Steel for low-yield-ratio structure with yield strength larger than or equal to 690MPa and production method of steel

Steel for a low-yield-ratio structure with the yield strength larger than or equal to 690MPa comprises, by weight, 0.01-0.05% of C, 0.41-0.60% of Si, 2.10-3.50% of Mn, less than 0.010% of P, less than 0.005% of S, 0.10-0.50% of Mo, 0.51-0.80% of Cr, 0.10-0.80% of Cu, 0.05-0.50% of Ni, 0.052-0.11% of Nb, 0.053-0.10% of V, 0.008-0.025% of Ti, 0.015-0.060% of Als, 0.0010-0.0080% of Ca, 0.0005-0.0050% of Mg, 0.0010-0.0030% of N and 0.0005-0.0025% of O. A production method includes the steps of steel making, continuous casting for billet formation, heating of cast billets, temperature preservation, rough rolling, fine rolling, relaxing and slow cooling of a steel plate, and straightening. The mechanical properties of the steel include RP0.2 is larger than or equal to 690MPa, Rm is larger than or equal to 800MPa, the ductility A is larger than or equal to 22%, KV2 at the temperature of -20 DEG C is larger than or equal to 180J, and RP0.2/Rm is smaller than or equal to 0.83. The steel after welding is not preheated or the preheating temperature is not higher than 50 DEG C, heating processing is not carried out after welding, welding efficiency is greatly improved, and the steel has good cold and hot machining performance and high deformation resistance.