Yonglin Kang

Morphology and precipitation kinetics of AlN in hot strip of low carbon steel produced by compact strip production

Abstract Morphology and precipitation kinetics of AlN were investigated both experimentally and using a predictive model in hot strip of low carbon steel produced by compact strip production (CSP), a process for the production of hot strip from a continuously cast thin slab using direct charging. By an electrolytic dissolution technique and TEM analysis, the shapes of AlN were cube or plate, the precipitates were uniform and very fine, with average size less than 8 nm. The paper presents a model to describe the precipitation kinetics of AlN under thermomechanical process conditions of CSP, which predicted volume fraction evolution and the precipitate size of strain induced precipitation on the basis of classical nucleation theory. For the process conditions of this paper, about 3 s time was sufficient for appreciable strain induced precipitation occurred immediately and entirely on dislocations after the sixth finishing rolling pass, and the computing radius of precipitate was 2 nm, which was in good agreement with experimental data. The differences of AlN precipitation between CSP and conventional process are also discussed.

Precipitation Behavior and Its Strengthening Effect of X100 Pipeline Steel

Using TEM (transmisson electron microscopy), electron diffraction, EDX (energy dispersive X-ray) analysis and physicochemical phase analysis, the morphology, crystal structure, size distribution and chemical composition of precipitates in the microstructure of high strength Nb-microalloyed X100 pipeline steel were investigated, and the strengthening effect of precipitation was quantitatively calculated with Ashby-Orowan correction model. The precipitates obtained in X100 pipeline steel can be divided into two kinds: “complex” and “single” particles by morphology. The EDX analysis of “single” precipitates reveals that the chemical composition matches well with particle dimensions, especially the Nb to Ti ratio regularly decreases with the increase of particle size. The yield strength increments in the way of precipitation strengthening of X100 pipeline steel reached about 52 MPa, suggesting that the precipitation strengthening is not the dominative strengthening mechanism for X100 pipeline steel.

Comparison and analysis of dislocation density, morphology and evolution in microstructure of low-carbon steel produced using different technologies

Three kinds of specimens were produced from hot strips of similar composition and same thickness (nominal gauge 4.0 mm) but produced using different technologies, and the dislocation density of these strips was quantitatively measured by positron annihilation technique test. The dislocation morphology and evolution in microstructure of each pass for producing the 1.9 mm hot strip using CSP (compact strip production) technology were observed under an H-800 transmission electron microscope; its density was also quantitatively measured using the positron annihilation technique test, and the factors influencing the dislocation density during the production process were analyzed. The experimental results show that the dislocation density in the microstructure produced using CSP technology is higher than that in the microstructure produced using conventional technology. This result was discussed and confirmed on the basis of the finite element simulation and the theory relevant to dislocations.

Effect of Cooling Start Temperature on Microstructure and Mechanical Properties of X80 High Deformability Pipeline Steel

The effect of cooling (laminar cooling) start temperature on the phase constitution was analyzed by quantitative metallography. The martensite/austenite (M/A) island distribution was fixed by colour metallography. The strength and uniform elongation of the steels were tested with quasi-static tensile testing machine. The in-coordinate deformation of the soft and hard phases was analyzed using FEM. The results indicate that when the cooling start temperature is 690 °C, the mechanical properties are the best, meeting the requirements of X80 high deformability pipeline steel.

Microstructure and Properties of Ti and Ti＋Nb Ultra-Low-Carbon Bake Hardened Steels

Hot rolling, cold rolling and continuous annealing processes of Ti bearing and Ti+Nb stabilized ultra-low-carbon bake hardened steels were experimentally studied. The microstructure and texture evolution, as well as the morphology, size and distribution of second phase precipitates during hot rolling, cold rolling and continuous annealing were also analyzed. The results showed that the size of NbC precipitates in Ti + Nb stabilized ultra-low-carbon bake hardened steel was smaller than that of TiC precipitates in Ti bearing ultra-low-carbon bake hardened steel, which made the average grain size of Ti + Nb stabilized ultra-low-carbon bake hardened steel finer than that of Ti bearing ultra-low-carbon bake hardened steel: for the yield strength, the former was higher than the latter; but for the r value which reflects the deep-drawing performance, the former was lower than the latter.

Precipitation Characteristic of High Strength Steels Microalloyed with Titanium Produced by Compact Strip Production

Transmission electron microscopy (TEM) and physics-chemical phase analysis were employed to investigate the precipitates in high strength steels microalloyed with Ti produced by compact strip production (CSP). It was seen that precipitates in Ti microalloyed steels mainly included TiN, Ti4C2S2, and TiC. The size of TiN particles varied from 50 to 500 nm, and they could precipitate during or before soaking. The Ti4C2S2 with the size of 40-100 nm might precipitate before rolling, and the TiC particles with the size of 5-50 nm precipitated heterogeneously. High Ti content would lead to the presence of bigger TiC particles that precipitated in austenite, and by contrast, TiC particles that precipitated in ferrite and the transformation of austenite to ferrite was smaller. They were less than 30 nm and mainly responsible for precipitate strengthening. It should be noted that the TiC particles in higher Ti content were generally smaller than those in the steel with a lower Ti content.

Effect of boron on hot strips of low carbon steel produced by compact strip production

Abstract The effect of boron on hot strips of low carbon steel produced by compact strip production (CSP) to reduce the strength to a certain degree was investigated, which is quite different from that of high-strength low alloy steel. The mechanical properties and microstructural evolution of the hot strip were studied using optical microscopy and tensile tests. By means of an electrolytic dissolution technique and Thermo-Cal calculation, the precipitates containing boron were analyzed and detected. From the electron back-scattered diffraction analysis, it can be deciphered whether the microstructure has recrystallized or not. Furthermore, the effect of boron segregation on the recrystallization or non-recrystallization conditions can be distinguished. The segregation behavior of boron was investigated in boron-containing steel. The nonequilibrium segregation of boron during processing was discussed on the basis of the forming complexes with vacancies that migrate to the boundaries prior to annihilation, which was confirmed by the subsequent cold rolling with annealing experiments.

Research on Microstructural Evolution and Dynamic Recrystallization Behavior of JB800 Bainitic Steel by FEM

Abstract Single pass compression tests were conducted on Gleeble1500 thermal simulator. The effect of different deformation parameters on the grain size of dynamically recrystallized austenite was analyzed. A mathematical model of dynamic recrystallization and a material database of JB800 steel, whose tensile strength is above 800 MPa, were set up. A subprogram was compiled using Fortran language and called by Marc finite element software. A thermal coupled elastoplastic finite element model was established to simulate the compression process. The grain size of recrystallized austenite obtained by different recrystallization models was simulated. The results show that the optimized dynamic recrystallization model of JB800 bainitic steel has a higher precision and yields good agreement with metallographic observations.

Dynamic CCT Diagram of Automobile Beam Steel With High Strength Produced by FTSR Technology

The dynamic continuous cooling transformation (CCT) diagram and phase transformation rules of 510 MPa automobile beam steel, which is produced by a continuous casting of thin slab of FTSR technology in Tangshan Iron and Steel Co. Ltd., are researched by thermal simulation experiment. The microstructure characteristics of the beam steel under different test conditions are studied by means of optical microscope and scanning electron microscope, The test results show that the critical temperatures of phase transformation Ar3 and Ar1 will all decrease with the increase of the cooling rate. When the cooling rate is lower than 20 °C ° s−1, the ferrite and pearlite phase transformations are the main parts; when the cooling rate is higher than 20 °C ° s−1, the bainite phase appears. Moreover, the microstructures of 510 MPa automobile beam steel produced by FTSR technology are also studied, and the results are basically in accordance with the CCT diagram gained from the test.

Microstructure and Mechanical Properties of Automobile Beam Steels Produced by EAF-CSP Process

Abstract The microstructure, mechanical properties, and misorientation of automobile beam steels produced by EAF-CSP process were studied using optical microscopy (OM) and electron back-scattered diffraction (EBSD). It is shown that the microstructure of strips is mainly polygonal ferrite, and the average grain size is about 5–8 μm. The electron back-scattered diffraction results show that grain boundaries in ferrite are basically high-angle grain boundaries without remarkable preferred orientation. Hot strips of automobile beam steels possess a good combination of strength and plasticity because of their fine microstructures and low quantity of impurities.