The solidification of Ag-Zn alloy, which has similar shrinking properties to Fe-C alloy during peritectic phase transformation, was studied in this paper. The unidirectional solidification experiment was performed to investigate the solidification microstructure, the relationship between the shrinkage during peritectic phase transformation and the generation of crack on the slab was analyzed. The results indicate that the crack is mainly cause by shrinkage during perirtectic phase transformation(δ→γ) ,and for this reason, the subperitectic steel bears more crack than other kinds of steel.

包晶相变是包晶钢较其它碳含量钢种特有的相变，该相变由体心立方结构的δ相转变为面心立方结构的γ相，由于晶格致密度的增加，伴随该相变产生较大的体积收缩。据此选用具有相似收缩特征的Ag-Zn合金作为研究对象，进行定向凝固实验。对其凝固组织进行研究，分析了包晶相变收缩与裂纹产生之间关系，认为包晶相变过程中包晶转变(δ→γ)阶段引起的收缩是导致铸坯裂纹的主要原因，据此解释了为什么连铸生产中亚包晶钢较其它钢种裂纹严重。

Driven by the demand for the improving mechanical properties of steel products and the cost reduction in alloys, steels falling within the peritectic composition range are designed recently. However, notoriously cast surface defects such as cracks, deep oscillation mark formation and breakouts are found to occur frequently during continuous casting of steels, particularly at high casting speeds. This phenomenon is closely related to the shrinkage of phase transformation caused by the peritectic transformation. In order to understand the effects of cooling rate on the contraction of the peritectic transformation, the initial solidification processes of a peritectic steel (Fe-0.1C-0.21Si-1.2Mn, mass fraction, %) were observed using high-temperature confocal laser scanning microscopy under different cooling rates, and then variations in surface roughness were measured to reflect the degree of peritectic transformation contraction. The results show that the peritectic transformation occurs a massive transformation when the cooling rate exceeds the critical value. The massive transformation results in a sudden peritectic transformation contraction and surface roughness variations, which directly cause the occurrence of surface longitudinal cracks of slabs at high casting speeds. The contraction increases first and then decreases with the cooling rate increasing and the maximum surface roughness at the middle cooling rate (20 ℃/s) is about 2.8 times more extensive than that which occurs at the low cooling rate of 2.5 ℃/s. The phenomenon that the peritectic transformation contraction decreases under the high cooling rate may provide a new strategy to reduce cracks occurring in high speed casting.

通过高温激光共聚焦显微镜模拟观察了Fe-0.1C-0.21Si-1.2Mn (质量分数,%)包晶钢在不同冷却速率下的包晶相变过程,然后利用试样表面粗糙度变化反映了包晶转变收缩程度的不同。结果显示,冷却速率超过临界值后包晶转变能够发生快速相变,快速相变引起突然的包晶转变收缩和表面粗糙度变化。随冷却速率的增加包晶钢的包晶转变收缩呈先增加后减小的趋势,在冷却速率为20 ℃/s时表面粗糙度达到最大值,此时的表面粗糙度约是低冷却速率(2.5 ℃/s)时表面粗糙度的2.8倍。当冷却速率足够大后包晶转变收缩又开始减小,这一变化为高拉速下减少包晶钢连铸坯表面纵裂纹的发生提供了新策略。

The casting behaviour, microstructure formation and resulting mechanical properties of materials that undergo a peritectic phase transition during solidification are largely determined by the kinetics of this high-temperature phase transformation during the production process. However, current nucleation models do not accurately predict the nucleation behaviour and phase transition kinetics in many polycrystalline materials. Utilizing a newly developed experimental technique, we have performed in situ observations to study the nucleation behaviour of a newly forming intermediate phase by using the peritectic phase transition in Fe-C and Fe-Ni alloys as examples. In our experiments as well as by using thermodynamic and kinetic arguments we demonstrate that nucleation of a new intermediate (peritectic) phase can be constrained in the presence of solute diffusion fields that form during primary solidification due to an increase in the Gibbs free energy barrier for nucleation. We found a strong correlation between the magnitude of these diffusion fields and and the resulting nucleation undercooling required for the formation of a new phase. Our study casts new light on, and clarifies for the first time, the much-debated underpinning reason for the occurrence of massive phase transformations occurring during solidification processing at large nucleation undercoolings. These new insights contribute to the improvement of nucleation theory and allow more accurate predictions on nucleation events and, in turn, physical properties of materials that undergo phase transitions in the course of materials production processes. (c) 2014 Acta Materialia Inc. Published by Elsevier Ltd.

The mechanism and kinetics of the peritectic phase transition in steel were studied by using a concentric solidification technique in a high-temperature laser-scanning confocal microscope. Strong solute diffusion fields are established during solidification of the primary delta phase, which leads to suppression of nucleation of the austenite phase, and hence the extent to which a steel can be cooled to a temperature below the equilibrium peritectic temperature before the peritectic reaction will occur. The magnitude of these diffusion fields was also determined as a function of carbon content, alloying additions, cooling rate and the fraction of primary delta phase that solidified before the peritectic reaction occurred. Depending on the extent of undercooling required to nucleate the first austenite platelet on the liquid/delta-ferrite interface, three different modes of the solid-state peritectic transformation of delta to austenite have been observed: a diffusion-controlled phase transformation that occurred by the progression of a planar interface; a diffusion-controlled transformation with cellular/dendritic morphology; and a massive-type of transformation. Under certain circumstances, a changeover between these modes has also been observed. The underpinning thermodynamic considerations that lead to these different modes of transformation are discussed and a new explanation is provided for the occurrence of a massive transformation of delta-ferrite to austenite. The relevance to and implications of these new findings to the continuous casting of steel of near-peritectic steel composition are also discussed. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd.

Directional transformation of a hypo-peritectic Fe-17.5 at.% Co alloy was studied. Two consecutive phase transformations solidification (liquid to delta ferrite) and solid-state transformation (delta ferrite to gamma austenite)-were observed and compared with theory. In all experiments, the solidification front was planar and in the steady-state, and therefore produced a homogeneous parent phase for the following delta-gamma transformation. Depending on the growth conditions, gamma transformed from delta as cells or as a plane front. The cell tip radius decreased with growth rate from V = 1-5 mu m s(-1). At higher velocities, between 7 and 10 mu m s(-1) the delta/gamma interface morphology became planar. In order to explain this morphological transition, volume diffusion-controlled plane front growth and dendrite growth theory was applied. Good agreement was obtained between theory and experiments. It is concluded that plane front stabilization with increasing velocity is due to absolute stability, with a concentration spike at the transformation front. In the steady-state, this leads to composition invariance, typical for massive transformation. Computed interface velocities for quenching in heat treatment, which can be as high as several centimeters per second, show that, in certain cases, the controlling mechanism of massive transformation is steady-state plane front growth with a narrow concentration spike and not complete solute trapping. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd.