Continuous casting exemplifies the process of feeding heated metal into an open-ended mould that is water-cooled in order to solidify the metal in the shape of a bar, slab or an ingot. The benefit of this technique, compared to older methods of solidifying metal from molten states, is not only its high production rate, but also its ability to generate components of uniform shape with improved surface finish and reduced scrap. Furthermore, continuous casting can eliminate internal porosity since oxygen flux is diminsihed due to a more even temperature distribution. Cold-rolled steel in particular, largely attains its properties from continuous casting.
A key part in this process is the ‘hot-charge-heat-treatment’ (HCHT) of the stock prior to casting. The primary benefit of this technique is that it serves to homogenise the temperatures of the heated metal; thus producing a ‘potentially’ more uniform transfer of heat into the mould, reducing distortion and deformation of the solidified product. The technique is not without its drawbacks; but with careful process considerations these can be mitigated to produce an optimal quality product.
A key consideration when using the «hot-charge-heat-treatment» technique is the reduction of thermal stresses to the point where genuine equality of solidification is achieved. With regards to this consideration, two major points should be taken into account: the flow speed of the steel and its heat conduction (also dependent on the speed of flow); and the heat variation in the molten stock itself.
The flow speed of the steel is largely dependent upon the furnace temperature; and so, a very high temperature can produce a higher flow rate that encourages an uneven rate of cooling of the solidifying bar and slab due to the dynamic thermal gradients. Considerations of this flow speed should be made in relation to the diameter (or thickness) of the bar or slab, as the higher flow rate also implies higher cooling rates which subsequently leads to greater stresses in thinner sections, impedes thermal homogeneity, and causes significant tension, distortion and a non-uniform temperature cross-section in the final product.
The other major factor to consider is the thermal variation in the molten stock, which is largely dependent upon homogenisation and uniformity between the stock and than the hot charge. It is important to ensure that all of the materials used in the charge have a consistent temperature and composition to avoid any non-uniformities or inconsistencies in the temperature gradients of the final product which could also potentially lead to distortion, tension and deformation.
It is also important to note that it is generally easier to homogenise a steel charge of smaller dimensions, and so these should be used if possible to help minimise the thermal gradients. Furthermore, to also help ensure a good homogenisation of temperatures at the onset of casting; it is also recommended that a certain volume of stock is used as a ‘hot-charge-heat-treatment’ before the start of the casting cycle; so that the metal specimens are heated uniformly, and then, when the cycle begins, the metal specimens are simultaneously heated at the same rate of the incoming metal from the furnace. This then, largely helps to minimise thermal gradients and improve the overall quality of the output.
The ‘hot-charge-heat-treatment’ technique is an important part of the continuous casting process and offers some useful benefits if implemented correctly. By carefully considering the flow and thermal conditions of the metal being cast, it is possible to achieve a product with improved surface finish, reduced scrap and improved overall quality. Additionally, external and internal distortions can be minimised; and the resulting product should have an even temperature distribution, reducing oxygen and porosity related issues.
