Structure-properties relationship in TRIP type bainitic ferrite steel austempered at different temperatures

Nasr El-Din et al. International Journal of Mechanical and Materials Engineering (2017) 12:3 DOI 10.1186/s40712-017-0071-9 ORIGINAL ARTICLE Open Access Structure-properties relationship in TRIP type bainitic ferrite steel austempered at different temperatures Hoda Nasr El-Din1, Ezzat A. Showaib2, Nader Zaafarani2 and Hoda Refaiy1* Abstract Background: Attractive properties of TRIP-type bainitic ferrite (TBF) steel ascribe to its unique microstructure of lath structure bainitic ferrite matrix and interlath retained austenite films. This work is concerned with obtaining ultra high-strength hot forged TBF steel with high elongation and excellent strength-elongation balance. Methods: The effect of austempering temperature on the microstructure along with its retained austenite characteristics and tensile properties of a hot forged TBF steel was studied. A detailed investigation correlating the steel structure and its tensile properties was carried out. Results: Tensile strength ranging from 1058 to 1552 MPa was achieved when the hot forged steel was austempered at (325 - 475 °C). Conclusions: Ultra high tensile strength of 1058 MPa, large total elongation of 29% and excellent strengthelongation balance of 30 GPa % were attained when the steel was austempered at 425 °C. The large total elongation of this steel is mainly due to the uniform fine lath structure matrix and the pronounced TRIP effect of a large amount of retained austenite films which prevents a rapid decrease of strain hardening rate at low strain and leads to a relatively high strain hardening at high strain level. Rapid transformation of blocky retained austenite at low strain in the hot forged TBF steel austempered at higher temperatures results in a rapid increase of initial strain hardening. In addition, the coarse microstructure that contains large blocks of retained austenite / martensite and the insufficient numbers of bainitic ferrite lathes and retained austenite films deteriorate the total elongation and the strength-elongation balance of the TBF austempered at 475 °C. Keywords: Ultrahigh-strength steels, TRIP-aided steels, Austempering heat treatments, Microstructure, Retained austenite characteristics, Tensile properties Background The reduction in weight of vehicle body can improve the fuel efficiency and environmental control. Therefore, there is an international attention to develop advanced high-strength steel (AHSS). The TRIP-aided multiphase (TMP) steel as a class of AHSS exhibits an excellent combination of strength and stretch-formability (Sugimoto et al. 1995), good deep drawability (Matsumura et al. 1992) and high fatigue strength (Sugimoto et al. 1997). The microstructure of this steel is mainly composed of bainitic * Correspondence: 1 Plastic Deformation Department, Central Metallurgical R& D institute (CMRDI), Cairo, Egypt Full list of author information is available at the end of the article ferrite (bf) and carbon-enriched retained austenite (γr) embedded in a matrix of polygonal ferrite (Sugimoto et al. 1992). The ideal energy absorption behaviour of TMP steels, which can be attributed to the transformation of metastable retained austenite into martensite under stress and strain (TRIP effect) (Bleck 2002; Sugimoto et al. 2006), and the high work-hardening response improve the crashworthiness of a vehicle through good distribution of strain during crash deformation (Bleck 2002).The superior formability of TMP steel (Sugimoto et al. 2006) can also be attributed to its high work-hardening properties. TMP steel has been applied to some impact members (Ojima et al. 1998) due to its high ability of energy © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Nasr El-Din et al. International Journal of Mechanical and Materials Engineering (2017) 12:3 absorption under dynamic load. In spite of the excellent mechanical and technological properties of TMP steel, it cannot be applied to the automotive underbody press parts (e.g. lower arms and members) (Emadoddin et al. 2009) due to its poor stretch-flangability (Sugimoto et al. 1999). Also, this steel lacks sufficient performance in bendability and edge formability (De Cooman et al. 2004). Based on the fact that the bainitic steel exhibits an excellent stretch-flangability due to its uniform fine lath structure, (Sugimoto et al. 2000) have developed a new type of high-strength TRIP type bainitic ferrite (TBF) steel. The microstructure of TBF steel is characterized by bainitic ferrite lath matrix and interlath-retained austenite films. TBF steel is characterized by excellent balance between low edge crack susceptibility and high elongation (Sugimoto et al. 2002, 2006). It develops better stretchability compared to TMP steel with the same chemical composition and amount of retained austenite (Bhadeshia et al. 2001). It also exhibits good bendability and edge formability (Sugimoto et al. 2006). For these excellent properties, TBF steel can be applied to the applications that require high localized strain realization. Moreover, TBF steel shows high fatigue strength (Demeyer et al. 1999) and high impact energy (Hojo et al. 2008). Recently, (Sugimoto et al. 2010a) have developed a new type of hotforged TBF steel with high hardenability. The present work is aimed at producing ultrahigh-strength hot-forged TBF steel with high elongation. Tensile properties of TRIP steel are affected by heat treatment conditions. The specific purpose of this investigation is to study the effect of austempering temperature (TA) on the microstructure along with its retained austenite characteristics, and tensile properties of the investigated hot-forged TBF steel. The structure-properties relationship is also discussed. Methods One hundred kilogramme Y-blocks of the steel alloy were produced in a medium-frequency induction furnace. Chemical composition of the steel is shown in Table 1. The charge was made up of steel scrap and the required Mn, Si, P, Mo and Nb were added as ferroalloys (Fe–80% Mn), (Fe–75% Si), (Fe–28% P), (Fe–70% Mo) and (Fe–65% Nb). The heats were cast into 300 mm × 200 mm × 50 mm Y-blocks. Each leg of Y-blocks was machined and sectioned into 200 mm × 60 mm × 40 mm specimens. The specimens were homogenized at 1250 °C for 2 h in a muffle furnace, and then furnace cooled. The martensite start temperature was estimated to be 371 °C by the following equation (Tamura 1970): Page 2 of 9 MS ðo CÞ ¼ 550–361xð%CÞ–39xð%MnÞ–17xð%VÞ –20xð%CrÞ–17xð%NiÞ–10xð%CuÞ –5xð%Mo þ %WÞ–0xð%SiÞ þ 15xð%CoÞ þ30xð%AlÞ ð1Þ Fee-forging as one of the forming technologies is commonly used to refine the (...truncated)