Hydrogen fatigue resistant ferritic steel and manufacturing method thereof

Abstract

A ferritic steel having tensile properties and fatigue properties capable of withstanding use in a hydrogen environment and a method of manufacture thereof are provided. By adding one or more element selected from among vanadium (V), titanium (Ti) and niobium (Nb) so that the steel includes, together with at least ferrite grains in the structure, a carbide or carbides of one or more element selected from among V, Ti and Nb, the reduction of area and the fatigue crack propagation rate of the ferritic steel in a hydrogen environment are improved. The advantages of the invention were confirmed in cases where the ferrite grains are small grains of 1 μm or less in size, and in cases where the ferrite grains are coarse grains from several micrometers to 20 μm in size, and moreover in cases where the ferrite grains are coarse grains from several micrometers to 60 μm in size.

1 . A hydrogen fatigue-resistant ferritic steel, comprising, with one or more element selected from among vanadium (V), titanium (Ti) and niobium (Nb) being added, a carbide or carbides of one or more element selected from among V, Ti and Nb in a structure composed primarily of ferrite grains, wherein
 the ferritic steel exhibits improvement in reduction of area and in a fatigue crack propagation rate in a hydrogen atmosphere.   
     
     
         2 . The hydrogen fatigue-resistant ferritic steel of  claim 1 , wherein the structure is composed primarily of fine ferrite grains having a diameter of 1 μm or less. 
     
     
         3 . The hydrogen fatigue-resistant ferritic steel of  claim 1 , wherein the structure is composed primarily of coarse ferrite grains having a diameter of from several micrometers to 20 μm. 
     
     
         4 . The hydrogen fatigue-resistant ferritic steel of  claim 1 , wherein the structure is composed primarily of coarse ferrite grains having a diameter of from several micrometers to 60μ. 
     
     
         5 . The hydrogen fatigue-resistant ferritic steel of any one of  claims 1  to  4 , wherein the one or more element selected from among V, Ti and Nb has been added in at least the amount required to fix all carbon (C) in the structure as the carbide or carbides. 
     
     
         6 . The hydrogen fatigue-resistant ferritic steel of any one of  claims 1  to  4 , comprising carbides of all the elements V, Ti and Nb, wherein
 the V, Ti and Nb have been added in a collective amount which is substantially the same as the amount required to fix all carbon (C) in the structure as the carbides. 
 
     
     
         7 . The hydrogen fatigue-resistant ferritic steel of any one of  claims 1  to  4 , wherein the one or more element selected from among V, Ti and Nb has been added in an amount which is less than, but at least one-fourth of, the amount required to fix all carbon (C) in the structure as the carbide or carbides. 
     
     
         8 . The hydrogen fatigue-resistant ferritic steel of any one of  claims 1  to  4 , wherein, of the V, Ti and Nb, the only element that has been used as an additive is V, and the V has been added in an amount which is less than the amount required to fix all carbon (C) in the structure as a carbide of V. 
     
     
         9 . The hydrogen fatigue-resistant ferritic steel of any one of  claims 1  to  4 , wherein, of the V, Ti and Nb, the only element that has been used as an additives is V, and the V has been added in an amount which is less than, but at least one-fourth of, the amount required to fix all carbon (C) in the structure as a carbide of V. 
     
     
         10 . A method of manufacturing a hydrogen fatigue-resistant ferritic steel, the method comprising a step of adding one or more element selected from among vanadium (V), titanium (Ti) and niobium (Nb) so as to include a carbide or carbides of one or more element selected from among V, Ti and Nb in a structure composed primarily of ferrite grains, and thereby improving the reduction of area and fatigue crack propagation rate of ferritic steel in a hydrogen environment. 
     
     
         11 . The method of manufacturing a hydrogen fatigue-resistant ferritic steel of  claim 10 , wherein the one or more element selected from among V, Ti and Nb is added in at least the amount required to fix all carbon (C) in the structure as the carbide or carbides. 
     
     
         12 . The method of manufacturing a hydrogen fatigue-resistant ferritic steel of  claim 10 , wherein the ferritic steel includes carbides of all the elements V, Ti and Nb, and V, Ti and Nb are added in a collective amount which is substantially the same as the amount required to fix all carbon (C) in the structure as the carbides. 
     
     
         13 . The method of manufacturing a hydrogen fatigue-resistant ferritic steel of  claim 10 , wherein the one or more element selected from among V, Ti and Nb is added in an amount which is less than, but at least one-fourth of, the amount required to fix all carbon (C) in the structure as the carbide or carbides. 
     
     
         14 . The method of manufacturing a hydrogen fatigue-resistant ferritic steel of  claim 10 , wherein, of the V, Ti and Nb, the only element to be added is V, said V being added in an amount which is less than the amount required to fix all carbon (C) in the structure as a carbide of V. 
     
     
         15 . The method of manufacturing a hydrogen fatigue-resistant ferritic steel of  claim 10 , wherein, of the V, Ti and Nb, the only element to be added is V, said V being added in an amount which is less than, but at least one-fourth of, the amount required to fix all carbon (C) in the structure as a carbide of V.