Hot-rolled Steel Plates Research Articles

Study on Microstructure and Mechanical Properties of Hot Rolled Steel Plate Strengthened Complexly by Titanium and Molybdenum

Microstructure and mechanical properties of hot rolled steel plate strengthened complexly by titanium and molybdenum at different final rolling temperature and coiling temperature were studied by optical microscope (OM) and material testing machine. The precipitation behaviors of precipitates phase were obtained using transmission electron microscope (TEM) with EDS analysis. The results showed that at final temperature 850°C, microstructure of experimental steel were fine, uniform polygonal ferrite and acicular ferrite. With the coiling temperature decreasing from 620°C to 580 °C, the grain of polygonal ferrite became finer, and the amounts of acicular ferrite became more; precipitates phase mainly maintained both coarse TiN and small square round sheet (Ti,Mo)C. On the condition of the final rolling temperature (850°C) and coiling temperature (580°C), experimental steel acquired properties as follows: yield strength 608 MPa, tensile strength 720 MPa, elongation 24%. Ti-Mo composite strengthening effect had achieved, and significantly improved the strength of ferrite steel.

Study on Precipitation Behavior of High Strength Steel Plate Strengthened Complexly by Titanium and Molybdenum

The precipitation behaviors of hot rolling and cold rolled annealing steel plates strengthened complexly by titanium and molybdenum were studied in the paper. The microstructures and precipitate phases were analyzed using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) with energy disperse spectroscopy (EDS). The results showed that the coarsening square TiN phase and the fine roundness (Ti,Mo)C phase were precipitated mostly in the hot rolled steel plate. As the finishing temperature decreased and coiling holding time increased, the quantity of fine precipitates increased. And also the fine round precipitates increased, dispersion expanded and shape of the phase being uniformed as the annealing temperature increased. Therefore, the strengthen effects can be improved effectively by a reasonable control toward titanium and molybdenum precipitation behaviors.

Temperature Correction for Thickness Measurements of Hot Rolled Steel Plates

Abstract Measuring the thickness of steel plates during the hot rolling process is crucial for getting the necessary feedback to obtain the desired values at room temperature. The dilatation and the phase transformations of steel occur during heating and subsequent cooling and rolling strongly influences the results of thickness measurements. The radiometric method provides the necessary means for making real-time and fully automatized measurements of steel thickness during rolling, if only the temperature correction is precisely known and taken into consideration. This article proves that the experimental results concerning the relative variation of steel thickness and corresponding mass-thickness variation can be correctly explained by means of the theory of metal dilatation correlated with the theory of nuclear radiation absorption when passing through a metal or metallic alloy, such as steel.

The Development and Application of the Hot-Rolled Plate Ultra-Fast Cooling Equipment

The ultra-fast cooling technical is a newly developed thermal technical in recent years for the hot-rolled steel plate. The key to develop the ultra-fast cooling equipment are the design of the nozzle structure and the cooling process. The heat exchange mechanism of the jetting impingement heat transfer mode was illustrated during ultra-fast cooling process for the hot-rolled steel plate in this paper. The flow field characteristics and the cooling technological process during the plate ultra-fast cooling process were simulated. And the newly-developed ultra-fast cooling equipment for the hot-rolled plate and its application were also introduced in this paper.

Study on the Production of Ti Micro-Alloyed High Strength Hot Rolled Steel by CSP Process

Ti micro-alloyed high strength hot rolled steel was developed in Valin Lianyuan Steel in the CSP line. The cleanliness of liquid steel was good enough for thin slab casting after LF refining. The mould powder was adjusted for stabilizing the heat flux of thin slab continuous casting mould. Homogeneous microstructure consisting of ferrite and pearlite was obtained in the hot rolled steel plates by the improving of rolling process. The nano-scale precipitates of Ti(C, N) and Nb(C, N) is the main strengthening mechanism. The yield strength of developed hot rolled plate is higher than 660 MPa and the tension strength is 760 MPa. The ductile-brittle transition temperature is below -60 degree Celsius. The developed Ti alloyed steel with designed composition fulfils the requirements of 600 MPa grade steel for engineering machinery.

Considerations about the welding repair effects on the structural integrity of an airframe critical to the flight-safety

Structures critical to the flight-safety are commonly submitted to several maintenance repairs at the welded joints in order to prolong the in-service life of aircrafts. The aim of this study is to analyze the effects of Tungsten Inert Gas (TIG) welding repair on the structural integrity of the AISI 4130 aeronautical steel by means of experimental fatigue crack growth tests in basematerial, heat-affected zone (HAZ) and weld metal. The tests were performed on hot-rolled steel plate specimens, 0.89 mm thick, with load ratio R=0.1, constant amplitude, at 10 Hz frequency and room temperature. Increase of the fracture resistance was observed in the weld metal but decreasing in the HAZ after repair. The results were associated to microhardness and microstructural changes with the welding sequence.

Repair Welding Effects on the Bending Fatigue Strength of AISI 4130 Aeronautical Steel Used in a Critical to the Flight-Safety Structure

The aim of this study was to analyze the effect of successive TIG (tungsten inert gas) welding repairs on the reverse bending fatigue strength of AISI 4130 steel, which is widely used in components critical to the flight-safety. In order to simulate the abrupt maneuvers, wind bursts, motor vibration and helixes efforts, which generate cyclic bending loadings at the welded joints of a specific aircraft component called "motor cradle", experimental reverse bending fatigue tests were carried out on specimens made from hot-rolled steel plate, 1.10 mm (0.043 in) thick, by mean of a SCHENK PWS equipment, with load ratio R = -1, under constant amplitude, at 30 Hz frequency and room temperature. It was observed that the bending fatigue strength decreases after the TIG (Tungsten Inert Gas) welding process application on AISI 4130 steel, with subsequent decrease due to re-welding sequence as well. Microstructural analyses and microhardness measurements on the base material, heat-affected zone (HAZ) and weld metal, as well as the effects of the weld bead geometry on the obtained results, have complemented this study.

Considerations on corrosion and weld repair effects on the fatigue strength of a steel structure critical to the flight-safety

The aim of this study is to analyze the effects of corrosion and successive tungsten inert gas (TIG) welding repairs on the reverse bending fatigue strength of AISI 4130 steel used in components critical to the flight-safety. The tests were performed on hot-rolled steel plate specimens, 1.10 mm and 1.60 mm thick, by means of a SCHENK PWS equipment, with load ratio R = −1, constant amplitude, 30 Hz frequency and room temperature. It was observed that the reverse bending fatigue strength of AISI 4130 steel decreases due to the corrosion and the TIG welding and re-welding processes.

Investigation of the mixed-mode fatigue crack growth of a hot-rolled steel plate with a circular microdefect

The interaction between cracks and microdefects under mixed-mode loading conditions is important for precisely estimating the fatigue lifetime of a structure. In this study, the fatigue crack growth behavior of a crack with a circular microdefect is investigated under mixed-mode loading conditions experimentally using modified compact tension shear (CTS) specimens for various loading angles with/without a circular microdefect. The fatigue crack growth behavior observed through experiments is analyzed by linear elastic finite element (FE) analyses and the simulated crack path using the maximum tangential stress (MTS) criterion is compared and discussed with the crack path observed through experiments.

Correlation of Microstructure and Cracking Phenomenon Occurring during Hot Rolling of Lightweight Steel Plates

An investigation was conducted into the correlation of microstructure and the cracking phenomenon that often occurred in hot-rolled lightweight steel plates. Two kinds of steels were fabricated with varying Mn and Al contents, and their microstructures, tensile properties, and high-temperature transformation behavior were investigated. In the two steels, banded structures containing ferrite grains and κ-carbides were well developed along the rolling direction. Detailed microstructural analyses showed that cracks initiated at film-type κ-carbides continuously formed interfaces between bands, while the band populated with κ-carbides did not play an important role in initiating cracks. Thus, the formation of band structures and film-type interfacial κ-carbides must be minimized to prevent the cracking. The decreased content of hardenability elements, including aluminum, higher finish-rolling temperature, reduced central segregation during the slabmaking process, and decreased material variation during hot rolling, were suggested as practical methods for preventing the cracking.

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.

Modelling and experimental model validation for a pusher-type reheating furnace

The slab reheating process turns out to play a key role in dealing with the steadily increasing demands on the quality of hot rolled steel plates. Improvements both in the throughput of the furnace as well as the accurate realization of reheating paths for the slabs require to incorporate modern model-based control design techniques into the furnace automation. For this, suitable mathematical models with manageable dimension and complexity have to be determined for the furnace and slab dynamics. In this contribution, first principles are applied for the derivation of a physics-based model of the reheating process in a so-called pusher-type reheating furnace. Thereby, a discontinuous mode of furnace operation is considered, which is characterized by a varying number of slabs with variable geometry being discontinuously moved through the furnace. This, in particular, results in a hybrid structure of the mathematical model. The accuracy of the mathematical model is validated by a comparison with experimental data obtained from a measurement campaign with a test slab performed at an industrial pusher-type reheating furnace.

On the Occurrences of “Frizzle-Type” Surface Defects in a Hot-Rolled Steel Plate

Metallurgical investigations were directed to probe into the genesis of “frizzle” and fissure-type surface defects on 10-mm hot-rolled steel plates meant for application in flat-bed wagons for carrying heavy machinery. The thin hairline fissures on the steel plates were identified as skin laminations associated with long, shallow, and branched cracks, intruding from the plate surfaces to the interior with curved contours, replete with fragmented oxide scale entrapments and debris. The incidence of these superficial defects on the plates was linked to surface damage to the solidifying skin of continuously cast steel slabs, induced by the extensive pitting and cavitation of caster pinch rolls. It is presumed that during hot rolling of the steel slabs, surface blemishes like indentations and folds got rolled over by metal flow, entrapping copious amounts of primary as well as secondary scales underneath the laminated skin. Under the influence of shear forces during rolling, the defects are believed to have ingressed further into the plate interior leading to the formation of long, shallow, and branched cracks with curved contours and entrapped fragmented oxide scales.

Analysis of corrosion of hot-rolled X70 steel plate during storage

In this work, analysis was performed to investigate the reason resulting in corrosion, especially pitting corrosion, of a stock of hot-rolled X70 steel plates after 4 months of storage in a warehouse. Results demonstrated that corrosion and pitting corrosion of X70 steel plate during storage is attributed to the remnant and concentration of cooling water on the top surface of the plate. Pitting corrosion occurs locally at defects on surface deposit that is generated during corrosion of the steel. The small anode vs. big cathode geometry enhances the pitting corrosion process. Chloride ions enter the pits to maintain the charge-neutrality, while ferrous ions that are generated during corrosion diffuse out of the pits to result in corrosion product layer at pit mouth.

Artificial Neural Network: Some Applications in Physical Metallurgy of Steels

Artificial neural networks are parameterized nonlinear models used for empirical regression and classification modelling. Their flexibility enables them to discover very complex relationships among large number of variables with complex interdependencies. Hence it is a very appropriate technique for developing predictive models in the short term. This article discusses the development of neural network models based on a Bayesian framework and some applications of the same. These examples include: (i) prediction of yield and tensile strengths of hot-rolled low-carbon ferrite-pearlite steel plates as a function of composition and rolling parameters, (ii) estimation of bainite plate thickness, (iii) estimation of retained austenite as a function of process parameters in Transformation Induced Plasticity aided (TRIP-aided) steels, and (iv) analysis of strain-induced transformation behavior of retained austenite during uniaxial tensile testing of TRIP-aided steels. While examples (i), (ii), and (iv) provide an overview of the work reported earlier, example (iii) is reported here in open literature for the first time. In all these four cases, it has been shown that the results are consistent with the established physical metallurgy principles.

鋼板の精密穴抜き加工におけるPWパンチの耐焼付き性評価

In piercing, a smooth shearing surface can be obtained by reducing the clearance between the punch and the die. However, continuous piercing in a small clearance is difficult during mass production, since adhesion may easily occur. A Press Working punch (PW punch) is a punch for piercing a steel plate developed on the basis of cold backward extrusion technology. However, the dimensions of the PW punch were not evaluated accurately. In this study, we evaluated the adhesion resistance of the PW punch when used for the fine piercing of a steel plate. Experiments on continuous piercing using a PW punch with a 10 mm diameter and 0.5 % clearance are performed on a hot-rolled steel plate having a thickness of 4.5 mm. Experiments using a conventional right angle punch are also performed for comparison. A mirrorlike shearing surface was obtained using a ceramic-coated PW punch without causing adhesion in 600 continuous piercings. As a result, the PW punch was found to be highly advantageous for the piercing of steel plates.

Experimental Study on the Material Response of Laser Weldment under Dynamic Loading

The materials response of a laser beam welded joint from hot rolled DP600 steel plate to the dynamic loading has been experimentally studied by instrumented Charpy impact testing. Based on the impact force-displacement curve, Critical Crack Length, i.e. CCL and dynamic crack extension resistance curve, i.e. J-a were determined, describing the welding joint’s crack formation toughness and the dynamic crack behavior. It is shown that the difference of crack initiation and propagation between the HAZ and BM is not significant. In addition, the CCL of Welding Area, i.e. WA is multiple higher than that of HAZ and BM, which is also less dependent of low temperature. Moreover, the J-a curve of WA consists of two segments, the initial sudden ascending with linear feature and the subsequent slow rising. Furthermore, tearing module derived from the slope of J-a curve was discussed, indicating the material microstructure mechanism affected by the thermodynamic process of laser beam welding.

Fatigue behavior of hot-rolled steel plate at various stages of fabrication

In the present work, the effect of fabricating process on the fatigue performance of hot-rolled steel plate for automobile structure uses (according JIS standard) was studied. The crack growth behavior of specimens at various stages of fabricating process was also investigated. Rectangular bar specimens were machined from base metal and from weld metal of the fabricated components after rolling, forming and cold spinning processes. Four-point bending fatigue tests of the specimens with the as-fabricated and with the machined surfaces were performed at a stress ratio of 0.1 and a frequency of 20 Hz with a sinusoidal waveform. The fatigue test results indicated that surface morphology and surface defects that were induced by each stage of fabricating process strongly influenced the fatigue limit. The results of fatigue crack growth test indicated the influence of fabricating process on crack growth behavior in the threshold region. However, the fabricating process did not affect the fatigue crack growth behavior in Paris regime.

The effect of welded boundaries on the response of rectangular hot-rolled mild steel plates subjected to localised blast loading

A series of experiments examining the effects of welded boundaries on the localised blast load response of mild steel plates is presented in this short communication. Two types of welding are examined, TIG and MIG welding. The welded boundary response is compared with clamped boundary and straight and chamfer machined boundary responses. The overall trend of midpoint deflection–thickness ratio versus dimensionless damage number for all the plates follows previously reported empirical trends. The tearing threshold (Mode II*) in terms of the damage number is lowest for the machined plates followed by the welded plates with the clamped plates having the highest resistance to tearing.

Effect of Pressure on the Behavior of Cathode Spots in Oxide Removal by Arc Discharge

The effects of pressure on the cleaning action of the cathode spots were investigated in a wide pressure range from 10 to 20 000 Pa of inert atmosphere.Experiments were carried out by using hot rolled steel plates, the surfaces of which were covered with thick oxide of 5–10 µm.Experimental observation revealed the following results:(1) The cleaning action by cathode spots is confirmed even under a pressure as high as 20 000 Pa.(2) The number of cathode spots on the oxide surface increased with the increase of pressure in the range of 10–20 000 Pa.(3) The removal rate of oxide decreased with the increase of pressure.(4) The energy density of the cathode spot decreases with the increase of pressure.(5) The higher the pressure increases, the smaller area the migration of the cathode spot was restricted to. The migration area decreased with the increase of pressure.