High temperature SANS experiments on Nb(C,N) and MnS precipitates in HSLA steel
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GRAIN refinement is a powerful metallurgical mechanism to improve the mechanical properties of a material. Both strength and toughness properties improve with decreasing grain size. In hot rolling of high-strength lowalloy (HSLA) steels, a combination of two metallurgical mechanisms is applied to reduce the final grain size of the material: thermomechanical rolling and accelerated cooling. Thermomechanical rolling involves rolling in the temperature region where the austenite no longer recrystallizes after each rolling pass in the finishing mill. This results in accumulated strains over several passes, which is a favorable condition for the following accelerated cooling on the run-out table, usually with water. The heavily deformed austenite has a large interfacial area, and sometimes pronounced intragranular deformation bands, which act as additional sites for ferrite nucleation during the allotropic phase transformation. By adding small amounts of niobium (usually less than 0.05 wt pct) to low-alloy steels, the temperature below which no recrystallization of the austenite takes place is increased. This allows higher accumulated strains in the austenite range and hence leads to smaller ferrite grain sizes being formed during accelerated cooling. Whether the recrystallization is retarded as a result of the drag by solute niobium atoms[1] or by the pinning forces exerted by small Nb(C,N) precipitates[2] is still a matter of debate. The theory combining both explanations is that the solute drag retards recrystallization and this allows sufficient time N.H. VAN DIJK and W.G. BOUWMAN, Scientists, S.E. OFFERMAN, Ph.D. Student, and M.Th. REKVELDT, Senior Scientist, are with the Interfaculty Reactor Institute, Delft University of Technology, 2629 Delft, The Netherlands. Contact e-mail: [email protected] J. SIETSMA, Senior Scientist, and S. VAN DER ZWAAG, Professor, are with the Laboratory of Materials Science, Delft University of Technology, 2628 AL Delft, The Netherlands. A. BODIN, Researcher, is with CORUS Research, Development & Technology, 1970 CA IJmuiden, The Netherlands. R.K. HEENAN, Instrument Responsible, is with the Rutherford Appleton Laboratory, ISIS, Chilton, Didcot, OX11 0QX, United Kingdom. Manuscript submitted May 16, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
for the Nb(C,N) precipitates to form.[3] Further precipitation may occur upon slow cooling to ambient temperature after coiling of the steel in the hot strip mill. The formation and dissolution of the precipitates in the austenite has been studied quite extensively using ex-situ techniques such as transmission electron microscopy (TEM) and chemical dissolution.[4–8] A major complication in ex-situ observations is the unavoidable austenite-ferrite phase transformation. Nb(C,N) precipitates, which were not present in the austenite, may form upon cooling due to the reduced solubility in ferrite. Hence, it would be highly desirable to obtain in-situ information on the Nb(C,N) precipitates in austenite. In this study, small-angle neutr
