The Relationship Between Microstructural Evolution and Mechanical Properties of Heavy Plate of Low-Mn Steel During Ultra
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, Post-doctor, ZHAO-DONG WANG, Professor, BING-XING WANG, Lecturer, and GUO-DONG WANG, Academician, are with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, P.R. China. Contact E-mail: [email protected] R.D.K. MISRA, Professor, is with the Center for Structural and Functional Materials Research and Innovation and Department of Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX 79968-0521. Manuscript submitted January 19, 2015. Article published online April 30, 2015 2834—VOLUME 46A, JULY 2015
Thermomechanical controlled processing (TMCP) involving ultra fast cooling (UFC) technology is being currently applied to industrial production,[1–3] with the aim to reduce the consumption of alloying elements and make the steel making process economically viable.[4,5] Heavy plate products of C-Mn steel are processed by TMCP for structural applications.[6] However, there is difficulty in obtaining uniform microstructure in heavy plates because of non-uniform deformation and nonuniform distribution of accelerated cooling along the thickness direction that leads to inhomogeneous microstructure across the plate thickness.[7,8] In order to obtain near-uniform microstructure and similar mechanical properties from the surface to the center of plate, fast and effective cooling process is necessary. In this regard, the ongoing developments in UFC technology,[9,10] with strict control and faster cooling rate on the run-out table, provides a high degree of undercooling and potential for control of microstructure and phase transformation in water-cooled plates during the cooling process.[11] In the present study, the potential of UFC in the processing of C-Mn heavy plates together with reduction in Mn-content is illustrated. UFC factors such as UFC stop temperature and the accompanying relationship between microstructural evolution, mechanical properties, and cooling rate is elucidated. The nominal chemical composition of steel (in wt pct) was Fe-0.16 pctC-0.18 pctSi-1.0 pctMn-0.015 pctP0.003 pctS. Heavy steel plates of 20 and 40 mm thickness were industrially processed using the UFC process. Keeping in mind the microstructural and mechanical property benefits that may be derived from the UFC process, Mn-content was reduced by ~0.3 to 0.5 wt pct in relation to the conventional composition of 1.3 to 1.5 pct specified in grade Q345B steel. The rolling temperature was ~1373 K (1100 C) and finishing rolling temperature was controlled at ~1123 K (850 C). The parameters of hot rolling for 20 mm plate are presented in Figure 1. The start-cooling and stopcooling temperature of UFC was ~1073 K and 873 K (800 C and 600 C), respectively. To define the limits or boundaries of microstructural evolution that can be obtained in UFC, simulation experiments were carried out using F450 mm experimental rolling mill equipped with ultra fast cooling equipment. Here sheets were processed rather than plates because of the load capacity of the mill. The microstructural evolution experi
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