钛微合金化钢中铁素体相变及纳米相析出行为与机理研究

发布时间：2018-05-31 09:29

本文选题：钛微合金钢 + 铁素体相变　；参考：《东北大学》2015年博士论文

【摘要】：我国经济、社会的快速增长带动了钢铁行业的高速发展,同时也对钢铁材料的品种和质量等提出了更高的要求,新一代钢铁材料向着高强度、高韧性、低成本、减量化的方向发展。特别是随着人们对环境污染和矿产资源枯竭等问题的重视,微合金高强度钢的开发和研究受到了越来越多的关注,细晶强化和析出强化作为改善钢材综合力学性能的主要途径已经广泛的应用在微合金高强度钢的开发和生产中,因此,如何充分发挥微合金元素在细化晶粒和析出强化方面的作用成为控轧控冷工艺开发的一个关键问题。在中央高校基本科研业务费项目研究生科研创新项目(N110607006)的经费支持下,本文以添加Ti、Nb、Mo等元素的微合金钢为研究对象,利用热模拟实验系统研究了奥氏体高温变形行为、连续冷却相变行为、铁素体相变及纳米析出行为,丰富了钛微合金钢研究的一些基础理论,讨论了析出形式对铁素体相微观力学性能的影响,同时利用热轧实验研究了控轧控冷工艺参数对钛微合金钢力学性能和析出行为的影响,为现场工业生产提供了理论基础。本文的主要工作和研究成果如下： (1)通过单道次压缩实验研究了钛微合金钢奥氏体高温变形行为,分析了微合金元素以及变形参数对奥氏体动态再结晶的影响,同时建立了实验钢的变形抗力模型。结果表明,Ti、Nb、Mo的加入能够提高奥氏体的流变应力和热变形激活能,抑制奥氏体的动态再结晶,计算得到C-Mn、Ti、Ti-Nb、Ti-Nb-Mo实验钢的奥氏体热变形激活能分别为351kJ/mol、458kJ/mol、483kJ/mol、497kJ/mol。 (2)通过连续冷却相变实验研究了奥氏体在连续冷却中的相变行为,绘制了实验钢的CCT曲线,分析了微合金元素、冷却速率以及变形在奥氏体相变过程中的作用。结果表明,Ti、Nb、Mo的加入能够提高奥氏体在连续冷却过程中的稳定性,抑制铁素体和珠光体相变,促进贝氏体相变；变形降低了奥氏体的稳定性,提高了相变开始温度,促进了铁素体相变,使铁素体相变的C曲线向左移动。 (3)利用热模拟技术系统研究了微合金元素、等温时间、卷取前冷却速率、卷取温度、卷取后冷却速率以及变形对钛微合金钢铁素体相变和纳米析出行为的影响。研究结果表明,在卷取温度为640℃时,Ti、Nb、Mo等元素的加入能细化铁素体晶粒,同时在铁素体中形成大量的纳米析出相,显著提高铁素体的硬度；640℃等温时,随着等温时间的增加,铁素体的尺寸和体积分数逐渐增加,铁素体中相间析出的面间距增大；随着卷取前冷却速率的增大,铁素体的晶粒尺寸和铁素体中纳米析出的粒子尺寸减小,铁素体的显微硬度逐渐增大；随着卷取温度的降低,实验钢的组织从铁素体+珠光体向贝氏体转变,铁素体的晶粒尺寸逐渐减小,基体的显微硬度先升高后降低,卷取温度为640℃基体的显微硬度最大,在卷取温度为640℃和700℃时,铁素体中发现了排列规则的相间析出,卷取温度越高,相间析出的粒子尺寸和面间距越大；640℃卷取后,随着冷却速率的增大,实验钢的显微组织从铁素体+珠光体向贝氏体转变,实验钢基体的显微硬度呈现降低的趋势,卷取后采用较小的冷却速率同样能获得尺寸细小的铁素体,而且能够促进微合金碳氮化物在铁素体中的析出；随着变形程度的增加,铁素体的显微硬度先升高后降低,变形为22%时铁素体的显微硬度最大,变形的增加使相间析出的面间距增大。 (4)利用纳米压痕实验研究了铁素体中析出对其微观力学性能的影响。结果表明,相同工艺下,C-Mn实验钢和Ti-Nb实验钢铁素体的纳米硬度分别为2.64GPa和4.19GPa,铁素体中纳米析出相将铁素体的纳米硬度提高了1.55GPa；卷取温度为600℃、640℃和700℃时铁素体的纳米硬度分别为3.90GPa、4.19GPa和3.60GPa。存在相间析出的铁素体晶粒的载荷-深度曲线在压入的初始阶段会出现一个平台,平台的长度与相间析出面间距的大小有关。不同卷取温度下铁素体晶粒内部纳米硬度的变化规律不同,600℃时铁素体晶界附近纳米硬度最大,晶粒内部纳米硬度变化不大；640℃时铁素体晶粒内部的纳米硬度基本不变；700℃时由于先形核的析出的消失和粗化,纳米硬度随离晶界距离的增加逐渐降低。 (5)控轧控冷工艺研究结果表明,随着终轧温度的升高,抗拉强度和屈服强度先升高后降低,840℃终轧时实验钢的强度最高；卷取温度从460℃升高到675℃时,强度先降低后升高,在675℃卷取时,由于析出强化作用的加强,具有很好的综合力学性能；冷却速率的增大能够同时提高细晶强化和析出强化作用从而提高实验钢的强度：Mo的加入细化了铁素体晶粒和纳米析出粒子,从而将屈服强度提高了25-35MPa；与卷取后石棉冷却相比采用保温+炉冷工艺屈服强度提高了94MPa,抗拉强度提高了54MPa。在国内某钢厂成功试制了不同厚度规格的600MPa和700MPa级别微合金高强度钢,具有较好的综合力学性能,其中析出强化作用可以超过300MPa。
[Abstract]:The rapid growth of China's economy and society has led to the rapid development of the iron and steel industry. At the same time, it has also raised higher requirements for the variety and quality of steel materials. The new generation of steel materials is developing towards the direction of high strength, high toughness, low cost and reduction, especially with the attention of people to environmental pollution and the depletion of mineral resources. More and more attention has been paid to the development and research of microalloy high strength steel. As the main way to improve the mechanical properties of steel, the main way to improve the comprehensive mechanical properties of steel has been widely used in the development and production of microalloy high strength steel. Therefore, how to make full use of the microalloying elements in refining grain and precipitation strengthening is made. With the support of the graduate scientific research and innovation project (N110607006) for the basic scientific research business fee project of the Central University, this paper takes the microalloy steel adding Ti, Nb and Mo as the research object, and studies the high temperature deformation behavior of austenite and continuous cooling phase by the thermal simulation experiment system. Change behavior, ferrite transformation and nanometer precipitation have enriched the basic theory of titanium microalloyed steel and discussed the influence of precipitation form on the micromechanical properties of ferrite phase. At the same time, the influence of controlled rolling and controlled cooling process parameters on the mechanical energy and precipitation behavior of titanium microalloy steel was studied by hot rolling experiment. The main work and research results in this paper are as follows:
(1) the high temperature deformation behavior of austenite in titanium microalloy steel was studied by single channel compression test. The influence of Microalloy Elements and deformation parameters on the dynamic recrystallization of austenite was analyzed, and the deformation resistance model of the experimental steel was established. The results showed that the addition of Ti, Nb and Mo could increase the rheological and thermal deformation activation energy of austenite, and suppress the activation energy of the austenite. The dynamic recrystallization of austenite is made and the activation energy of the austenite thermal deformation of C-Mn, Ti, Ti-Nb, Ti-Nb-Mo experimental steels is 351kJ/mol, 458kJ/mol, 483kJ/mol, 497kJ/mol., respectively.
(2) the phase transition behavior of austenite in continuous cooling was studied by continuous cooling phase transformation experiment. The CCT curve of experimental steel was plotted. The effect of Microalloy Elements, cooling rate and deformation on the phase transformation of austenite was analyzed. The results showed that the addition of Ti, Nb and Mo could improve the stability of austenite during continuous cooling and restrain iron. The phase transition of the prime body and pearlite promotes the bainite phase transformation, and the deformation reduces the stability of the austenite, improves the starting temperature of the phase transition, promotes the ferrite transformation, and moves the C curve of the ferrite transformation to the left.
(3) the effects of microalloying elements, isothermal time, cooling rate before coiling, coiling temperature, cooling rate and deformation on the phase transition and nano precipitation of titanium microalloy were investigated by thermal simulation. The results showed that the addition of Ti, Nb, Mo and other elements could refine ferrite grain when the coiling temperature was 640. At the same time, with the increase of isothermal time, the size and volume fraction of ferrite gradually increased with the increase of isothermal time, and the interphase separation between ferrite in ferrite increased. With the increase of cooling rate before coiling, the grain size of ferrite and the content of ferrite in ferrite increased with the increase of cooling rate before coiling. With the decrease of the particle size and the microhardness of the ferrite, the microstructure of the experimental steel is changed from ferrite and pearlite to bainite with the decrease of coiling temperature. The grain size of the ferrite decreases gradually. The microhardness of the matrix increases first and then decreases. The microhardness of the matrix is maximum at 640 C, and the coiling temperature is at the coiling temperature. At 640 C and 700 C, the arrangement rules are found in the ferrite. The higher the coiling temperature is, the larger the particle size and the surface spacing are. The microstructure of the experimental steel turns from ferrite + pearlite to bainite with the increase of cooling rate at 640 C, and the microhardness of the experimental steel matrix decreases. After coiling, small size of ferrite can be obtained with smaller cooling rate, and it can promote the precipitation of microalloy carbonitride in ferrite. With the increase of the deformation degree, the microhardness of ferrite first increases and then decreases, and the microhardness of ferrite is the largest when the deformation is 22%, and the increase of deformation makes the interphase precipitated between each other. The distance increases.
(4) the effect of precipitation on micromechanical properties of ferrite in ferrite was studied by nano indentation test. The results showed that the nano hardness of C-Mn experimental steel and Ti-Nb experimental steel was 2.64GPa and 4.19GPa under the same process. The nano precipitation of ferrite in ferrite increased the nanoscale hardness of ferrite by 1.55GPa, and the coiling temperature was 600, 640. The nanoscale hardness of ferrite at 700 C is 3.90GPa, and the load depth curve of ferrite grain that precipitates between 4.19GPa and 3.60GPa. will appear a platform in the initial stage of pressure entry. The length of the platform is related to the size of the interphase gap between the phases. The change law of the hardness of the ferrite grain inside the ferrite grain under different coiling temperatures At 600, the nanoscale hardness in the ferrite grain boundary is the largest and the nano hardness in the grain is not changed. At 640 C, the nano hardness in the ferrite grain is basically unchanged, and the nano hardness decreases gradually with the increase of the distance from the grain boundary at 700 C.
(5) the research results of controlled rolling and controlled cooling process show that the tensile strength and yield strength increase first and then decrease with the increase of final rolling temperature. The strength of the experimental steel is the highest at the end of 840 degrees centigrade. When the coiling temperature rises from 460 to 675 C, the strength decreases first and then increases. At 675 centigrade, it has good comprehensive strength due to the strengthening of precipitation strengthening. The increase of cooling rate can increase the strength of the fine grain and precipitate to increase the strength of the experimental steel at the same time. The addition of Mo has refined the ferrite grain and the nano precipitated particles, thus increasing the yield strength by 25-35MPa, and increasing the yield strength of 94MPa by using the yield strength of the asbestos cooling process after the coiling. The tensile strength is improved by 54MPa. in a steel plant in China, which has successfully developed 600MPa and 700MPa grade microalloy high strength steel with different thickness specifications. It has better comprehensive mechanical properties, and the precipitation strengthening effect can exceed 300MPa..
【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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