阅读说明：本技术 药芯焊丝用冷轧钢板及其制造方法 (Cold-rolled steel sheet for flux-cored wire and method for manufacturing same ) 是由 金在翼 于 2018-12-17 设计创作，主要内容包括：根据本发明一实施例的药芯焊丝用冷轧钢板,以重量％计包含：0.005至0.08％的碳(C)、0.05至0.25％的锰(Mn)、0.05％以下(0％除外)的硅(Si)、0.0005至0.01％的磷(P)、0.008％以下(0％除外)的硫(S)、0.001至0.035％的铝(Al)、0.0005至0.003％的氮(N)、0.3至1.7％的镍(Ni)、0.05至0.5％的钼(Mo)、余量铁以及不可避免的杂质,以面积％计包含1至10％的渗碳体以及残余的铁素体。(According to an embodiment of the present invention, a cold-rolled steel sheet for a flux-cored wire includes, in wt%: 0.005 to 0.08% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.05% or less (excluding 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.008% or less (excluding 0%) of sulfur (S), 0.001 to 0.035% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.3 to 1.7% of nickel (Ni), 0.05 to 0.5% of molybdenum (Mo), the balance being iron and inevitable impurities, including 1 to 10% of cementite and the remainder being ferrite in area%.)

1. A cold-rolled steel sheet for a flux-cored wire, comprising in wt.%: 0.005 to 0.08% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.05% or less (excluding 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.008% or less (excluding 0%) of sulfur (S), 0.001 to 0.035% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.3 to 1.7% of nickel (Ni), 0.05 to 0.5% of molybdenum (Mo), the balance being iron, and unavoidable impurities,

comprising 1 to 10% cementite in area% and the balance ferrite.

2. The cold rolled steel sheet for flux-cored wire of claim 1, wherein W defined as the following formula 1_AIs in the range of 0.30 to 1.80,

[ formula 1]

W_A＝(35×[C]+0.8×[Mn]+34×[Al])×(1.1×[Ni])×(1.5×[Mo])

(in the formula 1, [ C ], [ Mn ], [ Al ], [ Ni ] and [ Mo ] represent the contents of C, Mn, Al, Ni and Mo, respectively).

3. The cold-rolled steel sheet for flux-cored wire of claim 1, wherein the cold-rolled steel sheet has an elongation of 40% or more.

4. The cold-rolled steel sheet for flux-cored wire of claim 1, wherein the weld segregation index of the cold-rolled steel sheet is less than 0.15%.

5. The cold-rolled steel sheet for flux-cored wire of claim 1, wherein the cold-rolled steel sheet has an impact energy of 50J or more at-40 ℃.

6. The cold-rolled steel sheet for flux-cored wire according to claim 1, wherein a yield strength of a welded part of the cold-rolled steel sheet is 500MPa or more.

7. A method for manufacturing a cold-rolled steel sheet for a flux-cored wire, comprising: a step of manufacturing a slab comprising, in weight%: 0.005 to 0.08% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.05% or less (excluding 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.008% or less (excluding 0%) of sulfur (S), 0.001 to 0.035% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.3 to 1.7% of nickel (Ni), 0.05 to 0.5% of molybdenum (Mo), the balance being iron, and unavoidable impurities;

heating the slab;

a step of hot rolling the heated slab so that a hot finish rolling temperature reaches 800 to 900 ℃, thereby obtaining a hot rolled steel sheet;

a step of curling the hot rolled steel sheet at a temperature range of 550 to 700 ℃;

a step of obtaining a cold-rolled steel sheet by cold-rolling the curled hot-rolled steel sheet at a reduction ratio of 50 to 85%; and

a step of annealing the cold rolled steel sheet at a temperature range of 700 to 850 ℃.

8. The method of manufacturing a cold rolled steel sheet for flux-cored wire of claim 7, wherein in the slab, W defined as the following formula 1_AIs in the range of 0.30 to 1.80,

[ formula 1]

W_A＝(35×[C]+0.8×[Mn]+34×[Al])×(1.1×[Ni])×(1.5×[Mo])

(in the formula 1, [ C ], [ Mn ], [ Al ], [ Ni ] and [ Mo ] represent the contents of C, Mn, Al, Ni and Mo, respectively).

9. The method of manufacturing a cold rolled steel sheet for flux-cored wire of claim 7, wherein the step of heating the slab is heating at 1100 to 1300 ℃.

10. The method of manufacturing a cold rolled steel sheet for a flux cored wire as set forth in claim 7, further comprising the step of pickling the rolled hot rolled steel sheet before the cold rolling.

11. The method of manufacturing a cold rolled steel sheet for a flux cored wire of 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.

12. A flux-cored wire comprising a skin composed of the cold-rolled steel sheet of claim 1 and a flux filled in the skin.

Technical Field

The present invention relates to a cold-rolled steel sheet for a flux-cored wire and a method for manufacturing the same. More particularly, the present invention relates to a cold-rolled steel sheet for flux-cored wires, which is excellent in strength, low-temperature toughness, welding workability, and workability by adding an appropriate amount of Ni and Mo, and a method for manufacturing the same.

Background

Generally, a Flux Cored Welding (FCW) method is a Welding method in which Welding productivity is the highest and Welding is easy in various places. The welding material used in this welding method is a flux-cored wire, and a Strip steel (Strip) is formed by drawing a welding rod with a cold-rolled steel sheet, and is processed into a U-shape, and then a solder resist is added to the processed U-shaped pipe. In this case, in order to secure weldability and obtain the use characteristics of the welding rod, the flux to be used is mixed in a powder form, and a flux component including an oxidizing agent and the like and an alloy element such as manganese (Mn) are added thereto, and then processed and manufactured in an O-shape. That is, the flux component is added to ensure the soldering workability, and the alloying element is added to ensure the characteristics suitable for the use of the welding rod.

At this time, various characteristics required for the material of the welding rod are secured by changing the kind and the amount of the alloy component in the core wire added in a powder state. For example, in order to produce welded parts having excellent low-temperature toughness, it is necessary to mix and incorporate alloying elements and fluxes for improving low-temperature toughness in a processed wire core portion.

On the other hand, as cold rolled steel for a wire used for manufacturing a flux-cored wire, low carbon steel is generally used, and stainless steel is used in some special applications.

Cold rolled steel for welding rods based on low carbon steel has excellent elongation, so that the steel does not tear during drawing, and also, because of low work hardening, continuous manufacturing is possible without additionally increasing a heat treatment process in the manufacturing process from forming to final welding wire, and thus, it is used in various applications.

However, since the cold rolled steel material for a carbon steel welding rod is a low alloy steel, in order to ensure the welding rod characteristics based on the use environment of the welding rod, it is necessary to add a large amount of alloy elements for ensuring the use characteristics in the core wire in addition to the basic flux components as the flux components to be filled in the processed material for a welding rod. However, in order to ensure the welding workability of the welding rod, it is necessary to add an appropriate amount of flux, and there is a limitation in arbitrarily adjusting the amount of the alloying elements to be put into the core. That is, a large amount of an oxidizing agent, a slag forming agent, an arc stabilizer, an alloy component, and the like must be added to the center of the wire steel, but the wire steel is usually filled only to a volume of about 30 to 60% including the flux, and the weight ratio is about 15 to 25% although there is a difference in filling powder. In this case, when the content of the alloying element for securing the use characteristics of the welding rod is increased, since flux components and the like are limited, there is a problem that it is difficult to secure the stability of the welding characteristics. Further, there are also the following problems: since these alloying elements are added in the form of high-concentration powder, not only is the cost increased, but also the segregation of the weld portion due to the molten core wire component during the welding operation is caused by the increase in the specific gravity of the added alloying components, which becomes a factor of the poor welding.

Basically, stainless steel for a welding wire is used only for special applications and the like because it is basically a high alloy material and the raw plate material is expensive, because the amount of alloying elements such as chromium (Cr) added together with flux can be reduced because the content of the alloying elements such as chromium (Cr) in the carbon steel component is large as compared with that of ordinary carbon steel. In addition, in the case of the steel material for a welding rod of the stainless steel base material, since the possibility of occurrence of fracture due to work hardening is high when the welding rod wire is processed, an annealing heat treatment is additionally performed between the manufacturing processes, which becomes a factor of increasing the manufacturing cost due to the additional process.

Conventionally, in a steel material for a welding wire which is required to have workability, particularly drawability, strength and toughness, in order to secure strength and low-temperature toughness at the time of loading flux after molding using a low-carbon steel, expensive alloying elements are prepared in the form of high-purity powder and are charged together with other flux components added for securing weldability, thereby improving strength and low-temperature toughness. Further, the expensive alloy element added at this time causes segregation in the solder resist, and there is a problem that the soldering workability is deteriorated.

For example, a method for manufacturing a steel sheet for a flux-cored wire by adding Cr, Ti, or the like to manufacture a steel for a welding rod having excellent impact toughness and strength characteristics is disclosed. However, since a large amount of expensive alloying elements must be added, there is a problem that the manufacturing cost is increased and the ductility is low, which makes it difficult to secure the drawing property.

Further, there is disclosed a technique for reducing welding defects by adding Ti, Mg, or the like to flux raw materials to promote a deoxidation reaction of molten metal. However, although a large amount of alloying elements must be added to the flux in order to sufficiently obtain the deoxidizing effect of the molten metal, as described above, when a large amount of alloying elements is added to the flux, there is a problem that spattering (scatter) phenomenon in which fine particles are scattered around during soldering often occurs, and soldering workability is deteriorated.

Therefore, it is required to develop a welded steel strip using a cold-rolled steel sheet for a cored wire welding rod excellent in welding workability and drawing workability, which can obtain a welded portion excellent in strength and low-temperature toughness in an ultra-low temperature environment, and a method for manufacturing the same.

Disclosure of Invention

[ problem ] to provide a method for producing a semiconductor device

The invention aims to provide a cold-rolled steel plate for a flux-cored wire and a manufacturing method thereof. Specifically, the present invention aims to provide a cold-rolled steel sheet for flux-cored wires, which is excellent in strength, low-temperature toughness, welding workability, and workability by adding an appropriate amount of Ni and Mo, and a method for manufacturing the same.

[ MEANS FOR SOLVING PROBLEMS ] A method for producing a semiconductor device

According to an embodiment of the present invention, a cold-rolled steel sheet for a flux-cored wire includes, in wt%: 0.005 to 0.08% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.05% or less (excluding 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.008% or less (excluding 0%) of sulfur (S), 0.001 to 0.035% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.3 to 1.7% of nickel (Ni), 0.05 to 0.5% of molybdenum (Mo), the balance being iron and inevitable impurities, including 1 to 10% of cementite and the remainder being ferrite in area%.

In the cold rolled steel sheet for flux-cored wire according to an embodiment of the present invention, W defined as the following formula 1_AAnd may be 0.30 to 1.80.

[ formula 1]

W_A＝(35×[C]+0.8×[Mn]+34×[Al])×(1.1×[Ni])×(1.5×[Mo])

(in formula 1, [ C ], [ Mn ], [ Al ], [ Ni ] and [ Mo ] represent the contents of C, Mn, Al, Ni and Mo, respectively.)

The cold-rolled steel sheet for a flux-cored wire according to an embodiment of the present invention may have an elongation of 40% or more.

The cold-rolled steel sheet for a flux-cored wire according to an embodiment of the present invention may have a weld segregation index of less than 0.15%.

The cold-rolled steel sheet for a flux-cored wire according to an embodiment of the present invention may have an impact energy of 50J or more at-40 ℃.

In the cold-rolled steel sheet for a flux-cored wire according to an embodiment of the present invention, the yield strength of the welded parts may be 500MPa or more.

The method for manufacturing the cold-rolled steel plate for the flux-cored wire according to the embodiment of the invention comprises the following steps: a step of manufacturing a slab comprising, in mass%: 0.005 to 0.08% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.05% or less (excluding 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.008% or less (excluding 0%) of sulfur (S), 0.001 to 0.035% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.3 to 1.7% of nickel (Ni), 0.05 to 0.5% of molybdenum (Mo), the balance being iron, and unavoidable impurities; heating the slab; a step of hot rolling the heated slab so that a finish hot rolling temperature reaches 800 to 900 ℃, thereby obtaining a hot-rolled steel sheet; a step of curling the hot rolled steel sheet at a temperature range of 550 to 700 ℃; a step of obtaining a cold-rolled steel sheet by cold-rolling the curled hot-rolled steel sheet at a reduction ratio of 50 to 85%; and a step of annealing the cold-rolled steel sheet at a temperature of 700 to 850 ℃.

In the slab, W is defined as formula 1_AAnd may be 0.30 to 1.80.

[ formula 1]

W_A＝(35×[C]+0.8×[Mn]+34×[Al])×(1.1×[Ni])×(1.5×[Mo])

(in formula 1, [ C ], [ Mn ], [ Al ], [ Ni ] and [ Mo ] represent the contents of C, Mn, Al, Ni and Mo, respectively.)

The step of heating the slab may be heating at 1100 to 1300 ℃.

Further comprising the step of pickling the curled hot rolled steel sheet before cold rolling.

Further comprising the step of temper rolling the annealed cold-rolled steel sheet after the step of annealing the cold-rolled steel sheet.

The flux-cored wire according to an embodiment of the present invention includes a skin composed of the cold-rolled steel sheet described above and a flux filled in the skin.

[ PROBLEMS ] the present invention

The cold-rolled steel sheet for a flux-cored wire according to an embodiment of the present invention has excellent strength, low-temperature toughness, welding workability, and workability.

According to an embodiment of the present invention, a cold-rolled steel sheet for a flux-cored wire, which is applicable to shipbuilding industry, materials industry, construction industry, and the like and can perform all-round welding, can be provided.

Drawings

Fig. 1 is a cross-sectional photograph of a flux-cored wire manufactured by a cold-rolled steel sheet of invention example 2 observed with a Scanning Electron Microscope (SEM).

Fig. 2 is a photograph of a surface portion of a flux-cored wire manufactured by using a cold-rolled steel sheet of invention example 2, which is observed by a Scanning Electron Microscope (SEM).

Fig. 3 is a cross-sectional photograph of a flux-cored wire manufactured by using a cold-rolled steel sheet of comparative example 5, which is observed by a Scanning Electron Microscope (SEM).

Fig. 4 is a photograph of a surface portion of a flux-cored wire manufactured by a cold-rolled steel sheet of comparative example 5 observed with a Scanning Electron Microscope (SEM).

Detailed Description

The terms first, second, third, etc. herein are used to describe various parts, components, regions, layers and/or sections, but these parts, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first part, component, region, layer and/or section discussed below could be termed a second part, component, region, layer and/or section without departing from the scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, regions, integers, steps, actions, elements, and/or components, but do not preclude the presence or addition of other features, regions, integers, steps, actions, elements, and/or components.

If a portion is described as being "on" or "over" another portion, it can be directly "on" or "over" the other portion or there can be other portions between them. When a portion is described as being "directly on" another portion, there are no other portions between them.

Although not otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. To the extent that a term defined in a dictionary is referred to as having a meaning that is consistent with that disclosed in the pertinent art documents and this document, it should not be construed in an idealized or overly formal sense unless expressly so defined herein.

Further, in the case where not specifically mentioned,% represents% by weight, and 1ppm is 0.0001% by weight.

Further inclusion of the additional element in one embodiment of the present invention means that a part of the balance of iron (Fe) is replaced with the additional element in an amount equivalent to the added amount of the additional element.

The following detailed description of the embodiments of the present invention is provided to enable those skilled in the art to easily practice the present invention. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

According to an embodiment of the present invention, a cold-rolled steel sheet for a flux-cored wire includes, in wt%: 0.005 to 0.08% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.05% or less (excluding 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.008% or less (excluding 0%) of sulfur (S), 0.001 to 0.035% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.3 to 1.7% of nickel (Ni), 0.05 to 0.5% of molybdenum (Mo), the balance being iron and inevitable impurities, including 1 to 10% of cementite and residual ferrite in area%.

The reason for limiting the components of the cold-rolled steel sheet will be described below.

C: 0.005 to 0.08% by weight

Carbon (C) is an element added to improve the strength of steel, and is an element added to impart characteristics similar to those of the base material to the welding heat-affected zone. When the C content is too small, the effect is insufficient. However, when the C content is too large, there occurs a problem that the drawing process is broken due to high strength or work hardening. In addition, not only does low-temperature cracking of the welded joint occur or impact toughness is reduced, but also, since hardness is high, it is necessary to process the welded joint into a desired final product through multiple heat treatments. Therefore, the content of C may be 0.005 to 0.08 wt%, and more specifically, in order to improve the characteristics of the welding heat affected zone, the content of C may be 0.008 to 0.05 wt%.

Mn: 0.05 to 0.25% by weight

Manganese (Mn) is a solid solution strengthening element, and functions to increase the strength of steel and improve hot workability. However, when too much is added, a large amount of manganese sulfide (MnS) precipitates are formed, thereby hindering the ductility and workability of the steel. When the Mn content is too small, it becomes a cause of red hot brittleness and is not favorable for stabilizing austenite. On the contrary, when the Mn content is too much, it becomes a main factor that ductility is decreased and center segregation is caused, thereby causing breakage during a drawing operation of a welding rod manufacturing process. Accordingly, the content of Mn may be 0.05 to 0.25 wt%, and more specifically, may be 0.07 to 0.23 wt%.

Si: 0.05 wt% or less

Silicon (Si) combines with oxygen and the like to form an oxide layer on the surface of the steel sheet, which not only deteriorates the surface properties and reduces the corrosion resistance, but also promotes the hard transformation in the weld metal to reduce the low-temperature impact properties. Therefore, the content of Si is limited to 0.05 wt% or less. More specifically, the content of Si may be 0.001 to 0.03 wt%.

P: 0.0005 to 0.01% by weight

Phosphorus (P) is an element that exists as a solid solution element in steel and improves strength and hardness by causing solid solution strengthening. When the content of P is too small, it may be difficult to maintain a certain level of rigidity. When the content of P is too large, center segregation is caused during casting and ductility is reduced to deteriorate workability of the wire. Accordingly, the content of P may be 0.0005 to 0.01 wt%. More specifically, the content of P may be 0.001 to 0.009 wt%.

S: 0.008 wt% or less

Sulfur (S) combines with manganese in steel to form non-metallic inclusions and becomes a main factor of red shortness, and therefore it is preferable to reduce the content thereof. Further, when the S content is high, there arises a problem that toughness of the steel plate base material is lowered. Therefore, the content of S may be 0.008% or less, and more specifically, the content of S may be 0.0005 to 0.007 wt%.

Al: 0.001 to 0.035 wt.%

Aluminum (Al) is an element added as a deoxidizer in aluminum killed steel (aluminum killed steel) and to prevent deterioration of the material due to aging, and is an element contributing to securing ductility, and this effect is more remarkable at ultralow temperatures. When the content of Al is too small, the effect is not sufficient. On the contrary, when the content of Al is excessive, it is due to alumina (Al)₂O₃) The rapid increase of surface inclusions, etc., causes problems such as deterioration of surface properties of the hot rolled material and deterioration of workability, and also causes problems such as deterioration of mechanical properties due to local formation of ferrite at grain boundaries of a weld heat affected zone and deterioration of bead shape after welding. Therefore, the content of Al may be 0.001 to 0.035% by weight, more specifically, the content of Al may be 0.005 to 0.033% by weight.

N: 0.0005 to 0.003 wt.%

Nitrogen (N) is an element that exists in a solid solution state inside steel and contributes to material strengthening. When the content of N is too small, it is difficult to secure the target rigidity. However, if the N content is too high, not only the aging property is drastically deteriorated, but also the burden due to the denitrification is increased in the steel production step, thereby deteriorating the steel production workability. Accordingly, the content of N may be 0.0005 to 0.003 wt%, and more specifically, the content of N may be 0.001 to 0.0027 wt%.

Ni: 0.3 to 1.7% by weight

Nickel (Ni) is an element effective in stabilizing austenite and improving drawability by increasing ductility, and is an essential element for improving low-temperature impact properties by forming a stable structure at ultra-low temperatures. When the Ni content is too small, it is difficult to obtain the above-described effects and to achieve stable operation of the flux composition. Conversely, when the Ni content is too large, drawability may be deteriorated due to an increase in strength, and surface defects may be caused. Therefore, the content of Ni may be 0.3 to 1.7 wt%, and more specifically, the content of Ni may be 0.5 to 1.5 wt%.

Mo: 0.05 to 0.5% by weight

Molybdenum (Mo) is an element advantageous for ensuring the strength of a welded joint by improving hardenability. When the content of Mo is too small, the above-described effects are hardly obtained. When the content of Mo is too high, the increase in the production amount of molybdenum carbide causes brittleness, and thus a problem of deteriorating workability may occur. Therefore, the content of Mo may be 0.05 to 0.5 wt%. More specifically, the content of Mo may be 0.1 to 0.45 wt%.

The balance of the present invention is iron (Fe). However, impurities are inevitably mixed from raw materials or the surrounding environment in a general manufacturing process, and thus cannot be excluded. These impurities are predictable by all skilled in the usual manufacturing processes and therefore not all aspects of them are specifically described in this specification.

In addition, the cold-rolled steel sheet according to an embodiment of the present invention has a fine structure including cementite and residual ferrite in an area of 1 to 10%. If the cementite fraction is too small, precipitation of carbide cannot be promoted. Therefore, the solid solution elements in the steel become factors showing strain aging defects. On the contrary, when the cementite fraction is too high, not only cracks are caused in drawing but also there is a problem that corrosion resistance is deteriorated. Thus, the fraction of cementite may range from 1 to 10 area%. More specifically, the fraction of cementite may be 1.3 to 7.5 area%. The residual component is ferrite.

In addition, the cold rolled steel sheet of the present invention satisfies not only the alloy composition described above but also W defined by the following formula 1_AAnd may be 0.30 to 1.80.

[ formula 1]

W_A＝(35×[C]+0.8×[Mn]+34×[Al])×(1.1×[Ni])×(1.5×[Mo])

(in formula 1, [ C ], [ Mn ], [ Al ], [ Ni ] and [ Mo ] represent the contents of C, Mn, Al, Ni and Mo, respectively.)

W_AIs designed in consideration of the mutual relationship between the elements that affect the welding workability and the drawing workability. When W is_AIf the amount of transformation of the normal temperature structure into the hard phase is too small, the workability is improved, but if the amount of addition of the alloying element added as a component in the flux is increased to secure the strength and the low temperature toughness, the welding workability is deteriorated. Conversely, when W_AWhen too large, the fraction of the phase change hardened structure increases, so that the problem of cracking of the welded part occurs during the forming and drawing processes. Thus, W_AThe range of 0.30 to 1.80 may be satisfied. More specifically, W_AAnd may be 0.33 to 1.70.

The cold-rolled steel sheet according to an embodiment of the present invention has excellent elongation. Specifically, the elongation may be 40% or more. Since the physical properties are satisfied, it can be preferably used as a material for a flux cored wire welding rod. When the elongation is too low, the reduction rate of the fracture is lowered during the drawing of the wire to deteriorate the formability, and cracks such as tearing may occur during the processing. More specifically, the elongation may be 40 to 50%.

In addition, the cold-rolled steel sheet according to an embodiment of the present invention has an excellent weld segregation index. The weld segregation index is a segregation index of a weld welded using a flux-cored wire manufactured by using the cold-rolled steel sheet according to the embodiment of the present invention. The weld segregation index may be represented by the ratio of the area occupied by the segregation portion by adding an element to the entire area of the weld. Specifically, the weld segregation index may be 0.15% or less. More specifically, the weld segregation index may be 0.01 to 0.13%.

In addition, the cold-rolled steel sheet according to an embodiment of the present invention may have excellent low-temperature impact energy at-40 ℃. Specifically, the low-temperature impact energy at 40 ℃ may be 50J or less. In a low-temperature environment, cracks are generated in a welded portion or the like due to low-temperature impact or the like, and a safety problem of a welded structure may be caused.

In addition, the cold-rolled steel sheet according to an embodiment of the present invention has excellent weld yield strength. The weld yield strength is the yield strength of a weld welded using a flux-cored wire manufactured by a cold-rolled steel sheet according to an embodiment of the present invention. The yield strength of the welded portion is required to be maintained at an appropriate level regardless of the base material, and when used as a structural member, high strength characteristics should be ensured to be 500MPa or more in view of stability of the welded portion.

The method for manufacturing the cold-rolled steel plate for the flux-cored wire according to the embodiment of the invention comprises the following steps: a step of manufacturing a slab comprising, in weight%: 0.005 to 0.08% of carbon (C), 0.05 to 0.25% of manganese (Mn), 0.05% or less (excluding 0%) of silicon (Si), 0.0005 to 0.01% of phosphorus (P), 0.008% or less (excluding 0%) of sulfur (S), 0.001 to 0.035% of aluminum (Al), 0.0005 to 0.003% of nitrogen (N), 0.3 to 1.7% of nickel (Ni), 0.05 to 0.5% of molybdenum (Mo); the balance being iron and unavoidable impurities; heating the slab; a step of hot rolling the heated slab to obtain a hot-rolled steel sheet; curling the hot rolled steel sheet; a step of cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

The following steps are specifically described.

First, a slab is manufactured. In the steel making step, C, Mn, Si, P, S, Al, N, Ni, Mo are controlled to appropriate contents. In the steel making step, the molten steel whose composition is adjusted is made into a slab by continuous casting.

The components of the slab have been described in detail in the aforementioned cold rolled steel sheet for a flux cored wire, and thus, a repetitive description thereof will be omitted herein. The aforementioned formula 1 is also satisfied in the alloy composition of the slab. In the manufacturing process of the cold-rolled steel sheet for flux-cored wire, the alloy composition is not substantially changed, and the slab and the finally manufactured cold-rolled steel sheet for flux-cored wire may have the same alloy composition.

Next, the slab is heated. This allows the subsequent hot rolling process to be smoothly performed and the slab to be homogenized. Specifically, the slab may be heated at a temperature of 1100 to 1300 ℃. If the heating temperature of the slab is too low, there is a problem that the load is rapidly increased in the subsequent hot rolling process, and conversely, if the heating temperature of the slab is too high, not only the productivity cost is increased, but also the loss of material is caused by the increase in the amount of surface scale. More specifically, the heating temperature of the slab may be 1150 to 1250 ℃.

Then, the heated slab is hot-rolled to produce a hot-rolled steel sheet. At this time, the finish rolling temperature of the hot rolling may be 800 to 900 ℃. When the finish rolling temperature is too low, the phenomenon of uneven crystal grains rapidly occurs in the low temperature region along with the end of hot rolling, which may lead to the reduction of hot rolling performance and workability. On the contrary, when the finish rolling temperature is too high, the peeling property of the surface scale is lowered and the hot rolling results in non-uniform thickness as a whole, so that the grain refinement is insufficient, and the impact toughness due to the coarsening of the grains is lowered. More specifically, the finish rolling temperature may be 810 to 890 ℃.

Then, the hot rolled steel sheet is curled. At this time, the curling temperature may be 550 to 700 ℃. Before curling after hot rolling, cooling of the hot rolled steel sheet may be performed at a Run-out table (ROT). When the curling temperature is too low, the behavior of low-temperature precipitates during cooling and holding shows a difference due to the non-uniformity of the temperature in the width direction, thereby causing material variation to have an adverse effect on workability. On the contrary, when the curling temperature is too high, the surface texture is softened and the formability is deteriorated as the texture of the final product is coarsened. More specifically, the crimping temperature may be 610 to 690 ℃.

The method may further include, after the hot rolled steel sheet is curled, a step of pickling the curled hot rolled steel sheet before the step of cold rolling the curled hot rolled steel sheet.

Then, the hot-rolled steel sheet after the curling is cold-rolled to manufacture a cold-rolled steel sheet. At this time, the rolling reduction may be 50 to 85%. When the reduction ratio is too small, local growth of a structure or the like occurs due to low recrystallization driving force, which is not only disadvantageous in obtaining a uniform material but also in consideration of the thickness of the final product, the thickness of the hot-rolled steel sheet needs to be reduced during the work, thus causing a problem of significantly reducing the hot-rolling workability. On the contrary, if the reduction ratio is too high, the material is hardened, which not only causes cracks in drawing but also causes a problem that the cold rolling workability is lowered by the load of the rolling mill. Therefore, the rolling reduction can be 50 to 85%. More specifically, it may be 65 to 80%.

Then, the cold rolled steel sheet is annealed. Annealing is performed from a state in which the strength is improved due to deformation occurring during cold rolling, so that the target strength and workability can be secured. At this time, the annealing temperature may be 700 to 850 ℃. When the annealing temperature is too low, there is a problem that workability is remarkably lowered because deformation due to cold rolling is not sufficiently removed. Conversely, when the annealing temperature is too high, a problem of annealing passability such as plate breakage may occur. More specifically, the annealing temperature may be 730 to 845 ℃. The annealing may be continuously performed without curling the cold-rolled steel sheet.

After the step of annealing the cold-rolled steel sheet, a step of temper rolling the annealed cold-rolled steel sheet may be further included. The temper rolling may be performed at a reduction of 10% or less.

After the hot rolled plate is annealed, the hot rolled plate can be used for manufacturing the flux-cored wire.

The flux-cored wire according to an embodiment of the present invention includes a skin composed of the cold-rolled steel sheet and a flux filled in the skin. The effect of the flux-cored wire according to an embodiment of the present invention is independent of the filled flux, and is based on the effect of the cold-rolled steel sheet. Therefore, the flux may use a conventional flux used in the field of flux-cored wires without limitation. Since the flux is widely known, a detailed description thereof will be omitted herein.

Preferred examples and comparative examples of the present invention are described below. However, the following embodiment is only a preferred embodiment of the present invention, and the following embodiment is not intended to limit the present invention.