The low alloy steel is added elements with a small amount of Mn and Si, which are increasingly being developed with the propose, improving the mechanical properties and technology of this steel. However, to improve the quality of steel not only concern the composition but also constantly improve the technical processing of steel. After melting and solidification, thermal processing was carried out to create a multiphase structure with highly plasticity ferrite, the martensite structure, bainite, and a certain amount of austenite thereby improving ductility compared to AHSS steel. The TRIP (Transformation Induced Plasticity) aided bainite ferrite (TBF) steels is a group of steel that has been developed recently in terms of phase transformation due to plastic deformation and transformed into bainite, martensite or residual austenite. Besides, when combined with the cold deformation, it transforms into a hard martensite phase of the austenitic phase when plastic deformation occurs.^1-5

To receive the multi-phase structure, special mechanical-heat treatment steel includes (Fig. 1).

Fig. 1 
The heat treatment process of TBF steel.⁵ (Adapted from Ref. 5 on the basis of OA) A₃ is completely austenite temperature. A₁ the temperature that starts austenite transformation and B_s is bainite transformation temperature. M_s is the martensite transformation temperature

(1) The steel is heated to austenitizing temperature then keep the heat at a certain time to ensure the process is uniform. The austenitizing temperature is larger than the completely austenite temperature (A₃). The temperature starts austenite transformation which calls A₁.

(2) Then, cooling steel to the Bainite transformation zone (Between Bainite (B_s) and the Martensite (M_s) Transformation Temperature), keeping the appropriate heat to obtain a three-phase structure, such as Ferit, Bainite, and Austenite. Finally, cool down in the water to avoid degradation of the austenite.

When retaining heat in the Bainite transition zone, the conversion of austenite to bainite occurs simultaneously, while the carbonation of the austenite has not yet changed (Second Carbon Enrichment) without the occurrence of austenite decomposition to bainite due to the significant presence of Si-1.5% element that inhibits the secretion of carbide.^3,6-8 Element C-0.2% and Mn-1.5% in TBF steel are sufficiently high to lower the starting temperature of the excess austenite martensite, increasing the mechanical stability of this phase. To achieve the multi-phase microstructure for this steel, it is necessary to determine the Bainite phase transition temperature. According to the Bainite phase transition, the temperature is determined according to the continuous cooling transformation (CCT) diagram, and the conversion temperature is about 400^oC (Fig. 2).⁶

Fig. 2 
CCT diagram of developed steel. F: ferrite, B: bainite, and M: martensite

With the TBF process, effects on the deformation process on the microstructure and mechanical properties of steel have not been studied. In this article, we present the results about the effect on the deformation with 20, 40, and 80% and heat treatment to the microstructure and mechanical properties of this steel.

Cast post-cast billets are cut into blocks of size 80 × 70 mm, annealed for uniform composition caused by castings, tempering temperature is 1000^oC, keeping heat 3 hours.

After uniform annealing, steel billets are hot forged. The steel blocks were forged into a 20 × thick 70 mm wide plate. The temperature begins to forge about 1200^oC and finish forging at a temperature of about 900^oC.

Then hot rolling with relative pressures of 50% to 10 mm thickness to produce the initial castings. The microstructure of the steel has a small grain size.

Steel billets are then cut into slabs of different thicknesses and cold rolled to a thickness of 2 mm to ensure a correspondingly cold rolling rate of 20, 40, and 80%. The post-cold steel samples were subjected to two-stage heat treatment, phase-1 heating at 900^oC for 10 minutes. In the temperature range of 400^oC, the temperature of Bainite (Bainite, B_s, and Martensite, M_s) starts to change and the temperature is kept for 10 minutes. country. Iodine medium is a mixture of molten salt of 45% NaOH+30% NaNO₃+25% NaNO₂.

Forged, cold-rolled and heat-treated samples were photographed on the Axiovert 25 A optical microscope and X-Ray analysis, SEM as well as strength testing.

TBF heat treatment for low alloy steels CMnSi received mixed polyurethanes containing excess ferrite, bainite, and austenite to improve the strength and ductility of steel.

When increasing cooling deformation, the grain size of ferrite is decreased but increased in the percentage of austenite, the strength of the steel, and the modulus of elongation in small increments.

When the previous 80% cold-rolled, the grain size of ferrite is 6.4 μm, a large austenite fraction of 0-16%, and the best combination of strength and ductility.