Method of making ultra high strength steel having good toughness

A method of forming a steel tube, comprising:

• providing a steel rod comprising:
• about 0.25 wt. % to about 0.35 wt. % carbon;
• about 0.30 wt. % to about 0.70 wt. % manganese;
• about 0.10 wt. % to about 0.30 wt. % silicon;
• about 0.90 wt. % to about 1.70 wt. % chromium;
• about 0.60 wt. % to about 1.00 wt. % molybdenum;
• about 0.050 wt. % to about 0.150 wt. % vanadium;
• about 0.01 wt. % to about 0.04 wt. % aluminum;
• less than or equal to about 0.50% nickel;
• less than or equal to about 0.040 wt. % niobium less than or equal to about 0.015 wt. % titanium;
• and
• less than or equal to about 0.05 wt. % calcium;
• forming the steel rod into a tube in a hot forming operation at a temperature of about 1200° C. to 1300° C.;
• heating the formed steel tube in a first heating operation to a first temperature of about 900° C. to 950° C. for about 10 to 30 minutes;
• quenching the formed steel tube in a first quenching operation from the first temperature at a first rate such that the microstructure of the quenched steel after the first quenching operation is greater than or equal to about 95% martensite by volume and is substantially free of carbides;
• heating the formed steel tube after the first quenching operation in a second heating operation to a second temperature, lower than the first temperature, of about 880° C. to 930° C. for about 10 to 30 minutes;
• quenching the formed steel composition in a second quenching operation from the second temperature at a second rate such that the microstructure of the quenched steel after the second quenching operation is greater than or equal to about 95% martensite by volume and is substantially free of carbides and having a smaller grain size as compared to the grain size after the first quenching operation;
• and
• tempering the formed steel tube after the second quenching operation by heating the formed steel tube to a third temperature between about 450° C. to about 550° C. for between about 5 minutes to about 30 minutes;
• wherein the steel tube after tempering has a yield strength greater than about 175 ksi and wherein the Charpy V-notch energy is greater or equal to about 50 J/cm2 in the traverse direction and 65 J/cm2 in the longitudinal direction at about room temperature.