Quantitative analysis of fine nano-sized precipitates in low-carbon steels by small angle neutron scattering

The small angle neutron scattering (SANS) technique was used to determine the nano-sized precipitates in interstitial free (IF) and low-carbon (LC) steels with a hot-rolling temperature. The real-space direct model fitting method was applied to quantitatively analyze the SANS data. The magnetic and chemical properties of precipitates in the samples were also investigated by separation and comparison of nuclear and magnetic SANS scattering data. The size distribution of precipitates in the IF steel is in good agreement with the microstructure observation using transmission electron microscopy. The results revealed that the IF steel had two types of non-magnetic precipitates, Ti₄C₂S₂ and TiC, with the average size of about 30 nm in diameter and little difference in chemical composition. In case of commercial LC steel, the fine and large precipitates are identified as MnS and Fe₃C, respectively. The average size of spherical MnS precipitates was about 4.8 nm in radius and the distribution is isotropic. It is likely that the LC steels have almost the same composition and a similar size as precipitates such as MnS and Fe₃C with different finishing delivery temperatures. Interestingly, the average size and volume fraction of fine precipitates showed no significantly changes under the different finishing delivery temperatures.