JPH07109522A - Production of seamless tube of martensitic stainless steel - Google Patents

Abstract

PURPOSE:To produce a seamless tube of martensitic stainless steel minimal in anisotropy and excellent in strength and stress corrosion cracking resistance, which is equivalent to the characteristic of the seamless tube made by means of conventional quench-and-temper process, though the amounts of expensive elements to be added are not increased and a treatment process is simplified. CONSTITUTION:A martensitic stainless steel, having a composition consisting of 0.15-0.25% C, 0.10-1.0% Si, 0.10-1.0% Mn, <=0.03% P, <=0.01% S, 10-14% Cr, <=0.15% V, and the balance Fe with inevitable impurities, is used. Finish rolling is applied so that the reheating temp. T after rolling of this steel by a mandrel mill takes a value in the range satisfying 950 deg.C<=T<=1150 deg.C and T>=1.8R+930, where T means reheating temp. ( deg.C) and R means finish rolling draft (%). Then, cooling is done down to room temp. to form a martensitic structure, and the required strength is obtained by means of tempering heat treatment alone.

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 seamless pipe of martensitic stainless steel which has excellent low temperature toughness, mechanical properties including low temperature toughness and stress corrosion cracking resistance.

[0002]

2. Description of the Related Art Generally, martensitic stainless steel seamless pipe is widely used as a material for oil country tubular goods and the like, which is required to have strength, toughness and corrosion resistance, and it is well known that it is particularly excellent in CO ₂ corrosion resistance. ing.

Conventionally, this type of seamless pipe is heated to a temperature at which a steel slab can be pierced, and then pierced and rolled using, for example, a piercer and a mandrel mill, and then reheated to an austenite region and then a reducer or the like is used. It is manufactured by finishing rolling to the final product size, and then subjected to quenching and tempering treatment to impart predetermined mechanical properties.

Conventionally, as shown in the heat history of FIG. 3, it was usual to reheat a tube once cooled to room temperature and quench it after completion of the tube production, but in recent years, rationalization of process and energy saving For this reason, a direct quenching method is being adopted in which quenching is performed immediately after the pipe manufacturing process.

As the means of this direct quenching method, there are methods disclosed in, for example, JP-A-63-238217 and JP-A-2-277720. However, the methods disclosed in these publications are as follows. It was found that the toughness in the circumferential direction was not so good with respect to the rolling direction, instead of considering the anisotropy of toughness.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to prevent the conventional quenching / tempering from happening without unnecessarily increasing the amount of expensive alloying elements added and simplifying the treatment process. It is an object of the present invention to provide a method for producing a martensitic stainless steel seamless pipe having a small anisotropy equivalent to that of a process and excellent in strength and stress corrosion cracking resistance.

[0007]

By the way, it is generally the size of austenite grains during quenching and the precipitation of carbides during tempering that control the toughness and stress corrosion cracking resistance of steels having a martensitic structure. As a result, when it becomes a structure composed of fine recrystallized grains and finely dispersed carbide,
The toughness and stress corrosion cracking resistance are improved.

Therefore, the inventors of the present invention have made detailed studies on the thermomechanical treatment of martensitic stainless steel and its structure and toughness, and as a result, the reheating temperature before the final finish rolling was changed according to the degree of finish rolling. , Knowing that by selecting an appropriate temperature, it is possible to obtain a structure composed of fine crystal grains and finely dispersed carbide, and to manufacture a martensitic stainless steel seamless pipe with good anisotropy and toughness,
The present invention has been completed.

That is, in the present invention, C: 0.15-0.25%, Si: 0.10-1.0%, Mn: 0.10-
1.0%, P: 0.03% or less, S: 0.01% or less, Cr: 10-14
%, V: 0.15% or less, and when the martensitic stainless steel seamless pipe having a steel composition with the balance being Fe and inevitable impurities is manufactured by the Mannesmann mandrel mill method, before the final rolling, the reheating temperature (T ° C ) Is heated to a temperature defined by the following formula, and then 950 ℃ ≤ T ≤ 1150 ℃ (1) T ≥ 1.8 R + 930 (2) where T: reheating temperature (℃), R = finishing Rolling degree
(%), Finish rolling, immediately cool to room temperature and quench by air cooling,
This is a method for producing a seamless martensitic stainless steel pipe having a low-temperature toughness, which has a martensitic structure and is tempered at a temperature not higher than the Ac ₁ transformation point to obtain a predetermined strength without re-quenching heat treatment.

[0010]

FIG. 4 is a graph showing the steps of the method according to the present invention as a heat history, and in the case of the present invention, it is sufficient to perform only the tempering process without performing the quenching process as compared with the conventional method.

Next, the reason why the steel composition and the processing conditions are limited as described above in the present invention will be described in detail below together with its action. First, the reasons for limiting the steel composition are as follows. In addition, "%" is% by weight unless otherwise specified.

C: C is an element that enhances the strength of the steel material used in the present invention, but if it is less than 0.15%, δ ferrite precipitates and the hot workability deteriorates. If it exceeds 0.25%, coarse carbides increase and the toughness decreases significantly. Therefore, from 0.15
It is limited to 0.25%, preferably 0.18 to 0.22%.

If Si: Si is less than 0.10%, deoxidation is insufficient. If it exceeds 1.0%, precipitation of carbides at the grain boundaries is promoted and toughness and corrosion resistance deteriorate. It is preferably 0.15 to 0.50%.

If Mn: Mn is less than 0.10%, the strength is not increased. If it exceeds 1.0%, the toughness and corrosion resistance are deteriorated. It is preferably 0.50 to 0.80%.

P, S: P and S are all impurity elements, and the lower the content, the more desirable. In the present invention, 0.
Limit to less than 03% and less than 0.01%. Especially corrosion resistance, toughness,
In order to secure hot workability, P: 0.020% or less, S: 0.0
It is preferable to limit it to 05% or less.

Cr: Cr is necessary for improving the corrosion resistance of stainless steel. If it is less than 10%, the corrosion resistance is deteriorated, and if it exceeds 14%, δ ferrite is formed and the hot workability is deteriorated and martensitic transformation becomes difficult. .

V: V is an element effective in improving strength and toughness, and if it exceeds 0.15%, the strength becomes too high and the toughness decreases.

Next, the reason for limiting the pipe making conditions will be described. Since the structure after finish rolling becomes a structure extending in the rolling direction at a reheating temperature of less than 950 ° C. and a temperature not satisfying the formula (2), The toughness in the circumferential direction is significantly reduced. On the other hand, if the reheating temperature exceeds 1150 ° C, the crystal grains after finish rolling become coarse and the toughness decreases.

The rolling workability of the formula (2) is determined by the area reduction rate. In the present invention, after finishing rolling, predetermined properties can be obtained only by tempering heat treatment. Next, the function and effect of the present invention will be described more specifically based on Examples.

[0020]

Example A billet having a diameter of 150 mm was manufactured from a sample steel having a steel composition shown in Table 1 through a normal melting and rolling process, and processed under the conditions shown in Table 2, a reheating temperature and a final finishing process. Martensitic stainless steel seamless pipes were manufactured while changing the degree.

[0021]

[Table 1]

[0022]

[Table 2]

Specimens were taken from each seamless pipe produced in this manner, and the following tests were conducted. Tensile test: 19 mm wide arc-shaped GL5 0.8 mm tensile test piece 0.
2% proof stress test: 10 × 5 × 55 mm, transition temperature by 2 mm V notch Charpy test piece Hardness test: HRC Fig. 1 shows 0.2% proof stress and HRC hardness measurement result by tensile test, and Fig. 2 shows impact test measurement. The results are shown together for each pipe making condition.

The numbers in the figure are strength, hardness, and transition temperature, respectively. Strength is 80 to 95 ksi, hardness is 23 HRC or less, and transition temperature is rolling direction -20 ° C or less, circumferential direction. Passed 20 ° C or less. Regarding the strength and hardness, in the example of the present invention, tempering is performed at a higher temperature than in the conventional example, but the strength substantially equal to that of the conventional method is obtained under the conditions of the dotted line region or more.

Further, regarding the toughness in the rolling direction, the toughness equal to or higher than that of the conventional method is obtained in the region above the dotted line, which is similar to the strength and hardness. On the other hand, with respect to the toughness in the circumferential direction, it has been found that the toughness equal to or greater than that of the conventional example can be achieved in the region above the solid line.

Then, a stress corrosion cracking resistance test was conducted on the examples of the present invention in the following manner. A C-ring test piece was attached as shown in FIG. 5, stress was applied, and stress corrosion cracking resistance was evaluated. Stress is a stress 0.2% yield strength × 0.80, test conditions _{0.5% CH 3 COOH + 5%} NaCl + 0.1atm H 2 S, 25 ℃, 720Hr
Then, it was judged whether or not cracking occurred.

[0027]

As described above, according to the present invention, the low temperature toughness anisotropy of the martensitic stainless steel seamless pipe exhibiting excellent corrosion resistance not only in the CO ₂ environment but also in the H ₂ S environment is significantly improved. Can be improved.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between 0.2% proof stress and HRC hardness under the pipe making conditions (reheating temperature, finish rolling processability) of the present invention.

FIG. 2 is a graph showing the relationship between pipe making conditions and impact properties (rolling direction, circumferential direction).

[Fig. 3] Conventional method for manufacturing seamless tubes (quenching and tempering)
It is a graph which shows the heat history of.

FIG. 4 is a graph showing heat history in the method for producing a seamless pipe of the present invention.

FIG. 5 is an explanatory diagram of a procedure of a stress corrosion cracking resistance test.

Claims (1)

[Claims] 1. By weight%, C: 0.15 to 0.25%, Si: 0.10 to 1.0%, Mn: 0.10
~ 1.0%, P: 0.03% or less, S: 0.01% or less, Cr: 10 ~
14%, V: 0.15% or less, when producing a seamless pipe of martensitic stainless steel having a steel composition with the balance being Fe and inevitable impurities, the reheating temperature T after rolling by a mandrel mill is 950 ℃ ≤ T ≦ 1150 ℃ T ≧ 1.8 R + 930 However, T: reheating temperature (℃), R: finish rolling degree
(%), Finish rolling is applied so that it is in the area
Then, after cooling to room temperature to form a martensite structure,
A method for producing a martensitic stainless steel seamless pipe having excellent low-temperature toughness, which is characterized by obtaining the required strength only by tempering heat treatment without performing quenching heat treatment again.