DEVELOPMENT AND STRUCTURE-PROPERTY CORRELATIONS IN HIGH TENSILE WIRE RODS AND BARS

Abstract

In low alloy steels, high strength is attained by increasing the carbon and manganese contents, through the addition of microalloying elements to induce grain refinement and/or precipitation hardening and through controlled cold. deformation and/or thermomechanical controlled processing (TMCP). Which amongst the options is preferred for manufacturing high tensile wire rods depends upon the service requirements and techno-commercial considerations. Evidently, the basis of making a selection depends upon the structure property relations as they provide the basic frame work for identifying the microstructure of interest. Additionally, establishing such empirical correlations assists in arriving at the precise manufacturing sequence to follow for obtaining a desired product with requisite properties. In the case of high tensile wire rods / bars which are generally used either for manufacturing structural components such as high strength fasteners (nuts, bolts, rivets, pins, screws etc.) or as reinforcement in reinforced and prestressed concrete structures, the possible routes comprise, i) C-Mn chemistry, ii) microalloyed versions and iii) the use of TMCP practices. Quite evidently the basis of • arriving at the most preferred route out of the three will depend upon the two major considerations as already stated. The interrelation between the practice and the product developed within the frame work of the structure property relations, in essence, constitute the key aspect of the present study. Each of the above mentioned routes has three major aspects, i) alloy design, ii) evolution of processing methodology and finally, iii) characterisation of the product obtained. In order to arrive at an integrated understanding of the related aspects, a comprehensive review of literature was carried out and comprised strengthening mechanisms, phase transformations in the Fe-C system and their impact on properties and the influence of thermomechanical controlled processing on properties of interest. The experimental work V consisted of producing high strength wire rods on an industrial scale through the routes mentioned above. While the C-Mn and TMCP routes were employed for manufacturing concrete reinforcement wire rods / bars, the microalloyed route was utilised for producing cold heading quality steel. The wire rods thus produced were subjected to various tests keeping their end applications in mind and the results obtained were analysed with a view to establishing empirical correlations between microstructure and properties. For the wire rods manufactured through C-Mn chemistry route the alloy was designed keeping in view the property requirements as laid down in BIS:1786 specification. Aiming the designed chemistry, five commercial heats were made at the Bhilai Steel Plant in a basic open hearth furnace and the liquid metal cast into 8 ton ingots. They were subsequently processed into 8 & 10 mm diameter wire rods. The chemical composition of these heats was found to be within the designed limits. Carbon varied between 0.55 to 0.65 while, manganese was observed to be within the range of 0.85-1.50 wt.% to ensure that a wide variation in strength could be catered to. Microstructure and mechanical properties of the hot rolled wire rods were evaluated and analysed with reference to the requirements as laid down in the relevant specifications. Dilatometric studies were undertaken to optimise cooling parameters, especially the rate, for developing Fe415 as well as Fe500 and Fe550 grade reinforcement rods. Gripping performance of the designed ribbed pattern of the reinforcement rods in concrete was evaluated through the pull-out test by comparing the bond strength of the experimental wire rods with that attained in plain round wire rods of a similar diameter. Optical metallographic studies revealed that as rolled wire rods contained 80-93% pearlite, the remainder being grain boundary ferrite. Prior austenite grain size varied between 28-40 μm. With this microstructure, an yield strength of 422-585 MPa, tensile strength of 790-955 MPa, % elongation of 15-22, % reduction in area of 35-45% and hardness of 257-294Hv were vi attained. The tensile properties thus attained were in conformity with the requirements of BIS:1786 specification. Further, due to a fine dispersion of lamellar carbides constituting the pearlitic microstructure, the material exhibited a two stage work-hardening response. On analysing the tensile results it was found that stress-strain behaviour could be modelled as follows : 0=17156 0.26 (for E <_0.05, knee strain), and 0=10656 0.09 (for s>0.05) Since hardness is considered to be a reasonably accurate measure of the tensile strength, a relationship between the two was also established which was of the form : UTS (MPa) = 4.42 Hv - 354 The ductility of the hot rolled material was evaluated by performing bend test as per the BIS:1786 specification. It was found that the wire rods could be bent around a mandrel of diameter 3d (where 'd' is the nominal diameter of wire rod) without observing any crack on the bent surface. Presence of dimples in the tensile fractured surfaces, as revealed under SEM, agreed well with the good ductility found in the C-Mn wire rods. Bond strength in these wire rods, on embedding in concrete, was evaluated —76% more than the plain round of a similar diameter which is much above the specified limit (BIS:1786). During the dilatometric studies a series of microstructures were generated by employing different cooling rates which were subsequently characterised quantitatively. Hardness was also measured which represented the strength levels within the material. With the help of this data a microstructure-cooling rate (MCR) diagram was developed by plotting cumulative volume fraction of different constituents formed, as a function of the cooling rate. This diagram, as a matter of fact,. proved useful in predicting the vii cooling rate(s) required for achieving a particular level of strength. For example, the diagram revealed that increasing the cooling rate from —1° to —2° C/sec a fine pearlitic microstructure, additionally containing about 5-10% bainite, proved capable of producing strength levels found in Fe550 grade reinforcement rods (BIS:1786). For producing microalloyed wire rods a C-Mn-Nb-B chemistry was selected and the amounts of different elements were arrived at in a manner so as to facilitate achieving of properties equivalent to the ones indicated in ISO:898-I specification. A commercial heat was made in LD converter and cast into 8 ton ingots. It was subsequently processed and finally rolled down to 8 mm diameter wire rods. Chemical composition of the product was analysed to be within the designed chemistry range - C:0.13, Mn:1.05, Si:0.05, Nb:0.023, B:0.004, S:0.025, P:0.036, AI:0.04 and N:0.007 wt.%. As before, the microstructure of hot rolled wire rods was quantitatively characterised using an optical microscope while Auger electron spectroscopy (AES) was employed for locating B which was added specifically to meet with the hardenability requirements. Since the steel, as mentioned earlier, was proposed to be subjected to cold forming operations, deformation characteristics were analysed through wire drawing and upsetting tests. Heat treatments were carried out by using five different austenitising temperatures. At each of the temperature four different cooling rates were employed. The combined effect of varying the austenitising temperature and cooling rate on the mechanical properties was studied to understand the effects produced by Nb and B additions. These studies were supplemented by using dilatometry and differential thermal analysis. Effect of these elements on hardenability was studied through the end quench test using a sub-standard specimen. The microstructure of microalloyed wire rods in the as rolled condition contained about 12-15 % non-lamellar degenerate pearlite, the balance being polygonal ferrite. Average ferrite grain size was measured as —12 μm. AES viii studies revealed the presence of B within the pearlitic colonies and also at grain boundaries in the most probable form of M23(C,B)6 precipitate particles. Enthalpy calculations using DTA plots suggested the possible presence of Nb as NbC distributed throughout the matrix. Due to the formation of the aforesaid microstructure / microconstituents, the as rolled material attained a hardness of ~190Hv, YS of 360 MPa, UTS of 550 MPa, total elongation of 35% and % reduction in area of -70. Work hardening parameters were also analysed through computational techniques and the following typical relationship between stress and strain was established : a = 850 s0.167 During cold. working through wire drawing, different reductions were given to. the 8 mm die. wire rods and their tensile properties evaluated. Through an analysis it was established that on achieving 60-65 % reduction during drawing the material attained a tensile strength of -900 MPa with -11 % elongation and —32% reduction in area. The level of strength and the ductility thus attained is in conformity with the requirements of class 9.8 bolts as specified in ISO:898-I. During an analysis of the work-hardening behaviour it was also revealed that upto a wire drawing elongation of -0.4 (true strain), flow stress in the 'drawn' wire rods followed the same power law relation as obtained through conventional tensile tests, i.e. 6=850F,0.167. Under these conditions the yield strength of the subsequently cold drawn wire rods could be computed based on the tensile behaviour of the as rolled wire rods. However, at higher levels of 'cold drawing reduction' (8>0.4) the above mentioned inter-relation did not hold. This was attributed to anomalous strain hardening occurring probably due to the formation of 'deformation textures'. In this range of cold deformation, wire drawing elongation (true strain) and flow stress of the drawn wire rods followed a relation : 6 = 1156 60.47 Thus the extent of anomalous strain hardening is equivalent to 1.3660.303 ix Under compressive loading conditions used during upsetting test, it was observed that the extent of deformation differed from location to location, i.e. it was maximum at the centre and minimum at the parallel plain surfaces. Based upon this, different hardness levels varying from 200-303 Hv could be developed in the material. The material flow which is a measure of deformation, was expressed in terms of grain axle ratio, Raxle (where Raxle is grain length to width ratio) and it was related with the hardness through the relation: Hv = 3.06 + 0.47 Rax;e In so far as the increase in hardness with cold deformation was concerned, it is interesting to note that hardness levels in the material did not exceed the specified limit of 320Hv, thereby qualifying the material as suitable for cold heading applications. For the experimental microalloyed chemistry critical transformation temperatures namely Ac3, Act, Ms and Mf etc. were evaluated as 844°C, 717° C, 447°C and 232°C, on the basis of dilatometry and differential thermal analysis. These temperatures proved useful in identifying the reheating temperature(s) and in arriving at the rolling and cooling parameters for obtaining desired set of properties. Different cooling rates employed during dilatometry generated a series of microstructures which were quantitatively characterised as mentioned earlier. Cumulative fractions of these phases on being plotted against the cooling rate proved helpful in developing the MCR diagram for this chemistry also. Hardness values corresponding to each cooling rate were also plotted in the same diagram. As in the C-Mn chemistry, the MCR diagram for the microalloyed chemistry also proved helpful in identifying cooling rates required for achieving a particular level of hardness in the material. Hardness values thus obtained were interrelated with the UTS, TEL and RA with the help of the following models : UTS (MPa) = -50 + 3.2 Hv x TEL (%) = 40 - 0.075 Hv RA (%) = 84 - 0.075 Hv Through heat treating experiments it could also be established that the maximum effect of Nb and B alloying on strength was achieved by austenitising the steel in the temperature range of 1020-1050°C. With the help of the MCR diagram it was derived that by adjusting the cooling rate to about 20°C/sec, which may be attained through forced industrial air cooling / mist cooling in 8 mm dia. wire rods, a hardness of -197Hv could be obtained in the final product. This hardness level based on the aforesaid models corresponds to a UTS of -580 MPa, TEL of -25% and RA of -69%. The corresponding microstructure is expected to comprise -14% bainite, -80% acicular ferrite and -6% grain boundary ferrite. This level of strength is what is required to be attained in reinforcing steels as per BIS:1786 specification. It is therefore evident that Fe415 grade reinforcement wire rods could also be manufactured by using the microalloyed chemistry. For attaining strengths equivalent to those in the Fe500 and Fe550 grades convincingly, it is suggested that the amount of Nb and C in the presently designed microalloyed chemistry be slightly raised. Studies on high tensile wire rods / bars produced through TMCP route were conducted on the material collected from Krivoy Rog Steel Plant in the Ukraine Republic. The 18, 14, 20, 16 and 10 mm dia. reinforcement rods conforming to A-III, At-III, At-IV, At-V, and At-VI classes were produced from steel grades 35MnSi, 4KP, 27MnSi, 08Mn2Si and 35MnSi, respectively (GOST:5781). The rods were subjected to tensile testing, hardness measurements, optical metallography and transmission electron microscopy for arriving at the structure property correlations. i Optical metallography revealed that in the TMCP rods a microstructural gradient existed across the diameter due to partial quenching and subsequent auto-tempering. TEM investigations at three different depths helped in confirming that the TMCP rods, in general, contained tempered lath martensite near the surface (rim), bainite and/or degenerate pearlite at some intermediate depth and pearlite-ferrite structure within the centre. Hardness measurements across the diameter also indicated the presence of harder phases in the layers near to the surface and comparatively softer microconstituents away from it. In rods of different classes, the level of average hardness varied from 200 to 450 Hv depending upon the chemical composition and the rim thickness. Similarly, the tensile properties also varied with the volume fraction of martensite; YS varied between 459-1143 MPa, UTS between 590-1459 MPa and total elongation from 25 to 12 %, respectively. For this class of material a linear relationship, as given below, existed : UTS (MPa) = 3.22 Hvay. + 10 The fraction of cross section transformed to martensite has been expressed in terms of an index, named as 'rim thickness ratio, (w)' which was interestingly found to be related to the tensile properties. On analysing the results it was revealed that upto a rim thickness ratio of -0.20, tensile properties were controlled by volume fraction of pearlite while beyond this value, it was the volume fraction of martensite which predominantly influenced the properties. Accordingly, the nature of correlations in the two stages differed as would be evident by looking at the regression models : i) For thickness ratio _< 0.20 YS (MPa) = 460 + 926y~ UTS (MPa) = 607 + 892W1 TEL (%) = 25 + 54yi - 459912 ii) For thickness ratio > 0.20 xii YS (MPa) = 284 + 1693'v UTS (MPa) = 270 + 2394W TEL (%) = 25.7 - 604w + 6542w2 With the help of the above relationships it could be derived that a rim thickness ratio of about 0.15 is useful in attaining YS of -600 MPa, UTS of -P700- MPa and total elongation of -21 %. This level of tensile properties is consistent with the requirements of Fe550 grade reinforcement (BIS:1786). Thus, the TMCP bars originally desired to produce Fe415 and Fe500 grades of reinforcing material, could also be effectively used to produce a still higher strength of reinforcing material without a change in chemistry. Production costs as estimated on the basis of Standard Cost Statement 1995-96, as put out by the Bhilai Steel Plant came out to be Rs 8839/ton, Rs 9122/ton and Rs 9085/ton for 10 mm dia. wire rods processed through C-Mn, microalloyed and TMCP routes, respectively. These figures indicated that for producing wire rods not intended to be used in welded condition, the C-Mn route seems to be most economical. However, for producing weldable quality reinforcing steel, either of the remaining two routes may be adopted since their production costs are comparable. On comparing the expected life span of the microalloyed and TMCP rods, however, it emerges that the latter are prone to stress corrosion cracking due to presence of surface strains developed during martensitic transformation. This in turn shortened the life span by about 45%, thereby increasing the steel consumption. Steel cost to the consumer in that case comes out to be Rs 13174/ton which is the highest amongst the three options. It is thus implied that the most useful option for producing high tensile weldable quality wire rods is the microalloyed route. XII'