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Effect of ferrite-soluble alloying elements
Oct 17, 2018

Most alloying elements are added to low carbon steel in order to produce solid solution hardened steel at certain ambient temperatures, increasing the lattice frictional stress δi. However, it is currently not possible to predict lower yield stresses using only formulas unless the grain size is known. Although the determinants of yield stress are normalizing temperature and cooling rate, this method of research is still important because the range of toughness can be reduced by predicting a single alloying element by increasing δi.


The regression analysis of the non-plastic transition (NDT) temperature and the Charpy transition temperature of ferritic steel has not been reported so far, but these are also limited to the qualitative discussion of the effect of adding a single alloying element on toughness. The following is a brief introduction to the effects of several alloying elements on the properties of steel.


1) Manganese


The vast majority of manganese is about 0.5%. The addition of a deoxidizer or a sulfur-fixing agent prevents thermal cracking of the steel. The following effects are also found in low carbon steel.


◆The carbon content of 0.05% steel has a tendency to reduce the formation of cementite film at the grain boundary after air cooling or furnace cooling.


◆ The ferrite grain size can be slightly reduced.


◆ Can produce a large number of small pearlite particles.


The first two actions indicate that the NDT temperature decreases as the amount of manganese increases, and the latter two effects cause the peak of the Charpy curve to be sharper.


When steel has a high carbon content, manganese can significantly reduce the transition temperature by about 50%. The reason may be due to the large amount of pearlite, rather than the distribution of cementite at the boundary. It must be noted that if the carbon content of the steel is higher than 0.15%, the high manganese content plays a decisive role in the impact properties of normalized steel. Because of the high hardenability of steel, austenite transforms into brittle upper bainite rather than ferrite or pearlite.


2) Nickel


The effect of adding steel to manganese is to improve the toughness of the iron-carbon alloy. The size of the action depends on the carbon content and heat treatment. In steels with a low carbon content (about 0.02%), the addition of 2% can prevent the formation of cemented carbides in the hot-rolled state and normalized steel, while substantially reducing the initial transition temperature TS and increasing the Charpy impact. Curve peak.


Further increase in nickel content and improvement in impact toughness are reduced. If the carbon content is low until no carbide occurs after normalizing, the effect of nickel on the transition temperature will become very limited. The addition of nickel to normal-fired steel containing about 0.10% carbon has the greatest benefit of refining the grains and reducing the free nitrogen content, but the mechanism is still unclear. It may be due to the fact that nickel acts as a stabilizer for austenite, thereby lowering the temperature at which austenite decomposes.


3) Phosphorus


In a pure iron-phosphorus alloy, phosphorus segregation due to ferrite grain boundaries reduces the tensile strength Rm and causes intergranular embrittlement. In addition, since phosphorus is also a stabilizer for ferrite. Therefore, adding steel will greatly increase the δi value and the ferrite grain size. The combination of these effects will make phosphorus an extremely harmful embrittlement agent, undergoing transgranular fracture.


4) Silicon


Silicon is added to the steel for deoxidation and is beneficial for improving impact properties. If both manganese and aluminum are present in the steel, most of the silicon is dissolved in the ferrite, and δi is increased by solid solution hardening. The combined effect of this effect and the addition of silicon to enhance the impact properties is that silicon is added in weight percent in a stable grain size iron-carbon alloy, raising the 50% transition temperature by about 44 °C. In addition, silicon is similar to phosphorus and is a stabilizer for ferrite, which promotes ferrite grain growth. Addition of silicon to normalized steel by weight percent will increase the average energy conversion temperature by about 60 °C.


5) Aluminum


There are two reasons for the addition of alloys and deoxidizers to the steel: first, the formation of AlN with nitrogen in the solution to remove free nitrogen; second, the formation of AlN refines the ferrite grains. The result of both of these effects is that for every 0.1% increase in aluminum, the transition temperature is lowered by about 40 °C. However, when the amount of aluminum added exceeds the need, the effect of "cure" free nitrogen will be weakened.


6) Oxygen


Oxygen in the steel causes segregation at the grain boundaries resulting in intergranular fracture of the iron alloy. The oxygen content of the steel is as high as 0.01%, and the fracture occurs along a continuous channel created by the grain boundaries of the embrittled grains. Even if the oxygen content in the steel is very low, the crack will nucleate at the grain boundary and then diffuse through the crystal. The solution to the problem of oxygen embrittlement is to add deoxidizers carbon, manganese, silicon, aluminum and zirconium to combine with oxygen to form oxide particles, and to remove oxygen from the grain boundaries. Oxide particles are also advantageous materials for retarding ferrite growth and increasing d-/2.

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