Effect of hot‐rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

TRIP‐assisted multiphase steels have been thoroughly studied in the cold‐rolled and annealed state. The effects of hot‐rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP‐assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low‐carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium‐carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non‐recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low‐carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work‐hardening capabilities. In the case of the medium‐carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low‐carbon steel, the improvement of the work‐hardening behaviour was attributed to an Interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.     

连铸高铝氮积齿轮钢第三脆性区形成机制与控制

钢中的Al、N含量对连铸及其后续加工热塑性和奥氏体晶粒度控制有重要影响,这也是高温渗碳钢与各种Al脱氧钢广泛关注的问题。使用Gleeble 3800热/力学模拟试验机测定了一种轨道交通用高铝氮积齿轮钢(SCM420H)的高温热塑性,并结合差示扫描量热仪(DSC)分析、AlN析出热力学模型以及Schwerdtfeger热塑性特征值计算模型揭示了其第三脆性区的形成机制与调控途径。结果表明,高铝氮积齿轮钢第三脆性区低谷温度范围为750~850 ℃,这是由应力诱导先共析铁素体膜的产生与AlN粒子的大量析出共同导致的。Schwerdtfeger热塑性特征值计算模型可以较准确地预测高铝氮积齿轮钢第三脆性区的上限温度与最小面缩率,但由其预测的热塑性曲线下限温度偏高,应进一步考虑先共析铁素体膜析出的影响,并依据Ar3温度对其进行修正。高Al高N齿轮钢第三脆性区的下限温度取决于其先共析铁素体开始析出温度,主要与钢种成分和铸坯冷却速率相关,连铸生产中可控性有限;但其上限温度则与铸坯应变速率、冷却速率以及钢中的Al、N含量和AlN析出行为均有关联,调控空间较大,应该是连铸生产中合理控制铸坯热塑性与表面裂纹倾向的正确途径。     

连铸高铝氮积齿轮钢第三脆性区形成机制与控制

钢中的Al、N含量对连铸及其后续加工热塑性和奥氏体晶粒度控制有重要影响,这也是高温渗碳钢与各种Al脱氧钢广泛关注的问题。使用Gleeble 3800热/力学模拟试验机测定了一种轨道交通用高铝氮积齿轮钢(SCM420H)的高温热塑性,并结合差示扫描量热仪(DSC)分析、AlN析出热力学模型以及Schwerdtfeger热塑性特征值计算模型揭示了其第三脆性区的形成机制与调控途径。结果表明,高铝氮积齿轮钢第三脆性区低谷温度范围为750~850 ℃,这是由应力诱导先共析铁素体膜的产生与AlN粒子的大量析出共同导致的。Schwerdtfeger热塑性特征值计算模型可以较准确地预测高铝氮积齿轮钢第三脆性区的上限温度与最小面缩率,但由其预测的热塑性曲线下限温度偏高,应进一步考虑先共析铁素体膜析出的影响,并依据Ar3温度对其进行修正。高Al高N齿轮钢第三脆性区的下限温度取决于其先共析铁素体开始析出温度,主要与钢种成分和铸坯冷却速率相关,连铸生产中可控性有限;但其上限温度则与铸坯应变速率、冷却速率以及钢中的Al、N含量和AlN析出行为均有关联,调控空间较大,应该是连铸生产中合理控制铸坯热塑性与表面裂纹倾向的正确途径。     

Diffusion interaction of vanadium carbide with powdered steels

Conclusions The processes of diffusion interaction of vanadium carbides with cast and powdered steels are largely analogous. Differences are mainly due to unequal diffusional mobility of the components in cast and powdered materials.In cast and powdered steels equal boundary concentration of vanadium are established on the boundary with the carbides, and these concentrations are determined by the carbon content of the steel and by the temperature.The interdiffusion coefficient in ferrite does not depend on the carbon content, and it is the same for cast and powdered steels. The interdiffusion coefficient in austenite increases with increasing carbon content, and it is noticeably larger in powdered steels than in cast steels.The activation energy of interdiffusion in the austenite of powdered steels (220–230 kJ/mole) is noticeably lower than in cast steels (260–270 kJ/mole); this is due to the substantial contribution of the processes of surface diffusion to the overall mass transfer in porous materials.Translated from Poroshkovaya Metallurgiya, No. 11(323), pp. 40–45, November, 1989.     

超细晶粒钢及其力学性能特征

探索了在新一代钢中获得超细晶粒的方法。通过低温轧制和应变诱导铁素体相变，可以在碳素结构钢中获得晶粒尺寸小于5μm的超细晶粒，屈服强度大于400MPa。采用应变诱导铁素体相变可以在微合金钢中得到晶粒尺寸为1μm的超细晶粒。低碳微合金钢的屈服强度达到了600MPa，超低碳微合金钢的屈服强度超过了800MPa。采用微合金化和循环热处理可以在合金结构钢中获得2μm的奥氏体晶粒，1500MPa级抗拉强度下改善了耐延迟断裂性能。     

A New Series of Mo-free 215Cr-35Ni-xW-02N Economical Duplex Stainless Steels

A new series of economical Mo-free duplex stainless steels 21.5Cr-3.5Ni-xW-0.2N(x=1.8-3.0,mass%) have been developed.The effects of W on mechanical properties and corrosion resistance were investigated,and the microstructures were analyzed by optical microscopy,X-ray diffraction,transmission electron microscopy and electron backscatter diffraction.The designed steels have a balanced ferrite-austenite relation and are free of sigma phase after solution treatment at 750-1 300℃for 30min followed by water-quenching,whereas a small number of Cr23 C6 precipitates were found after solution treatment at 750℃.After solution treatment at 1 050℃,the steel with 1.8%(mass percent)W exhibits the highest room temperature tensile strength due to the strongest work hardening effect, while the steel with 3.0%(mass percent)W exhibits the highest fracture elongation owing to the transformation-induced plasticity(TRIP)effect.The ductile-brittle transition(DBT)and martensite transformation are respectively found in the ferrite and austenite,which deteriorates the impact properties of the steels with the increase of W content.The corrosion resistance of the designed steels is improved with the increase of W content.The pitting resistance of austenite is obviously better than that of ferrite for the designed alloys.Among the designed steels,the steel with 1.8%(mass percent)W is found to be an optimum steel with excellent comprehensive properties and lowest production cost.     

低碳锰(铌)钢控轧控冷实验研究

通过TMCP研究了低碳锰钢和添加微量Nb的低碳锰铌钢的组织和力学性能的影响因素.研究了Nb对实验钢显微组织和力学性能的影响.Nb的添加能够细化铁素体晶粒,促进铁素体转变,对贝氏体形成有一定的抑制作用.相对低碳锰钢来说,铌的加入减少了贝氏体的体积分数,贝氏体强化效果减弱,钢的屈强比升高.通过增加冷速和降低卷取温度,可以使低碳锰铌钢获得一定量的贝氏体,综合性能较佳.低碳锰铌钢的主要强化机制有细晶强化、贝氏体相变强化和析出强化.     

Characteristic properties of continuous annealed low-carbon low-aluminium steels

Continuous annealed low-carbon low-aluminium steel grades with good aging resistance and bake-hardenability were investigated. Taking into account the amounts of solute carbon and nitrogen in supersaturated ferrite it was tried to optimize the steel composition. Aging sensitivity depends mainly on solute nitrogen though the contribution to aging of solute carbon cannot be neglected. A reduced nitrogen aging requires a low nitrogen content (20 ppm) as well as Al/N ratios not lower than 5 and carbon contents higher than 0,025%. Both, low amounts of solute carbon and decrease of ferrite grain sizes of steels coiled at 650°C increase the effect of bake hardening. In comparison with aged steels the yield strength of overaged steels is conservative after baking. Bake-hardenable steels with good aging resistance are obtained in the carbon range 0,025 to 0,030%. Optimization according to the r?-value is not achieved in this range.     

The isothermal decomposition of austenite in hot-rolled microalloyed steels

The isothermal decomposition of austenite has been examined in a set of 0.1 C, 1.4 Mn steels containing small amounts of Ti, V, or Nb. The volume fraction of ferrite was measured as a function of transformation temperature and holding time, after hot rolling. Precipitation of carbonitrides, in both the austenite and the ferrite, was examined by electron microscopy of extraction replicas. The decomposition is slowest in the Nb-alloyed steel, in which the start of transformation is delayed and ferrite growth rates are much lower than in the other steels. In the V-alloyed steels, ferrite growth rates are lower than in the plain carbon or Ti alloyed steels. These results are discussed in terms of the effects of carbonitride precipitation in the austenite during high temperature deformation and in the ferrite during transformation. The roles of V and Nb in solution are also considered.     

Effect of Ultra Fast Cooling on the Alloy Cost Reduction of the X80 Pipeline Steel

The microstructures and mechanical properties of X80 pipeline steels produced by both novel ultra fast cooling and conventional‐accelerated continuous cooling modes are investigated. Results showed that different levels of Mo addition had a remarkable effect on the microstructures and mechanical properties of the investigated pipeline steels. The proeutectoid ferrite and pearlite formation is inhibited in the high‐Mo steel and acicular ferrite is obtained over a wide range of cooling rates, whereas the dominant acicular ferrite microstructure can only be obtained when the cooling rates reach up to 5 C s^?1. Very similar microstructures and mechanical properties are obtained in the low‐Mo steel produced with ultra fast cooling and in the high‐Mo steel produced by the conventional‐accelerated continuous cooling. It was proved by simulation and industrial trials that high‐strength low‐alloy steels such as pipeline steels, can be produced using the novel ultra fast cooling which also reduce alloy cost.

     

Tensile deformation behavior of high strength anti-seismic steel with multi-phase microstructure

To investigate the tensile deformation behavior of high strength anti-seismic steel with multi-phase micro-structure, tensile tests with strains of 0.05, 0.12 and 0.22 were performed at room temperature.Micro-structure of tested steels was observed by means of optical microscopy (OM), transmission electron mi-croscopy (TEM) and scanning electron microscopy (SEM).Tensile mechanical properties of tested steels were obtained, and the influence of bainite content on deformation behavior was also discussed.Mean-while, the deformation mechanism of steel with three kinds of microstructures of bainite, pearlite and fer-rite was analyzed.Results show that tested steel with high volume fraction of bainite exhibits a continuous deformation behavior, and this may be attributed to a higher bainite volume fraction and a lower mobile dislocation density.The morphology of microstructure will influence the mechanical properties of tested steels.An increasing content of bainite can improve the tensile strength, but reduce the plasticity and toughness of the tested steels.In the deformation process of 0.039Nb steel, the ferrite and bainite have priorities to deform, and the deformation exhibits co-deformation of all microstructures in the later stage of deformation.In the deformation process of 0.024Nb-0.032V steel, the ferrite and pearlite have priori-ties to deform, and the deformation exhibits co-deformation of all microstructures in the later stage of de-formation.     

高性能汽车钢组织性能特点及未来研发方向

 介绍了先进汽车用钢的组织和性能特点,认为具有较低强塑积的第一代汽车钢主要是通过铁素体、马氏体等多种基体组织的选取和配合对强度和塑性进行调控,第二代汽车钢通常具有单相奥氏体组织,表现出超高的强塑积,在汽车轻量化和安全性方面都有明显促进作用,但是其高合金质量分数提高了生产成本和难度,不利于规模化生产和应用。而第三代汽车钢则是通过在马氏体或超细晶铁素体基体上引入大量的亚稳奥氏体来提高汽车钢的强度和塑性,从而大幅度提高钢的强塑积。第三代汽车钢综合性能比第一代汽车钢提高1倍以上,其强塑积达到了25～50 GPa·%,接近或达到了第二代汽车钢的强塑积。新型合金化设计、高强度基体组织调控和大量亚稳奥氏体控制是第三代汽车钢的重要研究内容。基于轻量化与高安全性要求,低密度化与高强化将是未来第三代汽车钢的一个主要发展方向。     

Effect of magnesium addition on microstructure and mechanical properties in wheel steel

In order to reveal the effect of Mg in low carbon microalloy steel, low carbon microalloy steel of HR60 wheel steel was smelted in vacuum induction furnace and industrial field respectively. The characteristics of typical non- metallic inclusions and microstructure of experimental steels were both compared by OM, SEM- EDS and INCA Feature with automatically scanning inclusions function. The mechanical properties of the experimental steels were also measured. The results show that alumina inclusions are modified to spinel inclusions with small size after Mg addition. Furthermore, acicular ferrite can be induced effectively by inclusions containing magnesium. The microstructures of experimental steels are changed from ??polygonal ferrite(PF) + pearlite(P)??to ??polygonal ferrite(PF) + degenerate pearlite(DP) + acicular ferrite(AF)?? and refined by Mg treatment. The strength of experimental steels is improved with Mg addition. In industrial experiments, the fatigue limit of Mg- treated steels is greater than 460MPa, while the fatigue limit of Ca- treated steels is about 450MPa. In addition, the fatigue life of Mg- treated steels is generally higher than that of Ca- treated steels under the condition that the stress is greater than the fatigue limit. In laboratory experiments, the contents of Nb and Ti are reduced while Mg content in steel is 18??10-6, the strength of the wheel steel is close to the reference steel. Therefore, the project to reducing production cost by taking advantage of the microalloy role of Mg is feasible.     

Effect of Composition and Processing Parameters on the Formation of Nano-Bainite in Advanced High Strength Steels

The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel after controlled thermomechanical processing.The microstructure for both steels consisted of bainite.The microstructural characteristics of bainite,such as the morphology of the nano-bainite and thicknesses of bainitic ferrite and retained austenite layers,as a function of steel composition and processing was studied using transmission electron microscopy (TEM).It was found that the nano-bainitic structure can be formed in the low alloy steel through thermomechanical processing.Atom probe tomography (APT) was employed as a powerful technique to determine local composition distributions in three dimensions with atomic resolution.The important conclusions from the APT research were that the carbon content of bainitic ferrite is higher than expected from paraequilibrium level of carbon in ferrite for both steels and that Fe-C clusters and fine particles are formed in the bainitic ferrite in both steels despite the high level of Si.     

Influence of Vanadium on Grain Growth of Deformation Induced Ferrite in Low Carbon Steel during Continuous Cooling Process

The effects of vanadium on the deformation induced ferrite transformation (DIFT) in low carbon steel during heavy deformation at the temperature slightly higher than Ar₃ and on the coarsening of DIFT microstructure during the continuous cooling processes after deformation were investigated using a thermo‐simulator. The results show that vanadium has little effect on the volume fraction of DIFT microstructure under heavy deformation, whereas the deformation induced ferrite (DIF) grains are refined with increasing vanadium content. The steel containing a small amount of vanadium exhibits similar velocity of grain growth compared to vanadium free steel, while the vanadium remarkably inhibits grain growth in a steel containing a high amount of vanadium during the continuous cooling process. The diffusion activation energy of grain boundaries for all the tested steels is calculated and the influencing mechanism of vanadium on the grain growth during the continuous cooling process is discussed.     

Hot Mechanical Properties of 24Cr‐14Ni High Alloy Billets

24Cr‐14Ni alloys have gained importance in high temperature applications. Because of δ‐ferrite and α phase formation, 24Cr‐14Ni austenitic stainless steel billets are difficult to hot work. The mechanical properties at high temperature of such stainless steels are investigated on a hot tensile test machine according to hot‐rolling conditions, under different time and temperature regimes. These 24Cr‐14Ni stainless steels were also hot rolled under various reduction ratios. The influences of the reduction ratio on the hot mechanical properties and phase transformation from δ‐ferrite into σ phase in 24Cr‐14Ni stainless steels are discussed in detail. The results obtained can be a contribution to improve the hot rolling of this high alloy stainless steel.     

Effect of V–Nb Composite Microalloying on Microstructure and Properties of Non-Quenched and Tempered Forged Steel

Herein, non-quenched and tempered forging steels containing V and V–Nb are designed, and the mechanical properties and microstructure of two steels are compared and analyzed. The comprehensive mechanical properties of V–Nb containing steel are as follows: the yield strength is 525.1 MPa, the impact energy A_kV is 62.1 J at ambient temperature, and the elongation is 26.1%. It is shown in the results that the addition of Nb element can refine the grain size (17.2 μm), increase the ferrite content (54.1%), and refine the lamellar spacing of pearlite (274 nm). The formation of V (C, N) particles on MnS inclusions can promote fine ferrite nucleation and growth, and Nb element can further promote ferrite nucleation by forming coarser (V, Nb) (C, N) particles. The difference of yield strength and hardness between the two steels is mainly caused by the difference of precipitation strengthening, the precipitation-strengthening increment of V–Nb containing steel is 18.31 MPa higher than that of V containing steel, which is because the coarser-size (V, Nb) (C, N) particles produce stronger precipitation-strengthening effect. But the large-sized MnS inclusions are beneficial to the increase of crack driving force and reduce the plasticity and toughness.     

铌微合金化对高铁车轮钢奥氏体形核和长大的影响

 针对碳质量分数为0.47%中碳高铁车轮钢,研究了铌微合金化对前驱体为铁素体-珠光体的组织发生奥氏体逆相变的影响。结果表明,铁素体-珠光体钢的逆相变是一个由碳原子扩散控制的过程,奥氏体优先在珠光体内的铁素体与渗碳体（α/Fe3C）片层界面处形核,并且沿平行于珠光体片层方向的长大速率比垂直于珠光体片层方向更快。含铌车轮钢细化的珠光体组织可以提高奥氏体的形核率,有利于细化奥氏体晶粒。随着再加热温度的提高,含铌车轮钢的奥氏体混晶温度（960 ℃）比不含铌的钢高80 ℃,因此通过铌微合金化可扩大再加热奥氏体化温度窗口。结合Thermal-Calc热力学计算和透射电镜分析,铌在中碳钢中主要以析出物的形式存在,析出钉扎作用是其细化奥氏体晶粒、推迟混晶现象出现的主要机制。     

A Study about the Influence of Carbon Content in the Steel on the Casting Behavior

In the casting process of steels with a C‐content ranging from 0.09 to 0.53 mass%, austenite is formed as secondary crystal phase by peritectic reaction between crystal of δ ferrite and residual melt. For unalloyed or micro‐alloyed steels the C‐content or C‐equivalent influences the casting behavior of steel in the mould, such as strand shell growth, crack formation, heat transfer, temperature fluctuation in the copper plate, mould level fluctuation and oscillation marks formation. The negative casting behavior like the uneven strand shell growth, the deep oscillation mark formation, the high mould level fluctuation, the crack formation on the strand surface were found mostly for steel with C‐content or Cp between 0.10–0.13 mass%. The strand shell structure (strand shell growth, mushy zone, δ + γ phase transformation) and shrinkage of the strand shell were simulated depending on the C‐content by means of mathematical simulation. On the basis of the simulation results and of the measured high temperature strength of steel the dependence of stiffness and the irregularity of the shrinkage of strand shell on the C‐content was investigated. It was found that the stiffness and irregularity of the shrinkage of the strand shell reach the maximum value at a C‐content of about 0.12 mass%.     

高锰高铝钢凝固组织的实验研究

为研究高锰高铝钢的凝固组织,在实验室条件下采用真空感应炉冶炼了三种不同成分的高锰高铝钢。研究结果表明,高锰高铝钢的柱状晶组织比较发达,其晶体尺寸比较粗大;增加钢中的Al含量可以降低柱状晶区比例和晶体尺寸,随着钢中Al含量的增加,钢的热塑性也明显增强;在奥氏体晶界析出的铁素体会降低钢的热塑性,是诱发连铸坯角部裂纹的主要原因。