Relative Influences of Aluminium and Silicon on the Kinetics of Bainite Formation from Intercritical Austenite

Bainite formation from intercritical austenite is of great practical importance for the production of TRIP‐assisted steels. Silicon and aluminium play important roles during this transformation by delaying carbide precipitation, thus favouring the carbon enrichment of untransformed austenite, which makes its stabilisation down to room temperature possible. Previous studies have shown a strong dependence of bainite formation kinetics on both chemical composition and transformation temperature. In the present work, the effect of silicon and aluminium contents on bainite formation kinetics is investigated experimentally using dilatometry combined with microscopical observations. The experimental results are analysed by comparison with thermodynamic parameters, such as the activation energy G¹ for nucleation of bainite and the carbon content C_to corresponding to the T_o‐curve. It is shown that the faster transformation kinetics induced by the substitution of silicon by aluminium can be ascribed (i) to a higher driving force for nucleation, (ii) to a higher carbon content C_to at the To‐curve and (iii) to the precipitation of carbide in austenite in steels with a low Al content.     

Fe-6.5%Si合金温轧后退火过程中再结晶行为

用电子背散射技术观察了700℃温轧板在退火过程中的组织及织构演变以了解其再结晶行为.结果表明,温轧织构由强的（111）〈112〉、较弱的〈110〉∥RD及Goss组成,再结晶织构与之相似.〈110〉∥RD及（111）〈112〉新晶粒首先形成于与之构成小角度晶界的形变晶粒的晶界附近,而在角隅及组织不均匀区等位置孕育出与周围晶粒构成大角度晶界的晶核,择优取向不明显.退火过程中（111）〈112〉在形变组织中累积,最终转化为（111）〈112〉再结晶晶粒.分析认为,温轧后退火是不均匀组织在低储存能驱动下的再结晶过程.（112）〈110〉及（111）〈112〉形变拉长晶粒多发生连续再结晶从而退火织构与形变态相似.在角隅区形成核心进而发生不连续再结晶,核心取向的统计性及不连续晶核的长大弱化再结晶织构,其中Goss晶粒多以此方式形成于（111）〈112〉晶粒内部.     

Finite Element Modelling of TRIP Steels

A constitutive model that describes the mechanical behaviour of steels exhibiting “Transformation Induced Plasticity” (TRIP) during martensitic transformation is presented. Multiphase TRIP steels are considered as composite materials with a ferritic matrix containing bainite and retained austenite, which gradually transforms into martensite. The effective properties and overall behaviour of TRIP steels are determined by using homogenization techniques for non‐linear composites. The developed constitutive model considers the different hardening behaviour of the individual phases and estimates the apportionment of plastic strain and stress between the individual phases of the composite. A methodology for the numerical integration of the resulting elastoplastic constitutive equations in the context of the finite element method is developed and the constitutive model is implemented in a general‐purpose finite element program. The prediction of the model in uniaxial tension agrees well with the experimental data. The problem of necking of a bar in uniaxial tension is studied in detail.     

A Semi‐Empirical Model for the evolution of Retained Austenite via Bainitic Transformation in Multiphase TRIP Steels

Multiphase TRIP steels exhibit unique combinations of strength and cold formability, characteristics especially desirable in automotive applications. This behaviour is largely determined by the amount and stability of the retained austenite dispersion in the microstructure, produced by a two‐stage heat‐treatment, consisting of intercritical annealing followed by an isothermal bainitic treatment. The amount and stability of retained austenite is mainly determined by the proper selection of the temperature and temporal duration of the bainitic treatment. In the present work an approach is presented which allows for the calculation of the amount of retained austenite in the microstructure, as a function of bainitic treatment conditions. The approach is based on the physical characteristics of the bainitic transformation and on the stabilizing effects of the formation of bainitic ferrite in austenite. Each bainitic ferrite platelet is considered to chemically stabilize a part of the surrounding austenite due to carbon rejection. The spatial and temporal extent of this stabilization is determined by solving the corresponding carbon‐diffusion problem, and thus the amount of retained austenite contributed by any individual platelet is determined. Subsequently, the evolution of the population of the platelets in the entire microstructure is determined and, thus, the volume fraction of retained austenite as a function of transformation time is calculated. Application and comparison of calculations with experimental results, obtained from four different multiphase TRIP steel compositions, exhibited very good qualitative and quantitative agreement.     

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.     

Rheological Model for Simulation of Hot rolling of New Generation Steel Strips for Automotive Applications

The main objective of this paper is the development of a rheological model for automotive steels for the conditions of hot strip rolling and implementation of this model in a finite element program is. Three types of steels were investigated, IF, dual phase and TRIP steel. Plastometric tests were performed on a Gleeble 3800 simulator for the temperature range 850‐1200°C and strain rates 3‐150 s^?1. Inverse analysis was applied to eliminate the influence of disturbances occurring in the plastometric tests and to determine the real flow stress of the material. The coefficients in the flow stress equation were evaluated and this equation was implemented in the FEM code as the constitutive law. The model was validated by comparison of measured and calculated loads in the compression tests and by strip rolling experiments conducted in the laboratory mill. Validation confirmed a good predictive capability of the rheological model.     

高等级管线钢的超细晶粒控制

论述了高强度管线钢超细品粒控制的材料学原理和关键技术.分析了应变诱发相变和超细奥氏体强化相变这两种超细品粒控制方法的原理、工艺设计原则以及工业化应用需要注意的问题.讨论分析的结果表明,轧制过程中积累足够大的应变是实现超细晶粒控制的关键.在工业生产中只能通过多道次轧制工艺逐个道次的应变累加实现所需的应变积累.而抑制道次之...     

09MnVTiN钢再结晶奥氏体连续冷却转变的研究

本文利用Formastor-Press压力膨胀仪测试09MnVTiN钢热变形后再结晶奥氏体的连续冷却转变动力学曲线,并用金相法分析不同阶段淬火试样的组织。结果表明,再结晶控轧后,奥氏体区的冷却速度(1000～820℃)对再结晶奥氏体的晶粒尺寸和随后的连续冷却转变动力学都有影响。     

Cold-rolling technologies of advanced high strength steels in Ansteel

Low - carbon becomes a high - frequency and fashionable word which gets the greatest concern in the world.Low - carbon refers to a minimal output of greenhouse gas emissions into the biosphere, specifically refers to the greenhouse gas carbon dioxide.To reduce energy consumption of automobile,more and more high strength steels are used by vehicle companies.To meet the request of vehicle companies, various high strength steels are developed in steel companies all over the world.Ansteel can provide,ultra-low carbon bake hardening(BH) steels,dual phase(DP) steels and transformation - induced plasticity (TRIP) steels with the grades of under 780 MPa,up to now.AHSS steels have much different composition, microstructure and strenght than conventional vehicle steels,so there are some trouble in producing in cold rolling mills,for example,difficult to join,poor thickness and flatness,accurate temperature and velocity control and so on.To reduce the opportunity of strip breakage,we have done many welding experiments and special research.Now,DP and TRIP steels can be continuously produced in Ansteel.To assure thickness and flatness of strips,we optimized the hot rolling parameter to get low deformation resistance,optimized rolling oil to get fit frictional coefficient and fix on the targat rolling curve.To get more accurate and repetitive results over the production shifts,the Mathematical Model(MM) is used in Ansteel.The MM analyses information transmitted by various sensors and transmitters,compares the collected data with the chosen parameters and adjusts the settings of the various pieces of equipment to hone on the parameter setpoints.At present,the key production technologies of AHSS were grasped by cold rolling mill Ansteel,and Ansteel is the first company to apply the TRIP with the grade of 780 MPa.     

Determination of Optimal Recrystallization Annealing Temperature of Nb-Ti Microalloyed High Strength IF Steel

The Nb-Ti microalloyed high strength IF steel sheet was used to study the effect of annealing temperature on the microstructures,mechanical properties and textures.The experimental results show that experimental steel is incomplete recrystallization at 750℃ annealing,but complete recrystallization from 780℃ to 870℃ under experimental conditions.When the annealing temperature was increased,the yield strength and tensile strength would gradually reduce,the plastic strain ratio and yield point elongation would gradually increase.The yield strength,tensile strength,elongation,the plastic strain ratio and the strain hardening exponent were approximate 300MPa,410MPa,36.5%,1.7 and 0.22 respectively under annealing temperature 810℃ to 840℃.When the annealing temperature was increased,the α-textures and γ-textures were gradually weakened,and the α-textures have a trend to {111} texture.Therefore,the suggestion of the optimal recrystallization annealing temperature is about 810℃ to 840℃ in industrial production.     

Metallurgical Phenomena during Processing of Cold Rolled TRIP Steel

Metallurgical phenomena taking place during processing of TRIP Steel are investigated and described with the aim of achieving better understanding of the microstructure development throughout the entire integrated processing routes. Different TRIP steel structure sizes were created by controlling the hot rolling process prior to cold rolling. After that the specimens were intercritically annealed under different conditions to obtain prescribed austenite fractions, and subsequently quenched in salt bath at the bainite transformation temperature. The microstructures had been investigated using light optical microscopy (LOM) and the amount of retained austenite was determined by magnetometry.     

310S奥氏体耐热不锈钢热轧变形抗力研究

通过现场实际数据利用数学回归的方法建立了310S热轧变形抗力模型,模型计算的轧制力与实测值符合良好。利用所建立的模型对比分析了310S和304热轧变形抗力的影响因素,结果表明,随着温度的降低,310S变形抗力的增加速度较304快。