This article focuses on the three main factors affecting the properties of steel castings.
The Impact of Common Impurity Elements in Steel
The Impact of Common Elements
Common impurity elements in steel include phosphorus (P), sulfur (S), hydrogen (H), nitrogen (N), and oxygen (O). These elements generally have a detrimental effect on steel properties, but some can also have beneficial effects under specific conditions and serve as alloying elements (for example, sulfur and phosphorus can improve machinability).
The Impact of the "Five Harmful Elements"
Lead, tin, antimony, bismuth, and arsenic are often referred to as the "five harmful elements" in steel. These "five harmful elements" in steel share several common characteristics:
① Their melting points are relatively low compared to that of steel. They remain liquid even when the steel is solid, and are therefore often referred to as low-melting-point elements.
② When their content in steel exceeds a certain limit, they significantly reduce high-temperature mechanical properties, increase high-temperature brittleness, reduce strength and toughness, and render the steel brittle.
③ They often coexist, causing severe segregation. They rarely exist alone, and therefore have a more destructive effect on steel.
Impact of Non-Metallic Inclusions in Steel
Non-metallic inclusions in steel include oxides, sulfides, oxysulfides, nitrides, and silicate compounds. These inclusions originate from two main sources: exogenous and autogenous. Exogenous inclusions include impurities carried over from the charge during the steelmaking process, such as refractory material that falls off the furnace lining due to corrosion. Autogenous inclusions are formed during the steelmaking and pouring processes due to oxidation or other chemical reactions of elements in the molten steel. Inclusions reduce the mechanical properties of steel, particularly toughness. The extent to which inclusions weaken steel properties depends on their shape and distribution: Sharp, polygonal inclusions cause significant stress concentrations in the steel, easily forming crack sources under external forces; granular and spherical inclusions pose less of a threat; strip-shaped inclusions, when distributed in a network or discontinuous network along the grain boundaries, can cause a fracture in the steel, reducing its mechanical properties; isolated island-like inclusions have less of a fracture effect.
To mitigate the harmful effects of inclusions, two approaches can be taken:
1. Inclusion Removal
Generally, this can be effectively removed by ensuring that the molten steel is well-boiled during the oxidation phase of steelmaking. A period of calming (5-10 minutes) is then allowed after tapping and before pouring to allow inclusions to float out of the molten steel (larger, lighter-density inclusions float more quickly). Using refining treatment outside the furnace can even more effectively remove inclusions from the molten steel.
2. Improving Inclusion Morphology
Improving the shape and distribution of inclusions can also effectively mitigate the harmful effects of inclusions. For example, treating the molten steel with rare earth alloys can transform polygonal oxides and strip-shaped sulfides into spherical rare earth oxysulfides.
The Effect of Cast Steel Solidification Rate on Microstructure and Properties
The microstructure and properties of steel castings are determined not only by their chemical composition but also by the solidification rate. Castings with different wall thicknesses cast from the same furnace, or with varying thicknesses within the same casting, can exhibit significant microstructural differences due to varying solidification rates. This difference is mainly manifested in the primary crystallization structure (such as dendrite size and dendrite spacing), structure density, and non-metallic inclusion morphology (for example, when the solidification rate is low, inclusions tend to aggregate, resulting in larger inclusion sizes). These differences in structure lead to differences in performance.
Post time: Aug-21-2025