硼的固溶和析出行为对低碳钢性能影响的研究

发布时间：2018-03-18 16:04

本文选题：硼　切入点：BN　出处：《北京科技大学》2015年博士论文　论文类型：学位论文

【摘要】：目前,硼对低碳钢高温力学性能及低碳钢晶粒尺寸影响的观点尚不统一。本文计算了低碳钢中微量硼在凝固前沿硼的偏析、BN析出的热力学和动力学及冷速对BN长大的影响、分析了硼在不同合金元素匹配下对高温力学性能的影响机理、冷速对含硼钢高温力学性能的影响及硼对低碳钢相变规律和强化机制的影响,探讨了低碳钢中硼的存在形式及其对钢性能的影响机理,为含硼钢的开发提供了理论基础。本研究为扩大合金元素硼在钢铁行业中的应用,稳定并提高含硼钢性能提供了坚实的理论基础。首先,通过热力学计算分析了凝固前沿B的偏析、BN的生成及冷速对凝固前沿BN长大的影响,并通过实验对理论计算结果进行了验证。研究表明,在不考虑溶质偏析的情况下,凝固前沿BN无法生成。在考虑凝固偏析的情况下,凝固前沿硼的偏析十分明显,BN生成的限制性环节为N的扩散。冷速对凝固前沿B、N的偏析没有影响。BN尺寸随着冷速降低粗化的原因为凝固时间相对延长。采用高温热拉伸实验测定了不同硼含量低碳钢和低碳Nb、Ti钢的高温力学性能。通过对钢中析出物、断口形貌及组织的分析,讨论了硼对低碳钢和低碳Nb、Ti钢高温力学性能的影响机理。结果表明,钢中固溶硼能够促进动态再结晶、抑制沿晶铁素体的析出,对改善铸坯高温塑性有利。低碳钢中硼固N的作用,减少了细小氮化物的生成量,促进了动态再结晶,提高了铸坯的高温塑性；低碳Nb、Ti微合金钢中硼固N的作用没有降低钢中细小(Ti,Nb)(C,N)的析出,对改善高温塑性不起作用。通过对冷速和复合加入Nb、Ti对低碳含硼钢高温塑性影响的研究发现,提高冷速和复合加入Nb、Ti会导致低碳含硼钢的高温塑性下降。采用热模拟实验测定了不同硼含量低碳钢的连续冷却转变曲线以及不同轧钢工艺参数对低碳含硼钢组织的影响,分析了硼对低碳钢相变规律的影响和轧钢工艺对组织的影响。通过模拟轧制实验研究了硼对铁素体、珠光体型低碳钢板强韧化机制的影响。结果表明,铁素体和珠光体型低碳含硼钢中,硼固定钢中N,降低了细小氮化物生成所需的N含量,钢材的强度和韧性同时下降；当硼含量大于与钢中N平衡的硼含量时,继续提高硼含量,固溶硼促进了动态再结晶,晶粒细化提高了钢材的强度和韧性。提高冷速和终轧变形量、降低终轧温度和终冷温度均有利于组织细化。提高冷速和降低终冷温度,钢中组织由铁素体和珠光体向贝氏体和马氏体转变。
[Abstract]:At present, the effect of boron on the mechanical properties of low carbon steel at high temperature and grain size of low carbon steel is not uniform. In this paper, the thermodynamics and kinetics of boron segregation in low carbon steel at the solidification front and the effect of cooling rate on BN growth are calculated. The influence mechanism of boron on high temperature mechanical properties under different alloying element matching, the effect of cooling rate on high temperature mechanical properties of boron steel and the effect of boron on phase transformation and strengthening mechanism of low carbon steel were analyzed. The existence form of boron in low carbon steel and its influence mechanism on the properties of steel are discussed, which provides a theoretical basis for the development of boron steel. This study is intended to expand the application of alloying element boron in iron and steel industry. Stabilizing and improving the properties of boron steel provides a solid theoretical basis. Firstly, the formation of segregation BN of solidification front B and the effect of cooling rate on the growth of solidification frontier BN are analyzed by thermodynamic calculation. The theoretical calculation results are verified by experiments. The results show that the segregation of solute is not considered. Solidification frontier BN cannot be generated. When solidification segregation is considered, Boron segregation at solidification front is obviously restricted by N diffusion. The reason why the size of BN is coarsened with the decrease of cooling rate is the relative prolongation of solidification time. The high temperature mechanical properties of low carbon steel with different boron content and low carbon NB Ti steel were measured by high temperature hot tensile test. The precipitates, fractography and microstructure of the steel were analyzed. The effect of boron on the high temperature mechanical properties of low carbon steel and low carbon NB Ti steel is discussed. The results show that the solid solution boron in the steel can promote dynamic recrystallization and inhibit the precipitation of intercrystalline ferrite. The effect of boron fixation in low carbon steel reduces the amount of fine nitride, promotes dynamic recrystallization and improves the high temperature plasticity of billet. The effect of boron fixation on nitrogen fixation in low carbon NbsTi microalloyed steel does not reduce the precipitation of the fine TiNbNbCN in the steel, but does not contribute to the improvement of the high temperature plasticity. It is found that the effect of cooling rate and composite addition of NB + Ti on the high temperature ductility of low carbon boron steel is studied. The high temperature plasticity of low carbon boron steel can be decreased by increasing cooling rate and adding NB Ti. The continuous cooling transformation curve of low carbon steel with different boron content and the effect of different rolling process parameters on the microstructure of low carbon boron steel were measured by thermal simulation experiment. The effect of boron on the transformation law of low carbon steel and the effect of rolling process on microstructure were analyzed. The effect of boron on the strengthening and toughening mechanism of ferrite and pearlite low carbon steel plate was studied by simulated rolling experiment. In ferrite and pearlite type low carbon boron steel, the N content of boron fixed steel decreases the N content required for the formation of fine nitrides, and the strength and toughness of steel decrease simultaneously, and when the boron content is larger than the boron content in equilibrium with N in the steel, the boron content continues to increase. Solid solution boron promotes dynamic recrystallization, grain refinement improves the strength and toughness of steel, increasing cold rate and final rolling deformation, decreasing finish rolling temperature and final cooling temperature are beneficial to microstructure refinement, increasing cooling rate and decreasing final cooling temperature, The microstructure of steel is transformed from ferrite and pearlite to bainite and martensite.
【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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