High production cost affects the product competitiveness of steel factory, for the production of high-strength steel bars, there is a workable solution to reduce alloy cost to employ Ti replacing N-V for microalloying.
The strengthening effect of niobium on steel is mainly fine grain strengthening and diffusion strengthening. Niobium added to steel as a microalloying element does not change the structure of steel, but combines with carbon and nitrogen in the steel to form stable carbides and carbon-nitrides, changing the microstructure of the steel, while achieving a diffuse distribution of carbides and carbon-nitrides by inducing precipitation and controlling the cooling rate, thus producing steel with fine crystallization.
There are three downsides to add Nb into steel:
1) The high drawing speed of the steel in the high temperature brittle zone during solidification is prone to transverse cracking on the surface of the continuous casting billet;
2) The instability of the generated carbonitrides leads to no yield platform for the steel;
3) The high cost of the Nb alloy used to ensure that the steel reaches a certain yield strength. The use of N-V alloy can effectively solve the above problems, but the cost of the alloy is relatively high.
Micro-alloying with Ti and other rare earth elements instead of Nb and N-V in the continuous casting billet, there are a large number of diffuse fine precipitation phases inside the titanium-containing steel, and the precipitation strengthening is achieved by the diffuse fine TiN obstructing the dislocation movement, and the fine grain strengthening is achieved by the TiN obstructing the austenite grain growth in the heating and holding process and inhibiting the austenite recrystallization in the rolling process.
This solution can solve the problem of surface cracking of continuous casting billets in the high temperature brittle zone under high drawing speed conditions of low-alloy steel, improve the strength and toughness of steel bars, and achieve cost optimization on the basis of ensuring stable performance of seismic steel bars. Processing Ti and other rare earth elements into cored wire and adding them to the steel through a wire feeder to micro-alloy relative to Nb or N-V can reduce the cost of the alloy.
