1500MPa级直接淬火马氏体钢的组织控制与强化机理研究

【作者】 赵英利；

【导师】 谢刚； 时捷；

【作者基本信息】 昆明理工大学 ， 冶金物理化学， 2010， 博士

【摘要】 直接淬火工艺具有缩短生产流程、降低成本、节约能源等优点。目前国内外采用直接淬火技术生产钢板的抗拉强度一般在490-980MPa,对1500MPa级直接淬火马氏体钢的研究还较少。随着钢铁材料朝高强度方向发展,有必要对超高强度直接淬火马氏体钢的生产工艺及其强化机理进行深入的研究。本文以中碳微合金钢为研究对象,采用直接淬火-低温回火工艺,系统研究了钢坯再加热制度、热变形工艺、冷却工艺对直接淬火钢微观组织与力学性能的影响,分析了直接淬火马氏体钢的晶体学形态,讨论了直接淬火马氏体钢的强化机理。基于直接淬火-回火-重新奥氏体化工艺,研究了α→γ逆相变再结晶的晶粒细化机制,在此基础上,探索了利用纳米级析出相控制得到细晶奥氏体的工艺。为了确保合金元素能够较充分的固溶,同时组织中又不出现明显的粗大晶粒,实验钢的加热温度不宜高于1150℃,保温时间不宜超过1h。建立了实验钢在加热过程中的奥氏体晶粒长大动力学模型。通过研究实验钢的热变形行为,为实际生产提供控轧工艺参数及理论依据。计算得到了动态再结晶热变形激活能为477.7 kJ/mol,静态再结晶激活能为299.3KJ/mol,建立了动态再结晶本构方程及静态再结晶动力学方程；根据绘制的动态再结晶加工状态图,发现在变形量较小及应变速率较大的情况下,完全动态再结晶很难发生；奥氏体未再结晶温度为900℃；碳化物在奥氏体区的等温析出动力学曲线(PTT)呈典型的C型,随着应变速率的增大,产生的位错储能逐渐增大,使析出曲线向左平移,析出孕育期缩短,但不改变鼻尖温度。采用电子背散射技术(EBSD),对直接淬火马氏体钢的晶体学形态进行了分析。直接淬火马氏体的微观结构依次由原奥氏体晶粒、板条束(Packet)、板条块(Block)和板条(Lath)组成。板条束为原奥氏体晶粒内具有相同惯习面的马氏体板条晶区,板条块为板条束内大角度晶界所包围的区域,由具有相似晶体学取向的板条组成。随着未再结晶区变形量的增大,直接淬火马氏体钢的原奥氏体晶粒由等轴晶粒变成扁平化的晶粒,板条束(Packet)尺寸与原奥氏体晶粒的扁平宽度相等,板条块(Block)宽度逐渐减小：当未再结晶区变形量占总变形量的比例由10.5%增大到100%,原奥氏体晶粒尺寸由12.4μm减小到4.4μm,板条块宽度从2.5μm减小到1.3μm。研究发现直接淬火马氏体与母相奥氏体符合K-S关系,与传统再加热淬火马氏体相比,热变形后直接淬火得到的马氏体并未改变其取向分布。实验室轧制结果表明,与传统的再加热淬火-回火钢(RQT)相比,再结晶区轧制直接淬火-回火钢(RCR-DQT)、再结晶区-未再结晶区两阶段轧制直接淬火-回火钢(RCR-CR-DQT)的冲击韧性与之相当,横向抗拉强度分别提高了9.4%、14.6%,达到1570MPa、1645MPa。研究表明直接淬火钢较再加热淬火钢强度提高的主要机制是位错强化,是由于直接淬火马氏体继承了热变形过程中产生的大量晶体缺陷,导致位错密度增大的缘故。对RCR-DQT、RCR-CR-DQT两种直接淬火-回火钢,随未再结晶区变形量的增大,原奥氏体晶粒逐渐细化,板条块(Block)宽度逐渐减小,强度逐渐提高,屈服强度与原奥氏体晶粒尺寸、板条块宽度的-1/2次方都存在线性关系,说明细晶强化是强度提高的主要机制,板条块宽度为控制强度的“有效晶粒尺寸”。传统再加热淬火-回火钢的横纵向性能差异不明显,而直接淬火-回火钢由于继承了热轧时的织构,导致横纵向性能差异较大。在工业生产线上,热轧后分别采用间断式直接淬火工艺(IDQ)与直接淬火工艺(DQ),结果发现直接淬火工艺(DQ)较间断式直接淬火(IDQ)工艺具有更好的强韧性配比。通过精确控制钢坯加热制度、轧制工艺及冷却工艺参数,得到的直接淬火钢板板形良好。基于直接淬火-回火-重新奥氏体化工艺,通过降低热变形温度、增大变形量、增大轧后冷却速率、缩短回火时间可以增大逆转变奥氏体的形核率,从而细化奥氏体晶粒。在此基础上,提出了利用纳米级析出相细化奥氏体晶粒的新思路,总结了Ti在生产流程中的演变规律。经实验室轧制及后续热处理,得到的奥氏体晶粒尺寸约为5μm,晶粒细化效果明显,证明设计思路是科学可行的,具有较好的工业化应用前景。更多还原

【Abstract】 The direct quenching process shorts the production step, reduces cost, save energy and so on. Tensile strength of steel plate produced by directed quenching technology is usually 490-980MPa at home and abroad but few studies were carried out on the 1500MPa level steel through direct quenching. With development of material toward ultra-high strength, it is necessary to study the production process and strengthening mechanism of direct quenching martensite steel aiming at development ultrahigh strength direct quenching martensite steel.In this paper, the effects of reheating technique, hot deformation, cooling process on the micro structure and mechanical properties of direct quenched steel were carried out quantitatively on medium carbon microalloyed steel through direct quenching and low temperature tempering process. The crystallography of direct quenched martensitic steel was analyzied, and the strengthening mechanism of the direct quenching martensitic steel was discussed. Grain refinement mechanism of austenite throughα→γreverse phase transformation was studied by means of direct quenching, tempering and reaustenizing. The austenite grain refinement was studied through nano-precipitates to retard the boundary movement during reaustenization process.It is found that reheating temperature should be lower than 1150℃and holding time shorter than lh to obtain finer austenite grain for experimental Nb microalloyed steel. The kinetic equations of austenite grain growth in soaking process for tested steels are constructed.The thermal deformation behaviors were studied to provide controlled rolling parameters and theoretic understanding. The dynamic recrystallization activation energy and the static recrystallization activation energy are about 477.7 kJ/mol and 299.3KJ/mol, respectively. The kinetic equations of dynamic recrystallization and static recrystallization for tested steels were built, and the dynamic recrystallization process state diagram was obtained. According to the dynamic recrystallization process state diagram, it was found that the large deformation and low strain rate is essential for the full dynamic recrystallization. The strain induced carbide precipitation curves (PTT curves) have also been determined assuming the typical "C" typed shape. The precipitation process is obviously accelerated with strain rate increases without changing the nose-tip temperature.The microstructure of direct quenched martensite steel has a multi-scale structure including prior austenite grain, packet, block and lath structure. Packet is the group of laths with the same habit plane in a prior austenite grain and each packet is further subdivided into blocks which have high angle boundaries (the group of laths with a similar orientation). The equiaxed grains of direct quenched martensite steels become into pancake grains with increasing the amount of deformation in the non-recrystallization region, the packet size is equal to the thickness of the pancaked austenite grain and the block size is reduced due to deformation structure. The average austenite grain size decreased from 12.4μm to 4.4μm and the average block width decreased from 2.5μm to 1.3μm, when the fraction of the amount of deformation in the non-recrystallization increased from 10.4% to 100%. The results showed that the K-S orientation relationship between austenite and martensite is observed in direct quenched steels by electron backscattering diffraction (EBSD).Comparing with conventional reheat quenching-tempering (RQT), the tensile strengths of RCR-DQT and RCR-CR-DQT steels increases by 9.4% and 14.6%, whereas the toughness is almost same. Analysis on the relationship between microstructure and mechanical properties revealed that dislocation strengthening is the major mechanism to account for the strength increment of DQT steels comparing with the RQT steels, because the dislocation density in DQT steels was higher than that of RQT steels due to the inheritance of deformed substructures of austenite through transformation. The strength increases with increasing the amount of deformation in the non-recrystallization region can be attributed to the finer block, which was found to be the effective grain size controlling the strength. The linear relationship between yield strength and the minus square-roots of prior austenite grain size and block size, suggesting that grain refinement is the main reason to strength enhancement and the block width is the effective size controlling strength. Anisotropy of the DQT steels were obvious than that of RQT due to the texture inheritance from deformed austenite.In the industrial production line, compared with the intermittent direct quenching (IDQ), direct quenching (DQ) has better combination of strength and toughness. Direct quenching steel plates have good plate shape by means of precise control of reheating temperature, rolling and cooling process parameters.Based on the direct quenching-tempering-reaustenitizing process, by means of lowering hot deformation temperature, increasing the amount of deformation, increasing the cooling rate after hot deformation and shorting the temper time, the nucleation rate of austenite reverse transformation was increases and in turn the austenite grain size was significantly refined. For further refinement of austenite, the idea of grain refinement by nanosized precipitates to suppress the austenite grain growth was introduced and the effects of Titanium on microstructure evolution and mechanical properties was summarized and analyzed. The Lab controlled rolling and following heat treatment result in a clear refined microstructure with austenite grain size about 5μm. It was turned out to be true that austenite grain refinement is feasible by controlled rolling and nanosized precipitation, both of which would assume good prospects for industrial applications.更多还原