Influence of isothermal aging on σ precipitation in super duplex stainless steel

The time-temperature-transformation (TTT) curve of the 00Cr25Ni7Mo4N duplex stainless steel was obtained with a Formastor-digital thermal dilatometer, and the influence of isothermal aging on σ precipitation was studied by metallographic observation, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results show that the decomposition of ferrite phase is accompanied by the formation of σ phase at 750–1000℃, especially in the range of 800–900℃. The longer the aging time, the higher the amount of σ precipitation. The area fraction of various phases remains at a certain value upon the completion of ferrite deformation. The temperature of 850℃ is the most sensitive transaction temperature, the incubation time for the formation of σ precipitation is less than 1 min, and aging for 20 min leads to the complete transformation of ferrite. The σ phase is formed preferentially at the α/α/γ junction, and then grows along the α/α boundary in the matrix.     

含Nb-Ti低碳微合金钢纳米碳化物析出行为

采用热膨胀仪、显微硬度计和透射电镜研究了等温温度对Nb-Ti微合金钢组织性能及析出行为的影响规律.结果表明,在不同温度下等温淬火均得到铁素体与马氏体的双相组织,且随着等温温度的降低,铁素体的体积分数逐渐增大.实验钢中存在两种主要的析出形态,分别为相间析出和弥散析出,且随着等温温度的降低,析出形态逐渐从相间析出向弥散析出转变.通过HRTEM确定相间析出碳化物具有Na Cl型晶体结构,晶格常数为0.432 nm,且与铁素体基体满足Baker-Nutting(B-N)关系.铁素体相的维氏硬度随着等温温度的降低而逐渐增大.     

Analysis of the formation conditions and characteristics of interphase and random vanadium precipitation in a low-carbon steel during isothermal heat treatment

The characteristics of nanosized precipitates in steels depend on the heat-treatment parameters. The effects of characteristics of vanadium precipitates formed during isothermal heat treatment on the hardness of the ferrite matrix in low-carbon vanadium-alloyed steel were investigated through analysis of transmission electron microscopy images and microhardness measurements. The results show that, during isothermal holding in the temperature range from 675 to 750℃, only interphase precipitation occurs, whereas only random precipitation occurs in the ferrite matrix during holding at 600℃. Furthermore, during isothermal heat treatment between 600 and 675℃, both random and interphase precipitates occurred in the ferrite. Nanoscale vanadium carbides with different atomic ratios of vanadium (V) and carbon (C) were the dominant precipitates in the random and interphase precipitates. The sizes of random precipitation carbides were smaller than those of interphase ones. Also, the sample isothermally heat treated at 650℃ for 900 s exhibited a higher hardness with a narrower hardness distribution.     

热轧低碳钒钢强韧化机制的研究

采用不添加Mo,Cr,Ni,低成本V-N微合金化的成分设计,对实验钢进行控轧控冷(TMCP)实验,探讨其相变机理与析出行为,利用光学显微镜、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等测试手段,系统地研究了热轧微观组织和综合力学性能.结果表明,显微组织为针状铁素体、准多边形铁素体和粒状贝氏体及少量三角状M/A岛,析出的细小V(C,N)粒子呈不规则的椭球状,较均匀弥散地分布于铁素体基体内部.实验钢的屈服和抗拉强度分别为618,701 MPa,断后延伸率19%,冷弯性能合格,扩孔率达到94%,延伸凸缘性能及低温冲击性能良好,满足轮辐用钢的加工要求.细晶强化、固溶强化、析出强化、相变强化为主要强化机制.     

正火过程中V--N微合金化钢的第二相行为

采用物理化学相分析、高分辨透射电镜等手段研究V-N微合金化钢在正火过程中第二相行为,并进行相应的理论计算,讨论该行为对材料性能产生的影响.正火加热保温过程中,V-N钢有约32.9%的V( C,N)未溶解,阻止奥氏体晶粒长大.在正火冷却过程中,未溶解的V( C,N)诱导晶内铁素体形核,细化铁素体晶粒,而溶解的V( C,N)重新析出,起到析出强化作用. V( C,N)析出相行为的变化导致材料力学性能的改变.与热轧态V-N钢相比,正火态V-N钢细晶强化贡献值增加31 MPa,而析出强化贡献值减少45 MPa.     

Effects of ausforming strain on bainite transformation in nanostructured bainite steel

The effects of ausforming strain on bainite transformation in high-carbon low-alloy nanobainite steel were investigated using a Gleeble 3500 thermomechanical simulator machine. The bainite transformation speed at 300℃ was found to be accelerated by ausforming at 300, 600, and 700℃ under applied strains ranging from 10% to 50% followed by isothermal transformation at 300℃. The ausformed bainite volume fraction varied with the ausforming strain because of the mechanical stabilization of the deformed austenite. Ausforming at low temperatures not only enhanced the bainite ferrite volume fraction but also refined the microstructure substantially. Although the amount of bainite ferrite might have been reduced with increasing strain, the microstructures were refined by ausforming.     

The precipitation strengthening behavior of Cu-rich phase in Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel for USC power plant application

Copper has been used as a strengthening element in newly developed Fe–Cr–Ni type austenitic heat resistant steel for inducing Cu-rich phase precipitation to meet high temperature strength requirement for 60°C Ultra Super-Critical (USC) coal fired power plants for many years. However, the precipitation behavior and strengthening mechanism of Cu-rich phase in these advanced austenitic heat resistant steels is still unclear. In order to understand the precipitation strengthening behavior of Cu-rich phase and to promote high strength austenitic heat resistant steel development, 18Cr9 NiCuNb steel which is a Cu-added Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel has been selected for this study to be aged at 650°C till to 10,000 h. Micro-hardness and room temperature tensile test were conducted after long-time aging. SEM,TEM, HRTEM and three dimensional atom probe (3DAP) technology accompanying with thermodynamic calculation have been used to investigate the Cu-rich phase precipitation behavior during 650°C aging. The experimental results showed that Cu atoms can quickly concentrate in clusters at very early precipitation stage to form the fine nano-size Cu-rich ‘‘segregation areas’within less than 1 h at 650°C. With increasing aging time at 650°C Cu atoms continuously concentrate to Cu-rich segregation areas (clusters) and simultaneously other kinds of atoms such as Fe, Cr and Ni diffuse away from Cu-rich segregation areas to austenitic matrix, and finally to complete the transformation from Cu-rich segregation areas to Cu-rich phase. However, there is only Cu atoms concentration but not crystallographic transformation from early stage of Cu-rich clusters forming to the final Cu-rich phase formation. Even the Cu atom becomes the main composed element after 500 h aging at 650°C the Cu-rich phase still keeps coherent relationship with austenitic matrix. According the experimental results in this study, Cu-rich phase precipitation sequence which starts from the Cu atom segregation followed by the Cu diffusing from matrix to segregation areas and Fe, Cr and Ni atoms diffuse out from Cu-rich areas to matrix without crystallographic transformation is proposed. The Cu-rich phase is the most dispersed phase and contributes the most important strengthening effect among all precipitated phases (M23C6, MX and Cu-rich phase). It has been found that Cu-rich phase is very stable and still keeps in nano-size even for 10,000 h aging at 650°C. The unique precipitation strengthening of Cu-rich phase in combination with nano-size Nb-rich MX phase and grain-boundary M23C6carbide contributes excellent strengthening effect to 18Cr9 NiCuNb austenitic heat resistant steel.     

Forming condition and control strategy of ferrite decarburization in 60Si2MnA spring steel wires for automotive suspensions

The ferrite decarburization behavior of 60Si2MnA spring steel wires for automotive suspensions, including the forming condition and the influence of heating time and cooling rate after hot rolling, was investigated comprehensively. Also, a control strategy during the reheating process and cooling process after rolling was put forward to protect against ferrite decarburization. The results show that ferrite decarburization, which has the strong temperature dependence due to phase transformation, is produced between 675 and 875℃. The maximum depth is found at 750℃. Heating time and cooling rate after rolling have an important influence on decarburization. Reasonable preheating temperature in the billet reheating process and austenitizing temperature in the heat-treatment process are suggested to protect against ferrite decarburization.     

Nb-V低碳微合金钢等温淬火过程析出行为

利用透射电镜和纳米压痕仪对Nb-V低碳微合金钢中纳米碳化物的析出行为进行研究.研究结果表明,在700℃等温60min试样中,可同时观察到相间析出和弥散析出,在其余试样中均未观察到相间析出,此规律可以通过相变过程中的扩散准则和台阶机制来解释.另外,纳米压痕结果显示在600℃等温20min试样中,平均硬度为3.87GPa,650℃等温20min试样中,平均硬度为4.10GPa,且通过TEM观察可以看出,650℃等温20min中试样析出物数量密度较大且分布均匀.利用Ashby-Orowan机制对析出强化量进行计算,可以得出在650℃等温20min试样中,析出强化对整个屈服强度的贡献量可以达到110MPa.     

Element distribution and diffusion behavior in Q&P steel during partitioning

Carbon, manganese, and silicon distribution in quenching and partitioning （Q＆P） steel during partitioning process was investigated to reveal the diffusion behavior. The microstructure and chemical composition were analyzed by means of scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and three-dimensional atom probe. It is shown that the studied Q＆P steel consisted of martensite laths and thin, film-like retained austenite showing extraordinary phase transformation stability. Carbon atoms mostly diffused to the retained austenite from martensite at a higher partitioning temperature. In the experimental steel partitioned at 400℃ for 10-60 s, carbides or cementite formed through carbon segregation along martensite boundaries or within the martensite matrix. As a result of carbon atom diffusion from martensite to austenite, the carbon content in martensite could be ignored. When the partitioning process completed, the constrained carbon equilibrium （CCE） could be simplified. Results calculated by the simplified CCE model were similar to those of CCE, and the difference between the two optimum quenching temperatures, where the maximum volume fraction of the retained austenite can be obtained by the Q＆P process, was little.     

高碳马氏体不锈钢8 Cr13 MoV钢铸态组织及碳化物

利用Thermo-Calc软件对8Cr13MoV马氏体不锈钢的凝固过程进行计算,利用光学显微镜、扫描电子显微镜和X射线衍射分析仪对铸态组织和碳化物形貌以及类型进行观察与分析,利用Gleeble热模拟试验机测定材料的静态连续冷却转变曲线.结果表明,8Cr13MoV在平衡凝固条件下组织为铁素体和M23 C6型碳化物,而在实际的凝固条件下,组织为铁素体、马氏体、残余奥氏体、M7 C3型和M23 C6型碳化物,由于偏析导致最终组织中碳化物以M7 C3型为主,少量M23 C6以薄片或树枝状分布在晶界上.由于较高的C和Cr含量,以0.1℃·s-1的冷却速率冷却时,奥氏体也会发生马氏体转变.     

Study on precipitation and transition mechanisms from the magnetic properties of silicon steel during annealing

Precipitates play an important role in determining the mechanical and magnetic properties of silicon steel. This paper aims to investigate the growth kinetics of precipitates in commercial silicon steel by analyzing its magnetic properties during isothermal annealing at 200℃. The growth of precipitates was studied by optical microscopy, scanning electron microscopy, transmission electron microscopy, and magnetic measurements. In combination with the coercive field and initial susceptibility, this technique offers the advantage of being non-destructive and providing quantitative information about the number, mean radius of precipitates, and fraction of transformation. An observed decrease in the number of precipitated particles indicates that the transformation starts from particles of appreciable initial size.     

等温温度对超细贝氏体钢组织演变规律的影响

通过热膨胀试验研究实验钢的等温转变动力学,采用盐浴等温淬火工艺制备超细贝氏体组织,利用扫描电镜、透射电镜和X射线衍射仪定量分析工艺参数对微观组织结构的影响.结果显示:实验钢室温组织由大量超细板条状贝氏体铁素体和板条间分布的薄膜状奥氏体的复相组织构成,210℃等温淬火得到的贝氏体板条间距细化到约60 nm,硬度约为HBW610;实验钢的最终组织特征取决于发生贝氏体转变的等温温度和等温时间,等温温度越低时贝氏体转变完成需要的等温时间越长.     

TSCR流程生产钛微合金化高强耐候钢中的析出物

运用电子显微镜和化学相分析等多种实验手段研究了Ti微合金化高强耐候钢中的析出物,并在热力学计算的基础上分析了其析出过程. 结果表明:钢中主要存在TiC,TiCN,Ti4C2S2,TiN等析出物,连轧前TiN的析出过程已基本完成;大量纳米尺寸的TiC球形析出物粒子在铁素体的位错线上分布;Ti含量增加改变了MC相的粒度分布,小尺寸粒子的体积分数显著增加,增强了沉淀强化的效果.     

等温处理过程热轧TRlP钢残余奥氏体的分解行为

以低Si含Al热轧TRIP钢为研究对象,采用扫描电子显微镜、透射电子显微镜、X射线衍射仪和拉伸实验等实验方法,研究了贝氏体区等温处理过程中残余奥氏体的分解行为.结果表明:在不同的等温温度下,随着等温时间的增加,残余奥氏体逐渐分解为铁素体和碳化物,随着等温温度的升高,残余奥氏体发生分解所需要的时间减少;实验钢的抗拉强度、断后延伸率和强塑积在不同的等温温度下,随着等温时间的增加呈现不断降低的变化趋势;在不同的等温处理工艺下,残余奥氏体的体积分数呈现降低的趋势,而碳含量没有明显的变化.     

时效处理对2205 DSS组织及力学性能的影响

首先对2205 DSS进行了1 100℃固溶处理,随后将试样分别在650,700,750,800,850和900℃下进行不同时间的时效处理,探究2205 DSS中σ相的析出规律及其对材料组织和力学性能的影响.研究结果表明:2205 DSS中σ相的析出分为有碳化物伴随和无碳化物伴随两种方式,前者发生在α-γ相界上,后者则主要发生在α相的晶内和晶界;2205 DSS在850℃时效时σ相的析出行为最严重;在析出σ相后,合金元素Cr和Mo在各相中会发生不同程度的偏聚;2205 DSS中析出少量的σ相对材料的塑性影响不大,但会显著降低材料的冲击韧性,而σ相的大量析出则会使两者均发生严重恶化;σ相的析出对材料的屈服强度影响不大,对材料的抗拉强度有略微的提高作用.     

Precipitation behaviors of X80 acicular ferrite pipeline steel

The precipitation behaviors of X80 acicular ferrite pipeline steel were investigated by using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The results show that dendritic precipitates in the as-cast steel slabs precipitate mainly in grain boundaries, and these dendritic precipitates dissolve and re-precipitate to two kinds of carbonitrides: Ti- and Nb-rich (Ti, Nb)(C, N) carbonitrides during reheating. Four types of precipitates mainly exist in the hot rolled plate: Ti-rich carbonitrides resulted from the dendritic carbonitrides undissolved during the reheating process; Ti-rich carbonitrides re-precipitated along austenite grain boundaries during the reheating process; NbC carbides mainly heterogeneously nucleated on the small pre-existing Nb-rich carbonitrides in the hot rolling process; and NbC carbides precipitated on dislocations during hot rolling.     

Effects of pre-strain and baking parameters on the microstructure and bake-hardening behavior of dual-phase steel

In a typical process, C-Mn steel was annealed at 800°C for 180 s, and then cooled rapidly to obtain the ferrite-martensite microstructure. After pre-straining, the specimens were baked and the corresponding bake-hardening (BH) values were determined as a function of pre-strain, baking temperature, and baking time. The influences of pre-strain, baking temperature and baking time on the microstructure evolution and bake-hardening behavior of the dual-phase steel were investigated systematically. It was found that the BH value apparently increased with an increase in pre-strain in the range from 0 to 1%; however, increasing pre-strain from 1% to 8% led to a decrease in the BH value. Furthermore, an increase in baking temperature favored a gradual improvement in the BH value because of the formation of Cottrell atmosphere and the precipitation of carbides in both the ferrite and martensite phases. The BH value reached a maximum of 110 MPa at a baking temperature of 300°C. Moreover, the BH value enhanced significantly with increasing baking time from 10 to 100 min.     

590MPa级热轧V-N高强车轮钢组织性能控制

对一种低成本V-N微合金化钢进行了控轧控冷实验,探讨了相变机理与析出行为,并系统地研究了其综合力学性能.结果表明,显微组织为多边形铁素体、粒状贝氏体及少量的针状铁素体,纳米尺度V(C,N)析出质点弥散地分布于铁素体或贝氏体铁素体基体内部.抗拉强度615MPa实验钢具有优良的断后延伸率,冷弯性能合格,扩孔率达到95%,满足轮辐用钢的加工要求,低温冲击性能良好.细晶强化、位错强化、析出强化、相变强化为主要强化机制.     

热处理工艺对60CrNiMo钢组织与力学性能的影响

研究采用多步低温等温贝氏体转变工艺处理后60CrNiMo钢组织与力学性能,用金相显微镜、扫描电镜及透射电镜观察60CrNiMo钢相组织,并进行硬度、拉伸和冲击等力学性能测试。结果表明,经淬火+亚温淬火+高温回火处理的60CrNiMo钢可得到细小均匀的二次回火马氏体+铁素体混合组织,其力学性能得到改善;采用三步低温等温贝氏体转变工艺可有效减少材料块状残余奥氏体和细化贝氏体晶粒,从而提高60CrNiMo钢力学性能。