JP2002096105A - Method for manufacturing high-strength steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a high-strength steel pipe which restrains inner flaws from being caused and which is excellent in resistance to sulfide, stress and corrosion cracking. SOLUTION: Seemless pipes are manufactured by the direct hot rolling of billets made by continuously casting low-alloy steel containing C of 0.2-0.3 wt.%, Si of 0.1-0.3 wt.%, Mn of 0.1-0.3 wt.%, P of 0.15 wt.% or less, S of 0.002 wt.% or less, Cr of 0.5-1.5 wt.%, Mo of 0.2-1.0 wt.%, Nb of 0.01-0.05 wt.%, B of 0.0005-0.005 wt.%, O of 0.005 wt.% or less and H of 0.0005 wt.%, and the seemless pipes are then subjected to heat treatment such as quenching and tempering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel pipe excellent in sulfide stress corrosion cracking resistance, and more particularly, to direct drilling and hot rolling using a round billet by continuous casting with a specific component. The present invention relates to a method for manufacturing a seamless pipe having a high yield strength of 77 kg / mm ² or more by heat treatment after forming a steel pipe.

[0002]

2. Description of the Related Art With the tightening of the energy situation in recent years, oil wells or gas wells containing hydrogen sulfide, which have not been developed so far, are also to be developed. Therefore, it is desired to produce a seamless pipe having excellent sulfide stress corrosion cracking resistance. Such high-strength steels having excellent sulfide stress corrosion cracking resistance are disclosed in JP-A-55-134156, JP-A-58-42722,
JP-A-62-14981, JP-A-6-322478
And Japanese Unexamined Patent Publication No. 9-249935. Further, as shown in Japanese Patent Application Laid-Open No. 7-41856, there are two methods for producing a billet as a raw material, including a continuous casting method and a steel ingot-ingot method. From the viewpoint, the production using the billet by the continuous casting method is more advantageous, and the use ratio of the billet by the continuous casting in the low alloy steel is increasing. However, even in the case of low-alloy steel, as the amount of alloy addition increases, the incidence of flaws on the inner surface of the tube increases, and the yield decreases significantly. In particular, the high-strength steel pipe excellent in sulfide stress corrosion cracking resistance described above has a high rate of generation of flaws on the inner surface of the pipe due to the large amount of Cr, Mo, etc., and offsets the effect of omitting the process by continuous casting. Productivity is falling.

[0003]

As a method for suppressing the generation of flaws on the inner surface of a pipe, Japanese Patent Application Laid-Open No. Hei 8 (1994) -108 focuses on shifting the center of the billet, which causes flaws, from the center by controlling the cooling pattern. -52555, "Eccentric solidification continuous casting method of seamless pipe material and method of manufacturing seamless pipe"
Also, Japanese Unexamined Patent Publication (Kokai) No. 3-124352, which discloses a method for producing a continuous cast slab excellent in internal quality, has been proposed, which requires special equipment.
Also, alloy steel with a small amount of alloying elements does not cause any flaws on the inner surface even when a billet formed by continuous casting is used, but in a high-strength steel containing 0.5% or more of Cr and 0.2% or more of Mo, The occurrence of internal flaws becomes remarkable. This is because the center porosity and the cracks formed in the center of the billet appear on the inner surface of the pipe in the process of drilling the billet and become flaws. Therefore, a method for reducing the generation of flaws by shifting or crushing the flaws that cause these flaws from the central portion is proposed in the above-mentioned publication. However, these methods cannot cope with existing equipment, and all require the introduction of new equipment.

In view of this situation, the present inventors have studied the generation of internal surface flaws and the components contained in steel by using a billet formed by continuous casting. It was found that it greatly affected the occurrence. Therefore, the present invention is based on such knowledge, and can control the occurrence of inner surface flaws by controlling the composition of steel even in existing equipment, and has excellent resistance to sulfide stress corrosion cracking. It proposes that a high strength steel pipe can be manufactured.

[0005]

The gist of the first invention is that, by weight%, C: 0.2-0.3%, Si: 0.1-0.3.
%, Mn: 0.1 to 0.3%, P: 0.15% or less,
S: 0.002% or less, Cr: 0.5 to 1.5%, M
o: 0.2-1.0%, Nb: 0.01-0.05%,
B: 0.0005 to 0.005%, O: 0.005% or less, H: 0.0005% or less, and hot rolling directly from a billet of the low alloy steel by continuous casting (Hot) Direct Rolling), which is a method for producing a high-strength steel pipe, which comprises subjecting a seamless pipe to heat treatment such as quenching and tempering.

The gist of the second invention is that, in the first invention, Ti, Zr, and V, which are carbide-forming elements, are added by weight% to Ti:
0.05-0.03%, Zr: 0.005-0.03
%, V: one or more of 0.01 to 0.1%.

[0007] The gist of the third invention is that, in the first invention, C that fixes S, which is a cause of generation of internal flaws, and reduces the generation of flaws
a, Mg, REM, by weight%, Ca: 0.0005 to
0.005%, Mg: 0.0005 to 0.003%, R
EM: characterized by containing one or more of 0.005 to 0.01%.

[0008] The gist of the fourth invention is that, in the second invention, C that fixes S, which is a cause of the generation of internal flaws, and reduces the generation of flaws
a, Mg, REM, by weight%, Ca: 0.0005 to
0.005%, Mg: 0.0005 to 0.003%, R
EM: characterized by containing one or more of 0.005 to 0.01%.

Hereinafter, the reasons for limiting the components to the above ranges in the present invention will be described.

C: 0.2-0.3% C is an element that increases the quenching hardness, forms carbides by tempering, and controls the strength of steel. .2% or more, and if contained excessively, cracks will occur during quenching, so the C content should be 0.2-0.3%.

Si: 0.1 to 0.3% Si is added as a deoxidizing agent, but it needs to be 0.1% or more for sufficient deoxidation. Since the stress corrosion cracking property is reduced, the upper limit is 0.3%.

Mn: 0.1-0.3% Mn is added for the purpose of deoxidizing and desulfurizing.
If less than the above, a sufficient effect cannot be obtained. If the content exceeds 0.3%, the carbide after tempering is coarsened and the sulfide stress corrosion cracking resistance is reduced. Therefore, the range of Mn is set to 0.1 to 0.3%.

P: 0.015% or less P is contained in steel as an unavoidable impurity, but is an element that promotes the generation of inner surface flaws at the time of drilling a billet, and has an effect of increasing susceptibility to sulfide stress corrosion cracking. Because there is,
015% or less.

S: 0.002% or less S is contained as inevitable impurities in steel like P, and if the content is large, internal flaws are frequently generated during billet drilling.
Especially when drilling directly from an as-cast billet,
Since the microsegregation at the center of the billet is larger than when the billet is converted from the rolling process, its content must be limited as much as possible, and the upper limit of S is set to 0.002%.

Cr: 0.5-1.5% Cr is an important element for strength because it enhances hardenability and forms carbides. , Excessive addition coarsens grain boundary carbides and lowers sulfide stress corrosion cracking resistance. Therefore, the range of Cr is set to 0.5 to 1.5%.

Mo: 0.2 to 1.0% Mo has the effect of improving both hardenability and corrosion resistance. In addition, it forms carbides by tempering and contributes to strength, but is an expensive element. If the content exceeds 1.0%, its effect is saturated, so the content is set to 0.2 to 1.0%.

Nb: 0.01-0.05% Nb forms fine carbides at the time of tempering and improves the strength. However, if added excessively, it becomes already coarse carbides at the time of quenching, and the effect on the strength is reduced. To do so, the range of Nb is made 0.01-0.05%.

B: 0.0005 to 0.005% B is an element which enhances the quenching sensitivity and is effective for improving the strength. However, if it exceeds 0.005%, the grain boundary carbide after tempering becomes coarse. In order to significantly reduce sulfide stress corrosion cracking resistance, the range of B is set to 0.0005 to 0.005%.

O: 0.005% or less O is an element which greatly affects the generation of internal flaws at the time of billet perforation. In particular, when drilling a billet directly as cast, it is necessary to reduce the amount of O. As deacidification,
Mn, Si, Al and the like are effective, and any deoxidizing agent may be used. However, unless the amount of O added is 0.005% or less, a favorable inner surface state without flaws cannot be obtained.

H: 0.0005% or less H, like O, is an element that affects the generation of inner surface flaws during billet perforation. In the case of making a billet in the slab-rolling process, the amount of hydrogen in the steel is reduced by heating and rolling, but when using a directly cast billet that does not go through such a process, the amount of H is previously determined in the billet stage. It is necessary to reduce it sufficiently. Further, hydrogen may increase the susceptibility to sulfide stress corrosion cracking, so that the amount of H is desirably small, and is set to 0.0005% or less.

Ti: 0.005 to 0.03% Zr: 0.005 to 0.03% V: 0.01 to 0.1% All of Ti, Zr and V are fine carbides in carbon and steel. Is an element that contributes to the strength, but the effect is saturated when added excessively, so that the content of each element is set to 0.1.
005-0.03%, 0.005-0.03%, 0.0
1% to 0.1%.

Ca: 0.0005 to 0.005% Mg: 0.0005 to 0.003% REM: 0.005 to 0.01% These fix S and O in steel as sulfides and oxides. ,
Although it is an element having the effect of suppressing the generation of internal flaws, it generates a low melting point compound when added in excess, decreases hot workability and increases the generation of flaws, and the content thereof is 0.0005, respectively. ~ 0.005%, 0.0005 ~ 0.
003%, 0.005 to 0.01%.

[0023]

An embodiment of the present invention will be described. After smelting the steel of the components shown in Table 1 and casting it into a square mold of about 100 × 100 × 400 mm, one end 35 m from the center of the steel ingot
A test piece 1 (see FIG. 2) having a diameter of mφ and 42 mmφ and a length of 200 mm was sampled, and a hot workability test was carried out by a tilt rolling mill 2 as shown in FIG. The remainder of the steel ingot was hot-rolled into a 15 mmt plate for the purpose of examining strength and sulfide stress corrosion cracking resistance (abbreviated as “stress corrosion cracking” in the table). And a heat treatment for tempering.

[0024]

[Table 1]

The hot workability is shown in FIG.
Were evaluated by the degree of difficulty of rotary forging rolling by a test machine (inclined rolling mill) shown in FIG. Specifically, when the inclined rolling is performed without using the plug, the position where the volume of the center portion of the billet where the internal crack 3 does not occur (the portion of Lcr in the figure) is equal to the volume of the original billet. (= 20
× Lcr / L) (Dcr).
The critical draft was calculated by the formula (Dcr-D1) × 100 / Dcr.

In the heat treatment, the steel sheet was water-cooled from a quenching temperature of 900 ° C. and tempered at a temperature at which the yield strength was about 80 kg / mm ² . The sulfide stress corrosion cracking resistance was measured using a 0.5% CH ₃ COOH + 5% NaCl solution saturated with H ₂ S and applying a constant load test in which a stress of 85% of the yield stress was applied. Was evaluated by the presence or absence of breakage.

The criterion for determining the occurrence of internal flaws is the critical rolling reduction of the steel. If this value is 5% or less, a crack occurs at the center of the billet due to the effect of rotary forging before drilling, which causes a flaw in the inner surface of the tube when the tube is manufactured. From Table 1, the steels of the present invention all show a good critical rolling reduction of 5% or more, but 3 to 7 of comparative steels have a critical rolling reduction of 5% or less. It is determined that an inner surface flaw occurs in the pipe. These comparative steels have high O and H gas components in the steel or high S and P which are considered to produce low melting point compounds, suggesting that control of these components is an important factor in preventing the occurrence of internal flaws. are doing.

The sulfide stress corrosion cracking resistance of the high-strength material is improved by suppressing the coarse precipitation of grain boundary carbides.
It is considered that when Cr and Mn are high as in No. 2, carbides are coarsened and sulfide stress corrosion cracking resistance is reduced. Also, from Comparative Steel 6, it is determined that H in the steel also reduces sulfide stress corrosion cracking resistance.

[0029]

As described above, according to the present invention, internal surface flaws do not occur by using a continuously cast billet made of a low-alloy steel in which the gas composition and the amount of low-melting element in the steel are adjusted. A seamless pipe having a high yield strength of 77 kg / mm ² or more and excellent sulfide stress corrosion cracking resistance can be manufactured. According to the present invention, an oil country tubular good used for an oil well containing hydrogen sulfide can be provided more economically.

[Brief description of the drawings]

FIG. 1 is a view showing a taper billet emptying test method in an example.

FIG. 2 is a view showing a test piece of a taper billet.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B22D 11/12 B22D 11/12 A C21D 9/08 C21D 9/08 E // C22C 38/00 301 C22C 38 / 00 301Z 38/32 38/32 (72) Inventor Shuji Hashizume 1-2-1 Marunouchi 1-chome, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. F-term (reference) 4E002 AA04 AB06 BB08 BD02 CB03 CB10 4K042 AA06 BA01 BA07 CA02 CA03 CA06 CA08 CA09 CA12 CA13 CA14 DA01 DA02 DC02 DD02 DE02

Claims (3)

[Claims] C .: 0.2 to 0.3% by weight, S:
i: 0.1 to 0.3%, Mn: 0.1 to 0.3%, P
: 0.15% or less, S: 0.002% or less, Cr:
0.5-1.5%, Mo: 0.2-1.0%, Nb:
0.01-0.05%, B: 0.0005-0.00
5%, O: 0.005% or less, H: 0.0005%
A low-alloy steel containing the following, a seamless pipe is manufactured by hot rolling directly from a billet of the low-alloy steel by continuous casting, quenching, and a high-strength steel pipe characterized by being subjected to a heat treatment of tempering. Production method. 2. The composition according to claim 1, further comprising:
And Ti: 0.05-0.03%, Zr: 0.005-
A method for producing a high-strength steel pipe, comprising one or more of 0.03% and V: 0.01 to 0.1%. 3. In addition to the components according to claim 1 or 2, Ca: 0.0005 to 0.005% by weight,
Mg: 0.0005 to 0.003%, La + Ce: 0.
A method for producing a high-strength steel pipe, comprising one or more of 005 to 0.01%.