MXPA99012053A - Continuous casting process for producing low carbon steel strips and strips so obtainable with good as cast mechanical properties - Google Patents

Abstract

A process for the production of low carbon steel strips having a good combination of strength an formability, as cast, and a good weldability after the pickling by usual processes, comprising the following steps:casting, in a twin rolls continuous casting machine (1) comprising pinch rolls (3), a strip with a thickness comprised between 1 and 8 mm, having the following composition as weight percentage of the total weight:C 0. 02-0.10;Mn 0.1-0.6;Si 0.02-0.35;Al 0.01-0.05;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 the strip in the area comprised between the casting-rolls and the pinch rolls (3);hot deforming the strip cast through said pinch rolls (3) at a temperature comprised between 1000 and 1300°C until reaching a thickness reduction less than 15%, in order to encourage the closing of the shrinkage porosities;cooling the strip at a speed comprised between 5 and 80°C/s down to a temperature (Tavv) comprised between 500 and 850°C;and coiling into a reel (5) the so obtainable strip.

Description

CONTINUOUS CASTING PROCESS TO PRODUCE STEEL STRIPS WITH A LOW CARBON CONTENT AND STRIPS THAT CAN BE OBTAINED WITH MECHANICAL PROPERTIES, BOTH GOOD AND STRAIGHT DESCRIPTION The present invention relates to a process for the production of steel strips with a low carbon content, which have a good combination of strength and cold formability, as a casting. Different methods for producing carbon steel strips through twin roll continuous casting devices are already known. These methods point to the production of carbon steel strips that have good strength and ductility properties. In particular, EP 0707908 Al shows a twin roll continuous casting apparatus, where a carbon steel strip is emptied, and then subjected to a hot rolling line with a reduction of 5-50 percent. of its thickness, and that it cools down successively. The flat thin product thus obtained has good strength and ductility properties, thanks to the reduction in the grain dimension obtained with the hot lamination. WO 95/13155 shows an in-line thermal treatment for cast carbon steel strips, which aims to control a strip microstructure as a casting. In particular, the cast strip is cooled below the temperature where the transformation of austenite to ferrite occurs, and is heated successively until the material becomes riaustenitic (in line normalization). In this way, for the effect of a phase of double transformation to solid phase, the austenitic grains become thinner, and by means of the control of the conditions of the final cooling and of the coiling of the strip, it is possible to develop very thin structures that have good strength and ductility. However, the processes mentioned above require additional installations and higher energy consumption (for example, rolling lines, an intermediate heating furnace, etc.), and usually require a large space, and therefore less unit of the entire installation from the casting machine to the winding reel. In addition, the objective of the processes points to the thickness of the final structure of the strip, trying to make it as similar as possible to that of a hot rolled strip of a conventional cycle, and these do not teach how to obtain a product with the properties mechanical and technological requirements, by taking advantage of the peculiarities of phase transformation characteristics for steels such as castings with large austenitic grains (usually 150-400 millimeters).

Therefore, an object of the present invention is to provide a process for the production of steel strips with a low carbon content having, as castings, a good combination of strength and ductility and good weldability, without undergoing the steps of the rolling and / or thermal cycles. Another object of the present invention is to provide a strip of steel with carbon having, as a casting, improved mechanical properties, in particular a ratio of permanent deformation / relatively low fracture stress, and a continuous pattern of the stress-strain curve , in order to make the material particularly suitable for cold molding applications, such as bending and stretching. Therefore, an object of the present invention is a process for the production of steel strips with a low carbon content, having a good combination of strength and formability, as castings, and a good weldability after cleaning with acid by means of the usual processes, consisting of the following steps: casting, in a twin roll continuous casting machine, comprising compression rollers, a strip with a thickness comprised between 1 and 8 millimeters, having the following composition as a percentage by weight of the total weight: C 0.02-0.10; Mn 0.1-0.6; Yes 0.02-0.35; At 0.01-0.05; S < 0.015; P < 0.02; Cr 0.05-0.35; Not 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 the strip in the area between the rolling rolls and the compression rolls; deforming by heat the cast strip through these compression rollers, at a temperature between 1000 and 1300 ° C, until a thickness reduction of less than 15 percent is reached, in order to encourage the closing of the shrinkage porosities; cooling the strip at a speed between 5 and 80 ° C / second down to a temperature between 500 and 850 ° C; and - winding the strip thus obtainable on a reel. In the process of the present invention, the phase transformation characteristics of coarse-grained austenite, which was formed during the continuous casting process without hot rolling and / or on-line normalization, are used to produce, by means of a controlled cooling and coiling, divisions of previously determined volume of the constituents of the microstructure in the material as a casting in steels with a low carbon content. These final microstructures, consisting of equidimensional ferrite, acicular ferrite and / or bainite, provide a typical stress-strain diagram of the material, with a continuous pattern, which has an improved deformability to make the strip particularly suitable for molding applications. cold Another objective of the present invention are also steel strips with a low carbon content obtainable by means of the aforementioned process. These strips can provide a ratio of permanent deformation / low fracture stress, and a continuous pattern of the stress-strain curve of the material, as well as good weldability after acid cleaning. Hereinafter, the present invention will be described in accordance with a present embodiment thereof, given as a non-limiting example. Reference will be made to the figures in the appended drawings, in which: Figure 1 is a simplified schematic of the twin roll continuous casting machine for thin strips, and of the controlled cooling areas of the strips, in accordance with the present invention. Figure 2 is a schematic diagram of in-line cooling cycles applied as cast strips. Figure 3 is a photographic illustration in the optical microscope of the microstructure of a first type of a steel strip as casting, cooled in accordance with the present invention. Figure 4 is a photographic illustration in the optical microscope of the microstructure of a second type of steel strip as casting, cooled in accordance with the present invention. Figure 5 is a photographic illustration in the optical microscope of the microstructure of a third type of steel strip as a cast, cooled in accordance with the present invention. Figure 6 (a) is a photographic illustration in the optical microscope of a ferrite of the acicular type, obtained in particular in a strip according to the present invention. Figure 6 (b) is a photographic illustration in the electron microscope of a particular ferrite of the acicular type, obtained in particular in a strip according to the present invention. Figure 7 is a photographic illustration in the optical microscope of the microstructure of a second type of steel strip as a cast, cooled in accordance with the present invention. Figure 8 is a photographic illustration in the optical microscope of the microstructure of a third type of steel strip as casting, cooled in accordance with the present invention.

Figure 9 is a photographic illustration in the optical microscope of the microstructure of a fourth type of steel strip produced with a traditional cycle. Figure 10 is a tension stress diagram of a strip of a steel type. Figure 11 is a photographic illustration in the optical microscope of the microstructure of the steel strip as a casting, produced in accordance with the process of the present invention. Figure 12 is a diagram of the tension stress diagram in a continuous pattern of a steel strip as casting, obtained in accordance with the process of the present invention. Figures 13 (a) and 13 (b) are diagrams representing the weldability lobes of two types of steel strips cleaned with acid, obtained in accordance with the process of the present invention. Figure 14 is a diagram showing the weldability lobes of a steel strip with a low content of carbon, cleaned with acid, obtained with a conventional cycle. With reference to Figure 1, the process of the present invention provides for the use of a twin roll continuous casting apparatus 1. Immediately downstream of the rolls 1, two cooling devices 2a and 2b are provided for controlled cooling of the strip that passes continuously between them. Successively to the two cooling devices mentioned above, compression rollers 3 of a known structure are provided. At the outlet of the compression rollers 3, a final modular cooling device 4 is provided, wherein the strip passes through to reach a cooling device 5. During solidification and extraction of the casting device 1, it is subjected to to the strip at a suitable controlled pressure by working on the twin rollers that contragrate, in order to limit the formation of shrinkage porosities. Then, the cast strip undergoes cooling by water, or cooling mixed by water-gas on both sides to slow down the increase in growth of both the austenitic grains and the layers of surface oxides. By using compression rollers, the thickness is reduced to less than 15 percent at a temperature that varies between 1000 and 1300 ° C, to close the porosities due to shrinkage to acceptable dimensions. The cooling cycles of the steel strips as castings are established by working on the casting speed, the water flows and the number of active cooling areas. The final cooling cycle, after the compression rollers 3, is defined on the basis of the phase transformation characteristics of the steels, which depend largely on the initial dimensions of the austenitic grains, and on the content of C, Mn and Cr, in order to obtain the desired structures. Different implantation experiments were performed at laboratory and complete scale, using steels whose composition was defined as follows: C 0.02-0.10; Mn 0.1-0.6; Yes 0.02-0.35; At 0.01-0.05; S < 0.015; P < 0.02; Cr 0.05-0.35; Not 0.05-0.3; N 0.003-0.012: Ti < 0.03; V < 0.10; Nb < 0.035, the remaining part being substantially Fe. From these experiments it was evident that, by controlling the chemical analysis of the steel and the cooling modes in line, it is possible to develop suitable final microstructures, characterized by defined fractions in equidimensional ferrite volume and of acicular ferrite and / or bainite. The different division of the constituents of the microstructure thus obtained gives the strips as castings different combinations of strength, ductility and cold formability, which can be evaluated through the stress and Erichsen experiments. In particular, the inventors evaluated the properties connected with the formation of the acicular or bainite ferrite structures, characterized by a high density of dislocations, compared with traditional polygonal thin-grain ferrite structures. In accordance with the process of the present invention, in a steel strip with a low carbon content, as a casting, different types of structures and properties can be obtained, and those properties for each different type can be summarized as follows ( the following capital letters mean different types of steels with carbon): A) Predominance of equidimensional ferrite, acicular ferrite and / or bainite: < 20 percent by volume thick equidimensional grain ferrite: = 70 percent by volume perlite: 2-10 percent by volume permanent deformation stress: Rs = 180-250 MPa fracture stress: Rm = 280 MPa ratio of Rs / Rm = 0.75 total elongation: = 30 percent Erichsen index: = 12 millimeters B) Mixed structure of equidimensional ferrite and acicular ferrite, acicular ferrite and / or bainite: 20-50 percent by volume, coarse equidimensional grain ferrite: < 80 percent perlite volume: < 2 percent by volume permanent deformation stress: Rs = 200-300 MPa fracture stress: Rm = 300 MPa ratio of Rs / Rm = = 0.75 total elongation: - = 28 percent Erichsen's index: = 11 mm C) Predominance of acicular-bainite ferrite acicular and / or bainite ferrite: > 50 percent by volume thick equidimensional grain ferrite: < 50 percent perlite volume: < 2 percent by volume permanent deformation stress: Rs = 210-320 MPa fracture stress: Rm > 330 MPa ratio of Rs / Rm = 0.8 total elongation: = 22 percent Erichsen index: = 10 millimeters It was found that C, Mn and Cr, in the weight concentrations defined in the scope of the present invention, and the austenitic grains whose dimensions are more than 150 μm, as well as a cooling rate of > 10 ° C / second in the temperature range 750-480 ° C, promote the formation of non-equidimensional ferrite. Other experiments conducted in accordance with the process described in the present invention showed that it is possible to take advantage of the larger distribution and uniformity of concentration of the alloy components in cast strips with a high speed of solidification (low entity of segregation), in order to homogenize the distribution of the microstructures, and to avoid the formation of undesired structures, of the martensíico type, reducing the ductility and the formability of the material. On the other hand, the inventors discovered that the energetic cooling of the cast strip is effective to obtain a surface oxide scale, whose thickness and nature are such as to be removed, using the traditional acid cleaning processes. Through spot welding experiments of the acid-stripped specimen obtained with the process of the present invention, the weldability of the materials was verified positively, which, as is well known, is strongly influenced by the condition surface of sheet steel. On the other hand, the inventors observed how the addition of elements such as vanadium and niobium, increased the hardenability of austenite, and delayed the formation of equidimensional ferrite, facilitating the development of acicular ferrite and bainite. In addition, niobium and titanium, which form carbon nitrides, inhibit the dimensional growth of austenitic grains in high temperature heating processes, ensuring, for example, better ductility in the thermally altered area of a weld. The present illustrative and comparative examples of the microstructures and the properties of the strips obtained both by the process of the present invention and with conventional technologies, given as a non-limiting example, will be described hereinafter. For the sake of clarity, the tables mentioned in the following examples are all illustrated together after the last example (Example number 4).

EXAMPLE 1 Some cast strips having a thickness comprised between 2.2 and 2.4 millimeters were obtained, in accordance with the process of the present invention, by the use of type A steel (as already described above), whose analysis is reported in FIG. Table 1. The liquid steel was emptied in a vertical twin roll continuous casting machine (Figure 1), and by using an average separation stress of 40 t / m. The strips were cooled at the outlet of the casting machine until they reached a temperature of 1210-1170 ° C in the vicinity of the compression rollers 3. At these temperatures the thickness was reduced by about 10 percent. Subsequently, the cooling was modulated, as indicated schematically in Figure 2, to have a cooling rate comprised between 10 and 40 ° C / second in the range between 950 ° C and the winding temperature. The latter became variable between 780 and 580 ° C. Table 2 shows the main cooling and winding conditions, together with some characteristics of the microstructure of the strips produced. Table 3 reports the mechanical properties of the strips concerning the permanent deformation stress Rs, defined as ReL or RpO.2 (depending on whether the permanent deformation was continuous or discontinuous), the fracture stress, Rm, the proportion of Rs / Rm, the total elongation, A%, and the Erichsen index (I.AND.), The measure of the cold formability of the materials. In Figures 3-5, the typical microstructures are shown, respectively, of the strips emboiled at 760-730 ° C (strips 9 and 4) and at 580 ° C (strip 5) that can be observed in the optical microscope. It is observed how, when the winding temperature decreases and the average cooling speed of the strip increases, the perlite practically disappears, and the acicular and / or bainite ferrite structures develop, the details of which are shown in Figure 6. microstructures lead to a permanent deformation of the material of the continuous type (Table 3).

EXAMPLE 2 Other strips were obtained having a thickness of 2. 0-2.5 millimeters with the process of the present invention, by using steel types B and C of Table 1, which had a higher carbon content (0.052 percent and 0. 09 percent, respectively. Table 4 shows the main cooling and winding conditions, together with some characteristics of the microstructure of the so obtained. Table 5 reports the mechanical properties of the strips and the Erichsen index, the measure of the cold formability of the materials. Figures 7 and 8 show the typical microstructures respectively of strips 7 (steel B) and 14 (steel C), as observed in the optical microscope. Furthermore, in this case, by taking advantage of the phase transformation characteristics of thick austenitic steels, it is possible to obtain mixed structures containing equidimensional ferrite and also acicular ferrite and bainite. The resistance values are higher than those shown in example 1, related to steel that has 0.035 percent carbon, and the ductility and cold formability remain at good values.

EXAMPLE 3 In this comparative example, the microstructures and the mechanical properties of a strip having a thickness of 2 millimeters are reported, and that was obtained with type D steel (table 1), produced with a traditional cycle, and comparing with those of a strip as casting, having the same chemical analysis, produced in accordance with the process of the present invention. Clearly, the microstructure of the traditional strip consists of thin grains of polygonal ferrite and perlite (Figure 9), with a tension stress diagram of a discontinuous pattern (Figure 10). Table 6 shows the typical mechanical properties of this conventional strip. The use of relatively low winding temperatures (table 7), with the process of the present invention allows the use of materials with acicular structures of the type shown in Figure 11, which are characterized by similar values of fracture stress, with a continuous pattern of permanent deformation (Figure 12). ), and therefore with a lower rate of permanent deformation / fracture stress (table 8).

EXAMPLE 4 Some strips obtained in accordance with the process of the present invention, and made by means of type steels A and B, were cleaned with acid and passed through solderability experiments. The resistance spot welding experiments were performed with electrodes that had a diameter of 8 millimeters, adopting an effort of 650 kilograms, and by varying the current. In FIGS. 13 a and 13 b are shown respectively diagrams which at the level of the "number of current intensity per cycles" provide weldability lobes, that is, the field where the steel sheets can be welded without problems. The comparison with an acid-cleaned sheet steel having a similar thickness, in the steel with a low carbon content obtained by means of a conventional production cycle (Figure 14), shows how the strips obtained with the process of the present invention maintains good weldability characteristics, such as to indicate an acceptable surface condition.

Table 1 Chemical analysis of the steels used in the examples Table 2 Cooling conditions and final microstructures of type A steel strips as castings that were used in the examples Table 3 Mechanical properties of type A of steel strips as castings used in the examples Table 4 Cooling conditions and final microstructures in types B and C of steel strips as castings that were used in the examples Table 5 Mechanical properties of types B and C of steel strips as castings Table 6 Mechanical properties of strips of a conventional cycle in steel D Table 7 Cooling conditions and final microstructures in type D steel strips as castings, and having a thickness of 2 and 4 millimeters Table 8 Mechanical properties of type D steel strips as castings

Claims (5)

1. CLAIMS 1. A process for the production of steel strips with a low carbon content, which have a good combination of strength and formability, as castings, and good solderability after acid cleaning by means of the usual processes, which consists of the following steps: casting, in a twin roll continuous casting machine (1), comprising compression rollers (3), a strip with a thickness comprised between 1 and 8 millimeters, which has the following composition as a percentage by weight of the total weight: C 0.02-0.10; Mn 0.1-0.6; Yes 0.02-0.35; At 0.01-0.05; S < 0.015; P < 0.02; Cr 0.05-0.35; Not 0.05-0.3; N 0.003-0.012: and, optionally, Ti < 0.03; V < Ó, 10; Nb < 0.035, the remaining part being substantially Fe; cooling the strip in the area between the rolling rolls and the compression rolls (3); heat-deforming the cast strip through these compression rollers (3), at a temperature between 1000 and 1300 ° C, until a thickness reduction of less than 15 percent is reached, in order to encourage the closure of the porosities of shrinkage; cooling the strip at a speed between 5 and 80 ° C / second down to a temperature (Tavv) between 500 and 850 ° C; and winding the strip thus obtainable on a reel.
2. 2. A strip of steel as casting with a low carbon content, characterized in that it can be obtained by the process according to claim 1, and because it has microstructures that provide a ratio of permanent deformation / low fracture stress, and a continuous pattern of the stress-strain diagram of the material, as well as good weldability after acid cleaning.
3. 3. A strip of steel with a low carbon content, according to claim 2, having the following final microstructure and mechanical properties: acicular ferrite and / or bainite: < 20 percent by volume coarse equidimensional grain ferrite: > 70 percent by volume perlite: 2-10 percent by volume permanent deformation stress: Rs = 180-250 MPa fracture stress: Rm = 280 MPa ratio of Rs / Rm < 0.75 total elongation: > 30 percent Erichsen index: = 12 millimeters
4. 4. A steel strip with a low carbon content, according to claim 2, which has the following final microstructure and mechanical properties: acicular ferrite and / or bainite: 20-50 per cent by volume of coarse equidimensional grain ferrite: < 80 percent perlite volume: < 2 percent by volume permanent deformation stress: Rs = 200-300 MPa fracture stress: Rm > 300 MPa ratio of Rs / Rm = = 0.75 total elongation: = 28 percent Erichsen index: > 11 millimeters
5. 5. A strip of steel with a low carbon content, according to claim 2, having the following final microstructure and mechanical properties: acicular ferrite and / or bainite: > 50 percent by volume thick equidimensional grain ferrite: < 50 percent perlite volume: < 2 percent by volume permanent deformation stress: Rs = 210-350 MPa fracture stress: Rm > 330 MPa ratio of Rs / Rm = 0.8 total elongation: = 22 percent Erichsen index: = 10 millimeters.