Abstract:The authors design two kinds of Mo–Nb and Ni–Mo–Ti microalloyed steels based on C–Mn–Si steel using quenching and partitioning (Q&P) processing. The results indicate that the addition of microalloying elements increases retained austenite fraction, however, the average carbon concentration in retained austenite is reduced. Nano‐sized NbC and TiC precipitates are separately observed in the Mo–Nb and Ni–Mo–Ti microalloyed steel. Both precipitates can refine prior austenite grains as well as martensite packets an… Show more
“…Remarkably, this suggests that when welding high-strength low alloy steel (HSLA), an increase in the cooling rate leads to an increase in the formation of M. This increase in M has a negative effect on the mechanical properties of the welds. Yan et al [26] confirmed this argument by analyzing the micro-alloying elements, such as carbone, manganese, and silicon, on the microstructure of quenching and partitioning (Q&P) steels. They noted that an increase in the yield strength of molybdenum (Mo)-niobium (Nb) micro-alloyed steel, treated by Q&P, was caused by the precipitation strengthening of NbC (niobium-carbon) and a refinement of the martensitic structure.…”
The effect of heat input on the microstructure and mechanical properties of dissimilar S700MC/S960QC high-strength steels (HSS) using undermatched filler material was evaluated. Experiments were performed using the gas metal arc welding process to weld three samples, which had three different heat input values (i.e., 15 kJ/cm, 7 kJ/cm, and 10 kJ/cm). The cooling continuous temperature (CCT) diagrams, macro-hardness values, microstructure formations, alloy element compositions, and tensile test analyses were performed with the aim of providing valuable information for improving the strength of the heat-affected zone (HAZ) of both materials. Micro-hardness measurement was conducted using the Vickers hardness test and microstructural evaluation by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The mechanical properties were characterized by tensile testing. Dissimilar welded samples (S700MC/S960QC) with a cooling rate of 10 • C/s (15 kJ/cm) showed a lower than average hardness (210 HV5) in the HAZ of S700MC than S960QC. This hardness was 18% lower compared to the value of the base material (BM). The best microstructure formation was obtained using a heat input of 10 kJ/cm, which led to the formation of bainite (B, 60% volume fraction), ferrite (F, 25% volume fraction), and retained austenite (RA, 10%) in the final microstructure of S700MC, and B (55%), martensite (M, 45%), and RA (10%), which developed at the end of the transformation of S960QC. The results showed the presence of 1.3 Ni, 0.4 Mo, and 1.6 Mn in the fine-grain heat-affected zone of S700MC. The formation of a higher carbide content at a lower cooling rate reduced both the hardness and strength.
“…Remarkably, this suggests that when welding high-strength low alloy steel (HSLA), an increase in the cooling rate leads to an increase in the formation of M. This increase in M has a negative effect on the mechanical properties of the welds. Yan et al [26] confirmed this argument by analyzing the micro-alloying elements, such as carbone, manganese, and silicon, on the microstructure of quenching and partitioning (Q&P) steels. They noted that an increase in the yield strength of molybdenum (Mo)-niobium (Nb) micro-alloyed steel, treated by Q&P, was caused by the precipitation strengthening of NbC (niobium-carbon) and a refinement of the martensitic structure.…”
The effect of heat input on the microstructure and mechanical properties of dissimilar S700MC/S960QC high-strength steels (HSS) using undermatched filler material was evaluated. Experiments were performed using the gas metal arc welding process to weld three samples, which had three different heat input values (i.e., 15 kJ/cm, 7 kJ/cm, and 10 kJ/cm). The cooling continuous temperature (CCT) diagrams, macro-hardness values, microstructure formations, alloy element compositions, and tensile test analyses were performed with the aim of providing valuable information for improving the strength of the heat-affected zone (HAZ) of both materials. Micro-hardness measurement was conducted using the Vickers hardness test and microstructural evaluation by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The mechanical properties were characterized by tensile testing. Dissimilar welded samples (S700MC/S960QC) with a cooling rate of 10 • C/s (15 kJ/cm) showed a lower than average hardness (210 HV5) in the HAZ of S700MC than S960QC. This hardness was 18% lower compared to the value of the base material (BM). The best microstructure formation was obtained using a heat input of 10 kJ/cm, which led to the formation of bainite (B, 60% volume fraction), ferrite (F, 25% volume fraction), and retained austenite (RA, 10%) in the final microstructure of S700MC, and B (55%), martensite (M, 45%), and RA (10%), which developed at the end of the transformation of S960QC. The results showed the presence of 1.3 Ni, 0.4 Mo, and 1.6 Mn in the fine-grain heat-affected zone of S700MC. The formation of a higher carbide content at a lower cooling rate reduced both the hardness and strength.
“…It should be noted that the carbon content in the retained austenite was much higher than the initial average alloy composition. This higher carbon content and the presence of other alloying elements stabilized the retained austenite and/or martensite/austenite constituents to room temperature 1 – 3 , 13 , 14 . Figure 4 X-ray diffraction peaks of quench and partitioning heat treated microalloyed low carbon steel for ferrite ( a ) and austenite ( b ) used to estimate the amount of retained austenite.…”
Section: Resultsmentioning
confidence: 94%
“…Microalloyed steels are in high demand in industries, including oil and gas transportation sectors as well as automobile industries. Alloying elements affects the phase transformation behavior during processing and ensues better mechanical properties 1 – 3 . Demand by the oil and gas transportation pipeline industry for steel with better mechanical properties, mainly strength and toughness, for more effective and efficient oil and gas transportation has led to the development of a series of pipeline steels.…”
In this study, a microalloyed low carbon steel was subjected to quenching and partitioning (Q&P) heat treatment processes. The primary ferrite-pearlite microstructure of the steel was transformed into a bainitic microstructure containing interlath and sporadic blocks of retained austenite. The applied heat treatment process partitioned the carbon into the retained austenite to a weight percentage of 0.136. The microalloyed low-carbon steel acquired a continuous yield with high yield strength, a gigapascal level of ultimate tensile strength (i.e., ~ 1.1 GPa), and a very low yield ratio (i.e., 0.55) while retaining reasonable ductility and toughness when compared to the preheat-treated values.
“…This approach is principally limited to thin-gage strip products and requires specific adaptations of continuous annealing lines or galvanizing lines to allow fast reheating. Alloying concepts, including the benefits of molybdenum alloying, are similar to those for TBF steel [53]. Like martensitic steels, TBF as well as Q&P steels are sensitive to hydrogen embrittlement.…”
Considerable progress in developing flat-rolled steel grades has been made by the Chinese steel industry over the recent two decades. The increasing demand for high-performance products to be used in infrastructural projects as well as in production of consumer and capital goods has been driving this development until today. The installation of state-of-the-art steel making and rolling facilities has provided the possibility of processing the most advanced steel grades. The production of high-performance steel grades relies on specific alloying elements of which molybdenum is one of the most powerful. China is nearly self-sufficient in molybdenum supplies. This paper highlights the potential and advantages of molybdenum alloying over the entire range of flat-rolled steel products. Specific aspects of steel property improvement with respect to particular applications are indicated.
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