Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

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.

Complex Precipitation Mechanism of Ti-Nb-V Microalloyed Bainitic Base High Strength Steel

The addition of high Ti（>0.1%） in microalloyed bainitic high strength steel was designed, and the precipitation morphology of steels with different Ti, Nb, and V contents was studied by utilizing transmission electron microscopy（TEM）. Based on the classical nucleation-crystal growth theory and the Johnson-Mehl-Avrami equation, the precipitation thermodynamic and kinetic model of second phase particles in austenite was established in the form of（Nbx,Vy,Tiz）C, and the complex precipitation mechanism of second phase particles was emphatically studied. The experimental results show that the complex precipitation particles could be divided into two categories: the coarser particles with about 100 nm grain size and the independent complex precipitation particles in the form of（Nb,V,Ti）C with 35-50 nm grain size. The latter has a better precipitation strengthening effect, and the calculated PTT curve shows a typical "C" shape. When the deformed storage energy is 3 820 J?mol-1, the fastest precipitation temperature of calculated PTT curve is 925 °C, and the calculated result is essentially consistent with experimental values. The increase of Ti content increased the nose point temperature and expanded the range of fastest precipitation temperature.

Microstructural Transformation and Precipitation of an Ultra-high Strength Steel under Continuous Cooling

We investigated phase transition and precipitation of ultra-high strength steel（UHSS）in a new ＂short process＂ with controlled rolling and controlled cooling.Thermalexpansion test combined with metallographic observation was used to research the continuous cooling transformation（CCT）curve.Moreover,the microstructuraltransformation and precipitation law was revealed by morphologicalobservation and alloying elements by electron probe micro-analyzer（EPMA）.Transmission electron microscopy（TEM）was utilized to analyze the composition and grain orientation of microstructure.The study showed that the measured criticaltransformation temperatures of Ac1 and Ac3 were 746 and 868 ℃,respectively.The CCT curve indicated that the undercooled austenite was transformed into proeutectoid ferrite and bainite with HV 520 in a broad range of cooling rate 0.1^（-1） ℃·s^（-1）.When subjected to a cooling rate of 1 ℃·s^（-1）,the undercooled austenite was divided into small-sized blocks by formed martensite.With further increase of cooling rate,micro-hardness increased dramatically,the microstructure of specimen was mainly lathe bainite（LB）,granular bainite（GB）,lath martensite（LM）and residualaustenite.By diffraction test analysis,it was identified that there was K-S orientation relationship between martensite and austenite for {110}_α//{111}_γ,{111}_α//{101}_γ.EPMA clearly showed that carbon diffused adequately due to staying for a long time at high temperature with a lower cooling rate of 2 ℃·s-1.Phase transition drive force was lower and the residualaustenite existed in the block form of Martensite austenite island（M-A）.With the increase of cooling rate to 10 ℃·s^（-1）,the block residualaustenite reduced,the carbon content of residualaustenite increased and α phase around the residualaustenite formed into a low carbon bainite form.

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].

Development of grain boundary allotriomorphic ferrite/granular bainite duplex steel

A new hot-rolled low alloy high strength steel with grain boundaryallotriomorphic ferrite/granular bainite duplex microstructure has been developed through novelmicrostructure and alloying designs without any noble metal elements such as nickel and molybdenum.Its as-rolled microstructure and mechanical properties, fatigue crack propagation behavior comparedwith single granular bainitic steel as well as continuous cooling transformation, were investigatedin detail. The measured result of CCT (continuous cooling transformation) curve shows that suchduplex microstructure can be easily obtained within a wide air-cooling rate range. More importantly,this duplex microstructure has much better combination of toughness and strength than the singlegranular bainite microstructure. It is found that the grain boundary allotriomorphic ferrite in thisduplex microstructure can blunt the microcrack tip, cause fatigue crack propagation route branchingand curving, and thus it increases the resistance to fatigue crack propagation, improves steeltoughness. The mechanical properties of the above commercial duplex steel plates have achieved orexceeded 870 MPa ultimate tensile strength, 570 MPa yield strength, 18 percent elongation and 34 JCharpy V-notch impact energy at -40 deg C, showing good development potential.

FATIGUE PROPERTIES OF META-BAINITE IN STEEL 40CrMnSiMoVA

The microstructure,strength,toughness and fatigue properties of an ultra-high strength steel 40CrMnSiMoVA have been investigated.The so-called meta-bainite,composed of thin re- tained austenite films within or between the bainitic ferrite lathes was found in the steel after isothermally quenched at 300℃ for 1h.In comparison with the martensite structure obtained by isothermally quenching in martensite range,the meta-bainite has more excellent strength and plasticity,lower notch sensitivity,stronger strain harden ability,higher fatigue strength, longer strain or impact fatigue life,slower crack propagation rate and more remarkable overload effect on increasing fatigue life.

New bainite kinetics of high strength low alloy steel in fast cooling process

Based on Kolmgorov-Johnson-Mehl-Avrami analysis, a new bainite kinetics of high strength low alloy steel in fast cooling process was developed by utilizing different experimental methods. Upper bainite transformation morphological evolutions at a cooling rate of 8.3 K/s were directly observed by laser scanning confocal microscopy. This qualitative analysis suggests that bainite packet is more suitable to give a one-dimensional growth model if it is considered as a transformation unit. The nucleation rate of bainite packets in fast cooling process is assumed to give an a priori item. One-dimensional growth model with constant growth rate which is assumed as a function of cooling rate is adopted as well. Thus, the devel- oped new bainite kinetics is simple in expression and contains an adjustable parameter and an empirical pa rameter. Experimental results show upper bainite and lower bainite transformations in fast cooling processes. Their referential phase volume fractions are calculated by the expanded lever rule on the first derivative dilatometer curves. For the similar transformation mechanisms, upper bainite and lower bainite are considered to give the same kinetics. With considering the Nakamura＇s equation, the bainite kinetics is fitted with experimental data. Results show that bainite volume fractions and bainite transformation rates can be expressed precisely bY the newly developed bainite kinetics.

Effects of Rolling Processes on Bainite and Mechanical Properties of Ultra-High Strength Pipeline Steel

Ultra-high strength pipeline steels were rolled by thermal mechanical controlling process (TMCP),and effects of the volume fraction,the size and microstructure morphologies of three different bainite (AF,GB and LB) on mechanical properties were investigated by optical microscope.The results showed that,X120 ultra-high strength pipeline steel was rolled through the reasonable thermal mechanical controlling process (TMCP),and the yield strength and the low temperature charpy impact energy (-30℃) were higher than 840MPa and above 230J,respectively,meanwhile,the ratios of tensile strength to yield strength were lower below 0.82.As for X100 pipeline,of which the mechanical property were higher than that by X100 pipeline steel of ISO standard,yield strength of the steel was 715 MPa,ultimate tensile strength 963 MPa,impact energy 282J,and yield ratio 0.74.

Effect of Thermomechanical Treatment Temperature on Structure and Properties of CFB/M Ultra-High Strength Steel

Modified CCT diagram of carbide-flee bainite-martensite （CFB/M） ultra-high strength steel was established by applying controlled cooling of small samples. In addition, the influence of thermomechanical treatment tem- perature on the structure and properties was discussed. The experimental results showed that when deformed at 860℃ and below, ferrite transformation occurred due to strain. With the decrease of ausforming temperature, the quantity of ferrite increased and strength and toughness were deteriorated. Therefore, certain information was provided for optimizing technical parameter of ausforming process., firstly, the thermomechanical treatment temperature should not be lower than 860 ℃ in order to avoid ferrite formation induced by deformation; secondly, rapid cooling rate is also significant after deformation in order to avoid ferrite precipitation during subsequent cooling stage.

Low temperature impact toughness of laser hybrid welded joint of high strength low alloy steel

High strength low alloy steel with 16 mm thickness was welded by using high power laser hybrid welding. Microstrueture was characterized by using optical microscopy, scanning electron microscopy （ SEM ） , transmission electron microscopy （TEM） and selected area electron diffraction （SAED）. Low temperature impact toughness was estimated by using Charpy V-notch impact samples selected from the upper part and the lower part at the same heterogeneous joint. Results show that the low temperature impact absorbed energies of weld metal are （202,180,165 J） of upper samples and （178,145,160 J） of lower samples, respectively. All of them increase compared to base metal. The embrittlement of HAZ does not occur. Weld metal primarily consists of refined carbide free bainite and a little granular bainite since laser hybrid welding owns the character of low heat input. Retained austenite constituent film ＂locates among the lath structure of bainitie ferrite. Refined bainitic ferrite lath and retained austenite constituent film provide better low temperature impact toughness compared to base metal.

Latest Development and Application of High Strength and Heavy Gauge Pipeline Steel in China

Over the past twenty years, significant advances have been made in the field of microalloying and associated applications, among which one of the most successful application cases is HTP practice for heavy gauge, high strength pipeline steels. Combined the strengthening effects of TMCP and retardation effects of austenite recrystallization with increasing Nb in austenite region, HTP conception with low carbon and high niobium alloy design has been successfully applied to develop X80 coil with a thickness of 18.4 mm used for China＇s Second West-East pipeline. During this process, big efforts were made to further develop and enrich the application of microalloying technology, and at the same time the strengthening effects of Nb have been completely unfolded and fully utilized with improved metallurgical quality and quantitative analysis of microstructure. In this paper, the existing status and strengthening effect of Nb during reheating, rolling and cooling have been analyzed and characterized based on mass production samples and laboratory analysis. As confirmed, grain refinement remains the most basic strengthening measure to reduce the microstructure gradient along the thickness, which in turn enlarges the processing window to improve upon low temperature toughness, and finally make it possible to develop heavy gauge, high strength pipeline steels with more challenging fracture toughness requirements. As stated by a good saying that practice makes perfect. Based on application practice and theoretical analysis, HTP has been extended to develop heavy gauge and high strength pipeline steels with increasing requirements, including X80 SSAW pipe with a thickness of 22.0 mm and above, X80 LSAW pipe combining heavy gauge and large diameter, heavy gauge X80 LSAW pipe with low temperature requirements, as well as X90 steels. In this paper, alloy design, production processing, as well as mechanical properties and microstructure used for these products would be illustrated.

Microstructure,crystallography and nucleation mechanism of NANOBAIN steel

The microstructure of bainite ferrite in NANOBAIN steel transformed at different temperatures was investigated by scanning electron microscopy, transmission electron microscopy, electron back-scattered diffraction, and vickers hardness tester in detail. It is found that the average width of bainitic ferrite （BF） plates can be refined to be thinner with the reduction of temperature （473-573 K）, and the bainitic ferrite plates can reach up to 20-74 nm at 473 K. Crystallographic analysis reveals that the bainitic ferrite laths are close to the Nishiyama-Wasserman orientation relationship with their parent austenite. Temperature shows a significant effect on the variant selection, and a decrease in temperature generally weakens the variant selection. Thermodynamic analyses indicates that the Lacher, Fowler and Guggenheim （LFG） model is more suitable than the Kaufman, Radcliffe and Cohen （KRC） model dealing with NANOBAIN steel at a low temperature range. The free energy change △G^γ→BF is about -1500 J.mol^-1 at 473 K, which indicates that nucleation in NANOBAIN steel is the shear mechanism. Finally, the formation of carbon poor regions is thermodynamically possible, and the existence of carbon poor regions can greatly increase the possibility of the shear mechanism.

B950CF钢超窄间隙激光填丝焊接头疲劳性能

超窄间隙激光填丝焊(ultra-narrow gap laser welding,Ultra-NGLW)是一种先进的厚贝氏体钢焊接技术,具有热输入小,焊接接头低应力、变形小等优点.通过对不同“激光填丝-激光打底”比例的Ultra-NGLW接头高频拉-拉疲劳试验,绘制了10 mm厚不同填丝深度Ultra-NGLW接头S-N曲线,并通过对比分析接头的S-N曲线和疲劳断口特征,得到影响UltraNGLW接头疲劳性能的主要原因.试验结果表明,随着激光填丝深度的加深,接头的疲劳性能逐渐降低,填丝深度为0 mm时,接头疲劳强度为337.5 MPa,当填丝深度达到5 mm后,疲劳强度趋于稳定在270 MPa左右,Ultra-NGLW接头的疲劳源主要为焊缝粗晶区和熔合区的气孔、夹杂等焊接缺陷,在高频拉-拉疲劳循环载荷的作用下,粗晶区马氏体脆硬组织中形成较多裂纹,使其疲劳源裂纹快速扩展并连通脆硬组织中的裂纹直至接头失效.

1000 MPa级高强贝氏体钢的研制

针对汽车轻量化发展需求,河钢集团邯钢公司成功研制出1000 MPa级高强贝氏体钢。本文考察了退火模拟实验中各关键工艺参数对实验钢性能的影响,根据退火规律制定工艺方案,并应用于工业化生产。结果表明,贝氏体含量随着快冷完结温度升高而逐渐增多;当快冷完结温度为400℃时,实验钢的强度随均热温度和缓慢冷却完结温度的降低而减小。通过工业试制成功开发出屈强比大于0.8的1000 MPa级贝氏体钢,其成形极限实验结果表明,在单向拉伸情况下,此材料最大主应变和最大次应变分别为0.29和-0.07,其成形极限危险点(FLD_(0))的应变值为0.13;在等双向胀形情况下,主次应变为0.36。

Microstructure and Mechanical Properties of 1,000 MPa Ultra-High Strength Hot-Rolled Plate Steel for Coal Mine Refuge Chamber

The 1,000 MPa ultra-high strength hot-rolled plate steel with low-carbon bainitic microstructure was developed in the laboratory for coal mine refuge chamber. The static recrystallization behavior, microstructure evolution, and mechanical properties of this hot-rolled plate steel were investigated by the hot compression, continuous cooling trans- formation, and tensile deformation test. The results show that the developed steel has excellent mechanical properties at both room and elevated temperature, and its microstructure mainly consists of lath bainite, granular bainite, and ferrite after thermal-mechanical control process （TMCP）. The ultra-high strength plate steel is obtained by the TMCP process in hot rolling, strengthened by bainitic transformation, microstructure refinement, and precipitation of alloying elements such as Nb, Ti, Mo, and Cu. The experimental steel has relatively low welding crack sensitivity index and high atmospheric corrosion resistance index. Therefore, the developed steel has a good balance of strength and ductility both at room and elevated temperature, weldability and corrosion resistance, and it can suffice for the basic demands for materials in the manufacture of coal mine refuge chamber.

Development of JG785E High Strength High Toughness Steel Plates

A high strength structure steel plate --brand JG785E, which with more than 690MPa yield strength ,more than 47 joules toughness at minus 40℃ has been developed by Jigang of Shandong Steel Group P. R. China. The steel plates can be easily welded in engineering structure due to its lower carbon equivalent value. The maximum thickness of heavy plate is 50.88mm (2 inch), the yield strength is 710-860MPa, the toughness of steel plate is 139~336J at the temperature of - 40℃ . The microstructure of steel plate is lower carbon Bainite. The main solid dissolve elements are silicon and manganese. All parameters of reheating, rolling and accelerating cooling are controlled strictly. This TMCP procedure can ensure to get better mechanical properties of steel plates, and to keep the market competitive power and lower cost of manufacture. The cleanness of steel is high by refined in ladle furnace (LF), the contents of P and S is lower. It is the low carbon Bainite microstructure that possesses the high strength, excellent lower temperature toughness and better weld-ability without preheat process. The JG785E is typical brand of the Jigang’s high strength steel brands as the S690QL conforms to EN10025-6 and as the ASTM A514M conforms to USA quenching and tempering steel specification.

Research on microstructural evolution and dynamic recrystailization behavior of JB800 bainitic steel by FEM

Single pass compression tests were conducted on Gleeblel500 thermal simulator. The effect of different deformation parameters on the grain size of dynamically recrystallized austenite was analyzed. A mathematical model of dynamic recrystallization and a material database of JB800 steel, whose tensile strength is above 800 MPa, were set up. A subprogram was compiled using Fortran language and called by Marc finite element software. A thermal coupled elastoplastic finite element model was established to simulate the compression process. The grain size of recrystallized austenite obtained by different recrystallization models was simulated. The results show that the optimized dynamic recrystallization model of JB800 bainitic steel has a higher precision and yields good agreement with metallographic observations.

冷却速率对高强钢焊接粗晶热影响区组织和性能的影响

利用热模拟机、金相显微镜、扫描电子显微镜、电子背散射衍射仪,以及维氏硬度测试与夏比V型冲击试验分析了冷却速率对高强钢粗晶热影响区组织和性能的影响。结果说明:当冷却速率处于0.5~4℃/s时,粗晶区组织主要为粒状贝氏体;当冷却速率处于6~20℃/s时,粗晶区组织主要为板条贝氏体;当冷却速率处于30~60℃/s时,粗晶区组织为马氏体,马氏体转变起始温度Ms的测定值为437℃。当冷却速率为1℃/s时,对应粗晶区的室温冲击功为4.9 J,大角度晶界的占比为28.6%,断裂类型为准解理断裂;当冷却速率为10、60℃/s时,对应粗晶区的室温冲击功分别为20.2、18.9 J,大角度晶界的占比分别为41.4%、31.4%,断裂类型均为韧性断裂。总之,粗晶区组织为粒状贝氏体时韧性最差,粗晶区组织为板条贝氏体时韧性最佳。

低温回火对U20Mn高强贝氏体钢轨氢脆敏感性的影响

利用氢热分析(TDS)试验研究了U20Mn贝氏体钢轨试样的氢吸附特征。通过电化学充氢和慢应变速率拉伸试验(SSRT)研究了低温回火对高强贝氏体钢轨氢脆敏感性的影响,采用扫描电镜(SEM)、透射电镜(TEM)、电子背散射衍射(EBSD)技术及X射线衍射仪(XRD)等研究了U20Mn贝氏体钢回火前后的显微组织的变化。结果表明:在同等氢含量时,回火后的U20Mn贝氏体钢轨氢脆风险比回火前降低了近两倍,即其氢脆敏感性大幅降低。SEM断口观察表明,SSRT充氢拉伸断口在回火前存在沿晶开裂趋势和少量的沿晶裂纹,回火后断口则主要为准解理断裂形式。回火后贝氏体钢轨氢脆敏感性显著降低主要与钢中块状残留奥氏体(M/A岛)、可动位错及小角度晶界等的数量降低以及组织稳定性提高等因素有关。