【作者】 肖福仁；

【导师】 廖波； 杨柯；

【作者基本信息】 燕山大学 ， 材料学， 2003， 博士

【摘要】 现代钢铁材料的发展趋势是如何在现有材料的基础上，通过冶炼、浇铸、热加工过程的控制，降低材料的有害元素的含量、减小偏析、细化组织，使材料的综合力学显著提高，从而提高材料的使用寿命。对长距离油气管线用钢，高强韧性要求是保证管线安全、降低运营成本的关键。而X80级别针状铁素体管线钢是未来20内最有可能大量应用的高性能管线钢。本文以X60钢为基础成分的洁净钢为研究对象，通过对洁净低碳微合金管线钢的相变动力学的系统研究，确定如何鉴别管线钢中的针状铁素体以及针状铁素体管线钢的组织控制和细化工艺方法，探索提高针状铁素体管线钢的性能的关键工艺技术。测定低碳微合金管线钢过冷奥氏体连续冷却转变曲线及等温转变动力学曲线。在低碳微合金钢的TTT曲线中含有三条独立的“C”曲线，即多边形铁素体“C”曲线；块状铁素体“C”曲线；贝氏体“C”曲线。块状转变“C”曲线与铁素体、贝氏体相互重叠，转变区的大小与钢中含碳量和合金元素含量有关。碳含量降低，块状铁素体转变区增加。Mo降低碳活度，增加块状铁素体转变的孕育期。根据低碳微合金管线钢的相变动力学及轧板的组织分析，确定管线钢中针状铁素体组织为连续冷却条件下形成的块状铁素体、粒状铁素体和贝氏体铁素体的混合组织，并含有一定量岛状组织。其组织特征呈为无明显铁素体边界，不规则非等轴状的铁素体块，在状铁素体块内部含有无规则排列或有方向性排列的岛状组织。其形成的冷却速度范围介于多边形铁素体与贝氏体铁素体转变的冷却速度之间。提出了针状铁素体组织的鉴别和定量方法在Gleeble-2000和Gleeble-3500型热模型试验机上，研究了B钢和E钢的热变形行为及变形后等温过程中的静态再结晶和应变诱导碳化物的析出，计算出试验用钢的动态再结晶激活能和静态再结晶激活能，并绘制了应变诱导碳化物析出曲线(PTT曲线)。测定了不同变形条件试验用钢的动态相变动力学曲线，结果表明，热变形显著加速针状铁素体和多边形铁素体相变，使针状铁素体开始相变温度显著提高。对针状铁素体相变开始温度的影响主要取决非再结晶区的变形量和变形温度。在两相区变形，B钢有应变诱导铁素体转变，使针状铁素体组织开始温度下降。通过在Gleeble-3500热模拟试验机上热模拟轧制工艺对组织及细化作用的影响表明，对针状铁素体组织采用热加工过程的全过程控制的方法，即精确控制再加热温度、初轧、精轧和冷却工艺参数，能获得最佳的组织细化效果。实验室轧机模拟轧制的钢板组织、性能分析结果表明，通过热加工过程控制的全程控制，获得了细化的针状铁素体组织，使以X60钢为基础成分的洁净钢钢板的力学性能达到X80级别管线钢的强度和韧性的水平，试验钢板的韧脆转变温度低于-160 ℃。 <WP=5>对比针状铁素体与超细铁素体的抗硫化氢应力腐蚀性能，结果表明，针状铁素体具有更高的抗硫化氢应力腐蚀性能。根据针状铁素体管线钢的组控制和细化的工艺技术方法，在1700 mm轧机上，轧制出符合西气东输要求的14.7×1550 mm的X70级别针状铁素体管线钢钢卷。钢卷的性能为：(s=560 MPa；(b=678 MPa；CVN=284 J(-20 ℃)；韧脆转变温度低于-80 ℃。更多还原

【Abstract】 The development trend of modern iron & steel materials is focused on how to improve the mechanical properties and raise the service life, based on the currently available material, by the ways of controlling the melting processes to reduce the content of harmful elements, controlling the processes of casting to reduce the segregation, and controlling the processes of hot-working to refine the microstructure. The pipeline steels with good match of strength and toughness for long distance oil and gas transportation is a key to ensure the security and reduce the operation cost, for which the X80 grade acicular ferrite pipeline steel will be extensively applied in the future. In this thesis, the discrimination method for acicular ferrite in pipeline steel, control and refinement acicular ferrite microstructure for clean steels that were developed based on the chemical composition of the commercial X60 grade pipeline steels bas been investigated by study on the phase transformation kinetics. The aim is to explore the key technology for improvement of mechanical properties of acicular ferrite pipeline steels.The continuous cooling phase transition kinetics and isothermal phase transition kinetics for low-carbon microalled steels have been constructed. There are three independent “C” curves in the TTT diagrams for low-carbon microallyed steel, polygonal ferrites/pearlite “C” curve, the mass ferrite “C” curve and the bainitic “C” curve, respectively. The mass ferritic “C” curve is overlapped with “C” curves of polygonal ferrite and bainite. The width of transformation region for mass ferrite is related with amount of carbon and alloying elements. The transformation region of mass ferrite increases with reduction of the amount of carbon in steels. Mo reduces the carbon activity, which results in increase of incubation period of mass ferrite.According to the phase transformation kinetics of low-carbon microallyed pipeline steels and the microstructure analysis of rolling plate, the acicular ferrite in pipeline steels has been defined as the mixted microstructure, under a continuous cooling condition, with mass ferrite, granular ferrite and bainite ferrite, in which some islands structure is contained. The microstructure characteristic of acicular ferrite in pipeline steels is the unregular and non equiaxed ferrite masses which are without clear ferrite grain boundary and some of which contain random arranged or regularly islands.The hot deformation behavior, static recrystallization and strain induced carbide precipitation for two test steels, steel B and steel E, have been studied by using Gleeble-2000 and Gleeble-3500 thermal simulation tests. The dynamic recrystallization activation energy and <WP=7>the static recrystallizational activation energy have been determined, respectively. The strain induced carbide precipitation curves (PTT curves) have also been determined.The dynamic phase transformation kinetics curves of the test steels on different deformation conditions have been constructed. The results show that the acicular ferrite phase transformation and polygonal ferrite phase transformation are accelerated observably, and the beginning point of phase transformation for acicular ferrite is raised notablely. The effect on acicular ferrite phase transformation point depends mainly on the amount of deformation and deformation temperature in the noncrystallization region. For deformation in the dual phase region deforming, the ferrite is observed at austenite grain boundary in steel B, which makes the acicular ferrite phase transformation point decrease. The influence of rolling parameters on microstructure and refinement has been studied by using Gleeble-3500 thermal simulation test. the results show that the best effect of controlling and refinement for acicular ferrite microstructure could be achieved by all controlling hot working processes, namely, accurate controlling the parameters of reheat temperature, rough rolling, finish rolling and cooling rate. The clean steel based on the chemical c更多还原