for a long time.
A company’s roll casting process flow:
5T and 2T medium frequency induction electric furnaces are used for smelting metal solutions, and horizontal centrifugal casting is used to produce high-speed steel rolls.
During use, it was found that high-speed steel rolls are often prone to peeling of the working layer. Poor bonding of the bonding layer is the main cause of spalling. Good control of the bonding layer is the key to achieving a good bonding of high-speed steel and ductile iron. An appropriate control process needs to be developed.
2. Roll material
The working layer of the company’s centrifugally composite cast high-speed steel roll is made of high-speed steel, which has good red hardness, wear resistance and hardenability. The core is made of high-strength ductile iron that meets toughness requirements. Two different materials are made into high-speed steel rolls through centrifugal composite casting. The material composition of the design is shown in Table 1.
Table 1: Material composition (ωb/%)
3. Quality process control measures for high-speed steel roll bonding layer
In view of the serious spalling problem of the bonding layer of the centrifugal composite cast high-speed steel roll, the following process control measures were taken during production.
3.1 Centrifuge stop temperature control
It is ideal to control the stop temperature of the centrifuge so that the outer layer of molten steel is in a critical state of complete solidification. If the stall temperature is too high and the outer layer of molten steel has not completely solidified, it will cause partial collapse of the outer layer of molten steel and create a collapse area. Part of the molten steel in the collapse area will be reoxidized, resulting in slag inclusion defects in the bonding layer. If the stop temperature is too low, the glass slag solution added to prevent oxidation of the surface of the molten steel will thicken, and the melting point of the slag thrown to the inner surface of the molten steel during the centrifugation process will increase, making it difficult to melt during core filling, resulting in insufficient fusion and serious consequences. At this time, there will be no bonding at all, which will directly affect the bonding quality of the outer layer and the core, and cause slag inclusion defects in the bonding layer.
In order to accurately control the stalling temperature, based on the production roll specifications and working layer thickness, the square root law of steel ingot solidification is used to estimate the stalling time, and then corrected based on experience. According to the composition calculation in Table 1, the solidus temperature of high-speed steel is approximately 1283°C. Therefore, the centrifuge stall temperature was determined to be 1260~1270°C, and a continuous infrared thermometer was used to monitor the temperature of the outer molten steel.
3.2 Core filling interval time control
If the core-filling interval is too long, the free surface temperature of the outer molten steel will be too low, causing the anti-oxidation glass slag and the slag on the inner surface to become less fluid and unable to float. To this end, it is necessary to reasonably control the core filling interval and reasonably arrange the distance between the centrifuge and the pouring pit. Depending on the specifications of the rolls produced, usually, 3 to 6 minutes can meet the moulding time requirements.
3.3 Core-filled molten iron pouring temperature control
If the pouring temperature of the core-filling molten iron is too high, the solidified outer molten steel will be eroded too much. Strong carbides in the outer molten steel will form elemental molybdenum, and chromium will enter the core molten iron, causing the total amount of core alloy to increase, making the core The excessive number of carbides in the structure, especially the bonding layer, causes a sharp reduction in the strength of the bonding layer and the core. If the pouring temperature is too low, oxidized slag and glass slag cannot effectively float, and the core metal cannot melt enough outer metal, resulting in poor bonding and a large number of excessive slag inclusions distributed on the bonding layer.
The molten iron filling the core provides heat energy for the melting and bonding of the bonding layer. The molten iron in the core generally undergoes a spheroidization and incubation process. It needs to be well coordinated with the operation of the working layer, and the core is notified to tap out the iron before the centrifuge stops. The core pouring temperature is ensured by controlling the tapping temperature. The tapping temperature is controlled at 1500~1520℃, and the core filling temperature is controlled at 1410~1430℃.
3.4 Centrifugal casting process control
The timely addition of anti-oxidation glass slag can protect the inner surface of the molten steel and prevent the production of high-melting point oxides on the inner surface of the molten steel. At the same time, the glass slag is required to have good fluidity and quickly form a protective film on the inner surface. During cold lifting Ensure that there is a thin layer adsorbed on the inner surface to maintain insulation and prevent oxidation, and the rest flows out. Reasonably control the core filling speed so that the adsorbed part can float immediately after contacting the molten iron in the core. According to production practice, the centrifuge speed is determined to be 850r/min, and the core filling speed is controlled at (7~10) kg/s.
3.5 Charge control
Use low-sulfur, low-phosphorus high-quality alloy materials and steel materials. Reduce the addition of rusty raw materials, reduce the generation of inclusions, and improve the purity of steel and molten iron. S and P are both surface-active elements. As their content increases within a certain range, the crack resistance of steel decreases, so the content should be strictly controlled below 0.020%.
3.6 Melting of the outer layer of molten steel
First, crush the high melting point alloy ferrochromium and ferrovanadium, and then add them into the furnace to speed up the melting of the alloy. At the same time, glass fragments are added into the furnace during smelting to prevent oxidation of the molten steel, and the slag inside and outside the furnace must be clean.
4. Test results of the mill roll (φ380×650mm)
4.1 Morphology of bonding layer
By observing the high-speed steel roll with the naked eye, it was found that the bonding layer between the outer layer and the core has no obvious physical interface and is basically in a continuous and consistent state. Cut the cross-section sample of the roll and perform metallographic analysis. It is found that the shape of graphite changes from worm-like to spherical from outside to inside. According to the change of graphite shape, the existence of two layers is observed, and the interface is an undulating and irregular shape, indicating that the appearance of the graphite is undulating and irregular. The layer and core achieve good metallurgical bonding.
4.2 Bonding layer flaw detection
In actual production, through rough machining of the roll body section, visually observe the bonding layers at both ends of the roll body to see whether the bonded molten layer is good and whether the working layer is deflected during the core filling process. Use a board ruler to measure the thickness of the working layer to initially judge the quality of the bonding layer, and then conduct further in-depth inspection. An ultrasonic flaw detector was used to conduct flaw detection on the bonding layer, and no defects were found, indicating that the bonding layer is well controlled.
4.3 Mechanical property testing
A sample containing the working layer, bonding layer and core was cut from the roll. The sample was processed into the shape as shown in the figure. The tensile strength of the sample was measured to be 420MPa and 470MPa. The fracture site is located at the root of the sample, indicating that the tensile strength of the bonding layer is greater than this value and greater than the roll neck strength (≥350MPa) requirement.
5. Usage effect of the mill roll
(1) Judging from the test results, the centrifugal composite cast high-speed steel roll (φ380×650mm) produced by this process meets the requirements and solves the problem of roll body peeling during use due to the poor quality of the bonding layer. The working layer peeling rate dropped by 90.2%.
(2) After optimizing the process, the working layer and core of the high-speed steel roll have achieved good metallurgical bonding, and the service life of the roll has been greatly increased.
(1) The centrifugal casting process is used, using high-speed steel as the working layer and ductile iron as the core material to produce high-speed steel rolls.
(2) By controlling the centrifuge stop temperature, core-filling interval time, core-filling molten iron pouring temperature, centrifugal casting process, furnace charge, outer layer molten steel melting conditions and other factors, the quality of the high-speed steel roll bonding layer is improved.
(3) The results of metallographic inspection, flaw detection and bonding layer tensile strength testing show that after optimizing the process, the quality of the roll bonding layer meets the production requirements, and the problem of roll working layer peeling has been solved.