Otomotiv endüstrisinde iç ve dış panellerin üretiminde kullanılan, çok az miktarlarda karbon ve azot içermeleri nedeniyle pres altında şekillendirmeye uygun çeliklerdir. Arayer atomsuz çeliklerde, şekillendirilebilme özelliğini olumsuz yönde etkileyen arayer atomlarının (C ve N), çelik üretim prosesi esnasında bileşimden kontrollü bir biçimde uzaklaştırılması Titanyum (Ti) ve/veya Niobyum (Nb) atomlarının C ve N atomları ile reaksiyona girerek çökelti fazları oluşturması ile olmaktadır. Sıcak haddeleme prosesinde slab fırınında belli sıcaklığa kadar ısıtılarak çökeltilerin çözeltiye alınması sağlanır. Bu işlem nihai sıcak band dokusunu ve tane büyüklüğünü belirleyici ilk prosestir. İkmal ve sarılma sıcaklıkları da arayer atomsuz çelik üretiminde önemli olan proses parametrelerdir. Her iki proses parametresi de çökelti ve tane büyüklüğünü önemli ölçüde etkileyerek çeliğin nihai mekanik özellikleri üzerinde büyük oranda etkili olmaktadır. Şekillendirilebilme özelliğinin önemli bir göstergesi olan r değerinin arttırılmasında en önemli parametre soğuk ezme oranıdır. Artan tavlama sıcaklığı tane büyüklüğünün artmasında etkili olmaktadır. Çelikhane prosesinden itibaren sıcak haddeleme ve soğuk haddeleme prosesleri arayer atomsuz çeliğin mekanik özellikleri üzerinde etkili olmaktadır. Endüstriyel ölçekte gerçekleştirilen bu çalışmada, çelik kimyasal bileşiminde bulunan Ti ve Nb elementleri ile soğuk haddeleme ve sürekli tavlama parametrelerinin optimizasyonu sonucunda mekanik özelliklerde oluşan değişimler irdelenmiştir. Yapılan çalışmaların sonucunda Ti+Nb elementleri içeren kompozisyonlarda, yüksek ezme oranları (%80) ve tavlama sıcaklıklarında (870°C) yüksek r değerleri elde edilmiştir. Otomotiv sektörünün önemli oranda kullandığı arayer atomsuz çeliklerin mekanik özelliklerinde iyileştirmeler sağlanarak endüstrinin kullanımına sunulmuştur.Anahtar Kelimeler: IF Çelikleri, soğuk haddeleme parametreleri, r değeri, mekanik özellikler.Interstitial free steels (if steels) are very suitable material for automotive industry and they are used for inner and outer body panel applications due to their high formability properties. They have extremely high formability characteristics under forming presses. Interstitials damages formability characteristics of steel. Elimination of interstitials (C and N) is achieved through careful control of the steelmaking process by the addition of titanium and/or niobium to react with carbon and nitrogen to form precipitates. In the hot rolling process, firstly, slabs are heated upto some temperature and those precipitates are taken into solid solution. This process affects grain size and texture of hot rolled product. The other important process parameters in hot rolling are; finishing temperature and coiling temperature. Both parameters mainly affect grain size and size of the precipitates in hot rolled coil.and thus mechanical properties are affected. The most important parameter for formability is r value. And it is mainly affected by cold deformation.ratio. Also increasing annealing temperature increases grain size and results in higher formability. All the process parameters through steelmaking to the cold rolling process affects mechanical properties of interstitial free steels and thus affects r value of interstitial free steel. In this study, mechanical properties related with steel chemical composition by optimizing Titanium (Ti) and Niobium (Nb) contents and continuous cold rolling and annealing parameters were investigated in industrial scale. As a result, the highest r values were obtained at 80 % cold deformation ratio and annealing at 870 C for steel with Ti+Nb alloying addition. In this study, experiments were done in industrial scale. Firstly, interstitial free steels containing titanium and titanium+niobium were produced at steel shop. And then slabs produced at continuous casting machines were hot rolled After hot rolling process, those coils were sent to cold mill for cold rolling and annealing process. Also some coils were sent to galvanising line after cold deformation. Each process are controlled by high level process computers in Erdemir plants. Therefore process parameters for trial coils are taken from this computer results. Chemical analysis of heats were done at steel shop chemical laboratories and they were produced according to instructions given by metallurgical department. Mechanical test samples were taken after continuous annealing line and continuous galvanising line processes. Mechanical properties including yield strength, tensile strength, and elongation at fracture strain hardening exponent and anisotropy coefficient were determined by mechanical tensile tests. Tensile specimens were cut along rolling direction, transverse direction and 45° to the rolling direction. Samples were taken from the middle section of each coil. Tests were done on Zwick Z-100 testing machine. All tests were made at a strain ratio of 2 % Lo/min. The tensile test samples were processed according to DIN EN 10002-1 type 2 standards.The effect of process parameters on mechanical properties were investigated according to these results. Optical microscopy was used for the microstructural examinations and grain size measurements of the samples. Microstructural examinations were conducted on the cross sections perpendicular to the rolling direction. Microstructures of materials including ferrite grain size, carbide type and distributions were investigated with a leco 500 optical microscope. Also scanning electron microscopy was used for the determination of the microstructure and carbide distribution of the samples. The scanning electron microscope was a jeol 5600 jsm type machine. As a result of microstructure analysis effect of process parameters on microstructures analysed. In this study, texture analysis was conducted with a rigaku d-max ultima x-ray diffractometer with a pole figure attachment. Test samples were prepared from annealed samples, punched to a 4mm diameter and then metallographically prepared. The next step was data collection from an x-ray diffractometer. The xrd measurements of samples were carried out in the range of 20-90 2teta degrees with 0.02 steps. Then pole figure measurements were done with different alfa and beta angles in 110, 111 and 200 diffraction lines. As a result of that study, the best composition of interstitial free steel containing titanium and niobium were determined by optimization of titanium and niobium content. Also the best process parameters at the cold mill determined through analysis of results taken from mechancal tests and microstructure analysis in which the highest r value obtained. And interstitial free steel produced after that study put into use for the automotive market as a new quality of steel.Keywords: Interstitial free steel, cold mill process parameters, r-value, mechanical properties
