Compact Strip Production Research Articles

Effect of high-temperature annealing on elemental distribution in CSP-produced automotive steel billets

The distribution of C, Si, Mn, P and S in Q&P 980 automotive steel billets produced via the Compact Strip Production (CSP) process was investigated using Electron Probe Microanalysis (EPMA). The segregation ratio of each element was calculated. A diffusion kinetics model was established using the finite element method, and the chemical potential equations of different elements were used as the driving conditions for diffusion. Fick's law was applied to numerically calculate the diffusion behaviour and final concentration distribution of C, Si and Mn during the annealing process at 1100 °C. Results indicate that element segregation in the as-cast billet before annealing decreases gradually from the surface to the core. After annealing, the segregation ratio of elements is reduced, leading to a more homogeneous elemental distribution. It was calculated that the diffusion of C at 1100 °C completes in about 10 s, with micrometer-level long-range diffusion occurring in a shorter period. However, the concentration of Mn does not reach equilibrium at the austenite interface even after 20 h of diffusion at 1100 °C.

Influence of operating parameters on metal flow and thermal characteristics in an EMBr-single-ruler controlled CSP funnel-shaped mould

Electro-magnetic brake (EMBr), as a necessary technique to adjust the metal flow within the mould, has been extensively applied in the continuous casting (CC) production. The EMBr technique, if appropriately applied, can take the benefit of braking effect to weaken jet impingement effect, suppress fluid disturbance and stabilize surface fluctuation. As a result, to obtain a reasonable flow pattern within the CC mould, the braking performance of EMBr technique needs to be flexibly adjusted according to the variation of operational parameters. In this article, to obtain a braking performance of a commonly used EMBr-single-ruler technique within an optimal range, the effect of operating parameters on the metal liquid flow and thermal characteristics in a compact strip production (CSP) thin slab mould is described through a 3-D mathematical modeling of multi-physical field coupling. The results illustrate that the braking performance of the EMBr-single-ruler device is weakened with the increase of the casting speed or the mould width. For the effect of the casting speed and mould width on the metal flow with the application of an EMBr device, an optimum braking performance of the EMBr-single-ruler device can be obtained under the matching parameters of a magnetic induction intensity of 0.30 T and a casting speed of 4.5 m ∙ min‒1 together with a mould width of 1500 mm. Relative to the absence of the EMBr-single-ruler device, the maximum surface fluctuation height is controlled at 6.2 mm, and the mean surface temperature is raised to 1803.6 K. Based on these findings, we conclude that the proper utilization of the EMBr-single-ruler device can avoid the entrainment of flux powder and improve the melting performance of flux powder.

Modelling Study of EMBr Effect on Interface Fluctuation and Melt Flow in a Thin Slab Mould

Electromagnetic brakes (EMBrs) are regularly used in continuous casting to control interface fluctuations in a mould. In the research, to appraise the damping effect of EMBr technique, namely EMBr-ruler technique, the influence of magnetic field intensity on behaviours of melt flow and interface fluctuation in a Compact Strip Production (CSP) thin slab funnel mould are numerically simulated. The numerical results illustrate that the existence of the EMBr-ruler technique is conducive to inhibiting the impact of the ascending circulation on the interface in the CSP funnel mould. With the gradual enhancement of magnetic intensity, the service efficiency of the EMBr-ruler on the fluctuation of two-phase steel/slag interface is enhanced. For instance, by applying a magnetic induction intensity of 0.5 T, the maximum fluctuation height of two-phase steel/slag interface is decreased to 6.4 mm. This can well prevent surface defects, such as slag entrapment.

Study on Corrosion Behavior and Mechanism of Ultrahigh-Strength Hot-Stamping Steel Based on Traditional and Compact Strip-Production Processes.

Hot-stamping steel is a type of high-strength steel that is mainly used in key safety components such as the front and rear bumpers, A-pillars, and B-pillars of vehicles. There are two methods of producing hot-stamping steel, i.e., the traditional process and the near net shape of compact strip production (CSP) process. To assess the potential risks of producing hot-stamping steel using CSP, the microstructure and mechanical properties, and especially the corrosion behavior were focused on between the traditional and CSP processes. The original microstructure of hot-stamping steel produced by the traditional process and the CSP process is different. After quenching, the microstructures transform into full martensite, and their mechanical properties meet the 1500 MPa grade. Corrosion tests showed that the faster the quenching speeds, the smaller the corrosion rate of the steel. The corrosion current density changes from 15 to 8.6 μA·cm-2. The corrosion resistance of hot-stamping steel produced by the CSP process is slightly better than that of traditional processes, mainly since the inclusion size and distribution density of CSP-produced steel were both smaller than those of the traditional process. The reduction of inclusions reduces the number of corrosion sites and improves the corrosion resistance of steel.

Distribution behavior of inclusions in compact-strip-produced martensitic steel and its influence on mechanical properties

The distribution behavior of inclusions in martensitic steel produced by compact strip production process (CSP-MS) and its influence on mechanical properties were systematically investigated. The inclusions in the CSP-MS specimen are mainly composed of spherical Al2O3-CaO-CaS, MnS with high aspect ratio and small-sized TiN, whereas many coarse cuboidal TiN inclusions do exist in conventional martensitic steel (Con-MS). The high inclusion density of the CSP-MS specimen resulted in lower total elongation and impact toughness, and the MnS inclusions with high aspect ratio led to significantly stronger mechanical anisotropy than that for Con-MS specimen. The in-situ tensile results indicated that when the fracture direction is parallel to MnS direction, the microcracks induced by MnS inclusions tend to propagate into the matrix, leading to the formation of valley-like features, which significantly deteriorate the properties such as the total elongation and impact toughness. The microcracks caused by TiN inclusions are sharper than those caused by spherical Al2O3-CaO-CaS inclusions, and are easy to propagate into the matrix. This work is expected to guide the optimization of the mechanical properties of martensitic steels produced by CSP process and provide a theoretical basis for the CSP process design.

Refinement mechanism and optimization for compact strip production process

To avoid the fracture of cold-rolled sheets, this study investigated the refinement mechanism for the compact strip production process and proposed an optimised method. The primary austenite grain size was 50–475 µm, and the grain size of the hot-rolled strips was 3.6–4.8 µm. The Gleeble-3500 simulator was applied to simulate the hot rolling process. Grain refinement was realised through dynamic recrystallization and the supply of more nucleation sites for transformation by sub-structures. Increasing the coiling temperature from 620 to 730 °C increased the ferrite fraction from 3.95 to 24.16%. The adjacent cementite distance changed from 0.33 to 0.84 µm, resulting in decreased yield and tensile strengths and increased elongation. Thus, the fracture of cold-rolled sheets was avoided.

Phase Transformation Behaviors of Medium Carbon Steels Produced by Twin Roll Casting and Compact Strip Production Processes

Medium carbon steels have been widely used in the fields of tool and die manufacturing due to their outstanding hardness and wear resistance. In this study, microstructures of 50# steel strips fabricated by twin roll casting (TRC) and compact strip production (CSP) processes were analyzed to investigate the influences of solidification cooling rate, rolling reduction, and coiling temperature on composition segregation, decarburization, and pearlitic phase transformation. The results show that a partial decarburization layer with a thickness of 13.3 μm and banded C-Mn segregation were observed in the 50# steel produced by CSP, leading to the banded distributions of ferrite and pearlite in the C-Mn poor regions and C-Mn rich regions, respectively. For the steel fabricated by TRC, owing to the sub-rapid solidification cooling rate and short processing time at high temperatures, neither apparent C-Mn segregation nor decarburization was observed. In addition, the steel strip fabricated by TRC has higher pearlite volume fractions, larger pearlite nodule sizes, smaller pearlite colony sizes and interlamellar spacings due to the co-influence of larger prior austenite grain size and lower coiling temperatures. The alleviated segregation, eliminated decarburization and large volume fraction of pearlite render TRC a promising process for medium carbon steel production.

Modeling Study on Melt Flow, Heat Transfer, and Inclusion Motion in the Funnel-shaped Molds for Two Thin-Slab Casters

For the purpose of studying compact strip production (CSP) funnel-shaped mold and flexible thin-slab rolling (FTSR) funnel-shaped mold, a three-dimensional (3D) multi-field coupling mathematical model was established to describe the electromagnetic braking (EMBr) continuous casting process. To investigate the metallurgical effect of EMBr in the CSP and FTSR funnel-shaped thin-slab molds, a Reynolds-averaged Navier–Stokes (RANS) turbulence model, together with an enthalpy–porosity approach, was established to numerically simulate the effect of ruler EMBr on the behaviors of melt flow, heat transfer, solidification, and inclusion movement in high-speed casting. The simulation results indicate that the application of ruler EMBr in the CSP and FTSR molds shows great potential to improve the surface temperature of molten steel and reduce the penetration depth of downward backflow. This contributes to the melting of the slag rim near the meniscus region and facilitates the floating removal of the inclusions in the molten pool. In addition, in comparison with the case of no EMBr, the parametric study shows that the braking effect of ruler EMBr with an electromagnetic parameter of 0.5 T can enhance the upward backflow in the two high-speed thin-slab molds. The enhanced upward backflow can successfully entrain the inclusions to the top of the mold and improve the activity of surface fluctuations to avoid the formation of the slag rim. For instance, for the ruler EMBr applied to the FTSR mold, the maximum amplitude of surface fluctuation and the floatation removal quantity of inclusions with a diameter of 100 μm are increased by 4.6 percent and 51 percent, respectively.

Characteristics of advanced high-strength steels obtained by the compact strip production route

Advanced high-strength steels are used in structural components of automobiles to reduce their weight, fuel consumption and green-house gas emissions. Their properties result from tailoring their chemical composition and processing conditions to obtain microstructures consisting of a variety of hard and soft components. This work presents the analyses conducted on steels produced by continuous casting into thin moulds and hot rolling directly to the final shape. The microstructure was studied by microscopical techniques; the phase transformations occurring were assessed with the aid of commercial and dedicated computer programs. The stress–strain curves were fitted to different constitutive equations. It was found that the mechanical characteristics differed due to the strategies in design and production followed by the steelmaker.

Texture Evolution during Recrystallization and Grain Growth in Non-Oriented Electrical Steel Produced by Compact Strip Production Process.

Evolution of texture and α*-fiber texture formation mechanism of Fe-0.65%Si non-oriented electrical steel produced by Compact Strip Production (CSP) process during all the thermo-mechanical processing steps were investigated using electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) techniques. Columnar crystal structure of cast slab is fine and well-developed. Textures of the hot-rolled band are quite different in the thickness direction. During annealing of cold-rolled sheet, γ-fiber texture grains would nucleate and grow preferentially, and α*-fiber texture grains mainly nucleate and grow in the shear zone of α-fiber texture of cold-rolled sheet. During the recrystallization process, γ-fiber texture gradually concentrated to {111}<112>, and γ and α*-fiber texture increased significantly. {111}<112> texture priority nucleation at the initial stage of recrystallization. Due to the advantages of nucleation position and quantity, the content of α*-fiber texture is greater than {111}<112> texture in the mid-recrystallization. During grain growth process, {111}<112> oriented grains would grow selectively by virtue of higher mobility, sizes and quantity advantages than that of {411}<148 > and {100}<120>, resulting in the gradual increase of γ-fiber texture and the decline of α *-fiber texture.

Optimum Magnetic Properties of Non-Oriented Electrical Steel Produced by Compact Strip Production Process

Optimum grain size and effects of crystallographic textures on magnetic properties of Fe-0.65%Si non-oriented electrical steel produced by compact strip production (CSP) process were investigated by optical microscope, electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) techniques. Magnetic induction and core loss show a decreasing trend with the increase of grain size, and grain sizes for optimal magnetic properties are in the range of 26–30 μm. Core loss would be mainly affected by grain size, whereas crystallographic texture would primarily affect magnetic flux density. Magnetic properties increase with increasing of texture factor (volume fraction ratio of {100}/{111}) and magnetic texture factor (volume fraction ratio of &lt;100&gt;/&lt;111&gt;), and increasing with the decrease of A-parameter (minimum angle between magnetization direction and the closest &lt;100&gt; direction) and A(h→), respectively. Simultaneously, with increasing of A-parameter and A(h→), a linear decrease of B50 was obtained.

Effect of Solidifying Structure on Centerline Segregation of S50C Steel Produced by Compact Strip Production

Medium-high carbon steels having a high quality are widely used in China. It is advantageous to produce high value-added hot-rolled plates with the crystal refined and chemical composition homogenized in the casting slabs. However, element segregation occurs easily during high-medium carbon steels’ production. Generally, the centerline segregation is improved by enlarging the equiaxed zone with low-superheat casting and electromagnetic stirring (EMS). Studies were conducted on centerline segregation of S50C steel slabs with a thickness of 52 mm produced by the compact strip production (CSP) process in China without EMS equipped. By sampling along the width at different position, the secondary dendrite arm spacing (SDAS) was measured after etching and picture processing, based on which the cooling rate was calculated. It was found that the cooling rate increased from the center to the surfaces of the slabs ranging in 1~20 K/s, 10 times faster than that of a conventional process. The faster cooling rate led to a refined solidifying structure and columnar dendrite through the center of the slabs. The SDAS tended to increase from surfaces to the center, ranging only 32~120 μm smaller than that of a conventional process in 100~300 μm, indicating a finer solidifying structure by the CSP process. Results by EPMA indicated that elements C, Si, and Mn distribute in dispersed spots, increasing towards the center, and the centerline segregation changed in a narrow range: for C mainly in 1.0~1.1, Si in 0.98~1.08, Mn in 0.96~1.02, respectively, meaning a more chemical homogenization than that of thick slabs. Elements’ segregation originated from solute redistribution between solid and liquid. According to thermodynamic calculation, δ region of S50C is so narrow that the solute redistribution mainly occurred between γ-Fe and liquid during solidification. As the equilibrium partition coefficient of element C was the smallest, it was easy for C to be rejected to the residual liquid in the inter-dendritic space, leading to obvious segregation, relatively. Besides, as a result of high-cooling intensity, the solidifying structure became so fine that the Fourier number increased and the volume of the residual liquid decreased, making centerline segregation alleviated effectively both in volume and degree. Although bulging was observed during the industrial experiment, the centerline segregation was still inhibited obviously as the refining solidifying structure with permeability ranged only in 0.1~2.3 μm2 from the surfaces to centerline, which showed a good resistance on the residual flow towards the centerline.

Evolution of solidification structure for Si–Mn bearing AHSS under typical cooling rates

In this paper, two near-net-shape continuous casting technologies (strip casting and compact strip production) were used to produce a typical Si–Mn bearing Advanced High Strength Steel (AHSS) slab/strip, aiming to provide a clear understanding on the cooling rate on the solidification structure and corresponding properties. The results showed that strip casting (SC) technology under sub-rapid cooling conditions owns a higher heat transfer efficiency, and the solidified grains have been obviously uniformed and refined. Meanwhile, a substituted quenching process has been completed under high cooling strength of SC, leading the structural transformation from the initial ferrite/pearlite to the composite structure of martensite and bainite, which provides a very good structural basis for the excellent comprehensive mechanical performance of Si–Mn AHSS.

Continuous cooling transformation behaviour of 30CrMo steel simulated by CSP process

ABSTRACT The continuous cooling transformation (CCT) diagram of 30CrMo steel simulated by compact strip production process was constructed. And the effect of austenite deformation on the CCT behaviour was investigated. The results showed that the microstructure gradually transformed from proeutectoid ferrite and pearlite to bainite and martensite with the increase of cooling rate, accompanying a refined microstructure and an increasing hardness. When the cooling rate was lower than 0.1°C s−1, the desired microstructure of ferrite and pearlite could be obtained. And the austenite deformation strongly promoted the formation of proeutectoid ferrite, which led to a decreasing hardness of deformed steel. When the cooling rate was larger than 10°C s−1, the fully martensite microstructure was obtained. And the austenite deformation inhibited martensite transformation, resulting in a decrease of the martensite transformation starting (M s) temperature from 391°C to 368°C.

Hybrid Intelligent Control Based on Condition Identification for Combustion Process in Heating Furnace of Compact Strip Production

The heating furnace is a sizeable energy-consuming device in iron and steel industry. In compact strip production (CSP), the size of the heating furnace is large, and the working conditions are complicated. Furthermore, the large fluctuation of furnace temperature leads to large loss of billet combustion and energy consumption. This article deals with the design of hybrid intelligent control based on condition identification for the combustion process of the CSP heating furnace. By analyzing the process of the heating furnace and existing problems, the structure of a hybrid intelligent control system is proposed, and the working conditions are divided into two categories: stable and fluctuating. A fuzzy controller is designed to improve the control accuracy of furnace temperature under the stable working condition, and an expert controller is used to adjust the temperature rapidly under the fluctuating working condition. Besides, calorific value compensation is introduced to reduce the influence of calorific value of gas fluctuation on temperature. After verifying the effectiveness, the proposed method has been used in a steel plant and achieved a good control result. The intelligent control system improves the control precision of the furnace temperature and reduces energy consumption compared with the traditional control system.

Precipitates in Compact Strip Production (CSP) Process Non-Oriented Electrical Steel

Nitrogen and Sulfur in non-oriented electrical steel would form precipitates, which would severely affect its magnetic properties. Precipitates in compact strip production (CSP) process non-oriented electrical steel were investigated using a transmission electron microscope (TEM) and scanning electron microscopy (SEM). The precipitation mechanism and influence on grain growth were analyzed experimentally and theoretically. The results showed that the main particles in steel were AlN, TiN, MnS, Cu2S, and fine oxide inclusions. The spherical or quasi-spherical of MnS and Cu2S were more liable to precipitate along grain boundaries. During the soaking process, the amount of MnS precipitated on the grain boundary was much larger than that of Cu2S. AlN and TiN in cubic shape precipitated inside grains or grain boundaries. Precipitates preferentially nucleated at grain boundaries, and TiN, MnS mainly precipitated during soaking. In the subsequent processes after soaking, AlN and Cu2S would precipitate unceasingly with the decrease in the average size. The distribution density, the volume fraction, and the average size of the precipitates in the annealed sheets were 9.08 × 1013/cm3, 0.06%, and 54.3 nm, respectively. Precipitates with the grain size of 30–500 nm hindered the grain growth, the grains with 100–300 nm played a major role in inhibiting the grain growth, and the grains with the grain size of 70–100 nm took the second place.

Comprehensive Study of CaO–SiO2‐Based MgO/Li2O‐Modified Mold Flux System for the Casting of Peritectic Steels

It is a challenge to produce peritectic steels through compact strip production (CSP) technology, due to the large volume shrinkage during the initial solidification of peritectic steels and dramatic thermodynamics variation caused by the significant improvement of casting speed. Herein, two mold fluxes are developed to meet the demands for producing peritectic steels by CSP technology. The results show that the developed mold flux MF‐2 achieves the balance between the heat transfer control and mold lubrication, by increasing the CaO/SiO2 ratio to a particular point at 1.25 and the addition of Li2O content. The oxides of CaO and Li2O act as network breakers that will provide more O2− ions to depolymerize the original silicate structure, which in turn promote the mold flux crystallization capability and an effective heat transfer control. Meanwhile, the system viscosity of MF‐2 is lowered; also a good flow fluidity and break temperature are achieved to guarantee an appropriate mold lubrication behavior. In addition, the further addition of Li2O could lower the melting temperature to ensure a certain thickness of liquid layer as well. Therefore, MF‐2 shows the best overall performance and could be applied to industrial operations.

Origin of Goss (110)〈001〉 Grains in Hot-Worked Grain-Oriented Steel

This study involved hot working, mimicking the first strand of a compact strip production, of grain-oriented (GO) and non-grain-oriented (NGO) electrical steel. The as-cast microstructures, between the grades, were similar: large (~ 550 μm average diameter and 1 to 7 mm length) columnar grains with ND//〈001〉 fiber texture. However, the developments in hot-worked microstructures were remarkably different. Firstly, the GO had higher carbon (0.06 vs 0.003 weight pct)—which led to the presence of austenite phase at the hot working temperatures. The GO also showed dynamic recrystallization (DRx) and the presence of Goss (110)〈001〉 grains in the hot-worked state. These were absent in the NGO. Further, the majority (~ 95 pct) of the Goss grains, in the hot-worked GO, originated from dynamic recrystallization (DRx) in the ferrite phase, surrounding the harder prior austenite regions. This study thus identified DRx by particle-stimulated nucleation as the primary source of Goss orientation in the hot-worked GO.

Precipitation behavior and strengthening mechanism of Ti micro-alloyed steel via CSP process

Two types of micro-alloyed steel with different carbon content and coiling temperature were produced by compact strip production (CSP) process. The microstructure characteristics and precipitation behavior of the Ti particles during casting, soaking, rolling, coiling and cooling stages were discussed to reveal the strengthening mechanism. The samples were composed of ferrite bands and discontinuous pearlite/bainite at the grain boundaries. The average grain sizes of the sample with low carbon content and high coiling temperature was confirmed as 5.6 μm by quantitative metallography method, while that with high carbon content and low coiling temperature was confirmed as 5.2 μm. Moreover, the TiN precipitated before soaking stage was large in size because of the high undercooling temperature, which had nearly no strengthening effect. The strength was mainly attributed by cooling and coiling stages with the TiC precipitated at the inter-phase or within the ferrite. Taking Q345B steel with yield strength of 505 MPa produced by CSP as contrastive sample, the strength increments of fine-grain/solution/dislocation/precipitation strengthening for the low carbon sample were 65, 0, 22 and 113 MPa, while those for the high carbon sample were 74, 33, 28 and 57 MPa. Fine-grain and precipitation strengthening were the main strengthening mechanisms for the high strength CSP plates.

Effective Hot Rolling Batch Scheduling Algorithms in Compact Strip Production

This paper studies a hot rolling batch scheduling problem in compact strip production (CSP), which is decomposed into a two-stage problem. The first stage is the strip combination problem aimed at determining the strip combination of each rolling turn and the number of rolling turns with the objective of minimizing the number of virtual strips, and the second is the strip allocation and sequencing problem aimed at optimizing the allocation and rolling sequence of the strips in each rolling turn. We first model this two-stage problem considering a set of production constraints and then design an optimal approach to solve the strip combination problem. Subsequently, we design an evolutionary algorithm (i.e., artificial bee colony algorithm) with a novel search strategy for employed bees, a dynamic strategy for onlooker bees, a variable neighborhood search strategy for a scout bee, and an enhanced strategy to solve the problem in the second stage. Computational experiments demonstrate the effectiveness of the proposed algorithms. Note to Practitioners —The hot rolling batch scheduling process is crucial in linking the casting and rolling processes of iron and steel productions. In the rolling batch scheduling problem of CSP, there is no buffer between the casting and rolling processes, and virtual strips must be added to satisfy production constraints. Most rolling batch scheduling methods do not consider the addition of virtual strips. In this paper, we mathematically characterize the hot rolling batch scheduling problem in CSP with flexible production constraints. We then show how the optimal approach and artificial bee colony algorithm are designed. Finally, the effectiveness of the proposed algorithms is demonstrated by comparisons with other well-known metaheuristic algorithms. This paper can be extended to other hot rolling batch scheduling problems with buffers and hybrid flowshop scheduling problems.