Interaction of Hydrogen and Retained Austenite in Bainite/Martensite Dual-Phase High Strength Steel

The hydrogen trapping phenomena in two bainite/martensite dual-phase high strength steels(U20Si and U20DSi)were investigated by electrochemical permeation technique.The hydrogen diffusivity was calculated from data of permeation delay time,and the diffusion coefficient in U20 Si is far less than that in U20 DSi.Moreover,the hydrogen diffusivity decreases as the volume percent of retained austenite increases.The experiment results show that there are different hydrogen trappings and different volume percents of retained austenite in U20 Si and U20 DSi.The retained austenite is precipitated as films.The trap binding energy for the retained austenite and hydrogen is calculated to be 40.4kJ·mol-1.

Effect of Tempered Martensite on Magnetomechanical Damping of Ferritic Matrix Dual Phase Alloys

The influence of the volume fraction of tempered martensite on magnetomechanical damping of the ferritic matrix in dual-phase alloys was studied.The experimental results showed that(1) dual-phase damping alloy is of good combination of strength and toughness: (2)good damping ca- pacity(Q^(-1)≥5.0×10^(-3))can be obtained in dual-phase alloys only when the volume fraction of tempered martensite was controlled properly.The effect of martensite on magnetomechanical damp- ing of the ferritic matrix was analysed.The damping capacity of dual-phase alloys was related to the volume fraction V_m of tempered martensite by Q_^(-1)=Q_^(-1) (1-V_m/1+BV_m)at small vibration strain amplitudes ε_A≤5.0×10^(-6).

Stress-strain partitioning analysis of constituent phases in dual phase steel based on the modified law of mixture

A more accurate estimation of stress-strain relationships for martensite and ferrite was developed, and the modified law of mixture was used to investigate the stress-strain partitioning of constituent phases in dual phase （DP） steels with two different martensite volume fractions. The results show that there exist great differences in the stress-strain contribution of martensite and ferrite to DP steel. The stress-strain partitioning coefficient is not constant in the whole strain range, but decreases with increasing the true strain of DP steel. The softening effect caused by the dilution of carbon concentration in martensite with the increase of martensite volume fraction has great influence on the strain contribution of martensite. The strain ratio of ferrite to martensite almost linearly increases with increasing the true strain of DP steel when the martensite volume fraction is 22%, because martensite always keeps elastic. But the strain ratio of ferrite to martensite varies indistinctively with the further increase in true strain of DP steel above 0.034 when the martensite volume fraction is 50%, because plastic deformation happens in martensite. The stress ratio ofmartensite to ferrite decreases monotonously with increasing the true strain of DP steel whether the martensite volume fraction is 22% or 50%.

Microstructure-Properties Correlation of Dual Phase Steels Produced by Controlled Rolling Process

The purpose of this research is to quantify the effects of compositional and processing parameters on the microstruc-ture and properties of dual phase steel produced directly by hot rolling and rapid cooling. Steels with the base composition of 0.1%C, 1.4%Si, and 1.0%Mn with additions of 0.5%Cr to influence hardenability, 0.04%Nb to retard recrystallization in the latter stages of rolling, or 0.02%Ti to inhibit grain growth during and after reheating were investigated. Investigation was made to predict microstructure evolution and to correlate microstructure with processing parameters. The effects of the important microstructure parameters such as ferrite grain size, martensite volume fraction (VM) and morphology (polygonal or fibrous) on the tensile and impact properties are discussed. Multiple linear regression analysis of the ultimate tensile strength has shown that, increasing VM and martensite microhardness and grain refinement of ferrite are the major contributions to increase the strength of the steel. It was found that the dual-phase steel produced by controlled rolling process, with a microstructure which consisted of fine grained ferrite (4 um) and 35%~40% fibrous martensite, presented optimum tensile and impact properties because of enhanced resistance to crack propagation.

Corrosion behavior of tempered dual-phase steel embedded in concrete

Dual-phase （DP） steels with different martensite contents were obtained by appropriate heat treatment of an SAE1010 structural carbon steel, which was cheap and widely used in the construction industry. The corrosion behavior of DP steels in concrete was investigated under various tempering conditions. Intercritical annealing heat treatment was applied to the reinforcing steel to obtain DP steels with different contents of martensite. These DP steels were tempered at 200, 300, and 400℃ for 45 min and then cooled to room temperature. Corrosion experiments were conducted in two stages. In the first stage, the corrosion potential of DP steels embedded in concrete was measured every day for a period of 30 d based on the ASTM C 876 standard. In the second stage, the anodic and cathodic polarization values of these steels were obtained and subsequently the corrosion currents were determined with the aid of cathodic polarization curves. It was observed that the amount of second phase had a definite effect on the corrosion behavior of the DP steel embedded in concrete. As a result of this study, it is found that the corrosion rate of the DP steel increases with an increase in the amount of martensite.

Effect of Holding Time on the Microstructure and Mechanical Properties of Dual-Phase Steel during Intercritical Annealing

Continuous annealing simulation tests were conducted by using a continuous annealing thermomechanical simulator. Holding times of 5, 60, 180, and 480 seconds for an intercritical annealing temperature of 820℃ were adopted to investigate the evolution of the mierostructure and mechanical properties of ferrite-bainite dual-phase steel. The ferrite-bainite dual-phase steel was characterized by high strength and low yield ratio due to the presence of the constituents （polygonal ferrite, bainite, martensite and retained austenite） of the steel microstructure. Specimen 3 exhibits the highest value of A50 （7.67%） and a product of Rm × A50 （10453MPa%） after a 180s holding. This is likely attributed to the presence of a C-enriched retained anstenite in the microstructure. And the effect of martensite islands and carbide precipitate is thought to be able to contribute in strengthening the present steel. It is expected that equilibrium of anstenite fraction would be reached for reasonable intercritical holding period, regardless of the heating temperature. The results suggest that long increasing holding times may not be needed because the major phase of the microstructure does not change very significantly. It is favorable for industrial production of DP steels to shorten holding times. Key words： ferrite-bainite dual-phase steel; holding time; martensite islands; mechanical properties

Effect of Microstructure on Corrosion Fatigue Behavior of 1500 MPa Level Carbide-Free Bainite/Martensite Dual-Phase High Strength Steel

Influence of microstructure of the experimental steels on the corrosion fatigue behavior in 3.5% of NaCl aqueous solution was studied.Experimental results show that compared with the full martensite（FM）steel,the carbide-free bainite/martensite（CFB/M）steel has higher corrosion fatigue strength and corrosion fatigue crack threshold（ΔKthcf）,and lower corrosion crack propagation rate [（da/dN）cf].

Influence of Martensite Volume Fraction on Mechanical Properties of High-Mn Steel

Elastic-plastic deformation behavior of austenitic, martensitic, and austenitic-martensitic high-Mn steels is investigated by using crystal plasticity theory. The development of expandable pipes made of two-phase steel for oil and gas well applications is needed for improved and efficient recovery of hydrocarbons from difficult reservoirs. The current research is aimed at improving the down-hole post-expansion material properties of expandable pipes. A mathematical model is first developed based on finite-deformation crystal plasticity theory assuming that slip is the prime mode of plastic deformation. The developed model is then numerically implemented by using the finite element software ABAQUS, through a user defined subroutine. Finite element simulations are performed for austenitic, martensitic, and austenitic-martensitic steels having different proportions of martensite in an austenite matrix. Three primary modes of loading are considered: uniaxial tension, compression and simple shear. The variation in yield strength, hardening pattern and dissipated energy is observed and analyzed.

Role of Martensite Structural Characteristics on Corrosion Features in Ni-Advanced Dual-Phase Low-Alloy Steels

The relationship between the corrosion resistance and martensite structure of Ni-advanced dual-phase weathering steel was studied using transmission electron microscopy,scanning electron microscopy,electrochemical analysis,and atomic force microscopy.The investigations indicate that the final microstructure of the dual-phase weathering steel was composed of a large amount of low-carbon lath martensite distributed in the ferrite matrix.The potential of the martensite phase is higher than that of ferrite,which acts as a microcathode.As the martensite volume fraction in the Ni-advanced dual-phase weathering steel increased,the corrosion rate increased owing to the greater galvanic couple formed between the ferrite and martensite from the increasing ratio of the cathode area to the anode area.In addition,this work provides a method to obtain advanced weathering steel with improved mechanical properties and corrosion resistance.

Microstructure failure in ferrite-martensite dual phase steel under in-situ tensile test

To investigate microstructure failure in ferrite-martensite dual phase steel,in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests. Microstructure evolution of the observed region was investigated in details. The experimental data showed that micro-cracks in various regions differed in the initiation time,and micro-failures mainly occurred from the locations with typical characteristics of stress concentration（ i. e. ferrite interiors,the interfaces of ferrite-martensite grains and the martensite-martensite interfaces）. Growth of micro-crack generally experienced the following stages： cracking from martensite boundaries,tiny particles in ferrite interiors,or martensite interiors,propagating in ferrite,bypassing martensite boundaries,or passing through martensite-martensite interfaces,finally ending on martensite boundaries. Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly. Microstructure deformation was inhomogeneous in the stage of plastic deformation.

Bake-Hardening Response of High Martensite Dual-Phase Steel with Different Morphologies and Volume Fractions

Bake-hardening behaviour of carbon steel with different martensite morphologies and volume fraction was investigated. The specimens with fibrous and bulky martensite were prestrained in tension by 4%. After this, they were unloaded and bake hardened at 180 ℃ for 10-160 min. It was found that dual-phase steel samples which were bake hardened at 180 ℃ for 20 rain showed an increase in the yield stress （YS） and ultimate tensile stress （UTS） but a decrease in ductility. Further increase in the bake-hardening time of 80 or 160 min has reduced the YS and UTS, but increased the ductility. △σ （increase in stress due to bake hardening）, YS and UTS values are higher for the microstructure containing fibrous martensite compared to the microstructure-containing bulky martensite. It was also observed that at a given baking temperature Aa, YS and UTS increased by volume of martensite.

微合金化低碳马氏体钢的显微组织及力学性能

制备了钛、铌和钼等元素掺杂的微合金化低碳马氏体钢,并对其进行热轧+880℃淬火+不同温度(170,190,210,230,250℃)回火处理,研究了不同热处理态试验钢的显微组织、拉伸性能和冲击性能,以190℃回火态钢为例分析了强韧化机制。结果表明:热轧态试验钢由铁素体和贝氏体组成,淬火态试验钢由铁素体和马氏体组成,不同温度回火态试验钢均由回火马氏体和少量铁素体组成,回火态试验钢的铁素体面积分数在15%~20%,低于热轧态和淬火态;淬火态试验钢的抗拉强度和布氏硬度最高;随着回火温度升高,回火态试验钢的抗拉强度和布氏硬度下降,断后伸长率、屈服强度和冲击吸收功均先增大后减小;190℃回火态试验钢的断后伸长率和冲击吸收功最高,拉伸断口和冲击断口处纤维区中韧窝占比最高,抗拉强度、布氏硬度和屈服强度适中,综合力学性能最好;190℃回火态试验钢中析出的纳米级小尺寸(Ti,Nb,Mo,Cr)C相起到沉淀强化作用,纳米级大尺寸(Ti,Nb,Mo,Cr)C相起细晶强化作用,大角度晶界占比较高,提高了裂纹扩展功,因此试验钢强度和韧性提高。

等温温度对Ti-Mo微合金高强双相钢相变及组织性能的影响

采用热模拟实验、光学显微镜、显微硬度仪和透射电镜等研究了等温温度对Ti-Mo微合金高强双相钢相变、组织及性能的影响。结果表明:当等温温度在750~725℃时,实验钢组织主要为马氏体+少量贝氏体或铁素体,随着等温温度降低(≤700℃),铁素体含量增多,排碳行为增加了过冷奥氏体的稳定性,抑制了中低温区贝氏体相变,在随后的冷却阶段,过冷奥氏体更倾向转变为马氏体。铁素体与马氏体的显微硬度均随等温温度降低而增加,但进一步降低温度,马氏体硬度基本保持稳定(361 HV0.1);650℃时铁素体显微硬度最高(242 HV0.1),这主要是由于铁素体内部产生了大量细小的相间析出,同时析出强化作用减小了铁素体与马氏体的硬度差异,降低了双相钢强度对马氏体体积分数的依赖性,等温温度较高时,相间析出形成被抑制。

Effect of post rolling stress on phase transformation behavior of microalloyed dual phase steel

The slow phase transformation of microalloyed dual phase steel makes the nonuniform stress and temperature fields during the post rolling cooling process have a significant impact on the phase transformation process.Given the relatively slow phase transformation of DP780 steel within the microalloyed dual phase steel series,the influence of stress on the phase transformation behavior of DP780 steel was investigated.To quantify the nonuniform thermal and stress conditions in the steel coil,a thermo-mechanical coupled finite element model of the hot-rolled strip cooling process was established.Based on the simulation data,DP780 steel was chosen as the research material,and Gleeble 3500 thermal simulation equipment was used for experimental validation.The thermal expansion curves were analyzed through regression to establish the dynamic model of DP780 steel phase transformation under stress.Subsequently,metallographic analysis was conducted to determine phase transformation type and grain size of DP780 steel.The results confirmed that the stress promotes the occurrence of semi-diffusion-type bainite transformation.Furthermore,an appropriate level of stress facilitates the growth of bainitic grains,while the increased stress inhibits the growth of ferritic grains.

两相区热处理对DP780双相钢组织与力学性能的影响

采用OM、拉伸试验机研究了不同温度、时间的两相区热处理对DP780双相钢组织及力学性能的影响。结果表明:冷轧态DP780双相钢经过870℃×60 min奥氏体化,再进行780℃不同时间的两相区保温热处理后,试验钢组织为马氏体和铁素体双相组织,随着保温时间的增加,试验钢中马氏体含量逐渐减少。经过780℃×5 min两相区热处理的试验钢,其伸长率达到最大值18%,抗拉强度接近1100 MPa,试验钢的强塑性能匹配最佳。

新型1500MPa级高强钢的氢脆敏感性研究

根据钢材组织设计的思想 ,通过优化成分和工艺设计、研制出一种新的 15 0 0MPa级高强钢。采用阴极电解充氢的方法对其氢脆敏感性进行了研究 ,并与同一强度级别的 4 2CrMo高强钢进行了对比。结果表明 ,所设计的 15 0 0MPa级高强钢的氢脆敏感性低于传统的 4 2CrMo高强钢。SEM断口观察显示 ,两者的断口形貌也不同 ,15 0 0MPa级高强钢为准解理断裂 ,而 4 2CrMo高强钢为沿晶断裂。断口金相表明 ,前者的裂纹主要沿着贝氏体 /马氏体 (B/M)边界扩展 ,断裂模式为板条界分离 ,后者的裂纹沿着晶界扩展。对 15 0 0MPa级高强钢进行了TEM观察 ,发现其组织为贝氏体 /马氏体复相组织 ,残留奥氏体以薄膜状存在于贝氏体内部及贝氏体条片。

无碳化物贝氏体/马氏体复相钢的新进展

所研制的具有稳定残留奥氏体薄膜的无碳化物贝氏体 /马氏体复相钢 ,出现第一类回火脆性的温度范围提高到 370℃以上。由此可以通过提高回火温度改善钢的韧性 ,降低钢的氢脆敏感性。经 36 0℃回火后 ,所研钢的σb 为 15 80MPa,AKU为 81J ,该钢的KISCC值超过 5 0MPa·m1/ 2 。

组织对含铜钢中析出行为的影响

为研究含铜钢在连续冷却过程中的析出行为,采用不同冷却速度及中间淬火温度获得不同的金相组织.利用金相显微镜和透射电镜研究不同组织对析出的影响.实验结果表明:当冷却后组织为铁素体时,随着铁素体的产生,在铁素体内产生第二相析出颗粒,当冷却产物为贝氏体及马氏体时,并未观察到第二相析出.当相变后产物为多种组织时,仅在铁素体中发现第二相析出颗粒.第二相颗粒为富铜相,以相间沉淀方式析出.

新颖的贝氏体/铁素体双相低碳微合金钢

利用特殊微合金设计及终轧控冷工艺得到超细贝氏体/铁素体双相低碳微合金钢.该钢的组织由原奥氏体晶界上及晶粒内部的约5μm的准多边形铁索体及超细化的贝氏体板条束组成.铁素体的体积分数约20％.该双相低碳微合金钢的强度比同成分的全贝氏体钢略低,但其延伸率却大幅度提高.采取适当的回火处理,该双相钢屈服强度可达到700MPa,而延伸率大于25％,是一种具有高强度、高塑性的新型低碳微合金双相钢.

微纳结构超高强度钢的现状与发展

微纳结构超高强度钢是兼具高强度、高塑性和高韧性的一类重要高性能钢铁结构材料。超细马氏体、贝氏体组织对于其强度起了决定性作用,对塑性和韧性起决定作用的为残留奥氏体。因此,如何通过调整化学成分和热处理工艺等手段调控残留奥氏体微结构,来实现精细组织结构和优良综合力学性能,成为新一代钢铁材料研究领域重要的课题。首先从钢铁材料强韧化机制出发,指出微观组织的精细化及硬相与软相的合理调控,是发展新一代钢铁材料的重要途径。然后针对近年来发展起来的超级贝氏体钢、Q&P钢、Q-P-T钢等具有微纳组织结构的超高强度钢,从合金设计、热处理工艺及组织结构与力学性能、磨损性能和疲劳性能的关系等方面进行综述与分析,最后指出微纳结构超高强度钢有待进一步深入研究与开发的问题。