Hot rolled steel sheet and its manufacturing method

Abstract

Steel plate in the hot rolled condition, the steel having the following composition, in millipercent, or where indicated, in ppm: C: 1-50 ppm; Mn: max 200; Si: max 100; Al: 10-200; Ti: 46-100; Nb: max 100; V: Max 100; P: max 20; S: max 20; N: max. 100ppm; Σ(Ti+Nb+V): up to 200 in total; Optionally, the following elements: Cr: Max 100; Ni: max 100; B: Maximum 5ppm; Ca: max 10; Cu: max 100; Mo:max 100; Sn: max 50; Fe and unavoidable impurities: balance and the hot rolling conditions are fine-tuned to obtain a SEP1941 waviness value Wsa of 0.30 μm or less on the steel surface of a finished shaped article made from a blank made from the steel sheet.

Description

[Technical field]

[0001] The present invention relates to a hot rolled steel sheet. In a further aspect, the present invention relates to a method for producing a hot rolled steel sheet. [Background technology]

[0002] The properties of steel sheets required for various parts of the car body, with or without deformation, can be represented by waviness parameters. There is a need in the automotive industry for steel sheets or strips that have high formability and low waviness values ​​after forming into final shaped products. Usually, they are suitable for manufacturing car body parts, such as automotive exterior panels. Due to their high formability, interstitial free steel grades can be the steel grades used for such automotive exterior panels. Furthermore, improving the paint bake cycle of vehicles is important in the automotive industry. This can achieve excellent paint appearance, improve the global environment, and reduce the process costs for the paint bake cycle. One of the problems in achieving excellent paint appearance is to reduce the paint layer thickness as desired while maintaining or improving good paint appearance. This can be achieved if the surface of the steel sheet under the paint is of good surface quality. Therefore, there is a need in the automotive industry to develop steel sheets that not only have good surface properties, such as low waviness values ​​after forming, but also have excellent formability.

[0003] It should be noted that the terms steel sheet and steel strip can in most cases be used interchangeably, and in this specification a steel sheet is defined as a part or former part of a steel strip. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a steel sheet having good formability.

[0005] It is also an object of the present invention to provide a steel sheet having good surface properties, such as low waviness values ​​after forming.

[0006] Another object of the present invention is to provide an optionally metal coated (e.g. galvanized) steel sheet that allows the realization of fully finished quality steel products with low waviness values ​​for good appearance.

[0007] Another object of the present invention is to enable the manufacture of automotive exterior panels that exhibit little waviness variation due to different degrees of deformation at different locations within the panel. [Means for solving the problem]

[0008] The prior art focuses on improving metal coating processes (e.g. hot dip galvanizing processes) and skin pass rolling. The object of the present invention is to improve hot rolled steel sheets so that after further processing (e.g. cold rolling, annealing and galvanizing) the outer surface of the resulting steel sheet, blank or panel has an optimal surface.

[0009] The present invention seeks to provide a robust solution for improving formability and for having good surface properties, such as low post-molding waviness values ​​and low waviness variation due to molding of the final product, such as an automobile body panel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] As used herein, millipercent or ppm means millipercent by weight or ppm by weight, respectively.

[0011] In a first aspect of the present invention, The steel sheet is in a hot rolled state, The steels shall have the following composition in millipercent, or ppm where indicated: C: 1-50 ppm; Mn: max 200; Si: max 100; Al: 10-200; Ti: 46-100; Nb: max 100; V: Max 100; P: max 20; S: max 20; N: max. 100ppm; Σ(Ti+Nb+V): up to 200 in total; Optionally, the following elements: Cr: Max 100; Ni: max 100; B: Maximum 5ppm; Ca: max 10; Cu: max 100; Mo:max 100; Sn: max 50; Fe and unavoidable impurities: balance having In order to achieve a SEP 1941 waviness value (Wsa) of 0.32 μm or less, preferably 0.29 μm or less, more preferably 0.28 μm or less in the final formed product made from the cold rolled, annealed and optionally metal coated hot rolled steel sheet, the composition of the steel sheet and the hot rolling conditions are: HRT=0.752-0.351P+0.004G+0.026T<0.10 (Formula A) [In the formula, HRT is a parameter related to the microstructure, texture and final thickness of steel in the hot rolled state; P is the texture component determined using electron backscatter diffraction (EBSD) on a specimen representing a cross section of the steel sheet parallel to the rolling direction with a length of at least 1.5 mm through the thickness (112). <110> and (554) <225> is the total volume fraction of , and to calculate the volume fraction, the texture coefficient determined by EBSD is used, G is the average grain size (μm) of the steel in the hot-rolled condition based on the EBSD scan area; T is the thickness of the hot-rolled steel sheet (mm). The present invention is characterized in that the above-mentioned is fine-tuned to satisfy the following: Delta waviness is defined as (Wsa cup - Wsa flat), where Wsa cup is the waviness of a cup made from a cold rolled, annealed, optionally metal coated hot rolled steel sheet by equibiaxial deformation of 4.5% using a Marciniak tool, and Wsa flat is the waviness of a cold rolled, annealed, optionally metal coated hot rolled steel sheet; The finishing temperature is higher than the Ar3 transformation temperature. (112) and (554) are related to the normal direction of the hot-rolled steel plate, <110> and <225> relates to the rolling direction of the hot-rolled steel sheet.

[0012] P is the texture component (112) <110> and (554) <225> is the total volume fraction of <110> and <225> is the direction relative to the rolling direction. (112) and (554) are relative to the normal direction. The volume fraction P is the ratio of the two directions; (112) <110> and (554) <225> The volume fraction of each component is expressed as a dimensionless value. An angular spread of 11 degrees is used in the calculation of the volume fraction. Due to thickness gradients of microstructure and texture, the volume fractions measured at different thickness locations may differ. All measurements taken here are throughout the thickness. The texture is determined using electron backscatter diffraction (EBSD) on samples representing a cross section of the plate parallel to the rolling direction with a length of at least 1.5 mm in the thickness direction, and the texture coefficients determined by EBSD are used to calculate the volume fraction. The average grain size (μm) of the steel in the hot rolled condition is based on the EBSD scan area. The intermediate stages after hot rolling are cold rolling and skin pass rolling, and the surface properties after the final cold rolling and skin pass rolling are important to achieve the desired waviness values. During the cold rolling part, the steel is cold rolled and the roughness Ra of the work roll of the last stand of the cold rolling mill (abbreviated as Ra2.5 in this specification) measured at a cut-off threshold of 2.5 mm is less than 4.5 μm but more than 0.6 μm. The skin pass rolling operation can be performed using electric discharge textured work rolls (EDT) whose work surface has a roughness Ra2.5 of 2.0 μm to 3.5 μm, preferably 1.8 μm to 3.5 μm, more preferably 1.8 μm to 2.5 μm. The elongation of the metal sheet during the skin pass rolling operation is 0.5% to 2%.

[0013] One of the objectives of the present invention is to enable the production of steel strips, sheets or blanks with low waviness values ​​in the cold rolled and annealed state or in the cold rolled, annealed and metal coated state, the waviness value being expressed as Wsa(1-5) according to the standard SEP1941:2012, "Measurement of the waviness characteristic value Wsa(1-5) on cold rolled metallic flat products", hereinafter abbreviated as "Wsa". The low waviness value Wsa of such steel strips, sheets or blanks allows a reduction in the thickness of the paint layer used to obtain a given quality of paint appearance or allows an improvement in the quality of the paint appearance when the paint layer is of a certain thickness. It is noted that in SEP1941, with regard to the change in the waviness parameters due to the forming operation, the description is stated to be valid for the case of non-deformed sheet metal. In one aspect of the present invention, it is this change in waviness value that is defined herein as the delta waviness value, which it is sought to minimize. It has been found that when the features of the invention claimed above are observed, an optimal steel sheet in the hot-rolled state is realized and can be advantageously used to manufacture cold-rolled, optionally metal-coated steel sheets, for example for the manufacture of automobile body parts. The Wsa in the final formed product is then 0.30 μm or less, preferably 0.29 μm or less, more preferably 0.28 μm or less, and / or the delta waviness value is then 0.1 μm or less, preferably 0.08 μm or less, more preferably 0.06 μm or less. For a given thickness T, the appropriate P or G needs to be selected to fit equation A (herein also referred to as Eqn(A)).

[0014] In a second embodiment of the invention, the steel sheet contains at least one of the following elements in the steel, in millipercentages or, where indicated, in ppm, within the following ranges: C: 1-30 ppm; Mn: 10 to 200, preferably 40 to 180; Si: 1 to 50, preferably 2 to 15; Al: 10-100; Ti: 46-95, preferably 46-90; Nb: up to 90, more preferably up to 10; V: maximum 90, preferably maximum 50, more preferably maximum 10; P: max 15; S: max 15; N: max 80, preferably max 60 ppm; Σ(Ti+Nb+V): Total up to 100, preferably up to 90 exists in Optionally, at least one of the following elements is present in the following ranges, in millipercent or, where indicated, in ppm: Cr: Max 60; Ni: up to 60; B: Maximum 4ppm; Ca: max 5; Cu: up to 60; Mo:max 60; Sn: Max 30 It exists in.

[0015] In one embodiment of the present invention, the value of Σ(Ti+Nb+V) is a total of up to 100, preferably up to 90. The advantage of having such a value is to avoid clogging during continuous casting.

[0016] In an alternative embodiment of the invention, the steel plate contains at least one of the following elements in the steel in the following ranges, in millipercentages or, where indicated, in ppm: C: 1-22 ppm; Mn: 10-150; Si: 1-13; Al: 20-80; Ti: 46-70; P: 1-13; S: 1-13; N: 10-60 ppm exists in Optionally, at least one of the following elements is present in the following ranges, in millipercent or, where indicated, in ppm: Nb: max 3; V: max 5; Cr: max 50; Ni: max 50; B: Maximum 3ppm; Ca: max 2; Cu: max 50; Mo:max 40; Sn: max 20 It exists in.

[0017] In another embodiment of the invention, the steel sheet contains at least one of the following elements in the steel in the following ranges, in millipercentages or, where indicated, in ppm: C: 1-21 ppm; Mn: 40-130; Si: 2-13; Al: 30-70; Ti: 50-70; P: 2 to 13, preferably 1 to 5; S: 3-13; N: 10-40 ppm exists in Optionally, at least one of the following elements is present in the following ranges, in millipercent or, where indicated, in ppm: Nb: max 2; V: max 4; Cr: max 40; Ni: up to 40; B: Maximum 2ppm; Ca: max 1; Cu: up to 40; Mo:max 20; Sn: Max 10 It exists in.

[0018] In a further aspect, the present invention provides a method for producing a method for the treatment of a cancer HRT=0.752-0.351P+0.004G+0.026T<0.09 The present invention relates to a steel sheet characterized by the above.

[0019] If the HRT value is smaller, the waviness and / or delta waviness of the resulting product will also be smaller. A further embodiment of the present invention relates to a steel sheet having a G of 22 μm or less, preferably 20 μm or less. If the value of G is smaller, the value of HRT will be smaller. This allows a fine grain size, which is favorable for the final product, to be achieved.

[0020] Further aspects of the invention relate to methods for manufacturing said steel sheets for each of the compositions described herein. Manufacturing methods for producing products from hot rolled steel sheets according to embodiments of the invention may include steps after hot rolling such as pickling, cold rolling, annealing, galvanizing and temper or skin pass rolling. Steels having compositions specified according to embodiments of the invention are refined, for example in a converter, and formed into slabs, such as by a continuous casting process.

[0021] The slabs used are preferably produced by a continuous casting process to prevent macrosegregation of the components. The slabs used can be produced by an ingot production method or a thin slab casting process. Alternatively, in addition to the conventional method in which the slabs are produced and then first cooled to room temperature and then heated again, energy-saving processes, such as hot direct rolling or direct rolling, may be applied. Energy-saving processes may include placing the slabs in a heating furnace while maintaining the slab temperature without cooling to room temperature, or performing rolling immediately after a short temperature hold.

[0022] The slab used in the hot rolling process may be heated. In the heating, the slab heating temperature is preferably as low as possible to save energy. However, if the heating temperature is lowered below 1150°C, the carbides do not dissolve sufficiently. From the viewpoint of increasing the amount of reduction due to the increase in the oxidation weight, the slab heating temperature is preferably 1250°C or less.

[0023] In hot rolling, the slab is rolled at a hot rolling finishing temperature that is equal to or higher than the Ar3 transformation temperature, and then cooled at an average cooling rate of 30°C / s or higher and coiled. Here, the Ar3 temperature is the temperature at which the ferrite transformation starts during cooling. If the hot rolling finishing temperature is reduced below the Ar3 temperature, both α and γ phases will be generated during rolling, resulting in larger ferrite grain size and less uniform grains in the thickness direction. Therefore, the hot rolling finishing temperature is equal to or higher than the Ar3 temperature.

[0024] The maximum hot rolling finishing temperature is preferably 970°C. In one embodiment of the present invention, the method comprises hot rolling the steel sheet, the hot rolling finishing temperature being less than 960°C. Thus, the method comprises hot rolling the steel sheet, the hot rolling finishing temperature being less than 960°C, the finishing temperature being higher than the Ar3 transformation temperature. The maximum hot rolling finishing temperature is preferably Ar3+70°C. A further embodiment of the present invention relates to a method for producing the above mentioned steel sheet, the hot rolling finishing temperature being less than 945°C.

[0025] Due to the temperature gradient in the thickness direction of the hot-rolled plate in the final hot-rolling process, it is recommended to reduce the preferred hot-rolling finishing temperature when the gauge thickness of the hot-rolled plate is thicker. For example, for a gauge thickness of 3.5 mm, a finishing temperature of Ar3+50°C is selected, and for a gauge thickness of 4.7 mm, a finishing temperature of Ar3+30°C is preferred.

[0026] A further embodiment of the present invention relates to a method for producing the above mentioned steel sheet, wherein the reduction in the last stand of the hot rolling finisher is more than 15%, preferably more than 20%. With increasing reduction, the grain size G becomes finer and the texture content P increases.

[0027] In a further embodiment, the present invention relates to a method for producing a steel sheet as described above, wherein the steel sheet is cold rolled and has a roughness Ra of the work rolls of the last stand of the cold rolling mill, measured at a cut-off threshold of 2.5 mm (abbreviated herein as Ra2.5), less than 4.5 μm but more than 0.6 μm.

[0028] The total cold rolling reduction during cold rolling is generally between 50% and 85%, which results in a substrate having a thickness of, for example, 0.2 mm to 2 mm. In some embodiments, an annealing step may be performed. The cold rolled substrate is then subjected to annealing, which is carried out in a conventional manner in an annealing furnace under a reducing atmosphere, with the aim of recrystallization after the work hardening suffered during the cold rolling operation. The annealing step consists in heating the cold rolled steel sheet to a temperature between 650°C and 900°C. The annealing step is preferably carried out at 780°C to 820°C. After the recrystallization annealing, the steel sheet is cooled to a temperature close to the bath temperature. After entering the bath, the two faces of the steel sheet or substrate are metal-coated by Zn-based plating. The coating weight per surface is between 35 and 45 g / m 2 The metal plate is then wiped by nozzles that emit wiping gas and are installed on both sides of the metal plate. The wiping gas is ejected from each nozzle along a direction that is horizontal and perpendicular to the metal plate. The running speed of the substrate in front of the nozzles in the production line is 80 m / min to 160 m / min. This may be preferably more than 100 m / min, or even more than 120 m / min.

[0029] The nozzle outlet is generally located at a distance of 6 mm to 12 mm from the metal sheet along the main injection direction. The outlet generally appears as a rectangular slot, extending perpendicular to the running direction of the steel sheet substrate and over a width at least equal to the width of the metal sheet. The distance between the nozzle outlet and the metal sheet is preferably equal to or less than 10 mm, more preferably less than 8 mm.

[0030] Once the metal plated steel sheet has completely cooled, the metal sheet may be subjected to a temper rolling or tension leveling operation to impart a texture to the outer surface of the metal coating. A transferred surface texture to the outer surface of the metal coating, having sufficient roughness, will enable the metal sheet to retain a sufficient amount of oil that has been applied to the metal sheet to ensure that the forming process will proceed properly. Another purpose of this transferred surface texture is to provide the metal sheet with the desired low waviness characteristics.

[0031] The skin pass rolling operation may be performed using an electric discharge textured work roll (EDT) with a work surface having a roughness Ra2.5 of 2.0 μm to 3.5 μm, preferably 1.8 μm to 3.5 μm, more preferably 1.8 μm to 2.5 μm. The elongation of the metal sheet during the skin pass rolling operation is 0.5% to 2%. All steel sheets are given a waviness value Wsa of less than 0.35 μm, preferably less than 0.30 μm. The optionally temper rolled or skin pass rolled steel sheet may be cut and subjected to a forming process (e.g., a forming process by drawing, drawing or bending) to form a part, and then a paint may be applied on each coating of the part. After the deformation process, the outer surface of the metal coating of the part has a waviness value Wsa of 0.32 μm or less, or 0.30 μm or less, or 0.28 μm or less. This waviness value may be measured, for example, after equibiaxial deformation of 4.5% in the rolling direction and transverse direction of the sheet surface, or approximately 9% through the thickness.

[0032] In a further embodiment, the present invention relates to a method for producing the above mentioned steel sheet, wherein the steel is hot-dip galvanized and temper rolled, and the roughness Ra2.5 of the work rolls of the last stand of the temper mill is between 1.0 μm and 5.0 μm. In a further aspect, the present invention relates to a steel sheet with a hot-rolled sheet gauge of between 3 mm and 5 mm thickness. EXAMPLES

[0033] The present invention is further illustrated by the following non-limiting examples: Steel having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was refined in a vacuum melting furnace to obtain a slab material having a thickness of 225 mm.

[0034] [Table 1]

[0035] In the embodiment, the slab thickness is about 225 mm, the transfer gauge is 35 mm to 40 mm, and the final gauge thickness of the hot rolled steel strip is 3 mm to 5 mm. The average target cooling rate after finish rolling is at least 30°C / s so that the microstructure of the hot rolled steel sheet is more uniform. The coiling temperature is below 750°C but above 550°C. Thus, the microstructure of the hot rolled steel sheet is more uniform and the burn-down caused by oxidation is reduced.

[0036] In all experiments, a pickling step is carried out. The pickling step is a step of removing oxide scale on the surface of the hot-rolled steel sheet obtained in the hot rolling step by carrying out pickling. The cold rolling step is a step of cold rolling the pickled sheet after the pickling step. In some experiments, at least the final cold rolling pass is carried out using a relatively smooth work roll. In such a case, for reliable deformation, the surface of the conditioned non-etched roll of the rolling mill, which is in direct contact with the steel sheet, has a roughness Ra2.5 of less than 0.5 μm. The target value of the waviness value Wsa(1-5) of the cold-rolled steel sheet is less than 0.40 μm. In the examples listed below, all steel sheets are cold rolled with such surface characteristics. In the examples listed below, the roughness Ra2.5 of the work surface of the EDT work roll for temper rolling is 2.50 μm to 3.0 μm. The elongation during the temper rolling operation is between 1.0% and 1.5%. Hot rolling was carried out using the process conditions shown in Table 2. The remaining processes (pickling, cold rolling, annealing, galvanizing and temper rolling) were carried out as described in the previous sections. The waviness values ​​after temper rolling and after the subsequent biaxial stretching at 4.5% strain using a Marciniak tool are also shown in Table 2. In Examples 7 to 15, the Ra2.5,CR (surface roughness of the work roll used for cold rolling CR in the final stand of the cold rolling mill) is 0.5 μm and the Ra2.5,TR (surface roughness of the work roll used for temper rolling TR in the temper rolling mill) is 2.8 μm.

[0037] [Table 2]

[0038] Depending on whether the microstructure and texture satisfy the HRT criteria defined by the present invention, the waviness value Wsa after biaxial stretching is low or high, resulting in a poor (B), good (G) or excellent (E) surface appearance, as shown in Table 2. The waviness value Wsa of the steel sheet surface is measured after the skin-pass rolling operation (flat) and after stretching (cup). The latter is performed by equibiaxial deformation of 4.5% using a Marciniak tool. The results of the Wsa measurements are summarized in Table 2. The use of a parameter HRT satisfying formula A makes it possible to achieve the above-mentioned waviness values ​​Wsa after skin-pass rolling and equibiaxial deformation of 0.30 μm or less. As shown in Table 2, if the hot band microstructure and texture do not satisfy the HRT criteria, the waviness values ​​become considerably large. For large values ​​of hot rolling T, low values ​​of finishing temperature can be selected. For samples 7, 14 and 15, the hot rolling T has large values. In such a case, the finishing temperature may be reduced to a lower value so as to comply with Eqn(A). For sample 8, the hot rolled T has a large value (4.69 mm), but a low finishing temperature such as 921°C is selected, so that Eqn(A) is complied with. The waviness value Wsa referred to here is as defined in the standard SEP1941:2012. The present invention has been described above with reference to some exemplary embodiments. Modifications and alternative embodiments of some parts or elements are possible and fall within the scope of protection defined in the appended claims.

Claims

1. A hot-rolled steel sheet in a hot-rolled state, wherein the steel has the following composition in millimeters or, if indicated, in ppm: C: 1 to 50 ppm; Mn: maximum 200; Si: maximum 100; Al: 10 to 200; Ti: 46 to 100; Nb: maximum 100; V: maximum 100; P: maximum 20; S: maximum 20; N: maximum 100 ppm; Σ(Ti + Nb + V): maximum 200 in total; Optionally, the following elements: Cr: maximum 100; Ni: maximum 100; B: maximum 5 ppm; Ca: maximum 10; Cu: maximum 100; Mo: maximum 100; Sn: maximum 50; Fe and unavoidable impurities: the balance having, in a final formed product made from a hot-rolled steel sheet cold-rolled, annealed, and optionally metal-coated, to achieve a SEP1941 waviness value (Wsa) of 0.32 μm or less, preferably 0.29 μm or less, more preferably 0.28 μm, or to achieve a delta waviness value of 0.1 μm or less, preferably 0.08 μm or less, more preferably 0.06 μm or less, the composition and hot-rolling conditions of the steel sheet are HRT = 0.752 - 0.351P + 0.004G + 0.026T < 0.10 wherein, HRT is a parameter related to the microstructure, texture, and final thickness of the steel in the hot-rolled state, P is the total volume fraction of the texture components (112)<110> and (554)<225> determined using the electron backscatter diffraction method (EBSD) for a sample representing a cross-section of the steel sheet parallel to the rolling direction and having a length of at least 1.5 mm in the thickness direction. The texture coefficients determined by EBSD are used to calculate the volume fraction, G is the average grain size (μm) of the steel in the hot-rolled state based on the EBSD scan area, T is the final thickness (mm) of the hot-rolled steel sheet. ]] characterized in that it is finely adjusted to satisfy The delta waviness value is defined as (Wsa cup - Wsa flat), where Wsa cup is the waviness value of a cup made by 4.5% equibiaxial deformation using a Marciniak tool from a hot-rolled steel sheet cold-rolled, annealed, and optionally metal-coated, and Wsa flat is the waviness value of a hot-rolled steel sheet cold-rolled, annealed, and optionally metal-coated, The finishing temperature is higher than the Ar3 transformation temperature, A steel sheet in which (112) and (554) are with respect to the normal direction of the sheet surface of the hot-rolled steel sheet, and <110> and <225> are with respect to the rolling direction of the hot-rolled steel sheet.

2. In the steel, at least one of the following elements is in the following ranges in weight percent or, if indicated, in ppm: C: 1 to 30 ppm; Mn: 10 to 200, preferably 40 to 180; Si: 1 to 50, preferably 2 to 15; Al: 10 to 100; Ti: 46 to 95, preferably 46 to 90; Nb: at most 90, more preferably at most 10; V: at most 90, preferably at most 50, more preferably at most 10; P: at most 15; S: at most 15; N: at most 80, preferably at most 60 ppm; Σ(Ti + Nb + V): in total at most 100, preferably at most 90 is present, Optionally, at least one of the following elements is in the following ranges in weight percent or, if indicated, in ppm: Cr: at most 60; Ni: at most 60; B: at most 4 ppm; Ca: at most 5; Cu: at most 60; Mo: at most 60; Sn: at most 30 is present, the steel sheet according to claim 1.

3. In the steel, at least one of the following elements is in the following ranges in weight percent or, if indicated, in ppm: C: 1 to 22 ppm; Mn: 10 to 150; Si: 1 to 13; Al: 20 to 80; Ti: 46 to 70; P: 1 to 13; S: 1 to 13; N: 10 to 60 ppm is present, Optionally, at least one of the following elements is in the following ranges in weight percent or, if indicated, in ppm: Nb: at most 3; V: at most 5; Cr: at most 50; Ni: at most 50; B: at most 3 ppm; Ca: at most 2; Cu: at most 50; Mo: at most 40; Sn: at most 20 is present, the steel sheet according to claim 1.

4. In the steel, at least one of the following elements is in the following ranges in weight percent or, if indicated, in ppm: C: 1 to 21 ppm; Mn: 40 to 130; Si: 2 to 13; Al: 30 to 70; Ti: 50 to 70; P: 2 to 13, preferably 1 to 5; S: 3 to 13; N: 10 to 40 ppm is present, Optionally, at least one of the following elements is in the following ranges in weight percent or, if indicated, in ppm: Nb: at most 2; V: at most 4; Cr: at most 40; Ni: at most 40; B: at most 2 ppm; Ca: at most 1; Cu: at most 40; Mo: at most 20; Sn: at most 10 is present, the steel sheet according to claim 1.

5. HRT = 0.752 - 0.351P + 0.004G + 0.026T < 0.09 The steel sheet according to claim 1, characterized in that the above is satisfied.

6. The steel sheet according to claim 1, wherein G is 22 μm or less, preferably 20 μm or less.

7. The steel sheet according to claim 1, wherein the gauge of the hot-rolled steel sheet is 3 mm to 5 mm thick.

8. A method for manufacturing the steel sheet according to any one of claims 1 to 7, including a step of hot-rolling the steel sheet, wherein the finishing temperature of the hot rolling is less than 960 °C and higher than the Ar3 transformation temperature.

9. The method according to claim 8, wherein the finishing temperature of the hot rolling is less than 945 °C.

10. The method according to claim 8, wherein the reduction ratio in the final stand of the hot-rolling finishing mill is more than 15%, preferably more than 20%.

11. The method according to claim 8, wherein the steel sheet is cold-rolled and the roughness Ra2.5 of the work roll in the final stand of the cold rolling mill is less than 4.5 μm.

12. The method according to claim 8, wherein the steel sheet is hot-dip galvanized and temper-rolled, and the roughness Ra2.5 of the work roll in the final stand of the temper rolling mill is 1.0 μm to 5.0 μm.