奥氏体不锈钢的组织超细化技术

在试验的基础上研究了获得奥氏体不锈钢超细组织的方法,研究结果表明,原始晶粒尺寸为100 μm的304N不锈钢经1道次等径角挤压变形 退火工艺处理后,晶粒尺寸可显著细化到2～7 μm,强度可提高50%以上,而塑性并不降低;增加等径角挤压变形的道次,经退火后可获得更细小、均匀的再结晶晶粒.     

用等径角挤压变形法进行奥氏体不锈钢锭开坯的研究

对铸态奥氏体不锈钢的等径角挤压变形(ECAP)试样进行了高温固溶处理,并与锻造试样 固溶处理后的组织、性能进行了对比,研究表明:室、高温ECAP试样在1150℃固溶处理后均可得到完全再结晶组织;增加ECAP道次对完全再结晶后的晶粒尺寸影响不大,但略微增加晶粒尺寸的均匀性;铸态奥氏体不锈钢采用ECAP 固溶处理新工艺后,可以获得与锻造 固溶处理工艺类似的组织、力学性能,有可能实现铸锭的开坯.     

铸态304L奥氏体不锈钢等径角挤压变形的力学性能

 研究了经1~4道次等径角挤压变形(ECAP)后,铸态304L奥氏体不锈钢微观结构的演变,同时测定了ECAP变形后的力学性能。结果表明,经4道次变形后,铸态粗大晶粒破碎形成细小的大角度晶粒,平均晶粒尺寸约202 nm;抗拉强度和屈服强度大大提高(Rp02=1 002 MPa,Rm=1 100 MPa),但均匀塑性变形能力(A＜3%)和加工硬化指数(n=0060)却显著下降。     

铸态304L奥氏体不锈钢等径角挤压变形研究

 研究了铸态304L奥氏体不锈钢在等径角挤压(ECAP)变形过程中显微组织的演变过程。结果表明,经4道次剪切变形后树枝晶破碎、原始粗大晶粒碎化。显微组织的变化过程可归纳为：原始粗晶粒→晶粒被滑移带分割→位错发展形成高密度位错墙,与滑移带共同作用形成胞块结构→应变增加形成层片状界面→形成大角度晶界的细小晶粒。表明铸态304L奥氏体不锈钢经ECAP变形后塑性变形机制主要由滑移完成。     

基于BP神经网络不锈钢锻造再结晶的晶粒尺寸模型

金属的热变形是一个非常复杂的非线性过程,热变形过程中的晶粒尺寸变化直接决定着变形后金属的组织和性能。利用BP神经网络处理了304不锈钢的热变形非线性系统,从试验数据中自动总结出规律。采用人工神经网络技术对304奥氏体不锈钢锻造工艺参数（变形温度和变形速率）,再结晶（包括静态再结晶、动态再结晶）和晶粒长大进行建模,分析了静态、动态再结晶晶粒尺寸,并对模型的预测性能进行了研究。     

等通道热挤压变形制备奥氏体不锈钢纳米级组织

 通过采用700 ℃等通道挤压法(ECAP法)对00Cr19Ni10奥氏体不锈钢实施变形,制备出晶粒尺寸在200~300 nm的超细晶组织,由此可使其抗拉强度与屈服强度显著增加。同时探讨了ECAP细化机理,对试验钢在等通道挤压变形中的微观组织演变过程进行了分析,发现其组织演变与滑移、孪晶以及动态再结晶有关。     

低碳马氏体钢的微观组织及其对强度的影响

利用光学显微镜、扫描电镜、背散射电子衍射(EBSD)以及透射电镜对17CrNiMo6钢经淬火和低温回火后的马氏体组织进行了定量分析,并且研究了其对强度的影响.结果表明:17CrNiMo6钢Packet(板条束)尺寸和Block(相似取向的板条束)宽度随原奥氏体晶粒尺寸的减小而减小,而Lath(板条)宽度在0.3μm左右,对原奥氏体晶粒尺寸变化不敏感.17CrNiMo6钢板条马氏体的屈服强度与原奥氏体晶粒尺寸、Packet尺寸以及Block宽度都符合Hall-Petch关系,而Block宽度是对强度起作用的组织控制因素.     

线材超细晶钢的生产技术和超快速冷却工艺

1950年代,Hall-Petch提出了晶粒尺寸和屈服强度关系。世界上目前生产的低碳钢,铁素体晶粒度一般为ASTMNo.8～9级,即晶粒尺寸相当于14～20μm,根据Hall-Petch公式,现在大量生产的碳素结构钢,只要把晶粒细化到小于5μm,其强度就可以由200MPa增强到400MPa以上。把低合金钢晶粒细化至2μm左右,强度可增强到800MPa以上。线材超细晶钢的核心技术:利用大变形量细化加热后的粗大奥氏体晶粒;防止大变形量后晶粒再长大,轧制过程中通过快速穿水冷却,防止轧制升温(线材精轧     

碳锰钢压缩过程中非均匀应变与再结晶之间关系的研究

采用有限元方法模拟了热模拟试验的变形过程,分析了热模拟变形过程中的非均匀应变对奥氏体动态再结晶及晶粒尺寸的影响.结果表明,在等效应变最大的区域,奥氏体动态再结晶并非最完全,而剪应变对动态再结晶的影响则较大,在剪应变最大的区域,再结晶最完全,晶粒最细小.在试验所设定的最大变形量为62%的变形条件下,等效应变对晶粒细化的影响存在一个临界值,当等效应变大于0.96时,不完全动态再结晶区域的奥氏体晶粒得不到进一步细化,而随着剪应变的增加,奥氏体晶粒不断细化,可见剪应变对奥氏体晶粒尺寸的影响更大.因此,用等效应变等于实际应变处的晶粒尺寸来考察实际晶粒尺寸的方法,存在着不合理性.     

剧烈塑性变形后超细晶粒耐蚀钢的强度

给出了Cr-Ni奥氏体耐蚀钢经剧烈塑性变形可得到纳米级或亚微晶结构的研究结果。经高压扭转变形后，其平均晶粒尺寸约为50nm，而经等径弯曲通道变形后，一般可获得带有被分割晶粒（其尺寸为100-250nm）的取向结构。采用高压扭转变形或等径弯曲通道变形法进行剧烈塑性变形可促进马氏体转变，相应得到的Cr-Ni奥氏体钢表现出较强的加工硬化性能。     

The study of grain size dependence of yield stress of copper for a wide grain size range

Room temperature yield strength of copper has been measured by means of miniaturized disk-bend test as a function of grain size ranging from about 30 nm to 180 μm. It has been established that grain size dependence of strength does not obey the Hall-Petch relation in the entire range of grain sizes studied. The results obtained suggest that a gradual change of deformation mechanism takes place with decreasing grain size. Nanostructured samples appear to be rather ductile.     

Effect of Grain Size on Mechanical Properties of Nickel-Free High Nitrogen Austenitic Stainless Steel

The fine grained structures of nickel-free high nitrogen austenitic stainless steels had been obtained by means of cold rolling and subsequent annealing.The relationship between microstructure and mechanical properties and gain size of nickel-free high nitrogen austenitic stainless steels was examined.High strength and good ductility of the steel were found.In the grain size range,the Hall-Petch dependency for yield stress,tensile strength,and hardness was valid for grain size ranges for the nickel-free high nitrogen austenitic stainless steel.In the present study,the ductility of cold rolled nickel-free high nitrogen austenitic stainless steel decreased with annealing time when the grain size was refined.The fracture surfaces of the tensile specimens in the grain size range were covered with dimples as usually seen in a ductile fracture mode.     

Effect of Martensite Fine Structure on Mechanical Properties of an 1 100 MPa Grade Ultra-high Strength Steel

An 1100 MPa grade ultra-high strength steel with different martensite fine structures,characterized by prior austenite grain size,martensite packet size,block width and lath width,was studied by various heat treatment processes.The result shows that with decreasing prior austenite grain size,both the packet size and block width decrease,while the lath width has virtually no change.Accordingly,both strength and toughness increase,while total elongation decreases.The yield strength has a Hall-Petch type relationship with the prior austenite grain size,packet size and block width,and the block width may be regarded as a key factor influencing strength.On the other hand,the ductile to brittle transition temperature(DBTT)is found to be more related to the packet size,which may be considered as a dominant factor influencing toughness.     

Effect of equal-channel angular pressing and annealing conditions on the texture,microstructure, and deformability of an MA2-1 alloy

Equal-channel angular pressing (ECAP) of am MA2-1 alloy according to routes A and Bc is used to study the possibility of increasing the low-temperature deformability of the alloy due to grain refinement and a change in its texture. To separate the grain refinement effect from the effect of texture on the deformability of the alloy, samples after ECAP are subjected to recrystallization annealing that provides grain growth to the grain size characteristic of the initial state (IS) of the alloy. Upon ECAP, the average grain size is found to decrease to 2–2.4 μm and the initial sharp axial texture changes substantially (it decomposes into several scattered orientations). The type of orientations and the degree of their scattering depend on the type of ECAP routes. The detected change in the texture is accompanied by an increase in the deformability parameters (normal plastic anisotropy coefficient R, strain-hardening exponent n, relative uniform elongation δ_u) determined upon tensile tests at 20°C for the states of the alloy formed in the IS-4A-4Bc and IS-4Ao-4B_cO sequences. The experimental values of R agree with the values calculated in terms of the Taylor model of plastic deformation in the Bishop-Hill approximation using quantitative texture data in the form of orientation distribution function coefficients with allowance for the activation of prismatic slip, especially for ECAP routes 4B_c and 4B_cO. When the simulation results, the Hall-Petch relation, and the generalized Schmid factors are taken into account, a correlation is detected between the deformability parameter, the Hall-Petch coefficient, and the ratio of the critical shear stresses on prismatic and basal planes.     

Effects of Prior Cold Rolling and Post Temper Rolling on the Properties of Continuously Annealed Low Carbon Dual Phase Steel

The effects of two processing variables, prior cold reduction and temper rolling, on the mechanical properties of a continuously annealed 0.05 pct C low alloy dual phase steel were studied. It was found that cold rolling affected the strength of the dual phase steel through its influence on the recrystallized ferrite grain size. The correlation between yield strength and mean free path in ferrite was shown to follow a Hall-Petch type relationship. Similarly, temper rolling also had a pronounced effect on the yield strength of the steel. The effect was strongly dependent upon the previous overaging treatment; the lower the overaging temperature, the stronger the effect. Basically, the effect of temper rolling on dual phase steel was not much different from that on mild steel and HSLA in terms of the change in yield strength and yield point elongation. Another interesting observation was the influence of quenching rate on the strength of the dual phase steel. A slower quenching rate in the heavier gauge dual phase steel caused pearlite rims to form around martensite colonies and thus decreased the strength of the steel.     

The role of grain size on the ductile-brittle transition of a 2.25 Pct Cr-1 Pct Mo steel

The ductile-brittle transition temperature of a 2 1/4 pct Cr-1 pct Mo steel has been meas-ured using ‘V’ notch Izod impact specimens for an unembrittled and embrittled 2 1/4 pct Cr-1 pct Mo steel with prior austenite grain sizes within the range 40 to 150 μm. The mi-crostructure of this steel was upper bainite. The variation of yield strength with grain size obeys a Hall-Petch relationship. The ductile-brittle transition temperature was found to have a pronounced grain size dependence for both unembrittled, 15 K mm^1/2, and embrittled, 19 K mm^1/2, specimens. The bainite colony size was found to vary as the prior austenite grain size. From the low temperature quasi-cleavage facet size, together with metallographic observations of crack path, it has been concluded that bainite colony size rather than prior austenite grain size is the effective grain size.     

Formation of a submicrocrystalline structure in St10 and 08R steels during equal-channel angular pressing

The effect of cold equal-channel angular pressing (ECAP) on the structure and properties of low-carbon St10 and 08R steels in the initially ferritic-pearlitic and bainitic states is studied. Directly after ECAP, only a partially submicrocrystalline structure with a grain size of 150–300 nm (depending on alloying and the initial state) can be obtained. Along with a granular structure, a subgrain and/or cellular structure, including an oriented structure, are observed. The finest structure forms in a boron-containing 08R steel with a structural-element size of 190 nm. The strength of the 08R steel subjected to cold ECAP (σ_u = 805–1235 MPa) corresponds to a hardware strength class of 8.8–12.9. The strength of the deformed St10 steel is close to a strength class of 8.8.     

等径弯曲通道变形对超低碳钢组织及性能的影响

 研究了室温下C方式等径弯曲通道变形(ECAP)对超低碳钢组织及性能的影响。结果表明：第1道次ECAP变形后，组织细化效果最显著；随变形道次的增加，组织由取向差小的板条状亚晶演变成取向差大的等轴晶；第4道次ECAP变形后，晶粒平均尺寸约03 μm；变形道次继续增加，晶粒尺寸变化不显著，而晶粒取向差不断增大。这表明第4道次ECAP变形为超低碳钢细化极限；ECAP变形可大幅度提高超低碳钢的强度，并保持较高的塑性。     

Influence of microstructure on tensile properties of spheroidized ultrahigh-carbon (1.8 Pct C steel

Ultrahigh-carbon steel (UHCS) containing 1.8 pct carbon was processed to create microstructures consisting of fine-spheroidized carbide particles (0.2- to 1.5-μm size range) within a fine-grained ferrite matrix (0.3- to 5-μm range) through a variety of thermomechanical processing and heat-treatment combinations. Tensile ductility, yield, and fracture strengths, and strain-hardening behavior were evaluated at room temperature. Yield strengths ranged from 640 to 1450 MPa, and uniform tensile elongation ranged from 3 to 23 pct. Quantitative analyses revealed that a Hall-Petch type relationship exists between the yield strength and the ferrite grain size and carbide particle size within grain interiors. The fracture strength, on the other hand, was found to be uniquely dependent on the coarse carbide particle size typically found at grain boundaries. Data from other investigators on spheroidized carbon steels were shown to correlate well with the data for the UHCS (1.8 pct C) material. It was shown that the tensile ductility will increase when the difference between the fracture strength and the yield strength is increased and when the strain-hardening rate is decreased. The basis for the trends observed is that the tensile ductility is limited by the fracture process that appears to be dictated by the nucleation of cracks at large carbide particles. The results obtained indicate that UHCSs have significant potential for sheet applications where high strength and good ductility are primary requirements.     

A composite model for the grain-size dependence of yield stress of nanograined materials

Based on the observation that, in a nanograined material, a significant portion of atoms resides in the grain-boundary region and grain-boundary activity plays a key role for its plastic behavior, a micromechanics-based composite model is developed to calculate the transition of yield stress as the grain size decreases from the coarse grain to the nanograin regime. The development makes use of a generalized self-consistent scheme in conjunction with the secant moduli of the constituent phases and a field-fluctuation approach. The constituent grains are modeled by inclusions with a grain-size-dependent plastic property and, in order to reflect the atomic sliding inside the grain boundaries observed in molecular dynamic simulations, the grain-boundary phase is modeled as a soft, ductile material with a pressure-dependent property. Applications of the developed model to a high-density copper showed the distinctive—some experimentally observed—features: (1) the yield stress initially increases following the Hall-Petch equation, but as the grain size reduces to the nanorange, it will depart and decrease; (2) when the grain size drops to a critical value (called the critical equicohesive grain size), the slope turns negative, (3) there is a tension-compression asymmetry (or strength-differential effect) in the yield stress, and (4) parametric calculations for materials whose grains deform only elastically indicate that the Hall-Petch plot will exhibit a continuously decreasing negative slope over the entire range of grain size. Further application of the theory to palladium in the nanorange shows a continuous decrease of the yield strength with decreasing grain size. It can be generally concluded that the range following the Hall-Petch equation is dominated by the deformation of the grains, and the range with a negative slope is controlled by the plasticity of the grain boundaries. During the transitional stage, both grains and grain boundaries deform competitively.