Continuous Annealing Process Research Articles

Comprehensive tension setting optimisation technique for a cold rolled strip during continuous annealing

ABSTRACTThe previous tension setting for the continuous annealing process (CAP) is not comprehensive; that is, it does not synthetically consider the factors of strip deviation, buckling, flatness and width narrowing. To address this issue, this study integrated the mechanical and technological characteristics of a continuous annealing line to present a comprehensive control index that can characterise the stability of a travelling strip. Meanwhile, strip deviation, buckling, strip shape and width narrowing were regarded as constraints in each process section. A comprehensive optimisation technique for the tension setting during CAP was then established, and a related theory was applied during production. In addition, a comprehensive tension setting optimisation software was developed for a cold rolled strip during continuous annealing, and the comprehensive optimisation of the tension parameters in each process section was realised. A practical implementation proves that the developed technique exhibits good application effects and considerable application values.

Mechanical properties, formability and corrosion resistance of thermomechanically controlled processed Ti-Nb stabilized IF steel

Aim of the present study is to examine the possibilities of achieving a better combination of yield strength (YS), ductility and formability with a better corrosion resistance of a Ti-Nb stabilize IF steel through microstructural refinement by simple phase controlled thermomechanical rolling. The phase controlled thermomechanical rolling could be an industrially reliable method for the production of hot strips, which can substitute cold rolled steel sheets. Accordingly, the phase controlled multi-pass rolling was performed in 3 selected phase regimes (γ-recrystallization, γ→α transition & α-regions) on the basis of Ac3/Ar3, Ac1/Ar1 (obtained through Thermo-Calc & Gleeble-3800) and Tnr (from Boratto equation) followed by air cooling. The volume fraction of precipitate(s) correspond to the deformation temperature was estimated using Thermo-Calc Software and the morphology of the precipitates was analyzed by TEM. The strain induced phase transformation of unstable γ occurred during rolling at a high reduction of ~80%, ɛ=1.6 at γ→α transition region. Thus, dynamically recovered stable bimodal equiaxed ferrite structures (fine ferrite ~5µm embedded with larger size ferrite grains ~32µm) were obtained after air cooling to room temperature. In case of the rolling at α-region, improvement of the YS (>3-fold) is attributed to the formation of ultrafine ferrite grains (1–3µm) through subgrain structures, strain-induced precipitation of nanosize NbC and/or TiC and micro-shear bands. Very short-annealing (~100s) at 850°C followed by forced air cooling was employed in order to simulate continuous annealing process and was found to improve the formability without much affecting its YS. The avoidance of FeTiP phase formation (which deteriorates to form {111} recrystallization texture) and nucleation of ferrite grains within the deformation bands (studied through EBSD study) by a short-annealing treatment are accountable for regaining the formability. The role of strain hardening exponent (n) and plastic-strain-ratio (r) on the deformation characteristics of the thermomechanically treated IF steel were also investigated to correlate the YS and uniform elongation. Furthermore, the rolled (at α-region) + short-annealed samples showed an excellent corrosion resistance due to the formation of dense oxide film on the surface. This is attributed to the dissolution of Fe(Ti+Nb)P precipitates (which are the potential sites for initiation of pits), and formation of fine grains (which facilitate to form dense oxide film on the large surface area).

Modelling of strip buckling in the continuous annealing process

This paper considers the equipment and technological characteristics of continuous annealing process (CAP) to solve the issue of strip buckling. It analyses the mechanism of hot buckling and transverse compressive stresses from internal and external forces. A basic description of strip buckling is given together with a new concept of a buckling index. The critical buckling condition is presented, and the model of strip buckling index in the CAP is proposed. The distribution of strip buckling index in different sections of the process is analysed quantitatively along with the influence of incoming strip shape, transverse temperature difference, roller profile, friction coefficient, set tension and strip processing speed on the strip hot buckling index in CAP. Results show that with the influence of thermal stress, the transverse compressive stress that affects the middle part of the strip is largest in the heating section, which is also where hot buckling occurs most easily; centripetal force is dominant in the soaking and slow cooling sections, and in these sections buckling occurs at the roller shoulder. In different sections, the strip buckling index distribution differs, whereas the influence of other parameters on buckling index remains similar.

Research of the Continuous Annealing Process to Eliminate the "Band" Feature of Dual Phase Steels

A new continuous annealing process which can eliminate the "band" feature of dual phase steels has been designed. The sheets were annealed in MULTIPAS annealing simulator under variation of annealing temperature. The microstructure evolution of dual-phase steel sheets annealed at intercritical temperatures, were analyzed by optical microscopy and scanning electron microscopy. And the properties were compared with the sheets which annealed by the existing annealing process. The results show that: the "band" feature of dual-phase steel can be effectively eliminated by the optimizing continuous annealing process,and the mechanical properties are improved.

Nonlinear Process Monitoring Using Regression and Reconstruction Method

In this paper, a new regression and reconstruction method for process monitoring is proposed. The main contributions of the proposed approaches are as follows: 1) a new nonlinear regression algorithm is proposed to extract the output-relevant variation, which, compared with the conventional algorithm, builds a more direct relationship between the input and output variables; 2) the fault direction is determined by possible fault magnitude of every possible principal component; and 3) the fault is effectively diagnosed compared with the conventional kernel partial least-squares (KPLS) method. The proposed method is applied to a continuous annealing process and is compared with the KPLS method. Experiment results show that the proposed method can more effectively detect fault compared with the KPLS method. In addition, the selection of fault direction is more accurate using the proposed reconstruction algorithm compared with the KPLS reconstruction approach.

Microstructure Characterization and Mechanical Properties of TRIP-aided Steel under Rapid Heating for Different Holding Time

In order to develop a comprehensive understanding about the effect of different holding time under rapid heating on the microstructural evolution and mechanical properties of transformation-induced plasticity (TRIP) steel, continuous annealing process simulations were performed using a thermal system with resistance heating method. The morphology and distribution of all phases present in the microstructure and the mechanical properties of TRIP steel were revealed. It appeared that the final tensile strength of the TRIP steel increased and retained austenite carbon content decreased with increasing holding time. An overlap between ferrite recrystallization and austenitization was observed during intercritical holding. In addition, the work hardening of the samples was evaluated by calculating the instantaneous n value as a function of the true strain. The difference in work hardening behavior corresponds to the rate of the retained austenite transformation during straining, which can be attributed to the carbon content and the morphology of the retained austenite.

Study on Continuous Annealing Process for Ultrathin Cold-Rolled SPCC Steel

The continuous annealing process was simulated on the heat treatment system for thin steel sheet (CCT-AY-II). As the recrystallization was known, the annealing temperature and holding time were taken into account to study the influence on microstructure and mechanical properties of 0.08mm ultrathin cold-rolled SPCC steel. Combined with transmission electron microscope (TEM), the precipitation behavior was analyzed. The optical micrographs show that the microstructure of the samples are more similar to batch-annealing steel rather than general specification SPCC steel through continuous annealing. With 5°C/s heating rate and annealed at 860°C for 80s, the sample performs best: the yield strength is 161MPa, the elongation is 29% and the r-value is 1.51. AlN and MnS precipitations are observed in the inner grain and at grain boundary. The annealed textures mostly composed of {111} texture. The highest intensity is centered around the {111}<110> orientation.

Development of the multi-scale model of cold rolling based on physical and numerical investigation of ferritic–pearlitic steels

With the aim of developing dual phase (DP) microstructure using a continuous annealing process, four steels with different chemical compositions were investigated in the axisymmetrical plastometric tests and during the cold rolling process. The rheological model for these steels was developed using the results of the plastometric tests and application of the inverse analysis. Load–displacement curves were used to identify parameters of the rheological model, which was incorporated into the finite element code for rolling. First, the model was validated with experiments realized on the laboratory cold rolling mill. Calculated loads were compared with experimental data and good agreement was obtained. Beyond this, metallographic analysis was performed and deformation of the ferritic–pearlitic microstructure during rolling was investigated. Finally, the generated results were combined in the form of the multi-scale numerical model based on the digital material representation approach capable of investigating local microstructural inhomogeneities.

Non-linear model predictive control of throughput and strip temperature for continuous annealing line

A non-linear model predictive controller (MPC) was designed to automate and optimise the control of a continuous annealing process. The process is known as a continuously transitional and time delay system. By employing a first principle furnace model, the controller computes the required furnace load and line speed for every control step. A non-linear constrained dynamic optimisation problem is solved in real time by applying piecewise linear models over a receding horizon. The new controller has been tested and implemented on a 400 000 t continuous annealing line. Production using this controller has proved its reliability and yields a broad improvement in temperature accuracy, throughput increase and energy conservation. The details of the furnace model and the validation results are described, followed by the MPC problem formulation, system execution and implementation results.

Effect of Continuous Annealing Process on Microstructure and Properties of B250P Steel

Effect of Continuous Annealing Process on Microstructure and Properties of B250P Steel

Microstructure Evolution and Mechanical Properties of C-Mn Cold Rolled Dual Phase Steel after Continuous Annealing Process in Laboratory Conditions

Abstract The article deals with the influence of annealing parameters on evolution of microstructure and mechanical properties of dual phase steel. Dual phase steel was annealed in laboratory conditions according to the three chosen cycles of annealing: into intercritical region (780°C), into austenite region (920°C) and into austenite region (920°C) by subsequently cooling into intercritical region (780°C) with the hold at the temperature of 495°C. Simulation of annealing regimes by thermo-mechanical simulator Gleeble was done. The obtained microstructure consists from three phases: ferritic matrix, martensite and martensite/ bainite grains. For the microstructure identification the TEM and nanoindentation experiments were performed.

Continuous Annealing Process and Microstructure Property of 1200mpa High Strength Steel by Ultrafast Cooling

The different dilatometric curves of continuous cooling transformation have been determined by DIL805 thermal mechanical simulate, through metallographic and hardness method to study the effect of different cooling rate on the microstructure of transition. The critical point Ac1 and Ac3 of the tested steel are 709°C and 865°C. With the increase of cooling rate, the hardness of the steel and the content of martensite increase. In the laboratory conditions, the steel in this experiment was heated to 780°C, 800°C, 820°C, 840°C and 860°C, for 80s, then slowly cooled to 680°C, and water quenched to room temperature finally. The aging temperature was 240°C for 300s, and the last the sample was air cooled to room temperature. The results show that the microstructure of the annealed experimental steel belongs to martensite and ferrite. With the increase of annealing temperature, the content of martensite, the tensile strength and yield strength of the experimental steel increase, and the elongation decreases continuously. The sample was annealed at 800°C for 80s, then slowly cooled to 680°C and finally water quenched to room temperature. After overaging at 240°C, the samples were obtained with high mechanical properties. The tensile strength, yield strength and elongation are 1223MPa, 605MPa and 9.2%, respectively.

Creep‐Induced Microstructure Evolution of Type 2205 Duplex Stainless Steel during Continuous Annealing Processing

Superplastic behavior at 1323 K (1050°C) results in the pronounced creep deformation of cold rolled type 2205 duplex stainless steel during continuous annealing process. This superplastic behavior was evaluated by strain rate sensitivity measurements carried out in the 1273–1373 K (1000–1100°C) temperature range. The superplastic behavior was associated with the grain boundary diffusion in the steady state creep stage. Continuous dynamic recrystallization of ferrite was observed during the creep deformation and static recrystallization of austenite was observed in the early stage of the creep deformation. The 3D phase morphology was investigated in specimens annealed with and without applied stress. The stress applied during continuous annealing had a clear influence on the phase morphology which changed from an elongated and flat “pancaked” microstructure into a microstructure with island‐like austenite embedded in a ferrite matrix.

Evolution of microstructure and mechanical properties of cold-rolled SUS430 stainless steel during a continuous annealing process

This study investigates the effect of the annealing temperature on the microstructure, strength, elongation, and anisotropy properties of cold-rolled SUS430 stainless steel by a continuous annealing experiment. The SUS430 stainless steels of cold-rolled reductions of 84% and 60% were used in this study. The evolution of the microstructure and mechanical properties of SUS430 stainless steel during the annealing process was obtained. As continuous annealing proceeds, the softening behavior of the cold-rolled SUS430 stainless steel strip presented a two-stage trend characterized as recovery and recrystallization. Recovery mainly occurred at a temperature ranging from 600°C to 700°C. The hardness of the strip presented a certain decrease and unobvious change in microstructure was observed in this stage. Recrystallization of the deformed grains initiated at the temperature higher than 700°C, and the hardness of the strip decreased significantly. The mean recrystallized grain size of the sample cold rolled to 84% and 60% reductions are 13.5µm and 20µm, respectively. The tensile and yield strength of the specimens decrease as annealing temperature increases, and the elongation displays a sigmoidal increase with annealing temperature. Annealing temperature has little influence on the mean Lankford value (rm); however, an increase in cold-rolled reduction from 60% to 84% increases the value of rm from 0.9 to 1.3.The yield strength Re, the elongation δ, and the quantitative relationship between the yield strength Re and the softening fraction X were determined for cold-rolled SUS430 stainless steel in the range of εCR=1.0–2.2. The results of our study provide a theoretical basis for the prediction of the mechanical properties of cold-rolled SUS430 stainless steel as well as for the development of new annealing processes.

Experimental Study for Ultra High Strength Strapping Flat Steel

Ultra high strength strapping flat steel is used in packaging for papermaking, tobacco, compressed cotton and wool. At present, there is only one company can produce it by Lead bath quenching line. In this paper, the feasibility to produce new ultra high strength strapping flat steel of which tensional strength is 1350 MPa and elongation is 12% by continuous annealing process was investigated. The experiment result of steelmaking, hot rolling, pickling, cold rolling and continues heat treatment showed, the steel could be got using new chemical composition. The microstructure and X-ray diffraction spectrum of the steel showed, the steel was strengthened and toughened by martensite and retained austenite. The effect of continues heat treatment on microstructure and mechanical properties was discussed.

Quality-Relevant Monitoring and Diagnosis with Dynamic Concurrent Projection to Latent Structures

In this paper, a data-driven dynamic concurrent projection to latent structures (DCPLS) approach is proposed for quality-relevant fault diagnosis of dynamic processes. First, a novel DCPLS algorithm is proposed for dynamic modeling which captures the dynamic correlations between quality variables and process variables. Quality-specific variations, process-specific variations, and quality-process covariations of dynamic processes are monitored respectively. Secondly, a multi-block extension of DCPLS is designed to compute the contributions according to block partition of the lagged variables, in order to help localize faults. Finally, the application results on strip-thickness relevant fault diagnosis for a practical cold rolling continuous annealing process (CAP) demonstrate the effectiveness of the proposed methods.

Effect of the Continuous Annealing Process on Microstructure and Mechanical Properties of DP590 Steel

The microstructure and mechanical properties of the continuous annealing DP590 steel produced by a cold rolled plant were studied by utilizing a Gleeble thermal simulator tested machine. The results show that, increasing annealing temperature can not effectively improve the martensite content in the steel and annealing temperature be set at 780 °C and 800 °C is rational. Martensite decomposition at high overaging temperature will decrease tensile strength of the samples. As the overaging temperature sets at 250 °C ~ 300 °C, the samples could achieve tensile strength more than 600 MPa, which meet the requirements of DP590.

Fault Detection of Non-Gaussian Processes Based on Model Migration

In this paper, a new modeling approach is proposed for common and specific feature extraction. The original space of a mode can be separated into two different parts, namely, the common and specific ones. There are both non-Gaussian similarity and dissimilarity in the underlying correlations of different modes. After two different non-Gaussian blocks are separated, one can obtain the common and specific blocks, respectively. They play different roles in industrial batch processes, which are referred to as repetitive and complementary effects, respectively. Then, the common block and specific block are analyzed. A new multiblock monitoring method is proposed and the monitoring process is carried out in each block. The proposed method is applied to process monitoring of a continuous annealing process. Application results indicate that the proposed approach effectively captures the non-Gaussian relations to build the process model and improves the detection ability.

Fundamentals for Controlling the Microstructure and Properties of Cold Rolled and Continuously Annealed Sheet Steels

The strict control of microstructure is indispensable for meeting the demands for sheet steels with superior properties. This paper reviews the state-of-the-art related to continuous annealing process from the viewpoint of physical metallurgy. For mild steel, control of texture is one of the most important issues ensuring deep drawability. Therefore, the fundamental guiding principles on texture control are reviewed. Heterogeneity in cold rolled structures is discussed, with emphasis placed on grain boundary, shear band and cementite in terms of preferential nucleation of recrystallized grains. Furthermore, C-X interactions in the recovery stage are discussed in conjunction with scavenging effects. The ND// texture usually evolves according to the grain growth, which is enhanced by reducing the pinning effect. The ductility in low-C steels is significantly influenced by the state of C, which implies the importance of C control in the continuous annealing process. As for high strength sheet steels, the basic guiding principles for dual phase and transformation induced plasticity (TRIP) steels are reviewed in terms of phase transformation and TRIP effect. Because the strength of steels currently in practical use is considered to be only a part of their potential, steels are materials with many advantages and technical problems that remain to be solved.

Decentralized Fault Diagnosis of Continuous Annealing Processes Based on Multilevel PCA

Process monitoring and fault diagnosis of the continuous annealing process lines (CAPLs) have been a primary concern in industry. Stable operation of the line is essential to final product quality and continuous processing of the upstream and downstream materials. In this paper, a multilevel principal component analysis (MLPCA)-based fault diagnosis method is proposed to provide meaningful monitoring of the underlying process and help diagnose faults. First, multiblock consensus principal component analysis (CPCA) is extended to MLPCA to model the large scale continuous annealing process. Secondly, a decentralized fault diagnosis approach is designed based on the proposed MLPCA algorithm. Finally, experiment results on an industrial CAPL are obtained to demonstrate the effectiveness of the proposed method.