Effect of Rolling Temperature and Ultrafast Cooling Rate on Microstructure and Mechanical Properties of Steel Plate
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UCTION
OVER the last three decades, thermomechanical controlled processing (TMCP) has been successfully applied to manufacture large quantities of steel for diverse applications, such as shipbuilding, pipelines, and machinery.[1] TMCP provides flexibility in terms of controlled rolling, accelerated cooling, and addition of microalloying elements.[2] In the 1990s, a new fast cooling technology called super-OLAC (super-online accelerated cooling), which gives a two to five times higher cooling rate than conventional accelerated cooling, was developed by JFE company in Japan.[2] Recently, a new generation of TMCP was developed in our laboratory based on ultrafast cooling (UFC) and applied on industrial scale to meet increasing demands for high-performance steels.[3–6] In an attempt to improve production efficiency, higher finish rolling temperatures (FRTs) are used in UFC than in conventional TMCP.[6] This could reduce the rolling load, with lower yield strength (YS) and austenite recrystallization at higher temperatures.[7] This change in rolling strategy, in combination with the higher cooling rate in UFC, alters the final microstructure transformed from austenite. It has been shown that QIBIN YE, Doctor, and ZHENYU LIU and GUODONG WANG, Professors, are with The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819, P. R. China. Contact e-mail: [email protected] YU YANG, Senior Engineer, is with the Institute of Iron and Steel Research, Ansteel Group Corporation, Anshan, 114009, P. R. China. Manuscript submitted June 12, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
low-temperature transformed constituents, such as bainitic ferrite, acicular ferrite, and even martensite/ austenite (M/A) constituent, can be formed in conventional ferrite–pearlite steels when UFC is applied.[8,9] Moreover, there is a distinct possibility that UFC can result in microstructural variation through thickness in heavy-gauge plates. In the centerline regions of steel plates, the local hardenability is dramatically increased, stimulating the occurrence of abnormal banding which is susceptible to hydrogen-induced cracking and delamination fracture, leading to disastrous failures of steel structures.[10,11] Microsegregation-induced ferrite–pearlite banding has been extensively studied, and the general conclusion is that its formation can be effectively suppressed by increasing the cooling rate.[10,12–18] However, little investigation has been done on microstructural banding in centerline regions of hot-rolled steel plates. The objective of this study is to explore the effect of process variables associated with UFC in an attempt to minimize the microstructural variation through thickness of plates and consequently their mechanical properties. An accompanying objective is to study the impact of UFC on microstructural banding in centerline segregation regions.
II.
EXPERIMENTAL
Figure 1 shows a schematic of the processing details applied for three plates of AH36 grade steel for shipbuilding. The chemical comp
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