JP2004292857A - Non-heat treated seamless steel tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-heat treated seamless steel tube by which high strength and high toughness can be combined without requiring strict control of tube-making draft and tube-making temperature in the course of tube making and further superior weldability can be secured. <P>SOLUTION: The non-heat treated seamless steel tube has a composition which consists of, by weight, 0.10 to <0.20% C, 0.05 to 1.0% Si, 0.5 to 2.5% Mn, ≤0.03% P, ≤0.05% S, 0.5 to 1.5% Cr, 0.03 to 0.3% V, 0.003 to 0.10% Al, 0.001 to 0.02% N, ≤0.003% O and the balance Fe with impurities and in which the carbon equivalent Ceq.(%) defined by equation Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 is made to 0.60 to 0.85%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５４】
表１中、試料番号（試番）１〜２１が本発明例であり、引張強度は、最低でも７００ＭＰａを超えており、最高９００ＭＰａ近くにまで達している。このように高強度であっても、衝撃値で評価した靭性は、５０Ｊ以上を確保でき、また溶接割れも発生しなかった。
【００５５】
試料番号（試番）２２〜２８は比較例である。試番２２、２３は、従来のフェライト・パーライト鋼となる中炭素系成分の例である。この場合、強度は確保できるものの、靭性は低く、溶接割れも発生した。また、試番２４〜２７は、Ｍｎ、Ｃｒ、Ｖが複合添加されていないか、規定の範囲から外れた例である。溶接割れは起こらなかったが、靭性が不足した。試番２８は、成分組成範囲を満足するものの、炭素当量Ｃｅｑ．が低い例である。強度が７００ＭＰａを大幅に下回り、高強度非調質継目無鋼管の用途には使用できない。
【００５６】
【発明の効果】
以上のようにこの発明によれば、途中の製管加工度や製管温度を厳しく制御しなくても、高強度と高靭性とを両立し、さらに良好な溶接性を確保できる非調質継目無鋼管を得ることができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a seamless steel pipe used for a machine structure or the like, and more particularly to a non-heat treated seamless steel pipe which can be used as it is as a hot pipe, has high strength and high toughness, and is excellent in weldability. Things.
[0002]
[Prior art]
Conventionally, seamless steel pipes used for applications requiring high strength and toughness have been manufactured as follows. That is, first, a seamless steel pipe is manufactured from a billet through a piercing step and a rolling step in hot working. Thereafter, the steel pipe is quenched and tempered to give a predetermined strength level and toughness, thereby obtaining a product.
[0003]
In the production of the above-mentioned seamless steel pipe, a heat treatment step is required after the pipe production, so that the cost increases and the delivery time becomes longer. In order to solve such problems, there is an increasing demand for a seamless steel pipe that does not undergo heat treatment, that is, has high strength and high toughness without heat treatment.
[0004]
Non-heat-treated, high-strength, seamless steel pipes having high toughness include, for example, Japanese Patent Application Laid-Open Nos. 05-20247, 09-25541, 10-130783, and 10-204571. JP-A-10-324946, JP-A-11-36017, JP-A-2000-328192, JP-A-2001-323338, JP-A-2001-247793, and JP-A-2001-262275. ing.
[0005]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 05-20247
[Patent Document 2]
JP-A-09-25554
[Patent Document 3]
Japanese Patent Application Laid-Open No. Hei 10-130783
[Patent Document 4]
JP-A-10-204571
[Patent Document 5]
JP-A-10-324946
[Patent Document 6]
JP-A-11-36017
[Patent Document 7]
JP 2000-328192 A
[Patent Document 8]
JP 2001-323338 A
[Patent Document 9]
JP 2001-247931 A
[Patent Document 10]
JP 2001-262275 A
[Problems to be solved by the invention]
JP-A-Heisei 05-20247, JP-A-09-25541, JP-A-10-130784, JP-A-10-204571, JP-A-10-324946, JP-A-11-36017 and JP-A-11-36017 Japanese Patent Application Laid-Open No. 2000-328192 discloses a method of adjusting components and hot pipe production in order to achieve both high strength and toughness without heat treatment. Further, what is common to the technologies disclosed in these publications is that carbon (C) is added in an amount of 0.2% or more and a medium carbon component is designed. Since these conventional technologies contain 0.2% or more of carbon, the toughness is not sufficient for the strength level. In particular, quenching and hardening in the welded portion lowers the toughness and causes weld cracking.
[0016]
Japanese Patent Application Laid-Open No. 2001-323338 discloses that a steel with a wide range of carbon addition is compatible with hot workability, machinability and toughness. However, it can be pointed out that sufficient strength cannot be ensured because vanadium (V) is not added to the steel pipe disclosed in this publication.
[0017]
JP-A-2001-247931 and JP-A-2001-262275 are directed to steels with a wide range of carbon addition, and strength and toughness are controlled by controlling the metal structure by regulating the hot pipe-forming temperature. It discloses a technology for securing and a technology for securing hot workability. However, in order to carry out the low-temperature pipe production proposed in these publications, motor power is insufficient in the conventional equipment, and the equipment needs to be modified. In addition, in order to form a pipe by reheating after cooling once, a facility such as a reheating furnace is required.
[0018]
In addition, in Japanese Patent Application Laid-Open No. 2001-247931, although a wide range of the amount of added carbon is specified in the claims, in the examples, the amount of added carbon is 0.2% or more.
[0019]
SUMMARY OF THE INVENTION An object of the present invention is to provide a non-heat treated seamless steel pipe which has both high strength and high toughness without heat treatment and can further secure good weldability. In particular, the present invention aims at achieving both high strength and high toughness and preventing cracks and a decrease in toughness at a welded portion without severely limiting the degree of pipe forming and the pipe forming temperature in the middle.
[0020]
[Means for Solving the Problems]
The present inventor has found that the following is effective in achieving the above object.
[0021]
(1) Reduce the carbon content. Then, manganese (Mn), chromium (Cr), and vanadium (V) are added in a composite manner in order to compensate for the strength accompanying the decrease in the carbon content. Thereby, high strength is obtained and good toughness including a weld is obtained.
[0022]
(2) After reducing the carbon content, the carbon equivalent (Ceq.) Is adjusted to a predetermined range.
[0023]
Ferritic / pearlite steel is the standard metal structure of non-heat-treated steel, but it has been found that toughening is reduced when carbonization is performed to meet the demand for higher strength. Then, the inventor of the present application added Mn, Cr, and V in combination to reduce the carbon content and supplement the strength. The metal structure of the non-heat treated steel obtained in this way becomes a structure mainly composed of bainite, and high strength and high toughness can be secured. The "structure mainly composed of bainite" naturally includes a structure in which 100% of bainite is present, but also includes a mixed structure of bainite and ferrite in which ferrite is 50% by volume or less.
[0024]
The gist of the present invention lies in the following points.
[0025]
(1) To suppress the carbon content to less than 0.2%.
[0026]
(2) Complex addition of manganese (Mn), chromium (Cr) and vanadium (V).
[0027]
(3) The metal structure becomes a structure mainly composed of bainite, and among these, as a component composition range capable of achieving both high strength and high toughness, a carbon equivalent Ceq. (%) Is not less than 0.60 and not more than 0.85.
[0028]
Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
In order to secure the strength, Ceq. Should be in the range of 0.60 to 0.85, and a method for satisfying the condition can be adjusted by adjusting the amount of addition of various alloying elements. The inventor of the present application has found that, among them, Mn, Cr, and V in particular, when combined, simultaneously improve the balance between strength and toughness. That is, when the carbon content is the same, the target carbon equivalent Ceq. Is achieved, V is added while decreasing the amounts of Mn and Cr, and the target carbon equivalent Ceq. , Good toughness can be obtained. Therefore, it is important to add Mn, Cr and V in combination.
[0029]
In view of the above, the non-heat-treated seamless steel pipe according to the present invention has, on a weight basis, C: 0.10 to less than 0.20%, Si: 0.05 to 1.0%, and Mn: 0.5. 2.5%, P: 0.03% or less, S: 0.05% or less, Cr: 0.5 to 1.5%, V: 0.03 to 0.3%, Al: 0.003 to 0.10%, N: 0.001 to 0.02%, O: 0.003% or less, the balance being Fe and impurities, and the carbon equivalent Ceq. Defined by the following formula. (%) Is 0.60 to 0.85%.
[0030]
Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
Part of Fe is Ni: 0.05 to 1.5%, Mo: 0.05 to 1.5%, Cu: 0.05 to 1.5%, and B: 0.0003 to 0.01%. One or two or more selected from them may be substituted.
[0031]
Alternatively, part of Fe may be replaced with one or two selected from Ti: 0.005 to 0.2% and Nb: 0.005 to 0.2%.
[0032]
Further, a part of Fe is Ni: 0.05 to 1.5%, Mo: 0.05 to 1.5%, Cu: 0.05 to 1.5%, B: 0.0003 to 0.01. %, One or two or more selected from Ti: 0.005 to 0.2%, and Nb: 0.005 to 0.2%. You may.
[0033]
The reasons for the above limitation will be described below.
[0034]
C: 0.10 to less than 0.20% C is an element that increases strength, but is an element that reduces toughness and weldability. Therefore, in order to ensure toughness and weldability at a high strength level, it is necessary to limit the content of C to less than 0.20%. From the viewpoints of toughness and weldability, the lower the C content, the better. However, if it is less than 0.10%, it becomes difficult to secure strength. For this reason, the content of C is limited to the range of 0.10 to less than 0.20%. From the viewpoint of ensuring the best balance between strength and toughness, the content of C is preferably in the range of 0.13 to 0.17%.
[0035]
Si: 0.05 to 1.0%
Si acts as a deoxidizing agent and has an effect of improving strength. However, if the content is less than 0.05%, the effect cannot be obtained, and if it exceeds 1.0%, the toughness is reduced. For this reason, the content of Si is limited to the range of 0.05 to 1.0%. From the viewpoint of ensuring the best balance between strength and toughness, the content of Si is preferably in the range of 0.1 to 0.4%.
[0036]
Mn: 0.5-2.5%
Mn is an element that increases the strength without lowering the toughness by adding it together with Cr and V to steel with reduced C. In order to secure a predetermined strength, the content must be 0.5% or more. On the other hand, if it exceeds 2.5%, the weldability and toughness decrease. For this reason, the Mn content is limited to the range of 0.5 to 2.5%. From the viewpoint of ensuring the best balance between strength and toughness, the content of Mn is preferably in the range of 1.5 to 2.0%.
[0037]
P: 0.03% or less P is an impurity element that is concentrated near the final solidification position during solidification and segregates at grain boundaries to reduce hot workability and toughness. Therefore, it is preferable to reduce the content as much as possible. However, the content of P is set to 0.03% or less because it is allowable up to 0.03%. However, in order to further secure high toughness, the content is preferably set to 0.02% or less, more preferably 0.01% or less.
[0038]
S: 0.05% or less S, like P, is an impurity element that segregates at the grain boundary during solidification and reduces hot workability and toughness. Therefore, it is preferable to reduce the content as much as possible, but the content of S is set to 0.05% or less because it is allowable up to 0.05%. However, if the content is excessively reduced, the machinability may be reduced. Therefore, when the machinability is emphasized, the lower limit of S is preferably set to 0.01%. On the other hand, when the toughness is more important than the machinability, the S content is preferably set to 0.02% or less, more preferably 0.01% or less.
[0039]
Cr: 0.5 to 1.5%
Cr is an element that increases the strength without decreasing the toughness by adding it in combination with Mn and V to steel with reduced C. In order to secure a predetermined strength, the content must be 0.5% or more. On the other hand, if it exceeds 1.5%, the weldability and the toughness decrease. For this reason, the content of Cr is limited to the range of 0.5 to 1.5%. From the viewpoint of ensuring the best balance between strength and toughness, the content of Cr is preferably in the range of 0.9 to 1.4%.
[0040]
V: 0.03-0.3%
V is added because it precipitates fine V carbides and increases the strength. If Mn and Cr are added in combination, the decrease in toughness can be suppressed to a small level while having high strength. In order to obtain this effect, it is necessary to add 0.03% or more. On the other hand, if it exceeds 0.3%, the toughness decreases, so the V content is limited to the range of 0.03 to 0.3%. From the viewpoint of ensuring the best strength-toughness balance, the V content is preferably in the range of 0.05 to 0.15%.
[0041]
Al: 0.003 to 0.10%
Al acts as a deoxidizing agent. In order to obtain this effect, the content of 0.003% or more is necessary. If it exceeds 0.10%, there is a concern that alumina-based inclusions increase and surface defects frequently occur. For this reason, the Al content is limited to the range of 0.003 to 0.10%. In order to secure stable surface quality, the range of 0.003 to 0.05% is preferable.
[0042]
N: 0.001 to 0.02%
N coexists with Al and Ti to refine the crystal grains and has the effect of improving the toughness. However, if the content is less than 0.001%, the effect is small, and if it exceeds 0.02%, the toughness is reduced. For this reason, the content of N is limited to the range of 0.001 to 0.02%.
[0043]
O: 0.003% or less When O is contained in excess of 0.003%, toughness and fatigue strength are reduced. Therefore, the content of O is set to 0.003% or less.
[0044]
Ni: 0.05 to 1.5%, Mo: 0.05 to 1.5%, Cu: 0.05 to 1.5%, B: 0.0003 to 0.01% Any one or more of Ni, Mo, Cu, and B are elements that improve the hardenability and increase the strength of steel, and one or more of them can be added as necessary. When added, the effect is exhibited at 0.05% or more for Ni, Mo, and Cu, and 0.0003% or more for B. On the other hand, when Ni, Mo, and Cu are added in excess of 1.5%, Ni saturates the effect of increasing strength and increases cost, Mo decreases weldability and toughness, and Cu decreases hot workability. Let it. Therefore, the upper limits of Ni, Mo, and Cu are each set to 1.5%. If B is added in excess of 0.01%, the toughness decreases, so the upper limit was made 0.01%.
[0045]
One or two kinds of Ti and Nb selected from Ti: 0.005 to 0.2% and Nb: 0.005 to 0.2% all form carbides to refine the structure and improve toughness. It is an element that improves the strength and precipitates in the matrix to increase the strength and contribute to the high strength. Therefore, if necessary, one or two of the above can be added. The effect can be obtained by adding all elements at 0.005% or more. On the other hand, if added in excess of 0.2%, the toughness decreases. For this reason, the range is limited to the range of 0.005 to 0.2%.
[0046]
Carbon equivalent Ceq. (%): 0.60 to 0.85%
Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
The alloy element amounts are each defined by mass%. In addition to the above-mentioned limitation of the component composition, from the viewpoint of maintaining good strength, toughness and weldability, Ceq. Is preferably limited to the range of 0.60 to 0.85%. Ceq. Is less than 0.60%, the strength cannot be secured, and Ceq. Exceeds 0.85%, the toughness is reduced and weld cracks are liable to occur. In addition, from a viewpoint of high strength, 0.65 to 0.85% is preferable, and more preferably 0.70 to 0.85%.
[0047]
The balance is Fe and impurities.
[0048]
Among the impurities, each of Ca, Mg, and REM (rare earth metal) can be contained with an upper limit of 0.01%. These elements do not significantly affect the strength, toughness, and weldability, but may be added at the time of casting, particularly when casting into a round billet, because they prevent nozzle clogging of the tundish. If the content of these elements exceeds 0.01%, the surface properties are deteriorated and the yield is reduced. Therefore, each may be contained as an impurity with the upper limit being 0.01%.
[0049]
The seamless steel pipe according to the present invention can be manufactured as follows. That is, steel having the above composition is melted in a converter, an electric furnace, or a vacuum melting furnace, and solidified by a continuous casting method or an ingot-making method. This solidified product is obtained as it is or by agglomerating into a steel pipe raw material, and then subjected to an ordinary seamless steel pipe manufacturing process to form a steel pipe, followed by air cooling.
[0050]
The cooling after hot rolling is preferably air cooling by natural cooling, but warm cooling such as blast (cooling while covering with a windshield cover) or wind cooling (cooling while sending some wind) may be performed.
[0051]
【Example】
Steel having the chemical composition shown in Table 1 was melted and cast into an ingot, and then a billet was formed by forging. These billets were heated to 1250 ° C. and formed into a tube with a Mannes mandrel type mill to form a seamless steel tube having an outer diameter of 150 mm and a wall thickness of 24.2 mm. These steel tubes were air cooled after rolling. The as-made pipes were examined for mechanical properties (tensile properties, impact properties by Charpy test) and weld cracking properties of these steel pipes.
[0052]
In addition, welding crackability was examined by a covered arc welding method with a heat input of 20 kJ / cm without preheating in accordance with JIS Z 3158 "Y-type welding crack test" to check for cracks. Table 1 also shows the results of these tests. In the table, TS (MPa) indicates tensile strength. 2uE20 (J) indicates a Charpy rupture energy value based on JIS Z 2202 and JIS Z 2242 as an index of toughness. Corresponding.
[0053]
[Table 1]

[0054]
In Table 1, sample numbers (test numbers) 1 to 21 are examples of the present invention, and the tensile strength exceeds 700 MPa at the minimum and reaches nearly 900 MPa at the maximum. Even with such a high strength, the toughness evaluated by the impact value could secure 50 J or more, and no welding cracks occurred.
[0055]
Sample numbers (test numbers) 22 to 28 are comparative examples. Test Nos. 22 and 23 are examples of medium carbon-based components that become conventional ferrite-pearlite steels. In this case, although the strength could be ensured, the toughness was low and welding cracks also occurred. Test numbers 24 to 27 are examples in which Mn, Cr, and V are not added in a composite manner or are out of the specified range. No weld cracking occurred, but the toughness was insufficient. Test No. 28 satisfies the component composition range, but has a carbon equivalent Ceq. Is a low example. The strength is significantly lower than 700 MPa and cannot be used for high strength non-heat treated seamless steel pipe applications.
[0056]
【The invention's effect】
As described above, according to the present invention, a non-refined seam that can achieve both high strength and high toughness and secure good weldability without strictly controlling the degree of pipe forming and the pipe forming temperature in the middle. Steel-free pipe can be obtained.

Claims (4)

In mass%,
C: less than 0.10 to 0.20%,
Si: 0.05 to 1.0%,
Mn: 0.5-2.5%,
P: 0.03% or less,
S: 0.05% or less,
Cr: 0.5 to 1.5%,
V: 0.03-0.3%,
Al: 0.003 to 0.10%,
N: 0.001 to 0.02%,
O: contains 0.003% or less,
The balance consists of Fe and impurities,
The carbon equivalent Ceq. (%) Is 0.60 to 0.85%, non-heat-treated seamless steel pipe characterized by the above-mentioned.
Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Part of Fe,
Ni: 0.05-1.5%,
Mo: 0.05-1.5%,
Cu: 0.05-1.5%,
B: The non-heat-treated seamless steel pipe according to claim 1, wherein the pipe is replaced with one or more kinds selected from 0.0003 to 0.01%. Part of Fe,
Ti: 0.005 to 0.2%,
The non-heat-treated seamless steel pipe according to claim 1, wherein one or two kinds selected from Nb: 0.005 to 0.2% are substituted. Part of Fe,
Ni: 0.05-1.5%,
Mo: 0.05-1.5%,
Cu: 0.05-1.5%,
B: one or more selected from 0.0003 to 0.01%, and Ti: 0.005 to 0.2%,
The non-heat-treated seamless steel pipe according to claim 1, wherein one or two kinds selected from Nb: 0.005 to 0.2% are substituted.