Abstract��The microstructures and precipitates of high strength low alloy ��HSLA�� automotive steels with five different kinds of strength level were investigated by the optical microscope�� scanning electron microscope and transmission electron microscope. The results showed that microstructures of the samples were all composed of ferrites and perlites�� perlite was distributed on the ferrite grain boundary. The perlite was mainly composed of lots of aberration perlites and small amount lamellar perlites. With the increase of the strength level�� the grain size decreased�� the level of grain degree were 10.5�� 11�� 11.5�� 12 and 12.5 levels�� respectively. When the strength level was 260 MPa level�� the number of precipitates Nb��C�� N�� with the size of 10-70 nm was more than that in the steel with the strength level of 340 MPa. Thus�� the stronger precipitation strengthening effect resulted in the higher yield strength for the former one�� while the weaker precipitation strengthening resulted in the lower yield strength for the later one.
2018, Vol. 36 
