Hot-rolled steel sheet and its manufacturing method

Abstract

The present invention relates to a hot-rolled steel sheet suitable for use in automobile chassis structural members and the like, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet suitable for use in automobile chassis structural members and the like, and a method for manufacturing the same. [Background technology]

[0002] In recent years, in order to reduce global warming, there has been a rapid shift in the automobile market from internal combustion engine vehicles, which are the mainstream, to environmentally friendly vehicles such as electric vehicles.

[0003] With the shift from internal combustion engine vehicles to electric vehicles, the types of components that make up vehicles are changing, and so is the weight of vehicles. For example, when comparing the weight of an internal combustion engine vehicle and an electric vehicle released as the same model, it is known that the weight of the electric vehicle is heavier than that of the internal combustion engine vehicle by the weight of the battery.

[0004] Meanwhile, chassis parts of automobiles are important parts that support the vehicle body and absorb vibrations and shocks from the road surface while driving, ensuring ride comfort and driving stability. As the weight of the automobile increases, the fatigue load applied to the chassis parts increases, so steel materials used in chassis parts of electric vehicles and the like are required to have excellent fatigue strength.

[0005] The fatigue strength of steel is proportional to its tensile strength and yield strength. Therefore, steel used in chassis parts for electric vehicles and other vehicles needs to have improved tensile strength and yield strength. Steel sheets used in the manufacture of chassis parts are therefore gradually becoming stronger.

[0006] Chassis parts are mainly manufactured by press forming. Although it is possible to reduce the weight of parts by reducing the thickness of steel plates through the use of high-strength steel, the shape of the parts cannot be changed significantly, so ensuring formability suitable for press forming of parts is the most important factor when manufacturing high-strength steel.

[0007] Various techniques have been proposed to improve the strength and formability of hot-rolled steel sheets.

[0008] As an example, Patent Document 1 discloses a method for manufacturing a high-strength hot-rolled steel sheet with excellent hole expandability, in which the main phase is bainitic ferrite, which does not contain carbides in the microstructure, for a steel material containing 0.01 to 0.05% carbon by weight. According to Patent Document 1, although it is possible to manufacture a steel sheet with excellent hole expandability and a tensile strength of 980 or more, there are concerns that the effect of reducing the weight of parts is minimal due to low yield strength and poor fatigue properties, and that part formability is poor due to the poor elongation rate.

[0009] Therefore, in order to ensure the running stability of chassis parts for environmentally friendly vehicles such as electric vehicles, it is necessary to develop steel materials that not only have high tensile strength and yield strength and excellent fatigue life, but also excellent formability such as elongation rate and hole expandability for easy press forming. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 2008-255484 Summary of the Invention [Problem to be solved by the invention]

[0011] An object of one aspect of the present invention is to provide a hot-rolled steel sheet that not only has high strength and excellent fatigue performance but also has excellent formability and is suitable for press forming, and a method for manufacturing the same.

[0012] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire contents of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the contents described in the specification of the present invention. [Means for solving the problem]

[0013] One aspect of the present invention is a sintered body containing, by weight, 0.09 to 0.25% carbon (C), 0.5 to 2.3% silicon (Si), 1.5 to 3.5% manganese (Mn), 0.001 to 1.0% aluminum (Al), 2.5% or less (inclusive of 0%) chromium (Cr), 2.0% or less (inclusive of 0%) molybdenum (Mo), 0.01 to 0.20% titanium (Ti), 0.0005 to 0.005% boron (B), 0.0001 to 0.05% phosphorus (P), 0.0001 to 0.05% sulfur (S), 0.0001 to 0.05% nitrogen (N), and the remainder including iron (Fe) and inevitable impurities, The microstructure contains, by area percentage, 75-90% martensite and austenite composite and 10-25% bainitic ferrite. The above austenite relates to a hot rolled steel sheet containing 3 to 10%.

[0014] The above-mentioned hot-rolled steel sheet may further contain niobium (Nb): 0.01 to 0.2%.

[0015] The bainitic ferrite may have an average particle size of 2.0 μm or more.

[0016] The average spacing of the bainitic ferrite may be 3 μm or more.

[0017] The hot-rolled steel sheet has a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation rate of 9% or more, and a hole expansion rate of 45% or more.

[0018] Another aspect of the present invention is a method for producing a steel slab containing, by weight, 0.09-0.25% carbon (C), 0.5-2.3% silicon (Si), 1.5-3.5% manganese (Mn), 0.001-1.0% aluminum (Al), 2.5% or less (inclusive of 0%) chromium (Cr), 2.0% or less (inclusive of 0%) molybdenum (Mo), 0.01-0.20% titanium (Ti), 0.0005-0.005% boron (B), 0.0001-0.05% phosphorus (P), 0.0001-0.05% sulfur (S), 0.0001-0.05% nitrogen (N), the balance being Fe and other unavoidable impurities, the method comprising: reheating a steel slab at a temperature range of 1100-1350°C; hot rolling the reheated steel slab to produce a hot rolled steel sheet; The above hot-rolled steel sheet is B S a step of performing primary cooling at a cooling rate of 50°C / s or more to a temperature below; After the first cooling (B S +M S ) / 2 or more at a cooling rate of 25 ° C. / s or less for ts time (seconds); After the above secondary cooling (M S a third cooling step at a cooling rate of 30°C / s or less to a temperature range of -20°C to 200°C; and winding the cooled product in the third cooled temperature range. During the hot rolling, finish hot rolling is performed within a temperature range of 750 to 1150°C so that the value of Du defined by the following relational expression 1 satisfies the range of 2 to 10, The above primary and secondary cooling relate to a method for producing a hot-rolled steel sheet that satisfies the conditions of the following relational expressions 2 to 4. [Equation 1] Du=(FDT+(7.4×[C])-(24.7×[Si])-(4.7×[Mn])-(3.9×[Cr])-(5.2×[Mo])-(560×[Ti])-(1110×[Nb]))×0.049-34.2 (In the above Relational Formula 1, FDT means the rolling finish temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] each represent the weight percent content of the element in parentheses.) [Equation 2] 5.0×10 6 ≦Du×Bat×2.968×10 10 ≦2.0×10 7 (In the above Relational Formula 2, Du is defined as in Relational Formula 1, and Bat represents 55.845 × [B] / (1080.6 + 45.04 × [B]), where [B] represents the weight content (%) of boron (B).) [Equation 3] 0.75≦exp(-k(T)×(ts) 2 )≦0.9 (The above k(T) is a value defined by the following relational expression 4, and ts represents the secondary cooling time.) [Equation 4]

number

[0019] During the hot rolling, the total reduction in the final two passes may be 10 to 40%.

[0020] After the winding, a final cooling step to room temperature may be further included.

[0021] After the final cooling, the method may further include pickling and oiling.

[0022] After the pickling and oil coating, the method may further include a step of hot dip galvanizing. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide a steel sheet having a high strength of 980 MPa or more and excellent formability, and a manufacturing method thereof, which can be suitably applied to automobile chassis structural members, etc.

[0024] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a graph showing the relationship between boron content and Du where Relational Formula 1 and Relational Formula 2 are simultaneously satisfied, and the microstructure intended by the present invention can be secured within the solid line connecting ABCDEF. [Figure 2] Photographs (a), (b), and (c) show the microstructures of Example 4, Comparative Example 2, and Comparative Example 3, respectively, observed with a scanning electron microscope in the examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified in various different forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0027] The terms used herein are for the purpose of describing particular embodiments and are not intended to limit the present invention. Furthermore, the singular forms used herein include the plural forms unless the relevant definition clearly indicates otherwise. The meaning of "comprises" as used herein embodies a configuration and does not exclude the presence or addition of other configurations.

[0028] In order to improve hole expandability in high-strength steel with a tensile strength of 980 MPa or more, conventional hot-rolled steel sheet manufacturing methods, which prevent deterioration of hole expandability due to differences in interphase hardness by adjusting the microstructure of the steel to a specific microstructure fraction of 90% or more, have been unable to ensure excellent elongation. On the other hand, when retained austenite is used to ensure excellent elongation, although it is possible to ensure elongation, there is a problem in that it is difficult to simultaneously ensure hole expandability.

[0029] Therefore, the inventors have found that by using martensite and austenite as a matrix structure to ensure high strength and elongation, while uniformly dispersing bainitic ferrite as a secondary phase within the structure, excessive concentration of stress at specific locations during deformation can be prevented, suppressing the occurrence of microcracks and improving hole expandability, thereby completing the present invention. The present invention will be described in more detail below.

[0030] A steel sheet according to one embodiment of the present invention will be described. The steel sheet contains, by weight, 0.09 to 0.25% carbon (C), 0.5 to 2.3% silicon (Si), 1.5 to 3.5% manganese (Mn), 0.001 to 1.0% aluminum (Al), 2.5% or less (inclusive of 0%) chromium (Cr), 2.0% or less (inclusive of 0%) molybdenum (Mo), 0.01 to 0.20% titanium (Ti), 0.0005 to 0.005% boron (B), 0.0001 to 0.05% phosphorus (P), 0.0001 to 0.05% sulfur (S), and 0.0001 to 0.05% nitrogen (N).

[0031] Carbon (C): 0.09~0.25% Carbon (C) is an important element that diffuses into austenite after martensitic transformation to stabilize the austenite, thereby forming retained austenite. As the C content increases, the fraction of retained austenite increases, resulting in improved elongation and tensile strength. If the C content is less than 0.09%, the fraction of retained austenite is low, making it difficult to ensure the elongation and tensile strength. On the other hand, if the C content exceeds 0.25%, the Ms temperature becomes excessively low, preventing carbon diffusion and resulting in excessive formation of fresh martensite, resulting in poor hole expandability. Therefore, in the present invention, the C content is preferably 0.09 to 0.25%. It is more advantageous that the C content is 0.090 to 0.250%. Even more advantageously, the lower limit of the C content may be 0.12%, or the upper limit of the C content may be 0.23%.

[0032] Silicon (Si): 0.5 to 2.3% Silicon (Si) is an important element that delays the formation of carbides after martensitic transformation and forms retained austenite. Si also plays a role in improving strength through its solid solution strengthening effect. If the Si content is less than 0.5%, carbides are formed, the fraction of retained austenite is low, and it is difficult to ensure elongation. On the other hand, if the Si content exceeds 2.3%, Fe-Si composite oxides are formed on the slab surface during reheating, which not only deteriorates the surface quality of the steel sheet but also reduces weldability. Therefore, in the present invention, the Si content is preferably 0.5 to 2.3%. It is more advantageous that the Si content is 0.50 to 2.30%. Even more advantageously, the lower limit of the Si content may be 0.7%, or the upper limit of the Si content may be 2.1%.

[0033] Manganese (Mn): 1.5-3.5% Manganese (Mn) is an element that improves the hardenability of steel, prevents the formation of ferrite during cooling after finish rolling, and facilitates the formation of low-temperature transformation structures. If the Mn content is less than 1.5%, the hardenability is insufficient and the ferrite fraction is excessively increased, whereas if the Mn content exceeds 3.5%, the hardenability increases significantly, the maintenance time required to sufficiently form the bainitic ferrite desired in the present invention is excessively increased, and the hole expandability is reduced.

[0034] Therefore, in the present invention, the Mn content can be 1.5 to 3.5%, and more advantageously, the Mn content is 1.50 to 3.50%. Even more advantageously, the lower limit of the Mn content may be 1.6%, or the upper limit of the Mn content may be 3.0%.

[0035] Aluminum (Al): 0.001 to 1.0% Aluminum (Al) is an element that is usually added to deoxidize molten steel, and some of it may remain in the steel after deoxidation. However, like Si, it also plays a role in delaying the formation of carbides after martensitic transformation and forming retained austenite.

[0036] If the Al content is less than 0.001%, carbides are formed, the fraction of retained austenite decreases, and it becomes difficult to ensure elongation. On the other hand, if the Al content exceeds 1.0%, oxide and nitride inclusions increase in the steel, deteriorating the formability of the steel sheet. Therefore, in the present invention, the Al content is preferably 0.001 to 1.0%. More advantageously, the lower limit of the Al content may be 0.01%, or the upper limit of the Al content may be 0.5%.

[0037] Chromium (Cr): 2.5% or less (including 0%) Chromium (Cr) is an element that improves the hardenability of steel and suppresses the formation of ferrite during cooling after finish rolling. If the Cr content exceeds 2.5%, the hardenability increases significantly, the bainitic transformation does not occur smoothly during the cooling zone, the maintenance time required to maintain the bainitic ferrite fraction increases excessively, and hole expandability deteriorates. Therefore, in the present invention, the Cr content may be 2.5% or less, more preferably 2.50% or less, and even more preferably 1.5% or less.

[0038] On the other hand, the present invention includes the case where the Cr content is 0%, since the intended physical properties can be ensured without the inclusion of Cr. However, it should be made clear that when Cr is intentionally added, it is effective to add a minimum of 0.01%.

[0039] Molybdenum (Mo): 2.0% or less (including 0%) Molybdenum (Mo) is an element that improves the hardenability of steel, improves strength through solid solution strengthening, and suppresses the formation of ferrite during cooling after finish rolling. If the Mo content exceeds 2.0%, the hardenability increases significantly and bainitic transformation does not occur smoothly during cooling. This excessively increases the maintenance time required to maintain the bainitic ferrite fraction, resulting in reduced hole expandability. Therefore, in the present invention, Mo may be contained in an amount of 2.0% or less, more preferably 1.0% or less, and even more preferably 0.5% or less.

[0040] On the other hand, the present invention includes the case where the Mo content is 0%, since there is no significant problem in ensuring the intended physical properties even if Mo is not contained. However, it should be made clear that when Mo is intentionally added, it is effective to add a minimum of 0.01%.

[0041] Titanium (Ti): 0.01 to 0.20% Titanium (Ti) is an element that forms carbonitrides in steel and is widely used in applications where it induces the formation of precipitates in this way to ensure the strength of steel. In the present invention, however, titanium (Ti) is used to remove nitrogen (N) from steel and suppress the formation of BN, thereby concentrating boron (B) at the austenite grain boundaries and controlling the austenite grain size before rolling.

[0042] To fully achieve the effects of the present invention, the Ti content is preferably 0.01% or more, and in order to remove nitrogen (N) from the steel, it is preferable to contain at least 2.9 times the nitrogen (N) content. However, if the Ti content exceeds 0.20%, oxides may form during continuous casting, which may cause problems such as clogging of the casting nozzle.

[0043] Therefore, in the present invention, the Ti content can be 0.01 to 0.20%, and more advantageously, the lower limit of Ti may be 0.015%, or the upper limit of the Ti content may be 0.12%.

[0044] Boron (B): 0.0005 to 0.005% Boron is an element that concentrates at austenite grain boundaries to reduce grain boundary energy and thereby improve the hardenability of steel. In the present invention, boron plays a role in suppressing the phase transformation between ferrite and upper bainite, which occurs at austenite grain boundaries through diffusional transformation and nucleation of phase transformation, thereby ensuring a composite structure of martensite and austenite as the main phase.

[0045] To fully achieve the effects of the present invention, the B concentration is preferably 0.0005% or more. However, if the B content exceeds 0.005%, the hardening ability increases significantly, the maintenance time required to fully form the bainitic ferrite desired in the present invention increases excessively, and the hole expandability decreases.

[0046] Therefore, in the present invention, the above B can be contained in an amount of 0.0005 to 0.005%, and more advantageously, the lower limit of the B content may be 0.001%, or the upper limit of the B content may be 0.0025%.

[0047] Phosphorus (P): 0.0001 to 0.05% Phosphorus (P) is an impurity that is inevitably contained in steel and is the element that is the main cause of impairing the workability of steel due to segregation, so it is preferable to control its content as low as possible.

[0048] Theoretically, it is advantageous to limit the P content to 0%, but controlling the P content to less than 0.0001% would require excessive production costs, so the lower limit can be set to 0.0001%. However, if the P content exceeds 0.05%, workability may be reduced, so the upper limit can be limited to 0.05%. However, more advantageously, the lower limit of the P content may be 0.0005%, or the upper limit of the P content may be 0.02%.

[0049] Sulfur (S): 0.0001 to 0.05% Sulfur (S) is an impurity that is inevitably contained in steel and has the problem of combining with Mn and other elements to form non-metallic inclusions, which can reduce the workability of the steel. Therefore, it is preferable to control the S content as low as possible.

[0050] Theoretically, it is advantageous to limit the S content to 0%, but controlling the S content to less than 0.0001% would require excessive manufacturing costs, so the lower limit can be set to 0.0001%. However, if the S content exceeds 0.05%, there is a risk of reduced workability, so the upper limit of S can be limited to 0.05%. However, more advantageously, the lower limit of the S content may be 0.0005%, or the upper limit of the S content may be 0.005%.

[0051] Nitrogen (N): 0.0001 to 0.05% Nitrogen (N) is an impurity that is inevitably contained in steel, and has the problem of combining with Al and other elements to form nitrides, which impair the workability of the steel. Therefore, it is preferable to control the N content as low as possible.

[0052] Theoretically, it is advantageous to limit the N content to 0%, but controlling the N content to less than 0.0001% would require excessive manufacturing costs, so the lower limit can be set to 0.0001%. However, if the N content exceeds 0.05%, workability may be reduced, so the upper limit can be limited to 0.05%. However, more advantageously, the lower limit of the N content may be 0.001%, or the upper limit of the N content may be 0.006%.

[0053] The hot-rolled steel sheet of the present invention may further contain niobium (Nb) in addition to the alloy composition described above.

[0054] Niobium (Nb): 0.01-0.2% Niobium (Nb) is an element that forms carbonitrides in steel and is widely used to induce the formation of precipitates and thereby ensure the strength of steel. In the present invention, however, Nb serves to control the austenite grain size by delaying recrystallization during hot rolling. If the Nb content is less than 0.01%, the effect of controlling grain size is low, and if the Nb content exceeds 0.2%, the austenite grain size becomes excessively fine, resulting in poor formability. Therefore, in the present invention, the Nb content may be 0.01 to 0.2%.

[0055] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, all of the details of these impurities will not be specifically mentioned in this specification.

[0056] The hot-rolled steel sheet of the present invention includes a composite structure of martensite and austenite as a matrix structure in order to simultaneously secure a tensile strength of 980 MPa or more and an elongation of 9% or more. Therefore, according to one embodiment of the present invention, the composite structure composed of martensite and austenite may account for 75 to 90% by area.

[0057] In the present invention, in the tertiary cooling process performed after hot rolling, a part of the untransformed austenite is converted into M. S The austenite transforms to martensite below this temperature. After coiling, the steel sheet is gradually cooled and maintained at a near-isothermal temperature, during which an isothermal phase transformation of austenite to martensite occurs, increasing the martensite fraction. Martensite is formed by displacive phase transformation, which creates a high dislocation density within the structure due to screw dislocations that reduce shear deformation during transformation and edge dislocations that accommodate volume expansion due to phase transformation. The presence of fine carbides within the structure also contributes to improving the yield strength and tensile strength of steel. However, the high dislocation density and fine carbides hinder the movement of dislocations within the structure, resulting in a deterioration in elongation.

[0058] Therefore, it is preferable to include austenite in the matrix structure to improve the elongation rate of high-strength steel through the plasticity-induced transformation phenomenon. Immediately after martensitic transformation, carbon atoms supersaturated in martensite diffuse and migrate to austenite, gradually increasing the carbon concentration within the austenite. The austenite, whose stability has increased due to the carbon enrichment, does not undergo phase transformation even when cooled to room temperature, but remains in the microstructure, thereby improving the elongation rate of the steel sheet. Therefore, in the present invention, from the perspective of ensuring tensile strength and elongation rate, it is preferable that the martensite-austenite composite structure be included at an area fraction of 75% or more. On the other hand, in order to ensure the hole expandability described below, it is preferable that the area fraction of the martensite-austenite composite structure be limited to 90% or less.

[0059] In this case, the austenite may be 3 to 10% by area. If the area fraction of austenite is less than 3%, the effect of improving the elongation rate due to the plasticity-induced transformation phenomenon is minimal. On the other hand, if the area fraction of austenite exceeds 10%, the carbon (C) content to be added to the steel must be increased, which causes a problem of deterioration in the weldability of the steel.S If the temperature is too low, carbon diffusion is difficult, and the amount of fresh martensite generated in the final cooling stage to room temperature after coiling increases excessively, deteriorating hole expandability.

[0060] The hot-rolled steel sheet of the present invention may have bainitic ferrite as a secondary phase in the microstructure, and the secondary phase may be contained in an amount of 10 to 25% by area.

[0061] The present invention avoids ferrite phase transformation during the primary cooling after hot rolling and S After cooling to a temperature below the bainite transformation start temperature, the bainite transformation progresses through slow cooling during the subsequent secondary cooling. Because the bainite transformation occurs in the high-temperature bainite transformation range, bainitic ferrite is formed and carbon diffuses into the untransformed austenite, resulting in the absence of carbides within the bainitic ferrite. Meanwhile, the bainitic ferrite formed by shear transformation contains a large number of dislocations, but the recovery phenomenon during the secondary cooling reduces the dislocation density to an appropriate level, resulting in soft properties.

[0062] On the other hand, martensitic transformation occurs when M S Because the transformation process occurs over a wide temperature range, from the initial temperature to the end temperature of the third cooling, the temperature at which phase transformation begins varies depending on the location within the steel plate. The transformation stress remaining within the steel plate varies depending on the temperature at which martensite is formed, so the transformation stress is distributed unevenly depending on the location within the steel plate, and this stress remains within the steel plate even after it has cooled to room temperature. When external deformation is applied during part formation, the deformation concentrates in areas of high residual stress within the steel plate, facilitating the growth and propagation of cracks, resulting in poor hole expandability.

[0063] On the other hand, if soft bainitic ferrite is evenly distributed with an appropriate size within a high-strength matrix structure with non-uniform stress distribution, it can evenly accommodate deformation during forming, preventing localized stress concentration and improving hole expandability.

[0064] Therefore, in the present invention, if the fraction of the secondary phase, bainitic ferrite, is less than 10%, it becomes difficult to ensure hole expandability, while if the fraction exceeds 25%, it becomes difficult to ensure a composite structure of martensite and austenite, which plays a role in improving strength.

[0065] Meanwhile, the average grain size of the bainitic ferrite may be 2.0 μm or more, and the average spacing of the bainitic ferrite may be 3 μm or more.

[0066] In this case, the average grain size of the bainitic ferrite means the circle equivalent diameter, and the average spacing of the bainitic ferrite means the average of the distances between the five structures nearest to each microstructure.

[0067] If the average grain size of the soft structure, bainitic ferrite, is less than 2.0 μm, the deformation accommodation effect is low and improvement in hole expandability cannot be expected. Furthermore, if the average spacing of the bainitic ferrite is less than 3.0 μm, the soft steel fraction increases excessively, which may result in a deterioration in yield strength and tensile strength. While there are no particular restrictions on the upper limits of the average grain size and average spacing of the soft structure, provided that the soft structure fraction is in the range of 10 to 25%, the average grain size of the soft structure can preferably be 20 μm or less. Furthermore, the average spacing of the soft structure can be 20 μm or less.

[0068] The hot-rolled steel sheet of the present invention may contain carbides and fresh martensite as other structures in addition to the structure described above, but it is preferable that the area fraction of these is controlled to less than 5%.

[0069] The hot-rolled steel sheet may generate carbides during the manufacturing process. Some carbon atoms immediately after martensitic transformation can form fine carbides within the laths, improving strength. However, since the present invention utilizes austenite to improve elongation, the generation of carbides may cause a decrease in the austenite fraction. In other words, excessive carbide generation inhibits the improvement in elongation targeted in the present invention. However, when Ti and Nb are present in the phase, alloy carbonitrides can be formed, which can be expected to further strengthen the grains by refining them. However, since coarse carbides inhibit the toughness of the steel, it is preferable that the carbides present in the hot-rolled steel sheet of the present invention be less than 5%.

[0070] According to one embodiment of the present invention, the hot-rolled steel sheet may include fresh martensite as a microstructure. In the present invention, the martensite that is isothermally transformed during the tertiary cooling process and immediately after coiling transforms before carbon enrichment in the austenite begins, and therefore has a lath morphology within the composition range of the present invention. On the other hand, if the coiling temperature is too low, carbon diffusion is not easy and cooling may end before the austenite is sufficiently stabilized. In this case, the M of the austenite where carbon enrichment has occurred may be lost. S If the temperature is above room temperature, it may transform into fresh martensite during cooling. Fresh martensite formed during the final cooling process accommodates shear deformation during phase transformation by twin formation rather than dislocation, and therefore has a plate shape and twins are observed in the microstructure, making it easily distinguishable from the martensite of the present invention. Fresh martensite with a high carbon concentration has excessively high hardness and deteriorates hole expandability, so the fraction of fresh martensite present in the hot-rolled steel sheet of the present invention is preferably less than 5%.

[0071] The hot-rolled steel sheet of the present invention, which has the above-mentioned alloy composition and microstructure, has high strength with a yield strength of 800 MPa or more and a tensile strength of 980 MPa or more, while having an elongation of 9% or more, a product of tensile strength and elongation of 13,000 MPa·%, and a hole expansion ratio of 45% or more, and is characterized by excellent formability.

[0072] Next, a method for manufacturing a hot-rolled steel sheet according to yet another embodiment of the present invention will be described in detail below. However, this does not necessarily mean that the hot-rolled steel sheet of the present invention must be manufactured by the following manufacturing method.

[0073] The above manufacturing method can produce a steel slab that satisfies the alloy composition proposed in the present invention by carrying out a series of steps of [reheating - hot rolling - cooling - coiling].

[0074] The conditions for each of the above steps will be described in detail below.

[0075] steel slab reheating Before carrying out the rolling step described below, the steel slab is preferably subjected to a step of reheating and homogenizing the slab, which can be carried out at a temperature in the range of 1100 to 1350°C.

[0076] If the temperature during reheating of the steel slab is less than 1100°C, there is a problem that the alloy elements will not be sufficiently homogenized. On the other hand, if the temperature exceeds 1350°C, excessive oxides will be formed on the slab surface, which may deteriorate the surface quality of the steel sheet.

[0077] hot rolling The reheated steel slab can be hot-rolled to produce a hot-rolled steel sheet. At this time, the hot-rolling is preferably carried out in a temperature range of 750 to 1150°C, and the total reduction in the final two passes is preferably controlled to 10 to 40%.

[0078] First, if hot rolling is started at a temperature above 1150°C, excessive oxides are formed on the surface of the rolled steel sheet, which cannot be effectively controlled even by a pickling process, resulting in deterioration of surface quality.On the other hand, if hot rolling is performed at a temperature below 750°C, the rolling load increases excessively, reducing workability, and ferrite is generated during rolling, resulting in deterioration of anisotropy.

[0079] Hot rolling is usually performed in multi-stage rolling in order to reduce the rolling load and precisely control the thickness. When hot rolling is performed in such a multi-stage rolling process, if the total reduction rate of the final two passes (the latter two passes) exceeds 40%, the rolling load in the final two passes becomes excessive, resulting in a problem of deterioration in workability. On the other hand, if the total reduction rate of the final two passes is less than 10%, the temperature of the steel sheet drops rapidly, resulting in a problem of poor shape.

[0080] On the other hand, the austenite grain size after hot rolling is affected by the alloying elements and the rolling end temperature, which affect the bainite formation behavior in the subsequent cooling process and the final microstructure. In addition, the fraction and size of bainitic ferrite, which is the main constituent phase in the present invention, are significantly affected by the austenite grain size after hot rolling.

[0081] In equiaxed ferrite and pearlite, the grains grow due to the diffusion of elements during phase transformation, and the size of the structure after phase transformation is affected by the phase transformation temperature and maintenance time. In contrast, bainitic ferrite, which is generated by shear transformation such as bainite, grows only within austenite grains, and its size is never larger than that of the austenite before transformation. Therefore, in order to control the size of bainitic ferrite, it is advantageous to control the grain size of austenite after hot rolling.

[0082] Therefore, in the present invention, the effective grain size of austenite after hot rolling is derived as the relationship between the rolling finish temperature (FDT) and a specific alloy composition, and specifically, is defined by the following relational expression 1. That is, during hot rolling, finish hot rolling is performed within a temperature range of 750 to 1150°C so that the value of Du, defined by the following relational expression 1, falls within the range of 2 to 10. [Equation 1] Du=(FDT+(7.4×[C])-(24.7×[Si])-(4.7×[Mn])-(3.9×[Cr])-(5.2×[Mo])-(560×[Ti])-(1110×[Nb]))×0.049-34.2 (In the above Relational Formula 1, FDT means the rolling finish temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] each represent the weight percent content of the element in parentheses.)

[0083] The Du value is an index showing the effective grain size of austenite immediately before the primary cooling after hot rolling. If the Du value defined by the above-mentioned relational expression 1 is 2 or more, the average grain size of bainitic ferrite will be 2.0 μm or more, and a hole expansion ratio of 45% or more can be ensured. On the other hand, if the Du value defined by the above-mentioned relational expression 1 exceeds 10, the grain boundary concentration of boron element will increase excessively, delaying the phase transformation during the secondary cooling, which will result in a problem of not being able to ensure a sufficient bainitic ferrite fraction and deteriorating hole expandability. The Du value is more preferably 2.0 to 10.0.

[0084] According to one embodiment of the present invention, boron segregates at austenite grain boundaries to stabilize them, thereby delaying the nucleation of ferrite and upper bainite and slowing the phase transformation rate. In order to ensure the desired area fraction and average spacing of bainitic ferrite, it is important to control the concentration of boron segregated at austenite grain boundaries. The concentration of boron segregated at austenite grain boundaries varies from grain boundary to grain boundary depending on microsegregation during casting and the austenite grain size. Among these, nucleation of bainitic ferrite occurs selectively at austenite grain boundaries with low boron concentrations during the secondary cooling stage. Generally, when the austenite grain size is small, the concentration of boron segregated at each grain boundary is low, facilitating nucleation. However, when the austenite grain size is large, the concentration of boron segregated at the grain boundaries is high, which is expected to delay nucleation. Therefore, the boron concentration at the grain boundaries and the phase transformation behavior of bainitic ferrite during secondary cooling are affected by the boron content added to the steel and the austenite grain size, as shown in Equation 2.

[0085] Relational formula 2 is an index showing the concentration of boron (B) distributed at the grain boundaries of austenite just before cooling, and is expressed as Du × Bat × 2.968 × 10 10 The value is 5.0×10 6 If the ratio is less than Du × Bat × 2.968 × 10, the fraction of bainitic ferrite becomes excessive, and the yield strength and tensile strength cannot be ensured. 10 The value is 2.0×10 7 If the cooling time exceeds 100°C, the secondary cooling time required to ensure the bainitic ferrite fraction becomes excessively long, resulting in a problem of poor hole expandability. [Equation 2] 5.0×10 6 ≦Du×Bat×2.968×10 10 ≦2.0×10 7 (In the above Relational Formula 2, Du is defined as in Relational Formula 1, and Bat represents 55.845 × [B] / (1080.6 + 45.04 × [B]), where [B] represents the weight content (%) of boron (B).)

[0086] FIG. 1 is a graph showing the relationship between boron content and Du, where the above-mentioned Relational Formula 1 and Relational Formula 2 are simultaneously satisfied. The microstructure intended in the present invention can be secured within the solid line connecting ABCDEF.

[0087] Cooling and winding The hot-rolled steel sheet produced as described above is cooled, and it is preferable to cool it stepwise depending on the temperature to which it is cooled.

[0088] Specifically, the above hot-rolled steel sheet is B S After the first cooling at a cooling rate of 50°C / s or more to the temperature below (B S +M S It is preferable to perform secondary cooling at a cooling rate of 25°C / s or less to a temperature of (Ms-20°C) / 2 or higher for a time (seconds) ts defined in Relation 3, and then perform tertiary cooling at a cooling rate of 30°C / s or less to a temperature range of (Ms-20°C) to 200°C.

[0089] The hot-rolled steel sheet manufactured as above is heated to a temperature B S The steel is cooled rapidly below the bainite start temperature B to prevent the formation of ferrite (granular ferrite). S and martensite start temperature M S By gradually cooling to an intermediate temperature or higher for a time (ts) (seconds), it is possible to ensure an area fraction of 10 to 25% bainitic ferrite.

[0090] After the above hot rolling is completed, B S When primary cooling is performed at a temperature below the above range, if the cooling rate is less than 50°C / s, there is a problem that a ferrite phase is formed during cooling. In this case, there is no particular upper limit to the primary cooling rate, but if the steel sheet is cooled too quickly, the sheet shape may be distorted, so it can be limited to 200°C / s or less.

[0091] There is no particular restriction on the lower limit of the cooling end temperature during the primary cooling. However, if the temperature is too low, the cooling time during the subsequent secondary cooling may not be sufficient. S It should be made clear that it can be limited to -100°C.

[0092] The temperature of the hot-rolled steel sheet reaches B S When the temperature drops below this level, the rapid cooling is stopped and the temperature is cooled at a rate of 25°C / s or less (B S +M S ) / 2 or higher, secondary cooling can be performed.

[0093] Bainitic ferrite growth occurs in the primarily cooled hot-rolled steel sheet while it is cooled from the primarily cooled temperature to the target temperature for secondary cooling. In particular, in order to obtain the target fraction in the present invention, it is preferable to maintain the secondary cooling for a time ts (seconds (sec)) that satisfies the following Relational Expression 3:

[0094] In Equation 3, k(T) is an index showing the growth rate of bainitic ferrite, and is affected not only by the alloying elements of the steel, but also by the phase transformation temperature and the grain size after hot rolling. Therefore, the value of Equation 3, i.e., the relationship between k(T) and the holding time (exp(-k(T)×(ts)) 2 If the ratio is less than 0.75, the proportion of bainitic ferrite becomes excessive and the target level of strength cannot be ensured, whereas if the ratio exceeds 0.9, the hole expandability deteriorates. [Equation 3] 0.75≦exp(-k(T)×(ts) 2 )≦0.9 (The above k(T) represents the value defined by the following relational expression 4.) [Equation 4]

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[0095] During secondary cooling according to the above-mentioned conditions, the temperature of the steel sheet may rise due to transformation heat generated by the bainite phase transformation. At this time, excessive heat generation may cause an excessive decrease in dislocation density. Therefore, to minimize the temperature rise of the steel sheet due to transformation heat generation, the cooling rate during secondary cooling may be controlled to 25°C / s or less. If the cooling rate exceeds 25°C / s, the sheet shape may be distorted. It should be noted that in the present invention, the secondary cooling also includes an air cooling process.

[0096] After the secondary cooling is completed, the hot-rolled steel sheet is S It is preferable to carry out tertiary cooling at a cooling rate of 30°C / s or less to a temperature range of -20°C to 200°C, and then winding at that temperature. SMartensitic transformation proceeds at the following temperatures, and a part of the untransformed austenite can further grow into martensite under isothermal conditions even after coiling.

[0097] If the cooling rate is too high during martensitic transformation, the plate shape may be distorted due to rapid volume expansion, which may further lead to cooling imbalances and uneven material distribution. Therefore, in the present invention, the cooling rate in the third cooling stage, which involves rapid phase transformation, is set to 30°C / s or less to prevent shape distortion during cooling and the resulting uneven material distribution within the plate. On the other hand, if the cooling rate is too slow, bainitic ferrite will grow during cooling, excessively increasing the fraction of secondary phases and making it difficult to ensure the strength of the steel. Therefore, in the present invention, the cooling rate in the third cooling stage can be set to 5°C / s or more.

[0098] Meanwhile, the maximum carbon concentration that can be dissolved in austenite varies depending on the temperature at which carbon enrichment progresses, and generally, the lower the temperature, the higher the carbon solubility limit in austenite. Therefore, if the coiling temperature at which carbon enrichment occurs is excessively high, austenite will not be sufficiently carbon enriched, and the phase stability required for plasticity-induced transformation will not be ensured. Even if it remains at room temperature, it will disappear due to stress-induced transformation in the early stages of deformation, and an improvement in elongation cannot be expected. Therefore, in the present invention, the upper limit of the tertiary cooling end temperature is set to M in order to ensure sufficient phase stability. S Preferably, the tertiary cooling end temperature is -20°C. On the other hand, if the temperature at which enrichment progresses is too low, carbon diffusion is not smooth and the carbon concentration inside austenite does not reach the solid solubility limit. In this case, the stability of austenite is insufficient, and there is a risk that the austenite may transform into fresh martensite during cooling, resulting in deterioration of hole expandability. Therefore, in the present invention, the lower limit of the tertiary cooling end temperature is preferably 200°C.

[0099] In the present invention, Bs and Ms can be calculated by the following formula, and each element represents a weight content. B S(℃)=830-(320×[C])-(90×[Mn])-(35×[Si])-(70×[Cr])-(120×[Mo]) M S (℃)=550-(330×[C])-(41×[Mn])-(20×[Si])-(20×[Cr])-(10×[Mo])+(30×[Al])

[0100] Final cooling After the cooling and coiling steps are completed as described above, the target hot-rolled steel sheet can be obtained by final cooling. At this time, the final cooling can be completed by air-cooling to room temperature.

[0101] On the other hand, as described above, the hot-rolled steel sheet of the present invention obtained after the completion of final cooling can be further pickled and oiled.

[0102] The pickled and oiled hot-rolled steel sheet can be heated to a temperature range of 420 to 740°C and subjected to a hot-dip galvanizing process.

[0103] The hot dip galvanizing step can use a zinc-based plating bath, and the alloy composition in the zinc-based plating bath is not particularly limited. [Example]

[0104] The present invention will be described in more detail through the following examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0105] (Example) A steel slab having the alloy composition (wt %, the remainder being Fe and unavoidable impurities) shown in Table 1 below was prepared.

[0106] Each prepared steel slab was reheated to 1200°C, and then hot-rolled steel sheets with a thickness of 2.5 mm were manufactured through the processes of hot rolling, cooling, coiling, and final cooling (air cooling) under the conditions shown in Table 2 below. The total reduction rate of the final two passes during hot rolling was the same at 25%, and the cooling rate during the first cooling was uniformly 70°C / s, and the cooling rate during the third cooling was uniformly 20°C / s.

[0107] For each of the hot-rolled steel sheets, the mechanical properties were measured and the microstructure was observed, and the results are shown in Tables 3 and 4 below.

[0108] Among the mechanical properties, yield strength, tensile strength, and elongation were measured at room temperature using a universal tensile tester after JIS-5 standard test pieces were taken from a direction perpendicular to the rolling direction. The yield strength, tensile strength, and elongation were expressed as 0.2% off-set yield strength, maximum tensile strength, and breaking elongation, respectively.

[0109] The hole expandability was measured based on the ISO TS16630 standard method using the same test pieces as those used in the tensile test.

[0110] The microstructure of each hot-rolled steel sheet was also examined by etching the same test specimens as those used in the tensile tests using Nital etching, then observing them at 10,000x magnification using a scanning electron microscope and an image analyzer, and calculating the fraction of each phase. The average size of bainitic ferrite was expressed as the equivalent circle diameter, and the average spacing was the average distance between the five nearest neighboring structures for each bainitic ferrite phase.

[0111] The austenite fraction was calculated using the integrated intensity of the diffraction peak of each phase using a Bruker X-ray Diffractometer.

[0112] At this time, the microstructure was observed at a point at a thickness position t / 4 on the cross section of the test piece, that is, on the cross section perpendicular to the rolling direction.

[0113] [Table 1]

[0114] [Table 2] FDT is the finishing rolling temperature (℃) Du=(FDT+(7.4×[C])-(24.7×[Si])-(4.7×[Mn])-(3.9×[Cr])-(5.2×[Mo])-(560×[Ti])-(1110×[Nb]))×0.049-34.2 Va=Du×Bat×2.968×10 10 (Bat represents 55.845 × [B] / (1080.6 + 45.04 × [B]), where [B] represents the weight content (%) of boron (B). [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight percent contents of the elements in parentheses.) Equation 3 is exp(-k(T)×(ts) 2 ) (The above k(T) is a value defined as follows, and ts is the secondary cooling time.)

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[0115] [Table 3] M: Martensite A: Austenite FM: Fresh martensite BF: Bainitic ferrite

[0116] [Table 4] YS: Yield strength TS: Tensile strength El: Stretching rate TS*El: product of tensile strength and elongation rate HER: Hole expansion rate

[0117] As shown in Tables 1 to 4 above, Examples 1 to 11 of the present invention, which satisfy all of the alloy compositions and manufacturing conditions proposed in the present invention, have a base structure composed of a composite structure of martensite and austenite with an area fraction of 75 to 90% and a second phase of bainitic ferrite with an area fraction of 10 to 25%, thereby ensuring the target strength and formability.

[0118] On the other hand, in Comparative Example 1, which did not contain boron and did not satisfy the alloy composition proposed by the present invention, excessive bainitic ferrite was formed during secondary cooling, making it impossible to achieve the target strength, and this low strength tended to result in a relatively high elongation.In Comparative Example 2, which did not contain titanium, sufficient bainitic ferrite was not obtained, making it difficult to achieve the hole expandability required by the present invention.

[0119] On the other hand, in Comparative Examples 3 to 5, the alloy compositions satisfy the present invention, but the manufacturing conditions deviate from the present invention.

[0120] In Comparative Examples 3 and 4, the secondary cooling time was too long, so Relational Expression 3 was not satisfied, and the fraction of the secondary phase, bainitic ferrite, was excessive, so a tensile strength of 980 MPa or more could not be ensured.

[0121] In Comparative Example 5, the tertiary cooling end temperature was too low, making it difficult for carbon to diffuse and move, resulting in a low austenite fraction and making it impossible to ensure elongation, and the amount of fresh martensite was excessive, resulting in poor hole expandability.

[0122] In Comparative Example 6, Si was not added, and the austenite fraction could not be ensured, resulting in a deterioration in elongation ratio.

[0123] 1 is a graph showing the relationship between boron content and Du, which satisfies both Relation 1 and Relation 2. The microstructure intended by the present invention can be secured within the solid line connecting ABCDEF.

[0124] Figure 2 shows photographs of the microstructures of Example 4, Comparative Example 2, and Comparative Example 3, observed with a scanning electron microscope. As shown in Figure 2(a), Example 4 has an appropriate matrix structure and secondary phase, which are the microstructures that the present invention aims to achieve. On the other hand, as shown in Figure 2(b), it can be seen that Comparative Example 2 does not have a sufficient amount of soft structure, which is a secondary phase. On the other hand, as shown in Figure 2(c), Comparative Example 3 has an excessive amount of soft structure, which is a secondary phase.

Claims

1. In weight percent, carbon (C): 0.09 to 0.25%, silicon (Si): 0.5 to 2.3%, manganese (Mn): 1.5 to 3.5%, aluminum (Al): 0.001 to 1.0%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.20%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.05%, and the remainder including iron (Fe) and inevitable impurities, The microstructure contains, by area percentage, 75 to 90% of a composite structure of martensite and austenite, and 10 to 25% of bainitic ferrite; The hot-rolled steel sheet contains 3 to 10% austenite.

2. The hot-rolled steel sheet according to claim 1, further containing niobium (Nb): 0.01 to 0.2%.

3. The hot-rolled steel sheet according to claim 1, wherein the bainitic ferrite has an average grain size of 2.0 μm or more.

4. The hot-rolled steel sheet according to claim 1, wherein the average spacing of the bainitic ferrite is 3 μm or more.

5. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation rate of 9% or more, and a hole expansion rate of 45% or more.

6. reheating a steel slab containing, by weight, 0.09-0.25% carbon (C), 0.5-2.3% silicon (Si), 1.5-3.5% manganese (Mn), 0.001-1.0% aluminum (Al), 2.5% or less (including 0%) chromium (Cr), 2.0% or less (including 0%) molybdenum (Mo), 0.01-0.20% titanium (Ti), 0.0005-0.005% boron (B), 0.0001-0.05% phosphorus (P), 0.0001-0.05% sulfur (S), 0.0001-0.05% nitrogen (N), the balance being iron (Fe) and other unavoidable impurities, in a temperature range of 1100-1350°C; hot rolling the reheated steel slab to produce a hot rolled steel sheet; The hot-rolled steel sheet is B Ｓ a step of primarily cooling at a cooling rate of 50° C. / s or more to a temperature below: After the primary cooling (B Ｓ +M Ｓ ) / 2 or more at a cooling rate of 25 ° C. / s or less for a time (ts) (seconds); After the secondary cooling (M Ｓ a third cooling step to a temperature range of -20°C to 200°C at a cooling rate of 30°C / s or less; and winding the material in the tertiary cooled temperature range. During the hot rolling, finish hot rolling is performed within a temperature range of 750 to 1150 ° C. so that the value of Du defined by the following relational expression 1 satisfies the range of 2 to 10, The method for manufacturing a hot-rolled steel sheet, wherein the primary and secondary cooling satisfy the conditions of the following relational expressions 2 to 4. [Relationship 1] Du=(FDT+(7.4×[C])-(24.7×[Si])-(4.7×[Mn])-(3.9×[Cr])-(5.2×[Mo])-(560×[Ti])-(1110×[Nb]))×0.049-34.2 (In the above Relational Formula 1, FDT means the rolling finish temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] each represent the weight percent content of the element in parentheses.) [Relationship 2] 5.0×10 ６ ≦Du×Bat×2.968×10 １０ ≦2.0×10 ７ (In the above-mentioned Relational Formula 2, Du is defined as in the Relational Formula 1, and Bat represents 55.845 × [B] / (1080.6 + 45.04 × [B]), where [B] represents the weight content (%) of boron (B).) [Relationship 3] 0.75≦exp(-k(T)×(ts) ２ )≦0.9 (The k(T) is a value defined by the following relational expression 4, and ts represents the secondary cooling time.) [Relationship 4] [Equation 1] (In the above-mentioned relational formula 4, Du is defined as in relational formula 1, and Bat is defined as in relational formula 2. Furthermore, T1 represents the primary cooling end temperature [°C], and T2 represents the secondary cooling end temperature [°C]. Furthermore, [C], [Si], [Mn], [Cr], and [Mo] each represent the weight percent content of the element in parentheses.)

7. The method for producing a hot-rolled steel sheet according to claim 6, wherein a total reduction in the final two passes during the hot rolling is 10 to 40%.

8. The method for manufacturing a hot-rolled steel sheet according to claim 6, further comprising a step of final cooling to room temperature after the coiling.

9. The method for manufacturing a hot-rolled steel sheet according to claim 8, further comprising the steps of pickling and oiling after the final cooling.

10. The method for manufacturing a hot-rolled steel sheet according to claim 9, further comprising the step of hot-dip galvanizing after the pickling and oiling.

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