Recrystallization controlled rolling and accelerated cooling for high strength and toughness in V-Ti-N steels
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I.
INTRODUCTION
THE
desirable properties of high-strength low-alloy (HSLA) steels are achieved basically through ferrite grain refinement and by precipitation of microalloy carbonitride particles. For economic production, it is necessary to maximize these effects, which is the aim of various types of thermomechanical controlled processing (TMCP). The most frequently referred to of these processes is lowtemperature controlled rolling (CR), i.e., below the recrystallization stop temperature. Niobium is frequently used in such HSLA steels, and a low nitrogen content ( < - 4 0 ppm) is usually recommended. Excellent results can be obtained by CR processing, but productivity tends to be poor due to delays while cooling to the lower temperature, and powerful mills are required because of the steel's high resistance to deformation under these conditions. However, as was shown previously, I~1 deformed austenite is not a prerequisite for structure refinement since the ferrite grain sizes produced from recrystallized or unrecrystallized austenite are similar if the austenite grain size is small (20/xm or less) t21 and nitrogen may be considered as a useful alloying element in microalloyed steels. 13"41This provides an alternative to CR, a recrystallization controlled rolling process (RCR), in which the microstructure of austenite is refined by a sequence of static recrystallization stages after each pass at high temperature. The most effective way of controlling the austenite grain size during hot rolling and, at the same time, STANISLAW ZAJAC, Senior Researcher, Mechanical Metallurgy Department, TADEUSZ SIWECKI, Group Leader, Thermomechanical Processes and Properties, Mechanical Metallurgy Department, and BEVIS HUTCHINSON. Head of Department, Mechanical Metallurgy Department, are with the Swedish Institute for Metals Research, S-114 28 Stockholm, Sweden. MA_RTEN ATTLEGA,RD, Senior Metallurgist, is with Svenskt St~l AB, Oxel6sund, Sweden. Manuscript submitted November 13, 1990. METALLURGICAL TRANSACTIONS A
maintaining a fine initial as-reheated grain size is provided by a dispersion of fine titanium nitrides. I51 For an effective prevention of grain coarsening, the volume fraction of the pinning particles should be high and their size small) 6~ The optimum ratio of T i : N for best grain refinement appears to be an open question. Some data I7,8~ suggest that this should be close to the stoichiometric value of 3.42: 1, whereas other data t9] indicate that an excess of N gives improved austenite grain stability. Excess titanium, on the other hand, appears to be deleterious. II~ It would be expected that precipitation will occur at the lowest temperatures (after casting) in the stoichiometric case, thereby giving the finest scale precipitation. However, if grain coarsening on reheating is controlled by coarsening of the TiN particles, then an excess of nitrogen, by reducing the content of free Ti, should provide for greater stability. When the titanium content is optimized for grain size control by TiN, it is obvious that th
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