High-strength thick hot-rolled steel plate with excellent elongation and method for producing the same

Abstract

The present invention provides a high-strength thick hot-rolled steel plate with excellent elongation and a method for manufacturing the same. The high-strength thick hot-rolled steel plate of the present invention comprises, by weight, 0.05-0.15% C, 0.01-1.0% Si, 1.0-2.0% Mn, 0.01-0.1% acid-soluble aluminum (Sol.Al), 0.005-1.0% Cr, 0.001-0.02% P, 0.001-0.01% S, 0.001-0.01% N, 0.001-0.01% Nb, 0.005-0.07% Ti, and the remainder iron and inevitable impurities, and satisfies the following relational expression 1. The high-strength thick hot-rolled steel plate has a fine structure, and by area %, the fine structure comprises: 25-50% polygonal ferrite, 30-50% bainitic ferrite and acicular ferrite, and 20% or less bainite. 2 ) with a diameter of 0.5 μm or more and the sum of the area fractions of the carbides and pearlite structure observed within: less than 5%, and the MA phase (martensite-austenite component): less than 5%.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing high-strength hot-rolled steel plates with a thickness of 10-14 mm for structural parts such as wheel discs, rims, components and frames of commercial vehicles. In more detail, the present invention relates to a high-strength thick hot-rolled steel plate and a method for manufacturing the same. The high-strength thick hot-rolled steel plate is characterized in that the high-strength thick hot-rolled steel plate has an excellent tensile strength of 590 MPa or more and an excellent elongation of 30% or more, thereby preventing cracks from occurring during the molding of components such as wheel discs. Background Art

[0002] Traditionally, structural components for commercial vehicles and heavy equipment primarily used sheet metal with a tensile strength of 440 MPa or greater and a thickness of 13-20 mm. However, as part of the development of high-strength, thin-walled steel for lightweighting, technology is currently underway to utilize high-strength steel with a tensile strength of 590 MPa or greater and a thickness of 10-15 mm. However, when using a phase transformation structure in the hot rolling process to manufacture such high-strength, thick steel, it is difficult to ensure a uniform microstructure throughout the thickness. Consequently, problems exist: ensuring stable ductility and yield strength is difficult, cracks are easily generated during component manufacturing, and durability is poor due to localized stress concentration during use.

[0003] In this regard, the following have been proposed: Conventional steel is hot-rolled in the normal austenite region and then coiled at high temperature to ensure strength and ductility by forming fine precipitates with the ferrite phase as the matrix (Patent Document 1); or, to prevent the formation of coarse pearlite, a two-stage cooling process is used after rolling, cooling the coiling temperature to a temperature at which a bainite phase is formed before coiling (Patent Document 2). Furthermore, a technique has been proposed to refine austenite grains by significantly reducing the austenite content by 40% or more during hot rolling using Ti, Nb, etc. in the unrecrystallized region (Patent Document 3).

[0004] However, while alloying elements such as Si, Mn, Al, Mo, and Cr, primarily employed in these technologies to produce thick, high-strength steel, are effective in increasing strength, excessive addition of these elements can lead to segregation and microstructural inhomogeneity, resulting in poor formability. Furthermore, microcracks generated on shear surfaces are more likely to propagate in fatigue environments, leading to component damage. In particular, increasing thickness increases the microstructural inhomogeneity between the surface and deeper layers, increasing local stress concentration and the crack propagation rate in fatigue environments, thus deteriorating durability.

[0005] Furthermore, in order to refine the grains of thick steel and achieve precipitation strengthening effects, it is effective to utilize precipitate-forming elements such as Ti, Nb, and V. However, if coiling is performed at high temperatures of 500-700°C, where precipitates are easily formed, or if the cooling rate of the steel sheet is not controlled during the cooling process after hot rolling, coarse carbides will form in the center of the thickness of the thick steel, resulting in poor shear surface quality.

[0006] Furthermore, to prevent the formation of coarse pearlite, the coiling temperature is cooled to the bainite-forming temperature through a two-stage cooling control method. This cooling control method is limited by the length of the ROT zone, and as the unit weight required by the customer decreases, its effectiveness over the entire coil length becomes limited. Furthermore, maintaining a cooling rate of 70°C / second during the first cooling stage places a heavy load on the equipment, directly leading to reduced productivity. Furthermore, applying a high reduction of 40% in the unrecrystallized zone during hot rolling degrades the shape quality of the rolled sheet and places additional strain on the equipment, making it difficult to implement in practice.

[0007] Prior art literature

[0008] (Patent Document 1) Japanese Patent Publication No. 2002-322541

[0009] (Patent Document 2) Korean Patent Publication No. 10-2020-0062422

[0010] (Patent Document 3) Japanese Patent Publication No. 1997-143570 Summary of the Invention

[0011] Technical problems to be solved

[0012] The object of the present invention is to provide a thick hot-rolled steel plate and a method for manufacturing the same, wherein the thick hot-rolled steel plate ensures a uniform distribution of microstructure in the thickness direction of the steel material by controlling the steel composition, hot rolling and cooling conditions, thereby achieving a constant hardness distribution in the thickness direction and excellent strength and ductility.

[0013] The technical problems of the present invention are not limited to the above-mentioned contents. The technical problems of the present invention can be understood from the entire contents of this specification, and any person skilled in the art with ordinary knowledge in the technical field to which the present invention belongs will not have difficulty in understanding the additional problems of the present invention.

[0014] Technical Solution

[0015] One aspect of the present invention relates to a high-strength thick hot-rolled steel plate with a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher. The high-strength thick hot-rolled steel plate comprises, by weight, C: 0.05-0.15%, Si: 0.01-1.0%, Mn: 1.0-2.0%, acid-soluble aluminum (Sol.Al): 0.01-0.1%, Cr: 0.005-1.0%, P: 0.001-0.02%, S: 0. 0.001-0.01%, N: 0.001-0.01%, Nb: 0.005-0.07%, Ti: 0.005-0.11%, the balance being iron and inevitable impurities, and satisfying the following [Relationship 1], the high-strength thick hot-rolled steel plate has a fine structure, which, in terms of area %, comprises: polygonal ferrite: 25-50%, bainitic ferrite and acicular ferrite: 30-50%, bainite: less than 20%, and per unit area (1cm 2 ) with a diameter of 0.5 μm or more and the sum of the area fractions of the pearlite structure: less than 5%, and the MA phase (Martensitic-austenitic constituents): less than 5%.

[0016] [Equation 1]

[0017] 0.3≤R≤1.0

[0018] R=[C]*+0.7×[Mn]+8.5×[P]+7.5×[S]-0.9×[Si]-1.5×[Nb]

[0019] [C]*=[C]-[C]×Q

[0020] Q=([Nb] / 93+[Ti] / 48) / ([C] / 12)

[0021] In the above formula 1, C, Mn, P, S, Si, Nb, and Ti are the weight % of the corresponding alloying elements.

[0022] The hot rolled steel plate has an El×TS×0.5Hv 最大(max) The / ΔH value can meet 140000 or above.

[0023] In addition, another aspect of the present invention relates to a method for manufacturing a high-strength thick hot-rolled steel plate having a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher, the method comprising the following steps:

[0024] Reheating the steel billet satisfying the composition and the following [Relationship 1] at 1200-1350° C.;

[0025] The reheated steel slab is subjected to hot finish rolling within a temperature range satisfying the following [Relationship 2];

[0026] Cooling the hot-rolled steel sheet to a temperature in the range of 450-550° C. at a cooling rate satisfying the following [Relationship 3], and then coiling the steel sheet at a temperature in the range of 400-550° C.; and

[0027] The coiled steel plate is air-cooled or water-cooled to a temperature ranging from room temperature to 200°C.

[0028] [Equation 1]

[0029] 0.3≤R≤1.0

[0030] R=[C]*+0.7×[Mn]+8.5×[P]+7.5×[S]-0.9×[Si]-1.5×[Nb]

[0031] [C]*=[C]-[C]×Q

[0032] Q=([Nb] / 93+[Ti] / 48) / ([C] / 12)

[0033] In the above formula 1, C, Mn, P, S, Si, Nb, and Ti are the weight % of the corresponding alloying elements.

[0034] [Equation 2]

[0035] Tn-70≤FDT≤Tn

[0036] Tn=750+92×[C]+70×[Mn]+45×[Cr]+647×[Nb]+515×[Ti]-50×[Si]-2.4×(t-5)

[0037] In the above formula 2, C, Mn, Cr, Nb, Ti, and Si are the weight % of the corresponding alloying elements.

[0038] FDT in the above-mentioned relational expression 2 is the temperature (° C.) of the hot-rolled sheet at the end of hot rolling.

[0039] In the above relational expression 2, t is the thickness (mm) of the finished rolled plate.

[0040] [Equation 3]

[0041] CR 最小(Min) ≤CR*≤60

[0042] CR 最小=65-157×[C]-25.2×[Si]-14.1×[Mn]-27.3×[Cr]+61×[Ti]+448×[Nb]+1.4×(t-5)

[0043] In the above formula 3, C, Si, Mn, Cr, Ti, and Nb are the weight % of the corresponding alloy elements.

[0044] CR* in the above relational expression 3 is the cooling rate (° C. / second (sec)) when the rolled sheet is cooled after hot rolling.

[0045] After the air cooling or water cooling, the steel plate may be pickled and oiled.

[0046] Beneficial effects

[0047] The present invention, as described above, can provide a high-strength thick hot-rolled steel plate having a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher, wherein the high-strength thick hot-rolled steel plate has a fine structure comprising: polygonal ferrite: 25-50 area%, bainitic ferrite and acicular ferrite: 30-50 area%, and bainite: 20 area% or less, and a microstructure of 10-14 mm thick. 2 ) The sum of the area fractions of carbides and pearlite with a diameter of 0.5 μm or more observed in the 1.5 μm-diameter structure is less than 5 area %, and the MA phase (martensite-austenite component) is less than 5 area %, and El×TS×0.5Hv 最大 The / ΔH value is greater than 140,000. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 These are photographs showing the cross-sectional microstructures of the steel plates (surface layer and 1 / 4t) of Inventive Example 3 and Comparative Example 9 in the embodiment of the present invention.

[0049] Figure 2 This is a graph showing the distribution of hardness values ​​in thickness cross sections of Inventive Example 3 and Comparative Example 3 in the embodiment of the present invention.

[0050] Figure 3 It is El×TS×0.5Hv that shows the elongation level of the invention example and the comparative example in the embodiment of the present invention 最大 / ΔH value distribution diagram.

[0051] Best Practice

[0052] Hereinafter, the present invention will be described.

[0053] The present inventors investigated the distribution of microstructures and detailed material changes in the thickness direction of thick rolled steel sheets with various microstructures having different compositions, depending on the composition, hot rolling, and cooling conditions. Based on these results, they explored a method for achieving excellent strength and ductility in thick hot rolled steel sheets. In particular, they derived equations 1 to 3, described below, and based on these equations, they confirmed that if thick high-strength steel with a thickness of 10-14 mm is produced, uniformity of the microstructure in the thickness direction can be ensured, and a constant hardness distribution in the thickness direction can be achieved, with El×TS×0.5Hv 最大 The present invention was proposed based on the fact that the / ΔH value satisfies 140,000 or more.

[0054] The high-strength thick hot-rolled steel plate of the present invention comprises, in weight %, the following: C: 0.05-0.15%, Si: 0.01-1.0%, Mn: 1.0-2.0%, acid-soluble aluminum: 0.01-0.1%, Cr: 0.005-1.0%, P: 0.001-0.02%, S: 0.001-0.01%, N: 0.001-0.01%, Nb: 0.005-0.07%, Ti: 0.005-0.11%, and the balance is iron and inevitable impurities, and satisfies the following [Relationship 1]. In terms of area %, the steel microstructure comprises: polygonal ferrite: 25-50%, bainitic ferrite and acicular ferrite: 30-50%, bainite: 20% or less, and the ratio of the basalt content per unit area (1 cm) is 0. 2 ) The sum of the area fractions of carbides and pearlite structures with a diameter of 0.5 μm or more observed in the composite structure is less than 5%, and the MA phase (martensite-austenite component) is less than 5%.

[0055] The following describes the reasons for limiting the steel components and their contents in the present invention, wherein "%" used herein means "% by weight."

[0056] C: 0.05-0.15%

[0057] C is the most economical and effective element for strengthening steel. Increasing its addition increases the precipitation strengthening effect and the fraction of bainite, leading to higher tensile strength. Furthermore, as the thickness of the hot-rolled steel sheet increases, the cooling rate in the center of the thickness slows during cooling after hot rolling. Consequently, when the C content is high, coarse carbides or pearlite are more likely to form. Therefore, when the C content is less than 0.05%, a sufficient strengthening effect is difficult to achieve. When the C content exceeds 0.15%, coarse carbides or pearlite and banded structures form in the center of the thickness, leading to reduced formability and durability, and consequently, reduced weldability. Therefore, in the present invention, the C content is preferably limited to 0.05-0.15%. More preferably, it is limited to 0.06-0.12%.

[0058] Si: 0.01-1.0%

[0059] The Si can deoxidize the molten steel, has a solid solution strengthening effect, and delays the formation of coarse carbides, which is beneficial to improving formability. However, when the Si content is less than 0.01%, the solid solution strengthening effect is small, and the effect of delaying carbide formation is also small, making it difficult to improve formability. In addition, when the Si content exceeds 1.0%, the phase transformation temperature increases. When extremely thick materials are hot rolled in the low temperature zone, coarse grains caused by rolling in the local ferrite zone are easily formed in the surface layer, and red oxide scale is formed on the surface of the steel plate. Therefore, not only is the surface quality of the steel plate very poor, but there is also a problem of reduced ductility and weldability. Therefore, in the present invention, it is preferred to limit the Si content to the range of 0.01-1.0%. More preferably, it is limited to the range of 0.1-0.9%.

[0060] Mn: 1.0-2.0%

[0061] The Mn, like Si, is an element that effectively strengthens the steel through solid solution, increases the hardenability of the steel, and thus facilitates the formation of a bainite phase during the cooling process after hot rolling. However, when the Mn content is less than 1.0%, the effect of the addition cannot be achieved. When the Mn content exceeds 2.0%, the hardenability will increase significantly, and the martensitic phase transformation will easily occur, which will promote the formation of pearlite during high-temperature coiling. In addition, in the continuous casting process, when the slab is cast, the segregation part will be greatly developed in the center of the thickness. When cooling after hot rolling, the fine structure in the thickness direction is unevenly formed, thereby deteriorating the formability and durability. Therefore, in the present invention, it is preferred to limit the Mn content to the range of 1.0-2.0%. More advantageously, it is limited to the range of 1.1-2.0%.

[0062] Cr: 0.005-1.0%

[0063] Cr strengthens the steel by solid solution, delaying the ferrite transformation during cooling, and thus contributing to the formation of bainite. However, when the Cr content is less than 0.005%, the effect of the addition cannot be achieved. When the Cr content exceeds 1.0%, the ferrite transformation is excessively delayed, and the elongation deteriorates due to the formation of the martensite phase. In addition, similar to Mn, the segregation area in the center of the thickness is large and developed, making the microstructure uneven in the thickness direction, thereby deteriorating the formability and durability. Therefore, in the present invention, it is preferred to limit the Cr content to the range of 0.005-1.0%. More preferably, it is limited to the range of 0.1-0.9%.

[0064] P: 0.001-0.02%

[0065] Like Si, P has the effects of both solid solution strengthening and promoting ferrite transformation. However, controlling the P content to below 0.001% requires significant manufacturing costs, which is economically disadvantageous and insufficient to achieve sufficient strength. Furthermore, when the P content exceeds 0.02%, brittleness due to grain boundary segregation occurs, making microcracks more likely to form during molding, significantly degrading formability and durability. Therefore, in the present invention, the P content is preferably limited to the range of 0.001-0.02%.

[0066] S: 0.001-0.01%

[0067] S is an impurity present in steel. When the S content exceeds 0.01%, it combines with Mn and other materials to form non-metallic inclusions. This can easily cause microcracks during steel cutting, significantly reducing formability and durability. Furthermore, controlling the S content to less than 0.001% requires a long steelmaking process, potentially reducing productivity. Therefore, in the present invention, the S content is preferably limited to 0.001-0.01%.

[0068] Acid soluble aluminum: 0.01-0.1%

[0069] Acid-soluble aluminum is primarily added for deoxidation. When its content is less than 0.01%, its effect is insufficient. When its content exceeds 0.1%, it combines with nitrogen to form AlN, which can easily cause corner cracks in the slab during continuous casting and defects caused by the formation of inclusions. Therefore, in the present invention, the content of acid-soluble aluminum is limited to 0.01-0.1%.

[0070] N: 0.001-0.01%

[0071] N, along with C, is a representative solid solution strengthening element, forming coarse precipitates along with Ti, Al, and other elements. While N generally exhibits a superior solid solution strengthening effect compared to carbon, increasing the amount of N in steel significantly decreases toughness. Furthermore, controlling the N content to less than 0.001% requires a significant amount of time during steelmaking, resulting in reduced productivity. Therefore, in light of this, the present invention preferably limits the N content to 0.001-0.01%.

[0072] Ti: 0.005-0.11%

[0073] Ti is a representative precipitation strengthening element. It has a strong affinity with N and forms coarse TiN in steel. TiN has the effect of inhibiting grain growth during the heating process for hot rolling. In addition, the remaining Ti after reacting with nitrogen dissolves in the steel and combines with carbon to form TiC precipitates. Therefore, Ti is a useful component for improving the strength of steel. When the Ti content is less than 0.005%, the above-mentioned effect cannot be achieved. When the Ti content exceeds 0.11%, coarse TiN is produced, and due to the coarsening of the precipitates, there is a problem that local stress concentration is easily caused during forming, which easily leads to cracks. Therefore, in the present invention, it is preferred to limit the Ti content to 0.005-0.11%. More advantageously, it is limited to 0.01-0.1%.

[0074] Nb: 0.005-0.07%

[0075] Nb, along with Ti, is a representative precipitation strengthening element. It precipitates during hot rolling and effectively improves the strength and impact toughness of steel due to the grain refinement effect brought about by delayed recrystallization. When the Nb content is less than 0.005%, this effect cannot be achieved. When the Nb content exceeds 0.07%, excessive recrystallization delay during hot rolling leads to the formation of elongated grains and coarse composite precipitates, which deteriorates formability and durability. Therefore, in the present invention, the Nb content is preferably limited to 0.005-0.07%. More preferably, it is limited to 0.01-0.07%.

[0076] Relationship 1

[0077] The present invention is characterized in that the contents of C, Mn, P, S, Si, Nb, and Ti are controlled so that the R value defined by the following [Relational Formula 1] satisfies the range of 0.3 to 1.0.

[0078] The reason for setting this [Relationship 1] in the present invention is to minimize the segregation of C, Mn, P, S, etc. and the formation of MnS generated during the solidification of steel and cooling of the slab in the continuous casting process, thereby suppressing segregation, coarse carbides, and the formation of uneven pearlite in the thickness direction of the rolled plate, thereby improving the uniformity of the microstructure. When the R value defined by the above [Relationship 1] is less than 0.3, the fraction of segregation, coarse carbides, and uneven pearlite in the thickness direction of the rolled plate is sharply reduced, but there is a problem that the strength level proposed in the present invention cannot be ensured. When the R value defined by the above [Relationship 1] exceeds 1.0, there is a problem that excessive pearlite is formed in the center of the microstructure of the rolled plate, or the unevenness of the microstructure in the thickness direction increases due to unnecessary segregation and the formation of coarse carbides.

[0079] [Equation 1]

[0080] 0.3≤R≤1.0

[0081] R=[C]*+0.7×[Mn]+8.5×[P]+7.5×[S]-0.9×[Si]-1.5×[Nb]

[0082] [C]*=[C]-[C]×Q

[0083] Q=([Nb] / 93+[Ti] / 48) / ([C] / 12)

[0084] In the above formula 1, C, Mn, P, S, Si, Nb and Ti are the weight percentages of the corresponding alloying elements.

[0085] The remaining component of the present invention is iron (Fe). However, during typical manufacturing processes, undesirable impurities may inevitably be introduced from the raw materials or the surrounding environment, and therefore cannot be eliminated. These impurities are well known to those skilled in the art of typical manufacturing processes, and therefore all of them are not specifically mentioned in this specification.

[0086] In addition, the high-strength thick hot-rolled steel sheet of the present invention has a steel microstructure, which contains, in terms of area%, polygonal ferrite: 25-50%, bainitic ferrite and acicular ferrite: 30-50%, and bainite: less than 20%. 2 ) The sum of the area fractions of carbides and pearlite structures with a diameter of 0.5 μm or more observed in the composite structure is less than 5%, and the MA phase (martensite-austenite component) is less than 5%.

[0087] In the present invention, when the fraction of polygonal ferrite as the high-temperature ferrite structure is less than 25%, there is a problem that sufficient ductility cannot be ensured. When the fraction of polygonal ferrite as the high-temperature ferrite structure exceeds 50%, an appropriate fraction of low-temperature phases including bainitic ferrite cannot be ensured, and there is a problem of deterioration in strength and durability due to the coarsening of the microstructure and the increase in the fraction of grain boundary carbides due to the relatively slow cooling rate.

[0088] Furthermore, the present invention limits the combined fractions of bainitic ferrite and acicular ferrite to 30-50%. However, if the combined fractions exceed 50%, sufficient polygonal ferrite cannot be achieved, leading to reduced ductility. Furthermore, for thick materials (10-14 mm), controlling the cooling rate and coiling temperature to ensure a bainitic ferrite content exceeding 50% imposes significant equipment limitations, including cooling equipment and the length of the ROT section. This can directly lead to reduced productivity. Furthermore, if the combined fractions of bainitic ferrite and acicular ferrite are less than 30%, polygonal ferrite and pearlite may be unnecessarily formed, the fractions of low-temperature phases such as bainite and martensite may increase dramatically, and strength, ductility, and durability may deteriorate.

[0089] Furthermore, in the present invention, the bainite fraction is limited to 20% or less. If the bainite fraction exceeds 20%, the fraction of polygonal ferrite in the structure decreases sharply, thereby causing a problem of unnecessary deterioration of ductility.

[0090] In addition, in the present invention, the unit area (1cm 2 ) is limited to less than 5% by area of ​​the sum of the carbides and pearlite structures with a diameter of 0.5 μm or more observed in the steel and to less than 5% by area and the MA phase (martensite-austenite component) is limited to less than 5% in order to suppress the fraction of the MA phase which would locally increase the deformation rate due to excessive concentration of carbon, and instead form bainite and bainitic ferrite in appropriate proportions, thereby improving ductility and durability by inducing uniform deformation behavior along the thickness direction.

[0091] Hot rolled steel sheets with the above steel composition and microstructure, a tensile strength of 590 MPa or higher and an elongation of 30% or higher with a thickness of 10-14 mm, can form a uniform structure in the thickness direction. The El×TS×0.5Hv coefficient, which is related to elongation, strength, and material uniformity, is 最大 The / ΔH value can meet 140000 or above.

[0092] Next, the method for producing a high-strength hot-rolled steel sheet of the present invention will be described in detail.

[0093] In the present invention, the steel composition must satisfy the following [Equation 1], and the steel manufacturing process must simultaneously satisfy the following [Equations 2] and [Equations 3] to ensure appropriate steel strength, ductility, and material uniformity. This can be effectively achieved by minimizing the segregation of C, Mn, P, S, and other elements, as well as the formation of MnS, that occurs during steel solidification and slab cooling during the continuous casting process.

[0094] Delaying recrystallization during hot rolling refines the microstructure during phase transformation and promotes ferrite transformation, thereby helping to ensure uniform yield strength and formability through the thickness. Furthermore, the reduction of untransformed phases reduces the fraction of coarse MA and martensite phases. This reduces the amount of coarse carbides and pearlite in the center of the thickness, where cooling rates are relatively slow, thereby eliminating uneven microstructure in the hot-rolled steel sheet.

[0095] However, conventional hot rolling is difficult to achieve uniform microstructure in the center of thick materials (10-14 mm thick). If hot rolling is performed at excessively low temperatures to achieve a recrystallization delay effect in the center, the deformed structure develops strongly from the t / 4 position just below the surface of the rolled plate, which in turn increases the inhomogeneity with the microstructure in the center. This increases the likelihood of microcracks forming in these inhomogeneous areas during forming, and also reduces ductility. Therefore, as shown in the following [Equation 2], to achieve the desired effect for thick materials, the hot finish rolling finish temperature is controlled between Tn, the temperature at which recrystallization delay begins, and Tn-70, and a total reduction of 10-60% is applied between Tn and the hot finish rolling finish temperature. However, a total reduction of less than 10% makes it difficult to achieve the recrystallization delay effect, resulting in the formation of uneven, coarse grains. A total reduction exceeding 60% results in an overly elongated microstructure, which forms along carbide grain boundaries within the microstructure, making cracks along these grain boundaries more likely to form during forming. In addition, fine precipitates are reduced, and the precipitation strengthening effect is also reduced.

[0096] In addition, when cooling after hot rolling, the cooling rate of the thickness center portion is slower than that of the thickness surface portion of the rolled plate, so that coarse ferrite phase and coarse carbide are formed, which may have an uneven microstructure. Therefore, when cooling the plate rolled immediately after hot rolling, in order to prevent the thickness center portion from being excessively maintained in the ferrite transformation region, the cooling rate of the following [Relationship 3] must be greater than the specific cooling rate (CR 最小 ) Faster cooling. In addition, limiting the CT as the cooling termination temperature to 400-550°C not only suppresses the formation of coarse ferrite phase and coarse carbides but also minimizes the formation of MA phase and martensite phase.

[0097] The method for manufacturing high-strength thick hot-rolled steel plates of the present invention comprises the following steps: reheating a steel billet satisfying the composition and [Relationship 1] at 1200-1350°C; hot-rolling the reheated steel billet within a temperature range satisfying the following [Relationship 2]; cooling the hot-rolled steel plate to a temperature in the range of 450-550°C at a cooling rate satisfying the following [Relationship 3], and then coiling at a temperature in the range of 400-550°C; and air-cooling or water-cooling the coiled steel plate to a temperature in the range of room temperature to 200°C.

[0098] First, in the present invention, the steel slab satisfying the composition and the [Relationship 1] is reheated at 1200-1350°C.

[0099] At this point, if the reheating temperature is lower than 1200°C, the precipitates will not fully redissolve, reducing the formation of precipitates in the post-hot rolling process, resulting in residual coarse TiN. Furthermore, if the reheating temperature exceeds 1350°C, abnormal grain growth of austenite grains may lead to a decrease in strength. Considering this, in the present invention, the reheating temperature is preferably limited to the range of 1200-1350°C.

[0100] Next, in the present invention, the reheated steel slab is subjected to hot finish rolling within a temperature range satisfying [Relational Expression 2].

[0101] In the present invention, the hot rolling start temperature is preferably 1150°C or lower, and the finish hot rolling temperature is adjusted to satisfy the following [Equation 2] in consideration of the fine structure of the steel. Starting hot rolling at a temperature higher than 1150°C increases the temperature of the hot-rolled steel sheet, resulting in coarsening of the grain size and possible degradation of the surface quality of the hot-rolled steel sheet.

[0102] In particular, in the present invention, when rolling is completed at a temperature higher than Tn, the temperature set forth in the following [Relationship 2], the steel's microstructure becomes coarse and non-uniform. Furthermore, when rolling is completed at a temperature lower than Tn-70°C, the relatively low temperature surface layer of thick high-strength steel corresponding to a steel plate thickness of 10-14 mm promotes ferrite transformation, increasing the fraction of fine ferrite phase. However, this phase has an elongated grain shape, potentially causing rapid crack propagation. Furthermore, a non-uniform microstructure may remain in the center of the thickness, potentially impairing durability.

[0103] In the present invention, a reduction of 10-60% is required within the hot finish rolling temperature range to achieve the aforementioned effects. When the reduction is less than 10%, the recrystallization delay effect is difficult to achieve, resulting in the formation of uneven, coarse grains. Furthermore, when the reduction exceeds 60%, an overly stretched microstructure forms, which forms along the grain boundaries of carbides within the microstructure, making cracks along these grain boundaries more likely to form during forming. Furthermore, the number of fine precipitates decreases, reducing the precipitation strengthening effect.

[0104] [Equation 2]

[0105] [Equation 2]

[0106] Tn-70≤FDT≤Tn

[0107] Tn=750+92×[C]+70×[Mn]+45×[Cr]+647×[Nb]+515×[Ti]-50×[Si]-2.4×(t-5)

[0108] In the above formula 2, C, Mn, Cr, Nb, Ti, and Si are the weight % of the corresponding alloying elements.

[0109] FDT in the above-mentioned relational expression 2 is the temperature (° C.) of the hot-rolled sheet at the end of hot rolling.

[0110] In the above relational expression 2, t is the thickness (mm) of the finished rolled plate.

[0111] Subsequently, in the present invention, the hot-finished rolled steel sheet is cooled to a temperature in the range of 450-550° C. at a cooling rate satisfying the following [Relationship 3], and then the cooled steel sheet is coiled at a temperature in the range of 400-550° C.

[0112] The temperature range from FDT, which is the temperature after hot rolling, to CT, which is the cooling termination temperature, corresponds to the temperature range where ferrite transformation occurs during cooling. Since the cooling rate of the thickness center is slower than that of the thickness surface of the rolled plate, coarse ferrite phase and coarse carbide are formed in the thickness center, resulting in an uneven microstructure. Therefore, in order to suppress this, it is necessary to cool the steel sheet at a lower temperature than the specific cooling rate (CR 最小) faster cooling. In addition, when the average cooling rate of the surface portion of the hot-rolled steel sheet (the region from the steel sheet surface to 1-2 mm in the thickness direction) in this temperature range exceeds 60°C / second (sec), the difference in cooling rate between the surface portion and the deep portion is too large, resulting in a significant increase in the difference in hardness between the surface portion and the deep portion, which may deteriorate formability and durability. There are also disadvantages such as the equipment load caused by extreme cooling in the previous step of the equipment and the difficulty in achieving a slab with a small cut. Therefore, as shown in the following [Relationship 3], it is necessary to cool the steel in consideration of the steel composition to meet the set cooling rate.

[0113] Furthermore, in the present invention, when the coiling temperature exceeds 550°C, the pearlite phase forms a banded structure or a large amount of coarse carbides, resulting in insufficient formability and durability of the steel. When the coiling temperature is below 400°C, excessive martensite and MA phases form, deteriorating formability and durability. Therefore, in the present invention, the coiling temperature is preferably limited to 450-550°C.

[0114] [Equation 3]

[0115] CR 最小 ≤CR*≤60

[0116] CR 最小 =65-157×[C]-25.2×[Si]-14.1×[Mn]-27.3×[Cr]+61×[Ti]+448×[Nb]+1.4×(t-5)

[0117] In the above formula 3, C, Si, Mn, Cr, Ti, and Nb are the weight % of the corresponding alloying elements.

[0118] CR* in the above relational expression 3 represents the cooling rate (° C. / second) when the rolled sheet is cooled after hot rolling.

[0119] Furthermore, in the present invention, the coiled hot-rolled steel sheet is air-cooled or water-cooled to a temperature ranging from room temperature to 200°C.

[0120] Air cooling of steel plates refers to cooling in ambient air at a cooling rate of 0.001-10°C / hour. Even if the cooling rate exceeds 10°C / hour, as long as the coiling temperature and cooling conditions are adhered to, some of the untransformed phase in the steel can be suppressed from transforming to the MA phase, so water cooling is also acceptable. Water cooling involves cooling coils in a room-temperature water tank. Furthermore, controlling the cooling rate to less than 0.001°C / hour requires additional heating and heat preservation equipment, which is economically disadvantageous.

[0121] Next, in the present invention, the steel sheet may be pickled and oiled after the air cooling or water cooling, thereby manufacturing a pickled and oiled (PO) steel sheet.

[0122] The high-strength thick hot-rolled steel plate of the present invention with a thickness of 10-14 mm manufactured by the manufacturing process as described above has a steel microstructure, which comprises, in terms of area %, polygonal ferrite: 25-50%, bainitic ferrite and acicular ferrite: 30-50%, and bainite: less than 20%. 2 ) is less than 5% by area fraction of carbides and pearlite structures with a diameter of 0.5 μm or more, and less than 5% by area fraction of MA phase (martensite-austenite component), and the high-strength thick hot-rolled steel plate can show a tensile strength of 590 MPa or above and an elongation of 30% or above.

[0123] Furthermore, the hot rolled steel sheet of the present invention has an El×TS×0.5Hv 最大 The / ΔH value can meet 140000 or above. DETAILED DESCRIPTION

[0124] Hereinafter, the present invention will be described in more detail with reference to examples.

[0125] (Example)

[0126] Table 1 below shows steel slab compositions having the chemical compositions shown in the present invention. Hot-rolled steel sheets were produced from each of these steel slabs using the manufacturing process conditions shown in Table 2 below, including the rolling thickness, hot finish rolling temperature (FDT), coiling temperature (CT), reduction (%) applied within the non-recrystallization temperature range (Tn-70°C or lower), and cooling rate (CR) from the coiling temperature to the cooling end temperature after hot finish rolling. The cooling rate for the steel sheets after coiling, not shown in Table 2, was maintained at a constant 1°C / hour.

[0127] The microstructure fractions of the surface layer (1 mm below the surface) and the center of the thickness (2 / t) of the produced hot-rolled steel sheets were measured and are shown in Table 3 below. The area fraction of the MA phase is the result of analysis at 1000x magnification using an optical microscope and an image analyzer after etching using the Lepera etching method. Furthermore, the phase fractions of martensite (M), polygonal ferrite (PF), bainitic ferrite (BF), acicular ferrite (AF), bainite (B), and pearlite (P) were measured based on analysis at 3000x and 5000x magnification using a scanning electron microscope (SEM) after etching using a nitric acid etchant.

[0128] The mechanical properties and hardness of the produced hot-rolled steel sheets were measured, and the results are shown in Table 4 below. YS, TS, YR, and E1 refer to the 0.2% offset yield strength, tensile strength, yield ratio, and elongation at break, and are the results of testing using JIS No. 5 test pieces taken parallel to the rolling direction.

[0129] In addition, the hardness measurement is carried out using a Micro-Vickers hardness tester. The Vickers hardness value is measured in the thickness direction at intervals of 0.5 mm from the surface (1 mm below the surface) to 1 / 2 t, while applying a load of 500 g. The highest Vickers hardness value measured is defined as H 最大 , the minimum hardness value is defined as H 最小 , ΔHv refers to H 最大 -H 最小 .

[0130] [Table 1]

[0131]

[0132] [Table 2]

[0133]

[0134] [Table 3]

[0135]

[0136] * In Table 3, P refers to pearlite and 2 ) is the sum of the fractions of carbides with a diameter of 0.5 μm or more observed in the sample.

[0137] [Table 4]

[0138]

[0139] *In Table 4, A* is E1×TS×0.5Hv 最大 / ΔH

[0140] As shown in Tables 1 to 4, it can be seen that Invention Examples 1 to 7, which simultaneously satisfy the composition ranges, manufacturing conditions, and Relational Formulas 1 to 3 proposed in the present invention, have the target microstructures and material properties.

[0141] In contrast, Comparative Example 1 shows a case where the C content is outside the range proposed by the present invention, and [Relational Expression 1], which takes segregation into account, exceeds the range of the present invention. In this case, it is found that excessive pearlite is formed in the center and surface layers of the microstructure, resulting in insufficient ductility and large variations in hardness across the thickness.

[0142] Comparative Example 2 shows a case where the Si content does not meet the range of the present invention and does not satisfy [Relational Expression 1]. It can be seen that the MA phase fraction is high in both the surface and deep layers. This is because excessive Si addition increases the transformation temperature, forming a ferrite phase in the surface during hot rolling, resulting in two-phase rolling. Part of the untransformed austenite phase is converted to the MA phase, resulting in poor elongation.

[0143] Comparative Examples 3 to 4 are cases where [Relationship 1] is not satisfied while departing from the composition range of Mn proposed in the present invention. Specifically, Comparative Example 3 is a case where the Mn content is too high, pearlite structure is excessively formed, and extended MnS inclusions are also observed. In particular, when measuring the hardness in the thickness direction, a high hardness difference is locally shown, and ductility is also insufficient. In addition, in Comparative Example 4, the Mn content is small, and segregation or coarse carbides and uneven pearlite structure along the thickness direction of the rolled plate are not formed, but due to insufficient yield strength and tensile strength, the result of departing from the present invention is shown.

[0144] Comparative Examples 5 and 6 are cases where the steel composition meets the range of the present invention, but the FDT range does not meet the range of the present invention. Comparative Example 5 leads to the formation of coarse ferrite (polygonal ferrite), which does not meet the target strength. On the other hand, Comparative Example 6 is rolled in the two-phase region to form a large amount of extended coarse ferrite (polygonal ferrite), so the structural heterogeneity increases and the strength also deteriorates.

[0145] Comparative Examples 7 and 8 do not meet the coiling temperature requirements of the present invention. Specifically, in Comparative Example 7, where the coiling temperature was above the range specified in the present invention, pearlite was locally formed within the structure, with pearlite bands observed particularly in the center of the thickness. Consequently, when measuring hardness through the thickness, a high localized hardness difference was observed. Furthermore, in Comparative Example 8, where the coiling temperature was below the range specified in the present invention, it was confirmed that excessive martensite formation within the structure led to degraded elongation.

[0146] Comparative Example 9 is a case where the cooling rate during cooling after hot rolling is slower than the range proposed in the present invention. It can be confirmed that the fraction of coarse ferrite (polygonal ferrite) in the structure is high, which not only deteriorates the strength, but also forms pearlite and coarse carbides in the center of the thickness, causing hardness deviation.

[0147] in addition, Figure 1 The photographs show the cross-sectional microstructures of the steel plates (surface layer and 1 / 4t) in the examples of the present invention in Inventive Example 3 and Comparative Example 9. It can be seen that Inventive Example 3 has a more homogeneous structure than Comparative Example 9.

[0148] and, Figure 2 Graph showing the distribution of hardness values ​​in thickness cross sections of Inventive Example 3 and Comparative Example 3 in the examples of the present invention. It can be seen that the hardness variation at each thickness is smaller in Inventive Example 3 than in Comparative Example 3.

[0149] Figure 3 The elongation levels of the invention examples and comparative examples are shown in the examples of the present invention. 最大 / ΔH value distribution diagram.

[0150] As described above, the preferred embodiments of the present invention are described in the detailed description of the present invention. However, those skilled in the art will readily appreciate that various modifications can be made without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments but should be determined by the claims and their equivalents.

Claims

1. A high-strength thick hot-rolled steel plate with a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher, wherein the high-strength thick hot-rolled steel plate comprises, in weight percent, C: 0.05-0.15%, Si: 0.01-1.0%, Mn: 1.0-2.0%, acid-soluble aluminum (Sol.Al): 0.01-0.1%, Cr: 0.005-1.0%, P: 0.001-0.02%, S: 0.001-0.01%, N: 0.001-0.01%, Nb: 0.005-0.07%, Ti: 0.005-0.11%, and the balance being iron and unavoidable impurities, and satisfies the following relationship 1: The high-strength thick hot-rolled steel plate has a fine structure, which, in terms of area %, comprises: polygonal ferrite: 25-50%, bainitic ferrite and acicular ferrite: 30-50%, bainite: less than 20%, and 1 cm per unit area. 2 The sum of the area fractions of carbides and pearlite with a diameter of 0.5 μm or more observed in the interior is less than 5%, and the MA phase, i.e., martensite-austenite component, is less than 5%. [Equation 1] 0.3≤R≤1.0 R=[C]*+0.7×[Mn]+8.5×[P]+7.5×[S]-0.9×[Si]-1.5×[Nb] [C]*=[C]-[C]×Q Q=([Nb] / 93+[Ti] / 48) / ([C] / 12) In the above formula 1, C, Mn, P, S, Si, Nb, and Ti are the weight % of the corresponding alloying elements.

2. The high-strength thick hot-rolled steel plate with a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher according to claim 1, characterized in that: The hot rolled steel plate has an El×TS×0.5Hv 最大 / ΔH value satisfies 140000 or more, wherein the hardness measurement is to use a micro Vickers hardness tester to measure the Vickers hardness value along the thickness direction from 1mm to 1 / 2t of the thickness directly below the surface at intervals of 0.5mm. At this time, a load of 500g is applied, and the highest Vickers hardness value measured at this time is defined as H 最大 , the minimum hardness value is defined as H 最小 , ΔHv refers to H 最大 -H 最小 .

3. A method for producing a high-strength thick hot-rolled steel plate having a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher, comprising the following steps: The steel slab is reheated at 1200-1350° C., and comprises, in terms of weight%, the following: C: 0.05-0.15%, Si: 0.01-1.0%, Mn: 1.0-2.0%, acid-soluble aluminum (Sol.Al): 0.01-0.1%, Cr: 0.005-1.0%, P: 0.001-0.02%, S: 0.001-0.01%, N: 0.001-0.01%, Nb: 0.005-0.07%, Ti: 0.005-0.11%, and the balance is iron and unavoidable impurities, and satisfies the following relationship 1; The reheated steel slab is subjected to hot finish rolling at a reduction of 10-60% within a temperature range satisfying the following relational expression 2; Cooling the hot-rolled steel plate to a temperature in the range of 450-550° C. at a cooling rate satisfying the following relational expression 3, and then coiling the steel plate at a temperature in the range of 400-550° C.; and The coiled steel plate is air-cooled or water-cooled to a temperature ranging from room temperature to 200°C. [Equation 1] 0.3≤R≤1.0 R=[C]*+0.7×[Mn]+8.5×[P]+7.5×[S]-0.9×[Si]-1.5×[Nb] [C]*=[C]-[C]×Q Q=([Nb] / 93+[Ti] / 48) / ([C] / 12) In the above formula 1, C, Mn, P, S, Si, Nb and Ti are the weight percentages of the corresponding alloying elements. [Equation 2] Tn-70≤FDT≤Tn Tn=750+92×[C]+70×[Mn]+45×[Cr]+647×[Nb]+515×[Ti]-50×[Si]-2.4×(t-5) In the above formula 2, C, Mn, Cr, Nb, Ti and Si are the weight percentages of the corresponding alloying elements. The FDT in the relational expression 2 is the temperature of the hot-rolled plate at the end of hot rolling, wherein the unit of the temperature is ° C. In the relational expression 2, t is the thickness of the final rolled plate, wherein the unit of the thickness is mm. [Equation 3] CR 最小 ≤CR*≤60 CR 最小 =65-157×[C]-25.2×[Si]-14.1×[Mn]-27.3×[Cr]+61×[Ti]+448×[Nb]+ 1.4×(t-5) In the above formula 3, C, Si, Mn, Cr, Ti and Nb are the weight percentages of the corresponding alloy elements. CR* in the above equation 3 is the cooling rate when the rolled plate is cooled after hot rolling, wherein the unit of the cooling rate is °C / second. In the relational expression 3, t is the thickness of the final rolled plate, wherein the unit of the thickness is mm.

4. The method for manufacturing a high-strength thick hot-rolled steel plate with a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher according to claim 3, characterized in that: After the air cooling or water cooling, the method further includes the steps of pickling and oiling the steel plate.

5. The method for manufacturing a high-strength thick hot-rolled steel plate with a thickness of 10-14 mm, a tensile strength of 590 MPa or higher, and an elongation of 30% or higher according to claim 3, characterized in that: The hot-rolled steel plate has a fine structure, which comprises, in terms of area %, polygonal ferrite: 25-50%, bainitic ferrite and acicular ferrite: 30-50%, bainite: less than 20%, and 1 cm2 per unit area. 2 The sum of the area fractions of carbides and pearlite with a diameter of 0.5 μm or more observed in the inner cavity is less than 5%, and the MA phase, i.e., martensite-austenite component, is less than 5%, and El×TS×0.5Hv 最大 / ΔH value satisfies 140000 or more, wherein the hardness measurement is to use a micro Vickers hardness tester to measure the Vickers hardness value along the thickness direction from 1mm to 1 / 2t of the thickness directly below the surface at intervals of 0.5mm. At this time, a load of 500g is applied, and the highest Vickers hardness value measured at this time is defined as H 最大 , the minimum hardness value is defined as H 最小 , ΔHv refers to H 最大 -H 最小 .

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