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2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)Influence of cold working on Mechanical andmetallographic characteristics of low carbon steel(S355)Dr. O.P UpadhyayDepartment of Mechanical Engineering,MJP, Rohilkhand UniversityBareilly, Uttar Pradesh, India.Abstract:In the era of rapid industrialization various grades of steel is extensively utilized. This class of material isessentially imbibed with specific characteristics such as corrosion resistance behavior, good formability andhigh yield strength etc. The specific aim of this work is to analyze the effect of cold rolling process on themechanical properties of low carbon steel grade S355. The prepared specimens undergoing the cold rollingprocess with 0 to 30% cold work percentage and was annealed at the temperature of 400 C for 1 hour. Theultimate tensile strength (tensile test), toughness (Charpy impact test) and hardness (Vickers test) of eachspecimen was conducted to analyze the Mechanical properties. Further microstructure analysis of the S355steel before and after cold rolling treatment was conducted to characterize the material. The resultsenumerates that both tensile strength and toughness of material is augmented with increasing percentreduction in specimen thickness for 0 to 30 percent. Furthermore, the microscopic observation implies microstructural modification (elongation and compression of grains) and increase in the mechanical propertieswhich justifies its application as a structural material.Key words: Steel grade S355, Cold rolling, Tensile strength, Mechanical properties, Microstructure.IntroductionLow carbon steel is a type of carbon steel with a low amount of carbon – it is actually also known as “lowcarbon steel.” Although ranges vary depending on the source, the amount of carbon typically found in lowcarbon steel is 0.05% to 0.25% by weight, whereas higher carbon steels are typically described as havingcarbon content from 0.30% to 2.0%. Low carbon content and additions of elements that have a strong affinityfor carbon, such as niobium or titanium; have long been known to promote recrystallization textures favorablefor severe forming operations with a high degree of tolerance to processing variables [1]. If any more carbonthan that is added, the steel would be classified as cast iron. Low carbon steel is not an alloy steel andtherefore does not contain large amounts of other elements besides iron; you will not find vast amounts ofchromium, molybdenum, or other alloying elements in mild steel. Since its carbon and alloying elementcontent are relatively low, there are several properties it has that differentiate it from higher carbon and alloysteels [2]. Less carbon means that low carbon steel is typically more ductile, machinable, and weldable thanhigh carbon and other steels, however, it also means it is nearly impossible to harden and strengthen throughheating and quenching. The low carbon content also means it has very little carbon and other alloyingJETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1321
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)elements to block dislocations in its crystal structure, generally resulting in less tensile strength than highcarbon and alloy steels [3]. Low carbon steel also has a high amount iron and ferrite, making it magnetic. Thelack of alloying elements such as those found in stainless steels means that the iron in low carbon steel issubject to oxidation (rust) if not properly coated. But the negligible amount of alloying elements also helpsmild steel to be relatively affordable when compared with other steels. The material used in this project is lowcarbon steel grade S355.This grade of material show great value for structural purpose. However, its maindrawback is low tensile strength as compared to medium and high carbon steel. By using proper metalforming technique the mechanical properties can be improved. Among most of the metal forming processescold rolling is one of the most suitable technique which results in improvement in mechanical properties aswell as microstructure of material. Structure and resulting material properties are significantly influenced bycold rolling because in the given terms no recrystallization can occur. Extension of grains in the direction ofrolling occurs and the arrangement of crystallographic lattice gets a directional character [4]. Bandingcharacter of other structural phases, such as of inclusions, pearlitic blocks, etc. has been developed too. Threetypes of texture (i.e. deformation, structural and crystallographic texture) arise, which yields in a directionalcharacter of mechanical properties. Heat treatment is included after cold forming for removal of anisotropy ofproperties. To factors influencing the microstructure after annealing are: the initial character of materialstructure before cold forming, the total cold reduction, annealing conditions (temperature and time) and alsocooling speed. More and more progressive types of material have been used in this field of processing [5-8].Many metals need to undergo plastic deformation (i.e. rolling, drawing, forging, etc.) to generate useful finalproducts. In this study, attention will be given to the cold rolling process where the metal is passed betweencounter-rotating rolls. Cold rolling is carried out below 0.3 Tm, where Tm is the absolute melting point ofthe metal. This process is widely used to make sheets and strips with superior surface finish and dimensionaltolerance [9-13]. During deformation, the internal structure of a metal changes in several ways; the grainschange their shape, total grain boundary area increases.Many metals need to undergo plastic deformation (i.e. rolling, drawing, forging, etc.) to generate useful finalproducts. In this study, attention will be given to the cold rolling process where the metal is passed betweencounter-rotating rolls [14]. Cold rolling is carried out below 0.3 Tm, where Tm is the absolute melting pointof the metal. This process is widely used to make sheets and strips with superior surface finish anddimensional tolerance [15]. During deformation, the internal structure of a metal changes in several ways; thegrains change their shape, total grain boundary area increases, an internal substructure forms within each grainand point defects are generated. These structural changes are associated with the formation and accumulationof dislocations [17].Most of the energy expended in cold deformation appears in the form of heat, with only a very small amountstored in the metal as strain energy associated with various lattice defects created by the deformation [18]. Theamount of energy retained depends on the deformation process and a number of other variables, for examples,composition of the metal as well as the rate and temperature of deformation [8]. In cubic metals, there are twoprinciple modes of deformation: slip and twinning. If the stacking fault energy is relatively high as in the caseof low carbon steels, plastic deformation at normal conditions is only due to crystallographic slip (i.e. slip onplanes and along directions fixed with respect to the crystal axis). Typical slip systems for BCC materials areon the {110}, {112} or {123} planes in the close packed directions [19]. During deformation, each grain tendsto change its orientation with respect to the direction of the applied deformation which leads to a preferredorientation or texture as deformation proceeds. However, these changes in grains‟ orientation are notuniformly distributed since each grain of an aggregate is subjected to the constraints exercised by neighborgrains, each of which is deforming in a unique manner, thereby generating micro structural heterogeneities inJETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1322
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)the form of deformation bands, transition bands and shear bands [20]. The hierarchy of microstructure inpolycrystalline metal deformed by slip was defined by, Ueji et al. [5]. At the smaller scale, dislocation cells,which comprise the smallest volume elements, are surrounded by single walled Dense Dislocation Walls(DDW) at low strain or double walled Micro Bands (MB) at medium strain. At high strain, the cell blocksbecome elongated and surrounded by lamellar dislocation boundaries (LB) which replaces the DDW and MBstructure.Based on the literature review, the present work is carried out to meet few specific objectives such as: analysisof important mechanical properties and microscopic observation on the low carbon steel S355 when subjectedto different percentage of cold work. Further the observations recorded from known cold rolled material assistus to gain some insight on microstructure texture evolution in order to justify its viability as a structuralmaterial.Material and method:A sheet of low carbon steel with dimension 610*40*20 (L*W*H) is chosen to prepare test specimens. Forperform cold rolling process at different reduction stages (10, 20, and 30) cut that sheet into four parts with thehelp of power hacksaw. Three sheet were cold rolled to three different reductions; 10%, 20% and 30%. Size ofeach sheet is equal with dimension 150*40*20 (L*W*H). A small amount of error in thickness was allowedfor due to the difficulties of obtaining the exact desired amount of reduction. Cold Rolling was performedusing a four-high rolling mill. Initial thickness of sheet is 20mm. Reduction in per pass is 0.5mm.The detailsof cold roiling process on low carbon steel is mentioned in table. 1 as:Table. 1 The details of cold roiling process on low carbon steel specimens.S. No123Material selectedLow carbon steelReduction (%)102030Passes4812Thickness (mm)181614Static annealing was carried out for each sample from all the different reductions. Annealing is done aftermachining because during machining stress generated for minimize these stress annealing is done. Theannealing temperatures used 400 C for low carbon steel. After annealing process furnace cooling is done for25 C.SPECIMEN PREPARATIONThere are different types of specimen depending on the type of the grips and on the form of the availablematerial (sheet, rod, etc.). Generally all specimens have two main parts, the gage section and the ends. Thedimensions of the specimens are standardized (TS, DIN, ASTM, British standard etc.). Each standard maycontain a variety of test standards suitable for different materials, dimensions and fabrication history. Astandard specimen is prepared in a round or a square section along the gauge length as, depending on thestandard used. Both ends of the specimens should have sufficient length and a surface condition such that theyare firmly gripped during testing. It has two shoulders and a gage section in between.JETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1323
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)Overall lengthShoulder distanceGauge lengthReduced sectionWidth of grip sectionDia. or WidthFigure: 1 Specimen terminology and prepared specimen of low carbon steel S35Figure: 2 Specimen of low carbon steel S355 for Vickers and Charpy testTable. 2 Actual dimensions of prepared specimen of low carbon steel S355.Overall length Gap length152mm80mmWidth10mmThickness4mmGrip SectionWidth20mmLength24mmTest methodInitially all the length and diameter of the specimen are measured then mark the location of the gauge lengthalong the parallel length of each specimen for subsequent observation of necking and strain measurement.After these initial fitments, grip the specimen on to the Universal testing machine (UTM) and tests werecarried out by gradually applying the load by moving the upper jaw upwards with hydraulic piston. Record thegraph of the rupturing load. Then open the data file in a graphing or spreadsheet program (e.g. Excel)checking that the load and displacement data columns are intact. Then, develop the following columns inaddition to the data columns: engineering stress, engineering strain, true stress, and true strain. Develop andpresent the following plots such that in each plot both materials are shown: Load versus displacement for the entire displacement range. Engineering Stress versus engineering strain for the entire strain range tested.JETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1324
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162) True stress versus true strain up to necking.Determine the following for each data set (material) noting the proper units for each: 0.2% off set yieldstrength, ultimate strength, ruptures strength, modulus of elasticity, modulus of toughness, percent elongation.Determine percent reduction in area only for the specimen tested by the group.Vickers and Charpy hardness testThe Vickers hardness test method consists of indenting the test material with a diamond indenter, in the formof a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a loadof 1 to 100 kgf. The Vickers hardness value (VHN) can be calculated from the applied load divided by areasof indentation, at which the latter is derived from the diagonals of the pyramid as expressed in the equationbelow.𝑉𝐻𝑁 2𝑃𝑠𝑖𝑛(𝛼 2)𝑑2 1.8544𝑃/𝑑2The Pyramid indentation of test specimen is shown in figure.2 for measurements.Impact tests determine impact toughness, a material property, most commonly by measuring the workrequired to fracture a test specimen under impact. The most common impact test is the Charpy impact test.The Charpy impact test is a high strain rate test that measures the work required to rupture a specimen inflexure. Charpy specimens are uniform, rectangular prismatic specimens with one notch per specimen toencourage rupture. The Charpy testing machine is comprised essentially of a hammer with a striking head (awedge shaped head was used in this laboratory) attached to a nearly frictionless pendulum with a knownpotential energy. This test is performed on. The prepared test specimens are shown in figure.2.MICROSCOPIC OBSERVATIONMetallographic is the study of the structure of metals and of metal alloys through the examination ofspecimens with a metallurgical microscope. The structures observed in the microscope are often recordedphotographically. The metallographic examination of specimens allows the metallographer to observe andrecord the crystalline structures and to interpret from them the history of manufacture and use of the material.The metallurgical microscope for metallographic observations is shown in Figure. 3.Figure. 3. Representing Vickers hardness tester and optical micrograph.JETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1325
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)RESULT AND DISCUSSIONThe mechanical properties mainly affected by the percentage of cold rolling, as the case may be, for lowcarbon steel.Effect of cold rolling on hardness of low carbon steelFigure. 4, shows the hardness of low carbon steel samples after various stages of cold rolling and thecorresponding values are depicted in table.3. From this figure it can be seen that as the level of reductionincreases the hardness increases due to the increase in the dislocation density.Table. 3 Data table for hardness of test specimen%cold reduction0102030Hardness(in HV10)135208218235300HardnessHardness (HV 10)25020015010050012345678% cold reductionFigure. 4: Variation in Hardness of low carbon steel samples after various stages of cold rolling.The hardness of the samples after various stages of cold rolling and subsequent annealing were measuredincreases the hardness of low carbon steel due to the increasing of deformation. This is in turn corresponded tothe strain hardening as a result of cold rolling which leads to reduce the thickness of steel sample. Moreover,the interactions between the particles and the dislocations involved in the deformation of low carbon steel areresponsible for the greater increase in hardness compared with the IF steel.Effect of cold rolling on toughness of low carbon steelThe toughness of the samples after various stages of cold rolling and subsequent annealing were measured.Figure.5, shows the toughness of low carbon steel samples after various stages of cold rolling and theJETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1326
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)corresponding values are depicted in table.4 . From this figure it can be seen that as the level of reductionincreases the toughness decreases due to the increase in the dislocation density.Table. 4 Data table for toughness of test specimen%cold reduction0102030Toughness (Joules)4234282450ToughnessToughness (Joules)45403530252015105012345678% cold reductionFigure. 5: Variation in Toughness values of low carbon steel samples after various stages of cold rolling.The toughness of the samples after various stages of cold rolling and subsequent annealing were measuredwhich was found to decrease the toughness value of low carbon steel due to the increasing of deformation.Effect of cold rolling on tensile strength of low carbon steelThe tensile strength of the sample after various stages of cold rolling and subsequent annealing were measuredon UTM.Table. 5 Data table for tensile strength of test specimen%cold reduction0102030Tensile strength (MPa)602776816841JETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1327
2015 JETIR July 2015, Volume 2, Issue 71000www.jetir.org (ISSN-2349-5162)Tensile strength900Tensile strength (MPa)800700600500400300200100012345678% cold reductionFigure. 6: Variation in tensile strength values of low carbon steel samples after various stages of cold rolling.Hardness900ToughnessTensile strength800Tensile 01520253035Figure. 7: Effect of cold rolling on the mechanical properties on low carbon steelFigure.6, show relationship between the tensile strength vs. cold rolling reduction at various stages (10, 20,and 30) and the corresponding values are depicted in table.5. When material is cold worked, the crystalstructure of the metal is deformed (bent, twisted, compressed, etc., resulting in the relatively uniformcrystalline plains (from a recrystallization anneal) moving over and past one another. This movement createsimperfections; these discontinuities in the structure are dislocations. These dislocations provide furtherresistance to deformation, which can be seen as an increase in hardness, as measured by the penetration of anindenter under load. The ultimate tensile strength, and yield strength also increase due to the "locking effect"of those distorted and twisted grains (metallic crystals).The elasticity limit is defined as the point of transformation of the elastic deformation intoplastic deformation. The yield strength of the low carbon steel minimizes with the incrementing percentage ofcold rolling, which in turn truncates the ductility of material and induct the brittleness in material because ofthe strain hardening as a result of cold rolling, which in turn truncates the ductility of material and induct thebrittleness in material because of the strain hardening as a result of cold rolling. Further, the incrementing ofpercentage cold rolling abbreviates the tensile strength due to strain hardening and makes the material brittleJETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1328
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)as well as for the same reasons deal with yield strength. Figure.7, show the incrementing in percentage of coldrolling leads to increase in hardness and tensile strength but decline in material toughness.MICROSCOPIC OBSERVATIONSteels were cold rolled to reductions of 10%, 20%, and 30%. The effect of different reductions on themicrostructure can be seen in Figure for the low carbon steel, respectively. These figures show that the grainsbecame more elongated along the rolling direction as the reduction level increases. The elongated sub grainsare separated by much sharper boundaries than the other grain which indicate higher disorientations betweenneighboring sub grains and, hence, higher stored energy.(A)(B)Figure. 8 (A, B). Microscopic image of annealed low carbon steel at (A) 100μm and (B) 50μm.(C)(D)Figure. 8 (C, D). Microscopic image of 10% cold rolled annealed low carbon steel at (A).100μm and (B) 50μm.(E)(F)Figure. 8 (E, F). Microscopic image of 20% cold rolled annealed low carbon steel at (E). 100μmand (F) 50μm.(G)(H)Figure. 8 (G, H). Microscopic image of 30% cold rolled annealed low carbon steel at (G).100μm and (H) 50μm.Figure. 8. Microscopic image of 10%, 20% and 30% cold rolled annealed low carbon steel at 100μm and50μm.JETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1329
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)During cold rolling, elastic deformation initially occurred as progressive grain elongation in the paralleldirection of cold rolling. As a percentage of cold rolling increases this may leads to thetransition from the elastic deformation to the plastic deformation as a point of elasticity as a yield point isdisunited between them. The specimens to evaluate the structure by optical microscope after cold rolling wereconsisted of the fine grains of the ferrite with minor grains of pearlite. Incrementing the percentage of algidrolling leads to increment the elongation of ferrite and pearlite grains, which in turn prodigiously affected themechanical properties; this is concurred with the earlier research work. [12].Despite ductility decrease or increase accompanied by loss of strain-hardening capacity, the fracture featurespresent a ductile characteristic with numerous diminutive and sizably voluminous dimples (Fig. 8 A-H).Spherical crack areas at the surface fractures designate that dimple fractures are controlled by nucleation,magnification, and micro-void coalescence during the necking phenomenon of tensile deformation. Fracturemorphology of the sample in Fig. (8A-D) pellucidly shows numerous dimples with some more diminutivevoids at the grain boundary and some more astronomically immense voids in the pearlite regions. The voidsnucleated and grew in regions of high stress concentration at the boundaries between the ferrite matrix and thegrains [35] that consist of numerous carbide precipitates with high strain energy [21]. However, only a fewfine dimples optically canvassed under high magnification (Fig. 8 E-H) can be attributed to the minimizedcapacity for plastic deformation, as denoted by the low ductility during cold rolling process.ConclusionImpact of cold rolling on mechanical properties and microstructure of low carbon steel was studied. TheAnnealed specimens with refined microstructure showed typical tensile properties of ultra-fined steels withlack of excessive work hardening and increased ductility. The tensile strength dropped after annealing and recrystallization due recovery from stressed conditions. However, increased fracture strains reflect theirimproved formability essentially required for structural applications of steel. The optical metallography of therolled specimen showed elongation of the grains and increase of aspect ratio along the rolling direction. Themicro structural observations of the annealed specimens clearly revealed the re-crystallizations. The stressneeded to increase the strain beyond the proportionality limit in the material continues to rise beyond theproportionality limit indicating an increasing stress requirement to continue straining. It has been observedthat mechanical properties like tensile strength and hardness increases with increasing percentage of coldreduction while the toughness of material reduces with subsequent increase in percentage cold reduction. Thedifference in yield strength was attributed to the strain hardening, resulting from the different degrees of colddeformation.References[1] K.K. Singh et. al, (2011). Detection of defects on Cold Rolling Mill (CRM) rolls with Ultrasonic & Eddycurrent law detectors, , NDESAI ,Jamshedpur, India.[2] Soszyński, A. Studnicka, BIPROMET S.A. ul. Graniczna (2012) A review of contemporary solutions forcold rolling that allow quality improvement,W. 29, 40-956 Katowice, Poland, International OCSCO worldpress[3] Shen et. al., (2008), Influence of Rolling Chemicals on temper rolling process and anti rust performance ofcold rolled steels, China Technical Report, No. 21, pp. 45-51JETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1330
2015 JETIR July 2015, Volume 2, Issue 7www.jetir.org (ISSN-2349-5162)[4] Mackel (2000) Maintenance and quality related condition monitoring in rolling mill, AISE, Chicago,Illinois/USA[5] R. Ueji, N. Tsuji, Y. Minamino, Y. Koizumi, Ultragrain refinement of plain low carbon steel by coldrolling and annealing of martensite, Acta Mater. 50 (2002) 4177–4189.[6] D.A. Hughes, N. Hansen, Microstructure and strength of nickel at large strain, Acta Mater. 48 (2000)2985–3004.[7] N. Tsuji, R. Ueji, Y. Ito, Y. Saito, In-situ recrystallization of ultrafine grains in highly strained metallicmaterials, Proceedings of the 21st Riso International Symposium on Materials Science, 2000, pp. 607–616.[8] G. Krauss, Martensite in steel: strength and structure, Mater. Sci. Engng A273–275 (1999) 40–57.[9] Q. Liu, et al., Mirostructure and strength of commerial purity aluminum (AA1200) cold rolled to largestrain, Acta Mater. 50 (2002) 3789–3802.[10]Manufacturing Engineering And Technology by Serope Kalpakjian (Illinois Institute of Technology)[11] Introduction to Basic Manufacturing Process and Workshop Technology by Rajender Singh[12] Prof. Satish V. Kailas, Department of Mechanical Engineering Indian Institute of Science, Bangalore,“Material Science” 55.[13] Aaronson, H. I. (1993), Atomic Mechanisms of Diffusional Nucleation and Growth and Comparisonswith Their Counterparts in Shear Transformations, Metall. Trans., 24A, 241-76.[14] Beck, P. A. (1954), Annealing of Cold-worked Metals, Adv. Phys., Vol. 3, 245-324[15] Bhadeshia, H. K. D. H. (1981), A Rationalisation of Shear Transformations in Steels, Acta Metall., Vol.29, 1117-1130.[16] Cohen, M. and Hansen, S. S. (1979), Microstructural Control in Microalloyed Steels, inMiCon78:Optimisation of Processing, Properties, and Service Performance Through Microstructural Control,ASTM STP 672,[17] Cottrell A. H. (1953), In: Theory of Dislocations. Chalmers, B. (ed.), Progress in Metal Physics. Vol. 4,251-255.[18] Courtney, T. H. (1990), Mechanical Behavior of Materials, McGraw Hill, 173-184. Dieter, G. (1986),Mechanical Metallurgy, McGraw Hill, 203-207.[19] Dunn, C. G. and Walter, J. L. (1966), Recrystallization, Grain Growth and. Textures, ASM, Metals Park,OH, 461[20] Farrar, R. A. and Harrison, P. L. (1987), Acicular Ferrite in Carbon –Manganese Weld Metals: anOverview, J. of Mater. Sci., Vol. 22, 3812-3820.[21] Fernández, A. I., Uranga, P., López, B. and Rodriguez-Ibabe, J. M. (2003), Dynamic Recrystallizationbehaviour covering a wide austenite grain size range in Nb and Nb- Ti microalloyed steels, Mater. Sci. andEng., Vol. 361A, 367-376.JETIR1701603Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org1331
Key words: Steel grade S355, Cold rolling, Tensile strength, Mechanical properties, Microstructure. Introduction Low carbon steel is a type of carbon steel with a low amount of carbon - it is actually also known as "low carbon steel." Although ranges vary depending on the source, the amount of carbon typically found in low
