Thick steel plate has been widely used in domestic and international market demand, used in construction, machinery, wind power, water and electricity and other fields, especially in more than 50 mm thick and high strength low alloy structural steel, mainly used in the key part of large equipment, sometimes also required thickness direction (Z) performance, on the uniformity of product performance, testing, quality requirement is high, the production technology is difficult.
SS355JR+N structural steel plate development standard requirements
SS355JR+N is a European standard structural steel, its composition and performance requirements meet the "EN10025-2" standard, and also meet the "EN10164" standard in the Z direction performance requirements, 50mm thick SS355JR+N steel plate chemical composition and mechanical properties requirements.
Chemical composition design:
SS355JR+N normalizing extra-thick steel plate requires not only high strength and plasticity, but also good lamellar tear resistance.
In order to guarantee the performance of the steel plate after normalizing, satisfying the requirements of the standard, to overcome the thickness effect at the same time, and has good welding performance, in jinan steel composition design using the appropriate C, Mn content increases the strength of the steel, at the same time adding strong carbonitride formation elements, such as Nb, Ti by microalloy element of fine-grain strengthening and precipitation strengthening effect to improve strength and toughness can, Improve the comprehensive performance of extra-thick steel plate.
Production process: KR→ converter smelting →CAS→LF→RH→CCM→ slow cooling of continuous casting billet → composite billet making → heating → high pressure water descaling → rough rolling → fine rolling →ACC weak cooling → slow cooling of steel plate → normalizing → cutting → defect detection → inspection → spray number → storage of qualified products.
In order to ensure the performance of extra-thick steel plate and the pass rate of flaw detection, clean steel smelting technology is adopted to strictly control the content of HARMFUL elements S, P and H in steel. Low sulfur iron was obtained by KR iron pretreatment, S≤0.005%; Control terminal C content in converter smelting process to reduce slag amount in drawing process; LF process deep desulfurization, and ensure the composition and temperature of molten steel uniform; When RH vacuum degassed, the time of vacuum treatment was ≥15min at 100Pa, and Ca treatment was carried out after the vacuum was broken. During the soft blowing process, the flow of argon gas was adjusted to the liquid steel surface slightly fluctuated to prevent secondary oxidation caused by bare liquid steel. The whole process of continuous casting is protected by argon, the low superheat of molten steel is strictly controlled, and the drawing speed is stable in the continuous casting process. The continuous casting billet is slowly cooled for 48h or more, and the thickness of the continuous casting billet is 250mm.
(1) Through composition design, composite billet making, rolling, heat treatment process design, using Nb, V, Ti microalloying, high temperature and low speed large pressure, rolling steel plate slowly cooling, normalizing treatment and other technological means, developed excellent performance 50mm thick SS355JR+N steel plate, performance in line with "EN10025-2" and "EN10164" standards. The yield strength of the steel plate is more than 40MPa, the tensile strength is more than 50MPa, the average impact energy at 20℃ is more than 200J, and the average area shrinkage is more than 45%.
(2) Through strict control of smelting, continuous casting and composite billet making processes, and strict implementation of high temperature, low speed and large pressure process in the steel plate rolling process, slow cooling after rolling, to ensure that the rolled steel plate has good internal quality, inspection results meet the standard requirements of "EN10160S1/E1".
(3) The bonding of the composite interface was analyzed by means of flaw detection, Z-tensile, low power, metallography and scanning electron microscopy. The results show that there is no bonding trace at the interface of 1/2 thickness of the steel plate, forming a grain structure consistent with the matrix structure, and the steel plate meets the requirements of flaw detection in z-direction performance.
(4) Practice has proved that combining with the characteristics of excellent internal quality of continuous casting slab, the new technology of composite billet making can prepare high-quality large-thickness billets, provide high-quality raw materials for the production of extra-thick steel plate, and expand the thickness specification range of extra-thick steel plate, and the product performance is stable.