低合金高强钢针状铁素体组织特征和形成机理

针状铁素体是一种具有大角度晶界、高位错密度的板条状中温转变组织,该组织能有效细化晶粒并具有良好的强韧性匹配.因此,通常在低合金高强度钢焊缝和粗晶区中,利用细小的夹杂物来诱导针状铁素体形成,形成有效晶粒尺寸细小的针状铁素体联锁组织或者针状铁素体和贝氏体的复合组织,使其具有良好的韧性.然而,相关研究对针状铁素体组织的形成机理和控制原理的解释并不十分清楚,对于针状铁素体的定义和理解也存在差异.总结了针状铁素体的本质、相变、形核、形态、晶体学取向关系、长大行为、细化机理和力学性能等方面的特征,归纳了奥氏体晶粒尺寸、转变温度、冷却速度、夹杂物类型和尺寸等对针状铁素体形成的影响,提出了针状铁素体组织形态和转变机理方面几个仍需深入研究的问题和方向.     

Influence of V(C,N) Precipitates on Microstructure and Mechanical Properties of Continuous Cooled C-Mn-V

The effect of chemical composition and processing parameters on the formation of acicular ferrite and/or bainite has been investigated.In particular,this paper deals with the influence that N through its combination with V,as V(C,N) precipitates,has on the decomposition of austenite.Likewise,the intragranular nucleation potency of V(C,N) precipitates is analyzed through the continuous cooling transformation diagrams (CCT) of two C-Mn-V steels with different contents of N.Results reported in this work allow us to conclude that acicular ferrite can only be achieved alloying with vanadium and nitrogen,meanwhile bainite is promoted in steels with a low level of nitrogen.It is concluded that higher strength values are obtained in acicular ferrite than in bainitic steel but a similar brittle-ductile transition temperature (BDT),and lower values of impact absorbed energy (KV) has been recorded in nitrogen-rich steel.     

Bainite in steels

The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstätten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.     

Effects of B and Cu Addition and Cooling Rate on Microstructure and Mechanical Properties in Low-Carbon, High-Strength Bainitic Steels

The effects of B and Cu addition and cooling rate on microstructure and mechanical properties of low-carbon, high-strength bainitic steels were investigated in this study. The steel specimens were composed mostly of bainitic ferrite, together with small amounts of acicular ferrite, granular bainite, and martensite. The yield and tensile strengths of all the specimens were higher than 1000?MPa and 1150?MPa, respectively, whereas the upper shelf energy was higher than 160?J and energy transition temperature was lower than 208?K (?C65?°C) in most specimens. The slow-cooled specimens tended to have the lower strengths, higher elongation, and lower energy transition temperature than the fast-cooled specimens. The Charpy notch toughness was improved with increasing volume fraction of acicular ferrite because acicular ferrites favorably worked for Charpy notch toughness even when other low-toughness microstructures such as bainitic ferrite and martensite were mixed together. To develop high-strength bainitic steels with an excellent combination of strength and toughness, the formation of bainitic microstructures mixed with acicular ferrite was needed, and the formation of granular bainite was prevented.     

Phase transformation behaviour in two C–Mn–Si based steels under different cooling rates

Abstract

The continuous cooling transformation (CCT) behaviour of two C–Mn–Si based steels was investigated. The effects of chemical composition and cooling rate on γ→α transformation were studied using dilatometric measurements. Quantitative phase analysis was carried out in order to determine the effect of cooling rate on the precise phase distribution after transformation. Presence of Cr and Mo in the experimental steels appears to retard pearlitic transformation and promotes formation of acicular products (combination of acicular ferrite, Widmanstatten ferrite and bainite). Martensitic transformation also starts at a perceptibly lower cooling rate in the Mo containing alloy as compared with the one without any Mo.     

Bainitic chromium-tungsten steels with 3 Pct chromium

Previous work on 3Cr-1.5MoV (nominally Fe-3Cr-2.5Mo-0.25V-0.1C), 2.25Cr-2W (Fe-2.25Cr-2W-0.1C), and 2.25Cr-2WV (Fe-2.25Cr-2W-0.25V-0.1C) steels indicated that the impact toughness of these steels depended on the microstructure of the bainite formed during continuous cooling from the austenitization temperature. Microstructures formed during continuous cooling can differ from classical upper and lower bainite formed during isothermal transformation. Two types of nonclassical microstructures were observed depending on the cooling rate: carbide-free acicular bainite at rapid cooling rates and granular bainite at slower cooling rates. The Charpy impact toughness of the acicular ferrite was considerably better than for the granular bainite. It was postulated that alloying to improve the hardenability of the steel would promote the formation of acicular bainite, just as increasing the cooling rate does. To test this, chromium and tungsten were added to the 2.25Cr-2W and 2.25Cr-2WV steel compositions to increase their hardenability. Charpy testing indicated that the new 3Cr-W and 3Cr-WV steels had improved impact toughness, as demonstrated by lower ductile-brittle transition temperatures and higher upper-shelf energies. This improvement occurred with less tempering than was necessary to achieve similar toughness for the 2.25Cr steels and for high-chromium (9 to 12 pct Cr) Cr-W and Cr-Mo steels.     

Analysis Microstructure of Weld Metal for HQ130+QJ63 High Strength Steel

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Analysis of Microstructure and Orientation in X80 Line Pipe Steels

Microstructures in X80 line pipe were classified by SEM analysis.The experimental results showed that the microstructures in X 80 line pipe steels were complicated consisting of polygonal ferrite,bainite and acicular ferrite.Orientation relation within acicular ferrite was investigated systematically by means of EBSD-OIM.The sub-structures were observed maximum in acicular ferrite which gives high strength and high toughness to line pipe steels.The K-S orientation relation was generally observed between acicular ferrite and austenite during phase transformation.The low energy CSL boundary characterized by Σ3 orientation relation according to Brandon criterion appeared with higher probability,which was benefit to improve the mechanical properties of line pipe steels.The orientations or texture of initial austenite grains could be deduced based on the Σ3 orientation relationship of acicular ferrite variants.     

Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS

The isothermal transformation vs time of a medium-carbon microalloyed steel at 450 °C, following austenitization at 1250 °C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase. The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of upper bainite formation.     

Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS

The isothermal transformation vs time of a medium-carbon microalloyed steel at 450°C, following austenitization at 1250°C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase. The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of upper bainite formation.     

高钢级管线钢的组织特征和强韧性

背散射(EBSD)和扫描(SEM)电子显微镜及力学性能试验表明,微合金化X70、X80和X100管线钢的组织由针状铁素体、粒状贝氏体和少量下贝氏体组成;随钢的有效晶粒尺寸降低、贝氏体含量增加以及组织均匀性提高,高钢级管线钢的强韧性明显增加。     

Effects of Zr on acicular ferrite formation in HAZ of Ti-bearing low carbon steels with high heat input of weld thermal simulations

To research the effect of Zr addition on inhibiting austenite grain growth of Ti-bearing low carbon steels,two steels with different Zr contents were prepared using a laboratory vacuum induction furnace. The performance of HAZ under weld thermal simulations was investigated. The impact toughness,microstructure and the second-phase particle performance of HAZ under weld thermal simulations were investigated. The HAZ toughness was improved from 13 J to 87 J by addition of 0. 010 % Zr into the steel,with the fracture mechanism changing from cleavage fracture to toughness fracture,which was mainly attributed to the second-phase particles that were potent to nucleate acicular ferrite in HAZ during welding. It was concluded that the second-phase particles TiO x + MnS,ZrO 2 + MnS or TiO x + ZrO 2 + MnS were nucleated on ZrO 2 or TiO x ( x =1. 5,2) . This method can be applied to grain refinement by promoting the acicular ferrite formation and growth during large-scale welding,as in the cases of thick steel plates requiring higher heat inputs during welding.     

Ti对大线能量焊接焊缝组织和性能的影响

对不同Ti含量的气电立焊焊缝组织及力学性能进行了对比研究。结果表明,Ti的质量分数在0.028%~0.038%范围内时,焊缝中获得大量细小的针状铁素体,焊缝组织及低温韧性得以明显改善。当Ti过量时,焊缝中的针状铁素体减少,组织以贝氏体为主,低温韧性相应下降。焊缝组织中观察到块状和条状的M-A组元,随着焊缝Ti含量增加,其总量增加。焊缝夹杂物多为以氧化物为核心,外层包裹着MnS的复合夹杂物,并随夹杂物Ti含量的增加,由Mn-Si-Al-O型向Ti-Mn-Al-O型转变,有利于促进针状铁素体形成。而当焊缝中Ti过量时,主要夹杂物又转变为对针状铁素体形核无效的Ti-Al-O型,促进了贝氏体转变。     

铬微合金化高碳重载车轮钢的连续冷却

 采用Gleeble 3500模拟试验机、金相显微镜和扫描电镜研究了两种含铬微合金高碳重载车轮钢的连续冷却组织转变规律,分析了冷速对组织及性能的影响。试验结果表明,铬元素含量高的高碳重载车轮钢的临界冷却速度低,淬透性高,珠光体域和珠光体片层间距小,洛氏硬度高;冷速为5.0、80、10.0℃/s时有少量的贝氏体和针状铁素体出现,且它们的体积分数随铬含量的增加而增加。     

Diffusion in growth of bainite

Edgewise growth rates for Widmanstätten ferrite and bainite in low alloy steels can be represented with an empirical equation showing proportionality to the square of the supersaturation of the austenite. The proportionality constant has a value in reasonable agreement with the assumption of rate control by carbon diffusion. The growth rates are too low to give a noticeable supersaturation of carbon in the growing ferrite. The experimentalB _s for low alloy steels does not seem to be related to theT _s line, nor doesBs evaluated from the incomplete transformation to bainite for an alloy steel. By assuming rate control by carbon diffusion, the empirical equation can be used to calculate the growth rate under paraequilibrium or no partition, local equilibrium (NPLE) conditions. Experimental growth rates for a similar steel falls in-between. The fact that paraequilibrium does not seem to apply is taken as an indication that the α/γ interface for Widmanstätten ferrite and bainite is not of a purely martensitic type.     

The origin of transformation textures in steel weld metals containing acicular ferrite

The present investigation is concerned with basic studies of the development of transformation textures in steel weld metals, using the electron backscattering pattern (EBSP) technique. It is shown that the acicular ferrite (AF) plates exhibit an orientation relationship with both the austenite and the prior delta ferrite columnar grains in which they grow. The observed orientation relationship lies within the Bain orientation region and can be described by three texture components,i.e., a 〈100〉 component and two complementary 〈111〉 components. Each of these texture components is orientated approximately parallel with the original cell/dendrite growth direction. Measurements of the spatial misorientation between neighboring plates confirm that the morphology of AF in low-alloy steel weld metals bears a close resemblance to upper bainite.     

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.     

Ferrite and bainite in alloy steels

The addition of alloying elements even in small concentrations can alter the properties and structure of ferrite and bainite. The various morphologies of ferrite-carbide aggregates are surveyed including alloy pearlite, fibrous carbide eutectoids and precipitation of fine alloy carbides atγ-α interfaces. Modern ideas on the morphology and growth kinetics of ferrite and upper and lower bainite are also summarized. Using this information, an attempt is made to rationalize subcritical transformations of austenite in low alloy steels. Basic factors influencing the strength of alloy ferrites are discussed, leading to an examination of structure-mechanical property relationships in ferrite and bainite. Finally the exploitation of the ferrite and bainite reactions to produce useful alloy steels by direct transformation of austenite is explored.     

Effect of Dissolution and Precipitation of Nb on Phase Transformation,Microstructure, and Microhardness of Two High-Nb Pipeline Steels

Niobium (Nb) as an important microalloying element is widely applied in high strength pipeline steels. In this work, the continuous cooling transformation diagrams of two high-Nb steels with and without hot deformation were studied using a Gleeble 3500 thermal simulator. The amounts of dissolved Nb, undissolved Nb, and precipitated Nb were determined by inductively coupled plasma-atomic emission spectrometry. Results show that the increasing of Nb content in the high-Nb steels can restrain the prior austenite grain growth, dynamic, and/or static recrystallization; moreover, it can suppress polygonal ferrite transformation and promote acicular ferrite and bainite transformation, refining the microstructure and increasing the microhardness as a consequence. Nevertheless, the amplified Nb content in steels escalates trends of strain-induced Nb(C,N) precipitation. The increase in the amount of Nb(C,N) precipitates promote the polygonal ferrite and acicular ferrite transformation, while also decrease the microhardness. The results from this work show that the higher Nb content of 0.13% in the tested steel is unnecessary.