The influence of the stress state on the plasticity of transformation induced plasticity-aided steel

The influence of the stress state on the plastic deformation of CMnSi, CMnSi(Nb), and CMnAlSi transformation induced plasticity (TRIP)-aided steel has been analyzed. Imposing hydrostatic pressures up to 800 MPa during tensile deformation made it possible to change the stress state of the tensile testing specimens. It was found that the ratio of normal to shear stresses has a pronounced effect on the evolution of the microstructure, the austenite volume fraction change during straining, and the fracture surface morphology. The CMnAlSi TRIP steel, which has the largest uniform elongation and the smallest equivalent strain at fracture in the absence of the hydrostatic pressure, had a more pronounced improvement of all plastic characteristics at increasing hydrostatic pressure. An increased austenite stabilization, brought about by the high hydrostatic pressure, was clearly observed. The austenite stabilization results in a decrease of 20 °C to 25 °C of M _s for an increase of 100 MPa of the hydrostatic pressure. The implications of the observations could be far-reaching for new sheet forming technologies, such as hydroforming, as the full transformation potential is available for crashsensitive structural parts by avoiding the formation of the martensite during forming operations.     

高碳钢固态相变与组织

采用THERMECMASTOR-Z热/力模拟试验机研究了w(C)为0.67%～0.83%的高碳钢在连续冷却过程中组织与力学性能变化规律和特征,测量了高碳钢的相变温度,建立了高碳钢的连续冷却转变曲线(CCT).根据试验结果制定了轧后控冷工艺,使高碳钢线材的珠光体平均片层控制在150 mm左右,具有良好的抗拉强度和拉拔性能.     

Ti微合金钢热变形后连续冷却相变及第二相析出行为

采用热力模拟试验技术、金相显微镜(OM)、扫描电镜(SEM)及透射电镜(TEM),并结合宏观硬度测试,研究了钛微合金钢形变奥氏体的连续冷却相变和组织演变规律,建立了试验钢连续冷却转变曲线,探讨了不同冷速对第二相析出的影响。结果表明,不同冷速下,试验钢获得了不同的微观组织,随冷速增加,第二相析出量增多,尺寸更细小,但冷速过高,析出被抑制。     

CCSE船板钢连续冷却转变行为及动力学研究

 热轧在线组织性能预报系统的准确性很大程度上取决于钢的相变动力学模型,因此需要合理的模型来描述钢的相变动力学行为。采用热膨胀仪对CCSE船板钢的静态连续冷却转变曲线进行测定,并结合室温金相组织及硬度得出CCT曲线。采用Matlab编程拟合相变开始及结束温度与冷却速率之间的关系曲线,并对相变动力学模型参数进行优化,最后对比试验钢连续转变过程中的试验数据和回归模型的动力学行为曲线。结果表明,模拟计算值与试验值拟合程度很高,进一步说明该模型的合理性。     

Comparison of Austenite Decomposition Models During Finite Element Simulation of Water Quenching and Air Cooling of AISI 4140 Steel

An indigenous, non-linear, and coupled finite element (FE) program has been developed to predict the temperature field and phase evolution during heat treatment of steels. The diffusional transformations during continuous cooling of steels were modeled using Johnson–Mehl–Avrami–Komogorov equation, and the non-diffusion transformation was modeled using Koistinen–Marburger equation. Cylindrical quench probes made of AISI 4140 steel of 20-mm diameter and 50-mm long were heated to 1123 K (850 °C), quenched in water, and cooled in air. The temperature history during continuous cooling was recorded at the selected interior locations of the quench probes. The probes were then sectioned at the mid plane and resultant microstructures were observed. The process of water quenching and air cooling of AISI 4140 steel probes was simulated with the heat flux boundary condition in the FE program. The heat flux for air cooling process was calculated through the inverse heat conduction method using the cooling curve measured during air cooling of a stainless steel 304L probe as an input. The heat flux for the water quenching process was calculated from a surface heat flux model proposed for quenching simulations. The isothermal transformation start and finish times of different phases were taken from the published TTT data and were also calculated using Kirkaldy model and Li model and used in the FE program. The simulated cooling curves and phases using the published TTT data had a good agreement with the experimentally measured values. The computation results revealed that the use of published TTT data was more reliable in predicting the phase transformation during heat treatment of low alloy steels than the use of the Kirkaldy or Li model.     

An acoustic emission study of martensitic transformation of retained austenite in intercritically annealed HSLA steels

The martensitic transformation of retained austenite particles in an intercritically annealed high-strength low-alloy steel has been studied using the acoustic emission technique. It was found that the stability of austenite particles was mainly dependent on the particle size and to a lesser extent on the enrichment of austenite by carbon and manganese. We have demonstrated that a substantial fraction of the retained austenite particles in the as-annealed condition already contains martensite embryos which can initiate martensitic transformation during cooling below 300 K. The martensite embryos are believed to form from a suitable group of dislocations. Those austenite particles which do not contain such dislocations will not transform by supercooling alone. The particle size stabilization effect is viewed as a decreasing probability of finding dislocations in a particle with decreasing particle volume. Plastic deformation can bring about the transformation by introducing dislocations in the austenite particles. The transformation of the retained austenite particles is shown to enhance the ductility of the HSLA steel.     

奥氏体等温淬火工艺对冷轧高强钢扩孔性能的影响

双相钢钢板以其碳当量低、综合性能好的优点在汽车制造中获得广泛应用。但双相钢钢板的扩孔率不够理想,在一些翻边类的应用中表现不够好。以原本用于冷轧双相钢生产的钢板为研究对象,研究不同的奥氏体等温淬火温度对试验用钢扩孔性能的影响。研究结果表明:在Ms点附近的温度区间进行等温淬火,获得回火马氏体+贝氏体的多相组织,可以得到较理想的扩孔性能;在Ms以上的较高温度区间进行奥氏体等温淬火,由于等温过程中奥氏体不能完全转变为贝氏体,在后续冷却时转变成为较多的未回火马氏体,扩孔率反而下降。     

Conditions of Liquid Steel Treatment for Near Net‐Shape Casting Processes

Slab casting for hot rolled steel and strip casting using the twin roll casting (TRC) method are compared in terms of inclusions evolution. There are differences of the processes, mainly the use of casting flux in CC of slabs, or pool shapes and sizes in terms of TRC, particularly when roll diameters vary. The inclusion evolution of alumina in a low carbon steel grade was estimated. By modelling particle growth rate, coagulation and deposition, an ‘agglomeration index’ was created to describe the probability of clogging in the SEN. Similarly the growth of secondary alumina precipitation during cooling of the melt in the pool by Stokes collision and turbulent collision was estimated in terms of forming large particles which are able to float‐up. Influences of melt superheat or caster size were taken into account.     

低合金钢焊接粗晶区连续冷却铁素体相变规律

 利用焊接粗晶区连续冷却淬火方法,对比分析了钛处理钢和普通C Mn钢焊接粗晶区连续冷却不同阶段的相变组织,研究了铁素体相变规律。结果表明,C Mn钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体;钛处理钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体+晶内铁素体组织。在钛处理钢中,晶界铁素体、魏氏组织铁素体和晶内铁素体的相变开始温度相同,但各自长大的动力学条件不同。当晶内铁素体和魏氏组织铁素体竞争发生相变时,晶内铁素体在晶内弥散分布氧化物夹杂上的非均质形核抑制了魏氏组织铁素体向晶内的长大。     

帘线钢盘条索氏体片层间距控制生产实践

对帘线钢盘条索氏体片层间距控制进行了理论分析和生产实践研究。根据理论分析确定了控制索氏体片层间距所需的盘条相变过程的转变温度;通过测定帘线钢的相变过程连续冷却曲线,确定了盘条吐丝后冷却速度。在生产实践中采用了增加风机风量的方法,增加了盘条相变过程的冷却速度,保证了盘条相变温度,使盘条索氏体片层间距得到了有效控制。     

FEM technique to study residual stresses developed in continuously cast steel during solid–solid phase transformation

Abstract

Solid–solid phase transformation of as cast steel may generate stress concentration zones in the microstructure due to the accumulation of thermodynamic (cooling and phase transformation processes) stresses in the microstructure at the interface between phases. These stress concentration zones are vulnerable regions to the formation of microcracks or the growth of flaws in these regions. In the present investigation, a finite element model was created to simulate the cooling of ASTM-SAE grade 1010 steel with different cooling rates. The phase transformation simulations were based on the continuous cooling transformation diagram. Therefore, they were quasi-real models. The models predict analytically the generation of stress concentration regions due to thermodynamic strains during the cooling of a sample from the austenite temperature range with different cooling rates.     

The use of thermal analysis techniques for the validation of phase transformation models

In the study of the phase transformation of steel several methods can be used to obtain information about the behaviour of steel during heating and cooling. In this research several conventional methods, e.g. dilatometry, were studied together with an in-situ thermal analysis method (ISTA). This method, which can be applied to any cooling curve, enables us to draw conclusions as to the transformation behaviour of the steel. Using a continuous annealing simulator (Casim) a range of cooling rates can be investigated with the resulting mechanical properties. It will be demonstrated that ISTA can be performed on the results of a variety of experimental techniques and therefore under very different experimental conditions. It is possible to investigate transformation behaviour and mechanical properties on a single test piece. Using the pressurized air cooling of the Casim proves to be a good compromise between the possibilities of determining the mechanical properties and flexibility in cooling conditions of the specimens.     

Cooling Rate Dependence of Boron Distribution in Low Carbon Steel

The behavior of boron (B) segregation to austenite grain boundaries in low carbon steel was studied using particle tracking autoradiography (PTA) and secondary ion mass spectroscopy (SIMS). An effective time method was used to compare the cooling rate (CR) dependence of this segregation during continuous cooling and its time dependence during isothermal holding. Comparison of these segregation behaviors has confirmed that the CR dependence of B segregation agrees well with its time dependence and is mainly a result of the phenomenon of nonequilibrium segregation. Based on the CR dependence of B segregation, the continuous cooling transformation behavior of B-bearing steel as compared with B-free steel was also investigated using dilatometry and microstructural observations. The addition of a small amount of B to low carbon steel retarded significantly the austenite-to-ferrite transformation and finally expanded the range of cooling programs that result in the formation of bainitic microstructures. Analysis of the B distribution has confirmed that this retardation effect of B on ferrite transformation is attributed to the CR dependence of B segregation to austenite grain boundaries during cooling after austenitization.     

Influence of stresses on the kinetics of pearlitic transformation during continuous cooling

The effect of stress/strain on the kinetics of pearlitic transformation is reviewed. Our results on the pearlitic transformation of an eutectoïd carbon steel under applied uniaxial tensile stresses are analyzed. From these results, we have modelized the effect of the internal stresses, generated during cooling in a solid specimen, on the transformation kinetics. This model is based on a shifting in time of the IT curves of the steel as a function of the internal stress condition (deviatoric part of the stress tensor). A coupled thermal, phase transformation, stress calculation, which includes this model, is applied to the cooling of an eutectoïd carbon steel cylinder. The calculated results show that the internal stresses have an important effect on the kinetics of transforamtion and on the temperature evolutions. The calculated cooling laws are compared with those obtained by experiment and the validity of the model is discussed.     

核岛用P280GH钢的冷却相变过程及数值模拟

 为了研究核岛用P280GH钢管热轧成形后的冷却过程组织演变,采用物理模拟方法建立了CCT和TTT相变动力学曲线。结果表明,低冷速会形成粗大铁素体和珠光体,增大冷速可细化低温相组织,当冷速超过10 ℃/s时即开始出现贝氏体组织。运用叠加原理,基于有限元法,建立了试验钢冷却过程数值模型,分析了[?]219.1 mm×18.26 mm规格P280GH钢管在空冷和水冷两种冷却过程沿壁厚方向温度场、组织演变规律。结果显示,空冷条件相变组织为均匀的珠光体、无贝氏体和残余奥氏体;水冷条件获得相变组织为马氏体组织,管内外表面体积分数相差3.6%,计算结果与实际热处理工况基本一致。研究结果可为热轧P280GH钢管生产的控冷工艺提供指导。     

Predicting the onset of transformation under noncontinuous cooling conditions: Part I. Theory

On cooling a steel, the temperature at which a new phase forms is an important parameter in the genesis of final microstructure. For diffusional transformation processes, prediction of this temperature, until now, has relied upon empirical equations which are based on the cooling rate or degree of undercooling. Modern steel processing procedures involve a range of continuous and step cooling schedules, and these equations are not appropriate in many situations. A method that is capable of predicting the onset of transformation during the accelerated cooling of steel, regardless of how complex a thermal path may be, was derived. The concept of an “ideal” isothermal transformation curve for the start of transformation was introduced based on the assumption of the consumed fractional incubation time being additive. Mathematical relationships between the experimental time to the start of transformation and the ideal incubation time were quantified, and methods for deriving an ideal time-temperature-transformation (TTT) curve from experimental data were established in this study. Details of theoretical derivations are presented in the first article of this two-part series. Application of this prediction method to an austenite-to-pearlite transformation process was studied to show the validity of the derivation. Experimental procedures and results of this application are given in PartII.     

Influence of Strain History and Cooling Rate on the Austenite Decomposition Behavior and Phase Transformation Products in a Microalloyed Steel

The effect of simple strain path changes as well as post-deformation continuous cooling rate during thermomechanical-controlled processing of microalloyed steel was studied using laboratory physical simulation. The phase transformation characteristics were directly analyzed by dilatometry under various cooling rates. The microstructures of the transformation products were characterized quantitatively using EBSD. The results have shown that while strain path changes impose a considerable influence on the hot flow behavior of the austenite, the cooling rate following hot deformation is the determining factor of the phase transformation mechanism and behavior which establishes the final transformation products and subsequent mechanical properties.     

微量铜和砷对Ti-IF钢铁素体相变行为和组织的影响

利用Gleeble 3800热模拟试验机,结合膨胀仪研究了微量铜和砷对连续加热过程和冷却过程中Ti-IF钢相变动力学的影响;结合光学显微镜和扫描电子显微镜,模拟研究了微量铜和砷对不同热轧终轧温度条件下TiIF钢的铁素体晶粒尺寸的影响。结果表明:与几乎不含铜和砷的Ti-IF钢相比,铜、砷的质量分数为0.08%、0.04%的Ti-IF钢中的铜和砷显著提高连续加热过程中Ti-IF钢奥氏体相变的开始温度和连续冷却过程中铁素体相变的结束温度。这主要是由于砷是封闭铁基奥氏体区的元素。由于微量砷提高铁素体相变的结束温度,导致铁素体晶粒尺寸反常粗大。因此,对于残余砷含量较高的Ti-IF钢,应适当控制较高的热轧终轧温度,以避免形成粗大的铁素体组织。     

形变对Cr-Mo低合金钢相变的影响

 Cr-Mo低合金钢在工业生产中有着重要的应用。CCT曲线是研究过冷奥氏体相转变的重要依据。通过Gleeble-3500热模拟试验机及DIL805A淬火变形膨胀仪模拟了Cr-Mo低合金钢的变形及冷却工艺,并利用超组元模型进行热力学计算分析。理论计算结果表明,形变通过提高了Cr-Mo低合金钢相变过程中的自由能进而影响了碳在奥氏体中的活度及相界面碳平衡摩尔分数,相界面碳平衡摩尔分数的变化带来了相变驱动力与形核驱动力的不同,进而影响相变的孕育期与过冷度。结果表明,相变孕育期与过冷度的变化与理论计算结果一致,热力学计算很好地解释了形变对相变的影响。同时表明形变可以提高铁素体相变临界冷速,当冷速为0.3 ℃/s时,Cr-Mo低合金钢可获得最为均匀细小的铁素体晶粒。适当的变形与冷却工艺对改善Cr-Mo低合金钢组织与性能有着重要的作用。     

In situ observation of growth behaviour of acicular ferrite in low-carbon steel containing 13 ppm magnesium

SS400 steel with 13?ppm magnesium was prepared to study the effects of supercooling degree on the formation behaviour of acicular ferrite (AF) and microstructures of Mg-containing low-carbon steel. The inclusions characterised using an automated inclusion analyzer, ASPEX, were mostly MnS, MgAl₂O₄ and MgO with sizes of 1–2?μm. The growth behaviour of AF during the cooling process for austenite transformation to ferrite and continuous cooling temperature diagram was investigated using in situ observation with high-temperature confocal laser scanning microscopy. The initial formation temperature of AF decreased with increasing cooling rate. The temperature of AF transformation was in a narrow range of around 100–150?K. The probability of AF nucleation from inclusions and the average AF lath growth rate increased with increasing cooling rate due to the larger driving force and thermal strain energy around the inclusions during the cooling processes.