BN粒子的生长机理及BN新型易切削钢应用基础研究

发布时间：2018-06-28 20:55

本文选题：BN + 夹杂物　；参考：《北京科技大学》2017年博士论文

【摘要】：易切削钢是一类重要的特殊钢,被广泛应用于汽车工业,机械加工等领域。目前市场上的易切削钢主要是硫系和铅系易切削钢,这两种钢都含有对环境污染严重的易切削元素(S,Pb)。而且硫的使用造成钢材质量的下降。因此急需开发新型的高质量环境友好型易切削钢,提升钢材附加值,减少环境污染。本文利用热力学,动力学理论计算,结合实验室热态实验,真空感应炉实验,切削实验,机械性能测试实验及热塑性实验,同时借助一系列表征手段,对以下四个方面的内容进行了详细研究:钢中BN粒子形成机理研究;BN钢切削性能与机械性能研究;BN-S复合易切削钢夹杂物共存机理及复合易切削钢性能研究;BN易切削钢热塑性研究。钢中BN粒子形成机理研究表明:BN在钢液凝固末端1380℃～1430℃范围内大量快速析出。B,N在凝固前沿大量富集,局部浓度增加,达到临界浓度,BN匀质形核析出,钢液中存在成核点时,BN在其表面以异质形核的方式均匀析出,钢中的Al2O3,球形的MnS粒子可以作为BN的形核核心。基于扩散作用BN长大到2～3μm,后期以碰撞结合(自身碰撞或和其他粒子碰撞)的方式长大。根据实验结果和理论分析,提出了基于显微偏析和动力学的BN析出扩散长大模型,基于碰撞和凝聚作用的长大模型,模型计算结果与实际结果相符。BN钢切削性能与机械性能研究表明:BN可以改善钢的可加工性,控制BN粒子尺寸在2～4μm或12～14μm可达到优异·的切削加工性能改善效果。纳米探针压痕实验辅助揭示了BN改善钢的切削性能机理:BN破坏基体的连续性,充当钢基体内部缺口,引起应力集中以降低切削抗力;BN具有良好塑性变形能力,有助于刀具前面上的塑性流动,起到润滑作用;BN可以包裹钢中Al2O3,SiO2等超硬夹杂物质点,减轻了刀具磨损;BN易切削钢中含A1元素时,在刀具刀面上生成AlN覆盖膜,减轻刀具的扩散磨损。钢中的BN粒子对钢的强度等常温力学性能影响不大,但是对钢的冲击性能有所影响。BN-S复合易切削钢火杂物共存机理及复合从切削钢性能研究表明:BN与MnS析出温度区间相差不大,不会阻碍彼此的析出。BN+MnS复合易切削钢中,易切削粒子或单独存在或复合存在。复合存在时,BN,MnS相互碰撞或彼此包裹。BN可以显著改善钢的切削性,1000ppm S和115ppmB,240ppm N的复合切削效果,包括钻削和车削,皆优于2000ppm S的效果。BN+MnS复合易切削钢的屈服强度和抗拉强度高于MnS易切削钢,但是复合易切削钢的冲击性能却较MnS钢有所降低。BN易切削钢热塑性研究表明:BN易切削钢在实验温度700～1200℃范围内断面收缩率基本皆在60%以上,较C45对比钢有较大幅度的提高,BN易切削钢拉伸试样皆为韧性断裂,而C45对比钢主要为沿晶界脆性断裂。提出了BN易切削钢热塑性改善机制:由于B原子较小,可快速偏聚到晶界,抑制晶界滑移,从而提高热塑性;B优先偏聚于晶界,占据了原本作为TiN,(Fe,Mn)S等第二相形核核心的空位,抑制了第二相在晶界的析出;B偏聚于晶界,抑制了晶界铁素体的析出。热加工过程中冷速对BN易切削钢热塑性影响较大。20℃/s的冷速条件下,800～900℃温度范围内热塑性恶化严重。因此冷速的控制是实现BN易切削钢热塑性改善的关键。总之,通过对BN粒子的生长机理及BN新型易切削钢的系统性研究表明BN粒子作为易切削粒子生产BN系新型易切削钢可行,BN粒子对钢材切削性能改善作用明显,对钢材机械性能影响较小。因此,BN新型易切削钢潜在商业价值巨大,对提高钢材附加值,减少环境污染意义重大。
[Abstract]:Easy cutting steel is a kind of important special steel, which is widely used in the automotive industry, mechanical processing and other fields. At present, the most easy cutting steel in the market is mainly sulfur and lead based easy cutting steel. These two kinds of steel contain easy cutting elements (S, Pb) for serious environmental pollution. Moreover, the quality of steel is reduced by sulfur. Therefore, new type of steel is urgently needed to develop a new type of steel. The high quality and environment-friendly easy cutting steel improves the added value of steel and reduces environmental pollution. In this paper, we use thermodynamics, dynamics theory, laboratory thermal experiment, vacuum induction furnace experiment, cutting experiment, mechanical performance test and thermoplastic experiment, and use a series of characterization means to advance the contents of the following four aspects. The research on the formation mechanism of BN particles in steel, the research on the cutting properties and mechanical properties of BN steel, the coexistence mechanism of the inclusions in the BN-S composite free cutting steel and the study of the properties of the composite easy cutting steel, the study of the thermal plasticity of the BN easily machined steel. The study on the formation mechanism of BN particles in the steel shows that BN is fast in the range of 1380 C to 1430 C at the end of the solidification of molten steel. The precipitation of.B, N is enriched at the cutting edge, the local concentration increases, the critical concentration reaches the critical concentration, the BN homogeneous nucleation is precipitated, and when there is nucleation point in the molten steel, the BN is uniformly precipitated in the form of heterogeneous nucleation on the surface of the steel. The Al2O3 and spherical MnS particles in the steel can be used as the nucleation core of BN. The BN is grown to 2~3 u m based on the diffusion effect and the later collision binding ( According to the experimental results and theoretical analysis, the BN precipitation diffusion growth model based on microsegregation and dynamics is proposed. The model based on the effect of collision and condensation is based on the impact and condensation. The results of the model calculation agree with the actual results of the.BN steel cutting performance and mechanical properties. The results show that BN can be improved. The machinability of steel can be achieved by controlling the size of BN particles at 2~4 mu m or 12~14 u m. The nano probe indentation test helps to reveal the mechanism of BN to improve the cutting performance of steel: BN destroys the continuity of the matrix, acts as the internal gap in the steel matrix, causes the stress concentration to reduce the cutting resistance; BN has good plasticity. The ability of sexual deformation helps to lubricate the plastic flow on the front of the tool and plays a lubrication role. BN can wrap the particles of superhard inclusions, such as Al2O3 and SiO2 in steel, and reduce the tool wear. When the A1 element in the BN free cutting steel is contained, the AlN cover film is formed on the cutter surface to reduce the spread wear and tear of the tool. The mechanical properties of the steel in steel and the strength of the steel are also reduced. The impact properties of the steel can not be affected, but the impact properties of steel have some influence on the coexistence mechanism of the.BN-S composite free cutting steel and the properties of the composite from the cutting steel. It is shown that the difference of the precipitation temperature range between BN and MnS is small, and it will not hinder the precipitation of.BN+MnS composite free cutting steel. MnS collisions or each other wrapping.BN can significantly improve the cutting properties of steel, 1000ppm S and 115ppmB, 240ppm N compound cutting effects, including drilling and turning, are superior to 2000ppm S, the yield strength and tensile strength of.BN+MnS composite Machinable steel are higher than that of MnS Machinable steel, but the impact properties of the composite easy cutting steel are more than that of MnS steel. The study on the thermal plasticity of.BN free cutting steel shows that the section shrinkage rate of the BN easily machined steel is basically above 60% in the range of 700~1200 C, compared with the C45 contrast steel, the tensile specimens of the BN easy cutting steel are all ductile fracture, and the C45 contrast steel is mainly brittle fracture along the grain boundary. The thermal plasticity of the easy cutting steel of BN is put forward. Improvement mechanism: because B atom is small, it can quickly get to grain boundary, inhibit grain boundary slip, inhibit grain boundary slip and improve thermal plasticity; B preferentially segregates at grain boundary, occupies the original space of the two phase nuclear core, which is originally as TiN, (Fe, Mn) S and inhibits the precipitation of the second phase in grain boundary; B biased at grain boundary, restraining the precipitation of grain boundary ferrite. The effect of speed on the thermal plasticity of BN easy cutting steel is larger at the cold speed of.20 C /s. The thermal plastic deterioration in the temperature range of 800~900 C is serious. Therefore, the control of the cold speed is the key to the improvement of the thermal plasticity of the BN easy cutting steel. In a word, the mechanism of the growth of BN particles and the systematic study of the BN new easy cutting steel show that the BN particles are the easy cutting particles. The production of BN new type of easy cutting steel is feasible, and the effect of BN particles on the cutting properties of steel is obvious, and the mechanical properties of steel are less affected. Therefore, the potential commercial value of BN new type of easy cutting steel is huge, which is of great significance to increase the added value of steel and reduce environmental pollution.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG142.1

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