2020 Volume 61 Issue 10 Pages 1987-1993
High-speed steel type cast iron rolls (HSS-rolls) used in the hot rolling process of steel are composite rolls manufactured by the CPC method or the centrifugal casting method (CCM). Comparing the two, the centrifugal cast HSS-roll is advantageous in terms of production cost, and since the shaft material is ductile cast iron, the thermal crown during rolling is small, and the plate threading property is excellent. For this reason, Centrifugal cast HSS-rolls are often used as standard type HSS-rolls, and some of them are also used in the later stands of the hot strip mills. On the other hand, forged steel is used for the shaft material of CPC-HSS-rolls, and the strength of the shaft material is superior to that of ductile cast iron, so there is very little risk of fracture accident (thermal breakage) from the center. The thermal breakage of the roll is defect that occurs when the tensile residual stress of the shaft and the thermal stress generated during rolling exceed the material strength of the shaft. Therefore, in order to prevent thermal breakage, it is considered extremely effective to suppress the tensile residual stress of the shaft portion. Therefore, in this study, an attempt was made to significantly reduce the tensile residual stress of the shaft by improving the heat treatment conditions. Furthermore, the influence of heat treatment conditions on the compressive residual stress of the outer layer was also investigated. As a result, it was confirmed that by applying the surface rapid heating method to heating conditions before quenching, the tensile residual stress inside the roll can be reduced by about 30% compared to the conventional uniform heating method. Effective results were obtained by deciding the manufacturing conditions for significantly improving safety against thermal breakage in centrifugal cast HSS-rolls.
This Paper was Originally Published in Japanese in J. JFS 92 (2020) 144–150. Figures 10 and 11 were slightly changed.
Fig. 8 Comparison results of residual stress distribution in axial direction.
In the hot rolling process of steel, continuous rolling under high pressure and rolling volume per unit time are increasing due to the high strength of the rolled material. For this reason, there are problems related to wear and deterioration of the roll material and reduction of the replacement frequency of the roll. In response to these problems, there is an increasing demand for quality improvements such as wear resistance, surface deterioration resistance, and crack resistance of hot-rolled roll materials.1–4) Recently developed high-carbon high-speed steel type cast iron rolls (hereinafter referred to as “HSS-rolls”) are cast iron rolls that use alloys similar to high-speed steel. This HSS-roll crystallizes high-hardness and finely dispersed MC-type carbides and eutectic M2C-type carbides in the primary austenite gaps by containing carbide-forming elements (Cr, Mo, W, V, etc.). In addition, it is a high wear-resistant roll in which secondary carbides are finely precipitated in the mother phase by heat treatment.5–9) HSS-rolls are widely applied mainly to the work rolls in the former stands of hot strip mills. Generally, HSS-rolls are composite rolls manufactured by the CPC (Continuous Pouring process for Cladding) method using forged steel as the shaft material or the Centrifugal Cast Method (CCM) using ductile cast iron as the shaft material. Centrifugal cast HSS-rolls (hereinafter referred to as “CCM-HSS-rolls”) are widely used as standard HSS-rolls because they are extremely advantageous in terms of manufacturing cost and can be manufactured with existing centrifugal casting equipment. In addition, CCM-HSS-rolls use ductile cast iron as the shaft material. Therefore, compared with CPC-HSS-rolls, thermal crown during rolling is small and threading property is excellent. For this reason, they are also used in some rolling mills in the latter stands of the hot strip mills. On the other hand, CCM-HSS-rolls have lower shaft strength than CPC-HSS-rolls. For this reason, there is a high risk of fracture accident (thermal breakage) from the center. Thermal breakage is considered to occur when the sum of the residual tensile stress of the shaft and the thermal stress/rolling stress generated during rolling exceeds the material strength of the shaft. Therefore, to prevent the occurrence of thermal breakage, it is considered effective to reduce the tensile residual stress at the shaft. However, the residual stress of HSS-roll is considered to be complicatedly related to transformation stress associated with phase transformation of each material (outer layer and inner layer) during heat treatment, thermal stress due to shrinkage accompanying cooling and creep, etc. For this reason, it is difficult to say that we have a firm grasp on the residual stress in the HSS-roll shaft. On the other hand, the compressive residual stress generated in the outer layer (HSS layer) of HSS-rolls is considered to have a large effect on the progress of cracks generated on the roll surface when a rolling incident called cobble incident occurs during rolling.
Therefore, in this study, we focused on the heating conditions during quenching, which is considered to be one of the factors controlling the residual stress of HSS-rolls. In order to clearly understand the effect of heating conditions during quenching on the residual stress distribution of HSS-rolls, actual rolls with different heating conditions applied during quenching were manufactured, and the residual stress distribution inside the rolls was investigated.
As shown in Fig. 1, HSS-rolls used for work rolls in finishing stands of hot strip mills are composite rolls in which different materials are applied to the part to be rolled (outer layer) and the shaft part (inner layer). Some CCM-HSS-rolls may have an intermediate layer between the outer and inner layers. As shown in Table 1, the HSS-roll material used for rolling rolls is white cast iron using an alloy similar to high-speed steel. Table 2 shows the comparison of the characteristics of CPC-HSS-rolls and CCM-HSS-rolls. When manufacturing a composite roll by combining different materials in the outer and inner layers by metallurgical bonding, it is necessary to combine the materials so that the final solidified part is not formed at the boundary between the outer and inner layers. For this reason, in the case of the CPC method in which the inner layer (shaft material) is solidified first, it is difficult to apply high-speed steel type cast iron material to the outer layer and to apply ductile cast iron material having a lower melting point than the outer layer material to the inner layer. In addition, in the case of the CPC method, the shaft material must be locally heated to the melting temperature of the high-speed steel type cast iron material, and the material must be able to withstand the thermal stress generated at this time. For this reason, forged steel such as SCM is applied to the shaft material of CPC-HSS-rolls. On the other hand, in the case of the centrifugal casting method (CCM), the outer layer must be solidified first, so it is necessary to apply a material with a lower melting point to the inner layer. For this reason, ductile cast iron is usually applied to the inner layer material of CCM-HSS-rolls Ductile cast iron has lower material strength than forged steel. However, when applied to the inner layer material of a hot rolling roll, the thermal crown during rolling is small and the roll has excellent threading properties. Therefore, it is considered necessary to improve the safety against thermal breakage by reducing the residual stress in the inner layer (shaft core) in order to make CCM-HSS-rolls with excellent threading properties to higher quality rolls.
Schematic diagram showing structure of high-speed steel type cast iron rolls for hot strip mills.
Since hot rolling rolls undergo thermal cycles during rolling, an appropriate compressive residual stress is applied to the roll surface to suppress the occurrence of thermal cracks.10) On the other hand, tensile residual stress is generated inside the roll. CCM-HSS-rolls have low material strength of shaft material (ductile iron). Therefore, it has been reported that when a large tensile residual stress occurs in the shaft core, a thermal breakage occurs from the center during rolling. Therefore, it is important to understand the residual stress distribution inside CCM-HSS-rolls. In the case of composite rolls with different materials in the outer layer (rolling layer) and inner layer (shaft) such as HSS-rolls, transformation stress of each material, thermal stress due to shrinkage due to cooling, creep, etc. are generated in the heat treatment process. It is thought that these residual stresses are complicated and the final residual stress is determined. Its theoretical analysis and measurement are considered to be difficult.11) In recent years, analysis results of residual stress distribution of composite rolls have been reported,10,12,13) but there are very few reports on the results of actual measurement of residual stress distribution in actual rolls. Figure 2 schematically shows the transformation characteristics of the outer and inner layers. In the case of HSS-rolls, the inner layer material (ductile cast iron) undergoes pearlite transformation (partially graphite + ferrite precipitates) during quenching and cooling. However, the outer layer (high-speed steel type white cast iron) shrinks to the bainite transformation start temperature, and transformation expansion occurs when the bainite transformation temperature is reached. As one of the factors controlling the tensile residual stress of the inner layer, it is considered that the inner layer shrinkage during quenching has a large effect. In other words, if the inner layer temperature during quenching can be reduced, the amount of thermal shrinkage can be reduced & thereby a reduction the tensile residual stress of the inner layer may be possible. Therefore, by changing the heating conditions at the time of quenching, the temperature distribution inside the roll at the start of quenching was changed and the effect on the residual stress distribution was investigated.
Schematic diagram showing transformation characteristics of high-speed steel type cast iron rolls for hot strip mills.
As a heating method that can reduce the inner layer temperature during quenching, the application of induction heating or heater type electric furnace was examined. First, in order to confirm the effect of the inner layer temperature reduction during quenching on the residual stress, the outer layer part is rapidly heated using a mobile induction heating device that is thought to be able to reduce the inner layer temperature at the beginning of the bainite transformation of the outer layer most. Then, a quenching test was performed in which only the outer layer was heated to the quenching temperature (from 1000 to 1100°C). In addition, the residual stress distribution was compared with the case of heating to the quenching temperature (from 1000 to 1100°C) by the whole uniform heating method using a conventional cart furnace. For safety reasons, considering the thermal stress during heating, we decided to use CPC-HSS-rolls with forged steel applied to the shaft material in the heating test using a mobile induction heating device. The size of the CPC-HSS-rolls used in the test was 615 mm for the outer diameter and about 65 mm for the outer layer thickness. The outer layer materials were compared with the rolls using the same composition HSS material from the range shown in Table 1. The heat treatment conditions after heating up to the quenching temperature were aimed at almost the same conditions, and tempering was completed.
Subsequently, the heat treatment test in CCM-HSS-rolls was performed. Table 3 shows the sizes of the rolls used in this residual stress distribution measurement test. The rolls used for the test were manufactured by the centrifugal casting method (CCM) using the high-speed steel type white cast iron shown in Table 1 for the outer layer material and ductile cast iron for the inner layer (shaft material). In the case of CCM-HSS-rolls, the heat treatment conditions (surface rapid heating method) that can reduce the inner layer temperature at the start of the outer layer bainite transformation were studied considering the material strength of the shaft. As a result, the electric heater furnace shown in Fig. 3 was adopted, which can rapidly heat the entire outer layer of the roll by radiant heat from the surface. The heating conditions before quenching were applied to the surface rapid heating method and the case where the entire roll was heated uniformly at the quenching temperature (overall uniform heating method) using a conventional cart furnace for comparison. When the surface rapid heating method was applied, we measured the temperature at the place in which thermocouples were inserted into the roll in advance to confirm the temperature distribution inside the roll at the start before quenching. We also measured the temperature distribution when the roll surface rose to the maximum temperature (1100°C) that was assumed as the quenching temperature. Figure 4 shows the results of this temperature measurement test. It was confirmed that the temperature of the shaft core was 800°C or less.
Electric heater furnace to ensure surface rapid heating conditions.
Measurement results of temperature distribution before quenching of rolls using surface rapid heating conditions.
After reaching the quenching temperature in each heating method, cooling with a mixed mist of water and air was performed until the outer layer material became below the temperature at which the pearlite nose could be swept after exiting the furnace. Thereafter, it was gradually cooled until the quenching end temperature was reached. During this cooling, the outer layer undergoes bainite transformation and the inner layer undergoes pearlite transformation. Tempering was performed twice after reaching the quenching end temperature.
3.2 Measurement method of residual stress distributionThe Sachs method is known as a conventional method for measuring residual stress, but it is not suitable for long rolling rolls because it requires the entire workpiece to be processed.11,13) Therefore, in recent years, the residual stress was measured using the Disk Method11) devised by Higashida et al. The outline of Disk Method is shown in Fig. 5. First, a 25 mm thick disc is cut out from the center of the body of the roll, and a strain gauge is attached to the side of the disc, and then divided into elements (cubes). Then, using the strain ε(R), ε(θ) measured by this strain gauge and the eqs. (1) to (3) derived from the Axisymmetric elasticity theory of the cylinder, each direction (circumferential direction (θ), radial (R), axial (Z)) stresses in the roll state are estimated.
| \begin{equation} \sigma(R) = \frac{E}{1 - \nu}\left(-\frac{1}{r^{2}}\int\nolimits_{0}^{r}\varepsilon^{*}\,rdr + \frac{1}{b^{2}}\int\nolimits_{0}^{b}\varepsilon^{*}\,rdr\right) \end{equation} | (1) |
| \begin{equation} \sigma(Z) = \frac{E}{1 - \nu}\left(-\varepsilon^{*} + \frac{2}{b^{2}}\int\nolimits_{0}^{b}\varepsilon^{*}\,rdr\right) \end{equation} | (2) |
| \begin{equation} \sigma(\theta) = \frac{E}{1 - \nu}\left(\frac{1}{r^{2}}\int\nolimits_{0}^{r}\varepsilon^{*}\,rdr - \varepsilon^{*} + \frac{1}{b^{2}}\int\nolimits_{0}^{b}\varepsilon^{*}\,rdr\right) \end{equation} | (3) |
| \begin{equation*} \varepsilon^{*} = \frac{[\varepsilon(R) + \varepsilon(\theta)]}{1 - \nu} \end{equation*} |
Residual stress measurement method by Disc Method.
For CPC-HSS-rolls that had been heat-treated under two types of heating conditions, a 25 mm thick and 615 mm diameter disc was cut out from the body of each roll, and the residual stress distribution in the roll state was measured using the Disk Method described above. Figure 6 shows the measurement results of the residual stress distribution when only the outer layer is rapidly heated to the quenching temperature (from 1000 to 1100°C) using a mobile induction heating device. Figure 7 shows the measurement results of residual stress distribution when heating to the quenching temperature (from 1000 to 1100°C) by the whole uniform heating method using a conventional cart furnace. In addition, Fig. 8 shows the results of extracting only the axial residual stress σ(Z), which directly affects the thermal breakage of the roll, for comparison under both heating conditions. As a result, it was found that when the outer layer was heated rapidly, the maximum tensile residual stress in the axial direction in the inner layer (shaft) was reduced by about 35% compared to the conventional overall heating method. The reason is shown in the schematic diagram of Fig. 9. By applying the surface rapid heating method to reduce the inner layer temperature at the start of quenching, the amount of thermal shrinkage of the inner layer from the beginning to the end of quenching was reduced. In addition, the shrinkage of the inner layer after the outer layer expanded by bainite transformation greatly decreased. It is considered that the tensile residual stress in the inner layer was greatly reduced by these effects. It was also found that the residual stress reduction effect near the center of the shaft is significant. The reason for this is thought to be that the temperature at the start of quenching decreases as the outer layer is heated rapidly. On the other hand, the compressive residual stress in the outer layer showed almost the same stress distribution, and no effect of heat treatment conditions (heating conditions before quenching) was observed. It is estimated that the compressive residual stress of the outer layer (HSS layer) is dominated by transformation stress. The compressive residual stress in the outer layer is thought to affect the progress of cobble cracks generated on the roll surface by the rolling incident. Ideally, the compressive residual stress should be reduced to the same level (150–250 MPa) as indefinite chilled rolls (also referred to as high-nickel grain rolls) to improve the rolling incident resistance of the outer layer. However, if it is a precondition to have the same material, and to ensure the same level of hardness, this result suggests that it is difficult to control the compressive residual stress of the outer layer of HSS-rolls by heat treatment conditions.
Residual stress distribution measurement result of the CPC-HSS-roll heat treated under surface rapid heating conditions.
Residual stress distribution measurement result of CPC-HSS-roll heat-treated under conventional uniform heating conditions.
Comparison results of residual stress distribution in axial direction.
Schematic diagram of significant decrease in tensile residual stress in inner layer due to rapid surface heating.
For CCM-HSS-rolls, a disk with a thickness of 25 mm and a diameter shown in Table 3 was cut out from the roll body after the heat treatment was completed, and the residual stress distribution in the roll state was measured by the disk method described above. Figure 10 shows the residual stress distribution of the roll using the uniform heating method. Focusing on the residual stress σ(Z) in the axial direction related to heat bending during rolling, the maximum value is 236 MPa at the center of the roll. Contrarily, Fig. 11 shows the residual stress distribution of the roll using the surface rapid heating method. The residual stress σ(Z) in the axial direction had a more uniform stress distribution than that of the entire uniform heating type roll, and reached a maximum value of 164 MPa in the middle of the inner layer. By applying the surface rapid heating method, it can be confirmed that the axial residual stress σ(Z) affecting the thermal breakage during rolling is reduced by about 30% compared to the conventional uniform heating method. The same tendency as the experimental result of CPC-HSS-rolls was observed, but the effect of reducing the residual stress was slightly less. This is thought to be due to the difference in the internal temperature distribution due to the difference in the heating equipment under rapid surface heating conditions. Since the thermal stress generated during rolling is generated due to the temperature distribution of the roll, it is considered to be the largest at the center of the roll. When the surface rapid heating method was applied, the axial tensile residual stress in the inner layer of the roll showed the maximum value in the middle of the inner layer, similar to the experimental results by CPC-HSS-roll. The thermal stress generated during rolling is maximized at the roll axis, so this is also considered to be extremely effective as a countermeasure against thermal breakage.
Measurement results of residual stress distribution of CCM-HSS-roll heat-treated under conventional uniform heating conditions.
Residual stress distribution measurement result of CCM-HSS-roll heat treated under surface rapid heating conditions.
Hot rolling rolls undergo a thermal cycle during rolling. For this reason, moderate compressive residual stress is applied to the roll surface to suppress the occurrence of thermal cracks. However, the excessive compressive residual stress of the outer layer is thought to affect as to whether or not the cracks generated on the roll surface develop into the roll when a cobble incident occurs.14) On the other hand, there is a report that the compressive residual stress of the roll does not promote crack propagation if it is about 200 MPa.15) Currently, about 30 years have passed since the development of HSS-rolls. However, only a small percentage of CCM-HSS-rolls with excellent threading properties are used in latter stands of hot strip mills where cobble incidents frequently occur during rolling, and application of HSS-rolls has hardly progressed. Therefore, in order to improve the rolling incident resistance of the outer layer of HSS-rolls, it is necessary to reduce the residual stress to the same level (150–250 MPa) as indefinite chilled rolls widely used in latter stands of hot strip mills.
According to the results of CPC-HSS-rolls this time, the compressive residual stress value of the outer layer was almost the same stress distribution (Fig. 8) without being affected by changing the heat treatment conditions. On the other hand, in the results of CCM-HSS-rolls, there were differences between the two heat treatment conditions. As a reason for this, it was estimated that the CCM-HSS-rolls in this test were largely influenced by the fact that only the rolls with different composition systems could be prepared. Since the compressive residual stress of the outer layer was considered to depend on the transformation stress, the relationship between the composition system of the outer layer material and the compressive residual stress of the outer layer surface measured by X-ray was investigated from the production data. It was found that there was a correlation with ΔC (C%-0.24V%) of the outer layer material. ΔC (C%-0.24V%) indicates the amount of carbon remaining when all V is assumed to crystallize into VC, and was used to simply evaluate the amount of matrix in which transformation stress occurs. Figure 12 shows the relationship between ΔC (C%-0.24V%) and the compressive residual stress value on the outer layer surface. The compressive residual stresses on the outer surface of the current HSS-rolls are all over 300 MPa. In addition, the test results using the CCM-HSS-rolls were added to Fig. 12. From these results, it was concluded that the difference in the distribution of compressive residual stress in the outer layer between the different heat treatment conditions of CCM-HSS-rolls was due to the difference in ΔC of the outer layer. Therefore, when combined with the results of CPC-HSS-rolls, if the same material and the same hardness are preconditions, it is considered difficult to control the compressive residual stress value of the outer layer with the heat treatment conditions. On the other hand, the compressive residual stress value of the outer layer that can be controlled by ΔC is 300 MPa or more even at the lowest composition level of the current HSS-rolls material. The outer layer compressive residual stress value of indefinite chilled rolls widely used in latter stands of hot strip mills is 150–250 MPa level. Therefore, the current outer layer compressive residual stress value of HSS-rolls cannot be reduced to the same level as indefinite chilled rolls even when controlled by ΔC.
Relationship between compressive residual stress on surface of HSS-rolls and chemical composition ΔC (C%-0.24V%).
Compared with CPC-HSS-rolls, CCM-HSS-rolls have excellent threading properties due to their small thermal crown during rolling. In this CCM-HSS-rolls, the effect of heat treatment conditions on the residual stress distribution was investigated with the aim of greatly improving the safety against thermal breakage. The following results were obtained.
From the above results, it became possible to significantly improve the safety against thermal breakage of CCM-HSS-rolls by improving the heat treatment conditions. On the other hand, the compressive residual stress generated in the outer layer (HSS layer), which is said to have a significant effect on the progress of cobble cracks generated during a rolling incident, was considered difficult to control under heat treatment conditions. Therefore, it was thought that it was extremely difficult to improve the rolling incident resistance (cobble resistance, suppression of progress of surface cracks) to the level that can be suitably used in the latter stands of hot strip mills in CCM-HSS-rolls. The results suggest that it is necessary to develop a new high wear-resistant roll in order to improve the wear resistance of the roll for latter stands of hot strip mills.
