Research On Tensile Stratification Of Low-alloy High Strength Steel HG70C Hot Rolling Plate

Steel manufacture is one of the most important basic industry of the national economy andthe significant symbol of economy level and comprehensive international strength. Thedevelopment of iron and steel industry affects national defence industry, construction, machinery,shipbuilding, automobile and household appliances. With the expansion of the numbers of steelproducts, steel manufacture has entered a stage, accelerating structural adjustment, enhancingquality of products and improving overall competitiveness. But at present, many varieties of steelsurface quality defects, center segregation and center stretch layered still exist, influenced thesteel delivery rate seriously, so it is imperative to improve the quality of steel.In this paper, the metallographic and electron microscopy examination has been used,related data of the smelting technology of steel grade (chemical composition), continuous castingtechnology (such as degree of superheat), rolling technology (heating system, rolling system, etc.)have been analysed by means of statistics. Finally, HG70C slab hot simulation experiment hasbeen carried out in laboratory, studying the Nb, Ti’s carbon nitrides precipitation behavior underlow temperature, including the amount of precipitation, particle shape, size and distribution ofthe precipitation.The conclusions through the analysis were as following：(1) The center banded structure of pearlite and massive carbon titanium nitride (niobium)inclusions are the direct cause of HG70C tensile layer.(2) The element content in the second phase particles affects the shape, larger Ti content andsmaller Nb content in the square size particles.(3) When finishing temperature was900℃and relaxation time was30s, the number ofprecipitation was more and the size of particles was uniform and fine.Based on the above analysis the following production improvement measures are putforward:(1) Improving billet quality, reducing enrichment of inclusion content,minimizing the levelof center segregation as much as possible by EMS and making deformation processing toinclusions.(2) Improving the roughing mill rolling temperature, reducing the deformation resistance, orextending soaking time, increasing the thickness of intermediate billet, namely increasingdeformation of finishing mill could reduce the level of banded structure.(3) The pearlite transformation temperature is usually between700~650℃, so in order toreduce pearlite banded structure, controlled rolling process of controlled cooling is the key.