Study On The Effect Of Mn On The Microstructure And Properties Of Low-carbon Titanium Microalloyed Steels

As an important microalloying element in steel,Ti can refine the grain of steel and have good second-phase strengthening effect,and also has a good cost advantage.Therefore,(ultra)low carbon titanium microalloyed steel has been widely used in engineering construction field.As an important deoxidation element in steel,Mn can refine the grain by reducing the phase transition temperature,also plays a certain part in solid solution strengthening,in addition,Mn prevents the thermal cracking tendency of sulfur.Ti is a strong carbide forming element,Mn is a weak carbide formation elements,many studies have shown that Mn can increase the austenite Ti-C or Ti-C-N solid solubility,and have a certain impact on the strength of this material.SM490 A is one kind of engineering structural steel that with good-welding capability,one steel plant produce ultra-low carbon micro-titanium alloys,there are two composition different by content of Mn,one is 0.4%Mn the other is 0.8% Mn.During production found that the strength fluctuation of the steel SM490 A exceeds limits:1)0.4% Mn type,the yield strength is low;2)0.8% Mn type,the yield strength is high.The intensity fluctuation value of the two types is even over 100 MPa,and the performance of SM490 A steel is obviously weakened by change in content of Mn,when other contents of Ti and C keep in the same,This also means the production process to be unstable.So this paper through the statistical analysis of SM490 A steel performance,combined with observation the micro organization of the steel,the results show that:1)The factors of 0.4% Mn type with low strength is as follow:The small of rolling reduction result in small deformation,and the process hardening effect is also low.2)Mn,Ti content using the lower limit control,the fine grain strengthening and second phase strengthening effect is low.The factors of 0.8% Mn type with high strength is as follow:Mn and Ti content are controlled by upper limit,which results in high solid solution strengthening,fine grain strengthening and second phase strengthening.The rolling reduction has little effect on the tensile strength.Based on comprehensive data analysis,also fully consideration of urgent need for cost reducing and efficiency improvement,This paper give a optimization proposal of SM490 Asteel production process.Comfirmed by actual products testment,it found that steel strength fluctuation problems has been solved,objections for product quality and related economic losses also been reduced.Thick≤9.7mm,Mn Be controlled within 0.5%~0.6%,Goal:0.55%,Ti Be controlled within 0.055%~0.07%,Goal:0.06%。Thick>9.7mm,Mn Be controlled within 0.068%~0.08%,Goal is: 0.072%Ti keep in the same range as before.