Microalloying of low carbon steel with boron and a method for determining the effective concentration of dissolved boron

Abstract

Goal. The effect of boron on the properties of steel is considered. It has been established that hardenability improves due to the influence of boron, which is not bound in oxides, nitrides or other compounds. This boron is in solid solution in austenite and is called an “effective” boron. The mechanism of increasing the hardenability of steel due to such boron is associated with inhibition of the occurrence of crystallization centers of ferrite and bainite. Methodology.The effective fraction of boron in dissolved state in austenite is determined by the method of electrochemical phase analysis. The microalloying of low-carbon steel 08kp with boron is tested on industrial heats under conditions of Martin workshop of JSC «Zaporozhstal». Smelting and teeming of steel were performed according to the requirements of operating technology. The input of ferroboron into metal took place while letting out the heats into the ladle after completion of the ferromanganese input. All the experimental heats were blown through in the ladle with argon to appearance of the furnace slag. For microalloying the ferroboron of the grade FB20 was used. At the by-sheet sorting of the experimental ingot of the heat (the average content of boron in the heat - 0,00138%), sampling of metal rolled stock was brought off on 5 horizons by the height. In every sample the chemical analysis for the content of boron was executed in 3th points, corresponding to the edge of roll, ¼ on the width and axis. Results of the chemical analysis are presented in the table 1. At the average content of boron in the heat - 0,00138%, the content of boron was distributed along the height and width of the ingot was distributed from 0,0008% (-0,00058%) to 0,0024% (+ 0,00102%). Results. The technology of microalloying low-carbon steel with boron has been mastered with obtaining a guaranteed content of at least 0,0008%. Putting a ferroboron into the ladle with a flow rate of 0.26 - 0.31 kg / t followed by purging the metal in a ladle with pure argon provides the boron content in the steel 0.0009 - 0.0015%. The absorption of boron during microalloying depends on the fractional composition of boron-containing material. For a fraction of less than 3 mm, assimilation is 29%, and for a fraction of 5-10 mm, 76% of the assimilation values obtained for an optimal fraction of 20-40 mm. Boron in 08kp steel increases the tensile strength by 4.5% and the yield strength by 2.6% compared with the average values of this steel without boron.