High-tension high-toughness steel having excellent resistance to delayed fracture and method for producing the same

A high-tension high-toughness steel excellent in resistance to delayed fracture, having a tempered martensitic structure obtained by quenching and tempering and having a tensile strength of at least 100 kg/mm.sup.2 comprising the steps of

• preparing a steel consisting essentially, by weight, of 0.15 to 0.50% of C, up to 1.50% of Si, 0.20 to 1.50% of Mn, up to 2.00% of Cr, 0.0005 to 0.0030% of B, 0.005 to 0.10% of acid-soluble Al, up to 0.010% of P, up to 0.0020% of N, 0.010 to 0.050% of Ti and the balance iron and inevitable impurities,
• austenitizing said steel at a temperature of not lower than A.sub.3 transformation point,
• quenching the thus austenitized steel in water, oil or a salt bath, and
• tempering the thus quenched steel at a temperature of not higher than A.sub.1 transformation point.

A method for producing a high-tension high-toughness steel excellent in resistance to delayed fracture, and having a tensile strength of at least 100 Kg/mm.sup.2 comprising the steps of

• preparing a steel consisting essentially, by weight, of 0.15 to 0.50% of C, up to 1.50% of Si, 0.20 to 1.50% of Mn, up to 2.00% of Cr, 0.0005 to 0.0030% of B, 0.005 to 0.10% of acid-soluble Al, up to 0.010% of P, up to 0.0020% of N, 0.010 to 0.050% of Ti and the balance iron and inevitable impurities,
• austenitizing said steel at a temperature of not lower than A.sub.3 transformation point,
• quenching the thus austenitized steel in water, oil or a salt bath, and
• tempering the thus quenched steel at a temperature of not higher than A.sub.1 transformation point.

A method for producing a high-tension high-toughness steel excellent in resistance to delayed fracture, and having a tensile strength of at least 100 kg/mm.sup.2 comprising the steps of

• preparing a steel consisting essentially, by weight, of 0.15 to 0.50% of C, up to 1.50% of Si, 0.20 to 1.50% of Mn, up to 2.00% of Cr, 0.0005 to 0.0030% of B, 0.005 to 0.10% of acid-soluble Al, up to 0.010% of P, up to 0.0020% of N, 0.010 to 0.050% of Ti and at least one member selected form the group consisting of up to 0.50% of Mo, up to 0.20% of V, up to 0.10% of Nb and up to 0.50% of Cu, the balance being iron and inevitable impurities,
• austenitizing said steel at a temperature of not lower than A.sub.3 transformation point,
• quenching the thus quenched steel in water, oil or a salt bath, and
• tempering the quenched steel at a temperature of not higher than A.sub.1 transformation point.