CN109804092B

CN109804092B - Cold-rolled steel sheet for flux-cored wire and method for manufacturing same

Info

Publication number: CN109804092B
Application number: CN201780062649.8A
Authority: CN
Inventors: 金在翼
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-10-11
Filing date: 2017-10-11
Publication date: 2020-10-20
Anticipated expiration: 2037-10-11
Also published as: WO2018070753A1; CN109804092A; JP2019534382A; KR101795970B1; JP6811854B2

Abstract

One aspect of the present invention relates to a cold-rolled steel sheet for a flux-cored wire, comprising, in wt%: c: 0.01-0.15%, Mn: 0.1 to 0.5%, 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: 0.5 to 2.0% and the balance Fe and unavoidable impurities, wherein the fine structure comprises ferrite in an amount of 93 to 98% by area fraction, and needle bainite and cementite in an amount of 2 to 7% in total.

Description

Cold-rolled steel sheet for flux-cored wire and method for manufacturing same

Technical Field

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

Background

For the material for the welding rod, steel plate and Flux (Flux) materials are being compositely developed and applied to satisfy various uses. As typical applications, many welded parts for special purposes have been developed, such as welded parts of high Mn steel having excellent wear resistance, welded parts for extremely low temperatures having excellent toughness at extremely low temperatures, and welded parts for vibration-proof steel having excellent vibration-proof performance. Therefore, a material for a welding rod which meets these steels for special welding has been developed.

Among the general Welding methods, a Flux Cored Welding (FCW) method is a Welding method that is most productive and easily welds various positions. The welding material used in this welding method is a flux cored Wire (flux cored Wire), a Strip steel (Strip) obtained by drawing a general cold-rolled steel sheet is processed into a U-shape, a flux component for securing welding workability and an alloy element such as manganese (Mn) and nickel (Ni) mixed in a powder form according to the purpose in order to secure characteristics suitable for the use of the welding rod are added to the processed U-shaped tube in an amount of about 5 to 50% by weight, and a circular material for the welding rod is manufactured.

At this time, various characteristics required for the electrode material are secured by changing the kind and the addition amount of the alloy component in the core added in the powder form. Therefore, in order to produce welded parts requiring low-temperature toughness, such as extremely low-temperature welded parts, it is necessary to separately incorporate an element for improving low-temperature toughness in the wire core as an alloying element added to the flux.

On the other hand, as a cold rolled steel for a welding wire surrounding a core for manufacturing a flux-cored welding wire, a general carbon steel to which a large amount of alloying elements is 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 is excellent in elongation so that a tearing phenomenon of the steel material does not occur at the time of drawing, and can be continuously manufactured without a separate heat treatment process even during the period from molding to manufacturing of a final welding wire due to a low degree of work solidification, and thus is widely used in many applications. However, the above-mentioned carbon steel welding steel is low alloy steel, and therefore, in order to ensure the characteristics of the welding rod, it is necessary to add a flux for filling the inside of the welding wire and a core alloying element, but in order to ensure the welding workability, basically, the addition amount of the flux needs to be optimized, and thus there is a limitation in increasing the core alloying element. 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 wire steel₂、SiO₂、Al₂O₃、ZrO₂MnO, etc.), arc stabilizer (K, Na, etc.) and alloy component(s) ((ii)Si, Mn, Ni, Zr, Cr, etc.) and a flux is filled in the wire steel material, and the limit of filling the above components is about 30 to 60% by volume, but varies depending on the powder to be filled, but the limit is about 15 to 25% by weight. In this case, when the content of the alloying element for ensuring the characteristics is increased, it is difficult to ensure the welding characteristics because the flux component and the like are limited. Further, as the alloying elements are added in the form of powder, the molten core component causes segregation in the weld zone during the welding operation, which also causes poor welding.

In the steel material for a welding wire using stainless steel, the amount of alloying elements such as nickel (Ni) and chromium (Cr) present in the low-carbon steel component is substantially larger than that of ordinary carbon steel, and therefore, the amount of addition of core alloying elements added together with flux can be reduced. Furthermore, with these stainless steel welding raw plates, there is a high possibility that wire breakage occurs due to solidification of the work when the welding wire is worked, so that an annealing treatment needs to be additionally performed during the manufacturing process, thereby increasing the manufacturing cost.

At present, for a steel material for an extremely low temperature welding wire requiring workability, particularly drawability and low temperature toughness, in order to secure low temperature toughness when a flux is charged after a pipe is manufactured using a normal carbon steel, an expensive element is prepared in the form of a high purity powder and is charged together with other flux components to improve the low temperature toughness. In addition, the expensive alloying elements added at this time cause a segregation phenomenon in the flux, and thus the welding workability is also deteriorated.

For example, patent document 1 discloses a method for producing a steel sheet for a flux-cored wire, the method comprising the steps of adding Mn: 1.4-2.4%, Si: 0.2 to 0.4%, Ni: a method for producing a steel for 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, in patent document 1, although a large amount of expensive alloying elements are added, which leads to an increase in cost, and high strength can be ensured by adding alloying elements, it is difficult to ensure drawing workability due to poor ductility.

Patent document 2 discloses a technique of adding Ti, Mg, or the like to a flux raw material to promote a deoxidation reaction of a molten metal, thereby reducing welding defects. However, although a large amount of alloying elements must be added to the flux in order to obtain a sufficient deoxidizing effect of the molten metal, when a large amount of such alloying elements are added to the flux, there is a problem that welding workability is deteriorated, such as a spatter (scatter) phenomenon in which fine particles are scattered around during welding.

Therefore, there is a need for development of a cold-rolled steel sheet for flux-cored wires, which can obtain a welded portion having excellent low-temperature toughness in a very low-temperature environment and is excellent in welding workability and drawing workability, and a method for manufacturing the same.

Documents of the prior art

(patent document 1) Korean laid-open patent publication No. 2006-107910

(patent document 2) Japanese laid-open patent publication No. 60-46896

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 welding workability and drawing workability, and a method for manufacturing the same.

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 contents of the entire specification, and additional problems to be solved by the present invention can be easily understood by those skilled in the art to which the present invention pertains.

(II) technical scheme

One aspect of the present invention relates to a cold-rolled steel sheet for flux-cored wire excellent in low-temperature toughness, comprising, in wt%: c: 0.01-0.15%, Mn: 0.1 to 0.5%, 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: 0.5 to 2.0% and the balance Fe and unavoidable impurities, wherein the fine structure comprises ferrite in an amount of 93 to 98% by area fraction, and needle bainite and cementite in an amount of 2 to 7% in total.

In addition, another aspect of the present invention relates to a method for manufacturing a cold-rolled steel sheet for a flux-cored wire, the method comprising the steps of: heating a slab to 1100-1300 ℃, wherein the slab comprises, by weight percent: c: 0.01-0.15%, Mn: 0.1 to 0.5%, 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: 0.5-2.0% and the balance of Fe and inevitable impurities; hot rolling the heated plate blank until the hot finish rolling temperature reaches 880-950 ℃ to obtain a hot rolled steel plate; rolling the hot rolled steel plate within the temperature range 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.

Moreover, not all features of the present invention are set forth in the foregoing description. The various features of the invention, together with the advantages and effects based thereon, may be understood in more detail with reference to the following detailed description.

(III) advantageous effects

According to the present invention, a cold-rolled steel sheet for flux-cored wires excellent in welding workability and drawing workability, and a method for manufacturing the same can be provided.

Drawings

FIG. 1 is a photograph showing a microstructure of invention example 2.

Fig. 2 is a photograph of the microstructure of comparative example 6.

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 forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to further fully explain the present invention to those skilled in the art.

Cold-rolled steel plate for flux-cored wire

Next, a cold-rolled steel sheet for a flux-cored wire according to an aspect of the present invention will be described in detail.

A cold-rolled steel sheet for a flux-cored wire according to an aspect of the present invention includes, in wt%: c: 0.01-0.15%, Mn: 0.1 to 0.5%, 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: 0.5 to 2.0% and the balance Fe and unavoidable impurities, wherein the fine structure comprises ferrite in an amount of 93 to 98% by area fraction, and needle bainite and cementite in an amount of 2 to 7% in total.

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

C：0.01～0.15％

Carbon (C) is an element added to improve the strength of steel in general and to impart characteristics similar to those of the base material to the welding heat-affected zone.

When the C content is less than 0.01%, the effect is insufficient. On the other hand, when the C content exceeds 0.15%, problems such as wire breakage may occur at the time of drawing due to high strength or work curing. In addition, the welded joint portion is subjected to low-temperature cracking or a reduction in impact toughness, and requires a plurality of heat treatments due to high hardness to be processed into a final product. Therefore, the content of C is preferably 0.01 to 0.15%, and more preferably 0.02 to 0.13% in order to improve the characteristics of the welding heat-affected zone.

Mn：0.1～0.5％

Manganese (Mn) is a solid solution strengthening element and has an effect of improving the strength of steel, reducing Ar3, and improving hot workability. However, when Mn is added excessively, a large amount of manganese-sulfide (MnS) precipitates are formed, possibly hindering the ductility and workability of the steel.

When the Mn content is less than 0.1%, it becomes a cause of red hot brittleness, and it is difficult to contribute to stabilization of austenite. On the other hand, when the Mn content exceeds 0.5%, ductility is reduced, and due to addition of a large amount of alloying elements, cost is increased and center segregation occurs, and wire breakage may be caused at the time of drawing operation. Therefore, the Mn content is preferably 0.1 to 0.5%, and more preferably 0.2 to 0.45%.

Si: below 0.05% (except 0%)

Silicon (Si) bonds with oxygen and the like to form an oxide layer on the surface of the steel sheet, which causes deterioration of surface properties and reduction of corrosion resistance, and promotes transformation of a hard phase in the weld metal to cause reduction of low-temperature toughness, so that the amount of Si added is limited to 0.05% or less. More preferably, the Si content may be 0.04% or less, and may be further more preferably 0.02% or less.

P：0.0005～0.01％

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

S: less than 0.008% (except 0%)

Sulfur (S) combines with Mn in steel to form a non-metallic inclusion and causes red shortness, and therefore it is preferable to reduce the S content as much as possible. Further, since the base material toughness of the steel sheet is lowered when the S content is high, the S content is preferably 0.008% or less, and more preferably 0.007% or less.

Al：0.005～0.06％

Aluminum (Al) is an element added to aluminum killed steel for the purpose of preventing deterioration of the material quality due to a deoxidizer and aging, and is an element advantageous for ensuring ductility, and such an effect is more remarkable than that at extremely low temperatures.

When the Al content is less than 0.005%, the effect is insufficient. On the other hand, when the Al content exceeds 0.06%, alumina (A)l₂O₃) And the surface inclusions are sharply increased to deteriorate the surface characteristics of the hot rolled material, to reduce workability, and to form local ferrite in the weld heat affected zone grain boundary to deteriorate the mechanical characteristics, and the shape of the weld bead may be deteriorated after welding. Therefore, the Al content is preferably 0.005 to 0.06%. More preferably, the Al content may be 0.01 to 0.05%, and still more preferably 0.01 to 0.04%.

N：0.0005～0.003％

Nitrogen (N) is an element that exists in a solid solution state inside steel and is effective for material strengthening, and it is necessary to add 0.0005% or more of N in order to secure a target rigidity. On the other hand, when the N content exceeds 0.003%, the aging property is drastically deteriorated, and the burden due to denitrification is increased in the manufacturing step of steel, so that the steel manufacturing workability is deteriorated. Therefore, the N content is preferably 0.0005 to 0.003%. More preferably, the content of N may be 0.001 to 0.0027%.

Ni：0.5～2.0％

Nickel (Ni) is an element necessary to increase ductility to effectively improve drawing workability, and to form a stable structure at an extremely low temperature to improve low-temperature toughness. When Ni is added as a component of the flux instead of as a component of the steel sheet, it is necessary to produce Ni in a high-purity powder form, and therefore, it is advantageous in terms of cost to add Ni as a component of the steel sheet. Further, since the percentage of the welding rod that can be added as flux is limited, the addition of Ni as a component of the steel sheet can increase the amount of other flux elements that affect weldability, and thus weldability and the like can be improved.

When the Ni content is less than 0.5%, the effect is insufficient. On the other hand, when the Ni content exceeds 2.0%, the drawing workability may be deteriorated due to the increase in strength, and surface defects may be caused, and the manufacturing cost is increased since Ni is an expensive element. Therefore, the Ni content is preferably 0.5 to 2.0%. More preferably, the Ni content may be 0.6 to 1.8%.

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

In this case, W defined by the following relational expression 1, which satisfies not only the ranges of the contents of the respective elements described above_FCCan be 0.5 to 4.5. The unit of the content of each element in the following relational expression 1 is weight%.

Relation 1: w_FC＝(25*C+0.4*Mn+26*Al)*Ni

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_FCIf the amount of the alloy added as the flux is less than 0.5, the amount of transformation from the normal temperature structure to the hard phase is small, and the workability is advantageous, but the welding workability is deteriorated as the amount of the alloy added as the alloy element of the flux is increased in order to secure the low temperature toughness. Thus, W_FCThe lower limit of (b) is preferably 0.5, and a more preferred lower limit may be 0.505.

On the other hand, when W_FCWhen the amount exceeds 4.5, the fraction of the hard transformation structure increases, so that the problem of breakage of the welded part occurs during pipe forming and drawing, and the manufacturing cost increases as a large amount of expensive alloy elements are added. Thus, W_FCThe upper limit of (b) is preferably 4.5, a more preferred upper limit may be 4.0, and a still more preferred upper limit may be 3.5.

The fine structure of the cold-rolled steel sheet according to the present invention contains ferrite in an amount of 93 to 98% by area fraction, and needle bainite and cementite in an amount of 2 to 7% in total.

When the ferrite fraction is less than 93%, the material solidifies to cause fracture during tube forming and drawing, and when the ferrite fraction exceeds 98%, the rigidity decreases with softening of the material, so that the thickness of the flux-cored wire increases. Therefore, the fraction of ferrite is preferably 93 to 98%, and may be more preferably 93.5 to 97.5%.

Further, the sum of the acicular bainite and cementite as the hard phase needs to be controlled to 2 to 7%. When the total of the needle bainite and the cementite is less than 2%, the thickness of the flux-cored wire increases as the rigidity decreases, and when the total of the needle bainite and the cementite exceeds 7%, the workability deteriorates. Therefore, the total of bainite and cementite is preferably 2 to 7%, and more preferably 2.5 to 6.5%.

In this case, the cold-rolled steel sheet according to the present invention may have a yield strength of 200 to 300MPa and an elongation of 40% or more. By satisfying these physical properties, it can be preferably applied to a material for a flux cored wire.

When the yield strength is less than 200MPa, the pipe may be bent, and when the yield strength exceeds 300MPa, although it is advantageous in terms of the pressure resistance characteristics of the pipe, the pipe formability is lowered due to the increase in strength, and the manufacturing cost is increased due to the increase in the degree of wear of the processing tools.

When the elongation is less than 40%, pipe formability is deteriorated, and cracks such as tearing may occur during processing.

In addition, the weld segregation index of the cold-rolled steel sheet according to the present invention may be 0.15% or less.

More specifically, it represents the segregation index of a welded portion welded by a flux-cored wire manufactured using a cold-rolled steel sheet according to the present invention. The weld segregation index may represent a ratio of the entire area of the weld to the area occupied by the segregation portion based on the additive element.

When segregation occurs in the welded portion, stress concentrates on the segregation portion during machining, which causes fracture. In order to prevent tearing caused by segregation of the welded portion in the second working after welding, the segregation index of the welded portion is preferably 0.15% or less.

In the conventional flux-cored wire, in order to secure low-temperature toughness, elements such as nickel (Ni) are added as alloying elements of the flux instead of the base metal, and thus the segregation index of the weld portion is increased, but when the cold-rolled steel sheet according to the present invention is used, the segregation causes as described above are significantly reduced, and thus the segregation index can be secured to 0.15% or less.

Method for manufacturing cold-rolled steel plate for flux-cored wire

Next, a method for manufacturing a cold-rolled steel sheet for a flux cored wire according to another aspect of the present invention will be described in detail.

The method for manufacturing a cold-rolled steel sheet for a flux-cored wire according to another aspect of the present invention includes the steps of: heating the plate blank with the alloy composition to 1100-1300 ℃; hot rolling the heated plate blank until the hot finish rolling temperature reaches 880-950 ℃ 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.

Step of heating the slab

Heating the slab with the alloy composition to 1050-1300 ℃. This is to smoothly perform the subsequent hot rolling process and to perform the homogenization process on the slab.

When the slab heating temperature is less than 1050 ℃, the load is sharply increased at the time of the subsequent hot rolling, and on the other hand, when the slab heating temperature exceeds 1300 ℃, the energy cost is increased and the amount of surface scale is increased, thereby possibly causing material loss.

Step of Hot Rolling

And carrying out hot rolling on the heated plate blank until the hot finish rolling temperature reaches 880-950 ℃ to obtain the hot rolled steel plate.

When the finish rolling temperature is less than 880 ℃, the grain mixing and graining are sharply performed as the hot rolling is terminated in the low temperature region, resulting in a reduction in hot rolling property and workability. On the other hand, when the finish rolling temperature exceeds 950 ℃, uniform hot rolling is not achieved over the entire thickness, and thus grain refinement is insufficient, so that a decrease in impact toughness due to coarsening of the grains may occur.

Winding step

And rolling the hot rolled steel plate at the temperature of 550-700 ℃. At this time, cooling of the hot rolled steel sheet after hot rolling and before rolling may be performed on a Run-out-table (ROT).

When the rolling temperature is less than 550 ℃, the tendency to generate low-temperature precipitates during cooling and holding shows a difference due to unevenness of the temperature in the width direction, thereby causing material variation, and thus adversely affecting workability. On the other hand, when the take-up temperature exceeds 700 ℃, the surface texture softens and the tube formability deteriorates as the texture of the final product becomes coarse.

Step of Cold Rolling

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%, the driving force for recrystallization is low, and local structure growth occurs, so that it is difficult to secure a uniform material, and the thickness of the hot-rolled steel sheet needs to be reduced in consideration of the thickness of the final product, so that hot-rolling workability is significantly reduced. On the other hand, when the reduction ratio exceeds 85%, the material is solidified and becomes a cause of cracking at the time of drawing, and not only, 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%.

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

Continuous annealing step

The cold-rolled steel sheet is continuously annealed in order to secure workability and rigidity. Annealing for removing distortion is performed in a state where the strength is increased by distortion introduced in cold rolling, thereby securing the target strength and workability.

At this time, the continuous annealing may be performed at a temperature range of 700 to 850 ℃.

At annealing temperatures less than 700 ℃, the workability decreases significantly with insufficient removal of the deformation. On the other hand, at an annealing temperature exceeding 850 ℃, a problem may occur in the pass-through property of the continuous annealing furnace based on high-temperature annealing.

In this case, the continuously annealed cold-rolled steel sheet may be cooled at a cooling rate of 30 to 60 ℃/sec. This is because, when the cooling rate exceeds 60 ℃/sec, a large amount of needle-like bainite and cementite are generated, which may cause breakage during tube forming and drawing, and when the cooling rate is less than 30 ℃/sec, the rigidity may decrease as the material is softened, and therefore the thickness of the flux-cored wire may increase.

Detailed Description

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

After heating slabs having the composition shown in the following table 1 to 1250 ℃, cold-rolled steel sheets were manufactured according to the manufacturing conditions described in the following table 2. The cooling rate after the continuous annealing was 40 ℃/sec.

The cold rolled steel sheets were measured for pass properties, yield strength, elongation, workability, and microstructure and are described in table 3 below.

Further, the segregation index of the welded portion welded by the flux-cored wire for low temperature toughness manufactured by using the cold-rolled steel sheet was measured and is described in table 3 below. In each case, a flux composition was set so that the Ni content in the entire alloy composition of the flux-cored wire was 1.5%, and a welding wire having a diameter of 1.4mm was manufactured and subjected to a welding test using a Pilot (Pilot) welding machine under conditions of a voltage of 29V, a current of 150 to 180A, and a welding speed per minute of 14cm, using a steel strip for shipbuilding.

The yield strength and elongation shown in table 3 are represented as "O" if they satisfy the respective target standards of yield strength in the range of 200 to 300MPa, elongation of 40% or more, and segregation index of less than 0.15%, respectively, and are represented as "X" if they cannot satisfy the respective characteristic standards.

In addition, regarding workability, when a cold-rolled steel sheet is subjected to drawing at a cross-sectional reduction rate of 45%, if a processing defect such as tearing occurs, it is indicated as "poor", and if no processing defect occurs, it is indicated as "good".

In the pass-through properties, if there is no rolling load at the time of cold rolling and hot rolling and no defect such as thermal buckling (Heat buckle) occurs at the time of continuous annealing, it is represented as "O", and if there is a rolling load or a defect such as thermal buckling occurs at the time of continuous annealing, it is represented as "X".

[ Table 1]

[ Table 2]

[ Table 3]

The invention examples 1 to 9 satisfying the alloy composition and the production conditions proposed in the present invention satisfy the yield strength of 200 to 300MPa and the elongation of 40% or more, which are the target material standards, and are excellent in drawability and pass-through property. In addition, the segregation index of the welded portion is less than 0.15%, and the welded portion is not torn or cracked in the second working, so that excellent workability can be ensured.

As can be confirmed from fig. 1 in which the microstructure of invention example 2 was photographed, the alloy composition and the production conditions of the present invention were satisfied, and the microstructure of the present invention could be secured.

Although comparative examples 1 to 4 satisfied the alloy composition proposed in the present invention, they did not satisfy the production conditions, and it was confirmed that comparative examples 1 to 4 had high yield strength, poor elongation, and poor drawing workability. Further, it was observed that the passing properties of comparative examples 1 and 2 were also poor, and the microstructure of comparative example 1 was formed into deformed grains, i.e., unrecrystallized ferrite (deformed), and the portion other than 42.3% in comparative example 2 was deformed grains.

Comparative examples 5 to 10 satisfied the production conditions proposed in the present invention, but did not satisfy the alloy composition. In all cases, the material quality and the segregation index of the welded portion are not satisfied, and therefore, tearing or cracking occurs during the drawing of the welding wire and the second processing of the welded portion.

Further, the steel sheet does not ensure the pass-through property (comparative example 10), the yield strength is out of the range of 200 to 300MPa (comparative example 5 and comparative examples 7 to 10) which is required in the present invention, and the elongation does not satisfy the target level (comparative examples 6 to 10), and thus the steel sheet cannot satisfy the target characteristics of the cold-rolled steel sheet for flux-cored wire which requires the workability and the low-temperature toughness.

Referring to fig. 2, which shows the microstructure of comparative example 6, it was confirmed that the area% of ferrite formed was more than 98%.

As described above, according to the present invention, the occurrence of segregation in the welded portion is significantly improved by controlling the alloy composition and the manufacturing conditions, and the content of other flux elements is increased by reducing the Ni content in the flux, so that the characteristics of the cold-rolled steel sheet for flux-cored wire excellent in low-temperature toughness and welding workability can be secured. Therefore, the cold-rolled steel sheet of the present invention can reduce the Ni content in the flux, which causes an increase in process cost, and can reduce cracks generated in the welded portion by significantly reducing segregation in the welded portion, and also can ensure stable workability of the product, and therefore can reduce variations in the material quality of the product, and is effective in cost saving and improvement in workability.

The present invention has been described above with reference to the embodiments, but it will be apparent to those skilled in the art that various modifications and variations can be made without departing from the technical idea and the scope of the present invention described in the claims.

Claims

1. A cold-rolled steel sheet for a flux-cored wire, comprising, in weight%: c: 0.01-0.15%, Mn: 0.1 to 0.5%, Si: 0.05% or less and 0% or less, P: 0.0005 to 0.01%, S: 0.008% or less and 0% or less except, Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.5 to 2.0% and the balance Fe and unavoidable impurities, the fine structure comprising 93 to 98% of ferrite by area fraction and 2 to 7% of needle-like bainite and cementite in total,

wherein W is defined by the following relation 1_FCIs in the range of 0.5 to 4.5,

relation 1: w_FC＝(25*C+0.4*Mn+26*Al)*Ni，

The unit of the content of each element in the relation 1 is weight%.

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

the cold-rolled steel sheet has a yield strength of 200-300 MPa and an elongation of 40% or more.

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

the weld segregation index of the cold-rolled steel sheet is less than 0.15%.

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

heating a slab to 1100-1300 ℃, wherein the slab comprises, by weight percent: c: 0.01-0.15%, Mn: 0.1 to 0.5%, Si: 0.05% or less and 0% or less, P: 0.0005 to 0.01%, S: 0.008% or less and 0% or less except, Al: 0.005-0.06%, N: 0.0005 to 0.003%, Ni: 0.5 to 2.0% and the balance of Fe and inevitable impurities, W defined by the following relation 1_FCIs in the range of 0.5 to 4.5,

relation 1: w_FC＝(25*C+0.4*Mn+26*Al)*Ni，

The unit of the content of each element in the relational expression 1 is weight percent;

hot rolling the heated plate blank until the hot finish rolling temperature reaches 880-950 ℃ 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

and continuously annealing the cold-rolled steel sheet.

5. The method of manufacturing a cold rolled steel sheet for a flux cored wire as set forth in claim 4,

the continuous annealing is performed at a temperature ranging from 700 to 850 ℃.

6. The method of manufacturing a cold rolled steel sheet for a flux cored wire as set forth in claim 4,

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

7. The method of manufacturing a cold rolled steel sheet for a flux cored wire as claimed in claim 5,

and cooling the continuously annealed cold-rolled steel sheet at a cooling rate of 30 to 60 ℃/sec.