【作者】 王玉辉；

【导师】 王天生； 黄晓旭；

【作者基本信息】 燕山大学 ， 材料学， 2018， 博士

【摘要】 鉴于降低能耗与提高安全性的要求,高强高韧一直是低温用钢铁材料的研发热点。具有优异室温和低温性能的9%Ni钢(GB 24510-2009规定的9%Ni钢主要性能指标包括:室温屈服强度≥590 MPa,拉伸强度680~820 MPa,延伸率≥18%,-196 ℃冲击功≥80 J)在液化天然气(LNG)的储运工程中得到了成功应用,比如在LNG运输储罐建造上大量使用,但因昂贵的Ni含量高以及生产工艺复杂,造成材料成本高,而且我国还主要依赖进口。近年来,开发成本相对较低的高锰(Mn)孪晶诱发塑性(TWIP)奥氏体低温用钢得到了广泛关注。目前国际上开发的低温用高Mn奥氏体TWIP钢的典型成分是Fe-(15~25)%Mn-Al-C,由于其低层错能诱发的TWIP效应,其室温和低温延伸率(50%)均高于9%Ni钢,但这类钢的室温屈服强度只有约450 MPa,而且随着温度降低或塑性变形,马氏体将在奥氏体晶界和晶内形成,这大大降低了低温下的冲击性能(-196 ℃的冲击功约40 J),因此这两项性能指标均低于9%Ni钢。另一方面,对二元Fe-Mn奥氏体钢的研究表明,当Mn含量在(30-40)%范围时,其层错能较高,在-196 ℃至室温的温度范围内,其塑性变形机制为位错滑移,马氏体的形成得到了抑制,室温拉伸延伸率(>40%)和-196 ℃时的冲击功(>200 J),均远优于9%Ni钢。但是,这类高Mn奥氏体钢的室温屈服强度较低(<200 MPa),不能满足设计LNG储罐等低温应用的要求。本研究的目的就是以这一类高Mn钢为研究对象,通过微观组织调控,实现大幅提高其室温和低温力学性能,使其综合力学性能达到9%Ni钢的水平。本研究微观组织调控的前提是实现强化而不降低塑性和韧性。对于高Mn钢而言,微观组织细化一方面可以提高强度,另一方面可以进一步抑制马氏体的形成。最近的研究也发现,超细的层状复合结构可进一步提高材料的强塑性和冲击性能。因此,细化组织和制备超细层状复合组织是本研究微观组织调控的两个基本指导思想。本研究自主开发了一种成分简单且室温轧制(冷轧)性能优良的高Mn奥氏体钢(Fe-34.5Mn-0.04C),通过90%冷轧与退火处理相结合,成功制备了三类样品,它们分别具有不同的微观组织(晶粒形貌和尺寸):(1)完全层片组织,其片层间距在0.047~0.067mm范围变化;(2)由再结晶和回复组织构成的层状复合组织,晶粒尺寸为0.45~2.0mm;(3)完全再结晶的等轴晶组织,晶粒尺寸为2.3~21.0mm。对上述具有不同微观组织的样品进行室温拉伸,结果表明,随着晶粒尺寸增加,屈服强度降低,延伸率提高。完全层片状组织的屈服强度达890~1060 MPa,但延伸率低于13%;层状复合组织的屈服强度为460~783 MPa,而且其延伸率达20~40%;完全再结晶组织的屈服强度为210~440 MPa,延伸率达30~40%。同时发现,微观组织对拉伸流变行为有重要影响,晶粒尺寸d<2.0mm时,拉伸曲线呈连续流变,当晶粒尺寸在2.0mm≤d<3.8mm范围时,拉伸曲线出现屈服平台,当晶粒尺寸d≥3.8mm时,拉伸曲线又转变为连续流变。低温拉伸和对拉伸样品的微观组织表征结果表明,材料的屈服强度随温度降低而提高,但晶粒尺寸和形貌不同样品的拉伸塑性随温度没有呈现单调变化的规律,当温度降低到一定程度时样品出现早期断裂。对于晶粒尺寸小于2μm的完全层片组织和层状复合组织,马氏体相变得到了有效抑制,低温强度和塑性得到大幅提高。比如平均晶粒尺寸为2μm的层状复合组织样品,-180℃下其屈服强度达600 MPa,延伸率达50%,均匀延伸率与拉伸强度的乘积大于50 GPa·%。但是,当晶粒尺寸大于3.8mm时,-180 ℃拉伸时沿晶界发生马氏体相变而塑性显著降低,生成的马氏体不连续,断口呈混合断口(韧窝+解理+沿晶)。晶粒尺寸增大到21.0mm时,-180℃拉伸晶界和晶内均发生马氏体相变,生成的马氏体片层厚度约1mm,引起穿晶断裂和沿晶断裂,断口呈沿晶和穿晶状,严重恶化低温性能,延伸率降低到小于10%。对屈服强度和晶粒尺寸的Hall-Petch关系分析结果表明,当细晶样品拉伸曲线上呈现屈服平台时,其Hall-Petch斜率比粗晶区的斜率高,即出现了Hall-Petch斜率的正偏离,这与文献里报道的细晶Al、Ti和IF钢的Hall-Petch关系相似。对呈现屈服平台的晶粒尺寸为2mm的层状复合组织样品施加3~5%的附加应变后,在晶粒内部形成位错结构,不仅有效地消除了屈服平台,而且在保证良好塑性的条件下,有效地提高了屈服强度。通过分别建立完全层片组织和完全再结晶组织的Hall-Petch关系,利用混合法则,计算了由再结晶晶粒与层片组织构成的层状复合组织的强度,揭示了约束效应强化和位错源强化的影响。发现在室温时屈服强度为461 MPa的层状复合组织样品(平均晶粒尺寸2μm)中,存在约90 MPa的约束效应强化和约50 MPa的位错源强化,从而在实验上证实了在单相层状复合组织中存在约束强化效应。总之,本研究通过超细化、层状结构设计以及与附加变形处理相结合,在成分简单和成本较低的Fe-34.5Mn-0.04C中,获得了室温屈服强度为600~783 MPa,延伸率为20~35%,以及优异的低温拉伸性能。这些指标达到或超过了传统的低温用9%Ni钢。下一步的研究将集中在制备微观组织相似的大尺寸样品,以证实其低温冲击性能的改善,为推广这一低温钢的工业应用奠定基础。更多还原

【Abstract】 In view of energy saving and high safety requirements,the development of high strength and high toughness steels has always been the hot research topic in cryogenic materials science.9%Ni steel possesses excellent ambient and cryogenic temperature properties(the main properties of 9%Ni steel in GB 24510-2009 provisions include:yield strength≥590 MPa,tensile strength 680~820 MPa and elongation≥18%at room temperature,Charpy impact energy≥80 J at-196 ℃)and has been used successfully in the storage and transportation of liquefied natural gas(LNG),such as building LNG tanks.Because of the high content Ni and expensive manufacturing processes,the cost of 9%Ni steel is very high.Furthermore,the 9%Ni steel used in China to date is mainly imported.In recent years,great effort has been made to develop high manganese(Mn)twinning-induced plasticity(TWIP)austenitic steels,which have relatively low cost,for cryogenic-temperature applications.The typical compositions of such TWIP steels are Fe-(15~25)%Mn-Al-C.The TWIP effect caused by the low stacking fault energy in these steels results in good tensile elongation at ambient and cryogenic temperature(50%),higher than that of 9%Ni steel.But the yield strength of these steels is only about 450 MPa at room temperature.Furthermore,at low temperature or under plastic deformation,martensite will form within austenite grains and at grain boundaries,which greatly reduces the impact performance at low temperatures(-196 ℃ Sharpy impact energy of about 40 J).It is apparent that these two properties are lower than that of 9%Ni steel.On the other hand,previous studies on binary Fe-Mn austenitic steels have shown that when the Mn content increases to 30~40%,the dominant plastic deformation mechanism is dislocation slip and the martensite transformation is suppressed.The tensile elongation of these steels is more than 40%and the impact energy is higher than 200 J at-196 ℃,which are better than that of 9%Ni steel.However,the yield strength of this kind of high Mn austenite steels is typically low(<200 MPa),which cannot meet the requirements for the design of cryogenic applications such as LNG tanks.The objective of the present research is thus to enhance the mechanical properties of high Mn steel(30~40%)at ambient and cryogenic temperatures through microstructural control to the level comparable to that of 9%Ni steel.The precondition of the microstructural control in this project is to strengthen the material without sacrificing its ductility and toughness.For the high Mn steel,microstructural refinement is an approach that not only increases the strength,but also suppresses the martensitic transformation that is beneficial for maintaining high toughness.Recent studies have also reported that steels with ultrafine lamellar composite structures have improved strength,ductility and toughness.Therefore,two basic strategies will be applied for the microstructural control in the present study:refinement of microstructure and design of ultrafine lamellar composite.A high Mn austenitic steel with a simple composition,Fe-34.5Mn-0.04C,and good cold-rollability has been developed independently.By combining 90%cold rolling with annealing treatments,three kinds of specimens with different microstructures(different grain morphologies and grain sizes)have been successfully prepared:(1)specimens with a full lamellar structure where the lamellar boundary spacings vary within the range of 0.047~0.067mm;(2)specimens with a laminated composite structure composited of recovered and recrystallized layers where the grain sizes vary within the range of 0.45~2.0mm;(3)specimens with a fully recrystallized equiaxed grain structure where the average grain sizes change from 2.3 to21.0mm.When the samples were tensile tested at room temperature,it was found that the yield strength decreases and the elongation increases with increasing grain size.The yield strength of the full lamellar structure is 890~1060 MPa,but the elongation is less than13%.The yield strength of the laminated composite structure is 460~783 MPa,and the elongation is as large as 20~40%.The yield strength of the fully recrystallized microstructure is 210~440 MPa,and the elongation reaches 30~40%.The microstructure has an important influence on the tensile behavior.The tensile curves showed a continuous flow when the grain sizes are less than 2.0mm.When the grain size is the range of 2.0~3.8mm the tensile curves showed a discontinuous feature.When the grain sizes are larger than3.8mm the tensile curves return to a continuous flow.Tensile tests at low temperatures and corresponding microstructural characterization of the tensile-tested samples were carried out.It was found that the yield strength increases with decreasing temperature,but the tensile ductility of samples with different morphologies and grain sizes does not show monotonic change with temperature,and premature failure occurs when the temperature is below a critical temperature for a given grain size.The martensitic transformation has been effectively suppressed for the full lamellar structure and the laminated composite structure with grain sizes less than 2μm,at the same time the strength and ductility at low temperatures have been greatly improved.For example,the laminated composite structure specimen with an average grain size of 2μm has a yield strength of 600 MPa and an elongation of 50%at-180 ℃.The product of uniform elongation and tensile strength is more than 50 GPa·%.When the specimens with grain sizes larger than 3.8μm were tested at-180 ℃,it was found that martensite forms along grain boundaries,causing a significant decrease in tensile elongation.The martensite formation was not continuous at the grain boundary,a mixed fracture was observed(fractographs showed a mixture of dimple,cleavage and intergranular fractures).When the grain size was increased to 21μm,martensitic transformation occurred both in austenite grain interiors and at grain boundaries with a layer thickness of about 1μm when tensile tested at-180 ℃,which caused transgranular fracture and intergranular fracture of the material.Accordingly the elongation decreased to less than 10%at-180 ℃.The Hall-Petch relationship between the yield strength and the grain size was analyzed.It was found that the Hall-Petch slope for the samples showing yield plateau is higher than that for the samples of coarser grain sizes.This means a positive deviation of the Hall-Petch slope in the fine grained samples,which is similar to the previous observations in fine-grained Al,Ti and IF steel.To eliminate the yield plateau of the sample with an ultrafine laminated composite structure(2μm in average grain size),an additional cold rolling of 3~5%was applied.Such a slight deformation has introduced dislocation structures in the grain interior,which not only removed the yield plateau but also increased the yield strength while ensuring good ductility.The Hall-Petch relationships for the samples with the full lamellar structure and the fully recrystallized structure were established.The strength of a laminated composite structure consisting of recrystallized grains and recovered lamellar structures was calculated using the rule of mixture to evaluate the contributions of constraint effect and dislocation source strengthening effect.It was found that for the laminated composite structure(average grain size of 2μm)with a yield strength of 461 MPa at room temperature,the strength contribution originated from the constraint effect is about 90MPa and the dislocation source strengthening is 50 MPa.These results confirmed the existence of strengthening effect of constraint in the single phase laminated composite structure.In summary,through structural refinement,design of laminated composite structure design and additional deformation treatment,excellent tensile properties at ambient and cryogenic temperatures have been achieved in the Fe-34.5Mn-0.04C steel studied which has a relatively simple composition and low cost.The obtained good combinations are the yield strength of 600~783 MPa and tensile elongation of 20~35%at room temperature,which are comparable or even better than the conventional 9%Ni steel.The future research will focus on the preparation of large scale samples with similar microstructures to evaluate the low temperature impact properties,and pave a way for industrial applications of this cryogenic steel.更多还原