Continuous casting process for producing low carbon steel strips and low carbon steel strips produced by this process

Abstract

In the process of the invention, a strip with a thickness of between 1 and 8 mm is cast in a continuous casting device (1) containing two stripping rollers (3), the strip is cooled to a temperature between 1000 and 1300 °C in the region between the casting rollers and the stripping rollers (3); the pores created during casting shrinkage are closed by hot forming between the stripping rollers (3) with a reduction of up to 15%; then the strip is cooled to a temperature between 480 and 900 °C at a speed between 5 and 80 °C / s and wound up.The steel strip according to the invention has the following mass% composition: 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, and the remainder Fe; and has a combined microstructure in which, in addition to pearlite and coarse globular ferrite, there is acicular ferrite / or bainite ensuring a low yield strength / rupture stress ratio, a continuous elongation curve and good weldability after descaling. à.

Description

Iroda Kft., Budapest (54) Continuous casting process for the production of low-carbon steel strips and low-carbon cast steel strips EXTRACT In the process of the invention, a strip with a thickness of between 1 and 8 mm is cast in a continuous casting device (1) containing two stripping rollers (3), the strip is cooled to a temperature between 1000 and 1300 °C in the region between the casting rollers and the stripping rollers (3); the pores created during casting shrinkage are closed by hot forming between the stripping rollers (3) with a reduction of up to 15%; then the strip is cooled to a temperature between 480 and 900 °C at a rate of between 5 and 80 °C / s and wound up. The composition by weight of the steel strip according to the invention is as follows: 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=O,O5-O,3; N=0.003-0.012 and optionally Ti<0.03, V<0.10, Nb<0.035, and the remainder Fe; and has a combined microstructure in which, in addition to pearlite and coarse globular ferrite, there is acicular ferrite and / or parasite, which provides a low yield strength / rupture stress ratio, a continuous elongation curve and good weldability after descaling. Figure 1 EN 222 856 B1 The description is 20 pages long (including 12 pages of illustrations). HU 222 856 Bl The invention relates to a continuous casting process for the production of low-carbon steel strips and to low-carbon steel strips produced by the process, which have adequate strength and cold formability in the cast state. Various processes are known for the production of low-carbon steel strips in a two-roll continuous casting plant. These processes are intended to produce carbon steel strips with good strength and cold formability properties. EP 0707908 A1 describes a two-roll continuous casting apparatus in which a low carbon steel strip is cast, the thickness of the steel strip is reduced by 5-50% by hot rolling, and then cooled. The resulting flat, thin product has good strength and cold formability properties due to the grain size being reduced by hot rolling. WO95 / 13155 describes a solution for the in-line heat treatment of low carbon steel strips, with the aim of controlling the microstructure of the cast strip. According to the solution, the cast strip is cooled to a temperature at which the austenite transforms into ferrite, and then the material is heated until it is re-austenitized (normalizing heat treatment). In this way, due to the fact that a double phase transformation occurs, at the end of which a solid phase is obtained, the austenite grains become smaller and, by controlling the final cooling conditions and the cooling of the strip, very thin materials can be produced with good strength and cold formability properties. However, the above-mentioned processes require additional equipment (e.g., roller trains, furnaces for intermediate heating, etc.), higher energy consumption and usually require a large space for their placement, and therefore the entire system from the casting equipment to the winding roll is less uniform. Furthermore, the processes described aim to produce the final structure and thickness of the strip so that it is as similar as possible to that of a strip produced by hot rolling in a conventional cycle, and do not provide any teaching on how to produce a product with the desired mechanical and technological properties by exploiting the specificities of the phase transformation properties of cast steels containing large (usually 150-400 pm) austenitic grains. The aim of the invention is therefore to develop a process for producing low-carbon steel strips that have good strength, cold formability and weldability properties in the as-cast state, without undergoing multiple rolling and / or heat treatment operations. We also aimed to develop a low-carbon steel strip that has good mechanical properties in the as-cast state, a relatively low yield strength / breaking stress ratio, and a continuous elongation curve, making the material particularly suitable for cold forming applications such as bending and drawing. We achieved the set goal by: - in a two-roll, continuous casting machine with stripping rolls, a strip with a thickness between 1 and 8 mm is cast, the mass% composition of which is as follows: 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, and the remainder Fe; - the strip is cooled in the region between the casting rolls and the stripping rolls; - the cast strip is hot-formed between the stripping rollers at a temperature between 1000 and 1300 °C until its thickness reduction is less than 15%, thereby closing the pores created during casting shrinkage; - the strip is cooled to a temperature of between 480 and 900 °C at a rate of between 5 and 80 °C / s; and - the resulting tape is wound onto a winding device. In the process of the invention, the phase transformation properties of coarse-grained austenite, which is formed without continuous, preforming hot rolling and / or heat treatment on the production line, are exploited in order to achieve a predetermined volume ratio of the microstructure constituents in the low-carbon cast steel material by controlled cooling and coiling. The final microstructure, which is formed by globular ferrite, acicular ferrite and / or benite, gives the material a characteristic, continuous stress-strain characteristic curve and the material's formability is improved, making the strip particularly suitable for cold forming applications. The invention further aims to produce low carbon steel strips by the above-mentioned process. These strips provide the material with a low yield strength / tensile strength ratio, the tensile-elongation curve of the material is continuous, and the weldability after descaling is also good. Below, the method and steel strip according to the invention are described in more detail in connection with an exemplary embodiment, based on the attached drawing, where the Figure 1 shows a two-roll, continuous casting device for casting thin strips according to the invention, as well as the controlled cooling areas of the strips, Figure 2 is a schematic diagram of the cooling cycles of cast strips on a production line, Figure 3 is a light microscope photograph of the microstructure of a cast-based steel strip (labeled A) cooled by the method according to the invention, Figure 4 is a light microscope photograph of the microstructure of another cast steel strip (labeled B) cooled by the method according to the invention, HU 222 856 B1 Figure 5 is a light microscope photograph of the microstructure of a further cast steel strip (labeled C) cooled by the method according to the invention, Figure 6(a) is a light microscope photograph of a needle-type ferrite obtained in a tape produced by the process of the invention, Figure 6(b) is an electron microscope photograph of a type of needle-type ferrite in a strip produced according to the invention, the Figure 7 is a light microscope photograph of the microstructure of a cast steel strip (labeled B) cooled by the method according to the invention, Figure 8 is a light microscope photograph of the microstructure of a cast steel strip (labeled C) cooled by the method according to the invention, Figure 9 is a light microscope photograph of the microstructure of a conventionally produced steel strip (marked D), Figure 10 is a tensile stress diagram of a steel strip, Figure 11 is a light microscope photograph of the microstructure of a cast steel strip produced by the method of the invention, Figure 12 is a tensile stress diagram of a continuous section of a cast steel strip obtained by the process according to the invention, Figures 13(a) and 13(b) are diagrams illustrating the weldable zones of two different types of descaled steel strip obtained by the process according to the invention, and Figure 14 is a diagram illustrating the weldable ranges of a descaled, low-carbon steel strip obtained by a conventional process. Figure 1 illustrates the process according to the invention. A two-roll, continuous casting machine 1 is used for the process. Immediately after the draw-off rolls of the casting machine 1, cooling units 2a and 2b are located, which serve to cool the strip continuously passing between them. After the two cooling units 2a and 2b, there are pull-off rollers 3 of known construction, and behind them there is a further cooling unit 4, through which the strip reaches a winder 5. The solidified strip leaving the casting device 1 is cooled to a temperature between 1000 and 1300 °C in the cooling units 2a and 2b, and then hot-formed by means of the two counter-rotating draw-off rollers 3, so that the size of the pores formed during the casting shrinkage is reduced to an acceptable level. The reduction applied is maximum 15%. The cast strip is then cooled on both sides with water or a mixture of water and gas to slow down the growth of austenitic grains and surface oxide layers. The desired cooling cycles are set by selecting the casting speed and the cooling water flow rate and by switching on the appropriate number of cooling zones. The last cooling cycle is formed after the 3 draw-off rolls based on the phase transformation properties of the steels. These depend mainly on the initial austenitic grain size and the C, Mn and Cr content. Various laboratory and field experiments were carried out using steels whose composition is defined as follows: 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; Ti<0.03; V<0.10; Nb<0.035, the remainder was Fe. It was evident from these experiments that by controlling the chemical analysis of the steel and the cooling conditions during production, it was possible to achieve final microstructures characterized by a specific volume ratio of globular ferrite to acicular ferrite and / or ferritic iron. The different distributions of microstructural components thus obtained gave the cast strips different combinations of strength, ductility and cold formability, which could be evaluated by tension and Erichsen tests. The inventors have evaluated the properties associated with the formation of globulitosferrite or bainite structures as being characterized by a high dislocation density compared to the conventional structures of polygonal fine-grained ferrite. The process according to the invention can produce strips of various types, structures and properties from low-carbon steel strips, and their properties can be summarized as follows according to the different types - A, B and C (see the examples): A) Predominantly globular ferrite, acicular ferrite and / or paralite: <20 vol.%, polygonal coarse-grained ferrite: >70 vol.%, pearlite: 2-10 vol.%, yield strength: Rs=180-250 Mpa, fracture stress: Rm>280 Mpa, Rs / Rm ratio: <0.75, total elongation: >30%, Erichsen number: >12 mm, B) Mixed structure of globular and acicular ferrite globular ferrite and / or pearlite: 20-50 vol.%, coarse-grained globular ferrite: <80 vol.%, pearlite: <2 vol.%, yield strength: Rs=200-300 MPa, fracture stress: Rm>300 MPa, Rs / Rm ratio: <0.75, total elongation: >28%, Erichsen number: >11 mm, C) Predominantly acicular ferrite-bainite acicular ferrite and / or bainite: >50 vol.%, coarse-grained globular ferrite: <50 vol.%, pearlite: <2 vol.%, yield strength: Rs: 210-320 MPa, ultimate tensile strength: Rm>330 MPa, Rs / Rm ratio: <0.8, total elongation: >22%, Erichsen number: >10 mm. HU 222 856 Bl It has been found that C, Mn and Cr in the mass concentrations specified within the scope of the invention, as well as austenitic grains with sizes larger than 150 pm, and a cooling rate greater than 10 °C / s in the temperature range of 750-480 °C promote the formation of non-globulized ferrite. Further experiments with the process of the invention have shown that the greater uniformity of distribution and concentration of alloy components in cast strips, together with a high solidification rate (with low precipitation), can be exploited to homogenize the microstructure and avoid the formation of undesirable phases, such as martensite, which reduce the ductility and formability of the material. Furthermore, the inventors have found that intensive cooling of the cast strip is effective in obtaining a surface oxide scale layer of such thickness and structure that it can be removed using conventional descaling methods. Spot welding experiments on descaled strip samples obtained by the method according to the invention have yielded positive results with regard to weldability, which, as is well known, is strongly influenced by the surface quality of the strip material. It is known to the skilled person that elements such as vanadium and niobium increase the hardenability of austenite and delay the formation of globular ferrite, thereby facilitating the formation of acicular ferrite and ferritic iron. Furthermore, niobium and titanium, which form carbon nitrides, prevent the size increase of austenite grains during high-temperature heating processes, thus providing, for example, better ductility in a thermally altered area due to welding. The following are illustrative and comparative examples of the microstructures and properties of the tapes obtained by the process according to the invention and by conventional technologies, which are not intended to limit the scope of the invention. For the sake of clarity, the tables mentioned in the following examples will be included together after the last example (after example 4). Example 1 Using type "A" steel, several cast strips were produced by the process of the invention, the thickness of which was 2.2-2.4 mm, the chemical composition of which is given in Table 1. The liquid steel was poured into a vertical, two-roll, continuous casting machine 1 (see Figure 1) and an average specific linear parting pressure of 40 t / m was applied. The strips were cooled after the casting machine 1 until a temperature of 1210-1170 °C was reached at the draw-off rolls 3. At such temperatures, the thickness was reduced by approximately 10%. The cooling was then carried out as schematically indicated in Figure 2 to achieve a cooling rate Vr of between 10 and 40 °C / s in the range between the temperature of 950 °C and the coiling temperature Tavv. The coiling temperature was generally 580-760 °C. The cooling rates (Vr) and the coiling temperatures (Tavv) are given in Table 2, together with the microstructural characteristics of the strips produced.The mechanical properties of the strips: the yield strength Rs (denoted by R^L or Rp0 2 depending on whether the elongation is continuous or discontinuous), the breaking stress Rm, the ratio Rs / Rn, the elongation A and the IE Erichsen index indicating the degree of cold formability of the materials are shown in Table 3. Figures 3-5 illustrate the characteristic microstructures of the strips wound at 730-760 °C (strips 9 and 4), as seen under a light microscope. It was observed that as the winding temperature decreases and the average cooling rate of the strip increases, pearlite practically disappears and acicular, ferrite and / or bainite structures develop, the details of which are shown in Figure 6. These microstructures lead to the material having a continuous type of strain curve (see Table 3). Example 2 Further strips of 2.0-2.5 mm thickness were produced using the process according to the invention, using steels B and C according to Table 1, which had a higher carbon content (0.052 and 0.09%). The cooling and winding conditions are illustrated in Table 4, together with some microstructural properties of the resulting strips. The mechanical properties of the strips and the Erichsen index, i.e. the cold formability characteristics of the materials, are given in Table 5. Figures 7 and 8 show the typical microstructures of strip 7 (steel B) and strip 14 (steel C) as seen under a light microscope. By exploiting the phase transformation properties of coarse-grained austenitic steels, combined microstructures containing globular ferrite and acicular ferrite and bainite were also obtained. The strength values ​​are higher than those shown in Example 1 for steel containing 0.035% Ct, and the ductility and cold formability are also satisfactory. Example 3 In this comparative example, the microstructure and mechanical properties of a 2 mm thick strip of steel D (see Table 1) produced by a conventional manufacturing process are compared with the microstructure and mechanical properties of a cast strip produced by the process according to the invention. The composition of the two materials was completely identical. It can be seen that the microstructure of the conventionally manufactured strip is formed by thin polygonal ferrite grains and pearlite (see Figure 9) and that its elongation diagram is discontinuous (see Figure 10). The microstructure of this conventionally manufactured strip The typical mechanical properties of strip HU 222 856 Bl are shown in Table 6. The use of relatively low winding temperatures (see Table 7) in conjunction with the process according to the invention allows the production of materials with an acicular structure corresponding to that shown in Figure 11 and which are characterized by similar values ​​of breaking stress, with a continuous elongation curve (see Figure 12) and consequently a lower yield strength / breaking stress ratio (see Figure 8). Example 4 Some strips of steels A and B made by the process according to the invention were descaled and weldability tests were carried out. The spot welding tests were carried out with electrodes having a diameter of 8 mm, applying a pressing force of 650 kg and with varying current. The curves shown in Figures 13a and 13b show the number of cycles (which is proportional to the welding time) as a function of the current. Here (between the two curves) the ranges where the steel sheets can be welded without problems are shown. A comparison with a descaled steel made of a similar thickness of low carbon steel'Ί 0 by a conventional manufacturing process (see Figure 14) shows that the strips obtained by the process according to the invention retain their good weldability properties, indicating an acceptable surface condition. Table 1 Chemical analysis of the steels used in the examples Steel c Mn Si Cr Ni SP Al NA 0.038 0.48 0.16 0.31 0.13 0.008 0.016 0.044 0.01 B 0.052 0.45 0.16 0.22 0.08 0.004 0.008 0.021 0.0086 C 0.090 0.59 0.31 0.09 0.07 0.014 0.008 0.010 0.0088 0.034 0.22 0.02 0.05 0.06 0.003 0.008 0.035 0.0080 Table 2 Cooling conditions and final microstructures of cast steel strips A used in the examples Experimental strip number Vr (°C / s) Taw (°C) Microstructure (in vol.%) globular ferrite acicular ferrite-t-baenite pearlite 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 cast steel strips A used in the examples Number of experimental strips Vr (°C / s) Taw (°C) ReL (Mpa) Rp0.2 (Mpa)r Rm (Mpa) Rs / Rm A (%) IE (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 HU 222 856 Bl Table 4 Cooling conditions and final microstructures of cast steel strips B and C used in the examples Steel type / strip Vr(°C / s) Taw (°C) Microstructure (in vol%) globular ferrite acicular ferrite+bainite perlite IB / 8 20 860 67 2 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 cast steel strips AB and C Steel type / strip Vr (°C / s) Taw (°C) ReL (Mpa) Rp0.2 (Mpa)r 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 strips made of AD steel using conventional processes Steel type / 1 strip Thickness (mm) Vr (°C / s) Taw (°C) ReL (Mpa) Rm (Mpa) Rs / Rm A (%) IE (mm) I 0 / 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 4 mm thick D-type cast steel strips 1 Steel type / strip Thickness (mm) Vr(°C / s) Taw (°C) Microstructure (in vol%) globular ferrite acicular fenite+bainite pearlite 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 cast steel strips marked D Steel type / strip Vr (°C / s) Taw (°C) ReL (Mpa) Rp0.2 (Mpa)r Rm (Mpa) Rs / Rm A (%) IE (mm) D / 3 50 720 287 - 390 0.74 26 - D / 5 80 720 - 238 356 0.67 31 - HU 222 856 Bl Table 8 (continued) Steel type / strip Vr (°C / s) Tavv (°C) RcL (Mpa) Rp0.2 (Mpa)r Rm (Mpa) Rs / Rm A (%) IE (mm) 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 - PATENT CLAIMS

Claims

1. Continuous casting process for the production of steel strips with low carbon content, easily weldable even in the cast state, with high strength and easily weldable after descaling, characterized in that - in a continuous casting device (1) containing two draw-off rollers (3), a strip with a thickness between 1 and 8 mm is cast, the composition of which in mass% is as follows: 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, and the remainder 25 Fe; - the strip is cooled to a temperature between 1000 and 1300 °C in the area between the casting rolls and the stripping rolls (3); - the pores created during casting shrinkage are closed by hot forming between the stripping rolls (3) with a reduction of up to 15%; then - the strip is cooled to a temperature between 480 and 900 °C at a rate between 5 and 80 °C / s and wound up.

2. Low-carbon cast steel strip, characterized by the following composition in mass%: 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; 40 Ni = 0.05-0.3; N = 0.003-0.012 and optionally Ti < 0.03, V < 0.10, Nb < 0.035, and the remainder Fe; and having a combined structure in which, in addition to pearlite and coarse globular ferrite, there is acicular ferrite and / or parasite providing a low yield stress / rupture stress ratio, a continuous elongation curve and good weldability after 45 descale.

3. Steel strip according to claim 2, characterized in that its structure contains less than 20 vol.% acicular ferrite and / or paralite and at least 70 vol.% polygonal coarse-grained ferrite, as well as 2-10 vol.% pearlite, and its mechanical properties are as follows: yield strength: Rs=180-250 Mpa, breaking stress: Rm>280 Mpa, Rs / Rm ratio: <0.75, total elongation: >30%, Erichsen number: >12 mm.

4. Steel strip according to claim 2, characterized in that its structure contains 20-50 vol.% acicular ferrite and / or paralite and less than 80 vol.% coarse-grained globular ferrite, and less than 2 vol.% pearlite, and its mechanical properties are as follows: yield strength: Rs=200-300 MPa, tensile strength: Rm>300 MPa, Rs / Rm ratio: <0.75, total elongation: >28%, Erichsen number: >11 mm.

5. Steel strip according to claim 2, characterized in that its structure contains more than 50 vol.% acicular ferrite and / or paralite and less than 50 vol.% coarse-grained globular ferrite, and less than 2 vol.% pearlite, and its mechanical properties are as follows: yield strength: Rs: 210-320 MPa, tensile strength: Rm>330 MPa, Rs / Rm ratio: <0.8, total elongation: >22%, Erichsen number: >10mm. HU 222 856 Bl Int. Cl.7: B 22 D 11 / 06 HU 222 856 Bl Int. Cl.7: B 22 D 11 / 06 Vr - 20 - 60 C / s Figure 2 HU 222 856 Bl Int. Cl.7: B 22 D 11 / 06 Figure 3