JPS62218516A - Manufacture of high strength steel superior in sulfide stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To obtain the titled steel even in severe corrosive environment of H2S existence, by hot working, water cooling and tempering steel having a prescribed component compsn. under respectively prescribed temp. ranges. CONSTITUTION:Steel contg. by weight, 0.10-0.40% C, 0.05-0.50% Si, 0.30-1.5% Mn, 0.1-2.0% Cr, 0.2-7.5% Mo and restricted quantities of <=0.005% S, <=0.005% P is treated as follows: Namely, the steel is heated to Ac3 transforma tion point + >=50 deg.C-<=1,150 deg.C, to obtain uniform bainite structure, then hot worked at >=Ar3 transformation point by >=30% draft. The steel after working is immediately water cooled to 600 deg.C- room temp. by 5-60 deg.C/sec rate for preventing coarsening of austenite grain due to recrystallization and growth, then tempered at temp. of 400 deg.C-Ac1 transformation point to obtain the aimed titled steel.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for manufacturing steel with excellent H2S resistance, and more specifically, to a method for manufacturing steel with excellent H2S resistance, and more specifically to a method for manufacturing steel with excellent H2S resistance. This invention relates to a method for manufacturing high-strength steel with excellent corrosion cracking resistance.

(Prior art) Sulfide stress corrosion cracking is a phenomenon that occurs when stress is applied to steel materials used in humid environments containing hydrogen sulfide, such as in sour oil wells and sour gas wells.In general, the higher the material strength, the better the sulfide resistance. It is known that stress corrosion cracking of materials deteriorates, and it has been confirmed that high tensile materials crack even when subjected to extremely low stress.

Conventionally, oil country tubular goods used in the above-mentioned sour environment have been made of quenched and tempered 4135 steel of the artificial S engineering standard, or p-, Ti-based products with this composition but with Mo added up to 0.5%. -B treatment was used to improve sulfide stress corrosion cracking resistance and strength.

Furthermore, regarding hardening and tempering, JP-A-59-1
Usually, heat treatment as described in 73245 is performed, but in that case, it is difficult to obtain steel with high strength and excellent sulfide stress corrosion cracking resistance. There is no established method for producing dams that meet the contradictory demands of having excellent sulfide stress corrosion cracking properties.

(Problems to be Solved by the Invention) In view of the current situation, the present invention provides a method for obtaining steel with high strength and excellent resistance to sulfide stress corrosion cracking even in a severe corrosive environment where H2S exists. The purpose is to

(Means for solving the problems) The gist of the present invention is that C: 0.10% by weight.
~0.40%, St: 0.05~0.50%, Mn
: 0.30-1.5%, Cr2 0.1-2.0%, M
o: Contains 0.2-0.75%, S: 0.0
Ti: 0.002 to 0.08%, Nb: 0.05% or less, P: 0.005% or less, as necessary.
005-0.1%, B: 0.0005-0.004%,
A steel containing one or more Cu: 0.01 to 1.0%, with the remainder being Fe and unavoidable impurities.
After heating to transformation point +50°C to 1150°C or less, hot working at a processing rate of 30% or more at a temperature above ArB transformation point, immediately after processing, water cooling to a temperature of 5~B, and then 400°C~A c 1 transformation point. This is a method for producing high-strength steel with excellent resistance to sulfide stress corrosion cracking, which is characterized by tempering at a high temperature.

The present invention will be explained in detail below.

(Function) First, the reason for limiting the range of each component in the present invention as described above will be described below.

First, C is an element that ensures the strength of steel, but if its content exceeds 0.40%, the amount of martensite and carbides increases, and stress corrosion cracking resistance deteriorates.

On the other hand, if it is less than 0.10%, ferrite tends to appear and it is difficult to form the desired structure, so the C component range is reduced to 0.
．． It was set at 10% to 0.40%.

Si is added for deoxidation or strength adjustment.

If the Si content is less than 0.05%, a deoxidizing effect cannot be obtained, while if it exceeds 0.50%, embrittlement occurs, so the Si content was set at 0.05 to 0.50%. .

Mn has the function of deoxidizing agent, the function of improving strength and toughness, and the function of obtaining a uniform payinite structure.
The above is necessary. On the other hand, if the Mn content exceeds 1.5%, the toughness of the steel will deteriorate, so the Mn content should be reduced to 0.
It was set at 3 to 1.5%.

Cr is effective in improving strength, toughness, hardenability, and corrosion resistance, but if it is less than 0.1%, this effect cannot be obtained;
．． Since the effect of improving hardenability is saturated even if the Cr content exceeds 0%, the Cr content is set at 0.1 to 2.0%.

Mo tends to improve the hardenability of steel and increase its strength, and is also effective in improving sulfide stress corrosion cracking resistance, but if it is less than 0.20%, the effect is not sufficient. on the other hand,
Even if the MO content exceeds 0.75%, the effect of improving hardenability and the effect of improving sulfide stress-corrosion cracking resistance will be saturated, and it will increase the cost and be uneconomical. Therefore, the MO content should be reduced to 0.20%.
It was set at ~0.75%.

P is an element that inevitably exists in steel as an impurity, but if it exceeds 0.005%, it not only causes deterioration of toughness but also increases susceptibility to stress corrosion cracking, so the upper limit should be set to 0.00%.
It was set at 5%.

Like P, S is an element that inevitably exists in steel as an impurity, and it is desirable to suppress its content to a small amount as much as possible. Especially 0. If QO exceeds 5%, the sulfide stress corrosion cracking resistance deteriorates, so it is limited to 0005% or less.

The above are the basic components of the steel of the present invention, but in the present invention, in addition to these, Ti,
One or more of Nb, B, and Cu can be contained, but the range should be as follows.

First, along with B, Ti has the effect of improving the hardenability of steel, or it has the effect of improving stress corrosion cracking resistance by refining the grains and forming a bainite structure.
．． If it is less than 0.002%, the effect cannot be obtained, and on the other hand, if it is added in excess, it will cause embrittlement due to burning.
．． It was set at 0.8%.

Nb has the effect of refining the steel structure, improving tempering resistance, and improving sulfide stress corrosion cracking resistance, but if its content is less than 0.005%, this effect cannot be obtained; If it exceeds .1%, the toughness will be impaired, so N
The content of b was determined to be 0.005 to 0.1%.

B has the effect of improving the hardenability of steel, but if its content is less than o, ooo; s%, this effect cannot be obtained.
On the other hand, since the hardenability improvement effect is saturated even if the B content exceeds 0.004%, the B content is adjusted to between o, o o o s and 0.004%.
It was set as 0.004%.

Cu is effective for solid solution strengthening and improving corrosion resistance, but if added in excess, many defects will occur on the steel surface, so Cu
The upper limit when adding is 1. Let it be OcX. On the other hand, Cu
Since the addition effect of is not obtained at less than 0.01%,
The content of Cu was set at 0.01 to 1.0%.

In the present invention, hot working is performed after melting steel containing the above-mentioned components.Next, the hot rolling process will be described.

The reason why the heating temperature of the slab according to the steel composition of the present invention is limited is that in order to obtain a uniform paynite structure, it is necessary to perform hot working in a completely austenite region. To prevent this, the temperature should be 1150°C or less.

A hot working rate of 30% or more is required to obtain a microstructure that does not deteriorate sulfide stress corrosion cracking resistance. The processing temperature is required to be processed in a completely austenite region in order to obtain a uniform payinite structure, and the processing temperature should be higher than the A r z transformation point.

In order to prevent grain coarsening due to recrystallization and growth of austenite grains after processing, it is necessary to start cooling as soon as possible after processing. The cooling rate must be 5°C/sec or more to obtain a uniform fine payinite structure, and 60°C/sec or more is required.
6 to become a martensitic structure at temperatures exceeding ℃/Sec.
The temperature should be 0°C/sec or less. Stopping water cooling when the cooling temperature is over 600℃ has no effect on uniform paynight, so
Cool to at least 600°C.

The reason for limiting the tempering temperature is as follows.

Tempering is unavoidable to remove internal stress that is harmful to sulfide stress corrosion cracking resistance, and is also effective in increasing financial strength.The effective tempering range is from 400℃ to Ac1 transformation point temperature. .

As detailed above, according to the manufacturing method of the present invention, the steel structure becomes an ultra-fine-grained, even-paynite structure, and the resistance to sulfide stress corrosion cracking can be significantly improved.

(Example) For each steel manufactured by melting steel having the composition shown in Table 1 using a vacuum melting furnace and treating it under the manufacturing conditions shown in Table 1,
API tensile test and NACE-SSC test were conducted. The results are also shown in Table 1. Steels 1 to 25 are made by the method of the present invention, and steels 26 to 29 are comparative materials made by the conventional method.

The subsulfide stress corrosion cracking test was carried out using the NACE constant load tensile test shown in FIG. 1, and the minimum stress at which breakage did not occur even after 720 hours of applying a certain amount of stress was determined as the critical stress σth.

In Fig. 1, reference numeral 3 denotes a fixed tension jig for fixing the test piece 1, and the load of the weight 5 is transferred to the corrosion cell 6 by the support rod 4.
the test piece 1 by the fixing jig 2.
Apply tensile stress to.

As is clear from the results shown in Table 1, the steel produced by the method of the present invention has high strength and significantly improved resistance to sulfide stress corrosion cracking, whereas the comparative steel It can be seen that the sulfide stress corrosion cracking resistance is poor.

(Effects of the Invention) The present invention is a low-cost manufacturing method that does not require a reheating and quenching process for steel that has both high strength and excellent sulfide stress corrosion cracking resistance, and can be used in sour environments. It enables the production of steel that can be effectively used for oil well steel pipes, thick plates, etc.

[Brief explanation of drawings]

FIG. 1 is a side view showing the structure of a constant load tensile tester that applies tensile stress to a test piece. DESCRIPTION OF SYMBOLS 1... Test piece, 2... Fixing jig, 3... Fixed tension jig, 4... Support rod, 5- Weight, 6... Corrosion cell. Agent: Patent attorney Masamitsu Akizawa and 1 other person, 1 illustration

Claims (2)

[Claims] (1) In weight%, C: 0.10-0.40%, Si: 0.05-0.50%, Mn: 0.30-1.5%, Cr: 0.1-2.0% , Mo: 0.2 to 0.75%, S: 0.005% or less, P: 0.005% or less, and the balance is Fe and unavoidable impurities.
c_3 After heating above transformation point +50℃ and below 1150℃, A
Hot-worked at a processing rate of 30% or more at a temperature above the r_3 transformation point, and immediately after processing at a cooling rate of 5 to 60°C/sec to 600°C.
Water cooled to a temperature of ℃~room temperature, then 400℃~Ac_
A method for producing high-strength steel with excellent resistance to sulfide stress corrosion cracking, characterized by tempering at a temperature of 1 transformation point. (2) In weight%, C: 0.10-0.40%, Si: 0.05-0.50%, Mn: 0.30-1.5%, Cr: 0.1-2.0% , Mo: 0.2-0.75%, S: 0.005% or less, P: 0.005% or less, Ti: 0.002-0.08%, Nb: 0. Ac_3 transformation of steel containing one or more of the following: 005 to 0.1%, B: 0.0005 to 0.004%, and Cu: 0.01 to 1.0%, with the balance consisting of Fe and inevitable impurities. Point +50℃ or more 115
After heating to 0℃ or less, hot working at a temperature of Ar_3 transformation point or higher with a processing rate of 30% or more, and immediately after processing at 5 to 60℃/se
A method for producing high-strength steel with excellent resistance to sulfide stress corrosion cracking, characterized by water cooling to a temperature of 600° C. to room temperature at a cooling rate of c and then tempering at a temperature of 400° C. to Ac_1 transformation point.