A Study on the Heat Transfer Characteristics of Steel Plate in the Matrix Laminar Cooling Process

Accurate prediction and control of the steel plate temperature in the laminar cooling process are very challenging. In this research, an experimental platform was built to measure the heat transfer characteristics of the steel plate in the process of matrix laminar spray cooling when the steel plate is one millimeter away from the upper surface. The “buried couple method” was used, including the cooling temperature and cooling rate. Then, the temperature and the integrated heat transfer coefficient at the steel plate surface were calculated by the time-sequential function method (TSFM). The obtained results show that the fast cooling stage under the water cooling condition occurred in the first 1.5 s, and the measuring point temperature decreased by 8%. The “re-reddening” phenomenon of the steel plate appeared with time, and the measuring point temperature increased by 37%. Second, the maximum calculated difference between the surface temperature and the measuring point temperature was 0.75 °C, and the integrated heat transfer coefficient conformed to the periodic boundary features. The comprehensive convective heat transfer coefficient on the surface was in agreement with the periodic boundary characteristics, and its value exhibited oscillatory attenuation with the cooling process, and the oscillatory peak period was about 6 seconds. Two methods, sequential function method (SFM) and finite difference method (FDM), were used to verify the correctness of TSFM.

Effect of Cooling Process on Microstructure Especially Precipitation Behavior of High-Strength Pipeline Steel

The effect of different cooling processes on microstructure especially precipitation behavior and properties of high grade pipeline steel was investigated in this paper. The results showed that: By two stage controlled cooling process, ferrite-bainite phase was obtained in X80 pipeline steel with high deformability, the average size of Martensite and Austenitic islands (M/A) was approximately 1μm, the Uniform Elongation (UEl) was 10%. Whereas granular bainite was obtained after conventional laminar cooling process and fine M/A of 1∼2μm size dispersed in grains or along grain boundaries. Compared with conventional pipeline steel, high deformability pipeline steel through two stage controlled cooling process had higher Elongation (El), impact toughness and lower yield ratio, due to the existence of ferrie+bainite phase structure which reduced yield strength (YS) and enhanced deformation performance and fracture toughness. Additionally, the effect of precipitation strengthening of high grade pipeline steel produced by Ultra-fast cooling (UFC) process was superior to that produced by conventional laminar cooling process. A significant amount of nano particles precipitated with the size of 1nm to 5nm were obtained through the UFC process. The application of UFC made the precipitation process avoid the nose temperature of precipitation temperature time (PTT) curve, thus the size of nano particles were smaller and the distribution of precipitation was more dispersive.

The Impact of Interphase Precipitation on the Mechanical Behavior of Fire-Resistant Steels at an Elevated Temperature

In this study, we address the challenge of obtaining high strength at ambient and elevated temperatures in fire-resistant Ti–Mo–V steel with ferrite microstructures through thermo-mechanical controlled processing (TMCP). Thermally stable interphase precipitation of (Ti, Mo, V)C was an important criterion for retaining strength at elevated temperatures. Electron microscopy indicated that interphase precipitation occurred during continuous cooling after controlled rolling, where the volume fraction of interphase precipitation was controlled by the laminar cooling temperature. The interphase precipitation of MC carbides with an NaCl-type crystal structure indicated a Baker–Nutting (B–N) orientation relationship with ferrite. When the steel was isothermally held at 600 °C for up to 3 h, interphase precipitation occurred during TMCP with high thermal stability. At the same time, some random precipitation took place during isothermal holding. The interphase precipitation increased the elastic modulus of the experimental steels at an elevated temperature. It is proposed that fire-resistant steel with thermally stable interphase precipitation is preferred, which enhances precipitation strengthening and dislocation strengthening at elevated temperatures.

Study on rolling process and heat treatment of high strength ship plate steel EH40

Means of a tensile test studied the mechanical properties and microstructure of the experimental steel plate under different rolling processes, Charpy impact test, optical microscopy and scanning electron microscopy (SEM). The results show that the optimum thermomechanical control process (TMCP) is a heating temperature of 1200 °C, the best rolling temperature of 1180 °C. The thickness of the ship plate steel was rolled from 170 mm to 40 mm in the recrystallization zone by multi-channel time deformation, and then the thickness was decreased from 40 mm to 15mm in the non-recrystallization zone, the temperature waiting for a range 980 °C ~ 920 °C, the finish rolling temperature of 830 °C. After rolling and being cooled rapidly by laminar cooling, the cooling rate is about 12 °C/s and the final target temperature of 600 °C, which maintains the best state of steels. All data of the experimental steels have accelerated the international level, high-strength ship plate EH40 has been successfully trialed and met the practical requirements, all of these provide a solid foundation for further scientific research.