EP1565587B1 - Ready-use low-carbon steel mechanical component for plastic deformation and method for making same - 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

The invention relates to mechanical parts made of low carbon steel with high characteristics, such as the ball joints of land vehicle wheels, pivots, pins, suspension triangles, connecting rods, or other similar mechanical parts ready for use obtained by plastic deformation. a long steel product (wire, bar ...)

It is known that steels for plastic deformation must have properties of both deformability and strength. Thus, during the manufacture of mechanical parts to which some of them are intended, they must be able to withstand without significant changes in shape while sometimes sometimes having high mechanical characteristics. In fact, in some cases, the required characteristics of the 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 yield strength of 900 MPa. In addition, these steels must have good machinability characteristics because a majority of applications require ultimate machining for 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 plastic deformation (punching, forging ...), the plots are cold-formed to the press, if necessary after a globular 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 1000-1200 ° C, shaped hot and cooled. The parts thus obtained are then thermally treated by quenching and tempering, the quenching being able to be done directly during the cooling after forging.

Achieving these various 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. In recent years, we have also been looking for shades of steel that make it possible to get rid of them and to obtain high-performance parts "ready for use" that can be used for the intended application without having to undergo a heat treatment to modify their metallurgical structure after the plastic deformation operation.

As regards the cold stamping for example, it is already known, for example, to use steel shades of essentially bainitic structure (ie containing more than 50% of bainite), having a good compromise between deformability and final mechanical characteristics. However, in view of the capacities of the cooling means generally available on a hot rolling line, these grades make it possible to obtain an essentially bainitic structure only on wires or rolled bars of relatively small diameter, rarely exceeding 8 mm. in fact. 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 strong dispersion of the mechanical characteristics within the same ring or between several coiled wire rings or between several bars or within the same bar. the outcome of 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 reach the core cooling rate required to obtain the bainitic structure referred to in the mass. In addition, the periphery of the part is inevitably cooled much more vigorously than the core, resulting in internal tensions that can lead to permanent deformities crippling.

We therefore see that we are looking classically, in applications of shades for plastic deformation, bainitic structure that offers a good compromise between deformability and mechanical characteristics, at the same time as a good machinability. In any case, the success of obtaining this bainitic structure is subject to the cooling constraints of the core steel, whether this cooling occurs before the plastic deformation or after. These constraints imposed on the 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 a heat treatment after their shaping.

US-A-5 554233 relates to the manufacture of hot-rolled and cold-deformable bars for obtaining parts without the need for a heat treatment before or after the cold deformation. The steel used comprises in particular from 0.10% to 0.14% of carbon and from 0.01% to 0.1% of molybdenum.

The objective of the invention is the provision of 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. use both cold press and hot forging.

More specifically, the object of the invention is the development of a low carbon steel grade specific to the manufacture of mechanical parts having a bainitic or essentially bainitic structure that can be obtained already with a low cooling rate at the core, which can go down to 1 ° C / s, and offering both good deformation and good machinability for the realization of these parts by cold or hot deformation, without heat treatment after shaping, said grade having high mechanical characteristics enabling said pieces to be in grades 8.8 to 12.9 according to ISO 898.

The invention thus relates to a mechanical part with high characteristics low carbon steel ready for use from the plastic processing of a long rolled steel product, according to claim 1.

In a first preferred embodiment, the cold-deformed steel mechanical part defined above is characterized in that the long product from which it is derived by plastic transformation is a heat-treated laminated wire or bar by cooling in the hot rolling mill. at a cooling rate sufficient to confer a bainitic or essentially bainitic structure.

In a second preferred embodiment of the invention, the mechanical part made of hot-forged steel defined above is characterized in that the long product from which it is derived by plastic transformation is a bar or a rolled wire whose billet of forge which has been extracted has been thermally treated by quenching under a sufficient cooling rate to give it a bainitic structure to the core, since a quenching temperature of the order of 1200 ° C and more to which the billet has undergoes a plastic transformation by forging bringing it to its desired final shape.

In the two embodiments mentioned above, the heat treatment involved in the preparation of the mechanical part comprises a final phase of cooling at low speed, which can go down to about 1 ° C / s, at heart.

It will be noted that this cooling of the room is a mild cooling, in any case different from a cooling operation which would soak the steel, which in any case would be, in normal practice, followed by an income.

In a variant, the mechanical part is made of a steel whose carbon content is between 0.06% and 0.10%.

In another variant, the mechanical part is made of steel whose molybdenum content 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 to use low carbon steel having a breaking strength of more than 800 MPa, which comprises the steps of claim 6.

As will be understood, 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 acquire a bainitic (or essentially bainitic) homogeneous structure in the mass of the room with few requirements for cooling. This structure can indeed be obtained already from a low core cooling rate that can go down to about 1 ° C / s, a speed that can be achieved, as we know, directly in the hot rolling mill. even for wires and bars of diameter of the order of 20 mm and more depending on the installations.

Therefore, the invention opens to the large diameters the production range of hot-rolled long products for cold-forging or cold-forging workshops, and, for those reserved for hot forging, it provides the economy of a additional final tempering-tempering heat treatment. To better fix ideas, note that with the usual rolling hot, the limiting diameters are around 20 to 25 mm for the shades according to the invention.

• "fils ou petites barres" les produits laminés sous des diamètres allant jusqu'à 30 mm environ (que l'on conditionne souvent d'ailleurs sous forme de couronnes pour livraison aux transformateurs);
• et "barres" ceux laminés à partir de 18 mm de diamètre et qui sont livrés rectilignes après découpe à longueur à la sortie du train.

The vocabulary habits in the steel profession are so-called

• "wire or small bars" means products rolled in diameters up to about 30 mm (which are often packaged in the form of crowns for delivery to processors);
• and "bars" those rolled from 18 mm in diameter and which are delivered rectilinear after cutting to length at the exit of the train.

In addition, for the sake of clarity of the disclosure, the expression "bainitic structure" will refer to a "bainitic or essentially bainitic structure".

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, long semi-finished products (billets or blooms) are produced from a steel having, in addition to iron, the following composition, in weight content relative to iron:

From 0.02 to 0.10%, and preferably 0.08%, of carbon. The carbon at these contents serves to obtain a bainitic structure having the required mechanical properties. It makes it possible to obtain 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 0.04 to 0.10%, and preferably 0.06 to 0.08%, of niobium. Niobium works synergistically with molybdenum and boron to expand the bainitic transformation domain. It increases boron hardenability by increasing the effective boron content in steel. Indeed, the formation of carbides Fe ₂₃ (CB ₆ ) (trapping boron and passive as to the hardenability of steel) is made more difficult by the action of niobium which stabilizes the austenite and delays the diffusion of carbon. Moreover, it makes it possible to increase the recrystallization temperature of the austenite which allows to obtain a finer bainitic structure during controlled rolling, and thus to increase the resilience of the parts.

0.001 to 0.005% boron. Boron inhibits the germination of ferrite thus favoring the formation of a bainitic structure. It works synergistically with niobium and molybdenum to expand the bainitic domain.

From 0.10 to 0.35%, and preferably less than 0.30% molybdenum. Molybdenum is a carburigenic element that broadens the bainitic domain by retarding the germination of ferrite. In addition, at these levels, its action on the quenchability of steel makes it possible to obtain a steel of greater mechanical strength by lowering the bainitic transformation start temperature. It tends to compensate for the low carbon content needed to obtain good ductility. Moreover, it acts in synergy with boron and with niobium, whose role it reinforces. In addition, at these levels, it acts synergistically with niobium to increase the recrystallization temperature of austenite.

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

From 0.007 to 0.010% 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. Titanium is used to fix nitrogen and thus protect boron. Without titanium, boron will lose its quenching power by reacting with nitrogen. Titanium also provides a fine austenitic grain which improves cold forming and ductility.

Less than 0.08% aluminum. This residual dissolved aluminum, coming from the calming of the steel before casting, is a good deoxidant 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 semifinished product, and thus to give the steel good resilience properties.

Possibly 0.001 to 0.1% sulfur. This sulfur combines with manganese to form plastic and ductile manganese sulphides. It allows to obtain 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.

This steel also presents the inevitable impurities and residual elements resulting from its production, in particular the phosphorus whose content must remain less than 0.02% to guarantee good ductility during and after the setting in cold form, as well as copper and nickel, the content of which must be less than 0.30%.

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.

Most of the time, it will also be possible to limit the molybdenum content to 0.30% and the manganese content to 1.80% in order to avoid the risk of the appearance of a martensitic quenching structure in certain cases. given local conditions.

An essential aspect of the invention is that the mechanical parts have a homogeneous bainitic structure in the mass at low core cooling rate of hot forged parts, or wires or bars from which they are cold-stamped, which can descend up to about 1 ° C / s.

When, according to an implementation of the invention, the mechanical part is cold-forged (or cold-forged), the bainitic structure is obtained before shaping. The steel, after deformation, then 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 800 MPa.

In this first embodiment, the part is indeed obtained by cold plastic deformation of the steel already having a bainitic structure. A long half-product consisting of an analysis steel according to the invention is supplied which is hot-rolled, if necessary after reheating above 1100 ° C., in accordance with the usual hot-rolling practice. obtaining a rolled wire of 10 mm in diameter for example. The wire removal temperature is less than 1000 ° C. The laminated wire obtained is then cooled in air in the hot rolling itself in the usual manner ("Stelmor" process for example), at a low core speed which can go down to about 1 ° C./s. to obtain a homogeneous bainitic structure.

The laminated wire is then delivered (or deliverable) to the transformer as a crown. The transformer receiving the crown unwinds the wire, raises it if necessary, before cutting it into pieces of the desired length. Each piece is then subjected to a usual operation of cold plastic deformation to obtain the ready-to-use final part (ball joints, shafts, connecting rods, screws, etc.), after a nominal setting machining if necessary. . The final mechanical characteristics will naturally be obtained by the work hardening resulting from the shaping.

In a second embodiment, the part is deformed under heat and the bainitic structure is obtained after this plastic deformation operation: a long half-product consisting of an analysis steel according to the invention is supplied. while hot until a rolled bar of 30 mm diameter for example. After cooling if necessary, the cut-to-length bar is available straight to the blacksmith with its ordinary metallographic structure naturally acquired during hot rolling.

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 at low core cooling rate which can go down to 1 ° C / s about. 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. all in the end, after hot shaping and controlled cooling.

It is recalled that the mechanical parts according to the invention are obtained by plastic deformation of rolled products without additional tempering and tempering heat treatment.

Laboratory tests were carried out on a casting of following composition: % VS % Mn % Nb % Cr % B % MB % Ti % N ₂ % Yes % S % Al 0.08 1.6 0.08 0.2 0,003 0.2 0,029 0.006 0.25 0,004 0,028

The billet from the casting was hot rolled after reheating above 1100 ° C to form a 12 mm diameter wire. The temperature of removal of the wire after rolling 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 5 ° C / s. A homogeneous bainitic structure is obtained on the whole of the wire, at the periphery as well as at the core.

The mechanical characteristics of the wire are as follows: Rm (MPa) Rp _0.2 (MPa) AT (%) Z (%) 857 683 17.4 71.4

• Rm représente la résistance à la rupture correspondant à la force maximale avant rupture rapportée à la section initiale du fil.
• Rp _0.2 représente la limite d'élasticité conventionnelle correspondant à la force rapportée à la section initiale du fil provoquant un allongement plastique de 0,2 %.
• A représente l'allongement à la rupture.
• Z représente la striction correspondant à la réduction de section du fil après rupture.

It is recalled that:

• Rm represents the breaking strength corresponding to the maximum force before breaking compared to the initial section of the wire.
• Rp _0.2 represents the conventional yield strength corresponding to the force referred to the initial section of the yarn causing a 0.2% plastic elongation.
• 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 undergone by the wire is as follows: Reduction rate (%) Rm (MPa) Rp ₀ . ₂ (MPa) AT (%) 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 parts with high characteristics according to the invention are remarkable in that they make it possible in particular to save quenching and tempering treatments currently used in cold-forging or forging operations.

On the other hand by imposing less drastic cooling conditions, they are less likely to deform during the cooling operation, or to equivalent cooling fluid they may have larger diameters or thicknesses.

They are also remarkable for the very good machinability characteristics they present, 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 ability.

It goes without saying that the invention can not be limited to the examples just described.

Thus, for example, in hot forging applications, one skilled in the art may choose to improve machinability by varying the sulfur content. Likewise, although intended more particularly for striking or cold forging or hot forging applications, the invention also applies to other applications of plastic deformation such as drawing, drawing, stamping, etc. ...

Claims (6)

1. A ready-for-use low-carbon steel mechanical component with elevated characteristics obtained by the plastic transformation a long rolled metallurgical product, comprising the following characteristics:

   - the chemical composition of said steel complies with the following analysis, given in percentages by weight in relation to iron:

   0.02 % ≤ C ≤ 0.10 %

   0.04 % ≤ Nb ≤ 0.10 %

   0.001 % ≤ B ≤ 0.005 %

   0.10 % ≤ Mo ≤ 0.35 %

   1.3 % ≤ Mn ≤ 2.0 %

   0.10 % ≤ Si ≤ 1.30 %

   0.01 % ≤ Al ≤ 0.08 %

   N ≤ 0.015% with Ti ≥ 3.5 x % N;

   finally from 0.001 to 0.1 % sulphur,

   the remainder iron and the unavoidable residual impurities arising from the processing of steel, of which less than 0.02 % is P and less than 0.30 % is Cu and Ni,

   - said long product is obtained from a semi-finished product derived from continuous casting and hot-rolled in the austenitic range, then treated thermally to obtain a bainitic or essentially bainitic structure, and worked, either by cold plastic transformation after said thermal treatment or by hot plastic transformation during said thermal treatment, so as to give it its final shape with an ultimate tensile strength higher than 800 MPa, without thermal treatment subsequent to this working.
2. A low-carbon steel mechanical component deformed by a cold process according to Claim 1, characterised 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 hot rolling at a cooling rate sufficient to impart a bainitic or essentially bainitic structure thereto.
3. A steel mechanical component forged according to Claim 1, characterised in that the long product, from which it is derived by a hot plastic transformation, is a rolled rod or a wire, whose forged billet, which has been extracted therefrom, has been treated thermally by quenching at a cooling rate sufficient to impart thereto a bainitic structure through to the core, this a quenching being from a temperature of the order of 1200°C and above, at which the billet has undergone plastic transformation by forging, bringing it into the desired final shape.
4. A steel mechanical component according to Claim 2 or 3, characterised in that the heat treatment used in its manufacture comprises a final slow cooling phase, the rate of which can fall to as low as 1°C/s at the core.
5. A steel mechanical component according to any one of Claims 1 to 4, characterised in that the steel from which it is composed has a molybdenum content not exceeding 0.30 % and a manganese content of less than 1.80 %.
6. A process for manufacturing a ready-for-use low-carbon steel mechanical component with elevated characteristics exhibiting an ultimate tensile strength of more than 800 MPa, characterised in that it comprises the following steps:

   - starting from a long semi-finished product whose composition least complies with the following analysis, given in percentages by weight, in relation to the iron:

   0.02 % ≤ C ≤ 0.10 %

   0.04 % ≤ Nb ≤ 0.10 %

   0.001 % ≤ B ≤ 0.005 %

   0.10 % ≤ Mo ≤ 0.35 %

   1.3 % ≤ Mn ≤ 2.0 %

   0.10 % ≤ Si ≤ 1.30 %

   0.01 % ≤ Al ≤ 0.08 %

   N ≤ 0.015% with Ti ≥ 3.5 x % N;

   finally from 0.001 to 0.1 % sulphur,

   the remainder being iron and the unavoidable residual impurities arising from the processing of steel, of which less than 0.02 % is P and less than 0.30 % is Cu and Ni,

   a long product (wire or rod) is hot-rolled, the settling temperature of the long product after rolling being below 1000°C;

   - the resultant rolled long product then undergoes thermal treatment, said thermal treatment comprising a final slow cooling phase, the rate of which can fall to as low as approximately 1°C/s at the core to obtain a bainitic or essentially bainitic structure, said long product being deformed plastically to bring it to its desired final shape, the plastic deformation process being carried out either by cold plastic transformation after said thermal treatment or by hot plastic transformation during said thermal treatment, so as to give it its final shape with an ultimate tensile strength higher than 800 MPa, without thermal treatment subsequent to this working.