Fe-C-Mn-Si系组织梯度中碳纳米高强钢制备基础研究

The strength of nano-structured steel is usually several times that of counterparts. Only low-carbon nano-structured steel can be fabricated by cold rolling and annealing technology and the elongation is poor. A new approach combining the treatment of carbon gradient and cold rolling and annealing to fabricate the Fe-C-Mn-Si medium carbon nano-structured high strength steel and improve the elongation of nano-scale steel is proposed.The graded carbon distribution from surface to core in sample is obtained by gradient treatment at austenization range. The gradient microstructure of bainite in surface and martensite in core is obtained by queching the sample with gradient distribution of carbon.Finally,the gradient microstructure of nano-scale ferrite grain in core to micron-scale ferrite grain on surface can be produced after cold rolling and annealing. Medium carbon nano-structured steel can be produced by this new method, which will expand the nano-scale steel from low-carbon steel to medium-carbon steel through cold rolling and annealing technology. Meanwhile,the elongation of nano-structured steel with gradient microstructure of grain size can be improved to meet the engineering requirment in the condition of that strength of steel is still more than two times that of counterparts. The influences of composition distribution and technology parameters on gradient microstructure will be ascertained by this study. At the same time, the mechanism of elongation improvement by gradient microstructure will be clarified. The study of this project will not only provide the theoretical basis for the industry production of medium carbon nano-structured high stregth steel, but also enrich the basic theory of elongation improvement by gradient microstructure in nano-structured steels.

纳米钢铁材料强度是相同成分普通钢铁材料的数倍以上，目前采用冷轧和退火工艺只能制备低碳纳米钢，并且制备出的低碳纳米钢延伸率较低。本项目提出结合碳梯度分布处理和冷轧退火工艺，制备晶粒尺寸梯度分布Fe-C-Mn-Si系中碳纳米高强钢并改善延伸率的新方法。通过碳梯度处理使钢中含碳量从中心到表面呈梯度分布，淬火后获得芯部马氏体和表面贝氏体复相组织，然后采用冷轧和退火工艺，得到晶粒尺寸由芯部纳米级逐渐向表面微米级连续变化的铁素体梯度组织，制备出中碳纳米钢，将冷轧和退火工艺制备纳米钢由低碳钢扩大到中碳钢范围。同时，通过晶粒尺寸梯度分布组织改善中碳纳米钢延伸性能。揭示碳元素分布和冷轧退火工艺参数对纳米晶粒梯度组织的影响规律，阐明晶粒梯度组织对纳米碳锰钢强塑性的影响机理，探索改善纳米钢铁材料延伸性能的新方法。不仅能为中碳纳米钢工业化生产提供理论指导，而且可以丰富纳米钢铁材料梯度组织改善延伸性能的基础理论。

纳米钢铁材料强度是相同成分普通钢铁材料的两倍以上，目前制备大块纳米钢铁材料最有效的方法是冷轧+退火工艺，但目前还不能制备含碳量大于0.2wt.%的中碳纳米钢，并且制备出的低碳纳米钢由于纳米级晶粒加工硬化能力低，导致延伸率很低，达不到工程用钢的要求。针对这一难题，本项目提出结合碳梯度分布处理和冷轧退火工艺，制备晶粒尺寸梯度分布Fe-C-Mn-Si系中碳纳米高强钢并改善延伸率的新方法。项目采用脱碳、轧制、退火等方法制备了晶粒尺寸从表面到中心逐渐减小的梯度组织中碳钢，对具有梯度纳米结构的中碳钢组织和力学性能进行了系统的研究。此外，还进行了脱碳钢的温轧退火实验，探明了轧制温度对梯度组织及其性能的影响规律。结果表明，通过脱碳淬火可成功实现碳梯度分布，对中碳马氏体实施等效应变为1.5的大压下轧制，试样表面无微裂纹产生，相比未脱碳均质马氏体钢具有更好的塑性。脱碳淬火试样中存在碳梯度变化和马氏体尺寸梯度变化。表层由于C扩散速度更快，提高了碳化物的溶解速率，使得奥氏体晶界迁移速率更快，表层的奥氏体晶粒更粗大，最终得到了奥氏体晶粒尺寸的梯度分布，从而进一步获得马氏体的尺寸梯度分布，即表层粗大的低碳马氏体组织逐渐过渡到芯部细小的中碳马氏体组织。另外，随着退火时间的延长，获得的梯度组织屈服强度、抗拉强度逐渐降低，总延伸率逐渐降低。当组织中存在较多超细晶组织时，拉伸曲线没有表现出明显的加工硬化能力。梯度组织板条马氏体钢在进行压下量为70%的温轧后，在550~650℃退火可获得梯度组织，当低温段时间退火处理后，芯部为细小纳米晶粒组织，边部为较粗大的微米级晶粒组织，实现了从表层到芯部的组织梯度变化，改善了纳米结构钢铁材料的延伸率。本项目研究结果不仅能为中碳纳米钢的工业化生产提供理论指导，而且可以丰富纳米钢铁材料梯度组织改善延伸性能的基础理论。