This study deals with a relationship between strength and coiling temperature of high strength hot-rolled sheet steels consisting of ferrite and nanometer-sized carbides in order to evaluate the stability of the strength against the variation of the coiling temperature. Ti-Mo-bearing and Ti-bearing steels were prepared to form (Ti,Mo)C and TiC in ferrite matrix, respectively. Ti-Mo-bearing steel exhibited the high strength even under the high temperature coiling while the strength of Ti-bearing steel decreased significantly. Ti-bearing steel just after transforming at 923K had the same hardness as that at 898K. In addition, hardness of Ti-bearing steel coiled at 898K decreased significantly by holding at 923K for 8.64ks while Ti-Mo-bearing steel did not represent a large change in hardness. These results confirm that (Ti,Mo)C is not coarsened easily by Ostwald ripening at the high coiling temperature unlike TiC. Consequently the retardation of Ostwald ripening of (Ti,Mo)C is attributed to the small amount of titanium in solution in Ti-Mo-bearing steel.
Sequential transformation phenomena of α → γ → α during intercritical annealing and subsequent cooling were investigated to achieve a more advanced control of mechanical properties in a low-carbon coldrolled DP steel sheet with a chemical composition of 0.13mass%C-1.4mass%Si-2.0mass%Mn. The steel was intercritically annealed at 1 073 K for 0-1 000 s, then air-cooled to 873-1 073 K (quenching start temperature: Tq), followed by water-quenching. The tensile strength increased with an extension of the annealing time, especially at the low Tq, corresponding to the increase in the volume fraction of martensite. This means the γ → α transformation during air-cooling was delayed by extending the annealing time. Microstructural observation and elemental analysis by EPMA indicated that the volume fraction of γ during annealing was almost saturated after annealing for 250 s, whereas the Mn content in γ was still increasing at that time. These results suggest that the retardation of the γ → α transformation during air-cooling by extending the annealing time results from the chemical stabilization of γ by the enrichment of Mn during intercritical annealing. In order to obtain the less scattering of mechanical properties in cold-rolled DP steel sheets, precise microstructural control considering the partitioning of substitutional alloying elements during intercritical annealing is quite important.
The design concepts and properties of three unique high strength steel sheets developed utilising nanoengineering are reviewed. The first steel is developed by optimising the distribution of nanoprecipitates and exhibits low yielding ratio in spite of being strengthened by grain refinement and precipitation hardening. The second is the ferrite single phase tensile strength 780 MPa grade advanced high strength steel sheet utilising the thermally stable nanosized precipitates, which possesses significantly well balanced elongation and stretch flangeability. These two were already commercialised. The last is the ultrahigh strength steel, of which the formability is enhanced by optimising the combination of hard phases. The steel consists of bainite, retained austenite and tempered martensite and exhibits 35% of elongation with 1470 MPa of tensile strength. Although further optimisation of the composition and the processing are needed to produce and commercialise the steel, the results indicate that the approach has the potential to improve formability dramatically.
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