EP4640897A1

EP4640897A1 - Hot-rolled steel sheet and method for manufacturing same

Info

Publication number: EP4640897A1
Application number: EP23907586.4A
Authority: EP
Inventors: Tae-Jin Song; Young-Roc Im; Yong-Jin Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-12-14
Publication date: 2025-10-29
Also published as: CN120344702A; EP4640897A4; MX2025007287A; WO2024136300A1; JP2025541896A; KR20240098492A

Abstract

The present invention relates to: a hot-rolled steel sheet which can be suitably applied to a chassis structural member of a vehicle; and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a hot-rolled steel sheet suitable for use in a structural chassis member of an automobile and the like and a method for manufacturing the same.

Background Art

Recently, to reduce global warming, the transition from internal combustion engine vehicles, which are the mainstream in the automobile market, to eco-friendly vehicles such as electric vehicles and the like is rapidly taking place.
As the transition from internal combustion engine vehicles to electric vehicles and the like is taking place, the types of components that make up automobiles are also changing, and the weight of automobiles is also changing. For example, when comparing the weights of internal combustion engine vehicles and electric vehicles released in the same model, it is known that the weight of electric vehicles increases by approximately the weight of the battery, as compared to internal combustion engine vehicles.
Meanwhile, the chassis parts of automobiles play a role in supporting the body of the automobile and are important parts in ensuring ride comfort and driving stability by absorbing road vibrations and shocks during driving. As the weight of the automobile increases, the fatigue load applied to the chassis parts increases, and thus the steel applied to the chassis parts of electric vehicles and the like is required to have excellent fatigue strength.
Since the fatigue strength of steel is proportional to tensile strength and yield strength, steel used for chassis parts such as electric vehicles and the like needs to have improved tensile strength and yield strength, so the steel sheets used for the manufacturing of chassis parts are gradually becoming higher in strength.
In addition, chassis parts are mainly manufactured by press forming. Although the weight of parts may be reduced by reducing the thickness of the steel sheet through the application of high-strength steel, the shape of the parts cannot be significantly changed, and thus securing formability suitable for press forming of parts is becoming more important than anything else in the manufacture of high-strength steel.
Various technologies have been proposed to improve the strength and formability of hot-rolled steel sheets so far.
For example, Patent Document 1 discloses a method for manufacturing a high-strength hot-rolled steel sheet with excellent hole expandability in which the microstructure is comprised of bainitic ferrite without carbides as the main phase, using steel containing 0.01 to 0.05% of carbon in weight %. According to patent document 1, it is possible to manufacture a steel plate with excellent hole expandability of tensile strength 980 grade or higher, but the yield strength is low, so the fatigue characteristics are inferior, the effect of weight reduction of the part is minimal, and the elongation is inferior, and thus there is concern that the formability of the part will be inferior.
Therefore, to secure the driving stability of chassis parts of eco-friendly vehicles and the like such as electric vehicles, it is necessary to develop a steel material that has excellent formability such as elongation, hole expandability and the like, as well as excellent fatigue life due to high tensile strength and yield strength, so that press forming is easy.
(Patent document 1) Japanese Patent Laid-Open Publication No. 2008-255484

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a hot-rolled steel sheet having high strength and excellent fatigue property, as well as excellent formability, and suitable for press forming, and a method for manufacturing the same.
Meanwhile, the subject matter of the present disclosure is not limited to the above-described contents. The subject matter of the present disclosure may be understood from the entire contents of this specification, and a person having ordinary knowledge in the technical field to which the present disclosure belongs will have no difficulty in understanding the additional task of the present disclosure.

Solution to Problem

According to an aspect of the present disclosure, a hot-rolled steel sheet comprises, in weight %, 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%, a remainder of Fe and unavoidable impurities,

wherein a microstructure comprises, in area %, 75-90% of a composite structure of martensite and austenite, and 10-25% of bainitic ferrite, and
wherein the microstructure comprises 3-10% of the austenite.

The hot-rolled steel sheet may further comprise niobium (Nb): 0.01 to 0.2%.
An average grain size of the bainitic ferrite may be 2.0 µm or more.
An average spacing of the bainitic ferrite may be 3 µm or more.
The hot-rolled steel sheet may have a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation of 9% or more, and a hole expansion ratio of 45% or more.
According to an aspect of the present disclosure, a method for manufacturing a hot-rolled steel sheet comprises, an operation of reheating a steel slab at a temperature within a temperature range of 1100 to 1350°C, the steel slab comprising, in wt%, 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%, a remainder of Fe and other unavoidable impurities;

an operation of hot-rolling the reheated steel slab and manufacturing the hot-rolled steel sheet;
an operation of first cooling the hot-rolled steel sheet to a temperature of B_S or less at a cooling rate of 50°C/s or more;
an operation of second cooling for ts time (seconds) at a cooling rate of 25°C/s or less to a temperature of (B_S+M_S)/2 or more after the first cooling;
an operation of third cooling at a cooling rate of 30°C/s or less to a temperature within a temperature range of (M_S-20°C) to 200°C after the second cooling; and
an operation of coiling at the temperature within a temperature range of the third cooling,
wherein in the hot rolling, finishing hot rolling is performed at a temperature within a temperature range of 750 to 1150°C so that a value of Du defined by the following Relationship 1 satisfies a range of 2 to 10, and
the first cooling and the second cooling satisfy conditions of the following Relationships 2 to 4.

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 Relationship 1 above, FDT represents a finishing delivery temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent weight% contents of elements in parentheses, respectively.) $5.0 \times 10^{6} \leq Du \times Bat \times 2.968 \times 10^{10} \leq 2.0 \times 10^{7},$
(In Relationship 2 above, Du is the same as the definition in Relationship 1, and the Bat represents 55.845×[B]/(1080.6+45.04×[B]), and [B] represents a weight content (%) of boron (B).) $0.75 \leq \exp (- k (T) \times {(ts)}^{2}) \leq 0.9,$
(The k(T) is a value defined by Relationship 4 below, and ts represents a second cooling time.) $k (T) - \frac{20}{Du} \times \exp (- {(\frac{(T 1 + 2 \times T 2) / 3 - 557 + 320 \times [C] + 35 \times [Si] + 90 \times [Mn] + 70 \times [Cr] + 120 \times [Mo] + 160000 \times [Bat]}{112})}^{1.92})$
(In Relationship 4 above, Du is the same as the definition in Relationship 1, Bat is the same as the definition in Relationship 2, and also, T1 represents a first cooling end temperature[°C], and T2 represents a second cooling end temperature [°C], and also, [C], [Si], [Mn], [Cr], and [Mo] represent weight% contents of elements in parentheses, respectively.)
In the hot rolling, a total reduction amount of final 2 passes may be 10 to 40%.
The method for manufacturing a hot-rolled steel sheet may further comprise an operation of final cooling to room temperature after the coiling.
The method for manufacturing a hot-rolled steel sheet may further comprise an operation of pickling and oiling after the final cooling.
The method for manufacturing a hot-rolled steel sheet may further comprise an operation of hot-dip zinc plating after the pickling and oiling.

Advantageous Effects of Invention

According to the present disclosure, a steel sheet having a high strength of 980 MPa or more and excellent formability and a method for manufacturing the same may be provided. Accordingly, it may be suitably applied to structural chassis members of automobiles, and the like.
The various and beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and may be more easily understood in the process of explaining detailed embodiments of the present disclosure.

Brief Description of Drawings

FIG. 1 is a graph illustrating the relationship between the boron content and Du, simultaneously satisfying Relationship 1 and Relationship 2, in which the microstructure intended in the present disclosure may be secured within the solid line connecting A-B-C-D-E-F.
FIG.2 illustrate photographs of the microstructures of Invention Example 4, Comparative Example 2 and Comparative Example 3, respectively, observed using a scanning electron microscope in the embodiments of the present disclosure.

Best Mode for Invention

Hereinafter, preferred embodiments of the present disclosure will be described. However, the embodiments of the present disclosure may be modified in various other forms, and the scope of the present disclosure is not limited to the embodiments described below. In addition, the embodiments of the present disclosure are provided to more completely explain the present disclosure to a person having average knowledge in the relevant technical field.
Meanwhile, the terms used in this specification are for describing specific embodiments, and are not intended to limit the present disclosure. For example, the singular forms used in this specification also include plural forms unless the relevant definition clearly indicates a meaning contrary thereto. In addition, the meaning of "comprising or including" used in the specification specifies a configuration, and does not exclude the presence or addition of other configurations.
To improve hole expandability in high-strength steel with a tensile strength of 980 MPa or more, the hot-rolled steel sheet manufacturing method of the related art, which prevents the deterioration of hole expandability due to the difference in hardness between phases by making the fraction of a specific microstructure of the steel 90% or more, could not secure excellent elongation. On the other hand, when utilizing retained austenite to secure excellent elongation, it was possible to secure elongation, but there was a problem that it was difficult to secure hole expandability at the same time.
Accordingly, the inventors of the present disclosure confirmed that hole expandability may be improved by securing high strength and elongation by using martensite and austenite as a matrix structure, while uniformly dispersing bainitic ferrite as a secondary phase within the structure to prevent excessive stress concentration at a specific location during deformation and suppress the occurrence of microcracks, thereby completing the present disclosure. The present disclosure will be described in more detail below.
A steel sheet according to an embodiment of the present disclosure will be described. The steel sheet may comprise, in wt%, 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%, and nitrogen (N): 0.0001 to 0.05%.

Carbon (C): 0.09 to 0.25%

Carbon (C) is an important element that forms retained austenite by stabilizing austenite by diffusing into austenite after martensite phase transformation. As the content of C increases, the fraction of retained austenite increases, thereby simultaneously improving elongation and tensile strength. If the content of C is less than 0.09%, the fraction of retained austenite is low, so elongation and tensile strength cannot be secured. On the other hand, if the content exceeds 0.25%, the B_S temperature is excessively low, making carbon diffusion difficult, resulting in a problem of excessive fresh martensite generation and poor hole expandability. Therefore, in the present disclosure, the content of C is preferably 0.09 to 0.25%. It is more advantageous that the content of C is 0.090 to 0.250%. 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%.

Silicon (Si): 0.5 to 2.3%

Silicon (Si) is an important element that delays the formation of carbides after martensite transformation and forms retained austenite. In addition, Si plays a role in improving strength through the solid solution strengthening effect. If the content of Si is less than 0.5%, carbides are formed and the fraction of retained austenite is low, making it difficult to secure elongation. On the other hand, if the content exceeds 2.3%, Fe-Si composite oxides are formed on the surface of the slab during reheating, which not only deteriorates the surface quality of the steel sheet, but also deteriorates the weldability. Therefore, in the present disclosure, the content of Si is preferably 0.5 to 2.3%. It is more advantageous that the content of Si is 0.50 to 2.30%. 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%.

Manganese (Mn): 1.5 to 3.5%

Manganese (Mn) is an element that improves the hardenability of steel, and prevents the formation of ferrite during cooling after finish rolling, thereby facilitating the formation of low-temperature transformation structures.
If the content of Mn is less than 1.5%, there is a problem that the hardenability is insufficient and the fraction of ferrite increases excessively. On the other hand, if the content exceeds 3.5%, the hardenability increases significantly, so that the holding time for sufficiently forming the bainitic ferrite to be obtained in the present disclosure excessively increases, and hole expandability deteriorates.
Therefore, in the present disclosure, the Mn may be included in an amount of 1.5 to 3.5%, and it is more advantageous that the Mn content is 1.50 to 3.50%. 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%.

Aluminum (Al): 0.001 to 1.0%

Aluminum (Al) is an element that is usually added to deoxidize molten steel, and may exist in the steel to some extent after deoxidation. However, similar to Si, it also plays a role in forming retained austenite by delaying the formation of carbides after martensite transformation.
If the content of Al is less than 0.001%, carbides are formed and the fraction of retained austenite is low, making it difficult to secure elongation. On the other hand, if the content exceeds 1.0%, it causes an increase in oxide and nitride inclusions in the steel, which deteriorates the formability of the steel sheet. Therefore, in the present disclosure, it is preferable that the content of Al is 0.001 to 1.0%. More advantageously, the lower limit of the Ai content may be 0.01%, or the upper limit of the Al content may be 0.5%.

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 content of Cr exceeds 2.5%, the hardenability increases significantly, so that the bainite transformation does not occur smoothly in the cooling zone, and the holding time for securing the fraction of bainitic ferrite increases excessively, thereby deteriorating hole expandability. Therefore, in the present disclosure, the Cr may be included in an amount of 2.5% or less, and more advantageously in 2.50% or less, and more advantageously, may be included in an amount of 1.5% or less.
Meanwhile, the present disclosure includes a case where the Cr content is 0% because there is no significant problem in securing the intended properties even if the Cr is not included. However, it should be noted that when intentionally adding the Cr, it is effective to add at least 0.01%.

Molybdenum (Mo): 2.0% or less (including 0%)

Molybdenum (Mo) is an element that improves the hardenability of steel, plays a role in improving strength through the solid solution strengthening effect, and suppresses the formation of ferrite during cooling after finish rolling. If the content of Mo exceeds 2.0%, the hardenability increases significantly, and the bainite transformation does not occur smoothly in the cooling zone. As a result, the holding time for securing the fraction of bainitic ferrite increases excessively, and hole expandability decreases. Therefore, in the present disclosure, the Mo may be included in an amount of 2.0% or less, more advantageously at 1.0% or less, and even more advantageously at 0.5% or less.
On the other hand, the present disclosure includes a case where the Mo content is 0% because there is no significant problem in securing the intended properties even if the Mo is not included. However, it should be noted that when the Mo is intentionally added, it is effective to add in an amount of at least 0.01%.

Titanium (Ti): 0.01 to 0.20%

Titanium (Ti) is an element that forms carbonitride in steel, and is widely used to secure the strength of steel by inducing the formation of precipitates in this way. However, in the present disclosure, it is used to control the grain size of austenite before rolling by removing nitrogen (N) in the steel to suppress the formation of BN, thereby concentrating boron (B) in the austenite grain boundary.
To sufficiently obtain the effect intended for the present disclosure, it is preferable to contain the Ti at 0.01% or more, and it is preferable to contain 2.9 times or more the nitrogen (N) content to remove nitrogen (N) in the steel. However, if the content exceeds 0.20%, problems such as clogging of the casting nozzle or the like may occur due to the formation of oxides during continuous casting.
Therefore, in the present disclosure, the Ti may be included in an amount of 0.01 to 0.20%, and more advantageously, the lower limit of the Ti may be 0.015%, or the upper limit of the Ti content may be 0.12%.

Boron (B): 0.0005 to 0.005%

Boron is an element that improves the hardenability of steel by concentrating in austenite grain boundaries and reducing grain boundary energy. In the present disclosure, the phase transformation of ferrite and upper bainite may be suppressed, where the nucleus of phase transformation occurs by diffusion transformation in the austenite grain boundaries, so that a composite structure of martensite and austenite may be secured as the main phase.
To sufficiently obtain the desired effect in the present disclosure, it is preferable to contain the concentration of B at 0.0005% or more. However, if the content exceeds 0.005%, the hardenability increases significantly, so that the holding time for sufficiently forming the bainitic ferrite to be obtained in the present disclosure increases excessively, and hole expandability decreases.
Therefore, in the present disclosure, the B may be included 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%.

Phosphorus (P): 0.0001 to 0.05%

Phosphorus (P) is an impurity that is inevitably contained in steel, and is an element that is the main cause of deteriorating the workability of steel due to segregation. Therefore, it is desirable to control the content as low as possible.
In theory, it is advantageous to limit the content of the P to 0%, but since excessive manufacturing costs are required to control the content of the P to less than 0.0001%, the lower limit may be set to 0.0001%. However, since there is a concern that the workability may deteriorate if the content exceeds 0.05%, the upper limit of the P may be limited to 0.05%. However, more advantageously, the lower limit of the content of the P may be 0.0005%, or the upper limit of the content of the P may be 0.02%.

Sulfur (S): 0.0001 to 0.05%

Sulfur (S) is an impurity that is inevitably contained in steel, and forms non-metallic inclusions by combining with manganese or the like, which causes a problem of deterioration in the workability of the steel. Therefore, it is desirable to control the content as low as possible.
In theory, it is advantageous to limit the content of the S to 0%, but since excessive manufacturing costs are required to control the content of the S to less than 0.0001%, the lower limit may be set to 0.0001%. However, since there is a concern that the workability may deteriorate if the content exceeds 0.05%, the upper limit of the S may 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%.

Nitrogen (N): 0.0001 to 0.05%

Nitrogen (N) is an impurity that is inevitably contained in steel, and there is a problem that it forms nitrides by combining with Al or the like, thereby hindering the workability of the steel. Therefore, it is desirable to control the content as low as possible.
In theory, it is advantageous to limit the content of the N to 0%, but since excessive manufacturing costs are required to control the content of the N to less than 0.0001%, the lower limit may be set to 0.0001%. However, since there is a concern that workability may deteriorate if the content exceeds 0.05%, the upper limit of the N may 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%.
The hot-rolled steel sheet of the present disclosure may further contain niobium (Nb) in addition to the alloy composition described above.

Niobium (Nb): 0.01 to 0.2%

Niobium (Nb) is an element that forms carbonitrides in steel, and is widely used to secure the strength of steel by inducing the formation of precipitates, but in the present disclosure, it delays recrystallization during hot rolling and controls the grain size of austenite. If the content of Nb is less than 0.01%, the effect of controlling the grain size is low, and if the content exceeds 0.2%, there is a problem that the grain size of austenite is too fine and formability is poor. Therefore, the content of Nb in the present disclosure may contain 0.01 to 0.2%.
The remaining component in the present disclosure is iron (Fe). However, unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during a normal manufacturing process, which cannot be ruled out. These impurities are known to anyone skilled in the art of manufacturing processes of the related art, and thus are not specifically mentioned in this specification.
The hot-rolled steel sheet of the present disclosure includes a composite structure of martensite and austenite as a matrix structure to simultaneously secure a tensile strength of 980 MPa or more and an elongation of 9% or more. Therefore, according to an embodiment of the present disclosure, the composite structure comprised of martensite and austenite may be included in an amount of 75 to 90% in terms of area %.
In the third cooling process performed after hot rolling in the present disclosure, a portion of the untransformed austenite is transformed into martensite at the M_S temperature or less. After coiling, the steel sheet is gradually cooled and maintained in a state close to isothermal, and at this time, an isothermal phase transformation of austenite into martensite occurs, so that the fraction of martensite increases. Martensite is formed by shear transformation (displacive phase transformation), so there is a high level of dislocation density in the structure due to the screw dislocations formed in the structure to reduce the shear strain generated during transformation and the blade dislocations formed to accommodate the volume expansion due to phase transformation, and it is suitable for improving the yield strength and tensile strength of steel due to the fine carbides present in the structure. On the other hand, there is a characteristic that the high level of dislocation density and fine carbides hinder the movement of dislocations in the structure, and thus the elongation is inferior.
Therefore, it is desirable to improve the elongation of high-strength steel by the transformation-induced plasticity phenomenon by including austenite in the matrix structure. Immediately after martensite transformation, carbon atoms superabsorbed in martensite diffuse into austenite, and thus the carbon concentration inside austenite gradually increases, and the austenite with increased stability due to carbon enrichment does not undergo phase transformation even when cooled to room temperature, but remains in the microstructure, playing a role in improving the elongation of the steel sheet. Therefore, in terms of securing tensile strength and elongation, the present disclosure preferably includes the martensite and austenite composite structure at an area fraction of 75% or more. Meanwhile, to secure the hole expandability to be described later, it is preferable to limit the area fraction of the martensite and austenite composite structure to 90% or less.
At this time, the austenite may be 3 to 10% in area %. If the area fraction of austenite is less than 3%, the elongation improvement effect due to the transformation-induced plasticity phenomenon is minimal. On the other hand, in order for the area fraction of the austenite to exceed 10%, the carbon (C) content that should be added to the steel should increase, so there is a problem that the weldability of the steel is inferior, and since the M_S temperature is excessively low and carbon diffusion is not easy, the amount of fresh martensite generated in the final cooling stage to room temperature after coiling increases excessively, which causes inferior hole expandability.
The hot-rolled steel sheet of the present disclosure may comprise bainitic ferrite as a secondary phase of the microstructure, and may contain 10 to 25% of the secondary phase in terms of area %.
In the present disclosure, by avoiding ferrite phase transformation during the first cooling after hot rolling, cooling to a temperature of B_S (bainitic transformation initiation temperature) or less is performed, and then slow cooling is performed during the subsequent second cooling, thereby progressing the bainite transformation. Since the bainite transformation at this time occurs in the hightemperature bainite transformation range, the formation of bainitic ferrite and the diffusion of carbon into untransformed austenite occur, and there is a characteristic that no carbide is formed inside the bainitic ferrite. Meanwhile, although a large amount of dislocations exist inside the bainitic ferrite formed by shear transformation, the dislocation density decreases to an appropriate level due to the recovery phenomenon during the second cooling, so there is a characteristic that it has a soft characteristic.
On the other hand, since martensite transformation occurs in a wide temperature range from the M_S temperature to the third cooling end temperature, the temperature at which the phase transformation starts varies depending on the location in the steel sheet. Since the transformation stress remaining in the steel sheet varies depending on the temperature at which martensite is formed, the transformation stress is distributed unevenly depending on the location in the steel sheet, and remains in the steel sheet even after cooling to room temperature. When deformation is applied from the outside during component forming, the deformation is concentrated in the area where the residual stress is high in the steel sheet, which facilitates the growth and propagation of cracks, resulting in poor hole expandability.
On the other hand, when soft bainitic ferrite is evenly distributed in an appropriate size within a high-strength matrix structure with an uneven stress distribution, it has the effect of preventing local stress concentration by evenly accommodating deformation during forming, thereby improving hole expandability.
Therefore, in the present disclosure, if the fraction of bainitic ferrite, the secondary phase, is less than 10%, there is a problem that it is difficult to secure hole expandability, and on the other hand, if the fraction exceeds 25%, there is a problem that it is difficult to secure martensite and austenite composite structures that play a role in improving strength.
Meanwhile, the average grain size of the bainitic ferrite may be 2.0 µm or more. In addition, the average spacing of the bainitic ferrite may be 3 µm or more.
At this time, the average grain size of the bainitic ferrite refers to the equivalent circular diameter, and the average spacing of the bainitic ferrite refers to the average distance between the five most adjacent structures for respective microstructures.
If the average grain size of the soft structure, bainitic ferrite, is less than 2.0 µm, the effect of strain acceptance is low, and thus improvement in hole expandability cannot be expected. In addition, if the average spacing of the bainitic ferrite is less than 3.0 µm, the fraction of soft steel increases excessively, so the yield strength and tensile strength may be inferior. The upper limit of the average grain size and the upper limit of the average spacing of the soft structure are not separately regulated, but under the condition that the fraction of the soft structure satisfies the range of 10 to 25%, the average grain size of the soft structure may be preferably 20 µm or less. In addition, the average spacing of the vertical structure may be 20 µm or less.
The hot-rolled steel sheet of the present disclosure may include carbides and fresh martensite as other structures in addition to the above-described structures, but it is preferable that these are controlled to an area fraction of less than 5%.
The hot-rolled steel sheet may generate carbides during the manufacturing process. Immediately after martensite transformation, some carbon atoms may form fine carbides inside the lath, which may play a role in improving strength. On the other hand, since the present disclosure seeks to improve the elongation by utilizing austenite, the formation of carbides may cause a decrease in the austenite fraction. In detail, excessive formation of carbides inhibits the improvement of elongation targeted by the present disclosure. However, if Ti and Nb are present in the phase, alloy carbonitrides may be formed, and in this case, an additional strengthening effect due to grain refinement may be expected, but 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 disclosure be less than 5%.
In addition, according to an embodiment of the present disclosure, the hot-rolled steel sheet may include fresh martensite as a microstructure. In the present disclosure, the martensite that is transformed isothermally during the third cooling process and immediately after coiling is transformed before the carbon concentration inside the austenite begins, and therefore has a lath form within the component range of the present disclosure. Meanwhile, if the coiling temperature is excessively low, carbon diffusion is not easy, so cooling may be completed before the austenite is sufficiently stabilized. In this case, if the M_S temperature of the austenite in which carbon is concentrated is room temperature or more, it may transform into fresh martensite during cooling. The fresh martensite generated during the final cooling process accepts shear deformation as the creation of twins rather than dislocations during phase transformation, so it has a plate shape and has the characteristic of twins being observed within the microstructure, so it may be easily distinguished from the martensite in the present disclosure. Since fresh martensite with a high carbon concentration has excessively high hardness, which makes hole expandability inferior, the fraction of fresh martensite present in the hot-rolled steel sheet of the present disclosure is preferably less than 5%.
The hot-rolled steel sheet of the present disclosure having the above-described alloy composition and microstructure has high strength with a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation of 9% or more, a product of tensile strength and elongation being 13000 MPa·% or more, and a hole expansion ratio of 45% or more, and thus has excellent formability.
Next, a method for manufacturing a hot-rolled steel sheet, which is another embodiment of the present disclosure, will be described in detail below. However, it does not mean that the hot-rolled steel sheet of the present disclosure should be manufactured by the manufacturing method described below.
The manufacturing method may be performed by manufacturing a steel slab satisfying the alloy composition proposed in the present disclosure by performing a series of processes of [reheating - hot rolling - cooling - coiling].
The conditions of respective processes described above will be described in detail below.

Reheating of Steel Slab

Before performing the rolling process described below, it is desirable to reheat the steel slab and perform a homogenization process. This may be performed at a temperature within a temperature range of 1100 to 1350°C.
If the temperature of the reheating of the steel slab is lower than 1100°C, there is a problem that the homogenization of the alloy elements is not sufficient. On the other hand, if the temperature exceeds 1350°C, excessive oxides are formed on the surface of the slab, which may deteriorate the surface quality of the steel sheet.

Hot Rolling

The reheated steel slab may be hot-rolled to manufacture a hot-rolled steel sheet. At this time, the hot rolling is performed at a temperature within a temperature range of 750 to 1150°C, and it is desirable to control the total reduction amount of the final two passes to 10 to 40%.
First, if the hot rolling is initiated at a temperature exceeding 1150°C, oxides are excessively formed on the surface of the steel sheet after rolling, and even if the pickling process is performed, they cannot be effectively controlled, resulting in poor surface quality. On the other hand, if the hot rolling is performed at a temperature lower than 750°C, the rolling load increases excessively, resulting in poor workability, and ferrite is generated during rolling, resulting in poor anisotropy.
Usually, multi-stage rolling is performed during hot rolling to reduce the rolling load and precisely control the thickness. When performing hot rolling with this multi-stage rolling, if the total reduction ratio of the final two passes (the last two passes) exceeds 40%, the rolling load of the final two passes becomes excessive, resulting in poor workability. On the other hand, if the total reduction ratio of the final two passes is less than 10%, the temperature of the steel sheet rapidly decreases, resulting in a problem of poor shape.
Meanwhile, the grain size of austenite after hot rolling is affected by the alloy composition and finishing delivery temperature, which affects the bainite formation behavior and final microstructure in the subsequent cooling process. In addition, the fraction and size of the main constituent phase, bainitic ferrite, in the present disclosure are greatly affected by the austenite grains after hot rolling.
Since the grains of equiaxed ferrite and pearlite grow by the diffusion of elements during phase transformation, the size of the structure after phase transformation is affected by the phase transformation temperature and holding time, whereas bainitic ferrite generated by shear transformation such as bainite grows only within the austenite grains, and its size cannot be larger than the austenite size before transformation. Therefore, to control the size of bainitic ferrite, it is advantageous to control the grain size of austenite after hot rolling.
Accordingly, the present disclosure derives the effective grain size of austenite after hot rolling as a relationship between the finishing delivery temperature (FDT) and a specific alloy composition, which is specifically defined as Relationship 1 below. That is, during hot rolling, finishing hot rolling is performed so that the value of Du defined by Relationship 1 below satisfies the range of 2 to 10 within a temperature range of 750 to 1150°C. 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 Relationship 1, FDT refers to the finishing delivery temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight% contents of the elements in parentheses, respectively.)
The Du is an index indicating the effective grain size of austenite immediately before the first cooling after hot rolling. If the Du value defined by the above Relationship 1 is 2 or more, the average grain size of bainitic ferrite becomes 2.0 µm or more, so that hole expandability may be secured as 45% or more. On the other hand, if the Du value defined by the above Relationship 1 exceeds 10, the grain boundary concentration of boron element increases excessively, which delays the phase transformation during the second cooling, so that a sufficient fraction of bainitic ferrite cannot be secured, which causes a problem in that hole expandability is inferior. The Du value is more preferably 2.0 to 10.0.
Meanwhile, according to an embodiment of the present disclosure, boron segregates at the grain boundaries of austenite and stabilizes the austenite grain boundaries, thereby delaying the nucleation of ferrite and upper bainite, thereby lowering the phase transformation speed. To secure the area fraction and average spacing of bainitic ferrite intended in the present disclosure, it is important to control the concentration of boron segregated at the grain boundaries of austenite. The concentration of boron segregated in the austenite grain boundaries exhibits different values for respective grain boundaries due to the microsegregation during casting and the grain size of the austenite, and thereamong, bainitic ferrite nucleation selectively occurs at the austenite grain boundaries with a low concentration of boron element during the second cooling stage. In general, when the austenite grain size is small, the concentration of boron segregated in each grain boundary is low, making nucleation easy, and when the grain size is large, the concentration of boron segregated in the grain boundary is high, and thus it can be expected that nucleation is delayed. Therefore, the concentration of boron existing in the grain boundaries and the phase transformation behavior of bainitic ferrite during the second cooling are affected by the content of boron added to the steel and the grain size of the austenite, as illustrated in Relationship 2.
Relationship 2 is an indicator of the concentration of boron (B) distributed at the grain boundaries of austenite immediately before cooling. If the value of Du×Bat×2.968×10¹⁰ according to Relationship 2 below is less than 5.0×10⁶, the fraction of bainitic ferrite is excessive, and the yield strength and tensile strength cannot be secured. On the other hand, if the value of Du×Bat×2.968×10¹⁰ according to Relationship 2 below exceeds 2.0×10⁷, the second cooling time for securing the fraction of bainitic ferrite becomes excessively long, which causes a problem of poor hole expandability. $5.0 \times 10^{6} \leq Du \times Bat \times 2.968 \times 10^{10} \leq 2.0 \times 10^{7}$
(In the above Relationship 2, Du is the same as the definition in Relationship 1, and the Bat represents 55.845×[B]/(1080.6+45.04×[B]), and [B] represents the weight content (%) of boron (B).)
FIG. 1 is a graph illustrating the relationship between the boron content and Du, which simultaneously satisfies the above Relationship 1 and Relationship 2, and the microstructure intended in the present disclosure may be secured within the solid line connecting A-B-C-D-E-F.

Cooling and Coiling

The hot-rolled steel sheet manufactured as described above is cooled, which is preferably performed in stages according to the cooling temperature.
In detail, it is preferable to first cool the hot-rolled steel sheet to a temperature of B_S or less at a cooling rate of 50°C/s or more, then to second cool the same to a temperature of (B_S + M_S)/2 or more at a cooling rate of 25°C/s or less for a ts time (second) defined in Relationship 3, and then to perform a third cooling at a cooling rate of 30°C/s or less to a temperature within a temperature range of (Ms-20°C) to 200°C.
The hot-rolled steel sheet manufactured as described above is rapidly cooled to a temperature (B_S) or less at which bainite begins to form, thereby suppressing the formation of ferrite (granular ferrite). Subsequently, by slowly cooling for a time (seconds) of ts to a temperature intermediate between the bainite initiation temperature (B_S) and the martensite initiation temperature (M_S) or a temperature higher than the temperature, bainitic ferrite having an area fraction of 10 to 25% may be secured.
When first cooling is performed at a temperature of B_S or less after the hot rolling is completed, if the cooling rate is less than 50°C/s, there is a problem that a ferrite phase is formed during cooling. At this time, the upper limit of the first cooling rate is not particularly limited, but since there is a concern that the plate shape may be distorted if the steel sheet is cooled too rapidly, it may be limited to 200°C/s or less.
The lower limit of the cooling completion temperature during the first cooling is not particularly limited, but since there is a concern that the cooling time for the subsequent second cooling may not be sufficient if it is lowered excessively, it is stated that it may be limited to B_S-100°C.
When the temperature of the hot-rolled steel sheet becomes Bs or less by the first cooling, the steel cooling is terminated, and second cooling may be performed at a temperature of (B_S + M_S)/2 or more at a cooling rate of 25°C/s or less.
While the first-cooled hot-rolled steel sheet is cooled from the first-cooled temperature to the target temperature of the second cooling, bainitic ferrite growth occurs. To obtain the target fraction of the present disclosure in particular, it is preferable to maintain the second cooling for a time (ts, seconds) satisfying the following Relationship 3.
In Relationship 3, k(T) is an indicator of the growth rate of bainitic ferrite, which is affected not only by the alloy composition of the steel but also by the phase transformation temperature and the grain size after hot rolling. Accordingly, if the value of Relationship 3, for example, the relationship (exp(-k(T)×(ts)²)) between k(T) and the holding time, is less than 0.75, the fraction of bainitic ferrite becomes excessive, and thus the target level of strength may not be secured. On the other hand, if the value exceeds 0.9, there is a problem of deterioration of hole expandability. $0.75 \leq \exp (- k (T) \times {(ts)}^{2}) \leq 0.9$
(The k(T) represents a value defined by Relationship 4 below.) $k (T) = \frac{20}{Du} \times \exp (- {(\frac{(T 1 + 2 \times T 2) / 3 - 557 + 320 \times [C] + 35 \times [Si] + 90 \times [Mn] + 70 \times [Cr] + 120 \times [Mo] + 160000 \times [Bat]}{112})}^{1.92})$
(In the Relationship 4 above, Du is the same as defined in Relationship 1, and Bat is the same as defined in Relationship 2. In addition, T1 represents the first cooling end temperature[°C], and T2 represents the second cooling end temperature[°C]. In addition, [C], [Si], [Mn], [Cr], and [Mo] represent the weight% contents of the elements in parentheses, respectively.)
During the second cooling according to the above-mentioned conditions, the temperature of the steel sheet may increase due to transformation heat generation caused by the bainite phase transformation. At this time, since the dislocation density may be excessively reduced due to excessive heat generation, the cooling rate may be controlled to 25°C/s or less during the second cooling to minimize the temperature rise of the steel sheet due to transformation heat generation. If the cooling rate exceeds 25°C/s, there is a concern that the plate shape may be distorted. It is disclosed that the second cooling in the present disclosure also includes the process of air cooling.
According to the above, it is preferable to perform third cooling at a cooling rate of 30°C/s or less to a temperature within a temperature range of (M_S-20°C) to 200°C for the hot-rolled steel sheet for which the second cooling is completed, and then to coil at that temperature. During the third cooling, martensite transformation progresses at the temperature of M_S or less, and a portion of the untransformed austenite may additionally grow into martensite under isothermal conditions even after coiling.
When the cooling rate is excessive during the martensitic transformation, the plate shape may be distorted due to rapid volume expansion, which may in turn cause cooling imbalance and result in uneven material distribution. Therefore, in the present disclosure, by setting the cooling rate in the third cooling stage, which is accompanied by rapid phase transformation, to 30°C/s or less, shape distortion during cooling and resulting uneven material deviation within the plate may be prevented. On the other hand, if the cooling rate is excessively slow, bainitic ferrite grows during cooling, and the fraction of the secondary phase increases excessively, and thus it may be difficult to secure the strength of the steel. Therefore, in the present disclosure, the cooling rate in the third cooling stage may be performed at 5°C/s or more.
Meanwhile, the maximum carbon concentration that may be dissolved in austenite exhibits different values depending on the temperature at which carbon enrichment proceeds, and generally, the solubility limit of carbon within austenite increases as the temperature decreases. Therefore, if the coiling temperature at which carbon enrichment occurs is too high, austenite cannot secure phase stability for transformation-induced plasticity because carbon enrichment is not sufficient, and even if it remains at room temperature, it is lost due to stress-induced transformation at the initial stage of deformation, so improvement in elongation cannot be expected. Therefore, in the present disclosure, it is preferable that the upper limit of the third cooling end temperature is M_S-20°C to secure sufficient phase stability. On the other hand, if the temperature at which enrichment occurs is too low, carbon diffusion is not smooth, so that the carbon concentration inside the austenite does not reach the solid solution limit, and in this case, the stability of the austenite is insufficient, so that phase transformation into fresh martensite occurs during cooling, which may deteriorate hole expandability. Therefore, in the present disclosure, it is preferable that the lower limit of the third cooling end temperature is 200°C.
In the present disclosure, Bs and Ms may be derived by the following equations, and each element represents a weight content. B_S(°C)= 830-(320×[C])-(90×[Mn])-(35×[Si])-(70×[Cr])- (120×[Mo]) Ms(°C)= 550-(330×[C])-(41×[Mn])-(20×[Si])-(20×[Cr])- (10×[Mo])+(30×[Al])

Final Cooling

After completing the cooling and coiling process as described above, the target hot-rolled steel sheet may be obtained by final cooling. At this time, the final cooling may be completed by performing air cooling to room temperature.
Meanwhile, the hot-rolled steel sheet of the present disclosure obtained by completing the final cooling as described above may be additionally pickled and oiled.
In addition, the hot-rolled steel sheet that has been pickled and oiled may be heated to a temperature within a temperature range of 420 to 740°C in a hot-dip galvanizing process.
The hot-dip galvanizing process may utilize a zinc-based plating bath, and there is no particular limitation on the alloy composition in the zinc-based plating bath.

Mode for Invention

Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the invention. This is because the scope of the present disclosure is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

(Example)

Steel slabs having the alloy composition (weight %, the remainder being Fe and inevitable impurities) illustrated in Table 1 below were prepared.
Each of the prepared steel slabs was reheated at 1200°C, and then hot-rolled, cooled, coiled, and finally cooled (air-cooled) under the conditions illustrated in Table 2 below to manufacture hot-rolled steel sheets having a thickness of 2.5 mm. The total reduction ratio of the final two passes in the hot rolling was applied equally at 25%, and the cooling rate was uniformly applied at 70°C/s during the first cooling, and uniformly applied at 20°C/s during the third cooling.
The mechanical properties of each hot-rolled steel sheet were measured and the microstructure was observed, and the results are illustrated in Tables 3 and 4 below.
Among the mechanical properties, the yield strength, tensile strength, and elongation were measured at room temperature using a universal tensile tester after collecting JIS-5 standard test specimens in a direction perpendicular to the rolling direction. At this time, the yield strength, tensile strength, and elongation were expressed as 0.2% off-set yield strength, maximum tensile strength, and fracture elongation, respectively.
In addition, hole expandability was measured according to the ISO TS16630 standard method for the same specimens as those used in the tensile test.
In addition, the microstructure of each hot-rolled steel sheet was observed at 10,000x magnification using a scanning electron microscope and an image analyzer after etching the same specimen as the above tensile test using the Nital etching method, and the fraction of each phase was calculated. The average size of bainitic ferrite was expressed as the equivalent circular diameter, and the average spacing was expressed as the average distance between the five closest structures for respective bainitic ferrite phases.
The austenite fraction was calculated using the integrated intensity of the diffraction peak of each phase using an X-ray diffractometer by Bruker.

At this time, the microstructure was observed at the thickness location t/4 point with respect to the cross-section of the specimen, for example, the cross-section perpendicular to the rolling direction.

[Table 1]

Class ifica tion	C	Si	Mn	P	S	Al	Cr	Mo	Ti	Nb	B	N
Steel 1	0.15	0.9	2.3	0.01 2	0.00 2	0.02 5	0	0	0.08	0	0.001 3	0.00 4
Steel 2	0.20	1.5	2.1	0.01 2	0.00 3	0.02 3	0	0	0.08	0	0.001 4	0.00 4
Steel 3	0.18	1.2	2.1	0.01 2	0.00 3	0.01 8	0.2	0.1 5	0.08	0.02	0.000 9	0.00 3
Steel 4	0.15	2.0	1.8	0.01 1	0.00 3	0.02 5	0	0	0.06	0	0.001 8	0.00 4
Steel 5	0.15	1.5	2.8	0.01 2	0.00 2	0.02 2	0	0	0.07	0.02	0.002 1	0.00 4
Steel 6	0.22	0.9	1.8	0.01 2	0.00 2	0.02 8	0	0.1	0.08	0.02	0.001 5	0.00 3
Steel 7	0.12	1.5	2.5	0.01 2	0.00 3	0.02 5	0.9	0	0.12	0	0.001 2	0.00 4
Steel 8	0.18	0.9	2.2	0.01 2	0.00 2	0.02 5	0	0.1	0.02	0.05	0.002 1	0.00 4
Steel 9	0.12	1.2	2.1	0.01 2	0.00 2	0.02 5	0.2	0.1	0.08	0	0.001 5	0.00 4
Steel 10	0.15	1.2	2.3	0.01 2	0.00 3	0.02 3	0	0	0.08	0	0	0.00 4
Steel 11	0.10	1.2	2.3	0.01 2	0.00 3	0.02 5	0	0	0	0	0.001 8	0.00 4
Steel 12	0.12	0.8	1.7	0.01 2	0.00 2	0.02 2	0	0	0.12	0.05	0.000 8	0.00 4
Steel 13	0.10	0.1	2.1	0.01 2	0.00 3	0.01 8	0	0	0.08	0	0.001 5	0.00 4

[Table 2]

Ste el	Hot Rolling			1st Cooling		2nd Cooling					3rd Cool ing	Remar ks
Ste el	FDT	Du	Va	End temp erat ure T1 (°C)	B_S (°C)	End temp erat ure T2 (°C)	(B_S+M_S) /2 (°C)	Coo lin g Tim e ts (s)	Rel ati ons hip 3	Cool ing Rate (°C/s )	End temp erat ure T3 (°C)	Remar ks
Ste el 1	920	7.1	1.42 ×10⁷	500	543. 5	480	466.2	2.0	0.7 9	10.0	345	Inven tion Examp le 1
Ste el 2	920	6.5	1.39 ×10⁷	480	524. 5	460	446.5	2.0	0.7 7	10.0	320	Inven tion Examp le 2
Ste el 3	920	5.6	7.80 ×10⁶	480	509. 4	460	442.5	2.0	0.8 2	10.0	320	Inven tion Examp le 3
Ste el 4	900	5.5	1.51 ×10⁷	520	550. 0	500	468.7	2.0	0.8 5	10.0	335	Inven tion Examp le 4
Ste el 5	910	5.0	1.60 ×10⁷	450	477. 5	440	416.9	2.5	0.8 3	4.0	280	Inven tion Examp le 5
Ste el 6	925	6.4	1.47 ×10⁷	520	554. 1	500	469.8	2.0	0.8 4	10.0	320	Inven tion Examp le 6
Ste el 7	910	4.6	8.43 ×10⁶	440	451. 1	420	405.9	2.5	0.8 3	8.0	320	Inven tion Examp le 7
Ste el 8	910	5.6	1.79 ×10⁷	520	530. 9	500	456.5	3.5	0.8 0	5.7	320	Inven tion Examp le 8
Ste el 9	920	6.8	1.55 ×10⁷	520	534. 6	500	465.3	3.0	0.8 3	6.7	320	Inven tion Examp le 9
Ste el 1	900	6.1	1.22 ×10⁷	520	543. 5	500	466.2	2.5	0.8 1	8.0	345	Inven tion Examp le 10
Ste el 1	920	7.1	1.42 ×10⁷	520	543. 5	500	466.2	2.5	0.8 3	8.0	350	Inven tion Examp le 11
Ste el 10	925	7.0	0.00	520	533. 0	500	457.9	3.0	0.7 6	6.7	350	Compa rativ e Examp le 1
Ste el 11	950	10. 4	2.87 ×10⁷	520	549. 0	500	474.2	3.0	0.8 3	6.7	350	Compa rativ e Examp le 2
Ste el 12	880	1.6	1.95 ×10⁷	550	610. 6	530	518.0	2.0	0.1 3	10.0	340	Compa rativ e Examp le 3
Ste el 7	910	4.6	8.43 ×10⁷	440	451. 1	420	405.9	4.5	0.5 4	4.4	320	Compa rativ e Examp le 4
Ste el 1	920	7.1	1.42 ×10⁷	500	543. 5	480	466.2	1.5	0.8 8	13.3	20	Compa rativ e Examp le 5
Ste el 13	925	8.4	1.92 ×10⁷	550	605. 5	530	517.5	1.5	0.8 6	13.3	380	Compa rativ e Examp le 6

FDT is the finishing delivery temperature (°C) 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 $Va = Du \times Bat \times 2.968 \times 10^{10}$
(Bat represents 55.845×[B]/(1080.6+45.04×[B]), and [B] represents the weight content (%) of boron (B). [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight% contents of the elements in parentheses, respectively.)
Relationship 3 is exp(-k(T)×(ts)²).
(The k(T) is a value defined as below, and ts is the second cooling time) $k (T) = \frac{20}{Du} \times \exp (- {(\frac{(T 1 + 2 \times T 2) / 3 - 557 + 320 \times [C] + 35 \times [Si] + 90 \times [Mn] + 70 \times [Cr] + 120 \times [Mo] + 160000 \times [Bat]}{112})}^{1.92})$

(T1 represents the first cooling end temperature[°C], and T2 represents the second cooling end temperature[°C]. In addition, [C], [Si], [Mn], [Cr], and [Mo] represent the weight% contents of the elements in parentheses, respectively)

[Table 3]

Remarks	Microstructure
	Area%				BF Distribution
	M	A	FM	BF	Average Size (µm)	Average Spacing (µm)
Invention Example 1	72.7	5.7	0	21.6	6.7	13.6
Invention Example 2	71.1	7.2	0	21.8	5.5	9.8
Invention Example 3	74.1	6.9	0	19.1	5.3	11.2
Invention Example 4	79.8	5.3	0	14.9	4.8	10.8
Invention Example 5	74.3	5.6	2.5	17.6	4.8	10.7
Invention Example 6	76.1	8.1	0	15.8	6.1	14.2
Invention Example 7	78.0	4.3	0	17.7	4.2	9.0
Invention Example 8	73.0	6.7	0	20.3	5.0	9.7
Invention Example 9	79.1	4.0	0	16.8	6.1	13.1
Invention Example 10	74.4	5.7	0	19.9	5.7	11.8
Invention Example 11	77.7	5.5	0	16.7	6.3	13.2
Comparative Example 1	52.4	5.6	0	42.0	18.0	22.8
Comparative Example 2	90.6	3.4	0	6.0	10.0	38.3
Comparative Example 3	11.4	4.1	0	84.5	1.6	1.6
Comparative Example 4	50.2	4.1	1.2	44.6	9.2	13.0
Comparative Example 5	81.6	0.3	6.0	12.1	6.7	17.6
Comparative Example 6	85.0	0.2	0	14.8	7.6	17.7

M: Martensite
A: Austenite
FM: Fresh Martensite
BF: Bainitic Ferrite

[Table 4]

Remarks	Mechanical Properties
Remarks	YS (MPa)	TS (MPa)	El (%)	TS*El (MPa ·%)	HER (%)
Invention Example 1	923	1104	13.5	14925	56
Invention Example 2	963	1135	14.6	16562	61
Invention Example 3	975	1156	14.4	16616	58
Invention Example 4	1018	1210	13.6	16445	57
Invention Example 5	1045	1124	14.7	16521	60
Invention Example 6	962	1152	16.3	18772	54
Invention Example 7	1031	1217	11.4	13910	60
Invention Example 8	951	1133	15.4	17432	61
Invention Example 9	1002	1198	11.6	13851	54
Invention Example 10	934	1119	14.9	16730	58
Invention Example 11	951	1145	14.5	16638	55
Comparative Example 1	801	925	15.5	14298	52
Comparative Example 2	1041	1270	11.8	14949	42
Comparative Example 3	515	820	18.7	15358	38
Comparative Example 4	851	962	14.5	13983	63
Comparative Example 5	1249	1426	8.6	12203	29
Comparative Example 6	861	1005	9.2	9249	57

YS: Yield Strength
TS: Tensile Strength
El: Elongation
TS*El: Product of tensile strength and elongation
HER: Hole Expandability

As illustrated in Tables 1 to 4 above, Invention Examples 1 to 11, which satisfy all of the alloy compositions and manufacturing conditions proposed by the present disclosure, were comprised of a composite structure of martensite and austenite with an area fraction of 75 to 90% as a matrix structure and a second phase of bainitic ferrite with an area fraction of 10 to 25%, thereby securing the target strength and formability.
On the other hand, Comparative Example 1, which does not satisfy the alloy composition system proposed by the present disclosure because boron was not added, was unable to secure the target strength due to excessive generation of bainitic ferrite during the second cooling, and showed a relatively high tendency for elongation due to this low strength. Comparative Example 2 did not obtain sufficient bainitic ferrite because titanium was not added, and thus it was difficult to secure hole expandability required by the present disclosure.
Meanwhile, in Comparative Examples 3 to 5, alloy compositions satisfy the present disclosure, but the manufacturing conditions are outside the present disclosure.
Comparative Examples 3 and 4 did not satisfy Relationship 3 because the second cooling time was too long, and thus the fraction of the secondary phase, bainitic ferrite, was excessive, and thus the tensile strength of 980 MPa or more could not be secured.
In Comparative Example 5, the third cooling end temperature was too low, so the diffusion of carbon was not easy, and accordingly, the fraction of austenite was low, and thus elongation could not be secured, and hole expandability was also inferior due to excessive fresh martensite.
In Comparative Example 6, Si was not added, so the fraction of austenite could not be secured, and thus the elongation was inferior.
FIG. 1 is a graph illustrating the relationship between the boron content and Du that simultaneously satisfies Relationship 1 and Relationship 2. The microstructure intended by the present disclosure may be secured within the solid line connecting A-B-C-D-E-F.
FIG. 2 exhibits photographs of the microstructures of Invention Example 4, Comparative Example 2, and Comparative Example 3, respectively, observed using a scanning electron microscope. As illustrated in (a) of FIG. 2 in Invention Example 4, the matrix structure and the secondary phase intended to be implemented in the present disclosure were appropriately formed in the microstructure. On the other hand, as illustrated in (b) of FIG. 2, it can be confirmed that in Comparative Example 2, the secondary phase, the soft structure, was not sufficiently formed. On the other hand, as illustrated in (c) of FIG. 2, in Comparative Example 3, the secondary phase, the soft structure, was excessively formed.

Claims

A hot-rolled steel sheet comprising:
in weight %, 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%, a remainder of Fe and unavoidable impurities,

wherein a microstructure comprises, in area %, 75-90% of a composite structure of martensite and austenite, and 10-25% of bainitic ferrite, and

wherein the microstructure comprises 3-10% of the austenite.
The hot-rolled steel sheet of claim 1, wherein the hot-rolled steel sheet further comprises niobium (Nb): 0.01 to 0.2%.
The hot-rolled steel sheet of claim 1, wherein an average grain size of the bainitic ferrite is 2.0 µm or more.
The hot-rolled steel sheet of claim 1, wherein an average spacing of the bainitic ferrite is 3 µm or more.
The hot-rolled steel sheet of 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 of 9% or more, and a hole expansion ratio of 45% or more.
A method for manufacturing a hot-rolled steel sheet, comprising:
an operation of reheating a steel slab at a temperature within a temperature range of 1100 to 1350°C, the steel slab comprising, in wt%, 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%, a remainder of Fe and other unavoidable impurities;

an operation of hot-rolling the reheated steel slab and manufacturing the hot-rolled steel sheet;

an operation of first cooling the hot-rolled steel sheet to a temperature of B_S or less at a cooling rate of 50°C/s or more;

an operation of second cooling for ts time (seconds) at a cooling rate of 25°C/s or less to a temperature of (B_S+M_S)/2 or more after the first cooling;

an operation of third cooling at a cooling rate of 30°C/s or less to a temperature within a temperature range of (M_S-20°C) to 200°C after the second cooling; and

an operation of coiling at the temperature within a temperature range of the third cooling,

wherein in the hot rolling, finishing hot rolling is performed at a temperature within a temperature range of 750 to 1150°C so that a value of Du defined by the following Relationship 1 satisfies a range of 2 to 10, and

wherein the first cooling and the second cooling satisfy conditions of the following Relationships 2 to 4, 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,

where FDT represents a finishing delivery temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent weight% contents of elements in parentheses, respectively, $5.0 \times 10^{6} \leq Du \times Bat \times 2.968 \times 10^{10} \leq 2.0 \times 10^{7},$

where Du is the same as the definition in Relationship 1, and the Bat represents 55.845×[B]/(1080.6+45.04×[B]), and [B] represents a weight content (%) of boron (B), $0.75 \leq \exp (- k (T) \times {(ts)}^{2}) \leq 0.9,$

where the k(T) is a value defined by Relationship 4 below, and ts represents a second cooling time, and $k (T) - \frac{20}{Du} \times \exp (- {(\frac{(T 1 + 2 \times T 2) / 3 - 557 + 320 \times [C] + 35 \times [Si] + 90 \times [Mn] + 70 \times [Cr] + 120 \times [Mo] + 160000 \times [Bat]}{112})}^{1.92})$

where Du is the same as the definition in Relationship 1, Bat is the same as the definition in Relationship 2, and also, T1 represents a first cooling end temperature[°C], and T2 represents a second cooling end temperature[°C], and also, [C], [Si], [Mn], [Cr], and [Mo] represent weight% contents of elements in parentheses, respectively.
The method for manufacturing a hot-rolled steel sheet of claim 6, wherein in the hot rolling, a total reduction amount of final 2 passes is 10 to 40%.
The method for manufacturing a hot-rolled steel sheet of claim 6, further comprising an operation of final cooling to room temperature after the coiling.
The method for manufacturing a hot-rolled steel sheet of claim 8, further comprising an operation of pickling and oiling after the final cooling.
The method for manufacturing a hot-rolled steel sheet of claim 9, further comprising an operation of hot-dip zinc plating after the pickling and oiling.