摘要:
为保证制成品的质量,需要尽可能地破少连铸产品的峡陷。随菅热装、薄披连轧及直轧的推广,生产无缺陷的连铸产品也日益重要。这是由于在这些条件下,缺陷的检查与清理变得更加困难的缘故。
在连铸产晶的各项缺陷中,横向表面裂纹受微合金元衰的影响较大。铌是非常有害的,0.01%的铌就会促成裂纹的出现。尽管在钒含量0.15%,氮含量0.02%的场合,也有出现横向裂纹的报道,但含钒锕中,氮低于0.005%时,横向裂纹就不会发生。
据信,横向裂纹形成于结晶器,随后在连铸过程,特别是矫直过程中扩张。微合金钢在某一温度范圈内,显示出较低的延展率,当在此低延展率区间进行矫直时,就会产生裂纹。铌在深化延農槽,井使其向高温区城扩股方面影晌显著。铌的这一特点要归因于碳氮化铌的析出。铌的沉积促成低延展失效,井迟滞了再结晶过程。钒对热延展性的影响不那么显著,仅在钢中钒、氮含较高的情况下,其延展性才接近含铌钢的水平。向含铌钢中添加钒,可通过使析出相粗化稍微改普延展性。钛对热延展性的影响较复杂，到目前还不完全为人们所知。
通过适当选择钢的成分,如降低铌的含量、以钒和氮取代铌、或向含铌钢中添加钒,可将横向裂紋的出现降至最低。设备操作条件如二次冷却策略,对避免横向裂纹的出现也很重要。将矫直湿度选在低延股率温度区间以外,也可降低裂纹的出现。
Abstract:
To ensure the appropriate quality in finished products, it is important that defects in continuously cast products are minimised. As the use of hot charging and thin slab casting coupled with direct rolling becomes more common, it is increasingly important to produce defect-free continuously cast products, as inspection and repair in these situations become more difficult.
Of the many types of defect in continuously cast products, only transverse surface cracking is strongly influenced by the presence of microalloying elements. Nb has a particularly strong detrimental effect, and Nb additions of as low as 0.01% can promote cracking. For V steels with <0.005%N, transverse cracking does not appear to occur, although at high levels of V and N(0.15%V,0.02%N), transverse cracking has been reported.
It is believed that transverse cracks form in the mould, and propagate later in the continuous casting process, particularly during the straightening process. Microalloyed steels can exhibit low ductility over certain temperature ranges, and when the straightening process is carried out in this low ductility region, cracking can occur. In this respect, Nb has a strong effect in deepening the ductility trough, and extending it to higher temperatures. This behaviour is due to the presence of Nb(CN) precipitates, which promote low ductility failures, and retard recrystallisation. The effect of V on hot ductility is much less marked, and only at high levels of V and N does their ductility approach that found in Nb steels. V additions to Nb steels appear to slightly improve hot ductility, by promoting coarser precipitates. The effects of Ti on hot ductility are complex and still not completely understood.
Transverse cracking may be minimised by appropriate selection of steel composition, such as minimising Nb, replacing Nb by V and N combinations, or by making V additions to Nb steels. Machine operating conditions such as secondary cooling strategy are also important in avoiding transverse cracking. By selecting straightening temperatures which are outside the temperature range of low hot ductility, cracking can be reduced.
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