Multi-phase transformation kinetics of HSLA steels during continuous cooling: experiments and cellular automaton (CA) simulation

ABSTRACT

Kinetics of multiply ferrite/bainite phase transformation of HSLA steels is investigated by experiments and cellular automaton (CA) simulation. Peak-differentiation method to elucidate the sequential ferrite and bainite phase transformation individually, which is verified by the CA simulation. Such CA modelling executed using classic JMAK theory, but also gives an insight of microstructure evolution of the multi-phase transformation routine on different cooling rate. From that, it enables classic JMAK modelling to capture the detached phase transformation with different growth models and interface-migration mechanisms. Also, we find that the final phase constitution is sensitive to the cooling rate. With increasing the cooling rate, bainite sheaves nucleated at prior austenite boundaries and ferrite/austenite interfaces are significantly facilitated, which seriously inhibits the growth of prior ferrites. The scenario can be interpreted by the CA simulation and the influence of the cooling rate on sequential multi-phase transformation can be also obtained.     

γ-α相变中不同晶界特征下铁素体生长形貌的相场模拟

利用多相场模型模拟了奥氏体(γ)-铁素体(α)相变过程中不同晶界特征下铁素体晶粒的形貌与生长动力学.模型中通过能量梯度系数和耦合项系数的协同变化定量表达晶界能与晶界迁移率的各向异性,同时固定相场界面宽度来保证计算精度.模拟结果显示:随着原奥氏体晶界能与铁素体-奥氏体晶界能比值σ_(γ,γ)/σ_(α,γ)的增加,三叉相界面处的平衡角β减小,铁素体晶粒沿原奥氏体晶界与垂直于奥氏体晶界方向的生长速率差变大.铁素体与奥氏体晶粒间的晶粒取向越接近,铁素体生长越缓慢.模拟结果可描述铁素体晶粒生长形貌的多样性,与实验结果符合.     

钢铁材料中形变诱导相变超细化机理研究

通过计算机编程建立奥氏体相中12[1 1 0]刃位错、奥氏体相中非形变区和形变区奥氏体/铁 素体相界模型.用实空间的连分数方法计算了非形变区和形变区奥氏体/铁素体相界界面能， 计算了碳、氮及微合金元素在完整晶体及位错区引起的环境敏感镶嵌能，进而讨论形变过程 中铁素体形核的难易程度，碳、氮及合金元素在位错区的偏聚及析出与铁素体细化的关系. 计算结果表明：α-Fe易于在高密度位错区（形变带、亚晶界、晶界）形核，在奥氏体形变 过程中，就会大大提高α-Fe形核率，细化铁素体晶粒；碳、氮和微合金元素易于单独或共 同 关键词： 奥氏体/铁素体相界 刃位错 形变 晶粒细化     

Five-parameter crystallographic characteristics of the interfaces formed during ferrite to austenite transformation in a duplex stainless steel

The crystallography of interfaces in a duplex stainless steel having an equiaxed microstructure produced through the ferrite to austenite diffusive phase transformation has been studied. The five-parameter interface character distribution revealed a high anisotropy in habit planes for the austenite–ferrite and austenite–austenite interfaces for different lattice misorientations. The austenite and ferrite habit planes largely terminated on (1 1 1) and (1 1 0) planes, respectively, for the austenite–ferrite interfaces associated with Kurdjumov–Sachs (K–S) and Nishiyama–Wasserman (N–W) orientation relationships. This was mostly attributed to the crystallographic preference associated with the phase transformation. For the austenite–ferrite interfaces with orientation relationships which are neither K–S nor N–W, both austenite and ferrite habit planes had (1 1 1) orientations. Σ3 twin boundaries comprised the majority of austenite–austenite interfaces, mostly showing a pure twist character and terminating on (1 1 1) planes due to the minimum energy configuration. The second highest populated austenite–austenite boundary was Σ9, which tended to have grain boundary planes in the tilt zone due to the geometrical constraints. Furthermore, the intervariant crystallographic plane distribution associated with the K–S orientation relationship displayed a general tendency for the austenite habit planes to terminate with the (1 1 1) orientation, mainly due to the crystallographic preference associated with the phase transformation.     

Magnetic-field-induced microstructural features in a high carbon steel during diffusional phase transformation

In this work, a high purity, high carbon steel was heat treated without and with a 12-T magnetic field. The microstructural features induced by magnetic field during its diffusion-controlled austenite decomposition were investigated by means of optical microscopy and SEM/EBSD. It is found that the magnetic field increases the amount of the abnormal structure, which is composed of proeutectoid cementite along the prior austenite boundaries and ferrite around it, because magnetic field increases the austenite grain size and promotes the transformation of carbon-depleted austenite to ferrite. No specific orientation relationship between abnormal ferrite and cementite has been found in the non-field- or the field-treated specimens. Magnetic field evidently promotes the spheroidization of pearlite, due to its effect of enhancing carbon diffusion through raising the transformation temperature and its effect of increasing the relative ferrite/cementite interface energy. As magnetic field favors the nucleation of the high magnetization phase-pearlitic ferrite, the occurrence of the P-P2 OR that corresponds to the situation that ferrite nucleates prior to cementite during pearlitic transformation is enhanced by the magnetic field.     

Multi-phase field simulation of competitive grain growth for directional solidification

The multi-phase field model of grain competitive growth during directional solidification of alloy is established. Solving multi-phase field models for thin interface layer thickness conditions, the grain boundary evolution and grain elimination during the competitive growth of SCN-0.24-wt% camphor model alloy bi-crystals are investigated. The effects of different crystal orientations and pulling velocities on grain boundary microstructure evolution are quantitatively analyzed. The obtained results are shown below. In the competitive growth of convergent bi-crystals, when favorably oriented dendrites are in the same direction as the heat flow and the pulling speed is too large, the orientation angle of the bi-crystal from small to large size is the normal elimination phenomenon of the favorably oriented dendrite, blocking the unfavorably oriented dendrite, and the grain boundary is along the growth direction of the favorably oriented dendrite. When the pulling speed becomes small, the grain boundary shows the anomalous elimination phenomenon of the unfavorably oriented dendrite, eliminating the favorably oriented dendrite. In the process of competitive growth of divergent bi-crystal, when the growth direction of favorably oriented dendrites is the same as the heat flow direction and the orientation angle of unfavorably oriented grains is small, the frequency of new spindles of favorably oriented grains is significantly higher than that of unfavorably oriented grains, and as the orientation angle of unfavorably oriented dendrites becomes larger, the unfavorably oriented grains are more likely to have stable secondary dendritic arms, which in turn develop new primary dendritic arms to occupy the liquid phase grain boundary space, but the grain boundary direction is still parallel to favorably oriented dendrites. In addition, the tertiary dendritic arms on the developed secondary dendritic arms may also be blocked by the surrounding lateral branches from further developing into nascent main axes, this blocking of the tertiary dendritic arms has a random nature, which can have aninfluence on the generation of nascent primary main axes in the grain boundaries.     

铸造奥氏体不锈钢中铁素体与奥氏体位向关系及其对声衰减的影响

研究了铸造奥氏体不锈钢中铁素体与奥氏体位向关系及其对超声散射衰减的影响.利用电子背散射衍射技术表征了两相的晶体取向及其位向关系,基于真实的铁素体形貌建立了二维声传播各向异性模型并利用时域有限差分法进行了计算,分析了不同位向关系、铁素体形貌特征对声衰减系数的影响规律并进行了实验验证.结果表明：铸造奥氏体不锈钢奥氏体晶粒中散布着形状复杂的铁素体,典型铁素体形貌为条状和岛状;铁素体与奥氏体的位向关系以Kurdjumov-Sachs关系为主,少量满足Nishiyama-Wassermann关系.对声传播过程进行计算,发现两相位向关系和铁素体形貌协同作用影响超声波传播,在较高检测频率（15 MHz）下对散射衰减的影响不能忽略.结合“原位”实验对奥氏体<101>柱状晶粒的声衰减影响因素进行了定量分析,发现对于单一铸造奥氏体晶粒,晶粒内部取向不均匀性、奥氏体-铁素体位向关系以及奥氏体晶粒内铁素体形态都是超声散射衰减的主要原因.     

The formation of α at triple junctions of parent β phase in titanium alloys

We have examined the formation of α phase at grain boundary triple junctions of parent β in a metastable β titanium alloy with orientation imaging microscopy based on electron backscattered diffraction (EBSD). As in the case of α formed at grain boundaries of parent β grains, α at a triple junction also forms with the Burgers orientation relationship with one of the three neighbouring β grains. The experimental results are analyzed in terms of the deviation of the 36 possible α variants that can form at a triple junction from the Burgers orientation relationship with neighbouring grains.     

The influence of morphology on grain-boundary and twin-boundary bainite growth kinetics at the bay in Fe–C–Mo

The growth kinetics morphology and crystallography of grain-boundary bainite (GBB) and twin-boundary bainite (TBB) formed in Fe–0.24?wt%?C–4?wt%?Mo were reassessed. Optical microscopy and transmission electron microscopy observations of these coalesced allotriomorphic slabs showed that the bainite–austenite growth fronts were rough, even spiky in place, characteristic of Widmanstätten growth. Furthermore, a significant substructure of regions having different ferrite orientations (subunits) was revealed, the GBB subunit arrangement was quite complex, suggestive of competitive growth between subunit, whereas TBB had a simpler subunit arrangement that yielded less subunit competition. These crystallographic and interfacial structure effects are instrumental for explaining the more rapid thickening of GBB relative to TBB. The differing subunit arrangements in GBB versus TBB originate in the differing ferrite–austenite orientation relationships established during nucleation at austenite grain boundaries versus twin boundaries. The complexities inherent in a multicrystalline allotriomorphic slab migrating with a rough growth front prevents direct interpretation of thickening kinetics data in terms of a fundamental growth process.     

Influence of magnetic field on ferrite transformation in a Fe-C-Mn alloy

The kinetics of ferrite transformation in a Fe-0.10mass%C-2.94mass%Mn alloy in a strong magnetic field of 8 T were studied with regard to alloying element-partitioned and partitionless growth. According to the theory of diffusion-controlled growth, the slow Mn diffusion dictates partitioned growth that occurs at a low undercooling, whereas partitionless growth at a larger undercooling is rate-controlled by fast carbon diffusion. The alloy was austenitized and isothermally reacted at temperatures that encompass the two growth modes. The nucleation and growth rates of ferrite increased at all temperatures in the magnetic field, whereas the amount of increase was somewhat greater at lower temperatures. In the region of slow growth, besides its sluggish diffusion Mn possibly destabilizes the ferrite phase due to the influence on the magnetic moment and the Curie temperature of bcc Fe solid solution, and partially offsets the accelerating effect of transformation. The temperature of transition from the slow to the fast growth is predicted to increase, due to the shift in the ferrite/austenite phase boundaries in the presence of magnetic field.     

Abnormal austenite–ferrite transformation behaviour of pure iron

The isochronal and isothermal austenite (γ) → ferrite (α) transformation of pure iron was measured by high-resolution dilatometry and differential thermal analysis. Both abnormal and normal transformation kinetics were recognized for the first time in pure iron according to the variation in the ferrite formation rate. The occurrence of the type of γ?→α transformation strongly depends on the grain size; the transformation type changes from abnormal to normal with decreasing grain size. The abnormal transformation process involves the occurrence of additional peaks in the transformation rate for the first stage of the transformation. A phase transformation model, involving repeated nucleation (autocatalytic nucleation), interface-controlled continuous growth and incorporating correction for impingement, has been employed successfully to describe the observed kinetics of the abnormal transformation.     

Fe-C合金中形变诱导动态相变的蒙特卡罗模拟

采用蒙特卡罗（MC）方法模拟了Fe-C合金在奥氏体-铁素体相变的平衡温度之上的形变诱导动态相变过程.通过建立合适的MC规则,在一个MC模型中同时实现了奥氏体-铁素体相变、铁素体-奥氏体逆相变以及奥氏体动态再结晶过程的模拟.同时,一个基于矢量变换的拓扑模型被嵌入此MC相变模型,用来跟踪由于塑性变形导致的晶粒形貌变化.在此基础上模拟分析了动态相变过程中铁素体的形成特点,讨论了由于相变、逆相变和动态再结晶交互作用所带来的影响. 关键词： 形变诱导动态相变 蒙特卡罗模型 动态再结晶 介观模拟     

Effect of annealing prior to cold rolling on magnetic and mechanical properties of low carbon non-oriented electrical steels

The effects of annealing prior to cold rolling on the microstructure, magnetic and mechanical properties of low-C grain non-oriented (GNO) electrical steels have been investigated. The grain structure of hot-rolled electrical steel strips is modified by annealing at temperatures between 700 and 1050 °C. Annealing at temperatures less than the ferrite to austenite+ferrite transformation temperature on heating (Ac₁) causes a marginal effect on the grain size. However, annealing in the intercritical region at temperatures between Ac₁ and Ac₃ (the ferrite+austenite to austenite transformation temperature on heating) causes rapid decarburization and development of large columnar ferrite grains free of carbide particles. This microstructure leads, after cold rolling and a fast annealing treatment, to carbide free, large ferrite grain microstructures with magnetic and mechanical properties superior to those observed typically in the same steel in the industrially fully processed condition. These results are attributed to the increment in grain size and to the {1 0 0} fiber texture developed during the final annealing at temperatures up to 850 °C. Annealing at higher temperatures, T>Ac₃, results in a strong {1 1 1} fiber texture and an increase of the quantity of second phase particles present in the microstructure, which lead to a negative effect on the final properties. The results suggest that annealing prior to cold rolling offers an attractive alternative processing route for the manufacture of fully processed low C GNO electrical steels strips.     

Wetting Properties of Grain Boundaries in Solid 4He

We have observed boundaries between hcp 4He crystal grains in equilibrium with liquid 4He. We have found that, when emerging at the liquid-solid interface, a grain boundary makes a groove whose dihedral angle 2theta is nonzero. This measurement shows that grain boundaries are not completely wet by the liquid phase, in agreement with recent Monte Carlo simulations. Depending on the value of theta, the contact line of a grain boundary with a solid wall may be wet by the liquid. In this case, the line is a thin channel with a curved triangular cross section, whose measured width agrees with predictions from a simple model. We discuss these measurements in the context of grain boundary premelting and for a future understanding of the possible supersolidity of solid 4He.     

Grain growth and phason-strain field in quasicrystalline Al-Li-Cu

We report on grain growth and related structure change in single phased Al-Li-Cu quasicrystals. The icosahedral phase grains have been investigated using scanning ion microscopy and transmission electron microscopy. Regular boundaries between large grains have been observed both before and after high temperature annealing. The electron diffraction study shows that the grain growth is accompanied by a reduction of the phason-strains. The orientation relation between grains sets the 2-fold icosahedral axes parallel, and the coincidence of the planes depends on the phason strain-field. The effect of phason-strain field on these boundaries is discussed. It is proposed that the phason strain elimination can play a role in the grain growth. Received 1 February 1999 and Received in final form 12 May 1999     

Exploring the origin of variant selection through martensite-austenite reconstruction

An approach of near neighbour correlation, with manual intervention, was developed for reconstructing parent austenite microstructure in a martensitic stainless steel. This was validated in a ferrite-austenite dual structure. Two-hundred and twenty randomly selected austenite grains were reconstructed from the experimental EBSD (electron backscattered diffraction) measurements. From these reconstructions, martensite variant selection was quantified as the number of variants (n_V) and the variant selection index (VSI: a statistical index for the relative area fractions of the variants). For each prior austenite grain, both n_V and VSI appeared to depend on the associated transformation (austenite-martensite) strain. Selection of common variants between two neighbouring austenite grains was related to the presence of 60°<111?>?or Σ3 boundary in the austenite phase and corresponding minimisation of the transformation strain.     

Crystalline damage growth during martensitic phase transformations

A thermomechanical model is developed within a large deformation setting in order to simulate the interactions between martensitic phase transformations and crystalline damage growth at the austenitic grain level. Subgrain information is included in the model via the crystallographic theory of martensitic transformations. The damage and transformation characteristics are dependent of the specific martensitic transformation systems activated during a loading process, which makes the model strongly anisotropic. The state of transformation for the individual transformation systems is represented by the corresponding volume fractions. The state of damage in the austenite and in the martensitic transformation systems is reflected by the corresponding damaged volume fractions. The thermodynamical forces energetically conjugated to the rate of volume fraction and the rate of damaged volume fraction are the driving forces for transformation and crystalline damage, respectively. The expressions for these driving forces follow after constructing the specific form of the Helmholtz energy for a phase-changing, damaging material. The model is used to analyze several three-dimensional boundary value problems that are representative of microstructures appearing in multiphase carbon steels containing retained austenite. The analyses show that the incorporation of damage in the model effectively limits the elastic stresses developing in the martensitic product phase, where the maximum value of the stress strongly depends on the toughness of the martensite. Furthermore, in an aggregate of randomly oriented grains of retained austenite embedded in a ferritic matrix the generation of crystalline damage delays the phase transformation process, and may arrest it if the martensitic product phase is sufficiently brittle. The response characteristics computed with the phase-changing damage model are confirmed by experimental results.     

The austenite microstructure evolution in a duplex stainless steel subjected to hot deformation

Abstract

The austenite microstructure evolution and softening processes have been studied in a 23Cr–6Ni–3Mo duplex stainless steel, comprising equal fractions of austenite and ferrite, deformed in uniaxial compression at 1000 °C using strain rates of 0.1 and 10 s^?1. The texture and microstructure evolution within austenite was similar in character for both the strain rate used. The observed large-scale subdivision of austenite grains/islands into complex-shaped deformation bands, typically separated by relatively wide transition regions, has been attributed to the complex strain fields within this phase. Organised, self-screening microband arrays were locally present within austenite and displayed a crystallographic character for a wide range of austenite orientations. The microband boundaries were aligned with the traces of {1?1?1} slip planes containing slip systems having high, although not necessarily the highest possible, Schmid factors. The slightly lower mean intercept length and higher mean misorientation obtained for the sub-boundaries at the higher strain rate can be ascribed to the expected more restricted dynamic recovery processes compared to the low strain rate case. Dynamic recrystallisation within austenite was extremely limited and mainly occurred via the strain-induced migration of the distorted original twin boundaries, followed by the formation of multiple twinning chains.     

定向倾斜枝晶生长规律及竞争行为的相场法研究

采用多相场模型对定向凝固过程中倾斜枝晶生长进行了研究, 模拟了单一取向枝晶列的演化规律及不同取向枝晶列汇聚生长竞争淘汰行为. 结果表明, 枝晶尖端过冷度随倾斜角度的增大而增大, 即相同条件下倾斜枝晶尖端位置总是低于非倾斜枝晶; 汇聚生长时择优取向枝晶总是阻挡非择优取向枝晶, 但在抽拉速度较低时, 由于溶质扩散场的相互重叠, 晶界处择优取向枝晶的生长受到相邻非择优枝晶的影响而延缓, 这可能导致非择优取向枝晶淘汰择优取向枝晶.     

Modeling of the γ → α transformation in steels

A mathematical physically based model of the decomposition of undercooled austenite (γ-phase) with the formation of ferrite (α-phase) and pearlite has been developed. The model differs from the currently existing analogs by a new approach to the inclusion of the effect of complex alloying of steels on the nucleation rate of ferrite grains and on the mobility of the α/γ-phase boundary. In the model, the effect of alloying of steels with substitutional elements on the diffusion coefficient of carbon in the bulk of austenite is taken into account. The results of the modeling of the kinetics of austenite decomposition and the calculation of the final ferrite grain size are in good quantitative agreement with the experimental data obtained for a set of steels with a wide range of chemical compositions.