双相变回温温度对钢组织及其热塑性的影响

Effect of cyclic phase transformation cooling process at returning temperature on microstructure and plasticity of steels

含铌钢连铸过程极易产生铸坯角部横裂纹。对连铸坯角部实施γ→α→γ双相变控冷工艺,可提高其组织的高温热塑性而减少裂纹产生。其中,α→γ相变阶段的回温温度是影响双相变控冷工艺实施效果的重要参数。通过Gleeble热模拟与金相观察、析出物透射以及断口扫描相结合的检测手段,研究分析了双相变过程回温温度对Q345D-Nb钢组织演变及其热塑性的影响规律。结果表明,回温温度为850℃时的奥氏体晶粒相比传统冷却工艺下的晶粒尺寸未产生细化,平均晶粒尺寸为502.2μm;回温温度升至900℃时,回温奥氏体出现了明显的混晶现象;当回温温度达到950℃时,晶粒细化至61.2μm;当回温温度达到1000℃时,回温奥氏体晶粒出现了一定程度粗化,相比950℃回温温度下的奥氏体平均晶粒尺寸增加了38.07%。传统冷却工艺和不同回温温度时的双相变控冷工艺(回温温度为850、900、950、1000℃),钢组织在700~900℃温度区内的断面收缩率最低值分别为29.6%、45.0%、56.3%、68.2%、63.2%。在传统冷却工艺下,钢组织在750℃时晶界铁素体膜的厚度为20~25μm,且碳氮化物呈大尺寸链状分布,其断裂形式为沿晶脆性断裂。而相同拉断温度下,双相变控冷工艺的钢组织奥氏体晶界的铁素体膜厚度减小至5~10μm,碳氮化物呈弥散细小析出,断裂形式逐渐转为塑性断裂,塑性显著提高。采用双相变控冷工艺生产Q345D-Nb钢,铸坯角部皮下5 mm与10 mm深处的奥氏体平均晶粒尺寸细化至186μm与362μm,增大铸坯抵抗裂纹的能力。

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 the stage ofα→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 502.2μm,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 61.2μm.When the returning temperature rose to 1000℃,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 1000℃,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 20-25μm,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 5-10μm,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 186μm and 362μm,which increased the ability of slab to resist cracks.