JP3589066B2 - Manufacturing method of high strength and high toughness seamless steel pipe - Google Patents

Description

【００４２】
【表２】

【００４３】
表２の結果から明らかなように、発明例の試験１〜９では、最終圧延での仕上げ温度が１０５０℃以上で、圧延加工度を４０％以下として圧延を実施し、次いで保熱を９３０℃以上にて行った後、直ちに直接焼入れ＋焼戻し処理にて製品鋼管を製造している。このため、焼入前のオーステナイト結晶粒を粗大の状態で保つことができ、充分な焼入れ性が確保できる。また、素材鋼である鋼Ａ〜ＥはＰ、Ｓの低減により、粗大な結晶粒であっても、低温靱性の著しく改善している。したがって、高強度で、かつ高靱性と言える油井用鋼管の目安となる、ＹＳ：７００Ｍｐａ以上、およびｖＴｒｓ：−５０℃を、いずれの発明例も達成している。
【００４４】
これに対し、比較例の試験１０〜１２は、本発明で規定する素材鋼を用いたが、圧延仕上げ温度、最終圧延加工度、または保熱温度の何れかが本発明の規定範囲から外れているため、靱性劣化が著しい。
【００４５】
比較例の試験１３〜１９は、最終圧延および直接焼入れの条件は本発明の規定を具備するものであるが、素材鋼の化学組成のいずれかが本発明の規定範囲を外れるものであるから、製品鋼管での靱性劣化が著しい。特に、試験１７では焼入れ性を確保するために最小限必要なＣ含有量が不足しているため、必要な強度も確保することができなかった。
【００４６】
【発明の効果】
本発明の高強度高靱性継目無鋼管の製造方法によれば、焼入れ性向上に有効であるが高価な合金元素を添加することなく、高強度で、かつ高靱性の特性を兼備する油井用鋼管を製造することができる。しかも、製管連続プロセスを前提とする「直接焼入れ法」による処理であるため、均一で優れた特性を有する継目無鋼管を高い生産効率で製造することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a seamless steel pipe used as a steel pipe for oil wells, and more particularly, to a method for manufacturing a seamless steel pipe having both high strength and high toughness required as properties for oil wells with high production efficiency. How to do it.
[0002]
[Prior art]
In recent years, seamless steel pipes for oil wells used for exploring natural gas fields and crude oil fields with a depth of several thousand meters and producing natural gas and crude oil have secured safety and workability. For this reason, it is increasingly required to satisfy the characteristics of high strength and high toughness.
[0003]
Conventionally, such a seamless steel pipe is provided with a quenching device and a tempering device separately from a rolling line, and re-heats a steel pipe manufactured in a rolling line and once cooled to room temperature to perform a quenching-tempering process. (Hereinafter, this method is referred to as “reheating-quenching method”). And, as a steel pipe raw material applied to this manufacturing method, an expensive alloy element such as Mo is added to secure the strength of the steel pipe and to design a component that combines necessary properties.
[0004]
For example, Japanese Patent Publication No. 2-25969 discloses a method for producing a high-strength seamless steel pipe having excellent low-temperature toughness as well as sulfide stress corrosion cracking resistance on the premise of deep wells under the trend of sourcing. , Cr: 1.0-4.0%, Mo: 0.2-1.0%, Nb: 0.01-0.1%, etc. A method of manufacturing by the "method" has been proposed. However, in the proposed method, it is necessary to contain a large amount of expensive elements in order to combine high strength and low temperature toughness.
[0005]
In Japanese Patent Application Laid-Open No. 60-33312, expensive elements such as Mo or the like are not added, and even if they are added, the amount of these alloys added is kept to a minimum, and the "reheating-quenching method" is used. There is disclosed an oil well steel pipe having the same strength and toughness as a case where an expensive alloy component is added by quenching. However, V and Nb are added to the steel pipe disclosed here to secure the strength, which is insufficient for not adding expensive elements. Furthermore, in this manufacturing method, since the tempering temperature conditions are limited, when applied to the production of a seamless steel pipe, there is a problem that a complicated tempering temperature control is required and a reduction in productivity is inevitable. is there.
[0006]
[Problems to be solved by the invention]
As described above, conventionally, in order to manufacture a high-strength seamless steel pipe by the "reheating-quenching method", an expensive alloy element effective for improving hardenability has been added. However, recently, as a method of obtaining a high-strength seamless steel pipe with high production efficiency, a continuous process of hot pipe making has been studied, and the "direct quenching method" has been adopted as a premise. The "direct quenching method" is a method in which quenching is performed by utilizing the retained heat of a rolled material without performing substantial reheating.
[0007]
On the other hand, regarding the toughness, it is effective to control the rolling finishing temperature as low as possible to make the austenite crystal grains fine. However, in the production of a seamless steel pipe by rolling, it is difficult to lower the rolling finishing temperature due to difficulties in lubrication and the like, and usually the finishing temperature is maintained at 1000 ° C. or higher. If so, it is difficult to refine the austenite crystal grains in the rolling step in which the finishing temperature is 1000 ° C. or higher, and when this is manufactured by ordinary “direct quenching”, the low-temperature toughness is inferior. .
[0008]
The present invention has been made in view of the problems that occur during the above-described “direct quenching method”, and uses the “direct quenching method” that is a premise of the continuous pipe making process, and furthermore, has been made to improve the hardenability. It is an object of the present invention to provide a method for producing a seamless steel pipe having high strength and high toughness with high production efficiency without adding an effective but expensive alloy element.
[0009]
[Means for Solving the Problems]
The present inventors have conducted various studies on the component design of the raw steel, the piercing rolling conditions and the direct quenching conditions in order to achieve the above object, and as a result, added an expensive alloy element for improving the hardenability. Even without this, it was clarified that a high-strength and high-toughness seamless steel pipe can be manufactured by combining appropriate rolling conditions and thermomechanical treatment. The findings obtained from this study are the following (1) to (3).
[0010]
{Circle around (1)} In the finishing rolling mill, which is the final rolling step, the austenite crystal grains of the steel pipe after the rolling is completed can be made coarse by performing the low working with the cross-sectional compression ratio of 40% or less at the finishing temperature of 1050 ° C. or more. . When quenching is performed while the crystal grains are coarse, quenching properties are improved, and high strength is obtained.
[0011]
(2) By keeping the heat at as high a temperature as possible after the end of rolling, the austenite crystal grains can be kept coarse, and the uniformity can be improved over the entire length of the steel pipe, thereby preventing variation in quenching performance. .
[0012]
{Circle around (3)} The coarseness of austenite crystal grains improves quenching properties, but remarkably deteriorates toughness. Further, regarding toughness, P and S contained as impurities in steel have an adverse effect. Therefore, by reducing P and S contained in the base steel, the toughness of the steel pipe can be significantly improved when the crystal grains are coarsened.
[0013]
The present invention has been completed based on the above findings, and has a gist of the following method for manufacturing a seamless steel pipe.
[0014]
That is, in weight%, C: 0.15 to 0.5%, Si: 0.1 to 1.0%, Mn: 0.1 to 1.5%, P: 0.02% or less, S: 0 0.002% or less, Cr: 0.1 to 1.5%, Ti: 0 to 0.5%, B: 0 to 0.01%, Al: 0.005 to 0.5%, N: 0.01 % Or less, O (oxygen): 0.01% or less, the remainder being a material consisting of Fe and unavoidable impurities, heated and pierced and rolled to produce a seamless steel pipe. Processing at a compression ratio of 40% or less is performed at a finishing temperature of 1050 ° C. or more, then immediately at 930 ° C. or more, and directly quenched in a state where the minor axis of austenite grains before quenching is 40 μm or more on average. This is a method for producing a high-strength, high-toughness seamless steel pipe.
[0015]
When a seamless steel pipe is manufactured by hot piercing and rolling, dimensions of the perforated hollow shell are adjusted by a stretching rolling mill and a finishing rolling mill, as described later. There are several types of equipment for performing this rolling. For example, in the Mannesmann mandrel mill method, piercing and rolling are performed by a piercer mill, and elongation rolling is performed by a mandrel mill, and finish rolling is performed by a sizer or a reducer. Therefore, the final rolling step in the present invention means both the above elongation rolling and the finish rolling step using a sizer or a reducer.
[0016]
In the seamless steel pipe of the present invention, high strength means that the yield stress is 700 Mpa or more, and high toughness means that the impact fracture transition temperature (vTrs) exceeds -50 ° C.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the requirements of the present invention will be sequentially described in terms of the chemical composition of the raw steel and the method of manufacturing the seamless steel pipe. In addition,% which shows content of a chemical component means "weight%."
[0018]
(A) Chemical composition C of material steel: 0.15 to 0.5%
C is an element necessary for improving the hardenability and improving the strength of the steel pipe. If it is less than 0.15%, the hardenability is insufficient and the strength cannot be secured. On the other hand, if it exceeds 0.5%, sintering cracks and delayed fracture are likely to occur, making it difficult to manufacture a seamless steel pipe. Therefore, the C content is set to 0.15 to 0.5%.
[0019]
Si: 0.1 to 1.0%
Since Si has the effect of improving the strength of steel material in addition to the deoxidizing effect of steel, it needs to be added in an amount of 0.1% or more. On the other hand, if the addition exceeds 1.0%, toughness is deteriorated. For this reason, the Si content is set to 0.1 to 1.5%.
[0020]
Mn: 0.1-1.5%
Mn also has a deoxidizing and desulfurizing action on steel, and to achieve this purpose, it is necessary to add 0.1% or more. On the other hand, if the addition exceeds 1.5%, toughness is deteriorated, so the content is set to 0.1 to 1.5%.
[0021]
P: 0.02% or less P is an impurity inevitably contained in steel, and segregates at grain boundaries to deteriorate toughness, so that the content is reduced as much as possible. In the method of the present invention, even when the austenite crystal grains are coarsened after rolling, the toughness of the steel pipe can be significantly improved by reducing the P content. Therefore, the upper limit is set to 0.02%. Further, by setting the upper limit to 0.005%, the toughness can be further improved.
[0022]
S: 0.002% or less S is an impurity unavoidably contained in steel, like P, and exists as inclusions to deteriorate toughness. Therefore, the content of S is reduced as much as possible. In the method of the present invention, even if the austenite crystal grains become coarse after rolling, the toughness can be significantly improved by reducing the S content. Therefore, the S content is set to 0.002% or less. More preferably, it is 0.0008% or less.
[0023]
Cr: 0.1-1.5%
Cr has the effect of securing the quenchability and increasing the strength, but if its content is less than 0.1%, the effect cannot be obtained, and if it exceeds 1.5%, the toughness deteriorates. Therefore, the Cr content is set to 0.1 to 1.5%.
[0024]
Ti: 0 to 0.5%
Ti does not need to be added, but is added when higher strength is required because it is an element effective for hardenability. However, if the addition exceeds 0.5%, the toughness of the steel pipe is reduced, so the Ti content is set to 0.5% or less.
[0025]
B: 0 to 0.01%
B does not need to be added, but is added when higher strength is required because hardenability is improved by adding a small amount. However, if the addition exceeds 0.01%, the toughness deteriorates and the susceptibility to quenching cracking increases, so the B content is set to 0.01% or less.
[0026]
Al: 0.005 to 0.5%
Al is an element useful as a deoxidizing agent for steel, but if its content is less than 0.005%, its effect cannot be obtained. If it exceeds 0.5%, inclusions increase and the toughness decreases. Therefore, the Al content is set to 0.005 to 0.5%.
N: 0.01% or less N is present in steel as an impurity and lowers the toughness of the steel pipe.
[0027]
O (oxygen): 0.01% or less O, like N, exists as an impurity in steel and lowers the toughness of a steel pipe.
[0028]
(B) Method of Manufacturing Seamless Steel Pipe Hereinafter, a method of manufacturing a seamless steel pipe of the present invention will be described in the order of steps.
[0029]
1. Heating and drilling of raw steel:
The material steel is a so-called billet, such as a steel slab that is slab-rolled into a round bar or a slab cast by a continuous casting machine having a mold having a circular cross section. In order to save energy, the billet is preferably charged into a heating furnace after being subjected to slab rolling or continuous casting and before completely cooled to room temperature.
[0030]
The heating temperature of the billet may be a temperature at which perforation can be performed by hot working, and may be determined in consideration of the high-temperature ductility and high-temperature strength of the material. Usually, it heats in the range of 1100-1300 degreeC. In the piercing step, a hollow shell (hollow shell) is manufactured by making a through hole in a solid billet by using a piercer such as an inclined roll rolling machine.
[0031]
2. Final rolling process:
The perforated hollow shell is finally rolled by a stretching rolling mill and a finishing rolling mill to produce a seamless steel pipe of a predetermined size. As described above, in the case of the Mannesmann mandrel mill system, the final rolling step of the present invention means both the mandrel elongation rolling and the finish rolling step using a sizer or a reducer.
[0032]
In order to refine the austenite crystal grains of the product steel pipe and increase the low-temperature toughness, it is only necessary to perform the rolling with a high working ratio at the lowest possible temperature in the final rolling. This is because the rolling at a high degree of processing increases processing strain and promotes refining by recrystallization, and also lowers the rolling temperature to suppress grain growth after recrystallization. is there.
[0033]
However, when the austenite crystal grains of the steel pipe are small, the strength is low, so that high strength cannot be ensured, and a property combining high strength and high toughness cannot be satisfied. On the other hand, if the rolling temperature is too low, the rolling productivity will be impaired due to an increase in the rolling load, such as occurrence of seizure when the mandrel bar is pulled out after rolling. Therefore, in the method of the present invention, in consideration of increasing the production efficiency in the rolling, the rolling at a low working ratio is performed at a high finishing temperature in the final rolling step. Therefore, specifically, the rolling at a low working ratio is 40% or less in cross-sectional compression ratio, and the high-temperature finishing temperature is 1050 ° C. or more.
[0034]
As described above, according to the method of the present invention, the austenite crystal grains of the steel pipe become coarse, but at the same time, the hardenability can be significantly improved. Can be obtained. Next, regarding the toughness, even if the austenite crystal grains become coarse after rolling, the toughness of the steel pipe can be remarkably improved by reducing the contents of P and S, so that there is no problem. .
[0035]
3. Heat retention after final rolling:
The steel tube after the final rolling is immediately kept at 930 ° C. or higher. In general, placing something like a regenerative furnace in the rolling line may not be a viable option in terms of equipment costs. However, in order to secure the uniform heat of the steel pipe before quenching and to suppress the variation in quenching strength and toughness, it is necessary to maintain uniformity of the structure and performance in the longitudinal and circumferential directions of the steel pipe. A furnace is needed.
[0036]
The temperature was kept at 930 ° C. or higher in order to keep the heat as high as possible in order to prevent the crystal grains from being refined by recrystallization. The heat retention time is at least one minute in order to make the temperature of the entire steel pipe uniform. However, if the heat is kept for more than 60 minutes, the effect is saturated and only the productivity is reduced.
[0037]
4. Direct quenching:
The steel pipe whose temperature has been made uniform by heat retention is directly quenched by water cooling or the like. At this time, when the crystal grain size is large, the ferrite transformation becomes difficult, the martensite transformation becomes easy, and the hardenability improves, so that the minor axis of the austenite crystal grains before quenching should be 40 μm or more on average. I have. At this time, it is not necessary to set an upper limit because the earlier the cooling, the more uniform the tissue can be. However, at a temperature of 10 ° C./sec or less, the strength decreases, the structure becomes coarse, and the toughness also decreases.
[0038]
After the rapid cooling described above, the air may be cooled to an appropriate temperature, for example, room temperature. Even with the above cooling, the product steel pipe has excellent characteristics. However, if this is subjected to a tempering treatment, the hardness of the structure can be reduced and the toughness can be further improved. Therefore, if necessary, the tempering may be performed.
[0039]
【Example】
A seamless steel pipe for an oil well is manufactured using a raw steel having a chemical composition shown in Table 1. These material steels were charged into a heating furnace, heated to 1300 to 1280 ° C. for 2 hours or more, and then pierced and rolled with a piercer to obtain hollow shells. As a final rolling step, mandrel elongation rolling was performed, followed by finishing with a sizer, heat retention, direct quenching, and tempering to produce a product steel pipe. At this time, a test was conducted in which the rolling finish temperature, the degree of rolling, the heat retention temperature (quenching temperature), the tempering temperature, and the minor axis average value of the austenite crystal grains before quenching were changed as shown in Table 2. . In addition, the particle size of the austenitic crystal was measured by ASTM E112.
[0040]
Specimens were cut out from the tempered product steel pipe and subjected to a tensile test and a Charpy test to investigate the yield strength (YS) and tensile strength (TS) as strength and the fracture surface transition temperature (vTrs) as toughness. Table 2 shows the results.
[0041]
[Table 1]

[0042]
[Table 2]

[0043]
As is clear from the results in Table 2, in Tests 1 to 9 of the invention examples, rolling was performed with the finishing temperature in the final rolling being 1050 ° C. or more and the rolling degree being 40% or less, and then keeping the heat at 930 ° C. Immediately after the above, the product steel pipe is manufactured by direct quenching and tempering. Therefore, the austenite crystal grains before quenching can be kept in a coarse state, and sufficient hardenability can be secured. Further, the steels A to E, which are the material steels, have remarkably improved low-temperature toughness due to the reduction of P and S, even in the case of coarse crystal grains. Therefore, all of the invention examples achieve YS: 700 Mpa or more and vTrs: −50 ° C., which are indications of a high-strength and high-toughness oil-well steel pipe.
[0044]
In contrast, Tests 10 to 12 of Comparative Examples used the raw steel specified in the present invention, but either the rolling finish temperature, the final rolling degree, or the heat retention temperature was out of the specified range of the present invention. Therefore, the toughness deteriorates remarkably.
[0045]
Tests 13 to 19 of the comparative examples show that the conditions of final rolling and direct quenching satisfy the requirements of the present invention, but any one of the chemical compositions of the raw steel is out of the specified range of the present invention. Significant deterioration in toughness of product steel pipe. In particular, in Test 17, the minimum necessary C content for securing hardenability was insufficient, and the required strength could not be secured.
[0046]
【The invention's effect】
According to the method for producing a high-strength, high-toughness seamless steel pipe of the present invention, an oil-well steel pipe having high strength and high toughness characteristics without adding an expensive alloy element, which is effective for improving hardenability. Can be manufactured. Moreover, since the treatment is performed by the “direct quenching method” on the premise of a continuous pipe production process, a seamless steel pipe having uniform and excellent characteristics can be manufactured with high production efficiency.

Claims (1)

By weight%, C: 0.15 to 0.5%, Si: 0.1 to 1.0%, Mn: 0.1 to 1.5%, P: 0.02% or less, S: 0.002 %, Cr: 0.1 to 1.5%, Ti: 0 to 0.5%, B: 0 to 0.01%, Al: 0.005 to 0.5%, N: 0.01% or less , O (oxygen): 0.01% or less, the remainder being a material consisting of Fe and unavoidable impurities, heating and hot piercing and rolling to produce a seamless steel pipe. High strength characterized in that processing of 40% or less is performed at a finishing temperature of 1050 ° C. or more, followed by immediate heat retention at 930 ° C. or more, and direct quenching with the minor axis of austenite grains before quenching being 40 μm or more on average. Manufacturing method of high toughness seamless steel pipe.