Hot Rolling Conditions Research Articles

Rational configuration of finishing group and winding unit for a continuous broad-strip mill

7In hot rolling on a continuous broad-strip mill, the final structure and mechanical properties of the strip are mainly formed in the finishing group and the winding unit. A number of factors limit the development of new hot-rolling conditions for low-carbon steel strip on the basis of the austenite recrystallization diagram [1] and optimal metal structure [2]. The chosen conditions are often somewhat nonoptimal on account of the technological, energy, and configurational constraints of the mill. Thus, the equal intercell distances L int in the finishing group of the mill and the actual distance from the last cell of the finishing group to the first section of the sprayer unit L fg-su do not permit the required optimization. For example, for the 2000 mill at OAO NLMK, L int = 6 m and L fg-su = 28 m. Less severe reduction in the first cells of the finishing group (at high temperature and low speed of the strip) in optimizing thick-strip rolling only partially limits the collective growth of the austenite grains in the intervals between the cells, after the end of primary recrystallization. Increasing the reduction in the subsequent cells (at lower temperatures and higher rolling speeds) ensures complete primary recrystallization but there is an upper limit on the rolling force in the penultimate cell. Moreover, in establishing the relative reduction, its critical value must be taken into account ( e < e cr ). In the present work, we develop a rational configuration of the finishing group and winding system for a continuous broad-strip mill, by analysis of the conditions of structure formation in the rolled strip. The completeness of recrystallization of the austenitic structure in eU 3 OO low-carbon steel at the beginning of the next reduction in finishing-group cell ( i + 1) is described by the equation [1]

Effect of microstructure on drop weight tear properties and inverse fracture occurring in hammer impacted region of high toughness X70 pipeline steels

In the present study, the effect of microstructure on drop weight tear test (DWTT) properties and inverse fracture occurring in the hammer impacted region of high toughness API X70 pipeline steels was investigated. Six kinds of steel specimens were fabricated under varying hot rolling conditions. The DWTT was conducted on the hot rolled steel specimens and the results were discussed in comparison with the Charpy V-notch (CVN) impact test data. The DWTT results indicated that all the specimens showed excellent DWTT properties as the per cent shear area well exceeded 85% at –20°C or lower, irrespective of microstructures. The specimens rolled in the single phase region had the lower 85% shear appearance transition temperature (SATT) than the specimens rolled in the two phase region, which was similar to the CVN test results. This 85%SATT had a good correlation with the energy transition temperature (ETT) obtained from CVN test. Inverse fracture started to occur at –20°C and its size increased with decreasing test temperature. The strain hardened hammer impacted region after DWTT became thicker and harder with decreasing temperature, thereby deteriorating the toughness of the specimens.

Mechanism of Oxidation of Austenitic Stainless Steels under Conditions of Hot Rolling in Steckel Mills

The development of oxide scales on hot-rolled austenitic stainless steels under conditions imposed by the industrial Steckel Mill operation introduces particular characteristics that impact downstream on the surface quality of the hot-rolled strip product. In this research, the development of these particular surface structures were studied on 302 austenitic stainless steel by means of laboratory process simulation involving mechanical deformation in a multipass hot rolling schedule and long interpass time inside equalizing furnaces. Surface analysis using a set of complementary techniques that included field emission gun scanning electron microscopy (FEG-SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and glow discharge optical spectroscopy (GDOS) provided a full characterization of the resulting surface structures. These structures consisted in multiple oxide scale and metal layers that were developed after each rolling pass and heavily modified by the following rolling operation establishing a single repetitive mechanism of surface development. The mechanism of formation of these complex multilayer surface structures has been fully determined as being associated with a cyclic oxidation pattern involving successive stages of protective oxidation, chemical breakaway, and the progress of duplex oxidation.

On Factors Affecting the Phase Transformation and Mechanical Properties of Cold-Rolled Transformation-Induced-Plasticity–Aided Steel

Two Mo-Nb microalloyed transformation-induced-plasticity (TRIP) steels, with Al contents of 0.23 and 0.65, were subjected to several hot-rolling conditions designed to generate different ferrite morphologies and grain sizes. These structures were then cold rolled and TRIP annealed under different heat-treatment conditions. To further develop TRIP steel in terms of strength and ductility, stabilizing retained austenite by isothermal bainitic transformation was studied in detail. Microstructure observation and tensile tests were conducted, and volume fractions of retained austenite were measured. It was observed that increasing the aluminum content enhances the transformation rate and increases the total amount of bainite fraction at the expense of retained austenite. The latter effect enhances formability by increasing ductility. Furthermore, it was observed that the hot-rolling schedule, prior to cold rolling and heat treatment, has a decisive effect on structure refinement, which enhances the strength-ductility balance of the final product. To study the transformation behavior, dilatometer testing was conducted under conditions similar to that of the heat treatment. Thermodynamic calculations were used to verify the results.

Production of rolls of low-alloy rolled steel sheet for electrowelded pipe

The structure of an algorithm used in developing new production technologies for hot-rolled sheet with unique properties is proposed. As an example, the algorithm is applied to 17Γ1C-Y steel. The corresponding changes in the hot-rolling conditions and their influence on the mechanical properties of the product are considered.

A Laboratory Steckel Mill Simulation

The recrystallisation of AISI430 ferritic stainless steel during hot working is considered crucial for developing suitable product properties for sheet metal forming. Attention has focused on the ability to accurately reproduce the process route conditions of hot Steckel mill rolling at the laboratory scale. The variability was considered through five complex simulations on the IMMPETUS thermomechanical compression (TMC) test machine. This paper presents the methods employed to translate the industrial practice into laboratory based experiments, and the development of the simulation to a satisfactory level. This paper includes a discussion of the quality of the simulation, particularly with regard to its use as a tool for analysing metallurgical changes.

Modeling ductile to brittle fracture transition in steels—micromechanical and physical challenges

Both scientists and engineers are very much concerned with the study of ductile-to-brittle transition (DBT) in ferritic steels. For historical reasons the Charpy impact test remains widely used in the industry as a quality control tool to determine the DBT temperature. The transition between the two failure modes, i.e. brittle cleavage at low temperature and ductile fracture at the upper shelf occurs also at low loading rate in fracture toughness tests. Recent developments have been made in the understanding of the micromechanisms controlling either cleavage fracture in BCC metals or ductile rupture by cavity nucleation, growth and coalescence. Other developments have also been made in numerical tools such as the finite element (FE) method incorporating sophisticated constitutive equations and damage laws to simulate ductile crack growth (DCG) and cleavage fracture. Both types of development have thus largely contributed to modeling DBT occurring either in impact tests or in fracture toughness tests. This constitutes the basis of a modern methodology to investigate fracture, which is the so-called local approach to fracture. In this study the micromechanisms of brittle cleavage fracture and ductile rupture are firstly shortly reviewed. Then the transition between both modes of failure is investigated. It is shown that the DBT behavior observed in impact tests or in fracture toughness specimens can be reasonably well predicted using modern theories on brittle and ductile fracture in conjunction with FE numerical simulations. The review includes a detailed study of a number of metallurgical parameters contributing to the variation of the DBT temperature. Two main types of steels are considered : (i) quenched and tempered bainitic and martensitic steels used in the fabrication of pressurized water reactors, and (ii) modern high-toughness line-pipe steels obtained by chemical variations and optimized hot-rolling conditions. An attempt is also made to underline the research areas which remain to be explored for improving the strength-toughness compromise in the development of steels.

Influence of hot rolling conditions on the mechanical properties of hot rolled TRIP steel

Influence of hot rolling conditions on the mechanical properties of hot rolled TRIP steel was investigated. Thermomechanical control processing (TMCP) was conducted by using a laboratory hot rolling mill, in which three different kinds of finish rolling temperatures were applied. The results show that polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes. The finer ferrite grain size is produced through the deformation induced transformation during deformation rather than after deformation, which affects the mechanical properties of hot rolled TRIP steel. Mechanical properties increase with decreasing finish rolling temperature due to the stabilization of retained austenite. Ultimate tensile strength (UTS), total elongation (TEL) and the product of ultimate tensile strength and total elongation (UTS×TEL) reaches optimal values (791 MPa, 36% and 28 476 MPa%, respectively) when the specimen was hot rolled for 50% reduction at finish rolling temperature of 700 °C.

７８０ＭＰａ級Ｄｕａｌ Ｐｈａｓｅ鋼の静的，動的強度特性に及ぼす炭素と珪素の影響

Various types of high strength steel have been developed to improve the impact safety and reduce the weight of cars. For the steels used in anti-crash equipments, mechanical properties, especially, at high strain rates such as 103/s is of importance. In this study, 0.110%C–1.44%Si–1.29Mn–0.65%Cr–0.29%Mo containing hot rolled dual phase steel with 780 MPa grade in tensile strength was employed as a base steel and the effects of carbon and silicon on static (strain rate: 10−3/s)/ dynamic (strain rate: 103/s) mechanical properties of the dual phase steel were investigated. Carbon and silicon contents were changed in a range of 0.076–0.190% and 1.44–2.39%, respectively. Grain size of the steels was varied by hot rolling reduction: 53% (named coarse grain process), 73% (middle grain process) and 88% (fine grain process). Dynamic absorbed energy up to 10% tensile strain had a linear relationship with tensile strength, regardless of microstructures, i.e., neglecting carbon and silicon contents, and hot rolling conditions. All absorbed energy to fracture had a close relationship with tensile strength–ductile balance parameter (tensile strength×total elongation), reflecting microstructural change through chemical and rolling conditions. All the processed 0.190% C steels, and the fine grain processed 1.93% Si and 2.39% Si steels showed the highest all absorbed energy of all the steels tested. The 0.190% C steel was characterized by almost 100% martensite with some content of retained austenite, and the 1.93% Si and 2.39% Si steels were fine grained ferrite+martensite. It was found that carbon improves all absorbed energy through increase in volume fraction of martensite and silicon raises it through solid solution hardening of ferrite matrix.

Kinetics of Austenite Formation in Dual Phase Steels

In this research study a low carbon steel with composition of C=0.11 %; Mn=1.30%; Cr=0.45%; Si=0.20%; Al=0.03% and Fe=bal. was cast and used in hot rolled condition. After intercritical annealing at different temperatures for different times, dual phase structures with various volume fraction of martensite were produced. Then the volume fraction of martensite for different samples were measured. Study of the relationship between temperature and time of intercritical annealing and martensite volume fraction showed that a new equation as f γ /f e =1 -exp(-kt) can be used to give variation of martensite volume fraction with temperature and time of annealing. Also it is found that k coefficient changes exponentially with annealing temperature.

Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

Ti 添加 Dual Phase 鋼の疲労限に及ぼす熱間圧延条件の影響

The effects of hot rolling conditions on the size of TiC precipitates in Ti-added ferrite-martensite hot-rolled steel sheets and precipitated TiC size on fatigue limit were investigated. Steel sheets were hot-rolled at 1153 K or 1073 K and rapidly cooled to 953 K after hot-rolling and air-cooled for several seconds in order to accelerate ferrite transformation and rapid cooled again to coiling temperature. It was found that TiC particles were coarsened with the decline of finish-rolling temperature (FRT) and the increase of air-cooling time after hot rolling. This is presumably because the decrease of FRT raises the ferrite transformation temperature and TiC precipitation temperature; and the increase of air-cooling time from 953 K results in the coarsening of TiC particles due to growth or Ostwald growth of TiC. As for the relationship between TiC particle size and fatigue limit, it was found that fatigue limit was maximized when the average size of TiC particle was about 7 nm. This indicates that the fatigue limit of Ti added ferrite-martensite hot-rolled steel sheets depends upon precipitation strengthening of precipitates which are not cut by dislocations.

The processing and fracture analysis on transmission shafts of a peanut harvester

Abstract In this study, the lifespan of the transmission shafts is discussed from the view of microstructure. The transmission shafts are made by the processes of hot rolling, machining, heat treatment and grinding. It shows a great difference of lifespan varied from 2 months to 12 months under the same using condition, with the three transmission shafts made by different hot rolling and heat treatment process. Metallographic examination, and micro-hardness measurement along transverse and longitudinal cross-sections of the shafts reveal the forming and heat treatment process of the three kinds of failed transmission shafts. Furthermore, fracture surface examination and mechanics analysis can be used to determine the fracture mode of failed transmission shafts. Scanning electron microscope can conduct the fracture surface examination of failed shafts, and the finite element analysis is used to analyze the stress distribution of transmission shafts. Based on this study, the facts of fracture mode of the transmission shaft, and the best hot rolling condition as well as the heat treatment condition to get the better lifespan can be found.

Effect of cold rolling reduction and intercritical annealing temperature on the bulk texture of two TRIP-aided steel sheets

High strength TRIP-aided steel sheets with high formability and better ductility are of industrial interest. Texture control and retained austenite characterization are considered as the main factors with respect to the formability and ductility. Processing parameter such as initial conditions of hot rolling, cold rolling to final size of sheet, intercritical annealing after cold rolling and subsequent isothermal heat treatment to achieve TRIP specification in product, affect bulk texture of sheets. Moreover, chemical composition of steel should be noted especially because it can change the intensity of orientation and final macro texture of sheet. In this work, the effect of cold rolling and intercritical annealing on texture development has been investigated for two TRIP-aided steel sheets, which are different in C, Si and Al content. Results show that the cold rolling extends the γ-fiber on these grades of steel, whereas any intensive α-fiber components were not observed by change of cold rolling reduction. Also the effect of cold rolling on final texture of experimented materials is different and depends on chemical composition of steel. In contrary, intercritically annealing of cold rolled sheets decreases the intensity of γ-fiber so that the chemical composition of TRIP steel would influence effectively.

The Effect of Restoration Process on the Mechanical Behavior of Ultra-Fine Grain Size Nb-Ti Steel Processed by Warm Rolling and Sub and Intercritical Annealing

The ferrite grain size refining is the unique mechanism for increasing both mechanical strength and toughness of steels. Steel with an ultra-fine ferrite grained structure must show a good relation between mechanical strength, ductility and toughness, while the low carbon content enhances good welding characteristics. The objective of this work is to investigate the influence of restoration process on the mechanical behavior of a microalloyed low carbon-manganese (0.11%C, 1.41%Mn, 0.028%Nb and 0.012%Ti) steel with ultra-fine grains produced through out quenching, warm rolling, followed by sub and intercritical annealing. The evolution of restoration process -recovery and recrystallization - was followed by optical and scanning microscopy. The x-rays diffraction was employed to evaluate the degree of restoration after subcritical annealing. The mechanical behavior of the steel was estimated using microhardness tests. After subcritical annealing, the microstructure was formed by spheroidal iron carbides and a ferritic recovered matrix. Otherwise, after intercritical annealing, the microstructure was composed mainly by ultra-fine grain polygonal ferrite, MA constituent and carbides. The hardness after processing has shown a 20% increase compared with the steel under hot rolling condition.

An investigation of descaling spray on microstructural evolution in hot rolling

High pressure water spray is used to remove oxide scales in hot rolling of steel plate. This would reduce the temperature temporarily on the workpiece surface. To investigate the effect of temperature variation due to the descaling spray on the microstructural evolution, a thermal-mechanical coupled FE model has been established. The heat loss due to the spray is experimentally measured and modelled using the established FE model. The heat transfer coefficient is determined using an inverse engineering method. A set of unified viscoplastic constitutive equations is implemented into the commercial FE solver MARC through the user-defined subroutine CRPLAW. The effect of descaling water spray on the surface temperature variation is numerically studied. Furthermore, the effect of surface temperature variation on the evolution of dislocation density, recrystallisation and grain growth is investigated for various hot rolling conditions.

High Fugacity Hydrogen Effects in Beta-21S Titanium Alloy

Beta-21S titanium alloy is ranked among the most important advanced materials for a variety of technological applications, due to its combination of a high strength/weight ratio, good corrosion behavior and oxidation resistance. However, in many of these technological applications, this alloy is exposed to environments which can act as sources of hydrogen, and consequently, severe problems may arise. The objective of this paper is to investigate the influence of high fugacity hydrogen on Beta-21S alloy in as-received (mill-annealed and hot-rolled) condition. Hydrogen effects on the microstructure are studied using X-ray diffraction and electron microscopy, while the absorption and desorption characteristics are determined respectively by means of a hydrogen determinator and thermal desorption spectroscopy. Preliminary results at room temperature revealed hydrogen-induced straining and expansion of the lattice parameters. However, neither second phases formation (hydrides), nor hydrogen-induced cracking, were observed after hydrogenation. The main characteristics of hydrogen absorption/desorption behavior, as well as hydrogen-induced microstructural changes in both microstructures are discussed in detail.

Aging response and strength formability parameters of hot rolled copper alloyed interstitial free steel

Laboratory investigations have been carried out on a copper alloyed interstitial free steel to develop a high strength steel for automotive applications in hot rolled conditions. The aging characteristics for a wide range of temperature and time have been documented and discussed in terms of diffusivity and solubility of copper in α iron. Coiling temperature, in general, influences the strength formability parameters of the hot rolled steel owing to its effect on copper precipitation. A high strength hot rolled steel for automotives with high strain hardening rate can be developed by adopting low temperature coiling at 550°C, without using any additional step in the normal processing schedule of interstitial free steels. Deep drawability or the Lankford parameter appears not to be influenced much by the coiling temperature.

Investigation into damage nucleation and growth for a free-cutting steel under hot-rolling conditions

The spatial variations in the inclusion size and distribution are examined for a free-cutting steel bloom. Chemical analysis has been carried out on the inclusions using energy-dispersive X-ray analysis and the main findings are reported. It has been observed that inclusions are often associated with a lead tail. Thus an assumption is made that the lead layer becomes liquid or gaseous at hot-forming temperatures. Based on this assumption, a micromechanical finite element model is established and used to model the damage evolution process. Microstructure examinations of hot-tensile-tested specimens are used to compare experimentally the observed inclusion shape and void growth features with those obtained from the micromechanical finite element method. It is concluded that there is no significant inclusion debonding process for the material deformed under hot-forming conditions. The damage growth is directly related to the inclusion size and spacing.

A Microstructure Evolution Model for Hot Rolling of a Mo-TRIP Steel

A comprehensive study of microstructure evolution for a Mo-TRIP (transformation-induced-plasticity) steel under hot strip rolling conditions has been conducted. This investigation includes austenite grain growth during reheating, deformation behavior, and static recrystallization kinetics of austenite as well as the effect of cooling rate and austenite conditioning on the continuous cooling transformation (CCT) behavior. The physically based Kocks–Mecking model has been employed to describe the deformation behavior of austenite, while the Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach has been used to predict static recrystallization, and an empirical equation has been formulated for the recrystallized austenite grain size. Ferrite transformation start is described by a model considering early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous cooling is predicted by the semiempirical JMAK approach in combination with Scheil’s equation of additivity. The effect of carbon enrichment on ferrite transformation kinetics is explicitly included in the model. In addition, a phenomenological model for the bainite formation has been proposed. Martensite transformation start is described by an empirical equation taking into account carbon enrichment of remaining austenite. Finally, the entire hot strip rolling and controlled cooling process have been simulated by hot torsion tests, and the optimum coiling temperatures for the formation of TRIP microstructures have been determined.