AUTHOR: Christian KlinkenbergNiobium Products Company GmbH

Microalloyed steel has become a standard materialfor line pipe, automotive and construction use.

Since the end of the 1960s the development of thesesteel grades has resulted in ever-increasing strength levelscombined with excellent toughness and weldingproperties. However, until recently, the potential ofmicroalloyed high-strength steel has not been used to thesame extent in structural hot-rolled steel sections or inforged engineering steels. However, customers and steelproducers are showing growing interest in this proventechnology and this paper provides the metallurgicalbackground and examples for the use of niobiummicroalloyed steel in hot-rolled beams, rebar and forgings.

Niobium has a threefold influence on the mechanicalproperties of steel: grain size refinement duringthermomechanical hot forming, lowering the γ to αtransition temperature (Ar3), and precipitation hardening.Grain refinement is the only mechanism thatsimultaneously increases strength, toughness and ductility.This makes Nb the most effective microalloying element,even if added in small quantities, as shown in Figure 1.

The grain refining effect of Nb is due mainly todelaying or preventing recrystallisation in the last hotforming steps. Flattened grains and a high dislocationdensity of the austenite enhance ferrite nucleation. Bylowering the γ to α transformation temperature, Nbsimultaneously enhances the ferrite nucleation rate andreduces the grain growth rate, and the combined effectleads to a particularly fine-grained transformationstructure. In order to make optimum use of itsmetallurgical potential Nb has to be in solid solution byheating at an adequate reheating furnace temperatureto dissolve Nb(C,N) precipitates before hot forming. Thesolubility of Nb(C,N) is given in Figure 2.

BEAMSH beams are taken as an example of hot-rolled sections,as they represent a large part of this product group.

When compared to flatproducts, due to the greaterloads involved in producingthe complex shape, beamsrequire higher rollingtemperatures where the flowstress is lower. In addition,deformation is rather in-homogeneous, ranging fromabout 4% in the web toflange transition area, to50–80% in the web and flanges. Therefore,thermomechanical treat-ment, applying well-definedtemperatures and hot-forming schedules is often difficult to realise. Hot rollingis usually performed in the recrystallisation-controlledrolling (RCR) mode in order to obtain a homogeneousfine grain size and high strength in the final product.

Beams are usually rolled in the upper austenite regionafter reheating to 1,250°C in order to benefit from the

The use of Niobium as a microalloying element is particularly beneficial in increasing strength andtoughness through its ability to control austenite grain size during reheating, and via grainrefinement and precipitation hardening after austenite transformation. Steels with lower carboncontents can thus be utilised, further enhancing toughness and weldability.

Niobium in microalloyed structural and engineering steels

FORMING PROCESSES

a

163

MIL

LEN

NIU

M S

TEEL

2006

r Fig.1 Contribution of microalloying elements to toughness andstrength of 0.08%C, 0.90% Mn steel

r Fig.2Solubility ofNb(C,N)

5.3 MS06-11 pp163-167 4/6/06 11:54 am Page 163

MIL

LEN

NIU

M S

TEEL

2006

164

austenite to ferrite phase transformation and via precipi-tation hardening in ferrite. A lower phasetransformation temperature results in finer ferrite grainsand bainite formation during air cooling. Laboratorysimulation has shown that a deformation of 15% perpass is sufficient to achieve an average grain sizediameter of 16μm (ASTM 9). For a similar rollingschedule the C-Mn steel without Nb has an averagegrain diameter of 32μm (ASTM 7). The typicalmicrostructure of the Nb microalloyed steel consists of60% ferrite and the rest being bainite and somepearlite.

The contribution of different microalloying elementsto precipitation hardening can be taken from Figure 1,highlighting that Nb is the most effective alloyingelement even when added in small quantities.

With higher strength steels and/or flange thicknessabove 20mm, hot rolling in the upper austenite regionis not sufficient to meet the specified strength. Inaddition, beams may have to match toughnessrequirements at -20°C or even lower temperatures, suchas for offshore application. Further grain refinement isthe most effective method to fulfill these requirementsand can be achieved by lower finish rollingtemperatures.

Figure 3 gives the grain sizes and toughness (in termsof Charpy impact energy) in the flange for variousdistances from the web after conventional hot rollingapplying a 1,040°C FRT and for the so-called HighTemperature Process (HTP) concept with an FRT of960°C [1]. For HTP hot rolling start temperature was1,080°C as in conventional processing then after the5th pass, a delay time of around 90 seconds wasintroduced in order to allow the hottest parts of thebeam to cool down below 1,000°C. The last two passeswere performed with a finish rolling temperature of≤960°C. This hot forming below the austeniterecrystallisation stop temperature leads to the finemicrostructure of the HTP beam. The chemicalcomposition is given in Table 1. The mechanicalproperties given in Table 2 correspond to the web toflange transition area, ie, the most critical position.

REINFORCING BAR (REBAR)Formable and weldable high strength rebar is producedby fast cooling after rolling or by microalloying.

Steel Flange Yield Tensile Elongation Impactthickness strength strength A5 energy ave

[mm] [MPa] [MPa] [%] [J/cm2]Nb 11 389 540 32.2 166 (room temp)HTP 40 425 510 32 220 (-20°C)

r Table 2Mechanicalproperties

of grade 50 (S 355) in the

web to flangetransition

area in the hot rolledcondition

Steel C Si Mn P S Al N Nb Ti Ti/NC-Mn 0.140 0.012 0.7 0.029 0.012 0.030 0.009 – – –Nb 0.079 0.20 1.5 0.019 0.014 0.027 0.009 0.02 0.016 1.78HTP 0.060 0.43 1.6 0.018 0.016 0.032 0.008 0.14 – –

r Table 1 Chemical composition of grade 50 (S 355) in wt.%

reduced flow stress and rolling force, consequently staticrecrystallisation takes place after each pass. The finalgrain size depends on the initial austenite grain size,which can be controlled by fine TiN precipitates. Thebest results have been achieved by using an under-stoichiometric Ti/N ratio plus Nb. Table 1 shows thechemical compositions of grade 50 (S355) steelsaccording to different concepts. The N content reflectssteel production by the EAF route.

For the Nb grade in Table 1, with the elevated finishrolling temperature (FRT) of 1,050–1,080°C, Nb willstay in solid solution and adds strength by delaying the

r Fig.3 Variation of grain size and toughness ofwide flange beam (Re=425MPa)

5.3 MS06-11 pp163-167 4/6/06 11:55 am Page 164

165

MIL

LEN

NIU

M S

TEEL

2006

FORMING PROCESSES

Vanadium has so far typically been used for thispurpose, but given the high price of vanadium, use ofNb is being reconsidered.

Use of low carbon content and Nb alloying provide agood combination of high strength, excellent ductility,toughness and weldability. Earthquake resistance andlow temperature properties for cryogenic applicationssuch as liquid natural gas (LNG) storage tanks (-100 to-125°C) are particularly improved by use of Nbmetallurgy.

Conventional production applies air cooling after hotrolling, which is typically completed at 1,000–1,050°Cand strength is mainly controlled by the C and Mncontents. Lowering the C content will improvetoughness while the associated loss in strength can becompensated for by microalloying. Figure 4 gives theyield strength for different Nb and V additions in air-cooled 0.18% carbon steel having a ferrite-pearlitemicrostructure [2]. About 0.03% Nb will generate a

yield strength of about 480MPa. For the same effect,more than 0.06% V will be necessary. Nb will also boostthe effect of V when used in combination.

European specifications limit the carbon content to≤0.24% and the carbon equivalent CE to ≤0.52.Therefore, reinforcing bar producers generally applywater cooling followed by self-tempering after hotrolling, as in the TEMPCORE® process. In such a process,microalloying is limited to high strength or larger bardiameter application where about 0.03% Nb willgenerate 600–650MPa minimum yield strength.

ENGINEERING STEELSEngineering steels comprise forging steel grades such asfor case hardening, quench and tempering (QT) as wellas for cold heading, springs and roller bearings.Although engineering steels usually have an elevatedcarbon content, the basic principles of microalloyingalso apply to this steel group and will be explainedusing the example of case carburising steel.

Automotive components such as transmission gearshave to meet complex requirements. The surface offorged parts is often carburised and quenched to add ahard surface to a tough core, combining excellent wearresistance and fatigue strength. The conventionalcarburisation process applies austenitising temperaturesbetween 880 and 930°C giving a carburisation depthof about 2mm after 20 hours. Increasing the carburisingtemperature to 1,050–1,100°C leads to an almost 60%reduction of the processing time. However, theseelevated temperatures will result in considerableaustenite grain growth. Table 3 gives an example of atypical carburising steel grade, 16MnCr4, as specified inthe European standard EN10084, as well as differentmicroalloyed variants. The specification allows theaddition of microalloying elements by agreement withthe customer.

The base steel plus Al, N and Nb shows grain sizestabilisation up to a simulated carburising temperatureof 1,075°C (Figure 5), relying on the combined action of a

Steel C Si Mn Ni Cr Mo Al N Ti NbBase 0.18 0.22 1.05 0.14 0.92 0.06 0.033 0.008 – –(16MnCr5)Base + Nb 0.16 0.28 1.12 0.12 1.01 0.03 0.022 0.013 – 0.032

Base + 0.19 0.23 0.75 – 1.19 0.02 0.040 0.026 – 0.045Al, N, NbBase + 0.17 0.30 1.35 0.15 1.12 0.04 0.016 0.019 0.016 0.055N, Ti, Nb

[compositions in wt.%]

r Table 3 Chemical composition of carburising steel grades

r Fig.4 Yield strength of microalloyed ferrite-pearlite rebar steel

5.3 MS06-11 pp163-167 4/6/06 11:55 am Page 165

MIL

LEN

NIU

M S

TEEL

2006

166

Nb(C,N) and AlN [3]. The AlN particles prevent graingrowth until they become ineffective by Ostwaldripening at about 1,075°C. The formation of coarse Nbnitrides at high temperature will not effectively preventgrain growth once the AlN particles have dissolved. Incontrast, fine Nb precipitates significantly reduceaustenite grain growth during high temperatureannealing. The 16MnCr5 base steel plus Nb containsfine Nb(C,N) precipitates that are most effective in grain

size stabilisation. This is illustrated in Figure 5, whichalso shows the benefits of a combined Nb and Tiaddition to the base steel composition, where fine TiNparticles act as a nucleus for Nb precipitation.

Microalloyed ferritic-pearlitic steels Ferritic-pearlitic steel grades with precipitation hardening fromthe hot working temperature have been developed onthe basis of microalloyed low and medium carbon steels.These steels are controlled cooled after forging andhave partially replaced quench and tempered (QT)steels. Benefits have been experienced throughelimination of heat treatment, reduced distortion,improved machinability and more consistent properties.The European standard EN 10267 specifies thetechnical delivery conditions. Vanadium is typically usedto provide strength via precipitation hardening,however, EN 10267 allows the substitution of V by Nb.As demonstrated in Figure 1, the addition of Nb willgenerate comparable increase in strength by aconsiderably reduced addition of alloying element.Figure 2 shows that reheating before forging to about1,250°C brings the Nb into solid solution.

The combined addition of Nb and Ti controls theaustenite grain size during austenitising. Besidesincreased strength and toughness, a fine homogeneousmicrostructure offers improved fatigue resistance. Asfatigue crack propagation takes place along the softferrite network on the former austenite grainboundaries, lowering the austenite grain size andavoiding inner-granular ferrite nucleation will help toovercome this problem. Raising the silicon contentenhances inner-granular ferrite nucleation.

Dispersion hardening reduces the toughness of theforged part. This can be compensated for by a smallergrain size and reduced carbon, and thus pearlite,content. In the course of an optimisation project inChina [4], toughness was increased by reducing the Ccontent, thus increasing the amount of ferrite from 19to 54%. The resulting decrease in strength wascompensated for by addition of 0.04% Nb to theexisting 0.10% V content. Almost 100% of the Nb wasfound as precipitates. Furthermore, the quantity ofprecipitated V was increased from 40 to 65%. Thus theaddition of 0.04% Nb doubled the volume fraction ofprecipitates.

The development of microalloyed ferritic-pearliticsteels took advantage of the triple role of Nb, namelygrain refining, reducing pearlite interlamellar spacing bylowering the γ to α transition temperature andprecipitation strengthening. Table 4 gives the exampleof the NbV microalloyed Metasafe (trade name) steelgrades that have been developed in France [5].

Tensile C Si Mn Nb V Cu Ni Applicationstrength [MPa]

800 0.20 0.3 1.8 0.07 0.06 - - Suspension1,000 0.45 0.3 1.5 0.04 0.12 - - Wheel hub1,200 0.22 0.6 1.7 0.06 0.07 1.6 1.2 Axle part

[compositions in wt.%]

Reference BHS [6] IMAFORM [7] KOBE [8]Composition [mass-%]C 0.10 0.06 0.12Mn 1.0 0.9 2.0Mo 0.6 – 0.25Cr 0.3–0.5 1.1 1.0Nb 0.05 0.04 0.12[ppm] B – 50 20 Tensile strength 950 950 1,250[MPa]CVN impact 90 60 75energy [J]

r Table 4 Examples of Metasafe grades

r Table 5 Direct quenched martensitic forgings steels (US, Europeanand Japanese automotive applications)

FORMING PROCESSES

r Fig.5 Effect of case hardening temperature on grain size(Austenitised for 2hr 47mins then oil quenched)

5.3 MS06-11 pp163-167 4/6/06 11:55 am Page 166

Conventional forging is performed well above 1,000°C.However, Nb is most effective in refining the finalmicrostructure by delaying austenite recrystal-lisationduring hot forming in the lower or metastable austeniteregion. The addition of 0.04–0.08% Nb and 0.1% V to aCMn steel (0.30% C, 1% Mn, 0.6% Cr, and 0.007% S)resulted in an excellent combination of toughness(Charpy V notch =135J/cm2 at room temperature) andstrength (TS=960 MPa) after controlled forging at 900°C.

Low carbon martensitic forgings Direct quenching(DQ) after forging is an economic technology where themartensitic microstructure is allowed to auto temperwithout any further heat treatment. Martensiteformation is enhanced by elements like Mn, Mo, Cr andB, but also Nb. The fine-grained auto temperedmartensitic steels display high yield strength(945–1225MPa) coupled with excellent toughness.Some examples of DQ steel grades are given in Table 5.

SUMMARYThe broad experience of Nb metallurgy in flat products canbe extended to structural and engineering steel grades.Rising requirements for fatigue strength, toughness andweldability can be met by reduced C content with theresulting decrease in strength compensated by theaddition of a small amount of Nb. MS

[1] K Hulka, H-G Hillenbrand, F Heisterkamp, and K ANiederhoff, Microalloying 1995, ISS publ., pp235–248.[2] H Sekine et al, Tetsu to Hagane, 56, 5, p569 (1970).[3] W Bleck, G Pariser, S Trute, and C Klinkenberg, Proc.of THERMEC 2003, Ed: T Chandra, J M Torralba, and TSakai, Madrid (ES), Mat. Sci. Forum Vols. 426–432, TransTech Publications, pp1,201–1,206 (2003).[4] H Zhang et al, THERMEC 88, ISIJ, Tokyo, Japan,pp527–534 (1988).[5] L Bäcker et al, HSLA Steels Technology andApplications, ASM, Metals Park, OH, pp1,025–1,035(1984).[6] C I Garcia, E J Palmiere and A J DeArdo, MechanicalWorking and Steel Processing Proceedings, pp79–86(1987).[7] V Ollilainen and J Kohopåå, Microalloyed Bar andForging Steels, CIM, Montreal, Canada, pp237–248(1990).[8] M Katsumata et al, Materials Trans. JIM 8,pp715–728 (1991).

Christian Klinkenberg is Managing Director at NiobiumProducts Company GmbH, Düsseldorf, Germany.

CONTACT: ck@niobium.de

Defining quality for tomorrow’s needsYour company's success depends on the quality of yourproducts. In order to ensure this quality, constantchecking throughout the production process is required.pfaff aqs is a reliable partner in this vitally importantfield of continuous quality assurance and improvement.We have the expertise you need for innovative systemsolutions and patented in-house developments.

Automatic quality control systemsfor the metal industry

PAL-MILAutomatic milling machine for metal samplesThe pfaff aqs PAL-MIL milling machine automaticallyprepares metal samples for X-ray and OE-analysis. Themajor advantage of PAL-MIL is its one-step milling preparation of the surface to be analyzed. No liquid cooling required!

Sampling.......Transportation........................Preparation................................Analysis

www.pfaff-aqs.com

pfaff aqs GmbH Lenneper Straße 23-25DE-42289 Wuppertal, Germanyphone +49 (0)202 26 284-0metal@pfaff-aqs.com

5.3 MS06-11 pp163-167 4/6/06 11:56 am Page 167