Hot-rolled Dual-phase Steel Research Articles

Correlation between microstructure, mechanical properties, and slurry erosion behavior of hot-rolled dual-phase steel

A low alloy dual-phase steel was produced via a thermo-mechanical control process to improve production efficiency and reduce costs. The study investigates the influence of martensite volume fractions on the mechanical properties and slurry erosion behavior of the hot-rolled dual-phase steel, and the correlation between the mechanical properties and slurry erosion behavior was revealed to provide insights into the material's wear resistance. Microstructure of dual-phase steel with varying martensite were characterized, and nanohardness, Vickers hardness, tensile propertiy, and slurry erosion were tested. Result indicates that martensite content significantly affects the mechanical propertiy of the hot-rolled dual-phase steel. Hardness and strength increase with increased martensite content, exhibiting a linear relationship. The main strengthening mechanisms are dislocation and solid solution strengthening. Slurry erosion result reveals that matrix hardness, martensite content/size, and work hardening ability significantly impact the slurry erosion performance of hot-rolled dual-phase steel. Martensite plays a crucial role in resisting wear failure, with higher content and larger size providing better protection to the soft ferrite and reducing erosion weight loss. Work hardening ability also leads to a decrease in periodic erosion weight loss.

HAZ Effects in Hot-Rolled Dual-Phase Steel During Flash Butt Welding of Wheel Rims

HAZ Effects in Hot-Rolled Dual-Phase Steel During Flash Butt Welding of Wheel Rims

The formability and application of hot-rolled dual-phase steel under nonlinear strain paths

The hot-rolled dual-phase steel 700DP was developed to reduce weight and improve safety in automobile industry. However, strain path changes inevitably occur for most of components during sheet forming process. Therefore, it’s valuable to predict the cracking accurately under nonlinear strain paths for the application of 700DP steel with finite element method (FEM). In this paper, the mechanical properties of hot-rolled dual-phase steel 580DP and 700DP were studied with tensile test. Nakazima test and two-step nonlinear loading test were used to establish the modelling with the method of equivalent plastic strain-based FLC (ep-FLC). Furthermore, the ep-FLC was applied to predict the disc forming result of three-stage process, which agreed well with the experimental results for both 580DP and 700DP compared with conventional FLC.

Formability analysis of hot-rolled dual-phase steel during the multistage stamping process of wheel disc

For reduction of vehicle weight and higher fatigue property, the hot-rolled dual-phase steel 700DP was developed to produce wheel disc with multistage stamping process. The wheel disc is one critical component of wheel system whose forming precision is extensively concerned. The critical area of the part characterizes nonlinear strain path during the multistage stamping process, which makes it difficult to accurately evaluate the formability through standard forming limit curve (FLC) method. The concept of equivalent plastic strain was reported to be strain path independent. Hence, in this paper, the formability of 700DP steel was studied through tensile test, Nakazima test, and two-step nonlinear formability test. It was certified that the modeling using the equivalent plastic strain–based FLC (ep-FLC) had better agreement with the two-step nonlinear test results. Then based on current die shape, the feasibility of 700DP steel for wheel disc with three-stage stamping process was studied by finite element method (FEM) with both traditional FLC and ep-FLC. Furthermore, the corresponding experiments were conducted for validation. The results show that the location and critical condition for the occurrence of fracture can be more exactly predicted by the ep-FLC compared with the traditional FLC. The process parameters such as the radius of die corner were then optimized by the modeling analysis. Finally, the wheel disc was successfully manufactured with high precision and sound mechanical property.

2020 Technology and Operation of a Hot Rolling Mill

The state-of-the-art of both technology and operational practice of a hot rolling mill are discussed. The focus is first on the control of entry temperature in the finish mill. Transfer bar cooling and induction heating are presented. Next, the safe keeping in the finishing mill of surface quality, crown and flatness are discussed. Important tools to achieve these properties are lubrication, friction measurement, roll cooling, inter stand and skin cooling, bending, shifting and prediction of roll forces. The third and final topic is the run- out table cooling. Ultra-fast cooling is compared with laminar cooling. The cooling strategies and their importance to realise the required micro-structure and properties are discussed for hot rolled Dual Phase steel, pipe line grades and rebar.

Development of Tailored Structure and Tensile Properties of Thermomechanical Treated Micro Alloyed Low Carbon Dual Phase Steel

Direct hot rolled dual phase steel production represents a challenging route, compared with cold rolled and intercritical annealing process, due to complex and sophisticated control of the hot strip mill processing parameters. Instead, high technology compact slab production plant offers economic advantages, adequate control and prompt use of the advanced thermomechanical controlled rolling. The current work aims to obtain different structures and tensile properties by physical simulation of direct hot rolled niobium micro alloyed dual phase low carbon steel by varying the metallurgical temperatures of hot strip mill plant. This starts with adaptation of the chemical analysis of a low carbon content to fall far from the undesired peritectic region to avoid slab cracking during casting. Thermodynamic and kinetics calculations by Thermo-Calc 2020 and JMat pro software are used to define the transformation’s temperatures Ae1 and Ae3 as well as processing temperatures; namely of reheating, finishing rolling, step cooling and coiling temperatures. The results show that the increase of finishing rolling temperature from 780°C to 840°C or decreasing either of step cooling duration at ferrite bay from 7 to 4 seconds, enhances yield and tensile strengths, all due to more martensite volume fraction formation. The yield and tensile strengths also increase with decreasing coiling temperature from 330°C to 180°C, which is explained due to the increase of dislocation densities resulted from the sudden shape change during martensite formation at the lower coiling temperature in additional to the self-tempering of martensite formed at higher coiling temperatures which soften the dual phase steel.

Effect of Transformation Temperature on the Ferrite–Bainite Microstructures, Mechanical Properties and the Deformation Behavior in a Hot-Rolled Dual Phase Steel

The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainite can be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing the bainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% with the ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel with the lower ferrite transformation temperature after expanding. The microstructure separation along the boundary between bainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appeared in the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7%. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio.

Effect of cooling process on mechanical properties of thin-gauge hot-rolled dual-phase steel

The high strength hot-rolled dual-phase steel suitable for a thin slab continuous casting and rolling (TSCR) process was developed by a low C and Nb micro-alloyed design with the controlled rolling and cooling technology. The effect of the isothermal temperature and holding time on the microstructure and mechanical properties of the dual-phase steel was studied using a uniaxial tensile test and optical microscope (OM). The results showed that with the decrease of the isothermal temperature in the ferrite phase transformation region, the thermodynamic conditions of the ferrite transformation were enhanced. The proportion of the ferrite increased while that of martensite decreased, resulting in the reduction of the yield strength and tensile strength and the increase of elongation. With the rise in the isothermal time, the kinetic conditions of the ferrite transformation were enhanced and the proportion of the ferrite increased. The ferrite transformation has completed after 8 seconds of the isothermal holding time, and it was not apparent for increased amount of the transformed ferrite to increase the isothermal time.

Effect of Scale Spallation During Coiling on the Electrochemical and Pickling Behavior of a Hot-Rolled Dual-Phase Steel

Tertiary oxide scale consists of magnetite and hematite, and in dual-phase steel strips were observed after the exit of final rolling stand in a thin slab casting rolling (TSCR) mill. Microscopic observation confirmed that these scales cracked and spalled off from many places of surface during coiling causing a non-uniform oxide layer on the steel surface. The present work discusses the synergistic effect of such non-uniform oxide layer and run-out-table cooling water on the electrochemical behavior of the dual-phase steel. The electrochemical response was assessed with the help of dynamic and linear polarization tests considering cooling water as electrolyte. The presence of non-uniform scale produced high cathodic (scaled zone) to anodic (spalled-off zone) area ratio that eventually led to extensive pitting corrosion in the present steel. Furthermore, the pickling behavior of the steel was also studied in detail. It was observed that the steel surface, with spalled-off area of about one-tenth of total surface area, corroded preferentially 3-4 times faster at spalled-off area than that of the uniformly pickled surface. The characterization study was performed using scanning electron microscopy, confocal laser 3D scanning microscopy, x-ray diffraction and Raman spectroscopy in order to establish the reasons for pitting corrosion due to the scale spallation.

Effect of Cr and Al additions on the development of interphase-precipitated carbides strengthened dual-phase Ti-bearing steels

The development of Ti-Cr-bearing hot-rolled dual-phase steel with an excellent combination of mechanical properties (yield strength of 655MPa, ultimate tensile strength of 911MPa, and elongation of 30%) has been achieved. Due to the significant strengthening obtained from interphase-precipitated nanometer-sized carbides in ferrite, this dual-phase steel has yield ratio of about 0.72, which is much higher than that of conventional dual-phase steels. Isothermal transformations on a dilatometer have been performed to investigate the effect of Cr and Al additions on austenite-to-ferrite transformation in Ti-based steels. It was found that an addition of 0.6wt.% Cr obviously retards the transformation rate of ferrite, as compared with an addition of 0.1wt.% Al. The results from dilatometry and transmission electron microscopy revealed that due to the lower transformation rate in Ti-Cr-bearing steel, a much higher density of nanometer-sized interphase-precipitated carbides in ferrite occurred during the isothermal holding at 650°C, leading to a significant increase in the strength of ferrite.

Model Algorithm Research on Cooling Path Control of Hot-rolled Dual-phase Steel

With the development of advanced high strength steel, especially for dual-phase steel, the model algorithm for cooling control after hot rolling has to achieve the targeted coiling temperature control at the location of downcoiler whilst maintaining the cooling path control based on strip microstructure along the whole cooling section. A cooling path control algorithm was proposed for the laminar cooling process as a solution to practical difficulties associated with the realization of the thermal cycle during cooling process. The heat conduction equation coupled with the carbon diffusion equation with moving boundary was employed in order to simulate temperature change and phase transformation kinetics, making it possible to observe the temperature field and the phase fraction of the strip in real time. On this basis, an optimization method was utilized for valve settings to ensure the minimum deviations between the predicted and actual cooling path of the strip, taking into account the constraints of the cooling equipment's specific capacity, cooling line length, etc. Results showed that the model algorithm was able to achieve the online cooling path control for dual-phase steel.

Effect of Mo on Transformation, Microstructure and Property of Hot-Rolled DP600 Steel

In according with the transformation rules of C-Si-Mn-Cr and C-Si-Mn-Cr-Mo tested steels, rolling experiment was carried out in lab. to analyze the effect of Mo on transformation, microstructure and property of high strength hot rolled dual phase steels. The results showed that the addition of element Mo decreased start temperature of ferrite transformation and restrains pearlite transformation. There was a metastable austenite zone between ferrite and bainite transformation zone of No.2 steel with 0.35% of Mo. After controlled rolling and step cooling process, dual phase microstructures in which fine martensite islands dispersed in soft ferrite matrix were obtained by two kinds of tested steel. With the addition of Mo, yield strength and tensile strength increased by 80MPa and 60MPa respectively, meanwhile, elongation increased slightly.

THE PRODUCTION OPPORTUNITIES OF THE HOT-ROLLED DUAL-PHASE STEEL WITH SINGLE-STAGE COOLING ON THE RUNOFF TABLE OF HOT-ROLLING MILL 2000 AT JSC “NLMK”

The purpose of these studies was to find opportunities to provide the mechanical properties of hot-rolled dual-phase steel in the range of strength class DP 450–600 at “NLMK”. In this case decisive role in the formation of the final structure and properties acquires a one-stage cooling strip strategy on the run-out table in the conditions of hot rolling mill 2000 at “NLMK”. The laboratory investigations have been conducted on the influence of hot rolling parameters on the structure and mechanical properties of steel grade SPRC440R, used by “NLMK” for the production of cold-rolled dual-phase steels. The temperature-speed parameters of accelerated cooling on the run-out table of hot-rolling mill 2000 at “NLMK” were calculated by mathematical model in order to realize the rolling experiment. The results of laboratory investigations and the carried out calculations were used for the development and implementation at experimental-industrial rolling conditions. In this article authors give recommendations for the possibility of providing the mechanical properties of hot-rolled dual-phase steel in the range of strength class DP 450–600.

Formation of dual-phase structure during cooling on the exit conveyer of the 2000 broad-strip rolling mill at OAO NLMK

The step-by-step development in the production of HDT580X hot-rolled dual-phase steel (according to the EN 10338 standard) by means of the 2000 broad-strip hot-rolling mill at OAO Novolipetskii Metallurgicheskii Kombinat is considered. The analysis is based on theoretical and experimental results regarding topics such as the influence of alloying elements and the cooling parameters on structural and phase transformations in dual-phase steel for the auto industry. Physical modeling of the deformational parameters and the rolling and cooling temperatures and speeds on the broad-strip mill also proves helpful. Rolling and cooling rates are proposed for the 140 FGA semicontinuous hot-rolling mill.

Production of auto-body dual-phase steel with single-step cooling on the output conveyer of the 2000 continuous broad-strip mill at OAO NLMK

The goal is to produce hot-rolled dual-phase steel of strength class DP 450–600 at OAO Novolipetskii Metallurgicheskii Kombinat (NLMK). The steel’s final structure and properties is greatly affected by single-stage strip cooling on the output roller conveyer of the 2000 continuous broad-strip mill. The influence of the hot-rolling parameters on the structure and mechanical properties of the hot-rolled SPRC440R steel from which cold-rolled dual-phase steel is produced at OAO NLMK is studied in the laboratory. The temperatures and rates for accelerated cooling on the output roller conveyer of the 2000 continuous broad-strip mill at OAO NLMK are calculated by means of a mathematical model. The laboratory data and simulation results are used in developing trial conditions for industrial rolling. Recommendations are made regarding the production of hot-rolled dual-phase steel corresponding to strength class DP 450–600.

A Study of the Microstructure and Mechanical Properties of C-Si-Mn Hot Rolled Dual Phase Steel

The effects of the microstructure properties on the mechanical properties of C-Si-Mn hot rolled dual phase steel are reported. It was observed that both the amounts of the martensite and the particle size of the ferrite significantly influence the mechanical properties of hot rolled dual phase steel. The aim of our works is to develop a modeling method for controlling the microstructure properties of C-Si-Mn hot rolled dual phase steel through the experimental trend comparison of the aforementioned two factors on their mechanical properties.

Effect of C on Microstructure and Mechanical Property of Hot Rolled DP Steel

Two kinds of C-Si-Mn-Cr-Mo-Nb tested steels with different C content were disigned, and controlled rolling and controlled cooling experiment of tested steels have been carried out in lab to analyze the effect of C on microstructure and mechanical property of high strength hot rolled dual phase steel. It is shown that two kinds of tested steels obtained ferrite and martensite microstructure under controlled rolling and step cooling process. With the increase of C content from 0.032wt% to 0.064wt%, ferrite grain size refines from 9.8μm to 5.0μm, width of martensite colony reduces from 10μm to 5.8μm, and martensite volume fraction increases from 11% to 25%. Yielding strength of high C tested steel is higher than that of low C steel by 55MPa, and amplification of tensile strength is about 155MPa. Elongation ratio decreases about 6.4%. Increasing of yielding strength is determined by strengthening of ferrite grain refining. The amplification of tensile strength is effect by volume fraction and refining of strengthening phase.

Thermo-Mechanical Behaviour of Dual-Phase Steels in Various Structural Morphologies: Experiments and Modelling

The thermo-mechanical behaviour of the hot rolled dual-phase steel 10MnSi7 has been determined as a function of temperature and time within the two-phase field α+γ as well as at lower deformation temperatures down to Ms-temperature. The corresponding microstructures, phase hardness, state of recrystallisation have been determined using quantitative metallography. Using this information, a finite element model of a representative volume element (RVE) has been set up. Each element represents a subdomain of a grain that transforms according to the kinetics found in the measurements. The RVE has been subjected to the same loading history as in the accompanying experiments and the overall stress-strain response is monitored during ongoing transformation. The effect of a preferred orientation of the two phase structure has been studied taking into account the effects of a significant plastic deformation. The model is validated by comparison with the experimental evidence.

Potential of Hot Rolled Dualphase Steel from Compact Strip Production CSP<sup>®</sup> in Automotive Applications

Industrial thin slab casting and direct rolling processing started in 1989 with the world’s first CSP® plant at Crawfordsville (USA). Since this time CSP® and competing thin slab casting and direct rolling concepts have been developed to a standard process for hot strip production [1]. Typical features of the CSP® process are the homogeneous structural and mechanical properties all along the strip. Direct hot rolling of thin slabs may be followed by a well defined cooling pattern to produce hot strip from high strength multiphase steel, like dualphase (DP) grades, on the runout table. These steel grades are characterized by a favorable combination of strength and ductility based on hard martensitic particles embedded in a ductile ferritic matrix. This paper highlights the mechanical properties of hot rolled DP steel from CSP® production. To this purpose, multiple tests and modeling have been applied to determine e.g. r-values, forming limit curves and yield locus. In addition, forming simulation as well as laboratory and industrial deep drawing tests have been performed.

The influence of aluminium on the ferrite formation and microstructural development in hot rolled dual-phase steel

Abstract The influence of intentional aluminium additions on the ferrite phase transformation of hot rolled ferrite–martensite dual-phase steel is investigated. A range of isothermal holding temperature experiments indicates acceleration of the rate of ferrite transformation, with suggestions made to utilise aluminium additions to increase the range of holding temperatures which are currently employed during production. Through adequate alloying, it is shown that aluminium additions have the potential to maximise the initial range of holding temperatures used during three-stage cooling, enabling easier processing and limiting the distribution of mechanical properties within the final material.