Steel plate with excellent bendability and manufacturing method thereof

Abstract

The present invention relates to a steel sheet and a manufacturing method thereof, and more specifically to a steel sheet having excellent bendability and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly to a steel sheet having excellent bendability and a manufacturing method thereof. [Background technology]

[0002] Automobile safety regulations are becoming stricter to ensure passenger safety in the event of a collision. To meet these requirements, automotive steel sheets must be strong or thick. However, due to environmental concerns, automobile manufacturers are continually demanding lighter car bodies to improve fuel efficiency. Therefore, in order to simultaneously ensure the collision stability and light weight of automobiles, it is necessary to increase the strength of steel sheets.

[0003] Conventional methods for strengthening steel include solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening. Among these, precipitation-hardened high-strength steels are produced by either strengthening steel sheets by precipitating carbon and nitrides through the addition of carbon- and nitride-forming elements such as Cu, Nb, Ti, and V, or by refining grains through the suppression of grain growth by fine precipitates. While this technology offers the advantage of easily achieving high strength at low manufacturing costs, it also has the disadvantage of requiring high-temperature annealing to induce sufficient recrystallization and ensure ductility due to the sharp increase in recrystallization temperature caused by fine precipitates. Another drawback of precipitation-hardened steels, which are strengthened by the precipitation of carbon and nitrides in a ferrite matrix, is that it is difficult to obtain high-strength steels of 600 MPa or higher.

[0004] Meanwhile, various types of transformation-strengthened high-strength steel have been developed, including ferrite-martensite dual-phase steel, which contains hard martensite in a ferrite matrix; TRIP (Transformation Induced Plasticity) steel, which utilizes the transformation-induced plasticity of retained austenite; and CP (Complexed Phase) steel, which is composed of ferrite and hard bainite or martensite.

[0005] Recently, there has been a demand for stronger steel sheets for automobiles to improve fuel efficiency and durability, and from the viewpoint of collision safety and passenger protection, high-strength steel sheets with a tensile strength of 780 MPa or more are increasingly being used for vehicle body structures and reinforcement materials.

[0006] However, as strength continues to increase, cracks and wrinkles occur during the press forming process of automotive parts, reaching a limit for the manufacture of complex parts. In particular, if the ductility (El) and bendability of DP steel, the most widely used transformation-strengthened high-strength steel, could be improved, processing defects such as cracks and wrinkles that occur during press forming could be prevented, expanding the application of high-strength steel to complex parts.

[0007] An example of prior art for such high-tensile steel sheets is the invention disclosed in Patent Document 1. The prior art describes a manufacturing method for cold-rolled steel sheets having a composite structure containing ferrite, bainite, martensite, and retained austenite, in which Si is added to the steel and retained austenite is introduced into the final annealed steel sheet through bainite transformation, thereby ensuring the ductility of the steel sheet. However, the addition of Si may result in the occurrence of in-furnace dents during continuous annealing and liquid metal embrittlement during spot welding by customer companies of the plated steel sheet. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 2019-0076258 Summary of the Invention [Problem to be solved by the invention]

[0009] According to one aspect of the present invention, there is provided a steel sheet having excellent bendability and a method for manufacturing the same.

[0010] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the entire content of this specification. [Means for solving the problem]

[0011] According to one embodiment of the present invention, the composition is, in weight percent, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, the balance being iron (Fe) and other unavoidable impurities, The T value defined by the following relational expression 1 is 1648 or more, The microstructure of the steel sheet can contain, in area percentages, 50 to 80% ferrite, 5 to 25% bainite, 10 to 30% fresh martensite, and 5% or less retained austenite. [Equation 1] T=279*[C]+711*[Mn]+474*[Nb]+177*[Ti]-75*[Cr] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element.)

[0012] The steel sheet may have an RT value defined by the following relational expression 2 of 0.01 or more. [Equation 2] RT = [Si] + [Nb] + [Ti] (In the formula, [Si], [Nb], and [Ti] are the weight percentages of each element.)

[0013] The steel sheet may have a tensile strength (TS) of 780 MPa or more and an elongation (El) of 14.0% or more.

[0014] The above steel plate may have a bending angle (°) / thickness (mm) value of 50° / mm or more during a 180° bending test (here, the bending angle (°) means a bending angle at which no cracks occur in the bent portion during a 180° bending test).

[0015] The steel sheet may further include a hot-dip galvanized layer or a hot-dip galvannealed layer on the surface.

[0016] According to another embodiment of the present invention, a step of reheating a hot slab containing, by weight, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, the remainder being iron (Fe) and other inevitable impurities, and the step of reheating the hot slab having a T value of 1648 or more as defined by the following Relation 1: hot rolling the reheated steel slab; a step of coiling the hot-rolled steel sheet and then cooling it; cold rolling the cooled steel sheet; Heating the cold-rolled steel sheet to a T1 temperature of 800 to 850 ° C., cooling it to a T2 temperature of 400 to 600 ° C. at an average cooling rate of 20 ° C. / s or less, and then holding it for 50 seconds or more to perform continuous annealing; and The continuously annealed steel sheet is cooled to room temperature, A method for manufacturing a steel sheet having an R value defined by the following relational expression 3 of 1797 to 1850 can be provided. [Equation 1] T=279*[C]+711*[Mn]+474*[Nb]+177*[Ti]-75*[Cr] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element.) [Equation 3] R=174*[C]+680*[Mn]+370*[Nb]+177*[Ti]-86*[Cr]+0.33*[T1]-0.05*[T2] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element, and T1 and T2 are the heating temperature (°C) and cooling end temperature (°C) during continuous annealing, respectively.)

[0017] The steel slab may have an RT value defined by the following relational expression 2 of 0.01 or more. [Equation 2] RT = [Si] + [Nb] + [Ti] (In the formula, [Si], [Nb], and [Ti] are the weight percentages of each element.)

[0018] The reheating is carried out in the temperature range of 1100 to 1300°C. The hot rolling is carried out at a finish rolling temperature of 800 to 950°C. In the cooling step after the coiling, the coiling is performed at a temperature range of 400 to 700°C, and then cooled to room temperature at an average cooling rate of 0.10°C / s or less. The cold rolling can be carried out at a reduction ratio of 40 to 70%.

[0019] The method may further include pickling the steel sheet before the cold rolling.

[0020] After the continuous annealing step and before the cooling step, the method may further include a step of hot-dip galvanizing the steel sheet at a temperature range of 430 to 490°C.

[0021] After the hot dip galvanizing step, the steel sheet may be further subjected to an alloying heat treatment at a temperature range of 460 to 530° C. before being cooled. [Effects of the Invention]

[0022] According to one aspect of the present invention, a steel sheet having excellent bendability and a method for manufacturing the same can be provided.

[0023] According to one aspect of the present invention, it is possible to provide a steel sheet that can be used for automotive structural members, has excellent formability, and can be used to form complex shapes during press forming, and a manufacturing method thereof. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a photograph of the microstructure of Example 13 according to one embodiment of the present invention, observed with an electron microscope. [Figure 2] 1 is a photograph of the microstructure of Comparative Example 6 according to one embodiment of the present invention, observed with an electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to further explain the present invention in detail to those skilled in the art to which the invention pertains.

[0026] According to one embodiment of the present invention, by optimizing the alloy composition with minimal or no Si addition, it has been confirmed that the occurrence of in-furnace dents and liquid metal embrittlement during spot welding can be reduced and excellent bendability can be obtained while still satisfying the physical properties of conventional DP steel, leading to the completion of the present invention.

[0027] The present invention will be described in detail below.

[0028] The steel composition of the present invention will be described in detail below.

[0029] In the present invention, unless otherwise specified, the percentage representing the content of each element is based on weight.

[0030] A steel plate according to one embodiment of the present invention may contain, by weight, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, the balance being iron (Fe) and other unavoidable impurities.

[0031] Carbon (C): 0.05~0.20% Carbon (C) is a very important element added to strengthen the transformation structure. Carbon (C) increases strength and promotes the formation of martensite in dual-phase steels. As the carbon (C) content increases, the amount of martensite in the steel increases. However, if the carbon (C) content exceeds 0.20%, the strength of martensite increases, but the strength difference with ferrite, which has a low carbon concentration, may increase. This strength difference may lead to increased fracture at the interphase interface when stress is applied, resulting in reduced bendability. According to one embodiment, carbon (C) may be contained in an amount of 0.17% or less. Furthermore, poor weldability may result in welding defects during component processing by customer companies. On the other hand, if the carbon (C) content is less than 0.05%, it may be difficult to achieve the desired level of strength. In one embodiment, carbon (C) may be contained in an amount of 0.07% or more.

[0032] Silicon (Si): 0.10% or less Silicon (Si) is a ferrite-stabilizing element that promotes ferrite transformation and contributes to martensite formation by promoting carbon enrichment in untransformed austenite. It also has excellent solid-solution strengthening properties, effectively increasing the strength of ferrite and reducing interphase hardness differences. This makes it a useful element that ensures strength without reducing the ductility of steel sheets. However, if the silicon (Si) content exceeds 0.10%, it may induce surface scale defects, degrading the coating surface quality, and potentially inducing liquid metal embrittlement during spot welding of the coated material. According to one embodiment, the silicon content may be 0.05% or less.

[0033] Manganese (Mn): 1.0-3.0% Manganese (Mn) refines particles without impairing ductility, completely precipitates S in steel as MnS, and strengthens the steel. It also reduces the critical cooling rate for martensite formation in dual-phase steels, making martensite easier to form. A manganese (Mn) content of less than 1.0% can make it difficult to achieve the strength targeted by the present invention. In one embodiment, Mn can be contained at 1.6% or more. On the other hand, a manganese content exceeding 3.0% can lead to problems with weldability and hot rolling, unstable material properties due to excessive martensite formation, and the formation of intrastructural Mn bands (Mn oxide bands), which can increase the risk of processing cracks and sheet fractures. Furthermore, Mn oxides can dissolve on the surface during annealing, significantly impairing galvanic properties. In one embodiment, Mn can be contained at 2.5% or less.

[0034] Aluminum (sol.Al): 1.00% or less Aluminum (sol.Al) is an element added to refine the grain size and deoxidize steel. Similar to Si, it is a ferrite stabilizing element. It is also an effective component for distributing C in ferrite to austenite to improve martensite hardening. It is also a useful element for improving the ductility of steel sheets by effectively suppressing carbide precipitation in bainite while maintaining the bainite region. However, if its content exceeds 1.00%, while it is beneficial for increasing strength through its grain refinement effect, it can increase the likelihood of surface defects in plated steel sheets due to excessive inclusion formation during continuous steel casting operations, as well as increase production costs. According to one embodiment of the present invention, the aluminum content may be 0.50% or less.

[0035] Chromium (Cr): 0.1 to 1.0% Chromium (Cr) is an element that can be added to improve the hardenability of steel and ensure high strength. It also plays a very important role in the formation of martensite, minimizing the decrease in elongation rate while increasing strength, which is advantageous for producing dual-phase steel with high ductility. In particular, Cr is used in the hot rolling process. 23 Cr forms Cr-based carbides such as C6. These carbides partially dissolve during annealing, while the remaining carbides remain undissolved. This allows the amount of solute C in martensite after cooling to be controlled below an appropriate level, thereby suppressing the occurrence of yield point elongation (YP-El). This makes Cr an advantageous element for producing dual-phase steels with low yield ratios. Therefore, in the present invention, the Cr content can be limited to 0.1% or more. According to one embodiment of the present invention, Cr can be contained at 0.8% or less. However, if the Cr content exceeds 1.0%, not only will the above-mentioned effects saturate, but there will also be problems with excessive increases in hot rolling strength, resulting in poor cold rolling performance. Furthermore, the increased proportion of Cr-based carbides can lead to coarsening, which can coarsen the size of martensite after annealing, resulting in reduced elongation. According to one embodiment, Cr can be contained at 0.2% or more.

[0036] Niobium (Nb): 0.05% or less Niobium (Nb) is an element that segregates at austenite grain boundaries, inhibits coarsening of austenite grains during annealing, and forms fine carbides, thereby contributing to increased strength. However, if the niobium (Nb) content exceeds 0.05%, coarse carbides are precipitated, reducing the carbon content in the steel, which can result in reduced strength and elongation, and also increases manufacturing costs. According to one embodiment of the present invention, the niobium (Nb) content may be 0.04% or less.

[0037] Titanium (Ti): 0.05% or less Titanium (Ti) is a fine carbide-forming element that contributes to ensuring yield strength and tensile strength. Titanium (Ti) also forms nitrides, causing N in steel to precipitate as TiN, suppressing AlN precipitation, and thus reducing the risk of cracks during continuous casting. However, if the titanium (Ti) content exceeds 0.05%, coarse carbides are precipitated, reducing the carbon content in the steel and resulting in reduced strength and elongation, which may lead to nozzle clogging during continuous casting. In one embodiment of the present invention, the titanium (Ti) content may be 0.03% or less.

[0038] Phosphorus (P): 0.100% or less Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect, improving in-plane anisotropy and ensuring strength without significantly impairing formability. However, excessive addition of P can significantly increase the likelihood of brittle fracture, potentially causing slab breakage during hot rolling and acting as an element that impairs the properties of the coating surface. Therefore, in the present invention, its content can be limited to 0.100% or less. However, 0% is excluded in consideration of the level of P that is inevitably added during the manufacturing process.

[0039] Sulfur (S): 0.010% or less Sulfur (S) is an unavoidably added impurity element in steel, and since it reduces ductility and weldability, it is important to keep its content as low as possible. In particular, since it increases the possibility of red shortness, it is preferable to control its content to 0.010% or less. However, 0% is excluded in consideration of the level of S that is inevitably added during the manufacturing process.

[0040] Nitrogen (N): 0.010% or less Nitrogen (N) is an element that effectively stabilizes austenite, but if its content exceeds 0.010%, it can cause problems such as a sharp increase in steel refining costs. Furthermore, since the risk of cracks occurring due to the formation of AlN during continuous casting increases significantly, it is preferable to limit its upper limit to 0.010%. However, 0% is excluded in consideration of the level of N that is inevitably added during the manufacturing process.

[0041] In addition to the above-described composition, the steel material of the present invention may contain the remaining iron (Fe) and inevitable impurities. The inevitable impurities cannot be excluded because they may be unintentionally mixed in during the normal manufacturing process. Since such impurities are known to anyone skilled in the field of normal steel manufacturing, the details of all of them will not be specifically mentioned in this specification.

[0042] The steel sheet according to an embodiment of the present invention may have a T value defined by the following Relation 1 of 1648 or more. [Equation 1] T=279*[C]+711*[Mn]+474*[Nb]+177*[Ti]-75*[Cr] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element.)

[0043] The above-mentioned relational expression 1 quantitatively expresses the contribution of the additive elements in the steel sheet to the strength and bendability of the steel sheet. In the present invention, the contents of the representative components of the steel sheet, namely, C, Mn, Nb, Ti, and Cr, can be limited so as to satisfy the following relational expression 1.

[0044] Specifically, C and Mn have the effect of increasing the strength of steel sheets through the solid solution strengthening effect of steel. However, each element contributes to the strength of steel sheets in different ways, and the constant value multiplied by each element in the relevant relationship represents the relative contribution of each element to strength. Furthermore, Nb and Ti have a precipitation strengthening effect, contributing to improved strength. They precipitate in the ferrite matrix of DP steel to strengthen the ferrite, and they reduce the interphase hardness difference between ferrite and martensite, thereby improving the bendability of the steel sheet. Therefore, the constant value multiplied by each element is a positive value. On the other hand, Cr has the least solid solution strengthening effect among the above elements and significantly increases hardenability. Therefore, adding too much Cr can result in the formation of a large amount of martensite, which can reduce bendability, and therefore the constant value can be a negative value.

[0045] If the T value defined by Relational Formula 1 is less than 1648, the strength and bendability of the steel sheet targeted by the present invention may not be ensured. According to one embodiment of the present invention, the T value defined by Relational Formula 1 may be 1650 or more. Although not particularly limited in the present invention, when an excessive amount of an element is added, there may be a problem that the strength increases excessively and the elongation decreases below the targeted level. Taking this into consideration, the upper limit of the T value may be effectively limited to 1800 or less.

[0046] The steel sheet according to one embodiment of the present invention may have an RT value defined by the following Relation 2 of 0.01 or more. [Equation 2] RT = [Si] + [Nb] + [Ti] (In the formula, [Si], [Nb], and [Ti] are the weight percentages of each element.)

[0047] In a steel sheet according to an embodiment of the present invention, the Si content is minimized because adding a large amount of Si can cause problems such as dent defects in the steel sheet in the annealing furnace, phosphate treatment properties of cold-rolled steel sheets, and liquid metal embrittlement and deterioration of plating properties of plated steel sheets. However, reducing the amount of Si can cause deterioration of mechanical properties, and to overcome this, Nb and Ti are added to prevent deterioration of mechanical properties due to carbide precipitation.

[0048] If the RT value defined by the above Relational Formula 2 is less than 0.01, the desired level of physical properties cannot be ensured in the present invention. According to one embodiment of the present invention, the RT value defined by the Relational Formula 2 may be 0.02 or more. Although not particularly limited in the present invention, the upper limit of the RT value may be limited to 0.2%, which is the same as the maximum addition of each component within its limited range.

[0049] The steel microstructure of the present invention will now be described in detail.

[0050] In the present invention, unless otherwise specified, the percentage representing the fraction of the microstructure is based on the area.

[0051] The microstructure of the steel sheet according to one embodiment of the present invention may contain, in area percentages, 50 to 80% ferrite, 5 to 25% bainite, 10 to 30% fresh martensite, and 5% or less retained austenite.

[0052] The ferrite is a soft structure that contributes to the ductility of the steel sheet. If the area fraction of the ferrite relative to the entire microstructure contained in the steel sheet is less than 50%, it may be difficult to ensure the target bendability. On the other hand, if the area fraction exceeds 80%, it may be difficult to ensure the strength level desired in the present invention.

[0053] Bainite is a phase with intermediate hardness between ferrite and martensite, and can be appropriately contained. If the bainite area fraction is less than 5%, ferrite and martensite are dominant, which may result in poor bendability. On the other hand, if the bainite area fraction exceeds 25%, there may be a problem of reduced strength.

[0054] The fresh martensite is a phase that contributes to increasing strength, and if its area fraction is less than 10%, the target strength cannot be ensured. On the other hand, if its area fraction exceeds 30%, the bainite area fraction is relatively reduced, which may cause a problem of poor bendability.

[0055] The above-mentioned retained austenite can be generated in a small amount of 5% or less during the final cooling process, and in the case of a plated steel sheet with a high area fraction of retained austenite, it tends to be vulnerable to liquid metal embrittlement during spot welding in the assembly of automobile parts, so it is preferable to control the area fraction of retained austenite in the steel sheet to 5% or less.

[0056] According to an embodiment of the present invention, the microstructure fraction can be determined by analyzing the matrix structure at a 1 / 4 point in the sheet thickness of a continuously annealed steel sheet, and specifically, the area fraction of the microstructure can be measured using an FE-SEM, an image analyzer, and XRD.

[0057] The method for producing a steel sheet according to the present invention will be described in detail below.

[0058] The steel sheet according to one embodiment of the present invention can be produced by reheating a steel slab having the above-mentioned alloy composition, hot rolling, coiling, cooling, cold rolling, continuous annealing, and cooling.

[0059] reheating A steel slab satisfying the alloy composition of the present invention can be reheated in the temperature range of 1100 to 1300°C.

[0060] Reheating can be performed to smoothly carry out the subsequent rolling process and to sufficiently obtain the target physical properties of the steel sheet. The present invention is not particularly limited to such reheating conditions, and ordinary reheating conditions are possible. However, the preferred reheating temperature range may be 1100 to 1300°C.

[0061] If the reheating temperature is less than 1100°C, the re-solubility of precipitated elements such as Nb and Ti may decrease, reducing the effect of adding the elements.On the other hand, if the reheating temperature exceeds 1300°C, problems may arise such as an increase in process costs and a deterioration in the surface quality of the steel sheet due to the generation of a large amount of hot-rolled oxides.

[0062] hot rolling The reheated steel slab can be hot rolled at a finish rolling temperature of 800 to 950°C.

[0063] In the present invention, the reheated steel slab can be hot-rolled at a normal hot-rolling temperature. Hot rolling can produce a hot-rolled steel sheet in which carbides, which serve as austenite nucleation sites, are finely dispersed. Uniform dispersion of fine carbides during this hot-rolling process has the effect of finely dispersing austenite generated as the carbides are dissolved during annealing. As a result, martensite generated during cooling after annealing can be finely and uniformly dispersed, which can contribute to improving the strength and elongation of the final steel sheet.

[0064] If the finish rolling temperature during hot rolling is less than 800°C, the hot rolling temperature is low, which can cause problems such as an increased hot rolling load, while if the temperature exceeds 950°C, the grains become coarse, reducing the strength of the steel sheet, and the hot rolling oxides in the surface layer increase, which can cause problems such as a deterioration in the surface quality of the steel sheet.

[0065] Winding and cooling The hot-rolled steel sheet is coiled in the temperature range of 400 to 700°C, and then cooled to room temperature at an average cooling rate of 0.10°C / s or less.

[0066] If the coiling temperature is less than 400°C, a large amount of low-temperature structures such as martensite or bainite is generated, significantly increasing the strength of the hot-rolled steel sheet and creating a rolling load during cold rolling. On the other hand, if the temperature exceeds 700°C, the hot-rolled microstructure becomes coarse, reducing the strength of the final annealed steel sheet, and the increase in oxides on the steel sheet surface may deteriorate the surface quality and galvanizability of the steel sheet.

[0067] Furthermore, if the average cooling rate after coiling exceeds 0.10°C / s, the cold rolling load increases due to the generation of low-temperature structures, and the shape of the hot-rolled steel sheet deteriorates due to the high cooling rate, which may result in sheet breakage during cold rolling.

[0068] cold rolling The cooled steel sheet can be cold rolled at a reduction ratio of 40 to 70%.

[0069] If the reduction rate during cold rolling is less than 40%, it may be difficult to obtain the target thickness and also difficult to correct the shape of the steel sheet. On the other hand, if it exceeds 70%, there is a high possibility that cracks will occur at the edge of the steel sheet, which may cause a cold rolling load. Therefore, in the present invention, it is preferable to limit the reduction rate to 40 to 70%. As one embodiment of the present invention, a pickling process of pickling the steel sheet before cold rolling may be further included.

[0070] continuous annealing The cold-rolled steel sheet can be heated to a T1 temperature of 800 to 850°C, cooled to a T2 temperature of 400 to 600°C at an average cooling rate of 20°C / s or less, and then held for 50 seconds or more for continuous annealing.

[0071] In the present invention, continuous annealing can be performed to simultaneously recrystallize and form ferrite and austenite, and distribute carbon.

[0072] If the heating temperature (T1) during continuous annealing is below 800°C, not only will sufficient recrystallization not occur, but it will also be difficult to form sufficient intercritical austenite, making it impossible to ensure the desired martensite and bainite fractions after annealing. On the other hand, if the temperature exceeds 850°C, productivity will decrease and excessive austenite will be formed, resulting in a significant increase in the fractions of bainite and martensite after cooling, which may increase yield strength and reduce ductility. Furthermore, surface segregation of elements that reduce the wettability of hot-dip galvanizing, such as Si, Mn, and B, may become severe, resulting in reduced coating surface quality. In consideration of this, in the present invention, it is preferable to limit the heating temperature during continuous annealing to 800-850°C. Within this temperature range, the fractions of intercritical austenite and ferrite in the steel sheet are determined, and the strength of the final steel sheet varies depending on these fractions. Generally, the strength of the final annealed steel sheet tends to increase as the fraction of intercritical austenite increases. However, subsequent processes may also affect the final microstructure, resulting in changes in the physical properties of the steel sheet.

[0073] The austenite in the heated steel sheet can transform into different fractions of ferrite depending on the cooling end temperature T2. If the cooling end temperature T2 exceeds 600°C during continuous annealing, there may be a problem that ferrite transformation occurs excessively during heat treatment, resulting in a decrease in strength. On the other hand, if the temperature is less than 400°C, there may be a problem that bainite is excessively generated during the process of maintaining the temperature for 50 seconds or more, resulting in a decrease in martensite formation and a decrease in strength.

[0074] Thereafter, the annealed steel sheet can be cooled to room temperature. When cooling to room temperature, the cooling conditions are not particularly limited, but air cooling can be used as an example.

[0075] The steel sheet according to one embodiment of the present invention may have an R value defined by the following relational expression 3 of 1797 to 1850 during continuous annealing. [Equation 3] R=174*[C]+680*[Mn]+370*[Nb]+177*[Ti]-86*[Cr]+0.33*[T1]-0.05*[T2] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element, and T1 and T2 are the heating temperature (°C) and the cooling end temperature (°C) during continuous annealing, respectively.)

[0076] In the present invention, the elements and annealing conditions are specified to simultaneously achieve the target strength and bendability of the steel sheet during continuous annealing, and the contents of C, Si, Mn, and Al elements and the T1 and T2 conditions are optimized.

[0077] The T1 temperature refers to the heating temperature during the continuous annealing process. This temperature determines the fraction of intercritical austenite and ferrite in the steel sheet, which in turn affects the strength of the final annealed steel sheet. Generally, the strength of the final annealed steel sheet tends to increase as the fraction of intercritical austenite increases. However, subsequent processes can also affect the final microstructure and change the physical properties of the steel sheet, making it difficult to describe the effect of the annealing temperature alone. During the subsequent cooling process, intercritical austenite can undergo additional transformation with different fractions of ferrite depending on the cooling end temperature, T2, making it an important factor influencing the physical properties of steel sheet. Furthermore, the fractions of bainite, retained austenite, and martensite in the final annealed structure can vary depending on the T2 temperature.

[0078] If the T2 temperature is higher than the bainite transformation start temperature or lower than the martensite transformation temperature, bainite cannot be introduced into the steel sheet structure. Therefore, the T2 temperature must be set between the bainite transformation start temperature and the martensite transformation start temperature. The T1 and T2 temperatures, along with the steel sheet's chemical composition, affect the microstructure of the final annealed steel sheet, which ultimately affects the steel sheet's physical properties. To ensure the target physical properties, the optimized Relational Formula 3 must be satisfied. This allows for the production of high-tensile steel sheets with the target strength and excellent bendability, even when the Si content is minimized.

[0079] As described above, the final properties of a steel sheet are affected by its chemical composition and the temperature and time of each important heat treatment process. Therefore, when the following relational expression is satisfied, a high-tensile steel sheet with an optimal combination of physical properties and excellent bendability can be manufactured. On the other hand, if the R-value defined by the following relational expression 3 is less than 1797, the strength of the steel sheet may not be achieved. On the other hand, to ensure the target bendability, the upper limit of the R-value can be limited to 1850.

[0080] Hot-dip galvanizing According to one embodiment of the present invention, the continuously annealed steel sheet can be hot-dip galvanized in a temperature range of 430 to 490°C.

[0081] The steel sheet manufactured according to the present invention can be galvanized by a galvanizing method in which the steel sheet is immersed in a hot-dip galvanizing bath. The hot-dip galvanizing conditions are not particularly limited in the present invention, and hot-dip galvanizing can be performed under common conditions applicable in the same technical field. Through hot-dip galvanizing, the steel sheet according to one embodiment of the present invention can include a hot-dip galvanized layer on its surface. Furthermore, if necessary, the steel sheet can be subjected to an alloying heat treatment after the hot-dip galvanizing step. In one embodiment, the hot-dip galvanized steel sheet can be subjected to an alloying heat treatment at a temperature range of 460 to 530°C and then cooled to room temperature. The steel sheet can also include an alloying hot-dip galvanized layer on its surface through the alloying heat treatment.

[0082] The steel sheet of the present invention manufactured in this manner has a tensile strength (TS) of 780 MPa or more, an elongation (El) of 14.0% or more, and a bending angle (°) / thickness (mm) value of 50° / mm or more during a 180° bending test (here, the bending angle (°) means a bending angle at which no cracks occur in the bent portion during the 180° bending test), thereby ensuring excellent properties in strength and bendability. [Example]

[0083] The present invention will be described in more detail below with reference to 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.

[0084] (Example) Steel slabs having the compositions shown in Table 1 below were produced, then reheated under the conditions shown in Table 2 below, and finish hot rolled. The hot-rolled steel sheets were coiled under the conditions shown in Table 2 below, and cooled to room temperature to produce steel sheets. The steel sheets were then pickled, cold-rolled at a reduction of 50%, heated to temperature T1 as shown in Table 2 below, cooled to temperature T2, and held for 50 seconds or more. After that, they were hot-dipped at a hot-dip galvanizing temperature of 460°C, and finally cooled to room temperature.

[0085] [Table 1] [Equation 1] T=279*[C]+711*[Mn]+474*[Nb]+177*[Ti]-75*[Cr] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element.) [Equation 2] RT = [Si] + [Nb] + [Ti] (In the formula, [Si], [Nb], and [Ti] are the weight percentages of each element.)

[0086] [Table 2] [Equation 3] R=174*[C]+680*[Mn]+370*[Nb]+177*[Ti]-86*[Cr]+0.33*[T1]-0.05*[T2] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element, and T1 and T2 are the heating temperature (°C) and cooling end temperature (°C) during continuous annealing, respectively.)

[0087] The mechanical properties of the steel sheets manufactured above were measured, and the results are shown in Table 3 below. Tensile tests were performed on each specimen in the L direction according to ASTM standards to evaluate room temperature tensile properties. In particular, bendability was measured by dividing the bend radius at which cracks did not occur in the bent portion by the thickness (mm) of the specimen in a 180° bending experiment. The term "bend portion" refers to the portion of the steel sheet where a bend angle is applied, or the portion where bending is typically applied. The microstructure fraction was determined by analyzing the matrix structure at a quarter-thickness point of the continuously annealed steel sheet. Specifically, the fractions of ferrite (F), bainite (B), fresh martensite (M), and retained austenite (A) were measured using FE-SEM, an image analyzer, and XRD.

[0088] [Table 3]

[0089] As shown in Table 3, in the case of the inventive example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructural characteristics proposed in the present invention were satisfied and the physical properties targeted in the present invention were also secured. Figure 1 is a photograph of the microstructure of inventive example 13 according to one embodiment of the present invention, observed with an electron microscope.

[0090] On the other hand, Comparative Examples 1 and 2 are examples in which the T1 temperature during continuous annealing did not satisfy the conditions of the present invention, and also did not satisfy Relational Expression 3. As a result, the elongation and bendability did not reach the desired levels.

[0091] In Comparative Example 3, Relational Expression 3 was satisfied, but the T1 temperature did not satisfy the conditions of the present invention, and the elongation rate was deteriorated.

[0092] In Comparative Example 4, the T1 and T2 temperatures were satisfied during continuous annealing, but the condition of Relational Formula 3 proposed in the present invention was not satisfied, and bainite was formed in an excessive amount compared to the area fraction targeted in the present invention, and as a result, the targeted elongation and bendability properties could not be secured.

[0093] In Comparative Examples 5 and 6, Relational Expressions 1 and 2 do not satisfy the conditions of the present invention, and martensite was formed in an excessive amount compared to the area fraction targeted in the present invention, resulting in deterioration of bendability. Figure 2 is an electron microscope photograph of the microstructure of Comparative Example 6 according to an embodiment of the present invention, and it can be seen that excessive martensite was formed.

[0094] In Comparative Examples 7 and 8, the alloy composition conditions satisfied the conditions of the present invention, but the T1 temperature was not reached, and ferrite was formed in excess of the area fraction targeted in the present invention, and bainite was insufficient, resulting in a decrease in elongation.

[0095] In Comparative Example 9, T2 did not meet the requirements of the present invention, and bainite was formed in excess of the level suggested, and martensite was reduced, making it impossible to ensure the desired strength.

[0096] Although the present invention has been described in detail with reference to the above examples, other embodiments are possible, and the spirit and scope of the claims set forth below should not be limited to the examples.

Claims

1. In weight percent, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol. Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.0100% or less, nitrogen (N): 0.010% or less, the balance being iron (Fe) and other unavoidable impurities, The T value defined by the following relational expression 1 is 1648 or more, The microstructure of the steel plate contains, in area percentages, 50 to 80% ferrite, 5 to 25% bainite, 10 to 30% fresh martensite, and 5% or less retained austenite. [Relationship 1] T=279*[C]+711*[Mn]+474*[Nb]+177*[Ti]-75*[Cr] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight percentages of each element.)

2. The steel sheet according to claim 1, wherein the steel sheet has an RT value defined by the following relational expression 2 of 0.01 or more. [Relationship 2] RT=[Si]+[Nb]+[Ti] (In the formula, [Si], [Nb], and [Ti] are the weight percentages of each element.)

3. The steel sheet according to claim 1, wherein the steel sheet has a tensile strength (TS) of 780 MPa or more and an elongation (El) of 14.0% or more.

4. 2. The steel sheet according to claim 1, wherein the steel sheet has a value of bending angle (°) / thickness (mm) of 50° / mm or more in a 180° bending test (here, the bending angle (°) means a bending angle at which no cracks occur in the bent portion in the 180° bending test).

5. The steel sheet according to claim 1 , further comprising a hot-dip galvanized layer or a galvannealed layer on a surface thereof.

6. a step of reheating a steel slab containing, by weight, carbon (C): 0.05 to 0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0 to 3.0%, aluminum (sol. Al): 1.00% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, the balance being iron (Fe) and other inevitable impurities, and having a T value of 1648 or more as defined by the following relational expression 1; hot rolling the reheated steel slab; coiling the hot-rolled steel sheet and then cooling it; cold rolling the cooled steel sheet; The cold-rolled steel sheet is heated to a T1 temperature of 800 to 850 ° C., cooled to a T2 temperature of 400 to 600 ° C. at an average cooling rate of 20 ° C. / s or less, and then held for 50 seconds or more for continuous annealing; and cooling the continuously annealed steel sheet to room temperature; A method for manufacturing a steel sheet, wherein the R value defined by the following relational expression 3 is 1797 to 1850. [Relationship 1] T=279*[C]+711*[Mn]+474*[Nb]+177*[Ti]-75*[Cr] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight percentages of each element.) [Relationship 3] R=174*[C]+680*[Mn]+370*[Nb]+177*[Ti]-86*[Cr]+0.33*[T1]-0.05*[T2] (In the formula, [C], [Mn], [Nb], [Ti], and [Cr] are the weight percentages of each element, and T1 and T2 are the heating temperature (°C) and cooling end temperature (°C) during continuous annealing, respectively.)

7. The method for producing a steel plate according to claim 6, wherein the steel slab has an RT value defined by the following relational expression 2 of 0.01 or more. [Relationship 2] RT=[Si]+[Nb]+[Ti] (In the formula, [Si], [Nb], and [Ti] are the weight percentages of each element.)

8. The reheating is carried out in a temperature range of 1100 to 1300°C, The hot rolling is carried out at a finish rolling temperature of 800 to 950°C, In the step of cooling after winding, the sheet is wound at a temperature in the range of 400 to 700°C, and then cooled to room temperature at an average cooling rate of 0.10°C / s or less, The method for producing a steel sheet according to claim 6, wherein the cold rolling is performed at a reduction ratio of 40 to 70%.

9. The method for manufacturing a steel sheet according to claim 6, further comprising the step of pickling the steel sheet before the cold rolling step.

10. The method for producing a steel sheet according to claim 6, further comprising the step of hot-dip galvanizing the steel sheet at a temperature in the range of 430 to 490°C after the continuous annealing step and before the cooling step.

11. The method for manufacturing a steel sheet according to claim 10, further comprising the step of subjecting the steel sheet before cooling to an alloying heat treatment in a temperature range of 460 to 530°C after the hot dip galvanizing step.

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