With the rapid growth of China's equipment manufacturing industry, the demand for construction machinery steel has significantly increased, offering promising development prospects. Qinhuangdao Shouqin Metal Materials Co., Ltd. (Shouqin Company) has established a comprehensive production system that includes magnesium particle hot metal desulfurization, top-and-bottom blowing converter, LF furnace, RH vacuum treatment, slab continuous casting, 4300 mm wide and heavy plate rolling mills, and a quick cooling system. The company also features a heat straightener and heat treatment furnace after the rolling process. In 2009, Shouqin Company identified high-strength steel as one of its top ten 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 involves a relatively high alloy content and usually requires quenching and tempering. However, as the demand for construction machinery steel increases, there is a growing need for cost-effective solutions without compromising performance. In this context, Shouqin Company developed Q690E steel plates by optimizing composition design, utilizing controlled rolling and cooling techniques, along with tempering treatment to achieve superior mechanical properties.
According to GB/T 16270-1996, Q690E steel must meet specific requirements for strength, low ductile-brittle transition temperature, and elongation. To achieve this, Shouqin Company adopted a low-carbon composition combined with Mo-Ni-Cu-B micro-alloying, and further enhanced the properties using Nb, V, and Ti for grain refinement. Elements like Cr, Mo, and B improved hardenability, while Ni enhanced toughness. Additionally, P and S levels were minimized, and Ca was used to spheroidize inclusions, ensuring better overall quality.
The production process involved several stages: hot metal pretreatment, converter top and bottom re-blowing, LF furnace refining, RH vacuum treatment, continuous casting, billet inspection, heating, descaling, rough rolling, finishing rolling, ACC water cooling, straightening, cooling bed, shot blasting, tempering, and final inspection. This approach ensured high steel cleanliness, uniform austenitization, and optimal microstructure through controlled rolling and cooling.
After rolling, the steel underwent tempering at temperatures between 650–680°C for 25–30 minutes. The results showed that the yield strength met the national standard, with some samples exceeding 790 MPa. Tensile strength and impact toughness at -40°C also met the required standards. Microstructural analysis revealed a fine low-carbon bainite structure after tempering, leading to improved elongation from 14% to 17%.
Compared to traditional quenching and tempering methods, the TMCP (Thermomechanical Control Process) combined with tempering offered better strength and toughness. The retained dislocations during rolling, along with rapid cooling, contributed to a finer microstructure. Tempering further refined the grains and improved mechanical properties.
In conclusion:
1. The Q690E steel produced using the TMCP + tempering process exhibits a low-carbon bainite microstructure, providing excellent strength, toughness, and weldability.
2. The developed Q690E fully complies with GB/T 16270-1996, with a significant reduction in production costs compared to conventional quenching and tempering.
3. The steel not only meets but exceeds the standard requirements for strength and low-temperature impact toughness, marking a successful breakthrough for Shouqin Company in the high-strength steel market.
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