EP0748877B1 - Process for manufacturing a hot rolled steel sheet with very high elastic limit and steel sheet produced accordingly - Google Patents

Abstract

Steel sheet is made by hot rolling a steel containing max 0.1% C; 1-1.5 Mn; max 0.3 Si; max 0.03 P; max 0.01 S; 0.01-0.1 Al; 0.04-0.06 Nb; 0.1-0.15 Ti 0.1-0.2 Mo. After hot rolling it is cooled at 20-100 degrees C/sec until it reaches 550-750 degrees C.

Description

The invention relates to a method for producing a sheet metal strip. hot rolled steel with very high yield strength, particularly for the shaping of parts and having a brittle / ductile transition to a temperature below -50 ° C.

In the field of hot-rolled steel sheet production in steel, the characteristics of which are obtained by controlled rolling, knows products in a range of steels with high yield strength, that is to say between 315 MPa and 700 MPa.

Controlled rolling keeps sheet steel in a low equivalent carbon level and obtain mechanical properties wanted by refining of the ferritic structure grain and coherent precipitation niobium carbonitride, niobium can be combined with titanium or vanadium.

For example, a product such as a ULCB steel sheet (bainitic structure low carbon) has a higher yield strength at 700 MPa and a brittle / ductile transition temperature of approximately - 60 ° C. However, flawless 180 ° bending of the steel sheet is limited to internal bending diameters greater than 2.5 times the thickness of the sheet in the rolling direction and greater than 3 times the thickness of the sheet in the cross direction of rolling.

In another example, a product such as a micro-alloy steel sheet has a yield strength greater than 700 MPa and can be bent 180 ° without generating a defect for a bending diameter less than twice the thickness of the sheet but this steel has the disadvantage of having a temperature of ductile / brittle transition between -20 ° C and -40 ° C.

We know from US-A-3,976,514 and FR-A-2,133,952 steels with high resistance and having good resilience in particular at low temperature. To obtain these properties, the steel compositions must contain addition elements such as rare earths or simultaneously zirconium, titanium and niobium. These additions can be costly and require special production conditions.

The object of the invention is to present a method for producing a strip of hot-rolled steel sheet with very high yield strength especially for the shaping of parts and presenting a possibility faultless 180 ° bending under a bending diameter of less than twice sheet thickness and brittle / ductile transition at a lower temperature at -50 ° C.

• carbone ≤ 0,1%,
• 1 % ≤ manganèse ≤ 1,5 %
• silicium ≤ 0,3 %
• phosphore ≤ 0,03 %,
• soufre ≤ 0,01 %,
• 0,01 % ≤ aluminium ≤ 0,1 %,
• 0,04 % ≤ niobium ≤ 0,06 %,
• 0,1 % ≤ titane ≤ 0,15%,
• 0,1 %≤ molybdène ≤ 0,2 %,

le reste, à l'exception des impuretés inhérentes à la fabrication, étant constitué par du fer,
est, après laminage à chaud, soumis à un refroidissement contrôlé à une vitesse de refroidissement comprise entre 20°C par seconde et 100°C par seconde jusqu'à une température de fin de refroidissement comprise entre 550°C et 750°C, de façon à obtenir une microstructure de ferrite et bainite granulaire durcie par la précipitation des éléments de micro-alliage niobium-titane.The subject of the invention is a process for producing a strip of hot-rolled sheet steel of thickness between 2.5 mm and 10 mm, the elastic limit of which is greater than 700 MPa and which can be folded 180 ° without defect under an internal bending diameter less than twice the thickness of the sheet, usable in particular for shaping parts and having a brittle / ductile transition at a temperature below -50 ° C, characterized in that the steel of the following weight composition:

• carbon ≤ 0.1%,
• 1% ≤ manganese ≤ 1.5%
• silicon ≤ 0.3%
• phosphorus ≤ 0.03%,
• sulfur ≤ 0.01%,
• 0.01% ≤ aluminum ≤ 0.1%,
• 0.04% ≤ niobium ≤ 0.06%,
• 0.1% ≤ titanium ≤ 0.15%,
• 0.1% ≤ molybdenum ≤ 0.2%,

the rest, with the exception of the impurities inherent in manufacturing, being constituted by iron,
is, after hot rolling, subjected to controlled cooling at a cooling rate of between 20 ° C per second and 100 ° C per second to an end of cooling temperature between 550 ° C and 750 ° C, so as to obtain a granite ferrite and bainite microstructure hardened by the precipitation of the niobium-titanium micro-alloy elements.

• la température de fin de laminage à chaud est de préférence supérieure à 850°C ou même 900°C ;
• le temps séparant la fin du laminage à chaud et le début du refroidissement contrôlé est de préférence inférieur à 10 secondes et par exemple de l'ordre de 1,5 seconde.

Other characteristics of the invention are as follows:

• the temperature at the end of hot rolling is preferably greater than 850 ° C or even 900 ° C;
• the time between the end of hot rolling and the start of controlled cooling is preferably less than 10 seconds and for example of the order of 1.5 seconds.

The invention also relates to a steel sheet as defined in claim 4.

• carbone ≤ 0,1 %,
• 1 % ≤ manganèse ≤ 1,5 %,
• silicium ≤ 0,3 %,
• phosphore ≤ 0,03 %,
• soufre ≤ 0,01 %,
• 0,01 % ≤ aluminium ≤ 0,1 %,
• 0,04 % ≤ niobium ≤ 0,06 %,
• 0,1 % ≤ titane ≤ 0,15 %,
• 0,1 % ≤ molybdène ≤ 0,2 %.

Après laminage à chaud, la tôle est soumise à un refroidissement contrôlé depuis une température de préférence supérieure à 850°C ou même 900°C à une vitesse de refroidissement comprise entre 20°C par seconde et 100°C par seconde jusqu'à une température de fin de refroidissement comprise entre 550°C et 750°C, le temps séparant la fin du laminage à chaud et le début du refroidissement étant de préférence inférieur à 10 secondes.According to the invention, the steel of the sheet has the following weight composition:

• carbon ≤ 0.1%,
• 1% ≤ manganese ≤ 1.5%,
• silicon ≤ 0.3%,
• phosphorus ≤ 0.03%,
• sulfur ≤ 0.01%,
• 0.01% ≤ aluminum ≤ 0.1%,
• 0.04% ≤ niobium ≤ 0.06%,
• 0.1% ≤ titanium ≤ 0.15%,
• 0.1% ≤ molybdenum ≤ 0.2%.

After hot rolling, the sheet is subjected to controlled cooling from a temperature preferably above 850 ° C or even 900 ° C at a cooling rate of between 20 ° C per second and 100 ° C per second to a end of cooling temperature between 550 ° C and 750 ° C, the time between the end of hot rolling and the start of cooling is preferably less than 10 seconds.

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

Figure 1 is a curve showing schematic, according to the method of the invention, the variation of temperature as a function of time, imposed on the strip of hot-rolled sheet steel.

Figure 2 shows transition curves brittle / ductile for a 5 mm thick steel sheet, brittle / ductile transition measured in the direction long and crosswise of the rolled sheet strip hot.

• 0,06 % ≤ carbone ≤ 0,08 %,
• 1,4 % ≤ manganèse ≤ 1,5 %,
• 0,2 % ≤ silicium ≤ 0.26 %,
• phosphore ≤ 0,02 %,
• soufre ≤ 0,005 %,
• 0,02 % ≤ aluminium ≤ 0,06 %,
• 0,055 % ≤ niobium ≤ 0,06 %,
• 0,110 % ≤ titane ≤ 0,14 %,
• 0,130 % ≤ molybdène ≤ 0,170 %.

The method according to the invention relates to the production of a strip of hot-rolled steel sheet with a very high elastic limit usable in particular for shaping parts and having a brittle / ductile transition at low temperature. In an application example according to the invention, the base steel has the following weight composition:

• 0.06% ≤ carbon ≤ 0.08%,
• 1.4% ≤ manganese ≤ 1.5%,
• 0.2% ≤ silicon ≤ 0.26%,
• phosphorus ≤ 0.02%,
• sulfur ≤ 0.005%,
• 0.02% ≤ aluminum ≤ 0.06%,
• 0.055% ≤ niobium ≤ 0.06%,
• 0.110% ≤ titanium ≤ 0.14%,
• 0.130% ≤ molybdenum ≤ 0.170%.

The basic composition provides a ferrite microstructure and hardened granular bainite by precipitation of niobium-titanium microalloy elements.

The rolling process according to the invention aims to obtain a good recrystallization of the austenitic grains at the exit of the rolling mill stands thus allowing obtaining an equiaxed structure.

Steel structure of sheet metal strip allows high elongations greater than 15 % up to 20% and bending at 180 ° without defect with smaller inside bending diameters twice the thickness of the sheet.

In the process, the temperature at the end of rolling is greater than 850 ° C, and preferably 900 ° C in order to limit the precipitation of micro-alloying elements induced by hot rolling and secondly, to make the most of their effect hardening during controlled cooling.

Controlled cooling is imposed on the belt hot-rolled sheet after a time interval less than 10 seconds between the end of hot rolling and the start of controlled cooling. Preferably, the time interval is of the order of 1.5 seconds, this which ensures a very fine grain size and a limit high elasticity.

The controlled cooling rates are chosen between 20 ° C per second and 100 ° C per second in depending on the thickness of the sheet strip to be treated. For example, the controlled cooling rate is, on average 40 ° C per second for a strip of sheet metal about 5 mm thick. Under these conditions, there are hardening precipitation of microalloy elements, which provides an upper yield strength at 700 MPa, or 740 MPa.

End of controlled cooling temperature is between 550 ° C and 750 ° C and preferably 650 ° C, which ensures hardening precipitation of microalloy elements.

Figure 1 is a diagrammatic curve, according to the process of the invention, the temperature variation in time function imposed on the sheet steel strip after hot rolling. After rolling to a temperature greater than 850 ° C represented by the portion of curve "a", a period of less than 10 seconds, represented by the portion of curve "b", is requested before controlled cooling of the rolled sheet strip hot which is represented by the portion of curve "c".

When the sheet metal strip reaches the temperature end of cooling from 550 to 750 ° C, it is wound as shown schematically in "d" in the figure.

Figure 2 shows two transition curves A and B brittle / ductile, i.e. curves giving the breaking energy of a test piece taken from a sheet metal as a function of temperature. Curves A and B relate to a steel sheet 5 mm thick and the brittle / ductile transition is measured respectively in the long direction and in the cross direction of the sheet hot rolled. The sheet steel strip according to the invention does not have a brittle / ductile transition in the temperature range between + 20 ° C and -80 ° C. All the ruptures are therefore ductile up to -80 ° C.

The brittle / ductile transition temperature is therefore less than -80 ° C.

The method according to the invention allows the realization thick hot-rolled steel sheet between 2.5 and 10 mm, sheet of which the steel has an elastic limit greater than 700 MPa. Prison steel can be subjected to a 180 ° bend without defects, the inside diameter of the folding can be less than twice the thickness of the sheet.

0.4% equivalent low carbon content and the low manganese content give the steel a excellent weldability.

Titanium participates in precipitation hardening in the form of TiC.

Molybdenum allows after precipitation microalloy elements in the form of carbide, limit carbon diffusion at high temperatures thus ensuring obtaining fine precipitates in the ferritic matrix and lowering the temperature of fragile / ductile transition at a temperature always located below -50 ° C in the case of a steel according to the composition defining the invention.

The hot-rolled sheet strip according to the invention can be used for parts manufacturing folded profiled or stamped with reduced thickness for their lightening and / or pieces having better mechanical characteristics in fatigue. The low temperature of brittle / ductile transition offers the possibility operation of pieces of equipment, entering through example in the construction of cranes, in a field of very low temperature without risk of fragile breakage.

Claims (4)

1. Process for manufacturing a strip of hot rolled steel sheet of thickness lying between 2.5 mm and 10 mm, of which the elastic limit is greater than 700 MPa and which can be bent through 180° without failure with an inside diameter of bending less than twice the thickness of the sheet, capable of use in particular for making shaped articles and having a brittle/ductile transition at a temperature lower than -50°C, characterised in that the steel of the following composition by weight:

   carbon ≤ 0.1%

   1% ≤ manganese ≤ 1.5%

   silicon ≤ 0.3%

   phosphorus ≤ 0.03 %,

   sulfur ≤ 0.01%,

   0.01% ≤ aluminium ≤ 0.1 %,

   0.04 % ≤ niobium ≤ 0.06 %,

   0.1% ≤ titanium ≤ 0.15 %,

   0.1% ≤ molybdenum ≤ 0.2 %,

   the balance, apart from impurities inherent in manufacture, being constituted by iron,
   is, after hot rolling, subjected to controlled cooling at a cooling rate lying between 20°C per second and 100°C per second to a final cooling temperature lying between 550°C and 750°C, in such a way as to obtain a microstructure of ferrite and granular bainite hardened by the precipitation of the niobium-titanium micro-alloy elements.
2. Process according to Claim 1, characterised in that the temperature at the end of hot rolling is higher than 850°C.
3. Process according to Claims 1 and 2, characterised in that the time interval between the end of hot rolling and the start of controlled cooling is less than 10 seconds.
4. Hot rolled steel sheet having a thickness lying between 2.5 mm and 10 mm and having an elastic limit greater than 700MPa and which can be bent through 180° without failure with an inside diameter of bending less than twice the thickness of the sheet, capable of use in particular for shaping articles and having a brittle/ductile transition at a temperature below -50°C, the sheet being obtained by the process according to Claims 1 to 3, characterised by the following composition by weight:

   carbon ≤ 0.1 %

   1 % ≤ manganese ≤ 1.5 %

   silicon ≤ 0.3%

   phosphorus ≤ 0.03 %,

   sulfur ≤ 0.01 %,

   0.01 % ≤ aluminium ≤ 0.1%,

   0.04 % ≤ niobium ≤ 0.06%,

   0.1 % ≤ titanium ≤ 0.15 %,

   0.1% ≤ molybdenum ≤ 0.2%,

   the balance, apart from the impurities inherent in manufacture, being constituted by iron, and having a microstructure of ferrite and granular bainite hardened by the precipitation of the niobium titanium micro-alloy elements.

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