development of high mn–n duplex stainless steel for automobile structural components

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www.elsevier.com/locate/corsci

Corrosion Science xxx (2007) xxx–xxx

Development of high Mn–N duplex stainless steel forautomobile structural components

Ihsan-ul-Haq Toor, Park Jung Hyun, Hyuk Sang Kwon *

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST),

373-1 Guseong-dong, Yuseong-gu, Daejon 305-701, Republic of Korea

Received 24 November 2006

Abstract

A new high Mn–Ni free (duplex stainless steel) DSS containing 18Cr–6Mn–1Mo–0.2N has been developed by examining the effects ofmanganese on the corrosion and mechanical properties of high Mn SSs containing 18Cr–4 � 11Mn–0 � 2Ni–0 � 1Mo–0.2N. The alloywith 45% ferrite is found to be an optimum alloy with much higher mechanical strength and similar corrosion resistance compared withthose of standard SS304. In addition, the alloy was free of precipitation of sigma phase and Cr-nitride when exposed to high tempera-tures due primarily to relatively low contents of Cr, N and Mo. With an increase in Mn content, the resistance to pitting and metastablepitting corrosion of high Mn DSS decreased since the number of (Mn, Cr) oxides, acting as preferential sites of pitting, increased with theMn content.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Duplex stainless steel; A. Manganese; B. ENA; B. Thermocalc; C. pitting corrosion

1. Introduction

Austenitic stainless steels such as 310 and 304 are attrac-tive as structural materials for load frame materials inautomobile and transportation industries because of theirhigh strength, toughness, formability and corrosion resis-tance which make automobiles lighter and corrosion resis-tant. Additionally, they are environmentally friendlymaterial that is easy to recycle. However due to their highcost ( because of high Ni content, 8–12 wt.%), these steelshave been limited to applications where they would other-wise be an ideal choice, so attempts are being made todevelop cheap stainless steels while still maintaining rela-tively high corrosion resistance. For such a reason, highMn–N austenitic stainless steels replaced expensive Ni withlow cost Mn and are actively being developed by many spe-cial steel companies like AK steel, Carpenter, Allegheny,POSCO. These steels contain 6–11 wt.% Mn (c-stabilizer)

0010-938X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2007.07.004

* Corresponding author. Tel.: +82 42 869 3326; fax: +82 42 869 3310.E-mail address: [email protected] (H.S. Kwon).

Please cite this article in press as: I.-u.H. Toor et al., Developmentdoi:10.1016/j.corsci.2007.07.004

which is 7 to 8 times cheaper than Ni at an equivalentweight [1–6]. But Mn is not as effective c-stabilizer as Ni,and this problem can be solved by combining Mn with Nin these stainless steel alloys. Nitrogen, in addition of func-tioning as an austenite stabilizer, also improves resistanceto localized corrosion, and has several other benefits suchas increased strengthening and retardation of sensitization[7–12]. The solubility of nitrogen in stainless steels increaseswith an addition of Mn in stainless steels. The addition ofMn to stainless steels, however, decreases the corrosionresistance significantly [4], and this problem can be mini-mized by reducing the Mn content by changing the struc-ture of stainless steel from austenite to duplex (50%ferrite +50% austenite) phases. The industrial use of duplexstainless steel is rapidly increasing due to the combinedadvantages of better mechanical and corrosion properties.The second generation duplex stainless steels with higherlevels of Cr, Mo and N have an increased localized corro-sion resistance and mechanical properties [13–18]. Accord-ing to Lunarska et al. [19], Mn containing alloys areextremely sensitive to the presence of minor constituents

of high Mn–N duplex stainless steel for ..., Corros. Sci. (2007),

mailto:[email protected]

2 Ihsan-ul-Haq Toor et al. / Corrosion Science xxx (2007) xxx–xxx

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such as C, N, S, P and due to this sensitivity, different con-troversial effects of manganese on the pitting corrosionresistance are reported. Hsaio et al. [20] reported that nitro-gen along with manganese improves the strength andtoughness of the stainless steels. To develop the high Mnand N duplex stainless steel, it is essential to examine thecorrosion and mechanical properties of the alloys that areinfluenced by the alloying elements such as Cr, Mn, Moand N.

The objective of this study is to develop a new high Mnand N duplex stainless steel with good corrosion andmechanical properties by examining the effects of alloyingelements on the structure, corrosion and mechanical prop-erties and precipitation of intermetallic compounds such assigma phase and Cr2N.

2. Experimental procedures

Three alloys designated as Alloy 1, Alloy 2, and Alloy 3,were designed to have their alloy compositions as shown inTable 1a. The Ni and Cr equivalents calculated with thehelp of Schaeffler diagram suggested, that Alloy 1 andAlloy 2 have a duplex structure composed of ferrite andaustenite while the third one has an austenitic structure[21]. The melting of the alloys was carried out in a vacuuminduction furnace, and the ingots obtained were hot rolledto 6 mm thick plates. The hot rolled plates were solutionannealed, water quenched, cold rolled (35%) and subse-quently solution annealed and cold rolled again. To getthe desired microstructure of 55% austenite and 45% fer-rite, three alloys were solution annealed at 1050 �C,1120 �C, 1020 �C, respectively as shown in Fig. 1. Thesesolution annealing temperatures were calculated using thecommercial Thermocalc software [22].

The specimens were spot welded; mounted, polished upto 2000 grit emery paper and sealed in silicon tape for elec-trochemical analysis. A three electrode cell (a specimen as aworking electrode, a Pt counter electrode and a saturatedcalomel reference electrode) was used for the tests. Thespecimens were cathodically cleaned for 5 min at�0.8 VSCE to remove the primary oxide film. All the elec-trode potentials are referred to the SCE scale.

Potentiodynamic polarization tests were performedaccording to ASTM G 5 at a scan rate of 0.5 mV/s, andmetastable pitting events density was determined in termsof electrochemical noise analysis (ENA) at 23.5 �C in0.5% NaCl solution under potentiostatic condition [23–27]. The solution was deaerated for 2 h before the experi-ment started and then continuous deaeration wasperformed throughout the experiment. The specimens withexposed surface area of 0.22 cm2 were used for polarizationtests and electrochemical noise analysis (ENA). To mini-mize the external noise during ENA, experiments were per-formed in a Faraday cage. According to Burstein [24], Epit

is notoriously irreproducible, and sensitive to specimensize, so experiments were performed 3–5 times to determineEpit under exactly similar experimental conditions. The


critical pitting temperature of the alloys was measuredaccording to ASTM G48A.

Subsize tensile specimens (ASTM E8) were used todetermine the mechanical properties such as YS, UTSand elongation of the alloys. An Instron tensile testingmachine (model 4206, load capacity 150 kN) was used forthese mechanical tests under following operating parame-ters (strain rate = 1 mm/min, load = 30 kN, temp. =25 �C, humidity = 50%).

3. Results and discussion

3.1. Microstructure

Fig. 2 shows the optical microstructure of alloys 1, 2, 3after being electrochemically etched for 5–10 s at 5 V in20 wt.% KOH solutions. The ferrite fraction of the alloys1, 2, and 3 was measured by Feritscope and also calculatedusing the Thermocalc [22], and is presented in Table 1b.The Thermocalc software performs standard equilibriumcalculations and calculation of thermodynamic quantitiesbased on thermodynamic databases, and Feritscope, onthe other hand, through the use of a plug-in Smart probe,provides quick and precise, non-destructive, on-site mea-surement of the ferrite fraction of the specimens. Table1b shows that the ferrite content of Alloys 1and 2 is about45%, and that of Alloy 3 is 10%. It was found that the con-tents of Cr and Mo are higher in the alpha phase comparedwith those in the bulk, while Mn content is higher in aus-tenite than in the bulk, which is due primarily to the factthat Cr and Mo are ferrite stabilizers while Mn is an aus-tenite stabilizer. Although there is some controversy onthe effects of manganese on the microstructure of stainlesssteels, manganese has traditionally been accepted as anaustenite former [4]. There are some reports, however,showing manganese as a ferrite forming element [28,29].

3.2. Resistance to pitting corrosion

Fig. 3 shows the polarization response of the designedalloys in deaerated 0.5% NaCl solution at 23 � C along withstandard SS304 for comparison. The results showed thatAlloy 1, Alloy 2, and Alloy 3 have lower values of icorr thanSS304, while SS304 has higher Ecorr than rest of the alloys,due primarily to its high Ni content. Table 1c shows thatpitting susceptibility of high Mn DSS alloys was increasedwhen compared with the value of Epit in the order of Alloy2 (4% Mn) > Alloy 1 (6% Mn) > Alloy 3 (11% Mn). TheEpit of the Alloy 3 was lowest among the three alloys dueprobably to its highest Mn content, i.e. the increase inMn content of the alloys decreased the resistance to pittingcorrosion. The resistance to pitting corrosion is also associ-ated with nonmetallic inclusions present in the alloys.BS-SEM images of Alloys 1, 2, 3 in Fig. 4 show that thenumber and size of nonmetallic inclusions increased withthe Mn content of the alloys. These inclusions, which con-firmed to be (Mn, Cr) oxides, may act as pitting initiation


Table 1Table showing the chemical composition in wt.% and properties of the designed high Mn duplex stainless steel alloys

Alloy designation Fe Cr Mn Mo Ni N S C

(a) Chemical composition in wt.% of the three high Mn DSS alloys

Alloy 1 Bal. 18.08 5.77 1.01 0 0.19 0.005 <0.03Alloy 2 Bal. 18.01 4.0 1.01 1.03 0.19 0.002 <0.03Alloy 3 Bal. 17.88 10.75 1.04 1.99 0.22 0.005 <0.03

Thermocalc (a content (%)) Feritscope (a content (%))

(b) Comparison of Ferite content calculated and measured by Thermocalc and Feritscope

Alloy 1 45 45.7Alloy 2 45 57.8Alloy 3 10 15.6

# Solution annealed 304

Alloy 1 Alloy 2 Alloy 3

(c) The va1ues of Ecorr, Epit and icorr determined from polarization test in 0.5solution at 23.5 �C

Ecorr (mVSCE) �160 �150 �140 �110Epit (mVSCE) 310 400 270 330icorr (A/cm2) 1.40 � 10�s 1.0 � 10�s 1.6 � 10�s 1.2 � 10�s

Solution annealed


(d) The metastable pitting events density of the thee alloys calculated from Fig. 5 by electrochemical noise analysis

Metastable events/cm2 37.74 22.72 Stable pitting corrosion

Alloy 1 Alloy 2 Alloy 3 304 SS

(e) Critical pitting temperature measured according to ASTM G48A in FeCl3 � 6H2O

0 �C No No No No2.5 �C No No No No5 �C No No No No7.5 �C No No No No10 �C No No Pitting No12.5 �C Pitting No Pitting15 �C Pitting

Solution annealed Carbon stee1 (AISI 1005) 304 SS


(f) Tensile test results of the alloys in an Instron tensile testing machine

V.S. (MPa) 426.9 466.4 397.2 284.7 290.0UTS (MPa) 947.4 1029.7 770.2 376.3 663.1Elongation (%) 35.0 24.0 71.2 38.8 65.4

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Fig. 1. Thermocalc calculated equilibrium fractions of each phase versustemperature for Alloy 1.

Fig. 2. Optical microstructures of the alloys 1, 2, 3 after being annealedfor 1 h at (1050, 1120, 1020) �C, respectively.

1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01-400

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Fig. 3. Potentiodynamic polarization behaviour of the alloys in deaerated0.5% NaCl solution at 23.5 �C along with standard SS304.


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sites, thereby reducing the resistance to pitting corrosion asconfirmed by the decrease in Epit. According to G.T. Bur-stein [24] pitting potentials are notoriously irreproducible,so the potentiodynamic polarization experiments were per-


formed 3–5 times to measure the Epit values and a variationof ±20 � 40 mV was found (Table 1c). Fig. 3 shows thepolarization curves close to the average polarization behav-iour of the designed alloys.

It is significant that many current peaks were appearedin the anodic polarization of alloys 1, 2, and 3 even beforeattaining pitting potential of each alloy, as shown in Fig. 3.These peaks are associated with events of metastable pit-ting. Analysis of current noise generated during metastablepitting has been proved very successful to study the forma-tion, growth and repassivation of a microscopic pit. Previ-ous work [25–27] has showed that growth of corrosion pitsoccurred in two consecutive stages, characterized by meta-stable growth in the early period, followed by stablegrowth. The metastable pitting events of the alloys areshown as current peaks in current transient curves of thealloys under a potentiostatic condition as shown inFig. 5. Each current spike means an initiation and repass-ivation of a single metastable pitting event. The numberof current spikes per unit area was defined as metastablepitting event density, and a high value of metastable pittingevents density means high susceptibility to metastable pit-ting. It is clear from Fig. 5 and Table 1d, that an increasein Mn content of the alloys increased the metastable pittingevent density, due probably to the increase in the numberof (Mn, Cr) oxide inclusions [30–32]. From the aboveresults, it is concluded that Mn content is an important fac-tor in determining the resistance to pitting corrosion. Theseresults are in agreement with those obtained previously,which showed that an increase in manganese content ofthe alloys decreased the general corrosion resistance andpitting potential of the alloys [4,33,34].

The CPT of the alloys was measured according toASTM G48A in 10 wt.% FeCl3 � 6H2O solution. The solu-tion temperature was increased by 2.5 �C/24 h from 0 �C–15 �C . The CPT results for alloys 1, 2, 3 and SS304 inTable 1e showed that pitting corrosion occurred at 10–15 �C. Especially, the CPT of Alloy 1 was 12.5 �C that isequivalent to that of SS304. These results also show thatas Mn content is increased, the CPT value is decreased in


Fig. 4. Back scattered SEM images of alloys 1, 2, 3, showing the inclusions (Mn–Cr oxide).

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the following order; alloy 2 (4% Mn) > Alloy 1 (6%Mn) > Alloy 3 (11% Mn).

3.3. Mechanical test

The mechanical properties of the designed alloys weremeasured by tensile tests, and compared with those ofSS304 and carbon steel (AISI 1005) as shown in Table 1f.The results show that elongation of SS304 and Alloy 3(both having austenitic FCC structure) is higher than Alloy1, 2, while the YS & UTS of alloys 1, 2 (duplex alloys) wasmuch greater than that of SS304 as well as that of Alloy 3.These results suggest a clear relationship between tensileproperties and ferrite fraction of the alloys. Alloy 2 withhighest ferrite fraction (57.8%) has exhibited the highestmechanical strength but worst elongation (24%). Alloy 3with lowest ferrite fraction (15.6%), showed the lowestmechanical strength and best elongation. Alloy 1 having45.7% ferrite, showed optimum mechanical propertiescombined with high mechanical strength (YS = 426 MPa,UTS = 947.4 MPa) and a reasonable elongation (35%).These results spotlight the importance of control over theferrite fraction in duplex stainless steels to achieve the opti-mum mechanical properties.

3.4. Analysis of secondary phases in alloy 1

Alloy 1 with optimum mechanical properties was selectedfor further tests to determine the precipitation behaviour ofsecondary phases (nitrides, sigma phase etc) which maydeteriorate mechanical and corrosion properties in theduplex stainless steels [35–37]. Fig. 6 shows the equilibrium


fractions of each phase versus temperature for Alloy 1 thatwas derived using the Thermocalc program. To analyzethe precipitation behaviour, Alloy 1 was aged for 3 h at800 �C, 700 �C & 600 �C, respectively. XRD analysisshowed that sigma phase was not precipitated at these tem-peratures as shown in Fig. 7. For further confirmation of theprecipitation of sigma phase, specimens of alloy 1 were agedfor 50 & 100 h at 600 �C and 800 �C. The former tempera-ture (600 �C) is that at which Thermocalc showed 20% pre-cipitation of sigma phase in Fig. 6, and the later temperature(800 �C) was selected because it is widely known to be thetemperature at which the sigma phase precipitates relativelyfast in many duplex stainless steels. However the sigmaphase was not observed in these specimens. The absence ofsigma phase in alloy 1 is due to low amounts of Cr andMo that contribute to the formation of sigma phase.

Precipitation of Cr2 N has bee reported to have detri-mental effects on impact toughness and corrosion proper-ties of stainless steels [38]. The Thermocalc calculation inFig. 6 showed precipitation of Cr2N around 900 �C, soAlloy 1 was aged at (900, 800, 700) �C for 3 h and thenwas examined by X-ray diffraction technique. The XRDpatterns in Fig. 8 showed that no nitride phase was presentin this alloy at these temperatures, which lead to the con-clusion that the absence of nitride phase is due to theappropriate content of nitrogen (0.2%) in this alloy. Previ-ous studies by Merello [1] and others [39,40] also showedthat nitrogen retards the formation of harmful intermetal-lic phases, such as (r, v, etc,). On the other hand anincrease in the N content promotes the formation ofnitrides in Fe–Cr stainless steels [41], so only a proper bal-ance of these two elements in an alloy system can avoid the


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Fig. 5. Current time transients showing metastable pitting events of thealloys at 200 mVSCE in 0.5% NaCl solution at 23.5 �C.

Fig. 6. Equilibrium fractions of each phase versus temperature for Alloy 1derived using Thermocalc program.

Fig. 7. XRD patterns of Alloy 1 aged for 3 hours at (600, 700, 800) �C.

Fig. 8. XRD patterns of the Alloy 1 aged for 3 h at (700, 800, 900) �C.


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formation of nitride, and this was the case in this newlydeveloped DSS alloy.

4. Conclusions

1. With an increase in Mn content of the high Mn DSSalloys, the resistance to pitting corrosion was decreased,as confirmed by the decrease in Epit or by the increase inmetastable pitting event density.

2. With an increase in ferrite content of the high Mn DSSalloys, mechanical strength of the alloys was increasedwhereas their elongation was decreased. Alloy 1 having45% ferrite showed the best combination of yieldstrength (427 MPa) and elongation (35%).

3. XRD results showed that alloy 1 was free of sigma phaseprecipitation even when aged for 100 h at 600 �C–800 �C, and also free of Cr-nitride precipitation whenaged for 3 h at 700 �C–900 �C. This was due primarilyto relatively low contents of Cr, N and Mo that contrib-ute to the formation of sigma phase and Cr-nitride.

4. Among the designed DSS alloys, Alloy 1 cotaining18Cr–6Mn–1Mo–0.2N is found to be an optimum alloywith higher mechanical strength and similar corrosionresistance compared with those of standard SS304, atan economical cost.


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Acknowledgements

The authors gratefully acknowledge the financial sup-port from NGV. This work was also partially supportedby Brain Korea 21 Project.

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development of high mn–n duplex stainless steel for automobile structural components

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