Feb. 04, 2021
There are three main states, namely hot rolled steel, normalized steel and tempered steel.
The alloy system of hot rolled steel includes C-Mn system, C-Mn-Si system, etc., which mainly depends on the solid solution strengthening of Mn and Si to improve the strength, and can also add trace amount of V, Nb or Ti to further improve the strength of steel and improve the plasticity and toughness by precipitation and grain refinement of its carbides and nitride.Its structure has fine grain ferrite and pearlite.The yield strength is mostly below 400MPa.
Normalizing steel can give full play to the effect of precipitation strengthening, by normalizing the precipitated phase from the solid solution to fine particles, dispersed in the grain boundary and grain, and refine the grain, effectively improve the strength, and has good plasticity and toughness.The alloying elements added to normalizing steel are Mn, Si, Ni, Mo, Cr, V, Nb, Ti and so on. Most of the structure of normalizing steel is fine grain ferrite and pearlite.After normalizing the steel containing molybdenum, its structure is upper bainite and a small amount of ferrite. In order to improve the plasticity and toughness of this kind of steel, it is required to normalize and then temper.The yield strength of normalized steel is 420~540MPa.
Quenched and tempered steel is low carbon quenched and tempered steel, the alloying design principle of it - the pearl size of hot rolled steel and ferrite and normalized steel is different, its strength is not directly depends on the content of alloy elements, and depends on the organization, by quenching martensite structure to obtain high strength, after tempering treatment improves as the tempered martensite, make its plasticity and toughness is improved.
The added alloying elements are Cr, Ni, Mo, Cu, V, Nb, Ti, B, etc., the purpose is to ensure the penetration, and some elements (such as Mo) can also improve the anti-tempering performance of steel.
In this kind of steel, nickel is a very important alloying element, it can improve the toughness of steel, reduce the brittle transition temperature of steel.Chromium can obviously improve hardenability, and good comprehensive mechanical properties can be obtained when chromium and nickel are added together, thus developing into high strength and high toughness steel series, such as HY80, HY100, HY130 steel, etc.With the increase of strength and toughness, the content of nickel should be increased continuously, ω-Ni can reach 5%-10%;However, with the upper limit of 1.6%, the increase of Cr has no effect on hardenability, but makes the toughness of steel decrease.In order to improve the ability of hydrogen cracking resistance and low temperature toughness of steel, it is very effective to reduce the carbon content and add trace titanium (ω (Ti) is about 0.01%).
In order to make up for the strength loss caused by carbon reduction melting, a variety of trace elements can be added, especially the elements that have a strong effect on hardenability like trace boron, so it has been developed into a tempering and tempering high strength steel with a very low carbon content (ω (C) ≤0.09%), that is, welded crack free steel (CF steel for short).The alloy series of quenched and tempered high strength steels is more complex, and many alloying elements are added, but the amount of addition is generally not high.Its structure belongs to low carbon tempered martensite, also known as lath martensite.
