本文選題：S355鋼　切入點：第三脆性區(qū)　出處：《江西理工大學》2017年碩士論文　論文類型：學位論文

【摘要】：S355鋼屬于低合金高強度結構鋼,具有良好的綜合性能,廣泛應用于橋梁、海洋平臺、電力設備、船舶、壓力容器等領域。由于微合金鋼中第二相析出與組織的裂紋敏感性等原因,導致連鑄過程鑄坯表面橫裂紋缺陷頻繁發(fā)生。而且橫裂紋常在鑄坯的彎曲、矯直溫度區(qū)(700~900℃)產生。因此開展相關基礎研究工作,可為上述難題的解決奠定理論基礎。通過Gleeble熱/力模擬實驗,研究了微合金化S355鋼的熱塑性,測定了S355鋼的第三脆性溫度區(qū)范圍。通過熱膨脹法,結合金相-硬度法,測定了微合金化S355鋼的CCT曲線。通過Thermo-Calc軟件,分析了第二相的熱力學析出行為。采用SEM、TEM觀察分析了S355鑄坯中析出物的形貌及析出位置,確定了鑄坯中存在的第二相類型。通過Gleeble熱/力模擬試驗、結合顯微組織觀察及斷口形貌分析,分析了溫度歷程對鑄坯塑性的影響。綜合分析S355鋼的高溫力學性能、相變規(guī)律、第二相析出規(guī)律,得出了微合金化S355鋼第三脆性區(qū)的脆性機理;通過工藝改進提高了S355鋼在第三脆性溫度區(qū)的塑性。具體試驗結果如下:(1)S355鋼第三脆性區(qū)溫度范圍約為:667~850℃,其中750℃為脆性槽低谷。當溫度低于750℃時,塑性隨著溫度的降低而升高;當溫度高于750℃時,塑性隨著溫度的升高而升高。低溫端塑性的回升是由于鐵素體的大面積析出,體積分數(shù)為40%的鐵素體可作為低溫端塑性回升的判據。(2)S355鋼第三脆性區(qū)的脆性機理:(1)網狀鐵素體沿著奧氏體晶界析出。在脆性槽低谷時,網狀鐵素體膜厚度約為20μm。沿著奧氏體晶界析出的網狀鐵素體處會產生應力集中,使得裂紋優(yōu)先在晶界處擴展,產生沿晶脆性斷裂。(2)第二相沿著奧氏體晶界析出。第二相主要從加劇應力集中,促使網狀鐵素體形成等方面惡化S355鋼的熱塑性。(3)S355鋼的過冷奧氏體在不同冷速(0.2~20℃/s)下分別發(fā)生了鐵素體轉變,珠光體轉變(轉變結束的臨界冷速為2℃/s),貝氏體轉變(開始轉變的臨界冷速為1℃/s)和馬氏體轉變(開始轉變的臨界冷速15℃/s)。S355鋼的Ac1、Ac3溫度分別為774、886℃,Fs在755~591℃溫度范圍,Ps、Pf分別在637~551℃、593~552℃溫度范圍,Bs、Bf分別在564~455℃、388~494℃溫度范圍,Ms為393℃。(4)S355鋼中Ti的第二相析出溫度較高,Nb的第二相析出溫度次之,V的第二相析出溫度更低;氮化物析出溫度普遍高于碳化物。各析出相的種類及其開始析出溫度分別為:TiN(1388℃)、NbN、NbC(1100℃)、TiC(1080℃)、AlN(936℃)、VNC(900℃)。(5)在3~10℃/s冷速范圍,僅改變冷速無法改善S355鋼在第三脆性區(qū)的塑性。通過返溫工藝,可明顯提高S355鋼的熱塑性,尤其是使得脆性槽低谷的塑性明顯提高。塑性的改善主要是因為返溫工藝下奧氏體晶粒細化、鐵素體晶粒的細化及網狀特征的弱化。
[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，

本文編號：1633984