A hot-rolled sheet of 18% Cr stainless steel was quenched from 1200°C, a temperature in (α+γ) region, and annealed at 800°C to provide a variety of microstructures with ferrite and transformation products from austenite in order to correlate the ridging phenomena and the microsegregation of Cr and C.
(1) Microsegregation of Cr, i.e., enrichment in ferrite and depletion in prior austenite, is preserved after annealing for 20hr.
(2) Carbon, segregated in prior austenite region, is gradually homogenized, and precipitation of chromium carbide proceeds in the prior austenite region in the annealing process.
(3) Inhomogeneous distribution of carbides and microsegregation of Cr are not homogenized in the single stage cold rolling and annealing process.
(4) The annealing process induces softening and tempering of martensite, but the microstructural changes do not affect the ridging profile after cold rolling and annealing.
(5) Size and spacing of the microsegregation region of Cr and C are nearly in the same order to the smaller undulation in the ridging profile.

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(I) Reduction of molten iron oxide and FeO bearing slag by H₂-Ar plasma was studied using water cooled Cu crucible. The sample weights were 25 to 75g, the flow rate of mixture-gas was 20l/mm and DC electric power of plasma was 8.3kW. Results obtained were as follows:
(1) The reduction of molten iron oxides proceeds linearly with time and the reaction rate is proportional to the partial pressure of atomic hydrogen. Therefore, it is considered that the rate determining step is the chemical reaction between FeO and the atomic hydrogen formed by thermal dissociation in the plasma.
(2) The rate of reduction of FeO bearing slag is lower than that of molten iron oxide and is proportional to the FeO concentration in slag. It is presumed that the reduction rate is controlled by both the chemical reaction rate of FeO with atomic hydrogen at the gas-solid interface and the mass transport rate of FeO across the boundary layer between the interface and the molten slag bulk.
(3) The reduction of molten iron oxide and FeO bearing slag by H₂-Ar plasma takes place only on the cavity formed at the surface of melt by the momentum of plasma jet gas.
(II) Continuous melting of pre-reduced ore powder, obtained by a fluidized bed reduction was examined using MgO crucible and H₂-Ar plasma.
Following results were obtained:
(1) Carry-over loss of the pre-reduced ore powder during the melting in plasma arc furnace was small, when the condition of powder feeding and plasma arc were properly chosen.
(2) Reduction of FeO in slag, accompanied in fed material, by H₂-Ar plasma, could be described by a simple model of continuous melting and reduction, based on experimental results of the reduction of FeO bearing slag as described (1-2).
With this model, the rate of reduction during continuous melting was determined.

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Effect of metallurgical factors on the bake hardenability (BH) of low-C Al-killed steel is studied. The _yield stress change due to baking (ΔYS) is determined by grain size and dissolved (C+N) content. On the other hand, the ultimate tensile stress change due to baking (ΔTS) depends mostly on the dissolved (C+N) content. The locking mechanism of dislocation is discussed in relation to grain size dependence of these properties. The influence of alloying elements (Mn, Si, P) on BH through their metallurgical factors is also studied. Mn decreases ΔYS through the reduction of dissolved C content. Si gives rise to the increase of dissolved C content and grain refining up to 0.5% Si content. Over 0.5% Si content, the grain becomes coarser in turn. Consequently, ΔYS increases up to 0.5% Si content and levels of after that. Though the addition of 0.06% P does not affect the dissolved (C+N) content, it gives rise to a little increase of ΔYS through grain refining. Larger grain size dependence of ΔYS is observed at -5ppm of dissolved (C+N) content for 0.06% P containing steel.

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A newly developed freezing-cycle simulator which permits in situ melting, freezing and tensile testing was used to investigate the high temperature ductility and strength of various materials, including ferritic low alloy steels, austenitic stainless steels and Ni base superalloys. Based on the results, the correlation between those high temperature properties and the severity of weld high temperature cracking was described.
It was demonstrated that embrittlement of simulated weld occurred in the melting temperature region as well as in the austenite temperature region, and that both cases of embrittlement might be related qualitatively to the severity of high temperature cracking in welding.
An embrittlement parameter, ΔT=NST-NDT, where NST and NDT are the nil strength and nil ductility temperatures, respectively, was introduced for the melting temperature region embrittlement. Good correlation was found between ΔT and solidification cracking susceptibility of weld metals.
It was also demonstrated that the austenite region embrittlement was associated with the melting of grain boundaries, and that it was promoted by the elements which produce a wide γ+L region in the constitutional phase diagram.

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The possibility for the production of solid iron containing little phosphorus from molten iron oxide containing phosphorus oxide was examined experimentally.
Molten iron oxide containing phosphorus, lime and silica was reduced by CO gas in an iron crucible. The samples containing 20 and 60% FeO were used as “model” substances for steelmaking slag and iron ore, respectively, in the case of smelting reduction.
Reduction temperature were at 1350 and 1450°C. Phosphorus contents of produced iron are always below 0.2%. Lower phosphorus content of produced iron can be obtained, with the sample containing 60% FeO than with that containing 20% FeO, with the sample with CaO/SiO2 ratio of 0.5 than with unity.
The reduction of iron oxide in the sample begins after an incubation period of a few minutes while that of phosphorus oxide in the sample has no incubation period. Namely, at very early stage of the reduction, phosphorus oxide alone is reduced. The reduced phosphorus is diluted with produced iron after beginning of reducing of iron oxide. The content of phosphorus increases again when the reducing rate of the iron oxide decreases.
The reduction rates of oxides of iron and phosphorus can be described as time-linear equations.
It is speculated that the rate determining step of the reduction is the transport process of the substances through the molten oxide.

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The phosphorus distribution ratios between CaO-MgO_sat.-Fe_tO-SiO₂-P₂O₅(≤3wt%)-MnO(2-5wt%) or CaO-MgO_sat.-Fe_tO-SiO₂-P₂O₅(4-5wt%) Slags and liquid iron have been obtained in the temperature range from 1550 to 1650°C. The dephosphorization quotients can be expressed by the following three different equations.
(i) log (P₂O₅)/([P]²(Fe_tO)⁵)=0.145[(%CaO)+0.3(%MgO) -0.5(%P₂O₅)+0.6(%MnO)]+22810/T-20.506
(ii) log (P₂O₅)/([P]²(Fe_tO)⁵)=7.87 log[(%CaO) +0.3(%MgO)-0.05(%Fe_tO)-0.5(%P₂O₅) +0.6(%MnO)]+22240/T-27.124
(iii) log (P)/([P](T.Fe)^5/2)=0.0720[(%CaO)+0.3(%MgO) +0.6(%P₂O₅)+0.6(%MnO)]+11570/T-10.520
The activity coefficient of P₂O₅ by Turkdogan and Pearson's treatment can be expressed as follows: log γP₂O₅=-1.02(23N_CaO+17N_MgO+8N_FetO -26N_P2O5+13N_MnO)-22990/T+9.490
In the plot of logarithms of activity coefficient of Fe_tO against mole fraction of SiO₂, the values for log γFe_tO tend to increase with increase of the P₂O₅ content and with the addition of MnO in the range of N_SiO2<0.1.

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Experiments have been made to study the effect of Na₂O(7-13wt%) and BaO(4wt%) additions on the phosphorus distribution between liquid iron and CaO-MgO-Fe_tO-SiO₂-P₂O₅(≤1wt%) slags at 1550°C. The effect of Na₂O and BaO on the logarithm of the equilibrium quotient, k_p=(%P₂O₅)/([%P]²(%Fe_tO)⁵), was found to be 1.2 and 0.9 times as large as that of CaO on weight percent basis, respectively. The values for log k_p can be expressed as log k_p=0.145[(%CaO)+0.3(%MgO)-0.5(%P₂O₅) +1.2(%Na₂O)+0.9(%BaO)]+22 810/T-20.506
The effect of Na₂O or BaO on the activity coefficients of P₂O₅ was also studied by the Turkdogan and Pearson's treatment. The following expression for the log γP₂O₅ is obtained for the Na₂O- or BaO-containing slaps. log γP₂O₅=-1.02(23N_CaO+17N_MgO+8N_FetO-26N_P2O5 +33N_Na2O+42N_BaO)-22990/T+9.490
It was found from the plot of activity coefficients of Fe_tO vs. mole fraction of SiO₂ that activity coefficients of Fe_tO decrease with the addition of Na₂O, while it does not change with the addition of BaO. The dephosphorization equilibrium was discussed on the basis of the theoretical optical basicity proposed by Duffy and Ingram. The following equation was obtained as a function of optical basicity. log N_PO^3-4/([P][O]^5/2)=22.22∧_th+30890/T-28.752 where, N_PO^3-4: the anionic fraction of PO^3-₄ ion [ ]: weight percent in metal phase.

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To improve the quality of continuously cast carbon steels, the characteristics of the hot ductility were studied via in-situ melted and solidified tensile testing method. Special emphases were placed on the effect of C and P on the hot ductility in the temperature range between melting temperature and 700°C.
It was found that zero strength temperature (ZST_c) and zero ductility temperature (ZDT_c) have a linear relationship with the equilibrium solidus temperature in Fe-C binary alloys ranging from 0.003 to 1.6% C and does not have the singularity between 0.1 to 0.2% C content. The embrittlement due to P exists only in the carbon steels having C content higher than 0.25%, which are subjected to the melting thermal history. This embrittlement is caused by the segregation of P along the austenite grain boundary.
Mild cooling in the secondary cooling stage is very effective to diminish the embrittlement resulting in producing defect free cast slabs.

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Precipitation of M₂₃C₆ and σ-phase during isothermal aging of δ/γ duplex stainless steels has been studied mainly by means of transmission electron microscope coupled with energy dispersive X-ray analyser. M₂₃C₆ particles form at both γ/γ and γ/δ boundaries. The particles are crystallo-graphically related to either one of the γ grains or the γ grain corresponding to the γ/γ or the γ/δ boundary. The M₂₃C₆ particles nucleating at γ/δ interphase boundaries grow into the untransformed δ-ferrite as an M₂₃C₆/new γ eutectoid structure dragging the γ/δ boundaries. The habit plane is the {111}γ//{111}_M23C6 which is most parallel to the edgewise growth direction. Since C atoms to form M₂₃C₆ are supplied from the γ behind the eutectoid structure, the reaction will finally stop after consumption of C atoms. Then σ-phase particles nucleate on the δ/new γ interphase boundary in Nenno orientation relationship with the new γ, and grow into the δ-ferrite as a σ/γ cellular structure with the (001)σ//(111)γ habit plane. In some cases, inpingements between σ and γ grains nucleating separately occur, and result in a tangled eutectoid structure without any specific orientation relationship. Since the partition of alloying elements between γ and δ-ferrite phases accelerates precipitations of both M₂₃C₆ and σ-phase, the solution treatment in the δ-ferrite single phase region largely retards these reactions.

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In plate mills without facilities for the control of plate crown and flatness such as a roll bender, the thickness control system has been generally based on the Force Correction Factor (actual rolling force/calculated rolling force) Learning Method: i.e., Thickness Control Method. In this method the control of plate crown and fatness is sacrificed to a certain extent especially for thin plate under 6mm.
In this paper, the authors present a new flatness control method; Predicted Force Aiming Method, that can afford flatness and thickness accuracy.

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The Muroran No. 3 Continuous Casting Machine, which started operation in November 1981, is a slab/bloom combination caster (with twin-strand operation for bloom). The time required for the process from tapping molten steel from the BOF to coiling strip into hot coils is 3.5hr.
Integrated process control of the continuous caster (CC) and the heat holding furnace (HF) is performed by a single process computer.
The process computer control functions for the CC-DR process are described below.
1) Casting speed guidance
2) Present control for each steel grade
3) Secondary cooling control models
4) Calculation and setting of cutting length
5) Collection of data on quality
6) HF pacing
7) Automatic operation of the HF
The CC/HF process computer is contributing greatly to the stable operation and quality improvement of the CC-DR process.

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