Effects of rolling process parameters on the mechanical properties of hot rolled St60Mn steel Case Studies in Construction Materials 6 (2017) 134–146 Contents lists available at ScienceDirect Case Stu[.]
Case Studies in Construction Materials (2017) 134–146 Contents lists available at ScienceDirect Case Studies in Construction Materials journal homepage: www.elsevier.com/locate/cscm Case study Effects of rolling process parameters on the mechanical properties of hot-rolled St60Mn steel Peter U Nwachukwu*, Oluleke O Oluwole Department of Mechanical Engineering, University of Ibadan, Ibadan, Nigeria A R T I C L E I N F O Article history: Received 20 July 2016 Received in revised form 24 December 2016 Accepted 17 January 2017 Available online February 2017 Keywords: Rolling strength St60Mn steel finish rolling temperature % total deformation Rolling strain rate A B S T R A C T This work studied the effect of rolling process parameters at different rolling strain rates, % total deformations and finish rolling temperatures on the mechanical properties of hotrolled St60Mn steel The rolling process parameters studied included finish rolling temperature, % total deformation and rolling strain rates The results were compared with existing literature on rolling carbon steels The tensile strength, yield strength, hardness, young’s modulus of elasticity, toughness, bendability, % enlongation and % reduction in area of the hot-rolled product were obtained The results showed that the rolling process parameters remarkably influenced the mechanical properties of St60Mn steel The trend in property change was dictated by rolling strain rate, % total deformation and finish rolling temperature It was concluded that increasing the rolling strain rate from 6.02851 103s-' to 6.10388 103s-', using % total deformations of 99% and finish rolling temperature of 958 C enhanced the mechanical properties of St60Mn steel © 2017 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Introduction It had be concluded so far that in hot rolling, increase in height of roll grooves which was a function of its expansion, caused by the process parameters, resulted in increase in thickness of rolled stock, which affected the mechanical properties of the rolled samples such as ultimate tensile strength, yield strength, bendability, modulus of elasticity, % reduction in area, hardness, toughness and % elongation, depending on the diameter of rebar being rolled Dutta [19_TD$IF]stated that during hot-rolling, a metal billet or bloom/slab with a thickness hi enters the rolls at the entrance plane x-x with a velocity vi It passes through the roll gap and leaves the exit plane y-y with a reduced thickness hf and at a velocity vf Given that there is no increase in width, the vertical compression of the metal is translated into an elongation in the rolling direction Since there is no change in metal volume at a given point per unit time throughout the process, bhivi = bhv = bhfvf Where, b is the width of the metal stock, v is the velocity at any thickness h intermediate between hI and hf Obikwelu [2], in his study on the optimization of mechanical properties of rolled products, discovered that most mills in developing nations of the world still operated on the basis of conventional rolling which was devoid of modern facilities * Corresponding author E-mail addresses: ebubedikeugwu@gmail.com (P.U Nwachukwu), lekeoluwole@gmail.com (O.O Oluwole) http://dx.doi.org/10.1016/j.cscm.2017.01.006 2214-5095/© 2017 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/) P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 135 Table Chemical composition of the material specimen used Steel grade Chemical composition % C Si Mn P S Cr Ni Cu N ST60Mn 0.41 0.24 1.12 0.021 0.008 0.02 0.03 0.03 0.010 offered by controlled rolling According to his findings, conventional mill operations were not executed along with the necessary temperature monitoring with a view to controlling the evolved microstructure Saroj [5] further stated that steel bars produced through conventional rolling often exhibited abysmally low mechanical properties From their findings, control of inter-stand temperature such that the desired initial austenite grain size is achieved at the last stand is imperative This would ensure that appropriate phase transformation of the right grain size, morphology and texture is obtained during cooling of the bars Table Effects of rolling strain rates on the mechanical properties of St60Mn steel at constant finish rolling temperatures,changing % total deformations Sample ID Rolling strain rate (S-') % Total deformation Finish rolling temperature ( C) Ultimate tensile strength (MPa) Yield strength (MPa) % Elongation Toughness (J/mm2) Bendability % Reduction in area Hardness (HB) Young’s modulus of elasticity (GPa) 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 6.02851 103 6.02851 103 6.02851 103 6.03713 103 6.03713 103 6.03713 103 6.06754 103 6.06754 103 6.06754 103 6.07132 103 6.07132 103 6.07132 103 6.0981 103 6.0981 103 6.0981 103 6.10388 103 6.10388 103 6.10388 103 6.02851 103 6.02851 103 6.02851 103 6.03713 103 6.03713 103 6.03713 103 6.06754 103 6.06754 103 6.06754 10 6.07132 103 6.07132 103 6.07132 103 6.0981 103 6.0981 103 6.0981 103 6.10388 103 6.10388 103 6.10388 103 6.02851 103 6.02851 103 6.02851 103 6.03713 103 6.03713 103 6.03713 103 6.06754 103 6.06754 103 6.06754 103 6.07132 103 6.07132 103 6.07132 103 6.0981 103 6.0981 103 6.0981 103 6.10388 103 6.10388 103 6.10388 103 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 99 98 96 922 922 922 922 922 922 922 922 922 922 922 922 922 922 922 922 922 922 939 939 939 939 939 939 939 939 939 939 939 939 939 939 939 939 939 939 958 958 958 958 958 958 958 958 958 958 958 958 958 958 958 958 958 958 612 569 509.5 614 569.6 511 614.6 570 512.8 622 572.3 520.1 629 580 539 635.7 600 598.6 610 568 509 613 570 510 614 572 512.7 621 579 520 628 583 538 634.7 598 580 508 566 507 611 568 505 613 570 511 620 577 518 626 582 536 633 596 570 445.3 425 423 450.5 426 424 456.5 428.5 425 477 440 429 479 443.3 441 483 445 443 445 424 421 448 425 423 456 428.4 424 464 436 428 476 443.2 440 480 444 442 443 423 419 446 424 420 454 427 422 463 435 425 475 442 439 478 443 441 18.6 19 19.5 18.4 18.7 19.2 18.1 18.4 18.9 16 17.3 17.7 15.7 16 16.9 15.4 15.9 16.3 18.7 19.5 19.9 18.5 18.8 19.4 18.2 18.5 19 17 17.4 17.9 16.7 16.3 17 16.4 16 16.5 19.4 20.4 21.2 19.1 19.6 20.5 18.9 19.4 20.2 18 18.6 19.8 17.7 18.3 19.2 17.3 17.9 18.6 0.4607 0.5089 0.5195 0.4606 0.5086 0.519 0.4605 0.5076 0.5185 0.4605 0.502 0.5135 0.4527 0.501 0.5127 0.4525 0.498 0.512 0.4608 0.509 0.5196 0.4607 0.5087 0.5192 0.4606 0.5077 0.5186 0.4605 0.503 0.5136 0.4528 0.502 0.5128 0.4526 0.499 0.513 0.461 0.515 0.5199 0.4609 0.5095 0.5197 0.4608 0.5085 0.5187 0.4607 0.5039 0.5156 0.453 0.5029 0.5135 0.4528 0.501 0.5131 45.95 44.48 42.75 46 44.49 42.78 46.05 44.5 42.9 46.25 44.53 43.45 46.28 45 43.52 46.34 45.2 43.6 44.93 44.47 42,74 45 44.48 42.77 46.04 44.49 42.8 46.24 44.52 43 46.27 44.98 43.51 46.33 45.1 43.5 44.91 43.47 42.72 44 43.5 42.75 45 44.46 42.8 45.23 44.5 43 46.24 44.90 43.49 46.3 45 43.5 30.2 36 40 30 34.9 39 29.8 34.5 39.9 26.8 34.2 39.8 26.3 34.1 39 26 33.5 37.8 30.3 37 43 30.1 35 41 29.9 34.7 40 26.9 34.3 39.9 26.5 34.2 39 26 33.8 37.9 30.8 39 48 30.5 37 45 30 34.8 43 28.6 34.4 41.1 26.6 34.3 40 26.1 33.9 38.8 222 221 216 224 223 217 229 224 218 229 225 220 230 226 221 231 227 222 221 220 215 223 222 216 227 223 217 228 224 219 229 225 220 230 226 221 220 219 214 222 221 215 226 222 216 227 223 218 228 224 219 229 225 220 57 53 40 60 53 40 61 53 42 61 55 42 62 55 44 65 56 44 56 53 40 59 53 40 60 53 41 60 54 41 61 54 43 64 55 43 55 51 39 58 52 39 59 52 40 59 53 40 60 53 43 63 54 43 [(Fig._1)TD$IG] 136 P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 1a 2a 1b 2b Fig 1a: Ultimate tensile strength versus rolling strain rate at a constant finish rolling temperature of 922 C, changing % total deformation 1b: Ultimate tensile strength versus rolling strain rate at a constant finish rolling temperature of 939 C, changing % total deformation 2a: Bendability versus rolling strain rate at a constant finish rolling temperature of 922 C, changing % total deformation 2b: Bendability versus rolling strain rate at a constant finish rolling temperature of 939 C, changing % total deformation 3a: Toughness versus rolling strain rate at a constant finish rolling temperature of 922 C, changing % total deformation 3b: Toughness versus rolling strain rate at a constant finish rolling temperature of 939 C, changing % total deformation 4a: Hardness versus rolling strain rate at a constant finish rolling temperature of 922 C, changing % total deformation 4b: Hardness versus rolling strain rate at a constant finish rolling temperature of 939 C, changing % total deformation 5a: % Enlongation versus rolling strain rate at a constant finish rolling temperature of 922 C, changing % total deformation 5b: % Enlongation versus rolling strain rate at a constant finish rolling temperature of 939 C, changing % total deformation 6a: % Reduction in area versus rolling strain rate at a constant finish rolling temperature of 922 C, changing % total deformation 6b: % Reduction in area versus rolling strain rate at a constant finish rolling temperature of 939 C, changing % total deformation 7a: Young’s modulus of elasticity versus rolling strain rate at a constant finish rolling temperature of 922 C, changing % total deformation 7b: Young’s modulus of elasticity versus rolling strain rate at a constant finish rolling temperature of 939 C, changing % total deformation 2c: Bendability versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 1c: Tensile strength versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 3c: Toughness versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 8c: Yield strength versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 8a: Rolling strain rate versus yield strength at a constant finish rolling temperature of 922 C, changing % total deformation 8b: Rolling strain rate versus yield strength at a constant finish rolling temperature of 939 C, changing % total deformation 7c: Young’s modulus of elasticity versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 6c: % Reduction in area versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 5c: % Enlongation versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 4c: Hardness versus rolling strain rate at a constant finish rolling temperature of 958 C, changing % total deformation 9a: Variation of rolling strain rate of 6.07132 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 958 C 9b: Variation of rolling strain rate of 6.07132 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 939 C 9c: Variation of rolling strain rate of 6.07132 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 922 C 9d: Variation of rolling strain rate of 6.03713 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 958 C 9e: Variation of rolling strain rate of 6.03713 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 939 C 9f: Variation of rolling strain rate of 6.03713 103s-;' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 922 C g: Variation of rolling strain rate of 6.0981 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 958 C h: Variation of rolling strain rate of 6.0981 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 922 C 9i: Variation of rolling strain rate of 6.02851 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 958 C 9j: Variation of rolling strain rate of 6.02851 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 939 C 9k: Variation of rolling strain rate of 6.02851 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 922 C 9l: Variation of rolling strain rate of 6.06754 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 958 C 9m: Variation of rolling strain rate of 6.06754 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 939 C 9n: Variation of rolling strain rate of 6.06754 103s-' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 922 C P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 [(Fig._1)TD$IG] 3a 3b 4a 4b 5a 5b 137 Fig (Continued) Perelom et al [3] in their study of hot rolling of steel, discovered that temperature was the dominant parameter controlling the kinetics of metallurgical phenomena such as flow stress, strain-rate and recrystallization (both static and dynamic) The mechanical properties of the final product were determined by a complex sequence of microstructural changes conferred by thermal variations According to their findings, temperature also aided the softening mechanism by which rolling stocks (billet) were prevented from brittle fracture due to work hardening effect of the rolling forces Panigrahi [120_TD$IF][19] investigated on the processing of low carbon steel plate and hot strip—an overview and found that the soaking temperature, drafting schedule, finish rolling and coiling temperatures all played importantroles in processing of low carbon plate and strip They controlled the kinetics of various physical and metallurgical processes, viz austenitization, recrystallization and precipitation behaviour The final transformed microstructures depended upon these processes and their interaction with each other In view of increasing cost of input materials, new processing techniques such as recrystallized controlled rolling and warm rolling had been developed for production of plates and thinner hot bands with very good deep drawability respectively Besides hybrid computer modelling was used for production of strip products with tailor made properties Although there had been few reviews on low carbon microalloyed steels in the past the present one dealt with new developments 10ai: Rolled sample 10aii: Rolled sample 10aiii: Rolled sample 10aiv: Rolled sample 10av: Rolled sample 10avi: Rolled sample 10avii: Rolled sample 10aviii: Rolled sample 10aix: Rolled sample 10ax: Sample 10 10axi: Sampl 11 10axii: Sample 12 10axiii: Rolled sample 13 10axiv: Rolled sample 14 10axv: Rolled sample 14 10axvi: Rolled sample 17 10axvii: Rolled Sample 16 10axviii: Rolled sample 15 10axix: Rolled sample 18 10axx: Rolled sample 19 10axxi: Rolled sample 20 10axxii: Rolled sample 21 10axxiii: Rolled sample 22 10axxiv: Rolled sample 23 10axxv: Rolled sample 24 10axxvi: Rolled sample 25 10axxvii: Rolled sample 26 10axxviii: Rolled sample 28 10axxix: Rolled sample 27 10axxx: Rolled sample 29 10axxxi: Rolled sample 31 10axxxii: Rolled sample 32 10axxxiii: Rolled sample 30 10axxxiv: Rolled sample 33 10axxxv: Rolled sample 34 10axxxvi: Rolled sample 35 10axxxvii: Rolled sample 36 10axxxviii: Rolled sample 37 10axxxix: Rolled sample 38 10axl: Rolled sample 40 10axli: Rolled sample 41 10axlii: Rolled sample 39 10axliii: Rolled sample 42 10axliv: Rolled sample 43 138 P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 [(Fig._1)TD$IG] 6a 6b 7a 2c 3c 8a 7b 1c 8c 8b Fig (Continued) P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 [(Fig._1)TD$IG] 7c 6c 5c 4c 139 Fig (Continued) Choi [14] also established that property sensitive parameters of hot rolled steel bar depended largely on the finishing temperatures Barrett and Wilshire [12] employed the idea in the early 1980s, in the production of ferritic hot rolled interstitial free steel to eliminate temperature control problems This was accomplished by reducing the finishing temperature from the conventional 1030–810 C Laasraoui and Jonas [16] further stated that control of temperature during rolling was more important at the finishing than at the roughing stage [12_TD$IF]Usually desired, the best practice was to ensure a much lower working temperature at the last pass This would drastically reduce grain growth during cooling Granbom [7] investigated on the structure and mechanical properties of dual phase steels and found that microstructure and consequently mechanical properties of dual phase steels were impacted not only by the chemical composition of the steel but also by a large number of process parameters such as soaking temperature, cooling rate to quenching, quench and temper annealing temperature Daramola [12_TD$IF]studied the effects of heat treatment on the mechanical properties of rolled medium carbon steel Their result showed that the steel developed had excellent combination of tensile strength, impact strength and ductility which was very attractive for structural use Balogun [6] investigated on the influence of finishing temperature on the mechanical properties of conventional hot rolled steel bar and discovered that detailed temperature tracking of a conventional rolling operation, recorded improvements in the bars mechanical properties within 840–860 C finishing temperature It was for further understanding of the effects of rolling process parameters on the mechanical properties of hot-rolled St60Mn steel that the present study was devised 140 [(Fig._1)TD$IG] P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 Fig (Continued) Materials and methodology 2.1 Materials Starting materials were St60Mn steel Billets of initial dimension of 120 120 12,000 mm, which were obtained from the billet yard box at the Osogbo Steel Rolling Company Limited Nigeria The compositions are presented in Table 2.[123_TD$IF] 2.2 Methodology The st60mn steel billets were charged into the furnace and heated to the rolling temperatures in the range 1150 C– 1250 C They were then rolled into 12 mm, 14 mm, 16 mm and 25 mm diameters of rebars (Table 1[124_TD$IF]) Fifty four rolling cycles, were selected, set by set, for this study Fifty four rolling cycles of steel billets, in three sets of six samples each, were investigated in the final instance thus:first set of six samples each were inspected at rolling strain rates of 6.02851 103s-', 6.03713 103s-', 6.06754 103s-', 6.07132 103s-', P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 [(Fig._1)TD$IG] 800 600 400 200 0 0.02 0.04 Stress(MPa) 800 Stress(MPa) Stress(MPa) 800 700 600 500 400 300 200 100 0.06 600 600 400 200 0.2 Strain (mm/mm) 0.1 Strain (mm/mm) 600 400 200 0.2 0.05 0.1 0.15 0.2 Strain (mm/mm) 600 600 800 Stress(MPa) 800 400 200 0.02 0.04 Strain (mm/mm) 10avii 0.2 10avi 800 Stress(MPa) Stress(MPa) 10av 200 0.1 Strain(mm/mm) 800 200 400 10aiii 400 10aiv 200 0.06 Stress(MPa) 800 0.04 Stress(MPa) Stress(MPa) 800 0.02 10aii 0.1 400 Strain(mm/mm) 10ai 600 Strain (mm/mm) 141 0.05 600 400 200 0.1 Strain (mm/mm) 10aviii 0.05 0.1 Strain (mm/mm) 10aix Fig (Continued) 6.0981 103-', 6.10388 103s-', and constant finish temperature of 922 C, changing deformations to 99%, 98% and 96% Second set of six samples each were inspected at the above same strain rates and constant finish temperature of 939 C, changing deformations as in the above Final set of six rolled samples each were inspected at the above same rolling strain rates, and constant finish rolling temperature of 958 C, while changing to % total deformations of 99%, 98% and 96% respectively At the end of the rolling, eighteen samples were collected from each set and taken to the laboratory for test and measurement 2.3 Mechanical test 2.3.1 Tensile tests In carrying out tensile evaluation properties on the bars, the entire test specimens were prepared according to the British standard (BS 4449) Relevant clauses of the Nigerian Industrial Standards (NIS 117-42/50HD 2004) were also complied with A universal materials testing machine type upds100s, was used to obtain the test specimens’ % elongation, yield strength, tensile strength, % reduction in area and young’s modulus of elasticity characteristics in the laboratory 2.3.2 Impact testing The Charpy test specimens were prepared by cutting them to the appropriate sizes with lathe machines The dimensional analysis of the test specimen were:55 mm length with a v-notch at the center and 10 mm square cross section The Charpy P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 800 700 600 500 400 300 200 100 Stress(MPa) 800 700 600 500 400 300 200 100 0.05 Strain (mm/mm) 800 0.05 0.1 0.2 500 400 300 200 100 0 Stress(MPa) 400 200 200 600 0.06 10axviii 800 400 200 Strain (mm/mm) 0.05 Strain (mm/mm) Stress(MPa) 800 600 Stress(MPa) 800 0.04 400 0.05 10axvii 0.02 0.04 600 Strain (mm/mm) 200 0.02 Strain (mm/mm) 10axv 600 0.05 400 0.2 800 0 0.06 800 Stress(MPa) Stress(MPa) Stress(MPa) 0.02 0.04 Strain (mm/mm) 700 600 500 400 300 200 100 10axiv 10axvi 10axix 0.1 10axii Strain (mm/mm) 0 10axi 10axiii 200 Strain (mm/mm) Strain (mm/mm) 700 600 500 400 300 200 100 400 Strain (mm/mm) 700 600 600 Stress(MPa) Stress(MPa) 10ax 700 600 500 400 300 200 100 Stress(MPa) Stress(MPa) [(Fig._1)TD$IG] Stress(MPa) 142 600 400 200 0 0.02 0.04 0.1 0.2 Strain (mm/mm) Strain (mm/mm) 10axx 10axxi Fig (Continued) test method was adhered to by holding the specimen horizontally and breaking it, using a Pendulum impact testing machine PSW 30 The consumed striking energy or impact energy and toughness of the specimen were determined at the end of the test respectively 2.3.3 Bendability test The Alba Automatic Bar Bending machine was used to determine the Bendability of each test sample Mandrel diameters for rebend test in accordance with the British Standards 4449, were selected; for nominal diameters less than or equal to 16 mm, maximum mandrel diameter was 4d whereas for nominal diameters greater than 16 mm, maximum mandrel P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 600 Stress(MPa) Stress(MPa) 800 400 200 0 0.1 700 600 500 400 300 200 100 0.2 10axxii 200 200 200 600 400 200 0 0.02 0.04 Strain (mm/mm) 10axxviii 0.04 0.06 600 Stress(MPa) 200 0.02 10axxvii 800 400 Strain (mm/mm) 10axxvi Stress(MPa) Stress(MPa) 400 Strain (mm/mm) 600 0.04 600 0.05 Strain (mm/mm) 800 0.02 Strain (mm/mm) 800 400 0.04 10axxv 10axxiv 600 0.02 200 Stress(MPa) Stress(MPa) Stress(MPa) 400 400 0.2 800 600 0.1 10axxiii 800 0 600 Strain (mm/mm) Strain (mm/mm) 143 800 Stress(MPa) [(Fig._1)TD$IG] 0.06 400 200 0 0.02 0.04 0.06 0.1 0.2 Strain (mm/mm) Strain (mm/mm) 10axxx 10axxix Fig (Continued) diameter was 7d respectively, where ‘d’ is the nominal diameter of the test sample The angle selector was used to select an acute angle, since the grade of the test sample was medium carbon steel At the end of the test, the final angle of the rebend test for each sample was determined 2.3.4 Hardness test The Brinell hardness of each test sample was measured with the Hardness Testing Machine A Spherical indentation was made on each test sample using a hardened steel ball indenter by an applied load Each load was applied for 15 seconds and removed The diameter of the indentation was measured and the Brinell hardness was calculated using the values of the applied load and the diameter of the indentation [125_TD$IF]Results and discussion 3.1 Data from the effect of rolling process parameters on the mechanical properties of St60Mn steel The curves of the effect of rolling strain rate on yield strength, tensile strength,% elongation, % reduction in area, young’s modulus of elasticity, bendability, hardness and toughness at constant finish rolling temperature of 922 C, changing to different % total deformations of 99%, 98% and 96% respectively, are shown in Fig 11a–8a Fig 11b–8b show the effect of rolling strain rate on the mechanical properties at constant finish temperature of 939 C, changing deformations Fig 11c–8c P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 0.1 600 500 400 300 200 100 0.2 800 0.02 Strain (mm/mm) 400 200 0 0.02 0.04 700 600 500 400 300 200 100 Strain (mm/mm) Stress(MPa) Stress(MPa) 0.05 700 600 500 400 300 200 100 Strain (mm/mm) 10axxxvii 0.02 0.04 0.06 700 600 500 400 300 200 100 0.04 0.02 0.04 0.06 Strain (mm/mm) 10axxxvi 10axxxv 0.02 Strain (mm/mm) Strain (mm/mm) 10axxxiv 700 600 500 400 300 200 100 0 10axxxiii Stress(MPa) Stress(MPa) Stress(MPa) 600 200 0.04 10axxxii 800 400 0 Strain (mm/mm) 10axxxi 600 Stress(MPa) 700 600 500 400 300 200 100 Stress(MPa) Stress(MPa) [(Fig._1)TD$IG] Stress(MPa) 144 0.02 0.04 0.06 700 600 500 400 300 200 100 Strain (mm/mm) 10axxxviii 0.1 Strain (mm/mm) 0.2 10axxxix Fig (Continued) show the effect of rolling strain rates on the mechanical properties at constant finish rolling temperature of 958 C, changing deformation Table show the effect of rolling strain rate on the mechanical properties of st60Mn steel 3.1 Influence of process parameters on the mechanical properties of st60mn steel at constant finish rolling temperature of 922 C, changing to different % total deformations of 99%, 98% and 96% At 922 C constant finish rolling temperature and % total deformations of 96%, 98% and 99%, St60Mn steel showed improved tensile strength, yield strength, hardness, bendability and young’s modulus of elasticity The mechanical properties increased as the % total deformations increased;with the highest values recorded at 99% total deformation (Fig 11a, 8a, 4a, 2a, 7a) The observed trend in the change in strength of the steel can be attributed to effective deformation of the rolled stock inside the rolls caused by high rate of strain hardening.[126_TD$IF] There is very high dislocation density and fine dispersion of alloy carbide in the rolled stock Strain hardening is due to dislocation movement impeded by various obstacles such as interstitial atoms, precipitated secondary phase, and other dislocations The stress field around a dislocation therefore, interacts elastically with the stress field around the obstacle, and slippage in a given crystallographic plane is thus hindered This gives rise to improved tensile strength, yield strength, hardness, modulus of elasticity and bendability The high values of modulus of elasticity at higher rolling strain rate and % total deformations indicated that the stiffness of the rolled stocks were very high, and this stiffness decreased as the rolling strain rate and % total deformations decreased;this was also shown by the values of bendability Simultaneously, decrease in toughness,% reduction in area and % elongation (Fig 13a, 6a and 5a) was due to the presence of internal stress, non-uniform dislocation structures and some quantities of P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 145 -phase which resulted in coarser grains and extensive slippage [127_TD$IF]and as the temperature increased, the grains become coarser and the slippage increased progressively The observed property trend also supports the theory that the tensile strength, yield strength, hardness, modulus and bendability, for a given metallurgical structure increase with increasing strain rate or rate of loading, whearas toughness, % enlongation and % reduction in area decrease with increasing strain rate or rate of loading [137_TD$IF][17,18] 3.2 Influence of process parameters on the mechanical properties of St60Mn steel at a constant finish rolling temperature of 939 C, changing deformations to 99%, 98% and 96% At 939 C constant finish rolling temperature and % total deformations of 96%, 98% and 99%, the same pattern above was repeated, but at decreased tensile strength, yield strength, hardness, bendability and young’s modulus of elasticity (Fig 11b, 8b, 4b, 2b and 7b), and increased % enlongation, toughness and % reduction in area (Fig 15b, 3b and 6b) This also showed that, the higher the finish rolling temperature, the lower the tensile strength, yield strength, hardness, modulus of elasticity and bendability,; whereas the higher the toughness, % elongation and % reduction in area, which also supports the theory that the tensile strength, yield strength, hardness, modulus and bendability, for a given metallurgical structure increase with increasing strain rate or rate of loading, whearas toughness, % enlongation and % reduction in area decrease with increasing strain rate or rate of loading [137_TD$IF][17,18] At 96% total deformation, the minimal values of tensile and yield strength of the control sample of St60Mn steel are not exceeded (Fig 11b and 8b) 3.3 Influence of process parameters on the mechanical properties of St60Mn steel at a constant finish rolling temperature of 958 C, changing deformations to 99%, 98% and 96% At 958 C constant finish rolling temperature and % total deformations of 96%, 98% and 99%, the same pattern above was repeated, but at more decreased tensile strength, yield strength, hardness, bendability and young’s modulus of elasticity (Fig 11c, 8c, 4c, 2c and 7c), and more increased % enlongation, toughness and % reduction in area (Fig 15c, 3c and 6c) % Enlongation, toughness and % reduction in area are highly influenced by increasing finish rolling temperature;whereas the yield strength, tensile strength, hardness, bendability and young’s modulus of elasticity are influenced by increasing rolling strain rates and % total deformation (Fig 11c and 6c) 0.1 600 500 400 300 200 100 0.2 0.04 800 600 600 400 200 0.02 0.04 Strain (mm/mm) 0.06 600 400 200 0 400 200 0 0.02 0.04 Strain (mm/mm) 10axliv Fig (Continued) 0.02 Strain (mm/mm) 10axlii 800 Stress(MPa) Stress(MPa) 0.02 10axli 10axl 10axliii Strain (mm/mmm) Strain (mm/mm) 800 Stress(MPa) 700 600 500 400 300 200 100 Stress(MPa) Stress(MPa) [(Fig._1)TD$IG] 0.06 0.04 146 P.U Nwachukwu, O.O Oluwole / Case Studies in Construction Materials (2017) 134–146 Conclusion From the results it could be concluded that hot-rolling at finish rolling temperature of 958 C and % total deformation of 99% produced better results especially with increasing rolling strain rates, because higher toughness, % enlongation and % reduction values are obtained Also good values of tensile strength, yield strength, hardness, bendability and young’s modulus of elasticity that exceed the minimal values of the mechanical properties of the control sample of St60Mn steel, which could not lead to the breaking of the rolls during the actual rolling process, are obtained It is therefore recommended that for effective strengthening of St60Mn steel and to avoid breaking of rolls, hot-rolling at 6.02851 103s-' 6.10388 103s-', a % total deformation of 99% and finish rolling temperature of 958 C should be employed for St60Mn steels [129_TD$IF]References [2] D.O.N Obikwelu, Metallurgical consideration in the optimization of mechanical properties of rolled products, Seminar Paper Presented at the Metallurgical and Research Department, Delta Steel Company, Aladja, Warri, Nigeria, 1987 [3] E.V Perelom, Strain-induced precipitation behaviour in hot rolled strip steel, Mater Sci Eng A299 (2001) 27–37 [130_TD$IF][5] K Saroj, Optimal temperature tracking for accelerated cooling processes in hot rolling of steel, J Dyn Control (4) (2000) 327–340 Nov [6] S.A Balogun, Influence of finishing temperature on the mechanical properties of conventional hot rolled steel bar, J Eng Technol Res (October (11)) (2011) 307–313 [7] [13_TD$IF]Ylva Granbom, Structure and mechanical properties of dualphase steels, [132_TD$IF]Int J Sci Res (2010) i-56 [12] C.J Barrett, B Wilshire, The production of ferritically hot rolled interstial free steel on modem hot strip mill, J Mater Process Technol 122 (1) (2002) 56–62 [13_TD$IF][14] Y Choi, Integrated Model for Thermo-Mechanical Controlled Process in Bar Rolling, Mater Process Technol 125–126 (September) (2002) 678–688 [134_TD$IF][16] A Laasraoui, J.J Jonas, Prediction of temperature distribution flow stress and microstructure during multipass hot rolling of steel plate and strips, ISIJ Int 31 (2007) 95–105 [17] [135_TD$IF]M Mihalikova, et al., Influence of loading and strain rates on the strength properties and formability of higher strength sheet, Metalurgija 46 (2007) 107–110 [18] Y Fakher, et al., Effect of the rolling direction and draft on some of the mechanical properties [138_TD$IF]for medium carbon steel, IJSR (2014) 2425–2431 [19] Panigrahi, Processing of low carbon steel plate and hot strip-an overview, Bull Mater Sci 24 (4) (2001) 361–371 ... rolling strain rate on the mechanical properties of st60Mn steel 3.1 Influence of process parameters on the mechanical properties of st60mn steel at constant finish rolling temperature of 922 C, changing... using the values of the applied load and the diameter of the indentation [125_TD$IF]Results and discussion 3.1 Data from the effect of rolling process parameters on the mechanical properties of St60Mn. .. investigated on the influence of finishing temperature on the mechanical properties of conventional hot rolled steel bar and discovered that detailed temperature tracking of a conventional rolling operation,