Influence of the Size and Volume Fraction of TiN Particles on Hot Strength and Dynamic Recrystallisation in Structural Steels

Abstract

The low solubility of titanium nitrides (TiN) in austenite is taken advantage of in structural steels to control the evolution of the microstructure in the hot rolling process and in the heat affected zone (HAZ) in processes involving the application of heat, such as welding. However, the quantitative influence of the precipitation state of these particles on hot strength and dynamic recrystallisation kinetics, given by the precipitate size distribution and the precipitated volume, is practically unknown. The present work studies the influence of various Ti and N compositions, which give rise to different precipitation states at the reheating temperatures, on the aforementioned phenomena. The influence of the precipitation state on hot strength has been quantified by changes in the peak stress and the activation energy. A maximum has been obtained for the activation energy which corresponds to a Ti/N ratio of approximately 1.3. The model used to predict the flow curve and dynamic recrystallisation kinetics has been improved, extending it to include microalloyed steels containing Ti.

Assessment of austenite static recrystallization and grain size evolution during multipass hot rolling of a niobium-microalloyed steel

“…It has been proposed that the precipitates formed during hot working or heat treatment in austenite are rather coarse (around 20 nm in diameter) and thus they have only a limited effect for precipitation hardening compared to the precipitation strengthening effect caused in ferrite by the very fine particles formed during cooling [38]. Nevertheless, it is known that fine particles can hinder the climb of dislocations that governs the plastic flow of the austenite and can contribute in that way to strengthen the austenite [39]. The hardening effect of precipitates can be seen for example in the points of Fig.…”

Role of microalloying elements in the microstructure of hot rolled steels

“…For example, a relationship between the precipitation state and the maximum strength of Ti microalloyed steels during hot deformation can be described and the precipitation state affects the value of the activation energy for deformation Q d [40]. The increase of activation energy due to precipitation (ΔQ d ) reaches a maximum when the Ti/N ratio is between 1 and 2, as Fig.…”

Microalloying Effect on the Activation Energy of Hot Deformation

“…The function of flow stress, f ( σ ), in Equation can be described as hyperbolic sine law for a wide range of hot deformation stress: where n and α are material constants. Note that there are different descriptions of f ( σ ) depending on the stress level; i.e., a power‐law function for relatively low stresses ( ασ < 0.8) and exponential‐law function for relatively high stresses ( ασ > 1.2) .…”

“…Obtained Q values are summarized in Figure (a) where literature Q value of C‐Mn non‐microalloyed steel (280 kJ mol −1 ) is also provided. It is obvious that microalloying significantly affects Q .…”