Showing papers on "Microalloyed steel published in 2003"

Analysis of different acicular ferrite microstructures in low-carbon steels by electron backscattered diffraction. Study of their toughness behavior

Abstract: Acicular ferrite formation, promoted by the intragranular nucleation of ferrite plates, is well known to be beneficial for achieving a good combination of mechanical properties. However, the set of microstructures that can be obtained during the subsequent development of the transformation from the primary plates generated at particles can be quite complex and depends on a certain number of variables: steel composition, temperature range, prior austenite grain size, and particle density. In the present work, acicular ferrite microstructures have been produced by isothermal treatments in three different steels with different active particle types and densities. The morphology of the obtained intragranular microstructures has been found to depend on the steel composition, the prior austenite grain size, and the density of particles able to promote intragranular nucleation. Electron backscattered diffraction (EBSD) techniques have been used to define the microstructural unit controlling toughness in these types of microstructures.

Dynamic recrystallization behavior covering a wide austenite grain size range in Nb and Nb–Ti microalloyed steels

Abstract: The dynamic recrystallization (DRX) behavior of Nb and Nb–Ti microalloyed steels has been investigated. It has been observed that the initial austenite grain size, the amount of microalloying elements in solid solution, and the deformation conditions (temperature and strain rate), affect dynamic recrystallization kinetics. To characterize the dynamic recrystallization behavior of the microalloyed austenite, continuous torsion tests were carried out after reheating the specimens at different temperatures between 1000 and 1420 °C, leading to a wide range of initial grain sizes, from 16 to 805 μm. It has been observed that decreasing the values of the Zener–Hollomon parameter and the initial grain size, promotes dynamic recrystallization. Microalloying elements in solid solution produce a retardation of dynamic recrystallization, resulting in higher values of the characteristic critical, e c , and peak, e p , strains. A corrective factor has been applied to quantify the retardation produced by the increase in the amount of Nb and Ti dissolved as the reheating temperature increases. In this manner, it has been possible to propose a unique relationship to predict the e p peak strain for both steels.

Continuous cooling transformation of undeformed and deformed low carbon pipeline steels

Abstract: The continuous cooling transformation (CCT) behaviors of three low carbon pipeline steels containing the different carbon and alloy additions such as Mn, Nb, V, Ti and/or Mo were investigated in the undeformed and deformed conditions, respectively. The corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed, which almost involved the formation curves of bainitic ferrite, acicular ferrite, polygonal ferrite, and pearlite. It was found that with the exception of V, the aforementioned alloy additions played a significant role in suppressing the formation of polygonal ferrite and promoting the formation of acicular ferrite. Furthermore, hot deformation could also strongly promote the formation of acicular ferrite, that is, the temperature zone of acicular ferrite transformation was enlarged from 400-600 degreesC in the static CCT diagrams to 450-700 degreesC in the dynamic CCT diagrams. The corresponding cooling rate range of acicular ferrite transformation was significantly increased, and the island constituents in acicular ferrite became finer due to hot deformation. (C) 2003 Elsevier Science B.V. All rights reserved.

Hydrogen Trapping Behavior in Vanadium-added Steel

Abstract: Hydrogen trapping and de-trapping behavior was investigated for steels with and without V. The de-trapping of hydrogen is very slow while the trapping presumably proceeds rapidly for steels containing VC precipitates. The activation energy for de-trapping is in the range of 33 to 35 kJ/mol. The trapped-hydrogen content and diffusible-hydrogen content in the steady state increase with increasing hydrogen entry rate into the steel. The density of hydrogen trapping sites decides the maximum trapped-hydrogen content; 9 ppm for 1 % V steel tempered at peak secondary hardening temperature. Analysis of hydrogen embrittlement cracking tests in terms of hydrogen contents such as the critical hydrogen content should be performed on the specimens with uniform hydrogen distribution and must consider the nature of hydrogen whether it is trapped or diffusible.

Transition between static and metadynamic recrystallization kinetics in coarse Nb microalloyed austenite

Abstract: The recrystallization behaviour of a Nb microalloyed steel, under deformation conditions corresponding to the initial stands of a thin slab hot rolling process which is characterized by coarse initial austenite grain sizes and large strains per pass, has been investigated. A range of strain levels were studied ranging from below the critical strain for the onset of dynamic recrystallization, e c , well into the steady state for dynamic recrystallization, e > e ss . There is a transition strain, e c e T e ss , separating the post-dynamic softening strain-dependent range from that which is strain-independent. At strains above e T , the kinetics of recrystallization is independent of strain but strongly dependent on strain rate. Under these conditions, metadynamic recrystallization operates as the prime softening mechanism and the kinetics have been derived. At strains between e c and e T , both metadynamic and classical static recrystallization are involved in post-dynamic softening. A model is derived to predict the evolution of softening as a function of the holding time for different strain ranges, based on the contributions of the individual components to the overall softening.

Low cycle fatigue behavior of a multiphase microalloyed medium carbon steel: comparison between ferrite–pearlite and quenched and tempered microstructures

Abstract: In an attempt to improve fatigue and fracture resistance, a multiphase (ferrite–bainite–martensite) microstructure was developed in a V-bearing medium carbon microalloyed steel using a two-step cooling and annealing (TSCA) treatment following finish forging. The monotonic, cyclic stress–strain and low cycle fatigue behavior of this steel are reported. These results are compared with those of ferrite–pearlite and tempered martensite microstructures obtained by air cooling (AC) and quenching and tempering (Q&T), respectively. The tensile properties of the multiphase microstructure are superior to those of the ferrite–pearlite and the Q&T microstructures. Under cyclic loading, the ferrite–pearlite microstructure showed hardening at higher total strain amplitudes (≥0.7%) and softening at lower total strain amplitudes (<0.7%). The quenched and tempered and the ferrite–bainite–martensite (TSCA) microstructures displayed cyclic softening at all total strain amplitudes employed. Despite the cyclic softening, the ferrite–bainite–martensite structure was cyclically stronger than the ferrite–pearlite and the Q&T microstructures. Bilinearity in the Coffin–Manson plots was observed in Q&T and the multiphase TSCA conditions. An analysis of fracture surface provided evidence for predominantly ductile crack growth (microvoid coalescence and growth) in the ferrite–pearlite microstructure and mixed mode (ductile and brittle) crack growth in Q&T and the multiphase TSCA microstructures.

Modeling of AGS and recrystallized fraction of microalloyed medium carbon steel during hot deformation

Abstract: In this paper, we have developed the equations for predicting austenite grain size (AGS) of microalloyed medium carbon steel during hot rolling Dynamic recrystallization, which plays a major role in reducing the flow stress and AGS, was described by modifying Avrami's equation In addition, for better AGS prediction of material during rolling, the fraction of dynamic recrystallization was expressed as a function of strain together with the Zener–Hollomon parameter Torsion tests were performed in the temperature range of 900–1100 °C and the strain rate range of 50×10 −2 –50×10 0 s −1 to study the softening behavior To examine the validity of the developed equation, we have then applied it to a four-pass continuous deformation process simulated by hot torsion test For the exact prediction of microstructural evolution during hot torsion test, the change of temperature was calculated using finite difference method It was found that the calculated grain size of the alloy is in good agreement with the observed grain size It is expected that a fine grained steel showing high strength and high toughness can be obtained by controlling deformation conditions on the basis of the newly developed equation

High conductive copper -steel composite cast cooling wall of blast furnace

Abstract: A cooling wall of blast furnace is made up of pure copper and microalloyed steel through combined casting. It has high thermal conductivity, rigidity, high-temp strength and thermal flushing and heatshock resistance. The surface of water tube inlaid in it is plated by Zn or Sn. The inlaid copper part is welded to the cooling water tubes.

Prevention of Slab Surface Transverse Cracking by Microstructure Control

Abstract: Slab surface transverse cracking is well known to be induced by strain concentration at film-like primary ferrite, i.e. allotriomorphs of ferrite formed along the austenite grain boundaries. In the present study, a new concept for the prevention of transverse cracking by means of microstructure control at continuous casting strand is examined. Three kinds of examinations in charge of each objective were conducted; (a) ingot cooling tests for microstructure control with secondary cooling; (b) hot tensile tests for hot ductility with the microstructure; and (c) continuous casting tests for cracking susceptibility on continuously cast slab. Results obtained are concluded as follows. (1) Slab surface microstructure could be controlled by secondary cooling condition. Surface structure control (SSC) cooling, providing intensive cooling until less than A 3 transformation temperature just below mold and subsequently reheated up to 1 250 K in secondary cooling, brings film-like ferrite free structure. (2) Hot tensile tests subsequent to in-situ remelting and solidification prove that hot ductility is much improved and ductility trough almost disappeared with that microstructure control. The results also confirm that in-situ remelting of specimen is indispensable on the hot tensile test to evaluate the effect of microstructure on susceptibility to transverse cracking. (3) Continuous casting test confirms that susceptibility to transverse cracking could be alleviated with this microstructure control. (4) The prevention of transverse cracking and microstructure control is a result of uniform fine precipitates dispersion, such as (Ti, Nb)(C, N), according to SSC cooling.

Dynamic recrystallization of austenite in microalloyed high carbon steels

Abstract: Hypereutectoid steels of 1% carbon, alloyed with high silicon and microalloying levels of vanadium were subjected to dynamic recrystallization. It was found that an increase in carbon and vanadium content leads to smaller grain size. To characterize the dynamic recrystallization behavior of these steels, compression tests were performed over the temperature range 900–1050 °C using strain rates of 0.01, 0.1 and 1 s −1 . Equations were generated that can be used to predict the critical strain for dynamic recrystallization. Of interest is the finding that there is an activation energy for deformation specifically associated with dynamic recrystallization (i.e. peak strain). This activation energy associated with the peak strain is lower than that associated with the steady state stress. This is contrary to Sellar's original observation that the peak strain is a function of the activation energy for deformation according to the Zener–Hollomon relationship.

Modelling of Phase Transformation Kinetics by Correction of Dilatometry Results for a Ferritic Nb-microalloyed Steel

Abstract: Using the dilatometry technique, γ→α transformation kinetics has been determined at different cooling rates in a steel with low carbon and low niobium contents (0.09 and 0.017 mass% respectively). First of all the real and the conventional transformation temperatures of the steel were determined. The real start temperature for proeutectoid ferrite formation (A' r3 ) corresponds to the point where the dilatometric curve starts to diverge from the straight during cooling. The conventional start and finish temperatures for proeutectoid ferrite formation (A r3 and A r1 ) are given by two points close to the minimum and the first maximum of the curve, respectively. The real start and finish eutectoid transformation temperatures -(A' r1 ) s and (A' r1 ) f - correspond to the second point of inflection and a point close to the second relative maximum of the curve, respectively. Carbon enrichment of the remaining austenite, as the transformation to ferrite advances, is corrected taking into account the dependence on the carbon content of the atomic volume of austenite. On the other hand, the dilatometric data have also been corrected with regard to the different expansion coefficients of austenite and ferrite. In this way it has been seen that the lever-rule method applied to the dilatometric curve is useful for determining transformation temperatures, but not for determining transformation kinetics, since the amount of proeutectoid ferrite calculated with this method was up to 10% greater than the real amount measured with an image analyser. Finally a model based on Avrami's law has been developed for the real γ→α transformation kinetics.

Role of microstructure on sulfide stress cracking of oil and gas pipeline steels

Abstract: Sulfide stress cracking (SSC) behavior of three microstructures, i.e., ferritic-pearlitic microstructure, ultrafine ferrite microstructure, and acicular ferrite dominated microstructure, was investigated using the bent-beam test in aqueous hydrogen sulfide (H2S) environments. The critical stress (Sc) values of these three microstructures were determined experimentally to be 1008, 1190, and more than 1260 MPa, respectively. As a result, the acicular ferrite-dominated microstructure possessed the best SSC resistance, the ultrafine ferrite microstructure was in a second position, and the ferritic-pearlitic microstructure was relatively the worst. It was analyzed that hydrogen embrittlement (HE) was the main failure mechanism in SSC cracking for high-strength pipeline steels, and preferential hydrogen accumulation within the plastic zone of the main crack tip accounted for the exhibited embrittlement. It was remarkable that the strength values of pipeline steels were not the only factor to determine their SSC susceptibilities. Microstructure played an important role in the SSC initiation and propagation of pipeline steels. In particular, both the fine dispersed precipitations of carbonitrides and the high-density tangled dislocations in acicular ferrite, which behaved as the hydrogen traps, should be attributed to the optimal SSC resistance of pipeline steels.

Effect of cooling rate on the as-quenched microstructure and mechanical properties of HSLA-100 steel plates

Abstract: The effect of cooling rate on the as-quenched microstructure and mechanical properties of a 14-mm-thick HSLA-100 steel using various cooling media such as brine, water, oil, air, and furnace has been studied. While quenching in brine, water, and oil resulted in lath martensite structures, the granular bainite and martensite-austenite (M-A) constituents were found in air- or furnace-cooled specimens. The average lath spacing increased slightly on decreasing the cooling rate (300 nm in brine-quenched specimen to 400 nm in oil-quenched specimen). The precipitates of Cu and Nb(C, N) were observed in all the quenching conditions except in the brine-quenched specimen. The as-quenched strength and toughness of the brine-, water-, and oil-quenched specimens were higher (yield strength: 894 to 997 MPa, ultimate tensile strength: 1119 to 1153 MPa, and Charpy V-notch energies: 65 to 70 J at −85 °C) than those of air- and furnace-cooled specimens (yield strength: 640 to 670 MPa, ultimate tensile strength: 944 to 1001 MPa, and Charpy V-notch energies: 10 to 20 J at −85 °C). For industrial production of HSLA-100 steel plates, oil or water quenching is recommended in lower thickness plates (<25 mm). For production of thicker plates, however, water quenching is more suitable.

Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

Abstract: The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T 5pct) and the recrystallization-limit temperature (T 95pct), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T 5pct

On coherent carbonitride precipitation in commercial microalloyed steels

Abstract: Fine vanadium carbonitride particles nucleated during interphase precipitation have been investigated in the transmission electron microscope under controlled conditions of diffraction contrast. On dark-field micrographs taken with a carbonitride reflection, particle sizes ranged from 5 nm down to below 2 nm in diameter. No matrix strain around these particles could be detected when matrix reflections were used to produce diffraction contrast under two-beam conditions. It was concluded that the vanadium carbonitrides had precipitated in the form of incoherent particles.

Strain induced precipitation effect on austenite static recrystallisation in microalloyed steels

Abstract: Using torsion tests and applying the back extrapolation method, the strain induced precipitation effect on austenite static recrystallisation in vanadium and niobium microalloyed steels has been studied and a model has been constructed. This model takes account of precipitation and its influence on recrystallisation kinetics, in particular on the activation energy, which is increased. The model is applied at temperatures below the temperature at which inhibition of recrystallisation commences owing to the induced precipitation. The new values of activation energy can be three times higher than the activation energy before precipitation has started, depending on the contents of elements responsible for the precipitation (Nb, V, N, C).

Flow stress behaviour, static recrystallisation and precipitation kinetics in a Nb-microalloyed steel after a strain reversal

Abstract: In the present work, a Nb microalloyed steel has been hot worked in torsion mode following multipass deformation sequences. Some of these sequences involve a reversion of the strain. The effect of this reversion on the mechanical response of the steel on reloading after the pre-straining has been analysed. For pre-strains within the strain hardening deformation range, after a transient, the normal flow curve shape corresponding to austenite exhibiting a single peak dynamic recrystallisation is reached. However, the new position of the peak indicates that the dynamic recrystallisation is clearly delayed by the reversal. Deformation sequences including holding times after different strain paths have been used to study the effect of the reversal on the static softening and the strain induced Nb(C,N) precipitation. A complex relationship between the kinetics of both processes and the strain path is observed.

Effect of microstructure on hydrogen embrittlement of weld-simulated HSLA-80 and HSLA-100 steels

Abstract: HSLA-80 and HSLA-100 steels have been subjected to weld-simulated grain-coarsened heat-affected zone (GCHAZ) and grain-refined heat-affected zone (GRHAZ) treatments at peak temperatures of 1350 °C and 950 °C, respectively, followed by varying cooling rates to approximate the weld heat inputs of 10 to 50 kJ/cm. Subsequent slow strain rate testing in synthetic seawater has been employed to assess the hydrogen embrittlement (HE) propensity of the materials. It is indicated that in spite of an increase in strength after weld simulation, further ductility deterioration, compared to the base material under similar testing conditions, did not occur in GCHAZ HSLA-100 steel and for low heat input condition of GRHAZ HSLA-80. This has been attributed to their HE resistant microstructures. Predominant acicular ferrite or lath martensite or a combination of both imparts resistance to HE, as observed in the case of grain-coarsened HSLA-100 and for the low heat input grain-refined HSLA-80 steels. The deleterious effect of bainitic-martensitic microstructure has been reflected in the ductility values of grain-coarsened HSLA-80, which is in agreement with the observation of higher susceptibility of the as-received HSLA-100 steel having a similar structure. However, contrary to its beneficial effect in the as-received HSLA-80, an acicular ferrite structure has shown vulnerability toward HE for high heat input grain-refined HSLA-80. This has been attributed to the presence of polygonal ferrite and to the development of an HE susceptible substructure on GRHAZ weld simulation.

Grain Refinement of As Cast Austenite by Dynamic Recrystallization in HSLA Steels

Abstract: Recent progress of steel manufacturing process necessitates to refine an extremely coarse austenitic microstructure evolved in a strand cast steel. Grain refinement of as cast austenite by dynamic recrystallization in HSLA steels was studied by using a hot working simulator. The specimens were prepared from a strand cast slab and the hot rolled steel plate of 0.09%C-1.14%Mn-2.26Ni-0.54Mo-0.045%V steel supplied from a steel plant, and also laboratory heat ingots of 0.14%C-1.45%Mn and 0.14%C-1.45%Mn-0.018%Ti steels. Variations of the true stress-true strain curve and dynamically recrystallized grain size with the deformation temperature, strain rate and the initial γ grain size were investigated by hot compression test. It was confirmed that dynamically recrystallized grain size in as cast steels was determined simply by steady state flow stress or Zener-Hollomon parameter, but was not influenced by the initial grain size. Austenitic grain size variation with the reheating temperature in the as cast 0.09%C-2.26Ni-Mo-V steel was very small, and flow stress in the as cast Ti-bearing steel was markedly higher compared with those of the hot rolled plate of this steel or the C-Mn steel. These appeared to be caused by grain growth suppression due to the interdendritic phase enriched with carbon or alloying elements, and dispersion of the micro segregation region with high hardness, respectively. Finally, the direct hot deformation experiment after levitation melting and solidification was conducted, where the Ti-bearing steel was reheated at the temperature from 1 743 to1 773 K and hot deformed by tensile strain at 1 523 K. It was confirmed that very coarse γ grain size in an order of mm was much refined down to 130 to 170 μm by dynamic recrystallization.

Precipitate Dissolution and Grain Growth in the Heat Affected Zone of HSLA-100 Steel

Abstract: An attempt has been made to predict the dissolution of Nb(CN) precipitates in the heat affected zone (HAZ) of a Nb containing HSLA-100 steel. An invariant size approximation model along with existing solubility product data have been applied to continuous heating and cooling situation to ascertain the kinetic strengths from heat input-peak temperature conditions at which dissolution of precipitates take place. Peak temperatures beyond which precipitates rapidly dissolve and cause grain growth have been identified, and they are in consonance with the austenite grain growth data.

Microstructure refinement of structural Steel in China

Abstract: In Oct. 1998 Chinese government opened national program: Fundamental Research on New Generation of Iron and Steel Materials in China (N.G. Steels). The progress of on-going project has been briefly reported in this paper. In order to get microstructure refinement in structural steels, the following 5 ways have been studied: For Ferrite+Pearlite microstructure in plain carbon steels and low-alloy steels, (1) Purified steel-making→Fully equiaxed continuous casting→Rough-mill with higher Zener-Hollman parameter (y-DRX refinement)→Finishing-mill rolling by Deformation Induce Ferrite Transformation (DIFT) technology. (2) For thin-slab continuous casting and rolling (CSP) process, ultra-fine sulfides (20-60 nm) and oxides (5∼20nm) controlling technology. (3) For Low Carbon Bainite (and Ultra Low Carbon Bainite) microstructure (LCB/ULCB) in microalloyed steels, combination of Deformation Induced Precipitation (DIP) and subsequent middle temperature phase transformation controlling. For quench-temperated martensite microstructure in alloy structural steels: (4) Ultra-fine y grains by innovated heat treatment and alloy design; (5) Carbide-free bainite/martensite microstructure with retained y films along lath/sub-lath boundaries.

Influence of phosphorus and boron on hydrogen embrittlement susceptibility of high strength low alloy steel

Abstract: Hydrogen-charged miniaturized Charpy-type specimens were subjected to three-point bending tests to investigate an influence of phosphorus and boron on the hydrogen embrittlement (HE) susceptibility of high strength low alloy (HSLA) steels for bolts. The tests were carried out under wide variety of deformation rate to examine an effect of deformation rate on the susceptibility also. The experimental results revealed that the HE susceptibility increased with decreasing deformation rate. This dependence of susceptibility on deformation rate seemed to be associated with velocity relations between the deformation rate and the diffusion rate of hydrogen. The susceptibility was more pronounced by the addition of phosphorus, even though the phosphorus segregation was not sufficient by itself to cause temper embrittlement. On the other hand, boron had almost no influence by itself on the susceptibility. A cyclic voltammetry and thermal desorption spectroscopic analysis were conducted for understanding the change in HE susceptibility with phosphorus and/or boron from the point of view of hydrogen adsorption/absorption characteristics, respectively. However, it was hardly observed that those elements influenced the hydrogen content and the trapping site of hydrogen in the steel under the present hydrogen charging condition. Consequently, it was considered that the increase in HE susceptibility with phosphorus mainly resulted from the reduction in grain boundary strength due to phosphorous, rather than the variation in hydrogen adsorption/absorption characteristics.