Cold-rolled Strip Research Articles

Sliver defects in an ultra-low carbon Al-killed steel caused by low steel level in the tundish

ABSTRACT Sliver defects were detected in galvanized and uncoated cold-rolled strips, and their causes were investigated. Most of these strips were produced from transition ladle slabs, which were obtained in a sequence of twenty heats of an ultra-low carbon Al-killed steel. A significant reduction in the steel level in tundish occurred during these slabs’ casting. The strip defects, and as a comparative, a mould flux inclusion found in a first slab of the sequence, were analysed with a scanning electron microscope (SEM). Also, each of the twenty heats was sampled in tundish, quenched in water, and manually analysed with SEM to characterize its dominant inclusions. Regarding the sliver defects, mainly silica together with alumina or Al–Ti–O systems particles were found in them. The primary sources for these particles are likely tundish flux and reoxidation products. Finally, a mechanism was proposed for the formation of these inclusions.

Estimation of depth and degree of mill roll surface hardening during grit texturing

The paper presents shot blasting (SB) as the most common method of arrangement of temper mills rolls surface microgeometry, providing required roughness of the cold-rolled strip. The surface riffing studies were carried out at the Willibrator industrial unit; metallographic studies were performed using MEIJI 2700 optical microscope and JSM-6490LV scanning electron microscope. Refinement of the surface layer structure during interaction of the roll material with the grit was revealed. In the work the lower bound method, one of the methods of plasticity theory, is used for theoretical analysis of the SB process. To make computations easier, the roll microcave is approximated to spherical shape. Quantitative estimates of deformation degree and the depth of hardened layer were determined by speed of the grit and hardness of the roll surface. The depth of hardened layer is obtained by the size of used grit, and to a lesser extent, by the speed of grit impact on the roll surface and textured surface hardness. It has been searched out in literature that increase in hardness by one unit leads in average to 3 % increase in resistance. Thus, SB application allows reduction of time of rolls rehandling of temper mills by 6.0 – 10.5 %, depending on application modes of rolls riffing with grit. Analysis of microstructure have shown that increase in hardness of the surface layer is a consequence of refinement of its structure in process of interaction with the grit. It has been revealed that increase in hardness of the surface layer leads to an increase in its wear resistance and fatigue strength. The authors of the work have found that at speed of 60 m/s the hardness increases by 3.5 units.

Influence of the Cross-Sectional Shape of Hot Semifinished Rolled Products on the Formation of the Plot of Specific Tension in Cold-Rolled Strips

In view of the increasing competition in the market of rolled products, the problem of evaluation of the quality of strips in various stages of the process of rolling seems to be quite urgent. As the main geometric criteria of the quality of rolled strips, we can mention the cross-sectional profile of hot-rolled semifinished products and the plot of specific tension in the strips after cold rolling. We discuss the problems of formalization of measurements of the cross-sectional profiles, classification of hot-rolled profiles into characteristic types, and the influence of these criteria on the formation of the plots of specific tension in cold-rolled strips.

Profiles’ classifier of hot-rolled rolling

The geometric parameters describing the features of the cross-sectional profile of the hot-rolled strips do not give a complete picture of the flatness acquired by the cold-rolled strips rolled from these strips. An additional analysis showed that there are four characteristic classes of cross-sectional profiles of hot rolled strips that have a significant effect on the shape of the strips during cold rolling, three of which negatively affect the flatness of the cold rolled strips. The cross-sectional profiles of hot-rolled strips with a concave middle part and (or) marginal thickenings lead to the appearance of edge waviness, peak-like cross-sectional profiles cause central warping. Therefore, the actual task is to determine the factual shape of cross-sectional profile. Sixth order polynomials were used to digitalize and parameterize hot-rolled profile. As a result, we developed analytic function of the transverse profile, which keeps important information about its near-edge areas and features in the middle part. To assign a specific cross-sectional profile of a hot-rolled strip to one of four characteristic classes of cross-sections, mathematical software was developed, called a classifier, and implemented with the programming environment R. To classify the profiles of the hot-rolled cross-section according to characteristic classes, a linear discriminant method was used as a machine learning method analysis. The result is an adequate mathematical model for recognizing the shape of the cross-sectional profile.

Analysis of geometrical parameters of hot-rolled rolling

The shape of cross-sectional profile is the most important characteristic of the quality of hot rolled steel, because it is accurately reproduced after subsequent cold rolling and thereby has a significant effect on the flatness of cold rolled strips. Traditional method to evaluating the accuracy of cross-sectional profile by geometric dimensions is the least squares approximation of hot-rolled strip thickness by second order polynomial. A number of parameters that characterize the macro-features of hot-rolled strips’ profile are found from the obtained second order parabola coefficients; this is wedge, convexity, displacement, coefficient of determination and the recently introduced saddle-shape coefficient, showing the presence of edge thickenings. Edge thickenings are formed due to wear of the work rolls in the areas corresponding to the edges of the rolled strips. An analysis of the existing geometrical parameters of the cross-sectional profile of the hot-rolled strips showed that with their help in some cases it is practically impossible to predict the flatness of cold-rolled strips. Analysis of the flatness of cold-rolled strips has identified four specific classes of cross-sectional profiles of hot rolled strips that have a significant effect on the shape of cold-rolled strips, and three of them negatively affect the flatness of cold rolled strips. Profiles with a concave middle and (or) near-edge areas lead to waviness, pike-shaped profiles cause warping.

Investigation of Self-Oscillations and the Development of a Procedure of Stabilization of the Process in a Continuous Mill for Strip Cold Rolling

We study the problem of operation of sheet rolling mills under the conditions of nonstationary technology, the stochasticity of the parameters in the presence of negative self-oscillation vibration processes associated with the “chatter” phenomenon. We determine the parameters of stability of the process of sheet rolling, reveal the main causes of the chatter phenomenon, and describe the mechanism of their initiation. We also propose a procedure for the identification of this phenomenon by continuous statistical processing of the parameters of rolling process. As a result of the performed investigations, we developed a method aimed at the adjustment of the technology, which excludes vibration and enables us to enhance the productivity of rolling mills by means of increasing the average speed of rolling.

Multi-objective optimization of rolling schedule for tandem cold strip rolling based on NSGA-II

Abstract A multi-objective optimization model of rolling schedule based on NSGA-II algorithm was presented, and three objective functions are defined as motor power balance, good flatness and preventing slippage. The experimental results show the excellent performance of NSGA-II in obtaining non-dominated front which could suggest several alternative solutions to rolling schedule in a single run of the algorithm. Additionally, considering that the rolling load distribution strategy dynamically adjusts are required to balance the impact of roll surface condition changes on the rolling process, a model of weighted-sum approach with variable weight coefficients based on rolling length was proposed. In this model, a roll change cycle was divided into four optimization stages which are given different weight coefficients. Moreover, a smooth correction model of weight coefficient was proposed to ensure the quality of strip flatness. Compared to the traditional weighted-sum approach, the presented model on the basis of variable weight coefficients can make full use of Pareto set, and has better adaptability to changing rolling conditions. Finally, the proposed model was applied to the multi-objective optimization model above, the results showed that the final optimal solutions have better performance in almost all objective dimensions than the empirical one. The practical application has further confirmed the effectiveness of the multi-objective optimization for rolling schedule.

Improved mechanical properties of V-microalloyed dual phase steel by enhancing martensite deformability

A good combination of ultimate tensile strength (UTS) up to 1365 MPa and total strain to failure (StF) to 15.5 % has been achieved due to deformable martensite in the invented vanadium-microalloyed dual-phase (DP) steel, which was manufactured by two-stage annealing of cold rolled steel strip. The employed extensive characterizations revealed that the ductile martensitic phase in this DP steel differentiated from ordinarily low-carbon martensitic lath in both morphology and lattice structure. Complex coherent orientation relationships between ferrite, reverse austenite, martensitic phase and vanadium carbide (VC) do exist, leading to a new martensitic transformation mechanism and resultant dual-phase microstructure. Besides, a detailed characterization including essential phase transformation analysis in combination with in situ TEM observation, shows that, all the essential processing including recrystallization, reverse austenitic and martensitic transformation, in debt to the particular effects of VC, can be recognized as phase transformations with higher thermodynamic driving force and higher kinetic energy barrier as compared to previously common processing, which actually changes the microstructure and, indirectly leads to higher strength and higher ductility. This synergy of thermodynamics and kinetics can be generalized to improve mechanical properties of present steels.

Multi-faceted modelling for strip breakage in cold rolling using machine learning

In the cold rolling process of steel strip products, strip breakage is an undesired production failure which can lead to yield loss, reduced work speed and equipment damage. To perform a root cause analysis, conventional physics-based approaches which focus on mechanical and metallurgical principles have been applied in a retrospective manner. With the advancement of data acquisition technologies, numerous process monitoring data is collected by various sensors deployed along this process; however, conventional approaches cannot take advantage of these data. In this paper, a machine learning-based approach is proposed to characterise and model strip breakage in a predictive manner. First, to match the temporal characteristic of strip breakage which occurs instantaneously, historical multivariate time-series data of a cold rolling process were extracted in a run-to-failure manner, and a sliding window strategy was adopted for data annotation. Second, breakage-centric features were identified from three facets – physics-based approaches, empirical knowledge and data-driven features. Finally, these features were used as inputs for strip breakage modelling using recurrent neural networks (RNNs), which are specialised in discovering underlying patterns embedded in time-series data. An experimental study using real-world data collected from a cold-rolled electrical steel strip manufacturer revealed the effectiveness of the proposed approach.

Effects of Abrasive Particles on Lubricating Property of Oil-in-Water (O/W) Emulsions for Cold-Rolled Strip

The abrasive particles in the working emulsion were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The wettability and lubricating properties of different emulsion samples were tested by MRS-10E four-ball friction and wear testing machine. The results show that the average particle size of the abrasive particles in the working emulsion is 2.72 μm, the smallest particle size of the abrasive particles is 0.36μm, and the largest size of the abrasive particles is 6.57μm. Furthermore, the abrasive particles can increase the internal friction of emulsions, which leads to the increasing wetting Angle. The abrasive particles cause the lubrication performance of working emulsion to decrease, which eventually leads to larger wear scar diameter and the worse morphology of wear scar on the tested ball.

Research on Tribological Properties of Environment-Friendly Nano-MoS<sub>2 </sub>Water-Based Rolling Liquid

In this research, the four-ball tribometer was examined to evaluate the tribological behavior of environment-friendly nano-MoS2 water-based rolling liquid. This experimental method is used to measure the maximum non-seizure load (PB) and the coefficient of friction (μ). The newly introduced parameter ω can be combined with PB and μ to evaluate the tribological performances of the rolling liquid comprehensively. The rolling lubrication performance of nano-MoS2 water-base lubricant was investigated using a single stand 4-high reversing rolling mill. The surface morphologies and rolling textures of the samples were measured by laser scanning confocal microscope after cold rolling. The results show that when the concentration of nano-MoS2 is 0.4wt.%, the water-based rolling liquid has excellent tribological and lubricating properties. Meanwhile, compared with the surface quality of the rolled strip under other lubrication conditions, the surface quality of the cold-rolled strip using 0.4wt% nano-MoS2 rolling liquid as the lubrication condition is smooth without visible surface defects.

Effect of Annealing Conditions on the Evolution of the Grain Structure and Intermetallic Phases in the Cold-Rolled Strip of Aluminum–Magnesium Alloy

The effect of annealing conditions of the cold-rolled strip made of a new aluminum–magnesium alloy 1565ch, containing zinc and zirconium, on the evolution of the grain structure has been analyzed in this work. This material was recently developed by Russian scientists and has already been used in transport engineering, but has not yet been sufficiently studied. Recrystallization processes occurring in this alloy and their connection with intermetallic particles are of particular interest. This work is aimed at studying the effect of the temperature and annealing time on the grain size in the 1565ch alloy. The grain structure has been studied by optical microscopy, and the intermetallic particles have been studied by scanning electron microscopy. In addition, the phase composition of the 1565ch alloy has been calculated using Thermo-Calc software. The grain size after achieving the recrystallization threshold has been shown to be insensitive to the annealing time and directly depends only on the annealing temperature, which no longer has a noticeable effect after 350°C. The 1565ch alloy tends to recrystallization with involving coarse intermetallic particles of the second phase. These particles are Al6(Mn, Fe) and Mg2Si. In addition, the effect of fine intermetallic particles on the recrystallization process has been investigated. The dissolution and coagulation of the fine particles in this alloy provoke rapid recrystallization.

Influence of Pulling Modes and Temperature of Coiling Cold-Rolled Strips

The formative patterns of gaps between turns of cold-rolled strips in rolls and the ways these gaps affect the stress-and-strain state (SSS) of these rolls are disclosed. The effects of surface roughness, temperature, physical mechanical properties, strain degree, and hardening of cold-rolled steel on the interturn gaps in and the roll’s SSS are analyzed in detail. Possibilities of affecting the roll’s SSS by adjusting the cold-rolling temperature of fine strips and tinplate are exposed. Recommendations on choosing thermal-speed modes of strip rolling and coiling are presented.

Recycling prospect and sustainable lubrication mechanism of water-based MoS2 nano-lubricant for steel cold rolling process

The water-based MoS2 nano-lubricant was prepared and tentatively applied in cold rolling lubricant recirculation systems to investigate its recycling prospect and sustainable lubrication mechanism. The tribological properties and rolling lubrication performance of recycled nano-lubricants with different cycle stages were preliminarily evaluated using four-ball tribotester and two-high rolling mills. To determine the phase transformation of MoS2 in recycled nano-lubricants, the already used MoS2 nano-lubricant was collected, dried and then analyzed by means of X-ray diffraction (XRD). Molecular dynamics simulation was conducted to analyze the specialized lubrication mechanism of reused MoS2 nano-lubricant. The results revealed that the MoS2 was considered to gradually transform into MoO3 during cold rolling process. Those oxidizing metamorphic MoS2 nano-lubricants still had excellent friction reducing and wear resistance. That indicated MoS2 nano-lubricant could be recycled in cold rolling process. The recirculation of MoS2 nano-lubricant reduced the use of lubricants, the discharge of waste liquid and the production cost. Not only the pollution to environment was relieved, but also resources and costs were saved. The calculation results of adsorption energy between nanoparticles and steel surface further demonstrated the absorbing type of lubricating film in different cycle stages. A lubrication model suitable for MoS2 nano-lubricants under steel strip cold rolling lubrication circulation condition was established to explain their sustainable lubrication mechanism.

Effect of carbon on isothermal reduction of high-strength steel oxide scale in 30%H2–N2 atmosphere

To replace the traditional pickling process, an environmentally friendly gaseous reduction method was used for the removal of the oxide scale of steel in this work. Isothermal hydrogen reduction of high-strength steel was investigated in the temperature range from 500 °C to 800 °C in 30%H2–N2 atmosphere. The obtained results revealed a clear relationship between the oxide phase composition and the holding temperature during the heating stage, prior to hydrogen reduction. With increasing temperatures, the Fe2O3 and Fe3O4 components of the oxide scale gradually transform into FeO. When the sample was reduced under H2–N2 atmosphere, the surface morphology of the high-strength steel was different from that of the traditional hot-rolled steel strip. It was observed that the surface pore size and porosity increased with increasing temperatures and internal and external oxidation occurred simultaneously in the reduction layer. The addition of carbon to the reduction system decreased the water pressure and clearly promoted the reduction at 800 °C. The surface of the sample was reduced to irregular but uniformly distributed small particles of pure iron. Defects in the oxide scale such as cracks and gaps and carbon reacting with water play a vital role in the reduction process. Since the use of hydrogen reduction of hot-rolled strip eliminates the need for the acid pickling pollution process and the pre-oxidation process in the annealing of cold-rolled strips, it can significantly reduce the energy consumption of steel production and improve the production line efficiency. Thus, this approach is a promising green production technology in hot-dip galvanizing.

Simulation modelling of cold rolled metal strip by asymmetric technology

In the article provided the results of studies the optimization of energy-saving technologies in mechanical engineering using modern methods of simulation modeling is becoming an urgent area for the informatization of engineering production on the process of cold asymmetric rolling of metal sheets is accompanied by an inhomogeneous stress-strain state.The research considers the noncanonical areas of strain, an approach is proposed for determining the optimal technological parameters and studying the stress state of the cold rolling process using asymmetric rolling technology in the active zone of elastoplastic strains based on simulation using the application software LS-DYNA by LS-PrePost (R) version V4.6.1. To optimize energy-saving for rolling technologies, considered a stationary regime of symmetrical rolling, the kinetic energy does not change over time, and the potential energy in the active zone of elastic-plastic strains tends to increase. For the case of asymmetric rolling, a high level of kinetic energy is observed, and there is a fluctuation over time, explained by different coverage of asymmetric rollers in the zone of active elastic-plastic strains, as a result of which the stationary nature of the metal sheet rolling mode is violated.

Investigation of the structure and properties of cold-rolled strips from experimental alloy 1580 with a reduced scandium content

The structure and properties of cold-rolled strips with a thickness of 3 mm from an experimental aluminum alloy 1580 with a lower scandium content of 0.03% (wt.) relative to the grade of the aluminum alloy and a 5083 similar alloy without scandium were studied. Ingots obtained at the laboratory installation of semicontinuous casting of the foundry department of the Siberian Federal University (Siberian Federal University, Russia, Krasnoyarsk) were used. The preparation of ingots for rolling included milling up to sizes of 40 × 100 × 145 mm and annealing according to a two-stage mode: at a temperature of 350 °C for 3 h (first stage) and at a temperature of 425 °C for 4 h (second stage). Hot rolling was carried out in the laboratory of the Department of Metal Forming at the Siberian Federal University at a temperature of 450 °C on a two-roll hot rolling mill with a roll diameter of 330 mm and a barrel length of 520 mm to a thickness of 5 mm, which corresponded to a total reduction of 88% with a single reduction of 2–5%. For cold rolling, a two-roll sheet rolling mill with a roll diameter of 200 mm and a barrel length of 400 mm grade LS 400 AUTO was used. Rolling was carried out to a thickness of 3 mm with a single reduction of 2–5%, and then the strip was annealed. The results of determining the mechanical properties by tension by the universal LFM400 machine showed that with an increase in the annealing temperature in the range from 250 to 350 °C for 3 h, ultimate tensile strength of the cold-rolled strips of aluminum alloy 1580 decreases from 385 to 365 MPa. For aluminum alloy 5083, a decrease in this strength characteristic is also observed from 345 to 320 MPa. A decreasing tendency with increasing annealing temperature was also observed for the conditional yield strength Rp, and over the entire range of annealing temperatures, the values ​​of Rp for strips of alloy 1580 were higher than for alloy 5083 by 35–40 MPa, which amounted to 14–17% The values ​​of the plastic properties, for which the value of the elongation to failure was analyzed, were close throughout the range of annealing temperatures for both alloys. The results of the analysis of micro- and fine structure allowed to conclude that an increase in the strength properties of cold-rolled and annealed sheets from the experimental alloy 1580, compared with alloy 5083, is a consequence of the addition of scandium in the experimental alloy 1580, which leads to an increase in the temperature of recrystallization of the alloy, preserving it contains a subgrain structure and dispersion hardening caused by precipitation of Al3(Sc, Zr) phase particles during the decomposition of a solid solution.

Modeling and Optimization of Rolling Process: A Multi-Objective Approach

In a high-speed cold strip rolling process, it is necessary to optimize the process parameters for improved quality in the product. In this study, two separate multi-objective optimization problems for a cold rolling process are formulated. The objectives in one of the cases are minimum isothermal film thickness and film temperature rise in the inlet zone and in another case it is minimum thermal film thickness and film temperature rise in the inlet zone. Particle swarm optimization algorithm has been used for solving the optimization problem. The key input parameters for the cold rolling process are identified and prioritized through the convergence study and the coefficient of variation analysis. A response analysis is performed on the critical input variables. This study assists the process engineer to understand the lubrication in cold strip rolling at high speed and select an appropriate lubricant for a given combination of strip and rolls.

Microstructure and Mechanical Properties of an Austenitic CrMnNiMoN Spring Steel Strip with a Reduced Ni Content

The article presents the mechanical properties of the austenitic stainless steel X5CrMnNiMoN16-4-4 after deformation by cold rolling and subsequent short-term tempering (deformation and partitioning (D&P) treatment). Tensile strengths of 1700–900 MPa and beyond were achieved both after work hardening and in the D&P-treated strip. The initial state of austenite in terms of grain size and pre-strengthening, as well as the selected cold rolling temperature significantly influenced the deformation-induced formation of α’ martensite and thus the flow and hardening behavior of the steel. The usage of two different rolling temperature regimes showed that the strength properties in the cold strip can be specifically adjusted. Lower deformation-induced martensite fractions enabled a larger thickness reduction of the strip without increasing the rolling force, while high deformation-induced martensite fractions led to strong hardening at low deformation levels. The D&P-treatment permits the strength of the cold-rolled strip with a predominantly austenitic microstructure to be increased to the required level. The total elongation of such a D&P strip was well over 2%. The D&P treatment of the spring steel strip is a cost-effective alternative to conventional tempering treatment.

Dynamic rolling force modeling of cold rolling strip based on mixed lubrication friction

Dynamic rolling force modeling of cold rolling strip based on mixed lubrication friction