The new heat treatment process of GCr15 steel roll
1 Problems and Analysis in the Use of GCr15 Steel Rolls In the rolling process of thin metal sheets, the rolls are subjected to complex and heavy stresses. With the increase of rolling speed and the development of heavy-duty continuous rolling technology, the surface quality of the rolls themselves The requirements are getting higher and higher, but the modern heat treatment process generally can not meet the requirements on the use of the roll surface pitting, Le, pits, peeling and other phenomena, there are three reasons, namely the level of wear resistance, effective quenching The depth of the hard layer and the size of the crack resistance. These three factors are related to each other and are contradictory to each other. Generally, the common phenomenon of skinning on the surface of the roll is due to the fact that the force applied to the roll is larger and the effective hardened layer is shallower and the hardness of the surface layer is lower. Although the surface hardness is high, the wear resistance can be improved but the crack resistance is poor. The surface skinning phenomenon of the roll body is easy to occur, and the work roll is scrapped early.
If the surface hardness of the roller body is relatively low or the hardness distribution is not uniform, although this can improve the crack resistance but the wear resistance is significantly reduced, pitting, nicks, grid pits, etc. often appear on the surface of the roller body, so that the roller needs to be changed. Heavy grinding seriously affects production efficiency. The cold roll used by Taian Bicycle Parts Factory adopts medium frequency processing, effective hardened layer is shallow, hardness is not uniform, pits, marks, and peeling are easy to occur. The roll can only be maintained for one shift and it needs to be replaced and ground once or even Serious can be scrapped. A large number of scrap rolls cause great waste.
In order to save the used rollers, reduce the production cost and increase the service life of the rollers, we have carried out a lot of practice and explored a new heat treatment process for roll production and return.
2 The renewed heat treatment process of cold roll uses 75kW or 90kW gas carburizing furnace. One end of the roll is welded with a ring (removed after quenching). The furnace is installed by an upside-down method. The workpiece is heated in a vertical position by its own weight to ensure that the bending deformation is minimal when the workpiece is heated. Both ends of the roller are equipped with protective sleeves. The protective sleeve is filled with crushed asbestos clay to prevent cracking at both ends due to chilling during cooling.
During the heating process, kerosene is dripped to prevent oxidative decarburization of the workpiece.
2. 2 Quenching and tempering process Take the 5190mm cold roll as an example. The heat treatment process of GCr15 steel roll production and returning is shown in Figure 1.
2. 2. 1 quenching heating temperature and holding time In order to reduce thermal stress, reduce deformation, prevent cracking, heating with the furnace heating, and to perform preheating, in 600 appropriate to increase the quenching heating temperature of large-section workpiece can be obtained Deeper hardened layer. By the well furnace hanging into the cooling tank, part of the heat loss on the surface of the workpiece, so the quenching heating temperature to a certain height is appropriate. The quenching heating temperature is 85 860 e and the time is 13 hours. The preheating holding time is 1.25 times the holding time of the quenching heating.
2. 2. 2 Selection of Quench Cooling and Cooling Agent After heating and cooling the cold roll, it is suspended in a 5 10 NaCl aqueous solution. The coolant temperature is 20 40 e and the cooling time is 50 55 minutes. During the quenching process, the operation must be rapid, especially from the first cooling tank to the second cooling tank. The residence time in the space must not exceed 5 seconds. The cold roll must be constantly oscillated from left to right and up and down in the cooling tank.
The purpose of the double liquid quenching of the GCr15 steel roll is to make its surface energy obtain high hardness martensite and avoid the risk of cracking. Therefore, in the high temperature zone where the austenite transformation speed is the highest, the 5 10 NaCl aqueous solution with a very high cooling rate is used as the coolant, and in the low temperature zone where the martensite transformation occurs, the 30 35 CaCl aqueous solution is selected as the coolant.
The aqueous solution has a large cooling rate (450 e P sec.) in the high temperature region and a small cooling rate (20 30 e P sec.) in the low temperature region. When the large-section workpiece is quenched and cooled, the temperature difference between the workpiece surface and the core is large. When the surface layer is cooled to a low-temperature region, the core portion may still be in a high-temperature region. With this coolant, it is possible to achieve slow cooling of the martensitic surface while allowing the heart to cool rapidly. If oil is selected as the coolant, the heat transferred from the high temperature zone of the heart to the surface layer cannot be rapidly cooled, and the surface layer will inevitably increase in temperature and decrease its hardness.
2.2.3 Control of Cooling Time The key to double liquid quenching is to precisely control the residence time of the workpiece in the second medium, or the temperature at which the workpiece is transferred from the first medium to the second medium. Because if the residence time in the first medium is too short, the austenite will decompose when slowly cooling after being taken out, so that the purpose of quenching cannot be achieved. If the time is too long, martensite transformation has occurred and the significance of the double liquid quenching has been lost.
According to production practice, the cooling time of the roller can be determined according to the following empirical formula.
In the formula, T) cooling time, D) cold roll diameter.
Large-diameter cold-rolled rolls take the upper limit of a given coefficient, and small-diameter cold-rolled rolls take the lower limit of a given coefficient.
2.2.4 Tempering method When large section work is quenched and cooled, due to the non-penetrating hardening property, the internal stress caused by the non-uniformity of the metallographic structure is extremely large and must be tempered in time, otherwise cracking may occur.
The selection of tempering temperature and holding time is based on the removal of the internal stress and the reduction of the surface hardness obtained.
Tempering temperature is 180 e, time is 20 hours, air cooling.
After tempering, the cold roll was lifted into a boiling water bath for 1 hour. After the slot has to be wiped clean. The Roll Work Section 3 Conclusion We accepted the commission of the Taian Bicycle Parts Factory to perform the practice of returning cold roll rolls. In the treatment process, the double liquid quenching is deeper than the original medium frequency quench hardened layer, and the hardness distribution is even and no waste products appear. The rollers work well in use, have high production efficiency, long service life, and have a lifetime of about 23 times that of intermediate frequency quenching. It has been proved by practice that the process parameters of the roll returning process are appropriate.
Zhao Rengao: New Heat Treatment Process for GCr15 Steel Rolls
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