采用NETZSCH DIL 402C高温热膨胀仪对三种不同化学成分的钢种在30-1150℃温度范围内的热膨胀特性进行了测量，得到线性热膨胀和瞬时线膨胀系数随温度的变化曲线。对实测的热膨胀值进行定量计算和对比分析后发现，钢在加热过程中的线性热膨胀及瞬时线膨胀系数随温度的变化过程可分为室温至相变区、固态相变区和高温奥氏体区三个阶段，每个阶段碳含量对热膨胀的影响程度不同。在整个升温过程中，物理热效应引起的热膨胀占主导作用，不同钢种的物理热膨胀总量基本相同，而相变引起的收缩量大约占整个膨胀绝对变化量的16%，且收缩总量随碳含量的增加而减少，导致最终不同钢种试样的膨胀量不同。
Two kinds of low carbon bainitic steels,Nb-free Mo bearing and Nb + Mo addition steels,were cold rolled and annealed to investigate the effect of micro-alloying element Nb on the microstructure and properties of Mo microalloyed low carbon high strength bainitic steel. No precipitates were observed in Nb-free Mo bearing steel,whereas,two types of precipitates,i.e.,Nb( C,N) and composite( Nb,Mo)( C,N),were observed in the Nb + Mo microalloyed steel,resulting in precipitation strengthening. The strength of Mo bearing steel was improved by addition of Nb under the same annealing conditions. The grain size of Nb addition steel was almost the same as Nb-free steel. Unlike the obvious grain refinement and precipitation strengthening in hot rolling,the increase in yield strength of Nb addition steels in cold rolling and annealing mainly results from the precipitation strengthening,while the effect of grain refinement strengthening can be almost ignored.
The dynamic observations of bainitic transformation in a Fe-C-Mn-Si superbainite steel were conducted on a high temperature laser scanning confocal microscope. It is indicated that the mutual intersection of bainite sheaves often occurs during growth of bainite ferrite, resulting in an interlocked bainite microstructure. Moreover, bainite transformation is promoted by higher austenization temperature and the longer and finer bainite platelets are obtained. Further, The average growth rate of bainite after austenization at 1 100 ℃ is calculated as 5.8 μm·s-1. In situ observation investigation makes it possible to identify bainite transformation in real time during isothermal holding.
