锰对热轧态双相钢显微组织和力学性能的影响

用薄晶体透射电镜研究锰对热轧空冷后低碳Si-Mn双相钢的组织和力学性能的影响。实验结果表明:钢中锰含量为1.79%时,显微组织中出现珠光体。拉伸工程应力-应变曲线有明显物理屈服延伸。钢中锰含量越少、珠光体量越多时,应力-应变曲线上屈服平台越长。锰含量大于2.09%时,轧态组织中不再出现非马氏体型转变产物珠光体。轧态组织中的马氏体岛区,由几个微区组成。这些微区分别为内孪晶马氏体区和位错板条马氏体区。     

Correlation of Tensile Properties and Strain Hardening Behavior with Martensite Volume Fraction in Dual-Phase Steels

In the present study, tensile properties, strain hardening and fracture behavior of dual-phase (DP) steels were correlated with martensite volume fraction (V _M ). A series of DP steels with different amounts of V _M (28–50 %) were produced by cold rolling and subsequent intercritical annealing of a ferrite-pearlite starting structure. Hardness and tensile tests results of DP steels showed that variation of hardness, uniform elongation and total elongation with V _M was linear and obeyed the rule of mixtures, whereas yield strength and ultimate tensile strength exhibited a nonlinear variation with V _M . Analysis of strain hardening behavior of DP steels by the Hollomon analysis showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (n _I ) increased with increasing V _M , while the strain hardening exponent of second stage (n _II ) as well as transition strain between the deformation stages decreased.     

Influence of Martensite Distribution on the Mechanical Properties of Dual Phase Steels: Experiments and Simulation

The application of multiphase steels in the automotive industry has been rapidly increased according to economic, environmental and safety reasons. To determine an optimal combination of high strength and good formability of multiphase steels by using the FE modelling, their complex microstructures have to be considered. Two‐dimensional Representative Volume Elements (RVEs) were currently developed based on real microstructures for dual phase (DP) steels. In general, the microstructure of DP steels contains hard martensite particles and a soft ferritic matrix. The strain hardening behaviour of the individual phases was described in the model taking the microstructural constituents and the carbon partitioning during intercritical annealing into account. Two dual phase microstructures with same martensitecontent but different martensite distributions were investigated in experiment as well as in FEM simulation by means of the RVE. The resulting mechanical properties of these steels are strongly influenced by the phase distribution and interaction. As validation, calculated flow curves were compared with the experimental results from quasi‐static tensile tests. In addition, the local stress and strain partitioning between both phases depending on the spatial phase distribution and morphology is discussed.     

Tempering of Martensite in Dual-Phase Steels and Its Effects on Softening Behavior

The isothermal and nonisothermal tempering of martensite in dual-phase (DP) steels was investigated mainly by analytical transmission electron microscopy, and the effect on softening behavior was studied. The isothermal tempering resulted in coarsening and spheroidization of cementite and complete recovery of laths. However, nonisothermal tempering manifested fine quasi-spherical intralath and platelike interlath cementite, decomposition of retained austenite, and partial recovery of laths. The distinct characteristic of nonisothermal tempering was primarily attributed to the synergistic effect of delay in cementite precipitation and insufficient time for diffusion of carbon due to rapid heating that delays the third stage of tempering. The finer size and platelike morphology of cementite coupled with partial recovery of lath resulted in reduced softening in nonisothermal tempering compared to severe softening in isothermal tempering due to large spheroidized cementite and complete recovery of lath substructure. The substitutional content of precipitated cementite in nonisothermal tempering was correlated to the richness of particular steel chemistry. Softening resistance during nonisothermal tempering was related to DP steel chemistry, i.e., Cr and Mn content. Fine cementite and less decomposed martensite in rich chemistry confer high resistance to softening compared to leaner chemistries, which indicated severe decomposition of martensite with coarser cementite.     

Failure during Sheared Edge Stretching of Dual-Phase Steels

The use of dual-phase steels has been limited in a number of applications, due to failure during sheared edge stretching. Previous investigations have studied the properties of dual-phase steels, especially regarding the mechanical properties of the individual phases or constituents, the strain partitioning to the microconstituents during loading, and the decohesion at the interface during loading. On the basis of the literature review, a hypothesis is developed in which failure in sheared edge stretching is the result of a sequence of events. Cracking first develops in the hard constituent, cracks grow in the interface between the hard constituent and ferrite, and relative movement of ferrite relative to the hard constituent increases the rate of cracking. In the present study, a single steel was heat treated to produce different amounts of hard constituent within the ferrite matrix in order to better understand the behavior of dual-phase steels during sheared edge stretching. The results of the study are consistent with the proposed hypothesis. It was found that in contrast to other studies, increased strength of the hard constituent retards crack initiation. Crack growth increased with increasing surface area of hard constituent–ferrite interfaces and increasing movement of ferrite relative to the hard constituent.     

On the Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Ferrite/Martensite Dual-Phase Steels

Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material.     

Microstructure and Properties of 980MPa UltrafineGrained Cold Rolled Dual-Phase Steels

Two kinds of 980MPa grade cold rolled dual phase steels have been developed by designing C-Si-Mn and C-Si-Mn-Nb alloy systems.The microstructure of martensite in Nb-free steel is consisted of lath martensite and twined martensite with the volume fraction of 67%.However,the main hard phase in Nb-containing one is twined martensit with the volume percent of 59%.The size of martensite islands in Nb-containing steel is from 1μm to 3μm,and the size of NbC precipitates is from 15nm to 40nm.As to Nb-containing steel,the yield strength,tensile strength,yield ratio and elongation are 501MPa,1035MPa,0.48 and 17.5% respectively.Futhermore,Nb-containing steel has higher working hardening rate value above the critical strain 6.5%.And it decreases slowly with increasing the strain.This is mainly because of ultrafine grain size and nano-precipitates in ferrite,which improves the compatibility of phases and reduces the stress concentration at the phase interface.     

Influence of Thermal Aging on the Microstructure and Mechanical Behavior of Dual-Phase, Precipitation-Hardened, Powder Metallurgy Stainless Steels

The effects of thermal aging on the microstructure and mechanical behavior of dual-phase, precipitation-hardened, powder metallurgy (PM) stainless steels of varying ferrite–martensite content were examined. Quantitative analyses of the inherent porosity and phase fractions were conducted on the steels, and no significant differences were noted with respect to aging temperature. Tensile strength, yield strength, and elongation to fracture all increased with increasing aging temperature reaching maxima at 811 K (538 °C) in most cases. Increased strength and decreased ductility were observed in steels of higher martensite content. Nanoindentation of the individual microconstituents was employed to obtain a fundamental understanding of the strengthening contributions. Both the ferrite and martensite nanohardness values increased with aging temperature and exhibited similar maxima to the bulk tensile properties.     

MnS添加方式对Fe-Cu-C-MnS烧结钢力学性能影响

比较了混合法及预合金法添加MnS的Fe-Cu-C-MnS烧结钢中MnS分布特点和钢的力学性能的差异。结果发现:采用混合法添加MnS时,存在于铁颗粒间的MnS将影响铁颗粒间的烧结,降低了铁颗粒间连接强度,降低了烧结钢的力学性能;而采用预合金法添加MnS时,MnS均匀分布于烧结体内部,对铁颗粒间烧结影响很小,故不影响Fe-Cu-C-MnS烧结钢的力学性能。     

Influence of Carbide Modifications on the Mechanical Properties of Ultra-High-Strength Stainless Steels

The mechanical properties of ultra-high-strength secondary hardened stainless steels with varying Co, V, and C contents have been studied. A reduced-Co alloy based on the chemical composition of Ferrium S53 was made by increasing the V and C content. This changed the M₂C-strengthened microstructure to a MC plus M₂C-strengthened microstructure, and no deteriorative effects were observed for peak-aged and over-aged samples despite the large reduction in Co content from 14 to 7 wt pct. The mechanical properties according to alloying modification were associated with carbide precipitation kinetics, which was clearly outlined by combining analytical tools including small-angle neutron scattering (SANS) as well as an analytical TEM with computational simulation.     

1 500 MPa级贝氏体/马氏体复相高强度钢的疲劳特性

对于一种C-Si-Mn-Cr合金,通过900 ℃奥氏体化20 min、空冷及280 ℃和370 ℃回火2 h,获得了抗拉强度为1 500 MPa的新型贝氏体/马氏体(B/M)复相组织高强度钢.利用疲劳实验研究了B/M复相高强度钢的疲劳极限,并采用C-T试样测定了疲劳裂纹扩展速率曲线.结果表明,B/M复相高强度钢的疲劳极限超过700 MPa,疲劳裂纹门槛值约为12.7 MPa*m1/2;经370 ℃回火的B/M复相高强度钢疲劳裂纹扩展速率较280 ℃回火时低;这种钢的疲劳性能较常规调质处理的高强度钢有所改善.     

Influence of Texture on the Mechanical Properties of Hot Rolled Duplex Stainless Steels

This paper reports an investigation of bulk and micro texture in the as cast condition and in hot rolled conditions (50 and 75% thickness reduction) of nitrogen alloyed duplex stainless steel. Bulk texture measurements were carried out using orientation-imaging microscopy (OIM). The micro texture measurements were carried out using electron backscattered diffraction (EBSD) system for the dual phases; ferrite (alpha) and austenite (gamma). The amount of alpha fiber formation for each reduction is reported under the conditions of before heat treatment (BHT) and after heat treatment (AHT). Individual texture components for both 50 and 75% reduction under the condition of BHT and AHT are also reported. The results reveal that there is a definite improvement in the mechanical properties after reducing the specimen thickness to 75% by hot rolling, followed by heat treatment (solutionizing). This improvement in the mechanical property is correlated with the volume of low angle grain boundaries (EBSD analysis) and with the cleavage facets (SEM analysis).     

经济型铁素体/贝氏体高扩孔钢的组织与性能

 通过TMCP工艺实验,研究了Si、Mn含量对低碳Si Mn钢显微组织、力学及成形性能的影响,探讨了铁素体/贝氏体双相钢(FB钢)在扩孔过程中的裂纹形成及扩展行为。研究结果表明,增加Si含量,实验钢中等轴铁素体的体积分数增加,扩孔性能得到改善;而增加Mn含量,实验钢的强度和韧性显著提高,但塑性和扩孔性能有所下降。FB钢中的裂纹扩展主要是以微孔聚集机制进行,当遇到贝氏体时,裂纹通过铁素体 贝氏体相界面并剪断铁素体进行扩展。合理选择Si、Mn含量和TMCP工艺参数,可以获得690 MPa级的经济型热轧FB高扩孔钢,扩孔率达到了95%,综合性能较好。     

磷和晶粒尺寸对低碳钢力学性能的影响

研究了磷含量和晶粒尺寸对低碳钢的屈服强度、冲击韧性和韧脆转变温度的影响。结果表明,磷在铁素体中的固溶使含磷钢的强度明显增加;磷在铁素体晶界的偏聚提高了钢的韧脆转变温度。晶粒细化在提高强度的同时减轻了磷在晶界的偏聚程度,从而提高了钢的韧性并降低了钢的韧脆转变温度。因此可以通过晶粒细化来弥补钢中因磷的加入而造成的韧性损失。     

Tempering of Mn and Mn-Si-V Dual-Phase Steels

Changes in the yield behavior, strength, and ductility of a Mn and a Mn-Si-V dual-phase (ferrite-martensite) steel were investigated after tempering one hour at 200 to 600 °C. The change in yield behavior was complex in both steels with the yield strength first increasing and then decreasing as the tempering temperature was increased. This complex behavior is attributed to a combination of factors including carbon segregation to dislocations, a return of discontinuous yielding, and the relief of residual stresses. In contrast, the tensile strength decreased continuously as the tempering temperature was increased in a manner that could be predicted from the change in hardness of the martensite phase using a simple composite strengthening model. The initial tensile ductility (total elongation) of the Mn-Si-V steel was much greater than that of the Mn steel. However, upon tempering up to 400 °C, the ductility of the Mn-Si-V decreased whereas that of the Mn steel increased. As a result, both steels had similar ductilities after tempering at 400 °C or higher temperatures. These results are attributed to the larger amounts of retained austenite in the Mn-Si-V steel (9 pct) compared to the Mn steel (3 pct) and its contribution to tensile ductility by transforming to martensite during plastic straining. Upon tempering at 400 °C, the retained austenite decomposes to bainite and its contribution to tensile ductility is eliminated.     

Strain Hardening Behavior of Dual-Phase Steels

A detailed qualitative and quantitative examination of the microstructure and mechanical properties of three different classes of DP600 and DP450 dual-phase (DP) steels was carried out. The tested DP steels are characterized by different alloying elements: aluminum, boron, and phosphorus. Among them, aluminum DP steels showed the lowest percentages of hard phases, while phosphorus DP steels exhibited the highest resistance values. The Hollomon, Pickering, Crussard–Jaoul (CJ), and Bergstrom models were used to reproduce the strain hardening behavior of DP steels. Relationships that correlate the fitting parameters with the chemical composition and the thermal cycle parameters were found, and the predictive abilities of different models were evaluated. The Pickering equation, among the tested models, is the best one in the reproduction of the experimental stress-strain data.     

硅对低碳Si-Mn双相钢纤维组织的影响

研究了硅对低碳Si-Mn双相钢纤维状马氏体形成、变化的影响。结果表明:在过热度△T_Ⅱ(奥氏体化温度与A_C1的温度差值)相近的条件下,钢中硅含量提高有阻止马氏体纤维粗化的作用。原始组织为高温淬火态的钢,在(α+γ)两相区二次淬火组织中,纤维马氏体开始消失的温度随钢中硅含量增多而升高,钢中硅含量每增加一倍,该温度相应升高约40℃。     

Fe-Mn-Si-Al钢力学性能与显微结构研究

对3种成分Fe-Mn-Si-Al钢的力学性能、显微结构和重位晶界进行了研究,结果表明:随着钢中Mn、Si和Al元素含量的降低,材料的屈服强度和抗拉强度逐渐升高,伸长率逐渐降低,低∑值重位晶界出现频率依次为83.4%、36%和21%,并且钢中形变孪晶的数量逐渐减少。Fe-30Mn-3Si-3Al钢中形变孪晶分为一次孪晶和二次孪晶,一次孪晶和二次孪晶生长方向成60°夹角,大部分二次孪晶分布在一次孪晶的两孪晶界之间,但在高应力的形变带内二次孪晶可穿过一次孪晶的孪晶界。