Development of grain boundary allotriomorphic ferrite/granular bainite duplex steel

A new hot-rolled low alloy high strength steel with grain boundaryallotriomorphic ferrite/granular bainite duplex microstructure has been developed through novelmicrostructure and alloying designs without any noble metal elements such as nickel and molybdenum.Its as-rolled microstructure and mechanical properties, fatigue crack propagation behavior comparedwith single granular bainitic steel as well as continuous cooling transformation, were investigatedin detail. The measured result of CCT (continuous cooling transformation) curve shows that suchduplex microstructure can be easily obtained within a wide air-cooling rate range. More importantly,this duplex microstructure has much better combination of toughness and strength than the singlegranular bainite microstructure. It is found that the grain boundary allotriomorphic ferrite in thisduplex microstructure can blunt the microcrack tip, cause fatigue crack propagation route branchingand curving, and thus it increases the resistance to fatigue crack propagation, improves steeltoughness. The mechanical properties of the above commercial duplex steel plates have achieved orexceeded 870 MPa ultimate tensile strength, 570 MPa yield strength, 18 percent elongation and 34 JCharpy V-notch impact energy at -40 deg C, showing good development potential.

Non-Steady Equilibrium during Pro-Eutectoid Ferrite Formation in Fe-0.2C Alloy

As rolled microstructure and mechanical properties of a new kind of grain boundary allotriomorphic ferrite/granular bainite duplex steel plate and its crack propagation behavior were investigated in comparison with simple granular bainitic steel plate. These new duplex plate steels possess better combination of strength and toughness than granular bainite plate steels under the conditions of conventional rolling and air cooling. The observation of fatigue crack propagation behaviors showed that the existence of proper grain boundary allotriomorphic ferrite increases the compatible deformation ability of duplex microstructure and leads to the formation of crack branching and curving route, and it has an evidently blunting effect on microcrack tip and results in higher impact toughness. In addition, the new duplex steel plate also has good weldability as hot rolled high strength low alloy structural steel.

Effects of chromium on the microstructure and hot ductility of Nb-microalloyed steel

It is well-known that the surface quality of the niobium microalloy profiled billet directly affects the comprehensive mechanical properties of the H-beam.The effects of chromium on theγ/αphase transformation and high-temperature mechanical properties of Nb-microalloyed steel were studied by Gleeble tensile and high-temperature in-situ observation experiments.Results indicated that the starting temperature of the γ→αphase transformation decreases with increasing Cr content.The hot ductility of Nb-microalloyed steel is improved by adding 0.12wt% Cr.Chromium atoms inhibit the diffusion of carbon atoms,which reduces the thickness of grain boundary ferrite.The number fractions of high-angle grain boundaries increase with increasing chromium content.In particular,the proportion is up to 48.7% when the Cr content is 0.12wt%.The high-angle grain boundaries hinder the crack propagation and improve the ductility of Nb-microalloyed steel.

Effect of Aluminum and Titanium Treatment on Non-Metallic Inclusions and Microstructures of CGHAZ in HSLA Steel

The Effect of A1 and Ti treatment on non-metallic inclusions and microstructures of coarse-grain HAZ in HSLA stee1 was investigated in this paper based on experiments and thermodynamic calculations.The results showed that the inclusions in A1 treated steel were mainly aluminum oxides and titanium nitrides which could not promote the formation of acicular ferrite microstructures.Microstructure of coarse-grain HAZ in A1 treated steels consists of heavy grain boundary ferrite and ferrite side plate.The inclusions in Ti treated steel were A1,Ti,Mg,Ca composite oxides with size in the range of 0.5-3μm and titanium nitrides with size less than 0.3μm.Ti composite oxide could promote the formation of acicular ferrite and microstructures of coarse-grain HAZ in Ti treated steel consists of grain boundary ferrite,small amounts of ferrite side plate and large amounts of intragranular acicular ferrite.The size of grain boundaries ferrite was increased and the amount of ferrite side plate was decreased with the increase of soaking time at the peak temperature.The amount of grain boundary ferrite and the size of acicular ferrite were also increased with the increase of cooling rate during ferrite phase formation.

Influence of Nb on Microstructure and Property of Low-Carbon Mn-Series Air-Cooled Bainitic Steel

The effect of Nb on the microstructure evolution of low-carbon Mn-series air cooled bainitic steels has been studied by using thermomeehanical simulation machine, SEM, and TEM observation. The results show that the amount of ferrites is relatively higher in the steel without Nb than that of Nb-bearing steel under same deformation conditions, and the ferrites in Nb-bearing steel are more finer. The mechanical properties of the Nb-bearing steel are higher than those of the steel without Nb at the same finishing rolling temperature （FRT）, and the toughness of the Nb- bearing steel is about 100J higher than that of the steel without Nh at the finishing rolling temperature 750℃. The toughness of Nb microalloyed steel will be improved more effectively than that of the steel without Nb with the decrease of FRT.