Low-carbon Microalloyed Steel Research Articles

Evolution of Carbides during Prestrain and Tempering

The effect of thermo-mechanical treatment on the microstructural evolution of low carbon micro-alloyed high strength steel was studied by combining prestrain with tempering (PST) in this paper. It was found that the prestrain causes the dislocation to plug up around the grain boundary and carbide, resulting in carbide boundary fragmentation. Moreover, it breaks the thermo-dynamic equilibrium between the matrix and carbide, induces the dissolution of carbon in the high energy state, and then changes the distribution of carbon in the matrix. In the subsequent tempering process, the precipitation regularity of carbide was changed, which promoted the precipitation carbide at low temperature. The influence of carbide precipitation on dislocation can be divided into two stages: the first stage was precipitation induced creep, which promoted stress relaxation; the second stage was precipitation pinning dislocation, which improved material strength and inhibited stress relaxation.

Study of Microstructural Evolution and Mechanical Properties of 1000 Mpa Low-Carbon Microalloyed Steel Prepared by Multiple Quenching Strategies Near Ms

Study of Microstructural Evolution and Mechanical Properties of 1000 Mpa Low-Carbon Microalloyed Steel Prepared by Multiple Quenching Strategies Near Ms

Heterogeneous Microstructure of Low-Carbon Microalloyed Steel and Mechanical Properties

The microstructure plays a major role in the performance of metallic materials, which can be tailored through the composition and/or processing technique. In this study, a heterogeneous microstructure was developed for low-carbon microalloyed API X65 steel, the most commonly used pipeline steel for oil and gas transportation, using a heat treatment process. The heat treatment process involved intercritical heating of the steel followed by high-temperature isotheral cooling, allowing for phase transformation, as well as alloying element partitioning. The heat treatment transformed the banded ferrite–pearlite microstructure of rolled steel to a quasi-polygonal ferrite microstructure, with the sporadic presence of austenite at the grain boundaries. The quasi-polygonal ferrite was distributed in a heterogeneous form with a fine-grain shell surrounding the coarse-grained core. The heterogeneity in the microstructure, despite being single phase, led to a significant improvement in the tensile yield strength, ultimate tensile strength, ductility and toughness of the steel, with a marginal reduction in microhardness values.

Structural nonuniformity: Impact on the cold resistance of low carbon microalloyed steel for building cоnstructions

The nonuniformity of macro- and microstructure of low-carbon microalloyed steel pipes across their wall thickness was estimated comprehensively using optical, scanning and transmission electron microscopy. The texture of the steel was studied by X-ray diffraction analysis. Both the actual and prior austenite structures were analyzed. The possible impact of all peculiarities of the structure, namely, substructure, texture and nonmetallic inclusion as well on the cold resistance of the steel was considered and discussed. The cold resistance of steel was estimated based on the ductile-brittle transition temperature (DBTT) determined in using Сharpy impact tests performed in a wide temperature range (from ambient and subzero temperatures). Correlations between the structural features of samples, which were cut from the near-surface areas of steel pipes and at the pipe-wall midthickness, and the DBTT were found. The high nonuniformity of ductility was demonstrated, which can be characterized by the fact that the DBTT of midthickness areas of pipe wall exceeds that of near-surface areas by 30°. The lower DBTT (the higher cold resistance) corresponds to the steel characterized by thin (less than 20 ​μm) fragmented lath-like bainite bands.

The effect of the initial microstructure of the X70 low-carbon microalloyed steel on the heat affected zone formation and the mechanical properties of laser welded joints

In this paper, the heat affected zone (HAZ) of laser welded joints of the X70 steel were studied by the transmission electron microscopy method. The effect of the initial microstructure (coarse-grained hot-rolled and fine-grained after cross-helical rolling) on the HAZ formation and the mechanical characteristics of the welded joints were shown. It was found that the microstructure in the inter-critical HAZ of the steel after cross-helical rolling was more dispersed, homogeneous, and uniform compared to that of the coarse-grained hot-rolled one due to the initial fine-grained ferrite-bainitic-pearlite microstructure and the absence of pronounced ferrite-pearlite banding in the base metal. The character of the microhardness value distribution in the HAZ of the steel after cross-helical rolling was smooth with the gradual decrease from 370 down to 185 HV as shifted towards the base metal. In the HAZ of the coarse-grained hot-rolled steel, the heterogeneous microhardness value (up to 640–670 НV) distribution was revealed. The reason was the upper degenerate bainite microstructure with high residual stresses, characterized by laths up to 2.0–2.5 μm long and a high martensitic-austenitic constituent fraction (10–16%) of a slender shape along the boundaries of bainite laths. The conclusion was drawn that one of the ways to reduce the brittleness of the laser welded joints could be using the initially fine-grained steels possessing the homogeneous (mainly bainitic) microstructure.

Hot deformation behavior and constitutive modelling of low carbon micro-alloyed steel YQ450NQR1 during isothermal compression

In this paper, the flow stress curves of YQ450NQR1 low carbon (wt. 0.12%) vanadium micro-alloyed YQ450NQR1 steel are obtained by conducting isothermal compression tests using Gleeble-1500 thermal simulation machine covering the temperature range of 1143K-1433K. To better describe the complex and abrupt deformation during the hot rolling process of the “Z-shaped” YQ450NQR1 products used for train beams, three deformation strain rates (1s−1, 10 s−1 and 30s−1) are selected to study the hot deformation behavior. On this basis, a constitutive model considering deformation activation energy is established from the experimental stress-strain data. The model is further validated against multiple sets of hot compression test data and shows a good accuracy between predicted and tested flow stress with a correlation coefficient of 0.988 and an average error of 8.4%, which indicates the accuracy and reliability of the constitutive model for YQ450NQR1 steel.

Correlative analysis of interaction between recrystallization and precipitation during sub-critical annealing of cold-rolled low-carbon V and Ti–V bearing microalloyed steels

In this paper a new insight into fundamentals of static recrystallization, precipitation and their interaction during sub-critical annealing of three cold-rolled low-carbon microalloyed steel grades is presented. The grades under investigation are a base grade containing V as a microalloying element, a Ti + grade containing Ti as microalloying element added into the base grade, and a Ti + Mn + grade containing additional Mn added into the Ti + grade. The cold-rolled steels are sub-critically annealed inside a muffle furnace to simulate industrial continuous annealing parameters in order to investigate the interaction between recrystallization and precipitation across transient stages of the annealing process as a function of temperature and time. The Zener pinning of precipitates and solute drag force of Mn on the recrystallization process are calculated and compared with measured values obtained from experimental studies on the recrystallization kinetics. Results suggest that the recrystallization kinetics is fastest in the base grade. For the Ti + grade, fine (<15 nm) (Ti,V)(C/N) particles retard the recrystallization kinetics. For the Ti+ and Ti + Mn + grades, solute drag effect of Mn solute atoms for dwell time longer than 2 min at annealing temperature of 800 °C is negligible.

New Approach for Multiplying the Strength and the Formability of Cold Rolled BCC Based Structure Steel through Ultra Fine Structure Technique

Reduction in grain size of bcc based structure steel is still highly concerned in the cold rolled sheet to attain superior mechanical properties. As long as, the reduction of weight is much considered in the structure purposes, the strength/weight ratio of steel is highly demanded. In this study, an innovative technique was applied to attain ferrite grain size with hundreds of nanometer, in tandem with preserving the mechanical properties. In this approach, the micro-alloyed low carbon steel resulted from the thermomechanical process was followed by subcritical annealing regime prior to the first critical transformation temperature. To identify the effect of a micro-alloying element as vanadium, and the effect of subcritical annealing regime on the low carbon steel, two low carbon steel was subjected to studying in this research. The results refer that applying a subcritical annealing regime for the micro-alloyed low carbon steel after hot compression at intercritical annealing temperature can lead for attaining hundreds of nanometer ferrite grain size, which has a powerful effect on promoting the strength of the steel to exceed 1200 Mpa, in one hand with preserving the formability up to 20% as uniform elongation. Unexpectedly, the fine grain size obtained after the innovative technique promotes the impact toughness at room temperature, which is attributed to the fineness and the spheroid morphology of the secondary phase in conjugation with bcc ferrite structure.

Mechanical Properties and Strength Prediction of Ti Microalloyed Low Carbon Steel with Different Ti Content

The effects of Ti on microstructures, recrystallization and mechanical properties of microalloyed low carbon steels were studied. Hall-Petch relationship and Ashby-Orowan equation were applied to analyse the reason for the variation of strength. The results show that the recrystallization activation energy increases parabolically as the Ti content increases. Besides, the yield strength, tensile strength and the yield ratio increase in varying degrees with the increasing of Ti content, while the elongation decreases. The fine grain strengthening makes the greatest contribution to the strength, and the precipitation strengthening of TiC particles is the main reason why the strength of test steel increases significantly when the Ti content exceeds 0.042%. In addition, the strength prediction model was established for Ti microalloyed low carbon steel. The relative error of predicted yield strength and tensile strength of Q345B steel are within ± 8% and ± 7% respectively.

Study of combined effect of zirconium and titanium on microstructure and mechanical properties of micro-alloyed cast steels

Aim of the present work is to study the influence of zirconium as well as titanium in combinations on the microstructures and mechanical properties of low carbon micro-alloyed cast steels and to compare with non micro-alloyed low carbon steels. The quantum of research work on the zirconium bearing micro-alloyed steels with respect to grain refinement and mechanical properties is scanty. The micro-alloyed steels with 0.05 wt% to 0.10 wt% Zr and Ti were melted by a conventional air induction melting furnace and they were further subjected to normalizing at a temperature of 1000 °C for 1 h followed by cooling in atmospheric air. The metallurgical properties of the heat treated samples were observed by optical microscope and TEM. It is observed that steel with Zr and Ti shows the presence of acicular ferrite with a very fine grain size of around 5–10 microns as compared to the non micro-alloyed steel. In addition, the inter-lamellar spacing of pearlite as well as presence of intermetallic compounds were seen clearly in the heat treated samples by TEM. It is also observed that the presence of precipitates accounts for the improvement of strength and toughness of steels containing micro alloying content in the heat treated condition. The steel with 0.1 wt% Zr + 0.1 wt% Ti content showed tensile strength with nearly two times higher in comparison to steel without micro alloy addition in steel. However the elongation of both steels is almost equal (percentage of elongation is found to be around 36%). This can be attributed due to the presence of zirconium carbo-nitrides causing the formation of fine grains in steel with Zr and Ti contents.

The Effect of Initial Microstructure on Microstructure Evolution and Mechanical Properties of Intercritically Rolled Low‐Carbon Microalloyed Steel Plates

Steel plates rolled in the intercritical region have ultrafine and elongated‐grained microstructures, and excellent toughness. The effect of the initial microstructure on microstructure evolution and the mechanical properties of intercritically rolled (IR) low‐carbon microalloyed steel plates is investigated herein. Coarse‐grained initial microstructure mainly containing lath bainite and fine‐grained initial microstructure mainly containing quasi‐polygonal ferrite are produced by austenitization treatments at different temperatures. The results show that the coarse‐grained initial microstructure restrains continuous dynamic recrystallization during intercritical rolling, which leads to a less fraction of high‐angle grain boundaries (HAGBs). However, initial microstructure has little influence on the average size of ferrite grains surrounded by HAGBs. Moreover, coarse‐grained initial microstructure results in a relatively higher strength improving about 15 MPa due to the lower fraction of HAGBs. Fine‐grained initial microstructure is beneficial to the Charpy impact toughness such that the upper shelf energy is improved from 108 to 118 J, and low‐temperature toughness is also enhanced. However, the effect of the initial microstructure on the microstructure and mechanical properties of IR rolled steel plates is not great, which indicates that coarse‐grained initial microstructure can also be used to develop this kind of steel plates with excellent toughness.

The Influence of the Strain Aging on the Mechanical Properties upon Extension in the Metal of X80 Strength Grade Pipes

The influence of the natural aging for ten years and the subsequent heating to 680°C on the magnitude of the strain aging effect in low-carbon microalloyed 05Mn2Nb grade steel for trunk pipelines of the X80 grade pipes has been studied. A method for the assessment of the strain aging effect by the parameters of the stress–strain curve, namely, strength increment Δσy on the yield plateau, change in the σy/σu ratio, and proportional elongation δp, has been proposed.

Effect of Carbide Precipitation on the Evolution of Residual Stress during Tempering

The evolution of microstructure and residual stress during the tempering of 700 L low-carbon micro-alloyed steel was studied using a crack compliance method for measuring residual stress. Additionally, a non-isothermal tempering dilatation test, Vickers micro-hardness test, and transmission electron microscopy were used. The evolution of residual stress during tempering consists of two stages. The first stage coincided with cementite precipitation. Under the initial residual stress, the transformation plasticity due to cementite precipitation leads to partial relaxation of the micro-stress evoked by the austenite-to-ferrite transformation during quenching. It also caused the material surface and the core to exhibit different residual stress evolution trends. After tempering at 300 ∘ C for 30 min, the residual stress was reduced from 487 MPa to 200 MPa; however, the elastic strain energy remained unchanged. The second stage coincided with alloy carbide precipitation and Mn partitioning, but the precipitation of the alloy carbide only reduced the elastic strain energy by 8.7%. Thus, the change in activation energy was the main reason for the relaxation of residual stress at this stage. After tempering at 600 ∘ C for 30 min, the residual stress was reduced to 174 MPa, the elastic strain energy was reduced by 72.72%, and the residual stress was controlled.

Determining role of microstructure on crack arrest and propagation phenomenon in low-carbon microalloyed steel

The relationship between microstructure and crack arrest characteristics of a microalloyed steel with a yield strength of 460 MPa was elucidated in this study with particular focus on the effect of the microstructure on the propagation of brittle cracks. The study emphasized that the acicular ferrite (AF) content could be increased to 41% by lowering the final rolling temperature and final cooling temperature, which reduces the average effective grain size (EGS) to as small as ~4.1 μm and increases the high-angle grain boundaries (HAGB) proportion to as high as ~48.8%. The presence of AF in large numbers could reduce the ductile-to-brittle transition temperature to approximately −87 °C through refinement of the microstructure. The presence of HAGB between adjacent grains could significantly arrest cracks by changing the propagation direction of brittle cracks and was a decisive factor in determining the characteristics of brittle fracture. Moreover, the fracture roughness varied directly with the proportion of the HAGB and inversely with the average EGS. Martensite/austenite islands with low content (0.92%) and small size (~1.5 μm) reduced the stress concentration significantly at the crack tip. The excellent crack arrestability was attributed to AF, which minimized the expansion of cleavage facets by the split nail effect ahead of the growth direction, so that the cleavage facets were divided into two or more branches, considerably reducing the stress concentration and retarding brittle crack growth during crack propagation at low ambient temperature.

Effect of Heat Treatment on Microhardness and Fracture Resistance of the NWZ of a Welded Joint of Microalloyed Steel EH36 with Thickness 150 mm for Offshore Oil-Gas Platforms

The microhardness, microstructure and susceptibility to brittle fracture with respect to the CTOD parameter are studied for industrial specimens of welded joints of sheet low-carbon microalloyed steel VL EH36 Z352 with thickness 150 mm after normalizing and subsequent post weld heat treatment. It is shown that the treatment affects positively the lowering of local microhardness and elevation of resistance to brittle fracture in the coarse-grained region of the near-weld zone of the studied type of welded joint.

Influence of pre-tempering treatment on microstructure and mechanical properties in quenching and partitioning steels with ferrite-martensite start structure

The low carbon micro-alloyed steel with start microstructure of ferrite-martensite was pre-tempered to be ferrite-tempered martensite microstructure before cold rolling. Then an identical quenching and partitioning process was applied to the cold rolled sheet with start microstructures of deformed ferrite-martensite (Q&P-UFC) and deformed ferrite-tempered martensite (Q&P-T). It revealed that the austenitization of Q&P-T was retarded and much more austenite with higher stability was obtained after annealing. In both cases, the austenite formed at recrystallized martensite boundaries was significantly fine, while that formed at recrystallized ferrite boundaries was much coarser. Although the retained austenite fraction (5–6%) and corresponding carbon content (1.1–1.2 wt.%) were identical in both cases, a significant Mn-enrichment of retained austenite was only observed in Q&P-T related to the pre-existed Mn-enriched cementite and hence the corresponding TRIP effect in Q&P-T occurred at relatively larger strain. Moreover, abundant martensite variant combination with twin boundary was observed and exhibited dispersive distribution in Q&P-T. The work hardening behaviors of both cases showed three-stage variation, successively ascribed to the dislocation glide of ferrite, TRIP effect of retained austenite and dislocation intersection/tangle of ferrite and martensite, respectively. Both samples exhibited excellent combination of strength and ductility with UTS larger than 1000 MPa and TEL larger than 20%, and Q&P-UFC was favorable to obtain high strength level, while Q&P-T possessed better ductility and excellent combination of strength and ductility.

Investigation of the influence of external loading on distribution of plastic deformation in the structure of cutting microwave steels after cooled cooling

Purpose. Investigate the effect of external load on the plastic deformation of the structural components of low-carbon micro-alloyed steels after accelerated cooling. Methodology. Analysis of the behavior of the metal under tension to a certain level of load; further experimental study of sample structure. Findings. The influence of external loading on the plastic deformation of the structural components of low-carbon alloyed steels after accelerated cooling has been investigated for steels St3sp, 09G2S, 10CHSND, 10G2FB. Originality. The results obtained in the work show the relationship between the external load and the corresponding distribution of plastic deformation taking into account the structural state. Practical value. Development of ways to stabilize mechanical properties, in particular impact strength, sheet steel rolled products from low-carbon microlagic steels of domestic production on the basis of establishing the relationship between the parameters of the structure and the surfaces of fracture.

Local corrosion of austenitic steels and alloys for heat exchanger tubes of nuclear power stations steam generators

Elaboration of modern domestic structural materials with increased corrosion resistance in contact with advanced heatcarriers of future reactor plants is an important problem at development of innovation projects of nuclear power engineering. Heatexchanging tubes are the critical components, which influence on the safety and reliability of steam generators operation. Corrosion properties of non-stabilized nitrogen-containing corrosion resistant steels of austenite class after cold deformation, thermal treatment and long-term thermal aging studied. It was shown, that silicon introducing into chrome-nickel steel, alloyed by nitrogen and molybdenum, results in increasing of its resistance against local kinds of corrosion and equated it on resistance against intercrystallite and pitting corrosion with particularly low-carbon steels and alloys. But the experimental 03Х18Н13С2АМ2ВФБР-Ш low carbon micro-alloyed steel, proposed for operation at a heat-carrier temperature of 450–500 о С, in the first version had a tendency to a decrease of resistance against local corrosion and impact resistance after long-term thermal aging at temperatures of 360 о С and higher. At present specifying of technological parameters of production and balanced alloying element content takes place, which prevents heat exchanging tubes properties degradation. Steel 03Х17Н13С2АМ2 which has higher resistance against local corrosion and strength comparing with 316LN and 08Х18Н10Т grades, can be taken as a candidate material for production of heat-exchanging tubes of steam generators of nuclear power stations having power reactors of water-water type. The new 03Х17Н9АС2 steel, resistant against inter-crystallite corrosion in high-oxidizing media, was proposed for tests of its operation under conditions of contact with lead heat-carriers instead of 10Х15Н9С3Б1-Ш (ЭП 302-Ш) steel.

A Model and Experimental Validation to Predict Heating Rates for Overlap Between Ferrite Recrystallization and Austenite Transformation in Dual Phase Steel Manufacture

A systematic theoretical and experimental study has been conducted to predict the heating rates required to obtain a pre-defined percentage of overlap between the ferrite recrystallization process and the austenite formation process in dual phase steel manufacture. Isothermal recrystallization kinetics for three different cold-reduced low-carbon micro-alloyed steels (50%, 60%, and 75%) with ferrite-pearlite-bainite initial microstructures was evaluated. Using various experimentally determined rate kinetic constants and critical temperatures, a continuous heating rate model which predicts the heating rate required for a predefined amount of overlap was successfully developed. The model predicted the heating rates required for a predefined 1%, 15%, 34%, 67%, 88% and 99% of overlap to be 0.2°C/s, 0.9°C/s, 1.8°C/s, 7°C/s, 50.5°C/s and 511°C/s, respectively. The experimentally determined recrystallization percentage values validated the predicted heating rates.

The evolution of strain pattern induced by banded structure under uniaxial tension in low-carbon microalloyed steel

The presence of banding inhomogeneities is detrimental to the mechanical behaviors of steels due to the strain anisotropy. In this paper, combined with the in-situ tensile stage, digital image correlation (DIC) method was used to investigate the influence of banding pattern including the size, quantity and morphology on the continuous localized deformation and fracture behavior of low-carbon microalloyed steel under the uniaxial tension load. A zero-deformation test was introduced at the beginning of the experiment, where the accuracy and stability of DIC for this application were validated. The results show that the effect of banded structure on local deformation and damage evolution is significant. Compared to the rolling direction (RD) tensile, the local deformation of banded structure exhibits a higher sensitivity to the transverse direction (TD), and the development of shear bands is clearly affected by the local high strains caused by the deformation of soft banding. The coarse banding (CB) in specimen could decrease the mechanical properties of material. Meanwhile, the mechanical anisotropy is explained by the evolution model of local deformation. This work is helpful to understand the effect of banded structure on the mechanical behavior of steel.