中锰钢组织演变规律与相变诱导塑性行为

发布时间：2018-06-25 08:25

本文选题：中锰钢 + 组织演变　；参考：《北京科技大学》2015年博士论文

【摘要】：相变诱导塑性(transformation induced plasticity, TRIP)钢是利用亚稳态的残留奥氏体在应力应变作用下发生相变诱导塑性效应而研发的一种先进高强钢,具有优良的综合力学性能。传统TRIP钢是一种多相钢,其微观组织由铁素体、贝氏体和一定数量的残留奥氏体(5%-15%)组成。而当Mn元素含量适当增加后,淬透性提高,可得到由铁素体和残留奥氏体组成的两相组织,并且残留奥氏体的含量可以达到20%～30%。为了获得高强度和高塑性,不仅要控制各相所占的比例,得到较高的残留奥氏体体积分数,还必须控制各相的晶粒尺寸、形貌和分布,细化晶粒是相变诱导塑性钢组织控制的热点之一。在残留奥氏体增多及晶粒细化后,相变诱导塑性行为将表现出独有的特点,因此本文采用综合细化晶粒技术制备了超细晶中锰钢,通过定量拉伸实验深入分析了其相变诱导塑性行为,并利用ABAQUS有限元模拟软件建立了中锰钢变形过程的计算模型,研究了残留奥氏体的相变规律。结果表明：采用中锰钢合金成分体系,控制Mn元素含量为5%-7%,增加Mn元素含量能降低Ac1和Ac3温度,实现低温临界区退火。设计了预淬火+退火的两段式退火工艺,即在常规退火工艺前进行一次预淬火处理,随后进行两相区退火工艺,控制回复和再结晶过程获得超细晶组织,同时得到较多的残留奥氏体。对10Mn7钢(0.1C-7Mn-0.04Nb)退火后的晶粒尺寸进行统计发现铁素体基体的晶粒尺寸基本在1μm以下,而残留奥氏体的晶粒尺寸约0.5μm,残留奥氏体的体积分数最高可达40.29%。此工艺还能够明显缩短最优退火时间,提高实验钢的力学性能。10Mn7钢在625℃保温4h后即可达到最佳力学性能,抗拉强度为1177MPa,延伸率为30.92%,强塑积为36.39GPa·%。分析了退火过程中微观组织的演变规律,发现与常规的中锰钢退火工艺相比,加入预淬火工艺后能够明显消除实验钢微观组织中Mn元素的显微偏析,并在退火后得到两种形貌的残留奥氏体——长条状和块状。对退火过程中奥氏体的演变规律进行分析发现,第一阶段预淬火后获得马氏体组织,并在马氏体板条间有细小的残留奥氏体。在第二阶段低温临界区退火时,马氏体板条间细小的残留奥氏体将沿板条界长大,成为长条状奥氏体：同时碳化物沿马氏体板条界或原奥氏体晶界析出,成为块状奥氏体的核心。随后冷却过程中,由于奥氏体中C、Mn元素较多,两种形貌的奥氏体均被保留至室温。由FCC相和BCC相的反极图可发现,在同一原奥氏体晶粒内的长条状残留奥氏体具有相同的取向,并且与周围基体保持K-S关系。而在富碳区形成的块状残留奥氏体则与基体没有固定的位向关系。并且两种形貌的残留奥氏体的形成方式不同,导致其化学成分也存在差异,长条状残留奥氏体内的Mn元素含量高于块状残留奥氏体。研究了热轧后的钢板在加热和保温过程中碳化物的析出和分布规律,发现在500℃保温时碳化物尺寸最细小、分布最弥散,并且在500℃变形时,动态回复和再结晶等软化作用与加工硬化作用相当,变形抗力较低。因此优化了冷轧工艺,采用温轧工艺。首先将热轧酸洗后的钢板加热至500℃保温1h后进行轧制,每道次轧制后放入炉中保温5min再进下一道次的轧制,如此循环,总压下率为55%左右。采用温轧工艺后15Mn7钢(0.15C-7Mn-0.04Nb)的最终抗拉强度由1021MPa上升至1135MPa,延伸率由31.16%上升至35.30%,强塑积由31.81GPa·%上升至40.06GPa·%。并且采用温轧工艺后,实验钢的最终显微组织中长条状残留奥氏体所占比重上升,残留奥氏体的体积分数由39.86%增加至44.68%。对中锰钢的塑性变形规律进行了研究,发现10Mn7钢在不同温度退火后的工程应力-应变曲线分为两种情况：在580℃和600℃退火温度较低时,由于实验钢中的晶粒尺寸小且残留奥氏体的含量少,加工硬化作用较弱,塑性变形主要依靠Liiders应变。而退火温度升高至625℃和650℃后,钢中的残留奥氏体体积分数增加,变形时有较多的马氏体生成,加工硬化作用明显,瞬时n值较大。采用定量拉伸的方法研究了10Mn7钢625℃退火后实验钢内TRIP效应的发生过程和影响。发现在变形初期即有大量的残留奥氏体发生马氏体相变,转变机制可归结为应力诱导马氏体相变：当应变量在0.03～0.12之间时处于Luders变形阶段,应力值和残留奥氏体的体积分数基本保持不变,变形主要依靠铁素体基体的Luders应变;应变量大于0.12后,进入具有明显的加工硬化阶段,残留奥氏体的体积分数随应变量的增加逐渐减少。与常规退火工艺相比经预淬火+退火工艺后,残留奥氏体在变形时能够实现渐进式的转变,在应变量较大时仍能产生TRIP效应。采用TEM和EBSD技术对不同应变量变形后实验钢的显微组织进行观察,发现大角度晶界附近的残留奥氏体具有更高的稳定性,小角度晶界处的残留奥氏体首先发生转变：长条状残留奥氏体比块状残留奥氏体稳定性更强,块状奥氏体会优先发生相变；但由于马氏体相的生成对周围产生“保护”作用,同时马氏体相变时会引起体积膨胀使静水压力增强等原因会使块状残留奥氏体的中心区域稳定性增加。利用ABAQUS有限元软件对中锰钢的变形过程进行了模拟计算,发现在单向拉伸条件下马氏体相变首先发生在残留奥氏体内的尖角或两相交界处,并在新生马氏体周围出现应力集中。随着变形量的增加,相变和应力集中主要沿着拉伸方向逐渐扩展,较不容易发生在横向,应变量首先在铁素体晶粒内增大并与拉伸方向成45°。变形量较大时首先在个别新生马氏体相的尖角位置出现局部应变量过大,继续变形时此位置将成为微孔的形核区域。残留奥氏体的稳定性降低后屈服强度和抗拉强度变化不明显,但会降低材料的延伸率。无论是升高马氏体的强度,还是降低铁素体的强度,材料的塑性都大大降低。在双轴拉伸条件下相变发生早、扩展迅速,应力主要集中在新生马氏体相内,应变主要集中在软相铁素体内或与硬相的边界处。最终断裂时,在水平和垂直方向出现两条断裂带。
[Abstract]:The phase transition induced plasticity (transformation induced plasticity, TRIP) steel is a kind of advanced high strength steel developed by the residual austenite under the stress-strain effect of metastable austenite. It has excellent comprehensive mechanical properties. The traditional TRIP steel is a kind of multiphase steel, and its microstructure is composed of ferrite, bainite and certain. The amount of residual austenite (5%-15%) is made up, and when the content of Mn is increased properly, the hardenability is improved, and the two phase structure composed of ferrite and retained austenite can be obtained. The content of retained austenite can reach 20% ~ 30%. to obtain high strength and high plasticity, and not only control the proportion of each phase, but also obtain higher residual austenite. The volume fraction of the body must also be controlled by the grain size, morphology and distribution of each phase. Grain refinement is one of the hot spots in the microstructure control of phase transition induced plastic steel. After the increase of retained austenite and grain refinement, the ductile behavior of phase transition will show unique characteristics. Therefore, this paper uses a comprehensive grain refinement technique to prepare the manganese in ultrafine crystals. Steel, through the quantitative tensile test, the phase transition induced plastic behavior was analyzed, and the calculation model of the deformation process of middle manganese steel was established by using the ABAQUS finite element simulation software. The phase transformation law of the retained austenite was studied. The results showed that the phase transformation of the retained austenite was studied.
Using the alloy composition system of middle manganese steel, the content of Mn element is 5%-7%, the content of Mn element can be increased, the temperature of Ac1 and Ac3 can be reduced and the temperature of the critical region is annealed at low temperature. The two stage annealing process of pre quenching + annealing is designed, that is, a pre quenching treatment is carried out before the conventional annealing process, and the two phase annealing process is carried out after the annealing process, and the recovery and recrystallization are controlled. The grain size of 10Mn7 steel (0.1C-7Mn-0.04Nb) annealed at the same time was obtained. The grain size of the annealed 10Mn7 steel (0.1C-7Mn-0.04Nb) was calculated. The grain size of the ferrite matrix was below 1 u m, while the grain size of the retained austenite was about 0.5 mu, and the volume fraction of retained austenite was up to 40.29%.. The optimum annealing time is shortened and the mechanical properties of the experimental steel are improved. The best mechanical properties of.10Mn7 steel can be reached at 625 C for 4H. The tensile strength is 1177MPa, the elongation is 30.92%, and the strong plastic product is 36.39GPa%.
The evolution of microstructure in the annealing process is analyzed. It is found that compared with the conventional annealing process of medium manganese steel, the microsegregation of Mn elements in the microstructure of the experimental steel can be eliminated obviously after adding the pre quenching process. After annealing, the retained austenite, the long strip and the lump of two kinds of morphologies, is obtained after annealing. It is found that martensitic structure is obtained after the first stage of quenching, and there are small retained austenite between martensitic plates. In the second stage, the small retained austenite between martensitic plates will grow up and become long strip austenite during the second stage low temperature critical zone. The austenite grain boundary precipitates and becomes the core of the massive austenite. In the process of cooling, the austenite of two forms of austenite in the austenite is retained to the room temperature because of the more C and Mn elements in the austenite. It is found that the long retained austenite in the same original austenite grain has the same orientation and is preserved with the surrounding matrix. The bulk retained austenite formed in the rich carbon region has no fixed relationship with the matrix, and the formation of retained austenite in the two morphologies is different, which leads to the difference in the chemical composition, and the content of the Mn element in the oblong retained austenite is higher than that of the massive retained austenite.
The carbide precipitation and distribution of hot rolled steel plate during heating and heat preservation are studied. It is found that the size of carbide is the finest and the distribution is most dispersed at 500 C, and the softening effect of dynamic recovery and recrystallization is equal to the working hardening, and the deformation resistance is low at 500 C. So the cold rolling process is optimized. Hot rolling process. First, the hot rolled steel plate was heated to 500 C for 1H and then rolled. After each rolling, it was put into the furnace and then kept in the furnace for 5min and then into the next rolling. The total pressing rate was about 55%. The ultimate tensile strength of 15Mn7 steel (0.15C-7Mn-0.04Nb) was increased from 1021MPa to 1135MPa after the rolling process, and the elongation rate was 3. 1.16% up to 35.30%, the strong plastic product rises from 31.81GPa% to 40.06GPa%. And after the warm rolling process, the proportion of the retained austenite in the final microstructure of the experimental steel increases and the volume fraction of the retained austenite increases from 39.86% to 44.68%..
The plastic deformation law of medium manganese steel is studied. It is found that the stress strain curve of 10Mn7 steel is divided into two kinds of stress-strain curves at different temperatures. When the annealing temperature is low at 580 and 600, the grain size in the experimental steel is small and the residual austenite content is few, the work hardening is weak, and the plastic deformation depends mainly on the Liiders. After the annealing temperature rises to 625 and 650 C, the residual austenite volume fraction in steel increases, and more martensite is formed when the deformation is deformed, and the working hardening effect is obvious, and the instantaneous n value is larger.
The process and influence of TRIP effect in 10Mn7 steel annealed at 625 C were studied by quantitative tensile method. It was found that a large number of retained austenite occurred martensitic transformation at the early stage of deformation, and the transformation mechanism could be attributed to stress induced martensitic transformation: when the strain was between 0.03 and 0.12, the strain was in the phase of Luders deformation and stress. The volume fraction of the value and residual austenite remains basically the same, and the deformation depends mainly on the Luders strain of the ferrite matrix. After the strain is greater than 0.12, the entry has an obvious stage of processing hardening. The volume fraction of retained austenite decreases with the increase of the strain. The progressive transformation can be achieved when the body is deformed, and the TRIP effect can still be produced when the strain is large. The microstructure of the experimental steel after the deformation of different variables is observed by TEM and EBSD. It is found that the retained austenite near the large angle grain boundary is more stable, and the retained austenite at the small angle grain boundary is first turned. The long strip retained austenite is more stable than the massive retained austenite, and the block like austenite takes precedence of phase transition. However, the formation of martensite phase produces a "protection" effect on the surrounding area, while the martensitic phase change causes volume expansion to increase the static water pressure and other reasons for the stability of the central region of the massive retained austenite. Increase.
The deformation process of middle manganese steel is simulated by ABAQUS finite element software. It is found that the martensitic transformation occurs at the sharp angle or two phase boundary in the residual austenite under uniaxial tension, and the stress concentration around the new martensite. With the increase of the deformation amount, the phase transition and stress concentration are mainly along the tensile direction. Gradually expanding, it is not easy to occur in the transverse direction. The strain increases first in the ferrite grain and is 45 degrees in the direction of the tensile. First, the local strain is too large in the sharp angle position of the new martensite phase when the deformation amount is large, and this position will become the nucleation area of the micropore when the deformation is continued. The stability of the retained austenite is reduced. The change of strength and tensile strength is not obvious, but it can reduce the elongation of the material. Whether it is the strength of martensite or the strength of the ferrite, the plasticity of the material is greatly reduced. In the condition of biaxial tension, the phase transition occurs early, the expansion is rapid, the stress is mainly concentrated in the new martensite phase, and the strain is mainly concentrated in the soft ferrite. At the end of the fracture, there are two fracture zones in the horizontal and vertical directions.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

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