Study On Precipitation' Law Of Copper During Aging Process In Cu-containing High Purity Steels

High strenghth low alloy steel containing Cu has many excellent performances, i.e high strength, high toughness, well weldablity, corrode resistance and so on, so it is widely used in the automobile industry, the structural purpose steel products, the heavy engineering structure, the high pressure conduit, the bridge, the high-pressure vessel, the containers, the ships, the chemical industry oil equipment ,et. Study on aging strengthening behavior and the precipitation'law of Cu in steel is significant.Microhardness of the solution state and the different state during aging had been tested and analyzed by microhardness tester. By means of the optical microscope, PHILIPS PW1700 X-ray diffractionmeter and JEM-2010 transmission electron microscope, the solution state and the microstructure under the different state during aging and the copper clusters were observed. The influence of different Cu-containing quantity on the structure and hardness was studied. The precipitation particle's type, appearance, microstructure, size, distribution, volume fraction, nucleation mechanism and its orientation relations with the parent phase were studied during aging treatment. The precipitation sequence during aging process of high purity low-carbon steel bearing copper was researched. The strengthened mechanism of Cu in the steel was analysed.The results showed that at the same aging temperature, with Cu-containing quantity increasing, aging process was accelerated, the holding time arriving at aging peak was shorter and the aging peak hardness was lower during aging in different Cu-containing quantity. After solution treatment, when the rate of cooling achieves definite value, we can obtain the tinier massive ferrite crystal grain. Different copper content's specimen, under same aging temperature, microstructures not have change obviously. Copper is not distributed equally in matrix after solution treatment, but exists as a kind of ordering domain, Cu atoms show cluster state, which microstructure effects on the following aging behavior greatly. Cu-rich clusters in Cu–containing high purity steel during aging were precipitated firstly in ferrite crystal grain, Cu atoms were clustered on (001)αplanes. It was also shown that they were Cu-rich G.P zone particles at aging peak, whose appearance was layered and round cake-shaped. Cu-rich layer and Cu-poor layer distributed alternately on Cu-rich G.P zone particles. The G.P zone kept half-coherency with ferrite matrix. A high density of dislocations and faults was involved in or about the Cu-rich layers and Cu-poor layers of the G.P zone. At the early of over-aging, with Cu-rich clusters particles growing up gradually, the number of Cu-rich layers and Cu-poor layers increased in these clusters, but the thickness of Cu-rich layer decreased, the density of dislocations was reduced. There has notε-Cu particles precipitated at aging peak. The new structure has not found by X-ray diffraction analysis and high-resolution electron microscope observation at 550℃aging 50h. Only after long-time aging treatment at 550℃, or enhancing aging temperature,ε-Cu particles can be precipitated. The factors affecting precipitating process are the temperature, copper content, vacancy, dislocation and so on. The higher aging temperature can accelerate the diffusion and precipitating of copper in steel, namely Cu-rich particles are precipitated at 650℃aging than at 550℃aging.The important reason for aging strengthening of high purity steel containing copper is the distortion, the half-coherency interface, the high density dislocations and faults by the copper-containing G.P area causing, and its initiating stress field.