Research On High Strength Q&P And Q-P-T Steels

Currently in China, the annual output of steels is more than 400 million tons, but the general strengths of structural steel in production and application are between 400MPa ~ 800MPa. In order to save energy and raw materials as well as on environment protection, there is an urgent need to develop high or ultra-high strength steel. For gainning higher strength (1500MPa-2000MPa) accompanying adequate plasticity, toughness and low-cost structural steel, the principle of microstructure design of high-strength steel and a new Quenching-Partitioning-Tempering(Q-P-T ) process was proposed by Xu Zuyao(T. Y. Hsu)according to the principle of microstructure design of high-strength steel based on the recently developed Quenching and Partitioning(Q&P) treatment by Speer et al. Two strength level steels: 1500MPa level and 2000 MPa level have been designed, respectively. The microstructure of the Q-P-T steels were characterized by means of optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy(TEM) and the origin of good combination of high strength and adequate ductility of Q-P-T steels has been revealed. Furthermore, the stability of retained austenite during the Quenching and Partitioning process has been studied in order to control the microstructure of the Q&P and Q-P-T steel.The Q&P steel with composition of 0.2C-1.53Si-1.46Mn has been designed and developed, and the heat treatment process parameters have been proposed as following: the initial quenching temperature is calculated as 250℃and the carbon partitioning temperature is calculated as 480℃in Constrain Paraequelibrium (CCE) conditions by using the J-Mat Pro software. Tensile strengths of the 0.2C-1.53Si-1.46Mn steel developed are about 1000MPa, elongation of 10 ~ 20% after the different Q&P treatments. The tensile strength of one-step (the initial quenching temperature and the carbon partitioning temperature are the same) treated Q&P steel is higher than that treated by two-step process(the initial quenching temperature is lower than carbon partitioning temperature), but the elongation of one-step Q&P steel is only about half of two-step's. The microstructure of Q&P steel has been characterized by OM, SEM, TEM and XRD, and the results show that the Q&P steel consists of martensite and retained austenite, and its tensile strength depends primarily on the volume fraction and the carbon content of martensite, the retained austenite plays an important role in plasticity and toughness. The experimental result indicates that the volume fraction of cementite and transition carbides (unstable) precipitated in martensite matrix increase, while the volume fraction of the retained austenite decreases with increasing partitioning time. The main reasons are: 1) the migration of martensite / austenite interface during the Q&P process; 2) the decomposition of austenite into cementite at relatively high temperature and longer period during the Q&P process. By comparing with other advanced structural steels (such as dual-phase steel, TRIP steel, martensitic steel, etc.), Q&P steel has a better combination of high strength and good plasticity.In order to further raise the strength of steels, based upon modifying the recently developed Q&P treatment, a Quenching-Partitioning-Tempering process is proposed for ultra-high strength steels containing certain amount of carbide forming elements such as Nb and Mo. A designed and developed steel with composition Fe-0.2C-1.5Si-1.5Mn-0.053Nb-0.13Mo shows tensile strength about 1500MPa and total elongation 15% after subjected to Q-P-T treatment(the initial quenching temperature is designed to be 220℃and the partitioning/tempering temperature is designed to be 400℃) . The microstructure characterization shows the parent austenite grains are much finer than that of Q&P steel with similar composition. The addition of Nb results in the existence of finer martensitic packets and thinner laths in the Q-P-T steel after quenching and fine complex carbide dispersively distributed in martensite matrix with high density dislocation and the existence of a certain amount (about 4～6% volume fraction as XRD measured) of retained austenite between martensite lath due to the addition of Si. The above microstructural features are expected by Hsu'ideal of microstructure design for good combined mechanical properties.In order to attain ultra-high strength steel with the tensile strength level of 2000MPa accompanying the elongation of 10% for the steel containing less than 0.5wt%C , and a medium-carbon Q-P-T steel is designed and developed according to the principle of design of composition and structure of high-strength steel. The corresponding process is designed as: the initial quenching temperature is 95℃and the partitioning/tempering temperature is 400℃. The medium carbon Q-P-T steel exhibits tensile strength as high as over 2000MPa and total elongation over 10%. The microstructure in the Q-P-T steel is determined as follows. The average width of martensite laths are several tens nanometers, and the width of film-like retained austenite around martensite as well as the average size of stable carbides dispercively distributed in martensite matrix are both several nanometers, such an ultra-high strength fully nano-scaled martensitic steel had never been reported before. The present work shows that the tensile strength of medium carbon Q-P-T steel depends on the refinement of packets and laths of martensite and the stable carbide precipitates in the martensite matrix, and the plasticity depends primarily on content of the retained austenite and the softening of martensite matrix (the carbon concentration decreased in matrix and the effect of solid solution strengthening of carbon decreased after Q-P-T process). The investigation of this work verifies the feasibility of Q-P-T process (including both low-carbon and medium-carbon Q-P-T steels) in stead of Q&P process and demonstrates its more extensive application for Si-alloying structural steels than Q&P process. As a results, Q-P-T steels studied in the work is a new family of advanced structural steels whose comprehensive mechanical properties are overwhelming to dual-phase (DP), transformation induced plasticity (TRIP), twinning induced plasticity(TWIP) and Q&P steels.Migration of interface between martensite and austenite directly affects volume fraction of retained-austenite during Q&P process, in turn does the microstructure and mechanical properties of steels. The migration of martensite/austenite interface is observed in this work and the thermodynamic condition allowing iron atom diffusion across the interface is theoretically verified, which argues the constrained condition of interface between martensite and austenite proposed in the"Constrained Carbon Paraequilibrium"(CCE) model by Speer et al. The migration velocity of martensite/austenite interface is thermodynamically calculated as 3.08nm/s, and the migration distance of interface is high up to tens of nanometers during the Q&P process as the numerical calculation.