【作者】 王俊峰；

【导师】 王利； 金学军；

【作者基本信息】 上海交通大学 ， 材料科学与工程， 2017， 博士

【摘要】 随着能源危机和环境污染问题日益严重,汽车行业正在探究通过多种途径实现钢质白车身轻量化。一种方法是使用先进高强钢和超高强钢,如双相钢(DP钢)、相变诱发塑性钢(TRIP钢)、淬火-配分钢(Q&P钢)、马氏体钢(MS钢)等去替代低强度级别钢种,使汽车零部件减薄而实现减重。另一种潜在方法是在C-Mn钢的基础上添加适量Al元素,在确保钢板具有较高强韧性特征的同时降低钢板密度,从而使其具有高比强度(强度与密度之比)来实现车身轻量化目标。上述具有高比强度的新型钢铁材料称为低密度钢。按微观组织不同,低密度钢可划分为铁素体低密度钢、铁素体-奥氏体双相低密度钢和奥氏体低密度钢。近年来,富含3~6 wt%Al的铁素体低密度钢由于具有低密度、优异的力学性能和相对良好的可制造性而受到广泛关注。因其微观组织中包含主要特征组元相δ铁素体和适量残余奥实体,铁素体低密度钢通常又称为δ-TRIP钢。本论文以Fe-1.1Mn-4.1Al-0.35C-0.38Si(wt.%)δ-TRIP钢为研究对象,利用材料显微表征方法(OM、SEM、EBSD和TEM)、相分析方法(XRD和磁性法)和力学表征方法(单向拉伸和纳米压痕法)分别对钢板的微观结构、相组成和宏微观力学性能进行深入分析,重点研究了:热轧卷取温度对热轧钢板以及冷轧退火钢板的微观组织和力学性能的影响;冷轧退火钢板基体中残余奥氏体的稳定性及其在不同变形温度条件下的应变诱导相变行为;通过在δ铁素体内部形成弥散分布的析出物来强化δ-TRIP钢的可行性。论文主要研究结果如下:(1)热轧卷取温度(介于400~700°C之间)显著影响热轧δ-TRIP钢的微观组织和力学性能。一般地,热轧组织包含铁素体条带和第二相条带,两者均沿轧向分布。铁素体条带由δ铁素体晶粒组成,而第二相条带则由高温奥氏体相变而来,且相变产物与卷取温度直接相关。当卷取温度低于450°C,第二相条带为贝氏体条带。具体的说,当卷取温度为400°C,贝氏体条带主要由下贝氏体和块状残余奥氏体组成;而当卷取温度为450°C时,贝氏体带主要为板条状铁素体和奥氏体构成的无碳贝氏体结构。当卷取温度高于500°C时,第二相条带主要是由珠光体形成的碳化物条带。在热轧组织中存在三种类型的碳化物:下贝氏体内的过渡型?碳化物、位于碳化物条带内部和碳化物条带-铁素体条带界面处的渗碳体以及δ铁素体晶界处的κ-碳化物。热轧板残余奥氏体含量在450°C卷取时可获得最大值9.6%,而500°C以上卷取时降至0。450°C卷取时,由于亚稳残余奥氏体具有较多体积分数和板条状形貌(即较好力学稳定性),拉伸变形过程中TRIP效应显著提高了均匀延伸率,使均匀延伸率和总延伸率分别达到25%和32%。(2)高温卷取时,热轧钢板中碳化物易发生粗化,促进后序冷轧变形中裂纹萌生。但卷取温度的提高同时增强铁素体基体回复和降低冷轧变形抗力,从而有效抑制裂纹扩展和边裂的发生。从可制造性出发,实验用δ-TRIP钢的卷取温度选用700°C。(3)热轧卷取温度对冷轧退火钢板力学性能影响不明显。冷轧钢板经830°C临界退火后可获得最佳强塑性。退火钢板的屈服强度、抗拉强度和延伸率分别为500MPa、805 MPa和33%。退火组织中残余奥氏体Ms点小于-150°C,sMs温度约为-10°C。(4)室温拉伸时,冷轧退火钢板基体中残余奥氏体发生应变诱发马氏体相变,68%的残余奥氏体转变形成孪晶马氏体(厚度100~200 nm),未转变的残余奥氏体晶粒内形成了大量的形变孪晶,钢板抗拉强度达805 MPa。在25~150°C范围内,残余奥氏体稳定性随变形温度升高而增加,马氏体相变逐渐被抑制,应变硬化率降低,强度逐渐降低;至150°C,马氏体相变被完全抑制,残余奥氏体在拉伸变形过程中发生应变诱发贝氏体相变(形成位错型贝氏体),但由于形成的贝氏体量较少,应变硬化率较低,故强度降至700 MPa。随着变形温度进一步升高,更多的残余奥氏体转变为贝氏体,抗拉强度逐渐升高。当变形温度升高至300°C时,90%的残余奥氏体转变为贝氏体,且贝氏体铁素体尺寸与室温形成的马氏体相当,剩余残余奥氏体以薄膜状分布于贝氏体铁素体片层间(Al和Si元素抑制了渗碳体形成);另外,在该较低变形温度下,碳原子扩散受到限制,贝氏体铁素体中仍然含有大量过饱和碳原子,由应变诱发的贝氏体相变使δ-TRIP钢在该变形温度下具有最高加工硬化率,钢板抗拉强度达到最大值890 MPa。随变形温度进一步升高,贝氏体相变驱动力降低;另外,碳原子的扩散能力增强,碳原子从贝氏体铁素体迅速扩散至相邻奥氏体并使之稳定。两者均使奥氏体向贝氏体转变被抑制,变形组织中有大量形变奥氏体存在。相应地,钢板抗拉强度显著降低。以上结果表明,随变形温度增加,δ-TRIP钢的变形强化机制依次为:应变诱发马氏体相变(SIMT)和孪晶诱发塑性(TWIP)→应变诱发贝氏体相变(SIBT)→位错滑移(DG)。(5)在合适的热力变形和热处理条件下,Nb/Ti微合金化和Cu合金化使δ-TRIP钢基体弥散分布细小析出物,从而为开发高强δ-TRIP钢提供一种潜在方法。对于Nb/Ti微合金化而言,添加少量Nb/Ti可以略微细化冷轧退火钢板基体铁素体晶粒尺寸,但却不能消除其晶粒尺寸不均匀分布和微观组织呈条带状分布的组织特征。铁素体晶粒内部形成不同尺度析出物,并且析出物所起的作用也不尽相同。研究表明:50~200 nm尺度Ti(C,N)和小于20 nm尺度(Nb,Ti)C细小颗粒通过钉扎位错和阻碍晶界移动抑制铁素体的再结晶,使δ铁素体内形成大量亚晶界;5~10μm尺寸(Nb,Ti)(C,N)粗大颗粒容易诱发裂纹在形变钢板基体中萌生。碳化物和碳氮化物析出降低了退火钢板内部残余奥氏体体积含量及其稳定性。力学性能上,由于Nb/Ti微合金化,上述微观结构特征使冷轧退火δ-TRIP钢的屈服强度提高65 MPa,但δ-TRIP钢的抗拉强度提高不明显且延伸率降低。对于Cu合金化而言,添加1.0 wt%Cu虽然不能细化热轧钢板基体中铁素体和残余奥氏体晶粒尺寸,但却延缓了卷取过程中贝氏体相变过程,使热轧板中残余奥氏体含量达17.1%(相同热轧工艺条件下,无Cu添加的热轧δ-TRIP钢板中残余奥实体含量为9.6%);尺寸约2~4 nm的ε-Cu颗粒均匀弥散析出显著强化了铁素体,使δ-TRIP钢热轧板的屈服强度提高了83 MPa(17%)。相比较不含Cu的δ-TRIP钢,析出强化与TRIP效应共同作用使热轧含Cu的δ-TRIP钢具有更高的加工硬化率,其抗拉强度增加至805 MPa(提高130 MPa,即19%),且延伸率基本不变(近30%)。以上研究结果为低密度δ-TRIP钢的开发与应用提供了理论基础与指导。更多还原

【Abstract】 The automotive industry has been exploring approaches to achieving structural lightweighting of steel-based car body-in-white in order to cope with the increasingly serious issues of energy shortage and environmental pollution.One approach is to utilize advanced high strength steels(AHSS)and ultrahigh strength steels(UHSS)such as dual phase(DP)steels,transformation-induced plasticity(TRIP)steels,quenched & partition(QP)steels and martensite(MS)steels to replace low-strength mild steels,thereby thinning of the gauge of auto parts and weight savings realized.Another potential way is to reduce the density but maintain high strengths and toughness of C-Mn steels by alloying with abundant Al,thus increasing specific strengths of steels.The newly-designed steels with enhanced specific strengths are named Al-enriched lightweight steels.In view of the microstructural difference,lightweight steels can be generally categorized into three types: ferrite-based lightweight steels,ferrite-austenite duplex steels and austenite-based lightweight steels.In recent years,ferrite-based lightweight steels containing 3~6 wt% Al have attracted increasing attention because of their lower densities,excellent combinations of strength and ductility,and appreciable producibility.Since the microstructure consists of a major characteristic phase of δ-ferrite embedded with appreciable volume fraction of metastable retained austenite,ferrite-based lightweight steels are often called δ-TRIP steels.In the dissertation,a δ-TRIP steel with chemical compositions of Fe-1.1Mn-4.1Al-0.35C-0.38Si(wt.%)was studied in terms of microstructure,phase constitution as well as macroscopic and microscopic mechanical properties,using microstructural characterization techniques including optical microscopy(OM),scanning electron microscopy(SEM),electron back-scattered diffraction(EBSD)and transmission electron microscopy(TEM),phase analysis techniques including X-ray diffraction(XRD)and magnetic probing,and mechanical characterization techniques including uniaxial tensile test and nanoindentation test.Investigative emphasis was placed on:(i)the effect of coiling temperature on the microstructure and mechanical properties of the hot rolledδ-TRIP steel as well as the cold-rolled and continuously-annealed steel;(ii)the stability of retained austenite in the cold-rolled and continuously-annealed steel sheet and the strain-induced transformation behavior of retained austenite during tensile deformation at different temperatures;(iii)the feasibility of strengthening the δ-TRIP steel by forming dispersed precipitates inside the phase of δ-ferrite.Main results were obtained as follows:(1)Coiling temperature ranging between 400 °C and 700 °C significantly affects the microstructure and mechanical properties of the δ-TRIP steel.Basically the hot-rolled microstructure consists of ferrite bands and secondary phase bands,both bands aligned with the rolling direction.Ferrite bands are aggregates of δ-ferrite grains,while secondary phases bands are originated from the decomposition of antecedent austenite.The secondary phase band could show different variants and morphologies at different coiling temperatures.The secondary phase band is a bainite band at coiling temperatures lower than 450 °C.More specifically,the bainite band mainly consists of lower bainite together with blocky retained austenite at the coiling temperature of 400 °C,while it primarily contains carbide-free bainite being an aggregate of lath-shaped ferrite and austenite at the coiling temperature of 450 °C.The secondary phase band is a carbide band which mainly contains a pearlite structure at coiling temperatures higher than 500 °C.There are three types of carbides in the microstructure of hot-rolled plate: transitional ?-carbide present inside lower bainite,cementite present within carbide bands as well as at the boundaries between carbide bands and δ-ferrite bands,and κ-carbide present at δ-ferrite grain boundaries.The volume fraction of retained austenite reaches the peak value of 9.6 pct at the coiling temperature of 450 °C,and abruptly drops to zero when the coiling temperatures are higher than 500 °C.Metastable lath-shaped retained austenite with a higher volume fraction contributes to significant enhancement of elongation through the TRIP effect,leading to a uniform elongation of 25% and an elongation-to-failure of 32% at the coiling temperature of 450 °C.(2)At high coiling temperatures,carbides in the hot-rolled steel readily coarsen,promoting cracking initiation in the steel plate during subsequent cold rolling deformation.In the meantime,an increase in coiling temperature facilitates recovery of the δ-ferrite matrix and then reduces the resistance to cold deformation of the steel plate,which further effectively suppresses the crack propagation in the steel matrix and the occurrence of noticeable edge cracking.Taking manufacturability into consideration,the coiling temperature of the δ-TRIP steel plate after hot rolling was selected to be 700 °C for the present study.(3)Coiling temperature does not significantly affect mechanical properties of the cold-rolled and continuously-annealed steel sheet.Intercritical annealing at 830 °C gives rise to an optimal combination of strength and ductility for the annealed sheet.The yield strength,tensile strength and elongation are 500 MPa,805 MPa and 33%,respectively.The Ms temperature of the retained austenite in the annealed sheet is below-150 °C and thesMstemperature is about-10 °C.(4)During room temperature tensile deformation,retained austenite in the annealed steel sheet is subjected to strain-induced martensite transformation,with 68 percent of retain austenite transformed into twin martensite(martensite plates being 100~200 nm thick).Deformation twins are formed in many untransformed retained austenite grains.The corresponding tensile strength of the annealed sheet is 805 MPa.When tensile deformation is performed in the range of 25 to 150 °C,the stability of retained austinite increases with deformation temperature,and the martensitic transformation is gradually suppressed.Correspondingly,the strain hardening rate and tensile strength of the annealed sheet decrease.At the deformation temperature of 150 °C,the strain induced martensite transformation of retained austenite is completely inhibited and starts to be replaced by the strain induced bainite transformation.However,a small quantity of evolved bainite results in a low strain hardening rate and a local minimum tensile strength of 700 MPa.With deformation temperature further increasing,retained austenite increasingly transforms into bainite and tensile strength concurrently increases until the deformation temperature of 300 ° C reaches.At this temperature,90 percent of retained austenite transform into bainite in which the thickness of bainitic ferrite plates is similar to the thickness of martensite plates formed during room temperature deformation,and retained austenite films are situated in between bainitic ferrite plates(the formation of cementite inhibited due to the presence of Al and Si).In addition,the bainite ferrite is supersaturated with carbon at this relatively low deformation temperature because of restricted carbon diffusivity.As such,the strain induced bainite transformation induces the highest strain hardening rate up to high strains and leads to the peak tensile strength of 890 MPa at 300 °C for the present annealed steel sheet.As deformation temperature continuously increases,the driving force for bainitic transformation becomes remarkably reduced.On the other hand,due to its significantly increased diffusivity,carbon repelled from bainitic ferrite readily partitions into adjacent austenite and then contributes to stabilizing the austenite region.These two factors cause the progressive suppression of the austenite-to-bainite transformation with deformation temperature increasing,and a deformed microstructure is visible in retained austenite.Accordingly,the tensile strength of the annealed sheet decreases constantly.The above findings indicate that the strengthening mechanism of the δ-TRIP steel changes from strain-induced martensitic transformation(SIMT)and twin induced plasticity(TWIP)to strain-induced bainitic transformation(SIBT)to dislocation glide(DG)in sequence as deformation temperature increases.(5)Under appropriate conditions of thermomechanical processing and heat treatment,by alloying with Nb/Ti as well as Cu,fine precipitates form in the matrix of the δ-TRIP steel,thus providing a potential approach to developing high-strength δ-TRIP steel.For the Nb/Ti microalloyed δ-TRIP steel,the addition of Nb/Ti could slightly refine ferrite grains of the cold-rolled and annealed steel sheet,but can not eliminate the inhomogeneous grain size distribution and the banded microstructure.Precipitates formed inside ferrite grains have various sizes and appear to play different roles.Ti(C,N)fine particles in the sizes of 50~200 nm and(Nb,Ti)C fine particles in the sizes less than 20 nm inhibit recrystallization of ferrite grains by pinning dislocations and hindering grain boundary movement,resulting in the formation of a large number of sub-grain boundaries within the δ-ferrite grains.Coarse(Nb,Ti)(C,N)precipitates in the sizes of 5~10 μm are observed to easily initiate cracking in the deformed steel sheet.The precipitation of carbide and carbonitride reduces the volume fraction and stability of retained austenite in the annealed steel sheet.Mechanically,the above microstructural features due to the Nb/Ti microalloying lead to an increase in the yield strength of the cold-rolled and annealed sheet by 65 MPa,while they cause insignificant variation of the tensile strength and a decrease of the elongation.For the hot-rolled Cu alloyed δ-TRIP steel,the addition of 1.0 wt.% copper could not refine ferrite and retained austenite grains,but it delays bainitic transformation during the coiling procedure subsequent to the hot rolling operation,leading to 17.1% volume fraction of austenite retained in the microstructure(In comparison,under the same hot-rolling process condition,the volume fraction of retained austenite in the hot-rolled Cu-free δ-TRIP steel is 9.6%).The homogeneous precipitation of ε-Cu particles in the sizes of about 2 to 4 nm strengthens ferrite and thus increases the yield strength of the hot-rolled Cu-alloyed steel by 83 MPa(i.e.,17% increment).As compared to the Cu-free steel,the composite effects of precipitation hardening and the transformation-induced-plasticity mechanism result in the higher work hardening rate,higher tensile strength of 805 MPa(increased by 130 MPa,namely 19% increment),and remarkable elongation-to-failure(~30%)for the copper-alloyed steel.The present study provided a theoretical basis and guidance for further development and application of low-density δ-TRIP steels.更多还原