时间：2013-11-20 09:25　作者：admin　点击： 次

The reduction in volume and wire size of steel during solidification and cooling is called shrinkage.It has a great influence on the formation of defects such as hot cracking, shrinkage cavity and porosity of the cast slab (steel ingot).
The shrinkage during the solidification of molten steel can be divided into the following types according to the temperature range in which the shrinkage occurs.
(1) Liquid shrinkage e_liquid, The shrinkage of the molten steel when it cools from the superheated temperature to the liquidus temperature;
(2) Solidification shrinkage e_液； The shrinkage of the molten steel when it cools from the liquidus temperature to the solidus temperature;
(3) Solid state shrinkage e_液； The shrinkage that occurs when molten steel cools from the solidus temperature to room temperature.
Generally, when carbon solidifies at normal casting temperature, the approximate value of volume shrinkage is; liquid shrinkage, about 1%; solidification shrinkage, about 4%, solid state shrinkage, 7% to 8%.
Usually the liquid shrinkage of steel is very small.It depends on the extreme temperature and superheat of the molten steel.The shrinkage of liquid shrinkage increases with the increase of carbon content; when the composition of molten steel is constant, the higher the degree of superheat, the greater the amount of liquid shrinkage.
The solidification shrinkage of steel depends on the composition of the steel and the solidification temperature range.The greater the solidification temperature range, the greater the shrinkage.When the carbon content in steel is increased to w[C]=0%, the solidification shrinkage increases significantly.When you continue to increase the carbon content, the solidification shrinkage decreases instead.In general, the solidification shrinkage of medium and high-carbon steel is larger than that of low-carbon steel, and its shrinkage and porosity are also more serious than that of low-carbon steel.
The solid-state shrinkage of steel depends on the temperature change from the solidus to room temperature and the change of the phase structure (for example, the volume shrinkage caused by the peritectic reaction, and the volume expansion occurs when austenite transforms into pearlite).
In the case of different carbon content, the total shrinkage of carbon steel varies from 10% to 14%, which is reflected in the casting billet (steel ingot) consisting of two parts, one part is linear shrinkage, that is, after the casting billet (steel ingot) solidifies , The reduction of the linear dimensions (such as length, width, height) of the outline.It occurs in the range of solidification shrinkage and solid state shrinkage, but mainly solid state shrinkage.The gap between the billet and the mold wall (steel ingot and mold wall) is the surface of the billet (steel ingot) linear shrinkage, which worsens the cooling conditions, and is also an important factor for the surface cracks of the billet (steel ingot).The other part is volume shrinkage, which is mainly caused by liquid shrinkage and solidification shrinkage. It does not affect the shell size of the cast billet (steel ingot), but it can cause shrinkage holes, porosity or internal cracks inside.Since the total contraction amount is constant, and the total contraction amount is the sum of the volume contraction and the linear contraction, when the volume contraction amount is large, the linear contraction amount is small, and vice versa.