基于铁素体温变形微碳深冲双相钢晶内剪切带形成机理与织构调控

Since traditional deep drawing steels with single phase matrix of ferrite cannot satisfy the requirement of higher strength for advanced high strength steel (AHSS), multi-phase strengthening has been proved to be an effective and feasible solution. To produce AHSS with good combination between strength, elongation and deep-drawing ability, textural modification in complex phase microstructure has always been a major technical bottleneck. This program intends to design a class of Cr-Nb micro-alloying dual phase steels, combine with warm deformation of ferrite and inter-critical annealing process in view that coordinately control recrystallization texture of <111>//ND based on in-grain shear band and precipitation. A deep drawable dual phase steel with ferrite matrix and martensite distributed grain boundaries can be obtained. Meanwhile, the mechanism of interstitial atoms and alloy element extend dynamic strain aging will also be investigated. In this program, the formation mechanism of ferrite in-grain shear bands will be discussed, and effects of interstitial atoms and alloys on microstructures and textures. After analyze the interaction effects between precipitates, texture and shear bands, the mechanism of composite strengthening for micro-carbon steels could be clear and effective textural modification technology would also be developed. Based on the above works, it is hopeful to achieve a further expands of AHSS in outer cover parts and accelerate the “light-weight” progress of automobile.

传统铁素体基深冲钢已不能满足先进高强钢（AHSS）的更高强度需求，复合强化是获得高强塑积的有效措施，然而，基于复相组织下深冲织构的控制一直以来是AHSS优化深冲性能的技术瓶颈。本项目拟通过Cr、Nb复合微合金化，利用铁素体低温变形结合两相区退火工艺，基于铁素体晶内剪切带和第二相粒子析出协同调控<111>//ND再结晶织构，实现在铁素体基体加马氏体双相组织特征下的深冲性能优化。通过研究间隙原子与合金元素延缓动态应变时效行为的作用机制，揭示铁素体晶内剪切带的形成机理，明确第二相-织构-剪切带间的交互作用过程，阐明微碳钢复合强化机理与织构调控思想，进一步扩大AHSS在汽车外覆盖件上使用比例，有效推进汽车轻量化进程。

本项目研究了Cr-Nb系和Cr-Ti-B系两种微碳深冲钢，重点探讨了两种钢的动态应变时效行为，分析了温变形工艺对剪切带和织构的影响，研究了第二相-织构-剪切带间的交互作用，揭示了微碳钢温变形过程中深冲织构优化的最佳工艺，以及热处理过程中的织构演变过程，主要研究内容和结果有：（1）Cr-Nb系微碳钢动态应变时效（DSA）机制是含Nb的纳米级析出相粒子与高密度可动位错的交互作用，且DSA效应会阻碍变形过程中晶内剪切变形带的形成，从而不利于退火再结晶过程中γ纤维织构的产生；（2）Cr-Ti-B系更适合采用温变形工艺获得剪切带，且在300-450℃变形时会因溶质原子与可动位错的交互作用以及析出相钉扎位错而产生DSA；（3）Cr的添加会降低锯齿的临界激活能，延缓DSA产生温度区间，高含量Cr元素的添加抑制了在350 ℃轧制温度下的γ织构和α织构，促进了在450 ℃、550 ℃下γ织构，而该轧制温度下α织构变化不明显；（4）Cr-Ti-B系热轧板由铁素体加珠光体组成，450℃温轧时形变γ织构最强，剪切带含量达到57%，二次冷轧后，剪切带含量增加了7%，基体中固溶C原子含量降低；（5）Cr-Ti-B系微碳钢在两相区快速加热退火时，部分碳化物的回溶促进马氏体的形成，840℃为最佳退火温度；（6）Cr-Ti-B系微碳钢在快速加热中可分为四个阶段：640℃前为回复期， 640-740℃再结晶阶段；740-780℃为晶粒长大期，780℃开始发生相变。再结晶初期以定向形核机制为主导，{111}取向晶粒首先在晶界及剪切带处发生再结晶，再结晶后期主要为大角度晶界迁移，且后期的强高斯织构来源于回复期在晶界附近的高斯织构，并将逐渐吞噬变形组织中{111}<112>。项目研究将为微碳深冲钢中马氏体相的引入以及铁素体基体织构的强化提供理论依据，为抑制微碳钢温变形过程中的DSA行为，拓宽温变形在微碳钢中应用提供实践参考。