PROCEDURE FOR THE PRODUCTION OF LOW-CARBON STEEL CASTING RAW STRIPS AND THE STRIPS OBTAINABLE THUS

Description

R M97 A 03 67 SIB 91340 DESCRIPTION of the industrial invention entitled: PROCEDURE FOR THE PRODUCTION OF ROUGH CASTING STRIPS IN LOW-CARBON STEEL AND STRIPS OBTAINABLE THUS by the companies 1) VOEST-ALPINE Industrieanlagenbau GmbH with headquarters in LINZ (AUSTRIA) 2) ACCIAI SPECIALI TERNI SpA with headquarters in TERNI (ITALY) >>>><<<< The present invention relates to a process for the production of low carbon steel strips, with a good combination of mechanical strength and cold formability, in the as-cast condition. Several methods are known for producing carbon steel strip using continuous casting machines with counter-rotating roller molds. These methods are designed to produce carbon steel strip with good mechanical strength and ductility. In particular, EP 0707908 Al describes a continuous casting apparatus S'* IR * 0 » ROME - Two counter-rotating rollers cast a carbon steel strip, which is then hot-rolled in-line, reducing its thickness by up to 50-70%, and then cooled. The resulting thin flat product has good mechanical strength and ductility thanks to the reduction in grain size achieved by hot-rolling. WO 95 / 13155 describes a method for the in-line heat treatment of cast carbon steel strips aimed at controlling the strip's microstructure in the as-cast condition. Specifically, the cast strip is cooled below the temperature at which austenite transforms into ferrite and then heated until the material re-austenitizes (in-line normalization). In this way, the austenitic grains are refined through a double phase transformation from solid to solid, and by controlling the final cooling and strip winding conditions, it is possible to develop fairly fine structures with adequate mechanical strength and ductility. The above mentioned procedures require, however, Yes! * I * -3 additional equipment and energy consumption (e.g. rolling stand(s), intermediate heating furnace, etc.) and generally require more space and, therefore, less compactness of the entire production plant, from the casting machine to the coiler. Furthermore, they are aimed at refining the final structure of the strip, trying to make it as similar as possible to that of a hot-rolled strip from a conventional cycle, and do not provide lessons on obtaining a product that develops the desired mechanical and technological properties, exploiting the peculiarities of the phase transformation characteristics of cast raw steels with a large austenitic grain size (typically 150-400 Hm). The aim of the present invention is therefore to provide a process for the production of low carbon steel strips which, in the as-cast condition, without introducing rolling stages and / or thermal cycling, present a good combination of mechanical strength and ductility and adequate weldability. -4Another object of the present invention is to provide a carbon steel strip which exhibits, in the as-cast condition, improved mechanical properties, in particular a relatively low yield stress / tensile strength ratio and a continuous-type stress-strain curve, which makes the material particularly suitable for cold forming applications such as bending and drawing. The object of the present invention is in fact a process for the production of raw casting strips, made of low carbon steels, with a good combination of mechanical strength and formability, comprising the following operations: - continuous casting, in a continuous casting machine with a counter-rotating roller ingot mold, of strips with a thickness between 1 and 8 mm, having the following composition expressed as a percentage by mass: C 0.02-0.10; Mn 0.1-0.6; Si 0.02-0.35; Al 0.010.05; S<0.015; P<0.02; Cr 0.05-0.35; Ni 0.05-0.3; N 0.003-0.012; and, possibly, Ti<0.03; V<0.10; Nb<0.035, the remaining part being substantially Fe; and IR W * a iSr λ ROME - cooling of the strip in the section between the casting rollers and the gripping rollers; - hot deformation of the cast strip using said gripping rollers at a temperature between 1000 and 1300 °C until a reduction in thickness of no more than 15% is achieved to facilitate the closing of the shrinkage cavities; - controlled cooling of the strip at a rate of between 5 and 80 °C / s to a temperature of between 500 and 850 °C; and winding of the strip thus obtained at the aforementioned temperature. In the process of the present invention, the phase transformation characteristics of coarse-grained austenite, formed during the continuous casting process without hot rolling and / or in-line normalizing, are exploited to produce, in low-carbon steels, through controlled cooling and coiling, predetermined volumetric distributions of the microstructural constituents in the as-cast material. These final microstructures, consisting of eguiaxic ferrite, acicular ferrite, and / or bainite, give us the stress curve Si 8*^ .IB ROME -6def orination, characteristic of the material, a continuous trend, with improved deformability so as to make the strip particularly suitable for cold forming applications. The present invention also includes low-carbon steel strips obtainable with the aforementioned process. These strips possess low segregation, predetermined mixed microstructures containing acicular ferrite and / or bainite, capable of providing a low yield strength / tensile strength ratio and a continuous-type stress-strain curve of the material, as well as adequate weldability after pickling. The present invention will be better illustrated below by the description of a preferred embodiment thereof, given by way of example and not by way of limitation, and by referring to the attached drawings, in which: Figure 1 is a simplified diagram of the counter-rotating roller continuous casting machine for thin strips and of the controlled cooling regions of the strips, according to the present invention; RÓMA figure 2 is a diagram of the in-line cooling cycles applied to raw casting strips; Figure 3 is a photographic optical microscope illustration of the microstructure of a first type of cooled cast steel strip according to the present invention; Figure 4 is a photographic illustration under an optical microscope of the microstructure of a raw steel strip cast, cooled in accordance with the present invention; Figure 5 is a photographic optical microscope illustration of the microstructure of a raw steel strip cast, cooled in accordance with the present invention; Figure 6(a) is a photographic illustration under an optical microscope of a detail of the acicular type ferrite of a ribbon according to the present invention; Figure 6(b) is a photographic illustration of a transmission electron microscope detail of the acicular ferrite of a ribbon according to the present invention; Figure 7 is a photographic illustration of the optical microscopic microstructure of a -8second type of raw steel strip casting, cooled according to the present invention; Figure 8 is a photographic illustration of the optical microscopic microstructure of a third type of cast steel strip, cooled in accordance with the present invention; Figures 9(a) and 9(b) are photographic illustrations of the optical microscopic microstructure of a fourth type of steel strip produced with a traditional cycle; Figure 10 is a tensile strength curve of a strip of one type of steel; Figure 11 is a photographic illustration of the optical microstructure of a raw steel casting strip produced according to the process of the present invention; Figure 12 is a continuous tensile curve diagram of a cast raw steel strip produced according to the process of the present invention; Figures 13(a) and 13(b) are representative plots of the weldability lobes of two types of pickled steel strip produced according to the process of the present invention; and -9Figure 14 is a representative plot of the weldability lobes of a low carbon pickled steel strip produced by a conventional cycle. With reference to figure 1, the process of the present invention provides for the use of a continuous casting apparatus with a counter-rotating roller mould 1. Immediately downstream of the rollers 1, two cooling devices 2a and 2b are provided to cool in a controlled manner the strip which passes continuously through them. Following the two aforementioned cooling devices, gripping rollers 3 of already known construction are provided. At the exit of station 3 there is a modular final cooling device 4 through which the strip is passed before reaching a winding device 5. During solidification and extraction from the ingot mold 1, the strip is subjected to an adequate force acting on the counter-rotating rollers, so as to limit the formation of shrinkage cavities. The cast strip is then subjected to | to MW n ROME -10 Both faces are cooled with water, or with a combination of water and gas, to slow the growth of both the austenitic grains and the surface oxide layer. Using gripper rollers, a thickness reduction of less than 15% is achieved at a temperature ranging between 1000 and 1300°C to close the residual porosity caused by shrinkage to an acceptable size. The cooling cycles of the as-cast steel strips are set by adjusting the casting speed, water flow rate, and the number of active cooling zones. The final cooling cycle, after the nip rolls, is defined based on the phase transformation characteristics of the steels, which depend primarily on the initial austenitic grain size and the C, Mn, and Cr contents, in order to develop the desired structures. Various laboratory and full-scale plant tests were conducted, using steels whose composition was defined in the following range: C 0.02-0.10; Mn 0.1-0.6; Si 0.02-0.35; Al 0.010.05; S<0.015; P<0.02; Cr 0.05-0.35; Ni 0.05-0.3; N 0.003-0.012; Ti<0.03; V<0.10; Nb<0.035, the Sa 80¾ | K « 3 « SmP a ROME - 11 remaining part being substantially Fe. These tests showed that by controlling the steel's chemical analysis and in-line cooling methods, it is possible to develop suitable final microstructures, characterized by defined volume fractions of equiaxed ferrite and acicular ferrite and / or bainite. The different distribution of microstructural constituents thus achievable gives the raw casting strips different combinations of mechanical strength, ductility, and cold formability, which can be assessed with tensile and Erichsen tests. In particular, the inventors evaluated the properties related to the formation of acicular ferrite or bainite structures, characterized by a high dislocation density, compared to traditional fine-grained polygonal ferrite structures. With the process of the present invention, various types of structures and properties are created on a raw casting strip, made of low carbon steel, which can be summarised in the following diagram: A) Prevalence of equiaxed ferrite Yes £3 « 1 Are you? « ROME - 12 acicular ferrite and / or bainite: <20% by volume coarse-grained equiaxed ferrite: ^70% by volume pearlite: 2-10% by volume yield strength: Rs = 180-250 MPa unit tensile strength at break: Rm 280 MPa Rs / Rm ratio s. 0.75 elongation at break: 30% Erichsen index: 12 mm B) Mixed structure of equiaxed and acicular ferrite acicular ferrite and / or bainite: 20-50% by volume coarse-grained equiaxed ferrite: <80% by volume pearlite: <2% by volume yield strength: Rs = 200-300 MPa unit tensile strength at break: Rm 300 MPa Rs / Rm ratio = <_ 0.75 elongation at break: 28% Erichsen index: 11 mm C) Prevalence of acicular ferrite-bainite acicular ferrite and / or bainite: >50% by volume Sat aaa. .LB. ROME - 13 coarse-grained equiaxed ferrite: <50% by volume pearlite: <2% by volume yield strength: Rs - 210-320 MPa unit tensile strength at break: Rm > 330 MPa Rs / Rm ratio 0.8 elongation at break: 22% Erichsen index: 10 mm It has been found that C, Μη and Cr, in the mass concentrations defined in the scope of the present invention, and austenitic grains larger than 150 pm, as well as cooling rates > 10 °C / s in the temperature range 750-480 °C, favor the formation of non-equiaxed ferrite. Further tests conducted according to the process described in the present invention have shown that it is possible to exploit the greater uniformity in terms of distribution and concentration of the alloying elements in a strip cast with a high solidification rate (low degree of segregation) to homogenize the distribution of the microstructures and avoid the formation of unwanted martensitic-type structures, which reduce the ductility and formability of the material. R M 9 Ϊ» «tfr 4 ROME - 14Furthermore, the inventors discovered that vigorous cooling of the cast strip is effective in producing a surface oxide scale of sufficient thickness and nature to be removed using traditional coil pickling processes. Spot resistance welding tests on pickled strip samples manufactured using the process of the present invention demonstrated positive weldability of the materials, which, as is known, is strongly influenced by the surface condition of the laminations. Furthermore, the inventors observed that the addition of elements such as vanadium and niobium, by increasing the hardenability of austenite and retarding the formation of equiaxed ferrite, facilitates the development of acicular ferrite and bainite. Furthermore, niobium and titanium, by forming carboxy-nitrides, inhibit the growth of austenitic grain size during high-temperature heating processes, ensuring, for example, improved ductility in the heat-affected zone of a weld. Illustrative and comparative examples will be given below, given for information purposes only. ROME -15 examples, not limited to, of microstructures and properties of tapes manufactured both with the process of the present invention and with conventional technologies. For the sake of clarity, the tables mentioned in the examples are reported all together after the last example (Example No. 4). EXAMPLE 1 Some strips with a thickness between 2.2 and 2.4 mm were produced according to the process of the present invention, using steel A, the analysis of which is reported in table 1. The liquid steel was cast in a vertical continuous casting machine (Figure 1) with a counter-rotating roller mold and an average separation force of 40 t / m. The strips were cooled at the mold exit until they reached temperatures of 1210-1170 °C near the gripper rollers. At these temperatures, a thickness reduction of approximately 10% was imparted. Subsequently, the cooling was modulated, as schematically shown in Figure 2, so as to achieve cooling rates between 10 and 40 °C / s in the range of Yes R ROME - 16 temperatures between 950 °C and the winding temperature. The latter was varied between 780 and 580 °C. The main cooling and winding conditions are shown in Table 2, together with some microstructural characteristics of the manufactured strips. The mechanical properties of the strips in terms of yield strength Rs, expressed as ReL or Rp0.2 depending on whether the yield strength is discontinuous or continuous, unit tensile strength at break, Rm, ratio Rs / Rm, elongation at break, A%, and Erichsen index (IE), a measure of the cold formability of the materials, are reported in Table 3. Figures 3-5 show the typical microstructures of the tapes wound at 760-730 °C (tapes 9 and 4) and at 580 °C (tape 5), respectively, as observable under the optical microscope. It can be observed that as the winding temperature decreases and the average cooling speed of the tape increases, the pearlite practically disappears and acicular ferrite and / or bainite structures develop, the details of which are shown in figure 6. These microstructures lead to a yielding of the I 1¾ ROME -17 continuous material (Table 3). EXAMPLE 2 Other strips of thickness 2.0 - 2.5 mm were manufactured with the process of the present invention, but using the steels B and C of table 1, having a higher carbon content (0.052% and 0.09%, respectively). The main cooling and winding conditions are shown in Table 4, along with some microstructural characteristics of the fabricated strips. The mechanical properties of the strips and the Erichsen index, a measure of the cold formability of the materials, are reported in Table 5. Figures 7 and 8 show the typical microstructures of strips 7 (steel B) and 14 (steel C), respectively, as observed under an optical microscope. Also in this case, by exploiting the phase transformation characteristics of steels with a coarse austenitic grain, it is possible to develop mixed structures containing, in addition to equiaxed ferrite, acicular ferrite and bainite. The mechanical strength values ​​are higher than those shown in example 1 relating to SXX ROME - loyal steel with 0.035% C, but ductility and cold formability remain at good levels. EXAMPLE 3 In this comparative example, the microstructures and properties of a 2 mm thick strip made of D steel (Table 1) produced with a traditional cycle are reported, compared with those of a raw casting strip, of the same chemical analysis, processed according to the process of the present invention. Clearly, the microstructure of the traditional strip is made up of fine grains of polygonal ferrite and pearlite (Figure 9), with a discontinuous tensile curve (Figure 10). The typical mechanical properties of this conventional strip are shown in Table 6. The use of relatively low winding temperatures (Table 7), allows, with the process of the present invention, to move on to materials with acicular structures of the type shown in Figure 11, characterised by similar values ​​of the unit tensile strength at break, but with a continuous yield strength (Figure 12) and, therefore, with a lower yield strength / tensile strength ratio (Table 8). YES .Ld. ROME -19EXAMPLE 4 Some strips of the present invention, in steels A and B, were pickled and subjected to weldability tests. The spot welding tests were carried out with 8 mm diameter electrodes, adopting a load of 650 kg, and varying the current intensity. Figure 13 shows the diagrams that in the "number of cycles-current intensity" plane give the weldability lobes, i.e. the domain in which the sheets can be welded without problems. The comparison with a pickled sheet of similar thickness, in low carbon steel from a conventional cycle (Figure 14), shows how the strips produced with the process of the present invention retain adequate weldability characteristics, indicating an acceptable surface condition. Table 1 Chemical analysis of the steels used in the examples Sa ! R * 8 * «ài « ROME Steel C Mn Si Cr Ni SP Al A 0.038 0.48 0.16 0.31 0.13 0.008 0.016 0.044 B 0.052 0.45 0.16 0.22 0.08 0.004 0.008 0.021 C 0.090 0.59 0.31 0.09 0.07 0.014 0.008 0.010 D 0.034 0.22 0.02 0.05 0.06 0.003 0.008 0.035 Table 2 Cooling conditions and final microstructures of the raw steel casting strips used in the examples Ribbon Vr (°C / s) Taw (°C / s) MICROSTRUCTURE (% by volume) EQUIAXIC FERRITE ACICULAR FERRITE + BAINITE PERLITE 9 15 760 56 40 4 4 34 730 40 58 2 3 30 680 50 50 2 11 15 620 50 50 1 5 26 580 10 90 0 Table 3 Mechanical properties of the raw casting strips in the A steel used in the examples Ribbon Vr (°C / s) Taw (°C / s) ReL (MPa) RpO. 2 (MPa) Rm (MPa) Rs / Rm (-) A (%) I .E. (mm) 9 15 760 - 250 351 0.71 30 12.7 4 34 730 - 264 351 0.75 28 12.5 3 30 680 - 250 338 0.74 28 12.6 11 15 620 - 251 355 0.70 28 11.4 5 26 580 - 306 384 0.79 22 11.0 Table 4 Cooling conditions and final microstructures of the raw casting strips in the B and C steels used in the examples Steel / strip Vr (°C / s) Taw (°C / s) MICROSTRUCTURE (% by volume) EQUIAXED FERRITE ACICULAR FERRITE + BAINITE PERLITE B / 8 20 860 67 27 6 B / 6 20 610 40 59 1 B / 7 25 500 20 80 0 C / 13 20 820 80 15 5 C / 14 25 620 30 70 0 Table 5 Mechanical properties of raw casting strips in B and C steels I or * L 4 ROME -22nd. - Steel / Strip Vr (°C / s) Taw (°C / s) ReL (MPa) Rp0.2 (MPa) Rm (MPa) Rs / Rm (-) A (%) IE (mm) B / 8 20 860 258 - 343 0.75 26 12.5 B / 6 20 610 267- 353 0.76 24 12.4 B / 7 25 500 - 320 406 0.79 22 12.2 C / 13 20 820 202 - 310 0.65 30 11.4 C / 14 25 620 - 253 344 0.73 22 10.3 Table 6 Mechanical properties of conventional steel D-stainless steel strips Steel / Strip Thickness mm Vr (°C / s) Taw (°C / s) RgL (MPa) Rm (MPa) Rs / Rm (-) A (%) IE (mm) D / 7 2 30 640 323 383 0.84 30 13.3 D / 8 4 20 650 303 372 0.81 35 - Table 7 Cooling conditions and final microstructures of 2 and 4 mm thick cast strips in D steels STEEL / THICKNESS Vr Taw STRIP MICROSTRUCTURE mm °C / s °C EQUIAXED FERRITE ACICULAR FERRITE + BAINITE PERLITE D / 3 2 50 720 30 70 0 D / 5 2 80 720 40 60 0 D / 2 2 15 620 50 50 0 D / 4 2 80 620 25 75 0 D / 6 4 50 620 40 60 0 Table 8 Mechanical properties of raw casting strips in steel D Steel / Strip Vr <°C / S) Taw (°C / s) ReL (MPa) Rp02 (MPa) Rm (MPa) Rs / Rm (-) A (%) I .E. (mm) D / 3 50 720 287 - 390 0.74 26 - D / 5 80 720 - 238 356 0.67 31 - D / 2 15 620 - 223 366 0.61 27 - D / 4 80 620 - 259 380 0.68 25 13.0 D / 6 50 620 - 196 338 0.58 38 - Gilberto Tonon (her. Register n. 83 BM) S3 « 3 * 0 ROME

Claims

-24R Μ 97 A 03G7 CLAIMS 1 . Process for the production of low carbon steel strips having a good combination of mechanical strength and formability, in the as-cast condition, and adequate weldability after pickling by usual processes, comprising the following operations: - casting, in a continuous casting machine with a counter-rotating roller ingot mould, of strips with a thickness of between 1 and 8 mm, having the following composition expressed as a percentage by mass: C 0.02-0.10; Mn 0.1-0.6; Si 0.02-0.35; Al 0.010.05; S<0.015; P<0.02; Cr 0.05-0.35; Ni 0.05-0.3; N 0.003-0.012; and, optionally, Ti<0.03; V<0.10; Nb<0.035, the remaining part being substantially Fe; - cooling of the strip in the section between the casting rollers and the gripping rollers; - hot deformation of the cast strip by means of said gripping rollers at a temperature between 1000 and 1300 °C until the thickness is reduced by no more than 15% to facilitate the closing of the shrinkage cavities; cooling of the strip at a speed between 5 and 80 °C / s to a temperature between 500 and 850 °C; and - winding of the strip thus obtained. 2.Raw casting strip made of low carbon steel, characterised by being obtainable through the process according to claim 1 and by having low segregation, pre-set mixed microstructures containing acicular ferrite and / or bainite, capable of conferring a low yield strength / tensile strength ratio and a continuous trend to the stress-strain curve of the material, and adequate weldability after pickling.

3. Low carbon steel strip according to claim 2, with the following final microstructure and related properties: acicular ferrite and / or bainite: <20% by volume coarse-grained equiaxed ferrite: ^.70% by volume pearlite: 2-10% by volume yield strength: Rs = 180-250 MPa unit tensile strength at break: Rm 280 MPa Rs / Rm ratio 0.75 elongation at break: > 30% -26 Erichsen index: 12 mm 4.Low carbon steel strip according to claim 2, with the following final microstructure and related properties: acicular ferrite and / or bainite: 20-50% by volume coarse-grained equiaxed ferrite: <80% by volume pearlite: <2% by volume yield strength: Rs = 200-300 MPa ultimate tensile strength: Rm 300 MPa Rs / Rm ratio 0.75 elongation at break: 28% Erichsen index: 11 mm 5. Low carbon steel strip according to claim 2, with the following final microstructure and related properties: acicular ferrite and / or bainite: >50% by volume coarse-grained equiaxed ferrite: <50% by volume pearlite: <2% by volume yield strength: Rs = 210-350 MPa ultimate tensile strength: Rm > 330 MPa Rs / Rm ratio = 0.8 and ! RI » «mJ ft ROME -27 elongation at break: > 22% Erichsen index: > 10 mm 6.Process for the production of low-carbon steel strips with a good combination of mechanical strength and formability in the as-cast condition and weldable low-carbon steel strips, as previously described, illustrated and claimed. pp 1) VOEST-ALPINE Industrieanlagenbau GmbH. 2) ACCIAI SPECIALI TERNI SpA Gilberto To non (Register no. 83 BM)