JP2834276B2 - Manufacturing method of high strength steel with excellent sulfide stress cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention is suitable as a steel material used for oil wells or gas wells and peripheral equipment, and has a sulfide stress crack (SSC) generated under a combination of wet hydrogen sulfide and stress. High-strength steel with a high yield strength of 70 kg f / mm ²
The present invention relates to a method for producing a steel having the above high strength and excellent SSC resistance in which the ratio between the critical stress of cracking of SSC and the yield strength (hereinafter referred to as _RS value) is 80% or more.

(Prior Art) In recent years, with the urgency of the energy situation, steel materials have been increasingly used for drilling, transportation, and storage of crude oil containing hydrogen sulfide. In particular, steel used for oil well pipes used for crude oil drilling will be exposed to severe corrosive environments with the tendency to deepen wells, and steel that has both high yield strength and excellent SSC resistance is required. I have. Most demands must be met with low synthetic steel for economic reasons.

In response to such a request, JP-A-55-73848 discloses
As introduced in JP-A-55-134156, etc.,
Many types of SSC-resistant steel tubes have been developed.

(Problems to be Solved by the Invention) Sulfide stress cracking due to hydrogen sulfide is caused by hydrogen embrittlement caused by diffusion of hydrogen generated when a steel material surface is corroded into the steel material. In a steel material having a chemical composition based on a low alloy, as the strength of the steel material increases, the susceptibility to embrittlement increases, so that it is difficult to simultaneously provide the strength of the steel material and the excellent SSC resistance.

Specifically, high-strength steel pipe excellent in SSC resistance in the prior art is considered the upper limit is C95 class (yield strength 66.5~77kg / mm ^2), is required steels can be further manufactured at a low cost I have.

(Means for Solving the Problems) As a result of a detailed study of the metallurgical factors governing the SSC resistance, the present inventors have found that the decrease in the SSC resistance is caused by the former austenite grain boundary cracking, and Excellent SSC resistance
It has been found that it is necessary to take measures to increase the grain boundary strength in order to simultaneously provide the properties. As a conventional method of increasing the grain boundary strength, low alloy steel contains Mn and P in a certain relationship.

Although there is a technique for optimizing the steel composition as disclosed in JP-A-61-6208 and JP-A-62-17721 in which P and Mo are contained in a certain relationship, the present inventor uses other methods. As a result of studying the fundamental method of strengthening the grain boundary of steel, it was found that the grain boundary strength was improved by increasing the dislocation density in the vicinity of the crystal grain boundary as compared to the inside of the grain.

Based on such knowledge, the present invention has determined constituent components in consideration of the fact that a steel material having a sufficient thickness can be manufactured. The gist is as follows: C: 0.15-0.35%, Si: 0.05-0.50%, Mn: 0.20-1.0%, P: 0.015% or less, S: 0.010% or less, N: 0.008% or less, Mo: 0.10-0.80%, Nb : 0.010 to 0.050%, Ti: 0.028% or less and -0.005% ≦ Ti−3.4N ≦ 0.01
%, Al: 0.005 to 0.10%, B: 0.0005 to 0.0025%, or Cr: 1.5% or less and the balance substantially consisting of iron. After being reheated from the low temperature immediately after the end of the cold rolling to the austenite temperature, the cross-sectional area is reduced by 15 to 40% in the temperature range of 950 to 700 ° C, and the volume ratio is increased by 90% or more by quenching. This is a method for producing a high-strength steel with high resistance to sulfide reaction cracking, characterized by transforming into martensite and then tempering in a temperature range of 580 to 720 ° C.

 Hereinafter, the present invention will be described in detail.

First, the reason why the steel composition of the present invention is limited as described above will be described.

C is an element essential for ensuring the required strength of the low alloy steel material, and its content is set to 0.15% or more. But 0.35
% May cause cracking during quenching, so the upper limit was set to 0.35%.

It is desirable to reduce the amount of Si as a component that lowers the grain boundary strength of steel, and the upper limit is set to 0.50%.

Mn is an effective component for improving hardenability and preventing red-hot brittleness, and is added in an amount of 0.2% or more. However, if contained in a large amount, the grain boundary strength is reduced, so the upper limit was made 1.0%.

P is a harmful component that lowers the grain boundary strength, and its content must be reduced. Therefore, in the present invention
Limited to 0.015% or less.

S is an impurity that cannot be completely removed from steel production, and when contained in a large amount, forms MnS, which serves as a crack initiation point. Therefore, the upper limit of the S content is set to 0.010%.

Although it is desirable that the N content is low in order to reduce the total Ti content,
As an effective component for suppressing grain boundary cracking and making austenite grains fine, the upper limit was made 0.008%.

Mo is contained as a component having the effect of suppressing the segregation of P at the grain boundary and improving the grain boundary strength. However, the effect is small at 0.10% or less, and no further effect can be expected even if it exceeds 0.8%. . Therefore, the content of Mo is set to 0.10 to 0.80%.

Al is an effective component for improving the toughness of steel by acting to refine the crystal grains, and its content is set to 0.005% or more. In addition, an excessive content exceeding 0.10% increases Al ₂ O ₃ and causes
Reduces SC performance. Therefore, the content of Al is 0.005 to 0.1
0%.

Nb forms Nb carbonitride and has the effect of making fine grains of reheat-quenched steel or fine grains recrystallized during rolling.
%, The effect is not sufficient, and even if added in a large amount, not only a further fine-graining effect cannot be expected, but also there is a possibility that scratches are likely to occur during hot working. Therefore, the upper limit of the Nb content is set to 0.05%.

Ti fixes N as TiN, maintains the function of improving the hardenability of B, and has the effect of suppressing surface cracking during casting of B-containing steel. However, excessive addition of Ti promotes precipitation of coarse TiN,
Since the SC property is reduced, in the relationship of Ti-3.4N-
0.005 to 0.01% and the upper limit of the total Ti content was set to 0.028%.

B is an element that significantly improves hardenability, but 0.0005
%, The effect is not sufficient, and even if it is added in a large amount, not only the effect is saturated, but also the occurrence of cracks and flaws during hot working is concerned, so the upper limit was made 0.0025%. Also B
The effect of reducing the content of Mn and Cr suppresses the tendency of reducing the SSC resistance.

Cr is an effective component that enhances hardenability and strengthens steel, and is selectively contained. Cr is an element that does not relatively lower the SSC resistance when its content is small, but when added in a large amount, it obviously lowers the SSC resistance, so the upper limit is 1.5.
%.

Steel of the above component composition, a converter, a melting furnace such as an electric furnace or a molten steel that has been subjected to further degassing and melted to form a slab by ingot-bulking method or continuous casting method. It is manufactured through hot rolling. After the finish rolling of the hot rolling or after the completion of the hot rolling, the steel sheet is reheated from a low temperature to an austenite region temperature, subjected to 15 to 40% hot working in a temperature range of 950 to 700 ° C, and then subjected to a quenching treatment. . This step is the most important point of the present invention. The dislocations introduced by this processing are densely deposited near the grain boundaries, and the dislocations are inherited by martensite generated during the quenching process. That is, the martensite obtained by this processing contains high-density dislocations peculiar to the structure and has a particularly high dislocation density near the prior austenite grain boundary. Here, the reason why the processing temperature is limited to 950 to 700 ° C. is that at 950 ° C. or higher, dislocations introduced by the processing do not effectively remain and do not contribute to the grain boundary strength due to the dislocation. 700
At temperatures below ℃, ferrite transformation occurs during processing to reduce the strength, exhibit a mixed structure of martensite structure and ferrite structure, and degrade SSC resistance. Therefore 9
Limited to 50-700 ° C. The reason for setting the processing rate to 15% or more is 1
If it is less than 5%, the effect of strengthening by sufficient dislocation will not be obtained.
Since processing exceeding 0% may induce recrystallization and reduce dislocations, the content is set to 15 to 40%. Here, the processing rate is a cross-sectional area reduction rate defined by {(cross-sectional area before processing) − (cross-sectional area after processing)} / (cross-sectional area before processing) × 100 (%).

The processed steel is transformed into martensite by quenching. When the diffusion transformation occurs, the dislocations introduced by the processing disappear, and the effect intended by the present invention cannot be obtained.
Therefore, it is necessary to perform martensitic transformation by non-diffusion transformation. However, it is generally difficult to completely transform martensite into steel of a certain thickness, so that the working effect is almost obtained, and the martensite is 90% or more, which can be easily achieved industrially.

It is necessary to adjust the as-quenched steel thus obtained to a desired strength and to perform tempering to improve toughness and SSC resistance. At 580 ° C or lower, it is difficult to lower the strength to improve the SSC resistance, and when heated at a temperature of 720 ° C or higher, an austenite phase precipitates, transforms to ferrite after cooling, and sometimes transforms to martensite. It is not preferable for SSC resistance because it has an uneven structure. Therefore, the tempering temperature was set at 580 to 720 ° C.

The steel manufactured as described above has extremely excellent SSC resistance.

(Examples) Next, the present invention will be described based on examples.

It was manufactured using the steel shown in Table 1 under the conditions shown in Table 2.

Each steel was evaluated for strength (yield strength, YS) and sulfide stress cracking characteristics. The sulfide stress cracking characteristic is that the diameter of the parallel part is 5% NaCl + 0.5% CH ₃ COOH aqueous solution saturated with 1 atm of H ₂ S.
Tensile stress was applied to a 6.4 mm round bar tensile test piece, and the maximum stress (σ _th ) that did not break in 720 hours was determined. As shown in Table 2, when compared at the same strength level, the steel of the present invention is excellent in high sulfide stress cracking resistance R _S (σ _th / YS). The effect of the working before quenching according to the present invention can be seen in steels other than the present invention, but is low in absolute value.

(Effect of the Invention) As described above, according to the method for producing steel according to the present invention, steel having both high strength and sulfide stress cracking at a high level, which cannot be achieved by the conventional method, can be obtained. It is extremely large where it contributes to the top.

Claims (2)

(57) [Claims] [Claim 1] C: 0.15 to 0.35%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.0%, P: 0.015% or less, S: 0.010% or less, N: 0.008% or less, Mo: 0.10 to 0.80% , Nb: 0.010 to 0.050%, Ti: 0.028% or less and -0.005% ≤ Ti-3.4N ≤ 0.01%, Al: 0.005 to 0.10%, B: 0.0005 to 0.0025%, with the balance being substantially iron Hot-working after completion of hot rolling or after re-heating from low temperature to austenite temperature after completion of hot rolling, the cross-sectional area reduction rate in the temperature range of 950 to 700 ° C is 15 to 40% A method for producing a high-strength steel sheet having a high resistance to sulfide stress cracking, wherein the steel sheet is subjected to a quenching treatment, followed by tempering in a temperature range of 580 to 720 ° C. 2. C: 0.15 to 0.35%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.0%, P: 0.015% or less, S: 0.010% or less, N: 0.008% or less, Mo: 0.10 to 0.80% , Nb: 0.010 to 0.050%, Ti: 0.028% or less and -0.005% ≤ Ti-3.4N ≤ 0.01%, Al: 0.005 to 0.10%, B: 0.0005 to 0.0025%, and Cr: 1.5% The steel containing the following and substantially consisting of iron is reheated from the low temperature to the austenite temperature after the completion of the hot rolling or after the completion of the hot rolling, and then cut in a temperature range of 950 to 700 ° C. A method for producing a high-strength steel sheet with high resistance to sulfide stress cracking, characterized by hot working with an area reduction rate of 15 to 40%, quenching, and then tempering at a temperature in the range of 580 to 720 ° C. .