700 MPa级热轧双相钢的组织和性能

 采用热模拟并借助光学显微镜、SEM技术研究了双相钢的相变规律及不同工艺参数下的组织演变规律。根据热模拟结果在实验室试制出700 MPa级热轧双相钢,优化了轧制和冷却工艺参数。实验结果表明：热轧双相钢组织为多边形铁素体+马氏体岛,抗拉强度730 MPa,屈强比062,伸长率236%,达到了DP700级双相钢的性能要求,并讨论了热轧卷取温度对双相钢最终力学性能的影响。     

Model for Microstructure Prediction in Hot Strip Rolled Steels

Phase transformations from work‐hardened austenite after hot rolling of steel are of paramount importance for achieving the required mechanical properties. Moreover, it influences the control of cooling process in the cooling table. This paper proposes semi‐empirical mathematical models for predicting such transformations in carbon‐manganese and microalloyed steels, subjected to transformations conditions leading to a microstructure constituted by ferrite and pearlite. To develop the models, dilatometric tests were carried out in a Gleeble 3500 machine, simulating the industrial processing conditions as close as possible, including the effect of coiling. Emphasis was placed on the application of a criterion for determining the start of pearlite formation, which was based on carbon enrichment of austenite during the ferrite formation. Based on comparisons between measured and calculated ferrite fractions in tested specimens, it was proposed to consider a transition of the thermodynamic conditions prevailing during ferrite formation, which depend on steel chemistry and transformation conditions. As a preliminary assessment of its predictability the model was applied to a typical industrial process condition for microalloyed steels in hot strip rolling. It was shown that the model was capable of predicting reasonably well the evolution of ferrite and pearlite during the cooling in and also during the subsequent coiling process.     

600 MPa级热轧双相钢组织与性能优化分析

 通过热轧厂实际生产试制,研究了钛、铌微合金元素对600 MPa级低成本低温卷取型铁素体/马氏体双相钢组织和性能的影响,并与同强度级别中温卷取双相钢进行对比。研究结果表明,沿晶界分布的纳米级（Nb,Ti）C第二相显著细化了铁素体/马氏体两相组织,由此解决了不含钛、铌元素的低温卷取双相钢马氏体岛粗大的问题,提高其强度和塑性。此外,试制生产对比发现,中温卷取双相钢存在晶粒尺寸较粗,马氏体体积分数较少,强度相对略低等特征,并提出了相应的热轧工艺改进思路。     

含钼双相钢DP600相变研究和工艺实践

采用热模拟试验研究了含钼双相钢DP600在不同冷却模式、转变温度和冷却速率时的显微组织转变,分析了相变后的马氏体比例和晶粒度级别,根据热模拟结果设计了DP600钢的生产工艺,并探讨了钼元素对双相钢的影响。结果表明,DP600钢在热轧组织转变时,两段式冷却工艺比一段式工艺形成的马氏体细小,且晶粒度提升1级。奥氏体向铁素体转变过程中,存在最佳相变温度平衡点;590 ℃以上减缓DP600钢铁素体＋珠光体的过冷转变速率,可以细化晶粒、增加马氏体比例。生产的DP600钢金相显微组织为铁素体＋马氏体,马氏体比例为17%,晶粒度为11级;纵向、横向抗拉强度分别为592和620 MPa,伸长率分别为28.5%和26.5%。钼元素可以强烈抑制C- Si- Mn- Cr- Mo系DP600钢的铁素体转变,缩小铁素体转变区。     

冷作硬化非调质钢的连续冷却相变规律

钢的连续冷却相变曲线(CCT)是组织调控的基本依据,为了优化紧固件用冷作硬化非调钢热轧态的组织和力学性能,采用DIL805A相变仪测定了试验钢在0.1～50℃/s不同冷却速率下的热膨胀曲线,结合金相硬度法确定相变类型,并绘制了试验钢的CCT曲线.结果 表明,试验钢马氏体转变点(Ms)为280℃,在不同冷速范围内均有铁素...     

Advances in the Optimization of Thin Strip Cast Austenitic 304 Stainless Steel

This study is about the latest advances in the optimization of the microstructure and properties of thin strip cast austenitic stainless steel (AISI 304, 1.4301). Concerning the processing steps the relevance of different thin strip casting parameters, in‐line forming operations, and heat treatments for optimizing microstructure and properties have been studied. The microstructures obtained from the different processing strategies were analysed with respect to phase and grain structures including the grain boundary character distributions via EBSD microtexture measurements, the evolution of deformation‐induced martensite, the relationship between delta ferrite and martensite formation in austenite, and the texture evolution during in‐line deformation. It is observed that different process parameters lead to markedly different microstructures and profound differences in strip homogeneity. It is demonstrated that the properties of strip cast and in‐line hot rolled austenitic stainless steels are competitive to those obtained by conventional continuous casting and hot rolling. This means that the thin strip casting technique is not only competitive to conventional routes with respect to the properties of the material but also represents the most environmentally friendly, flexible, energy‐saving, and modern industrial technique to produce stainless steel strips.     

Q&P钢在冷却过程中铁素体的相变规律

 为了研究冷却过程中Q&P钢(quenching and partition steel)铁素体的相变规律,在热膨胀仪上以846 ℃均热200 s为冷却的初始条件,检测了成分(质量分数)为0.2%C、1.25%Si、2.0%Mn的Q&P钢在不同冷却速率下铁素体的相变热膨胀数据,应用杠杆定律将数据处理为相变规律与温度的关系,通过光学显微镜检测热处理后金相中的铁素体相体积分数和铁素体晶粒尺寸,得出了饱和位置形核条件下铁素体的形核率,基于混合控制模型得出了铁素体相变的相界迁移速率。结合相变开始温度,利用混合控制模型计算了相变结束温度和铁素体晶粒尺寸在相变过程中的演变规律,铁素体晶粒尺寸计算值与实测值吻合程度较高,相变结束温度的计算值与实测值的误差在±15 ℃以内,所获得的铁素体相变规律可以用于控制Q&P钢在冷却过程中的铁素体相变体积分数。     

Effect of Strengthening Phase on Deformation Behaviour during Uniaxial Tension of Hot-rolled Dual Phase Steel

Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite （F） ＋martcnsite （M） or ferrite （F）-bainite （B） with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength （YS/TS） and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible de formation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.     

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

Thermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature.     

800 MPa级双相组织低屈服比钢厚板试制

对基本成分为Fe-0.1C-Mo的微合金HSLA钢进行了变化轧制与冷却参数的试验,分析了钢的组织形貌及微观结构;研究了变形和冷却参数与钢的屈服比、缺口冲击功、延伸率等的关系.指出:通过轧制、冷却工艺的组合与变化,可以实现钢的强度、塑性、屈服比的控制;抗拉强度800 MPa级的厚钢板,其屈服比可控制在0.75以下,低温冲击韧性良好.钢的组织主要由针状铁素体和马氏体两相组成.     

Study of Bake Hardening Effect on Laser Welded Hot Rolled Ferrite‐Bainite Dual Phase Steels

The influence of bake hardening (BH) effect on laser welded hot rolled ferrite‐bainite dual phase steel (FBS) with respect to the process conditions was investigated. The samples were firstly laser welded and then pre‐strained. Pre‐strained the samples with defined degrees of deformation and a subsequent aging treatment leads to enhanced hardness and strengthening for both conditions. The microstructure of the fusion zone (FZ) and heat affected zone (HAZ) was studied. A high volume fraction of martensite could be observed in the FZ as well as in the HAZ. Both steels exhibited a clear BH effect in both, the as‐received and the laser welded conditions. The BH effect is more pronounced in the pre‐strained laser welded condition.     

980MPa级冷轧双相钢组织性能研究

为研究980 MPa级C-Si-Mn-Nb系冷轧双相钢组织性能,在试验室冶炼该钢并采用临界区保温+两段式冷却+过时效处理的工艺进行热处理。研究表明,试验钢的屈服强度为476 MPa,抗拉强度为1 021 MPa,伸长率为15%,n值为0.29;试验钢热轧组织为(F+P),铁素体晶粒尺寸约为3.3μm;退火组织为(F+M),马氏体体积分数约为63%。微合金元素Nb的添加,起到细晶强化和析出强化的作用。与热轧组织相比,连续退火板带状组织得到明显改善,试验钢表现出良好的强韧性匹配。     

Effects of Process Parameters prior to Annealing on the Formability of Two Cold Rolled Dual Phase Steels

This study concerns the effects of coiling temperature after hot rolling and the degree of reduction during cold rolling on formability‐related properties of high strength cold rolled dual phase (DP) steels. The effect of coiling temperature on the final structure and properties of two cold rolled and annealed DP‐steels is investigated. Further, the effect of cold rolling reduction and its impact on the final properties of the material is studied. Aspects of the impact of the different process parameters on the ferrite to austenite and austenite to martensite transformation are discussed based on results from production scale experiments, tensile testing and metallographic examinations of the materials.     

含铌冷轧DP780钢的连退工艺对组织及性能的影响

 通过成分工艺优化,在传统冷轧铁素体和马氏体双相钢DP780的显微组织上引入了一定体积分数的残余奥氏体,研究了冷轧退火工艺参数对双相钢DP780的显微组织和力学性能的影响。通过调整连续退火工艺来控制显微组织中一次铁素体、二次铁素体、马氏体、残余奥氏体的比例、尺寸、形貌、分布,同时获得了连退工艺参数-显微组织-力学性能的本质关系。结果表明,通过在传统冷轧铁素体和马氏体双相钢的组织上引入了体积分数为5%～7%的残余奥氏体,不仅可以获得[ReL/Rm≤0.5]的超低屈强比型冷轧DP780,也改善了成型性能。     

Anisotropy of Retained Austenite Stability during Transformation to Martensite in a TRIP‐Assisted Steel

Retained austenite as a key constituent in final microstructure plays an important role in TRansformation Induced Plasticity (TRIP) steels. The volume fraction, carbon concentration, size, and morphology of this phase are the well‐known parameters which effects on the rate of transformation of retained austenite to martensite and the properties of steel, are studied by many researchers. Of the transformation of retained austenite to martensite under strain in a TRIP steel is studied in this paper. The experimental results show that the transformation rate of retained, austenite with similar characteristics, to martensite in differently processed TRIP steel samples, exhibits an anisotropic behavior. This phenomenon implies a kind of variant selection of martensitic reaction of retained austenite under strain and is explained by ferrite texture developed in steel.     

Nb-Ti微碳深冲双相钢的连续冷却转变规律

摘要：通过连续冷却实验研究了Nb Ti微碳深冲双相钢在不同冷却速率下的显微组织变化规律。并结合显微组织、热膨胀曲线以及实验钢的硬度值绘制出实验钢的CCT曲线。结果表明,实验钢的CCT曲线由铁素体、珠光体与贝氏体区组成,其中铁素体和贝氏体的区域较大,覆盖冷却速度范围较广。实验冷却速率下未出现马氏体组织。在05～1℃/s的慢冷速下,组织由铁素体和珠光体组成;当冷速增加至3℃/s时,贝氏体开始出现,珠光体消失。当冷速在5～10℃/s范围内时,获得铁素体+贝氏体双相组织;当冷速大于10℃/s时,铁素体相变消失,此时为纯贝氏体转变。热处理过程中若想获得一定量的马氏体组织,退火温度宜设置在820～900℃双相区较低温度范围,使合金元素充分富集于少量奥氏体中,在随后冷却过程中此奥氏体转变为马氏体组织。     

硅质量分数对CSP生产DP600相变规律及工艺的影响

基于C Si Mn Cr Mo系600 MPa级热轧双相钢的组分,设计了不同硅质量分数(0.55%和1.17%)的两种试验钢。采用Gleeble 3500热模拟试验机测定了两种试验钢的连续冷却转变曲线,分析了硅质量分数对试验钢连续冷却过程中组织转变的影响,并研究了硅质量分数对短流程生产中温卷取型热轧双相钢生产工艺的影响。结果表明,相对于w(Si)=1.17%,w(Si)=0.55%使铁素体开始转变温度降低40~50 ℃,明显缩短了铁素体转变的孕育期,并增加了铁素体的体积分数。在CSP线上生产时,低硅钢的终轧温度可控制为820~830 ℃,低的终轧温度使铁素体相变时间增加2.2 s左右,铁素体转变量增加,且后续相变过程中可避免非马氏体组织的出现。因此,低硅钢适合在CSP短流程线上生产中温卷取型热轧双相钢。     

缓慢冷却工艺对高强度冷轧双相钢组织性能的影响

研究了缓慢冷却速度和缓慢冷却终止温度对C—MnCr系冷轧双相钢的显微组织和力学性能的影响。结果表明,随缓慢冷却速度的增加,试验钢的强度增加,伸长率则减小,屈强比变化不大,均在0．46左右。在不同缓慢冷却速度下,试验钢的显微组织均由铁索体和马氏体组成,随着缓慢冷却速度的增加,晶粒尺寸逐渐减小,马氏体体积分数明显增多。铁素...     

A method of producing very fine ferrite grains by means of hot forming

It is well known that refining the grain size of metals should lead to an improvement in the mechanical properties both of strength and of toughness. Therefore, a method has to be improved for the formation of very fine ferrite grains during hot rolling. By means of hot compression tests using an IF‐type steel and a HSLA steel the effect of chemical composition, deformation temperature, strain and strain rate on the resulting ferrite grain size has been investigated. By optimising the different process parameters during hot deformation, a ferrite grain of ASTM 15 can be produced. By means of the Hall‐Petch relationship an estimation of the influence of ferrite grain refinement on the mechanical properties can be given.     

CSP流程生产经济型热轧双相钢的工艺与组织性能

 为了在CSP产线上开发新一代经济型热轧双相钢,并确定生产的最佳成分和工艺,介绍了在武钢CSP生产线进行580MPa级热轧双相钢的工业化生产试制情况。分别采用C-Mn-Si系和C-Mn-Si-Cr系钢为原料,通过控制轧制和基于超强冷却设备的控制冷却工艺,成功开发出抗拉强度580MPa级热轧双相钢。通过比较分析2种成分钢的力学性能和微观组织,结果表明：经济型的C-Mn-Si系钢相对于C-Mn-Si-Cr系钢具有屈服强度低、屈强比小、伸长率大的特点,虽然马氏体量相对较少,但具有马氏体呈岛状更加均匀分布在铁素体晶界上等典型双相钢的特征,同时提出了生产过程中控制铁素体析出量和促进马氏体形成的具体措施。