A New Role for Microalloyed Steels

8/10/2019 A New Role for Microalloyed Steels

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A New Role for Microalloyed Steels Adding Economic Value

Michael Korchynsky

Consultant in Metallurgy. U.S. Vanadium CorporationA Subsidiary of Strategic Minerals Corporation, Pittsburgh. Pennsylvania.

Abstract

Microalloyed (MA) steels have matured during the past 40 years into an important class ofhigh-strength structural materials. Their cost-effectiveness has been enhanced by the growthof electric-arc-furnace (A!) steelma"ing and the thin-slab-casting process. A recent pro#ectinvolving an ultra-light-steel auto body ($%&A') concluded that high-strength-steels are thematerials of choice for the automotive industry. This pro#ect showed that replacing cheapercarbon steels with high-strength steels allowed automa"ers to reduce the weight of an auto

body at the same or at potentially lower costs. The same economic principles can be applied

to other applications.

The strengthening effects of vanadium ma"e microalloyed steels particularly suited for high-strength-steel applications. 'y effectively combining grain refinement and precipitationhardening vanadium maimises the strengthening process and is compatible with currentsteel-processing technology.

To dramatise the cost effectiveness of these high-strength-steels in potentially newapplications a series of demonstration pro#ects is needed involving the cooperation of steel

producers fabricators and users. *n many applications the decision to replace plain-carbonsteel with higher strength vanadium-bearing microalloyed steel can be shown to improve the

profitability of both the steelma"er and the steel user.

1. ntroduction! "se of Microalloyed Steels Reduces #osts

The development of microa*loyed steels including their alloy design processing andapplications covers the last four decades.+-4),uring this period microalloyed high-strengthlow-alloy (&%A) steels became an indispensable class of structural steels. Their ability toachieve final engineering properties in as hot-ro*led conditions eliminated the need for heattreatments such as normalising. ield strengths ranging up to //0 to 00 M1a can beattained through small additions (less than 0.+2) of selected carbonitride formers withoutre3uiring costly alloying elements. The resulting cost-effectiveness of microalloyed steels ledto the successful displacement of heat-treated steels in applications such as truc" side railsand telescoping crane booms. ecent technological developments in steel melting and hot

rolling further reduced the cost and enhanced the competitiveness of microalloyed steels.

,espite these improvements the total consumption of microalloyed steel is currentlyestimated to be only +0 to +/2 of the world5s steel production (i.e. 60 to +70 million tons peryear). This tonnage is about evenly distributed between flat and long products. As a resultthere is plenty of room for growth. A ma#or #ump in the usage of microalloyed steels shouldhave strong economic benefits for both steel producers and steel users.

$. Microalloying! #om%limentary Strengthening Mechanisms

ot-rolled plain-carbon steel is the most popular material used in construction. *ts strengthcan be increased by raising the carbon content. *n fact its strength is proportional to the

carbon e3uivalent (8) which is essentially the combined effect of the carbon andmanganese content of steel based on the formula9 :8 ; 28 < 2Mn=>. ?hile raising the


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@rain refinement may be further enhanced by the intra-granular nucleation of ferrite inaustenite.l7)The precipitation of vanadium nitride in austenite provides the most effectiveintra-granular nucleation of ferrite. The highest rate of carbonitride precipitation in ferriteoccurs at 00F8 which is the customary sheet coiling temperature.

+. Making Nitrogen A 'riend, Not A 'oe

Two new developments in steelma"ing and steel processing the growth of the electric arcfurnace (A!) and processing by thin slab casting have contributed to further cost reductionin the production of microalloyed steels.+G)

A! steelma"ing is growing rapidly worldwide because it is less capital intensive than theconventional processes used by integrated steel producers. Hirtually all new steelma"ingcapacity added either by mini-mills or integrated producers uses electric arc furnaces. &oon/02 of the world5s steelma"ing or about 400 million tons annually will be made in thesefacilities.

*n a scrap based A! practice the nitrogen content is C0-+00 ppm or 7 to G times higher thanthat typical of the basic oygen or 'I! practice. The nitrogen level of steels made in an A!can be reduced by modifying the slag practice or changing the feed stoc". 'oth thesemethods can increase costs.

!ree nitrogen in solution in ferrite has serious detrimental effects such as aging andbrittleness. ,uring concasting ecessive nitrogen may increase possible transverse orlongitudinal crac"ing. !ears are also fre3uently epressed about the detrimental effects ofnitrogen on weldability.

owever the harmful effects of nitrogen may be neutralised by nitrogen binding elementswhich acting as scavengers remove nitrogen from solid solution in ferrite. Aluminium andtitanium are effective scavengersJ however niobium (columbium) is not an effectivenitrogen-binding element in high strength low alloy steels. *n niobium steels niobiumcarbonitrides are only present when the carbon to nitrogen ratio ranges between +9+ and 49*.Thus the effect of niobium depends on the nitrogen content of the steel.

Among the various microalloying elements vanadium has a uni3ue dual effect on nitrogen. +4)

Hanadium not only neutralises nitrogen by forming HE compounds but also uses nitrogen tooptimise the precipitation reaction. nhanced nitrogen increases the supersaturation in ferriteand promotes a more active nucleation of H(8E) particles as shown in !igure 7.8onse3uently the interparticle distance is reduced (!igure G) and the strengthening effect of

precipitation is increased. *n the presence of nitrogen less vanadium is needed to achieve thedesired yield strength. As a result vanadium effectively converts nitrogen previously

considered an impurity into a valuable alloy that helps strengthen steel as shown in !igure 4.

The pioneering efforts of the Eucor &teel 8orporation+/)in commercialising thin slab castinghave dramatically changed the economics of hot band production. The revolutionary effect ofthis new process can be compared to two previous developments which have changed theeconomics of steel production9 the switch of steelma"ing from open hearths to a basicoygen ('I!) converter and the replacement of ingot casting by continuous casting.

The thin slab casting process converts in-line li3uid steel into a mar"etable product. l)Theprocess incorporates a series of steps that contribute to either cost reductions or to propertyimprovements. The rapid solidification in the mould accounts for the small sie of globularinclusions which do not elongate during hot rolling. This promotes isotropic properties such

as bendability in longitudinal or transverse directions. Eear net-shape dimensions of the slab(/0 - C0 mm) facilitate rolling to an aim thic"ness of + mm (or less) allowing hot rolled steel


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to economically replace cold rolled sheet. *n-line processing permits the slab to be directlycharged into the rolling mill contributing to energy savings. The amount of deformation per

pass is 7 to G times higher than that on a hot strip mill rolling thic" slabs. cellentmicrostructure and properties are obtained in a +/-mm thic" strip for a total deformation ofless than 49+.

'ecause of lower hot rolling costs the mar"et share for hot bands produced by thin slabcasting is being increased at the epense of high cost integrated producers. *n developing theconcept of replacing carbon steels with microalloyed steels we will limit our choice initiallyto strip made by thin slab casting technology.

-. "ltra/ight Steel Auto 0ody! uantifying the Economic 0enefits ofMicroalloyed Steel

The pressure to reduce the weight of automobiles and the ever-present threat from light-weight materials such as aluminium or magnesium led to the creation of an internationalconsortium of steel producers whose goal was to produce a lighter-weight auto body. Iver G0

steel producers #ointly sponsored an ambitious pro#ect9 $ltra %ight &teel Auto 'ody($%&A').+C)The ob#ectives of the pro#ect were three-fold9 (+) design a stronger and saferauto body compared to best models available (7) lower the weight and (G) "eep costs thesame or less than auto bodies being built today. All three goals have been successfullyattained. Three factors contributed to the success of the pro#ect9 novel design conceptsmaterial selection and new fabrication methods.

*n the area of materials the most important change was the replacement of inepensivecarbon steel with higher value &%A steels. More than D02 of the $%&A' structure usedhigh strength steel ranging in yield strength from 7+0 to 470 M1a. Ine half of the steel usedhad G/0 M1a yield strength. 'oth cold and hot rolled sheet 0./ to 7.0 mm in thic"nesshave been used. The use of steel stronger than 470 M1a was minimal.

&%A steel emerged as the material of choice for modem automobile design. !or a costconscious automobile industry the use of more epensive &%A steel was found to beeconomically attractive as a replacement for cheaper carbon steel.

ears ago a @M eecutive made a controversial statement9 K?hat is good for @eneralMotors is good for America.B Today we may paraphrase this slogan9 B?hat is good for$%&A' may be good for many steel processing industries.B The $%&A' pro#ectdemonstrated that the competitiveness of steel hinges on the following parameters9engineering properties and fabricability weight reducing potential and cost.

Than"s to technology advances and the successful adaptation of a series of cost reduction

steps microalloyed steels have all the necessary attributes to successfu++y replace inefficientand often higher cost carbon steels in such areas as construction transportation and machinebuilding.

2. 3eight Reduction! 4he Key to Adding Economic Value

Microalloyed high strength low alloy steels may have yield strengths that are 7 to G timeshigher than hot rolled weldable carbon steels. The weight reduction achievable throughsubstitution depends not only on the difference in strength but also on the mode of loading.!or straight loading in tension the weight reduction is proportional to the difference instrength. An increase in yield strength by a factor of two may reduce the weight of steel bytwo - a situation found in concrete reinforcing bars. The range of weight savings is shown in

!igure /.


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!or other types of loading (e.g. bending) a two fold increase in strength may contribute to aweight reduction of G42 or more. 8onsidering a safety factor one may as same as the &%Asteel being twice as strong as carbon steel may reduce the weight by at least 7/2. Thefollowing simplified calculation illustrates the economic value obtained through substitution9

*t is evident that the substitution is economically attractive for both the producer and user.The producer may en#oy twice as much profit for the value added microalloyed steel. Theuser pays LGD less for the material. is additional benefits include ease of fabricationimproved overall properties (e.g. toughness ductility) and lower transportation cost. Therange of cost savings is shown in !igure .

5. Vanadium! )ffering the /owest #ost *er "nit of Strength

*n selecting the most economical microalloyed steel the following factors should beconsidered9 (+) alloy design that maimises the strengthening effect of the two cost effectivemechanisms grain refinement and precipitation hardening) (7) low processing cost affected

by reheat and finishing temperatures and (G) compatibility with the typical nitrogen contentof electric furnace steels.

If the three microalloying elements - niobium vanadium and titanium - only vanadiumcontributes to cost reductions in all three areas listed above. 'y incorporating vanadium in analloy design both grain refinement and precipitation hardening will be fully utilised

providing up to C02 of the total yield strength. *n addition vanadium permits the use of thelowest cost hot rolling practice. 'ecause of the high solubility of vanadium nitride (HE) in

austenite a low reheating temperature (++/0F8) may be used. This contributes to energysavings. At the same time the finishing temperature may be high (D00 to +000F8) since


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grain refinement is obtained by repeated recrystalliation. !inally vanadium not onlyneutralises the ill effects of BfreeB nitrogenJ it uses this BimpurityB as a valuable alloyingelement - a uni3ue characteristic among microalloying elements.

There are virtually unlimited sources of vanadium on earth.+6)Hanadium is etracted fromvanadium bearing ores (Australia) as a by-product of steel production from iron ore

containing both vanadium and titanium (&outh Africa ussia and 8hina) from spentcatalysts ($.&.A.) and from oil products ($.&.A.). At present the eisting industrial capacityfor vanadium production eceeds the demand. ?ith new sources of supply on the horionshortages are not li"ely to occur in the foreseeable future.

6. "nlocking the #ommercial 0enefits

'ased on the eample in the $%&A' pro#ect it can be seen that substituting a higher valuesteel in an eisting application brings economic and performance benefits. Ine way ofachieving this ob#ective is through the use of demonstration pro#ects. These demonstrationsmust be conducted on carefully selected products with a full understanding of the potential

benefits for the steel producer fabricator and the user.As an eample microalloyed steel might be substituted for commonly used carbon steel inthe production of spirally welded water pipe. !or steel producers this substitution wouldincrease their share of value-added products which command a higher profit margin. !or thefabricator the ease of welding a low carbon (0.04-0.02 8) microalloyed steel provides astrong incentive in the form of reduced labour costs. *n addition the user will benefit notonly from lower material cost but improved strength and fracture resistance for safeoperation.

To deliver these benefits each demonstration pro#ect must fully document and evaluate theservice performance and provide a detailed cost analysis. The pro#ect should also assure that

mandatory standards and specifications are met. !inally the results of the demonstrationpro#ect should be incorporated into pertinent engineering specifications and brought to theattention of designers and engineers through educational and promotional efforts.

7. New )%%ortunities for *roducers and "sers

The selection of high strength steels as the material of choice by the automotive industrysuggests new opportunities for microalloyed steels in a variety of applications. Their highstrength compared to hot rolled carbon steels offers an opportunity for significant weightreduction. This lower weight more than offsets the slightly higher unit cost of microalloyedsteels adding economic value to both steel producers and steel users. !or steelma"ers sellingmore value-added high strength steels improves their competitive position against light-

weight materials and allows increased profit margins. !or users value-added high strengthsteels offer lower material costs better product performance and reduced fabrication andtransportation costs.

The feasibility of satisfying all engineering needs with less steel is also advantageous for thenational economy. *t reduces the pressure of capital intensive demands for replacing obsoletetraditional steelma"ing facilities. *n addition any shortfall in the capacity of integrated

producers will be balanced by cheaper and more fleible electrical furnaces.

The path from a theoretically attractive concept of substitution to commercial reality re3uiresa dedicated effort mainly by the steel industry. Eevertheless advances achieved during the

past 40 years in the science technology and applications of microalloyed high strength

steels ma"e them ready for this economic challenge.


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*n fact the importance of weight reduction and greater emphasis on value-added products ares3uarely in line with the trends of the steel industry as reflected in the following 3uotations9

BThe steel industry5s weight paring campaign is an investment in the future.B (Al ?rigleyAmerican Metal Mar"et une 7 7000)

B*nnovative technology will help us to produce the high-value-added steels critical to oursuccess and the success of our customers.B (1aul . ?ilhelm 1resident $ .&. &teel Eew&teel une 7000 page /)

18 #onclusion

The new role of microalloyed steels in adding economic value through replacement of hotrolled carbon steels is feasible today. 1otential economic benefits to the steel industry and itscustomers could reach billions of dollars annually.

The competitive advantages of microalloyed steel compared to hot rolled carbon steel includesuperior fabricability weight reduction by at least 7/2 and lower overall cost. To fully

eploit these benefits re3uires vision and dedication on the part of the steel industry and itscustomers. The economic advantages to both producers and users - typical for a Bwin- winBsituation provide a strong incentive for initiating a drive in this direction.

RE'EREN#ES

+. &ymposium9 %ow-Alloy igh-&trength &teelsJ EurembergJ May 7+-7G +DC0J TheMetallurgy 8ompanies.

7. *nternational 8onference9 1roceedingsJ Micro- alloying 5C/J ?ashington ,.8.J $nion8arbide 8orporation.

G. *nternational 8onference9 Technology and Applications of igh-&trength &teelsJ

1hiladelphia 1a.J Ictober +D6GJ American &ociety for Metals.4. *nternational 8onference9 Microalloying 5D/J 1ittsburgh 1AJ une ++-+4 +DD/J *ron and

&teel &ociety.

/. !. '. 1ic"ering9 BB1hysical Metallurgy and the ,esign of &teelBJ Applied &cience1ublishers %ondon (+DC6).

. T. &iwec"i et al.9 Becrystalliation 8ontrolled olling of &%A &teelsBJ Microalloying5D/J une ++-+4 +DD/J *ron and &teel &ociety.

C. %. . 8uddy9 BThe ffect If Microalloy 8oncentration of Austenite during ot,eformationBJ 8onference 1roceedings9 Thermo-mechanical 1rocessing of MicroalloyedAusteniteJ The Metallurgical &ociety 1ittsburgh 1A (+D6+).

6. &. Na#ac et al.9 BThe ole of Eitrogen in Microalloyed &teelsBJ Microalloying 5D/J une ++-+4 +DD/J *ron and &teel &ociety.

D. . %agneborg et al.9 B*nfluence of 1rocessing oute and Eitrogen 8ontent onMicrostructure ,evelopment and 1recipitation ardening of H - Microalloyed &%A&teelsBJ 8onference 1roceedings9 Thermomechanical 1rocessing of MicroalloyedAusteniteJ The Metallurgical &ociety 1ittsburgh 1A (+D6+).

+0. M. Oorchyns"y9 BEew &teels for Eew MillsBJ &candinavian ournal of MetallurgyJ 28(+DDD) pp. 40- 4/.

++. T. &iwec"i and @. ngberg9 Becrystalliation 8ontrolled olling of &teelsJB 1roceedings9Thermomechanical 1rocessing in Theory Modelling and 1racticeJ A&M *nternational&toc"holm (+DD).


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+7. T. Oimura et al.9 Beavy @auge -&hapes with cellent &eismic-esistance for 'uilding&tructures 1roduced by Third @eneration *M81JB *nternational &ymposium 1roceedings9&teel for !abricated &tructuresJ 8incinnati IhioJ A&M (+DDD).

+G. M. Oorchyns"y9 B8ost ffectiveness of Micro- alloyed &teelsJB *nternational &ymposium1roceedings9 &teel for !abricated &tructuresJ 8incinnati IhioJ A&M (+DDD).

+4. . %agneborg at al.9 BThe ole of Hanadium in Microalloyed &teelsJB &candanavianournal of Metallurgy 26 (Ictober +DDD).

+/. 1. . %ubens"y et al.9 Bigh &trength &teel 1rocessing via ,irect 8harging $sing Thin&lab TechnologyJB Microalloying 5D/J une ++-+4 +DD/J *ron and &teel &ociety.

+. M. Oorchyns"y and &. Na#ae9 B!lat olled 1roducts from thin-&lab TechnologyJTechnological and conomical 1otentialJB 1roceedings9 Thermomechanical 1rocessing inTheory Modelling and 1racticeJ A&M *nternational &toc"holm (+DD).


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Fig. 1 The strength of hot-rolled steel depends on the five mechanisms shown above. *n highstrength low alloy steels microalloying elements such as vanadium and nitrogen

provide up to C02 of the strength by contributing to grain refinement and precipitationhardening./)

Fig. 2 *ncreasing the nitrogen content promotes nucleation forming smaller vanadium-nitrideparticles.6)


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Fig. 3 educing the particle diameter of precipitates from 4 to 7 nm gives eight times thenumber of precipitates in a given volume of steel. The larger number of small

precipitates gives more efficient strengthening by reducing interparticle spacing

Fig. 4 *n vanadium steels nitrogen acts as a valuable alloy that helps increase yieldstrength.+4)


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Fig. 5 The weight of components can be reduced substantially by substituting high-strengthsteel for low-strength carbon-manganese steel (&&A' &heet &teel andboo").

Fig. 6 'ecause of the cost effectiveness of microalloyed steels the weight reduction morethan offsets the difference in prices between microalloyed and carbon-manganesesteels (&&A' &heet &teel andboo").

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A New Role for Microalloyed Steels