Fig. 1, Fig. 2 and Fig. 3 show the linear scanning position and energy spectrum test results of carbon element in the austenite structure obtained at different isothermal transformation temperatures. It can be seen from the figure that the content of carbon in austenite on both sides of ferrite sheet is high.
The results show that after nucleation at grain boundary, ferrite grows rapidly in the form of sheet or needle driven by large undercooling. The carbon content in the undercooled austenite on both sides is further increased and the stability is improved. The ferrite structure is formed by alternating ferrite sheet and high carbon austenite sheet.
At the same time, combined with Fig. 1 (b), Fig. 2 (b) and Fig. 3 (c), it is found that the carbon content in austenite increases with the decrease of distance from ferrite / austenite grain boundary along the direction perpendicular to ferrite needle growth. Combined with Fig. 1 (c), Fig. 2 (c) and Fig. 3 (d), it is found that finer high carbon austenite flakes exist in acicular ferrite obtained at different isothermal transformation temperatures.
It can be seen from Fig. 3 (d) that ferrite flakes are interlaced in the strip austenite obtained after isothermal transformation. It can be seen from Fig. 4 (E) that the carbon content in the massive austenite obtained after isothermal transformation presents a “U” distribution, and the closer to the austenite / ferrite grain boundary, the higher the carbon content.