High-strength micro-alloy steel and process for making same

A process for producing steel having a desired microstructure, and precipitation strengthening particles of a desired particle size and volume fraction for enhanced precipitation strengthening, comprising:

• a) heating steel containing a precipitation strengthening substance to a selected dissolving temperature selected to dissolve substantially all of the precipitation strengthening substance in the steel;
• b) processing the steel to produce the desired microstructure;
• c) cooling the steel to a selected target temperature at which the desired microstructure is essentially stable and at which those precipitation strengthening particles that form tend to be of the desired particle size;
• and
• d) with the steel at the selected target temperature, introducing bending strains into the steel to introduce dislocations in the crystal structure of the steel so as to increase the kinetics of precipitation, and thus the volume fraction, of precipitation strengthening particles of the desired particle size.

A process for producing steel plate having a desired microstructure, and precipitation strengthening particles of a desired particle size and volume fraction for enhanced precipitation strengthening, comprising:

• a) heating steel containing a precipitation strengthening substance to a selected dissolving temperature selected to dissolve substantially all of the precipitation strengthening substance in the steel;
• b) processing the steel to produce a steel plate having the desired microstructure;
• c) cooling the steel plate to a selected target temperature at which the desired microstructure is essentially stable and at which those precipitation strengthening particles that form tend to be of the desired particle size;
• and
• d) with the steel plate at the selected target temperature, introducing bending strains into the steel by levelling the steel plate so as to introduce dislocations in the crystal structure of the steel to increase the kinetics of precipitation, and thus the volume fraction, of precipitation strengthening particles of the desired particle size.

A process for producing steel having a desired microstructure, and precipitation strengthening particles of a desired particle size and volume fraction for enhanced precipitation strengthening, comprising:

• a) heating steel containing a precipitation strengthening substance to a selected dissolving temperature selected to dissolve substantially all of the precipitation strengthening substance in the steel;
• b) processing the steel to produce the desired microstructure;
• c) cooling the steel to a selected target temperature at which the desired microstructure is essentially stable and at which those precipitation strengthening particles that form tend to be of the desired particle size;
• and
• d) with the steel at the selected target temperature, rolling the steel to reduce the thickness of the steel to introduce dislocations in the crystal structure of the steel so as to increase the kinetics of precipitation, and thus the volume fraction, of precipitation strengthening particles of the desired particle size.

A process for producing steel plate having a desired microstructure, and precipitation strengthening particles of a desired particle size and volume fraction for enhanced precipitation strengthening, comprising:

• a) heating steel containing a precipitation strengthening substance to a selected dissolving temperature selected to dissolve substantially all of the precipitation strengthening substance in the steel;
• b) processing the steel to produce a steel plate having the desired microstructure;
• c) cooling the steel plate to a selected target temperature at which the desired microstructure is essentially stable and at which those precipitation strengthening particles that form tend to be of the desired particle size;
• and
• d) with the steel plate at the selected target temperature, deforming the steel by rolling the steel plate to reduce the thickness of the steel plate so as to introduce dislocations in the crystal structure of the steel to increase the kinetics of precipitation, and thus the volume fraction, of precipitation strengthening particles of the desired particle size.

A process for making high-strength micro-alloy steel with enhanced precipitation strengthening, the process comprising the steps of:

• a) heating steel containing a precipitation strengthening substance to a selected dissolving temperature selected to dissolve substantially all of the precipitation strengthening substance in the steel;
• b) with the steel at a selected roughing passes temperature, being above the temperature below which austenite does not recrystallize (Tnr), rolling the steel for a series of roughing passes to break down the austenite grains through multiple recrystallization cycling to produce an austenite grain size of no more than about 30 m;
• c) with the steel at a selected finishing passes temperature, being below the Tnr but above the temperature at which austenite begins to change to ferrite on cooling (Ar3), rolling the steel for a series of finishing passes to produce a heavily pancaked austenite structure in the steel;
• d) cooling the steel at a rate of at least about 15 C./sec and no more than about 20 C./sec, from a temperature above the Ar3 to a stop-cooling temperature of at least about 350 C. and no more than about 450 C.;
• and
• e) with the steel at a temperature of at least about 350 C. and no more than about 450 C., deforming the steel to introduce dislocations in the crystal structure of the steel so as to enhance precipitation of the precipitation strengthening substance.

A process for making high-strength micro-alloy steel with enhanced precipitation strengthening, the process comprising the steps of:

• a) heating a slab of steel having a chemistry of:
• at least about 0.01 to no more than about 0.1% wt. carbon;
• at least about 0.03 to no more than about 0.12% wt. niobium;
• at least about 0.008 to no more than about 0.03% wt titanium;
• at least about 1 to no more than about 1.9% wt. manganese;
• at least about 0.1 to no more than about 0.5% wt. molybdenum;
• a maximum phosphorus content of about 0.02% wt.;
• a maximum sulfur content of about 0.015% wt.;
• a maximum nitrogen content of about 0.015%/wt.;
• and
• the balance being iron (Fe) and incidental impurities;
• to a selected dissolving temperature selected to dissolve substantially all of the niobium in the steel;
• b) with the steel at a selected roughing passes temperature, being a temperature above the temperature below which austenite does not recrystallize (Tnr), rolling the steel for a series of roughing passes so as to break down the austenite grains through multiple recrystallization cycling to produce an austenite grain size of no more than about 30 m;
• c) cooling the steel to a selected finishing passes temperature, being a temperature below the Tnr but above the temperature at which austenite begins to change to ferrite on cooling (Ar3);
• d) with the steel at the selected finishing passes temperature, rolling the steel for a series of finishing passes so as to produce a steel plate having a heavily pancaked austenite structure;
• e) cooling the steel at a rate of at least about 15 C./sec and no more than about 20 C./sec from a temperature above the Ar3 to a stop-cooling temperature of at least about 350 C. and no more than about 450 C.;
• and
• f) with the steel at a temperature at least about 350 C. and no more than about 450 C., deforming the steel so as to introduce dislocations in the crystal structure of the steel;
• wherein introducing dislocations in the crystal structure of the steel enhances the precipitation of fine precipitates containing niobium.

High-strength steel suitable for making line pipe and pressure vessels, the steel characterized by:

• a) a steel composition comprising:
• at least about 0.01 and no more than about 0.1% wt. carbon;
• at least about 0.03 and no more than about 0.12% wt. niobium;
• at least about 0.008 and no more than about 0.03% wt titanium;
• at least about 1.4 and no more than about 1.9% wt. manganese;
• at least about 0.1 and no more than about 0.5% wt. molybdenum;
• a maximum phosphorus content of about 0.02% wt.;
• a maximum sulfur content of about 0.015% wt.;
• a maximum nitrogen content of about 0.015% wt.;
• and
• the balance being iron (Fe) and incidental impurities;
• b) a microstructure comprising about 30% polygonal ferrite and about 70% acicular ferrite with an average grain size of no more than about 5 m;
• and
• c) precipitates containing niobium with a precipitate particle size of no more than about 5 nm.

High-strength steel suitable for making line pipe and pressure vessels, the steel characterized by:

• a) a steel composition comprising:
• at least about 0.01 and no more than about 0.1% wt. carbon;
• at least about 0.03 and no more than about 0.15% wt. titanium;
• at least about 1.0 and no more than about 1.9% wt. manganese;
• at least about 0.1 and no more than about 0.5% wt. molybdenum;
• a maximum phosphorus content of about 0.02% wt.;
• a maximum sulfur content of about 0.015% wt.;
• a maximum nitrogen content of about 0.005% wt.;
• and
• the balance being iron (Fe) and incidental impurities;
• b) a microstructure comprising about 30% polygonal ferrite and about 70% acicular ferrite with an average grain size of no more than about 5 m;
• and
• c) precipitates containing titanium with a precipitate particle size of no more than about 5 nm.

High-strength steel suitable for making line pipe and pressure vessels, the steel characterized by:

• a) a steel composition comprising:
• at least about 0.01 and no more than about 0.1% wt. carbon;
• at least about 0.03 and no more than about 0.15% wt. titanium, and a maximum niobium content of about 0.12% wt., such that the total combined amount of titanium and niobium is at least about 0.03 and no more than about 0.2% wt.;
• at least about 1.0 and no more than about 1.9%/wt. manganese;
• at least about 0.1and no more than about 0.5% wt. molybdenum;
• a maximum phosphorus content of about 0.02% wt.;
• a maximum sulfur content of about 0.015% wt.;
• a maximum nitrogen content of about 0.005% wt.;
• and
• the balance being iron (Fe) and incidental impurities;
• b) a microstructure comprising about 30% polygonal ferrite and about 70% acicular ferrite with an average grain size of no more than about 5 m;
• and
• c) precipitates containing titanium or niobium with a precipitate particle size of no more than about 5 nm.

High-strength steel suitable for making line pipe and pressure vessels, the steel characterized by:

• a) a steel composition comprising:
• at least about 0.01 and no more than about 0.1% wt. carbon;
• a maximum niobium content of about 0.12% wt. and a maximum vanadium content of about 0.12% wt., such that the total combined amount of niobium and vanadium is at least about 0.03% wt. and no more than about 0.2% wt.;
• at least about 0.008 and no more than about 0.03% wt titanium;
• at least about 1.0 and no more than about 1.9% wt. manganese;
• at least about 0.1 and no more than about 0.5% wt. molybdenum;
• a maximum phosphorus content of about 0.02% wt.;
• a maximum sulfur content of about 0.015% wt.;
• a maximum nitrogen content of about 0.015% wt.;
• and
• the balance being iron (Fe) and incidental impurities.;
• b) a microstructure comprising about 30% polygonal ferrite and about 70% acicular ferrite with an average grain size of not more than about 5 m;
• and
• c) precipitates containing vanadium or niobium with a precipitate particle size of no more than about 5 nm.