JPH0241566B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention is applied to the production of composite rolls used for rolling. BACKGROUND TECHNOLOGY The property most required for rolls is wear resistance, and it is effective to use hard martensite as the base structure. Therefore, materials with good hardenability are widely used. On the other hand, it is also necessary to prevent accidents such as cracking during use, so a composite type with an inner layer of pearlite base structure that has few internal defects and excellent toughness is often used to provide both wear resistance and accident resistance. . In the heat treatment of this type of roll, quenching is generally performed to form the aforementioned martensitic base structure. Problems to be Solved by the Invention In order to fully exhibit wear resistance, it is desirable that the base structure be completely martensite. In general, martensitic transformation starts at the Ms point when the quenching cooling rate is sufficient to prevent pearlite transformation, that is, the critical cooling rate, the amount of transformation increases as the temperature decreases, and martensite becomes completely uniform at the Mf point. Become an organization. However, since the Mf point of many materials is below 0°C, normal quenching and cooling to room temperature does not completely convert them to martensite, leaving some austenite remaining. Austenite is extremely soft compared to martensite and impairs wear resistance. Therefore, sub-zero processing is sometimes performed, but sub-zero processing requires a refrigerant such as dry ice, which is expensive, and requires special cooling equipment to lower the temperature. In the case of steel rolling rolls, the maximum is 70 tons.
It can be said that the work is extremely difficult due to the large unit weight. On the other hand, as shown in FIG. 2, martensitic transformation is accompanied by significant volumetric expansion, so internal stress is generated in a composite roll with an inner layer of pearlite structure that does not undergo expansion. That is, compressive stress is generated in the outer layer that undergoes martensitic transformation and expansion, and tensile stress is generated in the inner layer that does not expand. Therefore, if there is too much martensitic transformation, a large tensile stress will be applied to the inner layer and cracks will occur. Therefore, the present invention aims to provide a simple heat treatment method for this type of composite roll to prevent cracking due to internal stress and to completely convert the outer layer to martensite. Means for Solving the Problems The present invention provides a method for heat treating a composite roll consisting of an outer layer having a martensitic matrix structure and an inner layer having a pearlite matrix structure, from the martensitic transformation start temperature (Ms point) during quenching and cooling. , cooling is interrupted at room temperature or within the temperature range above room temperature up to the martensitic transformation end temperature (Mf point),
This is a heat treatment method for a composite roll, which is characterized in that quenching is performed at least once by reheating to a quenching temperature of 400 to 600°C, followed by quenching cooling, and then tempering is performed. This not only promotes the martensitic transformation of the outer layer, but also removes the residual stress caused by the martensitic transformation that occurred when the heating first cooled down, making it possible to reduce the internal stress and residual stress during heat treatment with the same amount of martensite. Can prevent cracking. Effect In the present invention, the outer layer is limited to a material having a martensite base structure because martensite is the hardest and most effective material compared to pearlite and bainite in terms of wear resistance, and the transformation is completed at high temperatures. This is because for pearlite, the low-temperature heat treatment operation that is a feature of the present invention has almost no effect on the final matrix structure. The reason why the inner layer is limited to a material having a pearlite base structure is that as the outer layer undergoes martensitic transformation and expansion, the pearlite base inner layer, which does not expand, contracts relatively and generates tensile stress, which poses a risk of cracking. This is because the stress reduction effect of the present invention becomes effective. As mentioned above, during quenching cooling, cooling is interrupted during quenching within the temperature range from the martensitic transformation start temperature (Ms point) to the martensitic transformation end temperature (Mf point) at or above room temperature. As shown, the amount of martensitic transformation in the outer layer is too large,
The amount of martensitic transformation is suppressed in order to prevent cracks from occurring due to a large expansion difference between the outer layer and the inner layer. Next, heating at 400 to 600°C is performed to activate the austenite remaining during the first quenching and cooling, raise the Ms point during the subsequent cooling, and perform martensitic transformation at a relatively high temperature. (edited by the Japan Iron and Steel Institute, "Heat Treatment of Steel" revised 5th edition)
(See page 67). Therefore, in the case of a material with an Mf point of 0°C or lower, austenite will necessarily remain after one quenching cooling to atmospheric temperature, but this heating will not complete the martensitic transformation even at temperatures above room temperature. It becomes possible. Therefore, it is used in some cases to heat treat single materials such as high-speed tool steels simply to increase their hardness. However, this heating temperature has a stress-relieving effect, and this is used to reduce the internal stress of the composite roll that was generated by the expansion of martensite in the outer layer due to the first quenching, and then martensitic transformation expansion is performed again. Ultimately, even with the same amount of martensite, it is possible to reduce internal stress and residual stress during heat treatment, thereby significantly reducing the risk of cracking in the composite roll. If the heating temperature is too low, the effect of activating the remaining austenite is small, martensitic transformation cannot be achieved at the high temperature side during subsequent cooling, and the effect of stress relief is also small. Even lower temperature 100~200℃
On the contrary, austenite becomes stable.
On the other hand, if the heating temperature is too high, the remaining austenite transforms into pearlite, and the initially formed martensite is also tempered and softened, resulting in a decrease in hardness. Of course, in this case no martensitic transformation occurs during subsequent cooling. Above, we have described the case where the martensitic transformation of the outer layer is performed in two steps, but when the material has a large martensitic transformation expansion, the outer layer cross-sectional area ratio is large, and a large internal stress is expected to occur. , by increasing the number of divisions and repeating this process, the effect will increase. Example: High chromium cast iron on the outer layer as a steel rolling roll.
Ductile cast iron was used for the inner layer, and a composite roll was manufactured under the conditions shown in Table 1. High chromium cast iron
A material whose main component is 10 to 25% Cr and other C and other appropriate alloying elements, and it is easy to obtain high hardness and has excellent wear resistance due to its hard chromium carbide and good hardenability. It is generally manufactured as a composite type with an inner layer of ductile cast iron or high-grade cast iron. Of course, the outer layer in this case has a martensite base structure, and the inner layer has a pearlite base structure. The conventional method and the example have the same shape and the same material, but in the conventional method, cracking occurred due to excessive internal stress during heat treatment despite the hardness (Hsc) of 85, whereas in the example, the hardness (Hsc) was 85. Hsc) 91.5
The effect of this method is remarkable, with no cracking occurring. FIG. 3 shows the thermal expansion (contraction) during quenching and cooling of the outer layer employed in the example. In the case of this material, the Ms point is 220°C, and expansion due to martensitic transformation is observed at temperatures below this temperature. When cooling was interrupted at 80°C, heated to 500°C, and then cooled again, martensitic transformation restarted at 150°C, which is 70°C higher than the temperature at which cooling was interrupted, and the transformation was completed at 80°C. This shows that the martensitic transformation progresses in parts, and that the second transformation occurs on the high temperature side, and that the transformation is finally completed above room temperature. Although the embodiment is an example in which the martensitic transformation is divided into two steps, it is easy to understand that the same effect can be obtained even if the martensitic transformation is divided into more steps. In both the example and the conventional example, after the martensitic transformation was completed, tempering was performed for the purpose of stabilizing the structure and removing stress.

[Table] Effects of the Invention The present invention has the following remarkable effects. The outer layer of martensitic matrix structure, which has high hardness and wear resistance, which are most required for rolls, can be used in rolls without internal defects, resulting in high quality with excellent wear resistance and long life. We can manufacture rolls. There is no need for large-scale and expensive sub-zero processing using a refrigerant such as dry ice, which involves quenching and cooling to the martensitic transformation end temperature (Mf point) below 0°C. Therefore, heat treatment is easy and can be done at low cost.

[Brief explanation of drawings]

FIG. 1 is a sectional view schematically showing the structure of a composite roll according to the present invention. FIG. 2 is a schematic diagram showing the expansion (contraction) of the outer layer and inner layer during quenching and cooling. FIG. 3 is a diagram showing the expansion (contraction) characteristics of the outer layer in an example according to the present method. FIG. 4 is a diagram showing the heat treatment curve of the conventional method shown in Table 1, and FIG. 5 is a diagram showing the heat treatment curve of the example shown in Table 1. 1...outer layer, 2...inner layer, 3...cooling, Ms point...martensite transformation start temperature, Mf point...martensite transformation end temperature.

Claims (1)

[Claims] 1. During heat treatment of a composite roll consisting of an outer layer with a martensitic matrix structure and an inner layer with a pearlite matrix structure, during quenching and cooling, the martensitic transformation start temperature (Ms point) reaches room temperature or above room temperature, where the martensitic transformation ends. Temperature (Mf point)
Interrupt cooling within the temperature range up to 400-600℃
A method for heat treatment of a composite roll, comprising: reheating to a quenching temperature, followed by quenching and cooling at least once, followed by tempering.