JPH042720A - Production of high strength steel wire for use in sour environment - Google Patents

Abstract

PURPOSE:To produce a high strength steel wire for use in sour environment excellent in SSC resistance by subjecting a steel having a specific composition consisting of C, Si, Mn, P, S, and Fe to specific cold working and spheroidizing annealing and then applying specific tensile strain to the above. CONSTITUTION:A steel which has a composition consisting of 0.40-0.70% C, 0.10-1% Si, 0.20-1% Mn, <=0.025% P, <=0.010% S, and the balance Fe with inevitable impurities and further containing, if necessary, 0.008-0.050% Al is formed into a wire rod. This wire rod is formed into uniform pearlite structure by means of patenting treatment and then cold-worked at 25-75% reduction of area. The resulting steel wire is subjected to spheroidizing annealing at 500-620 deg.C so as to be formed into spheroidal cementite structure. Subsequently, 0.2-2% tensile strain is applied to this steel wire by means of heat stretching at 250-400 deg.C. By this method, the high strength steel wire for use in sour environment having about 70-80kgf/mm<2> tensile strength and excellent in HIC resistance and SSC resistance can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing high-strength steel wire having a tensile strength of 70 kgf/- or more.
The present invention relates to a method for manufacturing high-strength steel wire used in a wet hydrogen sulfide environment.

[Conventional technology]

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

However, with the recent development of deeper wells, the environment surrounding oil wells has changed, and the environment for transporting crude oil and gas has become harsher.

In other words, sour environments with hydrogen sulfide are becoming more common. Therefore, among the properties required for deformed steel wires, stability against hydrogen penetrating into the steel wires from the usage environment, that is, hydrogen induced cracking.
d Cracking (hereinafter referred to as HIC) and Sulfide Stress Corrosion Cracking (hereinafter referred to as HIC) and Sulfide Stress Corrosion Cracking (hereinafter referred to as HIC)
error cracking, hereinafter referred to as SSC
There is now a special requirement that no such occurrences occur. Incidentally, HIC is a crack that occurs when hydrogen enters a steel wire under no load, while SSC
This is a crack that occurs when hydrogen enters the steel wire under high load.

In response to this trend, the present inventors have developed a tensile strength of 50
Two technologies are proposed for steel wires of kgf/- or more. One of them is as already disclosed in Japanese Patent Application Laid-open No. 1-279710 as "Production method of high strength steel wire with excellent hydrogen-induced cracking resistance" (hereinafter referred to as conventional method 1). This is a method of patenting a high carbon steel wire containing ~0.70% C, cold working with a reduction in area of 25% to 75%, and then annealing it to form a spheroid at 500 to 700°C.

The other one is ``Method for manufacturing high-strength steel wire for sour environments.''
(hereinafter referred to as conventional method 2), a patent application was filed on March 30, 1990. After applying a tensile strain of 0.2 to 2% to the steel wire manufactured by conventional method 1, 250
This is a method of blueing at ~400°C.

The steel wire manufactured according to Conventional Method 1 has excellent HIC resistance. However, in actual use environments, steel wires are subjected to strong tensile stress, so it is important that they have excellent SSC resistance as well as HIC resistance. From this point of view, Conventional Method 2 improves the SSC resistance characteristics of Conventional Method 1. However, in order to enable steel wire to be used more stably under higher loads and in sour environments, it is necessary to increase the strength of the steel wire and make it resistant to S.
It is necessary to further improve the SC characteristics.

[Problem to be solved by the invention]

An object of the present invention is to provide a method for producing a high-strength steel wire for use in sour environments, which has superior strength and SSC resistance properties compared to those produced by conventional methods.

[Means to solve the problem]

The gist of the present invention is as follows.

(1) C: 0.40-0.70%, Si: 0.
10-1%.

Mn: 0.20-1%, P: 0.025% or less, s: 0
．． A steel containing 0.010% or less and 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 620 'C,
After that, in the temperature range of 250-400"C, 0.2~
A method for producing a high-strength steel wire for use in a sour environment, characterized by imparting a tensile strain of 2%.

(2) C: 0.40~0.70%, Si: 0.10~
1%.

Mn: 0.20-1%, P: 0.025% or less, s20
．． 010% or less, AI: 0.008-0.050
%, with the remainder consisting of Fe and unavoidable impurities. After cold working with a reduction in area of 25 to 75%, it is annealed to form a spheroid at 500 to 620°C, and then
A method for producing a high-strength steel wire for use in a sour environment, characterized by imparting a tensile strain of 0.2 to 2% in a temperature range of 400°C.

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 C exceeds 0.70%, strong cold working becomes difficult, and fine cracks occur in the center of the steel wire during processing, deteriorating the HIC properties.
The upper limit was %.

St is required as a deoxidizing agent in a minimum amount of 0.10% or more. As the amount increases, the strength improves.

However, if it exceeds 1%, decarburization becomes severe and this causes frequent cracking of the steel wire during cold working, which is not preferable.

Mn is required to be 0.20% or more in order to prevent hot embrittlement. Additionally, since Mn improves hardenability, a large amount of Mn is desirable in order to obtain a uniform pearlite structure through patenting, but if it exceeds 1%, the frequency of HIC caused by center segregation increases. The upper limit is 1%.

Next, since P tends to segregate at grain boundaries, it reduces workability. Therefore, the smaller the amount, the better. However, in the case of manufacturing by continuous casting, the melting temperature is increased, which causes re-P to occur, so only the upper limit was set at 0.025%.

S has the same disadvantages as P, and from the viewpoint of corrosion resistance, it is preferable to have less S, but 0.010% S can be economically produced at present.
was set as the upper limit. Note that industrial production of S up to 0.001% is possible.

AI is optionally used as a deoxidizing agent and crystal refining element. In the case of adding AI, the lower limit of the amount of AI required for grain refinement is o, o o s%. On the other hand, AI is 0,0
When it exceeds 50%, the amount of nonmetallic inclusions increases, resulting in a decrease in yield due to surface defects.

In addition to the above-mentioned elements, if the hardenability of the deformed steel wire is insufficient due to the thick wall thickness, it is effective to add 0.6% or less of Cr. Furthermore, 0.3% or less of Cu and 0.02% or less of W have the effect of suppressing hydrogen intrusion into the steel, so adding these as necessary will further improve HIC resistance. be able to.

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

The steel wire of the present invention 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, here, the tensile strength after spheroidizing annealing is 70 to 80 kg.
f/- is called a high-strength copper wire. That is, unless the tensile strength is 70 kgf/- or more, it will not be able to withstand the internal and external pressures required in harsh usage environments, and will not be effective as an armored wire. On the other hand, if the tensile strength exceeds 80 kgf/-, SSC in a mode that connects spheroidized cementite is likely to occur, so the upper limit was set to 80 kgf/-.

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

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

The reason why the area reduction rate is limited to a range of 25 to 75% in the present invention will be explained.

When the area reduction rate is less than 25%, cementite becomes insufficiently spheroidized during annealing after processing, and HIC occurs. Moreover, if the area reduction rate exceeds 75%, cracks will occur on the end face and inside of the flat tension wire due to processing, and in particular, internal cracks will induce HIC, which is not preferable. Note that the cross-sectional reduction rate of the present invention is defined by the following formula.

S: Cross-sectional area of the deformed steel wire S0: Cross-sectional area of the strand (wire rod) The first main feature of the present invention is the cross-sectional area reduction rate of 25-75%.
After cold working, spheroidizing annealing is performed to remove processing strain and change the pearlite structure to a structure in which fine spheroidized cementite is dispersed in ferrite (matrix).

That is, it has been newly discovered that the spheroidized cementite structure obtained by annealing has significantly superior HIC properties compared to the conventional layered pearlite structure. Hydrogen atoms that penetrate into steel accumulate at the cementite/ferrite interface and form HIC nuclei there, but in the case of spheroidized cementite, the stress concentration is small, so HIC resistance is low.
It is considered to have excellent characteristics.

In order to find an appropriate spheroidizing annealing temperature range, a lead patented wire rod with a diameter of 9.5 ann was drawn and flat rolled into a flat wire, and then 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 70 kgf/- or more can be obtained with C00
42% steel wire (・marked: C0.42%, Si0.22%.

MnO, 75%, Po, 015%, 30.003%
, Al: o, o os%, area reduction rate 67%) is below 550°C, and steel wire with C0.65% (Δ mark: C0,
65%, Si0.24%, MnO, 71%, Po, 01
5%, So, 005%, Al: 025%, area reduction rate 58%), the temperature is 620°C or less. On the other hand, the spheroidizing annealing temperature that suppresses the tensile strength to 80 kgf/- or less is C
A temperature of 500°C or higher is required for a 0.42% steel wire, and a temperature of 520°C or higher is required for a 0.65% CO steel wire. Therefore, the spheroidizing annealing temperature range of the present invention is 500 to 620°C.

The second main feature of the present invention is that in order to improve the SSC resistance, the spheroidized annealed steel wire produced by the above method is further subjected to 0.2 to 2% tensile strength in a temperature range of 250 to 400°C. It is to apply strain.

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 caused by the applied stress and aggregates there, accelerating the yielding phenomenon and producing micro-cranks. Therefore, in order to improve the SSC resistance characteristics, it is important to prevent the local minute breakdown phenomenon before the macroscopic breakdown phenomenon begins.

In other words, increasing the yield strength of the steel material is effective.

However, for boat fishing, the tensile strength is increased by increasing the yield strength. As mentioned above, an increase in tensile strength is not desirable because it increases the risk of SSC occurrence.

The present inventor has researched a processing method that increases the yield strength but has less effect on the tensile strength and other mechanical properties, and as a result, the steel wire after spheroidizing annealing is heated to the blue brittle temperature. By applying a slight tensile strain at high temperature (hereinafter referred to as heat stretching), the target high yield strength can be obtained, and as a result, the SSC resistance properties are significantly improved. Ta.

The tensile strain in heat stretching is 0.2
%, the SSC improvement effect is insufficient.

As the tensile strain increases, the yield strength increases, and the lower limit stress for SSC generation also increases accordingly. However, if it exceeds 2%, HIC will occur and the lower limit stress for SSC generation will also tend to decrease, so the upper limit is set at 2%.

Regarding the heat stretching temperature, if it is lower than 250°C, HIC occurs, and the lower limit stress for SSC generation is also low.

On the other hand, if the temperature exceeds 400″C, the yield strength decreases, so
The SSC resistance properties are significantly reduced. For the above reasons, the heat stretching temperature is set at 250 to 400°C.

The heat stretching device must be capable of continuously processing a long coiled steel wire while heating it. In this sense, by increasing the rotation speed of the pulley on the side that winds the copper wire a little faster than the rotation speed of the pulley on the side that supplies the steel wire, a certain strain is applied to the copper wire.
It is desirable to have a device equipped with a mechanism that can simultaneously heat the tensioned steel wire between both pulleys using an induction heating method or an electrical heating method.

Example A wire rod with a diameter of 9.5 m made into a fine pearlite structure by lead patenting was drawn to a diameter of 5Ila.
fi steel wire, and then flat-rolled to a thickness of 0.9 to 2.
．． The wire was made of 85mmn0 flat pressure wire. After this was annealed to form a spheroid, a tensile strain of 0.2 to 2% was applied to the steel wire while induction heating it using a heat stretching device equipped with the mechanism described above.

HIC characteristics were evaluated using the following method. The above-mentioned flat pressure wire was cut to a length of 100 m++, and 5% NaCJ -0,
After being immersed in a 5% CH3CO0HHis saturated solution at 25° C. for 96 hours, it was polished at three locations and observed with an optical microscope for the presence or absence of microcracks.

The SSC characteristics were evaluated by the following method. The above-mentioned flat wire was used as a test piece as it was, and both ends were grasped to apply a tensile stress of 80 to 110% of the actual yield strength. The center 200mm of the test piece was placed in a sour environment, that is, the HI described above.
It was immersed in a solution with the same composition as in Test C. The temperature of the solution is 2
The temperature was set at 5°C. A load test was conducted for 720 hours under such conditions, and the maximum stress at which no breakage occurred, ie, the lower limit stress for SSC generation, was measured.

Table 1 shows the chemical composition of the steel wire used, manufacturing conditions such as cold working, annealing temperature, and heat stretching conditions, as well as the mechanical properties and sour resistance properties of the product.

NcL1-4, No, 10 and N(111, No, 1
8 to 21 and Nα24 to 27 are the results of a comparison between the method of the present invention and two conventional methods.After producing annealed steel wire in the same manufacturing process, the method of the present invention shows 0.
Heat stretching was performed to impart a tensile strain of 8 to 2.0%. All of the steel wires manufactured by the method of the present invention have a tensile strength of 75 kgf/- or more, and have a higher yield strength than steel wires manufactured by the conventional method. Furthermore, the lower limit stress for SSC generation is 70 kgf/- or more, which is a high level that could not be achieved by any of the conventional methods.

Nos. 5 to 9 show the effects of tensile strain during heat stretching, and Nos. 13 to 17 show the effects of temperature during heat stretching on the characteristics of the profiled wire. By selecting the heat stretching conditions within the range defined by the present invention, it is possible to produce a deformed wire that does not generate HIC and has excellent sour resistance characteristics with a lower limit stress for SSC generation of 70 kgf/- or more.

〔Effect of the invention〕

As explained above, according to the present invention, 70kgf/
- Has a tensile strength of more than
It is possible to produce a high-strength steel wire for sour environments with significantly improved C characteristics.

[Brief explanation of drawings]

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

Claims (2)

[Claims] (1) C: 0.40-0.70%, Si: 0.10-1
%, Mn: 0.20-1%, P: 0.025% or less, S
: A steel containing 0.010% or less and the remainder consisting of Fe and unavoidable impurities is subjected to cold working with a cross-section reduction rate of 25 to 75%, and then spheroidizing annealed at 500 to 620°C,
A method for producing a high-strength steel wire for use in a sour environment, the method comprising then applying a tensile strain of 0.2 to 2% at a temperature range of 250 to 400°C. (2) C: 0.40-0.70%, Si: 0.10-1
%, Mn: 0.20-1%, P: 0.025% or less, S
: 0.010% or less, Al: 0.008 to 0.050%
After cold working steel with a cross-section reduction rate of 25 to 75%, with the remainder consisting of Fe and unavoidable impurities,
Spheroidizing annealing at 500-620℃, then 250-4
A method for producing a high-strength steel wire for use in a sour environment, characterized by imparting a tensile strain of 0.2 to 2% in a temperature range of 0.000C.