Formulation of grain growth diagram in simulated weld HAZ of Ti-microalloyed steel

In this paper, the response of TiN particles to welding thermal cycle and the kinetic behavior of the grain growth in weld heat-affected zone of the Ti-microalloyed steel have been studied by using the simulation technology. Based on the theory of the TiN particle dissolution and coarsening kinetic models in weld thermal cycle and some metallographic data from thermal simulated samples, the grain growth diagram of Ti-microalloyed steel is presented. The experimental data from thermal simulated weld samples agreed well with the value of the diagram predicted.

Effect of Annealing Temperature on Recrystallization Behavior of Cold Rolled Ti-Microalloyed Steel

In order to develop cold rolled Ti-microalloyed steel strips, the effects of annealing temperature on recrys-tallization behavior of experimental steel were researched by optical microscopy （OM）, transmission electron micros-copy （TEM） and Vickers hardness test. The annealing treatment could be divided into three distinct stages： recovery, recrystallization and grain growth. Reerystallization 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, re crystallization took place because of precipitates coarsening caused by Ostwald ripening.

Metallurgical Interpretation on Grain Refinement and Synergistic Effect of Mn and Ti in Ti-microalloyed Strip Produced by TSCR

The grain refinement mechanism and synergistic effect of Mn and Ti involved in the Ti-microalloying technology of thin slab casting and direct rolling （TSCR） were elucidated. Because the inevitable precipitation of TiN in high Ti-containing liquid steel decreases the volume fraction of TiN precipitated from austenite and the rapid coarse- ning rate leads to a large size of TiN particles, a relatively weak inhibition effect on the recrystallized grain growth was obtained compared with that in the low Ti-containing steel. However, the ferrite grain size in high Ti-containing steel can be refined by the so-called non-recrystallization rolling. The complex addition of Mn and Ti can improve the strength and toughness of strip remarkably, and the mechanisms are that Mn decreases the transformation tempera- ture, refines the ferrite grains, and enhances the formation of bainite and TiC precipitation in ferrite.

Effect of controlled rolling and controlled cooling process on microstructure of Ti-alloyed pipeline for CSP

The pipeline steel is usually produced by adding niobium and vanadium alloying elements.The titanium alloyed pipeline steel was studied in the paper to reduce the production cost and enhance the competitive ability of pipeline steel.The steel containing 0.070%Ti was refined in the laboratory vacuum refine furnace.The dynamic CCT curve of developed steel was conducted on Gleeble-1500 thermal simulator and multi-pass deformation tests of studied pipeline steel were performed to simulate the CSP hot-rolling technology.Corresponding microstructures were observed and the influences of such technology parameters as deformation strain,deformation temperature, cooling rate and finishing temperature etc.on microstructure were analyzed.The dynamic CCT curve was obtained.The test results showed that the volume fraction of acicular ferrite in the microstructure of Ti-microalloyed X70 pipeline steel increased obviously with the increase of cooling rate after thermo-mechanical deformation.However,the volume fraction of acicular ferrite varies scarcely after the cooling rate of 20℃/s.In order to obtain Ti-microalloyed X70 pipeline steel with excellent compound mechanical properties which has acicular ferrite as the main ideal microstructure,the cooling rate should be controlled to be 20℃/s or more.The higher of the finishing temperature,the more of the volume of acicular ferrite and the less of the volume of polygonal ferrite.The quite fine microstructure can be obtained by lowering the finish-cooling temperature.The results have shown that it is possible to produce the Ti-addition X70 pipeline steel from the point of view of microstructure.

Influence of cooling rate on phase transformation and precipitation behavior of Ti-bearing steel in continuous cooling process

The influence of cooling rate on microstructural evolution and precipitation behavior in Ti,Ti–Nb and Ti-Mo low-carbon steels during the continuous cooling process was studied by dilatometer method,optical microscopy,and transmission electron microscopy.The results indicated that austenite transformation temperature decreased with the increasing cooling rate in three steels.The addition of Nb and Mo promoted bainite and martensite transformation and improved the hardenability of steels.In addition,precipitates formed in deformed austenite and ferrite can be observed simultaneously.Deformation in the austenite non-recrystallization zone can introduce a large number of deformation bands,and then,the precipitates preferentially nucleated in these deformation bands.In the following process,randomly distributed precipitates and interphase precipitates will be formed in ferrite.The precipitates formed in deformed austenite obey Kurdjumov-Sachs orientation relationship with the matrix,while the precipitates formed in ferrite obey Baker-Nutting orientation relationship with the matrix.The addition of Nb and Mo in Ti-bearing steels decreased the precipitates size and increased the number density of precipitates and then improved the precipitation hardening.And the effect of Mo addition is more obvious than that of Nb addition.