EP0835945B1 - Hot rolled steel sheet for deep drawing - Google Patents

Abstract

Hot-rolled steel sheet suitable for shaping by deep pressing contains by weight 0.010-0.080%, preferably 0.020-0.040% carbon, 0.1-0.5%, preferably 0.15-0.25% manganese, 0.02-0.08%, preferably 0.02-0.04% aluminium, less than 0.1%, preferably 0.02-0.04% silicon, less than 0.04%, preferably less than 0.02% phosphorous, less than 0.025%, preferably less than 0.005% sulphur, less than 0.05%, preferably less than 0.02% titanium, less than 0.009% nitrogen, 0.001-0.01%, preferably 0.002-0.004% boron, 0.1-0.8%, preferably 0.30-0.40% copper and 0.05-0.6%, preferably 0.15-0.20% nickel, the rest being iron and impurities. Preferably the Ni content is half that of the Cu. Also claimed is manufacture of the steel sheet.

Description

The present invention relates to a hot-rolled steel sheet for deep drawing, from a band train.

The shaping properties of steels are important for the production of stamped parts of complex shapes. In the range of hot rolled flat products, the mechanical characteristics of which are obtained by controlled rolling on a broadband train, steels with the best stamping properties are steels called 3C and 3C Ti.

These steels have compositions containing carbon, manganese, titanium and have very high levels of additives weak to soften the mechanical properties. They own nevertheless gamagenic elements such as carbon and manganese whose contents are high enough to have a temperature of relatively low ferritic transformation, such as, for example, AR3 transformation temperature of 840 ° C for a thickness of 4.5 mm. It is necessary to laminate above this temperature, i.e. in the austenitic area to avoid laminating in the area two-phase austenite-ferrite, rolling domain which degrades the properties steel shaping.

On the other hand, the sheets made with these steels can be coated continuously on a galvanizing line to protect them against corrosion. This coating method results in the sheets being subjected to a cycle thermal which causes in the steel of said sheets, by diffusion of carbon and nitrogen, an increase in the yield strength of steel and a decrease in its elongation.

The object of the invention is to provide a steel sheet having a on the other hand, high shaping properties for deep drawing, and secondly, comparable mechanical properties after rolling to hot and after continuous galvanizing.

0,010%< carbone < 0,080%

0,1 < manganèse < 0,5%

0,02% < aluminium < 0,08%

silicium < 0,1%

phosphore < 0,04%

soufre < 0,025%

titane < 0,05%

azote < 0,009%

0,001% < bore < 0,01%

0,1 < cuivre < 0,8%

0,05% < nickel < 0,6%.

The subject of the invention is a hot-rolled steel sheet for deep drawing, characterized in the following weight composition:

0.010% <carbon <0.080%

0.1 <manganese <0.5%

0.02% <aluminum <0.08%

silicon <0.1%

phosphorus <0.04%

sulfur <0.025%

titanium <0.05%

nitrogen <0.009%

0.001% <boron <0.01%

0.1 <copper <0.8%

0.05% <nickel <0.6%.

• la teneur en nickel est sensiblement égale à la moitié de la teneur en cuivre.

Another characteristic of the invention is:

• the nickel content is approximately equal to half the copper content.

• un laminage à chaud à une température supérieure à la température de transformation AR3,
• un refroidissement commençant dans un intervalle de temps inférieur à 10 secondes après le laminage à chaud, le refroidissement étant, d'une part, compris entre 3°C par seconde et 80°C par seconde, et, d'autre part, effectué jusqu'à une température comprise entre 600°C et 750°C.

The invention also relates to a process for manufacturing a hot-rolled steel sheet for deep drawing in which the composition of the steel is subjected after production to:

• hot rolling at a temperature higher than the transformation temperature AR3,
• cooling commencing in an interval of time less than 10 seconds after hot rolling, the cooling being, on the one hand, between 3 ° C per second and 80 ° C per second, and, on the other hand, carried out up to '' at a temperature between 600 ° C and 750 ° C.

• le laminage à chaud est effectué à une température comprise dans un intervalle de 10°C à 120°C au-dessus de la température de transformation AR3.

Another characteristic of the invention is:

• hot rolling is carried out at a temperature in the range of 10 ° C to 120 ° C above the transformation temperature AR3.

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

FIG. 1 shows the influence of the contents of carbon elements, boron, copper plus nickel on the lowering of the AR3 transformation point.

Figure 2 shows the evolution of AR3 as a function of temperature for a steel containing 0.002% boron and a steel not containing no boron.

Figure 3 shows the evolution of sheet metal processing in its production process.

0,010%< carbone < 0,080%

0,1 < manganèse < 0,5%

0,02% < aluminium < 0,08%

silicium < 0,1%

phosphore < 0,04%

soufre < 0,025%

titane < 0,05%

azote < 0,009%

0,001% < bore < 0,01%

0,1 < cuivre < 0,8%

0,05% < nickel < 0,6%.

le reste étant du fer et des impuretés inhérentes à l'élaboration, permet l'obtention d'une microstructure ferrite cémentite homogène.Hot rolled steel sheet for deep drawing, the composition of which is as follows:

0.010% <carbon <0.080%

0.1 <manganese <0.5%

0.02% <aluminum <0.08%

silicon <0.1%

phosphorus <0.04%

sulfur <0.025%

titanium <0.05%

nitrogen <0.009%

0.001% <boron <0.01%

0.1 <copper <0.8%

0.05% <nickel <0.6%.

the remainder being iron and impurities inherent in the preparation, makes it possible to obtain a homogeneous cementite ferrite microstructure.

The transformation point is lowered by the copper, nickel elements and boron without the hardening of the structure.

FIG. 1 shows the influence of the contents of carbon elements, boron, copper plus nickel on the lowering of the AR3 transformation point.

The addition of nickel in a content equal to half the copper content is necessary to attenuate the surface defects of the sheet.

Copper and nickel provide sheet steel with improved corrosion resistance.

Carbon, at a content of less than 0.08%, makes it possible to obtain good formatting properties. Low carbon content ensures a limitation of the hardening of the matrix due to a low rate of carburetted phases.

The main function of titanium is to combine with nitrogen to form very stable titanium nitride precipitates during the solidification of steel. Titanium in over-stoichiometry (3.4 <Ti / N <10) precipitates during cooling in the form of titanium carbide and thus traps part of the carbon in the steel. The Ti / N ratio must remain less than 10 to avoid precipitation hardening of carbide titanium.

The titanium content should therefore be limited to avoid hardening by the precipitates. In high content in the indicated range, titanium precipitated in the form of TIC can be an advantage for steels for enameling because it allows the conservation of mechanical properties after sheet metal shaping and thermal enamelling treatment.

The function of boron in particular is to control the germination and growth of ferrite and thus obtain good setting properties shape, properties which are characterized by an elongation of the steel improved. Boron on the other hand precipitates with carbon in the form of borocarbons or segregations at grain boundaries.

In the steel according to the invention, containing boron, the starting point of ferritic transformation decreases with increasing temperature rolling. This remark considerably lowers the ferritic transformation start temperature and thus prevents rolling two-phase, rolling below the start of processing temperature ferrite-bainite. Indeed, two-phase rolling leads to type defects orange peel on the surface linked to the magnification of the ferritic grain with degraded formatting properties. The phenomenon highlighted lowers the carbon and manganese contents and thus improves shaping properties thanks to a softer structure with a higher ferritic grain size and therefore higher elongation without risk of two-phase rolling.

Figure 2 shows the evolution of AR3 as a function of temperature for a steel containing 0.002% boron and a steel not containing no boron.

As shown in Figure 2, boron helps control the ferritic transformation start temperature in association with end of rolling temperature.

The combination of titanium and boron allows their precipitation to keep the mechanical properties obtained after hot rolling at during heat treatment on galvanizing line.

The rolling temperature is chosen so that it is 10 ° C to 120 ° C higher than the point value AR3 transformation to avoid rolling in the austenite domain ferrite unfavorable for shaping properties.

Figure 3 shows the evolution of the heat treatment of the sheet in its manufacturing process. A time of less than 10 seconds is necessary before the first cooling heat treatment, the cooling being carried out with a speed of between 3 ° C / s and 80 ° C / s depending on the thickness of the laminated sheet, which ensures 80 ° C / s depending on the thickness of the laminated sheet, which ensures controlled and homogeneous germination of ferrite. After cooling down sheet metal up to a temperature between 600 ° C and 750 ° C, the structure final composed of cementite ferrite ensures, on the one hand, resistance mechanical between 250 MPa and 370 MPa and, on the other hand, a limit between 180 MPa and 280 MPa as well as an elongation more than 30%.

0,020%< carbone < 0,040%

0,15 < manganèse < 0,25%

0,02% < aluminium < 0,04%

0,02 < silicium < 0,04%

phosphore < 0,02%

soufre < 0,005%

titane < 0,02%

azote < 0,009%

0,002% < bore < 0,004%

0,35% < cuivre < 0,45%

0,18% < nickel < 0,23%.

In an application example, a hot-rolled steel sheet for deep drawing is produced from a steel of the following composition by weight:

0.020% <carbon <0.040%

0.15 <manganese <0.25%

0.02% <aluminum <0.04%

0.02 <silicon <0.04%

phosphorus <0.02%

sulfur <0.005%

titanium <0.02%

nitrogen <0.009%

0.002% <boron <0.004%

0.35% <copper <0.45%

0.18% <nickel <0.23%.

The hot rolling temperature is chosen at the point value transformation AR3 plus 20 ° C. Cooling started 1.5 second after hot rolling is performed at 30 ° C per second until a temperature of 680 ° C. Elongations of hot rolled sheet according to the invention can reach 36% for sheet thicknesses between 1.8 and 2.8 mm and values greater than 40% for sheet thicknesses between 3 and 8 mm.

Table 1 shows two other compositions of the steel sheet according to the invention. VS mn Cu Or al Ti NOT B Sheet A 0.044 0.274 0.406 0.214 0.031 0,021 0.0042 0.0027 Sheet B 0,040 0.267 0.202 0.098 0,028 0,019 0.0042 0.0020

The temperature at the start of the AR3 ferritic transformation, respectively for sheet A and sheet B, is 818 ° C and 842 ° C.

The thermomechanical treatment of the two sheets according to the invention involves rolling at a temperature of 900 ° C, winding at a temperature of 700 ° C., the cooling of the sheets having been carried out at a speed of 25 ° C per second.

Table 2 presents the mechanical characteristics of the two examples of sheets A and B. Re (MPA) Rm (MPa) AT (%) Sheet A 246 344 43 Sheet B 244 328 43.4

Table 3 below presents, for a sheet A, the so-called raw mechanical characteristics obtained before thermal galvanizing treatment and the mechanical characteristics after thermal galvanizing treatment at 700 ° C. and 600 ° C. Raw sheet metal 700 ° C. 600 ° C Re (MPa) 246 262 246 Rm (MPa) 344 350 348 AT (%) 43 43.3 36.3

Conditions of heat treatment during galvanizing are as follows:

The rate of temperature rise is between 3 ° C / s and 20 ° C / s, speed generally being 8 ° C / s. The holding temperature is between 550 ° C and 850 ° C, the current temperature being 700 ° C, with a holding time of 20 s to 120 s and preferably 60 s. This temperature rise is followed by cooling at a rate between 3 ° C / s and 25 ° C / s, the typical speed value cooling being 10 ° C / s. Cooling is carried out until temperature of the galvanizing bath, at 450 ° C.

The steel sheet according to the invention comprises, for a thickness between 1.5 mm and 8 mm of the mechanical characteristics comparable between the raw state of hot rolling and the galvanized state.

Claims (5)

1. Hot rolled steel sheet for deep drawing, characterised by the following ponderal composition:

   0,010% < carbon < 0,080%

   0,1% < manganese < 0,5%

   0,02% < aluminium < 0,08%

   silicon < 0,1%

   phosphorus < 0,04%

   sulphur < 0,025%

   titanium < 0,05%

   nitrogen < 0,009%

   0,001% < boron < 0,01%

   0,1 % < copper < 0,8%

   0,05% < nickel < 0,6%

   the rest being iron and impurities resulting from steel making.
2. Sheet according to claim 1, characterised in that the content of nickel is about half the content of copper.
3. Sheet according to claims 1 and 2, characterised by the following composition:

   0,020% < carbon < 0,040%

   0,15% < manganese < 0,25%

   0,02% < aluminium < 0,04%

   0,02% < silicon < 0,04%

   phosphorus < 0,02%

   sulphur < 0,005%

   titanium < 0,02%

   nitrogen < 0,009%

   0,002% < boron < 0,004%

   0,30% < copper < 0,40%

   0,15% < nickel < 0,20%

   the rest being iron and impurities resulting from steel making.
4. Method for the manufacturing of a steel sheet according to claims 1 to 3, characterised in that the steel, which has the following ponderal composition:

   0,010% < carbon < 0,080%

   0,1% < manganese < 0,5%

   0,02% < aluminium < 0,08%

   silicon < 0,1%

   phosphorus < 0,04%

   sulphur < 0,025%

   titanium < 0,05%

   nitrogen < 0,009%

   0,001% < boron < 0,01%

   0,1% < copper < 0,8%

   0,05% < nickel < 0,6%

   is subjected, after its elaboration:

   to a hot rolling, at a temperature higher than the transformation temperature AR3,

   to a cooling, beginning in a time interval less than 10 seconds after the hot rolling, the cooling being, on the one hand, between 3°C per second and 80°C per second, and, on the other hand, carried out to a temperature between 600°C and 750°C.
5. Method according to claim 4, characterised in that the hot rolling is carried out at a temperature in an interval of 10°C to 120°C above the transformation temperature AR3.

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