【作者】 王超；

【导师】 王国栋； 王昭东；

【作者基本信息】 东北大学 ， 材料加工工程， 2017， 博士

【摘要】 在船舶、建筑等领域的大型钢质结构建造中,采用大线能量焊接工艺可显著提高工程施工效率,节约制造成本。常规钢材在大热输入条件下热影响区(HAZ)韧性严重恶化以致无法满足使用要求,因此必须研发出具备耐大线能量焊接特性的钢材。日本较早开展了大线能量焊接用钢研发工作,并处于国际领先水平。我国近年己取得显著研究进展,但产品级别以及性能稳定性较低,还无法满足各领域的迫切需求。氧化物冶金是开发大线能量焊接用钢的有效工艺手段,但由于其技术复杂性和知识产权保护等原因,国内仍未实现成熟的工业化应用。本课题以此为背景,围绕氧化物冶金型大线能量焊接用钢组织性能调控与生产工艺开展了研究,主要包括以下内容。为阐明氧化物冶金工艺下夹杂物析出规律,并为实验和工业研发提供理论指导,开展了氧化物冶金脱氧热力学分析。计算结果表明,在一般微合金条件下Ti的多种脱氧产物中Ti203稳定性最高,为避免Al203的析出,在0.01%Ti时需控制A1含量在0.004%以下。Zr和Mg脱氧能力极强,微量Zr可使A1203和Ti203还原,极微量Mg就可使A1203转化成MgAl204尖晶石。凝固过程中由于元素偏析,Ti脱氧反应将继续发展,MnS、TiN也将在凝固末期析出,提高凝固冷速可减弱元素偏析程度,有利于三次夹杂物细小分布。为明确大线能量焊接用钢组织特征设计路线,分析了不同组织类型对HAZ韧性的影响规律以及HAZ脆化和韧化机制,其中晶内针状铁素体(AF)体积分数和晶界片层状铁素体(GBF)尺寸是影响韧性的主要因素。根据实验结果指出,高韧性HAZ组织调控应包括如下特征:控制有效形核粒子微细多量分布,以充分促进晶内铁素体转变;抑制原奥氏体晶粒粗化,减小片层状铁素体尺寸;通过增强晶内转变和适当提高淬透性抑制晶界铁素体和侧板条铁素体(FSP)生成量,或提高晶界上形核率促进多边形铁素体转变。对TiO系和MgO系氧化物冶金工艺进行了系统实验研究,分析了各冶炼条件下夹杂物分布规律及HAZ组织转变特征,明确了各工艺控制要点。TiO钢中控制Ti脱氧前氧位约0.005%,缩短浇铸时间及提高凝固冷速有利于夹杂物微细多量分布,钢中TiOx-MnS夹杂促进AF组织转变。Ti和强脱氧剂M(Zr、Mg、Ca、REM)复合脱氧进一步促进夹杂物细化,生成的TiO,-MOy(M(O,S))-]MnS-TiN复相夹杂有效诱导AF形核。MgO钢中主要生成亚微米级MgO-TiN-MnS复相夹杂,晶界钉扎效果显著,但在晶界面积增加和缺乏有效晶内形核的条件下,晶界铁素体和侧板条组织转变量增加,影响韧性的大幅提高。针对MgO系钢存在的不足,对第3代氧化物冶金工艺进行了进一步改进。采用Ti-REM/Zr→Mg脱氧工艺可增加钢中含钛氧化物的体积分数,提高针状铁素体组织转变程度;对MgO钢进行V微合金化处理可促进MgO、TiN和V(C,N)的复合析出,利用界面共格机制提高夹杂物诱导铁素体形核能力,同时起到了钉扎奥氏体晶粒和促进晶内转变的两方面作用。综合采用两种处理工艺时,HAZ组织细化效果最佳,侧板条铁素体和粗大晶界铁素体基本消失,整体组织细化均匀,500kJ/cm线能量下-20℃冲击韧性达到200J以上,在常规MgO处理的HTUFF钢(0℃冲击韧性约为100J)基础上性能进一步提高。通过奥氏体连续和等温转变实验分析了粗晶热影响区(CGHAZ)组织演变规律及夹杂物诱导铁素体转变机理。在Ti-Zr脱氧钢粗晶奥氏体连续冷却转变中,低冷速时得到晶界铁素体和针状铁素体组织,高冷速时针状铁素体分割原奥氏体晶粒,显著细化贝氏体和马氏体板条束尺寸。随等温转变温度的降低,分别得到晶内多边形铁素体、较粗大针状铁素体、细化针状铁素体、晶内贝氏体组织。温度降低时相变驱动力增加,可激发形核的夹杂物尺寸减小,并且能同时生成多个细小板条。针状铁素体转变特征与贝氏体类似,具有不完全反应现象。贫锰区机制为Ti-Zir钢中夹杂物诱导铁素体形核的主导机制,对含Ni无Mn钢的考察结果验证了 Mn元素在晶内铁素体转变过程中的关键作用。考察了钢中常用合金元素对大线能量HAZ组织性能的影响规律。结果表明,低C高Mn有利于韧性的改善,但存在合适的碳当量范围,极低C时需抑制晶界铁素体生成;添加0.01%Nb时可保持较高的HAZ韧性,但过量的Nb促进贝氏体生成而导致韧性恶化;添加0.05%V时能提高基体强度并保持优良HAZ韧性,但更高含量时因大量析出而使韧性下降;Ni和Cu可综合改善强韧性能,在较高C含量下Cr和Mo对HAZ韧性具有不利影响。结合成分优化设计,实验室条件下研制了基于不同类型氧化物冶金工艺的Q345级、EH460级、X80级可大线能量焊接原型钢,分别满足100～800kJ/cm大线能量焊接性能。结合实验研究结果进行了氧化物冶金型大线能量焊接用钢工业化技术开发,对常规冶炼流程进行了改进,并研发出基于RH增氧和脱氧的氧化物冶金新技术,新技术更有利于氧化物的微细均匀分布和产品性能稳定性的提高。工业试制钢板在200kJ/cm气电立焊条件下,HAZ粗晶区-60℃冲击韧性达到200J以上,与国产钢板最佳实物水平180kJ/cm线能量下-20℃冲击韧性约170J相比,新研发钢板大线能量HAZ低温韧性显著提高。为进一步实现氧化物冶金对钢板基体组织性能的改善,本研究中提出“氧化物冶金+新一代TMCP”新型热轧钢材生产工艺路线。基于这一路线进行了实验探索,结果表明,氧化物冶金钢在奥氏体变形再结晶条件下仍可发生夹杂物诱导针状铁素体形核,提高变形温度和冷速有利于晶内铁素体转变量的增加。实验钢在“高温热轧+超快冷”新一代TMCP工艺下获得针状铁素体型细晶组织,与常规钢相比强韧性能显著提高。该工艺的实施将对厚板、管型材等不适于低温大变形的产品轧态性能的大幅提升具有特殊意义。更多还原

【Abstract】 Engineering construction efficiency can be significantly improved and the production cost can also be saved by applying high heat input welding technology during large-scale steel structures fabrication in the fields of shipbuilding,architecture and so on.Conventional steels cannot meet the requirement because the HAZ toughness deteriorates severely under high heat input welding condition.Thus,developing steels with good weldability under high heat input is urgently needed.Japan has started research on high heat input welding steels for a long period and keeps an internationally leading position.In recent years,obvious progress has been achieved in China.However,the product grade is not high enough and the property stability is insufficient so that extensive domestic demands cannot be fulfilled yet.Oxide metallurgy is an effective technology for developing high heat input welding steel.However,this technology has not been maturely applied in domestic steel industry because of its complication and technical secrecy.With this background,research on high heat input welding steel production process based on oxide metallurgy has been carried out in this study.Main contents are as follows.In order to clarify inclusion precipitation rules under oxide metallurgy technology to provide guidance for experimental and industrial research,deoxidization thermodynamics of oxide metallurgy was analyzed.Calculation results showed that Ti2O3 is the stablest among different Ti oxide types in common microalloy steel.In order to avoid Al2O3 precipitation,Al content should be controlled below 0.004%when Ti content is 0.01%.Zr and Mg are very strong deoxidizer such that a minute amount of Zr can reduce Al2O3 and Ti2O3 and a trace of Mg can change Al2O3 to MgAl2O4 spinel.Deoxidation reaction will proceed during solidification because of elements segregation.MnS and TiN will also precipitate at the end of solidification.The increase in solidification cooling rate could weaken segregation and is in favor of disperse distribution of third inclusions.Influence of different microstructures on HAZ thoughness was analyzed as well as the HAZ embrittlement and toughening mechanisms in order to specify microstructural design route of high heat input welding steel.It showed that intragranular acicular ferrite fraction and grain boundary allotriomorphic ferrite size are the main factors influencing toughness.According to the experiment results,microstructure control of high toughness HAZ includes the following characteristics.Controlling the effective particles for ferrite nucleation distributing finely and abundantly so as to sufficiently promote intragranular ferrite transformation;inhibiting prior austenite grain coarsening in order to decrease allotriomorphic ferrite size;restraining the formation of grain boundary ferrite and side plates through enhancing intragranular transformation and increasing hardenability properly,or promoting polygonal ferrite formation by raising nucleation rate at grain boundary.Systemic experimental research on TiO and MgO oxide metallurgy was carried out.Inclusion distribution for different oxide metallurgy types and high heat input HAZ characteristics were studied.The key points of process control were made clear.In TiO steels,it was favorable for fine inclusion dispersion to control oxygen about 0.005%before Ti addition,shorten killing time before casting and increase solidification cooling rate,TiOx-MnS inclusions in TiO steel induced intragranular ferrite formation.Complex deoxidization with Ti and strong deoxidizer M(Zr,Mg,Ca,REM)could further refine inclusion size.The resultant TiOx-MOy(M(O,S))-MnS-TiN complex inclusions were effective for inducing acicular ferrite nucleation.In MgO steel,the main inclusion type was submicron-sized MgO-TiN-MnS.Prior austenite grain was significantly pinned.However,under the condition of increase in grain boundary area and lack of effective intragranular nucleation,grain boundary ferrite and lath packet structure tended to increase,which will not benefit remarkable improvement of HAZ toughness.With regard to the deficiency of MgO steel,further development was made based on the 3rd generation oxide metallurgy,and HAZ microstructure and properties were further improved.Fraction of inclusions containing Ti oxide was increased by adopting Ti-REM/Zr一Mg deoxidation process,and acicular ferrite transformation was further improved.V microalloying of MgO steel could promote the complex precipitation of MgO,TiN and V(C,N),which would increase the ferrite nucleation potency by coherent interface mechanism,such that the inclusions obtained both effects in pinning austenite grains and promoting intragranular transformation.Combining the two treatment measures,HAZ microstructure received the best refining effect,coarse ferrite side plates and allotriomorphs disappeared and the entire micro structure was fine and uniform instead.The impact toughness reached above 200J at-20℃ under 500kJ/cm heat input,which got further improved on the basis of normal MgO treated HTUFF steel(一100J at 0℃).CGHAZ microstructure evolution behavior and inclusion-induced intragranular ferrite transformation mechanism were analyzed by performing austenite continuous cooling and isothermal transformation experiment.In the continuous cooling transformation of coarse-grained austenite of Ti-Zr deoxidized steel,grain boundary ferrite and acicular ferrite was formed at low cooling rates.At higher cooling rates,prior austenite was segmented by acicular ferrite resulting in great refinement of bainite or martensite packet size.With the decrease of isothermal temperature,intragranular idiomorphic ferrite,coarse acicular ferrite,finer acicular ferrite and intragranular bainite were obtained respectively.With the increase in chemical driving force,the size of effective inclusions became smaller,and several fine plates could form at one inclusion simultaneously.Transformation mechanism of acicular ferrite is similar to that of bainite,which exhibits incomplete reaction phenomenon.Mn depletion zone mechanism is predominant for inclusion-induced nucleation in the present Ti-Zr steel.Investigation of Mn-free and Ni-bearing steel also indicated the significance of Mn element in intragranular ferrite transformation.Influence of commonly used alloying elements on high heat input HAZ toughness was researched.Results showed low C and high Mn is in favor of toughness improvement,butthere exists a proper range of Ceq and C content.Grain boundary ferrite needs to be inhibited when C content is ultra low.Relatively high HAZ toughness could still be obtained with 0.01%Nb.However,excessive Nb will cause bainite formation and toughness deterioration.A percentage of 0.05%of V could increase matrix strength and keep good HAZ toughness,while a larger amount will impair toughness because of precipitation strengthening.Ni and Cu can improve combined properties of strength and toughness.Cr and Mo are detrimental to toughness when C content is relatively high.With optimized compositions,model high heat input welding steels of Q345,EH460 and X80 grade based on different oxide metallurgy processes were produced in laboratory scale.The steels fulfilled high heat input welding property of 100-800kJ/cm respectively.Combining experimental research results,industrial production technology of oxide metallurgical steel for high heat input welding was developed.Normal steelmaking process was improved and new oxide metallurgy technology based on supplying oxygen and deoxidization in RH was developed,which would further promote uniform distribution of fine oxide and the increase in property stability of steel product.Industrially trial-produced steel plates exhibited CGHAZ toughness of above 200J at-60℃ under 200kJ/cm EGW condition.In comparison with the best domestic product level of～170J at-20℃ under 180kJ/cm,low temperature HAZ toughness for high heat input of the new developed steel improved significantly.In order to realize the improving effect of oxide metallurgy on steel matrix microstructure and properties,a new process route of“Oxide Metallurgy + NG-TMCP" was proposed.Experimental research was carried out based on the process.It was indicated that inclusion could still promote acicular ferrite nucleation within deformed and recrystallized austenite grains.The increase in deformation temperature and cooling rate contributed to promoting acicular ferrite transformation.The experimental steel obtained fine-grained acicular ferrite type microstructure under NG-TMCP process of high temperature rolling and ultra-fast cooling,which resulted in notable increase in strength and toughness in comparison with conventional steel.The application of this process will have particular significance in improving as-rolled properties remarkably for steel products which are not proper for heavy reduction at low temperature such as heave plate,tube steel and shape steel.更多还原