FR2847592A1 - Steel for the fabrication of high performance precision mechanical components by either hot or cold deformation processes, has a bainitic structure - Google Patents

Abstract

A steel for the fabrication of a mechanical component by hot or cold plastic deformation has the following composition, in addition to iron and production impurities, (by wt %): (a) 0.02 at most C at most 0.15; (b) 1.3 at most Mn at most 2.0; (c) 0.04 at most Nb at most 0.10; (d) 0.10 at most Mo at most 0.35; (e) 0.001 at most B at most 0.005; (f) 0.15 at most Si at most 1.30; (g) 0.01 at most Al 0.08; (h) N at most 0.015 with Ti at least 3.5xN; (i) possibly up to 0.80 of chromium and/or up to 0.1 of sulfur. Independent claims are also included for: (a) the fabrication of a mechanical component by cold plastic deformation; (b) the fabrication of a mechanical component by hot plastic deformation; (c) a ready for use steel mechanical component.

Description

Steel for cold or hot deformation, mechanical part

  ready to use with this steel and its manufacturing process.

  The invention, which is in the field of metallurgy, relates to steels for forming by plastic deformation, cold or hot, for the manufacture of mechanical parts ready for use. Specifically, it relates to cold-forging or hot forging steels of high-precision mechanical components, such as land vehicle wheel bearings, pivots, axles, suspension triangles, rods, or other similar mechanical parts. as well as

  wire and metal bars allowing, after processing, obtaining such parts.

  It is known that steels for plastic deformation must have properties of both deformability and strength. Thus, during the manufacture of the mechanical parts to which they are intended, they must be able to withstand without major changes of shape while at the same time having high mechanical characteristics. In fact, the required characteristics of the parts obtained from these steels are close to those of class 10.9 according to the ISO 898 standard, namely a minimum breaking limit of 1000 MPa and a minimum yield strength of 900 MPa. In addition, these steels must have good machinability characteristics because a majority of the applications require ultimate machining to put into operation.

at the final ribs.

  It is recalled that the plastic deformation operations are on steel slugs from the cutting son or bars conventionally obtained by hot rolling of continuous casting products (billets or blooms). In cold stamping, the slugs 25 are cold-formed in the press, if necessary after a spherical annealing, and the parts obtained are then thermally treated by quenching and tempering. For hot forging, the plots are heated first to a temperature of about 1200 0C, shaped hot and cooled. The parts thus obtained are then thermally treated by quenching and tempering, the quenching being made directly during

cooling after forging.

  Achieving these different heat treatments requires operations, certainly controlled, but nevertheless expensive, whose intended results are not always achieved and which, in any case, increase the time and cost of production. Also, have we sought in recent years steel grades to overcome and obtain parts "ready for use", which can be used for the intended application without having to undergo a heat treatment to modify their metallurgical structure after the plastic deinking operation.

  Regarding the cold stamping, it is already known, for example, to use steel shades of essentially bainitic structure, that is to say containing more than 50% of bainite, having a good compromise between deformability and final mechanical characteristics. . However, given the capacity of the cooling means generally available on a hot rolling line, these grades make it possible to obtain an essentially bainitic structure only on relatively small diameter wires or rolled bars, rarely exceeding 8 mm in diameter. made. Beyond this, a degenerate or associated bainite with ferrite is obtained, which leads to a marked deterioration of the mechanical properties of the rolled products. In addition, the structure is not well controlled, there is a risk of high dispersion of mechanical characteristics within the same ring or between several rings son or bar wound after the hot rolling. Similar problems are encountered with steel grades for hot forging for which the thickness of the forging often imposes severe cooling constraints to achieve the core cooling rate necessary to obtain the desired bainitic structure. Moreover, since the periphery of the part is inevitably cooled much more strongly than the core, the result is

  internal tensions that can lead to permanent deformations.

  It can thus be seen that, in plastic deformation applications, a bainitic structure which offers a good compromise between deformability and mechanical characteristics, as well as good machinability, is conventionally sought in plastic deformation applications.

  In any case, the success of obtaining this bainitic structure is subject to the cooling constraints of the core steel, whether this cooling takes place before the plastic deformation or after. Since these imposed cooling constraints are important on the steel grades currently known and used, this structure may not be obtained directly in the hot rolling mill, or even after the forging operation, so that many mechanical parts must

  undergo an off-line heat treatment after their shaping.

  The object of the invention is to provide a steel grade which makes it possible to obtain a bainitic or essentially bainitic structure with low cooling stresses, both for the manufacture of parts by cold press and forge forging. hot.

  More specifically, the object of the invention is to present a steel grade

  allowing to obtain a bainitic or essentially bainitic structure already with a very low core cooling rate, of only 0.5 ° C / s, and offering both a good deformability and a good machinability in view of the manufacture of mechanical parts by cold or hot deformation, without post-processing heat treatment, said grade further offering high mechanical characteristics allowing said parts to be in the quality classes 8.8 to 10.9 according to the ISO 898 standard.

  For this purpose, the subject of the invention is a steel for the manufacture of a steel part shaped by cold or hot plastic deformation whose chemical composition includes, in addition to iron and the inevitable residual impurities coming from the development of steel, at least (in weight content relative to iron):

0.02% <C <0.15%

  1.30% <Mn <2.00% 0.04% <Nb <0.10% 0.10% <Mo <0.35%

0.001% <B <0.005%

  0.15% <Si <1.30% 0.01% <Al <0.08% N <0.015% with a Ti content of about 3.5 times this nitrogen content,

  and optionally up to 0.8% chromium and / or up to 0.1% sulfur.

  The invention also relates to a first method of manufacturing a ready-to-use metal part, shaped by cold plastic deformation, characterized in that it comprises the following successive stages: product, preferably derived from continuous casting, and consisting of a steel according to the analysis given above, the end-of-rolling temperature being less than about 1000 ° C, the rolled product obtained being preferably in the form of wire or bar diameter less than about 25 mm; to give it a bainitic or essentially bainitic structure, said rolled product is cooled (in the hot rolling itself, preferably) with a core cooling rate of at least 0.5 ° C / s;

  - And, usually later, is then cut in this cooled rolled product at least one piece, which is plastically deformed cold in the usual manner to obtain the piece ready for use in the desired shape.

  According to the invention, a second method for manufacturing a ready-to-use metal part by shaping by hot plastic deformation is characterized in that it comprises the following successive stages: product, preferably derived from continuous casting and consisting of a steel according to the analysis given above, the laminated product obtained being optionally cooled, and preferably being in the form of bar diameter greater than 20 mm; - And, subsequently most often, is then cut in said rolled product at least one piece which is plastically deformed hot until a piece having the desired shape; and, said part is cooled with a core cooling rate of

  less 0.5 'C / s to give it a bainitic or essentially bainitic structure, and thus obtain a ready-to-use part.

  It will be noted that this cooling of the part is a gentle cooling, different in any case from a cooling operation which would soak the steel, which

  moreover, in normal practice, it would be followed by income.

  The subject of the invention is also a wire or bar made of rolled steel for plastic deformation, characterized in that it consists of a steel of chemical composition according to the analysis given above.

  Finally, the subject of the invention is a ready-to-use steel part, obtained by plastic deformation, characterized in that it is obtained from a billet derived from a

  wire or rolled bar corresponding to the chemical analysis given above.

  As will probably be understood, the idea underlying the invention consists in the design of an analysis of a steel for cold or hot plastic deformation to obtain a bainitic (or essentially bainitic) structure. ) homogeneous with few cooling stresses. This structure is indeed obtained already from a core cooling rate of only about 0.5 'C / s, a speed that can be easily achieved, as we know, directly in the hot rolling itself for wires and bars of diameter of the order of 20 mm and more depending on the installations.

  Therefore, the invention opens up to the large diameters the production range of the factory intended for cold knapping workshops, and for those reserved for the forge, it provides the economy of a final tempering heat treatment-income . To better fix the ideas, we note that with the usual hot, the diameter limits are located

  around 20 to 25 mm for the shades according to the invention.

  Vocabulary habits in the profession make "rolled wire or small bars" products rolled in diameters up to about 25 mm (which is often packaged in the form of crowns for delivery to processors). ); - and "bars" those rolled from 20 mm in diameter and which are delivered rectilinear

  after cutting to length at the exit of the train.

  The invention will be well understood and other aspects and advantages will appear more clearly in view of the detailed description which follows, given as an example embodiment.

  At the steelworks, by continuous casting, semifinished products (billets or blooms) from a steel having, in addition to iron, the following composition, in weight content relative to iron: From 0.02% to 0.15%, and preferably 0.08%, of carbon. Carbon at this level serves to obtain a bainitic structure having the required mechanical properties. It provides a good work hardening ability during cold plastic deformation. Its low content also makes it possible to avoid the formation of large carbides unfavorable to

  ductility without the need for a globulization treatment.

  From 1.30% to 2.00%, and preferably between 1.60 and 1.80%, of manganese. This manganese then makes it possible to obtain sufficient quenchability, assists in the formation of bainite and makes it possible to obtain the desired mechanical characteristics.

  From 0.04% to 0.10%, and preferably 0.06 to 0.08%, of niobium. At this content, niobium makes it possible to promote the production of a bainitic structure in the

  hot rolling by significant lowering of the nucleation rate of ferrite.

  This effect is further reinforced by the presence of boron with which it acts synergistically to expand the bainitic transformation domain. From 0.10% to 0.35%, and preferably from 0.15 to 0.30% molybdenum. It is recalled that molybdenum increases the quenchability and promotes the transformation of

  austenite in bainite. It increases the quenching power of boron and niobium.

  0.001% to 0.005% boron. Boron inhibits the nucleation of ferrite thus promoting the formation of a fine bainitic structure. It acts in synergy with the

  niobium to expand the bainitic domain.

  From 0.10% to 1.30%, and preferably from 0.20% to 0.35%, of silicon. At these contents, it allows a moderate hardening of the steel. It can go up to a content of 1.30% if necessary, in particular to increase the mechanical strength of

  steel. Silicon also makes it possible to deoxidize the steel during casting.

  From 0.007% to 0.009% of nitrogen, associated with a titanium content of the order of 3.5 times this nitrogen content for sacrificial screen for the benefit of boron. Nitrogen controls grain size and increases tensile strength, hardness and machinability. Titanium is used to fix nitrogen and thus protect 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 cold forming ability and

to ductility.

  Less than 0.08% aluminum. This residual dissolved aluminum, coming from the quenching of the steel before casting, is a good antioxidant of protection of the titanium against the oxidation by the dissolved oxygen inevitably present, so that this titanium remains available to protect the boron against the nitrogen. This aluminum also serves to control the magnification of the austenitic grain during the hot rolling of the semi-finished product,

  and thus to give the steel good properties of resilience.

  Preferably from 0.001% to 0.1% sulfur. This sulfur combines with manganese to form plastic and ductile manganese sulfides. It makes it possible to obtain a good aptitude for shaping by cold plastic deformation and a

  good machinability. It is possible, if one wishes to further improve the machinability, to increase its content up to a maximum value of 0.1% but not beyond it if one wants to ensure good deformability Cold.

  Optionally, this grade may include from 0.10% to 0.80% chromium. This component at the content in question makes it possible to adjust the quenchability and to obtain the desired mechanical properties without too hardening the steel or lead to the formation of martensite. This steel also presents the inevitable impurities and residual elements resulting from its production, in particular the phosphorus whose content must remain

  preferably less than 0.02% to ensure good ductility during and after cold forming, as well as copper and nickel, the content of which should preferably be less than 0.25%.

  This optimized composition allows the steel to have a very good plastic deformation ability at the same time as a good machinability. Indeed, this shade not only promotes obtaining bainite, but also reduces the risk of obtaining martensite, the presence of which can be a serious obstacle to a good machining operation.

  An essential aspect of the invention is that this grade makes it possible to obtain a homogeneous bainitic structure on wires and bars for cold deformation or on hot forged parts as soon as the core cooling of these wires, bars or mechanical parts reaches 0.5 'C / s.

  When, in accordance with an implementation of the invention, the bainitic structure is obtained before forming mechanical parts (cold-forging steel) the steel, before deformation, has a good ductility, measured by a necking much greater than 50%, a tensile strength greater than 650 MPa, and a mechanical strength greater than 750 MPa.

  In this first embodiment, the bainitic structure is in fact obtained prior to the plastic deformation operation: a long half-product consisting of an analysis steel according to the invention is supplied and is hot-lined. , if necessary after reheating above 1100 ° C, until a rolled wire 10 mm in diameter for example. The end of rolling temperature is less than 1000 ° C. The laminated wire obtained is then cooled in air, in the hot rolling itself in the usual manner, at a minimum heart rate of 0.5 ° C / s, which is easily achievable, and which does not necessarily need to go beyond, to get

  a homogeneous structure essentially bainitic.

  The rolled wire is then delivered (or deliverable) to the transformer as a wound coil. The transformer receiving the crown unwinds the wire, raises it if necessary, before cutting it into pieces of the desired length. Each billet is then subjected to a usual operation of cold plastic deinking to obtain the final piece ready for use (ball joints, shafts, rods, etc.), after a nominal setting machining if necessary. The final mechanical characteristics will naturally be obtained by the work hardening resulting from the shaping. It will be noted that in this embodiment, it is the end-of-rolling conditions which make it possible to obtain an optimal action of the niobium in favor of the bainitic transformation already carried out at the rolling stage. . In a second embodiment, the bainitic structure is obtained after the plastic deformation operation: a long half-product consisting of an analysis steel according to the invention is supplied which is then hot-rolled to obtain 40 of a rolled bar 30 mm in diameter for example. After possible cooling, the cut-to-length bar is available straight to the blacksmith with its ordinary ferrito-pearlitic structure that it naturally acquires during rolling.

hot.

  The blacksmith who receives it delivers it in pieces and each piece is then brought to a temperature of about 1200 ° C before being subjected to a hot plastic deformation operation at the forge. The parts are then cooled in the usual manner, in two stages, with a first controlled cooling to a temperature below 1000 ° C and a second cooling with a minimum core cooling rate of 0.5 ° C / s. In this embodiment, the end-of-rolling conditions do not have any particular importance on the obtaining of the metallurgical structure, since the bainite, which gives the part the essential of its properties of use, is reached. at the end, after the hot forming The invention is intended, it is recalled, the manufacture of mechanical parts by plastic deformation of rolled products without heat treatment

1 5 additional.

  Laboratory tests were carried out on a casting of following composition:% C% Mn% Nb% Cr% B% Mo% Ti% N2% Sil% S% AI

  0.08 1.6 0.08 0.2 0.003 0.2 0.029 0.006 0.25 0.004 0.028

  The billets from the casting were hot rolled after reheating above 1100 ° C to form a 12 mm diameter wire. The end of rolling temperature was 820 ° C. The cooling rate of the wire in the hot end of rolling (cooling air blown type "stelmor") was of the order of 50C / s. A homogeneous bainitic structure, with more than 90% bainite, is obtained over the entire

  wire, periphery as at heart.

  The mechanical characteristics of the wire are the following: Rm (MPa) Rpo.2 (MPa) A (%) Z (%)

857 683 17.4 71.4

  Remember that: - Rm: represents the breaking strength corresponding to the maximum force before

  break reported to the initial section of the wire.

  - RpO.2: represents the conventional yield strength corresponding to the force

  relative to the initial section of the wire causing a plastic elongation of 0.2%.

  - A: represents the elongation at break.

  Z: represents the necking corresponding to the section reduction of the wire after breaking.

  The evolution of the mechanical characteristics as a function of the rate of deformation underwent by the yarn is as follows: Reduction rate (%) Rm (MPa) Rpo.2 (MPa) A (%) Z (%)

960 885 13.7 67

1030 982 13 65.5

1100 1020 11.5 61.5

1160 1115 10.8 60.5

1265 1220 10.6 57.7

  The invention is remarkable in that it makes it possible in particular to save the income treatments currently used during cold-knapping or hot forging operations.

  On the other hand by imposing less drastic cooling conditions,

  it reduces the risk of deformation of the parts during the cooling operation, or equivalent cooling fluid it allows the use of larger diameter plots.

  It is also remarkable for the very good machinability characteristics that it has, which makes it possible in cold applications to reduce the sulfur contents and thus to limit the detrimental influence of this element in the deformability aptitude. .

  It goes without saying that the invention can not be limited to the examples just described, but that it extends to multiple variants and equivalents in the measure

  o is respected its definition given by the appended claims.

  Thus, for example, in hot forging applications, those skilled in the art may choose to improve machinability by varying the sulfur content or by adding other machining promoting agents such as nickel or carbon. selenium. Similarly, although intended more particularly for cold stamping or hot forging applications, the invention is also applicable to other plastic deformation applications.

  such as drawing, drawing, stamping, etc.

Claims (7)

  1) Steel for the manufacture of a ready-to-use steel mechanical part, shaped by hot or cold plastic deformation, characterized in that its chemical composition, in addition to iron and the inevitable residual impurities resulting from the at least the following analysis, given in percentages by weight relative to iron: 0.02% <C <0.15% 1.3% <Mn <2.0% 0, 04% <Nb <0.10% 0, 10% <Mo <0.35% 0, 001% <B <0.005% 0.15% <If <1.30% 0.01% <AI <0.08% N <0, 015% with Ti> 3.5x% N   and optionally up to 0.80% chromium and / or up to 0.1% sulfur.   2) A method of manufacturing a ready-to-use mechanical steel part, shaped by "cold" plastic deformation, characterized in that it comprises the following successive stages: - a semi-finished product is hot-rolled consisting of a steel of chemical composition according to the analysis given by claim 1, the end of rolling temperature being less than 1000 ° C; said rolled product is cooled with a core speed of at least 0.5 ° C./s, so as to obtain a bainitic or essentially bainitic structure; - Then, is cut in said rolled product a piece which is plastically deformed cold until a piece ready to use having the shape and   the required mechanical characteristics.   3) Process according to claim 2, characterized in that the product is cooled laminated in the hot rolling.   4) Process according to claim 2 or 3, characterized in that it is hot rolled   a half-product until a rolled product in the form of a wire or bar of diameter less than about 25 mm.   ) A method of manufacturing a ready-to-use steel mechanical part, shaped by plastic deformation, characterized in that it comprises the following successive steps: - a semi-product consisting of a steel of chemical composition according to the analysis given by claim 1; - Then cut a piece in the rolled product obtained; said billet is brought to a temperature of approximately 1200 ° C., which is then plastically deformed while hot to form a part in the final form required; and said part is cooled to a core cooling rate of at least 0.5 ° C./s in order to thereby give it a bainitic or essentially bainitic structure and thus obtain a ready-to-use piece having the shape and the   mechanical characteristics required.   6) Process according to claim 5, characterized in that one hot rolled a   half-product until a product rolled bar diameter greater than or equal to 20 mm.   7) Hot rolled product for hot or cold plastic deformation, characterized in that it consists of a steel bar or wire of composition   chemical compound confers to the analysis of claim 1.   8) Mechanical piece of ready-to-use steel obtained by hot or cold plastic deformation, characterized in that it is obtained from a rolled product. hot water according to claim 7.