Continuous Annealing Research Articles

Cold-rolled multiphase boron steels: microstructure and mechanical properties

The influence of the boron concentration on phase transformation characteristics, microstructure and mechanical properties of multiphase steels was investigated using computational thermodynamics (Thermo-Calc®), dilatometry, quantitative metallography and tensile tests. Pilot scale 50kg steel ingots were prepared in an induction furnace operating under an argon gas atmosphere with boron contents between 0 and 47ppm. The ingots were cut into 35mm thick blocks, which were reheated to 1250°C for 1h and hot rolled for seven passes to attain a thickness of 7.0mm. The hot-rolled sheets were machined and then cold rolled to a final thickness of 1.2mm. Continuous annealing cycles were performed in a Bähr dilatomer and in a Gleeble machine. Continuous annealing laboratory simulations showed that boron did not significantly influence the amount of austenite formed during heating and soaking steps. However, boron influenced austenite transformation during the cooling step, which reduced the amount of ferrite and increased the amount of bainite. Regarding the mechanical properties, adding boron increased strength and decreased ductility of the product. The steels with boron concentrations up to 27ppm exhibited the greatest effect. The amount of austenite, which was calculated using Thermo-Calc®, was slightly overestimated compared with that obtained by dilatometry and metallography, particularly for soaking temperatures lower than 800°C.

Microstructure and mechanical properties of dual phase strip steel in the overaging process of continuous annealing

Microstructure and properties of DP590 steel in overaging process of continuous annealing were investigated in the laboratory. Effects of the overaging temperature (OT) in conventional process (Process A) and reheated overaging temperature (ROT) of the proposed process (Process B) on microstructure and mechanical properties were analyzed. The results showed that, in process A, with the increase of the OT from 280°C to 400°C, the amount of granular retained austenite increases, the tensile strength decreases, the yield strength and elongation both increase. In process B, with the increase of the ROT from 310°C to 400°C, the tensile strength decreases, whereas the yield strength changes little. Compared with the conventional process, the tensile strength of the experimental steel increases obviously and the ratio of yield-to-tensile strength decreases in process B. The variation of instantaneous work hardening index (n⁎) with tensile strain can be divided into three stages. The increase in the OT or ROT leads to the continuous increase of n⁎ in the third stage.

Effect of higher heating rate during continuous annealing on microstructure and mechanical properties of cold-rolled 590 MPa dual-phase steel

In this presentation, the effect of higher heating rate in continuous annealing on microstructure and mechanical properties of a cold-rolled 590 MPa ferrite-martensite dual-phase steel were investigated by using microstructural observation and mechanical property measurement. The results show that compared with the conventional continuous annealed steels heated at a rate of 5 ∘C/s (CA), the average ferrite grain sizes heated at a higher rate (300 ∘C/s, HRA) was obviously refined from 15.6 μm to 5.3 μm. The morphology of martensite is observed to shift from network along ferrite grain boundaries to uniformly dispersed in the final DP590 microstructure. Twinned substructure of martensite can be found when heated at a higher heating rate in continuous annealing. EBSD orientation maps show that the fraction of low angle grain boundary is increased in HRA sample compared to CA sample. The HRA sample has excellent mechanical properties when compared to the CA sample. The variations of strength, elongation, strain hardening behavior and fracture mechanism of the this DP590 steel with different heating routine were further discussed in relation to microstructural features.

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.

Multiblock Concurrent PLS for Decentralized Monitoring of Continuous Annealing Processes

In this paper, a data-driven multiblock concurrent projection to latent structures (CPLS) method is proposed for monitoring large-scale manufacturing lines, particularly for cold rolling continuous annealing processes (CAPs) fault diagnosis. The proposed method provides decentralized process monitoring and helps localize faults in both input variables and output variables concurrently. First, the CPLS-based process monitoring method is briefly reviewed. Second, a multiblock CPLS algorithm, which incorporates process block partition, is proposed to diagnose faults relevant to process inputs or outputs with a decentralized structure. For the CAP line application, tension-specific variations, roll-specific variations, and tension–roll covariations are analyzed in each partitioned block. Furthermore, within the roll-specific subspace of an abnormal block, a delay-alignment scheme based on strip transportation delay is proposed to diagnose defective processing materials.

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

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

Creep or tensile stress induced anisotropy in FINEMET-type ribbons?

Continuous stress annealing can be performed on FINEMET-type (Fe 73.5 Cu 1 Nb 3 Si 15.5 B 7 ) ribbons. Here we report experiments conducted in a cheap open tubular furnace as a function of pulling velocity ( v ). We achieved up to v =120 m/min that is almost 100 times higher than previously reported for this kind of equipments. In this report we show for the first time how the properties are depending on the pulling velocity. While the achieved relative permeability µ r remained constant without changing any other parameter than v , the elongation of the ribbon and the contraction of the ribbon width were monotonically changing as v was increased. The results support that the creep is not determined exclusively by the applied stress, but it is determined by the details of the amorphous–nanocrystalline transformation, which is influenced by the pulling velocity. The bigger the pulling velocity is, the higher the onset temperature of the amorphous–nanocrystalline transformation is, and the wider the associated exothermic peak is, resulting in increased elongation of the ribbon due to a short time formation of undercooled liquid state. • We report detailed investigations as a function of pulling velocity in a dynamical stress annealing process. • The inverse effective permeability is linearly depending on the applied stress, in accordance with Herzer’s back-stress theory. • The creep is not uniquely determined by the stress, but depends also on the pulling velocity. • The permeability is independent from the elongation of the ribbon. • The induced anisotropy does not depend on the creep.

Improved Processing Technique for Preparation of Non-Oriented Electrical Steels with Low Coercivity

Non-Oriented Electrical Steels with Low Coercivity I. Petryshynetsa,∗, F. Kovac, J. Marcin, I. ’korvanek Institute of Materials Research SAS, Watsonova 47, 040 01 Ko2ice, Slovakia Institute of Experimental Physics SAS, Watsonova 47, 040 01 Ko2ice, Slovakia In order to improve soft magnetic properties of vacuum degassed NO steels, an adjusted temper rolling process for development of particular textures {100} was used. The main idea here relies on a deformation-induced grain growth, which promotes preferable formation of the grains with desired orientation. Two vacuum degassed NO steels were chosen as an experimental material. In both cases, a coarse or columnar grained microstructure, with pronounced intensity of cube and Goss texture components, was achieved during a continuous nal annealing. The obtained microstructure leads to a signi cant decrease of coercivity, measured in DC magnetic eld. The coercivity of steel with silicon content 2.4 wt.% decreased from 42 A/m to 17 A/m. Even more remarkable improvement of the soft magnetic properties was observed for the steel with Si 0.6 wt.%, where the coercivity value dropped from 68 A/m to 12.7 A/m.

Effect of hot strip coiling temperature on microstructure and properties of boron containing enamel steel

The influence of hot strip coiling temperature on microstructural characteristics, mechanical properties and hydrogen permeation behaviour of boron containing enamel steel was investigated. The material was produced and processed in laboratory. It is found that with the increasing hot strip coiling temperature, strength of annealed steel sheet reduced, the rm value increased, high temperature coiling (720°C) was more suitable for cold working performance. Supported by subsequent heavy reduction cold rolling and rapid continuous annealing, a large number of dispersed cementite particles within grains were obtained, the hydrogen diffusions (DL) in annealed sheets of hot strips coiled at temperatures used in present work were extremely slow and far less than the critical value, and the fish scale resistance of the tested enamel steel is excellent. With the effect of degenerated pearlite chains, DL can be reduced by about 30% in the annealed sheet form hot strips with the increasing coiling temperature form 630°C to 720°C. Hot strip coiling at high temperature is more suitable for enamel use.

Effect of heating rate on microstructure and mechanical properties of TRIP-aided multiphase steel

Optimization of high strength and high formability of multiphase cold rolled TRIP-aided steel sheet is based on the continuous annealing conditions. The influence of heating rate on the microstructure and mechanical properties of TRIP steel was investigated with Gleeble-3500 and a thermal system using resistance heating method. Experiments with different heating rates (5 and 500K/s) and annealing temperatures from 883K to 1223K were applied to study effects on the progress of austenitization and recrystallization kinetics. Different initial heating rates were applied to examine the influence of heating rate on the mechanical properties of TRIP steel. It appeared that the final tensile strength of the TRIP steel increased with increasing heating rate held at 1073K. The phase-transformation kinetics during the intercritical annealing was investigated by means of dilatometry. The results indicated that samples with higher heating rates have a much higher austenite volume fraction for a given annealing time of 80s held at 1073K. In addition, the higher heating rate results in a larger volume fraction of retained austenite (RA) and lower retained austenite carbon content in comparison to the lower heating rate after held at the same intercritical temperature. The morphology of RA under different annealing conditions was also tentatively discussed.

Effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region

The effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region was investigated using a Gleeble 1500 thermomechanical simulator and a continuous annealing thermomechanical simulator. The results showed that the microstructure consisted of ferrite and bainite as the main phases with a small amount of retained austenite and martensite islands at cooling rate of 5 and 50 °/s, respectively. Fast cooling after continuous annealing affected all constituents of the microstructure. The mechanical properties were improved considerably. Ultimate tensile strength (UTS) increased and total elongation (TEL) decreased with increasing cooling rate in all specimens. The specimen 1 at a cooling rate of 5 °/s exhibited the maximum TEL and UTS×TEL (20% and 27 200 MPa%, respectively) because of the competition between weakening by presence of the retained austenite plus the carbon indigence by carbide precipitation, and strengthening by martensitic islands and precipitation. The maximum UTS and YS (1 450 and 951 MPa, respectively) were obtained for specimen 2 at a cooling rate of 50 °/s. This is attributed to the effect of dispersion strengthening of finer martensite islands and the effect of precipitation strengthening of carbide precipitates.

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.

Buckling of Long Thin Plates under Residual Stresses with Application to Strip Rolling

We present a simplified numerical method which can be used to predict efficiently the response of long thin plates under effects of residual stresses induced by production process such as rolling or continuous annealing. The principle consists in assuming harmonic buckling mode along the sheet length, and we consider Koiter-Budiansky post-buckling theory to compute the stress-deflection curve. In this way, only the width of the sheet has to be discretized by 1D finite elements. The size and shape of the flatness defects can be predicted efficiently and for a large number of cases. Various types of residual stresses and loadings can be accounted for. In particular, we will see the influence of the global traction on the buckling and post-buckling behavior. The numerical results are compared with experimental data and full numerical computations

Aspects of Thermomechanical Processing of 3<sup>rd</sup> Generation Advanced High Strength Steels

The production of advanced high strength steels (AHSS) has been rapidly expanding in recent years as these steels allow for considerable reduction in weight and enhancement of car safety due to the unique combination of high strength, toughness and formability. Driven by growing demand for sheet AHSS products from carmakers, steel producers are currently developing AHSS of the so called 3rd Generation to further facilitate weight reduction of critical safety parts while ensuring crash worthiness and high absorbed energy. Such steels not only possess tensile strength above 1000 MPa but also are being designed for exceedingly high formability: high elongation, bendability, hole expansion and strain hardening. These enhanced properties are to be achieved in final operations of continuous annealing and/or galvanizing. However, due to complicated alloy designs of 3G AHSS the role of each manufacturing stage becomes progressively significant due to its impact on the final microstructure. Therefore, hot strip rolling gains increasing importance as one of the most critical stages responsible for producing the microstructure optimal for achieving the final properties of the sheet products without impairing downstream operations. In other words, hot rolling of AHSS has to be viewed as thermomechanical processing.

Influence of New System Nano-Inhibitors on the Abnormal Grain Growth with Goss Crystallographic Orientation of Silicon Steels

In the present work some new approaches have proposed for the grain oriented steel fabrication. This approach employs the new system of VC nanoprecipitates in the combination with dynamic continuous annealing for secondary recrystallization. The new system of VC inhibitors and dynamic annealing was applied to the grain oriented steel in order to obtain abnormal grain growth with Goss crystallographic orientation and considerably reduces the preparation time as the whole process lasts only several minutes. The EBSD analysis shown that suggested procedure led to evolution of the sufficiently strong {110}<001> Goss texture, which is comparable to that obtained in the conventionally treated GO steels. Moreover, the steels treated by this new method have the comparable final magnetic properties as the materials passed the conventional long – time heat treatment.

EFFECT OF ULTRA-FAST CONTINIOUS ANNEALING ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LOW Si GRADE Nb-Ti MICROALLOYING TRIP STEEL

Si-containing transformation induced plasticity(TRIP) steel is noted for good balance of excellent formability and high strength as the advanced high strength steel(AHSS). The advantage of this steel can be attributed to the TRIP effect, which is the transformation of the retained austenite. Furthermore, the local increase in specific volume caused by the TRIP effect can help to close propagating cracks. It is favorable for the automotive structural components based on the high work hardening rate and energy absorption behavior. Low Si-containing can optimize the galvanized performance of the cold rolling TRIP steel, and the ferrite stabilization can be compensated by adding Al. Microalloying with Nb and Ti may provide effective means for further strengthening via grain refinement and precipitation strengthening. The ultra- fast continuous annealing comprised of rapid heating and short austempering is a new-style process for grain refinement and precipitation solidifying. However, the influences of the process on the cold rolling low Si TRIP steel, especially the austenite transformation characteristics and their effects on microstructure and mechanical properties, were rarely reported. Therefore, in this work the microstructures of low Si grade Nb-Ti microalloying TRIP steel under different ultra-fast continuous annealing conditions were observed via EBSD and TEM, and the tensile properties were discussed. The results show that the polygonal ferrite is refined by heating rate of 100 ℃/s and short asutempering procedure. The dispersive and fine microalloyed carbonitrides formed during the hot-rolling stage are reserved. Therefore, the strength and ductility are enhanced simultaneously. The slow cooling procedure can effectively contribute to eliminate the yield point, while the strength is slightly decreased. As the annealing temperature increasing, the strength is enhanced. When the annealing temperature is 830 ℃, the morphology of retained austenite consists of alternated film and bainite-ferrite plates, resulting in optimal combination of strength and ductility: tensile strength 748 MPa, yield strength 408 MPa, uniform elongation 21.3%, work hardening exponent 0.27, balance of strength and ductility is 15932.4 MPa· %.

Effects of inter-critical temperatures on martensite morphology, volume fraction and mechanical properties of dual-phase steels obtained from direct and continuous annealing cycles

Homogenizing and normalizing heat treatments were performed on low carbon–manganese steel. Then, direct and continuous annealing heat treatments were carried out at 800°C, 770°C, 750°C and 725°C. Finally; dual phase ferrite–martensite steel was obtained. Thereafter, hardness and tensile tests were applied at ambient temperature, and impact tests for the initial sample and the dual-phase steels obtained from continuous and direct annealing heat treatment in the temperature ranges of (−67 to +70),(−70 to +60), (−70 to +29), respectively, were accomplished. The ductile–brittle transition temperature (DBTT) and the fracture modes of the samples were obtained, and the fracture surface of the steel was observed through scanning electron microscopy (SEM). The results revealed that the best mechanical properties in dual-phase steels, like impact toughness and flexibility, appear at the inter-critical temperature of 725°C for both continuous and direct annealing cycles. The (DBTT) for the specimens obtained from direct and continuous annealing and the initial sample were −49°C, −6°C, and −34°C, respectively. The dual-phase specimen achieved through the direct annealing method had better toughness and impact properties than the initial specimen or the one obtained through continuous annealing.

Investigation of the Mechanical Properties of Intercritically Treated Ultra High Strength Steel

Intercritically treated ultra high strength steel was investigated using a continuous annealing simulator of the laboratory. The results have shown that the microstructure consists of a ferrite matrix with martensite. A satisfactory mechanical property was obtained by intercritical annealing and subsequent overaging. The specimen exhibited optimum Rm (1360MPa) and At (20%), a relatively low Rp0.2/Rm (0.72) and an ideal product of Rm and At (27200MPa%). Satisfactory mechanical properties of the present steel are attributed to the fact that the effect of dispersion strengthening of martensite islands as well as the contribution to strength by the softer phase, such as ferrite (retained austenite).

Effects of Heat Treatment Process Parameters on Microstructure and Mechanical Properties of DP440 Cold Rolled Strip

In order to optimize the production process, improve the production efficiency and accelerate the development and application of the domestic dual-phase steel, the effects of heat treatment process parameters on micro-structure and mechanical properties of DP440 cold rolled strip were studied by the CAS-300 simulated continuous annealing equipment. When the heating rate increased from 5 to 100 °C/s, both the tensile strength and yield strength increased because of the decreased grain size. When the intercritical annealing temperature increased from 780 to 850 °C, the martensite content decreased so that the tensile strength decreased first, then increased, and the yield strength increased. When the rapid cooling rate increased from 25 to 100 °C/s, because the martensite content increased, the tensile strength increased, while the yield strength decreased. When the overaging temperature increased from 260 to 400 °C, the tensile strength decreased, while the yield strength increased. When the overaging time increased from 240 to 480 s, the tensile strength decreased a little, while the yield strength increased a little.

Thermal anti-oxidation treatment of CrNi-steels as studied by EXAFS in reflection mode: the influence of monosilane additions in the gas atmosphere of a continuous annealing furnace

Surface sensitive, reflection mode EXAFS spectroscopy at the Cr and Fe K-edges was used to study the surfaces of stainless steel (EU alloy grade 1.4301 X5CrNi18-10) after different heat treatments. The thermal treatments were performed in a conveyor belt furnace at temperatures between 600 and 900 °C in different gas atmospheres containing inert carrier gases (N2 or Ar) and different additives such as hydrogen (H2) and monosilane (SiH4). The ex-situ EXAFS results show that the iron and chromium in the steel can only be reduced to a metallic state, if SiH4 was used as a reducing additive for temperatures of 900 °C. Lower temperatures and process atmospheres without silane always result in strongly oxidized steel surfaces.