Microalloying third-generation car spring flat steel and preparing method thereof

The invention relates to a microalloying third-generation car spring flat steel and a preparing method thereof. The microalloying third-generation car spring flat steel comprises, by weight percentage, 0.48%-0.60% of C, 0.4%-0.8% of Si, 0.60%-1.00% of Mn, 0.80%-1.20% of Cr, 0.01%-0.05% of Nb, 0.0005%-0.003% of B, not larger than 0.025% of P, not larger than 0.025% of S, not larger than 0. 25% of Cu, not larger than 0.35% of Ni, not larger than 0.02% of Ti and the balance Fe. The preparing method includes the steps of blast furnace molten iron production, molten iron desulphurization, converter smelting, LF refining, RH vacuum treatment, square billet protection pouring, square billet heating, rolling, cooling bed slow cooling and fixed-size shearing. According to the microalloying third-generation car spring flat steel and the preparing method, niobium and boron microalloying and the silica content more suitable for the spring steel high elasticity ultra-low decarburization trend are adopted, the proper clean