JP2840977B2 - Manufacturing method of high strength steel wire for sour environment - Google Patents

Description

The present invention relates to a method for producing a high-strength steel wire having a tensile strength of 70 kgf / mm ² or more, and more specifically, to a sour environment (Sour envir
onments, wet hydrogen sulfide environment).

[Conventional technology]

Conventionally, for example, for the armored wire of flexible pipes for high-pressure fluid transport of gas, crude oil, etc., after drawing a low-carbon steel wire of C 0.2% or less, the wire is deformed by deforming such as drawing, roller dies, and rolling. A steel wire (flat pressure wire or grooved wire) having a cross-sectional shape of, was used as it is, or after being subjected to low-temperature annealing at a temperature of less than 500 ° C, in a non-sour environment.

However, with the recent deepening of wells, the environment surrounding oil wells has changed, and the transport environment for crude oil and gas has become more severe. That is, the sour environment accompanied by hydrogen sulfide has increased. For this reason, among the characteristics required for deformed steel wires, stability against hydrogen penetrating into the steel wires from the use environment, that is, hydrogen-induced cracking (Hydrogen Induced Crack)
ing, hereinafter referred to as HIC) and sulfide stress corrosion cracking (Su
lfide Stress Corrosion Cracking (SSC)
It has become particularly required that no phenomena occur. By the way, HIC is a crack that occurs when hydrogen enters a no-load steel wire, while SSC is a crack that occurs when hydrogen enters a high-load steel wire. It is.

In response to such a trend, the present inventors have set a tensile strength of 50%.
We propose two techniques targeting the steel wire kgf / mm ^2. One of them is disclosed in Japanese Patent Application Laid-Open No. 1-279710, entitled "Method of Manufacturing High-Strength Steel Wire with Excellent Resistance to Hydrogen-Induced Cracking" (hereinafter referred to as Conventional Method 1).
After patenting a high carbon steel wire containing 40 to 0.70% C, cold-working with a reduction in area of 25 to 75% is performed, and then 500
This is a method of performing spheroidizing annealing at ~ 700 ° C. The other one is
A patent application was filed on March 30, 1990 for "Method of manufacturing high-strength steel wire for sour environment" (hereinafter referred to as Conventional Method 2). After applying a tensile strain of 2%, it is a method of bluing at 250 to 400 ° C.

The steel wire manufactured according to the conventional method 1 has excellent HIC resistance. However, in an actual use environment, since a strong tensile stress acts on a steel wire, it is important to have excellent SSC resistance besides HIC resistance. From this point of view, Conventional Method 2 is an improvement of the SSC resistance characteristics of Conventional Method 1. However, in order to use the steel wire in a higher load state and more stably in a sour environment, it is necessary to increase the strength of the steel wire and further improve the SSC resistance.

[Problems 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 a sour environment in which strength and SSC resistance are much better than those produced by a conventional method.

[Means for solving the problem]

 The gist of the present invention is as follows.

(1) C: 0.40 to 0.70%, Si: 0.10 to 1%, Mn: 0.20 to 1%,
P: 0.025% or less, S: 0.010% or less, with the balance being Fe and unavoidable impurities, after being cold-worked at a cross-sectional reduction rate of 25-75%, spheroidized at 500-620 ° C A method for producing a high-strength steel wire for a sour environment, comprising annealing and then giving a tensile strain of 0.2 to 2% in a temperature range of 250 to 400 ° C.

(2) C: 0.40 to 0.70%, Si: 0.10 to 1%, Mn: 0.20 to 1%,
P: 0.025% or less, S: 0.010% or less, Al: 0.008 to 0.050%, the balance is made of steel consisting of Fe and inevitable impurities. 500-620 ℃
Spheroidizing annealing in a temperature range of 250 to 400 ° C.
A method for producing a high-strength steel wire for a sour environment, wherein a tensile strain of about 2% is given.

 Hereinafter, the present invention will be described in detail.

If C is less than 0.40%, a predetermined strength cannot be obtained after spheroidizing annealing. On the other hand, if C exceeds 0.70%, it becomes difficult to perform strong working by cold working, and fine cracks are generated in the center of the steel wire during working to deteriorate the HIC characteristics. Therefore, the upper limit is set to 0.70%.

Si must be at least 0.10% or more as a deoxidizing agent. The strength increases as the amount increases. However, 1%
Exceeding the limit causes excessive decarburization, which is not preferred because cracks frequently occur in the steel wire during cold working.

Mn is required to be 0.20% or more to prevent hot brittleness. In order to improve the hardenability of Mn, it is desirable to increase the amount of Mn in order to obtain a uniform pearlite structure by patenting, but if it exceeds 1%, the frequency of HIC caused by central segregation can increase. The upper limit is 1%.

Next, P tends to segregate at the grain boundaries, and thus reduces workability. Therefore, the smaller the amount, the better.
However, in the case of manufacturing by continuous casting, since the re-P occurs to increase the melting temperature, only the upper limit is specified to 0.025%.

S has the same harmful effects as P and is preferably as small as possible in terms of corrosion resistance. However, the upper limit is 0.010% which can be economically manufactured at present. Industrial production is sufficiently possible up to 0.001% of S.

Al is used as necessary as a deoxidizing agent and a grain refinement element. In the case of adding Al, the lower limit of the amount of Al required for grain refinement is 0.008%. On the other hand, if the content of Al exceeds 0.050%, the amount of nonmetallic inclusions increases, causing a decrease in yield due to surface defects.

In addition to the above-described elements, when the hardenability is unpredictable due to the large thickness of the deformed steel wire, it is effective to add 0.6% or less of Cr. In addition, less than 0.3% Cu and 0.02%
% Or less of W has the effect of suppressing the intrusion of hydrogen into steel.
The characteristics can be improved.

The wire having the above composition is processed into a steel wire. The steel wire of the present invention is a general term for a wire having a circular or irregular (rectangular or groove-shaped) cross-sectional shape obtained by deforming a wire, performing roller dies, rolling, or the like.

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

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

The reason for limiting the reduction in area to the range of 25 to 75% in the present invention will be described.

If the cross-sectional reduction rate is less than 25%, spheroidization of cementite will be insufficient by annealing after processing, and HIC will occur. Also,
If the cross-sectional reduction rate exceeds 75%, for example, cracks due to processing occur on the end face and inside of the flat pressure line, and especially, internal cracks are HI
It is not preferable because it induces C. The cross-sectional reduction rate of the present invention is defined by the following equation.

S: Cross-sectional area of deformed steel wire S _O : Cross-sectional area of wire (wire) The first of the main features of the present invention is that after cold working with a reduction rate of 25 to 75%, spheroidizing annealing is performed. The purpose of the present invention is to remove the 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 found that the spheroidized cementite structure obtained by annealing has remarkably excellent HIC characteristics as compared with the conventional layered pearlite structure. Hydrogen atoms penetrating into steel are cementite /
It accumulates at the ferrite interface and forms HIC nuclei there. However, in the case of spheroidized cementite, it is considered that HIC resistance is excellent because stress concentration is small.

The reason for defining the spheroidizing annealing temperature range of 500 to 620 ° C. in the present invention is as follows.

When the spheroidization temperature range is lower than 500 ° C., as shown in the comparative example data of No. 12 in Table 1, the frequency of HIC occurrence increases,
When the HIC resistance deteriorates and the SSC generation lower limit stress cannot achieve the SSC generation lower limit stress of 70 kgf / mm ² or more, which is the target value of the present invention, and when it exceeds 620 ° C, the No. 1 in Table 1 also shows .
As shown by the data of Comparative Examples 22 and 23, the tensile strength, the yield strength, and the SSC lower limit stress cannot achieve the target values of the present invention.

The second major 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 a tensile strain of 0.2 to 2% in a temperature range of 250 to 400 ° C. It is to give.

It is considered that the cause of SSC is that hydrogen that has penetrated into the steel material from the sour environment diffuses into the small yield region generated by the applied stress and agglomerates there, thereby accelerating the yield phenomenon and causing microcracks. Therefore, in order to improve the SSC resistance, it is important to prevent the local minute breakdown phenomenon before the macro breakdown phenomenon starts. That is, increasing the yield strength of the steel material is effective.

However, in general, by increasing the yield strength,
The tensile strength increases. Increasing the tensile strength is not preferable because it increases the risk of SSC occurrence, as described above.

The present inventor studied a processing method that increases the yield strength but has little effect on the tensile strength and other mechanical properties, and as a result, heats the steel wire after spheroidizing annealing to a blue hot brittle temperature,
Giving a slight tensile strain at that temperature (hereinafter,
Heat-stretching) yielded the target high yield strength, and as a result, led to a new finding that SSC resistance was significantly improved. If the tensile strain in heat stretching is less than 0.2%, the effect of improving SSC is insufficient. As the tensile strain increases, the yield strength increases, and with it the SSC lower limit stress also increases. However, if it exceeds 2%, HIC occurs, and the lower limit stress for SSC occurrence also tends to decrease, so the upper limit is 2%.

Regarding the heat stretching temperature, if the temperature is less than 250 ° C., HIC occurs, and the lower limit stress for SSC occurrence is low. on the other hand,
If the temperature exceeds 400 ° C., the yield strength is reduced, and the SSC resistance is significantly reduced. For the above reasons, the heat stretching temperature is set to 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, a certain strain is given to the steel wire by slightly increasing the rotation speed of the pulley on the winding side of the steel wire with respect to the rotation speed of the pulley on the side supplying the steel wire, and at the same time, between the two pulleys. An apparatus having a mechanism capable of heating a tensioned steel wire by an induction heating method or an electric heating method is desirable.

Example A wire having a diameter of 9.5 mm, which was made into a fine pearlite structure by lead patenting, was formed into a steel wire having a diameter of 5 mm by wire drawing, and then flat rolled into a flat pressure wire having a thickness of 0.9 to 2.85 mm. After spheroidizing the steel wire, a tensile strain of 0.2 to 2% was given while the steel wire was induction-heated using a heat stretching device having the above-described mechanism.

The evaluation of the HIC characteristics was performed by the following method. The above-described flat pressure curve was cut to a length 100mm, 5% NaCl-0.5% CH 3 COOH-H 2 S
After being immersed in a saturated solution at 25 ° C. for 96 hours, three places were polished, and the presence or absence of microcracks was observed with an optical microscope.

Evaluation of SSC characteristics was performed by the following method. The above flat pressure line was used as a test piece as it was, and both ends were gripped to give a tensile stress of 80 to 110% of the actual yield strength. A 200 mm central portion of the test piece was immersed in a sour environment, that is, a solution having the same composition as that of the HIC test described above. The temperature of the solution was 25 ° C. In this state, a load test was performed for 720 hours to measure the maximum stress at which no rupture occurred, that is, the lower limit stress at which SSC occurred.

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

No.1-4, No.10 and No.11, No.18-21, No.24-27
Is a comparison between the method of the present invention and two conventional methods.After producing an annealed steel wire in the same manufacturing process, the method of the present invention uses 33
Heat stretching was performed at 0 to 360 ° C to impart a tensile strain of 0.8 to 2.0%. The steel wire produced by the method of the present invention has a tensile strength of 75 kgf / mm ² or more,
Yield strength is higher than steel wire manufactured by the conventional method. In addition, the lower limit stress for SSC occurrence is 70 kgf / mm ² or more, which is a high level that cannot be achieved by any of the conventional methods.

Nos. 5 to 9 show the effects of the tensile strain during the heat stretching, and Nos. 13 to 17 show the effects of the temperature during the heat stretching on the characteristics of the deformed wire.
By selecting heat stretching conditions within the range specified by the present invention, it is possible to produce a deformed wire having no SIC generation and having an SSC generation lower limit stress of 70 kgf / mm ² or more and excellent in sour resistance.

As has been described [Effect of the Invention According to the present invention, has a 70 kgf / mm ² or more tensile strength, high strength steel wire for sour environments HIC resistance and SSC resistance characteristics are greatly improved Can be manufactured.

Claims (2)

(57) [Claims] (1) C: 0.40 to 0.70%, Si: 0.10 to 1%, Mn: 0.20 to
1%, P: 0.025% or less, S: 0.010% or less, with the balance being Fe
Steel with inevitable impurities
% Cold working, then spheroidizing annealing at 500 to 620 ° C, and then giving a 0.2 to 2% tensile strain at a temperature in the range of 250 to 400 ° C. Wire manufacturing method. 2. C: 0.40 to 0.70%, Si: 0.10 to 1%, Mn: 0.20 to
1%, P: 0.025% or less, S: 0.010% or less, Al: 0.008 to 0.050%
, The balance of which consists of Fe and unavoidable impurities, after cold-working with a cross-sectional reduction rate of 25 to 75%,
A method for producing a high-strength steel wire for a sour environment, comprising performing spheroidizing annealing at -620 ° C, and thereafter giving a tensile strain of 0.2 to 2% in a temperature range of 250 to 400 ° C.