In recent years, the promotion of recycling of steels is desired from the viewpoints of effective usage of resources and restraint of CO2 emission. When steel scrap containing copper is used, surface cracking occurs during hot rolling process, which poses a serious problem. Copperenriched phase is formed at the steel/scale interface through selective oxidation of Fe during heating for hot working. The copper-enriched phase liquefies at the temperature beyond 1356K and penetrates into austenite grain boundaries to cause surface cracking by liquid embrittlement. This phenomenon is called surface hot shortness due to Cu. In this research, effect of C on the surface hot shortness and effects of B and P on the hot shortness of an IF steel are investigated using steels containing 0.2% Cu. The susceptibility to the hot shortness of an IF steel is larger than that of 0.1% and 0.5% carbon steels. Addition of boron and phosphorous in IF steel is effective on the suppression of surface hot shortness in IF steel. The reasons for the effects of carbon, phosphorus and boron were discussed from the viewpoints of the effects of these elements on the amount, the shape and the penetration easiness of Cu-enriched phase.
Heavily cold drawn pearlitic steel filaments containing small amount of micro alloying elements were investigated to stop delamination in torsional deformation. In Nb treated high carbon steel wire, the volume of secondary ferrite increased remarkably, and delamination occurred in the steel filament that was drawn up to 3.8 in drawing strain. But in Ti and boron treated high carbon steel filament, the secondary ferrite drastically decreased. And delamination did not occurr in this steel filament which was heavily drawn up to 4.2 in true strain. From microstructural and chemical analysis, free boron seemed to suppress to generate secondary ferrite at boundary between pro-eutectoid cementite and austenite during patenting. Patenting process has very short reheating condition and very fast cooling and transformation time. The authors understood that such a very short heat treatment process is quite favorable to utilize boron to control secondary ferrite generation in high carbon pearlitic steel.
The microstructural change of lath martensite during tempering was investigated in ultra-low carbon 9%Cr-1%Ni-(080) ppm B martensitic steels, and then the effect of boron (B) content on the recrystallization behavior was discussed in terms of the relation between driving force and restraining force for grain boundary bulging. The addition of small amount of B is very effective for the increase in dislocation density of martensite, but excess addition more than 10 ppm is not so effective. Besides, the B addition works to reduce the spacing of carbide particles precipitated on grain boundaries with the increase in volume of the particles. The condition for the occurrence of the recrystallization is decided by the energy balance between the driving force (dislocation density) and the restraining force (grain boundary pinning). The recrystallization of martensite is promoted in the low B steel (040 ppm B) because the driving force is enlarged owing to the increase in the dislocation density. On the contrary, in the high B steel (4080 ppm B), the restraining force becomes higher than the driving force because of the reduction of carbide spacing on grain boundaries, and this leads to a retardation of recrystallization.
The effects of boron addition on the heating processes of martensite in 0.1C-9Cr-3W-3Co steels have been studied. The steels with and without 100 ppm boron were prepared. The specimens were austenitized at 1200°C for 1.2 ks, and quenched into iced brine, resulting in formation of α' martensite laths. They were heated up to 1000°C at a rate of 10°C/min. Temperature derivative of electric resistivity, dρ/dT, decreased gradually around 500°C, increased slightly around 750°C, decreased around 810°C and largely decreased around 870°C in both steels. Observations by transmission electron microscopy revealed that the reactions were related to the formation of M7C3, dissolution of both cementite and M7C3, the precipitation of M23C6 and the austenite formation, respectively. The boron addition promotes M23C6 formation and retards the α→γ transformation.
In order to investigate the effect of the combined addition of W, Nb and V with B on the creep strengthening of high Cr ferritic heat resistant steel, the creep tests and the microstructural observation of crept specimen were carried out. The effect of adding B on the creep strengthening was discussed. In based steel and V free steel, the short time creep strength increases with adding B. In W free steel and Nb free steel, however, the short time creep strength of B adding steel is similar to that of B free steel. In based steel, W free steel and Nb free steel, the long term creep strengthening by adding B occurs. The suppression of microstructural recovery by adding B dose not occur in short time crept specimen of W free steel. It is suggested that the addition of B contributes to the creep strengthening through two mechanisms. One is the long term creep strengthening by stabilizing precipitates such as M23C6, Laves phase. And another is the short time creep strengthening by refining of MX, increasing the latent creep resistance and synergetic effect of alloying elements.
Effect of B addition on strain aging properties of ferritic steels was examined by Fe-B-C alloys by strain aging tests together with computer simulations. By the addition of B, the strain aging curve of the Fe-C alloy shifted to the longer aging time and the increase in the yield stress, Δσ, decreased at any aging time under the present conditions. The activation energy for the strain aging of the Fe-C alloy was similar to those previously reported and the activation energy for bulk diffusion of C in α-Fe. By the addition of B, the activation energy became larger than that of the Fe-C alloy. It suggests that B addition makes C diffusion slower. From the analysis by Yoshinaga's model, it is found that the maximum concentration of C directly below the extra half planes decreases and the formation of the C atmosphere is delayed with decreasing C concentration. These tendencies are the same as those obtained in the strain aging tests. Thus, it is concluded that the amount of interstitial solute atoms C and N is decreased by B addition. Thermodynamic calculations with Thermo-Calc showed that the C solubility was not affected by the B addition, and that the N solubility was decreased due to the precipitation of BN. From these results, it is concluded that the effect of B addition on the strain aging characteristics in ferritic steels is explained by the reduction of C diffusion rate and the reduction of the total amount of solute elements, C and N.
Effect of boron on microstructure and toughness of simulated HAZ in 0.03%C-2%Mn-0.6%Cr-0.2%Mo-0.02%Nb bainitic steel was investigated. It was found that morphology of bainitic ferrite of simulated HAZ is changed from granular into lath like morphology, and that toughness is deteriorated by increasing boron. It is thought that the change in bainitic ferrite morphology deteriorates toughness, considering that facet of Charpy impact surface is coarsened with increasing boron. It has also been clarified that bainitic transformation is retarded by increasing boron. The change of bainitic ferrite morphology and the depression of bainitic transformation by increasing boron could be explained by the decrease in γ grain boundary energy due to free boron segregation to γ grain boundary.
Alpha-particle track etching (ATE) method is most effective in observing boron distribution in steels. Previously, in Japan, neutron irradiation for this method was carried out in the reactor at the Institute of Atomic Energy, Rikkyo University. This reactor, however, was shut down in 1999. Therefore, the establishment of a new system for ATE method has been required and experimental research was performed using the reactor at the Japan Atomic Energy Research Institute (JAERI). It was clarified that the irradiation equipment for medical treatment of the reactor JRR-4 was most suitable for ATE method. The specimen trestle for low radioactive exposure was newly-developed. ATE image obtained by 12 h irradiation using this trestle showed a good quality similar to that obtained using Rikkyo's reactor and that obtained using the trestle of the old model. Using this new trestle, the amount of neutron which the worker suffers during the operation at the irradiation equipment decreases from 4μSv/h to 0-1μSv/h compared with the trestle of the old model. The total amount of thermal neutron after 12 h irradiation was almost same as that under the recommended condition of the reactor at Rikkyo University, 6.5×1014ncm-2.
Alpha-particle track etching (ATE) method is widely used for the observation of boron distribution in steels. In this method, cellulose films are essential. The cellulose film named CN85 has been used in almost all boron observations by ATE method as a standard film. Recently, however, the production of this film was stopped and the stock seems to be almost exhausted. Therefore, finding the alternative films is urgently required. For this reason, in the present research, the quality of the ATE image, the activation by neutron irradiation and other properties of various kinds of films were examined and the following results were obtained. Industrial nitrocellulose (INC-2 and INC-3) films preprocessed in a lithium nitric acid showed suitable ATE images and can be recommended as the first candidate for the alternative film. Another industrial nitrocellulose (INC-4) film shows acceptable ATE images without any pre-process or pre-heat treatment for surface modification. But after the irradiation of neutron this film becomes radioactive through the generation of 60Co. Therefore, it is necessary to handle this film in a hot laboratory as an activation material. Acetyl cellulose and cellulose triacetate films are the third candidate of the new recommendable film and the ATE images are inferior compared with the nitrocellulose films above mentioned.