발간논문

Home > KJMM 논문 > 발간논문

A compression type hot deformation simulator was used to investigate ferrite nucleation from deformed austenite of low carbon steels. In the specimen deformed at 725℃, just above the Ar3 temperature, austenite grains were elongated and unrecrystallized and a number of banded equiaxed ferrite grains, 2∼3㎛ in size along elongated austenite grain boundary appeared. These very fine ferrites were thought to nucleate and dynamically recrystallize during hot deformation. The effects of amount of deformation and deformation temperature on ferrite transformation were also investigated. Increasing the amount of deformation and lowering the deformation temperature caused the acceleration of strain-induced transformation to ferrite. Orientation changes between ferrite grains were studied by Kikuchi pattern analysis.

keyword :

The concept of oxide metallurgy was applied to Ti-containing low carbon steels in order to investigate the effect of fine Ti-oxide inclusions on the microstructures and mechanical properties of the steels. When Ti-containing low carbon steels were cooled at a rate of 3 to 6 K/sec after austenitized at 1623 K, fine intragranular acicular ferrite was formed instead of relatively coarse Widmansta¨tten ferrite which forms without Ti addition under the same austenitizing and cooling conditions. The effective grain size and volume fraction of acicular ferrite were found to be dependent of austenite grain size, cooling rate, and chemical composition of steels. It has been observed that once the acicular ferrite nucleation is initiated at the surface of titanium oxide inclusion, the so-called sympathetic nucleation seems to be prevalent in the rest of the nucleation process. The ultimate tensile strength, yield ratio, and elongation were about 600 ㎫. 0.8, and 24%. respectively, when the composition of steel is 0.14%C, 1.8%Mn, 0.18%Si, 0.011%Ti, 0.003%O, and 0.001%N.

keyword :

The effects of manganese sulfide and vanadium carbonitrides on the intragranular ferrite nucleation have been studied by calculating the equilibrium relationship between vanadium carbonitrides and manganese sulfide and by applying controlled cooling rate experiment in 0.26C-1.5Mn steels. Equilibrium calculation shows that there is little interaction between vanadium carbonitrides and MnS. The vanadium nitride forms as a vanadium carbonitride(VC_yN_(1-y)) below about 1100℃ and the carbon composition(y) rises with decreasing temperature. The manganese sulfide is expected to form at above 1500℃, i.e. from liquid phase. Controlled cooling rate experiments showed that MnS particles consist of bimodal size distribution. The relatively fine particles(<5㎛) continuously grow during solid-state cooling and their distribution is influenced by the cooling rate, faster cooling rate favoring ferrite distribution. MnS acts as an important intragranular ferrite nucleation site. Intermediate holding experiments during continuous cooling experiment suggests that Mn depletion around MnS particles is mainly responsible for intragranular ferrite nucleation. In the case of after-heat treatments, MnS particles were believed to act as nucleation site for vanadium carbonitrides, which largely promotes the intragranular ferrite nucleation.

keyword :

Small amount of aluminum addition to ultrahigh carbon steels (UHCS) leads to finer grain size during the established thermomechanical processing, and is known to improve the superplasticity of the steels. However, the effect of Al on the grain refinement in the hyper-eutectoid steels during thermomechanical processing is not well recognized and has never been quantitatively documented. Up to 5wt% aluminum was added to 1.2∼1.5wt%C steels and its influence on microstructural refinement has been studied. That is, the formation of proeutectoid carbide, austenite grain size, pearlite colony size, interlamellar spacing, equiaxed ferrite grain size and carbide particle sizes as a function of the Al-content were determined after hot-warm working plus an isothermal rolling or the divorced eutectoid transformation with associated deformation. Fine grain microstructure consisting of less than 1㎛ ferrite grains has been achieved by small Al-addition. It has been demonstrated that the dilute Al-alloying enhances the superplasticity of the UHCS: the optimum strain-rate has shifted to the faster rate region(at 800℃) and the tensile elongation of 545% has been observed at an initial strain-rate of 8.3×10^(-2)sec^(-1).

keyword :

Properties of fine grained steel weldments have been investigated with reference to welding process in order to establish the basis of high efficient welding technologies for the high strength structural steel for next generation, the so-called HIPERS-21. The steel used mainly consisted of very fine grained ferrite of about 5-7㎛ in size, which was made experimentally using the controlled rolling and accelerated cooling technology. However conventional GTA and GMA welding practices caused remarkable microstructural changes including grain coarsening up to a few hundreds ㎛ in size at HAZ particularly near the fusion line of the steel, depending on the heat input applied. Laser welding process characterized with low heat input resulted in much finer HAZ microstructural characteristics and higher hardness distribution at HAZ than the GTAW process studied. This indicates laser welding process to be an effective method for controlling HAZ microstructure of the fine grained steel.

keyword :