Refinement of packet size in low carbon bainitic steel by special thermo-mechanical control process

The packet size of bainitic steel can be refined by a special relaxation-precipitation-control phase transformation (RPC)technology. When processed by RPC process, the low carbon bainitic steel composes of two kinds of main intermediate transformation phases. One is ultra-fine lath-like bainitic ferrite and the lath is less than 1 μm in width and about 6 μm in length; the alignment of laths forms a refined packet, and the size of packets is about 5-7 μm in length and about 3-4 μm in width. The other is acicular structure. The morphology and distribution of these acicular structures are influenced by relaxation process, the thin and short acicular structures cut the prior austenite grain and refine the bainitic packet size. For the optimum relaxation time, the packet size can be refined to the finest. The mechanical properties are influenced by relaxation time and the 800 Mpa grade low carbon bainitic steel with excellent toughness can be obtained by RPC process.