Effects of deformation on the microstructure of a Ti-V microalloyed steel in the phase transition region

Abstract

Microalloyed steels are used in the forging of many automotive parts like crankshafts and connecting rods. They are hot worked in a sequence of stages that includes the heating to the soaking temperature, followed by forging steps, and finally the controlled cooling to define the microstructure and mechanical properties. In this work it was investigated the thermomechanical behavior and the microstructural evolution of a Ti-V microalloyed steel in the phase transition region. Torsion tests were done with multiple steps with true strain equal to 0.26 in each step. After each torsion step the samples were continuous cooled for 15 seconds to simulate hot forging conditions. These tests provided results for the temperature at the beginning of the phase transformation, and allowed to analyze the microstructural changes. Also, workability tests were held to analyze the microstructural evolution by optical and scanning electron microscopy. Results from the torsion tests showed that the temperature for the beginning of phase transformation is about 700 ºC. Workability tests held at 700 ºC followed by water-cooling presented microstructures with different regions: strain hardened, and static and dynamic recrystallized. Workability tests at 700 ºC followed by air-cooling showed a complex microstructure with ferrite, bainite and martensite, while tests at 650 and 600 ºC followed by water-cooling showed a microstructure with allotriomorphic ferrite present in the grain boundaries of the previous austenite.<br>Os aços microligados são usados em peças automotivas forjadas, tais como girabrequins e bielas. Essas peças são trabalhadas a quente em uma seqüência de estágios, que inclui o aquecimento até a temperatura de encharque, seguido por vários passes de forjamento, e, finalmente, há um resfriamento controlado para definir a microestrutura e propriedades mecânicas. Nesse trabalho, foram investigados o comportamento termomecânico e a evolução microestrutural de um aço microligado Ti-V na região de transição de fase. Os testes de torção foram feitos em múltiplos passos com deformação verdadeira de 0,26 em cada passo. Após cada passo de torção, as mostras foram resfriadas continuamente por 15 segundos para simular as condições de forjamento a quente. Esses testes forneceram resultados da temperatura de início de transformação de fase e permitiram analisar as mudanças de microestrutura. Foram, também, realizados os testes de trabalhabilidade para analisar a evolução microestrutural por microscopia ótica e eletrônica de varredura. Os resultados dos testes de torção mostraram que a temperatura de início de transformação de fase está em torno de 700ºC. Os testes de trabalhabilidade feitos a 700ºC seguidos por resfriamento em água apresentaram microestruturas com características distintas: regiões encruadas e de recristalização estática e dinâmica. Os testes de trabalhabilidade de 700ºC seguidos por resfriamento ao ar mostraram uma complexa microestrutura de ferrita, bainita e martensita, enquanto que os testes feitos em 650 e 600ºC ...

The study of central cracking mechanism and criterion in cross wedge rolling

“…Cross wedge rolling (CWR), an innovative metal forming process whereby a cylindrical billet is plastically deformed into a stepped shaft under the action of two dies with wedges moving tangentially to each other, is mostly used in manufacturing steel products such as camshafts and connecting rods in the automotive industry [1]. It has been promoted to other industries such as high-speed railways [2] and nuclear reactors [3] where materials such as titanium alloys [2,4], aluminium alloys [5] and 42CrMo/Q235 laminated materials [6,7] are used. It was reported that the annual production of CWR parts in China reached over 2 billion pieces with a gross weight of over 400,000 tons in 2018 [8].…”