【作者】 赵岩；

【导师】 王群骄；

【作者基本信息】 东北大学 ， 材料工程（专业学位）， 2023， 硕士

【摘要】 伴随着人类对低碳、环保的认识越来越深刻,很多大型钢铁公司都在努力提升车辆用钢的强度和韧性,以达到减轻重量,从而达到节能减排的目标。高扩孔钢板是一种新型的高强度高韧性钢材,其微观结构为铁素体-贝氏体(F-B)复合结构,不仅拥有高的强度,还拥有优异的延伸凸缘性能,可用于复杂形状的车用零部件,如车辆后轴悬挂摇杆、轮辐、轮辋等。本文以C、Si、Mn作为基本元素,同时加入少量Cr、Mo、Nb、Ti等合金元素,通过分析各元素在钢中的作用,对轧制高扩孔钢进行成分设计,并通过热模拟实验、轧制实验、热处理实验、力学性能检测和显微组织分析探讨成分不同的高扩孔钢奥氏体连续冷却组织变化规律,研究轧制工艺参数、热处理工艺对力学性能的影响,分析高扩孔钢的强韧化机理。结论如下:(1)实际热轧工艺可参考连续冷却曲线。在冷速为1～5℃/s,实验钢的微观组织为准多边形铁素体和贝氏体组织,有时会出现极少量的珠光体。当冷速提高到5～15℃/s,铁素体的转变量变少,贝氏体体积分数不断增加。当进入冷速为15～30℃/s时,以切变为转变机制的马氏体组织开始出现。变形引起CCT曲线的左侧偏移,使得B相变区域增大。Cr、Mo、Nb、Ti等合金元素的添加抑制了铁素体和珠光体的转变,使Ar3线下降,CCT曲线向右偏移,扩展贝氏体转变区。(2)热轧工艺参数和热处理对组织和性能有着很大的影响,终轧温度降低,铁素体晶粒也变得更加细小,铁素体含量增加;冷速的越大时,晶粒更加细小;卷取温度决定最终组织的构成,卷取温度在490℃以上时,显微组织为铁素体和粒状贝氏体的混合组织,卷取温度在350～490℃之间时,显微组织主要为铁素体和板条状贝氏体。轧制后的板材经过高温回火后,组织形貌并无改变,晶粒尺寸变小,析出物含量增大。(3)随着碳含量的增加,实验钢的强度增加,扩孔率下降,与3#实验钢相比,0.12C的4#实验钢抗拉强度提高了 95 MPa,扩孔率降低了 11%;与0Cr-0Mo的2#实验钢,添加了 0.41Cr-0.29Mo的3#实验钢抗拉强度提高了 305 MPa,扩孔率下降了 21.7%;0.04Ti的2#实验钢相比于0Ti的1#实验钢相比,抗拉强度提高了 145 MPa,扩孔率下降了 5.5%,而实验钢经过高温回火后,抗拉强度提高,伸长率变化不大,扩孔率上升。(4)高扩孔钢合适的生产工艺为:两阶段控制轧制+两阶段冷却+卷取保温+高温回火,具体如下:加热温度为1200～1250℃,开轧温度为1050～1100℃,终轧温度为830～850℃,空冷时间为10 s,层流冷速为61.9℃/s,卷取温度为340～360℃,回火温度为650℃,保温30min。采用该工艺生产的3#实验钢具有良好的综合力学性能:屈服强度可达1127 MPa,抗拉强度可达1144 MPa,伸长率达到17.3%,扩孔率达到48.2%,满足了 HR 780/980 HE要求,且扩孔率较国标提高了 54%。(5)高扩孔钢的裂纹扩展机理为微观孔隙聚集机理。裂纹萌生是由缺陷引起的应力集中,沿铁素体-贝氏体相界向外扩散,并在裂尖向外扩散到铁素体而形成的。本实验拟采用控轧控冷、加入合金元素等手段调控微观组织,以改善扩孔率和强度,在保证高扩孔的前提下获得更高的强度。更多还原

【Abstract】 With the increasing awareness of low-carbon and environmental protection,many large steel companies are trying to improve the strength and toughness of vehicle steel,in order to reduce weight,so as to achieve the goal of energy saving and emission reduction.High expansion steel is a new type of high strength and high toughness steel,its microstructure is ferrite-bainite(F-B)composite structure,not only has high strength,but also has excellent extended flange performance,can be used in complex shape of vehicle parts,such as rear axle suspension rocker,spoke,rim,etc.In this paper,C,Si,Mn as the basic elements,at the same time add a small amount of Cr,Mo,Nb,Ti alloy elements,through the analysis of the role of each element in steel,rolling high expansion steel composition design.Through thermal simulation experiment,rolling experiment,heat treatment experiment,mechanical property test and microstructure analysis,the change law of austenite continuous cooling structure of high expansion steel with different compositions was discussed,the influence of rolling process parameters and heat treatment process on mechanical properties was studied,and the strengthening and toughening mechanism of high expansion steel was analyzed.The conclusion is as follows:(1)The actual hot rolling process can refer to the continuous cooling curve.When the cooling rate is 1-5℃/s,the microstructure of the experimental steel is quasi-polygonal ferrite and bainite,and sometimes a very small amount of pearlite appears.When the cooling rate increased to 5～15℃/s,the ferrite transformation decreased,and the bainite volume fraction increased.When the cooling rate was 15～30℃/s,the martensite structure with shear as the transition mechanism began to appear.The left side of the CCT curve is shifted by the deformation,and the B phase transition region is enlarged.The addition of Cr,Mo,Nb,Ti and other alloyed elements inhibited the transformation of ferrite and pearlite,decreased the Ar3 line,and shifted the CCT curve to the right,extending the bainite transition zone.(2)Hot rolling process parameters and heat treatment have great influence on the microstructure and properties,the final rolling temperature decreases,the ferrite grain becomes finer,the ferrite content increases;The larger the cooling rate,the finer grain.The final microstructure is determined by the coiling temperature.When the coiling temperature is above 490℃,the microstructure is a mixture of ferrite and granular bainite.When the coiling temperature is between 350 and 490℃,the microstructure is mainly ferrite and lamellar bainite.After high temperature tempering,the microstructure of the rolled plate does not change,the grain size decreases,and the content of precipitates increases.(3)With the increase of carbon content,the strength of the experimental steel increases and the porosity decreases.Compared with the 3#experimental steel,the tensile strength of the 4#0.12C experimental steel increases by 95 MPa and the porosity decreases by 11%.Compared with the 2#test steel of 0Cr-0Mo,the tensile strength of the 3#test steel of 0.41Cr0.29Mo is increased by 305 MPa,and the porosity is decreased by 21.7%.Compared with the 1#test steel of 0Ti,the tensile strength of the 2#test steel of 0.04Ti is increased by 145MPa,and the porosity is decreased by 5.5%.After tempering at high temperature,the tensile strength of the test steel is increased,the elongation is not changed much,and the porosity is increased.(4)The appropriate production process of high expansion steel is as follows:two-stage controlled rolling+two-stage cooling+coiling insulation+high temperature tempering:The heating temperature is 1200～1250℃,the opening temperature is 1050～1100℃,the final rolling temperature is 830～850℃,the air cooling time is 10 s,the laminar flow cooling rate is 61.9℃/s,the coiling temperature is 340～360℃,the tempering temperature is 650℃,and the heat preservation is 30 min.The 3#test steel produced by this process has good comprehensive mechanical properties:the yield strength can reach 1127 MPa,the tensile strength can reach 1144 MPa,the elongation can reach 17.3%,and the porosity can reach 48.2%,which meets the requirements of HR 780/980 HE,and the porosity is increased by 54%compared with the national standard.(5)The mechanism of crack propagation in high expansion steel is microscopic pore aggregation.Crack initiation is caused by stress concentration caused by defects,which diffuses outward along the ferrite-bainite phase boundary and outward to ferrite at the crack tip.In this experiment,we plan to control the microstructure by means of controlled rolling and cooling,adding alloying elements and so on,so as to improve the porosity and strength,and obtain higher strength under the premise of ensuring high porosity.更多还原