High-strength low-alloy steel can be used for both skyscrapers and long-span bridges, as well as for line pipes, very large vessels, and near-shore pressure vessels. However, in order to manufacture these devices, the low alloy steel used is required to satisfy both high strength and impact toughness as well as good weldability. With TMCP, boron can be used to replace carbon and other alloying elements to enhance strength, even if a small amount of boron (below 100 ppm) can affect the microstructure and mechanical properties of steel. Due to the development of trace boron control and detection technology, the addition of boron to steel has begun to receive attention.

Boron can enhance the hardenability of steel by grain boundary segregation, and can also precipitate at grain boundaries or affect the precipitation rate of carbides and other precipitates. Experiments have shown that the segregation of boron is initially increased and then decreases as the heat input increases.

It is known that even a small amount of boron as an alloying element can increase the hardenability of steel by segregation. Although the boron content is very low, martensite can be formed under very slow cooling rate conditions, such as 2 ° C / s and 1 ° C / s.

Welding heat input is a very important factor in various welding parameters because thermal cycling, such as heating and cooling rates during welding, is determined by heat input. Therefore, the microstructure and mechanical properties of the weld can be greatly affected by the heat input. In addition, heat input can affect boron segregation because boron segregation behavior is determined by thermal cycling. Regardless of the external stress, the level of boron segregation at the grain boundary initially increases and then decreases as the heat input increases. This means that the highest level of boron segregation may occur at the intermediate heat input, ie there is a critical heat input. It is believed that such a result is due to the reverse diffusion of boron after the segregation of non-equilibrium grain boundaries with increasing exposure time at high temperatures. In other words, unbalanced segregation may initially occur at low heat input. Then, as the cooling rate decreases and the exposure time at high temperatures increases, the increase in the diffusion time of the vacancy boron complex can increase the level of boron segregation. Finally, due to the difference in boron concentration after critical heat input, the segregated boron atoms can diffuse from the grain boundaries to the inside of the grains.

According to previous studies, boron can effectively inhibit the formation of a ferrite phase. Hardenability can only be affected by segregation of boron grain boundaries. With the addition of boron, the hardenability increases, and the granular bainite phase based on the ferrite structure is effectively suppressed. Therefore, although cooling is performed at a slow cooling rate such as 5 ° C / s and 2 ° C / s, a harder phase such as martensite and bainitic ferrite is mainly observed at a lower temperature.

The study of the evolution of mechanical properties due to the addition of boron is mainly carried out by considering the microstructure relationship. Some researchers have also studied the effects of boron segregation on grain boundaries and grain interiors, and these work reveal non-equilibrium segregation behavior. Grain boundary strengthening due to boron segregation at the grain boundary. These results are also consistent with previous studies. Although the level of boron segregation is low, the strengthening effect of the martensite phase is higher than that of the bainite phase. In the presence of the bainite phase, although boron segregation occurs due to a very slow cooling rate, a second phase free of ferrite can be formed at the grain boundaries. Therefore, since the second phase free of ferrite exists at the grain boundary, the strengthening effect of boron segregation is lowered.