S. K. Chaudhuri

Development of API X-70 Grade Plates and Strips in SAIL

API X-70 grade plates and hot strips microalloyed with Nb, V, and Ti have been developed simultaneously from two units of SAIL at Bhilai and Bokaro Steel Plants, respectively. Suitably designed steel (C: 0.1% max., Mn: 1.7% max., Nb: 0.06% max., V: 0.04% max., and Ti: 0.02% max.) was processed through vacuum arc degassing (VAD), and cast into 240 mm thick slabs and hot rolled into 20 mm thick plates, deploying an appropriate thermomechanical controlled processing (TMCP) technology. The tensile properties (YS: 473–531 MPa, YS/UTS: 0.74–0.88, %El: 32–48%) and Charpy impact toughness properties (CIE: 113–159 J at 0°C) were found to be quite attractive, owing to their fine microstructure of ferrite (∼10 µm) and low pearlite (volume fraction of ∼12%). In case of hot strips, the chemistry was suitably modified with leaner alloy chemistry (C: 0.08% max., Mn: 1.5% max., Nb: 0.06% max., V: 0.05% max., and Ti: 0.02% max.) and slabs were control rolled into 10/12 mm thick strips followed by accelerated cooling prior to coiling. The mechanical properties of hot strips varied within a narrow range of YS: 535–557 MPa, YS/UTS: 0.86–0.88, %El: 33–50%, and Charpy impact toughness property: 150–180 J at 0°C showing appearance of finer ferrite grains: ∼8 µm. Examination of precipitates in both plates and hot strips revealed presence of fine NbV(CN) and complex M4C3 type coarse carbides. The mechanical properties of Electrical Resistance Welding (ERW) pipes manufactured using hot strips satisfied all specified requirements of American Petroleum Institute (API) norms. Both the products are now ready for their commercial exploitation.

Development and characterization of steel containing very fine ferrite grains

Ferrite grain sizes of the order of 1 to 2 µm were obtained by optimizing the strain, strain rate, the stage of cooling, as well as the cooling rate during hot rolling of 0.15C-0.92Mn-0.01Si-0.036S-0.04P-0.013Nb steel. It was found that in single-pass rolling of a 10 mm plate to a thickness of 3.5 mm with an entry temperature of 800 °C, and early-stage water cooling, very fine grains of ferrite (1–2 µm) were formed at the surface and in subsurface regions. It was also found that the threshold level of reduction during rolling, which is required for the refinement of ferrite grains, is >50%. The 3.5 mm thermomechanically processed plate was found to possess very attractive mechanical properties in terms of the yield strength (485 MPa), the ultimate tensile strength (UTS) (763 MPa), and particularly the yield strength to ultimate tensile strength (YS/UTS) ratio (0.63). This combination of properties can be explained on the basis of the composite microstructure consisting of ferrite and bainite that was obtained as a result of the thermomechanical processing.

Influence of Slab Reheating Practice on the Microstructural Evolution and Toughness Behavior of Quenched and Tempered Plates of Medium Carbon Microalloyed Steel

In the current study, granular bainite was found to be the major component in the microstructure of air cooled 80 mm thick plates of medium carbon microalloyed steel. The second constituent in this granular bainite was identified as cementite. It was further observed that (1) ferrite lath size and (2) amount of cementite in granular bainite varied with slab reheating time before plate rolling. Smaller ferrite laths and a lesser amount of cementite were found in the plate processed with the longer slab reheating time of 26 h. Contrary to this, very large sized ferrite laths and a larger population of cementite were formed in the plate processed with the shorter slab reheating time of 4 h. Subsequent quenching and tempering of these plates favored the formation of lower bainite and tempered martensite in the plate with 26 h slab reheating time. On the other hand, upper bainite and coarser cementite were formed after the quenching and tempering of the plate with 4 h of slab reheating time. The influence of different microstructures, formed due to varied slab reheating time, on the toughness property of tempered plates was evaluated under different test conditions. In tensile test and fracture toughness testing of thinner specimens, a ductile mode of fracture was observed, irrespective of varied microstructures in the tempered plates. However, in the three-point bend test of full thickness specimens, the mode of fracture was ductile in the tempered plate with 26 h slab reheating time, while the tempered plate from the slab with 4 h reheating time gave rise to a predominantly brittle mode of fracture. These observations showed that the toughness property of these tempered plates was sensitive to the microstructure only under the specific condition, which prevailed during the three-point bend test of full thickness specimens. Under this condition, coarse cementite and upper bainite became prone to cracking resulting in a lower toughness of the tempered plate associated with lower slab reheating time.

Microstructure and texture formation in high strength cold rolled and annealed sheet and their correlation with formability property

A comprehensive understanding was developed on the evolution of microstructure and texture during annealing of Nb bearing microalloyed drawing quality high strength cold rolled (HSCR) sheet with specific reference to cold rolling and annealing practices. Further, this study aimed at establishing the best combination of these processing parameters to achieve an YET value of 1.4 min. For a hot rolled microstructure of moderately coarse ferrite grain of size 16 µm, it was observed that 70% cold reduction guaranteed high intensity of (222) component accompanied with minimum intensity of (200) component. Further, investigation was carried out to understand the influence of annealing time on recrystallization behavior and texture development during intermediate annealing of 60% and 70% cold reduced specimens at 550 °C. It was found that recrystallization ceased after 12 h of intermediate annealing at 550 °C. The textures and microstructures produced during final annealing of 70% cold rolled specimens at various temperatures like 670, 690, 710, and 730 °C with varied duration of soaking (12–18 h) were critically examined. An YET value of ∼1.5 was achieved in HSCR microalloyed steel when 70% cold rolled specimens were intermediately annealed at 550 °C for 12 h followed by final annealing at 710 °C for 12 h.

Microstructural Control in Aluminium-Killed Low C-Mn Boron Containing Steel for Improved Formability and Cold Reducing Properties

Microstructural control through thermo-mechanical simulation has been attempted in aluminum-killed low carbon-manganese boron containing steel. The results from dilatometric studies were used to explain austenite decomposition characteristics under different soaking temperatures and cooling rates. A significant lowering in Ar3 temperature was observed when the steel was soaked at 1200°C followed by rapid cooling (20°C/sec). On the contrary, no appreciable change in Ar3 temperature was noticed when soaking temperature was brought down to 950°C. Hot rolling simulations were carried out both in austenitic and ferritic regions to understand microstructural evolution. Ferritic hot rolling at 750°C followed by coiling at 650°C exhibited formation of coarse recrystallized ferrite grains of 30 micron, which is ideally suited for cold forming and reducing applications.

Influence of alloying elements and microstructure on the corrosion behaviour of some low alloy steels

The influence of alloying elements (0.43Cr, 0.75Cr and 0.39Cr + 0.25Cu) and microstructure on the corrosion behaviour in both hot rolled as well as in quenched and tempered conditions was studied through polarization testin the Tafel region, time-potential measurement and weight gain measurement. All these tests revealed the superiority of 0.39 Cr + 0.25 Cu alloyed steel over others in hot rolled condition (ferrite + pearlite microstructure) and of the 0.43Cr steel in the quenched and tempered condition (tempered martensite microstructure). In general, tempered martensite microstructure resulted in more negative corrosion potential. However, the potential was found to decrease continuously towards the more negative side following a logarithmic behaviour for all the alloys and microstructure showing continued corrosion process in 3.5% NaCI solution. X-ray analysis of oxide formed on the surface of the steel after corrosion revealed it to be a mixture of Fe 3 O 4 , β-FeOOH and γ-FeOOH. However, scanning electron microscopy and electron probe microanalysis revealed the presence of Cr in the inner oxide layer, making it more compact / dense as compared to the outer oxide layer where no Cr was observed. A depth profile study carried out by Auger electron spectroscopy also revealed the presence of Cr in the oxide.