Alloy design by dislocation engineering

Ultra-high strength alloys with good ductility are ideal materials for lightweight structural application in various industries. However, improving the strength of alloys frequently results in a reduction in ductility, which is known as the strength-ductility trade-off in metallic materials. Current alloy design strategies for improving the ductility of ultra-high strength alloys mainly focus on the selection of alloy composition (atomic length scale) or manipulating ultra-fine and nano-grained microstructure (grain length scale). The intermediate length scale between atomic and grain scales is the dislocation length scale. A new alloy design concept based on such dislocation length scale, namely dislocation engineering, is illustrated in the present work. This dislocation engineering concept has been successfully substantiated by the design and fabrication of a deformed and partitioned (D&P) steel with a yield strength of 2.2 GPa and an uniform elongation of 16%. In this D&P steel, high dislocation density can not only increase strength but also improve ductility. High dislocation density is mainly responsible for the improved yield strength through dislocation forest hardening, whilst the improved ductility is achieved by the glide of intensive mobile dislocations and well-controlled transformation-induced plasticity (TRIP) effect, both of which are governed by the high dislocation density resulting from warm rolling and martensitic transformation during cold rolling. In addition, the present work proposes for the first time to apply such dislocation engineering concept to the quenching and partitioning (Q&P) steel by incorporating a warm rolling process prior to the quenching step, with an aim to improve simultaneously the strength and ductility of the Q&P steel. It is believed that dislocation engineering provides a new promising alloy design strategy for producing novel strong and ductile alloys.     

Current state of Fe-Mn-Al-C low density steels

Fe-Mn-Al-C steels, previously developed in the 1950s for replacing Fe-Cr-Ni steels, are currently generating a lot of interest with potential applications for structural parts in the automotive industry because they are lighter. This paper provides a review on the physical metallurgy, processing strategies, strengthening mechanisms and mechanical properties of Fe-Mn-Al-C steels from the published literature over a period of many years, and suggests avenues for future applications of these alloys in the automotive sector.The addition of Al to Fe-C steels leads to a reduction in both density and Young’s modulus. A 1.3% reduction in density and a 2% reduction in Young’s modulus are obtained per 1 wt% addition of Al. Due to the addition of the high amounts of Al, together with Mn and C, the physical metallurgy, general processing, microstructural evolutions and deformation mechanisms of these steels are largely different from those of the conventional steels.The addition of Al to high-Mn austenitic steels brings two other important effects: increasing the stacking fault energy (SFE) and producing short-range ordering (SRO) and/or κ′-carbide precipitation. Plastic deformation of low-density Fe-Mn-Al-C steels with a high SFE, which involves SRO, is dominated by planar glide. New deformation mechanisms such as the microband induced plasticity (MBIP), the dynamic slip band refinement (DSBR) and the shear band induced plasticity (SIP) are introduced to describe plastic deformation of Fe-Mn-Al-C austenitic steels in addition to the transformation-induced plasticity (TRIP) and the twinning-induced plasticity (TWIP), which are often observed in Mn TWIP steels. These new deformation mechanisms are related to the formation and uniform arrangement of the SRO or nano-sized κ′-carbides which are coherent with the austenitic matrix. The κ′-carbide precipitation is a unique strengthening mechanism in the austenitic Fe-Mn-Al-C steels bearing high amounts of Al and C.The lightweight Fe-Mn–Al-C alloys can produce a variety of microstructures and achieve a wide range of properties. These alloys can be classified into four categories: ferritic steels, ferrite based duplex steels, austenite based duplex steels and austenitic steels. The austenitic steels are the most promising in terms of properties and processing. The tensile properties of the austenitic lightweight steels are similar to those of high Mn TWIP steels. The impact toughness of these steels in the solution treated condition is slightly lower than that of Cr-Ni stainless steels but is higher than that of the conventional high strength steels. The energy absorption at high strain rate is similar to that of high Mn TWIP steels and higher than that of conventional deep drawing steels. The ferrite based duplex low-density steels is another promising alternative. A bimodal microstructure can be obtained here through process control for steels with lower alloying contents, in which the plastic deformation of the ferrite and the TRIP and/or TWIP effects from the retained austenite can be profitably used. This type of Fe-Mn-Al-C steels exhibits an improved combination of strength and ductility compared with the first generation advanced high strength steels. The ferritic Fe-Al steels have tensile properties comparable with HSLA steels of 400–500 MPa strength level. The corrosion behaviour of Fe-Mn-Al-C steels is not improved in comparison with the conventional high strength steels. The application properties such as the fatigue behaviour and formability of Fe-Mn-Al-C steels cannot be properly understood at this stage, because of the limited experimental results so far. Some other application aspects such as weldability, coatability are not well documented.The applications of the Fe-Mn-Al-C steels in the automobiles is still not prevalent due to the lack of knowledge related to application properties so far. Above all, the reduced Young’s modulus of these steels and the processing problems as a result of the high Al and high Mn contents are the main issues. The future developments will therefore have to concentrate on the alloying and processing strategies and also on the methods to increase the Young's modulus. An improved processing strategy and a high value for the Young’s modulus will go a long way towards upscaling these steels to real automotive applications.     

Initial microstructure and retained austenite in 8 Mn steel controlled by cooling rate

The present work investigates the effect of the initial microstructure on phase transformation after intercritical annealing by measuring the amount of austenite, which was obtained by X-ray diffraction and saturation magnetisation. Pieces of 8?Mn steel were austenitised at 1100°C for 1?h followed by different cooling rates: water, air, and furnace. Samples of each piece were subsequently intercritically annealed from 600 to 800°C followed by air cooling. The microstructure was characterised using scanning electron microscopy and electron backscatter diffraction. Results show how changing the cooling rate affects the temperature of intercritical annealing at which the highest content of retained austenite was obtained.     

TRIP800激光焊焊接接头组织与性能

本文通过改变激光焊的焊接功率、焊接速度以及离焦量的工艺参数对厚度为1.8mm的TRIP800钢板进行焊接,激光焊接时最佳工艺参数为焊接速度为7mm/s,最焊接功率为570W,离焦量为-1mm;在断口扫描照片分析得到为典型的韧性断裂,断口上有大量的韧窝存在,从断裂机理上来看属于典型的微孔聚集型断裂.     

钢的薄带铸轧技术的最新进展及产业化方向

对国内外钢的薄带铸轧技术发展情况进行了总结,结合实验室对耐侯钢、铁素体不锈钢和高锰TWIP／TRIP钢薄带铸轧的研究结论,对钢的薄带铸轧过程中的亚快速凝固和近终型成形引起的新的冶金学现象如表面负偏析、全等轴晶铸态组织以及消除带材边裂等进行了阐述,指出薄带铸轧有利于进一步提高材料性能。初步明确了钢的薄带铸轧技术的产业化发展方向。     

变压边力条件下TRIP钢板成形窗口的数值模拟

为提高TRIP高强度钢板的成形质量,提出通过改变刚性凸模胀形试样宽度来测定板料FLC,并对其进行优选。在此基础上,以盒形件为例,借助数值模拟研究其压边力成形窗口以及最优的压边力变化曲线。结果表明:随着冲头行程的逐步增加,TRIP钢板盒形件的压边力成形窗口高度逐渐减小,成形后期必须严格控制压边力的大小;当采用最优压边力变化曲线时,盒形件成形质量好、变形充分。     

高碳硅锰系TRIP钢控制冷却热处理的研究

研究了高碳Ｓｉ－Ｍｎ系（Ｔｒａｎｓｆｏｒｍａｔｉｏｎ－Ｉｎｄｕｃｅｄ Ｐｌａｓｔｉｃｉｔｙ（ＴＲＩＰ）钢的控冷热处理工艺（油冷至一定温度＋空气炉等温）对拉伸性能的影响。结果表明：控冷后得到了高的强度（σｂ＝１１６０￣１５３０ＭＰａ）和良好的塑性（δ５＝１４％￣２６％）。其组织主要是板条状贝氏体和１０％￣１６％的残留奥氏体。组织和性能取决于油冷后的温度（即油冷时间）,空气炉中等温温度的适当波动影响不