the demand of the day to require higher and higher strength by means of respectively limiting the content of elements to a specified ratio and minutely controlling the conditions of the heat treatment as follows carbon c is an essential element for increasing the tensile strength and the lower limit of its content for ensuring the tensile strength of 140 160 kgf mm 2 is 0 30 by weight when however the content thereof exceeds 0 50 by weight it deteriorates not only toughness but also resistance to delayed fracture obliging the upper limit to 0 50 by weight for particularly enhancing the resistance to delayed fracture in respect to relation with other elements it is desired to keep the c content within the range of 0 40 0 50 by weight silicon si must be held down to as low content as possible because it tends to promote internal oxidation and subsequently bring about the delayed fracture considering however its effect as a deoxidation element only the upper limit of the content thereof is defined as 0 15 by weight it is however preferable to keep its content below 0 10 by weight for preventing deterioration in the resistance to delayed fracture by means of more effectively deterring the oxidation in the grain boundaries manganese mn is like si preferable to be held down to the lowest possible content because of its inclination to promote undesirable oxidation of the grain boundaries considering however its role to make sure the tempering the upper limit of the content alone being defined here as 0 40 by weight.