KR101726134B1 - Wire rod having excellent weldability and method for manufacturing the same - Google Patents

Abstract

One aspect of the present invention comprises: 0.32-0.72 wt% of C; 0.2-0.3 wt% of Si; 0.6-0.9 wt% of Mn; 0.0001-0.002 wt% of Ca; 0.01 wt% or less of P; 0.01 wt% or less of S; and the remaining consisting of Fe and unavoidable impurities. The present invention relates to the wire material with excellent weldability, wherein Ca composite inclusions having an area of at least 700 m^2 in a zone within 1/4*D (D: a diameter of the wire material measured in mm) from a center of a wire material which exists to be below or equal to 200 units/g.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire rod having excellent weldability,

The present invention relates to a wire having excellent weldability and a method of manufacturing the wire. More particularly, the present invention relates to a wire having excellent weldability which can be preferably applied to an armor cable, a lock coil, and the like, and a method of manufacturing the wire.

Armor cables and lact coils are used for flexible pipes for transporting crude oil. They are used to support pipelines connected to the sea floor from floating structures such as Floating Production Storage and Outfitting (FPSO) or Production Platform. .

In general, the steel used for the armor cable is a piano steel having a carbon content of 0.3 to 0.7%. It is a JIS (Japanese Industrial Standards) steel having an Si content of 0.2 to 0.3% and a Mn content of 0.6 to 0.9% The SWRSs 32B to 72B included are representative. The diameter of wire rod is 10 ~ 20 mm and there is a difference in diameter depending on the application. The processing in the processing machine is the same by heat treatment -> dry drawing -> rolling -> twisting.

Although the required strength varies from product to product, it has a tensile strength in the range of 1100 ~ 1600 MPa. It can be said that the higher the strength, the more the use environment moves from the offshore to the deep sea. to be. As a method of increasing the strength, a method of processing a product by increasing only C effectively without increasing Cr or Si is being applied.

The steel used for armor cables does not require material cleanliness, and therefore does not place much constraint on steel production. However, because of the high carbon content and the production of Al-Si killed steel, clogging of the tundish nozzle can not be avoided. For this purpose, the molten steel is subjected to vacuum degassing by RH (Ruhrstahl Heraeus) Ca-Si wire is added. Therefore, Ca is present in the molten steel, and since the affinity with oxygen is much larger than that of Al, Si, Mg, etc., low melting point complex inclusions such as Al-Si-Mg-Ca-O are artificially formed. These jeoyong point Ca composite inclusions since the rapid formation than other inclusions easy to exist as nucleation sites for other inclusions formed, Ca tank having compared to when non-addition, and more than 700 ㎛ ² area when also the content is increased inclusion of Is likely to be formed. These inclusions affect the drawing and rolling, but they cause the fracture of the welded part when the tensile load is applied after welding the same working line, so that the required properties of the product are not satisfied.

Therefore, in order to improve the weldability of the final machined wire made by using the wire used in the armor cable and the lact coil casted by adding Ca in the molten steel inevitably, research and development of a method and an acceptable level for controlling the Ca composite inclusion It is a fact that is demanded.

One aspect of the present invention is to provide a wire having excellent weldability and a method of manufacturing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

An aspect of the present invention is a steel sheet comprising, by weight, 0.32 to 0.72% of C, 0.2 to 0.3% of Si, 0.6 to 0.9% of Mn, 0.0001 to 0.002% of Ca, 0.01% The balance of Fe and unavoidable impurities,

And a Ca composite inclusion having an area of 700 탆 ² or more in an area up to 1/4 * D (D: wire diameter measured in mm) from the center of the wire is 200 or less / g or less.

Another aspect of the present invention is to provide a method for producing a tantalum carbide which comprises, after vacuum degassing treatment of molten steel, adding 1 kg or less of Ca-Si wire per ton of molten steel and setting the retention time to 20 minutes or more before charging tundish, The molten steel containing 0.32 to 0.72% of C, 0.2 to 0.3% of Si, 0.6 to 0.9% of Mn, 0.0001 to 0.002% of Ca, 0.01% or less of P and 0.01% or less of S and remaining Fe and unavoidable impurities Preparing;

Producing a billet with the molten steel and rolling to obtain a billet; And

And heating and rolling the billet to obtain a wire rod.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, there is provided a wire material in which a Ca composite inclusion having an area of 700 mu m ² or more is present in an area up to 1/4 * D (D: wire rod diameter measured in mm) Thereby preventing breakage of the welded portion during welding between the machining lines manufactured by drawing and rolling it.

In addition, the weld defect can be predicted according to the number of Ca composite inclusions in the wire, thereby reducing manufacturing cost.

Fig. 1 shows coarse inclusions exceeding 700 탆 ² of Comparative Example 1. Fig.
Fig. 2 is a photograph of a crack in a welded portion generated when tensile load was applied in Comparative Example 1. Fig.
3 is a photograph in the longitudinal direction (1 / 2t) after the final rupture of Comparative Example 1 and Inventive Example 1. Fig.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other 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 more fully explain the present invention to those skilled in the art.

The present inventors have found that, in the case of representative JIS standard steels (0.32 to 0.72% by weight, 0.2 to 0.3% by weight Si, 0.6 to 0.9% by Mn, and 0.01% or less respectively by P and S) , It is necessary to inject a certain Ca in order to prevent clogging of the tundish nozzle during the secondary refining. Therefore, there is a high possibility that a coarse Ca composite inclusion having an area of 700 탆 ² or more is formed compared to when no Ca is added, The effect of the tensile load on the welded part was investigated in order to solve the problem.

As a result, it was confirmed that the weldability can be improved by controlling the number of Ca complex inclusions having an area of 700 탆 ² or more, and the present invention has been accomplished.

Hereinafter, a wire having excellent weldability according to one aspect of the present invention will be described in detail.

According to one aspect of the present invention, there is provided a wire rod excellent in weldability, comprising 0.32 to 0.72% of C, 0.2 to 0.3% of Si, 0.6 to 0.9% of Mn, 0.0001 to 0.002% of Ca, S: 0.01% or less, the balance of Fe and unavoidable impurities,

There exist 200 Ca / g inclusions of Ca composite inclusions having an area of 700 탆 ² or more in a region from the wire rod center to 1/4 * D (D: wire rod diameter measured in mm).

First, the alloy composition of the wire rod excellent in weldability according to one aspect of the present invention will be described in detail. Hereinafter, the unit of each element content is% by weight.

C (carbon): 0.32 to 0.72%

C is an element added to secure the strength of the material, and is an effective element that greatly increases the strength by forming a hardened cementite phase in the pearlite structure. It is generally known that the tensile strength increases by 100 MPa when the C content is increased by 0.1%.

When the C content is less than 0.32%, it is difficult to secure a high strength. When the C content is more than 0.72%, the hydrogen resistance due to the formation of the cornerstone cementite becomes poor.

Si (silicon): 0.2 to 0.3%

Si is easy to be solidified in ferrite and inhibits the formation of carbides, and Si has an effect of increasing strength when added. It is generally known that the tensile strength is increased by 14 ~ 16 MPa when the Si content is increased by 0.1%.

When the Si content is less than 0.2%, the above-mentioned effect is insufficient, and when the Si content is more than 0.3%, the effect of increasing the strength is not so large.

Mn (manganese): 0.6 to 0.9%

Mn is an austenite stabilizing element, and is known as an element which is effective in retarding growth and increasing strength in pearlite transformation.

When the Mn content is less than 0.6%, it is difficult to secure the entrapment property. When the Mn content is more than 0.9%, center segregation may occur.

Ca (calcium): 0.0001 to 0.002%

Ca is an element which is easy to vaporize, but it is injected into molten steel because it is excellent in suppressing clogging of tundish nozzle in carbon steel and Al-Si killed steel, and is not used for the purpose of improving mechanical properties. Though there is a difference depending on the case, Ca-Si wire is used after being vacuum degassed by the ladle RH method, and it is supplied at 1 kg or less per ton of molten steel.

If the Ca content is less than 0.0001%, the tundish nozzle may be clogged. If the Ca content is more than 0.002%, the formation of the Ca complex inclusion and the increase of the production cost may occur.

P: not more than 0.010%

P is an impurity and does not specifically specify the content, but is preferably 0.010% or less from the viewpoint of securing ductility as in the conventional steel wire.

S: not more than 0.010%

S, like P, is an impurity and does not specifically specify the content, but is preferably 0.010% or less from the viewpoint of securing ductility as in the conventional steel wire.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

In the wire material excellent in weldability according to the present invention, a Ca composite inclusion having an area of 700 탆 ² or more in an area from 1/4 * D (D: wire rod diameter measured in mm) do.

Since fine inclusions having an area less than 700 탆 ² are discharged out of the steel along the bond line during welding and the amount thereof is small even when melted at the time of welding, there is no significant influence on the weldability. Therefore, Ca inclusive inclusions having an area of 700 탆 ² or more Respectively.

In the case where the Ca composite inclusions having an area of 700 탆 ² or more are more than 200 pieces / g, when the wire made by drawing and rolling the wire is butt welded by electric resistance welding, as shown in Fig. 2, Cracks are formed, and fracture can be caused by such cracks.

On the other hand, the area from the wire center to 1/4 * D means the inner area of the circle having a radius of 1/4 * D with the wire center as the center. The reason for limiting the area from the wire rod center to 1/4 * D is because the population of Ca complex inclusions is most active in this area.

At this time, the type of the Ca composite inclusion is not particularly limited, and may include, for example, at least one of Al ₂ O ₃ -CaS, AlCaMgO, AlCaSiO, AlCaMgO and AlCaMgSiO.

In addition, the microstructure of the wire may include at least 70% by area of ferrite and the remaining cormorite ferrite.

Hereinafter, a method of manufacturing a wire having excellent weldability, which is another aspect of the present invention, will be described in detail.

According to one aspect of the present invention, there is provided a method of manufacturing a wire having excellent weldability, which comprises: subjecting a molten steel to a vacuum degassing treatment, adding Ca-Si wire to the molten steel at a rate of 1 kg or less, 0.30 to 0.002%, P: 0.01% or less, S: 0.01% or less, and the balance of Fe and unavoidable impurities, in terms of% by weight, of C: 0.32 to 0.72%, Si: 0.2 to 0.3%, Mn: 0.6 to 0.9% Preparing molten steel containing impurities; Producing a billet with the molten steel and rolling to obtain a billet; And heating and rolling the billet to obtain a wire rod.

The Ca-Si wire is added to prevent clogging of the tundish nozzle. Ca is an element with high volatility. Even if Ca-Si wire is added in excess of 1kg per ton of molten steel, Ca content remaining in the steel is saturated at about 20ppm.

However, if the Ca content is less than 0.0001 wt%, clogging of the tundish nozzle may occur. Therefore, the Ca-Si wire is added so that the Ca content of the molten steel is 0.0001 wt% or more after the Ca-Si wire is added. Can be added.

When the retention time is less than 20 minutes, the inclusion flotation separation time is not sufficient, and a Ca complex inclusion having an area of 700 탆 ² or more in a region from 1/4 * D (D: wire rod diameter measured in mm) / RTI > may be present at greater than 200 < RTI ID = 0.0 &

At this time, when the Ca content is 0.001 to 0.002% by weight, the residence time is preferably 25 minutes or more. When the Ca content is 0.0001 wt% or more and less than 0.001 wt%, the inclusion flotation separation time is sufficient even with a retention time of 20 minutes or more but less than 25 minutes. When the Ca content is 0.001 wt% or more, This is because the floating separation time may be insufficient.

On the other hand, the longer the residence time is, the more effective it is for separating the inclusion flotation. Therefore, the upper limit of the residence time is not particularly limited, but the productivity can be lowered for 40 minutes or longer.

Preparing a billet by rolling a billet with molten steel; And the step of heating and rolling the billet to obtain a wire rod can be carried out under ordinary conditions, so that the conditions are not particularly limited.

For example, a 400 (mm) × 500 (mm) performance bloom is heated to 1150 to 1250 ° C. in a steel pipe heating furnace and maintained for 300 to 400 minutes, ) Billet. ≪ / RTI > The billet is heated at 1000 to 1100 ° C in a wire furnace, maintained at 90 to 120 minutes, and then rolled at a temperature of 950 ° C or higher to obtain a wire rod.

When the heating temperature is lower than the lower limit of the heating temperature in the furnace heating furnace and the wire rod furnace, sufficient austenitization is difficult. If the upper limit is exceeded, the austenite grains may become coarse and the productivity may be reduced due to scale loss There is a problem. This tendency can be similarly applied to the lower limit and the upper limit of each holding time.

The wire material produced by the above-described manufacturing method has a Ca composite inclusion having an area of 700 mu m ² or more in an area up to 1/4 * D (the wire diameter measured in D: mm unit) from the wire rope center is 200 pieces / g or less , The wire rod is drawn and rolled under normal conditions in a fresh wire to produce a machining line, and then the final product obtained by welding between the machining lines is welded at the welding heat affected part, .

The above drawing, rolling and welding conditions are not particularly limited because they can be performed under ordinary conditions.

For example, in a drawing condition, a working line can be prepared by rolling a 16 mm wire rod at a drawing speed of 2 m / s or less at a drawing speed of 60%, and then rolling it to a thickness of 5 mm. In addition, the welding is performed by butt welding between the working lines by the electric resistance welding method, and the welding conditions may be a current density of 50 to 150 A / mm ² and a heating power of 300 to 500 kg / cm ² .

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

JIS-SWRS35B carbon steel, which is currently used as a representative armor cable having the composition shown in Table 1 below, was used. However, P and S are impurities. In both of the examples and comparative examples, P satisfies 0.010 wt% or less and S is 0.010 wt% or less and is not separately shown in Table 1.

The steel was subjected to secondary refinement under ordinary conditions after 300 tons of leaching, and the ladle was degassed by bubbling with a strong bubbling at a level of 3-5. After desulfurization, vacuum degassing treatment was performed by the RH method. In order to prevent clogging of the tundish nozzle, Ca-Si wire was added to control Ca content as shown in Table 1 below.

After the Ca-Si wire was inserted and stayed for the RH residence time shown in the following Table 1, the billet was rolled by 160 (mm) x 160 (mm) rolling of the bloom, and the billet was heated to 1050 캜 for 100 minutes , And then rolled at 1000 DEG C to produce 16 mm wire rods.

The ESAA (electrolysis in AA solution under ultrasonic wave), which is a method of extracting and analyzing inclusions by collecting test pieces in a region from the center of the produced wire to 1/4 * D (D: wire diameter measured in mm) Ca-Mg-O, or Al-Ca-Mg-O, or Al-Ca-Mg-Si-O The composite inclusions having an area of 700 mu m < ^{2 >} or more were classified according to area and are shown in Table 1 below. Since the fine inclusions do not affect the weldability, the number of Ca-based composite inclusions having an area of 700 탆 ² or more was measured.

The wire was rolled up to 10 mm thickness and then rolled to thickness 5 mm to manufacture the machined wire. The welded wire was butt welded using electric resistance welding method to evaluate the weldability. The welding conditions were a current density of 100 A / mm ² and a heating force of 400 kg / cm ² .

The weldability was evaluated by applying a strain rate of 20 m / m at a test specimen length of 500 mm including the welded part. The break origins were judged to be the welds or weld heat affected zones at the occurrence of fractures.

The hardness of the welded portion, the heat affected portion and the working line (base material) is shown in Table 2 below.

JIS-SWRS32B is a carbon steel composed of ferrite and pearlite. When drawn and rolled, the drawn ferrite and pearlite appear and their hardness is measured at 302 to 310 Hv as measured by Vickers hardness as shown in Table 2 below. In the case of welded joints, it is cooled after melting, so the structure appears to be tempered martensite. Its hardness is 315 ~ 325 Hv, which is harder than base metal. The welding heat affected zone is a mixed structure of tempered martensite, ferrite and pearlite, and has a hardness of 278 to 290 Hv, which is lower than that of the base material. Therefore, cracks are generated in the heat affected zone . However, if Ca composite inclusions are present in the light welded joint, cracks are formed in the welded joint when the tensile is applied.

As shown in the following Table 2, it can be confirmed that the microstructures of the comparative example and the inventive example are similar to each other due to the similar hardness of each of the welded part, the heat affected part and the base material. This is because the number of Ca complex inclusions does not greatly affect the mechanical properties such as hardness when the microstructure is similar. However, when the tensile load is applied, it may affect the elongation depending on the occurrence of cracks, and may affect the hardness fatigue and HIC characteristics.

division weight (%) RH
Retention time (minutes) Ca complex inclusions per area condition (number / g) C Mn Si Ca 700 to 1900 탆 ² 1900 to 3800 탆 ² 3800 탆 ²
Excess Total number Comparative Example 1 0.34 0.8 0.25 0.0003 15 152 64 10 226 Inventory 1 0.35 0.7 0.24 0.0002 20 116 52 2 170 Inventory 2 0.34 0.8 0.24 0.0003 25 121 41 0 162 Inventory 3 0.33 0.9 0.24 0.0004 30 113 29 0 142 Honorable 4 0.36 0.8 0.25 0.0003 40 95 15 0 110 Comparative Example 2 0.35 0.8 0.25 0.0011 15 194 73 15 282 Comparative Example 3 0.33 0.9 0.26 0.0010 20 181 68 7 256 Inventory 5 0.33 0.8 0.24 0.0012 25 142 42 3 187 Inventory 6 0.34 0.7 0.24 0.0010 30 105 29 One 135 Honorable 7 0.35 0.7 0.26 0.0011 40 76 18 0 94 Comparative Example 4 0.33 0.8 0.26 0.0018 15 179 83 18 280 Comparative Example 5 0.34 0.8 0.25 0.0019 20 166 78 10 254 Honors 8 0.34 0.9 0.25 0.0020 25 127 52 6 185 Proposition 9 0.35 0.8 0.26 0.0017 30 90 39 4 133 Inventory 10 0.35 0.8 0.26 0.0018 40 61 28 3 92

division Weldability evaluation Hardness of weld (Hv) Heat Affected Zone Hardness (Hv) Base material hardness (Hv) Breaking point Comparative Example 1 320 278 302 Weld Inventory 1 318 282 302 Heat affected part Inventory 2 325 285 310 Heat affected part Inventory 3 315 284 308 Heat affected part Honorable 4 320 286 305 Heat affected part Comparative Example 2 319 288 304 Weld Comparative Example 3 316 290 306 Weld Inventory 5 322 284 302 Heat affected part Inventory 6 315 285 300 Heat affected part Honorable 7 318 289 308 Heat affected part Comparative Example 4 320 284 302 Weld Comparative Example 5 317 285 304 Weld Honors 8 322 286 305 Heat affected part Proposition 9 318 290 308 Heat affected part Inventory 10 325 288 304 Heat affected part

In Comparative Example 1 and Inventive Examples 1 to 4, when the Ca content is less than 0.001 wt%, the change in the number of Ca complex inclusions with increase in RH retention time can be known.

In the case of Comparative Example 1, the RH retention time was 15 minutes, and as shown in Fig. 1, the number of Ca complex inclusions having an area of 700 탆 ² or more was 226 pieces / g and fracture occurred at the welded portion under application of tensile load. On the other hand, in Examples 1 to 3 in which the RH retention time was 20 minutes or longer, the number of Ca complex inclusions having an area of 700 탆 ² or more was 200 pieces / g or less, and fracture occurred in the heat affected zone (HAZ) under application of tensile load.

FIG. 2 is a photograph of a crack in a welded portion generated when a tensile load is applied in Comparative Example 1, and FIG. 3 is a photograph in the longitudinal direction (1 / 2t) after the final rupture of Inventive Example 1 and Comparative Example 1. In Comparative Example 1, cracks occurred in the welds generated at the time of tensile load application, and final fracture occurred at the welds.

In Comparative Examples 2 to 5 and Comparative Examples 5 to 10 in which the Ca content is not less than 0.001 wt%, it is determined whether the fracture origin is the welded portion or the welded heat affected portion, depending on whether or not the Ca composite inclusion having the area of 700 탆 ² or more is 200 pieces / .

When the Ca content is 0.001% by weight or more, it is found that the RH retention time is 25 minutes or more, which is advantageous in securing the wire material having the Ca composite inclusion having the area of 700 탆 ² or more at 200 g / g or less.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (6)

And the balance Fe and unavoidable impurities are contained in an amount of 0.32 to 0.72% of C, 0.2 to 0.3% of Si, 0.6 to 0.9% of Mn, 0.0001 to 0.002% of Ca, 0.01% Including,
A wire composite excellent in weldability in which the number of Ca composite inclusions having an area of 700 탆 ² or more is 200 or less / g or less in an area up to 1/4 * D (D: wire rod diameter measured in mm) from the wire rod center.
The method according to claim 1,
Wherein the microstructure of the wire comprises at least 70% by area of ferrite and the remaining corundum ferrite.
The method according to claim 1,
Wherein the Ca composite inclusions include at least one of Al ₂ O ₃ -CaS, AlCaMgO, AlCaSiO, AlCaMgO and AlCaMgSiO.
After the molten steel was subjected to a vacuum degassing treatment, a Ca-Si wire was added in an amount of 1 kg or less per ton of molten steel, and the residence time until charging by tundish was 20 minutes or more,
And the balance Fe and unavoidable impurities are contained in an amount of 0.32 to 0.72% of C, 0.2 to 0.3% of Si, 0.6 to 0.9% of Mn, 0.0001 to 0.002% of Ca, 0.01% Preparing molten steel containing the molten steel;
Producing a billet with the molten steel and rolling to obtain a billet; And
And heating and rolling the billet to obtain a wire rod.
5. The method of claim 4,
Wherein the retention time is 25 minutes or more when the Ca content is 0.001 to 0.002 wt%.
The method according to claim 4 or 5,
And the upper limit of the residence time is 40 minutes or less.

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