EP1138796A1 - High strength hot rolled steel with high yield strength for use in the car industry - Google Patents

Abstract

A hot rolled steel with a very high elastic limit and mechanical strength for the production of motor vehicle components has the following composition by weight;- 0.08 - 0.2% carbon, 1 - 2% manganese, 0.02 - 0.1% aluminium, less than 0.5% silicon, less than 0.03% phosphorus, less than 0.01% sulphur, less than 0.3% vanadium, less than 1% chromium, less than 0.015% nitrogen, less than 0.6% molybdenum, the remainder being iron and inherent production impurities. An Independent claim is included for a method for the production of a hot rolled steel strip with this composition.

Description

The invention relates to hot rolled steel with very high yield strength and mechanical resistance usable in particular for the production of vehicle parts automobiles.

In the field of production of hot-rolled steel sheets, steel of which the characteristics are obtained by a controlled rolling, it is known products to high yield strength, i.e. between 315 MPa and 700 MPa.

In the field of hot-rolled sheets from a strip train, the resistance in service parts obtained by shaping from these sheets is a criterion important since it defines the lifetime of the shaped parts, as for for example by stamping, profiling or hydroforming.

Service behavior linked to fatigue behavior defines the service life for specific charges.

In order to improve the fatigue resistance of shaped parts, a solution consists of the use of very high strength steels which have properties in significant fatigue because there is a first approximation, a proportional relationship between the endurance limit and the mechanical resistance. However, the steel must be suitable for stamping. However, in general, the formatting properties decrease with the increase in mechanical strength thus limiting the possibilities of setting form of workable parts with high strength steels.

Shock resistance is also an important property for reasons of safety, particularly in automotive applications, since the impact defines the resistance to sudden rupture of the parts. In order to improve this ownership of shaped parts, one solution consists in using steels with very high elastic limit because there is a first approximation, a linear relation between impact resistance and yield strength. In general, however, shaping properties decrease with increasing yield strength.

In the range of common hot rolled flat products, including mechanical characteristics are obtained by controlled rolling on a wide train strip, there are in particular four main families of steels with characteristics high mechanical.

HLE steels, steels with high yield strength, are micro-alloyed steels which have a yield strength of between 315 MPa and 700 MPa, but which have limited formability due in particular to a Re / Rm ratio high greater than 0.85. These steels have a ferrite-carburetted phase structure of cementite type. The yield strength level is obtained by controlled rolling and precipitation of micro alloy elements such as niobium, vanadium, titanium during of ferritic transformation.

Dual-Phase steels are ferrite-martensite structural steels with a remarkable shaping property. The mechanical strength levels are generally between 550 MPa and 800 MPa. The highest level is obtained by precipitation of micro-alloy elements during the ferritic transformation which comes complete the hardening effect of martensite.

HR steels are so-called high strength steels, carbon and manganese, undergoing after rolling, a relatively rapid cooling associated to a low temperature winding to give them a ferrito-bainitic structure. They have an intermediate formability characteristic between HLE steels and steels Dual-Phase. The resistance level varies between 450 MPa and 800 MPa.

Martensitic steels with the highest strength levels. These steels have a martensitic structure obtained by heat treatment after rolling. Achieving this type of structure on a broadband train is difficult due to the fragility of the martensite which leads to band ruptures after rolling. Martensitic steels achieve resistance levels greater than 1000 MPa but with a very low level of ductility and elongations less than 8%. It is also necessary to carry out a heat treatment after rolling.

The increase in the resistance level of all the above-mentioned steels is accompanied by an increase in rolling forces which limits the reduction thick not allowing to take full advantage of the possibilities relief.

The object of the invention is to present a hot rolled steel with a very high limit. of elasticity and mechanical resistance with good shaping properties for the production of parts by stamping, profiling, hydroforming in particular for the automotive industry.

0,08% < carbone < 0,2%

1% < manganèse < 2%

0,02% < aluminium < 0,1%

silicium < 0,5%

phosphore < 0,03%

soufre < 0,01%

vanadium < 0,3%

chrome < 1%

azote < 0,015%

Molybdène < 0,6%

   le reste étant du fer et des impuretés inhérentes à l'élaboration.
De préférence, l'acier est caractérisé en la composition pondérale suivante :

0,1%<carbone<0,14%

1,4% < manganèse < 1,8%

0,02% < aluminium < 0,08%

0,15% < silicium < 0,3%

phosphore < 0,03%

soufre < 0,008%

0,1% < vanadium < 0,3%

0,3% < chrome < 0,6%

azote < 0,012%

0,15 < molybdène < 0,4

   le reste étant du fer et des impuretés inhérentes à l'élaboration. L'invention concerne également un procédé de réalisation d'une bande de tôle d'acier laminée à chaud à très haute résistance utilisable notamment pour la réalisation de pièce de véhicules automobiles caractérisée en ce que l'acier de composition pondérale suivante :

0,08% < carbone < 0,2%

1% < manganèse < 2%

0,02% < aluminium < 0,1%

silicium < 0,5%

phosphore < 0,03%

soufre < 0,01%

vanadium < 0,3%

chrome < 1%

azote < 0,015%

molybdène < 0,6%

   le reste étant du fer et des impuretés inhérentes à l'élaboration est soumis à :

• un laminage à une température inférieure à 950°C et de préférence inférieure à 880°C,
• un refroidissement effectué à une vitesse supérieure à 20°C par seconde, et de préférence à une vitesse comprise entre 100°C et 200°C par seconde jusqu'à une température comprise entre 400°C et 600°C, et de préférence jusqu'à une température comprise entre 450°C et 500°C.

The object of the invention is a hot-rolled steel with a very high yield strength and mechanical strength which can be used in particular for the production of a motor vehicle part, characterized in the following weight composition:

0.08% <carbon <0.2%

1% <manganese <2%

0.02% <aluminum <0.1%

silicon <0.5%

phosphorus <0.03%

sulfur <0.01%

vanadium <0.3%

chromium <1%

nitrogen <0.015%

Molybdenum <0.6%

the rest being iron and impurities inherent in the preparation.
Preferably, the steel is characterized in the following weight composition:

0.1% <carbon <0.14%

1.4% <manganese <1.8%

0.02% <aluminum <0.08%

0.15% <silicon <0.3%

phosphorus <0.03%

sulfur <0.008%

0.1% <vanadium <0.3%

0.3% <chromium <0.6%

nitrogen <0.012%

0.15 <molybdenum <0.4

the rest being iron and impurities inherent in the preparation. The invention also relates to a process for producing a strip of hot-rolled steel sheet of very high resistance which can be used in particular for making parts for motor vehicles, characterized in that the steel of the following composition by weight:

0.08% <carbon <0.2%

1% <manganese <2%

0.02% <aluminum <0.1%

silicon <0.5%

phosphorus <0.03%

sulfur <0.01%

vanadium <0.3%

chromium <1%

nitrogen <0.015%

molybdenum <0.6%

the remainder being iron and impurities inherent in the preparation is subjected to:

• rolling at a temperature below 950 ° C and preferably below 880 ° C,
• cooling carried out at a speed greater than 20 ° C per second, and preferably at a speed between 100 ° C and 200 ° C per second to a temperature between 400 ° C and 600 ° C, and preferably up to '' at a temperature between 450 ° C and 500 ° C.

0,08% < carbone < 0,2%

1 % < manganèse < 2%

0,02% < aluminium < 0,1%

silicium < 0,5%

phosphore < 0,03%

soufre < 0,01%

vanadium < 0,3%

chrome < 1%

azote < 0,015%

molybdène < 0,6%

   le reste étant du fer et des impuretés inhérentes à l'élaboration, possède une structure entièrement bainitique. Sous cette forme, il est possible d'atteindre un niveau de résistance supérieur à 1000 MPa avec un allongement supérieur à 10%.
L'acier mis sous forme d'une bande laminée à chaud selon l'invention est soumis à:

• un laminage à une température inférieure à 950°C et de préférence inférieure à 880°C,
• un refroidissement effectué à une vitesse supérieure à 20°C par seconde, et de préférence à une vitesse comprise entre 100°C et 200°C par seconde jusqu'à une température comprise entre 400°C et 600°C, et de préférence jusqu'à une température comprise entre 450°C et 500°C.

The following description and the appended figures, all given by way of nonlimiting example, will make the invention clear.
Figure 1 is a diagram showing the temperature variation as a function of time during the cooling of the hot rolled steel strip.
FIG. 2 shows an elongation curve as a function of the stress for a steel according to the invention.
The steel according to the invention, the weight composition of which is as follows:

0.08% <carbon <0.2%

1% <manganese <2%

0.02% <aluminum <0.1%

silicon <0.5%

phosphorus <0.03%

sulfur <0.01%

vanadium <0.3%

chromium <1%

nitrogen <0.015%

molybdenum <0.6%

the rest being iron and impurities inherent in the production, has an entirely bainitic structure. In this form, it is possible to reach a resistance level greater than 1000 MPa with an elongation greater than 10%.
The steel formed in the form of a hot rolled strip according to the invention is subjected to:

• rolling at a temperature below 950 ° C and preferably below 880 ° C,
• cooling carried out at a speed greater than 20 ° C per second, and preferably at a speed between 100 ° C and 200 ° C per second to a temperature between 400 ° C and 600 ° C, and preferably up to '' at a temperature between 450 ° C and 500 ° C.

As shown in the diagram in Figure 1, the cooling cycle at from a temperature between 400 ° C and 600 ° C, and preferably up to a temperature between 450 ° C and 500 ° C is made on a reel.

• le carbone limité à 0,2% permet d'assurer une bonne soudabilité tout en permettant un durcissement par précipitation avec le vanadium.
• le manganèse permet d'abaisser les points de transformation AR3, Bs et Ms qui correspondent respectivement, à la température de début de la transformation ferritique, à la température de début de la transformation bainitique et à la température de début de la transformation martensitique. Il permet de par cet effet d'augmenter la trempabilité en évitant de former de la ferrite du fait des vitesses de refroidissement élevées et d'obtenir une structure entièrement bainitique. Le début de transformation bainitique (Bs) abaissé permet d'augmenter les propriétés mécaniques.
• l'aluminium est utilisé pour calmer l'acier.
• le silicium est conservé à des teneurs relativement faibles pour bénéficier du pouvoir durcissant en solution solide qu'il apporte sans dégrader l'état de surface après décapage, ni la revêtabilité du produit sur ligne de galvanisation ou d'électrozingage en continue. Le silicium est connu pour dégrader d'une part, l'aspect de surface des produits décapés par la formation de Fe₂O₃SiO₄ et d'autre part, la mouillabilité et donc, l'adhérence des revêtements.
• le molybdène, par son effet de trempabilité notamment une diminution de Bs, permet d'augmenter les propriétés mécaniques et cela par la formation d'une structure entièrement bainitique.
• le vanadium est l'élément nécessaire à la formation de précipité de type nitrures et carbures qui se forment à différentes températures au cours du traitement thermomécanique. Ces précipités très durcissants permettent d'atteindre le niveau de propriétés mécaniques très élevé. Cet élément permet ce durcissement par précipitation sans augmentation de la dureté à chaud. Cet effet est contradictoire avec les effets connus des éléments de micro alliage qui, par précipitation induite au cours du laminage, conduisent à une augmentation de ladite dureté à chaud. Cette constatation a permis aux inventeurs d'obtenir, avec l'élément vanadium contenu dans l'acier selon l'invention, de laminer de fines épaisseurs de tôle jusqu'à 1,4 mm d'épaisseur sans augmentation des efforts de laminage.

From the point of view of the composition of the steel according to the invention:

• carbon limited to 0.2% ensures good weldability while allowing precipitation hardening with vanadium.
• manganese makes it possible to lower the transformation points AR3, Bs and Ms which correspond respectively to the temperature at the start of the ferritic transformation, to the temperature at the start of the bainitic transformation and to the temperature at the start of the martensitic transformation. By this effect, it makes it possible to increase the quenchability by avoiding the formation of ferrite due to the high cooling rates and to obtain an entirely bainitic structure. The lowered bainitic transformation start (Bs) increases the mechanical properties.
• aluminum is used to calm steel.
• the silicon is stored at relatively low contents in order to benefit from the hardening power in solid solution which it provides without degrading the surface condition after pickling, or the coating of the product on a continuous galvanizing or electrogalvanizing line. Silicon is known to degrade on the one hand, the surface appearance of the pickled products by the formation of Fe ₂ O ₃ SiO ₄ and on the other hand, the wettability and therefore, the adhesion of the coatings.
• molybdenum, by its quenchability effect in particular a reduction in Bs, makes it possible to increase the mechanical properties and this by the formation of an entirely bainitic structure.
• vanadium is the element necessary for the formation of precipitates of the nitride and carbide type which are formed at different temperatures during the thermomechanical treatment. These very hardening precipitates make it possible to reach the very high level of mechanical properties. This element allows this precipitation hardening without increasing the hardness when hot. This effect is contradictory with the known effects of micro-alloy elements which, by precipitation induced during rolling, lead to an increase in said hardness when hot. This observation enabled the inventors to obtain, with the vanadium element contained in the steel according to the invention, to laminate thin sheet thicknesses up to 1.4 mm in thickness without increasing the rolling forces.

The examples below present the results obtained for an example B of application according to the invention framed by two comparison examples A and C, the analyzes of the two comparison examples comprising, one, a low vanadium content, l 'other high vanadium content.
The compositions of the examples are presented in the following table 1: Steel VS Mn Cr Mo Yes NOT V P AT 0.11 1.58 0.51 0.33 0.2 0.0035 0 0.02 B 0.11 1.58 0.51 0.32 0.2 0.0040 0.2 0.02 VS 0.11 1.58 0.51 0.34 0.2 0.0050 0.45 0.02 The following table 2 gives the conditions of the heat treatment after hot rolling. Steel Rolling time ° C Cooling rate. ° C / s Winding time. ° C AT 900 65 450 B 885 40 450 VS 890 50 450 Table 3 below presents the mechanical characteristics in mechanical resistance Rm, elastic limit Re, and elongation A, of the three exemplary embodiments. Steel Rm (MPa) Re (MPa) AT (%) AT 790 670 14 B 1090 990 10.4 VS 1125 1015 8.9 It can be noted that vanadium increases the mechanical resistance and reduces the elongation. Vanadium is the necessary element, in steel with bainitic structure, for the realization of a hardening effect, which is unexpected because the elements of micro alloy have a hardening effect by precipitation but this precipitation is completely finished at higher temperature and must be done in the ferritic field to be hardening. This effect cannot be obtained by other micro alloy elements such as titanium or niobium because these elements lead to an increase in hot hardness which limits the reduction rates in hot rolling and therefore the minimum thickness achievable for this type of sheet. It turns out that vanadium has no effect on hot hardness.

Other residual elements may be present and used depending on their properties known as Cu, Ni. The addition of alloying elements such as titanium or boron can be used to promote the precipitation of vanadium carbides at depends on vanadium nitrides. Titanium and boron form high nitrides temperatures which remain stable during the subsequent thermomechanical treatment.

Industrial tests were carried out on the basis of analysis B presented in table n ° 4. VS % Mn% Cr% NOT % V% MB% Al% Yes % 0.124 1,560 0.389 0.0051 0.189 0.280 0.031 0.185 An example of mechanical property obtained for a thickness of 1.7 mm is presented in Figure 2 through a tensile curve.
The characteristics of the steel are a mechanical resistance of 1015 MPa, an elastic limit of 880 MPa, an elongation of 12%.

The final structure of the steel according to the invention is a bainitic structure. This structure allows to obtain an elastic limit higher than 700 MPa, a resistance mechanical greater than 1000 MPa and an elongation greater than 10%. These values show the good shaping properties of the steel according to the invention.

The invention allows the rolling of a steel having a thickness between 1.4 and 5 mm which has both a high mechanical resistance, that is to say greater than 1000 MPa and remarkable shaping properties, thanks to a elongation greater than 10%.

The surface condition, without defects, after the pickling of the hot-rolled sheet is ensured, in the composition of the steel, by a silicon content of less than 0.5%.

The strip of hot-rolled steel sheet of the invention has an advantage in its use in business sectors such as the automotive and mechanical engineering in general, for stamped, bent, profiled or hydroformed, parts that can be lightened while ensuring performance in fatigue, improved shock performance and a combination of these benefits.

Claims (3)

Hot rolled steel with very high yield strength and mechanical strength which can be used in particular for the production of a motor vehicle part, characterized in the following weight composition: 0.08% <carbon <0.2% 1% <manganese <2% 0.02% <aluminum <0.1% silicon <0.5% phosphorus <0.03% sulfur <0.01% vanadium <0.3% chromium <1% nitrogen <0.015%. molybdenum <0.6% the rest being iron and impurities inherent in the preparation. Steel according to claim 1 characterized in the following weight composition: 0.1% <carbon <0.14% 1.4% <manganese <1.8% 0.02% <aluminum <0.08% 0.15% <silicon <0.3% phosphorus <0.03% sulfur <0.008% 0.1% <vanadium <0.3% 0.3% <chromium <0.6% nitrogen <0.012% 0.15 <molybdenum <0.4 the rest being iron and impurities inherent in the preparation. Process for producing a strip of hot-rolled steel sheet of very high resistance which can be used in particular for the production of motor vehicle parts, characterized in that the steel of the following composition by weight: 0.08% <carbon <0.16% 1% <manganese <2% 0.02% <aluminum <0.1% silicon <0.5% phosphorus <0.03% sulfur <0.01% vanadium <0.3 chromium <1% nitrogen <0.015%. molybdenum <0.6% the remainder being iron and impurities inherent in the preparation, is subjected to: rolling at a temperature below 950 ° C and preferably below 880 ° C, cooling carried out at a speed greater than 20 ° C per second, and preferably at a speed between 100 ° C and 200 ° C per second to a temperature between 400 ° C and 600 ° C, and preferably up to '' at a temperature between 450 ° C and 500 ° C.

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