Bauschinger Effect in Microalloyed Steels: Part I. Dependence on Dislocation-Particle Interaction

The Bauschinger effect (yield stress decreasing at the start of reverse deformation after forward prestrain) is an important factor in strength development for cold metal forming technology. In steels, the magnitude of the Bauschinger effect depends on composition, through the presence of microalloy precipitates, and prior processing, through the size and distribution of microalloy precipitates and presence of retained work hardening. In this article, the microstructures of two (Nb- and Nb-V-microalloyed) steel plates, in terms of (Ti,Nb,V,Cu)-rich particle distributions and dislocation densities, have been quantitatively related to the Bauschinger parameters for the same processing conditions. For the 12- to 50-nm effective particle size range, the Bauschinger stress parameter increases with the particle number density and dislocation density increase. The relative influence of these two microstructure parameters is discussed.     

Bauschinger effect in Nb and V alloyed line-pipe steels

Abstract

The UOE process is used for cold forming of large diameter steel line-pipes. Pipe strength has been found to increase (work hardening) or decrease (Bauschinger effect) after the UOE process compared to the plate depending on the steel grade, plate and pipe processing history. The steel chemistry, through the presence of microalloy precipitates, and prior processing, through the size and distribution of microalloy precipitates and presence of retained work hardening, affects the magnitude of the Bauschinger effect. In this paper the microstructures of two (Nb and Nb–V alloyed) steel plates, in terms of (Nb,V)(C,N) particle distributions and dislocation densities, have been related to the Bauschinger parameters in the as rolled and annealed initial conditions. The Bauschinger stress parameter increases with microalloy particle number density and dislocation density increase and the relative importance of the two effects is discussed.     

Evolution of Phases and Mechanical Properties of Thermomechanically Processed Ultra High Strength Steels

The present study concerns the development of two low carbon microalloyed ultra high strength steels on a pilot scale. This recent endeavour has been made towards the reduction of weight by achieving high strength to weight ratio together with improved weldability for the various prospective high performance defence applications such as explosive ammunition, gun barrel, missile skins, light-weight military bridges etc. These steels were thermomechanically processed and finished at different finish rolling temperatures followed by water quenching. Variation in microstructure and mechanically properties at different finished rolling temperatures was studied. The experimentally determined continuous cooling transformation diagrams have revealed that adequate hardenability is achievable in these steels usually at a cooling rate >5 °C/s. Lath martensite along with the microalloy (Ti, Nb) CN precipitate particles are the characteristic microstructural feature of the investigated steels. The high strength value obtained in the present steels is due to the accumulated contribution of fine grained pan-caked austenite, highly dislocated lath martensite along with the presence of tiny precipitates of microalloy carbide/carbonitride and Cu rich precipitates. The good combination of strength (1,364–1,538 MPa) and ductility (11–16 %) has been achieved for the selected range of finish rolling temperature. The Charpy impact toughness values (30–80 J) reveal approximately consistent fall with the lowering of testing temperature.     

The Bauschinger effect in precipitation strengthened aluminum alloys

The Bauschinger effect in precipitation strengthened Al-Cu-Mg, Al-Zn-Mg and Al-Cu polycrystals was measured as a function of applied strain. Alloys heat treated to contain easily shearable precipitates,i.e., GPB, GP and θ″ exhibited a small Bauschinger effect, on the order of that in pure aluminum. In contrast, alloys with nonshearable precipitates, S′, η and θ′ showed an anomolously large effect. A unique hysteresis loop shape, with a region of convex curvature between sharp inflection points, was observed in the nonshear-able precipitate alloys. The large Bauschinger effect and unusual hysteresis loop shape are due to internal elastic or back stresses exerted by the strong precipitates on the matrix. A nonlinear elastic hardening model is proposed in which the overall work harden-ing is partitioned into an elastic back stress component and a frictional dislocation forest hardening term. Plastic relaxation around the precipitates and inhomogeneous deformation in the polycrystal reduces the level of the internal stresses below that predicted theoretically by the Brown and Stobbs hardening theory.     

500 MPa级高延性方管用钢的开发及加工硬化行为

 为了开发满足二次加工性能要求的500 MPa级高延性方管用钢,采用OM、SEM和TEM等对500 MPa级高延性方管用钢制管前后的组织与性能进行分析,研究了其强化机制与加工硬化机理。结果表明,两种试验钢的组织均由铁素体和少量珠光体组成,低C-低Mn-Nb、Ti微合金化试验钢铁素体晶粒与珠光体球团尺寸更加细小,第二相析出物尺寸稍大,位错密度相似。两种试验钢制管前力学性能相似,低C-低Mn-Nb、Ti微合金化试验钢屈强比较高;制管后低C-低Mn-Nb、Ti微合金化试验钢加工硬化程度显著,屈服强度、抗拉强度分别增加了45与26 MPa,伸长率降低6.0%,高C-高Mn-Nb微合金化试验钢屈服强度、抗拉强度分别增加了22与10 MPa,伸长率降低4.0%。固溶强化与细晶强化是两种试验钢最主要的强化机制,由晶粒细化引起的强度增量占总强度的52.9%~61.8%,由固溶强化引起的强度增量占总强度的17.2%~25.3%;析出强化与位错强化对强度的贡献较小。制管后低C-低Mn-Nb、Ti微合金化试验钢位错强化增加显著,达到了82 MPa,明显高于高C-高Mn-Nb微合金化试验钢位错强化的贡献(65 MPa);对于制管用途而言,高C-高Mn-Nb微合金化试验钢制管后综合力学性能更加优异。     

Influence of cold forming on the tensile behavior and properties of low-carbon microalloyed steel

The tensile behavior and properties of cold formed low-carbon microalloyed steel with its microstructure of all ferrite and pearlite (F+P) were investigated.Bending and flattening deformations were carried out in the laboratory on hot-rolled sheets in order to simulate the cold forming process of steel sheets during pipe fabrication and sampling of high frequency straight bead welding pipes.A comparison of the tensile behavior and properties of the material made before and after cold forming indicates that cold deformation alters the tensile behavior and properties of the material to a certain degree depending on the manner of the cold deformation and the degree.The research on the Bauschinger effect indicates that for the steels investigated,when the plastic strain is small,the back stress increases rapidly with the increase of the plastic strain and then rapidly tends to saturation.The finite element analysis indicates that the change in the properties of the steel sheets due to cold forming is a result of the Bauschinger effect and work hardening.The mechanism of the change in the properties is also given in this study.     

Ti-V-Mo微合金化22MnB5钢中析出相及其强化作用

使用温成形替代热成形可以避免热成形过程中表面氧化等问题,但热成形常用22MnB5钢在高温回火后出现明显的软化现象.而通过向钢中添加Ti、V、Mo等微合金元素可以在钢中形成细小的析出相以及细化晶粒,起到提高强度的作用,从而可以解决该问题.因此,通过在22MnB5钢中添加Ti、V、Mo微合金元素,利用OM(光学显微镜)、F...     

Precipitation Effect on Mechanical Properties and Phase Stability of High Manganese Steel

High manganese (Mn) steels are attractive for automotive applications due to their excellent tensile strength and superior elongation. However, the relatively low yield strength of Mn steels compared to other advanced high-strength steels is a critical problem limiting their use in structural parts. In order to increase the yield strength, the precipitation hardening effect of Mn steels was investigated by the addition of carbide-forming elements. Changes in the austenite phase stability were also evaluated in terms of stacking fault energy (SFE). As a result, fine V(C,N) precipitates were found to increase the yield strength effectively but to lower the SFE by the consumption of matrix carbons. For achieving precipitation hardening without sacrificing austenite stability, the soluble carbon content was discussed.     

On minimizing the bauschinger effect in steels by dynamic strain aging

The Bauschinger effect was measured in the conventional tension-compression mode in five steels, AISI 1020, 1522, and 1035, ASTM A374 (Cor-Ten) and an HSLA Mn-Mo-Cb steel. The results were compared with those obtained by performing the forward strain at a temperature near the peak of dynamic strain aging for each of the steels and the subsequent reverse strain at room temperature. The dynamic strain aging process sharply reduced the extent of the Bauschinger effect and was particularly effective in re-storing the elastic portion of the flow curve during the reverse strain. We conclude that the straightening of steel parts after heat treatment should be done warm, in the dynamic strain aging range, whenever feasible. Formerly students at the University of Michigan.     

Microstructural Examination of a Grade 100 Microalloyed Steel and Correlation with Yield Strength

Abstract

The microstructure of a Grade 100 microalloyed steel was examined in terms of the iron matrix phases and microalloy precipitates using electron microscopy. Since microalloyed pipeline and structural steels are currently graded according to their yield strength, the different microstructural factors that affect the yield strength of the steel were assessed and their contributions to the strength were estimated. The microstructural factors include grain size, precipitate size and volume fraction, solid solution content and dislocation density. Accurate grain size measurements were only possible through high resolution electron microscopy imaging that made it possible to resolve grain/sub-grain boundaries of micron and sub-micron sized grains/subgrains. It was found that the increased strength was mainly due to the formation of bainitic structures with fine grain/sub-grain sizes. The contribution from other strengthening sources such as precipitates, dislocations and atoms in solid solution was significantly less.     

冷作强化非调质钢冷变形过程中的鲍辛格效应

 为了降低冷作强化非调质钢冷镦变形的变形抗力，研究了冷作强化非调质钢MFT8在冷变形过程中的鲍辛格效应。结果表明，鲍辛格效应随冷拔减面率γ增加而提高；γ=30%时，鲍辛格效应最大，压缩真应力最小；γ＞30%时，压缩真应力重新提高，这是鲍辛格效应与加工硬化共同作用的结果，即加工硬化抵消了鲍辛格效应。     

Austenite Grain Structures in Ti- and Nb-Containing High-Strength Low-Alloy Steel During Slab Reheating

Austenite-grain growth was investigated in a couple of microalloyed steels, one containing Ti and the other containing Nb, Ti, and V, using different reheating temperatures between 1273 K and 1523 K (1000 °C and 1250 °C). Nature and distribution of microalloy precipitates were quantitatively analyzed before and after reheating. Interdendritic segregation (or microsegregation) during casting can result in an inhomogeneous distribution of microalloy precipitates in the as-cast slabs, which can create austenite grain size variation (even grain size bimodality) after reheating. Ti addition reduced the grain size variation; however, it could not eliminate the grain size bimodality in Nb-containing steel, due to the differential pinning effect of Nb precipitates. A model was proposed for the prediction of austenite grain size variation in reheated steel by combining different models on microsegregation during solidification, thermodynamic stability, and dissolution of microalloy precipitates and austenite grain growth during reheating.     

Influence of Laves Phase Precipitation and Coarsening on High‐Temperature Strength of Ferritic Stainless Steels

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high‐temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe? Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms – precipitation–hardening, grain refinement, and solid‐solution strengthening – were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high‐temperature strength about 40%, also after long‐term annealing. The contribution of the Laves phase precipitates on the high‐temperature strength is rather small due to their rapid coarsening.     

Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part I: Yield Strength

Low carbon Nb-Mo microalloyed steels show interesting synergies between the “micro”-alloying elements when high strength–high toughness properties are required. Strain accumulation in austenite is enhanced, and therefore grain sizes are refined in the final microstructures. The presence of Mo facilitates the presence of non-polygonal phases, and this constituent modification induces an increment in strength through a substructure formation as well as through an increase in the dislocation density. Regarding fine precipitation and its strengthening effect, the mean size of NbC is reduced in the presence of Mo and their fraction increased, thus enhancing their contribution to yield strength. In this paper, a detailed characterization of the microstructural features of a series of microalloyed steels is described using the electron-backscattered diffraction technique. Mean crystallographic unit sizes, a grain boundary misorientation analysis, and dislocation density measurements are performed. Transmission electron microscopy is carried out to analyze the chemical composition of the precipitates and to estimate their volume fraction. In this first part, the contribution of different strengthening mechanisms to yield strength is evaluated and the calculated value is compared to tensile test results for different coiling temperatures and compositions.     

New approach to predict the long-term creep behaviour and evolution of precipitate back-stress of 9–12% chromium steels

Modern advanced 9–12 % Cr steels are complex alloys with excellent creep strength even at high temperatures up to 620°C. The mechanical properties of these steels are significantly influenced by the presence and stability of various precipitate populations. Numerous secondary phases grow, coarsen and, sometimes, dissolve again during heat treatment and service, which leads to a varying obstacle effect of these precipitates on dislocation movement. In this work, the experimentally observed creep rupture strength of an modified 9–12% Cr steel developed in the European COST Group is compared to the calculated maximal obstacle effect (Orowan stress) caused by the precipitates present in these steels for different heat treatment conditions. It is shown that the differences in creep rupture strength caused by different heat treatments disappear after long time service. This observation is discussed on the basis of the calculated evolution of the precipitate microstructure. The concept of boosting long-term creep rupture strength by maximizing the initial creep strength with optimum quality heat treatment parameters for precipitation strengthening is critically assessed.     

Bauschinger effect and residual phase stresses in two ductile-phase steels: Part I. The influence of phase stresses on the Bauschinger effect

The Bauschinger effect (BE) in dual-phase steels has been computationally simulated, and the influence of phase stresses, developed due to nonhomogeneous deformation during preloading, on the BE has been investigated. Isotropic-and anisotropic-hardening models were used in finite-element method calculation to produce the reverse flow stress-strain curves (compression) of dual-phase steels from the reverse stress-strain curves of single-phase materials. Aspects of the Bauschinger effect, including the rounding of the reverse flow curve, yielding at low reverse stresses, high initial work-hardening rates, and the absence of permanent softening,etc., were elucidated by the variation in phase stresses in the constituent phase.     

Effect of manganese on the recrystallization texture of P-containing low-carbon steel sheet

The effect of manganese on the development of 111 recrystallization textures of P-containing low-carbon Al-killed steel sheet has been investigated. A proper combination of phosphorus and manganese content in steel gave a strong 111 texture through a simulated box annealing cycle. The experimental data suggest that 111 recrystallization textures could be developed providing that the steels contained both high manganese and phosphorus. Phosphorus in steel was useful in enhancing the uniform dispersion of precipitates such as (FeMn)₃C and A1N. This might be attributed to softening the possible interaction between manganese and carbon or nitrogen atoms by the addition of phosphorus which is regarded as a repulsive element for carbon or nitrogen. The 0.2 pct P-1.2 pct Mn Al-killed steel sheet tested had a highr- value of 2.0 through the precipitates effect and a high strength of 430 MPa through the effect of a solid solution hardening due to phosphorus. Formerly Graduate Student, Kyushu University, is with Pohang Iron and Steel Company     

Effect of magnesium addition on microstructure and mechanical properties in wheel steel

In order to reveal the effect of Mg in low carbon microalloy steel, low carbon microalloy steel of HR60 wheel steel was smelted in vacuum induction furnace and industrial field respectively. The characteristics of typical non- metallic inclusions and microstructure of experimental steels were both compared by OM, SEM- EDS and INCA Feature with automatically scanning inclusions function. The mechanical properties of the experimental steels were also measured. The results show that alumina inclusions are modified to spinel inclusions with small size after Mg addition. Furthermore, acicular ferrite can be induced effectively by inclusions containing magnesium. The microstructures of experimental steels are changed from ??polygonal ferrite(PF) + pearlite(P)??to ??polygonal ferrite(PF) + degenerate pearlite(DP) + acicular ferrite(AF)?? and refined by Mg treatment. The strength of experimental steels is improved with Mg addition. In industrial experiments, the fatigue limit of Mg- treated steels is greater than 460MPa, while the fatigue limit of Ca- treated steels is about 450MPa. In addition, the fatigue life of Mg- treated steels is generally higher than that of Ca- treated steels under the condition that the stress is greater than the fatigue limit. In laboratory experiments, the contents of Nb and Ti are reduced while Mg content in steel is 18??10-6, the strength of the wheel steel is close to the reference steel. Therefore, the project to reducing production cost by taking advantage of the microalloy role of Mg is feasible.     

平整和自然时效对超低碳烘烤硬化钢板性能的影响

研究了平整率(0～3.3%)和自然时效对830℃退火0.7 mm超低碳烘烤硬化钢板(%:0.003 0C、0.008Nb-0.003 Ti、0.003 0N和0.0030C、0.012Nb-0.012Ti、0.004 2N)力学性能和烘烤硬化性能的影响。结果表明,最初随平整率增加,由于柯氏气团减少和位错密度增加,屈服强度降低;当0.008Nb-0.003 Ti钢平整率达到0.48%,0.012Nb-0.012Ti钢平整率达到0.26%时,屈服强度降至最低;平整率继续增加由于冷加工硬化增强,钢的屈服强度升高。平整率过低和过高,均会导致烘烤硬化性能下降。平整率控制在0.5%～1.5%时,钢板能获得较低的屈服强度、较高的断后伸长率和最大的烘烤硬化性能。钢板经自然时效3个月,烘烤硬化性能和延伸率下降。     

Local Use of Ageing Effects in Multiphase Steels for Designing Local Properties of Constructional Elements

This paper describes processes leading to local bake hardening (BH) effects in multiphase steels. The investigations are part of a comprehensive project which investigates the influence of the deformation path and of the temperature and duration of thermal treatments on strengthening in modern multiphase steels, in regard to both local and bulk properties of steel structures. Dual phase (DP), retained austenite steels (TRIP) ‐ both hot and cold rolled ‐ and complex phase (CP) steel are investigated to examine the effect of thermomechanical processing parameters on local bake hardening ability. For this purpose two ways to achieve a local BH effect, i.e. local deformation and local heat treatment, are studied, as well as ageing stability of the adjusted strength. Hardness increased after local deformation through bending as result of work hardening and bake hardening effect. The local heat treatment leads to an improvement of mechanical properties (hardness and strength) and to local strengthening of material. The stability of the local bake hardening effect could be confirmed.