Development Of High Performance Bridge Steel Q420qE In Qinhuangdao Shouqin Co., LTD

With the rapid development of traffic and transportation industries that lead to the increase of bridge construction, as well as the market requirement of high quality bridge steel. Qinhuangdao shouqin metal materials Co., LTD(Hereinafter referred to as shouqin company), according to the requirement of Baisha Tuo Yangtze River Bridge Project contracted to build by China Railway Bridge Bureau, the bridge steel needs the properties with high toughness and low yield ratio, Shouqin company undertook the project and develop a new high performance bridge steel Q420 q E instead of traditionzal bridge steel which have steel plate thickness effect, lower strength, bad welding and weatherability. In this paper, based on the consideration of the chemical composition optimization, TMCP, and tempering process, we design a 420 MPa high-performance bridge steel that the yield strength, toughness, weldability, weatherability resistance are all good with low carbon and alloy element. The results of design indicated that all indexes can perfectly achieve the new national standard GB714-2000 for bridge steel. The chief result is as follows:The chemical composition of Q420 q E steel was designed by concept of low carbon and microalloying, and the themal-mechanical control rolling processing(TMCP): the rough rolling temperature is 1140-1160℃, finishing rolling is 779-806℃, begin and finish cooling temperature are 749-773℃ and 481-522℃ with the colling rate 12-14℃/s. The microstructure of TMCP is granular bainite+quasi-polygonal ferrite+a few equiaxed ferrite.With the increasing of tempering temperature, yield strength, tensile strength and yield ratio of steel were improved, precipitation gradually increased. The precipitation mainly the Nb, Ti carbide. When tempering temperature at 600℃, cementite already precipitated. The best tempering temperature is 500℃, and the best tempering time is 140 min. Under this state, large dislocation mergered, transformation stress released and hardness has certain ascend.Rolling state of Q420 q E transformation stress mainly quasi-polygonal ferrite and bainite transition which produce a large number of dislocation. Tempered state’s is much better than TMCP, ductile-brittle tansition temperature of 16-40 mm thickness is below-100℃, another 50 mm thickness is below-60℃, Which indicate Q420 q E has good impact property. The inclusion level are class B inclusions≤2.0, class A inclusions ≤1.0, basicall no others inclusions. The inclusions can meet the requirement of control level.