The metallographic structure photo of the sample is given in Fig. 1. Under various process parameters, the experimental steel obtained a multiphase structure dominated by bainite and martensite, accompanied by a small amount of residual austenite. Figure 1a-f shows that at 850 ° C to 950 ° C, the microstructure mostly presents as fine lower bainite and martensite laths. When the quenching holding temperature reaches 1000 ° C, the lath size in Fig. 1g and H increases significantly, and the morphology of bainite is mainly upper bainite.
(a) 850 ° C, LH; (b) 850 ° C, 2h; (c) 900 ° C, 1 h; (d) 900 ° C, 2h; (E) 950 degrees C, LH; (f) 950 degrees C, 2h; (g) 1000 ° C, 1 h; (H) 1000 ° C, 2h
The microstructure of the experimental steel is more finely observed by EBSD and TEM. The results are shown in figures 2 and 3. The EBSD results further show that when the quenching holding temperature is increased from 850 ° C to 950 ° C, the size of the strip does not change significantly, while at 1000 ° C, the size of the strip increases significantly. The TEM photos in Fig. 3 show that the substructure of martensitic laths in the experimental steel is mainly high-density dislocations. At the same time, the distribution state of residual austenite in martensite lath can be obtained. The residual austenite in black film shape is distributed along the boundary of martensite lath. This is because in the process of martensite transformation, the transformed martensite will cause the cooperative deformation of adjacent untransformed austenite, and the volume expansion occurs during the formation of martensite, The untransformed austenite is under pressure, and the continuous transformation needs to overcome these compressive stresses, which hinders the further transformation, so a part of the remaining austenite will remain.
The research shows that in the lath martensite structure, an original austenite grain is divided into several packets, and the pulling bundle is further divided into blocks, and the lath block is composed of several laths. The lath bundle is composed of martensitic laths distributed roughly parallel in the original austenite grain. With the increase of austenite grain size, the size of lath bundle increases.
In this study, SEM photos were used to reconstruct the typical original austenite grains according to the above structural changes of martensite in the experimental steel. The results are shown in Fig. 6-6. The results show that the typical original austenite grain size is 121 when the quenching holding temperature is 850 ° C, 900 ° C, 950 ° C and 1000 ° C respectively μ m、１３５ μ m、１６７ μ m、２９５ μ m. That is, the original austenite grain size increases with the increase of Ping fire holding temperature, especially at 1000 ° C.
Comparing the martensite structures with different original austenite grain sizes, it can be seen that when the grain size is small, most martensite blocks are small, and some lath blocks are large. When the original austenite grain size is large, the strip width increases, but the whole is relatively uniform (see the TEM structure in Figure 4). Martensite nucleates preferentially at the grain boundary of the original austenite, and the lath block is a lath region according to the same orientation. When martensite nucleates and grows into laths at the austenite grain boundary, the growth of lath blocks is completed by the nucleation and growth of other parallel laths between the first formed laths. During the growth process, martensite variants coordinate with each other to reduce the strain energy. The laths formed in the early stage of transformation are relatively thick. When a lath block is formed, in order to reduce the strain energy, Slats containing different variants will grow immediately adjacent to the previously formed slats. Although the growth rate of lath is very fast, the size of lath bundle also increases with the increase of grain size, so there is enough time and space for the full growth of other structures without phase transformation. For fine austenite grains, the size of lath bundle is also small, and the remaining areas without phase transformation are also reduced. Therefore, one lath block may occupy most of the area in one lath bundle, and other lath blocks cannot grow completely, so as to maintain a relatively small state.
The above experimental results show that when the experimental steel is quenched, the original austenite grain size increases with the increase of austenitizing temperature, and the martensite lath bundle size and lath block width will also be greatly affected. However, the sensitivity of lath width to the change of original austenite grain size is not strong, and the lath size will increase significantly only when the original austenite grain changes significantly. It can also be said that the quenching and holding temperature directly affects the austenite grain size, and further affects the martensite lath bundle and lath block, which can become the substructure of effective grains, thus affecting the strength and toughness.