Materials Science Forum Vols. 561-565

Abstract: A ultra low carbon high strength steel heavy plate (60∼80mm thick) was produced with TMCP at Xiangtan Steel. The steel has a good combination of high strength (σb∼600MPa) and impact toughness (∼300J). The microstructural features were analyzed by optical microscopy and scanning electron microscopy (SEM). Results showed that microstructures consisted of a large amount of acicular ferrite and a small amount of polygonal ferrite. A very few amount of retained austenite was also observed in the microstructures. The formation of fine acicular ferrite resulted in the high strength of the steel plate. The good impact toughness of the steel plate is mainly attributed to the formation of acicular ferrite and a very few amount of retained austenite.

Abstract: The hot workability of modified 9Cr-1Mo, as a grade of heat resistant steels, is inferior to that of low-alloy steel, so the inner surface crack (ISC) easily occurs in seamless boiler tubes produced by the Mandrel Mill under improper rolling conditions. With the aid of FEM, the metal flow status during the rolling process was analyzed in 140mm 8-stand mandrel mill of Bao Steel. Both the metallographic shape and size of the ISC together with the result from the simulation show that the ISC of seamless tube forms at the elongation stage of shell. The mechanism of the ISC was discussed. With its initiation in stand No.1 and No.2 due to poor hot workability of modified 9Cr-1Mo steel, the ISC develops in subsequent passes. Based upon the mechanism devised was a special roll pass system which substantially upgraded the yield of qualified products.

Abstract: Dynamic continuous stiffness measurement (CSM) nanoindentation method was applied to measure nanohardness in low carbon microalloying steel. The matrix hardness for quenched phase and acicular ferrite calculated through analysis of curves is less fluctuated than the raw results, which shows that the analysis of the curves is helpful for determining matrix hardness.

Abstract: This study aims to analyze the potential of grain boundary corners (GBCs) as nucleation sites of bcc precipitates in an fcc matrix. By combined serial sectioning and electron backscatter diffraction analysis, the crystallography of GBCs and GBC precipitates was analyzed in a Co-Fe alloy.

Abstract: An investigation into the compression forming of cylinder using the commercial code SUPERFORM is developed. The cylinder billet compressed between the upper and lower dies is meshed by a quadrangle elastic-plastic element. The numerical simulation based on the FEM also compares with the slab method established by us. In the slab method analysis, the stress distributions are estimated by considering the coulomb friction between the dies and the cylinder. Throughout this study, the effects of frictional coefficient, rotating angular speed, reduction and aspect ratio etc upon the compression force, the effective stress and the effective strain, and velocity field are discussed systematically. For verifying the validity of two models, comparisons of compression forces based both modes are carried out to prove the feasibility of both models.

Abstract: Effects of Ti and Nb stabilization on the recrystallization and the pitting potential in Fe-21%Cr ferritic stainless steels were studied by using optical microscopy, scanning electron microscopy, transmission electron microscopy and polarization curve measurement. The results show that both Ti and Nb, either in solution or as precipitates, retard the recrystallization and enhance the recrystallization temperature. Substitution of Nb for Ti in Fe-21%Cr ferritic stainless steels increases the recrystallization temperature by 30 to 50°C. Nb and Ti stabilized ferritic steels present higher pitting potential than Ti stabilized steels.

Abstract: Critical high temperature components of machines and structures are often subjected to complicated load and temperature histories. The closest laboratory simulation of service loading conditions involves creep under nonsteady temperatures and stresses. For example, the start up and shut down cycles can be well simulated by temperature variation by use of intermittent heating tests. Such approach is illustrated by recent experimental results on advanced high creep strength 9- 12%Cr ferritic-martensitic steels (P91, P92 and E911). A comparison between the creep characteristics of nonsteady and monotonously creep specimens has revealed no significant deterioration of the creep strength and fracture resistance of these steels in power-law (dislocation) creep.

Abstract: At the Egyptian Iron and Steel Company, attention is constantly focused on improving basic steel making practice with the aim of improvement of blowing regime and the addition of forming slag materials system. A number of factors considered important in controlling the properties of steel products and affecting the residual manganese in the basic Oxygen furnace (BOF) have been investigated by changing some industrial parameters, aiming at optimizing the residual manganese in BOF. The studied factors were manganese oxide in the slag, iron oxide in the slag, tapping temperature, Slag basicity, Lance height, blowing time, and carbon content. It was found that residual manganese increased from 0.25 to 0.35 % wt, due to the reduction of both MnO in slag from 22% to 15% and FeO from 21 to18%, also the increase of tapping temperature from 1650 oC to 1670 oC caused an increase the residual manganese from 0.27% to 0.35%, and the slag basicity decrease from 4.25 to 3.8 led to an increase in the residual manganese from 0.25 to 0.37%. Also, the change of the lance height from 1050 mm to 825 mm caused an increase in the residual manganese from 0.27 % to 0.33 %. These results are believed to be reflected on the total energy consumption and ferromanganese additions needed for producing specific grades.

Abstract: Being examined by tensile tests at 820°C with initial strain rates of 2.5×10-4 s-1, 5.0×10-3 s-1and 1.0×10-1 s-1, the hot-rolled spay formed high speed steel (SF-HSS) had superplastic properties. With έ=2.5×10-4 s-1and 5.0×10-3 s-1, the σ-ε curves indicate that there has occurred dynamic recrystallization at the later stage of deformation.The tensile elongation decreases monotonously with strain rate increasing. The strain rate sensitivity m =dlogσ/dlog έ is about 0.23. In the sub-surface of fractured SF-HSS samples with έ=2.5×10-4 s-1,, most of holes on the subsurface are observed to distribute near the carbides and arranged along the tensile axis direction. The interface between coarse carbides and matrix is very weak sites where the holes are easy to nucleate and connect to cracks during deformation. In the necked region, fine carbide particles on the grain boundary have pinned the slip of dislocations and formed dislocation wall inside the grain. Dynamic recrystallization and some climb of dislocations has occurred. The superplastic deformation mechanism with έ=2.5×10-4 s-1 was dislocation creep mechanism controlled by dynamic recrystallization. During deformation, the role of some carbide in the materials was to retard the grain growth and keep SF-HSS having fine equiaxed grain size and remain stable.

Abstract: Creep tests and microstructural observations in several high Cr ferritic steels bearing V and Nb were carried out in order to investigate effects of addition of V and Nb on
, which is a coefficient of
method. Creep strength was improved with increasing the V content and was slightly lowered by increasing the Nb content.
in all the steels bearing V or Nb was lager than that in fundamental steel. The dependence of
on the amount of additions was different between V and Nb. In steels bearing V,
became larger with increasing V content. In steels bearing Nb,
became smaller. These results revealed that
corresponds to the creep strength, indicating that the value of
can be estimated using the strengthening effects of the additive element.