C-Mn钢中渗碳体纳米级析出的调控技术与形成机理研究

Cementite in steel is a kind of economical and common second phase constituent for strengthening. If cementites can be effectively refined to the scale of a few nanometers, it can also generate very strong precipitation strengthening effects. Therefore, cementite is viewed as a viable option to replace micro-alloying precipitates for green strategy in steels, in order to meet the requirements of reduction in alloy cost and maintain strength. However, according to the kinetic theory of precipitation, Ostwald ripening rate of cementite is larger than microalloy carbonitrides over 2.5 to 4 orders of magnitude, thus it is difficult for cementite to achieve the nanometer scale, with easily coarsening..This research provides an exciting new prospect and processing in obtaining nanoscale cementites. The ultra fast cooling (UFC) technology developed by RAL lab, with the high cooling capacity and high precision control, will be applied after the hot rolling experiments for the research of cementite precipitation and formation mechanism in low and medium carbon steels (0.04%-0.5%C). .Given that the cementites in carbon steels are usually to form lamellar pearlite structure in the near-equilibrium conditions, rather than to form nanoscale particles precipitation, the non-equilibrium precipitation of cementites can be considered by increasing the cooling rate after hot rolling. The high-energy state after rolling is kept to the dynamic phase transformation point during ultra fast cooling, leading to the increase of degree of austenite undercooling and transformation driving force. Finally, the precipitation nanoscale cementite (average size of 20-50nm) will be realized in C-Mn steels by the control of rolling temperature, cooling rate and final cooling temperature. .The Optimized processing for nanoscale cementite precipitation can be obtained, and microstructural evolution and formation mechanism of nanoscale cementite in a non- equilibrium state will be clarified, with the establishment of precipitation models. The impact of research is far reaching because of the need to pioneer a new frontier of high strength-low cost carbon steel for engineering.

渗碳体是钢中最为经济的强化相。有效细化渗碳体尺寸，实现纳米级渗碳体的析出强化作用符合钢铁材料高强度、低成本和易循环的绿色化发展要求。然而，根据析出动力学理论，渗碳体Ostwald熟化速率比微合金碳氮化物大2.5-4个数量级，极易粗化长大，难以达到纳米级尺度范围。.本项目应用极限冷却路径控制形变奥氏体相变温度和相变速率，通过加速相界面移动获得渗碳体在非平衡状态下不连续生长，在碳锰钢中得到纳米级渗碳体，实现弥散析出强化。通过分析超快速冷却过程中纳米渗碳体的析出条件及其关键影响因素，明确纳米渗碳体析出尺寸（20-50nm）、数量、分布等特征规律及其对强度的贡献作用，获得纳米渗碳体控制的最优化工艺。同时研究碳扩散和界面移动速率的竞争机制及其与冷却条件的协同关系，阐明纳米渗碳体在非平衡态下的演变规律和形成机理，并建立纳米渗碳体的析出模型。本研究为开发低成本、高强度的先进碳锰钢提供理论依据和技术原型。

渗碳体是钢中最为主要和最为经济的强化相。通过工艺控制将渗碳体有效地细化到纳米级尺寸，充分发挥了纳米渗碳体的析出强化作用，符合钢铁材料高强度、低成本和易循环的绿色化发展要求。.本项目利用超快速冷却工艺，提高过冷奥氏体相变驱动力加速相界面移动，促使渗碳体在非平衡状态下不连续生长，在无微合金添加的条件下，最终实现了纳米级渗碳体（平均尺寸20-50nm）在C-Mn钢组织中均匀弥散析出。通过分析超快速冷却过程中纳米渗碳体的析出条件及关键影响因素，明确了纳米渗碳体析出尺寸、数量、分布等特征规律及其对强度的贡献作用，获得了超快速冷却和形变热处理条件下纳米渗碳体析出的优化工艺，总结了钢中碳含量和不同冷却工艺参数对组织性能的影响规律，掌握了过冷奥氏体分解的相变规律，研究了碳扩散过程和界面移动速率的竞争机制，在计算相变驱动力和界面前沿碳浓度的基础上，从热力学角度阐明了非平衡条件下纳米渗碳体的形成机理，并建立了析出模型，为钢中渗碳体的控制理论提供实验依据，进一步丰富和完善C-Mn钢的强化理论，并将研究成果推广到工业化实践，圆满完成本项目的各项任务。.相关研究成果在MSEA、JMST、Steel Res. Int.和材料研究学报等国内外重要学术期刊发表高水平学术论文16篇，其中SCI收录6篇，EI收录3篇，授权国家发明专利2项，参编相关著作2部，协助培养研究生7名，获得冶金科学技术奖三等奖1项，获河北冶金科学技术奖一等奖1项，研究成果作为创新点之一，获得国家科技进步二等奖1项。项目所形成的纳米渗碳体析出相控制理论与技术，推广应用于国内南钢、宝武韶钢等钢铁企业，实现了普碳钢的减量化生产和产品升级，促进了钢铁企业“资源节约、节能减排”绿色化发展。