.1500 Mpa Grade Directly Quenched Martensite Steel Control Of The Organization And Strengthen The Mechanism

The direct quenching process shorts the production step, reduces cost, save energy and so on. Tensile strength of steel plate produced by directed quenching technology is usually 490-980MPa at home and abroad but few studies were carried out on the 1500MPa level steel through direct quenching. With development of material toward ultra-high strength, it is necessary to study the production process and strengthening mechanism of direct quenching martensite steel aiming at development ultrahigh strength direct quenching martensite steel.In this paper, the effects of reheating technique, hot deformation, cooling process on the micro structure and mechanical properties of direct quenched steel were carried out quantitatively on medium carbon microalloyed steel through direct quenching and low temperature tempering process. The crystallography of direct quenched martensitic steel was analyzied, and the strengthening mechanism of the direct quenching martensitic steel was discussed. Grain refinement mechanism of austenite throughα→γreverse phase transformation was studied by means of direct quenching, tempering and reaustenizing. The austenite grain refinement was studied through nano-precipitates to retard the boundary movement during reaustenization process.It is found that reheating temperature should be lower than 1150℃and holding time shorter than lh to obtain finer austenite grain for experimental Nb microalloyed steel. The kinetic equations of austenite grain growth in soaking process for tested steels are constructed.The thermal deformation behaviors were studied to provide controlled rolling parameters and theoretic understanding. The dynamic recrystallization activation energy and the static recrystallization activation energy are about 477.7 kJ/mol and 299.3KJ/mol, respectively. The kinetic equations of dynamic recrystallization and static recrystallization for tested steels were built, and the dynamic recrystallization process state diagram was obtained. According to the dynamic recrystallization process state diagram, it was found that the large deformation and low strain rate is essential for the full dynamic recrystallization. The strain induced carbide precipitation curves (PTT curves) have also been determined assuming the typical "C" typed shape. The precipitation process is obviously accelerated with strain rate increases without changing the nose-tip temperature.The microstructure of direct quenched martensite steel has a multi-scale structure including prior austenite grain, packet, block and lath structure. Packet is the group of laths with the same habit plane in a prior austenite grain and each packet is further subdivided into blocks which have high angle boundaries (the group of laths with a similar orientation). The equiaxed grains of direct quenched martensite steels become into pancake grains with increasing the amount of deformation in the non-recrystallization region, the packet size is equal to the thickness of the pancaked austenite grain and the block size is reduced due to deformation structure. The average austenite grain size decreased from 12.4μm to 4.4μm and the average block width decreased from 2.5μm to 1.3μm, when the fraction of the amount of deformation in the non-recrystallization increased from 10.4% to 100%. The results showed that the K-S orientation relationship between austenite and martensite is observed in direct quenched steels by electron backscattering diffraction (EBSD).Comparing with conventional reheat quenching-tempering (RQT), the tensile strengths of RCR-DQT and RCR-CR-DQT steels increases by 9.4% and 14.6%, whereas the toughness is almost same. Analysis on the relationship between microstructure and mechanical properties revealed that dislocation strengthening is the major mechanism to account for the strength increment of DQT steels comparing with the RQT steels, because the dislocation density in DQT steels was higher than that of RQT steels due to the inheritance of deformed substructures of austenite through transformation. The strength increases with increasing the amount of deformation in the non-recrystallization region can be attributed to the finer block, which was found to be the effective grain size controlling the strength. The linear relationship between yield strength and the minus square-roots of prior austenite grain size and block size, suggesting that grain refinement is the main reason to strength enhancement and the block width is the effective size controlling strength. Anisotropy of the DQT steels were obvious than that of RQT due to the texture inheritance from deformed austenite.In the industrial production line, compared with the intermittent direct quenching (IDQ), direct quenching (DQ) has better combination of strength and toughness. Direct quenching steel plates have good plate shape by means of precise control of reheating temperature, rolling and cooling process parameters.Based on the direct quenching-tempering-reaustenitizing process, by means of lowering hot deformation temperature, increasing the amount of deformation, increasing the cooling rate after hot deformation and shorting the temper time, the nucleation rate of austenite reverse transformation was increases and in turn the austenite grain size was significantly refined. For further refinement of austenite, the idea of grain refinement by nanosized precipitates to suppress the austenite grain growth was introduced and the effects of Titanium on microstructure evolution and mechanical properties was summarized and analyzed. The Lab controlled rolling and following heat treatment result in a clear refined microstructure with austenite grain size about 5μm. It was turned out to be true that austenite grain refinement is feasible by controlled rolling and nanosized precipitation, both of which would assume good prospects for industrial applications.