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1.
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. 相似文献
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Microstructural Evolution and Mechanical Properties of Nb-Ti Microalloyed Pipeline Steel 总被引:1,自引:0,他引:1
The correlation between microstructures and mechanical properties of a Nb-Ti microalloyed pipeline steel was investigated. The results revealed that with decreasing the finish rolling temperature and the cooling stop temperature, the matrix microstructure was changed from quasi-polygonal ferrite to acicular ferrite, as a result of improvement of both strength and low temperature toughness. By means of electron backscattered diffraction observation, an effective acicular ferrite packet contained several low angle boundaries or subboundaries plates which made important contributions to improvement of strength. It was found that many fine quasi-polygonal ferrite grains with high angle boundaries as the toughening structure were introduced into the acicular ferrite matrix to refine effective grain size and improve the toughness. 相似文献
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N. Isasti D. Jorge-Badiola M. L. Taheri B. López P. Uranga 《Metallurgical and Materials Transactions A》2011,42(12):3729-3742
Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior
to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both
strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure.
Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source
of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation
dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions
(0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite.
Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved
through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered
diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges
to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite
accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However,
an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary
units. 相似文献
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Andrii G. Kostryzhev Martin Strangwood Claire L. Davis 《Metallurgical and Materials Transactions A》2010,41(6):1399-1408
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. 相似文献
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采用实验室热轧、显微分析及力学性能检测手段,对Nb-Mo及Nb微合金化X100管线钢在不同工艺条件下的组织特征及力学行为的变化规律进行了研究.分析结果表明:工艺参数对Nb-Mo复合成分试验钢影响较大,控轧控冷工艺条件下Nb-Mo及Nb微合金化X100管线钢力学性能均能达到API 5L中X100管线钢要求,但Nb-Mo复合成分力学性能富余量较大,性能较优.随冷却速度的增加及终冷温度的降低,试验钢强度增加,韧性及塑性恶化.板条马氏体与贝氏体复相组织较板条马氏体可大大提高试验钢的塑性及低温冲击韧性. 相似文献
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In this article, a detailed study was conducted to evaluate the microstructural evolution and mechanical properties of microalloyed
steels processed by thermomechanical schedules incorporating cool deformation. Cool deformation was incorporated into a full
scale simulation of hot rolling, and the effect of prior austenite conditioning on the cool deformability of microalloyed
steels was investigated. As well, the effect of varying cooling rate, from the end of the finishing stage to the cool deformation
temperature, 673 K (400 °C), on mechanical properties and microstructural evolution was studied. Transmission electron microscopy
(TEM) analysis, in particular for Nb containing steels, was also conducted for the precipitation evaluation. Results show
that cool deformation greatly improves the strength of microalloyed steels. Of the several mechanisms identified, such as
work hardening, precipitation, grain refinement, and strain-induced transformation (SIT) of retained austenite, SIT was proposed,
for the first time in microalloyed steels, to be a significant factor for strengthening due to the deformation in ferrite.
Results also show that the effect of precipitation in ferrite for the Nb bearing steels is greatly overshadowed by SIT at
room temperature. 相似文献
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Dominic Bartels Andrea von Lattre-Hertel Tobias Novotny Andreas Mohr Horst Hill Carsten Merklein Michael Schmidt 《国际钢铁研究》2023,94(8):2200925
Low-alloyed steels are used for a variety of different applications like bearings or gears. Additive manufacturing technologies like directed energy deposition (DED-LB/M) allow for a fast and close-to-contour fabrication of sophisticated products without excessive waste of material. However, the DED-LB/M process cannot be considered as state-of-the-art for this group of materials. This study presents findings on the material properties of the additively manufactured low-alloyed steel Bainidur AM by means of DED-LB/M. This includes studies on the mechanical properties (hardness, compression strength) as well as the microstructural properties (scanning electron microscopy [SEM]). The microstructure in the as-built state appears like a bainitic–martensitic one with shares of retained austenite which is not fully transformed during cooling. As a differentiation is barely possible from the SEM images, a plethora of investigations is further used to assess the microstructure. As-built samples possess a good combination of ductility and hardness. Furthermore, the specimens are characterized by a good tempering stability up to 600 °C. This tempering stability is characterized by a homogeneous hardness of around 400 HV1 for all temperatures. In contrast, the conventionally hardened specimens show a drop-off in material hardness, further indicating the excellent material properties of additively manufactured Bainidur AM. 相似文献
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Different samples of TRIP (transformation induced plasticity) steel obtained by two different hot-rolling schedules are investigated by using a SEM (scanning electron microscope). The microstructure is characterized by using an OM (optical microscope) for phase distribution and by EBSD (electron backscatter diffraction) for texture and phase mapping. ODF (orientation distribution function) graphs are used to investigate the effect of recrystallization behavior of the hot-deformed austenite on phase transformation during the controlled cooling process. The mechanical behavior is interpreted in terms of the strength of both hard and soft phases, in combination with the quantity, location and transformation kinetics of the mechanically induced martensite (TRIP effect). The results show that more austenite grains exist in the steels obtained at finishing rolling temperature (FRT) of 750 °C, which inherited the deformation structure after the hot-rolling process. The instantaneous n value (ni) of those steels is kept high during a large range of strain before failure, while the tensile strength and total elongation of the steels with respect to the different finishing rolling temperatures do not show any significant differences. 相似文献
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Jingwei Zhao Taekyung Lee Jeong Hun Lee Zhengyi Jiang Chong Soo Lee 《Metallurgical and Materials Transactions A》2013,44(8):3511-3523
In the current study, the effects of tungsten (W) addition on the microstructure, hardness, and room/low [223 K and 173 K (?50 °C and ?100 °C)] temperature tensile properties of microalloyed forging steels were systematically investigated. Four kinds of steel specimens were produced by varying the W additions (0, 0.1, 0.5, and 1 wt pct). The microstructure showed that the addition of W does not have any noticeable effect on the amount of precipitates. The precipitates in W-containing steels were all rich in W, and the W concentration in the precipitates increased with the increasing W content. The mean sizes of both austenite grains and precipitates decreased with the increasing W content. When the W content was equal to or less than 0.5 pct, the addition of W favored the formation of allotriomorphic ferrite, which subsequently promoted the development of acicular ferrite in the microalloyed forging steels. The results of mechanical tests indicated that W plays an important role in increasing the hardness and tensile strength. When the testing temperature was decreased, the tensile strength showed an increasing trend. Both the yield strength and the ultimate tensile strength obeyed an Arrhenius type of relation with respect to temperature. When the temperature was decreased from 223 K to 173 K (from ?50 °C to ?100 °C), a ductile-to-brittle transition (DBT) of the specimen with 1 pct W occurred. The addition of W favored a higher DBT temperature. From the microstructural and mechanical test results, it is demonstrated that the addition of 0.5 pct W results in the best combination of excellent room/low-temperature tensile strength and ductility. 相似文献
14.
The Bauschinger effect (a reduced yield stress at the start of reverse deformation following forward prestrain) is an important
factor of 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
the microalloy precipitates and the presence of retained work hardening. In this article, the parameters of the Bauschinger
effect and work hardening (coefficient and exponent) in forward and reverse deformations were quantitatively related to the
particle number density and dislocation density for two high-strength low-alloy (HSLA) steels. An example of the application
of the obtained dependences is discussed with respect to the strength development during UOE forming of large diameter line
pipes. 相似文献
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用Mo来改善V-Nb微合金钢的性能 总被引:1,自引:0,他引:1
1 前言结构钢的发展特征是不断探索较高的强韧性及具有良好的焊接性能 ,这些要求的一个例子就是生产输送气体的管线钢。其基本趋势就是减少壁厚增大直径。厚度减少就必须增加强度且不影响韧性和焊接性 ,较大长度的焊接管线钢因工作压力的增加要求具有高的抗拉性能。现在人们对屈服强度超过 5 5 0 MPa的管道钢感兴趣并要求增加其断裂韧性。本文研究的目标是热机械控制工艺参数对Mo- V,Mo- Nb,Mo- V - Nb和 V - Nb微合金钢在加工过程中的硬度、机械性能以及最终显微组织的改善。2 试验过程2 .1 试验钢种分别对 Nb- Mo、V- Mo、Nb- V… 相似文献
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T. H. Courtney 《Metallurgical and Materials Transactions A》1977,8(5):685-689
During liquid phase sintering, microstructural coarsening takes place. One mechanism by which this occurs is Ostwald ripening.
Alternatively, particle coalescence also leads to a concomitant reduction in the solid particle surface area per unit volume.
In isolated structures in which particle-particle contacts are made, the rate of coarsening by coalescence is limited by the
time between particle contacts, for this is long compared to the time to fuse two particles together. In skeletal structures
the “coalescence time” limits coarsening by coalescence since this is long in comparison to the time between contacts. Expressions
for the rate of particle coarsening are developed for the different mechanisms and different particle morphologies. The results
of these calculations are combined with the microstructure maps developed in Part I of this paper to refine these maps so
that they predict both the morphology developed and the dominant mechanism of coarsening in liquid phase sintered systems. 相似文献
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Beatriz López Jose M. Rodriguez-Ibabe 《Metallurgical and Materials Transactions A》2017,48(6):2801-2811
As thin slab direct rolling technologies are moving to the production of higher quality steel grades, chemical compositions based on Nb-Ti and Nb-Mo become a good option. However, with the use of multiple microalloying additions, the as-cast austenite conditioning becomes more complex. This paper analyzes some of the microstructural features that should be taken into account during the as-cast austenite conditioning in Nb-Ti and Nb-Mo microalloyed steel grades. In the case of Nb-Ti grades, it has been observed that the process parameters during solidification and post-solidification steps affect the austenite evolution during hot rolling. This is due to the differences in the size and volume fraction of TiN particles that can be formed. Fine TiN precipitates have been shown to be able to delay recrystallization kinetics. Moreover, the solute drag effect of Ti cannot be ignored in the case of hyperstoichiometric Ti/N ratios. It is observed that Nb-Ti grades tend to have lower non-recrystallization temperatures compared to Nb grades, which means that pancaking of the austenite is more difficult for these steels. The opposite is observed for the Nb-Mo grades, although in both cases the behavior is affected by the nominal content of Nb. 相似文献
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Sudin V. V. Stepanov P. P. Bozhenov V. A. Kantor M. M. Efron L. E. Zharkov S. V. Chastukhin A. V. Ringinen D. A. 《Metallurgist》2021,65(5-6):500-516
Metallurgist - Microstructural mechanisms reducing the impact strength values of a coarse grained heat-affected zone are studied for two K60 microalloyed steels. Research is conducted on specimens... 相似文献