Materials technology Influence of TiN particles distribution on static recrystallisation in structural steels at reheating temperature Sebastian F. Medina, Marla Isabel Vega and Alberto Quispe Results of investigations concerning the influence of Ti and N contents on static recrystallisation kinetics are presented. By means of torsion tests, and applying the back extrapolation method, the statically recrystallised fraction has been determined for four structural steels with different Ti and N contents. at different temperatures, equivalent strain of 0.35 and strain rate of 3.63 S·1. Two reheating tem- peratures have been used to obtain different austenite grain sizes and different precipitate mean size in steels containing Ti. The results show that a low precipitated volume and a relatively large size of precipitates. at reheating temperature, did not impede grain boundary self-diffusion during the static recrystallisation, the main mechanism governing static recrystallisation. Nevertheless, the smaller size of the precipitates acted as an obstacle to grain boundary self-diffusion. These effects were measured by calculation of the activation energy for static recrystallisation. On the other hand. the influence of austenite grain size in steels containing Ti can not be correctly determined. as at each reheating temperature, in addition to a different austenite grain. the distribution of precipitates is also different, affecting the activation energy and making it impossible to clearly appreciate the influence of austenite grain size. EinfluB der Verteilung von TiN-Ausscheidungen auf die statische Rekristallisation in Baustahlen bei Nachwarmtemperatur. Forschungsergebnisse zum EinfluB des Gehaltes an Ti und N auf statische Rekristallisationskinetik werden vorgestell!. Der statisch rekristallisierte Anteil in vier Baustahlen mit unterschiedlichen Ti- bzw. N-Gehalten wurde in Torsionsversuchen und unter Anwendung der ROck-Extrapolationsmethode ermittel!. Die Tests wurden bei verschiedenen Temperaturen durchgefOhrt, bei einer Vergleichsformande- rung von 0.35 und einer Formanderungsgeschwindigkeit von 3,63 S·1 . Unter Anwendung zweier Nachwarmtemperaturen lieBen sich in den Ti-haltigen Stahlen verschieden groBe Austenltkorner und Ausschei- dungen mittlerer GroBe erzeugen. Bei Nachwarmtempsratur fOhren ein geringer Volumenanteil an und relativ groBe Ausscheidungen zu keiner Behinderung der KorngroBenselbstdiffusion - dem die statische Rekristallisation bestimmendem Hauptmechanismus - wahrend der statischen Rekristallisation. Dennoch wirken die kleineren Ausscheidungen als Hindernisse. Diese Effekte wurden durch Berechnung der Aktivierungsenergie fOr die statische Rekristallisation ermittel!. Andererseits laBt sich der EinfluB der Austenitkornqrofse in Ti-Iegierten Stahlen nicht korrekt bestimmen, da - zusatzllch zu einer unterschiedlichen AustenitkorngroBe - die Verteilung der Ausscheidungen fOr jede Nachwarmtemperatur wiederum anders lst, Das wirkt sich auf die Aktivierungsenergie aus und macht es unrnoqlich, den EinfluB der AustenitkorngroBe klar einzuschiitzen. During hot controlled rolling, the behaviour of steels with titanium as the only metallic element acting as former of precipitates, such as nitrides or carbides, is very differ- ent from that of other microalloyed steels with niobium or vanadium. While niobium precipitates, nitrides, carbides or carbonitrides, are dissolved at the reheating temperatures, titanium nitrides (TiN) are little soluble in austenite and their complete dissolution is not possible unless the re- heating temperature is very high. The low solubility of titanium nitrides in austenite [1...3] makes Ti and N the ideal elements for controlling the austenite grain in proc- esses where austenite undergoes reheating at high tem- peratures. Structural steels must have a very fine final ferrite structure in order for their yield strength and toughness to be high, and Ti is an element that helps to achieve this. The role of TiN as austenite grain size controllers - inhib- iting grain growth - is well known, and the mechanism by which they exercise this control can be explained and interpreted by Zener and Gladman's expressions, which relate precipitate grain sizes, precipitated fraction and austenite grain size [4]. Most authors [5...9] agree that for many purposes related with austenite grain control a hypostoichiometric Ti/N ratio, generally between I and 3, provides good results, Dr. Eng. Sebastian F. Medina: Maria Isabel Vega: Dr. Alberto Quispe. Physical Metallurgy Department, CENIM-CSIC, Madrid, Spain. 24 especially when titanium starts to precipitate in solid state, as the size of the precipitates would be much smaller [10]. On the other hand, the static recrystallisation experi- enced by austenite between successive passes in any hot rolling is affected by all the variables that intervene in hot working, such as the temperature, the reduction made in the pass, the strain rate and the austenite grain size. Many models on static recrystallisation kinetics, following Avrami's law and including in their equations the afore- mentioned variables, have been published [11...13], but the influence of the size of TiN, not dissolved at the reheating temperature, on static recrystallisation kinetics has not been studied. This work aims to investigate the effect of different Ti and N contents on static recrystallisation kinetics, Two steels with Ti and two others without Ti, but with different nitrogen contents, have been studied. In the case of the steels with Ti, a study of their precipitates (TiN) was car- ried out in order to satisfactorily explain static recrystalli- sation kinetics. Experimental procedure and materials The steels used in this study were manufactured in a vacuum induction furnace using high purity electrolytic iron. The compositions of these steels are shown in table 1, and on the basis of their nitrogen contents they can be classified in two groups: those with a low nitrogen content, steel research 72 (2001) NO.1