Chemical composition of steel substrate and its effect on properties of coated steel substrate

Chemical composition of steel substrate and its effect on properties of coated steel substrate

The main component in steel is iron, and iron-carbon alloys with carbon content less than 2.11% are called steel. However, industrial carbon steel has other elements in addition to carbon, such as silicon, manganese, phosphorus, sulfur, etc., which are divided into deoxygenated elements (such as silicon, manganese, etc.), impurity elements (such as smelting not exhausted phosphorus, sulfur, etc.) and alloying elements (added to improve performance such as molybdenum, chromium, rare earth, etc.).

(1) The effect of carbon

Carbon is the main element that determines the microstructure and properties of carbon steel after slow cooling. In general, the carbon content increases, the strength increases, the elongation decreases, in addition, the different structures (ferrite, cementite and pearlite) in the performance of the difference is also obvious. When the carbon content increases, the corrosion resistance of carbon steel decreases, and carbon also makes the welding performance and cold workability (stamping, drawing) of carbon steel worse.

(2) The effect of manganese

The content of manganese in carbon steel is generally 0.25% to 0.80%, and in carbon steel with higher manganese content, the manganese content can reach 1.2%. In carbon steel, manganese is a beneficial element. For the killed steel, manganese can improve the deoxidation effect of silicon and aluminum, and can also be combined with sulfur to form manganese sulfide, thereby eliminating the harmful effects of sulfur in steel to a considerable extent. Manganese has a good effect on the mechanical properties of carbon steel, which can improve the hardness and strength of steel after hot rolling. When the manganese content is not high, manganese can slightly increase or not reduce the surface shrinkage and impact toughness of carbon steel.

(3) The effect of silicon

Silicon content in carbon steel is less than or equal to 0.5%. Silicon is also a beneficial element in steel. In the boiling steel, the silicon content is very low, and silicon is added to the killing steel as a deoxidation element. Silicon increases the fluidity of molten steel. In addition to forming nonmetallic inclusions, silicon dissolves in ferrite. However, when the silicon content exceeds a certain value, the impact toughness of the material decreases significantly.

(4) The effect of sulfur

Generally speaking, sulfur is a harmful element, which mainly comes from pig iron raw materials. The harm of sulfur is to cause the steel to crack during hot processing, that is, to produce the so-called hot brittleness, the reason for the hot brittleness is due to the severe segregation of sulfur, even in the case of not very high sulfur content, there will be (Fe+FeS) eutectic. Manganese is generally added to prevent thermal brittleness. Sulfur affects the mechanical properties of steel by forming sulfide inclusions. The influence of sulfur on the mechanical properties of steel is not only related to the content, but also to the size, shape and structure of inclusions.

(5) The effect of phosphorus

Generally speaking, phosphorus is a harmful impurity element, which comes from steelmaking raw materials such as ore and pig iron. Phosphorus has a considerable solubility in pure iron, and phosphorus can improve the strength of steel, but the toughness is reduced, especially the brittle transition temperature of steel is sharply increased, that is, the cold brittleness of steel is increased. In the steel with relatively low carbon content, the cold brittleness of phosphorus is less harmful, in this case, phosphorus can be used to improve the strength of steel, in addition, phosphorus can improve the resistance to atmospheric corrosion, reduce the bonding of hot rolled sheet.

(6) The influence of oxygen

The solubility of oxygen in steel is very small, the oxide in steel appears in the form of inclusions, in general, the oxygen content increases, the inclusions are more, the inclusions are more, the toughness of the steel plate is reduced, other mechanical properties such as fatigue strength, wear resistance also have different degrees of decline in steelmaking is to remove the impurity elements in the liquid steel by oxygen, so oxygen has played a positive role. But it is harmful to solid steel. Therefore, it must be deoxidized.

(7) Nitrogen effect

The nitrogen in the steel comes from the charge, and the liquid steel also absorbs nitrogen from the furnace gas during smelting, which causes the quenching and deformation aging of carbon steel, which has a significant impact on the performance of carbon steel. Due to aging, the hardness and strength of steel is increased, but the toughness should be reduced, especially in the case of deformation aging, the reduction of toughness is more significant, for ordinary low-alloy steel, the aging phenomenon is harmful, so nitrogen is a harmful element.

Adding a sufficient amount of aluminum to the steel can form aluminum nitride, which can eliminate or weaken the aging phenomenon, because aluminum nitride is highly dispersed and stable in texture, it can play the role of refining the grain, from this aspect, nitrogen is a beneficial element.

(8) The effect of hydrogen

In the smelting process, the liquid steel can either be carried into hydrogen by the rusted water-bearing charge, or it can absorb hydrogen directly from the furnace gas, and the steel can absorb hydrogen during pickling, electroplating or electrolysis when heated in the reducing protective gas containing hydrogen. Hydrogen in the form of ions or atoms dissolved into liquid or solid steel, molten into solid steel to form intermittent solid melt, hydrogen is a harmful element in steel, into steel to reduce the toughness of steel, causing the so-called hydrogen embrittleness. On the other hand, when hydrogen precipitates from the steel, it causes defects of the nature of internal cracks, and the higher the strength of the steel, the greater the sensitivity of hydrogen embrittleness.

(9) The effect of aluminum

Aluminum is added to steel as a deoxidation element, and the aluminum part of the liquid steel is combined with oxygen to form Al2O3 or various inclusions containing Al2O3. In addition to the deoxidation effect of aluminum, the dispersive AlN particles formed by the combination of aluminum and nitrogen can prevent austenite grain growth. Therefore, it is necessary to ensure that there is a certain amount of acid-soluble aluminum in the steel.

(10) The influence of other residual elements

There are some elements brought into carbon steel by scrap steel or ore, common copper, nickel, chromium, etc., in order to make the performance fluctuation range of all kinds of carbon steel is not too large, their content has certain restrictions. However, the presence of these elements will improve the strength of hot rolling to a certain extent.