Dilatometric study of phase transformations in advanced high-strength bainitic steel

Abstract

The work deals with dilatometric studies of a new-developed advanced high-strength bainitic 3Mn-1.5Al steel. Ferritic, bainitic and martensitic phase transformations are investigated in detail in respect of their temperature range forming and microstructures produced under various conditions of both continuous and isothermal cooling. The equilibrium temperatures of A e1 and A e3 and phase composition of the investigated steel were initially calculated whereas critical temperatures of A c1 and A c3 as well as the decomposition of retained austenite were determined upon heating. The major tests consisted of controlled cooling of undeformed or plastically deformed austenite using the dilatometer within the cooling rate range of 2-0.5°C s -1 . The effects of the cooling rate and deformation at temperatures of 900 and 1,050°C on the phase transformation behaviour and microstructure were explained. The final experiment was carried out using a thermo-mechanical simulator under conditions of multistep deformation and isothermal holding of the steel at 400°C. Microstructural features were revealed using light microscopy and scanning electron microscopy techniques.

Phase Equilibrium and Austenite Decomposition in Advanced High-Strength Medium-Mn Bainitic Steels

“…The martensite start temperatures are relatively low: 340 • C for the 3 Mn steel and 251 • C for the alloy with the higher Mn content. The most interesting are bainitic bays in both steels because they constitute large technological temperature-time windows for realization of isothermal bainitic treatments, which are needed for production of carbide-free bainite-austenite mixtures [7,8,27].…”

“…New demands of the automotive industry for relatively low-cost steel sheets characterized by tensile strength above 1000 MPa require further searching for new chemical composition strategies. Advanced ultrahigh strength steels contain a higher fraction of hard phases (i.e., acicular ferrite, bainite, or martensite [7][8][9][10]) compared to AHSS containing polygonal ferrite as a matrix.…”

Optimization of mechanical properties of complex, two-stage heat treatment of Cu–Ni (Mn, Mo) austempered ductile iron

“…In this approach very useful is the analysis of the sequence of phase transformations proceeding during heating, reflecting decomposition of metastable phases produced during ADI heat treatment. First-order phase transformations, which are considered, are described by changes of entropy (or enthalpy) (DH = 0) and volume (DV = 0), so both calorimetric (DSC) and dilatometric methods are suitable and very useful here [29,30].…”

CCT diagram plotting based on the numerical analysis of dilatometric tests results

“…Кроме хими-ческого состава к таким характеристикам относятся дан-ные о положении критических точек и кинетике распада переохлажденного аустенита [2,3], на основании кото-рых, в частности, решаются вопросы прокаливаемости заготовок. От прокаливаемости применяемой стали и возможности обеспечения диапазона скоростей охла-ждения при закалке, в котором происходит образование однородной структуры, зависит получение заданных механических свойств во всем объеме заготовки, что яв-ляется, как указывалось выше, одной из составляющих безопасности изделия при эксплуатации [1,4,5].…”