连铸S355钢的热塑性及第二相析出行为研究

发布时间：2018-03-19 11:05

  本文选题：S355钢　切入点：第三脆性区　出处：《江西理工大学》2017年硕士论文　论文类型：学位论文

【摘要】：S355钢属于低合金高强度结构钢,具有良好的综合性能,广泛应用于桥梁、海洋平台、电力设备、船舶、压力容器等领域。由于微合金钢中第二相析出与组织的裂纹敏感性等原因,导致连铸过程铸坯表面横裂纹缺陷频繁发生。而且横裂纹常在铸坯的弯曲、矫直温度区(700~900℃)产生。因此开展相关基础研究工作,可为上述难题的解决奠定理论基础。通过Gleeble热/力模拟实验,研究了微合金化S355钢的热塑性,测定了S355钢的第三脆性温度区范围。通过热膨胀法,结合金相-硬度法,测定了微合金化S355钢的CCT曲线。通过Thermo-Calc软件,分析了第二相的热力学析出行为。采用SEM、TEM观察分析了S355铸坯中析出物的形貌及析出位置,确定了铸坯中存在的第二相类型。通过Gleeble热/力模拟试验、结合显微组织观察及断口形貌分析,分析了温度历程对铸坯塑性的影响。综合分析S355钢的高温力学性能、相变规律、第二相析出规律,得出了微合金化S355钢第三脆性区的脆性机理;通过工艺改进提高了S355钢在第三脆性温度区的塑性。具体试验结果如下:(1)S355钢第三脆性区温度范围约为:667~850℃,其中750℃为脆性槽低谷。当温度低于750℃时,塑性随着温度的降低而升高;当温度高于750℃时,塑性随着温度的升高而升高。低温端塑性的回升是由于铁素体的大面积析出,体积分数为40%的铁素体可作为低温端塑性回升的判据。(2)S355钢第三脆性区的脆性机理:(1)网状铁素体沿着奥氏体晶界析出。在脆性槽低谷时,网状铁素体膜厚度约为20μm。沿着奥氏体晶界析出的网状铁素体处会产生应力集中,使得裂纹优先在晶界处扩展,产生沿晶脆性断裂。(2)第二相沿着奥氏体晶界析出。第二相主要从加剧应力集中,促使网状铁素体形成等方面恶化S355钢的热塑性。(3)S355钢的过冷奥氏体在不同冷速(0.2~20℃/s)下分别发生了铁素体转变,珠光体转变(转变结束的临界冷速为2℃/s),贝氏体转变(开始转变的临界冷速为1℃/s)和马氏体转变(开始转变的临界冷速15℃/s)。S355钢的Ac1、Ac3温度分别为774、886℃,Fs在755~591℃温度范围,Ps、Pf分别在637~551℃、593~552℃温度范围,Bs、Bf分别在564~455℃、388~494℃温度范围,Ms为393℃。(4)S355钢中Ti的第二相析出温度较高,Nb的第二相析出温度次之,V的第二相析出温度更低;氮化物析出温度普遍高于碳化物。各析出相的种类及其开始析出温度分别为:TiN(1388℃)、NbN、NbC(1100℃)、TiC(1080℃)、AlN(936℃)、VNC(900℃)。(5)在3~10℃/s冷速范围,仅改变冷速无法改善S355钢在第三脆性区的塑性。通过返温工艺,可明显提高S355钢的热塑性,尤其是使得脆性槽低谷的塑性明显提高。塑性的改善主要是因为返温工艺下奥氏体晶粒细化、铁素体晶粒的细化及网状特征的弱化。
[Abstract]:S355 steel is a kind of low alloy high strength structural steel. It has good comprehensive properties and is widely used in bridges, offshore platforms, power equipment and ships. Due to the second phase precipitation in microalloyed steel and the crack sensitivity of microalloyed steel, the surface transverse crack defects occur frequently in the continuous casting process, and the transverse cracks often bend in the billet. Therefore, the related basic research work can lay a theoretical foundation for solving the above problems. The thermoplasticity of microalloyed S355 steel is studied by Gleeble thermal / force simulation experiment. The range of the third brittleness temperature range of S355 steel was measured. The CCT curves of S355 steel were measured by means of thermal expansion method and metallographic hardness method. By means of Thermo-Calc software, The thermodynamic precipitation behavior of the second phase was analyzed. The morphology and location of the precipitates in S355 billet were observed and analyzed by means of SEM-TEM. The type of the second phase in the billet was determined. The Gleeble thermal / force simulation test was carried out. Combined with microstructure observation and fracture morphology analysis, the effect of temperature history on billet plasticity was analyzed. The mechanical properties at high temperature, phase transformation and precipitation of the second phase of S355 steel were comprehensively analyzed. The brittleness mechanism of the third brittleness zone of S355 steel was obtained, and the plasticity of S355 steel in the third brittleness temperature region was improved by technological improvement. The concrete test results are as follows: the temperature range of the third brittleness zone of S355 steel is about: 667n 850 鈩，

本文编号：1633984