Effects of Plastic Warm Deformation on Cementite Spheroidization of a Eutectoid Steel

 The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 ℃ and in the strain rate of 0.01 s-1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.     

Constitutive Model of Warm Deformation Behavior of Medium Carbon Steel

The compressive behaviors of medium carbon steel specimens were investigated over a wide range of tem-peratures and strain rates using a Gleeble-3500 thermo-simulation machine.The results show that the flow stress in-creased with strain at first,and then gradually decreased after reaching a peak value.The flow stress softening rate at a high strain rate was larger than that at a low strain rate.The effects of deformation heating and friction on flow stress were analyzed.A new friction correction method,wherein the effect of strain on frictional coefficient was con-sidered,was established here.The stresses revised by the new method deviated from the measured stresses with in-creasing strain.Meanwhile,the apparent frictional coefficient variation law with strain was obtained.The frictional coefficient increased as the strain increased and then slightly decreased after maintaining a constant value.The stress was corrected by considering deformation heating.The accuracy of the temperature correction method was verified using a special experiment.The results of the verification experiment show that the temperature correction method exhibited a good accuracy in calculating the variation of stress caused by deformation heating.A constitutive model considering strain was proposed here to describe the deformation behaviors.Compared with experimental data,the modified constitutive model exhibited a good accuracy as to constitutive correlation.     

Warm Deformation Microstructure of a Plain Carbon Steel

 Grain refinement in a plain carbon steel under intercritical warm deformation was studied by torsion testing. Based on the experimental results, the warm flow behaviour and microstructural evolution of ferrite were researched with particular emphasis on the effect of the strain rate in controlling the grain refinement mechanism of ferrite. The deformed microstructures were investigated at various strain rates using optical microscopy and electron back-scattered diffraction (EBSD). The EBSD observations indicate that an increase in the strain rate leads to the development of new fine ferrite grains with high angle boundaries. Furthermore, it shows that the annihilation of dislocations occurs more readily at lower strain rate. The elongated ferrite grains continuously dynamically recrystallize to form the equiaxed fine ferrite grains. Thereby, the aspect ratio of elongated grains decreases with increasing the strain rate. Furthermore, the peak stress and steady state stress of ferrite both increase with increasing strain rate. Based on the study, the effect of strain rate on the development of fine ferrite grains during continuous dynamic recrystallization of ferrite was analyzed in detail.     

Warm Forged Medium Carbon V Steel

Nowadays there is a continuous demand,particularly from the automotive industry,for cheaper,lighter and more reliable components.It is not surprising then that steel research has been focused during the last decades in new qualities and processes.This paper is dealing with the use of vanadium microalloyed steels on one of those new processes,warm forging.For its low precipitation temperature and its recognised ability to strengthen steel microstructures via austenite grain growth control,precipitation hardening and interference of the static recrystallization process,vanadium in microalloyed steels seem to be an appropriate candidate for warm forging.     

高 碳 钢 的 温 变 形 行 为

 在Gleeble 3500热模拟试验机上，用热压缩方法研究了原始组织为层片状珠光体的高碳钢在温度为913~973 K、应变速率为001~10 s-1范围内的温变形行为。结果表明：实验条件下，流变应力及峰值应变随变形温度的降低和应变速率的提高而增大。另外，回归出了高碳钢的峰值应力及峰值应变与变形温度、应变速率之间的关系，得到了相应的形变激活能和温变形方程式，为高碳钢的温变形工艺优化提供了理论依据。     

中碳钢马氏体组织温塑性变形力学行为

 利用Gleeble 3500热模拟试验机对中碳钢淬火和完全退火组织的温加工流变应力在不同的变形参数条件下进行了对比研究,并根据其真σ ε曲线拟合了可反映该材料两种原始状态下压缩的稳态流变曲线和变形参数之间关系的中温压缩变形本构方程。结果表明：当为001 s-1时,室温压缩的σ ε曲线,淬火态的远高于退火态;350~550 ℃压缩,淬火态稍高于退火态;高于600 ℃压缩,淬火态的流变应力在变形初期略高于退火态,随应变的增加,淬火组织的流变曲线低于退火组织;在压缩温度为650 ℃,为1 s-1、01 s-1、001 s-1时,淬火和退火组织压缩的σ ε曲线有交点,在为0001 s-1时,淬火组织的σ ε曲线低于退火组织的σ ε曲线。     

Influence of Initial Microstructure on Warm Deformation Processability and Microstructure of an Ultrahigh Carbon Steel

Various isothermal compression tests are carried out on an ultrahigh carbon steel(1.2% C in mass percent), initially quenched or spheroidized,using a Gleeble-3500system.The true stress is observed to decrease with increasing temperature and decreasing strain rate.The true stress of the initially quenched steel is lower than that of the initially spheroidized steel at high deformation temperature(700℃)and low deformation strain rate(0.001s-1).The value of the deformation activation energy(Q)of the initially quenched steel(331.56kJ/mol)is higher than that of the initially spheroidized steel(297.94kJ/mol).The initially quenched steel has lower efficiency of power dissipation and better processability than the initially spheroidized steel.The warm compression promotes the fragmentation and the spheroidization of lamellar cementites in the initially quenched steel.The fragmentation of lamellar cementites is the spheroidizing mechanism of the cementites in the initially quenched steel.Results of transmission electron microscope investigation showed that fine grains with high angle boundaries are obtained by deformation of the initially quenched steel.     

Influence of Warm Tempforming on Microstructure and Mechanical Properties in an Ultrahigh-Strength Medium-Carbon Low-Alloy Steel

A 0.4 pct C-2 pct Si-1 pct Cr-1 pct Mo steel was quenched and tempered at 773 K (500 °C) and deformed by multi-pass caliber rolling (i.e., warm tempforming). The microstructures and the mechanical properties of the warm tempformed steels were investigated as a function of the rolling reduction. At rolling reductions of more than 28 pct, not only extension of the martensite blocks and/or the packets in the rolling direction (RD) but also a grain subdivision became more significant, and an ultrafine elongated grain (UFEG) structure with a strong ??110??//RD fiber deformation texture was formed after 78 pct rolling. The tensile deformation behavior became significantly anisotropic in response to the evolution of UFEG structure. The longitudinal yield strength (??_y) of the quenched and tempered sample increased from 1480 to 1860 MPa through the 78 pct rolling, while the transverse ??_y leveled off at around 1600 MPa up to 28 pct rolling. The transverse true fracture stress was also markedly degraded in contrast to the longitudinal one. Charpy impact properties were enhanced at a rolling reduction of 52 pct or more. The 52 pct-rolled sample underwent a ductile-to-brittle transition in the temperature range from 333 K to 213 K (60 °C to ?60 °C), while the 78 pct-rolled sample showed an inverse temperature dependence of the impact toughness because of brittle delamination. The tensile and Charpy impact properties are discussed in association with the microstructural evolution.     

中碳当量冷顶锻钢碳化物相快速球化的工业试验

ＭＬ４５钢钢坯在冶金工厂轧钢生产线通过三阶段控制形变后可将钢的奥氏体晶粒超细化至平均直径５μｍ，随后在短时等温过程中，奥氏体的共析转变和形变铁素体的再结晶得以完成，同时粒状渗碳体得到适度聚集，最终获得渗碳体球化优良的显微组织。这种钢材可直接用生产效率高的冷锻法制造紧固件成形坯件，进而通过热处理得到高强紧固件成品。     

高碳珠光体钢在温变形过程中的组织变化

用SEM、TEM等方法研究了一种原始组织为层片状珠光体的高碳钢在多向温变形(楔横轧)和单向温变形(压缩)条件下试样心部与表层显微组织的演变情况.结果表明,在多向温变形条件下,试样表层可获得铁素体晶粒与渗碳体颗粒尺寸均在0.3 μm以下的超微细(α θ)复相组织,心部渗碳体片虽已碎断并部分球化,但其排布形态与变形珠光体基本保持一致,而单向温变形条件下的情况则恰好相反;变形过程中试样所受应力、应变状态的不同是引起组织差异的根本原因.     

TMCP工艺在低合金钢生产中的应用

根据济钢宽厚板厂现场实际情况,采取低碳低合金化和TMCP工艺相结合,成功开发了低合金高强度结构钢Q345E,该钢的各项力学性能符合国际标准要求。不仅为后续高强钢Q550D-Z、Q690D-Z合金减炼化提供参考数据,同时也为轧制同级别钢种去合金化成份提供合理的数据依据,保证该钢种采用合理的化学成份满足客户需求的力学性能。     

Deformation Behavior of Cementite in Deformed High Carbon Steel Observed by X-ray Diffraction with Synchrotron Radiation

The deformation behavior of cementite in drawn pearlitic steel and spheroidal cementite steel, which have hypereutectoid composition, was investigated by X-ray diffraction using synchrotron radiation. A detailed analysis of diffraction peak profiles reveals that the deformation behavior strongly depends on the shape of cementite in steel. The unit cell volume of the cementite in the drawn pearlitic steel compressively and elastically deforms by 1.5 to 2 pct of the initial volume at the early stage of drawing, whereas that in the drawn spheroidal cementite steel is compressed by 1 pct of the initial volume even at a large true strain. The cementite in the drawn pearlitic steel fragments into small pieces with increasing the true strain, and these pieces change to amorphous cementite. The dislocation densities of the cementite in the drawn pearlitic steel and in the drawn spheroidal cementite steel are estimated to be ~10¹³/m² before drawing and ~10¹⁴/m² after drawing. Although the large strain is induced in the cementite by drawing, the maximum strain energy in the cementite is too small to contribute to the dissolution of the cementite.

     

Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Ferrite in Low Carbon Steels

 Abstract Effects of prior austenite deformation and non-metallic inclusions on the ferrite nucleation and grain refinement of two kinds of low carbon steels have been studied. The ferrites nucleation on MnS and V(C,N) is observed. The combination of thermal-mechanical processes with adequate amounts of non-metallic inclusions formed in low carbon steels could effectively refine the grain size and the microstructure. Ferrite nucleated on the single MnS or V(C,N) inclusions and complex MnS+V(C,N) inclusion. The proper addition of elements S and V could effectively promote the formation of ferrite and further refinement of ferrite grains.     

Effect of Microstructure of Cementite on Interphase Stress State in Carbon Steel

 The experiments related to stress states of ferrite and cementite in carbon steels were carried out including in situ four point bending and tensile test by X ray diffraction technique. Stresses in the cementite phase can be measured by conventional X ray diffraction instrument after a specific treatment on the specimen surface. In order to estimate the stress states in two phases, the X ray elastic constants of two phases in single phase state (PXEC) are determined by the experimental X ray elastic constants of them in composite state (CXEC). The effects of volume fraction and particle size of spheroidal cementite on the interphase stress state are estimated. The experimental results are in good agreement with the theoretical relationships reported in the previous studies.     

Dynamic Recrystallization Behavior of Medium Carbon Cr-Ni-Mo-Nb Steel during Hot Deformation

Hot compression deformation behaviors of medium carbon Cr-Ni-Mo-Nb steel were investigated at deformation temperatures ranging from 1223 to 1423 Kand strain rates of 0.1,1and 5s^-1.Dynamic recovery（DRV）and dynamic recrystallization（DRX）were observed during the hot compression deformation.For all of the samples,DRX occurred at deformation temperatures above 1323 Kat different strain rates,while below 1223 K,no DRX was observed.The activation energy of the tested steel was determined as 386.06kJ/mol.The ratio of critical stress to peak stress and the ratio of critical strain to peak strain were 0.835 and 0.37,respectively.Kinetic equations interpreting the DRX behavior of the tested steel were proposed,and the corresponding parameters including the volume fraction and the average grain size were determined.Moreover,the microstructures induced under different deformation conditions were analyzed.     

The Influence of Vanadium on Ferrite and Bainite Formation in a Medium Carbon Steel

The influence of vanadium additions on transformation kinetics has been investigated in a medium carbon forging steel. Using dilatometry to track transformation during continuous cooling or isothermal transformation, the impact of vanadium on both ferrite-pearlite and bainite has been quantified. Transmission electron microscopy and atom probe tomography have been used to establish whether vanadium was present in solid solution, or as clusters and precipitates. The results show that vanadium in solid solution has a pronounced retarding influence on ferrite-pearlite formation and that, unlike in the case of niobium, this effect can be exploited even during relatively slow cooling. The influence on bainite transformation was found to depend on temperature; an explanation in terms of the effect of vanadium on heterogeneous nucleation is tentatively proposed.     

Warm Deformation of Low Carbon Steel Using Forward Extrusion-Equal Channel Angular Pressing Technique

A combination of extrusion and equal channel angular pressing (ECAP) was used to deform a plain low carbon steel. This process consists of two successive deformations by extrusion and ECAP in a single die (Ex-ECAP). Cylindrical samples were heated to predefined temperatures (650 and 850 ℃) and then pressed through a die channel with crosshead speed of 10 mm/s. Microstructure and resultant mechanical properties of processed material were studied. The results showed that pressing temperature has a significant effect on the resultant microstructure. While at 650 ℃, the cold worked structure with elongated ferrite grains were obtained, and at 850 ℃ the microstructure consisted of elongated ferrite grains and very fine grains at their boundaries as a consequence of continuous dynamic recrystallization (CDRX) of ferrite phase. Also at 850 ℃, a particular microstructure consisted of cold worked ferrite and static recrystallized grains on shear bands was obtained.     

Spheroidization and Ostwald Growth of Carbide in Isothermal Process of Hot-Deformed High Carbon Chromium Cast Steel

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