Showing papers in "Tetsu To Hagane-journal of The Iron and Steel Institute of Japan in 2004"

BN Type Inclusions Formed in High Cr Ferritic Heat Resistant Steel

Abstract: Scanning electron microscopy observations of fractured steel rod samples were made to clarify the formation and dissolution behaviour of boron nitrides in high Cr ferritic heat resistant steels during heat treatment at high temperature. A large number of coarse size BN type inclusions of 2 to 5 μm were distributed at the bottom of the dimples. They did not dissolve during annealing at 1150°C. However, they had a tendency to begin dissolving and reducing its particle size with time at 1200°C and dissolved completely in a short time at 1250°C. From the chemical analyses of boron and nitrogen in many high Cr heat resistant steels and SEM observations of their fractured samples, critical boron and nitrogen concentrations for the formation of coarse size BN type inclusions of 0·001%B and 0·015%N were found, and the solubility limit of BN was represented as log [%B]=−2·45log [%N]−6·81 at 1150°C.

Microstructure Development and Control in Steel Welds

Abstract: Synopsis : In welded structures, welds are required to have performances equivalent to base metals in wrought and annealed conditions. To achieve the performance required, various microstructure controls have been developed and applied for steel weld metals and heat-affected zone to minimize the coarse, heterogeneous and brittle structures. In this article, these microstructure controls are reviewed with a particular emphasis on the toughness of carbon—manganese and low-alloy steel welds. The control of inclusions and the optimization of steel compositions for the improved microstructure refinement are discussed, since the refinement of weld microstructure has been achieved by the inclusion-assisted grain refinement of prior austenite and the enhancement of intra-granular ferrite formation. The discussions are extended to the role of inclusions as pinning particles and as preferential nucleation sites of ferrite, which is important for further improvement of the technology. Toughness degradation due to martensite-austenite constituent in welds is also referred. Current understandings of the microstructure controls in carbon—manganese and low-alloy steels welds and issues to be solved are summarized.

The Current Status and Future of Iron Ore and Coal Resources for Japanese Steel Mills

Abstract: Synopsis : There are iron ore and coal resources to supply raw materials for ironmaking for many years. Recent developments, such as the consolidation of raw materials suppliers and increase of iron ore imports in China, influence the present and future availability of raw materials. Carajas iron ore, pisolitic iron ore and \"semi -soft\" coking coal are the accomplishment of about 20 years' cooperative development of new sources and raw materials utilization by Japanese steel mills. To adopt the changes and natural resources availability, the next targets are Marra Mamba and high P Brockman iron ores from Western Australia for iron ore and less usage of prime coking coal for coal. In future, utilization of scrap is a key for steel industries.

Hydrogen Absorption of High Nb Bearing Steel

Abstract: Synopsis : Effects of niobium addition on strengthening, hydrogen absorption and hydrogen embrittlement have been investigated paying attentions to carbides morphologies by using laboratory melted clean steels. Addition of niobium and austenitizing at high temperature enhanced temper softening resistance due to fine niobium carbide precipitation during subsequent tempering. Addition of both niobium and vanadium improved temper softening resistance furthermore. High niobium steel absorbed less hydrogen in corrosive media than high vanadium steel. It would attribute to small size and incoherency of niobium carbide. High niobium steel showed good resistance to hydrogen embrittlement. It would be due to both uniform dispersion of cementite by high temperature tempering and low hydrogen absorption.

Influence of Limestone and Coke Breeze Distribution in the Quasi-particle on Permeability during Sintering and Sinter Quality

Abstract: Synopsis : To improve sinter productivity and quality, the effect of secondary admixing coke breeze and limestone in the drum mixer on the permeability and the reduction characteristics were investigated by the sintering pot and tablet tests. The following results were obtained. ( 1 ) The secondary admixing technique to coke breeze enhanced permeability at moisture zone with increasing the quasi-particle size and strength, and the secondary admixing technique to limestone enhanced permeability at melting zone by the improvement of melt fluidity. ( 2 ) The sinter reducibility was improved by secondary admixing limestone and coke breeze, because of increasing micro pore originated from relict ore. ( 3 ) The above improvements were decreased with extending the coating time, because of destruction of quasi-particles due to intrusion of secondary admixing material inside the granules.

Effect of Cementite Decomposition on Delamination in High Carbon Steel Wires

Abstract: Synopsis : In order to find out the effect of cementite decomposition on delamination, decomposition behavior of cementite in high carbon steel wires during drawing and aging was investigated by atom probe field ion microscope, transmission electron microscope and torsion testing. Fracture mode of normal fracture in torsion test is Mode III and that of delamination is Mode II. Microvoids or coarse grain boundary ferrite are not observed in the delaminated wires. Cementite decomposition proceeds with the drawing strain and aging, and the carbon concentration of lamellae ferrite remarkably increases, which can only be due to the dissolution of cementite and concurrent migration of carbon atoms into ferrite. The measured carbon concentration varies from region to region, depending on the lamellae spacing or the location within wires. It is considered that cementite decomposition proceeds heterogeneously in macroscopic and microscopic scale. Delamination occurs in the regions in which the maximum carbon concentration of ferrite exceeds 1 at%, irrespective of the dry drawn, the wet drawn and the aged wires. The heterogeneous decomposition of cementite suggests that the strength of ferrite is non-uniform. It may be concluded that delamination is caused by the non-uniformity of ferrite strength. Namely, delamination results from the local shear stress concentration in the low ferrite strength regions, which correspond to the locally low carbon concentration areas.

Strength and Structure of Heat Resisting Steels at High Temperatures

Abstract: Synopsis : The heat resistant steel is a structural material for the high temperature of thermal power generation etc. As mechanical property for the heat resistant steel, high temperature strength should be high, and the rupture time be long. But, the structure of the steel remakably change during long time creep deformation because diffusion easily occurs at the high temperature. Therefore, it is necessary to control the structural change during creep deformation to maintain strength for a long time at the high temperature. In this paper, at first the origin of high temperature sterngth of the heat resistant steel is described in view point of high-temperature deformation mechanism. Next, the grain boundaries in martensitic structure which are the fundamental structure of the heat resistant steel is described, and the change of these boundaries during creep deformation is described. The structure control of a further making of the heat resisting steel to high strength is described by using these findings.

Influence of Microstructure on Rolling Contact Wear in High Carbon Steels

Abstract: Synopsis : It has been reported that the characteristic of rolling contact wear of rail steels varies depending on the microstructure of steels . However, the reasons for such variations have not been fully elucidated yet. Accordingly, a two cylinder rolling contact wear test was conducted by using pearlitic steels, tempered martensite steels, spheroidal carbide steels and pro eutectoid cementite steels which contain pro-eutectoid cementite structures in pearlite structure. The relation between micro-structures and wear in high carbon steels was investigated , and the dominating factor of the rolling contact wear and the wear mechanism in high carbon steels was discussed . The main findings are as follows: ( 1) The wear property of high carbon steels is greatly influenced by the micro-structural morphology , and the wear resistance of pearlitic steels and pro eutectoid cementite steels with the lamella structure as pearlite are higher than the tempered martensite steels and spheroidal carbide steels with the carbide dispersion structure (the carbide are distributed in the matrix ferrite). (2) The reason why the wear resistance of steels with the lamella structure as pearlite improves compared with steels with the carbide dispersion structure is attributable to the prevention of adhesive wear due to increasing hardness (strength) of the rolling contact surface and to the prevention of the occurrence of fatigue wear due to flaking in the rolling contact surface.