Showing papers on "Microalloyed steel published in 1999"

Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels

Abstract: A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (Vm) were produced by intermediate quenching of a boron- and vanadium-containing microalloyed steel The volume fraction of martensite was varied from 03 to 08 by changing the intercritical annealing temperature The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at ∼055 Vm A further increase in Vm was found to decrease the yield and tensile strengths as well as the impact properties It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with Vm>055 Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation

The Influence of Composition on the Hot Ductility of Steels and to the Problem of Transverse Cracking

Abstract: A review of the literature has been made, concentrating on the influence of N, V, Ti and the residuals, Cu, Sn, S and P, on the hot ductility of steels Nitrogen is generally detrimental to ductility in Al containing and microalloyed steels; to avoid transverse cracking the N levels should be kept as low as possible When Ti additions are made to low N, C-Mn-Al steels, (0005 % N) the best ductility is likely to be given by a high Ti : N ratio of 4-5 : 1 For high N steels (001 % N), a low Ti level (001 %) is recommended to reduce the volume fraction of Ti containing particles, but allow precipitation to occur at high temperatures In addition a low soluble Al level is needed to prevent the excess N from combining to form AlN For C-Mn-Nb-Al steels, similar recommendations can be made with regard to adding Ti However, the presence of Nb and Al appears to have little influence on the ductility, since these elements coarsen the Ti containing precipitates Furthermore, once the Ti has combined with the N any remaining N now seems beneficial, possibly because high N contents encourage precipitation at higher temperatures V as a microalloying addition gives better hot ductility than Nb and the product of the V and N has to approach 12×10-3, eg 01 % V and 0012 % N before ductility deteriorates to that normally shown with a 003 % Nb addition S levels should be kept low to reduce the detrimental precipitation of fine sulphides at the γ boundaries Care must be taken with Ca treatment as if the steel is too "clean", none of the Nb may be able to precipitate out prior to straightening Residual levels of Cu> 015% and Sn are detrimental to surface quality and encourage transverse cracking It is found that Cu is deleterious to ductility, but Ni can be added to compensate for this loss It is recommended that the amount of Ni added should be greater than the residual Cu level to be sure of avoiding problems, (Ni : Cu ratio nearer to 15 to 2) Phosphorus although potentially beneficial to ductility can not be recommended unless its segregation during solidification can be reduced

The rôle of vanadium in microalloyed steels

Abstract: The overall objective of the present paper is to review the role of V in microalloyed steels with a particular address as to how it affects microstructural evolution and mechanical properties. Its role in thermomechanical controlled processing (TMCP) is emphasised. The review is largely based on work carried out at the Swedish Institute for Metals Research (SIMR) during the last 25 years, but also includes reference to other relevant, published work. A specific aim is to demonstrate the present scientific knowledge of the subject. Therefore the understanding and interpretation of essential phenomena related to microstructure formation and properties are thoroughly examined, ranging from the influence of microalloying on prevention of austenite grain growth and recrystallisation, to precipitation in ferrite and its effect on strength. Within the well-known thermodynamic database, Thermocalc, a special microalloy database has been developed at SIMR, allowing reliable predictions of phase equilibria and thermodynamic functions for phase transformations in microalloyed steels. A comprehensive account is given of the role of V in the most important processing steps that the microalloyed steels are subjected to, viz. TMCP, continuous casting and welding. Compared to the other microalloying elements, Nb and Ti, V exhibits essential differences. In particular, the solubility of its carbonitrides is much larger and the solubility of its nitride is about 2 orders of magnitude smaller than its carbide, contrary to Nb but similar to Ti. For optimal alloy design and thermomechanical processing, proper allowance must be made for these differences. To reach the maximum ferrite grain refinement, ∼4 μm, repeated recrystallisation in a series of rolling reductions is used in TMCP of V-microalloyed steels, so-called Recrystallisation-Controlled-Rolling (RCR), as opposed to traditional controlled rolling of Nb-steels where heavy rolling at low temperatures in the non-recrystallisation regime is the means of attaining grain refinement. RCR presents some important advantages, in particular a more economical hot rolling practice by allowing low reheating and high finishing temperatures. As compared to Nb, V has certain further advantages as a microalloying element due to its greater solubility in austenite. The tendency for hot cracking of cast slabs is much less pronounced and dissolution of coarse V(C,N) compounds is more easily achieved prior to hot rolling than for the corresponding NbC. It is demonstrated that the relatively large solubility of V(C,N) and the much lower solubility of VN than VC makes V an eminent choice for strong and easily controllable precipitation strengthening. A corollary of the difference in VN and VC solubilities is that N becomes an essential microalloying element in V-steels, because it largely determines the density of V(C,N) precipitation and thereby the degree of precipitation strengthening. Moreover, since pure ferrite dissolves more N than C, the total N-content of the steel is normally dissolved in the ferrite before V(C,N) precipitation, whereas only a fraction of the C-content, given by the austenite/ferrite or ferrite/ cementite equilibrium, is dissolved in ferrite. Hence, by precise additions of N, this circumstance facilitates the control of V(C,N) precipitation strengthening.

Modeling macro- and microstructures of gas-metal-arc welded HSLA-100 steel

Abstract: Fluid flow and heat transfer during gas-metal-arc welding (GMAW) of HSLA-100 steel were studied using a transient, three-dimensional, turbulent heat transfer and fluid flow model. The temperature and velocity fields, cooling rates, and shape and size of the fusion and heat-affected zones (HAZs) were calculated. A continuous-cooling-transformation (CCT) diagram was computed to aid in the understanding of the observed weld metal microstructure. The computed results demonstrate that the dissipation of heat and momentum in the weld pool is significantly aided by turbulence, thus suggesting that previous modeling results based on laminar flow need to be re-examined. A comparison of the calculated fusion and HAZ geometries with their corresponding measured values showed good agreement. Furthermore, “finger” penetration, a unique geometric characteristic of gas-metal-arc weld pools, could be satisfactorily predicted from the model. The ability to predict these geometric variables and the agreement between the calculated and the measured cooling rates indicate the appropriateness of using a turbulence model for accurate calculations. The microstructure of the weld metal consisted mainly of acicular ferrite with small amounts of bainite. At high heat inputs, small amounts of allotriomorphic and Widmanstatten ferrite were also observed. The observed microstructures are consistent with those expected from the computed CCT diagram and the cooling rates. The results presented here demonstrate significant promise for understanding both macro-and microstructures of steel welds from the combination of the fundamental principles from both transport phenomena and phase transformation theory.

Recrystallization-Precipitation Interaction Study of Two Medium Carbon Niobium Microalloyed Steels

Abstract: Recrystallization-precipitation-time-temperature (RPTT) diagrams for strains of 0.20 and 0.35 have been determined for two microalloyed steels with niobium percentages of 0.024 and 0.058, respectively, and equal percentages of carbon and nitrogen. The method known as "back extrapolation" has been used for the determination of static recrystallization kinetics and also for the plotting of the diagrams. While a single plateau was observed on the recrystallized fraction against time curves for the first steel, as a consequence of strain induced precipitation, these curves for the second steel showed the formation of a double plateau, whose interpretation, confirmed by calorimetric analysis, supposes the formation of two types of precipitates. The work uses transmission microscopy to show the precipitates which are formed in both steels, as well as the size most probably capable of inhibiting recrystallization. Finally, an analysis is made of the RPTT diagrams and of the large amount of information which they offer for designing a more appropriate rolling schedule in order to obtain finer precipitates and a better austenitic microstructure before the austenite→ferrite transformation.

The Effect of Chemistry on the Formation of Ultrafine Ferrite in Steel

Abstract: A thermomechanical process has been developed to produce ultrafine (1-2 μm) ferrite grains in hot rolled steel strip. This process involves rolling the steel at or just above the austenite to ferrite transformation temperature, and was applied to a variety of steel chemistries, including plain low and high carbon grades and steels containing microalloying elements such as Nb, Ti, B and Mo. Significant volume fractions of ultrafine grains were observed in all samples. However, it was found that the chemical composition of the steel slightly influenced the morphology and volume fraction of ultrafine grains formed in the surface layers of the strip, and significantly altered the microstructure formed in the core of the strip. In plain carbon grades, the level of ferrite refinement increased slightly as carbon content increased. In the case of the microalloyed steels, those containing Nb and Ti additions produced the greatest level of refinement. The volume fraction and level of refinement of the ultrafine ferrite grains also influenced the strengths of the steels.

Effect of epitaxial ferrite on yielding and plastic flow in dual phase steel in tension and compression

Abstract: A low carbon, microalloyed steel was heat treated to obtain dual phase microstructures containing constant levels of 18 and 25 vol.-% martensite at two levels of microstructural refinement and with varying epitaxial ferrite content. Tensile and compression tests were conducted at a strain sensitivity of 2 × 10-5. Elastic limits in tension and compression were indistinguishable and very low, suggesting that mobile dislocations were present in the ferrite as a consequence of stress relaxation processes. These mobile dislocations accommodated the volume increase accompanying the austenite to martensite transformation during heat treatment. Epitaxial ferrite had little effect on the 0·2% proof stress, but average proof stresses were generally higher in compression than in tension owing to residual stresses in the martensite and ferrite following heat treatment. The residual stresses calculated from this asymmetry in the proof stresses were small because of stress relaxation in the ferrite at the tempe...

Mechanical behaviour of strain aged dual phase steels

Abstract: Dual phase steels, characterised by good formability and excellent surface finish, are suitable for applications where processing involves cold deformation. In this context an investigation has been conducted into the cold deformation aging susceptibility of carbon steel API–5L –B and microalloyed steel API–5L –X52, both with dual phase microstructures. Changes in mechanical properties such as phase microhardness, ultimate tensile strength, and yield strength in both types of steel were observed at aging temperatures of 25, 80, and 150°C. This aging is associated with dislocation structures formed on ferrite grains in the vicinity of ferrite/martensite interfaces during intercritical treatments, which become preferential sites for solute atom diffusion.

Effects of induction hardening and prior cold work on a microalloyed medium carbon steel

Abstract: The torsional strength and microstructural response to induction hardening of a 10V45 steel with prior cold work was evaluated. The vanadium-microalloyed 1045 (10V45) steel was characterized in three conditions: as-hot-rolled, 18% cold-reduced, and 29% cold-reduced. Two of these evaluations, 10V45 as-hot-rolled and 10V45-18%, were subjected to stationary and progressive induction hardening to three nominal case depths: 2, 4, and 6 mm. All specimens were subsequently furnace tempered at 190 °C for 1 h. The martensitic case microstructures contained residual lamellar carbides due to incomplete dissolution of the pearlitic carbides in the prior microstructure. Torsional overload strength, as measured by maximum torque capacity, is greatly increased by increasing case depth, and to a lesser extent by increasing prior cold work level. Maximum torque capacity ranges from 2520 to 3170 N · m, depending upon induction hardening processing. Changing induction hardening processing from stationary (single-shot) to progressive (scan) had little effect on torque capacity.

The Effect of Thermal History on the Hot Ductility of Microalloyed Steels

Abstract: Transverse cracks in continuous cast steels can form if the hot ductility of the cast steel at the unbending stage is poor. To measure hot ductility, tensile specimens are usually reheated to a high temperature (preferably to the melting point), cooled to the test temperature and then isothermally fractured. In this work, high temperature tensile testing was used to determine the hot ductility of a Nb-Ti and a Ti-B microalloyed steel. However, instead of cooling directly to the test temperature after melting, the specimens were subjected to thermal histories typical of a continuously cast billet surface, and then, at the unbending temperature, subjected to a tensile test to fracture. In other words, physical simulations of the continuous casting procedure were performed. The results were compared with those generated by conventional isothermal tensile testing. For the isothermal tests, both steels exhibited a temperature range of low ductility. However, the physical simulations did not reveal such hot ductility behaviour. For both steels, almost all the physical simulation variants led to hot ductility values lower than predicted by the isothermal tests at the corresponding tensile test temperature. For the Nb-Ti steel, it was revealed that there is a critical minimum temperature, attained by the specimen during the thermal history, below which the hot ductility, measured at the tensile test temperature, is much reduced. It is assumed that this critical minimum temperature leads to the formation of grain boundary ferrite, which probably enhances the rate of formation of Nb precipitates, decreasing the hot ductility in this way. However, for the Ti-B steel, the effect of thermal history could not be explained in such a straightforward manner.

A mechanism for effect of vanadium on hardenability of medium carbon manganese steel

Abstract: The effect of additions of vanadium with simultaneous additions of other carbonitride forming microalloying elements (Ti, Nb, Zr) or Al on the hardenability of steels of composition Fe–0·4C–1·5Mn–(0·004–0·021)N (wt-%) was investigated. Using a thermodynamic model, the composition of the austenite and the amount of undissolved carbonitride formed by microalloying elements for each austenitising temperature were determined. The relationships between the hardenability parameters and the calculated austenite composition as well as the amount of undissolved carbonitride were studied. The optimum conditions for increasing the efficacy of vanadium as a hardenability agent were established.

Investigations of the solidification structure of continuously cast slabs

Abstract: A study of solidification phenomena performed under industrial conditions is presented. The solidification structure of high carbon, medium carbon and microalloyed steel grades was observed and correlated to casting parameters such as superheat and secondary cooling. Experimental data of dendrite morphology are related to calculated solidification variables. The calculation is based on the solution of the enthalpy balance equation. An expression for the dependence of secondary dendrite arm spacing on solidification variables is proposed for columnar growth. The results are compared to the findings of other authors. The effect of chemical composition on secondary dendrite arm spacing was observed. Therefore the developed relationships are extended to carbon content on the basis of the presented experimental data.

New thermomechanical strategy for production of high strength low alloyed multiphase steel showing transformation induced plasticity effect

Abstract: In the past few years a lot of research was carried out on the development of transformation induced plasticity (TRIP) assisted multiphase steels. Two principal ways were proposed: (i) controlled cooling during the hot rolling process to obtain hot rolled TRIP assisted multiphase steels and (ii) the combination of intercritical annealing and isothermal holding at bainite formation temperatures during continuous annealing resulting in cold rolled TRIP assisted steel products. Unfortunately, both thermomechanical methods proposed require a high silicon level to inhibit cementite precipitation to avoid a loss of stability for the metastable retained austenite. In addition, due to high silicon levels, red scale surface defects appear and only moderate hot dip galvanisability is possible. In this paper a new thermomechanical strategy for the production of high strength low alloyed TRIP assisted multiphase steels with good hot dip galvanisability and without red scale defects will be presented.

Hydrogen Embrittlement of a HSLA-100 Steel in Seawater

Abstract: Hydrogen embrittlement of a copper precipitation strengthened and niobium microalloyed HSLA-100 steel on cathodic changing in synthetic seawater has been studied using slow strain rate technique. The effects of potential applied for hydrogen changing, pre-charging with hydrogen and changes in strain rate have been studied. A loss in ductility in terms of drop in percent elongation and percent reduction in area has been observed, the effect being prominent at potentials beyond –900 mV (SCE), SEM fractography shows an increase in brittle quasi-cleavage features with decreasing potential. A hardening effect on hydrogen charging up to –700mV (SCE), followed by a softening effect, has been observed. Precharging has led to a similar behaviour, but an overall increase in the strength values compared to material without precharging.

Effect of strain on recrystallisation–precipitation interaction in low vanadium microalloyed steel

Abstract: Using torsion tests and applying the ‘back extrapolation’ method in isothermal conditions, recrystallisation–precipitation–time– temperature (RPTT) diagrams have been determined for a microalloyed steel with 0·35%C, 0·033%V, and 121 ppm N. The RPTT diagrams provide abundant information about the recrystallisation–precipitation interaction. Data such as the minimum incubation time for precipitates and the corresponding recrystallised fraction, the temperature for the minimum incubation time, and the time necessary for recrystallisation to be completed are deduced from the RPTT diagrams. The present study is completed with the determination of the activation energy for recrystallisation before and after precipitation, arriving at a new concept of the phenomenon that establishes discontinuous variation both in the derived function t0·5 against the inverse of the temperature and in the function itself, where t0·5 is the time corresponding to a 50% recrystallised volume fraction.

Sn segregation and its influence on electrical steel texture development

Abstract: During the recrystallization microalloyed Sn in non-oriented electrical steel segregates to the surface and on grain boundary and affects the texture development. In spite of the fact that the grain boundary segregation is much smaller compared to surface segregation, both have an influence on recrystallization and on texture development in electrical steel. Auger electron spectroscopy (AES) was used to measure the grain boundary and surface segregation of Sn in non-oriented electrical steels alloyed with different Sn weight contents (0.025, 0.05 and 0.1 %). The grain boundary segregation of the specimens, which were previously aged at 530°C for various times and were fractured under UHV conditions, was measured. The surface segregation temperature dependence and its kinetics were followed in polycrystalline specimens in the temperature range from 400 to 900°C on the grains of known crystallographic orientations: (100), (111) and (110). The textures were measured by X-ray texture goniometer and the results were presented as orientation distribution functions (ODF). By controlled surface and grain boundary segregation it is possible to achieve the selective grain growth which improves the electrical properties of non-oriented electrical steel. The best results were obtained by alloying it with 0.05 wt% Sn.

Evolution of substructure during continuous rolling of microalloyed steel strip

Abstract: With the aim of investigating the evolution of the substructure, the rolling of a steel strip containing 0·13%C, 0·75%Mn, and 0·048%Nb (all wt-%) was interrupted and the partially rolled strip water quenched. Using X-ray diffraction spectra, subgrain size and dislocation density were determined for specimens taken before and after the rolling passes. Both these microstructural characteristics were related to the rolling temperature and the amount of NbC precipitation. The distribution of precipitates was investigated via TEM. At lower rolling temperature, particles precipitated in rows in the austenite temperature range.

Microstructures of heat-affected zone in niobium containing steels

Abstract: In order to understand the effect of niobium additions on the microstructures of the HAZ (heat-affected zone) in mild steels, a plain C-Mn steel (without niobium content) and three niobium containing steels with 0.01, 0.02 and 0.04 mass%Nb, respectively were investigated through simulated HAZ experiments with the heat inputs of 2 × 10 6 , 5 × 10 6 and 8 × 10 6 J / m. The microstructures of simulated coarse-grained HAZ have been examined by optical metallography and transmission electron microscopy. It was found that the addition of niobium had a significant effect on the transformation. For high energy heat inputs (8 × 10 6 and 5 × 10 6 J/m), the additions of niobium evidently retarded the pearlite formation even in the case of 0.01 mass%Nb containing steel; the microstructures of coarse-grained HAZ of niobium-containing steels consisted mainly of secondary Widmanstatten ferrite, but that of the niobium-free steel comprised a large amount of pearlite besides secondary Widmanstatten ferrite. In the condition of low energy heat input (2 × 10 6 J/m), the microstructures of coarse-grained HAZ of niobium-containing steels were all similar and consisted mainly of interlocking ferrite plates with small amounts of bainite and Widmanstatten ferrite; while that of the niobium-free steel comprised Widmanstatten ferrite and martensite with a small quantity of pearlite. The results from Charpy impact test indicated that the niobium-containing steels treated with the heat input 2 × 10 6 J/m could possess higher toughness than those treated with the heat inputs of 5 x 10 6 and 8 × 10 6 J/m. It is proposed that the interlocking ferrite structure, which forms in niobium-containing steels under 2 × 10 6 J/m heat input, improves the toughness property.

New steels for new mills

Abstract: Thin-slab casting technology has profoundly changed the economics of steel production. In-line conversion of liquid metal into a finished product is accompanied by a series of cost-reducing steps. Because of low capital investment and high productivity, the new process has gained rapid worldwide acceptance. It is well suited for production of microalloyed high-strength low-allow steels. In steels containing vanadium, nitrogen is converted from impurity into a cost-effective alloy. In view of their low cost, and weight-reducing potential, vanadium-microalloyed steels may be an economically attractive replacement of plain carbon steels.

Computer Simulation and Control of Microstructure and Mechanical Properties in Hot Forging

Abstract: An analytical procedure for predicting the microstructure and mechanical properties of forged microalloyed steels is presented. Empirical relationships between recrystallized grain size, temperature and strain as well as the relationship between austenite grain size and ferrite grain size are obtained. These relations are expressed as an incremental formula and incorporated into thermo-coupled FEM code. By applying this analysis to practical multiple stage hot forging of flange companions or automotive parts made of microalloyed steel, the microstructure and hardness of forged parts can be predicted. The results of these predictions are in good agreement with the experimental results. The simulation is used to investigate the thermo-mechanical processes for forging functional parts with partially fine grain and high hardness.

Low cycle fatigue behaviour of low carbon microalloyed steel: microstructural evolution and life assessment

Abstract: In this paper the cyclic stress–strain response, low cycle fatigue (LCF) behaviour, and evolution of dislocation structures under LCF loading in the case of a low carbon microalloyed steel are discussed. The cyclic stress response revealed cyclic softening resulting from the propagation of Luders bands. The experimental LCF life was compared with the life predicted using Tomkins' model and the modified universal slopes (MUS) equation. While the life predicted by Tomkins' model showed good correlation with the experimental results, the life predicted using the MUS equation grossly overestimated the life. Inclusion induced delaminations under cyclic loading were thought to be responsible for the overestimation by the MUS equation. Low energy dislocation structures, i.e. cells, were observed near the fracture surfaces. Interrupted tests revealed cell formation after 10 cycles at a total strain amplitude of 0·3%.

Advanced microscopy techniques resolving complex precipitates in steels

Abstract: Scanning electron microscopy as well as analytical transmission electron microscopy techniques such as high resolution, electron diffraction, energy dispersive X-ray spectrometry (EDX), parallel electron energy loss spectroscopy (PEELS) and elemental mapping via a Gatan Imaging Filter (GIF) have been used to study complex precipitation in commercial dual phase steels microalloyed with titanium. Titanium nitrides, titanium carbosulfides, titanium carbonitrides and titanium carbides were characterized in this study. Both carbon extraction replicas and thin foils were used as sample preparation techniques. On both the microscopic and nanometric scales, it was found that a large amount of precipitation occurred heterogeneously on already existing inclusions/precipitates. CaS inclusions (1 to 2 μ m), already present in liquid steel, acted as nucleation sites for TiN precipitating upon the steel's solidification. In addition, TiC nucleated on existing smaller TiN (around 30 to 50 nm). Despite the complexity of such alloys, the statistical analysis conducted on the non-equilibrium samples were found to be in rather good agreement with the theoretical equilibrium calculations. Heterogeneous precipitation must have played a role in bringing these results closer together.