Compact Strip Production Process Research Articles

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.

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.

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.

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.

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.

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.

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 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.

Features of the ISO-25498: Method of Selected Area Electron Diffraction Analysis in Transmission Electron Microscopy

International standard ISO-25498 specifies the method of selected area electron diffraction (SAED) analysis in TEM. It is applicable to the acquisition of SAED patterns, indexing the patterns and calibration of diffraction constant. Several features of this standard are introduced. As an example of the applications, phosphide with nanometer scale in a low-carbon steel produced by compact strip production process was analyzed by SAED and EDX. The phosphide precipitates in the steel are identified as M x P, where x is 2-3 and M is Fe, Ti, Cr, or Ni. It possesses a hexagonal lattice with lattice parameter a = 0.609 nm and c = 0.351 nm.

Mechanism of Non-metallic Inclusion Formation and Modification and Their Deformation during Compact Strip Production (CSP) Process for Aluminum-Killed Steel

The formation and modification of non-metallic inclusions for Al-killed steel during Compact Strip Production Process (CSP Process) was studied by industrial experiments. The thermodynamics for the formation of Al2O3, MgO·Al2O3 spinel, various calcium aluminates and CaS bearing inclusion were analyzed with the help of calculation software FactSage. It is found that the oxygen activity in liquid steel during LF (Ladle Furnace) refining is confirmed to be determined by the equilibrium between dissolved Al and Al2O3 in inclusion. There are two manners for Al2O3 inclusion modification during LF refining: the first route is followed by Al2O3-low modified calcium aluminates-liquid calcium aluminates; and the other is as Al2O3– MgO·Al2O3 spinel-CaO–MgO–Al2O3 multi-component inclusion. In addition, the liquid calcium aluminates have the tendency to be multi-component inclusion. Two types of CaS bearing inclusion could precipitate in a slab during solidification. The favorable modified inclusions deform very well along with steel matrix during rolling process even if they are associated with CaS bearing layer, while the no or low modified Al2O3 based inclusion would be rolled into pieces and the micro-cracks might be generated around the inclusions.

EFFECTS OF Ti AND Mn CONTENTS ON THE PRECIPITATE CHARACTERISTICS AND STRENGTHENING MECHANISM IN Ti MICROALLOYED STEELSPRODUCED BY CSP

As an advanced manufacturing technology to produce hot rolled strips,compact strip production(CSP) process was developed at the end of the last century and has been widely applied due to its high efficiency and low cost.Compared with the traditional technology,the advantages of CSP technology benefit from the refinement of austenitic grains and precipitation strengthening in the steels.This is because cooling rate is higher during the solidification of slab and the direct charging slab temperature is also higher in the CSP process,resulting in much higher solute contents in the solid solution before hot rolling than expected by the experiences from traditional steel production. Therefore,the Ti microalloyed steels produced by the CSP process exhibit good performance and have drawn much attention to Ti precipitate behavior and strengthening mechanism.However,the influence of alloying element Mn on the Ti microalloyed steels produced by CSP.especially the synergistic effect of Mn and Ti,was rarely reported.Therefore,in this work the microstructure and precipitate characteristics of two Ti microalloyed steels with different Ti and Mn contents produced by the CSP process were studied by electron backscatter diffraction(EBSD) technology and high resolution transmission electron microscope(HRTEM).The results show that the steel with higher Ti and Mn contents has a higher fraction of small-angle grain boundaries.Furthermore,the weight fraction of TiC precipitates with particle size smaller than 10 nm increases significantly,from 7.6%in the lower Ti and Mn steel to 26.1%in the higher Ti and Mn steel.However,the amount of Fe_3C precipitates decreases markedly. In addition,the strengthening mechanism analysis of the two tested steels shows that grain refinement strengthening and dislocation strengthening make great contribution to the yield strength,while the precipitation strengthening is the primary reason for the difference in the strength between two tested steels.

Effects of thermo-mechanical treatment and microalloying with Cr, Nb and Ti on phase transformation in C–Mn steel strips produced by compact strip production process

Abstract The C–Mn steel strips microalloyed with Cr, Nb, Ti was produced by compact strip production process and then heat-treated under different conditions. Optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy were used to investigate phase transformations in the steel after different treatments. It was revealed that the phase transformations upon quenching were greatly affected by the austenization temperature and time. When the steel was annealed at 950 °C, carbides of Cr, Mn and Fe were dissolved dramatically, while carbides of Nb and Ti are relatively stable at this temperature. When the temperature increases to 1100 °C, the carbides of Nb were dissolved rapidly, while the carbides of Ti still show somewhat stable (partial dissolution). Annealing time influences both the amount and the shapes of carbides in the steel, which leads to different phase transformations in the following air cooling processes. Grain growth in the steel in annealing process strongly depends on the dissolution of carbides on grain boundaries. Additionally, a subsequent rolling after annealing treatment at 950 °C lead to obvious precipitation of carbides of Ti and Nb in the steel.

Distribution of arsenic on micro-interfaces in a kind of Cr, Nb and Ti microalloyed low carbon steel produced by a compact strip production process

Distribution of arsenic on micro-interfaces in the Cr, Nb and Ti microalloyed low carbon steel strip was investigated by SEM, TEM and EDS in a compact strip production (CSP) and a following reheating processes. It is discovered that, in the CSP processes, the arsenic content within grains at the half thickness of the strip was a little higher than that at the grain boundaries, which was attributed to a quicker occupying of vacancies by carbon, Nb and Ti atoms on grain boundaries. However, arsenic segregated strongly on grain boundaries at the interface between the base metal and the oxide scale, where it had a maximum arsenic content of 2.93 wt.%. The strongly segregation of arsenic on grain boundaries at the interface between the base metal and the oxide scale may lead to difficulties in the subsequent production galvanized sheet with the hot rolled CSP strip that contains arsenic. When the hot rolled strip (CSP) was reheated to 1100 °C, held for half an hour, arsenic segregated strongly on subgrain boundaries for the availability of more vacancies caused by the solid solution of alloy carbonitrides.

Effect of Rare Earth on Microstructure and Impact Toughness of Low Carbon Steel Based on Compact Strip Production Process

The effect of rare earth (RE) on microstructure and impact toughness of hot rolled low carbon structure steel based on the Compact strip production (CSP) process was investigated in this work. Scan electron microscope (SEM) were employed to characterize the microstructure and analyze the fracture surface. Moreover, hot compression experiments were also adopted on the Gleeble-1500 hot simulator as well. The results indicate that acicular ferrite and/or ultrafine ferrite morphologies can be induced when an appropriate amount of rare earth added into the low carbon steel, which possessed the satisfied strength and toughness behaviors. Compared with commercial low carbon structural steels, the additions of RE can improve dynamic recrystallization critical strain of low carbon structural steel as well. In addition, the role of RE on the formation of these acicular ferrite morphologies has been investigated.

Microstructure homogenization control of Nb-bearing X65 pipeline steel by the CSP process

In order to get the controlled methods of microstructure homogenization and high strengthening-toughening combination by compact strip production (CSP) rolling, the dynamic recrystallization characteristics of each pass were obtained during CSP rolling using a Gleeble-1500 thermal mechanical simulator. Then, the CSP process was simulated by laboratory rolling experiment. Through thermal mechanical simulation experiment and laboratory rolling experiment, a design idea of Nb-bearing pipeline steel by CSP can be obtained as the following: the level of dynamic recrystallization behavior should be increased through the reasonable balance of high deformation temperature and deformation amount in F1 and F2 passes, and F3-F7 passes should be controlled in the austenite non-recrystallization zone. Finally, X65 pipeline steel with microstructure uniformity and good properties was produced by CSP. The yield strength is up to 497 MPa, the tensile strength is up to 563 MPa, the elongation is 30% on average, and the toughness is very good whose Charpy impact value is 110 J on average and drop-weight tear test shearing areas are all 100% at −60, −40, −20, 0, and 20°C.