| 摘要 |
PROBLEM TO BE SOLVED: To form a stable nitrogen-enriched layer without causing deteriorating in material and ununiform coloring (blue) and to impart seizure resistance and scratch resistance at the time of secondary working such as press working to a steel sheet by heating a titanium thin sheet in a nitrogen atmosphere in a specified temp. range and forming a nitrogen-enriched layer having specified thickness. SOLUTION: A titanium sheet, preferably, a thin sheet in a cold-rolled state is heated at 500 to 700 deg.C in a nitrogen atmosphere to form a nitrogen-enriched layer having 0.5 to 5μm, preferably, 1.5 to 5μm thickness. This nitriding is executed in a nitrogen atmospher having >=99.999% purity in a batch annealing furnace, and it is allowed to serve also as annealing for the titanium thin sheet after cold rolling. The use of gaseous ammonia as the nitriding atmosphere is not suitable from the view point of hydrogen embrittlement, and furthermore, ion nitriding is not suitable since uniform electrolytic formation on the coiled whole surface is impossible. Moreover, in the case the thickness of the nitrogen- enriched layer is <=1.5% (3% in both faces) that of the titanium sheet, it hardly influences on its tensile characteristics and does not damage its formability even in the case of hydraulic bulging.
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