EP0748877A1 - 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 process for producing a strip of hot-rolled steel sheet with a very high yield strength which can be used in particular for shaping parts and having a brittle / ductile transition at a temperature below -50 ° C.

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

Controlled rolling makes it possible to maintain a low equivalent carbon level in the steel of the sheet and to obtain desired mechanical properties by refining the grain of ferritic structure and coherent precipitation of niobium carbonitride, the niobium being able to be associated with titanium. or vanadium.

For example, a product such as a ULCB steel sheet (low carbon bainitic structure) has an elastic limit which can be greater than 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 microalloyed steel sheet has an elastic limit greater than 700 MPa and can be bent at 180 ° without generating a defect for a bending diameter less than twice the thickness of the sheet metal but this steel has the drawback of having a ductile / brittle transition temperature of between -20 ° C and -40 ° C.

The object of the invention is to present a process for producing a strip of hot-rolled steel sheet with a very high elastic limit which can be used in particular. for the shaping of parts and having a possibility of folding at 180 ° without defect under a folding diameter less than twice the thickness of the sheet and a brittle / ductile transition at a temperature below -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 %,

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.The subject of the invention is a process for producing a strip of hot-rolled steel sheet with a very high elastic limit which can be used 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 composition by weight:

• 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%,

is, after hot rolling, subjected to controlled cooling at a cooling rate of between 20 ° C per second and 100 ° C per second to a cooling end temperature between 550 ° C and 750 ° C.

• 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.

• 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 %,

et, qu'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.The invention also relates to a steel sheet characterized in that the weight composition of the steel is as follows:

• 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%,

and, that 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 up to an end of cooling temperature between 550 ° C and 750 ° C, the time between the end of the hot rolling and the start of cooling is preferably less than 10 seconds.

The following description and the appended figures, all given by way of non-limiting example, will make the invention easier to understand.

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

FIG. 2 shows fragile / ductile transition curves for a steel sheet 5 mm thick, fragile / ductile transition measured in the long direction and in the transverse direction of the strip of hot-rolled sheet.

• 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 very high yield strength which can be used in particular for the shaping of 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 makes it possible to obtain a granite ferrite and bainite microstructure hardened by the precipitation of niobium-titanium microalloy elements.

The rolling process according to the invention aims to obtain good recrystallization of the austenitic grains at the outlet of the stands of the rolling mill, thus making it possible to obtain an equiaxed structure.

The steel structure of the sheet metal strip allows high elongations of more than 15% up to 20% and bending at 180 ° without defects with internal bending diameters less than twice the thickness of the sheet .

In the process, the temperature of the end of rolling is greater than 850 ° C., and preferably 900 ° C. in order on the one hand to limit the precipitation of the microalloy elements induced by hot rolling and on the other hand , make the most of their hardening effect during controlled cooling.

Controlled cooling is imposed on the hot-rolled sheet strip after an interval of 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, which ensures a very fine grain size and a high elastic limit.

The controlled cooling rates are chosen between 20 ° C per second and 100 ° C per second 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 sheet metal strip about 5 mm thick. Under these conditions, there is hardening precipitation of the microalloy elements, which makes it possible to obtain an elastic limit greater than 700 MPa, or 740 MPa.

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

Figure 1 is a diagram showing, according to the method of the invention, the temperature variation as a function of time, imposed on the strip of steel sheet after hot rolling. After rolling at a temperature above 850 ° C represented by the curve portion "a", a period of less than 10 seconds, represented by the curve portion "b", is required before the controlled cooling of the sheet strip hot rolled which is represented by the portion of curve "c".

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

FIG. 2 shows two curves A and B of brittle / ductile transition, that is to say curves giving the fracture energy of a test piece taken from a sheet as a function of the 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 hot rolled sheet. The sheet steel strip according to the invention does not exhibit a brittle / ductile transition in the temperature range between + 20 ° C and -80 ° C. All the ruptures are therefore ductile down to -80 ° C.

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

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

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

Titanium participates in hardening by precipitation in the form of TiC.

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

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

Claims (4)

Process for producing a strip of hot-rolled steel sheet with very high elastic limit which can be used in particular for shaping parts and having a brittle / ductile transition at a temperature below -50 ° C, characterized in what the following weight composition steel: - 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%, is, after hot rolling, subjected to controlled cooling at a cooling rate of between 20 ° C per second and 100 ° C per second to a cooling end temperature between 550 ° C and 750 ° C. Method according to claim 1, characterized in that the temperature at the end of hot rolling is greater than 850 ° C. Method according to claims 1 and 2, characterized in that the time between the end of hot rolling and the start of controlled cooling is less than 10 seconds. Hot rolled steel sheet with very high yield strength, usable in particular for shaping and having a brittle / ductile transition at a temperature below -50 ° C, characterized in that the weight composition of the steel is the next one : - 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%, and, that 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 up to an end of cooling temperature between 550 ° C and 750 ° C, the time between the end of the hot rolling and the start of cooling is preferably less than 10 seconds.

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