Patent Application
20240254610
The present invention relates to the use of a steel of determined composition for the manufacture of a mechanical part the mechanical features of which are obtained by natural cooling after a hot forming step (rolling, forging, etc.)
Some mechanical parts, such as crankshafts, wishbones, etc. require mechanical strengths on the order of 850-1000 MPa. Such requirements explain the use of micro-alloyed ferritic-pearlitic grades such as 38MnVS6. Such grades, which are described by the standard DIN EN 10267, use the phenomenon of interphase precipitation in vanadium in order to obtain mechanical features superior to the mechanical features of conventional ferritic-pearlitic steels. Such grades are used for obtaining, without any heat treatment after hot forming, the mechanical strengths mentioned hereinabove. Moreover, since the grades are relatively easy to implement (the mechanical properties are obtained without any subsequent heat treatment), said grades have become widely established since the 1990's.
However, such grades have two major drawbacks. On the one hand, the mechanical features obtained have an increased sensitivity to the cooling rate, and hence, for a natural cooling, to the size of the parts. On the other hand, the use of ferrovanadium raises environmental and economic questions. Thereby, the volatility of ferrovanadium prices led to peaks such as the cost of adding 0.2-0.3% vanadium alone could almost equal the basic cost of steel.
There is thus a great interest in developing alternative solutions to micro-alloyed ferritic-pearlitic steels, without, however, losing the advantages of the latter. Thereby, relatively little alloyed grades (37Cr4, 42CrMo4) could easily be used for achieving the required level of mechanical features, but would require a heat treatment the environmental and economic effects of which would completely cancel out the benefits won on the alloy.
Such solutions should this not only avoid the use of ferroalloys with a high environmental and economic impact (vanadium, molybdenum), but also not require heat treatment.
The last two decades have seen, moreover, the development of grades with bainitic microstructure (still sometimes called acicular ferrite), the main goal of which is to obtain mechanical features superior to the mechanical features obtained with micro-alloyed ferritic-pearlitic grades, without resorting to heat treatment. To achieve such goal, the chemical compositions have been adapted so as to obtain a microstructure which is no longer ferritic-pearlitic but bainitic, directly during cooling following rolling or hot forging. To make things simpler, the term “bainitic grade” will be used for referring to such grades.
The document SEW 605-Ed1 presents a representative overview of the development of bainitic grades in the field of long products. As shown upon reading said document, most of the detailed grades claim mechanical properties which are significantly higher than 950 MPa or even 1050 MPa. For this purpose, such grades use not only relatively high Mn and Cr concentrations, but also significant additions of Mo, Si and V, for all of the grades.
The same applies to the different patents. Thereby, EP 1 780 293 A2 presents a hot-formed bainitic-martensitic steel, and which is used for obtaining high mechanical features without any heat treatment (examples of mechanical strength at 1180 MPa are given). WO 2005/100618 presents a steel with an acicular ferrite structure (i.e., in theory, bainite formed intragranularly) used for obtaining, without any heat treatment, a mechanical strength of about 1150 MPa according to the examples. WO 2009/138586 A2 relates to a steel with a bainitic structure making it possible to obtain a mechanical strength of at least 1100 MPa without any heat treatment. WO 2016/151345 A1 is similar to the preceding example in that same describes a grade having a predominantly bainitic microstructure (70 to 100%) and having a mechanical strength higher than 1100 MPa in all the examples. The same applies to the WO 2019/180563 A1. Document EP 3061838A1 concerns a grade with a predominantly bainitic structure (minimum 60% bainite) having a mechanical strength higher than 1150 MPa.
It is thus clear, through the various documents mentioned hereinabove, that the development of so-called ‘bainitic’ grades has above all responded to a problem of increasing mechanical features compared to the widely used references that are vanadium micro-alloyed ferritic-pearlitic microstructure steels.
In the context of mass production, an increase in mechanical properties, if the increase is not imposed by the mechanical design, is often undesirable because same can lead to additional machining costs, which in turn can cancel the sought for economic and environmental benefits. From such point of view alone, the solutions mentioned hereinabove are thus not conceivable for the substitution of a grade such as 38MnVS6 or other grades such as described by the standard NF EN 10267.
A detailed examination of SEW 605-Ed1 suggests that, from the point of view of mechanical features, some of the proposed solutions might nevertheless be suitable. Such is the case for grades called 27MnCr6-3, and to a lesser extent 37Mn7 and 37MnCr4-3. Indeed, all such grades are likely to have mechanical strengths on the level of the mechanical strength of the 38MnVS6 (taken herein because the most used grade amongst the grades of NF EN 10267). However, the detail of the chemical compositions shows that for all said grades, the additions of V are needed. Such solutions are thus also not likely to answer the existing problems of a grade having a mechanical strength between 850 and 1000 MPa without any heat treatment and without a significant use of vanadium (or other critical ferroalloys).
The lack of solution is not fortuitous for two reasons. The first is that it would be going against the stream to consider a grade with a bainitic structure for the stated objectives. Indeed, as has been shown, the work on such grades has focused on obtaining high mechanical features. The second reason is technical and will be discussed in detail thereafter.
As disclosed e.g. in application WO 2011/124851, high mechanical features are obtained by reducing the temperature at which bainitic transformation begins. In the cited reference, said temperature is given, in degrees Celsius, by the formula 830−270*C %−90*Mn %−70*Cr % where C %, Mn % and Cr % are the concentrations of C, Mn and Cr in mass percentages. As can be seen, the lowering of said temperature involves the addition of alloy elements such as Mn and Cr (the C remaining limited because C greatly slows down the bainitic transformation). Now, as is well known to a person skilled in the art, the additions will also slow down the formation of ferrite and pearlite during cooling, and hence enhance the production of the sought for microstructure. In this way, the achievement of both goals (formation of a predominantly bainitic and high-performance structure) is enhanced by the addition of alloy elements such as Mn, Cr, etc.
On the other hand, in order to obtain a bainitic microstructure the mechanical strength of which will be moderate, the transformation temperature will need to be raised. For this purpose, the concentrations of alloying elements can be reduced. However, the above goes henceforth in the opposite direction to the direction sought for obtaining a bainitic microstructure (the formation of ferrite-pearlite being enhanced by such modifications). It has been indicated hereinabove that the grades of the document SEW 605-Ed1 having moderate mechanical features ((850-1000 MPa) all have additions of vanadium. Given the elements set out hereinabove, it is henceforth possible to show that such choice is not innocuous. Indeed, vanadium, just like molybdenum (which has the same disadvantages in terms of cost), has the particularity of greatly slowing down the formation of ferrite-pearlite without, however, influencing the kinetics of bainite formation. It is thus understandable that same are used for bainitic grades, especially when it is sought to limit the additions of Mn and Cr as is imposed by the present problems.
Returning to the defined problems, namely the development of a grade with a mechanical strength between 850 and 1000 MPa in the forged or rolled state, without heat treatment, and without significant use of vanadium or molybdenum, and given the elements set out hereinabove, it is thus clear not only that there is no solution, but also that the development of such a solution on the basis of a bainitic microstructure goes against the logic that has underpinned the development of bainitic grades to date.
The invention can now be described in detail, the goal of which is namely to propose a new grade of steel making little or no use of ferroalloys the costs of which are fluctuating (V, Mo, NB, Ti), and to be used for obtaining a predominantly bainitic microstructure for parts with typical dimensions (thickness or diameter) of 20-100 mm, by natural cooling after rolling or hot forging, said parts having to have a mechanical strength comprised between 850 and 1000 MPa.
It is further specified that the morphologies of bainite or bainite ferrite sometimes referred to as acicular ferrite or intragranular bainite are not considered distinct from the microstructure claimed for the steels of the invention, and that bainite should be understood in general, thus to the exclusion of ferrite-pearlite, Widmanstaetten ferrite, or martensite.
To this end, the subject matter of the invention is a hot-formed steel part, characterized in that the composition of the steel, in percentages by weight, consists of:
the remainder being iron and impurities resulting from the elaboration; and for which the following relations are verified:
The mixture of bainite ferrite and carbides or residual austenite forms bainite, bainite including the morphologies of bainite or bainite ferrite called acicular ferrite or intragranular bainite.
Preferentially, the residual austenite fraction is less than or equal to 5%.
The steel part according to the invention preferentially has one or a plurality of the following features, taken alone or in combination:
A further subject matter of the invention is a manufacturing method for a steel part, characterized in that:
The method according to the invention can comprise one or a plurality of the following features, taken individually or in combination:
The part concerned could be, but is not limited to, a crankshaft or an injection rail.
The choice of composition ranges for the various elements of the invention will now be justified. As already mentioned, all concentrations are given in weight percentages.
The concentration of carbon is comprised between 0.22 and 0.35%. Low carbon concentrations can be favorable to the rapid formation of bainite, and this point has been the subject matter of a specific patent (WO2011124851-A2); however, low carbon concentrations also facilitate the formation of pro-eutectoid ferrite. In the absence of vanadium in particular, such ferrite has a particularly low hardness and can be a preferred initiation site for fatigue failures. The low carbon concentrations also lead to the need for large quantities of ferroalloys, in order to maintain the mechanical properties. A low limit of 0.22% was thus retained, significantly more than many so-called bainitic grades, which favor concentrations on the order of 0.16-0.20%. Preferentially, the concentration of carbon is strictly higher than 0.25% and better higher than 0.26%. On the other hand, too high carbon concentrations lead to a significant slowdown of bainitic transformation while enhancing the formation of pearlite (which is not sought). The above forces, as indicated hereinabove, retaining alloying elements such as molybdenum or vanadium, which strongly slow down the transformation into ferrite-pearlite without affecting the kinetics of the bainitic transformation. An upper limit of 0.35% was thus retained, so as not to have to resort, in return, to the use of such elements. The choice of the preferred range makes it possible to avoid more definitely the absence of ferrite (for the low limit) and martensite (for the high limit). Preferentially, the carbon concentration is less than or equal to 0.30%.
The concentration of Mn is comprised between 0.50 and 1.7%, preferentially greater than or equal to 1.10% and/or less than or equal to 1.70%. Mn is used, together with Cr, for controlling the parameter Bs, an indicator of the temperature at which bainite formation begins during continuous cooling. In particular, an addition of Mn lowers the parameter Bs. If the effect on Bs can be obtained for relatively low concentrations (0.50%), it is also preferable to avoid too high concentrations (>1.70%) which lead to problems of excessive segregation and to a too great lowering of the parameter Bs.
The concentration of Cr is comprised [between] 0.50 and 1.70%. In the present invention, Cr is used in the same way as Mn for the purpose of controlling the parameter Bs. Same can be used as such for substituting a variably significant part of the manganese. However, a minimum concentration of 0.50% is required so as to guarantee that the microstructure of the invention is obtained, and a maximum concentration of 1.70% is imposed for limiting segregation phenomena and the cost of the grade.
The concentration of Mo is comprised between traces and 0.15% and preferentially between traces and 0.10%. The well-established role of Mo has already been mentioned as slowing down the ferritic-pearlitic transformation, a particularly favorable role for obtaining a bainitic microstructure. However, for reasons of cost and exposure to the fluctuating prices of ferromolybdenum, the additions of Mo are, in the present invention, limited to 0.15% or even to 0.10%. This latter concentration can be reached by the mere presence of residues in the scrap that is possibly used so that such concentration is not necessarily a deliberate addition. Preferentially, the concentration of Mo is strictly less than 0.10%.
The concentration of Si is comprised between traces and 0.40%, preferentially between traces and 0.35%. Advantageously, the concentration of Si is less than or equal to 0.25%, preferentially less than or equal to 0.15%. The above point is central to the present invention. The role of silicon is well known for enhancing the presence of residual austenite in the microstructure and thereby benefits from a so-called TRIP (Transformation Induced Plasticity) effect, an effect that simultaneously leads to higher mechanical strength and greater elongation during tensile testing. Such effect is generally observed for additions on the order of 1.2 to 1.5% for isothermal transformations. In the case of continuous cooling, document EP 0787 812 B1 specifically mentions that silicon has no stabilizing effect on residual austenite below 0.8%. Now, in the case of the present invention, the aforementioned TRIP effect is not desirable, since TRIP implies a mechanical strength in excess of the target (850-1000 MPa); it is thus a question of guaranteeing the absence thereof or of greatly limiting the TRIP effect. Surprisingly, the inventors were able to show that the views on the role of Si were erroneous, and that, within the range of claimed compositions and on components of typical dimensions of automotive parts, Si could contribute to the presence of residual austenite starting from 0.40% and even, in some cases, starting at 0.35%. The Si concentration has thus been deliberately limited to such values, the preferential concentration guaranteeing the result with greater certainty.
The concentration of Ni is comprised between traces and 0.50%, preferentially between traces and 0.35%. Nickel may be present only by being fed into the raw materials as a residual element, in which case Ni concentration will naturally be limited to 0.35%. The latter could be increased to 0.50% but additions in excess of said limit are prohibited for the same reasons as the additions of Mo and V (cost and fluctuation of the costs of ferroalloys, environmental impact).
The concentration of Cu is comprised between traces and 0.50%, preferentially between traces and 0.30%. Just like Ni and a small amount of Mo, Cu can be present exclusively by being fed into the raw materials, as a residual element. Additions are thus not required, but the concentration thereof is limited to 0.50%, better to 0.30% in order to avoid any difficulties of hot forming.
The concentration of V is comprised between traces and 0.08%, and preferentially between traces and 0.05%. Unlike molybdenum, copper and nickel, vanadium can be oxidized during the elaboration from scrap, so that a concentration of less than 0.015% is always conceivable. However, modest additions can be made for improving the hardenability and the strength to tempering, without however exceeding 0.08%, and preferentially 0.05%, in order to minimize the consequences on the cost and the variability of the cost of the grade.
Al is comprised between traces and 0.10% and preferentially between traces and 0.05%. Al is optionally added to deoxidize the steel. Additions are capped at 0.10% in order to limit the risk of formation of re-oxidation inclusions by contact of the liquid metal with air. Such limitation will be all the more effective if the preference interval is satisfied.
B is comprised between 0.001 and 0.010% and preferentially between 0.001 and 0.006%. Boron is a powerful retarder of the formation of pro-eutectoid ferrite that is indispensable to the invention. It is well known to a person skilled in the art that the addition of boron in very small quantities leads to obtaining a pronounced effect, but a reproducible effect requires a minimum concentration of 0.001% (10 ppm). Moreover, in order to prevent the formation of boron precipitates and the harmful effects thereof, the additions are limited to 0.010%. The absence of the precipitates will be guaranteed with greater certainty by limiting to additions of less than 0.006%.
Ti is comprised between 0.01 and 0.06%. Titanium is indispensable for fixing the nitrogen that is inevitably present during the elaboration and thereby allowing the boron to remain in solid solution. The inventors have established that, under the conditions of implementation of the invention, a concentration on the order of 0.01% could be sufficient. On the other hand, a concentration of more than 0.06% is not necessary from the point of view of the sought for effect and can also lead to the formation of precipitates harmful to the lifetime under fatigue. Additions beyond said limit are also not desirable for cost reasons. Finally, it should be noted that a minimum of Ti is imposed with respect to the N concentration (Ti %≥2.5 N %), in order to guarantee the effectiveness of the protection of B.
Nb is comprised between traces and 0.05%. Although not indispensable to the present invention, niobium can be added for improving the hardenability and/or for refining the austenitic grain at high temperature. However, the additions are limited to 0.05% for reasons similar to the reasons mentioned for Ti. Preferentially, for cost reasons, the concentration will be limited to 0.02%.
S is comprised between traces and 0.15%. As is well known, sulfur can be added for improving the machinability of the grade, concentrations between 0.05 and 0.15% are then imparted thereto. For the same reasons, the composition can comprise additions of Ca up to 0.010%, and/or Te up to 0.030%, and/or Se up to 0.050% and/or Bi up to 0.050% and/or Pb up to 0.100%.
P is comprised between traces and 0.100%. In general, the presence of P will be limited to 0.030% if it is sought to limit the consequences of the embrittling effect thereof. For certain applications, however, it is possible to envisage concentrations greater than or equal to 0.030%, either because ductility is not a sought for property, or even because brittleness is wanted (case of breakable connecting rods). However, the concentration will remain limited to 0.100%, beyond which the elaboration and rolling difficulties are significant.
N is comprised between traces and 0.013%, preferentially between traces and 0.010%. Although inevitably present, nitrogen should be limited in order to maintain the effectiveness of boron additions. To this end, a maximum concentration of 0.013% is imposed, noting that a limit of 0.010% will improve the guarantee of obtaining the result.
The other elements present in the steel of the invention are iron, and impurities resulting from the elaboration, present in usual concentrations taking into account the raw materials used and the mode of elaboration of the liquid steel (use of a converter or an electric arc furnace for obtaining the liquid metal, vacuum or non-vacuum treatment of liquid metal, etc.).
It should be understood that the possible preferential concentrations of each element taken in isolation are independent of one another. In other words, one can chose to be within a preferential range for one or a plurality of said elements and outside the preferential ranges for the other elements which include one preferential range.
The parameter Bs, estimating the temperature at which bainite begins to form during cooling, should also be comprised between 540 and 600° C. The parameter or “bainitic transformation start temperature” Bs is defined by the following formula:
wherein C %, Mn %, Cr %, Mo % and Ni % denote the concentrations of C, Mn, Cr, Mo and Ni, respectively, in the steel composition, expressed as percentages by weight.
Herein too, the inventors go against the stream of the approach which chooses said parameter to be mainly bounded by a higher value, and as low as possible. And yet, the parameter Bs according to the invention makes it possible to form bainite at high temperature during cooling, so that a coarse bainite structure is obtained. Thereby, bainite has a lower mechanical strength than in the steels according to the prior art mentioned hereinabove.
More particularly, a minimum value of 540° C. is needed for preventing a too high mechanical strength. On the other hand, the concentrations of C, Mn and Cr corresponding to values greater than 600° C., are not compatible with obtaining the sought for microstructure.
Preferentially, the parameter Bs is greater than 560° C., better still greater than 570° C.
The microstructure of the steel consists of, in surface fractions:
The mixture of bainitic ferrite and carbides or austenite forms bainite. More particularly, bainite is in the form of a matrix of bainite ferrite strips or plates, between which carbides and/or austenite are present.
In the microstructure of the steel according to the invention, bainite, in particular bainite ferrite, is formed during cooling, as soon as the temperature becomes lower than the start temperature Bs for bainite transformation. As described hereinabove, due to the high value of BS, bainite is formed at relatively high temperature. Thereby, bainite has a coarse structure, making it possible to limit the mechanical strength to at most 1000 MPa.
The residual austenite fraction has to be less than or equal to 10%, preferentially less than or equal to 5%. Indeed, as described hereinabove, the presence of residual austenite in the microstructure leads in particular to a higher mechanical strength. A residual austenite fraction greater than 10% would lead to a mechanical strength in excess of the target (850-1000 MPa). A high residual austenite fraction is thus detrimental to achieving the goal of the present invention. A residual austenite fraction less than or equal to 5% is thus preferred.
The structure can include martensite, pro-eutectoid ferrite and/or pearlite, but the sum of the fractions of the constituents has to remain at most 30% and the sum of the fractions of pro-eutectoid ferrite and pearlite at most 10% in order to achieve the sought for mechanical features.
A manufacturing method for a part according to the invention will now be described.
The part is produced by a hot-forming, in austenitic phase, of a steel semi-finished product with the composition described hereinabove.
The semi-finished product is e.g. a billet or a bar.
According to one embodiment, before hot-forming, the semi-finished product is subjected to initial forming by machining or cold forming.
The hot-forming is e.g. a hot forging or a hot rolling.
Hot-forming is carried out in the austenitic phase (typically between 1100 and 1250° C.).
After hot-forming, the part is cooled. Cooling is carried out, e.g. with still air, forced air, under a hood or in a container, depending on the sought for cooling rate.
According to the invention, the sought for mechanical features are obtained without the use of any heat treatments after the hot-forming, nor any very restrictive particular control of the rate of cooling which can be carried out naturally in still air.
During cooling, the cooling rate between 750° C. and 550° C. is preferentially greater than or equal to 0.15° C./s, in order to prevent or limit the formation of ferrite and pearlite, which are likely to form in said temperature range.
Preferentially, the cooling rate between 550° C. and 250° C. is comprised between 0.1 and 0.5° C./s. Indeed, given the parameter Bs according to the invention, bainitic ferrite is formed in said temperature range. The cooling rate should not be too high, in order to maximize bainite formation in said temperature range.
Below 250° C., the phase transformation is generally completed, so that the cooling rate is between 0.1° C./s and 100° C./s.
As indicated hereinabove, a still air cooling could be sufficient for obtaining the cooling rate ranges specified hereinabove.
Nevertheless, an adaptation of the cooling can be used in certain cases, in particular due to the diameter of the parts. For example, with parts of large dimensions, in particular with an equivalent diameter greater than or equal to 120 mm (i.e. such that the natural cooling rate of the parts is less than or equal to the cooling rate of a bar with a diameter of 120 mm), still air cooling could lead to too slow cooling, especially at the core of the parts, and lead to the occurrence of ferrite and/or pearlite in too large quantities. In such case, forced air cooling can be used in order to obtain a sufficient cooling rate.
On the other hand, cooling can be implemented under a hood or in a container, in order to reduce the cooling rate.
According to one embodiment, after cooling, a shaping is carried out by cold machining or cold deformation, in order to obtain the part, in particular in order to obtain the precise dimensions and surface features of the final part.
According to one embodiment, after cooling or after a possible cold forming, a surface treatment of the surface of the part is carried out by high-frequency induction in order to impart to same the benefits of such technique (increase in hardness, residual compression stresses, etc.). The surface treatment is generally carried out on a specific portion of the part.
After such a surface treatment, tempering can be carried out for adjusting the hardness of the treated zones of the part.
In addition, or as an alternative, a portion of the part is mechanically reinforced by a method such as roller burnishing, autofretting, or other methods aimed at obtaining local work hardening as well as residual compressive stresses in the part to be reinforced.
After cooling, according to one embodiment, the part is subjected to a deposition of a coating, e.g. by electrogalvanization or a deposition of paint, as well as, if appropriate, to heat treatments required by such a deposition.
The results obtained in the laboratory on compositions according to the invention (Inv1 to Inv4), compositions which are close but not according to the invention (Steel1 to Steel7) and the reference 38MnVS6 will now be presented. All the compositions retained are presented in [Table 1]. It should be understood that the complement to 100% of the compositions mentioned is Fe, the elements not mentioned in the table being present only in trace form, including the elements which could only be optionally present in the invention, such as the machinability elements Ca, Te, Se, Bi, Pb that have not been added in the examples considered. The underlined values are not according to the invention.
0.380
0.450
0.200
0.125
0.0001
0.001
0.423
0.212
0.420
0.183
0.180
0.194
0.091
0.0001
0.001
507
0.595
0.820
0.185
0.0001
0.001
0.300
498
The results discussed thereafter were obtained using billets with a diameter ˜40 mm, to which a heat treatment was applied such that the condition thereof was similar, or even identical, to that expected on a forged part such as an automotive crankshaft: austenitization at 1050° C. followed by still air cooling, resulting in cooling rates of typically 0.45-0.50° C./s between 800 and 550° C., 0.25-0.30° C./s between 550° C. and ambient temperature. Tensile testing was then carried out in order to determine the mechanical strength of the steel thereby treated, and a metallographic examination was carried out in order to determine the microstructure thereof.
~pearlite,
bainite
~pearlite,
bainite
The results are summarized in [Table 2] wherein the underlined values are the values not according to the invention. In an expected manner, the reference grade (38MnVS6) has the sought for mechanical strength, but not the target microstructure; as explained hereinabove, said grade uses the interphase precipitation of vanadium for obtaining the mechanical properties thereof, an element the addition of which is precisely to be avoided. Moreover, the absence of addition of B and Ti should be noted, as well as an insufficient concentration of Cr for the invention, which will explain in part that the microstructure remains entirely ferritic-pearlitic whereas a mainly bainitic microstructure is sought.
Alt1 steel has a microstructure and tensile strength in accordance as expected, but with carbon and molybdenum concentrations not according to the invention. As mentioned hereinabove, the use of molybdenum in significant quantities is prohibited, in order to limit the use of the corresponding ferroalloys. The comparison with Alt2 illustrates well the reason for the limitations on the concentration of carbon. Indeed, Alt2, for which the Mo concentration has been reduced compared to Alt1, has a microstructure which is not as per the requirements of the invention, with in particular ˜25% pearlite. Such result clearly illustrates the difficulty of preventing the formation of said constituent without the use of molybdenum in excess of 0.2%.
Alt3 steel has a non-conforming parameter Bs, with conforming microstructure and Si concentration. As a result of a Bs value lower than the minimum required, the mechanical strength is greater than the requirements of the invention, which may lead to machining difficulties during production.
The Alt4 and Alt5 steels have compositions very close to the compositions of the invention, with the exception of the Si concentration which is in excess of the maximum imposed by the invention. A mechanical strength significantly greater than the targeted mechanical strength (850-1000 MPa) results therefrom. Moreover, as emphasized hereinabove, it has been demonstrated using X-ray diffraction that said steels have residual austenite concentrations of 11 and 15%, respectively. It was also explained that the residual austenite was responsible for the high mechanical strengths due to the TRIP effect. The above result is central to the invention, as it is in opposition to the whole scientific corpus, generally estimating the limit for stabilization of residual austenite at 0.6-1 Si %.
Alt6 steel conforms at many points with the exception of the addition of Ti and B. A microstructure results therefrom, which is not according to the invention (the experience of the inventors is that mixed pearlite-bainite microstructures have a very high sensitivity to cooling conditions and are not desirable for use in industrial conditions).
Finally, Alt7 steel is also close to the steels of the invention but has a parameter Bs significantly lower than the parameter required, resulting in a mechanical strength much higher than the target.
Inv1 up to Inv 4 steels all have a structure consisting of at least 70% of a mixture of bainitic ferrite and carbides or residual austenite in surface fractions, the residual austenite fraction being less than or equal to 5%, at most 30% of martensite and/or pro-eutectoid ferrite and/or pearlite, the pro-eutectoid ferrite and/or pearlite fraction being less than or equal to 10%.
As shown by the results obtained on Inv1 to Inv4 steels, satisfying all the criteria defined by the present invention makes it possible to obtain the sought for microstructure and mechanical strength, while avoiding the use of vanadium and molybdenum in significant quantities. It should be noted that the molybdenum concentrations correspond to the concentrations which can be obtained by the mere presence of molybdenum as a residual element in the scrap used for the production.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2105812 | Jun 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/064945 | 6/1/2022 | WO |
