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Resistance spot welding studies at varying current with an 8 mm electrode on steel grades DP980, DP690, TRIP980 and TRIP780, established that peak strength with sound weld could be achieved at a current range of 9 to 10 kA, where the nugget diameter was between 6.9 and 7.5 mm. The joint efficiency, measured as hardening ratio was highest in TRIP690 at 2.22 and for higher strength grade it was between 1.37 to 1.51. The softening ratio in the HAZ associated with tempering of preexisting martensite, was 0.86 to 0.9. Higher fusion strength was associated with alloying content that increased resistance that increased weld pool fusion that enhanced the nugget diameter and hence the strength. The TRIP steel at high cooling rate showed martensite and retained austenite and the DP steel showed martensite content in higher grade and dispersed ferrite with martensite in the lower DP grade. The Tensile shear strength was lowest (16kN) for DP780, while for other grades it is between 20 to 22 kN. The strength enhancement in TRIP steel was due to higher weld diameter at higher current, where partial pull out failure mode was observed. At lower current, the interfacial pull out failure mode was observed with poor fusion and at high currents partial pull out at HAZ is observed. The Coach peel strength was highest at 3.2 kN for DP780, 2.2 kN for DP980, 2 kN for TRIP 690 and 1.7kN for TRIP 980 and follows the trend in TSS. The observed failure may be attributed to the high fusion strength in TRIP steel due to sound nugget and a microstructure of martensite with retained austenite. The lower strength with DP steels is attributed to low alloyed lath martensite in DP 980 and the lower grade DP780 is softer associated with ferrite dispersion in martensite. At very high currents, the fusion is good but there is pull out at HAZ due to the softening associated with the tempering of preexisting martensite that influenced the failure at HAZ failure mode.

Three low-carbon (~0.1 wt.% carbon) grades of structural steel with varying manganese content (0.75-1.5 wt.%) with ferrite-pearlite microstructure were used for the study. The 1.5 % manganese steel is a microalloyed steel with vanadium, niobium, and titanium content, which is strengthened by interphase precipitates. The steels were tensile tested at room temperature and in the temperature range between 200°C-700°C, at 100°C intervals. The elevated temperature flow strengths of the steels studied show higher strength values for the higher manganese steel. It was found that the manganese enhanced the strength at all ranges of test temperatures. The retention factor of the conventional structural steels was less than or equal to 0.50 against 0.66 for a fire-resistant steel grade. The study shows that the addition of elements such as chromium, molybdenum is required to achieve a higher retention factor to fulfill the requirement of fire resistance characteristics. [ABSTRACT FROM AUTHOR]

Mold flux entrapment during continuous casting of steel contributes to both surface and sub-surface defects in the final product. Continuous casting operating parameters such as casting speed, SEN immersion depth, SEN port geometry, argon flow, and mold EMS significantly affect the mold flow conditions and flow profile. During continuous casting operation, SEN immersion depth is continuously varied to avoid localized erosion of SEN, and it impacts the flow dynamics in the mold. In the present work, water modeling studies were carried out for a wide range of mold widths (1200-1800 mm) and casting speeds (0.8-1.4 m/min) on a 0.5 scaled down water model to optimize casting speed for different combinations of SEN immersion depth and mold width. Results from water modeling were further validated using nail board studies in the actual plant. A safe operating matrix was identified from these experiments to avoid mold slag entrapment during continuous casting.