强磁场下保温时间对Fe 0.76%C钢先共析 铁素体组织形貌的影响

 借助于光学显微镜研究了磁场（12 T）对Fe 0.76%C合金在807 ℃奥氏体化保温不同时间(10 min、30 min、60 min)后以2 ℃/min的冷速冷却后,先共析铁素体显微组织的形貌变化。结果表明：在相同奥氏体化保温时间下,经强磁场热处理样品的先共析铁素体的面积分数和晶粒数量明显高于无磁场热处理样品。这可归结为强磁场降低了先共析铁素体形核所需的驱动力。随着奥氏体化保温时间的延长先共析铁素体晶粒沿着强磁场方向伸长的趋势明显变弱。这主要是由于奥氏体晶粒随着奥氏体化保温时间的延长逐渐增大,导致铁素体晶核之间的距离增大,从而造成奥氏体中的Fe原子向先共析铁素体晶粒扩散的距离增大所致。     

形变诱导铁素体逆相变过程中的组织及取向变化

 通过对普碳钢Q235在Gleeble1500热模拟机上变形后的微观组织分析,研究了组织中形变诱导的铁素体在变形后保温阶段转变为奥氏体的逆相变现象;并利用背散射电子衍射(EBSD)技术分析了晶粒取向变化。结果表明,在变形后的保温过程中,形变诱导的铁素体先逆相变为奥氏体,同时伴随着诱导铁素体晶粒的长大;然后随着变形后保温时间的延长,逆相变后的奥氏体由马氏体相变逐渐过渡到铁素体的平衡转变,相应地铁素体由具有少量亚结构的形变诱导铁素体逐渐转变为具有较多亚结构的先共析铁素体。     

Crystallography of grain boundary proeutectoid ferrite

A study has been made of the crystallography of proeutectoid ferrite precipitated at high angle austenite grain boundaries in an Fe-0.47 pct C alloy, isothermally transformed above the eutectoid temperature. Using the Kossel X-ray microdiffraction technique, ferrite orientations have been determined in relation to the orientations of both matrix grains at the grain boundary; the austenite orientations were derived indirectly. It has been observed that the ferrite, irrespective of morphology, possessed an orientation relationship with respect to at least one matrix grain which approximated to the Kurdjumov-Sachs and, to a lesser extent, the Nishiyama relationships. At several of the boundaries the ferrite was allowed to possess an orientation relationship with both matrix grains and it has been shown that the ferrite orientation at these boundaries was often influenced by the orientation of both matrix grains. Several instances have been observed in which preferential growth occurred into the austenite grain with which the precipitate did not have a specific orientation relationship. The results have been compared with the work of Ryder, Pitsch and Mehl,5 and factors governing the observation of Widmanstatten sideplates have been discussed.     

Crystallography of grain boundary proeutectoid ferrite

A study has been made of the crystallography of proeutectoid ferrite precipitated at high angle austenite grain boundaries in an Fe-0.47 pct C alloy, isothermally transformed above the eutectoid temperature. Using the Kossel X-ray microdiffraction technique, ferrite orientations have been determined in relation to the orientations of both matrix grains at the grain boundary; the austenite orientations were derived indirectly. It has been observed that the ferrite, irrespective of morphology, possessed an orientation relationship with respect to at least one matrix grain which approximated to the Kurdjumov-Sachs and, to a lesser extent, the Nishiyama relationships. At several of the boundaries the ferrite was allowed to possess an orientation relationship with both matrix grains and it has been shown that the ferrite orientation at these boundaries was often influenced by the orientation of both matrix grains. Several instances have been observed in which preferential growth occurred into the austenite grain with which the precipitate did not have a specific orientation relationship. The results have been compared with the work of Ryder, Pitsch and Mehl,5 and factors governing the observation of Widmanstatten sideplates have been discussed.     

In-Situ Observation of Proeutectoid Ferrite Growth Process in Carbon Steel Under Continuous Cooling Conditions

The growth of proeutectoid ferrite in Fe-0.15%C-0.8%Mn carbon steel during continuous cooling process was observed in-situ and tracked dynamically by using a high-temperature confocal scanning laser microscope (HTCSLM), and the growing process was also investigated. The growth regularity of proeutectoid ferrite plates within austenite was obtained by analyzing the growth fashions and directions and quantitatively studying the growth rates. The results show that the proeutectoid ferrite plates grow in the fashion of creeping, twisting, branching off and deflection within austenite grain. The proeutectoid ferrite plates can grow up along preferential orientation but not strictly. Its growth direction fluctuates and changes in a small range. The growth rate also changes with the change in growth fashion and direction. When the proeutectoid ferrite grows in the fashion of deflection, the average growth rate of proeutectoid ferrite before its deflection is about 61.9, and 46.2 μm/s after the 15° deflection. At the beginning of proeutectoid ferrite growing, there is a fast growing stage. As the tip of proeutectoid ferrite extends forward continuously, the growth rate will slow down. Therefore, it will induce the proeutectoid ferrite to deflect and grow along another favorable direction, then it will grow rapidly again.     

Influence of magnetic field on the kinetics of proeutectoid ferrite transformation in iron alloys

The kinetics of proeutectoid ferrite transformation in Fe-C base alloys in a strong magnetic field (7.5 T) were studied. The transformation kinetics were accelerated at temperatures not only below but also significantly above the Curie temperature. The free energy and equilibrium ferrite/austenite phase boundaries in applied magnetic fields were calculated using the reported experimental magnetic susceptibility and Weiss molecular field theory. The persistence of magnetic field effects above the Curie temperature can be attributed to the rapidly diminishing difference in relative stability between ferrite and austenite toward the Ae ₃ temperature of iron.     

Microstructures of hot-rolled high-strength steels with significant differences in edge formability

The relationship between microstructure and hole expansion was investigated for three industrial mill-processed steels with similar yield strength (about 525 MPa) and total elongation (about 25 pct). The nominal steel composition was (in mass pct) 0.1C, 1.4Mn, 0.1Si, 0.02Al, 0.04Nb, and 0.02Ti; any variations in composition or processing history were unintentional. The microstructures of all steels consisted of about 80 pct of proeutectoid ferrite and 20 pct of a carbon-enriched, high-hardness, low-temperature transformation product (LTTP). Despite these similarities, the hole-expansion values for the steels were 44, 74, and 115 pct. Detailed microstructural characterization revealed significant differences in the LTTPs of the three steels, as well as several important differences in the proeutectoid ferrite grains. Previously reported negative effects of large quantities of martensite, microstructural banding, and a high hardness ratio (LTTP/ferrite) were validated. Different hardness ratios correlated with differences in (1) dislocation substructures of proeutectoid ferrite grains (2) grain-size distribution, and (3) the fine structure of bainitelike/pearlitelike regions. Superior hole-expansion performance (or edge formability) was associated with a microstructure consisting of 78 pct of uniformly fine-grained proeutectoid ferrite and 22 pct of a bainitelike microconstituent, a minimum amount of microstructural banding, and a low hardness ratio. Tensile-bar fracture surfaces of a material with this microstructure showed the largest amount of microplasticity. At the time the work was carried out R.D.K. Misra was at LTV Steel, Technology Center.     

Microstructures of hot-rolled high-strength steels with significant differences in edge formability

The relationship between microstructure and hole expansion was investigated for three industrial mill-processed steels with similar yield strength (about 525 MPa) and total elongation (about 25 pct). The nominal steel composition was (in mass pct) 0.1C, 1.4Mn, 0.1Si, 0.02Al, 0.04Nb, and 0.02Ti; any variations in composition or processing history were unintentional. The microstructures of all steels consisted of about 80 pct of proeutectoid ferrite and 20 pct of a carbon-enriched, high-hardness, low-temperature transformation product (LTTP). Despite these similarities, the hole-expansion values for the steels were 44, 74, and 115 pct. Detailed microstructural characterization revealed significant differences in the LTTPs of the three steels, as well as several important differences in the proeutectoid ferrite grains. Previously reported negative effects of large quantities of martensite, microstructural banding, and a high hardness ratio (LTTP/ferrite) were validated. Different hardness ratios correlated with differences in (1) dislocation substructures of proeutectoid ferrite grains, (2) grain-size distribution, and (3) the fine structure of bainitelike/pearlitelike regions. Superior hole-expansion performance (or edge formability) was associated with a microstructure consisting of 78 pct of uniformly fine-grained proeutectoid ferrite and 22 pct of a bainitelike microconstituent, a minimum amount of microstructural banding, and a low hardness ratio. Tensile-bar fracture surfaces of a material with this microstructure showed the largest amount of microplasticity. At the time the work was carried out R.D.K. Misra was at LTV Steel, Technology Center.     

Modeling for Formation of Proeutectoid Ferrite in Steel during Continuous Cooling

A novel model of the evolution of microstructure during continuous cooling with the formation of proeutectoid ferrite in steel was proposed from a Voronoi construction for the austenite grains, based on the Rappaz‘s integral nucleation model and the assumption that the ferrite nucleates at the edges of the original austenite grains and the successive growth of the ferrite grain is radial. The model can be used to calculate the fraction of ferrite as a function of time or temperature during continuous cooling, and to determine the microstructure of ferrite. The calculated results are in agreement with experimental results investigated in 0.38C-0.28Si-0.55Mn-0.92Cr-0.20Mo steel under continuous cooling using a Gleeble 1500 thermomechanical simulator.     

Mechanism of Crack Propagation during Hot-Rolling Process of a Medium-Carbon 40Cr Alloy Steel

In this work, the mechanism of crack propagation during hot-rolling process of a typical medium-carbon 40Cr alloy steel is investigated by cracks characterization, thermodynamic calculation, and confocal laser scanning microscopy (CLSM). The depth of cracks is about 2.5 mm and its length along with rolling direction can even reach 2000–3000 mm. Thermodynamic calculations show that the oxide phases including MnSiO₃ and MnCr₂O₄ can generate when the oxygen content is 0.3–1.0 wt%, suggesting that low oxygen is beneficial to the selective oxidation of Si, Mn, and Cr elements from the medium-carbon low alloy. Furthermore, in situ experiment by CLSM indicates that the submicron Cr–Mn–Si–O particles can refine austenite grains. In addition, the contents of chain proeutectoid ferrite in the steel containing Cr–Mn–Si oxides increase by 6.3% and 12.0% at the lower cooling rates of 5 and 10 °C min⁻¹, respectively, comparing with that of no-oxide particles steel. The submicron Cr–Mn–Si–O particles can refine austenite grains, which induces the precipitation of chain proeutectoid ferrite. Thus, the serious surface cracks propagate along the chain proeutectoid ferrite with the submicron Cr–Mn–Si–O particles during the hot-rolling process.     

The principle of additivity and the

This study critically examines the principle of additivity and the reason that the proeutectoid ferrite transformation is additive. Austenite-to-proeutectoid ferrite transformation kinetics were measured under isothermal and stepped-isothermal conditions for AISI 1010 and 1020 steel grades using a dilatometer and a Gleeble 1500 thermomechanical simulator. The additive nature of the austenite-to-proeutectoid ferrite transformation was experimentally assessed by measuring transformation kinetics partially at one temperature and after a rapid temperature change to another temperature. Results of the tests on the 1010 steel showed that the proeutectoid ferrite transformation with allotriomorphic morphology is additive. Transformation kinetics were mea- sured for the 1020 steel with the ferrite morphology changing from allotriomorphic to predom- inantly Widmanstätten, and the transformation was additive. However, the stepped-isothermal test in which the ferrite was transformed and equilibrated at the first temperature and then rapidly cooled to the second temperature was not additive. The second part of the study involved de- veloping mathematical models with planar and spherical interface geometries to theoretically assess the additivity of the proeutectoid ferrite transformation. Additivity of the proeutectoid ferrite transformation was tested by predicting the ferrite growth kinetics and the associated carbon gradients under stepped-isothermal conditions. The predictions were consistent with the observed experimental additivity of the proeutectoid ferrite transformation, providing an expla- nation for this behavior, although theory would suggest ferrite reaction to be nonadditive.     

New insights into the widmanstätten proeutectoid ferrite transformation: Integration of crystallographic and three-dimensional morphological observations

The crystallography and three-dimensional (3-D) morphology of Widmanstätten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit^[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstätten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations.     

强磁场对铁基合金相变温度和显微组织的影响

The effect of a high magnetic field up to 30 T on phase transformation temperature and microstructure of Fe-based alloys has been reviewed. A high magnetic field accelerates ferrite transformation, changes the morphology of the transformed microstructures and increases the As and A1 temperature. In a magnetic field of 30 T, the A1 temperature increases by about 37.1℃ for Fe-0.8C, the A3 temperature for pure Fe increases by about 33.1 ℃. The measured transformation temperature data are not consistent with calculation results using Weiss molecular field theory. Ferrite grains are elongated and aligned along the direction of magnetic field in Fe-0.4C and Fe-0.6C alloys by ferrite transformation, but elongated and aligned structure was not found in pure Fe, Fe-0.05C alloy and Fe-1.5Mn0.11C-0.1V alloy.     

热变形双相不锈钢的微区取向及组织分析

 在变形温度为1000和1100℃,应变速率为01s-1的条件下,利用MMS-200热模拟试验机,对S32750超级双相不锈钢进行了高温压缩试验。利用电子背散射衍射(EBSD)分析了其晶体取向和微观组织。研究结果表明,铁素体在两种试验条件下均可形成<001>和<111>∥压缩轴织构,在变形温度为1100℃时,<001>织构要强一些;奥氏体在两种变形温度下均形成了<001>织构,强度很弱。在变形温度为1100℃条件下,奥氏体中存在的以Σ3为主的CSL特殊晶界数量更多。两种试验条件下,S32750超级双相不锈钢中铁素体和奥氏体均发生了动态再结晶,降低变形温度有利于细化晶粒。在铁素体向奥氏体转变过程中,奥氏体可以在铁素体晶界处生成,也可以在铁素体晶粒内部形成。     

Effect of Magnetic Field Intensity on Abnormal Microstructure in Fe–1.1%C Alloy

The features of the abnormal microstructure during the austenitic decomposition in Fe–1.1%C alloy under different magnetic field intensity have been investigated by optical microscopy. It was found that the high magnetic field considerably affects the abnormal microstructure in Fe–1.1%C alloy. The high magnetic field increases the area fraction and width of the abnormal microstructure through remarkably decreasing the Gibbs energy needed for the ferrite transformation. Simultaneously, due to the fact that the Ae1 line shifts to high temperature side with the increase of magnetic field intensity, the abnormal microstructure transformation is imposed to stop and the pearlite transformation begins. Therefore, the area fraction of the abnormal microstructure firstly increases and then decreases with the increase of the magnetic field intensity.     

Analysis of Microstructure and Orientation in X80 Line Pipe Steels

Microstructures in X80 line pipe were classified by SEM analysis.The experimental results showed that the microstructures in X 80 line pipe steels were complicated consisting of polygonal ferrite,bainite and acicular ferrite.Orientation relation within acicular ferrite was investigated systematically by means of EBSD-OIM.The sub-structures were observed maximum in acicular ferrite which gives high strength and high toughness to line pipe steels.The K-S orientation relation was generally observed between acicular ferrite and austenite during phase transformation.The low energy CSL boundary characterized by Σ3 orientation relation according to Brandon criterion appeared with higher probability,which was benefit to improve the mechanical properties of line pipe steels.The orientations or texture of initial austenite grains could be deduced based on the Σ3 orientation relationship of acicular ferrite variants.     

V-N对中碳SiMn非调质钢显微组织的影响

用光学显微镜和透射电镜研究了0.130%V-0.021 5%N、0.130%V-0.0304%N和0.001%V- 0.020 7%N三种V-N含量的(%)0.37～0.38C、0.82～0.92Si、1.78～1.81Mn、0.06 Ti、0.015Nb非调质钢的组织,用Gleeble 1500热模拟机测定了该钢的应力-应变曲线,并用JMatPro 4.1软件计算了该钢的CCT曲线以及900℃和600℃平衡状态下钢中各相的含量。结果表明,该钢的组织为珠光体+先共析铁素体,随氮含量增加,珠光体增多,晶界铁素体变粗;随钒含量减少,珠光体数量显著增加,贝氏体及铁素体变粗;共析铁素体和先共析铁素体中的析出物尺寸≤3 nm;V和/或N含量高的钢,应变时应力大。     

塑料-锶铁氧体复合材料密封条制造新工艺

介绍了一种以"预压磁取向"法为基础制造塑料-锶铁氧体复合材料密封条(以下简称塑料磁条)的新工艺,并探索了工艺因素对制品磁性能的影响。本工艺具有产品质量好、操作简单、节能及经济效益显著的优点。     

Study of the ferrite grain coarsening behind the transformation front by electron backscattered diffraction techniques

The degree of ferrite grain refinement that can be reached in low-carbon microalloyed steels by thermomechanical processing is limited, to a certain extent, by the grain coarsening which can take place behind the transformation front. The coarsening of ferrite grains is the result of two different mechanisms: elimination of ferrite grains produced by normal grain growth after full impingement on the austenite grain boundary plane and/or coalescence between different ferrite grains with close orientation formed from the same crystallographic variant. The lack of experimental data to support either process is due to the experimental difficulties encountered when analyzing the phenomenon. Some transmission electron microscope (TEM) observations reveal that the ferrite grains formed along a prior grain boundary in deformed austenite are separated by a mixture of low and high angle grain boundaries upon impingement. In the present work, the electron backscattered diffraction (EBSD) technique has been applied to investigate the microstructural evolution during transformation, with special emphasis placed on the α-α grain boundary character as a means of investigating the contribution of coalescence/grain growth to coarsening.     

The kinetics of ferrite nucleation at austenite grain edges in Fe-C and Fe-C-X alloys

Nucleation kinetics of proeutectoid ferrite allotriomorphs at the edges of austenite grains in Fe-C and Fe-C-X alloys, where X is successively Mn, Ni, Co, and Si, have been measured using a modification of the techniques previously developed to study nucleation at grain faces. Analysis of these data with classical heterogeneous nucleation theory has shown that ferrite nuclei formed at grain edges have low energy interphase boundaries. An equivalent conclusion was reached during our previous studies of ferrite nucleation at austenite grain faces. The influence of alloying elements on nucleation rates was also found to follow a pattern similar to that demonstrated for grain face nucleation. Formerly Graduate Student with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University,