形变及冷却速率对热轧超高强汽车钢板中纳米析出的影响

为精确控制热轧780 MPa级Nb-Ti微合金化C-Mn钢中的纳米析出物(Nb,Ti)C,利用热力模拟实验技水,通过透射电镜观察及统计分析,研究形变及冷却速率对纳米析出的影响规律.结果表明,形变可显著地提高析出物形核率,并细化析出物平均直径;析出物数量随冷却速率的增大逐渐减小;既定的实验条件下,冷却速率达到15℃/s可完全抑制析出物在冷却过程中形核;随着冷却速率的增大,析出物的形核区间由奥氏体区形核向铁素体或贝氏区转变,析出物平均直径明显细化;在低冷却速率条件下的变形实验钢中,形变提高组织中的空位浓度,促进析出物空位形核的发生;晶界或亚晶界是过饱和空位的主要陷阱,但空位的扩散活性很高致使低冷却速率条件下晶界或亚晶界附近的空位浓度低于析出物形核的临界形核浓度,从而无法形核,形成晶界附近无析出带;无析出带宽度随冷却速率的增大而减小,这归因于空位扩散活力随冷却速率的增大而降低.

EFFECTS OF DEFORMATION AND COOLING RATE ON NANO-SCALE PRECIPITATION IN HOT-ROLLED ULTRA-HIGH STRENGTH STEEL

In order to control nano-scale precipitation(Nb,Ti)C in hot-rolled 780 MPa grade C-Mn steel micro-alloyed with niobium and titanium for automobile frames,the effects of deformation and cooling rate on nano-scale precipitation were investigated by using the thermal simulation experiment technology,then through the transmission electron microscopy observation and statistical analysis.The result indicated,deformation could significantly improve density of dislocation,subgrain boundary and vacancy etc in microstructure,and promote heterogeneous nucleation of precipitation,and improve nucleation rate of precipitation and decrease the average diameter of precipitation.Deformation could improve vacancy concentration and promoted vacancy nucleation.The induction period of precipitation nucleation decrease with the increase of deformation amount and strain rate,and precipitation more easily to nucleate.Precipitation nucleation driving force was mainly supersaturation of microalloy in undeformed experimental steel,and the nucleation mechanism was mainly homogeneous nucleation.However,the nucleation mechanism was mainly heterogeneous nucleation in deformed experimental steel.In one fixed experimental deformation condition, when the cooling rate below 5℃/s,there was(Nb,Ti)C-PFZ(precipitate free zone) nearby original austenitic grain boundaries or subgrain boundaries,and the width of PFZ at cooling rate of 0.5,1, 2 and 5℃/s were 46.9,30.2,28.1 and 0 nm,respectively,so the width of PFZ decreased with the cooling rate increasing.When the cooling rate reached 15℃/s,the nucleation of precipitation was totally inhibited during cooling process.The number of precipitation along with the cooling rate increases gradually decreases.With the increase of cooling rates,the nucleation zone of precipitation was transferred from austenite to ferrite or bainite,and the average diameter of precipitation was refined. Due to grain boundaries or the subgrain boundaries were main traps for supersaturated vacancy,but the diffusivity of vacancy was high,which made the vacancy concentration nearby grain boundaries or the subgrain boundaries lower than critical vacancy concentration for precipitation nucleation,so precipitate could not nucleate nearby grain boundaries or subgrain boundaries.Due to the diffusivity of vacancy was affected by temperature,when the cooling rate was slow,vacancy had enough time to diffuse and annihilate,which made wide PFZ formed.Whereas,when the cooling rate was high, the diffusivity of vacancy was reducing or disappearing,so the width of PFZ was small.In orde to ensure experimental steel had higher yield strength,austenite zone precipitation and(Nb,Ti)C-PFZ nearby boundaries should be inhabited,so the cooling rate should be more than 15°C/s in the practical rolling process.