Research On The Strength And Toughness Of Low Carbon Martensitic Steel Grain Control Unit

Low carbon martensite steel has received extensive attention due to its good combination of high strength and toughness. Grain refinement can not only improve the strength of steel, but also effectively decrease the ductile to brittle transition temperature, thus favoring the improvement of toughness of steel. However, there are some complex substructures with multi-scales in low carbon martenisite, and the controlling unit for strength and toughness of low carbon marstensite steel is still controversial. Two kinds of low carbon martenisite steels with the compositons of0.2C-2Mn(wt%) and0.02C-5Mn(wt%) were used in this paper. Nano-indentation instrument and electron backscatter diffraction (EBSD) were employed to study the strenghening effect of the substructure and the interfaces in0.2C-2Mn(wt%) martensite steel. The effects of high angle boundaries on the propagation of impact fracture cleavage cracks or the propagation of secondary cleavage cracks near the impact fracture in0.02C-5Mn (wt%) lath martensitic steel were studied by EBSD. The effect of substructure in low carbon martenisite steel on the strength and toughness was studied, with the aim of determining the controlling unit for strength and tougness.Low carbon lath martensite includs three grain structures:prior austenite grain, packet, and block, which were surrounded by high angle boundaries with misorientation angle larger than10°substructures, and the block can be also divdied into several sub-blocks with the misorientation angle of5-10°and laths with angle less than2°, the size of these substructure was successively in decreasing order. The strengthening effect of the substructure boundaries in the0.2C-2Mn(wt%) lath martensite steel was studied using nanoindentation.The results showed that in a packet, the blocks with the same orientation have the similar hardness value, but there are some difference in hardness for the blocks with different orientations, and block boundaries have strengthening effect. HV/Hn, HB/Hn and HB/HV value increase with the decrease of prior austenite grain size. It indicated that prior austenite grain boundary, packet boundary and block boundary can make contributions to the strength. In crystallography, the twin boundary has a small strengthening effect, and packet boundary and block boundary has a similar strengthening effect.The block in low carbon martensite steel is considered as the controlling unit for strengthening because it is the minimum grain unit.The effects of high angle boundaries on the propagation of impact fracture cleavage cracks or the propagation of secondary cleavage cracks near the impact fracture in0.02C-5Mn (wt%) lath martensitic steel were studied by EBSD. The results indicated that the cleavage cracks or secondary cleavage cracks were deflected largely from previous propagation direction when crossing the prior austenite grain boundaries packet boundaries and most of the block boundaries. Like the prior austenite grain and packet boundaries, block boundaries could hinder the propagation of cleavage cracks. In crystallography, the ability to hinder cleavage crack propagation of packet and block boundaries is similar. The block is the grain unit controlling the cleavage fracture and the ductile to brittle transition temperature of lath martensite. Block is toughness control unit in lath martensite. The main direction of crack propagation in a large block may be not changed after deflected largely twice when crossing another small block. If it was changed, the latter may be out of the vertical to the former. Therefore, the cleavage plane of low carbon lath martensitic steel is not unique and not necessarily{100} plane.