Compact Strip Production Research Articles

Influence of inclusions on hydrogen-induced delayed cracking in hot stamping steels

The hydrogen-induced delayed cracking (HIDC) behaviors of two types of 1500 MPa grade hot stamping steels (HSSs) have been investigated by the method of slow strain rate tensile test and hydrogen permeation, where one is manufactured by compact strip production (CSP) process which is a revolution to the traditional HSS and the other by the traditional cold rolling process. The results show that the performance of HSS produced by CSP is superior to that of the traditional HSS, due to lower hydrogen embrittlement index, lower hydrogen diffusion coefficient and lower hydrogen content. It has been found that HIDC behavior is closely associated with inclusions. The inclusions of HSS produced by CSP are mainly spherical Al–Ca–O and CaS, while the inclusions in the traditional HSS are TiN + Al2O3 + MnS with sharp edges and corners. Based on these results, the influence of composition, shape and distribution of inclusions in HSS on HIDC and the mechanism of HIDC from the perspective of inclusions were analyzed and discussed.

Microstructural Characterization and Softening Mechanism of Ultra-Low Carbon Steel and the Control Strategy in Compact Strip Production Process

In this paper, the microstructures and properties of hot rolled ultra-low carbon steel sheet produced by different compact strip production (CSP) processes were investigated. The softening mechanism was also discussed and the control strategy was proposed in order to obtain optimum properties. Result showed that the average ferrite grain sizes of austenite rolling sheet and multiphase rolling sheet were 31.0 μm and 74.6 μm, respectively. The sheet after austenite rolling had a slightly higher yield and tensile strength while had a 6.3% higher elongation than that of the sheet after multiphase rolling. The higher dislocation in the sheet after multiphase rolling increased the strength while decreased the elongation. The softening mechanism of the sheet after multiphase rolling was the coarsening of ferrite grain. The combined role of {001} and {111} orientation resulted in a slight increase of the r and $${\bar{\text{r}}}$$ value in the sheet after multiphase rolling. It was a wise choice to conduct rolling at the Ac1 temperature in CSP process to increase the grain size and decrease the dislocation density. Then, the strength of the sheets could be further reduced and the elongation could also be improved. The microstructures and properties of hot rolled ultra-low carbon steel sheet produced by different compact strip production (CSP) processes were investigated. The average ferrite grain sizes of austenite rolling sheet and multiphase rolling sheet were 31.0 μm and 74.6 μm, respectively. The sheet after austenite rolling had a slightly higher yield and tensile strength while had a 6.3% higher elongation than that of the sheet after multiphase rolling. The higher dislocation in the sheet after multiphase rolling increased the strength while decreased the elongation. The softening mechanism of the sheet after multiphase rolling was the coarsening of ferrite grain. It was a wise choice to conduct rolling at the Ac1 temperature in CSP process to increase the grain size and decrease the dislocation density. Then, the strength of the sheets could be further reduced and the elongation could also be improved.

A multi-objective hot-rolling scheduling problem in the compact strip production

• We study a new hot rolling scheduling problem from compact strip production. • We divide the problem into two sub-problems of minimizing virtual sheet strips and optimizing the sequence of sheet strips. • An improved heuristic is developed to minimize virtual sheet strips. • A multi-objective evolutionary algorithm is proposed to optimize the sequence of sheet strips. • The effectiveness of the presented algorithms is demonstrated by example instances from practice. This paper addresses a hot-rolling scheduling problem from compact strip production processes. At first, a mathematical model that consists of two coupled sub-problems is presented. The first sub-problem is the sheet-strip assignment problem that is about how to assign sheet-strips to rolling-turns with the objective of minimizing virtual sheet-strips. The second is the sheet-strip sequencing problem that is about how to sort the sheet-strips in each rolling-turn with the objective of minimizing the maximal changes in thickness between adjacent sheet-strips and the change times of the thickness so as to ensure high quality sheet-strips to be produced. And then, an improved hot-rolling scheduling heuristic is proposed to solve the sheet-strip assignment problem. A multi-objective evolutionary algorithm is developed to find the Pareto optimal or near-optimal solutions for the sheet-strip sequencing problem. Besides, the problem-specific knowledge is explored. The key operators including crossover operator, mutation operator and repair operator are designed for the multi-objective evolutionary algorithm. At last, extensive experiments based on real-world instances from a compact strip production process are carried out. The results demonstrate the effectiveness of the proposed algorithms for solving the hot-rolling scheduling problem under consideration.

Microstructures and Mechanical Properties of Low Carbon Steel Hot Rolled in Ferrite Region Based on CSP Line

In this paper, the effect of ferrite region rolling on the microstructure and properties of steel plate heat commercial (SPHC) steel produced by compact strip production (CSP) process was studied in detail. Comparing ferrite rolled sheet with austenite rolled sheet, it was found that the average ferrite grain size of low carbon steel sheet rolled in ferrite region increased from 10.6 to 27.6 µm and the dislocation density was slightly reduced from 1.79 × 1011 to 8.9 × 1010 cm−2. The yield strength of ferrite rolled sheet is 92 MPa lower than that of austenite rolled sheet. The theoretical calculation and experimental results indicates that the coarsening of grains contributes to the 70% reduction of yield strength of ferrite rolled sheet, and the reduction of dislocation density and other strengthening factors account for about 30%. Texture analysis results by electron backscattered diffraction (EBSD) shows that the growth of {001} <110> and {112} <110> component in the texture of ferrite rolled sheet lead to the reduction of and Δr.

Determination of non-metallic inclusions in a continuous casting slab of ultra-low carbon interstitial free steel by applying of metallographic method, electrolytic method and RTO technique

Metallographic, electrolytic method and RTO(room temperature organic) technique were applied in the present study to more accurately determine non-metallic inclusion in a ultra-low carbon interstitial free(IF) steel and further to confirm their origination in a Compact Strip Production Process (CSP Process) continuous casting (CC) slab. Results show that inclusions detected by metallographic method usually appear relative smaller size and are mainly Al2O3 based or TiN based in composition, whereas those extracted by electrolytic method usually have larger size and are much more calcium-silicate based in composition. In addition, inner structures of extracted inclusions were detected by applying of RTO technique. The large size calcium-silicate based inclusions are confirmed high possibilities originating from mold flux and/or tundish flux entrapment, which are less affected by the liquid steel composition; while the smaller ones are generally endogenous inclusion precipitating during the refining or solidification process that strongly depending on the liquid steel composition and temperature.

Effect of Original Microstructure on Microstructure and Mechanical Properties of High Strength Steel WHF1500H during Hot Forming

The press hardening steel sheets WHF1500H with different original structures produced by compact strip production (CSP) line in Wuhan Iron and Steel Ltd. were austenitized at 950 °C for 5 min, and then hot stamped and quenched by using the flat die. The microstructure and mechanical properties were investigated by optical microscope, transmission electron microscope and universal testing machine. The results show that the microstructure after stamping and quenching is fully composed of lath martensite. The mechanical properties of the steel samples with different original structures are different after hot stamping and quenching, and this difference is smaller than that before hot stamping and quenching. When the original structure consists of ferrite and pearlite, the austenite grains after austenitization are fine and uniform. In addition, the martensite structure obtained after hot stamping and quenching is also uniform and fine, leading to higher mechanical properties. With the increase of the strength of the original steel, when the original structure is martensite, the austenite grains after austenitization are coarse, and the martensite structures obtained after quenching is also coarse, and thereby decreasing the strength.

Effect of rapid heating process in hot stamping on compact strip production hot rolled plate

Compact strip production hot rolled plate is one of blanks with short process. Owing to low cost and less investment, its tendency of replacement to cold rolled plate is development in industry. The mechanical property of both compact strip production hot rolled plate and cold rolled plate after quenching at different heating rates are investigated. Results showed that the tensile strength of both hot rolled plate and cold rolled plate after quenching increased with the increase of heating rate, and the gap narrower and narrower. The tensile strength of both hot rolled plate is 1393MPa when heating rate is 10 ˚C/s, which can’t meet the requirement of mechanical properties of hot stamping parts, while tensile strength is 1587MPa when heating rate was 100 ˚C/s, this result indicated that hot rolled plate could be raw materials of hot stamping industry by means of conductive heating process.

Microstructure and Mechanical Properties of Heat-Treated 30CrMo Steels Produced by Compact Strip Production and Conventional Rolling Processes

In this paper, the microstructure and mechanical properties of heat-treated 30CrMo hot-rolled steels, manufactured by the compact strip production (CSP) process, and the traditional continuous casting and rolling (CCR) process were systematically investigated. The results show that the microstructure of hot-rolled 30CrMo steel is finer and more homogeneous when produced by the CSP process. Lath martensite and plate martensite were obtained in both of steels after oil quenching. However, the specimens produced by CSP process exhibited a finer microstructure and a higher fraction of plate martensite after quenching. After tempering, the mechanical properties of the CSP-produced specimen were better than the specimens derived from the hot-rolled steel produced by CCR process. When the specimen was tempered at 500 °C for 2 h, the optimized mechanical properties were obtained. The tensile strength, yield strength, and elongation were 1135 MPa, 1084 MPa, and 10%, respectively.

A hybrid variable neighborhood search algorithm for the hot rolling batch scheduling problem in compact strip production

Abstract This paper deals with a hot rolling batch scheduling (HRBS) problem arising from the compact strip production (CSP) process, which is one of the most popular production systems in the modern iron-steel industry to produce sheet strips. The HRBS problem aims to determine a sequence of the sheet strips in a predetermined number of rolling turns with the objective of minimizing average thickness change. In this paper, a mathematical model based on a comprehensive investigation is first given. Then a constructive heuristic based on the problem-specific characteristics is presented to generate initial feasible solutions. The heuristic can guarantee a minimum number of rolling turns to accommodate all the ordered sheet strips, but generally performs poorly in the objective of average thickness change. To improve the objective, a hybrid variable neighborhood search algorithm (HVNS) is proposed. In the HVNS, a thickness value permutation is used to encode the solution and four neighborhood structures are well designed. The fruit fly optimization algorithm, is integrated to improve search efficiency. Correspondingly, a neighborhood switching strategy is developed to improve local search ability. Moreover, a restart strategy based on the block swapping operator is used to help the algorithm escape from local optima. To investigate the effectiveness of the solution approach, two sets of instances are tested, including real-world instances and randomly generated instances. The performance of the proposed HVNS is evaluated by comparing with the other existing algorithms and the experimental results demonstrate that the proposed algorithm performs much better.

Industrial Production of Fine Grained Ferrite in Low-Carbon Steel

Refinement of the ferrite grains provides a promising approach to simultaneously improving both the ‎strength and the toughness of steels. Among recent techniques to obtain ultrafine ferrite, dynamic ‎strain induced transformation (DSIT) is used. This type of treatment is sensitive to steel ‎composition, deformation temperature, the prior austenite grain size, strain and cooling rate.‎This work is aiming to improve the mechanical properties of steels using ferrite grain refining ‎through trials on industrial scale using a compact strip production plant (CSP) with hot strip mill ‎‎ (HSM) containing six stands F1 – F6 in EZDK Steel Company. The chemical composition of the ‎trials was ~0.05% C, 0.05% Si and 0.5% ‎Mn. The effect of the finishing deformation temperature at the last rolling stand (F6) on the ‎microstructure and mechanical properties was studied. In addition the effect of changing strain at ‎the final two rolling stands (F5 and F6) was carried out during trials. Comparing the traditionally processed steel which resulted in a ferritic grain size of 11µm with the processed fine grained (~5µm) steel, the yield stress has increased by 23% and the tensile strength increased by 12% on the expense of only 5% decrease in the ductility. However, the impact energy has improved by 7.5 %.

More than 25 Years of Experience in Thin Slab Casting and Rolling Current State of the Art and Future Developments

Industrial thin slab casting and direct rolling processing started more than 25 years ago with the world's first compact strip production (CSP®). Since that time, CSP® and competing thin slab casting and direct rolling concepts have been developed to a standard process for hot strip production. Typical features of this process are the homogeneous structural and mechanical properties all along the strip. The product range covers low carbon, as well as medium and high carbon steel grades comprising IF, HSLA, API, electrical, and AHSS multiphase steel grades. Hot strip from thin slab can be easily further processed to cold rolled and/or surface treated strip. A special feature is thin hot strip that may substitute cold rolled strip. Today, process and material developments, for example, go for rise in productivity, thinner gauge hot strip, as well as thicker line pipe and 3rd generation AHSS grades combining higher strength and ductility. The quality- and product-related advantages of thin slab casting and direct rolling technology are explained in this contribution at the example of the CSP® process.

A mathematical model and two-stage heuristic for hot rolling scheduling in compact strip production

• We study a new hot rolling scheduling problem from compact strip production process. • We model the problem as a combination of a sheet strip assignment sub-problem and a sheet strip sequencing sub-problem. • We present a two-stage solution approach consisting of a sheet strip assignment heuristic and a sheet strip sequencing heuristic. • The effectiveness of the two-stage heuristic is demonstrated by extensive experiments on both synthetic and real-world instances. This paper addresses a new hot rolling scheduling problem from the compact strip production process, which is the mainstream production technology that is used worldwide for sheet strips. The problem is modeled as a combination of two coupled sub-problems. One sub-problem is a sheet strip assignment problem that assigns sheet strips to rolling turns with the constraints of safe values of different gauge levels, and the other is a sheet strip sequencing problem that decides the rolling sequence for all of the sheet strips in a rolling turn to form a particular parabolic shape in thickness. To solve this hot rolling scheduling problem, we present a novel approach that consists of a sheet strip assignment heuristic and a sheet strip sequencing heuristic. The sheet strip assignment heuristic minimizes the number of virtual sheet strips by generating rolling turns according to the ordered sheet strips with maximum gauge level and their safe values. The sheet strip sequencing heuristic minimizes the average change of the thickness of adjacent sheet strips by arranging a certain number of duplicate sheet strips to the increasing stage of a rolling turn. Extensive experiments based on both synthetic and real-world instances from a compact strip production process show the effectiveness of the proposed two-stage heuristic in solving the hot rolling scheduling problem.

Roughing of Thick, Coarse-grained High-temperature-processing Steels Processed via Compact Strip Production

The authors are grateful to CBMM and Dr. Malcolm Gray of the Microalloyed Institute for financial support.

Improvement in continuously variable crown work roll contour under CVC cyclical shifting mode

A method of calculating the continuously variable crown work roll contour and the relevant constraint conditions under CVC cyclical shifting mode are proposed. The finite element model is built to calculate the roll gap crowns and the adjustment coefficients of bending force for different strip widths. Improvement in the continuously variable crown work roll contour has been implemented on the 1800-mm compact strip production line. This has resulted in the shifting stroke for the main strip width increasing by 30 %, the work roll wear being more uniform and the strip cross-section shape being better. The qualified rate of strip crown has risen from 98 to 99.5 %, and the qualified rate of strip head flatness has increased from 80 to 91 %.

Research on strip hysteretic behavior and mill vertical vibration system nonlinear dynamics

Rolling mill vibration is a technical problem in the iron and steel industry for many years and has serious impact and harm on production. There were serious vibrations in the middle mills when rolling thin container strip for the compact strip production (CSP) strip hot rolling process. This paper studied the hysteretic characteristic of rolled strip and established the vertical vibration system single-degree-of-freedom dynamics model of the F3 mill rollers. The influence of parameters on the system characteristics was studied, such as the linear damping coefficient, linear stiffness coefficient, nonlinear displacement coefficient, nonlinear velocity coefficient and exciting force, and then, the vibration source and vibration-restraining measure were studied from the roll gap. The results show that with increasing linear stiffness, damping and hysteresis coefficient, it can reduce the possibility of chaotic system; the linear stiffness coefficient had the greatest influence, and hysteresis damping coefficient had minimal influence on chaotic threshold. In order to reduce rolling mill vibration amplitude, we should reduce the external excitation force firstly, and in order to improve the dynamic performance of the system, we should control the speed of nonlinear coefficient values. The contrast experiments were carried out at the production scene finally.

Recent Development of Thin Slab Casting and Rolling Technology in a Challenging Market

Thin Slab Casting and Rolling Technology, in particular CSP® technology (Compact Strip Production), is well established for hot coil production. From its introduction in 1989, CSP® technology has been continuously developed further. Today, on a CSP® plant, nearly the full range of high-quality steel grades can be produced with tightest geometrical and metallurgical tolerances under optimum economic conditions.

Study of Shell-Mold Thermal Resistance: Laboratory Measurements, Estimation from Compact Strip Production Plant Data, and Observation of Simulated Flux-Mold Interface

The slag film that forms between the shell and mold in steel continuous casting is key in regulating the heat transfer between them. Generally, the mechanisms proposed are related to the phenomena associated with the formation of crystals in the solid layer of the film, such as the appearance of internal pores and surface roughness, which decrease phononic conduction through the layer and interfacial gap with the mold, respectively, and the emergence of crystals themselves, which reduce the transmissivity of infrared radiation across the layer. Due to the importance of the solid layer, this study investigates experimentally the effective thermal resistance, R T, between a hot Inconel surface and a cold Cu surface separated by an initially glassy slag disk, made from powders for casting low and medium-carbon steels, denoted as A and B, respectively. In the tests, an initially mirror-polished disk is sandwiched for 10,800 seconds while the Inconel temperature, away from the disk face, is maintained steady at a value, T c, between 973 K and 1423 K (700 °C and 1150 °C)-below the liquidus temperature of the slags. The disks have a thickness, d t, between ~0.7 and 3.2 mm. Over the range of conditions studied, mold slag B shows R T 33 pct larger than slag A, and microscopic observation of disks hints that the greater resistance arises from the larger porosity developed in B. This finding is supported by high-temperature confocal laser scanning microscope observations of the evolution of the surface of slag parallelepipeds encased between Pt sheets, which reveal that during devitrification the film surface moves outward not inward, contrary with what is widely claimed. This behavior would favor contact of the slag with the mold for both kinds of powders. However, in the case of slag A, the crystalline grains growing at or near the surface pack closely together, leaving only few and small empty spaces. In slag B, crystalline grains pack loosely and many and large empty spaces arise in and below the surface. Estimation from plant data shows R T values smaller than measured ones, suggesting that the process film slag thickness must be considerably thinner than those of laboratory disks. However, the difference in thermal resistance of both powders, averaged over the mold length, is close to the dissimilarity found in laboratory.

Effects of TMCP Schedule on Precipitation, Microstructure and Properties of Ti-microalloyed High Strength Steel

Using the similar compositions of the Ti-microalloyed high-strength steels produced by the thin-slab casting process of compact strip production (CSP), four thermo-mechanical control processes (TMCP) after the simulated thick-slab casting, i. e. the two hot rolling routes and the two cooling processes, were designed, aiming at achieving the same mechanical properties as the thin strip products. The final microstructures after the four TMCP processes were examined by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile properties and Charpy impact energy were measured correspondingly. Strain-induced TiC precipitation was found in the two-stage rolling route with the finish rolling temperature at low levels, leading to grain refinement due to the pinning effect during austenite recrystallization. Precipitation hardening in ferrite was observed when a period of isothermal holding was applied after hot rolling. It could be concluded that both finish rolling temperature and the subsequent isothermal holding temperature were crucial for the achieved strength level due to the combined effect of grain refinement and precipitation hardening. At the same time, it was found that the isothermal holding led to poor impact toughness because of remarkable precipitation hardening. Therefore, it was suggested that the precipitation kinetics of titanium carbides in both austenite and ferrite should be investigated in future.

Effects of auto-tempering on microstructure and mechanical properties in hot rolled plain C-Mn dual phase steels

In this paper, the behavior of auto-tempering in hot rolled plain C-Mn dual phase steels was investigated. The steels with thickness of 11mm were fabricated on the compact strip production line and coiled at 150°C and 260°C, respectively. The auto-tempering in the steel with coiling temperature of 260°C caused the softening of ferrite and martensite and the reduction in hardness difference between these two phases, ultimately led to the decrease in tensile strength and strain hardening ability and the increase in post-necking elongation. The Charpy impact test results indicated that the toughness of auto-tempered steel was superior to the non-tempered steel. When the tensile test or impact test was conducted on the auto-tempered steel, both ferrite grains and tempered martensite islands were readily elongated as fibrous structure. The acquired fibrous structure could inhibit the formation of microvoids and propagation of cracks in tensile test, and delayed the tensile fracture. In addition, the fibrous structure could inhibit the propagation of main crack during impact test, whereas induced the branching cracks, leading to high impact energy.

CVC cyclical shifting mode and its working characteristics for the mills of CSP

The compact strip production (CSP) line has two different rolling schedules: rolling strips with same width or rolling strips with different widths alternately. However, the continuously variable crown (CVC) work rolls used in the CSP production line do suffer from non-uniform wear in conventional shifting mode. A cyclical shifting mode for CVC work rolls is developed to uniform the work roll wear and to improve the quality of strip cross section. In this shifting mode, the work rolls shift with fixed mode, while the changes of strip crown caused by shifting are compensated by the bending force. A mathematical model is introduced to explain the working characteristic of the CVC cyclical shifting mode. A 3D finite element model of roll stacks for 1800 mm CSP hot rolling mill is built to calculate the parameters in the mathematical model. The working principle for the CVC cyclical shifting mode is determined as well. The proposed shifting mode has been implemented in the 1800-mm CSP production line. The work roll wear is more uniform by using the new shifting mode. This has resulted in the rolling length increasing by 20 % and the consumption of work roll decreasing by 14 %.