Microstructure and Mechanical Properties of a TRIP-Aided Martensitic Steel

Metallogr. Microstruct. Anal. (2015) 4:344–354 DOI 10.1007/s13632-015-0221-5 TECHNICAL ARTICLE Microstructure and Mechanical Properties of a TRIP-Aided Martensitic Steel Koh-ichi Sugimoto1 • Ashok Kumar Srivastava2 Received: 28 May 2015 / Revised: 22 July 2015 / Accepted: 21 August 2015 / Published online: 3 September 2015  The Author(s) 2015. This article is published with open access at Springerlink.com Abstract This paper deals with the microstructural and mechanical properties of a transformation-induced plasticity-aided martensitic (TM) steel that is expected to serve as an advanced structural steel for automotive applications. The microstructure consisted of a wide lath-martensitestructured matrix and a mixture of narrow lath-martensite and metastable retained austenite of 2–5 vol% (MA-like phase). When 1%Cr and 1%Cr–0.2%Mo were added into 0.2%C–1.5%Si–1.5%Mn steel to enhance its hardenability, the resultant TM steels achieved a superior cold formability, toughness, fatigue strength, and delayed fracture strength as compared to conventional structural steel such as SCM420. These enhanced mechanical properties were found to be mainly caused by (1) plastic relaxation of the stress concentration, which resulted from expansion strain on the strain-induced transformation of the metastable retained austenite, and (2) the presence of a large quantity of a finely dispersed MA-like phase, which suppressed crack initiation or void formation and subsequent void coalescence. Keywords Automotive applications  Microstructure  Mechanical property  TRIP-aided martensitic steel  Retained austenite  Microalloying & Koh-ichi Sugimoto 1 Department of Mechanical Systems Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan 2 Department of Metallurgical Engineering, O. P. Jindal University, Punjipathra, Raigarh, Chhattisgarh 496001, India 123 Introduction In the past decade, some advanced high-strength and ultrahigh-strength sheet steels (AHSS and AUHSS, respectively) such as transformation-induced plasticity (TRIP) [1] -aided sheet steel [2–11], quench and partitioning steel [12, 13], and twinning-induced plasticity (TWIP) steel [14] have been developed in order to reduce the weight and improve the impact safety of automobiles. General ultrahigh-strength TRIP-aided steels such as TRIP-aided bainitic ferrite (TBF) steel are produced by an isothermal transformation (IT) process at a temperature above the martensite-start temperature (Ms) or at a temperature between Ms and the martensitefinish temperature (Mf) [6]. Sugimoto et al. [8] reported that the microstructure of TBF steel changed to a wide lathmartensite matrix and a large amount of finely dispersed narrow lath-martensite and metastable austenite (MA-like phase) when the IT process was carried out at temperatures lower than Mf. Such a TRIP-aided martensitic (TM) steel attained a superior combination of tensile strength and formability [8]. Combinations of tensile strength and total elongation of some of the abovementioned AHSS and AUHSS steels are controlled by the original volume fraction of austenite and retained austenite, as shown in Fig. 1. Because TM steel also possesses superior mechanical properties such as toughness [15–17], fatigue strength [18, 19], and hydrogen embrittlement resistance [20] as compared to conventional structural steel (Fig. 2), application of TM steel to automotive drivetrain components such as gears, drive shafts, CV joints, clutch plates, etc. is expected to reduce the weight and CO2 emission. This paper introduces the microstructural and mechanical properties of C–Si–Mn TM steels with different C, Mn, Cr, Mo, Ni, Nb, and B contents, which affect the hardenability of the steel, in order to assess the suitability of such Metallogr. Microstruct. Anal. (2015) 4:344–354 345 TSxTEl (GPa %) 100 80 40 20 TM Mar. 20%Mn-3%Si-3%Al Aus. TRIP/TWIP 60 TBF TDP 0 0 5-10%Mn TRIP Q&P 20 40 60 f (vol%) 80 100 0 Fig. 1 Relationship between a combination of the tensile strength and total elongation (TS 9 TEl) and the original volume fraction of austenite and retained austenite (fc0), in various TRIP and TWIP steels. TDP TRIP-aided dual-phase steel, TBF TRIP-aided bainitic ferrite steel, TM TRIP-aided martensitic steel, Q&P Quenching and partitioning steel, Mar conventional martensitic steel, Aus austenitic steel Carburizing property Tensile strength Conventional martensitic steel Cold formability Delayed fracture strength TM steel Impact toughness (Fracture toughness) Notch fatigue strength Fig. 2 Comparison of the mechanical properties of TM steel and conventional martensitic steel steels for application to automotive structural parts and drivetrain components. Materials and Experimental Procedure The chemical compositions of the steels used in this study are listed in Table 1. Steel A is a base steel with a chemical composition of 0.2%C, 1.5%Si, 1.5%Mn, and 0.05%Nb (mass%). The addition of Mn, Cr, Mo, Ni, Nb, and/or B to this created Steels B–H. In Steels I–M, Nb was omitted and the carbon content was varied. Figure 3 illustrates the heat-treatment diagram of TM steel, which consisted of austenitizing followed by an isothermal transformation (IT) process at temperatures below the martensite-finish temperature (Mf). In some instances, a final partitioning process was also conducted at 250–350 C for 1000 s in order to promote carbon enrichment of the retained austenite. The microstructures of the steels were observed by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FE-SEM), which was performed using an instrument equipped with an electron backscatter diffraction (EBSD) system. The steel specimens for the FE-SEM–EBSD analyses were first ground with alumina powder and colloidal silica, and then ion thinning was performed. The retained austenite characteristics of the steels were evaluated by x-ray diffractometry. The volume fractions of the retained austenite phases (fc, vol%) were quantified from the integrated intensity of the (200)a, (211)a, (200)c, (220)c, and (311)c peaks obtained by x-ray diffractometry using Mo-Ka radiation [21]. The carbon concentrations (Cc, mass%) of the retained austenite phases were estimated from the empirical equation proposed by Dyson and Holmes [22]. In this case, the lattice constant (ac, 90.1 nm) was measured from the (200)c, (220)c, and (311)c peaks of CuKa radiation. The tensile tests were performed at 298 K (25 C) using a tensile testing machine under a crosshead speed of 1 mm/ min (resulting in a strain rate of 6.67 9 10-4 s-1). Hole punch and hole expansion tests were carried out using a graphite-type lubricant [8]. A hole with a diameter of 4.76 mm was punched out at a punching rate of 10 mm/ min (at 25 C), with a clearance of 10% between the die and the punch. Successive hole expansion tests were performed at 25 C using a 60 conical die at a punching rate of 1 mm/min. (...truncated)