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%, 0.020 to 0.080% of C, 0.01 to 0.50% of Si, 0.8 to 1.8% of Mn, 0.010 to 0.100% of Al, 0.001 to 0.020% of P, 0.001 to 0.010% of S, 0.001 to 0.010% of N, 0.010 to 0.120% of Ti, and 0.010 to 0.050% of Nb, with the remainder being Fe and unavoidable impurities, has an X value defined by relational expression 1 satisfying 3.50 to 6.00, and has a microstructure containing 90 area% or more of the sum of ferrite and bainite phases and less than 10 area% of the sum of residual martensite and MA phases.

Description

Technical Field

[0001] The present invention relates to a high-strength composite texture hot-rolled steel sheet having excellent thermal stability and mainly applicable to automotive chassis components, and a manufacturing method thereof. Background Art

[0002] Conventional high-strength hot-rolled steel sheets for automotive chassis and frames are being thinned into high-strength thin sheets for weight reduction. At the same time, considering the component shape, excellent formability is required, and in order to maximize the durability of the components, a specified level of bake hardening amount is required, which represents the degree of hardening after painting. In addition, during the manufacturing process and use, for various purposes, sometimes part or the whole of the steel sheet and components are heated, and due to the heating process, the strength of the steel sheet and components may change, resulting in a problem of poor durability.

[0003] Generally, during the heating process, the amount of solid solution carbon in the structure increases, and at the same time, clusters are formed at dislocations, grain boundaries, etc., and carbides are formed. At the same time, the fine structures such as martensite, bainite, and retained austenite in the steel also change simultaneously, resulting in a sharp change in the strength of the steel and also affecting formability and durability. The change in the fine structure and the physical properties of the steel during this heating process depends on the composition and fine structure of the initial steel, and highly depends on heat treatment conditions such as heating temperature and holding time. Therefore, the existing technologies mainly focus on suppressing the reduction in strength at high temperatures above 600°C.

[0004] Patent Document 1 and Patent Document 2 propose technologies for ensuring high-temperature strength by adding elements such as Cr, Mo, Nb, and V and performing heat treatment on the steel sheet after hot rolling, but these technologies are applicable to the manufacturing process of thick steel sheets for construction. In addition, considering inevitable heating environment factors such as fires for construction steel, a large amount of alloying elements such as Cr, Mo, Nb, and V are added to the steel, so that even when exposed to a high-temperature environment above 600°C for a long time, a certain level of strength can be ensured. However, due to the use of expensive alloying elements and the need to perform a heat treatment process at the steel sheet stage to ensure physical properties, there is a problem of excessively high manufacturing costs. In particular, when used in the case of short-time exposure in a heating environment below 600°C, the above technologies have excessive thermal stability.

[0005] Patent Document 3 proposes a technique for ensuring the strength of the heat-affected zone of welding by adding Ti, Nb, Cr, Mo, etc. During arc welding, the part adjacent to the welding material melted due to the welding heat is heated to a high temperature of 600 °C or higher. In particular, it is sometimes heated to a temperature above the austenite region. Therefore, the addition of Cr and Mo can increase the hardenability of steel, and low-temperature phases such as bainite and martensite are formed during cooling, thereby ensuring strength. However, this solution based on maximizing hardenability has limitations when applied to automotive steel sheets that also need to ensure high formability after heat treatment as required after manufacturing the steel sheets.

[0006] [Prior Art Documents]

[0007] [Patent Documents]

[0008] (Patent Document 1) Authorized Patent No. KR10-0358939

[0009] (Patent Document 2) Authorized Patent No. KR10-1290382

[0010] (Patent Document 3) Authorized Patent No. KR10-0962745 Summary of the Invention

[0011] (1) Technical Problems to be Solved

[0012] The object of the present invention is to provide a high-strength composite structure hot-rolled steel sheet having excellent formability, bake hardening amount, and thermal stability, and a manufacturing method thereof.

[0013] In addition, the technical problems to be achieved by the present invention are not limited to the above-mentioned technical problems, and those of ordinary skill in the technical field to which the present invention pertains can clearly understand other technical problems not mentioned from the following description.

[0014] (2) Technical Solutions

[0015] One aspect of the present invention relates to a hot-rolled steel sheet which, by weight %, comprises: C: 0.020 - 0.080%, Si: 0.01 - 0.50%, Mn: 0.8 - 1.8%, Al: 0.010 - 0.100%, P: 0.001 - 0.020%, S: 0.001 - 0.010%, N: 0.001 - 0.010%, Ti: 0.010 - 0.120%, Nb: 0.010 - 0.050%, the balance being Fe and unavoidable impurities, the X value defined by the following relational expression 1 satisfies 3.50 to 6.00, the hot-rolled steel sheet has a microstructure comprising the sum of ferrite and bainite phases: 90 area % or more and the sum of retained martensite and MA phase: less than 10 area %, the hot-rolled steel sheet has a tensile strength of 590 MPa or more, an expansion hole rate (HER0) of 40% or more, a bake hardening amount (BH2) of 30 MPa or more, and the bake hardening amount (BH h ) remains 30 MPa or more, and when △TS is defined by the following relational expression 2, the absolute value of △TS × BH h -1 is 0.70 or less.

[0016] [Relational expression 1]

[0017] X = A B

[0018] A = 1.3 [Mn] + 200 [C]

[0019] B = (Nb / 93 + Ti / 48) / (C / 12 + N / 14)

[0020] Ti = Ti - 3.42N - 1.5S

[0021] wherein, Mn, C, Nb, Ti, N, and S respectively represent their weight %.

[0022] [Relational expression 2]

[0023] △TS = TS h - TS0

[0024] TS h : Tensile strength after heat treatment, TS0: Tensile strength before heat treatment

[0025] BH h : Bake hardening amount after heat treatment

[0026] The hot-rolled steel sheet may contain one or more of Mo, Cr, V, Ni, and B with a total content of 1.500% or less.

[0027] A hot-dip galvanized layer may be formed on the surface of the hot-rolled steel sheet.

[0028] In addition, another aspect of the present invention relates to a method for manufacturing a hot-rolled steel sheet, which includes the following steps: reheating a steel slab to a temperature range of 1100 - 1350 °C, the steel slab satisfying the above alloy composition components, and the X value defined by Relational Expression 1 satisfying 3.50 to 6.00; hot-rolling the reheated steel slab in the range of 850 - 1150 °C to manufacture a hot-rolled steel sheet; and cooling the hot-rolled steel sheet at an average cooling rate of 10 - 70 °C / sec to a temperature in the range of 400 - 550 °C, and then coiling.

[0029] It may further include a step of applying oil after pickling the coiled hot-rolled steel sheet.

[0030] It may further include a step 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, by weight%, the plating bath contains: Mg: 0.01 - 30%, Al: 0.01 - 50%, and the balance zinc.

[0031] (III) Beneficial Effects

[0032] The present invention configured as described above can effectively provide a hot-rolled steel sheet having a tensile strength of 590 MPa or more, a hole expansion rate (HER0) value of 40% or more, a bake hardening amount (BH2) of 30 MPa or more, and a bake hardening amount (BH h ) of 30 MPa or more after heat treatment at 300 - 600 °C, and the absolute value of △TS×BH h -1 is 0.70 or less, thus having excellent thermal stability.

[0033] Therefore, the hot-rolled steel sheet of the present invention can be effectively applied to components for automotive chassis parts, lower arms, reinforcement members, connecting members, and parts of vehicle frames. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a graph showing the changes in the tensile strength and bake hardening amount before and after heat treatment for the X value of the steel sheets of the example and comparative example of the present invention (the absolute value of △TS×BH h -1 ). BEST MODE FOR CARRYING OUT THE INVENTION

[0035] The present invention will be described below.

[0036] In order to expand the applicability of hot-rolled chassis components, the inventor of the present invention studied steels with different compositions and microstructures, and confirmed that the change in the room-temperature tensile strength measured after heat treatment in the temperature range of 100 - 600 °C depends on the slope of the dynamic strength value measured during the heating of the steel. From this result, the relational expression 1 for the component contents of C, Mn, Si, Ti, and Nb, which are the main components of the steel that make the hot-rolled steel sheet have excellent thermal stability, was derived. Based on the relational expression 1, the optimal steel microstructure was formed, and thus a high-strength composite structure hot-rolled steel sheet with a tensile strength of 590 MPa or more, a yield ratio of 0.7 or more, an expansion hole rate (HER0) value of 40% or more, and a bake hardening amount (BH2) of 30 MPa or more was manufactured, and thus the present invention was proposed. The hot-rolled steel sheet manufactured by this technology, even after heat treatment at 100 - 600 °C, the bake hardening amount (BH h ) also remains at 30 MPa or more, and the absolute value of △TS×BH h -1 is 0.70 or less, and it has excellent thermal stability. Therefore, when used as an actual component, it can be heat-treated at a relatively low temperature for a short time, so that the application of the use can be expanded, and it is easy to be used in the manufacture of hot-rolled steel sheets plated with molten zinc or the like.

[0037] This hot-rolled steel sheet of the present invention contains, by weight: C: 0.020 - 0.080%, Si: 0.01 - 0.50%, Mn: 0.8 - 1.8%, Al: 0.010 - 0.100%, P: 0.001 - 0.020%, S: 0.001 - 0.010%, N: 0.001 - 0.010%, Ti: 0.010 - 0.120%, Nb: 0.010 - 0.050%, the balance of Fe and inevitable impurities, the X value defined by the relational expression 1 satisfies 3.50 to 6.00, the hot-rolled steel sheet has a microstructure including the sum of ferrite and bainite phases: 90 area% or more and the sum of retained martensite and MA phases: less than 10 area%, the hot-rolled steel sheet has a tensile strength of 590 MPa or more, an expansion hole rate (HER0) of 40% or more, a bake hardening amount (BH2) of 30 MPa or more, and the bake hardening amount (BH h ) after heat treatment at 300 - 600 °C remains at 30 MPa or more, and when △TS is defined as in the relational expression 2, the absolute value of △TS×BH h -1 satisfies 0.70 or less.

[0038] Hereinafter, the composition of the steel sheet provided in the present invention will be described in detail. At this time, unless otherwise specifically stated, the content of each component is expressed in weight %.

[0039] C: 0.020 - 0.080%

[0040] The said C is the most economical and effective element for strengthening the steel. When the addition amount increases, the precipitation strengthening effect or the fraction of the low-temperature phase increases, thereby increasing the tensile strength. However, when the content of C is less than 0.020%, it is difficult to achieve sufficient precipitation strengthening effect and the formation of the low-temperature phase, thus unable to ensure the target strength and bake hardening amount. When the content of C exceeds 0.080%, too much low-temperature phase and carbides are formed, so the formability and weldability may deteriorate. In addition, when heat treatment is carried out in the range of 100 - 600°C after adding too much C, it causes the deterioration of the low-temperature phase and the formation of additional residual carbides, thus greatly reducing the strength and bake hardening amount after heat treatment, and the formability becomes worse. Therefore, in the present invention, it is preferred to limit the content of the said C to 0.020 - 0.080%. More preferably, the content of the said C is limited to the range of 0.030 - 0.072%.

[0041] Si: 0.01 - 0.50%

[0042] The said Si deoxidizes the molten steel and has a solid solution strengthening effect, delaying the formation of coarse carbides, which is beneficial to improving the formability. In addition, when heat treatment is carried out in the range of 100 - 600°C, it also has the effect of suppressing the formation of carbides. However, when the content of Si is less than 0.01%, the effect of delaying the formation of carbides is small, it is difficult to improve the formability, and the thermal stability is also reduced. On the other hand, when the content of Si exceeds 0.50%, red scale caused by Si is formed on the surface of the steel sheet during hot rolling, which not only makes the surface quality of the steel sheet very poor, but also has problems of reducing the ductility and weldability. Therefore, in the present invention, it is preferred to limit the content of the said Si to the range of 0.01 - 0.50%. More preferably, the content of the said Si is limited to the range of 0.10 - 0.43%.

[0043] Mn: 0.8 - 1.8%

[0044] Like Si, the Mn is an effective element for solid-solution strengthening of steel. By increasing the hardenability of the steel, it makes the low-temperature phase easy to form. However, when the content of Mn is less than 0.8%, the above effects brought by adding Mn cannot be obtained. When the content of Mn exceeds 1.8%, the hardenability is increased significantly, resulting in an increase in the martensite phase fraction, and the segregation part in the thickness center of the slab becomes significantly developed during continuous casting, thus deteriorating the formability. In addition, when heat treatment is carried out in the range of 100 - 600 °C, the formation of carbides becomes easy, and large changes occur in strength and bake hardening amount. Therefore, in the present invention, it is preferable to limit the content of the Mn to 0.8 - 1.8%. More preferably, the content of the Mn is limited to the range of 0.8 - 1.5%.

[0045] P: 0.001 - 0.020%

[0046] Like Si, the P has both the effects of solid-solution strengthening and promoting ferrite phase transformation. However, in order to make P less than 0.001%, a large amount of manufacturing cost is required, which is economically disadvantageous and the strength cannot be obtained sufficiently. When the content of P exceeds 0.020%, brittleness caused by grain boundary segregation occurs, fine cracks are likely to occur during forming, and the ductility and impact resistance characteristics are significantly reduced. Therefore, it is preferable to limit the content of the P to the range of 0.001 - 0.02%.

[0047] S: 0.001 - 0.010%

[0048] The S is an impurity present in the steel. When the content of S exceeds 0.010%, it combines with Mn, etc. to form non-metallic inclusions. Therefore, fine cracks are likely to occur during the shearing process of the steel. On the other hand, when the content of S is less than 0.001%, a large amount of time is required during steelmaking operations, thus reducing productivity. Therefore, in the present invention, it is preferable to limit the S content to 0.001 - 0.010%.

[0049] Acid-soluble aluminum (Sol.Al): 0.010 - 0.100%

[0050] The acid-soluble aluminum (Sol.Al) is mainly a component added for deoxidation. When the content of the acid-soluble aluminum (Sol.Al) is less than 0.010%, its addition effect is insufficient. When the content of the acid-soluble aluminum (Sol.Al) exceeds 0.100%, it combines with nitrogen to form AlN, so that corner cracks are likely to occur in the slab during continuous casting and casting, and defects caused by the formation of inclusions are likely to occur. Therefore, in the present invention, it is preferable to limit the content of acid-soluble aluminum (Sol.Al) to 0.010 - 0.100%.

[0051] N: 0.001 - 0.010%

[0052] The N, together with C, is a representative solid-solution strengthening element and forms coarse precipitates together with Ti, Al, etc. Generally, the solid-solution strengthening effect of N is superior to that of carbon, but as the N content in the steel increases, there is a problem of significantly reducing toughness. In addition, in order to make N less than 0.001%, a large amount of time is required during the steelmaking operation, thereby reducing productivity. Therefore, in the present invention, it is preferable to limit the N content to 0.001 - 0.010%.

[0053] Ti: 0.010 - 0.120%

[0054] The Ti, together with Nb and V, is a representative precipitation strengthening element. Due to the strong affinity with N, coarse TiN is formed in the steel. TiN has the effect of suppressing grain growth during the heating process for hot rolling. In addition, the remaining Ti after reacting with nitrogen will dissolve in the steel and combine with carbon to form TiC precipitates, which is a useful component for improving the strength of the steel. Therefore, when the Ti content is less than 0.010%, the above effects cannot be obtained, and 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, it is preferable to limit the Ti content to 0.010 - 0.120%. More preferably, the Ti content is limited to the range of 0.070 - 0.115%.

[0055] Nb: 0.010 - 0.050%

[0056] The Nb, together with Ti and V, is a representative precipitation strengthening element and precipitates during hot rolling. Through the effect of grain refinement by delaying recrystallization, it effectively improves the strength and impact toughness of the steel. However, when the Nb content is less than 0.010%, the above effects cannot be obtained, and when the Nb content exceeds 0.050%, recrystallization is overly delayed during hot rolling, resulting in the formation of elongated grains and coarse composite precipitates, thereby having a problem of poor formability. Therefore, in the present invention, it is preferable to limit the Nb content to 0.010 - 0.050%. More preferably, the Nb content is limited to the range of 0.015 - 0.040%.

[0057] In addition, in the present invention, one or more components of Mo, Cr, V, Ni, and B can be further included as needed. At this time, their total content is within 1.500%.

[0058] In the case of Mo and Cr, the ferrite phase transformation is delayed, which is beneficial for ensuring low-temperature transformation structures such as bainite. In addition, both of these elements combine with C, which helps to ensure the strength brought about by the formation of carbides. Ni is an austenite stabilizing element, and compared with the above two elements, its hardenability effect is very large. Therefore, by ensuring the low-temperature transformation structure, it is very beneficial for the improvement of strength. B is also a very effective hardenability element. Compared with the above elements, even in a significantly small amount (in the unit of dozens of ppm), it can exert the same effect. V is a precipitation element that precipitates in the low-temperature region compared with Nb and Ti, and has the advantage of being able to improve strength through the precipitation strengthening effect.

[0059] Relationship 1

[0060] The feature of the present invention is that by controlling the contents of C, Mn, Ti, Nb, and N, the X value defined by the following Relationship 1 satisfies 3.50 to 6.00.

[0061] In the present invention, the main factors determining the strength and fine microstructure of the steel plate are hardenability elements such as C and Mn, and precipitation elements such as Ti and Nb. In addition, in order to reduce the change in strength after heat treatment (in order to improve thermal stability), it is necessary to reduce the fraction of low-temperature phase transformations such as bainite and martensite in the formed fine microstructure. In particular, when the fraction of martensite, which is a very hard structure, is relatively high, serious strength reduction may occur depending on the heat treatment conditions. In addition, appropriately designing the precipitation strengthening elements is also important for reducing the strength deviation before and after heat treatment, because during the reheating, hot rolling, coiling processes, and further heat treatment processes, re-solution, formation, and size change of precipitates occur, which affect the strength of the material. The present invention is proposed by deriving the alloy composition design factor X for ensuring appropriate thermal stability from the main hardenability elements related to these fine microstructures and precipitation strengthening, namely C, Mn, and precipitation elements, namely Nb, Ti, etc.

[0062] In the present invention, when the X value is less than 3.50, there is a high possibility that insufficient hardenability elements or precipitation hardening elements are added. In this case, it is difficult to ensure the required tensile strength, and the material deviation caused by the deviation of the hot rolling manufacturing conditions may be large. In addition, when the X value exceeds 6.00, excessive addition of hardenability elements or precipitation hardening elements may lead to excessive increase in strength and consequent reduction in elongation, or strength reduction due to softening of secondary hard phases after additional heat treatment, or problems such as aggravated material change due to re-precipitation, etc.

[0063] [Relationship 1]

[0064] X = A B

[0065] A = 1.3 [Mn] + 200 [C]

[0066] B = (Nb / 93 + Ti / 48) / (C / 12 + N / 14)

[0067] Ti = Ti - 3.42N - 1.5S

[0068] Among them, Mn, C, Nb, Ti, N, and S represent their weight percentages.

[0069] In the present invention, the remaining components and the balance are Fe and inevitable impurities.

[0070] In addition, the hot-rolled steel sheet of the present invention may have a steel sheet microstructure including the sum of ferrite and bainite phases: more than 90% and the sum of retained martensite and MA phases: 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 retained pearlite or martensite and MA phases exceeds 10%, and there may be a problem of poor hole expansion rate. The hole expansion rate is greatly affected by the microstructure composition of the steel sheet. Especially when the steel sheet is composed of a soft phase and a hard phase in combination, due to the hardness difference between the constituent phases, the hole expansion rate will be significantly reduced. In particular, the higher the fraction of pearlite or martensite, which is a very hard microstructure, the easier it is to generate cracks at the phase interface during hole expansion, thus reducing the hole expansion rate. Therefore, it is necessary to limit the phase fraction.

[0071] The hot-rolled steel sheet of the present invention having the microstructure as described above may have a tensile strength of 590 MPa or more, a hole expansion rate (HER0) of 40% or more, and a bake hardening amount (BH2) of 30 MPa or more.

[0072] In addition, the hot-rolled steel sheet of the present invention may have a bake hardening amount (BH h ) maintained at 30 MPa or more after heat treatment at 300 - 600 °C, and when △TS is defined by the following relational expression 2, the absolute value of △TS × BH h -1 satisfies a high-temperature bake hardening property of 0.70 or less. That is, the hot-rolled steel sheet of the present invention can also maintain a bake hardening amount (BH h ) of 30 MPa or more after heat treatment at 300 - 600 °C, etc., so that a plated steel sheet can be effectively manufactured by using a subsequent hot-dip galvanizing process or the like.

[0073] [Relational expression 2]

[0074] △TS = TS h -TS0

[0075] TSh : Tensile strength after heat treatment, TS0: Tensile strength before heat treatment

[0076] Next, a method for manufacturing a hot-rolled steel sheet according to a preferred embodiment of the present invention will be described in detail.

[0077] The method for manufacturing a hot-rolled steel sheet according to the present invention includes the following steps: reheating a steel billet to a temperature range of 1100 - 1350 °C, the steel billet satisfying the above alloy composition and an X value defined by the relational expression 1 of 3.5 to 6.0; hot-rolling the reheated steel billet in the range of 850 - 1150 °C to manufacture a hot-rolled steel sheet; and cooling the hot-rolled steel sheet at an average cooling rate of 10 - 70 °C / second to a temperature in the range of 400 - 550 °C, and then coiling.

[0078] Reheating

[0079] First, in the present invention, a steel billet having the above alloy composition is reheated to a temperature range of 1100 - 1350 °C. At this time, when the reheating temperature is lower than 1100 °C, the re-solution rate of precipitates containing Ti, Nb, Mo, and V decreases, and the formation of fine precipitates in the subsequent processes after hot rolling will be reduced. When the reheating temperature exceeds 1350 °C, due to the coarsening of austenite grains, the strength decreases. Therefore, it is preferable to limit the reheating temperature to 1100 - 1350 °C.

[0080] Hot rolling

[0081] Next, in the present invention, the reheated steel billet is hot-rolled in the range of 850 - 1150 °C to manufacture a hot-rolled steel sheet. At this time, when hot rolling starts at a temperature higher than 1150 °C, the temperature of the hot-rolled steel sheet becomes high, the grain size becomes coarse, and the surface quality of the hot-rolled steel sheet deteriorates. In addition, when the hot rolling is terminated at a temperature lower than 850 °C, due to the excessive delay of recrystallization, elongated grains develop and anisotropy intensifies, resulting in poor formability.

[0082] Cooling and coiling

[0083] Then, in the present invention, the hot-rolled steel sheet is cooled at an average cooling rate of 10 - 70 °C / second to a temperature in the range of 400 - 550 °C, and then coiled.

[0084] When the hot-rolled steel sheet is cooled to below 400°C and coiled, low-temperature phases such as martensite and MA phase are unnecessarily formed in the steel, thereby reducing the thermal stability of the structure. The formability before and after heat treatment deteriorates, and there is a problem that the strength decrease amplitude after heat treatment increases. On the other hand, when cooled to above 550°C and coiled, an appropriate fraction of bainite, martensite, and MA phase cannot be ensured, so the BH value cannot be ensured before and after heat treatment.

[0085] In addition, when the average cooling rate during cooling is less than 10°C / second, the grains of the matrix structure become coarse and the fine structure becomes uneven. On the other hand, when the average cooling rate exceeds 70°C / second, due to the increase in the fraction of low-temperature phases, similar problems to those occurring when coiling at below 400°C may occur.

[0086] Preferably, the cooling temperature is limited to 400 - 500°C.

[0087] In addition, the present invention may further include a step of oiling after pickling the coiled hot-rolled steel sheet as needed.

[0088] In addition, as needed, it may further include a step of pickling the coiled hot-rolled steel sheet, then 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. Among them, in terms of weight%, the plating bath contains: Mg: 0.01 - 30%, Al: 0.01 - 50%, and the balance of zinc. Detailed Description of the Invention

[0089] Hereinafter, the present invention will be described in more detail by way of examples. However, the descriptions of these examples are only for illustrating the implementation of the present invention, and the present invention is not limited to the descriptions of these examples.

[0090] (Examples)

[0091] Steel billets having the alloy compositions shown in Table 1 below were prepared, and then these billets were reheated at 1200°C. Then, under the conditions shown in Table 2 below, the reheated billets were hot-rolled, cooled, and coiled to manufacture hot-rolled steel sheets. At this time, the cooling rate of the coiled hot-rolled steel sheet was maintained at a level of 0.5 - 10°C / second.

[0092] The fine structure composition and fraction of the hot-rolled steel sheet manufactured as described above were measured, and the results are shown in Table 2 below.

[0093] Specifically, the fractions of the ferrite phase (F), bainite phase (B), martensite phase (M), and pearlite phase (P) were measured as a result of analysis using SEM at magnifications of ×3000 and ×5000. To read the martensite and MA phases, after etching with nitric acid ethanol (Nital) and Lepera, analysis was performed using an optical microscope and an image analyzer at a magnification of ×1000.

[0094] In addition, the tensile strength (TS0), bake hardening amount (BH2), and hole expansion rate (HER0) of the manufactured hot-rolled steel sheet were measured, and the results are shown together in Table 3.

[0095] The tensile material and the bake hardening amount were the results of tests after taking specimens from the DIN standard specimens along the rolling direction, and the tensile evaluation was carried out at room temperature.

[0096] The measurement of the bake hardening amount was carried out by measuring the strength value at 2% pre-deformation at room temperature and the difference in strength values after heat treatment at 170 °C for 20 minutes after 2% pre-deformation and then cooling to room temperature. Specifically, when measuring the bake hardening amount, the strength after heat treatment at 170 °C for 20 minutes after pre-deformation was measured as the lower yield strength to measure the lower bake hardening amount.

[0097] Moreover, the hole expansion rate is shown as the average value of the results evaluated three times in total at room temperature. Specifically, in each test, the test was terminated when the generation of cracks was visually observed, and the major axis length of the part where the cracks occurred was measured to evaluate the hole expansion rate, and this measurement was independent of the rolling direction of the specimen.

[0098] In addition, under the conditions shown in Table 4 below, further heat treatment was performed on the steel sheets of Comparative Examples 1 to 9 and the steel sheets of Inventive Examples 1 to 5. That is, the hot-rolled steel sheet was heat-treated at a heat treatment temperature of 500 °C for 10 minutes and then air-cooled to room temperature. And after performing the heat treatment as described above, the mechanical and physical properties of the steel sheet before and after the heat treatment were evaluated and are also shown in Table 4. Specifically, the tensile strength and the bake hardening amount of the steel sheet after heat treatment were measured, and the results were compared with the tensile strength and the bake hardening amount of the steel sheet before heat treatment and shown. At this time, the measurement methods of the tensile strength and the bake hardening amount after heat treatment were as described above.

[0099] [Table 1]

[0100]

[0101] In Table 1, the balance is Fe and unavoidable impurities.

[0102] [Table 2]

[0103]

[0104] In Table 2, F represents ferrite, B represents bainite, M represents martensite, MA represents island martensite, and P represents pearlite.

[0105] [Table 3]

[0106]

[0107] In Table 3, TS0 and BH2 respectively represent the tensile strength before heat treatment and the bake hardening amount, and TS h and BH h represent the tensile strength and the bake hardening amount after heat treatment.

[0108] △TS = TS h -TS0

[0109] As shown in Tables 1 to 3 above, Invention Examples 1 to 5 that satisfy the steel plate composition range, X value, and manufacturing conditions proposed by the present invention can all ensure the target material. Specifically, the hot-rolled steel plates of Invention Examples 1 to 5 all have a tensile strength of 590 MPa or more, an expansion ratio (HER0) of 40% or more, and a bake hardening amount (BH2) of 30 MPa or more. In addition, after further heat treatment, the bake hardening amount (BH h ) remains 30 MPa or more, and the absolute value of △TS×BH h -1 satisfies 0.70 or less, and it can be confirmed that the present hot-rolled steel plate can be effectively applied to various plating processes.

[0110] On the other hand, Comparative Steels 1 to 9 are cases where the composition range and / or manufacturing process conditions proposed by the present invention are not satisfied.

[0111] Specifically, in Comparative Examples 1, 3, and 4, the contents of C and Mn are too high and do not satisfy the relational expression, and the martensite phase and the MA phase are formed unnecessarily, resulting in a poor expansion ratio value of the steel plate or a large decrease in the tensile strength after heat treatment.

[0112] In Comparative Examples 2 and 5, the contents of C and Mn are insufficient respectively. Due to the decrease in hardenability, a sufficient low-temperature phase fraction cannot be ensured, so the steel plate strength does not satisfy 590 MPa, and the bake hardening amounts before and after heat treatment are both poor.

[0113] Comparative Steel 6 and Comparative Example 7 are cases where the contents of Ti and Nb are too high. Due to the formation of excessive carbides, it is difficult to ensure the low-temperature phase fraction. Therefore, the bake hardening amounts before and after heat treatment are both poor. Moreover, due to the increase in coarse precipitates, the hole expansion rate also remains at the lower limit or fails to reach the target.

[0114] In addition, in Comparative Example 8 and Comparative Example 9, the X value according to Relational Expression 1 exceeds the scope of the present invention, and the coiling temperature exceeds the range proposed by the present invention. That is, as shown in Comparative Example 8, when the coiling temperature exceeds the scope of the present invention, it is difficult to form a low-temperature phase in the structure, so it is difficult to ensure the bake hardening amount before and after heat treatment. As shown in Comparative Example 9, when the coiling temperature does not reach the scope of the present invention, the low-temperature phase fraction in the structure increases unnecessarily, not only making the yield ratio poor, but also increasing the variation range of the strength and bake hardening amount values before and after heat treatment.

[0115] In addition, Comparative Examples 10 to 12 are cases where the alloy composition components are within the scope of the present invention, but the manufacturing conditions exceed the scope of the present invention. In Comparative Example 10, the cooling rate until the coiling temperature is too fast, resulting in the fraction of the hard phase martensite in the fine tissue phase exceeding 10%. It was confirmed that this led to a deterioration in the hole expansion rate and an exacerbation of the strength deviation after heat treatment. In Comparative Examples 11 and 12, the cooling rate until the coiling temperature is too slow, resulting in the pearlite fraction in the fine tissue phase exceeding 10%, leading to a deterioration in the hole expansion rate, bake hardenability, and a decrease in the strength / bake hardening amount after heat treatment.

[0116] In addition, as described above, Comparative Examples 13 to 14 are cases where the X value according to Relational Expression 1 exceeds the scope of the present invention, but the manufacturing conditions satisfy the scope of the present invention. It was confirmed that according to the change in the tensile strength / bake hardening value before and after heat treatment, the absolute value of △TS×BH h -1 failed to satisfy being 0.7 or less.

[0117] Figure 1 is a graph showing the changes in the tensile strength and bake hardening amount before and after heat treatment for the X value of the steel plates of the examples and comparative examples of the present invention (the absolute value of △TS×BH h -1 . As Figure 1 shown, in the case of the hot-rolled steel plates of the inventive examples of the present invention, the absolute values of △TS×BH h -1 are all 0.70 or less, having excellent thermal stability.

[0118] The above has been described with reference to the embodiments. However, for those skilled in the art in this technical field, within the scope of the basic idea of the present invention, various modifications and changes can be made to the present invention. In addition, it should be clear that the scope of the rights of the present invention should be interpreted based on the claims.

Claims

1. A hot-rolled steel sheet, by weight%, the hot-rolled steel sheet contains: C: 0.020 - 0.080%, Si: 0.01 - 0.50%, Mn: 0.8 - 1.8%, Al: 0.010 - 0.100%, P: 0.001 - 0.020%, S: 0.001 - 0.010%, N: 0.001 - 0.010%, Ti: 0.010 - 0.120%, Nb: 0.010 - 0.050%, the balance of Fe and inevitable impurities, The X value defined by the following relational expression 1 satisfies 3.50 to 6.00, The hot-rolled steel sheet has a microstructure comprising the sum of ferrite and bainite phases: more than 90 area% and the sum of retained martensite and MA phases: less than 10 area%, The hot-rolled steel sheet has a tensile strength of 590 MPa or more, an expansion hole ratio (HER0) of 40% or more, a bake hardening amount (BH2: bake hardening amount before heat treatment) of 30 MPa or more, and a bake hardening amount (BH h ) after heat treatment at 300 - 600 °C remains 30 MPa or more, and when ΔTS is defined by the following relational expression 2, the absolute value of ΔTS × BH h -1 is 0.70 or less. [Relational expression 1] X=A B A = 1.3 [Mn] + 200 [C] B=(Nb / 93+Ti / 48) / (C / 12+N / 14) Ti =Ti - 3.42N - 1.5S Among them, Mn, C, Nb, Ti, N, and S respectively represent their weight%, [Relational expression 2] △TS = TS h -TS0 TS h : Tensile strength after heat treatment, TS0: Tensile strength before heat treatment BH h : Bake hardening amount after heat treatment.

2. The hot-rolled steel sheet according to claim 1, wherein, The hot-rolled steel sheet contains one or more of Mo, Cr, V, Ni, and B with a total content of 1.500% or less.

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

4. A method for manufacturing a hot-rolled steel sheet, which includes the following steps: Reheat the steel billet to a temperature range of 1100 - 1350 °C. By weight%, the steel billet contains: C: 0.020 - 0.080%, Si: 0.01 - 0.50%, Mn: 0.8 - 1.8%, Al: 0.010 - 0.100%, P: 0.001 - 0.020%, S: 0.001 - 0.010%, N: 0.001 - 0.010%, Ti: 0.010 - 0.120%, Nb: 0.010 - 0.050%, the balance of Fe and inevitable impurities, and the X value defined by the following relational expression 1 satisfies 3.50 to 6.00; Hot-roll the reheated steel billet in the range of 850 - 1150 °C to manufacture a hot-rolled steel sheet; And Cool the hot-rolled steel sheet at an average cooling rate of 10 - 70 °C / second to a temperature in the range of 400 - 550 °C, and then coil it, [Relational expression 1] X=A B A = 1.3 [Mn] + 200 [C] B=(Nb / 93+Ti / 48) / (C / 12+N / 14) Ti =Ti - 3.42N - 1.5S wherein, Mn, C, Nb, Ti, N, and S respectively represent their weight%.

5. The manufacturing method of the hot-rolled steel sheet according to claim 4, wherein, The hot-rolled steel sheet contains one or more of Mo, Cr, V, Ni, and B with a total content of 1.500% or less.

6. The manufacturing method of the hot-rolled steel sheet according to claim 4, wherein, The manufacturing method further includes a step of pickling the coiled hot-rolled steel sheet and then applying oil.

7. The manufacturing method of the hot-rolled steel sheet according to claim 4, wherein, The manufacturing method further includes a step of pickling the coiled hot-rolled steel sheet, heating it to a temperature range of 450 - 750 °C, and then performing hot-dip galvanizing.

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