Hot-rolled Low-carbon Steel Research Articles

A simulation study of pearlite-to-austenite transformation kinetics in rapidly heated hot-rolled low carbon steel

The main aim of the present research was to obtain an optimized microstructure with adequate mechanical properties in a low carbon steel. The effect of microstructure on kinetics of austenite transformation was simulated. A 3.2mm hot rolled steel was subjected to continuous annealing to obtain properties of Dual Phase 590 grade. Kinetics of austenite transformation was studied with respect to the condition of just pearlite dissolution to form austenite under rapid heating. Annealing parameters were based on process conditions of dual phase steel production in a continuous annealing line. DICTRA was used to simulate heating rates of the order 10–500°C/s with peak temperatures in the range 750–850°C to predict isothermal annealing time required for complete dissolution of pearlite into austenite under different temperature-heating rate conditions. Simulation results showed dependency of temperature and heating rate on austenite transformation time. Interestingly, no significant effect of heating rate on complete pearlite dissolution into austenite was evident. Results were validated with limited experimentation on Gleeble. Microstructure analysis validated the simulation results to be accurate. The observations have pertinent inputs while designing industrial continuous annealing line parameters where rapid heating rates are generally encountered (10–20°C/s).

Improvement of strength and toughness for hot rolled low-carbon bainitic steel via grain refinement and crystallographic texture

Low-carbon bainitic steel with effective grain size of ~2.0µm and strong <110>//RD (rolling direction) transformation texture was obtained by adopting ultra fast cooling following non-recrystallization rolling. The steel exhibited higher tensile strength of ~840MPa and lower ductile-to-brittle transition temperature of ~−106°C in comparison with the steel that had no distinct texture with effective grain size of ~4.5µm. High intensity of {110} planes normal to the rolling direction were formed as a result of the strong {114}~{223}<110> transformation texture and effectively inhibited the main crack propagation by ductile fracture mechanism. The existing {100} planes parallel to the rolling plane led to the impact fracture delamination at −20~−100°C, which improved the low temperature toughness.

Effect of MnS inclusion and crystallographic texture on anisotropy in Charpy impact toughness of low carbon ferritic steel

The combined effect of microstructure and crystallographic texture on the anisotropy in Charpy impact toughness has been studied in two hot-rolled low-carbon steel plates having different sulphur contents: 0.03wt% S in high-sulphur, HS, steel and 0.01wt% S in low-sulphur, LS, steel. Sub-size Charpy impact specimens were prepared from the rolled plates along 0°, 30°, 60° and 90° to the rolling direction and tested over the temperature range of +40°C to −196°C. Microstructure, inclusions and crystallographic texture have been characterized on the plane parallel to the fracture plane of each sample. The variation in upper shelf energy (USE) was more severe (∼69%) in HS steel due to the presence of coarse and elongated MnS inclusions. Crystallographic texture, especially, higher fraction of alpha- to gamma-fibre texture and stronger cube texture resulted in stronger variation in ductile to brittle transition temperature (DBTT) in LS steel (by ∼26°C). Increase in the fraction of {001} planes of the crystals on the fracture plane of the sample increased the DBTT by helping the cleavage crack propagation. On the other hand, increase in the fraction of {110} planes of the crystals on the shear planes (inclined at 45° to fracture plane) increased the USE by promoting the plastic deformation.

Non-destructive Magnetic Evaluation of Laser Weld Quality in Hot Rolled Coils

Weld quality evaluation was conducted on laser welded thin sectsions (2 mm) of hot-rolled (HR) low-carbon steel coils during cold rolling process. The analysis revealed that the poor welds consisting of the weld defects like incomplete fusion, cluster of porosity, and large difference in hardness between the weld zone and base metal were responsible for the weld failures. Experiments were conducted by varying the welding parameters; laser power and welding speed to optimize the parameters for minimizing the weld defects. The optimized weld process parameters have helped elimination of weld defects and the results are verified with microscopy and microhardness measurements. As destructive evaluation techniques are time consuming and not always permitted in industrial applications, attempts have been made in the present investigation for the utilization of suitable non-destructive techniques for the evaluation of weld quality. Non-destructive magnetic techniques of magnetic hysteresis loop and magnetic Barkhausen emissions were used in the present investigation to establish possible correlations of magnetic properties across the weld seam with the mechanical property (microhardness) for evaluation of weld quality. It is inferred that the magnetic properties of coercivity and inverse of root mean square voltage can be effectively utilized to determine weld quality in HR steel coils.

TRIP Effect in a Hot-Rolled Low-Carbon High Strength Complex Phase Steel

In the present work, a study has been made of the hot-rolling process for a transformation induced plasticity (TRIP) steel Fe-0.12C-0.5Si-1.4Mn-0.5Cr (wt%). The volume fractions of retained austenite before and after a deformation were determined by X-ray diffraction (XRD). The microstructure was characterized by optical microscope, scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). A uniaxial tension text indicated that the steel possesses ultimate tensile strength of 748 MPa with yield ratio of 0.7 and elongation of 20%. The steel with the volume fraction of retained austenite of 12.5 % exhibits significant TRIP effect.

Mechanical properties and characteristics of nanometer-sized precipitates in hot-rolled low-carbon ferritic steel

The microstructures and properties of hot-rolled low-carbon ferritic steel have been investigated by optical microscopy, field-emission scanning electron microscopy, transmission electron microscopy, and tensile tests after isothermal transformation from 600°C to 700°C for 60 min. It is found that the strength of the steel decreases with the increment of isothermal temperature, whereas the hole expansion ratio and the fraction of high-angle grain boundaries increase. A large amount of nanometer-sized carbides were homogeneously distributed throughout the material, and fine (Ti, Mo)C precipitates have a significant precipitation strengthening effect on the ferrite phase because of their high density. The nanometer-sized carbides have a lattice parameter of 0.411–0.431 nm. After isothermal transformation at 650°C for 60 min, the ferrite phase can be strengthened above 300 MPa by precipitation strengthening according to the Ashby-Orowan mechanism.

Hot-dip Galvanizing of Carbon Steel after Cold Rolling with Oxide Scale and Hydrogen Descaling

A new processing method for producing hot-dip galvanized steel is designed and tested, in which pickling is skipped. Hot-rolled low carbon steel sheets are rolled with oxide scale in an experimental mill at room temperature, prior to annealing under a 20% hydrogen reducing atmosphere and galvanizing on a hot-dip galvanizing simulator. Micro-cracks formed in the oxide scale during cold rolling roughen the steel surface and enlarge the specific surface. Through-thickness cracks provide transport channels for hydrogen, and hence the reduction of oxide scale is enhanced. When the sheet is dipped in the zinc bath, cracks are submerged by liquated zinc and the defects are not distinct after hot-dip galvanizing. The overlay coating occludes with rough surface of the sheet, whereby a superior coating adherence is realized.

Nonuniform structure and properties in hot-rolled low-carbon steel coils

The austenite phase transitions in the industrial production of hot-rolled steel coils are studied magnetometrically. Nonuniformity of the structure and mechanical properties of low-carbon steel strip over its length and width may be attributed to incomplete phase transitions in the strip on the output roller conveyer and nonuniformity of the subsequent coil cooling in regular production conditions. Inheritance of the structural inhomogeneity formed after hot rolling may be responsible for the thickness variation observed after cold rolling of the strip. Recommendations are made for reducing or eliminating the structural inhomogeneity so as to improve product quality.

Adhesion of Thermal Oxide Scales on Hot-Rolled Conventional and Recycled Steels

The mechanical adhesion of thermally-formed oxide scales formed on industrial hot-rolled low carbon steel strips produced through the blast-furnace route (conventional steel) or the electric-arc-furnace route (recycled steel) was studied. A new macro-tensile test was compared to a micro-tensile test previously used. It was observed that spallation of scales during straining increased with increasing the tensile strain rate. A higher strain rate resulted in a lower strain inducing the first spallation. As a result, the mechanical adhesion energy of scales actually formed on the recycled steel was in the range 300–700 J m−2. Comparison at the same strain rate of the conventional and recycled steels showed higher scale adhesion for the recycled steel due to the presence of high amounts of interfacial silica.

Fatigue crack growth behaviors in hot-rolled low carbon steels: A comparison between ferrite–pearlite and ferrite–bainite microstructures

The roles of microstructure types in fatigue crack growth behaviors in ferrite–pearlite steel and ferrite–bainite steel were investigated. The ferrite–bainite dual-phase microstructure was obtained by intermediate heat treatment, conducted on ferrite–pearlite hot-rolled low carbon steel. This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy (SEM), fatigue crack growth (FCG) rate tests, and fatigue fractography analysis. Microscopy images arrested by in-situ SEM showed that the fatigue crack propagation in F–P steel could become unstable more ealier compared with that in F–B steel. The fatigue cracks in ferrite–pearlite were more tortuous and could propagate more freely than that in ferrite–bainite microstructures. However, frequent crack branching were observed in ferrite–bainite steel and it indicated that the second hard bainite phase effectively retarded the crack propagation. The variation of FCG rate (da/dN) with stress intensity factor range (ΔK) for F–P and F–B steels was discussed within the Paris region. It was shown that FCG rate of F–P steel was higher than that of F–B steel. Moreover, the fatigue fracture surface analysis proved that grain boundaries could also play a role in the resistance of crack propagation.

Prediction of Mechanical Properties of Low Carbon Steel in Hot Rolling Process Using Artificial Neural Network Model

This work deals with the prediction of mechanical properties of hot rolled steel slab in the hot rolling mill to avoid the manual working of preparing tension test samples in the mechanical testing lab. The time consumption for testing is avoided and the cost of product is decreased.A model for predicting mechanical properties of low carbon steel has been developed and Feed Forward Back Propagation (FFBP) as one type of algorithm of the Artificial Neural Network has been applied to the prediction system. Yield strength (YS), Ultimate tensile strength (UTS) and Elongation(EL) are the basic mechanical properties of low carbon steel are predicted as a function of thermo-mechanical process parameters. These properties mainly depend on the input parameters such as Dispatch Temperature (DISTEMP), Transfer-Bar/Rolling Temperature (TBART), Finishing Temperature (FINT), Coiling Temperature (COILT) and Carbon Equivalent (CEQ). The FFBP is a supervised system that requires a lot of input and output data pairs for training process. The data are acquired from Indian Public Sector Steel Company and preprocessed before training.Performance of the model is evaluated by the Normalized Root Mean Square Error (NRMSE) and the Coefficient of Correlation (R). The NRMSE and the R values of both training and validation parts show excellent values. Therefore, the model using the FFBP algorithm is appropriate to predict the mechanical properties of the hot rolled low carbon steel.

Influence of Ti and Nb on the Strength–Ductility–Hole Expansion Ratio Balance of Hot-rolled Low-carbon High-strength Steel Sheets

To satisfy the demand for high-strength steel sheets with excellent strength–ductility–hole expansion ratio balance, high-strength steels consisting of a ferrite or bainitic ferrite matrix strengthened by a large amount of finely dispersed precipitates have been developed.In this study, the influence of the precipitation-forming elements Ti and Nb on the strength–ductility–hole expansion ratio balance of these steels has been investigated. The strength–ductility–hole expansion ratio balance of steel with Ti added was superior to that with Nb added. The reason for the superiority of steel with added Ti was discussed from various viewpoints such as microstructure, texture, the condition of the pierced surface, sulphide formation, the nucleation and propagation behaviour of cracks during hole expansion, etc. We concluded that the inferior strength–ductility–hole expansion ratio balance of the steel with added Nb was mainly caused by the formation of large textural colonies and a detrimental influence of MnS. From these inferences, we proposed a combined addition of Ti and Nb, and succeeded in improving the strength–ductility–hole expansion ratio balance of the advanced high-strength steel.

OTIMIZAÇÃO DE AÇO LAMINADO A QUENTE PARA ESTAMPAGEM SEVERA PELO USO DE DESGASEIFICADOR RH E ADIÇÕES DE MICROLIGA

This work describes the development and use of a hot-rolled low carbon steel, semi stabilized with boron-titanium to reduce interstitial nitrogen and to act as inclusion morphology agent. The RH vacuum degassing process was used to refine the content range of carbon and manganese, as well to limit the content of residual elements and to optimize the use of this equipment. As performance, criteria was evaluated the mechanical properties, mainly the drawnability, estimated by a semi-quantitative equation. The drawing final performance at customers was monitored.

Mechanical Adhesion and Pickling Behaviour of Thermal Oxide Scales on Hot-Rolled Low Carbon Steel Strips Produced by Different Finishing Temperatures

Hot-rolled low carbon steel strips were produced using two different finishing temperatures at 910 and 820 °C in an industrial hot-rolling line. Mechanical adhesion of scale on the steel substrate at 40 mm from the edge was investigated by tensile test. It was found that the strain initiating the first spallation of scale produced at higher finishing temperature was lower. Spallation ratio which is a spalled area of scale divided by the total area of scale examined under an optical microscope was steeper when the scale was produced at higher finishing temperature. The lower values of strain initiating the first spallation and the higher values of spallation ratio of scale formed both at higher finishing temperature were due to higher thickness of that scale. Pickling behaviour of the hot-rolled steels was investigated by immersing the studied steels in a 10%v/v HCl solution at 80 °C. X-ray diffraction (XRD) peak of hematite relative to that of iron decreased with pickling time and approached zero during pickling periods from 3 to 10 s, while magnetite-and-iron ratio gradually decreased and tended to be zero at longer pickling time. This might indicate the existence of hematite as the outermost layer of scale and subscale containing magnetite as the inner part.

Preparation of a Low Carbon Steel with Ultrafine Grained Microstructure through Rare Earth Micro-Alloying

The effects of the rare earth (RE) element on the microstructure of a hot-rolled low carbon steel (B450NbRE) were investigated under different heat treatment conditions. It was found that the B450NbRE steel with ultrafine grained structure could be successfully prepared with a certain content of RE additive under an appropriate heat treatment temperature between A1 (717°C) and A3 (880°C). The corresponding critical temperature increases with decreasing the RE content. For instance, the critical temperature is 800°C for the RE content of 0.0140 wt.%, but it increases to 850°C as the RE content decreases to 0.0075 wt.%

Prediction of the Mechanical Properties of Hot-Rolled Low Carbon Steel Strips in Correlation to Chemical Compositions and Rolling Conditions

Seven thousand sets of data consisting of mechanical properties, chemical compositions, and rolling parameters of industrial hot-rolled coils were analysed using multiple regression. This was to establish empirical formulas to predict mechanical properties of steel as a function of chemical compositions and rolling parameters. The empirical formulas predicting yield strength (YS), ultimate tensile strength (UTS) and percentage of elongation (EL) of low carbon steel strip were obtained, e.g. YS = 461+ 418 C + 61.6 Mn + 796 P ¬– 303 S + 159 Si + 146 Cu + 204 Ni + 49.7 Cr + 1127 V + 1072 Ti + 3674 Nb – 266 Mo – 6299 B – 76.3 Al – 557 Sn – 3.54 THK – 0.00758 WID – 0.114 FT – 0.223 CT. The rolling parameters in equation included finishing temperature (FT), coiling temperature (CT), thickness (THK) and width (WID) of strip. R-Square values for the formulas predicting YS, UTS, and EL were 82.3%, 90.1%, and 75.8% respectively. These equations were validated by using another 120 hot-rolled coils. The averages of absolute values of the difference between the predicted and actual values of YS, UTS, and EL were 9.6 MPa, 7.8 MPa, and 2.7 % respectively. Correlation of chemical compositions and rolling conditions with mechanical properties was discussed in the paper.

Development of Tensile Test to Investigate Mechanical Adhesion of Thermal Oxide Scales on Hot-Rolled Steel Strips Produced Using Different Finishing Temperatures

The objective of this work was to carry out tensile tests to investigate the effect of finishing temperature on mechanical adhesion of thermal oxide scale on hot-rolled low carbon steel strips. Two hot-rolled low carbon steel strips were produced in an industrial hot rolling line by fixing a coiling temperature at 620 °C and varying finishing temperatures at 820 and 910 °C. Two testing methods were conducted. First, each of a number of samples was subjected to a given imposed strain with ex-situ imaging of scale surface after straining. Second, only one sample was strained in a test with ex-situ imaging of scale surface at every 2 mm elongation of the sample. A spallation ratio, an area where scale was spalled out and normalised by the total area observed by microscope, was plotted as a function of the imposed strain. These two methods gave the same tendency of results as follows. At a given strain, the spallation ratio of scale on steel produced using higher finishing temperature was larger. The gradient of spallation ratio with respect to the imposed strain of that scale was also steeper. This reflects the higher susceptibility of scale to spall out with increasing imposed strain. This behaviour might be related to the larger thickness of scale on steel produced using higher finishing temperature. For the second testing method, lowering the magnification of microscope to observe scale spallation from 50x to 20x increased R2 of the curve of spallation ratio versus the imposed strain, as well as improved the reproducibility of the test.

Research on Characteristics of Deposition Precipitation Process in Production of Low Carbon Aluminum-Killed Steel in FTSR Technology

Precipitation characteristics of second phase in low-carbon steel produced by FTSR technology have been researched by TEM and EDS in this article. And preliminary research of precipitation conditions of second phase particles in thermodynamics and dynamics have been took. The results indicate that: there’re second phase particles precipitated dispersively in hot rolled low-carbon steels by FTSR technology. These particles mainly contain particles of Al2O3, MnS and AlN, and the precipitate size is about tens to hundreds of nano. Thermodynamical analysis declares that most of the Al2O3 and all of the MnS, AlN particles are precipitated in solid state. That’s why the precipitation process is slowed down by the diffusion velocity of the elements in solid, and thinner particles are precipitated while the material is in solid state than in liquid state. Different from the traditional plate rolling production of Al-killed steel, the precipitation of AlN particles are mainly controlled by the continuous casting process in thin slab casting and rolling process, but almost no precipitation of AlN particles in the rolling process.

Prediction of carbon segregation on the surface of continuously annealed hot-rolled LCAK steel

An attempt has been made to predict the segregation of carbon in continuously annealed hot-rolled low-carbon aluminum-killed steel (LCAK) through thermodynamic calculations. It is found that the surface carbon content calculated by the Guttmann model is comparable with the experimental results for inert atmosphere condition, whereas a large discrepancy is observed for the conventional continuous annealing furnace atmosphere. The main reason for this difference is the water–gas reaction that takes place on the surface of the steel sheet. It is proposed that this reaction, along with furnace atmosphere, determines the extent of carbon content on the sheet's surface. Based on this, the surface carbon content for various furnace conditions could be predicted.

Mn-Series Low-Carbon Air-Cooled Bainitic Steel Containing Niobium of 0.02%

A new hot rolled low-carbon air-cooled bainitic steel containing Nb of 0.02% has been developed based on alloying design of the grain boundary allotriomorphic ferrite (F gba )/granular bainite (B g ) duplex steel. The as-rolled microstructure and mechanical properties of bainitic steel containing Nb of 0.02% were investigated by tensile test, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that adding 0.02% of Nb obviously improves the strength without sacrificing toughness of the F gba /B g steel. Adding 0.02% of Nb not only refines the grain boundary allotriomorphic ferrite grains but also promotes the refinement of granular bainite including its bainitic ferrite and M/A island. Any Nb(C,N) has been hardly observed in the steel containing Nb of 0.02%. It is suggested that the strengthening mechanism of Nb of 0.02% can be mainly attributed to the effect of Nb in solution (solute drag-like effect) on the phase transformation rather than the precipitation strengthening of Nb(C,N) particles.