Hot-rolled steel sheet and its manufacturing method

Abstract

The hot-rolled steel sheet of the present invention contains, by weight, 0.020-0.080% C, 0.01-0.50% Si, 0.8-1.8% Mn, 0.010-0.100% Al, 0.001-0.020% P, 0.001-0.010% S, 0.001-0.010% N, 0.010-0.120% Ti, 0.010-0.050% Nb, the balance being Fe and unavoidable impurities, and has a microstructure in which the X value defined by Relation 1 satisfies 3.50-6.00 and the total area percentage of ferrite and bainite phases is 90% or more, and the total area percentage of the remaining martensite and MA phases is less than 10%.

Description

[Technical Field]

[0001] The present invention relates to a high-strength dual-phase hot-rolled steel sheet having excellent thermal stability that can be applied mainly to chassis parts of automobiles, and a method for producing the same. [Background technology]

[0002] High-strength hot-rolled steel sheets used for conventional automobile chassis and frames are becoming thinner and stronger to reduce weight. At the same time, excellent formability is required to accommodate the shape of the part, and a certain level of bake hardening, which refers to the degree of hardening after painting, is also required to maximize the durability of the part. Furthermore, heat is often applied to part or all of the steel sheet and part for various purposes during manufacturing and use, which can cause a problem of changes in the strength of the steel sheet and part due to the heating process, resulting in deterioration of durability.

[0003] Typically, during the heating process, as the amount of solute carbon in the structure increases, clustering occurs at dislocations and grain boundaries, resulting in the formation of carbides. This simultaneously changes the microstructure of the steel, such as martensite, bainite, and retained austenite, resulting in a rapid change in the strength of the steel and affecting its formability and durability. The changes in the microstructure and physical properties of steel during the heating process vary depending on the initial steel composition and microstructure, and are highly dependent on heat treatment conditions such as heating temperature and holding time. Therefore, previous technologies have focused solely on preventing strength loss at high temperatures above 600°C.

[0004] Patent Documents 1 and 2 propose a technique for ensuring high-temperature strength by adding Cr, Mo, Nb, V, etc., followed by hot rolling and heat treating the resulting steel plate. This technique is suitable for the manufacturing process of thick steel plates for construction. Furthermore, taking into consideration environmental factors, such as the inevitable heating of steel for construction due to fires, the addition of large amounts of alloying elements, such as Cr, Mo, Nb, and V, to the steel ensures a predetermined level of strength even when exposed to high-temperature environments of 600°C or higher for long periods of time. However, this technique has the drawback of requiring expensive alloying elements and a heat treatment process to ensure physical properties at the steel plate stage, resulting in excessive manufacturing costs. In particular, the thermal stability of this steel is too high for use in environments exposed to temperatures of 600°C or lower for short periods of time.

[0005] Patent Document 3 proposes a technology for ensuring the strength of weld heat-affected zones by adding Ti, Nb, Cr, Mo, etc. During arc welding, the area adjacent to the molten welding material is heated to temperatures of 600°C or higher, and may even be heated to temperatures above the austenite range. Therefore, the addition of Cr and Mo increases the hardenability of the steel and forms low-temperature phases such as bainite and martensite during cooling, thereby ensuring strength. However, this concept based on maximizing hardenability has limitations in its application to automotive steel sheets, which must maintain high formability even after heat treatment as required after steel sheet production. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Registration No. 10-0358939 [Patent Document 2] Korean Patent Registration No. 10-1290382 [Patent Document 3] Korean Patent Registration No. 10-0962745 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a high-strength dual-phase hot-rolled steel sheet excellent in formability, bake hardening amount, and thermal stability, and a method for producing the same.

[0008] Furthermore, the technical problems that the present invention aims to solve are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those having ordinary skill in the technical field to which the present invention pertains from the following description. [Means for solving the problem]

[0009] One aspect of the present invention is The alloy contains, by weight, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.8 to 1.8%, Al: 0.010 to 0.100%, P: 0.001 to 0.020%, S: 0.001 to 0.010%, N: 0.001 to 0.010%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, the balance being Fe and unavoidable impurities, and the value of X defined by the following relational expression 1 satisfies 3.50 to 6.00, A microstructure including a total of ferrite and bainite phases: 90% by area or more, and a total of the remaining martensite and MA phases: less than 10% by area, The tensile strength is 590 MPa or more, the hole expandability (HER0) is 40% or more, the bake hardening amount (BH2) is 30 MPa or more, and the bake hardening amount (BH h ) is maintained at 30 MPa or more, and △TS is defined as in the following relational expression 2, △TS × BH h -1 The absolute value of the above formula (1) is 0.70 or less.

[0010] [Equation 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

[0011] Here, Mn, C, Nb, Ti, N and S indicate their weight percentages.

[0012] [Equation 2] △TS=TS h -TS0 TS h : Tensile strength after heat treatment, TS0: Tensile strength before heat treatment BH h : Amount of bake hardening after heat treatment

[0013] The hot-rolled steel sheet may contain one or more elements selected from the group consisting of Mo, Cr, V, Ni, and B in a total amount of 1.500% or less.

[0014] The surface of the hot-rolled steel sheet may be coated with hot-dip galvanizing.

[0015] Another aspect of the present invention is Reheating the steel slab, which satisfies the above-mentioned alloy composition and the X value defined by Relation 1 of 3.5 to 6.0, to a temperature range of 1100 to 1350 ° C.; hot rolling the reheated steel slab at a temperature in the range of 850 to 1150 ° C to produce a hot-rolled steel sheet; cooling the hot-rolled steel sheet to a temperature in the range of 400 to 550°C at an average cooling rate of 10 to 70°C / sec, and then coiling the sheet.

[0016] The method may further include the step of pickling the coiled hot-rolled steel sheet and then oiling it.

[0017] The method may further include a step of pickling the coiled hot-rolled steel sheet, then heating it to a temperature range of 450 to 750°C, and then immersing it in a coating bath containing, by weight, 0.01 to 30% Mg, 0.01 to 50% Al, and the balance being zinc, to form a hot-dip galvanized layer on the surface. [Effects of the Invention]

[0018] The present invention having the above-described configuration provides a tensile strength of 590 MPa or more, a hole expandability (HER0) value of 40% or more, a bake hardening amount (BH2) of 30 MPa or more, and a bake hardening amount (BH) after heat treatment at 300 to 600°C. h ) is 30 MPa or more, and △TS × BH h -1 The absolute value of β is 0.70 or less, and a hot-rolled steel sheet having excellent thermal stability can be effectively provided.

[0019] Therefore, the hot-rolled steel sheet of the present invention can be effectively applied to parts used in automobile chassis members, lower arms, reinforcing members, connecting members, and frames. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a graph showing the change in tensile strength and the amount of bake hardening before and after heat treatment (absolute value of ΔTS×BHh −1 ) versus the X value for steel sheets of invention examples and comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be described below.

[0022] The present inventors have studied steels with various chemical compositions and microstructures to expand the applicability of steels to hot-rolled chassis parts. They have confirmed that the change in room-temperature tensile strength measured after heat treatment in the 100 to 600°C temperature range depends on the slope of the dynamic strength measured during the heating process. From these results, they derived Relational Formula 1, which determines the contents of the main steel components C, Mn, Si, Ti, and Nb so that hot-rolled steel sheets have excellent thermal stability. They have also confirmed that by forming an optimal steel microstructure based on Relational Formula 1, it is possible to manufacture high-strength dual-phase hot-rolled steel sheets with a tensile strength of 590 MPa or more, a yield ratio of 0.7 or more, a hole expandability (HER0) value of 40% or more, and a bake hardening value (BH2) of 30 MPa or more. The present invention provides this technology. Hot-rolled steel sheets manufactured using this technology retain a high bake hardening value (BH2) even after heat treatment in the 100 to 600°C range. h) remains above 30MPa, and △TS×BH h -1 The absolute value of β is 0.70 or less, which means that the material has excellent thermal stability. When used in actual parts, it can be heat-treated in a short time at a relatively low temperature, which allows for a wide range of applications and makes it easy to use in the production of coated hot-rolled steel sheets using molten zinc or the like.

[0023] The hot-rolled steel sheet of the present invention contains, by weight %, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.8 to 1.8%, Al: 0.010 to 0.100%, P: 0.001 to 0.020%, S: 0.001 to 0.010%, N: 0.001 to 0.010%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, the balance being Fe and unavoidable impurities, The X value defined by Equation 1 satisfies 3.50 to 6.00, the microstructure contains 90 area % or more of ferrite and bainite phases in total, and less than 10 area % of the remaining martensite and MA phases in total, the tensile strength is 590 MPa or more, the hole expandability (HER0) is 40% or more, the bake hardening amount (BH2) is 30 MPa or more, and the bake hardening amount (BH2) after heat treatment at 300 to 600°C is 300 to 600°C. h ) is maintained at 30 MPa or more, and △TS is defined as in the following relational expression 2, △TS × BH h -1 The absolute value of is 0.70 or less.

[0024] First, the chemical composition of the steel sheet provided by the present invention will be described in detail below. Here, unless otherwise specified, the content of each component means % by weight.

[0025] C: 0.020 to 0.080% C is the most economical and effective element for strengthening steel. Increased C content increases the precipitation strengthening effect or the low-temperature phase fraction, thereby increasing tensile strength. However, if the C content is less than 0.020%, it is difficult to achieve sufficient precipitation strengthening and low-temperature phase formation, making it difficult to achieve the desired strength and bake hardening amount. If the C content exceeds 0.080%, the low-temperature phase and carbides are excessively formed, resulting in poor formability and weldability. Furthermore, if excessive C is added, heat treatment in the 100-600°C range significantly reduces the strength and bake hardening amount after heat treatment due to the deterioration of the low-temperature phase and the formation of additional excess carbides, further deteriorating formability. Therefore, in the present invention, the C content is preferably limited to 0.020-0.080%. More preferably, it is limited to the range of 0.030-0.072%.

[0026] Si: 0.01 to 0.50% The Si content is advantageous in that it deoxidizes molten steel, strengthens the solid solution, and retards the formation of coarse carbides, thereby improving formability. It also has the effect of suppressing the formation of carbides during heat treatment at temperatures between 100 and 600°C. However, if the Si content is less than 0.01%, the effect of retarding carbide formation is small, making it difficult to improve formability and reducing thermal stability. On the other hand, if the Si content exceeds 0.50%, red scale due to Si is formed on the surface of the steel sheet during hot rolling, significantly deteriorating the surface quality of the steel sheet and also reducing ductility and weldability. Therefore, in the present invention, the Si content is preferably limited to the range of 0.01 to 0.50%. More preferably, it is limited to the range of 0.10 to 0.43%.

[0027] Mn: 0.8 to 1.8% Like Si, Mn is an effective element for solid-solution strengthening of steel, increasing the hardenability of steel and facilitating the formation of low-temperature phases. However, if its content is less than 0.8%, the above-mentioned effects of its addition are not obtained. If its content exceeds 1.8%, the hardenability increases significantly, increasing the martensite phase fraction. This leads to the development of large segregations in the thickness center during slab casting in the continuous casting process, resulting in poor formability. Furthermore, during heat treatment in the 100 to 600°C range, carbides are easily formed, resulting in significant changes in strength and bake hardening. Therefore, in the present invention, the Mn content is preferably limited to 0.8 to 1.8%. More preferably, it is limited to the range of 0.8 to 1.5%.

[0028] P: 0.001 to 0.020% Like Si, P simultaneously strengthens the solid solution and promotes ferrite transformation. However, if the P content is less than 0.001%, the manufacturing cost will be high, which is economically disadvantageous, and the strength will be insufficient. If the P content exceeds 0.020%, embrittlement will occur due to grain boundary segregation, which will easily cause fine cracks during forming and will significantly deteriorate ductility and impact resistance. Therefore, it is preferable to limit the P content to the range of 0.001 to 0.02%.

[0029] S: 0.001 to 0.010% S is an impurity present in steel, and if its content exceeds 0.010%, it combines with Mn and other elements to form nonmetallic inclusions, which can easily cause fine cracks during cutting of the steel. On the other hand, if its content is less than 0.001%, steelmaking operations take longer, resulting in reduced productivity. Therefore, in the present invention, it is preferable to limit the S content to 0.001 to 0.010%.

[0030] Sol.Al: 0.010~0.100% The sol. Al is a component added mainly for deoxidation, and if its content is less than 0.010%, its effect is insufficient, while if it exceeds 0.100%, it combines with nitrogen to form AlN, which is likely to cause corner cracks in slabs during continuous casting and casting, and is likely to cause defects due to the formation of inclusions. Therefore, in the present invention, it is preferable to limit the sol. Al content to 0.010 to 0.100%.

[0031] N: 0.001 to 0.010% N, along with C, is a typical 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 the problem is that the toughness deteriorates significantly as the amount of N in steel increases. Furthermore, if N is added at less than 0.001%, the steelmaking operation takes a long time, resulting in reduced productivity. Therefore, in the present invention, it is preferable to limit the N content to 0.001 to 0.010%.

[0032] Ti: 0.010 to 0.120% Ti, along with Nb and V, 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, Ti remaining after reacting with nitrogen dissolves in the steel and bonds with carbon to form TiC precipitates, making it a useful component for improving the strength of steel. Therefore, if the Ti content is less than 0.010%, the above effect cannot be achieved. However, if the Ti content exceeds 0.120%, the formation of coarse TiN and the coarsening of TiC precipitates can lead to problems of degraded formability. Therefore, in the present invention, the Ti content is preferably limited to 0.010 to 0.120%, more preferably to 0.070 to 0.115%.

[0033] Nb: 0.010 to 0.050% Nb, along with Ti and V, is a typical precipitation strengthening element. It precipitates during hot rolling and has a grain refinement effect due to delayed recrystallization, effectively improving the strength and impact toughness of steel. However, if the Nb content is less than 0.010%, the above effect cannot be obtained. If the Nb content exceeds 0.050%, excessive delay in recrystallization during hot rolling causes the formation of elongated grains and coarse composite precipitates, resulting in poor formability. Therefore, in the present invention, the Nb content is preferably limited to 0.010 to 0.050%, more preferably to the range of 0.015 to 0.040%.

[0034] Furthermore, in the present invention, if necessary, one or more of Mo, Cr, V, Ni and B may be further contained, and in this case, the total content of these elements is 1.500% or less.

[0035] Mo and Cr are advantageous in delaying the ferrite transformation and ensuring a low-temperature transformation structure such as bainite, and both elements combine with C to form carbides, contributing to ensuring strength. Ni is an austenite stabilizing element and has a much greater hardening effect than the other two elements, making it extremely advantageous in ensuring a low-temperature transformation structure and increasing strength. B is also a very effective hardening element, and can achieve the same effect at much smaller amounts (tens of ppm) than the above elements. V is a precipitation element that precipitates at lower temperatures than Nb and Ti, and has the advantage of increasing strength through its precipitation strengthening effect.

[0036] Equation 1 The present invention is characterized in that the contents of C, Mn, Ti, Nb, and N are controlled so that the value of X defined by the following relational expression 1 satisfies 3.50 to 6.00.

[0037] In the present invention, the main factors determining the strength and microstructure of steel sheets are hardening elements such as C and Mn and precipitation elements such as Ti and Nb. Furthermore, to minimize changes in strength after heat treatment (to enhance thermal stability), the fraction of low-temperature transformation phases such as bainite and martensite must be reduced. In particular, when the fraction of martensite, a very hard structure, is high, significant strength degradation can occur depending on the heat treatment conditions. Furthermore, proper design of precipitation strengthening elements is also important for reducing the strength variation before and after heat treatment. This is because precipitates undergo redissolution, formation, and size change during reheating, hot rolling, coiling, and additional heat treatment, which affect the strength of the material. The present invention derives and provides an alloy composition design factor, X, to ensure optimal thermal stability from the microstructure and the main hardening elements related to precipitation strengthening, C and Mn, and precipitation elements, Nb and Ti.

[0038] In the present invention, if the X value is less than 3.50, it is highly likely that the hardening elements or precipitation hardening elements are not added sufficiently, making it difficult to ensure the desired tensile strength and resulting in large variations in material properties due to variations in hot rolling manufacturing conditions. Furthermore, if the X value exceeds 6.00, the hardening elements or precipitation hardening elements are added in excess, resulting in problems such as an excessive increase in strength and a resulting decrease in elongation, as well as significant changes in material properties due to softening of the secondary hard phase after additional heat treatment or reprecipitation.

[0039] [Equation 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

[0040] Here, Mn, C, Nb, Ti, N and S indicate their weight percentages.

[0041] In the present invention, the other components and the balance are iron and unavoidable impurities.

[0042] The hot-rolled steel sheet of the present invention may have a microstructure of the steel sheet containing a total of 90% or more of ferrite and bainite phases and a total of less than 10% of martensite and MA phases. If the total phase fraction of ferrite and bainite is less than 90% by area, the total of the remaining pearlite or martensite and MA phases exceeds 10%, which means that the hole expandability is degraded. Hole expandability is greatly affected by the microstructural structure of the steel sheet, but particularly when a steel sheet contains a composite of soft and hard phases, it is significantly degraded due to the difference in hardness between the constituent phases. In particular, the greater the fraction of pearlite or martensite, which are very hard microstructures, the more likely cracks are to occur at the interface between the phases during hole expansion, which degrades hole expandability. Therefore, it is necessary to limit the phase fraction.

[0043] The hot-rolled steel sheet of the present invention having the above-described microstructure may have a tensile strength of 590 MPa or more, a hole expandability (HER0) of 40% or more, and a bake hardening amount (BH2) of 30 MPa or more.

[0044] In addition, the hot-rolled steel sheet of the present invention has a bake hardening amount (BH) after heat treatment at 300 to 600°C. h ) is maintained at 30 MPa or more, and △TS is defined as in the following relational expression 2, △TS × BH h -1 In other words, the hot-rolled steel sheet of the present invention has high-temperature bake hardening properties in which the absolute value of BH satisfies 0.70 or less. h ) can be maintained at 30 MPa or more, and a plated steel sheet can be effectively produced using a subsequent hot-dip galvanizing process.

[0045] [Equation 2] △TS=TS h -TS0 TS h : Tensile strength after heat treatment, TS0: Tensile strength before heat treatment

[0046] Next, a method for producing a hot-rolled steel sheet according to a preferred embodiment of the present invention will be described in detail.

[0047] The method for producing a hot-rolled steel sheet of the present invention includes the steps of reheating a steel slab that satisfies the above-mentioned alloy composition and the X value defined by Relational Formula 1 of 3.5 to 6.0 to a temperature range of 1100 to 1350°C, hot-rolling the reheated steel slab at a temperature range of 850 to 1150°C to produce a hot-rolled steel sheet, and cooling the hot-rolled steel sheet to a temperature range of 400 to 550°C at an average cooling rate of 10 to 70°C / sec, and then coiling the hot-rolled steel sheet.

[0048] reheating First, in the present invention, a steel slab having the above-described alloy composition is reheated to a temperature range of 1100 to 1350°C. If the reheating temperature is less than 1100°C, the re-dissolution rate of precipitates containing Ti, Nb, Mo, and V decreases, and the formation of fine precipitates decreases in processes after hot rolling. If the reheating temperature exceeds 1350°C, the strength decreases due to coarsening of austenite grains, so the reheating temperature is preferably limited to 1100 to 1350°C.

[0049] hot rolling Next, in the present invention, the reheated steel slab is hot-rolled in the range of 850 to 1150°C to produce a hot-rolled steel sheet. If hot-rolling is started at a temperature higher than 1150°C, the temperature of the hot-rolled steel sheet increases, the grain size becomes coarse, and the surface quality of the hot-rolled steel sheet deteriorates. On the other hand, if hot-rolling is completed at a temperature lower than 850°C, elongated grains develop due to excessive delay in recrystallization, resulting in severe anisotropy and deterioration of formability.

[0050] Cooling and winding In the present invention, the hot-rolled steel sheet is cooled to a temperature in the range of 400 to 550°C at an average cooling rate of 10 to 70°C / sec, and then coiled.

[0051] If the hot-rolled steel sheet is cooled to below 400°C and coiled, low-temperature phases such as martensite and MA phases are unnecessarily formed in the steel, reducing the thermal stability of the structure, resulting in poor formability both before and after heat treatment and an increased decrease in strength after heat treatment.In contrast, if the hot-rolled steel sheet is cooled to above 550°C and coiled, the appropriate fractions of bainite, martensite, and MA phases are not ensured, making it impossible to ensure the BH value both before and after heat treatment.

[0052] Furthermore, if the average cooling rate during cooling is less than 10°C / sec, the crystal grains of the base structure become coarse and the microstructure becomes non-uniform, whereas if the average cooling rate exceeds 70°C / sec, the low temperature phase fraction increases, causing problems similar to those encountered when coiling at temperatures below 400°C described above.

[0053] Preferably, the cooling temperature is limited to 400 to 500°C.

[0054] Meanwhile, the present invention may further include a step of pickling the coiled hot-rolled steel sheet and then oiling it, if necessary.

[0055] If necessary, the method may further include a step of pickling the coiled hot-rolled steel sheet, then heating it to a temperature range of 450 to 750°C, and then immersing it in a coating bath containing, by weight %, 0.01 to 30% Mg, 0.01 to 50% Al, and the balance being zinc, to form a hot-dip galvanized layer on the surface. [Example]

[0056] The present invention will be described in more detail below using examples. However, the description of such examples is merely for illustrating the implementation of the present invention, and the present invention is not limited by the description of such examples.

[0057] (Example) Steel slabs having the alloy compositions shown in Table 1 below were prepared, and then reheated at 1200°C. Next, the reheated slabs were hot-rolled, cooled, and coiled under the conditions shown in Table 2 below to produce hot-rolled steel sheets. At this time, the cooling rate of the coiled hot-rolled steel sheets was maintained at a level of 0.5 to 10°C / s.

[0058] The composition and fraction of the microstructure of the hot-rolled steel sheet manufactured as described above were measured, and the results are shown in Table 2 below.

[0059] Specifically, the fractions of ferrite (F), bainite (B), martensite (M), and pearlite (P) phases were measured using SEM at magnifications of 3000x and 5000x. To identify the martensite and MA phases, the specimens were etched with Nital and Lepera, and then analyzed at 1000x magnification using an optical microscope and image analyzer.

[0060] The tensile strength (TS0), bake hardening amount (BH2) and hole expandability (HER0) of the produced hot-rolled steel sheets were measured, and the results are shown in Table 3.

[0061] The tensile properties and the amount of bake hardening were measured using DIN standard test pieces taken in the rolling direction, and the tensile evaluation was carried out at room temperature.

[0062] The amount of bake hardening was measured by measuring the difference between the strength value at room temperature after 2% pre-strain and the strength value after 2% pre-strain, heat treatment at 170°C for 20 minutes, and cooling to room temperature. In particular, the strength after pre-strain and heat treatment at 170°C for 20 minutes was measured as the lower yield strength to measure the amount of lower bake hardening.

[0063] The hole expandability was evaluated three times at room temperature and the average value was calculated. Specifically, for each test, if a crack was visible to the naked eye, the test was stopped and the length of the major axis of the cracked area was measured to evaluate the hole expandability. This measurement was performed regardless of the rolling direction of the sample.

[0064] Further, the steel sheets of Comparative Examples 1 to 9 and Invention Examples 1 to 5 were subjected to additional heat treatment under the conditions shown in Table 4 below. That is, the hot-rolled steel sheets were heat-treated at a heat treatment temperature of 500°C for 10 minutes and then air-cooled to room temperature. After such heat treatment, the mechanical properties of the steel sheets before and after the heat treatment were evaluated, and are also shown in Table 4 below. Specifically, the tensile strength and the amount of bake hardening of the steel sheets after the heat treatment were measured, and the results were compared with the tensile strength and the amount of bake hardening of the steel sheets before the heat treatment. Here, the methods for measuring the tensile strength and the amount of bake hardening after the heat treatment were as described above.

[0065] [Table 1]

[0066] *In Table 1, the balance is Fe and unavoidable impurities.

[0067] [Table 2]

[0068] *In Table 2, F indicates ferrite, B indicates bainite, M indicates martensite, MA indicates island martensite, and P indicates pearlite.

[0069] [Table 3]

[0070] *In Table 3, TS0 and BH2 indicate the tensile strength and the amount of bake hardening before heat treatment, respectively, and TS h and BH h indicates the tensile strength and the amount of bake hardening after heat treatment. △TS=TS h -TS0

[0071] As shown in Tables 1 to 3 above, all of the hot-rolled steel sheets of Examples 1 to 5, which satisfy the ranges of chemical composition, X values, and manufacturing conditions of the steel sheets proposed in the present invention, were able to achieve the targeted properties. Specifically, all of the hot-rolled steel sheets of Examples 1 to 5 had a tensile strength of 590 MPa or more, a hole expandability (HER0) of 40% or more, and a bake hardening amount (BH2) of 30 MPa or more. In addition, the bake hardening amount (BH2) after the additional heat treatment was 100% or more. h ) remains above 30MPa, and △TS×BH h -1 The absolute value of satisfies 0.70 or less, and it can be confirmed that this hot-rolled steel sheet can be effectively applied to various plating processes.

[0072] In contrast, Comparative Steels 1 to 9 do not satisfy the composition ranges and / or manufacturing process conditions proposed by the present invention.

[0073] Specifically, in Comparative Example 1, Comparative Example 3, and Comparative Example 4, the C and Mn contents were excessive, respectively, and the relational expressions were not satisfied. As a result, the martensite phase and the MA phase were unnecessarily formed, and the hole expandability value of the steel sheet deteriorated, or the tensile strength after heat treatment decreased significantly.

[0074] In Comparative Examples 2 and 5, the contents of C and Mn were insufficient, respectively, and the low-temperature phase fraction was not sufficiently secured due to a decrease in hardenability. As a result, the strength of the steel sheet did not satisfy 590 MPa, and the amount of bake hardening was poor both before and after heat treatment.

[0075] Comparative steel 6 and comparative example 7 were cases in which the contents of Ti and Nb were excessive, respectively, and it was difficult to ensure the low-temperature phase fraction due to the formation of excessive carbides. Therefore, the amount of bake hardening was poor both before and after heat treatment, and the hole expandability remained at the lower limit or did not achieve the target due to the increase in coarse precipitates.

[0076] Furthermore, in Comparative Examples 8 and 9, the X value according to Relational Formula 1 deviates from the range of the present invention, and the coiling temperature deviates from the range proposed by the present invention. That is, when the coiling temperature is exceeded as in Comparative Example 8, it is difficult to form a low-temperature phase in the structure, making it difficult to ensure the amount of bake hardening before and after heat treatment. When the coiling temperature is not achieved as in Comparative Example 9, the low-temperature phase fraction in the structure unnecessarily increases, resulting in a deterioration in the yield ratio and a large range of change in the strength and bake hardening value before and after heat treatment.

[0077] On the other hand, in Comparative Examples 10 to 12, the alloy composition components were within the range of the present invention, but the manufacturing conditions were outside the range of the present invention. In the case of Comparative Example 10, the cooling rate to the coiling temperature was very fast, so the hard phase martensite fraction of the microstructure phase exceeded 10%. This confirmed the poor hole expandability and the significant variation in strength after heat treatment. In the cases of Comparative Examples 11 and 12, the cooling rate to the coiling temperature was very slow, so the pearlite fraction of the microstructure phase exceeded 10%, resulting in poor hole expandability and bake hardenability, and a decrease in the strength / bake hardenability value after heat treatment.

[0078] In addition, as described above, in Comparative Examples 13 and 14, the X value according to Relational Formula 1 is outside the range of the present invention, but the manufacturing conditions are within the range of the present invention, and the tensile strength / bake hardening value before / after the heat treatment is changed, so that △TS × BH h -1 It was confirmed that the absolute value of was not 0.7 or less.

[0079] FIG. 1 shows the change in tensile strength and bake hardening amount (△TS × BH) before and after heat treatment with respect to the X value of the steel sheets of the present invention and comparative examples. h -1 As shown in FIG. 1, in the case of the hot-rolled steel sheet of the invention example, ΔTS × BH h -1 The absolute value of is 0.70 or less, and it can be confirmed that the film has excellent thermal stability.

[0080] Although the present invention has been described above with reference to the embodiments, it is understood that those skilled in the art can make various modifications and changes to the present invention within the scope of the basic idea of ​​the present invention, and it is also made clear that the scope of the present invention should be interpreted based on the claims.

Claims

1. In weight percent, it contains C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.8 to 1.8%, Al: 0.010 to 0.100%, P: 0.001 to 0.020%, S: 0.001 to 0.010%, N: 0.001 to 0.010%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, the balance being Fe and unavoidable impurities, and the value of X defined by the following relational formula 1 satisfies 3.50 to 6.00, A microstructure including a total of ferrite and bainite phases: 90 area % or more, and a total of the remaining martensite and MA phases: less than 10 area %, The tensile strength is 590 MPa or more, and the hole expandability (HER ０ ) is 40% or more, and the bake hardening amount (BH ２ Bake hardening amount (BH : amount of bake hardening before heat treatment) is 30 MPa or more, and bake hardening amount (BH : amount of bake hardening after heat treatment at 300 to 600 ° C. ｈ ) is maintained at 30 MPa or more, and ΔTS is defined as in the following relational expression 2, ΔTS × BH ｈ －１ The absolute value of the hot-rolled steel sheet is 0.70 or less. [Relationship 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 Here, Mn, C, Nb, Ti, N and S indicate their weight percentages. [Relationship 2] △TS=TS ｈ -TS ０ TS ｈ : Tensile strength after heat treatment, TS ０ : Tensile strength before heat treatment BH ｈ : Amount of bake hardening after heat treatment

2. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains one or more elements selected from the group consisting of Mo, Cr, V, Ni, and B in a total amount of 1.500% or less.

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

4. reheating a steel slab containing, by weight, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.8 to 1.8%, Al: 0.010 to 0.100%, P: 0.001 to 0.020%, S: 0.001 to 0.010%, N: 0.001 to 0.010%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, the balance being Fe and unavoidable impurities, and having an X value of 3.50 to 6.00 as defined by the following Relational Formula 1, to a temperature range of 1100 to 1350°C; hot rolling the reheated steel slab at a temperature in the range of 850 to 1150°C to produce a hot-rolled steel sheet; cooling the hot-rolled steel sheet to a temperature in the range of 400 to 550°C at an average cooling rate of 10 to 70°C / sec, and then coiling the hot-rolled steel sheet. [Relationship 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 Here, Mn, C, Nb, Ti, N and S indicate their weight percentages.

5. The method for producing a hot-rolled steel sheet according to claim 4, wherein the hot-rolled steel sheet contains one or more elements selected from the group consisting of Mo, Cr, V, Ni, and B in a total amount of 1.500% or less.

6. The method for manufacturing a hot-rolled steel sheet according to claim 4, further comprising the steps of pickling the coiled hot-rolled steel sheet and then oiling it.

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

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