基于快速退火的超高强冷轧双相钢再结晶-相变-析出协调及局部成形性能控制机理

High strength dual phase (DP) steel is dominantly applied in automotive steels, which highly contributes to weight reduction, energy-saving and emission reduction. Rapid transformation annealing (RTA) technology has great potentials in improving the strength grade of automotive DP steels in virtue of its high capability of heating and cooling. However, RTA would lead to the formation of banded martensite for conventional hot rolled microstructure. In addition, the inhomogeneous strain distribution exists between soft ferrite and hard martensite. The banded martensite and the inhomogeneous strain lead to cracking during local forming. New concepts are proposed, which include the regulation of microstructure uniformity and ferrite strength through controlling hot rolled microstructure and precipitation, as well as retained austenite control through over-aging route design. On this basis, the synthetical controlling mechanisms of austenite nucleation homogeneity and ferrite recrystallization-precipitation-austenite transformation kinetics will be clarified. And the effect of austenite stability and over-aging route on the fraction and morphology of retained austenite will be explored. Furthermore, the effect of microstructure and retained austenite on strain distribution and fracture behavior will be revealed. Finally, the key theories and technologies of the integrated control of hot rolling, cold rolling, RTA and local formability are to be established. This project may enrich the research field of high performance automobile steels and broaden the microstructure control concept as well.

高强度双相钢在汽车用钢中占比最高，对实现汽车轻量化、推动汽车工业节能减排具有重要意义。快速退火技术以快速加热和快速冷却为核心，对提高双相钢强度效果显著。但研究发现，传统热轧组织冷轧之后快速退火形成带状马氏体，加之双相钢自身软硬相应变分配不均，其局部成形开裂问题突出。为此，本项目提出基于热轧组织和析出控制的组织均匀性和铁素体强度调控以及基于过时效路径设计的残余奥氏体调控新思路。阐明快速加热过程中奥氏体形核弥散化控制机理；弄清铁素体再结晶-析出-奥氏体相变耦合控制机理；探究奥氏体稳定性及过时效路径对残余奥氏体分数、形态等的影响规律；揭示局部成形过程中两相组织特征及残余奥氏体对应变分配和失效机制的影响机理；提出热轧-冷轧-快速退火-局部成形性能一体化控制关键理论与技术。本项目提出一种新的高强度双相钢成形性能调控方法，对于丰富高性能汽车用钢开发领域的研究内容、拓宽组织控制思路具有实际意义。

高强度双相钢在汽车用钢中占比最高，对实现汽车轻量化、推动汽车工业节能减排具有重要意义。RTA技术以快速加热和快速冷却为核心，对提高双相钢强度效果显著。本项目针对传统热轧初始组织下运用RTA技术形成带状马氏体，导致的局部成形开裂问题，将传统的连续退火工艺参数控制调控双相钢的组织性能视角前移，从热轧初始组织和析出行为控制入手开展研究。基于不同初始组织，模拟与组织表针相结合，阐明了加热过程中的铁素体再结晶、奥氏体形核及奥氏体相变动力学规律，揭示出快速加热条件下，铁素体再结晶与奥氏体相变的相互作用机理；分析不同初始组织对应的析出状态及后续加热过程中的析出行为，探究快速加热条件下的析出-铁素体再结晶-奥氏体相变耦合控制机理；通过元素（C/Mn）分配调控，结合热力学分析，弄清RTA条件下残余奥氏体及其稳定性控制机理；通过力学性能与局部成形性能评价、断裂行为分析，借助EBSD、纳米压痕等分析手段，阐明了两相组织特征与局部成形性能之间的定量关系表征出局部成形性能与组织特征之间的定量关系。提出了热轧-冷轧-快速退火-局部成形性能一体化控制关键理论与技术，打通了基于RTA制备高强度双相钢组织和局部成形性能控制的工艺路线，对于丰富高性能汽车用钢开发领域的研究内容、拓宽组织控制思路具有实际意义。

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