新型汽车用高强度中锰钢研究现状及发展趋势

Medium Manganese High Strength Steel for Automotive Application:Status Quo and Prospects

随着全球能源危机的加剧以及环境问题的日益严重,汽车行业正向着轻量化设计发展,以满足节能、降耗、环保与安全的要求。实施汽车轻量化的途径主要有:(1)应用新材料、新工艺,即采用轻量化材料或轻量化成型技术,以达到减重的目的;(2)结构优化设计,即使零部件薄壁化、中空化、小型化、复合化以及对车身零部件进行结构和工艺方面的改进等。作为汽车的主要材料,高强钢的应用不但能够达到减薄车身用板厚度与减轻质量的目的,还可提高安全性能。目前,高强钢的发展经历了三个阶段。从显微组织上来看,经历了以BCC晶格的铁素体为基体的第一代到FCC的奥氏体的第二代,再到铁素体与残余奥氏体的第三代。尽管第一代高强钢的强度比较高,但其塑性受到限制。为了解决此问题,可在第二代高强钢中添加大量的Cr、Ni、Mn、Si和Al等合金元素,但这样会增加生产成本,并且在后续加工过程中产生一些问题,从而限制了其规模化生产进程。鉴于此,第三代高强钢通过降低合金元素的含量,以C、Mn、Al、Si元素为主,在降低成本的同时,兼具高强度与高塑性,能够达到较高的强塑积。第三代高强钢以中锰钢为代表,是利用热轧或冷轧板在退火过程中发生奥氏体逆转变形成亚微米级的奥氏体和铁素体双相组织,随后奥氏体在变形过程中发生相变诱导塑性(TRIP)或孪生诱导塑性(TWIP)效应来提高钢的塑性和强度。近些年来,研究主要集中在通过对加热速度、奥氏体化温度、退火温度、退火时间与冷却速度等工艺参数优化来获得适量而稳定的残余奥氏体,取得了丰硕的成果。然而,对中锰钢力学性能方面的研究仅局限在拉伸性能,缺乏对于其后续成形性能及断裂机理方面的研究。本文归纳了新型汽车用高强度钢的发展历史及研究现状,明确了中锰钢在生产成本与力学性能方面的优势,介绍了中锰钢化学成分设计的依据及各合金元素所起的作用,分析了奥氏体化温度、退火温度、退火时间、加热速率与冷却速度等临界区退火工艺参数对残余奥氏体调控的影响。并揭示了中锰钢形变过程中残余奥氏体发生TRIP与TWIP效应来提高其强韧性的变形机制,阐述了孔洞形核、长大及其断裂机制,分析了拉伸过程中吕德斯带产生的原因,介绍了中锰钢在热成形方面的应用前景,展望了中锰钢未来的发展趋势,以期为中锰钢的工业化生产及实际应用提供参考。

With the global energy crisis and increasingly serious environmental problems,the automotive industry has the trend towards lightweight design to meet thedemands of energy saving,consumption reduction,environmental protection and safety.The main approaches to implement automotive lightweighting are as follow.Ⅰ.Employ the advanced materials and processes,that is,adopt lightweight materials or forming technology to achieve the weight reduction goal.Ⅱ.Optimize the structural design,including thin wall,cavity,miniaturization,compounding and structural and technological improvement of body parts.,The application of high strength steel as the main material of the automobile can not only contribute to reducing of the body thickness and weight,but also improve the safety performance.At present,the development of high strength steel has experienced three stages.As regard to microstructure,the first generation advanced high strength steel consists of ferrite(BCC)as matrix,the second generation consists of austenite(FCC),and the third generation consists of ferrite and retained austenite.Although the strength of the first generation advanced high strength steel is relatively high,its plasticity is limited.For the sake of solving this problem,a large amount of alloying elements,such as Cr,Ni,Mn,Si and Al,are added in the second generation advanced high strength steel.Nevertheless,this approach increases the production cost,and also causes some problems in the subsequent processing,which blocks its widespread application in large-scale production.Accordingly,the content of alloying elements has been reduced in the third generation advanced high strength steel,and the main alloying elements are composed of C,Mn,Al,and Si.In this case,high strength and high plasticity can be achieved at a lower cost,as well as an excellent combination of elongation and tensile strength.As a representative of the third generation advanced high strength steel,medium manganese high strength steel can achieveimproved plasticity and strength through TRIP or TWIP effects of retained austenite during deformation.The medium manganese high strength steel consists of sub-micron austenite and ferrite which are formed by the austenite reverse transformation in the annealing process of hot or cold rolled sheets.In recent years,most researches have focused on how to obtain appropriate fraction of stable retained austenite by optimizing process parameters,such as heating rate,austenitizing temperature,annealing temperature,annealing time and cooling rate,and fruitful results have been achieved.However,the study on its mechanical properties is limited to tensile properties.There is a lack of research on subsequent forming properties and fracture mechanism of medium manganese steel.In this article,the development history and recent research situation of medium manganese high strength steel for automotive application are summarized.The advantages of medium manganese steel in material cost and mechanical properties are emphasized.Firstly,chemical composition design of medium manganese steel is introduced,as well as the effects of alloy element on the structures and properties of medium manganese steel.Secondly,the effects of intercritical annealing process parameters,including austenitization temperature,annealing temperature,annealing time,heating rate and cooling rate,on adjusting retained austenite are analyzed.Finally,the enhanced strength and toughness of medium manganese steel are revealed due to the TRIP and TWIP effects of retained austenite during the deformation process.The nucleation and growth of microvoid and fracture mechanism are explained.The cause of the lüders deformation behavior during stretching is studied.The application of medium manganese steel in hot forming process is introduced.In addition,the future development trend of medium manganese steel is predicted.It is believed that the industrial production and practical application of medium manganese steel can be realized in near future.