Physical simulation model of γFe → αFe grain nucleation & growth in isothermal solid-state transformation and the rule of grain nucleation & grain growth’s boundary migration are established. The Genetic Algorithm is introduced in the process of grain nucleation & growth. The grain boundary migration agrees with the theory of grain growth. The system free energy ΔG calculation formula is exported. The thermodynamic criterion of new nucleus growing up is that new grain average radius rc is bigger than theory critical average radius rc0 when ΔG is achieved ΔGmax, ΔG is continually decreased at the remaining transformation, the final energy ΔGf is less than initial energy at the end of transformation; be influenced by the limit of nucleation number N.

For the thick steel plate 13MnNiMoNbR, submerged arc welding was carried out, three kinds of wires were used H08Mn2MoA, H10Mn2MoA, H10Mn2Ni2MoA and together with two kinds of fluxes which are SJ101, SJ102 .After welding ,normalizing plus tempering heat treatment was added.Using V-shaped notch Charpy impact test and the fracture morphology was observed for researching. The toughness of weld and heat affected zone was lower than 27J at -20 °C,cleavage fracture exited in main proportion. Experiments proved that because of the slow colling speed of weld after normalizing a large amount of the upper bainite produced. When normalizing temperature fell in the critical zone where the ferrite and austenite two-phase can exist together (critical normalizing),upper bainite in weld was eliminated , strength and low temperature (-20 °C) toughness correspond with national standard.