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Guangzhou, China

Huo X.-D.,Jiangsu University | Mao X.-P.,Guangzhou Zhujiang Steel Co. | Lu S.-X.,Jiangsu University | Lin Z.-Y.,China Institute of Technology | Chen Q.-L.,China Institute of Technology
Beijing Keji Daxue Xuebao/Journal of University of Science and Technology Beijing | Year: 2011

Optical microscopy, electron microscopy and electrolytically extracted phase analysis, in combination with thermodynamic calculations, were used to study precipitates and their precipitation behavior in Ti-microalloyed high strength steel produced by CSP process. It is found that many micron-sized cubic TiN particles and a large number of nanometer precipitates exist in the steel. The mass fraction of MX phases (M=Ti, Mo, Cr; X=C, N) in the steel is 0.0927%, in which particles smaller than 10 nm account for 26.9%. Thermodynamic calculations show that titanium nitride has been almost completely precipitated from the steel during or before soaking, but titanium carbide hardly precipitates before tandem rolling. Decreasing the contents of nitrogen and sulfide and controlling the coiling temperature can increase the volume fraction of titanium carbide, and decreasing the γ→α transformation temperature can prevent fine carbides from growing. These results indicate that the precipitation hardening resulting from nanometer particles is about 156 MPa and the effect can be raised through controlling the chemical composition and rolling parameters. Source

Huo X.-D.,Jiangsu University | Mao X.-P.,Guangzhou Zhujiang Steel Co. | Lu S.-X.,Jiangsu University
Journal of Iron and Steel Research International | Year: 2013

In order to develop cold rolled Ti-microalloyed steel strips, the effects of annealing temperature on recrystallization behavior of experimental steel were researched by optical microscopy (OM), transmission electron microscopy (TEM) and Vickers hardness test. The annealing treatment could be divided into three distinct stages: recovery, recrystallization and grain growth. Recrystallization took place from 933 to 1033 K, during which a large number of recrystallized grains appear and hardness drops sharply. The morphology and size of TiN particles nearly remained unchanged at different stages of processing. With increasing annealing temperature, nanometer precipitates coarsened and the dislocation density was significantly reduced. In comparison with annealing time, annealing temperature was more crucial for recrystallization of cold rolled Ti-microalloyed steel. It could be concluded that the pinning force of nanometer particles on dislocations increased the recrystallization temperature. At higher annealing temperature, recrystallization took place because of precipitates coarsening caused by Ostwald ripening. © 2013 Central Iron and Steel Research Institute. Source

Huo X.-D.,Jiangsu University | Mao X.-P.,Guangzhou Zhujiang Steel Co. | Dong F.,Jiangsu University
Beijing Keji Daxue Xuebao/Journal of University of Science and Technology Beijing | Year: 2013

Keeping chemical composition and other parameters unchanged, the effects of coiling temperature (625 and 579°C) on the microstructure and properties of Ti-microalloyed high strength steel were investigated during compact strip production (CSP). Experimental results of mechanical properties show that compared with steel strips coiled at 625°C, the yield strength of steel strips coiled at 579°C decreases by 205 MPa, but the impacting energy at -20°C increases from 11.7 J to 47 J. Optical microscopy and electron microscopy were used to study the microstructure and precipitates in the steel. It is found that the primary microstructural constituent of steel strips coiled at 625°C is ferrite grains, but the microstructure of steel strips coiled at 579°C is finer and characterized with bainite grains. The volume fraction of nanometer carbides in steel strips significantly reduces with decreasing coiling temperature, which weakens the precipitation hardening effect and causes a marked reduction of strength. However, the toughness is improved due to grain refinement and volume fraction decreasing of precipitates. Coiling temperature needs to be strictly controlled, because it is a more important parameter of producing Ti-microalloyed high strength steel. Source

Mao X.-P.,Guangzhou Zhujiang Steel Co. | Mao X.-P.,Wuhan University of Science and Technology | Chen Q.-L.,Control Iron and Steel Research Institute, China | Li C.-Y.,Control Iron and Steel Research Institute, China
Kang T'ieh/Iron and Steel | Year: 2012

Thin slab casting and direct rolling process (hereinafter referred to as "TSCR"), characterized by high liquid steel solidification rate, low heating temperature and short stay time in the roller-hearth furnace and high pass reduction, was helpful to inhibit the segregation of chemical composition, refine the non-metallic inclusions, lower surface decarburization, reduce the inter-laminar distance of pearlite, and alleviate to some extent the problems that traditional production process had in producing high-medium carbon steel strips. Consequently, TSCR was a suitable process for producing high-quality high-carbon strips. The microstructure of high carbon steel produced by TSCR process was investigated by optical microscope, scanning electron microscopy and other methods. The results show that: maximum carbon segregation rate is 1.16, much lower than 2.0 of traditional process, its single face decarburization depth is less than 1.0% of strip's thickness, only 30%-60% of that of traditional process, its pearlite inter-laminar distance is much smaller than that of traditional process, which are favorable for increasing the overall performance of the materials. High-medium carbon steels, such as high quality carbon steel, high carbon tool steel, spring steel, alloy structure steel and alloy tool steel, with carbon content up to 1.0%, can be produced by TSCR process and can also be widely used in industries such as automotive manufacturing, engineering machinery, special equipment, high-end blade and special tools. Source

Mao X.,Guangzhou Zhujiang Steel Co. | Mao X.,China Institute of Technology | Chen Q.,China Institute of Technology
Proceedings of the 10th International Conference on Steel Rolling | Year: 2010

This paper expounds microalloyed steels produced by thin slab easting and direct rolling process (hereinafter referred to as TSCR), which focus on the physical metallurgy characteristics of Ti, V, Nb microalloyed steels, including; the precipitation ride of microalloying elements, microslruclure characteristics and strengthening mechanism. The research shows that Ti precipitates throughout the entire process from casting to coiling, in 700 MPa grade Ti microalloyed steel, the mass fraction of less than 10 nm particles accounts for 33.7% of the total mass fraction of MC phases, the microslruclure is quasi-polygonal territe wilh less pearlile, the smallest lerrile grain size is 2.9μm, grain refinement is the main way of strengthening; the great quantity of V(C, N) precipitates with less TiN or (Ti, V) (C, N) were found in the slab of V microalloyed steel, and its average grain size is about 40 nm, calculations show that these precipitates can inhibit the growth of deformed austenite in the subsequent hot rolling process and fine lerrile, the ferrile grain size of 550 MPa grade V microalloyed steel is 3.0 ∼4.0 μm; Mixed grain tend to appear in Mb microalloyed steel in TSCR, the key measures to solve the problem is to avoid deformation in the partially recrystallized zone. Currendy, 450 ∼ 700 MPa grade high-strength steel were developed by Ti microalloying, 275 ∼ 550 MPa high-strength steel were developed by V microalloying, and high-strength automotive structural steel QSlE340 ∼ 460 TMs, pipeline steel X52 ∼X65 as well as oil casing steel J55 were developed by Nb microalloying. Source

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