Challenges of the study on precipitation behaviors of MnS in oriented electrical steels

The studies of the thermo-mechanic principles and the kinetic analysis of MnS precipitation behavior in oriented electrical steels conducted in the last dozen years have been summarized. The problems and insufficiency in the fields concerning nucleation theory of MnS precipitation, the relationship between the precipitation and dynamic recrystallization of the matrix as well as the influence of high-energy boundary precipitation on the pinning effect of the boundary migration were discussed. Suggestions were proposed to modify researches concerning the initial position density for MnS nucleation, the mutual interaction between the precipitation and the dynamic recrystallization as well as the kinetic principles of precipitation nucleation on grain boundaries with different boundary energy, which would complete the related theory of MnS precipitation behaviors.     

无取向硅钢中第二相的析出行为

使用扫描电镜(SEM)和能谱仪(EDS)等手段并结合热力学和动力学计算,研究了无取向硅钢900～1000℃常化处理过程第二相的析出行为。结果表明,无取向硅钢中的第二相主要为AlN和少量MnS。AlN和MnS在不同基体相(α相、γ相及(α+γ)两相）中有三种析出形核机制(均匀形核、晶界形核和位错形核),其临界形核半径(d*)都随常化温度的提高而增大。在同一温度下,相对于其他基体相AlN在(α+γ)两相区中晶界形核的临界形核功最小,相对形核率最大,因此以晶界形核为主;而MnS在α相中位错线上临界形核半径最小,相对形核率大,开始析出温度低,因此以位错形核为主。     

高速钢高温变形再结晶规律研究

本文研究了W9Mo~3Cr~4V钢的热变形奥氏体动态再结晶规律。结果表明:由于形变诱发析出碳化物的影响,W9Mo~3Cr~4V钢热变形奥氏体动态再结晶方式与低碳低合金钢的晶界弓出机构和晶界生核机构不同,是通过亚晶合并形成再结晶晶粒的,即亚晶合并机构。当形变速率较低时,可由亚晶粒直接形成再结晶晶粒。形变温度和形变速率对W9Mo~3Cr~4V钢热变形奥氏体动态再结晶影响比较复杂,不能简单地用Z参数来统一描述。     

Hot deformation behavior of a Nb-containing 316LN stainless steel

A Nb-containing 316LN stainless steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s^?1. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. The microstructural evolution was observed and the mechanism of flow instability was discussed. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate. On the contrary, the efficiency of power dissipation increased with them; Flow instability was manifested as cracking and flow localization; The hot deformation equation and the relationships between deformation condition and dynamic recrystallization grain size and fraction were obtained; For Nb-containing 316LN stainless steel, the favorite nucleation sites for dynamic recrystallization are in sequence of triple point, grain boundary, twin boundary and intragranular deformation band; The suggested processing window is given.     

脉冲电流对2091铝锂合金动态再结晶动力学的影响

研究了脉冲电流对2091铝锂合金超塑变形中动态再结晶及动力学的影响,结果表明,脉冲电流加速动态再结晶,减少形核时的平均晶粒,脉冲电流能加快位错墙的形成并使其角度增大,使再结晶形核率提高,脉冲电流加快位错在晶界上的攀移及消失,减少形核界面两边的能量差,降低形核界面的迁移速率及再结晶形核的长大速率,分析了脉冲电流作用下的动态再结晶动力学行为。     

脉中电流对2091铝锂合金动态再结晶动力学的影响

研究了脉冲电流对2091铝锂合金超塑变形中动态再结晶及动力学的影响结果表明,脉冲电流加速动态再结晶,减小形核时的平均晶粒直径.脉冲电流能加快位错墙的形成并使其角度增大,使再结晶形核率提高.脉冲电流加快位错在晶界上的攀移及消失、减小形核界面两边的能量差,降低形核界面的迁移速率及再结晶形核的长大速率分析了脉冲电流作用下的动态再结晶动力学行为     

钢中MnS对BN析出行为的影响

采用α粒子径迹法和电子显微镜研究了含微量硼的低碳钢中MnS对BN析出行为的影响。结果表明,当加热温度低于1150℃时,BN多以球状的MnS为结晶核心在其周围析出,呈MnS+BN复合体形态;当加热温度超过1150℃时,未固溶的MnS大部分由球形变成多面体形,这种形状的MnS已失去BN结晶核心的作用。除了呈MnS+BN复合体存在的BN外,也有单体的BN,随着温度的提高和保温时间的延长,其尺寸变大,形貌各异。     

Effect of composition and process variables on Nb(C,N) precipitation in niobium microalloyed austenite

Abstract

Nucleation theory and the solubility product of niobium, carbon, and nitrogen in austenite have been used to derive equations for the start of Nb (C, N) precipitation as a function of temperature and composition. The predicted curves have been compared with the experimental observations of several authors to determine the effects of thermomechanical processing variables on the density of preferred nucleation sites and to incorporate these in the equations. Good agreement between the predicted and observed forms of precipitation curve is obtained with consistent constants in the equations when account is taken of the influence of different methods of detecting the onset of precipitation. Combining the calculated precipitation start curves with the dependence of recrystallization kinetics on composition and thermomechanical process variables when all niobium is in solution leads to prediction of the lower temperature limit for complete recrystallization and of the upper temperature limit for effective stoppage of recrystallization by precipitation. The predictions are in good agreement with observed results.

MST/495     

TB17钛合金β相区动态再结晶行为及转变机理EI北大核心CSCD

通过Gleeble-3800热压缩模拟试验机对TB17钛合金β相区进行热压缩实验,研究该合金β相区的动态再结晶行为及转变机理。结果表明:TB17钛合金在β相区变形时会发生动态回复(DRV)与动态再结晶(DRX)。不同应变速率下存在两种动态再结晶形核位置,低应变速率下主要在晶粒内部形核,高应变速率下主要在晶界附近形核。通过EBSD和TEM分析可知,在低应变速率下发生连续动态再结晶(CDRX),其发生的主要形式为亚晶合并转动。高应变速率下发生不连续动态再结晶(DDRX),发生的主要形式为晶界剪切伴随着亚晶转动。尽管两种动态再结晶的转变方式不同,其本质都是通过位错的增殖、滑移和胞状结构演化形成新的动态再结晶晶粒。     

A PARAMETRIC STUDY OF DYNAMIC RECRYSTALLIZATION USING THE MONTE CARLO METHOD

A parametric study of dynamic recrystallization was undertaken using the Monte Carlo method. Cells in the simulation domain were connected to six neighboring cells across interfaces whose energy varied from 0 to 5 depending on the difference in orientation between the cells. The internal energy of cells was increased during each Monte Carlo step (MCS). A cell was allowed to grow (take up the orientation of one of its neighbors), recrystallize (take up a new orientation), or remain unchanged. Three different energy input rates were used in the simulations. Results presented include the effect of energy input rate on the internal energy, boundary energy, nucleation and growth events, and the structural evolution of the domain. Some comparisons are made with phenomenon observed during dynamic recrystallization of metals.     

金属材料动态再结晶模型研究现状

动态再结晶是热塑性变形过程中重要的材料软化、晶粒细化、组织控制和塑性成形能力改善方法,而材料发生动态再结晶过程形成的组织结构直接决定其综合性能,因此,长期以来动态再结晶一直是热成形过程中的研究热点。概述了动态再结晶的物理机理,介绍了位错密度模型、动力学模型和微观组织演化数值模拟,并对目前研究现状进行分析,展望其未来发展前景。     

The microstructure evolution and nucleation mechanisms of dynamic recrystallization in hot-deformed Inconel 625 superalloy

Hot compressions tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator at different strains between 900 °C and 1200 °C with a strain rate of 0.1 s⁻¹. Optical microscope, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the microstructure evolution and nucleation mechanisms of dynamic recrystallization. It was found that both the size and fraction of dynamically recrystallized grains increase with increasing deformation temperature. However, the size of dynamically recrystallized grains almost remains constant with increasing deformation strain. The dominant nucleation mechanism of dynamic recrystallization in Inconel 625 superalloy deformed at 1150 °C is the discontinuous dynamic recrystallization, which is characterized by the bulging of the original grain boundaries accompanied with twining. The continuous dynamic recrystallization characterized by progressive subgrain rotation occurs simultaneously in dynamic recrystallization process, although it can only be considered as an assistant nucleation mechanism at the early stage of hot deformation.     

Effect of minor Sc addition on microstructure and stress corrosion cracking behavior of medium strength Al–Zn–Mg alloy

Influence of Sc content on microstructure and stress corrosion cracking behavior of medium strength Al–Zn–Mg alloy have been investigated by optical microscopy, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy and slow strain rate test. The results indicate that the addition of Sc results in the formation of the quaternary coherent Al_3(Sc, Zr, Ti) dispersoids during homogenization treatment, which will inhibit the dynamic recrystallization behavior. The number density of Al_3(Sc, Zr, Ti) particles increases with the increase of Sc content, and thus the recrystallization fraction of hot-extruded alloy is reduced and the peak strength in two-stage artificial aging sample is enhanced. At the same time, the wide of precipitation free zone is reduced, and the content of Zn and Mg in grain boundary particles and precipitation free zone is increased with the increase of Sc content. In peak-aged state, the 0.06 wt% Sc added alloy shows the better stress corrosion cracking resistance than the Sc-free alloy because of the reduction of recrystallization fraction and the interrupted distribution of grain boundary precipitates along grain boundary. However, the further addition of Sc to 0.11 wt% will result in the deterioration of stress corrosion cracking resistance due to the increase of electrochemical activity of grain boundary particles and precipitation free zone as well as hydrogen embrittlement.     

Dynamic Recrystallization and Precipitation Behavior of Mn-Cu-V Weathering Steel

The hot deformation behavior of a Mn-Cu-V weathering steel was investigated at temperatures ranging from 850 to 1050℃ and strain rates ranging from 0.01 to 5 s-1 using MMS-300 thermal-mechanical simulator. The activation energy for dynamic recrystallization and stress exponent were calculated to be 551 kJ/mol and 7.73, respectively. The accurate values of critical strain were determined by the relationship between work hardening rate and flow stress (θ-σ) curves. The hyperbolic sine constitutive equation was employed to describe the relationship between the peak stress and Zener-Hollomon parameter during hot deformation. The interaction between dynamic recrystallization and dynamic precipitation of V(C,N) at a low strain rate was analyzed. The results showed that precipitation particles size of weathering steel increased with increasing strain at deformation temperature 950℃ and strain rate 0.1 s-1. The calculation results of the recrystallization driving force and pinning force showed that dynamic precipitation could retard the progress of dynamic recrystallization but not prevent it while the pinning forces is less than driving force. On the contrary, dynamic precipitation can effectively prevent the progress of dynamic recrystallization.     

Kinetics of dissolution-precipitation reaction at the surface of small particles: modelling and application

In the framework of the theory of phase transformations with position-dependent nucleation rate, a model has been developed aimed at describing the dissolution-precipitation reaction at the surface of small particles. The precipitation reaction takes place by nucleation and growth processes under time-dependent supersaturation. Depending on the coverage of the particle surface by the new phase, the reaction kinetics exhibits high- and low-rate regimes. The computation is performed for both progressive and simultaneous nucleation. In the case of simultaneous nucleation, closed-form solutions are attained for diffusion- and interface-limited growth modes and for isotropic and anisotropic growths of the nuclei, as well. The scaling properties of the kinetics on particle size are also investigated. The kinetic model is employed for analysing experimental data and makes it possible to estimate the nucleation density on the particle surface and to have an insight into the microscopic growth law of nuclei.     

Early-stage nucleation of manganese sulfide particle and its processing evolution in Fe—3wt.%Si alloys

Manganese sulfide is often referred to as one of important inhibitors in grain-oriented electrical steels, which is of great importance to yield strong Goss texture. However, the early stage of nucleation for such inhibitors and their evolution during the processing has not been well understood. In present work we selected a Fe--3.12wt.%Si--0.11wt.%Mn--0.021wt.%S model system and used FE-SEM and atom probe tomography (APT) to investigate the precipitation behavior of MnS inhibitors at near atomic scale. It was found that the Si--S enriched clusters with sizes of 5--15 nm were formed close to the MnS particles. The density of inhibitors decreased after large pseudo-plane-strain compression because of the effect of dislocation motion, and then slightly increased again when sample was aged at 200°C for 48 h. The dislocations and grain boundaries can act as fast diffusion paths and assist the reemergence of Si--S enriched clusters.     

Effect of homogenization on recrystallization in a twin-roll cast Al–Fe–Si alloy

The effect of homogenization treatment on the recrystallization process in a twin-roll cast AlFeSi alloy was investigated by means of calorimetry, microstructural analysis, electrical conductivity, and hardness measurements and cupping tests. The response to annealing of cold-rolled AlFeSi sheet processed with a homogenization treatment at the cast gauge is a typical two-stage, recovery and recrystallization process, while that processed without homogenization softens without recovery. The rather limited precipitation capacity in the former allows recrystallization to occur largely discontinously, favoring the annealing texture. The nucleation rate and the volume fraction of the discontinously recrystallized grains are largely reduced in the sheet processed without homogenization, owing to extensive dynamic precipitation. This reduces the strength of the annealing texture components and gives a more or less random crystallographic texture after annealing. With a relatively finer-grain structure and a nearly random crystallographic texture, AlFeSi sheet processed to soft temper at 1 mm without a homogenization treatment is an attractive foil stock material.     

The recrystallization of nickel-base superalloys

The effects of recrystallization on the γ′ distribution in four nickel-base superalloys of varying γ′ volume fraction (Nimonics PE16, 80A and 115, and Udimet 720) have been studied by transmission electron microscopy. These effects are explained in terms of high solubility and diffusivity in the recrystallization interface, and it is suggested that high diffusivity assumes greater importance as the amount of solute dissolved in the boundary increases. Some attention is given to the nucleation of recrystallization. It is shown that in one of the alloys (Udimet 720), nucleation at grain boundaries involves subgrain coalescence. Subsequent growth of the nucleus occurs by strain-induced boundary migration.     

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.     

新型Al-Zn-Mg-Cu合金热变形组织演化

采用Gleeble-1500D热力模拟试验机研究新型Al-Zn-Mg-Cu高强铝合金在变形温度为300~450℃,应变速率为0.001~10s~(-1)条件下的热变形组织演化。利用光学显微镜(OM)和透射电子显微镜(TEM)观察合金不同热变形条件下的组织形貌特征。结果表明:随着变形温度的升高和应变速率的减小,位错密度减小,亚晶粒尺寸增大;合金热压缩变形过程中主要的软化机制为动态回复和动态再结晶。变形温度为300~400℃时,主要发生动态回复;变形温度为450℃,应变速率为0.001~10s~(-1)时,软化机制以动态再结晶为主,存在晶界弓出、亚晶长大、亚晶合并3种再结晶形核机制。