Cold-rolled steel sheet with excellent weldability and manufacturing method thereof

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof. The cold-rolled steel sheet according to one embodiment may contain, by weight percent, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten (W): 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the balance being Fe and unavoidable impurities, and the yield strength of the welded member may be 500 MPa or more.

Description

[Technical field]

[0001] The present invention relates to a steel sheet, and more particularly to a cold-rolled steel sheet having excellent weldability and a method for producing the same. [Background technology]

[0002] Generally, flux cored welding (FCW) is a welding method that has the highest welding productivity and is easy to weld at various positions. The welding material used in the flux cored welding method is a flux cored wire, which is made by processing a strip drawn from a cold rolled steel plate for a welding rod into a U-shape and adding flux to the processed strip. The flux is made by mixing and adding flux components including an oxidizer to ensure weldability and various alloy elements according to the purpose in the form of high purity powder to improve the use characteristics of the welding rod, and then processing it into an O-shape to manufacture a welding wire. In this way, the flux components are added to ensure welding workability, and the alloy elements are added to ensure characteristics suitable for the use of the welding rod.

[0003] In this case, various properties required for the welding rod material are ensured by changing the type and amount of alloying elements in the core, which are added in the form of high purity powder. For example, in order to produce welding parts that require excellent low-temperature toughness, alloying elements for improving low-temperature toughness and flux must be mixed and charged together into the processed wire core.

[0004] In addition, low carbon steel is used as the cold rolled steel for wire used to manufacture the flux cored wire, and stainless steel is used for some special applications. The cold rolled steel for welding rod based on low carbon steel is a low alloy steel, so a large amount of alloy elements must be added to the core to ensure the welding rod characteristics according to the environment in which the welding rod is used. However, in the case of materials used for welding rods, a proper level of flux must be added to ensure welding workability, so there is a problem that the amount of alloy elements added to the core is limited as desired. Specifically, a large amount of oxidizer, slag former, arc stabilizer, and alloy components must all be added to the center of the steel for welding rods, but generally, the wire steel can be filled at a volume of about 30 to 60% including flux, so the weight ratio is limited to about 15 to 25%, depending on the alloy elements to be filled.

[0005] In this case, if the amount of alloying elements added is increased to ensure the usability of the welding rod, the flux components are limited, making it difficult to ensure stable welding characteristics. In addition, since these alloying elements must be added in the form of high-purity powder, not only does this increase the manufacturing cost of the welding rod, but the specific gravity of most of the added alloying elements is high, so that the added elements melted during welding work cause segregation of the welded part, which can cause welding defects.

[0006] In the case of stainless steel for welding wire, the amount of nickel (Ni) alloy element present in the carbon steel composition is basically larger than that of ordinary carbon steel, so the amount of core alloy element added with flux can be reduced to a certain extent, but the price of the original plate material is high because it is a high alloy steel, so it is only used for special purposes. In addition, in the case of the stainless steel-based welding rod steel, breakage often occurs due to work hardening during welding rod wire processing, so it is necessary to solve the problem of increased manufacturing costs due to additional processes such as the need to perform a separate annealing heat treatment.

[0007] Therefore, currently, for cold-rolled steel materials for welding wires that require workability, specifically drawing workability, strength, and toughness, a method is generally used in which expensive alloy elements are prepared in the form of high-purity powder and added together with flux components in order to ensure strength and low-temperature toughness when charging flux after low-carbon steel is made into a pipe. As described above, there is a problem that not only is the alloy powder added to ensure the characteristics high-purity and expensive, but also that the amount added is large, which restricts the addition conditions of the flux components to ensure welding stability.

[0008] In addition, the expensive alloying elements added cause segregation in the flux, which deteriorates welding workability. For example, as a method for manufacturing a steel plate for flux cored wire, a method for manufacturing a steel for welding rods having excellent impact toughness and strength properties by adding an element such as titanium (Ti) has been attempted. However, this method has problems such as an increase in cost due to the addition of a large amount of expensive alloying elements, and difficulty in ensuring drawability due to low ductility of the steel material.

[0009] In addition, a technology has been proposed that reduces welding defects by promoting the deoxidation reaction of the molten metal by adding materials such as titanium (Ti) and magnesium (Mg) to the flux raw materials. However, in order to fully obtain the deoxidation effect of the molten metal, it is necessary to add many alloy elements to the flux, but adding such a large number of alloy elements to the flux causes a problem of frequent spattering, in which fine particles are scattered around during welding, which reduces the efficiency of welding work.

[0010] Therefore, there is a need for development of a welding steel material and a manufacturing method thereof that utilizes a cold-rolled steel plate for a flux cored wire welding rod, which can obtain a welded joint having excellent strength and low-temperature toughness in an extremely low-temperature environment and has excellent welding workability and drawing workability. Summary of the Invention [Problem to be solved by the invention]

[0011] The technical problem to be solved by the present invention is to provide a cold rolled steel plate for flux cored wire welding rod, which can obtain a welded joint having excellent strength and low-temperature toughness in an extremely low temperature environment, and has excellent welding workability and drawability.

[0012] Another technical problem to be solved by the present invention is to provide a method for producing a cold-rolled steel sheet having the above-mentioned advantages. [Means for solving the problem]

[0013] A cold rolled steel sheet according to one embodiment of the present invention may contain, by weight, 0.01 to 0.05% carbon (C), 0.6 to 1.2% manganese (Mn), 0.05% or less (excluding 0%) silicon (Si), 0.0005 to 0.01% phosphorus (P), 0.008% or less (excluding 0%) sulfur (S), 0.0005 to 0.015% aluminum (Al), 0.0005 to 0.003% nitrogen (N), 0.1 to 0.4% tungsten (W), 1.0 to 2.0% chromium (Cr), 0.05 to 0.15% zirconium (Zr), the balance being Fe and unavoidable impurities, and the yield strength of the welded member may be 500 MPa or more.

[0014] In one embodiment, the cold rolled steel sheet has a FCW of the following formula 1: ,A The value can range from 40 to 150.

[0015] <Expression 1> F.C.W. ,A =(2.5×[Mn]+1.8×[Cr]+24×[Al])×(18×[W])×(45×[Zr]) / (34×[C]) (In the above formula 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C, respectively, in weight percent.)

[0016] In one embodiment, the cold rolled steel sheet may have a structure fraction of at least one of cementite and bainite of 1 to 7% by area, and the balance of ferrite. In one embodiment, the cold rolled steel sheet may have an elongation of 35% or more. In one embodiment, the cold rolled steel sheet may have a segregation index of a weld of less than 0.15%. In one embodiment, the cold rolled steel sheet may have an impact energy of 50 J or more at -40°C.

[0017] A method for producing a cold-rolled steel sheet according to another embodiment of the present invention includes the steps of: carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten (W): 0.1 to 0.4%, chromium (Cr): 1. The method may include the steps of heating a steel slab containing 0.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the balance Fe and unavoidable impurities, hot rolling the heated slab at a finishing rolling temperature of 820 to 880°C, coiling the hot rolled hot rolled steel sheet at 580 to 700°C, cold rolling the coiled hot rolled steel sheet to 50 to 90%, and annealing the cold rolled cold rolled steel sheet in the range of 700 to 850°C.

[0018] In one embodiment, the slab comprises a FCW of Formula 1: ,A The value can range from 40 to 150.

[0019] <Expression 1> F.C.W. ,A =(2.5×[Mn]+1.8×[Cr]+24×[Al])×(18×[W])×(45×[Zr]) / (34×[C]) (In the above formula 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C, respectively, in weight percent.)

[0020] In one embodiment, the step of heating the steel slab may be performed at 1,180° C. or more. In one embodiment, the hot-rolled steel sheet may be pickled before the step of cold rolling the coiled hot-rolled steel sheet. In one embodiment, the method may further include a step of temper rolling the annealed cold-rolled steel sheet after the step of annealing the cold-rolled cold-rolled steel sheet. Effect of the Invention

[0021] The cold rolled steel sheet according to an embodiment of the present invention provides a cold rolled steel sheet having excellent strength, excellent low-temperature toughness, excellent weldability and processability by controlling the contents of major elements such as manganese, chromium, aluminum, tungsten, zirconium and carbon, and can provide a cold rolled steel sheet for weldable flux cored wire used in various industrial fields such as the shipbuilding industry, the materials industry and the construction industry.

[0022] A method for producing a cold rolled steel sheet according to another embodiment of the present invention can provide a method for producing a cold rolled steel sheet having the above-mentioned advantages. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Terms such as first, second and third are used to describe various parts, components, regions, layers and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Thus, a first part, component, region, layer or section described below can be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.

[0024] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms include the plural forms unless the phrase clearly indicates otherwise. As used herein, the meaning of "comprising" embodies certain features, regions, integers, steps, operations, elements and / or components and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements and / or components.

[0025] When an element is referred to as being "on" or "on" another element, it means that it is immediately on or above the other element, or there may be other elements in between. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements.

[0026] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having a meaning consistent with the relevant technical literature and the currently disclosed content, and are not interpreted as ideal or very formal unless otherwise defined.

[0027] Moreover, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight.

[0028] In an embodiment of the present invention, the term "additionally contain an additional element" refers to the additional amount of the additional element substituting for the remaining iron (Fe).

[0029] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0030] A cold rolled steel sheet according to one embodiment of the present invention contains, by weight percent, 0.01 to 0.05% carbon (C), 0.6 to 1.2% manganese (Mn), 0.05% or less (excluding 0%) silicon (Si), 0.0005 to 0.01% phosphorus (P), 0.008% or less (excluding 0%) sulfur (S), 0.0005 to 0.015% aluminum (Al), 0.0005 to 0.003% nitrogen (N), 0.1 to 0.4% tungsten (W), 1.0 to 2.0% chromium (Cr), 0.05 to 0.15% zirconium (Zr), with the balance being Fe and unavoidable impurities.

[0031] The reasons for limiting the alloy components are explained below. In the following, wt% can be expressed as %.

[0032] Carbon (C): 0.01 to 0.05% Carbon (C) is an element added to ensure the strength of steel and to allow the heat affected zone (HAZ) to have similar material properties to the base metal during welding. The carbon content may be in the range of 0.01 to 0.05%, specifically, 0.012 to 0.048%.

[0033] If the carbon content is outside the upper limit of the range, there is a problem that wire breakage occurs due to high work hardening during drawing. In addition, low-temperature cracks occur at the welded joint, reducing impact toughness, and there is a processing problem that an additional heat treatment must be performed due to the high strength. If the carbon content is outside the lower limit of the range, there is a problem that the effect of ensuring the strength of the steel and enabling the heat-affected zone to have similar material properties to the base metal during welding is not achieved.

[0034] Manganese (Mn): 0.6 to 1.2% Manganese (Mn) is a representative solid solution strengthening element that increases the strength of steel and improves hot workability. The manganese content may be in the range of 0.6 to 1.2%, specifically, 0.65 to 1.15%.

[0035] If the manganese content is outside the upper limit of the range, a large amount of manganese sulfide (MnS) precipitates are formed, which not only reduces the ductility and workability of the steel, but also acts as a factor in the occurrence of center segregation, which may cause wire breakage during drawing in the welding rod manufacturing process.If the manganese content is outside the lower limit of the range, it may cause red brittleness, which may not contribute to the stabilization of austenite.

[0036] Silicon (Si): 0.05% or less, Silicon (Si) can combine with certain elements such as oxygen to form an oxide layer on the surface of the steel sheet, deteriorating the surface properties and reducing the corrosion resistance, and can promote hard phase transformation in the weld metal, reducing the impact properties. The silicon content may be in the range of 0.0005 to 0.0045%.

[0037] Phosphorus (P): 0.0005 to 0.010% Phosphorus (P) is an element that exists as a solid solution element in steel and causes solid solution strengthening to improve strength and hardness. The phosphorus content may be in the range of 0.0005 to 0.01%, specifically, 0.0008 to 0.0095%.

[0038] If the phosphorus content is outside the upper limit of the range, there is a problem that center segregation occurs during casting, and ductility is reduced, resulting in reduced wire workability, whereas if the phosphorus content is outside the lower limit of the range, there is a problem that it is difficult to maintain a certain level of rigidity.

[0039] Sulfur (S): 0.008% or less, Sulfur (S) combines with manganese (Mn) in steel to form nonmetallic inclusions, which may cause red shortness. The content of sulfur (S) may be 0.008% or less, specifically, in the range of 0.0004 to 0.0075%.

[0040] If the sulfur content is outside the upper limit of the range, a large amount of the nonmetallic inclusions is formed, which may cause the red shortness to worsen and may also cause a decrease in the toughness of the base metal of the steel plate.

[0041] Aluminum (Al): 0.0005 to 0.0150% Aluminum (Al) is an element added to aluminum-killed steel to prevent deterioration of the material due to carbonation and aging, and is an element advantageous in ensuring ductility in an appropriate range, and the above-mentioned effect is remarkable at extremely low temperatures. The content of the aluminum may be in the range of 0.0005 to 0.015%, specifically, 0.0006 to 0.0148%.

[0042] If the content is outside the upper limit, not only will the surface inclusions such as aluminum oxide (Al2O3) increase rapidly, deteriorating the surface characteristics of the rolled material and reducing workability, but ferrite will form locally in the grain system of the heat-affected zone, reducing mechanical characteristics and deteriorating the shape of the weld bead after welding. If the content is outside the lower limit, the adhesive strength of oxygen will decrease, causing material deterioration due to aging.

[0043] Nitrogen (N): 0.0005 to 0.003% Nitrogen (N) is an element that exists in a solid solution state in steel and strongly induces material strengthening. The nitrogen content may be in the range of 0.0005 to 0.003%, specifically, 0.0008 to 0.0029%.

[0044] If the nitrogen content is outside the upper limit of the range, the aging property may be excessively deteriorated, and the burden of denitrification may increase during steelmaking, which may deteriorate steelmaking workability.If the nitrogen content is outside the lower limit of the range, it may be difficult to ensure the desired rigidity.

[0045] Tungsten (W): 0.1 to 0.4% Tungsten (W) is an element that is effective in improving strength, high-temperature properties, and drawing workability, and forms a stable structure even at extremely low temperatures to improve low-temperature impact properties. The tungsten content may be in the range of 0.1 to 0.4%, specifically, 0.12 to 0.39%.

[0046] If the tungsten content is outside the upper limit of the range, the strength increases, which reduces the workability and induces surface defects, whereas if the tungsten content is outside the lower limit of the range, the effects of high temperature properties and drawing workability are not easily exhibited, and stable operation of frack production is difficult.

[0047] Chromium (Cr): 1.0 to 2.0% Chromium (Cr) is an element that is advantageous for the strength of a welded joint and plays a role in forming a stable rust layer, thereby contributing to improving corrosion resistance. The chromium content may be in the range of 1.0 to 2.0%, specifically, 1.10 to 1.85%.

[0048] If the chromium content is outside the upper limit of the range, chromium-based carbides are formed, causing brittleness and making the steel difficult to process, whereas if the chromium content is outside the lower limit of the range, the effects of adding chromium to improve the strength of the welded joint, form a stable rust layer, and contribute to improving corrosion resistance are difficult to achieve.

[0049] Zirconium (Zr): 0.05 to 0.15% Zirconium (Zr) is an element that is advantageous in terms of securing low-temperature toughness of a weld through the formation of precipitates, and also contributes greatly to improving the workability of welding materials. The content of zirconium may be in the range of 0.05 to 0.15%, specifically, 0.07 to 0.14%.

[0050] If the zirconium content is outside the upper limit of the range, the amount of zirconium precipitates increases, resulting in a decrease in workability, and the operating characteristics decrease due to an increase in the working temperature.If the zirconium content is outside the lower limit of the range, the effect of ensuring the low-temperature toughness of the welded portion and improving the workability of the welding material through the formation of the precipitates is difficult to be achieved.

[0051] The balance is iron (Fe). Inevitable impurities may also be included. Inevitable impurities refer to impurities that are inevitably mixed in during the manufacturing process of the cold rolled steel sheet. Since unavoidable impurities are widely known, a detailed description will be omitted. In one embodiment of the present invention, the addition of elements other than the above-mentioned alloy components is not excluded, and various elements may be included within a range that does not impair the technical concept of the present invention. When an additional element is further included, it is included to replace the balance Fe.

[0052] In one embodiment, the cold-rolled steel sheet may further contain Cu, Ti, Nb, and Ni. The respective component contents may be Cu 0.03% or less, Ti 0.002% or less, Nb 0.002% or less, and Ni 0.02% or less.

[0053] In one embodiment, the cold rolled steel sheet has a FCW of the following formula 1: ,A The value is between 40 and 150.

[0054] <Expression 1> F.C.W. ,A =(2.5×[Mn]+1.8×[Cr]+24×[Al])×(18×[W])×(45×[Zr]) / (34×[C]) (In the above formula 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C, respectively, in weight percent.)

[0055] The FCW of the above formula 1 ,Ais related to the correlation of each element on the weldability and drawing workability, and a cold rolled steel sheet having excellent weldability for flux cored wire has an FCW index of the above formula 1. ,A The value may be in the range of 40 to 150, specifically, 45 to 145.

[0056] The FCW of the above formula 1 ,A If the value is outside the upper limit of the above range, the fraction of the hard phase transformation structure increases, which may cause the welded member to break during pipe making and drawing. ,A If the value is outside the lower limit of the above range, the amount of transformation of the room temperature structure into a hard phase is small, which is advantageous in terms of workability, but there is a problem in that the amount of alloy elements in the flux increases in order to ensure strength and low-temperature toughness, which reduces welding workability and increases manufacturing costs.

[0057] In one embodiment, the cold-rolled steel sheet includes a structure fraction composed of at least one of cementite and bainite, which is 1 to 7% by area, and the remainder being ferrite. Specifically, the cold-rolled steel sheet having excellent weldability of the present invention may have a microstructure including a structure fraction composed of cementite and bainite, which is 1 to 7% by area, and the remainder being ferrite. The structure fraction composed of the cementite and the bainite may be in the range of 1 to 7%, specifically 1.1 to 6.8%, by area.

[0058] When the fraction of the structure composed of the cementite and the bainite is outside the upper limit of the range, there is a problem that it may cause breakage during drawing and deteriorate corrosion resistance, whereas when the fraction of the structure composed of the cementite and the bainite is outside the lower limit of the range, there is a problem that it may act as a factor causing deformation aging defects due to solid solution elements in the steel since precipitation of carbides is not promoted.

[0059] In one embodiment, the cold rolled steel sheet having excellent weldability may have an elongation percentage of 35% or more. Specifically, the elongation percentage may be in the range of 37% or more.

[0060] When the elongation satisfies the above range, the cold rolled steel sheet can be used for flux cored wire welding rod. When the elongation is out of the above range, the area reduction rate is low during drawing of the welding wire, which deteriorates pipe-making workability and causes cracks such as breaks during processing.

[0061] In one embodiment, the cold rolled steel sheet having excellent weldability may have a segregation index of a welded portion of less than 0.15%, where the segregation index of the welded portion refers to the segregation index of a welded portion welded with a flux cored wire manufactured using the cold rolled steel sheet having excellent weldability according to one embodiment of the present invention.

[0062] The segregation index of the weld is expressed as a ratio of an area occupied by the segregation due to the added element to the total area of ​​the weld. The segregation index of the weld may be 0.15% or less, specifically, in a range of 0.001 to 0.13%. If the segregation index of the weld is out of the range, there is a problem that the weldability of the cold rolled steel sheet is deteriorated.

[0063] In one embodiment, the cold rolled steel sheet may have a low-temperature impact energy of 50 J or more at -40°C. The cold rolled steel sheet may cause cracks in members such as welds due to low-temperature shock in a low-temperature environment, which may cause problems in the safety of the welded structure. The cold rolled steel sheet satisfies the impact energy of 50 J or more at low temperatures, thereby maintaining safety even in a low-temperature environment.

[0064] In one embodiment, the cold rolled steel sheet may have a yield strength of 500 MPa or more. The yield strength of the welded member refers to the yield strength of a welded joint welded with a flux cored wire manufactured using the cold rolled steel sheet. The yield strength of the welded joint needs to be maintained within an appropriate range regardless of the base material. When the yield strength of the welded member satisfies 500 MPa or more, high strength characteristics can be ensured in terms of ensuring the stability of the welded joint when applied to structural members.

[0065] A method for producing a cold rolled steel sheet according to another embodiment of the present invention includes, by weight percent, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten (Ti): 0.0005 to 0.015%, and the like. The method includes the steps of heating a steel slab containing 0.1 to 0.4% W, 1.0 to 2.0% Cr, 0.05 to 0.15% Zr, the remainder Fe, and unavoidable impurities, hot rolling the heated slab, coiling the hot rolled hot rolled steel sheet, cold rolling the coiled hot rolled steel sheet, and annealing the cold rolled cold rolled steel sheet. The detailed description of the steel slab is the same as that of the cold rolled steel sheet described above to the extent that it does not contradict, so duplicated description will be omitted.

[0066] First, the steel slab is heated. The step of heating the steel slab may be performed at 1,180°C or higher. Specifically, the step of heating the steel slab may be performed at a temperature range of 1,180 to 1,280°C. Specifically, the steel slab may be heated at a temperature range of 1,190 to 1,270°C.

[0067] If the temperature is outside the upper limit of the range, not only will the energy costs increase, but the amount of surface scale will increase, which may lead to material loss, and if the temperature is outside the lower limit of the range, there will be a problem of a sudden increase in load during the subsequent hot rolling.

[0068] Then, in the step of hot rolling the heated slab, the heated slab is hot rolled to obtain a hot rolled steel sheet. In one embodiment, the finish rolling temperature may be in the range of 820 to 880°C. Specifically, the finish rolling temperature may be in the range of 830 to 875°C.

[0069] If the finish rolling temperature is outside the upper limit of the range, the peelability of the surface scale is reduced, uniform hot rolling is not performed throughout the thickness, and grain refinement is insufficient, which may cause a decrease in impact toughness due to coarsening of grains. If the finish rolling temperature is outside the lower limit of the range, the hot rolling is finished in a low temperature region, which may cause a rapid progress in grain mixing, resulting in a decrease in hot rollability and workability.

[0070] Then, the step of coiling the hot rolled steel sheet may coil the hot rolled steel sheet obtained through the hot rolling step. In one embodiment, the coiling step may be performed at a coiling temperature of 580 to 700°C. Specifically, the coiling step may be performed at a temperature range of 590 to 690°C.

[0071] If the coiling temperature is outside the upper limit, the structure of the final product may become coarse, which may cause problems such as softening of the surface material and deterioration of pipe formability.If the coiling temperature is outside the lower limit, the temperature may be non-uniform in the width direction during the cooling and maintaining steps, which may cause differences in the behavior of low-temperature precipitates, thereby inducing material deviations, resulting in a problem of reduced processability.

[0072] Then, in the step of cold rolling the coiled hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled to a target thickness to obtain a cold-rolled steel sheet. In one embodiment, the hot-rolled steel sheet may be pickled prior to the step of cold rolling the coiled hot-rolled steel sheet.

[0073] In one embodiment, in the cold rolling step, the cold reduction may be in the range of 50 to 90%. Specifically, the cold reduction may be in the range of 55 to 85%.

[0074] If the cold reduction is outside the upper limit of the range, the material hardens, which reduces cold rolling workability due to the load of the rolling mill, and cracks may occur during drawing. If the cold reduction is outside the lower limit of the range, the driving force for recrystallization is low, which causes localized structure growth, making it difficult to ensure uniform material. In addition, when considering the thickness of the final product, the thickness of the hot-rolled steel sheet must be reduced, which significantly reduces hot rolling workability.

[0075] Then, the step of annealing the cold-rolled steel sheet can ensure the target strength and workability by performing annealing in a state where the strength is increased by the deformation introduced by the cold rolling. The step of annealing the cold-rolled steel sheet may be performed at a temperature range of 700 to 850°C. Specifically, the step of annealing the cold-rolled steel sheet may be performed at a temperature range of 710 to 840°C.

[0076] If the annealing step is performed at a temperature outside the upper limit of the above-mentioned range, the risk of sheet breakage due to heat buckle increases, and the annealing passability may be reduced.If the annealing step is performed at a temperature outside the lower limit of the above-mentioned range, deformation caused by cold rolling may not be sufficiently removed, resulting in a significant reduction in workability.

[0077] The method may further include a step of temper rolling the annealed cold rolled steel sheet after the annealing step. The step of temper rolling the annealed cold rolled steel sheet may be applied to a rolling reduction of 3% or less. Specifically, the step of temper rolling the annealed cold rolled steel sheet may be applied to a rolling reduction of 0.3 to 2.5% or less.

[0078] In the step of temper rolling the annealed cold-rolled steel sheet, the reduction ratio satisfies the above range, so that the shape of the material can be controlled to obtain a target surface roughness. In the step of temper rolling the annealed cold-rolled steel sheet, if the reduction ratio is excessively high, the material is hardened, but there is a problem that the workability is reduced.

[0079] After the annealing step, the annealed sheet can be used for manufacturing a flux cored wire. In one embodiment, the cold rolled steel sheet for the flux cored wire may include an outer skin made of the cold rolled steel sheet or the cold rolled steel sheet manufactured by the cold rolled steel sheet manufacturing method described above, and flux filled in the outer skin.

[0080] The flux cored wire including the cold rolled steel sheet of the present invention can exhibit the effects of the flux cored wire, and the effects are exhibited by the cold rolled steel sheet or the cold rolled steel sheet manufactured by the cold rolled steel sheet manufacturing method described above, regardless of the type of flux. Therefore, the flux can be a flux generally used in the field of flux cored wires, and detailed description thereof will be omitted. EXAMPLES

[0081] <Invention steels 1 to 5, Comparison steels 1 to 6> Table 1 below shows the compositions of the main components and FCW for the invention steels 1 to 5 and the comparison steels 1 to 6. ,A The values ​​shown are those of the FCW ,A The values ​​were designed taking into consideration the correlation between the elements that affect the weldability and drawing workability. Specifically, FCW ,A The value is (2.5 × [Mn] + 1.8 × [Cr] + 24 × [Al]) × (18 × [W]) × (45 × [Zr]) / (34 × [C]), where [Mn], [Cr], [Al], [W], [Zr], and [C] are the respective contents expressed in weight percentages.

[0082] [Table 1]

[0083] As seen from Table 1, the invention steels 1 to 5 satisfy the steel sheet composition of the present invention, and therefore are not FCW. ,A It can be seen that the value satisfies the values ​​of 40 to 150.

[0084] <Examples 1 to 4 and Comparative Examples 1 to 5> Inventive steels 1 to 5 in Table 1 and comparative steels 1 to 6 were processed under the process conditions shown in Table 2 below to produce inventive materials 1 to 9 and comparative examples 1 to 10. Specifically, the slabs in Table 1 above were heated at 1,230°C, and then hot rolling, coiling, cold rolling, and continuous annealing were performed under the manufacturing conditions disclosed in Table 2 below. At this time, a temper reduction of 0.9% was applied to the annealed sheets.

[0085] [Table 2]

[0086] Referring to Table 2, inventive materials 1 to 9 indicate that they were produced under the manufacturing conditions of the present invention using steels included in the range of inventive steels in Table 1. In contrast, comparative materials 1 to 4 indicate cases in which steels included in the range of inventive steels in Table 1 were used, but the manufacturing conditions were outside the range of manufacturing conditions of the present invention. Comparative materials 5 to 10 indicate that the steel types of comparative steels 1 to 6 in Table 1 were used, and they were produced under the manufacturing conditions of the present invention.

[0087] <Characteristics of cold-rolled steel sheets> Table 3 below shows the results of measuring the type and fraction of microstructure, elongation, threading property, and drawability of the cold rolled steel sheets manufactured under the manufacturing conditions in Table 2. The threading property was rated as good (O) if there was no rolling load during cold and hot rolling and no defects such as heat buckle occurred during continuous annealing, and was rated as poor (X) if there was a rolling load or defects such as sheet breakage occurred during continuous annealing.

[0088] The drawing workability was evaluated as poor (X) when a processing defect such as a break occurred during the drawing process of the flux cored wire with a cross-sectional area reduction rate of 61%, and good (O) when no processing defect occurred. The cold-rolled steel sheet thus produced was then cut into a strip with a width of 14 mm, which was then cut into a U-shape and filled with flux components to produce an O-shaped welding material with a diameter of 3.1 mm. The welding material was then drawn to produce a flux cored wire with a diameter of 1.2 mm, and low-temperature impact and tensile tests were carried out using the wire, with the results shown in Table 3 below.

[0089] In addition, the segregation index of the welded parts welded with the flux cored wire was measured using a scanning electron microscope, and the measurement results are shown in the following Table 3. At this time, the welded parts were drawn to a wire with a diameter of 1.2 mm, and the experiment was carried out on the welded parts using a pilot welding machine under the conditions of a voltage of 29 volts, a current of 150 to 180 A, and a welding speed of 40 cm per minute.

[0090] The segregation size of the welded parts welded with the flux cored wire was measured, and the measurement results are also shown in Table 3. The welded parts were drawn to a wire with a diameter of 1.2 mm, and the welded parts were welded using a pilot welding machine under the conditions of a voltage of 29 volts, a current of 150 to 180 A, and a welding speed of 40 cm per minute. [Table 3]

[0091] From Tables 2 and 3, it can be seen that the invention materials 1 to 9, which satisfy all of the alloy composition, microstructure characteristics, and manufacturing conditions, not only have good threading properties, but also meet the material standard of 35% or more elongation for cold-rolled steel sheets for flux cored wire welding rods. In addition, the segregation index of the wire manufactured from the welded parts is less than 0.15%, and no breaks or cracks occur in the welded parts during secondary processing, ensuring excellent workability. It can also be confirmed that the impact energy at -40°C is 50J or more, and the yield strength of the welded parts is 500MPa or more, ensuring excellent strength and low-temperature toughness.

[0092] In contrast, Comparative Materials 1 to 4 satisfy the alloy composition range proposed by the present invention but do not satisfy the microstructural properties and manufacturing conditions, and it can be confirmed that Comparative Materials 1 to 3 have poor rolling passability, and Comparative Examples 1 to 4 have poor annealing passability. It can also be confirmed that the elongation is lower than the target value, the yield strength of the welded member is less than 500 MPa, the impact energy value at -40°C is -50 J or less, and the drawability of the welded member is poor, and the target properties cannot be secured overall.

[0093] In addition, Comparative Materials 5 to 10 satisfy the manufacturing conditions proposed in the present invention, but do not satisfy the alloy composition. It can be confirmed that the properties such as the target elongation, segregation index of the weld, impact energy, and yield strength of the weld do not reach those of Invention Materials 1 to 9, and in most cases, it can be confirmed that breaks or cracks occur during drawing.

[0094] The present invention is not limited to the above-described embodiments and / or examples, and can be manufactured in various different forms, and a person having ordinary skill in the art to which the present invention pertains should understand that the present invention can be embodied in other specific forms without changing the technical concept or essential features of the present invention. Therefore, the above-described embodiments and / or examples should be understood to be illustrative in all respects and not limiting.

Claims

1. In weight percent, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten (W): 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the balance being Fe and unavoidable impurities. A cold-rolled steel plate having a welded member with a yield strength of 500 MPa or more.

2. FCW of the following formula 1 ，Ａ 2. The cold rolled steel sheet according to claim 1, wherein the value satisfies 40 to 150. <Formula 1> FCW ，Ａ =(2.5×[Mn]+1.8×[Cr]+24×[Al])×(18×[W])×(45×[Zr]) / (34×[C]) (In the above formula 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C, respectively, in weight percent.)

3. 2. The cold rolled steel sheet according to claim 1, wherein a structural fraction constituted by at least one of cementite and bainite contains, in terms of area percentage, 1 to 7% and the balance is ferrite.

4. 2. The cold rolled steel sheet according to claim 1, having an elongation of 35% or more.

5. 2. The cold rolled steel sheet according to claim 1, wherein the segregation index of the weld is less than 0.15%.

6. The cold-rolled steel sheet according to claim 1, having an impact energy of 50 J or more at -40 ° C.

7. heating a steel slab containing, by weight, 0.01 to 0.05% carbon (C), 0.6 to 1.2% manganese (Mn), 0.05% or less (except 0%) silicon (Si), 0.0005 to 0.01% phosphorus (P), 0.008% or less (except 0%) sulfur (S), 0.0005 to 0.015% aluminum (Al), 0.0005 to 0.003% nitrogen (N), 0.1 to 0.4% tungsten (W), 1.0 to 2.0% chromium (Cr), 0.05 to 0.15% zirconium (Zr), the balance being Fe and unavoidable impurities; hot rolling the heated slab at a finish rolling temperature of 820 to 880°C; A step of coiling the hot-rolled steel sheet at 580 to 700°C; cold rolling the coiled hot-rolled steel sheet to 50 to 90%; and annealing the cold-rolled steel sheet at a temperature in the range of 700 to 850°C.

8. The slab has a FCW of the following formula 1: ，Ａ The method for producing a cold rolled steel sheet according to claim 7, wherein the value satisfies 40 to 150. <Formula 1> FCW ，Ａ =(2.5×[Mn]+1.8×[Cr]+24×[Al])×(18×[W])×(45×[Zr]) / (34×[C]) (In the above formula 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C, respectively, in weight percent.)

9. The method for producing a cold rolled steel sheet according to claim 7, wherein the step of heating the steel slab is carried out at 1,180°C or higher.

10. The method for manufacturing a cold rolled steel sheet according to claim 7, further comprising the step of pickling the hot rolled steel sheet prior to the step of cold rolling the coiled hot rolled steel sheet.

11. The method for manufacturing a cold rolled steel sheet according to claim 7, further comprising the step of temper rolling the annealed cold rolled steel sheet after the step of annealing the cold rolled steel sheet.

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