ACTA METALLURGICA SLOVACA 2020, VOL. 26, NO. 3, 116-121 116 DOI: 10.36547/ams.26.3.554 RESEARCH PAPER ELEMENT PARTITIONING IN LOW-CARBON Si2Mn2CrMoVNb TRIP-ASSISTED STEEL IN INTERCRITICAL TEMPERATURE RANGE Vasily Efremenko 1 , Roman Kussa 1 , Ivan Petryshynets 2 , Kazumichi Shimizu 3 , František Kromka 2 , Vadim Zurnadzhy 1 , Victoria Gavrilova 1 1 Pryazovskyi State Technical University, (PSTU) vul. Universytets’ka 7, Mariupol 87555, Ukraine 2 Institute of Materials Research, Slovak Academy of Sciences, Watsonova 1935/47, 040 01, Kosice, Slovakia 3 Muroran Institute of Technology, 27-1 Mizumotocho, Muroran, Hokkaido 050-8585, Japan *Corresponding author: vgefremenko@gmail.com, Materials Science, Physics, Pryazovskyi State Technical University, (PSTU) vul. Universytets’ka 7, Mariupol 87555, Ukraine Received: 14.05.2020 Accepted: 14.08.2020 ABSTRACT The present paper is aimed at the study of the kinetics of Mn, Si, Cr partitioning in 0.2wt%C-Si2Mn2CrMoVNb TRIP-assisted steel under the annealing at 770 o C and 830 o C to be within the intercritical temperature range. The work was fulfilled using SEM, EDX, dilatometry, and hardness measurements. It was found that under heating a redistribution of the alloying elements between ferrite and austenite took place. Specifically, silicon partitioned to ferrite while chromium diffused to austenite with distribution coefficient values of 1.12-1.21 (KSi) and 0.75-0.86 (KCr). Manganese was found to partition to a much greater extent resulting in a distribution coefficient of KMn=0.38-0.50 and 2.6 times higher concentration in austenite as compared to ferrite. As annealing temperature raised from 770 o C to 830 o C the elemental partitioning was accelerated, followed by the decrease in manganese content in austenite (by 1.44 time) and ferrite (by 1.34 time) caused by an increase in austenite volume fraction. Silicon featured uneven distribution within ferrite to be accumulated at the “martensite/ferrite” interface and near ferrite grain boundaries, while manganese was concentrated in MC carbides. The recommendation for annealing holding was formulated based on elemental partitioning kinetics. Keywords: TRIP-assisted steel; annealing; SEM/EDX; element partitioning; ferrite; martensite INTRODUCTION Low carbon Mn-Si(Al) steels with TRIP effect (TRIP-assisted steels) remain attractive for researchers due to improved mechanical properties achieved under the low content of alloying elements [1-3]. They belong to the group of Advanced High Strength Steels (AHSSs) [4]. The distinguishable feature of TRIP-assisted steels is a heterogeneous multiphase structure comprising ferrite, carbide-free bainite, and retained austenite (RA) [5, 6]. According to adopted technology, TRIP-assisted steels are subjected to bainitizing heat treatment with preliminary annealing at intercritical temperature range (ITR), where carbon partitioning between ferrite and austenite takes place [1, 7, 8]. Enriched with carbon austenite retains after bainitizing, holding thus volume fraction of RA in final structure and reaches up to 10-15 vol.% [1, 9, 10]. Retained austenite is the key structural constituent of heterogeneous constructional steels due to its higher ductility and its capability toward strain-induced martensite transformation (TRIP-effect) [11, 12]. TRIP-effect results in enhancement of steel strength/ductility combination [13, 14] as well as in improvement in exploitation behaviour such as abrasive/erosive wear resistance [15-17]. A carbon partitioning between the phase constituents is a common feature of heat processing of different steel grades (TRIP-assisted steels [1, 5], QP-steels [18, 19], nanobainite steels [20]). During the intercritical annealing, carbon partitions between ferrite and austenite in order to reach a thermodynamic equilibrium being driven by the big difference in carbon equilibrium solubility in -Fe and -Fe. A high diffusivity of carbon (as an interstitial element) under ITR makes its partitioning very fast. In addition to carbon, other (substitutional) elements (Si, Mn, Cr, etc.) could be involved in partitioning under ITR, affecting the stability of austenite to phase transformations upon bainitizing treatment or cooling [21]. This phenomenon was studied repeatedly to highlight its importance for steel transformation behaviour [22-26]. Zhang et al. [22] found that C, Mn, and Al were partitioning under ITR annealing in 0.05-0.15 wt% C -5 wt% Mn - 3 wt% Al dual- phase steels, greatly affecting austenite volume fraction and stability. Specifically, ITR annealing induced enrichment of austenite with manganese, while ferrite enriched with aluminum. The same results for manganese were reported in [23] for 6 wt% Mn - 1.4 wt% Si TRIP-assisted steel, however no partitioning of Si and Al was observed during inter-critical annealing. Lis et al. [24] found for 4 wt% Mn TRIP-steel that soft annealing at 625 °C led to the enrichment of proeutectoid cementite with Mn; besides this, a non-uniform distribution of Mn between polygonal ferrite/bainite and austenite was detected leading to increase in Mn content up to 10 wt% in martensite-austenite “islands” retained after water quenching. Lee et al. [25] showed for 6 wt% Mn steel that Mn partitioning to austenite increases the stability of retained austenite only when austenite is ultra- fine grained with grain size less than 500 nm. Luo et al. [26] demonstrated that austenite formation in 5 wt% Mn steel at the early stage of ITR annealing was controlled only by carbon diffusion. Furthermore, the newly formed 20 nm-thick austenite was formed without the Mn partitioning; however, after that manganese segregates at the γFe/αFe interface. Park et al. [27] revealed that alloying elements may partition in CMnSiAl TRIP steels not only under intercritical annealing, but under tempering at 400 o C as well. The literature analysis allows one to conclude that investigation into elements partitioning under ITR annealing is mostly focused on medium-Mn TRIP- assisted steels and dual-phase steels alloyed by 5-7 wt% Mn. At the same time, the distribution of substitutional elements in low-Mn TRIP-assisted steel remains not well studied. Therefore the objective of the present work is to investigate the kinetics of elemental partition in low-carbon 0.2wt%C-Si2Mn2CrMoVNb TRIP- assisted steel under annealing at intercritical temperature range. MATERIAL AND METHODS The TRIP-assisted steel with the composition 0.20 wt% С, 1.79 wt% Si, 1.73 wt% Mn, 0.55 wt% Cr, 0.20 wt% Mo, 0.11 wt% V, 0.045 wt% Nb, 0.009 wt% S, and 0.013 wt% Р was used in the present work. Steel was smelted in a 120-kg induction furnace and poured in 50 mm-diameter cast ingots which then were electro-slag remelted to produce an 80 mm diameter ingot. This ingot was subsequently forged and then rolled to a 15 mm thick strip with further soft annealing at 900 o C before machining. Then the specimens for dilatometric study (of 2 mm diameter and 20 mm length) and microscopic observation (of 3 mm x10 mm x10 mm size) were prepared. The critical points Ac1 and Ac3 were determined by optical dilatometer under the heating with a rate of 1.0 K/s. The isothermal heating was performed in an