【作者】 黄峰；

【导师】 徐祖耀； 郭正洪；

【作者基本信息】 上海交通大学 ， Materials Science and Engineering， 2016， 博士

【摘要】 塑性形变与热处理一体化既能显著提高钢的力学性能,又具有节能功效,是开发新型材料及加工工艺的优先考虑方向。目前广为研究的淬火-配分（Q&P）钢主要基于传统的离线热处理（一步法和两步法）工艺,其在线（塑性形变加热处理）加工工艺参数与显微组织和力学性能的关系尚未明了,需进一步研究。为此,本工作尝试在低碳钢的淬火-碳配分热处理工艺前引入塑性形变,分别通过冷轧后淬火-碳配分处理及热轧后直接淬火-碳配分处理改变钢的显微组织,关注工艺参数对马氏体细化、残余奥氏体含量以及马氏体基体碳贫化等显微组织的作用,揭示显微组织调控对材料强韧化的作用,提出Q&P钢强化、增塑、增韧和改善高速冲击性能的主导因素。主要的研究内容和成果如下:（1）对Fe-0.19C-1.47Mn-1.50Si试样经预先冷轧后分别进行Q&P和Q&T（淬火-回火）处理,结果表明:预先冷轧细化原奥氏体晶粒后能显著细化两者的马氏体领域/束的宽度,并导致马氏体板条中的位错密度增加,而马氏体条宽基本不变,同时Q&P试样中的残余奥氏体量也基本一致。相较于Q&T试样,Q&P试样不仅含有较多的残余奥氏体,而且马氏体基体中的碳含量显著下降,产生严重的碳贫化现象。对Q&T和Q&P试样的力学性能研究表明,细化马氏体组织能有效提高两者的强度和冲击韧性,但对延伸率影响较小。与Q&T试样相比,Q&P试样中由于马氏体碳的严重贫化和含有较多的残余奥氏体,导致其具有高的延伸率和冲击韧性及低的强度。因此将冷轧与Q&P处理相结合,可获得具有高强塑积和高韧性的细晶Q&P钢。进一步的分析表明,马氏体基体的碳贫化才是Q&P试样增塑和增韧的主导因素,残余奥氏体的相变诱发塑性（TRIP）、阻碍裂纹扩展（BCP）和吸收马氏体的位错（DARA）等效应对Q&P钢的增塑作用十分有限。也即配分的主要意义在于让残余奥氏体吸收从马氏体中扩散过来的过饱和的碳原子,使得马氏体基体软化和韧化,进而提高Q&P试样的综合力学性能,而残余奥氏体自身仅起补充作用。（2）初步研究了热轧后直接淬火-配分工艺中关键的两个因素:轧制温度和配分方式,对其组织和力学性能的影响。结果表明,改变轧制温度可分别获得再结晶细化的和未再结晶形变的原奥氏体组织,进而改变经两步法淬火-配分处理后所得试样的马氏体组织特征:经再结晶细化的原奥氏体组织中,马氏体的领域和束均得到细化;而未再结晶形变态的原奥氏体组织中,仅马氏体束得到细化而领域尺寸略有增加,同时,原奥氏体中残留的位错还会遗传到相变后的马氏体中。由于马氏体相变的起始温度M_s受原奥氏体状态的影响很小,因此所得试样中的残余奥氏体含量基本一致。力学性能研究表明,无论轧制后的原奥氏体是否再结晶,其经两步法淬火-配分处理后的试样强度和韧性均得到显著提高而不牺牲延伸率,尤其是马氏体束和领域均细化的试样提高幅度更为显著。当轧制后的配分冷却方式由两步法改为快冷至M_s温度后自然冷却时,因碳的配分程度不够,导致在室温试样中的残余奥氏体含量和马氏体基体中的碳贫化程度显著降低,最终导致Q&P试样的强度增加,但延伸率和冲击韧性显著降低。（3）分别对低碳（Fe-0.20C-1.49Mn-1.54Si）和中碳（Fe-0.38C-1.54Mn-1.58Si）试样在M_s温度以下进行不同冷速的实验,模拟工业生产条件下冷却方式:炉冷（⁰.5℃/s）、空冷（⁵℃/s）、气冷（⁵0℃/s）和水冷（³00℃/s）对Q&P钢残余奥氏体含量的影响。发现在水冷（水淬）条件下,两种试样中的残余奥氏体含量均很低（<0.5%）,但中碳试样较低碳试样高,与一般规律相符。同时,随着冷速降低,两种试样中的残余奥氏体含量均升高,但低碳试样中的残余奥氏体含量高于中碳试样,与水淬截然相反。分析认为,这主要在于较低冷速条件下碳配分与马氏体相变同时进行,低碳试样由于M_s温度较高,碳的配分能力强,相同冷速下残余奥氏体能获得更多的碳,更易于被稳定到室温残留下来。鉴于冷却过程中马氏体相变与碳扩散同时进行,描述变温马氏体相变的K-M和Magee方程计算得到的马氏体含量应高于实际值,对于冷速较低的情况应该考虑碳配分对未转变奥氏体稳定的作用,因此马氏体形成量与温度的关系应介于Magee方程和经徐祖耀修正的方程之间。（4）利用霍普金森压杆装置对比了低碳（Fe-0.19C-1.47Mn-1.50Si）Q&P试样和Q&T试样的高速压缩性能及剪切断裂特征。结果表明,应变速率为500-2000s^-1时,两者的流变应力和屈服强度随应变速率的增加而增加,形变过程中应变速率硬化起主导作用。而当应变速率为2500s^-1时,两者的流变应力和屈服强度开始降低,表明马氏体基体的绝热软化效应大于应变速率硬化效应。利用Johnson-Cook方程拟合得到的参数表明,Q&P试样较Q&T试样具有更低的应变速率敏感性。同时,冲击剪切实验表明,Q&P试样比Q&T试样具有更好的抗绝热剪切断裂能力。上述两种性能差异的主要原因在于Q&P试样中残余奥氏体低的应变速率敏感性和低的温度敏感性以及形变时发生TRIP效应。同时,残余奥氏体的稳定性也受应变速率的影响,应变速率较低时,残余奥氏体易于发生应力/应变诱发马氏体相变,而随着应变速率升高,绝热升温效果显著,降低了相变驱动力,马氏体相变受到抑制。更多还原

【Abstract】 The combination of plastic deformation and heat treatment is recently considered as a preferential processing manner to improve mechanical properties of steels,in addition to the energy saving.Since being proposed ten years ago,the novel quenching&partitioning（Q&P）treatment has been investigated extensively both in the involved transformation theory and in the selection of processing parameters.However,most of the progresses are based on off-line processing and,thus,it is necessary to combine plastic deformation into Q&P treatment to develop the on-line manufacturing one.In this paper,both cold rolling prior to austenitization and hot rolling during austenitization were carried out before Q&P treatment to explore the microstructural evolution in a low carbon Fe-0.19C-1.47Mn-1.50Si Q&P steel,along with the changes in rolling parameters and cooling manners.The deformation behavior and associated mechanical properties including strength,plasticity,impact toughness and strain-rate dependence from quasi-static to dynamic ranges were studied with particular focus on the roles of retained austenite and carbon–depletion in martensitic matrix.The main results are summarized as follows.（1）Cold rolling prior to austenitization at 910°C was applied to refine prior austenite grains of the experimental low carbon steel.Then,either traditional quenching and tempering（Q&T）or typical two-step Q&P treatment was employed to obtain different specimens for comparison.It has been shown that because of the rolling in advance,the grain size of prior austenite was reduced to less than 10 percent of the unrolled state,resulting in a great decrease in packet/block size and an increase in dislocation density in martensite in both Q&T and Q&P specimens,but the lath size was not obviously changed.Meanwhile,cold-rolling showed little dependence on the state of retained austenite including its volume fraction in both Q&T or Q&P specimens.However,the carbon depletion in martensitic matrix was very serious in Q&P state.It is shown that refining martensitic microstructure enhances strength and impact toughness without scarifying elongation very much for both Q&T and Q&P specimens.In addition,Q&P specimens are softer than Q&T ones due to the existences of considerable amount of retained austenite and the carbon-depleted martensitic matrix.As a result,the higher elongation and impact toughness with relatively lower strength were exhibited in Q&P specimens.Analyses indicated that carbon depletion in martensitic matrix plays more critical role than the amount of retained austenite in affecting the plastic deformation behavior and mechanical properties of Q&P specimens,because martensitic matrix is dominant microstructure with volume fraction close to⁹0%.Through such a combination of rolling and Q&P processes,the optimized microstructure including refined martensitic matrix with carbon depletion and proper amount of retained austenite can be obtained,contributing to a best comprehensive mechanical property with high product of strength and elongation and low ductile-to-brittle transition temperature.（2）Hot-rolling plus direct quenching and partitioning（HDQ&P）process was designed to simulate the on-line manufacturing manner with particular focus on the effects of rolling temperature and the final cooling manner.By changing the rolling temperature,either fine-grained or deformed prior austenite can be obtained.After typical two-step Q&P treatment immediately following rolling,both packets and blocks were refined significantly when martensite formed in the fine-grained prior austenite,but only blocks were refined when formed in the deformed prior austenite.The amount of retained austenite,however,shows little relationship with respect to prior austenite state.By changing the cooling manner from two-step Q&P treatment to natural cooling after quenching to M_s temperature,the amount of retained austenite stabilized at room temperature was decreased dramatically and the associated degree of carbon depletion in lath matrix was decreased simultaneously.The refinement of martensitic microstructure including blocks and packets by hot rolling is benefit to improve both strength and toughness without scarifying plasticity,presumably due to the stronger strain-hardening and energy absorption capabilities provided by the existence of retained austenite and refined carbon-depleted lath matrix.Contrarily,the smaller amount of retained austenite and higher carbon concentration in lath matrix due to natural air cooling resulted in the higher strength but lower plasticity and impact toughness even though the martensitic microstructures including packet and block were refined effectively.In general,the specimen rolled at a temperature higher than the dynamic recrystallization temperature followed by typical two-step Q&P treatment exhibits the best comprehensive mechanical property,because of refined packet and block,considerable amount of retained austenite and lath matrix with serious carbon depletion.（3）Because it is difficult to operate two-step Q&P treatment under the condition of industrial application,partitioning of carbon may occur accompanying martensitic transformation during continuous cooling.By taking a medium-carbon Fe-0.38C-1.54Mn-1.58Si as reference,the effect of cooling rate below M_s temperature on the amount of retained austenite was investigated in the experimental low-carbon steel（Fe-0.20C-1.49Mn-1.54Si）.In water-quenched state（³00 ^oC/s）,the amount of retained austenite in the medium-carbon specimen is higher than in low-carbon one,although the actual volume fraction is very low（<0.5%as measured by magnetism measurement）in either one,which is in agreement with the previous results from other scholars.However,when the cooling rate decreases,e.g.cooling in air（^{5 o}C/s）or in furnace（⁰.5 ^oC/s）,the amount of retained austenite in both specimens increases but the increment in low-carbon one is much higher.As a result,the amount of retained austenite in low-carbon specimen is always higher than in medium carbon one for a given cooling rate.The thermodynamic and kinetic analyses have indicated that the higher M_s temperature of low-carbon specimen should be responsible for this difference.That is to say,the carbon diffusivity relies on temperature strongly and the lower cooling rate would detain the low-carbon specimen more time at relatively high temperature,assisting carbon atoms to escape from newly formed martensite into neighboring austenite with very quick velocity and,finally,stabilizing more at room temperature.Since carbon partitioning is inevitable in most cases during cooling even under the water-quenched state（for example,the cooling rate at the center of thick specimen is low）,the traditional equations describing the relationship between undercooling and amount of martensite formed,such as those proposed by Koistinen-Marburger（K-M）and Magee,are not accurate enough.Therefore,the modification by Hsu concerning the effect of carbon partitioning should be considered.The actual volume amount of martensite may exist between those predicted by K-M（Magee）and Hsu equations.（4）The dynamic compression properties of the above-mentioned low-carbon Q&P steel were investigated over the strain rate range of 500 to2500 s^-1 using split-Hopkinson pressure bar equipment.Traditional quenched and tempered（Q&T）specimen with an identified composition was used for comparison.For both specimen types,the flow stress and yield strength gradually increase as the strain rate increases from 500 to 2000 s^-1,implying that strain-rate hardening dominates the deformation behavior.At a higher strain rate(2500 s^-1),the flow stress and yield strength begin to decrease,indicating that the thermal softening of martensite matrix caused by adiabatic heating begins to overcome the effect of the strain-rate hardening.Based on the Johnson-Cook equation describing the deformation behavior and the parameters calculated by fitting with this equation,the Q&P specimens exhibit less pronounced strain-rate dependencies than the Q&T ones.In addition,the impact shearing tests demonstrated that adiabatic shear failure is less likely to occur in Q&P specimens than in Q&T ones.Analyses revealed that a considerable amount of retained austenite in Q&P specimen is responsible for these differences and,in turn,its stability was affected by the strain rate.At a relatively low strain rate,stress/strain induced martensitic transformation would occur easier.With higher strain rates,martensitic transformation might be suppressed because adiabatic heating decreases its driving force.更多还原