Formability of Ultrafine-Grained Interstitial-Free Steels

The stretch formability of ultrafine-grained (UFG) interstitial-free steel (IF-steel) produced by equal-channel angular extrusion/pressing (ECAE/P) via various strain paths was investigated with a miniaturized Erichsen test. A coarse-grained (CG) sample demonstrated high formability with an Erichsen index (EI) of 4.5 mm. Grain refinement by ECAE decreased the formability, but increased the required punch load (F _EI) depending on the applied strain paths. The EI values were 0.35, 2.90, and 3.91 mm for 8A-, 8Bc-, and 8C-processed samples, respectively. Decrease in the biaxial stretch formability was attributed to the limited strain-hardening capacity of the UFG microstructure. Also, the grain morphology of the UFG microstructure was found to be very influential on stretch formability. Heavily elongated grain morphology in the 8A-processed microstructure resulted in the lowest formability due to the increased cracking tendency through elongated grain boundaries. However, the UFG microstructures with equiaxed grains obtained after 8C and 8Bc ECAE resulted in better formability compared to 8A. The UFG microstructure reduced the roughness (orange peel effect) of the free surface of the biaxial stretched samples by decreasing the non-uniform grain flow leading to the so-called orange peel effect. It should be noted that the strength and ductility values gained from uniaxial tensile tests are not comparable directly to the EI and F _EI values determined from the Erichsen tests. Finally, it is important to emphasize that the UFG microstructure produced by a suitable strain path leading to equiaxed grains below 1 μm could be highly deformed even under multiaxial stress conditions.     

Microstructure and Mechanical Properties of Ultrafine-Grained Interstitial-Free Steel Processed by ECAP

Equal-channel angular pressing (ECAP) of interstitial-free (IF) steel at equivalent strain, ε_vm = 12 has been employed to develop ultrafine-grained (UFG) microstructure with high fraction of low angle grain boundaries, that enhances strength significantly with reduced tensile ductility. ECAPed IF steel has been deformed further by cold rolling/cryorolling at ?50 °C to >90 % reduction in area. It is observed that the UFG structure gets refined with an improvement in high angle grain boundary fraction and heavily stressed non-equilibrium grain boundaries in cryorolled state resulting in significant strengthening. However, the decrease in grain size to an ultrafine level with the increased lattice strain lowers the work hardening ability of the material that limits its ductility. Hence, the rolled samples are flash annealed at 675 °C in order to recover the ductility of the material by achieving partially recrystallized structures. Consequently, the increased subgrain size as well as the grain size, the reduced residual lattice strain, lower hardness and strength with marginal recovery of ductility is maintained in order to attain the yield strength 2–3 times compared to that of as-received coarse-grained IF steel.     

On the Microstructural Stability of Ultrafine-Grained Interstitial-Free Steel under Cyclic Loading

The microstructural stability of ultrafine-grained (UFG) interstitial-free (IF) steel under cyclic loading was investigated. The samples were extracted from material processed along two different equal channel angular extrusion (ECAE) routes (4C and 4E) at room temperature. Low-cycle fatigue tests were carried out in addition to electron and optical microscopy in order to characterize the microstructural evolution induced by cyclic deformation. The results revealed substantial differences in microstructure resulting from different processing routes. Specifically, the volume fraction of high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs) varied significantly depending on the processing route. The different microstructural characteristics stemming from different ECAE routes expressively influence the fatigue response. Route-4C-processed material displays cyclic softening, while processing along route 4E leads to microstructural stability under cyclic loading. This highly route-dependent trend in the cyclic stress-strain response is attributed to the instability of the LAGBs and stability of HAGBs during cyclic deformation, which is further supported by electron backscattering diffraction results. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.     

High Strength and High Ductility of Ultrafine-Grained,Interstitial-Free Steel Produced by ECAE and Annealing

Interstitial-free steel (IF steel) underwent severe plastic deformation by equal-channel angular extrusion/pressing (ECAE/P) to improve its strength, and then it was annealed to achieve a good strength-ductility balance. The coarse-grained microstructure of IF steel was refined down to the submicron level after eight-pass ECAE. The ultrafine-grained (UFG) microstructure with high dislocation density brought about substantially improved strength but limited tensile ductility. The limited ductility was attributed to the small, uniform elongation caused by early plastic instability. The annealing at temperatures below 723 K (450 °C) for 1 hour did not lead to remarkable softening, whereas annealing at temperatures up to 923 K (650 °C) resulted in complete softening depending on the development of recrystallization. Therefore, the temperature of approximately 923 K (650 °C) can be considered as a critical recrystallization temperature for UFG IF steel. The annealing at 873 K (600 °C) for different time intervals resulted in different stress–strain response. Uniform tensile elongation increased at the expense of strength with annealing time intervals. After annealing at 873 K (600 °C) for 60 minutes, the yield strength, tensile strength, uniform elongation, and total elongation were found to be 320 MPa, 485 MPa, 15.1 pct, and 33.7 pct, respectively, showing the better combination of strength and ductility compared with cold-rolled samples.     

Impact Toughness of an NM400 Wear-Resistant Steel

An NM400 wear-resistant steel was hot rolled and then the plates were heat-treated by direct quenching and tempering (DQT) and reheat quenching and tempering (RQT) techniques, respectively. The Charpy impact test was carried out with an instrumented Charpy impact tester. The microstructure and fracture surface were investigated by a combination of optical microscopy, transmission electron microscopy and scanning electron microscopy methods. It was found that the impact toughness of DQT specimen was much higher than that of RQT specimen. The microstructure of both DQT and RQT specimens was characterized by a mixture of tempered lath martensite and lower bainite. The lower bainite in DQT specimen extended into prior austenite grains and the content was higher than that in RQT specimen. The lower bainite in DQT specimen improved the impact toughness by increasing the proportion of large-angle boundaries and relieving the stress concentration at the crack tip. A number of fine and dispersed carbides in DQT specimen also contributed to the improvement of the impact toughness.     

高强度管线钢的微观组织与冲击韧性

研究了具有不同微观组织类型的高强度管线钢的夏比冲击韧性.结果表明,针状铁素体型管线钢比多边形铁素体 少量珠光体型管线钢具有更高的冲击韧性.对于针状铁素体型管线钢,可通过减少夹杂物,获得均匀细小的铁素体晶粒、细小弥散的M/A岛和析出物,来获得优异的冲击韧性.     

Silicon Pick-up of Interstitial-Free Steel From MgO-SiC Refractories

The silicon pick-up of interstitial-free steel （IF steel） from resin bonded MgO-SiC refractories is studied via a medium frequency induction furnace. Samples of IF steel were heated in MgO-SiC crucibles with mould flux cover at 1 600 ℃ for 180 min. The silicon contents of IF steel samples taken from the molten steel in every 30 min were examined and the refractories after heating were investigated by electron probe microanalysis （EPMA） with energy dispersive spectrometer （EDS）. The silicon content of steel rapidly increased in the first 90 min of beating, and then decreased because of the formation of liquid layer on the refractory. Liquid layer separated the liquid steel from the bulk refractory and stopped the pick-up of silicon. At the same time, desilication via oxygen through the liquid layer and flux started to reduce the silicon content of the liquid steel     

中低碳含硅空冷贝氏体钢的冲击韧性

研究了新设计的含硅中低碳空冷贝氏体钢的热处理工艺与冲击韧性。研究发现，该钢经连续冷却获得的贝－马复相组织具有高的韧性。经３００℃回火后，钢的冲击韧性可达到９６Ｊ／ｃｍ＾２，其韧性高于同一温度回火的马氏体组织。硅将回火脆性温度推向高温。钢中出现粗大的魏氏组织时使钢的冲击韧性显著降低。     

Delamination Effect on Impact Properties of Ultrafine-Grained Low-Carbon Steel Processed by Warm Caliber Rolling

Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500 °C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong α-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 μm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100 °C to −40 °C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.     

Investigation of Impact Toughness of Electromagnetic Centrifugal Cast High Speed Steel

The effect of external magnetic field on the solidification structure and impact toughness of high-speed steel(HSS)produced by electromagnetic centrifugal casting(EMCC)have been investigated.It was found that the solidification structure could be improved effectively by electromagnetic stirring,which was characterized by elimination of the inclined columnar dendrites and refinement of the grain size.The results may be related to the changes of the fluid layers of liquid-solid phase interface.Moreover,the impact toughness of the test steel increased with the exciting current increasing from 12 to 18 A,but decreased when the current was above 18 A.SEM analysis showed that impact fracture surface was dimple fracture due to effect of the refined equiaxed grains when the current was 18 A,which was considered an important reason for the improvement of impact toughness.     

IF钢的二次加工脆性及其评定方法

随着现代真空冶炼技术的采用,钢中的碳、氮含量可以降到很低的水平,Ｃ＜５０×１０－６,Ｎ＜３０×１０－６;只需添加少量的钛或（和）铌来固定钢中的碳和氮,可生产出无间隙原子钢,即ＩＦ钢。ＩＦ钢是目前成形性能最好,级别最高的冲压用钢。它是继沸腾钢、铝镇静钢之后第三代冲压用钢。超低碳无间隙原子（ＩＦ）钢中的碳、氮原子被钛或铌固定钢质十分纯净。ＩＦ钢中的晶界上诸如碳、氮等固溶原子,使晶界的结合力大大降低,从而使深冲钢板在低温高速时发生晶界断裂现象。另一方面,为了生产高强钢常采用加磷固溶强化,加磷ＩＦ钢受到…     

超低碳贝氏体钢焊接热影响区冲击韧性的研究

用Ｇｌｅｅｂｌｅ－１５００热模拟机对含铜超低碳贝氏体钢进行不同焊接工艺下热模拟试验。研究了焊接热影响区的组织、性能。探讨了不同ｔｓ／５对钢中贝氏体组织组成及形态的影响以及组织与低温韧性的关系。用径迹显微照相技术（ＰＴＡ）显示了硼在热影响区的行为。结果表明：在不同焊接条件下,此类钢焊接热影响区均具有较好的低温韧性和较低的韧脆转变温度。当以中等速率冷却时,高温时生成的一定量的粒状贝氏体可有效地分割奥氏     

Development of a High-Strength Ultrafine-Grained Ferritic Steel Nanocomposite

This article describes the microstructural and mechanical properties of 12YWT oxide-dispersion-strengthened (ODS)-ferritic steel nanocomposite. According to the annealing results obtained from X-ray diffraction line profile analysis on mechanically alloyed powders milled for 80 hours, the hot extrusion at 1123 K (850 °C) resulted in a nearly equiaxed ultrafine structure with an ultimate tensile strength of 1470 MPa, yield strength of 1390 MPa, and total elongation of 13 pct at room temperature comparable with high-strength 14YWT ODS steel. Maximum total elongation was found at 973 K (600 °C) where fractography of the tensile specimen showed a fully ductile dimple feature compared with the splitting cracks and very fine dimpled structure observed at room temperature. The presence of very small particles on the wall of dimples at 1073 K (800 °C) with nearly chemical composition of the matrix alloy was attributed to the activation of the boundaries decohesion mechanism as a result of diffusion of solute atoms. The results of Charpy impact test also indicated significant improvement of transition temperature with respect to predecessor 12YWT because of the decreased grain size and more homogeneity of grain size distribution. Hence, this alloy represented a good compromise between the strength and Charpy impact properties.     

影响20g时效冲击因素研究

统计了近3年中板厂20g钢板的性能情况，针对最近出现的时效冲击值偏低进行分析，找出影响时效冲击韧性的主要因素，提出了改进措施。     

Asymmetric Rolling of Interstitial-Free Steel Using One Idle Roll

The effect of additional shear on the asymmetric rolling (ASR) of an interstitial-free (IF) steel was studied by modeling and experiments. The asymmetry was introduced by making one roll idle. A 66 pct of total thickness reduction was performed in 6 passes with less than 16 pct reduction per pass. ASR was performed in two ways: monotonically and by rotating the sheet between passes by 180 deg around the rolling direction (RD). Better grain fragmentation was obtained in the near surface layers. The results of monotonic asymmetric rolling are similar to symmetric rolling in terms of misorientation and cell size with the difference that the volume fraction of grains containing shear bands (SB) is larger for monotonic ASR. ASR with the sheet rotated 180 deg around the RD direction between passes showed the most promising results in terms of grain refinement, depth of the highly deformed layer, texture, and properties. The grain fragmentation process was also simulated with a recent grain refinement polycrystal model for strain hardening, texture development, grain size distribution, and grain misorientation distribution. The obtained simulation results showed strong agreement with the experiments.     

Pearlitic Transformations in an Ultrafine-Grained Hypereutectoid Steel

Common steels of both hypo- and hypereutectoid compositions form pearlitic or martensitic constituents when cooled from austenite temperature in air or in water. Here, we provide evidence that this is not the case in an ultrafine-grained hypereutectoid steel. In this system, when the grain size was reduced to a scale of 2 to 4 μm, normal pearlite could not be obtained when the steel was cooled in air; instead, nanometer-sized granular cementite and ferrite were formed in the eutectoid transformation. When the cooling rate was increased by quenching in saltwater, martensite was no longer formed; instead, fine lamellar pearlite was formed. This research indicates that these abnormal phase transformations were related to the rapid diffusion present in the ultrafine-grained steel, which changed the diffusive transformations.     

管线钢L450冲击韧性低原因分析

管线钢L450在CSP试生产初期,3.5 mm钢带冲击韧性低,不能满足石油管道制管要求。本文主要采用金相显微镜对其进行了显微组织分析。结果表明,厚规格L450管线钢冲击韧性低是由于内部出现混晶组织和铸态组织。提出提高终轧温度、增大压缩比可以减少混晶组织。     

不同微观组织高强度管线钢冲击韧性的研究

采用示波冲击试验方法研究了具有不同微观组织高强度管线钢的夏比冲击性能。据该试验的位移-载荷曲线,描述了其断裂能量吸收特征、微观组织中脆性相的存在特点,并以此为依据论证了针状铁素体组织管线钢较铁素体珠光体组织管线钢具有更高的止裂能力。     

Processing of Ultrafine-Grained Twinning-Induced Plasticity Steel Using Martensite Treatment

For the first time, martensite treatment was used to fabricate an ultrafine-grained (UFG) twinning-induced plasticity (TWIP) steel. The effects of cold rolling with 70 pct reduction at the liquid nitrogen temperature and subsequently annealing at 973 K (700 °C) for 5 to 20 minutes on the microstructure and mechanical properties of Fe-22Mn-0.4C-1.5Al-1Si TWIP steel were investigated. The results showed that a fully recrystallized UFG TWIP steel with a mean grain size of about 400 to 600 nm can be produced by the designed martensite treatment. The UFG TWIP steel exhibited high yield and tensile strengths and relatively high ductility.     

热轧钢板桩SY390BZ的低温冲击韧性

试验用热轧钢板桩SY390BZ材料(%,0.23C、0.44Si、0.91 Mn、0.06V、0.010P、0.002S)用55 kg真空感应炉冶炼并铸成Φ170 mm圆锭,轧成12 mm板,平轧最后一道次的压下量为20%,终轧温度880～900℃,空冷.测定了该钢的韧脆性转变温度,采用Quanta 400型扫描电镜和Olympus GX71型光学显微镜观察20～60℃低温冲击断口和组织形貌.结果表明,室温时断口为大而多的等轴韧窝,具有较高的韧性(130 J);-40℃时,断口形貌韧窝比较小,而且非常浅,其韧性降低至30 J;用能量法计算得出其脆性转变温度为-30℃;钢中出现了带状组织以及Al2O3夹杂导致冲击韧性较低.