FR2790009A1 - HIGH ELASTICITY DUAL-PHASE STEEL - Google Patents

Abstract

Steel including Mn, Si and Al is obtained by cold rolling a hot strip and continuous annealing of the cold strip. The steel has a martensite phase in sufficient amount such that it does not exhibit a plateau in the elasticity limit, and the cold strip after annealing is subjected to a skin pass operation with a reduction rate of 1-5%. The dual-phase steel comprises (in weight %) 0.05-0.4 C, 1.0-2.0 Mn, 0.1-0.8 Si, 0.02-0.1 Al, 0-0.08 Mo, below 0.01 Nb, below 0.02 Ti, below 0.01 V, and iron and unavoidable impurities, including below 0.04 S, below 0.02 P and below 0.007 N, the remainder. The steel may also contain 0.01-0.6 weight % Cr. The steel has at least 5%, preferably at least 10%, martensite phase in the annealed state. Continuous annealing is carried out at a temperature above that of perlite transformation A1, followed by rapid cooling at a rate above or equal to the rate of hardening of the steel, to a temperature below or equal to 500 deg C. The continuous annealing cycle includes a plateau less than or equal to 360 deg C. An Independent claim is given for a method for production of the dual-phase steel having improved mechanical properties.

Description

DUAL-PHASE STEEL WITH HIGH ELASTICITY LIMIT.

  The present invention relates to the field of steels for application in the automotive field, in particular steels intended to produce structural parts and safety parts, for example

  anti-intrusion reinforcements for doors or seat rails.

  To make certain structural parts, the material sought must have a high elastic limit Re, a very good

  weldability, acceptable formability, as well as strong consolidation.

  lO To produce parts such as door reinforcements intended for shock absorption, the material sought must have a high breaking strength Rm, high ductility and good weldability. In a concern of lightening of the parts, one seeks more and more to increase the elastic limit Re of the steels used in order to be able, with equivalent mechanical resistance, to reduce the thickness of these parts, without however degrading the other properties of the material, especially its

elongation at break A%.

  It is currently known to use two types of steel for these automotive applications: - ferritic steels with a high yield strength produced from grades microalloyed with titanium and / or niobium and / or vanadium, to name only the most common microalloy elements, hardened by the presence of fine dispersed precipitates; very high strength steels of the dual-phase type, hardened mainly by the presence of hard phases such as bainite and / or martensite. These two families of steels, as well as their properties, are for example described in the document: "High-Strength Automotive Sheet Steels For Weight Reduction And Safety Applications - Proceedings Of The Symposium On High-Strength Sheet Steels For The Automotive Industry Baltimore, 16 -October 19, 1994 - pages

-77 ".

  Ferritic microalloyed steels are high limit steels

  elasticity Re, which can range from 280 MPa to 500 MPa depending on the range.

  This high elastic limit is obtained by adding a small amount of microalloy elements also called dispersoldes, the most common of which are niobum, titanium, vanadium and boron. These elements have a strong affinity for carbon and nitrogen, which will allow the formation of precipitates of very fine carbides and dispersed in the structure, which will cause hardening. Thus, depending on the quantity of carbon and microalloy elements in the steel, it is possible to produce different

  steel ranges with elastic limits Re from 280 MPa to 500 Mpa.

  Some examples of ferritic steels microalloyed with niobium are summarized in Table 1 below, in which the content of

  different constituents are given in 10-3% by weight.

  Ref. C Si Mn PSN AI Other Re Rm Re / Rm A% steel _ _ (MPa) (MPa) A1 50 10 100 15 15 3 40 Nb 350 410 0.85 22 A2 60 400 1200 15 5 3 40 Nb 500 570 0, 88 12 A3 70 10 600 20 15 3 40 Nb 350 420 0.83 22 A4 120 400 1500 15 5 3 40 Nb 500 570 0.88 10

Table 1

  These steels were produced by hot rolling with an end of rolling temperature of around 875 OC, coiling at 600 C, cold rolling, continuous annealing and skin-pass. The A3 and A4 steels were also subjected to galvanizing annealing and a galvanizing operation. Compared to the same steels without microalloy elements, these steels have a higher yield strength level Re, and a

  typical Re / Rm ratio of around 0.85.

  These steels also have a hardening capacity when the paint is cured, commonly called bake-hardening, which

is around 25 MPa.

  But these steels have an elongation at break A% degraded, and also have a high yield strength plateau after annealing, whether the latter is a continuous annealing, a base annealing or a galvanizing annealing. In general, this bearing is not completely erased by the skin-pass operation in order to minimize the degradation of the elongation at break due to work hardening. This leads to a relatively large dispersion of the mechanical characteristics inside the coil

or between different coils.

  These microalloyed steels are also very sensitive to the conditions of hot denatutation and more particularly to the chemical composition and to the winding temperature. This makes their manufacture complex and also leads to a dispersion of the characteristics

important mechanical.

  Thus, these steels have an anisotropy of mechanical characteristics 1o in the plane of the sheet very important. For example, the yield strength varies by more than 20 MPa depending on the direction of measurement compared to

the rolling direction.

  Finally, these microalloyed steels require relatively hot and long anneals to ensure complete recrystallization because of the microalloy elements present in the steel. This decreases the productivity of the annealing lines and contributes to the increase in the dispersion of the

  mechanical characteristics in the coil and between different coils.

  Dual-phase steels are steels which make it possible to achieve a very varied resistance range Rm, going from less than 500 Mpa to more

1400 MPa.

  These dual-phase steels are characterized by the presence of hard martensite and / or bainite phases, which give them high strength, and a ductile ferritic phase which ensures their ductility. The limits of elasticity Re and resistance Rm depend on the quantity of hard phases and the hardness of these phases, and the proportion of these phases

  depends on the couple "chemical composition - manufacturing conditions".

  Three examples of dual-phase steels are summarized in Table 2 below, in which the content of the various constituents is

given in 10-3% by weight.

  B1 steel is a non-microalloyed dual-phase steel, and steels

  B2 and B3 are dual-phase steels microalloyed with niobium.

  C Si Mn P S N AI Re Rm Re / Rm A% _._ __ __ (Mpa) (Mpa)

  B1 100 200 700 50 5 3 40 350 600 0.58 16 _

  B2 120 400 1500 15 5 3 40 400 800 0.50 8

  B3 150 500 1500 15 5 3 40 600 1000 0.60 5

Table 2

  If we compare a dual-pass steel to a microalloyed ferritic steel in terms of mechanical characteristics, we note two fundamental differences: on the one hand a lower Re / Rm ratio, of the order of 0.60 for a dual phase steel against 0.85 for a microalloyed steel, and on the other hand a much lower elongation at break for a dual-phase steel

  than for microalloyed steel, at an equivalent yield strength level.

  It is also noted that to obtain a level of elastic limit equivalent to that of a microalloyed steel, it is necessary to use a steel much more loaded with additives, which is harmful for its weldability and its ductility. For example, if we compare steels A1 and B1 which each have an elastic limit level equal to 350 MPa, we see that the contents of carbon, silicon, manganese and phospore are more than doubled in the case of steel dual-phase, which results in degraded ductility for this dual-phase steel B1 (A% = 16%

  against 22% for microalloyed steel A1).

The object of the invention is twofold.

  On the one hand, it is to propose a steel which, at an equivalent elastic limit level, has better ductility than microalloyed steels and dual-phase steels, that is to say a higher elongation at break. On the other hand, it is to offer several ranges of steel in terms of mechanical characteristics, from the same hot sheet, and even from the same cold sheet annealed, and this for all steels dual-phase, from dual-phase DP 450 (Rm> 450 MPa) to dual-phase DP

1200 (Rm> 1200 MPa).

  To this end, the invention relates to a dual-phase steel comprising by weight between 0.05 and 0.4% of carbon, between 1.0 and 2.0% of manganese, between 0.1 and 0.8 % of silicon, between 0.02 and 0.1% of aluminum, between 0 and 0.08% of molybdenum, less than 0.01% of niobium, less than 0.02% of titanium, and less than 0, 01% vanadium, the rest being iron and unavoidable residual impurities including less than 0.004% sulfur, less than 0.02% phosphorus, less than 0.007% nitrogen, obtained by cold rolling a strip with hot and continuous annealing of the cold strip, and in that it comprises a martensitic phase in an amount sufficient to be free of yield strength in the annealed state, and in that the cold strip leaving the annealing is subject to a skin-pass operation, with a reduction rate

between 1 and 5%.

  According to other characteristics of the invention, it comprises at least 10% of martensitic phase in the annealed state; it comprises at least 5% of martensitic phase in the annealed state; - it contains between 0.01 and 0.6% chromium; - continuous annealing is carried out at a temperature higher than the pearlitic transformation temperature A1, followed by rapid cooling at a speed greater than or equal to the quenching speed of the steel, to a temperature less than or equal to 500 VS; - the continuous annealing cycle has a monitoring level

  at a temperature less than or equal to 360 C.

  The invention also relates to a process for manufacturing a dualphase steel with improved mechanical characteristics, according to which: - a hot plate is produced comprising by weight between 0.04 and 0.4% of carbon, between 1.0 and 2 , 0% manganese, between 0.1 and 0.8% silicon, between 0.02 and 0.1% aluminum, between 0 and 0.08% molybdenum, less than 0.01% niobium, less than 0.02% titanium, and less than 0.01% vanadium, the remainder being inevitable iron and residual impurities including less than 0.004% sulfur, less than 0.02% phosphorus, less than 0.007% nitrogen; - cold rolling of said sheet is carried out to bring it to the desired thickness; - the cold-rolled sheet is subjected to continuous annealing at a temperature above the pearlitic transformation temperature A1, followed by rapid cooling at a speed greater than or equal to the quenching speed of the steel, to a temperature less than or equal to 500 ° C., ensuring that a martensitic phase is formed in a sufficient quantity so that the steel is free from an elastic limit plateau in the annealed state; - the annealed cold sheet is subjected to a skin-pass operation at a controlled rate, of between 1 and 5%, depending on the mechanical characteristics sought. According to another characteristic of the process, the continuous annealing cycle has a monitoring level at a lower temperature or

equal to 360 C.

  The characteristics and advantages of the invention will become apparent on

  io following the description which follows, given solely by way of example, made in

  reference to the appended drawings in which: - Figure 1 shows the incidence of carbon, manganese, silicon and chromium contents on the resistance Rm of dual annealed steels continuously; - Figures 2a and 2b show the incidence of the annealing temperature respectively on the resistance Rm of the steel and on its elongation at break A%; Figures 3a and 3b show the impact of the rapid cooling start temperature respectively on the resistance Rm of the steel and on its elongation at break A%; - Figures 4a and 4b show the impact of the monitoring temperature respectively on the resistance Rm of the steel and on its

elongation at break A%.

  The invention in question, which allows on the one hand to produce several ranges of final cold sheets from the same cold sheet annealed outlet, and on the other hand to produce a final cold sheet which , at equivalent elastic limit level Re, has better ductility than microalloyed steels and the usual dual-phase steels, is a dual-phase steel of known type, of which care has been taken to avoid any microalloy element, which we will have taken care to give it a martensitic phase sufficient for this steel to be free from a limit of elastic limit at the outlet of the annealing, and which we submit after annealing, to a skin-pass with a reduction rate

  greater than 1%, preferably between 1 and 5%.

  Thus, care will be taken to limit the niobium content to less than 0.01%, the titanium content to less than 0.02%, the vanadium content to less than 0.01% and the sulfur content to less than 0.004%, to name a few

  four main microalloy elements.

  Care will also be taken to produce a cold annealed outgoing sheet having at least 5% martensitic phase, preferably 10%, in order to ensure that the cold annealed outgoing sheet obtained does not have any

yield point.

  It is the skin-pass rate which ultimately controls the level of the elastic limit Re, which thus makes it possible for example to produce four different cold sheets, each of them meeting the targets of the io four ranges mentioned in table 6, from the same sheet to

cold annealed outlet.

  The Applicant has realized and has shown that the mechanical characteristics of a dual-phase steel depend relatively little on the chemical composition and on most of the conditions for hot and cold denaturation. On the other hand, the most important manufacturing parameters are: - the continuous annealing maintenance temperature, - the monitoring temperature if the annealing cycle has a plateau,

  - and as previously indicated, the skin-pass rate.

  Thus, the holding temperature essentially acts on

  the elongation at break and the optimum value is between 780 and 820 C.

  The monitoring temperature conditions the nature and hardness of the steel components. The optimal value is between 320 and 360 C. When this increases, the level of breaking strength Rm decreases, the elongation at break A% increases, the resistance / ductility compromise remains satisfactory. The yield point appears

  at the annealing outlet if this temperature is 400 or 450 C.

  These observations can be shown, for example from a dual-phase steel of the dual-phase 600 type (Rm> 600 Mpa), whose chemical composition in 10-3% is as follows: c Mn Si Cr Mo PN AI S Zr

121 1488 361 225 62 8 4.8 89 9 91

Table 3

  This chemical composition is a typical composition of dualphase 600 steels, and is similar to that of B2 steel in Table 2,

  except for the absence of niobium in this composition.

  From the development of this steel, it was denatured along four different metallurgical routes, which are found in the

table 4.

  Reference Temp. Temp. end Temp. Thickness Rate LAF Coil thickness start LAC LAC winding hot sheet cold sheet (C) (C) (C) (mm) (mm) (mm) Bob. 1 1089 915 640 2.20 67 0.70 Bob. 2 1102 887 655 4.20 67 1.5 Bob. 3 1098 912 720 2.20 62 0.70 Bob. 4 1115 891 725 4.2 64 1.5 Table 4 We can notice that the steel is loaded with carbon and manganese, which induces a fairly low transformation point Ar3 , calculated at 776 C. As a result, the hot rolling was performed well above

of the transformation point Ar3.

  These different hot sheets were cold rolled, annealed

  and were subjected to a skin-pass operation.

  The mechanical characteristics were measured on

ISO 12.5x50 test tubes.

  Impact of the chemical composition Among the elements contained in steel, the most

  significant are carbon, manganese and silicon.

  Chromium is only present as a quenching element, which makes it possible to obtain the dual-phase structure with cooling rates.

  lower than for the same steel containing no chromium.

  If we refer to Figure 1 attached, we see the influence of elements such as carbon, manganese and silicon on the resistance Rm of steel. So for example, a variation of 10.10-3% in the carbon content or 50.10-3% in the manganese content will only lead to a

  variation of around 12 Mpa on the resistance level Rm.

  The incidence of other chemical elements, except sulfur, such as phosphorus or aluminum for example, is low

  significant on the resistance level Rm of the steel.

  It is therefore not necessary to define ranges

  particularly tight analytical for the constituents of steel.

  o0 Effect of hot rolling conditions Regardless of the thickness of the sheet (0.7 and 1.5 mm), the winding temperature has a moderate impact on the mechanical characteristics of the cold strip obtained at the end of the annealing. Indeed, the reduction of the winding temperature from 720 C (hot winding) to 640 C (warm winding), leads, for example after continuous annealing to 800 C with over-aging bearing, to an increase in the resistance Rm between 17 and 27 Mpa, the rest of the mechanical characteristics hardly changing, in particular the elongation at break A% and

Distributed stretching Ag%.

  Ref. Ep. Temp. Temp. Re Rm Ag A P Re / Rm Coil (mm) monitoring winding (Mpa) (Mpa) (%) (%) - = = Bob. 3 0.70 350 C 720 C 355 624 14.6 20.1 0 0.57 Bob. 1 0.70 350 C 640 C 317 651 14.4 20.0 0 0.49 Bob. 4 1.50 400 C 725 C 399 591 14.4 24.6 0.8 0.67 Bob. 2 1.50 400 C 655 C 433 608 15.2 24.7 0 0.71

Table 5

  Other hot rolling conditions have little impact

  on the mechanical properties of the cold sheet after annealing.

  Impact of annealing conditions Annealing of dual-phase steels is done in the two-phase or austenitic domain in order to allow the transformation of austenite into

  hard constituents during cooling.

  The study of the incidence of the annealing temperature on the mechanical characteristics of the cold strip in the annealing stock leads to a maximum ductility between 775 and 820 C. Regarding the resistance, there is no significant impact the annealing temperature, if not a slight increase in the resistance Rm beyond 800 C, of the order of 0.5 MPa per degree Celcius. If we consider the best compromise strength / ductility, it is in the temperature range between 775 and 8500C io that the product Rm x A% is the highest, and therefore that this compromise is the best. Figure 2 shows the result of the experimental tests with the steel considered in the example in Table 1. The maximum ductility is

obtained after annealing at 800 C.

  The study of the impact of the slow cooling temperature, between 620 and 680 C, on the mechanical characteristics of the raw cold annealing strip of a hot-wound dual-phase steel annealed at 800 C, does not evidence of significant incidence of the end of slow cooling temperature, which corresponds to the start temperature of

rapid cooling.

  Figure 3 shows the effect of the end of slow cooling (or early cooling) temperature on the resistance

  Rm and the elongation at break A% of the steel in table 1.

  The study of the incidence of the rapid cooling rate shows that a decrease in the speed of the production line has no significant effect on the mechanical characteristics. Thus, for example, a reduction in the speed of the line by half, which results in a reduction in half of the rapid heating and cooling speeds and a doubling of the maintenance times and, if necessary, over-aging, leads to a reduction in the elastic limit Re of approximately 10 Mpa, a reduction in the resistance Rm of approximately 15 Mpa, and a small

increased ductility.

  Regarding the impact of the temperature of the monitoring level, this is very important on the resistance Rm,

  The elongation at break A% and the elastic limit bearing P%.

  If we refer to Figure 4, we see that: II - a decrease of 50 C in the monitoring temperature leads to an increase in resistance of 40 MPa and a decrease in elongation at break of 1.7% ; - the strength / ductility compromise, represented by the product Rm x A% is satisfactory in all cases; - the elastic limit bearing is absent for temperatures

  monitoring below 350 C and appears from 400 C.

  From a metallurgical point of view, the impact of the monitoring temperature is explained by the fact that the temperature of this monitoring level can be considered as the quenching temperature, and there will be formation of martensite from austenite. only if the monitoring temperature is lower than the transformation temperature Austenite -> Martensite, that is to say approximately 360 C. Thus, in the case where the monitoring temperature is lower than 360 C, there will be formation of martensite during cooling fast and the landing of

  yield strength will not be observable in the annealed condition.

  Impact of the skin-pass rate The study of the incidence of the skin-pass rate shows that the elastic limit level Re increases very significantly with the rate

skin pass.

  Table 7 below shows that, thanks to the invention, it is possible, from the same cold sheet annealed outlet, by correctly adapting the skin-pass rate, to produce for example the four ranges of steel the most representative and most requested in the

automotive field.

  These four ranges are grouped in Table 6.

  Re (MPa) Rm (MPa) A% Range 1> 340 600> 20 Range2 420 - 500 600> 18 Range3 460 - 540> 600> 16 Range4 500 - 580 600> 14

Table 6

  Ref. Ep Temp. Temp. Temp. Rate Re Rm A% Ag% P% coil (mm) bob. annealed surveillance. skin-pass (Mpa) (Mpa) _____ (c> (C) (Oc) (%) Bob.3 0.70 720 800 350 0 355 624 20.1 14.6 0

1,453,643 20.4 14.0 0

2,513,649 18.6 12.6 0

3,541,634 17.8 11.5 0

4,572,648 15.9 10.4 0

572,641 15.9 9.7 0

6,590,645 14.1 8.2 0

7,600,651 13.4 7.4 0

615 663 9.2 4.5 0

  Bob. 3 0.70 720 800 400 0 407 595 21.3 14.6 1.1

1,434,592 20.2 13.6 0

2,491,603 18.3 12.1 0

3,523,603 17.6 10.9 0

4,556,617 17.3 9.9 0

577 622 14.4 8.2 0

6,588,626 13.0 6.9 0

7,599,635 12.3 5.2 0

616,646 8.3 3.8 0

  Bob. 4 1.50 725 800 400 0 399 591 24.6 14.4 0.8

1,407,601 24.1 13.0 0

2,461,608 20.9 11.2 0

3,493,613 19.9 9.9 0

4,512,618 19.6 10.2 0

527 627 18.5 8.5 0

6,541,628 17.3 7.7 0

7,553,635 16.4 7.3 0

582,645 13.2 4.2 0

  Bob. 1 0.70 640 800 350 0 317 651 20.0 14.4 0

1,434,647 18.4 13.0 0

2,501,647 18.2 11.9 0

3,536,660 16.6 10.7 0

4,556,679 16.0 10.1 0

574,669 15.1 9.1 0

6,582,671 14.1 7.0 0

7,599,673 12.7 5.5 0

628 672 8.3 4.0 0

  Bob. 1 0.70 640 800 400 0 421 614 21.8 14.2 1.1

1,422,626 19.5 13.3 0

2,481,629 18.8 12.1 0

3,517,638 17.7 11.1 0

4,554,645 16.3 9.6 0

570,648 16.0 8.8 0

6,587,651 14.5 6.9 0

7,603,654 13.4 5.3 0

618,656 10.3 3.7 0

  Bob. 2 1.50 655 800 400 0 433 608 24.7 15.2 0

1,426,616 23.4 13.5 0

2,470,622 19.9 11.7 0

3,491,628 18.4 10.2 0

4,515,641 18.4 9.3 0

532,637 17.5 8.7 0

6,540,645 17.0 7.9 0

7,554,645 16.1 6.9 0

580 646 13.5 4.7 0

Table 7

  It can be seen that for the same base steel, for example the steel of coil 3 annealed at 800 C with a monitoring level at 350 C, it is possible, simply by controlling the skin pass rate, to offer a whole range steels whose resistance Rm is greater than 600 MPa, steel whose ratio Re / Rm is equal to 0.57 and whose elongation is equal to 20.1% (steel not skin-passed), up to '' to a more resistant steel (Re = 572 MPa), whose Re / Rm ratio is equal to 0.89, and whose

  the elongation A% is not too degraded and is equal to 15.9 (5% of skin pass).

  Thus, the incidence of the monitoring temperature and of the skin-pass rate is very interesting, specifically in the case of continuous annealing lines because: - it allows the adjustment of the level of the mechanical characteristics of a quality from '' a single manufacturing parameter that can be precisely controlled: the monitoring temperature; - it is possible to develop several ranges of products from the same steel grade with a minimum of variable parameters; - and it guarantees significant robustness with regard to

  manufacturing parameters and chemical composition.

  These findings, demonstrated in detail in the example of

  realization which has just been described, starting from a basic steel of type dual-

  phase 600, are also valid for the other types of dualphase steel, dual-phase 450 steels, dual-phase 750 steels, and dual-phase 1200 steels. Application of the invention to these other dual- steels phase only requires adapting the composition of the base steel, the

  Typical compositions are indicated in Table 8 below in 10-3%.

  e> \\ p DP 450 DP 600 DP 750 DP1200 Carbon Element 50 - 80 100 - 150 120 - 180 300 - 400 Manganese 1100 - 1300 1300 - 1500 1800 - 2000 1000 - 2000 Silicon 100 - 200 250 - 400 150 - 250 400 - 800 Molybdenum 0-80 Chromium 400 - 600 100 - 300 150 - 250 0 - 500 Aluminum 20- 100 20- 100 20- 100 20 100 Sulfur <4 (residual) <4 (residual) <4 (residual) <4 (residual ) Phosphorus <20 (residual) <20 (residual) <20 (residual) <20 (residual) Nitrogen <7 (residual) <7 (residual) <7 (residual) <7 (residual)

Table 8

Claims (12)

  1 - Dual-phase steel characterized in that it comprises by weight between 0.05 and 0.4% of carbon, between 1.0 and 2.0% of manganese, between 0.1 and 0.8% of silicon , between 0.02 and 0.1% aluminum, between 0 and 0.08% molybdenum, less than 0.01% niobium, less than 0.02% titanium, and less than 0.01% of vanadium, the balance being iron and unavoidable residual impurities including less than 0.004% sulfur, less than 0.02% phosphorus, less than 0.007% nitrogen, obtained by cold rolling a hot strip io and continuous annealing of the cold strip, and in that it comprises a martensitic phase in a sufficient quantity so that the steel is free of yield strength in the annealed state, and in that the cold strip leaves of the annealing is subjected to a skin-pass operation, with a reduction rate included between 1 and 5%.   2 - Steel according to claim 1, characterized in that it contains   also between 0.01 and 0.6% chromium.   3 - Steel according to one of claims 1 or 2, characterized in that   that it contains at least 5% of martensitic phase in the annealed state.   4 - Steel according to claim 3, characterized in that it   contains at least 10% martensitic phase in the annealed state.   5 - Steel according to one of the preceding claims, characterized   in that the continuous annealing is carried out at a temperature higher than the pearlitic transformation temperature A1, followed by rapid cooling at a speed greater than or equal to the quenching speed of the steel, up to   a temperature less than or equal to 500 C.   6 - Steel according to claim 5, characterized in that the continuous annealing cycle has a monitoring level at a temperature less than or equal to 360 C.   7 - Steel according to one of the preceding claims,   characterized in that it comprises by weight between 0.05 and 0.08% of carbon, between 1.1 and 1.3% of manganese, between 0.1 and 0.2% of silicon, between 0.02 and 0.1% aluminum, less than 0.01% niobium, less than 0.02% titanium, and less than 0.01% vanadium, the remainder being iron and unavoidable residual impurities of which less than 0.004 % of sulfur, less than 0.02% of phosphorus, less than 0.007% of nitrogen 8 - Steel according to claim 7, characterized in that it contains   also between 0.4 and 0.6% chromium.   9 - Steel according to one of claims 1 to 6, characterized in that   that it contains by weight between 0.1 and 0.15% of carbon, between 1.3 and 1.5% of manganese, between 0.25 and 0.40% of silicon, between 0.02 and 0.1 % aluminum, between 0 and 0.08% molybdenum, less than 0.01% niobium, less than 0.02% titanium, and less than 0.01% vanadium, the rest being iron and unavoidable residual impurities of which less than 0.004% of sulfur,   less than 0.02% phosphorus, less than 0.007% nitrogen.   - Steel according to claim 9, characterized in that it   also contains between 0.1 and 0.3% chromium.   i 1 - Steel according to one of Claims 1 to 6, characterized in that   that it contains by weight between 0.12 and 0.18% of carbon, between 1.8 and 2.0% of manganese, between 0.15 and 0.25% of silicon, between 0.02 and 0.1 % aluminum, less than 0.01% niobium, less than 0.02% titanium, and less than 0.01% vanadium, the remainder being inevitable iron and residual impurities including less than 0.004% sulfur , less than 0.02% of   phosphorus, less than 0.007% nitrogen.   12 - Steel according to claim 1 1, characterized in that it   also contains between 0.15 and 0.25% chromium.   13 - Steel according to one of claims 1 to 6, characterized in   what it comprises by weight between 0.3 and 0.4% of carbon, between 1.0 and 2.0% of manganese, between 0.4 and 0.8% of silicon, between 0.02 and 0, 1% aluminum, less than 0.01% niobium, less than 0.02% titanium, and less than 0.01% vanadium, the rest being iron and unavoidable residual impurities of which less than 0.004% sulfur, less than 0.02% of   phosphorus, less than 0.007% nitrogen.   14 - Steel according to claim 13, characterized in that it also contains between 0 and 0.5% chromium. - Method for manufacturing a dual-phase steel with improved mechanical characteristics, characterized in that: - a hot plate is produced comprising by weight between 0.04 and 0.4% of carbon, between 1.0 and 2, 0% manganese, between 0.1 and 0.8% silicon, between 0.02 and 0.1% aluminum, between 0 and 0.08% molybdenum, less than 0.01% niobium, less 0.02% titanium, and less than 0.01% vanadium, the balance being iron and unavoidable residual impurities including less than 0.004% sulfur, less than 0.02% phosphorus, less than 0.007% d 'nitrogen; - cold rolling of said sheet is carried out to bring it to the desired thickness; - the cold-rolled sheet is subjected to continuous annealing at a temperature above the pearlitic transformation temperature A1, followed by rapid cooling at a speed greater than or equal to the quenching speed of the steel, to a temperature less than or equal to 500 ° C., ensuring that a martensitic phase is formed in a sufficient quantity so that the steel is free from an elastic limit plateau in the annealed state; - the annealed cold sheet is subjected to a skin-pass operation at a controlled rate, between 1 and 5%, depending on the characteristics mechanical sought.   16 - Method according to claim 15, characterized in that the continuous annealing cycle has a monitoring level at a   temperature less than or equal to 360 C.