| Weathering steel is one type of low alloy structural steel with good comprehensive performance and lower cost, which not only has good atmospheric corrosion resistance, but also has a well mechanical property and weldability. Untill now, in order to obtain the higher combination of strength-toughness and weldability, development of weathering steel is facing to decrease carbon content and increase strength by the method of microalloying or microstructure strengthening.According to present research, in this paper, on the basis of insuring atmospheric resistance and weldability, carbon content of weathering steel was decreased to increase toughness, at the same time, increasing Mn and Cr content, V-N microalloying were adopted to increase strength of weathering steel respectively. On the condition without TMCP, influence of above design viewpoints on strength and toughness of weathering steel were studied as well as influence and mechanisms of alloy elements on mechanical properties and corrsosion resistance of weathering steel were analyzed and discussed respectively. Those results could be provided to design and develop a new generation of weathering steel.Mechanical tests showed that, yield strength of two kinds of weathering steels with V-N alloying treatment and bearing 0.50%Mn and 1.31%Mn respectively achieved 555MPa and 610MPa, in addition, tensile strength reached 650MPa and 705MPa due to a relative high strain-hardening properties. Further, both plasticity and toughness satisfied the requirement of the high strength weathering steel.Thermodynamics analysis proved that, V-N alloying increased the precipitation performance of Vanadium at high temperature remarkably. For the weathering steels with 0.0320%N-0.096V and 0.0358%N-0.083V, initial precipitation temperature of Vanadium increased to more than 1130℃and precipitation rate of Vanadium exceeded 90% at 850℃. However, for a normal weathering steel bearing 0.0050%N, initial precipitation temperature of Vanadium was lower than 1000℃and precipitation rate of Vanadium was lower than 20% respectively at 850℃. Kinetics analysis showed that, total precipitation time of VN was lower than 110s at 850℃. TEM analysis showed, for the V-N alloying weathering steels, the grain size of VN particles was in the range of 20nm-300nm, which exceeded the critical precipitation size of 7.89nm. Precipitation strengthening and refining strengthening contributed to more than 70% of yield strength owing toVN particles precipitating at high temperature.Polarization curve test and corrosion mass lost experiment showed, the corrosion rate of weathering steel was decreased because of V-N alloying. EIS test and rust layer analysis results showed, corrosion reaction occurred on the surface of matrix more symmetrically because of pitting being suppressed by V-N alloying, furthermore, the rust layer had a relative high charge conducting resistance, which characterized that the insulating property of rust layer increased. Cr enrichment characteristic was changed by V-N alloying and Cr enrichment zone expanded from inner rust layer to outer rust layer, which was different from other experimental weathering steels. V-N alloying could accelerate to form thermodynamic stable rsut layer because of the relative highα-FeOOH content and ratio ofα-FeOOH andγ-FeOOH.When Mn mass fraction increased to 0.93%, the strengthening effect was much weak, which yield strength and tensile strength was 345MP and 485MPa respectively, however, when Mn mass fraction increased to 1.36%, the yield strength increased from 390MPa to 435MPa and tensile strength increased from 470MPa to 600MPa due to the relative high strain-hardening property, as well as comparatively high plasticity and ductility were acquired, which showed that the strengthening-toughening effect was stronger. Thermal simulation results showed that, increasing Mn mass fraction to 1.36%, Ar3 temperature decreased 60℃～80℃, accordingly, ferrite grain size was refined remarkably and the hardenability enhanced. At the highest cooling rate of 15℃·s-1 in this study, the volume fraction of bainite almostly increased to 100%. Polarization curve test and corrosion mass lost experiment showed, the corrosion rate had a slight increase with Mn content increasing. EIS test and the morphology of rust layer cross section showed, pitting characteristic increased with Mn content increasing, because Mn element has a strong corrosion tendence proved by thermodynamics analysis. EIS test showed, Mn content increase had little influence on insulating property of rust layer. EPMA results showed, there was no enrichment of Mn element in the rust layer, and Mn just distributed in inner and outer rust layer homogeneously. Phase analysis of rust layer showed, increasing Mn content, theα-FeOOH content in rust layer increased somewhat but ratio of α-FeOOH andγ-FeOOH decreased, which characterized that Mn had little effect on the formation of thermodynamic stable rsut layer.Thermal simulation results showed that, when Cr mass fraction increased from 0.62% to 1.50%, Ar3 temperature had little change but the hardenability enhanced somewhat. At the cooling rate of 15℃·s-1, the volume fraction of bainite increased to 23%. Mechanical tests showed, Cr mass fraction had a slight influence on strengthening, though strain-hardening property increased somewhat, so its yield strength and tensile strength only was 350MPa and 460MPa respectively, however, its plasticity and ductility was high. Polarization curve test and corrosion mass lost experiment showed, the corrosion rate decreased with increasing Cr mass fraction from 0.62% to 1.50%. EIS test showed, pitting was stronger at the initial stage of corrosion reaction but it was suppressed immediately, yet, the Bode plots figure showed, the rust layer with better insulating property formed. EPMA results showed, increasing Cr content had little effect on the structure of rust layer but phase analysis of rust layer revealed that,α-FeOOH content and the ratio ofα-FeOOH andγ-FeOOH increased remarkably, which characterized that the formation progress of thermodynamic stable rsut layer was accelerated.
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