A hypothesis is proposed for the yield point phenomena of soft steel When an aunaled soft steel is subjected to mechanical stress, it reveals a palier in the stress-strain diagram as well as Luders' lines on the surface of steel. These phenomena are particular for only anuealed soft steel, and almost all other alloys and metals do not show them. The author seeked the cause of the phenomena in the special structure of the soft steel, which is composed of soft ferrit and hard pearlite or cementite. A little amount of cementite segregates out along the boundary of the ferrite crystallines and forms honey-comb-work of hard boundary structure. When such structure is subjected to heavy load, the boundary strncture yields at first, so the thick mass of soft ferrite strains suddenly until it can sustain the same amount of stress. This transition of.stress appears as the palier in the stress-strain diagram, and the trace of the destruction of the honey comb strueture forms the Luders lines on the surface of the steel. The author explains this hypothesis from different directions of the charactets of soft steel.

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The teeming temperature was measured with the lapse of time, and the distribution of temperatures in the molten bath of the blast furnace considered.

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The author carried out the decarburization of Swedish steel by dry and wet .hydrogen current at high temperatures and observed decarburization from their microscopic structures.
Steels were decarburized by dry and wet hydrogen current (dry hydrogen and hydrogen containing water vapor) at temperatures above and below the A3-transition point of iron-carbon system equilibrium diagram and studied the effect of the presence of water vapor in hydrogen, upon decarburization.
The decarburized structures obtained by using wet hydrogen (hydrogen containing water vapor) at temperatures below the A3-transition point developed a columnar ferrite band with a very clear boundary, while the structures obtained by decarburizing with dry hydrogen did not develope such ferrite band.
The author considered the reason for the formation of such ferrite band and its mechanism by applying the iron-carbon system diagram and made clear the relation between the appearance or not of ferrite band on decarburizing with dry or wet hydrogen.

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