The influence of the applied type of cooling after eight-stage hot compression test on the structure and mechanical properties of TRIPLEX type steels

Abstract

The aim of the work was to analyse the impact of an eight-stage hot compression process carried out on the Gleeble3800 simulator, with three cooling variants after thermo-mechanical treatment of Fe-Mn-Al-C steels for their structure and mechanical properties. Performed research allowed to evaluate the impact on the structure and properties of simulation conditions for multi-stage rolling of difficult-to-treat thermomechanical steels for which this treatment is the final process of obtaining ready-to-use high-strength construction steels. Applied thermo-mechanical treatment causes that the main process of removing the effects of strain hardening is dynamic recovery, and static and metadynamic recrystallisation taking place after the last deformation but also between successive deformations, which was also confirmed on the basis of structural analysis of the tested steel after different cooling variants. As a result of the eight-stage hot compression test, the ferrite changed its distribution from fine grains occurring at the boundaries of austenite grains after forging, to elongated grains in a perpendicular direction to the compression direction. Isothermal heating at 850°C for 30s resulted in obtaining a fine-grained structure, statically or metadynamically recrystallised. The maximum tensile strength of the tested steels is about 1250 MPa, and the total elongation value is about 27%.

Structure of Fe-Mn-Al-C Steels after Gleeble Simulations and Hot-Rolling

“…In addition, the increase in the stress values in the last (i.e., fourth) stage of deformation could have been influenced by the fact that the material was already partially fragmented as a result of the dynamic recrystallization in the first and second stages of deformation. The applied conditions of the multistage deformation resulted in fragmentation occurring, mainly of austenite grains, and a change in the ferrite morphology in the tested TRIPLEX steels [32][33][34][35][36][37]44], which had already been noted in previous publications [46,47].…”

“…In X105 steel, Mn 7 C 3 carbide was identified in austenite (Figure 25), characterized by an orthorhombic crystal lattice (Pnma group) with lattice parameters a = 0.4546 nm, b = 0.6959 nm, and c = 1.197 nm. Mn 7 C 3 carbide has also been identified in X98 steel, as described in earlier publications [17,18,21,41,43,46,47]. Mn 7 C 3 carbides occur in both austenite and ferrite, and their size ranges from 100 to 600 nm.…”

Bulletin of the Polish Academy of Sciences: Technical Sciences

“…

are coherent with the matrix, hinder the dislocation movement. The result of this effect is a high speed of strain hardening during stretching, which prevents the formation of a neck in the sample during the static tensile test, giving a high value of the Rp 0.2 /Rm ratio in the analyzed steel [3,4,7,[10][11][12][13][14][15][16].The increase in the strengthening speed at the MBIP formation site in the above-mentioned steel is due to an increase in the total density of dislocation due to the formation of microbands [4,7,[13][14][15][16][17][18][19][20][21]. With deformation of 5%, the dislocation in the slip plane along the major crystallographic directions was revealed.

…”

“…This mechanism causes the formation of thin slip bands in the austenite grain. During deformation, the number of slip bands increases, which results in their densification [16,[20][21][22][23][24][25][26][27][28][29]. Table 1 presents a comparison of the strengthening mechanisms in Fe-Mn-Al-C type TRIPLEX steel.…”

“…are coherent with the matrix, hinder the dislocation movement. The result of this effect is a high speed of strain hardening during stretching, which prevents the formation of a neck in the sample during the static tensile test, giving a high value of the Rp 0.2 /Rm ratio in the analyzed steel [3,4,7,[10][11][12][13][14][15][16].…”