Static recrystallisation kinetics of austenite in Ti,Nb and V microalloyed steels during hot deformation

Abstract

Static recrystallisation kinetics during hot deformation of two microalloyed steels (C–Ti–V and C–Ti–Nb) has been quantified. Double hit compression test using a Gleeble 1500 thermomechanical simulator was conducted to determine the recrystallisation kinetics. The kinetics of static recrystallisation was found to be more sluggish in the case of microalloyed steels with niobium additions as compared with other grades of microalloyed steels. The rate of recrystallisation increases with increasing temperature, strain and strain rate. The results are compared with other grades of microalloyed steels, mainly with Ti, Ni and Cu additions, from literature. For niobium containing microalloyed steels, a higher temperature is required for recrystallisation as compared with other microalloyed steels.     

Comparison of Mn,Cr and Mo alloyed sintered steels prepared from elemental powders

Abstract

Molybdenum, chromium and manganese offer considerable potential as alloy elements in sintered steels, especially for PM precision parts used, for example, in automotive engines and transmissions. This holds in particular for recycling and health/safety aspects. Within this work, the influence of these elements as admixed alloy metals on the sintering behaviour and the properties of structural PM steels is discussed. The considerable differences in the homogenisation behaviour during sintering are described as well as the respective advantages and drawbacks. It is shown that for high density PM steels, Cr and Mo are better suited while for conventionally produced mass products Mn is attractive, its affinity to oxygen being less of a problem today than commonly assumed and since Mn addition promotes swelling, this element might be a replacement for Cu. For Mn, admixing is the more attractive route, for Cr, in contrast, prealloying offers advantages, while for Mo both alloying techniques are feasible.     

Bauschinger effect and residual phase stresses in two ductile-phase steels: Part II. The effect of microstructure and mechanical properties of the constituent phases on Bauschinger effect and residual phase stresses

A finite-element method has been employed to calculate the effect of the prestrain, particle size, volume fraction, and yield strength ratio of the constituent phases on the Bauschinger effect (BE) and residual phase stresses for ferrite-martensite two-phase steels. The relationships be-tween the BE parameters and residual phase stresses are given, and the basics of inelastic un-loading and the effect of reverse deformation on the BE and residual phase stresses are also discussed. Based on the decaying of the residual phase stresses (commonly called “back stresses”) during reverse loading, the relationship between back stresses and permanent softening has been elucidated. Some of the calculated results are compared with experimental ones, and good agree-ment between them is found.     

Role of carbon and nitrogen content on microstructural homogeneity in thin slab direct rolled microalloyed steels

Abstract

This paper analyses the effect of carbon and nitrogen content on the austenite microstructural homogeneity before transformation in the thin slab direct rolling of Nb and Nb–V microalloyed steels. The study was made with the help of a microstructural hot working model adapted to the metallurgical peculiarities associated with thin slab direct rolling. The results show that an increase in carbon content from 0·04 to 0·09% in 0·05%Nb microalloyed steels requires a significant increase in the initial rolling temperature in order to avoid the presence of isolated as cast austenite grains prior to transformation. Similarly, an increase in nitrogen content from 30 to 120 ppm does not imply changes as drastic as in the case of carbon. In both situations the changes required in the rolling temperature can be explained by the interaction between post-dynamic softening mechanisms and strain induced precipitation kinetics. In this context, the incidence of different final gauge thicknesses on microstructural homogeneity is evaluated.     

Influence of cooling and reheating on the evolution of copper rich liquid in high residual low carbon steels

Abstract

This study investigates some effects of austenite microstructure on processes leading to copper hot shortness. Low carbon steels containing 0˙55 wt-% copper were subjected to two thermal profiles in an infrared image furnace with attached confocal scanning laser microscope: hold at 1150°C for 60 s; hold at 1150°C for 60 s, quench to 400°C, reheat to 1150°C. Heat treatments were conducted in dried/deoxidised argon to image microstructures. Subsequent samples were oxidised in air. The oxide/metal interface was studied in a scanning electron microscope. Additional confocal scanning laser microscope experiments involved melting copper directly on the steel. After quench/reheat, austenite grain size decreased by a factor of ～1˙7 and grain boundaries were redistributed. Copper evolved during the first heating was no longer found at boundaries. Results from direct copper exposure reveal an apparent effect of boundary character on copper penetration rate. Possible mechanisms by which hot shortness is affected are discussed.     

High temperature sintering and its effect on dimensional and geometrical precision and on microstructure of low alloyed steels

ABSTRACT

The effect of high-temperature sintering (1180°C and 1250°C) on the dimensional stability, geometrical precision, density and microstructure of rings made of five different low alloy steels was studied. Density and dimensional shrinkage slightly increase with temperature, but the dimensional and the geometrical precision of parts, do not depend on sintering temperature. Sintering temperature maybe therefore increased up to 1250°C without impairing the dimensional and geometrical precision of the investigated rings. An estimation of the effect of the high sintering temperature on tensile properties is presented, based on the microstructure and on the fraction of the load-bearing section. A significant increase in both tensile strength and tensile ductility may be expected, in particular when the sintering temperature is increased up to 1250°C.     

包辛格效应的原理及其在冷轧带肋钢筋中的应用

通过对冷轧带肋钢筋包辛格效应及其影响因素的分析,讨论了包辛格效应对冷轧带肋钢筋质量的影响,论证了利用包辛格效应来提高冷轧带肋钢筋质量的可行性．     

Nb、Ti微合金化CSP汽车大梁钢用热轧钢带研制

通过对薄板坯高温变形奥氏体再结晶和未再结晶区变形的有效控制,在包钢CSP生产线上成功地解决了含Nb钢的混晶问题.利用Nb、Ti复合微合金化技术成功地开发了汽车冲压结构用高强度钢带QStE380TM.分析表明,开发的含Nb钢带具有优异的韧性和成形性能,其性能完全满足汽车车箱纵梁、横梁的冲压和装配要求.     

铌、钒、钛在微合金钢中的作用

微合金化元素Nb、V、Ti在钢中的作用,主要表现在在热加工过程中抑制奥氏体的形变再结晶并阻止其晶粒的长大,并通过它们的碳氮化合物的应变诱导析出,对钢进行沉淀强化。这3个元素虽然都是通过细化晶粒和沉淀强化来提高强度,但它们在钢中的作用机理及强化程度并不同,Nb在钢中具有最强的晶粒细化强化效果,而V在钢中具有最强的沉淀强化效果,Ti则介于Nb和V两者之间。     

The effect of processing variables on the mechanical properties and strain ageing of high-strength low-alloy V and V-N steels

Abstract

Laboratory experiments have been conducted on a series of vanadium and vanadium-nitrogen steels. The results indicate that the combination of vanadium plus nitrogen gives an excellent notch toughness in the normalized plates (EA TT < ?80°C). Controlled rolling gives about 100 MPa higher yield strength with somewhat reduced notch toughness (EATT? ?50°C). Aluminum does not improve the properties of a normalized or controlled-rolled vanadium-nitrogen plate. Vanadium is able to combine with nitrogen so effectively that the steel can be considered. to be unsusceptible to strain ageing.

Résumé

Les auteurs ont fait des essais de laboratoire sur une série d'aciers au vanadium et au vanadium-azote. Les résultats indiquent que la combinaison vanadium-azote donne d'excellentes propriétés de résilience pour des tôles fortes normalisées (EA TT < ?80 °C). Le laminage contrôlé conduit à une limite d'élasticité plus élevée d'environ 100 MPa et a une résilience quelque peu réduite (EATT ? -50°C). L'aluminium n'améliore pas les propriétés d'une tôle forte d'acier au vanadium-azote après normalisation ou après laminage contrôlé. L'efficacit;amp;#x00E9; de la réaction entre Ie vanadium et l'azote est telle que l'acier peut être consideéré comme n'étant pas susceptible au vieillissement après écrouissage.     

Surface self-densification in boron alloyed austenitic stainless steel and its effect on corrosion and impact resistance

Abstract

A boron alloyed AISI 316L, sintered in pure hydrogen at 1250°C, was investigated to study the effect of the microstructure, with particular reference to the near full dense and boride free surface layers, on corrosion resistance and impact properties. The near full dense and homogeneous austenitic surface layer results from the flow of the liquid towards the bulk of the specimens during sintering. It significantly improves the corrosion resistance, while impact properties are strongly influenced by the bulk microstructure, where the boride network creates favourable conditions for both nucleation and propagation of crack.     

铌、钨和硼在TiAl基合金中的分布及其对组织的影响

铌、硼和钨在TiAl基合金中的存：在形式、分布状态与该合金的制备片法、加工工艺及热处理制度有密切的联系，它们对TiAl基合金的性能又有重要影响。为进一步对有关问题进行研究，该文对铌、钨、硼在TiAl基合金中的作用进行了综合分析，对铸造法制得的Ti-Al-Nb-W-B合金进行了热等静压和均匀化处理，并对微观金相组织进行了观察比较。结果表明：Ti-45Al-7Nh合金中添加少量的钨和硼，能使晶团尺寸明冠细化，达40μm。随钨含量的增加，晶团尺寸越来越小，片层间距则越来越大，如Ti-45Al-7Nb-0．15B的晶团尺寸约为100μm，片层间距约为1．2μm；Ti-45Al-7Nb-0．2W-0．15B的晶团尺寸约为70μm，片层间距约为1．6μm；Ti-45Al-7Nb-0．4W-0．15B的晶团尺寸约为50μm，片层间距约为1．7μm；Ti-45Al-7Nb-0.7W-0．15B的晶团尺寸约为40μm，片层间距约为1．8μm。钨含量达到0．4％以后，晶团细化及片层间距增加的效果使渐趋稳定。