To investigate the role of Al2O3, in iron are sinters, constitutional analysis of high Al2O3 and low Al2O3 contained iron ore sinters was carried out by applying EPMA random sampling method. Measured compositions were mainly located around the two tie lines of CaO·SiO2-CaO·3(Fe, Al)2O3 and 2CaO·SiO2-(Fe, Al)2O3. It was found that Al2O3 had a tendency to concentrate in calcium ferrite, especially in that of composition of Fe2O3 from 40% to 60%. MgO content in sinter are decreased with SiO2, but CaO content increased with SiO2. This reverse tendency suggests that the role of CaO and MgO in iron ore sinters can be different.
In order to clarify degradation of coke in the cohesive zone and the dripping zone of a blast furnace, the fundamental experiment about coke degradation reacted to molten FeO was carried out. The difference of coke degradation reacted to molten FeO and CO2 gas is discussed. The results are as follows. (1) Coke reacted to molten FeO forms dimple and thin reacted layer locally. Coke reacted to CO2 gas forms thick reacted layer around lumpy coke. (2) Coke reacted to CO2 gas discharges more fine coke than coke reacted to molten FeO. Coke reacted to molten FeO discharges particles. (3) Fine ratio of coke reacted to molten FeO is suppressed with increasing coke strength.
In order to clarify the influence of micro pore in sinter on the reducibility and permeability of actual blast furnace, the properties of actual sinter sample above 1000°C were measured by use of the apparatus reported in the past and the micro pore volume distributions under 400 μm were measured by the mercury porosimeter. The properties of 10 kinds of actual sinter sample with almost the same chemical compositions were measured and the relation between the sinter properties and the operation results of blast furnace at which the sinter was used by 80% in burden was examined concerning the reducibility and permeability. As a result of these analyses. the mean diameter of micro pore under 400μm (MDMP) was derived as the controlling factor affected on the reducibility and permeability of blast furnace. As the micro pore under 1 μm is generated abundantly in the sinter with small MDMP during reduction, the reduction is accelerated above 1000°C, especially above 1200°C and the FeO contained melt is also reduced. As the FeO contained melt is difficult to enter into the micro pore under 1 μm, the reduced metal with abundant micro pore under 1 μm is generated and suppresses the metal agglomeration. The technology to reduce MDMP in sinter is expected to be developed from now on.
In order to investigate the decarburization behavior in the 250 ton RH degassing unit, effect of [C], [O] and pressure in vacuum vessel, P, on apparent rate constant of decarburization, Kc, was studied based on CO bubble formation model. 1) Kc showed lower value in the initial and final stage of decarburization process and higher value in the middle stage. Low Kc in the initial stage was supposed to be due to low circulation rate, Q, and low Kc in the final stage was supposed to be due to low CO bubble formation rate. 2) Volmetric mass transfer coefficient for decarburization, ak, and Kc in the middle and final stage showed a good correlation with [C]·[O]. This relation could be derived from CO bubble formation model. 3) Pressure in the vacuum vessel had an effect on Q in the initial stage. Calculated Q with empirical equation coincided with the value of commercial RH obtained using observed Kc and estimated ak. 4) A new decarburization model for RH was constructed. Calculated results showed a good agreement with observed [C]. And effect of exausting rate on Kc could be evaluated with this model.
Tatara was used to be a traditional process to produce solid steel, so called "Kera", and/or liquid pig iron, so called "Zuku", from iron sand and charcoal using the box type furnace with blowing cold air intermittently. A modified Tatara furnace has been examined on the production mechanisms of "Kera". Temeratures and oxygen partial pressures in the furnace were measured by means of thermocouples and oxygen sensors with solid electrolyte of zirconia. Iron sand and a piece of "Kera" produced in the bottom of furnace were sampled and analyzed chemically or by a electron probe micro analyser. Iron sand is reduced above tuyere and absorbs carbon on fired charcoals in front of the tuyere at about 1350°C. Many small balls of liquid steel with different carbon content were found in molten slag pool of FeO-saturated silica, so-called "Noro", and cohere each other to grow into a bloom. The characteristics of Tatara steelmaking process are low temperature for carbon absorption of steel at about 1350°C, high oxygen potential in the molten slag of FeO-saturated silica and hypo-eutectic carbon content in steel.
Cold forging has been widely used for forming of machine structural parts. Steels have been usually softened by carbide spheroidizing treatment before cold forging. In recent years, it has been demanded to shorten or to eliminate spheroidizing because it has usually needed many hours. The cold forgeability of medium carbon steels with insufficiently spheroidized microstructure made by shortened spheroidizing was studied. The cold forgeability was evaluated by cold upsetting test in which the end surfaces of cylindrical and flanged specimens were frictionally constrained. The effect of strain path on the cold forgeability was also investigated by measuring the surface strains both parallel and circumferential at the midheigh of the specimens during upsetting. The cold cracks initiated from voids formed at cementites just below bulged free surface in insufficiently spheroidized steels. On the other hand, the surface micro-cracks generated in proeutectoid ferrite, where strain concentrated, in the as-rolled steel with ferrite-pearlite microstructure. The decrease in cementite length caused the better cold forgeability in the insufficiently spheroidized steels. Fracture limit decreased with the increase in strain-path slope in the circumferential strain versus axial strain diagram, namely the shift in strain path from homogeneous compression to plain strain condition. The more improvement in cold forgeability by shortened spheroidizing was observed in the specimens with larger strain-path slope. This result means that the cold forgeability of insufficiently spheroidized steels can be improved by the suitable design of the strain path during cold forging.
The alloy phases of the galvanized steel have been examined in order to clarify the effect of phosphorous added in the steel substrate on the galvannealing behavior. SEM observation have revealed that the density of the ζ-ZnFe crystal decreases and that the size of the ζ-ZnFe crystal becomes large as a function of the P content in the steel. The amount of the Al-Fe alloy as an inhibition layer for the alloying between Fe and Zn have been increased as a function of the P content. The Al-Fe alloy layer has been observed by TEM. AES and XPS analyses have revealed that P in the steel segregates to the surface of the steel and that the amount of segregated P increases as a function of P content of the steel. P has been also detected in the Al-Fe alloy layer for the steel having a high P content. The results suggest that the alloying between Fe and Zn is suppressed because the amount of the Al-Fe alloy inhibition layer increased. Therefore the number of the nucleation site of the ζ-ZnFe crystal can be reduced, because the Al-Fe alloy is altered for the phosphorus added steel.
The formation of the"out-burst" products which is consisted of Al-Fe intermetallic compounds (IMCs) grown locally and abnormally, has been studied to compare with the structure of thin alloy layer normally formed on the sheet steel in a hot-dip Zn-5 mass % Al alloy coating at the bath temperature of 450 °C. The formation of the out-burst products is not due to the localized growth of thin alloy layer consisted of mainly Zn containing Al3Fe(Al13Fe4, ) but due to the growth of Al-Fe IMCs with mainly Zn containing Al5Fe2formed at ferrite grain boundaries near the steel surface. These Al-Fe IMCs are considered to be changed from Fe-Zn IMCs which have been formed by the grain boundary diffusion of Zn. And, during changing to Al-Fe IMCs, Zn discharged from Fe-Zn IMCs further diffuses to the adjacent ferrite grain boundaries and also into the adjacent ferrite grains. As a result, Fe-Zn IMCs are formed around Al-Fe IMCs which have been formed at the grain boundaries, and changes again to Al-Fe IMCs. This growth behavior of Al-Fe IMCs formed at the grain boundaries is induced from the results obtained in the previous works that the changing rate from Fe-Zn IMCs to Al-Fe IMCs in a molten Zn-5 mass % Al alloy bath at 450 °C is very high.
Toughness and related characteristics of microstructures are examined on heat affected zone (HAZ) of weldments of Ti-killed steel given single-or double-heat cycle treatment. Brittle fracture initiation and stable crack propagation behavior are examined from fracture toughness test, observation of tensile deformation microstructures and local strain distributions. It is revealed that resistance for the stable crack propagation of Ti-killed steel is hardly affected by the simulated thermal cycle conditions. However, the ductile-brittle fracture transition temperature increases in double-heat cycled specimens compared with single-heat cycled specimens. It is also revealed that higher strain concentration takes place at the prior austenite grain boundary in a double-heat cycled specimen. It is discussed that strain localization leads to the brittle crack nucleation.
We have investigated the mechanical properties and hot workabilities of the newly developed TiB2-reinforced high modulus steel (HMS). considering the potential application to automobile parts. In two kinds of the HMS prepared via conventional powder metallurgical process, TiB2 particles of a few microns in diameter were suc-cessfully incorporated into Fe-Cr ferrite (α) matrix using commercial TiB2 powders, and finer particles less than 1 micron were synthesized through the in-situ reaction of ferro-titanium and ferro-boron powders, respectively. Young's modulus evaluated by bending tests reached 289 GPa for the HMS prepared with 30 vol% commercial TiB2 powders, 293 GPa for the one with the same content of in-situ formed TiB2 particles. The wear resistance was largely improved by incorporating TiB2 particles, i. e., the wear resistance of the HMS containing more than 20 vol%TiB2 was far greater than that of α matrix steel, and that of 30 vol%TiB2 HMS corresponded to that of SUJ2 hard steel. The improvement of both the tensile and fatigue strength was much remarkable due to the TiB2 particles, especially when the HMS contained in-situ formed particles. The sintered HMS provided sufficient hot workabilities at temperatures higher than 1273K in upsetting tests, and the trial forged product demonstrated a high feasibility of making automobile parts by employing the conventional equipment.
780 MPa high strength steel with low cold cracking sensitivity was characterized by lowering carbon equivalent (Ceq) and weld cracking sensitivity (PCM), and made a strong thermo-mechanical control process (TMCP) treatment. Therefore, these new 780 MPa steels have a good weldability. This paper has investigated the evaluation of reheat cracking susceptibility during post-weld heat treatment (PWHT), especially the relation between the stress relaxation behaivor and high temperature deformability. Simulated-weld heat treatment was carried out a maximum temperature of 1623K. PWHT was done at a heating rate of 111K/ks. The results are summarized as follows : 1) From the results of stress relaxation test, reheat cracking occurred all HAZ microstructures. Especially, the reheat cracking susceptibility of Bainitic types structure were higher compared with Lath-Martensite structure. Threshold retained stress of reheat cracking was differed about 200 MPa values on both structures during PWHT. 2) From the results of the high temperature deformability obtained by constant-load test on heating, all HAZ microstructures shows the minimum embrittle value (about 5%) from 800K to 900K during PWHT. The reason of these embrittlement causes the carbide precipitation behavior during PWHT process, and exerted the effect on deformability in the matrix. 3) When the reheat cracking susceptibility and the brittleness of material were evaluated, the grasp of the deformability of HAZ microstructure in PWHT was necessary. It was important that the brittleness evaluation chart of the material was shown from the relation to a necessary the amount of displacement with the deformability of the microstructure during PWHT.