【作者】 蔡明晖；

【导师】 丁桦；

【作者基本信息】 东北大学 ， 材料加工工程， 2009， 博士

【摘要】 热轧双相钢具有强度高、屈强比低、初始加工硬化率高以及强度和韧性配合良好等优点,是目前应用最多的先进高强度钢（Advanced High Strength Steel,AHSS）之一。与传统的铁素体/马氏体双相（FMDP）钢相比,铁素体/贝氏体双相（FBDP）钢具有强度和冷成形性的匹配,且焊接、疲劳以及延伸凸缘性能优良,是底盘和车轮等部件用热轧高强度钢的理想材料。本文结合上海宝钢集团公司开发项目“铁素体/贝氏体高扩孔钢的相变及强化机理”,研究低碳硅-锰钢在连续冷却及等温过程中的相变行为,利用控轧控冷（Thermo-MechanicalControl Process,TMCP）工艺开发出经济型的热轧FBDP钢板,通过扩孔实验评价不同强度级别FBDP钢板的延伸凸缘性能,并对FBDP钢在单轴拉伸下的变形行为和断裂机理,以及扩孔成形中的裂纹形成及其扩展行为进行研究,取得了如下研究成果:（1）通过热模拟实验研究不同硅、锰含量的低碳钢在连续冷却过程中的相转变规律,定量分析合金元素、热变形工艺参数以及冷却速度对相变温度和转变组织的影响。结果表明:1)硅含量增加使Ar₃温度升高,锰含量增加使Ar₃温度降低,但锰对Ar₃温度的影响效果强于硅;2)硅对Ar₃温度的影响与变形条件有关,增加硅含量使未变形条件下Ar₃温度升高的更加明显;3)硅在促进多边形铁素体形成的同时,也抑制了贝氏体相变,而锰不仅细化了铁素体和贝氏体组织,还促进了贝氏体的形成;4)变形使低硅钢中贝氏体铁素体板条变得细而短,使高硅钢中铁素体以及M/A岛的分布更均匀。（2）通过等温转变实验研究硅含量和变形对铁素体和贝氏体转变的影响及机理,采用超组元方法,修正后的Zener两参数（考虑了碳、硅和锰等元素之间的交互作用）,对等温转变过程中相变热力学和相变动力学进行数值模拟。结果表明:1)随硅含量增加,γ→α转变的孕育期缩短,γ→B转变的孕育期延长;2)随着硅含量增加,组织中铁素体晶粒细化,且贝氏体铁素体的形态由细长条状变为短杆状;3)奥氏体变形后,组织中铁素体的形态多呈等轴状,铁素体晶粒明显细化,贝氏体的体积分数减少;4)增加硅含量或奥氏体变形增大了碳在γ/（γ+α）或α/（α+γ）相界面的平衡浓度,以及γ→α转变的驱动力|△G^γ→α|。（3）通过TMCP实验,在低碳硅-锰钢中获得具有铁素体和贝氏体组织的FBDP钢,研究合金元素和工艺参数对实验钢显微组织和力学性能的影响规律。结果表明:1)以低碳硅-锰钢为基础,合理选择硅、锰含量和TMCP工艺参数,可获得不同强度级别的高延伸凸缘型FBDP钢板:抗拉强度可达472.5～721.5MPa,伸长率可达27.4～43.8%,扩孔率可达95～117.8%,满足高延伸凸缘钢扩孔率λ≥80%和伸长率A≥20%的标准,且比较经济;2)增加锰含量,可以有效提高实验钢的强度和韧性,但塑性和延伸凸缘性能有所下降;3)在实验范围内,当硅含量由0.06%增加至0.57%时,实验钢的抗拉强度、韧性和延伸凸缘性能均提高;当硅含量增至0.92%时,实验钢的抗拉强度升高,但韧性和延伸凸缘性能下降;当硅含量为0.57%时,实验钢具有强度、韧性和延伸凸缘性能的良好匹配;4)与铁素体/珠光体钢和FMDP钢相比,FBDP钢具有良好的强度与延伸凸缘性能之间的平衡;5)终轧温度的降低虽然有利于细晶粒铁素体的形成,改善钢材的塑性和韧性,但也使组织中贝氏体的体积分数降低,导致钢材的抗拉强度降低以及屈强比升高。（4）研究FBDP钢在扩孔过程中的裂纹形成及扩展行为。结果表明,预制圆孔的边缘是扩孔成形的变形危险区,当该处的切向伸长超过材料的成形极限时,就会产生缩颈或裂纹。FBDP钢的裂纹扩展主要以微孔聚集模式进行,当遇到贝氏体时,裂纹通过F-B相界面并剪断铁素体进行扩展。在裂纹扩展中,大部分的等轴铁素体在应力场作用下沿垂直于裂纹方向被显著拉长,这表明裂纹扩展在FBDP钢中进行时,强度较低的铁素体在应力场的作用下会产生较大的塑性变形,在一定程度上减弱了裂纹附近的应力集中,阻止了裂纹扩展。（5）对FBDP钢在单轴拉伸下的变形行为和断裂特性的研究表明:在均匀塑性变形阶段,FBDP钢的n^*（瞬时加工硬化指数）值随真应变ε的增加呈逐渐减小趋势,且n*-ε曲线大体可以分为n^*较高,n^*随ε增加而缓慢下降,以及n^*随ε增加而迅速下降等三个阶段。与HSLA和FMDP钢相比,FBDP钢在低应变区域具有明显高的加工硬化值。在拉伸断裂的缩颈过程中,FBDP钢的孔洞或微裂纹多产生在铁素体内或F-B相界面附近。通过压缩-拉伸实验测定了FBDP钢的包辛格效应参量以及预压缩变形对FBDP钢的加工硬化速率dσ/dε和加工硬化指数n的影响:随着预压缩变形量（0～1.0mm）增加,FBDP钢在拉伸变形过程中的dσ/dε和n值基本呈先减小然后增大的趋势。更多还原

【Abstract】 Dual-phase steels are one of the most widely used AHSS（Advanced High Strength Steel） sheets,because of their characteristic mechanical properties,such as high strength,low yield ratio,high initial work hardening rate as well as good combination of strength and toughness. As compared to FMDP（Ferrite/Martensite Dual-Phase） steels,FBDP（Ferrite/Bainite Dual-Phase） steels exhibit a superior combination of strength and cold formability,especially excellent welding and fatigue properties as well as high stretch-flangeability.Therefore, FBDP steel may be an ideal candidate for the complex press-formed parts,such as automotive chassis and wheels.The work of this dissertation was carried out integrating with the project of ’Phase Transformation and Strengthening Mechanism of Ferrite/Bainite Dual-Phase Steels with High Hole-Expansion Ratio’,which is supported by the Baoshan Iron and Steel Group.The continuous cooling and isothermal transformation behavior of low carbon Si-Mn steels was studied.Economical hot-rolled FBDP steels were developed by TMCP（Thermo-Mechanical Control Process） technology and the stretch flangeability of these steels with different strength grades was evaluated by hole-expansion tests.Deformation behavior and fracture characteristics in the uniaxial tension,and crack formation and propagation behavior during the hole-expansion were also investigated.The main work involved as follows:（1） The continuous cooling transformation behavior of low carbon steels with different Si and Mn contents was investigated by thermal simulation experiment.Effects of alloying elements,hot-deformation parameters and cooling rates on ferrite transformation start temperature（Ar₃）,microstructure and properties were studied.The results showed:1) Ar₃ rises with the increase in Si,while it drops with the increase in Mn.The effect of Mn on Ar₃ is more obvious than that of Si;2)Influence of Si on Ar₃ is dependent on deformation,and such effect is more significant without the prior deformation of austenite;3) Si promotes the formation of polygonal ferrite,but inhibited the transformation of bainite.Mn not only refines the microstructure of ferrite and bainite,but also promotes the formation of bainite;4)In case of the deformation with true strain of 0.4,the length of bainitic ferrite laths is significantly decreased in the low Si steel,whereas,the M/A constituents become more uniform in the high Si steel.（2） The influence of Si and prior deformation on the isothermal transformation of ferrite and bainite in low carbon steels was studied.The numerical simulation of phase transformation kinetics and thermodynamics was carried out by the super-element method and modified Zener’s parameters.The results were showed as follows:1) With Si content increasing,the incubation time of ferritic transformation decreases,while that of bainitic transformation increases;2) The ferrite grains are refined and the bainitic ferrite laths become shorter with increasing Si content;3) After the prior deformation of austenite,the ferrite grains become equiaxed and more refined,and the volume fraction of bainite decreases;4) Deformation or increasing Si content increases the carbon equilibrium concentration onγ/（γ+α） orα/（α+γ） phase boundary and driving force for ferrite transformation.（3） The economical hot-rolled FBDP steels were developed by TMCP technology.Some factors affecting the microstructure and properties of FBDP steels were investigated.1) FBDP steels with high stretch-flangeability were exploited by designing Si and Mn contents as well as TMCP parameters.The tensile strength,total elongation and hole-expansion ratio may reach 472.5～721.5MPa,27.4～43.8%and 95～117.8%,respectively,which meets the standards of high stretch-flangeability steels（λ≥80%,A≥20%）;2) The increase in Mn content increases both the strength and toughness,but decreases the ductility and stretch-flangeability; 3) When Si content increases from 0.06%to 0.57%,the values of tensile strength,impact toughness and stretch-flangeability all increase.When Si content increases to 0.92%,the strength increases,whereas,the values of toughness and stretch-fiangeability decrease.The steel with 0.57%Si exhibits the best combination of all the three property parameters;4) Compared with ferrite/pearlite and FMDP steels,FBDP steels have a better balance between strength and stretch-flangeability;5) The decrease in finish rolling temperature promotes the formation of fine-grained ferrite and improves the ductility and toughness of experimental steels,but results in the lower tensile strength and higher yield ratio of steels due to the decrease in the volume fraction ofbainite.（4） The crack formation and propagation behavior during the hole-expansion were investigated.Since the edge of punched hole is the dangous zone of deformation,the necking or cracking will occur when the tangential elongation is more than the forming limit here.The crack of FBDP steels is formed by the mode of microvoid coalescence.When a microcrack meets the bainite,it mostly propagates along the phase interface between ferrite and bainite by cutting off ferrite grains.In the crack propagation,the majority of equiaxed ferrite grains are elongated along the direction perpendicular to the crack,which shows that ferrite grains with lower strength will have a greater plastic deformation.This helps to reduce the stress concentration near the crack and thus restrain the crack propagation.（5） The deformation behavior and fracture characteristics in the uniaxial tension were studied.The curves of the instantaneous work-hardening factor n^* value versus true strainεare made up with three stages during the uniform plastic deformation:n^* value is relatively higher at stageⅠ,decreases slowly withεin’stageⅡ,and then decreases quickly withεat stageⅢ.Compared to the ferrite/pearlite and FMDP steels,FBDP steels show higher n^* value in the low strain region.The uniaxial compression-tension testing was performed,and the Bauschinger effect parameter and work-hardening behavior were examined to understand the deformation mechanism of FBDP steels under complex loading conditions.The values of dσ/dεand n firstly increase and then decrease with the prior compressive deformation increasing from O to 1.0mm.更多还原