JP4196501B2 - Steel for seamless steel pipe with high strength and excellent toughness - Google Patents

Description

【００４７】
製造した各鋼板からは、ＪＩＳ４号シャルピー試験片および丸棒引っ張り試験片を、それぞれ長手方向が圧延方向と平行になるよう採取して、シャルピー衝撃試験と引っ張り試験に供した。
【００４８】
引張り試験片は１枚の圧延材の先端部、中央部および後端部の場所で表層各４本、中心部各４本の合わせて２４本採取した。降伏強度、引張り強度は２４本の平均とし、また、強度バラツキを評価するために、２４本の引張り強度の標準偏差を求めた。その結果は表２に示す通りであった。
【００４９】
【表２】

【００５０】
さらに、延性破面率が５０％になる温度（Ｔ５０）、同じく８０％になる温度（Ｔ８０）と、その温度差（Ｔ８０−Ｔ５０）も求め、表２に併せて示す。
【００５１】
表２から明らかなように、本発明例の極低Ｎ化した鋼番５、７および８や、微量Ｔｉ含有鋼の鋼番９〜１１および１３〜１５ではＴ８０−Ｔ５０が小さくなるので、良好な靭性が得られる。また、本発明例では強度バラツキも小さく押さえられていることが分かる。
【００５２】
一方、比較例においては、表２に示すように鋼番１６は低Ｐ（請求項１に記載の鋼）であるが、ＮおよびＴｉ含有量が本発明で規定する範囲外であり、鋼番１７はＰは高目、極低Ｎであり（請求項２に記載の鋼）、Ｔｉを過剰に含有しており靱性は良好であるものの強度バラツキが大きく、実用に耐えない。
【００５３】
鋼番１８〜２１は、Ｐ、Ｎは高目で請求項３記載の鋼に対応するが、鋼番１８ではＴｉ無添加、鋼番１９はＴｉが少なすぎてＴ８０−Ｔ５０が大きくて靱性が劣っている。鋼番２０では、数式で規定する量よりも多いＴｉを含有しているので、靱性は良好であるものの強度のバラツキが問題である。また、鋼番２１は、数式で規定する範囲内のＴｉを含有しているがその絶対量が多過ぎ、靭性が低下している。鋼番２２は、Ｐ含有量が規定範囲より多く、靭性が低下している。
【００５４】
【発明の効果】
本発明によれば、高強度で靱性に優れたシームレス鋼管をインライン熱処理で生産できるので、高生産効率および省エネルギーにより、安価に鋼管を供給できる。
【図面の簡単な説明】
【図１】延性破面率とシャルピー衝撃試験温度との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel for seamless steel pipes that can be manufactured by in-line heat treatment and has high strength and excellent toughness.
[0002]
[Prior art]
In many cases, seamless steel pipes used for high grade oil well pipes are required to have both high strength and toughness. Conventionally, this requirement has been dealt with by tempering by off-line heat treatment after pipe making and refining the crystal grains. However, in recent years, there has been an increasing demand for process saving in seamless steel pipe manufacturing methods, and in-line heat treatment has been studied.
[0003]
However, the in-line heat treatment has a problem that there is no reverse transformation caused by cooling and heating unlike the off-line heat treatment, and the crystal grains tend to be coarser than the off-line heat treatment, so that it is difficult to ensure toughness. As an improvement method, for example, Japanese Patent Application Laid-Open No. 08-13031 discloses hot rolling in which a low alloy steel added with Nb and Ti is not recrystallized at a low temperature and the cumulative reduction ratio is 50% or more. A method of manufacturing a steel sheet excellent in low-temperature toughness of a fine structure in which tempering is performed, followed by in-line accelerated cooling to 300 ° C. or less and tempering is disclosed. When cold rolling is possible as in the production of steel sheets, Nb and Ti can be positively added, and the recrystallization temperature can be increased by increasing the recrystallization temperature. It is extremely effective.
[0004]
However, the seamless steel pipe manufacturing method is generally a method of drilling a solid billet, inserting a mandrel inside the hollow shell and rolling, and considering the friction coefficient with the mandrel, the life of the mandrel, and the high temperature strength, It is difficult to cold-roll like a steel plate. Therefore, in the manufacture of seamless steel pipes, the heat treatment that performs finish rolling at a temperature higher than the recrystallization temperature range and immediately quenches has become the mainstream, and it is difficult to apply the steel sheet manufacturing method as described above.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a steel for seamless steel pipes, which has a yield strength of 758 MPa (110 ksi) or more and excellent toughness that can be produced by in-line heat treatment.
[0006]
[Means for Solving the Problems]
As an effective toughness improvement method in the in-line heat treatment process of seamless steel pipes, the present inventors have adequately reduced the amount of inclusions such as TiN and MnS, so that practically sufficient γ grains can be obtained even in a relatively coarse structure. 550 MPa (80 ksi) grade steel exhibiting an appropriate fracture surface transition temperature was proposed (Japanese Patent Application No. 10-114949).
[0007]
However, if the C content is increased to increase the strength based on the component system, and the tempering temperature is adjusted to obtain a high strength oil well pipe having a yield stress of 758 MPa or more, the fracture surface transition temperature itself moves to the high temperature side. At the same time, the test results revealed that there is a problem that a brittle fracture surface remains on the high temperature side that is far from the transition temperature (hereinafter referred to as a decrease in the rate of change of the ductile fracture surface ratio with respect to temperature).
[0008]
FIG. 1 is a diagram illustrating an example of a Charpy impact test result. Ti-B steel with good hardenability was used as an off-line heat treatment material for this test. As the in-line heat treatment material, Cr steel having a small variation was used because of its good hardenability. The chemical composition was as follows:
[0009]
Offline heat treatment material:% by mass, C: 0.23%, Si: 0.31%, Mn: 1.3%, Cr: 0.1%, Ti: 0.02%, B: 0.0014%, N: 0.004, sol. Al: 0.021%, P: 0.028%, S: 0.004%.
[0010]
In-line heat treatment material:% by mass, C: 0.23%, Si: 0.3%, Mn: 1.35%, Cr: 0.2%, P: 0.026%, S: 0.001%, sol.Al: 0.022%, N: 0.006%.
[0011]
Hot rolling simulated piercing and rolling at the time of pipe making, and started rolling after heating to 1250 ° C., with a rolling reduction of 50 to 78% and a rolling finishing temperature of 950 ° C.
[0012]
As the in-line heat treatment, the production line of the actual machine was simulated, the rolled steel material was heated to 960 ° C., held for 8 minutes, immediately quenched with water, and tempered at 500 ° C. Moreover, as an off-line heat processing, the normal temperature steel materials after rolling were heated to 920 degreeC, water-quenched, and the tempering process was performed at 520 degreeC. The Charpy impact test piece is prepared by adjusting the yield stress to be 758 MPa class by such treatment, and the test is performed by changing the test temperature in a temperature range of −100 to 100 ° C. Asked.
[0013]
FIG. 1 is a diagram in which the above test results are organized by the relationship between the test temperature and the ductile wavefront ratio. As is clear from FIG. 1, the offline heat treatment material, which is a conventional process, suddenly increases the ductile fracture surface ratio when the test temperature reaches −30 to −10 ° C. and the ductile fracture surface ratio starts to rise ( That is, the rate of change of the ductile fracture surface ratio with respect to temperature increases), and the temperature difference between the ductile fracture surface ratio of 50% and 80% is extremely small. However, a material having a C-level of 550 MPa (80 ksi) grade steel shown in Japanese Patent Application No. 10-114949 is increased, and the tempering temperature is further adjusted to a 758 MPa grade material. (VTrs: temperature at which the ductile fracture surface ratio is 50%) is obtained, but the rising slope of the ductile fracture surface ratio is small (that is, the change rate of the ductile fracture surface ratio with respect to temperature is small), and the ductile fracture surface ratio is 50. The temperature difference between% and 80% is large.
[0014]
The present inventors can sufficiently reduce the temperature at which a ductile fracture surface ratio of about 80 to 90% is obtained instead of the 50% fracture surface transition temperature, and even if in-line heat treatment is performed, it is inferior to the offline heat treatment material. We have intensively studied the chemical composition that can provide the impact characteristics that are not. As a result, the following knowledge was obtained.
[0015]
1) When the basic idea regarding cleaning shown in Japanese Patent Application No. 10-114949 is applied to steel for oil wells with a high C, the fracture surface transition temperature is lowered and improved, but the ductility to temperature Since the rate of change of the fracture surface rate is not improved and the rise of absorbed energy is moderately linked, there is a problem that the required value of absorbed energy at a temperature considerably higher than the fracture surface transition temperature is not satisfied. This is because a brittle fracture surface including grain boundary fracture remains and the absorbed energy decreases even at a high temperature far from the fracture surface transition temperature.
[0016]
2) When in-line heat treatment is performed, N in steel is the main cause for the ductile fracture surface rate and the rate of change of absorbed energy to be smaller than those of offline heat-treated materials. Therefore, reduction of N is effective.
[0017]
3) If P is reduced to 0.01% or less instead of reducing N, the ductile fracture surface rate and the rate of change of absorbed energy with respect to temperature change will be slightly increased, but the overall transition curve shape will be affected. Although not exerted, it has the effect of lowering the fracture surface transition temperature itself. Therefore, even when compared at a ductile transition temperature of 80 to 90%, toughness equivalent to that of an off-line heat-treated material can be obtained. Further, at the 50% fracture surface transition temperature, the temperature is significantly improved by shifting to a lower temperature side than the off-line heat-treated material.
[0018]
4) In the case where P or N is not reduced in order to reduce the refining cost, if N is fixed by adding a small amount of Ti, the ductile fracture surface rate and the rate of change of absorbed energy with respect to temperature can be increased. Ti is essentially an element that lowers toughness, but N has a remarkable effect of lowering the ductile fracture surface ratio and the rate of change of absorbed energy with respect to temperature. Even when added, a remarkable effect can be obtained.
[0019]
The present invention has been made based on these findings, and the gist thereof is as follows.
[0020]
(1) In mass%, C: 0.15% to 0.35%, Si: 0.1% to 1.5%, Mn: 0.3% to 1.5%, P: 0.015% -0.025% , S: 0.005% or less, Cr: 0.05% -1.5%, sol.Al: 0.001% -0.2%, N: less than 0.003% , Ti: 0.0082% or less , Mo: 0 to 1%, Nb: 0 to 0.005 %, V: 0 to 0.5%, B: 0 to 0.003%, Ca: 0 to 0.01%, Mg : Steel for seamless steel pipes including 0 to 0.01%, REM: 0 to 0.01%, consisting of the balance Fe and impurities and having a high yield strength and toughness with a yield stress of 758 MPa or more .
[0022]
(2) By mass%, C: 0.15% to 0.35%, Si: 0.1% to 1.5%, Mn: 0.3% to 1.5%, P: 0.015% -0.025%, S: 0.005% or less, Cr: 0.05% -1.5%, sol. Al: 0.001% -0.2%, Ti: 0.003% -0.015 %, And “3.4 × N% −0.002”% or less, N: 0.003% to 0.015%, Mo: 0 to 1%, Nb: 0 to 0.005%, V: 0 to 0.5%, B: 0 to 0.003%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, REM: 0 to 0.01%, the balance Fe and Steel for seamless steel pipes with high strength and excellent toughness with yield stress of 758 MPa or more, consisting of impurities .
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reason which prescribed | regulated the chemical composition of steel for seamless steel pipes of this invention is demonstrated in detail. In addition, all the% display of a chemical composition shows the mass%.
[0024]
C
C is an element necessary for ensuring the strength of the steel pipe. If it is less than 0.15%, it is difficult to ensure the required strength due to insufficient hardenability. On the other hand, if it exceeds 0.35%, cracks occur during quenching and the toughness is greatly deteriorated. Therefore, the C content is set to 0.15% to 0.35%.
[0025]
Si
Si is contained for the purpose of deoxidation of steel. It also contributes to an increase in strength by increasing the temper softening resistance. In order to acquire these effects, it is necessary to make it contain 0.1% or more. On the other hand, if it contains more than 1.5%, toughness will be reduced. Therefore, the Si content is set to 0.1 to 1.5%.
[0026]
Mn
Mn increases the hardenability of the steel and is an effective component for securing the strength of the steel pipe. When the content is 0.3% or less, a product that satisfies both strength and toughness cannot be produced due to insufficient hardenability. On the other hand, when it contains exceeding 1.5%, segregation will increase and toughness will fall. Therefore, the Mn content is set to 0.3 to 1.5%.
[0027]
P
P is unavoidably present in steel as an impurity, but in the steel of the present invention, P is an important element that affects impact characteristics and production cost. If the N content is not reduced, it is necessary to sufficiently reduce it to less than 0.015% in order to ensure toughness.
[0028]
When N is reduced or when a small amount of Ti is added, it is not necessary to reduce P, which increases the refining cost, and toughness can be ensured even within a range of 0.015 to 0.025%.
[0029]
S
S forms MnS or combines with Ca or REM to form oxysulfide and exists in steel as inclusions. If the content is large, the toughness decreases, so it is necessary to keep it to 0.005% or less.
[0030]
Cr
Cr is contained in order to ensure the necessary hardenability and temper softening resistance. If the content is less than 0.05%, the effect of increasing the temper softening resistance in addition to the hardenability cannot be obtained. However, if it exceeds 1.5%, the toughness decreases, so the upper limit is made 1.5%.
[0031]
sol.Al
Al is an indispensable element for deoxidation. If 0.001% or more is not contained in sol.Al, steel quality is deteriorated due to insufficient deoxidation. However, if the content exceeds 0.2%, the toughness is lowered, which is not preferable. Therefore, the sol.Al content is set to 0.001% to 0.2%.
[0032]
N
N lowers the high temperature ductility due to impurities, and at the same time lowers the rate of change of the ductile fracture surface ratio with respect to temperature in the impact test. The effect is particularly remarkable when in-line heat treatment is performed and the crystal grains are large. Therefore, N should be as low as possible. However, low N requires a long refining time and requires special refining equipment as well as low P, so the N content is adjusted as follows depending on the P content and the presence or absence of Ti addition. .
[0033]
If P cannot be reduced to less than 0.015% from the viewpoint of refining equipment and refining costs, N can be reduced to less than 0.003%, or a small amount of Ti can be added to fix N in steel as will be described later. Then, since the rate of change of the ductile fracture surface ratio with respect to the temperature can be increased, a predetermined ductile fracture surface ratio and absorbed energy can be ensured even if the fracture surface transition temperature itself does not move to a low temperature side.
[0034]
When toughness is ensured by reducing P to less than 0.015%, it is not necessary to keep the N content very low, and sufficient toughness can be secured at 0.15% or less.
[0035]
Ti
In the case where P is reduced to less than 0.015% and the transition temperature curve is shifted to the low temperature side to improve toughness, Ti need not be contained. However, Ti may be included to improve the toughness and continuous castability of the weld heat-affected zone, but if it is excessively contained, the effect of lowering P will be reduced, and N in the steel will be fixed. When the amount of TiN produced increases, the toughness is greatly reduced. Therefore, the upper limit of the Ti content in the case of inclusion is set to 0.015%.
[0036]
When N is reduced to less than 0.003%, it is not necessary to contain Ti. However, it can be contained for the purpose of improving the toughness of the weld heat affected zone and improving the surface cracking during continuous casting. However, when Ti is 0.0082% or more, excessive Ti precipitates as TiC during tempering and strength. Since the variation becomes large, the upper limit of Ti when N is reduced is set to 0.0082%.
[0037]
Further, when N is allowed to be a content of 0.003% or more and the rate of change of ductile fracture surface ratio with respect to temperature is improved, 0.003 to 0.015% of Ti is necessary. However, when excessive Ti that cannot be combined with N increases, TiC is generated and strength variation occurs. Therefore, the excess Ti needs to be 0.002% or less, and for that purpose, Ti needs to satisfy “3.4 × N% −0.002”% or less.
[0038]
Mo
Mo is an element that is included as necessary. In the case of a thick-walled steel pipe, it is preferable that Mo be added when the hardenability and temper softening resistance are further increased and high-temperature tempering is required. When it is contained, if the content is less than 0.05%, the above effect cannot be obtained. Therefore, the content is desirably 0.05% or more. However, if it exceeds 1%, the toughness deteriorates remarkably, so 1% or less.
[0039]
Nb
Nb is an essential additive element in steel materials manufactured by so-called controlled rolling, but in the present invention, Nb is not required to be contained because it basically does not use controlled rolling. However, Nb is effective in increasing the strength, but if it exceeds 0.01%, when the rolling is finished at a high temperature of 1000 ° C. or higher, the toughness is remarkably impaired through precipitation strengthening, or variation due to the location of the steel pipe Cause. For this reason, when it contains, it is good to set it as 0.005% or less .
[0040]
V
V increases the strength by precipitation strengthening, but has a relatively small adverse effect on toughness, and is added as necessary to increase the strength. In addition to temper softening resistance, there is an effect of improving hardenability. In order to obtain these effects when contained, the content is preferably 0.05% or more. However, if it exceeds 0.5%, the toughness deteriorates greatly, so it was made 0.5% or less.
[0041]
B
B can secure good hardenability when it is contained when hardenability is insufficient when producing a thick steel pipe. When it is contained, the content is preferably 0.0003% or more. However, if the content exceeds 0.003%, the formation of M ₂₃ C ₆ type carbide is promoted after tempering and the toughness is lowered, so the upper limit is 0.003%. It was.
[0042]
Ca, Mg
Ca and Mg are elements to be contained as necessary, and react with S in steel to generate a sulfate in molten steel. Unlike MnS and the like, this sulfate does not extend in the rolling direction by rolling and is spherical after rolling. For this reason, the anisotropy of the mechanical properties is improved, and particularly the impact properties in the direction perpendicular to the rolling direction are improved. In addition, it also has an effect of suppressing hydrogen-induced cracking starting from the tip of the elongated inclusion. If these effects are obtained, the content may be contained. However, if the content exceeds 0.01%, the toughness of the base material is lowered due to the decrease in cleanliness, so the upper limit is preferably made 0.01%.
[0043]
REM (rare earth element)
REM has the effect of improving the toughness by refining the structure and fixing S, and if necessary, it is contained, but if it exceeds 0.01%, the toughness is lowered, so if it is contained, 0.01% or less It is good to do.
[0044]
【Example】
Twenty-two kinds of steels having chemical compositions shown in Table 1 were melted in a vacuum melting furnace having a test capacity of 150 kg. After forging a 150 kg ingot obtained by casting into a round mold, rolling was performed simulating in-line thermomechanical heat treatment. The thermomechanical conditions are shown below.
[0045]
Forging heating temperature (corresponding to billet heating in pipe making method): 1250 ° C
Rolling (corresponding to piercing and rolling): processing degree 70%, finishing temperature 1050 ° C., 20 seconds after rolling, inserted into a furnace at 950 ° C. for 10 minutes, and immediately followed by water quenching. Furthermore, the strength is adjusted to 758 MPa grade by tempering.
[0046]
[Table 1]

[0047]
From each manufactured steel sheet, JIS No. 4 Charpy test piece and round bar tensile test piece were sampled so that the longitudinal direction was parallel to the rolling direction, and subjected to Charpy impact test and tensile test.
[0048]
A total of 24 tensile test specimens were collected, including 4 on each surface layer and 4 on each central part at the positions of the front end, center, and rear end of one rolled material. The yield strength and the tensile strength were averaged from 24 pieces, and the standard deviation of the 24 pieces of tensile strength was determined in order to evaluate the strength variation. The results were as shown in Table 2.
[0049]
[Table 2]

[0050]
Further, a temperature at which the ductile fracture surface ratio becomes 50% (T50), a temperature at which the ductile fracture surface ratio becomes 80% (T80), and a temperature difference (T80-T50) are also obtained and are shown in Table 2.
[0051]
As is apparent from Table 2, since T80-T50 is small in steel Nos. 5 , 7 and 8 with the extremely low N of the examples of the present invention and steel Nos. 9 to 11 and 13 to 15 of the trace amount Ti-containing steel, it is good. Toughness is obtained. Moreover, it turns out that intensity variation is suppressed small also in the example of this invention.
[0052]
On the other hand, in the comparative example, as shown in Table 2, the steel number 16 is low P (steel according to claim 1), but the N and Ti contents are outside the range defined in the present invention. No. 17 is high and extremely low N (steel according to claim 2), and contains excessive Ti and good toughness, but has great strength variation and cannot be practically used.
[0053]
Steel Nos. 18 to 21 correspond to the steel according to claim 3 with high P and N, but with steel No. 18 no Ti addition, Steel No. 19 has too little Ti, T80-T50 is large and toughness is Inferior. Steel No. 20 contains a larger amount of Ti than the amount specified by the mathematical formula, so that the toughness is good, but the strength variation is a problem. Moreover, although the steel number 21 contains Ti within the range prescribed | regulated by numerical formula, the absolute amount is too much and toughness has fallen. Steel No. 22 has a P content greater than the specified range and has reduced toughness.
[0054]
【The invention's effect】
According to the present invention, since a seamless steel pipe having high strength and excellent toughness can be produced by in-line heat treatment, the steel pipe can be supplied at low cost with high production efficiency and energy saving.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between a ductile fracture surface ratio and a Charpy impact test temperature.

Claims (2)

In mass%, C: 0.15% to 0.35%, Si: 0.1% to 1.5%, Mn: 0.3% to 1.5%, P: 0.015% to 0.00%. 025%, S: 0.005% or less, Cr: 0.05% to 1.5%, sol. Al: 0.001% to 0.2%, N: less than 0.003%, Ti: 0.0082 % Or less, Mo: 0 to 1%, Nb: 0 to 0.005 %, V: 0 to 0.5%, B: 0 to 0.003%, Ca: 0 to 0.01%, Mg: 0 to 0% A steel for seamless steel pipes with high yield strength and excellent toughness with a yield stress of 758 MPa or more, comprising 0.01%, REM: 0 to 0.01%, and the balance being Fe and impurities. In mass%, C: 0.15% to 0.35%, Si: 0.1% to 1.5%, Mn: 0.3% to 1.5%, P: 0.015% to 0.00%. 025%, S: 0.005% or less, Cr: 0.05% to 1.5%, sol.Al: 0.001% to 0.2%, Ti: 0.003% to 0.015% And not more than “3.4 × N% −0.002”%, N: 0.003% to 0.015%, Mo: 0 to 1%, Nb: 0 to 0.005 %, V: 0 -0.5%, B: 0-0.003%, Ca: 0-0.01%, Mg: 0-0.01%, REM: 0-0.01%, the balance consisting of Fe and impurities A steel for seamless steel pipes having a high strength and toughness with a yield stress of 758 MPa or more .

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