Effect of Boron on Hot Ductility and Room-Temperature Tensile Properties of Microalloyed Steels with Titanium and Niobium.

Qi Li,Weijie Liu

doi:10.3390/ma12142290

Abstract

Effect of boron on the hot ductility and room-temperature tensile properties of Ti-Nb-microalloyed steels containing 0.071 wt.% carbon was studied. The thermal stress and thermal strain of continuous casting billets during cooling were simulated via hot tensile tests at the deformation rate of (6 mm/11,000)/s, and the hot ductility of different microalloyed steels was evaluated according to the area reduction of hot tensile specimens. It was found that boron addition was beneficial to improve the hot ductility of continuous casting billets during straightening, and the reduction of area exceeded 60%. The addition of boron, as well as the removal of molybdenum and vanadium, can effectively lower the austenite-to-ferrite transformation temperature and restrain the formation of intergranular ferrite, so as to avoid the brittle zone. Moreover, the room-temperature tensile properties of the steels were explored at different cooling rates after the rolling process. The results showed that as the cooling rate increased from 0.0094 to 0.13 °C/s, the amount of carbonitride precipitate gradually decreased, such as titanium carbide, leading to the relatively low tensile strength. On the other hand, the addition of boron, as well as the removal of Mo and V, promoted the formation of bainite and acicular ferrite, playing an important role in structure strengthening, and compensated for the decrease of tensile strength caused by the low precipitation strengthening.

Highlights

The hot ductility loss of microalloyed steels during continuous casting is a severe problem and is largely related to the numerous surface cracks formed on the casting billets
The cracks in the third brittle zone are usually surface cracks, which is because the high plastic deformation caused by the thermal stress during the cooling process of the slab exceeds the temperature plastic deformation caused by the thermal stress during the cooling process of the slab fracture limit of the material
The thermal strain can be calculated from Equation (1), thermal strain in austenite, and εα is the thermal strain in ferrite

Summary

Introduction

The hot ductility loss of microalloyed steels during continuous casting is a severe problem and is largely related to the numerous surface cracks formed on the casting billets. At the straightening stage of continuous casting billets, hot ductility is a valid parameter for evaluating the crack sensitivity of steel billets. The hot ductility of continuous casting billets is affected by many factors, including the austenite/ferrite transformation, precipitate phase, and dynamic recrystallization [1,2,3,4]. The variation of hot ductility caused by these microstructure changes is critically associated with the microalloy elements of steels. The surface transverse cracks of casting billets were solved at early times by studying the hot ductility behaviors via high-temperature mechanical tests. Suzuki et al proposed and improved the mechanism of three brittle temperature ranges [5,6,7] in order to solve the cracking of casting billets

Methods

Results

Conclusion