Chemical Factors affecting Steel Properties (2)

Manganese: It's a beneficial element. In ordinary carbon steel, it is a weak deoxidizer, which can improve the strength of steel, eliminate the influence of sulfur on the hot brittleness of steel, improve the cold brittleness tendency of steel, and do not significantly reduce the plasticity and toughness. Manganese is the main alloying element of low alloy steel in China, and its content is 0.8% to 1.8%. However, manganese is not good for welding performance, so the content should not be too much.

Silicon: A beneficial element. In ordinary carbon steel, it is a strong deoxidizer, often co-deoxidizing with manganese to produce sedative steel. The appropriate amount of silicon can refine the grain and improve the strength of steel, but has no significant adverse effects on plasticity, toughness, cold bending and weldability. The content of silicon in general sedated steel is 0.12% to 0.30%, and the content in low alloy steel is 0.2% to 0.55%. Excess silicon deteriorates weldability and corrosion resistance.

Titanium, vanadium, mud and other elements: can form fine carbides in steel, the right amount of addition can play a fine grain and dispersion strengthening effect, so as to improve the strength and toughness of steel, but also to maintain good plasticity.

Aluminum: is a strong deoxidizer, can also refine the grain, can improve the strength of steel and low temperature impact toughness of qualified low-alloy steel, its content is not less than 0.015%.

Chromium, town: is an alloying element to improve the strength of steel, used in Q390 and above grades of steel, but its content should be limited so as not to affect the other properties of steel.

Copper and chromium, town, molybdenum and other elements: can form a protective layer on the surface of the metal matrix, improve the corrosion resistance of steel to the atmosphere, while maintaining the steel has good welding performance. In weathering steel for welding structure in China, the content of copper is 0.20% ~ 0.40%.

Rare earth elements such as lanthanum and cerium: can improve the oxidation resistance of steel and improve other properties. In low alloy steel, its content is controlled by 0.02% ~ 0.20%.

Oxygen and nitrogen: Harmful elements. Oxygen is similar to sulfur to make steel hot brittle, and nitrogen has a similar effect to phosphorus, so its content should be strictly controlled. However, when special alloy components are matched, nitrogen can be used as an alloying element to improve the strength and corrosion resistance of low-alloy steel, such as the 15MnVN steel successfully used in the Jiujiang Yangtze River Bridge, which is a nitrogen-containing steel in Q420, and the nitrogen content is controlled at 0.010% ~ 0.020%.

Hydrogen: Hydrogen is a harmful element. An extremely unstable atomic state dissolved in steel, its solubility decreases with the decrease of temperature, often in the structure of loose areas, holes, lattice dislocation and grain boundary enrichment, the generation of hydrogen molecules, resulting in huge internal pressure, steel cracking, known as hydrogen embrittleness. Hydrogen embrittlement is a kind of delayed failure, which usually takes place after a certain pregnancy development period under the action of tensile stress. White spots are often seen on the fracture surface, which are called hydrogen white spots. The steel with lower carbon content and less sulfur and phosphorus content has low hydrogen embrittlement sensitivity. The higher the strength level of steel, the more sensitive it is to hydrogen embrittlement.