Steel plate and its manufacturing method

Abstract

The steel sheet of the present invention contains, by weight %, C: 0.008~0.015%, Si: 0.200% or less, Mn: 1.30~2.00%, Cr: 0.5~1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.0020~0.0080%, Al: 0.010~0.060%, with the remainder being Fe and unavoidable impurities. The K value, as defined by the following relational formula 1, is 0~15.000. The microstructure may contain, by area %, 0.30~0.80% of the transformed structure and the remainder being ferrite. [Relationship 1] K=-651[C]-2.42[Mn]+25.7[Cr]-220[N] (In the formula, [C], [Mn], [Cr], and [N] are weight percent of each element.)

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet excellent in low-temperature baking hardenability and room-temperature aging resistance, and a method for manufacturing the same.

Background Art

[0002] In recent years, for the purpose of weight reduction to improve the fuel efficiency of automobiles, a reduction in thickness due to the increase in the strength of steel sheets has been continuously demanded. Bake hardening steel is a material for outer panels and is known as the most suitable steel material for such characteristics. The bake hardening phenomenon is a phenomenon in which, when painting and baking occurs at dislocations generated during pressing, activated dissolved carbon and nitrogen are fixed and the yield strength increases. Steel with excellent bake hardenability is easy to form before painting and baking, and in the final product, dent resistance is improved, and formability and high strength can be realized simultaneously.

[0003] However, since bake hardening steel may cause aging deterioration such as yield point elongation when held at room temperature for a long time due to dissolved elements in the steel, it is required to have a certain level of room-temperature aging resistance so that it can be guaranteed against aging for a certain period or more.

[0004] Generally, as a method for manufacturing a cold-rolled steel sheet having bake hardenability, a low-carbon P-added Al-killed steel is simply wound at a low temperature, that is, a steel having a bake hardening amount of about 40 to 50 MPa is mainly used by the box annealing method using low-temperature winding in a temperature range of 400 to 500 °C for the hot-rolled coiling temperature. This is because box annealing makes it easier to achieve both formability and bake hardenability. In the case of P-added Al-killed steel by the continuous annealing method, since a relatively fast cooling rate is used, it is easy to ensure bake hardenability, but there is a problem that formability deteriorates due to rapid heating and short-time annealing, and it is limited to only automotive outer panels that do not require workability.

[0005] In recent years, supported by the dramatic advancements in steelmaking technology, it has become possible to control the appropriate amount of solid solution elements in steel. By using Al-killed steel sheets to which strong carbonitride-forming elements such as Ti or Nb are added, bake-hardening cold-rolled steel sheets with excellent formability are being manufactured, and their use in automotive body panels, where dent resistance is required, is steadily increasing.

[0006] Recently, some automakers have been considering lowering the curing temperature after painting as a way to reduce costs and CO2 emissions. In particular, there is a growing trend among automakers to use non-ferrous lightweight materials such as aluminum or plastic (or CFRP) instead of steel for exterior panels in order to lighten automobiles. Until now, steel and non-ferrous materials have been pressed separately, then painted and baked, and finally assembled into parts of both materials. However, in order to improve the efficiency of the production process, reduce energy costs, and protect the environment, there is a growing attempt to assemble steel and non-ferrous materials after pressing and then bake them at the same temperature. At this time, there is a growing demand to lower the baking temperature to below 120°C, or even to 100°C, taking into account the curing temperature of non-ferrous materials such as plastics. Such low-temperature baking treatments lead to a sharp decrease in the bake hardness value (BH value) that can be obtained at the conventional 170°C, resulting in the problem that the proper dent resistance, which is the purpose of using bake hardened steel, cannot be ensured.

[0007] Normally, lowering the baking hardening temperature of steel sheets delays the amount and time required for the fixation of dissolved carbon and nitrogen, thus reducing the baking hardening properties. For the baking hardening phenomenon, it is common to add a few percent of pre-strain and then perform a heat treatment at 170°C for 20 minutes after painting, requiring a minimum hardening amount of 30 MPa or more. Therefore, in order to lower the baking hardening temperature while ensuring a baking hardening property above an appropriate level, it is necessary to maximize the baking hardening property at a high temperature. However, if the baking hardening property of the steel sheet increases beyond a certain point, the aging resistance of the steel sheet deteriorates, and the possibility of surface defects occurring during parts processing increases. Therefore, the most desirable phenomenon is to control the difference between the amount of baking hardening obtained from a heat treatment at 170°C, which is the normal baking treatment, and the BH value obtained from a low temperature, i.e., a baking heat treatment at 100°C, to a low level. However, considering that the dissolved elements in steel depend exponentially with temperature, it is very difficult to reduce the difference between the low temperature of 100°C and the BH value of 170°C.

[0008] Various methods have been proposed to ensure proper bake-hardening of steel at low temperatures, and simultaneously to ensure superior bake-hardening properties and corresponding aging resistance under normal conditions of 170°C.

[0009] For example, Patent Documents 1 to 3 propose a method to suppress deterioration during room-temperature aging by increasing the rolling ratio in temper rolling.

[0010] However, cold-rolled steel sheets manufactured using this technology aim to improve aging resistance by introducing movable dislocations into the steel sheet, which results in a very large amount of deformation required to achieve high BH (Back-Heat) properties. Increasing the amount of deformation requires increasing the rolling ratio of the temper rolling process, but there is a limit to the temper rolling ratio that can be applied to high-tensile steel sheets, making continuous line production virtually impossible.

[0011] On the other hand, Patent Document 4 presents a method for controlling the distribution of iron carbide precipitates as a technique to improve low-temperature bake-hardening properties. Such a technique uses precipitation strengthening at low temperatures to achieve hardening, and it describes a different method from the technique of improving yield strength obtained by solid solution carbon and nitrogen and dislocation fixation, which is necessary for dent resistance. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 7-75803 [Patent Document 2] Japanese Patent Publication No. 2001-140038 [Patent Document 3] Japanese Patent Publication No. 2001-200337 [Patent Document 4] Japanese Patent Application Publication No. 6-73498 [Overview of the project] [Problems that the invention aims to solve]

[0013] One embodiment of the present invention aims to provide a steel sheet and a method for manufacturing the same.

[0014] One embodiment of the present invention aims to provide a steel sheet with excellent low-temperature bake-hardening properties and room-temperature aging resistance, as well as a method for manufacturing the same.

[0015] The problems that the present invention addresses are not limited to those described above. A person of ordinary skill should have no difficulty understanding further problems that the present invention addresses from the general content of this specification. [Means for solving the problem]

[0016] According to an embodiment of the present invention, by weight%, C: 0.008 to 0.015%, Si: 0.200% or less, Mn: 1.30 to 2.00%, Cr: 0.5 to 1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.0020 to 0.0080%, Al: 0.010 to 0.060%, the balance being Fe and inevitable impurities, the K value defined by the following relational expression 1 is 0 to 15.000, it is possible to provide a steel sheet having a transformation structure of 0.30 to 0.80% and the balance ferrite in terms of area%.

[0017] [Relational Expression 1] K = -651[C] - 2.42[Mn] + 25.7[Cr] - 220[N] (In the formula, [C], [Mn], [Cr] and [N] are the weight% of each element.)

[0018] The above transformation structure can include martensite, bainite, and ferritic bainite.

[0019] The above steel sheet can have a tensile strength of 340 MPa or more, a yield strength of 180 to 250 MPa, and an elongation of 34.0% or more.

[0020] After heat treatment of the above steel sheet at 100 °C for 20 minutes, the amount of age hardening is 30.0 MPa or more, and the difference between the amount of age hardening after heat treatment at 100 °C for 20 minutes and the amount of age hardening after heat treatment at 170 °C for 20 minutes can be 20.0 MPa or less.

[0021] The yield point elongation (AI) of the above steel sheet after heat treatment at 100 °C for 1 hour can be 0.20% or less.

[0022] According to an embodiment of the present invention, a steel slab containing, by weight %, C: 0.008 to 0.015%, Si: 0.200% or less, Mn: 1.30 to 2.00%, Cr: 0.5 to 1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.0020 to 0.0080%, Al: 0.010 to 0.060%, and the balance being Fe and unavoidable impurities, and having a K value defined by the following relational expression 1 of 0 to 15.000, is reheated; The reheated steel slab is subjected to finish hot rolling; The finish hot rolled steel plate is cooled in a temperature range of 550 to 700 °C and then coiled; The coiled steel plate is cold rolled; and The cold rolled steel plate is continuously annealed in a temperature range of 760 to 830 °C; A method for manufacturing a steel plate including the above steps can be provided.

[0023] [Relational Expression 1] K = -651[C] - 2.42[Mn] + 25.7[Cr] - 220[N] (In the formula, [C], [Mn], [Cr], and [N] are the weight % of each element.)

[0024] The reheating step is performed in a temperature range of 1100 to 1250 °C, The finish hot rolling step is performed in a temperature range of 880 °C or higher, The cold rolling step can be performed at a reduction ratio of 60 to 90%.

[0025] Before the cold rolling step, a pickling step can be further included.

[0026] After the continuous annealing step, a plating step of immersing the continuously annealed steel plate in a zinc plating bath in a temperature range of 440 to 500 °C can be further included.

[0027] A step of alloying the plated steel plate in a temperature range of 450 to 540 °C can be further included.

[0028] The process may further include the step of temper-rolling the plated steel sheet using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm at a temper-rolling rate of 0.5 to 1.5%. [Effects of the Invention]

[0029] According to one embodiment of the present invention, a steel sheet and a method for manufacturing the same can be provided.

[0030] According to one embodiment of the present invention, a steel sheet with excellent low-temperature bake-hardening properties and room-temperature aging resistance, and a method for manufacturing the same can be provided.

[0031] According to one embodiment of the present invention, it is possible to provide cold-rolled steel sheets, plated steel sheets, and methods for manufacturing the same, which have excellent low-temperature bake-hardening properties and room-temperature aging resistance, can be used as materials for automobile exterior panels, and have good properties in press forming, deep drawing, and the like.

[0032] According to one embodiment of the present invention, a steel sheet and a method for manufacturing the same can be provided that have a low curing temperature, enabling cost reduction and CO2 reduction, and that can be painted and baked simultaneously with non-ferrous lightweight materials such as plastics which have a low curing temperature due to the low curing temperature, thereby improving the efficiency of the vehicle body manufacturing process. [Modes for carrying out the invention]

[0033] Preferred embodiments of the present invention are described below. Embodiments of the present invention can be modified in various ways, and the scope of the invention should not be construed as being limited to the examples described below. These embodiments are provided to explain the present invention in more detail to those ordinary in the art to which the invention pertains.

[0034] The inventors of this invention aimed to develop a steel material that could achieve proper bake-hardening properties even at low temperatures, for use in conjunction with non-ferrous lightweight materials as exterior body materials for automobiles. At the same time, they sought to ensure a minimum bake-hardening property of 30 MPa or more even when the paint baking temperature was lowered to 100°C, taking into account the curing temperature of materials such as plastics, and thus completed the present invention.

[0035] The present invention will be described in detail below.

[0036] A steel sheet according to one embodiment of the present invention may contain, by weight percent, C: 0.008 to 0.015%, Si: 0.2% or less, Mn: 1.3 to 2.0%, Cr: 0.5 to 1.0%, P: 0.03% or less, S: 0.01% or less, N: 0.002 to 0.008%, Al: 0.01 to 0.06%, with the remainder being Fe and unavoidable impurities.

[0037] The steel composition of the present invention will be described in detail below.

[0038] Unless otherwise specified in this invention, the percentages representing the content of each element are based on weight.

[0039] Carbon (C): 0.008~0.015% Carbon (C) is an interstitial solid solution element that effectively contributes to ensuring the strength of steel. Furthermore, it is an important element for increasing the hardening ability of steel and ensuring the martensite fraction. Therefore, in order to secure the desired amount of transformation structure as intended by this invention, a certain level or higher of carbon (C) must be added. For this reason, the lower limit of the carbon (C) addition can be limited to 0.008%. However, excessive carbon (C) addition can lead to the formation of a transformation structure exceeding the target fraction in the steel of this invention, resulting in increased strength and decreased elongation, and potentially increasing the likelihood of bending defects occurring on the product surface during parts processing at the customer's facility. Therefore, this invention can limit the upper limit of the carbon (C) content to 0.015%. According to one embodiment of the present invention, this upper limit may be 0.014%.

[0040] Silicon (Si): 0.200% or less Silicon (Si) is an element that contributes to increasing the strength of steel through solid solution strengthening. In this invention, the desired physical properties can be secured without adding silicon (Si), so it is not intentionally added. On the other hand, if the silicon (Si) content exceeds a certain level, there is a problem that the surface properties of the plating will deteriorate, so in this invention, the upper limit of the silicon (Si) content can be limited to 0.200%. According to one embodiment of the present invention, the upper limit may be 0.100%.

[0041] Manganese (Mn): 1.30~2.00% Manganese (Mn) is a solid solution strengthening element that not only contributes to increasing the strength of steel but can also play a role in precipitating sulfur (S) in the steel as MnS. In the present invention, along with carbon (C) and chromium (Cr), it can increase the hardening ability of the steel and contribute to securing the fraction of the transformed structure to be obtained in the steel of the present invention. In the present invention, it is preferable to add 1.30% or more of manganese (Mn) in order to secure the minimum fraction of the transformed structure and thereby ensure low-temperature bake hardening and room-temperature aging resistance. According to one embodiment of the present invention, the lower limit of the manganese (Mn) content may be 1.40%. However, if more than 2.00% of manganese (Mn) is added, the fraction of the transformed structure in the steel will deviate from the conditions presented in the present invention, and the BH100 value (baking hardening amount after heat treatment at 100°C for 20 minutes (lower BH value)) may not meet 30 MPa or more. Furthermore, excessive addition of manganese (Mn) can lead to the formation of annealing oxides, potentially causing problems on the surface of plated products, including reduced elongation and degraded workability. According to one embodiment of the present invention, the upper limit may be 1.80%.

[0042] Chromium (Cr): 0.5~1.0% Chromium (Cr) is a solid solution strengthening element and, along with the aforementioned C, Mn, and N, is one of the extremely important elements in the steel of this invention. Chromium (Cr) increases the hardening ability of steel and effectively contributes to the formation of martensite. Furthermore, when chromium (Cr) is added to steel, Cr during hot rolling... 23By forming coarse Cr-based carbides such as C6, the amount of dissolved carbon in the steel is controlled to below an appropriate level, suppressing the occurrence of yield point elongation (YPel), thereby enabling the production of composite structure steel with a low yield ratio. Furthermore, chromium (Cr) is an element that effectively contributes to ensuring the elongation of composite structure steel by minimizing the decrease in elongation rate with respect to strength increase. Therefore, the present invention may require the addition of chromium (Cr) to achieve these effects. If chromium (Cr) is added at a low level, such as less than 0.5%, the hardening ability of the steel will decrease, and it may become impossible to secure a sufficient fraction of the transformed structure desired in the present invention. This leads to a decrease in the BH100 value, and furthermore, the BH170-BH100 value (the difference between the amount of bake hardening after heat treatment at 100°C for 20 minutes and the amount of bake hardening after heat treatment at 170°C for 20 minutes) will not meet the 20 MPa or less requirement specified for the steel of the present invention. In contrast, if the chromium (Cr) content exceeds 1.0%, it excessively increases the proportion of martensite formation, exceeding the conditions for the fraction of the transformed structure presented in this invention. As a result, the BH100 value deteriorates, and the elongation may decrease due to the excessive addition of chromium (Cr). According to one embodiment of the present invention, the upper limit may be 0.9%.

[0043] Phosphorus (P): 0.030% or less Phosphorus (P) is an impurity present in steel and is an element that is inevitably added. It is also the most effective element for ensuring the strength of steel through solid solution strengthening without significantly impairing its drawability. However, if phosphorus (P) is added in excess, the possibility of brittle fracture increases, which can induce slab fracture during hot rolling, and can also significantly degrade the surface properties of plated steel sheets. Therefore, in this invention, the upper limit of phosphorus (P) content can be limited to 0.030%.

[0044] Sulfur (S): 0.010% or less Sulfur (S) is an impurity present in steel and is inevitably added, but in order to ensure excellent welding properties, it is preferable to control its content as low as possible. In particular, since sulfur (S) in steel can induce red-hot brittleness, the present invention allows for limiting the upper limit of sulfur (S) content to 0.010%.

[0045] Nitrogen (N): 0.0020~0.0080% Nitrogen (N) is an impurity present in steel and is an element that inevitably flows in. Therefore, it is preferable to control its content to be as low as possible, but in the present invention, it is utilized as a very important element for ensuring low-temperature bake hardening and room-temperature aging resistance. Nitrogen (N) has a very fast diffusion rate and can induce both bake hardening and aging degradation simultaneously. In order to suppress aging degradation caused by nitrogen (N), it is necessary to ensure a certain fraction of the transformed structure by an appropriate combination of C, Mn, and Cr, and in order to obtain low-temperature bake hardening at 100°C in steel containing such a transformed structure, it may be necessary to add a certain amount or more of nitrogen (N). If the nitrogen (N) content is less than 0.0020%, it may be difficult to obtain bake hardening with a BH100 value of 30 MPa or higher. According to one embodiment of the present invention, the lower limit may be 0.0050%. According to one embodiment of the present invention, the lower limit may be 0.0060%. On the other hand, if the content exceeds 0.0080%, it becomes difficult to simultaneously ensure low-temperature bake-curing properties and room-temperature aging resistance.

[0046] Aluminum (Al): 0.010~0.060% Aluminum (Al) is an ingredient added to steel for grain refinement and deoxidation. The present invention allows for limiting the lower limit of the aluminum (Al) content to 0.010% in order to produce stable Al-killed steel. According to one embodiment of the present invention, the lower limit may be 0.020%. However, if aluminum (Al) is added in excess, although the strength increases due to grain refinement, excessive inclusions may form during continuous casting operations, potentially degrading the surface quality of the steel sheet and increasing manufacturing costs. Therefore, the present invention allows for limiting the upper limit of the aluminum (Al) content to 0.060%. According to one embodiment of the present invention, the upper limit may be 0.050%.

[0047] The steel of the present invention may contain, in addition to the above-described composition, the remaining iron (Fe) and unavoidable impurities. Since unavoidable impurities can be unintentionally introduced during the normal manufacturing process, they cannot be eliminated. Such impurities are known to any engineer in the field of ordinary steelmaking, and therefore, not all of them are specifically mentioned in this specification.

[0048] The steel plate according to one embodiment of the present invention may have a K value of 0 to 15.000 as defined by the following relational expression 1.

[0049] [Relationship 1] K=-651[C]-2.42[Mn]+25.7[Cr]-220[N] (In the formula, [C], [Mn], [Cr], and [N] are weight percent of each element.)

[0050] The inventors have found that the interaction of C, Mn, Cr, and N is extremely important in order to ensure a bake hardening amount of 30 MPa or more at a low temperature of 100°C. In this invention, the aim is to simultaneously ensure low-temperature bake hardening and room-temperature aging resistance, and the microstructure is strictly limited. Therefore, C, Mn, and Cr have been confirmed as necessary components to obtain the desired transformation structure in this invention, and N is a component that can obtain a certain amount of bake hardening even at low temperatures.

[0051] In this invention, a solid solution element with a high diffusion coefficient is required to ensure proper bake hardening at low temperatures. N is presumed to be a very advantageous element for obtaining bake hardening at low temperatures. That is, N diffuses more than 100 times faster than C, making it advantageous for obtaining bake hardening even at low temperatures compared to C. On the other hand, solid solution nitrogen also affects room temperature aging resistance. That is, solid solution nitrogen can easily improve not only bake hardening but also yield point elongation, which indicates aging resistance. In this invention, the problem of deterioration of aging resistance induced by N can be improved by utilizing and controlling the transformation structure formed in the steel. C, Mn, and Cr are advantageous for the formation of the transformation structure, and since there are many mobile dislocations around the transformation structure, the movement of N can be hindered. That is, by appropriately utilizing N, C, Mn, and Cr, aging does not occur at room temperature, and bake hardening (BH property) can be obtained even at low baking temperatures. Therefore, in this invention, relational equation 1 is proposed using C, Mn, Cr, and N.

[0052] If the K value in relational expression 1 is less than 0, the fraction of the transformed structure will fall outside the range of the present invention, and the BH100 value will be low and the BH170-BH100 value will be high, meaning that the difference in BH values ​​depending on the baking temperature will be large. On the other hand, if the K value in relational expression 1 exceeds 15.000, this may, in most cases, include conditions in which the content of C, Mn, Cr, and N falls outside the criteria presented in the present invention. This will fall outside the conditions for the fraction of the transformed structure targeted in the present invention, and the BH100 value may not be satisfied. According to one embodiment of the present invention, the lower limit of the K value may be 1. According to one embodiment of the present invention, the lower limit may be 1.500.

[0053] The microstructure of the steel according to the present invention will be described in detail below.

[0054] In this invention, unless otherwise specified, the percentage representing the fraction of microstructure is based on area.

[0055] The microstructure of the steel sheet according to one embodiment of the present invention may include 0.30 to 0.80% of the transformed structure and the remainder being ferrite, in area percent. According to one embodiment of the present invention, the transformed structure may include martensite, bainite, and ferritic bainite. According to one embodiment of the present invention, the transformed structure may be martensite.

[0056] If a bake hardening performance of 30 MPa or higher is ensured for the BH100 value, a higher bake hardening performance can be obtained for the BH170 value at high-temperature bake temperatures than for the BH100 value. In this case, a problem may arise in which the room-temperature aging resistance deteriorates due to the high content of solid solution elements in the steel. Therefore, the smaller the difference in bake hardening values ​​between the BH170 value and the BH100 value, the more advantageous the aging resistance can be. For this reason, in this invention, we aim to ensure room-temperature aging resistance by limiting the difference in bake hardening performance between the BH170 value and the BH100 value to 20 MPa or less.

[0057] On the other hand, the presence of a certain fraction of transformation structures within the microstructure can improve aging resistance due to the abundant mobile dislocations surrounding the transformation structures. However, an excessive fraction of transformation structures requires an increase in alloying elements to produce them, which may also lead to an increase in strength. Furthermore, it may reduce low-temperature bake hardening properties. Therefore, in this invention, the fraction of transformation structures is limited to 0.30-0.80%.

[0058] If the fraction of transformed tissue is less than 0.30%, sufficient mobile dislocations cannot be obtained, and the BH100 value cannot meet the level proposed in this invention. Furthermore, the added carbon affects not only the BH properties but also the aging properties, and problems may arise such as an increase in the BH170-BH100 value due to an increase in the BH170 value, as well as a deterioration in room-temperature aging resistance. On the other hand, if the fraction of transformed tissue exceeds 0.80%, the aging properties will be excellent due to the excessive transformation tissue, but problems may arise such as a low BH100 value.

[0059] A steel sheet according to one embodiment of the present invention has a tensile strength of 340 MPa or more, a yield strength of 180 to 250 MPa, an elongation of 34.0% or more, a bake hardening amount of 30.0 MPa or more after heat treatment at 100°C for 20 minutes, a difference of 20.0 MPa or less between the bake hardening amount after heat treatment at 100°C for 20 minutes and the bake hardening amount after heat treatment at 170°C for 20 minutes, and a yield point elongation (AI) of 0.20% or less after heat treatment at 100°C for 1 hour, and can have excellent properties of low-temperature bake hardening and room-temperature aging resistance. According to one embodiment of the present invention, the tensile strength may be 400 MPa or less.

[0060] The BH value represents the amount of hardening achieved by baking at a specific temperature. It is the value obtained by measuring the increase in lower yield strength after baking at a specific temperature for a certain period of time, with the flow stress after 2% pre-strain as the baseline.

[0061] The steel manufacturing method of the present invention will be described in detail below.

[0062] A steel sheet according to one embodiment of the present invention can be manufactured by reheating, finishing hot rolling, cooling, winding, cold rolling, and continuous annealing a steel slab that satisfies the above-mentioned alloy composition.

[0063] reheating A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1100 to 1250°C.

[0064] The above reheating step can be performed to facilitate the subsequent hot rolling step and to obtain the desired physical properties of the steel sheet.

[0065] If the reheating temperature is below 1100°C, inclusions in the steel slab may not remelt sufficiently, potentially leading to material deviations and surface defects after hot rolling. On the other hand, if the reheating temperature exceeds 1250°C, excessive growth of austenite grains may reduce strength, and excessive scale formation may degrade the surface quality of the steel sheet.

[0066] Finishing hot rolling The reheated steel slab described above can be finished hot-rolled in a temperature range of 880°C or higher.

[0067] The above-described finish hot rolling, when performed in the austenite single-phase region, can be advantageous from the viewpoint of refining the final microstructure because it forms pancake-like austenite grains and deformation zones.

[0068] If the finish hot rolling temperature is below 880°C, it falls under two-phase rolling of austenite and ferrite, which can induce material non-uniformity and lead to excessive rolling load. Therefore, in the present invention, the finish hot rolling temperature can be limited to 880°C or higher so that hot rolling is completed in the single-phase austenite region. According to one embodiment of the present invention, a more preferable upper limit for the finish hot rolling temperature may be 950°C.

[0069] Cooling and winding The hot-rolled steel sheet described above can be cooled to a temperature range of 550-700°C and then wound into a coil.

[0070] If the winding temperature is below 550°C, the steel sheet shape may be poor, and a large amount of low-temperature transformation phases such as martensite or bainite may be formed, potentially leading to an excessive increase in the strength of the steel sheet. On the other hand, if the temperature exceeds 700°C, coarse ferrite crystal grains are formed, and coarse carbides and nitrides are more likely to form, which may degrade the material properties of the steel. In addition, high winding temperatures increase hot-rolled sheet oxides such as Mn and Si, and even if some oxides remain after the pickling process or are completely removed, concentrated oxides may form on the surface of the steel sheet, which can cause surface defects during plating.

[0071] According to one embodiment of the present invention, a pickling step to remove surface scale can be further performed before cold rolling, which is a post-processing step after winding. The conditions for the above pickling step are not particularly limited and any commonly used conditions can be applied.

[0072] cold rolling The rolled steel sheet described above can be cold-rolled with a reduction ratio of 60-90%.

[0073] During the cold rolling process described above, if the reduction ratio is less than 60%, the recrystallization driving force from cold rolling is insufficient, which can lead to problems such as the inability to complete ferrite recrystallization and the retention of unrecrystallized ferrite structures. On the other hand, if the reduction ratio exceeds 90%, the load on the rolling rolls becomes extremely heavy during the process, which can lead to problems such as deterioration of the steel sheet shape. In particular, cracks may develop at the edges of the steel sheet, causing increased load during cold rolling.

[0074] Continuous annealing The cold-rolled steel sheet described above can be continuously annealed in a temperature range of 760 to 830°C.

[0075] If the continuous annealing temperature is below 760°C, recrystallization may not be completed sufficiently, and an unrecrystallized structure may occur. Furthermore, this corresponds to ferrite single-phase annealing, which has the problem of not being able to secure the fine structure desired in this invention. On the other hand, if the temperature exceeds 830°C, excessive austenite formation may reduce the carbon concentration in the austenite, potentially lowering the stability of the austenite. As a result, reverse transformation back to ferrite is likely to occur during cooling after annealing, or even if some austenite transforms into martensite, the insufficient carbon concentration may result in insufficient hardening ability, making it impossible to expect sufficient bake-hardening properties and room-temperature aging resistance.

[0076] Plating According to one embodiment of the present invention, the continuously annealed steel sheet can be plated by immersing it in a zinc plating bath at a temperature range of 440 to 500°C.

[0077] In this invention, a hot-dip galvanized steel sheet can be manufactured by immersing a continuously annealed cold-rolled steel sheet in a zinc plating bath.

[0078] Alloying treatment Furthermore, according to one embodiment of the present invention, the plated steel sheet can be alloyed in a temperature range of 450 to 540°C.

[0079] If the alloying treatment temperature is below 450°C, unplated areas may occur across the entire width of the annealed steel sheet. On the other hand, if the temperature exceeds 540°C, the powdering properties may deteriorate due to the influence of brittle Fe-Zn intermetallic compounds (Γ) resulting from excessive alloying.

[0080] Temper rolling According to one embodiment of the present invention, temper rolling can be performed on the steel sheet after continuous annealing or on the galvanized steel sheet.

[0081] Specifically, according to one embodiment of the present invention, temper rolling can be performed with a temper rolling rate of 0.5 to 1.5% using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm.

[0082] If the above-mentioned temper rolling ratio is less than 0.5%, sufficient dislocations will not be formed, which is disadvantageous from the viewpoint of sheet shape and may lead to surface defects in the plating. It may also be disadvantageous from the viewpoint of aging resistance. On the other hand, if the rolling ratio exceeds 1.5%, material deterioration due to an excessive increase in dislocation density in the surface layer may occur, and side effects such as sheet fracture may occur due to the limits of the equipment capacity. [Examples]

[0083] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes to illustrate the present invention in more detail and are not intended to limit the scope of the rights of the present invention.

[0084] (Examples) After preparing steel slabs having the alloy composition shown in Table 1, they were reheated, finished hot-rolled, coiled, and cold-rolled under the manufacturing conditions shown in Table 2. The cold-rolled steel sheets were then continuously annealed under the temperature conditions shown in Table 2, cooled by furnace cooling, and subsequently immersed in a molten zinc plating bath maintained at a temperature of approximately 460°C for hot-dip galvanizing. After that, a temper rolling rate of 0.8% was applied to produce the final hot-dip galvanized steel sheets.

[0085] [Table 1]

[0086] [Relationship 1] K=-651[C]-2.42[Mn]+25.7[Cr]-220[N] (In the formula, [C], [Mn], [Cr], and [N] are weight percent of each element.)

[0087] [Table 2]

[0088] For each hot-dip galvanized steel sheet produced, the microstructure was analyzed using an optical microscope at a point 1 / 4 of the sheet thickness after Le Pera etching, and the results are shown in Table 3 below. The transformation structures included martensite, bainite, and ferritic bainite. Other structures observed included ferrite. Furthermore, tensile tests were performed in the rolling direction according to JIS-5 standards to measure the yield strength (YP), tensile strength (TS), and elongation (El) of the galvanized steel sheets, and the results are shown in Table 3 below. Furthermore, for the same specifications, the Lower Yield Stress (L-BH) was measured after 2% pre-straining, baking at 100°C for 20 minutes, and baking at 170°C for 20 minutes. The BH100 value (amount of bake hardening after heat treatment at 100°C for 20 minutes) and the BH170-BH100 value (the difference between the amount of bake hardening after heat treatment at 100°C for 20 minutes and the amount of bake hardening after heat treatment at 170°C for 20 minutes) are shown in Table 3 below. Specifically, the BH value represents the amount of bake hardening at a specific temperature, and is the value measured by measuring the increase in lower yield strength after baking at a specific temperature for a certain period of time, based on the flow stress after 2% pre-straining. Room temperature aging resistance (AI) was shown by measuring the yield point elongation (YPel) that appeared after holding the test specimen in a water bath maintained at 100°C for 1 hour and then performing a tensile test, as described above.

[0089] [Table 3]

[0090] As shown in Table 3 above, in the case of an example of the invention that satisfies the conditions of the present invention, the characteristics of the microstructure proposed in the present invention were met, and the physical properties targeted in the present invention were also secured.

[0091] In contrast, Comparative Example 1 is a case where, although the alloy composition and relational formula 1 satisfy the conditions of the present invention, the annealing temperature did not reach the range of the present invention. As a result, annealing was performed in the ferrite single-phase region, and the transformed phase was not obtained.

[0092] Comparative Example 2 shows a case where the annealing temperature was outside the range of the present invention. Due to insufficient carbon concentration in the austenite, most of the austenite underwent reverse transformation into ferrite during cooling, resulting in a low fraction of the transformed structure. Furthermore, the resulting transformed structure lacked stability, had a very low BH100 value, and exhibited poor aging resistance.

[0093] Comparative Example 3 is the case where the Cr content added was lower than the range proposed in the present invention. Due to the low Cr content, the fraction of the transformed structure was insufficient, the value of relational formula 1 did not satisfy the conditions of the present invention, and the BH100 value was not met. The aging resistance at room temperature also deteriorated.

[0094] Comparative Example 4 is a case where the Mn content exceeded the range proposed in the present invention, and Comparative Example 5 is a case where the Mn content was excessive and the Cr content was insufficient. As a result, excessive transformation of the tissue was formed, and the BH100 value did not meet the conditions of the present invention.

[0095] Comparative Example 6 shows a case where the Mn content did not meet the conditions of the present invention. The low Mn content resulted in insufficient curing ability, and as a result, a sufficient transformed structure could not be obtained. Because almost no transformed structure was formed, sufficient mobile dislocations could not be secured, resulting in high yield strength and low elongation. Furthermore, the BH properties also did not meet the conditions of the present invention.

[0096] Comparative Example 7 showed a case where the N content was excessively high, and it did not meet the BH100 value targeted in the present invention. Furthermore, it exhibited the phenomenon of increased yield strength with the addition of high levels of N.

[0097] Comparative Example 8 is an example with a very low C content, while Comparative Example 9 is an example with an excessive C content. In Comparative Example 8, sufficient curing ability could not be ensured due to the low C content, and as a result, the fraction of the transformed structure did not reach the target level. It could not meet the K value range proposed in the present invention, nor did it meet the BH100 value. In Comparative Example 9, the curing ability was greatly increased by the addition of excessive C, and an excessive amount of transformed structure was formed. As a result, the tensile strength increased and the BH property was suppressed.

[0098] Comparative Example 10 shows the case where the amounts of C, Cr, and N added deviated from the conditions of the present invention. In particular, the high C content resulted in increased curing ability and decreased elongation. Furthermore, the K value in relational formula 1 fell outside the range of the present invention, and the conditions for the presented BH value could not be met.

[0099] Comparative Example 11 is a case where the alloy composition range of the present invention is met, but the conditions of relational formula 1 are not met. As a result, it was not possible to secure the BH100 value at the level targeted by the present invention.

[0100] Comparative Example 12 is a case where the C content is low and the K value in relational formula 1 falls outside the range of the present invention. As a result, the fraction of the transformed tissue is insufficient, and the BH100 value also does not meet the proposed standard.

[0101] Although the present invention has been described in detail above with reference to examples, other forms of examples are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. In weight percent, C: 0.008-0.015%, Si: 0.200% or less, Mn: 1.30-2.00%, Cr: 0.5-1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.0020-0.0080%, Al: 0.010-0.060%, with the remainder being Fe and unavoidable impurities. The K value defined by the following relational equation 1 is between 0 and 15.

000. The microstructure of the steel sheet is such that, in area percent, it contains 0.30 to 0.80% transformed tissue and the remainder is ferrite. [Relationship 1] K=-651[C]-2.42[Mn]+25.7[Cr]-220[N] (In the formula, [C], [Mn], [Cr], and [N] are weight percent of each element.)

2. The steel sheet according to claim 1, wherein the transformed structure comprises martensite, bainite, and ferritic bainite.

3. The steel plate according to claim 1, wherein the steel plate has a tensile strength of 340 MPa or more, a yield strength of 180 to 250 MPa, and an elongation of 34.0% or more.

4. The steel sheet according to claim 1, wherein, after heat treatment at 100°C for 20 minutes, the bake hardening amount is 30.0 MPa or more, and the difference between the bake hardening amount after heat treatment at 100°C for 20 minutes and the bake hardening amount after heat treatment at 170°C for 20 minutes is 20.0 MPa or less.

5. The steel sheet according to claim 1, wherein the steel sheet has a yield strength elongation (AI) of 0.20% or less after heat treatment at 100°C for 1 hour.

6. The step of reheating a steel slab containing, by weight percent, C: 0.008 to 0.015%, Si: 0.200% or less, Mn: 1.30 to 2.00%, Cr: 0.5 to 1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.0020 to 0.0080%, Al: 0.010 to 0.060%, with the remainder being Fe and unavoidable impurities, and having a K value of 0 to 15.000 as defined by the following relational formula 1; The step of hot-rolling the reheated steel slab; The step of winding the finished hot-rolled steel sheet after it has been cooled to a temperature range of 550 to 700°C; The step of cold-rolling the rolled steel sheet; and A method for manufacturing a steel sheet, comprising the step of continuously annealing the cold-rolled steel sheet in a temperature range of 760 to 830°C. [Relationship 1] K=-651[C]-2.42[Mn]+25.7[Cr]-220[N] (In the formula, [C], [Mn], [Cr], and [N] are weight percent of each element.)

7. The aforementioned reheating step is performed in a temperature range of 1100 to 1250°C. The aforementioned finish hot rolling step is carried out in a temperature range of 880°C or higher. The method for manufacturing a steel sheet according to claim 6, wherein the cold rolling step is performed with a reduction ratio of 60 to 90%.

8. The method for manufacturing a steel sheet according to claim 6, further comprising a pickling step before the cold rolling step.

9. The method for manufacturing a steel sheet according to claim 6, further comprising a plating step of immersing the continuously annealed steel sheet in a zinc plating bath at a temperature range of 440 to 500°C after the continuous annealing step.

10. The method for manufacturing a steel sheet according to claim 9, further comprising the step of alloying the plated steel sheet in a temperature range of 450 to 540°C.

11. The method for manufacturing a steel sheet according to claim 9, further comprising the step of temper-rolling the plated steel sheet using a skin-pass roll having a roughness (Ra) of 1.0 to 1.6 μm at a temper-rolling rate of 0.5 to 1.5%.

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