André Barros Cota

Microstructure and mechanical properties of a microalloyed steel after thermal treatments

The properties of a microalloyed steel, with Nb and V in its composition, were studied, after different intercritical thermal treatments and at different austenitizing and tempering temperatures. The mechanical properties of the specimens were measured in a Vickers hardness tester, and their microstructures were analyzed by optical microscopy, with the aid of a digital image processor. After austenitizing at 1100 °C and tempering at 625 °C, the samples showed significantly higher tempering resistance, reflected by their retention of high hardness, which may be associated with a secondary hardening precipitation of Nb carbon nitrides. In the sample with dualphase microstructure, the martensite volume fraction varied from 18.2 to 26.3% and the ferrite grain size remained unchanged, upon the variation of the time length of the intercritical treatments. Tempered samples showed Vickers hardness (HVN) varying from 327 to 399, and dualphase samples showed HVN from 362 to 429.

The kinetics of austenite formation at high heating rates

Foi estudada por dilatometria a variacao das temperaturas criticas para a formacao de austenita, com o aumento da velocidade de aquecimento. A analise foi realizada a velocidades de aquecimento entre 10 e 90oC/s. Foram propostas equacoes empiricas para o calculo de Ac3, em funcao da velocidade de aquecimento. O aumento da velocidade de aquecimento nao afetou Ac1, mas Ac3 aumentou em 115oC. As equacoes propostas se revelaram de uso mais geral, uma vez que tambem previram os resultados obtidos em trabalhos para outros acos com altos coeficientes de correlacao.

Effect of processing parameters on scale formation during hot steel strip rolling.

The influence of processing parameters (slab thickness, water flow of interstand cooling and oil flow in roll gap lubrication system) on the thickness and composition of the tertiary scale formed during hot strip rolling, was studied in a low carbon steel in factory. The scale formed on the rolled surface was characterized by scanning electron microscopy and Mossbauer spectroscopy. It was observed that the combined effect of a greater rolling oil volume applied, larger bar thickness, and smaller amount of water flow during interstand cooling reduces the tertiary scale thickness. Besides, a smaller crack density in the samples is associated with greater rolling oil volume and smaller oxide scale thickness. The principal phase of the scale formed in hot-rolled steel strips is stoichiometric magnetite, without isomorphic substitutions.

Study of the Simultaneous Effects of the Hot Band Grain Size and Cold Rolling Reduction on the Structure and Magnetic Properties of Nonoriented 3% Si Steel

The simultaneous effects of the hot band grain size and cold reduction on the final grain structure, texture and magnetic properties of 3% Si steel were studied by variation of finishing temperature and hot band thickness during hot rolling, followed by cold rolling to final thickness and annealing in H2/N2 atmosphere. The results have shown that there is an optimum combination of hot band thickness and hot band grain size for maximum Eta/Gamma ratio and better magnetic properties (B50 and W1.5/60) after final annealing. For the thicker hot band, it was observed that higher finishing temperature, or larger hot band grain size, was necessary to achieve high Eta/Gamma and better magnetic properties. Too large hot band grain size, however, could result in smaller final grain size due to the intense nucleation on recrystallization in the shear bands and consequently poor core loss values. This behavior was explained based on the hot band grain structure and cold rolling effects on the texture evolution.

The Influence of Hot Rolling Finishing Temperature on the Structure and Magnetic Properties of 2.0%Si Non-Oriented Silicon Steel

Rougher bar samples of 2.0%Si and 0.25%Al Non Oriented Grain with 25mm thickness were rolled to 2.3mm in a pilot reversible mill with temperatures ranging from 920 to 1120°C and processed until the final annealing to study the evolution of grain size and crystallographic orientation. The results showed that higher hot rolling finishing temperature results in larger hot band grain size, but the texture, composed mainly by alpha, gamma, {100} , and eta fibers suffers little effect. As a consequence, after final annealing, an increase of fraction of the grains with eta fiber orientation and a reduction of grains with gamma fiber orientation are observed, improving the magnetic induction. The final grain size decreases with hot rolling finishing temperatures above 1040°C in spite of increasing hot band grain size, deteriorating the core loss results. This was explained by the increasing of shear bands when the grain size prior to cold rolling increases. More shear bands generates more sites for nucleation on recrystallization, then smaller final grain size results. The Goss orientation is generated mainly in the sites inside shear bands. The increasing of prior grain size also increases the fraction of grains with Goss orientation after final annealing and its fraction increases suddenly when the hot rolling finishing temperature achieves 1040°C, supporting that affirmation.

Effect of Hot Band Grain Size on the Texture Evolution of 2%Si Non-Oriented Steel During Final Annealing

Industrial rough bar samples of 2%Si were hot rolled in a pilot reversing mill at different finishing temperatures. Hot rolled bands 2.3 mm in thickness were cold rolled to 0.50 mm and final annealed at different temperatures under H2-N2 atmosphere. To study the effect of hot band annealing, some samples were annealed at 1030 °C before cold rolling. Crystallographic texture and grain size (GS) were measured at the different processing steps. Magnetic properties were measured after final annealing. The results have shown that in case of finishing reversible hot rolling, hot band annealing may not result in benefit for magnetic properties due to the effect of static recrystallization between passes that already generates coarse hot band structure. Hot band GS has strong effect on final structure and magnetic properties. The increase of hot band GS has a beneficial effect on texture and magnetic polarization J50 due to the increase of eta fiber fraction. However, large initial GS may result in smaller GS after final annealing due to intensive nucleation in shear bands on recrystallization. On further grain growth, the fraction of eta fiber grains tend to decrease, resulting in simultaneous decrease of J50 and core loss.

A study of isochronal austenitization kinetics in a low carbon steel

The austenite formation under isochronal conditions in Nb microalloyed low carbon steel was studied by means of dilatometric analysis and the data was adjusted to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, for different heating rates and for three initial microstructures. It was shown that the kinetics of austenitization of a pearlite+ferrite structure is faster than that of martensite (tempered martensite) at a heating rate of 0.1 o C/s. For heating rates higher than 0.1oC/s, the kinet ics of austenitization of a martensite structure is faster than of pearlite+ferrite one. The K parameter of the JMAK equation increases with the heating rate for the three previous microstructures and it is greater for the initial microstructure composed of ferrite+pearlite. At lower heating rates, the formation of austenite in this steel is controlled by carbon diffusion, independently of the initial microstructure. At higher heating rates, the formation of austenite from an initial microstructure composed of pearlite and ferrite is controlled by interface-controlled transformation.

Efeito da rápida austenitização sobre as propriedades mecânicas de um aço SAE1045

This work studied the effect of fast austenitization on the mechanical properties of SAE1045 steel, as quenched and tempered. The samples austenitized at 900 and 950oC, and immediately quenched after reaching these temperatures, showed microstructures of refined martensite, with small volume fractions of polygonal and acicular ferrites. The values of the tensile and yield strengths of the tempered samples are strongly dependent on the soaking time. The phase transformations during the tempering have similar effects on the tensile and yield strengths of the tempered samples. These samples have high yield ratio values (average of 0.91). The values of the product of the tensile strength for the percent elongation varied from 32589MPa% to 24150MPa%. The optimization of the mechanical properties of the SAE1045 steel, with high strength and ductility, was achieved for the austenitization at 900oC, immediately quenched after reaching that temperature, and tempering at 600oC for 100s.

Tratamento térmico por indução eletromagnética em tubos de aço SAE 1045 para produção de hastes de sondagem geológica

This work describes the production of geological drill rods through electromagnetic induction thermal treatment of tubes. The research aims at determining, through metalography, mechanical tests and field tests, the ideal induction hardening and tempering parameters, to produce drill rods with qualities similar to the ones available in the international market. The determination of the induction hardening and tempering parameters of SAE-1045 steel tubes, for the production of geological drill rods, was capable of producing hardness profiles with low fluctuations in values, ranging from a maximum value of 45HRc close to the extremity, to minimum values close to 35HRc at the end of the treated region.

Modelagem da formação não isotérmica da austenita em um aço de baixo carbono

The austenite formation under isochronal conditions in Nb micro-alloyed low carbon steel was studied using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, for different heating rates and by means of dilatometric curves. The functional relationship between the apparent activation energy and heating rate was used in addition to this model. It was shown that this functional relationship is logarithmic and that the apparent activation energy decreases from 152.5 to 118.1 kJ/mol when the heating rate increases from 0.1 to 16 K/s, with the n exponent of the JMAK equation equal to 1.0.