【作者】 王红涛；

【导师】 王国栋； 田勇；

【作者基本信息】 东北大学 ， 材料加工工程， 2022， 博士

【摘要】 随着集装箱船舶向超大型化发展,为保证船舶的安全航行,船体中部、顶部的舱口围顶板及上甲板边板等关键部位迫切需求100 mm厚高止裂韧性钢板。日韩等国启动研发较早,在止裂钢生产技术与评价方面处于领先地位。基于此背景,本文依托国家重点研发计划“极寒与超低温环境船舶用钢及应用”和“高止裂韧度集装箱船用止裂钢板的研制与开发”,针对高止裂韧性特厚钢板研究及工业产品存在阻滞脆性裂纹传播的因素不清、显微组织细化机制模糊、低压缩比轧制钢板韧性较差等问题,重点研究了 460 MPa级止裂钢板的显微组织特征与断裂行为,并提出低奥氏体化温度轧制工艺,为国内首块-10℃止裂韧性≥8000 N/mm3/2的100 mm厚EH47止裂钢板的开发提供了技术指导。主要的研究工作如下:(1)以具有不同止裂韧性的特厚钢板为研究对象,对比分析了特厚(厚度≥80 mm)EH47止裂钢板的显微组织特征与断裂行为。结果表明,止裂韧性优异的80 mm厚EH47钢板的心部显微组织以等轴铁素体、多边形铁素体(PF)和准多边形铁素体(QF)为主,高角晶界(HAGB)比例为62.1%,韧脆转变温度(DBTT)为-84℃,并在-80℃具有209 J的平均Charpy冲击吸收能量且冲击行为稳定。韧性断口分裂出现在冲击断口的中下部时,能够扩大断口边缘的塑性变形区域,避免严重的应力集中和晶粒破碎,使本该形成平面应变断裂特征的放射区依旧保持韧性断裂,增加吸收的能量(63 J),实现断口分裂增韧的效果。加载过程中试样心部的平面应变状态是-70℃无塑性转变温度落锤试验发生脆性断裂的根本原因。落锤试样边缘处于平面应力状态的显微组织所产生的塑性变形、中心出现的断口分裂、钢板心部较弱的{001}<110>织构和较强的γ-fiber取向线上的织构、1/4厚度的{110}<111>、{110}<112>织构等因素均有助于改善平面应变条件下的断裂行为。(2)针对高止裂韧性钢板具有较强的脆性裂纹止裂能力的特点,通过调控控轧控冷工艺获得两种不同类型的显微组织和第二相,研究了低碳微合金钢中阻滞脆性裂纹传播的关键因素。结果表明,41%的针状铁素体(AF)、48.8%的HAGB 比例和4.1μm的有效晶粒尺寸是钢板1/4厚度DBTT低至-87℃的关键。高比例的HAGB频繁改变脆性裂纹的扩展方向,阻滞脆性裂纹的传播,并将脆性断口的粗糙度由1.30提高至1.52。M/A岛的平均最大弦长从1.5 μm增加至2.1μm,临界断裂应力将从2601 MPa减小至2198 MPa,促进脆性裂纹的起裂。大量的AF使解理刻面生长方向前端产生“劈钉”效应,能够阻滞脆性裂纹的扩展。(3)针对低压缩比轧制时奥氏体晶粒尺寸和畸变状态调控难度大的问题,优化了轧制变形分配方案,研究了奥氏体未再结晶温度(Tnr)以下轧制变形对钢板显微组织、织构和低温韧性的影响。结果表明,轧制压缩比为3.57时,Tnr以下施加60%轧制变形,心部的铁素体贝氏体混合显微组织具有51.6%HAGB比例、3.4±4.2 μm有效晶粒尺寸、39.1%{110}滑移面和较强的{113}～{112}<110>、{112}<131>、{332}<113>织构,并使钢板在-80℃时具有相对优异的低温韧性(平均冲击吸收能量233 J)。低压缩比轧制时,随着Tnr以下轧制变形量的增加,奥氏体晶粒尺寸增大,压扁程度增加;晶界变长引起的单位体积有效界面积(Sv)减少,晶内变形带增加带来的Sv单调增加;总的Sv在Tnr以下施加60%轧制变形时具有较高值(115.5 mm-1),并实现晶内/晶界形核位置的合理分配。(4)针对低压缩比轧制时,铁素体贝氏体混合组织中HAGB比例偏低的问题,提出低奥氏体化温度轧制技术。结果表明,钢板在900℃奥氏体化时产生的18.5±10.5μm的细化奥氏体晶粒是形成高HAGB比例的关键。Tnr以下轧制过程中,低奥氏体化温度轧制技术能够提供高达192.9～228.7 mm-1的Sv,比低压缩比轧制钢板优化后的工艺高93.5%～129.4%。大量的铁素体晶粒在不同原奥氏体晶界处形成并相互接触,可以在20～47°范围内贡献33.6%～34.5%的HAGB 比例,并使HAGB的累计占比曲线呈近似线性的增长趋势。调整显微组织中M/A岛等脆性相与基体组织之间薄弱界面的长度和间距,控制少量的断口分裂出现在冲击断口的下半部分是实现断口分裂增韧的前提。(5)利用前述研究结果开发出国内首块-10℃止裂韧性≥8000 N/mm3/2的100 mm厚EH47止裂钢板,并与常规工艺试制的同类钢板进行比较,结合原奥氏体晶界腐蚀和EBSD表征,对比研究奥氏体晶粒压扁状态、轧后弛豫和冷速对铁素体贝氏体相变的影响,明确高止裂韧性特厚钢板工业生产的控制性环节。结果表明,EH47特厚钢板从表层到1/2厚度的显微组织类型发生“贝氏体为主→细化AF+PF+QF→粗化贝氏体+PF+QF”的转变。较大的冷却速度(过冷度)、适当的冷速和弛豫时间、轧制过程中较小的变形渗透与轧后较小的冷却速度和过长的弛豫时间是影响特厚钢板表层、1/4和1/2厚度显微组织发生不同相变行为的主要因素。特厚钢板全厚度细化、均匀化的压扁状奥氏体晶粒是全厚度细化相变显微组织的关键。钢板心部韧性组织和有利织构因素所形成的优异的低温韧性,使心部和1/4厚度的显微组织在钢板全尺寸断裂时具有相同的阻滞脆性裂纹传播的能力,形成“多峰”分布的宏观断口,提高了钢板全厚度断裂时的止裂韧性。更多还原

【Abstract】 With the rapid development of mega container carriers,the steel with the excellent crack arrest ability and the thickness of 100 mm is applied to the large cargo opening structure such as the upper deck,hatch side coamings,and sheer strakes to ensure the ship’s safety.Countries such as Japan and South Korea started to research and development earlier in the related fields,and are in a leading position in the production technology and evaluation of crack arrest steel.Against this background,the paper takes up investigations relying on the National key R&D plans "Marine Steel in Extreme Cold and Ultra-low Temperature Environment and Its Application" and "Research and Development of Crack Arrest Steel for Container Ships with High Crack Arrest Toughness".Aiming at the problems existing in the research and industrial products of heavy-gauge steel with high crack arrest toughness(e.g.,the unclear factors retarding brittle crack propagation,the fuzzy microstructural refinement mechanism,and the poor toughness of the steel produced by the limited slab-to-plate ratio and weakening cooling rate across thickness by water jet at low temperatures,etc.),the microstructures and fracture behavior of 460 MPa heavy-gauge steel were studied,and a low austenitizing temperature rolling process was introduced.As a result,the steel with a thickness of 100 mm and brittle crack arrest toughness of more than 8000 N/mm 3/2 at-10℃was developed in China firstly with our technical support.The main work and conclusions are as follows:(1)Using the industrial steels with different crack arrest toughness,the microstructures and fracture behavior of EH47 steel with a thickness of more than 80 mm were studied.The results show that the microstructures dominated by the equiaxed ferrite,polygonal ferrite(PF),and quasi-polygonal ferrite(QF)at 1/2 thickness can achieve 62.1%high angle grain boundary(HAGB),-84℃ ductile-brittle transition temperature(DBTT),and 209 J average Charpy impact absorbed energy at-80℃in the steel with the thickness of 80 mm and excellent crack arrest toughness.To realize the delamination toughening,the delamination should appear in the middle and lower part of the impact fracture.Thus,the enlarged plastic deformation area at the fracture edge and avoided stress concentration and grain breakage during the impact test can make the radial zone that should have formed a plane-strain fracture still maintain the ductile fracture and increase 63 J absorbed energy.The plane-strain state in the center of the drop weight specimen during loading is the fundamental reason for the brittle fracture at-70℃.However,the deformed microstructures in the plane-stress state at the edge,the fracture delamination in the center,the weak {001}<110>texture,the strong y-fiber texture at 1/2 thickness as well as high-intensity {110}<111>,{110}<112>texture at 1/4 thickness can improve the fracture behavior in plane-strain state.(2)Based on the characteristics of excellent brittle crack arrest ability in low-carbon microalloyed steel,the key factors of retarding brittle crack propagation were studied by adjusting the controlled rolling and cooling process.The results show that 41%acicular ferrite(AF),48.8%HAGB and 4.1 μm effective grain are the keys to the-87℃ DBTT at 1/4 thickness.The high proportion of HAGB between adjacent grains can arrest cracks by changing the propagation direction frequently and increasing the brittle fracture roughness from 1.30 to 1.52.When the average maximum chord length of the M/A islands increases from 1.5 to 2.1 μm,the critical fracture stress decreases from 2601 to 2198 MPa,which can promote the initiation of brittle cracks.The excellent crack arrestability is attributed to AF,which can minimize the expansion of cleavage facets by the split nail effect ahead of the growth direction,so that the cleavage facets were divided into two or more branches,considerably reducing the stress concentration and retarding brittle crack growth at low ambient temperature.(3)To optimize rolling schedules for producing heavy plates with a limited slab-to-plate reduction ratio(3.57),we investigated the effect of rolling reduction below the non-recrystallization temperature(Tnr)of austenite on microstructures,texture,and low-temperature toughness.The steel with 60%reduction below Tnr showed the best toughness down to-80℃ with the impact absorbed energy of 233 J for the minimum EGS of 3.4±4.2 μm,the maximum HAGB proportion of 51.6%,{110} slip planes of 39.1%,and the strongest texture of {113}～{112}<110>,{112}<131>,{332}<113>at 1/2 thickness with the mixed microstructures of ferrite and bainite.The theoretical calculation shows that the nucleation sites for austenite-ferrite transformation are mainly affected by the increased austenite size,decreased effective interfacial area per unit volume(Sv)at the elongated austenite grain boundary,and increased Sv inside the austenite grain with the increased rolling reduction below Tnr.As a result,the total Sv has a higher value(115.5 mm-1)and reasonable distribution at the austenite grain boundaries and inside the austenite grain.(4)A low austenitizing temperature rolling process was utilized to increase the HAGB proportion in hot-rolled steel mixed with ferrite and bainite microstructures.The high HAGB proportion was attributed to the abnormally refined prior-austenite grain of 18.5±10.5 μm caused by the austenitizing temperature of 900℃.Thus,93.5%～129.4%more Sv(192.9～228.7 mm-1)could be provided compared with the optimized conventional process during rolling below the non-recrystallization temperature of austenite.Abundant ferrite grains that form and impinge from the different prior-austenite grains obtain 33.6%～34.5%frequency distribution of the grain boundary misorientation angles which fall into 20～47°.The result is an approximately linear growth trend of the accumulated proportion as the function of the misorientation angles.The premises of delamination toughening are that the delamination cracks appear at the lower part of the impact fracture with a reduced number by adjusting the length and spacing of the weak interface between the brittle phase of M/A islands in the microstructures.(5)Based on EH47 crack arrest steel with a thickness of 100 mm and brittle crack arrest toughness of more than 8000 N/mm3/2 at-10℃,developed firstly in China under our technical support,and the similar industrial steel plates produced by conventional process,the effect of pancaked austenite grain,relaxation after rolling,cooling rate on ferrite/bainite transformation,and the control links of industrial steel plate with excellent crack arrest toughness were clarified using the corroded austenite grain boundaries and EBSD test.The microstructures of EH47 heavy-gauge steel from surface to 1/2 thickness changes from refined bainite→ refined AF+PF+QF→ coarsened bainite+ PF+QF.The large cooling rate(undercooling),appropriate cooling rate and relaxation time,small deformation during rolling,small cooling rate after rolling,and long relaxation time are the main factors affecting the different phase transformation behavior of the microstructures at the surface,1/4 thickness,and 1/2 thickness of heavy-gauge steel.The flattened,refined,and homogenized austenite grains with full-thickness are the key to the full-thickness refinement of microstructures in heavy-gauge steel.The excellent low-temperature toughness caused by the ductile microstructures and texture in the center of the steel makes the micro structure in the center and 1/4 thickness have the same ability to arrest the propagation of the brittle crack in the full-scale fracture of the steel.So that,the "multi peaks" are formed and the crack arrest toughness of steel in the full-thickness fracture is improved.更多还原