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[.]
Trang 1Effects of rolling process parameters on the mechanical
properties of hot-rolled St60Mn steel
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 9 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 andfinish rolling temperatures on the mechanical properties of hot-rolled 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 andfinish rolling temperature
It was concluded that increasing the rolling strain rate from 6.02851103s-' to 6.10388 103s-', using % total deformations of 99% andfinish rolling temperature of 958C 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/)
1 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[119_TD$DIFF]stated that during hot-rolling, a metal billet or bloom/slab with a thickness hienters 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 hfand 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 hIand 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/
Trang 2offered by controlled rolling According to hisfindings, 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 2
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/mm 2 )
Bendability %
Reduction
in area
Hardness (HB) Young’s modulus of elasticity (GPa)
Trang 31a 1b
Fig 1 1a: Ultimate tensile strength versus rolling strain rate at a constantfinish 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 958C, 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 10 3
s -' 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 10 3
s -' 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 10 3 s - ' 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 10 3
s -' 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 10 3
s -' 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 10 3
s
-;' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 922 C.
9 g: Variation of rolling strain rate of 6.0981 10 3
s -' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 958 C.
9 h: Variation of rolling strain rate of 6.098110 3
s -' 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 10 3 s - ' 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 10 3 s - ' 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 10 3
s -' 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 10 3
s -' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 958 C 9m: Variation of rolling strain rate of 6.0675410 3
s -' with the micrograph at a constant deformation of 99%, changing finish rolling temperature to 939 C.
Trang 4Perelom 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 asflow stress, strain-rate and recrystallization (both static and dynamic) The mechanical properties of thefinal product were determined by a complex sequence of microstructural changes conferred by thermal variations According to theirfindings, 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$DIFF][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 Thefinal 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 1 10aii: Rolled sample 3 10aiii: Rolled sample 2 10aiv: Rolled sample 5 10av: Rolled sample 6 10avi: Rolled sample 4 10avii: Rolled sample 7 10aviii: Rolled sample 8 10aix: Rolled sample 9 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
Fig 1 (Continued)
Trang 56a 6b
Fig 1 (Continued)
Trang 6Choi[14]also established that property sensitive parameters of hot rolled steel bar depended largely on thefinishing 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 1030–810C
Laasraoui and Jonas[16]further stated that control of temperature during rolling was more important at thefinishing than at the roughing stage
[121_TD$DIFF]Usually desired, the best practice was to ensure a much lower working temperature at the last pass This would drastically reduce grain growth during cooling
Granbom[7]investigated on the structure and mechanical properties of dual phase steels and found that microstructure and consequently mechanical properties of dual phase steels were impacted not only by the chemical composition of the steel but also by a large number of process parameters such as soaking temperature, cooling rate to quenching, quench and temper annealing temperature
Daramola[122_TD$DIFF]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 840–860Cfinishing 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
Fig 1 (Continued)
Trang 72 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 inTable 2.[123_TD$DIFF] 2.2 Methodology
The st60mn steel billets were charged into the furnace and heated to the rolling temperatures in the range 1150C–
1250C They were then rolled into 12 mm, 14 mm, 16 mm and 25 mm diameters of rebars (Table 1[124_TD$DIFF])
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 thefinal instance thus:first set of six samples each were inspected at rolling strain rates of 6.02851103s-', 6.03713 103s-', 6.06754103s-', 6.07132 103s-',
Fig 1 (Continued)
Trang 86.0981103-', 6.10388 103s-', and constantfinish temperature of 922C, changing deformations to 99%, 98% and 96% Second set of six samples each were inspected at the above same strain rates and constantfinish temperature of 939C, changing deformations as in the above Final set of six rolled samples each were inspected at the above same rolling strain rates, and constantfinish rolling temperatureof 958C, 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
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Trang 9test 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
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Trang 10diameter 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, thefinal 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
3.[125_TD$DIFF]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 constantfinish rolling temperature of 922C, changing to different % total deformations of 99%, 98% and 96% respectively, are shown inFig 11a–8a.Fig 11b–8b show the effect of rolling strain rate on the mechanical properties at constantfinish temperature of 939C, changing deformations.Fig 11c–8c
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