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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite

Definitions

• the invention relates to low-carbon steel mechanical components with elevated characteristics, such as wheel swivel joints of terrestrial vehicles, pins, shafts, suspension bars, links, or other ready-for-use analogous mechanical components obtained by plastic deformation of a long steel product (wire, rod . . . ).
• steels for plastic deformation must exhibit characteristics of both deformability and strenght. Thus, during the manufacture of mechanical components for which some of them are intended, they have to be able to endure major changes in shape without rupture, and at times exhibit good mechanical properties in the finished product. In fact, in certain cases, the required properties of the components obtained from these steels are close to those of Class 10.9 according to Specification ISO 898, i.e. a minimum tensile strength of 1000 MPa and a minimum yield strength of 900 MPa. Moreover, these steels must exhibit good machinability characteristics, as a majority of the applications requires a final finishing to meet the end dimensions.
• plastic deformation operations are carried out on blanks that result from cut wires or rods that have usually been obtained by hot rolling semi-finished products from continuous casting (billets or blooms).
• cold plastic deformation stamping, forging . . .
• the blanks are shaped cold in a press, optionally after a spheroidiziation annealing step, and the resulting components are then thermally treated by quenching and tempering operations.
• hot forging the blanks are first reheated to a temperature of about 1000-1200° C., shaped whilst hot and cooled down.
• the resulting components are then thermally treated by quenching and tempering operations, the tempering operation being able to be done immediately on cooling after forging.
• the object of the invention is to provide converters with a grade of low-carbon steel, capable of developing a bainitic or essentially bainitic structure with few constraints regarding cooling, for the manufacture of ready-for-use components by both cold stamping and hot forging.
• the object of the invention is to develop a grade of low-carbon steel specifically for the manufacture of mechanical components possessing a bainitic or essentially bainitic structure, which can be already obtained with low cooling rates at the core—as low as 1° C./s—and offering not only good deformation properties, but also a good machinability for the manufacture of components by cold or hot deformation, without a heat treatment after forming, said grade having high mechanical properties such that said components will meet the quality requirements of Classes 8.8 to 12.9 of ISO specification 898.
• an object of the invention is a ready-for-use, low-carbon steel, mechanical component with good characteristics, obtained by plastic transformation of a rolled, long steel product, characterized in that:
• composition of said steel, apart from the iron and the unavoidable, residual impurities that result from the steel process complies with the following analysis, given in percentages by weight, based on the iron:
• said long product being obtained from a semi-finished product from continuous casting and hot-rolled in the austenitic range, then treated thermally to obtain a bainitic or essentially bainitic structure, and formed by a cold or hot plastic transformation into its final shape, exhibiting a tensile strength at break greater than 800 MPa.
• the steel, mechanical component deformed by a cold process defined above is characterized in that the long product, from which it is derived by plastic transformation, is a rolled wire or rod, treated thermally by cooling during the rolling process, at a cooling rate sufficient to provide it with a bainitic or essentially bainitic structure.
• the steel mechanical component, hot forged as defined above is characterized in that the long product, from which it is derived by plastic transformation, is a rod or a rolled wire, whose forged blank (extracted there from) was treated thermally by quenching at a cooling rate sufficient to provide it with a bainitic or essentially bainitic structure through to the core, this from a quenching temperature of about 1200° C. and more, at which the blank was subjected to a plastic transformation by forging, bringing it to the final desired shape.
• the heat treatment used in the manufacture of the mechanical component comprises a final slow cooling step, whose rate can be as low as 1° C./s at the core.
• said cooling of the component is a mild cooling, different in all respects from a cooling step for quenching the steel, which anyway would be followed in normal practice by tempering.
• the mechanical component is made with a steel whose carbon content is comprised between 0.06% and 0.10%.
• the mechanical component is made with a steel whose molybdenum content does not exceed 0.30% and a manganese content of less than 1.80%.
• Another object of the invention is a process for manufacturing a ready-for-use, low-carbon steel, mechanical component with good characteristics, exhibiting a tensile strength at break of more than 800 MPa, characterized in that it comprises the following steps:
• the resulting long, rolled product is then treated thermally, this heat treatment comprising a final phase of slow cooling at a rate that can be as low as approx. 1° C./s at the core, to obtain a bainitic or essentially bainitic structure, and said long product is subjected to plastic deformation into its final shape, the plastic deformation step being able to be carried out after or during said heat treatment.
• a further object of the invention is a long steel product intended for transformation into a steel mechanical component such as defined above, characterized in that it has the shape-of a hot-rolled wire or rod and that the steel, which forms it, at least complies with the following analysis, given in percentages by weight, based on the iron:
• the essential characteristics of the invention consist of the definition of an analysis of a low-carbon steel, based on niobium, boron and molybdenum, which is specific to mechanical components with elevated characteristics and capable-of developing a homogeneous bainitic (or essentially bainitic) structure in the mass of the component with few constraints regarding cooling.
• this structure can already be obtained by means of a low rate of cooling at the core which can be as low as about 1° C./s, the latter being a rate that can be achieved, as one knows, directly in the as-rolled conditions of rolling for wires and rods with a diameter of the order of 20 mm and more, depending on the installations.
• the invention opens up production for the line of hot-rolled long products of large diameters, destined for stamping workshops or cold forges, and for those allotted to hot forges, it saves an additional final heat treatment of quenching/tempering.
• the limits of diameters are about 20 to 25 mm for the grades according to the invention.
• bainitic structure refers to a “bainitic or essentially bainitic structure”.
• carbon From 0.02 to 0.15%, and preferably 0.08% of carbon. At these levels, carbon produces a bainitic structure with the required mechanical properties. It affords a good aptitude towards strain hardening during cold plastic deformation. Its low level also avoids any formation of large carbides that are unfavorable to ductility, without the need for any treatment of spheroidization.
• Niobium acts in synergy with molybdenum and boron to extend the range of bainitic conversion. It increases the quenching effect of boron by increasing the active boron content in the steel. Indeed, the formation of carbides Fe 23 (CB 6 ) (trapping the boron and passive towards the steel quenchability) is made more difficult under the influence of niobium, which stabilizes the austenite and delays the diffusion of carbon. Moreover, it increases the crystallization temperature of the austenite, thus promoting the appearance of a more finer bainitic structure during controlled rolling, and thereby increases the resilience of the components.
• Molybdenum is a carburigenic element enabling the bainitic range to be extended by retarding the germination of ferrite.
• its action on the quenchability of the steel enables a steel to be obtained with a superior mechanical resistance by lowering the onset temperature of the bainitic conversion. It thereby tends to compensate for the low carbon content required to obtain a good ductility.
• it acts in synergy with boron and niobium by reinforcing their action.
• it acts in synergy with niobium to increase the recrystallization temperature of the austenite.
• Manganese From 1.30 to 2.00%, and preferably between 1.60 and 1.80% of manganese.
• Manganese also enables an adequate quenchability, facilitates the formation of bainite and enables the mechanical properties to be obtained.
• silicon From 0.10 to 1.30%, and preferably from 0.20 to 0.35% silicon. At these levels, it facilitates a moderate hardening of the steel. If necessary, the level may be increased to 1.30%, particularly for increasing the mechanical strength of the steel. Silicon also enables the steel to be deoxidized during the casting.
• Titanium fixes nitrogen and thereby protects the boron. Without titanium, the boron would lose its quenching power by reacting with the nitrogen. Titanium also permits the formation of a fine austenitic grain, thereby improving the cold shaping and the ductility.
• This residual, dissolved aluminum, resulting from the steel killing prior to casting, is a good deoxidizer to protect the titanium against oxidation by the unavoidably present dissolved oxygen, so that the titanium remains available to protect the boron against nitrogen.
• This aluminum also serves to control the enlargement of the austenitic grain during hot rolling of the semi-manufactured starting product, thereby lending good resilience properties to the steel.
• sulfur combines with manganese, forming plastic and ductile manganese sulfides. It enables a good machinability. Should the machinability need to be increased further, it is possible to increase the sulfur content up to a maximum of 0.1%, but no greater, in order to retain good cold deformation properties.
• This steel also contains the unavoidable impurities and residual elements resulting from its manufacture, notably phosphorus, the content of which should preferably remain less than 0.02%, to guarantee a good ductility prior to and after the cold forming, as well as copper and nickel, whose contents should preferably be less than 0.30%.
• This optimized composition enables the steel to have a very good aptitude for plastic deformation as well as a good machinability. In fact, this grade not only favors the formation of bainite, but also reduces the risk of forming martensite, whose presence can constitute a serious obstacle to a good machining step.
• the molybdenum content can be limited to 0.30% and that of manganese to 1.80% in order to obviate any risk of a quench structure of the martensite type appearing in certain cases due to the local conditions.
• One essential aspect of the invention is that the mechanical components exhibit a homogeneous bainitic structure in the mass with a low rate of cooling (that can be as low as about 1° C./s) at the core of the hot-forged components, or of the wires or rods that are made from them by cold stamping.
• the mechanical component is cold stamped (or cold forged), the bainitic structure is obtained prior to shaping. Then, after deformation, the steel possesses a good ductility, measured by a reduction in cross-section largely superior than 50%, a tensile strength greater than 650 MPa, and a mechanical strength greater than 800 MPa.
• the component is actually obtained by cold plastic deformation of the steel, which already exhibits a bainitic structure.
• a long semi-finished product, constituted by steel whose analysis conforms with the invention, is supplied and hot-rolled, if needed after reheating above 1100° C., using the customary hot-rolling technique until a rolled wire is obtained with a diameter of 10 mm, for example.
• the removal temperature of the wire is less than 1000° C.
• the resulting rolled wire is then cooled in air in the terminal area of the rolling mill itself, according to the customary manner (“Stelmor Process”, for example) at a low rate at the core, which can be as low as approx. 1° C./s, to obtain a homogeneous bainite structure.
• the rolled wire is then delivered (or deliverable) to the converter customer in the form of a reel.
• the converter who receives the reel of wire, straightens it out if needed, before cutting it up into blanks of the required length.
• Each blank is then subjected to the customary step of cold plastic deformation so as to obtain the final ready-for-use component (swivel joints, shafts, links, pins . . . ) after machining to size, when needed.
• the final mechanical properties are obtained naturally by the annealing, resulting from the shaping.
• the component is deformed hot and the bainitic structure is obtained after this step of plastic deformation: a long semi-finished product, constituted by steel, whose analysis conforms with the invention, is supplied and hot-rolled until a rolled rod is obtained with a diameter of 30 mm, for example. After optional cooling, the rod is cut up into lengths and is deliverable in straight lengths to the forge with its ordinary metallurgical structure resulting naturally from the hot rolling.
• the smith customer who receives it cuts it up into blanks and each blank is then brought to a temperature of about 1200° C. before being subjected to a step of hot plastic deformation in the forge.
• the components are then cooled in the customary manner, in two steps, with a first controlled cool down to a temperature of less than 1000° C. and a second cool at a low cooling rate at the core, which can be as low as approx. 1° C./s.
• the conditions at the end of rolling are not particularly important for the resulting metallurgical structure, because the bainite, which lends the component the crux of its end-use properties, is realized after the hot shaping and the controlled cooling.
• the billets from the casting were hot rolled after reheating above 1100° C., to form a wire of 12 mm in diameter.
• the removal temperature of the wire after rolling was 820° C.
• the rate of cooling of the wire in the terminal area of the rolling mill was the order of 5° C./s.
• a homogeneous bainitic structure was obtained for the whole wire on its periphery as well as at the core.
• the mechanical properties of the wire were as follows: Rm (MPa) Rp 0.2 (MPa) A (%) Z (%) 857 683 17.4 71.4
• the change in mechanical properties as a function of deformation of the wire is as follows: Reduction (%) Rm (MPa) Rp 0.2 (MPa) A (%) Z (%) 20 960 885 13.7 67 35 1030 982 13 65.5 50 1100 1020 11.5 61.5 60 1160 1115 10.8 60.5 75 1265 1220 10.6 57.7
• the mechanical components with good characteristics according to the invention are remarkable in that, in particular, they allow the quenching and tempering treatments, which are actually carried out during stamping or cold forging or hot forging, to be dispensed with.
• the person skilled in the art can choose to improve the machinability by varying the sulfur content or by adding other agents that improve the machinability, such as tellurium, lead or selenium.
• the invention is more particularly directed to applications of stamping or cold forging or hot forging, it also applies to other applications of plastic deformation, such as wire drawing, deep drawing, stamping, etc . . . .

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
• Forging (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Coating With Molten Metal (AREA)
• Carbon And Carbon Compounds (AREA)

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• 2002
  • 2002-11-27 FR FR0214838A patent/FR2847592B1/fr not_active Expired - Lifetime
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  • 2003-11-27 AT AT03796115T patent/ATE456685T1/de not_active IP Right Cessation
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