EP3704280B1 - Martensitic stainless steel and method for producing same - Google Patents

Description

The present invention relates to a martensitic stainless steel and its method of manufacture.

This steel is more particularly, but not exclusively, intended to be used in the automobile industry, to form parts such as bodywork elements, intended to deform during an impact by absorbing a maximum quantity of energy.

To form vehicle bodywork elements, it is possible to use martensitic stainless steels shaped by hot stamping from sheet metal. They have the advantage of having a very high mechanical resistance Rm, and thus of allowing a lightening of the bodywork, with equal performance compared to steels more conventionally used in the past. However, the martensitic stainless steels known for this purpose have the disadvantage of having a limited bending capacity. They therefore do not have the ability to satisfactorily absorb the energy collected by the vehicle during an impact and the requirements imposed by the regulations on this subject, verified during impact tests ("crash -tests"), are difficult to satisfy.

This bending capacity of the steel is usually assessed by three-point bending tests carried out according to standard NF EN ISO 7438 and the VDA 238-100 procedure.

At the start of the test, the punch is brought into contact with the sheet, itself supported by two rollers, with a pre-force of 30 N. Once contact is established, the displacement of the punch is indexed to zero. The test then consists in moving the punch so as to carry out the “three-point bending” of the sheet.

The test stops when a micro crack in the sheet leads to a drop in force on the punch of at least 30 N, or when the punch has moved by 14.2 mm, which corresponds to the stroke maximum allowed.

At the end of the test, the sheet metal sample is therefore bent. The ductility in service is then evaluated by measuring the bend angle at ₁₀ %, in degrees. The higher the ₁₀ % angle, the better the crimping or bendability of the sheet.

JP-A-2004-052060 describes a steel for turbine blades whose microstructure, very low in ferrite, consists essentially of frankly tempered martensite, and which contains carbides which increase its tensile strength but which would be unfavorable to its ability to bend. EP-A-0 083 254 is very comparable to him from all these points of view.

Moreover, CN 102260826 A describes a martensitic stainless steel for high temperature applications.

The object of the invention is to provide a martensitic stainless steel with a composition and microstructure which make it well suited to the use which has just been mentioned, in that it would have good V-bendability characterized by the aforementioned standard, and a high bending angle θ, as well as sufficiently high mechanical strength properties Rm.

To this end, the subject of the invention is a martensitic stainless steel and a process for manufacturing said steel as described in the appended claims.

As will have been understood, the invention is based on the coupling between a steel composition and a particular microstructure. This coupling makes it possible in particular to prepare sheets capable of being easily shaped by hot stamping, and of presenting a very good compromise between a high tensile strength Rm and a high aptitude for bending in the final martensitic state, resulting in an equally high bending angle θ, measured according to the NF EN ISO 7438 standard and the VDA 238-100 procedure. These sheets are therefore particularly well suited to use as automobile body parts having a high capacity for absorbing energy during impacts to which the vehicle is subjected.

The measurement of the product “Rm x θ/180”, Rm being expressed in MPa and θ in degrees, which should be as high as possible, would be a good indicator of the capacity of the steel to reach such a compromise.

It is in fact known that the energy absorbed by a tubular structure in bending or compression is proportional to the mechanical strength of the material that composes it, to the bending angles of the different zones which deform as well as to a geometric factor of the section: thickness, width height. Reference may be made to the articles of D.Kecman Int.J. Mech. Sci, Vol.25, N°9-10, pp 623-636, 1983 or of T.Wierzbicki Computers & Structures, Vol.51, N°6, pp625-641, 1994 published by Elsevier .

The invention will be better understood from the description which follows.

The minimum C content of 0.05% is justified by the need to obtain austenitization of the microstructure during the first stage of the manufacturing process, therefore before quenching. This will govern the mechanical properties of the sheet, in particular its ability to be shaped and its mechanical strength. A content according to the invention of more than 0.30% is not desirable so as not to embrittle the martensite formed as it is quenched (which would reduce the ability to bend) and degrade the weldability, which would be a disadvantage for the privileged application of the invention to the automobile. A maximum content of 0.20% is however preferred, to better ensure obtaining a completely satisfactory bend angle when other conditions required by the invention on the composition of the steel and/or the treatments thermals experienced are close to the set limits.

The minimum Mn content of 0.20% achieves the necessary austenitization. Above 2.0% oxidation problems are to be feared during heat treatments if these are not carried out in neutral or reducing atmospheres. The mandatory use of neutral atmospheres can lead to a high cost for the execution of heat treatments. In addition manganese in quantity greater than 2.0% is unfavorable to obtaining the desired martensitic structure because of the possible presence of residual austenite at the end of quenching.

Its Si content is between traces and 1.0%. Silicon can be used as a deoxidizer during production, just like Al, to which it can be added or substituted. Beyond 1.0%, it is considered that it excessively promotes the formation of ferrite and therefore makes austenitization more difficult, and that it weakens the sheet too much for the shaping of a complex part to be able be carried out satisfactorily.

Furthermore, the total content of Mn and Si must not exceed 1.5%. As Mn and Si are both segregating elements, too high a total presence of Mn and Si could adversely affect the homogeneity of the martensitic microstructure, in particular depending on the thickness of the part if this is relatively high. The minimum content of Mn + Si is 0.2%, since Mn is necessarily present at a content of at least 0.2%, as seen above.

Its S content is between traces and 0.01%, in order to guarantee suitable solderability and resilience to the final product. Beyond that, the corrosion resistance would be degraded and the bending angle reduced (because of the MnS precipitates in particular), and in order not to have excessive precipitation of MnS, it is important in the invention to have the following relationship where the Mn and S contents are expressed in mass % (if Mn and S are each present in the state of simple impurities, in other words as traces resulting from production, it is assumed that for this relationship, their contents are considered as equal to 0):
0 ≤ 10,000 x Mn x S ≤ 40

Its P content is between traces and 0.04%, to ensure that the final product will not be excessively brittle. P is also detrimental to weldability and increases the brittleness of ferrite and martensite at grain boundaries.

Its Cr content is between 10.5 and 17.0%. The minimum content of 10.5% is justified to ensure the stainless steel. A content higher than 17% would make austenitization difficult and would unnecessarily increase the cost of the steel.

Its Ni content is between traces and 4.0%.

The presence of Ni within the prescribed limit of 4.0% maximum is advantageous for promoting austenitization. It is necessary, simultaneously, to respect the condition [Cr - 10.3 - 80 x (C + N) ² ] ≤ (Mn + Ni), in other words (Mn + Ni) - [Cr - 10.3 - 80*(C+N) ² ] ≥ 0, where the various contents are in mass %.

Exceeding the 4.0% limit and/or not respecting the previous equation linking Cr, C, N, Mn and Ni would however lead to an excessive presence of residual austenite and an insufficient presence of martensite in the microstructure after cooling. Ni is also an expensive element, and its quantity should, as far as possible, be limited to what is strictly necessary to obtain the desired properties.

Its Mo content according to the invention is between traces and 2.0%, preferably 1.0%.

The presence of Mo is favorable to good corrosion resistance. Above 2.0%, austenitization would be hampered and the cost of steel unnecessarily increased. A maximum content of 1.0% is preferred, because Mo is an embrittling element in that it limits the diffusion of hydrogen, which can thus be removed from the metal with greater difficulty.

Its W content is defined according to the invention as a function of that of Mo by respecting the relationship traces ≤ Mo + 2W ≤ 2.0%, preferably traces ≤ Mo + 2W ≤ 1.0%. The advantages and disadvantages of W are qualitatively comparable to those of Mo.

Its Cu content according to the invention is between traces and a maximum of 2.0%, with a preference for a content ≤ 0.5% if the steel is intended to manufacture a part which will have to be welded.

These requirements on Cu are classic for this type of steel. In practice, this means that an addition of Cu is not useful and that the presence of this element is only due to the raw materials used. A content greater than 0.5%, which would generally correspond to a voluntary addition of Cu, is very often not desired, because it would degrade the weldability. Cu can help with austenitization, however, and up to 2.0% can be accepted if the final part is not intended to be welded. This content of 2.0% should not however be exceeded, so as not to risk finding too much residual austenite in the steel.

For this same reason related to the possible excessive presence of residual austenite, the sum Cu + Co must not exceed 2.0%, and the sum Cu + Co + Ni must not exceed 4.0%.

Its Ti content is between traces and 0.5%.

Ti is a deoxidizer, like Al and Si, but its cost and its lesser efficiency than that of Al for deoxidation, with an equal quantity added, makes its use generally unattractive from this point of view. Ti may, however, be of interest in that the formation of nitrides and carbonitrides of Ti may limit grain growth and favorably influence certain mechanical properties and weldability. However, this formation can be a disadvantage in the case of the process according to the invention, because Ti tends to interfere with austenitization due to the formation of carbides, and TiN degrades resilience and bendability. A maximum content of 0.5% should therefore not be exceeded.

Its V content is between traces and 0.3%.

Its Zr content is between traces and 0.5%.

Like Ti, V and Zr are embrittling elements which are liable to form nitrides, and must not be present in excessive amounts, alone and in combination. On this last point, it is necessary to satisfy the following limit:
traces ≤ Ti + V + Zr ≤ 0.5%

Its Al content is between traces and 0.2%.

Al is usually used as a deoxidizer during production. A quantity exceeding 0.2% must not remain in the steel after deoxidation, because there would be a risk of forming an excessive quantity of AIN degrading the mechanical properties, and also of having difficulty in obtain the predominantly martensitic microstructure because Al is very ferritizing.

Its O content is between traces and 400 ppm.

The requirements on the O content are those which are conventional for martensitic stainless steels, depending on the ability to form them without cracks starting from the inclusions, and on the quality of the mechanical properties or bends sought on the final part, and that the excessive presence of oxidized inclusions is likely to alter.

A maximum Ca content of 20 ppm is tolerated. The addition of this element is not justified for reasons which would be linked to the final properties of the steel. But it can be present following the elaboration of the liquid steel, if it has been used, as is conventional, for the deoxidation of the liquid steel and the control of the composition and the morphology of the inclusions oxidized.

It is essentially the addition of deoxidizers Al, Si, Ti, Zr during production, the possible addition of Ca, the care then taken in the decantation of oxidized inclusions within the liquid steel and the subsistence of these deoxidizers in the dissolved state in the solidified steel, which determine the final O content. them (most often Al and/or Si) is present in sufficient quantity to ensure that the O content of the final sheet will not be too high for trouble-free forming of the part, and for future applications of the room whose mechanical properties will be degraded by an excessive presence of oxide inclusions. These mechanisms governing the deoxidation of steels and the control of the composition and the quantity of their oxidized inclusions are well known to those skilled in the art, and apply within the scope of the invention in a perfectly conventional manner.

Its Nb content is between traces and 0.3%, as is its Ta content. Since Ta is an element very close to Nb in terms of its metallurgical effects, in particular the formation of carbonites, the relationship 0.25 ≤ (Nb + Ta )/(C + N) ≤ 8 will be satisfied.

Nb and Ta are important elements for obtaining good resilience and good bending capacity, and at least one of them must be significantly present. But since they can interfere with austenitization, they should not be present in quantities exceeding what has just been prescribed. Also, these elements capture C and N, and their total content must be adjusted according to the C and N contents actually present in the steel. If there is too little Nb and/or Ta in the steel when the C and N contents are relatively high, there will not be enough dissolved Nb and Ta left for these elements to improve toughness sufficiently.

0.25≤(Nb+Ta)/(C+N)≤8 is therefore required to obtain resilience at 20° C. of the order of 50 J/cm ² or more.

Its Co content is preferably between traces and 0.5%. This element is, like Cu, likely to help austenitization. However, too much should not be added so as not to deteriorate the weldability if the steel is intended to be transformed into a part to be welded. Otherwise, a Co content according to the invention of a maximum of 2.0% is acceptable, provided, as has been said, not to exceed a total Cu + Co content of 2.0% and a total Cu + Co + Ni content of 4.0%, so as not to end up with too high a residual austenite content.

Its Sn content is between traces and 0.05%. This element is not desired because it is detrimental to the weldability and the ability of the steel to be hot transformed. The 0.05% limit is a tolerance which, in practice, will most often correspond to an absence of voluntary addition.

Its B content is between traces and 0.1%.

B is not obligatory, but its presence is advantageous for the hardenability and for the forgeability of the austenite. It therefore facilitates hot forming. Its addition above 0.1% does not, however, provide any significant additional improvement on this point, and increases the risks of precipitation in the form of boron nitrides, which would be unfavorable to forming.

Its H content is comprised according to the invention between traces and 5 ppm (0.0005%), preferably no more than 1 ppm (0.0001%) and, better still, no more than 0.1 ppm (0.00001%). ).

An excessive H content tends to embrittle martensite and ferrite. It will therefore be necessary to choose a production method for the steel in the liquid state which can ensure this low presence of H. Typically, treatments ensuring extensive degassing of the liquid steel (by massive injection of argon into the liquid steel under atmospheric pressure, well-known process known as “AOD”, or by passage under vacuum) are recommended.

Its N content is between traces and 0.2%.

N is an impurity of which the same treatments which make it possible to reduce the H content contribute to limiting the presence, or even to reducing it appreciably. It is not always necessary to have a particularly low N content, but its content, considered together with those of elements with which it can combine to form nitrides or carbonitrides, must not be too high. This results in the relationship 0.25 ≤ (Nb + Ta)/(C + N) ≤ 8 seen above, compliance with which contributes to providing sufficient resilience.

Its total rare earth and Y content is between traces and 0.06%.

Rare earths and Y improve the properties of resistance to oxidation (lower oxidation kinetics, therefore less oxide formed), which can be an advantage during hot forming. On the other hand, if they are present in large quantities, the non-metallic inclusions they form generate serious problems during casting, or, further downstream, during pickling, due to the high reactivity of rare earths and Y. The possible total addition of rare earths and Y at 0.06% The price of these elements means that it is, in any case, generally preferable to use more common elements, such as Al or Si, for ensure deoxidation.

It should be understood that the sheets prepared according to the invention can be coated sheets, the coating (generally with Zn, or Al, or alloys of which one and/or the other are among the main components ) which can take place before or after the forming of the sheet. The coating techniques that can be used do not differ from those usually practiced on steels (immersion in a molten bath of Al, or Zn, or one of their alloys, electrodeposition, CVD/PVD). This coating, typically 1 to 200 μm thick and present on one or both sides of the sheet, may have been deposited by any technique conventionally used for this purpose. It is simply necessary that, if it was deposited before the austenitization, it does not evaporate during the stay of the sheet at the austenitization temperatures, which can sometimes happen even if this stay is short.

The choice and optimization of the characteristics of the coating and its mode of deposition, so that these conditions are met, do not go beyond what the person skilled in the art knows how to do, when he is called upon to shape conventionally already coated stainless steel sheets. If the coating takes place before the austenitization, we can, however, favor the coatings based on Al compared to those based on Zn, as Al has less tendency than Zn to evaporate at austenitization temperatures. .

The steel having the composition according to the invention is produced, cast and hot transformed in a conventional manner, for example put in the form of hot and/or cold rolled sheets in a typical range of thickness ranging from 0.5 mm at 12mm. However, in the preferred application of the invention to automotive sheets, the preferred thickness will be in the range 0.5 to 4 mm.

Then, according to the invention, the transformed product thus obtained first undergoes austenitization, which brings it to a range of temperatures between the temperature Ac1 at which austenite appears during heating and 1100° C., ideally between Ac1 + 100°C and 1050°C, for a duration varying from 10 s to 1 hour to both guarantee complete austenitization and limit oxidation of the product as well as the energy cost of austenitization. It must be understood that this austenitization temperature must concern the entire volume of the sheet, and that the treatment must be long enough so that, taking into account the thickness of the sheet and the kinetics of the transformation, the austenitization is complete throughout this volume. This is, of course, valid for a semi-finished product other than a sheet.

The furnace where the austenitization takes place can be a conventional furnace or an induction furnace which allows rapid and homogeneous heating, but the heating rates must be greater than 5°C/s to avoid coalescence of the carbide precipitates already present. , which would slow down their dissolution during austenitization. The desire not to impose too long a total duration on the austenitization stage also motivates this minimum heating rate. The atmosphere is, in the standard case, air, and the time and temperature conditions are optimized to limit oxidation while guaranteeing complete austenitization.

It is, however, preferable to avoid carrying out this austenitization in an oxidizing atmosphere such as air, in order more certainly to avoid significant surface oxidation and/or decarburization of the sheet in the heating atmosphere. Surface oxidation would lead to the need to mechanically or chemically descale the sheet before it is shaped to avoid incrustation of scale in the surface of the sheet, and would lead to a loss of material and a degradation of the final appearance. of the surface of the formed part. Excessive skin decarburization to a depth of a few tens of µm would reduce the hardness and tensile strength of the sheet. The risks of observing significant oxidation and/or decarburization depend, in a known manner, not only on the austenitizing temperature, but also on the treatment atmosphere of the furnace. A non-oxidizing atmosphere, therefore neutral or reducing (nitrogen, argon, CO, hydrogen and their mixtures), preferably to air, makes it possible to increase the treatment temperature without damage, which makes it possible to ensure complete austenitization in a minimum of time. If a hydrogenated atmosphere is used, it must be ensured that it would not lead to hydrogen being taken up by the metal, which would cause it to exceed the limits prescribed above.

Typically, austenitization takes place at a temperature between 925°C and 1000°C for a period of 10 s to 1 h (this period being that the sheet passes above Ac1), preferably between 2 min and 10 min for heating in a conventional oven and between 30 s and 1 min for an induction oven. An induction furnace has the advantage, known per se, of providing rapid heating up to the nominal austenitizing temperature. In addition, it immediately concerns the entire volume of the sheet, and therefore allows a shorter treatment than a conventional oven to achieve the desired result. These temperatures and durations make it possible to ensure that the continuation of the treatments will lead to a sufficient formation of martensite, and this for a reasonable duration allowing good productivity of the process.

The purpose of this austenitization is to change all of the metal from the initial ferrite + carbides microstructure to an austenitic microstructure containing a maximum of 0.5% carbides in volume fraction, and a maximum of 20% residual ferrite in volume fraction. One purpose of this austenitization is, in particular, to lead to a dissolution of at least the majority of the carbides initially present, so as to release C atoms to form the martensitic structure during the following stages of the process. The maximum residual ferrite content of 20%, which must remain on the final product, is justified by the resilience and the conventional elastic limit which it is desired to obtain. The duration of austenitization is adjusted so that this microstructure is obtained throughout the treated steel, and can therefore vary according to the precise dimensions of the semi-finished product at this stage. A person skilled in the art can easily carry out this adjustment by modeling and/or experiments carried out on the installation at his disposal, for a semi-finished product of given shapes and dimensions.

The Ac1 temperature depends on the chemical composition, and also on the heating rate. It is measured, as is known, by following the expansion of a sample when heating at a predefined heating rate between 10 and 100°C/s. Ac1 is the first temperature at which the slope of the “temperature = f(time)” curve cancels out, and which therefore corresponds to the appearance of the austenitic phase. It should be understood that in the invention, the temperature Ac1 is that which is obtained experimentally on the semi-finished product having the required composition, but Ac1 is a quantity which does not depend on the dimension of the semi-finished product, but only on its composition. , essentially.

Then after this austenitization, according to the invention the semi-finished products are quenched, from the austenitization temperature, either in the open air, calm or pulsed, or by immersing them in a tank of water or oil. at ambient temperature, or in shaping tools in the case of the use of a hot forming process such as hot stamping. The objective is to obtain, throughout the volume of the semi-finished product, a cooling rate of between 0.5 and 1000°C/s down to a temperature below the temperature Ms at the start of the martensitic transformation, which is typically around 300°C, followed by cooling between 0.5 and 20°C/s to room temperature. Ms depends on the composition of the steel and can be determined, conventionally, by formulas and modeling or dilatometric tests. See, for example, the document " FB Pickering, Physical Metallurgical Development of Stainless Steels", Stainless Steels '84, pp2-28, The Institute of Metals, London, 1985 . Cooling down to ambient temperature is the most usual case, special control of the cooling rate below Ms not being necessary.

• Résistance à la traction Rm : au moins 900 MPa ;
• Limite élastique conventionnelle Rp_0,2 : au moins 700 MPa ;
• Allongement à la rupture A : au moins 5%, mesuré suivant la norme ISO 6892 ;
• Résilience préférée si possible : pour les états ou le taux de martensite est supérieur à 75% au moins 50 J/cm² à 20°C ;
• Capacité d'angle de pliage, mesurée sur un échantillon d'épaisseur 1,5 mm selon la norme VDA 238-100 : au moins 50°.
• Capacité d'absorption en essai de choc mesurée par la relation (Rm x angle de pliage / 180°) supérieure à 450, où Rm est exprimé en MPa et l'angle de pliage en degrés, mesuré selon la norme VDA 238-100 sur un échantillon d'épaisseur 1,5mm.

A final product is thus obtained, for example in the form of a sheet, intended to be used in a structure to better absorb the energy due to the crash while resisting rupture by impact and which typically has the following properties at the ambient temperature :

• Tensile strength Rm: at least 900 MPa;
• Conventional elastic limit Rp _0.2 : at least 700 MPa;
• Elongation at break A: at least 5%, measured according to standard ISO 6892;
• Preferred resilience if possible: for states where the martensite content is greater than 75% at least 50 J/cm ² at 20°C;
• Bending angle capacity, measured on a 1.5 mm thick sample according to the VDA 238-100 standard: at least 50°.
• Absorption capacity in impact test measured by the relationship (Rm x bend angle / 180°) greater than 450, where Rm is expressed in MPa and the bend angle in degrees, measured according to the VDA 238-100 standard on a 1.5mm thick sample.

These properties are obtained thanks to the combination of the composition of the aforementioned steel and a suitable microstructure of this steel, obtained thanks to the austenitization-quenching treatment described, which is at least 75% martensitic, and contains at most 20% of ferrite whose grain size is according to the invention from 4 to 80 μm, preferably between 5 and 40 μm, and a volume fraction of carbides of at most 0.5%. The residual austenite fraction which can be tolerated after quenching is therefore at most approximately 5%, corresponding substantially to the difference between 100% and the sum of the martensite, ferrite and carbide fractions.

Also, an additional heat treatment can be carried out on the final part, therefore after cooling to ambient temperature, to improve its elongation at break and bring it to a value of more than 10% and without reducing the mechanical characteristics and the capacity. in folding. This treatment consists of keeping the final part between 90°C and 250°C for 10 s to 1 hour. This additional treatment can also be undergone during hardening by paint annealing (bake hardening), the temperatures and durations of which are in this range, typically 180° C. and 20 min. These treatments and the subsequent cooling are carried out in still air, therefore at a cooling rate of the order of a few °C/s.

• soit le traitement de référence suivant : recuit à 800°C pendant 15 min, sans trempe, suivi d'un décapage.
• soit le traitement thermique suivant selon l'invention : montée à 950°C à une vitesse de chauffage de 20°C/s, austénitisation à 950°C pendant 5 min, trempe jusqu'à 300°C à un vitesse de refroidissement de 10°C/s à l'air puisé ; ce traitement pouvant ou non être précédé du traitement de référence et d'un décapage.

Table 1 below shows the compositions of steels to which were applied, after elaboration and hot rolling carried out under similar and conventional conditions, then annealing in a furnace under an inert hydrogen atmosphere carried out at 800° C. for 5 hours on the hot rolled product, then cold rolling up to 1.5mm:

• or the following reference treatment: annealing at 800°C for 15 min, without quenching, followed by pickling.
• or the following heat treatment according to the invention: rise to 950°C at a heating rate of 20°C/s, austenitization at 950°C for 5 min, quenching to 300°C at a cooling rate of 10 °C/s forced air; this treatment may or may not be preceded by the reference treatment and by pickling.

The contents are given in % by weight. The elements not mentioned are present only in the state of traces resulting from the elaboration.

Table 2 shows, for the steels of Table 1, how they satisfy or not the relationships required by the invention. Non-inventive values are underlined.

Table 3 shows the metallurgical structures obtained after the heat treatments carried out on the various steels in Table 1. The underlined values are those which mean that the examples concerned are not considered to be in accordance with the invention, from the point of view of their microstructure. .

Table 4 shows the properties of the examples according to the invention, and those of the reference examples which do not satisfy all the relationships and do not achieve all the properties which the invention aims to obtain. The underlined values are those which are not satisfactory with regard to the criteria cited above. Impact tests were not carried out on the steels which had an insufficient martensite content which placed them outside the invention anyway.

The following remarks can mainly be deduced from these results.

The reference steels 9 to 13 are non-stainless martensitic steels (therefore not belonging to the class of steels of the invention and other reference steels 14 and 15) of known type, commonly used in the field of the automobile. They were tested with a view to showing how the properties of the steels of the invention were compared to theirs.

The reference steels 9 to 12 have compositions in accordance with the invention on the elements other than Cr taken individually. But they do not contain enough Nb + Ta compared to the sum of C + N, not enough Mn + Si (except 9, and just barely) and too much Mn compared to S. Those who have undergone heat treatment in accordance with the invention, at 950° C. for 5 min, nevertheless end up with a suitable microstructure. Their bending angles in the martensitic state are correct, but at the same time their Rm is not high enough to provide them with an absorption capacity in impact tests which would correspond to expectations. As for those which have undergone the reference heat treatment, they end up with a ferritic microstructure: their bending angle is high (120°) but the Rm being low due to the absence of martensite, the absorption capacity in crash test is far below the target.

The reference steel 13 also has too high a C content. As expected, the bend angle is even less than those of other examples of non-stainless martensitic steels which have undergone the same heat treatment. During the impact test, its very high Rm does not compensate for this mediocre bendability.

The reference steels 14 and 15 are martensitic stainless steels.

The 14 has a measured C content which is only slightly lower than the minimum required by the invention, and which could be assimilated to this minimum. On the other hand, it does not respect the relationship linking Mn, Ni, Cr, C and N. At the end of the heat treatment carried out under conditions which would be in accordance with the invention, it is left with an excessively high ferrite content. Consequently, if its bend angle is correct, its Rm is by far not high enough to ensure sufficient absorption capacity in impact testing.

The reference steel 15 has a C content frankly lower than that required by the invention, which gives it, after the heat treatment, an almost entirely ferritic microstructure. The virtual absence of Nb and Ta gives it a lower (Nb+Ta)/(C+N) ratio than required by the invention. The relation linking Mn, Ni, Cr, C and N is not respected either, which contributes to the very ferritic character of the microstructure. Consequently, Rm and Rp0.2 are very poor, and despite a very high bending angle, the absorption capacity in the impact test is insufficient.

Steel 1, whose composition is in accordance with the invention, has undergone three different heat treatments.

The treatment at 950° C. for 5 min followed by quenching is in accordance with the invention. The result is a steel which conforms to the requirements of the invention in all respects. In particular, its bend angle of 135° is very high, and since its Rm is correct, its absorption capacity in the impact test is excellent.

The reference heat treatment applied to this steel 1 also made it possible to obtain this high bending angle. But Rm is very insufficient (like Rp _0.2 ), and the absorption capacity in the impact test is frankly insufficient.

The heat treatment at 950°C for 2 hours followed by quenching applied to this steel is not satisfactory either. Its long duration led to an excessive increase in the size of the ferritic grains (130 μm against 30 μm for the treatment according to the invention and 35 μm for the reference treatment, carried out at a lower temperature but for longer than the treatment according to the invention). The consequence was that the steel has a lower bending angle than with the treatment according to the invention, and above all an Rm and an elongation at break A which are insufficient. The absorption capacity in the impact test is therefore not in accordance with the requirements of the invention.

These test results on steel 1 show that it is indeed the coupling between the composition of the steel and the austenitizing and quenching heat treatment in the specific conditions required which is important to obtain the desired results in terms of shock absorption capacity. And satisfactory mechanical properties.

The steel 3 according to the invention has also undergone different heat treatments, one according to the invention at 950° C. for 5 min, the other in accordance with the reference treatment. The treatment according to the invention made it possible to obtain on the steel 3 satisfactory properties from all points of view with regard to the objectives aimed for, with an almost entirely martensitic structure, to be linked to the lower presence of Si than in the steel 1, and a small ferritic grain size. But as for steel 1, the reference heat treatment only led to mediocre Rp _0.2 and Rm properties, insufficient to ensure good shock absorption capacity despite high bendability.

Steel 8 according to the invention, which is relatively rich in C, Nb and V, has also undergone the same two heat treatments as steel 3. With the treatment according to the invention, its structure is entirely martensitic. Its elongation at break and its bending angle are only correct, but its high Rm gives it sufficient shock absorption capacity. If the reference heat treatment is applied to it, its structure is entirely ferritic. The high bend angle is not accompanied by a sufficient Rm for the shock absorption capacity to be adequate.

Steel 6 according to the invention is distinguished from the other examples by a C content of 0.24%, therefore even higher than that of steel 8. Its structure is almost exclusively martensitic after application of the heat treatment according to the invention. Its bending angle is just sufficient due to the high C content which is not in the preferred range for the invention, but its very high Rm nevertheless gives it good shock absorption capacity.

Claims (15)

1. - Martensitic stainless steel, characterized in that its composition is, in weight percentages:

   - 0.05% ≤ C ≤ 0.30%;

   - 0.20% ≤ Mn ≤ 2.0%;

   - traces % ≤ Si ≤ 1.0%;

   - 0.20% ≤ Mn + Si ≤ 1.5%;

   - traces ≤ S ≤ 0.01 %, with 0 ≤ 10,000 x Mn x S ≤ 40;

   - traces ≤ P ≤ 0.04%;

   - 10.5% ≤ Cr ≤ 17.0%, with [Cr - 10.3 - 80 x (C + N)²] ≤ (Mn + Ni);

   - traces ≤ Ni ≤ 4.0%;

   - traces ≤ Mo ≤ 2.0%;

   - traces ≤ Mo + 2W ≤ 2.0%;

   - traces ≤ Cu ≤ 2.0%;

   - traces ≤ Ti ≤ 0.5%;

   - traces ≤ V ≤ 0.3%;

   - traces ≤ Zr ≤ 0.5%;

   - traces ≤ Al ≤ 0.2%;

   - traces ≤ O ≤ 400 ppm;

   - traces ≤ Ta ≤ 0.3%;

   - traces ≤ Nb ≤ 0.3%;

   - 0.25 ≤ (Nb + Ta)/(C + N) ≤ 8;

   - Nb ≥ [1.2 (C + N) - 0.1]%;

   - 0.009% ≤ N ≤ 0.2%;

   - traces ≤ Co ≤ 2.0%;

   - traces ≤ Cu + Co ≤ 2.0%;

   - traces ≤ Cu + Co + Ni ≤ 4.0%;

   - traces ≤ B ≤ 0.1%;

   - traces ≤ H ≤ 0.0005%;

   - traces ≤ rare earths + Y ≤ 0.06%;

   - traces ≤ Ca ≤ 20 ppm;

   - traces ≤ Sn ≤ 0.05% ;

   - Ti + Zr + V ≤ 0.5% ;

   the remainder being iron and impurities resulting from the processing; and in that its microstructure comprises at least 75% martensite, at most 20% ferrite, at most 0.5% carbides, and at most 5% residual austenite, corresponding to the difference between 100% and the sum of the fractions of martensite, ferrite and carbides, the size of the ferrite grains being between 4 and 80 µm.
2. - Martensitic stainless steel according to claim 1, characterized in that traces ≤ Cu ≤ 0.5%.
3. - Martensitic stainless steel according to claim 1 or 2, characterized in that traces ≤ Co ≤ 0.5%.
4. - Martensitic stainless steel according to one of claims 1 to 3, characterized in that 0.05% ≤ C ≤ 0.20%.
5. - Martensitic stainless steel according to one of claims 1 to 4, characterized in that traces ≤ Mo ≤ 1.0%.
6. - Martensitic stainless steel according to one of claims 1 to 5, characterized in that Mo + 2W ≤ 1.0%.
7. - Martensitic stainless steel according to one of claims 1 to 6, characterized in that traces ≤ H ≤ 0.00001 %.
8. - Martensitic stainless steel according to one of claims 1 to 7, characterized in that the size of the ferrite grains is comprised between 5 and 40 µm.
9. - Method for preparing a martensitic stainless steel product according to any one of claims 1 to 8, characterized in that:

   - a martensitic stainless steel with a composition as recited in one of claims 1 to 7 is produced, poured and transformed hot and/or cold;

   - an austenitization of said hot and/or cold transformed steel is performed by bringing it to a temperature between Ac1 and 1100°C for 10 s to 1 hour, with a reheating speed of at least 5°C/s, the duration of the austenitization being adjusted to obtain, throughout the steel, an austenitic microstructure containing at most 0.5% carbides by volume fraction and at most 20% residual ferrite by volume fraction;

   - and a quenching of said austenitized steel is performed, from its austenitization temperature to a temperature below its Ms temperature at the start of the martensitic transformation, at a cooling rate of between 0.5 and 1000°C/s.
10. - Method according to claim 9, characterized in that said steel is transformed into a hot-rolled and/or cold-rolled sheet.
11. - Method according to claim 10, characterized in that said hot-rolled and/or cold-rolled sheet has a thickness of 0.5 to 12 mm, preferably 0.5 to 4 mm.
12. - Method according to one of claims 9 to 11, characterized in that during the austenitization, the hot and/or cold shaped steel is brought to a temperature between Ac1 + 100°C and 1050°C for 10 s to 1 hour.
13. - Method according to one of claims 9 to 12, characterized in that an additional heat treatment is performed on the austenitized and quenched steel at a temperature of 90 to 250°C for 10 s to 1 h.
14. - Method according to one of claims 9 to 13, characterized in that the austenitization of said hot and/or cold transformed steel is performed by bringing it to a temperature between Ac1 and 1100°C for 2 min to 10 min in a conventional oven.
15. - Method according to one of claims 9 to 13, characterized in that the austenitization of said hot and/or cold transformed steel is performed by bringing it to a temperature between Ac1 and 1100°C for 30 s to 1 min in an induction furnace.