Xinjun Sun

Microstructure evolution during continuous cooling in niobium microalloyed high carbon steels

Effects of microalloyed niobium (Nb) on the austenite decomposition behaviors and microstructure evolution during continuous cooling in the near eutectoid steels were investigated. Compared to the Nb free steel, the Nb microalloyed steel was refined with regard to polygonal ferrite grain, pearlite block and colony sizes. This was because its austenite grain size was smaller. The volume fraction of polygonal ferrite transformed was more in the Nb microalloyed steels, which indicated the eutectoid carbon content exceeded that of pure carbon steel. The spheroidization of pearlite during continuous cooling was enhanced by Nb microalloying, mainly due to a higher critical transformation temperature and the finer pearlite structure with smaller colony size and narrower interlamellar spacing. Hot deformation right above the equilibrium eutectoid temperature accelerated the spheroidization kinetics of pearlite, especially in the Nb microalloyed steel.

Microstructure and Mechanical Properties of Precipitation Strengthened Fire Resistant Steel Containing High Nb and Low Mo

Through the thermo-mechanical control process (TMCP), a high Nb low Mo fire resistant steel with the yield strength (YS) of 521 MPa at room temperature (RT) and 360 MPa at elevated temperature (ET) of 600 °C was developed based on MX (M=Nb, V, Mo; X=C, N) precipitation strengthening. A series of tensile and constant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transformation (CCT) as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal simulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was determined as 653 °C, and the granular bainite was obtained when the cooling rate was higher than 10 °C/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates (Nb, Mo, V)C with size of less than 10 nm precipitated in ferrite at ET (600 °C), which resulted in precipitation strengthening at ET.

Solution and Precipitation of Secondary Phase in Steels: Phenomenon, Theory and Practice

The secondary phases exert strong influence on the mechanical properties, processing properties and performance of steels. The volume fraction, size, shape and distribution of the secondary phases must be effectively controlled in order to obtain the excellent properties and performance. On the basis of equilibrium solubility product formula, precipitation thermodynamics and kinetics, and classical nucleation and growth theory, the theoretical calculation method and procedure for the PTT (precipitation fraction–time–temperature) curve and NrT (nucleation rate–temperature) curve for main secondary phases precipitated in austenite and in ferrite have been proposed. The calculation results are well in agreement with practical experimental results. By analyzing the calculation results, some important deductions for the practical control of secondary phases in steels have been made.

PRECIPITATION BEHAVIOR OF NANOMETER-SIZED CARBIDES IN Nb-Mo MICROALLOYED HIGH STRENGH STEEL AND ITS STRENGTHENING MECHANISM

Recently, increasing attention has been focused on the high strength low alloy (HSLA) steels mircoalloyed with multiple miroalloying elements, such as Nb-Ti, Nb-V and Ti-Mo, which can form synthetic carbide in steel, such as (Nb, Ti)C, (Nb, V)C and (Ti, Mo)C. Compared with the simplex carbide, such as NbC, TiC, those synthetic carbides with nanometer size exhibiting a superior thermal stability to exert their powerful influence mainly through their precipitation hardening in ferrite. It is reported that the precipitation hardening of approximate 300 MPa which can be obtained in Ti-Mo-bearing steel was developed by JFE steel, attributing to the synthetic (Ti, Mo)C particle precipitated in ferrite. However, as common microalloying elements, Nb and Mo are added synchronously in steel. The strengthening mechanism of Nb-Mo mircoalloyed as-rolled steel and the role of the carbide pre*国家重点基础研究发展计划项目2015CB654803和国家高技术研究发展计划项目2015AA034302资助 收到初稿日期: 2015-09-15,收到修改稿日期: 2016-01-05 作者简介:张正延,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00482

Effect of dissolved niobium on eutectoid transformation behavior

The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied. Dissolved niobium is important in the eutectoid transformation behavior. It increases the eutectoid carbon content significantly (by ~0.0477% per 0.000 01% dissolved niobium), increases the hardenability of steel markedly, yields finer, more uniform, polygonal proeutectoid ferrite, induces a transition in morphology of eutectoid cementite from lamellar to somewhat spheroidal, and increases the misorientation angle of pearlite colonies from being focused near 0° to near 60°.

Effect of Mo Addition on the Precipitation Behavior of Carbide in Nb-Bearing HSLA Steel

The effect of Mo addition on the precipitation of carbides both in austenite and ferrite of Nb-bearing HSLA steel were investigated by stress relaxation and transmission electron microscopy (TEM). Experimental results showed that the addition of ~0.2 wt.% Mo into Nb-bearing steel slightly accelerated the precipitation kinetics of Nb carbides both in austenite and ferrite. The particles, (Nb, Mo)C, precipitated in Nb-Mo-bearing steel exhibit superior coarsening resistance compared to that of NbC particles in Nb-bearing steel. And the difference of carbides precipitated in anstenite and ferrite of Nb-Mo-bearing steel was discussed.

Grain Refinement and Toughening of Low Carbon Low Alloy Martensitic Steel with Yield Strength 900 MPa Grade by Ausforming

Instead of off-line quenching and tempering (QT), on-line ausforming (non-recrystallization controlled roling) and direct quenching (DQ) was employed to improve the toughness of low alloy steel with yield strength of 900 MPa Grade. Low carbon content ensured a high level of upper shelf energy, while the fine-grained martensite substructures decreased the ductile-to-brittle transition temperature and compensated the strength loss due to carbon reduction. Two mechanisms for the refinement of martensite substructure were proposed: one was the austenite grain refinement in the thickness direction, and the other was the self-accommodation of martensite variants due to austenite grain hardening. More than 200 J of Charpy V-notch impact absorbed energy at 233 K was obtained in the industrial ausformed plate, which was about the double in the traditional QT steel with the same strength grade.

A Novel Low-cost Hot Rolled High Strength Steel for an Automatic Teller Machine

A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. The low-cost C-Mn steel was microalloyed with 0.08 mass%–0.12 mass% Ti rather than noble alloying elements, such as Nb, V, Mo, and Cu, etc. The novel steel had a good surface quality and welding property. After the hot rolled steel coils were leveled, the steel plates, the length of which was even down to 1500 mm, had an excellent flatness. The effects of hot rolling parameters on mechanical performance, microstructure and recrystallization behavior were studied. The metallurgical concept for the steel production was also discussed. The result shows that decreasing the finish rolling temperature, increasing cooling rate in the first cooling stage and decreasing the cooling rate in the last cooling stage, together with coiling at a modestly high coiling temperature all resulted in the refined grains and TiC precipitates, thereby improving the strength and toughness of this new steel greatly.

Strengthening Mechanisms of V‐Ti‐Mo Microalloyed 900 MPa Hot‐Rolled Ferritic Steel

A new V-Ti-Mo complex micro-alloyed hot-rolled high strength sheet steel was developed by controlling a thermomechanical controlled processing (TMCP) schedule, in particular with variants in coiling temperature. At coiling temperature of 600 °C, the experimental steel has the best mechanical properties: ultimate tensile strength (UTS) 1000 MPa, yield strength (YS) 955 MPa and elongation (EL) 17%. Furthermore, the contributions of precipitation hardening and grain refinement hardening were high to about 310 and 361 MPa, respectively. The steels developed were mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in contribution of precipitation hardening increment caused a significant difference in YS.