Farklı kalitedeki galvanizli çeliklerde fırın sertleştirmesi parametrelerinin mekanik özelliklere etkisi
The effect of bake hardening parameters to the mechanical properties in variable galvanize steels
Yazar: GÜRKAN AYTAN
Danışman: DOÇ. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Makine Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Demir ve çelik icat edildiğinden bu yana insanoğlunun yaşamını sürekli kolaylaştırmıştır. Çeliğin kolay işlenebilirliği, mukavemeti, sünekliği, montaj kolaylığı insanoğlunun ufkunu açmış ve gündelik hayatımızın pek çok yerinde kullanılır olmuştur. Dünyadaki pek çok gelişme, teknolojik buluşlar çeliğin kullanımıyla paralel gitmiştir. Özellikle depreme karşı dayanıklılığı sebebiyle pek çok ülkede artık yapısal çelik kullanılmaktadır. Çeliğin bu üstün özellikleri yanında büyük bir zaafı korozyondur. Çelik yapısı gereği oksijene yönelimi dolayısıyla korozyon ortaya çıkar ve zamanla çeliğin kimyasal kompozisyonu bozulur. Fiziksel yapısı zayıflar. Ayrışma sonucu gittikçe yok olur. Bu zaafiyetin giderilmesinin en etkili yolu galvanizleme kaplama uygulamalarıdır. Biyolojik ve kimyasal esaslı kaplamalar süreli ve etkisiz bir kaplama sağlar. Sıcak daldırma galvanizleme metalik esaslı bir kaplama olup proses sonunda demir ile çinko arasında bağıl bir alaşım oluşturur. Sıcak daldırma galvanizleme basit anlamda tasarımı ve kimyasal kompozisyonu galvanizlemeye uygun demir ve çelik ürünlerinin ergimiş çinko banyosuna daldırılmasıyla oluşan difüzyon sonucu meydana gelen metalik tepkimeyle oluşan kaplama yöntemidir. Sıcak daldırma galvanizleme dünyada yaklaşık 150 yıldır demir ve çeliğin korozyona karşı korunması için kullanılan bir kaplama yöntemidir. Dünyada üretilen çinkonun büyük bir bölümü bu şekilde tüketilmektedir. Bunun en önemli nedenleri çinkonun normal ayrışma olaylarına karşı dirençli , demiri koruma özelliği ve ekonomik oluşudur.
Otomotiv gövdeleri ve panelleri gibi bir çok endüstriyel uygulamada çelikler iyi şekil verilebilirlik ve yüksek mukavemete sahip olması gerekir. Bununla birlikte, iki gereksinim birbirinin genellikle zıttıdır. Bu problem fırın sertleştirmesi tekniği kullanılarak üstesinden gelinebilir. Yüksek mukavemetle otomobil gövdeleri için kullanılan düşük karbonlu çelik üretmek için fırın sertleştirmesi çokça tercih edilen bir işleme tekniğidir. Fırın sertleştirmesi işleminde çözeltideki yeterli karbona sahip olmak için uygun tavlama işlemi gereklidir. Bu boyali pişirme işleminden sonra otomobil gövde ve panellerini daha mukavemetli hale getiririr. 
Fırın sertleştirmesi ilk şekil verme operasyonu sırasında çözünmüş arayer atomlarının dislokasyon bölgelerine göçüyle ortaya çıkan bir süreçtir. Şekil verme sırasında oluşan dislokasyon yoğunluğu ile arayer atomların etkileşimi mukavemet artışına sebep olur. Daha sonra boya işlemi için pişirilen malzemeler estetik görünümün yanında mukavette de artış sağlar.
Bu tez çalışmasında düşük karbonlu DX51D, DX52D ve DX54 sıcak daldırılmış galvanizli çeliklerin herbiri farklı ön deformasyona (%2, %5, %8) tabi tutulmuş  ardından farklı fırın sıcaklıklarında, 170 °C ve 210 °C, 20 dakika fırında bekletildikten sonra 30 dakika havada soğumaya bırakılmış ve fırın sertleştirmesi sağlanmıştır. Fırın sertleşmesi parametreleri değiştirilerek mekanik özelliklerin iyileştirilmesi amaçlanmıştır. Bu çeliklerde ilk olarak fırınlama sıcaklığı sabit tutularak ve farklı ön deformasyon uygulayarak mukavemetteki artış izlenmiştir. Daha sonra maksimum mukavemet artışıyla sonuçlanan ön deformasyon miktarı sabit tutulup fırınlama sıcaklığı değiştirilerek mukavemetteki değişiklik gözlemlenmiştir. Yapılan bu işlemlerden sonra malzemelerin mikroyapı görüntüleri incelenmiştir. Parametrelerin etkisini sertlik ile arasındaki bağlantı için sertlik değerleri ölçülmüştür.
Yapılan deneyler sonucunda akma mukavemetindeki en çok artışın DX52D malzemesinde  %8 ön deformasyonda ve 170⁰C de 20 dakika fırında pişirildikten sonra havada soğutulmasıyla gözlemlenmiştir. Artan ön deformasyon miktarlarının mukavemete olumlu yönde etkilediği, sıcaklığın ise belirli bir noktadan sonra mukavemetteki artış oranında azalmaya neden olduğu gözlemlenmiştir. Yapılan ön deformasyon ve fırınlama işleminin mikroyapıya etkisinin çok fazla olmadığı görülmüştür. Yapılan sertlik değeri ölçümlerinde ise nispeten mukavemet artışıyla doğru orantılı olduğu görülmüştür.

Abstract:
All over the world individuals use automobile on regular schedule to reach their destinations,  which  also  brings  the  risk  of  road  accidents.  Only  in  the  European  countries  40,000  people  die  every year due to automotive accidents. Although it sounds cruel to label human life with  a very high money value, this mishaps cost the administrations something around 160 billion  Euros. So  as  to  decrease  fatalities  or  damages,  and  even  diminish  the  dimension  of  mishaps,  auto  fabricators,  governments  and  private  establishments  around  the  globe  are  cooperating to discover conceivable answers for this colossal issue. So, the way to go ahead  towards this target is to get high strength crash resistance steel but obviously not at the cost of  increased weight. 
Steels are required to have a good formability and high strenghts in a lot of industrial implemantation such as car bodies and panels. On the other hand, good formability and high strenghts are generally contrary to each other. This obstacle can be overcome by using bake hardening (BH) technique. In order to produce low carbon steels used for car bodies with high strength, bake hardening is a highly preferred technique. A suitable batch annealing is necessary in order to have enough carbon in solution required for bake hardening. After paint baking treatment , this process makes automotive bodies and panels strengthened.
Although steel have a lot of feature, the biggest problem of steel is corosion. Due to this issue galvanize sustain to solve corrosion problem. The galvanizing process produces a durable, abrasion resistant coating of metallic zinc and zinc-iron alloy layers bonded metallurgically to the steel base and completely covering the work piece. No other coating for steel matches galvanizing's unique combination of properties and advantages likes ; for most classes of steelwork galvanizing provides the lowest long-term cost. In many cases galvanizing also provides lowest initial cost, the galvanized coating becomes part of the steel surface it protects, the unique metallurgical structure of the galvanized coating provides outstanding toughness and resistance to mechanical damage in transport, erection and service, the galvanized coating is subject to corrosion at a predictably slow rate, between one-seventeenth and one- eightieth that of steel, depending on the environment to which it is exposed, galvanizing cathodic protection for steel ensures that small areas of the base steel exposed through severe impacts or abrasion are protected from corrosion by the surrounding galvanized coating, an inherent advantage of the process is that a standard minimum coating thickness is applied, during galvanizing the work is completely immersed in molten zinc and the entire surface is coated, even recesses and returns which often cannot be coated using other processes. If required, internal surfaces of vessels and containers can be coated simultaneously, Galvanized coatings are virtually 'self-inspecting' because the reaction between steel and molten zinc in the galvanizing bath does not occur unless the steel surface is chemically clean. Therefore a galvanized coating which appears sound and continuous  is sound and continuous. Galvanizing is a highly versatile process. Items ranging from small fasteners and threaded components, up to massive structural members can be coated. The mechanical properties of commonly galvanized steels are not significantly affected by galvanizing. Galvanizing provides outstanding corrosion performance in a wide range of environments. Duplex' coatings of galvanizing-plus-paint are often the most economic solution to the problem of protecting steel in highly corrosive environments. Such systems provide a synergistic effect in which life of the combined coatings exceeds the total life of the two coatings if they were used alone.
Hot dip galvanising is used widely around us everyday. We are in constant contact with hot dip galvanised steel unconsciously from steel construction materials to aesthetic steel objects that are surrounding our daily lives. It is a highly effective & cost efficient method to protect fabricated steel, structural steel, castings, or small parts from corrosion. Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel, and can be identified by the crystallization patterning on the surface. Like all other corrosion protection systems, galvanizing protects steel by acting as a barrier between steel and the atmosphere. However zinc is a more electronegative metal in comparison to steel, this is a unique characteristic for galvanizing which means that when a galvanized coating is damaged and steel is exposed to the atmosphere, zinc can continue to protect steel through galvanic corrosion (often within an annulus of 5 mm above which electron transfer rate decreases).
When sending steel through the hot dip galvanising process, it under goes rigorous preparation work, chemical treatment to remove impurities before finally dipping into a kettle of molten zinc at a temperature of around 450°C. When exposed to the atmosphere, pure zinc reacts with oxygen to form zinc oxide, which further reacts with carbon dioxide to form zinc carbonate, a dull grey, fairly strong material that stops further corrosion in many circumstances, to deliver a reinforced metallurgically alloy bond that protects the steel from corrosion.
Selecting the most effective corrosion protection system is important, and your analysis of each method should include such things as the durability, maintenance schedule, coating's service life, and initial and life-cycle costs.
Hot-dip galvanizing, with its superior durability, life-cycle cost, and maintenance schedule make it suitable for most applications.
The parts of the auto body board pass through a paint baking cycle (bake hardening process)  that is carried out after all the framing operations are finished. In this paint heating cycle, the  painted auto body part is dealt with in an oven to dry and to permit the paint to adhere to the  substrate  steel  of  the  auto.  The  paint  baking  process  not  only  gives  the  car  an  aesthetically  pleasing  look,  but  also  positively  influences  on  the  strength  of  the  material  used  for  the  car  structure.  The  increment  of  strength  which  happens  during  such  paint  baking  process  is  known as bake hardening effect.
Bake hardening is an advanced processing technique to produce low carbon steels, used for car bodies, with high strength. An optimized batch annealing treatment is necessary in order to have enough carbon in solution required for bake hardening. This makes the automotive bodies and panels strengthen after the paint baking treatment. This effect is due to pinning of dislocations by solute carbon atoms which refers to Cottrell's atmosphere.
Using ultra-low-carbon vacuum-degassed steel, and precise alloying additions, partially stabilized steels can be produced that have a low amount of solute carbon available after precipitation reactions are completed on the galvanizing line.
Bake hardenable steel (BHS) takes advantage of the low solute carbon to produce controlled carbon strain aging to augment the yield strength of formed automotive panels, thus improving dent resistance or permitting some thickness reduction. The strain comes from press forming and the aging is accelerated by the paint baking treatment. BH steels contain enough supersaturated solute carbon that the aging reaction typically adds 4 to 8 ksi [27 to 55 MPa] to a stamped panel yield strength.
This approach to providing higher strength panels has the advantage of presenting formable low yield strength material to stamping operations so as to avoid panel shape problems due to elastic deflection associated with initial yield strengths exceeding 35 ksi [240 MPa]. BHS is the practical consequence of modern manufacturing technologies, which permit control of supersaturated solute carbon at a level which is just high enough to provide a useful amount of accelerated strain aging, without aging during transport/storage. The BHS process produces a coated product that will be free from stretcher strains for at least 2 to 3 months after its production, allowing stampers time to consume it before its mechanical properties begin to deteriorate due to aging.
In this study, in low carbon DX51D, DX52D and DX54D hot dip galvanize steels, it is aimed to improve their mechanical properties by changing the bake hardening parameters. First of all, in these steels  bake temperature is remained stable and increase  in  strenghts is observed appyling different prestrain. Then, the prestrain ration resulting in the increase in the maximum strenghts is remained staible and the difference in strenghts is observed by changing the bake temparature. After this process microstruce was investigated. This study has finished by checking micro hardness test.
The most incerement in yield point has been observed with DX52 after prestrain %8 and bake temperature 170⁰C bake time 20 minutes with air dry. Increasing with prestrain  it affects positively yield point but it cant say same for bake temperature. Microstructure wasnt affectted so much from bake hardening process. Although there is no big different after the bake hardening process, hardness test results were in direct proportion to yield point increment.