Azotlu vanadyum ilavesinin S550MC kalite çeliklerde nihai özelliklere etkisinin incelenmesi
Investigation of nitrogen containing vanadium alloying element addition effects on final properties in S550MC grade steels
Yazar: ÖZCAN KAYA
Danışman: DOÇ. DR. CEVAT BORA DERİN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Termomekanik haddelenmiş ve soğuk şekillendirmeye uygun HSLA grubuna ait çelikler otomotiv sanayisi de dahil olmak üzere günümüzde yaygın kullanım alanına sahiptir. Bu çeliklerin üretiminde konvansiyonel yöntemde Nb, Ti ve V elementleri mikro alaşımlama ile ilave edilmekte olup bu çalışmada alternatif olarak azotlu vanadyumun alaşım ilavesi olarak kullanılmasının etkileri araştırılmıştır.

Tasarlanan kimyasal kompozisyon ile ilk etapta faz dönüşüm sıcaklıkları ve geçiş bölgeleri JMatPro yazılımı desteği ile belirlenmiş olup aynı zamanda sürekli soğuma şartlarında farklı soğuma hızlarının nihai mikro yapıya etkisi de araştırılmıştır. Ayrıca ThermoCalc yazılımı desteği ile tasarlanan kimyasal kompozisyona göre sıcaklığa bağlı olarak oluşacak kararlı çökelti fazlar da belirlenmiştir. 

HSLA çeliklerinde azotlu vanadyum ilavesi tane sınırlarında vanadyum nitrür (VN) çökeltisi oluşturduğundan dolayı kritik bir öneme sahiptir. Bu çökeltiler ile hem dayanımın arttığı hem de şekillendirilebilirlik kabiliyetinin arttığı tek mukavemet arttırma işlemi olan tane inceltme işlemi gerçekleştirilir.

Vanadyum ilavesinin etkinliği yapı içerisindeki mevcut azotun bir fonksiyonudur. Bu nedenle artan azot içeriği ile birlikte oluşan VN çökeltileri daha küçük ve yapıda yayılmış bir şekilde oluştuğundan dolayı soğuk şekil verme işlemlerinde yüksek deformasyona izin verir.

Deneysel çalışmalar yazılım destekleri ve mikro yapısal, mekanik incelemeler olarak iki ana grupta gerçekleştirilmiştir. Yazılım destekleri ThermoCalc veJMatPro, mikro yapısal, mekanik incelemeler ise mikro yapı incelemesi, çekme testi, kesit boyunca mikro sertlik, Taramalı Elektron Mikroskobu incelemesi ve şekillendirilebilirlik çalışmalarını kapsamaktadır.

Yazılım destekleri ve daha önceki tecrübeler doğrultusunda sıcak haddeleme proses sıcaklıkları uygun şekilde belirlenmiş ve üretimler gerçekleştirilmiştir. Bu üretimlerde de konvansiyonel yöntemle üretilen çeliklerde görüldüğü gibi ferritik-beynitik mikro yapı elde edilmiş olup, sonuçlar mekanik özellikler bakımından S550MC kalite spesifikasyonlarına uygun gerçekleşmiştir. 

Ek olarak oluşan VN çökeltileri sıcak haddehanedeki şerit hadde bölgesinde daha düşük yüklerde haddeleme imkanı sunmaktadır. Şerit haddeleme bölgesinde mevcut olan haddeleme sıcaklığında VN çökeltileri termodinamik olarak aktif olmadığından ve yapıda çökelti oluşturabilecek başka bir mikro alaşım ilavesi olmadığından dolayı daha düşük hadde yüklerinden haddelemeyi mümkün kılmaktadır.

Son olarak şekillendirilebilirlik davranışları da konvansiyonel yöntem ile üretilen ve azotlu vanadyum ilavesi ile üretilen çeliklerde kıyaslanmış olup şekillendirme sınır diyagramlarında güvenli bölge alanları birbirine çok yakın olarak elde edilmiştir.

Abstract:
Thermomechanical rolled and cold formable HSLA steels are widely used in various industries including automotive industry. In conventional steel making process of this group of steels, micro alloying elements such as Nb, Ti and V are used to obtain required mechanical properties. In this study, nitrogen contained Vanadium used as an alloying element and effect of this addition on final properties were comparatively investigated, respectively. 

JMatPro software was used to determine phase transformation temperatures and transition regions as well as effect of cooling rate on final micro structure considering continuous cooling conditions. With the help of ThermoCalc software, stable precipitate phases were analyzed depended on temperature for both Nb, Ti, V micro alloyed steel and nitrogen containing Vanadium alloyed steel. 

V-N single microalloying in HSLA steels has a critical role to form vanadium nitride (VN) precipitations in grain boundaries. Precipitates provide enhancements on mechanical strength and formability behavior by fining grain size.

V-N single microalloying allows reaching the required mechanical properties in higher nitrogen contents and increased nitrogen content assists dispersion of VN precipitates in microstructure. Dispersion of precipitates gives higher homogenous elongation values thus relatively higher deformation conditions in cold forming is also operable considering high strength values in this grade steels.

Vanadium carbide has relatively higher solubility in austenite phase in comparison other micro alloying elements precipitates. This higher solubility provides limiting the grain size of austenite and ferritic transformation begins at finer grain size conditions. On the other hand, soluble vanadium nitride (VN) in austenite phase is similar to other precipitates as titanium carbide (TiC) and niobium carbide (NbC) thus grain size reducing by vanadium nitride (VN) precipitation is also valid.

High nitrogen affinity of vanadium helps reducing nitrogen content in reasonable levels by formation of vanadium nitride (VN) precipitates. Nitrogen fixation in HSLA steels is significant to dismiss strain aging. V-N single microalloying in this steels makes material more mechanically stable during time between hot rolling and cold deformation as known as aging time.
Experimental procedure of study could be divided in two main groups as software assisted studies and metallographic, mechanical examination studies. Software assisted studies includes ThermoCalc and JMatPro investigations where metallograhic and mechanical studies includes Scanning Electron Microscope and formability behavior examinations.

ThermoCalc software studies were performed to estimate the precipitation behavior of both type of steels. The  carbide and nitride precipitations of vanadium are known as the strengthening secondary phases, therefore it is significant to design the process parameters to obtain required amount and temperature of specific precipitate such as vanadium carbide (VC) and vanadium nitride (VN).

The effect of aluminum (Al) plays role in the solidification. It is known that 0,040% by weight is the critical addition level of Al considering embrittlement behavior Aluminum nitride (AlN) phase.

ThermoCalc examinations showed that VC precipitates are tend to form in conventionally microalloyed steel, however, VN precipitates are leading the strengthening mechanism in V-N single microalloyed S550MC steel.
In hot rolling process finishing delivery temperature and coiling temperature were defined in accordance with this ThermoCalc outputs and production has been carried out.
 
JMatPro simulations showed that bainitic transformation are possible in a large scale of cooling rate in V-N single microalloyed S550MC grade steel. In contrast, bainitic transformation cooling rate range is much narrower in conventionally Nb, Ti, V microalloyed steel.

Within the scope of metallographic and mechanical studies SEM examination, tensile test, micro hardness measurement are performed and Forming Limit Diagram (FLD) of V-N single microalloyed steel and conventionally Nb, Ti, V microalloyed steel are plotted.
Final ferritic-bainitic micro structure and mechanical properties were obtained similar to S550MC steel grade specification limits similiar to conventionally micro alloyed products. SEM examination in V-N single microalloyed steel are showed bainite phases in ferritic matrice. Operating condition for cooling rate in hot rolling process defined as 15°C/s to obtain final ferritic-bainitic microstructure. Bainitic transformation is considered for strengthening combined with precipitation hardening.

Mechanical values of both V-N single microalloyed and conventionally Nb, Ti, V microalloyed steel are meet  the limits of S550MC steel specification, despite different alloying practices similar strength values obtained. Strengthening mechanism of precipitation hardening with vanadium-interstitial phases are effectively performed with proper finishing delivery temperature and cooling temperature in hot rolling process. Results show that content of VN precipitates in V-N single microalloyed  steel are adequent to provide required strength just as VC precipitates in conventionally Nb, Ti, V microalloyed steel. Micro hardness measurements of both specimens along surface to cross section are homogeneous which is a sign of straight phase transformations. 

Formability behavior of both specimens are investigated via FLD tests. Appropriate mold geometry defined according to Nakajima methodology in tests. V-N single microalloying and conventionally Nb, Ti, V microalloying have similar effect on formability. Operable areas in diagram are inseperable.  

General evaluation of study could be summed up as V-N single microalloying in steel making for S550MC grade steel provides required final mechanical and metallographical properties. Final microstructures are ferritic-bainitic for both specimens. Bainite ratios expected different due to different micro alloying practices. Formability behavior of both specimens are similar due to ductility of dispersed VN precipitate phases. Precipitation dispersion enhanced with V-N single microalloying by dint of higher nitrogen content in steel. Further, since higher nitrogen content drives precipitation hardening in V-N single microalloying, vacuum degassing process could be eliminated for denitrification. Required final properties such as mechanical strength and formability for S550MC steel obtained with V-N single microalloying correspondingly commercial benefit provided in terms of no addition of Nb and Ti microalloying in steel making. 

HSLA steels are substantial for automotive industry nowadays and it is important to work on alternative steel making practices for this grade steels. Study showed that it is possible to obtain required final properties in S550MC grade steels with using V-N single microalloying. VN formation blocks the austenite growth and ferritic transformation occurs in finer grain size microstructures. This precipitation hardening method provides desired properties in mechanical strength and formability behavior.

Additively, VN precipitates has positive effect in hot rolling process by requiring lower rolling force in finishing mill. In finishing mill temperature conditions during hot rolling process, VN precipitates are thermodynamically inactive and with the assistance of different micro alloying element precipitate absence such as NbC and TiN, it is easier to perform desired reduction ratio at lower rolling forces.

Bainitic transformation is occurred in both processes and elongation values are relatively high considering these strength values This higher elongation values leads increased cold formability. Preclude to bainite phase embrittlement tempering is necessary to fine sharp edges of bainites. This temper heat treatment performed automatically in this study with the conduction heat transfer between wraps in final hot rolled coil product.