【作者】 刘栋；

【导师】 易红亮；

【作者基本信息】 东北大学 ， 材料工程（专业学位）， 2017， 硕士

【摘要】 随着产业迅速发展与升级,钢铁材料面临着成本、环保、能源等诸多挑战。为了解决此一系列等问题,增加材料强度、降低材料密度并实现材料轻量化已经成为必然要求。轻量化意味着在保证材料其他性能的前提下,以降低材料密度的方式降低材料重量,减少资源消耗和废气排放。由于钢铁材料约占汽车总重的60%,因此采用高强度钢板代替普通钢板以减薄车身板材厚度,是实现汽车轻量化的有效方法。通过向合金钢中添加一定含量的Al,在创新的成分设计及工艺优化的基础上,得到其它钢种不具备的高强度、高塑性、低密度三者合理匹配的优异性能,并已成为低密度材料的研究热点。第一代汽车钢已经无法满足未来汽车理想用材的全部需求,而第二代先进高强钢,如奥氏体钢和孪晶诱发塑性钢(TWIP)等,则因为较高的合金含量而导致成本偏高。因此,新一代高强度低密度汽车钢的开发会对未来汽车材料产生重要的影响,希望将来在汽车结构件上得到大量应用。但是,汽车用高强度高塑性低密度钢的研究才刚刚开始,尚有许很多难题亟待解决。基于以上背景,本文以高强度高塑性低密度中锰钢为研究对象,主要研究了热处理工艺参数对实验钢显微组织和力学性能的影响。本文设计了 Fe-Mn-Al-C-Nb和Fe-Mn-Al-C-Ni-Nb复合添加Q&P钢及中锰钢,试图通过对这些实验结果的分析和讨论,希望能够揭示高强塑积低密度钢组织变化规律及力学变化趋势,且为高强塑积低密度钢的组织和性能的控制、优化提供具有参考意义的方法。论文的主要工作及研究成果如下:(1)通过对不同合金成分的中锰低密度钢进行不同温度下的热处理实验,研究了合金钢在热处理过程中组织和力学性能的变化规律。基于Q-T&P工艺的设计理念,通过碳元素和锰元素的复合添加以降低马氏体相变结束温度,使之在室温以下,保证热轧板直接淬火并冷却至室温后,能保存较多的残余奥氏体。合金1组织中残余奥氏体含量约为20～21%,以薄膜状为主;合金2组织中残余奥氏体含量可达到26～48%,且主要呈块状。同时,随配分温度和配分时间的提高,残余奥氏体含量发生显著变化。合金1残余奥氏体含量随配分温度和配分时间的升高而增多,后者则呈相反态势。随着碳锰含量的升高,合金钢微观组织形态发生变化,马氏体类型由位错马氏体转变为孪晶马氏体。(2)基于残余奥氏体的体积分数与强塑积呈正相关这一观点,通过热处理工艺设计得到大量的残余奥氏体。碳化物的形成不仅会恶化钢的力学性能,而且会消耗大量的碳元素和锰元素。因此需要在较高温度退火达到减少碳化物的目的,且配分温度的升高有利于提高原子的扩散能力。通过成分及工艺路线设计,在较高温度短时间退火完成碳锰的配分和碳化物的溶解,综合锰和碳的作用,达到稳定大量残余奥氏体的目的。(3)热轧合金钢在650～800℃临界区退火后获得了优异的综合性能,优于一般冷轧低合金钢,且本合金钢既无需冷轧也只需要较短的热处理时间。实验结果表明:合金1热轧钢能获得875～1000MPa的抗拉强度和46～61%的伸长率;合金2热轧钢能获得875～1100MPa的抗拉强度和14～52%的伸长率,性能优异。(4)研究了不同临界退火温度对实验钢热轧组织性能的影响。热轧板经退火处理,合金1较低温度650℃退火时,由于碳化物仍大量存在,得到较高的强度及较低的塑性,中温度700℃退火时,碳化物大部分溶解,得到较高的强塑积,可达53152MPa%,高温800℃退火时,碳化物完全溶解,此时晶粒尺寸较大,导致强度塑性均有所下降。(5)通过对不同临界区退火温度下A25标准拉伸试样加工硬化行为的分析,发现形变诱发马氏体相变行为的延迟发生现象是由铁素体优先形变造成的,致使奥氏体发生TRIP效应前已经产生较大变形量,从而大大提高了实验钢的塑性并获得优异的强塑性匹配。(6)通过对临界区退火下A25标准拉伸试样应力应变曲线及加工硬化行为的研究分析,观察到不连续TRIP现象的发生,这是因为:(一)马氏体切变而产生的体积膨胀致使高温铁素体和临界铁素体发生形变,从而导致了局部应力松弛;(二)奥氏体因形态不同而具有分级稳定性,致使TRIP效应的发生需要不同等级的临界应力。更多还原

【Abstract】 With rapid developments of industry,steels are also facing challenges such as safety,environment,resource,energy,cost and so on.In this circumstance,lightweight becomes an important development trend in the future development of industry.Lightening means that the vehicle mass should be decreased on the premise of security restriction in order to reduce fuel consumptions and low exhaust emissions.Since steels account for about 60 percent of the total vehicle’s body weight,it is an effective way to achieve lightweight using thinner steel sheet with higher strength instead of common steel.A new class steel has been developed by adding Al to the medium manganese steels,and through optimization of the alloy composition design and heat treatment processes,obtaining low density,optimal high strength and high ductility combination.With a great potential and a promising prospect,high strength and high ductility low density steels may be widely used in automotive structures.Both the conventional high strength steels(HSS)and the 1st generation advanced high strength steels(AHSS)cannot meet the entire requirements of mechanical performance for automobile steel in the future,and the cost of the 2nd generation AHSS,such as austenitic stainless steel and twinning induced plasticity(TWIP)steel,is quite higher mainly because of the high level of alloy elements.However,the research on high strength and high ductility low density steels for automobile aopplications has just started,and there are many theoretical problems to be solved.On the basis of above background and reasons,the mechanical properties and the microstructural evolution and mechanical properties for high strength and high ductility low density medium Mn steels during annealing heat treatments were investigated.Fe-Mn-Al-C-Nb and Fe-Mn-Al-C-Ni-Nb Q&P steel and medium Mn steel were designed.To decode the experimental results for high strength and high ductility low density steels can provide us the understanding of the structure-propertie relationship of high strength and high ductility low density steels,and also provide some methods to optimize the microstructures and improve mechanical properties for high strength and high ductility low density steels.The chief original work and results are as follows:(1)Different heat treatments were carried out for two typical mediun manganese Fe-Mn-Al-C low density steels,microstructure and mechanical properties of these steels during heat treatment processes were studied.Based on the concept of Q-T&P,controlling the Mf temperature below room temperature,sufficient retained austenite was received at room temperature.The former alloy contains 20～21vol.%retained austenite with mainly film-like morphology,while the later consists of 3.5～9.6 vol.%retained austenite with blocky shape.And the content of retained austenite changed significantly with the increase of partitioning temperature and partitioning time.The retained austenite content of the first alloy increases with the temperature and the time of partitioning,and the later is opposite.With the increase of carbon and manganese content,the microstructure of alloy steel changed,and the martensite type changed from dislocation martensite to twin martensite.(2)The volume of retained austenite is direct proportion to product of tensile strength and elongation.We gained plenty of retained austenite benefiting from different heat treating.However,the carbon content in austenite decreases gradually,once the carbide is formed.Based on that,we designed the composition and routes of heating treatment.The steel was annealed at high temperature to partition more Mn and C to austenite and then it was annealing above the carbide dissolved temperature.Combined the effect of Mn and C,much more austenite was retained at room temperature.(3)The as-hot-rolled steels annealed in the range of 750～800℃ and had similar or better mechanical properties than those of low-alloy and medium Mn TRIP steels.urthermore,less cold-rolling work or annealing time was required in the present work.1 alloy teel exhibited combinations of UTS of 875～1000MPa and TE of 46～61%.2 alloy steel demonstrated a high strength range of 875～1100MPa and a large elongation range of 14～52%.(4)The different annealing process on the microstructure evolution and mechanical properties was studied.Annealing at low temperature 650℃,carbide was still exist and got higher strength and lower plasticity.Annealing at meidium temperature 700℃,carbide was mostly dissolved,and the experimental steel has a high elongation and good strength-ductility balance as 53152MPa%.Annealing at 800℃,the strength and toughness of steel are worsely matched,because of grain roughening.(5)Through the study of the strain hardening behavior of samples annealed from different temperature,it was found that the deformation of ferrite initially suppressed the TRIP effect,thus,TRIP effect occurred at a higher strain,leading to superior ductility.(6)Discontinuous TRIP effect was first observed and proposed by studing the strain hardening behavior of as-hot-rolled 1 alloy.There are two important factors for the discontinuous TRIP effect.First,the martensitic transformation leads to a volume expansion,which leads to plastic deformation of the δ-ferrite and the intercritical ferrite(IF)laths,and consequently introduces relaxation and transfer of localized stress.Second,austenite with different degree of stability is responsible for the TRIP effect that occurs discontinuously only when a critical stress is attained.Austenite with different degree of stability result form its different morphology and composition.更多还原