KR101674835B1 - High strength wire rod having excellent corrosion resistance and method for manufacturing thereof - Google Patents

Abstract

According to one aspect of the present invention, the present invention relates to a high-strength wire rod having excellent corrosion resistance, which comprises 0.04-0.25 wt% of C, 0.07-0.6 wt% of Si, 5.0-9.0 wt% of Mn, an amount of less than or equal to 0.030 wt% of P, an amount of less than or equal to 0.030 wt% of S, and the remainder consisting of Fe and inevitable impurities.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high strength wire rod having excellent corrosion resistance,

The present invention relates to a high-strength wire rod excellent in corrosion resistance and a manufacturing method thereof.

ARMOR CABLE for deep-sea crude oil transport is a reinforcing material that supports loads applied to flexible pipes that transport crude oil at sea, and it is known as a product that requires resistance to hydrogen organic cracking and corrosion as well as high strength .

Conventional armor cable is a general steel product with a carbon content of 0.3 to 0.8%. The remaining components are composed of 0.2 to 0.3% of Si and 0.3 to 0.6% of Mn, and P and S are 0.015 And 0.012% or less.

Armor cables, etc., are produced by using wire rods produced in various sizes such as 10 ~ 25mm in general, and they are thermally transformed through heat treatment at the customer's premises to secure fine pearlite composed of fine pro cornerstone ferrite and pearlite, Drawing is performed to reduce the size, and rolling is applied according to the application to produce the final product. If the heat treatment and drawing process are omitted, the productivity can be improved and the economic effect is expected to be great.

In addition, due to the depletion of energy in the continental shelf, the oil sampling environment is shifting to the deep sea, so the carbon content of steel species applied to armor cables is changing from hypo-eutectoid steels to eutectoid steels.

In other words, when using wire made of vacant steel, the tensile strength is about 1400 MPa, which is 400 MPa higher than that of porcelain steel. As the strength is high, the length of the product is increased due to the decrease of the final product thickness. However, due to the presence of H ₂ S in the wells, the resistance to hydrogen must be high. As the carbon content increases, the pearlite fraction increases, which is known to be a poor hydrogen resistance texture. Can be limited. Also, corrosion resistance is an important factor. Since the corrosion sensitivity increases with an increase in the carbon content, there is a problem that the corrosion resistance is deteriorated when the carbon content is increased.

Accordingly, there is a demand for development of a high-strength wire having excellent corrosion resistance while omitting the heat treatment and drawing process of the lead in the customer, which can be suitably applied to armor cables for deep sea oil transportation, and a manufacturing method thereof.

Japanese Patent Application Laid-Open No. 63-317626

One aspect of the present invention is to provide a high strength wire having excellent corrosion resistance, which can be preferably applied to an armor cable for deep sea oil transportation, 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 ferritic stainless steel comprising 0.04 to 0.25% of C, 0.07 to 0.6% of Si, 5.0 to 9.0% of Mn, not more than 0.030% of P, not more than 0.030% of S, To a high strength wire having excellent corrosion resistance.

Another aspect of the present invention is a ferritic stainless steel comprising 0.04 to 0.25% of C, 0.07 to 0.6% of Si, 5.0 to 9.0% of Mn, not more than 0.030% of P, not more than 0.030% of S, Holding the impregnated billet at Ae ₃ + 150 ° C to Ae ₃ + 250 ° C for at least 120 minutes;

Obtaining a wire and rolling the billet in more than Ae ₃ + 100 ℃, to the final rolling to the final rolling (RSM) inlet temperature to the _{Ae 3 + 100 ℃ ~ Ae 3} + 150 ℃;

Winding the wire rod at Ae ₃ + 50 ° C; And

Cooling the rolled wire rod to a cooling end temperature of 650 to 750 ° C at a cooling rate of 10 ° C / s or more, and then cooling to a cooling end temperature of 150 to 250 ° C at a cooling rate of 1 ° C / s or less To a method of manufacturing a high strength wire having excellent corrosion resistance.

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, it is possible to provide a high-strength wire rod excellent in corrosion resistance and a method of manufacturing the same, which can secure an excellent tensile strength even when only the sheet rolling is performed by omitting the heat treatment and drawing process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the percentages of FCC, BCC, and carbide formed per temperature when the Mn content is 6 wt%.
2 is a photograph of microstructure of Inventive Example 1. FIG.
3 is a photograph of the microstructure of Comparative Example 6. 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.

According to one aspect of the present invention, there is provided a high strength wire rod excellent in corrosion resistance, comprising 0.04 to 0.25% of C, 0.07 to 0.6% of Si, 5.0 to 9.0% of Mn, 0.030% or less of P, The balance of Fe and unavoidable impurities. Hereinafter, the unit of each alloy element is% by weight.

C (carbon): 0.04 to 0.25%

C is an element added to secure the strength of a material, and penetrates into the C axis direction of martensite formed at the time of casting on austenite to induce lattice warping and to have a high strength.

In order to ensure acicular martensite having a low carbon content and sufficient ductility or toughness, the C content is preferably 0.25% or less. When the C content is more than 0.25%, a structure in which mild acicular martensite or acicular martensite and plate martensite are mixed occurs, so that when the wire rod is rolled, breakage may occur or delamination may occur. On the other hand, when the C content is less than 0.04%, it is difficult to secure high strength.

Therefore, the C content is preferably 0.04 to 0.25%.

Si (silicon): 0.07 to 0.6%

Si is easily dissolved in the ferrite and suppresses the formation of carbide, and the effect of increasing the strength when Si is added occurs. It is generally known that the strength of 14 ~ 16 MPa is improved when 0.1% Si is added.

When the Si content is less than 0.07%, the above-mentioned effect is not sufficient, and when the Si content exceeds 0.6%, the strength increasing effect tends to be slowed down. Therefore, the Si content is preferably 0.07 to 0.6%.

Mn (manganese): 5.0 to 9.0%

Mn is used by being used as a solid solution in a microstructure and serves to increase the strength. Further, Mn is added in order to secure the entrapment property. In the case where Mn is added in the range controlled by the present invention, it is possible to secure sufficient entrapment properties, and bed-type martensite can be formed only by air cooling in the stelmo cooling bed.

When the Mn content is less than 5.0%, it is difficult to ensure high strength. When the Mn content exceeds 9.0%, Mn segregation occurs severely and delamination occurs in the longitudinal direction during rolling. Therefore, the Mn content is preferably 5.0 to 9.0%.

P and S: 0.030% or less, respectively

P and S are impurities, and the content is not particularly specified, but it is preferable that the content is 0.030% or less from the viewpoint of ensuring 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.

The microstructure of the wire according to the present invention preferably contains 95% by area or more of acicular martensite.

Since the martensite has high strength and excellent toughness, cracking sensitivity is low even if the martensite is deformed. The acicular martensite can be formed in an alloy system having a low carbon content. When the carbon content is low, the acicular martensite is excellent in terms of corrosion resistance, and therefore, it is preferable that the acicular martensite contains 95% by area or more of acicular martensite. And more preferably 98% by area or more.

In this case, the wire is (Fe, Mn) ₂₃ includes a space between C ₆ and cementite, and its size is 230nm or less in the longitudinal direction, and in the transverse direction less than 10nm, cementite can be equal to or less than 205nm. (Fe, Mn) ₂₃ C ₆ cementite can improve the strength as an argument, the tight spacing between the cementite microstructure more formed during cooling.

The tensile strength of the wire according to the present invention may be 1400 MPa or more. In addition, when rolled at a total reduction of 40 to 60%, an excellent tensile strength of 1600 MPa or more can be secured and an elongation of 10% or more can be secured.

Further, when the substrate is immersed in a 5% solution of H ₂ SO ₄ for 10 hours, the maximum depth of the corrosion pits formed is 24 μm or less, which is excellent in corrosion resistance.

High strength with excellent corrosion resistance Wire rod  Manufacturing method

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

A method for manufacturing a high strength wire having excellent corrosion resistance, which is another aspect of the present invention, includes the steps of: maintaining a billet having the above-described alloy composition at Ae ₃ + 150 ° C to Ae ₃ + 250 ° C for 120 minutes or more;

Rolling the billet at a temperature higher than Ae ₃ +100 ° C and finally rolling the final rolling (RSM) inlet temperature to Ae ₃ + 100 ° C to Ae ₃ + 150 ° C to obtain a wire rod;

Winding the wire rod at Ae ₃ + 50 ° C; And

Cooling the wound wire rod to a cooling end temperature of 650 to 750 ° C at a cooling rate of 10 ° C / s or more, and then cooling to a cooling end temperature of 150 to 250 ° C at a cooling rate of 1 ° C / s or less .

Billet  Heating step

The (Fe, Mn) ₂₃ C ₆ cementite formed in the grain boundary is melted by holding the billet having the above-mentioned alloy composition in the wire rod furnace at a temperature range of Ae ₃ + 150 ° C to Ae ₃ + 250 ° C for 120 minutes or longer.

The temperature range is a range in which the austenite grains are not coarsened while maintaining the austenite single phase, and is a temperature range effective for removing remaining carbide. If it exceeds A _e3 + 250 ℃ has therefore increase the coarsening tendency of the final microstructure that is as a very coarse austenite grains formed after cooling is preferably controlled to less, Ae ₃ When the temperature is lower than + 150 ° C, it is difficult to maintain the austenite single phase. Therefore, it is preferable to heat at A _e3 + 150 ° C to A _e3 + 250 ° C. If the heating time is less than 120 minutes, the residual carbide may not be sufficiently dissolved, so it is preferable to keep the temperature higher than the above. The upper limit of the heating time is not particularly limited, but the productivity is remarkably reduced upon prolonged maintenance, so that the holding time can be limited to 180 minutes or less.

Billet  Rolling and Coiling  step

The billet extracted from the heating furnace was rolled (rough rolling, intermediate rolling and rough rolling) at Ae ₃ +100 ° C. or higher and the inlet temperature of RSM (Reducing & Sizing Mill) was set to Ae ₃ + 100 ° C. to Ae ₃ + 150 ° C, and finally rolled at a temperature of Ae ₃ + 50 ° C.

When the rolling temperature is lower than Ae ₃ + 100 ° C, microstructure due to deformation may occur during rolling, and carbides may be precipitated at the grain boundaries, so it is preferable to control the rolling more. Controlling the final rolling (RSM) inlet temperature from Ae ₃ + 100 ° C to Ae ₃ + 150 ° C is then carried out in order to minimize material variation when cooled to a coiling temperature Ae ₃ + 50 ° C.

Cooling step

The wound wire rod is cooled to a cooling end temperature of 650 to 750 ° C at a cooling rate of 10 ° C / s or more and then cooled to a cooling end temperature of 150 to 250 ° C at a cooling rate of 1 ° C / s or less. The cooling can be performed in a stellum cooling rack.

Fig. 1 shows FCC, BCC and (Fe, Mn) 23C6 cementite formed at each temperature range when the Mn content is 6%. It is maintained in the FCC single phase up to 700 ° C, then transformed into BCC structure after cooling, and cermetite is formed from 500 ° C to form final martensite. Therefore, after being wound on a Stemore cooling rack, the steel sheet is quenched to a cooling end temperature of 650 to 750 ° C at a cooling rate of 10 ° C / s or more on the basis of an 11 mm wire rod, cooled at a cooling rate of 1 to 250 ° C Followed by slow cooling to the end temperature to form the desired needle-shaped martensite in the present invention.

The lower limit of the cooling rate during the slow cooling is not particularly limited. However, if the cooling rate is too slow, the productivity may be lowered to such an extent that practical operation is difficult, and the ductility may be drastically lowered due to formation of intergranular carbides by slow cooling. ℃ / s Can be set to the lower limit.

At this time, it is preferable that the wire material produced according to the above-described manufacturing method has a thickness of 8 to 10 mm.

Since the wire material according to the present invention can be rolled only by omitting the LP (Lead patenting) heat treatment and drawing process, the thickness of the wire is 8 to 10 mm in order to be suitably applied to the final product such as armor cable having a thickness of 3 to 6 mm .

Wire rod Rolled plate  step

On the other hand, a step of rolling (plate rolling) the wire rods manufactured by the above-described manufacturing method at a total reduction amount of 40 to 60% in order to secure the final product at the customer can be additionally performed. So as to secure the thickness and strength of the final product.

In order to secure a desired strength in the present invention, it is preferable to apply a total reduction amount of 40 to 60%. At this time, the thickness to be secured may be about 3 to 6 mm. If the total reduction amount exceeds 60%, defects in the martensite may be formed and breakage may occur during rolling. The total amount of reduction can be calculated by the following equation (1).

[Relation 1]

Total reduction amount (%) = 100 × (1 - cross sectional area of final product / cross sectional area of wire rod)

The wire product according to the present invention can be manufactured by omitting the heat treatment and the drawing process and by rolling only.

Generally, the heat treatment process performed by the customer is to secure a fine pearlite structure, and the drawing process is for size reduction and strength enhancement. In the wire material according to the present invention, Mn is added in a large amount and martensite Therefore, the heat treatment and the drawing process can be omitted. Therefore, it is possible to obtain a product having a tensile strength of 1600 MPa or more and a total elongation of 10% or more by forming rolled martensite in a wire rod and then rolling by plate rolling only.

On the other hand, the rolling speed may be 100 to 300 m / min. If the rolling speed is less than 100 m / min, the productivity may be lowered. If the rolling speed is more than 300 m / min, there is a possibility of hot cracking.

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 )

The billets having the composition shown in the following Table 1 were maintained at a heating temperature of 1150 ° C for 125 minutes, a rolling temperature of 1050 ° C, a final rolling inlet temperature of 1000 ° C, and a coiling temperature of 910 ° C. The tensile strength (TS) and microstructure of the wire rod were observed and are shown in Table 2 below.

Comparative Example 6 and Comparative Example 6 were produced so that the thickness of the wire rod was 11 mm. After the coiling, the steel sheet was cooled at a cooling rate of 10 캜 / s until 700 캜 and then cooled at a cooling rate of 0.8 캜 / Followed by air cooling. Thereafter, only the plate rolling was carried out at a total reduction amount of 50% without heat treatment and drawing processing to obtain a pressure-sensitive laminated sheet. The tensile strength, the elongation and the maximum corrosion pit depth of the pressure-sensitive laminate were measured and are shown in Table 2 below.

Comparative Example 6 was manufactured so as to have a thickness of 16 mm as a commercially available wire rod and cooled at a cooling rate of 5 캜 / s after winding. After that, heat treatment, drawing process and plate rolling were carried out to obtain a final pressure-pressure serial product. The tensile strength after heat treatment and after drawing, tensile strength of elongated plate, elongation and maximum corrosion pit depth were measured and are shown in Table 2 below.

Corrosion resistance was measured by measuring the maximum depth of corrosion pit after immersion in 5% H ₂ SO ₄ solution for 10 hours.

division C Si Mn P S Comparative Example 1 0.05 0.50 4.00 100 90 Inventory 1 0.05 0.50 6.00 120 95 Inventory 2 0.05 0.50 8.00 110 98 Comparative Example 2 0.05 0.50 10.00 90 80 Comparative Example 3 0.02 0.50 6.00 100 95 Comparative Example 4 0.05 0.80 6.00 110 98 Inventory 3 0.05 0.25 6.00 90 80 Honorable 4 0.10 0.50 6.00 98 85 Inventory 5 0.20 0.50 6.00 100 95 Comparative Example 5 0.30 0.50 6.00 105 92 Comparative Example 6 0.72 0.25 0.75 100 80

In Table 1, the units of C, Si and Mn are% by weight, and the units of P and S are ppm by weight.

division Wire rod
TS Microstructure of wire rods Heat treatment material
TS Fresh material
TS Pressure pressure series Perla
It Marten
site (Fe, Mn) ₂₃ C ₆ TS Elongation
(%) Maximum corrosion pit
Depth (㎛) Length width interval Comparative Example 1 1270 X couch 183 7 207 skip skip 1400 15 22 Inventory 1 1470 X couch 189 7 205 skip skip 1605 13 20 Inventory 2 1590 X couch 198 8 203 skip skip 1720 12 17 Comparative Example 2 1650 X couch 199 9 206 skip skip Fracture Fracture X Comparative Example 3 1400 X couch 125 6 213 skip skip 1530 13 19 Comparative Example 4 1495 X couch 188 8 205 skip skip Fracture Fracture X Inventory 3 1460 X couch 180 7 204 skip skip 1600 11 20 Honorable 4 1520 X couch 205 9 189 skip skip 1640 12 21 Inventory 5 1590 X couch 230 10 178 skip skip 1700 10 24 Comparative Example 5 1650 X couch
+ Plate 260 12 169 skip skip Fracture Fracture 30 Comparative Example 6 980 O X X X X 1070 1510 1610 11 37

In the above Table 2, the unit of tensile strength (TS) is MPa, and the unit of length, width and interval of (Fe, Mn) ₂₃ C ₆ is nm.

Examples 1 to 5 show that the tensile strength of the last pressure-pressure softened material is 1,600 MPa or more, the elongation is 10% or more, and the maximum corrosion pit depth is 24 μm or less, which is excellent in corrosion resistance. 2, which is a photograph of the microstructure of Inventive Example 1, shows that needle-shaped martensite is formed.

In the case of Comparative Example 6, the tensile strength of the 16 mm wire rod was 980 MPa as shown in Table 2, and 1070 MPa when LP heat treatment and 1510 MPa when drawing wire. After final rolling, the tensile strength was 1,610 MPa and the elongation was 11%. In Comparative Example 6, since the carbon content is high and the microstructure is pearlite, it is understood that the LP heat treatment and drawing process must be performed, and that the maximum corrosion pit depth is 37 mu m, have. 3, which is a photograph of the microstructure of Comparative Example 6, shows that pearlite is formed.

Comparative Examples 1 and 2 and Inventive Examples 1 and 2 show changes depending on the Mn content.

In Comparative Example 1, when 4% of Mn was added, acicular martensite was formed, but because the Mn content was low, the tensile strength of the pressure-sensitive laminate was reduced to 1400 MPa.

In Examples 1 and 2, the Mn content was 6% and 8%, respectively, and the strength of the wire rod was high and the strengths secured at the final plate rolling were 1605 and 1720 MPa, respectively. %.

In Comparative Example 2, when the Mn content was 10%, the wire rod strength was high, but rupture occurred during rolling.

Comparative Examples 3 and 5 and Inventive Examples 4 and 5 show changes depending on the C content.

In Comparative Example 3, when the C content was 0.02%, an acicular martensite structure was formed, but the strength was low in the final product.

In Examples 4 and 5, when the C contents were 0.1% and 0.2%, respectively, excellent tensile strengths were obtained at 1640 MPa and 1700 MPa, respectively, in the final plate rolling.

In Comparative Example 5, when the C content was 0.3%, a mixed structure of acicular martensite and platy martensite was formed and fracture occurred during rolling.

In addition, the size of (Fe, Mn) ₂₃ C ₆ cementite increases with increasing C content, but it can be seen that the interval between crystal grains of cementite decreases.

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 (9)

The steel sheet contains 0.04 to 0.25% of C, 0.07 to 0.6% of Si, 5.0 to 9.0% of Mn, 0.030% or less of P, 0.030% or less of S and the balance of Fe and unavoidable impurities.
The microstructure includes a high-strength wire rod having excellent corrosion resistance including needle-like martensite of 95% by area or more.
delete The method according to claim 1,
The wire material is (Fe, Mn) ₂₃ C ₆ comprises a cementite, and its size is 230nm or less in the longitudinal direction, and the width direction 10nm or less, cementite spacing is excellent in high-strength wire corrosion resistance, characterized in that not more than 205nm .
The method according to claim 1,
Wherein the wire has a tensile strength of 1400 MPa or more.
The method according to claim 1,
Wherein a maximum depth of corrosion pits formed when the wire rod is immersed in a 5% H ₂ SO ₄ solution for 10 hours is 24 μm or less.
By weight%, C: 0.04 ~ 0.25% , Si: 0.07 ~ 0.6%, Mn: 5.0 ~ 9.0%, P: 0.030% or less, S: a billet containing 0.030% or less, and balance of Fe and unavoidable impurities Ae ₃ + Maintaining at 150 캜 to Ae ₃ + 250 캜 for 120 minutes or more;
Rolling the billet at a temperature higher than Ae ₃ +100 ° C and finally rolling the final rolling (RSM) inlet temperature to Ae ₃ + 100 ° C to Ae ₃ + 150 ° C to obtain a wire rod;
Winding the wire rod at Ae ₃ + 50 ° C; And
Cooling the rolled wire rod to a cooling end temperature of 650 to 750 ° C at a cooling rate of 10 ° C / s or more, and then cooling to a cooling end temperature of 150 to 250 ° C at a cooling rate of 1 ° C / s or less A method of manufacturing a high strength wire having excellent corrosion resistance.
The method according to claim 6,
Wherein the wire rod has a thickness of 8 to 10 mm.
The method according to claim 6,
Further comprising the step of subjecting the cooled wire material to a rolling reduction to a total reduction amount of 40 to 60%.
9. The method of claim 8,
Wherein the rolling speed in the plate rolling step is 100 to 300 m / min.

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