JPS6160894B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing steel pipes with excellent sulfide stress corrosion cracking resistance, which are suitably used in oil fields, natural gas fields, pipelines for transporting fossil resources thereof, and the like. As is well known, steel pipes used in oil fields, natural gas fields, or in the transportation of fossil resources, such as oil country tubular goods or oil transmission pipes, are exposed to acid environments, especially sulfide atmospheres, and are at high risk of stress corrosion cracking. The steel used for this type of application must have excellent resistance to sulfide stress corrosion cracking. Conventionally, methods for imparting sulfide stress corrosion cracking resistance to steel materials include increasing the amount of C in the steel to 0.40% or more, adding alloying elements, especially large amounts of Mo and V, and heating the steel to high temperatures after quenching. A method is known in which the hardness of the steel material is adjusted to H _rc 23 or less by sufficiently spheroidizing the carbides after long-term tempering. However, this method has the problem that if the amount of C is increased, quenching cracks and quenching distortions are likely to occur during heat treatment.
If large amounts of alloying elements such as Mo and V are added, a problem arises in that the manufacturing cost increases significantly because these alloying elements are expensive. Furthermore, the long-time tempering in the above method usually lasts for one hour, which poses a problem of lowering the productivity of the steel material. Furthermore, the above method has a fatal drawback in that the strength of the sulfide corrosion cracking-resistant steel that can be manufactured is at most H _Rc 23 or less, that is, about 80 kg/mm ² or less in terms of tensile strength. Therefore, when higher strength is required, the reality is that expensive high-alloy steel, especially stainless steel, has to be used. In view of the above-mentioned circumstances, there is a strong demand for the development of a method for manufacturing steel pipes that have excellent resistance to sulfide stress corrosion cracking and excellent strength using inexpensive ordinary steel materials. By the way, the present inventors have developed a method for manufacturing seamless steel pipes in which a steel billet is first hot-worked to be pierced and rolled, then finished to adjust the outside diameter through a second hot-worked process, and then cooled. We are currently conducting research and development on a method for immediately quenching (direct quenching). The manufacturing method of seamless steel pipes using such direct quenching will be explained in more detail with reference to FIG.
It is heated to a temperature of about 30°F or higher, and then hot worked in a series of piercing and rolling processes 3 using a piercer, elongator, plug mill, reeler mill, etc. to a cross-sectional dimension close to that of the final product, and then processed using a walking beam furnace, etc. Charged to reheating furnace 4 and heated to 900℃
The reheated steel material (raw pipe) is reheated to a certain degree.
In this method, the material is finished to a predetermined outer diameter by a finish rolling step 5 using a sizer mill or a stretch reducer, and then immediately hardened by a hardening device 6. In this method, the process from heating the steel billet 1 to quenching is performed in one continuous process.
If the quenching device 6, which is the final stage of the continuous process, stops for some reason, the steel material that has been processed and sent from the upstream side is discharged from the system at a stage close to the quenching device 6 and temporarily stored. It is necessary to keep it. In this case, generally, the steel material is discharged from the system and stored at the stage where the finish rolling process 5 of the sizer mill, etc. is completed, that is, at the stage where it has been processed to the dimensions and shape of the final product, and then, as the quenching equipment is restored, the steel material is stored. It is considered normal to then reheat and harden the stocked steel material. However, in this case, the product is not so-called direct quenching, which is quenching immediately after hot working, but normal quenching, so the quality of the product obtained is inferior to that of direct quenching.For example, when the same tempering conditions are used, Tensile strength is significantly inferior. Under these circumstances, the inventors of the present invention have proposed that when the quenching device 6 is stopped as described above, immediately before finishing rolling using a sizer mill or the like,
That is, we considered sending the steel discharged from the reheating furnace 4 as it is to the cooling bed 7 side and storing it there. With the restoration of the quenching equipment, an experiment was carried out in which the stocked steel was reheated in the reheating furnace 4, then slightly hot rolled in a finish rolling process 5 such as a sizer mill, and immediately quenched. As a result, they discovered that it is possible to obtain a steel pipe that not only has excellent strength but also excellent resistance to sulfide stress corrosion cracking. After further repeated experiments, we found that by adjusting the processing ratio during hot rolling (finish rolling) immediately before quenching, it was possible to obtain a steel material with excellent stress corrosion cracking resistance even with normal composition.
In other words, it was discovered that the above requirements could be satisfied,
This led to this invention. In other words, in the method for producing a sulfide stress corrosion cracking-resistant steel pipe of the present invention, for example, the steel pipe is discharged from the system and stored immediately before the finish rolling process such as a sizer mill in the series of seamless steel pipe manufacturing processes as described above, and the Ar ₁ transformation point is reached. A steel pipe that has already been processed to a cross-sectional size and shape close to the cross-sectional size and shape of the final product, such as the cooled steel pipe described below, is heated, hot-rolled at a processing ratio within a specific range, and immediately quenched directly. It is something. More specifically, the method of this invention provides C 0.15
~0.40%, Si 0.1~1.0%, Mn 0.4~2.0%, Al
0.01 to 0.10%, the remainder substantially consisting of Fe and unavoidable impurities, and has been pre-processed to a cross-sectional size and shape close to that of the product, and is Ar ₁
A steel pipe that has been cooled to a temperature below its transformation point,
By heating to a temperature above the Ac ₃ transformation point and below the austenite grain coarsening start temperature, the cross-sectional shrinkage ratio R is
It is characterized by hot rolling with a sizer or stretch reducer to a thickness of 0.015 or higher, immediately quenching, and then tempering at a temperature below the Ac ₁ transformation point. However, here, the cross-sectional shrinkage ratio R is the area shrinkage rate of the cross section perpendicular to the main rolling direction before and after hot rolling, that is, more precisely, the area of the cross section before hot rolling is S ₁ , If the subsequent area is _S2 , it is expressed by R=1- _S2 / _S1 . The method of the present invention will be explained in more detail below. As mentioned above, the composition range of the steel material targeted by this invention is C 0.15-0.40%, Si 0.1-1.0%, Mn
0.4~2.0%, Al 0.01~0.10℃, balance essentially Fe
and unavoidable impurities, and the reason for limiting these components is as follows. If C is less than 0.15%, the strength will be insufficient, and it will be difficult to secure a martensite ratio of 90% or more, which is necessary to improve sulfide stress corrosion cracking resistance. Cracks and sintering distortions are more likely to occur. Si is added for the purpose of deoxidizing and increasing strength, but for this purpose 0.1% or more is required;
If Si exceeds 1.0%, toughness decreases rapidly. It is necessary to add Mn in an amount of 0.4% or more for the purpose of improving strength and toughness, but if it exceeds 2.0%, segregation and quench cracking are likely to occur. Al is added for the purpose of deoxidation and to combine with N in steel to refine the grains, but for this purpose, it needs to be at least 0.010%, while if it exceeds 0.10%, its effect will be saturated. . In addition, for the purpose of further improving sulfide stress corrosion cracking resistance, Cu 0.05~0.5%, Cr
0.05~2.5%, Mo 0.05~1.5%, Nb 0.01~0.1
%, V 0.01-0.2%, Ti 0.005-0.1%, B
0.0005~0.005%, Ca 0.002~0.005%, REM
1 or 2 selected from 0.005-0.05%
More than one species may be added depending on the purpose and the like. The manufacturing method of this invention uses steel having the above-mentioned composition range as a material, but the material used in the manufacturing method of this invention has been processed in advance to a shape and size close to the cross section of the product, and is Ar ₁ This is a raw tube that is cooled to a temperature below its transformation point. For example, in a series of manufacturing processes for seamless steel pipes as shown in _FIG . A raw pipe that has already been subjected to primary hot working such as rolling process 3, or a raw pipe that has been discharged from the system and cooled to below the Ar ₁ transformation point just before the quenching device 6 in Fig. 1, that is, a raw pipe that has been subjected to piercing rolling. The primary hot processing such as step 3 and the secondary hot rolling such as finish rolling step 5 have already been performed, and the cross-sectional shape and dimensions of the product as originally planned (however, in the present invention, some additional hot rolling may be performed) This applies to raw pipes that have been processed to have a cross-sectional shape and dimensions (different from the cross-sectional shape and dimensions of the product in the manufacturing method of the present invention). Here, the reason for using a raw tube that has been cooled to below the Ar ₁ transformation point is as follows.
The purpose is to make crystal grains finer by causing ferrite to austenite transformation during austenitizing heating performed before hot rolling. As will be described later, making crystal grains finer is effective in improving stress corrosion cracking resistance. In the method of the present invention, such a raw pipe is first heated to a temperature above the Ac ₃ transformation point and below the austenite crystal grain coarsening starting temperature. This heating requires a temperature above the Ac ₃ transformation point in order to uniformly transform the steel into austenite and to sufficiently dissolve the alloying elements in the steel.
In order to refine the austenite crystal grains after hot rolling, it is desirable to heat the austenite crystal grains to a temperature as low as possible in the austenite temperature range, and the temperature is at least below the austenite crystal grain coarsening temperature. The material heated in this way is immediately processed using a sizer or stretch reducer according to the shape and dimensions of the product to be obtained, to determine the cross-sectional shrinkage ratio, that is, the area S 1 before rolling and the area S ₁ after rolling in a cross section perpendicular to the main rolling direction. The value of (1-S ₂ /S ₁ ) determined by the area S ₂ of
Hot rolled to a value of 0.015 or higher. The present inventors found through experiments that this cross-sectional shrinkage ratio is appropriate, as shown in the examples described later, and by setting the cross-sectional shrinkage ratio to 0.015 or more, the steel material after quenching and tempering can be improved. has excellent sulfide stress corrosion cracking resistance and a low cross-sectional shrinkage ratio.
If it is less than 0.015, good sulfide corrosion cracking resistance cannot be obtained. The upper limit of the cross-sectional shrinkage ratio in hot rolling is not particularly limited, but as shown in the examples, the effect of sulfide stress corrosion cracking resistance depends on the cross-sectional shrinkage ratio.
It is saturated at about 0.025 to 0.030, and the effect does not increase even if the cross-sectional shrinkage ratio is increased beyond that, and in the first place, the material targeted by this invention is one that has been processed in advance to have a cross-sectional shape and dimensions similar to those of the product. Therefore, the cross-sectional shrinkage ratio is usually about 0.10 or less. Furthermore,
In this hot rolling, it is desirable to obtain a cross-sectional shrinkage ratio of 0.015 or more by applying compressive load as much as possible at once. It is desirable to roll the sheet to a cross-sectional shrinkage ratio of 0.015 or more. When rolling with 4 passes or more, the cross-sectional shrinkage ratio of the entire hot rolling process is
Even if it is 0.015 or more, the cross-sectional shrinkage ratio per pass becomes extremely small, and there is a possibility that sufficient sulfide stress corrosion cracking resistance cannot be obtained. The steel pipe hot rolled as described above is immediately water quenched. That is, it is hardened before being cooled to a critical temperature. It is preferable that this quenching method employs a method of cooling by flowing a water stream along the longitudinal direction on both the inner and outer surfaces, that is, an axial flow, but the method is not limited to this. This quenching is
Of course, this means rapid cooling to below the Ar ₁ transformation point, as in normal metallurgical heat treatment. After quenching, it is tempered to below the Ac ₁ transformation point. This tempering is usually preferably carried out at a temperature of 620°C or higher, and after tempering, it is rapidly cooled in a conventional manner. In this way, after hot rolling,
By quenching and tempering, a steel pipe, which is the final product of the method of this invention, is obtained. In addition, when using raw pipe discharged from the middle of the seamless steel pipe manufacturing line shown in FIG. 1 as a raw material in the manufacturing method of this invention, the walking beam furnace, etc. shown in FIG. It is desirable to use the reheating furnace 4 for heating before hot rolling in the manufacturing method of the present invention.
That is, for example, if the material is a raw tube that has been discharged and stored from the reheating furnace 4 to the cooling bed 7 side and cooled to below the Ar ₁ transformation point as shown by the broken line in FIG. As shown by the solid line, the cooled raw tube is reinserted into the reheating furnace 4 and heated to a temperature above the Ac ₃ transformation point and below the austenite grain coarsening starting temperature, and then finished using a sizer mill, stretch reducer, etc. In the rolling step 5, the material may be hot rolled to a cross-sectional shrinkage ratio of 0.015 or more, and then quenched and tempered. For example, as shown by the broken line in FIG.
Even when using steel pipes that have been discharged to and cooled as raw materials, the steel pipes are re-charged into the reheating furnace 4 as shown by the solid line in Fig. 3, and the cross-sectional shrinkage ratio is 0.015 or more in the finish rolling process 5 again. Hot rolled to give
It may be quenched and tempered in the same manner as above. The steel pipe obtained as described above has excellent resistance to sulfide stress corrosion cracking. The reason may be as follows. That is, the heating for hot rolling is carried out at a low temperature below the austenite grain coarsening starting temperature (however, Ac ₃ points or higher), and
Because hot rolling is performed with a relatively large cross-sectional shrinkage ratio of 0.015 or more, the austenite crystal grains after hot rolling (before quenching) are significantly refined, and the crystal grains after quenching and tempering are also refined. The first reason for this is that the propagation of cracks is inhibited, and sulfide stress corrosion cracking is caused by hydrogen penetrating into steel materials in an acidic corrosion environment, which is trapped especially in sulfide-based nonmetallic inclusions. is thought to occur and lead to cracking, but the cross-sectional shrinkage ratio
By hot rolling at a large processing rate of 0.015 or more, the nonmetallic inclusions are expanded and further divided, and the sites where hydrogen is captured are dispersed, and as a result, the hydrogen is dispersed. The reason for Fig. 2 is thought to be that cracks are less likely to occur due to adsorption. In addition, from the viewpoint of grain size alone, in order to obtain excellent sulfide stress corrosion cracking resistance, it is desirable that the austenite grain size (JIS) after hot rolling is 5.0 or more. If the amount of additive elements that refine the crystal grains added to the chemical composition of the steel material is small, the cross-sectional shrinkage ratio during hot rolling will increase.On the other hand, if the amount of those alloying elements added is large, the cross-sectional shrinkage ratio during hot rolling will increase. It may be better to make it smaller. However, according to experiments conducted by the present inventors, when comparing the case where the grain size is obtained by increasing the cross-sectional shrinkage ratio and the case where it is obtained by increasing the amount of alloying elements added, the former is better. It has been confirmed that this material exhibits good resistance to sulfide stress corrosion cracking. The reason for this is thought to be that, as mentioned above, the improvement in sulfide stress corrosion cracking resistance is not simply due to the grain size, but is largely influenced by the fragmentation of inclusions due to hot rolling. Examples of this invention are described below. Example Three types of steel materials A to C shown in Table 1 were pre-hot worked to a diameter of 50 to 130 mm and a wall thickness of 6 to 15 mm.
This material is processed into a raw material tube, and after being thoroughly heated to 890-925℃ using a walking beam furnace, it is processed into various cross-sectional shrinkage ratios in 1-3 passes using a slotted hot rolling mill. It was hot rolled. Next, the steel materials were immediately water quenched using an axial flow hardening device on both the inside and outside surfaces, and then tempered to approximately 620℃ to 680℃ to increase the yield strength (σy) of each steel material to 80Kgf/mm ²
Aligned.

[Table] For each steel material obtained in the above example,
A sulfide stress corrosion cracking test was conducted in accordance with the NACE method. In other words, the diameter taken from the center of the steel material
A 2.54 mm round bar smooth specimen was immersed in a hydrogen sulfide saturated aqueous solution prepared by adding 50 g of sodium chloride and 5 g of glacial acetic acid to 945 g of water.
After 30 days at room temperature and normal pressure with a tensile stress of 50 to 100% applied, the presence or absence of cracks was observed. Sulfide stress corrosion cracking resistance was evaluated as σth/σy, with the highest stress at which cracking does not occur as the critical stress (σth). The values are shown in FIG. 4 in correspondence with each cross-sectional shrinkage ratio R of the steel material of each component. If the value of sulfide stress corrosion cracking resistance σth/σy is 0.75 or more, it can be judged that the sulfide stress corrosion cracking resistance is good, but as is clear from Fig. 4, the cross-sectional shrinkage ratio R is
The value of σth/σy increased rapidly in the range from 0.010 to 0.020, and if the cross-sectional shrinkage ratio R was 0.015 or more, it was possible to secure σth/σy of 0.75 or more for steel materials of any composition. As another example, steels A, B, and C having the composition shown in Table 1 above are processed into raw tubes in the same manner as above and cooled to below the Ar ₁ transformation point. After heating the material steel pipe, it was hot rolled using a sizer or stretch reducer, and then quenched and tempered.
The specific conditions are shown in Table 2. Furthermore, the yield strength and sulfide stress corrosion cracking resistance (σ _th /σ _y ) of the obtained steel pipes were investigated in the same manner as described above, and the results are also shown in Table 2. Comparative examples include an example where the heating temperature before hot rolling is too high (No. 6), and an example where the cross-sectional shrinkage ratio R in the final hot rolling is less than 0.015 (No. 7, No. 8, No. 8). 11) Example of not cooling to below the Ar ₁ transformation point before hot rolling with a stretch reducer or sizer (No. 9, No. 10: However, No. 10
is also an example in which the cross-sectional shrinkage ratio R in the final hot rolling is less than 0.015) is also shown in Table 2. From Table 2, in the case of the method of this invention (No. 1
It is clear that all of No. 5) have excellent sulfide stress corrosion cracking resistance.

【table】

[Table] As is clear from the above description, according to the manufacturing method of the present invention, a steel pipe with excellent resistance to sulfide stress corrosion cracking can be simply and easily manufactured using inexpensive steel with ordinary components. Therefore, a sulfide stress corrosion cracking resistant steel material suitable for oil country tubular goods, oil transmission pipes, etc. can be provided at low cost.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of the manufacturing process of seamless steel pipes as the background of this invention, and FIGS. 2 and 3 are block diagrams showing the series in which the method of this invention is applied to the manufacturing process of seamless steel pipes. FIG. 4 is a graph showing the relationship between cross-sectional shrinkage ratio R and sulfide stress corrosion cracking resistance (σth/σy) in an example of the present invention.

Claims (1)

[Claims] 1 C0.15 to 0.40% (weight%, same hereinafter), Si0.1
~1.0%, Mn0.4~2.0%, Al0.01~0.10%, the remainder essentially consisting of Fe and unavoidable impurities, and has been processed in advance to have a cross-sectional shape and dimensions close to the cross-sectional shape and dimensions of the product, and Ar ₁ A steel pipe that has been cooled to a temperature below the transformation point is used as a material, and the material is heated to a temperature above the Ac ₃ transformation point and below the austenite grain coarsening start temperature, and the cross-sectional shrinkage ratio R determined by the following equation (1) is calculated. Excellent sulfide stress corrosion cracking resistance characterized by hot rolling with a sizer or stretch reducer so that _the Ac method for manufacturing steel pipes. R=1- _S2 / _S1 ...(1) However, _S1 is the area of the cross section perpendicular to the main rolling direction before hot rolling, and _S2 is the area perpendicular to the main rolling direction after hot rolling. Represents the area of the cross section. 2 The cross-sectional shrinkage ratio R in the hot rolling is 0.015.
2. The manufacturing method according to claim 1, wherein the manufacturing method is rolled to a temperature of 0.10 or less.