JPH01123024A - Manufacture of seamless stainless steel tube - Google Patents

Abstract

PURPOSE:To manufacture a seamless stainless steel tube having superior toughness and resistance to stress corrosion cracking as rolled by successively subjecting a martensitic stainless steel billet to heating up to a specified temp., piercing, rolling, cooling under specified conditions, reheating in two steps, finish rolling and cooling. CONSTITUTION:A martensitic stainless steel billet is heated up to 1,050-1,250 deg.C, pierced, rolled and cooled to a temp. below the martensitic transformation start temp. at >=30 deg.C/min cooling rate in a temp. range to at least 500 deg.C to form a structure contg. >=80vol.% martensite. The resulting tube is reheated in a temp. range below the Ac1 transformation point in which austenite is not practically formed under conditions satisfying an inequality (273+T)X(20-logt)<2,000 [where T is temp. ( deg.C) and t is time (hr)] and the tube is further heated to a temp. above the reheating temp. in the temp. range of the Ac1 transformation point - the Ac1 transformation point - 200 deg.C. The heated tube is immediately finish-rolled at >=5% reduction of area and air-cooled or forced to be cooled to obtain a seamless tube having superior characteristics as rolled as compared with the conventional product.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention has the same strength as rolled products that have been subjected to conventional quenching and tempering treatments, and has superior toughness and stress corrosion cracking resistance. This invention relates to a method of manufacturing martensitic stainless steel seamless pipes that is superior to conventional ones.

(Conventional technology and its problems) In general, martensitic stainless steel seamless pipes are widely used for oil country tubular goods, transportation pipes, etc. that require strength, toughness, and corrosion resistance, and are particularly excellent in stress corrosion resistance. This is well known.

Conventionally, this type of seamless pipe is as illustrated in Fig. 1.
A billet is heated to a temperature that allows perforation, for example, is perforated and rolled using a piercer and a mandrel, then reheated to the temperature of austenite grains, and finished rolled using, for example, a stretch reducer. Manufactured. After finish rolling, the tube is air-cooled to become a martensitic structure, but in order to give it the necessary strength and toughness, it is
A heat treatment of quenching from 0°C and tempering at 600 to 750°C is performed, and the final result is a tempered martensitic composite fiber.

In addition to the Mannesmann mandrel mill method mentioned above, there are various methods for making pipes, such as the Mannesmann plug mill method and the Mannesmann aracel mill method. For manufacturing, quenching and tempering treatments are essential after pipe making.

Martensitic stainless steel seamless pipes manufactured by the above conventional method have high strength, but in recent years
In increasingly harsh usage environments, toughness and stress corrosion cracking resistance may be insufficient. In other words, an environment containing C08 often also contains H and S, and seamless martensitic stainless steel pipes manufactured by conventional methods are highly susceptible to sulfide stress corrosion cracking, so their use is currently restricted. In an environment with a high Has concentration, it is necessary to use a high-cost, high-cost alloy with significantly higher alloying elements such as Cr, Ni, and Mo than ordinary martensitic stainless steel.

The object of the present invention is to use rolling martensitic stainless steel or its improved stainless steel without unnecessarily increasing the amount of expensive alloying elements, and without additional heat treatment after tube manufacturing. It is an object of the present invention to provide a method for manufacturing a martensitic stainless steel seamless tube which is superior to that by conventional manufacturing methods.

(Means for solving the problem) In general, what controls the toughness and stress corrosion cracking resistance of steel with a martensitic structure is the size of the blocks and packets that are the substructures of the martensitic structure. When the particle size of prior austenite is reduced, the size of this block or packet becomes smaller, improving various properties such as toughness and stress corrosion cracking resistance. However, in martensitic stainless steel, the solid solution temperature of precipitated carbides is relatively high, and the conventional method requires a high quenching temperature, which inevitably coarsens the austenite crystal grains. There is a limit to reducing the austenite grain size.

As a result of detailed studies on the processing heat treatment of martensitic stainless steel and its structure, the present inventor found that - when pre-quenched steel is tempered and then warm-processed, it becomes significantly finer than the units of blocks and packets. It was found that a ferrite structure could be obtained. Based on this knowledge, when manufacturing seamless pipes, we separately quenched and
Seamless martensitic stainless steel with no tempering treatment, i.e., as rolled, with the same strength as conventional manufacturing methods, but far superior in toughness and stress corrosion cracking resistance. It was confirmed that the tube could be manufactured.

Here, the gist of the present invention is to provide a method for manufacturing a martensitic stainless steel seamless pipe with excellent toughness and stress corrosion cracking resistance, which is characterized by sequentially processing and heat treating a martensitic stainless steel piece through the following steps. ,It is in.

[1] A step of heating the piece to 1050-1250°C, perforating and rolling it, and cooling it to at least 500°C at a cooling rate of 30°C for 7 minutes or more to a temperature below the martensitic transformation start temperature to 80% by volume. A process of forming a structure in which the above is dominated by martensite, ■ A process of reheating in Ac where substantially no austenite is generated, in a temperature range below the transformation point, and under conditions that satisfy the following formula, (273+T) 20+logt)<2
0000, where T is temperature (°C) and t is time (hours).

■After heating to above the reheating temperature in [3] in the temperature range of Ac+transformation point to (Ac, transformation point -200℃), immediately perform finish rolling with an area reduction rate of 5% or more, and then air cool or forced cooling. Process.

The present invention can be applied to all methods of manufacturing seamless pipes in which a steel billet is hot-pierced and rolled, including the various Mannesmann pipe manufacturing methods listed above.

Furthermore, the martensitic stainless steel that is the object of the present invention includes those that are well known to those skilled in the art, and those that have been improved by adding certain elements or reducing impurities. All stainless steels have a tempered martensitic microstructure. Hereinafter, the standard composition of martensitic stainless steel desirable as a subject of the present invention will be illustrated, and the reason for selecting the content will be explained. Note that all percentages regarding the content of elements are percentages by weight.

Cr: 8-15% Cr content is required to be 8% or more in order to maintain corrosion resistance as stainless steel. However, if it exceeds 15%, the Ferrai H5 region expands at high temperatures, making martensitic transformation difficult during subsequent cooling.

C: 0.4% or less C is an element related to the strength of martensitic stainless steel, but if the content exceeds 0.4%, coarse carbides increase and the toughness is significantly impaired.

Si: 0.01-1% Si is added as a deoxidizer and a reinforcing element.

If the content is less than 0.01%, there will be no such effect; if the content exceeds 1%, it will promote the formation of grain boundary carbides,
Deteriorates toughness and corrosion resistance. In particular, in order to improve toughness and corrosion resistance, it is preferable to suppress the upper limit to 0.2%.

An: 0.05-2% Mn improves strength and toughness, but if it is less than 0.05%, it has no effect, and if it exceeds 2%, it deteriorates the toughness.

s: o, o3% or less S is an impurity element, and the lower the content, the more desirable it is.
If it is too high, the amount of sulfides increases, impairing toughness and stress corrosion cracking resistance. Although 0.03% is the permissible upper limit, if it is suppressed to 0.001% or less, the stress corrosion cracking resistance is significantly improved.

p: o, x% or less P is also an impurity element like S, and the lower the content, the more desirable it is; if it is too high, the toughness and corrosion resistance will deteriorate. Although 0.1% is the allowable upper limit, suppressing it to 0.01% or less is effective in improving toughness and corrosion resistance, and also reduces the anisotropy of these properties.

The most desirable thing is to suppress P to 0.01% or less and S to 0.001% or less.

5o11. Al: 0.005 to 0.1%^l is added to deoxidize molten steel. 5offi, 0.0 as Al
It is necessary to add so that the content is 0.05% or more, but if the content exceeds 0.1%, oxide inclusions will increase and the toughness and corrosion resistance will deteriorate.

In addition to the above components, the standard composition is one in which the balance consists of Fe and unavoidable impurities. In addition to this, the following
One or more elements selected from one or both of the group and the second group may be included.

Elements of the first group 2.0% or less 10, 5% or less Ni, 0.5% or less Nb, 0.5% or less ■, 0.5% or less Ti, 0
．． 5% or less Zr, 0.01% or less B, and 0.1
N less than 5%.

Elements of the second group 0.001-0.05% Ca, 0.001-0.0
5% La, and 0.001-0.05% Ce.

The effects of these elements are as follows.

MO: Effective in improving corrosion resistance. However, if the content exceeds 2%, martensitic transformation during cooling becomes difficult.

Ni: In addition to improving corrosion resistance, it has the effect of thickening (improving) strength and toughness in combination with the effect of suppressing C content. However, even if it is contained in excess of 5%, the effect will not increase and costs will increase. Just invite.

Nb, V, Ti, Zr: These elements have the effect of improving strength and toughness, and at the same time have the effect of preventing a decrease in Cr in the matrix, which is effective for corrosion resistance. However, if each content exceeds 0.5%, the toughness will deteriorate.

B: It is effective in improving strength and promotes finer structure,
It also has the effect of improving toughness and corrosion resistance. However, if the content exceeds 0.01%, the toughness and corrosion resistance will be adversely affected.

N: N is an inexpensive element that improves strength, but the content is 0.
．． If it exceeds 15%, a significant decrease in toughness will result.

Ca, La, Ce: These elements improve the shape of sulfides in steel and improve stress corrosion cracking resistance. If the content is less than 0.001%, the effect cannot be obtained, and if it exceeds 0.05%, the toughness and corrosion resistance will deteriorate.

Next, the process of heat treatment will be explained along with one process diagram of the method of the present invention illustrated in FIG.

(a) Steel billet heating temperature This heating requires sufficient temperature and time to uniformly heat the steel billet to the center and remove micro-segregation before performing the next step of drilling and rolling.

If the heating temperature is lower than 1050° C., the deformation resistance in the next step will increase, which is not preferable. On the other hand, heating at a temperature higher than 1250° C. not only causes significant scale formation, resulting in a decrease in yield and rough surface, but also facilitates the formation of δ-ferrite, resulting in a decrease in pipe-making performance.

The heating time is determined by the size of the steel piece, but it is set to be a time necessary and sufficient to uniformly heat the steel piece to the center as described above.

(b) Perforation and rolling The perforation with a piercer and rolling with a mandrel mill or plug mill are carried out in a conventional manner. The piercer may be of an inclined rolling type or a press piercing type.

In this step, care must be taken to ensure that the rolling end temperature does not become too low. When rolling is performed in a low-temperature non-recrystallized region, a large amount of strain remains at the grain boundaries, which promotes the precipitation of coarse grain boundary carbides during cooling.

Grain boundary carbides have a negative effect on the properties of the seamless pipe product, particularly on the toughness.For this reason, it is desirable to finish rolling at a temperature of 900°C or higher, preferably 940°C or higher.

(c) Cooling conditions Cooling conditions after rolling are extremely important. This cooling causes martensitic transformation of 80% by volume or more, preferably 95% to improve toughness and stress corrosion cracking resistance.
A homogeneous structure in which more than % of the volume is occupied by martensite (
The rest is ferrite and/or retained austenite)
Select so that That is, the cooling end temperature is set to be below the Ms point and at a temperature at which martensite is transformed to 80% by volume or more, preferably 95% by volume or more. However, it is necessary to cool the steel as quickly as possible to 500° C., where carbides tend to precipitate. i.e. at least 30°C up to 500°C
The cooling rate shall be at least 1/min. If the cooling rate is slower than 30° C./min, coarse grain boundary carbides will precipitate, which will cause a decrease in toughness. The greater the cooling, the better the toughness, so rapid cooling, such as water cooling, is performed.

(d) Reheating temperature There are no particular restrictions on the above-mentioned warm working of tempered martensite as long as the temperature is below the Ae+ transformation point.

However, from an industrial perspective, there is also the problem of mill power, so ^c1
It is desirable to process at a temperature as high as possible below the transformation point.

However, although tempering at a high temperature reduces the deformation resistance during subsequent processing, the precipitated carbides become coarser and the effect of improving toughness and stress corrosion cracking resistance is diminished. Therefore, in the present invention, the reheating equivalent to tempering is performed in two stages, and in the -th stage, the reheating temperature is kept low to disperse fine precipitated carbides, and then the reheating is performed in a short period of time to prevent the carbides from becoming coarse. The temperature is raised to near the c1 transformation point, and rolling is performed with the deformation resistance lowered.

Since the purpose of the first stage of reheating is to precipitate fine carbides as described above, it is important to appropriately select the temperature and time. Based on numerous experimental results, the present inventors have confirmed that a desired finely dispersed state of carbides can be obtained if the temperature T (° C.) and time (hours) satisfy the following equation.

(273+T) X (20+ log t) < 2
0000 If this formula is not satisfied, the carbide becomes coarse and good toughness cannot be obtained.

The second stage of reheating is performed in order to reduce the deformation resistance of the subsequent finish rolling without coarsening the precipitated carbides.
Therefore, in principle, finish rolling is carried out immediately after heating to a higher temperature than the first stage, but in some cases where the power of the mill is sufficient and there is no need to heat to a higher temperature than the first stage. May be the same as the temperature of one step, provided that A
Since the deformation resistance is too large at a temperature lower than c+transformation point -200°C, the lower limit of the heating temperature in the second stage is Ae, which is the transformation point -200°C. In addition, both the first and second stages
If the heating temperature exceeds the Ac+ transformation point, austenite will form and the desired toughness and stress corrosion cracking resistance cannot be ensured, so the upper limit for both is set to the C1 transformation point.

(e) finish rolling Immediately after the second stage of reheating, finish rolling is performed;
Finish rolling is performed, for example, with a stretch reducer, but may also be performed with a sizer, reeler, or the like.

The processing rate in finish rolling is also important. The rolling process advances microscopic recrystallization of ferrite and fine dispersion of precipitated carbides, resulting in the above-mentioned excellent properties.
For this purpose, processing with a reduction in area of 5% or more is required. Note that the area reduction rate K (%) is defined by the following equation (a).

K"(1-(r!z-r+")/(R1!-R+")
)) X100... (a) Here, R1 and R2 are the inner radius and outer radius r before finish rolling.
1 and r are the inner radius and outer radius after finish rolling.

Cooling after finish rolling may be performed by air cooling, but the above characteristics are further improved if forced cooling such as water cooling is performed.

The martensitic stainless steel seamless tube manufactured in this way has a tempered martensitic structure macroscopically, and microscopically has a structure in which ferrite crystal grains are extremely fine and precipitated carbides are finely dispersed. As rolled, it has excellent toughness, stress corrosion cracking resistance, and especially sulfide stress corrosion cracking resistance.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

(Example) From steel with the composition shown in Table 1, 100
KakurinφX300++nj! of steel billets were manufactured. Using these steel pieces, martensitic stainless steel seamless pipes were manufactured under the conditions shown in Table 2.

For the purpose of measuring the 0.2% yield strength and tensile strength of these steel pipes, and evaluating the toughness, 5 mm x 10 mg +
A Charpy impact test was conducted using a 2 mm V-notch test piece of X55 to measure the Charpy fracture surface transition temperature.

Furthermore, for the purpose of evaluating the stress corrosion cracking resistance, a shell type test was performed, that is, a horizontal three-point bending test piece was saturated with H2S at a temperature of 20°C, 1 atm, and 1 atm, with different loads applied to the center point. The specimens were immersed in a 0.5% acetic acid aqueous solution for 500 hours to observe the occurrence of cracks, and the Sc value, which is an index of resistance to sulfide stress corrosion cracking, was determined.

The above measurement results are summarized in Table 2.

First, when comparing the test results of N [11 to 43 of the present invention method in Table 2 with the test results of N [1 to 21] of the conventional method (quenching and tempering treatment after pipe manufacturing), it is found that 0.2% The yield strength and tensile strength are almost the same, but the fracture surface transition temperature and Sc value are far superior to those obtained by the method of the present invention. Note that the comparative method Nal~4 is similar to the method of the present invention in that no separate quenching and tempering treatment is performed, but any of the drilling, cooling conditions after rolling, reheating conditions, and finish rolling conditions are the same as the present invention. This is an example where the condition is not satisfied. in this case,
Toughness and stress corrosion cracking resistance are extremely poor.

(Effects of the Invention) The present invention takes advantage of the metallurgical properties of martensitic stainless steel, precisely adjusts the processing and cooling conditions, and creates a seamless product that far exceeds that of conventional products even when rolled. This made it possible to manufacture pipes.

The steel pipe manufactured by the method of the present invention has a tempered martensitic structure as rolled, and its crystal grains and dispersed carbides are extremely fine, so it is superior to conventional products, especially in terms of toughness and resistance to sulfide stress corrosion cracking. This greatly contributes to the expansion of the fields of use of martensitic stainless steel seamless pipes.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating a conventional process for manufacturing a martensitic stainless steel seamless tube, and FIG. 2 is a diagram illustrating the present invention in detail.

Claims (4)

[Claims] (1) A method for manufacturing a martensitic stainless steel seamless pipe with excellent toughness and stress corrosion cracking resistance, which comprises sequentially processing and heat treating a martensitic stainless steel piece in the following steps. [1] The step of heating the piece to 1050 to 1250°C, drilling and rolling, [2] Cooling the piece to at least 500°C at a cooling rate of 30°C/min or more to a temperature below the martensitic transformation start temperature and 80°C. A step of forming a structure in which at least % by volume is occupied by martensite, [3] A step of reheating in a temperature range below the Ac_1 transformation point where no austenite is substantially formed, and under conditions that satisfy the following formula, (273+T )×(20+logt)<20000 where T is temperature (° C.) and t is time (hours). [4] Ac_1 transformation point ~ (Ac_1 transformation point -200℃)
After heating to the reheating temperature in [3] or above in the temperature range, immediately perform finish rolling with a reduction in area of 5% or more, and then air cooling or forced cooling. (2) The method for manufacturing a seamless pipe according to claim 1, wherein the martensitic stainless steel has a normal chemical composition. (3) The method for manufacturing a seamless pipe according to claim 1, wherein the martensitic stainless steel contains one or more of the following first and/or second group elements. The first group weight% is 2.0% or less Mo, 5% or less Ni, 0.
5% or less Nb, 0.5% or less V, 0.5% or less T
i, 0.5% or less Zr, 0.01% or less B, and 0.15% or less N. 2nd group wt% 0.001-0.05% Ca, 0.001
~0.05% La, and 0.001-0.05% Ce. (4) Any one of claims 1 to 3, wherein the martensitic stainless steel has one or both of the impurity elements P and S reduced to the following ranges. A method for manufacturing seamless pipes. P: 0.01% by weight or less S: 0.001% by weight or less