US3741822A - High strength steel - Google Patents

Abstract

A HIGH-STRENGTH FERROUS ALLOY CHARACTERIZED BY HIGH YIELD STRENGTH, HIGH DUCTILITY AND HIGH IMPACT STRENGTH. THE COMPOSITION INCLUDES AS ALLOYING ELEMENTS MANGANESE, NICKEL, CARBON, SILICON, CHROMIUM AND VANADIUM. THE BALANCE IS IRON. AFTER HOT ROLLING, THE BAR IS NORMALIZED AND THEN AIR COOLED. THE ALLOY IS PARTICULARLY, THOUGH NOT EXCLUSIVELY, USEFUL AS ANCHOR BOLT MATERIAL.

Description

June 26, 1073 ing A. T. GORTON HIGH-STRENGTH STEEL Filed July 1.4, 1971 United States Patent Ofice Patented June 26, 1973 3,714,822 HIGH-STRENGTH STEEL Alan T. Gorton, St. Paul, Minn., assignor to North Star Steel Company, St. Paul, Minn. Filed July 14, 1971, Ser. No. 162,451 Int. Cl. C21d 1/28; C22c 39/20 US. Cl. 148-36 1 Claim ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The invention resides in the field of metallurgy and, more particularly, ferrous alloys formulated and heat treated to achieve high yield strength, high ductility and toughness.

Prior art compositions produced by hot rolling generally have had the following properties:

(1) 75,000 p.s.i. minimum yield strength, 0.006 inch per inch strain (2) 100,000 p.s.i. minimum tensile strength (3) ft. lb. maximum Charpy V-notch impact strength at 20 F.

(4)- 5% minimum elongation (5) A typical carbon equivalent (CE) of .80

The present invention, through unique formulation and heat treatment, improves the elastic properties of the steel, improves ducitility, improves toughness, lowers the typical carbon equivalent to about .60 and improves weldability.

SUMMARY OF THE INVENTION Steel made according to thepresent invention has the following mechanical properties:

0.2% offset yield strength 75,000 p.s.i. minimum. Ultimate tensile strength 100,000 p.s.i. minimum. Elongation 8 inch gage length 12.5% minimum. Reduction in area 25.0% minimum.

Charpy V-notch impact strength at -20 F. per ASTM E23 15 ft. lb. minimum.

The primary advantage over the prior art is the increased toughness or impact strength brought about by the critical formulation and heat treatment described more particularly below.

The invention is particularly useful as anchor bolt material. It may be used in many other ways, however, including concrete reinforcing bars and other environments in which a high impact strength is desirable.

More particularly, the alloy of the present invention is characterized by a yield point in excess of 75,000 p.s.i. and a Charpy V-notch impact strength (per ASTM E23) at 20 -F. of at least 15 ft. lb., and is formulated as follows:

:Element: Percentage by weight Carbon 028-038 Manganese 1.00-1.40

Vanadium 0.035-0.065

Chromium 0.10-0.20 Nickel 0.50-0.80 Silicon 0.15-0.30 Iron and other elements which do not adversely affect the properties Balance The bar formed according to the above formulation is heat treated by normalizing for a period of not more than ten minutes at 1600-1800 F., and then air cooled. No further treatment is necessary.

The primary object of the invention is to provide an alloy which can be formulated and heat treated at reasonable cost, and which has a yield strength in excess of 75,000 p.s.i. and an impact strength of at least 15 ft. lb. at -20 F. (Charpy V-notch per ASTM E23).

DESCRIPTION OF DRAWINGS FIG. 1 is a micrograph in longitudinal section of a bar formulated and heat treated according to the present invention, magnified times.

FIG. 2 is a micrograph in transverse section of a bar formulated according to the present invention, but not normalized, magnified 100 times.

FIG. 3 is a micrograph in transverse section of a bar formulated according to the present invention and normalized at a temperature above the critical temperature range, magnified 100 times.

DESCRIPTION OF THE PREFERRED EMBODIMENT Composition The composition of the alloy of the present invention, in weight percentages, is as follows:-

Element: Percentage by weight Carbon 028-038 Manganese LOO-1.40 Vanadium 0035-0065 Chromium 0. 10-0.20 Nickel 0.50-0.80 Silicon 0.15-0.30 Iron and other elements which do not adversely affect the properties Balance Sulphur and phosphorous may be present in maximum amounts of 0.04% in the case of sulphur and 0.035% in the case of phosphorous without adversely affecting the properties of the alloy. Copper may be added for corrosion resistance.

The individual effect of each of the alloying elements is obscured by the presence of the others, which together provide the desired mechanical properties. In other words, the individual contribution of each element cannot be easily isolated from the combined effect of all the alloying elements. Nevertheless, there are certain ranges in the quantity of each of the alloying elements that appear to be critical in the formulation of the present invention.

Carbon must be kept within a range of about 0.28- 0.38% to achieve optimum properties. This may be seen with reference to Table I. An increase in carbon has the effect of increasing the ultimate strength of the steel for a given normalizing temperature when the other elements remain constant. Increasing carbon does, however, have an adverse etfect on ductility as is shown by the decrease in elongation. In general, to achieve a balance between cost, increased ultimate strength and ductility, the carbon must be in the range of 0.28-0.38%. The yield-tensile ratio does not appear to be affected by the amount of carbon.

TABLE I.-INFLUENCE OF CARBON ON TENSILE PROPERTIES Ultimate Normaliz- Yield tensile Yield] Percent ing temp. strength strength Elongation tensile Heat No. F.) (p,s.i.) (p.s.i. (percent) ratio Si Mn 1? Cu Cr Ni V TABLE 11.-INFLUENCE OF MANGANESE 0N TENSILE PROPERTIES Ultimate Normaliz- Yield tensile Yield! Percent ing temp. strength strength Elongation tensile Heat No F.) (p.s.i.) (p.s.i.) (percent) ratio 0 Si Mn S P Cu Cr Ni V TABLE IIL-INFLUENCE OF CHROMIUM AND MOLYBDENUM ON TENSILE PROPERTIES Ultimate Normaliz- Yield tensile Yield/ Per ent ing temp. strength strength Elongation tensile H t N F.) (p.s.i.) (p.s.i.) (percent) ratio 0 Si Mn S P Cu Cr Ni V Mo 1 Not visible.

Manganese is the chief hardenability intensifier in the effect, namely, that of increasing the hardenability of steel alloy. A comparison of the tensile properties of the three heats in Table II shows the effect of manganese. Manganese additions rapidly increase the ultimate tensile strength of the steel and conversely decrease elongation or ductility. The yield-tensile ratio remains approximately constant. Table II indicates that a range of LOO-1.40% yields optimum properties.

The effect of chromium and molybdenum is shown in Table HI. The steel in Table III was air hardened, that is, as it cooled after normalizing some of the austenite transformed to martensite as well as pearlite-ferrite. The mixed structure with untempered martensite resulted in a steel with extremely low ductility. A definite yield point was not observed, a typical characteristic of air hardened grades. The presence of chromium in the amount of 0.54% is clearly too high for reasonable ductility. A small quantity in the range of (MO-0.20% is desirable, however, to increase hardenability and in such quantity does not adversely affect ductility. No molybdenum is necessary.

Both silicon and copper are ferrite strengtheners and act to increase the tensile strength of the alloy. Silicon can be detrimental to the impact strength, however, if it is present in a quantity above about 0.30% The preferred range of silicon is 0.15-0.30%.

Copper has the additional eflect of increasing the atmospheric corrosion resistance of the steel in comparison to other carbon grades of steel. The presence of some copper is important in minimizing corrosion of exposed portions of the alloy, but it is not required for tensile properties or toughness.

Vanadium and aluminum are alloying elements used for controlling grain size. The form in which vanadium and/or aluminum is present may be aluminum-nitride, aluminum-oxide and vanadium-carbide. Disassociated vanadium-carbide or pure vanadium has a secondary in the same way as any alloy. Vanadium-carbide disassociates directly with time and temperature during normalizing. Vanadium increases the yield-tensile strength ratio. This is important because the design strength of steel is usually a direct function of the yield strength.

The influence of vanadium, aluminum and the absence of either in the composition of the present invention is shown in Table IV.

With no aluminum or vanadium (such as in Heat No.

35 A 8814), a poor alloy resulted. The steel had no observable yield strength and very low elongation. Upon normalizing the steel coarsened and transformed upon cooling to extremely large, brittle, pearlite colonies surrounded by thin bands of ferrite.

The importance of vanadium is also apparent from treated and normalized steels was absent. This is illustrated by the low yield to tensile strength ratio of 0.723 in comparison to the higher values in vanadium steel, such as 0.785 for Heat No. B 1517 (see Table IV).

While aluminum is not vital to the composition of the present invention, vanadium should be present in the range of 0.035-0.065%.

The steel is formulated according to the foregoing in conventional steel making and rolling facilities. The bars may be hot rolled with a deformation pattern as described in ASTM Specification A615-68. Hot rolling with this deformation pattern is not essential, however, and the steel may also be rolled into smooth bars or shapes of any desired cross-section. Diiferent section sizes require slight changes in the quantity of the various alloying elements within the ranges specified.

TABLE IV.INFLUENCE OF VANADIUM AND ALUMINUM ON TENSILE PROPERTIES Ultimate N ormallz- Yield tensi Yield/ Percent ing temp. strength strength Elongation tensile Heat No.- (F.) (p.s. (p.s.l (percent) ratio 0 Si Mn 5 P Ni V Cu Cr A1 1 Not visible.

Heat treatment After hot rolling the steel bars are normalized in a continuous flow-through furnace. The bar is heated to a normalizing temperature of 1600-1800 F. within a period of ten minutes and then allowed to air cool. An optimum normalizing temperature range for most crosssections to achieve optimum mechanical properties and ductility is 17001750 F. No further heat treatment such as tempering is required after normalizing. Within the range specified, the normalizing temperature may be varied to adjust mechanical properties of the alloy. A low normalizing temperature (1600 F.) causes the steel to have lower mechanical properties (tensile and yield strength) and higher ductility. High normalizing temperatures result in higher mechanical properties and lower ductility.

The effect of normalizing and the importance of the range of normalizing temperatures specified, namely, 1600-1800 F. on tensile properties, ductility and impact strength is apparent when analyzing the data in Tables V, VI, VII, and VIII and when comparing FIG. 1 to FIGS. 2 and 3. Tables V and VI and FIG. 2 represent as-rolled bars, without normalizing. The data in Tables VII and VIII was obtained with normalized bars. FIG. 1 represents the metallurgical grain structure of a bar formulated in accordance with the present invention and normalized at 1750" F., within the range specified. The bar of FIG. 3 was formulated according to the present invention but normalized at 1850 F., above the range specified. The effect of normalizing on tensile properties is apparent when comparing Tables V and VII. The eifect of normalizing on impact strength is apparent when Tables VI and VIII are viewed together.

TABLE V.AS-ROLLED (NOT NORMALIZED) TENSILE PROPERTIES Yield Tensile Yield/ Percent strength strength Elongation tensile Heat N0 (p.s.1.) (p.s.i.) (percent) (ratio) 0 Si Mn S P Cu 01' Ni V Not visible.

TABLE VI.AS-ROLLED (NOT NORMALIZED) CHARPY IMPACT STRENGTH CharpyV impact strength Percent at --15 F.

(ft. lb.) 0 Si Mn S P Cu Cr Ni V TABLE VIL-TENSILE PROPERTIES-NORMALIZED Yield Tensile Elonga- Yield/ Percent strength strength tion RA. tensile (p.s.i.) (p.s.i.) (percent) (percent) ratio 0 Si Mn S P Cu Cr V Notmeasureel. Indeterminate. Not visible.

TABLE VIIL-CHARPY V-NOTCH IMPACT STRENGTH AT --20 F.-NORMALIZED Percent Normalizing Charpy V at Heat No temp. F.) -20 F. (ft. lb.) Si Mn S P Cu Cr Ni V These tables indicate that normalizing within the temperature range of l600-1800 F.:

(1) Increases yield strength,

(2) Decreases ultimate tensile strength,

(3) Increases the ratio of yield strength to ultimate tensile strength,

(4) Increases ductility as measured by elongation and reduction in area, and

(5) Increases Charpy V-notch toughness.

Normalizing above the range of 1600-1800 F. does not yield the desired properties as illustrated by FIG. 3 and Heat B 1514 (normalized at 1850 F.) of Table VII.

The change in the microstructure after normalizing is apparent when comparing FIG. 2 with FIG. 1. The average grain size of the as-rolled steel (prior to normalizing) shown in FIG. 2 is ASTM-5 or 6. After normalizing in the range of 1600 1800 F. the grain size is reduced to ASTM-8 or 9 and this is shown in FIG. 1. Normalizing at a temperature above the critical range, namely, at 1850 F. does not produce the reduced grain size and this is shown in FIG. 3.

Conclusion-Composition and heat treatment variables The precise influence of each element in an alloy on impact strength is very diflicult to isolate and measure. In general, however, alloys with a low ductility appear to have low impact strength, although the behavior of ferritic steels under notched conditions cannot always be predicted from tensile properties. Some alloys that display normal ductility in a tensile test may, nevertheless, break in brittle fashion when tested or used in a notched condition.

Nickel is an essential metal for tensile strength and impact strength. Heat A 7742. in Tables VII and VIII is an example of an alloy with low tensile strength and low impact strength when no nickel is present. Nickel influences both the impact strength and the transition temper ature and is, therefore, an essential metal in the composition of the present invention.

The grain size of the alloy must also be controlled to achieve high impact strength and vanadium is essential for this purpose. Heat A 8814 shown in Tables IV and VIII was very coarse grained and had a very low impact strength (2.5 ft. lb.) in the absence of vanadium.

Heat A 8820 shown in Tables VII and VIII is an example of composition and heat treatment where optimum properties were achieved. In this case the tensile properties, as well as the impact strength exceeded the level sought. Heat A 8820 in all respects lies within the composition ranges and heat treatment range which define the present invention.

To achieve consistent heat-lot to heat-lot results, the specified chemistry ranges and the normalizing temperature range of l600-l800 F. are essential. A normalizing temperature range of 17001750 F. may be used to further insure consistent results and it should be emphasized that the normalizing temperature must extend to the core of the bar.

Steel made in accordance with the composition and heat treatment limits specified may be certified by heatlot consistently and reliably to meet specifications desired, namely, 75,000 p.s.i. minimum yield strength, 100,000 p.s.i. minimum ultimate tensile strength, 12.5% minimum elongation, 25% minimum reduction in area and i15 ft. lb. minimum impact strength at 20 F. The invention described above has proven to consistently yield the desired physical properties of strength, toughness, elongation and reduction in area.

Having thus described the invention, the following is claimed:

1. A high-strength steel characterized by a yield point in excess of 75,000 p.s.i. and a Charpy V-notch impact strength (per ASTM E23) at 20 F. of at least 15 ft. 1b., containing in weight percentages:

Carbon 0.28-0.38 Manganese 1.00-1.40 Vanadium 0035-0065 Chromium 0.10-0.20 Nickel 0.504180 Silicon t 0.15-0.30 Iron and other elements which do not adversely alfect the properties Balance References Cited UNITED STATES PATENTS 2,281,850 5/1942 McKinney 14836 3,110,586 1l/ 1963 Gulya et al. -128 V 3,110,635 11/1963 Gulya 14836 3,216,823 11/4965 Gulya et a1. 75---128 V 3,328,211 6/1967 Nakamura 148-36 X FOREIGN PATENTS 396,438 8/ 1933 Great Britain 75-128 R 987,184 3/1965 Great Britain 75128 R 746,188 6/1944 Germany 75-128 R CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 75128 V

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