Effect of cooling rate on the microstructure and mechanical properties of a low-carbon low-alloyed steel

Abstract

Heavy plate steels with bainitic microstructures are widely used in industry due to their good combination of strength and toughness. However, obtaining optimal mechanical properties is often challenging due to the complex bainitic microstructures and multiple phase constitutions caused by different cooling rates through the plate thickness. Here, both conventional and advanced microstructural characterization techniques which bridge the meso- and atomic-scales were applied to investigate how microstructure/mechanical property-relationships of a low-carbon low-alloyed steel are affected by phase transformations during continuous cooling. Mechanical tests show that the yield strength increases monotonically when cooling rates increase up to 90 K/s. The present study shows that this is associated with a decrease in the volume fraction of polygonal ferrite (PF) and a refinement of the substructure of degenerated upper bainite (DUB). The fine DUB substructures feature C-rich retained austenite/martensite-austenite (RA/M-A) constitutes which decorate the elongated micrograin boundaries in ferrite. A further increase in strength is observed when needle-shaped cementite precipitates form during water quenching within elongated micrograins. Pure martensite islands on the elongated micrograin boundaries lead to a decreased ductility. The implications for thick section plate processing are discussed based on the findings of the present work.

Estimation of Cooling Rate of High-Strength Thick Plate Steel during Water Quenching Based on a Dilatometric Experiment

“…Figure 4 shows the dilatation behavior of an AQ specimen during heating and cooling. With a decreasing cooling rate, volume expansion occurs at a higher temperature during cooling, which is consistent with the previous studies [ 11 , 12 , 13 ]. Therefore, martensite can be transformed at a higher cooling rate, and bainite or ferrite can be transformed as the cooling rate becomes slower in the AQ specimen.…”

“…As the thickness increases, it is difficult to obtain a uniform microstructure in the thickness direction, leading to a complex microstructure composed of various phases such as martensite, bainite, and ferrite inside thick plates after water quenching. Although many researchers have tried to predict cooling rates in the thickness direction of thick plate steels during water quenching, a systematic analysis of cooling rates in thick plates has yet to be reported [ 13 , 14 , 15 , 16 ]. H. Wang et al [ 13 ] studied the effect of cooling rate on microstructure and mechanical properties in low-carbon, low-alloyed steel plates using a dilatometric experiment.…”

“…Although many researchers have tried to predict cooling rates in the thickness direction of thick plate steels during water quenching, a systematic analysis of cooling rates in thick plates has yet to be reported [ 13 , 14 , 15 , 16 ]. H. Wang et al [ 13 ] studied the effect of cooling rate on microstructure and mechanical properties in low-carbon, low-alloyed steel plates using a dilatometric experiment. However, the authors focused on how the microstructure and mechanical property relationships of a low-carbon, low-alloyed steel are affected by phase transformations during continuous cooling, not on the estimation of the cooling rate in a thick steel plate.…”

Effect of Mo and Cr on the Microstructure and Properties of Low-Alloy Wear-Resistant Steels

“…The results show that the additions of Cr, Mo, and Nb increased the rate of the formed martensite, which improved the hardness, wear resistance, and tribological behavior. Some studies also suggest that Mo can delay the growth of austenite grains, promoting grain refinement, and that both Mo and Cr can reduce the critical cooling rate for austenite transformation, facilitating bainitic transformation at low cooling rates and martensitic transformation at high cooling rates [20][21][22][23]. The Mo element can effectively increase the hardenability of steel and is one of the alloying elements commonly used to improve the hardenability of thick-specification wear-resistant steel plates, but the Mo element belongs to the precious alloying elements and scarce resources.…”

Effect of Microstructural Evolution on the Mechanical Properties of Ni-Cr-Mo Ultra-Heavy Steel Plate

“…The different cooling rates across the thickness under the quenching process is the main factor that affects the microstructural homogeneity [ 5 , 6 , 7 ]. In general, a heavy plate under the roller quenching process mainly consisted of three types of microstructure: martensite (M), martensite/lath bainite (M/LB) and lath bainite/granular bainite (LB/GB) [ 8 , 9 , 10 ]. Tempered martensite usually has an excellent combination of strength and toughness [ 11 ].…”