易切削非调质钢中硫化物生成行为与均匀化控制研究

发布时间：2018-01-18 06:34

  本文关键词：易切削非调质钢中硫化物生成行为与均匀化控制研究　出处：《北京科技大学》2015年博士论文　论文类型：学位论文

  更多相关文章： 易切削钢 非调质钢 硫化物 形成机理 热力学计算

【摘要】：易切削非调质钢是硫系易切削钢的一种,由于具有优良的切削性能与力学性能,已在国外汽车零部件上得到广泛应用,并且随国产汽车行业的快速发展具有良好的国内应用前景。硫在非调质钢中主要以硫化物的形式存在,对比分析表明,国外钢中硫化物往往尺寸控制合理、分布均匀,而国产非调质钢中硫化物往往为大尺寸细长条形且群聚分布,导致力学性能与切削性能均较差,不能达到国内用户对高品质汽车用非调质钢的要求,因此尚需大量进口国外非调质钢。为提高我国非调质钢中硫化物控制水平,本课题系统地研究了成分设计、凝固过程、热加工变形及均质化处理对硫化物生成行为与形貌变化的影响。 利用热力学计算得到凝固过程中随Mn、S在液相中的富集,大部分MnS于平衡凝固末端析出,析出量占总量的83%。初始析出温度随C、Si、Al含量增加而降低,随Mn、S含量增加而升高。Fe-Mn-S三元系相图计算表明,MnS的析出类型主要包括共晶反应与偏晶反应。 通过控制非调质钢凝固冷却速率与钢中A1含量,研究了不同种类MnS的形成机理。非调质钢中第1类MnS为偏晶反应生成,较快冷速下与基体分离生长；第1I类MnS为共晶反应析出,与基体合作生长且聚集分布；块状第1II类MnS则表现为离异共晶形貌,分布较为均匀。冷速为0.24℃/s、酸溶铝含量为0.044%时可以促进大量均匀分布的第1II类MnS生成,有利于改善钢中硫化物的分布。200℃/s的较高冷速会促进第1I类MnS生成,不利于改善铸态非调质钢中MnS的分布。 热力学计算结果表明S是影响MnS析出行为的重要元素,针对国产非调质钢中S含量控制波动较大的现状,研究了S含量变化对硫化物析出的影响。在试验研究范围内(0.025%-0.065%),随S含量增加,第1II类MnS的比例下降,MnS析出温度提高、数量增多、尺寸增大且沿晶聚集分布的现象加剧。单独MnS以及MnS-V(C, N)复合硫化物均可作为晶内铁素体的有效形核核心,细化非调质钢铸态组织。 铸坯中各类MnS在热加工过程中容易发生伸长,拉伸原位观察表明这会导致钢材产生各向异性。与纵向拉伸相比,横向拉伸时钢中长条状MnS的受力情况决定了其与基体更易发生分离,分离处将成为裂纹的起始源,并促使裂纹沿MnS扩展,从而危害非调质钢横向性能。因此,需要采取合理的后续加工工艺以减小硫化物的尺寸并使其分布均匀化。 通过铸态试样高温压缩试验研究了变形参数对硫化物相对塑性与分布的影响。整体上,低应变速率(0.01s-1)、大变形量(80%)下基体容易发生动态再结晶。此变形条件下MnS的相对塑性较小,尤其在变形温度1050℃和1250℃下进行热加工可以使MnS发生充分碎化而减小尺寸,同时碎化后MnS的分布得到一定的改善,有利于提高非调质钢力学性能。 通过对热加工后非调质钢进行等温均质化处理,可以进一步改善钢中硫化物的尺寸与分布。随加热与保温时间的增加,非调质钢中长条状MnS的尖端首先发生圆化,整体形貌逐渐向圆柱形发展,随后部分位置发生径向的收缩,导致MnS最终断裂碎化,生成的较小尺寸MnS会进一步发生球化。计算表明MnS长大与碎化的控制因素为S元素在基体中的扩散。在1250℃和1050℃下分别保温3h和5h进行均质化处理可使非调质钢中大量的MnS发生碎化,降低了硫化物的尺寸并改善了硫化物的分布。
[Abstract]:Easy cutting non quenched and tempered steel is a kind of sulfur free cutting steel, due to its cutting performance and excellent mechanical properties, has been widely used in auto parts, and with the rapid development of the domestic automobile industry has a good application prospect in domestic. The sulfur non quenched and tempered steel is mainly in the form of sulfide, contrast the analysis shows that foreign steel sulfide often size reasonable control, uniform distribution, while the domestic non quenched and tempered steel sulfide is often large size and slender bar cluster distribution, which leads to the mechanical properties and cutting performance are poor, can not reach the domestic users of non quenched and tempered steel demand for high-quality cars, so still need to import large quantities of foreign non quenched and tempered steel. In order to improve the control level of nonsulfide steel, this paper studies the composition design, solidification process, thermal deformation and homogenization of sulfur compounds generated The effect of change in behavior and morphology.
The calculated solidification process with Mn using thermodynamics, accumulation of S in liquid phase, most of the MnS in the balance at the end of solidification precipitation, precipitation accounted for 83%. of the total initial precipitation temperature with C, Si, Al content increased, with Mn, S content increased three yuan,.Fe-Mn-S phase diagram calculation indicates that precipitation type MnS mainly includes the reaction of eutectic reaction and partial crystal.
By controlling the content of A1 in steel and steel solidification cooling rate of non quenched and tempered, forming mechanism of different types of MnS. First kinds of MnS non quenched and tempered steel for monotectic reaction, fast cooling speed and separation of matrix growth; class 1I MnS eutectic reaction precipitation and matrix cooperation growth and aggregation distribution; block the 1II MnS showed the divorced eutectic morphology, distribution is more uniform. The cooling rate is 0.24 DEG /s, the aluminum content of 0.044% soluble acid can promote the 1II MnS generated a large number of uniform distribution, is conducive to the improvement of the distribution of sulphide in steel.200 C /s high cooling rate will promote the formation of MnS 1I that is not conducive to improve the casting distribution of non quenched and tempered steel MnS.
The thermodynamic calculation results show that S is an important element affecting the MnS precipitation, the volatile content of the domestic situation of S control in non quenched and tempered steel, the effect of S content on sulfide precipitation. In the range of experimental study (0.025%-0.065%), with the increase of S content, the 1II MnS decreased the proportion of MnS precipitation temperature increase the number, size, and distribution of intergranular aggregation phenomenon intensified. MnS and MnS-V (C, N) composite sulfide can be used as the nucleation of intragranular ferrite, cast microstructure refinement of non quenched and tempered steel.
The slab of various MnS easily in the heating process occurred in situ observation showed that tensile elongation, which causes the steel material anisotropy. Compared with longitudinal tensile, transverse tensile stress of strip steel MnS decision matrix and more prone to separation, separation will become the source of crack initiation, and promote crack along the MnS extension, thereby endangering the transverse properties of non quenched and tempered steel. Therefore, need to follow a reasonable process to reduce the size of sulfide and the distribution uniformity.
The effect of as cast samples of high temperature plastic deformation parameters and relative distribution of sulfide. The overall compression test, low strain rate (0.01s-1), large deformation (80%) matrix to the dynamic recrystallization occurs. The smaller the relative plastic deformation under the conditions of MnS, especially the thermal processing can make MnS the fragmentation and reduce the size of the deformation temperature of 1050 DEG C and 1250 DEG C, and broken after the distribution of MnS was improved to a certain extent, is conducive to improve the mechanical properties of non quenched and tempered steel.
Based on the non isothermal thermal processing after homogenization treatment of quenched and tempered steel can be further improved, the size and distribution of sulphide in steel. With the increase of heating and heat preservation time, long strip MnS in non quenched and tempered steel tip of the first round, the overall shape gradually to the cylindrical development, radial shrinkage after part of the location, cause MnS final fracture fragmentation, the small size of MnS generated will further spheroidizing. The calculation shows that MnS growth and fragmentation factors for controlling the diffusion of S element in the matrix. In 1250 degrees and 1050 degrees respectively. 3H and 5h are homogenized at MnS can be a large number of non quenched and tempered steel in pieces and reduce the size and improve the sulfide sulfide distribution.

【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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