Control of inclusions in 55SiCr spring steel

Abstract

Hardened and tempered 55SiCr spring steel is manufactured to be suitable for working under high cyclic stresses, especially in environments where high strength, superior wear resistance, and fatigue strength are required. To improve cleanliness levels of spring steel, the investigation of inclusions characteristics (chemical composition, number density, and size distribution) and their formation mechanisms in the refining process are important.The first part of the study analyzed three different spring wire samples with microalloying V and Nb addition. Additions of Nb and V did not have observable impacts on the formation of inclusions. Electron probe X-ray microanalysis (EPMA) was used to accurately measure the compositions, sizes, and shapes of the inclusions. CaO-SiO2 inclusions in the steel were associated with the refining slag as they have the similar compositions. Most Al2O3-SiO2-CaO inclusions with high Al2O3 and SiO2 contents originated from the coalescence of CaO-SiO2 and Al2O3-SiO2 inclusions; those with high CaO and SiO2 contents were considered to form by reduction of CaO-SiO2 inclusions by Al dissolved in the steel. Wavelength-dispersive spectrometry (WDS) and Monte Carlo simulation were carried out to verify the existence of trace contents of inherent FeO in the inclusions.The second part of the study elaborated on the formation and evolution of various inclusions in Si-deoxidized spring steel through the refining process. Steel samples from a vacuum degassing (VD) furnace and a tundish (TD) furnace, as well as the wire rods after hot rolling (HR), were collected. The Al2O3-SiO2-CaO inclusions in the steel samples from the vacuum degassing and tundish furnaces were formed by reduction of CaO-SiO2 inclusions by Al dissolved in steel. The increased number density of Al2O3-SiO2-CaO inclusions in spring rods was primarily attributed to the entrapment of Na2O-contained mold flux particles and inclusions crush after hot rolling. Al2O3-SiO2-MnO and SiO2-MnO inclusions originated from inherent reactions between [Al], [Si], and [O] in the molten steel. The precipitation of Al2O3-SiO2-MnO inclusions was attributed to solubility decrease during the solidification process with the temperature drop.The third part of the study focused on the source, characteristics, and mechanism of Al2O3-containing inclusions in Al-deoxidized spring steel utilizing electron probe X-ray micro-analysis. Al2O3-CaO inclusions were considered to be formed by the reduction of entrapped slag by Al dissolved in the steel. The Al2O3/CaO ratio obviously decreased with the increase of inclusion sizes. The Al2O3-MgO and Al2O3-SiO2-MnO inclusions originated from inherent reactions between dissolved [Al], [Si], , [Mg] and [O] in the steel. Al2O3-MgO-CaO inclusions were resulted from coalescence between Al2O3-MgO and Al2O3-CaO inclusions.The fourth part of the study investigated the effects of different refractories on non-metallic inclusions characteristics in Si-deoxidized spring steel. Three types of refractories (MgO, Al2O3, and CaAl12O19) were utilized to explore the formation and characteristics of non-metallic inclusions with electron probe X-ray micro-analysis on a laboratory scale. MgO and CaAl12O19 refractory obviously reduced the number density and average sizes of the inclusions. CaO-SiO2-MgO and SiO2 inclusions were newly-formed in the steel with MgO refractory. Al2O3 refractory had a negative effect on the inclusions control owing to the widespread presence of Al2O3-SiO2-CaO inclusions, which resulted from CaO-SiO2 inclusions reduced by Al dissolved in the steel.The fifth part of the study explored the effect of refractory on the evolution mechanism of non-metallic inclusions in Al-deoxidized spring steel on a laboratory scale. Al2O3 and Al2O3-MgO inclusions were the primary inclusions and mainly attributed to the deoxidization products. Al2O3-MgO-CaO inclusions were resulted from coalescence between MgO and Al2O3-CaO inclusions as well as reduction of Al2O3-CaO by dissolved Mg. Dual-phase Al2O3-MgO-CaO inclusions were characterized by pure MgO cores surrounded by liquidus Al2O3-CaO layer, which was related to the substitution of dissolved Ca and Al in molten steel for MgO in the inclusions.The sixth part of the study explored the liquidus temperatures and phase equilibria in the (Al2O3+SiO2)-(CaO+SiO2)-MgO system at fixed MgO with high-temperature equilibration and quenching technique utilizing electron probe X-ray microanalysis. Isotherms in the interval of 20 °C between 1260 and 1560 °C were identified in the different phase fields of anorthite, wollastonite, spinel, and melilite. Effects of the mass ratio of (Al2O3+SiO2) /((Al2O3+SiO2)+(CaO+SiO2)) and MgO content on the liquidus temperatures have been discussed to assist inclusion control. To decrease the liquidus temperature of Al2O3-SiO2-CaO-MgO system inclusions, refining time and deoxidation production amounts ought to be reduced.