Ultra-high strength, weldable steels with excellent ultra-low temperature toughness

A method for preparing a steel plate having a tensile strength of at least about 930 MPa (135 ksi), an impact energy by Charpy V-notch test at −40° C. (−40° F.) of greater than about 120 J (88 ft-lb), a 50% vTrs of less than about −60° C. (−76° F.), and a microstructure comprising at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns, said method comprising the steps:

• (a) heating a steel slab to a temperature in the range of about 1050° C. (1922° F.) to about 1250° C. (2282° F.);
• (b) reducing said slab to form plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes;
• (c) further reducing said plate in one or more hot rolling passes in a second temperature range in which austenite does not recrystallize, wherein a reduction in thickness of more than about 50 percent occurs in said second temperature range and said hot rolling is finished at a finish rolling temperature greater than both about 700° C. (1292° F.) and the Ar3 transformation point;
• (d) quenching said plate at a rate of at least about 10° C./sec (18° F./sec) to a Quench Stop Temperature in the range of about 450° C. to about 200° C. (842° F.-392° F.);
• and
• (e) stopping said quenching and allowing said plate to air cool to ambient temperature, so as to facilitate completion of transformation of said steel plate to at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns.

A steel plate having a tensile strength of at least about 930 MPa (135 ksi), an impact energy by Charpy V-notch test at −40° C. (−40° F.) of greater than about 120 J (88 ft-lb), a 50% vTrs of less than about −60° C. (−76° F.), and a microstructure comprising less than about 8 volume percent of martensite-austenite constituent and at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns, and wherein said steel plate is produced from a reheated steel comprising iron and the following alloying elements in the weight percents indicated:

• about 0.05% to about 0.10% C,
• about 1.7% to about 2.1% Mn,
• less than about 0.015% P,
• less than about 0.003% S,
• about 0.2% to about 1.0% Ni,
• about 0.01% to about 0.10% Nb,
• 0% to 0.8% Cu,
• about 0.005% to about 0.03% Ti, and
• about 0.25% to about 0.6% Mo.

A method for preparing a steel plate having a tensile strength of at least about 930 MPa (135 ksi), an impact energy by Charpy V-notch test at −40° C. (−40° F.) of greater than about 120 J (88 ft-lb), a 50% vTrs of less than about −60° C. (−76° F.), and a microstructure comprising less than about 8 volume percent of martensite-austenite constituent and at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns, said method comprising the steps:

• (a) heating a steel slab to a temperature in the range of about 1050° C. (1922° F.) to about 1250° C. (2282° F.);
• (b) reducing said slab to form plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes;
• (c) further reducing said plate in one or more hot rolling passes in a second temperature range in which austenite does not recrystallize, wherein a reduction in thickness of more than about 50 percent occurs in said second temperature range and said hot rolling is finished at a finish rolling temperature greater than both about 700° C. (1292° F.) and the Ar3 transformation point;
• (d) quenching said plate at a rate of at least about 10° C./sec (18° F./sec) to a Quench Stop Temperature in the range of about 450° C. to about 200° C. (842° F.-392° F.);
• and
• (e) stopping said quenching and allowing said plate to air cool to ambient temperature, so as to facilitate completion of transformation of said steel plate to less than about 8 volume percent martensite-austenite constituent and at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns.

A steel plate having a tensile strength of at least about 930 MPa (135 ksi), an impact energy by Charpy V-notch test at −40° C. (−40° F.) of greater than about 175 J (129 ft-lb), a 50% vTrs of less than about −60° C. (−76° F.), and a microstructure comprising at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns, and wherein said steel plate is produced from a reheated steel comprising iron and the following alloying elements in the weight percents indicated:

• about 0.05% to about 0.10% C,
• about 1.7% to about 2.1% Mn,
• less than about 0.015% P,
• less than about 0.003% S,
• about 0.2% to about 1.0% Ni,
• about 0.01% to about 0.10% Nb,
• 0% to 0.8% Cu,
• about 0.005% to about 0.03% Ti, and
• about 0.25% to about 0.6% Mo.

A method for preparing a steel plate having a tensile strength of at least about 930 MPa (135 ksi), an impact energy by Charpy V-notch test at −40° C. (−40° F.) of greater than about 175 J (129 ft-lb), a 50% vTrs of less than about −60° C. (−76° F.), and a microstructure comprising at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns, said method comprising the steps:

• (a) heating a steel slab to a temperature in the range of about 1050° C. (1922° F.) to about 1250° C. (2282° F.);
• (b) reducing said slab to form plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes;
• (c) further reducing said plate in one or more hot rolling passes in a second temperature range in which austenite does not recrystallize, wherein a reduction in thickness of more than about 50 percent occurs in said second temperature range and said hot rolling is finished at a finish rolling temperature greater than both about 700° C. (1292° F.) and the Ar3 transformation point;
• (d) quenching said plate at a rate of at least about 10° C./sec (18° F./sec) to a Quench Stop Temperature in the range of about 450° C. to about 200° C. (842° F.-392° F.);
• and
• (e) stopping said quenching and allowing said plate to air cool to ambient temperature, so as to facilitate completion of transformation of said steel plate to at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns.

A steel plate having a tensile strength of at least about 930 MPa (135 ksi), an impact energy by Charpy V-notch test at −40° C. (−40° F.) of greater than about 175 J (129 ft-lb), a 50% vTrs of less than about −85° C. (−121° F.), and a microstructure comprising at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns, and wherein said steel plate is produced from a reheated steel comprising iron and the following alloying elements in the weight percents indicated:

• about 0.05% to about 0.10% C,
• about 1.7% to about 2.1% Mn,
• less than about 0.015% P,
• less than about 0.003% S,
• about 0.2% to about 1.0% Ni,
• about 0.01% to about 0.10% Nb,
• 0% to 0.8% Cu,
• about 0.005% to about 0.03% Ti, and
• about 0.25% to about 0.6% Mo.

A method for preparing a steel plate having a tensile strength of at least about 930 MPa (135 ksi), an impact energy by Charpy V-notch test at −40° C. (−40° F.) of greater than about 175 J (129 ft-lb), a 50% vTrs of less than about −85° C. (−121° F.), and a microstructure comprising at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns, said method comprising the steps:

• (a) heating a steel slab to a temperature in the range of about 1050° C. (1922° F.) to about 1250° C. (2282° F.);
• (b) reducing said slab to form plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes;
• (c) further reducing said plate in one or more hot rolling passes in a second temperature range in which austenite does not recrystallize, wherein a reduction in thickness of more than about 50 percent occurs in said second temperature range and said hot rolling is finished at a finish rolling temperature greater than both about 700° C. (1292° F.) and the Ar3 transformation point;
• (d) quenching said plate at a rate of at least about 10° C./sec (18° F./sec) to a Quench Stop Temperature in the range of about 450° C. to about 200° C. (842° F.-392° F.);
• and
• (e) stopping said quenching and allowing said plate to air cool to ambient temperature, so as to facilitate completion of transformation of said steel plate to at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about ⅔ of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns.