Reasonable Temperature Schedules for Cold or Hot Charging of Continuously Cast Steel Slabs
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I.
INTRODUCTION
THERE are some continuously cast steel slabs that are prone to transverse fracture upon transportation or handling from their room temperature storage state, including high-strength low-alloy (HSLA) hypo-peritectic steels (e.g., Q550D and SQ600 grades), high-strength alloyed tool steels (e.g., 28Cr2Mo grade), high manganese steel, and ferritic stainless steels (AISI 430 grade). It is also frequently observed in domestic steel plants that some slabs are sensitive to the formation of surface cracks during the following rolling process in a certain hot charging temperature range for grades such as C-Mn steels, case hardening steels, and microalloyed steels.[1] Additionally, since mixed charging operation is often practiced for slabs with different temperature histories during production, the quality of the rolled products under the same reheating schedule is unsteady, especially for the batch steels with different hot crack susceptibility. Experiments[2,3] show that the fracture surface of slabs during cold charging is usually transversely cleaving with almost no macroscopic plastic deformation, where a mass of upper brittle texture is observed, possibly along with shrinkage defects or internal cracks. It is shown that the internal stress and brittle structure can be eliminated if a reasonable slow cooling
YANG LI, Doctor Student, JING WANG and JIN WEN, Master Students, and JIAQUAN ZHANG, Professor, are with the Department of Metallurgical Engineering and State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 100083 Beijing, P.R. China. Contact email: [email protected] XIN CHEN, Metallurgy Engineer, is with the Shanghai Baosteel Trading Co., Ltd., 200940 Shanghai, P.R. China. KE LIU, Metallurgy Engineer, is with the Shougang Research Institute of Technology, 100043 Beijing, P.R. China. Manuscript submitted August 31, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
schedule is adopted for those steel slabs discharged from the caster, and thus the cold charging crack susceptibility would be reduced remarkably.[4] On the other hand, it is indicated that the charging temperature may affect the properties and microstructure of rolled products.[5] The major cause of the slab surface crack after hot charging during the rough rolling process is possibly the reduced ductility due to the precipitation of pre-eutectoid ferrite film or carbonitrides at the primary austenite grain boundaries. For the determination of reasonable temperature schedules during charging operation for given steel grades, the effect of temperature and cooling rate on both the solid phase transformation and precipitation is studied in this article. Additionally, the CCT diagrams and the sensitive hot charging temperature range of related steels are revealed for a better understanding of the microstructure related crack susceptibility.
II. QUALITY ISSUES OF SLABS UNDER DIFFERENT TEMPERATURE SCHEDULES A. Mechanism of Slab Fracture During Cold Charging Some continuously cast steel slabs are sensitive t
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