Precipitation of titanium nitride in low-alloyed steel during cooling and deformation

Abstract

The precipitation of titanium nitride in undeformed solid carbon-manganese steel has been investigated by means of metallography and electron microscopy at in-situ melted samples the cooling of which was interrupted at different temperatures for a defined time. The deformation-induced precipitation of titanium nitride was studied on samples which were in-situ melted in a torsion plastometer, deformed after solidification, re-deformed after a holding time, and quenched. Additionally, 50 mm slabs were investigated after direct rolling in a pilot plant, partially with a preceding soaking. In the range of delta ferrite fast homogenization of segregated titanium takes place. No TiN nuclei were formed. In undeformed austenite some large precipitates were formed indicating the solidification structure. In the 1200 to 1100 "C range particle sizes between 20 and 70 nm were reached. A high density of deformation-induced particles smaller than 10 nm could be obtained at or below 1050 "C. The density of the particles increased with decreasing deformation temperature. After fast cooling, below 1000 "C also in undeformed material finely dispersed precipitates were formed.Ausscheidung von Titannitrid in niedriglegiertem Stahl wiihrend des AbkOhlens und Umformens. Die Bildung von Titannitrid in unverformtem festem Kohlenstoff-Mangan-Stahl wurde metallographisch und elektronenoptisch an Proben, deren AbkOhlung nach dem Erstarren bei unterschiedlichen Temperaturen fOr vorgegebene Zeiten unterbrochen wurde, untersucht. Eine verformungsinduzierte Ausscheidung von Titannitrid erfolgte an Proben, die in einem Torsionsplastometer in-situ geschmolzen, nach dem Erstarren umgeformt, nach einer Haltezeit nochmals verformt und abgeschreckt wurden. Zusatzllch wurden 50-mm-Platinen nach Direktwalzen in einer Versuchsanlage untersucht, teilweise mit AusgleichsglOhung vor dem Walzen.

Precipitation in Interstitial Free High Strength Steels

“…13) As the solubility of TiN is very low in Fe the full stabilization of N is expected in the IFHS steels. [14][15][16] The amount of Ti (in wt%) required to tie up with N is 3.42 times of the N present in the steels. 6,17) In IFHS steels N is generally kept below 40 ppm and thus maximum Ti required to tie up with N is 0.014 wt%.…”

The Influence of Ti on the Hot Ductility of Nb-bearing Steels in Simulated Continuous Casting Process.

“…The solid lines in the figure are on the basis that bulk concentrations are equal to the initial concentrations since a certain supersaturation is necessary for precipitate nucleation. For instance, Kunze et al 17) reported that TiN particles only formed in the regions where the supersaturation was higher than 15 fold. The dash lines are also for equilibrium conditions with no concentration gradients in the matrix and precipitates, but on the assumption that (NbTi) (CN) precipitates in equilibrium at 1 150°C are removed from the system.…”

Effects of Titanium Addition on Precipitate and Microstructural Control in C-Mn Microalloyed Steels.

“…The above compositional variation predicted by the thermodynamic analysis is a consequence of the continuous mutual solubility of the carbides and nitrides formed by Ti, Nb and V and of the decreasing solubility of the different carbides and nitrides with temperature in the giron. [1][2][3][4][5] …”

“…A proper design of the carbonitride precipitation sequence through control of composition, solidification and cooling rate is therefore of great importance and it was the subject of a large number of research works in the last decades. [1][2][3][4][5][6][7] During recent years, particular attention was dedicated to optimisation of solidification conditions and avoidance of transversal and edge cracking, especially during continuous casting. From hot tensile testing and from industrial practice on microalloyed steel, it was established that a loss of ductility is often encountered at temperatures approximately ranging from 750 to 950°C.…”

“…It is now established that Ti, being a powerful nitride former, acts by removing most of the N from solid solution at very high temperatures, thus preventing subsequent grain boundary embrittlement to occur due to formation of nitrides with other microalloying elements. 1,5,10,11) A debate was raised in literature on optimal Ti : N ratio as a function of other alloying elements and on the actual mechanisms acting in reducing hot cracking susceptibility. 8,9,11,12) It is now widely accepted that optimal Ti : N ratio should lie on the hyperstoichiometric side, at values ranging from 4 to 10, depending on actual steel composition.…”