Research And Development Of Boron-added HSLA H-beams

H-beam, as an economic and efficient section-steel due to optimized cross-sectional area distribution and reasonable ratio of strength to weight, has been widely used in high-rise buildings, industrial plants, bridge structures, ship ports, oil drilling platforms, with the advantage of good flexural capacity, simple construction, saving cost and light structure weight. Therefore the development of H-beam is advocated in China. With the resource development in low temperature zone and the application in high-tech field, the demand of high strength and toughness (especially good low-temperature toughness) H-beam is growing.Before the research, the low temperature impact toughness of original H-beam fluctuates large and the impact toughness at-40℃is unsatisfactory. In order to find a composition and process route which is appropriate for current production conditions, boron is introduced to the development of cryogenic H-beam. After analysis of original product, research on experimental steels in lab and application on practical production line, a new type of boron-added H-beam as well as the production technology was successfully developed with proprietary intellectual property rights, which not only brings substantial economic benefits and social benefits, but also improves competitiveness of high-end H-beam of our country.In this research, through the comprehensive utilization of tensile test at room temperature, series of impact test, thermal simulation, optical microscope, stereomicroscope, electron probe, scanning electron microscope and TEM, etc, the structures and properties of experimental steels are studied systematically. The emphasis of the research is the effect of boron on structure and properties. Boron is usually used to improve the hardenability of steel in the former research, whereas it is used to improve the low temperature impact toughness in this study.The process of this research is divided into four steps. Firstly, the properties and microstructures of previous H-beams were analyzed to estimate its impact toughness at low temperature, factors of structure and metallurgy which effect the flexible of it were been discussed at the same time. Secondly, improving low temperature impact toughness by adding boron element is suggested, in order to check whether the method is feasible or not, effects of trace boron element on structure and property were researched through different boron content steelmaking in lab. Thirdly, thermal simulation test was used to research the phase transformation behavior of the new type steel, providing knowledge reserve to industrialization production and heating processing technology. Forthly, industrial application was proceeded based on study of lab, and research on structures and properties was carried out to study the malleableize mechanism of the new production.The introduction of boron reduced the brittle transition temperature of the steel much higher as well as improving low temperature impact toughness, meanwell, the impact toughness was made more than national standard, strength and elongation were maintained. As that the size and distribution of Nb(C,N) was changed to be more uniform and dispersive, ferrite grain size was refined, which lead to crystallization strength. Most of the boron element existed in solution condition, only very few in Fe23(C,B)6-The segregation of boron in grain boundary could prevent the segregation of phosphorus and sulphur, in consideration of the improvement on boundary bond of itself, brittle transition temperature was reduced and low temperature impact toughness was improved at the same time.By means of thermal simulation tests, phase transformation behavior of the new type steel was studied, and process route which was appropriate for current production conditions was developed to direct industrial production. Processing route of boron-added H-beam was just like this:soaking the billet at1250-1300℃, make sure the tapping temperature is above1250℃and finishing rolling temperature between890-930℃, cooling fast to750℃after rolling, then natural cooling on cooling bed. Rolling parameters just as total deformation, deformation per pass remain unchanged.The impact toughness at low temperature was improved in the commercial process under industrial production's guidance, which proved the feasibility of industrial production. Although the banded structure still existed, but impact toughness in low temperature was improved obviously, especially the impact toughness at-40℃was much more than national standard, which proved that banded structure is not the determinant of impact toughness. By comparing the low temperature impact toughness between boron-added steel and non-boron steel with the same grain size, intercrystalline strengthening of boron beneficial to low temperature impact toughness was confirmed. Thus the mechanism is summarized as follows:(1) Grain refinement:compound action of boron and niobium refines the grain size, which improves the toughness.(2) Intercrystalline strengthening:segregation of boron at grain boundaries prevents the segregation of phosphorus and sulphur and stops the low temperature brittleness induced by the segregation of phosphorus and sulphur, together with the improvement of boundary bond by boron itself, so that the low temperature impact toughness is improved.