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 mechanical parts made of low carbon steel with high characteristics, such as ball joints of land vehicle wheels, pivots, axes, suspension triangles, rods, or other similar ready-to-use mechanical parts obtained by plastic deformation of '' a long steel product (wire, bar ).
• steels for plastic deformation must have properties of both deformability and resistance. Thus, during the manufacture of the mechanical parts for which some of them are intended, they must be able to withstand without breaking significant changes in shape while sometimes having in the end high mechanical characteristics. In fact, in certain cases, the required characteristics of parts obtained from these steels are close to those of class 10.9 according to ISO 898, namely a minimum breaking limit of 1000 MPa and a minimum elastic limit of 900 MPa. In addition, these steels must have good machinability characteristics, since a majority of applications require ultimate machining for final ribs.
• plastic deformation operations are carried out on pieces of steel resulting from the cutting of wires or bars conventionally obtained by hot rolling of semi-continuous casting products (billets or blooms).
• cold plastic deformation stamping, forging, etc.
• the pieces are cold formed using a press, if necessary after a globulation annealing, and the parts obtained are then heat treated by quenching and tempering.
• quenching and tempering For hot forging, the pieces are first heated to a temperature of around 1000-1200 ° C, hot formed and cooled. The parts thus obtained are then heat treated by quenching and tempering, the quenching can be done directly during cooling after forging.
• a bainitic structure which offers a good compromise between deformability and mechanical characteristics, as well as good machinability.
• the success of obtaining this bainitic structure is subject to the cooling constraints of the steel at heart, whether this cooling occurs before the plastic deformation or after. These constraints imposed on cooling are so severe on the steel grades currently known and used that this bainitic structure may not be obtained directly in the hot rolling, or even after the forging operation, so that many mechanical parts must undergo heat treatment after they are shaped.
• the objective of the invention is to provide transformers with a low carbon steel grade capable of developing a bainitic, or essentially bainitic, structure with low cooling stresses, for the manufacture of parts ready for use both by cold press and hot forging.
• the object of the invention is the development of a low carbon steel grade specific to the manufacture of mechanical parts provided with a bainitic or essentially bainitic structure which can already be obtained with a low cooling rate at the core, which can go down to 1 ° C / s, and offering both good deformability and good machinability for the production of these parts by cold or hot deformation, without post-heat treatment, said grade having high mechanical characteristics allowing said parts to be in quality classes 8.8 to 12.9 according to ISO standard 898.
• the subject of the invention is therefore a mechanical part with high characteristics in low carbon steel ready for use coming from the plastic transformation of a long rolled steel product, characterized in that: - the composition of said steel, in addition to iron and the inevitable residual impurities resulting from the production of steel, meets at least the following analysis, given in weight percentages relative to iron:
• - Said long product is obtained from a semi-product from continuous casting and hot rolled in the austenitic field, then heat treated to give it a bainitic, or essentially bainitic, structure, as well as shaped by plastic transformation to cold or hot to give it its final shape with a breaking strength greater than 800 MPa.
• the mechanical cold-deformed steel part defined above is characterized in that the long product from which it is obtained by plastic transformation is a rolled wire or bar heat treated by cooling in the hot rolling at a sufficient cooling rate to give it a bainitic or essentially bainitic structure.
• the mechanical hot-forged steel part defined above is characterized in that the long product from which it is obtained by plastic transformation is a bar or a rolled wire whose piece of forge which has been extracted therefrom has been heat treated by quenching under a cooling rate sufficient to give it a bainitic structure to the core, from a quenching temperature of the order of 1200 ° C. and above at which the slug has underwent a plastic transformation by forging bringing it to its desired final shape.
• the heat treatment involved in the development of the mechanical part comprises a final phase of cooling at low speed, which can drop to around 1 ° C / s, to the core.
• this cooling of the part is a gentle cooling, different in any case from a cooling operation which would quench the steel, which moreover would, in normal practice, be followed by tempering.
• the mechanical part is produced with a steel whose carbon content is between 0.06% and 0.10%.
• the mechanical part is made of steel, the molybdenum content of which does not exceed 0.30%, and that of manganese is less than 1.80%.
• the invention also relates to a method of manufacturing a mechanical part with high characteristics ready for use in low carbon steel having a breaking strength of more than 800 MPa, characterized in that it comprises the following steps : - from a long semi-finished product in low carbon steel, the composition of which, in addition to iron and the inevitable residual impurities resulting from the production of steel, meets at least the following analysis, given in weight percentages relative to iron: C ⁇ 0.15%
• the long rolled product obtained is then heat treated, this heat treatment comprising a final low-speed cooling phase, which can drop to around 1 ° C / s, - to the core to obtain a bainitic, or essentially bainitic structure, and said long product is plastically deformed to bring it to the desired final shape, the plastic deformation operation being able to be performed after or during said heat treatment.
• the invention also relates to a long steel product intended to obtain a mechanical steel part as defined above, characterized in that it is in the form of wire or hot-rolled bar and in that the The steel of which it is composed corresponds at least to the following analysis, given in weight percentages relative to iron:
• the invention in its essential characteristics, consists in the definition of an analysis of low carbon steel based on niobium, boron and molybdenum, which is specific to mechanical parts with high characteristics and able to develop a homogeneous bainitic (or essentially bainitic) structure in the mass of the part with few cooling requirements.
• This structure can in fact already be obtained from a low core cooling rate which can drop to around 1 ° C / s, a speed which can be reached, as we know, directly in the hot rolling mill - even for wires and bars with a diameter of around 20 mm and more depending on the installation.
• the invention opens up to large diameters the production range of long hot-rolled products intended for cold forging or forging workshops, and, for those reserved for hot forging, it provides the economy of a additional final heat-quenching heat treatment.
• the limit diameters are around 20 to 25 mm for the grades according to the invention.
• bainitic structure will denote a “bainitic or essentially bainitic structure”.
• Niobium acts in synergy with molybdenum and boron to widen the area of bainitic transformation. It increases the quenching effect of boron by increasing the effective boron content contained in the steel. Indeed, the formation of the carbides Fe 23 (CB 6 ) (trapping the boron and passive as for the hardenability of the steel) is made more difficult under the action of niobium which stabilizes the austenite and delays the diffusion of carbon. Furthermore, it makes it possible to increase the recrystallization temperature of the austenite which makes it possible to obtain a finer bainitic structure during controlled rolling, and thus to increase the resilience of the parts.
• Molybdenum is a carburizing element allowing to widen the bainitic domain by delaying the germination of ferrite.
• its action on the hardenability of the steel makes it possible to obtain a steel of higher mechanical strength by lowering the temperature at the start of bainitic transformation. It thus tends to compensate for the low carbon content necessary for obtaining good ductility.
• it acts in synergy with boron and with niobium, which it strengthens its role.
• it acts in synergy with niobium to increase the recrystallization temperature of austenite.
• Titanium is used to fix nitrogen and thus protect the boron. Without titanium, boron would lose its quenching power by reacting with nitrogen. Titanium also makes it possible to obtain a fine austenitic grain which improves the ability to cold forming and to ductility.
• sulfur combines with manganese to form plastic and ductile manganese sulfides. It provides good machinability. It is possible, if one wishes to improve machinability further, to increase its content up to a maximum value of 0.1% but not beyond it if one wishes to guarantee good deformability. Cold.
• This steel also has the inevitable impurities and residual elements resulting from its production, in particular phosphorus, the content of which must preferably remain below 0.02% to guarantee good ductility during and after setting. in cold form, as well as copper and nickel, the content of which should preferably be less than 0.30%.
• This optimized composition allows the steel to have a very good capacity for plastic deformation at the same time as good machinability. Indeed, this shade not only promotes the production of bainite, but also reduces the risk of obtaining martensite, the presence of which can constitute a serious obstacle to a good machining operation.
• An essential aspect of the invention is that the mechanical parts have a homogeneous bainitic structure in the mass at low cooling speed at the heart of the hot forged parts, or of the wires or bars from which they are obtained by cold striking, which can descend up to approximately 1 ° C / s.
• the mechanical part is cold struck (or cold forged)
• the bainitic structure is obtained before shaping.
• the steel after deformation, then has good ductility, measured by a necking much greater than 50%, a tensile strength greater than 650 MPa, and a mechanical resistance greater than 800 Mpa.
• the part is in fact obtained by cold plastic deformation of the steel already having a bainitic structure.
• a long semi-finished product consisting of an analysis steel in accordance with the invention is supplied which is hot-rolled, if necessary after reheating above 1100 ° C., according to the usual practice of hot rolling up to obtaining a laminated wire 10 mm in diameter for example.
• the wire removal temperature is less than 1000 ° C.
• the laminated wire obtained is then air-cooled in the rolling hot water itself in the usual way ("stelmor" process for example), at a low core speed which can drop to around 1 ° C / s to obtain a homogeneous bainitic structure.
• the rolled wire is then delivered (or deliverable) to the transformer in the form of a crown.
• the transformer who receives the crown unwinds the wire, erects it if necessary, before cutting it into pieces of the desired length. Each piece is then subjected to a usual cold plastic deformation operation to obtain the final ready-to-use part (ball joints, pins, rods, screws ...), after a nominal rib machining if necessary. .
• the final mechanical characteristics will naturally be obtained by the work hardening resulting from the shaping.
• the parts are hot deformed and the bainitic structure is obtained after this plastic deformation operation: a long semi-product consisting of an analysis steel in accordance with the invention is supplied which is rolled hot until a 30 mm laminated bar is obtained diameter for example. After possible cooling, the bar cut to length by cutting can be delivered rectilinear to the blacksmith with its ordinary metallographic structure acquired naturally during hot rolling.
• the blacksmith who receives it cuts it into pieces and each piece is then brought to a temperature of around 1200 ° C. before being subjected to a hot plastic deformation operation at the forge.
• the parts are then cooled in the usual way, in two stages, with a first controlled cooling down to a temperature below 1000 ° C and a second cooling at low core cooling rate which can go down to 1 ° C / s approx.
• the end of rolling conditions have no particular importance on obtaining the metallurgical structure, since the bainite, which gives the part most of its working properties, is reached at the very end, after hot forming and controlled cooling.
• the mechanical parts according to the invention are obtained by plastic deformation of laminated products without additional heat treatment of quenching and tempering.
• the billets from the casting were hot rolled after reheating above 1100 ° C to form a wire 12 mm in diameter.
• the wire deposition temperature after rolling was 820 ° C.
• the rate of cooling of the wire in the hot end of rolling was of the order of 5 ° C / s.
• a homogeneous bainitic structure is obtained over the whole of the wire, at the periphery as well as at the core.
• the mechanical parts with high characteristics according to the invention are remarkable in that they make it possible in particular to save the quenching and tempering treatments currently used during striking or cold forging or hot forging operations.
• machinability for example, in hot forging applications, a person skilled in the art may choose to improve the machinability by varying the sulfur content or by adding other agents favoring machining such as tellurium, lead or selenium.
• machining such as tellurium, lead or selenium.
• stamping applications or cold forging or hot forging the invention also applies to other plastic deformation applications such as wire drawing, 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
• 2003
  • 2003-11-27 JP JP2004556435A patent/JP5036967B2/ja not_active Expired - Lifetime
  • 2003-11-27 US US10/536,455 patent/US20070051434A1/en not_active Abandoned
  • 2003-11-27 DE DE60331163T patent/DE60331163D1/de not_active Expired - Lifetime
  • 2003-11-27 ES ES03796115T patent/ES2338227T3/es not_active Expired - Lifetime
  • 2003-11-27 AU AU2003298375A patent/AU2003298375A1/en not_active Abandoned
  • 2003-11-27 EP EP03796115A patent/EP1565587B1/de not_active Expired - Lifetime
  • 2003-11-27 AT AT03796115T patent/ATE456685T1/de not_active IP Right Cessation
  • 2003-11-27 WO PCT/FR2003/003516 patent/WO2004050935A1/fr active Application Filing

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