Title:
Ultra-high strength weathering steel and method for making same
Kind Code:
A1


Abstract:
An air-cast ultra-high strength weathering steel prepared by combining C, Si, Cr, Cu, Mo, Ni, Co, and Fe, wherein the steel has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent. In a preferred embodiment, the air-cast ultra-high strength weathering steel includes at least one ingredient selected from the group consisting of V, W, N and mixtures thereof In another preferred embodiment C is present in the range of from about 0.16 to 0.23 percent by weight, Si is present in the range of from about 0.2 to 1.5 percent by weight, Cr is present in the range of from about 2 to 5 percent by weight, Cu is present in the range of from about 0.2 to 2 percent by weight, Mo is present in the range of from about 0.8 to 3 percent by weight, Ni is present in the range of from about 7 to 12 percent by weight, Co is present in the range of from about 13 to 17 percent by weight; and Fe is present in a remaining amount.



Inventors:
Nasserrafi, Rahbar (Chula Vista, CA, US)
Wyte, Michael (Culver City, CA, US)
Application Number:
11/288873
Publication Date:
04/20/2006
Filing Date:
11/28/2005
Assignee:
Gainsmart Group Limited
Primary Class:
Other Classes:
420/91
International Classes:
C22C38/52
View Patent Images:



Primary Examiner:
YANG, JIE
Attorney, Agent or Firm:
Lewis Roca Rothgerber Christie LLP (Glendale, CA, US)
Claims:
1. An air-cast ultra-high strength weathering steel prepared by combining C, Si, Cr, Cu, Mo, Ni, Co, and Fe; wherein C is present in the range of from about 0.16 to 0.23 percent by weight, Si is present in the range of from about 0.5 to 1.5 percent by weight, Cr is present in the range of from about 2 to 5 percent by weight, Cu is present in the range of from about 0.2 to 2 percent by weight, Mo is present in the range of from about 0.8 to 3 percent by weight, Ni is present in the range of from about 7 to 12 percent by weight, Co is present in the range of from about 13 to 17 percent by weight; and Fe is present in a remaining amount; wherein the steel is essentially free of Al and Ti; and wherein the steel has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent.

2. The air-cast ultra-high strength weathering steel as recited in claim 1 wherein the steel has a yield strength in the range of from about 255,000 to 300,000 psi, a tensile strength in the range of from between 255,000 and 320,000 psi, and an elongation of between about 10 to 14 percent.

3. The air-cast ultra-high strength weathering steel as recited in claim 1 further comprising at least one ingredient selected from the group consisting of V, W, N and mixtures thereof.

4. The air-cast ultra-high strength weathering steel as recited in claim 1 wherein C is present in the range of from about 0.16 to 0.23 percent by weight, Si is present in the range of from about 0.2 to 1.5 percent by weight, Cr is present in the range of from about 2 to 5 percent by weight, Cu is present in the range of from about 0.2 to 2 percent by weight, Mo is present in the range of from about 0.8 to 3 percent by weight, Ni is present in the range of from about 7 to 12 percent by weight, Co is present in the range of from about 13 to 17 percent by weight; and Fe is present in a remaining amount.

5. The air-cast ultra-high strength weathering steel as recited in claim 1 wherein the steel is essentially free of Al and Ti.

6. The air-cast ultra-high strength weathering steel as recited in claim 1 wherein the steel comprises: in the range of from about 0.17 to 0.22 percent by weight C; in the range of from about 0.5 to 1.2 percent by weight Si; in the range of from about 2.5 to 4 percent by weight Cr; in the range of from about 0.5 to 1.5 percent by weight Cu; in the range of from about 1 to 2 percent by weight Mo; in the range of from about 8 to 10.5 percent by weight Ni; and in the range of from about 14 to 16 percent by weight Co.

7. The air-cast ultra-high strength weathering steel as recited in claim 6 further comprising at least one ingredient selected from the group consisting of V, W, N and mixtures thereof.

8. The air-cast ultra-high strength weathering steel as recited in claim 1 wherein the steel comprises: in the range of from about 0.18 to 0.21 percent by weight C; in the range of from about 0.5 to 0.9 percent by weight Si; in the range of from about 3 to 4 percent by weight Cr; in the range of from about 0.7 to 1.5 percent by weight Cu; in the range of from about 1.3 to 1.7 percent by weight Mo; in the range of from about 9 to 10.5 percent by weight Ni; and in the range of from about 14 to 15.5 percent by weight Co.

9. The air-cast ultra-high strength weathering steel as recited in claim 8 further comprising at least one ingredient selected from the group consisting of V, W, N and mixtures thereof.

10. An ultra-high strength weathering steel consisting essentially of: in the range of from about 0.16 to 0.23 percent by weight C; in the range of from about 0.5 to 1.5 percent by weight Si; in the range of from about 2 to 5 percent by weight Cr; in the range of from about 0.2 to 2 percent by weight Cu; in the range of from about 0.8 to 3 percent by weight Mo; in the range of from about 7 to 12 percent by weight Ni; in the range of from about 13 to 17 percent by weight Co; and a remaining amount Fe.

11. The ultra-high strength weathering steel as recited in claim 10 further comprising at least one ingredient selected from the group consisting of V, W, N and mixtures thereof.

12. The ultra-high strength weathering steel a recited in claim 10 wherein the steel has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent.

13. An ultra-high strength weathering steel comprising: in the range of from about 0.16 to 0.23 percent by weight C; in the range of from about 0.5 to 1.5 percent by weight Si; in the range of from about 2 to 5 percent by weight Cr; in the range of from about 0.2 to 2 percent by weight Cu; in the range of from about 0.8 to 3 percent by weight Mo; in the range of from about 7 to 12 percent by weight Ni; in the range of from about 13 to 17 percent by weight Co; up to about 2 percent by weight V; up to about 2 percent by weight W; up to about 0.4 percent by weight N; and a remaining amount Fe.

14. The ultra-high strength weathering steel a recited in claim 13 wherein the steel has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent.

15. A casting made from a weathering steel that is prepared according to the steps of: hot isostatic pressing a cast part made from weathering steel at a temperature of above about 1,900° F. for about 3 to 5 hours at a pressure of greater than about 14,000 psi; homogenating the cast part at a temperature of above about 2,000° F. for a period of from about 1 to 6 hours; solution heat treating the cast part at a temperature in the range of from about 1,600° to about 2,100° F. for about 1 to 4 hours, followed by cooling to room temperature; cooling the cast part by refrigeration for about 1 to 8 hours at a temperature below about −50° F.; and aging the cast part at a temperature of between about 800° F. and about 1,000° F. for about 4 to about 5 hours.

16. The casting as recited in claim 15 wherein, prior to the step of pressing, the casting is formed by: combining in the range of from about 0.16 to 0.23 percent by weight C, with in the range of from about 0.2 to 1.5 percent by weight Si, in the range of from about 2 to 5 percent by weight Cr, in the range of from about 0.2 to 2 percent by weight Cu, in the range of from about 0.8 to 3 percent by weight Mo, in the range of from about 7 to 12 percent by weight Ni, in the range of from about 13 to 17 percent by weight Co, and a remaining amount Fe; melting the combined ingredients to form a weathering steel mixture; and pouring the weathering steel mixture to form the cast part.

17. The casting as recited in claim 16 further comprising, during the step of combining, adding one or more ingredient selected from the group consisting of V, W, N and mixtures thereof.

18. The casting as recited in claim 16 wherein the steps of melting and pouring are conducted in an air environment.

19. The casting as recited in claim 15 wherein the cast part after the step of aging has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent.

20. The casting as recited in claim 15 wherein the hipping step is conducted in the range of from about 2,000° F. to 2,240° F., the homogenating step is conducted in the range of from about 2,150° F. to 2,220° F., the solution heat treating step is conducted in the range of from about 1,800° F. to 1,950° F., the cooling step is conducted in the range of from about −110° F. to −75° F., and the aging steps is conducted in the range of from about 900° F. to 1,000° F.

21. The casting as recited in claim 15 wherein, prior to the step of pressing, the casting is formed by: combining in the range of from about 0.17 to 0.22 percent by weight C, with in the range of from about 0.3 to 1.2 percent by weight Si, in the range of from about 2.5 to 4 percent by weight Cr, in the range of from about 0.5 to 1.5 percent by weight Cu, in the range of from about 1 to 2 percent by weight Mo, in the range of from about 8 to 10.5 percent by weight Ni, in the range of from about 14 to 16 percent by weight Co, and a remaining amount Fe; melting the combined ingredients to form a weathering steel mixture; and pouring the weathering steel mixture to form the cast part.

22. The casting as recited in claim 21 further comprising, during the step of combining, adding one or more ingredient selected from the group consisting of V, W, N and mixtures thereof.

23. The casting as recited in claim 21 wherein the steps of melting and pouring are conducted in an air environment.

24. The casting as recited in claim 23 wherein the cast part after the step of aging has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent.

25. A method for making a cast part from a weathering steel comprising the steps of: combining in the range of from about 0.16 to 0.23 percent by weight C, with in the range of from about 0.5 to 1.5 percent by weight Si, in the range of from about 2 to 5 percent by weight Cr, in the range of from about 0.2 to 2 percent by weight Cu, in the range of from about 0.8 to 3 percent by weight Mo, in the range of from about 7 to 12 percent by weight Ni, in the range of from about 13 to 17 percent by weight Co, and a remaining amount Fe; melting the combined ingredients to form a weathering steel mixture; and pouring the weathering steel mixture to form the cast part.

26. The method as recited in claim 25 further comprising, during the step of combining, adding one or more ingredient selected from the group consisting of V, W, N and mixtures thereof.

27. The method as recited in claim 25 wherein the steps of melting and pouring are conducted in an air environment.

28. The method as recited in claim 25 further comprising the steps of hot isostatic pressing the cast part at a temperature of above about 1,900° F. for about 3 to 5 hours at a pressure of greater than about 14,000 psi; homogenating the cast part at a temperature of above about 2,100° F. for a period of from about 1 to 6 hours; solution heat treating the cast part at a temperature in the range of from about 1,600° to about 2,100° F. for about 1 to 4 hours, followed by cooling to room temperature; cooling the cast part by refrigeration for about 1 to 4 hours at a temperature below about −50° F.; and aging the cast part at a temperature of between about 800° F. and about 1,000° F. for about 4 to about 5 hours.

29. The method as recited in claim 28, wherein after the step of aging, the cast part has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

This is a coninuation of pending application Ser. No. 10/410,459, filed Apr. 8, 2003, entitled Ultra-High Strength Weathering Steel and Method for Making Same which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to ultra-high strength steels and, more particularly, to ultra-high strength steels that are air castable and that provide improved properties of strength, ductility, toughness, and corrosion resistance when compared to conventional ultra-high strength steels.

BACKGROUND OF THE INVENTION

Conventional ultra-high strength steels suffer from a number of drawbacks. For example, most martensitic and maraging steels are not resistant to atmospheric corrosion, particularly during the casting process. Conventional maraging steels are melted under vacuum conditions using high-purity raw materials in order to strictly control the presence of unwanted interstitial and tramp elements.

Maraging steels and other types of ultra-high strength precipitation hardening steels are melted and poured under vacuum conditions in an inert environment to protect essential and very reactive elements used in making such steels, such as titanium and aluminum, from exposure with atmospheric air. Air exposure during the melting and pouring process is intentionally avoided for the purpose of preventing the reactive elements from undergoing oxidation reactions. Presence of the reactive elements in the oxidized state is known to severely degrade the desired mechanical properties of the resulting steels.

However, this vacuum melting and pouring process is expensive. Additionally, many conventional ultra-high strength steels must be vacuum melted and poured multiple times in order to achieve a desired level of purity. Once the melting and pouring process is complete, the steels must go through significant wrought processing to achieve the desired mechanical properties. Because wrought processing is required, the steel manufacturer must have and use progressive dies to achieve the desired shape and mechanical properties, further adding to the cost and time involved in making the desired steel product. Still further, many conventional ultra-high strength steels require chrome or cadmium plating for most applications to provide a level of corrosion protection to protect the alloy from rusting.

Conventional ultra-high strength maraging steels typically include the following alloying elements provided below in Table I (in weight percent):

TABLE I
Maraging 200:  18 Ni3.3 Mo8.5 Co0.2 Ti0.1 AlBalance
Fe
Maraging 250  18 Ni  5 Mo8.5 Co0.4 Ti0.1 AlBalance
Fe
Maraging 300  18 Ni  5 Mo  9 Co0.7 Ti0.1 AlBalance
Fe
18 Ni (vacuum  17 Ni4.6 Mo 10 Co0.2 Ti0.3 AlBalance
cast)Fe
Cobalt Free18.5 Ni  3 Mo0.7 Ti0.1 AlBalance
250Fee

Titanium and aluminum are mandatory ingredients that are used in making maraging steels for the purpose of providing properties of increased strength and hardness to the resulting steel. However, any steels that are made using titanium and aluminum cannot be melted in air (as explained above) because of their susceptibility to form embrittling compounds. Thus, ultrahigh strength steels formed using titanium and aluminum must he subjected to vacuum melting and pouring.

Also, conventional martensitic ultra-high strength steels such as AISI 4340, H 11, and 300 M possess an insufficient level of toughness and poor corrosion resistance against atmospheric elements for acceptable use in many high strength steel applications calling for both a desired degree of toughness and some degree of corrosion resistance, e.g., use in an outdoor exposure application. In addition, the alloys used in making these steels make it generally difficult to weld the resulting steel, and produce steel having inferior mechanical properties as castings in comparison to their wrought counterparts.

Ultra-high strength maraging steels on the other hand are known to possess good properties of toughness and weldability. However, as discussed above, maraging steels are sensitive to atmospheric corrosion and related stress corrosion cracking. These steels are also are very expensive to acquire.

As briefly discuss above, the most common process for manufacturing ultra-high strength maraging steels is by double or triple melting process that includes a minimum of one vacuum melting/refining cycle. The melting/refining cycle is generally followed by forging and rolling into sheet or bar stock. Maraging steels achieve their best properties via wrought processing. Although casting maraging steels is know to provide a steel product having decent strength, casting into net shape results in inferior ductility (compared to its wrought and machined counterparts), as shown in Table 2 below.

TABLE 2
YieldTensile
StrengthStrengthReduction %
AlloyPSIPSIElongation %(Ductility)
Maraging 250
F(wrought)247,000260,000855
Maraging 250
(cast)242,102261,2335.810

It is, therefore, desired that new ultra-high strength steels be developed that provide improved combined properties of strength, toughness, ductility and weather or corrosion resistance when compared to those conventional ultra-high strength steels noted above. It is also desired that such new ultra-high strength steels be capable of being made without the need for expensive and time consuming vacuum melt and pour processing, in a manner that permits air casting.

SUMMARY OF THE PREFERRED EMBODIMENTS

In accordance with one aspect of the present invention there is provided an air-cast ultrahigh strength weathering steel prepared by combining C, Si, Cr, Cu, No, Ni, Co, and Fe, wherein the steel has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent. In a preferred embodiment, the air-cast ultra-high strength weathering steel includes at least one ingredient selected from the group consisting of V, W, N and mixtures thereof In another preferred embodiment C is present in the range of from about 0.16 to 0.23 percent by weight, Si is present in the range of from about 0.2 to 1.5 percent by weight, Cr is present in the range of from about 2 to 5 percent by weight, Cu is present in the range of from about 0.2 to 2 percent by weight, Mo is present in the range of from about 0.8 to 3 percent by weight, Ni is present in the range of from about 7 to 12 percent by weight, Co is present in the range of from about 13 to 17 percent by weight; and Fe is present in a remaining amount.

In accordance with another aspect of the present invention there is provided an ultra-high strength weathering steel consisting essentially of in the range of from about 0.16 to 0.23 percent by weight C; in the range of from about 0.2 to 1.5 percent by weight Si; in the range of from about 2 to 5 percent by weight Cr; in the range of from about 0.2 to 2 percent by weight Cu; in the range of from about 0.8 to 3 percent by weight Mo; in the range of from about 7 to 12 percent by weight Ni; in the range of from about 13 to 17 percent by weight Co; and a remaining amount Fe. In a preferred embodiment, the ultra-high strength weathering steel includes at least one ingredient selected from the group consisting of V, W, N and mixtures thereof

In accordance with yet another aspect of the present invention there is provided an ultra-10 high strength weathering steel including in the range of from about 0.16 to 0.23 percent by weight C; in the range of from about 0.2 to 1.5 percent by weight Si; in the range of from about 2 to 5 percent by weight Cc in the range of from about 0.2 to 2 percent by weight Cu; in the range of from about 0.8 to 3 percent by weight Mo; in the range of from about 7 to 12 percent by weight Ni; in the range of from about 13 to 17 percent by weight Co; up to about 2 percent by weight V; up to about 2 percent by weight W; up to about 0.4 percent by weight N; and a remaining amount Fe. In a preferred embodiment, the ultra-high strength weathering steel has a yield strength of greater than about 230,000 psi, a tensile strength of greater than about 250,000 psi, and an elongation of greater than about 6 percent.

In accordance with another aspect of the present invention there is provided a casting 20 made from a weathering steel that is prepared according to the steps of hot isostatic pressing a cast part made from weathering steel at a temperature of above about 1,900° F. for about 3 to 5 hours at a pressure of greater than about 14,000 psi; homogenating the cast part at a temperature of above about 2,000° F. for a period of from about 1 to 6 hours; solution heat treating the cast part at a temperature in the range of from about 1,600° to about 2,100° F. for about I to 4 hours, followed by cooling to room temperature; cooling the cast part by refrigeration for about 1 to 8 hours at a temperature below about −50° F.; and aging the cast part at a temperature of between about 800° F. and about 1,000° F. for about 4 to about 5 hours. In a preferred embodiment, the casting is formed by combining in the range of from about 0.16 to 0.23 percent by weight C, with in the range of from about 0.2 to 1.5 percent by weight Si, in the range of from about 2 to 5 percent by weight Cr, in the range of from about 0.2 to 2 percent by weight Cu, in the range of from about 0.8 to 3 percent by weight Mo, in the range of from about 7 to 12 percent by weight Ni, in the range of from about 13 to 17 percent by weight Co, and a remaining amount Fe; melting the combined ingredients to form a weathering steel mixture; and pouring the weathering steel mixture to form the cast part.

In accordance with yet another aspect of the present invention there is provided a method S for making a cast part from a weathering steel. The method includes the steps of combining in the range of from about 0.16 to 0.23 percent by weight C, with in the range of from about 0.2 to 1.5 percent by weight Si, in the range of from about 2 to 5 percent by weight Cr, in the range of from about 0.2 to 2 percent by weight Cu, in the range of from about 0.8 to 3 percent by weight Mo, in the range of from about 7 to 12 percent by weight Ni, in the range of from about 13 to 17 percent by weight Co, and a remaining amount Fe; melting the combined ingredients to form a weathering steel mixture; and pouring the weathering steel mixture to form the cast part.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to ultra-high strength weathering steels (hereafter 15 “weathering steel”) and methods of making the same. Weathering steels of this invention generally comprise cobalt, nickel, chromium, copper, molybdenum, silicon, and carbon as principle alloying elements. The specific constitution of the alloying ingredients used to form steels of this invention, and the thermal treatments used for processing the same, provide an air castable steel having improved combined properties of strength, ductility, toughness, and atmospheric corrosion resistance when compared to conventional ultra-high strength steels, If desired weathering steels of this invention can be vacuum melted and poured directly into a mold (investment cast) or made into ingots that could be subsequently forged to further improve its toughness and fatigue properties.

A key feature of weathering steels of this invention is the ability to achieve such 25 performance improvements while intentionally avoiding use of reactive elements such as titanium and/or aluminum. The use of these elements is common in making typical maraging steels for the purpose of providing contributions in hardness. Therefore, the ability to provide a steel having a high level of hardness while intentionally avoiding the use of these elements was something unexpected, surprising, and not before thought possible.

These reactive elements are intentionally avoided for the purpose of allowing weathering steels to be air melted. Other alloying elements are judicially selected and/or adjustments are made to the overall balance of alloying ingredients used to make weathering steels to achieve the desired combination of mechanical properties. The strategic substitution of these elements avoids having to conduct multiple melt and pour cycles in a nonreactive environment, thereby enabling formation of weathering steels in a manner that is less expensive and more time efficient.

Weathering steels can be formed from the following atomic elements: Fe, Ni, Mo, Co, Cr, C, Si and Cu. As discussed above, a key feature distinguishing steels of this invention from conventional ultra-high strength steels, such as maraging steels or the like, is the intentional avoidance of the reactive elements titanium and aluminum.

Weathering steels of this invention comprise alloying elements provided in respective weight percent ranges as recited below in Table 3. It is to be understood that all provided values are approximate.

TABLE 3
PreferredMost Preferred
ElementExample RangeRangeRange
C0.16 to 0.230.17 to 0.220.18 to 0.21
Si0.2 to 1.50.3 to 1.20.5 to 0.9
Cr2.0 to 5.02.5 to 4.03.0 to 4.0
Cu0.2 to 2.00.5 to 1.50.7 to 1.5
Mo0.8 to 3.01.0 to 2.01.3 to 1.7
Ni 7.0 to 12.0 8.0 to 10.5 9.0 to 10.5
Co13.0 to 17.014.0 to 16.014.5 to 15.5
V0.0 to 2.00.0 to 0.50.02 to 0.05
W0.0 to 2.00.0 to 1.50.025 to 1.0
N 0.0 to 0.040.01 to 0.03 0.01 to 0.025

The balance is essentially iron, optional additions, and the usual impurities found in commercial grades of maraging steels. For example, weathering steels of this invention may include trace elements such as up to 0.01 pbwt 5, up to 0.01 pbwt P, and up to 0.04 pbwt N.

The element C is used for the purpose of increasing strength & hardenability of the alloy by heat treatment. Weathering steels of this invention comprise C in the range of from 0.16 to 0.23 pbwt. The element Si is used for the purpose of providing improved properties of castability and fluidity during processing of the steel. Weathering steels of this invention comprise Si in the range of from 0.2 to 1.5 pbwt. The element Cr is used for the purpose of improving the corrosion resistance, strength and hardenability of these alloys. Weathering steels of this invention comprise Cr in the range of from 2 to 5 pbwt.

The element Cu is used for the purpose of providing improved corrosion resistance and 5 weatherability to the finished steel product, it along with the Cr, Ni and Mo improve the alloys corrosion resistance. Weathering steels of this invention comprise Cu in the range of from 0.2 to 2 pbwt. The element Mo is used for the purpose of improving hardenability of HSS. Weathering steels of this invention comprise Mo in the range of from 0.8 to 3 pbwt. The element Ni is used for the purpose of adding strength and toughness to the alloy. Weathering steels of this invention comprise Ni in the range of from 7 to 12 pbwt. The element Co is used for the purpose of increasing strength. Weathering steels of this invention comprise Co in the range of from 13 to 17 pbwt.

Use of the alloying elements V, W and N is understood to be optional. The element V is used for the purpose of increasing hardness, or to enhance toughness at a given hardness level. Weathering steels of this invention comprise up to about 2 pbwt V. The element W is used for the purpose of providing improved properties of strength and hardness. Weathering steels of this invention comprise up to about 2 pbwt W. The element N is used for the purpose of stabilizing the austenite phase while adding some strength to the alloy. Weathering steels of this invention comprise up to about 0.04 pbwt N.

Weathering steels of this invention are rendered more robust against atmospheric corrosion by substitution of the highly reactive elements, e.g., aluminum and/or titanium, contained in conventional maraging or other ultra-high strength precipitation hardening alloys with more benign elements. Purposeful substitution of the highly reactive elements with less reactive elements, together with judicious levels of C and Si, result in the creation of an alloy that can be single melted and poured in air while exhibiting superior stability and castability (fluidity).

In a preferred embodiment, weathering steels of this invention are investment cast into a desired form. Preferably, the weathering steel is melted and poured in a single operation that, unlike conventional maraging steel, does not have to take place in a non-reactive environment. It will be appreciated that weathering steels, prepared according to the principles of the invention, can be readily air cast into complex configurations.

It should be understood that although air melting is preferably used, a protective atmosphere can also be used. For example, Argon, used as either a liquid or in a gas state, can be applied over the surface of the charge material during melting in order to enhance its castability. Additionally, during the first ten minutes or so of the solidification of the casting, if air is eliminated from around the cooling shell by placing a can over the cooling shell a better casting surface may be obtained. However, this is not a limitation on the invention.

Generally speaking, weathering steels of this invention display the following desired combined mechanical properties: (i) yield strength of greater than about 230,000 psi, preferably in the range of from about 255,000 to 300,000 psi; (ii) tensile strength of greater than about 250,000 psi, preferably in the range of from about 255,000 to 320,000; and (iii) elongation of over about 6 percent, preferably from 10 to 14 percent. In addition to these mechanical properties, weathering steels of this invention also display good properties of castability, fluidity, weldability, and resistance to atmospheric corrosion.

Weathering steels of this invention can be formulated to provide desired levels of 15 strength, toughness, elongation, and corrosion resistance within the above-noted ranges as needed to accommodate a particular steel application. The different levels of performance in these areas can be obtained by selective use of alloying ingredients and/or by varying the amounts of such alloying ingredients.

As noted above, weathering steels of this invention can be melted and poured in an air 20 environment, i.e., air melted, poured, and air cast into a desired configuration. After the part is cast, the typical process for processing the weathering steel includes the following steps: (a) high-temperature/high-pressure consolidation; (b) homogenation; (c) solution heat treating; (d) cooling; and (e) aging.

The step of high-temperature/high-pressure consolidation is preferably carried out by hot 25 isostatic pressing (hipping). During the hipping step, the investment cast product is placed in a special pressure vessel that is capable of heating the product to specific temperatures under inert gas pressures of over about 14,000 psi. The high temperatures and pressures used in this process cause the collapse of internal casting discontinuities within the product, such as voids, gas, or shrinkage porosity Thereby, resulting in a casting that has substantially lower internal defects and better mechanical properties.

The cast product is preferably Hipped at a pressure of about 15,000 psi, and at a temperature of greater than about 1,900° F. and, more preferably within the range of from about 1,900° F. to 2,250° F. in an inert gas-filled chamber, e.g., using argon gas. A pressure chamber is filled or charged with argon gas to a specific pressure range before the chamber is heated. The temperature within the chamber is raised to a desired temperature, causing the pressure within the chamber to increase to a desired level. The desired temperature is held for a period of from about 3 to 5 hours followed by cooling of the castings to room temperature. Products are normally inspected after hipping to evaluate pore closure.

The homogenation step is conducted in a neutral atmosphere at a temperature range of from about 2,000° F. and about 2,225° F. for a period of about I to 6 hours. The homogenation step serves to reverse and or neutralize the micro-structural changes produced during the hip cycle and minimizes any chemical segregation.

The solution heat treating step is conducted in a neutral atmosphere at a temperature in the range of from about 1,600° F. to 2,100° F. for a period of from about 1 to 4 hours, followed by rapid gas fan cooling to room temperature. The solution heat treating step serves to put the micro-structure in an Austenitic condition and by rapid cooling transforming the structure into Martensite.

The cooling step is conducted by refrigerating the room temperature cast product within 24 hours of completing the solution heat treat for a period of from about 1 to 4 hours at a temperature below about −50° F. preferably (−90° F. to −100° F.) to complete the transformation of the Austenite into Martensite. The aging step is conducted at a temperature range of from about 800° F. to 1,000° F. for a period of from about I to 6 hours to achieve the desired combination of above-described mechanical properties.

Table 4, provided below, sets forth preferred, more preferred and most preferred ranges of the processing parameters used for making weathering steel according to the principles of this invention. It will be understood that all values are approximate.

TABLE 4
Process StepPreferredMost PreferredMost Preferred
Hipping1,900° F. to2,000° F. to 2,240° F.2,125° F. to
2,250° F.@ 14,000@ 14,000 to 16,000 psi2,220° F. @ 14,000
to 16,000 PSI forfor 3 to 5to 16,000 psi
3 TO 5 HoursHoursfor 3 to 5 Hours
Homogenation2,000° F. to2,150° F. to 2,220° F.2,205° F. to
2,225° F. for 1 tofor 2 to 5 Hours2215° F. for 4
6 HoursHours ± 15 Minutes
Solution1,6000° F. to1,800° F. to 1,950° F.1,875° F. to
Heat Treating2,100° F. for 1 tofor 1 to 3 Hours1,925° F. for 1
4 HoursHour ± 16 Minutes
Cooling−350° F. to −50° F.−110°to −75° F. for−110° F. to −90° F.
for 1 to 8 Hours1 to 3.5 Hoursfor 3 Hours ± 15 Minutes
Aging800° F. 1000° F. for900° F. to 1,000° F.925° F. to 950° F.
1 to 6 Hoursfor 3 to 5 Hoursfor 4 Hours ± 15 Minutes

Three example embodiments of weathering steels of this invention were prepared according to the practice of this invention. These examples are designated, RNR-2, RNR-3, RNR-4, RMR-5, RNR-6 and RNR-2L in Table 5 provided below, These examples were prepared by combining alloying ingredients in the amounts presented in Table 5, and air melting, 25 pouring and casting the combined alloy mixture into a desired cast part.

The east part was hipped at a temperature of approximately 2,125° F. and a pressure of approximately 15,000 psi for a period of about 3 hours, The hipped part was homogenized for a period of about 4 hours at a temperature in the range of from 2,125° F. to about 2200° F., followed by solution heat treating at a temperature of about 1,900° F. for a period of about 1.5 hours. The solution heat treated part was then rapid gas fan cooled to room temperature, and subsequently refrigerated at a temperature of about −100° F. for a period of about I to about 3 hours to transform retained austenite to martensite. The part was then aged at a temperature of about 950° F. for a period of from 4 to 6 hours to achieve the desired mechanical properties.

The mechanical properties for the so-formed examples are also provided in Table 5 below:

TABLE 5
ALLOYRNR2RNR3RNR4RNR5RNR6RNR2L
C0.180.170.160.160.160.16
Mn0.020.010.010.010.010.02
Si0.710.580.550.60.620.56
Cr2.672.072.063.152.053.20
Ni10.079.919.879.779.989.74
Mo1.190.962.2852.160.970.94
Cu1.440.640.650.60.640.63
Co15.8514.9214.6814.4214.7814.67
W0.10.010.020.021.110.01
N0.0040.0040.0040.0040.0040.004
V0.230.010.010.010.010.01
MECH.TEST
DATA
YS(KSI)278251277278260234
UST(KSI)303279304305292267
% EL101169.3107
% RA363524303320

The example weathering steels of this invention were tested and were shown to provide properties of yield strength and tensile strength that are comparable to conventional vacuum melted wrought ultra-high strength stainless steel alloys. However, the example weathering steels of this invention were provided in the form of investment castings that were air melted and air cast.

Table 6 below provides data comparing the physical/mechanical properties of the example weathering steels prepared as described above with other conventional steels:

TABLE 6
Strength
toCorrosionCastability
ALLOYYS, PSIUTS, PSI% 5ELDensityWeightRangRating
RNR2278,000303000100.287105836248
Avg.
RNR3251000279000II0.28797371058
Avg.
RNR52780003050009.30.287106445058
Avg.
RNR6260000292000100.287101908058
Avg.
17-415930016700080.28259219988
Cast
17-4178000192000120.2826808518
Wrought
Super17000018500070.28265602895
steel C
Ti-6-4140000150000100.1693750010
sheet
Ti-6-412200013400070.16837500103
Avg.
Custom234000257000120.28291134872
465
13-8 PH210000225000120.28279787282
3043000070000350.28224822797
stainless
IC 3163500075000350.282265957106
IC 8620120000140000100.28050000015

According to the performance data set forth above, weathering steels of this invention display properties of yield strength and tensile strength that are both superior to other compared conventional steels, while at the same time providing properties of percent elongation and corrosion resistance that are comparable to the other compared conventional steels.

Accordingly, the present invention provides weathering steels that are air-castable and that are specially designed to overcome the noted drawbacks of conventional ultra-high strength steels discussed above. Weathering steels of this invention are more resistant to atmospheric corrosion than their martensitic and maraging counterparts, and offer a better combination of mechanical properties than most wrought hardened and precipitation hardened stainless steels, providing a yield strength of over about 230,000 psi and tensile strength of more than about 250,000 psi with over 10% elongation and 35% reduction in area.

Weathering steels of this invention exhibit excellent fluidity and castability, which permits their use in being cast into complex thin-walled configurations. Weathering steels are less reactive than other maraging steels, enabling air melting and pouring, and achieve the desired combination of mechanical properties without the need for multiple vacuum melting and wrought processing. The less reactive nature of such steels results in less metal-mold reaction and porosity. Additionally, weathering steels of this invention can be produced to near net shape directly via investment casting. These alloys are significantly cheaper and easier to obtain than conventional maraging alloys. They can achieve their desired mechanical properties without the need for high purity raw material. They can easily be recycled to reduce costs and help the environment. They exhibit better weldability than their low alloy martensitic counterparts (4140 or 300M alloys). They do not require chrome or cadmium plating for most applications, in comparison to conventional ultra high strength steels, thereby making them more environmentally friendly.

Weathering steels of this invention can be single melted, thereby significantly reducing 25 processing costs. They can be air melted and air cast into complex configurations, thereby reducing the capital expenditure needed for vacuum casting furnaces. As an air cast alloy, productivity (output per hour) is significantly higher than that associated with using vacuum melted alloys. Weathering steels require lower acquisition lead-times, faster production cycle times, and lower manufacturing costs. Due to their simpler processing and faster production throughput and cycle times (set up and nun times), less inventory is required. Since they achieve their properties without the need for wrought processing, they do not require progressive dies to achieve the desired shape and mechanical properties. Wrought processing and vacuum melting can improve their properties for certain applications, at the expense of acquisition and processing costs.

While specific examples have been disclosed and illustrated, it is to be understood that alloys produced in accordance with the principles of this invention can have one of a number of different chemical make-ups, depending on the particular application for the final product. Furthermore, it should be understood that the temperatures, times and pressures, and the like described herein are only exemplary, and that those skilled in the art will be able to configure the process and chemistry differently than disclosed and illustrated without departing from the spirit of this invention.