Spheroidizing Kinetics and Optimization of Heat Treatment Parameters in CK60 Steel Using Taguchi Robust Design

For processing parts made from medium carbon steel,toughness and flexibility are of importance.Therefore,to achieve these properties,the cementite in the steels is spheroidized through heat treatment.Different parameters such as the time and temperature of spheroidizing and the initial microstructure of the steel affect the amount of spheroidized cementite.In the present work,the percent of contribution of two parameters,i.e.initial microstructure and spheroidizing time,to the percent of spheroidization in CK60 steel was investigated using Taguchi robust design.The initial microstructures consisted of martensite,coarse pearlite,fine pearlite and bainite and the chosen spheroidization times were 4,8,12,and 16h.Spheroidizing was done at the constant temperature 700℃.After spheroidizing was completed,the samples were prepared in order to observe their microstructure under an optical microscope and to determine the spheroidized percent using MIPTM(metallographic image processing)software.It was found that the spheroidizing time had the most influence(58.5%)on spheroidized percent and the initial microstructure only had a 31.1% contribution.Finally,the instantaneous growth rate of the carbide was also deduced.     

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.     

Effect of Cyclic Annealing on Microstructure and Mechanical Properties of Medium Carbon Steel

The microstructure and mechanical properties of medium carbon steel after cyclic heat treatment were in-vestigated.The effects of cyclic numbers and long time annealing on the microstructure and mechanical properties of the experimental steel were compared.A short-duration (5 min)holding at 1 023 K (above A1 temperature)and a short-duration (3 min)holding at 893 K are adopted in each cyclic heat treatment.The spheroidization is accelerated during cyclic heat treatment,and the spheroidizing ratio grows with cyclic numbers.After 1 2-cycle heat treatments, there are few incompletely spheroidized regions in the specimens,and cementite lamellae mostly change into cement-ite particles.The morphological character of cementite for 12 cycles is similar to that undergoing annealing for 10 h at 973 K.The strength of the experimental steel after 5-cycle heat treatment is the lowest in the following cyclic heat treatment,but it is still higher than that of specimens with subcritical annealing over a long period (10 h).After 12-cycle heat treatment,the strength of the experimental steel is close to that of the normalized steel,and the plasticity is the best in all heat-treated specimens.     

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.     

New Spheroidizing Technique of Ultra-High Carbon Steel With Aluminum Addition

A new spheroidizing process of ultra-high carbon steel （UHCS） containing C 1.55%, Cr 1.45%, and Al 1.5% in mass percent has been proposed. The effect of processing parameters on the microstructure was analyzed. The UHCS produced by this new process has a microstructure with recrystallized ferrite matrix and fine and uniform carbide particles. After this spheroidizing, the UHCS exhibits good mechanical properties at ambient temperature, for example σb= 1 100 MPa, σs =915 MPa, δ=8% and high ratio of σs/σb.     

Influence of Mo Content on Microstructure and Mechanical Properties of Fire-Resistant Construction Steel

In order to ensure fire-resistant property of the steel under the condition of the least molybdenum content,effects of molybdenum on microstructure and properties of the fire-resistant construction steel,specially on high temperature strength were investigated.The results show that the room temperature and high temperature strength of the steel is enhanced due to increased Mo content but the impact toughness deteriorates at the same time.The fraction of bainite in the microstructure increases with increasing Mo content while the fractions of polygonal ferrite and pearlite decrease.Comparing with the steel of adding Nb alone,it was obvious that combined addition of Mo and Nb could provide better effect in high temperature strengthening.Addition of 0.25mass% Mo in the steel containing Nb could meet fire-resistant requirements of the construction steel and provide better economical effectiveness.     

Microstructure Evolution and Microhardness of Ultrafine-grained High Carbon Steel during Multiple Laser Shock Processing

Surface microstructure and microhardness of(ferrite+cementite)microduplex structure of the ultrafinegrained high carbon steel after laser shock processing(LSP)with different impact times were investigated by means of scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and microhardness measurements.Equiaxed ferrite grains were refined from 400 to 150nm,and the cementite lamellae were fully spheroidized,with a decrease of the particle diameter from 150 to 100nm as the impact times increased.The cementite dissolution was enhanced significantly.Correspondingly,the lattice parameter ofα-Fe and microhardness increased with the impact times.     

The Cellular Automata Model of Electroslag Remelting Ingot Structure

Electroslag remelting (ESR) is an advanced process for the production of high quality steels. The microstructure of remelted steel which affect the mechanical properties and the performance of the ingot was determined by the technological parameters. A two-dimensional axisymmetric geometry which was established in this paper was divided into macro-grid finite element in order to compute temperature field; then the grid was divided into more detailed and uniform cells, and at last the continuous nucleation model based on the Gaussian distribution and KGT growth model was established for nucleation and growth calculations using cellular automaton method (CA) on the solidification of molten steel. The results show that: a vertical columnar grain zone and a inverted V-shaped columnar crystal zone appeared in the ESR ingot. In addition, the temperature field with different electrode melting rate and slag pool temperature parameters and the microstructure with different average nucleation under cooling and maximum grain density were studied in this paper. The simulation results agree well with the experimental results, so it is proved that the model and calculation method is reliable. To produce ideal solidified ingot and achieve the purpose of optimizing the production process, the production process was adjusted according to the simulation results.     

Constitutive Equation Models of Hot-Compressed T122 Heat Resistant Steel

 Based on dislocation reaction theory and Avrami equation, a constitutive equation model was developed to describe dynamic recovery and dynamic recrystallization during hot deformation of T122 heat resistant steel, which have taken the effect of dynamic strain aging into account. Uniaxial hot compression test had been carried out over a wide range of strain rate (001 to 10 s-1) and temperature (900 to 1200 ℃) with the help of Gleeble 3500. Obtained experimental data was applied to determine the material parameters in proposed constitutive equations of T122 steel, by using the non-linear least square regress optimization method. The calculated constitutive equations are quantitatively in good agreement with experimentally measured curves and microstructure observation. It shows that propose constitutive equation T122 steel is able to be used to predict flow stress of T122 steel during hot deformation in austenite temperature scope.     

Hot Stamping Parameters Optimization of Boron Steel Using a Response Surface Methodology Based on Central Composite Design

The effect of hot stamping parameters on the mechanical properties of 22MnB5 steel sheet with thickness of 1.1mm is studied.The considered parameters are austenization temperature(800-1 000 ℃),austenitizing soaking time(60-540s),initial deformation temperature(560-800 ℃)and tool temperature(20-220 ℃).In order to obtain hot stamped parts with optimal mechanical properties,response surface methodology based on the central composite design has been employed to design the experiment matrix.Tensile strength of hot stamped parts is determined as the relation in the mathematical model.The optimal condition and objective effects of parameters are determined via this relation.The statistical analysis showed that all four factors significantly affect the tensile strength of the hot stamped parts.The optimum austenization temperature is found to be 918.89 ℃ with the austenitizing soaking time,initial deformation temperature and tool temperature of 279.45 s,684.69 ℃ and 21.85 ℃,respectively.These optimal hot stamping parameters prove to have high tensile strength(1 631.84MPa)where deviation between predicted and actual response falls within 2%.     

轴承钢珠光体球化的研究现状及发展趋势

概述了轴承钢球化退火在轴承生产中的作用,分析了珠光体由片层状转变为颗粒状的变化规律,讨论了片层渗碳体的打断、短棒状向颗粒状转变以及颗粒状熟化长大的热力学机制。结果认为珠光体球化工艺的不足,一是存在碳化物分布不均匀、尺寸大小存在差异等问题;二是退火工艺固定化、格式化等问题,未能适应轴承钢的发展;三是球化退火后的组织检验粗糙化、不够细致。球化工艺的发展趋势,一是要在轧钢或锻压生产阶段通过引入塑性变形或增大冷却过冷度等方法来保证片层组织更加均匀、细化,无网状渗碳体;二是要开发新的球化工艺,例如在有可能的条件内引入电场、磁场以及高温高压应力场等方法来改善球化工艺;三是要将球化组织的定量化检验标准化。     

Deformation induced pearlite transformation and dynamic spheroidization in a eutectoid steel

The microstructure evolution of a eutectoid steel during the deformation induced pearlite transformation of undercooled austenite was investigated by uniaxial hot compression simulation experiment. The effects of different deformation degree, deformation rates and deformation temperature on the deformation induced pearlite transformation were explored. The results indicate that the induced pearlite transformation can occur rapidly during the deformation, for the stress accelerates phase transition. With the increase of the deformation degree, the dislocation density and phase transition driving force in the microstructure are improved, accelerating the occurrence of phase transition and the process of cementite spheroidization. For the diffusion- controlled phase transition, the deformation rates decrease to prolong the deformation time, so the carbon atoms can diffuse sufficiently to obtain spheroidized cementite. At lower deformation temperature from A1 to Ar1, significant refinement of the fragmentation of cementite occurs due to the increase of supercooling and spheroidized time. The ultrafine microstructure of cementite particles can be obtained through the high deformation degree, low deformation rates and low deformation temperature. It is also observed that the pro- eutectoid ferrite nucleates along the austenite boundary in the process of deformation.     

2%铝质量分数超高碳钢的球化退火工艺

 为获得完全球化的超高碳钢组织,基于离异共析转变机制对2%铝质量分数超高碳钢进行球化退火工艺研究。研究发现,由于成分的不均匀性,超高碳钢锻态组织由片层间距不一致的珠光体和网状碳化物组成,单纯使用离异共析工艺无法使其完全球化;2%铝质量分数超高碳钢锻态组织网状碳化物厚度在1 μm以下,[Acm]温度以下正火即可获得片层均匀细小的珠光体并消除网状碳化物;提高正火温度能显著减少正火组织中长条和短棒状碳化物的数量,利于获得较好的球化组织。2%铝质量分数超高碳钢经900～925 ℃正火后在830 ℃奥氏体化并在760 ℃等温4 h后获得了由超细铁素体＋细小球状渗碳体组成的完全球化组织。     

Pearlite spheroidization by thermomechanical treatment of directly charged thin slabs

Considering coarse initial austenite grain and the reduced thickness of directly charged thin slabs, a modified thermomechanical treatment was carried out on an unalloyed engineering steel with 0.66% C. For the laboratory simulation to determine the microstructural and mechanical properties a continuous casting simulator linked with the hot deformation simulator (Wumsi) were used. The aim of these tests was to study the process of the strain induced spheroidization of lamellar pearlite in order to improve the cold deformability of the steel, taking account of the particular conditions of direct charging. By the Variation of hot rolling deformation schedules, the influence of austenite grain size, pearlite interlamellar spacing as well as strain and strain temperature were investigated. The spheroidization process was mostly influenced by the strain applied just after finished pearlite transformation. Improving both strength and ductility by increasing fraction of spheroidized pearlite was supported by a smaller pearlite interlamellar spacing before deformation. A coarse austenite grain hardly affects the spheroidization process and brings about no impairment of mechanical properties, which makes this processing particularly attractive for direct charging of thin slabs. Moreover, a finely spheroidized pearlite exerts structural similarities to a tempered martensite, concerning distribution, shape and size of the cementite, accordingly leading to comparable mechanical properties. This justifies such modified hot rolling to be accepted as a potential substitution for the conventional post-rolling quench and tempering of high carbon steel products.     

一种具有优化组织和优异性能的超高碳钢

研究了超高碳钢不同组织结构与其力学性能之间的关系,分析了珠光体和球化组织各自的优点和缺点,并提出将这两种组织结合起来,进行优化组合的创新性的思路。研发了一种具有特别结构的超高碳钢,其晶内为细密珠光体,晶界邻区为球化组织。这种材料同时兼有珠光体所能提供的高强度和球化组织所能提供的高塑性,达到了抗拉强度1300MPa,伸长率18％的优异综合力学性能。     

Si在高碳钢盘条开发中的应用

研究了Si对高碳钢盘条珠光体相变及钢丝热稳定性的影响。研究表明,Si含量的增加可以明显地提高珠光体相变温度,且在高碳钢中增加相同含量的Si,不同相变温度下盘条硬度的增加值基本保持不变;三维原子探针分析结果表明,在高碳钢中添加一定量的Si可以减小盘条铁素体片层中C原子的偏聚程度,有利于提高C原子分布的均匀性;同时Si可以明显地提高渗碳体片层的球化温度,有利于钢丝热稳定性的提高。