High-yield-ratio high-strength steel with excellent impact resistance after cold forming and its manufacturing method

Abstract

The present invention relates to a high strength steel and a manufacturing method thereof, and more particularly to a high strength steel having excellent impact resistance after cold forming and a high yield ratio, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel and a manufacturing method thereof, and more particularly to a high-strength steel having excellent impact resistance after cold forming and a high yield ratio, and a manufacturing method thereof. [Background technology]

[0002] Due to vehicle characteristics, conventional commercial vehicle chassis components typically use steel materials with a thickness of 8 mm or more. High-strength hot-rolled steel sheets with a tensile strength of 600 MPa or more are used for chassis members, and hot-rolled steel sheets with a tensile strength of 440 MPa or more are used for wheel discs. However, in recent years, in order to reduce vehicle weight, high-strength steel sheets with a tensile strength of 700 MPa or more are being used for chassis members and 590 MPa or more for wheel discs, with a trend toward reducing the thickness of steel sheets and changing the design of components. Furthermore, while wheels were traditionally manufactured using a press forming process, they are now being manufactured using spinning and flow forming. Because these forming processes subject hot-rolled steel sheets to large deformations, hot-rolled steel sheets with higher elongation are required, and the formed parts must ensure durability and impact resistance during use.

[0003] However, when conventional high-strength steels are applied to the spinning and flow forming processes, the durability of the resulting parts must be equal to or greater than that of conventional steels. However, they are difficult to apply because they can cause fine cracks on shear surfaces during forming of the parts and have poor durability in areas with a high forming volume.

[0004] Conventional high-strength steels, as disclosed in Patent Documents 1 and 2, are hot-rolled in the normal austenite region, followed by coiling at high temperatures to form a ferrite matrix and fine precipitates. Patent Document 3, for example, employs a technique in which the coiling temperature is cooled to a temperature at which a bainite matrix is ​​formed, and then the steel is coiled, in order to prevent the formation of coarse pearlite. Patent Document 4 also proposes a technique for refining austenite grains by using Ti, Nb, or the like to reduce the roll width by 40% or more in the non-recrystallized region during hot rolling. Patent Document 5 recently proposed a technique for improving the uniformity of the microstructure between the surface and deep layers of the steel sheet and suppressing the formation of coarse carbides. Patent Document 6 also proposed a technique for simultaneously suppressing the formation of pearlite, MA (Martensite and Austenite) phases, and martensite, which adversely affect durability.

[0005] However, Patent Documents 1 to 4 fail to consider the occurrence of cracks on and around the sheared surfaces during shear forming of high-strength thick steel sheets, and are configured with cooling rate and large reduction conditions that are difficult to achieve during the production of thick steel sheets with a thickness of 8 mm or more. When using precipitate-forming elements such as Ti, Nb, and V to refine the grains of thick steel sheets and ensure strength, coiling at high temperatures of 500 to 700°C, where precipitates are likely to form, can lead to excessive ferrite growth, reducing yield strength and forming coarse pearlite. Furthermore, even when manufacturing at a low coiling temperature to utilize a bainite matrix structure, if the cooling rate in the width direction of the steel sheet during cooling after hot rolling is not uniformly controlled, a non-uniform microstructure will form in the hot-rolled steel sheet, making it difficult to achieve high elongation and stable high yield ratio properties. This also increases sensitivity to forming cracks, such as cracks on the sheared surfaces, during forming. Furthermore, because this technology is intended for hot-rolled steel sheets less than 5 mm thick used in passenger cars, the required cooling rate is too high, making it unsuitable for manufacturing thick steel sheets. Furthermore, applying a large reduction of 40% in the unrecrystallized region can degrade the shape quality of the rolled sheet and place a strain on the equipment, making it difficult to apply to thick steel sheets with a thickness of 8 mm or more. Patent Documents 5 and 6 are inventions aimed at thick steel sheets. First, Patent Document 5 proposes a manufacturing technology for thick high-strength steel sheets, in which the crystal grain shape in the deep thickness portion (1 / 4t to 1 / 2t) is equiaxed and has fine crystal grains, thereby suppressing the formation of MA phases and martensite, in order to improve the durability of the thick high-strength steel. Patent Document 6 proposes a manufacturing technology in which a hot-rolled coil is divided into three equal parts in the length direction using a relational expression derived for specific components, and the head, mid, and tail portions are cooled to different cooling finish temperatures at constant cooling rates, and then coiled. This technology produces a uniform microstructure by controlling the cooling rate after hot rolling through a relationship derived for specific components, taking into account the quality of the shear surface of the part. Although this technology is effective in improving the durability of commercial vehicle wheels, which contain many punching holes and are subjected to constant loads, it does not take into account the impact resistance after forming.Furthermore, it is difficult to uniformly control the cooling of the steel sheet after hot rolling over the entire width, and when the hot-rolled steel sheet is thick, such as 8 mm or more, it is difficult to control it at an actual cooling rate.

[0006] The cooling process after hot rolling is usually carried out within several tens of seconds from a 100-120 m long run-out table (ROT), but it is difficult to produce a hot-rolled steel sheet by cooling it to the cooling finish temperature or coiling temperature while the cooling rate at the deep part of the thickness of the hot-rolled steel sheet satisfies the proposed range. Therefore, in the above-mentioned conventional technology, it is difficult to obtain the effect of suppressing the formation of coarse carbides in a hot-rolled steel sheet having a thickness of 8 mm or more, and there are problems in that it is not possible to ensure a high level of impact resistance. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2010-0029138 (Published on March 15, 2010) [Patent Document 2] Japanese Patent Publication No. 2007-262487 (published October 11, 2007) [Patent Document 3] Korean Patent Publication No. 10-1528084 (Published June 10, 2015) [Patent Document 4] Japanese Patent Publication No. Hei 9-143570 (published June 3, 1997) [Patent Document 5] Korean Patent Publication No. 10-2020-0062422 (Published June 4, 2020) [Patent Document 6] Korean Patent Publication No. 10-2021-0068808 (Published June 10, 2021) Summary of the Invention [Problem to be solved by the invention]

[0008] An object of one aspect of the present invention is to provide a high-strength steel having excellent impact resistance after cold forming and a high yield ratio, and a method for producing the same.

[0009] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the entire content of this specification. [Means for solving the problem]

[0010] One aspect of the present invention is a steel sheet comprising, by weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, the balance being Fe and other unavoidable impurities, with the total of Nb and Ti being 0.04 to 0.1%, The microstructure of the surface layer within a thickness of 50 μm from the surface contains, by area%, 95% or more of equiaxed ferrite and 3% or less of pearlite, and contains a total of 5% or less of one or more of bainitic ferrite, bainite, MA (Martensite-Austenite constituent) phase, and martensite; It is possible to provide a steel sheet in which the microstructure in the center part of the 1 / 4 to 3 / 4 thickness range contains, in area percentages, 80 to 95% bainitic ferrite, 10% or less bainite, 3% or less pearlite, 5 to 10% in total of one or two of the MA (Martensite-Austenite constituent) phase and martensite, and the remainder contains equiaxed ferrite.

[0011] The steel plate may have a thickness of 8 to 25 mm.

[0012] The steel plate may have a tensile strength of 590 MPa or more, a breaking elongation of 25% or more, a yield ratio of 0.75 to 0.9, and an impact toughness of 70 J or more at -20°C after cold forming.

[0013] The steel plate may have a ratio (E / YS) of impact toughness (E) at −20° C. after cold forming to yield strength (YS) before cold forming of 0.15 or more.

[0014] The steel plate includes edge portions corresponding to 30% regions at both ends in the width direction and a central portion of a central 40% region corresponding to a region excluding both edge portions, and the difference in tensile strength between the edge portions and the central portion may be 10 MPa or less, the difference in fracture elongation may be 8% or less, and the difference in impact toughness at -20°C after cold forming may be 20 J or less.

[0015] Another aspect of the present invention is a process for reheating a steel slab containing, by weight, C: 0.05-0.15%, Si: 0.01-0.5%, Mn: 1.0-2.0%, Al: 0.01-0.1%, Cr: 0.001-1.0%, P: 0.001-0.05%, S: 0.001-0.01%, N: 0.001-0.01%, Ti: 0.03-0.08%, Nb: 0.01-0.05%, the balance being Fe and other unavoidable impurities, and the sum of Nb and Ti being 0.04-0.1%; hot rolling the reheated steel slab; and The hot-rolled steel sheet is cooled to a temperature range of 500 to 650°C at an average cooling rate of 1 to 30°C / s, which is equal to or greater than the CR value defined by the following Relation 1, and then coiled, In the cooling and winding step, the edge portions corresponding to 30% of the area at both ends in the coil width direction are cooled at a temperature of 550 to 650 ° C (TE), and the central portion of the central 40% area corresponding to the area excluding both edge portions in the width direction is cooled at a temperature of 500 to 550 ° C (TC), A method for manufacturing a steel sheet in which the average temperature difference between the edge portion and the center portion is 50 to 150°C can be provided. [Equation 1] CR=45-16.3×[C]-5.6×[Si]-16.3×[Mn]-2.9×[Cr]+15×[Ti]+23×[Nb]-0.9×(t-8) (Here, [C], [Si], [Mn], [Cr], [Ti] and [Nb] are the weight percentages of each element, and t is the thickness of the steel plate (mm).)

[0016] The reheating temperature is 1100 to 1350°C, The hot rolling temperature can be 800 to 1150°C.

[0017] The method may further include air-cooling the wound coil at a temperature range of 200°C or less.

[0018] The steel plate may have a thickness of 8 to 25 mm. [Effects of the Invention]

[0019] According to one aspect of the present invention, it is possible to provide a high-strength steel having excellent impact resistance after cold forming and a high yield ratio, and a method for producing the same.

[0020] According to one aspect of the present invention, it is possible to provide a high-strength steel that can be used for steel materials used in chassis members and wheels of medium and large commercial vehicles, and a method for manufacturing the same. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a photograph showing the microstructure of Example 2 according to an embodiment of the present invention, observed with a scanning electron microscope (×3,000). [Figure 2] 1 is a photograph showing the microstructure of Comparative Example 2 according to an embodiment of the present invention, observed with a scanning electron microscope (×3,000). DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to further explain the present invention in detail to those skilled in the art to which the invention pertains.

[0023] In order to solve the above-mentioned conventional problems and ensure excellent formability and impact resistance, the present inventors have studied the change in impact resistance after cold forming depending on the microstructural characteristics of the steel sheet. As a result, they have confirmed that the desired physical properties can be achieved by optimizing the alloy composition and manufacturing conditions to control the microstructure according to the thickness and width directions of the steel sheet, which led to the completion of the present invention.

[0024] In hot-rolled steel sheets typically manufactured in the form of coils, coarse carbides and pearlite are likely to form when maintained at high temperatures of approximately 500 to 700°C for a long period of time. In particular, if the ferrite phase transformation that begins during the cooling process after hot rolling proceeds slowly, the untransformed phases tend to contain a high amount of carbon, creating conditions that favor the formation of coarse carbides and pearlite. Furthermore, the cooling rate is slower in the center of the coil width than in the edge regions, which further promotes the development of these structures. Therefore, to prevent the formation of coarse carbides and pearlite in the center of the coil width, the coil must be cooled to room temperature by forced cooling, such as water cooling. However, this is undesirable because the edge regions, where the cooling rate is faster, experience excessive formation of martensite and MA (Martensite and Austenite) phases, resulting in a non-uniform microstructure, making it difficult to achieve high elongation, and increasing cracking on the shear surface. Therefore, the present invention proposes a method for preventing the formation of coarse carbides and pearlite without forced cooling of the coil.

[0025] The present invention will be described in detail below.

[0026] The steel composition of the present invention will be described in detail below.

[0027] In the present invention, unless otherwise specified, the percentage representing the content of each element is based on weight.

[0028] A steel according to one aspect of the present invention may contain, by weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, the balance being Fe and other unavoidable impurities, and the total of Ti and Nb may be 0.04 to 0.1%.

[0029] Carbon (C): 0.05~0.15% Carbon (C) is the most economical and effective element for strengthening steel. Increased addition of carbon increases the precipitation strengthening effect or the bainite phase fraction, making it easier to ensure strength. However, as the thickness of hot-rolled steel increases, the cooling rate at the center of the thickness during cooling after hot rolling slows, and a high carbon (C) content can easily lead to the formation of coarse carbides and pearlite. Therefore, if the carbon (C) content is less than 0.05%, sufficient strengthening effect is difficult to achieve. If the carbon (C) content exceeds 0.15%, the formation of pearlite and coarse carbides at the center of the thickness can lead to problems such as reduced impact resistance and poor weldability. The preferred lower limit is 0.055%, and the more preferred upper limit is 0.12%.

[0030] Silicon (Si): 0.01 to 0.5% Silicon (Si) is an element that is effective in deoxidizing molten steel and solid-solution strengthening steel, and also in delaying the formation of coarse carbides, thereby improving formability. However, if its content is less than 0.01%, the effects of solid-solution strengthening and delaying carbide formation cannot be maximized. If its content exceeds 0.5%, red scale is formed on the steel sheet surface during hot rolling, resulting in significantly inferior quality, as well as reduced ductility and weldability. Preferably, Si is contained in an amount of 0.05% or more, and more preferably 0.3% or less.

[0031] Manganese (Mn): 1.0-2.0% Manganese (Mn), like Si, is an effective element for solid-solution strengthening of steel, increasing the hardenability of steel and facilitating the formation of bainite during cooling after hot rolling. However, if the Mn content is less than 1.0%, the above effects of addition are not obtained. If the Mn content exceeds 2.0%, the hardenability increases significantly, making martensitic phase transformation more likely to occur, and there is a risk of large segregation developing from the center of the thickness of the slab during continuous casting. Preferably, Mn should be contained in an amount of 1.3% or more, and more preferably 1.8% or less.

[0032] Aluminum (Al): 0.01 to 0.1% Aluminum (Al) is a component added primarily for deoxidation, and if its content is less than 0.01%, its effect may be insufficient. On the other hand, if its content exceeds 0.1%, it combines with nitrogen to form AlN, which is likely to cause corner cracks in the slab during continuous casting and defects due to the formation of inclusions. It is preferable that Al be contained in an amount of 0.015% or more, and more preferably 0.06% or less.

[0033] Chromium (Cr): 0.001 to 1.0% Chromium (Cr), like Mn, strengthens steel through solid solution and slows the ferrite phase transformation during cooling, promoting the formation of bainite. However, if its content is less than 0.001%, the above effects are not achieved. If its content exceeds 1.0%, the ferrite phase transformation is excessively delayed, resulting in excessive martensite formation and reduced elongation. Furthermore, excessive Cr addition can lead to the development of large segregations in the center of the steel, resulting in a non-uniform microstructure across the thickness and reduced impact resistance. The preferred lower limit is 0.01%, and the more preferred upper limit is 0.5%.

[0034] Phosphorus (P): 0.001 to 0.05% Like Si, phosphorus (P) simultaneously strengthens the solid solution and promotes ferrite transformation. However, manufacturing with a phosphorus (P) content of less than 0.001% is economically disadvantageous due to the high manufacturing costs, and is insufficient to obtain sufficient strength. On the other hand, if the P content exceeds 0.05%, embrittlement occurs due to grain boundary segregation, which easily causes microcracks during molding and significantly reduces impact resistance.

[0035] Sulfur (S): 0.001 to 0.01% S is an impurity present in steel, and if its content exceeds 0.01%, it combines with Mn and other elements to form non-metallic inclusions, which can cause microcracks during forming and significantly reduce impact resistance. However, reducing the content to less than 0.001% requires a long time during steelmaking operations, resulting in reduced productivity.

[0036] Nitrogen (N): 0.001-0.01% Nitrogen (N), along with C, is a representative solid solution strengthening element and forms coarse precipitates with Ti, Al, etc. Generally, the solid solution strengthening effect of nitrogen (N) is superior to that of C, but the problem is that the toughness decreases significantly as the amount of nitrogen (N) in steel increases. Therefore, the upper limit of N content can be limited to 0.01%. However, manufacturing steel with a N content of less than 0.001% requires a long time during steelmaking operations, resulting in poor productivity.

[0037] Titanium (Ti): 0.03 to 0.08% Titanium (Ti) is a typical precipitation strengthening element and forms coarse TiN in steel due to its strong affinity with N. TiN has the effect of suppressing grain growth during the heating process for hot rolling. Furthermore, TiC precipitates formed when the remaining titanium (Ti) reacts with N and dissolves in the steel, bonding with C, are a useful component for improving the strength of steel. If the titanium (Ti) content is less than 0.03%, the above effect cannot be achieved. However, if the titanium (Ti) content exceeds 0.08%, the generation of coarse TiN and the coarsening of the precipitates can lead to problems such as reduced impact resistance during forming. Preferably, the titanium content is 0.04% or more, and more preferably 0.075% or less.

[0038] Niobium (Nb): 0.01-0.05% Niobium (Nb), along with Ti, is a representative precipitation strengthening element that precipitates during hot rolling and effectively improves the strength and impact toughness of steel by refining grains through delayed recrystallization. If the niobium (Nb) content is less than 0.01%, the above effect cannot be achieved. If the Nb content exceeds 0.05%, excessive delayed recrystallization during hot rolling can lead to the formation of elongated grains and coarse complex precipitates, deteriorating formability. The preferred lower limit is 0.015%, and the more preferred upper limit is 0.04%.

[0039] In addition to the above-described composition, the steel of the present invention may contain the remaining iron (Fe) and inevitable impurities. Since inevitable impurities may be unintentionally mixed in during normal manufacturing processes, they cannot be excluded. Since such impurities are known to anyone skilled in the field of normal steel manufacturing, the details of all of them will not be specifically mentioned in this specification.

[0040] In the steel according to one aspect of the present invention, the total content of niobium (Nb) and titanium (Ti) can be 0.04 to 0.1%.

[0041] Niobium (Nb) and titanium (Ti) precipitate as (Ti, Nb)(C, N)-based composite precipitates. Their precipitation during hot rolling significantly enhances grain refinement by delaying recrystallization. However, excessive formation of these composite precipitates increases the number of coarse composite precipitates, resulting in little strength improvement and poor formability. If the total content of niobium (Nb) and titanium (Ti) is less than 0.04%, the grain refinement and strength improvement effects may be insufficient. On the other hand, if the total content exceeds 0.1%, formability deteriorates and is economically disadvantageous. The preferred lower limit is 0.045%, and the more preferred upper limit is 0.09%.

[0042] The steel microstructure of the present invention will now be described in detail.

[0043] Unless otherwise specified in the present invention, the percentages representing the fractions of the microstructures are based on the area.

[0044] In one aspect of the steel of the present invention, the microstructure of the surface layer portion within a 50 μm thickness range from the surface contains, in area %, 95% or more equiaxed ferrite, 3% or less pearlite, and a total of 5% or less of one or more of bainitic ferrite, bainite, MA (Martensite-Austenite constituent) phase, and martensite, while the microstructure of the center portion within a 1 / 4 to 3 / 4 thickness range contains, in area %, 80 to 95% bainitic ferrite, 10% or less bainite, 3% or less pearlite, and a total of 5 to 10% of one or two of MA (Martensite-Austenite constituent) phase and martensite, with the remainder being equiaxed ferrite.

[0045] In the present invention, if the surface layer contains less than 95% equiaxed ferrite, the ductility may be insufficient during spinning and flow forming, which are used in manufacturing commercial vehicle wheels. This may result in severe work hardening in the surface layer, leading to the risk of microcracks occurring during forming. In particular, if more than 3% of highly brittle pearlite is formed, or if more than 5% of one or more of the highly hard bainitic ferrite, bainite, MA phase, and martensite is contained, cracks may easily propagate along the interface with the matrix phase. Therefore, to suppress the occurrence of microcracks formed in the surface layer during forming and prevent crack propagation, it is preferable that the surface layer contains less than 5% of bainitic ferrite, bainite, MA phase, and martensite in total. In the present invention, the microstructure of the surface layer may contain 100% equiaxed ferrite, and the total of pearlite, bainitic ferrite, bainite, MA phase, and martensite may be 0%.

[0046] Furthermore, if the bainitic ferrite content in the center is less than 80%, cracks easily occur on the sheared surface during punching and shear forming during wheel manufacturing, resulting in poor impact resistance after forming. Furthermore, during the cooling process of a steel plate after hot rolling, after bainitic ferrite, which is the matrix structure, is formed in the center of the thickness of the rolled plate, high concentrations of residual carbon remain in the untransformed austenite, which facilitates the formation of pearlite. However, if the pearlite content exceeds 3%, severe cracks occur on the sheared surface during forming, resulting in poor impact resistance after forming. When the pearlite fraction is 3% or less, cracks do not occur during forming, such as shearing, and excellent impact resistance at low temperatures can be achieved. In the present invention, pearlite may contain carbides and nitrides with a diameter of 1 μm or more.

[0047] In contrast, when the MA phase or martensite is contained in an amount of 5-10%, it can be advantageous for ensuring high impact resistance and strength after forming without affecting crack generation. The MA phase is advantageous for forming potential density around it to ensure high strength, and when formed together with a matrix structure composed of ferrite and bainite, it can provide excellent impact resistance even when the potential density increases after cold forming. However, if the MA phase or martensite is contained in an amount of less than 5%, the yield strength and tensile strength are insufficient, and if it is contained in an amount of more than 10%, there is a problem of insufficient ductility and poor formability. Bainite, too, may also have a problem of insufficient ductility if it is contained in an amount of more than 10%.

[0048] In the present invention, the microstructure at the center can contain 0% bainite and 0% pearlite, and can unavoidably contain equiaxed ferrite in addition to bainitic ferrite, bainite, pearlite, MA phase, and martensite.

[0049] In the present invention, the area fraction of the microstructure can be analyzed using an optical microscope and a scanning electron microscope (SEM), and the area fraction of the phase can be measured from an image observed at 3,000x magnification at a position corresponding to the center of thickness of the rolled cross section.

[0050] The steel production method of the present invention will be described in detail below.

[0051] The steel according to one aspect of the present invention can be produced by reheating, rolling, cooling and coiling a steel slab satisfying the alloy composition described above.

[0052] reheating A steel slab satisfying the alloy composition of the present invention can be reheated in the temperature range of 1100 to 1350°C.

[0053] If the reheating temperature is less than 1100°C, the precipitates are not fully redissolved, which may reduce the formation of precipitates in the process after hot rolling, and coarse TiN may remain.On the other hand, if the temperature exceeds 1350°C, abnormal grain growth of austenite grains may cause a decrease in strength.

[0054] hot rolling The reheated steel slab can be hot rolled in the temperature range of 800 to 1150°C.

[0055] If the hot rolling temperature exceeds 1150°C, the temperature of the hot-rolled steel sheet will be too high, the grain size will become coarse, and the surface quality of the hot-rolled steel sheet may deteriorate. On the other hand, if the temperature is less than 800°C, elongated grains will develop due to delayed recrystallization, which may cause severe anisotropy and poor formability. If the rolling temperature is below the austenite temperature range, the non-uniform microstructure may develop even more severely.

[0056] Cooling and winding The hot-rolled steel sheet can be cooled to a temperature range of 500 to 650°C at an average cooling rate of 1 to 30°C / s that is equal to or greater than the CR value defined by the following relational expression 1, and then coiled. During the cooling, the edge portions, which account for 30% of the coil width at both ends, can be cooled to a temperature of 550 to 650°C (TE), and the central portion, which accounts for 40% of the coil width excluding both edges, can be cooled to a temperature of 500 to 550°C (TC). At this time, the average temperature difference between the edge portions and the central portion can be 50 to 150°C.

[0057] In the present invention, Relational Equation 1 was derived to induce an appropriate level of ferrite phase transformation during cooling of a steel sheet, form a fine and uniform MA phase, and suppress excessive pearlite formation. If the cooling rate is below the CR value of Relational Equation 1, ferrite in the center of the thickness becomes coarse and pearlite is excessively formed, resulting in severe cracking on the shear surface and degraded impact resistance after forming. Furthermore, if the cooling rate exceeds 30°C / s, excessive formation of bainite, MA phase, and martensite occurs, resulting in insufficient ductility and poor shear surface quality. [Equation 1] CR=45-16.3×[C]-5.6×[Si]-16.3×[Mn]-2.9×[Cr]+15×[Ti]+23×[Nb]-0.9×(t-8) (Here, [C], [Si], [Mn], [Cr], [Ti] and [Nb] are the weight percentages of each element, and t is the thickness of the steel plate (mm).)

[0058] In order to suppress the formation of excessive carbides and pearlite, the cooling temperature after hot rolling must be lower than the cooling end temperature. However, excessive bainite formation can result in a decrease in ferrite or excessive formation of the MA phase and martensite, which can make it difficult to achieve the target elongation.

[0059] Therefore, in the present invention, in order to increase the cooling rate at the center of the coil width direction and reduce the time the coil is maintained at high temperature after coiling, the cooling end temperatures at the center and edge portions of the coil width direction can be set differently during cooling after hot rolling. However, in this case, the average temperature difference between the edge and center portions can be 50 to 150°C. If the average temperature difference is less than 50°C, it may be difficult to achieve the above effects. On the other hand, if the temperature exceeds 150°C, the above effects will not be further increased, but it may be difficult to control the temperature of each section of the coil.

[0060] In the present invention, the method for controlling the cooling end temperatures of the edge portion and the center portion during coiling is not particularly limited, but as an example, when cooling a hot-rolled steel sheet, a method can be applied in which cooling water injected into the edge portion is stopped before it reaches the steel sheet, or the amount of cooling water injected is adjusted to be different. Alternatively, the two methods can be used in combination.

[0061] In the present invention, in order to ensure the desired strength, formability, and impact resistance, it is preferable to satisfy all of the conditions of Relational Formula 1 and the cooling end temperature. When all of the cooling conditions are satisfied, the center portion in the thickness direction has a uniform and fine microstructure with bainitic ferrite as the matrix structure, and coarse carbides and pearlite are reduced in the inner winding portion and thickness center portion of the coil, where the cooling rate is slow, thereby eliminating the non-uniform structure of the hot-rolled steel sheet. Furthermore, the non-uniform formation of MA phases and the formation of coarse martensite can be suppressed in the outer winding portion and edge portion of the coil, where the cooling rate is relatively fast.

[0062] cooling The wound coil can be air cooled to a temperature range of 200°C or less.

[0063] In the present invention, the wound coil can be air-cooled at a temperature range of 200°C or less. Air-cooling the coil means cooling it in the air at room temperature at a cooling rate of 0.001 to 10°C / h. If the cooling rate exceeds 10°C / h, some untransformed phases in the steel at the outer winding portion of the coil are likely to transform into the MA phase, which can deteriorate the shear formability, punching formability, and durability of the steel. On the other hand, controlling the cooling rate to less than 0.001°C / h requires additional heating and heat retention equipment, which can be economically disadvantageous. The preferred lower limit is 0.01°C / h, and the more preferred upper limit is 1°C / h.

[0064] The steel of the present invention thus manufactured has a thickness of 8 to 25 mm, a tensile strength of 590 MPa or more, an elongation at break of 25% or more, a yield ratio of 0.75 to 0.9, an impact toughness at -20°C after cold forming of 70 J or more, and a ratio of the impact toughness after cold forming to the yield strength before cold forming of 0.15 or more, thereby providing excellent properties of impact toughness while maintaining a high yield ratio. Furthermore, the steel includes edge portions corresponding to 30% regions at both ends in the width direction and a central portion of a central 40% region corresponding to the region excluding both edge portions, and the difference in tensile strength between the edge portions and the central portion may be 10 MPa or less, the difference in elongation at break of 8% or less, and the difference in impact toughness at -20°C after cold forming of 20 J or less. [Example]

[0065] The present invention will be described in more detail with reference to the following examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0066] (Example) Hot-rolled steel sheets were manufactured from steel slabs having the alloy composition shown in Table 1 below under the conditions shown in Table 2 below. The steel slabs were reheated at temperatures of 1100 to 1350°C and then hot-rolled. Table 2 shows the cooling rates applied during manufacturing and the CR values ​​of Relation 1. The cooling end temperatures are the temperature (TC) within 40% of the widthwise center and the temperature (TE) within 30% of the widthwise end at each end. The difference between the temperature at the center and the temperature at the edge is also shown.

[0067] [Table 1]

[0068] [Table 2] [Relational expression 1] CR=45-16.3×[C]-5.6×[Si]-16.3×[Mn]-2.9×[Cr]+15×[Ti]+23×[Nb]-0.9×(t-8) (Here, [C], [Si], [Mn], [Cr], [Ti] and [Nb] are the weight percentages of each element, and t is the thickness of the steel plate (mm).)

[0069] Table 3 below lists the microstructures of the manufactured steel sheets. The microstructures were measured at the surface and center in the thickness direction, and the fractions were also measured at the edge and center in the width direction. For the surface, the microstructure was observed from the surface to a thickness of 50 μm. For the center, the microstructure was observed from the steel sheet surface to 1 / 4 to 3 / 4 of the thickness (25 to 75% interval, where t is thickness (mm)) in the thickness direction. For the edge, the microstructure was observed in a region corresponding to 30% of the width at both ends, and for the center, the region corresponding to the central 40% of the width excluding the edge. The area fractions of the MA phase and martensite were measured using an optical microscope and an image analyzer at 1,000x magnification after etching using the Lepera etching method. The fractions of equiaxed ferrite (PF), bainitic ferrite (BF), bainite (B), and pearlite (P) were measured using a scanning electron microscope (SEM) at 3,000x and 5,000x magnifications. Here, PF refers to polygonal ferrite with an equiaxed crystal shape, and BF includes ferrites observed in the low-temperature range, such as acicular ferrite and bainitic ferrite. Furthermore, P includes pearlite and coarse carbides and nitrides with diameters of 1 μm or more.

[0070] [Table 3]

[0071] Table 4 below shows the physical properties of each manufactured test piece, measured at the center and edge in the width direction. Yield strength (YS), tensile strength (TS), yield ratio (YR), and fracture elongation (T-El) were evaluated by performing tensile tests on JIS No. 5 standard test pieces taken perpendicular to the rolling direction. In addition, the impact absorption energy (E) at -20°C after cold forming was measured, and the ratio of impact absorption energy to yield strength (E / YS) at -20°C after cold forming was shown. Impact absorption energy was measured using Charpy V-notch test pieces manufactured according to ASTM standard (ASTM A370) taken perpendicular to the rolling direction.

[0072] [Table 4]

[0073] As shown in Tables 3 and 4, inventive examples that satisfied the alloy composition and manufacturing conditions of the present invention, the microstructural characteristics proposed in the present invention were satisfied and the physical properties targeted in the present invention were secured. Figure 1 is a photograph showing the microstructure of the hot-rolled steel sheet of Inventive Example 2, observed with a scanning electron microscope (×3,000).

[0074] On the other hand, in Comparative Example 1, the total content of Ti and Nb exceeded the range of the present invention, and the impact resistance was deteriorated due to the formation of coarse precipitates and TiN caused by excessive precipitates in the ferrite grains.

[0075] In Comparative Example 2, the cooling rate did not reach the standard cooling rate suggested by Relation 1, and as shown in FIG. 2, excessive pearlite was formed in the microstructure.

[0076] As shown in Figure 2, this did not significantly reduce the strength, but it was not possible to ensure the desired impact resistance.

[0077] In Comparative Example 3, the coiling temperature in the widthwise central portion exceeded the range proposed by the present invention. In particular, excessive pearlite was formed in the thicknesswise central portion in both the central portion and the edge portions, and the target impact resistance properties could not be ensured.

[0078] In Comparative Example 4, the winding temperature of the widthwise edge portion exceeded the range proposed by the present invention. As a result, it was found that the impact resistance deteriorated due to the excessive formation of pearlite in the center of the thickness direction of the edge portion. This is because the temperature at the edge portion was high, and the heat transfer from the winding coil proceeded slowly at the edge portion.

[0079] In Comparative Example 5, the coiling temperature in the widthwise center portion did not reach the range of the present invention, and bainite was excessively formed in the center portion in the thickness direction of the center portion, and pearlite, MA phase, martensite, etc. were formed in the surface layer portion in excess of the levels proposed in the present invention, resulting in a deterioration in elongation. On the other hand, the edge portion satisfied the coiling temperature range and had relatively good elongation and impact resistance, but ferrite in the surface layer portion did not reach the range proposed in the present invention. This is thought to be because the temperature of the edge portion of the coil after coiling also rapidly decreased due to the low cooling end temperature in the widthwise center portion.

[0080] In Comparative Example 6, it was confirmed that when the coiling temperature of the widthwise edge portion did not reach the range of the present invention, excessive bainite was formed in the center portion in the thickness direction, resulting in a deterioration in elongation. In addition, the surface layer of the center portion had insufficient ferrite, and the center portion had insufficient bainitic ferrite, so the ratio of impact resistance to yield strength did not satisfy the level proposed in the present invention.

[0081] In Comparative Example 7, the steel thickness was less than 8 mm, and an excessive cooling rate was applied for the given steel composition, resulting in a lack of ferrite in the surface layer and a decrease in bainitic ferrite in the center of the thickness direction, while excessive pearlite formed. This is believed to be due to an increase in untransformed phases during the initial cooling process, resulting in the formation of pearlite in areas with a relatively high C content. As a result, the desired level of elongation could not be achieved.

[0082] In Comparative Example 8, the total content of Ti and Nb did not reach the range of the present invention. The phase fractions of the microstructure measured at each position satisfied the range proposed by the present invention, but the microstructure became coarse due to the decrease in precipitates during hot rolling, and the fine precipitates also decreased after cooling and coiling, resulting in insufficient strength and a significant decrease in impact resistance.

[0083] In Comparative Example 9, the cooling rate was equal to or greater than the CR value of Relational Formula 1, which satisfied the range proposed by the present invention, but exceeded 30°C / s. As a result, polygonal ferrite was insufficient in the surface layer, and bainite was excessively formed in the center portion in the thickness direction, making it impossible to ensure the desired level of elongation.

[0084] Although the present invention has been described in detail with reference to the above examples, other embodiments are possible, and the spirit and scope of the claims set forth below should not be limited to the examples.

Claims

1. The alloy contains, by weight, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, and Nb: 0.01 to 0.05%, with the balance being Fe and other unavoidable impurities, and the total of Nb and Ti being 0.04 to 0.1%; The microstructure of the surface layer portion within a thickness range of 50 μm from the surface contains, in area%, 95% or more of equiaxed ferrite and 3% or less of pearlite, and contains 5% or less in total of one or more of bainitic ferrite, bainite, MA (Martensite-Austenite constituent) phase, and martensite, The microstructure of the center portion within the range of 1 / 4 to 3 / 4 of the thickness contains, in area percentages, 80 to 95% bainitic ferrite, 10% or less bainite, 3% or less pearlite, 5 to 10% in total of one or two of an MA (Martensite-Austenite constituent) phase and martensite, and the remainder containing equiaxed ferrite.

2. The steel plate according to claim 1, wherein the steel plate has a thickness of 8 to 25 mm.

3. The steel plate according to claim 1, wherein the steel plate has a tensile strength of 590 MPa or more, a fracture elongation of 25% or more, and a yield ratio of 0.75 to 0.

9.

4. The steel plate according to claim 1, wherein the ratio (E / YS) of impact toughness (E) at −20° C. after cold forming to yield strength (YS) before cold forming is 0.15 or more.

5. The steel sheet includes edge portions corresponding to 30% regions at both ends in the width direction and a central portion of a central 40% region corresponding to a region excluding both edge portions, The difference in tensile strength between the edge portion and the center portion is 10 MPa or less, the difference in fracture elongation is 8% or less, and the difference in impact toughness at −20 ° C. after cold forming is 20 J or less. Steel plate according to claim 1.

6. reheating a steel slab containing, by weight, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, and Nb: 0.01 to 0.05%, with the balance being Fe and other unavoidable impurities, and the sum of Nb and Ti being 0.04 to 0.1%; hot rolling the reheated steel slab; and The hot-rolled steel sheet is cooled to a temperature range of 500 to 650°C at an average cooling rate of 1 to 30°C / s, the average cooling rate being equal to or greater than the CR (°C / s) value defined by the following Relation 1, and then coiled: In the cooling and winding step, the edge portions corresponding to 30% of the area at both ends in the coil width direction are cooled at a temperature of 550 to 650 ° C. (TE), and the central portion of the central 40% area corresponding to the area excluding both edge portions in the width direction is cooled at a temperature of 500 to 550 ° C. (TC), The method for producing a steel plate according to claim 1, wherein the average temperature difference between the edge portion and the center portion is 50 to 150°C. [Relationship 1] CR=45-16.3×[C]-5.6×[Si]-16.3×[Mn]-2.9×[Cr]+15×[Ti]+23×[Nb]-0.9×(t-8) (Here, [C], [Si], [Mn], [Cr], [Ti] and [Nb] are the weight percentages of each element, and t is the thickness of the steel plate (mm).)

7. The reheating temperature is 1100 to 1350°C, The method for producing a steel sheet according to claim 6, wherein the hot rolling temperature is 800 to 1150°C.

8. The method for manufacturing a steel sheet according to claim 6, further comprising the step of air-cooling the wound coil at a temperature range of 200°C or less.

9. The method for manufacturing a steel plate according to claim 6, wherein the steel plate has a thickness of 8 to 25 mm.

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