Met. Mater. Int., Vol. 21, No. 1 (2015), pp. 85~95 doi: 10.1007/s12540-015-1010-z
Characterisation of Microstructure, Texture and Mechanical Properties in Ultra Low-Carbon Ti-B Microalloyed Steels 1
2,
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R. Shukla , S. K. Ghosh *, D. Chakrabarti , and S. Chatterjee
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Research & Development Centre for Iron & Steel, SAIL, Ranchi - 834 002 2 Indian Institute of Engineering Science and Technology, Shibpur, Department of Metallurgy and Materials Engineering, Howrah - 711 103, India 3 Indian Institute of Technology Kharagpur, Department of Metallurgical and Materials Engineering, Kharagpur - 721 302, India (received date: 7 March 2014 / accepted date: 3 June 2014) In the present study, thermo-mechanical controlled processing followed by water quenching has been utilised to produce ultra low-carbon microalloyed steel in a laboratory scale. The variation in microstructure and corresponding mechanical properties at the selected range of finish rolling temperatures (FRT), (850-750 °C) has been evaluated. The microstructures of the steels consisted of polygonal ferrite, acicular ferrite as well as granular bainite with the average ferrite grain sizes less than 5 μm. Finish rolling at 850 °C produced weak texture. -fibre and -fibre intensified with the decrease in finish rolling temperature to 800 °C. Intensities of the beneficial texture components such as, {554} and {332} also reached the highest value at 800°C. Ferrite deformation texture i.e. -fibre dominated at 750°C FRT. The characteristic ferrite - bainite microstructure with fine ferrite grain size and uniform distribution of fine TiC particles (< 50 nm) resulted in high yield strength (405-507 MPa), moderate tensile strength (515-586 MPa) and high total elongation (19-22%) for the selected range of finish rolling temperatures. Fairly good impact toughness value in the range of 63-74J was obtained at subzero temperature (-40 °C) in the sub-size sample. The above strength - ductility - toughness combination boosts the potentiality of developed steel for the pipeline application. Keywords: alloys, thermo-mechanical processing, microstructure, transmission electron microscopy, mechanical properties
1. INTRODUCTION Modern pipeline steels used for the transportation of crude oil and natural gas over large distances under high pressure, need to satisfy the requirements specified by the American Petroleum Institute (API) [1-3]. These grades are processed by thermo-mechanical controlled rolling (TMCR) and accelerated cooling route with the prime aspiration to achieve the best possible combination of strength and toughness [1-3]. Besides high strength, high fracture toughness and impact toughness are essential to prevent premature ductile and brittle failure, while formability is required for pipe-bowing. Good weldability [2], resistance to hydrogen induced blister cracking in sour service environment [4,5], resistance to stress corrosion cracking resistance for underground service, especially in H2S environment [6-8], and fatigue resistance are the additional requirements for pipeline steels [9,10]. The microstructure of steel is influenced by alloy chemistry *Corresponding author:
[email protected] KIM and Springer
and thermo-mechanical processing parameters [11]. The alloying elements commonly added in pipeline steels include Mn, Si, Ti, Nb, V, B etc. [5,6,12-14]. However, judicious selection of the alloying elements is necessary to obtain beneficial effect on mechanical properties with reduced alloy cost. For instance, the addition of alloying elements should be restricted to maintain a low carbon equivalent (CE), which can ensure good weldability [15]. Low carbon level in steel is beneficial for ductility, toughness, formability and weldability but the loss in solid-solution strengthening accompanied by the decrease in carbon level needs to be compensated by the other strengthening mechanisms [16]. Mn is an important alloying element for solid solution strengthening, however, reduced Mn content in steels decreases the centreline microstructural banding [17]. Mn improves the hardenability and promotes the formation of intermediate transformation products such as, bainitic and acicular ferrite, however, too high Mn level can cause the problem of microstructural banding as a result of inter-dendritic segregation [18]. The addition of Si up to 0.6 wt% strengthens the ferrite matrix, [19] whilst, high Si addition has an adverse effect on the
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ductile-brittle transition temperature of the steel [20]. Ti in steel forms TiN, Ti(C, N), TiC and Ti4C2S2 particles, which provides precipitation strengthening, grain refinement and sulphide shape control [21]. On the other hand, brittle TiN particles, if coarse in size, can act as the cleavage crack initiators [22]. Other microalloying elements such as, Nb and V are extensively used in pipeline steel for achieving grain refinement and precipitation strengthening [1]. However, availability of those expensive elements, their role on increasing the flow-stress and hampering the weldability impose some industrial challenges. B is generally added in low and medium carbon ‘quenched and tempered steels’ for increasing the hardenability and achieving tempered martensite structure in the rolled plates, maintaining a low carbon equivalent [23]. In the present study, B is added along with Ti for strengthening purpose in ultra-low carbon steel (0.002 wt% C), aiming at pipeline application. In the processing of pipeline steels, TMCR is the preferred route because it provides the desirable fine-grained microstructure [24,25]. The final microstructure depends on the processing parameters including reheating temperature, percentage reduction, deformation temperature, cooling rate, and cooling temperature [24]. Higher cooling rate and lower interrupted cooling temperature encourage the formation of lower bainite, acicular ferrite-based microstructure with uniform distribution of martensite-austenite (MA) islands as the second phase, which are considered to provide the desired combination of strength and toughness [12,26]. Texture is directly related to the rolling schedule [27] and the influence of various rolling parameters on the development of texture in pipeline steels is yet to be understood fully [28]. This is of paramount importance as the mechanical properties of the rolled steel plate not only depend on the microstructure but also on the texture [29,30]. In view of the above aspects, efforts have been made in the present study to develop the ultra low-carbon steel with Ti and B microalloying through TMCR for the potential pipeline application. The role of finishing rolling temperature on the microstructure, texture and mechanical properties has been investigated. The grain size, phase fractions, grain boundary misorientation distribution, micro-texture and bulk texture were analyzed in the rolled samples to establish the correlation between microstructure, texture and the resulting mechanical properties.
2. EXPERIMENTAL PROCEDURE The investigated steel was manufactured in a 25 kVA air
induction furnace with the chemical composition as listed in Table 1. After cropping the top section of the ingot containing shrinkage/pipes, the remaining ingot of 200 mm × 50 mm × 50 mm size was hot forged down to 16 mm thick slab, which was further soaked at 1200 °C for 40 minutes and subsequently controlled rolled down to 6 mm thick plate as shown schematically elsewhere [31]. Three finish rolling temperatures (FRTs) were maintained at 850 °C, 800 °C and 750 °C and finally the rolled plates were subjected to water quenching. Temperatures at different stages of rolling were measured by inserting thermocouples into the steel slab. Gleeble 3500 thermo-mechanical simulator with the quartz tipped linear variable displacement transducer (LVDT) type dilatometer was used for dilatometric study. After soaking at 950 °C for 2 minutes the samples were cooled at different cooling rates (0.5, 5, 10, 30 and 150 °C/s) and the transformation start and finish temperatures were determined from the dilatometric cooling curves. Based on the dilatometric study, continuous cooling transformation (CCT) diagram was constructed for the investigated steel. Conventional grinding and polishing techniques were applied to prepare transverse cross-sectional specimens which were subsequently etched with 2% nital solution for optical microscopy. Quantitative image analysis was performed using ImageJ software (version 1.42). Thin foils for transmission electron microscopy (TEM) were prepared by twin-jet electro polishing using an electrolyte of mixture of 90% acetic acid and 10% perchloric acid. Thin foils were subsequently observed in Philips, CM 200 model TEM equipped with EDS facility from EDAX, at 200 kV operating voltage. Electron back scatter diffraction (EBSD) analysis characterized the size and orientation of the ferrite grains using Oxford HKL Channel 5 system attached with the Zeiss EVO 60 SEM. The sample preparation for EBSD consisted of standard mechanical polishing (up to 0.25 m grit), followed by electropolishing in 5 vol.% perchloric acid and 95 vol.% acetic acid solution. A step size of 0.3 m was used to construct the EBSD maps, over 200 m × 160 m microstructural area at the quarter thickness locations of the rolled plates. In the orientation imaging microscopy (OIM), the image boundaries having misorientation angle of 15° or more are considered as the highangle boundaries, i.e., ‘grain boundaries’. Boundaries having misorientation angles between 2 and 15° are considered as low-angle boundaries, i.e., ‘sub-boundaries’. Five frames from different area of each specimen were selected for mapping and a high pattern indexing quality of around 85% was achieved. Macro-texture of the samples was studied by high resolution
Table 1. Chemical composition of the investigated steel (wt%) C Mn Si S P Ti Al B 0.002 1.35 0.25 0.011 0.010 0.06 0.030 0.0024 *Pcm[33] = C + (Si)/30 + (Mn + Cr + Cu)/20 + (Ni)/60 + (Mo)/15 + (V)/10 + 5B (wt%). **TNR [34] = 887 + 464C + (6445Nb - 644Nb) + (732V - 230V) + 890Ti + 363Al - 357Si (wt%).
N 0.008
*Pcm 0.09
**TNR (C) 863
Characterisation of Microstructure, Texture and Mechanical Properties in Ultra Low-Carbon Ti-B Microalloyed Steels
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X’pert-PRO X-ray texture goniometer operated at 45 kV voltage, 40 mA current using Co target. Step size along and angles were 5° each and the time step for the scan was 0.5 s. ASTM E8M standard [32] was followed to prepare flat tensile specimens of 25 mm gauge length and 3.5 mm thickness from the rolled plate in longitudinal direction. All tests were conducted at room temperature at a crosshead speed of 0.5 mm/min in an Instron testing machine (Model 4204). For Charpy impact tests, sub-size Charpy V-notch (CVN) specimens (55 mm × 10 mm × 5 mm) were machined from T-L orientation of the rolled plates following ASTM E23 standard [32] and tests were conducted at +25 °C and -40 °C. The average values of three different measurements are recorded.
3. RESULTS 3.1. Microstructure evolution of continuous cooled and rolled samples The CCT behaviour of the investigated steel is shown in Fig. 1. The Ac3 and Ac1 temperatures are 870 °C and 710 °C, respectively, at 0.5 °C/s heating rate. The bainite start temperature measured at the cooling rate of 30 °C/s (BS = 680 °C) fairly agrees with the estimated temperature (BS = 708 °C) [35]. The variation in hardness can be explained from the optical micrographs of the specimens for different cooling rates, Figs. 2a-e. The microstructures obtained at cooling rates of 0.5 °C/s to 30 °C/s consisted of polygonal ferrite (PF) and quasi-polygonal ferrite (QPF) grains, Figs. 2a-d. The average ferrite grain size as measured from optical micrographs reduced with the increase in cooling rate, from ~130±20 µm at 5 °C/s to 80±10 m at 30 °C/s, which resulted in an increase in hardness value from 188 HV to 219 HV. Small amount of (15° misorientation angle), also contributed to the best impact toughness for 850 °C FRT sample, Table 4. In spite of the coarser grain size, the impact toughness of 800 °C FRT sample was comparable to 750 °C FRT sample, because of the dominance of -fibre components and {332} and {554} components in 800°C FRT sample, Table 3. -fibre components and {332} and {554} components are known to be beneficial for the toughness of lowcarbon steel [59,60]. These components possibly improved the strain hardening behaviour as well, resulting in the lowest YS/UTS ratio in 800 °C FRT sample. On the other hand, highest fraction of cube texture hampered the impact toughness of 750 °C FRT sample. The Cube and Goss texture has an undesirable effect on the delamination behaviour of steels and affect the toughness negatively [60]. At lower FRT of 750 °C, ferrite grains become acicular (AF) or plate shaped (thickness
