WO2005121384A2

WO2005121384A2 - High strength steel alloy

Info

Publication number: WO2005121384A2
Application number: PCT/US2004/040719
Authority: WO
Inventors: Drake H. Damerau
Original assignee: Chamberlian Manufacturing Corporation
Priority date: 2003-12-04
Filing date: 2004-12-02
Publication date: 2005-12-22
Also published as: WO2005121384A3

Abstract

Steel alloys having physical properties such as high impact toughness at -40°F while maintaining a yield strength of at least about 180,000 psi are disclosed. The properties of the inventive steel alloys are comparable to other steel alloys containing higher amounts of nickel but are considerably less expensive to make by substituting a portion of the nickel with specific amounts of vanadium.

Description

HIGH STRENGTH STEEL ALLOY

FIELD OF THE INVENTION [001] This invention relates to steel alloys, commonly designated as specialty steels, having relatively low levels of nickel but high strength properties and reduced costs as compared to prior art steel alloys of similar physical properties.

BACKGROUND OF THE INVENTION [002] Over the years, various steel alloys have been proposed having improvements in one or more physical properties. For example, there has been a continuing need in the metallurgical industry for steel alloy compositions, which have high strength and low temperature impact toughness combined with lower manufacturing costs or other economic benefits for manufacturers. [003] The mechanical properties of alloy steels can vary due to several factors, including the amounts of the alloying elements, impurities, etc. Many alloying systems and thermomechanical treatments, such as rolling, pressing, hammering, forging and various heat treatments can be used to alter the mechanical properties of alloy steels. The ability to make an alloy which achieves certain physical properties is not necessarily difficult when costs are not an issue. A common problem, however, is that the costs associated with doing so make the use of the alloy uneconomical for both the manufacturer and end user. [004] The military is a large consumer of steel alloys. Modern artillery shells and ordinance employ various steel alloys. The nature of such products infers that they are designed for a single use. Consequently, the cost per unit should be as low as possible. Improvements have been sought in the areas of providing ordinance which is capable of piercing fortified bunkers which often have reinforced concrete several feet thick. The ability to provide artillery shells or other steel products made with steel alloys which can meet the needs of the military for this use at reduced costs is an on-going need. The present invention addresses these and other needs.

SUMMARY OF THE INVENTION [005] Accordingly, it is an object of the present invention to provide low alloy steel compositions and articles made therefrom having unique blends of desirable properties, including high strength and high toughness at low temperature. [006] Another object of the present invention to provide high strength steel alloys and articles made therefrom for use in small diameter bomb programs. [007] These and other objects are achieved by the present invention, which in one aspect provides an improved steel alloy composition having high strength and high toughness at low temperature. [008] The steel alloy composition contains by weight percent: from about 0.28 to about 0.33 carbon, from about 0.60 to about 0.80 manganese, from about 0.10 to about 0.22 silicon, from about 3.00 to about 3.20 nickel, from about 0.96 to about 1.12 chromium, from about 0.30 to about 0.40 molybdenum, from about 0.10 to about 0.15 vanadium, less than about 0.25 copper, less than about 0.015 aluminum, less than about 0.010 phosphorus, less than about 0.008 sulfur, and less than about 0.006 calcium. The remainder of steel alloy composition includes iron and incidental impurities. [009] Methods of preparing steel alloys using techniques including austenitizing, tempering are also disclosed. [010] One advantage of the steel alloys of the present invention is the fact that they demonstrate superior strength when compared to other alloys having similar nickel content. Thus, significant cost savings are realized. For example, in preferred aspects of the invention, the steel is designed to have high impact toughness at -40 while maintaining a yield strength of at least about 180,000 psi, preferably at least about 185,000 and more preferably at least about 200,000 psi. This inventive steel alloy is comparable to some steel alloys containing high amounts of nickel but is considerably less expensive. This is accomplished by substituting a portion of the nickel with specific amounts of vanadium, refining the grains using calcium, and limiting the embrittling effects of antimony, tin, arsenic and sulfur. For example, without sacrificing the desirable toughness properties, the cost efficiencies of the inventive alloys are realized, in part, by reducing the amount of nickel in the alloy and eliminating expensive processes like vacuum arc remelting and/or resorting to the use of aircraft quality steel alloys. [011] The alloys of the invention are specifically designed for use in penetrating warheads, but have practical applications in any field where high strength, low temperature impact toughness and lower costs are desirable. [012] Other objects and advantages of the present invention will be apparent to those skilled in the art upon reference to the following detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE INVENTION [013] The low alloy steel of the present invention is characterized by the combination of properties set forth below: Yield Strength @ .2% offset: >180ksi, Elongation: >12%,

Reduction of Area: >35%, at room temperature,

CVN Charpy V Notch @ -40°F: >23 ft-lbs (impact toughness) when the alloy is forged using about a 6:1 reduction ratio and in a heat treated condition, as described in more detail below. The steel alloy compositions of the present invention contain the following elements by weight percent: [014] Carbon: from about 0.28 to about 0.33 Manganese: from about 0.60 to about 0.80 Silicon: from about 0.10 to about 0.22 Nickel: from about 3.00 to about 3.20 Chromium: from about 0.96 to about 1.12 Molybdenum: from about 0.30 to about 0.40 Vanadium: from about 0.10 to about 0.15 Copper: less than about 0.25; Aluminum: less than about 0.015 Phosphorus: less than about 0.010 Sulfur: less than about 0.008; and Calcium: less than about 0.006 with the balance being iron and incidental impurities.

[015] Although incidental impurities should be kept to the least amount possible, the alloys of the invention can also include from about 0 to less than about 0.011 wt % tin, from about 0 to less than about 0.009 wt % antimony, and from about 0 to less than about 0.009 wt % arsenic. The amount of the residual elements such as phosphorous, sulfur, copper and particularly the synergistic effects of tin, antimony and arsenic, should be kept within the amounts specified herein in order to avoid any possible adverse on the impact properties of the inventive alloys. [016] Most industry standard steel making processes well known to those of ordinary skill can employed in the preparation of the inventive steel alloys. Any suitable furnace may be employed but it is preferable that the alloy be made in an electric furnace due to the cost benefits thereof. The alloy also employs non-resulfu zed steel and will have an ASTM Grain Size 7 or finer, be fully aluminum killed, vacuum degassed, and calcium treated. The reduction ratio of the final product is preferably at least about 6:1 as obtained by hot working. A cast aspect ratio of 1.25 to 1.55 and strand stirring is also desirable. [017] Within the general parameters provided above, preferred alloys are of the following chemical composition (as determined by ladle analysis), with the remainder, again being iron and residual elements. The column entitled "Aim" provides preferred aspects of the inventive alloys.

TABLE 1 Element Minimum Maximum Aim Carbon 0.28 0.33 0.31 Manganese 0.60 0.80 0.70 Silicon 0.10 0.22 Nickel 3.00 3.20 3.10 Chromium 0.96 1.12 Molybdenum 0.30 0.40 Copper 0.25 Aluminum 0.015 Phosphorus 0.010 Sulfur 0.008 Vanadium 0.10 0.15 Calcium 0.006. [018] Aluminum content cannot be used as basis for grain size determination. Within this framework, the non-specified residual elements should not exceed 0.05% each or 0.15% combined. Total %wt of (Sn + Sb + As) should not exceed 0.021%. [019] The alloy compositions of the present invention are suitably prepared by any appropriate technique known in the art such as continuous cast metallurgy techniques. Thus, the alloys are made using an electric furnace, being fully aluminum killed, vacuum degassed, and calcium treated before being continuous casted into strands and rolled into bars, billets or other forms with a suitable reduction factor for subsequent hot working. [020] In preferred aspects of the invention and to obtain the highest possible properties form this alloy in finished goods, it should be first forged to obtain a uniform longitudinal grain flow. Next, The material should be heat-treated in a manor that austenitizes the material before quenching in oil. After fully hardened, the material should be tempered to achieve the desired properties. [021] Examples of suitable heat treating include normalizing at a temperature of about 1650° F, +/- 15 F, and austenitizing at temperatures of from about 1600 to about 1625°F and quenching in oil at temperatures of from about 120°F to about 140°F. Tempering can be carried out at temperatures of about 800 to about 900° F. Thus, in a further aspect of the invention, there is provided a method of preparing method of preparing a high strength steel. The method includes: a) providing a steel alloy composition as described above, b) austenitizing the steel alloy composition at a temperature of greater than about 1600°F; and c) quenching the resultant product of step b) in oil at a temperature of about 120°F to about 140°F. [022] In further aspects, and depending upon the needs of the artisan, the steel alloy is then tempered using any of the techniques known to those of ordinary skill at temperatures mentioned above and, if needed, quenched in water at about ambient temperature, so as to minimize the effects that residual elements such as phosphorous, sulfur, copper as well as tin, antimony and arsenic, can have on the impact properties and ductility. [023] The tempered steel alloys are then in condition for final shaping into artillery shells or whatever final product is desired. For example, hot rolled bars made from the inventive alloy can be heated and forged into any desired shape using processes known to those of ordinary skill in the art.

EXAMPLES [024] The following examples serve to provide further appreciation of the invention but are not meant in any way to restrict the effective scope of the invention.

Example 1 INVENTIVE ALLOY [025] Scrap iron was melted in an electric furnace until it reached a temperature of about 50°F above liquidous temperature of the alloy. Ladle analysis was performed and the necessary elements were added to achieve the chemical composition set forth below in Table 2 which was confirmed by ladle analysis. Thereafter, the ladle contents were vacuum degassed and poured into ingots. After the ingots were formed, they were hammer forged into bars. The alloy obtains hardenability through the use of a relatively high nickel content and a moderate level of chromium, coupled with a modest amount of vanadium. The levels of carbon, manganese, and molybdenum, are lower than most HSLA steel. Because of this, the susceptibility for quench cracking is lessoned significantly than that of other materials. The calculated Dl is well over 13 inches and the Jominy was observed to be above 51.5 out past J32.

TABLE 2 Actual Material Chemical Composition Used Element Wt % Element Wt % Carbon 0.28 Ti 0.001 Manganese 0.66 Tungsten 0.012 Silicon 0.14 Columbium 0.003 Nickel 3.03 Zirconium 0.001 Chromium 0.96 Boron 0.0002 Molybdenum 0.32 Lead 0.005 Copper 0.10 Calcium 0.0002 Aluminum 0.007 Tin 0.002 Phosphorus 0.007 Antimony 0.001 Sulfur 0.008 Arsenic 0.002 Vanadium 0.116 Nitrogen 0.0092 Cobalt 0.006 Oxygen 0.0036

Example 2 [026] The process of Example 1 is repeated except that instead of making ingots, a full, commercial scale continuous casting technique is used. The ladle containing the material of Table 2 is cast into blooms and the blooms are hot rolled to a minimum 6:1 reduction to form bars.

Example 3 [027] The ingots of example 1 are subjected to various tempering treatments and impact toughness was measured thereafter. For purposes of this description, "normalizing" shall be understood as heating to 1 ,650°F and air cooling. "Heat treated" shall be understood to include being austenitized at 1625°F, being quenched in oil and immediately tempered by heating the part to the stated temperature.

TABLE 3

[028] Turning now to the data provided above in Table 3, it can be seen that each of the heat treatments transformed the alloy into a product having an impact toughness of 23.5 or greater. Additional analysis of the product which was normalized, heat treated and tempered at 800°F yielded the following additional data:

Thus, the alloys of the invention have the properties set forth at the outset, e.g. : Yield Strength @ .2% offset: >180ksi, Elongation: >12%, Reduction of Area: >35%, at room temperature, CVN Charpy V Notch @ -40°F: >23 ft-lbs (impact toughness). [029] Although the results provided above are based on an alloy made on a small scale, those of ordinary skill will of course understand that continuous cast product made using industry standard techniques will have at least these physical properties and will probably exceed most measurements. , [030] While there have been described what are presently believed to be the preferred embodiments of the invention, those skilled in the art will realize that changes and modifications may be made without departing from the spirit of the invention. It is intended to claim all such changes and modifications as fall within the true scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A steel alloy composition characterized by the properties of: Yield Strength© .2% offset: >180ksi; Elongation: ≥ 12%;

Reduction of Area: > 35%;

CVN (Charpy V Notch) @ -40°F: > 20 ft-lbs; the composition comprising by weight percent: Carbon: from about 0.28 to about 0.33 Manganese: from about 0.60 to about 0.80 Silicon: from about 0.10 to about 0.22 . Nickel: from about 3.00 to about 3.20 Chromium: from about 0.96 to about 1.12 Molybdenum: from about 0.30 to about 0.40 Vanadium: from about 0.10 to about 0.15 Copper: less than about 0.25; Aluminum: less than about 0.015 Phosphorus: less than about 0.010 Sulfur: less than about 0.008; and Calcium: less than about 0.006 with the remainder being iron and incidental impurities.

2. The steel alloy of claim 1 , wherein said incidental impurities comprise Tin: less than about 0.011 wt %; Antimony: less than about 0.009 wt %; and Arsenic: less than about 0.009 wt %.

3. The steel alloy of claim 1 , wherein said incidental impurities do not exceed 0.05 wt % each.

4. ^' The steel alloy of claim 1, wherein the total amount of said incidental impurities is less than about 0.15 wt %.

5. The steel alloy of claim 2 wherein the combined total %wt of (Sn + Sb + As) is less than about 0.021%.

6. The steel alloy of claim 1 , wherein the carbon content is about 0.33 % wt.

7. The steel alloy of claim 1 , wherein the manganese content is about 0.70 % wt.

8. The steel alloy of claim 1 , wherein the nickel content is about 3.1 % wt.

9. The steel alloy of claim 1 , wherein the carbon content is about 0.33 % wt, the manganese content is about 0.70 % wt. and the nickel content is about 3.1 % wt.

10. The steel alloy of claim 1 , wherein said steel alloy composition has been austenitized at temperature of about at a temperature of about 1600°F.

11. The steel alloy of claim 9, wherein said austenitized steel alloy composition has been quenched in oil and tempered at temperature of about 800°F.

12. The steel alloy of claim 11, characterized by the properties of:

Yield Strength® .2%offset 188.6 ksi,

Tensile strength 211.0 ksi

Elongation 13%,

Reduction of Area 55.2%, and

CVN Charpy V Notch @ -40°F 26.5 ft-lbs.

13. The steel alloy of claim 1 having a quenched hardness of about 53.4HRC.

14. A method of preparing a high strength steel, comprising: a) providing a steel alloy composition comprising by weight percent: Carbon: from about 0.30 to about 0.35; Manganese: from about 0.60 to about 0.80; Silicon: from about 0.10 to about 0.22; Nickel: from about 3.00 to about 3.20; Chromium: from about 1.00 to about 1.15; Molybdenum: from about 0.30 to about 0.40; Vanadium: from about 0.10 to about 0.15; Copper: less than about 0.25; Aluminum: less than about 0.015; Phosphorus: less than about 0.010; Sulfur: less than about 0.008; and Calcium: less than about 0.006; with the remainder being iron and incidental impurities; b) austenitizing said steel alloy composition at a temperature of greater than about 1600°F; and c) quenching the resultant product of step b) in oil at a temperature of about 120F to 140F.

15. The method of claim 14, further comprising: d) tempering the resultant product of step c) at a temperature of from about 800 to about 900°F; and e) quenching the resultant product of step d) in water at about ambient temperature.

16. An artillery shell comprising the steel alloy of claim 1.