Hot-rolled Steel Bars Research Articles

Combined effects of corrosion and fire on load-carrying response of hot-rolled steel reinforcement

PurposeAlthough separate studies on the influence of corrosion and fire exposure on the constitutive relationship of concrete and steel have been done, there is still a gap in knowledge on the influence of corrosion-temperature superimposition as nonlinear phenomenon. The current study is focused to investigate the response of hot-rolled steel bars subjected to corrosion-temperature superimposition.Design/methodology/approachUsing the accelerated corrosion-impressed-current technique, hot-rolled specimens with different levels of corrosion were obtained. The hot-rolled rebars were first corroded to target levels such as (6, 12, 18, 24, 30 and 36%) and subsequently subjected to target temperatures (250 °C, 400 °C, 550 °C, 800 °C and 950 °C), before tensile tests were carried out to evaluate the residual mechanical response.FindingsThe outcomes showed a significant decline in the parameters governing the mechanical properties of steel reinforcement due to the combined damage due to corrosion and fire. Corroded reinforcement still showed ductile failure after exposure to fire. Moreover, the combined loss of load-bearing characteristics due to corrosion and fire has little influence on the modulus of elasticity. The outcomes of this investigation provide a theoretical database for the assessment of aged structural elements exposed to combination after exposure to fire.Originality/valueThe information concerning structural material's response to corrosion-temperature combined damage is still limited. The cover of the reinforcement is designed to safeguard the reinforcing bars from foreign agencies but is often damaged and spalled off due to corrosion, rendering the reinforcing bars directly exposed. The study aims at the experimental production of fire conditions in a corrosion-damaged infrastructure to cover the aforementioned research gap. The effects of corrosion being superimposed by exposure to elevated temperatures on key parameters affecting mechanical behavior were examined.HighlightsInfluence of corrosion-temperature superimposition on the mechanical properties of hot-rolled rebars.Influence of corrosion-temperature superimposition on the macro and microstructure properties of hot-rolled rebars.Influence of corrosion-temperature superimposition on stress-strain curves of hot-rolled rebars.Influence of corrosion-temperature superimposition on tensile strength, modulus of elasticity and elongation of hot-rolled rebars.

A hybrid radial basis function—Finite difference method for modelling two-dimensional thermo-elasto-plasticity, Part 2. Application to cooling of hot-rolled steel bars on a cooling bed

This paper represents Part 2 of the parallel paper Part 1, where the strong form hybrid RBF-FD method was developed for solving thermo-elasto-plastic problems. It addresses the industrial application of this novel meshless method to steel bars cooling on a cooling bed (CB) where the formation of residual stress is of primary interest. The study investigates the impact of the distance between the bars and the distance to the heat shield above the CB on radiative heat fluxes and, consequently, on thermo-mechanical response. The thermal model is solved on bars cross-section with a RBF-FD method where augmented polyharmonic splines are used for the local approximation. View factors, computed with a Monte-Carlo method, are included in radiative heat fluxes. The thermal solution is incrementally applied on a mechanical model that assumes a generalised plane strain state and captures bars bending. The study employs a hybrid RBF-FD method to resolve a nonlinear discontinuous mechanical problem successfully. The simulation of the process shows how different process parameters influence the thermo-mechanical response of the bars.

AN APPLICATION OF COMPUTER SIMULATION OF AUSTENITE DECOMPOSITION IN OPTIMIZATION OF HOT ROLLING OF LOW ALLOYED STEEL BARS

Modern technologies need more and more precise computer design of manufacturing processes. Especially it is visible in thermal processing of steel, such as hot rolling, hot forging, casting, welding, 3D printing, and so on. An application of computer simulation of austenite decomposition in optimization of manufacturing of low alloyed steel bars by hot rolling was studied. For this purpose the mathematical model and computer software for very precise anticipation of TTT diagrams data was developed. The calculation of TTT diagrams data was based on chemical composition of low alloyed steels. The study started with definition mechanisms of austenite decomposition at constant subcritical temperatures. The mechanism of γ→α phase change in steel is not yet been adequately clarified. Methods for calibration of kinetic equations of austenite phase transition at constant subcritical temperatures were developed in this paper. Mathematical modeling of TTT diagrams data consists both, prediction of kinetic of phase transformations and anticipation of steel hardness. It was found out that the hardness of both, microstructural constituents and total hardness of steel mostly depends on the carbon content and temperature of γ→α phase transformation. The model of TTT diagrams data of low alloyed steel was verified in purpose to apply the model of isothermal decomposition of austenite in very precise method for prediction of microstructure composition and hardness of hot rolled steel bars. The verification of developed model was done by comparison of the calculated results of TTT diagrams data with experimentally estimated TTT diagrams data. By the simulation of hardness and microstructural composition of hot rolled steel bar the optimal cooling regime in cooling beds can be designed.

Residual Mechanical Properties of Corroded Steel Bars after High-Temperature Exposure

Accidental fires in aged concrete structures can lead to combined damage from corrosion and fire. In this study, we aimed to explore the effects of corrosion and high temperature on the residual mechanical properties of hot-rolled plain and ribbed bars. The results indicated that the corroded steel bars still exhibited ductile failure after high-temperature exposure. Moreover, the combined action of high temperature and corrosion has little effect on the elastic modulus. The residual bearing capacity of the hot-rolled steel bar became smaller under the condition of temperatures above 600°C. When the temperatures experienced by the plain and ribbed steel bars were less than 600°C and 500°C, respectively, the high temperature accelerated the decrease in the nominal yield and ultimate strength. However, the yield and ultimate strengths based on the minimum cross-sectional area remained unchanged with an increase in the cross-sectional loss ratio. Similar to that at room temperature, the ultimate strain of steel bars subjected to high-temperature exposure decreased exponentially with an increase in the cross-sectional loss ratio. The new findings of this study can provide a theoretical basis for the safety assessment of aged reinforced concrete structures after exposure to fire.

Numerical study on upgrading beam-column connections in steel framed buildings for progressive collapse mitigation

This research used intensive numerical analysis to identify weak points and structural issues important to limiting the spread of collapse. As a consequence, four specimens of strengthened and unstrengthened steel beam-column joint assemblies were examined using scaled models. The data were used to validate numerical models. The control specimen was a simple shear joint from one of the four experimental assemblies. The second specimen was a bolted steel beam column joint used as a reference specimen to represent the ideal beam-column joint often used in intermediate moment-resisting frames in seismic zones across the globe. The third specimen, like the control, but it had two side plates welded together to strengthen the joint site, while the fourth specimen had high-strength, hot-rolled steel bars pretensioned inside the joint zone. ABAQUS software was used to create numerical finite element models to evaluate the behavior of steel frame assemblies before and after upgrading. The FEM matrix included 22 specimens with various characteristics, including plate thickness, steel grade, a contact between beam flange-strengthening plates, and a column that was either welded or not welded in the first scheme, number and diameter of hot-rolled steel bars were for the second scheme. The effectiveness of the strengthening techniques was established by comparing the mode of failure and load–displacement characteristics of the investigated specimens.

Experimental Study on Seismic Performance Improvement of Pile-Cap Joints in a Prestressed Concrete Solid Square Pile Using a Snap-In Mechanical Connection

ABSTRACT To improve the seismic performance of prestressed concrete solid square piles with a snap-in mechanical connection, effects of the prestressed tension control stress of PC steel rods, anchoring method and reinforcement ratio of hot-rolled steel bars on the seismic performance of the pile-cap joints in the prestressed concrete solid square piles using a snap-in mechanical connection were investigated in this article. Considering the above-mentioned effect factors, the quasi-static test was first carried out on the cap pile-cap joint specimens. Then, the seismic performance of the specimens was investigated. The results indicated that the yield load and ultimate load changed slightly, the displacement ductility increased, the stiffness decreased and the damage development rate slowed down as the prestressed tension control value decreased. When the prestressed tension control stress was reduced to 0.4 times of the standard value of tensile strength of the PC steel rods, the crack resistance moment could meet the requirements of construction and hoisting at the same time. In the process of pile cutting and anchoring, the 135° hook treatment on the end of the PC steel rods could reduce the strength degradation of the pile-cap joints during the loading process, improve the energy-dissipation capacity and slow down the damage development rate of the pile-cap joints. For the specimens with ordinary hot-rolled deformed steel bars added to the pile end, its ductility was improved, the horizontal bearing capacity, displacement ductility and energy dissipation capacity were all greatly improved, and the damage development rate was slowed down. However, the reinforcing ratio of the ordinary hot-rolled steel bars should be kept within a reasonable range. Finally, the improvement measures were proposed to improve the mechanical properties of the prestressed concrete solid square piles and pile-cap joints.

Optimising the corrosion performance of hot-rolled steel bar in concrete

ABSTRACT To optimising the corrosion resistance mechanism of hot-rolled steel bar, the effects of different tempering temperatures and high-oxygen-rich oxidation times were studied by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, electrochemical and atmospheric corrosion tests. The thickness of the oxide layer of the rebar could reach 10 μm and rarely corroded after high-oxygen-rich oxidation at 850°C for 1 s, which exhibits superior corrosion resistance. The electrochemical test results showed that the I corr value after 1 s of high-oxygen-rich oxidation at 850°C was about 0.02 mA cm−2, which was 56.4% lower than that of the heated specimen at 685°C (0.045 mA cm−2). The results of Raman spectroscopy showed that the rust layer of the heated specimen at 685°C mainly consisted of α-Fe2O3, γ-FeOOH and α-FeOOH, and the oxidised layer after 1 s of high-oxygen-rich oxidation at 850°C mainly consisted of α-Fe2O3 and Fe3O4, which maintained the original oxidised layer morphology.

Study of the properties of large-sized blanks of turbine blades made of 15Kh11MF-Sh steel depending on the method of obtaining the initial metal

We consider the manufacturing process of forgings for turbine blades of extra-large dimensions (length >2 m) from electroslag remelting steel 15H11MF-Sh. With an increase of the workpiece dimensions (weight >450 kg), we face the problem of choosing the technology for obtaining the original billet. The production technology is known to affect the structure of the material and, as a consequence, the final properties of the product. We present the results of studying the influence of the method of obtaining the original metal blanks (hot-rolled steel bars and forged billets) on the properties of large-sized forgings of turbine blades. The mechanical characteristics, microstructure properties, chemical composition and content of δ -ferrite were determined by examining the initial samples of long products and forged rods. It is shown that billet in the forged state exhibit higher strength characteristics. However, after the stamping process, the properties of the blades equalize, i.e., after stamping and heat treatment, the same structure is formed, regardless of the method of obtaining the original workpiece. The results obtained can be used in the serial production of large-sized turbine blades of a new profile.

Research on Controlled Jet Ultrasonic Inspection Method for LZ50 Hot Rolled Large Steel Bars

Ultrasonic inspection is the main method of on-line inspection for large size steel bars. For maximum Φ 600 mm in diameter, single maximum length up to 10 m of LZ50 hot rolled steel bar, to meet the requirement of the on-line inspection efficiency and precision at the same time, adopting controlled jet method that the probe and the workpiece is non-contact, coupled with one certain length water body, and its comprehensive inspection ability is stronger. In the application process of controlled jet method, the self-developed jet coupling module and the zonal inspection technology are used to effectively suppress the turbulence noise and structure echo. The test results show that under the premise of signal-to-noise ratio (SNR) not less than 8 dB, the controlled jet method can effectively identify Φ 3.0 mm artificial flat bottom hole of 5 mm and 300 mm in depth, and verify the effectiveness of the controlled jet method.

Experimental research on physical and mechanical performance of steel rebars in Chinese modern reinforced concrete buildings built during the Republic of China era from 1912 to 1949

In order to study the physical and mechanical performance of rebars in Chinese modern reinforced concrete buildings built in the Republic of China era from 1912 to 1949, 66 rebars including 36 rectangular rebars and 30 round rebars were collected from the different Chinese modern concrete buildings. Surface geometries, mechanical properties, chemical composition and metallographic microstructures of these rebars were tested and analyzed. The test results show that the rib heights of rectangular rebars meet the requirements of current standards, but the rib spaces, the sums of gaps between the ribs, and the relative rib areas do not. For the rectangular rebars, the average yield strength is 278.60 MPa, the average tensile strength is 375.86 MPa, the average ratio of tensile strength and yield strength is 1.35 and the average elongation is 32.25 %. For the round rebars, the average yield strength is 350.65 MPa, the average tensile strength is 464.37 MPa, the average ratio of tensile strength and yield strength is 1.32 and the average elongation is 25.08 %. These two kinds of rebars are all low-carbon and hot-rolled steel bars. According to the test results above, the physical and mechanical performance of rebars in Chinese modern concrete buildings obviously differs from that of the current rebars. Finally, the relationship formulas between strength and Leeb hardness of these two kinds of rebars are presented respectively. These results can be applied to analysis of the bearing capacity and the structural safety assessment of Chinese modern reinforced concrete buildings, which provide calculation basis of the conservation design of these buildings

Vision-based camber measurement system in the endless hot rolling process

In the endless hot rolling process, camber is a significant defect where camber is flatness asymmetries produced by nonuniform distribution of rolling pressure. This kind of defect may not only cause clogging of the finishing mill, but also visible defects, such as bar edge folds, edge cracks, holes, and scrapes. It is important to measure the exact camber values of the overall shape and head/tail part of steel bars to reduce camber for hot-rolled steel bars. We introduce a vision-based camber measurement system based on two-area-scan charge-coupled device (CCD) cameras and proposes a camber-detection algorithm to achieve the optimal cutting-line, the overall bar shape, and camber values from the obtained image. The proposed algorithm consists of three parts: optimal cutting-line detection, image-stitching, and camber detection. The optimal cutting-line detection part determines the cutting line of the head/tail part of the bar for continuous joining between steel bars. The image-stitching part obtains the overall shape of a hot-rolled steel bar with continuous image sequence obtained by two-area-scan CCD cameras. The camber-detection part measures head camber, tail camber, and whole camber of the steel bar. Through the proposed camber-detection algorithm, the system not only satisfies a time constraint for detecting the camber of the steel bar but also obtains the optimal cutting line, the overall bar shape, and the camber values.

The use of the ultrasound measurement technique for the evaluation of mechanical properties of the ASTM A36 steels

This work presents a study about the correlations between the characteristics of the acoustic waves and the mechanical properties of low carbon hot rolled steel bars, through the comparison between the wave velocity or attenuation and the tensile strength, yield point or elongation of the material. The aim is the possibility of using this technique to predict the production quality of ASTM A36 steels. Acoustic velocity and attenuation measurements by gain acquisition were carried out with the use of longitudinal waves and transducers of 5 and 15 MHz. Samples of hot rolled reinforcement steel bars with one inch diameter were used. A metallographic study of grain size and inclusion content was made along with chemical composition to verify the influence of these factors to the acoustic measurements. Results indicated that it is possible to use the presented methodology to estimate the mechanical properties, in especial, the tensile strength in steel rebars.

플래시버트 용접과 연속열간압연법으로 제조된 철근의 기계적 성질과 미세조직에 미치는 합금원소의 영향

Flash butt welding is applied in many industries. New technology was developed recently for joining billets which called EBROS (Endless Bar Rolling System). After reheating billets in furnace, two billets were joined using flash butt welding. The objective of this study was to investigate the effect of alloying elements on mechanical properties of flash butt welded zone of hot rolled steel bar. The tensile properties on welded zone of Fe-Mn steel and Fe-Mn-V steel were dropped as compared with non-welded zone. Fe-Mn-Nb steel was opposed to the former. It was found that the white band at the welded zone had high ferrite volume fraction and large ferrite grain size. The vertical white band between flash butt welded billets was transformed into an arrowhead it of steel bar. According to this band, softening has been appeared. There was a interesting phenomenon with HAZ of Fe-Mn-Nb Steel, 40nm scale of particles were observed and hardness of HAZ was higher than non-welded zone.

Influence of base composition on the strength of vanadium-microalloyed steel bars

In this work, the effect of base composition on the strength of hot-rolled V-microalloyed steel bars was studied. Industrial heats with a wide range of carbon contents from 0.15 to 0.3% and manganese contents from 0.65 to 1.3% in combination with V-microalloying up to 0.1%, were carried out. The grain size of the produced hot rolled steel bars was measured and the different strengthening mechanisms were analysed. Carbon and manganese were found to have pronounced effect on refining the microstructure. Vanadium-micraolloying up to 0.1% has a slight grain refinement. This slight grain refinement effect of vanadium decreases with increasing the carbon content. The strengthening potential of vanadium seems to be due to precipitation strengthening rather than grain refinement effect. Both carbon and manganese in combination with vanadium showed a significant effect on increasing the precipitation strengthening. The vanadiumprecipitation strengthening is correlated with carbon and manganese contents. Two modified equations are derived to predict the yield strength of hot-rolled steel bars, in terms of chemical composition and grain size or only chemical composition. Die vorliegende Arbeit befaßt sich mit dem Einfluß der Grundzusammensetzung auf die Festigkeit warmgewalzter, V-mikrolegierter Stahlknüppel. Aus Betriebsschmelzen mit Massengehalten zwischen 0.15 und 0.3%C sowie 0.65 und 1.3%Mn und V-Mikrolegierungsgehalten von bis zu 0.1% wurden Stahlknüppel hergestellt. Die Korngröße des warmgewalzten Produktes wurde gemessen und die verschiedenen Verfestigungsmechanismen wurden analysiert. Kohlenstoff und Mangan hatten einen deutlichen Einfluß auf die Kornfeinung im Mikrogefüge. Das Mikrolegieren mit Vanadium wirkte leicht kornfeinend, ein Effekt, der mit steigendem Kohlenstoffgehalt abnimmt. Die festigkeitssteigernde Wirkung von Vanadium läßt sich wohl eher auf die Ausscheidungsverfestigung als auch die Kornfeinung zurückführen. Sowohl Kohlenstoff als auch Mangan fördem in Kombination mit Vanadium die Ausscheidungsverfestigung beträchtlich, Die Verfestigung durch Vanadiumausscheidungen wird mit den Massengehalten an Kohlenstoff bzw. Mangan korreliert. Zwei modifizierte Gleichungen werden hergeleitet, um die Streckgrenze der warmgewalzten Stahlknüppel zu berechnen, die eine unter Berücksichtigung der chemischen Zusammensetzung und der Korngröße, die andere unter alleiniger Verwendung der chemischen Zusammensetzung.

Reliability of ultrasonic testing methods for hot-rolled steel bars of medium and small diameter

This article describes research on the reliability of ultrasonic testing methods for hot-rolled steel bars of 40–60 mm diameter. Comparison of testing methods is made using direct contact with the bars along the circumferential surface using a straight beam probe (‘S’ probe), a twin crystal probe where the probe face is contoured to the bar curvature (‘SR’ probe) and a straight beam probe with a delay block. The distribution state diagrams of the echo sound pressure and the effective testing range in the cross-section of the bars are obtained. The conclusion is that methods using the SR probe and the straight beam probe with the delay block produce better quality results for steel bars with diameters equal to or smaller than 60 mm, but the S probe is not suitable.

Calculation of microstructure and hardness of hot rolled steel bars

Calculation of microstructure and hardness of hot rolled steel bars

Calculation of microstructure and hardness of hot rolled steel bars

Abstract A procedure for computer aided calculation of the distribution of microstructure and mechanical properties of steel parts is outlined. The microstructural constitution in a certain position is calculated from the local cooling rate, grain size, and chemical composition by the Creusot–Loire regression model. The variation of mechanical properties is predicted from the microstructure and cooling rate distributions and the steel composition by empirical models. In the general case, numerical methods, e.g. the finite element method, must be applied to calculate the cooling rate at different positions of steel parts having complex geometrical form. Reasonable expressions for the temperature dependency of thermal properties, i.e. heat capacity and heat conductivity, of low alloy steels have been evaluated from literature data. An approximate expression for the calculation of cooling rates for air cooling of bars of small dimensions is derived. The procedure is used to predict microstructure and hardnes...

Anisotropy of bulk forming limits in hot-rolled steel bars

The anisotropy of bulk formability in hot-rolled AISI 1040 steel bars has been evaluated using upset and bend tests in longitudinal, circumferential, and radial directions. The index of workability is improved approximately 65 pct by aligning the critical tensile stresses parallel to inclusion plane and elongation (longitudinal direction). The lowest workability is attained in the radial direction where tensile stresses are perpendicular to the plane of inclusions. In the circumferential direction, tensile stresses are parallel to the plane of inclusions, though perpendicular to inclusion elongation, leading to intermediate workability. Tensile ductilities in the principal directions are compared with the three workability levels. Tension fracture data agree with the forming limit line in the radial direction. In the other directions, however, the tension test progressively overestimates workability as the workability level increases.

Defect inspection of hot-rolled steel bars without descaling

The use of a stray magnetic flux technique for detecting surface cracks in rolled steel bare is described. Results from comparative tests show the technique to be superior to both eddy current and ultrasonic testing in resolving cracks in ‘black’ bar material.

Duration of Impact of Bars.

Duration of Impact of Two Spheres.---The Hertz theory was verified by experiments with brass and cast-iron spheres of 3.8 and 7.6 cm. diameter respectively, using a condenser discharge method of measuring short time intervals which gave accurate results for intervals of the order of ${10}^{\ensuremath{-}4}$ second.Duration of Impact of Bars.---Experiments with hot-rolled steel bars, 2.86 cm. in diameter and 16 to 62 cm. long, prove that the compressional wave theory is inadequate since the duration of impact was found to be a function of the initial velocity and to be proportional not to the first power of the length of the shorter bar but more nearly to the two-fifths power. Moreover, the fundamental assumption of the compressional wave theory was shown to be wrong by means of a model in which the wave velocity was slow enough to be observed. A theoretical expression was then derived on the basis of certain assumptions as to the distribution of pressure and its variation with the displacement of the center of mass of the colliding body, which agrees in general with the experimental results.