JPS62149813A - Production of high-strength seamless steel pipe having excellent resistance to sulfide stress corrosion cracking - Google Patents

Abstract

PURPOSE:To produce a high-strength seamless steel pipe which is inexpensive and has excellent resistance to sulfide stress corrosion cracking by subjecting a steel having the specific compsn. which does not contain much of costly alloys and is suppressed in the content of impurities such as P and S to hot rolling then to hardening and adequate tempering. CONSTITUTION:The stock which contains 0.45-0.65wt% C, 0.20-0.35% Si, 0.8-1.6% Mn, 0.1-<0.8% Cr, 0.05-<0.40% Mo, 0.005-0.1% Al, 0.02-0.10% Ti, and 0.01-0.10% V, contains >=1 kinds of <=0.1% Nb, and <=0.1% Zr is suppressed to <=0.015% P and <=0.005% S among the inevitable impurities incorporated therein and consists of the balance substantially Fe is subjected to hot rolling and is made into a pipe. The steel pipe is then hardened to martensite structure. The steel pipe is thereafter tempered in the temp. range of 620 deg.C-Ac1 transformation point or below. The seamless steel pipe which has both of the excellent resistance to sulfide stress corrosion cracking and the high strength of about 75-120kgf/mm<2> yield strength is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) This invention is applicable to oil country tubular goods, which are becoming increasingly deeper due to the trend towards sour oil, and line pipes for sour gas and sour oil.
Furthermore, regarding the manufacturing method of high-strength seamless steel pipes with excellent sulfide stress corrosion cracking resistance suitable for use in applications such as chemical plant piping, etc. The present invention discloses the development results for effectively avoiding such high yield strength.

(Prior art) In response to the above-mentioned trend toward deeper and sour wells, the sulfide stress corrosion cracking resistance generally deteriorates as the strength increases. 64 to 74 kgf/mm2 class C-Mo steel (C
: 0.25 to 0.3!5wt% (hereinafter simply referred to as %).

Cr: about 1%, Mo: 0.2-0.8%) are considered to be the most excellent.

By the way, recently, according to Japanese Patent Application Laid-Open No. 53-78917,
Cr compared to the conventional 65kgf/mm 2nd grade Cr-Mog
-75 to 90 kgf/mm2 with increased Mo and a large amount of V added to improve sulfide stress corrosion cracking resistance
A grade steel has been developed, but it is expensive because it contains large amounts of expensive elements such as Mo and V, and because it contains a large amount of V, it is prone to cracking during hot working by continuous casting.

Also, JP-A-57-19322 and JP-A-57-1932
In each publication No. 3, a La-added steel with excellent sulfide stress corrosion cracking resistance was proposed, but the yield strength of this steel is about 80 kgf/mm2 at the highest, which is lower than conventional steel. There is virtually no improvement seen.

Furthermore, JP-A-57-35622 discloses a high-strength steel for oil wells with reduced p and s, but although this steel has considerably improved strength, it is suitable for use in alkaline environments and H2S Resistance to stress corrosion cracking is guaranteed only when it is contained in only trace amounts.

(Problems to be Solved by the Invention) As mentioned above, all conventional steels have had some problems in terms of price or properties.

The present invention advantageously solves the above problems, and is an inexpensive component system suitable for continuous casting that does not contain large amounts of the expensive elements listed above, and has excellent sulfide stress corrosion resistance. Breakability and yield strength are 75-120kgf/mm”
The purpose of this study is to propose an advantageous manufacturing method for seamless steel pipes that have high strength.

(Means for Solving the Problems) As a result of extensive research to solve the above problems, the inventors have obtained the knowledge described below.

(1) Compared to conventional steels whose strength is improved only with Cr and Mo, steels with high C content and the combined addition of small amounts of Ti and V have better resistance to sulfide stress corrosion cracking. Moreover, in this case, the amount of Cr+Mo added can be significantly reduced compared to conventional steel, resulting in cost reduction.

(2) In order to obtain excellent sulfide stress corrosion cracking resistance with a steel having a yield strength of 75 kgf/mm2 or more, the tempering temperature must be at least 620°C or higher.

(3) In order to obtain excellent sulfide stress corrosion cracking resistance,
It is necessary to harden completely, but for this purpose Cr,
In steel with a smaller amount of Mo added, it is necessary to add more Mn than in conventional sour-resistant steel.

(4) When the tempering temperature is 620°C or higher, 75
In addition to stably obtaining a yield strength of kgf/mm” or more,
In order to achieve complete hardening, C: 0.45% or more,
Mn: Must be 0.8% or more.

(5) In high-strength steels such as the steel of this invention containing 0.8% or more of Mn, in order to obtain good sulfide stress corrosion cracking resistance, the P content must be 0.015% or less. S is 0.
It is necessary to limit it to 0.005% or less.

(6) As shown in item (1) above, not only Cr and Mo, but also Cr.

Sulfide stress corrosion cracking resistance is improved by reducing Mo and adding Ti, V, etc. in combination. In other words, it is better to use as many different types of carbide-forming elements as possible, rather than just one type, for better results. This is because it is better to precipitate carbides of various compositions than to precipitate carbides of a single composition, because the precipitation temperature is also different, so the precipitated carbides are more finely and uniformly dispersed, thus achieving high strength. It is thought that the sulfide stress corrosion cracking resistance is improved at the same time.

Therefore, in addition to Cr+Mo+Ti and V, Nb,
By adding one or two types of Zr at the same time,
Furthermore, strength and sulfide stress corrosion cracking resistance can be further improved.

This invention is based on the above knowledge.

That is, this invention includes: C: 0.45-0.65% Si: 0.20-0.35% Mn: 0.8-1.6% Cr: 0.1% or more and less than 0.8%, Mo : 0
．． 05% or more, less than 0°40%, AI: 0.005
~0.1% Ti: 0.02~0.10% and V: 0.
01 to 0.10%, and at least one selected from Nb: 0.1% or less and Zr: 0.1% or less; 0
．． 0.015% or less S: suppressed to 0.005% or less, the remainder being essentially Fe, is made into a hollow material for seamless steel pipes by hot rolling, and then quenched to form a martensitic structure. After that, the temperature exceeds 620°C,
This is a method for producing a high-strength seamless steel pipe with excellent resistance to sulfide stress corrosion cracking, which is characterized by tempering at a temperature range below Ac, transformation point.

This invention will be specifically explained below.

First, in this invention, the reason why the material components are limited to the above range will be explained.

C: 0.45 to 0.65% C is an element useful in obtaining high strength even in high temperature tempering, and especially when added in a small amount as shown in this invention, high temperature tempering at 620°C or higher can be achieved. by 75
In order to obtain a yield strength of kgf/mm2 or more, a content of at least 0.45% is required, but if it exceeds 0.65%, cracks that cause quench cracking will occur, so the content should be between 0.45% and 0.45%.
It was decided to add it in a range of 65%. Si: 0.20~
0.35% Si is at least 0.35% for deoxidizing steel and improving strength.
Although 20% is required, if it exceeds 0.35%, the toughness deteriorates, so it is limited to a range of 0.20 to 0.35%.

Mn: 0.8 to 1.6% Mn is not only useful for improving hardenability and strength, and also for deoxidation, but is also a relatively inexpensive element. In particular, when other additive elements are contained in small amounts as in the steel of this invention, the content may be 0.
If the amount is less than 8%, the effect of the addition is poor. If Shikaji na Akura exceeds 1.6%, P.

Since it causes segregation of S, etc. and deteriorates sulfide stress corrosion cracking resistance, it should be contained in the range of 0.8 to 1.6%.
I made it.

Cr: 0.1% or more, less than 0.8% Cr forms carbides during quenching and tempering treatment, and effectively contributes to increasing strength and tempering resistance. For that, ゛0
．． It is necessary to add 1% or more, but if elements such as Mo, Ti, V, etc. coexist, the effect reaches saturation even if 0.8% or more is added, so 0.1 to 0.8%. limited to the range of

Mo: 0.05% or more, less than 0.40% MO is also Cr
Similarly, it increases strength and tempering resistance, and also effectively contributes to improving sulfide stress corrosion cracking resistance by preventing grain boundary segregation of P, but if the content is less than 0.05%, the The effect of addition is poor, and even if 0.40% or more is added, the effect reaches saturation when T i + V etc. are added simultaneously. Furthermore, the Mo content was limited to a range of 0.05 to 0.40% because it would increase the cost and go against the purpose of the present invention, which is to provide an inexpensive material.

＾7! : o, oos~0.1% AI is a useful element that not only contributes to deoxidation but also combines with N to refine crystal grains and improve toughness, strength, and resistance to sulfide stress corrosion cracking. be.

However, if the content is less than o,oos%, the effect of its addition is poor, while if it exceeds 0.1%, the effect not only reaches saturation, but also causes deterioration of toughness, so o,oos~0 .1% range.

Ti: 0.02~0.10% and V: 0.01~
0.10%These elements form carbides in the quenching and tempering treatment like Cr and Mo, and are equally effective in contributing to improving the hardenability and tempering softening resistance of steel. In this invention steel, carbide forming elements of various compositions are precipitated over a wide temperature range by reducing Cr and Mo compared to conventional Cr-Mo steels and adding a small amount of Ti and ν. It can be done. As a result, the precipitated carbides are fine, spherical, and uniformly distributed, making it possible to obtain a steel that has both high strength and excellent sulfide stress corrosion cracking resistance.

However, this effect cannot be obtained when Ti is less than 0.02% and ■ is less than 0.01%; on the other hand, when Ti is less than 0.10%,
(2) If it exceeds 0.10%, it not only becomes expensive, but also causes coarsening of carbides, which actually deteriorates the resistance to sulfide stress corrosion cracking.

Therefore, the amount of Ti added was set to 0.02 to 0.10%, and the amount of added Ti was set to 0.01 to 0.10%.

P: 0.015% or less, S: 0.005% or less Both P and S are harmful elements that cause a significant deterioration of the sulfide stress corrosion cracking resistance of steel. In order to ensure the desired strength and sulfide stress corrosion cracking resistance at higher temperatures, P≦0.015% and S≦0.005%, respectively.
need to be controlled.

Furthermore, in this invention, in order to further improve the strength and sour critical stress ratio, at least one of Nb and Zr is added in the following range.

Nb and/or Zr: 0.1% or less Nb and Zr
Like Cr and Mo + h + V, both form carbides in the quenching and tempering treatment, and effectively contribute to improving the hardenability and temper softening resistance of steel. However, if the amount added exceeds 0.1%, the precipitates become coarser, which not only deteriorates the sulfide stress corrosion cracking resistance but also reduces the workability and toughness.
Nb and Zr must be added in an amount of 0.1% or less, whether added alone or in combination.

After casting the molten metal prepared to have the above-mentioned preferred composition, a hollow material is obtained by drilling according to a conventional method.

The thus obtained hollow material for a seamless steel pipe is then subjected to a hot elongation process, and then subjected to a quenching and tempering process.

In this hardening treatment, it is desirable that 90% or more of the steel be made into a martensitic structure.

Further, the tempering treatment needs to be performed in a temperature range of 620° C. or higher, Ac, or lower than the transformation point. This is because in order to obtain a yield strength of 75 to 120 kgf/mm and good sulfide stress corrosion cracking resistance as expected in this invention, 620
This is because it is essential to perform the tempering treatment at a high temperature of ℃ or higher.On the other hand, when the temperature exceeds the Ac point, austenite is formed, and when cooled to room temperature, this austenite becomes martensite that does not undergo tempering, making it sulfur-resistant. This is because it causes a significant deterioration in chemical stress corrosion cracking properties.

In this way, a high-strength seamless steel pipe with excellent resistance to sulfide stress corrosion cracking can be obtained.

(Function) The reason why not only the strength but also the resistance to sulfide stress corrosion cracking is significantly improved by following the present invention has not yet been clearly elucidated, but it is thought to be as follows.

In other words, sulfide stress corrosion cracking occurs when hydrogen generated by corrosion of steel by an aqueous solution containing hydrogen sulfide penetrates into the steel.
It is considered to be a type of hydrogen embrittlement that accumulates in stress concentration areas such as inclusions, precipitates, and dislocations and embrittles the steel. However, in addition to lowering P and s to reduce inclusions, it is necessary to temper at a high temperature of 620℃ or higher. to reduce the dislocation density and make the precipitates spheroidal.
-M.

By reducing Cr and Mo content compared to steel, but by adding a small amount of Ti and V in combination, carbides of various compositions can be precipitated over a wide temperature range, resulting in a structure in which precipitated carbides are finely and uniformly distributed. This is thought to be due to the fact that stress concentration points where hydrogen accumulates are significantly reduced by .

Normally, as mentioned above, when high temperature tempering is performed to obtain low dislocation density and spherical precipitates, the strength of the steel material decreases.
It is believed that by combining the ingredients of this invention, a structure having precipitates with an appropriate shape and distribution can be obtained, and high strength can be obtained without impairing sulfide stress corrosion cracking resistance.

(Example) Hollow materials for seamless steel pipes having the component compositions shown in Table 1 were hot-worked and then quenched, and then tempered at the temperatures shown in Table 1 to produce products.

The yield strength of each seamless steel pipe thus obtained is Y.

Table 1 also shows the results of the investigation on tensile strength T, S, ), and resistance to sulfide stress corrosion cracking.

For stress corrosion cracking resistance, round bar tensile test pieces were tested by NACE.
It was evaluated by a test in which a stress of 80% of the yield strength was applied in a solution (0.5% acetic acid, 5% salt-added saturated hydrogen sulfide water), and those that did not break for 30 days in this test were marked with an ○. , and the broken ones are indicated with an X mark.

As is clear from the same table, all the seamless steel pipes (numbers 1 to 8) obtained according to the present invention have high Y, S, and T, S values as well as excellent sulfide stress corrosion cracking resistance. Obtained.

On the other hand, although the component composition satisfies the appropriate range,
The comparative steel (floor 9.10) whose tempering temperature is below the lower limit is
It had poor sulfide stress corrosion cracking resistance.

In addition, among the components (Nos. 11 to 24) in which any of the component compositions deviate from the appropriate range of this invention, No. 11 where the C content is below the lower limit and No. 12 where the MnN content is below the lower limit are sulfur resistant, respectively. Although the material stress corrosion cracking resistance was good, only low y, s, and T, S values were obtained.

All other columns 13 to 24 are Y, S, , T, S.
Although the values were good, the resistance to sulfide stress corrosion cracking was poor.

(Effects of the Invention) According to the present invention, it does not contain a large amount of expensive elements as in the past, is an inexpensive component system suitable for continuous casting, and has excellent resistance to sulfide stress corrosion cracking. Yield strength:
A seamless steel pipe having a high strength of 75 kgf/mm" or more can be obtained.

Claims (1)

[Claims] 1. C: 0.45 to 0.65 wt% Si: 0.20 to 0.35 wt% Mn: 0.8 to 1.6 wt% Cr: 0.1 wt% or more, 0.8 wt% Less than, Mo: 0
．． 05wt% or more, less than 0.40wt%, Al: 0.0
05 to 0.1 wt% Ti: 0.02 to 0.10 wt% and V: 0.01 to 0.10 wt%, and selected from Nb: 0.1 wt% or less and Zr: 0.1 wt% or less. A seamless steel pipe containing at least one type of P and S among unavoidable impurities, P: 0.015 wt% or less, S: 0.005 wt% or less, and the remainder being substantially Fe. A hollow material for use is hot-rolled to form a pipe, then quenched to form a martensitic structure, and then heated to a temperature of 620°C or higher, A
A method for producing a high-strength seamless steel pipe with excellent resistance to sulfide stress corrosion cracking, characterized by tempering at a temperature range below the c_1 transformation point.