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 成旻官 金珍儿 于 2018-07-05 设计创作，主要内容包括：根据本发明的一个方面，其目的在于提供一种低温韧性、焊接操作性及加工性优异的药芯焊丝用冷轧钢板及其制造方法。本发明的一个实施方式提供一种药芯焊丝用冷轧钢板及其制造方法，以重量％计，所述冷轧钢板包括：C：0.005～0.10％、Mn：0.05～0.25％、Si：0.05％以下(0％除外)、P：0.0005～0.01％、S：0.008％以下(0％除外)、Al：0.005～0.06％、N：0.0005～0.003％、Ni：0.8～1.7％、Cr：0.1～0.5％及余下的Fe和不可避免的杂质，由以下关系式1定义的W<Sub>N</Sub>是0.10～0.75。关系式1：W<Sub>N</Sub>＝(31×C+0.5×Mn+20×Al)×(Ni)×(0.6×Cr)，根据本发明的一个方面，提供一种低温韧性、焊接操作性及加工性优异的药芯焊丝用冷轧钢板，从而能够提供用于造船产业、材料产业、建筑产业等领域的可全方位焊接的药芯焊丝型焊棒钢带。(According to one aspect of the present invention, an object is to provide a cold-rolled steel sheet for a flux-cored wire excellent in low-temperature toughness, welding workability, and a method for manufacturing the same. One embodiment of the invention provides a flux-cored wireA cold-rolled steel sheet and a method for manufacturing the same, the cold-rolled steel sheet comprising, in wt.%: c: 0.005-0.10%, Mn: 0.05 to 0.25%, Si: 0.05% or less (except 0%), P: 0.0005 to 0.01%, S: 0.008% or less (except 0%), Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.8-1.7%, Cr: 0.1 to 0.5% and the balance Fe and inevitable impurities, W defined by the following relation 1 N Is 0.10 to 0.75. Relation 1: w N According to one aspect of the present invention, there is provided a cold-rolled steel sheet for a flux-cored wire having excellent low-temperature toughness, welding workability, and workability (31 × C +0.5 × Mn +20 × Al) × (Ni) × (0.6 × Cr), and thereby it is possible to provide a flux-cored wire type rod steel strip which can be used in the fields of shipbuilding industry, materials industry, construction industry, and the like and can be welded in all directions.)

1. A cold-rolled steel sheet for a flux-cored wire, comprising, in wt.%: c: 0.005-0.10%, Mn: 0.05 to 0.25%, Si: 0.05% or less (except 0%), P: 0.0005 to 0.01%, S: 0.008% or less (except 0%), Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.8-1.7%, Cr: 0.1 to 0.5% and the balance Fe and inevitable impurities, W defined by the relation 1_NIs 0.10 to 0.75,

relation 1: w_N＝(31×C+0.5×Mn+20×Al)×(Ni)×(0.6×Cr)

Wherein the content of each element in the relation 1 is represented by weight%.

2. The cold rolled steel sheet for flux cored wire as claimed in claim 1, wherein,

the cold-rolled steel sheet has a microstructure composed of 1 to 6% by area of iron carbide and the balance of ferrite.

3. The cold rolled steel sheet for flux cored wire as claimed in 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 as claimed in 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 as claimed in claim 1, wherein,

the cold-rolled steel sheet has an impact energy of 50J or more at-40 ℃.

6. A manufacturing method of a cold-rolled steel plate for a flux-cored wire comprises the following steps:

heating a slab to 1100-1300 ℃, the slab comprising in weight%: c: 0.005-0.10%, Mn: 0.05 to 0.25%, Si: 0.05% or less (except 0%), P: 0.0005 to 0.01%, S: 0.008% or less (except 0%), Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.8-1.7%, Cr: 0.1 to 0.5% and the balance Fe and inevitable impurities, W defined by the following relation 1_NIs 0.10 to 0.75;

hot rolling the heated plate blank to enable the hot finishing temperature to reach 880-950 ℃ so as to obtain a hot rolled steel plate;

rolling the hot rolled steel plate at the temperature of 550-700 ℃;

cold rolling the rolled hot rolled steel plate at a reduction rate of 50-85% to obtain a cold rolled steel plate; and

continuously annealing the cold-rolled steel sheet at a temperature of 700 to 850 ℃,

relation 1: w_N＝(31×C+0.5×Mn+20×Al)×(Ni)×(0.6×Cr)

Wherein the content of each element in the relation 1 is represented by weight%.

7. The method of manufacturing a cold rolled steel sheet for a flux cored wire as set forth in claim 6, further comprising:

and a step of pickling the rolled hot rolled steel sheet before the cold rolling.

8. The method of manufacturing a cold rolled steel sheet for a flux cored wire as set forth in claim 6, further comprising:

and a step of temper rolling the continuously annealed cold rolled steel sheet.

Technical Field

The invention relates to a cold-rolled steel plate for a flux-cored wire and a manufacturing method thereof.

Background

Steel sheets and Flux (Flux) materials used as base sheets have been developed and applied to steel strips for welding rods applied to Flux-Cored wires (Flux-Cored wires) and the like in order to meet various applications. As special applications, for example, various welding members for special purposes such as a welding member of high manganese (Mn) steel excellent in wear resistance, a welding member for extremely low temperature excellent in toughness at extremely low temperature, a welding member for dustproof steel excellent in dustproof performance, and the like have been developed. Therefore, a material for a welding rod is being developed which meets these special steels for welding.

In general, a Welding method that has the highest Welding productivity and is easy to weld at various positions is a Flux Cored Welding (FCW) method. The welding material used in the welding method is a flux-cored wire, which is manufactured by processing a Strip steel (Strip) drawn from a common cold-rolled steel sheet into a U shape, mixing a flux component and an alloy element such as manganese (Mn) and nickel (Ni) in an amount of about 5 to 50% by weight in a powder form, adding the mixture to the processed U-shaped tube, and then processing the tube into a round shape. 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 a welded component requiring excellent low-temperature toughness, it is necessary to mix and incorporate an alloying element and a flux for improving the low-temperature toughness in a processed wire core portion.

On the other hand, as cold rolled steel for a welding wire used for manufacturing a flux-cored wire, general carbon steel to which a large amount of alloying elements are not added is generally used, and stainless steel is used in some special applications.

A steel material for a welding wire based on a general carbon steel has an excellent elongation, so that the steel material is not torn during drawing, has a low work hardening degree, and can be continuously manufactured without a separate heat treatment process from molding to final welding wire manufacturing, and thus is applied to various applications. However, since the carbon steel welding steel is a low alloy steel, it is necessary to add a flux and an alloy element in a core wire to fill the inside of a welding wire in order to secure characteristics of a welding rod for various applications. However, since an appropriate flux needs to be added to ensure soldering workability, there is a limitation in increasing the amount of alloy elements to be put into the core wire. That is, it is necessary to add a large amount of an oxidizing agent (Ti, Mn, Zr, Al, etc.) and a slag forming agent (TiO) to the central portion of the steel material for the welding wire₂、SiO₂、Al₂O₃、ZrO₂MnO, etc.), arc stabilizer (K, Na, etc.) and alloy components (Si, Mn, Ni, Zr, Cr, etc.), but the flux is included in the wire steel and the amount of the flux is limited to about 30 to 60% by volume, and the amount is limited to about 15 to 25% by weight, depending on the powder to be filled. In this case, when the content of the alloying element for ensuring the characteristics is increased, since the flux component and the like are limited, it is difficult to ensure stable soldering characteristicsAnd (4) sex. Further, when these alloying elements are added in the form of powder, the molten core wire component causes segregation in the weld zone during the welding operation, which also causes poor welding.

Basically, the stainless steel for a welding wire has a large amount of alloying elements such as nickel (Ni) or chromium (Cr) in the carbon steel component compared with the ordinary carbon steel, and the amount of the alloying elements of the core wire added together with the flux can be reduced. In addition, these stainless steel welding raw plates have a high possibility of fracture due to work hardening when a welding rod wire is processed, and therefore, annealing heat treatment needs to be performed separately in the manufacturing process, which causes an increase in manufacturing cost.

At present, for a steel material for an extremely low temperature welding wire requiring workability, particularly drawability and low temperature toughness, a common carbon steel pipe is used, and then, when flux is loaded, in order to secure low temperature toughness, expensive alloy elements are prepared in the form of high purity powder and are added together with other flux components to improve low temperature toughness. Further, a segregation phenomenon is caused in the flux by the added high-valence alloying elements and a phenomenon of accumulating in a part of the welding rod causes a problem of deterioration in workability such as tearing in a welding rod processing step.

Therefore, in order to ensure the characteristics of a cold-rolled steel sheet for a flux-cored wire suitable for use in an extremely low temperature environment, for example, in order to ensure the characteristics of a flux-cored wire suitable for an extremely low temperature, it is necessary to develop a steel material for a wire having excellent low-temperature toughness and welding workability, and studies have been made to ensure an elongation of a material of 40% or more, a weld segregation index of less than 0.15%, and an impact energy value of-40 ℃ or more of 50J or more.

For example, as a method for manufacturing a steel sheet for a flux-cored wire, patent document 1 discloses a method for manufacturing a steel sheet for a flux-cored wire by adding Mn: 1.4-2.4%, Si: 0.2 to 0.4%, Ni: a method for producing a steel for a welding rod excellent in impact toughness and strength characteristics by adding Cr, Mo, Ti or the like to 2.8 to 6.4% of the steel. However, patent document 1 has a problem that the addition of a large amount of expensive alloying elements increases the production cost, and the addition of the alloying elements ensures high strength, but has a problem that the toughness is low and it is difficult to ensure workability.

Patent document 2 discloses a technique for promoting a deoxidation reaction of a molten metal and reducing welding defects by adding Ti, Mg, or the like to a flux material. 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, when a large amount of alloying elements is added to the flux as described above, the welding workability is deteriorated, for example, a phenomenon of scattering (scatter) in which fine particles are scattered around during welding occurs.

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

Disclosure of Invention

Technical problem to be solved

According to one aspect of the present invention, an object is to provide a cold-rolled steel sheet for a flux-cored wire excellent in low-temperature toughness, welding workability, and a method for manufacturing the same.

On the other hand, the technical problem to be solved by the present invention is not limited to the above. Technical problems to be solved by the present invention can be understood through the entire contents of the present specification, and additional technical problems to be solved by the present invention can be easily understood by those of ordinary skill in the art to which the present invention pertains.

(II) technical scheme

According to an embodiment of the present invention, a cold-rolled steel sheet for a flux-cored wire is provided to weighThe cold-rolled steel sheet includes: c: 0.005-0.10%, Mn: 0.05 to 0.25%, Si: 0.05% or less (except 0%), P: 0.0005 to 0.01%, S: 0.008% or less (except 0%), Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.8-1.7%, Cr: 0.1 to 0.5% and the balance Fe and inevitable impurities, W defined by the relation 1_NIs 0.10 to 0.75, and has a relation of 1: w_N(31 xc +0.5 xmn +20 × Al) × (Ni) × (0.6 × Cr) (wherein the unit of the content of each element in the relational expression 1 is wt%).

According to another embodiment of the present invention, there is provided a method of manufacturing a cold-rolled steel sheet for a flux-cored wire, including the steps of: heating a slab to 1100-1300 ℃, the slab comprising in weight%: c: 0.005-0.10%, Mn: 0.05 to 0.25%, Si: 0.05% or less (except 0%), P: 0.0005 to 0.01%, S: 0.008% or less (except 0%), Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.8-1.7%, Cr: 0.1 to 0.5% and the balance Fe and inevitable impurities, W defined by the following relation 1_NIs 0.10 to 0.75; hot rolling the heated plate blank to enable the hot finishing temperature to reach 880-950 ℃ so as to obtain a hot rolled steel plate; rolling the hot rolled steel plate at the temperature of 550-700 ℃; cold rolling the rolled hot rolled steel plate at a reduction rate of 50-85% to obtain a cold rolled steel plate; and continuously annealing the cold-rolled steel sheet at a temperature range of 700 to 850 ℃, wherein the relation of formula 1: w_N(31 xc +0.5 xmn +20 × Al) × (Ni) × (0.6 × Cr) (wherein the unit of the content of each element in the relational expression 1 is wt%).

Moreover, the technical solutions do not list all features of the present invention. Various features of the present invention and advantages and effects according to the features thereof can be understood in more detail with reference to the following detailed description.

(III) advantageous effects

According to one aspect of the present invention, a cold-rolled steel sheet for a flux-cored wire excellent in low-temperature toughness, welding workability, and workability is provided, and a flux-cored wire type rod steel strip which can be used in the fields of shipbuilding industry, materials industry, construction industry, and the like and can be welded in all directions can be provided.

Drawings

Fig. 1 is a photograph showing the microstructure of invention example 2 of the present invention example, (a) is a photograph showing the flux-cored wire manufactured in invention example 2, and (b) is a photograph showing the outer skin portion of (a) enlarged.

Fig. 2 is a photograph showing the microstructure of comparative example 5 of the present invention, (a) is a photograph of a flux-cored wire produced in example 2 of the present invention, and (b) is a photograph of an enlarged outer skin portion of (a).

Best mode for carrying out the invention

Preferred embodiments of the present invention will be described below. However, the embodiment of the present invention may be modified into other various embodiments, and the scope of the present invention is not limited to the embodiments described below. The embodiments of the present invention are provided to further fully explain the present invention to those skilled in the art to which the present invention pertains.

The cold-rolled steel sheet for flux-cored wire of the present invention will be described in detail below.

The cold-rolled steel sheet for flux-cored wire of the present invention is characterized by comprising: c: 0.005-0.10%, Mn: 0.05 to 0.25%, Si: 0.05% or less (except 0%), P: 0.0005 to 0.01%, S: 0.008% or less (except 0%), Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.8-1.7%, Cr: 0.1 to 0.5% and the balance Fe and inevitable impurities, W defined by the following relation 1_NIs 0.10 to 0.75.

First, the alloy composition of the present invention is explained in detail. Unless otherwise specified, the unit of each element content below represents wt%.

C：0.005～0.10％

Carbon (C) is an element that is generally added to improve the strength of steel, and is also an element that is added to impart characteristics similar to those of the base material to the welding heat-affected zone. When the C content is less than 0.005%, the effect is insufficient. However, when the C content exceeds 0.10%, problems such as breakage may occur during the drawing process due to high strength or work hardening. Further, low-temperature cracking or reduction in impact toughness of the welded joint occurs, and the welded joint can be processed into a desired end product through multiple heat treatments due to high hardness. Therefore, the content of C is preferably 0.005 to 0.10%, and more preferably 0.01 to 0.06% in order to improve the characteristics of the welding heat-affected zone.

Mn：0.05～0.25％

Manganese (Mn) is a solid solution strengthening element, and functions to increase the strength of steel and improve hot workability. However, when the manganese is excessively added, a large amount of manganese sulfide (MnS), precipitates, is formed, thereby possibly hindering the ductility and workability of the steel. When the Mn content is less than 0.05%, it becomes a cause of red hot brittleness and is not favorable for stabilizing austenite. However, when the Mn content exceeds 0.25%, ductility is reduced and center segregation is caused, and breakage may be caused during a drawing operation in a welding rod manufacturing process, and therefore, the Mn content is preferably 0.05 to 0.25%, and may be more preferably 0.06 to 0.24%.

Si: below 0.05% (except 0%)

Silicon (Si) combines with oxygen and the like to form an oxide layer on the surface of the steel sheet, which causes deterioration of surface properties and deterioration of corrosion resistance, and also promotes transformation of hard phases in the weld metal to cause deterioration of low-temperature impact properties, so the amount of addition is limited to 0.05% or less. The content of Si is more preferably 0.04% or less.

P：0.0005～0.01％

Phosphorus (P) is an element that exists as a solid solution element in steel and causes solid solution strengthening to improve strength and hardness, and is preferably added in an amount of 0.0005% or more in order to maintain a predetermined level of rigidity, but when the content thereof exceeds 0.01%, center segregation may be caused at the time of casting and ductility may be reduced, thereby deteriorating wire workability thereof. Therefore, the content of P is preferably 0.0005 to 0.01%, and more preferably 0.001 to 0.009%.

S: less than 0.008% (except 0%)

Since sulfur (S) combines with manganese in steel to form a non-metallic inclusion and causes red hot shortness (red short), it is preferable to reduce the content thereof as much as possible. Further, since the toughness of the base material of the steel sheet is lowered when the S content is high, the S content is preferably 0.008% or less, and more preferably 0.0075% or less.

Al：0.005～0.06％

Aluminum (Al) is an element added as a deoxidizer in aluminum deoxidized steel and for preventing deterioration of material due to aging, and is also an element advantageous for ensuring ductility, and such an effect is remarkable at extremely low temperatures. When the Al content is less than 0.005%, the effect is insufficient. However, when the Al content exceeds 0.06%, such as alumina (Al)₂O₃) The surface inclusions of (2) increase rapidly, deteriorating the surface characteristics of the hot rolled material and reducing workability, and local ferrite is formed in the weld heat affected zone grain boundary, possibly deteriorating the mechanical characteristics thereof, and also possibly causing problems such as deterioration of the shape of the welded bead after welding. Therefore, the content of Al is preferably 0.005 to 0.06%, and more preferably 0.007 to 0.050%.

N：0.0005～0.003％

Nitrogen (N) is an element that exists in a solid solution state inside steel and contributes to material strengthening, and it is necessary to add 0.0005% or more of nitrogen in order to secure a target rigidity. However, if the N content exceeds 0.003%, the aging properties are drastically deteriorated, and the burden due to denitrification is increased in the steel production step, thereby deteriorating the steel production workability. Therefore, the content of N is preferably 0.0005 to 0.003%, and more preferably 0.008 to 0.0029%.

Ni：0.8～1.7％

Nickel (Ni) is an effective element for improving drawability by increasing ductility, is an element necessary for forming a stable structure at extremely low temperatures to improve low-temperature impact properties, and is added in an amount of 0.8% or more in order to obtain the above-mentioned effect and to achieve stable operation of a flux composition. However, when the Ni content exceeds 1.7%, drawability may be deteriorated due to an increase in strength, and surface defects may be caused. In addition, since Ni is an expensive element, there is a problem that the production cost increases. Therefore, the content of Ni is preferably 0.8 to 1.7%, and more preferably 0.085 to 1.65%.

Cr：0.1～0.5％

Chromium (Cr) is an element that contributes to the strength of the welded joint and also contributes to the improvement of corrosion resistance by performing the action of forming a stable rust layer, and in order to ensure the effect, it is preferable to add 0.1% or more of chromium. However, if the amount of Cr added exceeds 0.5%, chromium carbide may be formed and brittleness may be caused, which may result in problems such as non-workability. Therefore, the content of Cr preferably satisfies the range of 0.1 to 0.5%, and more preferably 0.13 to 0.45%.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, undesirable impurities may be inevitably mixed from raw materials or the surrounding environment, and thus cannot be excluded. These impurities are well known to those skilled in the conventional manufacturing process and, therefore, not all of them are specifically mentioned in this specification.

On the other hand, the cold rolled steel sheet of the present invention satisfies not only the above alloy composition but also W defined by the following relational formula 1_FCPreferably 0.10 to 0.75. Wherein the unit of the content of each element in the following relational expression 1 is weight%.

Relation 1: w_N＝(31×C+0.5×Mn+20×Al)×(Ni)×(0.6×Cr)

The relational expression 1 is designed in consideration of the correlation of the elements affecting the welding workability and the drawing workability. When W is_NIf the amount is less than 0.10, the amount of transformation from the normal temperature structure to the hard phase is small, which is advantageous in terms of workability, but if the amount of alloy added as an alloying element of the flux is increased in order to secure low temperature toughness, the welding workability is deteriorated. However, when W_NAbove 0.75, the fraction of hard transformation structure increases, causing breakage of welded parts at the time of pipe making and drawing, and the manufacturing cost rises due to the addition of a large amount of high-priced alloy elements, and therefore, W_NThe range of (3) preferably satisfies the range of 0.10 to 0.75, and more preferably 0.11 to 0.73.

On the other hand, the cold-rolled steel sheet of the present invention preferably has a fine structure composed of 1 to 6% by area of iron carbide and the balance ferrite. When the fraction of the iron carbide is low and less than 1%, precipitation of carbides is not promoted and strain aging defects are caused by solid solution elements in the steel, but when the fraction of the iron carbide exceeds 6%, not only cracks are caused during drawing but also corrosion resistance is deteriorated, so that the fraction of the iron carbide is preferably in the range of 1 to 6%, and more preferably 0.13 to 5.8%.

The cold-rolled steel sheet of the present invention may have an elongation of 40% or more. By satisfying such physical properties, it can be preferably used as a material for a flux cored wire. When the elongation is less than 40%, the shrinkage of the cross section becomes small at the time of drawing processing of the welding wire, so that there may occur problems that the tube-making workability is deteriorated and cracks such as tearing occur at the time of processing.

In addition, the cold-rolled steel sheet manufactured according to the present invention may have a weld segregation index of 0.15% or less and a low-temperature impact energy at-40 ℃ of 50J or more. More specifically, the segregation index of the weld portion welded using the flux-cored wire manufactured using the cold-rolled steel sheet of the present invention is represented by the ratio of the area occupied by the segregation portion caused by the additive element in the entire area of the weld portion. When the weld portion is segregated, stress concentrates on the segregated portion during machining, causing fracture. In order to prevent tearing due to segregation of the welded portion in the 2 nd process after welding, the segregation index of the welded portion is preferably 0.15% or less, and may be more preferably 0.125% or less. In the conventional flux-cored wire, in order to secure low-temperature toughness, an element such as nickel (Ni) is added as an alloying element of a flux that is not a base material, and a problem occurs in that the segregation index of the welded portion increases, but when the cold-rolled steel sheet of the present invention is used, the segregation causes as described above can be significantly reduced, and the segregation index of the welded portion can be secured to 0.15% or less. Further, it is necessary to ensure that the impact energy at the time of the impact test for evaluating the low temperature stability of the weld rod is 50J or more at the test temperature of-40 ℃. When the impact energy value obtained in an impact test at-40 ℃ is reduced to 50J or less, cracks may be caused in the welded portion or the like by low-temperature impact or the like in a low-temperature environment, and the safety of the welded structure may be affected, so that it is necessary to secure 50J or more of impact energy. The low-temperature impact energy at-40 ℃ is more preferably 55J or more.

The method for producing a cold-rolled steel sheet for a flux cored wire of the present invention will be described in detail below.

The manufacturing method of the cold-rolled steel plate for the flux-cored wire comprises the following steps: heating the plate blank satisfying the alloy composition to 1100-1300 ℃; hot rolling the heated plate blank to enable the hot finishing temperature to reach 880-950 ℃ so as to obtain a hot rolled steel plate; rolling the hot rolled steel plate at the temperature of 550-700 ℃; cold rolling the rolled hot rolled steel plate at a reduction rate of 50-85% to obtain a cold rolled steel plate; and continuously annealing the cold-rolled steel sheet at a temperature of 700 to 850 ℃.

Firstly, heating the slab to 1100-1300 ℃. This is to smoothly perform the subsequent hot rolling process and to homogenize the slab. When the slab heating temperature is less than 1100 ℃, there is a problem in that the load is sharply increased at the subsequent hot rolling, but when the slab heating temperature exceeds 1300 ℃, the energy cost is increased and the loss of material is caused due to the increase in the amount of surface scale. Therefore, the heating temperature of the slab is preferably 1100-1300 ℃, and can be more preferably 1150-1280 ℃.

And carrying out hot rolling on the heated plate blank so that the hot finishing temperature of the plate blank reaches 880-950 ℃, thereby obtaining the hot rolled steel plate. When the finish rolling temperature is less than 880 ℃, as the hot rolling in the low temperature region is finished, the intermixing of crystal grains occurs rapidly, resulting in a reduction in hot rolling property and workability. However, when the finish rolling temperature exceeds 950 ℃, the grain refinement is insufficient because uniform hot rolling is not performed over the entire thickness, and the coarsening of the grains causes a decrease in impact toughness. Therefore, the hot finish rolling temperature is preferably 880-950 ℃, and can be more preferably 885-940 ℃.

And rolling the hot rolled steel plate at the temperature of 550-700 ℃. In this case, the hot-rolled steel sheet may be cooled on a Run-out table (ROT) after hot rolling and before rolling. When the take-up temperature is less than 550 ℃, the formation of low-temperature precipitates varies due to temperature unevenness in the width direction during cooling and holding, resulting in material variation and thus adversely affecting workability. However, when the take-up temperature exceeds 700 ℃, the texture of the final product becomes coarse, and there is a problem that the surface texture is softened and the tube formability is deteriorated. Therefore, the rolling temperature is preferably 550-700 ℃, and can be more preferably 555-690 ℃.

And cold rolling the rolled hot-rolled steel sheet at a reduction ratio of 50-85% to obtain the cold-rolled steel sheet. When the reduction ratio is less than 50%, a driving force for recrystallization is low, local texture growth occurs, and the like, and it is difficult to secure a uniform material, and in consideration of the thickness of a final product, it is necessary to reduce the thickness of a hot-rolled steel sheet, and thus hot rolling workability is significantly reduced. However, when the reduction ratio exceeds 85%, the material is hardened, which causes cracks in drawing, and the cold rolling workability is lowered by the load of the rolling mill. Therefore, the reduction ratio is preferably 50 to 85%, and more preferably 65 to 80%.

In this case, a step of pickling the rolled hot rolled steel sheet before cold rolling may be further included.

The cold-rolled steel sheet is continuously annealed in order to secure workability and rigidity. The target strength and workability are ensured by applying the strain removal annealing from the state of high strength by introducing the strain in the cold rolling. The continuous annealing may be performed at a temperature ranging from 700 to 850 ℃. When the annealing temperature is less than 700 ℃, workability is significantly reduced because deformation formed by cold rolling is not sufficiently removed. However, when the annealing temperature exceeds 850 ℃, problems may occur in the pass-through property of the continuous annealing furnace due to high-temperature annealing. Therefore, the continuous annealing is preferably 700 to 850 ℃, and may be more preferably 730 to 845 ℃.

Then, a step of temper rolling the continuously annealed cold rolled steel sheet may be further included, and the temper rolled steel sheet may be used for manufacturing a welding wire.

Detailed Description

The present invention will be described in more detail below with reference to examples. However, the following examples are only for illustrating the present invention in further detail, and do not limit the scope of the present invention. This is because the scope of the present invention is determined by the matters recited in the claims and reasonably derived therefrom.

(examples)

A cold rolled steel sheet was manufactured according to the manufacturing conditions described in the following Table 2 after heating a slab having the composition shown in the following Table 1 to 1250 ℃, and it was observed that the microstructure of the cold rolled steel sheet had a ferrite structure, the kind and fraction of the microstructure, elongation, pass formability, and drawing formability of the cold rolled steel sheet were measured and are shown in the following Table 3, for the pass formability, if there was no rolling load at the time of cold rolling and hot rolling and a defect such as a hot buckling phenomenon (Heat buckling) did not occur at the time of continuous annealing, it was shown as "○", if there was a rolling load or a defect such as a hot buckling phenomenon occurs at the time of continuous annealing, it was shown as "x", for the drawing formability, if a processing defect such as tearing occurs at a section shrinkage of 61% at the time of drawing of the flux cored wire, it was shown as "bad", and if a processing defect does not occur, it was shown as "good".

Further, a strip steel having a width of 14mm was manufactured using the manufactured cold rolled steel sheet, and then the strip steel was bent and filled with flux and alloying components, and then a welding material having a diameter of 3.1mm was manufactured. The welding material manufactured as described above was drawn to manufacture a flux-cored wire having a diameter of 1.2mm, and a low-temperature impact test was performed, and the results thereof are shown in table 3.

Also, the weld segregation index of the welded parts welded using the flux cored wire was measured, and the results thereof are shown in table 3. In this case, the welded member was drawn using a wire having a diameter of 1.4mm, and a Pilot (Pilot) welding machine was used to perform a test on the welded member manufactured under conditions of a voltage of 29 volts, a current of 150 to 180A, and a welding speed of 14cm per minute.

[ Table 1]

[ Table 2]

[ Table 3]

As is apparent from tables 1 to 3, inventive examples 1 to 9 satisfying the alloy composition and the production conditions proposed by the present invention not only have good pass-through properties, but also satisfy an elongation of 40% or more, which is a standard material for a desired cold-rolled steel sheet for a flux-cored wire. Further, since the segregation index of the wire produced as a welded part is also less than 0.15%, the welded portion is not torn or cracked at the 2 nd processing, and excellent workability can be ensured. Further, the impact energy at-40 ℃ is 50J or more, and excellent low-temperature toughness can be secured.

On the other hand, comparative examples 1 to 4 satisfied the alloy composition proposed by the present invention and did not satisfy the production conditions, and it was confirmed that the rolling pass property (comparative examples 1 to 3) and the annealing pass property (comparative example 4) thereof were deteriorated, and the elongation thereof was lower than the target or the impact energy at-40 ℃ was 50J or less or the drawing workability was poor.

Comparative examples 5 to 9 satisfied the production conditions proposed by the present invention but did not satisfy the alloy composition, and comparative example 10 did not satisfy the alloy composition and the production conditions. It was confirmed that most of comparative examples 5 to 10 did not satisfy the target elongation, weld segregation index, impact energy, and the like of the present invention, and had poor pass-through properties. In addition, tearing or cracking occurs at the time of drawing.

Fig. 1 and 2 are photographs of the fine groups of invention example 2 and comparative example 5, respectively, (a) is a photograph of the flux-cored wire manufactured by using invention example 2, and (b) is a photograph of the outer skin portion of (a) enlarged. As is clear from fig. 1, the outer layer is relatively homogeneous, and therefore, it is understood that good drawability can be ensured. On the other hand, as can be seen from fig. 2, since the outer layer is not homogeneous, it is difficult to ensure good drawing workability.

As described above, according to the present invention, by appropriately controlling the alloy composition and the manufacturing conditions, the occurrence of weld segregation can be significantly improved, and the content of the flux for improving the welding workability can be increased by reducing the alloying elements in the flux, so that a cold-rolled steel sheet for flux-cored welding excellent in low-temperature toughness and welding workability can be obtained. Therefore, the cold-rolled steel sheet according to the present invention can reduce the amount of alloying elements in the flux, which increases the process cost, and remarkably reduce segregation in the welded portion, thereby reducing the occurrence of cracks in the welded portion, and also can ensure stable workability of the welded member, thereby reducing variations in the material quality of the product, and is also effective in cost saving and improvement of workability.

The present invention has been described in detail with reference to the embodiments, but it should be understood that various modifications and changes can be made by those skilled in the art without departing from the technical idea and the technical field of the present invention described in the claims.