基于临界区加速冷却的(B+F)X80大变形管线钢的组织和性能研究

采用热模拟、力学性能测试和材料显微分析等试验技术,对X80管线钢在临界区加速冷却工艺下的组织性能变化规律进行了研究。结果表明,通过临界区加速冷却,X80管线钢可获得贝氏体+铁素体(B+F)双相组织。随着始冷温度的上升,试验钢的贝氏体含量增加,铁素体含量降低,导致屈服强度增高,塑性降低。当始冷温度为840℃时,显微组织以细小、多位向分布的贝氏体为主,辅以高密度位错的多边形铁素体。这种(B+F)双相组织使得试验钢的屈强比为0.80、均匀伸长率为10.0%、形变强化指数为0.12,满足了大变形管线钢的技术要求。     

1470MPa级双相钢的性能特征与强韧化机制

对0.16C-1.38Si-3.2Mn双相钢进行轧制和退火处理,用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、电子背散射衍射(EBSD)等手段表征试验钢的微观组织和断口形貌,分析试验钢经退火后钢板的力学性能和加工硬化行为,重点研究了试验钢晶粒细化的强韧化机制。结果表明:试验钢在800℃退火后的显微组织主要由8.8%铁素体和91.2%回火马氏体构成。退火后的钢板具有良好的综合力学性能,屈服强度为873 MPa,表现为连续屈服特征,抗拉强度为1483 MPa,总伸长率为11%,屈强比为0.58;试验钢的Mn含量、退火前的初始组织、冷轧大变形以及退火过程中关键工艺参数等都有利于试验钢退火板的晶粒细化,铁素体尺寸为1-2μm,马氏体板条束的有效晶粒尺寸为0.2-1.5μm。细小的晶粒有利于阻碍位错的运动和增加裂纹扩展的阻力,从而提高了钢板的强度和塑韧性。     

新型贝氏体钢的组织和冲击疲劳性能研究

通过显微组织观察和冲击疲劳实验,研究了不同热处理新型贝氏体钢的组织和冲击疲劳性能.结果表明:新型贝氏体钢正火低温回火的组织由贝氏体铁素体和奥氏体组成,淬火低温回火组织为回火马氏体和残余奥氏体,正火低温回火热处理的冲击疲劳寿命高于淬火低温回火热处理的冲击疲劳寿命.分析了多冲疲劳裂纹扩展的行为,讨论了正火低温回火提高冲击疲劳的原因.     

热轧TRIP钢组织性能的研究

采用实验室热轧机对高硅和低硅TRIP钢(A钢和B钢)进行控制轧制试验,研究了热轧后等温淬火对热轧TRIP钢组织性能的影响.通过显微组织观察,力学性能分析,探讨了两种钢的应变诱导相变和相变诱发塑性行为.研究表明:A、B钢均能够获得铁素体、贝氏体和大量稳定残余奥氏体的混合组织,具有较高的力学性能;残余奥氏体稳定性是提供TRIP的重要因素,B钢中贝氏体和残余奥氏体较多,相变诱发塑性效果更好,其性能优于A钢;等温时间影响热轧TRIP钢的力学性能,随等温时间的延长,A、B钢的伸长率增加,等温时间超过120 min,导致碳化物析出,残余奥氏体的稳定性降低;B钢经热轧后在400 ℃等温25 min,抗拉强度和伸长率分别达到了784 MPa和36%的最高值.     

450~780 MPa系列双相钢扩孔性能实验

目的避免双相钢在成形过程中经常出现的翻边或扩孔开裂的问题,提高生产效率。方法以几种强度级别为450~780 MPa的双相钢为实验对象,测试材料的单向拉伸和扩孔性能,并从微观组织上分析影响双相钢强度和扩孔性能的因素。结果 450~780 MPa强度级别双相钢的屈服强度随铁素体晶粒尺寸的减小而增加,抗拉强度随马氏体体积分数的增加呈近似线性增加。双相钢的扩孔性能随着材料强度的提高呈下降的趋势。DP500与DP450和DP600相比,抗拉强度(579 MPa)居中,伸长率最高,但由于其马氏体形态和分布的差异,其扩孔性能反而最低。结论双相钢的扩孔性能会受到马氏体的含量、尺寸、形态和分布的影响,与材料强度和伸长率没有必然的关系,当马氏体呈颗粒状均匀分布时,具有更好的扩孔性能。     

氮含量对钒微合金钢粗晶热影响区组织和韧性的影响

采用焊接热模拟的方法,研究了氮含量对实验钢焊接粗晶热影响区(CGHAZ)显微组织和韧性的影响规律。结果表明:随着氮含量的增加,CGHAZ的组织从晶界铁素体、贝氏体和侧板条铁素体转变成针状铁素体、多边形铁素体和少量的贝氏体,且铁素体晶粒细化;CGHAZ韧脆转变温度(FATT₅₀)先降低后升高,屈服强度升高。氮含量从0.004 4%增加到0.009 4%时,有效晶粒尺寸减小,导致CGHAZ的FATT₅₀降低;氮含量从0.009 4%增加到0.019 0%时,CGHAZ中固溶氮、屈服强度增量对FATT₅₀的综合作用大于晶粒的细化作用,导致FATT₅₀升高。      

超快冷条件下Mn-Nb-B系低碳贝氏体高强钢组织与性能研究

采用Mn-Nb-B减量化成分设计的低碳贝氏体高强钢为研究对象,通过热模拟实验研究实验钢热变形行为和相变行为。结合中厚板生产线特点制定控制轧制与超快速冷却相结合生产工艺路线,充分利用超快速冷却条件下的细晶强化、析出强化等综合强化机制,实现综合力学性能优良的低成本高强工程机械用钢的试制和生产。产品屈服强度和抗拉强度分别达到678MPa和756MPa,伸长率A50为33%,-20℃低温冲击达到261J。产品显微组织由粒状贝氏体、针状铁素体和板条贝氏体组成,基体组织内弥散分布着细小的点状、粒状M/A岛和均匀细小的(Nb,Ti)(C,N)析出粒子以及大量位错组织。     

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。     

N含量对Cr-Mo-V系超低碳贝氏体钢组织性能和析出行为的影响

通过光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和能谱仪(EDS)等实验方法,研究了三种不同N含量的超低碳贝氏体钢的显微组织和析出相的成分、尺寸、形貌以及分布等特征。结果表明:低氮含量的钢组织为粒状贝氏体,高氮含量的钢组织为粒状贝氏体+少量的针状铁素体。当实验钢中V/N比为3.4时,通过细晶强化和沉淀强化综合作用,可以使材料的屈服强度和抗拉强度分别增加231MPa和95MPa。与氮含量低的钢相比,高氮含量的钢具有更细小的贝氏体铁素体板条亚结构,且析出相尺寸减小,体积分数增加。基体中存在两种尺寸的纳米级析出相:一种尺寸在10～15nm之间,为V(C,N)析出相,弥散分布在贝氏体板条内部;另一种是含有Cr和V尺寸在10nm以下,具有面心立方结构的(V,Cr)(C,N)复合析出相。     

轧制工艺对CSP生产低碳贝氏体高强钢组织和性能的影响

研究了CSP工艺生产低碳贝氏体高强钢的工艺控制与显微组织和力学性能的关系.通过比较不同终轧温度和卷取温度对低碳贝氏体高强钢组织性能的影响,得出钢板最终组织为不同比例的铁素体加贝氏体复相组织,且随着终轧温度和卷取温度尤其是卷取温度的降低,组织中贝氏体的体积分数增多,组织更加细小均匀,并且可以获得细小弥散的M-A组织和析出粒子,从而提高钢的屈服强度和韧性.     

Microstructures and Mechanical Properties of Si-AI-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP （transformation induced plasticity） steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10^-2 to 10^-4 s^-1, the volume fraction of transformed martensite from retained austenite, as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

Microstructures and Mechanical Properties of Si-Al-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation induced plasticity) steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10-2 to 10-4 s-1, the volume fraction of transformed martensite from retained austenite,as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

ULCB钢相变产物粒状与板条状贝氏体组织性能研究

对ULCB钢的主要相变组织粒状贝氏体与板条贝氏体进行了力学性能比较,结果显示粒状贝氏体与板条贝氏体是完全不同的两种组织,较高温度相变产物粒状贝氏体组织较为粗大,对强度和韧性有不利影响,而较低温度相变产物板条贝氏体组织对强度和韧性是有益的.     

热轧C-Si-Mn系TRIP钢的组织与力学性能

为了探讨热轧TRIP钢的制备工艺与其组织及力学性能的关系,采用热轧控冷工艺在实验室制备了C-Si-Mn系TRIP钢,利用光学显微镜、扫描电镜及透射电镜对试验钢的组织进行了观察,利用能谱仪对试验钢中的夹杂进行了观察.研究得到试验钢的力学性能为:σb=605 Pa,σs=440 Pa,δ=28.4%,σs/σb=0.73.定量金相检测结果表明,试验钢中三相含量分别为:残余奥氏体5.6%,铁素体67.6%,贝氏体26.8%.     

The effect of processing parameters on the microstructure and mechanical properties of low-Si transformation-induced plasticity steels

A base low Si, high-Al transformation-induced plasticity (TRIP) steel and one with 0.03Nb and 0.02Ti (wt%) additions were subjected to thermo-mechanical processing (TMP) and galvanising simulations. The microstructure and mechanical properties were analysed using a combination of optical and electron microscopy, X-ray diffraction and tensile testing and the results compared with those from intercritically annealed–galvanised steels. The addition of Nb and Ti results in microstructure refinement and an increase in the amount of the retained austenite after TMP which in turn, leads to increases in the tensile strength (~750 MPa) and the total elongation (TE) (~29 %). A deterioration in the volume fraction of retained austenite and the mechanical properties was noted in both steels after the additional galvanising simulation. For the base steel, all TMP and galvanised samples presented with continuous yielding during tensile testing. The Nb–Ti steel exhibited discontinuous yielding and extended Lüders banding when TMP was followed by a longer coiling time. Both steels returned discontinuous yielding after the intercritical annealing–galvanising treatment. The discontinuous yielding behaviour was associated with the much finer ferrite grain size in the intercritically annealed steels and the ageing processes that take place during galvanising.     

Mechanical properties and hot-rolled microstructures of a low carbon bainitic steel with Cu-P alloying

A low carbon bainitic steel with Cu-P alloying was developed. The new steel aims to meet the demand of high strength, high toughness and resistance to chloride ion corrosion for the components used in the environment of sea water and oceanic atmosphere. Mechanical properties of the steel were tested and strengthening and toughening mechanisms were analyzed by comparing hot-rolled microstructures of the low carbon bainitic steels with and without Cu-P alloying. The results show that Cu-P alloying provided strong solution strengthening with weak effect on ductility. The toughness loss caused by Cu-P alloying could be balanced by increasing the amount of martensite/remained austenite (M/A island) at lower finishing temperature. The static recovery process during rolling interval was delayed by the interaction of phosphorous, copper atoms with dislocations, which was favorable to the formation of bainitic plates. Super-fine Nb(C, N) particles precipitated on dislocations had coherency with bainite ferrite at 830 °C finishing temperature. Raising finishing temperature to 880 °C, Nb(C, N) particles were prone to coarsening and losing coherency. It was also found that no accurate lattice match relationship among retained austenite, martensite and bainite in granular bainitic microstructure.     

Study on carbide-bearing and carbide-free bainitic steels and their wear resistance

Carbide-free and carbide-bearing bainitic steels have been obtained. The relationship between the bainitic microstructure and wear resistance has been studied. Results show that carbide-free upper and lower bainitic microstructures obtained in the steel with Si?+?Al mainly consist of bainitic ferrite and retained austenite. Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si?+?Al consist of bainitic ferrite, carbide and trace amounts of retained austenite. The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite, especially the toughness. Under lower wear loading, carbide-bearing lower bainite (LB) exhibits higher wear resistance. Under higher wear loading, carbide-free LB exhibits higher wear resistance, which results from the improved surface hardness due to strain-induced martensitic transformation from the retained austenite.     

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。     

Development in high-grade dual phase steels with low C and Si design

Cold rolled dual phase steels with low C and Si addition were investigated in terms of combination of composition and processing in order to improve mechanical properties and workability including welding and galvanizing. Mo and Cr could be used as alloying elements to partially replace C and Si to assure enough hardening ability of the steels and also give solute-hardening. Mo addition is more effective than Cr addition in terms of obtaining the required volume fraction of martensite and mechanical strength. The ferrite grain was effectively refined by addition of Nb microalloying, which gives optimized mechanical properties. The experimental results show that it is possible to obtain the required mechanical properties of high grade 800 MPa dual phase steel, i.e., tensile strength > 780 MPa, elongation > 15%, and yield/tensile strength ratio < 0.6 in the condition of low carbon (C < 0.11 wt.%) and low silicon design (Si < 0.05 wt.%) through adequate combination of composition and processing.     

Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat-affected zone

The microstructure of acicular ferrite and its formation for the grain refinement of coarse-grained region of heat-affected zone of high strength low-alloy bainite steels were studied using three-dimensional reconstruction technique. Crystallographic grain size was analyzed by means of electron backscatter diffraction. It was revealed that the microstructure in the coarse-grained region of the heat-affected zone consisted of predominantly bainite packets and a small proportion of acicular ferrite. Acicular ferrite was of lath or plate-like rather than needle or rod-like morphology. Tempering of the coarse-grained region of heat-affected zone showed that the acicular ferrite was more stable than the bainite, indicating that the acicular ferrite was formed prior to bainite. The acicular ferrite laths or plates divided the prior austenite grains into smaller and separate regions, and confining the bainite transformed at lower temperatures in the smaller regions and hence leading to the grain refinement in the coarse-grained region of the heat-affected zone.