Method of improving the toughness of low-carbon, high-strength steels
View Patent ↗A method of post-solidification processing to minimize the content of extremely coarse grain-refining precipitates that may form during solidification is low-alloy and alloy high-strength steels containing approximately 0.09-0.17% by weight C so as to provide improved toughness in a wrought and heat-treated product. The method entails cooling an as-cast steel at a reduced rate in a furnace held at a temperature in excess of the equilibrium solution temperature for AlN in austenite. The steel is maintained at this temperature for a sufficient amount of time to effect the dissolution of coarse AlN precipitates in the microstructure, and the so-treated steel is then cooled at any desired rate to room temperature or to a hot-rolling temperature.
1. A method for improving the toughness of a high-strength steel containing in % by weight about 0.09% to 0.17% C, 0.005% to 0.05% Al, and more than about 60 ppm N, comprising the steps of:
(a) casting said steel to provide a hot, solidified steel shape;
(b) cooling the hot solidified steel shape in a thermally controlled environment from a temperature near a solidification temperature of the steel at a rate of 1° C./second or less to a temperature in excess of an equilibrium solution temperature for AlN in austenite for a sufficient amount of time to effect the dissolution of coarse AlN precipitates in a microstructure of said hot solidified steel shape; and
(c) further cooling the hot solidified steel shape to room temperature or to a primary hot-rolling temperature.
2. The method of claim 1 in which the thermally controlled environment is a furnace maintained at a temperature in the 1250°-1350° C. range.
3. The method of claim 1 in which the steel also contains one or more grain-refining elements selected from the group consisting of Ti, Nb, and V, wherein equilibrium solution temperatures for both AlN and a least soluble species of carbonitride precipitate in austenite are maintained below about 1300° C.
4. A method for improving the toughness of low-alloy and alloy high-strength steels containing in % by weight about 0.09-0.17% C, 0.005-0.05% Al, and greater than about 60 ppm N, comprising:
(a) casting the steel in an ingot mold to provide a solidified steel ingot;
(b) stripping the solidified steel ingot from the ingot mold to provide a hot steel ingot;
(c) placing the solidified hot steel ingot in a furnace maintained at a temperature equal to or greater than 1250° C. and also in excess of an equilibrium solution temperature for AlN in austenite;
(d) cooling the hot steel ingot in the furnace for a sufficient amount of time from a temperature near a solidification temperature of the steel to said temperature in step (c) to dissolve coarse AlN precipitates in a microstructure of the steel; and
(e) further cooling the hot steel ingot at any desired rate from said temperature in step (c) to room temperature or to a primary hot-rolling temperature.
5. The method of claim 4 in which the furnace in step (c) is maintained at a temperature of about 1250°-1350° C.
6. The method of claim 4 in which the steel also contains one or more grain-refining elements selected from the group consisting of Ti, Nb, and V, wherein equilibrium solution temperatures for both AlN and a least soluble species of carbonitride precipitate in austenite are maintained below about 1300° C.
7. A method for improving the toughness of low-alloy and alloy-strength steels, comprising the steps of:
(a) providing a high-strength melt containing in % by weight about 0.09-0.17% C, 0.005-0.05% Al, greater than about 60 ppm N, and none or one or more grain-refining elements selected from the group consisting of Ti, Nb, and V;
(b) casting the steel in an ingot mold to provide a solidified hot steel ingot;
(c) stripping the hot solidified steel ingot from the ingot mold to provide a hot steel ingot;
(d) placing the hot steel ingot in a furnace at a temperature of about 1250°-1350° C.
(e) cooling said solidified hot steel ingot in said furnace from a temperature near a solidification temperature of the steel to said temperature in step (d) for a sufficient amount of time to dissolve coarse AlN precipitates in a microstructure of said steel; and
(f) further cooling the hot steel ingot to room temperature or to a primary hot-rolling temperature.
8. The method of claim 7 wherein equilibrium solution temperatures for AlN and a least soluble species of carbonitride precipitate in austenite are maintained below about 1300° C.
9. A method for improving the toughness of a high-strength steel article, comprising the steps of:
(a) providing a high strength steel containing in % by weight about 0.09% to 0.17% C, 0.005% to 0.05% Al, and more than about 60 ppm N;
(b) casting said steel to provide a hot, solidified steel shape;
(c) cooling the hot solidified steel shape in a thermally controlled environment from a temperature near a solidification temperature of the steel at a rate of 1° C./second or less to a temperature in excess of an equilibrium solution temperature for AlN in austenite for a sufficient amount of time to effect the dissolution of coarse AlN precipitates in a microstructure of said hot solidified steel shape; and
(d) further cooling the hot solidified steel shape to room temperature or to a primary hot-rolling temperature; and
(e) forming said article.
10. The method of claim 9 in which the thermally controlled environment is a furnace maintained at a temperature in the 1250°-1350° C. range.
11. The method of claim 9 in which the steel also contains one or more grain-refining elements selected from the group consisting of Ti, Nb, and V, wherein equilibrium solution temperatures for both AlN and a least soluble species of carbonitride precipitate in austenite are maintained below about 1300° C.