Development of high strength low alloy TRIP‐aided steels with annealed martensite matrix

Formable high‐strength low‐alloy TRIP‐aided sheet steels with annealed martensite matrix or TRIP‐aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate‐like retained austenite along annealed martensite lath boundary, the stability of which against the strain‐induced transformation was higher than that of the conventional TRIP‐aided dual‐phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the TRIP‐aided annealed martensitic steels exhibited superior large elongation and reduction of area. In addition, the steels possessed the same excellent stretch‐flangeability and bendability as TRIP‐aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress in matrix and so on.     

Macromodelling of Transformation Induced Plasticity combined with Viscoplasticity for Low‐Alloy Steels

Metal forming processes are important technologies for production of engineering metal components. In order to optimize the resulting material properties, it becomes necessary to simulate the entire forming process by taking into account physical effects such as phase transformations. In this work we concentrate on the phase change from austenite to martensite and present a macroscopic material model, which combines the effects of viscoplasticity with the effect of transformation induced plasticity (TRIP). An extensive experimental data basis for a low‐alloy steel is used for parameter identification, thus taking into account the effects of uniaxial compressive and tensile stress on the kinetics of phase transformation at different temperatures. In a finite element simulation the austenite to martensite phase transformation within a shaft subjected to thermal loading is investigated.     

30SiMnCrB5热成形钢的微观组织和力学性能

为了提高热成形钢的综合性能,设计了一种C-Si-Mn-Cr-B系热成形钢,采用热膨胀仪测定并研究了30SiMnCrB5热成形钢的连续冷却转变曲线和相变规律.分析了经轧制、退火及热成形模拟后钢板的微观组织形貌和力学性能,结合等密度线极图的方法,判定了热成形模拟后钢板中马氏体变体与母相的取向关系.30SiMnCrB5热成形钢具有较好的淬透性,临界冷速为5℃·s^-1,有效抑制了珠光体和贝氏体的形成,完全马氏体组织的硬度可达600 HV以上.热成形模拟后的微观组织由板条马氏体和残余奥氏体构成,残余奥氏体主要以薄膜状分布在马氏体板条间,质量分数为6%~8%,抗拉强度为1800 Mpa左右,总伸长率可达10%以上,强度和塑性的匹配较好.热成形模拟后30SiMnCrB5热成形钢板中马氏体变体与母相的取向关系更接近N-W关系,12种变体没有都出现在原始奥氏体内.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Microstructual Evolution of a Nb‐Microalloyed Advanced High Strength Steel Treated by Quenching‐Partitioning‐Tempering Process

The recently developed “quenching and partitioning” heat treatment and “quenching‐partitioning‐tempering” heat treatment are novel processing technologies, which are designed for achieving advanced high strength steels (AHSS) with combination of high strength and adequate ductility. Containing adequate amount of austenite phase is an important characteristic of the above steel, and the partitioning treatment is a key step in Q&P or Q‐P‐T process during which the austenite phase is enriched with carbon and achieves thermal stability. However, the microstructural evolution of the steel during the partitioning process is rather complicated. In present study, evolution of complex microstructure in a low carbon steel containing Nb during the Q‐P‐T process has been studied in detail. The microstructural evolution of the steel was investigated in terms of X‐ray diffraction, scanning electron microscope and transmission electron microscope. The experimental results show that the Nb‐microalloyed steel demonstrates a complex multiphase microstructure which consists of lath martensite with high dislocation density, retained austenite, alloy carbide, transition carbide, and a few twin martensite after the Q‐P‐T process. The experimental results can be helpful for the design of Q‐P‐T heat treatment and for the control of mechanical properties of Q‐P‐T steel.     

Transformation Induced Martensite Evolution in Metal Forming Processes of Stainless Steels

Because of their high corrosion resistance and deformation characteristics, the industrial application of stainless steels is of high importance. During deep drawing processes, phase transformation of austenite to martensite occurs, which leads to an increased strain hardening of the material. The phase transformation depends on alloying constituents, transformation temperatures, stresses and strains. Consequently, in the design of deep drawing processes of stainless steels the phase transformation has to be considered. This paper presents a mathematical model for the calculation of the martensite evolution depending on temperatures, stresses and strains. The precise simulation of deep drawing processes of stainless steels can be enabled by the implementation of this model into commercial FE‐programs.     

Effect of Pre-strain and High Stresses on the Bainitic Transformation of Manganese-boron Steel 22MnB5

During the last decade, the use of press-hardened components in the automotive industry has grown considerably. The so-called tailored tempering, also known as partial press hardening, employs locally heated tools seeking to obtain bainitic transformations. This leads to (seamless) zones within the formed parts with higher ductility. Due to the intrinsic nature of this process, phase transformations happen under the influence of high loads and in pre-deformed austenite. The austenite pre-strain state and applied stresses affect the kinetics of the bainitic transformation. Moreover, stresses have an additional relevant effect in this process, the so-called transformation plasticity. Linear transformation plasticity models have been successfully used to predict the behavior in the presence of low stresses. Nonetheless, because of the process’s severe conditions, these tend to fail. A strong nonlinearity of the transformation plasticity strain is observed for applied stresses above the austenite yield strength. Using thermomechanical tests on sheet specimens of a manganese-boron steel (22MnB5), widely utilized in the industry, the effect on the bainitic transformation of various degrees of deformation in the range of 0 to 18 pct, applied stresses in the range of 0 to 250 MPa and the transformation plasticity effect are investigated in this work.     

Use of electrical resistance testing to redefine the transformation kinetics and phase diagram for shape-memory alloys

The phase-transformation temperatures of a nickel-titanium-based shape-memory alloy (SMA) were initially evaluated under stress-free conditions by the differential scanning calorimetric (DSC) technique. Results show that the phase-transformation temperature is significantly higher for the transition from detwinned martensite to austenite than for that from twinned martensite (or R phase) to austenite. To further examine transformation temperatures as a function of initial state, a tensile-test apparatus with in-situ electrical resistance (ER) measurements was used to evaluate the transformation properties of SMAs at a variety of stress levels and initial compositions. The results show that stress has a significant influence on the transformation of detwinned martensite, but a small influence on the R-phase and twinned martensite transformations. The ER changes linearly with strain during the transformations from both kinds of martensite to austenite. The linearity between the ER and strain during the transformation from detwinned martensite to austenite is not affected by the stress, facilitating application to control algorithms. A revised phase diagram is drawn to express these results.     

Modeling tensile deformation of dual-phase steel

An analytical model has been developed which can describe the tensile deformation behavior of dualphase steel containing retained austenite which transforms to martensite during deformation. The model takes into account the internal back stresses created in the material as a result of the deformation. The influence of various metallurgical factors, such as the amounts of the secondary phases (martensite and retained austenite), strength ratio of the phases, work hardening coefficients, and the stability of retained austenite with respect to the strain-induced transformation, was analyzed. The strongest influence on both strength and ductility was found to result from a large work hardening coefficient of the martensite. Increasing the stability of retained austenite to strain-induced transformation improved the ductility remarkably. The model developed was used to predict the tensile deformation behavior of a commercial dual phase steel fairly accurately.     

Forming response of retained austenite in a C‐Si‐Mn high strength TRIP sheet steel

TRIP sheet steels typically consist of ferrite, bainite, retained austenite, and martensite. The retained austenite is of particular importance because its deformation‐induced transformation to martensite contributes to excellent combinations of strength and ductility. While information is available regarding austenite response in uniaxial tension, less information is available for TRIP steels with respect to the forming response of retained austenite in complex strain states. Therefore, the purpose of this work was to study the austenite transformation behaviour in different strain paths by determining the amount of retained austenite before and after forming. Forming experiments were performed on a high strength 0.19C‐1.63Si‐1.59Mn TRIP sheet steel 1.2 mm in thickness in two different strain conditions, uniaxial tension (ε₁ = ‐2ε₂) and balanced biaxial stretching (ε₁ = ε₂). Specimens were formed to strains ranging from zero to approximately 0.2 effective (von Mises) strain. Specimens were tested both longitudinally and transverse to the rolling direction in uniaxial tension, and subtle mechanical property differences were found. The volume fraction of austenite, determined with X‐ray diffraction subsequent to forming, was found to decrease with increasing strain for both forming modes. Some modification in the crystallographic texture of the ferrite was observed with increasing strain, in specimens tested in the balanced biaxial stretch condition. This trend was not evident in the uniaxial tensile test results. Slight differences were found in the transformation behaviour of the austenite when formed in different strain conditions. More austenite transformed in specimens tested parallel to the rolling direction than transverse to the rolling direction in uniaxial tension. The amount of austenite transformed during biaxial stretching was determined to be greater than the amount transformed in uniaxial tension for specimens tested transverse to the rolling direction at an equivalent von Mises strain. The amount of austenite that transformed in biaxial tension, however, was comparable to the amount of austenite that transformed in specimens tested longitudinal to the rolling direction in uniaxial tension.     

高强韧性海洋平台用E550钢板生产工艺研究

介绍了E550钢板的主要生产工艺和技术难点,通过理论分析设计了E550的成分体系,采用Thermal-Calc和经验公式,获得了其热力学相图和相变点温度等热力学数据。根据E550的热力学特性,设计了两阶段轧制工艺,精轧的终轧温度控制在再结晶温度附近,利用奥氏体再结晶充分细化晶粒。淬火奥氏体化温度选择为920℃,回火温度设计为630℃,利用碳化物的析出强化效果和缺陷密度变化的位错强化获得良好的强韧性匹配。50 mm厚钢板的淬火态1/4厚度处的微观组织为马氏体,中心的组织为马氏体和少量贝氏体的混合物。回火热处理后,马氏体板条界面减少,碳化物在马氏体板条界面析出,钢板1/4到中心的组织均匀化。30和50 mm厚E550钢板的力学性能达到了船级社标准要求,并有较大的富裕量。热输入能量为15和50 kJ/cm焊接后,钢板具有良好的强度性能,熔合线和热影响区的冲击功较高。     

Q&P钢热处理工艺的开发进展

Q&P钢的工艺概念已提出了有些年了,它作为一种热处理工艺,生产的组织具有马氏体和富C的亚稳残余奥氏体.工艺包括:控制淬火形成部分的马氏体,然后热处理以使C从过饱和的马氏体中转移到残余奥氏体中.潜在的应用包括具有好的成型性和抗断裂的高强度钢.应用该工艺可生产薄板和厚板,以及用从长锻材生产热处理元件,甚至于铸铁.自2003年一直在实验室研究,早期的工作结果已有发表.近期的研究主要集中于机理方面的细节.国际范围的独立团队在此研究领域也有很多进展.     

Mechanical Property and Microstructural Characterization of C-Mn-Al-Si Hot Dip Galvanizing TRIP Steel

 Mechanical properties and microstructure in high strength hot dip galvanizing TRIP steel were investigated by optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD), dilatometry and mechanical testing. On the heat treatment process of different intercritical annealing (IA) temperatures, isothermal bainitic transformation (IBT) temperatures and IBT time, this steel shows excellent mechanical properties with tensile strength over 780 MPa and elongation more than 22%. IBT time is a crucial factor in determining the mechanical properties as it confirms the bainite transformation process, as well as the microstructure of the steel. The microstructure of the hot dip galvanizing TRIP steel consisted of ferrite, bainite, retained austenite and martensite during the short IBT time. The contents of ferrite, bainite, retained austenite and martensite with different IBT time were calculated. The results showed that when IBT time increased from 20 to 60 s, the volume of bainite increased from 14.31% to 16.95% and the volume of retained austenite increased from 13.64% to 16.28%; meanwhile, the volume of martensite decreased from 7.18% to 1.89%. Both the transformation induced plasticity of retained austenite and the hardening of martensite are effective, especially, the latter plays a dominant role in the steel containing 7.18% martensite which shows similar strength characteristics as dual-phase steel, but a better elongation. When martensite volume decreases to 1.89%, the steel shows typical mechanical properties of TRIP, as so small amount of martensite has no obvious effect on the mechanical properties.     

Stability and Constitutive Modelling in Multiphase TRIP Steels

Multiphase TRIP steels are a relatively new class of steels exhibiting excellent combinations of strength and cold formability, a fact that renders them particularly attractive for automotive applications. The present work reports models regarding the prediction of the stability of retained austenite, the optimisation of the heat‐treatment stages necessary for austenite stabilization in the microstructure, as well as the mechanical behaviour of these steels under deformation. Austenite stability against mechanically‐induced transformation to martensite depends on chemical composition, austenite particle size, strength of the matrix and stress state. The stability of retained austenite is characterized by the M^σ_S temperature, which can be expressed as a function of the aforementioned parameters by an appropriate model presented in this work. Besides stability, the mechanical behaviour of TRIP steels also depends on the amount of retained austenite present in the microstructure. This amount is determined by the combinations of temperature and temporal duration of the heat‐treatment stages undergone by the steel. Maximum amounts of retained austenite require optimisation of the heat‐treatment conditions. A physical model is presented in this work, which is based on the interactions between bainite and austenite during the heat‐treatment of multiphase TRIP steels, and which allows for the selection of treatment conditions leading to the maximization of retained austenite in the final microstructure. Finally, a constitutive micromechanical model is presented, which describes the mechanical behaviour of multiphase TRIP steels under deformation, taking into account the different plastic behaviour of the individual phases, as well as the evolution of the microstructure itself during plastic deformation. This constitutive micromechanical model is subsequently used for the calculation of forming limit diagrams (FLD) for these complex steels, an issue of great practical importance for the optimisation of stretch‐forming and deep‐drawing operations.     

1C－1．5Cr钢等温马氏体的TEM观察及其对残余奥氏体的影响

１Ｃ－１．５Ｃｒ钢奥氏体化后在稍低于Ｍｓ点的热浴中等温，可获得较常规工艺更多的残余奥氏体组织。随等温时间延长，因等温马氏体的生成，使室温组织中残余奥氏体量相应增加，当等温进入下贝氏体转变区后，由于下贝氏体的形成降低了奥氏体的稳定性，使残余奥氏体量又趋下降。通过ＴＥＭ观察表明，等温马氏体形成时，将优先以原马氏体片共格长大的方式进行。     

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

Effects of Process Parameters prior to Annealing on the Formability of Two Cold Rolled Dual Phase Steels

This study concerns the effects of coiling temperature after hot rolling and the degree of reduction during cold rolling on formability‐related properties of high strength cold rolled dual phase (DP) steels. The effect of coiling temperature on the final structure and properties of two cold rolled and annealed DP‐steels is investigated. Further, the effect of cold rolling reduction and its impact on the final properties of the material is studied. Aspects of the impact of the different process parameters on the ferrite to austenite and austenite to martensite transformation are discussed based on results from production scale experiments, tensile testing and metallographic examinations of the materials.     

Anisotropy of Retained Austenite Stability during Transformation to Martensite in a TRIP‐Assisted Steel

Retained austenite as a key constituent in final microstructure plays an important role in TRansformation Induced Plasticity (TRIP) steels. The volume fraction, carbon concentration, size, and morphology of this phase are the well‐known parameters which effects on the rate of transformation of retained austenite to martensite and the properties of steel, are studied by many researchers. Of the transformation of retained austenite to martensite under strain in a TRIP steel is studied in this paper. The experimental results show that the transformation rate of retained, austenite with similar characteristics, to martensite in differently processed TRIP steel samples, exhibits an anisotropic behavior. This phenomenon implies a kind of variant selection of martensitic reaction of retained austenite under strain and is explained by ferrite texture developed in steel.     

Influence of austenite strength on martensite start temperature Ms

It is known that austenite strength determines the morphology of the new phase during martensitic transformation. As the strength of austenite influences the growth of a martensite crystal, i.e. the movement of the austenite/martensite interface, a correlation between strength of the parent phase and M_s has to exist. M_s depends on thermodynamical and mechanical properties of the alloys. To distinguish the individual variables, austenite strength was changed by different hardening mechanisms: solid solution hardening, plastic deformation or both.     

Effect of Testing Temperature on Retained Austenite Stability of Cold Rolled CMnSi Steels Treated by Quenching and Partitioning Process

The effect of testing temperature on retained austenite (RA) stability of industrially cold rolled CMnSi sheet steel treated by quenching and partitioning (Q&P) process has been investigated by observing the deformation and transformation behavior of RA at different testing temperatures. Uniaxial tensile properties at different temperatures were determined and a correlation between RA stability and mechanical properties were also established. Ultimate tensile strength increases monotonously when temperature decreases, while total elongation reaches an optimum value between 0 and 20°C, where RA exhibits the greatest TRIP effect. Work hardening rate was calculated to decrease through three different stages in an oscillation manner, leading to significant enhancement in both strength and ductility. The kinetic of deformation‐induced martensite transformation is also studied and the stability of RA can be evaluated by comparing the kinetic parameter β.