addition of up to about 2 0 is acceptable with respect to the weldability but the machinability is decreased if the molybdenum content exceeds about 1 0 therefore the molybdenum content in the steel of this invention is selected from within the range of about 0 01 to 1 0 vanadium and niobium are both helpful in strengthening the austenite matrix and also promote precipitation of the carbides in the matrix which greatly tends to enhance the strength of the steel if no precipitation of carbides takes place at the grain boundary weldability is not impaired therefore blending of these elements with their grain refining effect is rather advantageous for obtaining rolled materials with fine crystal structure also amost no deterioration of machinability takes place with this increase in strength therefore it is recommended to add vanadium and niobium in amounts of not less than about 0 01 each however excessive addition of these elements causes excessive precipitation of carbides at the grain boundary during the heat treatment after welding which results in inferior mechanical properties and machinability therefore it is preferable to add one or both of them in amounts of not more than 0 5 titanium and zirconium have no direct effect on strength and machinability within the ranges defined in this invention however they are beneficial in refining the columnar structure and improving ductility at high temperatures which has a favorable effect on the weldability addition of these elements proves particularly effective where welding must be conducted under the constraints imposed by large heat input according to this invention therefore one or both of these elements are added in amounts of about 0 005 to 0 5 for titanium and about 0 01 to 1 0 for zirconium calcium serves as a deoxidizer or.