高强塑性中锰钢的组织演变及力学性能的研究
发布时间：2018-09-11 12:04
【摘要】：轻质高强汽车用钢因其兼具出色的强度和塑性,且能满足节能环保的要求而成为研究热点。强塑积(抗拉强度与总伸长率的乘积)是衡量汽车用钢综合性能的指标。传统的汽车用钢的强塑积为15±10GPa%,代表性钢种有无间隙原子(Interstitial Free, IF)钢和双相钢等,为第一代汽车用钢；而以奥氏体钢为代表的第二代汽车用钢的强塑积为60±10GPa%。这两代汽车用钢在工业使用上都存在限制,如第一代钢强塑积过低,而第二代钢太昂贵。近年来,很多关于中锰钢(Mn含量为4～12%)的研究中发现其强塑积为30GPa%,是理想的第三代汽车用钢。 本文主要研究Mn含量为8%和11%的TRIP钢的组织演变和力学性能。研究表明TRIP钢的力学性能取决于奥氏体的含量和稳定性。优化的淬火+回火的工艺使实验钢获得了大量的奥氏体,从而保证了拉伸过程中发生明显的TRIP效应。此外,本文还研究了奥氏体稳定性的影响因素,如晶粒尺寸、形貌、化学成分等。本文获得的实验结果归纳如下： (1)热轧态实验钢在750～800℃淬火并回火后获得的力学性能优于或与其他冷轧低合金TRIP钢和中锰钢相当,但本实验钢未经冷轧工序且需要的热处理时间较短。8Mn热轧钢能获得810～1000MPa的抗拉强度和32～39%的伸长率；11Mn热轧钢能获得880～1100MPa的抗拉强度和34-40%的伸长率；11Mn-Nb热轧钢能获得960～1160MPa的抗拉强度和28-40%的伸长率。 (2)通过比较热轧实验钢在800℃淬火并回火和未回火试样的拉伸性能可知,回火可以显著地提高实验钢的塑性,主要是由于在回火过程中6铁素体的碳原子向临近的奥氏体扩散,提高了奥氏体的稳定性,从而表现出更好的伸长率。对于在850～900℃淬火后的试样,回火显著提高塑性,降低强度,这主要是因为回火马氏体的生成使内应力降低。 (3)冷轧实验钢经过淬火后均能获得优秀的力学性能。8Mn冷轧钢在730℃C淬火后能获得873MPa的抗拉强度和57%的伸长率；11Mn冷轧钢在750℃淬火后能获得998MPa的抗拉强度和67%的伸长率；11Mn-Nb冷轧钢在750℃C淬火后能获得979MPa的抗拉强度和63%的伸长率。11Mn钢和11lMn-Nb的综合力学性能是目前所报道的中锰钢中最好的。 (4)通过研究11Mn热轧实验钢拉伸过程中的变形行为首次观察到不连续TRIP效应,并阐明了其产生的主要原因：第一,马氏体相变产生体积膨胀导致6铁素体和临界铁素体的变形,最终引起局部应力松弛和转移；第二,奥氏体具有不同等级的稳定性使得只有达到某一临界应力时TRIP效应才能发生。而且,研究发现临界铁素体的分割使奥氏体由块状变成不同厚度和长度的薄膜状,因而具有不同等级的稳定性。 (5)通过对11Mn热轧实验钢拉伸前后的组织进行EBSD分析可知,奥氏体的晶粒取向在一定程度的影响奥氏体的稳定性,具有大施密特因子的晶粒能优先发生相变；但是,施密特因子并不是决定奥氏体稳定性的决定性因素,形貌对奥氏体稳定性的影响更大。 (6)通过研究不同温度淬火后的拉伸试样的应变硬化行为发现,拉伸过程中铁素体的优先变形能有效地推迟TRIP效应的发生,使奥氏体在较大的应变下发生TRIP效应,从而使实验钢获得优秀的伸长率。此外,实验获得的应变硬化行为与Crussard-Jaoul (C-J)分析的结果相一致。 (7)通过对1lMn冷轧实验钢的研究表明,其应变硬化中第三阶段的锯齿波动行为主要是由于不连续TRIP效应的作用。而不连续TRIP效应主要是由于锰元素的不均匀分布使得奥氏体具有不同等级的稳定性。研究发现,延长热处理时间可以使得锰元素分布更加均匀,但是提高热处理的温度的作用刚好相反。晶粒尺寸是影响冷轧实验钢奥氏体稳定性最主要的因素。
[Abstract]:Light-weight and high-strength automotive steel has become a research hotspot because of its excellent strength and plasticity, and can meet the requirements of energy-saving and environmental protection. Strength-plasticity product (product of tensile strength and total elongation) is an index to measure the comprehensive performance of automotive steel. Free, IF steel and dual-phase steel are the first generation automotive steel, while the second generation automotive steel represented by austenitic steel has a Strength-plasticity product of 60 6550 It is found that its strong plastic product is 30GPa%, which is the ideal third generation automobile steel.
The microstructure evolution and mechanical properties of TRIP steel with Mn content of 8% and 11% were studied in this paper. The results show that the mechanical properties of TRIP steel depend on the content and stability of austenite. The optimized quenching and tempering process makes the experimental steel obtain a large amount of austenite, which ensures the TRIP effect in the tensile process. The factors affecting the stability of austenite, such as grain size, morphology and chemical composition, were investigated.
(1) The mechanical properties of hot-rolled as-cast steel quenched and tempered at 750-800 C are superior to or equal to those of other cold-rolled low alloy TRIP steel and medium manganese steel, but the experimental steel has not been cold-rolled and needs shorter heat treatment time. The tensile strength of 810-1000 MPa and the elongation of 32-39% can be obtained by hot-rolled 11Mn steel. Tensile strength of 80-1100 MPa and elongation of 34-40%; 11Mn-Nb hot-rolled steel can obtain 960-1160 MPa tensile strength and 28-40% elongation.
(2) By comparing the tensile properties of the hot-rolled steel quenched and tempered at 800 C with that of the non-tempered steel, it can be seen that tempering can remarkably improve the plasticity of the test steel, mainly because the carbon atoms of 6 ferrite diffuse to the adjacent austenite during tempering, thus improving the stability of the austenite and thus showing better elongation. Tempering significantly increases plasticity and decreases strength of quenched specimens at 0-900 C, mainly because the formation of tempered martensite reduces internal stress.
(3) Cold-rolled steel can obtain excellent mechanical properties after quenching. 8Mn cold-rolled steel can obtain 873 MPa tensile strength and 57% elongation after quenching at 730 C; 11Mn cold-rolled steel can obtain 998 MPa tensile strength and 67% elongation after quenching at 750 C; 11Mn-Nb cold-rolled steel can obtain 979 MPa tensile strength and 97 9 MPa tensile strength after quenching at 750 C. The mechanical properties of.11Mn steel and 11lMn-Nb at 63% elongation are the best among reported medium manganese steels.
(4) Discontinuous TRIP effect was observed for the first time by studying the deformation behavior of 11Mn hot-rolled experimental steel during tensile process, and its main causes were clarified. Firstly, volume expansion of martensitic transformation resulted in the deformation of 6 ferrite and critical ferrite, which eventually led to local stress relaxation and transfer. Secondly, austenite had different grades. The TRIP effect can only occur when a critical stress is reached, and it is found that the separation of critical ferrite makes the austenite change from lump to thin film with different thickness and length, thus resulting in different levels of stability.
(5) EBSD analysis of the microstructure of 11Mn hot-rolled experimental steel before and after tensile shows that the grain orientation of austenite affects the stability of austenite to a certain extent, and the grain with large Schmidt factor can have preferred phase transformation; however, Schmidt factor is not the decisive factor determining the stability of austenite, and the morphology of austenite is stable. Qualitative impact is even greater.
(6) By studying the strain hardening behavior of the tensile specimens quenched at different temperatures, it is found that the preferential deformation of ferrite can effectively postpone the TRIP effect and make the austenite produce the TRIP effect under larger strain, so that the experimental steel can obtain excellent elongation. The results of d-Jaoul (C-J) analysis are consistent.
(7) The study of 1Mn cold-rolled steel shows that the sawtooth fluctuation in the third stage of strain hardening is mainly due to discontinuous TRIP effect. The discontinuous TRIP effect is mainly due to the uneven distribution of manganese elements, which makes the austenite have different stability grades. The distribution of manganese is more uniform, but the effect of increasing the heat treatment temperature is just the opposite. Grain size is the most important factor affecting the Austenite Stability of cold-rolled experimental steel.
【学位授予单位】：东北大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1
本文编号：2236642
[Abstract]:Light-weight and high-strength automotive steel has become a research hotspot because of its excellent strength and plasticity, and can meet the requirements of energy-saving and environmental protection. Strength-plasticity product (product of tensile strength and total elongation) is an index to measure the comprehensive performance of automotive steel. Free, IF steel and dual-phase steel are the first generation automotive steel, while the second generation automotive steel represented by austenitic steel has a Strength-plasticity product of 60 6550 It is found that its strong plastic product is 30GPa%, which is the ideal third generation automobile steel.
The microstructure evolution and mechanical properties of TRIP steel with Mn content of 8% and 11% were studied in this paper. The results show that the mechanical properties of TRIP steel depend on the content and stability of austenite. The optimized quenching and tempering process makes the experimental steel obtain a large amount of austenite, which ensures the TRIP effect in the tensile process. The factors affecting the stability of austenite, such as grain size, morphology and chemical composition, were investigated.
(1) The mechanical properties of hot-rolled as-cast steel quenched and tempered at 750-800 C are superior to or equal to those of other cold-rolled low alloy TRIP steel and medium manganese steel, but the experimental steel has not been cold-rolled and needs shorter heat treatment time. The tensile strength of 810-1000 MPa and the elongation of 32-39% can be obtained by hot-rolled 11Mn steel. Tensile strength of 80-1100 MPa and elongation of 34-40%; 11Mn-Nb hot-rolled steel can obtain 960-1160 MPa tensile strength and 28-40% elongation.
(2) By comparing the tensile properties of the hot-rolled steel quenched and tempered at 800 C with that of the non-tempered steel, it can be seen that tempering can remarkably improve the plasticity of the test steel, mainly because the carbon atoms of 6 ferrite diffuse to the adjacent austenite during tempering, thus improving the stability of the austenite and thus showing better elongation. Tempering significantly increases plasticity and decreases strength of quenched specimens at 0-900 C, mainly because the formation of tempered martensite reduces internal stress.
(3) Cold-rolled steel can obtain excellent mechanical properties after quenching. 8Mn cold-rolled steel can obtain 873 MPa tensile strength and 57% elongation after quenching at 730 C; 11Mn cold-rolled steel can obtain 998 MPa tensile strength and 67% elongation after quenching at 750 C; 11Mn-Nb cold-rolled steel can obtain 979 MPa tensile strength and 97 9 MPa tensile strength after quenching at 750 C. The mechanical properties of.11Mn steel and 11lMn-Nb at 63% elongation are the best among reported medium manganese steels.
(4) Discontinuous TRIP effect was observed for the first time by studying the deformation behavior of 11Mn hot-rolled experimental steel during tensile process, and its main causes were clarified. Firstly, volume expansion of martensitic transformation resulted in the deformation of 6 ferrite and critical ferrite, which eventually led to local stress relaxation and transfer. Secondly, austenite had different grades. The TRIP effect can only occur when a critical stress is reached, and it is found that the separation of critical ferrite makes the austenite change from lump to thin film with different thickness and length, thus resulting in different levels of stability.
(5) EBSD analysis of the microstructure of 11Mn hot-rolled experimental steel before and after tensile shows that the grain orientation of austenite affects the stability of austenite to a certain extent, and the grain with large Schmidt factor can have preferred phase transformation; however, Schmidt factor is not the decisive factor determining the stability of austenite, and the morphology of austenite is stable. Qualitative impact is even greater.
(6) By studying the strain hardening behavior of the tensile specimens quenched at different temperatures, it is found that the preferential deformation of ferrite can effectively postpone the TRIP effect and make the austenite produce the TRIP effect under larger strain, so that the experimental steel can obtain excellent elongation. The results of d-Jaoul (C-J) analysis are consistent.
(7) The study of 1Mn cold-rolled steel shows that the sawtooth fluctuation in the third stage of strain hardening is mainly due to discontinuous TRIP effect. The discontinuous TRIP effect is mainly due to the uneven distribution of manganese elements, which makes the austenite have different stability grades. The distribution of manganese is more uniform, but the effect of increasing the heat treatment temperature is just the opposite. Grain size is the most important factor affecting the Austenite Stability of cold-rolled experimental steel.
【学位授予单位】：东北大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1
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