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Showing papers on "Microalloyed steel published in 1998"


Journal ArticleDOI
TL;DR: In this paper, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined nonrecrystallization temperature (Tnr), and specimens are cooled at a constant cooling rate of 1 °C/s.
Abstract: It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (Tnr). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined Tnr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.

77 citations


Journal ArticleDOI
TL;DR: In this paper, the role of vanadium, nitrogen and carbon in controlling the microstructures and strength of steels designed for hot rolled long products was investigated, and it was confirmed that nitrogen is a very reliable alloying element, increasing the yield strength of V-microalloyed steels by some 5 MPa for every 0.001% N, essentially independent of processing conditions.
Abstract: The present work has concentrated on the roles of vanadium, nitrogen and carbon in controlling the microstructures and strength of steels designed for hot rolled long products. Effects of cooling rate and additional microalloying with titanium have also been included. The degree of precipitation strengthening of ferrite at a given vanadium content depends on the available quantities of carbon and nitrogen. The nitrogen content of the ferrite is approximately the same as that of the austenite from which it forms, i.e. the total nitrogen content in steel. It was confirmed that nitrogen is a very reliable alloying element, increasing the yield strength of V-microalloyed steels by some 5 MPa for every 0.001% N, essentially independent of processing conditions. Carbon content, on the other hand, has usually been considered not relevant to precipitation strengthening when the precipitation occurs in ferrite because of the very small carbon content in solution in ferrite at equilibrium. We demonstrate that the effective carbon for precipitation in ferrite may be much greater than this during the period of phase transformation, which in turn has a great effect on precipitation strengthening. Such behaviour is explained on the basis that the activity of carbon in ferrite is abnormally high in the presence of under-cooled austenite and before cementite nucleation so that profuse nucleation of vanadium carbonitride is encouraged. This new mechanism for precipitation is particularly significant for medium carbon steels typically used for hot rolled bars and sections. The total carbon content of the steel also contributes to the yield strength by increasing the volume fraction of pearlite. It is shown that the contribution from pearlite is stronger than generally recognised.

67 citations


Journal ArticleDOI
TL;DR: In this paper, a thermomechanical process was developed to produce ultrafine (1 μm) equiaxed ferrite grains in hot-rolled steel strip, which is applicable to a wide range of steel chemistries, including low and high carbon and microalloyed steels.
Abstract: A new thermomechanical process has been developed to produce ultrafine (1 μm) equiaxed ferrite grains in hot rolled steel strip. This process is remarkably simple and is applicable to a wide range of steel chemistries, including low and high carbon and microalloyed steels. Strips are reheated to produce a coarse austenite grain size, then rolled in a single pass at or just above the austenite to ferrite transformation temperature. It is suggested that the observed refinement is due to strain induced transformation from austenite to ferrite. The requirements for this appear to be high strain induced by shear in the strip surface layers, and thermal gradients created by heavy quenching of the strip surface by the work rolls. The yield strength was markedly higher than conventionally processed strip, although there was little work hardening even though total elongation of over 20% was achieved.

66 citations


Journal ArticleDOI
TL;DR: In this paper, a review of the metallurgical principles on which microalloyed forging steels are based, including the relationships between steel composition, thermomechanical processing, microstructure and resulting properties, highlighting the various strengthening mechanisms that are invoked.
Abstract: Microalloyed forging steels have been developed to improve the competitiveness of wrought steel components, especially in the automotive sector, by achieving the desired properties in the as-forged condition, thus eliminating the need to subsequently heat treat, straighten and stress relieve the previously specified low alloy steels. Significant cost reductions are realised by adopting microalloyed steels. This paper reviews the metallurgical principles on which microalloyed forging steels are based, including the relationships between steel composition, thermomechanical processing, microstructure and the resulting properties, highlighting the various strengthening mechanisms that are invoked. The properties, characteristics and applications of the initial development grade, 49MnVS3, are described. Research and development then focussed on increasing the strength and/or the toughness of this steel to improve its appeal to the market, especially for safety critical applications. the various metallurgical options are described and discussed. Attention has also been placed on maximising the machinability of these steels by controlled additions of sulphur, the adoption of inclusion modification techniques and other free machining additives. The fatigue properties and toughness of microalloyed steel forgings have been demonstrated to be fit for purpose, but compared with heat treated low alloy steels their fracture toughness is lower, albeit still significantly superior to castings. A wide range of forged automotive applications had been successfully converted to air cooled microalloyed steels over the past 25 years, with a large proportion of crankshaft and connecting rods now being made by this route. Future challenges have been identified to further extend the attainable properties and to improve the combination of strength and toughness, to broaden the market applications and the product range to include bar and rod. The use of warm near net shape forming processes for microalloyed steel is also anticipated. Greater exploitation of computer aided modelling and design techniques is encouraged to facilitate rapid prototyping, in order to improve further the competitiveness of forged engineering steels.

55 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of thermomechanical processing on the volume fraction and stability of retained austenite in a Nb microalloyed Mn-Si TRIP steel was investigated.
Abstract: The effect of thermomechanical processing on the volume fraction and stability of retained austenite in a Nb microalloyed Mn-Si TRIP steel was investigated. In one set of tests, specimens were isothermally held at 400 °C for a series of different times, then air cooled to room temperature. In another set of tests, specimens were continuously cooled to room temperature at a cooling rate of 2 °C/s or 10 °C/s. The former generated a mixed microstructure of ferrite + bainite + retained austenite, while the latter produced a microstructure of bainite + martensite + retained austenite. It was found that the stability of retained austenite against a strain induced transformation to martensite increases with increasing carbon concentration, decreasing size and hard surrounding phase.

53 citations


Journal ArticleDOI
TL;DR: In this paper, the isothermal transformation vs time of a medium-carbon microalloyed steel at 450 °C, following austenitization at 1250 °C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy and transmission electron microscope (TEM).
Abstract: The isothermal transformation vs time of a medium-carbon microalloyed steel at 450 °C, following austenitization at 1250 °C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase. The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of upper bainite formation.

53 citations


Journal ArticleDOI
TL;DR: In this paper, the micromechanisms involved in cleavage propagation in the brittle and ductile-brittle behavior of microalloyed steels are analyzed and the cleavage process is divided into three different steps which must take place dynamically in order to succeed: nucleation of a micro-crack in a microstructural feature, propagation of the microcrack across the particle-matrix boundary and finally, propagation across the matrix surmounting high angle boundaries.
Abstract: The micromechanisms involved in cleavage propagation in the brittle and ductile-brittle behaviour of microalloyed steels are analysed. The cleavage process is divided into three different steps which must take place dynamically in order to succeed: nucleation of a microcrack in a microstructural feature, propagation of the microcrack across the particle-matrix boundary and finally, propagation across the matrix surmounting high angle boundaries. The microstructural parameters involved in the three steps are considered, together with their effective surface energy values. Depending on testing temperature, the microstructural feature controlling cleavage fracture changes. In relation to the ductile-brittle transition, the main parameter is the volume fraction of coarse microstructural units surrounded by high angle boundaries.

41 citations



Journal ArticleDOI
TL;DR: In this paper, the effects of thermomechanical processing (TM P), accelerated cooling and composition (Mn+Si levels) on γ transformation start temperature (Ar 3 ), phase transformation kinetics, CCT diagrams and microhardness are investigated.
Abstract: Continuous cooling transformation (CCT) behaviour of high strength microalloyed steels containing two different levels of Mn+Si additions is investigated in undeformed and thermomechanically processed conditions using quench and deformation dilatometry respectively. The deformation schedule used in the dilatometer is designed to simulate the industrial controlled rolling procedures for the production of plates as closely as possible in laboratory. CCT diagrams for the undeformed and thermomechanically processed steels are constructed. Effects of thermomechanical processing (TM P), accelerated cooling and composition (Mn+Si levels) on γ transformation start temperature (Ar 3 ), phase transformation kinetics, CCT diagrams and microhardness are investigated. The results show that TMP accelerates the onset of γ/α transformation (Ar 3 is raised), but the progress of γ/α transformation is retarded considerably in deformed samples. Significant retardation is observed during the final 30% of the phase transformation reaction. Increase in cooling rate lowers the Ar 3 significantly and accelerates the progress of transformation. The steel with a higher level of Mn+Si addition (1.96%) exhibits lower Ar 3 , sluggish transformation kinetics and higher hardnesses in undeformed and thermomechanically processed conditions as compared with the steel with a lower level of Mn+Si addition (1.17%). These effects are explained in terms of the effects of Mn and Si contents on the carbon partitioning and the subsequent phase transformation behaviour of these steels during continuous cooling. Increase in cooling rate increases the microhardnesses of both steels while TMP lowers them.

37 citations


Journal ArticleDOI
TL;DR: In this article, the effect of low-Sigma CSL boundaries on the nucleation and propagation of cracks during deep-draw forming operations in ultra-low carbon microalloyed steels was evaluated.

35 citations


Journal ArticleDOI
TL;DR: In this paper, the solubility of niobium in microalloyed austenite has been analyzed using a thermodynamic description of the Fe-Nb-C-N system.
Abstract: The solubility of niobium in microalloyed austenite has been analyzed using a thermodynamic description of the Fe-Nb-C-N system. The description of this system forms an important basis for calculations of precipitation of niobium carbonitrides in microalloyed steels. Previously presented thermodynamic descriptions are combined with a new description of the Fe-Nb-N system, and equilibria in the quaternary system are calculated. New experiments were performed on Nb, Nb-Ti, and Nb-Ti-V microalloyed steels to confirm the calculated results. The results of theoretical calculations show good agreement with the experimental data on dissolution/precipitation of Nb carbonitrides in microalloyed austenite.

Journal ArticleDOI
TL;DR: In this paper, the effect of zirconium carbonitrides on grain coarsening behavior in controlled rolled Al-killed Zr and Zr-Nb microalloyed HSLA steels with various Zr/N ratios (2.8-22) has been studied, and compared with those of Ti-bearing carbonit rides in Ti and Ti Nb-Zr steels.
Abstract: The effect of zirconium carbonitrides on austenite grain coarsening behaviour in controlled rolled Al–killed Zr and Zr–Nb microalloyed HSLA steels with various Zr/N ratios (2.8–22) has been studied, and compared with those of Ti-bearing carbonitrides in Ti–Nb and Ti–Nb–Zr steels. TEM observations and PEELS analysis showed that, in the Zr and Zr–Nb steels, with hypo Zr additions(Zr/N=2.8–6.3), the Zr-bearing particles were large ZrN-rich carbonitrides with sizes from ≈100 nm to several microns and of irregular shape, while with hyper Zr additions(Zr/N=15–22), which were far from stoichiometry (Zr/N=6.5), fine spherical particles of Zr-rich Zr carbonitrides(10–100 nm) were formed together with many more large ZrC-rich carbonitrides. Moreover, only in the steel with the lowest Zr/N ratios, i.e. 2.34 for the 0.011 Zr steel, and 2.75 for the 0.022Zr–Nb steels, was a significant fraction of AlN formed. Austenite grain coarsening occurred around 1050–1100°C in all the hypo Zr and Zr–Nb steels because very few small carbonitrides(

Journal ArticleDOI
TL;DR: In this paper, the effect of Mo in Si-Mn TRIP steels was investigated and it was shown that Mo has an important retardation effect on the formation of both ferrite and pearlite.
Abstract: Experimental studies were performed to determine the effect of Mo in Si-Mn TRIP steels, as well as its potential for reducing the levels of Si. Three compositions were investigated. Bainite transformation conditions were investigated on the final mechanical properties. Results revealed that Mo has an important retardation effect on the formation of both ferrite and pearlite. The Mo and reduced Si level steel generated excellent mechanical properties (as high as: UTS=1269, T.El.=36%) in the range observed by previous investigators.

Journal ArticleDOI
TL;DR: CENIM's authors acknowledge financial support from the Spanish Comisio´n Interministerial de Ciencia y Tecnologi´a (CICYT) (project-PETRI 95-0089-OP) as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, the influence of finish deformation temperature in the range of 650-820°C and strain on microstructure and final mechanical properties of Nb steel is investigated.

Journal ArticleDOI
TL;DR: In this article, a combined diffusion model has been suggested to describe the growth mechanism of titanium nitride precipitates in the CGHAZ of a high nitrogen Ti-V microalloyed steel weld.
Abstract: A high nitrogen, Ti–V microalloyed steel was found to produce coarse-grained heat affected zone (CGHAZ) with impact toughness superior to a similar Ti–V steel with lower nitrogen content. The CGHAZ microstructures of the high nitrogen steels were investigated to determine that precipitation in this region played an important role for the improvement of the mentioned toughness. The nitrogen-enhanced steel, with 130 parts per million (ppm) nitrogen, yielded a large number of fine nitride precipitates with sizes ranging from 30 to 900 A (3 to 90 nm). These cubic or rectangular-prism shaped, titanium-rich particles limited the austenite grain growth at high temperatures. The average austenite grain diameter of 50 μm at 1350°C was about three times smaller than that observed in the low nitrogen steel with 30 ppm nitrogen. The small grain size in the CGHAZ of the nitrogen-enhanced steel controlled the toughness. Instead of nitrides, the low nitrogen steel CGHAZ exhibited a large number of titanium or aluminum oxide particles. An important observation was that the particle number density of the nitrogen-enhanced steel decreased slowly, from 4.3×10 6 to 3.3×10 6 per mm 2 , with increasing holding time at 1350°C. Ostwald ripening alone was unable to explain the particle density change and precipitate growth. It was then determined that most TiN particles smaller than 420 A (42 nm) were located along the austenite grain boundaries. Thus, coarsening of these finer TiN precipitates at the grain boundaries would be determined by grain boundary diffusion and not lattice diffusion. In the case of welding, with thermal cycles characterized by high peak temperatures and short holding times, much of the titanium and nitrogen atoms would be expected to remain in solution, albeit in supersaturation. Hence, nitride particles larger than 420 A (42 nm) and located inside the austenite grains would receive solute atoms precipitating directly from the supersaturated matrix. Only after this supersaturated concentration reaches the equilibrium solubility that the effect of Ostwald ripening would become predominant in regulating the size distribution of the precipitates. In this paper, a combined diffusion model has been suggested to describe the growth mechanism of titanium nitride precipitates in the CGHAZ of a high nitrogen Ti–V microalloyed steel weld.

Journal ArticleDOI
TL;DR: In this article, the authors used torsion testing and thermal analysis to obtain information about the behavior of the austenite transformation after the samples had been deformed by torsions according to a rolling schedule.

Journal ArticleDOI
TL;DR: In this paper, the effect of a strain rate change on the flow stress and the subsequent softening kinetics was investigated, and a reasonable agreement was obtained between the results of both the stress relaxation and double-compression methods.
Abstract: Data on the static and post-dynamic recrystallization have been determined in five Ti-microalloyed steels. Both the stress relaxation and interrupted deformation techniques have been employed. The effect of a strain rate change on the flow stress and the subsequent softening kinetics was also investigated. A reasonable agreement is obtained between the results of both the stress relaxation and double-compression methods, which further confirms the reliability of the stress relaxation technique. The results indicate that steels with plain Ti or with Ti-Ni-V or Ti-Ni-Cu alloying recrystallize at temperatures above 900°C (pass reduction ≥0.15) for interpass times characteristic of plate rolling, but Nb (ca. 0.03%) retards the recrystallization rate so that the final rolling temperature should be about 1000°C for full recrystallization between passes. The characteristics of static and metadynamic recrystallization are distinctly different. Softening becomes independent of strain and highly dependent on the strain rate even at strains leading to a small fraction of dynamic recrystallization. Nb has only a small retarding effect in metadynamic recrystallization. The flow stress level and softening kinetics are independent of the strain rate history only being dependent on the final strain rate.

Journal ArticleDOI
TL;DR: In this paper, two low carbon microalloyed steels were investigated: 0.07C-0.33Mn and 0.028Nb-1.1Mn.
Abstract: Two low-carbon microalloyed steels were investigated: 0.07C-0.02Nb-0.33Mn and 0.07C-0.028Nb-1.1Mn. The two steels were hot rolled in a similar way, followed by either air cooling to room temperature (ACRT) or accelerated cooling (AC) to 680°C and held isothermally at 650°C for various times prior to ACRT. The mechanical properties of the steels in their fully-processed condition were evaluated. As expected, the steel with the higher Mn content showed the highest tensile (YS and UTS) properties for all processing conditions. The results of this study also indicated that the tensile properties of both steels remained unchanged, prior to and after the isothermal treatments. The central goal of this study was to conduct a detailed microstructural analysis which could help in the understanding of the observed mechanical properties. The results from the ACRT samples after hot rolling revealed two distinct precipitation sequences: (i) precipitation of fine NbC (5 nm in average size) in austenite, and (ii) interphase and multivariant precipitation of NbC in ferrite. The samples which were AC and then held isothermally also exhibited fine NbC precipitates. All the precipitates observed in these samples had formed in austenite. The absence of the Baker-Nutting (B-N) orientation relationship clearly confirmed that precipitation in ferrite did not take place. Atom Probe analysis of the precipitates revealed that the stoichiometry of the precipitates was close to NbC 0.9 . From the detailed TEM microstructural analysis, all the hardening increments contributing to the yield strength of the steels were computed the results showed a good agreement with the measured tensile properties.

Journal ArticleDOI
TL;DR: In this article, the relationship between the structure and the mechanical properties of the carburized and heat-treated specimens was investigated by means of optical microscopy, surface hardness and microhardness measurements, X-ray diffractometry and impact tests.

Journal ArticleDOI
TL;DR: In this paper, a technique was developed to simulate the cooling conditions experienced during the casting of thin slabs 50mm in thickness in small laboratory ingots, and the cast ingots were deformed in a three point bend mode using strain rate conditions expected during the straightening of thin slab.
Abstract: One of the problems associated with the thin slab casting process route is the formation of surface defects such as transverse cracks in the continuously cast slab. The objective of this work was to determine the likelihood of developing transverse surface cracks for various microalloyed steel types under thin slab casting conditions. A technique was developed to simulate the cooling conditions experienced during the casting of thin slabs 50mm in thickness in small laboratory ingots. To simulate the slab straightening procedure, the cast ingots were deformed in a three point bend mode using strain rate conditions expected during the straightening of thin slabs. To generate significant cracking, surface strains greater than those expected during continuous casting had to be applied. The tests were conducted over a range of temperatures, and the degree of surface cracking assessed. Five steel types were examined based on 0.06%C, 1.5%Mn and with additions of V, Nb and N. Small cracks, up to 2mm long, were observed at austenite grain boundaries for all the steels across a wide temperature range. However, the Nb, V-N, and V-Nb steels exhibited cracks which were longer than this in some parts of the temperature range. The Nb steel exhibited the longest cracks (17mm at 820°C) over the widest temperature range (725-875°C) while the V-N and V-Nb steels exhibited cracks which were up to 15mm and 12mm long, respectively, in the temperature range 775-825°C. The most severe cracking was associated with the austenite-ferrite transformation, together with detrimental precipitate distributions. It was concluded that transverse cracking would be most likely to occur in the colder regions of the slab such as the slab corner, or in areas with a high level of water impaction.

Journal ArticleDOI
TL;DR: In this paper, the authors focused on the particular contributions of different microalloying additions with optimized deformation schedules to improve the mechanical properties achievable through this new technology, and presented a new post forging treatment (two-step cooling, TSC) as a promising alternative to the CC of medium carbon micro-alloyed steels.
Abstract: In the last years, the automotive industry has been demanding a continuous reduction of weight in order to compensate for the introduction of new constructive components according to higher requirements on passenger safety and driving comfort, as well as to accomplish with new strict environmental regulations. The adoption of new materials with improved mechanical properties and/or lower density, together with modern cost effective processing technologies have contributed greatly to reach the above objectives [1]. Particularly microalloyed steels, which may provide a higher strength with small alloying additions and low heat treatment expenses, have evolved into an important category of engineering materials suitable for the manufacturing of hull, frame and motor components. As an example, the classical quench and tempering (QT) of alloyed steels followed by straightening operations and stress relieving cycles has been progressively substituted by the continuous cooling (CC) of more economical microalloyed steels from the forging temperature. An accurate control of the cooling rate adjusts the properties of the ferritic-pearlitic microstructure obtained. This technology results in lower processing costs and eliminates product rejectances due to quench-cracking. However there are limitations in the mechanical properties achievable by this processing route if compared with the QT-operation [2], and hardly any cooling rate may be encountered which would lead to a balance of strength and ductility comparable to that after QT. In this context a new post forging treatment (two-step cooling, TSC) has been presented elsewhere [3] as a promising alternative to the CC of medium carbon microalloyed steels. This paper is focused on the particular contributions of different microalloying additions with optimized deformation schedules to improve the mechanical properties achievable through this new technology.

Journal ArticleDOI
TL;DR: In this paper, a finer effective grain size than typical for thermomechanically rolled steels with a ferritic-pearlitic microstructure is achieved with microstructures of acicular bainite or martensite.
Abstract: Costs and time savings in the fabrication of constructions by using steel grades with improved weldability and higher strength lead to an increased realisation of steel structures. But the higher strength can only be used, provided the construction is safe, by avoiding any brittle fracture as well as the propagation of existing flaws by ductile fracture. Besides a high cleanliness of the steel, a low carbon content and the maximisation of grain refinement as strengthening mechanisms are the fundamentals of modem HSLA steel production. As result of the thermomechanical processing of microalloyed steels, high strength levels with a rather low carbon content can be produced. Actually a chemical composition with less than the 0.09% C threshold value is aimed for, in order to avoid the peritectic reaction during solidification, which is responsible for microsegregations and thus a deterioration of the heat affected zone toughness. A finer effective grain size than typical for thermomechanically rolled steels with a ferritic-pearlitic microstructure is achieved with microstructures of acicular bainite or martensite. The alloy design of such steels, which can guarantee yield strength levels of 690 MPa and more, asks for an overall higher alloy content or a faster cooling rate or combinations of both. In any case, austenite processing before transformation adds to the refinement of these microstructures. The industrial verification relies on interrupted accelerated cooling or direct quenching after thermomechanical rolling. Furthermore, the toughness in the heat affected zone is improved with a microstructure of low carbon acicular bainite.

Journal ArticleDOI
TL;DR: Using torsion tests, the no-recrystallisation temperature Tnr was determined for two cooling rates in a Nb-V-Ti microalloyed steel.
Abstract: Using torsion tests, the no-recrystallisation temperature Tnr was determined for two cooling rates in a Nb-V-Ti microalloyed steel. The same tests made it possible to deduce the Ar3 phase transformation temperature. When Tnr is known, it is possible to establish approximately the conditions for which the strip rolling or plate rolling of this steel must be performed to obtain the appropriate microstructure. With the aid of a recrystallisation model and the results obtained in other work, a discussion is made of the concept of Tnr the conclusion being that it represents the temperature above which the fraction recrystallised between consecutive passes reaches a value close to 50% and not 100%. This parameter, and the value of Ar3, can be determined at minimal cost and in a relatively short time. Both economic and time aspects are of interest to any steel manufacturer producing hot rolled microalloyed steels.

Journal ArticleDOI
TL;DR: In this paper, the transformation behavior of a C-Mn-Nb steel deformed by torsion at temperatures both above and below the non recrystallisation temperature for this steel has been studied in detail.
Abstract: In the present work the transformation behaviour of a C-Mn-Nb steel deformed by torsion at temperatures both above and below the non recrystallisation temperature for this steel has been studied in detail. After deformation, specimens were cooled at a constant cooling rate of 1°C/s, and interrupted quenching from different temperatures was used to observe different stages of transformation. In the case of transformation from deformed austenite, in addition to the austenite grain boundaries, the deformation bands and twin boundaries act also as preferential nucleation sites for the ferrite. It has been observed that in all the cases when impingement becomes important, the number of grains per unit volume decreases significantly during transformation denoting that ferrite coarsening is taking place.

Journal ArticleDOI
TL;DR: In this paper, the constitution of austenite in thin-slab cast, low-C, Mn steel microalloyed with 0.007wt%Ti, 0.04wt%Nb and 0.011wt%V was simulated in small, laboratory castings.
Abstract: The constitution of austenite in thin-slab cast, low-C, Mn steel microalloyed with 0.007wt%Ti, 0.04wt%Nb and 0.011wt%V was simulated in small, laboratory castings. Two carbon levels, 0.007wt% and 0.22wt%C, each at two nitrogen levels, 0.003wt% and ∼0.013wt%N, were investigated. After solidification the castings were quenched rapidly from ∼1400°C in order to investigate the eutectic carbonitrides. Samples of the castings were reheated to a range of temperatures in the austenite phase field to investigate the fine precipitates that then appeared, and the re-solution of the eutectic carbonitrides. The compositions of precipitates were determined using high resolution electron microscopy with EDX and PEELS, and compared with the results of a computer model of the solution thermodynamics of (Ti x Nb v v 1-x-v )(C y N 1-y ) in austenite. The eutectic carbonitride formed only in the higher-C steels independently of N content. However, increasing the nitrogen content of the steel increased the N:C ratio in the eutectic carbonitride. At both nitrogen levels the carbonitride was almost pure Nb(CN). After allowing for strong segregation of Nb and weak segregation of Ti to the liquid during freezing, these results agreed with the thermodynamic model, assuming the eutectic carbonitride to be in equilibrium with austenite. The presence of the eutectic carbonitride had little effect on subsequent precipitation in reheated austenite. The new precipitates were Nb-rich at low and intermediate temperatures, with some V at the lowest reheating temperatures. With increasing reheating temperature, first the V content and then the Nb content of these precipitates decreased, and their Ti content increased, as they also became N-rich, in approximate agreement with the computer model. At temperatures above approximately 1200°C the Ti tended to form Ti-rich Ti-Mn-oxides. The eutectic precipitates partially dissolved, but remained Nb-rich on reheating.

Journal ArticleDOI
TL;DR: In this paper, a series of V, Nb/V and Nb containing steels having a base composition of 0.1 %C, 1.4%Mn, 0.3 %Si have been examined.
Abstract: The hot ductility behaviour of a series of V, Nb/V and Nb containing steels having a base composition of 0.1 %C, 1.4%Mn, 0.3 %Si have been examined. Two levels of V, (0.05 and 0.1%), N (0.005 and 0.01%), and Nb, (0.015 and 0.03%Nb) were selected. Tensile samples were heated to 1330°C and cooled at 50°C/min to test temperatures in the range 700 to 1000°C and tested to failure using a strain rate of 3x10 -3 s -1 . Heat treatment was chosen to closely follow the commercial continuous casting conditions and the strain rate used was that applied during the straightening operation. In addition, to make the tests more relevant to continuous casting, tensiles from selected steels were melted, re-solidified and cooled to the test temperature as for the samples solution treated at 1330°C. In all cases a ductility trough was obtained in the temperature range 750 to 950°C. The 0.030%Nb containing steel gave the worst hot ductility of all the steels examined. For samples heated to 1330°C, increasing the V and N levels in the V containing steels caused the ductility to deteriorate but the product of the total V and N concentrations had to be as high as 1.2 x 10 -3 , i.e. 0.1%V and 0.012%N, before approaching the low values of reduction of area exhibited by the Nb containing steel. The hot ductility curves for all the Nb/V containing steels at the 0.005% N level were very similar. Adding V at this low N level to the 0.03%Nb containing steel was found to improve ductility in the temperature range 800 to 900°C. For the as-cast samples, again the V containing steels were found to give better ductility than the 0.03%Nb containing steel. The hot ductility behaviour was found to be mainly related to the size of the precipitates, the coarser the precipitation the better being the ductility.

Journal ArticleDOI
TL;DR: In this article, the role of carbon in controlling the microstructure and strength of V-N structural steels containing 0.04-0.22%C has been investigated.
Abstract: The present work has concentrated on the role of carbon in controlling the microstructure and strength of V-N structural steels containing 0.04-0.22%C. Carbon content has usually been considered not relevant to precipitation strengthening when the precipitation occurs in ferrite because of the very small carbon content in solution in ferrite at equilibrium. We demonstrate that the effective carbon for precipitation in ferrite is much greater than this during the period of phase transformation, which in turn has a great effect on precipitation strengthening. Such behaviour is explained on the basis that the activity of carbon in ferrite is abnormally high in the presence of under-cooled austenite and before cementite nucleation so that profuse nucleation of vanadium carbonitride is encouraged. This new mechanism for precipitation is particularly significant for medium carbon steels typically used for hot rolled bars and sections. The total carbon content of the steel also contributes to the yield strength by increasing the volume fraction of pearlite. It is shown that the contribution from pearlite may be stronger than generally recognised.

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TL;DR: In this article, it has been shown that the high temperature behavior of medium carbon microalloyed steels can be accurately described by the classical hyperbolic sine relation provided the stresses are normalised by Young's modulus E(T) and the strain rates by the self-diffusion coefficient D(T).
Abstract: Two different behaviours are classically observed during the high temperature deformation of metals: i) power law creep and ii) exponential law creep. The first is observed at relatively low stresses and is considered as a deformation process controlled by diffusion. At higher stresses the above behaviour is converted into an exponential one, i.e. the power law breaks down. Both phenomena can be described by a single expression of the form: e = A(sinhασ) n .exp(-Q/RT) Here the parameters A, n, α and Q depend on the material being considered, and are usually referred to as apparent values because no account is generally taken of the internal microstructural state. In the particular case of microalloyed steels, a broad range of values have been reported in the literature for the latter constants, and clear trends have not always been evident. In recent work, it has been shown that the high temperature behaviour of medium carbon microalloyed steels can be accurately described by the classical hyperbolic sine relation provided the stresses are normalised by Young's modulus E(T) and the strain rates by the self-diffusion coefficient D(T). According to this formulation, only two parameters need to be determined to characterise the hot flow behaviour: A and α (n can be set equal to 5 for carbon steels). In the present work, the latter expression is extended to plain carbon and low carbon microalloyed steels, and applied to the peak and steady stresses of the flow curve. To attain this goal, experimental results corresponding to several different steels reported by many authors are employed. The effect of chemical composition on the above constants is derived statistically.

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TL;DR: In this paper, it is suggested that recrystallisation kinetics should be linearly, rather than quadratically, related to austenite grain size, and that above a limiting carbon content the addition of Nb and Ti results in excess, eutectically precipitated, Nb-rich carbonitride due to segregation during freezing.
Abstract: The direct hot rolling of the as-cast austenite of thin slab castings requires the microstructural changes within the austenite phase field that are necessary for a satisfactory product to be accommodated within a smaller degree of total reduction, within a smaller number of passes, and within a shorter time scale. Particularly in the case of microalloyed steel, the constitution of the as-cast austenite prior to rolling differs from that of austenite of thick slabs reheated for conventional hot rolling. Consequently, the models developed for conventional rolling employing reheated austenite cannot be relied upon for predicting microstructural change during the rolling of as-cast austenite. The as-cast austenite of microalloyed steels has larger grains, is segregated, and carbonitride precipitates may be present that are not in equilibrium with their solid solution. These factors are discussed in the light of recent research. It is suggested that recrystallisation kinetics should be linearly, rather than quadratically, related to austenite grain size, and that above a limiting carbon content the addition of Nb and Ti results in excess, eutectically precipitated, Nb-rich carbonitride due to segregation during freezing. The former leads to more rapid recrystallisation than would be predicted by the conventional models, and helps to ensure that plain C-Mn steels can be satisfactorily processed. The presence of eutectic carbonitrides means that the supersaturation of the austenite with respect to microalloy constituents is unknown, and recrystallisation-stop temperatures cannot be modelled satisfactorily.