2023 年 64 巻 2 号 p. 500-505
A model experiment for in-line hot rolling was proposed, and its effectiveness in reducing surface cracking and improving the mechanical properties of cast strips of AC7A Al–Mg alloy was investigated. The strips were cast using an unequal-diameter twin-roll caster at 60 m/min. The strips had a width of 100 mm, and were immediately cut to a length of 200 mm and hot rolled at temperatures of 300, 350, 400, 450 and 500°C. The temperature of the strip surface was measured using a K-type thermocouple. The center area in the width direction of the strips was hot rolled. The thickness of the as-cast strips was 2 mm, and the width of the rolled region was 20 mm. The roll diameter was 70 mm, and the rolling speed was 15 m/min. The thickness reduction caused by hot rolling ranged from 19 to 42%. Surface cracking was reduced by the in-line hot rolling process. The hot-rolled strips were cold rolled down to a thickness of 0.5 mm and then annealed, and tensile tests were conducted. The in-line hot rolling process induced shear deformation of the microstructure. The most appropriate conditions for in-line hot rolling of AC7A were a thickness reduction of 27% and a temperature of 350°C, judging from the tensile strength and elongation results.
An unequal-diameter twin-roll caster (UDTRC) can cast aluminum alloy strips at speeds from 10 to 60 m/min.1–3) In comparison, the casting speed of a conventional twin-roll caster for aluminum alloys (CTRCA) is usually less than 2 m/min.4–6) Thus, the UDTRC exhibits much higher productivity than the CTRCA. Al–Mg alloys show a greater tendency for surface cracking than other aluminum alloys such as Al–Si alloys. These surface defects cannot be removed by cold rolling. Hot rolling by heating the as-cast strip may be useful to overcome this problem. Off-line hot rolling requires heating equipment and prolonged heating times, which lead to increased production cost. The performance of the in-line hot rolling process proposed in the present study was investigated in a model experiment using AC7A, which is an Al–4.8%Mg alloy. Considering the cost of a hot-rolling mill, high-speed roll casting is essential to reduce the cost per strip. The UDTRC is more suitable for in-line hot rolling than the CTRCA with regard to casting speed. However, the effect of in-line hot rolling on the microstructure of cast strips is still not clear. The rolling temperature is higher than the recrystallization temperature, and the flow stress is smaller than that at room temperature. The thickness reduction is also greater than that for cold rolling. The microstructure after in-line hot rolling may be different from that after cold rolling, and improvement of the mechanical properties is expected. In the present study, the effects of the strip temperature and thickness reduction during in-line hot rolling were investigated by tensile tests, and the most appropriate temperature and thickness reduction for AC7A were determined.
A schematic illustration of the model experiment for in-line hot rolling is shown in Fig. 1. The temperature was measured at strip surface. The as-cast strip was cut to 200 mm in length and was then hot rolled at designated temperatures and thickness reduction levels. The rolls in the UDTRC were made from copper and no mold release material was used, in order to increase the casting speed. The diameters of the upper and lower rolls were 300 and 1000 mm, respectively. Both rolls had a width of 100 mm. The roll speed was 60 m/min, and roll loads of 13, 83 and 167 N/mm were used. Tensile testing and measurement of the cooling curves were conducted on a strip cast at 13 N/mm. Strips cast at 13, 83 and 167 N/mm were used to investigate surface cracking. The mill was small and the motor power was not sufficient to roll the full width of the strip. The width of the roll was 20 mm, so that only the center area of the strips was rolled. The thickness of the as-cast strips was 2 mm and the thickness reduction was 19, 27, 36, and 42%. The strip temperature was set at 300, 350, 400, 450 and 500°C. The hot-rolled strips were cold rolled down to a thickness of 0.5 mm, and were then annealed at 360°C for 5.4 ks. A tensile test was conducted on the annealed strips. The gage length and width of the parallel part and the thickness of the test piece were 8, 4 and 0.5 mm, respectively. The chemical composition of the AC7A alloy, which is a typical Al–Mg alloy used for casting, is shown in Table 1. The AC7A was poured into the caster at 720°C. The cooling curve for the as-cast strip was measured before the experiment to determine the maximum temperature for in-line hot rolling.
Schematic illustration showing model experiment for in-line hot rolling.
The cooling curve for an as-cast strip is shown in Fig. 2. The temperature of the strip at 10 and 20 s after leaving the roll was 508 and 482°C, respectively. The time required to cut the strip and insert the cut strip into the mill was less than 10 s. The highest temperature for in-line hot rolling was set at 500°C. In actual equipment, the required distance between the caster and the hot rolling mill is about 20 m. Figure 2 shows that in-line hot rolling at 450°C is feasible.
Cooling curve for as-cast strip. Strip temperature was measured at the strip surface.
The effect of the roll load and temperature during in-line hot rolling on the surface of the strip is shown in Fig. 3. Penetrant inspection was conducted on the surface. The thickness reduction was 42%. Cracks occurred on the surface of the as-cast strip, and the cracking became worse as the roll load of the caster increased. The cracks could be removed by in-line hot rolling under the conditions shown in Fig. 3. The effect of the thickness reduction during in-line hot rolling on the surface of the strip is shown in Fig. 4. The roll load was 13 N/mm. No cracking was observed on the surface. A 19% thickness reduction was sufficient to alleviate the surface cracking that occurred during casting, and new defects did not occur during in-line hot rolling. These results show that the surface condition does not limit the choice of roll load during roll casting or the temperature and thickness reduction during in-line hot rolling.
Effect of roll load of caster and temperature during in-line hot rolling on surface of hot-rolled strip. A penetrant inspection was conducted on the surface. The thickness reduction during hot rolling was 42%.
Effect of thickness reduction during in-line hot rolling on surface condition. The roll load during casting was 13 N/mm.
The microstructure of cross-sections of an as-cast strip, a cold-rolled strip, and a cold-rolled and annealed strip is shown in Fig. 5. A 30% thickness reduction was attained by cold rolling of three times. The annealing condition was 360°C for 5.4 ks. The microstructure of cross-sections of in-line hot-rolled strips is shown in Fig. 6, and that after in-line hot rolling, cold rolling and annealing is shown in Fig. 7. Figure 5 is shown for comparison with Figs. 6 and 7. In the cross-section of the in-line hot-rolled strip, shear deformation occurred for grains near the surface, as shown in Fig. 6, and was greater than that produced by cold rolling, as shown in Fig. 5. There were several important reasons for this. One is that the flow stress at 400 and 500°C was smaller than that at room temperature, and the friction between the strip and roll might be greater for in-line hot rolling than for cold rolling. As shown in Fig. 6, the shear deformation was small for a 19% thickness reduction. For cold rolling, a 30% thickness reduction was attained by cold rolling three times. The sum of these small thickness reductions might not induce large shear deformation near the surface. For these reasons, the shear deformation near the surface produced by in-line hot rolling was greater than that produced by cold rolling. The shear deformation increased with increasing thickness reduction during in-line hot rolling. The shear deformation reached the center in the thickness direction when the temperature was 400 and 500°C for a thickness reduction of 27%.
Cross-sections of as-cast strip, cold-rolled strip and cold-rolled and annealed strip. A 30% thickness reduction was attained by cold rolling of three times. Annealing: 360°C for 5.4 ks.
Cross-sections of in-line hot rolled strips.
Cross-sections of strips after in-line hot rolling, cold rolling and annealing. Annealing: 360°C for 5.4 ks.
As shown in Fig. 6, recrystallization occurred at temperatures and thickness reductions of 400°C and 36%, and 500°C and 27%, respectively. The higher temperature of the strip and the induced strain caused this recrystallization. Grain growth occurred for temperature and thickness reduction values of 400°C and 42%, and 500°C and 36%, respectively. The thickness reduction at which recrystallization and grain growth occurred decreased as the temperature increased. It seems that the larger the reduction is, the smaller the percentage of scattered coarse crystals becomes. It also seems that the larger the reduction is, the clearer the grain boundaries are. The grain boundaries are clear evidence that recrystallization has been completed. It was not clear whether recrystallization and grain growth occurred immediately after in-line hot rolling or some time later.
The microstructure became uniform and very fine in the thickness direction after in-line hot rolling, cold rolling and annealing, as shown in Fig. 7. However, the influence of temperature and thickness reduction during in-line hot rolling is not clear from this figure.
3.4 Results of tensile testingThe tensile strength and elongation of test samples subjected to different treatments are shown in Figs. 8 and 9, respectively. The results of annealing after cold rolling, of in-line hot rolling alone, and of annealing after in-line hot rolling and cold rolling are shown. It is concluded that both the tensile strength and elongation for in-line hot rolled strips were inferior to those for cold-rolled and annealed strips. For the in-line hot-rolled strips, a temperature at which the tensile strength became lower than that at lower and higher temperatures existed. For example, when the thickness reduction was 27, 35 and 42%, the temperature where this occurred was 350, 350 and 400°C, respectively. The temperature at which the tensile strength was a minimum appeared to increase as the thickness reduction increased. The magnitude of the drop in tensile strength increased as the thickness reduction increased. The dependence of the elongation on temperature was different from that for the tensile strength. This shows that the minimum in the tensile strength curve was not influenced by the elongation. The cause of this minimum is not clear at present. There was a rough tendency for the elongation to increase with increasing temperature. The elongation increased from 19 to 36% thickness reduction, and decreased for 47% thickness reduction.
Effect of temperature and thickness reduction during hot rolling on tensile strength. Roll load during casting was 13 N/mm.
Effect of temperature and reduction during hot rolling on elongation. Roll load during casting was 13 N/mm.
The tensile strength after in-line hot rolling, cold rolling and annealing was higher than that after in-line hot rolling alone. For thickness reductions of 19 and 27%, the tensile strength after in-line hot rolling, cold rolling and annealing was higher than that after cold rolling and annealing at some temperatures. The tensile strength, at the temperature at which the tensile strength was lower, was not lower but rather than higher than that at other temperature. A detailed investigation is required to clarify the reason for these results. When the thickness reduction was 36 or 42%, the tensile strength after in-line hot rolling, cold rolling and annealing was lower than that after cold rolling and annealing. The thickness reduction during cold rolling increased with that during in-line hot rolling. It is possible that the tensile strength increases as the thickness reduction during cold rolling increases.
The elongation of the strips after in-line hot rolling, cold rolling and annealing was superior to that after cold rolling and annealing. No correlation could be found between the elongation of the strips after in-line hot rolling, cold rolling and annealing and that after in-line hot rolling alone. No influence of the temperature and thickness reduction during in-line hot rolling on the elongation was observed.
From Figs. 8 and 9, it was judged that the most appropriate conditions for in-line hot rolling of the AC7A alloy are a thickness reduction of 27% and a temperature of 350°C.
