| High strength meteallic-coated cold-rolled structural steel sheet such as 320MPa and 350MPa grade developed by C-Mn steel can't satisfy special application where formability and weldability must be guaranteed, so it's urgent to develope high strength meteallic-coated cold-rolled structural steel sheet of low carbon equivalent by rephosphorized steel microalloyed with Nb, Ti or Nb+Ti . This paper aims at determining proper compositions and processing parameters by laboratory experiment, laying basis for further commercial developement.Firstly, recrystallization kinetics during annealing of specimens with different compositions and coiling temperatures was evaluated by testing hardness value, then recrystallization activation energy was calculated by Avrami equation;secondly, the influence of compositions and coiling temperatures on tensile properties of hot-rolled specimens was evaluated, subsequently, the influence of composition, coiling temperature and annealing temperature on tensile properties of annealed specimens was evaluated, at the same time, the reason of tensile properties variation was analysed by optical microscopy and Transmission electron microscopy(TEM) test.Experiment results and conclusions were summarized as follow:1.The base rephosphorized steel is easy to recrystallize during annealing, the recrystallization process of which isn't influenced by coiling temperature, in contrast, its recrystallization is obviously delayed by microalloying and is influenced greatly by coiling temperature. the recrystallization of microalloyed steel with 0.020%Nb, 0.020%Nb+0.012%Ti, 0.015%Ti is most difficult when they were coiled at 600℃, in contrast, the recrystallization of microalloyed steel with 0.036%Nb is most difficult when they are coiled at 650℃.2. The influence of coiling temperature on as-rolled microstructure and properties was summarized as follow:For microalloyed steel containing 0. 020%Nb> 0. 015%Ti, sufficient and fine precipitation was obtained when they were coiled at 600°C,precipitation was gradually coarsened when they were coiled at 650°C> 700°C;for microalloyed steel containing 0. 036%Nb, little precipitation was obtained when they were coiled at 600°C, sufficient and fine precipitation was obtained when they were coiled at 650 °C, precipitation was coarsened when they were coiled at 700°C;for microalloyed steel containing 0.020%Nb+0.012%Ti, sufficient and fine precipitation was obtained when they were coiled at 600°C, precipitation was coarsened to some extent when they were coiled at 650°C, precipitation won't continue to be coarsened when they were coiled at 700°C.The maximum strength of microalloyed steel containing 0. 020%Nb> 0. 015%Ti was obtained when they were coiled at 600°C, the strength of which decreased as the coiling temperature increased;The maximum strength of microalloyed steel containing 0. 036%Nb was obtained when they were coiled at 650°C;The maximum strength of microalloyed steel containing 0.020%Nb+0. 012%Ti was obtained when they were coiled at 600°C, same strength was obtained when they were coiled at 650 °C and 700 °C.3. The influence of coiling temperature and annealing temperature on as-annealed microstructure and properties was summarized as follow:As annealing temperature increased from 750°C to 780°C the grain size of base rephosphorized steel and microalloyed steel containing 0. 015%Ti grew up to some extent, whereas that of microalloyed steel containing 0.020%Nb -. 0.020%Nb+0. 012%Ti> 0. 036%Nb didn't grow up.Precipitation in microalloyed steel containing 0.020%Nb -. 0.015%Ti > 0. 020%Nb+0. 012%Ti coiled at 600°C were still fine when they were annealed at 750 °C for 30s , and it coarsened a little when they were annealed at 780°C for 30s, as a result .strength decreased as annealing temperature increased, whereas the precipitation in microalloyed steel containing 0. 036%Nb increased as annealingtemperature increased , as a result the strength decreased a little and then increased.4.compositions and parameters to develope high strength meteallic-coated cold-rolled structural steel sheet of low carbon equivalent was commended as follow:For 350MPa object, microalloyed steel containing 0. 020%Nb+0.012%Ti -coiled at 650 °C-annealed at 800 °C for 30s;for 320MPa object, microalloyed steel containing 0. 020%Nb-coiled at 650°C-annealed at 780°C for 30s;for 280MPa object, microalloyed steel containing 0.015%Ti -coiled at 600°C-annealed at 750°C for 30s.
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