Characterization of precipitation kinetics of Nb-HSLA steels in the Thin Slab Direct Rolling

Abstract

The precipitation kinetics of nitrides and carbides during the TSDR (Thin Slab cast and Direct rolling) processing of Nb-HSLA (High Strength and Low Alloys) steel containing Ti were studied in this thesis. The Ti content ranged from 0.0012 mass% to 0.01 mass% in four selected Nb-HSLA steels. A low nitrogen Nb-HSLA steel containing 0.0007mass% of nitrogen was used for one of the steels. A high level of nitrogen ranging from 0.008 mass% to 0.01 mass% was used for the other steels to simulate the composition of steel produced by EAF(Electric Arc Furnace) used for TSDR. The carbon replica technique combined with TEM (Transmission Electron Microscopy) including EDS (Energy Dispersive X-ray Spectrometry) and EELS (Electron Energy Loss Spectrometer) were used to analyze the precipitates. Ti-rich (TixNb1-x)N/Nb(CyN1-y) type particles were often observed consisting of a cuboidal core and cap-shaped deposits. The composition of the cuboidal core was TixNb1-xN (0.7≥x>0.2). The most likely composition of the cap deposit was NbCyN1-y (1.0≥x>0.4). TiN precipitated even in when only a trace amount of Ti was present, Ti-rich precipitates were always nucleated prior to niobium carbonitride formation, and acted as a nucleation site for Nb-rich precipitates. In the interface area between the cuboidal core and the cap deposit, the Ti/(Nb+Ti) atomic ratio evolved to a low value of Ti/(Nb+Ti)≈0 in the cap area. A high value of Ti/(Nb+Ti)≈0.8 was obtained for the cuboidal cores. The distribution of the precipitation particles appeared to be random in the matrix and at dislocations in all the steels. The precipitation kinetics were simulated with MatCalc. The interfacial energy of γ/TiN and γ/NbC interfaces was shown to be of major importance when comparing the simulations comparing with the experimetal result. The hot deformation during the rolling process was also simulated experimentally. The precipitation size distribution was affected by strain. Fine particles were observed after precipitation in the deformed state. The high temperature mechanical properties such as TnRex, mean flow stress (MFS), critical strain and stress, as well as peak strain and stress were measured by hot torsion tests.