JPH03281725A - Production of high strength steel wire for use in sour environment - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing high-strength steel wire with a tensile strength of 50 kgf/+nm2 or more, and more specifically, to a method for manufacturing a high-strength steel wire with a tensile strength of 50 kgf/+nm2 or more, and more specifically, a method for manufacturing a high-strength steel wire with a tensile strength of 50 kgf/+nm2 or more, and more specifically, a method for manufacturing a high-strength steel wire with a tensile strength of 50 kgf/+nm2 or more. The present invention relates to a method for manufacturing a high-strength steel wire with a good shape.

[Conventional technology]

Conventionally, for example, armored wire for flexible pipes for transporting high-pressure fluids such as gas and crude oil has been produced by drawing low-carbon steel wire with a carbon content of 0.2% or less, and then processing it into irregular shapes, such as irregular drawing, roller die processing, and rolling. The steel wires were made into deformed steel wires (flat pressure wires or groove wires) with a predetermined cross-sectional shape, and then used as they were or after being annealed at a low temperature of less than 500° C. for use in a non-sour environment.

However, with the recent trend toward deeper wells, the environment surrounding oil wells has changed, and the environment for transporting crude oil and slag has become harsher. In other words, there are more and more environments in which hydrogen sulfide is present.

For this reason, among the properties required for deformed steel wires, stability against hydrogen penetrating into the steel wire from the usage environment, namely hydrogen-induced cracking (hereinafter referred to as IC) and sulfide stress corrosion cracking (hereinafter referred to as IC), are required. In particular, it has become necessary to avoid the occurrence of SSC (SSC). By the way, HIC is a crack that occurs when hydrogen enters a steel wire under no load, while SSC is a crack that occurs when hydrogen enters a steel wire under a high load. .

In response to such trends, the present inventors have developed a method for reducing 0.0.
We have developed a method of patenting high carbon steel wire containing 40 to 0.70% C, cold working with a reduction in area of 25 to 75%, and then annealing it to spheroidize at 500 to 700°C. (Hereinafter referred to as conventional method 1).

The steel wire manufactured according to Conventional Method I has excellent HrC resistance properties. However, in actual usage environments, steel wires are subjected to strong tensile stress, so it is important that they have excellent SSC resistance as well as HIC resistance. Therefore, the inventors of the present invention have conducted many experiments with the aim of further improving the HIC resistance characteristics, and have already filed a patent application for the results (hereinafter referred to as conventional method 2). Conventional method 2 is
The SSC resistance was improved by applying a tensile strain of 0.3 to 5% to the steel wire manufactured by the conventional method. A common feature of these two conventional methods is that although HIC resistance and SSC resistance are excellent, dimensional accuracy and shape are not necessarily sufficient. Since the deformed wires constituting the flexible pipe are used in a mutually interlocked state, variations in the dimensions and shapes of the deformed wires pose a serious hindrance to the molding and usability of the flexible pipe. That is, in the steel wire manufactured by Conventional Method 1, winding may interfere with forming due to bending and twisting that occur during annealing.

In addition, although the steel wire manufactured by conventional method 2 has good straightness, it tends to cause looseness between the deformed wires due to non-uniformity of tensile strain. This is a problem because it reduces the fatigue properties of the pipe.

That is, the deformed wire for sour environments is required to have high dimensional accuracy and correct shape in addition to strength and sour resistance characteristics.

[Problem to be solved by the invention]

An object of the present invention is to provide a method for manufacturing a high-strength steel wire for sour environments that has good HIC resistance and SSC resistance, and is also superior to conventional methods in terms of dimensional accuracy and shape.

[Means to solve the problem]

In the present invention, C: 0°40~0.70%, Si: 0.1~0.
1%, Mn: 0.20-1%, P: 0.025
% or less, S: contains 0.010% or more, and optionally contains Al: 0.008% to 0.050%,
After cold working the steel, the remainder of which is Fe and unavoidable impurities, with a reduction in area of 25 to 75%,
This is a method for manufacturing a high-strength steel wire for use in a sour environment, which is characterized by performing spheroidizing annealing at 0° C. and then drawing at a reduction in area of 5 to 15%.

[For production]

The present invention will be explained in detail below.

If C is less than 0.40%, the desired strength cannot be obtained after spheroidizing annealing. In addition, if CO exceeds 70%, it will be difficult to perform strong cold working, and fine cracks will occur in the center of the steel wire during processing, resulting in f (0.7
The upper limit was 0%.

Si is required as a deoxidizing agent in an amount of at least 0.10%.

As the amount increases, the strength improves. However, 1
%, decarburization becomes severe and this causes frequent cracking of the steel wire during cold working.

0.2% or more of Mn is required to prevent hot brittleness. Furthermore, since Mn improves hardenability, a large amount of Mn is desirable in order to create a uniform pearlite texture through patenting, but if it exceeds 1%, the formation of IIIC due to center segregation may occur. 1% due to higher frequency
Let be the 1-corner.

Next, (2) tends to segregate at grain boundaries, reducing workability. Therefore, the smaller the amount, the better. However, when manufacturing by continuous casting, the melting temperature is increased, which causes the recurrence (1) to occur, so only the upper limit was set at 0.025%.

In addition to the same disadvantages as p, S is preferable as it is less in terms of Fa4 corrosivity, but o and ot can be produced economically at present.

The upper limit was %. Note that S is 0. Industrial production is fully possible up to o%.

A1 is used as a deoxidizing agent and a crystal grain refining element as required. In the case of Ae addition 6, the lower limit of the amount of Al required for grain refinement is 0.008%. On the other hand, if Al exceeds 0.050%, the number of nonmetallic inclusions increases, resulting in a decrease in yield due to surface defects.

- In addition to each of the two elements, it is effective to add 0.6% or less of C if the hardenability is insufficient due to the thick wall thickness of the deformed steel wire. .3% or less Co and 0.02% or less W have the effect of suppressing hydrogen intrusion into the steel, so if these are added as necessary, the HIC resistance can be further improved. can.

A wire rod having the above composition is processed into a steel wire.

In the present invention, the steel wire is a general term for wire rods that have a circular or irregular cross-sectional shape (rectangular or grooved) by processing such as irregular drawing, roller die processing, or rolling. In addition, the tensile strength after spheroidizing annealing is 50 to 80 kgf.
/++n+" is called high-strength steel wire. In other words, unless the tensile strength is 50 kg [711 m2 or more, it cannot withstand internal and external pressure and is not effective as an armored wire. On the other hand, If it exceeds 80kgf/mm2, the hardness will be H.
The upper limit was set to 80 kgf/mm2 because the RC is 22 or more and SSC occurs under low stress.

Next, the processing method according to the present invention will be explained.

Usually, wire rods are heat treated before processing, but in the present invention, a patenting treatment is performed. As a result, the structure of the wire becomes a uniform fine pearlite structure, and the cross-section reduction rate is 25.
Provides performance that can withstand up to 75% processing.

The reason why the area reduction rate is limited to a range of 25 to 75% in the present invention is that if the area reduction rate is less than 25%, the cementite will not be sufficiently spheroidized during annealing after processing, HIC will occur, and the area reduction rate will decrease. If it exceeds 75%, for example, cracks will occur on the end face and inside of the flat tension wire due to processing, and internal cracks in particular will cause HI.
This is to induce C. Note that the cross-sectional reduction rate of the present invention is defined by the following formula.

Area reduction rate (%') = (1--"-') X 1'
OO8゜S: Cross-sectional area of shaped steel wire So = Cross-sectional area of strand (wire rod) In the present invention, after cold working with a cross-section reduction rate of 25 to 75%, spheroidizing annealing is performed to remove processing strain. At the same time, the pearlite structure is changed to a structure in which fine spheroidized cementite is dispersed in ferrite (matrix). That is,
The spheroidized cementite structure obtained by annealing has significantly better HIC properties than the conventional layered pearlite structure. Hydrogen atoms that penetrate into the steel accumulate at the cementite/ferrite interface and form HIC nuclei there.
In the case of spheroidized cementite, stress concentration is small, so
It is considered to have excellent HIC resistance.

In order to find the appropriate spheroidizing annealing temperature range, a lead patented wire rod with a diameter of 9.5 mm (components are shown in Table 1) was drawn and rolled to make it a flat wire.
Spheroidizing annealing was performed. Spheroidizing annealing was performed under the conditions of heating for 2 hours and keeping the temperature for 4 hours.

The results are shown in Figure 1. A tensile strength of 50 kgf/nm2 or more can be obtained at 690° C. or lower for a steel wire containing 42% CO, and at 700° C. or lower for a steel wire containing 65% CO. On the other hand, the spheroidizing annealing temperature that suppresses the tensile strength to 80 kgf/w2 or less is 500°C or higher for CO, 42%, CO, 6
At 5%, a temperature of 540°C or higher is required. Therefore, the spheroidizing annealing temperature range of the present invention is 500 to 700°C. However, considering the range of ingredients and variations in furnace temperature, etc., 550 to 680°C is industrially optimal.

Table 1: In order to improve the SSC resistance and improve the dimensional accuracy and formability, the present invention further improves the spheroidized annealed steel wire produced by the following two methods with an area reduction rate of 5 to 15. % drawing processing is applied. The cause of SSC is thought to be that hydrogen that has entered the steel material from the sour environment diffuses into the micro-yield region created by the applied stress and aggregates there, accelerating the yielding phenomenon and generating micro-cracks. Therefore, in order to improve the SSC resistance characteristics, it is important to prevent local minute breakdown phenomena before macroscopic breakdown phenomena occur. That is, it is effective to increase the yield strength of the steel material, and the present inventor has found that it is effective to apply plastic strain to the steel wire after spheroidizing annealing for that purpose.

On the other hand, with regard to the dimensional accuracy and shape of steel wire, drawing using a die or Turks head is the best. Therefore, by drawing the steel wire after spheroidizing annealing, favorable results such as not only improvement in size and shape but also improvement in SSC resistance properties can be obtained. If the cross-sectional area reduction rate during drawing is less than 5%, the effect of improving the size and shape will be insufficient, whereas if it exceeds 15%, HIC will easily occur.

Therefore, the area reduction rate is set to 5 to 15%.

〔Example〕

A wire rod with a diameter of 9.5 rnn made into a fine pearlite structure by lead patenting was made into a steel wire with a diameter of 5 M by wire drawing, and then flat-rolled into a flat wire with a thickness of 0.9 to 2.85 M [7]. After this was annealed to form a spheroid, drawing was performed using a Turk's head.

The I-i 1C characteristics were obtained by cutting the above-mentioned flat wire into lengths of 100 squares, 5% NaCj7-0.5% CH3CO0H-H
, S saturated solution at 25° C. for 96 hours, polished at three locations, and evaluated by observing with an optical microscope the presence or absence of microcracks.

The SSC characteristics are determined by using the above-mentioned flat wire as a test piece, grasping both ends to apply a tensile stress of 80 to 110% of the actual yield strength, and placing the central 200 mm of the test piece in a sour environment, that is, as described above. It was immersed in a solution with the same composition as the HIC test, and the temperature of the solution was 25°C.
A 20-hour load test was conducted, and the maximum stress at which no breakage occurred, that is, the lower limit stress for SSC generation, was measured and evaluated.

Table 2 shows the manufacturing conditions such as the chemical composition of the steel wire used, cold working, annealing temperature, and cross-section reduction rate during drawing, as well as the mechanical properties, sour resistance characteristics, and variations in product width.

No. 1-4, NO, 1O and 1], No. 18-
21. Nos. 24 to 27 are comparisons between the method of the present invention and the conventional method. After producing spheroidized annealed steel wires in the same manufacturing process, the method of the present invention achieved a cross-section reduction rate of 7.4 to 12.2%. A drawing process was performed. On the other hand, in Conventional Method 1, the spheroidizing annealing is still performed, and in Conventional Method 2, a stretching process of 0.8 to 4.4% is applied. All steel wires manufactured using the conventional method have a tensile strength of 60 kg f/rnm 2
Although there is no occurrence of HIC, the lower limit stress for SSC occurrence is less than 55 kgf/mm', which is lower than that produced by the method of the present invention. The steel wire manufactured according to Conventional Method 2 has almost the same tensile strength and HIC resistance properties as those manufactured according to Conventional Method 1, but the lower limit stress for SSC occurrence is 58
~60kgf/ff1m7, which is improved compared to the conventional method. However, the dimensional accuracy of the product is significantly inferior to the steel wires manufactured according to Conventional Method 1 and the method of the present invention. On the other hand, the steel wire manufactured by the method of the present invention has HI
There is no occurrence of C, and the lower limit stress for SSC occurrence is 60 kgf/I.
Im' or higher, which is higher than those produced by conventional methods, and the dimensional accuracy of the product is superior to that which could not be achieved by any conventional method.

Na 5 to 9, No. 13 to 17 were obtained by examining the influence of the area reduction rate during drawing on the characteristics of the deformed wire. By performing drawing processing with an appropriate area reduction rate,
No HIC occurs, SSC generation minimum stress is 60kgf
It was possible to manufacture a deformed wire for use in a sour environment with a high resistance value of /n+o'' or more and excellent dimensional and shape properties.

〔Effect of the invention〕

As explained in 42 below, according to the method of the present invention, the HI
It is possible to produce a high-strength steel wire for sour environments that has good C properties and SSC resistance properties, and has significantly improved dimensional viscosity and shape properties compared to conventional ones.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between annealing temperature and tensile strength.

Claims (1)

[Claims] (1) C: 0.40-0.70%, Si: 0.10-1
%, Mn: 0.20-1%, P: 0.025% or less, S
:0.010% or less, and if necessary, Al:0.010% or less.
A steel containing 008 to 0.050%, with the remainder consisting of Fe and unavoidable impurities, is subjected to cold working with an area reduction rate of 25 to 75%, and then spheroidizing annealed at 500 to 700 °C,
A method for producing a high-strength steel wire for use in a sour environment, which comprises thereafter performing drawing processing at a cross-section reduction rate of 5 to 15%.