【作者】 王红涛；

【导师】 田勇； 叶其斌； 邓想涛；

【作者基本信息】 东北大学 ， 材料加工工程， 2017， 硕士

【摘要】 极寒环境下船舶用低温钢要求钢材具有优异的强韧性能匹配和良好的裂纹止裂能力。本文采用某钢厂试轧生产的EH40和EH47级别船板钢,重点研究了试验钢低温韧性的影响因素,并对裂纹止裂机理进行了详细的分析。论文主要工作如下:(1)利用热模拟实验机,测定高温奥氏体在900℃时分别经未变形和30%变形后的CCT曲线。结果显示,未变形时,在较低冷速下,显微组织主要是多边形铁素体、铁素体和极少量的珠光体组织;在2～5℃/s的冷速范围内,出现粒状贝氏体;在10～30℃/s的高冷速下,主要得到粒状贝氏体和贝氏体铁素体组织。经30%变形后,在2～30℃/s全部冷速范围内,都可以获得贝氏体组织,且随着冷速的增大,贝氏体种类并未发生改变,均以粒状贝氏体为主,但M/A岛分布更加均匀细小。900℃时的变形处理,能够促进铁素体相变而抑制贝氏体相变。和静态相变相比,动态CCT曲线会向左上角移动,两个CCT曲线形状稍有不同,但相变产物基本对应,形变诱发相变能够使相变区域左移20℃/s左右。(2)高温淬火实验主要研究了 EH40船板钢在不同加热温度时奥氏体晶粒的长大规律,结果表明奥氏体粗化温度在1250℃以上,在保证微合金元素完全固溶的条件下,钢坯加热到1200～1250℃为宜。(3)单道次压缩实验结果表明,试验钢在各温度条件下都要经历加工硬化和回复软化两个过程,在1 100℃以上时,发生动态再结晶,利用origin软件回归建立试验钢的变形抗力模型,模型的建立对指导EH40高强船板钢实际控轧过程中变形抗力的预测具有重要的指导意义。(4)双道次压缩实验结果表明,试验钢的静态未再结晶临界温度Tnr在900～950℃之间;静态再结晶激活能为2.115× 105J/mol。(5)对某钢厂试轧生产的86 mm EH40、EH47船板钢进行组织性能检验,并重点分析了其低温冲击韧性,通过组织性能比较确定了影响船板钢低温强韧性的机理。检验结果显示两种试轧钢均是由针状铁素体和准多边形铁素体组成的复合组织,这种复合组织正是试验钢强韧性能良好匹配的关键所在;针状铁素体条束之间和两种组织之间的界面都是大角晶界,大角晶界的存在能够显著阻滞裂纹的扩展,增加裂纹扩展功;针状铁素体和部分准多边形铁素体基体上还分布有高密度的位错,而且部分位错被细小的析出物所钉扎,这些有利因素的叠加效果正是试验钢具有优异低温强韧性的关键。更多还原

【Abstract】 Cryogenic steels used for ships under extreme cold conditions require high strength,excellent low-temperature toughness and good crack arresting ability.The paper adopts a test rolling steels EH40 and EH47 produced by a steel plant and focus on the factors which affect the low temperature toughness of steel,the crack arresting mechanism is also analyzed in detail here.The main work are as follows.(1)The continuous cooling transformation(CCT)curve after no deformation and 30%deformation of austenite at 900℃ is determined by MMS-200 thermal simulation machine.Results show that when there is no deformation,the micro structure is dominated by polygonal ferrite,ferrite and very small amount of pearlite at lower cooling rates.At the same time,Granular bainite occurs in the range of 2～5℃/s and when the cooling rate is 10～30℃/s,the granular bainite and bainitic ferrite structure are obtained.But after 30%deformation of austenite at 900℃,bainite can be obtained during all cooling rates.The bainite species did not change with the increase of cooling rate and all of them were mainly granular bainite.However the distribution of island M/A component was more uniform and small.Results also show that the deformation treatment at 900℃ can accelerate the transformation of ferrite and restrain the transformation of bainitic.Comared with the static transformation,dynamic CCT curve mobiles to the left corner.Also the shape of two CCT curves are slightly different,but the transformation product corresponds roughly,The deformation induced transformation can make the transition region left 20℃/s.(2)Through the high temperature quenching test,we studied the austenite grain growth behavior of EH40 steel.The results show that the austenite coarsening temperature is over 1250℃,which means that is appropriate to heat the billet up to 1200～1250℃ during production under the condition that the microalloying elements are completely solubled.(3)Single pass compression test display that the experimental steel has undergone two processes of work hardening and recovery softening under various temperature conditions and the dynamic recrystallization occurs at temperatures above 1100℃.The deformation resistance model of test steel is established by Origin through mathematical regression.The establishment of the model will have high valus for the steel EH40 during actual prediction of deformation resistance in controlled rolling process.(4)Double channel compression test shows that the static nonrecrystallization critical temperature Tnr of the test steel is between 900 and 950℃,and that the activation energy of the static recrystallization is 2.115×105J/mol.(5)The micro structure properties of EH40 and EH47 steel with thickness of 86 mm were examined.And the low temperature impact toughness was analyzed,and the mechanism of low temperature strength and toughness of ship plate steel was determined by comparison of organization and performance.The test results show that the two kinds of test steel are acicular ferrite and quasi-polygonal ferrite composite structure and this composite organization is the key to the good performance of the test steel.The interface between the acicular ferrite strips and the two tissues is a large angle grain boundary,which has a good effect on the crack propagation and increases the crack propagation.Acicular ferrite and partially quasi-polygonal ferrite have high density dislocations,and partial dislocations are pinned by fine precipitates,all of which are key to the excellent low temperature toughness of the test steel.更多还原