JP6250750B2 - High strength wire rod excellent in corrosion resistance and method for producing the same - Google Patents

Description

The present invention relates to a high-strength wire excellent in corrosion resistance and a method for producing the same, and more specifically,
Corrosion resistance that requires hydrogen-induced cracking resistance, corrosion resistance, etc. in addition to high strength as a reinforcing material to support the load on flexible pipes that transport crude oil at sea, such as armor cable for deep-sea crude oil transportation (ARMOR CABLE) The present invention relates to a high-strength wire excellent in the above and a method for producing the same.

Armor Cable for Deep Sea Crude Oil Transportation (ARMOR CABLE) is a reinforcing material that supports the load on a flexible pipe that transports crude oil at sea. Products that require hydrogen-induced crack resistance and corrosion resistance in addition to high strength Known as.
Conventionally, the steel type applied to the armor cable is a general hard steel product having a carbon content of 0.3 to 0.8%, and the other remaining component system is Si of 0.2 to 0.00. 3%, Mn is 0.3 to 0.6%, and P and S are 0.015 and 0.012% or less, which are normal levels.

The process of producing armor cables, etc., generally uses wires produced in various sizes such as 10 to 25 mm, and is subjected to isothermal transformation through lead bath heat treatment on the customer side to produce fine proeutectoid ferrite and pearlite, or After securing fine pearlite, wire drawing is performed to reduce the size, and then rolling according to the application is performed to produce the final product. If the lead bath heat treatment and the wire drawing step can be omitted, productivity can be improved and a great economic effect can be expected.
In addition, because the oil well collection environment has moved to the deep sea due to the depletion of the continental shelf energy, the carbon content of the steel grade applied to the armor cable varies from hypo-eutectoid steel to eutectic steel. Has changed.

That is, when a wire made of eutectic steel is used, the tensile strength is 1400 MPa, which is 400 MPa higher than hypoeutectoid steel. In addition, because the strength is high and the length is increased by reducing the thickness of the final product, it is possible to collect oil wells even in deeper seas. However, since H ₂ S exists in the oil well, the resistance to hydrogen must also be high, but the pearlite fraction increases as the carbon content increases. This is known as a structure with poor hydrogen resistance, which limits the use of steel with a high carbon content. Corrosion resistance is also an important factor, but there is a problem that the corrosion resistance decreases as the carbon content increases because the carbon sensitivity increases and the corrosion sensitivity increases.
Therefore, it can be applied to armor cables for transporting deep-sea crude oil (ARMOR CABLE), etc., and development of a high strength wire rod with excellent corrosion resistance and a manufacturing method thereof that can omit the lead bath heat treatment and wire drawing process on the customer side. Is required.

Japanese Patent Laid-Open No. 08-170149

The object of the present invention is to provide hydrogen-induced cracking in addition to high strength as a reinforcing material for supporting a load on a flexible pipe for transporting crude oil on the sea such as armor cable for transporting deep sea crude oil (ARMOR CABLE). To provide a high-strength wire rod excellent in corrosion resistance that requires resistance, corrosion resistance, and the like, and a method for producing the same.

In the present invention, by weight, C: 0.04 to 0.25%, Si: 0.07 to 0.6%, Mn: 5.0 to 9.0%, P: 0.030% or less, S : 0.030% or less, balance Fe and inevitable impurities ,
The fine structure is characterized by containing 95% or more of acicular martensite .

Moreover, this invention is weight%, C: 0.04-0.25%, Si: 0.07-0.6%, Mn: 5.0-9.0%, P: 0.030% or less , S: maintaining a billet composed of 0.030% or less, the balance Fe and unavoidable impurities at Ae ₃ + 150 ° C. to Ae ₃ + 250 ° C. for 120 minutes or more,
Rolling the billet at Ae ₃ + 100 ° C. or higher, final rolling (RSM) entry side temperature at Ae ₃ + 100 ° C. to Ae ₃ + 150 ° C., and finally rolling to obtain a wire;
Winding the wire at Ae ₃ + 50 ° C .;
Cooling the wound wire 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. viewing including the door,
The microstructure is characterized by containing 95% or more of acicular martensite .

According to the present invention, a lead wire heat treatment and a wire drawing step are omitted on the customer side, and a high strength wire rod excellent in corrosion resistance while securing excellent tensile strength and a method for manufacturing the same are provided even when only sheet rolling is performed. There is an effect that can.

When Mn content is 6 weight%, it is a graph which shows the phase fraction of FCC formed by temperature, BCC, and a carbide | carbonized_material. 4 is a photograph of the microstructure of Invention Example 1. 6 is a photograph of the microstructure of Comparative Example 6 taken.

Hereinafter, preferred embodiments of the present invention will be described.
The high-strength wire rod having excellent corrosion resistance according to the present invention is, by weight, C: 0.04 to 0.25%, Si: 0.07 to 0.6%, Mn: 5.0 to 9.0%, P : 0.030% or less, S: 0.030% or less, remaining Fe and inevitable impurities are included. Hereinafter, the unit of each alloy element is% by weight.

C (carbon): 0.04 to 0.25%
C is an element added to ensure the strength of the material, and it penetrates in the C-axis direction of martensite formed when quenching in the austenite phase, and induces lattice strain to give high strength. do.
In order to secure acicular martensite having a low carbon content and sufficient flexibility or toughness, the C content is preferably 0.25% or less. When the C content is more than 0.25%, a hard acicular martensite or a mixed structure of acicular martensite and plate martensite is generated. Can induce delamination. On the other hand, when the C content is less than 0.04%, there is a problem that it is difficult to ensure high strength.
Therefore, the C content is preferably 0.04 to 0.25%.

Si (silicon): 0.07 to 0.6%
Si easily dissolves in ferrite and plays a role of suppressing the formation of carbides. If Si is added, an effect of increasing the strength occurs. Generally, it is known that if 0.1% Si is added, the strength of 14 to 16 MPa is improved.
When the Si content is less than 0.07%, the above-described effects are insufficient, and when the Si content exceeds 0.6%, the strength increasing effect tends to slow down. Therefore, the Si content is preferably 0.07 to 0.6%.

Mn (manganese): 5.0 to 9.0%
Mn is used as a substitutional solid solution in the fine structure and plays a role of increasing the strength. Mn is added to ensure hardenability. When Mn is added within the range controlled by the present invention, sufficient hardenability can be ensured, and acicular martensite can be formed only by air-cooling in a stealmore cooling zone.
When the Mn content is less than 5.0%, there is a problem that it is difficult to ensure high strength. When the Mn content exceeds 9.0%, Mn segregation occurs too much and breaks in the length direction during rolling. There is a problem that delamination occurs. 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. However, as with conventional steel wires, each is made 0.030% or less from the viewpoint of ensuring flexibility. It is preferable.
The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed from the raw materials or the surrounding environment, and thus cannot be excluded .

Moreover, it is preferable that the fine structure of the wire according to the present invention includes 95% by area or more of acicular martensite.
This is because acicular martensite is high in strength and excellent in toughness, and is less susceptible to cracking even when deformed. In addition, acicular martensite can be formed of an alloy system having a low carbon content, and if the carbon content is low, it is excellent in terms of corrosion resistance. More preferably, it is 98 area% or more.
In this case, the wire includes (Fe, Mn) ₂₃ C ₆ cementite, the size thereof is 230 nm or less in the length direction, 10 nm or less in the width direction, and the interval between the cementites is 205 nm or less. It may be. (Fe, Mn) ₂₃ C ₆ cementite is a factor formed during cooling, and the strength can be improved as the spacing between cementite is finer.

The tensile strength of the wire according to the present invention is 1400 MPa or more. Moreover, when plate-rolling with the total reduction / exemption amount of 40 to 60%, an excellent tensile strength of 1600 MPa or more can be secured, and the stretching ratio can be secured to 10% or more.
Further, when immersed in a 5% H ₂ SO ₄ solution for 10 hours, the maximum depth of the formed corrosion pits is 24 μm or less, and the corrosion resistance is excellent.

Method for producing excellent in corrosion resistance high strength wire rod <br/> hereinafter be described in detail the method of producing a high strength wire rod excellent in corrosion resistance of the present invention.
The method for producing a high-strength wire rod excellent in corrosion resistance according to the present invention includes a step 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, and the billet at Ae ₃ + 100 ° C. or more. rolled, and obtaining a final rolled to wire the final rolling (RSM) entry side temperature of _{Ae 3 + 100 ℃ ~Ae 3 +} 150 ℃, the steps of winding the wire in _Ae 3 + 50 ° C., was taken the winding Cooling the wire 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 stage A billet having the above-described alloy composition was maintained at a temperature range of Ae ₃ + 150 ° C to Ae ₃ + 250 ° C for 120 minutes or more in a wire heating furnace and formed at grain boundaries (Fe, Mn ) ₂₃ C ₆ cementite is melted.
The temperature range is a range in which the austenite crystal grains are not coarsened while maintaining the austenite single phase, and is an effective temperature range for removing remaining carbides. When Ae ₃ + 250 ° C. is exceeded, the austenite crystal grains become very coarse, and the final microstructure formed after cooling tends to become coarse. Therefore, it is preferable to control the temperature below this value, and less than Ae ₃ + 150 ° C. In this case, since it is difficult to maintain the austenite single phase, it is preferable to heat at Ae ₃ + 150 ° C. to Ae ₃ + 250 ° C. When the heating time is less than 120 minutes, the remaining carbides may not be sufficiently dissolved, so it is preferable to maintain it longer than that. The upper limit of the heating time is not particularly limited. However, if maintained for a long time, the productivity is remarkably reduced, so that the maintenance time can be limited to 180 minutes or less.

Billet rolling and winding stage The billet extracted in the heating furnace is rolled at Ae ₃ + 100 ° C or higher (rough rolling, intermediate rolling and finish rolling), and the final rolling (RSM) inlet side temperature is Ae ₃ + 100 ° C. ~ Ae ₃ + 150 ° C is finally rolled to obtain a wire, and then wound at Ae ₃ + 50 ° C.
When the rolling temperature is less than Ae ₃ + 100 ° C., a fine structure due to deformation may be generated during rolling, and carbides may be precipitated at the grain boundaries. To control the final rolling (RSM) entry side temperature _{Ae 3 + 100 ℃ ~Ae 3 +} 150 ℃ are hereinafter minimizes the material deviation in controlling the cooling to the winding temperature _Ae 3 + 50 ° C. in a water-cooled base Because.
Cooling step The wound wire is cooled to a cooling end temperature of 650 to 750C at a cooling rate of 10C / s or more, and then cooled to 150 to 250C at a cooling rate of 1C / s or less. Cool to end temperature. The cooling can be performed in a Stealmore cooling zone.

FIG. 1 shows FCC, BCC and (Fe, Mn) ₂₃ C ₆ cementite formed at each temperature interval when the Mn content is 6%. Up to 700 ° C., it is maintained in the FCC single phase, and during the subsequent cooling, transformation to the BCC structure is performed, and it can be confirmed that cementite is formed from 500 ° C. and is composed of the final martensite. Therefore, after winding in the Stealmore cooling zone, it is rapidly cooled to a cooling end temperature of 650 to 750 ° C. at a cooling rate of 10 ° C./s or more as a 11 mm wire standard, and 150 to 250 ° C. at a cooling rate of 1 ° C./s or less. Then, the desired acicular martensite is formed in the present invention.

The lower limit of the cooling rate during the slow cooling is not particularly limited, but if the cooling rate is too slow, the productivity may decrease as actual operation becomes difficult. Since the softness may decrease rapidly, a cooling rate of 0.5 ° C./s may be set as the lower limit.
At this time, it is preferable that the wire manufactured by the manufacturing method as described above has a thickness of 8 to 10 mm.
The wire rod according to the present invention is preferable for an armor cable or the like that is a final product having a thickness of 3 to 6 mm because it can omit only the LP (Lead patenting) heat treatment and the wire drawing process on the customer side and can carry out rolling only. In order to apply, it is preferable to control the thickness to 8 to 10 mm.

Step of rolling the wire rod On the other hand, the step of rolling (sheet rolling) the wire rod manufactured by the above-described manufacturing method with a total reduction amount of 40 to 60% in order to secure the final product on the customer side. Further can be done. This is to ensure the thickness and strength of the final product.
In the present invention, in order to ensure the target strength, it is preferable to apply a total rolling reduction of 40 to 60%. At this time, the secured thickness may be about 3 to 6 mm. If the total cross-section reduction rate exceeds 60%, defects may be formed in martensite, and breakage may occur during rolling. The total reduction amount can be calculated by the following (Equation 1).
[Equation 1]
Total cross-section reduction rate (%) = 100 × (1-cross-sectional area of final product / cross-sectional area of wire rod)
The wire rod according to the present invention can produce a final product by omitting the heat treatment and wire drawing steps and performing only rolling.

Generally, in the customer, the heat treatment process is performed to ensure a fine pearlite structure, and the wire drawing process is performed to reduce the size and improve the strength. Since it is added in a large amount and the microstructure is hard martensite, the heat treatment and wire drawing steps can be omitted. Therefore, after forming acicular martensite with a wire, a product is formed by applying only plate rolling, and at this time, a tensile strength of 1600 MPa or more and a total draw ratio of 10% or more can be ensured.
On the other hand, a rolling speed shall be 100-300 m / min. This is because when the rolling speed is less than 100 m / min, the productivity can be lowered, and when it exceeds 300 m / min, hot cracks may occur.

Hereinafter, the present invention will be described in more detail through examples.
[Example]
The billet having the composition shown in Table 1 below is maintained at a heating furnace temperature of 1150 ° C. for 125 minutes, the rolling temperature is 1050 ° C., the final rolling entry temperature is 1000 ° C., and the winding temperature is 910 ° C. did. The tensile strength (TS) and microstructure of the wire were observed and are shown in Table 2 below.
The remaining comparative examples and invention examples except for comparative example 6 were manufactured so that the thickness of the wire was 11 mm, and after winding, after cooling to 700 ° C. at a cooling rate of 10 ° C./s, 200 ° C. Was cooled with air at a cooling rate of 0.8 ° C./s. Thereafter, only sheet rolling was performed at a total cross-section reduction rate of 50% without performing heat treatment and wire drawing to obtain a sheet rolled material. The tensile strength, stretch ratio, and maximum corrosion pit depth of the sheet rolled material were measured and shown in Table 2 below.

In Comparative Example 6, a wire rod currently in regular sale was manufactured to a thickness of 16 mm, and after winding, it was cooled at a cooling rate of 5 ° C./s. Thereafter, heat treatment, wire drawing and plate rolling were performed to obtain a final rolled sheet. The tensile strength after heat treatment and wire drawing, the tensile strength of the rolled sheet material, the stretch ratio, and the depth of the maximum corrosion pit were measured and are shown in Table 2 below.
The corrosion resistance was evaluated by comparative analysis by measuring the maximum depth of the formed corrosion pits after dipping in a 5% H ₂ SO ₄ solution for 10 hours.

上記表１において、Ｃ、Ｓｉ及びＭｎの単位は重量％であり、Ｐ及びＳの単位は重量ｐｐｍである。

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

上記表２において、引張強度（ＴＳ）の単位はＭＰａであり、（Ｆｅ，Ｍｎ）_２３Ｃ_６の長さ、幅及び間隔の単位はｎｍである。

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

In Invention Examples 1 to 5, it can be seen that the final sheet rolled material has a tensile strength of 1600 MPa or more, a stretching ratio of 10% or more, a maximum corrosion pit depth of 24 μm or less, and excellent corrosion resistance. Moreover, when FIG. 2 which is the photograph which image | photographed the fine structure of the example 1 of an invention is seen, it can confirm that the acicular martensite was formed.
In the case of Comparative Example 6, as shown in Table 2 above, the tensile strength of the wire currently being sold on a regular basis, as shown in Table 2 above, was 980 MPa, 1070 MPa during LP heat treatment, and 1510 MPa during wire drawing. The tensile strength after final plate rolling was 1610 MPa, and the stretch ratio was 11%. In Comparative Example 6, since the carbon content is high and the microstructure is pearlite, it is found that the LP heat treatment and the wire drawing process must be performed, and the maximum corrosion pit depth is 37 μm. It can be confirmed that the corrosion resistance is poor. Moreover, when FIG. 3 which is the photograph which image | photographed the fine structure of the comparative example 6 is seen, it can confirm that pearlite was formed.

Comparative Examples 1 and 2 and Invention Examples 1 and 2 show changes due to the Mn content.
In Comparative Example 1, 4% of Mn was added and acicular martensite was formed. However, since the Mn content was low, the tensile strength of the sheet rolled material was inferior at 1400 MPa.
Invention Examples 1 and 2 are cases where the Mn content is 6 and 8%, respectively, the wire strength is high and the strength secured at the time of final plate rolling is 1605 and 1720 MPa, respectively, which is similar to or higher than existing steel materials. The stretching ratio is also excellent at 10% or more.
In Comparative Example 2, the Mn content was 10% and the wire strength was high, but breakage occurred during rolling.
Comparative Examples 3 and 5 and Invention Examples 4 and 5 show changes depending on the C content.
In Comparative Example 3, the acicular martensite structure was formed when the C content was 0.02%, but the strength was low in the final product.

Inventive Examples 4 and 5 were cases where the C contents were 0.1% and 0.2%, respectively, and the strengths secured at the time of final plate rolling were 1640 MPa and 1700 MPa, respectively, and excellent tensile strength could be secured. .
In Comparative Example 5, when the C content was 0.3%, a mixed structure of acicular martensite and plate martensite was formed, and fracture occurred during rolling.
Moreover, although the magnitude | size of (Fe, Mn) ₂₃ C ₆ cementite increases as the C content increases, it can be confirmed that the inter-grain spacing of cementite tends to decrease.

As mentioned above, although preferred embodiment regarding this invention was described, this invention is not limited to the said embodiment, All the changes in the range which does not deviate from the technical field to which this invention belongs are included.

Claims (8)

% By weight, C: 0.04 to 0.25%, Si: 0.07 to 0.6%, Mn: 5.0 to 9.0%, P: 0.030% or less, S: 0.030 % Or less, balance Fe and inevitable impurities ,
A high-strength wire excellent in corrosion resistance, characterized in that the microstructure contains acicular martensite of 95 area% or more .   The wire includes (Fe, Mn) 23C6 cementite, the size thereof is 230 nm or less in the length direction, 10 nm or less in the width direction, and the interval between cementites is 205 nm or less. The high-strength wire excellent in corrosion resistance according to claim 1. The tensile strength of the said wire is 1400 Mpa or more, The high-strength wire excellent in corrosion resistance of Claim 1 characterized by the above-mentioned. 2. The high strength excellent in corrosion resistance according to claim 1, wherein when the wire is immersed in a solution of 5% H 2 SO 4 for 10 hours, the maximum depth of the formed corrosion pits is 24 μm or less. wire. % By weight, C: 0.04 to 0.25%, Si: 0.07 to 0.6%, Mn: 5.0 to 9.0%, P: 0.030% or less, S: 0.030 %, Maintaining the billet composed of Fe and unavoidable impurities at Ae ₃ + 150 ° C. to Ae ₃ + 250 ° C. for 120 minutes or more,
Rolling the billet at Ae ₃ + 100 ° C. or higher, final rolling (RSM) entry side temperature at Ae ₃ + 100 ° C. to Ae ₃ + 150 ° C., and finally rolling to obtain a wire;
Winding the wire at Ae ₃ + 50 ° C .;
Cooling the wound wire 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. viewing including the door,
A method for producing a high-strength wire rod excellent in corrosion resistance, characterized in that the fine structure contains 95% by area or more of acicular martensite . The method for producing a high-strength wire excellent in corrosion resistance according to claim 5 , wherein the wire has a thickness of 8 to 10 mm. The method for producing a high-strength wire with excellent corrosion resistance according to claim 5 , further comprising rolling the cooled wire at a total cross-section reduction rate of 40 to 60%. The method for producing a high-strength wire with excellent corrosion resistance according to claim 7 , wherein a rolling speed is 100 to 300 m / min in the stage of rolling the plate.

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