材料期刊网

在高铬不锈钢的生产中,影响质量最严重的是钢锭中的轴心裂纹和钢材中的髪纹。本文的研究结果指出,这两种缺陷属于同一来源,没有轴心裂纹的钢锭在锻成钢材后也没有髪纹。 钢锭中轴心裂纹是在凝固过程中形成的,因而冷凝条件对它有影响。利用耐火材料做锭模可以得到内部坚实的钢锭,减小锭模模壁厚度,增大钢锭圆锥度或降低浇铸温度等只能减轻而不能完全消除裂纹。 熔炼过程中的脱氧方法对钢中夹杂物的性质和形状分布有影响。改善脱氧方法并采用薄模浇铸,可以大大地减少轴心裂纹和髪纹,从而提高钢的收得率。我们认为夹杂物的性质、多寡及其形状分布可能对钢液的流动性和表面张力有影响;改善钢液的流动性或减小其表面张力可以促进钢液的充填性能,从而弥补凝固过程中由于冷凝收缩而产生的缺陷。 在一定压力以下的真空中熔化和浇铸可以得到内部良好的钢锭。通过真空熔铸所得到的钢锭,其柱状晶区大小缩小,而柱状晶的缩小对避免钢锭中轴心裂纹有利。得到这种效果的原因可能是多方面的,真空有去气作用,在真空中钢锭的凝固散热与在一般情况下有所不同,真空作用使钢液的性质发生变化等等。虽然,上述几方面的理论基础还有待于进一步研究,但从目前实际效果来说,真空熔化和浇铸应该认为是提高不

In the manufacture of stainless steel of the high chromium type, axialcracking or porosity in ingots and ghost lines in forged billets were frequentlyencountered. It is shown that these two kinds of defects are of the same origin, billets made from ingots free from axial cracking were also immune from ghostlines. The formation of axial cracking in ingots occurs during solidification, there-fore cooling conditions which affect the interval of solidification must play animportant role in assessing the extent of cracking. The use of refractory ingot-mould led to complete immunization of axial cracking, whereas the adoption ofother alternative means such as increasing the tapering of the ingot or decreasingthe wall thickness of the ingot-mould was not so effective. To certain extents axial cracking in ingots or ghost lines in billets wereassociated with the presence of nonmetallic inclusions in steel. Through an im-provement of deoxidation procedure during smelting, the nature and the distri-bution of inclusions were favourably changed. This, supplemented by the adoptionof thin-walled ingot-mould, has been tried in works practice, the severity of axialcracking in ingots or ghost lines in billets being greatly lessened. It is consideredthat the fluidity and surface tension of liquid steel may vary with the amount,the nature and the shape of inclusions present, any change in the latter whichwill lead to an improvement of the filling properties of the liquid steel must havea favourable influence towards the immunization of solidification defects. Smelting and casting in vacuum below certain pressure range resulted in per-fectly sound ingots, such ingots showed an external zone of columnar crystalsmuch smaller than those smelted and cast under similar conditions but in air.There may be many reasons for the above observation, for example, vacuum pro-motes degasification, the transfer of heat between ingot and mould is altered byvacuum, the properties of the liquid steel in vacuum and in air are different, etc.Some of these problems undoubtedly deserve further investigation. In the mean-time, it can be said that vacuum smelting and casting appear to be the bestmethod to ensure the production of high quality stainless steel.