【作者】 张寒；

【导师】 白秉哲；

【作者基本信息】 清华大学 ， 材料科学与工程， 2011， 博士

【摘要】 本论文以Mn-Si-Cr系超高强钢为对象，系统研究了该系超高强钢的超塑性变形行为﹑性能优化方案及工程应用实例。在研究超塑性变形行为时，首先通过小试样变形模拟得到应变速率敏感性系数（m值），而后按照所设计的变形工艺路线利用小型轧机进行轧制获得变形后板材，从板材上取板式拉伸样进行拉伸实验，得到延伸率，再结合m值和延伸率来评价Mn-Si-Cr系超高强钢的超塑性变形能力。在进行小试样变形模拟时，由于温变形过程中存在组织演变，首次提出了“双m值”测定方法，即在温变形初期和后期分别利用速率突变过程（Backofen法）来测定温变形初期和后期的m值，进而分析组织演变对超塑性变形能力的影响。论文成功利用Mn-Si-Cr系中高碳钢设计了三种获得等轴细晶（铁素体+球状碳化物）复相组织的工艺，并深入研究了各工艺过程及超塑性变形过程中的组织演变。研究表明，适当的Mn-Si-Cr合金化和工艺过程可有效提高超高强钢的超塑性变形能力。Mn-Si-Cr系超高碳钢在1023K时延伸率可达900%。在大量实验的基础上，论文还首次提出了“超塑性潜能”的概念，并解释了该“潜能”在Mn-Si-Cr系超高强钢的超塑性变形中的实现机制。研究还发现，马氏体温变形过程中在较小的应变量下（真应变0.7）可形成晶粒尺寸的动态平衡，动态平衡时稳定的第二相可抑制基体晶粒长大。在不同的变形温度下可形成有利于超塑性的不同形态的动态平衡组织，如等轴细晶的（铁素体+球状碳化物）复相组织和（奥氏体+铁素体）复相组织。其中，（奥氏体+铁素体）复相组织中的奥氏体是形变诱导产生的，在超塑性变形后冷却过程中可以转变为马氏体，从而保证了高强度。由此我们为设计较为简易的超塑性变形工艺路线提供了思路。通过Mn-Si-Cr合金化，显著提高了超高强钢的淬透性，油淬生产的直径为170mm的1600MPa级液压支架铰接轴表面和心部的硬度差仅为HRC5。研究发现，Mn-Si-Cr系超高强钢中的粒状组织不具有回火脆性，这在很大程度上有利于简化回火工艺，同时保证了较佳的强韧性配合。此外，结合V微合金化开发了屈服强度高于1200MPa的Mn-Si-Cr系超高强钢筋，屈强比低，使用安全。更多还原

【Abstract】 The paper studied the superplastic behavior﹑performance optimization andengineering applications of Mn-Si-Cr series ultrahigh strength steels systematically.The superplasticity of Mn-Si-Cr series ultrahigh strength steels was evaluated bythe strain rate sensitivity index (i.e. m value) and elongation obtained through smallspecimen thermomechanical testing and plate-shape tensile testing respectively. In orderto study the effect of microstructure evolution on superplastic characteristics duringsmall specimen thermomechanical testing, a “double m value” method was firstly putforward by introducing the strain rate change method at the initial and later stages ofwarm deformation.The paper successfully developed three technologies to obtain equiaxed and fine(ferrite+spherical carbides) duplex microstructures in Mn-Si-Cr series medium andhigh carbon steels. The microstructure evolution during the technologies andsuperplastic deformation was intensively studied. It was found that appropriateMn-Si-Cr alloying and technology improved the superplasticity of ultrahigh strengthsteels effectively. The Mn-Si-Cr series ultrahigh carbon steel could exhibit superplasticcharacteristics and high elongations (~900%at1023K). Based on repeated lab tests, wepresented a new concept of “superplastic potential” and explained the realizationmechanism of the “potential” during superplastic deformation of Mn-Si-Cr seriesultrahigh strength steels. Study also showed that a dynamic equilibrium of grain sizecould be reached at a low strain (~0.7) during warm deformation of martensite and thestable second phase in this equilibrium suppressed the growth of matrix grains. Duringwarm deformation at different temperatures, the dynamic equilibrium microstructureswith different morphologies (e.g. equiaxed and fine (ferrite+spherical carbides) duplexmicrostructures and (austenite+ferrite) duplex microstructures) were obtained. Theaustenite in the (austenite+ferrite) duplex microstructures was formed bystrain-induced transformation and could transform to martensite during cooling afterwarm deformation. As a result, an ultrahigh strength was ensured. Based on this, wepresented an idea for designing relatively simple superplastic deformation schemes.Through Mn-Si-Cr alloying, the hardenability of the designed ultrahigh strength steels was greatly improved. The hardness difference between the surface and center of1600MPa oil-quenched articulated shafts of hydraulic support in a diameter of170mmis only HRC5. The granular structure of Mn-Si-Cr series ultrahigh strength steels doesnot exhibit temper brittleness. Therefore, the tempering used for the performanceoptimization can be simplified to a great extent. In addition, combined with Vmicroalloying, Mn-Si-Cr series ultrahigh strength finished rolled rebars with relativelylow yield ratio were developed.更多还原