JPH0421718A - Production of high strength steel excellent in sulfide stress cracking resistance - Google Patents

Abstract

PURPOSE:To produce a high strength steel excellent in sulfide stress cracking resistance property by subjecting a steel having a specific composition consisting of C, Si, Mn, P, S, N, Mo, Nb, Ti, Al, B, and Fe to hot rolling and then to respectively specified heat treatment and working. CONSTITUTION:A steel having a composition which consists of 0.15-0.35% C, 0.05-0.50% Si, 0.20-1.0% Mn, <=0.015% P, <=0.010% S, <=0.008% N, 0.10-0.80% Mo, 0.010-0.050% Nb, <=0.028% Ti, 0.005-0.10% Al, 0.0005-0.0025% B, and the balance essentially Fe and further contains, if necessary, <=1.5% Cr and in which -0.005<=Ti-3.4N<=0.01% is satisfied is hot-rolled. Directly after the above or after reheating to a temp. in the austenite region, the steel is worked at 950-700 deg.C at 15-40% reduction of area. Then, this steel is hardened and successively tempered at 580-720 deg.C. By this method, the high strength steel having high sulfide stress cracking resistance property can be obtained at a low cost.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is suitable for use as a steel material for oil or gas wells and their peripheral equipment, and is suitable for use in sulfide stress cracking (hereinafter referred to as SSC), which occurs under the combination of wet hydrogen sulfide and stress. High-strength steel with high resistance to
The present invention relates to a method for manufacturing a steel having high strength of f/- or more and excellent SSC resistance such as a ratio of SSC cracking limit stress to yield strength (hereinafter referred to as Rs value) of 80% or more.

(Prior Art) In recent years, as the energy situation has become more urgent, steel materials are increasingly being used for drilling, transporting, and storing crude oil containing hydrogen sulfide. In particular, steel used in oil country tubular goods used for crude oil drilling is exposed to a severe corrosive environment due to the trend toward deeper wells, and there is a need for steel that has both high yield strength and excellent SSC resistance. There is. Also, due to economical requirements, it is necessary to meet most of the requirements with low-alloy steel.

In response to such demands, many types of SSC-resistant steel pipes have been developed, as introduced in Japanese Patent Laid-Open Publications No. 55-73848 and Japanese Patent Laid-open No. 55-134158.

(Problems to be Solved by the Invention) Sulfide stress cracking caused by hydrogen sulfide is said to be caused by hydrogen embrittlement caused by hydrogen generated when the steel surface corrodes and diffuses into the steel material. In steel materials with chemical compositions based on low alloys, as the steel material strength increases, this embrittlement susceptibility increases, so it has been difficult to simultaneously provide steel material strength and excellent SSC resistance.

Specifically, high-strength steel pipes with excellent SSC resistance using conventional technology have a C95 class (yield strength of 66.5 to 77 kg).
/m() is considered to be the upper limit, and steel that can be manufactured at even lower cost is required.

(Means for Solving the Problems) As a result of a detailed study by the present inventors on the metallurgical factors governing SSC resistance, we found that the decrease in SSC resistance is caused by prior austenite intergranular cracking, and therefore high strength Excellent S resistance
It was found that in order to simultaneously provide SC properties, it is necessary to take measures to increase grain boundary strength. Conventionally, as a method of increasing grain boundary strength, low alloy steel contains Mn and P in a certain relationship.

There are techniques for optimizing the steel composition, such as JP-A-61-621)8 and JP-A-62-17721, in which P and Mo are contained in a certain relationship. As a result of investigating a drastic grain boundary strengthening method for steel using this method, it was found that grain boundary strength can be improved by increasing the dislocation density near the grain boundaries compared to inside the grains.

The present invention is based on such knowledge, and the structural requirements have been determined taking into consideration the ability to manufacture a steel material of sufficient thickness. The gist is C: 0.15-0.35%, Si: 0.05-050%
, Mn: 0.20-1.0%, P: 0. [1
15% or less, S: 0.010% or less, N2O,
oog% or less, Mo: 0. lO~0.80%, Nb:
0.010-0.050%, Ti: 0.0211
1% or less and -0,005%≦Ti-3,4N≦0.
01%, 8Ω・0.005~010%, B:0.0005~0
．． 0025%, or further contains Cr: 1.5% or less, with the remainder being substantially iron. After being reheated to 950 to 700°C, the cross-sectional area reduction rate is 15 to 40%, and then quenching is carried out to transform it into martensite with a volume fraction of 90% or more. This is a method for producing high-strength steel with high resistance to sulfide stress cracking, which is characterized by tempering in a temperature range.

The present invention will be explained in detail below.

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

C is an essential element for ensuring the required strength of low alloy steel, and its content was set to 0.15% or more.

However, a large content exceeding 0.35% may cause cracks during quenching, so the upper limit was set at 0.35%.

As a component that reduces the grain boundary strength of steel, it is desirable to reduce the amount of Si, and the upper limit of Si is set to 0.50%.

Mn is an effective component that improves hardenability and prevents red brittleness, and is added in an amount of 0.2% or more. However, if it is included in a large amount, grain boundary strength is reduced, so the upper limit was set at 1.0%.

P is a harmful component that lowers grain boundary strength, so it is necessary to reduce its content. Therefore, in the present invention, 0
．． It was limited to 0.015% or less.

S is an impurity that cannot be completely removed in steelmaking, and if it is included in a large amount, it forms MnS, which becomes a starting point for cracks.

Therefore, the upper limit of the S content was set to 0.010%.

A lower amount of N is desirable in order to reduce the total amount of Ti, but
As an effective component for suppressing grain boundary cracking and refining austenite grains, the upper limit was set at o and oos%.

Mc) is included as a component that has the effect of suppressing the grain boundary segregation of P and improving the grain boundary strength, but if it is less than 0.10%, the effect is small, and if it is added in excess of 0.8%, the effect is even worse. The effect cannot be expected. Therefore, the content of Mo is 0.
The content was set at 10% to 0.80%.

Allium is an effective component that improves the toughness of steel by refining grains, and its content was set to 0.005% or more. In addition, excessive content exceeding 0.10% is Al2O
3 and decreases SSC resistance.

Therefore, the content of AI was set to 0.005 to 0.10%.

Nb has the effect of forming Nb carbonitrides and making finer grains of reheated and hardened steel or recrystallized grains during rolling, but 0
．． If the amount is less than 0.01%, the effect will not be sufficient, and even if added in a large amount, not only will it not be possible to expect a further grain refinement effect, but there is also a risk that scratches will be more likely to occur during hot working. Therefore N
The upper limit of b content was set to 0.05%.

Ti fixes N as TiN, maintains the hardenability improvement function of B, and has the effect of suppressing surface cracking during casting of B-containing steel. However, excessive addition of Ti promotes the precipitation of coarse TiN and reduces SSC resistance, so in the relationship of Ti-3,4N, the total T
The upper limit of Hit was set to 0.028%.

B is an element that significantly improves hardenability, or 0.00
If it is less than 0.5%, the effect is not sufficient, and even if it is added in a large amount, the effect will only reach saturation, which will reduce cracking during hot working,
Due to concerns about scratches, the upper limit was set at 0.0025%. Further, the inclusion of B has the effect of reducing the contents of Mn and Cr and suppressing the tendency for SSC resistance to decrease.

Cr is an effective component that improves hardenability and strengthens steel, and is included selectively. Cr is an element that does not relatively reduce SSC resistance when the content is small, but when a large amount Since adding it to the water obviously lowers the SSC resistance, the upper limit was set at 1.5%.

Steel with the above-mentioned composition is produced by turning molten steel into slabs by using an ingot-making/blowing method or a continuous casting method, using a melting furnace such as a converter or an electric furnace, or by further degassing treatment. Manufactured through hot rolling.

After finish rolling of the hot rolling or after reheating from low temperature to austenite range temperature after hot rolling, 950 to 700°C
After performing hot working of 15 to 40% in a temperature range of , quenching treatment is performed. This step is the most important point of the present invention. The dislocations introduced by this processing are deposited at a high density near the grain boundaries, and these dislocations are inherited by martensite generated during the quenching process.

That is, the martensite obtained by this processing contains a high density of dislocations peculiar to the structure, and the dislocation density near the prior austenite grain boundaries is especially high.

The reason why the processing temperature was limited to 950 to 700℃ is 9.
At temperatures above 50°C, dislocations introduced by processing do not remain effectively and do not contribute to grain boundary strengthening due to dislocations.

On the other hand, at temperatures below 700° C., ferrite transformation occurs during processing, resulting in a decrease in strength and a mixed structure of martensitic and ferrite structures, deteriorating the SSC resistance.

Therefore, the temperature was limited to 950 to 700°C. Processing rate 15
The reason why it is set at 15 to 40% is because if it is less than 15%, there will be no sufficient strengthening effect due to dislocations, while if it is processed more than 40%, recrystallization may be induced and dislocations may be reduced. The machining rate here is ((section ratio before machining) -
(Cross-sectional area before processing / (Cross-sectional area before processing) x 100 (%
) is the cross-sectional area reduction rate defined by

Processed steel is transformed into martensitic material by quenching. When diffusion transformation occurs, the dislocations introduced by processing disappear, and the effects aimed at by the present invention cannot be obtained. Therefore, it is necessary to carry out martensitic transformation without diffusion. However, since it is generally difficult to completely transform steel of a certain thickness into martensitic material, the processing effect was almost achieved, and the martensite content was 90% or more, which can be easily achieved industrially.

It is necessary to temper the as-quenched steel thus obtained in order to adjust it to a desired strength and improve its toughness and SSC resistance. At temperatures below 580°C, it is difficult to reduce the strength to a level that improves the SSC resistance;
When heated at a high temperature of 20° C. or higher, an austenite phase precipitates and transforms into ferrite after cooling, and in some cases transforms into martensite, resulting in a non-uniform structure, which is unfavorable for SSC resistance. Therefore, the tempering temperature is 580~
The temperature was set at 720°C.

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

(Example) Next, the present invention will be explained 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 properties. Sulfide stress cracking characteristics are the maximum that does not break in 720 hours when tensile stress is applied to a round bar tensile test piece with a parallel part diameter of 6.4 in a 5% Na C9 + 0.5% CH3CO0H aqueous solution saturated with 1 atm H2S. Stress (σIh)
I asked for As shown in Table 2, when compared at the same strength level, the steel of the present invention has sulfide stress cracking resistance Rs (σ, , /YS
) is high and excellent. The effect of the processing before quenching according to the present invention is also seen in steels other than those according to the present invention, but the absolute value is low.

(Effect of the invention) As described above, according to the method for manufacturing steel according to the present invention,
It is possible to obtain steel that has both high strength and resistance to sulfide stress cracking, which could not be achieved using conventional methods, and this makes a huge contribution to industry.

Claims (2)

[Claims] (1) 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: 0.010-0
．． 050%, Ti: 0.028% or less and -0.00
5%≦Ti-3.4N≦0.01%, Al: 0.005-0.10%, B: 0.0005-0
．． 950~ after reheating a steel containing 0.025% and the remainder being substantially iron from a low temperature immediately after hot rolling or from a low temperature immediately after hot rolling to an austenite region temperature.
Cross-sectional area reduction rate of 15-40% in the temperature range of 700℃
A method for producing high-strength steel with high sulfide stress cracking resistance, which comprises quenching the steel after processing and then tempering it at a temperature in the range of 580 to 720°C. (2) 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: 0.010-0
．． 050%, Ti: 0.028% or less and -0.00
5%≦Ti-3.4N≦0.01%, Al: 0.005-0.10%, B: 0.0005-0
．． 0025%, and Cr: 1.5% or less, and the remainder is substantially iron. Immediately after hot rolling or at a low temperature immediately after hot rolling, a steel is processed into the austenitic region. Sulfiding characterized by processing the cross-sectional area reduction rate by 15-40% in a temperature range of 950-700°C after reheating to a temperature, followed by quenching treatment, and then tempering in a temperature range of 580-720°C. A method for manufacturing high-strength steel with high stress cracking resistance.