两相区退火温度对TRIP钢组织性能的影响

 针对600 MPa级别TRIP钢,进行了760、780、800、820、840、860 ℃两相区退火温度试验,利用扫描电镜和拉伸试验机等设备,分析了其对应的组织比例和力学性能检验结果,得出结论：随着两相区退火温度的升高,铁素体体积分数逐渐减少,钢板的抗拉强度值不断增加,但伸长率值却先下降再升高,在820 ℃伸长率有最大值,这与820 ℃时较高残余奥氏体体积分数和最大残奥中碳质量分数相对应,说明TRIP效应可以改善钢板的塑性指标,获得最佳强塑组合;在800～820 ℃的两相区转变温度范围内,强塑积可以达到2.17×104 MPa·%,为600 MPa级TRIP钢退火工艺提供了实际指导。     

The effect of niobium on the hardenability of microalloyed austenite

The powerful effect that varying the extent of niobium-carbide dissolution has on the “hardenability” of microalloyed austenite is demonstrated using dilatometric measurement of the critical cooling rate required to from microstructures containing >95 Pct martensite. The results can be rationalized on the hypothesis that the hardenability of austenite is enhanced by niobium in solid solution, possibly by its segregation to austenite grain boundaries, but is decreased by precipitation of niobium-carbide particles. This effect appears analogous to that of boron in steels and is found to be independent of variations in the austenite grain size.     

临界区退火温度对Nb-Cr-RE系微碳DP钢组织及性能的影响

实验室设计了Nb-Cr-RE系微碳DP钢,利用扫描电镜以及拉伸试验机,观察与分析了退火温度对微碳DP钢组织与性能的影响.结果表明:微碳DP钢退火板主要由铁素体及其晶界上附着的细小马氏体所构成;随退火温度的上升,DP钢的抗拉强度先升后降,屈服强度逐渐降低,伸长率先增加后降低;铁素体再结晶越充分,晶粒尺寸分布越均匀,等轴化越明显,DP钢屈服强度越小,伸长率越高;另外高温退火时的较大尺寸短棒状碳化物会明显恶化伸长率.      

Influence of intercritical annealing temperature on microstructure and mechanical properties of medium Mn TRIP steel

The transformation, microstructure and mechanical properties of the 0. 2C- 5Mn TRIP steel after intercritical annealing were investigated using dilatometer, scanning electronic microscopy (SEM), transmission electron microscopy(TEM), X- ray diffraction (XRD), and tensile testing machine. The phase transformation thermodynamics of the investigated steel after intercritical annealing was calculated by Factsage software and the characteristics of the transformation were discussed. The results show that the reversed austenite content increases with the increasing of the intercritical annealing temperature, the carbon content in reversed austenite firstly increases and then decreases, manganese content in reversed austenite decreases, which results in the decreasing of the thermal stability of reversed austenite. When the intercritical annealing temperature is 700??, an obvious martensitic transformation occurs during the cooling process. With the increasing of intercritical annealing temperature, cementite is gradually dissolved, but it cannot be completely dissolved due to the short transformation time. When the intercritical annealing temperature is 600-675??, the microstructure after intercritical annealing consists of ferrite, cementite and retained austenite. When the intercritical annealing temperature is 700??, the microstructure after intercritical annealing consists of ferrite, retained austenite, martensite and a small amount of undissolved cementite. The engineering stress and strain curves of the investigated steel are significantly changed with increasing intercritical annealing temperature. At the same time, the optimal mechanical properties with tensile strength of 1138MPa and total elongation of 23% can be obtained after annealed at 675?? for 3min.     

Dynamic strain aging effects in niobium microalloyed steel

Abstract

The dynamic strain aging behaviour of a niobium microalloyed steel has been examined. Hot tensile testing was carried out on heat treated and as received specimens. Heat treated specimens were austenitised at 1000°C for 1 h, and then cooled in air or in a stainless steel cylinder to obtain various amounts of free or uncombined interstitial solutes in solid solution, to examine the effect on the dynamic strain aging behaviour of the steel. It was found that dynamic strain aging takes place in niobium microalloyed steel during tensile testing at temperatures ranging from ambient temperature to 450°C at a crosshead displacement rate of 2 mm min^-1. As a result, the ultimate tensile strength and initial work hardening rate exhibit maximum values at temperatures between 200 and 350°C. Also, load-extension graphs for tested specimens show serrated behaviour and yield points at 200, 250, and 300°C. It is believed that dynamic strain aging in niobium steel is caused by interaction between dislocations and interstitial solutes (nitrogen and carbon) or solute pairs consisting of one interstitial and one substitutional solute atom (for example Mn-C and Mn-N).     

Study on a new type nitriding steel microalloyed with niobium

A niobium microalloyed nitriding steel has been introduced to meet the high demand in modern engine applications,based on compositions of traditional nitriding steels,and on phase calculation and prediction with JmatPro software.The new nitriding steel BTHJ-1 shows a better nitriding tendency,as well as a good combination of strength and toughness,while compared with 38CrMoAl and H13 steels.BTHJ-1 steel also shows a better thermal stability when being held at 620℃.Investigations with OM,TEM,XRD have been made so as to get a better understand on the characteristics of BTHJ-1 steel.The quantities optimization of alloy elements and the grain refinement of niobium seem to be reasons for the improved properties of the nitriding steel.     

Influence of intercritical annealing on the texture formation in low-carbon steel strips

Cold rolled low carbon steels are often recrystallized in continuous annealing lines. This procedure is accompanied by partial austenitization. Up to 40 % phase transformation the {111} texture is enhanced, whereas at higher austenitization grades this texture is weakened. The underlying mechanisms of texture formation by phase transformation are investigated by means of quantitative texture analysis.     

Austenitization during intercritical annealing of an Fe-C-Si-Mn dual-phase steel

Partial austenitization during the intercritical annealing of an Fe-2.2 pct Si-1.8 pct Mn-0.04 pct C steel has been investigated on four kinds of starting microstructures. It has been found that austenite formation during the annealing can be interpreted in terms of a carbon diffusion-limited growth process. The preferential growth of austenite along the ferrite grain boundaries was explained by the rapid carbon supply from the dissolving carbide particles to the growing fronts of austenite particles along the newly formed austenite grain boundaries on the prior ferrite grain boundaries. The preferential austenitization along the grain boundaries proceeded rapidly, but the austenite growth became slowed down after the ferrite grain boundaries were site-saturated with austenite particles. When the ferrite grain boundaries were site-saturated with austenite particles in a coarse-grained structure, the austenite particles grew by the mode of Widmanstätten side plate rather than by the normal growth mode of planar interface displacement.     

Influence of rolling temperature on the austenite structure and properties of low-carbon microalloyed steel

In laboratory conditions, the influence of the temperature interval in the finishing stage of controlled rolling on the structure of hot-deformed austenite, the final microstructure, and the properties of low-carbon microalloyed steel is studied. Experiments show that reducing that temperature interval within the range T _nr-A _r3 reduces the grain size in the direction transverse to rolling and increases the number of deformational twins within the hot-deformed austenite grains. The effect is to improve the grain size and uniformity of the ferrite-bainite microstructure and hence to improve the mechanical properties of the thick sheet produced.     

Effect of intercritical temperature and cold-deformation on the kinetics of austenite formation during the intercritical annealing of dual-phase steels

The objective of this investigation was to study the effect of the intercritical temperature and percentage of cold-deformation on the kinetics auf austenite formation during the intercritical annealing in the alpha + gammy (α + γ) phase field of the iron-carbon phase diagram. This investigation was carried out on an Fe–0.11 C–1.58Mn–0.4 Si ferritic-pearlitic alloy with different structures of 0% (hot-rolled), 25% and 50% cold-deformation. The intercritical annealing temperatures were 735, 750°C and the intercritical annealing time ranged from 15 to 1815 s. It has been observed that recrystallization of the deformed ferrite was completed before any austenite formation. Surprisingly, it was noted that the recrystallized ferrite grain size was independent of percentage cold-deformation. Furthermore, it was expected that cold-deformation accelerates the kinetics of austenite formation. Nevertheless, the amounts of austenite formed from pearlite dissolution were mostly equal, irrespective of the starting condition. As has been previously reported, increasing the intercritical annealing temperature was found to increase the amount of austenite.     

Effect of intercritical annealing temperature and cooling rate on the microstructure and mechanical properties of low-carbon aluminum killed steel

In this study,the intercritical annealing process for a typical low-carbon aluminum killed steel is investigated.A cold-rolled sheet was annealed at intercritical temperatures ranging from 730 ℃ to 770 ℃ and then cooled in air or water.The annealed steel was then baked at 210 ℃,and its mechanical properties and microstructures were analyzed in detail.It is shown that after the air-cooling process,the strength of steel decreased and ductility increased with an increase in the annealing temperature.However,after water-cooling,the strength and ductility both increased with the increase of annealing temperature.These results are attributed to the propertyoptimization of the steel.     

Enhanced mechanical properties of dual-phase steel by repetitive intercritical annealing

The effect of repetitive intercritical annealing on the mechanical properties and work-hardening response of a typical C–Mn dual-phase (DP) steel was studied by consideration of different pre-intercritical annealing microstructures and martensite volume fractions. It was revealed that the DP steels produced from the initial martensitic microstructure have much better mechanical properties compared with those originated from the ferritic–pearlitic banded microstructure. The repetitive intercritical annealing of the initial martensitic microstructure was effective for the enhancement of strength–ductility balance, where at a comparable ductility; a twofold increase in the tensile strength was obtained. Finally, these results were discussed based on the Crussard–Jaoul work-hardening rate analysis.     

Development of ultra-fine grained dual phase microalloyed steels through severe cold rolling and intercritical annealing

A Nb-microalloyed structural steel with ferrite-pearlite microstructure was subjected to cold rolling and intercritical annealing to produce ultra-fine grained dual phase microstructure. Optical and transmission electron microscopy techniques were employed to characterise the microstructure. Initial results showed that the intercritical annealing (at 790°C for 90s) of samples rolled to a true strain of 2.4 resulted in a significant grain refinement from the average initial grain size of 20 μm to 1–2 microns. The microstructure primarily consisted of UFG ferrite matrix with homogeneously distributed islands of plate martensite with volume fraction of 27%.     

Nb-V-Ti微合金钢中奥氏体两相区变形过程组织演变

利用循环加热-淬火工艺制备超细晶奥氏体的基础上,将超细晶奥氏体快冷至两相区实施不同应变速率下的单道次变形,分析其在两相区的动态组织演变特征。结果表明：奥氏体晶粒尺寸为1～3μm的Nb-V-Ti微合金,在两相区不同应变速率变形下可获得尺寸小于500 nm铁素体晶粒,且变形过程中铁素体由初始的不均匀条带状分布逐渐演变为均匀弥散化分布。     

中锰钢两相区退火奥氏体逆转变的数值分析

根据中锰钢热轧组织结构确立两相区奥氏体化的几何模型和初始条件,利用DICTRA动力学分析软件对中锰钢马氏体基体奥氏体化过程进行计算分析.在奥氏体化初期的形核过程中,马氏体中过饱和的碳锰元素从铁素体迅速转移到奥氏体并在相界面奥氏体一侧聚集.后续的相变过程中,碳在奥氏体中快速均化,但锰在相界面奥氏体一侧的聚集加剧.相变初期奥氏体界面推移速度比中后期高出若干个数量级,但随时间推移迅速衰减.相变初期相界面推移是碳扩散主导,相变后期界面推移受到锰在奥氏体中扩散速度制约.温度升高可显著提高相界面推移速度.达到相同数量奥氏体的情况下,低温长时退火有利于锰从铁素体向奥氏体转移并提高其在奥氏体中的富集度,从而提高奥氏体的稳定性.     

临界区加热温度对低碳当量冷轧超高强双相钢组织及性能的影响

利用CCT-AWY型设备进行连续退火模拟试验,研究了不同临界区加热温度对冷轧超高强双相钢显微组织及力学性能的影响。试验结果表明,随着退火温度的升高,马氏体的体积分数不断增长;在适当的退火温度下,可以获得近等轴铁素体和均匀分布马氏体的双相组织。试验用钢的强度、延伸率、显微硬度以及断口形貌与加热温度、显微组织紧密相关。     

Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T _5pct) and the recrystallization-limit temperature (T _95pct), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T _5pct<T<T _95pct), as determined in this study. The Nb(CN) precipitation kinetics have been calculated using a model proposed in an earlier work, and these results are used to estimate the precipitate pinning force under the given processing conditions. Based on these estimations, a criterion has been proposed to predict the onset of recrystallization. The predicted results are found to be in reasonably good agreement with the experimental measurements.     

Effect of intercritical annealing on phosphorus segregation in hot- rolled medium- manganese TRIP steel

Field emission scanning electron microscope and transmission electron microscope combined with energy dispersive spectrometer were used to observe the phosphorus segregation behavior in hot- rolled medium- manganese TRIP steel. The samples were annealed at different temperatures, followed by the same aging process at 560?? for 50h. The results show that particle- like phosphorus is dispersively distributed in the samples annealed at 650??, while the sample annealed at 750?? shows uneven narrow- band- like phosphorus distribution. The entire phosphorus segregation zone overlaps with carbides precipitation. Both 650 and 750?? are intercritical region (ferrite + austenite) temperatures. The segregation usually occurs at ferrite grain boundaries. At the same temperature, the diffusion coefficient of phosphorus in ferrite is 26 times than in austenite. However, it is insufficient to explain the effect of annealing temperature on phosphorus segregation by diffusion coefficient. According to the Kikuchi line analysis in the electron backscatter diffraction, the phosphorus- containing precipitate phase is mainly FeP in orthogonal crystal system and MnxFe1-xP solid solution, which is in accord with the calculation results in Fe- Mn- C- P system by Factsage software.     

Recrystallization kinetics of microalloyed steels deformed in the intercritical region

A plain carbon and two microalloyed steels were tested under interrupted loading conditions. The base steel contained 0.06 pct C and 1.31 pct Mn, and the other alloys contained single additions of 0.29 pct Mo and 0.04 pct Nb. Double-hit compression tests were performed on cylindrical specimens of the three steels at 820 °C, 780 °C, and 740 °C within the α + γ field. A’softening curve was determined at each temperature by the offset method. In parallel, the progress of ferrite recrystallization was followed on quenched specimens of the three steels by means of quantitative metallography. It was observed that, in the base steel, a recrystallizes more slowly thany. The addition of Mo retards recrystallization and has a greater influence on γ than on α recrystallization. This effect is in agreement with calculations based on the Cahn theory of solute drag. Niobium addition has an even greater effect on the recrystallization of the two phases. In this steel, the recrystallization of ferrite was incomplete at the three intercritical temperatures. Furthermore, the austenite remained completely unrecrystallized up to the maximum time involved in the experiments (1 hour). The metallographic results indicate that the nucleation of recrystallization occurs heterogeneously in the microstructure, the interface between ferrite and austenite being the preferred site for nucleation.     

Fe-C-Mn系冷轧双相钢两相区奥氏体化过程模拟

在对双相钢两相区奥氏体化过程进行热力学与动力学分析的基础上,建立了两相区奥氏体化过程的扩散模型,并采用显式有限体积法对740℃与780℃下的奥氏体化过程进行了数值求解.模拟结果表明:奥氏体长大初期受C元素在奥氏体中的扩散控制并很快达到亚平衡.该阶段奥氏体长大速度较快.奥氏体长大后期受Mn元素在铁素体中的扩散控制.该过程由于Mn元素的扩散速率比C元素的扩散速率低几个数量级而持续数千秒.当Mn元素在两相中的扩散通量相等时,奥氏体停止长大,Mn元素继续从铁素体向奥氏体中转移以完成其在两相中的均化.