EP0564309B1 - Process for manufacturing of steel sheets and steel sheets obtained by this process - Google Patents

Description

The present invention relates to a process for producing a sheet from a steel slab, having very high mechanical strength, a formability and excellent weldability.

The present invention also relates to a steel sheet obtained by this process.

In certain industries, in particular the automobile industry, certain metallic parts are required to have specific mechanical characteristics, namely high resistance to fatigue, great forming possibilities as well as excellent weldability.

For example, on passenger vehicles and heavy goods vehicles, the wheel sails have thinned parts which constitute zones of weakness for these wheels. Thus, manufacturers are moving towards steel having an increasingly high mechanical resistance as well as excellent stamping and welding properties.

The steel sheets used to make this type of part are generally part of the range of hot-rolled flat products whose mechanical characteristics are obtained by controlled rolling on a rolling mill with wide strips.

Several steels with high mechanical properties are known in this range, the first of which is a high yield strength steel, known as HLE.

HLE steel is a microalloyed steel which has an elastic limit between 315 and 690 MPa, but which has a poor formability due in particular to a ratio between the elastic limit Re and the high mechanical resistance Rm , i.e. greater than 0.85.

DUAL PHASE steels are also known, the structure of which is of the ferrite-martensite type and which have remarkable formability.

On the other hand, these steels have poor weldability and a mechanical resistance which does not exceed 600 MPa.

So-called HR steels are also part of steels with high mechanical characteristics and are carbon-manganese steels which undergo a sufficiently rapid cooling after rolling associated with a winding at around 400-450 ° C.

The steel sheets thus produced have a ferrito-bainitic structure of intermediate formability between the HLE steel sheets and the DUAL PHASE.

Despite, good drawing ability, due to a ratio between the elastic limit Re and the mechanical resistance Rm of between 0.75 and 0.80 and a very good weldability, it is found that these steels have a mechanical strength close to 600 MPa which does not give them sufficient fatigue behavior.

TRIP steels are also known, that is to say with induced plastic transformation, which combine a very high mechanical resistance, greater than 800 MPa with high elongations which give them excellent formability.

However, due to the high contents of carbon and alloying elements, such steel is very difficult to weld.

For example, processes for the manufacture of such steel sheets according to the prior art are given in documents EP-A-0 080 809 and GB-A-2 019 439.

The object of the present invention is to remedy the drawbacks mentioned above by proposing a method for producing a steel sheet having at the same time a very high mechanical strength, a good formability and an excellent weldability.

• carbone moins de 0,05%
• manganèse de 1,5 à 2%
• silicium de 0,1 à 0,5%
• molybdène de 0,2 à 0,5%
• phosphore moins de 0,03%
• soufre moins de 0,01%
• niobium de 0,04 à 0,06%
• titane de 0,02 à 0,05%
• aluminium de 0,01 à 0,06%
• bore de 10 à 40 ppm,

les teneurs en azote et en titane satisfaisant à la relation 3,5 N ≤ Ti, le reste étant constitué par du fer et des impuretés,

• on effectue un laminage à chaud de la brame d'acier dans un train de laminoir à larges bandes avec un taux d'écrouissage dans un train de laminoir finisseur compris entre 70 et 95% de façon à obtenir, à la sortie dudit laminage, une tôle d'acier dont la température est comprise entre 800 et 900°C,
• on refroidit la tôle à l'eau et à l'air avec une vitesse de refroidissement comprise entre 50 et 150°C/s dans le domaine austénitique,
• et on effectue un bobinage de la tôle refroidie à une température comprise entre 500 et 600°C.

The present invention thus relates to a process for producing a sheet from a slab steel, characterized in that a steel is used whose weight composition is as follows:

• carbon less than 0.05%
• manganese 1.5-2%
• 0.1 to 0.5% silicon
• 0.2 to 0.5% molybdenum
• phosphorus less than 0.03%
• sulfur less than 0.01%
• niobium from 0.04 to 0.06%
• 0.02 to 0.05% titanium
• aluminum from 0.01 to 0.06%
• boron from 10 to 40 ppm,

the nitrogen and titanium contents satisfying the 3.5 N ≤ Ti relationship, the remainder consisting of iron and impurities,

• hot rolling of the steel slab is carried out in a rolling mill train with wide strips with a hardening rate in a finishing rolling mill train of between 70 and 95% so as to obtain, at the exit of said rolling, a sheet steel whose temperature is between 800 and 900 ° C,
• the sheet is cooled with water and air with a cooling rate of between 50 and 150 ° C / s in the austenitic range,
• and winding of the cooled sheet is carried out at a temperature between 500 and 600 ° C.

• on utilise un acier dont la composition pondérale des éléments suivants est de préférence :
• carbone de 0,03 à 0,05%
• manganèse 1,7 à 2%
• silicium de 0,2 à 0,4%
• molybdène de 0,2 à 0,4%
• bore de 10 à 20 ppm,
• on soumet, en outre, la tôle à un décapage chimique dans de l'acide chlorhydrique,
• on effectue le bobinage de la tôle de préférence à une température comprise entre 560 et 590°C.

According to other characteristics of the invention:

• a steel is used whose weight composition of the following elements is preferably:
• carbon from 0.03 to 0.05%
• manganese 1.7 to 2%
• 0.2 to 0.4% silicon
• 0.2 to 0.4% molybdenum
• boron from 10 to 20 ppm,
• in addition, the sheet is subjected to a chemical pickling in hydrochloric acid,
• the sheet metal is preferably wound at a temperature between 560 and 590 ° C.

• la limite d'élasticité Re de l'acier est comprise entre 620 et 720 MPa, la résistance mécanique Rm de l'acier est comprise entre 800 et 900 Mpa, la limite d'élasticité et la résistance mécanique vérifiant la relation 0,7 ≤ Re/Rm ≤ 0,82,
• les allongements mesurés sur des éprouvettes proportionnelles de type Lo = 5,65 V So, où Lo et So sont respectivement la longueur et la section initiales desdites éprouvettes, prélevées en sens long et en sens travers sur ladite tôle, sont respectivement supérieurs à 15 et à 13%.

The present invention also relates to a steel sheet obtained by the process mentioned above and whose characteristics are the following:

• the elastic limit Re of the steel is between 620 and 720 MPa, the mechanical resistance Rm of the steel is between 800 and 900 Mpa, the elastic limit and the mechanical resistance verifying the relationship 0.7 ≤ Re / Rm ≤ 0.82,
• the elongations measured on proportional test pieces of type Lo = 5.65 V So, where Lo and So are respectively the initial length and section of said test pieces, taken in the long direction and in the cross direction from said sheet, are respectively greater than 15 and at 13%.

Other advantages and characteristics of the present invention will appear during the description which follows and which is given solely by way of example.

The steel sheets which are the subject of the present invention are obtained by hot rolling of slabs from, for example, a continuous casting installation.

• carbone 0,04%
• manganèse 1,77%
• silicium 0,24%
• molybdène 0,29%
• phosphore 0,01%
• soufre 0,004%
• niobium 0,054%
• titane 0,03%
• aluminium 0,045%
• bore 21 ppm.

The weight composition of the steel constituting the slabs is for example as follows:

• carbon 0.04%
• manganese 1.77%
• 0.24% silicon
• 0.29% molybdenum
• phosphorus 0.01%
• sulfur 0.004%
• niobium 0.054%
• 0.03% titanium
• aluminum 0.045%
• boron 21 ppm.

The nitrogen content of 80 ppm verifies the 3.5N ≤ Ti relationship, the rest being made up of iron and impurities.

The steel slabs from continuous casting are reheated in an oven at a temperature between 1200 and 1250 ° C and, more precisely at 1250 ° C, then introduced into a rolling mill with wide strips comprising a roughing train and a finishing train, each consisting of several rolling mill stands.

At the entry of the roughing train, there is for example a slab of 220mm thickness which becomes at the exit of said train a blank of thickness between 35 and 40mm and for example equal to 38mm.

The essential function of this rolling is to reduce the thickness of the slab.

The blank obtained is then engaged in the finishing train in which a work hardening is carried out with a reduction rate varying from 70 to 95% thus producing a refining of the austenite grains.

Unlike rolling in the roughing train, the rolled blank in the finishing train is permanently engaged in one of the cages of said train where the recrystallization phenomenon is delayed.

At the exit from the finishing train, the sheet that is obtained has a thickness of between 2 and 10 mm and its temperature, also called the end of rolling temperature, is between 800 and 900 ° C. and for example equal to 845 °. vs.

It should be noted that the passage of the slab in the roughing train of the rolling mill is not an essential operation and one can on the contrary produce by a die different from the conventional die of continuous casting of thin slabs which it is no longer necessary to rough.

The casting of thin slabs makes it possible in particular to suppress the passage of slabs in a roughing train.

In this case, the thin slab is directly engaged in the finishing train in which the hardening is carried out with a reduction rate varying from 70 to 95%.

After the sheet has been laminated, air and water cooling is carried out using spray bars.

During this cooling operation, it is known to see a ferritic phase appearing at the grain boundaries as well as a pearlitic phase.

Advantageously, the combined effect of boron and niobium makes it possible to inhibit the germination of the ferrite and thus the austenite begins its transformation into bainite at temperatures lower than the usual temperatures.

Niobium is completely effective from 0.04% but, beyond 0.06%, the shaping properties of steel deteriorate.

The optimum boron is between 10 and 40ppm.

Until now, it has been difficult to imagine a microalloyed steel for stamping because the formation of precipitates is detrimental to shaping.

However, the microalloy elements of steel, according to the present invention, are not used as above, but to inhibit ferritic and pearlitic transformation.

In addition, the choice of the end of rolling temperature makes it possible to generate more or less defects, called dislocations, in the crystal lattice and in which the bainite germinates.

For example, for a temperature of 800 ° C the elongation of the austenite grains is higher than at 900 ° C, therefore the dislocations will be more numerous which leads to a finer bainitic structure than at 900 ° C.

During this cooling, the austenite is cooled at a speed of between 50 and 150 ° C / s, preferably equal to 80 ° C / s.

The cooled sheet is then wound at a temperature between 500 and 600 ° C, preferably between 560 and 590 ° C, and for example equal to 580 ° C.

At a too low winding temperature, a martensitic phase forms in the sheet which tends to degrade the ductility.

At a higher temperature, that is to say between 400 and 500 ° C., a very fine bainitic structure is obtained which is particularly suitable for so-called HLE or high elastic limit steels.

The properties obtained due in particular to the high winding temperature, namely greater than 500 ° C., are completely unexpected. Indeed, it is known that the winding of a steel sheet at high temperature causes a reduction in the elastic limit Re and also a reduction in the mechanical resistance Rm.

However, the Applicant has had the merit of realizing that molybdenum allows the formation of highly fragmented complex carbides at a high winding temperature above 500 ° C and which maintain a high mechanical resistance Rm between 800 and 900 MPa. The mechanical strength of the steel according to the embodiment of the present invention is equal to 803 MPa measured on a proportional test piece Lo = 5, 65 V So where Lo and So respectively designate the initial length and section of the test piece before traction, taken in the long direction from a steel sheet.

It has also been found that a molybdenum content greater than 0.5% degrades the elongation of the steel.

A silicon content of less than 0.5% makes it possible to improve the elongation and therefore the shaping properties and makes it possible to avoid the formation of surface defects which are called fayalite during the heating of the slabs in the ovens .

This defect is unacceptable when pickled sheets are used.

Indeed, the fayalite Fe ₂ SiO ₄ forms with FeO an eutectic whose melting point is at 1177 ° C, thus this eutectic penetrates along the gamma grain boundaries and the incrustations during rolling in the roughing train are revealed during pickling.

By choosing a manganese content ranging from 1.5 to 2%, the point of beginning of bainitic transformation was lowered and it was thus possible to obtain the targeted mechanical characteristics.

The elastic limit Re of the steel according to the present invention is generally between 620 and 720 MPa and for example equal to 632 MPa measured on a proportional test piece of type Lo = 5.65 V So where Lo and So respectively designate the initial length and section of the test piece, taken in the long direction from a steel sheet.

The elongation of such a proportional test piece taken in the long direction from a steel sheet is greater than 15% and for example equal to 18.2%.

For a proportional test piece of the same type and taken crosswise from the same sheet of steel, the elongation is greater than 13% and for example equal to 15%.

These characteristics give this steel an excellent resistance to fatigue linked to its maximum resistance before high rupture, as well as a good aptitude for shaping and in particular for stamping thanks to its Re / Rm ratio of between 0.7 and 0.82.

The low Re / Rm ratio of the steel thus obtained and its elongation favorably allow its use for the manufacture of hollow profiles and in particular of tubes with high mechanical resistance.

To improve the shaping properties of steel sheets, the sulfur content must be less than 0.01% and this element must be globularized.

Furthermore, the presence of a low carbon content gives the steel excellent weldability, but it is advisable not to exceed the value of 0.05% beyond which there is an increase in the dispersion of the mechanical characteristics.

The low carbon content also prevents the formation of martensite which is rather harmful to the ductility of steel.

In addition, titanium is present in steel to protect boron from nitrogen and allow synergy between boron and niobium.

To this end, it is necessary to add titanium in an amount at least stoichiometric with respect to nitrogen, which leads to a Ti / N ratio ≥3.5.

After the winding operation, the sheet has all its mechanical characteristics of use.

As soon as the sheet is cooled, pickling is carried out, for example by immersion in one or more hydrochloric acid baths in order to remove all traces of oxide on the face of said sheet.

The previously mentioned measures have been performed on a sheet having undergone pickling.

Depending on the applications envisaged and the specifications of the customers to whom the sheets are delivered, the sheet can be cut into strips by shears before delivery.

The strips thus obtained also have a tight flatness for which the corrugation deflection is less than 1% of the length of the corrugation and in all cases less than 10mm.

The advantage of using these steel sheets for stamped, bent or profiled parts in all sectors of activity, such as the automobile and mechanical engineering, consists in the possibility of a reduction in thickness. allowing a reduction in parts or an improvement in fatigue performance or even a combination of these two advantages.

Claims (6)

1. A process for producing a sheet from a steel slab, characterised in that a steel is used which has the following composition by weight:

   - carbon less than 0.05%

   - manganese from 1.5 to 2%

   - silicon from 0.1 to 0.5%

   - molybdenum from 0.2 to 0.5%

   - phosphorus less than 0.03%

   - sulphur less than 0.01%

   - niobium from 0.04 to 0.06%

   - titanium from 0.02 to 0.05%

   - aluminium from 0.01 to 0.06%

   - boron from 10 to 40 ppm

   the nitrogen and titanium contents satisfying the expression 3.5 N ≤ Ti, the remainder being iron and impurities,

   the steel slab is hot-rolled in a wide-strip rolling mill train, with a work-hardening coefficient of from 70 to 95% in a finishing mill train such that when the rolling is completed a steel sheet is obtained with a temperature of from 800 to 900°C,

   the sheet is air- and water-cooled at a cooling rate of from 50 to 150°C/second in the austenitic range,

   and the cooled sheet is coiled at a temperature of from 500 to 600°C.
2. A process according to claim 1, characterised in that a steel is used which preferably has the following composition by weight:

   - carbon from 0.03 to 0.05%

   - manganese from 1.7 to 2%

   - silicon from 0.2 to 0.4%

   - molybdenum from 0.2% to 0.4%

   - boron from 10 to 20 ppm.
3. A process according to claim 1 or 2, characterised in that the sheet is also chemically pickled in hydrochloric acid.
4. A process according to claims 1 and 2, characterised in that the sheet is preferably coiled at a temperature of from 560 to 590°C.
5. A steel sheet produced by a process according to any of the preceding claims, characterised in that the elastic limit Re of the steel is from 620 to 720 MPa, the mechanical strength Rm of the steel is from 800 to 900 MPa, the ratio Re to Rm satisfying the expression 0.7 ≤ Re/Rm ≤ 0.82.
6. A steel sheet according to claim 5, characterised in that the elongations measured on proportional test pieces of type Lo = 5.65 V So, where Lo and So are respectively the initial length and the initial section of the test pieces taken longitudinally and transversely from the steel sheet, are respectively greater than 15 and 13%.