A Mixed-control Mechanism Model of Proeutectoid Ferrite Growth under Non-equilibrium Interface Condition in Fe-C Alloys

By combining the α/γ interface migration and the carbon diffusion at the interface in Fe-C alloys, a mathematical model is constructed to describe the mixed-control mechanism for proeutectoid ferrite formation from austenite. In this model, the α/γ interface is treated as non-equilibrium interface, i.e., the carbon concentration of austenite at γ/α interface is obtained through theoretical calculation, instead of that assumed as the local equilibrium concentration. For isothermal precipitation of ferrite in Fe-C alloys, the calculated results show that the rate of interface migration decreases monotonically during the whole process, while the rate of carbon diffusion from γ/α interface into austenite increases to a peak value and then decreases. The process of ferrite growth may be considered as composed of three stages: the period of rapid growth, slow growth and finishing stage. The results also show that the carbon concentration of austenite at γ/α interface could not reach the thermodynamic equilibrium value even at the last stage of ferrite growth.

THEORETICAL PREDICTION OF PROEUTECTOID FERRITICTRANSFORMATION IN HYPO-PROEUTECTOID STRUCTRAL STEELS

Proeutectoid ferrite with carbon content xo precipitating from austenite in a multicomponent steel at temperature T is supposed to be equivalent to proeutectoid ferrite with the same carbon content precipitating from austenite in Fe-C binary system at temperature T'.is described as the temperature difference of proeutectiod ferrite formation, and can be calculated from the Fe-X diagrams and the equilibrium temperature A3. By introducing Tf and basing on the thermodynamic model for Fe-C binary alloy, the driving force for phase transformation from austenite to proeutectoid ferrite in multicomponent steels has been successfully calculated. Through the Johnson-Mehl equation and using the data hem known TTT diagrams, the relationship between the chemical composition and the intedecial edenly packeter as well as activation energy for proeutectoid ferrite formation can be calculated. The starting curves of proeutectoid ferritic transformation calculated in this way in some hypo-proeutectoid structural steels agree well with the erperimental data.

Mn-S与Mn-V-S非调质钢热塑性规律对比

采用Gleeble-3800热模拟试验机测定了Mn-S、Mn-V-S两种非调质钢不同高温条件下的热塑性,并分析了不同温度区间两种钢的断裂失效机制。结果表明,两种钢在800~850℃温度区间均出现了抗拉强度下降幅度最小的现象,主要原因为800~850℃温度区间处于部分再结晶区,温度升高再结晶程度升高,晶粒得到进一步细化,强度得到补充。Mn-S钢在850~1200℃温度区间平均每升高100℃,抗拉强度降低53.9 MPa。Mn-V-S钢的抗拉强度变化总体较为均匀,平均每升高100℃,抗拉强度降低53.9 MPa。Mn-V-S钢中V元素在950℃以下析出第二相粒子,减少了固溶C含量,抗拉强度低于Mn-S钢,在950℃以上V发生回溶,两者强度趋于相近。两种非调质钢断面收缩率变化可分为线性上升和平台期两部分,以1000℃为分界线,Mn-S钢750~850℃温度区间断面收缩率随温度升高增长幅度较小,850~950℃温度区间塑性得到较大幅度提升,900~1200℃范围内钢断面收缩率均超过90%。Mn-V-S钢750~1000℃温度区间断面收缩率随温度升高增长幅度较为均匀,在900℃后断面收缩率均在90%以上,1200℃达到最高,最高值为99.4%。750~850℃温度区间两种钢的断面收缩率低于80%,主要原因是先共析铁素体沿晶界呈网状分布,导致拉伸时产生应力集中,建议实际生产中避开该温度范围变形。

In-Situ Observation of Proeutectoid Ferrite Growth Process in Carbon Steel Under Continuous Cooling Conditions

The growth of proeutectoid ferrite in Fe-0.15%C-0.8 % Mn carbon steel during continuous cooling process was observed in situ and tracked dynamically by using a high temperature confocal scanning laser microscope （ HTC SLM）, and the growing process was also investigated. The growth regularity of proeutectoid ferrite plates within austenite was obtained by analyzing the growth fashions and directions and quantitatively studying the growth rates. The results show that the proeutectoid ferrite plates grow in the fashion of creeping, twisting, branching off and de flection within austenite grain. The proeutectoid ferrite plates can grow up along preferential orientation but not strictly. Its growth direction fluctuates and changes in a small range. The growth rate also changes with the change in growth fashion and direction. When the proeutectoid ferrite grows in the fashion of deflection, the average growth rate of proeutectoid ferrite before its deflection is about 61.9, and 46.2 μm/s after the 15°deflection. At the begin ning of proeutectoid ferrite growing, there is a fast growing stage. As the tip of proeutectoid ferrite extends forward continuously, the growth rate will slow down. Therefore, it will induce the proeutectoid ferrite to deflect and grow along another favorable direction, then it will grow rapidly again.

球化退火冷却速率对SR19热作模具钢组织和硬度的影响

利用DEFORM软件模拟得到大尺寸(200 mm×800 mm×2 000 mm)SR19热作模具钢在退火降温过程中不同深度位置的温度场,根据降温曲线设计该钢小尺寸试样的球化退火工艺,如下:将试样加热至860℃保温2 h,分别以不同速率(15,30,45,60℃·h^(-1))缓冷至740℃保温4 h,再炉冷至500℃后空冷至室温。分析了冷却速率对试样组织和硬度的影响。结果表明:随着冷却速率增大,试样中的先共析铁素体含量和碳化物分布均匀程度均先减小后增大,球化率先保持平稳后快速下降,退火硬度先减小后增大;当冷却速率为45℃·h-1时得到的显微组织最佳,碳化物分布均匀,球化率较高,先共析铁素体含量较少且出现了硬度的回落。先共析铁素体产生的原因主要是退火过程中冷却速率的控制不当导致奥氏体优先转变为铁素体并保留在最终组织中。

Growth Kinetics of Proeutectoid Ferrite in an Fe-0.09C-1.5Mn-0.2Si Steel

Growth kinetics of proeutectoid ferrite, including grain boundary face nucleated ferrite, grain boundary edge nucleated ferrite allotriomorph and intragranular ferrite idiomorph, were experimentally measured in an Fe- 0.09C-1.5Mn-0.2Si steel and compared with theoretical calculation in local equilibrium and paraequilibrium modes. Grain boundary edge nucleated ferrite exhibited larger growth rate than grain boundary face nucleated ferrite and in- tragranular ferrite idiomorph. Experimental kinetics of proeutectoid ferrite was within the window defined by the lo- cal equilibrium and paraequilibrium limits. A transition of growth kinetics from paraequilibrium to local equilibrium was observed in the temperature range of 650--750 ℃, which can be explained in terms of solute drag.

Calculation on the Incubation Period of Proeutectoid Ferrite Transformation for Si-Mn TRIP Steel

The incubation period of proeutectoid ferrite transformation for Si-Mn transformation induced plasticity (TRIP) steel has been calculated by the Aaronson's incubation period model for transformation.The influences of chemical compositions and hot deformation of austenite on the incubation period have been taken into consideration in the calculation,and some parameters have been proposed and validated with the measured time temperature transformation (TTT) curves from dilation tests.The calculation results show that it is essential to take into account of the effect of solute atoms on the interfacial energy in the austenite grain boundaries.For hypoeutectoid steel,the incubation period of ferrite transformation increases with the increase of C and Mn contents,and C has a greater impact than that of Mn,while the incubation period of ferrite transformation decreases with the increase of Si content.Hot deformation shortens the incubation time and promotes austenite to ferrite transformation.

X70钢级弯管焊缝冲击性能劣化成因分析

X70钢级Φ711 mm×11 mm规格弯管弯曲段采用淬火+全管体回火工艺生产时,出现直管段焊缝回火后冲击韧性无法满足技术要求的情况。研究X70钢级弯管焊缝冲击性能劣化的原因,认为:焊态焊缝中有来源于母材的强淬透性元素Cr,而Cr有利于韧性较差的粗大板条状先共析铁素体的析出,从而造成焊态焊缝冲击韧性不高;直管段焊缝回火后,先共析铁素体明显长大,从先共析铁素体中析出的C、N形成了碳氮化物,造成焊缝冲击韧性的继续降低。

含铝超高碳钢等温球化工艺的研究

在对超高碳钢进行铝合金化的基础上,利用成分不均匀奥氏体化加热控制,提出了锻态超高碳钢(UHCs-1 6Al)有效的无形变球化工艺,并利用扫描电镜对先共析碳化物的析出及球化进行了观察,分析了碳化物的球化机理.结果表明,通过对UHCs-1 6Al成分不均匀奥氏体化加热,使先共析碳化物在随后冷却时以粒状形式在基体上弥散析出,利用等温过程使其球化长大,并使奥氏体继续冷却时发生离异共析转变,从而获得球化组织;碳化物颗粒的尺寸可以通过等温温度和等温时间控制;当奥氏体化进行充分时,先共析碳化物析出的孕育期延长,部分碳化物在随后共析转变中以片状形式形成;UHCs-1 6Al最佳球化工艺的奥氏体化透烧温度和等温温度分别为850～870℃和780～800℃.

含铌微合金低碳钢的连续冷却过程的相变

用Gleeble-1500热力模拟实验机研究了含铌微合金低碳钢在不同变形条件下连续冷却过程的相变规律,利用热膨胀法结合金相法建立了静态和动态的连续冷却转变曲线,分析了变形参数对组织的影响规律.研究表明,高温变形促进了珠光体相变,在950℃以上,变形温度的升高导致铁素体转变区减少;从贝氏体转变开始温度看,950℃变形促进了贝氏体相变;在相同变形温度下,随着变形量的增加,先共析铁素体的量增多,贝氏体量减少;在900℃以下变形促进了高温等轴铁素体的形成,抑制了贝氏体的相变.

热轧高强钢氧化动力学和氧化铁皮结构控制

通过实验室氧化增重实验和热模拟实验系统研究了不同时间和温度条件下高强钢(610L)的氧化动力学行为和FeO的转变行为,推导出了变温条件氧化动力学模型,在此基础上可实现热轧高强钢的厚度演变规律研究;测定了高强钢610L的氧化铁皮的等温转变曲线,指出FeO等温转变包含两个转变过程,即第一个转变过程为析出先共析Fe3O4,第二个转变过程发生共析反应(FeO→Fe+Fe3O4),同时给出610L钢FeO层发生先共析转变和共析转变的"鼻温"范围分别为350～500℃和350～450℃,为控制氧化铁皮结构提供了理论依据.

铌微合金化高碳钢的连续冷却转变

在Geeble 1500热/力模拟试验机上测定了铌微合金化高碳钢与普通高碳钢变形后的连续冷却转变曲线,分析比较了它们的组织演变规律。实验结果表明,与普通高碳钢相比,含铌高碳钢的转变温度较高,A+P两相区的温度范围较大(在所测定的冷速下,温度范围均在100℃以上),且在相同冷速下,先共析铁素体的数量明显增多,而奥氏体晶粒较小。在本实验条件下,当冷速大于10℃/s时,含铌高碳钢转变后的珠光体片层间距较小;在冷速大于30℃/s时,其基体中出现贝氏体组织。

共析钢过冷奥氏体动态相变过程中先共析铁素体的生成

通过在Gleeble1500热模拟试验机上进行单轴热压缩试验，研究了共析钢过冷奥氏体在A1～Ar1之间变形时的动态相变特征。结果表明：共析钢过冷奥氏体在变形过程中动态相变与等温相变具有明显差异。动态相变过程中首先沿原始奥氏体晶界形成先共析铁素体，随后生成的珠光体与等温相变得到的珠光体相比，其片层间距和渗碳体厚度较小。提高应变速率将导致动态相变完成时先共析铁素体含量增加，珠光体片层间距减小而渗碳体厚度略有提高。由此提出过冷奥氏体变形使共析点右移的观点，并给予初步解释。

奥氏体变形对铌微合金钢贝氏体相变的影响

用Gleeb le-1500热模拟机研究了铌微合金钢奥氏体变形后连续冷却条件下贝氏体相变规律.研究表明,随变形温度的升高,先共析铁素体量较少,贝氏体的板条束变宽.在连续冷却条件下,贝氏体的转变量随变形量的增加而减少,随应变速率的增加而减少,但应变速率对贝氏体转变量的影响随冷却速度的增加而减弱.随着变形后保温时间的延长,奥氏体稳定性增加,在较快冷却条件下转变产物中存在残余奥氏体.

C和N含量对V-N-Ti微合金非调质钢组织的影响

研究了C含量0．26wt％～0．42wt％及N含量0．0041wt％～0．021wt％变化对中低碳v—N—Ti微合金非调质钢锻态组织的影响。结果表明，C、N含量对V—N—Ti微合金非词质钢锻态组织中先共析铁素体的数量及分布存在显著影响。钢中的N含量一定，C含量约为0．33wt％时，组织中形成的晶内铁素体数量最多、组织最细小均匀；钢中的C含量一定，锻态组织中先共析铁素体的数量随着N含量的增加而增加，但N含量对先共析铁素体数量的影响要远小于C含量的影响。采用基于经典形核和长大理论模型计算所得析出动力学结果很好地解释了上述现象。

铝合金化对超高碳钢先共析碳化物析出行为的影响

通过对不同铝含量超高碳钢的相平衡热力学计算、热处理工艺试验以及扫描电镜的微观组织观察,研究了铝合金化对超高碳钢先共析碳化物的数量和析出行为的影响。结果表明,对碳含量为 1.6%的超高碳钢,添加 2%以上的铝,可以显著抑制先共析网状碳化物的析出。铝的添加,导致平衡状态下的先共析碳化物数量减少,尤其在 2%Al附近,更为显著,是铝合金化抑制超高碳钢网状碳化物的主要原因。

含Nb低碳钢相变温度A_(r3)预测模型

根据热膨胀实验测定的4种不同Nb含量低碳钢在不同连续冷却条件下先共析铁素体相变温度(A_(r3)),建立了一个新的铁素体形核和长大模型,用来预测含Nb低碳钢的A_(r3),模型中考虑了Nb的溶质拖曳作用.结果表明,先共析铁素体相变形核长大的临界碳浓度随钢中Nb含量的增加而呈线性增大,但对温度和冷却速率的变化不敏感;在连续冷却过程中钢中Nb含量的增加会降低先共析铁素体形核温度,但当Nb含量大于0.023%之后形核温度并不会继续降低;固溶Nb会抑制铁素体相变的发生,NbC的析出会促进铁素体形核;本模型适合于Nb含量低于0.04%的低碳钢,使用该模型预测的A_(r3)与实测值吻合.

帘线钢表面氧化优化工艺及控制机理研究

采用OM和SEN分析方法研究了帘线钢空气气氛下连续氧化行为以及高线精轧、吐丝温度对热轧盘条表面氧化铁皮厚度和相组成的影响，同时采用热模拟方法分析了FeO层发生先共析、共析转变生成Fe3O4的条件。实验结果表明，945℃是帘线钢快速氧化起点温度，随着温度升高，氧化速率急剧增加；而当温度降低到700℃时，氧化过程极其缓慢。控制700～945℃范围内的冷速是控制氧化铁皮厚度及相组成的关键工艺。高线生产控制精轧和吐丝温度在900～940℃，氧化铁皮厚度7～20μm，FeO质量分数为65％～80％，具有最佳除鳞性能。热模拟分析表明，400～500℃是FeO层发生共析转变的“鼻尖”温度，孕育期最短。提高400～500℃区间冷速可抑制FeO层发生共析转变，使机械除鳞率提高3％左右。

GCr15轴承钢相变规律及合金元素的影响

利用Gleeble3500分析了GCr15轴承钢冷却过程相变规律及合金元素的影响,基于热力学理论计算分析了先共析渗碳体析出行为。结果表明:渗碳体析出、珠光体转变和全部马氏体转变的临界冷速分别为5、10和15℃/s;Mo元素的添加和Si、Mn元素含量的提高,均使得相转变区间向右移动,提高了试验钢的淬透性;C、Cr元素含量的变化对先共析渗碳体析出温度影响较大,其它元素含量的增加并没有明显的影响。

药芯焊丝焊缝中先共析铁素体的数量预测

合理控制低合金高强钢药芯焊丝焊缝金属中的先共析铁素体含量 ,对获得理想的焊缝性能具有重要意义。将焊缝凝固过程中的偏析效应引入基于扩散控制增长的先共析铁素体生长机制 ,指出成份偏析效应改变了先共析铁素体生长温度和时间区间 ,从而影响偏析微区内先共析铁素体的体积比例 ,建立了预测和分析药芯焊丝焊缝中先共析铁素体体积比例的模型 。