In order to clarify the high-temperature deformation mechanism in solution-hardened iron alloys, the work-hardening rate without dynamic restoring effect, h, and the softening rate without work-hardening effect, r, were measured by the stress-change test during creep of an Fe-3.5at%Mo alloy in a temperature range 1063-1138K and a stress range 5.4-18.8 MPa.
It was found that the measured effective-stress for dragging the solute atmosphere around dislocations agreed well with that derived from the Cottrell-Jaswon theory and the measured internal-stress was described by the attractive junction theory. From the changing rate of internal stress after the stress change, h and r were determined as a function of the internal stress and temperature.
The parameters h and r are shown to be useful for the prediction of mechanical behavior along an arbitrary deformation path.

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The existence of a sharp transition of high-temperature creep behavior in bcc solid solutions has been confirmed by studying steady-state creep characteristics of an Fe-2.1mol% (3.56mass%)Mo alloy in the temperature range T from 1 100 to 1200K under the tensile stress range a from 4 to 50MPa. In the lower stress region, the creep characteristics are those typical in creep controlled by viscously-gliding dislocations which drag the solute (Cottrell) atmosphere. Steady-state creep-rates ε_s can be represented by the equation, s^M_s=1.21×10^-2 (Gb/kT) D_Mo (σ/G)^3.45 (G: the shear modulus, b: Burgers vector, D_Mo: the diffusion coefficient of Mo). In the higher stress region, the creep characteristics are different from those observed in the lower stress region and ε^H_s=5.7×10⁷(Gb/kT). D_Fe (σ/G)^5.3 (D_Fe: the diffusion coefficient of Fe). Results obtained in conventional tensile test at 1 100K with strain-rates ranging from 3× 10^-5 s^-1 to 3×10^-3s^-1 coincide well with creep behavior. The solute atmosphere seems to be broken at some locations under high stresses and a part of dislocations can move as a bare dislocation in the matrix where immobile solute atoms are distributed randomly. The transition observed in this investigation is essentially the same as the upper transition, i.e., the transition between regions M and H, observed in Al-Mg (fcc) alloys and α-Ti-Al (hcp) alloys.

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The deformation structures of work-hardened austenite were investigated in austenitic Fe-Ni-C alloy and a commercial Nb-V HSLA steel. It was made clear that the deformation structures of hot deformed fcc poly-crystals were classified into three types according to their morphology; (1) transition band, (2) microband and (3) heavily deformed regions near the grain boundary. Transition band was defined as the deformation structure, in which there is an orientational change across the deformation structure, and microband was defined as the deformation structure, in which there is no orientational change across it. The effects of deformation conditions and austenite grain size on the morphology and densities of deformation structures were investigated. It was found that the densities of deformation structures increase with increasing amount of deformation. The density of transition bands was higher than that of micro bands in all the deformation conditions studied. Transition bands can be clearly observed even in a small amount of deformation and act as preferential nucleation sites for recrystallization. As the amount of deformation increases, the microbands become sharp. The deformation structures of grain interior become less clear as the austenite grain size is reduced.

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The stress-strain behavior in the constant strain rate test has been examined for the Zn-Al eutectoid while comparing with that of the constant velocity test. It has been revealed in results that a remarkable workhardening took place at 473K and favorite strain rates in the constant E test.
Activation energies calculated using the maximum stress values at different temperatures were 121.6, 61.3 and 111.2kJ/mol for regions I to III. These values for regions I and III are considered to correspond to that for volume diffusion of Zn or Al, while that for region II is near to that grain boundary diffusion of Zn or Al.
The examination of strain dependence of the activation energy has revealed that the energy for region I lowered with an increase of strain, while not so big change for regions II and III. This implies a possibility that the deformation is controlled by a similar process from the initial stage to just before the fracture in region II or III. On the contrary, it is very possible that the rate controlling mechanism may be altered during the deformation of region I.

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Flow stress of a Si-Mn steel in ferrite plus metastable austenite region and austenite plus ferrite two phase region has been examined by single-stage and multi-stage tension tests. Flow stress of “ferrite” (92% ferrite plus 8 metastable austenite) has been found to be expressed by an equation similar to that applicable to austenite. Strain hardening exponent is smaller in “ferrite” than in austenite region, while strain rate exponent is larger in “ferrite” region. The hot deformation resistance in austenite ferrite two phase region can be approximated by the law of mixture for the hot deformation resistance of austenite and “ferrite”. By adding a term expressing release of strain concurrent with phase transformation to the conventional formula describing static restoration process in austenite, the kinetic of static restoration in austenite-ferrite two phase region can be formulated as a simple mathematical equation. On the basis of experimental results a mathematical model of hot deformation resistance in austenite ferrite two phase region has been developed and successfully applied to roll force prediction in a production plate mill.

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A critical condition for static recrystallization of austenite (γ) in plate rolling process has been formulated in terms of the change in average dislocation density. The latter was calculated from the decrease in stress due to recovery and recrystallization observed by the double deformation tests. The main results are summarized as follows:
(1) The relation among stress, strain and average dislocation density has been formulated. By the present formulation, deformation stress for practical conditions of the controlled rolling of plate can be calculated as a function of temperature, strain, strain rate and γ grain size.
(2) The behavior of static recovery and recrystallization taking place during holding period after deformation has also been formulated as a function of dislocation density. This formulation makes it possible to estimate the critical condition for static recrystallization during an interval time between the successive rolling passes of plate.
(3) Deformation conditions such as temperature, strain, strain rate, γ grain size and interval time between passes affect the critical condition for recrystallization. The shorter interval time elevates the temperature limit of non recrystallization.

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The isothermal recrystallization behavior of 17% Cr stainless steel at the temperature range from 900 to 1250°C was investigated with a plain compression type hot deformation simulator. Special emphasis was put on the effects of α_??_γ partial transformation on recrystallization, which were studied by varying the amount of γ phase prior to deformation. In the partial transformation region, the effects of transformation on recrystallization were markedly large. When specimens were cooled down from the a single phase region to the α+γ phase region, recrystallization was suppressed by the strain enhanced transformation of α→γ. When specimens were cooled down from the α+γ phase region to the α+carbides phase region, recrystallization was suppressed by the strain enhanced decomposition of γ phase. From the above results, the suppression of recrystallization was attributed to the depletion of recrystallization nucleation sites by the strain enhanced partial transformation.

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A calculation model for predicting the strength of hot rolled low carbon steel sheets from their chemical compositions and processing variables has been developed as a combination of the hot deformation model, the transformation model, and the relationship between the strength and the microstructure. To make up a comprehensive model based on the previously reported hot deformation model and transformation model, the effect of stored strain on transformation kinetics and the prediction of ferrite grain size from the transformation model are studied for inclusion in the comprehensive model. Further, the relationships between the microhardnesses and the microstructures are examined. The microhardness of each microconstituent in a mixed microstructure is found dependent on its transformation temperature. These relationships are formulated for the present model. The model has been applied to the calculation of the microstructure and the tensile strength of steels. Good agreement is obtained between the values calculated and observed, and the applicability of the model for hot strip rolling process has been confirmed.

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The effect of various alloying elements on static recovery and recrystallization kinetics in Ni-Cr austenitic stainless steels was investigated by an intermittent two stage hot compression test. Types 304 and 316 stainless steels showed a marked retardation of recrystallization kinetics compared with that of austenite in 0.25%Si-1.25%Mn steel. This retardation was found to arise from solid solution effect of Cr, but not of Ni. The finish-rolling temperature to attain the completely recrystallized structure in as-rolled plate was examined by hot rolling experiment, and this recrystallization temperatures in 304 and 316 steels were 975 and 1650°C, respectively. The retarding rate of solute atoms was quantitatively evaluated, and it increased in the following ascending order: Ni<Cr< V<Ti<Mo<Nb. This retarding effect of solute atoms resulted in a consistent correlation to the lattice constant change measured by high temperature X-ray dif fractometry. This effect appeared to be caused by the interaction between dislocations and lattice distortion.

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A low carbon steel and a vanadium and niobium microalloyed steel were deformed by torsion with the true strain rates from about 10^-3 to 10s^-1 in the temperature range from 900 to 1200°C. Through the determination of their flow behavior and the observation of the deformed microstructure, dynamic recrystallization behavior was analyzed.
Austenite grain refinement by dynamic recrystallization and final ferrite grain refinement due to microalloying elements in the course of thermomechanical treatments were simulated by using a torsion machine. The final grain size in air-cooled specimen was much reduced in the schedule including only one stage deformation at 800°C particularly in the microalloyed steel. The microstructures in the thermomechanical processes revealed the characteristic feature of dynamic restoration process in the two phases region of steels. Especially in the microalloyed steel, austenite recrystallized partly at grain boundaries, whereas grain fragmentation like subgrain formation proceeded in ferrite.

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Using 2.5% Ni steel plates for LPG storage tanks, the effect of C content on crack initiation toughness of the fusion boundary of the MIG welded joint and the effects of processing conditions in controlled rolling and accelerated cooling on mechanical properties of base plate were investigated.
The experimental results indicated that the minimum crack initiation toughness of the welded joint was improved by decreasing C content. Furthermore, the crack arrestability of the 0.01%C-2.5%Ni steel plate was improved by reducing ferrite grain diameter and decreasing solute Nb. The ferrite grain was refined by lowering the slab repeating temperature and increasing the cooling rate after hot rolling. Refinement of ferrite grain made by increasing the cooling rate after hot rolling improved carck arrestability, even though solute Nb increased. On the basis of these experimental results, a 0.01%C-2.5%Ni steel plate with good crack arrestability in base plate and also good initiation toughness in welded joint was produced for LPG storage tanks using the thermo-mechanical control process.

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The effect of reheating temperature on tensile strength of controlled rolled steels and on grain coarsening of austenite during solution treatment were investigated for three series of Ti bearing HSLA steels (Ti, V-Ti and Nb-Ti), by comparing with the stability and composition of precipitates in each steel. It was found that addition of 0.03wt% Nb decreases the tensile strength of Ti bearing steels reheated at a low temperature and retards austenite grain coarsening, whereas addition of 0.05wt% V increases the tensile strength and scarcely retards austenite grain growth. These differences arise from the different stability of precipitates in Nb-Ti and V-Ti bearing steels. The equilibrium thermodynamic model of quaternary compounds is proved to predict the solubility and composition of precipitates in Ti bearing steels. It is concluded that the difference in solubility of the precipitates in austenite corresponds to the different solubility between Nb and V in the precipitates.

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The effects of carbon content, rolling condition and cooling rate on the mechanical properties were examined for development of non heat-treated steels of 80-100kgf/mm² tensile strength grade.
Titanium, boron, and niobium microalloyed 0.1%C-1.8%Mn steel was found to provide a good combination high strength, 80-100kgf/mm², and high ductility at a cooling rate of 5-15°C/s after hot rolling. The controlled rolling produces a fine acicular ferrite structure and consequently improves the toughness more effectively than any other rolling conditions.

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The effect of thermo-mechanical treatment on the mechanical properties of Cu bearing age hardenable steels was examined. Accelerated cooling and direct quenching after controlled rolling enhanced to produce the microstructure dominant of low carbon bainite which caused the improvement of strength and toughness. Addition of Cu over 1% was effective in both ε-Cu precipitation strengthening and microstructural control through its effect on hardenability. Studies were focussed on the specific effects of Cu in thermo-mechanical process. It was revealed that the improvement of mechanical properties was partly attributed to the retardation of recrystallization in hot working due to Cu addition over 1% and the suppression of ε-Cu precipitation during cooling due to rapid cooling.

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The effects of high-reduction hot working on microstructure, deformation behavior, and mechanical properties in 18-8 stainless steel and 42% Ni austenitic alloy have been investigated. A newly developed 150 t thermo-mechanical treatment simulator is used, which can provide a wide range of hot working condition. It is shown that the working temperature range in which recrystallized microstructure is obtained is broadened toward the lower side by high-reduction hot working. Recrystallized microstruc-ture is remarkably refined by higher-reduction hot working at lower working temperature, and a very fine grain size of below 10μm is obtained at 950°C for 18-8 stainless steel. High-reduction hot working is shown to enhance dynamic restoration due to temperature increase and accelerate static recrystallization after the working as well. The results obtained by the simulator experiments are also compared with the microstructure of the seamless pipe produced by hot extrusion, and a new thermo-mechanical processing technology in hot extrusion process is discussed.

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Large deformation for a common C-Mn steel at the temperatures around Ar₃ results in an ultrafine-grained ferrite structure. This is due to dynamic transformation and recrystallization of ferrite during deformation. To apply the new finding to an actual plant rolling, multi-pass deformations which substitute for a single pass heavy deformation have been studied.
It was realized that when the interpass time during successive deformations is shorter than 2s, the effect of accumulative strain can make ferrite grains fine.
A trial of strip rolling in plant provided a mean ferrite grain size of ASTM No. 13.5.

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In order to understand effects of straightening points of continuous caster on surface cracking susceptibility of slabs, stress relaxation during hot deformation of low alloy steels and austenitic steels has been studied by means of intermittent tensile testing at temperatures ranging from 700 to 1300°C, in which the processes of deformation to given strains at a slow strain rate of 4×10^-4s^-1 and the subsequent stress relaxation for 3 min are repeated. Stress relaxation is largely suppressed by lowering the deformation temperature especially in Nb bearing steels because of the dynamic precipitation of NbC. Although the dynamically nucleated carbonitride precipitates can grow into considerably coarse particles in the subsequent relaxation process, the dynamic precipitation introducing the ductility loss occurs successively in the following deformation processes. Thus the ductility in the intermittent deformation becomes almost the same as in the continuous deformation, indicating that increase of the straightening points will enhance the dynamic precipitation because of decrease in the average strain rate, resulting in significant enhancement of surface cracking frequency. While at higher temperatures, since stress relaxation can occur much easily, the straightening point increase could reduce inner cracking which is usually formed at temperatures close to the solidus points.

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For the sake of understanding the mechanism of the cracking in continuous casting and hot direct rolling, the mechanical behavior at elevated temperatures and in a wide range of strain rates must be investigated. Many previous reports described the hot ductility in low speed tensile tests, but only a few researchers have been concerned with high speed tensile tests.
Hot ductility of austenite in low and plain carbon steels has been examined at strain rates up to 200/s by using a hot working simulator. When Mn: S ratio is less than 20 in carbon steels, or sulfur content is greater than 30ppm in the carbon steel without addition of manganese, the embrittlement with intergranular fracture is observed. The embrittlement is restrained if the sulfur content is less than the order of 10ppm or Mn: S ratio is greater than 50. Thin-layered precipitates of MnS are observed on the fracture surfaces in a 0.19 % carbon steel with Mn: S ratio of 35. The effect of carbon content on the embrittlement is poorly understood. For a low carbon steel containing 0.26Mn and 0.0155, the ductility increases with decrease of the cooling rate from the solution-treating temperature or decrease of the strain rate when the solution-treating temperature is 1573K. If the temperature is up to 1673K, however, the recovery of ductility is not achieved despite of slow cooling and low strain rate. On the other hand, the ductility in ferrite and austenite-ferrite two phase region is good at the strain rate of 10^-2/s.

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Monte Carlo simulation study of transformation textures has been conducted on martensitic steels transformed from rolled austenite. A new variant selection concept, called as Bain Strain (BS) model, has been proposed: i.e., such a transformation variant is selected that the generation of Bain strain is most effectively assisted by the rolling stress.
Transformation textures simulated in this way have been successfully utilized to explain typical Fe-Ni textures experimentally determined by other authors; with this model, the deformation induced transformation textures presented by Grewen et al. have been satisfactorily reproduced by a simulation under a compressive stress during rolling, and the texture of martensite transformed after rolling reported by Abe et all has been simulated in terms of an internal stress state in rolling.
The merits of BS model are discussed in comparison with the existing models. Certain possibilities of the texture control of steels are pointed out for applications in thermomechanical rolling process.

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The effect of the inhomogeneous shear strain through the thickness of rolled sheet on the recrystallization behavior and texture formation has been investigated. Type AISI 304 stainless steel sheets were rolled to various reductions at temperatures ranging from 800-1100°C at a rolling speed ranging from 15-37m/s without lubrication. The rolled specimen was quenched at an interval of 3.5-250ms after rolling.
The redundant strain by friction produces a severely sheared region beneath the surface. A band of extremely fine grains is formed by dynamic or metadynamic recrystallization above a critical reduction which is dependent on rolling temperature.
The deformation texture consists mainly of {001}<110>, {111}<110> and {112}<110> in the severely sheared region, and mainly {110}<112> in the midthickness. The recrystallization texture in severely sheared region retains the deformation texture except for the {001}<110> component, and is unchanged in the subsequent annealing. Static recrystallization occurs in the region other than severely sheared region by holding or subsequent annealing after rolling. The texture randomizes with progress of the static recrystallization.

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