We Claim:

1.

A method for preparing a steel plate having a microstructure comprising (i) predominantly finegrained lower bainite, finegrained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) up to about 10 vol% retained austenite, said method comprising the steps of : (a) heating a steel slab to a reheating temperature sufficiently high to (i) substantially homogenize said steel slab, (ii) dissolve substantially all carbides and carbonitrides of niobium and vanadium in said steel slab, and (iii) establish fine initial austenite grains in said steel slab; (b) reducing said steel slab to form steel plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes; (c) further reducing said steel plate in one or more hot rolling passes in a second temperature range below about the Tnr temperature and above about the Ar3 transformation temperature; (d) quenching said steel plate at a cooling rate of at least about 10°C per second (18°F/sec) to a Quench Stop Temperature below about 550°C (1022°F); and (e) stopping said quenching, said steps being performed so as to facilitate transformation of said microstructure of said steel plate to (i) predominantly finegrained lower bainite, finegrained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) up to about 10 vol% retained austenite.

2.

The method of claim 1 wherein step (e) is replaced with the following: (e) stopping said quenching, said steps being performed so as to facilitate transformation of said microstructure of said steel plate to a predominantly microlaminate microstructure comprising finegrained lath martensite, finegrained lower bainite, or mixtures thereof, and up to about 10 vol% retained austenite film layers.

3.	The method of claim 1 wherein step (e) is replaced with the following: (e) stopping said quenching, said steps being performed so as to facilitate transformation of said microstructure of said steel plate to a predominantly fine granular bainite (FGB).

4.	The method of claim 1 wherein said reheating temperature of step (a) is between about 955°C and about 1100°C (1750°F2010°F).

5.	The method of claim 1 wherein said fine initial austenite grains of step (a) have a grain size of less than about 120 microns.

6.	The method of claim 1 wherein a reduction in thickness of said steel slab of about 30% to about 70% occurs in step (b).

7.	The method of claim 1 wherein a reduction in thickness of said steel plate of about 40% to about 80% occurs in step (c).

8.	The method of claim 1 further comprising the step of allowing said steel plate to air cool to ambient temperature from said Quench Stop Temperature.

9.	The method of claim 1 further comprising the step of holding said steel plate substantially isothermally at said Quench Stop Temperature for up to about 5 minutes.

10.	The method of claim 1 further comprising the step of slowcooling said steel plate at said Quench Stop Temperature at a rate lower than about 1.0°C per second (1.8°F/sec) for up to about 5 minutes.

11.

The method of claim 1 wherein said steel slab of step (a) comprises iron and the following alloying elements in the weight percents indicated: about 0.03% to about 0.12% C, at least about 1 % Ni, up to about 1.0% Cu, up to about 0.8% Mo, about 0.01% to about 0.1% Nb, about 0.008% to about 0.03% Ti, up to 0.05% Al, and about 0.001% to about 0.005% N.

12.	The method of claim 11 wherein said steel slab comprises less than about 6 wt% Ni.

13.	The method of claim 11 wherein said steel slab comprises less than about 3 wt% Ni and additionally comprises up to about 2.5 wt% Mn.

14.	The method of claim 11 wherein said steel slab further comprises at least one additive selected from the group consisting of (i) up to about 1.0 wt% Cr, (ii) up to about 0.5 wt% Si, (iii) about 0.02 wt% to about 0.10 wt% V, (iv) up to about 2.5 wt% Mn, and (v) up to about 0.0020 wt% B.

15.	The method of claim 11 wherein said steel slab further comprises about 0.0004 wt% to about 0.0020 wt% B.

16.	The method of claim 1 wherein, after step (e), said steel plate has a DBTT lower than about62°C (80°F) in both said base plate and its HAZ and has a tensile strength greater than about 830 MPa (120 ksi).

17.

A steel plate having a microstructure comprising (i) predominantly finegrained lower bainite, finegrained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) up to about 10 vol% retained austenite, having a tensile strength greater than about 830 MPa (120 ksi), and having a DBTT of lower than about62°C (80°F) in both said steel plate and its HAZ, and wherein said steel plate is produced from a reheated steel slab comprising iron and the following alloying elements in the weight percents indicated: about 0.03% to about 0.12% C, at least about 1% Ni, up to about 1.0% Cu, up to about 0.8% Mo, about 0.01% to about 0.1% Nb, about 0.008% to about 0.03% Ti, up to about 0.05% Al, and about 0.001% to about 0.005% N.

18.	The steel plate of claim 17 wherein said steel slab comprises less than about 6 wt% Ni.

19.	The steel plate of claim 17 wherein said steel slab comprises less than about 3 wt% Ni and additionally comprises up to about 2.5 wt% Mn.

20.	The steel plate of claim 17 further comprising at least one additive selected from the group consisting of (i) up to about 1.0 wt% Cr, (ii) up to about 0.5 wt% Si, (iii) about 0.02 wt% to about 0.10 wt% V, (iv) up to about 2.5 wt% Mn, and (v) from about 0.0004 to 0.0020 wt % B.

21.	The steel plate of claim 17 further comprising about 0.0004 wt% to about 0.0020 wt% B.

22.	The steel plate of claim 17 having a predominantly microlaminate microstructure comprising laths of finegrained lath martensite, laths of finegrained lower bainite, or mixtures thereof, and up to about 10 vol% retained austenite film layers.

23.

The steel plate of claim 22, wherein said microlaminate microstructure is optimized to substantially maximize crack path tortuosity by thermomechanical controlled rolling processing that provides a plurality of high angle interfaces between said laths of finegrained martensite and finegrained lower bainite and said retained austenite film layers.

24.	The steel plate of claim 17 having a microstructure of predominantly fine granular bainite (FGB), wherein said fine granular bainite (FGB) comprises bainitic ferrite grains and particles of mixtures of martensite and retained austenite.

25.

The steel plate of claim 24, wherein said microstructure is optimized to substantially maximize crack path tortuosity by thermomechanical controlled rolling processing that provides a plurality of high angle interfaces between said bainitic ferrite grains and between said bainitic ferrite grains and said particles of mixtures of martensite and retained austenite.

26.

A method for enhancing the crack propagation resistance of a steel plate, said method comprising processing said steel plate to produce a predominantly microlaminate microstructure comprising laths of finegrained lath martensite, laths of finegrained lower bainite, or mixtures thereof, and up to about 10 vol% retained austenite film layers, said microlaminate microstructure being optimized to substantially maximize crack path tortuosity by thermomechanical controlled rolling processing that provides a plurality of high angle interfaces between said laths of finegrained martensite and finegrained lower bainite and said retained austenite film layers.

27.

The method of claim 26 wherein said crack propagation resistance of said steel plate is further enhanced, and crack propagation resistance of the HAZ of said steel plate when welded is enhanced, by adding at least about 1.0 wt% Ni and at least about 0.1 wt% Cu, and by substantially minimizing addition of BCC stabilizing elements.

28.

A method for enhancing the crack propagation resistance of a steel plate, said method comprising processing said steel plate to produce a microstructure of predominantly fine granular bainite (FGB), wherein said fine granular bainite (FGB) comprises bainitic ferrite grains and particles of mixtures of martensite and retained austenite, and wherein said microstructure is optimized to substantially maximize crack path tortuosity by thermomechanical controlled rolling processing that provides a plurality of high angle interfaces between said bainitic ferrite grains and between said bainitic ferrite grains and said particles of mixtures of martensite and retained austenite.

29.

The method of claim 28 wherein said crack propagation resistance of said steel plate is further enhanced, and crack propagation resistance of the HAZ of said steel plate when welded is enhanced, by adding at least about 1.0 wt% Ni and at least about 0.1 wt% Cu, and by substantially minimizing addition of BCC stabilizing elements.

30.

A method for preparing a steel plate having a microlaminate microstructure comprising about 2 vol% to about 10 vol% of austenite film layers and about 90 vol% to about 98 vol% laths of predominantly finegrained martensite and finegrained lower bainite, said method comprising the steps of : (a) heating a steel slab to a reheating temperature sufficiently high to (i) substantially homogenize said steel slab, (ii) dissolve substantially all carbides and carbonitrides of niobium and vanadium in said steel slab, and (iii) establish fine initial austenite grains in said steel slab; (b) reducing said steel slab to form steel plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes; (c) further reducing said steel plate in one or more hot rolling passes in a second temperature range below about the Tnr temperature and above about the Ar3 transformation temperature; (d) quenching said steel plate at a cooling rate of about 10°C per second to about 40°C per second (18°F/sec72°F/sec) to a Quench Stop Temperature below about the Ms transformation temperature plus 100°C (180°C) and above about the Ms transformation temperature; and (e) stopping said quenching, said steps being performed so as to facilitate transformation of said steel plate to a microlaminate microstructure of about 2 vol% to about 10 vol% of austenite film layers and about 90 vol% to about 98 vol% laths of predominantly finegrained martensite and finegrained lower bainite.

31.

A method for controlling the mean ratio of pancake length to pancake thickness during processing of a an ultrahigh strength, ausaged steel plate in order to enhance transverse toughness and transverse DBTT of said steel plate, said method comprising the steps of : (a) heating a steel slab to a reheating temperature sufficiently high to (i) substantially homogenize said steel slab, (ii) dissolve substantially all carbides and carbonitrides of niobium and vanadium in said steel slab, and (iii) establish fine initial austenite grains in said steel slab; (b) reducing said steel slab to form steel plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes; (c) further reducing said steel plate in one or more hot rolling passes in a second temperature range below about the Tnr temperature and above about the Ar3 transformation temperature; (d) quenching said steel plate at a cooling rate of at least about 10°C per second (18°F/sec) to a Quench Stop Temperature below about 550°C (1022°F); and (e) stopping said quenching, so as to produce a mean ratio of pancake length to pancake thickness of less than about 100 in said steel plate.