Cold-rolled steel sheet, hot-dip galvanized steel sheet, and method for manufacturing thereof

Abstract

One aspect of the present invention provides a cold-rolled steel sheet and a hot-dip galvanized steel sheet, each having high strength and excellent formability and suitable for automotive materials, and a method for manufacturing same.

Description

Cold rolled steel sheet, hot-dip galvanized steel sheet, and manufacturing method thereof

[0001] The present invention relates to a steel sheet suitable as an automobile material, and more specifically, to a high-strength cold-rolled steel sheet and a hot-dip galvanized steel sheet having excellent formability and a method for manufacturing the same.

[0002] Recently, efforts are being made to secure technology for manufacturing high-strength steel sheets to reduce the weight of automobiles. High-strength steel sheets for cold forming, particularly those with formability, offer significant cost advantages due to increased productivity and offer advantages in terms of final component safety. In particular, steel sheets with high tensile strength (TS) can withstand high loads before fracture, leading to a growing demand for AHSS steels with tensile strengths exceeding 590 MPa.

[0003] Accordingly, various attempts have been made to improve the strength of steel, but it has been discovered that simply improving strength has the disadvantage of reducing formability.

[0004] As a means of improving the formability of steel, methods for increasing elongation are being applied, and in particular, a method that utilizes the TRANSFORMATION INDUCED PLASTICITY (TRIP) phenomenon by introducing a retained austenite phase into the steel is widely used. However, in the case of TRIP steel, it is common to add a large amount of elements such as Si, Mn, and Al to the steel to introduce retained austenite. However, steel containing these elements has a problem in that the elements form oxides on the steel surface during the annealing heat treatment process, which lowers the plating property and causes plating peeling.

[0005] One aspect of the present invention is to provide a high-strength cold-rolled steel sheet and a hot-dip galvanized steel sheet having excellent formability and a method for manufacturing the same.

[0006] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall content of this specification.

[0007] According to one aspect of the present invention, a cold-rolled steel sheet comprises, in wt%, carbon (C): 0.080 to 0.30%, silicon (Si): 0.50 to 2.3%, aluminum (Al): 0.0030 to 2.0% or less, manganese (Mn): 1.0 to 3.5%, phosphorus (P): 0.0400% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), and nitrogen (N): 0.01000% or less (excluding 0%), the remainder being Fe and unavoidable impurities, and comprises, as a microstructure, 5 to 25 area% of retained austenite and 25 to 90% of a hard phase, the remainder being ferrite, and the hard phase comprises at least one or more of bainite and martensite, and the area fraction of the hard phase having an R value of 4.0 or more derived by the following [Relational Expression 1] is The area may exceed 60% of the total area.

[0008] [Relationship 1] R = length of the major axis of the crystal grain / length of the minor axis of the crystal grain

[0009] The cold-rolled steel sheet described above may contain, in weight percent, at least one of niobium (Nb), chromium (Cr), nickel (Ni), and molybdenum (Mo) in an amount of 2.0 wt% or less (excluding 0%).

[0010] The cold-rolled steel sheet described above may additionally contain at least one of titanium (Ti) of 0.050 wt% or less (excluding 0%) and boron (B) of 0.003000 wt% or less (excluding 0%).

[0011] The cold rolled steel sheet described above may have a tensile strength (TS) of 590 MPa or more, and a product of the tensile strength and total elongation (TS x T-El) of 23000 or more.

[0012] According to one aspect of the present invention, a hot-dip galvanized steel sheet may have a zinc-based plating layer formed on at least one surface of the cold-rolled steel sheet described above.

[0013] In addition, the alloyed hot-dip galvanized steel sheet according to one aspect of the present invention can have an alloyed zinc-based plating layer formed on at least one surface of the cold-rolled steel sheet described above.

[0014] According to another aspect of the present invention, there is provided a method for manufacturing a cold-rolled steel sheet, the method comprising the steps of: heating a steel slab comprising, in wt%, carbon (C): 0.080 to 0.30%, silicon (Si): 0.50 to 2.3%, aluminum (Al): 0.0030 to 2.0% or less, manganese (Mn): 1.0 to 3.5%, phosphorus (P): 0.0400% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), and nitrogen (N): 0.01000% or less (excluding 0%), with the remainder being iron and unavoidable impurities; after the heating, performing a finish hot rolling on the steel slab to obtain a hot-rolled steel sheet; after the finish hot rolling, coiling at a temperature of 100°C or more and Ms or less, and then air cooling; The method may include the steps of: cold rolling the hot-rolled steel sheet at a cumulative reduction ratio of 10% or more and 31% or less to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at a temperature of 750°C or more and less than Ae3; cooling the cold-rolled steel sheet to a temperature range of 200 to 500°C at a cooling rate of 10°C / s or more and 150°C / s or less after the continuous annealing; and reheating the cold-rolled steel sheet to 300 to 560°C and then performing an overaging heat treatment for 0.5 minutes or more.

[0015] The method for manufacturing the cold-rolled steel sheet described above may include at least one of niobium (Nb), chromium (Cr), nickel (Ni), and molybdenum (Mo) in an amount of 2.0 wt% or less (excluding 0%).

[0016] The method for manufacturing the cold-rolled steel sheet described above may additionally include at least one of titanium (Ti) of 0.050 wt% or less (excluding 0%) and boron (B) of 0.003000 wt% or less (excluding 0%).

[0017] The above-described heating can be performed in a temperature range of 1100 to 1300°C, and the above-described finishing hot rolling can be performed in a temperature range of 850 to 1000°C.

[0018] The method for manufacturing the above-described hot-dip galvanized steel sheet may additionally include a step of obtaining a hot-dip galvanized steel sheet by immersing the cold-rolled steel sheet in a hot-dip galvanizing bath at 450 to 500°C after the above-described over-aging heat treatment.

[0019] The method for manufacturing the above-described alloyed hot-dip galvanized steel sheet may include a step of alloying heat treating the above-described hot-dip galvanized steel sheet at 480 to 550°C.

[0020] The method for manufacturing the above-described cold-rolled steel sheet may additionally include a step of temper rolling after completing the above-described alloying heat treatment.

[0021] The present invention can provide a high-strength cold-rolled steel sheet and a hot-dip galvanized steel sheet having excellent formability and suitable as an automobile material, and a method for manufacturing the same.

[0022] In addition, the present invention can provide a method for manufacturing a high-strength cold-rolled steel sheet having excellent formability without excessive addition of expensive elements.

[0023] Figure 1 is a diagram briefly showing the R value defined in one embodiment of the present invention.

[0024] Figure 2 is a SEM image showing a tempered martensite structure with fine carbides precipitated after coiling.

[0025] Figure 3 is a SEM image photograph observing the microstructure of Example 4.

[0026] Figure 4 is a SEM image photograph observing the microstructure of Comparative Example 7.

[0027] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0028] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0029] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0030] The inventor of the present invention, in order to provide a steel sheet suitable as an automobile material, conducted in-depth research on a steel sheet that can secure excellent formability by improving not only strength but also ductility, while also having excellent plating properties.

[0031] As a result, it was confirmed that a steel plate having a microstructure advantageous for securing the target properties could be provided by optimizing the alloy composition and manufacturing conditions of the steel plate, and the present invention was completed.

[0032] Hereinafter, a cold rolled steel sheet according to one embodiment of the present invention will be described in detail.

[0033] A cold-rolled steel sheet according to an example of the present invention may include, in weight %, carbon (C): 0.080 to 0.30%, silicon (Si): 0.50 to 2.3%, aluminum (Al): 0.0030 to 2.0% or less, manganese (Mn): 1.0 to 3.5%, phosphorus (P): 0.0400% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), and nitrogen (N): 0.01000% or less (excluding 0%), with the remainder being iron and unavoidable impurities.

[0034] Below, the reason for limiting the alloy composition of the steel plate provided in the present invention as described above is explained in detail.

[0035] Carbon (C): 0.080~0.30%

[0036] Carbon (C) is an element that is effective in strengthening steel, and in the present invention, it is an important element added to stabilize retained austenite and secure strength. In order to obtain the above-mentioned effect, it is preferable to add 0.080% or more, but if the content exceeds 0.30%, the risk of occurrence of cast defects increases. In addition, if the content exceeds 0.30%, arc weldability and laser weldability deteriorate, the risk of occurrence of weld cracks due to low-temperature brittleness increases, and the problem of inferior hole expandability may occur. Therefore, the cold-rolled steel sheet according to an example of the present invention may have the C content of 0.080 to 0.30%, and in another example, the C content may be 0.10 to 0.12%, and in another example, 0.18 to 0.24%.

[0037] Silicon (Si): 0.50~2.3%

[0038] Silicon (Si) is an element that suppresses the precipitation of carbides within ferrite and promotes the diffusion of carbon within ferrite into austenite, thereby contributing to the stabilization of retained austenite. If the Si content is less than 0.50%, there is a concern that the retained austenite phase in the steel will be insufficient, which may lower the ductility. On the other hand, if the content exceeds 2.3%, the weld properties may deteriorate due to the formation of LME cracks, and the surface properties and plating properties of the steel may deteriorate. Therefore, in one example of the present invention, the Si content may be 0.50 to 2.3%. More specifically, the Si content may be 1.2 to 1.6%.

[0039] Aluminum (Al): 0.0030~2.0%

[0040] Aluminum (Al) is an element that combines with oxygen in steel to have a deoxidizing effect. For this purpose, it is desirable to maintain its content at 0.0030% or more. In addition, Al, like Si, contributes to the stabilization of residual austenite by suppressing the formation of carbides within ferrite. If the content of Al exceeds 2.0%, it becomes difficult to manufacture a sound slab through a reaction with the mold flux during casting, and there is also a problem of forming surface oxides, which hinders the plating property. Therefore, in the present invention, the content of Al may be 0.0030 to 2.0%, and in another embodiment, the Al content may be 0.15 to 0.30%.

[0041] Manganese (Mn): 1.0~3.5%

[0042] Manganese (Mn) is an element that is effective in controlling the transformation of ferrite and in forming and stabilizing retained austenite. If the content of Mn is less than 1.0%, a large amount of ferrite transformation occurs, making it difficult to secure the target strength. On the other hand, if the content exceeds 3.5%, the phase transformation is excessively delayed during the annealing heat treatment stage, resulting in the formation of a large amount of martensite structure, making it difficult to secure the intended ductility. Therefore, the content of Mn in the present invention may be limited to 1.0 to 3.5%. In another embodiment, the Mn content may be 1.5 to 2.0%.

[0043] Phosphorus (P): 0.0400% or less (excluding 0%)

[0044] Phosphorus (P) is an impurity that is inevitably contained in steel, so it is advantageous to control its content to the lowest possible, but it is also intentionally added to increase the strength of the steel. However, if the content of P is excessive, the toughness of the steel deteriorates, so in the present invention, the P content is limited to 0.0400% or less. More advantageously, it may be 0.0200% or less, and even more advantageously, 0.0150% or less, and 0% may be excluded considering the level of unavoidable addition.

[0045] Sulfur (S): 0.0100% or less (excluding 0%)

[0046] Sulfur (S) is an impurity that is inevitably contained in steel, so it is advantageous to control its content to the lowest possible. In addition, if the content of S is excessive, there is a concern that the ductility and impact properties of the steel may be inferior. Considering this, the present invention may contain S at 0.0100% or less. More advantageously, it may contain S at 0.00800% or less, and even more advantageously, it may contain S at 0.00500% or less. Considering the level of unavoidable addition, 0% may be excluded.

[0047] Nitrogen (N): 0.01000% or less (excluding 0%)

[0048] Nitrogen (N) is an impurity that is inevitably contained in steel. If its content exceeds 0.01000%, there is a concern that it may deteriorate the performance quality by combining with Al in the steel to form AlN. Therefore, the N may be contained in an amount of 0.01000% or less, more advantageously 0.007000% or less, and even more advantageously 0.005000% or less.

[0049] A cold rolled steel sheet according to one embodiment of the present invention may further include the following components in addition to the components described above.

[0050] A cold-rolled steel sheet according to an example of the present invention may contain at least one of niobium (Nb), chromium (Cr), nickel (Ni), and molybdenum (Mo) in an amount of 2.0 wt% or less (excluding 0%).

[0051] The above Nb, Cr, Ni, and Mo are elements that increase the strength of steel. Although the above elements are advantageous in increasing the strength and hardenability of steel, if the content is excessive, the target strength is exceeded, and there is a problem that the manufacturing cost increases significantly due to the expensive elements. Therefore, the cold-rolled steel sheet according to one embodiment of the present invention may include at least one or more of the above Nb, Cr, Ni, and Mo in a total amount of 0.20% or less.

[0052] In addition, the cold-rolled steel sheet may additionally include at least one of titanium (Ti) of 0.050 wt% or less (excluding 0%) and boron (B) of 0.003000 wt% or less (excluding 0%).

[0053] The above boron (B) segregates in an elemental state at grain boundaries within the steel, thereby enhancing the steel's hardenability. If the B content exceeds 0.003000%, BC precipitates are formed at the grain boundaries, which can actually worsen the steel's hardenability.

[0054] Therefore, when adding the above B, it can be included at 0.003000% or less. However, in order to sufficiently obtain the effect of improving hardening ability by adding the above B, it can be included at 0.0002000% or more, and more advantageously, it can be included at 0.001300% or more.

[0055] Meanwhile, when B is added in combination with the above Ti, if the content of B is 0.0002000% or more, it is advantageous to add Ti in an amount of 0.015% or more. This is to sufficiently obtain the effect of B, and since the effect of B cannot be obtained if the B combines with N in the steel and is lost as BN, it is advantageous to induce precipitation of TiN by adding Ti in an amount exceeding a certain amount. However, if the content of Ti exceeds 0.050% at this time, there is a concern that defects such as nozzle clogging may occur due to the formation of coarse TiN, which may lower the continuous castability.

[0056] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0057] A cold-rolled steel sheet according to one embodiment of the present invention may have a microstructure comprising 5 to 25 area% of retained austenite and 25 to 90 area% of a hard phase, with the remainder being ferrite. In this case, the hard phase may include at least one of bainite and martensite.

[0058] In the present invention, the retained austenite is an important structure for securing ductility. Therefore, the cold-rolled steel sheet according to an example of the present invention may include 5 area% or more of retained austenite to secure excellent ductility. However, if the area fraction of the retained austenite is excessively high, there may be a problem of poor delayed fracture resistance. Therefore, in one embodiment of the present invention, the upper limit of the area fraction of the retained austenite may be 25 area%. In another example, the area fraction of the retained austenite may be 10 to 15 area%, and in another example, 16 to 18 area%.

[0059] As a non-limiting example, the retained austenite may be needle-shaped. Previously, to secure a certain level of retained austenite, a large amount of austenite-stabilizing alloying elements, such as carbon, had to be added to the steel, resulting in increased costs and reduced weldability. However, according to one example of the present invention, when the retained austenite is needle-shaped, the stability of the austenite can be significantly increased even with the same chemical composition. Thus, even without including a large amount of austenite-stabilizing alloying elements, an appropriate level of retained austenite can be secured.

[0060] In addition, the cold-rolled steel sheet according to one embodiment of the present invention may include a hard phase of 25 to 90 area% in its microstructure, and the hard phase may include at least one of bainite and martensite. In this case, the martensite may be a term that includes both fresh martensite and tempered martensite.

[0061] That is, the cold rolled steel sheet according to an example of the present invention can secure high strength by including the hard phase at 25 area% or more. On the other hand, if the hard phase is excessively large, a problem of reduced ductility may occur, so the cold rolled steel sheet according to an example of the present invention can have the hard phase area fraction at 90 area% or less. In another example, the hard phase can be included at 40 to 60 area%, and in another example, it can be included at 65 to 75 area%.

[0062] In particular, as a non-limiting example, most of the bainite and martensite structures included in the above hard phase may be needle-shaped.

[0063] More specifically, in the cold-rolled steel sheet according to one embodiment of the present invention, the area fraction of the hard phase having an R value of 4.0 or more, as derived by the following [Relational Expression 1], may be greater than 60 area% with respect to the entire area fraction of the hard phase. Fig. 1 briefly illustrates the R value defined in one embodiment of the present invention in a diagram.

[0064] [Relationship 1] R = length of the major axis of the crystal grain / length of the minor axis of the crystal grain

[0065] When the area fraction of the hard phase having the R value of 4.0 or more exceeds 60 area%, the overall grain size can be maintained small, thereby suppressing the propagation of cracks within the cold-rolled steel sheet. In addition, since the bainite and martensite structures included in the hard phase form a boundary with the surrounding ferrite and retained austenite, as described above, when the grain size is maintained small, the stability of the retained austenite can be secured by inducing carbon diffusion into austenite in the overaging section. As a result, high ductility and high strength of the steel can be secured at the same time, and therefore, the present invention can set the area fraction of the hard phase having the R value of 4.0 or more to exceed 60 area%. In this way, since the higher the area fraction of the needle-like hard phase, that is, the higher the R value is of 4.0 or more, the more advantageous it is for achieving the purpose of the present invention, the upper limit of the area fraction of the needle-like hard phase is not specifically limited, but considering that some recrystallization may inevitably occur during the annealing heat treatment, the upper limit of the area fraction of the needle-like hard phase may be set to 99 area%. In another embodiment, the area fraction of the hard phase may be 60 to 85 area% or 70 to 75 area%.

[0066] According to an example of the present invention, the microstructure of a cold-rolled steel sheet may be composed of a remainder of ferrite. In this case, the ferrite may include both coarse polygonal ferrite and acicular ferrite. As a non-limiting example, the ferrite may be included in the cold-rolled steel sheet at an area% of 10 to 80% to ensure excellent ductility and strength, but is not necessarily limited thereto.

[0067] Aside from the aforementioned ferrite, retained austenite, bainite, and martensite phases, the microstructure may include precipitates of several tens of nanometers in size. However, since the formation of pearlite phase is very disadvantageous in terms of securing ductility, it is necessary to suppress the formation of pearlite phase as much as possible. More specifically, the area fraction of the pearlite phase may be 3 area% or less.

[0068] The steel plate of the present invention composed of the above-described microstructure has the characteristics of high strength and high formability, and specifically, can have a tensile strength (TS) of 590 MPa or more, a total elongation (T-El) of 20% or more, and a product of tensile strength and total elongation (TS x T-El) of 23000 or more.

[0069] Meanwhile, the steel sheet of the present invention may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the cold-rolled steel sheet, or an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.

[0070] Although not particularly limited, the zinc-based plating layer may be a zinc-plated layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0071] Hereinafter, a method for manufacturing a cold-rolled steel sheet provided in the present invention, which is another aspect of the present invention, will be described in detail.

[0072] Briefly, the present invention can manufacture a desired steel plate through the process of [steel slab heating - hot rolling - coiling and air cooling - cold rolling - annealing - cooling - overaging heat treatment].

[0073] The conditions for each step are explained in detail below.

[0074] [Heating of steel slabs]

[0075] First, a steel slab satisfying all of the aforementioned alloy compositions is prepared, then heated. This process facilitates the subsequent hot rolling process and ensures the desired steel plate properties are achieved.

[0076] The above heating process can be performed at a temperature range of 1100 to 1300°C. If the heating temperature is lower than 1100°C, friction between the steel sheet and the rolling mill increases, which causes a rapid increase in the load applied to the rollers during hot rolling. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required to raise the temperature increase, but the amount of surface scale increases, which can lead to material loss.

[0077] Accordingly, the above heating process can be performed in a temperature range of 1100 to 1300°C. More advantageously, it can be performed at 1170°C or higher, 1180°C or higher, and 1230°C or lower, 1220°C or lower.

[0078] [Hot rolling]

[0079] A hot-rolled steel sheet can be obtained by hot-rolling a steel slab heated as described above, and at this time, finishing hot rolling can be performed in a temperature range of 850 to 1000°C.

[0080] By performing final hot rolling within the temperature range described above, both the rigidity and formability of the steel sheet can be improved. However, if the temperature falls below 850°C, the rolling load increases, resulting in increased shape defects and reduced productivity. Conversely, if the temperature exceeds 1000°C, the surface quality deteriorates due to increased oxides caused by excessive high-temperature operation.

[0081] Therefore, the finishing hot rolling during the above hot rolling can be performed in a temperature range of 850 to 1000°C. More advantageously, it can be performed at 890°C or higher, and can be performed in a temperature range of 950°C or lower, or 930°C or lower.

[0082] [Winding and air cooling]

[0083] The hot-rolled steel sheet manufactured as described above can be coiled, and at this time, it can be done after rapidly cooling to the coiling temperature. Preferably, it can be cooled to a temperature of 100℃ or more and Ms℃ or less at a cooling rate of 20 to 100℃ / s. At this time, the Ms temperature can be checked by inputting the steel grade component using the commercial program J-mat pro. In the present invention, in Jmat pro ver. 13.1 version, considering the hot rolling temperature, the austenitization temperature was set to 1000℃, and the grain size of high-temperature austenite before cooling was input as 30μm, and the derived Ms was used as the standard.

[0084] When cooling to coiling temperature, cooling rates below 20°C / s reduce hot rolling productivity and necessitate the deliberate use of a cooling medium with low cooling capacity during actual production. Conversely, cooling rates exceeding 100°C / s can lead to uneven temperature variations within the steel, resulting in poor shape and excessively high strength.

[0085] After the above cooling, the method for manufacturing a cold-rolled steel sheet according to an example of the present invention can perform a coiling process on the hot-rolled steel sheet and then air-cool it. Accordingly, the method for manufacturing a cold-rolled steel sheet according to an example of the present invention can form a martensite phase in the steel at an appropriate level. At this time, the area fraction of the formed martensite phase can be 80% or more. In this way, the martensite phase formed during the coiling process can be transformed into fine needle-shaped austenite and ferrite during the subsequent annealing process, which can play a beneficial role in improving the ductility and strength of the steel.

[0086] In addition, as a non-limiting example, the coiling can be performed in a temperature range of 100℃ or more and Ms℃ or less. That is, in one embodiment of the present invention, by performing the coiling in a temperature range of 100℃ or more and Ms℃ or less, the martensite structure of the hot-rolled steel sheet can be softened so that the subsequent cold rolling process can be performed while sufficiently securing the martensite structure. In addition, the martensite formed as described above can have a microstructure in which fine carbides are precipitated through a tempering process during the coiling. Fig. 2 is an SEM image photograph showing a martensite structure in which fine carbides are precipitated in this way.

[0087] If the coiling temperature is below 100℃, the hot-rolled steel sheet obtained after coiling has low ductility and poor impact properties, which may cause the sheet to burst or tear during the cold rolling process. On the other hand, if the coiling temperature exceeds Ms℃, the martensite phase is not sufficiently formed, making it impossible to achieve the desired ductility and strength enhancement.

[0088] In another embodiment, the coiling temperature may be (Ms-50)°C to (Ms-10)°C, and in another embodiment, (Ms-150)°C to (Ms-100)°C.

[0089] [Cold rolling]

[0090] Typically, the cold reduction ratio during cold rolling to obtain a cold rolled steel sheet generally exceeds 31%. However, in a non-limiting embodiment of the present invention, cold rolling may be performed at a reduction ratio of 31% or less so that the martensite structure formed during the coiling process is not destroyed during the subsequent annealing process, and fine needle-like austenite and ferrite are formed along the martensite interface. If the cold reduction ratio exceeds 31%, the recrystallization driving force becomes greater than the transformation driving force, which destroys the hot rolled structure, ultimately forming coarse austenite and ferrite, which results in poor formability. More specifically, if the reduction ratio during cold rolling becomes excessively large, a large number of dislocations are introduced into the tempered martensite phase formed during the hot rolling, and a phenomenon in which the austenite reverse transformation, called mechanical stabilization, is delayed during the subsequent annealing heat treatment may occur. As a result, austenite reverse transformation may not occur up to a high temperature range exceeding 700℃ in the subsequent continuous annealing process, and since recrystallization of the acicular martensite microstructure formed during hot rolling occurs preferentially, the acicular structure may not remain after the continuous annealing. Therefore, ultimately, the area fraction of the hard phase having an R value of 4.0 or higher, which is aimed at in the present invention, may not be secured in excess of 60 area%.

[0091] On the other hand, for shape correction after coiling, the method for manufacturing a cold-rolled steel sheet according to an example of the present invention can set the cold rolling reduction ratio to 10% or more. This allows the cold-rolled steel sheet to be easily rolled into a sheet during subsequent annealing. Specifically, the cold rolling reduction ratio can be measured based on an SPM or tension leveler equipment used for shape correction during the cold rolling process or before the cold rolling process.

[0092] As another example, the cold rolling reduction ratio may be 14 to 21%, and as another example, 21 to 25%.

[0093] [Sodun]

[0094] The cold rolled steel sheet obtained as described above can be annealed. As an example, it can be performed using a continuous annealing process, but is not limited thereto, and any known annealing method can be used.

[0095] The purpose of the annealing process is to raise the temperature of the steel sheet above the austenite transformation temperature to form a sufficient fraction of the austenite phase and to cause carbon diffusion into the austenite.

[0096] The present invention can be performed at a temperature range of 750℃ or more and less than Ae3℃ during annealing of the cold rolled steel sheet. In the present invention, the Ae3 was derived by inputting an austenitization temperature of 1000℃ and a grain size of high-temperature austenite before cooling of 30㎛ in consideration of the hot rolling temperature in Jmat pro ver.13.1. If the annealing temperature is less than 750℃, sufficient transformation into austenite does not occur, and thus, after annealing is completed, the martensite and bainite phases cannot be secured at the target level. On the other hand, if the temperature exceeds Ae3℃, coarse austenite is formed, destroying the needle-like structure, and the ferrite microstructure fraction necessary for securing ductility is insufficient, which may deteriorate the ductility.

[0097] As another example, the annealing temperature may be (Ae3-30)°C to (Ae3-10)°C, and as another example, (Ae3-60)°C to (Ae3-40)°C.

[0098] [Cooling and Maintenance]

[0099] The cold-rolled steel sheet annealed as described above can be cooled, and at this time, the cooling can be performed to a temperature range of 200 to 500°C, which is a temperature range where transformation of bainite and martensite phases occurs, and then a holding process can be performed. Preferably, the cold-rolled steel sheet can be cooled to a temperature range of 200 to 500°C at a cooling rate of 10°C / s to 150°C / s, and then reheated to a temperature range of 300 to 560°C, and then subjected to an overaging heat treatment for 0.5 minutes or longer.

[0100] That is, the method for manufacturing a cold-rolled steel sheet according to an example of the present invention can first cool the cold-rolled steel sheet to a temperature range of 200 to 500°C at a cooling rate of 10°C / s or more and 150°C / s or less after the continuous annealing.

[0101] If the cooling end temperature exceeds 500°C, the amount of bainite transformation may be insufficient, which may be detrimental to securing the retained austenite fraction, and the pearlite phase may be formed, which may deteriorate ductility. On the other hand, if cooling is performed below 200°C, the temperature deviation within the sheet increases, and the martensite phase, which has a very fast transformation rate, may be excessively introduced. As another example, the cooling end temperature may be 275 to 325°C, and as another example, 375 to 425°C.

[0102] In addition, when cooling to the above-described temperature range, it is advantageous to perform the cooling process at a critical cooling rate that can minimize the formation of ferrite, pearlite, etc., and for this purpose, a cooling rate of 10°C / s or more can be applied in the present invention. If the cooling rate is less than 10°C / s, even if the target cooling temperature is reached, high-temperature phase transformations such as ferrite and pearlite may be generated during cooling, making it difficult to secure high-strength characteristics. On the other hand, if the cooling rate is too high, overcooling may occur excessively, which may aggravate the temperature deviation in the width direction of the plate, resulting in a poor shape due to a difference in the fraction of the hard phase introduced in the width direction. Therefore, in a manufacturing method according to an example of the present invention, the upper limit of the cooling rate may be 150°C / s. As another example, the cooling rate may be 15 to 30°C / s, and as another example, 35 to 45°C / s.

[0103] The cooling process after the above continuous annealing can utilize a conventional rapid cooling facility, and is not particularly limited. However, as an example, it is to be noted that a rapid cooling facility using mist or hydrogen can be used.

[0104] The cold-rolled steel sheet cooled to the above-described temperature range can be maintained at a specific temperature to induce a target level of bainite phase transformation, and at this time, the cold-rolled steel sheet can be reheated to a temperature slightly lower or higher than the cooling temperature and then maintained. Alternatively, tempered martensite and bainite can be secured simultaneously by cooling to below Ms to introduce martensite and then reheating to a temperature slightly higher than Ms. In consideration of this, in the method for manufacturing a cold-rolled steel sheet according to an example of the present invention, an overaging process can be performed at a temperature range of 300 to 550°C for 0.5 minutes or more. During the overaging process, if the temperature is lower than 300°C or the time is shorter than 0.5 minutes, carbon partitioning into retained austenite due to sufficient carbon diffusion may not occur, and thus a sufficient retained austenite fraction may not be secured. Meanwhile, if the temperature exceeds 550°C, the bainite phase transformation may be slowed down, so that sufficient retained austenite may not be secured, or ultimately, excessive pearlite may be formed, making it impossible to secure sufficient ductility. As another example, the temperature range during the overaging process may be 380 to 400°C, and as another example, it may be 400 to 420°C. In addition, the present invention does not specifically limit the upper limit of the holding time in the overaging process, but as an example, the overaging may be performed for 100 minutes or less.

[0105] The cold rolled steel sheet obtained by completing a series of processes as described above can further undergo the process of [hot-dip galvanizing - alloying heat treatment] as needed.

[0106] First, a galvanized steel sheet having a plating layer on at least one side can be manufactured by subjecting the cold-rolled steel sheet to a plating treatment as described below.

[0107] [Hot-dip galvanizing]

[0108] A steel sheet manufactured through the above-described series of processes can be immersed in a hot-dip galvanizing bath to manufacture a hot-dip galvanized steel sheet.

[0109] At this time, hot-dip galvanizing can be performed under normal conditions, but for example, it can be performed in a temperature range of 450 to 500°C. In addition, there is no particular limitation on the composition of the hot-dip galvanizing bath during the hot-dip galvanizing, and it can be a pure zinc plating bath or a zinc alloy plating bath containing Si, Al, Mg, etc.

[0110] [Alloying heat treatment]

[0111] If necessary, an alloyed hot-dip galvanized steel sheet can be obtained by performing alloying heat treatment on the above hot-dip galvanized steel sheet.

[0112] In the present invention, there are no particular restrictions on the alloying heat treatment process conditions, and any conventional conditions may be used. For example, the alloying heat treatment process may be performed at a temperature range of 480 to 550°C.

[0113] Meanwhile, if necessary, after completing the above alloying heat treatment, further temper rolling treatment may be performed to correct the shape of the steel plate and adjust the yield strength.

[0114] The above temper rolling treatment can be performed at a reduction ratio of less than 1% after cooling the alloyed hot-dip galvanized steel sheet obtained by alloying heat treatment to room temperature.

[0115] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0116] (Example)

[0117] A steel slab having an alloy composition shown in Table 1 below was heated in a temperature range of 1100 to 1300°C, and then each cold-rolled steel sheet was manufactured through the process of [hot rolling - coiling and air cooling - cold rolling - annealing - cooling and maintenance] according to the conditions shown in Tables 2 and 3 below.

[0118] The microstructure, physical properties, etc. of the cold-rolled steel sheet manufactured as described above were measured, and the results are shown in Tables 4 and 5 below.

[0119] At this time, the microstructure of each steel plate was observed through a scanning electron microscope (SEM) and then measured using a point counting method. However, the fraction of the retained austenite phase was measured using XRD.

[0120] The tensile strength (TS), yield strength (YS), and elongation (El) of each steel plate were evaluated through a tensile test in the direction perpendicular to the rolling, and a test specimen specification of 50 mm in gauge length and 25 mm in width was used.

[0121] Steel gradeCSiMnPSAlNCrNiMoNbTiBA0.111.51.60.0070.0020.0030.0090.50.02000.0230.0015B0.1461.631.270.0060.0010.0030.008000000C0.220.45 1.50.0070.0020.750.009000000D0.2231.691.570.0060.0030.0300.009 0.50.020000.002E0.251.510.010.0020.030.008000000F0.20.531.830. 0060.0030.0300.0090.010.11000.0250.0017G0.221.52.30.0060.0030 .0450.0090000.020.0150.002H0.191.42.70.0060.0030.0330.0090.020 .020000J0.282.12.30.0060.0031.030.009000.1000K0.10.351.80.0060 .00300.009000000L0.041.41.550.0070.0020.0030.00800000.020.0015

[0122] ClassificationSteel gradeFDT(℃)Ms(℃)Coiling temperature(℃)Cold reduction ratio(%)Steel gradeExample 1A91025024015.0GAComparative example 1A89925015050.0GAComparative example 2A92025024015.0GAExample 2B93039529024.0CRComparative example 3B91539565042.0CRComparative example 4C91040336529.8CRExample 3D92034425015.0CRComparative example 5D90034451010.0CRComparative example 6D91534415040.0CRExample 4E91037224012.0CRComparative example 7E93037215050.0CRExample 5F90036530510.0CRComparative Example 8G91532730115.0CRComparative Example 9G92032733021.0GIExample 6H90032015010.0CRExample 7J89032118512.5CRComparative Example 10K89540715015.0CRComparative Example 11L90042834015.0CR

[0123] Classification Ae3 (℃) Annealing temperature (℃) Cooling rate (℃) Cooling stop temperature (℃) Reheating temperature (℃) GI FURNACE (℃) GA heattreatment (℃) Overaging time (min) Example 1882780124004004905103 Comparative example 1882780114004004805304 Comparative example 288278044004004705106 Example 288682011350440--5 Comparative example 388682015350440--20 Comparative example 489687020400400--10 Example 384584021400400--4 Comparative example 584584012400400--15 Comparative Example 684584013400400--14 Example 485984011200300--30 Comparative Example 785984012200300--13 Example 580680015360450--62 Comparative Example 882481020610590-21.0 Comparative Example 982481017350410500-0.3 Example 681079016400400--44 Example 793983012210450--16 Comparative Example 1082386013400400--2 Comparative Example 11908834114004004905483

[0124] Classification Steel grade Microstructure F (area %) Residual γ (area %) B (area %) M (area %) Note R 4.0 or more Hard phase ratio (%) Example 1A5 25.64 11.47 5.0 Comparative example 1A4 86.42 817.62 1.0 Comparative example 2A6 44.84 11 Residual pearlite 24.0 Example 2B 3 86.43 124.66 7.5 Comparative example 3B 3 45.44 119.62 1.0 Comparative example 4C 3 13.92 837.14 1.0 Example 3D 4 115.63 49.477.0 Comparative example 5D 4 08.53 21.525.0 Comparative example 6D 3 77.63 25.414.0 Example 4E3220.11235.971.0Comparative Example 7E309.43030.610.5Example 5F3519.5405.572.0Comparative Example 8G481.4213Residual Pearlite 21.5Comparative Example 9G393.81740.248.0Example 6H3117.53813.574.0Example 7J1416.81752.265.0Comparative Example 10K416.83022.210.5Comparative Example 11L294.23135.854.0

[0125] Classification Property YS (MPa) TS (MPa) T-El (%) U-El ​​(%) TS x T-El (% MPa) Example 15 106 104 2.02 3.02 5620 Comparative Example 129 049 031.01 7.01 5190 Comparative Example 23 105 2032.52 1.61 6900 Example 25 857 6535.72 7.42 7311 Comparative Example 34 326 4728.01 9.01 8086 Comparative Example 45 506 9324.53 0.61 6979 Example 37 158 2636.42 6.33 0099 Comparative Example 55 109 4019.51 6.8 18330 Comparative Example 659892021.017.519320Example 469081028.421.523004Comparative Example 742472819.614.314269Example 560677631.012.424045Comparative Example 8585108614.819.416073Comparative Example 9554118917.010.320213Example 6798121521.815.426487Example 7910141018.014.025380Comparative Example 1048075517.012.012835Comparative Example 1148061521.823.013407

[0126] As shown in Tables 1 to 5 above, Examples 1 to 7, which satisfy both the alloy composition and manufacturing conditions proposed in the present invention, all have TS x T-El exceeding 23000 due to the formation of the microstructure proposed in the present invention, and high strength and high formability are secured simultaneously.

[0127] On the other hand, Comparative Examples 1, 6 and 7 had cold reduction ratios exceeding 31%, which resulted in destruction of the initial martensite structure and failure to obtain sufficient needle-like structures, making it difficult to secure excellent strength and ductility.

[0128] In Comparative Example 2, the cooling rate was applied slowly at 4°C / s during cooling after annealing, so pearlite was generated and the residual austenite fraction was insufficient, resulting in poor elongation.

[0129] In Comparative Examples 3 and 5, the coiling temperature was higher than Ms, and the cold reduction ratio exceeded 31%, so a hard needle-like microstructure was not obtained, and the strength and elongation were also inferior.

[0130] In Comparative Example 4, the manufacturing conditions were applied in the manner suggested in this patent, but the Si content was low at 0.45%, so carbon partitioning was limited in the over-aging section, and sufficient retained austenite could not be secured.

[0131] In Comparative Example 8, the cooling temperature of continuous annealing was high at 610℃ and the reheating temperature was high at 590℃, so residual pearlite was formed and the residual austenite fraction was low, resulting in poor ductility.

[0132] In Comparative Example 9, the overaging heat treatment time of continuous annealing was short at 0.3 minutes, so that excessive fresh martensite was introduced during cooling after the overaging heat treatment, and retained austenite was not sufficiently formed, resulting in poor elongation.

[0133] In Comparative Example 10, the annealing temperature of the continuous annealing exceeded Ae3, and recrystallization proceeded after reverse transformation in the annealing heat treatment area, destroying the initial martensite structure and ultimately failing to secure a sufficient needle-shaped hard phase fraction, resulting in inferior elongation.

[0134] In Comparative Example 11, the manufacturing conditions were applied in the manner proposed in the present invention, but the C content was low at 0.04%, so sufficient retained austenite was not secured.

[0135] Figures 3 and 4 are SEM images observing the microstructures of Example 4 and Comparative Example 7, respectively. As shown in Figure 3, it can be confirmed that in Example 4, the area fraction of the needle-like hard phase with an R value of 4.0 or more derived from [Relationship 1] exceeds 60 area%, and thus the overall grain size becomes smaller. On the other hand, as shown in Figure 4, in Comparative Example 7, as a spherical hard phase is formed, it is disadvantageous for securing ductility and strength in the same component system.

Claims

1. Contains, by weight%, carbon (C): 0.080 to 0.30%, silicon (Si): 0.50 to 2.3%, aluminum (Al): 0.0030 to 2.0% or less, manganese (Mn): 1.0 to 3.5%, phosphorus (P): 0.0400% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), and nitrogen (N): 0.01000% or less (excluding 0%), with the remainder being iron and unavoidable impurities. The microstructure contains 5 to 25 area% of retained austenite and 25 to 90% of hard phase, with the remainder being ferrite. The above hard phase includes at least one of bainite and martensite, A cold rolled steel sheet having an area fraction of the hard phase having an R value of 4.0 or more derived by the following [Relational Formula 1] exceeding 60 area% with respect to the area fraction of the entire hard phase. [Relationship 1] R = length of the major axis of the crystal grain / length of the minor axis of the crystal grain 2. In paragraph 1, Cold rolled steel sheet containing at least one of niobium (Nb), chromium (Cr), nickel (Ni), and molybdenum (Mo) in an amount of 2.0 wt% or less (excluding 0%).

3. In paragraph 1, Cold rolled steel sheet additionally containing at least one of titanium (Ti) of 0.050 wt% or less (excluding 0%) and boron (B) of 0.003000 wt% or less (excluding 0%).

4. In at least one of paragraphs 1 to 3, A hot-dip galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the cold-rolled steel sheet.

5. In at least one of paragraphs 1 to 3, An alloyed hot-dip galvanized steel sheet having an alloyed zinc-based plating layer formed on at least one surface of the cold-rolled steel sheet.

6. In at least one of paragraphs 1 to 3, Cold rolled steel sheet having a tensile strength (TS) of 590 MPa or more, and the product of the tensile strength and total elongation (TS x T-El) of 23000 or more.

7. A step of heating a steel slab comprising, by weight%, carbon (C): 0.080 to 0.30%, silicon (Si): 0.50 to 2.3%, aluminum (Al): 0.0030 to 2.0% or less, manganese (Mn): 1.0 to 3.5%, phosphorus (P): 0.0400% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), and nitrogen (N): 0.01000% or less (excluding 0%), with the remainder being iron and unavoidable impurities. After the above heating, a step of final hot rolling the steel slab to obtain a hot rolled steel sheet; After the above finishing hot rolling, a step of coiling at a temperature of 100℃ or higher and Ms or lower, and then air cooling; A step of obtaining a cold rolled steel sheet by cold rolling the hot rolled steel sheet at a cumulative reduction ratio of 10% or more and 31% or less; A step of continuously annealing the above cold rolled steel sheet at a temperature of 750℃ or higher and less than Ae3℃; After the above continuous annealing, a step of cooling the cold rolled steel sheet to a temperature range of 200 to 500°C at a cooling rate of 10°C / s or more and 150°C / s or less; and A method for manufacturing a cold rolled steel sheet, comprising the step of reheating the cold rolled steel sheet to 300 to 560°C and then performing an over-aging heat treatment for 0.5 minutes or longer.

8. In paragraph 7, A method for manufacturing a cold rolled steel sheet, comprising at least one of niobium (Nb), chromium (Cr), nickel (Ni), and molybdenum (Mo) in an amount of 2.0 wt% or less (excluding 0%).

9. In paragraph 7, A method for manufacturing a cold rolled steel sheet additionally containing at least one of titanium (Ti) of 0.050 wt% or less (excluding 0%) and boron (B) of 0.003000 wt% or less (excluding 0%).

10. In paragraph 7, The above heating is performed at a temperature range of 1100 to 1300℃. The above finishing hot rolling is a method for manufacturing cold rolled steel sheets performed in a temperature range of 850 to 1000°C.

11. In paragraph 7, A method for manufacturing a cold rolled steel sheet, wherein the above-mentioned heat treatment step is performed for 100 minutes or less.

12. In at least one of paragraphs 7 to 11, A method for manufacturing a hot-dip galvanized steel sheet, further comprising the step of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath at 450 to 500°C after the above-mentioned overheating heat treatment to obtain a hot-dip galvanized steel sheet.

13. In paragraph 12, A method for manufacturing an alloyed hot-dip galvanized steel sheet, comprising the step of performing alloying heat treatment on the above-mentioned hot-dip galvanized steel sheet at 480 to 550°C.

14. In paragraph 13, A method for manufacturing an alloyed hot-dip galvanized steel sheet, further comprising a step of temper rolling after completing the above alloying heat treatment.

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