针状铁素体管线钢的低温韧性与“有效晶粒尺寸”的新定义

Low temperature toughness of acicular ferrite pipeline steel and redefinition of“effective grain size”

研究了试验钢的系列温度夏比冲击和落锤撕裂(DWTT)韧性性能,统计了沿解理裂纹扩展方向上的各种微观结构晶界的位向差角,计算了5个不同轧制方向上(0°、30°、45°、60°和90°)解理裂纹扩展路径上{100}解理面的夹角,重新定义了高等级针状铁素体试验钢的有效晶粒尺寸(晶界位相差大于15°),并定量分析了重新定义的有效晶粒尺寸和低温韧性性能(夏比冲击和DWTT)的关系。结果表明,不同方向上试验钢的微观组织相同,都是典型的针状铁素体(AF)+多边形铁素体(QF)+M/A岛。不同方向上大角度晶界都主要分布在45°~65°区间范围内。随着重新定义的有效晶粒尺寸(晶界间{100}解理面小于35°的晶粒组成的解理单元)的细化,5个方向的冲击吸收能量(ASTM A370-2017)和DWTT撕裂剪切面积均增大,而韧脆转变温度也随之降低。重新定义的有效晶粒晶界可以强烈阻碍解理断裂扩展,是断裂裂纹解理的有效组织单元。

Toughness of the tested steel was investigated by series temperature charpy impact test and drop-weight tear test(DWTT),themisorientation angles of the grain boundaries of various microstructures along the cleavage crack propagation direction were statistized,and the angles between{100}cleavage planes on the cleavage crack propagation path were calculated in five rolling directions(0°,30°,45°,60°and 90°).The effective grain size defined as grain with misorientation angle greater than 15°for high grade acicular ferritic pipeline steel was redefined as the size of cleavage units consisting of grains with{100}cleavage plane less than 35°between grain boundaries,and the quantitative influence of the redefined“effective grain size”on the low temperature toughness properties from Charpy impact test and DWTT were synthetically analyzed.The results show that the microstructure presents a typical acicular ferrite(AF)characteristic with somepolygonal ferrite(QF)and M-A islands,the distribution of high angel grain boundaries mainly distributes in the range of 45°-65°in different directions.The impact absorbed energy(ASTM A370-2017)and percent shear area of DWTT in the five directions increase with the refinement of the above redefined“effective grain size”,the ductile-brittle transition temperature also decreases with the refining of redefined“effective grain size”.The redefined effective grain boundaries can strongly hinder the fracture propagation,and thus the redefined“effective grain”can act as the effective microstructure unit for cleavage.