WO2003062476A1

WO2003062476A1 - In-line process for the recrystallization of solidified coarse strips in carbon steel and in low-alloyed steel and so obtainable strips having a highly checked microstructure

Info

Publication number: WO2003062476A1
Application number: PCT/IT2002/000728
Authority: WO
Inventors: Ettore Anelli; Giorgio Porcu; Luca Sartini
Original assignee: Thyssenkrupp Acciai Speciali Terni S.P.A.
Priority date: 2001-11-15
Filing date: 2002-11-15
Publication date: 2003-07-31
Also published as: ITRM20010678A1; ITRM20010678A0; DE60203733T2; CN1296498C; EP1444371A1; CN1606629A; ATE293174T1; EP1444371B1; DE60203733D1

Abstract

The invention refers to an in-line process for the making of strips in carbon steel and in low-alloyed steel by a recrystallization (rolling and annealing) treatment of solidified coarse strips, having a coarse structure, aimed at inducing an advanced recrystallization of the material with homogenizing, refining and checking of the end microstructure, and the entailed improvement of the mechanical properties, in terms of a mix of strength and ductility/cold formability, and of the reproducibility thereof. This process comprises the following steps: casting, in a twin-roll continuous casting machine (A) of strips having a thickness ranging from 1 to 6 mm and a composition, expressed as percent by mass, comprising the following components: 0.02-0.20 C; 0.1-1.6 Mn; 0.02-2.00 Si; <0.05 A1; <0.03 S; <0.1 P; 0.01-1.5 Cr; 0.01-0.5 Ni; <0.5 Mo; 0.003-0.012 N, with substantially Fe q.s. to 100; controlled cooling of the strip in the section comprised between the casting rolls and a rolling system (E); holding the strip at a controlled temperature in an insulated system (F) comprised between an outlet of the rolling system (E) and an inlet in a heating system (G); heating the strip with said heating system (G) at temperatures ranging from 670 to 1150 °C for times ranging from 5 to 40 s, so as to attain a >95% b/v strip recrystallization; ▪ controlled cooling in the section comprised between an outlet of said heating system (G) and a wrapping system.

Description

IN-LINE PROCESS FOR THE RECRYSTALLIZATION OF SOLIDIFIED COARSE STRIPS IN CARBON STEEL AND IN LOW-ALLOYED STEEL AND SO OBTAINABLE STRIPS HAVING A HIGHLY CHECKED MICROSTRUCTURE DESCRIPTION

The present invention applies to both Carbon and low-alloyed steel strips, made by direct continuous casting with a twin-roll or single-roll and strip machine, all characterized by the generation of large-size grains (from 150 to 1500 μm) in the solidified coarse product. The invention refers to an in-line controlled recrystallization treatment carried out on a solidified coarse strip during its roll path transfer, prior to its wrapping on a coiler. In particular, there are carried out in-line and in rapid succession a rolling and an annealing (rapid heating with holding at preset temperatures for short times) so as to attain a practically complete recrystallization of the austenite or of the other microstructural components (e.g., ferrite) work hardened in the preceding deformation. Both the rolling and the annealing can be carried out under conditions in which the austenite or the ferrite are stable, or in a mixed-phase field, in which both the austenite and the ferrite are stable. This invention refers to a process for the making of steel strips and sheets, having a thickness ranging from 4.5 to 0.7 mm, corresponding to common- or special-type hot-rolled or cold-rolled and annealed products, using as starting material continuous casting coarse strips and the present in-line recrystallization process. Several processes for improving the mechanical properties of continuously cast strips via the homogenizing and the refining of the primitive austenitic grains by thermomechanical treatments have already been advanced and patented.

In particular, EP 0707908 Al teaches a twin-roll continuous casting apparatus with which a Carbon steel strip is cast under an atmosphere of inert gas, like Ar or N₂. The strip is then subjected in-line to hot rolling(from 850 to 1350 °C) with a thickness reduction ranging from 5 to 50% and cooled. The thin flat product thus obtained has good strength and ductility properties by virtue of the grain size reduction attained by hot rolling.

Other structure refining methods (JP 61689846 and JP 63115654) refer to in-line thermal treatments, without hot rolling, comprising at least a double phase transition prior to the final cooling and wrapping. Said phase transition is effected subjecting the strip to single or multiple heat cycles of the type: cooling, heating, cooling. With specific reference to Carbon steel and low-alloyed steel in a cast coarse product state, in EP 0707908, EP 0818545 and JP 3249126 the processes and the apparatuses for carrying out in-line thermal treatments are described in which, prior to or after the optional hot reduction of the thickness of the cast coarse strip, a cooling and a subsequent in-furnace heating of the strip is carried out, aimed at refining the structure through the nucleation mechanism of new austenitic grains after phase transformation in the ferritic field. Some of the abovereported processes are aimed, besides at refining the structure in order to improve mechanical characteristics like strength, ductility and toughness, at attaining an adequate surface finishing of the strip (roughness). Also WO 95/13155 (ISHIKAWAJIMA and BHP) teaches an in-line thermal treatment of cast Carbon steel strip aimed at checking the microstructure of the as- cast strip. In particular, the cast strip, after an optional single-pass thickness reduction (20-50%) at 900-1100 °C, is cooled below the temperature at which the transformation of austenite into ferrite takes place, and subsequently heated in- furnace at <1200°C temperatures until reaustenizing the material (in-line normalization). The austenitic grains are thus refined, and, by checking the strip final cooling and wrapping, sufficiently refined structures providing adequate strength and ductility can be developed.

EP 0776984 (Nippon Steel) reports a method and the related equipment for continuously manufacturing metal coils, from a continuous casting system of strips, comprising also the steps of: hot deformation, with a >30% thickness reduction; in- furnace heating thermal treatment under non-oxidizing atmosphere at temperatures of 800-1250 °C (preferably of 1100-1250 °C); cooling, at a 20-40 °C/s rate and down to 100 °C; pickling; and strip wrapping/shearing. The main object of this invention is the connection between the main casting line and the strip pickling-edge trimming- end thermal treatment line, so as to carry out a single continuous step according to which the pickling, the edge trimming, the strip wrapping/shearing lines may independently be connected to the strip casting-rolling-thermal treatment line. The sole example reported relates to a stainless steel, although the metals usable in the invention comprise common and special steels. Also EP 0760397 (Nippon Steel) discloses an apparatus for stainless steelmaking realizing a process comprising the steps of: casting, cooling, hot deformation (rolling) at various temperatures, complete recrystallization by heating and cooling. Some of the above processes and other alike ones require the introduction of one or more optional cooling systems and of a rolling stand, as well as of a intermediate reheat furnace, located upstream or downstream of the rolling stand onto the roll path transferring the strip to the coiler. Most of the preceding experiences with Carbon steel and low-alloyed steel relate to in-line thermal or thermomechanical treatments exploiting sequential phase transformations of the austenite-ferrite-austenite type to attain, prior to the cooling preceding the strip wrapping, a homogenizing and a refining of the austenitic structure, so as to make it as similar as possible to that of a conventionally hot-rolled strip.

The state of the art provides no teachings about the attainment of a Carbon steel or low-alloyed steel product that, from a continuously cast solidified coarse strip, develop the desired mechanical and technological properties, related to hot-rolled or cold-rolled and annealed strips and sheets, exploiting a controlled in-line recrystallization treatment, by rolling in a ferritic, austenitic or mixed-phase field, and direct annealing, also in a subcritical field (<Ac₃ in-furnace treatment temperature).

The few occurrences of in-line recrystallization refer to high-alloyed (stainless) steels that always recrystallize at a very slow rate, as much so as to systematically require treatment temperatures of well above 1050 °C in order to attain a significant recrystallization over short times.

Also for Carbon steel, generally having recrystallization kinetics significantly faster than those of the stainless steels, the presence of very coarse grains in the solidified coarse strips inhibits the recrystallization process. Hence, though deforming at high temperatures the solidified strip and imposing elevated deformation in a single pass, the recrystallization is merely partial, with several grains practically conserving their initial geometry. If in the Carbon steel there are introduced also other alloy elements like Si, Cr, Ni, and/or microalloy elements like V and Nb, often required in order to attain certain mechanical properties and microstructures, the recrystallization at <950°C temperatures is practically non-existent. It has to be borne in mind that in the state-of-the-art systems the contact with the water-cooled rolls causes sensible strip cooling, also because the rolling rates are very low (elevated contact times) and the reductions imposed by single pass are often elevated (greater length of the arc of contact). Immediately post-rolling, the typical temperatures for casting coarse thin strips (<3.5 mm) are generally of <1050 °C, often of <1000 °C, dropping to <950°C within a few seconds even in case of a still-air cooling of the strip. Hence, to date in the known processes the recrystallization of cast coarse structures in Carbon steel and in low-alloyed steel is always partial, with non-homogeneous structures having mainly large-sized grains unsuitable for developing the desired end properties. An object of the present invention is that of providing an in-line process for the making of strips in Carbon steel and in low-alloyed steel by a controlled recrystallization treatment performed on a solidified coarse strip during the roll path transfer, prior to the wrapping on the coiler.

In particular, there are carried out in-line and in rapid succession a rolling, an annealing (rapid heating with holding at preset temperatures for short times in order to attain a near-complete recrystallization of the austenite and/or of the ferrite work hardened during the deformation), and a controlled cooling (optional direct galvanizing included) prior to the wrapping on the coiler.

This object is attained by a process for the making of steel strips as defined by claim 1.

According to a preferred embodiment, this novel process comprises the following steps:

■ casting, in a twin-roll continuous casting machine (A), of strips having a thickness ranging from 1 to 6 mm and a composition, expressed as percent by mass, comprising the following components:

0.02-0.20 C; 0.1-1.6 Mn; 0.02-2.00 Si; <0.05 Al; <0.03 S; <0.1 P; 0.01-1.5 Cr; 0.01- 0.5 Ni; <0.5 Mo; 0.003-0.012 N, and substantially Fe q.s. to 100;

^■ controlled cooling of the strip in the section comprised between the casting rolls and a rolling system (E);

^■ hot deformation of the strip cast with said rolling system (E) at a temperature ranging from 1150 to (Ari - 100) °C, until effecting a 15 to 80% thickness reduction thereto, said hot deformation being selected from the group comprising:

- rolling, at >Ar₃ temperatures and in a completely austenitic phase, Ar₃ being the transformation start temperature of the continuously cooled austenite;

- rolling, at a <Arl temperature and in a completely ferritic phase; - rolling, at temperatures ranging from Ar₃ to Ari and under coexistence conditions of the austenitic phase and of the ferritic phase;

^■ holding the strip at a controlled temperature in an insulated system (F) comprised between an outlet of the rolling system (E) and an inlet in a heating system (G);

^■ heating the strip with said heating system (G) at temperatures ranging from 670 to 1150 °C for times ranging from 5 to 40 s, so as to attain a >95% b/v strip recrystallization;

^■ controlled cooling in the section comprised between an outlet of said heating system (G) and a wrapping system, optionally having plural coilers (N, P), of the strip so obtained. Another object of the present invention is to provide strips of Carbon steel and of low-alloyed steel, manufactured with the hereto detailed process, having an end thickness ranging from 4.5 to 0.7 mm and preset microstructure characteristics, capable of developing the following properties, suitable for various application fields:

■ Cold formability for non-severe applications like bending and drawing, for the manufacturing of components for electrical household appliances (e.g., caps for refrigerators, radiators, refrigerating systems) and for the building industry (e.g., gutters, window frames, garage doors, ceilings, etc.).

^■ Cold forming even for severe applications (e.g., wheel rims).

■ Structural employ (e.g., guard-rail, car components, containers, etc.).

The present invention will hereinafter be better illustrated by the description of an embodiment thereof given by way of example and without limitative purposes, with the aid of the attached drawings, wherein:

* FIG. 1 is a simplified scheme of the continuous casting machine for thin strips and of the devices for the in-line recrystallization and the highly controlled cooling of the strips, used for the present invention; * FIG. 2 is a scheme of the in-line recrystallization and of the cooling cycles applied to cast coarse strips;

* FIGGS. 3 and 4 show strip microstructures, as detectable by optical microscope. With reference to FIG. 1, the process of the present invention provides the use of a continuous casting machine having a twin-roll or a single-roll/belt ingot mold (A), all characterized by the formation of large-size grains (150 to 1500 μm) in the solidified coarse product. Directly downstream, there are provided cooling devices (B and D) for cooling in a controlled manner the strip continuously crossing them, guided by the pinch rolls (C) of a per se already known construction. During the solidification and the extraction from the ingot mold (A) the strip is subjected to an adequate force, e.g. acting on the twin rolls, so as to limit the generation of shrinkage cavities. Then, the cast strip is subjected on both faces thereof to cooling in order to slow down the growth both of the austenitic grains and of the surface oxide layer. The cooling cycles of the as-cast steel strips are set acting on the casting rate, the flow rates and the number of active cooling areas (modules). The pair of said cooling systems B and D has modules, individually actuable, variable in the individual sections and capable of effecting onto the strip cooling rates ofup to 200 °C/s.

In said modules, the cooling is attained with cooling modes selected from the group comprising: natural air, forced air, inert gas jets in an inertized chamber, air-water, water and combinations thereof.

Downstream of the cooling system D there is provided a rolling system consisting of one or more stands (E), capable, in one or more passes, of effecting a 15-80% total thickness reduction in order to reduce to acceptable sizes the residual porosities due to the solid contraction and to induce a work hardening of the structure extant at the instant of deformation.

The stands are located as close as possible, so as to attain quite short interpass times (shorter than the time required to attain the 50% b/v recrystallization) and to allow deformation accumulation in the material (the greater the accumulated deformation, the more rapid the material recrystallization and the softening kinetics). In particular, the hot deformation of the cast strip can take place by means of two consecutive stands, each providing a 15-40%) thickness reduction. Moreover, there is provided a rapid heating system (R), aimed at controlling the temperature of the strip inletted in the rolling system (E), according to the steel composition and to the thermomechanical cycle to be carried out. The rapid heating system (R) is located downstream of the controlled cooling system (D). At the outlet of E there is located a further system (F), comprising a tunnel insulated under natural air, or inert gas, or combinations thereof, and aimed at holding the temperature of the strip which is about to enter the furnace (G) located nearby E. In particular, in the case of austenitic-phase rolling, the insulated section prevents temperature from dropping below Ar₃ and to start a phase transformation before the austenite recrystallization.

The furnace (G), of an induction-, gas- or the like type, is capable of operating under a reducing or a non-oxidizing atmosphere, and of rapidly (in less than 12 s) bringing the strip temperature to temperature values ranging from 670 to 1150 °C, apt to attain a near-complete recrystallization of the material (>95 % b/v) with temperature holding times of 5-40 s. The annealing temperature is selected so as to fall within a completely austenitic field (austenitic field rolling) or a ferritic field (ferritic field rolling) , or within a mixed-phase field in which ferrite and austenite coexist (ferritic or mixed-phase field rolling), as it is indicated in FIG. 2. Right at the furnace outlet there is located the cleaning system (H), which removes oxides or oxide residues partially or totally reduced by the furnace atmosphere from the strip surface.

Downstream of the system H, along the roll path leading to the coilers (N, P) there is located a set of at least three controlled cooling modules (I, L, M). These modules are capable of effecting on the strip cooling rates variable in the individual sections and ranging from 800 °C/s (ultrarapid cooling) to 0.01 °C/s (insulation), and.

Upon detecting the strip temperature at the furnace outlet by metering with a suitable device, the strip cooling cycle is defined according to the steel phase transformation characteristics, which mainly depend on the actual size of the austenitic grains and on the chemical analysis of the steel, so as to develop the desired structures. Onto the roll path, among the various modules, there are located other temperature meters (e.g., pyrometers), enabling monitoring of the thermal cycle. The latter can be quite complex (e.g., accelerated cooling, in-air cooling, wrapping) in case multiphase microstructures, made of various constituents like polygonal ferrite, bainite, martensite and/or residual austenite, are to be developed. One of the cooling modules is also capable of galvanizing the strip. However, preferably the strip is wrapped at a temperature of from 900 to 150°C. Various laboratory and full-scale plant tests were conducted, employing steels whose composition in percentage by mass was defined in the following field: 0.02-0.20 C; 0.1-1.6 Mn; 0.02-2.00 Si; <0.05 Al; <0.03 S; <0.1 P; 0.01-1.5 Cr; 0.01- 0.5 Ni; <0.5 Mo; 0.003-0.012 N; and, optionally, <0.03 Ti; <0.10 V; O.035 Nb; O.005 B, with substantially Fe q.s. to 100. These tests highlighted that: a) The grains of the cast coarse strip have coarse mean sizes (>150 μm) and often a column-type shape resembling the solidification macrostructure. b) The dynamic recrystallization, with these sizes of the initial grains and for the aboveindicated chemical analyses, is not activated for the typical in-line rolling conditions of cast coarse strips (15-40% reduction, 10-35 s^_1 deformation rate,

1000-1150 °C deformation temperature). c) A 30-40% reduction at 1000-1150 °C induces a static, yet partial, recrystallization, there remaining a significant coarse grain fraction (>50% b/v). In fact, the austenite recrystallization kinetics, already intrinsically slow, become slower with the dropping of the strip temperature just after the end of the hot rolling. A microstructural nonuniformity ensues, which cannot be compensated acting on the subsequent checked/controlled cooling and wrapping. d) By lowering the deformation temperature in the austenitic field the static recrystallization is inhibited, and in the subsequent phase transformation, besides at the edges of the primitive austenitic grains, ferrite nucleation ensues at the deformation bands inside of the coarse grains. The end result is a certain, yet nonhomogeneous, refining of the structure. e) The grains located near the surfaces of the cast and rolled coarse strip are often thinner than those at midthickness, due to the dishomogeneity of thickness deformation and of temperature gradients in the in-line rolling. f) The missed or incomplete recrystallization after the in-line rolling, combined to the scanty refining and to the low uniformity of the resulting microstructure, is a critical factor for the development of strips for structural employ (requiring high strength and adequate toughness) and for direct application, replacing the cold- rolled strips used in the building and household appliance fields. g) A near-complete recrystallization (>95% b/v) of the austenite having a very coarse initial structure is attained in less than 40 s even in low-alloyed steel having a composition which falls within the indicated limits, introducing an inline annealing, at 1050-1100 °C temperatures, immediately after the hot rolling stage (>15% deformation at 950-1150 °C). h) Much higher temperatures, do accelerate the static recrystallization, leading however to overly large sizes (>120 μm) of the recrystallized austenitic grains. i) After the in-line controlled recrystallization treatment, the austenitic grains are equiaxic and uniform, with mean sizes ranging from 50 to 120 μm, according to the deformation accumulated in rolling and at the annealing temperatures adopted. These austenitic grains, after the checked cooling, generate ferritic grains having 15-30 μm sizes, in case of >10 °C/s cooling rates and of wrappings at >700 °C temperatures. In the latter case, also pearlite islands are observable whose volume fraction relates to the Carbon content in the steel. j) The in-line recrystallization allows, by homogenizing the austenitic structure and the strip temperatures lengthwise as well as widthwise, to reduce the variety of structures, polygonal, acicular and otherwise unavoidable, entailing advantages of reproducibility of the mechanical characteristics, in particular for the direct- employ products, replacing the traditional cold-annealed strips. k) The rolling of the cast coarse strip, in the case of low-Carbon strips (C <0.06 %), may be carried out without the problem of excessive rolling forces even in the ferritic field, i.e. at temperatures ranging from Ari to Ari - 100 °C. For >25 % deformations in the ferritic field and in-line annealing conducted at a 670-720 °C temperatures, always for <40 s times, with rapid wrapping of the strip at such temperatures, ferrite structures with grains or subgrains having 30-60 μm sizes (low yield value) and a homogeneous carbide distribution (absence of lamellar pearlite) are obtained, suitable for nonsevere cold forming steps, like bending and drawing.

In case of rolling in a ferritic or mixed-phase field, even 0.06 - 0.2% Carbon steel strips can be annealed over short times. In this case, a subcritical field (Aci - Ac₃) annealing is resorted to. The temperature is set according to the quantity of austenite in the strip and to the desired Carbon enrichment of the latter at the furnace outlet, prior to the controlled cooling which is modulated according to the desired quantities and typologies of the end structures dispersed in the extant ferrite (e.g., bainite and high-Carbon martensite islands, or bainite and residual austenite islands). The present innovative in-line recrystallization treatment of cast coarse strips enables, by selecting the chemical analysis of the steel, to check the rolling and annealing temperatures and the in-line cooling cycles, to develop suitable end microstructures, having definite volume fractions of equiaxic (polygonal) ferrite, pearlite or carbides, of acicular and/or bainitic ferrite and of high-Carbon martensite/residual austenite islands. The different distribution of the microstructural components so obtained, highly checkable through the thermal and microstructural homogenization taking place during the in-furnace recrystallization treatment, provides the strips with different combinations of strength, ductility and cold formability.

In particular, there were evaluated the properties related to the generation of ferrite structures having a 30-80 μm equiaxic grain and pearlite or nonlamellar carbides, obtained by in-line recrystallization of the austenite or of the ferrite, respectively, deriving from cast coarse strips, in low-Carbon (C <0.06 %) steels having a very coarse primitive structure.

Treatments for the in-line recrystallization and the controlled cooling of the cast coarse strips in low-alloyed steel, containing Mn, Si and optionally other elements like Cr, were carried out in order to develop multiphase structures containing ferrite (>55%), bainite (5-40%), and a scattering of high-Carbon (1-15% C) martensite islands. The latter are characterized by having, at the ferrite interface, a high dislocation density with respect to the traditional polygonal ferrite/pearlite structures. The end material yielded has a continuous-type stress-deformation curve, provided with a good mix of strength and ductility. Exploiting a subcritical recrystallization treatment in continuously cast strips, microstructures exhibiting significant quantities of residual austenite (5-20 %) were made.

According to the object of the present invention, hereinafter some embodiments thereof are disclosed. EXAMPLE 1 An 1.6 mm thickness strip was made according to the process of the present invention and using steel A, the analysis of the latter being reported in Table 1.

Table 1 Chemical analysis of steel A

The molten steel was cast in a vertical continuous casting machine (Fig. 1) having a twin-roll ingot mold and a 6 t/m mean separation force. The strips were cooled at the ingot mold outlet until reaching a 1080-1100 °C temperature at the rolling system inlet. A 35% total thickness reduction was effected. The subsequent cooling and heating steps were carried out as it is schematically shown in FIG 2, so as to attain a > 850 °C minimum temperature Tm, a 10 °C/s heating rate, a 1050 °C maximum temperature of the in-furnace strip, with 10 s holding times thereat. 15 °C/s cooling rates were detected at the furnace outlet and up to the wrapping. The latter was carried out at >750 °C temperatures. The microstructural characteristics and the mechanical properties of the end strip, in terms of mean size (d) of the ferrite grains, pearlite (P) %, lowest yield value (RerJ, failure strength (Rm), Re_L/R ratio, ultimate elongation (A), are reported in Table 2. The ratio between the standard deviation and the mean value for the failure strength and for the elongation is of the 2.5% and of the 3%, respectively. These values indicate a high checkability of the end microstructure, induced by the introduction of the in-line recrystallization.

Table 2 Microstructural characteristics and mechanical properties of the 1.6 mm end thickness A steel strip

FIG. 3 shows the typical microstructure of the strip, as observable at the optical microscope.

Apparently, a ferrite structure having equiaxic-grain and pearlite islands was generated. EXAMPLE 2

An 1.6 mm end thickness strip having was made according to the process of the present invention and using steel B. The analysis of the latter is reported in Table 3.

Table 3

Chemical analysis of steel ] B

Steel C Mn Si Cr Ni Cu Ti Mo S P Al N

B 0.11 0.80 0.29 0.06 0.04 0.05 0.005 0.01 0.003 0.007 0.027 0.0042

The molten steel was cast in a vertical continuous casting machine having a twin-roll ingot mold and a 5.5 t/m mean separation force. The strips were cooled at the ingot mold outlet until reaching a 1080-1100 °C temperature at the rolling system inlet. A 35% total thickness reduction was effected. The subsequent cooling and heating steps were carried out as it is schematically shown in FIG. 2, so as to attain a >850 °C minimum temperature Tm, an 8 °C/s heating rate, a 1050 °C maximum temperature of the in-furnace strip, with 12 s holding times thereat. 15 °C/s cooling rates were detected at the furnace outlet and up to the wrapping. The latter was carried out at >750 °C temperatures. The microstructural characteristics and the mechanical properties of the end strip, in terms of mean size (d) of the ferrite grains, pearlite % (P) lowest yield value (Re_L), failure strength (Rm), ReiTRm ratio, ultimate elongation (A), are reported in Table 4. The ratio between the standard deviation and the mean value for the failure strength is of the 3% and of the 3.5%, respectively.

Table 4 Microstructural characteristics and mechanical properties of the 1.6 mm end thickness B steel strip

FIG. 4 shows the typical microstructure of the strip, as observable at the optical microscope.

EXAMPLE 3 An 1.6 mm thickness strip was made, according to the process of the present invention and using steel B. The analysis of the latter is reported in Table 1. The molten steel was cast in a vertical continuous casting machine having a twin-roll ingot mold and a 6 t/m mean separation force. The strips were cooled at the ingot mold outlet until reaching a 690 °C temperature at the rolling system inlet. A 37% total thickness reduction was effected. The subsequent cooling and heating steps were carried out as it is schematically shown in FIG. 2, so as to attain a >670 °C minimum temperature Tm 670 °C, a 5 °C/s heating rate, a 720 °C maximum temperature of the in-furnace strip, with 15 s holding times thereat. <0.5 °C/s cooling rates were detected at the furnace outlet and up to the wrapping. The latter was carried out at > 700 °C temperatures. The microstructural characteristics and the mechanical properties of the end strip, in terms of mean size (d) of the ferrite grains, lowest yield value (Re_L), failure strength (Rm), ReiTRm ratio, ultimate elongation (A), are reported in Table 5.

Table 5 Microstructural characteristics and mechanical properties of the 1.6 mm end thickness in-line recrystallized, ferritic-phase rolled A steel strip

Optical microscope observation highlighted a ferrite structure having dispersed nonlamellar carbides onto the strip so obtained. EXAMPLE 4

A 2.4 mm end thickness strip was made, according to the process of the present invention and using steel C. The analysis of the latter is reported in Table 6.

Table 6 Chemical analysis of steel C

The molten steel was cast in a vertical continuous casting machine having a twin-roll ingot mold and a 6.5 t/m mean separation force. The strips were cooled at the ingot mold outlet and then heated until reaching an 840-860 °C temperature at the rolling system inlet. A 40% total thickness reduction was effected. The subsequent cooling and heating steps were carried out as it is schematically shown in FIG. 2, so as to attain a >880 °C minimum temperature Tm, an 8 °C/s heating rate, a 1050 °C maximum temperature of the in-furnace strip, with 12 s holding times thereat. Downstream of the furnace the cooling rates were of from 50 °C/s to 700-680 °C/s, 5 s natural air cooling, <400 °C forced cooling (40-80 °C/s). Wrapping at 400-380 °C temperatures

A mixed ferrite/bainite structure having residual austenite islands was generated. The percent by volume of residual austenite, measured by X-ray diffraction, was of the 12%. This structure confers the mechanical properties shown in Table 7.

Table 7 Microstructural characteristics and mechanical properties of the 2.2 mm end thickness C steel strip

The elevated yield value (Rpo_.2 ) and failure strength value Rm are anyhow accompanied by a low yield/failure ratio, a good ductility (>22% elongation), and a particularly high (> 16000 MPa%) product of the elongation X failure strength (A X Rm) parameter.

These characteristics are conferred by the good structural homogeneity and by the presence of residual austenite, attained with the in-line recrystallization of continuous casting coarse strips.

Claims

1. A process for the making of strips in Carbon steel and in low-alloyed steel having a highly checked microstructure, comprising in sequence the following steps:

^■ Casting, in a twin-roll continuous casting machine (A), of strips having a thickness ranging from 1 to 6 mm and a composition, expressed as percent by mass, comprising the following components: 0.02-0.20 C; 0.1-1.6 Mn; 0.02-2.00 Si; <0.05 Al; <0.03 S; <0.1 P; 0.01-1.5 Cr; 0.01- 0.5 Ni; <0.5 Mo; 0.003-0.012 N, and substantially Fe q.s. to 100;

^■ hot deformation of the cast strip by said rolling system (E) at a temperature ranging from 1150 to (An - 100) °C, until effecting a 15 to 80% thickness reduction thereto, said hot deformation being selected from the group comprising:

- rolling, at a <Ari temperature and in a completely ferritic phase;

- rolling, at temperatures ranging from Ar₃ to Ari and under coexistence conditions of the austenitic phase and of the ferritic phase; ^■ holding the strip at a controlled temperature in an insulated system (F) comprised between an outlet of the rolling system (E) and an inlet in a heating system (G);

^■ heating the strip with said heating system (G) at temperatures ranging from 670 to 1150 °C for times ranging from 5 to 40 s, so as to attain a >95 % b/v strip recrystallization; ^■ controlled cooling in the section comprised between an outlet of said heating system (G) and a wrapping system, optionally having plural coilers (N, P), of the strip so obtained.

2. The process for the making of strips in Carbon steel and in low-alloyed steel according to claim 1, wherein the composition of said strip in steel provides at least another component selected from the group comprising:<0.03 Ti; <0.10 V; O.035

Nb and O.005 B.

3. The process for the making of strips in Carbon steel and in low-alloyed steel according to claim 1 or 2, wherein said controlled cooling in the section comprised between the casting rolls and the pinch-rolls C, and between the pinch rolls (C) and the rolling system (E) takes place by means of a pair of systems (B and D), having modules, individually actuable, variable in the individual sections and capable of effecting onto the strip cooling rates of up to 200 °C/s.

4. The process for the making of strips in Carbon steel and in low-alloyed steel according to claim 3, wherein said cooling is attained with cooling modes selected from the group comprising: natural air, forced air, inert gas jets in an inertized chamber, air-water, water and combinations thereof.

5. The process for the making of strips in Carbon steel and in low- alloyed steel according to any one of the claims 1 to 4, wherein said heating system (G) operates under a reducing or a non-oxidizing atmosphere.

6. The process for the making of strips in Carbon steel and in low-alloyed steel according to claim 5, wherein said heating system (G) operates at temperatures selected from the group comprising: temperatures corresponding to that of the existence of the sole austenitic phase, temperatures corresponding to that of the existence of the sole ferritic phase, temperatures coπesponding to that of coexistence of the ferritic and of the austenitic phase.

7. The process for the making of strips in Carbon steel and in low-alloyed steel according to claim 6, wherein said heating system (G) is followed by a system (H) for cleaning the strip surface and for removing therefrom oxides or oxide residues partially or totally reduced by the furnace atmosphere.

8. The process for the making of strips in Carbon steel and in low-alloyed steel according to any one of the claims 1 to 7, wherein the strip is held at a controlled temperature at the outlet of the rolling system (E) by a system (F) comprising an insulated tunnel containing air, or inert gas, or combinations thereof.

9. The process for the making of strips in Carbon steel and in low-alloyed steel according to any one of the claims 1 to 8, wherein at the outlet of the heating system (G) the strip is subjected to a cooling with at least three modules (I, L, M), located along a roll path, capable of effecting on the strip cooling rates variable in the individual sections and ranging from 800 °C/s (ultrarapid cooling) and 0. °C/s (insulation).

10. The process for the making of strips in Carbon steel and in low-alloyed steel according to any one of the preceding claims, wherein in one of said modules a strip galvanizing can be carried out.

11. The process for the making of strips in Carbon steel and in low-alloyed steel according to any one of the preceding claims, wherein the hot deformation of the cast strip takes place by means of two consecutive stands, each providing a 15-40% thickness reduction.

12. The process for the making of strips in Carbon steel and in low-alloyed steel according to any one of the preceding claims, wherein the interpass time between consecutive stands is shorter than the time required to attain a 50% b/v recrystallization.

13. The process for the making of strips in Carbon steel and in low-alloyed steel according to any one of the preceding claims, wherein there is provided a system (R) for heating the strip in the section comprised between the pinch rolls (C) and the rolling system (E).

14. The process for the making of strips in Carbon steel and in low-alloyed steel according to the preceding claim, wherein said heating system (R) is located downstream of the controlled cooling system (D).

15. The process for the making of strips in Carbon steel and in low-alloyed steel according to any one of the preceding claims, wherein the strip is wrapped at temperatures ranging from 150 to 900 °C.

16. The strips in Carbon steel and in low-alloyed steel, having an end thickness ranging from 0.7 to 4.5 mm, obtainable by a process as defined in any one of the preceding claims.