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It is well known that an iron oxide layer formed on a surface of work piece plays an important role of lubricating properties in hot rolling. However, a few papers fully reported the influence of the iron oxide layer on lubricating properties. Therefore, we investigated the influence of compositions and thickness of the iron oxide layer on lubricity by means of a simulation test of semless pipe hot rolling.First, some iron-oxide layers of various compositions and thickness were formed on three different steel materials by oxidizing treatment under different conditions: where the work pieces were exposed to air until the oxide layer grew to the set thickness. Then, the hot work piece was rolled between a cylindrical roll and a flat tool lubricated with graphite-based lubricant. The friction coefficient was measured during rolling, and the flat tool surface was observed after rolling.Summaries of the results are shown below.① In case the rolled material was carbon steel, scoring didn't occur regardless of its elongation in the test condition. On the other hand, in case the rolled material was 10% Cr steel or 18%Cr–8%Ni stainless steel, scoring occurred when the elongation was high.② The abovementioned results are thought to be caused that the scale of FeO generated in the case of carbon steel has high plasticity at elevated temperature.

The thermal fatigue (TF) behavior of a new type high-Cr cast hot work die steel was investigated. The results indicate that TF cracks initiate at the grain boundaries or at the interface between inclusion and matrix, which had been oxidized, and then propagated along the grain boundaries. The thermal fatigue crack initiation and propagation have been accelerated greatly by oxidation. The new type cast high Cr hot work die steel has a better thermal fatigue resistance compared to 4Cr5MoSiV1(8407) steel under thermal cycling temperatures of 650-25°C. Good oxidation and temper resistances are the main causes for the better TF resistance of the new type high-Cr cast hot work die steel.

The near-net-shape casting technology is an important process unattainable with traditional die-making processes. The key of this process is how to guarantee the safety application of dies. In this paper, the microstructures and thermal fatigue (TF) behavior of Cr-Ni-Mo hot work die steel modified by rare earth (RE) were investigated. The grains of cast Cr-Ni-Mo hot work die steel are refined by RE modification. With the increase of RE addition, both grain size and inclusion amount are reduced. The morphology and distribution of inclusions are improved by appropriate RE modification. RE modification favors fine plate martensite. When the residual RE content reaches 0.02 %, no obvious changes in strength and hardness are found, while fracture toughness is increased. The impact toughness, elongation and reduction of cross sectional area are increased by a factor of two. The influence of microstructure on initiation and propagation of thermal fatigue crack was also studied. The results show that the microstructures have apparent effect on resistance to TF crack initiation and propagation. Steel with microstructure of martensite and bainite dual phase have higher resistance to thermal fatigue than that of martensite owing to the loosing stress increase. Wrought process increases TF crack initiation resistance, however, lowers TF crack propagation resistance compare to cast process.

Hot work tool steel with an application in the die-casing industry was chosen as the test material. Most of the failure of dies depends on the heat check due to cyclic thermal stress. So, the present work aims to find a repair method of the failed tool. The microstructure of the Nd:YAG laser-processed material is formed by dendritic structure hardened into martensite. The steel shows secondary hardening by tempering and fatigue softening by cyclic loading even at room temperature. The rotating bending fatigue strength can be recovered to almost initial value by heat treatment at secondary hardening temperature after laser-processing. As the result, it is concluded that the laser processing is suitable for the repair of heat check.

Tool steel used in hot work such as die-casting is required to have the resistance to heat checking which results in fine shallow cracking on the part surface. Laser melt processing, which can produce a localized shallow melted zone, has been adapted to repair such kind of cracks. However, the melted zone has a detrimental effect on impact toughness because the laser heating may generate hard structures during the processing. In this paper, the effect of laser melt processing on repairing cracks was evaluated by means of the Charpy impact test and fractography. The test results suggested that when this laser melt processing can eliminate an extensive damaged volume through heat treatment after laser melt processing, the impact toughness is able to recover again to the initial state after heat treatment.

Effects of atmosphere and strain amplitude on low cycle fatigue behavior of AlSl H11 hot work tool steel were investigated. The fatigue life in vacuum was twice as long as that in air. From the result of the fatigue test in vacuum, it became clear that slip bands on the specimen surface were formed during fatigue and they grew up with increasing number of cycles. The initial fatigue cracks were generated at the intersection of slip bands and were vertical to loading direction. The fatigue cracks mainly initiated at internal inclusions when tested in vacuum, whereas superficial cracks were observed on the same fracture surface when applied higher strain amplitude. In contrast, nucleation sites of fatigue cracks when tested in air were specimen surface irrespective of the magnitude of strain amplitude. The extension of low cycle fatigue life in vacuum was mainly due to the decreasing of growth rate of superficial and internal cracks to the critical crack length.

The relationship between the strip surface flaw occurrence and rolling conditions in upstream stands together with the chemical compositions of rolled material are examined in the skinpass mill line. The ratio of the mean rolling pressure and the work roll surface in roll bite has a good correlation with the occurrence of the strip surface flaw. The strip surface flaw occurs frequently when the ratio exceeds the threshold value fixed by the chemical compositions of rolled strips. A slight addition of Si to Al-killed steel reduces remarkably the occurrence of the strip surface flaw. In order to clarify this phenomena, the influence of Si content on the behavior of scale formed on strip surface is investigated through laboratory tests. Moreover, the influence of the friction coefficient on the velocity difference between roll surface and strip surface is calculated by rigid-plastic FEM, as the friction coefficient changes with Si content.

The hardness of adamite roll materials for hot work rolls depends on their microstructures; the amount and hardness of coarse carbides and the hardness of matrix. The high-hardened adamite roll materials are excellent in wear resistance when the amount of coarse carbides is about 8%, but the wear resistance deteriorates when the amount of coarse carbides \exceeds a critical level.The reason is considered to be as follows: As the volume of coarse carbides increases, the spacing between coarse carbides decreases and the area of coarse carbides in contact with hot strip surfaces also increases, to induce a partial disintegration of brittle coarse carbides.The high-hardened adamite rolls with about 8% of coarse carbides have a hardness from Hs 65 to 70 and can be used for hot work rolls with satisfactory results.

Microstructures of hot work tool steels, AISI H13, HIO, and H19 tempered after quenching at various cooling rates are studied for improvements of the toughness of these steels. It is found that the development of upper bainite with decreasing rate of quenching is accompanied by the following microstructural changes.1) Increase in the width and length of bainite grains and the effective grain size2) Preferential precipitation of carbides along prior austenitegrain boundaries3) Dispersion of fine carbides in matrixThese microstructural changes lead to reduction of the toughness in all the steels; deterioration occurs in H10 and H19 at a higher cooling rate than in H13. The toughness value reduces in the order of H13, H10, and H19, in good correlation with increasing order of the density of frne carbides in matrix and the fraction of retained carbides.

Effects of hot or cold deformation and thermal history on theσ phase0 precipitation in (A) a 25Cr-7Ni-3Mo and (B) a 23Cr-6Ni steels with ferrite/austenite (α/γ) duplex structure have been studied. Results obtained are as follows:(1) The incubation time for the σ phase precipitation in Steel B was a couple of orders of magnitude longer than that in Steel A. This should be attributed to the difference in chemical composition between two steels, especially in a phase, where the majority of σ phase is known to precipitate.(2) σ phase precipitation is enhanced by hot work or thermal strain as well as cold work. The enhancing effect of cold work is predominant, and the precipitation of σ particles is observed in γ grains as well as in α grains.(3) The precipitation of σ phase is retarded by the prior heating history before aging; (a) heating at the α single phase region or (b) remelting. Especially the effect of the treatment (b) is remarkable. These retardation effects can be ascribed to homogenization of alloying elements by these heat treatments. The difference between the cases of (a) and (b) is thought to result from the degrees of their homogeneities.