超快速冷却条件下碳素钢中纳米渗碳体的析出行为和强化作用

PRECIPITATION BEHAVIOR AND PRECIPITATION STRENGTHENING OF NANOSCALE CEMENTITE IN CARBON STEELS DURING ULTRA FAST COOLING

利用超快速冷却技术对碳素钢中渗碳体的纳米析出行为和强化作用进行了研究.实验结果表明,在超快速冷却条件下,0.17%C和0.33%C钢的组织中形成了大量弥散的纳米级渗碳体析出,颗粒尺寸为10-100 nm,实现了在无微合金元素添加的条件下渗碳体的纳米级析出.随着超快速冷却终冷温度的降低,钢的屈服强度和抗拉强度都逐渐增加,当超快速冷却的终冷温度从890℃下降到600℃时,0.17%C和0.33%C钢的屈服强度提高超过了100 MPa.在超快速冷却之后采用形变热处理工艺,可以进一步增加钢的位错密度,促进渗碳体均匀形核,实现了纳米级渗碳体颗粒在整个组织中更加均匀弥散的分布,从而更好地实现均匀强化的效果.在超快速冷却和形变热处理工艺条件下,0.17%C钢的屈服强度提高到600 MPa以上.

In recent years, the precipitation strengthening by cementite, which is a common and economical second phase constituent in steels, has drawn renewed attention in the context of precipitation strengthening, because if cementites can be effectively refined to the scale of a few nanometers, they can induce significant precipitation strengthening effect. Therefore, nanoscale cementite is viewed as a viable option to replace precipitates of microalloying elements for reducing alloy costs in steel products. Given that cementites are usually to form lamellar pearlite structure in a traditional cooling process and generally tend to coarsen at relatively high temperatures, the thermodynamic feasibility for the formation of nanoscale cementite precipitates during cooling has been determined in the previous study, and the non-equilibrium precipitation of nanoscale cementite can be realized by increasing the cooling rate after hot rolling. Thus, the ultra fast cooling（UFC） technology was applied after the hot strip rolling for the research of precipitation behavior and precipitation strengthening of nanoscale cementite in carbon steels. The experimental results demonstrated that the UFC technology shows the unique effects on strengthening in carbon steels and a large number of dispersed nanoscale cementite precipitates with the size of 10-100 nm have been formed in 0.17%C and 0.33%C steels. The nanoscale precipitation of cementite was realized in the microstructure by UFC without the microalloy elements addition. Both the yield strength and tensile strength of the steels increased gradually with the stop temperature of UFC decreasing, and the yield strength increments of 0.17%C and0.33%C steel were more than 100 MPa, when the stop temperature of UFC decreased from 890 ℃ to 600 ℃. Besides, thermomechanical treatment（TMT） process was introduced after UFC to explore uniform nucleation of cementite in hot-rolled carbon steels, and it is a feasible way to realize the uniform precipitation of nanoscale cementite in the entire miscrostructure for the further strengthening improvement. This was accomplished by subjecting the UFC cooled steel to a small degree of plastic deformation, with the aim to increase the dislocation density evidently. By combining UFC and TMT processing, the yield strength of 0.17%C steel is greater than 600 MPa, leading to a superior strengthening effect.