EP0046423B1 - Method of executing the skin pass at a regulated rate in the rolling of steel strips annealed in a furnace - Google Patents

Description

The invention relates to a method of work hardening by rolling strips of mild steel intended in particular for the manufacture of packaging, this work hardening taking its place in the manufacture of strips, where sheets in coils from steel grade intended for continuous casting are brought to substantially strip thickness by a first cold rolling, annealed so as to remove the structural anisotropies generated by the first rolling, then hardened by a second rolling so as to bring the strip metal to hardness chosen. More specifically, the method is intended to give strips annealed in a bellows a hardness comparable to that of strips annealed continuously and work-hardened by dry rolling with an elongation of less than 2%.

- The invention takes its place in the technological development of the development of steel strips intended mainly for the manufacture of packaging, in the form of tinplate and black iron, that is to say laminated strips when cold, with nominal thicknesses ranging from 0.15 to 0.49 mm, in accordance with ISO 1111/11 (1976). By tinplate is meant bands or sheets covered with tin, and by black iron corresponding bands not tinned, either at the stage preceding tinning, or intended for the manufacture of articles protected by various surface treatments. The invention relates to the production of strips in the state of black iron, at a stage preceding the tinning of white iron or any other surface treatment.

The description of the state of the art which follows refers broadly to the "Steel Products Manual, Tin Mill Products", published by the American Iron and Steel Institute (1000 16th Street, NW, Washington, DC 20036 ), May 1979 update, especially pages 14 and 15.

The strips are produced from coils of sheets of the order of 2 mm, which undergo a first cold rolling, during which the sheets are reduced in thickness with a corresponding elongation, to give strips whose thickness is substantially the final thickness of the strip of black iron. The strips are then annealed under a protective atmosphere, to remove the structural anisotropies caused by the first rolling, where the metal has undergone considerable elongation giving it a fibrous crystalline structure. However, after annealing, the metal is too soft for processing by deformation. It is therefore subjected to a second rolling with low elongation, to work harden and give it the hardness required for subsequent shaping. This second rolling, hardening, also perfects the finish of the strips.

The sheets themselves were produced by hot rolling of slabs.

For a long time, slabs were obtained from steel cast in molds. The annealing of the strips after the first cold rolling was carried out in a closed coil. The grades of steel used were such that the second rolling, hardening, gave the strips a suitable hardness with an elongation of approximately 1 to 2%.

In parallel with the development of continuous casting of steel, which allows greater regularity in the composition of the metal and hence greater regularity in the mechanical and structural properties of the products produced, sheets and strips, and also allows higher production rates. , continuous annealing techniques have been developed, where the strip, after having undergone a first rolling, passes through an annealing furnace, before re-reeling for the second hardening rolling. This continuous annealing is carried out much faster than the conventional annealing in a coil under chloche, and notably includes brief heating and cooling phases. As a result, the crystalline structure of the steel after continuous annealing is much finer than after annealing in a bell, with as a consequence a higher hardness, which obviously affects the hardness of the strip after hardening rolling. Consequently, the grades of continuous casting steels of the coil sheets intended for the production of black iron strips have been modified by reduction of the content of hardening elements, in order to obtain suitable hardnesses of the black irons in the strip produced according to the continuous casting processes of steels, and the continuous annealing of strips after the first rolling. It will be noted that the use of milder grades of steel has an obvious advantage for cold rolling, by reducing the energy required to obtain the desired thickness reduction.

However, the installation of a cold rolling installation for the production of strips with the incorporation of a continuous annealing furnace requires very heavy investments, which are profitable only if the tonnage produced is sufficiently large. It has been estimated that the tonnage corresponding to the break-even point is, in any event, in excess of 100,000 tonnes per year. For tonnages below the break-even tonnage, annealing in a closed coil is necessary, and we are faced with the following dilemma: either give up the advantages of regularity of the metal, and ease of cold rolling procured by sheets coming from steels of continuous casting, and using sheets coming from steels cast in ingot molds, of higher hardness or else producing black iron strips of hardness lower than the hardness of continuously annealed strips, usually required for the subsequent shaping. In both cases, the commercial competitiveness of the bands which have been annealed under cover is compromised.

The main objective of the invention is a method of work hardening of strips coming from steel of grade intended for continuous casting, and annealed in a bell, which gives them a hardness comparable to that of corresponding strips annealed continuously.

The object of the invention is also a method of strip hardening which is applicable to conventional rolling mill trains without major transformations.

For these purposes, the invention provides a hardening rolling process for the manufacture of mild steel strips intended in particular for the manufacture of packaging, where the coil sheets are brought to substantially strip thickness by a first cold rolling, annealed so as to remove the structural anisotropies generated by the first rolling, then work hardened by a second rolling with low elongation so as to bring the strip metal to a chosen hardness, characterized in that it is intended to impart to strips annealed under a bellows a hardness comparable to that of strips continuously annealed and cold-worked by dry rolling with an elongation of less than 2%, a process in which the steel strips are led (which were of normal quality to be annealed continuously, but in fact which were annealed under cover) on two successive cages so that the resulting elongation is adjusted to a value chosen between 4 and 20% and obtained mainly in the first cage, the strip is lubricated at the inlet of the first cage by spraying an oil-in-water emulsion, the strip is washed with water at a temperature above room temperature between the cages, and the strip is dried by blowing air at the outlet of the second cage.

It is known to lubricate the sheets at the entry of cold rolling cages, to facilitate the reduction of thickness of the sheet in the cage. Generally used emulsions of oils, called organic in terms of profession, which are triglycerides, of vegetable or animal origin. In the preparation of so-called double reduction strips, which undergo a second thickness reduction after annealing after the first rolling, accompanied by an elongation of more than 30%, lubrication of the strips is also carried out. entry of second reduction cages, by using either emulsions of organic oils, or often emulsions of mineral or synthetic oils, treated to easily give stable emulsions. It is specified that the double reduction strips have high work hardening rates and are reserved for special uses, different from the uses of black iron strips (single reduction).

It is known to treat strips in hardening rolling, with lubrication similar to that practiced in double reduction. The aim is essentially an improvement in the finishing of the strips, by reducing the adhesion of residual deposits after annealing in order to avoid superficial encrustation of the residues on the strip, and by reducing the fouling of the rolling mill rolls. Correlatively it has been observed that lubrication increases the elongation of the strip during work hardening, in particular at high running speeds, while the hardness of the work hardened strip decreases somewhat at equal elongation. In order not to deviate from the mechanical properties conferred on dry-worked strips in the usual process with elongations of less than 2%, it is accepted that the elongations in wet work hardening must not exceed 3%. The opinion of those skilled in the art is that this wet hardening is only of interest for cleaning dirty cylinders.

However, the Applicant has found that it is possible to adjust the rate of work hardening of the strips after annealing under a bell, by correlatively adjusting the rate of elongation when passing through work hardening cages, the final hardness of the strip increasing with the elongation rate, and this final hardness can be reproducibly adjusted with good precision provided that the elongation rate is closely controlled within a range of 4 to 20%. A careful study of the parameters governing the constancy of the elongation rate revealed that the lubrication at the entrance of the first cage, where the elongation must be carried out for the most part, joined to the washing between the two cages so that the band is very little lubricated at the entrance to the second cage, was essential for the control of the elongation. Furthermore, the finished strip must be rolled up dry to avoid oxidation, and the drying results from the use of heated water for washing, with a view to its rapid subsequent evaporation, and from the blowing at the outlet of the second cage for minimize the amount of water to evaporate.

Preferably, the roughness of the first cage cylinders is between 3.5 and 1 micrometers Ra, while that of the second cage cylinders is between 0.75 and 0.4 micrometers Ra. These roughnesses, which correspond to a relatively rough surface state for the first cage, and a finely rectified state for the second cage, have proved useful so that the elongation is mainly obtained in the first cage, with effective control. of the total elongation.

Preferably, moreover, the elongation at the passage of the first cage represents 85 to 95% of the total elongation at work hardening.

It should be noted that elongations of 4 to 20% correspond to reductions in thickness respectively from 3.8 to 16.7%, that is to say variations in thickness, for a strip of 0.2 mm thickness respectively from 7.6 to 33.4 micrometers. The control to 1% near the elongation, if it were to be carried out on the thickness of the strip, would impose a thickness control to 2 micrometers. It is clear that the only thickness control, as it is practiced for the reduction of thickness, would be inoperative to adjust the hardness of the strip at work hardening in the required range. Furthermore, the classic work hardening with elongations of less than 2% is located in an area where the final hardness depends little on the elongation, which therefore does not require tight control.

Preferably, the oil in water emulsion used for lubrication is at a content of between 1 and 10% of oil.

Preferably also the washing water between the cages is approximately 50 ° C., which corresponds to a good compromise between evaporation rate requirements, and the absence of prematurely dried areas.

Furthermore, the washing water will preferably contain approximately 0.2% by weight of sodium nitrite, to passivate the surface of the strip and avoid subsequent oxidation.

• la figure 1 est un schéma de laminoir à deux cages, aménagé pour la mise en oeuvre du procédé suivant l'invention;
• la figure 2 est un diagramme de dureté finale en fonction de l'allongement.

The advantages of the invention will moreover emerge from the description which follows, by way of example, with reference to the appended drawings in which:

• Figure 1 is a diagram of rolling mill with two stands, arranged for the implementation of the method according to the invention;
• FIG. 2 is a diagram of final hardness as a function of the elongation.

As can be seen in FIG. 1, the rolling mill has two successive stands 11 and 12, the first and second stands in the direction of travel of the strip 10 indicated by the arrow 10a. These cages respectively comprise upper cylinders 11a and 12a and lower cylinders 11b and 12b. The strip 10 to be hardened is cut from a coil 1, which has been previously annealed in a protective atmosphere under a conventional bell. The strip 10 passes over a pair of tension rollers 2, braked to put the strip under tension. Leaving the tension rollers 2, the strip engages between the cylinders 11a a and 11b b of the first cage 11, then between the cylinders 12a and 12b of the cage 12, before passing between a pair of tension rollers 4, then to be wound on a take-up reel 5. As is conventional, the torques exerted on the axes of the tension rollers 2 and 4 are adjusted by servo-control to give the strip a regulated tension, and the speeds and driving torques of the rolls cages 11 and 12 are adjustable in cooperation with the tightening of the cages to adjust the elongations.

To implement the method according to the invention, the cylinders 11a a and 11b are preferably rectified with a roughness of 3 micrometers Ra, while the cylinders 12a and 12b are rectified polished with a roughness of 0.6 micrometers Ra. It is recalled that the roughness Ra is the quadratic mean of the deviations from the real surface to the theoretical surface. Furthermore, the rolling mill was equipped with spraying booms 6 at the entrance to the cage 11, and 7 in the vicinity of the outlet from this cage 11, and with blowing booms 8 at the exit from the cage 12. The booms 6 are supplied at a pressure adjustable from 2 to 5 bars, with water added with approximately 5% of oil, in this case a synthetic oil sold under the name FCF 106 by the company CRODA. The flow rate of the ramps is regulated by the supply pressure, to provide, in order of magnitude, 250 g of oil per 1000 m ² of exposed strip surface (that is to say, counting the two sides).

The ramps 7 are supplied with water at a temperature of approximately 50 ° C., at a rate of approximately 0.2 to 0.4 m ³ of eua per 1000 m ² of exposed strip surface. 5 kg of sodium nitrite are added to the water per cubic meter. Of course, the water which flows from the strip after washing is taken up and treated to be recycled.

The blowing by the ramps 8 is carried out with compressed air, and adjusted so that beyond the strip is wet, but not wet. The film deeua which remains on the surface beyond the blowing ramps 8 gradually evaporates to disappear before being wound up on the take-up reel 5.

The traction on the strip 10 between the tensioning rollers 2 and 4, the peripheral speeds of the cage cylinders 11 and 12, and the tightening of the cages are adjusted, according to the rules of the art, so that the elongation has a chosen value , and that approximately 90% of this elongation occurs in the first cage 11. The cage 12 essentially has the role of leveling and improving the finish of the strip 10, of regulating the traction on the strip (between 20 and 40 kN normally) on the side of the cage 11, and incidentally wring the strip.

It goes without saying that the parameter values indicated above are average values, determined by the Applicant during numerous tests. However, the most important parameter, to which the other parameters are subordinate, is the elongation imparted to the strip, depending on the state of the strip after annealing in a hood and the desired final hardness.

One presents hereafter results of hardening tests intended to specify the variation between elongation and final hardness, for two steels cast continuously; A and B, the compositions of which are given in the following table (content of constituents in ppm).

Figure 2 shows the hardness, expressed in Rockwell HR 30T of the work hardened strips, as a function of the elongation imparted. We see that the relationship between hardness and elongation is substantially linear between 4 and 20% elongation. The shade band A goes from 61 Rockwell HR 30T for 4% elongation to 69 HR 30T for 20% elongation, and the shade band B goes from 64 to 71 HR 30T for these limit elongations.

Consequently, for a black iron with a specified hardness of 65 ± 4 HR 30T, the shade band A will be hardened with an elongation of approximately 13%, and the shade band B with an elongation of approximately 7%.

It is moreover clear that the establishment of work hardening calibration diagrams, according to the grades of steel used and the annealing conditions under a bell is part of routine tests.

However, it should be noted that the tests confirmed that the roughness of the first cage cylinders should be higher the lower the chosen elongation, a roughness Ra of 3.5 micrometers suitable for elongations of 4%, while for elongations of 20%, a roughness of 1 micrometer Ra was suitable.

It will be noted that, parallel to the transience of the first cage cylinders, the lubricating power of the oil emulsion has an effect on the elongation. It is indicated to use an oil with a rather reduced intrinsic lubricating power, but with a relatively high content in the emulsion, which makes it easier to control the oil content of the emulsion and therefore the regularity of the lubricating power of the emulsion. 'emulsion.

The tests and inspections on pilot productions of work-hardened strips at a regulated rate revealed that the work-hardening rolling of the invention not only made it possible to adjust the surface hardness of continuous casting steels (quenched steels) annealed in a bell, but also gave them advantageous properties in the transition zone between the zone of elastic deformation and that of plastic deformation, the steels annealed under bell and hardened according to the process of the invention having a behavior closer to that of steels annealed continuously and hardened by the classic process than that of steel annealed in a bell and cold worked at low elongation.

Indeed, if one draws the stress / elongation diagrams relating to steels cast in continuous chosen in nuances such that the final surface hardnesses are practically identical after processes of continuous annealing or under chloche, and classic hardening or according to the invention, the latter applied of course to a steel annealed under a chloche, it can be seen that the diagrams substantially coincide in the zones with frankly elastic and frankly plastic behavior, while the diagrams clearly deviate in the transition zone . The transition connection has large radii of curvature for conventionally hardened and bell-hardened steels, a bearing or elbow with a very small radius of curvature for continuously annealed and conventionally hardened steels, and an elbow with a small radius of curvature for steels annealed under chloche and work-hardened according to the process of the invention. At the conventional point of elastic limit, corresponding to a plastic deformation of 0.2%, a stress difference of the order of 50 N / mm ² between the conventional processes is recorded, while the work hardened steels according to the invention have, compared to steels continuously annealed, a stress difference of less than 10 N / _MM ^2.

The advantage of the diagram with small transition radius is that, during the processes of machining of the work hardened strips, the accidental or inescapable stresses give place to irreversible deformations only if these stresses exceed the field of elastic behavior, so that the uncontrolled deformations are all the more reduced the narrower the transition zone and the radius of curvature of the diagram.

Of course, the invention is not limited to the examples described. It is understood in particular that the work hardening can be carried out on a rolling mill comprising more than two cages, using only two operating cages. Furthermore, one could implement the process with two rolling mills with one cage, respecting the operating conditions specific to each cage according to the teachings presented above, although the use of a rolling mill with at least two cages is preferred. .

Claims (6)

1. A temper rolling process for producing mild steel strips intended for the manufacture of packaging, in which coiled metal sheets are brought substantially to the strip thickness by a first cold rolling, annealing to eliminate structural anisotrophies generated by the first rolling, an then tempering by a second rolling with only slight elongation so as to bring the strip metal to a selected hardness, characterised in that for imparting to the strips which have been box-annealed a hardness comparable to that of strips which have been subjected to continuous annealing and tempered by dry rolling with an elongation of less than 2%, the steel strips, which were of normal quality for continuous annealing, but which have been box-annealed, are conveyed to two successive stands (11, 12), so that the resulting elongation is regulated to a selected value between 4 and 20% and produced for the most part by the first stand (11), the strip is lubricated at the entrance of the first stand by projection (6) of an oil-in-water emulsion, the strip is washed with water (7) at a temperature above the ambient temperature between the stands (11, 12), and the strip is dried by air-blasting (8) at the exit from the second stand (12).

2. A process according to Claim 1, characterised in that the roughness of the rolls of the first stand is between 3.5 and 1 micrometres Ra, while that of the rolls of the second stand is between 0.75 and 0.4 micrometre Ra.

3. A process according to Claim 1, characterised in that the elongation by the first stand is 85 to 95% of the total elongation.

4. A process according to Claim 1, characterised in that the emulsion is 1 to 10% of oil in water.

5. A process according to Ctaim 1, clraracterised in that the washing water is at approximately 50" C

6. A process according to Claim 1, characterised in that the washing water contains approximately 0.2% by weight of sodium nitrite.

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