Analysis of development status of high-strength structural steel Q690E

With the rapid growth of China's equipment manufacturing industry, the demand for construction machinery steel has significantly increased, offering a promising market outlook. Qinhuangdao Shouqin Metal Materials Co., Ltd. (referred to as Shouqin Company) has established a comprehensive production system, including magnesium particle hot metal desulfurization, top-and-bottom double-blowing converters, LF and RH vacuum treatment, slab continuous casting, 4300 mm wide plate rolling mills, and heavy plate rolling lines. The company is also equipped with heat straighteners and heat treatment furnaces after rolling. Since 2009, Shouqin Company has prioritized high-strength steel as one of its key products in the wide and thick plate category. Traditional high-strength steel plates with yield strengths above 600 MPa typically rely on solid solution and precipitation strengthening, which often requires higher alloy content and quenching and tempering processes. However, with the growing demands for cost reduction and enhanced performance in construction machinery steels, new approaches have emerged. In this study, Shouqin Company developed Q690E steel plates by optimizing composition design and utilizing controlled rolling, cooling, and tempering techniques. The technical requirements for high-strength Q690E steel include high strength, low ductile-brittle transition temperature, and sufficient elongation. According to GB/T 16270-1996, the mechanical properties of Q690E are specified in Table 1. During development, Shouqin Company adopted a low-carbon design with micro-alloying elements such as Mo, Ni, Cu, and B. Additionally, Nb, V, and Ti were used to refine grains and enhance both strength and toughness. Elements like Cr, Mo, and B improved hardenability, while Ni contributed to better impact resistance. The levels of harmful elements like P and S were minimized, and Ca was introduced to spheroidize inclusions. The production process followed a strict route: hot metal pretreatment, converter top and bottom re-blowing, LF furnace, RH vacuum treatment, continuous casting, billet inspection, heating, high-pressure water descaling, rough rolling, finish rolling, ACC water cooling, straightening, cooling bed, finishing, shot blasting, tempering treatment, and final inspection. The use of LF, Si-Ca-Ba feeding lines, and RH vacuum treatment ensured high steel cleanliness. Controlled rolling and cooling were optimized to refine grain structure and improve mechanical properties. After rolling, the steel underwent tempering at temperatures between 650–680°C for 25–30 minutes. Trial production results showed that the hot-rolled Q690E steel met the national standard for yield strength, though tensile strength varied slightly. Some samples exceeded the standard, but elongation and impact toughness at -40°C were within acceptable limits. After tempering, the mechanical properties improved further. The yield strength reached up to 790 MPa, exceeding the required 690 MPa, and the tensile strength also met the standard. Tempering refined the microstructure, leading to finer grains and enhanced toughness. Microstructural analysis revealed that the Q690E steel before and after tempering exhibited a low-carbon bainite structure, with some ferrite present initially. Post-tempering, the structure became more refined, increasing elongation from 14% to 17%. Compared to traditional quenching and tempering methods, the TMCP + tempering process offered superior strength and toughness. The retained deformation dislocations during rolling and rapid cooling led to a finer structure, resulting in better mechanical performance. In conclusion, the Q690E steel produced using the TMCP + tempering process exhibits excellent strength, toughness, and weldability. It fully complies with GB/T 16270-1996, with significant performance margins. The process also reduces costs compared to conventional methods. The successful development of Q690E marks an important milestone for Shouqin Company in the high-strength steel market.

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