Dynamic observation of bainite transformation in a Fe-C-Mn-Si superbainite steel

The dynamic observations of bainitic transformation in a Fe-C-Mn-Si superbainite steel were conducted on a high temperature laser scanning confocal microscope. It is indicated that the mutual intersection of bainite sheaves often occurs during growth of bainite ferrite, resulting in an interlocked bainite microstructure. Moreover, bainite transformation is promoted by higher austenization temperature and the longer and finer bainite platelets are obtained. Further, The average growth rate of bainite after austenization at 1 100 ° is calculated as 5.8 µm·s^?1. In situ observation investigation makes it possible to identify bainite transformation in real time during isothermal holding.     

In-situ observation of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by Zr-Ti combined deoxidation

The effect of Zr-Ti combined deoxidation on the grain refinement in the simulated coarse-grained heat-affected zone of a high-strength low-alloy steel was investigated by means of analytical characterization techniques such as in-situ microscopy, transmission electron microscopy, and electron backscattered diffraction analysis. Owing to the Zr-Ti combined deoxidation, a large amount of fine Zr-Ti oxide particles were formed in the steel and retarded the austenite grain growth during simulated welding thermal cycle. The austenite grains were small and uniform. The Mn can diffuse spontaneously from austenite to Zr-Ti oxide inclusion and MnS precipitated on ZrO₂, which can form Mn depleted zone in the vicinity of inclusion. The acicular ferrite grains nucleated on intragranular Zr-Ti oxide inclusions in austenite grains grew in different directions and effectively divided the austenite grain into several finer and separate regions at intermediate temperature. The crystallographic grain size became small in the simulated coarse-grained heat-affected zone of Zr-Ti-killed steel due to the effective pinning effect by Zr-Ti oxide particles and acicular ferrite formation.     

Effect of Zr-deoxidation on microstructure and mechanical behavior of microalloyed heavy plates with low impurity content

The significance of different deoxidation practises on the ductility and impact toughness of next generation of microalloyed heavy plates was elucidated to explore the best deoxidation practice in obtaining required mechanical properties,which was judged by the combined effects of composition,size and number density of inclusions on the ductility of the experimental high-strength low alloy steel.The impurity contents,i.e.,total O+N+S contents,of 82×10^-6(Al-killed)and 118×10^-6(Zr-killed)have been induced to characterize both the steels.Ductility was characterized using tensile and Charpy V-notch testing.The number,size and composition of the inclusions were characterized using a field emission scanning electron microscope with an energy dispersive spectrometer.In the Al-killed steel,the inclusion structure consisted of titanium nitrides,stringer calcium aluminates and elongated manganese sulfides,whereas in the Zr-killed steel,the inclusion structure consisted of mainly fine spherical oxide inclusions with sulphide shells.The impurity content did not have a significant effect on the number density of inclusions,as with higher and lower impurity content,the number of inclusions was 83.7 and 78.8 mm^-2,respectively.However,the size distribution of the inclusions,especially the coarse inclusions with their longest length greater than 8μm,differsmuch from each other.The number density of coarse inclusions differs from 0.8 to 1.1 mm^-2 with processing,and in Al-killed steel,55.5% of the coarse inclusions were titanium nitrides or manganese sulfides,whereas in Zr-killed steel,only 22.5% of the coarse inclusions were titanium nitrides and manganese sulfides.Coarse titanium nitrides were especially detrimental to the impact toughness.The number density of them should be below 0.33 mm-2 in order to guarantee the best possible toughness in the steel in question.The average crystallographic grain size detected by electron backscattered diffraction of Zr-killed steel(4.28±2.70μm)was smaller than that of Al-killed steel(6.00±4.80μm).As a result from the grain refinement and sulphide shape control,Zr-killed steel exhibited superior impact toughness(223±70 J)at -80℃ as compared with Al-killed steel(153±68 J).Thus,Zr-killed steel was observed to provide good performance in terms of mechanical properties as compared with Al-killed steel.     

Effect of Ni Addition on Bainite Transformation and Properties in a 2000 MPa Grade Ultrahigh Strength Bainitic Steel

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.     

残留奥氏体对微纳贝氏体钢塑韧性的影响

采用高碳和中碳低温贝氏体转变工艺（095C钢为200℃等温10d,030C钢为320℃等温1d）研究了残留奥氏体对微纳结构钢塑韧性的影响,对不同试样的显微组织、各相体积分数、伸长率和冲击韧性进行观察、检测和分析。试验结果表明,中碳钢贝氏体转变的塑韧性明显高于高碳钢贝氏体转变,主要原因是中碳钢贝氏体转变中存在一定的亚微米级薄膜状残留奥氏体,在拉伸或冲击过程中引起的残留奥氏体的塑性变形,使断裂的能量增加,可以显著提高样品的塑韧性。     

Fe-1C钢离异共析转变行为

离异共析转变(DET)是高碳钢实现快速球化退火的重要理论依据。为了深入理解离异共析转变机理,采用定量金相方法,通过奥氏体中断淬火和等温转变试验对Fe-1C钢离异共析转变行为进行研究。结果表明,在共析温度以下,Fe-1C钢中离异共析转变与珠光体转变存在竞争, DET相体积分数与奥氏体化后残留的渗碳体粒子间距L_θ以及珠光体片层间距λ有关,DET相体积分数随λ/L_θ值的增加而增大。用基于JMAK方程的离异共析转变动力学模型计算了DET相体积分数随λ/L_θ的变化关系并与试验结果进行了比较拟合,试验测量值与计算拟合曲线吻合较好。λ/L_θ的值越大,离异共析转变时碳原子扩散所需距离相对于珠光体转变越短,就越有利于离异共析转变的发生。     

轧制压下量对低碳超细晶粒钢Q235B组织及性能的影响

低碳钢Q235B奥氏体化后淬火得到马氏体组织,然后在室温下进行多道次轧制,总压下量分别为50%与70%,随后在500～650℃退火2 min制备低碳超细晶粒钢,研究轧制压下量对超细晶粒钢组织及性能的影响。采用光学显微镜和扫描电子显微镜观察显微组织演变,用能谱仪分析析出物颗粒化学成分,在Instron-5969拉伸试验机上进行拉伸实验。结果表明,冷轧+退火马氏体起始组织可以制备超细晶钢,强度相比原始钢强度提高近一倍。此外,随着压下量的增加,再结晶温度降低,渗碳体颗粒易长大,不利于超细晶钢机械性能的提高。相同退火工艺下,压下量增加,铁素体晶粒及渗碳体尺寸长大,其综合机械性能降低。     

含Mg夹杂物对低合金高强度钢CGHAZ韧性的影响

采用Ca、Mg处理工艺研究不同夹杂物粒子对200kJ/cm大线能量焊接低合金高强度钢CGHAZ冲击韧性的影响,对各个试样CGHAZ的显微组织,晶粒尺寸和冲击功进行观察和分析.试验结果表明,在低合金高强度钢CGHAZ中,Mg处理钢的冲击性能明显高于Ca处理钢,主要原因是含Mg夹杂物粒子析出能力比含Ca夹杂物粒子强,在焊接...     

合金碳化物M23 C6的磁性及磁致磁性

利用第一性原理研究了碳化物M23 C6 (M=Fe,Cr)的晶胞结构、磁矩和态密度,并结合魏氏分子场理论分析了磁场作用下一些碳化物的磁致磁性.结果 显示:Fe20 Cr3 C6碳化物中的Fe3和Fe4原子连接组成稳定的晶格框架,Cr1、Cr2和C原子位于框架中心;Fe20 Cr3 C6的磁性主要来源于Fe和Cr原子的3...