EP1725689A2

EP1725689A2 - Forged or stamped average or small size mechanical part

Info

Publication number: EP1725689A2
Application number: EP05739467A
Authority: EP
Inventors: Marie-Thérèse PERROT-SIMONETTA; Mario Confente
Original assignee: Mittal Steel Gandrange SA
Current assignee: ArcelorMittal Gandrange SA
Priority date: 2004-03-18
Filing date: 2005-03-16
Publication date: 2006-11-29
Also published as: FR2867785A3; FR2867785B3; WO2005100618A3; WO2005100618A2

Abstract

The inventive average or small size mechanical steel part is produced by hot forging or cold stamping i.e. by plastic processing of a long ferrous semi-product which is obtainable by continuous casting and hot rolling in the austenitic phase and, afterwards is shaped by plastic deformation and heat treated in order to obtain a metallographic structure substentially containing an acicular ferrite at least in mechanical toughness and fatigue stress areas. The composition of said steel, apart from iron and inevitable residual impurities resulting from steel production, corresponds at least to the following analysis: 0.2-0.5 % C, 0.5-2.0 % Mn, 0.05-0,5 % V, 0.6 1.5 % Si, 0.05 1.0 % Cr, 0.01-0.5 % Mo, 0.02-0.10 S, preferably from 0.01 to 0.02 % Ti and/or up to 0.20 % Al, and from 5 to 30 ppm of Ca.

Description

Medium or small size mechanical part from forging or striking.

The invention relates to mechanical parts of medium or small size made of micro-alloyed medium carbon steel, such as wheel hubs, connecting rods or ball joints for cars, or other similar mechanical parts obtained by hot or cold plastic deformation of a long steel semi-finished product and for which we are looking above all for fatigue resistance and toughness properties. By medium or small size, we mean here pieces whose diameter does not exceed approximately 80 mm. To produce such parts, it is known to use specially alloyed steels to obtain a metallographic structure of the bainitic or essentially bainitic type. By “essentially”, it is conventionally necessary to understand 80% and more by volume of the bainitic structure at the location of the room where this structure is sought. Their manufacture indeed requires being able to withstand without breaking or cracking significant shape changes while ultimately having good brittle breaking strength (toughness) and fatigue in view of the repetitive stress cycles to which the parts are subjected in use, as well as shocks (high resilience). Furthermore, these steels must have good machinability characteristics in order to allow precise final ribs by machining the part ready for use required in many applications. Conventionally, the manufacturing process can take a plastic deformation operation when cold (striking or forging), or hot (forging), the choice of hot or cold way often being made according to the final size of the parts. In all cases, this operation will be carried out on pieces of steel cut from bars produced from long steel semi-finished products, continuously cast and hot-rolled. When the plastic deformation is "hot", the pieces of steel are previously reheated to a temperature of about 1000 to 1200 ° C., then hot formed in the forge. The parts obtained are then cooled and heat treated by quenching and tempering. When the plastic deformation is "cold", the pieces are cold formed in the press, possibly after having undergone a globulation annealing. The parts obtained are then heat treated by quenching and tempering. It is recalled that in service, these parts are usually subjected to variable mechanical stresses, even cyclic, which generate significant fatigue work. The fatigue of the steel results in the appearance of microcracks which propagate until rupture, even if the stress is lower than the tensile strength or the elastic limit of the metal which composes the part. Today, it is estimated that fatigue is responsible for almost 90% of breaks in the service of mechanical parts. Likewise, the shocks that a mechanical part can undergo in service cause the appearance of microcracks which can cause the part to rupture prematurely if special care is not taken with the resilience properties of the metal which constitutes it. However, the bainitic structure of steel is usually in the form of parallel slats which consequently offer few obstacles to the propagation of microcracks. This structure, although sought for its mechanical strength and ductility properties, therefore does not necessarily have a toughness or a satisfactory fatigue life. It is for example known, from document EP 0 787 812, to improve the fatigue life of forgings thanks to the presence of residual austenite within the bainite, obtained by means of adequate controlled cooling associated with the choice a steel grade whose composition has been specially enriched in silicon. The object of the invention is to provide another solution to improving the fatigue life and toughness of forged or struck mechanical parts which retains their high mechanical characteristics, of resistance, ductility and resilience for example. To this end, the subject of the invention is a mechanical steel part obtained from hot forging or cold striking, of medium or small size, coming from the plastic transformation of a long steel semi-finished product, characterized in that the steel of which it is made has a composition 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: 0.2 < C <0.5 0.5 <Mn <2.0 0.05 <N <0.5 0.6 ≤ Si <1.5 0.05 ≤ Cr <1.0 0.01 <Mo <0.5 0.02 <S <0.10 and possibly up to 50 ppm of boron and in that said part is obtained from a long semi-product obtained from continuous casting and hot rolled in the austenitic domain, then put shaped by plastic deformation and heat treated to obtain a metallographic structure containing essentially acicular ferrite, at least in the stressed areas mechanical toughness and fatigue. By "essentially" here is meant at least 50% and preferably 60%, or even advantageously even 80% and more of acicular ferrite by volume. The invention also relates to a steel for the manufacture of a mechanical part by plastic deformation, characterized in that, in addition to the inevitable residual impurities from the production of steel, its chemical composition includes at least, expressed in weight content: 0.2 <C <0.5 0.5 <Mn <2.0 0.05 <N <0, 5 0.6 ≤ If <1.5 0.05 ≤ Cr <1.0 0.01 <Mo <0.5 0.02 <S <0.10 and possibly up to 50 ppm of B. and in this that the metallographic microstructure which it will present, once said part is implemented, is essentially composed of acicular ferrite at least in the zones of the part subjected to mechanical stresses in toughness and in fatigue. As regards both the mechanical part and the steel grade defined above, in order to facilitate obtaining acicular ferrite, the steel preferably also comprises from 5 to 30 ppm of Ca, and or from 0, 01 to 0.02% Ti, with possibly up to 0.2% Al. The subject of the invention is also a method of manufacturing such a mechanical steel part, characterized in that, in order to obtain acicular ferrite at least locally on said part, it comprises the following steps: - supplies a continuous casting billet of steel of composition in accordance with the analysis given above, which is hot rolled at a temperature above 1000 ° C in bar or wire before being cooled to ambient after rolling; - The wire being subjected to a controlled cooling before its crowning in order to obtain a metallographic structure composed essentially of acicular ferrite, wire which is then cut into pieces and which is cold struck into a ready finished part. in use; - the bar being, it, naturally cooled in the hot rolling before its cutting into pieces which are then hot forged -in a blank part which is cooled by controlled cooling to obtain a structure essentially composed of acicular ferrite at least in the stressed areas of the part, blank which is then machined if necessary to the desired ribs to make a finished part ready for use. As a variant, controlled cooling is natural cooling to the ambient. In practice, in fact, it happens that the forgings are immediately stored in bulk in buckets on top of each other. The pieces on top of the pile will cool faster than the pieces below. It is therefore not sought at this stage a controlled cooling of each part, since these will moreover more often then be heat treated. In the process according to the invention on the other hand, the parts can certainly cool naturally (that is to say without air blowing), but this cooling must nevertheless be controlled in order to ensure the formation of needle-like ferrite. This cooling control can be done for example by depositing the pieces one by one, distant from each other, directly after the forging operation on a conveyor belt, which routes them to the reception area of the workshop in view of their storage before shipment. However, according to a preferred variant of the invention, the controlled cooling is forced cooling, for example with blown air, ensuring a surface cooling rate of 0.5 to 15 ° C / s approximately. It is recalled that the vocabulary habits in the steel industry mean that products rolled in diameters up to approximately 30 mm in diameter are called "wire" (which is often packaged in the form of crowns), and "bars" those rolled from 18 mm in diameter and which are delivered rectilinear after cutting to length at the exit of the train. The invention finally relates to a long, medium carbon steel semi-finished product, intended to be transformed by forging or cold striking into a mechanical part with high characteristics, of small size or of medium size, characterized in that the steel which the compound corresponds at least to the following analysis, given in weight percentages: 0.2 <C <0.5 0.5 <Mn <2.0 0, O5 ≤ N <0.5 0.6 ≤ Si <1 , 5 0, O5 ≤ Cr <1,0 0, Ol <Mo <0,5 0, O2 <S <0,10 and possibly up to 50 ppm of boron and in that the metallographic microstructure which it will present after transformation will be essentially composed of acicular ferrite at least in the areas of the part subjected to mechanical stresses in toughness and fatigue. As will no doubt have been understood, the invention in fact consists in proposing the manufacture of a tenacious and resilient mechanical part endowed with a micro structure essentially composed of needle-like ferrite at least in the zones of the part mechanically stressed in fatigue. , this from a medium carbon steel associated, in the ranges of analyzes given in these elements, with manganese (also gamma) for resistance to rupture, and micro-alloyed with vanadium assisted by sulfur in order to promote the development of acicular ferrite and associated, on the one hand, with molybdenum to improve resilience and harden ferrite even more than vanadium alone, on the other hand, with chromium to facilitate the efficiency of controlled cooling during the transformation operation, and, for a third part, to silicon, alphagene him, to increase the resilience, but also to favor the precipitation at the grain boundaries of a ferrite which will prevent the bainite from invading everything and thus allow the acicular ferrite to be born to take the place which is due to it. It should be recalled here that acicular ferrite is a metallographic constituent known in the steel industry. It is already used for example, as shown in EP-N n ° 0288054, to facilitate the manufacturing process of fine grain sheets for low temperature use (of shore, etc.) by eliminating the reheating step intermediate between casting and hot rolling. Similarly, as USP No. 6,669,789 shows, it is known to use, alongside the usual polygonal-perlite ferrite, an acicular ferrite structure (which germinates on carbides) for the manufacture of sheet metal. made of high strength titanium steel and high elongation to limit the size of the austenitic grain from thin hot-rolled slabs. The invention will be well understood and other aspects and advantages will appear more clearly in the light of the detailed description which follows, given by way of exemplary embodiment. Long semi-finished products (billets or blooms) produced from steel having, in addition to iron, the following composition in weight content relative to iron are produced at the steelworks by continuous casting: From 0.2 to 0, 5% carbon. At these contents, carbon makes it possible to obtain good mechanical strength characteristics. In particular, the required resilience is ensured by the 0.2% minimum. On the other hand, its content must not be too great (0.5% max approximately) so as not to favor the formation of bainite instead of the acicular ferrite sought. From 0.5 to 2.0% manganese. Manganese is conventionally used here to increase the hardenability of steel under the above-mentioned carbon contents. However, its content is preferably less than 2.0% in order to avoid its segregation which would harm the homogeneity of the structure. - From 0.05 to 0.5% vanadium. Vanadium favors the development of acicular ferrite, as already said, by making it possible to increase the size of the bainitic domains and by shifting them towards high temperatures. It also decreases the range of appearance of perlite ferrite. From 0.02 to 0.10% sulfur. Sulfur not only improves the machinability of parts, but fulfills a role mainly sought here in the nucleation mechanism of acicular ferrite. It has in fact been discovered that it is the sulphides, and not the carbides as in the case of the document USP 6,669,789 cited above, which in fact constitute essential anchoring points on which the acicular ferrite grains will develop, the development of which is going to be promoted by vanadium, combined with silicon. From 0.6 to 1.5% silicon. Silicon is conventionally used to deoxidize steel. Its content must however remain below 1.5% here in order not to weaken the steel. It plays an essential role here in the controlled increase of the bainitic domain in which the acicular ferrite is formed by precipitating as already indicated primary ferrite at the grain boundaries and thus allowing vanadium to promote the development of acicular ferrite. From 0.05 to 1.0% chromium. The chromium makes it possible to adjust the hardenability of the shade and thus to accompany the increase in size of the parts to be produced. It also acts with silicon in order to increase the range of existence of acicular ferrite. From 0.01 to 0.5% molybdenum. The molybdenum contributes to obtaining the final structure by adjusting the hardenability of the shade. In fact, if the content of quenching elements is too low, a ferrito-pearlitic structure will be obtained, and conversely, a shade that is too quenching can lead to obtaining martensite or residual austenite. Optionally, but highly recommended in practice, from 0.01 to 0.02% of titanium in order to protect the elements from nitrogen, and in particular to keep sufficient free vanadium which would otherwise easily form precipitated nitrides. Similarly, optionally but conventionally widely used in practice, from 5 to 30 ppm of calcium in order to improve the flowability of the steel and its implementation. It facilitates the obtaining of inclusions of oxides which can enter into the nucleation mechanism of acicular ferrite. Optionally up to 50 ppm of boron which will act in synergy with molybdenum to widen the bainitic domain in which acicular ferrite is formed. Possibly up to 0.2% aluminum to control the size of the austenitic grain, but it will also play a role in the preservation of vanadium. This optimized composition allows the steel to present, following controlled cooling, a structure essentially composed of acicular ferrite. By essentially, it will be understood an acicular ferrite content of more than 50% and preferably more than 60%, and advantageously about 80% or even more. Such a metallographic structure allows the steel to present good mechanical characteristics of resistance, hardness and ductility, but also a resistance to shocks and to work in increased fatigue. As will be seen, the acicular ferrite is obtained before or after the shaping of the part, but in any case by means of controlled cooling of the steel. In the first case, the deformation takes place cold on a steel which already has a structure essentially composed of acicular ferrite. A long semi-product is supplied, consisting of an analysis steel conforming to the invention which is hot rolled, if necessary after reheating above 1100 ° C., according to the usual practice of hot rolling, until a laminated wire 10 mm in diameter, for example, is obtained. The wire removal temperature is of the order of 900 to 950 ° C. The laminated wire obtained is cooled with blown air in the "hot" rolling itself in the usual way ("Steelmor" process for example). If its diameter allows, the wire can also be naturally cooled to ambient air. The laminated wire is delivered in the form of a crown to the transformer who will cut it into pieces of the desired length and subject them to cold stamping to obtain the desired parts. The final mechanical characteristics are naturally obtained by the work hardening resulting from the shaping. In the second case, the plastic shaping is done "hot" and the metallographic structure is obtained directly on the forge blanks. A long semi-finished product consisting of an analytical steel is supplied, giving the invention that it is hot rolled to give it a diameter of 35 mm for example. After possible cooling, which does not need to be checked at this level, the bane is cut to length by cutting and delivered to the blacksmith. The bars are then cut into pieces. Each piece is brought to a temperature of at least 1100 ° C by means of an induction furnace. This heating can also be done more conventionally but the heating conditions (time, speed of heating, etc.) must then be optimized in order to obtain a homogeneous austenitic structure having a grain size favorable to the formation of acicular ferrite. The austenitic grain size is then estimated at 80 μm. The pieces are subjected to a hot plastic deformation operation. The forging ends at a temperature above 1100 ° C. The blanks of parts thus obtained are then subjected to forced cooling down to ambient temperature at a cooling rate of between approximately 0.5 and 15 ° C./s, depending on the diameter of the part and the optimization of the composition. steel. The part can also be cooled in a natural but controlled way, by placing the blanks at the forge outlet one by one on a conveyor belt for example. The part is then machined to respect the final target dimensions. Optionally, instead of machining, the blank can be subjected to a second plastic knockout. This additional operation can be carried out cold without risking cracking the part due to the ductile nature given by the microstructure to the steel. It is not necessary to carry out a thermal quenching and tempering treatment to obtain the targeted mechanical characteristics. The steel grade gives the invention a pennet to obtain a part of metallographic structure essentially composed of acicular ferrite. It has the mechanical characteristics of breaking strength and hardness required by its use properties, and meets the machinability requirements. In addition, it has increased toughness by its very structure, the intertwining of the slats serving as an obstacle to the appearance and propagation of cracks. This increased toughness allows it in fact to present suddenly also better impact resistance and better resistance to fatigue. In addition, it also allows a second cold forming by striking for example. Obtaining acicular fenite also increases the mechanical resistance of the shade by the high density of dislocation of its slats. Tests were carried out in the laboratories of the producer of semi-finished products for forge from continuous casting. A wheel hub was forged there from a steel according to the invention, the chemical composition of which, in addition to the iron and the impurities resulting from the production, corresponds to the following analysis:

Before forging, this piece was heated to 1200 ° C by induction. The end of forging temperature is 1100 ° C. After forging the blank is cooled at a speed of 2 ° C / s directly in the hot. No other thermal treatment is applied. The structure obtained on this test hub is 80% of the acicular ferrite, it also has the following mechanical characteristics:

It is recalled that: Rm represents the breaking strength corresponding to the maximum force before breaking referred to the initial section of the wire. Rpn_ represents the conventional elastic limit corresponding to the force related 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 reduction in cross-section of the wire after breaking.

It goes without saying that the invention cannot be limited to the example which has just been described, wheel hub, but that it extends to multiple variants or equivalents, in type of parts and in size and dimension, as long as its definition given in the appended claims is respected.

Claims

1 - Mechanical steel part from hot forging or cold pressing, of medium or small size, and coming from plastic transformation of a long steel semi-finished product, characterized in that the steel of which it is made has a composition 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: 0.2 <C <0.5 0.5 < Mn <2.0 0.05 <V <0.5 0.6 ≤ Si <1.5 0.05 ≤ Cr <1.0 0.01 <Mo <0.5 0.02 <S <0.10 and optionally up to 50 ppm of boron and in that said part is obtained from a long semi-product resulting from continuous casting and hot rolled in the austenitic field, then shaped by plastic shaping and heat treated for obtain a metallographic structure essentially containing acicular ferrite at least in the areas of mechanical stresses in tenacity and in fa Tigue.

2- Mechanical part according to claim 1 characterized in that the steel which composes it also contains from 0.01 to 0.02% of titanium and / or up to 0.20% of aluminum.

3- Mechanical part according to claim 1 or 2 characterized in that the steel which composes it further comprises between 5 and 30 ppm of calcium.

4- Steel for the manufacture of a mechanical part by plastic defonation, characterized in that, in addition to the inevitable residual impurities coming from the production of steel, its chemical composition comprises at least, expressed in weight content: 0.2 <C <0.5 0.5 <Mn <2.0 0.05 <V <0.5 0.6 ≤ If <1.5 0.05 <Cr <1.0 0.01 <Mo <0, 5 0.02 <S <0.10 and possibly up to 50 ppm of B. and in that the metallographic microstructure which it will present, once said part is implemented, is essentially composed of acicular fenite at least in the zones of the part subjected to mechanical stresses in tenacity and in fatigue.

5- Steel according to claim 5 or 6 characterized in that, to protect the vanadium, it also contains from 0.01 to 0.02% of titanium and / or up to 0.20% of aluminum.

6- Steel according to claim 4 or 5 characterized in that it further comprises between 5 and 30 ppm of calcium.

7- A method of manufacturing a mechanical steel part characterized in that, in order to obtain acicular ferrite at least locally on said part, it comprises the following steps: - a continuous steel billet is supplied of composition gives the analysis given above, which is hot rolled at a temperature above 1000 ° C in bar or wire before being cooled to room temperature after rolling; - the wire being subjected to a controlled cooling before its setting in crown to obtain a metallographic structure composed essentially of acicular ferrite, wire which is then cut into pieces and which is cold struck in ready finished part in use; - the bane being, it, naturally cooled in the hot rolling before its cutting into pieces which are then hot forged into a blank part which is cooled by controlled cooling to obtain an essentially composed structure of acicular ferrite at least in the stressed areas of the part, blank which is then machined if necessary to the desired ribs to make a finished part ready for use.

8-A method according to claim 7 characterized in that the controlled cooling is natural cooling to the ambient.

9- A method according to claim 7 characterized in that the controlled cooling is forced cooling ensuring a surface cooling rate of 0.5 to 15 ° C / s approximately.

10- Long carbon steel semi-finished product, intended to be transformed by forging or by striking into a mechanical part with high characteristics, of small size or of medium size, characterized in that with the aim that said part has a metallographic micro structure essentially composed of acicular ferrite at least in the zones of the part subjected to mechanical stresses in tenacity and in fatigue, the steel which composes it meets at least the following analysis, given in weight percentages: 0, 1 <C <0.5 0.5 ≤ Mn <2.0 0.05 <N <0.5 0.6 <Si <1.5 0.05 ≤ Cr <1.0 0.01 <Mo <0.5 0.02 <S <0.10 and possibly up to 50 ppm of boron and in that the metallographic microstructure which it will present after transformation will be essentially composed of acicular fenite at least in the areas of the part subjected to mechanical stresses in toughness and fatigue.