【作者】 李显达；

【导师】 王作成；

【作者基本信息】 山东大学 ， 材料加工工程， 2011， 硕士

【摘要】 低合金高强度钢(High Strength Low Alloy Steel)是典型的低碳低合金结构钢,它是通过微量合金元素的添加,利用细晶强化和沉淀析出强化等原理而生产的具有较高的强度、韧性和良好的焊接性能的一种钢材。它在建筑、桥梁、船舶、管线、海洋石油平台等领域有着广泛的应用。但是,目前生产的热轧低合金高强钢的低温冲击韧性较低,韧脆转变温度较高,尚不能满足高寒、高纬度地区的使用要求。热处理作为提高材料性能的重要手段,广泛应用于钢铁材料的生产中。因此通过适当的热处理方法,提高低合金高强度钢的冲击韧性,特别是低温冲击韧性,对于扩展该钢材的使用范围具有重要的意义。本课题研究了淬火以及回火工艺对硼镍复合添加的含铌HSLA钢组织性能的影响规律,确定了最佳的调质处理方案,得到了强度500MPa左右,伸长率25%左右的具有良好强韧性配合的钢材。特别是在低温冲击韧性方面,通过调质处理后的试验钢,其韧脆转变温度下降到-70℃以下,在-90℃时,其冲击功仍在100J左右。利用硼(10ppm)和镍(0.5%)在热处理中的复合作用,在成本增加不大的情况下,达到了3.5Ni钢的性能水平。本文首先通过热物理模拟试验得到试验钢的相变温度及CCT曲线；并通过淬透性试验确定了淬透层深度。然后通过对淬火温度、淬火保温时间以及回火温度三个工艺参数的调整,通过大量试验,利用金相法研究了各个工艺参数对钢材组织的影响规律,制定了最佳的热处理工艺方案。最后实施具体方案,通过硬度试验、拉伸试验以及系列温度冲击试验对调质后的钢材进行了力学性能检验。利用光学显微镜、扫描电镜、X射线衍射、高分辨透射电镜以及能谱分析等方法,对调质后试验钢的组织、相组成以及二相粒子的分布、形貌和成分进行了分析。结果发现,回火后在晶界析出的复杂碳化物对于抑制再结晶晶粒长大有重要影响,细小弥散的晶界碳化物没有使韧性降低,反而可能抑制裂纹在晶粒之间的传播,提高钢材的韧性。更多还原

【Abstract】 High Strength Low Alloy Steel is a type of low-carbon low-alloy structural steel, which, as a result of grain boundary hardening and precipitation hardening with micro alloying, exhibits high strength, high impact toughness, good weld-ability, and great resistance to corrosion. HSLA steel is widely used in structures that are designed to handle large amounts of stress or need a good strength-to-weight ratio, such as offshore drilling platforms, bridges and large ships. However, most hot-rolled HSLA steels can not provide high impact toughness at low temperature due to their relatively high fracture appearance transition temperature (FATT), and thus do not perform well in extremely cold regions. Heat treatment is often associated with increasing the strength of material, especially steels, since they respond well to change of temperature. Our research group has used heat treatment on a novel HSLA steel to increase its impact toughness, specifically the low-temperature-impact-toughness. Therefore, the improvement significantly extends the scope of applications of the steel.In our research, we have discovered the relationship between heat treatment process and the properties of the niobium added HSLA steel. Eventually, with an optimized quenching-tempering schedule, we have produced a new type of steel with a tensile strength at about 500Mpa, elongation at about 25%. The overall performance of the steel, especially the low-temperature-impact-toughness has significantly improved, as the FATT of the steel is below -70℃, and impact toughness is about 100J at -90℃. Along with the help of compound effect of boron (10ppm) and Nickel (0.5%), the new type of steel is able to replace 3.5Ni steel in certain circumstances since it is much more cost efficient.In this paper, we first built up a CCT (continuous cooling transformation) diagram using thermal simulation technology. Based on the analysis of the diagram, we are able to determine the phase transition temperature of the steel. After a quenching test and a hardness test, the depth of hardening was measured. By varying parameters of quenching and tempering, we studied how quenching temperature and time of maintaining, and tempering temperature influenced the microstructure of the steel. So an optimum quenching and tempering process control was obtained. Hardness tests, tensile tests and impact toughness tests at a series of low temperatures were performed afterwards to further examine the result. Several characterization techniques were used to analyze the microstructures, phase composition, and distribution of second phase particles of the sample, including SEM, TEM, XRD and photoelectron spectroscopy. The results indicate that complicated ingredient carbides generated along grain boundaries after tempering considerably inhibit grow of grains during recrystallization. Those fine distributive carbides do not damage the toughness of the steel. However, they have prevented fractures from spreading. Consequently, the toughness of the steel is enormously increased.更多还原