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Univerzitet u Zenici
  Doctoral theses defended at the University in Zenica

Date and place of thesis defense:
20.09.2010. Faculty of Metallurgy and Materials Science
Candidate:
mr. sc. Besim Baručija
Mentor:
prof. dr. Sreto Tomašević

Thesis title:

Impact mechanisms of aluminum nitride onto a cold deformability of low-carbon steel quenched with aluminum with the nitrogen content of > 70 ppm

Summary:
For the purpose of discovering the cause of low plasticity of the f6-25 mm hot-rolled wire during the cold heading test of aluminum-deoxidized steels, as prescribed by DIN 1654/Part2/80 or ISO 4954, thirty-two QSt.steel test heats were produced in Zenica Iron and Steel Works (now ACELOR MITTAL Ltd Zenica). Particularly low plasticity of the cold-state QSt.steel was observed on the hot-rolled wire produced on the Wire Rod Mill, while somewhat better plasticity was observed on the hot-rolled wire produced on the Fine Section Rolling Mill.
The low plasticity results obtained during the cold heading tests carried out on the sixteen QSt.steel test heats (the first group) with lower carbon and manganese content show that the existing rolling mill production technology under consideration is not the best in relation to the contemporary metallurgical practice in the production of these steel grades. Current best metallurgical practices in the production of aluminum-deoxidized steels should ensure an optimum chemical composition, interior homogeneity, higher purity, an optimum microstructure (coarser grains and as low pearlite content as possible), as well as a smooth surface of the wire rod.
The working hypothesis predicts that the uncontrolled development of aluminum nitride deposits takes place during the process of aluminum deoxidization of the molten steel. According to Zener, this blocks austenite grain growth during heating, the process of dynamic and static re-crystallization, as well as the grain growth after the hot rolling deformation, which results in the development of the fine-grained structure, with the grain size up to 10 (according to ASTM). The fine-grained structure and the high values of the upper yield stress (ReH) are harmful to this steel grade, i.e. it is coarse grain microstructure and lower values of the upper yield stress that are needed in order to achieve satisfactory cold-state plasticity. This has been achieved by means of the new production technology. The new thermomechanical treatment gave the reduction of the initial rolling temperature, longer holding time of the batch in the rocker-bar heating furnace obtained by accelerating the rolling rhythm, as well as slower cooling after hot rolling as compared to the existing production technology.
As for the author's hypothesis predicting the uncontrolled development of aluminum nitride deposits in the QSt.steel, a new production technology has been created for both rolling mills under consideration. Using the new thermomechanical treatment the second group of the sixteen test heats with higher carbon, manganese and aluminum content has been processed. The nitrogen content in the second group of the test heats was between 88 and 119 ppm, which exceeds the allowable content of 70 ppm.
The tensile test of the mechanical properties and microstructure carried out on the hot-rolled wire, which was produced using the new thermo-mechanical production technology, revealed lower values of the upper yield stress (ReH) by 15 MPa to 24 MPa, as well as higher values of the cross section contraction (Z) which were between of 1.6 and 2.8%, in comparison to the hot-rolled wire produced using the old technology. The secondary grain size was smaller by 1-2 degrees (according to ASTM) in relation to the values of the secondary grain size of the hot-rolled wire produced using the old production technology.           
The lower values of the upper yield stress (ReH), the higher values of the cross section contraction (Z), as well as coarser secondary grain microstructure were beneficial to a relatively satisfactory increase in QSt.steel plasticity during the cold heading tests by 45% to 83.5% depending on the rolled wire dimension, as well as the particular rolling mill.
The results of the microstructure investigation using transmission electron microscope (TEM) confirmed the hypothesis about the existence of aluminum nitride deposits in the QSt.steels. Thanks to the new production technology, these deposits are coarser, and create minor barriers to the grains boundary motion during the processes of dynamic and static re-crystallization, as well as less obstruction of the grain growth subsequent to the hot rolling treatment, which results in a coarser austenite, that is ferrite grain in the steel microstructure.
The aluminum nitride deposits which are not coherent with the matrix crystal system should reach Ostwald's maturity during the process of the thermo mechanical treatment of the Qst. steels so that, during the cold-state plastic deformation, they may create minor barriers to the motion of dislocations at the yield point according to Orowan mechanism. In view of this, lower values of the upper yield point (ReH) are acquired.

Key words:
thermomechanical processing, re-crystallization, deposits, secondary grain size/structure, plasticity


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