*4.2.1 Processing of TRIP steels*

*Engineering Steels and High Entropy-Alloys*

ment is achieved during rolling.

third-generation AHSS [5].

**4.2 Transformation-induced plasticity (TRIP) steels**

while **Figure 6b** is a micrograph of a typical TRIP steel (TRIP 700).

method (Route A).

remaining austenite to martensite [20]. The properties obtained by this method include lower tensile strength and higher ductility than those of gained by the first

The third method for producing DP steels (Route C in **Figure 5**) involves hot rolling of steel, followed by first slow cooling to the intermediate temperature, followed by second cooling at a very fast rate and finally slow cooling (i.e., coil cooling) to room temperature. This method of cooling is known as ultrafast cooling (UFC), and the processing route is referred to as new-generation thermomechanical controlled processing [21]. The properties obtained by Route C are better (as compared to those obtained by Route A and Route B) because higher grain refine-

Several authors have reported that DP steels with ultrafine bainite and fine ferrite-bainite/martensite microstructure with precipitation hardening can achieve good strength without loss of ductility, making this steel category more suitable for

Advanced high-strength transformation-induced plasticity (TRIP) steels are well suited for lightweighting car body construction with added advantage to reduce the safety problems. TRIP steels can be found already in the 1st+ generation AHSS as shown in **Figure 2**. One of the main features of TRIP steels that the strain or stress-induced transformation of retained austenite present in the microstructure in a sufficient amount can substantially harden the steel during deformation depending on the processing route and therefore results in a higher ductility [22]. The microstructure of TRIP steels contains retained austenite embedded in a primary matrix of ferrite. **Figure 6a** shows schematic microstructure of TRIP steel,

In addition to a minimum of 5 vol.% of retained austenite, hard phases such as martensite and bainite are present in varying amounts. TRIP steels typically require an isothermal hold at an intermediate temperature, which produces some bainite. TRIP steels are characterized by a relatively low content of alloying elements. For example, in TRIP 790 steel (UTS ≈ 790 MPa), the total content of alloying elements is about 3.5 wt.%. Thus, the selection of suitable alloying elements and the amount required to produce the intended properties is critical in the alloy design stage. The carbon content in TRIP steels is higher than in DP steels. Carbon

*Schematic view and micrograph of the microstructure of TRIP steel. (a) Schematic view of the microstructure* 

*of a TRIP steel and (b) micrograph of a typical TRIP steel (TRIP 700).*

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**Figure 6.**

The main processing of TRIP steels consists of heating the steel to the austenitic zone, cooling down to the intercritical region followed by deformation here, and quick transfer to the bainitic zone with subsequent soaking there, and finally quenching to room temperature (as shown in **Figure 7**).

The deformation in the intercritical region increases the rate of austenite (γ) to ferrite (α) transformation. The remaining austenite is enriched with carbon content, which stabilizes the γ phase. Furthermore, this deformation increases the nucleation rate of bainite but decreases its growth rate that results in small plates of bainite. This part of the T–t cycle also helps to enrich the γ phase with carbon and further increases the stability of γ phase. The stability of retained austenite is enhanced by the high carbon content, and the more carbon in γ phase results in more stability of γ during the TRIP effect, too, since more stable austenite needs more time to transform into martensite; these processes contribute to the increase of the ductility. The austenite to martensite transformation increases the tensile strength of the final microstructure. With this process, an improved strength–ductility combination is achieved [24]. Obviously, this processing route of TRIP steels is more time-consuming. This is because it needs special arrangements to deform the material at high temperature, to hold the specimen in the bainite region, and so on. This limits the use of TRIP steels in industrial applications. Some authors [25] using this route reported that rolling in the intercritical region improves TRIP steel

**Figure 7.** *Conventional processing route of TRIP steels.*

properties by enhancing the carbon content and dislocation density, decreasing the grain size, and resulting in a granular type morphology.
