Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese

Definitions

• This invention relates to new steels having a duplex microstructure of bainite and martensite upon air-cooling after hot forming, as by casting or hot forging or rolling and exhibit high hardenability without quenching, together with high strength, toughness and wear resistant properties.
• Such characteristics suit the steels, for example, to the economical manufacture of structural and equipment parts, fasteners, and dies and other wear-resistant articles.
• Steels used for structural and wear-resistant applications include, for example, high manganese steels and certain medium carbon steels with or without the hardening and strengthening elements chromium, nickel or molybdenum--such as SAE 4140, SAE 3140 and SAE 1345.
• High manganese alloy steels are expensive and require complicated heat treatment to develop required properties. For example, such steels commonly are reheated, for example to around 1100°C, after hot working and then water quenched to form austenite. Heat treatment of SAE 3140, SAE 4140 and SAE 1345 steels also is complicated, requiring oil quenching and high temperature tempering. The strength, toughness and wear-resistance properties of the less expensive steels such as SAE 1345 are quite low.
• Use of such steels provides full section hardenability of bars with a cross-section diameter of at least 30 mm.
• hardenable diameter is commonly used to describe the maximum depth dimension throughout which an article is hardenable to a particular hardness level. This term refers to the diameter of a test specimen, normally in the form of a rod or bar having a uniform cross-section normal to the specimen length.
• Table 1 C Mn Si Cr Ni Mo B High Manganese Steel 1.0 to 1.4% 11 to 14% 0.3 to 0.9% SAE 3140 0.37 to 0.44% 0.5 to 0.8% 0.2 to 0.4% 0.45 to 0.75% 1.0 to 1.4% SAE 4140 0.38 to 0.45% 0.5 to 0.8% 0.2 to 0.4% 0.9 to 1.2% 0.15 to 0.25% SAE 1345 0.42 to 0.49% 1.4 to 1.8% 0.2 to 0.4% Chinese Appln. No. 86103008 0.31 to 0.46% 2.1 to 3.4% 0.1 to 1.5% 0.0005 to 0.005% Chinese Appln. No.
• Table 1 steels optionally may contain up to 1.5% of tungsten or chromium, up to 1% molybdenum and up to 0.15% vanadium.
• duplex bainite-­martensite steels of this invention contain, as essential elements, carbon, silicon, manganese and boron.
• the steels also contain chromium, although in one embodiment of the invention chromium may be omitted if the manganese, carbon and silicon contents are present in sufficiently large amounts to provide the desired structure and properties, as hereinafter described.
• the steels are useful either in the forged or rolled or in the cast condition, followed by air-cooling from above the austenitizing temperature, e.g. about 820-950 deg.
• the steels of this invention utilize only relatively small amounts of low-cost elements such as manganese, silicon and boron, and the element chromium which is relatively less scarce and expensive as compared to molybdenum and tungsten which are used in many prior art steels for such applications.
• a broad composition range of the new steels, in weight percent, is given in Table 2.
• a more limited range of the Table 2 steels includes at least 0.15% carbon, at least 0.10% silicon and at least 0.10% chromium.
• one or more other alloying elements optionally may be added as follows: element composition range, wt% W up to 1.5 Mo up to 1.0 V up to 0.15 S up to 0.2 Ca up to 0.1 Pb up to 0.1 Ti up to 0.1 rare earth elements up to 0.2
• Chromium preferably is provided in an amount of at least 0.6% and preferably over 1% and up to 2%, especially in steels containing under about 0.5% carbon. If chromium is omitted, or when it is present in an amount under 1%, a combined manganese and silicon content of at least about 3% is used; and the silicon content of such steels should be at least 0.6% where carbon is under about 0.5%, and at least about 0.8% where the carbon content of such low chromium or chromium-free steels is under 0.2%. Such proportioning of the elements, manganese, silicon and chromium, together with carbon and boron, provides enhanced hardenability in the present steels by air-cooling only.
• chromium is 1% or more and the steel composition is balanced as above-described, the hardenable diameter is at least 35 mm. Hardenable diameter up to about 80 to 100 mm. is achievable. If Cr is over 1.0% and Si is over 0.8%, in the lower or medium carbon ranges from 0.10 to about 0.46%, Rockwell hardnesses upwardly of about R c 20 to R c 40 or 50 are obtainable. As carbon content of the new steels is increased to the medium high range of 0.47 to 0.7%, attainable hardness of the steels exceeds R c 50 to R c 58.
• the steel composition can be varied within the above-described element ranges. Proper balance of carbon with other alloying elements provides a good combination of strength and toughness. If carbon is less than 0.10%, steel strength is too low; if higher than about 0.70%, toughness of the steel is too low. If carbon and chromium are too low, for example, below about 0.47% and 1% respectively, hardenability is adversely affected unless manganese and silicon are used in the minimum amounts above-described.
• Formation of bainite after air-cooling depends upon addition of the proper amounts of manganese and boron which influence the position of the time-temperature-transformation (the "T-T-T”) and the continuous-cooling-transformation (the "C-C-T”) curves of the steel.
• Hardenability of the steel also can be further enhanced by use of the optional element molybdenum which also aids in avoiding temper brittleness.
• the carbide-forming elements vanadium and titanium can be added for grain refinement.
• the new steels are easily machined. Machinability can be further enhanced by additions of sulfur, calcium or lead. Rare earths may be added for spheroidizing sulfide inclusions.
• compositional ranges are given in Tables 3 to 22, wherein the aforesaid principles are to be taken into account, including the described balancing of the required elements C, Cr, Si and Mn.
• Table 3 A composition as in Table 2 wherein the steels contain: element composition range, wt% C 0.10 to 0.25 Mn 2.1 to 2.7
• Table 4 A composition as in Table 2 wherein the steels contain: element composition range, wt% C 0.10 to 0.25 Mn 2.4 to 3.5
• Table 5 A composition as in Table 2 wherein the steels contain: element composition range, wt% C 0.10 to 0.25 Mn 2.1 to 2.7 Cr 0.1 to 1.5
• Table 6 A composition as in Table 2 wherein the steels contain: element composition range, wt% C 0.10 to 0.25 Mn 2.1 to 2.7 Cr 1.6 to 3.5
• Table 7 A composition as in Table 2 wherein the steels contain: element composition range, wt% C 0.10 to 0.25 Mn 2.4 to 3.5 Cr 0.1 to 1.5
• the low to medium carbon steels of Tables 3 to 14 are particularly useful for the manufacture of cast articles such as liner plates and shock plates of crushers and grinders, as well as rolled or forced structural and machine parts such as oil pump sucker rods, reinforcing rods; bolts, nuts and other fasteners, and automotive axles and connecting rods.
• the medium carbon steels of Tables 15-20 are useful, for example, in the production of gear racks, various springs, cutting and other elements for mining machines, dies, and wear-resistant pieces.
• the higher carbon steels of Tables 21-26 capable of hardening to over R c 50, are especially useful as applied, for example, to dies for plastics, rubber and metals, for grinding balls and rods, other wear-resistant pieces, and for hard-facing welding rods.
• Exemplary properties of these new steels are illustrated by the following: Steel Type Tensile Strength kg/mm2 0.2% Off-Set Yield Strength, kg/mm2 Impact Strength AK, KJ/M2 (U-notch) Hardness Rc Low Carbon 1 ⁇ 70 ⁇ 50 ⁇ 700 ⁇ 21 2 ⁇ 82 ⁇ 63 ⁇ 580 ⁇ 24 3 ⁇ 110 ⁇ 85 ⁇ 450 ⁇ 33 4 (free machining) ⁇ 70-110 ⁇ 50-83 ⁇ 700-450 ⁇ 21-40 Medium Carbon 1 ⁇ 130 ⁇ 120 ⁇ 300 ⁇ 40-50 2 ⁇ 90-130 ⁇ 70-120 ⁇ 300 ⁇ 30-50 Medium-High Carbon - - ⁇ 100 ⁇ 52 Casting Steel 1 ⁇ 120 - ⁇ 400 ⁇ 40 2 - - ⁇ 130 ⁇ 50 3 - - ⁇ 70 ⁇ 54 Welding Rod Steel - - - ⁇ 52
• the present steels can be smelted in oxygen-blown converters and in electric furnaces.
• casting temperature is in the range of about 1500° to 1650° C. After casting, the cast article is reheated and air-cooled and the casting used either directly or after tempering.
• Forging, rolling and other hot-forming of the new steels is carried out by heating the steel to or above the austenitizing temperature, for example, to about 1050° C to about 1250° C, finishing at a temperature over about 800° C, and air-cooling.
• Table 27 No. C Cr Si Mn B Mo V W S Ca Pb Ti 1. 0.10 0.8 0.7 2.8 0.002 2. 0.18 1.5 0.8 2.3 0.003 3. 0.20 2.0 1.5 2.5 0.002 0.08 0.09 4. 0.22 1.5 0.8 2.2 0.003 5. 0.25 1.6 0.8 2.9 0.001 0.07 0.09 6. 0.28 1.8 1.5 2.6 0.002 0.20 7. 0.29 1.6 0.7 2.4 0.002 8. 0.30 3.0 0.8 2.2 0.003 9. 0.30 1.8 1.0 2.3 0.002 0.3 10. 0.32 2.0 0.8 2.7 0.003 0.08 11. 0.34 2.5 0.6 2.9 0.001 0.07 0.09 12.
• Automobile springs and railway springs were made of steels with compositions as in Examples 14 to 24 of Table 27.
• Rods for fabrication of the springs were rolled or forged at 1200-850° C, subsequently cooled either in still air or by use of simple fan cooling, and then tempered in the range of 150 to 500° C. Thereafter, the rods were reheated to forging temperature, hot worked to final form, air-cooled and then tempered at 150 to 500° C. After such processing, the steels had a duplex bainite-martensite structure and exhibited yield strengths of at least 120 Kg/mm2 and tensile strengths of at least 130 Kg/mm2.
• the toughness and fatigue properties of these steels are exemplified in Tables 29 and 30.
• Table 29 Fracture Toughness property this invention (1) comparison steel (2) KIC (3) at least 280 Kg.mm -3/2 200 to 260 Kg.mm -3/2 KISCC (4) at least 110 Kg.mm -3/2 at least 98 Kg.mm -3/2 (1) Example No. 14 of Table 27. (2) 60Si2Mn (0.56-0.64% C, 1.5-2.0% Si, 0.6-0.9% Mn), quenched from 870°C in oil and tempered at 480-500°C. (3) KIC is fracture toughness. (4) KISCC is fracture toughness per stress corrosion cracking test (in 3% NaCl solution). Table 30 Fatigue Properties Test load, Kg/mm2 Fatigue Life, No.
• N maximum minimum this invention 100 10 9-12 X 104 comparison steel (2) 100 10 5-7 X 104 (1)
• Example No. 14 of Table 27 (2) 60Si2Mn, quenched from 870°C in oil and tempered at 480-500°C.
• ingots of the Table 28 compositions were forged or rolled at 850°C to 1250°C into the form of die blanks. After cold working, the dies were heated to austenitizing temperature, 800-950°C, and air-cooled and tempered. Bending strengths, ⁇ bb of at least 260 Kg.mm2 were obtained. Alternatively, the die blanks may be tempered to obtain a hardness of R c 35 to R c 40, and then machined to final shape in which form they can be directly used, without quenching or further tempering.
• the invention provides new steels having an excellent combination of hardenability, strength, toughness and fatigue- and wear-resistance. Due to their superior hardenability, the steels can be used for making various types of heavy machinery parts and other large size articles in either forged or cast condition.
• the steels are air-hardenable after hot working or casting. Hence, conventional quenching or quenching-tempering treatments are not needed. Amenability of the steels to various forming procedures during air-cooling after the Previous hot working (for example, in the production of large springs) combines the formation of bainite/martensite microstructure and other benefits of hot working.
• the new steels are useful in production of articles in which final forming is done by working the steel at a temperature below that previously used for hot-working the steel prior to air-cooling (cold working or semi-hot working).
• Steels wherein the carbon content is up to about 0.46% are particularly useful in this respect, especially in case of articles having relatively large thickenesses.
• Smaller section articles such as wire, for example, for reinforcing mesh or springs, may be made by cold-working, following hot-working and air-cooling, the steels of higher carbon contents within the above-described broad range.
• the inventive steels may be produced with lower hardness and strength than exhibited by the bainite-containing microstructure by cooling the hot worked steel more slowly than the cooling rate in still air, for example less than about 300°C per hour.
• the resulting, softer pearlite or pearlite plus ferrite structure is more easily cold worked than the harder, stronger bainite or bainite/martensite structure.
• these new steels are useful in the manufacture of cold heading wire and rod.
• the hot worked steel may be slowly cooled by known means in an environment reducing rate of heat loss from the cooling steel.
• the hot rolled rod may be laid in loop form on a conveyor which is insulated or to which heat may be added to suitably slow the cooling rate to an extent to provide the softer pearlite or pearlite/ferrite structure.
• products such as rolled or forged die blocks or flats, or fastener stock, can be slow cooled to avoid bainite formation. After cold working such articles, they may be heated above the austenitizing temperature and then air-cooled to form the hard, strong bainite or bainite/martensite structure.
• an article of the new steels having a pearlite or pearlite/ferrite structure can be heated and air-cooled to form a hard, strong bainite-containing surface.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

Applications Claiming Priority (4)

Publications (1)

Family

ID=25742440

Family Applications (1)

Country Status (2)

Cited By (6)

Citations (6)

• 1989
  • 1989-04-28 EP EP89107783A patent/EP0348633A1/fr not_active Withdrawn
  • 1989-05-02 BR BR898902473A patent/BR8902473A/pt not_active Application Discontinuation

Patent Citations (6)

Also Published As

Similar Documents

Legal Events