Effect of B and B + Nb on the bainitic transformation in low carbon steels

Development of new, advanced high and ultra-high strength bainitic steels requires the selection of the optimum balance of bainite promoting elements allowing the production of the desired bainitic microstructure over a wide range of cooling rates. The addition of boron or a combined addition of boron and niobium is well known to retard strongly the polygonal ferrite formation but very little knowledge has been acquired on the bainitic transformation. Therefore, the purpose of this study is to investigate the influence of boron and boron plus niobium on the bainite transformation kinetics, microstructural evolution and mechanical properties in a low carbon steel (Fe-0.05C-1.49Mn-0.30Si). Isothermal and continuous cooling transformation diagrams were determined and followed by a detailed quantitative characterisation of the bainite microstructure and morphology using complementary advanced metallographic techniques (FEG-SEM-EBSD, SIMS and TEM). The relationship between microstructure and hardness has been evaluated. Finally, results of SIMS and TEM analyses coupled with microstructural investigations enable to propose a mechanism to explain the effect of the synergy between boron and niobium on the bainitic transformation and the resultant microstructure.     

The effects of Nb and Mo addition on transformation and properties in low carbon bainitic steels

Four low carbon bainite steels were designed to investigate the effects of Mo and Nb addition on bainitic transformation, microstructures and properties by metallographic method and dilatometry. The results show that single Nb addition retards bainitic transformation in low carbon bainite steels, although it can improve strength by refining microstructures. Moreover, Mo addition is effective to improve the strength of low carbon bainite steel by promoting bainitic transformation and single Mo addition has a better strengthening effect than single Nb addition. Further, in Mo bearing steel, Nb addition refines bainite sheaves, but meanwhile hinders bainitic transformation because of smaller austenite grains. Consequently, the composite strengthening effect of Mo and Nb addition has little improvement compared with individual addition of Mo in low carbon bainite steels.     

Critical ausforming temperature to promote isothermal bainitic transformation in prior-deformed austenite

The effects of deformation temperature on phase transformation and microstructure in nanostructured bainite steel were studied. The results indicate that the deformed austenite with a strain of 0.3 at 300°C presents accelerated kinetics of bainitic transformation. However, the amount of bainite in ausformed austenite then reduces with the increase in deformation temperature. A critical deformation temperature, determining whether the bainitic transformation can be promoted, was found in deformed austenite. In addition, the thickness of bainite plate in deformed austenite reduces with the decrease in ausforming temperature. The adjacent bainite ferrite plates grow up interactively, and the intersection angle is about 60–73°. A lower ausforming temperature contributes to a more serious cross-growth phenomenon of bainite plates.     

Deformation induced ferrite transformation in low carbon steels

Deformation induced ferrite transformation (DIFT) is a kind of solid state transformation induced through deformation, which can be applied to be as the effective method to produce fine or ultrafine ferrite grains. This paper reviews the research progress in the theory and application of DIFT from five aspects: evidence and study methods, thermodynamics and kinetics, transformation mechanisms, factors influencing DIFT, application of DIFT in production of fine grained C–Mn steel and ultrafine-grained microalloyed steel.     

Microstructure and mechanical properties of bainitic low carbon high strength plate steels

This paper reports the results of an investigation of plates produced by the advanced thermomechanical processing (TMP) schedules, which were designed using the results of a laboratory study. There were two steel compositions that corresponded to X-80 with carbon contents 0.04 and 0.07 wt.%, respectively. The variation in microstructure, hardness, tensile properties and Charpy impact properties with TMP schedule were determined, and compared with the expected requirements for X-100 linepipe steel. The relationships between microstructure and mechanical properties were experimentally obtained and discussed.     

Hardenability effect of boron on carbon steels

Abstract

An attempt has been made to investigate the effects of aluminium, titanium, nitrogen, and boron on the hardenability of boron-bearing medium-carbon steels. It was found that not only titanium but also boron had the scavenging effect for dissolved nitrogen. A parameter designated as boron potential (B_p = x_B?11/14(x_N?14/47·9X_Ti), where x_B, x_N, and x_Ti are the total boron, nitrogen, and titanium content, respectively) was chosen to evaluate and predict the boron hardenability effect. From the results of this study, boron potential is a parameter useful in discerning the optimization of contents of titanium, nitrogen, and boron for the development of boron steels having excellent hardenability.

MST/655     

Models for transformation plasticity in loaded steels subjected to bainitic and martensitic transformation

Abstract

The parameter k in the transformation plasticity model for bainitic and martensitic transformations is determined by experiments on four types of steel. The relationships between k and stress and chemical composition of the steels subjected to bainitic and martensitic transformations are obtained. In addition, based on experiments using 26Cr2Ni4MoV steel, the value of k is found first to increase with the increment of stress and then to remain unchanged when predeformation before transformation occurs.     

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.     

The effect of alloying elements on the structure and mechanical properties of ultra low carbon bainitic steels

Microalloying with Nb and B leads to a granular bainite microstructure which is composed of a bainitic ferrite matrix and a uniformly distributed martensite/austenite-constituent in the as-rolled condition. Due to this transformation strengthening mechanism, high strength and toughness could be achieved even though the C content was extremely low. It was found that both dissolved Nb in austenite and free B are prerequisites for granular bainite formation. Furthermore, there is a critical B content to achieve the complete bainitic transformation strengthening effect. The critical B content increases with C content. C thus diminishes the effect of B in promoting bainite transformation, due to the formation of boron carbides or the depletion of dissolved Nb in austenite. The effect of Mn, Mo and Ni on the decomposition of austenite is similar. A parameterMneq which relates the effect of these alloying elements on the Bs temperature was derived. It was confirmed that the strength of bainitic steels is inversely proportional to theBs temperature.     

The mechanism of the formation of boron ineffective zone and its effect on the properties of Ultralow carbon bainitic steels

In the manufacturing of ultralow carbon bainitic (ULCB) steels, boron is an alloying element which is essential to promote the desired bainitic transformation. In order to obtain this hardenability effect, boron must be in solution and it must segregate to the austenite grain boundary and where it decreases the contribution of the boundary's interfacial energy to ferrite nucleation. During the development of ULCB steels in China Steel Corporation, a small boron ineffective zone was found at the centre of steel plates. From electron-probe X-ray microanalysis (EPMA) and boron autoradiograph analysis, it was found that the formation of the boron ineffective zone was due to the centre-line segregation of inclusions which strongly combined with boron and formed a boron-free zone in its vicinity. The microstructure of the boron ineffective zone was a conventional ferrite with a strength which was much lower than that of the surrounding bainite. This resulted in crack separation in the tensile and impact specimens. It was found from a hydrogen-induced-cracking (HIC) test, that the HICs had a propensity to propagate along the boron ineffective zone. From a welding y-grooved test, a higher cold-cracking sensitivity at this boron ineffective zone was also found.     

Effect of quench aging on drawability in low carbon steels

Abstract

The equilibrium solubilities of C and N decrease sharply between 721°C and room temperature, although equilibrium is rarely approached during the cooling cycles employed industrially. The resulting supersaturated ferrite is thus susceptible to the precipitation of C and N during storage and subsequent processing. Such ‘quench aging’ effects are evaluated for three typical post-hot rolling cooling rates employed in the processing of low C steel wire rod. The investigation was carried out on steels containing 32 ppm N (low N) and 102 ppm N (high N); these were heated to 900°C, cooled at 8,3, or 1·5 K S^?l, and then aged at room or freezer temperatures for periods of between 1 h and 532 days. The specimens subjected to the fastest cooling rate exhibited considerable quench aging effects, as did industrially processed specimens that had been cooled in the mill and aged for several weeks at room temperature. It is shown that the high N steel is more susceptible to quench aging than the low N material. The implications regarding the occurrence of breaks during wire drawing are discussed.

MST/3175     

Influence of the chemical composition on transformation behaviour of low carbon microalloyed steels

In order to design thermomechanical schedules for processing low carbon microalloyed steels, the various critical transformation temperatures, i.e. the start and finish of the austenite transformation (A_r3, A_r1) and the non-recrystallization temperature (T_nr), must be determined. Continuous cooling torsion and compression testing are useful ways to measure these values. In this study six low carbon microalloyed steels with different additions (Nb, Cu, Si and Mo) were examined using these techniques. Moreover, the equilibrium phase diagrams for each alloy were calculated using FactSage. The comparison of the thermomechanical testing results with the thermodynamic calculations leads to a better understanding of the effect of the different elements on the transformation behaviour of pipeline steels. Regarding transformation temperatures, Cu in residual contents showed a strong effect on decreasing both A_r3 and A_r1, which indicates a hardenability effect of this element. On the other hand, increasing Nb contents increased T_nr by accelerating Nb(C,N) precipitation. However, when Si was added to a Nb-microalloyed steel, the T_nr decreased.     

Quantitative research on effects of stresses and strains on bainitic transformation kinetics and transformation plasticity

Abstract

In this paper, the effect of stress and strain on bainitic transformation kinetics and transformation plasticity have been studied quantitatively by means of experiments on a Gleeble-1500 testing machine. It is concluded that applied stresses will promote the evolution of bainitic transformation and increase the maximum volume fraction of the new phase and increase the value of the transformation plasticity parameter. Moreover, predeformation due to applied stress affects both the maximum volume fraction of the new phase and the value of the transformation plasticity parameter with increasing applied stress; but it has little effect on the variation of transformation kinetics parameter with the applied stresses.     

Effect of microstructure of low carbon steels on ultrasonic attenuation

The ultrasonic attenuation in low carbon steel with 0.04 wt% C to 0.80 wt% C was measured over a frequency range of 5 to 15 MHz, and the effects of the carbon content and normalizing temperature were analyzed. In pure iron, the attenuation is determined from the average grain size, which increases as the normalizing temperature increases; there is a noticeable effect caused by a few large grains. In the case of the hypoeutectoid steels, the proeutectoid ferrite grain, the size of which depends on prior austenite grain size, acts as the main scatterer. The prior austenite grain size increases as the carbon content decreases and the normalizing temperature increases. The colony is responsible for scattering in the eutectoid steel; scattering by pearlite is greater than that by ferrite.     

Effect of composition on kinetics of athermal martensite formation in plain carbon steels

Abstract

The kinetics of martensite formation in three Fe–C–Mn alloys has been determined using dilatometry, and is compared with the data for other steels reported in literature. Each curve can be well described by the Koistinen–Marburger equation using a composition dependent start temperature and rate parameter α _m. The empirical relationship derived for α _m as a function of the chemical composition can improve predictions with the Koistinen–Marburger model of the volume fraction martensite at a certain temperature.     

Effect of bainitic transformation on microstructure of Si-Mn steel

Several Si-Mn steels with similar Si and Mn levels and carbon contents, ranging from 0.25 to 0.75 wt %, were studied to determine the effect of bainitic transformation on the microstructure of Si-Mn steel. The microstructure was categorized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction. The results showed the existence of an optimum transformation time to produce the maximum content of retained austenite, though the retention of a large amount of retained austenite was encouraged as a result of bainitic transformation. The microstructure consisted of carbon-free upper bainite whose individual ferrite was separated by the thin-film type of retained austenite, while the blocky type of austenite was also found. The results also showed that carbide precipitation occurred in the residual austenite after the optimum time, which decreased the retained austenite content. The retained austenite stability is discussed in relation to the carbon content and morphology of the retained austenite.     

Effect of boron on hot ductility oflow carbon low alloyed steel

Abstract

The influence of boron on the hot ductility of C-Mn-Al-Cr steel has been investigated. At <980°C M(CB)₃ precipitated out and about half of the boron content was in solution in austenite at >900°C. It was found that solute boron atoms segregate to austenite grain boundaries and occupy the vacancies induced by deformation. This prevents the formation and propagation of microcracks at boundaries and results in improved hot ductility and a reduced dynamic recrystallisation temperature.     

Relationship between stress-induced martensitic transformation and impact toughness in low carbon austenitic steels

The effect of test temperature, which controls the stability of austenite, on the impact toughness of a low carbon Fe-Ni-Mn-C austenitic steel and 304 stainless steel, has been investigated. Under impact conditions, stress-induced martensitic transformation occurred, in a region near the fracture surface, at test temperatures below 80°C for the Fe-Ni-Mn-C steel and below –25°C for 304 stainless steel. The former shows significant transformation toughening and the highest impact toughness was obtained at 10°C, which corresponds to the maximum amount of martensite formed by stress-induced transformation above the Ms temperature. The stress-induced martensitic transformation contributes negatively to the impact toughness in the 304 stainless steel. Increasing the amount of stress-induced transformation to martensite, lowered the impact toughness. The experimental results can be well explained by the Antolovich theory through the analysis of metallography and fractography. The different effect of stress-induced transformation on the impact toughness in Fe-Ni-Mn-C steel and 304 stainless steel has been further understood by applying the crystallographic model for stress-induced martensitic transformation to these two steels.