Mechanism of generation of large(Ti,Nb,V)(C,N)-type precipitates in H13 + Nb tool steel

The characteristics and generation mechanism of（Ti,Nb,V）（C,N） precipitates larger than 2 μm in Nb-containing H13 bar steel were studied. The results show that two types of（Ti,Nb,V）（C,N） phases exist—a Ti-V-rich one and an Nb-rich one—in the form of single or complex precipitates. The sizes of the single Ti-V-rich（Ti,Nb,V）（C,N） precipitates are mostly within 5 to 10 μm, whereas the sizes of the single Nb-rich precipitates are mostly 2–5 μm. The complex precipitates are larger and contain an inner Ti-V-rich layer and an outer Nb-rich layer. The compositional distribution of（Ti,Nb,V）（C,N） is concentrated. The average composition of the single Ti-V-rich phase is（Ti_（0.511）V_（0.356）Nb_（0.133））（CxNy）, whereas that for the single Nb-rich phase is（Ti_（0.061）V_（0.263）Nb_（0.676））（C_xN_y）. The calculation results based on the Scheil–Gulliver model in the Thermo-Calc software combining with the thermal stability experiments show that the large phases precipitate during the solidification process. With the development of solidification, the Ti-V-rich phase precipitates first and becomes homogeneous during the subsequent temperature reduction and heat treatment processes. The Nb-rich phase appears later.

Static Softening Behaviour of Hot-worked Austenite in Microalloyed Structural Steel StE460

Double-hit compression tests were performed on StE460 steel containing microalloying elements niobium and vanadium over a range of temperatures and strain rates penment to hot rolling. The fractional softening was evaluated by use of the offset method, which was confirmed to be a very reasonable method. Appropriate expressions are given for the static recrystallization kinetics as a function of temperature and strain rate. Particular attention is paid to the effect of strain rate on static recrystallization. It can be shown that the static softening is apparently accelerated by strain rate. Recrystallization in low temperature austenitic region is stopped due to precipitation of carbonitrides of microalloying elements, which is reflected in the form of a plateau in the curves of static softening.

Effect of Precipitation on Intergranular Corrosion Resistance of 430 Ferritic Stainless Steel

With Nb-Ti-stabilized 430 ferritic stainless steel（NTS430FSS） and SUS 430 ferritic stainless steel（SUS430FSS） as experimental materials, the influence of precipitation on intergranular corrosion resistance was investigated. A series of aging treatment were carried out. The free-exposure corrosion test and double loop electrochemical potentiokinetic reactivation（DL-EPR） test with a scan rate of 1.67 m V/s at 26 °C were applied to evaluate the intergranular corrosion（IGC） resistance. Metallographic observation, scanning electron microscope（SEM）, transmission electron microscope（TEM） with energy dispersive spectroscopy（EDS） and X-ray diffraction（XRD） analysis were conducted. The results show that IGC occurred in SUS430 FSS aged above 700 °C, while it occurred in NTS430 FSS as the temperature was improved to 1 050 °C. The critical degree of sensitization Ir/Ia reaches 0.305 in SUS430 FSS, which is higher than that of NTS430 FSS, i.e. 0.010, aged at 950 °C for 2 h. The TEM, EDS and XRD results show that a large amount of Cr23C6 precipitates with size of 60 nm×22 nm are located at the SUS430 FSS grain boundaries as chains. With the addition of Nb and Ti and reduction of C, the amount of precipitates reduces significantly in NTS430 FSS. A majority of Cr23C6 were replaced by Ti C and Nb C. Only a small amount of spherical Ti C（R=186 nm） and square Ti N（312 nm×192 nm） with Nb and Cr adsorbed are left along grain boundaries. Due to the dual stabilization of Nb and Ti, the precipitation of Cr23C6 is restrained, the chromium depleted region is avoided and accordingly the resistance to the intergranular corrosion is improved.