A New Role for Microalloyed Steels

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A New Role for Microalloyed Steels
  A New Role for Microalloyed Steels Adding Economic Value Michael Korchynsky Consultant in Metallurgy. U.S. Vanadium Corporation  A Subsidiary of Strategic Minerals Corporation, Pittsburgh. Pennsylvania. Abstract Microalloyed (MA) steels have matured during the past 40 years into an important class of high-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 cheaper carbon 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 appliedto 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 maimises 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-strengthlow-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 hotrolling 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 per year). This tonnage is about evenly distributed between flat and long products. As a resultthere 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 thecarbon 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  carbon e3uivalent increases strength it also drastically reduces other engineering propertiessuch as ductility toughness and weldability. &ince welding is irreplaceable as a method of fabrication the carbon-e3uivalent mechanism of steel strengthening cannot be used in manyapplications re3uiring weldability.As shown in !igure + the success of microalloyed steels is due to complimentarystrengthening mechanisms specifically grain refinement and precipitation hardening. /) 1recipitation hardening increases strength but may contribute to brittleness. @rain refinementincreases strength but also improves toughness. As a result grain refinement counteracts anyembrittling caused by precipitation hardening.*n practice grain refinement can be achieved during hot rolling by the interaction betweenmicroalloying elements (niobium vanadium or titanium) and hot deformation. ,uring theallotropic transformation ferrite nucleates on austenitic grain boundaries. Maimum grainrefinement can be achieved by increasing the austenitic grain-boundary area. This can beaccomplished by either producing fine grains of austenite through repeated recrystalliation between passes )  or by flattening non-recrystallied austenite grains into Bpanca esB. The first process is generally used for vanadium steels and the second for columbium (niobium)steels. C) @rain refinement may be further enhanced by accelerating cooling after the completion of hot rolling. The undercooling of austenite enhances the rate of ferrite nucleation and slowsdown the rate of growth. A combination of these two factors contributes to the formation of smaller grains.&ignificant strengthening is obtained by the precipitation of microalloying elementsappearing as carbonitrides (or carbides) in ferrite. 6.D)  &ince their solubility in ferrite is muchless than in austenite there is strong supersaturation which provides the driving force for  precipitation. The most desirable are those microalloys which contribute to both grainrefinement and precipitation hardening. The combined effect of these two strengtheningmechanisms may provide as much as C02 of the yield strength accounting for theremar able cost-effectiveness of microalloyed steels.'ecause these two dominant strengthening mechanisms operate in microalloyed steels their carbon content (or 8) may be very low. A yield strength of //0 M1a can be obtained in asteel containing only 0.04 to 0.02 carbon. +0)  This low-carbon content contributes toecellent weldability. &. A New   'unction for (ot Rolling! )%timising Material *ro%erties Traditionally the main ob#ective of the hot-rolling process was to change the geometry of aslab or billet to meet the dimensions of the final product. !or this purpose the temperature of rolling was not well controlled with the unwritten rule being Bthe hotter the betterB.Accomplishing the BmiracleB of converting ordinary carbon steel into a sophisticated &%Asteel re3uires an understanding of the evolution of the austenite microstructure during hotrolling. At high temperatures the sie of a recrystallied grain after each deformation passdepends on the initial grain sie temperature and the amount of deformation. ++)  Thetendency of grains to coarsen between passes can be prevented by precipitated particleswithin the grain boundaries. !inely dispersed titanium nitrides (TiE) formed by titaniumadditions as low as 0.00/ to 0.00C2 effectively prevent grain coarsening. ?hen the rollingtemperature is low enough to prevent recrystalliation austenite is flattened into a Bpanca eB.The temperature at which this occurs depends on the type of microalloy. ecrystalliation issuppressed at a much higher temperature in niobium (columbium) steels than in vanadiumsteels. C)  @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 oygen 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 and brittleness. ,uring concasting ecessive nitrogen may increase possible transverse or longitudinal crac ing. !ears are also fre3uently epressed about the detrimental effects of nitrogen 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(8E) 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 previouslyconsidered 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 of this 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 basicoygen ('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)  The process incorporates a series of steps that contribute to either cost reductions or to propertyimprovements. The rapid solidification in the mould accounts for the small sie of globular inclusions which do not elongate during hot rolling. This promotes isotropic properties suchas 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  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 of less than 49+.'ecause of lower hot rolling costs the mar et share for hot bands produced by thin slabcasting is being increased at the epense 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 of Microalloyed 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 G0steel 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 safer auto 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 conceptsmaterial selection and new fabrication methods.*n the area of materials the most important change was the replacement of inepensivecarbon 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 nesshave 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 epensive &%A steel was found to beeconomically attractive as a replacement for cheaper carbon steel.ears ago a @M eecutive 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 reductionsteps microalloyed steels have all the necessary attributes to successfu++y replace inefficientand often higher cost carbon steels in such areas as construction transportation and machine building. 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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