Abstract：Continuous casting of Nb-bearing steels is easy to produce transverse cracks on slab corners. Applying a cyclic phase transformation cooling process(γ→α→γ),which can greatly improve the structure ductility of steel at high temperature,to the slab corners during continuous casting,the cracks could be greatly reduced. As one of most important parameters,the returning temperature at thestage of α→γphase transformation is an important factor affecting the applying effect of the process. In the present work,the detection methods,such as Gleeble thermal simulation,OM,TEM,as well as the fracture scanning are used to analyze the micro-structure evolution and plasticity of a Q345D-Nb steel under the different returning temperatures of the cyclic phase transformation cooling process. The results showed that the austenite grains would not be refined at the returning temperature of 850 ℃. The average size of the grains was about 502.2 μm,which was similar to that of the conventional cooling process. When the returning temperature rose to 900 ℃,the austenite showed mixed growth. When the returning temperature rose to 950 ℃,the grain size was refined to 61.2 μm. When the returning temperature rose to 1 000 ℃,the austenite grain coarsely grew. The average size of austenite grains increased 38.07 % compared with that of the returning temperature to 950 ℃. Under the conventional cooling process and the cyclic phase transformation cooling process at the returning temperature of 850,900,950 and 1 000 ℃,respectively,the minimum reduction of area (RA) at the temperature range of 700-900 ℃ were 29.6%,45.0%,56.3%,68.2%,and 63.2%. Under the conventional cooling process,the thickness of ferrite film at grain boundary of the steel at 750 ℃ was 20-25 μm,and the carbonitride precipitated with large size and chain structure distribution,and the fracture mode was intergranular brittle fracture. At the same tensile fracture temperature,the thickness of ferrite film of austenite grain boundary under the cyclic phase transformation cooling process decreased to 5-10 μm,and the carbonitride precipitates dispersedly and finely. The fracture mode transforms to the plastic fracture gradually. The plasticity was significantly improved. Applying the cyclic phase transformation cooling process to the practice,the average grain sizes of austenite at the depths of 5 mm and 10 mm below the corner of slab were refined to 186 μm and 362 μm,which increased the ability of slab to resist cracks.
程彪, 蔡兆镇, 安家志, 朱苗勇. 双相变回温温度对钢组织及其热塑性的影响[J]. 钢铁, 2023, 58(1): 67-77.
CHENG Biao, CAI Zhao-zhen, AN Jia-zhi, ZHU Miao-yong. Effect of cyclic phase transformation cooling process at returning temperature on microstructure and plasticity of steels[J]. Iron and Steel, 2023, 58(1): 67-77.
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2023, Vol. 58 
