More than 50 years ago, J.F. Wallace and his research team studied the role of various materials as grain refiners in cast steel .The steels used are low carbon steel (US No. 1018) and Ni-Cr-Mo low alloy high strength steel (US No. 4330) with 0.30%~0.35% C content.
It is found that Ti has obvious grain refinement effect, but there are many problems in the performance of steel.
1. After adding 0.6% Ti into the molten steel
With the addition of 0.6% Ti (about 0.46% residual Ti in steel), the grain size of as-cast structure of 4330 steel is significantly refined and the secondary dendrite spacing is greatly reduced (about 40%).
At that time, they thought that this was because TiC could be used as a heterogeneous nucleus precipitated from the crystallization of delta-ferrite and austenite in liquid steel.
After the conventional quenching and tempering treatment of steel with Ti 0.6%, the grains are refined, but there are many problems in mechanical properties, such as: the tensile strength of steel is not improved, but slightly decreased; the toughness decreases greatly; the section shrinkage decreases from 30.3% to 3.6%.At that time, they believed that the substantial decrease in toughness was due to the formation of large amounts of titanium sulfide films between dendrites, which not only obscured the strengthening effect of grain refinement, but also led to brittleness of steel.
In order to solve the brittle problem of titanium sulfide film, they adopted a special heat treatment process.Before quenching and tempering of low alloy steels, homogenize the steels at high temperature (holding at 1350 C for 24 h).The toughness of the homogenized steel can be basically restored and its strength is slightly higher than that of the steel without Ti.However, this process is not feasible in actual production.
2. Brittleness of Titanium Sulfide Film
To counter the brittleness of titanium sulfide films, they have tested low-sulfur steel (S content 0.007%) with 0.1% Ti.Although the section shrinkage of steel is significantly increased, the tensile strength and yield strength are not improved.
Recently, Tuttle also studied grain refinement of Ti on medium carbon steel .Adding Ti content is 0.1% and 0.3% and adding Ti with iron titanium.The effect of Ti on the structure and properties of steel is roughly as follows:
(1) Steel without Ti is acicular in ferrite and granular in ferrite after Ti addition.It is difficult to judge the difference of grain size due to the difference of grain shape.
(2) With the increase of Ti content, the yield strength and tensile strength of steel are slightly increased, but the elongation data are scattered.
Tuttle observed with scanning electron microscope, transmission electron microscope and analyzed with energy dispersive spectrometer to identify carbon and nitrogen compounds containing TiC, TiN and other Ti in steel.However, the results obtained by Schiel solidification analysis method show that precipitation of Ti carbon and nitride occurs only after about 60% of the liquid steel solidification.It can be seen that although Ti carbon, nitride and delta-ferrite lattices do not fit well, they can not be used as heterogeneous nuclei in steel, but only precipitate out of grain boundaries, pin down and inhibit grain growth.