Hot-rolled steel sheet and method for producing same

Abstract

Provided are a hot-rolled steel sheet and a method for manufacturing the same. The hot-rolled steel sheet according to the present invention contains, in wt%, 0.050 to 0.100% of C, 0.01 to 1.00% of Si, 1.4 to 2.0% of Mn, 0.010 to 0.100% of Al, 0.005 to 0.500% of Cr, 0.005 to 0.300% of Mo, 0.001 to 0.050% of P, 0.001 to 0.010% of S, 0.001 to 0.010% of N, 0.005 to 0.050% of Nb, 0.005 to 0.120% of Ti, and the balance Fe and unavoidable impurities, and has a microstructure containing 90% by area or more of the sum of ferrite and bainite phases, and less than 10% by area of the sum of residual pearlite, martensite, and MA phases.

Description

Technical Field

[0001] The present invention relates to a high-strength composite hot-rolled steel plate having excellent formability, bake hardenability and material uniformity, which can be mainly applied to components of automobile chassis parts and lower arms, reinforcements, connectors and vehicle frames, and a manufacturing method thereof. Background Art

[0002] Conventional high-strength hot-rolled steel sheets used in automotive chassis and frames are being reduced to thinner sheets for lightweighting. This requires excellent formability considering component shape, and furthermore, bake hardening, which increases yield strength, is required after coating heat treatment to maximize component durability.

[0003] Furthermore, in recent years, the use of hot-rolled plated steel has increased to improve the corrosion resistance of hot-rolled chassis components. This can only be achieved by performing an additional heat treatment after hot rolling. This additional heat treatment can alter the microstructure of the existing hot-rolled steel sheet. Therefore, appropriately selecting the alloy composition, hot-rolled microstructure, and heat treatment conditions is crucial to achieving the desired final microstructure and physical properties.

[0004] Patent Document 1 relates to a technology for producing a composite steel having a matrix structure of bainitic ferrite and granular bainitic ferrite, a ferrite phase generated in a low temperature region. However, in order to ensure further strength, Cu must be used, so surface defects may occur and high-temperature brittleness may occur during hot rolling. Ni must be added to prevent these problems.

[0005] Patent Document 2 describes a technology that ensures the strength of the heat-affected zone (HAZ) of a weld by adding Ti, Nb, Cr, Mo, and other additives. Specifically, during arc welding, the HAZ, adjacent to the weld material melted by the welding heat, is heated to temperatures exceeding 600°C, sometimes exceeding the austenite zone. Therefore, Cr and Mo are added to increase the hardenability of the steel, forming low-temperature phases such as bainite and martensite during cooling, thereby ensuring strength. However, solutions based on maximizing hardenability, like the present invention, have limitations when applied to automotive steel sheets, which require high formability even after heat treatment as required after production.

[0006] [Prior art literature]

[0007] [Patent Document]

[0008] (Patent Document 1) Authorized Patent No. KR10-1114672

[0009] (Patent Document 2) Authorized Patent No. KR10-0962745 Summary of the Invention

[0010] (1) Technical issues to be resolved

[0011] An object of the present invention is to provide a high-strength composite structure hot-rolled steel sheet having excellent formability, bake hardenability and material uniformity, and a method for producing the same.

[0012] In addition, the technical problems to be achieved by the present invention are not limited to the technical problems involved above, and ordinary technicians in the technical field to which the present invention belongs can clearly understand other technical problems not mentioned from the following description.

[0013] (2) Technical solution

[0014] Therefore, one aspect of the present invention relates to a hot rolled steel plate, comprising, in weight %, C: 0.050-0.100%, Si: 0.01-1.00%, Mn: 1.4-2.0%, Al: 0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P: 0.001-0.050%, S: 0.001-0.010%, N: 0.001-0.010%, Nb: 0.005-0 The hot-rolled steel sheet comprises 0.050% Ti, 0.005-0.120% Ti, and the balance Fe and inevitable impurities, the hot-rolled steel sheet having a fine structure comprising 90 area % or more of the sum of ferrite and bainite phases and less than 10 area % of the sum of retained pearlite, martensite, and MA phases, the hot-rolled steel sheet having a tensile strength of 760 MPa or more and a hole expandability (HER0) of 40% or more, a bake hardening capacity (BH2) of 30 MPa or more, and a ΔTS1 defined by the following Relational Formula 1 satisfying a value of 100 MPa or less.

[0015] [Equation 1]

[0016] △TS1=TS 最大 -TS 最小

[0017] TS 最大 : The maximum TS values at seven locations in the width direction including the extreme edge within 30m of the rear end of the hot rolled coil produced

[0018] TS 最小 : The minimum TS values at seven locations in the width direction including the extreme edge within 30m of the rear end of the hot rolled coil produced

[0019] The bake hardening capacity (BH) of the hot-rolled steel sheet after heat treatment at 300-600°C is h ) can be maintained above 30MPa, as shown in the following equation 2: △TS2×BH h -1The absolute value of can be 0.7 or less.

[0020] [Equation 2]

[0021] △TS2=TS h -TS0

[0022] TS h : tensile strength after heat treatment, TS0: tensile strength before heat treatment

[0023] The hot-rolled steel sheet may further contain one or more components selected from V, Ni, and B in a total amount within a range of 1.5% or less.

[0024] In the present invention, a hot-dip galvanizing layer may be formed on the surface of the hot-rolled steel sheet.

[0025] In addition, another aspect of the present invention relates to a method for manufacturing a hot-rolled steel plate, which comprises the following steps: reheating a steel billet having the above-mentioned alloy composition to a temperature range of 1100-1350°C; hot rolling the reheated steel billet in the range of 850-1150°C to manufacture a hot-rolled steel plate; cooling the hot-rolled steel plate once at an average cooling rate of 50-100°C / second to a temperature in the range of 550-650°C; stopping cooling the steel plate that has been cooled once for 3-7 seconds; and secondarily cooling the hot-rolled steel plate that has stopped the primary cooling at an average cooling rate of 1-30°C / second to a temperature in the range of 400-500°C, and then coiling it.

[0026] The method may further include cooling the coiled hot-rolled steel sheet to a temperature ranging from room temperature to 200° C. at an average cooling rate of 0.1-25° C. / hour (hr).

[0027] The method may further include the following steps: pickling the coiled hot-rolled steel sheet and then applying oil thereto.

[0028] The method may further include subjecting the coiled hot-rolled steel sheet to a pickling treatment, heating the coiled hot-rolled steel sheet to a temperature range of 450-750° C., and then immersing the coiled hot-rolled steel sheet in a plating bath to form a hot-dip galvanized layer on the surface thereof, wherein the plating bath comprises, by weight%, 0.01-30% Mg, 0.01-50% Al, and the balance zinc.

[0029] (3) Beneficial effects

[0030] The present invention as constructed above can effectively provide a hot-rolled steel sheet having a tensile strength of 760 MPa or more, a hole expansion ratio (HER0) value of 40% or more, a bake hardening value (BH2) of 30 MPa or more, and a bake hardening value (BH2) of 30 MPa or more after heat treatment at 300-600°C. h ) is above 30MPa and has excellent material uniformity.

[0031] Therefore, the present steel sheet can be effectively used for components of automobile chassis parts, lower arms, reinforcements, connectors, and vehicle frame parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 ΔTS2×BH before and after 520°C heat treatment of the steel sheets of Comparative Examples 1 to 5 and Inventive Examples 6 to 10 according to the embodiment of the present invention is shown. h -1 A graph showing the change in the absolute value of . Best Practice

[0033] Hereinafter, the present invention will be described.

[0034] In order to expand the applicability of hot-rolled chassis components, the inventors studied various steels with different compositions and microstructures, and concluded that controlling the microstructure through precise temperature and time control in the hot rolling manufacturing process is the most important factor in determining the level of material deviation within the coil. Based on the results, the main component range of the steel was set, so that the hot-rolled steel sheet has excellent material uniformity, and the structure consists of the sum of martensite, martensite and austenite (MA) phases and pearlite: less than 10% and the balance ferrite and bainite phases: more than 90%. This makes it possible to produce high-strength composite structure hot-rolled steel sheets that ensure a tensile strength of more than 760 MPa, a hole expansion (HER0) value of more than 40%, and a bake hardening capacity (BH2) of more than 30 MPa, thereby proposing the present invention. The hot-rolled steel sheet of the present invention has no material deviation and can maintain a bake hardening capacity (BH2) of more than 30 MPa even after heat treatment at 300-600°C. h ), so it can be effectively used when manufacturing hot-rolled steel sheets coated with molten zinc or the like.

[0035] The hot rolled steel sheet of the present invention comprises, by weight%, C: 0.050-0.100%, Si: 0.01-1.00%, Mn: 1.4-2.0%, Al: 0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P: 0.001-0.050%, S: 0.001-0.010%, N: 0.001-0.010%, Nb: 0.005-0.050%, Ti: 0.005-0.120%, with the balance being Fe and inevitable impurities, the hot-rolled steel sheet having a fine structure comprising 90 area % or more of the sum of ferrite and bainite phases and less than 10 area % of the sum of retained pearlite, martensite, and MA phases, the hot-rolled steel sheet having a tensile strength of 760 MPa or more, a hole expandability (HER0) of 40% or more, a bake hardening capacity (BH2) of 30 MPa or more, and a ΔTS1 defined by Relational Formula 1 of 100 MPa or less.

[0036] Hereinafter, the composition of the steel sheet provided by the present invention will be described in detail. Herein, the content of each component is expressed in weight % unless otherwise specified.

[0037] C: 0.050-0.100%

[0038] The C is the most economical and effective element for strengthening steel. When the addition amount increases, the precipitation strengthening effect or the low-temperature phase fraction increases, thereby increasing the tensile strength. However, when the C content is less than 0.050%, it is difficult to form a sufficient precipitation strengthening effect and low-temperature phase, thereby making it difficult to ensure the target strength and bake hardenability. When the C content exceeds 0.100%, excessive low-temperature phases and carbides are formed, the formability deteriorates, and the carbon equivalent is high, which usually has the disadvantage of poor weldability. In addition, according to the characteristics of the fine structure generated when adding too high a C content, the additional heat treatment after hot rolling causes the low-temperature phase to deteriorate and form additional excessive carbides, and the tensile strength after heat treatment may be significantly reduced. Therefore, in the present invention, the C content is preferably 0.05-0.10%, and more preferably, the C content can be in the range of 0.050-0.080%.

[0039] Si: 0.01-1.00%

[0040] The Si deoxidizes the molten steel and has a solid solution strengthening effect, delaying the formation of coarse carbides, which is beneficial to improving formability. In addition, when heat treated in the range of 300-600°C, it also has the effect of inhibiting the formation of carbides. However, when the Si content is less than 0.01%, the effect of delaying the formation of carbides is small, it is difficult to improve formability, and the effect of improving strength is also small. When the Si content exceeds 1.00%, red oxide scale caused by Si is formed on the surface of the steel plate during hot rolling, which not only seriously reduces the surface quality of the steel plate, but also has the problem of reduced ductility and weldability. Therefore, the Si content is preferably limited to 1.00%. More preferably, the Si content is controlled in the range of 0.02-0.08%.

[0041] Mn: 1.4-2.0%

[0042] The Mn, like Si, is an effective element for solid solution strengthening of steel, further improves the hardenability of steel, delays ferrite transformation under the same cooling rate, and easily forms low-temperature phases such as bainite and martensite. However, when the Mn content is less than 1.4%, the effect of improving solid solution strengthening and hardenability is smaller, and it is impossible to obtain the desired strength improvement effect. On the other hand, when the Mn content exceeds 2.0%, hardenability is greatly improved, and the martensitic phase fraction can exceed expectation, resulting in the segregation part in the thickness center portion during slab casting in the continuous casting process. The formability variation further causes the welding quality to deteriorate. Therefore, in the present invention, the content of the Mn is preferably limited to 1.4-2.0%, and more preferably, is limited to the scope of 1.5-1.9%.

[0043] P: 0.001-0.050%

[0044] Like Si, P has both the effects of solid solution strengthening and promoting ferrite transformation. However, reducing the P content to less than 0.001% requires significant manufacturing costs, which is economically disadvantageous and insufficient to achieve sufficient strength. When the P content exceeds 0.050%, brittleness due to grain boundary segregation occurs, microcracks are easily generated during molding, and ductility and impact resistance are significantly reduced. Therefore, the P content is preferably limited to 0.001-0.050%. More preferably, the P content is limited to the range of 0.002-0.004%.

[0045] S: 0.001-0.010%

[0046] S is an impurity present in steel. Reducing the S content to less than 0.001% requires significant steelmaking time, resulting in reduced productivity. On the other hand, if the S content exceeds 0.010%, S combines with Mn and other materials to form non-metallic inclusions, which can easily cause fine cracks during steel cutting. Therefore, the S content is preferably limited to 0.001-0.01%. More preferably, the S content is limited to 0.002-0.008%.

[0047] Acid soluble aluminum (Sol.Al): 0.010-0.100%

[0048] Acid-soluble aluminum (Sol.Al) is primarily added for deoxidation. When the content of acid-soluble aluminum (Sol.Al) is less than 0.010%, its addition effect is insufficient. When the content of acid-soluble aluminum (Sol.Al) exceeds 0.100%, it combines with nitrogen to form AlN, which can easily cause corner cracks in the slab during continuous casting and casting, and defects caused by the formation of inclusions. Therefore, in the present invention, the content of acid-soluble aluminum (Sol.Al) is preferably limited to 0.010-0.100%.

[0049] N: 0.001-0.010%

[0050] The N, along with C, is a representative solid solution strengthening element and forms coarse precipitates together with Ti, Al, etc. Generally, N has a better solid solution strengthening effect than carbon, but as the N content in the steel increases, there is a problem of significantly reducing toughness, so its upper limit is limited to 0.010%. On the other hand, in order to make the N content less than 0.001%, a lot of time is required during the steelmaking operation, which may reduce productivity. Therefore, in the present invention, the N content is preferably limited to 0.001-0.010%. More preferably, the N content is limited to the range of 0.001-0.008%.

[0051] Ti: 0.005-0.120%

[0052] Ti, together with Nb and V, is a representative precipitation strengthening element. Due to its strong affinity with N, it 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 will be dissolved in the steel and combined with carbon to form TiC precipitates, which is a useful component for improving the strength of the steel. However, when the Ti content is less than 0.005%, the above-mentioned effect cannot be obtained. When the Ti content exceeds 0.120%, due to the generation of coarse TiN and the coarsening of TiC precipitates, there is a problem of poor formability. Therefore, in the present invention, the Ti content is preferably limited to 0.005-0.120%. More preferably, the Ti content is limited to the scope of 0.010-0.100%.

[0053] Nb: 0.005-0.050%

[0054] The Nb, together with Ti and V, is a representative precipitation strengthening element. It precipitates during hot rolling and effectively improves the strength and impact toughness of steel by delaying the grain refinement effect of recrystallization. However, when the Nb content is less than 0.005%, the above-mentioned effect cannot be obtained. When the Nb content exceeds 0.050%, recrystallization is excessively delayed during hot rolling, resulting in the formation of elongated grains and the formation of coarse composite precipitates, thereby having the problem of poor formability. Therefore, in the present invention, the Nb content is preferably limited to 0.005-0.050%. More preferably, its content is limited to the range of 0.007-0.040%.

[0055] Cr: 0.005-0.500%

[0056] The Cr strengthens the steel by solid solution and delays the ferrite phase transformation during cooling, thereby playing a role in helping to form bainite. However, when the Cr content is less than 0.005%, the above-mentioned effect brought about by the addition cannot be obtained. When the Cr content exceeds 0.500%, the ferrite phase transformation is excessively delayed, a martensite phase is formed, and the elongation deteriorates. In addition, similar to Mn, Cr will significantly develop the segregation part at the center of the thickness, resulting in uneven microstructure in the thickness direction, thereby reducing ductility and flangeability. In addition, when the Cr content is too high, the corrosion resistance of the material may deteriorate. Therefore, in the present invention, the Cr content is preferably limited to 0.005-0.500%. More preferably, the Cr content is limited to the range of 0.010-0.400%.

[0057] Mo: 0.005-0.300%

[0058] Mo improves the hardenability of steel and promotes the formation of bainite. However, when the Mo content is less than 0.005%, the aforementioned effects are not achieved. When the Mo content exceeds 0.300%, the hardenability is excessively increased, martensite is formed, and formability rapidly deteriorates. Furthermore, this is economically disadvantageous and detrimental to weldability. Therefore, in the present invention, the Mo content is preferably limited to 0.005-0.3%. More preferably, the Mo content is limited to the range of 0.007-0.250%.

[0059] Furthermore, the present invention may contain one or more of V, Ni, and B as appropriate, with the total content being within 1.5%. The other components and the balance are composed of iron and unavoidable impurities.

[0060] Ni and B are elements that effectively improve the hardenability of ferrous materials. When adding these elements, ferrite transformation is delayed in cooling process, thereby being easy to ensure low temperature transformation phase (bainite, martensite etc.). Like this, by increasing the score of low temperature transformation phase, there is the ultimate strength advantage that can improve material. In addition, V is similar to Ni, Ti etc., is combined with C to generate precipitate, can add V as the precipitate element that utilizes precipitation strengthening effect to improve the intensity of material.

[0061] In addition, the hot-rolled steel sheet of the present invention may have a steel sheet microstructure comprising the sum of ferrite and bainite phases: more than 90% and the sum of residual pearlite, martensite and MA phase: less than 10%. When the phase fraction of the sum of ferrite and bainite phases is less than 90 area%, it means that the sum of residual pearlite or martensite and MA phase exceeds 10%, and there may be a problem of poor hole expansion rate. The hole expansion rate is greatly affected by the microstructure of the steel sheet, especially when the steel sheet has a composite structure with soft phases and hard phases, the hole expansion rate will be significantly reduced due to the hardness difference between the constituent phases. In particular, the higher the fraction of pearlite or martensite, which are very hard microstructures, the easier it is to generate cracks at the interface between the phases during hole expansion, thereby reducing the hole expansion rate, so it is necessary to limit the phase fraction.

[0062] The hot-rolled steel sheet of the present invention having the above-described microstructure can have a tensile strength of 760 MPa or more, a hole expansion ratio (HER0) of 40% or more, and a bake hardening amount (BH2) of 30 MPa or more.

[0063] Furthermore, it is possible to provide an excellent hot-rolled steel sheet having ΔTS1 defined by the following Relational Formula 1 of 100 MPa or less and no material variation.

[0064] [Equation 1]

[0065] △TS1=TS 最大 -TS 最小

[0066] TS 最大 : The maximum TS values at seven locations in the width direction including the extreme edge within 30m of the rear end of the hot rolled coil produced

[0067] TS 最小 : The minimum TS values at seven locations in the width direction including the extreme edge within 30m of the rear end of the hot rolled coil produced

[0068] In addition, the hot rolled steel sheet of the present invention may have a bake hardening capacity (BH) after heat treatment at 300-600°C. h ) is maintained at 30 MPa or above, and △TS2 is defined as follows: △TS2×BH h -1The absolute value of satisfies the high temperature bake hardening property of less than 0.7. That is, the bake hardening amount (BH) of the hot rolled steel sheet of the present invention after heat treatment at 300-600°C is h ) is 30 MPa or more, so that the plated steel sheet can be efficiently manufactured in the subsequent hot-dip galvanizing process, etc.

[0069] [Equation 2]

[0070] △TS2=TS h -TS0

[0071] TS h : tensile strength after heat treatment, TS0: tensile strength before heat treatment

[0072] Next, a method for producing a hot-rolled steel sheet according to a preferred embodiment of the present invention will be described in detail.

[0073] The method for manufacturing a hot-rolled steel plate of the present invention comprises the following steps: reheating a steel billet having the above-mentioned alloy composition to a temperature range of 1100-1350°C; hot rolling the reheated steel billet in the range of 850-1150°C to manufacture a hot-rolled steel plate; primary cooling the hot-rolled steel plate at an average cooling rate of 50-100°C / second to a temperature range of 550-650°C; stopping cooling the steel plate that has been subjected to the primary cooling for 3-7 seconds; and secondary cooling the hot-rolled steel plate that has stopped the primary cooling at an average cooling rate of 1-30°C / second to a temperature range of 400-500°C, and then coiling it.

[0074] Reheating

[0075] First, in the present invention, the steel slab having the above-described alloy composition is reheated to a temperature range of 1100-1350°C. At this point, when the reheating temperature is lower than 1100°C, the redissolution rate of precipitates containing Ti, Nb, Mo, and V decreases, reducing the formation of fine precipitates in processes subsequent to hot rolling. When the reheating temperature exceeds 1350°C, the austenite grains coarsen, reducing strength. Therefore, the reheating temperature is preferably limited to 1100-1350°C.

[0076] Hot Rolling

[0077] Next, in the present invention, the reheated steel slab is hot rolled within a temperature range of 850-1150°C to produce a hot-rolled steel sheet. However, starting hot rolling at a temperature above 1150°C increases the temperature of the hot-rolled steel sheet, coarsening the grain size and deteriorating the surface quality of the hot-rolled steel sheet. Furthermore, terminating hot rolling at a temperature below 850°C excessively delays recrystallization, leading to the development of elongated grains and increased anisotropy, thus deteriorating formability.

[0078] One-time cooling and holding

[0079] In addition, the hot-rolled steel sheet is primarily cooled at an average cooling rate of 50-100° C. / s to a temperature in the range of 550-650° C., and then the primarily cooled steel sheet is stopped for 3-7 seconds.

[0080] Specifically, in the present invention, the hot-rolled steel sheet is subjected to primary cooling at an average cooling rate of 50-100°C / second to a temperature within the range of 500-650°C. More preferably, the primary cooling is performed to a temperature within the range of 550-600°C. Primary cooling to a temperature below 500°C reduces the amount of ferrite and precipitates within ferrite during the formation of the final microstructure, potentially leading to a decrease in strength. On the other hand, primary cooling exceeding 650°C partially causes pearlite transformation during the formation of the final microstructure, potentially resulting in poor formability.

[0081] Furthermore, in the present invention, the average cooling rate during the primary cooling is preferably controlled within a range of 50-100°C / sec. If the cooling rate is less than 50°C / sec, the fraction of the formed ferrite phase may be too high, thereby hindering strength. If the cooling rate exceeds 100°C / sec, the fraction of the ferrite phase decreases significantly in the region where the primary cooling end temperature is low, thereby potentially leading to insufficient elongation.

[0082] Next, the steel sheet that has undergone the primary cooling is stopped for 3-7 seconds. Taking into account internal latent heat and phase transformation heat, the temperature of the hot-rolled steel sheet is adjusted to a range of 550-700°C. If the cooling stop time is less than 3 seconds, the ferrite transformation and precipitation effects are minimal. If the cooling stop time exceeds 7 seconds, the ferrite phase fraction in the microstructure increases significantly, while the bainite and bainitic ferrite phases, which are hard phases, decrease, ultimately failing to achieve the desired microstructure.

[0083] Secondary cooling and winding

[0084] Next, in the present invention, the hot-rolled steel sheet after stopping the primary cooling is subjected to secondary cooling at an average cooling rate of 1-30° C. / s to a temperature in the range of 400-500° C., and then coiled.

[0085] The cooling end temperature during the secondary cooling is preferably in the range of 400-500° C., more preferably 430-470° C. If the secondary cooling end temperature is too high, bainite may not be fully formed, making it difficult to ensure strength. On the other hand, if the secondary cooling end temperature is too low, bainite, martensite, and MA phases may be excessively formed, resulting in poor ductility, elongation, and flangeability of the steel.

[0086] Furthermore, the average cooling rate during the secondary cooling is preferably 1-30°C / second. Excessively high cooling rates can easily lead to the formation of an MA phase, excessive bainite, and reduced elongation. While there is no particular lower limit for the cooling rate, controlling the cooling rate to a lower rate of less than 1°C / second requires separate cooling and heat-insulating equipment, which can be economically disadvantageous. Therefore, considering this, the lower limit can be set at 1°C / second.

[0087] Normal temperature cooling

[0088] Subsequently, in the present invention, the coiled sheet can be cooled at an average cooling rate of 0.1-25°C / hour to a temperature ranging from room temperature to 200°C to produce the final hot-rolled steel sheet. Cooling rates exceeding 25°C / hour tend to cause the formation of MA phase in the steel, deteriorating its ductility and flangeability. Controlling the cooling rate to less than 0.1°C / hour requires separate heating equipment, which is economically disadvantageous. Preferably, cooling is performed at 1-10°C / hour.

[0089] In addition, the present invention may further include the following steps as needed: pickling the coiled hot-rolled steel sheet and then applying oil thereto.

[0090] In addition, as needed, the process may further include the steps of pickling the coiled hot-rolled steel sheet, heating it to a temperature range of 450-750° C., and then immersing it in a plating bath to form a hot-dip galvanized layer on its surface, wherein the plating bath contains, by weight%, Mg: 0.01-30%, Al: 0.01-50%, and the balance zinc. DETAILED DESCRIPTION

[0091] Hereinafter, the present invention will be described in more detail with reference to Examples. However, the description of these Examples is only for illustrating the practice of the present invention, and the present invention is not limited to the description of these Examples.

[0092] (Example)

[0093] Steel slabs having the alloy compositions shown in Table 1 below were prepared, and these slabs were then reheated at 1200°C. Subsequently, under the conditions shown in Table 2 below, the reheated slabs were hot rolled, subjected to primary cooling and holding, secondary cooling and coiling, and finally cooled to room temperature at a cooling rate of 8°C / hour to produce final hot-rolled steel sheet products.

[0094] The microstructure composition and fraction of the hot-rolled steel sheets produced as described above were measured, and the results are shown in Table 3 below.

[0095] Specifically, the fractions of ferrite (F), bainite (B), martensite (M), and pearlite (P) were measured using SEM analysis at magnifications of ×3000 and ×5000. To read the martensite and MA phases, etching with Nital and Lepera was performed, followed by analysis at ×1000 magnification using an optical microscope and an image analyzer.

[0096] In addition, the tensile strength (TS0), bake hardening amount (BH2) and hole expansion ratio (HER0) of the produced hot-rolled steel sheets were measured, and the results are shown together in Table 3.

[0097] The tensile material and bake hardening amount are the results of testing after taking DIN standard test pieces along the rolling direction. The tensile evaluation is performed at room temperature.

[0098] The bake hardening amount is measured by measuring the difference between the strength value at room temperature after 2% pre-deformation and the strength value after heat treatment at 170°C for 20 minutes after 2% pre-deformation and room temperature cooling. Specifically, when measuring the bake hardening amount, the strength after heat treatment at 170°C for 20 minutes after pre-deformation is measured as the lower yield strength to measure the lower bake hardening amount.

[0099] The hole expansion ratio is the average of three evaluations at room temperature. Specifically, the test was stopped when cracking was visually observed during each test, and the major axis length of the cracked portion was measured to evaluate the hole expansion ratio. This measurement was made regardless of the rolling direction of the test piece.

[0100] Furthermore, in order to measure the degree of material deviation within the same hot-rolled steel sheet coil, ΔTS1 was calculated based on Relational Formula 1, and the results are also shown in Table 3 below.

[0101] [Table 1]

[0102]

[0103] [Table 2]

[0104]

[0105] Table 2 FDT hot finishing temperature

[0106] [Table 3]

[0107]

[0108] In Table 3, F represents ferrite, B represents bainite, M represents martensite, MA represents island martensite, and P represents. Furthermore, ΔTS1 represents the difference in tensile strength based on Relational Formula 1.

[0109] As shown in Tables 1 to 3 above, Inventive Examples 6 to 10, which meet the composition ranges and manufacturing conditions proposed by the present invention, all ensure the target material quality. Specifically, it was confirmed that the hot-rolled steel sheets of Inventive Examples 6 to 10 all exhibited a tensile strength of 760 MPa or greater, a hole expandability (HER0) of 40% or greater, and a bake hardening capacity (BH2) of 30 MPa or greater. Furthermore, the ΔTS1 value defined by Equation 1 was 100 MPa or less, resulting in excellent hot-rolled steel sheets with no material variation.

[0110] In contrast, Comparative Examples 1 to 5 exceed the composition ranges proposed in the present invention. Specifically, these comparative examples do not meet the appropriate content ranges of C, Si, and Mn, which have the greatest impact on the microstructure and mechanical properties of the steel composition. Consequently, martensite and MA phases are often unnecessarily formed, preventing the proper precipitation strengthening effect from being achieved. This results in poor strength of the steel sheet, or poor hole expansion and bake hardenability. Specifically, Comparative Examples 2 and 5, respectively, have insufficient C and Mn contents. Due to reduced hardenability, a sufficient low-temperature phase fraction cannot be ensured, and the strength of the steel sheet cannot meet 760 MPa. Furthermore, Comparative Examples 1 and 4, respectively, have excessively high C and Mn contents. Due to excessively high hardenability, the low-temperature phase fraction exceeds the target value, resulting in excessively high strength and, consequently, poor hole expansion.

[0111] In addition, Comparative Examples 11 to 15 satisfy the component ranges proposed in the present invention but do not satisfy the production conditions.

[0112] Specifically, in Comparative Example 11, it was confirmed that pearlite formed within the microstructure due to the excessively high primary cooling temperature. The formation of pearlite significantly increased the hardness differences between microstructure phases, leading to inhomogeneity and, consequently, poor hole expandability and formability. This inhomogeneity within the microstructure also poses the problem of causing material variation in the resulting rolled sheet.

[0113] In Comparative Examples 12 and 15, the holding time after primary cooling was either too long or insufficient, respectively. When the holding time after primary cooling was too long, the bainite transformation was reduced due to excessive phase transformation of ferrite, a soft phase, and the strength decreased due to coarsening of precipitates. On the other hand, when the holding time was too short, the fraction of the low-temperature transformation phase increased excessively, the formation of precipitates decreased, and the hardness difference between microstructure phases increased, resulting in poor hole expandability.

[0114] Furthermore, in Comparative Example 13, it was confirmed that the winding temperature was higher than the appropriate production conditions proposed in the present invention, and thus the desired tensile physical properties could not be ensured.

[0115] (Example 2)

[0116] The steel plates of Comparative Examples 1 to 5 and Inventive Examples 6 to 10 of Example 2 were subjected to additional heat treatment under the conditions shown in Table 4 below. Specifically, the hot-rolled steel plates were heat treated at a heat treatment temperature of 520°C for 8 minutes and then air-cooled to room temperature. Furthermore, after this heat treatment, the mechanical properties of the steel plates before and after the heat treatment were evaluated and are shown in Table 4 below. Specifically, the tensile strength and bake hardening of the steel plates after the heat treatment were measured, and the results were compared with the tensile strength and bake hardening of the steel plates before the heat treatment of Example 1 and are shown. The methods for measuring the tensile strength and bake hardening after the heat treatment were the same as those in Example 1 above.

[0117] [Table 4]

[0118]

[0119] In Table 4, TS0 and BH2 represent the tensile strength before heat treatment and the bake hardening amount, respectively.

[0120] TS h and BH h Indicates the tensile strength after heat treatment and the amount of bake hardening.

[0121] △TS2=TS h -TS0

[0122] As shown in Table 4, it can be confirmed that in the case of the hot-rolled steel sheets of Examples 6 to 10 of the present invention, the bake hardening amount (BH h ) is maintained above 30MPa, the above △TS2×BH h -1 The absolute value satisfies 0.7 or less, and the hot-rolled steel sheet can be effectively used in various plating processes.

[0123] In contrast, Comparative Examples 1 to 5 failed to achieve the desired microstructure, and thus, it is clear that the changes in strength and bake hardenability after heat treatment are relatively large compared to Inventive Examples 6 to 10. In particular, as in Comparative Examples 1 and 4, when the hardenability element content is too high, the fraction of low-temperature transformation phases in the microstructure increases, and it can be confirmed that these phases significantly reduce strength during additional heat treatment (300-600°C) due to tempering effects, etc. Therefore, hot-rolled steel sheets with such microstructure characteristics are difficult to use in applications requiring additional heat treatment for plating.

[0124] Figure 1 ΔTS2×BH before and after heat treatment at 520°C for the steel sheets of Comparative Examples 1 to 5 and Inventive Examples 6 to 10 according to the present invention h -1 A graph showing the change in absolute value. Figure 1 As shown, in the case of the hot-rolled steel sheet of the present invention, it can be confirmed that ΔTS2×BH h -1 Excellent properties with an absolute value of 0.7 or less.

[0125] The above description is made with reference to the embodiments. However, for those skilled in the art, various modifications and changes can be made to the present invention within the scope of the basic idea of the present invention. In addition, it should be clear that the scope of rights of the present invention should be interpreted based on the claims.

Claims

1. A hot-rolled steel plate, comprising, by weight, C: 0.050-0.100%, Si: 0.01-1.00%, Mn: 1.4-2.0%, Al: 0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P: 0.001-0.050%, S: 0.001-0.010%, N: 0.001-0.010%, Nb: 0.005-0.050%, Ti: 0.005-0.120%, and the balance being Fe and unavoidable impurities. The hot-rolled steel sheet has a fine structure comprising 90 area % or more of the sum of ferrite and bainite phases and less than 10 area % of the sum of retained pearlite, martensite, and MA phases. The hot-rolled steel sheet has a tensile strength of 760 MPa or more and a hole expandability (HER0) of 40% or more, a bake hardening amount (BH2) of 30 MPa or more, and satisfies ΔTS1 defined by the following relational formula 1 of 100 MPa or less. [Equation 1] △TS1=TS 最大 -TS 最小 TS 最大 : The maximum TS values at seven locations in the width direction including the extreme edge within 30m of the rear end of the hot rolled coil produced TS 最小 : Minimum TS values at seven positions in the width direction including the extreme edge within 30 m of the rear end of the hot rolled coil produced.

2. The hot-rolled steel sheet according to claim 1, wherein Bake hardening after heat treatment at 300-600℃ (BH h ) is maintained above 30MPa, as defined in the following equation 2: △TS2×BH h -1 The absolute value of is less than 0.7, [Equation 2] △TS2=TS h -TS0 TS h : tensile strength after heat treatment, TS0: tensile strength before heat treatment.

3. The hot-rolled steel sheet according to claim 1, wherein: The hot-rolled steel sheet contains one or more components selected from V, Ni, and B in a total amount within a range of 1.5% or less.

4. The hot-rolled steel sheet according to claim 1, wherein A hot-dip coating is formed on the surface of the hot-rolled steel sheet.

5. A method for manufacturing a hot-rolled steel plate, comprising the following steps: reheating the steel slab to a temperature range of 1100-1350° C., wherein the steel slab comprises, in terms of weight %, C: 0.050-0.100%, Si: 0.01-1.00%, Mn: 1.4-2.0%, Al: 0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P: 0.001-0.050%, S: 0.001-0.010%, N: 0.001-0.010%, Nb: 0.005-0.050%, Ti: 0.005-0.120%, and the balance of Fe and unavoidable impurities; hot rolling the reheated steel billet at a temperature of 850-1150° C. to produce a hot-rolled steel plate; Cooling the hot-rolled steel plate to a temperature in the range of 550-650° C. at an average cooling rate of 50-100° C. / s; The steel plate that has undergone the primary cooling is stopped from cooling for 3-7 seconds; as well as The hot-rolled steel sheet after stopping the primary cooling is subjected to secondary cooling at an average cooling rate of 1-30° C. / second to a temperature in the range of 400-500° C., and then coiled.

6. The method for producing a hot-rolled steel sheet according to claim 5, wherein: The total amount of one or more components selected from V, Ni, and B is contained in a range of 1.5% or less.

7. The method for producing a hot-rolled steel sheet according to claim 5, wherein: The manufacturing method further includes the following step: cooling the coiled plate to a temperature ranging from room temperature to 200° C. at an average cooling rate of 0.1-25° C. / hour.

8. The method for producing a hot-rolled steel sheet according to claim 5, wherein: The manufacturing method further comprises the following steps: pickling the coiled hot-rolled steel sheet and then applying oil.

9. The method for producing a hot-rolled steel sheet according to claim 5, wherein: The manufacturing method further comprises the following steps: pickling the coiled hot-rolled steel sheet, heating it to a temperature range of 450-750° C., and then hot-dip galvanizing it.

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