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Effect of hot rolling conditions to produce deep drawing quality steels for continuous annealing process
Journal of Materials Processing Technology 170 (2005) 17–23
Effect of hot rolling conditions to produce deep drawing quality steels for continuous annealing process G. Erdem a,∗ , Y. Taptik b b
a Eregli Iron and Steel Works Co., Quality and Technology Department, Kdz. Eregli, Turkey Istanbul Technical University, Chemical and Metallurgical Faculty, Metallurgical and Materials Engineering Department, 80626 Maslak, Istanbul, Turkey
Received 14 July 2003; received in revised form 15 February 2005; accepted 11 April 2005
Abstract Most of cold rolled deep drawability steel sheets finds application in areas where forming is critical. Deep drawing sheet steel requires good mechanical properties and microstructures, as well as sufficient anti-aging characteristics. Therefore, deep drawing steels are produced by controlling these metallurgical factors and optimizing steelmaking, hot rolling, cold rolling and annealing processes. This paper explains the effect the microstructure and mechanical properties of hot rolling conditions on continuously annealed sheet properties in certain chemical composition. Experiments have been conducted to investigate the effect of hot rolling conditions. Consequently, it became clear that hot rolling conditions play an important role in producing continuous annealed deep drawing quality steels. © 2005 Elsevier B.V. All rights reserved. Keywords: Hot rolling condition; Deep drawing steel; Annealing process
1. Introduction Cold rolled steel sheet remains the most important material for manufacture of numerous products. Cold rolled steel sheet combining uniformity of mechanical properties and low levels of surface contamination following the annealing process is desired to meet increasingly stringent product quality requirements. There are two processes for annealing cold rolled sheet steels, batch annealing and continuous annealing [1]. Continuous annealing process for cold rolled steel sheets greatly increases the productivity and improves the uniformity of products. Various kinds of new continuous annealing processes have been developed to produce steel sheet having the same quality as in the batch-annealed product. However, in order for continuously annealed steel to be competitive with the batch-annealed product, the structure must be optimized with regard to three important factors [2]: 1. Development of strong (1 1 1) annealing texture components to enhance deep drawability. ∗
Corresponding author.
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.04.097
2. Achievement of adequately large grain size to confer a low yield stress and good work hardening characteristics. 3. Control of secondary carbide precipitation to control aging tendency and improve ductility. All these factors are being influenced in all stages from steelmaking, hot rolling, cold rolling and final annealing. In order to have good mechanical properties and microstructure, each production stage has to be optimized. It is clear that continuous annealing process is very sensitive to impurity levels in steel. Therefore, it is very important to adjust the steel composition of continuous annealed Alkilled steel. Also, hot rolling conditions are very effective for the improvement of microstructure and mechanical properties. Softening and improvement in deep drawability are attributed to carbide coarsening and ferrite grain growth in hot band. It has been clarified that coiling hot bands at a high temperature is effective for the improvement in deep drawability of continuous annealed sheet steels [3]. Therefore, high temperature coiled hot bands have been cold rolled in tandem mill and annealed in continuous annealing line. The normal procedure for compensating heat losses is to control the water
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
18
Table 1 The chemical analysis of the steels used (wt%) Trial no. C% I
Mn% P%
S%
Si%
Al%
N%
O%
0.035 0.250 0.012 0.010 0.025 0.045 0.0078 0.0058
II A B
0.025 0.250 0.014 0.016 0.010 0.045 0.0070 0.0050 0.040 0.270 0.017 0.010 0.011 0.059 0.0073 0.0035
A B C D
0.040 0.050 0.050 0.025
III
IV A B
0.240 0.280 0.320 0.250
0.012 0.014 0.012 0.014
0.009 0.014 0.010 0.016
0.010 0.013 0.014 0.010
0.059 0.049 0.055 0.045
0.0058 0.0059 0.0069 0.0059
0.0045 0.0081 0.0062 0.0060
0.050 0.240 0.010 0.010 0.015 0.064 0.0070 0.0060 0.040 0.290 0.017 0.017 0.014 0.061 0.0073 0.0063
cooling on the run-out table so that the ends of the strip are kept hotter than middle [4]. In the present study, the effect of hot rolling conditions on deep drawing Al-killed steels was investigated. The effects of mechanical properties and microstructure of these steels were determined.
2. Materials and experimental procedures The materials used in this investigation were taken from continuously cast slabs. Table 1 shows the chemical analysis of the materials used in this study. The deep drawing quality Al-killed steel slabs were rolled at hot strip mill No. 2 at Erdemir. These slabs with the thickness of 200 mm and width of 940–1225 mm and length of 12,000 mm were heated to 1100 and 1200 ◦ C in slab reheating furnace (slab exit temperature). The difference in slab reheating temperature is related
to the variation in finishing temperatures obtained. Roughing was carried out on 2 Hi (one pass) and 4 Hi reversing rougher (five passes). The exit temperature after rougher was about 1000–1100 ◦ C. The seven-stand finishing mill was used for the further hot rolling process. The mill is equipped with a coil box which allows to have homogeneous temperatures throughout the coil and prevents heat losses from head and tail of the strip [3]. The temperature difference between the edges and center are reduced with the coil box. The schematic illustration of experimental procedure is shown in Fig. 1. After hot rolling both surfaces of steels were inspected in recoiling line. All materials were hot rolled to 2.80 mm thickness in the same reduction schedule and specimens were taken from hot rolled coils at 10, 20, 40 and 80 m from the head end of the coil, middle of the coil and at 10, 20, 40 and 80 m of tail end from the coil in the recoiling line. Hot rolled specimens based on Japan International Standard (JIS—width 50 mm; length 50 mm) were machined for tensile testing to determine the yield stress (YS) at 0.2% elongation, tensile strength (TS) and elongation (El). Ferrite grain size and carbide morphology of hot rolled specimens were determined. The tensile properties were determined by SATEC Systems (UTM type, 55 tonnes, MII-120 HVL Model) and optical metallography was conducted in LECO 2001 Image Analyzing System.
3. Results and discussion 3.1. Microstructural observation The effects of finishing and coiling temperatures on the microstructure in the hot band sheet steel are shown in Fig. 2. In order to get the data for microstructure, 63 microstructures
Fig. 1. Schematic illustration of the experimental procedure.
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
19
Fig. 2. Effects of finishing and coiling temperatures on the microstructure of the hot band sheet steel.
were analyzed for each trial. The microstructures shown in Fig. 2 are the representative microstructures of trials. As expected [5,6], the change in the grain size is dependent on finishing temperature and, on the other hand, the morphology of cementite is strongly dependent on coiling temperature. The ferrite grain size decreases with decreasing finishing temperature and mixed grain structure is obtained when hot rolling was conducted at a temperature below Ar3 [7,8]. Trial I has a relatively coarse grain structure and resulted in mixed grain sizes. When the hot bands were rolled at 930 ◦ C (finishing temperature), large sizes on ferrite grain were observed. The flaky cementite particles were obtained along grain boundaries with increasing coiling temperature. On the other hand, when hot bands were coiled at 730 ◦ C, very coarse cementite particles were observed. Trial IV (B) has coarse cementite particles and coarse grain structure due to high finishing and coiling temperatures. This diagram also shows the relation between change in microstructure of hot rolled sheet steel and the rolling process parameters. Effects of finishing and coiling temperatures for the ferrite grain and carbide size are shown in Figs. 3 and 4. For plotting the grain size and carbide index, a mean grain size and carbide index were calculated. It can be seen in Fig. 2
that ferrite ASTM No. decreases with increasing finishing and coiling temperatures. In Fig. 3, Carbide Index No.1 decreases with increasing finishing and coiling temperatures. Carbide index number is similar to ferrite ASTM number. Carbide size increases with decreasing index number. According to 700 ◦ C annealing temperature in CAL process, hot band microstructure with high coiling temperature and finishing temperature (Trial IV, B) resulted in the final annealed microstructure shown in Fig. 5. After hot rolling both surfaces of steels were inspected in recoiling line and then they were pickled in HCl acid (18%, 65 ◦ C, 240 m/min) in order to remove the surface oxides. Because of the high coiling temperature for these steels, tension leveller was used before the pickling to ensure full-scale removal. The coils were rolled to a final gauge of 0.80 mm on four-stand tandem cold rolling mill and the amount of deformation in cold rolling process is 71%. It can be seen that sufficient recrystallization is completed in case of these process conditions. Finally, the full hard materials were annealed in continuous annealing line after electrolytic cleaning process. Fig. 1 shows the schematic illustration of experimental procedure.
1
It is based on NKK CAL Process Knowhow Book, January 1995.
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
20
Fig. 4. Effect of finishing temperature and coiling temperature on carbide size. Fig. 3. Effect of finishing temperature and coiling temperature on ferrite grain size.
3.2. Mechanical properties The mechanical properties of steels after rolling in the hot roll mill are given in Fig. 6. In this figure, it can be concluded that the decrease in finishing temperature causes an increase in yield strength and tensile strength but, results in a decrease in elongation. The increase in the coiling temperature causes the decrease in yield strength and tensile strength but results in the increase in elongation.
The requirement for high coiling temperature of hot rolled strip means that special handling is necessary to counteract heat losses from the inner and outer end. Excessive cooling leads to undesirable carbide morphologies and finely dispersed aluminium nitride precipitation which degrades the formability and heterogeneous mechanical properties in the final product. Fig. 7 shows the application of high finishing and coiling temperature with no water cooling 80 m from head and tail ends of the coil. The data in Fig. 7 are corresponding to the average values of the coils used in Fig. 6. Heat losses
Fig. 5. Annealed microstructure of hot band with high coiling and finishing temperature: (a) ferrite grain size (100×) and (b) carbide size (500×).
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
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Fig. 6. The effect of finishing temperature for (a) yield strength, (b) tensile strength and (c) elongation. The effect of coiling temperature for (d) yield strength, (e) tensile strength and (f) elongation.
are severe at the head and tail end of the coil because of heat radiation. This excessive cooling leads to undesirable carbide morphologies and finely dispersed aluminium nitride precipitation which degrades the formability of the final product at these positions [9]. By applying high finishing and coiling temperature with no water, the coarsening of carbide and the growth of ferrite grain were achieved. The large grain size of hot band aids the grain growth after cold rolling and
annealing. The coarsening of carbide was found to contribute greatly to the grain growth by weakening the dragging effect of the precipitates. Although the middle of the coil has of a lower temperature than the head or tail of the coil, it is possible to uniform properties when the coil was cooled because heat losses are severe at the head and tail of coil. The mechanical tests were performed after the coil was cooled. The temperature in Fig. 6
22
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
Fig. 7. The change of yield strength (a and d), tensile strength (b and e) and elongation, (c and f) depending on finishing and coiling temperature for whole coil length. (O) Middle of coil, (B) head of coil and (S) tail of coil.
correspond to the average values is used for obtaining Fig. 7. Therefore, uniform mechanical properties for the whole coil length were observed as seen in Fig. 7. Besides, the temperature difference between the edges and center was reduced to 10–20 ◦ C from 40 to 50 ◦ C with coil box. Although mechanical properties of continuously annealed products are largely affected by a heating temperature, the mechanical properties of continuously annealed cold rolled
low carbon Al-killed steel sheets which are finished and coiled at high temperature in hot rolling lie within range of 210–230 N/mm2 for yield strength, 310–330 N/mm2 for tensile strength, 45–46% for elongation and 1.4–1.6 for r mean value at 700 ◦ C constant heating temperature. Therefore, mechanical properties with low yield and tensile strength, and high r mean value and elongation for continuously annealed steel are obtained.
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
23
4. Conclusion
References
Using deep drawing 0.025–0.050% C Al-killed steels, the effects of hot rolling conditions on the mechanical properties and microstructure were investigated. The following results have been obtained:
[1] H. Katoh, H. Takachi, N. Takahashi, M. Abe, Cold-rolled steel sheets produced by continuous annealing, Technol. Continuously Annealed Cold-Rolled Steel (1984) 37–38. [2] N. Mizui, A. Okamoto, Effects of manganese content and hotband coiling temperature on deep drawability of continuous-annealed Al-killed sheet steels, Dev. Annealing Sheet Steels (1992) 247– 259. [3] B. Seter, U. Bergstr¨om, W.B. Hutchinson, Extra deep drawing quality steels by continuous annealing, Technol. Continuously Annealed Cold-Rolled Steel (1984) 111–112. [4] K. Matsudo, K. Osawa, K. Kurihara, Metallurgical aspects of the development of continuous annealing technology at Nippon Kokan, Technol. Continuously Annealed Cold-Rolled Steel (1984) 3– 36. [5] F. Ala, B. Atesli, C. Yesil, G. Erdem, O. Baydar, Development and design of Ti-containing interstitial free steels at Eregli iron and steel works, Mod. LC ULC Sheet Steels Cold Forming: Process. Prop. 2 (1998) 461–472. [6] L.E. Samuels, Optical microscopy of carbon steels, Am. Soc. Met. (1992) 81. [7] N. Mizui, A. Okamot, Effects of manganese content and hotband coiling temperature on deep drawability of continuous-annealed Al-killed sheet steels, Dev. Annealing Sheet Steels (1991) 247– 259. [8] O. Kwon, G. Kim, R.W. Chang, Effect of hot rolling conditions on the mechanical properties of cold rolled and annealed ultra-low carbon steels, Metall. Vac.-Degassed Steel Prod. (1990) 215–228. [9] F.H. Samuel, S. Yue, J.J. Jonas, Recrystalization characteristics of a Ti-containing interstitial-free steel during hot rolling, Metall. Vac.Degassed Steel Prod. (1990) 395–413.
(1) Hot rolled sheet steels for continuous annealing process with excellent formability can be produced a high coiling and finishing temperatures. Large ferrite grain sizes and coarse cementite particles were obtained with 930 ◦ C finishing temperature and 730 ◦ C coiling temperature. (2) The effect of finishing temperature and coiling temperature on hot rolled sheet steels microstructure in order to produce hot rolled sheet steels for continuous annealing process was developed. (3) A uniform mechanical property and microstructure throughout the coil for hot rolled material were achieved. (4) It is shown that with increasing finishing and coiling temperature, the yield strength and tensile strength decrease but elongation increases for hot rolled materials.
Acknowledgement The authors would like to thank the Eregli Iron and Steel Works for permission to study the present work and providing the materials.
Effect of hot rolling conditions to produce deep drawing quality steels for continuous annealing process G. Erdem a,∗ , Y. Taptik b b
a Eregli Iron and Steel Works Co., Quality and Technology Department, Kdz. Eregli, Turkey Istanbul Technical University, Chemical and Metallurgical Faculty, Metallurgical and Materials Engineering Department, 80626 Maslak, Istanbul, Turkey
Received 14 July 2003; received in revised form 15 February 2005; accepted 11 April 2005
Abstract Most of cold rolled deep drawability steel sheets finds application in areas where forming is critical. Deep drawing sheet steel requires good mechanical properties and microstructures, as well as sufficient anti-aging characteristics. Therefore, deep drawing steels are produced by controlling these metallurgical factors and optimizing steelmaking, hot rolling, cold rolling and annealing processes. This paper explains the effect the microstructure and mechanical properties of hot rolling conditions on continuously annealed sheet properties in certain chemical composition. Experiments have been conducted to investigate the effect of hot rolling conditions. Consequently, it became clear that hot rolling conditions play an important role in producing continuous annealed deep drawing quality steels. © 2005 Elsevier B.V. All rights reserved. Keywords: Hot rolling condition; Deep drawing steel; Annealing process
1. Introduction Cold rolled steel sheet remains the most important material for manufacture of numerous products. Cold rolled steel sheet combining uniformity of mechanical properties and low levels of surface contamination following the annealing process is desired to meet increasingly stringent product quality requirements. There are two processes for annealing cold rolled sheet steels, batch annealing and continuous annealing [1]. Continuous annealing process for cold rolled steel sheets greatly increases the productivity and improves the uniformity of products. Various kinds of new continuous annealing processes have been developed to produce steel sheet having the same quality as in the batch-annealed product. However, in order for continuously annealed steel to be competitive with the batch-annealed product, the structure must be optimized with regard to three important factors [2]: 1. Development of strong (1 1 1) annealing texture components to enhance deep drawability. ∗
Corresponding author.
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.04.097
2. Achievement of adequately large grain size to confer a low yield stress and good work hardening characteristics. 3. Control of secondary carbide precipitation to control aging tendency and improve ductility. All these factors are being influenced in all stages from steelmaking, hot rolling, cold rolling and final annealing. In order to have good mechanical properties and microstructure, each production stage has to be optimized. It is clear that continuous annealing process is very sensitive to impurity levels in steel. Therefore, it is very important to adjust the steel composition of continuous annealed Alkilled steel. Also, hot rolling conditions are very effective for the improvement of microstructure and mechanical properties. Softening and improvement in deep drawability are attributed to carbide coarsening and ferrite grain growth in hot band. It has been clarified that coiling hot bands at a high temperature is effective for the improvement in deep drawability of continuous annealed sheet steels [3]. Therefore, high temperature coiled hot bands have been cold rolled in tandem mill and annealed in continuous annealing line. The normal procedure for compensating heat losses is to control the water
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
18
Table 1 The chemical analysis of the steels used (wt%) Trial no. C% I
Mn% P%
S%
Si%
Al%
N%
O%
0.035 0.250 0.012 0.010 0.025 0.045 0.0078 0.0058
II A B
0.025 0.250 0.014 0.016 0.010 0.045 0.0070 0.0050 0.040 0.270 0.017 0.010 0.011 0.059 0.0073 0.0035
A B C D
0.040 0.050 0.050 0.025
III
IV A B
0.240 0.280 0.320 0.250
0.012 0.014 0.012 0.014
0.009 0.014 0.010 0.016
0.010 0.013 0.014 0.010
0.059 0.049 0.055 0.045
0.0058 0.0059 0.0069 0.0059
0.0045 0.0081 0.0062 0.0060
0.050 0.240 0.010 0.010 0.015 0.064 0.0070 0.0060 0.040 0.290 0.017 0.017 0.014 0.061 0.0073 0.0063
cooling on the run-out table so that the ends of the strip are kept hotter than middle [4]. In the present study, the effect of hot rolling conditions on deep drawing Al-killed steels was investigated. The effects of mechanical properties and microstructure of these steels were determined.
2. Materials and experimental procedures The materials used in this investigation were taken from continuously cast slabs. Table 1 shows the chemical analysis of the materials used in this study. The deep drawing quality Al-killed steel slabs were rolled at hot strip mill No. 2 at Erdemir. These slabs with the thickness of 200 mm and width of 940–1225 mm and length of 12,000 mm were heated to 1100 and 1200 ◦ C in slab reheating furnace (slab exit temperature). The difference in slab reheating temperature is related
to the variation in finishing temperatures obtained. Roughing was carried out on 2 Hi (one pass) and 4 Hi reversing rougher (five passes). The exit temperature after rougher was about 1000–1100 ◦ C. The seven-stand finishing mill was used for the further hot rolling process. The mill is equipped with a coil box which allows to have homogeneous temperatures throughout the coil and prevents heat losses from head and tail of the strip [3]. The temperature difference between the edges and center are reduced with the coil box. The schematic illustration of experimental procedure is shown in Fig. 1. After hot rolling both surfaces of steels were inspected in recoiling line. All materials were hot rolled to 2.80 mm thickness in the same reduction schedule and specimens were taken from hot rolled coils at 10, 20, 40 and 80 m from the head end of the coil, middle of the coil and at 10, 20, 40 and 80 m of tail end from the coil in the recoiling line. Hot rolled specimens based on Japan International Standard (JIS—width 50 mm; length 50 mm) were machined for tensile testing to determine the yield stress (YS) at 0.2% elongation, tensile strength (TS) and elongation (El). Ferrite grain size and carbide morphology of hot rolled specimens were determined. The tensile properties were determined by SATEC Systems (UTM type, 55 tonnes, MII-120 HVL Model) and optical metallography was conducted in LECO 2001 Image Analyzing System.
3. Results and discussion 3.1. Microstructural observation The effects of finishing and coiling temperatures on the microstructure in the hot band sheet steel are shown in Fig. 2. In order to get the data for microstructure, 63 microstructures
Fig. 1. Schematic illustration of the experimental procedure.
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
19
Fig. 2. Effects of finishing and coiling temperatures on the microstructure of the hot band sheet steel.
were analyzed for each trial. The microstructures shown in Fig. 2 are the representative microstructures of trials. As expected [5,6], the change in the grain size is dependent on finishing temperature and, on the other hand, the morphology of cementite is strongly dependent on coiling temperature. The ferrite grain size decreases with decreasing finishing temperature and mixed grain structure is obtained when hot rolling was conducted at a temperature below Ar3 [7,8]. Trial I has a relatively coarse grain structure and resulted in mixed grain sizes. When the hot bands were rolled at 930 ◦ C (finishing temperature), large sizes on ferrite grain were observed. The flaky cementite particles were obtained along grain boundaries with increasing coiling temperature. On the other hand, when hot bands were coiled at 730 ◦ C, very coarse cementite particles were observed. Trial IV (B) has coarse cementite particles and coarse grain structure due to high finishing and coiling temperatures. This diagram also shows the relation between change in microstructure of hot rolled sheet steel and the rolling process parameters. Effects of finishing and coiling temperatures for the ferrite grain and carbide size are shown in Figs. 3 and 4. For plotting the grain size and carbide index, a mean grain size and carbide index were calculated. It can be seen in Fig. 2
that ferrite ASTM No. decreases with increasing finishing and coiling temperatures. In Fig. 3, Carbide Index No.1 decreases with increasing finishing and coiling temperatures. Carbide index number is similar to ferrite ASTM number. Carbide size increases with decreasing index number. According to 700 ◦ C annealing temperature in CAL process, hot band microstructure with high coiling temperature and finishing temperature (Trial IV, B) resulted in the final annealed microstructure shown in Fig. 5. After hot rolling both surfaces of steels were inspected in recoiling line and then they were pickled in HCl acid (18%, 65 ◦ C, 240 m/min) in order to remove the surface oxides. Because of the high coiling temperature for these steels, tension leveller was used before the pickling to ensure full-scale removal. The coils were rolled to a final gauge of 0.80 mm on four-stand tandem cold rolling mill and the amount of deformation in cold rolling process is 71%. It can be seen that sufficient recrystallization is completed in case of these process conditions. Finally, the full hard materials were annealed in continuous annealing line after electrolytic cleaning process. Fig. 1 shows the schematic illustration of experimental procedure.
1
It is based on NKK CAL Process Knowhow Book, January 1995.
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
20
Fig. 4. Effect of finishing temperature and coiling temperature on carbide size. Fig. 3. Effect of finishing temperature and coiling temperature on ferrite grain size.
3.2. Mechanical properties The mechanical properties of steels after rolling in the hot roll mill are given in Fig. 6. In this figure, it can be concluded that the decrease in finishing temperature causes an increase in yield strength and tensile strength but, results in a decrease in elongation. The increase in the coiling temperature causes the decrease in yield strength and tensile strength but results in the increase in elongation.
The requirement for high coiling temperature of hot rolled strip means that special handling is necessary to counteract heat losses from the inner and outer end. Excessive cooling leads to undesirable carbide morphologies and finely dispersed aluminium nitride precipitation which degrades the formability and heterogeneous mechanical properties in the final product. Fig. 7 shows the application of high finishing and coiling temperature with no water cooling 80 m from head and tail ends of the coil. The data in Fig. 7 are corresponding to the average values of the coils used in Fig. 6. Heat losses
Fig. 5. Annealed microstructure of hot band with high coiling and finishing temperature: (a) ferrite grain size (100×) and (b) carbide size (500×).
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
21
Fig. 6. The effect of finishing temperature for (a) yield strength, (b) tensile strength and (c) elongation. The effect of coiling temperature for (d) yield strength, (e) tensile strength and (f) elongation.
are severe at the head and tail end of the coil because of heat radiation. This excessive cooling leads to undesirable carbide morphologies and finely dispersed aluminium nitride precipitation which degrades the formability of the final product at these positions [9]. By applying high finishing and coiling temperature with no water, the coarsening of carbide and the growth of ferrite grain were achieved. The large grain size of hot band aids the grain growth after cold rolling and
annealing. The coarsening of carbide was found to contribute greatly to the grain growth by weakening the dragging effect of the precipitates. Although the middle of the coil has of a lower temperature than the head or tail of the coil, it is possible to uniform properties when the coil was cooled because heat losses are severe at the head and tail of coil. The mechanical tests were performed after the coil was cooled. The temperature in Fig. 6
22
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
Fig. 7. The change of yield strength (a and d), tensile strength (b and e) and elongation, (c and f) depending on finishing and coiling temperature for whole coil length. (O) Middle of coil, (B) head of coil and (S) tail of coil.
correspond to the average values is used for obtaining Fig. 7. Therefore, uniform mechanical properties for the whole coil length were observed as seen in Fig. 7. Besides, the temperature difference between the edges and center was reduced to 10–20 ◦ C from 40 to 50 ◦ C with coil box. Although mechanical properties of continuously annealed products are largely affected by a heating temperature, the mechanical properties of continuously annealed cold rolled
low carbon Al-killed steel sheets which are finished and coiled at high temperature in hot rolling lie within range of 210–230 N/mm2 for yield strength, 310–330 N/mm2 for tensile strength, 45–46% for elongation and 1.4–1.6 for r mean value at 700 ◦ C constant heating temperature. Therefore, mechanical properties with low yield and tensile strength, and high r mean value and elongation for continuously annealed steel are obtained.
G. Erdem, Y. Taptik / Journal of Materials Processing Technology 170 (2005) 17–23
23
4. Conclusion
References
Using deep drawing 0.025–0.050% C Al-killed steels, the effects of hot rolling conditions on the mechanical properties and microstructure were investigated. The following results have been obtained:
[1] H. Katoh, H. Takachi, N. Takahashi, M. Abe, Cold-rolled steel sheets produced by continuous annealing, Technol. Continuously Annealed Cold-Rolled Steel (1984) 37–38. [2] N. Mizui, A. Okamoto, Effects of manganese content and hotband coiling temperature on deep drawability of continuous-annealed Al-killed sheet steels, Dev. Annealing Sheet Steels (1992) 247– 259. [3] B. Seter, U. Bergstr¨om, W.B. Hutchinson, Extra deep drawing quality steels by continuous annealing, Technol. Continuously Annealed Cold-Rolled Steel (1984) 111–112. [4] K. Matsudo, K. Osawa, K. Kurihara, Metallurgical aspects of the development of continuous annealing technology at Nippon Kokan, Technol. Continuously Annealed Cold-Rolled Steel (1984) 3– 36. [5] F. Ala, B. Atesli, C. Yesil, G. Erdem, O. Baydar, Development and design of Ti-containing interstitial free steels at Eregli iron and steel works, Mod. LC ULC Sheet Steels Cold Forming: Process. Prop. 2 (1998) 461–472. [6] L.E. Samuels, Optical microscopy of carbon steels, Am. Soc. Met. (1992) 81. [7] N. Mizui, A. Okamot, Effects of manganese content and hotband coiling temperature on deep drawability of continuous-annealed Al-killed sheet steels, Dev. Annealing Sheet Steels (1991) 247– 259. [8] O. Kwon, G. Kim, R.W. Chang, Effect of hot rolling conditions on the mechanical properties of cold rolled and annealed ultra-low carbon steels, Metall. Vac.-Degassed Steel Prod. (1990) 215–228. [9] F.H. Samuel, S. Yue, J.J. Jonas, Recrystalization characteristics of a Ti-containing interstitial-free steel during hot rolling, Metall. Vac.Degassed Steel Prod. (1990) 395–413.
(1) Hot rolled sheet steels for continuous annealing process with excellent formability can be produced a high coiling and finishing temperatures. Large ferrite grain sizes and coarse cementite particles were obtained with 930 ◦ C finishing temperature and 730 ◦ C coiling temperature. (2) The effect of finishing temperature and coiling temperature on hot rolled sheet steels microstructure in order to produce hot rolled sheet steels for continuous annealing process was developed. (3) A uniform mechanical property and microstructure throughout the coil for hot rolled material were achieved. (4) It is shown that with increasing finishing and coiling temperature, the yield strength and tensile strength decrease but elongation increases for hot rolled materials.
Acknowledgement The authors would like to thank the Eregli Iron and Steel Works for permission to study the present work and providing the materials.