Mikroalaşımlı dövme çeliklerde mikroyapısal karakterizasyon ve mikroyapı-mekanik özellikler ilişkisi
Microstructural characterization and microstructure mechanical properties relationship in microalloyed forging steels
- Tez No: 19376
- Danışmanlar: PROF.DR. H. ERMAN TULGAR
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 93
Özet
ÖZET Mikroalaşımlı çelikler (yüksek mukavemetli, düşük ala şımlı, çelikler alarak da adlandırılır: High Strength, Lou Allayed Steel : HSLA çelikler) geliştirildikleri yapı çeliklerine oranla çak daha yüksek mukavemet ve tokluk gös terirler. IMiabyum, vanadyum ve titanyum gibi karbür, nit- rür ve karbonitrür oluşturan alaşım elementlerinide düşük miktarda içeren bu çeliklerde mikroalaşımlama ve kontrollü haddeleme birlikte uygulanarak tane küçültme ve çökelti sertleşmesi sağlanmıştır. Mukavemet ve tokluğun ortak ar tımı öncelikle mekanik özellikleri iyileştirmiştir. Gaz ve petrol boru hattı,, off-shore konstrüksiyonları ve oto karoserlerinde kullanılan saç ve levhalardan otomotiv sanayiinde kullanılan dövme parçalara kadar oldukça geniş bir uygulama alanı içeren bu çelikler ortalama yirmi sene lik geçmişlerine rağmen-öncelikle mikroalaşım elementleri nin kombine rollerinin anlaşılması doğrultusunda yoğun bir kullanım potansiyeli içerirler.
Özet (Çeviri)
MICROSTRUCTURAL CHARACTERIZATION AND MICRO-STRUCTURE MECHANICAL PROPERTIES RELATIONSHIP IN MICROALLQYED FORGING STEELS SUMMARY The continuous wide-spread use of microalloyed steels is related to their higher strength and tougness than plain carbon steels from which they are developed. The attractive mechanical properties are obtained due to the combination of an advanced metallurgical processing. Small additions of alloying elements like 1Mb, V and Ti result in the formation of carbonitrides in the micro- structure. These very fine precipitates are effective in preventing grain growth. By the use of controlled rolling, recrystallization is retarded during the last passes. The microstructure after the termomechanical treatment consists of fine acicular ferrite and trans formation phases; their amount is determined by the cooling rate. The development of the impressive properties profile characterizing modern high-strength low-alloy steels must be regarded as one of the most significant metallurgical advances over the last two decades. The controlled rolling technique is most effective with steels containing microalloying elements. Micro- allaying additions increase the temperature where complete recrystallization occurs and retard the recrystallization of austenite below this temperature. Also the precipitate particles introduced by microalloy ing additions restrict the grain growth of austenite. The effectivness of microalloying elements in grain growth has been shown to be a function of the solubility of their precipitates in austenite. VINiobium, vanadium and titanium are the mast commonly used micraallaying elements because of their strong tendencies to form carbides, nitrides, and carbanitrides. Gf the three, niobyum has the strangest effect on retarding the recrystallizatian af austenit, followed by titanium. Vanadium is not so effective because it has high solubility in austenite and precipi tates at law temperatures. However, it has been shown that high vanadium content associated with optimum nitrogen content can also retard the recrystallizatian af austenite. Titanium has an additional adventage of sulf ide-shape control. The controlled-rDlling process is often divided into two or three stages : deformation in the austenite recrystallization region, deformation in the austenite non-recrystallization region and deformation in the two phase austenite-f errite region. Optimum mechanical properties of HSLA steels can be obtained only by careful control of microstructural changes in each stage of controlled railing. The primary abjective af control railing an HSLA steel is ta achive a very fine, uniform ferrite grain size resulting in an increase in yield strength and an improvement in taugness properties due ta gain size refinement. The latter improvement is demonstrated, far example, by achieving a lower ductile-to-brittle-trans- formation temperature. Conventional hot rolling differs from controlled rolling in that the nucleatian af ferrite occurs exclusively at austenite grain boundaries in the former, while it occurs in the strain hardened grain interiors as well as at grain boundaries in the latter, leading to more refined ferritic grain structures. Grain refinement in steels is enhanced through a combination of controlled rolling and micraallaying. The primary grain refinement mechanism in controlled rolling is the recrystallization of austenite during hot deformation, and is known as dynamic recrystalliza tion, where as static recovery and recrystallizatian take place between railing passes and during coaling at the completion of the rolling process. VllGrain refinement, through micraallDying, can inhibit austenite grain growth during reheat, and retard auatenite recrystallization during subsequent rolling. Nucleation sites for ferrite include deformation bands recovered substructural boundaries, and undissolved carbides and nitrides. In f errite-perlite steels, the main role of microal- loying elements is two fold: 1-to refine the grains of ferrite, and 2- to contribute to precipitation streng thening. In pearlitic steels, microallaying may also influence the inter-lameller spacing. Usually, micro- alloying involves a small addition of the fallowing carbonitride forming elements: niobiun, titanium or vanadium, either seperately or in combination. In“conditioning”the austenite during the termo- mechanical treatmen (TMT), these microalloying elements may perform one or more of the following functions: 1)- Maintain fine austenite grain size during reheating by increasing the grain coarsening temperature. 2)- Slow down the growth of recriystallized grains by pinning the grain boundaries: 3)- Retard or suppress recrystallization. The effect of microallaying elements on bath the kinetics af recrystallization and grain boundary motion is mainly the result of precipitation. Particles of microallaying compounds are either present in austenite, or precipitate on coaling in consequence of increasing supersaturation. The rate of precipitation is greatly enhanced by plastic deformation (strain induced precipi tation). The effectivness of microallaying precipitation depends on volume fraction, degree of dispersion and stability. By comparision, the influence of micros ? alloying elements in solution on kinetics of austenite recrystallization is very weak. Pinning of grain boundaries by precipitated particles is by far more Vllleffective than the salute drag, for that reason, in assessing the effectiveness of a micraalloying addition, the nature of the precipitated phase is of pronounced importance. The second function of microalloying elements, besides austenite“conditioning”is to provide precipi tation strengthening. Depending on the microalloying elements and TMT conditions, part of dissolved raicroal- loying elements precipitates in austenite, the rest, retained in solution, precipitates during or after the transformation. The effectiveness of micraalloying elements, both in austenite or in ferrite, depends on the size and stability of the precipitated particles. Thase tuo parameters are improved for compounds of lower solubility. In the last decade efforts have been made to develope microalloyed steels for forgins to be used in the automotive industry. By having a fine mixed structure of ferrite and paarlite uith controlled precipitation hardening due to microalloying this neui generation steel uill exhibit following economical advantages over the traditional guenched and hardened (DJ) steels: * After forging the controlled cooling is performed in air/pressed air. Conventional grades obtain their properties after QT-teatment. Cost a for past treatment straightening are also eliminated. * Microalloyed forging stepls hava a better machinability than QT steels, which in the tempered state consists of tempered martensite. This type of micrastructure is harder to machine than a ferritic pearlitic structure. Advantage is achieved due to higher cutting performance (higher cutting speed, higher feed, lower, cutting forces and wear of tool material) and better surface quality of the work material. IX* Micralloying with V,Ti or N additions up ta 0.15% cast less than higher allaying additions like l\li,Ma,Cr in the QT-steels. Nowadays the micraalloyed Forging steels ( AFP ausscheidungshaertende Ferritische-Perlitische Stahle: AFP çelikleri) are used widely in parts, statidy stressed (Crank shafts e.g.). Resently developed AFP steels with lower % C are, mainly for dynamically stressed autmotive parts (safety parts like as spindles e.g. ).
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