【作者】 王晓；

【导师】 王作成；

【作者基本信息】 山东大学 ， 材料加工工程， 2012， 博士

【摘要】 低合金高强度(HSLA)H型钢作为一种典型的低碳、低合金结构钢,具有较高的强度、韧性和优良的焊接性能。在高层建筑、桥梁、大型船舶以及海上石油平台等领域有着广泛的应用。近年来,随着高寒、高海拔地区的开发,具有良好低温韧性的高等级H型钢受到广泛的关注。但是,目前生产的低合金高强度H型钢的韧脆转变温度普遍偏高,低温下易发生脆性断裂,难以满足在低温地区使用的要求。因此,提高低合金高强度H型钢的低温韧性,降低其韧脆转变温度,具有重要的现实意义和理论价值。本文在添加痕量硼元素的低合金高强度H型钢的基础上,进行了合金成分设计,确定了硼镍复合添加的工艺方案。利用热模拟试验、常温拉伸试验、系列温度冲击试验、淬透性试验、显微硬度试验、光学显微镜和扫描电镜、高分辨透射电镜以及X射线衍射等试验方法和手段,系统研究了试验钢在不同状态下的组织和性能,获得了在-50℃下具有良好冲击韧性的热轧H型钢；并在此基础上,研究了该试验钢的热处理工艺,获得了最佳的淬火、回火工艺参数,利用淬火回火处理,进一步将试验钢的韧脆转变温度降低到-96℃,使其具备了优异的低温韧性。热物理模拟试验显示,试验钢在10℃/s加热时,其Ac1和Ac3温度分别为770℃和906℃,在接近平衡条件下冷却时,其Ar3和Ar1温度为775℃和560℃。随冷却速度由0.1℃/s增加到50℃/s时,试验钢的组织逐渐由多边形铁素体+珠光体转变为仿晶界铁素体+板条贝氏体、板条贝氏体+板条马氏体以及全部马氏体结构；其硬度由50HRA增加到73HRA。随变形量由0%增加到40%,试样中的铁素体含量逐渐增加,形变诱导相变作用逐渐加强,相变开始温度增加了55℃。对现场试生产的硼镍复合添加低温用H型钢进行了力学性能和微观组织分析。结果发现,所有样品的强度、塑性指标均满足国标要求,其-50℃低温冲击韧性达到80J左右,达到国标要求的2倍多。试样断口上较深的孔洞表明试样发生了韧窝断裂,从而使冲击韧性获得提高。镍元素对试验钢的夹杂物、组织以及二相粒子影响不明显,其细化珠光体片层以及对铁原子晶格结构的影响是提高低温韧性的主要原因。硼镍复合添加的HSLA钢在不同温度下进行的淬透性试验发现,在870到1000℃范围淬火时,试验钢的淬透性不随温度的增加而持续增加,950℃时淬透性最高；经1070℃高温处理后,试验钢的淬透性不升反降,铁素体含量增加。通过与只加硼不加镍以及硼、镍均不添加的试样对比,发现上述现象是由于硼促进淬透性的作用因高温处理而消失以及硼细化碳氮化铌颗粒所致。根据端淬曲线换算的950℃时试验钢浸水冷却最大可淬透板厚达19.34mm。通过对不同工艺热处理后试验钢的组织、性能研究发现,试验钢淬火后的抗拉强度最高可达到1000MPa。高温回火后抗拉强度仍保持在500-600MPa之间,较热轧试样的抗拉强度提高100MPa以上,伸长率下降约10%,但试样的韧性大幅提高到200J以上,韧脆转变温度最低达到-96℃。使调质0.5Ni钢的最低许用温度降低20℃以上。低温韧性的大幅升高是基体回复再结晶与碳化物球化共同作用带来的。颗粒状碳化物依赖晶界获得长大,但并非沿晶界析出,而是钉扎晶界,有利于细化晶粒,未对韧性造成破坏。通过对热处理参数对试验钢韧性的影响研究发现,淬火温度主要影响淬火回火后试样的韧脆转变温度,对韧性影响不大,而回火温度主要影响韧性的高低,对韧脆转变温度影响不大。这说明,淬火后残余铁素体的含量是影响韧脆转变温度的主要因素,而影响韧性高低的主要因素是淬火组织的再结晶程度。更多还原

【Abstract】 High strength low alloy H-beam, as a typical low-carbon low-alloy structural steel, exhibits an outstanding combination of high strength, resistance to brittle fracture and good weldability. It has been extensively applied to high-rise buildings, bridges, construction of large ships and offshore oil drilling platforms. Recently, with exploration of high latitude and cold regions, significant attention has been focused on high-grade H-beam with good toughness at low temperature. However, ductile-brittle transition temperature (DBTT) of existing hot-rolled H-beam product is too high, products have a tendency to brittle fracture, which does not meet the request for utilization in cold areas. Therefore, improving low-temperature toughness and reducing DBTT of HSLA H-beams are not only of remarkable realistic significance but also of theoretical value.Based on chemical composition of boron-added HSLA H-beam, composition of the new H-beam was designed, and0.5%nickel was added in new H-beams together with trace of boron so as to reduce its DBTT further. Mechanical properties and microstructures of experimental steels were investigated by means of thermomechanical simulation, room temperature uniaxial tensile tests, instrumented Charpy impact test, Jominy test, micro-hardness test, optical microscopy (OM), scanning electronic microscopy (SEM), high resolution transmission electronic microscopy (HRTEM) and X-ray diffraction (XRD). The H-beam with good impact toughness at-50℃was producted. Optimal heat treatment process of this steel was studied. Ductile to brittle transition temperature (DBTT) of the steel reduced to-96℃by quenching and tempering. Its low temperature impact toughenss is excellent.Thermomechanical simulation results show that Ac1temperature and AC3temperature of experimental steel are770℃and906℃respectively, when heating rate is10℃/s. In phase equilibrium conditions, the Ar3temperature and Ar1temperature are775℃and560℃respectively. When cooling rate increases from0.1℃/s to50℃/s, microstructures of this new type of steel gradually transform from polygonal ferrite and pearlite, grain-boundary ferrite and lath bainite, lath bainite and martensite to single martensite. Accordingly, its hardness increases from50HRA to73HRA. When deformation increases from0%to40%, more ferrite is found in specimens and Ar3temperature increases by55℃, due to effect of deformation induced phase transformation.Mechanical properties and microstructures of boron-nickel added cryogenic H-beam, produced by Laiwu Iron and Steel Company, were investigated. The results show that strength and plasticity of all producets meet requirements, especially, the toughness tested at-50℃reaches80J, which is twice more than requirement of national standard. Deep hole in fracture surface means dimple fracture that increases impact toughness significantly. Addition of nickel has little influence on inclusions, microstructures and second-phase particles. Refining pearlite and transformation of lattice structures are main causes of increasing low temperature toughness by nickel.Results of Jominy tests show that hardenability of boron-nickel added HSLA steel does not increase linearly when quenching temperature increases from870℃to1000℃. Its hardenability peaks at950℃. After heated at1070℃, the hardenability does not increase but decrease and volume fraction of ferrite increase. Compared with boron added steel and no boron or nickel steel, decreasing of hardenability is due to disappearance of anxo-action of boron and refined carbonitride niobium. Maximum quenching thickness equivalent through Jominy test reaches19.34mm.Mechanical properties and microstructures of experimental steel through different heat treatments were investigatied. The results show that tensile strength of quenched specimens reaches1000MPa. After high temperature tempering, the tensile strength remains from500MPa to600MPa, which is100MPa higher than that of hot-rolled steel. Compared with hot-rolled steel, elongation of quenched and tempered specimens decreases by10%but its impact toughenss reaches as high as above200J. Ductile brittle transition temperature decreases to-96℃. The allowable temperature of steel contents0.5%nickel decreases by20℃. Increasing of low temperature toughness is due to recrystallization and spheroidization of carbides. Carbides precipitated along grain boundaries do not harm to toughness, on the contrary, they prevent grain coarsening during recrystallization.Effect of heat treatment parameters on toughness was studied. Results show that quenching temperature has great influence on ductile brittle transition temperature but has little influence on toughness. Tempering temperature has great influence on toughness but has little influence on ductile brittle transition temperature. Residual ferrite after quenching is major influence of ductile brittle transition temperature. Recrystallization is the main factor of toughness.更多还原