Nb含量和变形量对水电站用800 MPa高强钢淬火再加热奥氏体晶粒尺寸及其分布的影响

Effects of Nb content and deformation on reheated austenite grain size and distribution of 800 MPa high strength steel for hydropower station before quenching

利用Gleeble 3500热/力模拟试验机,通过1000℃+820℃两阶段热变形+900℃淬火再加热联合模拟试验,研究了Nb含量和不同热变形量对水电站用800 MPa级高强度试验钢淬火再加热晶粒尺寸及其分布的影响规律,并通过透射电镜(TEM)对形变诱导析出的Nb(C,N)的粒子尺寸、分布进行了观测。结果表明,热变形态奥氏体晶粒尺寸(D)对于再加热淬火态奥氏体晶粒尺寸(D′)具有重要遗传性影响,二者以及900℃再加热保温时间t之间存在函数关系D′=(1.0057D-6.9785)×(t/300)^(0.215),用于预测800 MPa高强钢再加热淬火态晶粒尺寸时具有较高精度。增加Nb含量可同时细化晶粒尺寸D和D′,并改善晶粒尺寸分布、显著降低个别粗大晶粒出现的概率。在常用的淬火加热制度下,添加0.03%Nb和0.05%Nb的晶粒细化效果基本相当,兼顾其经济性应优选0.03%Nb。TEM观测结果表明,含Nb变形态试样中存在大量10~30 nm尺寸的Nb(C,N)粒子,其数量和密度随Nb含量增加而增加,但粒子尺寸并未随之明显增大。通过热力学计算并综合粒子尺寸和形成时间推断,这些Nb(C,N)粒子在1000℃形变诱导析出。

Effects of Nb content and hot deformation amount on austenite grain size and distribution of 800 MPa grade high strength tested steel for hydropower station during reheating for quenching were studied by using a Gleeble 3500 testing machine to carry out the combined simulation test of two-stage deformation at 1000℃+820℃and subsequent reheating at 900℃for different time,and the particle size and distribution of deformation-induced Nb(C,N)precipitates were observed by transmission electron microscopy.The results show that the grain size of as-deformed austenite(D)has an important heredity effect on the grain size of as-reheated austenite(D'),and there is a functional relationship between the two and the reheated holding time(t)at 900℃:D'=(1.0057D-6.9785)×(t/300)^(0.215),which can be used to predict the grain size(D')for the 800 MPa high-strength steel with high accuracy.The increase of Nb content reduces both the grain sizes D and D',and also improves grain size distributions and reduces the probability of individual coarse grains.Under the common industrial quenching heating system,adding 0.03%Nb and 0.05%Nb have very similar grain refining effect,then considering its economy,adding 0.03%Nb is preferred.The TEM observation results show that in the as-deformed specimens,a large number of Nb(C,N)particles with the size of 10-30 nm are observed,the number and density of particles increase with the increase of Nb content,but the particle size does not increase significantly.Based on thermodynamic calculation and comprehensive consideration of particle size and formation time,it is inferred that these particles are deformation-inducted Nb(C,N)precipitates formed at 1000℃.