Effect of Thermomechanical Processing on Microstructures of TRIP Steel

 In order to control retained austenite, the effect of hot deformation in the intercritical region on the microstructure of hot rolled transformation induced plasticity (TRIP) steel was studied on a Gleeble 1500 hot simulator. Compressive strains varying in amounts from 0 to 60% were imposed in the intercritical region, and effects on the formation of polygonal ferrite, carbide free bainite and retained austenite were determined. With increasing the hot deformation amount and the ferrite content and decreasing the carbide free bainite content, the volume fraction of retained austenite decreases. Increased dislocation density, grain refinement of ferrite and carbon enrichment are the main factors which control retained austenite stability.     

基于动态相变的热轧Nb-V-Ti微合金化TRIP钢组织及性能

通过单轴热压缩试验,结合扫描电镜以及X射线衍射技术,研究了动态相变前奥氏体晶粒状态对基于动态相变的热轧Nb-V-Ti微合金化TRIP钢复相组织状态及力学性能的影响.与动态相变前奥氏体晶粒为等轴状条件下相比,动态相变前奥氏体晶粒为拉长状条件下,动态相变得到的铁素体转变量较大,最终复相组织中贝氏体含量较少且团径较小,马氏体含量较少,但对残余奥氏体含量及其含碳量影响不明显.与不含微合金化元素的基于动态相变的热轧TRIP钢相比,Nb-V-Ti微合金化TRIP钢的屈服强度和抗拉强度明显提高,而延伸率有所降低.      

基于退火路径的中锰钢组织转变与力学性能

为了更好地指导中锰钢工业试制,利用扫描电镜、电子背散射衍射、透射电镜和拉伸试验机等研究了不同退火路径下低碳中锰钢的组织转变及合金元素配分行为,并评价了其对力学性能的影响.结果表明,热轧中锰钢的显微组织主要由铁素体、板条马氏体、粒状贝氏体和残余奥氏体构成.经冷轧变形后,原组织中的铁素体和马氏体晶粒破碎,残余奥氏体和M/A...     

Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, A1 1.3, and Nb-kV 0.13 （mass%） with annealed martensite （TAM steel） were investigated using optical microscopy, field emission gun scanning electronic microscope （FEG SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and mechanical testing. The mierostructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix （TPF steel）, the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain harden- ing behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.     

不同工艺下低碳Mn-Si钢的组织与力学性能

通过Gleeble-1500热模拟压缩试验,借助光学显微镜、扫描电镜、X射线衍射及拉伸试验等,研究一种低碳Mn-Si钢在基于热轧动态相变的热轧TRIP钢工艺和基于贝氏体等温处理工艺下的组织与力学性能,比较了通过两种工艺获得的不同复相组织状态对材料的加工硬化能力的影响.结果表明:实验钢在基于动态相变的热轧TRIP钢工艺下获得了以细晶铁素体为基体和贝氏体、残余奥氏体组成的复相组织,而在基于贝氏体等温处理工艺下得到了以板条贝氏体为基体和残余奥氏体组成的复相组织,前者中残余奥氏体含量较高且其碳含量也较高.实验钢具有以板条贝氏体为基体的复相组织时屈服强度和抗拉强度较高;但由于残余奥氏体稳定性较差,实验钢的加工硬化能力较弱,导致其均匀延伸率和总延伸率较小.      

控轧控冷工艺对900 MPa级热轧带钢组织和性能的影响

通过动态CCT曲线测试和实验室控轧控冷试验,分析了900 MPa级热轧带钢连续冷却过程中的相变过程以及不同卷取温度下显微组织、析出相和力学性能的关系。试验结果表明:随着冷却速度提高,显微组织中多边形铁素体比例下降,贝氏体组织比例升高,冷速大于15℃/s时,显微组织全部为贝氏体;随着卷取温度升高,显微组织中针状铁素体比例下降,多边形铁素体比例升高;当卷取温度为600℃时,组织为铁素体+少量珠光体,此时析出相细小弥散,可获得抗拉强度达到1 000 MPa,延伸率17%的热轧产品。     

Microstructure and Mechanical Properties of a Low-Carbon Mn-Si Multiphase Steel Based on Dynamic Transformation of Undercooled Austenite

The microstructure evolution of 0.20C-2.00Mn-2.00Si steel treated by the thermomechanical process to manufacture hot-rolled, transformation-induced plasticity (TRIP) steel based on dynamic transformation of undercooled austenite was investigated using a Gleeble 1500 (Dynamic Systems, Inc., Poestenkill, NY) hot simulation test machine in combination with light microscope (LM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The mechanical properties of this steel with different multiphase microstructures were also analyzed using room-temperature tensile tests. The results indicated that the multi-phase microstructures consisting of fine-grained ferrite with a size of 1–3 μm, bainite packets, and retained austenite and martensite were formed for the used steel by a thermo-mechanical process involving dynamic transformation of undercooled austenite, controlled cooling, isothermal bainite treatment and water-quenching. With the increase in the strain of hot deformation of undercooled austenite, the fraction of ferrite increased, that of bainite decreased, and that of martensite increased. At the same time, the fraction of retained austenite (RA), as well as the carbon content of RA, first increased and then decreased. For the used steel treated by such process, the tensile strength is about 1200 MPa with a total elongation of about 20 pct, and the product of tensile strength and total elongation can be up to 25,000 MPa × pct.     

1000MPa级TRIP钢的组织性能及强化机理

鉴于热镀锌TRIP钢在成分和工艺上的特殊性,试验采用低Si含P以及V微合金化的成分设计思路,并进行镀锌连续退火工艺模拟,分析其微观组织和性能。探讨了残余奥氏体稳定性的影响因素及强化机理。结果表明,添加P和采取较高的冷轧变形量均有利于提高残余奥氏体量及其稳定性。在残余奥氏体的形变诱导马氏体相变、贝氏体和V析出的强化作用以及铁素体晶粒细化的共同作用下,抗拉强度和强塑积分别达到1 035 MPa和25 875 MPa·%。     

Interrelations of cooling rate,microstructure, and mechanical properties in four HSLA steels

The present study was carried out on four steels containing 0.1 pct C-1.5 pct Mn-0.003 pct B* in common, with additions of 1 pct Cr, 0.5 pct Mo, 0.25 pct Mo + 1 pct Cr, 0.2 pct Ti + 1 pct Cr. They were designated, accordingly, as Cr, Mo, Mo-Cr, and Cr-Ti steels. All the steels exhibited a complete lath martensite microstructure with thin interlaths of retained austenite (≈0.05 pct) in the quenched condition. The normalized microstructures, granular bainite, contained massive areas of ferrite and granules of bainite laths. Both microconstituents contained a fine dispersion of cementite particles (size ≈50 Å) together with high dislocation densities. A mechanism explaining their for-mation has been given. The Cr steel, due to its low hardenability, showed in addition polygonal ferrite in the neighborhood of the so-called M-A constituent (twinned martensite and/or austenite). The annealed microstructure (using a cooling rate of 0.033 °C s^?1) of the Cr steel consisted of coarse ferrite-pearlite. Addition of 0.2 pct Ti to the Cr steel markedly refined the structure, whereas an addition of 0.25 pct Mo altered the microstructure to ferrite-lower bainite. In the 0.5 pct Mo steel, polygonal ferrite was found to be completely missing. The mechanical properties of the four steels after quenching, normalizing, and annealing were investigatedvia hardness and tensile test mea-surements. An empirical equation, relating the ultimate tensile strength to the steel composition, for steels that had granular bainite microstructures in the normalized condition, was proposed. The fracture surfaces exhibited cleavage and variable-size dimples depending on the microstructure and steel composition.     

Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel

The influence of hot deformation on the microstructure of a hot-rolled Si-Mn transformation-induced-plasticity (TRIP) steel was evaluated in an effort to better control retained austenite content. In this study, axial compressive strains varying in amounts from 0 to 60 pct were imposed in the austenite phase field, and effects on the formation of polygonal ferrite, bainite, and retained austenite were determined. In addition, modifications in simulated coiling temperature from 420 °C to 480 °C and cooling rates from the rolling temperature, between 10 °C/s and 35 °C/s, were assessed. Fast cooling rates, low coiling temperatures, and low degrees of hot deformation were generally found to decrease the amount of polygonal ferrite and increase retained austenite fraction. Unexpectedly, a sharp increase in polygonal ferrite content and decrease in retained austenite content occurred when the fastest cooling rate, 35 °C/s, was coupled with extensive hot deformation and high coiling temperatures. This effect is believed to be due to insufficient time for full recovery and recrystallization of the deformed austenite, even in the absence of intentional microalloying additions to control recrystallization kinetics. The resultant decrease in hardenability allowed the ferrite transformation to continue into the holding time at high (simulated) coiling temperatures.     

终轧温度对低硅含磷系TRIP钢组织性能的影响

通过实验室热轧试验,研究了不同终轧温度下低硅含磷系热轧TRIP钢的组织特征及性能特点。结果表明：终轧温度由900℃降低到790℃,铁素体体积分数增加,贝氏体体积分数降低,残余奥氏体体积分数变化不太明显;终轧温度900和820℃时,得到贝氏体为基体的室温组织;终轧温度降低到790℃时,低温变形促进了奥氏体到铁素体的相变率...     

Effects of Niobium Addition on Microstructures and Formability in Si‐Al‐Mn TRIP Cold‐rolled Steels

The effects of Nb addition on microstructures and formability in Si‐Al‐Mn TRIP cold‐rolled steels were investigated. These steels were intercritical annealed at 770 °C for 5 min, and isothermally treated at 400 °C for 3 min. Microstructural observation, tensile tests and forming limit diagram (FLD) tests were conducted, and the changes of retained austenite volume fraction as a function of tensile strain were measured by using an X‐ray diffractometer. The results showed that Nb addition makes grain size refined, the volume fraction of ferrite increase and that of bainite decrease, however, obviously it does not affect the volume fraction and carbon content of retained austenite. The Nb addition increased the stability of retained austenite owing to grain refinement. With Nb addition, increase in strength, ductility, strain hardening exponent and formability could be achieved simultaneously. These findings indicate that Nb addition can be a new direction of microalloying design for the low carbon TRIP steels with excellent formability and high stability of retained austenite.     

Transmission electron microscopy studies of thermomechanically control processed multiphase medium-carbon microalloyed steel: Forged, rolled, and low-cycle fatigued microstructures

A ferrite-bainite-martensite (F-B-M) microstructure was produced in a medium-carbon microalloyed (MA) steel through two routes, namely, low-temperature finish forging and rolling, followed by a two-step cooling (TSC) and annealing. Transmission electron microscopy (TEM) was employed to study the microstructural evolution in control forged and rolled material after TSC followed by annealing (TSCA). A TEM investigation was also carried out on samples low-cycle fatigue (LCF) tested at low and high total strain amplitudes of 0.4 and 0.7 pct in case of the forged steel (F-B-M(F)TSCA) and 0.55 and 0.8 pct for the rolled steel (F-B-M(R)TSCA), respectively. Microstructural changes accompanying the LCF testing were identified. The two-step cooled microstructure processed through forging (F-B-M(F)TSC) as well as rolling (F-B-M(R)TSC) revealed a complex multiphase microstructure, along with films and blocks of retained austenite. In both microstructural conditions, vanadium carbide precipitates were too fine to be identified after the TSC treatment. Annealing after TSC produced a stress-free microstructure. The F-B-M(F)TSCA microstructure predominantly consisted of granular/lower bainite, lath martensite, and polygonal ferrite with interlath films as well as blocks of retained austenite, while the F-B-M(R)TSCA microstructure predominantly consisted of lath martensite, granular/lower bainite, and polygonal ferrite with interlath strips/films of retained austenite. Lath martensite content was higher in the F-B-M(R)TSCA condition than in the F-B-M(R)TSCA condition. In both conditions, vanadium carbide precipitates could be seen after annealing. Fatigue-tested F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.4 pct and F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.55 pct were stable. Lath martensite did not undergo deformation and in both microstructural conditions dislocation cell structures were not observed in the ferrite or bainite regions. The interlath retained austenite strips/films played a significant role in preventing the softening during fatigue loading. First, it was stable up to a total strain amplitude of 0.4 and 0.55 pct in the respective microstructures. Second, it underwent heavy deformation during fatigue loading at high total strain amplitudes, thereby accommodating the strain. Fatigue-tested F-B-M(F)TSCA microstructure at a total strain amplitude of 0.7 pct and F-B-M(R)TSCA microstructure at a total strain amplitude of 0.8 pct revealed deformed bainite/martensite laths, dislocation cells, and slip bands in the ferrite regions, which are characteristic features of cyclic softening. The retained austenite transformed to martensite through a strain-induced transformation mechanism and, at that stage, the microstructure contained in addition dislocation-rich bainite and ferrite.     

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.     

Effect of Nb Microalloying and Hot Rolling on Microstructure and Properties of Ultrathin Cast Strip Steels Produced by the CASTRIP<Superscript>®</Superscript> Process

The microstructure and corresponding tensile properties of both plain and Nb-microalloyed grades of ultrathin cast strip (UCS) low alloy steel produced using the CASTRIP^® process were studied. Both as-cast and hot-rolled strip cast steels with various levels of Nb microalloying were manufactured and investigated in this study. Hot rolling had little effect on the yield strength of Nb microalloyed UCS specimens for a given chemical composition, but resulted in a slightly finer microstructure. The effect of Nb microalloying was significant, and this is attributable to the promotion of finer, tougher austenite transformation products such as bainite and acicular ferrite at the expense of large polygonal ferrite grains. A fine dispersion of Nb solute clusters was observed in all Nb-containing steels following hot rolling, and it is suggested that this also contributes to the observed strengthening.     

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.     

铁素体基Ti-Mo高强钢连续冷却相变及组织性能

摘要：为了深入了解铁素体基Ti-Mo高强钢在连续冷却相变过程中组织及硬度的变化及其原因,通过热膨胀法、金相及硬度等实验研究了Ti-Mo微合金钢在连续冷却条件下组织及性能的变化,探讨了冷却速率对组织、硬度及相变行为的影响机理,揭示了（Ti,Mo）C在奥氏体和铁素体中Ti/Mo原子比变化的原因。结果表明,随着冷却速率由0.06℃/s增加至17.9℃/s,组织依次为多边形铁素体+珠光体→多边形铁素体+粒状贝氏体→粒状贝氏体,硬度由144HV逐渐增大至228HV。当冷速由0.14℃/s增大至0.90℃/s时,组织中多边形铁素体比例不断增大,珠光体比例不断降低,硬度的提高主要来自于铁素体晶粒尺寸的细化及纳米级（Ti,Mo）C粒子的增多;当冷速由1.79℃/s增大至17.9℃/s时,组织中多边形铁素体比例不断降低,贝氏体比例不断提高,硬度的提高主要是由于贝氏体组织的细化及其比例的增加。（Ti,Mo）C粒子主要有2类：一类是奥氏体中析出的10～20nm的粒子,Ti原子数分数约为88%,另一类是铁素体中析出的小于10nm的粒子,Ti原子数分数约为68%,EDS测量结果与计算结果大致相当。     

Effect of Cooling Method on Microstructure and Mechanical Properties of Hot-Rolled C-Si-Mn TRIP Steel

 The controlled cooling technology following hot rolling process is a vital factor that affects the final microstructure and mechanical properties of the hot-rolled transformation induced plasticity (TRIP) steels. In the present study, low alloy C-Si-Mn TRIP steel was successfully fabricated by hot rolling process with a 450 hot rolling mill. To maximize the volume fraction and stability of retained austenite of the steel, two different cooling methods (air-cooling and ultra-fast cooling “AC-UFC” and ultra-fast cooling, air-cooling and ultra-fast cooling “UFC-AC-UFC”) were conducted. The effects of the cooling method on the microstructure of hot-rolled TRIP steel were investigated via optical microscope, transmission electron microscope and conversion electron Mssbauer spectroscope. The mechanical properties of the steel were also evaluated by conventional tensile test. The results indicated that ferrite and bainite in the microstructure were refined with the cooling method of UFC-AC-UFC. The morphology of retained austenite was also changed from small islands distributing in bainite district (obtained with AC-UFC) to granular shape locating at the triple junction of the ferrite grain boundaries (obtained with UFC-AC-UFC). As a result, the TRIP steel with a content of retained austenite of 1152%, total elongation of 32% and product of tensile strength and total elongation of 27552 MPa·% was obtained.     

微合金化热轧TRIP钢的工艺模拟

 借助MMS-300热模拟试验机研究了控轧温度区间、终冷温度、贝氏体区等温处理以及冷却路径对微合金化热轧TRIP钢组织演变规律的影响。结果表明,随着控轧温度区间“下调”,组织中的铁素体晶粒越来越细小,铁素体量逐渐增加,残余奥氏体量则先增加后减少。终冷温度升高时,组织中的残余奥氏体量也呈现出先增加后减少的变化趋势,而贝氏体温度范围等温时间的延长使残余奥氏体量增加。相对于“缓冷+快冷”,轧后采用“快冷+缓冷+超快冷”冷却路径更有助于铁素体晶粒的细化和奥氏体的残留。在“快冷+缓冷+超快冷”冷却路径下,当控轧温度区间为900~840℃,缓冷温度范围为710~680℃,贝氏体等温处理制度为450℃×5min时,组织中的残余奥氏体量达到最高值113%。     

Mechanical Properties and Retained Austenite Transformation Mechanism of TRIP-Aided Polygonal Ferrite Matrix Seamless Steel Tube

 Through the comparison of microstructure for polygonal ferrite (PF) matrix transformation induced plasticity (TRIP) seamless steel tube at different positions before and after tensile rupture, the transformation behavior of retained austenite (RA) was studied. The results showed that there were no yield points in tensile process and the splendid elongation and tensile strength were contributed by the uniform ferrite/bainite grains and the transformation of RA. The stability of RA was to some extent in inverse proportion with the ability of transformation induced plasticity. The coarse retained austenite located in ferrite and ferrite/bainite laths were all transformed into martensite during the tensile process.