Structure-properties relationship in TRIP type bainitic ferrite steel austempered at different temperatures
© The Author(s). 2017
Received: 24 October 2016
Accepted: 10 January 2017
Published: 19 January 2017
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.
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.
Tensile strength ranging from 1058 to 1552 MPa was achieved when the hot forged steel was austempered at (325 - 475 °C).
Ultra high tensile strength of 1058 MPa, large total elongation of 29% and excellent strength-elongation 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.
KeywordsUltrahigh-strength steels TRIP-aided steels Austempering heat treatments Microstructure Retained austenite characteristics Tensile properties
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 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 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 stretch-ability 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 hot-forged 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.
Chemical composition of the investigated steel alloy
Fee-forging as one of the forming technologies is commonly used to refine the microstructure and increase the strength values. The homogenized steel was reheated at 1200 °C for 30 min and then hot forged into rods of 16–18 mm in diameter. The hot forging was performed using Pneumatic Hammer (150 Kp).
The microstructure was identified by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction measurements. Specimens for optical microscopy were etched with 5% nital and rinsed with water followed by etching in a 10% sodium meta-bisulphite solution. With these etchants, retained austenite appears white, ferrite appears grey and bainite and martensite appear black (Jeong 1994). For microstructure investigation using SEM, samples were etched with 2% nital.
Tensile test was carried out on a universal testing machine (1000 KN) at room temperature with a cross head speed of 0.5 mm min-1.
Results and discussion
Influence of austempering temperature on retained austenite characteristics
Influence of austempering temperature on the tensile properties
It is well known that the discontinuous yielding behaviour is produced in the steel due to the presence of interstitial atoms and the yield point phenomenon in the steel is a function of the composition and heat treatment of the material. In the investigated steel, the carbide formers (Mo and Nb) and nitride formers (Al, Si and Nb) reduce the level of the interstitial atoms of carbon and nitrogen, which are strong lockers of dislocation. Consequently, the continuous yielding is promoted as shown in Fig. 3.
Influence of austempering temperature on strain-hardening behaviour
Microstructure and tensile properties
TBF steel austempered at temperatures lower than Ms
Microstructure of the TBF steel austempered at 325 and 350 °C, lower than Ms, consists of bainitic ferrite/martensite mixed matrix of very fine lath structure and interlath-retained austenite films as shown in Figs. 6a, b and 7a. This fine microstructure clearly shown in Fig. 7a has been attributed to the increase in transformation driving force and suppression of coalescence of fine bainite laths (Bhadeshia et al. 2001). The fine prior austenite grain size shown in Fig. 7 can be attributed to the combined addition of Nb and Mo which has a better refining effect than single Nb addition (Huab et al. 2015). Recently, (Hojo et al. 2010) have shown that the complex addition of Al-Nb-Mo to TBF steel tends to refine the lath structure and γr films.
TBF steel austempered at temperatures lower than its Ms indicates higher tensile strength “up to 1552 MPa” compared to the steel austempered at temperatures higher than Ms (Fig. 4a). This is due to its finer lath structure and its bainitic ferrite/martensite (αm) mixed matrix. Tensile strength of this TBF steel is decreased with increasing austempering temperature, from 325 to 350 °C (Fig. 4a) due to the decrease of martensite lathes in the mixed matrix. The strengthening effect of martensite lathes is more dominant in this steel than a small TRIP effect of (3%) γr. TBF steel austempered at temperatures lower than Ms exhibits a smaller total elongation compared to that austempered at 425 and 450 °C, higher than the Ms (Fig. 4b). This is due to steep fall of strain hardening at an early stage which may be caused by small internal stress hardening and initial martensite hardening resulting from a small volume fraction of second phase (Sugimoto et al. 2000), followed by great decrease of strain hardening at low strain (Fig. 5) which in turn results from insignificant TRIP effect of small γr volume fraction.
TBF steel austempered at temperatures higher than Ms
TBF steel austempered at 400 °C
The fine uniform microstructure of TBF steel austempered at 400 °C shown in Fig. 7d contains a large amount of γr (γr = 16%). However, this TBF steel exhibits a smaller total elongation compared to TBF steel austempered at 425 °C (Fig. 4b). This can be attributed to very high stability of γr (Cγ = 1.7 wt%) which prevents high amount of γr from transforming during tensile deformation. This results in a considerable decrease of strain hardening at high strain levels (Fig. 5). Therefore, the total elongation and strength-elongation balance are greatly decreased (Fig. 4b, c). According to (Chiang et al. 2011), the very high levels of carbon in γr > 1.8 wt % C prevent the γr from transforming completely.
TBF steel austempered at 425 °C
Figures 6e and 7d show the microstructure of TBF steel austempered at 425 °C. This fine uniform microstructure mainly consists of bainitic ferrite matrix of lath structure and interlath γr films. It is evident from the figures (Fig. 6d, e and 7c, d) that the matrix structure is changed in some zones from bf lathes to granular bainitic ferrite and the retained austenite films are changed to a few of coarse and island type γr. K. Sugimoto et al. (2010a) have also detected and reported these morphologic changes. It is also obvious from these figures that the pro-eutoctoid ferrite (αPE) and upper bainite are developed. The grain refinement through Nb addition leads to promotion of transformation during cooling from austenitization which causes ~20% transformed phase fraction prior to the overaging step (Hausman 2013). Ultrahigh-strength TBF steel austempered at 425 °C exhibits high total elongation. As shown in Fig. 4b, c, the total elongation and strength-elongation balance were considerably increased at 425 °C to the maximum values of 29% and 30 GPa%, respectively. The large total elongation can be attributed to the uniform microstructure and fine lath structure matrix and to the pronounced TRIP effect of a large amount of stable retained austenite films which gradually transforms upon loading to martensite. The strain-induced transformation of γr simultaneously relaxes the localized stress concentration at the matrix/second phase interface to suppress the void initiation (Sugimoto et al. 2000). Recently, (Zhao et al. 2014) have reported that the film type γr transforms into martensite and hardens the bf matrix. The resultant compressive stress on the bf matrix prevents crack propagation. Consequently, the gradual transformation of high amount of stable γr films in the TBF steel austempered at 425 °C prevents the rapid decrease of strain hardening rate at low strain and permits for the strain hardening to continue up to a high strain (Fig. 5) which results in a considerable increase of the total elongation (Fig. 4b).
The pro-eutectoid soft phases of ferrite and upper bainite contribute to enhanced elongation. The complex addition of Al and Nb to the investigated ultrahigh-strength TBF steel contributes also to enhanced total elongation (Sugimoto et al. 2010b). The reduced strain induced martensite formation at low strain levels in the steel austempered at 425 °C results in increasing the work hardening at high strain levels (Fig. 5). This result has been also detected and reported by (Hausman 2013).
TBF steel austempered at 450 and 475 °C
When the steel is austempered at higher temperatures of 450 and 475 °C, the numbers of bainitic ferrite lathes and the fraction of interlath γr films are greatly reduced while the amount of granular bainitic ferrite and both of blocky and island type γr/αm are increased due to the morphologic changes (Fig. 6f, g), which can be attributed to Nb addition (Sugimoto et al. 2006; Hausman 2013; Bleck et al. 2001). Increasing austempering temperature leads to an increase in the thickness of bainitic ferrite lathes (Fig. 7e, f) due to the reduced nucleation kinetics of bainite formation at higher TA. Increase in lath thickness is also attributed to low dislocation density due to the high temperatures (Zhao et al. 2014). Austempering at these higher temperatures leads to a small degree of super cooling that reduces the driving force of bainite formation. Consequently, the numbers of bainitic ferrite lathes are reduced (Fig. 6f, g) as well as the carbon content of the residual austenite. Upon cooling the steel after austempering, a high amount of lower carbon content residual austenite could not be stabilized at room temperature and both blocky and island type retained austenite/fresh martensite are formed (Figs. 6f, g and 7e, f).
As shown in Fig. 4b, c, the total elongation and strength-elongation balance of TBF steel are decreased with increasing austempering temperature from 425 to 450 °C due to the morphologic change of high amount of γr from film type to a less stable blocky one (compare Fig. 6e and f). Presence of high amount of initial martensite and reduced numbers of bainitic ferrite lathes (compare Fig. 7d and e) also lead to the decrease of total elongation of TBF steel austempered at 450 °C.
Enhanced strain hardening rate of TBF steels austempered at higher temperatures (450 and 475 °C) over a small strain range (Fig. 5) is attributed to the high long-range internal stress resulting from both the initial martensite hardening of original martensite and the difference in strength between the granular bainitic ferrite soft matrix and the γr hard second phase (Sugimoto et al. 2006). The total elongation and strength-elongation balance of TBF steel austempered at 475 °C are seriously decreased (Fig. 4b, c). The decrease of total elongation is firstly attributed to the rapid transformation of blocky γr at low strain which results in a rapid increase of initial strain hardening (Fig. 5). In addition, the coarse microstructure consisting of high amount of large blocks of γr/αm and insufficient numbers of bainitic ferrite lathes (Fig. 6g) results also in the decrease of total elongation. Martensite blocks act as the source of voids initiation and insufficient numbers of bf lathes lead to the decrease of γr stability (Sugimoto et al. 2002).
Ultrahigh-strength hot-forged TBF steels with tensile strength ranging from 1058 to 1552 MPa were attained when TBF steel is austempered at 325–475 °C.
Tensile strength of the hot-forged TBF steel was considerably increased, when austempered at temperature lower than Ms due to its hard fine lath-like bainitic ferrite/martensite mixed matrix. Relatively, small total elongation at these temperatures is due to both the steep fall of strain hardening at an early stage and the great decrease of strain hardening at low strain which resulted from insignificant TRIP effect of small amount of γr.
The considerable decrease of strain hardening at high strain levels and the great decreases of total elongation and strength-elongation balance in TBF steel austempered at 400 °C are due to very high stability of γr (Cγ = 1.7 wt%) which prevents high amount of γr from transforming.
Total elongation and strength-elongation balance of ultrahigh-strength hot-forged TBF steel were greatly increased to 29% and 30 GPa%, respectively, when austempered at 425 °C. Increase of total elongation is due to uniform fine bainitic ferrite lathes, interlath γr films and pronounced TRIP effect of large amount of γr which prevents rapid decrease of strain hardening rate at low strain and leads to a relatively high strain hardening at high strain levels. Pro-eutectoid soft phases and granular bainitic ferrite contribute also to enhanced total elongation.
Rapid transformation of blocky γr at low strain in hot-forged TBF steels austempered at 450 and 475 °C resulted in rapid increase of initial strain hardening. Additionally, the large blocks of martensite and insufficient numbers of bf lathes and γr films resulted in serious decreases of the total elongation and strength-elongation balance, especially in TBF steel austempered at 475 °C.
All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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