High-strength steel sheet with excellent thermal stability and method for manufacturing the same

Abstract

The present invention relates to a steel plate applicable to automobile chassis parts and the like, and more particularly to a high-strength steel plate having excellent thermal stability and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a steel plate applicable to automobile chassis parts and the like, and more particularly to a high-strength steel plate having excellent thermal stability and a method for manufacturing the same. Background Art

[0002] High-strength hot-rolled steel sheets, traditionally used in automotive chassis and frames, are being reduced in thickness to meet lightweighting requirements while also requiring excellent formability to accommodate component shapes. Furthermore, to maximize component durability, a certain bake hardening value (BH), which indicates the degree of hardening after painting, is required.

[0003] Furthermore, during the manufacturing and use of steel, heat is sometimes applied to part or all of steel sheets and components for various purposes. This heating process can cause changes in the strength of the steel sheets and components, leading to a decrease in durability.

[0004] Generally, when heated, the amount of dissolved carbon in the structure increases, causing clustering at dislocations and grain boundaries, and ultimately forming carbides. Simultaneously, the structure of the steel, including martensite, bainite, and retained austenite, also changes, leading to a dramatic change in the steel's strength and affecting its formability and durability.

[0005] As mentioned above, changes in the structure, physical properties, etc. of steel during heating vary depending on the alloy composition and microstructure of the initial steel and are largely dependent on heat treatment conditions such as heating temperature and holding time. Therefore, until now, technology has focused solely on suppressing strength reduction when heating at high temperatures above 600°C.

[0006] For example, Patent Documents 1 and 2 propose technologies for ensuring high-temperature strength by adding Cr, Mo, Nb, and V and then heat-treating the steel sheet after hot rolling. However, this technology is only suitable for manufacturing thick steel plates for construction. Furthermore, considering the environmental factors in which construction steel inevitably heats up due to fires and other factors, adding large amounts of Cr, Mo, Nb, and V to the steel can ensure a certain level of strength even when exposed to high temperatures above 600°C for long periods of time. However, the use of expensive alloying elements and the required heat treatment processes to ensure the desired physical properties lead to high manufacturing costs. In particular, the disadvantage of excessive thermal stability is present when used in environments exposed to temperatures below 600°C for short periods of time.

[0007] Patent Document 3 discloses a technique for ensuring the strength of the heat-affected zone (HAZ) by adding Ti, Nb, Cr, and Mo. Specifically, this technique involves heating the area adjacent to the weld material melted by the welding heat during arc welding to a high temperature of 600°C or higher. During this high-temperature heating, when heated to temperatures above the austenite zone, the addition of Cr and Mo to the steel increases its hardenability, forming low-temperature phases such as bainite and martensite during subsequent cooling, thereby ensuring strength. However, this technique for maximizing the hardenability of steel sheets has limitations in its application to automotive steel sheets, which require high formability even after heat treatment as required after production.

[0008] (Patent Document 1) Korean Patent Publication No. 1997-0043167

[0009] (Patent Document 2) Korean Patent Publication No. 2013-0002176

[0010] (Patent Document 3) Korean Patent Publication No. 2005-0085873 Summary of the Invention

[0011] Technical problems to be solved

[0012] One aspect of the present invention provides a high-strength steel sheet having excellent physical properties, particularly excellent formability, bake hardenability, and thermal stability, which is required to be suitable for use in automobile chassis parts and the like, and a method for producing the same.

[0013] The technical problem of the present invention is not limited to the above content. The technical problem of the present invention can be understood from the overall content of this specification, and those skilled in the art of the technical field to which the present invention belongs can easily understand the additional technical problems of the present invention.

[0014] Technical Solution

[0015] One aspect of the present invention provides a high-strength steel plate with excellent thermal stability, wherein the steel plate comprises, in terms of weight percent, carbon (C): 0.02-0.08%, silicon (Si): 0.01-0.5%, manganese (Mn): 0.8-1.8%, aluminum (Al): 0.01-0.1%, phosphorus (P): 0.001-0.02%, sulfur (S): 0.001-0.01%, and nitrogen (N): 0.001-0. 01%, titanium (Ti): 0.01-0.12%, niobium (Nb): 0.01-0.05%, molybdenum (Mo): 0.001-0.2%, and the balance Fe and other inevitable impurities, the steel plate satisfies the following Relationship 1 and Relationship 2, as a fine structure, the sum of the area fractions of the ferrite phase and the bainite phase is 90% or more (excluding 100%), and contains the balance of one or more of the martensite phase and the MA phase.

[0016] [Equation 1]

[0017] |K|≤0.85

[0018] (where K = -0.6-0.87[C]+0.03[Si]-0.14[Mn]+0.09[Ti]+0.01[Nb] 2 Each element is expressed as weight content.)

[0019] [Equation 2]

[0020] 5≤A≤20

[0021] (where A = ([Ti] / 48 + [Mo] / 96) × ([Nb] / 93) -1 Each element is expressed as weight content.)

[0022] Another aspect of the present invention provides a method for manufacturing a high-strength steel plate with excellent thermal stability, comprising the following steps: preparing a steel billet satisfying the above-mentioned alloy composition and Relationships 1 and 2; heating the steel billet within a temperature range of 1100-1350°C; hot rolling the heated steel billet within a temperature range of 850-1150°C to manufacture a hot-rolled steel plate; and cooling the hot-rolled steel plate to a temperature range of 400-550°C at an average cooling rate of 10-100°C / second (s) and coiling it.

[0023] Beneficial effects

[0024] According to the present invention, a steel sheet can be provided that has high strength and excellent thermal stability, and thus has excellent strength and bake hardenability even after heat treatment at a relatively low temperature.

[0025] Compared to the relatively high temperature heat treatment required for conventional steel sheets, the steel sheet can be heat-treated at a low temperature, thereby having the effect of expanding the range of applicable uses. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The graph shows the relationship between the |K| value of the alloying element and the strength change (ΔTS) before and after heat treatment and the bake hardenability [ΔTS×BH h -1 ] is a graph of the correlation between .

[0027] Best Practice

[0028] The inventors of the present invention measured the change in room temperature tensile strength after heat treating steels with various alloy compositions and different microstructures in the temperature range of 100-600°C. They confirmed that the change in room temperature tensile strength depends on the slope of the dynamic strength value measured during the heating of the steel.

[0029] Based on this, the present inventors confirmed that as a solution for ensuring excellent thermal stability of steel sheets, by optimizing the content relationship of specific elements and controlling the conditions of the steel sheet manufacturing process, a steel sheet with excellent thermal stability can be provided, thereby completing the present invention.

[0030] Hereinafter, the present invention will be described in detail.

[0031] The high-strength steel sheet with excellent thermal stability according to one aspect of the present invention may contain, in terms of weight%, carbon (C): 0.02-0.08%, silicon (Si): 0.01-0.5%, manganese (Mn): 0.8-1.8%, aluminum (Al): 0.01-0.1%, phosphorus (P): 0.001-0.02%, sulfur (S): 0.001-0.01%, nitrogen (N): 0.001-0.01%, titanium (Ti): 0.01-0.12%, niobium (Nb): 0.01-0.05%, and molybdenum (Mo): 0.001-0.2%.

[0032] Hereinafter, the reasons for limiting the alloy composition of the steel sheet provided by the present invention as described above will be described in detail.

[0033] In addition, unless otherwise specified, the content of each element in the present invention is based on weight, and the proportion of the structure is based on area.

[0034] Carbon (C): 0.02-0.08%

[0035] Carbon (C) is the most economical and effective element for strengthening steel. As the carbon content increases, the precipitation strengthening effect increases or the fraction of the low-temperature microstructure phase increases, thereby inducing an increase in tensile strength.

[0036] When the C content is less than 0.02%, the precipitation strengthening effect and the formation of low-temperature phases are insufficient, making it difficult to ensure the desired strength and bake hardenability. On the other hand, when the C content exceeds 0.08%, excessive low-temperature phases are formed, and there is a problem of poor formability and weldability due to the formation of carbides. In addition, the addition of excessive C causes degradation of the low-temperature phases and the formation of additional residual carbides during heat treatment in the range of 100-600°C, resulting in a significant decrease in strength and bake hardenability after heat treatment, and a problem of even worse formability.

[0037] Therefore, the C content may be 0.02-0.08%, and more preferably may be 0.03% or more and 0.07% or less.

[0038] Silicon (Si): 0.01-0.5%

[0039] Silicon (Si) deoxidizes molten steel, has a solid solution strengthening effect, and helps improve formability by delaying the formation of coarse carbides. It also has the effect of suppressing the formation of carbides during heat treatment in the 100-600°C range.

[0040] When the Si content is less than 0.01%, the effect of delaying carbide formation is low, making it difficult to improve formability and reducing thermal stability. On the other hand, when the Si content exceeds 0.5%, red scale caused by Si forms on the surface of the steel sheet during hot rolling, which not only significantly deteriorates the surface quality of the steel sheet but also reduces ductility and weldability.

[0041] Therefore, the Si content may be 0.01-0.5%, and more preferably may be above 0.05%.

[0042] Manganese (Mn): 0.8-1.8%

[0043] Like Si, manganese (Mn) is an element effective for solid solution strengthening of steel, and increases the hardenability of steel, thereby facilitating the formation of a low-temperature phase.

[0044] When the Mn content is less than 0.8%, the above effects are difficult to fully achieve. On the other hand, when the Mn content exceeds 1.8%, the hardenability is excessively increased, resulting in an increase in the fraction of the martensite phase. Furthermore, when the slab is cast in a continuous casting process, a large segregation area develops in the center of the thickness, resulting in poor formability. In addition, carbides are easily formed during heat treatment in the 100-600°C range, which may cause significant changes in strength and bake hardening values.

[0045] Therefore, the content of Mn may be 0.8-1.8%.

[0046] Aluminum (Al): 0.01-0.1%

[0047] Aluminum (Al) is an element added primarily for deoxidation. When the aluminum content is less than 0.01%, sufficient deoxidation is not achieved. On the other hand, when the aluminum content exceeds 0.1%, it combines with nitrogen (N) in the steel to precipitate AlN, increasing the risk of corner cracks in the slab during continuous casting and casting, and easily causing defects due to the formation of inclusions.

[0048] Therefore, the Al content may be 0.01-0.1%, more preferably 0.013% or more.

[0049] In the present invention, it is indicated that the Al is acid-soluble aluminum (Sol.Al).

[0050] Phosphorus (P): 0.001-0.02%

[0051] Similar to Si, phosphorus (P) is beneficial for simultaneously exhibiting solid solution strengthening effects and promoting ferrite transformation. However, when the phosphorus (P) content exceeds 0.02%, brittleness is generated due to grain boundary segregation, and microcracks are easily generated during molding, significantly degrading ductility and impact resistance.

[0052] Furthermore, controlling the P content to less than 0.001% requires excessive manufacturing costs, which is disadvantageous economically and also disadvantageous in terms of ensuring the target level of strength.

[0053] Therefore, the P content may be 0.001-0.02%.

[0054] Sulfur (S): 0.001-0.01%

[0055] Sulfur (S) is an impurity present in steel. When its content exceeds 0.01%, it combines with Mn and other substances in the steel to form non-metallic inclusions, which can easily cause microcracks during steel cutting. Furthermore, controlling the S content to less than 0.001% requires excessive steelmaking time, resulting in reduced productivity.

[0056] Therefore, the content of S may be 0.001-0.01%.

[0057] Nitrogen (N): 0.001-0.01%

[0058] Nitrogen (N), along with C, is a representative solid solution strengthening element. Nitrogen combines with Ti, Al, and other elements in steel to form coarse precipitates. While N generally exhibits a superior solid solution strengthening effect compared to C, increasing the N content in steel significantly reduces toughness. To account for this, the N content can be limited to 0.01% or less. Furthermore, controlling the N content to less than 0.001% requires excessive steelmaking time, resulting in reduced productivity.

[0059] Therefore, the N content may be 0.001-0.01%.

[0060] Titanium (Ti): 0.01-0.12%

[0061] Titanium (Ti) is a representative precipitation strengthening element along with Nb and V. Titanium forms coarse TiN in steel due to its strong affinity with N. This TiN has the effect of inhibiting grain growth during the heating process used for hot rolling. In addition, the remaining Ti after reacting with N is dissolved in the steel and combines with C to form TiC precipitates, which contributes to the improvement of steel strength.

[0062] When the Ti content is less than 0.01%, it is difficult to fully obtain the above-mentioned effects. On the other hand, when the Ti content exceeds 0.12%, coarse TiN and TiC are precipitated, which causes a problem of poor formability.

[0063] Therefore, the Ti content may be 0.01-0.12%, more preferably 0.115% or less.

[0064] Niobium (Nb): 0.01-0.05%

[0065] Niobium (Nb) is a representative precipitation strengthening element along with Ti and V. Niobium precipitates during hot rolling and effectively improves the strength and impact toughness of steel by refining the grains due to delayed recrystallization.

[0066] In order to achieve the above-mentioned effects, a Nb content of 0.01% or more is advantageous. However, when the Nb content exceeds 0.05%, elongated grains and coarse composite precipitates are formed during hot rolling due to excessive delay in recrystallization, resulting in poor formability.

[0067] Therefore, the Nb content may be 0.01-0.05%, more preferably 0.011% or more and 0.049% or less.

[0068] Molybdenum (Mo): 0.001-0.2%

[0069] Molybdenum (Mo) increases the hardenability of steel, thereby facilitating the formation of bainite in the steel and has the effect of refining the precipitates in the ferrite grains. Therefore, it is effective in improving the strength and thermal stability of the steel.

[0070] In order to obtain the above-mentioned effects, a Mo content of 0.001% or more is advantageous. However, when the Mo content exceeds 0.2%, martensite is formed due to an increase in hardenability, resulting in a sharp decrease in thermal stability, which is disadvantageous in terms of economy and ensuring weldability.

[0071] Therefore, the Mo content may be 0.001-0.2%, more preferably 0.002% or more and 0.19% or less.

[0072] In addition to the above alloy composition, the steel sheet of the present invention may further contain one or more of chromium (Cr), vanadium (V), nickel (Ni), and boron (B) in a total content of 1.5% or less.

[0073] Adding one or more of chromium (Cr), vanadium (V), nickel (Ni), and boron (B) can further contribute to the precipitation effect and facilitate the formation of an appropriate fraction of bainite.

[0074] Among the above elements, the chromium (Cr) content can be up to 1.0%. However, if the chromium content exceeds 1.0%, the hardenability becomes too high, leading to a sharp increase in the martensite fraction in the structure. This not only deteriorates the thermal stability of the steel but also significantly increases the cost of the alloy iron, thus posing an economic disadvantage. Therefore, the Cr content can be reduced to 1.0% or less, and more preferably, to 0.8% or less.

[0075] The remaining component of the present invention is iron (Fe). However, undesirable impurities may inevitably be introduced from the raw materials or the surrounding environment during conventional manufacturing processes, and such impurities cannot be eliminated. These impurities are well known to those skilled in conventional manufacturing processes, and therefore, all of them are not specifically described in this specification.

[0076] The relationship between the contents of specific elements in the steel of the steel sheet of the present invention having the above-mentioned alloy composition preferably satisfies the following Relational Formulas 1 and 2.

[0077] [Equation 1]

[0078] |K|≤0.85

[0079] (where K = -0.6-0.87[C]+0.03[Si]-0.14[Mn]+0.09[Ti]+0.01[Nb] 2 Each element is expressed as weight content.)

[0080] [Equation 2]

[0081] 5≤A≤20

[0082] (where A = ([Ti] / 48 + [Mo] / 96) × ([Nb] / 93) -1 Each element is expressed as weight content.)

[0083] In the present invention, |K| represented by the relational expression 1 is the slope of the dynamic strength value measured in a temperature increase process for heating steel to a specific temperature, and is based on the deformation resistance of the steel to an external force applied to the steel at a specific temperature.

[0084] For example, when testing steel through a high-temperature compression test or a high-temperature tension test, the force per unit area acting on the material can be measured by applying an external force at a constant deformation rate while heating the material at a constant heating rate. The resulting stress-temperature curve indicates the steel's sensitivity to temperature. In particular, the slope, or K value, can be used to determine the steel's inherent physical properties.

[0085] In the present invention, when the |K| value exceeds 0.85, the thermal stability of the steel sheet is insufficient, and the change in yield strength before and after heat treatment in the range of 100-600° C. increases.

[0086] In addition, since the change in yield strength before and after heat treatment within a specific temperature range satisfies the aforementioned Relationship 2, a more stable trend can be exhibited.

[0087] When the value of A in Relationship 2 is less than 5, the number of precipitates with a diameter of 50 nm or greater increases in the steel sheet's microstructure, while the fraction of precipitates within the grains decreases, leading to reduced thermal stability. This is because the decrease in precipitates with coherent interfaces with the matrix structure increases sensitivity to thermal changes. Furthermore, when the value of A exceeds 20, the effect of further improving thermal stability decreases, and the addition of large amounts of high-cost alloying elements is required, resulting in disadvantages in terms of economics.

[0088] In addition to the alloy composition described above, the steel sheet of the present invention that satisfies all of Relationships 1 and 2 can achieve desired physical properties when used as a practical component even after a short heat treatment at a relatively low temperature (e.g., below 600°C), thereby expanding its application range. Furthermore, it can also be used to obtain a steel sheet that can be plated.

[0089] In the steel sheet of the present invention that satisfies all of the above alloy compositions and Relationships 1 and 2, the microstructure comprises ferrite and bainite phases as main phases, and the sum of their area fractions is preferably 90% or greater (excluding 100%). If the sum of the ferrite and bainite fractions is less than 90%, excessive formation of the martensite phase and MA phase in the microstructure deteriorates formability and makes it difficult to ensure desired thermal stability.

[0090] The area fraction of the ferrite phase in the main phase is preferably 30-80%, and the area fraction of the bainite phase is also preferably 10-60%.

[0091] The remaining structure of the steel plate of the present invention other than the main phase may include one or more of martensite and MA (a mixed structure of martensite and austenite) phases, and their area fractions are respectively less than 5% (except 0%), which can advantageously play a role in ensuring the thermal stability of the steel plate.

[0092] However, when the fraction of each of the martensite and MA phase exceeds 5%, the thermal stability of the steel sheet deteriorates, local stress concentration becomes more likely during deformation, and there is a risk of cracking.

[0093] Furthermore, the steel sheet of the present invention may further include a pearlite phase, and the area fraction of the pearlite phase may be 5% or less (including 0%).

[0094] As described above, the steel sheet of the present invention includes ferrite and bainite phases as main phases and has the characteristics of having high strength while being excellent in bake hardenability and hole expandability.

[0095] Specifically, it is characterized by having a tensile strength of 590 MPa or more and a yield ratio of 0.7 or more, a hole expandability (HER) of 40% or more, and a bake hardening value (BH) of 30 MPa or more.

[0096] In particular, the steel plate of the present invention has excellent thermal stability. Unlike existing steel plates that require heat treatment at a high temperature of more than 600°C, the steel plate of the present invention can be heat treated in the temperature range of 100-600°C. After such heat treatment, the steel plate can maintain a bake hardening value (BH) of more than 30 MPa. h ) effect.

[0097] Furthermore, the effect is as follows: the strength change before and after the heat treatment in the temperature range is minimized, so the strength change before and after the heat treatment (ΔTS) and the bake hardening value after the heat treatment (BH h ) relationship [ΔTS×BH h -1 ] has an absolute value of less than 0.7.

[0098] Hereinafter, a method for producing a high-strength steel sheet having excellent thermal stability provided by the present invention, which is another aspect of the present invention, will be described in detail.

[0099] The high-strength steel sheet according to the present invention can be manufactured by subjecting a steel billet satisfying the alloy composition and component relationship formula proposed in the present invention to a series of processes of [heating-hot rolling-cooling-coiling].

[0100] The following is a detailed description of the various process conditions.

[0101] [Heating of steel billet]

[0102] In the present invention, before hot rolling, the steel slab is preferably subjected to a process of heating and homogenizing, wherein the heating process is preferably carried out at 1100-1350°C.

[0103] When the heating temperature is lower than 1100°C, the precipitates are not fully redissolved, and the formation of fine precipitates in the process after hot rolling is reduced. On the other hand, when the heating temperature exceeds 1350°C, there is a problem of reduced strength due to coarsening of austenite grains.

[0104] Therefore, the heating of the steel slab may be performed within a temperature range of 1100-1350°C.

[0105] [Hot Rolling]

[0106] The steel slab heated as described above may be hot rolled to produce a hot rolled steel plate, wherein the hot rolling may be performed at a temperature in the range of 850-1150°C.

[0107] During hot rolling, if the final temperature, or finishing rolling temperature, is below 850°C, elongated grains develop due to excessively delayed recrystallization, leading to increased anisotropy and poor formability. On the other hand, if hot rolling is initiated at a temperature exceeding 1150°C, the temperature of the hot-rolled steel sheet increases, causing coarsening of the grain size and deteriorating the surface quality of the hot-rolled steel sheet.

[0108] [Cooling and Winding]

[0109] The hot rolled steel sheet manufactured as described above can be cooled to a specific temperature before being coiled. Specifically, the cooling can be performed at a cooling rate of 10-100°C / s to a temperature range of 400-550°C and then the coiling process can be performed within this temperature range.

[0110] If the coiling temperature, at which cooling is completed, is lower than 400°C, low-temperature phases such as martensite and MA phases are unnecessarily formed in the steel, reducing the thermal stability of the structure. This deteriorates formability before and after heat treatment and increases the magnitude of the drop in strength after heat treatment. On the other hand, if the coiling temperature exceeds 550°C, it is impossible to maintain the appropriate fractions of bainite, martensite, and MA phases, making it difficult to achieve the target bake hardening value (BH) before and after heat treatment.

[0111] In addition, when cooling is performed within the above-mentioned temperature range, when the cooling rate is less than 10°C / s, there is a problem that the grains of the matrix structure become coarse and an uneven structure is generated. On the other hand, when the cooling rate exceeds 100°C / s, the low-temperature phase fraction increases, and problems such as reduced thermal stability occur.

[0112] [Final Cooldown]

[0113] The hot rolled steel sheet cooled and coiled as described above may be cooled to about room temperature, more specifically, may be cooled to room temperature to 200° C. at a cooling rate of 10-50° C. / hour.

[0114] During cooling, if the cooling rate exceeds 50°C / hour, some of the untransformed phases in the steel are likely to transform into martensite and MA phases, thereby deteriorating thermal stability. Furthermore, if the cooling rate is controlled to less than 10°C / hour, excessive ferrite phases form in the structure, making it difficult to achieve the desired bake hardenability (BH). Furthermore, additional heating equipment is required to control the slow cooling, which is uneconomical.

[0115] The present invention may further include the steps of pickling and oiling the steel plate obtained after the final cooling, and further, may further include the step of heating the pickled and oiled steel plate to a temperature range of 450-740° C. for hot-dip galvanizing.

[0116] The hot-dip galvanizing may utilize a zinc-based plating bath. Although the alloy composition of the plating bath is not particularly limited, as an example, the plating bath may include 0.01-30 wt% magnesium (Mg), 0.01-50 wt% aluminum (Al), and the balance Zn and inevitable impurities.

[0117] The present invention is described in more detail below using examples. However, it should be noted that the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the contents of the claims and any reasonable deductions therefrom. DETAILED DESCRIPTION

[0118] (Example)

[0119] Steel slabs having the alloy composition shown in Table 1 below were prepared, heated at 1100-1350°C, and then hot finish rolled according to the temperature conditions shown in Table 2 below to produce hot-rolled steel sheets. The hot rolling process was initiated at a temperature lower than the heating temperature. The hot-rolled steel sheets produced as described above were cooled at the cooling rates shown in Table 2 below to the temperatures shown in Table 2 below, and then coiled at these temperatures. Finally, the steel sheets were cooled at a cooling rate of 10-50°C / hour to room temperature to produce the respective steel sheets.

[0120] The mechanical properties and microstructure of each steel plate were measured, and the results are shown in Table 3 below.

[0121] First, DIN standard test pieces were taken in the rolling direction, and then the tensile strength (TS) and elongation at break (E1) of these test pieces were measured at room temperature.

[0122] The hole expandability and bake hardenability of the same test piece were also evaluated. The hole expandability was expressed as the average value of three measurements at room temperature.

[0123] For the hole expandability, a square test piece with a width × length of 120 mm was prepared, and a hole with a diameter of 10 mm was punched in the center of the test piece by punching. Then, with the burr facing upward, a cone was used to push upward. The hole expandability was calculated by calculating the percentage of the hole diameter that expanded until cracks were generated in the circumferential portion relative to the initial hole diameter (10 mm).

[0124] The bake hardening property is expressed by measuring the bake hardening value, wherein the bake hardening value (BH) is measured as follows: the yield strength value (MPa, YS) at room temperature when 2% pre-deformed and the yield strength value (MPa, YS) after heat treatment at 170°C for 20 minutes and cooling to room temperature after 2% pre-deformed are measured. h ) and calculate the difference between the above values.

[0125] In order to measure the strength change before and after heat treatment, the tensile strength (TS) before heat treatment and the tensile strength (TS) after heating at 500°C for 10 minutes and then cooling to room temperature were measured. h ), and the bake hardening value (BH) after heat treatment was measured according to the same method as the above-mentioned bake hardening value (BH) measurement method. h ).

[0126] Then, the difference in strength before and after heat treatment at 500°C (ΔTS = TS h -TS) and the relationship between the bake hardenability after the heat treatment [ΔTS×BH h -1 ] is expressed as an absolute value.

[0127] In order to analyze the microstructure of each steel sheet, the steel sheet was observed using a SEM at magnifications of ×3000 and ×5000, and the fraction of each phase was measured.

[0128] At this time, the martensite phase and the MA phase were measured by performing Nital etching and Lepera etching on the test piece and then analyzing the sample at a magnification of ×1000 using an optical microscope and an image analyzer.

[0129] [Table 1]

[0130]

[0131] (In Table 1, the alloy compositions of Comparative Steels 8 and 9 satisfy the present invention but are outside the range of the following production conditions, and therefore are classified as comparative steels.)

[0132] [Table 2]

[0133]

[0134]

[0135] [Table 3]

[0136]

[0137] As shown in Tables 1 to 3, it can be confirmed that the invention steels 1 to 5 satisfying all the alloy composition systems and production conditions proposed in the present invention have a desired microstructure and thus ensure desired physical properties.

[0138] In addition, Comparative Steels 1 to 7 are examples that do not satisfy the alloy composition proposed in the present invention. Among them, it can be confirmed that in Comparative Steels 1, 3, and 4, the contents of C, Si, and Mn are respectively too high to satisfy Relationship 1. As a result, the martensite phase and MA phase as the steel structure are unnecessarily formed, resulting in poor hole expandability of the steel plate and a significant decrease in tensile strength after heat treatment.

[0139] Comparative Steel 2 and Comparative Steel 5 have insufficient C and Mn contents, respectively. Due to the reduced hardenability of the steel plate, the low-temperature phase fraction cannot be fully formed, so the strength is ensured to be less than 590 MPa, and the bake hardenability before and after heat treatment is poor.

[0140] Comparative Steel 6 and Comparative Steel 7 respectively have excessive Ti and Nb contents. It can be confirmed that due to the formation of excessive carbides, a certain fraction of low-temperature phase cannot be ensured. Therefore, the bake hardening values ​​before and after heat treatment are poor, and the increase in coarse precipitates leads to a decrease in hole expandability.

[0141] Comparative Steels 8 and 9 are examples of alloy compositions meeting the requirements of the present invention, but whose coiling temperatures fall outside the present invention's range. In Comparative Steel 8, coiling at excessively high temperatures prevented sufficient formation of low-temperature phases within the microstructure, making it difficult to ensure bake hardenability before and after heat treatment. As shown in Comparative Steel 9, at significantly low coiling temperatures, unnecessary formation of low-temperature phases resulted in a poor yield ratio and significant variations in strength and bake hardening values ​​before and after heat treatment.

[0142] Figure 1 The relationship between the |K| value of the alloying element and the strength change (ΔTS) before and after heat treatment and the bake hardenability [ΔTS×BH h -1 ] is a graph of the correlation between .

[0143] like Figure 1As shown, it was confirmed that the invention steel produced solely by the present invention had a |K| value of 0.85 or less and exhibited minimal change in strength before and after heat treatment, indicating excellent thermal stability. On the other hand, it was confirmed that the comparative steels with |K| values ​​outside the range proposed by the present invention exhibited significant changes in physical properties before and after heat treatment. Furthermore, in Comparative Steels 8 and 9, which had |K| values ​​of 0.85 or less, the desired physical properties could not be achieved due to manufacturing conditions (coiling temperature) outside the range of the present invention.

Claims

1. A high-strength steel plate having excellent thermal stability, comprising, in weight percent, 0.02-0.08% carbon (C), 0.01-0.5% silicon (Si), 0.8-1.8% manganese (Mn), 0.01-0.1% aluminum (Al), 0.001-0.02% phosphorus (P), 0.001-0.01% sulfur (S), 0.001-0.01% nitrogen (N), 0.001-0.01% titanium (Ti), 0.01-0.12% niobium (Nb), 0.01-0.05% molybdenum (Mo), and the balance Fe and other unavoidable impurities. The steel plate satisfies the following equations 1 and 2: As a microstructure, the sum of the area fractions of the ferrite phase and the bainite phase is 90% or more and excluding 100%, and the remainder includes one or more of the martensite phase and the MA phase, [Equation 1] |K|≤0.85 in, K=-0.6-0.87[C]+0.03[Si]-0.14[Mn]+0.09[Ti]+0.01[Nb] 2 Indicates that each element represents weight content, [Equation 2] 5≤A≤20 Wherein, A=([Ti] / 48+[Mo] / 96)×([Nb] / 93) -1 Indicates that each element represents the weight content.

2. The high-strength steel sheet having excellent thermal stability according to claim 1, wherein The steel sheet further includes one or more of chromium (Cr), vanadium (V), nickel (Ni), and boron (B) in a total content of 1.5% or less.

3. The high-strength steel sheet having excellent thermal stability according to claim 1, wherein The steel sheet includes a martensite phase and an MA phase, each of which has an area fraction of 5% or less and excluding 0%.

4. The high-strength steel sheet having excellent thermal stability according to claim 1, wherein The steel plate has a tensile strength of 590 MPa or more, a yield ratio of 0.7 or more, a hole expansion efficiency (HER) of 40% or more, and a bake hardening value (BH) of 30 MPa or more.

5. The high-strength steel sheet having excellent thermal stability according to claim 1, wherein The bake hardening value BH of the steel plate after heat treatment at 100-600°C h The strength change ΔTS before and after the heat treatment is greater than 30 MPa and the bake hardening value BH after the heat treatment is greater than 1 MPa. h The relationship [ΔTS×BH h -1 ] has an absolute value of less than 0.

7.

6. A method for manufacturing a high-strength steel plate having excellent thermal stability, comprising the following steps: A steel slab is prepared, comprising, in weight %, carbon (C): 0.02-0.08%, silicon (Si): 0.01-0.5%, manganese (Mn): 0.8-1.8%, aluminum (Al): 0.01-0.1%, phosphorus (P): 0.001-0.02%, sulfur (S): 0.001-0.01%, nitrogen (N): 0.001-0.01%, titanium (Ti): 0.01-0.12%, niobium (Nb): 0.01-0.05%, molybdenum (Mo): 0.001-0.2%, and the balance Fe and other inevitable impurities, and satisfying the following equations 1 and 2; heating the steel billet within a temperature range of 1100-1350° C.; hot rolling the heated steel slab at a temperature ranging from 850° C. to 1150° C. to produce a hot-rolled steel plate; as well as The hot rolled steel sheet is cooled to a temperature range of 400-550°C at an average cooling rate of 10-100°C / s and then coiled. [Equation 1] |K|≤0.85 Among them, K=-0.6-0.87[C]+0.03[Si]-0.14[Mn]+0.09[Ti]+0.01[Nb] 2 Indicates that each element represents weight content, [Equation 2] 5≤A≤20 Wherein, A=([Ti] / 48+[Mo] / 96)×([Nb] / 93) -1 Indicates that each element represents the weight content.

7. The method for producing a high-strength steel sheet having excellent thermal stability according to claim 6, wherein: The method further comprises cooling the coiled hot-rolled steel plate to a temperature between room temperature and 200°C.

8. The method for producing a high-strength steel sheet having excellent thermal stability according to claim 7, wherein: After the cooling, the method further includes the steps of pickling and oiling the hot rolled steel plate.

9. The method for producing a high-strength steel sheet having excellent thermal stability according to claim 8, wherein: After the pickling and oiling, the method further includes heating the hot-rolled steel sheet to a temperature range of 450-740° C. and then hot-dip galvanizing.

10. The method for producing a high-strength steel sheet having excellent thermal stability according to claim 9, wherein: The hot-dip galvanizing uses a plating bath containing 0.01-30 wt % of magnesium (Mg), 0.01-50 wt % of aluminum (Al), the balance of Zn, and inevitable impurities.

11. The method for producing a high-strength steel sheet having excellent thermal stability according to claim 6, wherein: The steel slab further includes one or more of chromium (Cr), vanadium (V), nickel (Ni), and boron (B) in a total content of 1.5% or less.

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