| Recently,the high speed railways with design speed of 400 Km/h have been developed rapidly, the higher properties of steel were proposed by designer. Therefore,the high strength spring steel has been a research subject of interest. The results show that the niobium microalloyed combined with controlled rolling technology can effectively improve the strength and toughness of steel. However, the current research of the niobium microalloyed technology is mainly focused on the field of low carbon steels, and the public reports on the study of high carbon steel containing Nb are still few, which lead to the lack of necessary theoretical basis for industrial production.Based on the existing 60Si2 CrVAT spring steel, the effect of the niobium element on the transformation characteristics, decarburization sensitivity, microstructure and properties, recrystallization, precipitation kinetics and grain coarsening temperature of the 60Si2 CrVAT spring steel were investigated with using a single double pass compression test, stress relaxation test, transmission electron microscopy, material performance test method in this paper. In addition, the niobium segregation and carbide dissolution behavior in as-casting microstructure of the containing-niobium high strength spring steel were also studied with using Thermo-Calc calculation and experiments. According to the results, a scheme for the industrial production of niobium microalloyed high strength spring steel was proposed, and the quality of the products were analyzed.The results showed that the phase transformation characteristics of 60Si2 CrVAT spring steel was changed to a certain extent by the addition of trace Nb elements. Compared with the spring steel without niobium element, the CCT curves of the containing spring steel move to the bottom. The ferrite, pearlite transformation temperature is delayed by about 15℃ under the same cooling rate. The Nb element has little effect on martensite transformation. The Nb elements reduce significantly the pearlite lamellar spacing. The pearlite interlamellar spacing is refined from 209 nm to 198 nm when the cooling rate is 1℃/s, and the pearlite interlamellar spacing is refined from 194 nm to 179 nm when the cooling rate is 2℃/s. The Nb elements can change the morphology of pearlite, and the cementite interface is bent, intermittently.The Nb element has a significant influence on the microstructure and properties of 60Si2 CrVAT steel. And the Nb steel has refined microstructure of the hot rolling state and quenching and tempering. Finally, the Nb steel shows a high strength and plasticity. The tensile strength,the yield strength,the percentage elongation,the percentage reduction of area and impacting absorbed energy of the containing-niobium steel reaches 2024 MPa,1815MPa,12%,38% and 33 J,respectively. Besides the containing-niobium spring steel exhibits superior fatigue properties,and its fatigue endurance(σ-1) is 775 MPa,reaching closely to the fatigue properties of high strength spring steel by documented publicly.The results also show that the containing-niobium spring steel exhibits superior decarburization resistance, and its decarburization sensitive temperatures are between 950℃ to 1000℃. The decarburization depth of the containing-niobium steel is less than that of the steel without Nb element under the same reheating conditions. Besides, the decarburization depth of the containing-niobium steels increase with increasing the reheating temperatures and times.The compression experiments indicate that the addition Nb can effectively retard dynamic and static recrystallization of experimental steel because of the pinning effect of strain induced precipitates. Moreover,the activity energy of the containing-niobium steel and the free-Nb spring steel were determined as 354.27KJ/mol、319.56KJ/mol with using peak value stress methods,respectively. The stress relaxation experiments show that the precipitation-time-temperature(PTT) diagrams of the carbides in containing-niobium steel was a type of C-curve,with nose temperature of 900℃. In contrast,the precipitation kinetics of carbides in the containing-niobium steel is slower than that of the plain carbon steels,which is attributed to reduced activity of Nb. In addition,The result indicates that the precipitate is a Nb,V,Ti and C containing complex carbides at deformation temperatures ranging from 850℃ to 1050℃. Large parts of precipitates are(Nb,V)C,as well as tiny amount Ti C. Besides, the carbides are dispersed distributed below 900℃,and cluster distribution near dislocations and grain boundaries above 950℃.Furthermore, The reheating experiments indicate that the grain-coarsening temperature of the Nb-bearing spring steel was 1050℃,and this temperature is higher 50℃ than the free-Nb steel. Thermodynamic calculation indicates that both carbon and niobium content in spring steel has very important effect on the Nb segregation. Besides,the results also indicate that the precipitates distribute randomly in as-casting structures as thick strip and irregular sharps. These precipitates are usually in the width of about 1-3μm and the length of about 2-40μm. There are two accumulation areas in steel matrix for precipitates. One is located at grain boundary,where the precipitates are the mixture of niobium,vanadium and titanium carbonitrides. The other distributes randomly in steel,where the precipitates may be Fe2 Nb eutectic in terms of Thermo-Calc thermodynamic calculation and EDS analysis. The Nb-bearing carbides dissolve gradually into austenite with increasing temperatures,but they can not to be dissolved even at 1250℃.Based on above theory study,a kind of Nb-bearing high strength spring steel industry process is proposed in this paper. In order to evaluate the properties of industry production,the mechanical properties of steel were tested. The results indicate that the tensile strength,the yield strength,the percentage elongation,the percentage reduction of area of the industry steel reaches 1996.9MPa,1791.7MPa,9.9%,34.9%,respectively. These properties of the industry steel are lower lightly than that of the laboratory steel, which attributes to the core elements segregation of product in terms of fracture analysis,micro-hardness analysis and EPMA analysis. Though, the property of industry product is lower than that of laboratory product, this approach contributes to the achievement of an ultrafine grain, homogeneous higher carbon microstructures that exhibit superior toughness, high strength, less mechanical property variation in the final hot-rolled product, and reduced cost of quality. |
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