EP1099769A1 - Process for manufacturing high tensile strength hot rolled steel sheet for forming and especially for deep drawing - Google Patents

Abstract

Producing a hot rolled steel sheet includes forming the sheet ≤ 880 degrees C, initially cooling for ≤ 10 s, further cooling at 20-150 degrees C/sec to 700-750 degrees C, further cooling at 3-20 degrees C/sec, depending on the sheet thickness, to 640-700 degrees C and then controlled cooling at 20-150 degrees C/sec to 350-550 degrees C. The steel comprises 0.12-0.25% C, 1-2 % Mn, 0.03-2.5% Al, 0.04-2% Cr, 0.02-0.09 % P and ≤ 0.01% S. An Independent claim is included for a hot rolled steel sheet produced as above. Preferred Features: The steel may also include ≤ 0.15 % titanium, ≤ 0.15% Nb and ≤ 0.15% V.

Description

The invention relates to a method for producing a strip of sheet metal laminated to hot with very high resistance, usable for shaping and in particular for stamping.

In the field of mechanical engineering and more specifically the automobile, equipment including safety, comfort, and the need energy saving led to a search for relief while retaining the in-service properties of the stamped parts. Fatigue behavior, particular, is an essential criterion since it defines the life of these parts. In order to improve this fatigue resistance, one solution consists in using steels with very high resistance. There is indeed a linear relationship between the limit endurance and mechanical strength. It is then possible to use sheets with reduced thicknesses, which contributes to lightening while keeping the held in service. However, the steel must be suitable for stamping. Now, in generally, the formatting properties decrease with increasing mechanical resistance.

• Les aciers HLE dits à haute limite élastique qui sont les aciers microalliés présentant une limite d'élasticité comprise entre 315 MPa et 700 MPa, mais une aptitude au formage limitée, du fait en particulier, d'un rapport Re/Rm compris entre 0,85 et 0,9.
• Les aciers Dual-Phase, pour leur part, sont des aciers de structure ferritique martensitique ayant des propriétés de mise en forme remarquables, mais présentant des niveaux de résistance mécanique ne dépassant pas 600 MPa.
• Les aciers dits HR qui sont des aciers au carbone et au manganèse subissant après laminage un refroidissement rapide associé à un bobinage à basse température pour leur conférer une structure ferrito bainitique. Ces aciers ont des propriétés de mise en forme intermédiaires entre les aciers HLE et les aciers Dual-phase. Par exemple, l'acier HR 55 a un niveau de résistance minimal de 540 MPa, et présente une bonne aptitude à l'emboutissage, avec un rapport Re/Rm compris entre 0,75 et 0,8. De plus, cet acier est soudable et possède une excellente aptitude à subir une mise en forme du type relevé de collerette. L'obtention d'un acier du type HR60 nécessite de recourir soit, à l'ajout d'un élément de microalliage, par exemple le niobium, qui donne à cet acier des caractéristiques proches de celles d'un acier HLE soit, d'augmenter les teneurs en carbone ou en manganèse de l'acier du type HR55 conduisant à une composition pouvant entraíner des difficultés dans le domaine du soudage par résistance.

In the range of hot-rolled steels, the mechanical characteristics of which are obtained by controlled rolling on a wide-band train, there are in particular three types of hot-rolled steels having high mechanical characteristics with an elastic limit between 315 MPa and 700 MPa.

• HLE steels called high elastic steels which are microalloyed steels having an elastic limit between 315 MPa and 700 MPa, but a limited formability, due in particular to a Re / Rm ratio between 0, 85 and 0.9.
• Dual-phase steels, for their part, are steels of martensitic ferritic structure having remarkable shaping properties, but having mechanical strength levels not exceeding 600 MPa.
• The so-called HR steels which are carbon and manganese steels undergoing rapid cooling after rolling associated with a low temperature winding to give them a bainitic ferrito structure. These steels have intermediate forming properties between HLE steels and Dual-phase steels. For example, HR 55 steel has a minimum strength level of 540 MPa, and has good drawing ability, with a Re / Rm ratio of between 0.75 and 0.8. In addition, this steel is weldable and has an excellent ability to undergo shaping of the raised flange type. Obtaining a steel of the HR60 type requires either recourse to the addition of a microalloy element, for example niobium, which gives this steel characteristics close to those of an HLE steel, or increase the carbon or manganese contents of HR55 type steel leading to a composition which can cause difficulties in the field of resistance welding.

The families of steels mentioned above therefore have limits in their mechanical characteristics and their behavior.

A metallurgical solution to improve the strength compromise mechanical and elongation consists of the use of TRIP steels of ferrite-bainite structure- residual austenite. In this type of structure, the resistance compromise mechanical and elongation is significantly improved by the presence, in the microstructure, residual austenite. In this case, the amount of austenite residual is greater than 5%.

On the other hand, the presence of martensite in such a microstructure prevents the improvement of the drawability due to the presence of austenite residual.

A first possibility of obtaining TRIP steels is the use of steels C-Mn-Si type composition> 1%. These compositions have the disadvantage generate the formation of fayalite due to the presence of silicon.

Another possibility is the use of compositional steels of the C-Mn-AI type. This composition has an insufficient residual austenite.

Obtaining residual austenite is only possible for an interval of restricted winding temperature between 350 ° C and 400 ° C for both steels of the TRIP C-Mn-AI type only for TRIP C-Mn-Si steels.

A winding temperature below 350 ° C causes the appearance of martensite, which degrades in particular the formability of steels. A temperature of too high winding leads to a purely ferito-bainitic structure without residual austenite therefore without improvement in formability. Indeed, the presence residual austenite must be greater than 5% to have an effect on the formability of the steels produced. Below this value, its influence is practically zero.

Industrially, the winding temperatures in the area specified above are particularly difficult to obtain. Indeed, the temperature range winding between 350 ° C and 400 ° C corresponds to a zone of exchange instability between the steel strip and the cooling water, due to the rupture of the film of water vapor forming a screen between the hot metal and the cooling water. This phenomenon leads to a sudden increase in the exchange coefficient thermal in the area concerned which results, on the rolled steel strip, a heterogeneity of microstructure detrimental to the regularity of the properties mechanics of the finished product. The obligation to use winding temperatures bass associated with the allied nature of TRIP compositions causes difficulties of achievement. It is therefore sought to increase the temperature of winding to take advantage of higher ductility at high temperature.

The object of the invention is to develop a method for producing a very high strength TRIP type steel strip with good properties formatting.

0,12% ≤ carbone ≤ 0,25%,

1% ≤ manganèse ≤ 2%,

0,03% ≤ aluminium < 2,5%,

0,03% ≤ silicium ≤ 2%,

0,04% ≤ chrome ≤ 2%,

0,02% ≤ phosphore ≤ 0,09%,

soufre ≤0,01%, et de manière optionnelle,

titane ≤ 0,15%,

niobium ≤ 0,15%,

vanadium ≤ 0,15%, le reste étant du fer et des impuretés résiduelles,

est soumis à :

• un laminage à une température inférieure à 880°C,
• un premier refroidissement court, effectué dans un temps inférieur à 10 secondes,
• un deuxième refroidissement contrôlé avec une vitesse de refroidissement V ref1 comprise entre 20°C/ seconde et 150°C/seconde en fonction de l'épaisseur de la bande d'acier laminée, la température de fin de deuxième refroidissement étant au dessous du point Ar3 de la transformation de l'austénite en ferrite, la température de la fin du deuxième refroidissement étant comprise entre 700°C à 750°C,
• un maintien sur un palier de température associé à un refroidissement lent, la vitesse de refroidissement étant comprise entre 3°C/seconde et 20°C/seconde jusqu'à une température de fin de palier comprise entre 700°C et 640°C,
• un troisième refroidissement également contrôlé dont la vitesse est comprise entre 20°C/seconde et 150°C/seconde, refroidissement liée à l'épaisseur de la bande de tôle; la température de la fin du troisième refroidissement étant comprise entre 350°C et 550°C.

The object of the invention relates to a process for producing a strip of hot-rolled steel sheet with very high resistance usable for shaping and in particular drawing which is characterized in that the steel of composition following weight:

0.12% ≤ carbon ≤ 0.25%,

1% ≤ manganese ≤ 2%,

0.03% ≤ aluminum <2.5%,

0.03% ≤ silicon ≤ 2%,

0.04% ≤ chromium ≤ 2%,

0.02% ≤ phosphorus ≤ 0.09%,

sulfur ≤0.01%, and optionally,

titanium ≤ 0.15%,

niobium ≤ 0.15%,

vanadium ≤ 0.15%, the rest being iron and residual impurities,

is subject to:

• rolling at a temperature below 880 ° C,
• a first short cooling, carried out in a time of less than 10 seconds,
• a second controlled cooling with a cooling rate V ref1 of between 20 ° C / second and 150 ° C / second depending on the thickness of the rolled steel strip, the temperature at the end of the second cooling being below the point Ar3 of the transformation of austenite into ferrite, the temperature at the end of the second cooling being between 700 ° C to 750 ° C,
• maintaining a temperature level associated with slow cooling, the cooling rate being between 3 ° C / second and 20 ° C / second up to an end of temperature level between 700 ° C and 640 ° C,
• a third cooling, also controlled, the speed of which is between 20 ° C / second and 150 ° C / second, cooling linked to the thickness of the sheet metal strip; the temperature at the end of the third cooling being between 350 ° C and 550 ° C.

• la composition pondérale comprend moins de 0,5% de silicium,
• les refroidissements sont effectués sous air,
• l'acier est laminée à chaud pour obtenir une bande de tôle laminée à chaud dont l'épaisseur est comprise entre 1,4 mm et 6 mm.

The other characteristics of the invention are:

• the composition by weight comprises less than 0.5% of silicon,
• the coolings are carried out in air,
• the steel is hot rolled to obtain a strip of hot rolled sheet whose thickness is between 1.4 mm and 6 mm.

0,12% ≤ carbone ≤ 0,25%,

1% ≤ manganèse ≤2%,

0,03% ≤ aluminium ≤ 2,5%,

0,03% ≤ silicium ≤ 2%,

0,04% ≤ chrome ≤ 2%,

0,02% ≤ phosphore ≤ 0,09%,

soufre ≤ 0,01%, et de manière optionnelle,

titane ≤ 0,15%,

niobium ≤ 0,15%,

vanadium ≤ 0,15%, le reste étant du fer et des impuretés résiduelles,

The invention also relates to a hot-rolled steel sheet obtained by the process comprising in its weight composition:

0.12% ≤ carbon ≤ 0.25%,

1% ≤ manganese ≤2%,

0.03% ≤ aluminum ≤ 2.5%,

0.03% ≤ silicon ≤ 2%,

0.04% ≤ chromium ≤ 2%,

0.02% ≤ phosphorus ≤ 0.09%,

sulfur ≤ 0.01%, and optionally,

titanium ≤ 0.15%,

niobium ≤ 0.15%,

vanadium ≤ 0.15%, the rest being iron and residual impurities,

• la tôle d'acier laminée à chaud comprend dans sa composition pondérale moins de 0,5% de silicium,
• la tôle laminée à chaud a une épaisseur comprise entre 1,4 mm et 6 mm.

The other characteristics of the invention are:

• the hot-rolled steel sheet comprises in its composition by weight less than 0.5% of silicon,
• the hot rolled sheet has a thickness of between 1.4 mm and 6 mm.

The following description and the attached figures, all given by way of example non-limiting will make the invention well understood.

Figure 1 shows a diagram of the cooling of the sheet metal strip hot rolled according to the invention.

Figure 2 shows the variation of the austenite content as a function of the winding temperature for examples of steels according to the invention in comparison with reference steels TRIP C-Mn-Si and TRIP 0% Cr.

0,12% ≤ carbone ≤ 0,25%,

1% ≤ manganèse ≤ 2%,

0,03% ≤ aluminium ≤ 2,5%,

0,03% ≤ silicium ≤ 2%,

0,04% ≤ chrome ≤ 2%,

0,02% ≤ phosphore ≤ 0,09%,

soufre ≤ 0,01%, et de manière optionnelle,

titane ≤ 0,15%,

niobium ≤ 0,15%,

vanadium ≤ 0,15%, le reste étant du fer et des impuretés résiduelles,

est soumis à un laminage à chaud à une température inférieure à 880°C afin d'affiner sa structure par écrouissage.According to the invention, a steel whose weight composition is as follows:

0.12% ≤ carbon ≤ 0.25%,

1% ≤ manganese ≤ 2%,

0.03% ≤ aluminum ≤ 2.5%,

0.03% ≤ silicon ≤ 2%,

0.04% ≤ chromium ≤ 2%,

0.02% ≤ phosphorus ≤ 0.09%,

sulfur ≤ 0.01%, and optionally,

titanium ≤ 0.15%,

niobium ≤ 0.15%,

vanadium ≤ 0.15%, the rest being iron and residual impurities,

is subjected to hot rolling at a temperature below 880 ° C in order to refine its structure by work hardening.

A first short cooling, for example with air, is carried out in a time less than 10 seconds to obtain fine grains and to avoid the appearance of the perlite phase during cooling. The steel is then subjected to a second controlled cooling whose speed is between 20 ° C / second and 150 ° C / second, depending on the thickness of the steel strip laminated treated. The cooling rate, controlled according to the invention, ensures significant germination of the ferritic phase. The temperature at the end of second cooling is within a temperature range varying from 700 ° C and 750 ° C, i.e. below the Ar3 point of austenite formation in ferrite.

The sheet is then maintained on a temperature level where it undergoes a slow cooling, for example in air, with a cooling rate between 3 ° C / second and 20 ° C / second to reach an end temperature between 700 ° C and 640 ° C. Maintaining the steel strip on this bearing ensures the formation of a ferrite rate between 40% and 70%. It allows enrich the residual austenite with carbon, not transformed into ferrite, delaying its formation during cooling.

Hot rolled sheet steel, after temperature maintenance on the bearing is subjected to a third also controlled cooling, the speed of which is between 20 ° C / second and 150 ° C / second, linked to the thickness of the strip treated sheet metal and this up to a temperature between 350 ° C and 525 ° C so to complete the enrichment of the residual austenite during the transformation which starts at a temperature of around 640 ° C.

For example, the cooling rates Vref1 and Vref2 are included between 20 ° C / s and 50 ° C / S for sheet thicknesses between 4.5 mm and 6 mm and between 50 ° C / S and 150 ° C / s for thicknesses between 1.4 mm and 4.5 mm.

The final structure of hot rolled steel is composed of ferrite, bainite and residual austenite at a content higher than 5%, which allows to reach a mechanical resistance greater than 700 MPa, with elongation values distributed greater than 10% and an elongation at break greater than 25%.

From the point of view of the elements contained in the composition, according to the invention, the carbon stabilizes the austenite. Manganese lowers the points of transformation Ar3, Bs and Ms respectively corresponding to the temperature of start of ferritic transformation, at the start of transformation temperature bainitic and the temperature at the start of the martensitic transformation.

Aluminum and silicon prevents the diffusion of carbon and ensures the stabilization of austenite by their effect on carbon. Silicon and aluminum have the same effect complementing each other. However, it is preferable to keep the silicon at low contents to avoid the formation of fayalite generating surface defects that appear after pickling. The presence of phosphorus and chromium, alphagenic elements, promotes the formation of the ferritic phase at during the hold on the temperature level. The proportion of ferrite formed is important and the carbon enrichment of the residual austenite allows the stabilization of this phase in a large winding temperature range. The titanium, niobium and vanadium elements introduced into the composition so optional are micro alloy elements which can be added to the composition of the steel to obtain precipitation hardening and to refine the grain size of the ferrite. This provides more mechanical strength high by slightly reducing the distributed elongation.

The composition of the steel according to the invention makes it possible to obtain a microstructure residual austenite bainite ferrite type, hot rolling ensuring on the one hand, good recrystallization of the austenite grains at the outlet of the rolling mill stands and on the other hand, an equiaxed texture.

• la température de laminage est de 850°C,
• le premier refroidissement à l'air est de 1,5 secondes, suivi d'un deuxième refroidissement contrôlé à une vitesse de 80°C/seconde jusqu'à la température de 720°C, température en dessous du point Ar3,
• la bande d'acier obtenu est ensuite maintenue en température, à l'air, sur un palier de température où elle est refroidie jusqu'à la température de 680°C,
• le troisième refroidissement également contrôlé, est effectué à une vitesse de 80°C/seconde jusqu'à une température correspondant à la température de bobinage,
• le bobinage est effectuée dans l'exemple, à différentes températures, qui sont: 400°C, 450°C, 500°C, 550°C, 600°C. composition (x10^-3%) C Al Mn Si P Cr N 200 1330 1500 250 48 852 <2 Aux différentes températures de bobinage, il a été mesuré, comme présenté sur les tableaux suivants, les différentes caractéristiques mécaniques obtenues. Bobinage à 400°C. Rp02 Rm Ag * Re/Rm n MPa MPa (%) (4-8%) 418 799 14,6 0,52 0,22

In an application example, the steel, the composition of which is presented in Table 1, is subjected to the temperature treatment according to the invention in which:

• the rolling temperature is 850 ° C.,
• the first air cooling is 1.5 seconds, followed by a second controlled cooling at a speed of 80 ° C / second to the temperature of 720 ° C, temperature below the point Ar3,
• the steel strip obtained is then maintained at temperature, in air, on a temperature level where it is cooled to the temperature of 680 ° C.,
• the third cooling, also controlled, is carried out at a speed of 80 ° C / second up to a temperature corresponding to the winding temperature,
• the winding is carried out in the example, at different temperatures, which are: 400 ° C, 450 ° C, 500 ° C, 550 ° C, 600 ° C. composition (x10 ^-3 %) VS Al Mn Yes P Cr NOT 200 1330 1500 250 48 852 <2 At the different winding temperatures, the different mechanical characteristics obtained were measured, as presented in the following tables. Winding at 400 ° C. Rp02 Rm Ag * Re / Rm not MPa MPa (%) (4-8%) 418 799 14.6 0.52 0.22

Note:

Ag * represents the distributed elongation, corresponding to the elongation of the test piece traction when the start of necking appears.

Rm: breaking strength of the steel of the test piece.

Re: elastic limit of steel.

n: consolidation coefficient.

At the microstructure level, the bainite is slightly in majority compared to the ferrite which is present in fine grains. The residual austenite is present in the form of blocks between the ferrite grains, with an average of 12.8%. Winding at 450 ° C. Rp02 Rm Ag Re / Rm not MPa MPa (%) (4-8%) 519 728 11.9 0.71 0.20

Note: The microstructure is ferritic bainitic. One can observe austenite beaches in the form of islands between the slats of blessing. The average residual austenite is 7%. Winding at 500 ° C. Rp02 Rm Ag Re / Rm not MPa MPa (%) (4-8%) 458 779 14.4 0.59 0.21

Note: The microstructure is of the bainite ferrite type where the bainite is predominant in the form of large areas. Austenite is mainly in the form of blocks between the ferrite grains. The average residual austenite is 9.4%. Winding at 550 ° C. Rp02 Rm Ag Re / Rm not MPa MPa (%) (4-8%) 569 758 9.5 0.75 0.15

Note: The microstructure has very little residual austenite, the average residual austenite is 0.2%. Winding at 600 ° C. Rp02 Rm Ag Re / Rm not MPa MPa (%) (4-8%) 487 655 12.8 0.74 0.22

Note: the microstructure is of the ferrite bainite type and does not present an austenite residual.

In general, we note that steel with a ferrite-bainite-austenite microstructure residual with the following mechanical characteristics: Rm> 700 MPa, Re / Rm ratio <0.7, Ag> 10% and A%> 25% can only be achieved for winding temperatures between 400 ° C and 500 ° C thanks to a quantity residual austenite greater than 5%.

For the two highest winding temperatures, the quantity of residual austenite is zero or almost zero and the mechanical properties are not not conform with Ag% elongation or with a limit at rupture Rm acceptable, the Re / Rm ratio being moreover too high.

Figure 2 shows the residual austenite rate as a function of the winding temperature for different reference TRIP steel compositions and according to the invention. It shows that the process according to the invention has compared, for example, to steel A taken as a reference, TRIP C-Mn-Si a quantity higher austenite for a wider winding temperature range and higher in temperature. Figure 2 shows, for comparison with steel A on steel 1 of the example, and two steels 2 and 3 according to the invention and comprising 0% Cr and 2% Cr respectively. According to the process, we can obtain the rate of austenite desired in a wide range of winding temperature, which ensures regularity of the mechanical characteristics of the sheet produced, regularity without which the use of sheet metal for a stamped part would be impossible. The possibility according to the process of winding at higher temperature allows an industrial realization of the sheet without strengthening the capacities of the industrial tool.

The proposed invention allows the production of a rolled steel strip thickness between 1.4 mm and 6 mm which has both a high mechanical resistance greater than 700 MPa and setting properties important shape thanks to a Re / Rm ratio of less than 0.7, to an elongation distributed greater than 10% and an elongation at break greater than 25%.

When the silicon content is less than 0.5%, an appearance of surface of the sheet metal strip, faultless, after pickling.

According to the invention, the method makes it possible to obtain a strip of sheet steel hot rolled comprising a residual austenite bainite austenite structure with more 5% by carrying out in the process an extended winding in an interval of temperature between 350 ° C and 525 ° C. It is thus possible to leave the domain instability of the winding temperature below 400 ° C. This is possible in particular by the use in the composition of the base steel of a content determined in chromium and phosphorus.

The sheet metal strip according to the invention can be introduced in use for stamped, bent or profiled parts in the construction sector mechanical and automotive. Its use gives the possibility of reducing thicknesses parts ensuring their lightening and or an improvement in their performance tired. The parts that can be produced include absorbers, reinforcement, structural parts, wheels requiring good resistance to fatigue and also good stampability.

Claims (7)

Process for producing a strip of hot-rolled sheet metal with very high resistance, usable for shaping and in particular for stamping, characterized in that the steel of the following composition by weight: 0.12% ≤ carbon ≤ 0.25%, 1% ≤ manganese ≤ 2%, 0.03% ≤ aluminum ≤ 2.5%, 0.03% ≤ silicon ≤ 2%, 0.04% ≤ chromium ≤ 2%, 0.02% ≤ phosphorus ≤ 0.09%, sulfur ≤ 0.01%, and optionally, titanium ≤ 0.15%, niobium ≤ 0.15%, vanadium ≤ 0.15%, the rest being iron and residual impurities, is subject to: rolling at a temperature below 880 ° C, a first short cooling, carried out in a time of less than 10 seconds, a second controlled cooling with a cooling rate V ref1 of between 20 ° C / second and 150 ° C / second depending on the thickness of the rolled steel strip, the temperature at the end of the second cooling being below the point Ar3 of the transformation of austenite into ferrite, the temperature at the end of the second cooling being between 700 ° C to 750 ° C, maintaining a temperature level associated with slow cooling, the cooling rate being between 3 ° C / second and 20 ° C / second up to an end of temperature level between 700 ° C and 640 ° C, a third cooling, also controlled, the speed of which is between 20 ° C / second and 150 ° C / second, cooling linked to the thickness of the sheet metal strip, the temperature at the end of the third cooling being between 350 ° C and 550 ° C. Method according to claim 1 characterized in that the weight composition contains less than 0.5% silicon. Method according to claim 1 characterized in that the coolings are performed in air. Method according to claim 1 characterized in that the steel is hot rolled to obtain a strip of hot-rolled sheet whose thickness is between 1.4 mm and 6 mm. Hot-rolled steel sheet obtained by the process according to one of claims 1 to 4 characterized in that it comprises in its weight composition: 0.12% ≤ carbon ≤ 0.25%, 1% ≤ manganese ≤ 2%, 0.03% ≤ aluminum ≤ 2.5%, 0.03% ≤ silicon ≤ 2%, 0.04% ≤ chromium ≤ 2%, 0.02% ≤ phosphorus ≤ 0.09%, sulfur ≤ 0.01%, and optionally, titanium ≤ 0.15%, niobium ≤ 0.15%, vanadium ≤ 0.15%, the remainder being iron and residual impurities. Sheet according to claim 5 characterized in that it comprises in its weight composition less than 0.5% silicon. Sheet according to claim 5 characterized in that it has a thickness comprised between 1.4 mm and 6 mm.

Images