2 GPa中碳中锰纳米贝氏体钢的相变和塑性机理

 高碳(质量分数为0.78%~0.98%)高硅(质量分数约为1.5%)钢采用低温贝氏体转变(通常为150~250 ℃),可获得不小于2.0 GPa超高强度,但塑性较低(通常不大于8.0%);同时需要非常长的贝氏体相变时间(通常不小于4 d)。采用降低碳含量(Fe-0.30C-1.5Si-1.5Ni)的成分设计,可以显著加速贝氏体相变(300 ℃等温0.5 d),获得优良强度(抗拉强度(1 138±6) MPa)和塑性(伸长率为18.5%±1.5%)匹配的性能;但很难达到超高强度(1 500 MPa)级别。参考高/中碳贝氏体钢的合金设计、显微组织和力学性能特点,采用“中碳、以铝代硅、以锰代镍”的合金成分(Fe-0.30C-1.2Al-5.0Mn)体系,在M_s(马氏体开始转变温度)温度(300 ℃)附近进行贝氏体相变,可以获得强度为2.0 GPa级((2 029±9) MPa),伸长率超过10.0%(11.5%±1.0%)的高塑性纳米贝氏体钢,同时贝氏体相变时间适中(等温2 d),合金制造成本低廉(镍质量分数约为0.5%)。Fe-0.30C-1.2Al-5.0Mn钢具有超高强度主要是由于硬相组织贝氏体铁素体和马氏体总体积分数为85.1%,其中贝氏体铁素体板条宽度为(85±30) nm。具有较高塑性主要是由于软相组织残留奥氏体的体积分数为14.9%,碳质量分数为1.12%,位于贝氏体铁素体板条之间的薄膜状残留奥氏体尺寸为(30±15) nm;同时碳、锰元素能够增加残留奥氏体稳定性,特别是相对于低锰含量,5%中锰元素对残留奥氏体有更显著的稳定性作用,使其在低应力作用下不容易发生相变,但在高应力过程中持续发生TRIP效应以提高塑性。

Microstructural transformation and plasticity mechanism of 2 GPa medium-carbon medium-manganese nano-bainitic steel

High C(0.78%-0.98%)(mass percent) and high Si (about 1.5%) steels adopt low temperature bainite (usually 150-250 ℃) to obtain 2.0 GPa ultra-high strength,but with low plasticity (usually ≤8.0%).At the same time,a very long bainite transformation time (usually≥4 d) is required. The composition design of reducing the C content (Fe-0.30C-1.5Si-1.5Ni) can significantly accelerate the bainite transformation (isothermal at 300 ℃ for 0.5 d) and obtain excellent strength (tensile strength for (1 138±6)MPa) and plasticity (elongation for 18.5%±1.5 %) matching performance,but it is difficult to achieve ultra-high strength (≥1 500 MPa) level. Referring to the alloy design,microstructure and mechanical properties of high/medium C bainitic steel,the alloy composition of "medium C,replacing Si with Al,and replacing Ni with Mn"(Fe-0.30C-1.2Al-5.0Mn) was adopted,isothermal transformation near M_s temperature can obtain high-plastic nano-bainitic steel with strength of 2.0 GPa ((2 029±9) MPa) and elongation of more than 10.0 %,and the bainite transformation time is moderate (300 ℃isothermal for 2 days),and the fabrication cost of the alloy is low(about 0.5% Ni mass percent).The ultra-high strength of Fe-0.30C-1.2Al-5.0Mn steel is mainly to the total volume percent of hard-phase bainitic ferrite and martensit is 85.1%,which the width of bainitic-ferrite lath is (85±30) nm. The high plasticity is mainly to the volume percent of the soft-phase retained austenite of 14.9%,which the C mass percent of 1.12%,and the retained austenite film between in the bainitic-ferrite laths is (30±15)nm. At the same time,the pair of C and Mn elements can increase the stability of retained austenite. Especially compared with low Mn content,the 5% Mn element has a more obvious stabilization effect on retained austenite,making it more stable under low stress and the phase transformation is not prone to occur,but the TRIP effect persists to improve plasticity during high stress processes.