Precipitation of complex carbonitrides in a Nb–Ti microalloyed plate steel
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Complex carbonitrides precipitated in base metal and heat-affected zone (HAZ) in Nb–Ti hot-rolled microalloyed steel plates have been identified to be Ti-rich (Nb, Ti)(C, N). As the reheating temperature is decreased from 1,200 to 1,150 °C, the average particle size in base metal is decreased from 40 to 20 nm. The morphology of complex carbonitrides in the HAZ, however, is transformed from cuboidal to rectangle shape with length of over 500 nm. Reheating at low temperature 1,150 °C may improve the toughness of HAZ by reducing the austenite size at large heat input welding.
KeywordsAustenite Base Metal Impact Toughness Prior Austenite Particle Volume Fraction
It is well established that the microalloying elements, such as Nb, Ti, and V, independently or in combination, can trigger the grain refinement through the precipitation of carbides, nitrides, and carbonitrides in austenite during reheating or hot rolling. In particular, titanium is most commonly used to control austenite grain size by the formation of titanium nitride with high thermal stability during welding, especially after high heat inputs. Jun et al.  observed three types, i.e., dendritic, semi-dendritic, and rod-like, Nb-rich (Nb, Ti)(C, N) complex carbonitrides in a continuously cast Nb–Ti bearing steel. Hong et al.  studied the effect of Ti addition on strain-induced precipitation kinetics of NbC in Nb-microalloyed steels. Poths et al.  found that the core of carbonitrides is mainly based on titanium nitride, which is decorated by a layer or cap of niobium carbide. Similar results were also observed by Craven et al. , who made a detailed study of complex precipitates in Al-killed Nb–Ti HSLA steels. They found that the core of carbonitrides is also based on TiN but with a spherical, cubic, or cruciform shape. In addition, the precipitation of carbonitrides in Nb–V and Ti–V  as well as Nb–Ti–V [6, 7] microalloyed steels was also studied by several authors.
Besides, the thermodynamic calculations on Nb–Ti [8, 9, 10] and Nb–Ti–V  steels were also performed. For example, Zou and Kirkaldy  carried out a thermodynamic study on Nb–Ti steels. Nearly at the same time, Okaguchi et al.  gave a computer model for predicting the carbonitride precipitation during hot working in Nb–Ti bearing steels. Recently, Liu  made a description of the phase equilibria between the austenite matrix and carbonitride precipitates thermodynamically. In addition, Inoue et al.  calculated the equilibria between the austenite and carbonitrides in Nb–Ti–V steels. The response of carbonitride particles to welding was studied by Suzuki et al. , and three types of behaviors were observed in Nb–V, Ti, and Nb–Ti microalloyed steels.
Very recently, many investigations are still concentrated on the precipitation in the Nb–Ti and Nb–Ti–V containing steels. Zeng et al.  made a quantitative description for the precipitation of carbonitrides in Nb–Ti microalloyed steels during hot deformation on the basis of thermodynamics and kinetics. Cao et al.  studied comparatively the precipitates in Nb–Mo and Nb–Ti containing steels from the viewpoints of the morphology, size distribution, composition, and crystal structure. They found that the fine and uniformly distributed MC-type carbides in Nb–Mo containing steels are superior to coarse and sparsely distributed carbonitrides in Nb–Ti containing steels in terms of increasing yield strength. Similar study was also performed by Davis et al. , who studied the inhomogeneous precipitate distributions from different positions in Nb–Ti–V containing steels with varying Nb levels. In the area of weld thermal simulation, Bang et al.  gave an investigation of the effects of nitrogen content and weld cooling time on the toughness of heat-affected zone (HAZ) in a Ti-containing steel. And an optimal condition, ranging from 60 to 100 s for welding cooling time and 0.006% for nitrogen content, was obtained for high toughness of HAZ at −20 °C. This indicates that, although there have been some previous investigations, the precipitation behavior in Nb–Ti steels has not been well understood. Thus, the purpose of this study is to identify the strain-induced precipitates in matrix and HAZ and to examine the effect of reheat temperature and weld thermal cycle on the precipitation behavior in Nb–Ti bearing HSLA steels.
Chemical composition of the steel examined in this study (wt.%)
Precipitation in the base metal
Precipitation in the HAZ
Precipitate composition and evolution in the base metal
As mentioned before, non-equilibrium precipitates with dendritic, semi-dendritic, and rod-like morphologies were observed by Jun et al.  during continuous casting of Nb–Ti bearing steels. It is of interest to note that such precipitates disappeared during reheating process in the temperature range of 1,100–1,400 °C, followed by the formation of cubic shape precipitates . It is well known that the evolution of precipitates is closely related to the dissolution temperature. In general, the dissolution temperature of complex carbonitrides is lower than that of carbides and/or nitrides. In Nb–Ti bearing steels, the TiN is the most stable phase among the four phases, i.e., NbC, NbN, TiC, and TiN. Based on the chemistry of Jun et al. , the contents of Ti and N are 0.018 and 0.004, in combination with the solubility product of TiN, i.e., log10[Ti][N] = 4.94–14,400/T, we can obtain the equilibrium dissolution temperature of TiN as 1,312 °C. It was, however, observed that the complex precipitates in slabs almost completely disappeared at the lower temperature of 1,100 °C. This behavior means that the complex carbonitrides are more prone to be dissolved than pure TiN nitrides.
It is well known that defects such as grain boundaries, interfaces, dislocations, and vacancies are favorable for the nucleation of precipitates. As discussed by Tian et al. , the particles tend to be present in clusters if they form on subgrain boundaries, tangled dislocation lines, and/or at the corners of grains. However, in the course of a welding thermal cycle, the subgrain boundaries and dislocations are almost completely absent due to very high temperature (e.g. 1,350 °C). It is thus the undissolved precipitates which are responsible for the grain refinement in the coarse-grained HAZ. In the subsequent cooling, these pre-existing precipitates may act as nucleation sites or continue to grow. As a consequence, the grouping tends to take place and to be further intensified by a weld thermocycle, particularly in Fig. 5c and e. The rectangle precipitates in Fig. 5 are suggested to occur along grain boundaries, and they are more effective than cuboidal ones to restrict the prior austenite grain size. The initial microstructural differences in both steel plates are thought to be eliminated during welding. Precipitates in the HAZ undergo a rapid dissolution and reprecipitation process after the high heat input welding. It is believed that the austenite grain size does not influence the precipitation kinetics of complex carbonitrides, as reported by Dutta et al. . A consequence of this is that the precipitation kinetics in the HAZ during cooling leg of weld thermal cycle is mostly affected by the undissolved precipitates during rapid heating process.
Impact toughness, microstructure, and precipitate evolution in the HAZ
To summarize, the precipitates in Nb–Ti bearing steels are identified as Ti-rich (Nb, Ti)(C, N) carbonitrides both in base metal and HAZ. As the reheating temperature is decreased from 1,200 to 1,150 °C, the particle size of cuboidal carbonitrides, which is uniformly distributed in the matrix in base metal, is reduced from 40 to 20 nm. For the Nb–Ti bearing steels reheated at 1,200 °C, the size of complex carbonitrides in the HAZ is increased in comparison with those in the base metal, but the morphology remains basically unchanged. In contrast, for the Nb–Ti bearing steels reheated at 1,150 °C, the morphology of carbonitrides in the HAZ is transformed from cuboidal into rectangular shape with the largest edge length of exceeding 500 nm. When viewed from the practical production, reheating at relatively low temperature is beneficial to reduce the product cost.
The authors would like to express their gratitude to Dr. Jiaqiang Gao at the Testing Centre of Baosteel Research Institute for his help in TEM operations and valuable discussions. The authors are also indebted to Engineer Guobin Song for performing the weld simulations in a Gleeble 3800 system.