Metallography of AHSS steels with retained austenite

L. Kučerová*1, K. Opatová1, A. Jandová1 1 Regional Technological Institute, University of West Bohemia in Pilsen, Univerzitni 8, 30614, Pilsen, Czech Republic

Advanced grades of high strength steels have been developed in the last decades. Several of them relay on excellent performance of controlled volume of retained austenite during plastic deformation. Retained austenite plays a crucial role in mechanical properties of these steels and therefore its volume fraction, morphology and distribution must be analysed. Two types of these steels are used in this chapter, namely multiphase transformation induced plasticity steel (TRIP) and martensitic steel obtained by quenching and partitioning (QP). Analysed samples were processed by various heat and thermo-mechanical treatments to produce higher variety of final microstructures, possessing around 10-18% of retained austenite. TRIP an QP processed microstructures were characterised by the means of light microscopy, scanning and transmission electron microscopy and X-ray diffraction phase analysis. Distribution of retained austenite was also confirmed by EBSD analysis. Colour etching was used to visualise retained austenite in TRIP steels and the amount was subsequently quantified by image analysis of light micrographs.

Keywords: AHSS steel; retained austenite; metallography; TRIP; QP

1. Introduction

Retained austenite has been recently acknowledged as a vital phase in several low alloyed advanced high strength steel (AHSS) grades. Controlled volume fraction of retained austenite with suitable morphology and distribution in the final microstructure can significantly improve mechanical properties of these steels [1]. The benefit of 5-15 % of stabilized retained austenite in the final microstructure was firstly proven for TRIP (transformation induced plasticity) steels. TRIP steels could be obtained by either thermo-mechanical processing or by heat treatment with intercritical annealing and bainitic hold. TRIP steels are recently mostly processed by rolling [2, 3], however there have also been attempts for bulk processing by for example free forging [4] or rotary spin extrusion [5]. Resulting microstructures are in both cases multiphase and rather complex, consisting of ferrite, bainite, retained austenite [6]. To avoid martensitic transformation during heat or thermo-mechanical treatment, austenite has to be stabilized by chemical composition (high carbon and manganese contents), size effect and surrounding phases [7].

It has been long known that retained austenite effect on mechanical properties of high strength steels is rather complex and sufficient volume fraction of retained austenite does not necessarily guarantee good performance of the steel. Properly stabilized retained postpones the necking and thus increases homogeneous plastic deformation and it also enhances strain hardening by gradual deformation induced transformation to martensite. This has been known as “TRIP effect”. However, when the austenite is over-stabilized it would transform reluctantly or not at all and TRIP effect would not occur in the steel. On the other hand, if the austenite is not stable enough, it tends to transform in early stages of plastic deformation and does not improve strength to ductility balance of the steel either [7, 8]. Morphology, chemical composition and distribution of retained austenite are therefore of the utmost importance when evaluating TRIP steel microstructure. Retained austenite typically forms two morphologies in TRIP steels, occurring either as bulky islands placed between polygonal ferrite grains, or as laths in bainitic areas. Bainite in TRIP steel is commonly carbide-free microstructure consisting of the laths of bainitic ferrite and retained austenite [9]. It has been long agreed that smaller austenitic areas are generally more stable against martensitic transformation and it can be often seen that larger bulky islands transformed into M-A (martensitic-austenitic) constituent during the processing of TRIP steel. This means that original remaining austenite was not stable enough during the cooling and parts of the island already transformed to martensite, producing typical morphology of the island with martensitic core, while the edges remain austenitic to room temperature.

The same benefit of retained austenite can be used also for high strength steels with martensitic matrix prepared by quenching and partitioning (QP). The steel is in this case quenched from soaking temperature to a temperature between martensite start and martensite finish temperature to create controlled ratio of martensite and remaining austenite. In the next step, the material is heated up above martensite start temperature for a partitioning hold, which enables excessive carbon to diffuse into remaining austenite and stabilize it against martensitic transformation during final cooling [10]. In this case, around 15% of retained austenite is always present in the form of very thin films lining martensite laths. The typical thickness of the film is in the region of tens of nanometres. It is therefore very difficult to distinguish these areas using standard metallographic techniques, such as light microscopy and even standard scanning microscopy and diffraction techniques are usually applied to provide reliable data. The original QP processed steels were defined as purely martensitic microstructures with retained austenite produced by dedicated heat treatment [11], however several other sub-grades have been developed in the last decade. Various bainitic morphologies form very often important part of the microstructure of QP [12] steels and QP process has been also used in connection with intercritical annealing

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[13], resulting in significant fraction of proeutectoid ferrite in the final microstructures. QP microstructures have been also produced by thermo-mechanical treatments [14] either with individual or incremental deformations aiming to practical applications for example in hot stamping processing [15, 16] of thin sheets or production of rotary hollow semi-products [17].

2. Materials and processing

Low carbon low alloyed CMnSiNb steel was used for TRIP processing and middle carbon low alloyed 42SiCr steel was chosen for QP processing (Table 1). Both steels represent typical chemical compositions used for each processing method. As both steels should benefit from controlled retained austenite fraction in the final microstructure, both need to be alloyed by manganese and silicon, which help to stabilize retained austenite. TRIP steel is further micro-alloyed by niobium which turned out to have a positive effect on opening the window of processing parameters, microstructure refinement and precipitation hardening of the steel. The steel for QP processing has on the other hand the addition of chromium [18], which increases hardenability and strength of the steel. Higher carbon content supports austenite stabilization in QP steels and makes it easier to achieve fully martensitic matrix. Table1 Chemical composition of used steels CMnSiNb and 42SiCr in weight %, Martensite start (Ms) and finish (Mf) temperatures

C Mn Si P S Cr Nb Ms [°C] Mf [°C] 42SiCr 0.4 0.6 2.0 0.009 0.004 1.3 0.03 298 178 CMnSiNb 0.2 1.5 1.8 0.008 0.005 - 0.06 370 257

Microstructure analysis was performed using light microscopy and image analysis of colour-etched samples by NIS

elements software, scanning electron microscopy (SEM) by Crossbeam Zeiss with Oxford Instruments EBSD (electron back scattered diffraction) detector and transmission electron microscopy (TEM) of extraction carbon replicas and thin foils at Jeol 1200EX. Colour etching was performed in LePera and Klemm etchant (Table 2). Volume fraction of retained austenite was established by X-ray diffraction phase analysis using AXS Bruker D8 Discover with HI-STAR detector and Co lamp (λKα = 0.1790307 nm). Focusing polycapillary lens was used to achieve X-ray spot with 0.5 mm diameter.

All heat and thermo-mechanical treatment was carried out at thermo-mechanical simulator, which enables precise control and repeatability of heat and thermal schedules. Details of the optimisation of processing parameters for TRIP and QP treatment and their effects on microstructure and mechanical properties can be found elsewhere [19-22]. Table 2 Chemical composition of used colour etchants

Etchant Solutions LePera (i) 1% Na2S2O5 in distilled H2O, (ii) 4% (NO2)3C6H2OH (picric acid in ethanol)

Before use mix: 50% (i) + 50%(ii) Klemm (i) 1000 g Na2S2O3·5H2O in 300 ml H2O. Add 2g of K2SO4 to 100 ml of solution (i) before use

2.1 TRIP processing

Heat and thermo-mechanical treatments (TMT) were used to obtain microstructures typical for TRIP steel (Table 3) at CMnSiNb steel. In all three cases, full austenitization was carried out at the beginning of the treatment. Lower heating temperature of 900 °C was chosen to keep the processing cost effective and was followed either by simple heat treatment with the hold in bainite region, at 425 °C, or by thermo-mechanical treatment with two compressive deformations carried out at 900 °C and 720 °C and subsequent bainitic hold at 425 °C.

One TMT processing used high heating temperature of 1200 °C to dissolve niobium particles into austenite. Four deformation steps were subsequently necessary to refine coarse austenite grain during the cooling. Given deformation sizes were always calculated with respect to the actual sample length. First, 25% tensile and 25% compressive deformations were carried out at 1100 °C. The third, 50% tensile deformation was applied at 720°C, and the last 50% compressive deformation was performed at 680 °C, well below recrystallization temperature of the steel. Finally, TRIP processing was also applied to 42SiCr steel, with the same temperature schedule, as in the case of lower heating temperature processing of CMnSiNb steel, only an-isothermal deformation was carried out in temperature interval of 900 - 720 °C during the cooling of the steel. Incremental deformation steps were used in this case, applying small tensile and compressive deformation steps in quick succession, with tensile step being always slightly larger to change the shape of the sample as well.

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Table 3 Processing parameters, mechanical properties and retained austenite volume fraction. Deformation temperatures are given for TMT, in the brackets are numbers of deformations if more than one was carried out at the temperature

Steel TA/tA [°C/s]

Deformations temp. [°C]

Cooling rate [°C/s]

Holds [°C/s]

Rm [MPa]

A [%]

RA [%]

CMnSiNb

900/100 - 14 425/600 803 34 14 900/100 900, 720 14 425/600 859 34 13 1200/3 1100 (2x), 720, 680 3 425/300 1041 15 10

42SiCr

900/100 900-720 (40x) 14 425/600 1700 9 18 900/100 - 5 200/15-250/600 1930 19 19 900/100 900-720 (40x) 14 200/15-250/600 1959 15 16

2.2. QP processing

Heat and thermo-mechanical treatments were also used for 42SiCr steel to obtain microstructures typical for QP processing. Relatively low heating temperature of 900 °C was used again for both samples. Both samples were quenched to 200 °C to obtain austenite-martensite mixture. After 15s at this temperature, the steel was heated up to 250°C, where partitioning hold occurred. During 600 s hold at this temperature, carbon is supposed to diffuse into super-saturated austenite and thus stabilize it against martensitic transformation during final cooling. In TMT schedule, forty incremental deformations were applied to the steel during the cooling from heating temperature of 900 °C to 720 °C. The same set of tensile and compressive deformation steps as in the case of TRIP steel processing of 42SiCr was used in this case as well.

3. Microstructure of TRIP steel

Light and scanning electron microscopy were used to analyse all microstructures of CMnSiNb steel. The samples with lower heating temperature of 900 °C were further characterised by EBSD analysis and LePera and Klemm colour etching, while the microstructure resulting from thermo-mechanical processing with higher heating temperature of 1200°C was studied by transmission electron microscopy of extraction carbon replicas and thin foils. Volume fraction of retained austenite was further measured by X-ray diffraction phase analysis.

Sample for preparation of extraction carbon replicas was mechanically grinded and polished and then electrolytically polished at room temperature and 15 V in Morris solution using Lectropol. Standard 3% Nital etching was carried out in the last step to reveal microstructure.

Thin foils for transmission electron microscopy were cut from the central part of processed sample and mechanically grinded down to around 0.1 mm thickness. Circular discs with 3mm diameters were cut from thin foils and further electrolytically polished by jet method using Tenupol III in HClO4 electrolyte at temperatures around -55 °C and 25 V voltage.

Samples for EBSD analysis were also prepared first by mechanical grinding and polishing and they were finished by electrolytic polishing with 32V, flow rate 10, applied for 14 s to an electrolyte consisting of 500 ml of ethanol, 25 ml of 60% HClO4, and 7.5 ml of HNO3 to obtain deformation free surfaces.

All samples prepared from CMnSiNb steel had typical TRIP microstructures containing ferrite, bainite, retained austenite and M-A constituent (Fig. 1-Fig. 10). Heat treatment resulted in coarser microstructure than thermo-mechanical treatment with the same heating temperature of 900 °C (Fig. 1-Fig. 6). Average ferrite grains size established by image analysis of light micrographs in heat treated steel was 4.8 micrometres compared to 3.1 micrometres in thermo-mechanically processed steel (Table 4). The same trend was confirmed by grain sized established from EBSD analysis. Heat treated steel had average ferrite grain size 2.5 micrometres and retained austenite grain size 1 micrometre, while steel after thermo-mechanical treatment possessed finer ferrite grain size of 1.8 micrometres and retained austenite grain size only 0.8 micrometre. The reason why EBSD analysis determined significantly finer grains for both samples can be attributed to the fact, that grain size count was based on lattice type only. Bainitic ferrite laths were therefore also included in this evaluation, while only proeutectoid ferrite grains were considered for quantitative image analysis. The shape of free ferrite grains was rather irregular in both samples. Bainitic areas in heat treated steel were slightly more compact and were made of more laths, while thermo-mechanical treatment produced more granular bainite in the final microstructure. Finer microstructure was reflected in 50 MPa increase in ultimate tensile strength of TMT sample to 850 MPa. The same ductility 34% was measured for both samples with 900 °C heating temperature.

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Table 4 Retained austenite (RA) fraction and ferrite grain size in TRIP steels measured by various techniques RA phase fraction [%] Ferrite grain size [μm] CMnSiNb TRIP steel Image analysis X-ray diffraction EBSD Image analysis EBSD Heat treatment 12 14 8 4.8 2.5 TMT with 900°C heating 10 13 7 3.1 1.8

It is rather difficult to quantify phase fractions using image analysis of microstructures obtained by standard 3% Nital etching, as free ferrite has the same colour as bainitic ferrite and retained austenite. It is sometimes also difficult to distinguish martensite from retained austenite in M-A constituent. Both colour etchant LePera and Klemm can be successfully used for retained austenite revelation, as both colour all other phases and structural components leaving retained austenite white (Fig. 3 - Fig. 4). Image analysis was applied to these images and it provided very good agreement with X-ray diffraction phase analysis. Above 10% of retained austenite was determined in both samples with heating temperature 900 °C by image analysis and 13% and 14% were established by X-ray diffraction measurement. EBSD analysis confirmed distribution of retained austenite in bainitic areas and also in the form of individual bulky islands (Fig. 5 – Fig. 6). However, this method did not detect sufficient amount of retained austenite in either microstructure and its volume fraction in both samples was around 7.5%. This difference between retained austenite fraction determined by X-ray diffraction and EBSD method was observed also for other TRIP steels [23].

Fig. 1 CMnSiNb steel, TRIP heat treatment, SEM Fig. 2 CMnSiNb steel,TRIP TMP, SEM

Fig. 3 CMnSiNb steel, TRIP heat treatment, colour etching by LePera, light microscopy

Fig. 4 CMnSiNb steel,TRIP thermo-mechanical processing, colour etching by Klemm, light microscopy

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Fig. 5 CMnSiNb steel, TRIP heat treatment, EBSD phase map (FCC-green, BCC -yellow)

Fig.6 CMnSiNb steel, TRIP thermo-mechanical processing, EBSD phase map (FCC-green, BCC-yellow)

Proeutectoid ferrite grains size distribution in the microstructure of TRIP steel with high heating temperature of

1200 °C was very heterogeneous (Fig. 7) in comparison to thermo-mechanically treated steel with lower heating temperature of 900 °C (Fig. 8), with larger pancaked gains surrounded by extremely fine polygonal recrystallized grains with the size around 1.5 micrometres. Higher heating temperature also resulted in slightly coarser bainitic blocks, 41% of bainite and only 10% of retained austenite in the final microstructure, compared to 49% of finer bainite areas in 900 °C heated sample. As can be seen from comparison of the two light micrographs of TRIP steels (Fig. 7 - 8), electrolytic polishing prior to Nital etching made microstructure features more pronounced (Fig. 7). Extraction carbon replicas of this microstructure show rather large islands of M-A constituent (Fig. 9a), thinner lath of retained austenite between adjacent laths of bainitic ferrite and the presence of fine particles placed mainly in ferrite grains (Fig. 9b). Fine bulky island of retained austenite at triple point of surrounding ferrite grains was also detected by transmission electron microscopy of thin foil (Fig. 10). Higher amount of martensite is responsible for higher ultimate tensile strength of 1041 MPa, however it also deteriorated ductility to 15%.

Fig. 7 CMnSiNb steel, TRIP high temperature 1200 °C TMP, electrolytical polishing, 3% Nital, light microscopy

Fig. 8 CMnSiNb steel, TRIP, 900 °C TMP, 3% Nital, light microscopy

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Fig. 9 CMnSiNb steel, TRIP, high temperature 1200 °C TMP, electrolytic polishing, extraction carbon replicas, TEM

Fig. 10 CMnSiNb steel, TRIP, high temperature 1200 °C TMP, thing foil, TEM of bulky retained austenite

Fig. 11 42SiCr, TRIP, TMP with 40 deformation steps, colour etching by Klemm, light microsocpy

TRIP steel processing with incorporated 40 incremental deformations applied at 42SiCr steel resulted in a multiphase

microstructure, which was not very suitable for further utilisation of TRIP effect (Fig. 11). It consisted of bands of martensite distributed between larger bands of bainite-like microstructure. This microstructure consisted of the laths of bainitic ferrite and retained austenite combined with larger islands of M-A constituent or martensite. Klemm etchant was in this case very useful in distinguishing bluish bainitic areas from brown martensitic bands (Fig. 11). Even individual martensitic grains within bainitic areas are coloured brown. X-ray diffraction phase analysis still determined 18% of retained austenite in this microstructure, which was found mostly in the form of bainitic laths. The lack of proeutectoid ferrite is most probably responsible for very low ductility of this steel, which reached only 9%. On the other hand, high amount of martensite resulted in very high strength of 1700 MPa.

4. Microstructure of QP steel

QP microstructures consisting of predominantly martensitic matrix with 16-19% of retained austenite were prepared by heat and thermo-mechanical treatment of 42SiCr steel (Fig. 12 - Fig. 17). The strength of these steels was twice higher, while ductility reached only half of the value obtained in the case of CMnSiNb TRIP steels. Slightly better combination of the high ultimate tensile strength of 1930 MPa and 19% ductility was provided by the microstructure obtained by QP heat treatment. Colour etching did not help to reveal retained austenite in the final microstructures in these steel, as it forms mainly thin films whose size is below resolution limit of light microscopy. Only few small white ferrite islands can be seen in the micrograph (Fig. 12). Even detailed scanning electron microscopy imaging failed to capture retained austenite area (Fig. 13). EBSD analysis was more successful, however only few percent of retained austenite were detected (Fig. 14). It is apparent from quality pattern micrograph (Fig. 15) that larger martensite laths are surrounded by lath-like areas consisting of a network of much finer sub-grains. These fine areas had the highest amount of zero solution, which correspond to the points, where lattice parameters were not identified and are shown in the phase map by black spots (Fig. 13). The phase map of heat treated QP microstructure also proves that most of the detected retained

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austenite was found exactly in these areas of fine sub-grains. Localisation of very thin films of retained austenite along martensitic laths is also supported by evidence of extraction carbon replicas (Fig. 16).

Fig. 12 42SiCr steel, QP, TMP, colour etching by Klemm, light microscopy

Fig. 13 42SiCr steel, QP, heat treatment, SEM

Fig. 14 42SiCr steel, QP, heat treatment, EBSD phase map Fig. 15 42SiCr steel, QP, heat treatment, band contrast, SEM

Fig. 16 42SiCr steel, QP, heat treatment, extraction carbon replica, TEM

Fig. 17 42SiCr steel, QP, TMT, extraction carbon replica, TEM

Very fine carbides were also detected in the microstructure of heat treated QP steel inside individual bainitic laths

(Fig. 13). Silicon alloying is known to suppress cementite formation during partitioning hold; however it has been also demonstrated that it does not hinder as efficiently precipitation of epsilon carbides and their presence in the final microstructure is therefore not uncommon [24]. Coarser carbide needles were also observed at extraction carbon replicas of the microstructure after TMP (Fig. 17). Extremely fine grains were also found at martensite lath boundaries

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and according to the results of EBSD analysis it can be assumed, that significant amount of retained austenite will be hidden in these areas.

5. Conclusions

Microstructures typical for two AHSS grades containing retained austenite were prepared by heat and thermo-mechanical treatment of CMnSiNb steel and 42SiCr steel. Processing methods resulting in TRIP and QP steels were used. All the microstructures possessed 10-19% of retained austenite, which was measured by X-ray diffraction phase analysis. The amount, morphology and distribution of retained austenite were observed by light microscopy of colour etched samples, detailed scanning electron microscopy and EBSD analysis, transmission electron microscopy of extraction carbon replicas and thin foils. QP steel matrix was predominantly martensitic with occasional occurrence of lower bainite. TRIP steel microstructures were rather complex consisting of free proeutectoid ferrite, carbide-free bainite, retained austenite and M-A constituent.

Colour etching by LePera and Klemm was successfully applied to TRIP steels, tinting all phases and structural components in dark colours, except of retained austenite, which remained white. Image analysis performed at etched micrographs calculated very similar amounts of retained austenite, as X-ray diffraction phase analysis. Due to the typical size of retained austenite in QP steels, colour etching did not help in visualisation of retained austenite.

EBSD analysis of TRIP steels was able to detect about half of the retained austenite that was calculated from X-ray diffraction phase analysis. EBSD evaluation of QP processed steel determined only very low volume fractions of retained austenite compared with X-ray diffraction. Even though EBSD analysis did not detect the same amount of retained austenite as X-ray diffraction analysis, it offered valuable detailed information about morphology and distribution of retained austenite in both types of steels in relatively large areas of microstructure, which would be difficult to obtain by other metallographic methods.

Acknowledgement The present contribution has been prepared under project LO1502 ‘Development of the Regional Technological Institute‘ under the auspices of the National Sustainability Programme I of the Ministry of Education of the Czech Republic aimed to support research, experimental development and innovation.

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