KR20230092609A - Cold-rolled steel sheet with excellent weldability and method of manufacturing thereof - Google Patents

Abstract

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

Description

Cold-rolled steel sheet with excellent weldability and method for manufacturing the same

The present invention relates to a steel sheet, and more particularly, to a cold-rolled steel sheet having excellent weldability and a manufacturing method thereof.

In general, the flux cored welding (FCW) method is the welding method with the highest welding productivity and easy welding in various positions. The welding material used in the flux-cored welding method is a flux-cored wire, and a strip drawn using a cold-rolled steel sheet for welding rod is processed into a U-shape, and flux is added to the processed holding ball. The flux is added by mixing various alloy elements in the form of high purity powder according to the purpose to improve the use characteristics of the flux component including an oxidizing agent and the welding rod to secure weldability, and then processed into an O shape for welding made of wire As such, the flux component is added to secure welding workability, and the alloy element is added to secure characteristics suitable for the use of the welding rod.

At this time, various characteristics required for the welding electrode material are secured through changes in the type and amount of alloy elements in the core added in the form of high-purity powder. For example, in order to produce a welded member requiring excellent low-temperature toughness, an alloy element and a flux must be mixed and charged together to improve the low-temperature toughness of the processed wire core part.

In addition, as a cold-rolled steel for wire used for manufacturing the flux-cored wire, low-carbon steel is used, and stainless steel is used for some special purposes. Since the low-carbon steel-based cold-rolled steel for welding rod is a low-alloy steel, a large amount of alloying elements must be added to the core in order to secure the characteristics of the welding rod according to the environment in which the welding rod is used. However, in the case of the material used for the welding rod, since it is necessary to add an appropriate level of flux to secure welding workability, there is a problem in that there is a limit to the desired amount of alloying element input into the core. Specifically, a large amount of oxidizing agent, slag forming agent, arc stabilizer, and alloy components should all be added to the central part of the steel for welding rod, but it is limited to unilaterally filling the wire steel with about 30 to 60% of the volume including the flux. Therefore, although there is a difference depending on the alloy element to be filled, the weight ratio is limited to about 15 to 25%.

In this case, when the addition amount of the alloy element is increased to secure the use characteristics of the welding rod, the flux component is limited, making it difficult to secure stable welding characteristics. In addition, since these alloying elements must be added in the form of high-purity powder, they not only increase the manufacturing cost of the welding rod, but also cause segregation of the melted additives during welding as the specific gravity of most of the added alloying elements increases. There is a problem that acts as a factor of welding defects.

In the case of stainless steel for welding wire, since the amount of nickel (Ni) alloying element present in the steel component is fundamentally greater than that of general carbon steel, the amount of core alloying element added together with the flux can be reduced to some extent, but high alloy steel As a result, there is a problem that the price of the original plate material is high and is applied only for special purposes. In addition, in the case of the steel for the stainless steel base welding rod, there are many cases where disconnection occurs due to work hardening during processing of the welding rod wire, and it is necessary to solve the problem of increasing manufacturing cost due to additional processing, such as requiring separate annealing heat treatment. there is

Therefore, as a cold-rolled steel for welding wire that requires current processability, specifically drawing processability, strength, and toughness, expensive alloying elements are prepared in the form of high-purity powder to secure strength and low-temperature toughness when flux is charged after forming low-carbon steel. A method of adding together with flux components is generally used. As described above, not only is the alloy powder added to secure properties high-purity and expensive, but also there is a problem in that conditions for adding flux components to secure welding stability are restricted according to the large input amount.

In addition, there is a problem in that the expensive alloying elements added cause segregation in the flux and degrade welding workability. For example, as a method for manufacturing the steel sheet for the flux-cored wire, a method of manufacturing a steel for a welding rod having excellent impact toughness and strength characteristics by adding a component such as titanium (Ti) has been attempted. However, as the method adds a large amount of expensive alloying elements, there is a problem of cost increase and low ductility of the steel, making it difficult to secure high drawability.

In addition, a technique for reducing welding defects by accelerating a deoxidation reaction of molten metal by adding raw materials such as titanium (Ti) and magnesium (Mg) to a flux raw material has been proposed. However, in order to obtain sufficient deoxidation effect of molten metal, it is necessary to add many alloying elements to the flux. However, when many alloying elements are added to the flux, spatter phenomenon in which fine particles protrude to the surroundings during welding occurs. There is a problem in that welding workability is deteriorated due to a large number of occurrences.

Therefore, it is possible to obtain a welded part having excellent strength and low-temperature toughness in a cryogenic environment, and the development of a welded steel material using a cold-rolled steel sheet for a flux-cored wire welding rod having excellent welding workability and drawing workability and a manufacturing method thereof is required. The situation is.

The technical problem to be solved by the present invention is to obtain a welded part having excellent strength and low-temperature toughness in a cryogenic environment, and to provide a cold-rolled steel sheet for a flux-cored wire welding rod having excellent welding workability and drawability.

Another technical problem to be solved by the present invention is to provide a method for manufacturing a cold-rolled steel sheet having the above advantages.

According to an embodiment of the present invention, the cold-rolled steel sheet contains, by weight, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%) , phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten (W) : 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the remaining Fe and unavoidable impurities may be included, and the yield strength of the welding member may be 500 MPa or more.

In one embodiment, the cold-rolled steel sheet may satisfy FCW and _A values of 40 to 150 in Equation 1 below.

<Equation 1>

FCW, _A = (2.5 × [Mn] + 1.8 × [Cr] + 24 × [Al]) × (18 × [W]) × (45 × [Zr]) / (34 × [C])

(In Equation 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C in weight%, respectively. )

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

According to another embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet contains, by weight, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (0% excluding), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten Heating a steel slab containing (W): 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the balance Fe and unavoidable impurities, Hot rolling at a finish rolling temperature of 880 ° C., winding the hot-rolled hot-rolled steel sheet at 580 to 700 ° C., cold-rolling the coiled hot-rolled steel sheet at 50 to 90%, and cold-rolled cold-rolled steel sheet It may include the step of annealing in the range of 700 to 850 ℃.

In one embodiment, the slab may satisfy FCW and _A values of 40 to 150 in Equation 1 below.

<Equation 1>

FCW, _A = (2.5 × [Mn] + 1.8 × [Cr] + 24 × [Al]) × (18 × [W]) × (45 × [Zr]) / (34 × [C])

(In Equation 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C in weight%, respectively. )

In one embodiment, heating the steel slab may be performed at 1,180 °C or higher. In one embodiment, prior to cold rolling the rolled hot-rolled steel sheet, the hot-rolled steel sheet may be pickled. In one embodiment, after the step of annealing the cold-rolled cold-rolled steel sheet, the step of temper rolling the annealed cold-rolled steel sheet may be further included.

The cold-rolled steel sheet according to an embodiment of the present invention has excellent strength, excellent low-temperature toughness, welding workability and workability by controlling the content of major elements such as manganese, chromium, aluminum, tungsten, zirconium, and carbon. By providing this excellent cold-rolled steel sheet, it is possible to provide a cold-rolled steel sheet for flux-cored wire capable of being welded used in various industrial fields such as the shipbuilding industry, the material industry, and the construction industry.

A cold-rolled steel sheet manufacturing method according to another embodiment of the present invention may provide a method for manufacturing a cold-rolled steel sheet having the above advantages.

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

The terminology used herein is only for referring to specific embodiments and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. The meaning of "comprising" as used herein specifies particular characteristics, regions, integers, steps, operations, elements and/or components, and the presence or absence of other characteristics, regions, integers, steps, operations, elements and/or components. Additions are not excluded.

When a part is referred to as being “on” or “on” another part, it may be directly on or on the other part or may be followed by another part therebetween. In contrast, when a part is said to be “directly on” another part, there is no intervening part between them.

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having meanings consistent with related technical literature and currently disclosed content, and are not interpreted in ideal or very formal meanings unless defined.

In addition, unless otherwise specified, % means weight%, and 1ppm is 0.0001 weight%.

In one embodiment of the present invention, the meaning of further including an additional element means replacing and including iron (Fe) as much as the additional amount of the additional element.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein.

In the cold-rolled steel sheet according to an embodiment of the present invention, by weight, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%), Phosphorus (P): 0.0005 to 0.01%, Sulfur (S): 0.008% or less (excluding 0%), Aluminum (Al): 0.0005 to 0.015%, Nitrogen (N): 0.0005 to 0.003%, Tungsten (W): 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the balance Fe and unavoidable impurities.

In the following, the reason for limiting the alloy components will be explained. Hereinafter, wt% may be expressed as %.

Carbon (C): 0.01 to 0.05%

Carbon (C) is an element added to secure the strength of steel and to make the Heat Affected Zone (HAZ) have material characteristics similar to those of the parent material during welding. The carbon content may range from 0.01 to 0.05%, specifically, from 0.012 to 0.048%.

When the carbon content is out of the upper limit of the range, there is a problem in that disconnection occurs due to high work hardening in the drawing process. In addition, low-temperature cracks occur at welded joints, impact toughness is lowered, and additional heat treatment is required due to high strength. When the carbon content is outside the lower limit of the range, there is a problem in that the effect of securing the strength of the steel and having the heat-affected zone having material characteristics similar to those of the base material during welding is not expressed.

Manganese (Mn): 0.6 to 1.2%

Manganese (Mn) is a representative solid-solution strengthening element that increases strength of steel and improves hot workability. The manganese content may range from 0.6 to 1.2%, specifically from 0.65 to 1.15%.

When the content of manganese is outside the upper limit of the range, a large amount of manganese-sulfide (MnS) precipitates are formed to reduce the ductility and workability of the steel, and also act as a factor in the occurrence of center segregation, resulting in deterioration in the manufacturing process of the welding rod. There is a problem that causes disconnection during operation. When the content of manganese is out of the lower limit of the range, it causes red-hot brittleness and does not contribute to the stabilization of austenite.

Silicon (Si): 0.05% or less

Silicon (Si) combines with a specific element such as oxygen to form an oxide layer on the surface of the steel sheet, deteriorating the surface properties and acting as a factor in reducing corrosion resistance, and degrading impact properties by promoting hard phase transformation in the weld metal. may act as a contributing factor. The silicon content may range from 0.0005 to 0.0045%.

Phosphorus (P): 0.0005 to 0.010%

Phosphorus (P) is an element that exists as a solid solution element in steel and causes solid solution strengthening to improve strength and hardness. The phosphorus content may range from 0.0005 to 0.01%, specifically from 0.0008 to 0.0095%.

When the phosphorus content is outside the upper limit of the range, center segregation occurs during casting, and ductility is lowered, thereby reducing wire workability. When the phosphorus content is out of the lower limit of the range, it is difficult to maintain a certain level of stiffness.

Sulfur (S): 0.008% or less

Sulfur (S) combines with manganese (Mn) in steel to form non-metallic inclusions and may be a factor in red shortness. The sulfur (S) content may be 0.008% or less, specifically in the range of 0.0004 to 0.0075%.

When the sulfur content is out of the upper limit of the range, a large amount of the non-metallic inclusions are formed, and the red heat brittleness is intensified. In addition, there is a problem of lowering the base material toughness of the steel sheet.

Aluminum (Al): 0.0005 to 0.0150%

Aluminum (Al) is an element added for the purpose of preventing material deterioration due to carbonate and aging in aluminum killed steel, and is an element advantageous for securing ductility within an appropriate range, and the above-mentioned effects occur remarkably in the case of cryogenic temperatures. The aluminum content may range from 0.0005 to 0.015%, specifically, from 0.0006 to 0.0148%.

When the content is out of the upper limit, surface inclusions such as aluminum-oxide (Al ₂ O ₃ ) rapidly deteriorate the surface properties of the rolled material, degrade workability, and locally ferrite at the grain boundary of the heat-affected zone of the weld. There is a problem in that the mechanical properties are deteriorated and the shape of the weld bead is deteriorated even after welding. If the content is out of the lower limit, there is a problem that the adhesion of oxygen is lowered and material deterioration due to aging may occur.

Nitrogen (N): 0.0005 to 0.003%

Nitrogen (N) is an element that strongly induces material strengthening while existing in a solid solution state inside the steel. The nitrogen content may range from 0.0005 to 0.003%, specifically, from 0.0008 to 0.0029%.

When the nitrogen content is outside the upper limit of the range, the aging property is excessively deteriorated, and the burden due to denitrification increases in the steel manufacturing step, which may deteriorate steelmaking workability. When the nitrogen content is out of the lower limit of the range, it is difficult to secure the target stiffness.

Tungsten (W): 0.1 to 0.4%

Tungsten (W) is effective in improving strength, high-temperature properties, and drawability, and is an element for improving low-temperature impact properties by forming a stable structure even at cryogenic temperatures. The content of tungsten may range from 0.1 to 0.4%, specifically, from 0.12 to 0.39%.

When the content of tungsten is out of the upper limit of the range, there is a problem in that workability is deteriorated due to strength increase and surface defects are caused. When the content of tungsten is out of the lower limit of the range, there is a problem that the effect on the high-temperature characteristics and the drawing processability is difficult to express, and the stable operation of the flux composition may be difficult.

Chromium (Cr): 1.0 to 2.0%

Chromium (Cr) is an element that is advantageous to the strength of welded joints, plays a role in forming a stable rust layer, and contributes to improving corrosion resistance. The chromium content may range from 1.0 to 2.0%, specifically from 1.10 to 1.85%.

When the content of chromium is outside the upper limit of the range, there is a problem in that chromium-based carbides are formed to cause brittleness and processing is not easy. When the content of chromium is out of the lower limit of the range, there is a problem in that the effect of improving the strength of the welded joint secured by adding the chromium, forming a stable rust layer, and contributing to the improvement of corrosion resistance is difficult to express.

Zirconium (Zr): 0.05 to 0.15%

Zirconium (Zr) is an element that is not only advantageous in terms of securing low-temperature toughness of a welded part through the formation of precipitates, but also greatly contributes to improving workability of a welding material. The zirconium content may range from 0.05 to 0.15%, specifically, from 0.07 to 0.14%.

When the content of zirconium is out of the upper limit of the range, there is a problem in that the amount of zirconium precipitates increases, resulting in deterioration in processability and deterioration in operating characteristics as the working temperature rises. When the content of zirconium is out of the lower limit of the range, there is a problem in that the effect of securing the low-temperature toughness of the welding part and improving the workability of the welding material through the formation of the above-described precipitate is difficult to express.

It contains iron (Fe) as the remainder. In addition, it may contain unavoidable impurities. The unavoidable impurities refer to impurities that are unavoidably mixed in the manufacturing process of the cold-rolled steel sheet. Since unavoidable impurities are widely known, detailed descriptions are omitted. In one embodiment of the present invention, the addition of elements other than the above-described alloy components is not excluded, and may be variously included within a range that does not impair the technical spirit of the present invention. When additional elements are included, they are included in place of Fe, which is the remainder.

In one embodiment, the cold-rolled steel sheet may further include Cu, Ti, Nb, and Ni. The content of each component may be 0.03% or less of Cu, 0.002% or less of Ti, 0.002% or less of Nb, and 0.02% or less of Ni.

In one embodiment, the cold-rolled steel sheet satisfies FCW and _A values of 40 to 150 in Equation 1 below.

<Equation 1>

FCW, _A = (2.5 × [Mn] + 1.8 × [Cr] + 24 × [Al]) × (18 × [W]) × (45 × [Zr]) / (34 × [C])

(In Equation 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C in weight%, respectively. )

FCW, _A in Equation 1 relates to the correlation of each element affecting welding workability and drawing workability, and the cold-rolled steel sheet having excellent weldability for flux-cored wire has a value of FCW, _A , which is an index of Equation 1, is 40 to 150, specifically 45 to 145.

When the FCW and _A values of Equation 1 are out of the upper limit of the range, the fraction of the hard transformed structure increases, causing breakage of the welding member during pipe manufacturing and drawing. When the values of FCW and _A in Equation 1 are outside the lower limit of the range, it is advantageous in terms of processability because the amount of transformation into a hard phase is small in the room temperature structure, but the amount of alloying elements in the flux increases to secure strength and low-temperature toughness. Therefore, there is a problem in that welding workability is lowered and manufacturing cost is improved.

In one embodiment, the cold-rolled steel sheet has a structure fraction composed of at least one of cementite and bainite, as an area %, of 1 to 7%, and the balance includes ferrite. Specifically, the cold-rolled steel sheet having excellent weldability according to the present invention may have a microstructure including cementite and bainite of 1 to 7% and a balance of ferrite as an area %. The tissue fraction composed of the cementite and the bainite may be in the range of 1 to 7%, specifically 1.1 to 6.8%, as an area %.

If the fraction of the structure composed of the cementite and the bainite is out of the upper limit of the range, it may cause breakage during drawing and deteriorate corrosion resistance. When the structure fraction composed of the cementite and the bainite is out of the lower limit of the above range, there is a problem of acting as a factor in generating strain aging defects due to solid solution elements in the steel as the precipitation of carbides is not promoted.

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

When the elongation ratio satisfies the above range, it can be utilized as a cold-rolled steel sheet for a flux-cored wire welding rod. When the elongation is out of the range, the cross section reduction rate is lowered during the drawing process of the welding wire, resulting in deterioration in pipe making workability and the occurrence of cracks such as tearing during processing.

In one embodiment, the cold-rolled steel sheet having excellent weldability may have a weld zone segregation index of less than 0.15%. The welded part segregation index means the segregation index of a welded part welded with a flux-cored wire manufactured using a cold-rolled steel sheet having excellent weldability according to an embodiment of the present invention.

The welded portion segregation index is expressed as a ratio of an area occupied by a segregated portion due to additive elements to the total area of the welded portion. The weldment segregation index may be 0.15% or less, specifically, in the range of 0.001 to 0.13%. When the weld zone segregation index is out of the above range, there is a problem in that the weldability of the cold-rolled steel sheet is deteriorated.

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

In one embodiment, the cold-rolled steel sheet may have a welded member yield strength of 500 MPa or more. The yield strength of the welding member refers to the yield strength of a welded portion welded with a flux-cored wire manufactured using a cold-rolled steel sheet. The yield strength of the welded part needs to be maintained within an appropriate range regardless of the base material. Since the yield strength of the welding member satisfies 500 MPa or more, high-strength characteristics can be secured in terms of securing the stability of the welded portion when applied to the structural member.

According to another embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet contains, by weight, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (0% excluding), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten Heating a steel slab containing (W): 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the balance Fe and unavoidable impurities, and hot rolling the heated slab It includes the steps of: winding the hot-rolled hot-rolled steel sheet, cold-rolling the coiled hot-rolled steel sheet, and annealing the cold-rolled cold-rolled steel sheet. A detailed description of the steel slab is the same as that of the cold-rolled steel sheet described above to the extent that it does not contradict, and thus, duplicate descriptions will be omitted.

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

Outside the upper limit of the temperature range, not only energy cost increases, but also the amount of surface scale increases, which may lead to material loss. When out of the lower limit of the temperature range, there is a problem that the load increases rapidly during the subsequent hot rolling.

Next, in the step of hot-rolling the heated slab, a hot-rolled steel sheet may be obtained by hot-rolling the heated slab. In one embodiment, the finish rolling temperature may be carried out in the range of 820 to 880 ℃. The finish rolling temperature may be specifically, carried out in the range of 830 to 875 ℃.

When the finish rolling temperature is out of the upper limit of the range, the peelability of the surface scale is lowered, and uniform hot rolling is not performed throughout the thickness, so grain refinement is insufficient, resulting in impact toughness due to grain coarsening. may cause degradation. When the finish rolling temperature is out of the lower limit of the range, as hot rolling is finished in a low temperature region, crystal grains are rapidly grain-aggregated, resulting in deterioration in hot rolling properties and workability.

Next, in the step of winding the hot-rolled hot-rolled steel sheet, the hot-rolled steel sheet obtained through the hot-rolling step may be wound. In one embodiment, the winding may be performed at a winding temperature of 580 to 700 °C. Specifically, the winding may be performed at a temperature range of 590 to 690 °C.

If the coiling temperature is out of the upper limit value, as the structure of the final product is coarsened, it may cause a problem of softening the surface material and deteriorating the tubularity. When the coiling temperature is out of the lower limit value, in the cooling and maintaining step, the temperature non-uniformity in the width direction causes a difference in the formation behavior of low-temperature precipitates and causes material deviation, thereby reducing workability.

Next, in the step of cold rolling the coiled hot-rolled steel sheet, a cold-rolled steel sheet may be obtained by cold-rolling the hot-rolled steel sheet to a target thickness. In one embodiment, prior to cold rolling the rolled hot-rolled steel sheet, the hot-rolled steel sheet may be pickled.

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

When the cold rolling reduction ratio is out of the upper limit of the range, there is a problem in that the material is hardened, lowering the cold rolling workability due to the load of the rolling mill, and causing cracks during drawing. If the cold reduction ratio is out of the lower limit of the range, there is a problem in that it is difficult to secure a uniform material because the recrystallization driving force is low and local tissue growth occurs. In addition, considering the thickness of the final product, since the thickness of the hot-rolled steel sheet must be lowered, the hot-rolling workability is significantly deteriorated.

Next, in the step of annealing the cold-rolled cold-rolled steel sheet, target strength and workability can be secured by performing annealing in a state in which strength is increased due to strain introduced in cold rolling. Annealing the cold-rolled cold-rolled steel sheet may be performed at a temperature range of 700 to 850 °C. Specifically, the step of annealing the cold-rolled cold-rolled steel sheet may be in the temperature range of 710 to 840 °C.

In the step of annealing, when the upper limit of the temperature range is out of range, the risk of plate breakage due to heat-buckle increases, and the annealing passability may be reduced. When the annealing step is out of the lower limit of the temperature range, there is a problem in that workability is significantly deteriorated as the strain formed by cold rolling is not sufficiently removed.

After the step of annealing, a step of temper rolling the annealed cold-rolled steel sheet may be further included. The step of temper rolling the annealed cold-rolled steel sheet may be applied at a reduction ratio of 3% or less. Specifically, the step of temper rolling the annealed cold-rolled steel sheet may be applied at a reduction ratio of 0.3 to 2.5% or less.

In the step of temper rolling the annealed cold-rolled steel sheet, when the reduction ratio satisfies the above range, it is possible to control the shape of the material and obtain a target surface roughness. In the step of temper rolling the annealed cold-rolled steel sheet, when the reduction ratio is excessively high, the material is hardened but workability is deteriorated.

After the annealing step, the annealed sheet may be used for manufacturing a flux cored wire. In one embodiment, the cold-rolled steel sheet for flux-cored wire may include an outer shell composed of the aforementioned cold-rolled steel sheet or a cold-rolled steel sheet manufactured by the cold-rolled steel sheet manufacturing method, and a flux filled in the outer shell.

The flux-cored wire comprising the cold-rolled steel sheet of the present invention can exhibit the effect of the flux-cored wire, and the effect is independent of the type of flux. It is an effect expressed by Therefore, since the flux generally used in the flux cored wire field can be used, a detailed description thereof will be omitted.

<Invention Steels 1 to 5, Comparative Steels 1 to 6>

Table 1 below shows the composition and FCW _{, A} values of the main components for inventive steels 1 to 5 and comparative steels 1 to 6. The FCW _,A value is designed considering the correlation of each element affecting welding workability and drawing workability. Specifically, the FCW _,A value is (2.5 × [Mn] + 1.8 × [Cr] + 24 × [Al]) × (18 × [W]) × (45 × [Zr]) / (34 × [C]), [Mn], [Cr], [Al], [W], [Zr], and [C] represent weight% as each content.

steel grade Chemical composition (% by weight) C Mn Si P S Al N W Cr Zr FCW, _A value invention steel 1 0.024 0.76 0.019 0.005 0.005 0.012 0.0024 0.36 1.26 0.072 109.55 invention steel 2 0.031 0.68 0.011 0.007 0.002 0.009 0.0018 0.21 1.78 0.096 75.80 invention steel 3 0.038 0.94 0.008 0.004 0.004 0.011 0.0013 0.16 1.39 0.104 51.00 Invention Steel 4 0.028 1.08 0.028 0.005 0.006 0.004 0.0025 0.23 1.49 0.124 127.02 invention steel 5 0.041 0.82 0.009 0.003 0.003 0.003 0.0016 0.27 1.55 0.131 96.47 comparative steel 1 0.003 1.01 0.021 0.006 0.006 0.068 0.0015 0.25 1.60 0.002 26.70 comparative steel 2 0.021 0.31 0.016 0.005 0.004 0.012 0.0074 0.71 1.41 0.081 224.50 comparative lecture 3 0.043 0.72 0.007 0.042 0.006 0.002 0.0026 0.01 1.32 0.270 6.04 comparative lecture 4 0.027 1.54 0.008 0.006 0.034 0.007 0.0019 0.12 0.65 0.053 27.82 comparative steel 5 0.019 0.82 0.015 0.029 0.007 0.039 0.0022 0.25 0.03 0.024 21.85 comparative steel 6 0.086 0.90 0.540 0.005 0.004 0.005 0.0014 0.18 2.47 0.091 29.55

Looking at Table 1, it can be seen that inventive steels 1 to 5 satisfy the steel sheet composition of the present invention, and thus satisfy the FCW _,A values of 40 to 150.

<Examples 1 to 4 and Comparative Examples 1 to 5>

Inventive materials 1 to 9 and Comparative Examples 1 to 10 were prepared by working the inventive steels 1 to 5 and comparative steels 1 to 6 of Table 1 under the process conditions shown in Table 2 below. Specifically, after heating the slabs of Table 1 to 1,230 ° C., hot rolling, winding, cold rolling, and continuous annealing processes were performed under the manufacturing conditions described in Table 2 below. In this case, a temper reduction of 0.9% was applied to the annealed sheet.

division Steel grade No. Finish hot rolling temperature (℃) winding temperature
(℃) cold rolling reduction
(%) annealing temperature
(℃) Invention 1 Invention Steel 1 860 680 65 750 Invention 2 Invention Steel 1 860 680 70 790 Inventive 3 Invention Steel 1 860 680 75 830 Inventive 4 Invention Steel 2 840 600 75 780 Inventive 5 Invention Steel 2 840 600 80 820 Inventive 6 Invention Steel 3 840 660 75 800 Inventive 7 Invention Steel 4 840 620 70 760 Inventive 8 Invention Steel 5 870 640 65 760 Inventive 9 Invention Steel 5 870 640 80 820 comparative material 1 Invention Steel 1 700 680 75 580 comparative material 2 Invention Steel 1 960 680 40 780 comparative material 3 Invention Steel 2 840 500 93 820 comparative material 4 Invention Steel 3 840 760 75 890 comparative material 5 Comparative Lecture 1 880 640 70 820 comparative material 6 comparative steel 2 860 640 70 820 comparative material 7 comparative lecture 3 860 640 70 820 comparative material 8 comparative lecture 4 860 640 70 800 comparative material 9 comparative steel 5 860 640 70 800 comparative material 10 Comparative Lecture 6 860 660 70 800

Referring to Table 2, in the case of inventive materials 1 to 9, steels included in the range of inventive steels of Table 1 and those performed under the manufacturing conditions of the present invention are shown. In comparison, Comparative Materials 1 to 4 used steels included in the range of inventive steels in Table 1, but show cases outside the range of manufacturing conditions of the present invention in manufacturing conditions. In addition, Comparative Material 5 to Comparative Material 10 indicate that the steel grades of Comparative Steel 1 to Comparative Steel 6 in Table 1 were performed under the manufacturing conditions of the present invention.

<Characteristics of cold rolled steel>

Table 3 below shows the result of measuring the type and fraction of microstructure, elongation, sheet permeability, and drawing workability of the cold-rolled steel sheet manufactured through the manufacturing conditions of Table 2 above. Sheet permeability was marked as good (O) when there was no rolling load during cold and hot rolling and no defects such as heat-buckles occurred during continuous annealing, and poor when rolling load occurred or defects such as plate breakage occurred during continuous annealing ( X).

The drawing processability was marked as poor (X) if processing defects such as tearing occurred during the drawing process of the flux-cored wire with a section reduction rate of 61%, and it was marked as good (O) if processing defects did not occur. In addition, after manufacturing a strip having a width of 14 mm using the manufactured cold-rolled steel sheet, the strip was processed into a U-shape to fill the flux component, and then an O-shaped welding material having a diameter of 3.1 mm was made. Thereafter, a flux-cored wire having a diameter of 1.2 mm was prepared by drawing the welding material, and the results of the low-temperature impact and tensile tests are shown in Table 3 below.

In addition, after measuring the segregation index of the welding part through a scanning electron microscope with respect to the welding members welded with the flux-cored wire, the measurement results are shown in Table 3 below. At this time, the welding member was drawn with a wire having a diameter of 1.2 mm, and the welding member worked under the conditions of a voltage of 29 volts, a current of 150 to 180 A, and a welding speed of 40 cm per minute using a pilot welding machine. This is the result.

After measuring the segregation dimension of the welded part with respect to the welded member welded with the flux-cored wire, the measurement result is also shown in Table 3 below. At this time, the welding member was drawn with a wire having a diameter of 1.2 mm, and the quality of the welding member worked under the conditions of a voltage of 29 volts, a current of 150 to 180 A, and a welding speed of 40 cm per minute using a pilot welding machine. is the result of evaluation.

division Cementite, bainite fraction
(area%) permeability Elongation (%) weld
Segregation Index (%) impact toughness
(J, @-40℃) welding member
yield strength
(MPa) drawing
machinability Invention 1 3.1 ○ 41 0.07 92 578 Good Invention 2 2.8 ○ 43 0.09 88 584 Good Inventive 3 2.1 ○ 43 0.06 101 567 Good Inventive 4 4.8 ○ 39 0.03 79 592 Good Inventive 5 4.2 ○ 42 0.05 94 599 Good Inventive 6 5.2 ○ 40 0.02 106 604 Good Inventive 7 4.7 ○ 42 0.04 69 592 Good Inventive 8 6.4 ○ 44 0.08 113 574 Good Inventive 9 5.2 ○ 40 0.05 109 632 Good comparative material 1 0.4 X 15 0.10 42 392 error comparative material 2 1.5 X 24 0.19 44 458 error comparative material 3 0.5 X 31 0.09 38 409 error comparative material 4 0.8 X 43 0.22 46 422 Good comparative material 5 0.0 ○ 34 0.51 24 346 error comparative material 6 0.8 ○ 29 0.32 35 401 error comparative material 7 2.6 ○ 29 0.30 42 452 error comparative material 8 3.4 ○ 27 0.21 39 397 error comparative material 9 0.9 ○ 31 0.24 40 468 error comparative material 10 9.2 X 24 0.33 47 471 error

Looking at Tables 2 and 3, the invention materials 1 to 9 satisfying all of the alloy composition, microstructure characteristics, and manufacturing conditions not only have good sheetability, but also the material of the target cold-rolled steel sheet for flux-cored wire welding rods. It satisfies the standard elongation of 35% or more. In addition, since the segregation index of the wire made of the welding member is less than 0.15%, tearing or cracking of the welded part does not occur during secondary processing, so that excellent workability can be secured. In addition, it can be confirmed that excellent strength and low-temperature toughness with an impact energy of 50 J or more at -40 ° C. and a yield strength of 500 MPa or more of the welded member can be secured.

In contrast, Comparative Materials 1 to 4 satisfied the range of alloy composition presented in the present invention, but did not satisfy the microstructure characteristics and manufacturing conditions. And, in Comparative Examples 1 to 4, it can be confirmed that the annealing sheet passability is lowered. In addition, it can be confirmed that the elongation is lower than the target value, the yield strength of the welding member is less than 500 MPa, the impact energy value at -40 ° C is less than -50 J, and the welding member drawing workability is poor. properties cannot be obtained.

In addition, Comparative Materials 5 to 10 satisfy the manufacturing conditions presented in the present invention, but do not satisfy the alloy composition, and have characteristics such as target elongation, weld segregation index, impact energy, and weld yield strength of the present invention. It was confirmed that the inventive materials 1 to 9 did not reach, and in most cases, it was confirmed that tearing or cracking occurred during the drawing process.

The present invention is not limited to the above embodiments and / or embodiments, but can be manufactured in various different forms, and those skilled in the art to which the present invention belongs may change the technical spirit or essential features of the present invention. It will be appreciated that it may be embodied in other specific forms without Therefore, it should be understood that implementations and/or examples described above are illustrative in all respects and not restrictive.

Claims (11)

In weight percent, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten (W): 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the remainder including Fe and unavoidable impurities,
Cold-rolled steel sheet with welded member yield strength of 500 MPa or more.
According to claim 1,
A cold-rolled steel sheet having FCW and _A values of Equation 1 below satisfies 40 to 150.
<Equation 1>
FCW, _A = (2.5 × [Mn] + 1.8 × [Cr] + 24 × [Al]) × (18 × [W]) × (45 × [Zr]) / (34 × [C])
(In Equation 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C in weight%, respectively. )
According to claim 1,
A cold-rolled steel sheet comprising a structure fraction composed of at least one of cementite and bainite in an area % of 1 to 7% and the remainder containing ferrite.
According to claim 1,
Cold-rolled steel sheet with an elongation of more than 35%.
According to claim 1,
Cold-rolled steel sheet with a weld zone segregation index of less than 0.15%.
According to claim 1,
- Cold-rolled steel with an impact energy of 50 J or more at 40 °C.
In weight percent, carbon (C): 0.01 to 0.05%, manganese (Mn): 0.6 to 1.2%, silicon (Si): 0.05% or less (excluding 0%), phosphorus (P): 0.0005 to 0.01%, sulfur (S): 0.008% or less (excluding 0%), aluminum (Al): 0.0005 to 0.015%, nitrogen (N): 0.0005 to 0.003%, tungsten (W): 0.1 to 0.4%, chromium (Cr): 1.0 to 2.0%, zirconium (Zr): 0.05 to 0.15%, the balance of Fe and unavoidable impurities;
hot rolling the heated slab at a finish rolling temperature of 820 to 880 ° C;
winding the hot-rolled hot-rolled steel sheet at 580 to 700 ° C;
Cold-rolling the coiled hot-rolled steel sheet at 50 to 90%; and
Cold-rolled steel sheet manufacturing method comprising the step of annealing the cold-rolled cold-rolled steel sheet in the range of 700 to 850 ℃.
According to claim 7,
The method of manufacturing a cold-rolled steel sheet in which the slab satisfies FCW and _A values of 40 to 150 in Equation 1 below.
<Equation 1>
FCW, _A = (2.5 × [Mn] + 1.8 × [Cr] + 24 × [Al]) × (18 × [W]) × (45 × [Zr]) / (34 × [C])
(In Equation 1, [Mn], [Cr], [Al], [W], [Zr], and [C] represent the contents of Mn, Cr, Al, W, Zr, and C in weight%, respectively. )
According to claim 7,
The step of heating the steel slab is a method of manufacturing a cold-rolled steel sheet performed at 1,180 ℃ or more.
According to claim 7,
The cold-rolled steel sheet manufacturing method of pickling the hot-rolled steel sheet before the step of cold-rolling the coiled hot-rolled steel sheet.
According to claim 7,
After the step of annealing the cold-rolled cold-rolled steel sheet, the cold-rolled steel sheet manufacturing method further comprising the step of temper rolling the annealed cold-rolled steel sheet.