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Effects of Heat Treatment on Morphology, Texture, and Mechanical Properties of a Mnsial Multiphase Steel with TRIP Behavior

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Effects of Heat Treatment on Morphology, Texture, and Mechanical Properties of a Mnsial Multiphase Steel with TRIP Behavior metals Article Effects of Heat Treatment on Morphology, Texture, and Mechanical Properties of a MnSiAl Multiphase Steel with TRIP Behavior Alvaro Salinas 1, Alfredo Artigas 1, Juan Perez-Ipiña 2 , Felipe Castro-Cerda 1 , Nelson Garza-Montes-de-Oca 3, Rafael Colás 3, Roumen Petrov 4 and Alberto Monsalve 1,* 1 Departamento de Ingeniería Metalúrgica, Universidad de Santiago de Chile, Av. Ecuador 3735, Estación Central, Santiago 9170124, Chile; [email protected] (A.S.); [email protected] (A.A.); [email protected] (F.C.-C.) 2 GMF Universidad Nacional del Comahue—CONICET, Neuquén 8300, Argentina; [email protected] 3 Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Monterrey 66455, Mexico; [email protected] (N.G.-M.-d.-O.); [email protected] (R.C.) 4 Department of Electrical Energy, Metals, Mechanical Constructions & Systems, Ghent University, Technologiepark 903, 9052 Gent, Belgium; [email protected] * Correspondence: [email protected]; Tel.: +56-9-6847-5721 Received: 3 November 2018; Accepted: 27 November 2018; Published: 5 December 2018 Abstract: The effect that the microstructure exerts on the Transformation-Induced Plasticity (TRIP) phenomenon and on the mechanical properties in a multiphase steel was studied. Samples of an initially cold-rolled ferrite–pearlite steel underwent different intercritical annealing treatments at 750 ◦C until equal fractions of austenite/ferrite were reached; the intercritical treatment was followed by isothermal bainitic treatments before cooling the samples to room temperature. Samples in the first treatment were heated directly to the intercritical temperature, whereas other samples were heated to either 900 ◦C or 1100 ◦C to obtain a fully homogenized, single-phase austenitic microstructure before performing the intercritical treatment. The high-temperature homogenization of austenite resulted in a decrease in its stability, so a considerable austenite fraction transformed into martensite by cooling to room temperature after the bainitic heat treatment. Most of the retained austenite transformed during the tensile tests, and, consequently, the previously homogenized steels showed the highest Ultimate Tensile Strength (UTS). In turn, the steel with a ferritic–pearlitic initial microstructure exhibited higher ductility than the other steels and texture components that favor forming processes. Keywords: TRIP-assisted steel; microstructure; mechanical properties 1. Introduction Transformation-Induced Plasticity (TRIP)-assisted steels belong to the Advanced High-Strength Steels family, which combine high ductility and strength. The TRIP effect consists of the transformation of metastable austenite into martensite during deformation [1]. The energy absorption capacity of TRIP-assisted steels makes them attractive for the automotive industry. TRIP-assisted steels have a complex multiphase microstructure consisting mainly of ferrite, bainite, and retained austenite. Martensite and carbides may be present in some cases [2,3]. Improvements in the mechanical properties of TRIP-assisted steels are related to the chemical composition and microstructure (grain size, phase morphology, and others) and the stability of the retained austenite [4]. The stabilization of austenite at room temperature is enhanced by carbon enrichment during heat treatment [5]. There are several studies that have shown the effect that morphological aspects have on retained austenite and its stability against transformation [3,6–8]. Van Dijk et al. [3] noticed that the austenite Metals 2018, 8, 1021; doi:10.3390/met8121021 www.mdpi.com/journal/metals Metals 2018, 8, 1021 2 of 13 volume fraction, its carbon concentration, and the grain size of the retained austenite play a crucial role in the TRIP properties as they significantly affect the mechanical stability of the retained austenite. They found that the stability of retained austenite decreases when the carbon content decreases. OtherMetals studies 2018, [ 68,– x8 FOR] reported PEER REVIEW that the increase of the grain size of the retained austenite decreases2 of 13 the austeniteThere stability are several and, studies consequently, that have increasesshown the effect the TRIP that morphological effect. Wang etaspects al. [7 have] related on retained austenite stabilizationaustenite to and the its extra stability interfacial against (austenite/martensite) transformation [3,6–8]. Van energy Dijk et required al. [3] noticed for fine that austenite the austenite grains. That is,volume increasing fraction, austenite its carbon grain concentration, size increases and thethe grain MS temperature. size of the retained Sugimoto austenite et al. play [9] studieda crucial the effectsrole of silicon in the andTRIP manganese properties contentsas they significantly on the volume affect fraction the mechanical and stability stability of retained of the austeniteretained in carbon–manganese–siliconaustenite. They found TRIP-assistedthat the stability dual-phase of retained steels, austenite finding de thatcreases the when volume the fraction carbon ofcontent retained austenitedecreases. increased Other with studies increasing [6–8] report siliconed that and the manganese increase of contents. the grain Perelomasize of the etretained al. [10 austenite] suggested that Aldecreases decreases the the austenite carbon stability activity and, coefficient consequently, in ferrite, increases consequently the TRIP increasing effect. Wang the et solubilityal. [7] related of C in austenite stabilization to the extra interfacial (austenite/martensite) energy required for fine austenite ferrite, and at the same time inhibits the precipitation of Fe carbides, leading to higher enrichment in grains. That is, increasing austenite grain size increases the MS temperature. Sugimoto et al. [9] carbon of retained austenite. However, Al increases the M temperature, making the retained austenite studied the effects of silicon and manganese contents onS the volume fraction and stability of retained less stable.austenite They in also carbon–manganese–silicon found that 1.5% Mn content TRIP-assis assurested dual-phase hardenability steels, in finding TRIP-assisted that the steels. volume Accordingfraction of toretained De Cooman austenite [11 increased], some alloyingwith increasing elements silicon have and an manganese important contents. influence Pereloma on the et TRIP effect.al. Si [10] and suggested Al inhibit that cementite Al decreases formation, the carbon increasing activity coefficient the carbon in ferrite, content consequently of retained increasing austenite becausethe of solubility the extremely of C in ferrite, low solubility and at the ofsame Si andtime Alinhibits in cementite. the precipitation Si significantly of Fe carbides, increases leading theto C activityhigher coefficient enrichment in both in ferritecarbon and of retained austenite austenite. and reduces However, the solubility Al increases of C the in ferrite.MS temperature, On the other hand, Mnmaking stabilizes the retained austenite; austenite decreases less stable. the They activity also coefficient found that of1.5% C inMn ferrite content and assures austenite, hardenability increasing the C solubilityin TRIP-assisted in ferrite steels. and is soluble in cementite. According to De Cooman [11], some alloying elements have an important influence on the TRIP In a previous work on a multiphase low-alloy TRIP steel, Guzmán [12] proposed that an optimum effect. Si and Al inhibit cementite formation, increasing the carbon content of retained austenite combinationbecause of of microstructure the extremely low and solubi propertieslity ofcan Si and be obtainedAl in cementite. with an Si intercritical significantly treatment increases ofthe 10 C min ◦ at 750 activityC (just coefficient above A 1inin both Figure ferrite1), and reaching austenite an andα/ reducesγ proportion the solubility near to of 1/1. C in ferrite. Matsumara On the et other al. [13] showedhand, that Mn heating stabilizes just austenite; above A decreases1 ensures the a activity large content coefficient of retainedof C in ferrite austenite and austenite, with good increasing stability that providesthe C solubility for the in best ferrite combination and is soluble of strength in cementite. and ductility. Figure 1. Schematic representation of heat treatments to obtain multiphase Transformation-Induced Figure 1. Schematic representation of heat treatments to obtain multiphase Transformation-Induced PlasticityPlasticity (TRIP)-assisted (TRIP)-assisted steels. steels. BasedIn on a the previous facts that work (a) on the a TRIPmultiphase effect islow-alloy significant TRIP for steel, large Guzmán austenite [12] grain proposed size [6 –that8]; (b)an the globaloptimum mechanical combination response ofof a multiphasemicrostructure TRIP-assisted and properties steel can is also be influencedobtained with by the an interaction intercritical with the othertreatment microstructural of 10 min constituents;at 750 °C (just and above (c) heatA1 in treatmentFigure 1), hasreaching a strong an α influence/γ proportion on microstructural near to 1/1. features,Matsumara which has et al. an [13] effect showed on that austenite heating stability just above (chemical A1 ensures and a large mechanical) content of retained and the austenite features of other constituents,with good stability an optimum that provides heat for treatment the best combination must exist that,of strength considering and ductility. both effects, maximizes
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