| Anew class of Slim Bainitic ferrite-Austenite Dual Phase (SBA) steel (0.98C-1.59Si-1.94Mn-1.33Cr-0.30Mo-0.02Ni-0.11V) obtained by isothermal treated at 200～300℃ for tens of hours gained importance with increasing interest, on account of its extremely thin (<100nm) bainitic ferrite lath separated by carbon-rich austenite microstructure, low alloy costs, easily controlled processing and of course high strength (2.5GPa), large hardness (600HV) and excellent toughness (30MPa· m1/2). However, long time heat treatment limited its application. In this paper, two solutions have been proposed to shorten the manufacturing period which contain composition optimization and pre-deformation that can accelerate bainitic transformation. It has been proved that SBA can be produced within 1-3 hours, then the research processes and main results are as follows:A carbon poor regions(CPRs) thermodynamic model modified with KRC and LFG has been built to describe the nucleation of bainitic ferrite, the Gibbs free energy of nucleation below 300℃ is -1900 J·mol-1 to -2000 J·mol-1. From the thermodynamic point of view, bainitic ferrite could be transformed in a shear mechanism. With the help of MUCG 83 solftware, three classes of high silicon (1.5～2.5wt%) alloy have been designed, including 0.5Mn-0Cr (various carbon content), 0.8C-1Mn-0Cr and 0.8C-1Mn-1Cr,0.9-C-2Mn-1.5Cr-1.5Co-lAl (reference alloy). The TTT curves were measured by dilatometer (DIL805). Experiment results are in agreement with theoretical analysis, and it is indicated that increasing C content could decrease the Ms and Bs temperatures significantly. Mn is the key alloy of delaying the phase transformation start time (incubation time), it is also slowing down the transformation speed. The transformation time of alloy with Mn and Cr is longer than that with only Mn or Cr addition.The growth behavior of bainite influenced with pre-deformation was conducted by using a thermal and mechanical testing mechine (Gleeble 3500). The results show that bainitic transformation speed in a deformed austenite grain is extremely larger than in a normal grain, and the final volum fraction of bainitic ferrite (BF) could be increased if took the deformation condition in an optimal range. Final BF volum fraction would be decreased when pre-deformation occurring at higher temperature, such as 700℃, with a one step compression strain of 50%. When pre-deformation occurring at a temperature rangeing from 600℃ to Ms temperature, the bainitic transformation speed was accelerated, and the completion time decreased with increasing strain or became shorter with lower deformation temperature. When compress at 300℃ with a strain of 20%, the incubation time of bainitic transformation at 230℃ shorend form 5 hours to 30 minutes.Multi-step ausforming process and fast fabrication of SBA by thermo-mechanical control process (TMCP) were performed on cold rolling mechine. The results showed that SBA with BF lath thickness of -50nm, ultimate tensile strength of 2154MPa and total elongation of 14% can be produced by being isothermally treated at 300℃ within 2 hours after multi-step ausformd. Because of mechanical stability, austenite lath thickness increased to 100nm. SBA with ultimate tensile strength of 2600MPa and total elongation of 7%(Rm·A=18.2GPa·%) can be obtained by warm rolled the new designed alloy with chemical composition of 0.8C-2.5Si-0.5Mn-1 Al and then cooled at air without isothermal treated. The above warm rolling and cooling control process is called TMCP, with careflully controlled that SBA with ultimate strength of 2000MPa, elongation of 13%(26GPa·%) can be obtained by using TMCP.The mechanism of retained austenite forming in blocky morphology has been elucidated and the method of blocky austenite avoiding was proposed. Only the primary slip system can be activated under the influence of multi-step pre-deformation with small strain per-step (<10%). The orientation selection occurring in a single deformed austenite which affected the followed bainitic transformation that lead to a decreasing number of BF packet, so that film-like retained austenite formed between the BF lath. Also because the pre-deformation provides more nucleation site nearing the slip plain, the BF laths formed at high temperature like 300℃ for 1 hour are as thin as that of formed at low temperature (200℃) for tens of hours. These results change the concept that SBA can be only obtained by low temperature (<250℃) isothermal heat treatment for a long time, it makes the commercial production possible... |
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