Steel sheet and manufacturing method thereof

Abstract

The present invention provides: a steel sheet suitable for vehicle exterior panel materials and specifically having excellent low-temperature bake hardening properties and room-temperature aging resistance; and a manufacturing method thereof.

Description

Steel plate and method for manufacturing the same

[0001] The present invention relates to a steel sheet suitable as a material for automobiles, and more specifically, to a steel sheet suitable as a material for exterior panels and a method for manufacturing the same.

[0002] Recently, demand for lightweight automobiles has been increasing worldwide to meet stricter fuel efficiency regulations for automobile production. To achieve this, steel sheets are required to be both stronger and thinner. Among these, materials for automotive exterior panels require a certain level of bake hardenability (BH).

[0003] Bake hardening in steel sheets refers to the phenomenon of increased yield strength due to the fixation of dissolved carbon and dissolved nitrogen at dislocations formed during processing, resulting from the formation of a Cottrell atmosphere during paint baking. Steel sheets with excellent bake hardening properties facilitate the forming process prior to paint baking, and the final product exhibits enhanced dent resistance. For this reason, steel sheets with excellent bake hardening properties are considered ideal materials for automotive exterior plating.

[0004] The conventionally performed pre-hardening process involves adding a few percent of pre-strain to the steel plate, painting it, and then heat treating it at 170°C for several tens of minutes. The amount of pre-hardening under these conditions is generally secured to be approximately 30 MPa based on the yield strength.

[0005] These automotive exterior materials also require a certain level of aging resistance to ensure they can withstand aging for a certain period of time.

[0006] In general, as the bake hardenability of steel sheets increases, the aging resistance of the steel sheets tends to decrease. Therefore, even if bake hardenability of steel sheets is secured, aging occurs over a certain period of time, increasing the possibility of surface defects and other issues during component processing. For this reason, automotive exterior panel materials must possess both an appropriate level of bake hardenability and aging resistance.

[0007] Meanwhile, some automobile manufacturers are considering lowering the temperature during baking after painting of steel plates to reduce CO2 emissions and reduce costs during automobile manufacturing.

[0008] When the temperature is lowered during the hardening of steel plates, the amount of carbon and nitrogen adsorbed and the time required for adhering are delayed, which generally results in a decrease in hardening properties.

[0009] Nonetheless, to meet increasingly stringent regulations, it is essential to ensure a high level of bake hardenability at lower temperatures during bake hardening. This fundamentally requires high bake hardenability at high temperatures. However, this also reduces the aging resistance of steel sheets, increasing the likelihood of surface defects during machining.

[0010] Patent Document 1 proposes a technique for improving the bake hardenability of steel plates by adding tin (Sn) to the alloy composition of steel. However, it does not offer a fundamental solution to the problem of deterioration in aging resistance due to this improved bake hardenability.

[0011] Therefore, there is a need to develop a steel sheet suitable as a material for the exterior of automobiles by securing a certain level of hardenability, especially hardenability at low temperatures, and satisfying the required level of aging resistance.

[0012] (Patent Document 1) Japanese Patent Publication No. 1994-306531

[0013] According to one aspect of the present invention, it is an object to provide a steel sheet suitable as an automobile exterior panel material, specifically a steel sheet having excellent low-temperature bake hardening properties and excellent room-temperature aging properties, and a method for manufacturing the same.

[0014] In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0015] According to one aspect of the present invention, a steel sheet is provided, which contains, in wt%, carbon (C): 0.0010 to 0.0300%, manganese (Mn): 1.500 to 2.500%, chromium (Cr): 1.000% or less (excluding 0%), phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), aluminum (Al): 0.010 to 0.060%, the remainder being Fe and unavoidable impurities.

[0016] In one embodiment of the present invention, the steel plate may include a ferrite phase with an area fraction of 95% or more and a secondary phase as the remainder in the microstructure, and may satisfy the following relational expression 1.

[0017] [Relationship 1]

[0018] 0.01 ≤ D second / D grain ≤ 0.15

[0019] (In relation 1, D grain refers to the average grain size (㎛) of ferrite, and D second refers to the radius (㎛) based on the equivalent diameter of the secondary phase.)

[0020] Steel plates having such alloy composition and microstructure can have excellent low-temperature bake hardening properties and room-temperature aging resistance.

[0021] A steel plate according to one embodiment of the present invention can have a hardening amount of 30 MPa or more when hardened at 150°C for 20 minutes.

[0022] A steel sheet according to one embodiment of the present invention may further include silicon (Si) of 0.100% or less as an alloy composition, and may further include at least one of boron (B) of 0.004% or less and molybdenum (Mo) of 0.4% or less.

[0023] A steel sheet according to one embodiment of the present invention may include a plating layer or an alloy plating layer on at least one surface. Accordingly, the steel sheet according to one embodiment of the present invention may be a cold-rolled steel sheet, or a plated steel sheet obtained by plating the cold-rolled steel sheet.

[0024] According to another aspect of the present invention, a method for manufacturing a steel sheet is provided, comprising the steps of: heating a steel slab in a temperature range of 1100 to 1300°C; finishing hot-rolling the slab after the heating at a temperature of 880°C or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet to obtain a hot-rolled coil; air-cooling the hot-rolled coil; cold-rolling the hot-rolled coil after the air-cooling at a cold reduction ratio of 50 to 90% to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet at a temperature range of 750 to 850°C and then cooling it.

[0025] In one embodiment of the present invention, the steel slab may have the alloy composition described above.

[0026] In one embodiment of the present invention, the winding process can be performed under conditions satisfying the following relational expression 2.

[0027] [Relationship 2]

[0028] 18 ≤ (T coil / 100)×((100×[C])+[Mn]+[Cr]) ≤ 35

[0029] (In equation 2, [C], [Mn], and [Cr] represent the weight contents of the corresponding elements, and Tcoil represents the coiling temperature (℃).)

[0030] In this way, by controlling the winding process, the microstructure of the manufactured steel plate can have characteristics that satisfy the above-described relational expression 1.

[0031] In one embodiment of the present invention, a step of obtaining a hot-dip galvanized steel sheet by hot-dip galvanizing a cooled cold-rolled steel sheet may be further included, and a step of alloying heat treating the hot-dip galvanized steel sheet may be further included, if necessary.

[0032] According to the present invention, a steel sheet having excellent low-temperature hardenability and room-temperature aging resistance can be provided, and such a steel sheet can be advantageously applied as a material for the exterior panels of automobiles.

[0033] In addition, the present invention can lower the hardening temperature, thereby reducing manufacturing costs and CO2 emissions during steel sheet manufacturing. Furthermore, the steel sheet according to the present invention can be applied as a multi-material, which will further contribute to the weight reduction of automobile bodies.

[0034] The inventors of the present invention have conducted in-depth research on a method for providing a steel sheet suitable as a material for automobile exterior panels, while simultaneously achieving both bake hardening and aging resistance, and realizing continuously strengthened fuel efficiency regulations.

[0035] As a result, it was confirmed that a steel sheet could be provided that could secure equal or higher hardenability even at a lower temperature than the conventional hardening conditions, while also securing the target level of aging resistance, and thus the present invention was completed.

[0036] Hereinafter, the present invention will be described in detail.

[0037] A steel sheet according to one aspect of the present invention may contain, in wt%, carbon (C): 0.0010 to 0.0300%, manganese (Mn): 1.500 to 2.500%, chromium (Cr): 1.000% or less (excluding 0%), phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), and aluminum (Al): 0.010 to 0.060%.

[0038] Below, the reasons for limiting the alloy composition of a steel sheet according to one embodiment of the present invention are described in detail. Unless otherwise specified, the content of each element is based on weight, and the ratio of the structure is based on area. Furthermore, unless otherwise specified, "steel sheet" refers to cold-rolled steel sheet.

[0039] Carbon (C): 0.0010~0.0300%

[0040] Carbon (C), an interstitial solid-solution element, generally contributes effectively to the strength of steel. Furthermore, C increases the hardenability of steel and contributes to the formation of the martensite phase, a microstructure that constitutes steel. A certain level of C is required to achieve a composite structure.

[0041] In one embodiment of the present invention, if the C content is less than 0.0010%, there is a limit to implementing a composite structure, and it is also difficult to secure low-temperature bake hardenability. Therefore, the C may be included in an amount of 0.0010% or more. However, if the C content exceeds 0.0300%, the strength of the steel may increase excessively, thereby reducing the ductility of the steel, and the plastic anisotropy may deteriorate, thereby increasing the possibility of defects such as bending occurring on the surface of the product when processed into a part.

[0042] Therefore, in one embodiment of the present invention, C may be included in an amount of 0.0010 to 0.0300%. In another embodiment of the present invention, C may be 0.0030% or more, or 0.0040% or more. In yet another embodiment, C may be 0.0270% or less, or 0.0250% or less.

[0043] Manganese (Mn): 1.500~2.500%

[0044] Manganese (Mn) not only contributes to increasing the strength of steel as a solid solution strengthening element, but also prevents defects such as red-hot embrittlement caused by S by precipitating S present in the steel as MnS. In addition, similar to the above-mentioned C, it increases the hardenability of steel and contributes to the formation of martensite.

[0045] In one embodiment of the present invention, manganese (Mn) may be included in an amount of 1.500% or more to realize a composite structure and ensure low-temperature bake hardenability and aging resistance. However, if the content exceeds 2.500%, there is a risk that a large amount of annealing oxide will be generated on the surface during the annealing heat treatment process, which may cause surface defects in the steel. In addition, workability may be deteriorated as the strength increases excessively and ductility decreases.

[0046] Therefore, in one embodiment of the present invention, Mn may be included in an amount of 1.500 to 2.500%. In another embodiment of the present invention, the Mn may be 1.600% or more, or 1.700% or more. In yet another embodiment, the Mn may be 2.400% or less, or 2.300% or less.

[0047] Chromium (Cr): 1.000% or less (excluding 0%)

[0048] Chromium (Cr) is a solid solution strengthening element and has similar properties to the above Mn. In particular, Cr can effectively contribute to the formation of martensite phase by increasing the hardenability of steel. In addition, during hot rolling, Cr 23By forming coarse Cr-based carbides such as C6, the dissolved C content in steel can be controlled to an appropriate level, thereby suppressing the occurrence of yield point elongation (YP-El), making it effective in producing steel with a low yield ratio. In addition, Cr is also an element that effectively contributes to securing high ductility by minimizing the decrease in elongation relative to the increase in strength.

[0049] In one embodiment of the present invention, it is advantageous to add Cr as a prerequisite to obtain the aforementioned effect, and the content thereof may exceed 0%. However, if the content thereof exceeds 1.000% and is added in an excessive amount, not only may the ductility of the steel be reduced due to excessive formation of a martensite phase, but also the corrosion resistance of the steel may be reduced.

[0050] Therefore, in one embodiment of the present invention, Cr may be included in an amount of more than 0% and less than or equal to 1.000%. According to another embodiment of the present invention, the Cr may be greater than or equal to 0.001%, or greater than or equal to 0.005%, and according to yet another embodiment, the Cr may be less than or equal to 0.900%, or less than or equal to 0.800%.

[0051] Phosphorus (P): 0.100% or less (excluding 0%)

[0052] Phosphorus (P) is the element with the greatest strengthening effect, effectively enhancing the strength of steel without significantly impairing its drawability. Excessive P content increases the risk of brittle fracture, potentially leading to slab fracture during the hot rolling process. Furthermore, it can significantly impair the surface properties of galvanized steel sheets.

[0053] Taking this into account, according to one embodiment of the present invention, the content of P can be limited to 0.100% or less. However, considering the level that is inevitably introduced during the steel manufacturing process, 0% of the content can be excluded.

[0054] Sulfur (S): 0.0100% or less (excluding 0%)

[0055] Sulfur (S) is an unavoidable impurity added to steel. Because it impairs the ductility and weldability of steel, it is advantageous to keep its content as low as possible. In particular, S in steel is highly likely to cause red-hot embrittlement.

[0056] Taking this into consideration, according to one embodiment of the present invention, the content of S can be limited to 0.0100% or less. However, considering the level that is inevitably introduced during the steel manufacturing process, 0% of the content can be excluded.

[0057] Nitrogen (N): 0.010% or less (excluding 0%)

[0058] Nitrogen (N) is also an unavoidable impurity added to steel, so it is advantageous to keep its content as low as possible. However, a certain level of content may be permitted, taking into account the load and operating conditions during steelmaking to remove these impurities.

[0059] According to one embodiment of the present invention, even if N is contained at a maximum of 0.010%, the intended physical properties can be secured without difficulty, and thus the content can be limited to 0.010% or less. Furthermore, considering the level that is inevitably introduced during the steel manufacturing process, 0% can be excluded from the content.

[0060] Aluminum (Al): 0.010~0.060%

[0061] Aluminum (Al) is an element added to refine the grain size of steel and achieve a deoxidation effect.

[0062] In one embodiment of the present invention, to produce a stable aluminum-killed steel, aluminum may be included in an amount of 0.010% or more. However, if the content is excessive, while it is beneficial for improving strength through grain refinement, excessive inclusions may be formed during the steelmaking process, resulting in poor surface quality of the resulting steel sheet. This may also lead to increased manufacturing costs. Considering this, aluminum may be included in an amount of 0.060% or less.

[0063] Therefore, in one embodiment of the present invention, Al may be included in an amount of 0.010 to 0.060%. In another embodiment of the present invention, Al may be 0.020% or more, and in another embodiment, Al may be 0.050% or less.

[0064] Meanwhile, in one embodiment of the present invention, other elements may be further included to further improve the physical properties of the steel sheet. As one example, silicon (Si) may be further included, and as another example, at least one of boron (B) and molybdenum (Mo) may be further included. In this case, Si, B, and Mo may be included together, and the respective contents will be described in detail below.

[0065] Silicon (Si): 0.100% or less

[0066] Silicon (Si) is an element that contributes to increasing the strength of steel through solid solution strengthening. In the present invention, since the desired properties can be secured without adding Si, it is not intentionally added.

[0067] Meanwhile, in an attempt to obtain the effect of adding Si, if the content is excessive, there is a problem in that a large amount of Si oxide is formed, which deteriorates the surface properties of the plated steel sheet. Considering this, according to one embodiment of the present invention, when adding Si, the content can be limited to 0.100% or less. According to another embodiment, the Si can be 0.080% or less.

[0068] Boron (B): 0.004% or less

[0069] Boron (B) is an element that is prone to grain boundary segregation, and thus, B can be added to prevent secondary processing embrittlement that may occur when P is added for the purpose of solid solution strengthening. In addition, in composite phase steel, the hardenability can be significantly increased by adding a small amount of B, and thus, it can effectively contribute to the transformation of martensite during cooling after annealing.

[0070] In one embodiment of the present invention, a certain amount of B may be added to achieve the aforementioned effects. However, if the content exceeds 0.004%, there is a risk that the ductility of the steel may be reduced. Therefore, according to one embodiment of the present invention, the content of B may be limited to 0.004% or less when added.

[0071] Molybdenum (Mo): 0.4% or less

[0072] Molybdenum (Mo) is an element that enhances hardenability in steel sheets with composite structures, and is particularly important for forming the martensite phase of composite structures. When added alone or in combination with the aforementioned B, Mo induces the formation of uniform grains during annealing, making it effective in ensuring the formability of steel.

[0073] In one embodiment of the present invention, a certain amount of Mo may be added to achieve the aforementioned effects. However, if the content exceeds 0.4%, there is a risk that martensite phase may be formed excessively, which may actually reduce ductility. In addition, since it is an expensive element, manufacturing costs may increase significantly, thereby reducing economic feasibility. Therefore, according to one embodiment of the present invention, the content of Mo may be limited to 0.4% or less when added.

[0074] 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.

[0075] Hereinafter, the microstructural characteristics of a steel plate according to one embodiment of the present invention will be described in detail. At this time, the phase fraction of the microstructure constituting the steel plate may be a result measured for the entire thickness (t, mm) of the steel plate, and as an example, the result measured at the t / 4 point in the thickness direction may be representatively represented.

[0076] A steel sheet according to one embodiment of the present invention, i.e., a cold-rolled steel sheet, may have the aforementioned alloy composition, and may contain carbon (C) in the alloy composition at 0.0300% or less. Steel having such a C content corresponds to ultra-low carbon steel or low carbon steel, and its microstructure may be mainly composed of a ferrite phase. According to one embodiment of the present invention, the ferrite phase may be included at an area fraction of 95% or more.

[0077] Meanwhile, the steel sheet according to one embodiment of the present invention may have a composite structure, in which case it may be composed of ferrite and a secondary phase as a residual structure. As an example, the secondary phase may be included at an area fraction of 5% or less (excluding 0%), and may include one or more types of martensite and bainite phases corresponding to hard phases.

[0078] In one embodiment of the present invention, the lower the fraction of the ferrite phase in the composite structure, the higher the fraction of the secondary phase, i.e., the hard phase, relatively, and thus, it may be advantageous in implementing the composite structure. However, as the fraction of the hard phase increases, the yield strength and yield ratio inevitably increase, and when processing such a steel plate into a part, the possibility of defects such as waviness occurring on the surface increases. Therefore, in implementing the composite structure, it is advantageous to appropriately limit the fractions of the ferrite phase and the secondary phase, and in one embodiment of the present invention, the ferrite phase may be configured to have an area fraction of 95% or more, and the secondary phase may be configured to have an area fraction of 5% or less (excluding 0%).

[0079] Meanwhile, a steel plate according to one embodiment of the present invention in which the microstructure is composed of a composite structure can satisfy the following relational expression 1.

[0080] [Relationship 1]

[0081] 0.01 ≤ D second / D grain ≤ 0.15

[0082] (In relation 1, D grain refers to the average grain size (㎛) of ferrite, and D second refers to the radius (㎛) based on the equivalent diameter of the secondary phase.)

[0083] Through in-depth research, the inventors of the present invention have confirmed that not only is it advantageous to form a composite structure in order to secure the low-temperature hardenability and aging resistance of the steel plate to a level higher than required, but also that the grain size of such a composite structure affects the securing of low-temperature hardenability.

[0084] From this, the relational expression 1 according to one embodiment of the present invention factors the ratio of the crystal grain sizes of each phase constituting the composite structure. That is, the steel sheet according to one embodiment of the present invention may have a characteristic in which the size of the secondary phase is refined compared to the size of the ferrite, since the steel sheet has a composite structure as a microstructure. In the relational expression 1, the size of the ferrite and the size of the secondary phase refer to values ​​measured for a steel sheet that has not been bake-hardened.

[0085] A value of relational expression 1 according to an embodiment of the present invention being less than 0.01 may mean that the grains of the ferrite phase are coarse compared to the grain size of the secondary phase, or that the secondary phase is almost non-existent. In this case, it becomes difficult to secure bake hardenability at low temperatures. On the other hand, a value of relational expression 1 according to an embodiment of the present invention exceeding 0.15 may mean that the grains of the secondary phase are coarse compared to the grain size of the ferrite phase. In this case, not only may it be difficult to secure bake hardenability at low temperatures, but also workability may be poor due to the occurrence of yield point elongation (YP-El).

[0086] A steel sheet according to one embodiment of the present invention can have excellent bake hardenability and room temperature aging resistance by controlling the microstructural characteristics together with the alloy composition, and in particular, bake hardenability at low temperatures can be secured.

[0087] As an example, the steel plate may have a bake hardening amount of 30 MPa or more when baked at 150°C for 20 minutes. In this case, the bake hardening amount refers to the change in the lower yield strength before and after bake hardening based on the flow stress after applying 2% pre-strain.

[0088] The amount of bake hardening of steel plates was generally measured at about 170℃, and it was determined that the steel plate had the required properties if the change in yield strength at this temperature was 30MPa or more. However, the steel plate according to one embodiment of the present invention has a bake hardening amount measured at 150℃ that corresponds to the existing level, so it can be said to have a remarkable effect in terms of bake hardening properties. By securing bake hardening properties at a relatively low temperature in this way, the present invention not only has the economic effect of reducing the manufacturing cost of materials for automobile exterior panels, but also can be expected to have an environmentally friendly effect by reducing CO2, etc. generated during the process of manufacturing the target steel plate.

[0089] Furthermore, the steel plate according to one embodiment of the present invention can be applied to multi-materials, thereby further reducing the weight of the automobile body.

[0090] Meanwhile, the steel sheet according to one embodiment of the present invention may be a cold-rolled steel sheet, or may be a plated steel sheet including a plated layer on at least one surface of the cold-rolled steel sheet. In this case, the cold-rolled steel sheet may have the aforementioned alloy composition and microstructural characteristics, and the plated steel sheet may also have these characteristics.

[0091] In one embodiment of the present invention, the plating layer may be a zinc-based plating layer, and thus the plating steel sheet may be a zinc-plated steel sheet or an alloyed zinc-plated steel sheet obtained by alloying the zinc-plated steel sheet. The zinc-plated steel sheet at this time may be manufactured by any plating method, such as hot-dip galvanizing or electrogalvanizing.

[0092] Although not particularly limited, the plating layer may be, for example, a zinc-based plating layer, and as a non-limiting example, may be a zinc-plated layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc. Meanwhile, a plating steel sheet having a zinc alloy plating layer containing aluminum and magnesium in addition to zinc may be a ternary plating steel sheet of the Zn-Mg-Al system.

[0093] Hereinafter, a method for manufacturing a steel plate according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method is an example for manufacturing a steel plate according to one embodiment of the present invention.

[0094] According to one embodiment of the present invention, a steel plate can be manufactured by going through the process of [heating - hot rolling - cooling - coiling - cold rolling - annealing - cooling] for a prepared steel slab, and each process step is specifically described below.

[0095] [Heating of steel slabs]

[0096] After preparing a steel slab according to one embodiment of the present invention, the steel slab may be heated. The heating process of the steel slab is a process for smoothly performing the hot rolling process described below and sufficiently obtaining the target physical properties of the steel plate. As one example, the steel slab may have the same alloy composition as the steel plate according to one embodiment of the present invention, and the description of each alloy element is replaced with the above-mentioned matters.

[0097] In one embodiment of the present invention, the process of heating the steel slab may be performed under normal conditions, and as an example, may be performed at a temperature range of 1100 to 1300°C. If the heating temperature is lower than 1100°C, friction between the steel plate and the rolling mill increases, which causes a problem in that the load applied to the rollers during hot rolling increases rapidly. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required for the temperature increase, but the amount of surface scale increases, which may lead to material loss.

[0098] [Hot rolling]

[0099] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0100] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing finishing hot rolling at a temperature higher than the Ar3 transformation point during the hot rolling. In one embodiment of the present invention, if the finishing hot rolling process is performed at a temperature lower than the Ar3 transformation point, there is a concern that ferrite and austenite dual-phase rolling may occur, resulting in material non-uniformity.

[0101] As an example, the above finishing hot rolling can be performed in a temperature range of 880 to 1000°C, and if the temperature exceeds 1000°C, there is a concern that material unevenness may occur due to the formation of abnormal coarse grains caused by high-temperature rolling, and this causes a problem of coil distortion occurring during subsequent cooling.

[0102] [Winding]

[0103] The hot-rolled steel sheet manufactured above can be wound, and a hot-rolled coil can be obtained therefrom.

[0104] This coiling process can be performed in a certain temperature range, and according to one embodiment of the present invention, coiling can be performed at a temperature satisfying the following relational expression 2. That is, when setting the coiling temperature, it is controlled in relation to the contents of C, Mn, and Cr, which are elements that improve the hardenability of steel and induce the formation of secondary phases such as martensite phases.

[0105] [Relationship 2]

[0106] 18 ≤ (T coil / 100)×((100×[C])+[Mn]+[Cr]) ≤ 35

[0107] (In equation 2, [C], [Mn], [Cr] represent the weight contents of the corresponding elements, and T coil means the coiling temperature (℃).

[0108] During the coiling process, the coiling temperature affects the grain size of the hot-rolled steel. At this time, the higher the coiling temperature, the coarser the grain size of the hot-rolled steel, and since the precipitation of solid solution C occurs, the ferrite is in a relatively purified state, which has the characteristic of improving the workability. That is, the yield point elongation does not occur. Meanwhile, in order to secure the secondary phase according to one embodiment of the present invention, it is necessary to contain a certain amount or more of elements that affect the secondary phase transformation, that is, C, Mn, and Cr. Based on this, the inventors of the present invention confirmed that when the coiling temperature is increased to secure the workability of the steel, the possibility of generating a pearlite-like structure increases, but by appropriately controlling the content of hardenable elements contained in the steel, the fraction of pearlite can be suppressed even at high temperature coiling, thereby deriving the above-mentioned relational expression 2.

[0109] If the value of relational expression 2 according to one embodiment of the present invention is less than 18, it means that the coiling process is performed at a relatively low temperature, and in this case, it is difficult to secure processability, making it impossible to secure low-temperature bake hardening properties. On the other hand, if the value of relational expression 2 exceeds 35, there is a problem of yield point elongation occurring.

[0110] The inventors of the present invention have formed a composite structure of ferrite and a secondary phase as a microstructure in order to provide a steel sheet having room temperature aging resistance while securing bake hardenability, particularly bake hardenability at low temperatures, and have confirmed that the grain size between the composite structures has an effect on securing the intended physical properties.

[0111] [cooling]

[0112] The hot-rolled coil, which has undergone a coiling process under the above-mentioned specific conditions, can be cooled, and in one embodiment of the present invention, can be air-cooled to room temperature.

[0113] In one embodiment of the present invention, if a constant cooling rate is applied during cooling of a hot-rolled coil, there is a risk that uneven grain distribution may occur during the cooling process. Therefore, the cooling of the hot-rolled coil may be performed using air cooling.

[0114] [Cold rolling]

[0115] The above-mentioned coiled and cooled hot-rolled coil can be uncoiled and cold-rolled to produce a cold-rolled steel sheet. Meanwhile, a pickling process may be performed prior to the cold-rolling process to remove surface scale from the hot-rolled coil. The pickling process may be performed under normal conditions, and there are no particular limitations on the conditions.

[0116] In one embodiment of the present invention, cold rolling can be performed at a cold reduction ratio (total reduction ratio) of 50 to 90%. If the cold reduction ratio during cold rolling is less than 50%, not only will it be difficult to secure the target thickness, but also shape correction of the steel sheet will become difficult. On the other hand, if the cold reduction ratio exceeds 90%, the load on the rolling rolls may become excessive, which may deteriorate the shape of the steel sheet.

[0117] [Sodun]

[0118] The cold-rolled steel sheet manufactured above can be subjected to annealing heat treatment. As a non-limiting example, the continuous annealing treatment can be performed in a continuous alloying galvanizing furnace.

[0119] In one embodiment of the present invention, the annealing heat treatment can be performed at a temperature range of 750 to 850°C. That is, by performing the annealing treatment of a cold-rolled steel sheet in a two-phase region where ferrite and austenite coexist above the Ac1 point, not only austenite but also ferrite can be formed simultaneously with the recrystallization of the structure, while carbon can be distributed.

[0120] In the annealing heat treatment according to one embodiment of the present invention, if the temperature is lower than 750°C, not only will recrystallization not be sufficiently achieved, but the austenite phase will not be sufficiently formed either, making it impossible to secure the target microstructure phase composition, for example, a composite structure of ferrite and secondary phases, after the continuous annealing treatment. On the other hand, if the temperature exceeds 850°C, the austenite phase is excessively formed in the abnormal region, and it is difficult to secure the target level of secondary phase as the austenite with low stability is reversely transformed into ferrite during the subsequent cooling process. In this case, the yield strength increases and the ductility decreases, making it impossible to secure the intended bake hardenability and aging resistance. In addition, the surface enrichment of elements such as Si and Mn, which inhibit the wettability of the hot-dip galvanizing among the alloy compositions, may become severe, which may deteriorate the plating surface quality.

[0121] Therefore, in one embodiment of the present invention, the annealing heat treatment can be performed at a temperature range of 750 to 850°C. According to another embodiment of the present invention, the continuous annealing treatment can be performed at 770°C or higher.

[0122] [cooling]

[0123] The cold-rolled steel sheet annealed and heat-treated in the above two-phase temperature range can be cooled.

[0124] In one embodiment of the present invention, the cooling may be performed in stages, and as an example, the continuous annealing process may be a process in which the cold rolled steel sheet is first cooled to a temperature range of 600 to 700°C at a cooling rate of 3 to 10°C / s, and then the first-cooled cold rolled steel sheet is secondarily cooled to a temperature range of 400 to 600°C at a cooling rate of 5°C / s or more. At this time, the cooling rate of the second cooling may be performed faster than the cooling rate of the first cooling.

[0125] In this way, during the annealing heat treatment process according to one embodiment of the present invention, a cold-rolled steel sheet in which a certain fraction of a ferrite phase is formed along with austenite is cooled in stages at a specific temperature range according to the cooling rate, thereby controlling the steel sheet to have a composite structure.

[0126] In one embodiment of the present invention, by performing primary cooling on an annealed cold-rolled steel sheet at a cooling rate of 3 to 10°C / s to a temperature range of 600 to 700°C, a ferrite phase can be additionally introduced into the annealed cold-rolled steel sheet.

[0127] If the cooling rate during the first cooling exceeds 10°C / s, the additionally introduced ferrite phase may be insufficient, which may result in a decrease in the ductility of the steel sheet. On the other hand, if the cooling rate during the first cooling is less than 3°C / s, the fraction of the ferrite phase may be excessive, making it impossible to secure the secondary phase at the target level in the final microstructure. In addition, if the cooling end temperature during the first cooling is less than 600°C, the ferrite phase may not be sufficiently formed, whereas if the temperature exceeds 700°C, there is a problem in that the cooling rate must be unreasonably increased during the subsequent second cooling process, and the secondary phase may not be formed at the target level in the final microstructure.

[0128] In one embodiment of the present invention, after the first cooling of the continuously annealed cold-rolled steel sheet, secondary cooling can be performed at a cooling rate of 5°C / s or more to a temperature range of 400 to 600°C, and a secondary phase can be introduced during this process.

[0129] If the cooling rate during the secondary cooling is less than 5°C / s, pearlite may be generated during the cooling process, preventing the intended secondary phase from being completely formed. Meanwhile, the upper limit of the cooling rate during the secondary cooling is not particularly limited, and a person skilled in the art may appropriately select it in consideration of the specifications of the cooling equipment. As an example, it may be performed at 100°C / s or less. In addition, if the cooling end temperature during the secondary cooling is less than 400°C, there is a concern that the martensite phase among the secondary phases may be excessively formed, which may deteriorate the aging resistance of the steel sheet. On the other hand, if the temperature exceeds 600°C, there is a concern that the bainite phase among the secondary phases may be excessively formed, and the grains of the secondary phase may become coarse.

[0130] According to one embodiment of the present invention, a steel sheet (cold rolled steel sheet) having a composite structure can be obtained by controlling the alloy composition and manufacturing conditions. Specifically, the steel sheet according to one embodiment of the present invention can have a composite structure composed of ferrite and a secondary phase, and in particular, the crystal grain relationship between these phases can satisfy Relationship Equation 1. Therefore, the steel sheet according to one embodiment of the present invention can not only secure low-temperature bake hardenability, but also secure room-temperature aging resistance at a target level.

[0131] Meanwhile, according to one aspect of the present invention, a plated steel sheet can be provided. Specifically, a plated steel sheet can be obtained by plating a cold-rolled steel sheet manufactured through a series of processes according to one embodiment of the present invention. As one example, a cold-rolled steel sheet that has undergone annealing heat treatment and a step-by-step cooling process can be subjected to hot-dip galvanizing. As a non-limiting example, hot-dip galvanizing can be performed.

[0132] [Hot-dip galvanizing]

[0133] According to one embodiment of the present invention, a cold-rolled steel sheet, a cold-rolled steel sheet that has undergone annealing heat treatment and cooling processes, can be immersed in a molten zinc plating bath of a continuous plating line to produce a molten zinc-plated steel sheet.

[0134] In one embodiment of the present invention, the hot-dip galvanizing process may be performed under normal conditions, but as an example, may be performed at a temperature range of 430 to 480°C. In addition, the composition of the hot-dip galvanizing bath is not particularly limited, and may be a pure zinc plating bath or a zinc alloy plating bath containing Si, Al, Mg, etc.

[0135] [Alloying heat treatment]

[0136] In addition, if necessary, an alloyed zinc-plated steel sheet can be obtained by subjecting a hot-dip galvanized steel sheet manufactured by hot-dip galvanizing according to one embodiment of the present invention to an alloying heat treatment.

[0137] In one embodiment of the present invention, there are no particular restrictions on the alloying heat treatment process conditions, and any normal conditions may suffice. As an example, the alloying heat treatment process may be performed at a temperature range of 460 to 610°C. If the temperature during the alloying heat treatment is lower than 460°C, the alloying (e.g., Fe-Zn alloying) reaction does not proceed sufficiently. On the other hand, if the temperature exceeds 610°C, the alloying may proceed excessively, which may cause the plating layer to become brittle. In this case, problems such as the plating layer being peeled off during processing such as pressing may occur.

[0138] 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.

[0139] (Example)

[0140] After preparing steel slabs having the alloy composition shown in Table 1 below, they were heated at a temperature of 1200℃, and each heated slab was final hot-rolled at 900-950℃ to produce a hot-rolled steel sheet. Thereafter, a coiling process was performed according to the conditions shown in Table 2 to obtain a hot-rolled coil, which was then air-cooled to room temperature. Next, each hot-rolled coil was pickled under normal conditions and cold-rolled at a reduction ratio of 76% to obtain a cold-rolled steel sheet. Thereafter, the final cold-rolled steel sheet was manufactured through annealing heat treatment and cooling processes under the conditions shown in Table 2. Thereafter, the cold-rolled steel sheet was immersed in a hot-dip galvanizing bath at 460℃ to produce a hot-dip galvanized steel sheet, and then an alloying heat treatment was performed at 460-610℃ to obtain an alloyed hot-dip galvanized steel sheet. At this time, the process from annealing heat treatment to alloying heat treatment was performed using continuous equipment (e.g., continuous alloying plating line).

[0141] The microstructure of each alloyed hot-dip galvanized steel sheet manufactured as described above was measured and the mechanical properties were evaluated, and the results are shown in Table 3.

[0142] First, the microstructure was measured using an optical microscope after Lepera etching on specimens collected at a point 1 / 4t (where t represents the thickness (mm) of the cold-rolled steel sheet) in the thickness direction. The phase types were distinguished and the fraction and grain size were measured. The grain size was measured as an average value based on the equivalent circle diameter.

[0143] The amount of hardening (BH) was measured by applying 2% pre-strain to the same specimen as above, and then measuring the increase in the lower yield strength after sintering at 150℃ for 20 minutes based on the flow stress.

[0144] In addition, the yield point elongation (YP-El) was evaluated through a tensile test on the specimens after heat treatment at 100℃ for 1 hour, and the aging resistance was evaluated to be guaranteed based on the result that the length of the yield point elongation during the tensile test was 0.2% or less. The tensile test at this time was conducted in the C direction on specimens collected according to the JIS-5 standard.

[0145] Steel alloy composition (weight %)CMnSiCrAlPSNA0.00741.980.0090.3890.0270.0120.00130.004B0.01502.090.0050.4120.0230.0120.00100.004C0.02402.030.0220.4110.0250.0110 .00110.003D0.03401.820.0470.5080.0360.0110.00110.004E0.00150.5880.0 0500.0300.0100.00100.006F0.00160.5850.0040.4860.0180.0100.00100.006

[0146] Steel grade cancellation annealing 1st 2nd alloying relationship 2 Classification temperature (℃) Temperature (℃) End temperature (℃) Cooling rate (℃ / s) End temperature (℃) Cooling rate (℃ / s) Heat treatment temperature (℃) A7008006504.25608.553022 Invention example 1 A6508006504.25608.553020 Invention example 2 A6008006504.25608.553019 Invention example 3 B7008006205.150011.353028 Invention example 4 C7008006205.148013.253034 Invention example 5 D7008006504.25608.553040 Comparative example 1E7008006504.25608.55305Comparative Example 2F7008006504.25608.55309Comparative Example 3A5008006504.25608.553016Comparative Example 4A7007406502.85608.553022Comparative Example 5A7008606505.65608.553022Comparative Example 6

[0147] ClassificationSecondary phase fraction (area %)Relationship equation 1BH(150℃)YP-El Invention example 10.280.06360Invention example 20.300.06350Invention example 30.250.08330Invention example 41.370.07310Invention example 51.820.12310Comparative example 13.440.35230.5Comparative example 200120Comparative example 300290Comparative example 40.350.2270Comparative example 500280.2Comparative example 6<0.01 (approximately 0.005)0250In all examples, the remaining structure excluding the secondary phase fraction was a ferrite phase. In all examples, the secondary phase was one of martensite and bainite phases. It was ideal. Equation 1 is the relationship between the radius of the second phase equivalent circle diameter and the average grain size of ferrite, [D second / D grain ] means the calculated value.

[0148] As shown in Tables 1 to 3, invention examples 1 to 5, which satisfy both the alloy composition and manufacturing conditions according to one embodiment of the present invention, formed microstructures as intended. Accordingly, the bake hardenability (BH) at 150°C was shown to be 30 MPa or higher, and the aging resistance was also excellent.

[0149] On the other hand, Comparative Examples 1 to 6, which did not satisfy at least one of the alloy composition and manufacturing conditions according to one embodiment of the present invention, did not form the intended microstructure. As a result, the desired physical properties, particularly low-temperature bake hardenability, were not secured.

[0150] Specifically, Comparative Example 1 had an excessive C content in the alloy composition, which did not satisfy the condition of Equation 2 during the coiling process, resulting in the formation of coarse grains of the secondary phase relative to the grain size of the ferrite. As a result, not only the bake hardenability but also the aging resistance was inferior. Comparative Example 1 showed a yield point elongation of 0.5, and since there is a high possibility of stretcher-strain defects occurring when processing such a steel sheet, it is difficult to use it as a material for automobile exterior panels.

[0151] Comparative Examples 2 and 3 did not satisfy the condition of Equation 2 during the winding process due to insufficient Mn content, and the secondary phase was not formed at all, resulting in poor bake hardening properties. Among these, the bake hardening properties of Comparative Example 2, which had a lower content of the hardenable element, were even poorer.

[0152] Comparative Example 4 satisfies the alloy composition according to one embodiment of the present invention, but the coiling temperature was too low, exceeding the condition of Equation 2, so that the grain size of the secondary phase generated was coarse compared to the grain size of ferrite. As a result, the bake hardenability was inferior.

[0153] Comparative Examples 5 and 6 satisfy the alloy composition according to one embodiment of the present invention, but the annealing temperature is low or high, respectively, outside the range according to one embodiment of the present invention, so that the secondary phase is hardly generated, and thus the grain size characteristics corresponding to equation 1 cannot be secured. As a result, the bake hardenability was inferior.

Claims

1. Contains, in weight%, carbon (C): 0.0010 to 0.0300%, manganese (Mn): 1.500 to 2.500%, chromium (Cr): 1.000% or less (excluding 0%), phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), aluminum (Al): 0.010 to 0.060%, the remainder being iron and unavoidable impurities. A steel plate having a microstructure comprising a ferrite phase with an area fraction of 95% or more and a secondary phase as the remainder, and satisfying the following relationship 1. [Relationship 1] 0.01 ≤ D second / D grain ≤ 0.15 (In relation 1, D grain refers to the average grain size (㎛) of ferrite, and D second refers to the radius (㎛) based on the equivalent diameter of the secondary phase.) 2. In paragraph 1, The above steel plate is a steel plate further comprising, in weight %, silicon (Si): 0.100% or less.

3. In paragraph 1, The above steel plate further contains, in weight %, at least one of boron (B): 0.004% or less and molybdenum (Mo): 0.4% or less.

4. In paragraph 1, The above steel plate is a steel plate having a baking hardening amount of 30 MPa or more when baked at 150°C for 20 minutes. (Here, the amount of sub-hardening refers to the change in the lower yield strength before and after sub-hardening.) 5. In any one of paragraphs 1 to 4, The above steel plate is a steel plate including a plating layer or an alloy plating layer on at least one surface.

6. A step for preparing a steel slab containing, by weight%, carbon (C): 0.0010 to 0.0300%, manganese (Mn): 1.500 to 2.500%, chromium (Cr): 1.000% or less (excluding 0%), phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.0100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), aluminum (Al): 0.010 to 0.060%, the remainder being iron and unavoidable impurities; A step of heating the above steel slab in a temperature range of 1100 to 1300℃; A step of obtaining a hot-rolled steel sheet by final hot-rolling the slab after heating at a temperature of 880℃ or higher; A step of coiling the above hot-rolled steel plate to obtain a hot-rolled coil; A step of air cooling the above hot-rolled coil; A step of cold rolling the hot-rolled coil after the above air cooling at a cold reduction ratio of 50 to 90% to obtain a cold-rolled steel sheet; and It includes a step of annealing the above cold rolled steel plate at a temperature range of 750 to 850°C and then cooling it. A method for manufacturing a steel plate, wherein the above-mentioned winding is performed at a temperature satisfying the following relational expression 2. [Relationship 2] 18 ≤ (T coil / 100)×((100×[C])+[Mn]+[Cr]) ≤ 35 (In equation 2, [C], [Mn], and [Cr] represent the weight contents of the corresponding elements, and T coil refers to the coiling temperature (℃).

7. In paragraph 6, A method for manufacturing a steel plate, wherein the steel slab further contains, in weight %, silicon (Si): 0.1% or less.

8. In paragraph 6, A method for manufacturing a steel plate, wherein the steel slab further contains, in weight %, at least one of boron (B): 0.004% or less and molybdenum (Mo): 0.4% or less.

9. In paragraph 6, The step of cooling after the above annealing heat treatment is A method for manufacturing a steel sheet, comprising the step of first cooling a cold rolled steel sheet to a temperature range of 600 to 700°C at a cooling rate of 3 to 10°C / s and the step of second cooling the cold rolled steel sheet after the first cooling to a temperature range of 400 to 600°C at a cooling rate of 5°C / s or more.

10. In paragraph 6, A method for manufacturing a steel sheet further comprising the step of obtaining a hot-dip galvanized steel sheet by hot-dip galvanizing the cold-rolled steel sheet after the cooling.

11. In paragraph 10, A method for manufacturing a steel sheet further comprising the step of selectively performing alloying heat treatment on the above molten galvanized steel sheet.