Critical assessment 29: TRIP-aided bainitic ferrite steels

Transformation-induced plasticity (TRIP)-aided bainitic ferrite steels developed for automotive applications have attractive mechanical properties such as ductility, formability, toughness, fatigue strength and delayed fracture strength. These mechanical properties are principally associated with a ductile lath-structure matrix and the strain-induced transformation of the metastable-retained austenite films of 3–20 vol.%. In this paper, data on the microstructural and mechanical properties of the low-carbon TRIP-aided bainitic ferrite steels are critically assessed, as well as their deformation mechanism.     

Low temperature mechanical properties of quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling

Abstract

The mechanical properties of a quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling (CRP steel) have been studied over the temperature range 77–293 K with the aim of developing a CRP steel for low temperature ultrahigh strength applications. The results obtained were compared with those of a conventional quenched and tempered 0·4C–Ni–Cr–Mo steel (CHT steel). The CRP process was found to improve greatly the strength, ductility, and fracture and impact toughness for tempers at and below 473 K, independent of test temperature, but there was some concomitant deterioration in the transverse properties. It is postulated that the fine subcell structure, introduced during the CRP, is mainly responsible for the improved mechanical properties. However, there is an abrupt reduction in fracture energy of fatigue precracked steels for tempers above 473 K, so above this temperature there is little difference in the properties of the CRP and CHT steels. This is attributed to fine carbide precipitation, which promotes shear localisation and dimple fracture. Despite this, it is demonstrated by the present work that the CRP steel is attractive for low temperature ultrahigh strength steel applications.

MST/734     

Interface sheet-constrained groove pressing as a modified severe plastic deformation process

In this paper, a new severe plastic deformation (SPD) process entitled interface sheet-constrained groove pressing (ISCGP) as a new variant of conventional CGP has been developed for producing ultrafine-grained metallic materials. In this process, repetitive shear deformation is imposed into the sheet material by utilising symmetrically grooved die along with two interface sheet on both sides. To study the applicability, mild steel sheets were processed by both ISCGP and CGP processes, and mechanical and microstructural properties of the processed samples were investigated. The results show a considerable improvement in mechanical properties including hardness, yield strength, and ultimate tensile strength, though the ductility sacrifice was reduced. Comparing ISCGP and conventional CGP revealed interesting results, which are shown that ISCGP can result in better surface quality and ductility.     

Fatigue crack growth rates and fracture toughness in electroslag refined En 52 steel

En 52 steel has been electroslag refined and the resultant effects of refining on its mechanical properties have been assessed. It was found that refining caused a decrease in fatigue crack growth rates and increases in fatigue strength, fracture toughness, Charpy fracture energy and tensile ductility. Fatigue crack growth rates in region I and in region III were found to be considerably lower in the electroslag refined steel: they were unaffected in region II. The fracture toughness values for the electroslag refined steel are nearly twice those estimated for the unrefined steel. Measurements on heat-treated samples have shown that the electroslag refined steel has a better response to heat-treatment. The improvement in the mechanical properties is explained in terms of the removal of nonmetallic inclusions and a reduction in the sulphur content of the steel.     

Effect of multi-directional forging and ageing on fracture toughness of Al–Zn–Mg–Cu alloys

Strength, ductility and fracture toughness are the most important mechanical properties of engineering materials. In this work, an Al–Zn–Mg–Cu alloy was subjected to multi-directional forging (MF) and ageing treatment. Microstructural evolution was studied by optical and electron microscopy and strength, ductility and fracture toughness were researched. After MF, the dislocation density was increased and the microstructure was refined. The strength and fracture toughness were increased, while the ductility was decreased sharply. Without compromising the strength, the ductility was improved significantly after ageing. The fracture toughness was increased further. The coarse and discontinuously distributed grain boundary precipitates were found to be responsible for higher fracture toughness of the fine-grained structure Al–Zn–Mg–Cu alloy.     

Flexural toughness characteristics of steel–polypropylene hybrid fibre-reinforced oil palm shell concrete

Hybridization of steel–polypropylene leads to improvements of both the mechanical and ductility characteristics of concrete. In this investigation, the effect of steel, polypropylene (PP) and steel-PP hybrid fibres on the compressive strength, tensile strength, flexural toughness and ductility of oil palm shell fibre reinforced concrete (OPSFRC) was studied. The comparison on the above said properties between the specimens prepared with crushed and uncrushed oil palm shell (OPS) as lightweight coarse aggregate was also carried out. The experimental results showed that the highest compressive strength of about 50 MPa was produced by the mix with 0.9% steel and 0.1% PP hybrid fibres. The highest increments in the splitting tensile and the flexural strengths of the OPSFRC were found up to 83% and 34%, respectively. However, the mixes with 1% PP fibres produced negative effects on both the compressive and tensile strengths. The results on the toughness indices showed that the OPSC possess no post-cracking flexural toughness. Though, the flexural deflection and toughness of the OPSC was significantly enhanced by the addition of fibres; the dominance of the steel fibre on the first crack flexural deflection and toughness of OPSFRC was evident. The mixes with 0.9% steel and 0.1% PP hybrid fibres reported the highest improvement in toughness index and residual strength factor.     

低碳微合金直接淬火钢的组织与力学性能

为了提高低碳直接淬火钢的强韧性能,对一种低碳Nb-V微合金钢进行了轧后直接淬火(DQ)和再加热淬火(RQ)热处理实验,分析了低碳直接淬火钢的的强韧化机理.采用光学显微镜、透射电子显微镜、硬度计、拉伸试验机以及冲击试验机研究了轧后热处理工艺对低碳Nb-V微合金钢组织和力学性能的影响.结果表明,DQ工艺钢马氏体板条间距细小,含有较多的位错亚结构,因此具有较高的强度和韧性.DQ工艺钢马氏体中的大量位错,促进了碳化物弥散析出,产生了显著的二次硬化效果.由于基体中固溶的Nb、V等元素推迟淬火马氏体在回火过程中的各种转变,以及回火时析出的细小弥散碳化物抑制马氏体铁素体回复、再结晶过程,DQ工艺钢表现出较高的回火稳定性.     

Some characteristics of high strength fiber reinforced lightweight aggregate concrete

The effect of polypropylene and steel fibers on high strength lightweight aggregate concrete is investigated. Sintered fly ash aggregates were used in the lightweight concrete; the fines were partially replaced by fly ash. The effects on compressive strength, indirect tensile strength, modulus of rupture, modulus of elasticity, stress–strain relationship and compression toughness are reported. Compared to plain sintered fly ash lightweight aggregate concrete, polypropylene fiber addition at 0.56% by volume of the concrete, caused a 90% increase in the indirect tensile strength and a 20% increase in the modulus of rupture. Polypropylene fiber addition did not significantly affect the other mechanical properties that were investigated. Steel fibers at 1.7% by volume of the concrete caused an increase in the indirect tensile strength by about 118% and an increase in the modulus of rupture by about 80%. Steel fiber reinforcement also caused a small decrease in the modulus of elasticity and changed the shape of the stress–strain relationship to become more curvilinear. A large increase in the compression toughness was recorded. This indicated a significant gain in ductility when steel fiber reinforcement is used.     

Relative effect of C and Mn on strength-toughness of medium Mn steels

In order to find an alternative choice for structural load-bearing components, an effort was made to improve the impact toughness of medium Mn steel through inter-critical annealing treatment without significant reduction in strength. Besides, the relative effect of C and Mn contents on microstructure and mechanical properties of medium Mn steels was also studied. Fibrous microstructures with fine, alternate films of ferrite and martensite with retained austenite were obtained. The low-C, high-Mn steel showed a superior combination of yield strength, ductility and impact toughness due to finer microstructure and higher retained austenite fraction, as compared to high-C, low-Mn steel. Thus, the beneficial effect of Mn enrichment in stabilising retained austenite and improving mechanical properties by transformation induced plasticity effect becomes evident.     

Mechanical properties of high strength concrete reinforced with metallic and non-metallic fibres

This paper focuses on the experimental investigation carried out on high strength concrete reinforced with hybrid fibres (combination of hooked steel and a non-metallic fibre) up to a volume fraction of 0.5%. The mechanical properties, namely, compressive strength, split tensile strength, flexural strength and flexural toughness were studied for concrete prepared using different hybrid fibre combinations – steel–polypropylene, steel–polyester and steel–glass. The flexural properties were studied using four point bending tests on beam specimens as per Japanese Concrete Institute (JCI) recommendations. Fibre addition was seen to enhance the pre-peak as well as post-peak region of the load–deflection curve, causing an increase in flexural strength and toughness, respectively. Addition of steel fibres generally contributed towards the energy absorbing mechanism (bridging action) whereas, the non-metallic fibres resulted in delaying the formation of micro-cracks. Compared to other hybrid fibre reinforced concretes, the flexural toughness of steel–polypropylene hybrid fibre concretes was comparable to steel fibre concrete. Increased fibre availability in the hybrid fibre systems (due to the lower densities of non-metallic fibres), in addition to the ability of non-metallic fibres to bridge smaller micro cracks, are suggested as the reasons for the enhancement in mechanical properties.     

Einfluß einer thermomechanischen Behandlung auf die mechanischen Eigenschaften der Ti6Al4V-Legierung

Influence of thermomechanical treatment on the mechanical properties of Ti6Al4V Thermomechanical treatments of Ti6Al4V including deformation just below the β-transus followed by water quenching and aging were shown to substantially improve the mechanical properties. Especially the fatigue strength was raised well beyond levels known from conventional processed material, for both room and elevated temperatures. The paper shows how other properties like tensile strength, ductility, fracture toughness, creep strength fatigue crack growth and stress corrosion cracking are affected.     

超细晶1.73C超高碳钢的组织和性能

将1．73C％超高碳钢（UHCS-1．73C）的钢锭经过低应变、多道次锻造,加热淬火、高温回火后得到超细晶粒、球状碳化物组织,再进行循环感应热处理,得到超细晶粒马氏体基体上分布超细球状碳化物的组织,研究其组织和性能与循环感应热处理之间的关系．结果表明,随着感应加热淬火循环次数增加,组织中出现板条马氏体且数量增加,马氏体片变短、钝化,碳化物颗粒更圆整,压缩屈服强度升高,塑性增大．循环感应淬火4次后（不回火）屈服强度1105MPa,断裂强度1992MPa,压缩率9．8％．     

Overall stress–strain relationship of cold rolled transformation induced plasticity multiphase steels

Abstract

As a result of transformation induced plasticity, TRIP assisted steels possess favourable mechanical properties such as high strength, ductility and toughness. In the present work, the flow stress of a cold rolled TRIP multiphase steel has been calculated from the stress of individual constituent phases on the basis of a continuum model whereby the internal stress produced by the inhomogeneous distribution of plastic strain is considered. For the first time, account is taken of how the volume fraction of constituent phases changes with strain. A comparison of stress–strain curves determined from the model and experimentally derived stress–strain curves for an Si–Mn type TRIP800 steel gives satisfactory agreement. Variation of the strain hardening exponent of the TRIP steel with strain is also discussed.     

Effect of interlamellar spacing on the mechanical properties of 0.65% C steel

The mechanical properties of a steel containing a nearly fully pearlitic structure have been examined as a function of the interlamellar spacing. The steel had been heat-treated at different austenitization temperatures in order to obtain varying interlamellar spacings. It was observed that hardness and yield strength follow a Hall–Petch type of relationship with respect to the interlamellar spacing but the ultimate tensile strength (UTS), percent elongation and impact toughness did not do so. It was noted that, below a critical size of interlamellar spacing, the UTS, impact toughness and ductility remained invariant to the interlamellar spacing. The results have been explained on the basis of a microstructure–thermal residual stress relationship.     

Studies on influence of reheating temperature and cooling rate on structure–property behaviour of lean chemistry HSLA-100 steel

Abstract

Simulation studies on the influence of reheating temperature on austenite grain coarsening in lean chemistry high strength low alloy (HSLA)-100 steel were carried out to establish optimum soaking temperature before hot rolling. Experiments carried out in ‘Gleeble-3500’ dynamic thermomechanical simulator revealed that prior austenite grain sizes varied between 26 and 98 and 34 and 126 μm after soaking at 1150, 1200 and 1250°C for 1 and 5 min respectively; a soaking temperature of 1200°C was found to be optimum. Simulation experiments on the influence of cooling rate on microstructural changes and dilatometric studies indicated lowering of transformation temperature with faster cooling. Microstructural examination of dilatometric samples confirmed martensitic transformation at faster cooling rate. The martensite structure is desirable to achieve better strength and toughness. The findings of simulation studies were subsequently used for standardising thermomechanical treatments of Nb–Cu bearing lean chemistry HSLA-100 steels. One laboratory heat of Cu bearing HSLA steel containing 0·028%Nb was made. This heat was hot rolled into 12·5 mm thick plate by varying finish rolling temperature in the range of 800–1000°C. The soaking temperature was maintained at 1200°C. The rolled plates were heat treated by both conventional reheat quenching and tempering (RQT) as well as direct quenching and tempering (DQT) techniques. Evaluation of mechanical properties revealed that plates processed through DQT route were superior to those processed through RQT route. Transmission electron microscopy revealed that martensite structure and finer interlath spacing in DQT plates resulted in superior strength and impact toughness properties as compared to RQT steels.     

The influence of copper addition on microstructure and mechanical properties of thermomechanically processed microalloyed steels

The present study concerns the influence of Cu addition on the microstructural evolution and mechanical properties of directly air-cooled microalloyed thin-gauge steel. For this purpose, 1.5 wt% Cu was added to a Ti–B microalloyed steel. It is known that Ni addition to Cu-containing steel is beneficial to eliminate hot shortness caused by Cu. Therefore, the effect of Ni addition (half that of Cu in wt%) on the microstructure formation and mechanical properties has also been studied. Microstructures and mechanical properties of the directly air-cooled steels have demonstrated that addition of 1.5 wt% Cu along with 0.8 wt% Ni in Ti–B microalloyed steel results in a dual phase-like microstructure, which yielded attractive tensile strength (746 MPa) and ductility (31%). However, Cu addition deteriorated the impact toughness of the directly air-cooled Ti–B microalloyed steels.     

Effect of increasing cooling rate from normalising temperature on structure and properties of a Mn–Cr–Mo–V steel

Abstract

The work was undertaken to study the effect of increasing the cooling rate from the normalising temperature on the microstructure and mechanical properties of a Mn–Cr–Mo–V steel. The steel was to a chemical composition suitable for grade 271 in BS 1501 Part 2 and as such would be used in the normalised and tempered condition. Three linear rates of cooling (12, 36, and 120 K min ^?1) from the normalising temperature were used. With the tenfold increase in cooling rate, yield strength increases of about 20% and tensile strength increases of about 15% were obtained in the final tempered steel. Although these strength increases resulted in a loss in ductility and toughness, the values of these properties were still relatively high. The improvements in strength with increases in cooling rate have been related to an increase in the proportion of bainite and a decrease in the amount of ferrite in the resulting microstructure.

MST/683     

Effect of plastic strain state on ductility and toughness

The tensile ductility of seven structural steels was substantially reduced when the plastic strain state was changed from axisymmetric (round specimen) to plane strain. The ratio of plane-strain-tension ductility divided by axisymmetric-tension ductility declined from 72 percent for a mild steel to 17 percent for a steel with 248 ksi yield strength. The ductility (fracture strain) in the Charpy V-notch specimen was equal to the ductility in the plane-strain-tension specimen. Therefore, the Charpy toughness correlated with plane-strain-tension ductility, not with the ordinary axisymmetric-tension ductility. The reduction in ductility when the plastic strain state is changed from axisymmetric to plane strain is here defined as plane strain sensitivity. The decreased toughness that accompanies increased strength is caused by the increase in plane-strain sensitivity that occurs as the strength increases.     

Influence of grain refining elements on mechanical properties of AISI 430 ferritic stainless steel weldments – Taguchi approach

The effect of grain refining elements such as copper, titanium and aluminum on transverse tensile strength, ductility, impact toughness, microhardness and austenite content of AISI 430 ferritic stainless steel welds through Gas Tungsten Arc Welding (GTAW) process in as-welded condition was studied. Taguchi method was used to optimize the weight percentage of copper, titanium and aluminum for maximizing the mechanical properties and austenite content in the weld region of ferritic stainless steel welds. Based on Taguchi orthogonal array the regression equations were developed for predicting the mechanical properties of ferritic stainless steel welds within the range of grain refining elements. The observed mechanical properties and austenite content have been correlated with microstructure and fracture features.     

动态碳配分对先进高强钢组织与力学性能的影响

目的 为了使钢表现出更好的吸能特性,以具有较高的强度以及较好的塑性。方法 提出了一种新型一步法成形碳配分一体化工艺,即热冲压-动态碳配分(HS-DP)工艺。所提出的HS-DP工艺采用盐浴热处理的方式进行物理模拟。采用扫描电子显微镜(SEM)、X射线衍射(XRD)和拉伸试验等方法,研究了新工艺中的冷却速率对低碳先进高强钢的微观组织和力学性能的影响。针对冷却速率对残余奥氏体含量的影响进行了分析,重点研究了残留奥氏体的体积分数和碳含量对钢伸长率的影响。结果 经过HS-DP工艺处理的钢显微组织主要由初始淬火态马氏体相、最终淬火态马氏体相和残余奥氏体相共存组成。结论 实验钢表现出优异性能,说明了热冲压动态碳配分工艺前景广阔。