JP2016504498A - High strength precipitation hardening stainless steel - Google Patents

Abstract

A precipitation hardening martensitic stainless alloy steel is disclosed. The alloy steel is, in weight percent, C: about 0.03 or less, Mn: about 1.0 or less, Si: about 0.75 or less, P: about 0.040 or less, S: about 0.020 or less, Cr : About 10 to about 13, Ni: about 10.5 to about 11.6, Mo: about 0.25 to about 1.5, Cu: about 0.75 or less, Co: about 0.5 to about 1.5 , Ti: about 1.5 to about 1.8, Al: about 0.3 to about 0.8, Cb: about 0.3 to about 0.8, B: about 0.010 or less, N: about 0.8. Including 030 or less, the balance being iron and common impurities. The alloy according to the present invention has an excellent combination of strength, toughness and corrosion resistance.

Description

  The present invention relates to precipitation hardening type martensitic stainless alloy steel, and more particularly to martensitic stainless alloy steel having a novel combination of strength and corrosion resistance and an article made from the alloy.

The aerospace industry has long sought stainless steel alloys for landing gear. The main alloy currently used in civilian landing gear applications is the 300M alloy. The 300M alloy can be quenched and tempered to have a tensile strength (or maximum tensile strength) of at least 280 ksi and a fracture toughness (K _Ic ) of at least 50 ksi√in. However, 300M alloy does not have effective corrosion resistance. Accordingly, it was necessary to plate the landing gear components with a corrosion resistant metal such as cadmium. Cadmium is a highly toxic and carcinogenic substance and there are significant environmental risks associated with its use in the manufacture and maintenance of aircraft landing gear and other parts made from 300M alloys.

  Precipitation hardening stainless steel alloys having a combination of commercially acceptable strength and toughness are known and used in various aerospace applications. However, some of these alloys do not have the same strength as 300M and cannot be considered as “drop-in” instead of 300M alloy. Other known precipitation hardening stainless steels have adequate strength for landing gear applications, but are not sufficient in their corrosion resistance. Corrosion resistance required for aircraft landing gear applications includes general corrosion resistance, pitting corrosion resistance and stress corrosion resistance.

  In view of the above, there is a need for an alloy steel that has mechanical properties equivalent to 300M in various environments where civilian aircraft are used, thereby being used as an alternative drop-in and combining effective corrosion resistance.

  Most of the disadvantages associated with known precipitation hardened martensitic stainless steel alloys are solved by the alloys according to the invention. The alloy according to the present invention is a precipitation hardening type Cr—Ni—Ti—Mo martensitic stainless alloy steel having a combination of excellent (or unique, unique) strength, toughness and corrosion resistance.

合金の残部は、原則的に、このような鋼の商用の鋼種において見られる一般的な不純物（または、通常の不純物、usual impurities）を除くと、鉄と、千分の数パーセントの量から、当該合金が備える好ましい特性の組み合わせに悪影響を及ぼさない程度のより多くの量まで変動してもよい少量の追加元素とである。 The broad composition range, intermediate composition range and preferred composition range of the alloys according to the invention are given in weight percent as follows:

The balance of the alloy, in principle, from iron and a few thousandths of an amount, excluding common impurities (or usual impurities) found in commercial grades of such steels, A small amount of additional elements that may vary to a greater amount that does not adversely affect the preferred combination of properties that the alloy comprises.

  The foregoing table is provided as a brief overview and is therefore not intended to limit the upper and lower limits of each elemental range of the alloys of the present invention for use in combination. It is not intended to limit the range of elements used solely or in combination. Thus, a wide composition range of one or more elements and a preferred range of one or more other elements may be used together. Furthermore, a wide range of minimum or maximum amounts of one element of a preferred embodiment can be used for the minimum or maximum amount of the same element of other preferred embodiments. Further, the alloy according to the present invention may include the constituent elements described above and constituent elements described throughout the present application, and may basically be composed of these elements, or may be composed of these elements. Also good. In this specification, the symbol “percent” or “%” means% by weight or% by mass unless otherwise specified.

  The alloy according to the invention comprises an excellent combination of strength, toughness and corrosion resistance resulting from a novel balance (or balance) of elements of chromium, nickel, cobalt, molybdenum and elements of titanium, aluminum and columbium. . At least about 10%, preferably at least about 10.5%, more preferably at least about 11.0% chromium is present in the alloy and provides corrosion resistance similar to conventional stainless steel. Since nickel improves the strength and notch toughness of the alloy, at least about 10.5%, preferably at least about 10.75%, more preferably at least about 10.85% nickel is present in the alloy. Nickel also contributes to corrosion resistance by improving the repassivation ability of the alloy. Since cobalt contributes to the high strength and corrosion resistance provided by the alloy, the alloy comprises at least about 0.5% cobalt, preferably at least about 0.75%, more preferably at least about 0.9% cobalt. Since molybdenum contributes to the notch toughness of the alloy, the alloy has at least about 0.25%, preferably at least about 0.75%, more preferably at least about 0.9% molybdenum in the alloy. Molybdenum also improves the corrosion resistance of the alloy in the reducing medium and the corrosion resistance of the alloy in an environment that promotes pitting and stress corrosion cracking.

  The alloys of the present invention also contain at least about 1.5% titanium and have a beneficial effect on the strength of the alloy through precipitation of the nickel-titanium rich phase during aging. Columbium and aluminum also contribute to the strength of the alloy. Accordingly, the alloy includes at least about 0.3%, preferably at least about 0.4%, respectively, of columbium and aluminum. Preferably, the alloy includes at least about 0.45% aluminum.

  If chromium, nickel, cobalt, molybdenum, titanium, columbium and aluminum are not properly balanced, the ability of the alloy to completely transform into a martensitic structure is suppressed using conventional manufacturing techniques. Furthermore, the ability of the alloy to remain substantially martensite substantially when subjected to solution treatment and age hardening decreases. Under such conditions, the strength of the alloy is significantly reduced. Therefore, the amount of chromium, nickel, cobalt, molybdenum, titanium, columbium and aluminum in the alloy is limited. More specifically, chromium is limited to about 13% or less, preferably about 12.5% or less, more preferably 12.0% or less. Nickel is limited to about 11.6% or less, more preferably about 11.25% or less. Excess cobalt adversely affects the strength and toughness of the alloy. Accordingly, cobalt is limited to about 1.5% or less, preferably about 1.25% or less, more preferably 1.1% or less. Molybdenum is limited to about 1.5% or less, preferably about 1.25% or less, more preferably 1.1% or less.

  Excess titanium adversely affects the toughness and notch toughness of the alloy. Accordingly, titanium is limited to about 1.8% or less, preferably about 1.7% or less in the alloy. Excess aluminum adversely affects the toughness and corrosion resistance of the alloy. Therefore, aluminum is limited to about 0.8% or less, preferably about 0.7% or less, more preferably 0.65% or less in the alloy. Excess columbium is prone to undesired alloy segregation and undesired secondary phase precipitation such as the Laves phase. Accordingly, columbium is limited to about 0.8% or less, preferably about 0.7% or less, and more preferably 0.6% or less in the alloy.

  Additional elements such as manganese, silicon and boron may be present in controlled amounts so as to improve other desirable properties that the alloy comprises. More specifically, from the scrap source and deoxidation additive, about 1.0% or less, preferably about 0.5% or less, more preferably about 0.25% or less, and further preferably about 0.10% or less. Of manganese and / or about 0.75% or less, preferably about 0.5% or less, more preferably about 0.25% or less, and even more preferably about 0.10% or less of silicon is present in the alloy as a balance. May be. Such additions are beneficial if the alloy is not melted in vacuo. Manganese and / or silicon are preferably maintained at low concentrations in the matrix material because they adversely affect toughness, corrosion resistance and austenite-martensite phase equilibrium.

  In order to improve the hot workability of the alloy, no more than about 0.010% boron, preferably no more than about 0.005% boron, more preferably no more than about 0.0035% boron is present in the alloy. Also good. At least about 0.001%, preferably at least about 0.0015% boron may be present in the alloy to provide the desired effect.

  The balance of the alloy is basically iron, except for the common impurities that are inevitably found in commercial alloy types intended for similar services or applications. The concentration of such a plurality of elements is controlled so as not to adversely affect the desired characteristics.

  In particular, excess carbon and / or nitrogen reduces the corrosion resistance of the alloy and adversely affects the toughness of the alloy. Thus, about 0.03% or less, preferably about 0.02% or less, more preferably about 0.015% or less of carbon may be present in the alloy. Also, about 0.030% or less, preferably about 0.015% or less, more preferably about 0.010% or less of nitrogen may be present in the alloy. When carbon and / or nitrogen is present in large amounts, the carbon and / or nitrogen combines with titanium, aluminum, and / or columbium, and is an undesirable non-metallic material such as carbide, nitride, and / or carbonitride. Form inclusions. These reactions suppress the formation of an intermetallic phase of nickel-titanium / aluminum / columbium, which is a major factor in improving the high strength of the alloy.

  Phosphorus is maintained at a low concentration because it adversely affects toughness and corrosion resistance. Accordingly, about 0.040% or less, preferably about 0.015% or less, more preferably about 0.010% or less of phosphorus is present in the alloy.

  About 0.020% or less, preferably about 0.010% or less, more preferably 0.005% or less of sulfur is present in the alloy. Large amounts of sulfur, like carbon and nitrogen, promote the formation of non-metallic sulfide inclusions that suppress the desired strengthening effect provided by titanium, aluminum and columbium. These sulfide inclusions reduce the toughness of the alloy, especially the shear direction toughness.

  By selecting high-purity charge materials (or charge materials) or by utilizing alloy smelting techniques, sulfur and phosphorus can be reduced to very low concentrations, but their alloys The presence in it cannot be completely avoided under large scale production conditions. Thus, a small amount of calcium may be added in a controlled amount to combine with phosphorus and / or sulfur to facilitate removal and stabilization of these two elements in the alloy. Calcium is also used to deoxidize the alloy. When used, the amount of calcium remaining in the alloy is about 0.010% or less, preferably about 0.005% or less. As an alternative to calcium treatment, one or more rare earth metals (REM), especially cerium and lanthanum, may be added to the alloy. In this regard, the alloy may include at least about 0.001% REM, and preferably at least about 0.002% REM. Excess REM recovery adversely affects the hot workability and toughness of the alloy. Excess REM content also causes the formation of unwanted oxide inclusions in the alloy. Accordingly, the amount of REM present in the alloy is limited to about 0.025% or less, preferably about 0.015% or less, more preferably 0.010% or less. Further, it is considered that magnesium is added as an alternative to calcium or REM for desulfurization and deoxidation.

  Excess copper adversely affects the notch toughness, ductility and strength of the alloy. Accordingly, the alloy contains no more than about 0.75% copper, preferably no more than about 0.50%, more preferably no more than about 0.25% copper.

  No special technique is required in the melting process, casting process or processing process of the alloy of the present invention. Vacuum induction melting (VIM), and vacuum induction melting followed by vacuum arc remelting (VAR) are the preferred methods of melting and smelting the alloy, but other methods may be used. Further, the alloy may be made by powder metallurgy techniques as needed. Furthermore, the alloy of the present invention may be hot worked or cold worked, but cold working improves the mechanical strength of the alloy.

  A preferred method of supplying calcium into the alloy is to add a nickel-calcium compound during VIM. Nickel-calcium compounds, such as Ni-Cal® alloys sold by Chemalloy Co. Inc., are added in amounts that can effectively bind available phosphorus, sulfur and oxygen. Other techniques for adding calcium may be used. For example, elemental calcium capsules or calcium master alloys may be added to the melt. It is contemplated that slag containing calcium or calcium compounds may also be used. This chemical reaction results in the formation of secondary phase inclusions such as calcium sulfide, calcium oxide and calcium oxysulfide that are readily removed during primary and secondary dissolution. When used, REM melts in the form of misch metal, which is a mixture of multiple rare earth elements (eg, containing about 50% cerium, about 30% lanthanum, about 15% neodymium and about 5% praseodymium). Added to the alloy.

  The precipitation hardened alloy of the present invention is manufactured in a multi-step process so as to improve the desired combination of properties. In the first step, the alloy is solution annealed. The temperature of solution annealing is chosen to be high enough to ensure that essentially all the undesirable precipitates are dissolved in the alloy matrix material and that the granular structure is fully recrystallized. The Unrecrystallized grains can cause an increase in anisotropy of the mechanical properties (particularly ductility and toughness) of the alloy. However, if the solution annealing temperature is too high, the fracture toughness of the alloy is reduced by promoting excessive crystal grain growth. Preferably, the alloy according to the present invention is in the range of 1850 ° F. to 1950 ° F. (1010 ° C. to 1066 ° C.) and any precipitates are substantially completely dissolved in the alloy matrix and the grain structure Is solution annealed for a time sufficient to completely recrystallize. The time at the solution temperature depends on the thickness of the part. The alloy is then quenched, preferably oil quenched.

  A low temperature treatment (or refrigeration treatment) may be performed after quenching to further improve the high strength of the alloy. The low temperature treatment cools the alloy to a temperature well below the martensite transformation end temperature to ensure complete martensitic transformation. Preferably, the low temperature treatment includes cooling the alloy to −100 ° F. (−73 ° C.) or less for a sufficient time to ensure that the alloy is substantially completely transformed into martensite. The need for low temperature treatment is influenced, at least in part, by the martensitic transformation end temperature of the alloy. If the end temperature of the martensite transformation is sufficiently high, the transformation to the martensite structure can proceed without the need for low-temperature treatment. Furthermore, the need for low temperature processing may also depend on the cross-sectional dimensions of the parts being manufactured. As the cross-sectional area of the part increases, segregation in the alloy becomes more pronounced and the use of low temperature treatment becomes more effective. Furthermore, for large parts, it is necessary to lengthen the time for cooling the parts so as to complete the transformation to martensite. For example, in order to improve the high strength which is a characteristic of this alloy, it has been found that a low temperature treatment which lasts about 8 hours or more is preferable.

  The alloys of the present invention are age hardened by techniques known to those skilled in the art that are used in known precipitation hardening stainless steel alloys. For example, the alloy has sufficient time to ensure that the alloy is heated substantially uniformly at about 950-975 ° F. (510-524 ° C.) to an aging temperature that depends on the thickness of the part. In addition, it is generally preferred to age for an additional 4 to 8 hours to complete the aging reaction and reach the desired combination of strength and toughness. The specific aging temperature used is (1) the maximum tensile strength (or tensile strength) of the alloy decreases as the aging temperature increases, and (2) the alloy has the desired strength as the aging temperature decreases. It is selected taking into account that the time required to age harden to the level increases.

  The alloys according to the present invention can be formed into various product shapes for a wide variety of applications and are suitable for billet, bar, rod, wire, strip, plate or sheet forms used in general practice. . The alloys of the present invention are useful in a wide range of practical applications that require alloys having an excellent combination of corrosion resistance, strength and toughness. In particular, the alloys of the present invention can be used to manufacture structural parts for aircraft, including but not limited to landing gear parts and fasteners. The alloys are also well suited for use in medical and dental applications such as dental instruments and medical scrapers, cutters and suture needles.

Example A comparative test was conducted to demonstrate a novel combination of strength, toughness and corrosion resistance with the alloys according to the present invention. Seven 35 pound heats with the weight percent composition shown in Table 1 below were made by VIM.
(Table 1)

  The balance of each heat is iron and common impurities. Examples 1 and 2 are typical of alloys according to the present invention. Examples A to E are alloys as comparative examples. In particular, Example A is within the alloys disclosed in US Pat. No. 5,681,526.

  The VIM heat was melted and cast into a 4 inch square ingot. The ingot was then charged into a furnace operating at 1500 ° F and the furnace temperature was raised to 2300 ° F. The ingot was held at 2300 ° F. for 16 hours, after which the furnace temperature was reduced to 2000 ° F. The ingot was held at 2000 ° F. until the ingot temperature was substantially completely uniform. The ingot was then forged both ends into a 2-3 / 4 inch square billet from a starting temperature of 2000 ° F. (or double end forged) and then hot cut into three pieces. The piece was reheated at 2000 ° F. and forged at both ends to a 1-1 / 4 inch square. The bar was again hot cut into three pieces and reheated at 2000 ° F. The bar was then forged (or single end forged) to 11/16 squares without reheating. The bar was cooled in air, overaged (or overage annealed) at 1250 ° F. for 8 hours, and then air cooled.

Long, flat and notched (K _t = 3) tensile, longitudinal Charpy V-notched (CVN) and longitudinal rising step load (RSL) fracture toughness samples are Roughly processed from the bar. Samples from Examples 1, 2, B, C, D, and E were solution treated and oil quenched at 1900 ° F. for 1 hour. The sample from Example A was solution treated at 1800 ° F. by conventional means for the alloy. After solution treatment, all samples were refrigerated (or refrigerated) for 24 hours at −100 ° F. and then warmed to room temperature in air. The samples were then age hardened at various temperatures ranging from 900 ° F to 1000 ° F. Aging was performed by holding the sample in air for 4 hours at that temperature and then quenching the sample in water.

（表２Ｂ）

The results of a tensile test at room temperature for each heat sample were expressed as 0.2% offset yield strength (YS) and tensile strength (UTS) (unit: ksi), elongation ( % EL.), Cross-sectional shrinkage (% RA), and notch tensile strength (NTS) (unit is ksi), shown in Tables 2A and 2B below.
(Table 2A)

(Table 2B)

Rockwell C-scale hardness (HRC) and Charpy V-notch impact, showing aging temperature, feet pound (ft.-lbs) results for Charpy V-notch impact energy (CVN) for Examples 1, 2 and D The energy (CVN) is shown in Table 3 below. The CVN test was performed according to ASTM standard test procedure E23.
(Table 3)

Rising step load (RSL) samples for plane strain fracture toughness testing and stress corrosion cracking (SCC) testing were finished from the age hardened bars of Examples 1, 2, A and D. Two samples from each heat were tested in air to give a fracture toughness value (K _IC ). Additional samples were tested at room temperature in a 3.5% NaCl solution at a raw pH (natural pH) to define a critical stress intensity factor value (K _ISCC ). The test was conducted on a test machine that meets the standards of the ASTM Standard Test Procedure (or Standard Test Procedure) E1290. The room temperature fracture toughness test (K _IC ) and stress corrosion cracking test for Examples 1, 2, A and D were determined to be plane strain fracture toughness (K _IC ) in units of ksi√in and criticality in units of ksi√in. It is shown in Table 4 below, including the stress intensity factor values (K _ISCC ). In _KISCC , a record of each process section and a final value are recorded. According to standard test procedures, the lowest value measured for each example is expressed as the final value of K _ISCC . Tensile strength values for each example are also recorded in Table 4, indicating that fracture toughness and stress corrosion cracking were measured for alloys having the same level of strength.
(Table 4)

Two salt spray corrosion test cones (or cones) were machined from the bars of Examples 1, 2, A and D after age hardening. Cone-type samples were made by turning and hand polishing and finished to 600 grit. Prior to testing, all cones used for the salt spray were passivated with 20% nitric acid and 3 oz./gallon sodium dichromate for 30 minutes at a temperature of 120-140 ° F. Samples were run according to ASTM B117 with a 5% NaCl concentration, raw pH, and an experimental time of 200 hours at 95 ° F. The time until the first rusting was recorded for all samples, as well as the final evaluation after the end of the 200 hour test period. The results of the salt spray test are shown in Table 5 below, including the time until the first rust appears on the surface of the sample and the final evaluation after the test time is complete. Evaluation is defined as follows: 1: no rust appeared, 2: 1-3 rust appeared, 3: less than 5% rusted, 4: 5-10% rusted 5: 10-20% rusted, 6: 20-40% rusted, 7: 40-60% rusted, 8: 60-80% rusted, 9:80 More than% rusted.
(Table 5)

Cyclic polarization (pitting potential) samples were machined from the aged bars of Examples 1, 2, A and D. Scans for measuring pitting resistance were performed on two samples each from the examples. Samples were tested in a 3.5% NaCl solution at raw pH at room temperature and were not passivated but washed before testing. The test was performed according to the improved ASTM standard test procedure G61 described below. The voltage value at the protrusion of the curve (or knee) and the anticorrosion potential were measured for all samples. The results of the potentiodynamic pitting test (or potentiodynamic pitting test) are shown in FIG. 6 below, including pitting and anticorrosion potentials in millivolts (mV).
(Table 6)

Steel products made from the alloys described above and steel products processed by the processing steps described above include aircraft landing gear and other structural parts of aircraft (including but not limited to flap tracks and slat tracks), and For other applications that require both high strength and high corrosion resistance, it has a combination of properties that are particularly useful. In particular, steel products made from solution treated and quench hardened alloys, as described above, have a tensile strength of at least 280 ksi when tested on a test machine that meets the standards of the Standard Test Procedure E1290. And a fracture toughness (K _Ic ) of at least 45 ksi√in. The steel product of the present invention is also characterized by having a V-notch Charpy impact energy of at least about 4 ft-lbs when tested according to ASTM standard test procedure E23. Furthermore, the steel product according to the present invention is characterized by general corrosion resistance, and does not rust even when tested according to ASTM standard test procedure B117. The steel product according to the present invention is characterized by sufficient pitting corrosion resistance, and when tested according to the improved ASTM standard test procedure G61, the steel product has a pitting potential of at least 62 mV. The ASTM G61 test method was improved by using round bar samples rather than flat samples. The use of a round bar sample is considered a more rigorous test than the standard test G61 method, exposing the end grains.

  The terms and expressions used herein are explanatory terms and not restrictive terms. The use of these terms and expressions is not intended to exclude those features that have the same meaning. It will be appreciated that various modifications can be made within the scope of the invention described and claimed herein.

Claims (26)

In weight percent
C about 0.03 or less Mn about 1.0 or less Si about 0.75 or less P about 0.040 or less S about 0.020 or less Cr about 10 to about 13
Ni about 10.5 to about 11.6
Mo about 0.25 to about 1.5
Cu about 0.75 or less Co about 0.5 to about 1.5
Ti about 1.5 to about 1.8
Al about 0.3 to about 0.8
Cb about 0.3 to about 0.8
B Precipitation hardened martensitic stainless alloy having a combination of strength, toughness and corrosion resistance, characterized by comprising about 0.010 or less, N about 0.030 or less, and the balance being iron and general impurities steel.   The alloy steel of claim 1, comprising less than about 0.50% copper.   The alloy steel of claim 1, comprising at least about 0.75% cobalt.   The alloy steel of claim 1, comprising at least about 0.4% columbium.   The alloy steel of claim 1, comprising at least about 10.75% nickel.   The alloy steel of claim 1, comprising at least about 0.4% aluminum.   The alloy steel of claim 1, comprising at least about 10.5% chromium.   The alloy steel of claim 1, comprising about 12.5% or less chromium.   The alloy steel of claim 1, comprising less than about 1.7% titanium.   The alloy steel of claim 1, comprising about 1.25% or less of molybdenum.   The alloy steel of claim 1, comprising at least about 0.75 wt% molybdenum. The alloy steel of claim 1, comprising about 0.003% or less calcium. The alloy steel of claim 1, comprising about 0.025% or less cerium. In weight percent
C about 0.02 or less Mn about 0.25 or less Si about 0.25 or less P about 0.015 or less S about 0.010 or less Cr about 10.5 to about 12.5
Ni about 10.75 to about 11.25
Mo about 0.75 to about 1.25
Cu about 0.50 or less Co about 0.75 to about 1.25
Ti about 1.5 to about 1.7
Al about 0.4 to about 0.7
Cb about 0.4 to about 0.7
B about 0.001 to about 0.005
N Precipitation-hardening martensitic stainless alloy steel having an excellent combination of corrosion resistance, strength and toughness, characterized in that it contains about 0.015 or less and the balance is iron and general impurities.   The alloy steel of claim 14, comprising about 1.1% or less cobalt.   The alloy steel of claim 14, comprising at least about 0.9% cobalt.   The alloy steel of claim 14, comprising at least about 10.85% nickel.   The alloy steel of claim 14, comprising less than about 0.6% columbium.   The alloy steel of claim 14, comprising at least about 0.45% aluminum.   15. The alloy steel of claim 14, comprising less than about 0.65% aluminum.   The alloy steel of claim 14, comprising at least about 0.9% molybdenum.   The alloy steel of claim 14, comprising about 0.003% or less calcium.   The alloy steel of claim 14, comprising about 0.025% or less of cerium. In weight percent
C about 0.015 or less Mn about 0.10 or less Si about 0.10 or less P about 0.010 or less S about 0.005 or less Cr about 11.0 to about 12.0
Ni about 10.85 to about 11.25
Mo about 0.9 to about 1.1
Co about 0.9 to about 1.1
Cu about 0.25 or less Ti about 1.5 to about 1.7
Al about 0.45 to about 0.65
Cb about 0.4 to about 0.6
B about 0.0015 to about 0.0035
N Precipitation hardening type martensitic stainless alloy steel having an excellent combination of corrosion resistance, strength and toughness, characterized in that it consists essentially of about 0.010 or less and the balance is iron and general impurities.   25. The alloy steel of claim 24, comprising no more than about 0.003% calcium.   25. The alloy steel of claim 24, comprising no more than about 0.025% cerium.