PROCESS FOR THE PRODUCTION OF FINE-GRAINED CARBON STEEL STRIPS AND STRIPS THUS OBTAINABLE

DESCRIPTION

The present invention can be applied to carbon steel strips and coils, having thickness from 1.5 to 15 mm, produced with any process, optionally pickled, or pickled, cold-rolled and optionally annealed, which are subjected to a controlled rolling in ferritic field until bringing them to an end thickness of 0.3-7 mm. The invention relates to a non-conventional thermo- mechanical treatment of the strips and/or the corresponding coils which can be processed with existing plants, having a mainly ferritic structure, optionally also with cementite, pearlite, bainite and martensite. In particular, a rolling in ferritic phase with controlled accumulation of the deformation and an annealing is performed in line, or in parallel, or out of line, so as to promote the recrystallization of the work-hardened ferrite . Different methods have been already proposed for improving the mechanical properties of the strips, by homogenizing and refining the primitive austenitic grains by means of thermo-mechanical treatments.

Other methods resorting to high deformations (thickness reduction) during the hot rolling end phase are being added to the conventional thermo-mechanical treatment, applied to productive cycles starting both from traditional slab, with a thickness generally ranging from 180 mm and 360 mm, and from thin slab (thickness generally from 40 to 90 mm) or average slab (thickness generally from 90 to 120 mm) .

Usually, the conventional thermo-mechanical treatment, which has allowed an improvement of the strip properties in terms of mechanical resistance, weldability in field and toughness of the welds, provides a programmed hot rolling sequence so as to provide a certain reduction level of the rolled product's thickness

at relatively low temperatures, but generally higher than Ar ₃ . Under these conditions the austenite recrystallization is inhibited and the austenitic grains are squashed and elongated with deformation bands inside the austenitic grains, with consequent increase in the ferrite nucleation sites during the strip cooling. In this way the ferrite nucleation is promoted, with marked refining of the end ferritic grain.

The addition of micro-alloy elements such as Nb, Ti, V and Mo, alone or in proper combination, favours both the accumulation of the imparted deformation, by delaying the recrystallization kinetics, and therefore the increase in the ferrite nucleation sites, and the increase in the mechanical resistance by means of the hardening induced by the formation of nano-precipitates .

Also the introduction of high-efficiency cooling systems onto the roller path, with possibility both of lowering the temperatures of start transformation from the austenite into ferrite, with activation of a greater number of nucleation intra-granular sites for the ferrite, and of forming other microstructural constituents such as bainite and martensite, instead of pearlite, contributes to refine the end microstructure and to increase the mechanical resistance. The conditioning of the austenite transformation

(f.i. formation of ferrite and bainite, or ferrite and martensite, or mixtures thereof), so as to further refine the microstructure, besides the use of an ultra-rapid cooling soon after the hot-rolling, interrupted by a temperature interval well-defined according to the prevailing microstructure type which is wanted to be developed, often requires steels with hardenability improved by the addition of expensive alloy elements such as Mo, Cr, Ni. However, the state of art teaches that with these methods it is not possible to develop in a reproducible way, on the current rolling plants, starting from slabs

of conventional type or thin slabs, strips and coils with sizes of the ferritic grains lower than 5 μm.

Other recent methods, still not yet applied in systematic way, are based upon a process wherein the dynamic recrystallization of the austenite in the finisher is promoted, by means of an accurate control of the temperatures at values ranging from Ar ₃ (last stand) and Ar ₃ - 60 ⁰ C (first stand) .

For example, in EP 0945522 a method is described for manufacturing hot rolling raw strips with ferritic grains with an average size lower than 2 μm, based upon the temperature control in wholly austenitic field in the continuous finisher rolling mill, constituted by at least 5 stands, so that the temperature drop between the ingoing side of the first stand and the outgoing side of the last stand be lower than 60 ⁰ C. To this purpose systems for heating the strip are introduced to avoid excessive temperature drops and the risk of rolling in mixed field (austenite-ferrite) . This invention covers also the strips obtained by cold-rolling and subsequent annealing starting from the hot ones.

Additional recent methods, not yet applied in systematic way and extended on industrial scale, are based upon a process wherein the hot deformation and the phase transformation take place at the same time: ferrite transformation dynamically induced by the deformation (dynamic strain induced ferrite transformation, DIFT).

This process, the exact mechanisms thereof have not yet been defined, is able to develop ultrafine ferritic grains, but it requires to accumulate high deformations at temperatures near the transformation ones of austenite-ferrite phase with a high capability of controlling the rolling temperatures in the finisher in a very strict range. Such requirements are difficult to be met with the current plants, even of the last generation.

New processes to impart high hot deformations at controlled temperatures make reference to the

introduction of rolling techniques with cylinders with different diameter (asymmetric rolling) or both with small diameter.

For example, in EP 1279445 Al and in EP 1547700 Al, the inventions are described related to apparatuses for the continuous type rolling able to develop a mainly ferritic structure with relatively fine grains (from 3 to 7 μir.) , which are based upon the introduction of at least three stands with asymmetric cylinders (lesser diameter for the upper cylinder) or both with a very small diameter, only one or both motorized, with lubrication, suitable to impart rates of accumulated deformation higher than 0.6 and cooling systems of the "curtain-wall type cooler", able to control the rolling temperature in the desired range Ar ₃ T 50 ⁰ C (preferably, end rolling temperature = Ar ₃ ± 10 ⁰ C) .

Another procedure for forming fine grained ferrite with steels without niobium is described in JP59205447, wherein the end stage of the hot-rolling is performed proximate Ar ₃ , imparting a total reduction rate higher than 50%, in several passes, but with interpass times lower than 1 s, preferably in a temperature interval ranging from Ar: + 50 ⁰ C to Ar ₃ + 100 ⁰ C, wherein also austenite and ferrite can coexist. Soon after rolling the steel is cooled at a speed of 20 °C/s or greater up to temperatures of 600 ⁰ C or lower ones.

A similar procedure is proposed in US4466842 to form a structure constituted by at least 70% by fine grained ferrite (£ 4 μm) , by favouring the austenite dynamic transformation and the ferrite dynamic recrystallization. To this purpose, one or more passes with an interpass time lower than one second are imparted at temperatures proximate Ar ₃ , so as to give a thickness total reduction higher than 35% (preferably higher than 75%). Some variations (ex. JP59166651) provide within 2 s from the end rolling in mixed field, performed in a temperature interval ranging from ArI + 50 ⁰ C to Ar3 +

100 ⁰ C, giving a total reduction rate higher than 50% if with single pass or 60% if with multiple passes, a rapid cooling with speed of 20 °C/s or higher until a low temperature, so as to form a dual-phase structure of fine grained ferrite with average size of 5 μm or lower and by islands of hard phase (martensite) .

The state of art does not provide teachings as to how to obtain a microstructure mainly of fine grained ferrite in carbon steel strips (C 0.001-0,20; Mn 0.05- 1.9; Si 0.002-0,35; AKO, 06; S<0.03; P<0.1; N 0.003- 0.02), lacking in other alloy elements, obtained starting from conventional strips, which are subjected to a subsequent non-conventional thermo-mechanical treatment, in line or outside line, comprising as a main phase a rolling in wholly ferritic phase with controlled accumulation of the deformation imparted at values ranging from 0.7 and 1,6 and a direct annealing.

The object of the present invention is to provide a process for the production of carbon steels strips with thickness ranging from 0.3 to 7 mm, with mainly fine grained ferritic structure with average size lower than 5 μm, preferably comprised between 1 and 3 μm, with consequent improvement of the mechanical resistance and toughness, without the need of resorting to expensive microalloy elements such as Nb, Ti, B and alloy such as Cr, Mo, Ni, by means of a innovative thermo-mechanical treatment of strips and coils with thickness comprised between 1.5 and 15 mm.

In particular, a rolling in ferritic phase that is at temperatures lower than Ar ₁ - 20 ⁰ C, with controlled accumulation of the deformation imparted with one or more passes until total values ranging from 0.7 to 1.6 followed by an in-line annealing, is performed in line, or in parallel or outside line. Such object is achieved by a process for the production of carbon steel strips as defined in the enclosed claims.

The present invention will be better described hereinafter by the description of embodiments thereof, given by way of example and not with limitative purpose, with the help of the enclosed drawings, wherein: Figure 1 represents a simplified scheme for the rolling in ferritic phase with controlled accumulation of the deformation and in-line annealing;

Figure 2 represents a simplified scheme of the thermo-mechanical cycles for the rolling in ferritic phase with controlled accumulation of the deformation and in-line annealing.

By referring to figure 1 and figure 2, the process of the present invention provides the use, as starting material, of steel coils or strips having composition (expressed in mass percent) comprising: C 0.001-0.20; Mn 0.05-1.9; Si 0.002-0.35; AKO.06; S<0.03; P<0.1; N 0.003- 0.02, the remaining part being substantially Fe, apart from unavoidable impurities.

According to a preferred embodiment, such new process is constituted by the following stages for the production of fine grained carbon steel strips (figure 1 and figure 2 ) :

^■ transferring of a coil (1), said also bobin, formed starting from a strip with thickness ranging from 1.5 and 15 mm., obtained with any process, to a first (uncoiling/coiling) reel (2) placed near the ingoing side of a reversible rolling mill (3), having at least one stand, and equipped at the outgoing side thereof with a second (coiling/uncoiling) reel (4), also able to perform the uncoiling.

^■ uncoiling the bobin, by means of the reel (2) so that the uncoiled strip - optionally heated rapidly and lubricated - optionally under inert or non-oxidizing atmosphere, has a starting rolling temperature higher than 550°C and lower than (Ar ₁ - 2O) ⁰ C. If the coil (1) comes from a hot strip mill and is coiled on purpose at temperatures higher than 65O ⁰ C and transferred to the

reel (2) with an insulated system, it is possible limiting or avoiding the heating operation, otherwise necessary to start the rolling in ferritic phase at temperatures ranging from 55O ⁰ C and (Ari - 50) ⁰ C. ^■ controlled rolling in ferritic phase, wherein the rolled product's temperature is adjusted in a range of +/- 3O ⁰ C, by suitable heating (5), cooling (6) and/or holding (7) treatments, optionally under inert or non- oxidizing atmosphere, which can be performed by means of devices which can be positioned in the higher and/or lower area of the strip (arranged schematically in figure 1 only onto the upper portion of the strip) , so as to accumulate an overall deformation ε _Tot = In [1/(1 - R)] (being R the total thickness reduction, equalling to initial thickness minus end thickness, the whole divided by the end thickness), ranging from 0.7 and 1.6, in a temperature interval anyhow ranging from 550°C and (Ar: ~ 2O) ⁰ C; the deformation is accumulated by making the strip to pass through one or two rolling stands (3) once or more times, preferably by adopting a lubricating system (8) and other devices such as working cylinders with small diameter (9) or asymmetrical diameter (diameter of the upper working cylinder smaller than the lower one), suitable to reduce the forces necessary to rolling at relatively low temperatures; in case of repeated rolling sequence, the strip is coiled onto the reel (4) and subsequently uncoiled in the opposite direction, made to repass through the rolling mill operating in ferritic phase (3) and coiled again in the first reel (2) and subsequently uncoiled to be rolled again; this operation can be repeated at need several times until obtaining the end thickness.

■ coiling the strip onto the reel (4), upon reaching the desired end thickness, generally ranging from 0.3 and 7 mm, at a temperature equalling to the one of end rolling in ferritic phase or lower at maximum by 40 ⁰ C, so as to favour the recovery and re-

crystallization processes of the deformed ferrite, with formation of grains having a difference in the orientation between adjacent grains (disorientation) > 5°, with sizes lower than 5 μm, during the subsequent slow cooling of the coiled coil.

Another embodiment of the new process for the production of fine grained carbon steel strips comprises the operations shown previously, but wherein the strip after the last deformation in ferritic phase, before the end coiling is subjected (figure 2) to a quick controlled heating until a temperature < AcI so as to favour the re- crystallization of the deformed ferrite, with formation of grains having a disorientation > 5°, with sizes lower than 5 μm, during the heating and the subsequent slow cooling of the coiled coil.

Or the strip is subjected to (figure 2) : a) a quick and controlled heating up to a temperature ranging from AcI + 20 ⁰ C and Ac3 - 50 °C, wherein austenite and ferrite coexist, so as to favour, apart from the recrystallization of the deformed ferrite, with development of grains with disorientation > 5°, with sizes lower than 5 μm, the formation of austenite islands rich in carbon with sizes lower than 5 μm; b) a quick cooling, by means of suitable system able to carry out cooling speeds ranging from 10 and 500 °C/s, down to coiling temperatures lower than the one of the start of martensite formation (Ms), so as to develop a fine structure of dual-phase type, constituted by a matrix of ferrite with grain sizes lower than 5 μm (preferably ranging from 1 and 3 μm) , the volumetric fraction thereof is higher than 70% and by martensite islands with high carbon and residue austenite content (M _A constituent) with sizes lower than 5 μm (preferably ranging from 1 and 3 μm) , the volumetric fraction thereof is lower than 30%, and optionally by bainite in volumetric fraction lower than 15%.

Another embodiment of the new process for the

production of fine grained carbon steel strips comprises the operations shown previously, but wherein the coil to be subjected to the controlled rolling in ferritic phase has been only pickled or it has been pickled, cold-rolled and possibly annealed.

A further embodiment of the new process for the production of fine grained carbon steel strips comprises the operations shown previously, but wherein said roiling process in ferritic phase is applied directly in line or onto strip coming from a thermo-mechanical treatment performed in austenitic or mixed (austeno-ferritic) phase or onto strip coming from pickling line.

Subjects of the present invention are also the steel strips or sheets, with thickness < 7 mm, obtained by means of the above-mentioned process, able to develop the following properties, suitable for various application fields :

• good cold formability and low anisotropy, for non-severe applications such as folding and drawings. • high mechanical resistance and high toughness, with good ductility and isotropy, for use in the structural field.

Various laboratory and plant tests have been performed, by using steels the mass percent composition thereof was defined in the following field:

C 0.001-0.20; Mn 0.05-1.9; Si 0.002-0.35; AKO.06; S<0.03; P<0.1; N 0.003-0.012, the remaining part being substantially Fe, unavoidable impurities apart.

From these tests what follows was underlined. - Starting from an initial microstructure mainly constituted by ferrite and pearlite islands, a deformation equal to ε _Tθ ϊ = 0.5, imparted at temperatures of 500-690 ⁰ C, involves a refining of the ferritic grains which squash and elongate in the direction of the rolling in ferritic phase. Inside the ferritic grains sub-grains can be seen, the numerosity thereof increases by deformations around ε _Tot = 0.8. The pearlitic islands too

result to be elongated. Upon increasing the accumulated deformation, once exceeded a critical value of the deformation (generally proximate 0.7), an increase in the fraction of boundaries having a disorientation > 5° and an increasing cementite spheroidization, up to ε _Tot = 1.6 has been found.

- The annealing of the material subjected to rolling in ferritic phase, generally, does not lead to sensible variations in the average size of the ferrite grains, which even if they remain fine (1-5 μm) , they become equiassic and with boundaries with high corner. The globular cementite too results to be more uniformly distributed after annealing.

- The refining of the end microstructure, induced by the rolling in ferritic phase and in-line annealing, allows to raise the yield and to improve the toughness

(reduction in the ductile/brittle transition temperature), but usually it also increases the ratio yield strength/ultimate tensile strength and it reduces the ductility and the work hardening capability of the material .

- In order to preserve good levels of ductility and a certain work hardening capability in fine grained steels it is necessary introducing a second phase, much more harder than ferrite, such as cementite or fine islands of M _A constituent (martensite with high carbon and residual austenite content).

In conformity to what forms the subject of the present invention, hereinafter some examples of embodiments of the same are shown. EXAMPLE 1

As starting material, according to the process of the present invention, a coil with thickness 5.9 mm in steel A was used, the analysis thereof is shown in Table 1.

Table 1 Chemical analysis of steel A

* impurities

The starring coil had a ferrite (88%F) and perlite (12%P) structure, as it had been coiled at about 65O ⁰ C and cooled in natural way down to the room temperature.

The coil has been then heated up to a temperature of 630°C and subjected to rolling in ferritic phase with multiple passes. A total deformation E _:o r equal to 0.8 and 1.2, respectively, has been imparted. The obtained materials have been subjected to in-line annealing at the same end rolling temperature in ferritic phase (coiling at 63O ⁰ C and very slow cooling). The microstructural features and the mechanical properties, in terms of average size of the ferrite (d) grains, lower yield (Re _L ) , ultimate tensile strength (Rm) , ratio Re _L /Rm, elongation at rupture (A), are shown in Table 2.

Table 2

Microstructural features and mechanical properties of the A-steel strips after rolling in ferritic phase and inline annealing at the same end rolling temperature

" spheroidi z ed cement it e

EXAMPLE 2

As starting material, according to the process of the present invention, a coil with thickness 3.9 mm in steel B was used, the analysis thereof is shown in Table

3 .

Table 3 Chemical analysis of steel B

j 'impurities

The starting, pickled, coil had a ferrite (92%F) and pearlite (8%P) structure, as it had been coiled at about 64O ⁰ C and cooled in natural way down to the environment temperature.

10 The coil has been then heated in nitrogen atmosphere up to a temperature of 62O ⁰ C and subjected to rolling in ferritic phase with multiple passes. An overall deformation ε _rot equal to 1.2 has been imparted. The material soon after end rolling in ferritic phase has

15 been subjected, in non-oxidizing atmosphere (oxygen- lacking) , to in-line annealing by quickly heating it up to 760-780 ⁰ C. After keeping for 3-4 s, the strip has been quickly cooled (cooling speed higher than 40°C/s) . The microstructural features and the mechanical properties,

20 in terms of average size of the ferrite grains (d) , yield (Rpo. ₂ ), ultimate tensile strength (Rm), ratio Rpo. _∑ /Rrn, elongation at rupture (A) , are shown in Table 4. By comparison, Table 4 further shows the features of a dual- phase strip produced with conventional process (standard

25 DP) .

Table 4

Microstructural features and mechanical properties of the B-steel strips after rolling in ferritic phase and annealing (invention), compared to those of the starting

30 strip (total deformation = 0) and of a prior art dual- phase strip

1.2 (invention) 1 • 5 - 570 925 - 0.62 18

Standard DP 5. δ - 455 755 - 0.60 17 (prior art)

*MA constituent (about 2C % in volune) EXAMPLE 3

As starting material, according to the process of the present invention, a coil with thickness 14.9 mm in steel C was used, the analysis thereof is shown in Table 5.

Table 5 Chemical analysis of steel C

0 'impurities

The coil has been coiled at about 700 ⁰ C, transported with insulation system as far as the rolling line and processed in ferritic phase with multiple passes starting from a temperature of 62O ⁰ C. A total deformation ε _Tot 5 equalling to 0.95 was imparted. The material, soon after end rolling in ferritic phase, has been subjected to inline annealing, by heating it quickly up to 68O ⁰ C. After keeping for 2-3 s, the strip has been coiled and cooled slowly. The microstructural features and mechanical 0 properties, in terms of average size of the ferrite grains (d) , lower yield (Re _L ) , ultimate tensile strength (Rm), ratio Re^/Rm., elongation at rupture (A), toughness measured as fragile-ductile transition temperature (FATT) with test Charpy V, are shown in Table 6. By comparison 5 Table 6 shows also the features of a strip produced with conventional process.

Table 6

Microstructural features and mechanical properties of the C-steel strip after rolling in ferritic phase and annealing (invention), compared to those of a prior art strip

' sohe roidi z ed cerr.entite