Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel

The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15 vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS), about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10_-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100 °C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior and retained aus- tenite stability.     

Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel

Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20 °C to 200 °C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20 °C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.     

Effects of heat treatment and alloying elements on the microstructures and mechanical properties of 0.15 wt pct C transformation-induced plasticity-aided cold-rolled steel sheets

The main emphasis of this study has been placed on understanding the effects of manganese and silicon additions and of heat-treatment (intercritical annealing and isothermal treatment) conditions on the microstructures and mechanical properties of 0.15 wt pct C transformation-induced plasticity (TRIP)-aided cold-rolled steel sheets. The steel sheets were intercritically annealed and isothermally treated at the bainitic region. Microstructural observation and tensile tests were conducted, and volume fractions of retained austenite were measured. Steels having a high manganese content had higher retained austenite fractions than the steels having a low manganese content, but showed characteristics of a dual-phase steel such as continuous yielding behavior, high tensile strength over 1000 MPa, and a low elongation of about 20 pct. The retained austenite fractions and mechanical properties varied with the heat-treatment conditions. In particular, the retained austenite fractions increased with decreasing intercritical annealing and isothermal treatment temperatures, thereby resulting in the improvement of the elongation and strength-ductility balance without a serious decrease in the yield or tensile strength. These findings suggested that the intercritical annealing and isothermal treatment conditions should be established in consideration of the stability of austenite and the solubility of alloying elements in the austenite formed during the intercritical annealing.     

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.     

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.     

Crystallographic features of theγ-to-α transformation in a Nb-added transformation-induced plasticity steel

A Nb-bearing transformation-induced plasticity (TRIP) steel was control rolled and a certain amount of austenite was retained through appropriate heat treatment. Electron backscatter diffraction (EBSD) measurements were conducted on specimens deformed to various reductions, and the orientations of theα grains formed within individual prior-austenite grains were compared to those expected from the common correspondence relationships. While most of the bainite orientations cluster around the Bain circles formed by these relations, the polygonal (allotriomorphic) ferrite formed at the austenite grain boundaries does not obey any of these relations with respect to the neighboring austenite orientations. Grain-scale variant selection was observed in both the deformed and undeformed austenite grains. The crystallographic features of the deformation-induced ferrite transformation are also discussed.     

Phase transformation of stainless steel during fatigue

Transformation of austenite during cyclic loading was studied in AISI 301 and 304 alloys whose stability was adjusted by heat treatment and temperature changes. Fatigue life was determined under controlled strain amplitude tension-compression conditions. The amount of transformation to α’ (bcc) martensite was continuously indicated magnetically during testing, and the α’ and ∈ (hcp) phases were observed metallographically at failure. It was found in room temperature testing that at strain amplitudes in excess of 0.4 pct the formation of α’ (bcc) martensite was detrimental to the fatigue life. At 200°F (366 K) the fatigue life of an unstable alloy was increased, while in a completely stable austenitic alloy (20Cr, 6Ni, 9Mn), the life at 200°F (366 K) was less than that at room temperature for the same cyclic strain amplitude. The differing effect of temperature on life of these two types of alloy is attributed to the alteration of the austenite stacking fault energy and the relative free energies of the α’ (bcc), ∈ (hcp) and γ (fcc) phases in the unstable alloys. It has been observed that within the standard composition ranges of the two 300 series stainless steel grades there can be marked differences in the degree of transformation resulting from cyclic loading. This has the implication that for fatigue applications modifications in the specifications for the different grades of stainless would be advantageous. Formerly a Research Assistant     

Phase transformation of stainless steel during fatigue

Transformation of austenite during cyclic loading was studied in AISI 301 and 304 alloys whose stability was adjusted by heat treatment and temperature changes. Fatigue life was determined under controlled strain amplitude tension-compression conditions. The amount of transformation to α’ (bcc) martensite was continuously indicated magnetically during testing, and the α’ and ∈ (hcp) phases were observed metallographically at failure. It was found in room temperature testing that at strain amplitudes in excess of 0.4 pct the formation of α’ (bcc) martensite was detrimental to the fatigue life. At 200°F (366 K) the fatigue life of an unstable alloy was increased, while in a completely stable austenitic alloy (20Cr, 6Ni, 9Mn), the life at 200°F (366 K) was less than that at room temperature for the same cyclic strain amplitude. The differing effect of temperature on life of these two types of alloy is attributed to the alteration of the austenite stacking fault energy and the relative free energies of the α’ (bcc), ∈ (hcp) and γ (fcc) phases in the unstable alloys. It has been observed that within the standard composition ranges of the two 300 series stainless steel grades there can be marked differences in the degree of transformation resulting from cyclic loading. This has the implication that for fatigue applications modifications in the specifications for the different grades of stainless would be advantageous.     

控轧控冷TRIP钢的微观相组成及其与力学性能的关系

采用电子背散射衍射技术等实验方法,研究了控轧控冷工艺制备的铌钒微合金化C-Mn-Si系热轧TRIP钢的显微组织及相组成,并分析了与其对应的力学性能.奥氏体轧制过程中的热变形及随后的冷却工艺对最终各相组织的形貌、大小和分布都有直接影响,并决定TRIP钢最终的力学性能.对TRIP钢卷取温度的模拟结果显示,与450和350℃模拟卷取温度相比,400℃模拟卷取温度能使该钢获得更好的综合力学性能.     

38MnSiVS非调质钢的相变模型

 对于热轧非调质钢棒材,轧后相变组织对最终产品的力学性能有着重要影响。为了准确预测38MnSiVS非调质钢棒材热轧后的组织演变和性能,利用热膨胀与定量金相方法,在Gleeble-3500热模拟机及Dil805淬火变形膨胀仪上分别测定了试验钢动态连续冷却转变(CCT)和动态等温转变(TTT)曲线。研究分析了冷却速率对试验钢相变及珠光体片层间距的影响,基于Esake and Pietrzyk和Zener and Hillert模型,分别建立了铁素体晶粒尺寸d_α、珠光体片层间距S_P关系式。结合动态等温转变曲线数据和Scheil叠加原理对铁素体体积分数进行了理论计算,为实际热轧生产中的组织性能控制提供理论依据。     

Effect of bainite transformation and retained austenite on mechanical properties of austempered spheroidal graphite cast steel

Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Method for measuring transformation energy and quantitative characterization of transformation-induced plasticity

A method for measuring transformation energy (E _pt) of strain-induced martensite (SIM) and quantitative characterization of transformation-induced plasticity is developed using characteristics of the tensile curve of three metastable austenitic stainless steels, 10Cr18.5Ni8.5Mnl.9Si0.9, 19Cr17.5Ni7.4Mn2.3Si1.0, and 10Cr16.2Ni11.8Mn1.2Si0.7. The results show that the E _pt of tested materials at −196 °C is 11.3, 14.7, and 20.1×10⁶ J/m³, respectively; E _pt remains constant in the stages of elasto-plastic instability and stress plateau of tensile curves. As the E _pt, which mainly depends on chemical composition of materials, increases, M _s decreases, but the minimum strainproducing M transformation, e _ph, increases. The average plasticity increment (D) induced by M transformation is 0.17 to 0.20 for the metastable austenitic stainless steels, and it decreases with increasing carbon content of steels. The decrease of stacking fault energy (SFE) is beneficial to the D value.     

高铝TRIP钢的高温力学性能

利用Gleeble3500试验机研究汽车用C-Mn-Al系TRIP钢的高温力学性能,测定了零塑性温度和零强度温度,应用差示扫描量热法测定其相变区间,采用扫描电镜和光学显微镜分析了不同拉伸温度对应的断口宏观形貌及断口附近组织组成.该钢种零塑性温度和零强度温度分别为1425℃和1430℃,第Ⅰ脆性区间为1400℃-熔点,第Ⅲ脆性区间为800-925℃.第Ⅲ脆性区脆化的原因是α铁素体从γ晶界析出,试样从975℃冷却至700℃过程中,随着α铁素体析出比例的增大,断面收缩率先减小后增大.基体α铁素体比例为8.1%时（850℃）,断面收缩率降至28.9%;而拉伸温度在800℃以下时,基体α铁素体比例超过16.7%,断面收缩率回升至38.5%以上.该钢种在1275.6℃时开始析出少量粗大的Al N颗粒,但对钢的热塑性没有影响.     

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.     

Phase transformation of TRIP-aided multiphase steel during continuous cooling

The phase transformation characteristics of a high-strength TRIP-aided multiphase cold-rolled steel during continuous heating at different cooling rates were studied by means of dilatometry,and the critical temperatures were also determined.The samples were fully austenitized at 1 050 ℃ and then cooled at different cooling rates ranging from0.5 ℃/s to 100 ℃/s.The continuous cooling transformation(CCT)curves were obtained for the experimental steel.The experimental results showed that a high cooling rate depressed the formation of ferrite and pearlite and promoted the formation of bainite and martensite,leading to a higher hardness.A large amount of martensite in high-strength TRIP-aided multiphase cold-rolled steel can be obtained at cooling rates in excess of 50 ℃/s.The experimental results provide guidelines for cooling control and heat treatment in real steel production.     

Ti-V微合金化热轧高强钢的相变规律及组织性能

摘要：为进一步提升热轧高强钢的性能,利用热模拟试验机、扫描电子显微镜（SEM）、高分辨透射电子显微镜（HR-TEM）等设备系统研究了Ti-V微合金热轧带钢连续冷却相变规律、组织和性能随卷取温度的变化规律及强化机理。结果表明,当冷却速度低于1℃/s时,试验钢中的组织为铁素体和珠光体。当冷却速度为5～30℃/s时,基体组织由铁素体、珠光体和贝氏体组成,贝氏体相变开始温度介于580～600℃。当冷却速度增加至50℃/s时,试验钢中的奥氏体全部转变为贝氏体。此外,对不同卷取温度下试验钢的组织和性能研究表明：随着卷取温度的降低,试验钢的强度降低,塑性基本不变。当卷取温度为650℃时,力学性能最佳,其抗拉和屈服强度分别为716和653MPa,断后伸长率达到21.3%,主要是由于晶粒细化和沉淀强化所致。     

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 10¹⁷ m⁻³ when the dispersed austenite grain size is down to 1 μm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (M _s) temperature has been determined to be M _s (K)=273+545.8 · e ^−1.362w _c(mass pct). A function describing the M _s temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the M _s temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the M _s temperature when the grain size is as small as 0.1 μm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the M _s temperature with respect to the TRIP steels.     

Recrystallization and mechanical properties of microalloyed cold-rolled steel

Cold-rolled microalloyed steels have proven themselves in many applications. Here microalloying fulfills different metallurgical functions that can be used to produce high-strength or very mild deep-drawing steels. This article studies the recrystallization behaviour of microalloyed steels during anneals performed in the laboratory and in the production shop. The delay in recrystallization compared with unalloyed steels can be explained by the amount and distribution of precipitated carbonitrides and also by dissolved microalloying elements. The mechanical properties of the cold-rolled, high-strength steels are determined mainly by grain refinement and, depending upon the annealing process, to a smaller extent by precipitation hardening. With complete fixation of all the interstitial atoms and, at the same time, minimization of the amount of precipitates very mild special deep-drawing steels can be made.