Effect of Si,C and Mn on the formation of bainite in ferrite-bainite steel

High-strength steels have been widely applied to automotive chassis parts.In order to form complex shapes,high hole expansion rates and high formability are required.Dual phase (DP) steel has a good formability,but a poor hole expansion rate.In this circumstance,another kind of steel which has a microstructure of ferrite-bainite,rather than ferrite-martensite,has been found to be an alternative solution.It is called FB steel.This steel with Si,C and Mn additions are applied in this study.A two-step cooling process is used to get the desired F+ B microstructures.Continuous cooling transformation (CCT) diagrams are made with deformation and without deformation,and starting times and temperatures of the phase transformations of interest are obtained.It is shown that Si,C and Mn contents in the steel strongly affect the shapes and positions of the CCT diagrams,as well as the final microstructures of FB steel.An increase of the Si content can promote the formation of ferrite and move the CCT diagram toward the left.However,when Si content is too high,when comparing to carbon and manganese contents,the formation of bainite will be retarded because of the formation of more ferrite.It increases the amount of C in a solid solution in the untransformed austenite and promotes the formation of pearlite.C and Mn can inhibit the formation of ferrite and retard the accumulation of C in austenite.Therefore,the appropriate balance of C,Si and Mn contents in steels will be able to help in obtaining the desired microstructure.     

Optimization of Process Conditions for Ultralightweight Steel with More Than 13 wt% of Al and 5 wt% of Cr

An ultralightweight Fe–30Mn–13.2Al–1.6C–5Cr steel, which contains more than 13 wt% of Al and thereby reduces the density by 20%, is developed. The ultralightweight steel, which is very brittle due to high Al content, is fabricated by optimizing hot rolling and heat treatment conditions. Hot rolling is conducted after soaking at the temperature range of 1100–1200 °C for 2 h. The ultralightweight steel is hot-rolled successfully after soaking at 1100 °C, whereas specimens soaked at 1150 and 1200 °C are intergranularly cracked after hot rolling, resulting from coarse grain and a large fraction and size of ferrite, which is transformed to ordered DO₃ phase during cooling, at the grain boundaries. In homogenization heat treatment, water quenching and air cooling are performed, respectively, after holding at 1050 °C for 2 h. The air-cooled steel has inferior tensile property due to the formation of brittle ordered DO₃ phase at grain boundaries. Meanwhile, the water-quenched steel shows an excellent tensile property, which is attributed to a uniform microstructure comprising austenite, fine κ-carbide in austenite, and a very small fraction of the ordered DO₃ phase.     

新型TRIP钢热处理工艺初探

新型TRIP复相钢仅含C、Si、Mn等合金元素,采用临界区等温淬火热处理工艺,获得铁素体、贝氏体和残余奥氏体三相组织。该钢在Ms-Md温度之间菜变,应变诱导相变,相变诱发塑性（TRIP）,其力学性能指标特别是伸长率大幅度提高。     

405不锈钢高温组织转变研究

通过热膨胀法以及Thermo-Calc热力学计算软件对SA240-405不锈钢铁素体向奥氏体转变的温度进行了测量和计算。进一步结合淬火与回火热处理,分析了405不锈钢在高温下组织随温度与时间的变化关系。研究结果表明,405不锈钢铁素体向奥氏体开始转变的温度为795~832℃,转变终了温度为910~925℃。温度高于1 050℃,随温度升高,奥氏体逐渐向铁素体转变,淬火后的马氏体含量降低。在950及980℃淬火,得到的组织为马氏体与铁素体的双相组织,淬火时间为30~60 min得到的硬度较高;进一步延长淬火时间,硬度逐渐降低。在730℃回火后得到的组织为铁素体与回火马氏体,无明显残余奥氏体,回火后组织的硬度随时间延长逐渐降低。     

固溶处理对超级双相不锈钢S32750组织和性能的影响

研究了950～1 200℃60 min水冷的固溶处理对超级双相不锈钢S32750(/%:0.02C、0.49Si、1.03Mn、0.026S、0.001P、25.01 Cr、7.03Ni、3.80Mo、0.29N)12 mm板的组织、力学性能和耐蚀性的影响。结果表明,随固溶温度升高,钢中铁素体相增加,奥氏体相减少;在950℃加热时铁素体中析出大量σ-相,使钢的性能恶化,在1 050～1 100℃固溶处理后,钢中铁素体相和奥氏体相各占50%, S32750钢具有较好的综合力学性能和优良的耐蚀性能。     

C-Si-Mn系双相钢锰配分行为及其对组织性能的影响

采用场发扫描电镜和EPMA电子探针研究了C-Si-Mn系双相钢锰配分行为及其对马氏体形貌、体积分数和力学性能的影响。结果表明:试验用钢0.20C-1.28Mn-0.37Si在经双相区热处理后,在720℃的淬火温度下,随着保温时间的延长,锰元素由铁素体向奥氏体中的配分量不断增加,达到化学势平衡(即饱和状态)所需的保温时间为1500s;马氏体体积分数随着保温时间的延长而增加,达到饱和的时间是1300s;抗拉强度随着保温时间的延长而升高,当达到某一时间时,抗拉强度保持恒定。     

汽车大梁用钢WL510连续冷却过程的相变和组织

通过Thermecmastor-Z热模拟试验机研究了WL510钢(/%:0.090C、0.13Si、1.45Mn、0.005S、0.019P、0.040Al、0.020Ti、0.030Nb)粗轧后板坯(36 mm×1 500 mm)在1～36℃/s连续冷却条件下的相变和组织的变化,并用热膨胀法测定了试验钢连续冷却转变(CCT)曲线。结果表明,试验钢WL510在1～23℃/s低冷却速度下,主要形成多边形铁素体和少量珠光体;当冷却速度≥30℃/s时,主要组织为细针状铁素体、少量细珠光体和岛状马氏体/奥氏体(M/A)随着冷却速度的增加,试验钢组织明显变细。     

The effect of intercritical annealing temperature on the structure of niobium microalloyed dualphase steel

The structures produced in a Nb-microalloyed steel by oil quenching after intercritical anneals at 760 and 810 °C have been examined by light and transmission electron microscopy. After both anneals, the periphery of the austenite pool transforms on cooling to ferrite in the same orientation as the ferrite retained during intercritical annealing. Thus the ferrite forms by an epitaxial growth mechanism without the formation of a new interface or grain boundary. The new ferrite is precipitate-free in contrast to the retained ferrite which develops a very dense precipitate dispersion during intercritical annealing. In the carbonenriched interior of the austenite pool beyond the epitaxial ferrite only martensite forms in specimens annealed at 760 °C but various mixtures of ferrite and cementite form in specimens annealed at 810 °C. The latter structures include lamellar pearlite, a degenerate pearlite, and cementite interphase precipitation. All Nb is in solution in the austenite formed at 810 °C, and therefore the low hardenability of the specimens annealed at that temperature is best explained by the effect of low austenite carbon content.     

Banded Microstructure Formation and Its Effect on the Performance of 06Ni9 Steel

Focusing on the banded microstructure formed during the production of 06Ni9 steels for cryo-LNG,this paper examines its formation,distribution of alloying elements,structure,hardness,and low-temperature property.The results show that the banded microstructure formed after hot-rolling and cooling of the steel binct in which the element segregation occurred during solidification.The phase change during heat treatment also can cause the formation of the banded microstructure of 06Ni9 steel.The white bands are mainly composed of ferrite and reversed austenite,and the black bands are mainly composed of reversed austenite and a certain amount of ferrite.Element segregation and formation of more carbide caused some black regions to appear.Grain refinement of 06Ni9steel is beneficial to the formation of reversed austenite,the redistribution of alloying elements,improving the stability of austenite and the low-temperature toughness of steel.This steel easily undergoes nickel segregation;thus,undergoing a secondary quenching and tempering process is recommended.The refinement of martensite quenching above A c3,the martensite that is rich in nickel and carbon,residual austenite and a few little of ferrite after secondary quenching lower than A c3 are beneficial to the formation of high stability austenite.Thus,this can meet the strength and toughness requirement of the low temperature 06Ni9 steel.     

Towards an austenite decomposition model for TRIP steels

The current status of developing a fundamental model for describing the overall austenite decomposition kinetics to ferrite and carbide‐free bainite in low carbon TRIP steels alloyed with Mn and Si is reviewed. For ferrite growth, a model is proposed where both interface and carbon diffusion‐controlled ferrite formation are considered in a mixed‐mode approach. The kinetic model is coupled with Thermocalc to obtain necessary thermodynamic information. Spherical geometry with an outer ferrite shell is assumed to capture in a simple way the topological conditions for growth. The mixed‐mode modelling philosophy has been identified to permit a rigorous incorporation of the solute drag effect of substitutional alloying elements, in particular Mn. The Purdy‐Brechet solute drag theory is adopted to characterize the interaction of Mn with the moving austenite‐ferrite interface. The challenges of quantifying the required solute drag parameters are discussed with an emphasis on a potential solute drag interaction of Mn and Si. The model is extended to non‐isothermal processing paths to account for continuous and stepped cooling occurring on the run‐out table of a hot strip mill or on a continuous annealing line. The transformation start temperature during cooling is predicted with a model combining nucleation and early growth which had previously been validated for conventional low carbon steels. The overall model is evaluated by comparing the predictions with experimental data for the ferrite growth kinetics during continuous cooling of a classical TRIP steel with mass contents of 0.19 % C, 1.49 % Mn and 1.95 % Si. Extension of the model to include bainite formation remains a challenge. Both diffusional and displacive model approaches are discussed for the formation of carbide‐free bainite. It is suggested to develop a combined nucleation and growth model which would enable to capture a potential transition from a diffusional to a displacive transformation mode with decreasing temperature.     

退火后冷却方式对冷轧中锰钢微观组织和力学性能的影响

采用拉伸试验、扫描电镜、电子背散射衍射、透射电镜、X射线衍射等手段,研究了冷轧中锰钢(0.2C-5Mn)退火后不同冷却方式下的微观组织特点和拉伸性能.实验钢冷轧退火后为铁素体加逆转变奥氏体的双相组织.退火后空冷可以获得稳定性较高的逆转变奥氏体,且其体积分数也明显高于退火后炉冷.退火后空冷实验钢中的逆转变奥氏体在变形过程中产生持续的TRIP效应,提高强度的同时获得了较高的塑性,强塑积可达到26.5 GPa·%。      

Microstructural evolution of 405 stainless steel under high temperature

The ferrite-to-austenite phase transformation temperature of SA240-405 stainless steel was measured using the thermodilatometry method and calculated using Thermo-Calc. In addition,the effect of temperature and the soaking time on the microstructural evolution was investigated for various quenching and tempering treatments. The results indicated that the ferrite-to-austenite transformation of this steel started between 795 ℃and 832 ℃ and finished between 910 ℃ and 925 ℃. W hen the specimens were annealed above 1050 ℃,the austenite gradually transformed into ferrite; consequently,the content of as-quenched martensite decreased with increasing temperature. M oreover,when the specimens were quenched between 950 ℃ and 980 ℃,a microstructure of duplex phases comprising ferrite and martensite was obtained. Relatively high B-scale of Rockwell hardness( HRB) values were observed for quenching times of 30-60 minutes; then,the hardness gradually decreased with increasing quenching time. Tempering at 730 ℃ resulted in ferrite and tempered martensite,and no obvious residual austenite was observed. In addition,the hardness gradually decreased with increasing tempering time.     

Effect of Intercritical Annealing Parameters on Dual Phase Behavior of Commercial Low-Alloyed Steels

 It is known that dual phase (DP) heat treatments and alloying elements have a strong effect on martensitic transformations and mechanical properties. In the present work, the effects of some intercritical annealing parameters (heating rate, soaking temperature, soaking time, and quench media) on the microstructure and mechanical properties of cold rolled DP steel were studied. The microstructure of specimens quenched after each annealing stage, was analyzed using optical microscopy. The tensile properties, determined for specimens submitted to complete annealing cycles, are influenced by the volume fractions of multi phases (originated from martensite, bainite and retained austenite), which depend on annealing processing parameters. The results obtained showed that the yield strength (YS) and the ultimate tensile strength (UTS) increase with the increasing intercritical temperature and cooling rate. This can be explained by higher martensite volume ratio with the increased volume fraction of austenite formed at the higher temperatures and cooling rates. The experimental data also showed that, for the annealing cycles carried out, higher UTS values than ~ 800 MPa could be obtained with the S3 steel grade.     

304奥氏体不锈钢亚快速凝固组织演化和形成机理

通过感应炉熔化的304钢(/%:0.053C、0.55Si、1.50Mn、0.030P、0.002S、17.02Cr、8.01 Ni、0.50Cu、0.08Mo)直接浇铸在水冷铜模上得到厚7 mm直径25 mm的圆形试样,研究了Cr、当量/Ni当量和1.5～1 000℃/s的冷却速率对奥氏体不锈钢铸态凝固组织形态和分布的影响。结果表明,随冷却速率增加至75～90℃/s,该钢的凝固模式由FA(铁素体-奥氏体)模式向AF(奥氏体-铁素体)模式转变,初生相由枝晶铁素体转变成枝晶奥氏体,但冷却为～1 000℃/s时,观察到块状铁素体组织,并且枝晶状奥氏体转变成胞状奥氏体。     

Selective Oxidation and Sub‐Surface Phase Transformation of TWIP Steel during Continuous Annealing

The selective oxidation of Twinning Induced Plasticity (TWIP) steel during annealing at 800 °C in a N₂ + 10%H₂ gas atmosphere with a dew point of ?17 °C and ?3 °C was investigated by means of high resolution transmission electron microscopy of cross‐sectional samples. The annealing resulted in the selective oxidation of Mn and Si and the austenite‐to‐ferrite phase transformation of the sub‐surface region. In the low dew point atmosphere, the annealing resulted in the formation of a MnO layer at the surface. Crystalline c –xMnO · SiO₂ (x ≥ 2) particles and amorphous a –xMnO · SiO₂ (x < 0.9) particles were found at the interface between the MnO layer and the steel matrix. In a narrow zone of the sub‐surface, the Mn depletion resulted in the transformation of the initial austenite. In the high dew point atmosphere, a thicker MnO layer was formed on the surface and no mixed manganese‐silicon oxides particles were observed at the MnO/steel matrix interface. In the sub‐surface, Mn was significantly depleted in the range of 2–3 µm below the surface and the initial austenite in this zone was transformed to ferrite. MnO particles were found at the grain boundaries and in the interior of grains.     

新型S690低合金高强度钢Φ219 mm x 14mm无缝管的CCT曲线和调质工艺研究

测定了S690钢管（/%:0.15C,0.25Si,1.32Mn,0.012P,0.005S,0.20Cr,0.04Al,0.08V）0.01～30℃/s冷却速率下连续冷却转变（CCT）曲线,并研究了890～970℃淬火,600～690℃回火对其组织和力学性能的影响。结果表明,S690钢管相变点Ac₃为828℃,Ac₁为753℃,Ms为395℃,临界冷却速率为13℃/s,存在铁素体、珠光体、贝氏体,马氏体4个相变区;选择910～930℃淬火,钢管的组织较细小均匀,平均晶粒尺寸13.9μm,具有较高的硬度,HRC值42.5;在615～675℃回火,钢管可得到均匀的回火索氏体组织,其综合力学性能优良。     

奥氏体形变对50CrV4钢相变行为的影响

利用Thermecmastor-Z型热模拟试验机,结合金相显微镜(OM)、扫描电镜(SEM)、维氏硬度计等,系统研究了奥氏体区变形对50CrV4钢连续冷却相变和等温相变规律的影响。建立了试验钢动态CCT曲线。研究结果表明,奥氏体变形能促进连续冷却转变过程中铁素体-珠光体、贝氏体转变,但亦可提高奥氏体的机械稳定性,进而抑制马氏体转变,Ms点由331.6℃(奥氏体未变形)降低至291℃(950℃下变形50%+890℃下变形50%,变形速率均为5s-1,变形后冷速为20℃/s)。当轧后冷速小于0.5℃/s时,试验钢中可获得铁素体+珠光体组织。此外,在研究不同变形量对试验钢等温相变规律影响时发现,650℃等温时,试验钢中发生铁素体-珠光体相变。随着变形量的增加(由30%增加至50%),其等温相变动力学加快(相变完成时间由197.6s减小至136.5s),铁素体体晶粒尺寸、珠光体片层间距减小,硬度增加。     

车削非调质钢SG4201奥氏体晶粒的长大行为

采用Gleeble 3500热模拟试验机试验研究了直接车削用非调质钢SG4201（/%:0.42C,0.50Si,1.40Mn,0.009P,0.005S,0.02Nb,0.06V,0.015N）在1000~1250℃加热0~300 s的奥氏体晶粒长大行为,并建立了该钢奥氏体晶粒长大模型。试验结果表明,加热时间30 s时,奥氏体晶粒粗化温度和铌迅速大量固溶的温度为1100℃左右;奥氏体晶粒长大激活能约为110.8 kJ;确立SG4201钢铸坯均热不宜超过1150℃。工业生产结果表明,当铸坯均热温度≤1150℃,终轧温度800~850℃,轧后冷却速度30~35℃/s时,SG4201钢的力学性能为抗拉强度927 MPa,屈服强度687 MPa,延伸率23.5%,断面收缩率57%,U-冲击功48 J,HBW硬度值265。     

微合金化控轧控冷钢筋纵向金相组织研究

 对微合金化控轧控冷钢筋的纵向金相组织进行了研究,并分析了不同成分试验钢纵向“条带”组织的差异及形成原因。研究结果表明：偏析元素（P、Si、Mn等）在轧制过程中沿轧制方向呈条状分布,是20MnSi、20MnSiV钢产生带状组织的原因。铌及其碳氮化物的溶质拖曳和“钉扎”作用,使20MnSiNb钢的奥氏体未再结晶轧制温度提高到1050℃,在冷却过程中,先共析铁素体在形变奥氏体晶界和内部变形带均匀析出,随后沿形变奥氏体晶界（在先共析铁素体与奥氏体的界面上）生成珠光体带,最后在形变奥氏体晶粒内部形成贝氏体条。研究条件下优势形核点的排序为：形变奥氏体晶界和形变奥氏体晶内变形带、偏析元素和夹杂、再结晶奥氏体晶界。     

含铌冷轧DP780钢的连退工艺对组织及性能的影响

 通过成分工艺优化,在传统冷轧铁素体和马氏体双相钢DP780的显微组织上引入了一定体积分数的残余奥氏体,研究了冷轧退火工艺参数对双相钢DP780的显微组织和力学性能的影响。通过调整连续退火工艺来控制显微组织中一次铁素体、二次铁素体、马氏体、残余奥氏体的比例、尺寸、形貌、分布,同时获得了连退工艺参数-显微组织-力学性能的本质关系。结果表明,通过在传统冷轧铁素体和马氏体双相钢的组织上引入了体积分数为5%～7%的残余奥氏体,不仅可以获得[ReL/Rm≤0.5]的超低屈强比型冷轧DP780,也改善了成型性能。