US3666570A

US3666570A - High-strength low-alloy steels having improved formability

Info

Publication number: US3666570A
Application number: US842407A
Authority: US
Inventors: Michael Korchynsky; John David Grozier; John L Mihelich
Original assignee: Jones and Laughlin Steel Corp
Current assignee: Jones and Laughlin Steel Corp
Priority date: 1969-07-16
Filing date: 1969-07-16
Publication date: 1972-05-30
Anticipated expiration: 1989-05-30
Also published as: JPS518094B1; DE2034790A1; GB1278869A; CA922546A

Abstract

FULLY KILLED HIGH-STRENGTH LOW-ALLOY STEELS CONSISTING ESSENTIALLY OF .06% TO .20% CARBON, .50% TO 1.4% MANGANESE, .01% TO .08% COLUMBIUM OR .04% TO .12% VANADIUM, .05% MAXIMUM SILICON, .04% MAXIMUM SULFUR, .04% MAXIMUM PHOSPHORUS AND AN INCLUSION SHAPE-CONTROL AGENT COMPRISING EITHER .06% TO .20% ZIRCONIUM, .01% TO .10% OF A REARE EARTH OR.01% TO .10% MISCHMETAL ARE CHARACTERIZED IN A HOT-ROLLED FINISHED CONDITION BY YIELD STRENGTHS IN EXCESS OF 45,000 P.S.I., ULTIMATE TENSILE STRENGTHS IN EXCESS OF 60,000 P.S.I., DUCTILITIES AS MEASURED BY PERCENT ELONGATION (2 INCHES) IN EXCESS OF 20%, GOOD TOUGHNESS, SUPERIOR FORMABILITY AND REDUCED DIRECTIONALITY . THE STEELS ARE HOT-ROLLED FINISHED IN THE TEMPERATURE RANGE 1550*F. TO 1650*F., COOLED AT A RATEA WITHIN THE RANGE 20*F. TO 135*F. PER SECOND AND COLLECTED BY COILING OR PILING WITHIN A TEMPERATURE RANGE OF 1025*F. TO 1175*F.

Description

1972 I M. KORCHYNSKY L 5 5,570

HIGH-STRENGTH LOW-ALLOY STEELS HAVING IMPROVED FORIIABILITY Filed July 1e, 1969. z Sheets-Sheetfl mvamoks MICHAEL KORCHYNSKY I JOHN DAVID GROZIBR JOHN L. MIHELICH My 30, 1972- 3,666,570 amn swnmiwe'ru LOW-ALLOY STEELE HAVING IMPROVED FORMABILIIY Filed July 16 1969 M. KORCHYNSK Y- ET AL 2 Sheets-Sheet z 8 mm. F

INVENTORS MICHAEL KORCHYNSKY JOHN DAVID GROZIER JOHN L. MIHELICH United States Patent 3,666,570 HIGH-STRENGTH LOW-ALLOY STEELS HAVING IMPROVED FORMABILITY Michael Korchynsky, John David Grozier, and John L.

Mihelich, Bethel Park, Pa., assignors to Jones & Laughlin Steel Corporation, Pittsburgh, Pa. Filed July 16, 1969, Ser. No. 842,407

Int. Cl. C22c 39/54 s. Cl. 148-36 9 Claims ABSTRACT OF THE DISCLOSURE Fully killed high-strength low-alloy steels consisting essentially of .06% to .20% carbon, .50% to 1.4% manganese, .01% to .08% columbium or .04% to .12% vanadium, .05% maximum silicon, .04% maximum sulfur, .04% maximum phosphorus and an inclusion shape-control agent comprisingeither .06% to .20% zirconium, .01% to .10% of a rare earth or .01% to .10% mischmetal are characterized in a hot-rolled finished condition by yield strengths in excess of 45,000 p.s.i., ultimate tensile strengths in excess of 60,000 p.s.i., ductilities as measured by percent elongation (2 inches) in excess of 20%, good toughness, superior formability and reduced directionality.,The steels are hot-rolled finished in the temperature range 1550" F. to l*650 [F., cooled at a rate within the range 20 F. to 135 -F. per second and collected by coiling or piling within a temperature range of 1025 F. to 1175 F.

This invention relates to high-strength low-alloy steels characterized by a desirable balance and uniformity of physical properties and distinguished by their formability and reduced directionality.

- We have developed a class of high-strength low-alloy steels which in a hot-rolled finished condition exhibit good toughness, ductility and strength. In addition, the steels are of superior formability and reduced directionality, that is, the longitudinal (parallel to the rolling direction) and transverse (across the rolling direction) properties of'the steels, with respect to notch toughness and ductility, are more nearly the same. The improved formability and reduced directionality are brought about through the use of an inclusion shape-control agent comprising either zirconium, a rare earth, or mischmetal which, of course, is a mixture of rare earths. The use of an inclusion shape-control agent results in the formation of substantially spherically-shaped inclusions which retain their spherical shape in the finished material. This inclusion morphology results in a reduction of the directionality of the steels by improving their resistance to ductile fracture in the transverse direction and by making their longitudinal and transverse ductilities more nearly alike. In addition, the formability of the steels is improved.

The steels of the invention employ either vanadium or columbium as a strengthening agent and are processed within definite finishing and collecting temperature ranges to produce desired properties in the steel directly oil? the hot-mill.

Accordingly, an object of the present invention is to provide low-alloy steels having high strength in combination with goodtoughness and ductility, superior formability and reduced directionality. Another object of the invention is to provide such steels characterized in a hot-rolled finished condition byyield strengths in excess of 45,000 p.s.i., ultimate tensile strengths in excess of 60,000 p.s.i., ductilities as measured by percent elongation (2 inches) in excess of 20% and good toughness. Still another object of the invention is to provide such steels having improved resistance to ductile fracture in the transverse direction.

These and other objects and advantages of the present invention will become apparent from the following detailed description thereof with reference to the drawings wherein FIGS. 1 through 10 are photographic reproductions of steel specimens which have been subjected to bending tests and illustrate the improved formability of the steels of the invention.

The steels of the present invention are fully killed and have the following general chemistry: carbon, .06% to .20%; manganese, .50% to 1.4%; columbium, .01% to .08% or vanadium, .04% to .12% silicon, .5 maximum; sulfur, .04% maximum; phosphorus, 04% maximum; an inclusion shape-control agent comprising either 06% to .02% zirconium, .01% to .10% of a rare earth, or .0l% to 10% mischmetal; and balance iron.

The preferred steels of the invention consist essentially of .10% to .15% carbon, .9% to 1.2% manganese, .02% to .04% columbium or .04% to .07% vanadium, .05% maximum silicon, .025% maximum sulfur, .03% maximum phosphorus, .08% to .12% zirconium or .01% to .10% of a rare earth or mischmetal, balance iron. Rare earths which are employed in the steels of the invention are, for example, cerium, lanthanum, praseodymium, neodymium, yttrium and scandium.

The steels to possess the desired characteristics and properties of a yield strength in excess of 45,000 p.s.i., an ultimate tensile strength in excess of 60,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 20% and a superior toughness are hot-rolled finished in the temperature range of 1550" F. to 1650" F. and collected by coiling or piling within a temperature range of 1025 F. to 117 5 F. For the typical length of a modern hot-mill run-out table and conventional rolling speeds, the steel must be cooled at a rate within a range of 20 F. to 135 F. per second to maintain finishing and coiling temperatures within these ranges. Steels finished and/or collected at temperatures in excess of the temperatures set out above generally exhibit strengths below a yield strength of 45,000 p.s.i. and an ultimate tensile strength of 60,000 p.s.i. In addition, the steels do not have as good impact properties as steels hot rolled within the temperature ranges set out above. Steels finished or coiled below the desired temperature ranges exhibit ductilities as measured by percent elongation inferior to the ductilities of steels of the invention. In addition, low finishing temperatures result in production liabilities in that rolling speeds must be slower to achieve the lower finishing temperatures.

As noted above, the inclusion shape-control agents cause the sulfide inclusions in the steel to retain a spherical form, resulting in a significant improvement in the formability of the material and reducing the directionality of the steels. In the absence of an inclusion shapecontrol agent, the inclusions become elongated during hot rolling and aligned parallel to the rolling direction and contribute to the differences in ductility and impact energy absorbed ductile fracture) bet-ween longitudinal and transverse test sections of the steels.

Suflicient zirconium is added to the steels of the invention so that there is a minimum of .02% zirconium in the steel in excess of the zirconium which combines with the nitrogen in the steel to form nitrides. For a typical highstrength low-alloy steel containing .006% nitrogen, therefore, approximately a minimum of .06% zirconium is ice added to the steel. The minimum amount of zirconium re-- quired is given by the following formula: percent zirconium=0.02% zirconium+6.5 (wt. percent N). The zirconium is preferably added to the steel in the ingot mold during teeming. Zirconium additions are made when the mold is about one-third full and the additions completed by the time the moldis about two-thirds full. Typical zirconium recoveries achieved employing this method of addition are about 60%. The zirconium additions can also be made to the ladle after the heat is tapped. However, the steel in the ladle must be first fully killed toassure good zircon.- ium reCoveryQIn this techniquefit is important to employ goodteeming practice to minimize oxygen or nitrogen entrainment during teeming which adversely affects zirconium recovery.

. .The reduced directionality of the steels of the invention with respect to increasedtransverse impact shelf energies andmore nearly alike transverse and longitudinal ductilities is shown'in the table. All of the steels listed in the table were hot-rolled finished within 1550" F. to 1650 F. and collected within 1025 F. to l175 1 While Steel 1 contained .0l% zirconium it is considered to have not been treated with zirconium since that amount of zirconium is insuflicient to bring about the desired inclusion morphology. The specimens for which the data of the table were obtained comprised one-half size Charpy V- notch samples, except for Steel 5 where one-third size samples were employed. The impact energies set out in the table are at 100% ductile fracture of the specimens.

rare earths or' mischm'etalare used as the inclusion shape control. agent, additional carbon is not needed to maintain a given strength level.

We claim: I

1. A kill high-strength low-alloy steel which has been hot-rolled finished in the temperature range 1550 F. to 1650" E, cooled at a rate within the :r-ange bfZOiLIE. to 135 F. per second, and collected within a temperature range of 1025 F. to 1175 F., the steel beingcharacten ized in a hot rolled conditionby a yield strength inlexc'ess of 45,000 p.s.i., an ultimatetensile strength in excess of 60,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 20%, good toughness andjformability and reduced directionality, said steel consisting essentially of 06% to 20% carbon, .50% to 1.4% manganese, a strengthening agent 'selectedfroin the group consisting of .0l% to .08% columbiumand 04%" to .12% vanadium, .5 maximum silicon, sulfur in an amount up to 04%, .04% maximum phosphorus, asulfide inclusion shape-control agent selected .from the group consisting of 06% to 20% zirconium, .01%-to .10%"of a rare earth and .01% to .10% mischmetal, balance iron,

TABLE Chemistry (wt. percent) Yield iiiisii .Pereent Impact Steel 0 Mn Si Al V Cb Zr N Treatment ir ehtlon ii ii ii i i 2 i I l ti l b s 1 .10 .00 .052 .075 .047 .01 .007 None.- .-{gg gggg 22g Zifigg 33 2 .11 1.07 .047 .073 .05 .10 .007 Zr 32g g ggg 538 @3 8 12g 3 J2 m8 -----{%2E$t32i 13% 253%88 $53533 4 Zr "0 32553532033: 1528 333.133 351533 3323 ii is as? 20,233 2; a J04 Zr --i%?i$t32i..:: at an an: at "a 27 .071 .030 .087 .007 z: :ggg gfigg 338 gig;-

m i a: as: as; s

.021 .001 --.042 .11 .006 Zl' 74,000 igg 3 g The improved formability of the steels of the invention is shown by FIGS. 1 through 10 of the drawings. Samples were sheared from Steels 1 through 10 of the table and subjected to a 90 bend. The inside bend radius for all specimens except the specimens of Steel 5 was .250 inch. The inside bend radius for the specimens of Steel 5 was .125 inch. FIGS. 1 through 10 represent specimens taken from Steels 1 through 10 of the table, respectively. As shown in the drawings, the steels which did not contain zirconium, FIGS. 1, 3, 5, 7 and 9, cracked upon bending. Of the specimens from the steels containing zirconium, FIGS. 2, 4, 6, -8 and 10, only specimens from Steels 6 and 8 exhibited cracking, but to a very minor degree and substantially less than the specimens of the steels having approximately thesame chemistry but not containing zirconium. i 7

Equivalent reduced'directionality and-improved formability is obtained using rare earths and mischmetal rather thanzirconium" as the inclusion shape-control agent. In this regard, we have-found-that in order to maintain a given strength.- level for the steels of the invention containing vanadiumand employing zirconium as the inclusion shape control agent it is necessary to increase the carbon content of the steel. This is because the strength of the steel is derived fro'm'vanadium nitride precipitates and when-zirconium is added the nitrogen preferentially combines with the zirconium andstrengthening by the formation of vanadium nitrides does not occur. However, when the sulfide inclusions in the steel having a substantially spherical shape. I: 2. The steel of claim 1 wherein the strengthening v agent comprises 01% to .08% columbium.

3. The steel of claim 1 wherein the strengthening agent comprises .04% to .12% vanadium and the sulfide inclusion shape-control agent comprises 01% to .10% of a rare earth. i 4. A killed high-strength low-alloy steel which has been hot-rolled finished in the temperaturerange l550"F.' to 1650" F., cooled at a rate within the range-of 20 F. to F. per second and collected Within a temperature range of 1025 F. to 1175 F. the steelbeing charaeterized in a hot rolled condition by a yield strength in excess of 45,000 p.s.i., an ultimate tensile strengthin' excess; of 60,000 p.s.i., ductility as measured by' percent elongation (2 inches) in excess of 20%, good toughness and form'ability and reduced directionality, said stee'l consisting essentially of .10% to .15% carbon, .9% to 1.2% manganese, a strengthening agent selected from the group consisting of: .02% to .04% columbium and --.04-%' to .07% vanadium, 05% maximum' silicon, sulfur in an amount up to .025 %,'.03 maximum phosphorus, a sulfide inclusion shape-control agent-selectedfrom the group consisting of -.08% to".12% zirconium, .0l% to -.10% of a rare earth and 01% to .10% mischmetaljbala'nce iron, the sulfide inclusions in the steel having a substantially spherical shape.

5. The steel of claim 4 wherein the strengthening agent comprises .02% to .04% columbium.

6. The steel of claim 5 wherein the sulfide inclusion shape-control agent comprises .08% to .12% zirconium.

7. The steel of claim 5 wherein the sulfide inclusion shape-control agent comprises .01% to .10% of a rare earth.

8. The steel of claim 4 wherein the strengthening agent comprises .04% to .07% vanadium.

9. The steel of claim 8 wherein the sulfide inclusion shape-control agent comprises .01% to .10% of a rare earth.

References Cited UNITED STATES PATENTS 2,683,662 7/1954 Tisdale et al 75123 E 3,102,831 9/1963 Tisdale 148-12 3,333,987 8/1967 Schrader et a] 148-12.1 3,375,105 3/1968 Lynch 75123 J RICHARD O, DEAN, Primary Examiner US. Cl. X.R.

75123 E, 123 H, 123 N, 123 I; 148-l2, 12.3