Static Recrystallized Grain Size of Coarse-Grained Austenite in an API-X70 Pipeline Steel

The effects of initial grain size and strain on the static recrystallized grain size of coarse-grained austenite in an API-X70 steel microalloyed with Nb, V, and Ti were investigated using a Gleeble-3800 thermomechanical simulator. The results indicate that the static recrystallized grain size of coarse-grained austenite decreases with decreasing initial grain size and increasing applied strain. The addition of microalloying elements can lead to a smaller initial grain size for hot deformation due to the grain growth inhibition during reheating, resulting in decreasing of static recrystallized grain size. Based on the experimental data, an equation for the static recrystallized grain size was derived using the least square method. The grain sizes calculated using this equation fit well with the measured ones compared with the equations for fine-grained austenite and for coarse-grained austenite of Nb-V microalloyed steel.     

含Ti微合金钢中的第二相粒子对焊接粗晶热影响区组织及韧性的影响

利用碳萃取复型技术研究了含Ti微合金钢及其模拟粗晶区 (CGHAZ)中的第二相粒子 ,并利用OM(光镜 )、TEM(透射电镜 )及系列冲击试验对含Ti微合金钢及一种成分相近的不含Ti低合金高强钢焊接粗晶区的组织及韧性进行了研究。研究结果表明 ,含Ti微合金钢中含有大量的、尺寸细小的TiN粒子 ,这些粒子非常稳定 ,在焊接热循环过程中能有效地阻止奥氏体晶粒长大 ,抑制粗大贝氏体的形成 ,促进针状铁素体析出及M -A组元的分解 ,从而显著改善低合金高强钢焊接粗晶热影响区的韧性 ,t8/5(80 0～5 0 0℃冷却时间 )越大 ,这种改善作用越明显     

加热温度对含Nb中碳钢奥氏体晶粒长大的影响

利用微合金析出物与临界晶粒尺寸的定量关系,研究了Si含量较高的中碳Nb微合金钢在不同加热温度下的奥氏体晶粒长大规律。结果表明,随着加热温度的升高,试验钢中奥氏体晶粒逐渐长大,当温度高于1100℃时,晶粒开始粗化。由经验模型可得,随着温度的升高,析出相的体积分数逐渐减少,而颗粒半径逐渐增大,由于二者的共同作用导致了奥氏体晶粒在高于1100℃时迅速粗化;在实验的基础上,得到了适用于试验钢的晶粒长大模型。     

In situ measurement and modelling of austenite grain growth in a Ti/Nb microalloyed steel

Using a novel laser ultrasonics technique in situ measurements of austenite grain growth were conducted during continuous heating (10 °C s⁻¹) and subsequent isothermal holding at various temperatures in the range 950-1250 °C in a microalloyed linepipe steel. Based on the experimental results, a grain growth model was developed, which includes the pinning effect of precipitates present in the steel. Analyzing the grain growth behaviour and using the advanced thermo-kinetic software MatCalc, an approach was developed to estimate the initial distribution of precipitates in the as-received material and their dissolution kinetics. The evolution of the volume fractions and mean particle sizes of NbC and TiN leads to a time-dependent pinning pressure that is coupled with the proposed grain growth model to successfully describe the observed kinetics of austenite grain growth. The predictive capabilities of the model are illustrated by its application to independent grain growth data for rapid heat treatment cycles that are typical of the weld heat affected zone.     

低碳微合金Ti-Nb可焊钢中的N及其第二相粒子

研究了系列低碳微合金Ti-Nb可焊钢中的N含量对模拟焊接热影响区(HAZ)高温奥氏体晶粒尺寸和冲击韧度的影响及其第二相粒子的作用.对试样进行了大热输入焊接热模拟,测定了高温奥氏体晶粒尺寸和焊后试样中的冲击韧度值,并用透射电子显微镜萃取复型法观察了典型试样中第二相粒子(Ti,Nb)N的形态及分布特征.结果表明,钢中Ti、N含量及第二相粒子的尺寸和数目与高温奥氏体晶粒尺寸及冲击韧度值具有很好的对应关系;钢中的N由于生成了细小弥散分布的第二相粒子(Ti,Nb)N而细化了高温奥氏体晶粒,改善了焊后韧性.低碳微合金Ti-Nb钢中适宜的含氮量有一个范围.     

Enhanced mechanical stability of ultrafine grained steel through intercritical annealing cold rolled martensite

The ultrafine grained ferritic steels possess high strength but low ductility due to the shortage of work hardening.Fine grained ferrite-martensite dual phase microstructure was obtained in a microalloyed steel and low carbon steels through intercritical annealing of the cold rolled martensite.The dual phase microstructure was uniform and the ferrite grain size was smaller in the microalloyed steel resulted from the pinning effect of microalloyed precipitates.But ferrite grown apparently and the volume fraction of the martensite was much higher without the addition of microalloying elements.By introducing martensite into the fine grained ferrite,the work hardening was effectively improved,leading to better mechanical stability.As a result of the fine ferrite grain size as well as uniform distribution of the martensite,the work hardening was enhanced,showing better strength-ductility balance in the microalloyed dual phase steel.     

Analytical description of phase coarsening at high volume fractions

An analytical theory of volume fraction effects in diffusion-controlled phase coarsening is developed. It is based on the application of the Lifshitz–Slyozov–Wagner procedure of Ostwald ripening theory to a self-similar particle volume evolution equation which is approximated by a quadratic polynomial of the scaled particle size. A family of analytical particle size distributions with the scaled maximum particle size as a parameter is derived, which contains the size distribution of normal grain growth as a genuine limit distribution at ultra-high volume fraction without changing the t^1/3-Ostwald ripening kinetics. This reflects an important characteristic feature of particle coarsening at nearly space filling, which was first noted by Ardell and which is in principle in agreement with recent observations. The obtained analytical expressions for the maximum particle size, the particle size distribution and the coarsening rate constant are easy to use for evaluating coarsening data. The results compare well with a number of experimental and simulation results at intermediate and high volume fractions of the second phase.     

微合金化2000 MPa热成形钢的析出相热力学计算与强韧性

通过热力学计算软件Thermo-Calc计算了2000 MPa热成形钢的平衡相图、各相的析出温度、相中的元素含量、碳化物在不同温度下的长大规律以及不同Nb、V含量对其碳化物析出温度和析出量的影响规律。选定特定成分,利用50 kg真空炉进行了熔炼,并进行热轧和冷轧,利用平板模具淬火的方式模拟热成形工艺并进行了力学性能检测和三点弯曲性能检测。利用场发射扫描电镜和EBSD对组织进行了表征。结果表明,Nb、V微合金化2000 MPa热成形钢中的碳化物主要有NbC和VC,析出温度分别在1150 ℃以上及880 ℃以上,且其析出温度分别随着Nb和V含量的升高而升高。平板模具淬火后热成形钢板的抗拉强度超过2000 MPa,伸长率超过8%,拉伸断口为韧性断口,且三点弯曲角度超过66°。SEM和EBSD的结果表明,马氏体组织由马氏体束(packet)、马氏体块(block)和马氏体板条(lath)组成,原奥氏体晶粒约为10 μm,且马氏体块的尺寸<5 μm,马氏体块内部由马氏体板条组成,马氏体板条间为不连续的小角度晶界,晶界的取向差大部分小于5°。细小的原奥氏体晶粒和马氏体块组织是微合金化2000 MPa热成形钢具有高强度、高塑韧性的主要原因。     

BP网络模型在含硼微合金钢奥氏体晶粒尺寸及道次间软化程度预测中的应用

以实验数据为基础 ,建立了含硼微合金钢高温奥氏体晶粒尺寸及道次间软化程度与热加工工艺参数间的 BP网络 (人工神经网络 )模型。结果表明 :BP网络模型具有较高的预报精度     

MODELING OF AUSTENITE GRAIN SIZE IN LOW-ALLOY STEEL WELD METAL

The size of austenite grain has significant effects on components and proportions of various ferrites in low-alloy steel weld metal.Therefore, it is important to determine the size of austenite grain in the weld metal. In this paper, a model based upon the carbon diffusion rate is developed for computing austenite grain size in low-alloy steel weld metal during continuous cooling. The model takes into account the effects of the weld thermal cycles,inclusion particles and various alloy elements on the austenite grain growth.The calculating results agree reasonably with those reported experimental observations.The model demonstrates a significant promise to understand the weld microstructure and properties based on the welding science.     

Ti-V-Nb微合金钢第二相粒子在焊接热循环过程中的变化规律

利用萃取复型技术，对Ti-V-Nb微台金钢母材、焊缝及模拟粗晶区中第二相粒子进行了研究结果表明，母材中的粒子为含有Ti，V，Nb的碳氮化台物复合粒子，其尺寸均在100nm以下；粒子的形状不规则，不同粒子具有不同的成分．焊接过程中，溶池中的碳氮化合物粒了绝大部分发生溶解，溶解到溶池中的Ti通过冶金反应生成TiO，TiO与溶池中的其它脱氧(脱硫)产物结合成尺寸较大的夹杂物；而溶解到溶池中的Nb及V未重新忻出．分析表明，TiO位于夹杂物的表面，且其附近一般有MnAl2O4相，两者具有相同晶体位向．焊接热循环旨，焊接粗晶区中仍保留了较多的粒子，但粒子尺寸显著增大，粒子的形状由母材中的不规则状变为方形，粒子的Ti含量显著提高。     

Microstructure evolution during continuous cooling in niobium microalloyed high carbon steels

Effects of microalloyed niobium (Nb) on the austenite decomposition behaviors and microstructure evolution during continuous cooling in the near eutectoid steels were investigated. Compared to the Nb free steel, the Nb microalloyed steel was refined with regard to polygonal ferrite grain, pearlite block and colony sizes. This was because its austenite grain size was smaller. The volume fraction of polygonal ferrite transformed was more in the Nb microalloyed steels, which indicated the eutectoid carbon content exceeded that of pure carbon steel. The spheroidization of pearlite during continuous cooling was enhanced by Nb microalloying, mainly due to a higher critical transformation temperature and the finer pearlite structure with smaller colony size and narrower interlamellar spacing. Hot deformation right above the equilibrium eutectoid temperature accelerated the spheroidization kinetics of pearlite, especially in the Nb microalloyed steel.     

微合金低碳铁素体-珠光体钢力学性能的预报

在分析现有力学性能预报式的基础上,结合相关文献的试验数据,提出了用于微合金低碳铁素体一珠光体钢的由化学成分和晶粒尺寸计算屈服强度σa和抗拉强度σb的预报式,以及由化学成分和钢板厚度计算延伸率δ的预报式。实验室试制的四个试验用钢的试验结果与预报值基本相符。     

焊接热循环对X-60管线钢中第二相粒子的影响

利用萃取复型技术。对Ti-V-Nb微合金钢母材及模拟焊接粗晶热影响区中第二相粒子进行了研究。结果表明，母材中的粒子为含有Ti、Nb、V的碳氮化合物复合粒子，其尺寸均在100nm以下；粒子的形状不规则，不同粒子具有不同的成分。焊接热循环后，焊接粗晶区中仍保留了较多的粒子，但粒子尺寸显著增大，粒子的形状由母材中的不规则状变为方形，粒子的含Ti显著提高。     

Precipitation of NbC in a model austenitic steel

A model Fe–30 wt% Ni, 0.1 C, 1.61 Mn, 0.1 Nb microalloyed steel, that simulates conventional microalloyed C–Mn steels, but does not transform from the austenite phase on cooling, is reported. Plane strain compression testing was undertaken at 950°C at a constant true strain rate of 10 s⁻¹. Samples were deformed in a two stage process. An initial true strain of 0.25–0.45 was followed by unloading, a hold of 1–1000 s and a final deformation to a total true strain of 0.5–0.9. A single deformation was undertaken under identical conditions, but to the total true strain of the double deformation tests. Electron spectroscopic imaging (ESI) in the TEM was used to determine precipitate size and distribution. A 1 s hold time between equal strains of ϵ=0.25 was sufficient for appreciable strain induced precipitation, although 40% static recrystallisation occurred during the hold time. Precipitation occurred entirely on dislocations, present principally as microband walls but also as a rudimentary cell structure within the microbands. No evidence was found for NbC precipitation in the matrix, which therefore remains supersaturated with Nb. NbC particle diameter was in the range 2.5–15 nm, with a density of 3.8×10²¹ particles/m³ for a 100 s delay period between two strains of ϵ=0.45 at 950°C. Both the size and number density are consistent with those observed in conventional microalloyed C–Mn steels. The behaviour of the model microalloyed Fe–30 Ni steel is discussed in relation to the data on conventional microalloyed steels.     

低合金钢焊缝金属中奥氏体晶粒尺寸计算模型的研究

针对低合金钢焊缝金属奥氏体晶粒尺寸计算模型缺乏这一问题，通过对奥氏体晶粒长大情况的分析，从扩散的角度出发，提出并建立了基于碳原子扩散速率的低合金钢焊缝金属奥氏体晶料在连续冷却条件下长大的物理模型，进而得到了最终的奥氏体晶粒尺寸的计算模型。该模型在结合焊接热循环实际情况的基础上，充分考虑了焊缝金属中的合金元素对奥氏体晶粒长大的影响，从而可以更加准确地体现焊接条件下低合金钢焊缝金属奥氏体晶粒长大的真实情况。该模型的计算结果与试验测量结果能够符合得很好，表明了所建立的模型的有效性。     

高强韧工程机械用钢Q690D的淬火工艺优化

通过全自动相变仪、光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)等,研究880~1100 ℃淬火温度对30 mm厚Q690D钢显微组织、原始奥氏体晶粒尺寸、-20 ℃低温冲击性能和冲击断口形貌的影响。结果表明,当淬火温度低于950 ℃时,试验钢奥氏体平均晶粒尺寸小于10 μm,随着淬火温度的升高,Nb、V、Ti微合金碳化物溶入奥氏体量增加,-20 ℃低温冲击吸收能量逐渐升高;当淬火温度由950 ℃升高至1100 ℃,随着奥氏体晶粒快速长大,试验钢-20 ℃冲击吸收能量由最大值150 J降低至19 J;Q690D钢的最佳淬火工艺为950 ℃×20 min,水冷。     

Influence of production process chain on grain size stability of a microalloyed 20CrMo5 case hardening steel

Abstract

Within the scope of a research project in cooperation with steelmakers and steel users, the whole production process chain of gear parts was investigated concerning the influence on the grain size stability. Three different case hardening steels were investigated in a basic and a microalloyed state. This paper discusses the results gained with a case hardening steel with a new alloying concept, called 20CrMo5. The grain size stabilities of four different industrially produced states of the microalloyed material are compared to the standard material produced in a standard production chain. As a result, it emerged that additional annealing treatments above the eutectoid temperature within the production process chain reduce the grain stability of microalloyed steels.     

马氏体合金钢3J33b磨削力热载荷–晶粒尺寸演变的动态迭代作用机制研究

目的 探究微磨削过程中微磨削力热–晶粒尺寸演变的动态迭代作用机制,构建考虑动态迭代作用的磨削力热模型,提高微磨削力热的预测精度。方法 基于金属材料再结晶理论,探究磨削各阶段弹塑性变形作用下材料晶粒尺寸的演变规律。基于微观组织增强理论,分析微磨削加工过程中材料的流动应力,揭示多颗磨粒重复加载作用下晶粒尺寸演变–微磨削力热的动态迭代作用机制,通过微磨削实验进行实验验证。最后基于灵敏度分析,探究不同工艺参数组合下马氏体合金钢塑性变形和材料去除机理。结果 考虑动态迭代作用的磨削力预测模型的预测值在切向力的平均相对误差为31.51%,法向上的平均误差为27.40%,传统模型磨削力预测值切向力平均相对误差为40.82%,法向力平均相对误差为39.54%。考虑微磨削力热–晶粒尺寸演变的动态迭代作用的磨削温度最大温度预测值的平均相对误差为12.97%,而传统的磨削温度预测值的平均相对误差为16.14%。磨削力随着线速度的增大而减小,随着进给量、磨削深度和晶粒尺寸的增大而增大。磨削温度随着线速度和磨削深度的增大而增大,随着进给量和晶粒尺寸的增大而减小。结论 考虑微磨削力热–晶粒尺寸演变的动态迭代作用的磨...     

冷却速度对钒微合金钢的组织和析出相的影响

采用光学显微镜和透射电镜研究了不同冷却速度下钒微合金钢的微观组织和析出相变化规律。结果表明:当冷却速度小于或等于5℃/s时,钢的组织均为铁素体+珠光体,且随着冷却速度的增加,铁素体的晶粒尺寸明显变细。当冷却速度达到10℃/s时,钢的组织变为马氏体+少量铁素体。透射电镜研究显示:平衡态时析出相包含大量弥散分布的尺寸主要为45~100 nm的不规则形V(C,N)相和(V,Ti)(C,N)复合相,当冷却速度小于或等于5℃/s时,析出相数量无明显改变,但颗粒尺寸随冷却速度的增加不断减小;但当冷速达到10℃/s时,析出相的数量显著下降,尺寸变小。对含钒微合金钢而言,调整适当的冷却速度,不仅可以细化铁素体晶粒,还可以提高析出强化效果,从而提高钢材的强韧性。