EP0124501A1 - Method of improving the quality of steel sections - Google Patents

Abstract

Process for improving the quality of steel profiles leaving the rolling mill, in which the surface is subjected to fast cooling, firstly with a moderate heat flow until the surface reaches a temperature slightly above the Ms point of the steel in question, then by adjusting the moderate heat flow so that the temperature of the surface of the parts of the product to be treated remains substantially constant until the allotropic transformation of the austenite is approximately complete at the surface; during a third stage of fast cooling, a high-power heat flow is applied in order that the rate of cooling at the point of section where the rate is at a minimum should be sufficiently high for the austenite to be transformed into bainite. Finally, a homogenisation of the temperatures is carried out, for example in still air.

Description

The present invention relates to a method for improving the quality of steel profiles. Under this term of "profiles", one understands in the present invention the beams, the irons U, the angles, the plates, broad plates, largets and sheets, and generally any laminate having at least one element of a certain thickness.

The method according to the invention aims to obtain by heat treatment, at the outlet of the last stand of the rolling mill train, products with high elastic limit and having a structure and properties that are almost homogeneous over their entire thickness.

The process according to the invention is applicable both to quenched steels and to semi-quenched or effervescent steels.

Various processes are already known for manufacturing steel profiles, in particular sheets of a certain thickness, having inter alia a breaking load, an elastic limit and a resilience as high as possible for the grade of steel used, as well as '' Weldability, fatigue resistance and ductility satisfactory depending on the use for which the profile is intended.

It is known in particular that to improve the weldability of a steel, it is necessary to reduce its carbon and manganese content, or its equivalent carbon, which causes a reduction in its tensile strength. To remedy this drawback, the steel is subjected to an appropriate cooling treatment, preferably applied in the rolling hot, that is to say from the exit of the rolling mill train, which makes it possible to improve certain characteristics. mechanical profiles.

When the heat treatment, practiced according to techniques known up to now, only leads to an insufficient increase in the properties of the profiles, the action of this cooling can be completed by in particular the addition of dispersoid elements which refine the grain. of steel and cause hardening of the ferrite by precipitation of the elements in question, for example niobium or vanadium. This efficient method has the disadvantage of a cost which increases as the higher elastic limit is targeted.

Among the heat treatments recently developed, the surface hardening process followed by a self-tempering of the hardened layer under the influence of heat remaining in the core of the profiles after interruption of _'surface tempering phase, is now well known; this process makes it possible to remarkably increase the mechanical properties of steels while retaining very satisfactory weldability and ductility due to the low carbon and manganese contents.

For certain applications, however, this quenching and self-tempering process has drawbacks often resulting from the thickness of the products treated; thus for increasing thicknesses, the cooling rate in the layers below the surface decreases so that the transforma Allotropic tion begins before the martensite transformation temperature is reached. This results in a very significant decrease in the percentage of martensite for these high thicknesses. In addition, the process can cause a certain inhomogeneity depending on the thickness of the products treated; it follows that for certain uses one may prefer a more homogeneous character of the properties and for this reason to renounce the spectacular increases in the elastic limit which, even for weldable products, the quenching - self-tempering process provides.

The present invention has precisely. relates to a heat treatment process in the hot rolling, to obtain such steel products, of improved quality and having an almost homogeneous structure over the thickness, practically whatever it is.

The method according to the invention is particularly applicable to the fa _- nufacturing profiles in low alloy steel with low carbon content, for example containing less than 0.2%; its implementation leads to products not having ferrite-perlite balance structures, which do not provide high elastic limits or which at equal elastic limit do not have low equivalent carbons, nor. bainite-martensite quenching structures, which require income, with the vagaries of homogeneity that this treatment presents; on the contrary, the method of the invention aims to produce, in the profiles concerned, intermediate ferrite-bainite structures.

The process for improving the quality of the steel sections, object of the present invention, applied to products leaving the rolling mill, is essentially characterized in that the external surface of the parts of the product to be treated is subjected to cooling. fast, first with a moderate calorific flow until the surface reaches a temperature slightly higher than the point Ms of the steel considered, en.ce during a second phase of rapid cooling, one adjusts the calorific flow moderated in such a way that the temperature of the surface of the parts of the product to be treated remains substantially constant until the allotropic transformation of the austenite is substantially complete on the surface, in that during a third rapid cooling phase, a high power heat flow is applied so that the cooling speed iissement at the point of the section where this speed is minimum is high enough for the austenite to transform into bainite without the formation of perlite, and in that one then performs a homogenization of temperatures for example in still air.

According to the invention, the parameters of the heat treatment described above are chosen, as well as the chemical composition of the steel constituting the profile, so that the volume of pearlitic structure is substantially zero at any point in the section of the profile.

According to an advantageous embodiment of the process of the invention, there is interposed between the output of the product from the last rolling stand and the start of the accelerated-cooling phase, a slow cooling phase, for example at calm air, said slow cooling phase having a maximum duration such that the transformation of the austenite into ferrite does not exceed 30% in product skin.

According to another preferred mode of implementation, the heat flow is adjusted during the second rapid cooling phase while tolerating a slight decrease in the temperature of the external surface of the treated parts of the product; the cooling parameters are however adjusted so that the percentage by volume of martensite is substantially zero both on the surface of the profile and in the underlying areas.

According to the invention, the heat treatment can be interrupted before the transformation at the heart of the profile is complete, provided that this practice does not cause the appearance of pearlitic structures.

The four cooling phases of the process of the invention thus successively lead to the formation of the following structures: the two rapid cooling phases, but with moderate cooling power, ensure the formation of a surface layer of ferrite and / or bainite; the next phase of rapid cooling, but with high cooling power, then transforms the underlying layers also into ferrite and / or bainite, but without the formation of martensite; Finally, the air temperature homogenization and final cooling phase allows the transformation of the residual austenite into bainite, but without perlite formation.

In this regard, it may be interesting to note that, in addition to the homogeneity in the thickness presented by the products produced by implementing the process of the invention, the absence of perlite implies improved impact properties.

The following method. The invention can be implemented in cooling installations arranged at the outlet of the rolling mill, installations divided into continuous zones, arranged in such a way that cooling is uninterrupted, even on the underside of the sheet. and despite the presence of the carrier rollers. The cooling intensity of each zone is individually adjustable by varying the specific flow rate of water and / or the distance between the surface to be cooled and the cooling device.

Advantageously, the cooling devices are boxes provided with a large quantity of water / air sprayers, or water injectors, the water supply pressure and their distance from the pressure can be varied. surface of the product to be treated. These cooling devices allow a variation of the exchange coefficient, or heat flow, over a wide range, which allows the adaptation of the process to the conditions necessary for obtaining the product with the desired characteristics.

To illustrate the process which is the subject of the invention, the case of a sheet subjected to the treatment according to the invention will be considered from a temperature, at the outlet of the last stand of the rolling mill, of 850 ° C .; the thermal evolution of the surface of this sheet consists of cooling in 8 seconds from 850 to 450 ° C, followed by maintenance at 450 ° C (maintenance phase at substantially constant temperature). for 4 seconds, then rapid cooling at high power for 10 seconds and finally homogenization with calm air.

We can calculate, by known methods., The average cooling rate between 800 ° C and 600 ° C at heart, that is to say at the point of the section where the cooling rate is minimum, as a function of l 'thickness. Using the CCT diagram of the steel in question, it is possible to determine the maximum thickness for which the process can be applied without obtaining perlite at the core. Thus, in the case of a steel having the following analysis: carbon, 0.1 X, manganese: 0.8%, silicon: 0.37 X, sulfur: 0.015%, phosphorus 0.034%, and niohium: 0.027% , the maximum possible thickness is 83.5 mm.

Examination of an 80 mm thick steel sheet, of the composition which has just been described, sheet subjected to the above heat treatment, shows that between the plane of symmetry of the sheet and a depth of 6 mm the structure consists of ferrite-bainite; there is no formation of martensite in this range, since the temperature corresponding to this transformation is not reached at any point; there is also no formation of martensite on the surface since the surface has been maintained at a temperature above the point Ms for a time long enough to ensure - on the surface - a complete transformation of the austenite into bainite .

• - Première phase de refroidissement accéléré : application d'un débit spé- cifique d'eau de 11,6 1/m².sec. (flux calorifique de 1,4 MW/m²) dans une rampe de 5,5 m de longueur, ce qui, compte tenu de la vitesse de défilement du produit dans la rampe, donne une durée de 7,5 secondes.
• - Deuxième.phase : application d'un fluide de refroidissement avec un débit spécifique d'eau de 6,9 1/m².sec. (flux calorifique de 1 MW/m²) pour maintenir la température de la surface sensiblement aux environs de 450°C, dans une rampe de 3,5 m de longueur; à ce moment, soit après un trajet de 9 m et une durée de traitement de 12,5 secondes, toute l'austénite de surface se trouve transformée en ferrite/bainite.
• - La troisième phase consiste à appliquer un refroidissement avec un flux calorifique de 3,5 MW/m² (débit d'eau spécifique de 57,8 1/m².sec.) dans une rampe de 6 m de longueur.
• - Le traitement de refroidissement rapide est alors interrompu et on laisse la température s'homogénéiser, simplement en air calme, pour terminer la transformation de l'austénite résiduelle.

In the case of a 40 mm thick sheet, the conditions for implementing the process of the invention can be established as follows:

• - First accelerated cooling phase: application of a specific water flow of 11.6 1 / m ² .sec. (heat flow of 1.4 MW / m ² ) in a 5.5 m long ramp, which, taking into account the speed of travel of the product in the ramp, gives a duration of 7.5 seconds.
• - Second.phase: application of a cooling fluid with a specific water flow of 6.9 1 / m ² .sec. (heat flow of 1 MW / m ² ) to maintain the surface temperature substantially around 450 ° C, in a ramp 3.5 m long; at this point, after a 9 m journey and a treatment time of 12.5 seconds, all of the surface austenite is transformed into ferrite / bainite.
• - The third phase consists in applying cooling with a heat flow of 3.5 MW / m ² (specific water flow of 57.8 1 / m ² .sec.) In a ramp 6 m in length.
• - The rapid cooling treatment is then interrupted and the temperature is allowed to homogenize, simply in still air, to complete the transformation of the residual austenite.

By implementing one of the advantageous variants of the process of the invention, consisting in interposing between the end of the rolling and the start of the accelerated cooling treatment, air cooling, the limit thickness for which the treatment can be increased is applicable in the product; so by. example, by lowering the skin temperature to 780 ° C. by prior air cooling, the application of the method of the invention makes it possible to increase the limit thickness by approximately 10 to 15%; this initial cooling can be very effective in homogenizing the properties in the section.

By applying another method of implementing the method of the invention, consisting in admitting a core temperature at the end of the accelerated cooling treatment, for example 650 ° C. instead of 600 ° C. (increase in the average temperature of transformation), it is also possible to increase the limit thickness which the process can treat, under cooling conditions., for example of ramp length, identical.

Claims (3)

1. Method for improving the quality of steel profiles, applied to products leaving the rolling mill, characterized in that the external surface of the parts of the product to be treated is subjected to rapid cooling, first with a moderate heat flow until the surface reaches a temperature slightly higher than the points of the steel in question, in that during a second phase of rapid cooling, the moderate heat flow is adjusted so that the temperature of the surface of the parts of the product to be treated remains substantially constant until the allotropic transformation of the austenite is substantially complete on the surface, in that during a third phase of rapid cooling, a flux is applied high power calorific so that the cooling rate at the point of the section where this speed is minimum is high enough for the austenite to transform into bainite without the formation of perlite, and in that a homogenization of the temperatures is then carried out, for example in calm air. 2. Method according to claim 1, characterized in that one applies, between the exit of the product from the last rolling stand and the start of the accelerated cooling phase, a slow cooling phase, for example in air calm, said slow cooling phase having a maximum duration such that the transformation of austenite into ferrite does not exceed 30% in product skin. 3. Method according to either of claims 1 and 2, characterized in that one regulates the heat flow during the second rapid cooling phase by tolerating a slight decrease in the temperature of the outer surface of the treated parts of the product, ensuring that the percentage by volume of martensite is substantially zero both on the surface of the profile and in the underlying areas.