TMCP工艺对车轮钢组织及扩孔性能的影响

采用TMCP(Thermo Mechanical Controlled Process)实验,研究了不同冷却方式对汽车车轮用低碳FB钢的组织及扩孔性能的影响,以及FB钢扩孔时的断裂类型及断裂机理。结果表明,采用连续冷却的实验钢,得到了针状铁素体＋贝氏体组织,针状铁素体明显降低了两相硬度比,改善了扩孔性能。FB钢在扩孔过程中的断裂机理为微孔聚集的韧性断裂。实验结果可为进一步研究低碳FB钢在车轮上的应用提供实验依据。     

铁 素 体 贝 氏 体 钢 的 扩 孔 性 能

 通过力学拉伸实验和扩孔实验对铌、钛复合微合金化低碳热轧铁素体贝氏体钢板的力学性能和成形性能进行了研究,以了解控轧控冷工艺参数和微观组织对其性能的影响,并分析了影响扩孔率的因素以及铁素体贝氏体钢的裂纹扩展机制。研究结果可为开发高强度、高扩孔性能的汽车底盘用钢板提供实验依据。     

FB双相钢延伸凸缘性能与组织的关系

通过对比观察不同扩孔率双相钢的显微组织,研究了铁素体贝氏体双相钢的延伸凸缘性能与组织之间的关系。试验结果表明,当显微组织由多边形铁素体和粒状贝氏体组成时,FB双相钢能达到优异的延伸凸缘性。多边形铁素体有利于扩孔率的提高而下贝氏体和板条马氏体则有害;M-A的尺寸对延伸凸缘性影响很大,随着M-A岛尺寸的减小,扩孔率逐渐增大,当M-A岛的宽度为0.7μm时,FB双相钢的扩孔率能达到120%。     

含Nb低碳FB钢组织与扩孔性能的研究

 通过TMCP试验,研究了不同Nb含量低碳FB钢的微观组织和扩孔性能。试验结果表明,随着终轧温度或卷取温度的降低,试验钢强度提高,而扩孔性能降低。随着Nb含量的增加,试验钢的抗拉强度与扩孔性能明显改善。当终轧温度控制在860 ℃,冷却中间温度在710 ℃,卷取温度在450 ℃时,Nb的质量分数约为0.02%的试验钢的铁素体晶粒尺寸细化到6.3 μm,抗拉强度达到570 MPa,扩孔率为91%左右,获得了良好的综合性能。     

含有上贝氏体的ER8车轮钢的裂纹扩展行为

研究不同含量的上贝氏体对ER8车轮钢裂纹扩展行为的影响。利用激光共聚焦显微镜（LSCM）和扫描电镜（SEM）对ER8车轮钢的显微组织和裂纹扩展路径进行了研究。实验结果表明:ER8车轮钢中的组织除了有铁素体和珠光体,还存在上贝氏体;裂纹穿过上贝氏体和珠光体扩展,最终停止在珠光体区域;与珠光体组织相比,裂纹在上贝氏体中的扩展路径更曲折。利用扫描电镜（SEM）对ER8车轮钢的裂纹扩展变形进行原位观察。实验结果表明:含有80%上贝氏体的ER8车轮钢拉伸时,组织变形过程主要以铁素体和上贝氏体为主,裂纹在上贝氏体和珠光体中连续扩展,伴随着珠光体的变形;而含有50%上贝氏体的ER8车轮钢拉伸时,组织变形过程主要以铁素体和珠光体为主,并且上贝氏体对铁素体和珠光体的变形起到阻碍作用。上贝氏体能够有效地阻止裂纹扩展,在偏转裂纹路径和延缓裂纹扩展方面起着重要作用;并且对铁素体和珠光体的变形起到阻碍作用。      

回火对低碳贝氏体钢组织和性能的影响

研究了(250~500)℃×30 min回火热处理对低碳贝氏体钢组织和性能的影响。结果表明,TMCP状态试验钢的组织为粒状贝氏体、板条贝氏体和针状铁素体。回火温度升高,粒状贝氏体增加,板条贝氏体和针状铁素体减少,组织逐渐粗化。350℃回火时,试验钢的屈服强度(Rt0.5)为735~765 MPa,抗拉强度(Rm)为845~865 MPa,屈强比为0.87~0.88,-20℃冲击功为218~257 J,强韧性匹配最佳。300~400℃回火时,裂纹形成能(E1)、韧性裂纹扩展能(E2)及脆性裂纹扩展能(E3)+脆性裂纹扩展止裂能(E4)最大,止裂性能最佳。     

连续退火过时效工艺对碳锰钢组织和性能的影响

 在热模拟实验机上模拟了连续退火过时效工艺,研究了过时效温度和过时效时间对008C 015Si 150Mn钢组织和性能的影响。结果表明：过时效温度为350~550 ℃,保温时间1~20 min时,实验钢组织均由铁素体及分布在铁素体晶粒间的贝氏体组成;实验钢抗拉强度与贝氏体中M A岛含量和M A岛中的碳含量有关;随过时效温度升高,M A岛含量出现“峰值”,随过时效时间延长,M A岛中的碳含量也出现“峰值”。     

Si对热轧铁素体-贝氏体双相钢显微组织变化和机械性能的影响

系统的研究了si对铁素体-贝氏体双相（FBDP）钢显微组织变化、拉伸性能、冲击韧性和拉伸凸缘性的影响,从0到0．95％加人si可促进细的铁素体等轴晶的形成,高Si（0．95％）时也可导致块状马氏体岛和残余奥氏体的形成。随si含量的增加屈服强度、抗拉强度和延伸率提高,因此,由于加入si提高了应变硬化速率,改善了抗拉强度和韧性之间的平衡。孔扩张后的裂纹形貌表明FBDP钢有极好的拉伸凸缘性,这与显微裂纹在铁素体相内扩展以及铁素体晶粒沿与裂纹垂直方向延伸有关。含0．95％Si钢与含0．55％Si钢相似都有高强度与高冲击韧性组合,特别是含0．95％Si钢表现出极好的抗拉强度与拉伸凸缘性组合。     

热轧贝氏体双相钢中组织对性能的影响

通过超快冷工艺试验,得到了强韧性较好的热轧贝氏体双相钢,研究了双相钢中组织对性能的影响。研究结果表明,试验钢由贝氏体与铁素体组成,其抗拉强度可达到501 MPa,伸长率在34%左右。试验钢良好的机械性能与贝氏体组织有关,硬相贝氏体含量过多和珠光体均会恶化扩孔性能,大量的多边形铁素体和准多边形铁素体有效地提高了双相钢抗延迟断裂的能力,有利于试验钢伸长率和扩孔率的提高。     

低碳锰(铌)钢控轧控冷实验研究

通过TMCP研究了低碳锰钢和添加微量Nb的低碳锰铌钢的组织和力学性能的影响因素.研究了Nb对实验钢显微组织和力学性能的影响.Nb的添加能够细化铁素体晶粒,促进铁素体转变,对贝氏体形成有一定的抑制作用.相对低碳锰钢来说,铌的加入减少了贝氏体的体积分数,贝氏体强化效果减弱,钢的屈强比升高.通过增加冷速和降低卷取温度,可以使低碳锰铌钢获得一定量的贝氏体,综合性能较佳.低碳锰铌钢的主要强化机制有细晶强化、贝氏体相变强化和析出强化.     

Effect of Si,C and Mn on the formation of bainite in ferrite-bainite steel

High-strength steels have been widely applied to automotive chassis parts.In order to form complex shapes,high hole expansion rates and high formability are required.Dual phase (DP) steel has a good formability,but a poor hole expansion rate.In this circumstance,another kind of steel which has a microstructure of ferrite-bainite,rather than ferrite-martensite,has been found to be an alternative solution.It is called FB steel.This steel with Si,C and Mn additions are applied in this study.A two-step cooling process is used to get the desired F+ B microstructures.Continuous cooling transformation (CCT) diagrams are made with deformation and without deformation,and starting times and temperatures of the phase transformations of interest are obtained.It is shown that Si,C and Mn contents in the steel strongly affect the shapes and positions of the CCT diagrams,as well as the final microstructures of FB steel.An increase of the Si content can promote the formation of ferrite and move the CCT diagram toward the left.However,when Si content is too high,when comparing to carbon and manganese contents,the formation of bainite will be retarded because of the formation of more ferrite.It increases the amount of C in a solid solution in the untransformed austenite and promotes the formation of pearlite.C and Mn can inhibit the formation of ferrite and retard the accumulation of C in austenite.Therefore,the appropriate balance of C,Si and Mn contents in steels will be able to help in obtaining the desired microstructure.     

硅对Nb-V微合金化Si-Mn-Cr-Mo系贝氏体轨钢组织和性能的影响

研究了1 250 MPa级Nb-V 微合金化Si-Mn-Cr-Mo系贝氏体轨钢(/％:0.25 C,0.38～1.56 Si,1.69～1.76 Mn,1.34～1.37 Cr,0.32～0.33 Mo,0.034 0.036 Nb,0.11～0.12 V)硅含量对试验钢显微组织和力学性能的影响.试验结果表明:试验钢热...     

690 MPa级ULCB熔敷金属组织与强韧化规律

 采用传统的高强钢焊接材料焊接690 MPa级低碳铜沉淀强化钢时,仍需严格控制热输入、预热温度、层间温度,这使得低碳铜沉淀强化钢的优良性能和可节约生产成本的优势得不到很好地发挥。通过采用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电镜(TEM)等表征方法,研究了不同质量分数的Si/Mn/Ni配比对690 MPa级超低碳贝氏体(ULCB)熔敷金属的组织及强韧性能的影响,为690 MPa级低碳铜沉淀强化钢配套的焊接材料的工程化应用提供一定的技术支持和积累。结果表明,690 MPa级超低碳贝氏体(ULCB)熔敷金属组织主要由板条贝氏体、粒状贝氏体和针状铁素体组成。当Si质量分数为0.16%、Mn质量分数为1.46%时,熔敷金属组织细化,冲击韧性得以提升,但Si含量过低易使贝氏体铁素体呈块状,导致韧性提升有限。而当Si质量分数为0.29%、Mn质量分数为1.02%时,Ni含量增加,贝氏体铁素体板条呈细长条状,显微组织相互交错分布,使熔敷金属冲击韧性显著改善。相变位错强化受贝氏体开始转变温度(B_s)影响,这是影响ULCB熔敷金属强度的主要原因。ULCB熔敷金属中夹杂物主要分布在贝氏体铁素体的板条亚结构间,少量成为针状铁素体的形核质点,促进针状铁素体形核,因此,对熔敷金属中的夹杂物进行控制,可进一步发挥超低碳贝氏体熔敷金属的潜力,提高其韧性。     

Effect of Strengthening Phase on Deformation Behaviour during Uniaxial Tension of Hot-rolled Dual Phase Steel

Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite （F） ＋martcnsite （M） or ferrite （F）-bainite （B） with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength （YS/TS） and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible de formation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.     

Multiphase microstructure and property of low carbon bainite/ferrite steel by quenching and carbon partitioning

The multiphase microstructure and properties of low carbon steel by IQ&PB process under different partitioning temperatures and time were studied by means of SEM, EPMA, XRD and tensile testing. Results show that the microstructures of experimental steel consist of ferrite, bainite and retained austenite. With the increase of the holding time in two- phase region, the C and Mn contents apparently increase in martensite which transformed from austenite, and C and Mn content are 1. 47 times and 1. 16 times than average value of the base. With the decrease of the quenching and partitioning temperature, the volume fraction of the bainite increases, the microstructure is refined, and the content of M/A islands increases. With the temperature and time increased, the volume fraction of retained austenite at room temperature increases, the tensile strength decreases, the total elongation increases, and work hardening behavior occurrs continuously. When the partitioning time is 30min and the partitioning temperature is 400??, the tensile strength of the steel is 1107MPa, the elongation is 24%, the product of strength and elongation reaches above 26568MPa??%.     

C-Mn-Si系超级贝氏体钢连续冷却转变（CCT）曲线

采用热膨胀法和金相法,通过Gleeble-1500热模拟试验机测定C-Mn-Si系低碳(/%:0.11C、1.15Si、1.85Mn、0.032Al、0.003 Ti、0.002 4N)和中碳(/%:0.35C、1.11Si、1.82Mn、0.041Al、0.002 Ti、0.004 2N)贝氏体钢在0.5～30℃/s的冷却速度下连续冷却时的膨胀曲线,确定相变点,并结合显微组织,借助Origin软件分别绘制出两种钢的连续冷却转变(CCT)曲线。结果表明,0.11%C钢当冷却速度≤1℃/s时获得铁素体+贝氏体+马氏体组织,冷却速度≥2℃/s时为贝氏体+马氏体组织,0.35%C钢冷却速度≥0.5℃/s即可获得贝氏体+马氏体组织;随碳含量增加,贝氏体和马氏体转变温度均降低。     

L360NB管线钢连续冷却转变(CCT)曲线的测定和冷却速度对组织的影响

用Gleeble-1500热模拟试验机测定了L360NB管线钢圆铸坯(/%:0.14C、0.32Si、0.14Mn、0.020Ti、0.010V、0.038Nb、0.040Al)的连续冷却转变(CCT)曲线,并用光学显微镜观察了0.1～50℃/s冷却速度的组织。结果表明,当冷却速度为0.1℃/s时,钢的组织主要为板条贝氏体和准多边形铁素体,有少量针状铁素体,当冷却速度≥1.0℃/s,准多边形铁素体减少,板条状贝氏体和针状铁素体增加;当冷却速度≥10℃/s时试样的组织主要为板条状贝氏体和针状铁素体,准多边形铁素体很少,在50℃/s时未观察到块状准多边形铁素体,说明Nb、V、Ti复合微合金化促进贝氏体形成。     

Forming Limit Curve (FLC) and Fracture Mechanism of Newly Developed Low‐Carbon Low‐Silicon TRIP Steel

The Forming‐Limited Diagram (FLD) of intercritically annealed 0.11C‐1.65Mn‐0.62Si TRIP‐assisted steel was investigated. The high FLD₀ value of this new low carbon TRIP steel was indicative of a superior formability. The micro‐structural changes during deformation and fracture were studied in detail. The polygonal ferrite phase was found to plastically deform first and deformed most at larger strains. Fracture was initiated by micro‐voids nucleated at ferrite grain boundaries, within ferrite grains or at the interface between ferrite and the harder phases. Cracks were formed after micro‐voids grew, coalesced, and expanded in one direction. When crack tips reached the bainite phase or the martensite/austenite constituent, the cracks propagated along the boundary of these phases. Cracks reaching retained austenite islands caused stress‐induced martensite transformation at the crack tip. The direction of motion of the cracks also changed in this case.     

硅含量对低碳钢连续冷却中转变组织的影响

通过热模拟压缩实验,对C、Mn含量基本相同,w（Si）对低碳钢在850℃变形后、以30℃/s冷却到室温时的组织演变的影响。结果表明：w（Si）由O．50％增加到0．99％时,硅的增加促进了先共析铁索体的形成,而w（Si）由0．99％增加到1．41％时,硅的增加促进了粒状贝氏体的形成。同时Si对组织中先共析铁素体的晶粒尺寸以及体积分数的变化也有一定的影响。     

锰对低碳耐候钢组织与强韧性的影响

在真空感应炉冶炼了2炉不同锰质量分数的低碳耐候钢,利用热模拟机和金相显微镜分析了其组织特征和相变规律,并通过室温拉伸、冲击实验且结合断口分析表征了实验钢的强韧性。热模拟实验表明,低碳高锰耐候钢组织在低冷速下（＜1 ℃/s)为铁素体＋少量珠光体,而在较大冷速内(1～10 ℃/s)为贝氏体＋铁素体复相特征,随冷却速度的增加则钢中贝氏体增多。分析轧态组织表明,2组实验耐候钢中主要组织均为等轴铁素体;增加钢中锰则其强度明显增大,虽塑性和冲击韧性有所降低,但仍可获得良好的强韧性组合。