800MPa级冷轧耐候双相钢连续冷却相变及组织性能研究

为了探索一种800 MPa级冷轧耐候双相钢的连续冷却转变规律及退火后组织性能变化,利用For-master-FⅡ全自动相变仪及连续退火模拟实验机,进行了连续冷却转变(CCT)曲线的测定及连续退火实验.结果表明:实验钢的过冷奥氏体在很低的冷却速度(0.5℃/s)下即可发生马氏体转变,而珠光体转变较少.当冷速为80℃/s时,仅发生马氏体转变;退火后实验钢显微组织中的马氏体呈带状分布,经最优工艺退火后实验钢的显微组织为多边形铁素体(79%)+块状马氏体(16%)+细小的残余奥氏体(5%),残余奥氏体主要分布于马氏体晶粒内部或铁素体的晶界处;实验钢屈服强度为387 MPa,抗拉强度为863 MPa,延伸率为18%,强塑积达到15534.     

高强贝氏体基体钒微合金化TRIP钢的性能

对一种钒微合金化TRIP钢进行冷轧连续退火,研究了钢的组织特征和力学性能。结果表明,贝氏体基TRIP钢的组织由贝氏体/马氏体和少量的残余奥氏体组成。随着贝氏体区等温时间的延长,钢的抗拉强度下降,屈服强度和延伸率提高。残余奥氏体由块状向薄膜状转变,体积分数增加,薄膜状残余奥氏体主要分布在贝氏体板条间,厚度为50-90 nm。在400℃等温180 s连续退火钢板呈现出相对低抗拉强度(960 MPa)、高屈服强度(765 MPa)和高延伸率(22.0%)的特性,而且加工硬化指数(0.20)、各向异性指数(0.94)和强塑积(21120 MPa.%)也较为优良。     

0.23C-1.9Mn-1.6Si钢中的残余奥氏体对韧塑性的影响

采用优化的IQP工艺处理成分为0.23C-1.9Mn-1.6Si厚度为6.5 mm的低碳硅锰钢,制备出由亚温铁素体、马氏体以及残余奥氏体构成、抗拉强度为1000 MPa的多相组织高强钢。用SEM、XRD、拉伸以及示波冲击等手段对其显微组织和力学性能进行表征,并与同等抗拉强度的IQT钢和QT钢对比,研究了钢中残余奥氏体对韧塑性的影响。结果表明,在室温下IQP多相钢具有更高的冲击韧性、更好的延伸性能和强塑积,综合韧塑性要远优于其它钢种。该钢的性能,与其多相组织结构有密切的关系。大量弥散分布于铁素体和马氏体框架内的残余奥氏体在形变过程中发生TRIP效应,显著改善了钢的韧塑性,从而使其综合力学性能提高。     

正火温度对建筑耐火钢的组织与性能的影响

研究了正火温度对耐火钢组织与力学性能的影响。结果表明,当正火温度为700℃和750℃时,组织由针状或者条状铁素体和少量马氏体组成;升高正火温度至800℃,耐火钢基体组织中条状铁素体含量有所减少,而块状铁素体含量有所增多;继续升高正火温度至850℃,基体组织中主要为块状铁素体;当正火温度为700℃和750℃时,虽然耐火钢的强度较高,但是塑性较差,且屈强比高于0.8,无法满足耐火钢的使用要求;正火温度为800℃时,耐火钢的抗拉强度、屈服强度和断后伸长率分别为595 MPa、465 MPa和22%。     

冷却速度对低合金高强油罐用钢组织和硬度的影响

以含0.012Ti大型石油储罐用钢为研究对象,分析了不同冷却速度对石油储罐用钢显微组织和硬度的影响,结果表明在不同的冷却条件下,室温下得到的组织比较复杂,包含了多边形铁素体PF、珠光体P、退化珠光体PD、魏氏铁素体WF、块状铁素体BF、针状铁素体AF,粒状贝氏体GB、板条贝氏体LB、马氏体M及残余奥氏体等,相应的随着冷速的提高,硬度值明显增大。     

1 GPa级TRIP型BF钢的组织调控和强塑性机制

以节约化、经济化、轻量化为立足点，利用“TRIP效应”解决强塑性同步提高的矛盾，制备出一种1 GPa级TRIP型BF钢，研究了不同组织调控的微观形貌和强塑性机制。结果表明，合理的成分设计和将热轧初始组织调控为(20~30)%铁素体和(70~80)%针状贝氏体，有利于将最终成品的显微组织调控为(75~85)% BF和≥15%的第二相残γ组织，使1 GPa级TRIP型BF钢具有优良的综合性能。两相组织的合理配比及其形貌特征，对TRIP型BF钢的强塑性有显著的影响。在82%的BF组织基体中均匀分布着片层宽度为80~130 nm的第二相残γ组织，在两相协调变形机制作用下可达到优良的强塑性匹配：延伸率20.3%，强塑积达到22.0 GPa·%；当BF组织板条的宽度为0.15~0.45 μm且薄膜状残γ组织的宽度为50~90 nm时，强度达到1099 MPa，发生碰撞时剩余的残γ组织(约6%)发挥“TRIP效应”从而提高防撞件的吸能和安全性。     

热轧钛微合金化TRIP钢的组织与性能研究

对钛微合金化TRIP钢进行连续冷却转变曲线的测定,分析轧制与冷却工艺对其组织与性能的影响。结果表明:实验钢的奥氏体/铁素体、奥氏体/马氏体相变点分别在500~650℃和450℃左右;组织由铁素体/贝氏体及少量残余奥氏体组成;随着终轧温度的升高,实验钢的屈服强度和抗拉强度有所降低;随着空冷结束温度的降低,实验钢的屈服强度降低;当终轧温度和空冷结束温度分别为796℃和722℃时,实验钢的屈服强度,抗拉强度和强塑积分别为661,888MPa和25042MPa·%,其对应组织为细小的铁素体及板条贝氏体,铁素体基体上存在大量细小的析出物。     

基于修正C-J法和RVE模型的780MPa级冷轧双相钢的应变硬化行为

针对经不同工艺热处理的780 MPa级冷轧双相钢板,采用修正的C-J(Crussard-Jaoul)法分析了应变硬化行为,并基于真实组织形貌的代表性体积单元(Representative volume element,RVE)模型模拟其形变行为。结果表明:不同状态的实验钢均表现出较高的初始应变硬化率。岛状和针状马氏体表现出两阶段应变硬化特征,依次对应铁素体塑性变形和铁素体/马氏体协同塑性变形;当马氏体为粗大块状呈现三阶段应变硬化特征,其中第三阶段主要发生马氏体塑性变形时应变硬化能力下降明显。模拟结果表明:当块状和针状马氏体的应变集中分布在铁素体和马氏体界面,岛状马氏体的应变集中分布在马氏体块的连接部位。     

一种新型高强度钢—低含金TRIP钢

前言 在目前大量研究与应用的固溶强化、弥散析出强化,加工强化及组织强化等几种类型的高强度钢中,双相钢的强度高,同时塑性也较好,双相钢的强度取决于马氏体的数量与形态,如果双相钢的强度极限(σ_0)800MPa,马氏体的含量大约30％以上,此时钢的塑性较差,以冷轧薄钢板为例,当σ_b=80OMPa时,拉伸延伸率δ大约20％,当σ_b=1000MPa时,δ则只有15％。为了发展强度极限在800MPa上,而塑性也较好的钢种,国外近几年研制一种含有较多残留奥氏体的新型高强度钢—低含金TRIP钢。 在钢的Ms—Md温度范围内,应变诱发马氏体相变,此时钢呈现较高塑性,称为相变诱导塑性(Transformationinduced plasticity)。V.F.Zackay教授首先利用此效应研制高强度、高塑性的合金并命名为TRIP钢,但由于合金元素含量高,外理工艺复杂,一直没有工业应用。残留奥氏体也具有TRIP效应,也可利用残留奥氏体的TRIP效应发展高强度、高塑性钢。为了与V.F.Zackay等人研究的高合金化的TRIP钢相区别,我们称此种含有残留奥氏体高强度低合金钢为低合金TRIP钢。该钢目前主要     

Mn含量对低碳中锰TRIP钢组织性能的影响

为研究连续退火工艺生产中锰TRIP钢汽车板的可行性,采用CCT-AY-Ⅱ型钢板连续退火机模拟分析了不同锰含量对中锰TRIP钢组织性能的影响规律.采用SEM、TEM和EBSD等微观分析方法观察不同锰含量中锰TRIP的微观组织,利用XRD法测量了残留奥氏体量,实验测量其力学性能.结果表明:试验钢在650℃保温3 min时,随着锰质量分数(4.8%≤w(Mn)≤8%)的增加,屈服强度先增加后降低,抗拉强度持续升高,断后延伸率则基本不变,维持在20%左右,残余奥氏体含量也随着锰含量的增加而增加;当锰质量分数超过6%(含6%)时,真实应力-应变曲线由于动态应变时效而呈锯齿状,且加工硬化指数远大于5Mn钢.试验钢的高塑性由亚稳奥氏体的TRIP效应和超细晶铁素体或马氏体共同提供.     

前驱体对含Cu低碳钢I&Q&P处理后组织性能的影响

采用IQP工艺和EPMA、SEM和XRD等手段,研究了3种前驱体对含Cu低碳钢残余奥氏体含量及力学性能的影响。结果表明,双相区保温初期试验钢奥氏体长大由C配分控制,后期由合金元素Mn、Cu配分控制;双相区保温奥氏体化后,双相区配分后形成弥散分布的局部高浓度Mn、Cu区域仍保留富集效果,在随后的淬火-碳配分阶段易于形成残余奥氏体。经IQP处理后,前驱体为P+F的钢室温组织中马氏体板条较粗,原始奥氏体晶界并不明显;前驱体为F+M钢得到的马氏体板条有序细密;前驱体为M的钢室温组织中马氏体板条更加细密。其中,前驱体组织为M的钢中残余奥氏体量最高,延伸率为24.1%,强塑积可达25 338 MPa·%,综合性能最好。     

回火温度对两相区退火海工钢组织和性能的影响

对690 MPa级海工钢进行“淬火+两相区退火+回火”三步热处理,研究了回火温度对其组织和性能的影响、分析了力学性能变化与组织演变和残余奥氏体体积分数之间的关系。结果表明：回火后实验钢的显微组织为回火贝氏体/马氏体、临界铁素体和残余奥氏体的混合组织。随着回火温度的提高贝氏体/马氏体和临界铁素体逐渐分解成小尺寸晶粒,而残余奥氏体的体积分数逐渐增加;屈服强度由787 MPa降低到716 MPa,塑性和低温韧性明显增强,断后伸长率由20.30%增至29.24%,-40℃下的冲击功由77 J提升至150 J。残余奥氏体体积分数的增加引起裂纹扩展功增大,是低温韧性提高的主要原因。贝氏体/马氏体的分解和残余奥氏体的生成,引起组织细化、晶粒内低KAM值位错的比例逐渐提高和小角度晶界峰值的频率增大,使材料的塑性和韧性显著提高。     

Microstructure of as-quenched 3.5NiCrMoV rotor steel. Part I. General structure and retained austenite

The microstructural features have been examined for 3.5NiCrMoV steam turbine rotor steel, in the as-quenched state and tempered at 500 °C. Quenching produces lath martensite, with bands of retained austenite at the lath boundaries and, to a lesser extent, at prior austenite grain-boundaries. Autotempering occurs during the quench, resulting in loss of tetragonality of the martensite and extensive carbide precipitation in the matrix and to a lesser degree at prior austenite grain boundaries, but not at lath boundaries. Tempering at 500 °C leaves the lath structure largely intact, but causes retained austenite to transform to bands of ferrite and cementite. This transformation does not correlate with the reduction in stress corrosion crack velocity which occurs on tempering. The strength of 3.5NiCrMoV steel in the as-quenched and 500 °C tempered conditions is most probably due to the combination of carbide precipitation strengthening and substructure strengthening.     

Multiphase matrix structure of unalloyed austempered ductile iron

A multiple phase structure characterised by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and film retained austenite (RA) of an unalloyed ductile iron is developed. The designed austempering consists of initial rapid quenching to 210, 200 and 180°C respectively and finally austempering at 220°C for 240 min. The optimum mechanical properties, with a tensile strength of 1330?MPa, an elongation of 3.1% and 422HB, can be achieved by controlling the volume fraction of PM to 12.3% and the RA content to 18.1%. This is mainly attributed to PM that can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies.     

Microstructural characterization of controlled forged HSLA-80 steel by transmission electron microscopy

In this study, transmission electron microscopy (TEM) was used to evaluate the effect of controlled forging followed by cooling at various rates on microstructure of an HSLA-80 steel. The observations demonstrate that water-quenched steel has finer multiphase constituents of lath martensite, bainite and twined martensite, whereas air-cooling has resulted in a mixture of bainitic ferrite, retained austenite or MA constituents along with some Widmanstatten ferrite. When the steel is cooled in sand, the maximum volume fraction of polygonal ferrite (PF) was produced which, in turn, increased volume fraction of MA constituents. Precipitation of fine -Cu, Nb and Ti carbides and carbonitrides was observed and identified using energy dispersive spectrometric analysis (EDS) and electron diffraction.     

Microstructures and tensile properties of laser cladded AerMet100 steel coating on 300 M steel

A layer of AerMet100 steel was coated on the surface of forged 300 M steel using laser cladding technique. The chemical compositions, microstructures, hardness and tensile properties of this AerMet100/300 M material were systematically investigated. Results show that the composition of the AerMet100 clad layer is macroscopically homogeneous, and a compositional transition zone with width of 150 μm is observed between the clad layer and heat affected zone. Microstructures in transition zone transform from the fine needle-like bainite in 300 M steel to the lath tempered martensite in AerMet100 clad layer. Microstructures in heat affected zone also gradually change from the thick plate bainite and blocky retained austenite (unstable heat affected zone) to fine needle-like bainite and film-like austenite (stable heat affected zone) due to different thermal cycle processes. Thick plate bainite together with blocky retained austenite in unstable heat affected zone reduce the strength and ductility of AerMet100/300 M material. However, the tensile specimens, consisting of clad layer and stable heat affected zone, show slightly inferior mechanical properties to 300 M steel. Ductile fracture exists in AerMet100 clad layer while quasi-cleavage fracture occurs in the stable heat affected zone.     

An ultrahigh strength steel with ultrafine-grained microstructure produced through intercritical deformation and partitioning process

In this work, a novel design scheme in which deformation-induced ferrite transformation (DIFT) was applied to produce fine-grained steel and the quenching is controlled by quenching and partitioning (Q&P) process has led to the development of a new kind of steel. This steel possesses excellent mechanical properties and the ductility can be further improved without compromising strength because the refined microstructure contains martensite, retained austenite and deformation-induced ferrite. The highest elongation of 15% allied with strength of 1700 MPa is obtained through hot deformation followed by Q&P treatment at 300 °C. The microstructure evolutions are discussed in terms of the current knowledge of the Q&P process and the experimental observations. The results show that the designed multiphase steels are a promising candidate for the development of the third generation of advanced high strength steels.     

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240,360,and 400 ℃.The results show that martensite,bainite,and retained austenite (RA) are the main microstructural phases.The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix,and the transformation of ultrafine bainite into coarse lath bainite and granular bainite,respectively.Thick filmy RA was distributed between bainite laths.The polygonal martensiteaustenite islands and blocky RA formed along the grain boundaries.The average carbon concentration in the matrix decreased with the temperature increase,while the impact toughness initially increased and then dropped with temperature.The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps.The microcracks disappeared in the RA on the prior austenite grain boundaries.On the other side,the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns.The polygonal martensite-austenite islands or blocky RA constrained the microcracks.After austempered at 400 ℃,the brittle fracture was dominant,showing river patterns,and the microcracks propagated through the granular bainite without any resistance.     

一种新型高强塑积异质冷轧中锰钢的力学性能

使用原位电子背散射衍射(EBSD)和球差透射电镜(ACTEM)等手段,研究了新型异质结构中锰TRIP钢在拉伸过程中微观组织的演变机制和力学性能。结果表明,在680℃退火后的实验钢中生成了多形貌、多尺度的异质奥氏体结构(颗粒状、块状、片层状奥氏体)和铁素体组织,其抗拉强度为1272 MPa,总延伸率为54.5%,强塑积高达69.3 GPa·%。在拉伸过程中C/Mn含量较低的颗粒状奥氏体先发生相变,而C/Mn含量较高的块状和片层状奥氏体在较大的应变范围内逐渐发生相变,从而导致高强度与高塑性的良好匹配。结果还表明,马氏体相变优先在奥氏体晶界/相界附近的区域形核。与晶粒尺寸相比,C/Mn元素对奥氏体稳定性的作用更重要。