EP2134882B1 - Microalloyed steel with good resistance to hydrogen for the cold-forming of machine parts having high properties - Google Patents

Description

The invention relates to micro-alloyed steels for the cold forming, particularly by striking, of assembly parts, such as screws, bolts, etc., which the automotive industry commonly uses for the assembly of engines or ground connections of rolling stock.

As we know, the automotive industry is continuously aiming to increase the power of the engines at the same time as it seeks to reduce the weight, so to use parts of smaller and smaller size. These parts, which remain subject to the same mechanical stresses, must therefore have mechanical characteristics, including breaking strength, more and more high.

Until now, the vast majority of micro-alloyed steels, used for example in automotive fasteners, makes it possible to obtain screws in class 10.9, thus having a breaking strength of 1000 MPa and more. This resistance, already high, can be further artificially increased by about 100 to 200 MPa by tightening the screws at the very moment of assembly of parts. It will be understood, however, that such a practice can not be accepted as a solution in itself to the desired increase in the limit of rupture.

However, the other way, namely the "natural" one, which is aimed at the metallurgy of their manufacture, quickly comes up against problems of fragility linked to the presence of hydrogen in the steel. As we know, hydrogen in steel is indeed at the origin of mechanisms of rupture, delayed or even immediate sometimes, which is translated by the breakage of the part in service when the application of a certain level constraints.

Micro-alloy steel grades for screws with very high mechanical properties (1300 MPa and more resistance) have already been proposed to improve their resistance to hydrogen. This is the case, for example, of the nuance described in the document USP 5,073,338 of December 1991 and wherein molybdenum is added in an amount up to 1% by weight with a minimum of 0.5%.

The document US 2003/150529 describes a bolt of high strength excellent in resistance to delayed fracture, the steel has a good resistance to embrittlement by hydrogen for forming mechanical parts very high strength. Steel contains, in percentages by weight: 0.3-0.45% C, 0.35-1.5% Mo, 0.40-1.00% Mn, 0.5-1.5% Cr, 0.005 -0.030% Nb, 0.30-1.0% V, 0.005-0.030% Ti, S <0.010%, P <0.010%, Si <0.10%, 0.010-0.100% Al, the remainder being iron and unavoidable impurities.

However, it is to be feared that the heat treatments suffered by the steel during the striking process lead to an accumulation in some places of the metallic matrix of bulky molybdenum carbides which will weaken the structure of the steel and therefore will not always allow obtain the desired mechanical characteristics. Another disadvantage may be felt in a certain decrease in the cold-forming ability due to the increased hardness of the steel due to the presence of this high-grade hardening element. In addition, molybdenum is a This product is particularly expensive on the market, so that its introduction in quantity into the steel generates a significant additional cost of production.

Nevertheless, despite these disadvantages, the grades proposed in the literature for micro-alloyed steels intended for hardware seem to persevere in the sense of a strong presence of molybdenum in order to be able to reach levels in mechanical strength greater than 1300 MPa. This is the case, for example, with the nuance described in the document JPA 2001032044 published in February 2001 wherein the weight content of molybdenum is between 1.5 and 3%. This is still the case of the nuance described in the document EPA 1746177 published in January 2007 in which the molybdenum content can rise up to 6%, without being less than 0.5%.

We thus see, through this rapid panorama of the known state of the art, that it appears relatively easy to achieve, via metallurgy, micro alloyed steels for parts with high mechanical strength, without harming necessarily with the resistance to hydrogen, but that it is much less easy to obtain such a result if one assigns a deliberately low molybdenum content.

Going against the path drawn by the prior art, the object of the invention is to provide an economical micro-alloy steel, with a molybdenum content deliberately fixed for this purpose to less than 0.45% by weight, and having a good resistance to hydrogen, while achieving high mechanical characteristics on finished parts ready to use made from this steel.

$$\mathrm{0,20}\le \mathrm{Mo}\%<\mathrm{0,45}$$

$$\mathrm{0,4}\le \mathrm{Mn}\%\le \mathrm{1,0}$$

$$\mathrm{0,4}\le \mathrm{Cr}\%\le \mathrm{2,0}$$

$$\mathrm{0,04}\le \mathrm{Ni}\%\le 0.8$$

$$\mathrm{0,02}\le \mathrm{Nb}\%\le \mathrm{0,045}$$

$$\mathrm{0,03}\le \mathrm{V}\%\le \mathrm{0,30}$$

$$\mathrm{0,02}\le \mathrm{Ti}\%\le \mathrm{0,05};\mathrm{avec}\mathrm{Ti}>\mathrm{3,5}\mathrm{N}$$

$$\mathrm{0,003}\le \mathrm{B}\%\le \mathrm{0,005}\%$$

$$\mathrm{S}\%\le \mathrm{0,015}$$

$$\mathrm{P}\%\le \mathrm{0,015},$$

et optionnellement 0,05 ≤ Si % ≤ 0,20; Al % ≤ 0,05 et N % ≤ 0,015.For this purpose, the subject of the invention is a micro-alloyed steel with good resistance to hydrogen embrittlement for the cold forming of mechanical parts with high characteristics, characterized in that, in order to contain its weight content of molybdenum below 0.45%, its chemical composition, in addition to iron and the inevitable residual impurities resulting from the elaboration of steel, corresponds to the following analysis, given in percentages by weight: $$0.3\le \mathrm{VS}\%\le 0.5$$

$$0.20\le \mathrm{MB}\%<0.45$$

$$0.4\le \mathrm{mn}\%\le 1.0$$

$$0.4\le \mathrm{Cr}\%\le 2.0$$

$$0.04\le \mathrm{Or}\%\le 0.8$$

$$0.02\le \mathrm{Nb}\%\le \mathrm{0,045}$$

$$0.03\le \mathrm{V}\%\le 0.30$$

$$0.02\le \mathrm{Ti}\%\le 0.05;\mathrm{with}\mathrm{Ti}>3.5\mathrm{NOT}$$

$$\mathrm{0,003}\le \mathrm{B}\%\le 0.005\%$$

$$\mathrm{S}\%\le \mathrm{0,015}$$

$$\mathrm{P}\%\le \mathrm{0,015},$$

and optionally 0.05 ≤ Si% ≤ 0.20; Al% ≤ 0.05 and N% ≤ 0.015.

The subject of the invention is also a long rolled steel product (wire rod or rod) of microalloyed steel resulting from continuous casting in the form of billets or blooms and having a chemical composition in accordance with the analysis given above. in order to be able to present, after processing by cold forming and quenching and tempering heat treatment, a mechanical strength of 1200 to 1500 MPa and more, combined with good resistance to hydrogen.

The subject of the invention is also a ready-to-use mechanical part, formed cold, by striking in particular, and having high mechanical characteristics as well as good resistance to hydrogen, characterized in that it is micro-alloyed steel having the chemical composition given above and, preferably, produced from a long rolled steel product (bar or, more commonly, wire rod) from continuous casting in the form of billets or blooms.

More preferably, said mechanical part is an assembly screw for assembly in the automotive industry.

It will have already been understood that a range of 0.20 to 0.45% Mo is in fact sufficient, in the case of the invention, to obtain a synergy between this particular element and the other elements present in the chemical composition of the polymer. which are, on the one hand, niobium, vanadium and titanium (all of which act in the precipitated state in favor of a hardening of the grain of the steel structure and its refinement), and on the other hand, the boron present to increase the quenchability of the grade and which will ultimately allow to obtain a dominant martensite microstructure under the usual conditions of heat treatment suitable for cold forming, by striking or otherwise.

It is also important to note that the way in which the invention has been developed for producing such a low molybdenum grade has been to create a microalloyed steel capable of withstanding a higher amount of hydrogen than in the case of molybdenum. prior art. To do this, the nuance has been optimized to answer the problems related to hydrogen, no longer by the unique classical approach, namely that of the trapping of this element, but by three different joint ways. The research carried out has shown that the hydrogen resistance of steel can result from various independent factors, such as the chemical composition or the microstructure, but also, and it is easy to understand, the amount of hydrogen already present in the steel before commissioning the parts.

1. 1 - Le piégeage. La nuance selon l'invention présente la particularité de multiplier et diversifier les pièges à hydrogène de sorte à éviter une agglomération en un seul endroit de carbures du même type qui fragiliserait la structure et nuirait à la résistance mécanique de l'acier. Le molybdène n'est en effet plus le piège privilégié de l'hydrogène, puisque la nuance contient également à cet effet du niobium, du titane, du chrome et du vanadium.
2. 2 - La répartition. Les éléments, tels que le bore, le niobium, le molybdène, le vanadium et le titane sont favorisés, car ils permettent d'affiner le grain, ce qui permet d'accroître la tenue à l'hydrogène. En effet, l'accroissement de la finesse des grains induisant une augmentation de la surface des joints, l'hydrogène est alors mieux réparti dans l'acier et devient de ce fait moins nocif.
3. 3 - L'élimination. L'hydrogène, introduit dans l'acier lors des phases préparatoires de la matière en vue de la frappe, peut être en partie éliminé lors du traitement thermique final de trempe et revenu effectué sur les pièces frappées fabriquées avec de l'acier selon l'invention. L'augmentation de la température de revenu favorise ce dégazage. Cette augmentation est rendue possible par la présence d'éléments durcissants permettant d'aller en ce sens, tels que le vanadium, le titane, le molybdène, le niobium, mais également le bore par son effet synergique avec le niobium et le molybdène. La nuance selon l'invention permet d'atteindre des températures de revenu de l'ordre de 400°C ou plus.

Hydrogen, according to the invention, is therefore treated by the following three routes:

1. 1 - Trapping . The grade according to the invention has the particularity of multiplying and diversifying the hydrogen traps so as to avoid agglomeration in one place of carbides of the same type which would weaken the structure and adversely affect the mechanical strength of the steel. Molybdenum is no longer the preferred trap of hydrogen, since the grade also contains for this purpose niobium, titanium, chromium and vanadium.
2. 2 - The distribution . The elements, such as boron, niobium, molybdenum, vanadium and titanium are favored because they allow to refine the grain, which increases the resistance to hydrogen. Indeed, the increase in grain fineness inducing an increase in the surface of the joints, the hydrogen is then better distributed in the steel and thus becomes less harmful.
3. 3 - Elimination . Hydrogen, introduced into the steel during the preparatory phases of the material for the purpose of striking, may be partly eliminated during the final heat treatment of quenching and tempering performed on the struck pieces made of steel according to the invention. The increase of the temperature of income favors this degassing. This increase is made possible by the presence of hardening elements to go in this direction, such as vanadium, titanium, molybdenum, niobium, but also boron by its synergistic effect with niobium and molybdenum. The grade according to the invention makes it possible to achieve tempering temperatures of the order of 400 ° C. or more.

Therefore, in the case, for example, the production of cold-knuckle assembly screws, it has been possible to seek greater mechanical strength of the screws before tightening. The "ready-to-use" parts made with the steel grade according to the invention have in fact, without particular difficulties, a final ultimate strength of 1200 MPa, or even 1500 MPa (and even more, depending on the setting of the temperature that will be imposed for the final heat treatment), while initially displaying an intermediate resistance, at least half, or even a third only after a globularization annealing conducted preferentially just before the strike, for facilitate the work of this one

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

• de 0,3 à 0,5 % de carbone.

Pour des teneurs inférieures à 0,3 %, les très hautes résistances désirées ne peuvent être atteintes compte tenu de la teneur des autres éléments présents dans la nuance et des températures de revenu élevées visées. Pour des teneurs supérieures à 0,5 % le risque de fragilité augmente du fait de l'augmentation de la dureté.

• 0,20 % au moins de molybdène, mais sans jamais atteindre dépasser 0.45 % pour les raisons indiquées.

Le molybdène manifeste une forte interaction avec le phosphore dont il limite ainsi l'effet néfaste en limitant sa ségrégation aux joints de grains. De plus, il affiche un comportement carburigène marqué. Il autorise, pour des caractéristiques mécaniques données, des températures de revenu plus élevées, favorisant du coup le développement des carbures qui seront des pièges à hydrogène. C'est donc un élément qui renforce la résistance à la rupture différée.

• de 0,4 à 1,0 % de manganèse.

L'accroissement de la teneur en manganèse tend , en règle générale, à diminuer la résistance à la rupture différée de l'acier. Ceci pourrait provenir de son interaction avec le soufre conduisant à la formation de sulfures de manganèse. Lorsqu'on dépasse des seuils voisins de 1 % de manganèse, cette interaction avec le soufre pourrait même conduire à augmenter la fragilité de l'acier à l'hydrogène, ce, bien entendu, en l'absence de dispositions adéquates prises pour l'éviter. Le manganèse a cependant un effet bénéfique sur la trempabilité de l'acier et donc sur l'obtention des caractéristiques mécaniques finales recherchées sur les pièces réalisées.

• moins de 0,015 % de phosphore.

L'effet du phosphore est particulièrement nocif dans les aciers selon l'invention et ce pour plusieurs raisons. Par un effet contrariant de la recombinaison de l'hydrogène, il contribue à une plus haute concentration d'hydrogène atomique susceptible de pouvoir pénétrer dans le matériau, donc à un risque accru de rupture différée de la pièce en usage. De surcroît, en ségrégant aux joints de grain, il diminue leur cohésion. Sa teneur doit donc impérativement être maintenue très basse. On veillera à cet effet à ce que l'acier soit déphosphoré lors de son élaboration à l'état liquide.

• de 0,05 à 0,2 % de silicium.

Le silicium agit comme désoxydant de l'acier lors de son élaboration, à l'état liquide. Présent en solution solide dans le métal solidifié, il permet également d'augmenter la résistance de l'acier. Toutefois, à teneur trop élevée (plus de 0,2 %), il peut avoir un effet néfaste. Lors des traitements thermiques, tel un traitement de globulisation, le silicium a tendance en effet à former des oxydes intergranulaires et diminue ainsi la cohésion des joints de grains. Une trop forte teneur en silicium diminue également l'aptitude de l'acier à la déformation à froid en durcissant excessivement la matrice. C'est principalement pour cette raison que, dans le cas de la nuance d'acier selon l'invention, sa teneur maximale a été fixée à 0.2%.

• 0,05 % maximum d'aluminium.

L'aluminium est un désoxydant de l'acier à l'état liquide. Il contribue ensuite, sous forme de nitrures, à contrôler le grossissement du grain austénitique lors du laminage à chaud. En revanche, présent en trop grande quantité, il peut conduire à un grossissement des inclusions de type aluminates dans l'acier qui peuvent s'avérer néfastes aux propriétés du métal, notamment sa résilience.

• de 0,4 à 2,0 % de chrome.

Le chrome est recherché généralement pour son effet durcissant. Comme le molybdène, il retarde l'adoucissement au revenu, permettant des températures de revenu plus élevées ce qui favorise le dégazage mais aussi la formation de carbures piégeant l'hydrogène. A teneur trop élevée, en accroissant excessivement la dureté de l'acier, il rend délicat sa mise en forme par frappe.

• de 0.04 à 0.8% de nickel.

Cet élément procure une augmentation de la résistance du métal et a des effets bénéfiques sur la résistance à la rupture fragile. Il améliore également, de manière bien connue, la résistance de l'acier à la corrosion.

• de 0,02 à 0,045 % de niobium, de 0,03 à 0,30 % de vanadium, et de 0,02 à 0,05 % de titane.

Ces trois éléments sont souvent ajoutés à l'acier liquide pour accroître la dureté du matériau. Ici, dans les fourchettes indiquées, ils vont aussi accroître la résistance à la rupture différée de plusieurs façons. Ils vont aider à un affinement du grain austénitique et forment des précipités qui piègent l'hydrogène. En outre, le niobium piège le phosphore. Enfin, l'effet durcissant de chacun permet d'effectuer des revenus à plus haute température. Leur teneur maximale est fixée ici pour éviter l'obtention de précipités de taille trop importante qui serait alors néfaste vis-à-vis de la résistance de l'acier à la rupture différée.
Le niobium, en particulier, lorsqu'il est rajouté en trop forte quantité conduit à un risque accru de défauts de type "fissures" à la surface des billettes et des blooms brutes de coulée continue. Ces défauts, s'ils ne peuvent être totalement éliminés, peuvent s'avérer très néfastes au respect de l'intégrité des caractéristiques de la pièce finale, notamment pour ce qui concerne la tenue à la fatigue et la tenue à l'hydrogène. C'est la raison pour laquelle, dans le cas de la nuance selon l'invention, sa teneur a dû être contenue en dessous de 0.045 %.

• de 0,003 à 0,005 % de bore.

En ségrégant aux anciens joints de grains austénitiques, le bore, même à très faibles teneurs, permet d'accroître la résistance à la rupture différée induite par l'hydrogène. Il augmente fortement la trempabilité de l'acier et permet ainsi de limiter la teneur en carbone nécessaire pour l'obtention de la microstructure martensitique désirée. Il augmente la cohésion du joint de grain par son effet intrinsèque, mais également en rendant plus difficile la ségrégation du phosphore à ces joints de grain. Enfin, le bore agit en synergie avec le molybdène et le niobium, augmentant ainsi l'efficacité de ces éléments et leurs influences propres que permettent leurs teneurs respectives. Un excès de bore (au delà de 0.005%) conduirait toutefois à la formation de boro-carbures de fer fragiles.

• moins de 0,015 % de soufre.

Le soufre est, pour l'acier, un poison qui exprime toute sa nocivité en présence d'hydrogène, car il a un effet additif, c'est-à-dire coopératif avec lui en formant notamment du H₂S, qui en milieu humide en particulier conduit imparablement à une dégradation physique rapide des pièces. Son effet est d'ailleurs à cet égard bien plus marqué que celui du phosphore. Sa teneur doit donc être limitée tant que faire se peut, le plus proche de zéro si possible, en tous cas ne pas excéder la limite des 0,015 % édictée ici. L'acier doit donc être soigneusement désulfuré lors de son élaboration à l'état liquide à l'aciérie.

• moins de 150 ppm d'azote.

L'azote est considéré comme néfaste. Il piège le bore par formation de nitrures de bore, ce qui rend inefficace le rôle de cet élément sur la trempabilité de l'acier. Néanmoins, ajouté en faibles quantités, il permet, par formation notamment de nitrures de titane (TiN) et de nitrures d'aluminium (AlN), d'éviter un trop fort grossissement du grain austénitique lors des traitements thermiques subis par l'acier. De même, il permet aussi dans ce cas la formation de précipités de carbonitrures qui vont aider au piégeage de l'hydrogène.At the steelworks, by continuous casting, long semi-products (billets or blooms) are produced in a micro-alloyed steel having, besides iron, and less than 0.45% of molybdenum that is assigned, the following chemical composition, in terms of weight:

• 0.3 to 0.5% carbon.

For contents of less than 0.3%, the very high desired strengths can not be attained in view of the content of the other elements present in the grade and the targeted high-income temperatures. For contents above 0.5% the risk of brittleness increases because of the increase in hardness.

• At least 0.20% molybdenum but never reach more than 0.45% for the reasons indicated.

Molybdenum exhibits a strong interaction with phosphorus, limiting its harmful effect by limiting its segregation at the grain boundaries. In addition, it displays a marked carburigenic behavior. It allows, for given mechanical characteristics, higher temperatures of income, favoring suddenly the development of the carbides which will be traps with hydrogen. It is therefore an element that enhances the resistance to delayed fracture.

• from 0.4 to 1.0% manganese.

Increasing the manganese content tends, as a rule, to decrease the delayed fracture strength of the steel. This could be due to its interaction with sulfur leading to the formation of manganese sulphides. When we exceed thresholds of 1% of manganese, this interaction with sulfur could even lead to increase the fragility of steel with hydrogen, this, of course, in the absence of adequate provisions for the to avoid. However, manganese has a beneficial effect on the hardenability of steel and thus on the achievement of the final mechanical characteristics sought on the parts produced.

• less than 0.015% phosphorus.

The effect of phosphorus is particularly harmful in the steels according to the invention for several reasons. By a contrarian effect of the hydrogen recombination, it contributes to a higher concentration of atomic hydrogen likely to be able to penetrate into the material, thus to an increased risk of delayed rupture of the part in use. In addition, segregating at the grain boundaries, it decreases their cohesion. Its content must therefore be kept very low. To this end, it will be ensured that the steel is dephosphorized when it is prepared in the liquid state.

• from 0.05 to 0.2% silicon.

Silicon acts as deoxidizer of the steel during its elaboration, in the liquid state. Present in solid solution in solidified metal, it also increases the strength of steel. However, at too high a content (more than 0.2%), it can have a detrimental effect. During heat treatments, such as a globulization treatment, silicon tends to form intergranular oxides and thus reduces the cohesion of the grain boundaries. Too high a silicon content also decreases the ability of the steel to cold deformation by excessively hardening the matrix. It is mainly for this reason that, in the case of the steel grade according to the invention, its maximum content has been set at 0.2%.

• 0.05% maximum of aluminum.

Aluminum is a deoxidizer of steel in the liquid state. It then contributes, in the form of nitrides, to control the magnification of the austenitic grain during hot rolling. On the other hand, present in too great a quantity, it can lead to a magnification of inclusions of aluminates type in the steel which can be detrimental to the properties of the metal, in particular its resilience.

• 0.4 to 2.0% chromium.

Chromium is generally sought for its hardening effect. Like molybdenum, it delays the softening of the income, allowing higher tempering temperatures which favors degassing but also the formation of carbides trapping hydrogen. At too high a content, by increasing the hardness of the steel excessively, it makes delicate its formatting by striking.

• from 0.04 to 0.8% nickel.

This element provides an increase in the strength of the metal and has beneficial effects on the brittle fracture resistance. It also improves, in a well known manner, the resistance of steel to corrosion.

• 0.02 to 0.045% niobium, 0.03 to 0.30% vanadium, and 0.02 to 0.05% titanium.

These three elements are often added to the liquid steel to increase the hardness of the material. Here, within the indicated ranges, they will also increase the delayed breaking strength in several ways. They will help to refine the austenitic grain and form precipitates that trap hydrogen. In addition, niobium traps phosphorus. Finally, the hardening effect of each makes it possible to earn income at a higher temperature. Their maximum content is set here to avoid obtaining too large precipitates which would then be harmful vis-à-vis the resistance of the steel to delayed failure.
In particular, niobium, when added in excess, leads to an increased risk of "crack" defects on the surface of billets and continuous casting blooms. These defects, if they can not be completely eliminated, can be very detrimental to the respect of the integrity of the characteristics of the final part, in particular with regard to the resistance to fatigue and the resistance to hydrogen. This is the reason why, in the case of the grade according to the invention, its content had to be contained below 0.045%.

• from 0.003 to 0.005% boron.

By segregating with old austenitic grain boundaries, boron, even at very low levels, increases the hydrogen-induced delayed fracture strength. It greatly increases the hardenability of the steel and thus makes it possible to limit the carbon content necessary to obtain the desired martensitic microstructure. It increases the cohesion of the grain boundary by its intrinsic effect, but also by making it more difficult to segregate phosphorus at these grain boundaries. Finally, boron acts in synergy with molybdenum and niobium, thus increasing the efficiency of these elements and their own influences that allow their respective contents. An excess However, boron (above 0.005%) would lead to the formation of brittle iron boro-carbides.

• less than 0.015% sulfur.

Sulfur is, for steel, a poison that expresses all its harmfulness in the presence of hydrogen, because it has an additive effect, that is to say, cooperative with it, forming in particular H ₂ S, which in the middle wet in particular leads unstoppably to rapid physical degradation of parts. Its effect is in this respect much more marked than that of phosphorus. Its content must therefore be limited as far as possible, the closest to zero if possible, in any case not to exceed the limit of 0.015% enacted here. Steel must therefore be carefully desulphurized when it is prepared in the liquid state at the steelworks.

• less than 150 ppm nitrogen.

Nitrogen is considered harmful. It traps boron by forming boron nitrides, which renders ineffective the role of this element on the hardenability of steel. However, added in small amounts, it allows, particularly by formation of titanium nitride (TiN) and aluminum nitride (AlN), to avoid too high austenitic grain magnification during heat treatments on steel. Similarly, it also allows in this case the formation of carbonitride precipitates which will help the trapping of hydrogen.

This optimized composition makes it possible to have a very good resistance to hydrogen at the same time as a final mechanical strength of the steel, once transformed into a ready-to-use hammer after final heat treatment, greater than 1200 MPa and can even exceed 1,500 MPa, while retaining the same way as usual to carry out this transformation.

After reheating above 1100 ° C., if necessary, the steel semi-finished product (bloom, or more generally, billet) is then hot-rolled in the austenitic range, according to the usual practice, until it is obtained. a long rolled product, ready for shipment to customers after cooling to ambient. This long steel product is then in the form of bars, or more generally in the form of wire-wound machine for the selected applications.

The wire-machine is then transformed into screw by cold stamping, schematically in the following conventional manner:
The transformer receives the wire and after mechanical descaling (or chemical etching possibly followed by neutralization), it performs on the wire an annealing in a neutral atmosphere (under nitrogen for example). The yarn is then defatted before undergoing a first drawing, called drawing-roughing, for which a preliminary surface coating is provided, typically phosphating and soaping. During this drawing, the diameter of the wire is reduced by about 30%.

The wire-blank obtained is then subjected to a globulization treatment which, by providing a temporary drop in its hardness (intermediate Rm to about 500 MPa), will facilitate its subsequent forming, when striking, preserving the tool. This first heat treatment is followed by stripping, phosphating and soaping for a second drawing. This one is a finishing drawing, also called "final setting". The diameter reduction is more modest than before, generally less than 10%.

The wire, with a resistance temporarily weakened around 500 MPa, is then easily struck cold. The obtained raw stamping screws are first dephosphated, then subjected to a final quenching and tempering heat treatment, as well as to a final rolling operation to give the thread its final appearance. The rolling can be done either before the heat treatment or after. The income can advantageously operate at temperatures higher than the usual practice, namely of the order of 400 ° C and more, without compromising the achievement of the ultimate ultimate strength expected for screws produced ready. in use, with a Rm of 1200 to 1500 MPa and more .. Of course, the higher the income will be at high temperature, the lower the final Rm will be.

The surface of the screws is then cleaned and coated with a layer of phosphates or, if appropriate, with any other suitable chemical or electrochemical coating.

Note that if the grade of steel has been specially developed to provide good resistance to hydrogen, it is of course also desirable to introduce as little hydrogen as possible during the process of converting the wire rod. However, these processes of transformation into struck and coated parts are usually, by nature, generating hydrogen uptake. For example, during pickling, bath parameters (temperature, nature and acid concentration, iron pollution, inhibitor content, etc.) have an effect on the introduction of hydrogen into the steel. Similarly, the phosphating treatment being hydrogen generating, it will be necessary to optimize the treatment parameters to minimize the uptake of hydrogen by the metal at this stage of the transformation. The know-how of the skilled person will also play an important role during the austenization step before quenching. It has been shown that this step of the forming process can lead, when the proper precautions are not taken, to a non-negligible penetration of hydrogen into the steel.

We now give some figures, using the following tables of values, relating to the grade of micro-alloy steel according to the invention by positioning it with respect to known grades.

Laboratory tests were carried out on flows of the following chemical composition (in percentages by weight): VS mn P S Yes Or Cr MB Nb V Ti B AT 0.36 0.48 0.006 0,008 0.07 0.35 1.17 0.55 0,035 0.13 0.02 0.0025 B 0.37 0.79 0.014 0.01 0.08 0.25 1.20 0.31 0.033 0.11 0.02 0.0026 VS 0.36 0.64 0,013 0.01 0.08 0.39 1.11 0.45 0,037 0.11 0.02 0.0025 D 0.38 0.79 0.006 0,007 0.07 0.39 1.16 0.20 0,035 0.14 0.02 0.0024 42CD4 0.41 0.87 0,011 0.005 0.22 0.08 1.04 0.15 - - - -

With each time Al ≤ 0.05% and N ≤ 0.015%.

Note also that, according to its manufacturing process, and especially when it is made from scrap steel can contain up to 0.15% copper.

Castings A and 42CD4 are known steel shades of the prior art. Castings B, C and D are examples of the steel grade according to the invention.

The known grade A comprises in particular a molybdenum content greater than 0.5% and the known grade 42CD4 does not contain niobium, vanadium, titanium or boron.

The mechanical characteristics of the final pieces obtained are as follows, where Δ (Z) expresses the necking: Tr (° C) Rm (MPa) Δ (Z) in% AT > 400 1538 <5 B > 400 1532 <5 VS > 400 1545 <5 D > 400 1535 <5 42CD4 > 400 1505 16.5

The second column, Tr, indicates the tempering temperature after quenching of the final pieces. The third column, Rm, gives the tensile strength determined by pulling on standard specimens.

With respect to delayed fracture strength (last column), these results were obtained by slow tensile tests (0.005 to 0.01 mm / min versus 5 mm / min usually) on loaded and unloaded standard specimens. in hydrogen. The hydrogen loading conditions are identical for all five shades tested. The quantity of hydrogen introduced into the test pieces is greater than that introduced by the striking operation. Deferred breaking behavior is expressed by the Δ (Z), namely the average Z of the uncharged samples decreased by the mean Z of the loaded test pieces, Z being the measurement of the necking of the test piece during its rupture during its elongation. In other words, the greater the reduction in necking is important when the steel is loaded with hydrogen (and therefore the higher the Δ (Z)), the less the steel is resistant to delayed failure.

As can be seen, the grades of the invention B, C and D make it possible to obtain hydrogen withstand and strength results equivalent to the known grade A containing more than 0.5% molybdenum. The known 42CD4 grade, also containing little molybdenum, but containing no niobium, vanadium, boron or titanium, gives good results from a mechanical strength point of view, but does not offer a satisfactory performance at hydrogen.

The presence of elements such as titanium, boron, vanadium and niobium under the conditions defined by the invention is therefore essential for obtaining grades with high mechanical characteristics and having improved delayed breaking strength for shades. of low molybdenum steel.

The microalloyed steel according to the invention is therefore remarkable in that it exhibits both good aptitude for cold mechanical deformation (forging or forging) and good resistance to hydrogen (breaking strength). delayed) and in that it makes it possible to obtain, after tempering and tempering heat treatment, ready-to-use mechanical parts having a very high breaking strength.

It makes it possible, in fact, to temporarily maintain a low resistance (say less than 550 MPa) and a high ductility on the wire-machine that comes to the cold stamping, and then, after its transformation into pieces ready for use, to bring, by a conventional tempering heat treatment back, this same mechanical resistance to levels three times higher (1500 MPa and more) and to maintain good ductility.

Also, the steel grade of the invention is a raw material of choice for the industrial production of assembly parts with high mechanical properties required, such as screws for the automotive industry, when packaged in wire-machine or, more generally, in hot rolled long steel product resulting from continuous casting in the form of billets or blooms.

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 to the extent that its definition given in the appended claims is respected.

Thus, although it was initially designed to meet a specific need expressed by the automotive industry faced with the issue of the durability of the vital organs of motor vehicles, it nevertheless remains of more general application to the production of all mechanical parts of small and medium size, such as rivets, clips, staples, various fasteners, etc ... since it is sought a high normalized rupture limit (Rm of 1200 MPa and more) combined with good resistance to embrittlement by hydrogen .

Claims (7)

1. Micro-alloyed steel with good resistance to embrittlement by hydrogen for the cold forming of mechanical parts with very high characteristics, characterized in that, in order to contain its weight content of molybdenum below 0.45%, its chemical composition, in addition to iron and the inevitable residual impurities resulting from the preparation of the steel, corresponds to the following analysis, given in percentages by weight: $$0.3\le \mathrm{C}\%\le 0.5$$

   

   $$0.20\le \mathrm{Mo}\%<0.45$$

   

   $$0.4\le \mathrm{Mn}\%\le 1.0$$

   

   $$0.4\le \mathrm{Cr}\%\le 2.0$$

   

   $$0.04\le \mathrm{Ni}\%\le 0.8$$

   

   $$0.02\le \mathrm{Nb}\%\le 0.045$$

   

   $$0.03\le \mathrm{V}\%\le 0.30$$

   

   $$0.02\le \mathrm{Ti}\%\le 0.05,\mathrm{with}\mathrm{Ti}>3.5\mathrm{N}$$

   

   $$0.003\le \mathrm{B}\%\le 0.005\%$$

   

   $$\mathrm{S}\%\le 0.015$$

   

   $$\mathrm{P}\%\le 0.015,$$

   

   and optionally 0.05 ≤ Si% ≤ 0.20; Al% ≤ 0.05 and N% ≤ 0.015.
2. Steel according to claim 1, characterized in that it is in the form of a hot-rolled bar or wire rod from continuous casting in the form of blooms or billets.
3. Steel wire rod or bar, characterized in that it is made of micro-alloyed steel according to claim 1 in order to be able to present, by cold forming processing and quenching and tempering heat treatment, mechanical resistance of 1200 to 1500 MPa and above, combined with good resistance to hydrogen.
4. Mechanical part ready for use characterized in that it is obtained by cold forming a wire rod according to claim 3.
5. Mechanical part ready for use, cold formed and having high mechanical characteristics and resistance to hydrogen, characterized in that it is micro-alloyed steel having, in order to contain its weight content of molybdenum below 0.45%, a chemical composition which, in addition to iron and the inevitable residual impurities resulting from the preparation of steel, corresponds to the following analysis, given in percentages by weight: $$0.3\le \mathrm{C}\%\le 0.5$$

   

   $$0.20\le \mathrm{Mo}\%<0.45$$

   

   $$0.4\le \mathrm{Mn}\%\le 1.0$$

   

   $$0.4\le \mathrm{Cr}\%\le 2.0$$

   

   $$0.04\le \mathrm{Ni}\%\le 0.8$$

   

   $$0.02\le \mathrm{Nb}\%\le 0.045$$

   

   $$0.03\le \mathrm{V}\%\le 0.30$$

   

   $$0.02\le \mathrm{Ti}\%\le 0.05,\mathrm{with}\mathrm{Ti}>3.5\mathrm{N}$$

   

   $$0.003\le \mathrm{B}\%\le 0.005\%$$

   

   $$\mathrm{S}\%\le 0.015$$

   

   $$\mathrm{P}\%\le 0.015,$$

   

   and optionally 0.05 ≤ Si% ≤ 0.20; Al% ≤ 0.05 and N% ≤ 0.015.
6. Mechanical part according to claim 5, characterized in that it is an assembly screw.
7. Screw according to claim 6, characterized in that it is a component part of assemblies of motor elements or ground connections of motor vehicles produced by the automotive industry.