Precipitation Behavior of Boron in High Strength Steel

Abstract

The hardenability of boron steel is afected by the precipitation behavior of B. Precipitates in the B containing steel were studied by extraction replica technique at the successive manufacturing steps from hot-rolling to heat treatment.When steels are hot-rolled after heating at 1300°C for 1h, changes of B precipitates with the cooling rate after hot-rolling are as follows:(1) When steels are water-quenched (cooling rate of 90000°C/min) after hot-rolling, no precipitates can be observed on γ grain boundaries.(2) When steels are air-cooled (cooling rate of 300°C/min) after hot rolling, BN precipitates on γ grain boundary region.(3) When steels are furnace-cooled (cooling rate of 0.67°C/min) after hot-rolling, AIN grows radially from BN which has precipitated before the nucleation of AIN. AlN formed on BN during furnace-cooling is dendritic.By reheating at a rate of 15°C/min a specimen which has been waterquenched after hot-rolling (above-mentioned as (1)), the following B precipitates appear depending upon the temperature range:(1) Below 880°C, M23(CB)6 precipitates and coagulates on prior grain boundaries.(2) Above 900°C and below about 1000°C, BN appears on prior grain boundary region which has been previously occupied by M23(C, B)6. M23 (C, B)6 is no longer observed at this stage.(3) Heating at around 1000°C, AlN which is not dendritic but acicular grows radially from BN. The size of BN diminishes with growth of acicular AlN.The formation of AlN on BN is interpreted as a transit from the nonequilibrium to equilibrium state in Al-B-N system in steels.According to Pease, BN (hexagonal close packed) has the same crystal structure as that of graphite. Because BN in steels was revealed to have spherulite morphology such as spherical graphite, it is considered that [00.1] axes of BN are arranged in radial fashion and (00.1) planes are perpendicular to them, by an analogy to the crystal arrangement of spherical graphite. As the growth direction of AlN is [0.01]AlN, it is considered that an orientation relationship of (00.1)BN//(00.1)AlN is satisfied when AlN grows on BN.