High-strength cold-rolled steel sheet with excellent material uniformity, high-strength plated steel sheet and method for producing the same

Abstract

The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, wherein the steel sheet comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and further comprises MM with a total content of 0.0001 to 0.35% and the balance being Fe and unavoidable impurities, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr.

Description

Technical Field

[0001] One embodiment of the present invention relates to cold-rolled, coated steel sheets with excellent material uniformity. More specifically, one embodiment of the present invention relates to a method for collectively manufacturing high-strength, cold-rolled, coated steel sheets with excellent material uniformity. This method is suitable for use in household appliances such as televisions, washing machines, and air conditioner outdoor units. Background Art

[0002] The recent rise in demand for larger and thinner high-end video appliances has necessitated thinner frame materials, such as those used for TV backs and module covers. This thinning requires increased material strength to ensure dent and sag resistance. In particular, ensuring excellent shape retention and dimensional accuracy after stamping thin and wide sheet materials for large TVs over 65 inches requires, in addition to the aforementioned requirements, excellent uniformity and high strength of the steel sheet material.

[0003] Commonly used steel strengthening methods include solid solution strengthening, precipitation strengthening, phase transformation strengthening, processing strengthening, and grain refinement strengthening. However, the use of grain refinement strengthening methods in the production of high-strength steel has limitations. When performing solid solution strengthening and phase transformation strengthening, not only do large amounts of alloy components are required, but it is also difficult to ensure the processability required for stamping when performing processing strengthening.

[0004] On the contrary, precipitation strengthening can not only utilize carbide and nitride-forming elements such as Ti, Nb, Mo, V, and Cu for precipitation strengthening, but also utilize the strengthening effect of grain refinement, which has the advantage of achieving relatively high strength at low manufacturing cost.

[0005] As a typical method for manufacturing precipitation-strengthened high-strength steel, a low-carbon steel with a temperature below 0.15°C containing one or more components such as Ti, Nb, and V is used to control the final hot finishing temperature (hereinafter referred to as FDT) to be in the range of 750°C to 950°C and the coiling temperature (hereinafter referred to as CT) to be below 450°C to manufacture precipitation-strengthened high-strength steel. However, in this case, since the CT is too low, it is difficult to ensure the shape of the hot-rolled steel sheet or to ensure the cold rolling passability of the thin plate. In addition, there is a method for manufacturing high-strength precipitation-strengthened steel using Nb or V by performing hot rolling followed by accelerated cooling, but since the CT is set below 400°C, it has limitations in ensuring the shape and thinning. In addition, there is also a method for manufacturing high-strength precipitation-strength steel by adding more than 0.8% Cu and utilizing Cu precipitates, but there are problems such as surface defects and plating defects due to the high Cu content.

[0006] Since the conventional technology only aims at high strengthening of steel plates and does not ensure the uniformity of the material, deformation occurs after processing wide materials such as large televisions, making it impossible to obtain part accuracy and shape freezeability. Summary of the Invention

[0007] Technical problems to be solved

[0008] One embodiment of the present invention is to provide a cold-rolled, coated steel sheet having excellent formability and excellent material uniformity in the length and width directions of the steel sheet, and a method for manufacturing the same.

[0009] Technical Solution

[0010] According to an embodiment of the present invention, a cold-rolled steel sheet comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and further comprises MM with a total content of 0.0001 to 0.35% and the balance being Fe and unavoidable impurities, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr, and the alloying elements of the cold-rolled steel sheet simultaneously satisfy the following equations 1 and 2.

[0011] [Equation 1]

[0012] 0≤[Ti]-3.42[N]≤4.0[C]

[0013] [Equation 2]

[0014] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0

[0015] (In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM] and [N] are weight % of each element, wherein MM is at least one selected from the group consisting of Sn, B, Mo, Ni and Cr).

[0016] In the cold-rolled steel sheet, the area ratio of the ferrite structure as the microstructure is 85% or more.

[0017] In the cold-rolled steel sheet, ferrite grains as a microstructure have precipitates with a size of 100 nm or less accounting for more than 80% of the total number of precipitates.

[0018] The yield strength of the cold-rolled steel sheet is 240 to 530 MPa.

[0019] The ΔYS of the cold-rolled steel sheet in the longitudinal direction or the width direction is 30 MPa or less.

[0020] According to an embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet comprises: a step of reheating a slab, wherein the slab comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and a total content of The method comprises the following steps: comprising: a step of: comprising: a MM content of 0.0001 to 0.35% and a balance of Fe and inevitable impurities; and simultaneously satisfying the following equations 1 and 2, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr; a step of performing rough rolling and hot finish rolling on the reheated slab to produce a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet; a step of cold rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet; and a step of annealing the cold-rolled steel sheet.

[0021] [Equation 1]

[0022] 0≤[Ti]-3.42[N]≤4.0[C]

[0023] [Equation 2]

[0024] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0

[0025] (In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM] and [N] are weight % of each element, and MM is at least one selected from the group consisting of Sn, B, Mo, Ni and Cr).

[0026] According to an embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet comprises: a step of reheating a slab, wherein the slab comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and the like. ), further comprising: MM with a total content of 0.0001 to 0.35% and the balance of Fe and inevitable impurities, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr; the steps of performing rough rolling and hot finish rolling on the reheated slab to produce a hot-rolled steel sheet; the step of coiling the hot-rolled steel sheet; the step of cold rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet; and the step of annealing the cold-rolled steel sheet.

[0027] Wherein, in the step of coiling the hot rolled steel sheet, when the hot rolled steel sheet is cooled from the hot finishing temperature to the coiling temperature, the speed difference of the sheet on one belt of the run-out roller table (ROT) is less than 20%.

[0028] According to an embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet comprises: a step of reheating a slab, wherein the slab comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and the like. (In addition to the above, the present invention further comprises: MM having a total content of 0.0001 to 0.35% and the balance being Fe and unavoidable impurities, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni, and Cr; the steps of rough rolling and hot finish rolling the reheated slab to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet; and annealing the cold-rolled steel sheet. In the step of coiling the hot-rolled steel sheet, when cooling the hot-rolled steel sheet from the hot finish rolling temperature to the coiling temperature, the amount of cooling water at the edge portion is less than 50% of the amount of cooling water at the center portion. The edge portion is the portion within 15% of the total width of the hot-rolled steel sheet, extending from the left and right corners of the hot-rolled steel sheet toward the center of the steel sheet, based on the total width of the hot-rolled steel sheet. The center portion is the portion excluding the edge portion.)

[0029] The slab satisfies the following equations 1 and 2 simultaneously:

[0030] [Equation 1]

[0031] 0≤[Ti]-3.42[N]≤4.0[C]

[0032] [Equation 2]

[0033] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0

[0034] (In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM] and [N] are weight % of each element, and MM is at least one selected from the group consisting of Sn, B, Mo, Ni and Cr).

[0035] In the step of performing rough rolling and hot finish rolling on the reheated slab to produce a hot rolled steel sheet, the hot finish rolling temperature is 850°C to 950°C.

[0036] In the step of coiling the hot-rolled steel sheet, the coiling temperature is 600°C to 700°C.

[0037] The cold-rolled steel sheet further includes a plating layer containing at least one of Zn, Al and Mg as a plating metal.

[0038] The coating layer includes at least one selected from the group consisting of Zn, Zn-Al, and Zn-Al-Mg.

[0039] After the step of performing annealing heat treatment on the cold-rolled steel sheet, the method further includes the step of forming a coating, wherein the metal forming the coating includes at least one of Zn, Al and Mg.

[0040] The coating layer includes at least one selected from the group consisting of Zn, Zn-Al, and Zn-Al-Mg.

[0041] Beneficial effects

[0042] According to one embodiment of the present invention, a cold-rolled, plated steel having a thickness of 1 mm or less can be provided that has excellent formability, small yield strength deviation in the longitudinal or width direction of the steel plate, and excellent material uniformity. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a graph showing the measurement results of the yield strength and its variation of Examples and Comparative Examples according to the present invention.

[0044] Figure 2 Schematic diagram of a steel plate edge portion for controlling the amount of cooling water according to an embodiment of the present invention.

[0045] Figure 3FIG. 4 is a schematic diagram of an area of ​​YS measurement points on a steel plate for obtaining ΔYS according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] In this specification, terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another part, component, region, layer, and / or segment. Therefore, without departing from the scope of the present invention, the first part, component, region, layer, and / or segment described below can also be described as the second part, component, region, layer, and / or segment.

[0047] In this specification, the terms used are for describing specific embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, the singular forms used herein are intended to include the plural forms. As used in this specification, "comprising" may specifically refer to a certain characteristic, field, integer, step, action, element, and / or component, but does not exclude the presence or addition of other characteristics, fields, integers, steps, actions, elements, components, and / or groups.

[0048] In this specification, if a part is described as being on another part, it may be directly on the other part or there may be other parts therebetween. When a part is described as being directly on another part, there may not be other parts therebetween.

[0049] In addition, unless otherwise specified, % means weight %, and 1 ppm means 0.0001 weight %.

[0050] In one embodiment of the present invention, further comprising an additional element means that a portion of the remaining iron (Fe) is replaced by the additional element, and the amount of replacement is equivalent to the amount of the additional element added.

[0051] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms should be interpreted as having the same meaning as that in the relevant technical literature and disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0052] Hereinafter, the embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be implemented in various ways and is not limited to the embodiments described herein.

[0053] Hereinafter, embodiments of the present invention will be described in detail.

[0054] According to an embodiment of the present invention, the cold-rolled steel sheet comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and further comprises MM with a total content of 0.0001 to 0.35% and the balance being Fe and unavoidable impurities, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr.

[0055] The alloy elements of the cold-rolled steel sheet simultaneously satisfy the following equations 1 and 2.

[0056] [Equation 1]

[0057] 0≤[Ti]-3.42[N]≤4.0[C]

[0058] [Equation 2]

[0059] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0

[0060] (In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM] and [N] are weight % of each element, wherein MM is at least one selected from the group consisting of Sn, B, Mo, Ni and Cr).

[0061] First, each alloy component is described in detail.

[0062] C: 0.0005 to 0.2%

[0063] Carbon (C) is a precipitate-forming element. To strengthen the steel and ensure excellent material uniformity, it is preferable to uniformly disperse microscopic carbides within the grains for precipitation. When its content is less than 0.0005%, the precipitation of microscopic Ti-based carbides is insufficient, contributing little to the strengthening effect, and achieving high strength is difficult. When the C content exceeds 0.2%, a large amount of unprecipitated solid-solution C forms pearlite, leading to a problem of deteriorated formability. Therefore, in the present invention, it is preferable to limit its content to 0.0005 to 0.2%.

[0064] Mn: 0.05 to 0.2%

[0065] Manganese (Mn) is a solid solution strengthening element that not only helps to improve strength, but also plays a role in precipitating S in the steel in the form of MnS to suppress plate fracture and high-temperature embrittlement caused by S during hot rolling. However, if its content is too high, when the slab is cast during the casting process, a segregation portion in the center of the plate thickness is significantly developed, resulting in the formation of retained austenite around the segregation portion, which leads to a problem of reduced workability. Therefore, in the present invention, it is preferred to limit its content to 0.05 to 0.2%. In addition, if the manganese content is less than 0.05%, the amount of MnS precipitation is small and S remains in the steel plate. As S remains, high-temperature brittleness problems such as plate breakage occur during hot rolling.

[0066] Si: 0.15% or less (excluding 0)

[0067] Silicon (Si) is preferably limited to 0.15% or less, because exceeding this value is very detrimental to the surface characteristics of the electroplating and also causes problems in ensuring the surface quality.

[0068] Phosphorus (P): 0.03% or less (excluding 0)

[0069] Phosphorus (P) is an unavoidable impurity that impairs steel weldability and is the primary cause of temper brittleness due to segregation at grain boundaries. Therefore, it is essential to keep the phosphorus content as low as possible. While theoretically limiting the phosphorus content to 0% is advantageous, it is unavoidable during the manufacturing process. Therefore, the upper limit is preferably set to 0.03%.

[0070] Sulfur (S): 0.015% or less (excluding 0)

[0071] Sulfur (S), like phosphorus (P), is an unavoidable impurity. Since it combines with Mn and other substances to form non-metallic inclusions, significantly reducing the toughness of steel, its content must be minimized. While theoretically limiting the sulfur content to 0% is advantageous, its inclusion is unavoidable during the manufacturing process. Therefore, it is necessary to control its upper limit. In the present invention, the upper limit is preferably set to 0.015%.

[0072] Aluminum (Al): 0.01 to 0.05%

[0073] Aluminum (Al) is an element added to molten steel for deoxidation. If its content is less than 0.01%, it is impossible to produce killed steel in a stable state. If its content exceeds 0.05%, while it improves strength through grain refinement, it can cause nozzle clogging during the steelmaking process. In the present invention, the upper limit is preferably set to 0.01 to 0.5%.

[0074] Titanium (Ti): 0.01 to 0.1%

[0075] Titanium (Ti) reacts with dissolved C during hot rolling and coiling to form microscopic TiC-based precipitates, which contribute greatly to the strength of the steel. In this case, the size and distribution of the precipitates have a great influence on the material of the steel. The finer the size, the greater the strengthening effect, the more uniform the distribution in the grains, and the better the material uniformity. If its content is less than 0.01%, it is insufficient to form TiC-based precipitates. If it exceeds 0.1%, there are problems such as casting cracks and nozzle clogging during steelmaking, which lead to increased costs. Therefore, in the present invention, its content is limited to 0.01-0.1%.

[0076] Nitrogen (N): 0.01% or less (excluding 0)

[0077] Nitrogen (N) contributes to steel hardness but is difficult to control. Like phosphorus (P), it segregates at grain boundaries and contributes to the steel's brittleness. While theoretically limiting the nitrogen content to 0% improves brittleness resistance, its inclusion is unavoidable during the manufacturing process. Therefore, the upper limit is preferably set to 0.01%.

[0078] In addition to the above components, one or more MM elements selected from tin (Sn), boron (B), molybdenum (Mo), nickel (Ni), and chromium (Cr) are further added in a total content of 0.0001 to 0.35%.

[0079] Tin (Sn) preferentially segregates with phosphorus (P) at grain boundaries, competing with phosphorus for position. By reducing the number of phosphorus segregation sites, the occurrence of brittleness caused by phosphorus segregation is suppressed, contributing to improved impact resistance of steel.

[0080] Boron (B) can serve as an element replacing silicon (Si), and can improve hardenability and strengthen grain boundaries to increase strength even in a very small amount.

[0081] Molybdenum (Mo) improves impact toughness by increasing yield strength through solid solution strengthening and grain boundary strengthening. However, since it is an expensive element, a content exceeding 0.2% increases costs and reduces weldability.

[0082] Nickel (Ni) improves both the strength and toughness of the base metal. However, since it is an expensive element, a content exceeding 0.3% increases costs and reduces weldability.

[0083] Chromium (Cr) strengthens steel through solid solution and slows the transformation of bainite during cooling, facilitating the formation of ferrite. However, when the chromium content exceeds 0.3%, the ferrite transformation is excessively delayed, making it impossible to achieve the desired ferrite fraction, resulting in reduced elongation.

[0084] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment are unavoidable during typical manufacturing processes, these cannot be excluded. Since these impurities are known to those of ordinary skill in the art, not all of them are specifically mentioned in this specification.

[0085] Furthermore, in order to provide a cold-rolled steel sheet having excellent material uniformity, the alloy may satisfy both Relationship 1 and Relationship 2.

[0086] [Equation 1]

[0087] 0≤[Ti]-3.42[N]≤4.0[C]

[0088] [Equation 2]

[0089] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0

[0090] (In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM] and [N] are weight % of each element, wherein MM is at least one selected from the group consisting of Sn, B, Mo, Ni and Cr).

[0091] On the other hand, when the values ​​of the above-mentioned equations (1) and (2) are smaller than the respective set ranges, the desired yield strength cannot be achieved due to a lack of reinforcement effect. If the values ​​exceed the set ranges, there is a problem that the strength is too high and the formability is deteriorated, making it difficult to form the desired part shape.

[0092] Hereinafter, the microstructure and precipitates of the cold-rolled, plated steel of the present invention will be described in detail.

[0093] The microstructure of the cold-rolled, coated steel according to the present invention is composed of ferrite and secondary phases, which may include pearlite, bainite, martensite, and retained austenite.

[0094] In the cold-rolled steel sheet, the area ratio of ferrite as a microstructure is 85% or more. If the area ratio of ferrite is less than 85%, although the strength can be improved, it is disadvantageous in terms of workability. Therefore, it is necessary to limit the area ratio of ferrite to the above range.

[0095] In the cold-rolled steel sheet, ferrite grains having a size of 100 nm or less account for more than 80% of the total precipitates. This is because if the precipitates of 100 nm or less account for less than 80%, the precipitation strengthening effect is insufficient and it is difficult to ensure the required strength.

[0096] The yield strength of the cold-rolled steel sheet is 240 to 530 MPa.

[0097] The cold-rolled steel sheet has a material deviation in the length direction or the width direction satisfying ΔYS of 30 MPa or less, thereby achieving excellent material uniformity.

[0098] That is, the cold-rolled steel sheet may have a ΔYS of 30 MPa or less at any two points in the longitudinal direction, or may have a ΔYS of 30 MPa or less at any two points in the width direction.

[0099] The measurement of ΔYS is as follows (see Figure 3 ).

[0100] YS was measured on three specimens in the longitudinal direction (rolling direction) at the head, middle, and tail of the steel plate, and in the width direction (1 / 8 to 7 / 8 of the width). The difference between the maximum and minimum YS values ​​was defined as ΔYS. In this context, YS refers to 0-10% at the head, 45-55% at the middle, and 90-100% at the tail of the steel plate.

[0101] The cold-rolled steel sheet further includes a plating layer containing at least one of Zn, Al and Mg as a plating metal.

[0102] The coating layer includes at least one selected from the group consisting of Zn, Zn-Al, and Zn-Al-Mg.

[0103] Hereinafter, a method for producing a cold-rolled steel sheet according to an embodiment of the present invention will be described in detail.

[0104] In the following manufacturing method, the reasons for limiting the addition ratios of the various components in the slab are the same as those for the composition limitations of the cold-rolled steel sheet described above, and therefore, a repeated explanation thereof will be omitted. In each step of the manufacturing method described below, the composition of the slab remains substantially unchanged, and therefore, the composition of the slab is substantially the same as that of the cold-rolled steel sheet.

[0105] A method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention comprises: a step of reheating a slab, wherein the slab comprises, in weight percent, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and further comprises, a total content of The present invention relates to a steel sheet comprising: a steel sheet having an MM content of 0.0001 to 0.35% and a balance of Fe and unavoidable impurities, and simultaneously satisfying the following equations 1 and 2, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr; a steel sheet comprising: ...

[0106] According to an embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet comprises: a step of reheating a slab, wherein the slab comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and the like. The invention further comprises the following steps: (i) a slab comprising 0.0001 to 0.35% MM in total, and the balance being Fe and unavoidable impurities, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni, and Cr; (ii) rough rolling and hot finish rolling the reheated slab to produce a hot-rolled steel sheet; (iii) coiling the hot-rolled steel sheet; (iv) cold rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet; and (v) annealing the cold-rolled steel sheet. In the step of coiling the hot-rolled steel sheet, when cooling the hot-rolled steel sheet from the hot finish rolling temperature to the coiling temperature, the difference in sheet feed speed on one strip of the run-out table (ROT) is 20% or less.

[0107] When cooling hot-rolled steel sheets to coiling temperature, if the sheet feed speed on the run-out table (ROT) cannot be controlled at a constant speed and varies, material deviation may occur along the length of the steel sheet. Therefore, material deviation can be reduced by controlling the ROT feed speed difference within a strip to less than 20%.

[0108] The plate passing speed can be defined as follows.

[0109] (Entry speed - Exit speed) / Entry speed (Equation 3)

[0110] A method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention comprises: a step of reheating a slab, wherein the slab comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less (excluding 0), P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0), Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less (excluding 0), and the like. (a) The method further comprises the following steps: a total content of MM of 0.0001 to 0.35% and the balance of Fe and unavoidable impurities, wherein the MM is at least one element selected from the group consisting of Sn, B, Mo, Ni, and Cr; rough rolling and hot finish rolling the reheated slab to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet; and annealing the cold-rolled steel sheet. In the step of coiling the hot-rolled steel sheet, when cooling the hot-rolled steel sheet from the hot finish rolling temperature to the coiling temperature, the amount of cooling water at the edge portion is less than 50% of the amount of cooling water at the center portion.

[0111] Here, the edge portion is the portion within 15% of the total width of the hot-rolled steel plate from the left and right corners toward the center of the hot-rolled steel plate. The center portion is the portion excluding the edge portion.

[0112] When cooling hot-rolled steel sheets to coiling temperature, if the cooling rate across the width of the steel sheet is not uniformly controlled, material deviations may occur across the width. Therefore, the amount of cooling water used at the edges of a strip should be less than 50% of the amount used at the center.

[0113] In the step of cooling the hot-rolled steel sheet to the coiling temperature, if the ROT plate throughput speed and cooling water volume conditions are not met, the amount of precipitates formed in the ferrite grains will exceed the scope of the present invention, resulting in the absolute value of the in-plane anisotropy ΔYS of the yield stress being greater than 30 MPa.

[0114] The slab simultaneously satisfies the following Relationship 1 and Relationship 2. Since the detailed descriptions of Relationship 1 and Relationship 2 are the same as those of the cold-rolled steel sheet, their detailed descriptions are omitted.

[0115] [Equation 1]

[0116] 0≤[Ti]-3.42[N]≤4.0[C]

[0117] [Equation 2]

[0118] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0

[0119] (In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM] and [N] are weight % of each element, and MM is at least one selected from the group consisting of Sn, B, Mo, Ni and Cr).

[0120] During the reheating step, the slab is heated to a temperature of 1100°C to 1300°C. Reheating below 1100°C can lead to a sharp increase in rolling load during subsequent hot rolling. In particular, due to the uneven distribution of alloy components within the slab, a Mn segregation zone forms in the center, resulting in different microstructure phase fractions between the center and the surface, leading to material deviation. Conversely, if the temperature exceeds 1300°C, austenite locally coarsens due to abnormal grain growth, ultimately resulting in a coarsened or uneven structure.

[0121] In the step of performing rough rolling and hot finish rolling on the reheated slab to produce a hot rolled steel sheet, the hot finish rolling temperature is 850°C to 950°C.

[0122] If the hot finish rolling temperature is lower than 850°C, the rolling load may increase significantly. In particular, at the edges of the steel plate, where the temperature drop is significant, insufficient microprecipitates may form, potentially causing material deviations across the width. Conversely, temperatures exceeding 950°C cause coarsening of the steel plate structure, leading to brittle steel, thicker scale, and reduced surface quality, such as hot-rolled scale defects.

[0123] During the coiling step of the hot-rolled steel sheet, the coiling temperature is 600°C to 700°C. If the coiling temperature exceeds 700°C, even if the aforementioned ROT cooling conditions and other manufacturing conditions are met, the desired strengthening effect cannot be achieved due to the coarse growth of microscopic precipitates during the holding stage after coiling. Conversely, if the coiling temperature is below 600°C, the microstructure is mostly bainite or martensite, and the desired ferrite microstructure fraction, as guaranteed by the present invention, cannot be achieved.

[0124] In the step of cold-rolling the coiled hot-rolled steel sheet to produce the cold-rolled steel sheet, the cold rolling ratio is 50 to 90%. If the cold rolling ratio is less than 50%, the amount of annealing recrystallization nuclei generated is small, and the grain size during annealing is excessive, which may lead to reduced strength and workability. If the cold rolling ratio exceeds 90%, the amount of nuclei generated is excessive, and the annealing recrystallized grains are too fine, resulting in reduced ductility.

[0125] In the step of annealing the cold-rolled steel sheet, the annealing temperature is 700°C to 850°C. If the annealing temperature is lower than 700°C, recrystallization is incomplete, and the desired formability is insufficient. If the annealing temperature exceeds 850°C, shape defects may occur due to sagging during the annealing process.

[0126] The annealing is preferably continuous annealing, and the annealing time is preferably maintained until recrystallization is completed, and is performed in the range of 10 seconds to 30 minutes.

[0127] The cold-rolled steel sheet manufactured by the manufacturing method according to the embodiment of the present invention and the cold-rolled steel sheet can be used directly without further processing.

[0128] However, if necessary, the process may further include forming a coating layer after annealing the cold-rolled steel sheet. This process can provide a cold-rolled steel sheet with a coating layer and excellent material uniformity.

[0129] The metal contained in the plating layer is at least one of Zn, Al and Mg.

[0130] The coating layer includes at least one selected from the group consisting of Zn, Zn-Al, and Zn-Al-Mg.

[0131] The step of forming the plating layer may use a hot-dip galvanizing method or an electro-galvanizing method.

[0132] Furthermore, if necessary, after forming the plating layer on the cold-rolled steel sheet, an alloying heat treatment may be further performed to transform the plating layer into an alloy layer.

[0133] Hereinafter, the present invention will be described in further detail by way of examples. However, the following examples are merely provided to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the contents described in the claims and the contents reasonably inferred therefrom.

[0134] Example

[0135] Steel slabs having the compositions shown in Table 1 below (target steel composition values, wt%) were produced, and the slabs were reheated in a furnace at 1200° C. to produce steel plates according to the production method shown in Table 2.

[0136] The yield strength, tensile strength, and elongation of the cold-rolled steel sheets produced by the above-described method were measured, and the microstructures were observed and shown in Table 3 below.

[0137] As can be seen from the results in Table 3 below, when the conditions of the present invention are met, appropriate yield strength can be ensured and excellent material uniformity can be obtained.

[0138] In Table 1 below, formula (1) represents relational formula 1, and formula (2) represents relational formula 2.

[0139] [Equation 1]

[0140] 0≤[Ti]-3.42[N]≤4.0[C]

[0141] [Equation 2]

[0142] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0

[0143] (In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM] and [N] are weight % of each element, and MM is at least one selected from the group consisting of Sn, B, Mo, Ni and Cr).

[0144] In this example, the surface temperature of the steel sheet was measured, and the hot finishing temperature and the coiling temperature were shown in Table 2. The coiling temperature refers to the surface temperature of the steel sheet measured immediately before coiling.

[0145] The amount of cooling water for the edge is controlled based on the displayed value of the water cooling device.

[0146] YS, TS, and EL in Table 3 represent average values ​​of measurements repeated three times at the middle of the steel plate in the longitudinal direction and at a position of 1 / 4W in the width direction.

[0147] Samples were taken at nine locations in the longitudinal direction (rolling direction) of the steel plate: the head, middle, and tail, and at 1 / 4W, 1 / 2W, and 3 / 4W in the width direction. The difference between the maximum and minimum YS values ​​is expressed as ΔYS in Table 3.

[0148]

Table 1

[0149]

[0150]

[0151]

Table 2

[0152]

[0153]

Table 3

[0154]

[0155]

[0156] The present invention can be implemented in various ways and is not limited to the above-described embodiments. Those skilled in the art will appreciate that the present invention can be implemented in other specific ways without changing the technical concept or essential features of the present invention. Therefore, it should be understood that the above-described embodiments are illustrative in all respects and are not restrictive.

Claims

1. A cold-rolled steel sheet, wherein: The steel plate comprises, in weight%, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less and greater than 0, P: 0.03% or less and greater than 0, S: 0.015% or less and greater than 0, Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less and greater than 0, and further comprises MM with a total content of 0.0001 to 0.35% and the balance of Fe and unavoidable impurities, wherein: The MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr, The alloying elements of the cold-rolled steel sheet satisfy the following equations 1 and 2 simultaneously: wherein the area ratio of the ferrite structure as the microstructure in the cold-rolled steel sheet is 85% or more, wherein in the ferrite grains as the microstructure of the cold-rolled steel sheet, precipitates with a size of less than 100 nm account for more than 80% of the total number of precipitates, [Equation 1] 0≤[Ti]-3.42[N]≤4.0[C] [Equation 2] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0 In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM], and [N] are weight % of each element, wherein MM is at least one selected from the group consisting of Sn, B, Mo, Ni, and Cr.

2. The cold-rolled steel sheet according to claim 1, wherein: The yield strength is 240 to 530 MPa.

3. The cold-rolled steel sheet according to claim 1, wherein: The △YS in the length direction or width direction of the steel plate is less than 30MPa, Among them, the measurement of △YS is as follows: In the length direction, three sample specimens are taken from the head, middle and tail areas of the steel plate, and in the width direction, three areas between 1 / 8 and 7 / 8 of the width are measured. The difference between the maximum YS and the minimum YS is ΔYS. Among them, in the longitudinal direction of the steel plate, the head portion refers to 0-10%, the middle portion refers to 45-55%, and the tail portion refers to 90-100%.

4. A method for manufacturing a cold-rolled steel sheet, comprising: The step of reheating a slab, wherein the slab comprises, in weight %, C: 0.0005 to 0.2%, Mn: 0.05 to 0.2%, Si: 0.15% or less and greater than 0, P: 0.03% or less and greater than 0, S: 0.015% or less and greater than 0, Al: 0.01 to 0.05%, Ti: 0.01 to 0.1%, N: 0.01% or less and greater than 0, and further comprises MM in a total content of 0.0001 to 0.35% and the balance Fe and unavoidable impurities, and the slab simultaneously satisfies the following equations 1 and 2, wherein MM is at least one element selected from the group consisting of Sn, B, Mo, Ni and Cr; The step of performing rough rolling and hot finish rolling on the reheated slab to produce a hot-rolled steel plate; a step of coiling the hot-rolled steel plate; The step of cold rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet; as well as The step of performing annealing heat treatment on the cold-rolled steel sheet; wherein, in the step of coiling the hot rolled steel sheet, when the hot rolled steel sheet is cooled from the hot finishing temperature to the coiling temperature, the speed difference of the sheet on one belt of the run-out roller table (ROT) is less than 20%, wherein, in the step of coiling the hot rolled steel sheet, when the hot rolled steel sheet is cooled from the hot finishing temperature to the coiling temperature, the amount of cooling water at the edge portion is less than 50% of the amount of cooling water at the center portion; The edge portion is the portion within 15% of the total width of the hot-rolled steel plate from the left and right corners of the hot-rolled steel plate to the center of the steel plate, based on the total width of the hot-rolled steel plate. The center portion is the portion excluding the edge portion. The area ratio of ferrite as the microstructure in the produced cold-rolled steel sheet is 85% or more. Among them, in the ferrite grains as the microstructure of the manufactured cold-rolled steel sheet, precipitates with a size of less than 100 nm account for more than 80% of the total precipitates. [Equation 1] 0≤[Ti]-3.42[N]≤4.0[C] [Equation 2] 0.4≤1.2[C]+0.1[Mn]+0.2[Si]+0.9[P]+9.5[Ti]+190.9[MM]-31.6[N]≤1.0 In the above relationship, [C], [Mn], [Si], [P], [Ti], [MM], and [N] are weight % of each element, and MM is at least one selected from the group consisting of Sn, B, Mo, Ni, and Cr.

5. The method for manufacturing a cold-rolled steel sheet according to claim 4, wherein: In the step of performing rough rolling and hot finish rolling on the reheated slab to produce a hot rolled steel sheet, The hot finishing rolling temperature is 850°C to 950°C.

6. The method for manufacturing a cold-rolled steel sheet according to claim 4, wherein: In the step of coiling the hot rolled steel sheet, The coiling temperature is 600°C to 700°C.

7. The cold-rolled steel sheet according to any one of claims 1 to 3, wherein The cold-rolled steel sheet further includes a plating layer containing at least one of Zn, Al and Mg as a plating metal.

8. The method for manufacturing a cold-rolled steel sheet according to claim 4, wherein: After the step of annealing the cold-rolled steel sheet, Further comprising the step of forming a plating layer, The metal forming the plating layer includes at least one of Zn, Al, and Mg.

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