TW202039883A - Electric resistance welded steel pipe and method for manufacturing same, and steel pipe pile - Google Patents

Abstract

An electric resistance welded steel pipe having a base material part and also having a welded part in the tube axis direction, wherein the base material part has a specified component composition, a steel structure at a position corresponding to the depth 1/4t from the outer surface of the electric resistance welded steel pipe contains bainite at an area ratio of 70% or more wherein t represents the thickness of the base material part, the average effective grain diameter of the bainite is 10.0 [mu]m or less in terms of average equivalent circle diameter, the average aspect ratio of the bainite is 0.1 to 0.8, the tensile strength in the tube axis direction is 590 MPa or more, the 0.2% proof stress is 450 MPa or more, the yield ratio is 85 to 95%, the Charpy absorbed energy at -30 DEG C as determined employing the tube axis direction in the base material part as the length direction of a test specimen is 70J or more, and the residual stress of the outer surface of the steel pipe in the base material part as observed in the tube axis direction is 250 MPa or less; a method for manufacturing the electric resistance welded steel pipe; and a steel pipe pile.

Description

電焊鋼管及其製造方法以及鋼管樁Electric welded steel pipe and its manufacturing method and steel pipe pile

本發明係關於：很適合使用於作為構造物的基礎的鋼管樁之電焊鋼管及其製造方法、以及鋼管樁。尤其是本發明係關於：以熱軋鋼板(熱軋鋼帶)為素材，將素材在常溫下進行滾壓成形為鋼管而製得的電焊鋼管的高強度化、高韌性化、降伏比的最佳化、以及提高耐挫曲性能。The present invention relates to an electric welded steel pipe which is suitable for use as a steel pipe pile as a foundation of a structure, a manufacturing method thereof, and a steel pipe pile. In particular, the present invention relates to the high strength, high toughness, and best yield ratio of electric welded steel pipes made by using hot-rolled steel sheets (hot-rolled steel strips) as materials and roll-forming the materials into steel pipes at room temperature And improve the resistance to buckling.

近年來，作為對應於大規模地震的對策之一，針對於當作構造物的基礎來使用的鋼管樁，也強烈地要求鋼管樁的高強度化以及提高變形能量的吸收性能。一般而言，想要提高鋼管之變形能量吸收性能的話，採用具有高拉伸強度與低降伏比的鋼管是很有效的作法。但是，基於抑制鋼管樁在進行打樁時之鋼管變形的觀點考量，很難以將鋼管樁在管軸方向的降伏比設定成太低。再者，尤其是對於在寒冷地帶使用的鋼管樁而言，也必須具有很高的低溫韌性。此外，為了要能夠抗拒地震等所造成的變形，也必須具有很高的耐挫曲性能。In recent years, as one of the countermeasures against large-scale earthquakes, steel pipe piles used as the foundation of structures are also strongly required to increase the strength of steel pipe piles and improve the ability to absorb deformation energy. Generally speaking, if you want to improve the deformation energy absorption performance of steel pipes, it is very effective to use steel pipes with high tensile strength and low yield ratio. However, it is difficult to set the yield ratio of the steel pipe pile in the pipe axis direction to be too low from the viewpoint of restraining the steel pipe deformation during pile driving. Furthermore, especially for steel pipe piles used in cold regions, they must also have high low temperature toughness. In addition, in order to be able to resist deformation caused by earthquakes, it must also have high buckling resistance.

專利文獻1所揭示的是：耐局部挫曲性優異的耐震性焊接鋼管的製造方法。專利文獻1所揭示的製造方法，係將以重量%計，其組成分是含有C：0.03～0.15%、Mn：1.0～2.0%，且含有選自Cu：0.05～0.50%、Ni：0.05～0.50%、Cr：0.05～0.50%、Mo：0.05～0.50%、Nb：0.005～0.10%、V：0.005～0.10%、Ti：0.005～0.080%的其中一種以上，並且Pcm為0.10～0.25的鋼，進行熱軋，輥軋結束後，以5℃/s以上的冷卻速度進行冷卻至600℃以下而製得的鋼板，進行室溫下的成形加工而製作成鋼管。藉此，係可製得：進行管軸方向的拉伸試驗時的加工硬化指數為0.10以上之變形性能優異的鋼管，如此一來，能夠防止：鋼管受到來自橫向的外力的作用而發生局部挫曲之情事、以及因為這種局部挫曲所導致的發生脆性的龜裂和斷裂之情事。Patent Document 1 discloses a method for manufacturing a seismically resistant welded steel pipe excellent in local buckling resistance. The manufacturing method disclosed in Patent Document 1 will be based on weight %, and its composition will be C: 0.03 to 0.15%, Mn: 1.0 to 2.0%, and Cu: 0.05 to 0.50%, Ni: 0.05 to Steel with one or more of 0.50%, Cr: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Nb: 0.005 to 0.10%, V: 0.005 to 0.10%, Ti: 0.005 to 0.080%, and Pcm of 0.10 to 0.25 , Hot rolling is performed, and after the rolling is completed, the steel sheet is cooled to 600° C. or less at a cooling rate of 5° C./s or more, and formed into a steel pipe at room temperature. As a result, it is possible to obtain a steel pipe with excellent deformation properties with a work hardening index of 0.10 or more during a tensile test in the pipe axis direction. In this way, the steel pipe can be prevented from being locally frustrated by external forces from the lateral direction. The situation of music, and the occurrence of brittle cracks and fractures caused by this local setback.

專利文獻2所揭示的鋼管的製造方法，係將組成分以重量%計，含有C：0.02～0.20%、Si：0.02～0.50%、Mn：0.50～2.00%，並且含有選自Cu：0.10～1.5%、Ni：0.10～0.50%、Nb：0.005～0.10%及V：0.005～0.10%之其中一種或兩種以上，且Ceq的數值為0.38～0.45之鋼片，以在900℃以上的溫度域內的每一道次(pass)輥軋的軋縮率為小於等於4%的方式進行熱軋，而做成熱軋鋼板，將該熱軋鋼板在Ac1點以上且Ac3點以下的雙相溫度域進行再加熱，然後，從該雙相溫度域進行硬化淬火處理，並且又進行回火處理之後，才進行製管加工。以這種方法所製得的鋼管，是0.2%降伏強度為440MPa以上、拉伸強度為590～700MPa、降伏比為80%以下之低降伏比的高張力鋼管，很適合使用於建築物、橋樑、油槽之類的鋼構造物。The method of manufacturing a steel pipe disclosed in Patent Document 2 contains C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.50 to 2.00%, and Cu: 0.10 to 2.00%. 1.5%, Ni: 0.10～0.50%, Nb: 0.005～0.10% and V: 0.005～0.10% of one or two or more of them, and the value of Ceq is 0.38～0.45 for the steel sheet at a temperature above 900℃ The hot-rolled steel sheet is made into a hot-rolled steel sheet with a rolling reduction ratio of 4% or less per pass in the domain, and the hot-rolled steel sheet is at a dual phase temperature above Ac1 point and below Ac3 point The zone is reheated, and then the hardening and quenching treatment is carried out from the dual-phase temperature zone, and after the tempering treatment is carried out, the pipe making process is carried out. The steel pipe produced by this method is a high-tension steel pipe with a low yield ratio of 0.2% yield strength of 440 MPa or more, tensile strength of 590 to 700 MPa, and yield ratio of 80% or less. It is very suitable for use in buildings and bridges. , Steel structures such as oil tanks.

專利文獻3所揭示的製造方法，係在製造其組成分以質量%計，係含有C：0.10～0.18%、Si：0.1～0.5%、Mn：1～2%的鋼管時，依序地實施：加熱至Ac3點以上之後進行急速冷卻的工序、加熱至Ac1點～Ac3點的雙相溫度域之後進行室溫冷卻的工序、在室溫下進行成形為管狀的工序、再加熱至500～600℃的工序，來予以製作成低降伏比之建築構造用高張力鋼管。如此一來，無需使用昂貴的合金元素，即可製造出拉伸強度為590MPa以上的建築構造用鋼管。The manufacturing method disclosed in Patent Document 3 is carried out in order when manufacturing a steel pipe whose composition is calculated by mass% and contains C: 0.10 to 0.18%, Si: 0.1 to 0.5%, and Mn: 1 to 2%. : After heating to Ac3 point or higher, the process of rapid cooling is performed, the process of heating to the dual phase temperature range of Ac1 to Ac3, and the process of cooling at room temperature, the process of forming into a tube at room temperature, and then heating to 500-600 ℃ process to produce high-tensile steel pipes for building structures with low yield ratio. In this way, without using expensive alloy elements, steel pipes for building structures with a tensile strength of 590 MPa or more can be manufactured.

專利文獻4所揭示之適合鋼管樁用的低降伏比高強度電焊鋼管，其組成分以質量%計，係含有C：0.11～0.20%、Si：0.05～0.50%、Mn：1.00～2.00%、P：0.030%以下、S：0.010%以下、Al：0.01～0.08%，並且具有：以肥粒鐵相為主相，主相以外的第二相，以面積率計，是8～30%的波來鐵及/或擬似波來鐵，該主相與第二相的平均粒徑為4.0～10μm的金相組織，在管圓周方向及管軸方向的0.2%降伏強度YS為450MPa以上、拉伸強度TS為590MPa以上、且降伏比為90%以下。 [先前技術文獻] [專利文獻]Patent Document 4 discloses a low-yield ratio high-strength electric-welded steel pipe suitable for steel pipe piles. Its composition is based on mass% and contains C: 0.11 to 0.20%, Si: 0.05 to 0.50%, Mn: 1.00 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.01 to 0.08%, and have: the fertilizer grain iron phase as the main phase, and the second phase other than the main phase. The area ratio is 8-30% Polly iron and/or pseudo-wave-like iron, the main phase and the second phase have a metallographic structure with an average particle size of 4.0 to 10 μm, and the 0.2% yield strength YS in the pipe circumferential direction and the pipe axis direction is 450 MPa or more. The tensile strength TS is 590 MPa or more, and the yield ratio is 90% or less. [Prior Technical Literature] [Patent Literature]

專利文獻1：日本特開平11-6032號公報 專利文獻2：日本特許第2687841號公報 專利文獻3：日本特開2004-300461號公報 專利文獻4：日本特許第6123734號公報Patent Document 1: Japanese Patent Application Laid-Open No. 11-6032 Patent Document 2: Japanese Patent No. 2687841 Patent Document 3: JP 2004-300461 A Patent Document 4: Japanese Patent No. 6123734

[發明所欲解決之問題][The problem to be solved by the invention]

然而，以專利文獻1所揭示的技術來製造的鋼管，管軸方向的降伏比太低。因此，如果將該鋼管應用在鋼管樁的話，在進行打樁時，將會有因為進行打樁而發生挫曲等的問題之虞慮。However, the steel pipe manufactured by the technique disclosed in Patent Document 1 has too low a yield ratio in the pipe axis direction. Therefore, if the steel pipe is applied to steel pipe piles, there is a concern that problems such as buckling due to pile driving may occur during pile driving.

專利文獻2所揭示的技術，則是必須實施回火用的熱處理工序。又，專利文獻3所揭示的技術，除了必須採用大型的管用熱處理裝置之外，在製管之後，還要實施熱處理工序。這兩種還需要實施熱處理工序的技術，係有降伏比太低之問題。此外，也有工序變複雜而導致生產性降低之問題。又，會導致生產成本增加而難以較低廉的價格來提供產品。The technique disclosed in Patent Document 2 requires a heat treatment process for tempering. In addition, the technique disclosed in Patent Document 3 requires the use of a large-scale pipe heat treatment device, and also performs a heat treatment process after the pipe is manufactured. These two technologies, which also require a heat treatment process, have the problem of too low yield ratio. In addition, there is a problem that the process becomes complicated and the productivity is reduced. In addition, it will increase production costs and make it difficult to provide products at lower prices.

專利文獻4所揭示的技術，在熱軋之後的10秒～100秒的時間內，從最終輥軋結束溫度進行冷卻至550～700℃的溫度域，因而獲得以肥粒鐵和波來鐵為主體的金相組織，但卻無法獲得所期望的金相組織。此外，需要採用有非常長的冷卻區域的設備，因此，難以較低廉的價格來提供適合當作鋼管樁用的高強度高韌性電焊鋼管。The technique disclosed in Patent Document 4 cools from the end temperature of the final rolling to a temperature range of 550 to 700°C within 10 seconds to 100 seconds after hot rolling. The main body's metallographic structure, but it is unable to obtain the desired metallographic structure. In addition, equipment with a very long cooling area is required. Therefore, it is difficult to provide high-strength and high-toughness electric welded steel pipes suitable for steel pipe piles at a low price.

本發明，係有鑒於上述的技術課題而開發完成的，係以提供：具有最佳的降伏比和高耐挫曲性能，並且具備高強度及高韌性之電焊鋼管及其製造方法、以及鋼管樁為本發明之目的。The present invention was developed in view of the above-mentioned technical issues to provide: an electric welded steel pipe with the best yield ratio and high buckling resistance, and with high strength and toughness, its manufacturing method, and steel pipe piles For the purpose of the present invention.

此外，本發明主要是針對於使用板厚度為16mm以下的熱軋鋼板來作為素材的情況，提供可達成上述課題之電焊鋼管及其製造方法、以及鋼管樁。In addition, the present invention mainly aims at the case of using a hot-rolled steel sheet having a plate thickness of 16 mm or less as a material, and provides an electric resistance welded steel pipe, a manufacturing method thereof, and a steel pipe pile that can achieve the above-mentioned problems.

此處所稱的「高強度」，係指：在電焊鋼管的母材部中的管軸方向之0.2%降伏強度(YS)為450MPa以上、拉伸強度(TS)為590MPa以上的情況。此處所稱的「高韌性」，係指：將電焊鋼管的母材部中的管軸方向當作試驗片長邊方向時之在溫度-30℃時的夏比衝擊試驗的衝擊吸收能量為70J以上的情況，並且無論是在電焊鋼管的管圓周方向和管軸方向，都要符合上述的高韌性的電焊鋼管。此處所稱的「最佳的降伏比」，係指：0.2%降伏強度與上述拉伸強度的比值(YR)為85～95%的情況。此處所稱的「高耐挫曲性能」，係指：電焊鋼管的母材部中的鋼管外表面之在管軸方向上的殘留應力為250MPa以下，且降伏比為95%以下的情況。 [解決問題之技術手段]The "high strength" referred to here refers to the case where the 0.2% yield strength (YS) in the tube axis direction of the base material of the electric welded steel pipe is 450 MPa or more and the tensile strength (TS) is 590 MPa or more. The "high toughness" referred to here refers to the impact absorption energy of the Charpy impact test at a temperature of -30°C when the tube axis direction in the base material part of the electric welded steel pipe is taken as the longitudinal direction of the test piece is 70J or more Circumstances, and both in the circumferential direction of the electric welded steel pipe and the pipe axis direction, must meet the above-mentioned high toughness electric welded steel pipe. The "optimal yield ratio" referred to here refers to the case where the ratio of 0.2% yield strength to the above-mentioned tensile strength (YR) is 85-95%. The "high buckling resistance" referred to here refers to the case where the residual stress in the tube axis direction on the outer surface of the steel pipe in the base material of the electric welded steel pipe is 250 MPa or less, and the yield ratio is 95% or less. [Technical means to solve the problem]

本發明人等，為了達成上述目的，乃針對於會影響到：降伏比、0.2%降伏強度、拉伸強度、以及夏比衝擊特性之各種合金元素及製造條件，努力地進行了檢討。此外，也針對於所製得的鋼管(電焊鋼管)的耐挫曲性能，努力地進行了檢討。其結果，找到了既可維持較低的降伏比，又可兼具高強度和高韌性，且具有高耐挫曲性能之合適的組成分、鋼組織及製造條件。In order to achieve the above-mentioned object, the present inventors have diligently reviewed various alloy elements and manufacturing conditions that affect the yield ratio, 0.2% yield strength, tensile strength, and Charpy impact characteristics. In addition, the buckling resistance of the produced steel pipe (electrically welded steel pipe) has also been reviewed hard. As a result, a suitable composition, steel structure, and manufacturing conditions that can maintain a low yield ratio, have high strength and toughness, and have high buckling resistance have been found.

亦即，本發明人找到了一種創見，就是：針對於以特定的組成分，且限定了熱軋條件而製造出來的熱軋鋼板，在室溫下的滾壓成形所進行的滾壓成形工序中，進行焊接後，在特定的條件下實施縮徑輥軋。如此一來，係可製得：位於從電焊鋼管的母材部的鋼管外表面起算之板厚度t的1/4t深度位置處的鋼組織，以面積率計，係含有變韌鐵為70%以上；變韌鐵的平均有效粒徑，以平均圓當量直徑計，係10.0μm以下；且變韌鐵的平均長寬比，係0.1～0.8；0.2%降伏強度，係450MPa以上；拉伸強度很高，係590MPa以上；且降伏比，係85～95%；在-30℃溫度時的夏比衝擊試驗的衝擊吸收能量，係70J以上；母材部中的鋼管外表面之在管軸方向上的殘留應力，係250MPa以下之具有低降伏比、高強度、高韌性以及高耐挫曲性能的電焊鋼管。That is, the present inventors have found an original idea, which is: for a hot-rolled steel sheet manufactured with a specific composition and limited hot-rolling conditions, the roll forming process at room temperature is performed In this, after welding, diameter reduction rolling is performed under specific conditions. In this way, the system can be obtained: the steel structure located at the depth of 1/4t of the plate thickness t from the outer surface of the steel pipe of the base metal part of the electric welded steel pipe, in terms of area ratio, contains 70% toughened iron Above; the average effective particle size of the toughened iron is less than 10.0μm in terms of the average circle equivalent diameter; and the average aspect ratio of the toughened iron is 0.1～0.8; 0.2% yield strength is 450MPa or more; tensile strength Very high, above 590MPa; and the yield ratio is 85-95%; the impact absorption energy of the Charpy impact test at -30℃ is above 70J; the outer surface of the steel pipe in the base material is in the tube axis direction The residual stress above is an electric welded steel pipe with low yield ratio, high strength, high toughness and high buckling resistance under 250MPa.

本發明就是基於這種創見，再進一步地加以檢討而完成的，本發明的要旨如下所述。 [1]一種電焊鋼管，其係具有母材部以及在管軸方向上的焊接部之電焊鋼管， 母材部的組成分，係質量%計，係含有 C：0.020～0.11%、 Si：0.60%以下、 Mn：0.50～1.70%、 P：0.030%以下、 S：0.015%以下、 Al：0.010～0.060%、 Nb：0.010～0.080%、 V：0.001～0.060%、 Ti：0.010～0.050%、 N：0.006%以下，其餘部分由Fe及不可避免的雜質所組成， 假設前述母材部的板厚度為t時，位於從前述電焊鋼管的外表面起算之板厚度t的1/4t深度位置處的鋼組織， 以面積率計，係含有變韌鐵為70%以上； 前述變韌鐵的平均有效粒徑，以平均圓當量直徑計，係10.0μm以下；且前述變韌鐵的平均長寬比，係0.1～0.8； 管軸方向的拉伸強度，係590MPa以上；0.2%降伏強度，係450MPa以上；降伏比，係85～95%； 以前述母材部中的管軸方向當作試驗片長邊方向時之在-30℃溫度時之夏比衝擊試驗的衝擊吸收能量，係70J以上； 前述母材部中的鋼管外表面之在管軸方向上的殘留應力，係250MPa以下。 [2]如前述[1]所述的電焊鋼管，其中，除了前述組成分之外，以質量%計，還含有B：0.008%以下。 [3]如前述[1]或[2]所述的電焊鋼管，其中，除了前述組成分之外，以質量%計，還含有從Cr：0.01～1.0%、Mo：0.01～1.0%、Cu：0.01～0.50%、Ni：0.01～1.0%、Ca：0.0005～0.010%之中所選出的一種或兩種以上。 [4]一種電焊鋼管的製造方法，其係對於鋼素材依序地實施熱軋工序、冷卻工序而製作成熱軋鋼板，然後，對於該熱軋鋼板實施室溫下的滾壓成形工序而製作成電焊鋼管， 前述鋼素材，係具有如前述[1]至[3]中的任一項所記載的組成分； 前述熱軋工序，係對於前述鋼素材進行加熱到1100～1280℃的溫度之後，再以粗輥軋結束溫度為850～1150℃、最終輥軋結束溫度為750～850℃、且在進行粗輥軋與最終輥軋時之在930℃以下的合計軋縮率為65%以上的條件，來實施粗輥軋及最終輥軋而製作成熱軋鋼板的工序； 前述冷卻工序，係對於前述熱軋鋼板，以板厚度中心溫度為準，以開始冷卻至停止冷卻為止的平均冷卻速度為15～35℃/s、冷卻停止溫度為450～650℃的條件，來進行冷卻的工序； 前述室溫下的滾壓成形工序，係先對於將前述熱軋鋼板實施了滾壓成形加工後的鋼管素材進行焊接，再以對於焊接後的鋼管外表面的圓周長度而言的縮徑率為0.2～0.5%的條件，來實施縮徑輥軋。 [5]一種電焊鋼管的製造方法，其係對於具有：如前述[1]至[3]中的任一項所記載的組成分，假設板厚度為t時，位於從外表面起算之板厚度t的1/4t深度位置處的鋼組織，以面積率計，係含有變韌鐵為70%以上；前述變韌鐵的平均有效粒徑，以平均圓當量直徑計，係10.0μm以下；且前述變韌鐵的平均長寬比，係0.1～0.8的熱軋鋼板，實施室溫下的滾壓成形工序而製作成電焊鋼管， 前述室溫下的滾壓成形工序，係先對於將前述熱軋鋼板實施了滾壓成形加工後的鋼管素材進行焊接，再以對於焊接後的鋼管外表面的圓周長度而言的縮徑率為0.2～0.5%的條件，來實施縮徑輥軋。 [6]一種鋼管樁，其係使用如前述[1]至[3]中的任一項所記載的電焊鋼管。 [發明之效果]The present invention is completed based on this insight and further review. The gist of the present invention is as follows. [1] An electric welded steel pipe, which is an electric welded steel pipe having a base material portion and a welded portion in the pipe axis direction, The composition of the base metal part is calculated by mass% and contains C: 0.020～0.11%, Si: 0.60% or less, Mn: 0.50～1.70%, P: 0.030% or less, S: 0.015% or less, Al: 0.010～0.060%, Nb: 0.010～0.080%, V: 0.001～0.060%, Ti: 0.010～0.050%, N: 0.006% or less, the rest is composed of Fe and inevitable impurities, Assuming that the plate thickness of the base material portion is t, the steel structure at a depth of 1/4t of the plate thickness t calculated from the outer surface of the electric resistance welded steel pipe, In terms of area ratio, the system contains more than 70% toughened iron; The average effective particle size of the aforementioned toughened iron is 10.0μm or less in terms of average equivalent circle diameter; and the average aspect ratio of the aforementioned toughened iron is 0.1-0.8; The tensile strength in the axial direction of the pipe is above 590MPa; the yield strength of 0.2% is above 450MPa; the yield ratio is 85-95%; The impact absorption energy of the Charpy impact test at a temperature of -30°C when the tube axis direction in the aforementioned base material portion is taken as the long side direction of the test piece is 70J or more; The residual stress in the pipe axis direction on the outer surface of the steel pipe in the aforementioned base material portion is 250 MPa or less. [2] The electric resistance welded steel pipe according to the aforementioned [1], which further contains B: 0.008% or less in terms of mass% in addition to the aforementioned composition. [3] The electric resistance welded steel pipe according to [1] or [2], which, in addition to the aforementioned composition, contains, in terms of mass %, from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, and Cu ：0.01～0.50%, Ni: 0.01～1.0%, Ca: 0.0005～0.010% selected from one or two or more. [4] A method for manufacturing an electric welded steel pipe, which is a hot-rolled steel sheet by sequentially performing a hot rolling process and a cooling process on a steel material, and then performing a roll forming process at room temperature on the hot-rolled steel sheet. Into electric welded steel pipe, The aforementioned steel material has a composition as described in any one of [1] to [3]; The hot rolling process is to heat the steel material to a temperature of 1100 to 1280°C, and then to finish the rough rolling at a temperature of 850 to 1150°C and the final temperature to finish rolling at 750 to 850°C. The process of performing rough rolling and final rolling to produce a hot-rolled steel sheet under the condition that the total reduction ratio at 930°C or less during rolling and final rolling is 65% or more; The aforementioned cooling step is based on the temperature of the center of the thickness of the hot-rolled steel sheet, with an average cooling rate of 15 to 35°C/s and a cooling stop temperature of 450 to 650°C. Carry out the cooling process; The aforementioned roll forming process at room temperature is to first weld the steel pipe material after the aforementioned hot-rolled steel sheet has been roll-formed, and then take the diameter reduction rate relative to the circumferential length of the welded steel pipe outer surface Under the condition of 0.2 to 0.5%, reduce diameter rolling is performed. [5] A method for manufacturing electric resistance welded steel pipes, which has: the composition described in any one of [1] to [3], assuming that the plate thickness is t, the plate thickness from the outer surface The steel structure at the depth of 1/4t of t contains more than 70% of toughened iron in terms of area ratio; the average effective particle size of the aforementioned toughened iron, in terms of average circle equivalent diameter, is below 10.0μm; and The average aspect ratio of the aforementioned toughened iron is a hot-rolled steel sheet of 0.1 to 0.8, which is subjected to a roll forming process at room temperature to produce an electric welded steel pipe. The aforementioned roll forming process at room temperature is to first weld the steel pipe material after the aforementioned hot-rolled steel sheet has been roll-formed, and then take the diameter reduction rate relative to the circumferential length of the welded steel pipe outer surface Under the condition of 0.2 to 0.5%, reduce diameter rolling is performed. [6] A steel pipe pile using the electric resistance welded steel pipe described in any one of [1] to [3]. [Effects of Invention]

根據本發明，係可提供：很適合當作鋼管樁使用之具有最佳的降伏比及高耐挫曲性能，並且具備高強度及高韌性的電焊鋼管及其製造方法、以及鋼管樁。本發明的電焊鋼管可以很容易製造，因此能夠達成產業上的可利用性的效果。According to the present invention, it is possible to provide: an electric welded steel pipe with the best yield ratio and high buckling resistance, high strength and high toughness, and a method for manufacturing the steel pipe pile, which is very suitable for use as a steel pipe pile. The electric resistance welded steel pipe of the present invention can be easily manufactured, and therefore can achieve the effect of industrial applicability.

以下，就本發明進行詳細的說明。Hereinafter, the present invention will be described in detail.

首先，說明限定本發明的電焊鋼管的組成分之理由。在以下的說明中，如果沒有特別註明的話，都是將組成分的「質量%」簡單地以「%」來表示。First, the reasons for limiting the composition of the electric resistance welded steel pipe of the present invention will be explained. In the following description, unless otherwise specified, the "mass%" of the component is simply expressed as "%".

本發明的電焊鋼管，係具有母材部與焊接部，母材部的組成分，係含有C：0.020～0.11%、Si：0.60%以下、Mn：0.50～1.70%、P：0.030%以下、S：0.015%以下、Al：0.010～0.060%、Nb：0.010～0.080%、V：0.001～0.060%、Ti：0.010～0.050%、N：0.006%以下，其餘部分由Fe及不可避免的雜質所組成。The electric welded steel pipe of the present invention has a base metal part and a welded part. The composition of the base metal part contains C: 0.020 to 0.11%, Si: 0.60% or less, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015% or less, Al: 0.010～0.060%, Nb: 0.010～0.080%, V: 0.001～0.060%, Ti: 0.010～0.050%, N: less than 0.006%, the rest is composed of Fe and inevitable impurities composition.

此外，本發明的電焊鋼管，係在管軸方向上具有焊接部。又，後述的「熱軋鋼板」係包含：熱軋鋼板、熱軋鋼帶。In addition, the electric resistance welded steel pipe of the present invention has a welded portion in the pipe axis direction. In addition, the "hot rolled steel sheet" described later includes hot rolled steel sheet and hot rolled steel strip.

C：0.020～0.11% C係具有固溶強化作用，可增加鋼管(電焊鋼管)的強度，並且是對於變韌鐵之類的鋼組織的生成有相關性的元素。又，C係可藉由形成硬質組織來對於降低降伏比之有效的元素。板厚度較小的鋼管(例如：板厚度為16mm以下的鋼管)，因為外徑與內徑的差值很小，因此在進行製造鋼管時的加工度很小，降伏比不易上昇。因此，即使C含量只有0.020%，亦可將降伏比控制在95%以下。此外，板厚度較小的鋼管，在製造鋼管的素材(也就是熱軋鋼板)時的冷卻速度易於變大。因此，如果C含量超過0.11%的話，就很容易生成麻田散鐵而很容易導致韌性降低。C: 0.020～0.11% The C series has a solid solution strengthening effect, can increase the strength of steel pipes (electrically welded steel pipes), and is an element related to the formation of steel structures such as toughened iron. In addition, the C-based element is effective in reducing the yield ratio by forming a hard structure. Steel pipes with a small plate thickness (for example, steel pipes with a plate thickness of 16mm or less) have a small difference between the outer diameter and the inner diameter, so the degree of processing during the manufacture of the steel pipe is small, and the yield ratio is not easy to increase. Therefore, even if the C content is only 0.020%, the yield ratio can be controlled below 95%. In addition, a steel pipe with a small plate thickness tends to increase the cooling rate when manufacturing the material of the steel pipe (that is, the hot-rolled steel plate). Therefore, if the C content exceeds 0.11%, Asada scattered iron is easily produced and the toughness is easily reduced.

因此，為了獲得上述效果，C含量必須是0.020%以上。另一方面，C含量若超過0.11%的話，容易生成麻田散鐵，而無法獲得本發明想要的鋼組織。其結果，就無法確保本發明所欲獲得的高韌性。因此，將C含量設定在0.020～0.11%。C含量是設定在0.040%以上為宜，更好是設定在0.050%以上。又，C含量是設定在0.10%以下為宜。Therefore, in order to obtain the above effects, the C content must be 0.020% or more. On the other hand, if the C content exceeds 0.11%, Asada scattered iron is easily produced, and the steel structure desired in the present invention cannot be obtained. As a result, it is impossible to ensure the high toughness desired by the present invention. Therefore, the C content is set to 0.020 to 0.11%. The C content is preferably set to 0.040% or more, more preferably set to 0.050% or more. In addition, the C content is preferably set to 0.10% or less.

Si：0.60%以下 Si係具有作為脫氧劑的作用，並且是可增加鋼管的強度之元素。但是，Si含量太多的話，韌性會降低。基於這種理由，乃將Si含量設定在0.60%以下。Si含量設定在0.50%以下為宜，更好是設定在0.45%以下。又，Si含量的下限雖然並未特別地規定，但是基於電縫焊接性的觀點考量，將Si含量設定在0.01%以上為宜。更好是設定在0.02%以上。Si: 0.60% or less The Si series has the function as a deoxidizer and is an element that can increase the strength of the steel pipe. However, if the Si content is too large, the toughness will decrease. For this reason, the Si content is set to 0.60% or less. The Si content is preferably set to 0.50% or less, more preferably set to 0.45% or less. In addition, although the lower limit of the Si content is not specifically defined, it is preferable to set the Si content to 0.01% or more from the viewpoint of electric seam weldability. More preferably, it is set at 0.02% or more.

Mn：0.50～1.70% Mn係可藉由固溶強化來使鋼管的強度增加之元素。想要獲得這種效果來確保本發明所欲獲得的高強度，Mn含量必須設定在0.50%以上。另一方面，Mn含量若超過1.70%的話，鋼組織變成細微化，降伏強度變得太高，因而無法確保本發明所欲獲得的降伏比。因此，乃將Mn含量設定在0.50～1.70%。Mn含量是設定在0.55%以上為宜，更好是設定在0.60%以上。又，Mn含量是設定在1.65%以下為宜，更好是設定在1.60%以下。Mn: 0.50～1.70% Mn is an element that can increase the strength of steel pipes by solid solution strengthening. To obtain this effect and ensure the high strength desired by the present invention, the Mn content must be set at 0.50% or more. On the other hand, if the Mn content exceeds 1.70%, the steel structure becomes finer and the yield strength becomes too high, so that the yield ratio desired by the present invention cannot be ensured. Therefore, the Mn content is set at 0.50 to 1.70%. The Mn content is preferably set to 0.55% or more, more preferably set to 0.60% or more. In addition, the Mn content is preferably set to 1.65% or less, and more preferably set to 1.60% or less.

P：0.030%以下 P係會偏析在結晶粒界而導致韌性降低之元素，將P視為雜質的話，當然P含量是愈少愈好，但是在本發明中，可容許P含量至0.030%。基於這種理由，乃將P含量設定在0.030%以下。P含量是設定在0.025%以下為宜，更好是設定在0.020%以下。但是，過度降低P含量的話，將會導致精煉成本的上揚，因此將P含量設定在0.002%以上為宜，更好是設定在0.003%以上。P: 0.030% or less P is an element that segregates at the crystal grain boundary and reduces toughness. If P is regarded as an impurity, of course, the P content is as small as possible. However, in the present invention, the P content can be allowed to 0.030%. For this reason, the P content is set to 0.030% or less. The P content is preferably set to 0.025% or less, more preferably set to 0.020% or less. However, if the P content is excessively reduced, the refining cost will increase. Therefore, the P content is preferably set to 0.002% or more, and more preferably 0.003% or more.

S：0.015%以下 S係在進行製造鋼管的素材(也就是熱軋鋼板)時，以MnS的狀態存在於鋼中，在熱軋工序中被延展成很薄，而會對於鋼管的延性及韌性造成不良的影響。因此，在本發明中係將S視為雜質，而將其含量愈少愈好，但是S含量可容許至0.015%。因此，乃將S含量設定在0.015%以下。S含量是設定在0.010%以下為宜，更好是設定在0.008%以下。但是，過度降低S含量的話，將會導致精煉成本的上揚，因此將S含量設定在0.0002%以上為宜，更好是設定在0.001%以上。S: 0.015% or less When the S-based material (ie, hot-rolled steel sheet) is used to manufacture steel pipes, it exists in the steel in the form of MnS and is stretched very thin during the hot-rolling process, which adversely affects the ductility and toughness of the steel pipe. Therefore, in the present invention, S is regarded as an impurity, and its content is as small as possible, but the S content can be allowed to 0.015%. Therefore, the S content is set below 0.015%. The S content is preferably set to 0.010% or less, and more preferably set to 0.008% or less. However, excessive reduction of the S content will result in an increase in refining costs. Therefore, the S content is preferably set to 0.0002% or more, and more preferably 0.001% or more.

Al：0.010～0.060% Al係具有作為脫氧劑的作用，並且可與N結合而形成AlN而對於結晶粒的細微化有所助益。為了獲得這種效果，Al含量必須在0.010%以上。另一方面，Al含量太多而超過0.060%的話，將會降低鋼材(鋼管的素材也就是熱軋鋼板)的清淨度，因而降低鋼管的延性及韌性。因此，乃將Al含量設定在0.010～0.060%。Al含量是設定在0.015%以上為宜，更好是設定在0.020%以上。Al含量是設定在0.055%以下，更好是設定在0.050%以下。Al: 0.010～0.060% The Al system has a function as a deoxidizer, and can combine with N to form AlN, which is helpful for the refinement of crystal grains. In order to obtain this effect, the Al content must be above 0.010%. On the other hand, if the Al content is too large and exceeds 0.060%, the cleanliness of the steel (the material of the steel pipe is the hot-rolled steel plate) will be reduced, thereby reducing the ductility and toughness of the steel pipe. Therefore, the Al content is set at 0.010 to 0.060%. The Al content is preferably set to 0.015% or more, more preferably set to 0.020% or more. The Al content is set to 0.055% or less, more preferably 0.050% or less.

Nb：0.010～0.080% Nb係會與碳或氮相結合而形成細微的晶析物，利用晶析強化作用來增加鋼管的強度。為了獲得這種效果，必須將Nb含量設定在0.010%以上。另一方面，Nb含量若超過0.080%的話，在製造鋼管的素材(也就是熱軋鋼板)時，難以藉由熱軋工序中的加熱來使得Nb固溶化。其結果，將會成為粗大的晶析物而殘留下來，而導致韌性降低。因此，乃將Nb含量設定在0.010～0.080%。Nb含量是設定在0.015%以上為宜，更好是設定在0.020%以上。Nb含量是設定在0.075%以下為宜，更好是設定在0.070%以下。Nb: 0.010～0.080% The Nb system combines with carbon or nitrogen to form fine crystallization products, and uses the crystallization strengthening effect to increase the strength of the steel pipe. In order to obtain this effect, the Nb content must be set to 0.010% or more. On the other hand, if the Nb content exceeds 0.080%, it is difficult to solidify Nb by heating in the hot rolling process when manufacturing the material of the steel pipe (that is, hot rolled steel sheet). As a result, it will become a coarse crystallized product and remain, which will cause the toughness to fall. Therefore, the Nb content is set at 0.010 to 0.080%. The Nb content is preferably set to 0.015% or more, more preferably set to 0.020% or more. The Nb content is preferably set to 0.075% or less, more preferably set to 0.070% or less.

V：0.001～0.060% V係會與碳或氮相結合而形成細微的晶析物，利用晶析強化來增加鋼管的強度。為了獲得這種效果，必須將V含量設定在0.001%以上。另一方面，V含量若超過0.060%的話，晶析物將會變粗大化而導至韌性降低。因此，乃將V含量設定在0.001～0.060%。V含量是設定在0.002%以上為宜，更好是設定在0.003%以上。V含量是設定在0.055%以下為宜，更好是設定在0.050%以下。V: 0.001～0.060% The V series will combine with carbon or nitrogen to form fine crystallization products, and use crystallization strengthening to increase the strength of the steel pipe. In order to obtain this effect, the V content must be set to 0.001% or more. On the other hand, if the V content exceeds 0.060%, the crystallized product will be coarsened and the toughness will decrease. Therefore, the V content is set at 0.001 to 0.060%. The V content is preferably set to 0.002% or more, more preferably set to 0.003% or more. The V content is preferably set to 0.055% or less, and more preferably set to 0.050% or less.

Ti：0.010～0.050% Ti係會與碳或氮相結合而形成細微的晶析物，利用晶析強化來增加鋼管的強度。為了獲得這種效果，必須將Ti含量設定在0.010%以上。另一方面，Ti含量若超過0.050%的話，晶析物將會變粗大化而導至韌性降低。因此，乃將Ti含量設定在0.010～0.050%。Ti含量是設定在0.012%以上為宜，更好是設定在0.015%以上。Ti含量是設定在0.045%以下為宜，更好是設定在0.040%以下。Ti: 0.010～0.050% The Ti system combines with carbon or nitrogen to form fine crystallization products, and the strength of the steel pipe is increased by crystallization strengthening. In order to obtain this effect, the Ti content must be set to 0.010% or more. On the other hand, if the Ti content exceeds 0.050%, the crystallized product will be coarsened and the toughness will decrease. Therefore, the Ti content is set at 0.010 to 0.050%. The Ti content is preferably set to 0.012% or more, more preferably 0.015% or more. The Ti content is preferably set to 0.045% or less, more preferably set to 0.040% or less.

N：0.006%以下 N含量若是微量的話，係有增加鋼管的強度之效果，但是含量太多的話，會在高溫時形成粗大的晶析物而導至韌性降低。因此，乃將N含量設定在0.006%以下。N含量過度降低的話，將會導致精煉成本上揚，因此將N含量設定在0.001%以上為宜，更好是設定在0.002%以上。將N含量設定在0.005%以下為宜，更好是設定在0.004%以下。N: less than 0.006% If the N content is small, it has the effect of increasing the strength of the steel pipe, but if the content is too large, coarse crystallization products will be formed at high temperatures and the toughness will decrease. Therefore, the N content is set below 0.006%. If the N content is excessively reduced, it will lead to an increase in refining costs, so it is better to set the N content at 0.001% or more, and more preferably at 0.002% or more. The N content is preferably set to 0.005% or less, more preferably 0.004% or less.

其餘部分是Fe及不可避免的雜質。此外，在不減損本發明的效果的範圍內，係可容許0.0050%以下的含氧量當作不可避免的雜質。The rest is Fe and inevitable impurities. In addition, within a range that does not impair the effect of the present invention, an oxygen content of 0.0050% or less can be tolerated as an inevitable impurity.

上述的成分就是本發明的電焊鋼管之基本的組成分。根據上述的必要元素即可得到本發明所欲獲得之特性，但是，除了這個基本的組成分之外，亦可因應需求而又含有下列的元素。The above-mentioned components are the basic components of the electric welded steel pipe of the present invention. According to the above-mentioned essential elements, the desired characteristics of the present invention can be obtained. However, in addition to this basic composition, the following elements can also be contained according to requirements.

B：0.008%以下 B係可降低肥粒鐵開始變態的溫度而對於鋼組織的細微化有所助益的元素，可因應需求而含有B。但是，B含量若超過0.008%的話，很容易偏析在結晶粒界而有降低韌性之虞慮。因此，想要含有B的話，係將B含量設定在0.008%以下為宜，更好是設定在0.006%以下。又，係將B含量設定在0.0003%以上為宜，更好是設定在0.0005%以上。B: 0.008% or less B is an element that can reduce the temperature at which ferrite iron starts to change and is helpful for the refinement of the steel structure. B can be contained in accordance with demand. However, if the B content exceeds 0.008%, it is likely to segregate in the crystal grain boundaries, which may reduce the toughness. Therefore, if it is desired to contain B, the B content is preferably set to 0.008% or less, more preferably 0.006% or less. In addition, it is better to set the B content to 0.0003% or more, more preferably to 0.0005% or more.

從Cr：0.01～1.0%、Mo：0.01～1.0%、Cu：0.01～0.50%、Ni：0.01～1.0%、Ca：0.0005～0.010%之中選出的一種或兩種以上 Cr：0.01～1.0% Cr係可藉由提高淬火硬化性而可提昇鋼管的強度之元素，可因應需求而含有Cr。想要獲得這種效果，係將Cr含量設定在0.01%以上為宜。另一方面，Cr含量若超過1.0%的話，將會有降低韌性、焊接性之虞慮，因而將Cr含量設定在1.0%以下為宜。因此，想要含有Cr的話，係將Cr含量設定在0.01～1.0%為宜。Cr含量更好是設定在0.02%以上，更優是設定在0.03%以上。Cr含量更好是設定在0.8%以下，更優是設定在0.6%以下。One or two or more selected from Cr: 0.01～1.0%, Mo: 0.01～1.0%, Cu: 0.01～0.50%, Ni: 0.01～1.0%, Ca: 0.0005～0.010% Cr: 0.01～1.0% Cr is an element that can increase the strength of steel pipes by improving the quench hardenability, and can contain Cr according to demand. To achieve this effect, the Cr content should be set to 0.01% or more. On the other hand, if the Cr content exceeds 1.0%, the toughness and weldability may be lowered. Therefore, the Cr content is preferably set to 1.0% or less. Therefore, if it is desired to contain Cr, the Cr content should be set to 0.01 to 1.0%. The Cr content is more preferably set at 0.02% or more, and more preferably set at 0.03% or more. The Cr content is more preferably set to 0.8% or less, more preferably set to 0.6% or less.

Mo：0.01～1.0% Mo係可藉由提高淬火硬化性而可提昇鋼管的強度之元素，可因應需求而含有Mo。想要獲得這種效果，係將Mo含量設定在0.01%以上為宜。另一方面，Mo含量若超過1.0%的話，將會有降低韌性之虞慮，因而將Mo含量設定在1.0%以下為宜。因此，想要含有Mo的話，係將Mo含量設定在0.01～1.0%為宜。Mo含量更好是設定在0.02%以上，更優是設定在0.03%以上。Mo含量更好是設定在0.8%以下，更優是設定在0.6%以下。Mo: 0.01～1.0% Mo is an element that can increase the strength of the steel pipe by improving the quench hardenability, and it can contain Mo according to demand. To achieve this effect, the Mo content should be set to 0.01% or more. On the other hand, if the Mo content exceeds 1.0%, the toughness may be lowered, so it is appropriate to set the Mo content to 1.0% or less. Therefore, if it is desired to contain Mo, it is appropriate to set the Mo content to 0.01 to 1.0%. The Mo content is more preferably set at 0.02% or more, and more preferably set at 0.03% or more. The Mo content is more preferably set to 0.8% or less, more preferably set to 0.6% or less.

Cu：0.01～0.50% Cu係可藉由固溶強化來提昇鋼管的強度之元素，可因應需求而含有Cu。想要獲得這種效果，係將Cu含量設定在0.01%以上為宜。另一方面，Cu含量若超過0.50%的話，將會有降低韌性之虞慮，因而將Cu含量設定在0.50%以下為宜。因此，想要含有Cu的話，係將Cu含量設定在0.01～0.50%為宜。Cu含量更好是設定在0.02%以上，更優是設定在0.03%以上。Cu含量更好是設定在0.45%以下，更優是設定在0.40%以下。Cu: 0.01～0.50% Cu is an element that can increase the strength of steel pipes by solid solution strengthening, and can contain Cu according to demand. To obtain this effect, the Cu content is preferably set to 0.01% or more. On the other hand, if the Cu content exceeds 0.50%, there is a concern that the toughness may be reduced. Therefore, the Cu content is preferably set to 0.50% or less. Therefore, if it is desired to contain Cu, the Cu content should be set to 0.01 to 0.50%. The Cu content is more preferably set at 0.02% or more, and more preferably set at 0.03% or more. The Cu content is more preferably set to 0.45% or less, and more preferably set to 0.40% or less.

Ni：0.01～1.0% Ni係可藉由固溶強化來提昇鋼管的強度之元素，可因應需求而含有Ni。想要獲得這種效果，係將Ni含量設定在0.01%以上為宜。另一方面，Ni含量若超過1.0%的話，將會有降低韌性之虞慮，因而將Ni含量設定在1.0%以下為宜。因此，想要含有Ni的話，係將Ni含量設定在0.01～1.0%為宜。Ni含量更好是設定在0.02%以上，更優是設定在0.03%以上。Ni含量更好是設定在0.8%以下，更優是設定在0.6%以下。Ni: 0.01～1.0% Ni is an element that can improve the strength of steel pipes by solid solution strengthening, and it can contain Ni according to demand. To obtain this effect, the Ni content should be set to 0.01% or more. On the other hand, if the Ni content exceeds 1.0%, the toughness may be lowered, so it is appropriate to set the Ni content to 1.0% or less. Therefore, if Ni is desired to be contained, the Ni content should be set to 0.01 to 1.0%. The Ni content is more preferably set at 0.02% or more, and more preferably set at 0.03% or more. The Ni content is more preferably set to 0.8% or less, more preferably set to 0.6% or less.

Ca：0.0005～0.010% Ca係在進行製造鋼管的素材(也就是熱軋鋼板)時，可將在熱軋工序中被延展成很薄的MnS等的硫化物予以球狀化，因而對於提高鋼的韌性有所助益的元素，係可因應需求而含有Ca。想要獲得這種效果而含有Ca的話，是將Ca含量設定在0.0005%以上為宜。但是，Ca含量若超過0.010%的話，將會在鋼中形成Ca氧化物的群聚，而有導致韌性惡化之虞慮。因此，想要含有Ca的話，係將Ca含量設定在0.0005～0.010%為宜。Ca含量更好是設定在0.0010%以上，更優是設定在0.0015%以上。Ca含量更好是設定在0.005%以下，更優是設定在0.004%以下。Ca: 0.0005～0.010% Ca-based sulfides such as MnS, which are stretched to a very thin thickness during the hot rolling process, can be spheroidized when the material for manufacturing steel pipes (that is, hot-rolled steel sheets), which helps to improve the toughness of steel. The element of, can contain Ca according to demand. To obtain this effect, if Ca is contained, the Ca content should be set to 0.0005% or more. However, if the Ca content exceeds 0.010%, clusters of Ca oxides will be formed in the steel, which may cause deterioration in toughness. Therefore, if it is desired to contain Ca, the content of Ca should be set to 0.0005 to 0.010%. The Ca content is more preferably set to 0.0010% or more, more preferably set to 0.0015% or more. The Ca content is more preferably set to 0.005% or less, more preferably set to 0.004% or less.

其次，說明限定本發明的電焊鋼管的鋼組織之理由。Next, the reason for limiting the steel structure of the electric resistance welded steel pipe of the present invention will be explained.

假設在本發明的電焊鋼管中的母材部的板厚度為t時，位於從電焊鋼管的外表面起算之板厚度t的1/4t深度位置處的鋼組織，係為：以面積率計，是含有變韌鐵70%以上；變韌鐵的平均有效粒徑，以平均圓當量直徑計，係10.0μm以下；且變韌鐵的平均長寬比，係0.1～0.8。Assuming that when the plate thickness of the base material portion of the electric resistance welded steel pipe of the present invention is t, the steel structure at a depth of 1/4t of the plate thickness t calculated from the outer surface of the electric resistance welded steel pipe is: in terms of area ratio, It contains more than 70% of toughened iron; the average effective particle size of toughened iron is less than 10.0μm in terms of average circle equivalent diameter; and the average aspect ratio of toughened iron is 0.1-0.8.

此處所稱的板厚度t的1/4t的深度位置，係在進行控制鋼組織時之很重要的位置，因為其是位於在進行製造鋼管的素材(也就是熱軋鋼板)時的熱軋工序中之冷卻速度最大的最表層與冷卻速度最小的1/2t的深度位置的中間位置。此外，在本發明中，係採用：在板寬度W的1/4W位置處之與熱軋時的輥軋方向保持平行的剖面，來當作鋼組織的評判面。在本發明中，因為在熱軋後，不再實施熱處理等，因此，熱軋鋼板的組織係與鋼管(母材部)的組織相同的組織。The depth position of 1/4t of the plate thickness t referred to here is a very important position when controlling the steel structure, because it is located in the hot rolling process when the material of the steel pipe (that is, the hot rolled steel plate) is manufactured. It is the middle position between the outermost layer with the largest cooling rate and the 1/2t depth position with the smallest cooling rate. In addition, in the present invention, a section that is parallel to the rolling direction during hot rolling at a position of 1/4W of the sheet width W is adopted as the evaluation surface of the steel structure. In the present invention, since heat treatment or the like is not performed after hot rolling, the structure of the hot-rolled steel sheet is the same as the structure of the steel pipe (base material portion).

變韌鐵的面積率：70%以上 在本發明中，為了兼顧高強度和高韌性，以面積率計，含有變韌鐵70%以上的作法很重要。如果變韌鐵低於70%，就難以得到本發明想要獲得之強度。因此，位於從鋼管的外表面起算之板厚度t的1/4t深度位置處之母材部的鋼組織是設定為：以面積率計，含有變韌鐵70%以上。更好是72%以上。此外，變韌鐵的面積率太多的話，降伏比將會變得太高，因此，以面積率計，含有變韌鐵98%以下為宜。更好是95%以下。Area ratio of toughened iron: more than 70% In the present invention, in order to achieve both high strength and high toughness, it is important to contain at least 70% of toughened iron in terms of area ratio. If the toughened iron is less than 70%, it is difficult to obtain the strength desired by the present invention. Therefore, the steel structure of the base metal part located at a depth of 1/4t of the plate thickness t from the outer surface of the steel pipe is set to contain 70% or more of toughened iron in terms of area ratio. More preferably, it is 72% or more. In addition, if the area ratio of the toughened iron is too large, the yield ratio will become too high. Therefore, in terms of the area ratio, the content of the toughened iron is preferably 98% or less. More preferably, it is 95% or less.

變韌鐵以外的組織(其餘部分的組織)，可以是肥粒鐵、波來鐵、麻田散鐵、沃斯田鐵等。這些組織的面積率的合計，如果是鋼組織整體的30%以上的話，將會導致強度或韌性的不足、降伏比太低。因而，其餘部分組織之面積率的合計是設定在低於30%為宜。更好是低於28%。考慮到本發明想要獲得的降伏比的話，係將其餘部分組織之面積率的合計的下限設定在高過2%為宜，更好是超過5%。The organization other than the toughened iron (the rest of the organization) can be fat iron, polished iron, Asada loose iron, austenitic iron, etc. If the total area ratio of these structures is 30% or more of the entire steel structure, the strength or toughness will be insufficient, and the yield ratio will be too low. Therefore, it is advisable to set the total area ratio of the rest of the organization to be less than 30%. More preferably, it is less than 28%. In consideration of the yield ratio that the present invention intends to obtain, the lower limit of the total area ratio of the remaining tissues is preferably set to be higher than 2%, more preferably more than 5%.

此外，在本發明中，上述的各組織之面積率的測定，係可採用後述的實施例中所述的方法來進行測定。In addition, in the present invention, the above-mentioned measurement of the area ratio of each tissue can be measured by the method described in the examples described later.

變韌鐵的平均有效粒徑：以平均圓當量直徑計，係10.0μm以下 在本發明中，為了兼顧高強度和高韌性，將變韌鐵的平均有效粒徑設定為：以平均圓當量直徑計，係10.0μm以下的作法很重要。如果變韌鐵的平均有效粒徑，以平均圓當量直徑計，超過10.0μm的話，就難以得到本發明想要獲得之韌性。此外，無法得到本發明想要獲得之強度。因此，將其設定在8.0μm以下。此外，變韌鐵太過細微的話，降伏比將會變得太高，因此，將變韌鐵之平均有效粒徑設定在：以平均圓當量直徑計，係1.0μm以上為宜，2.0μm以上更好。The average effective particle size of toughened iron: in terms of average circle equivalent diameter, it is below 10.0μm In the present invention, in order to achieve both high strength and high toughness, it is important to set the average effective particle size of the toughened iron to be 10.0 μm or less in terms of average circle equivalent diameter. If the average effective particle size of the toughened iron exceeds 10.0 μm in terms of average circle equivalent diameter, it will be difficult to obtain the toughness intended by the present invention. In addition, the strength that the present invention intends cannot be obtained. Therefore, it is set to 8.0 μm or less. In addition, if the toughened iron is too fine, the yield ratio will become too high. Therefore, the average effective particle size of the toughened iron is set to: based on the average circle equivalent diameter, it is preferably 1.0μm or more, 2.0μm or more better.

此處，是先求出相鄰的結晶的方位差，將被相鄰的結晶的方位差(結晶方位差)達到15°以上的境界所圍繞的領域視為結晶粒時，將與該結晶粒的面積相等面積的圓的直徑視為變韌鐵的有效粒徑。從所取得的有效粒徑，求出粒徑的算術平均值，將其視為平均圓當量直徑(平均有效粒徑)。此外，在本發明中，結晶方位差、有效粒徑、以及平均圓當量直徑，係可採用後述的實施例中所述的方法來進行測定。Here, the azimuth difference of adjacent crystals is obtained first, and when the area surrounded by the boundary where the azimuth difference (crystal azimuth difference) of adjacent crystals is 15° or more is regarded as a crystal grain, the crystal grain is The diameter of a circle of equal area is regarded as the effective particle size of toughened iron. From the obtained effective particle size, the arithmetic average of the particle size is obtained, and this is regarded as the average circle equivalent diameter (average effective particle size). In addition, in the present invention, the crystal orientation difference, effective particle diameter, and average circle equivalent diameter can be measured by the method described in the examples described later.

變韌鐵的平均長寬比：0.1～0.8 在本發明中，為了將管軸方向的降伏比控制在85～95%的範圍內，必須將變韌鐵的平均長寬比設定在0.1～0.8。此處，係針對於上述的變韌鐵的結晶粒，計算出(在板厚度方向上的長度的平均值)/(在管軸方向上的長度的平均值)，將其視為變韌鐵的平均長寬比。如果變韌鐵的平均長寬比超過0.8的話，在管軸方向上的塑性變形性能將會降低，降伏比很容易就超過95%。另一方面，如果變韌鐵的平均長寬比低於0.1的話，管軸方向的強度將會降低，就無法得到本發明想要獲得的強度。Average aspect ratio of toughened iron: 0.1～0.8 In the present invention, in order to control the yield ratio in the tube axis direction within the range of 85 to 95%, the average aspect ratio of the toughened iron must be set to 0.1 to 0.8. Here, the (average value of the length in the thickness direction of the plate)/(the average value of the length in the tube axis direction) is calculated for the crystal grains of the above-mentioned toughened iron, which is regarded as toughened iron The average aspect ratio. If the average aspect ratio of the toughened iron exceeds 0.8, the plastic deformation performance in the tube axis direction will be reduced, and the yield ratio will easily exceed 95%. On the other hand, if the average aspect ratio of the toughened iron is less than 0.1, the strength in the tube axis direction will decrease, and the strength intended by the present invention cannot be obtained.

此外，在本發明中，變韌鐵的結晶粒中之在板厚度方向上的長度的平均值、在輥軋方向上的長度的平均值，係可採用後述的實施例中所述的方法來進行測定。In addition, in the present invention, the average value of the length in the thickness direction of the plate and the average value of the length in the rolling direction in the crystal grains of the toughened iron can be obtained by the method described in the following examples. Perform the measurement.

其次，說明本發明之其中一種實施方式的電焊鋼管的製造方法。Next, a method of manufacturing an electric resistance welded steel pipe according to one embodiment of the present invention will be explained.

本發明的電焊鋼管，例如：是對於具有上述的組成分的鋼素材，依序地實施熱軋工序、冷卻工序及捲取工序而製作成熱軋鋼板，然後對於該熱軋鋼板實施室溫下的滾壓成形工序而製作成的電焊鋼管。The electric welded steel pipe of the present invention is, for example, a hot-rolled steel sheet produced by sequentially performing hot rolling, cooling, and coiling processes on a steel material having the above-mentioned composition, and then subjecting the hot-rolled steel sheet to room temperature The electric welded steel pipe produced by the roll forming process.

此外，在以下之製造方法的說明中，有關於溫度的「℃」的標示，如果沒有特別地限定的話，都是表示鋼素材、鋼板(熱軋鋼板)的表面溫度。這些的表面溫度，係可採用：幅射熱溫度計等來進行測定。此外，鋼板之板厚度中心的溫度，係將鋼板剖面內的溫度分布，利用熱傳導解析方法來進行計算，再將該結果利用鋼板的表面溫度來進行補正而求出來的。又，「熱軋鋼板」係包含熱軋鋼板、熱軋鋼帶。In addition, in the following description of the manufacturing method, there are indications of "°C" for temperature, and unless specifically limited, they all indicate the surface temperature of the steel material or steel plate (hot rolled steel plate). These surface temperatures can be measured with a radiation heat thermometer. In addition, the temperature at the center of the thickness of the steel sheet is calculated by calculating the temperature distribution in the cross-section of the steel sheet using a thermal conduction analysis method, and then correcting the result using the surface temperature of the steel sheet. In addition, "hot rolled steel sheet" includes hot rolled steel sheet and hot rolled steel strip.

在本發明中，並未特別限定鋼素材(鋼胚材)的熔製方法。基於品質、生產性等之觀點的考量，是以將具有上述組成分的熔鋼利用轉爐、電爐、真空熔解爐等之常用的熔製方法來予以熔製，並且採用連續鑄造法等之常用的鑄造方法，來製作成胚材等的鑄片為宜。此外，即使採用造塊-分塊輥軋法來取代連續鑄造法也沒有問題。針對於熔鋼亦可實施盛鋼桶精煉等的二次精煉。In the present invention, the method of melting the steel material (steel blank) is not particularly limited. Based on the viewpoint of quality, productivity, etc., the molten steel with the above composition is melted using common melting methods such as converters, electric furnaces, and vacuum melting furnaces, and continuous casting methods are used. For the casting method, it is appropriate to make cast pieces such as blanks. In addition, there is no problem even if a block-dividing roll method is used instead of the continuous casting method. For molten steel, secondary refining such as ladle refining can also be implemented.

接下來，對於所製得的鋼素材(鋼胚材)實施熱軋工序。熱軋工序，係將鋼素材加熱到達1100～1280℃的加熱溫度後，實施粗輥軋結束溫度為850～1150℃的粗輥軋，然後實施最終輥軋結束溫度為750～850℃的最終輥軋，並且實施在進行粗輥軋及最終輥軋時之在930℃以下的合計軋縮率為65%以上的熱軋，而製作成熱軋鋼板的工序。Next, a hot rolling process is performed on the produced steel material (steel blank material). The hot rolling process is to heat the steel material to a heating temperature of 1100 to 1280°C, perform rough rolling with a rough rolling end temperature of 850 to 1150°C, and then perform a final roll with a final rolling end temperature of 750 to 850°C During the rough rolling and final rolling, hot rolling with a total reduction ratio of 65% or more at 930°C or less is performed to produce a hot-rolled steel sheet.

加熱溫度：1100～1280℃ 如果加熱溫度低於1100℃的話，就無法將在鑄造時所生成之存在於鋼素材中的粗大的碳化物予以固溶化。其結果，就無法充分地獲得含有碳化物形成元素的效果。另一方面，如果加熱溫度超過1280℃而處於高溫的話，結晶粒將會明顯地粗大化，鋼管的素材(也就是熱軋鋼板)的組織變得粗大化，將會難以確保本發明想要獲得的特性。因此，必須將鋼素材的加熱溫度設定在1100～1280℃。更好是設定在1120～1230℃。此外，將這個溫度當作加熱爐之爐內設定溫度。Heating temperature: 1100～1280℃ If the heating temperature is lower than 1100°C, the coarse carbides existing in the steel material formed during casting cannot be solid-soluted. As a result, the effect of containing carbide-forming elements cannot be sufficiently obtained. On the other hand, if the heating temperature exceeds 1280 ℃ and the temperature is high, the crystal grains will be significantly coarsened, and the structure of the material of the steel pipe (that is, the hot-rolled steel sheet) will be coarsened, and it will be difficult to ensure that the present invention is intended to obtain Characteristics. Therefore, the heating temperature of the steel material must be set at 1100 to 1280°C. More preferably, it is set to 1120-1230 degreeC. In addition, consider this temperature as the set temperature in the furnace.

粗輥軋結束溫度：850～1150℃ 如果粗輥軋結束溫度低於850℃的話，不會引發熱軋中的組織的回復，很容易生成朝向輥軋方向過度拉長的結晶粒。其結果，很容易導致變韌鐵的平均長寬比小於0.1。另一方面，如果粗輥軋結束溫度超過1150℃的話，在沃斯田鐵未再結晶溫度域的軋縮量不足，無法獲得細微的沃斯田鐵粒，其結果，難以確保本發明想要獲得的變韌鐵的平均有效粒徑。因此，將粗輥軋結束溫度設定在850～1150℃。更好是設定在860～1000℃。Finishing temperature of rough rolling: 850～1150℃ If the rough rolling end temperature is lower than 850°C, the structure recovery during hot rolling will not occur, and it is easy to generate crystal grains that are excessively elongated in the rolling direction. As a result, it is easy to cause the average aspect ratio of the toughened iron to be less than 0.1. On the other hand, if the rough rolling end temperature exceeds 1150°C, the amount of reduction in the austenitic iron non-recrystallization temperature range is insufficient, and fine austenitic iron grains cannot be obtained. As a result, it is difficult to ensure the desired value of the present invention. The average effective particle size of the toughened iron obtained. Therefore, the rough rolling end temperature is set at 850 to 1150°C. More preferably, it is set at 860 to 1000°C.

最終輥軋結束溫度：750～850℃ 如果最終輥軋結束溫度低於750℃的話，不會引發熱軋中的組織的回復，很容易生成朝向輥軋方向過度拉長的結晶粒。其結果，很容易導致變韌鐵的平均長寬比小於0.1。另一方面，如果最終輥軋結束溫度超過850℃的話，在沃斯田鐵未再結晶溫度域的軋縮量不足，無法獲得細微的沃斯田鐵粒，其結果，難以確保本發明想要獲得的變韌鐵的平均有效粒徑。因此，將最終輥軋結束溫度設定在750～850℃。更好是設定在770～830℃。Finishing temperature of final rolling: 750～850℃ If the final rolling end temperature is lower than 750°C, the structure recovery during hot rolling will not occur, and it is easy to generate crystal grains that are excessively elongated in the rolling direction. As a result, it is easy to cause the average aspect ratio of the toughened iron to be less than 0.1. On the other hand, if the final rolling end temperature exceeds 850°C, the amount of reduction in the austenitic iron non-recrystallization temperature range is insufficient, and fine austenitic iron particles cannot be obtained. As a result, it is difficult to ensure the desired value of the present invention. The average effective particle size of the toughened iron obtained. Therefore, the final rolling end temperature is set at 750 to 850°C. More preferably, it is set at 770 to 830°C.

粗輥軋與最終輥軋時之在930℃以下的合計軋縮率為65%以上 本發明，係在熱軋工序中將沃斯田鐵予以細微化，且將在後續的冷卻工序、捲取工序中所生成的變韌鐵及其餘部分組織予以細微化，藉此，能夠製得具有本發明想要獲得的強度及韌性之適合作為電焊鋼管的素材的熱軋鋼板。想要在熱軋工序中將沃斯田鐵予以細微化，必須提高在沃斯田鐵未再結晶溫度域的軋縮率來導入充分的加工變形。為了要獲得這種效果，在本發明中，係將在930℃以下之抵達最終輥軋結束溫度為止的溫度域中的合計軋縮率設定在65%以上。此處，所稱的「合計軋縮率」，係指：在930℃以下之抵達最終輥軋結束溫度為止的溫度域中的各個輥軋道次(pass)的軋縮率的合計。The total reduction ratio of rough rolling and final rolling below 930℃ is 65% or more In the present invention, the austenitic iron is refined in the hot rolling process, and the toughened iron generated in the subsequent cooling process and coiling process and the rest of the structure are refined, thereby making it possible to obtain A hot-rolled steel sheet suitable as a material for electric resistance welded steel pipes having the strength and toughness desired by the present invention. In order to miniaturize the austenitic iron in the hot rolling process, it is necessary to increase the rolling reduction ratio in the non-recrystallization temperature range of the austenitic iron to introduce sufficient processing deformation. In order to obtain this effect, in the present invention, the total rolling reduction ratio in the temperature range up to the final rolling end temperature at 930° C. or lower is set to 65% or more. Here, the "total rolling reduction ratio" refers to the total rolling reduction ratio of each rolling pass in the temperature range up to the final rolling end temperature below 930°C.

如果在930℃以下之抵達最終輥軋結束溫度為止的溫度域中的合計軋縮率低於65%的話，就無法在熱軋工序中導入充分的加工變形，因此，將會無法製得本發明想要獲得的鋼組織的變韌鐵的平均有效粒徑。在930℃以下之抵達最終輥軋結束溫度為止的溫度域中的合計軋縮率，更好是設定在70%以上。雖然並未特別地規定上限，但是如果超過80%的話，相對於軋縮率的上昇所帶來的提高韌性的效果變小，只會增加設備的負荷而已。因此，將在930℃以下之抵達最終輥軋結束溫度為止的溫度域中的合計軋縮率，設定在80%以下為宜。更好是設定在75%以下。If the total rolling reduction ratio in the temperature range up to the final rolling end temperature below 930°C is less than 65%, sufficient processing deformation cannot be introduced in the hot rolling process, and therefore, the present invention cannot be obtained. The average effective grain size of the toughened iron of the steel structure to be obtained. The total rolling reduction ratio in the temperature range up to the final rolling end temperature at 930°C or lower is more preferably set to 70% or more. Although the upper limit is not particularly specified, if it exceeds 80%, the effect of increasing the toughness with respect to the increase in the reduction ratio becomes small, and only increases the load on the equipment. Therefore, it is advisable to set the total rolling reduction ratio in the temperature range up to the final rolling end temperature at 930°C or less to 80% or less. It is better to set it below 75%.

在本發明中，將溫度設定在930℃以下的理由，是因為如果溫度超過930℃的話，在熱軋工序中沃斯田鐵將會再結晶，實施輥軋時所導入的轉位全部都消失，而無法獲得細微化的沃斯田鐵之緣故。In the present invention, the reason for setting the temperature below 930°C is that if the temperature exceeds 930°C, the austenitic iron will recrystallize during the hot rolling process, and all the index introduced during rolling will disappear. , And can’t get the miniaturized austenitic iron.

再者，在本發明中，在實施鋼素材的熱軋時，係可以採用：包含上述粗輥軋及最終輥軋在內之在930℃以下之抵達最終輥軋結束溫度為止的合計軋縮率設定在65%以上的熱軋；也可以採用：只有在最終輥軋時之在930℃以下之抵達最終輥軋結束溫度為止的合計軋縮率設定在65%以上的熱軋。在後者中，如果無法將最終輥軋時之在930℃以下之抵達最終輥軋結束溫度為止的合計軋縮率設定在65%以上的話，也是可以在粗輥軋的途中先將胚材冷卻到達930℃以下的溫度之後，再將粗輥軋與最終輥軋的兩者中之在930℃以下之抵達最終輥軋結束溫度為止的合計軋縮率設定在65%以上。Furthermore, in the present invention, when performing hot rolling of the steel material, it is possible to adopt the total rolling reduction ratio including the rough rolling and the final rolling mentioned above at a temperature of 930°C or less until the final rolling end temperature is reached. Set the hot rolling at 65% or more; it can also be used: only at the time of the final rolling, the total rolling reduction ratio is set to 65% or more at the temperature of 930℃ or less until the final rolling end temperature. In the latter, if it is impossible to set the total reduction ratio until the final rolling end temperature at the time of final rolling at 930°C or lower to 65% or more, it is also possible to cool the blank material to reach the end of the rough rolling. After the temperature is below 930°C, the total rolling reduction ratio of the rough rolling and the final rolling at 930°C or lower until the final rolling end temperature is set to 65% or more.

接下來，對於熱軋工序後的熱軋鋼板實施冷卻工序。冷卻工序，係將熱軋鋼板依據下列的條件來進行冷卻的工序，以板厚度中心的溫度為準，從開始冷卻至停止冷卻的平均冷卻速度為15～35℃/s、冷卻停止溫度為450～650℃。Next, a cooling step is performed on the hot rolled steel sheet after the hot rolling step. The cooling process is the process of cooling the hot-rolled steel sheet according to the following conditions. Based on the temperature at the center of the sheet thickness, the average cooling rate from the start of the cooling to the stop of the cooling is 15 to 35°C/s, and the cooling stop temperature is 450 ～650℃.

開始冷卻至停止冷卻的平均冷卻速度為15～35℃/s 以熱軋鋼板的板厚度中心的溫度為準，從開始冷卻至後述的停止冷卻的溫度為止的溫度域中的平均冷卻速度低於15℃/s的話，將會因為肥粒鐵的生成而導致變韌鐵的面積率降低，而無法得到本發明想要獲得的強度。另一方面，如果平均冷卻速度超過35℃/s的話，變韌鐵的平均長寬比將會超過0.8。其結果，會導致降伏比很容易超過95%。平均冷卻速度是設定在20℃/s以上為宜，且設定在30℃/s以下為宜。The average cooling rate from the start of cooling to the stop of cooling is 15～35℃/s Based on the temperature at the center of the thickness of the hot-rolled steel sheet, if the average cooling rate in the temperature range from the start of cooling to the temperature to stop cooling described later is lower than 15°C/s, it will be caused by the formation of ferrous iron The area ratio of the toughened iron decreases, and the strength desired by the present invention cannot be obtained. On the other hand, if the average cooling rate exceeds 35°C/s, the average aspect ratio of the toughened iron will exceed 0.8. As a result, the yield ratio can easily exceed 95%. The average cooling rate is preferably set at 20°C/s or more, and set at 30°C/s or less.

此外，在本發明中，如果沒有特別限定的話，所稱的平均冷卻速度，係指：((冷卻前的熱軋鋼板的板厚度中心溫度－冷卻後的熱軋鋼板的板厚度中心溫度)/冷卻時間)所求出的數值(冷卻速度)的平均值。冷卻方法，係可舉出例如：從噴嘴噴射出水之類的水冷方式，從噴嘴噴射出冷卻用氣體之類的氣冷方式。在本發明中，係採用：以對於熱軋鋼板的兩面都以相同條件進行冷卻的方式，來對於熱軋鋼板的兩面實施冷卻操作(處理)為宜。In addition, in the present invention, if there is no particular limitation, the average cooling rate refers to: ((the thickness center temperature of the hot rolled steel sheet before cooling-the thickness center temperature of the hot rolled steel sheet after cooling)/ Cooling time) is the average value of the calculated value (cooling rate). The cooling method includes, for example, a water cooling method such as spraying water from a nozzle, and an air cooling method such as spraying cooling gas from a nozzle. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the hot-rolled steel sheet by cooling both sides of the hot-rolled steel sheet under the same conditions.

冷卻停止溫度為450～650℃ 以熱軋鋼板的板厚度中心溫度為準，如果冷卻停止的溫度低於450℃的話，變韌鐵的平均長寬比將會超過0.8，其結果，降伏比將會很容易超過95%。另一方面，如果冷卻停止的溫度超過650℃的話，因為是高於變韌鐵的變態開始溫度，因而無法將變韌鐵的面積率維持在70%以上。冷卻停止的溫度，是設定在480℃以上為宜，且是設定在620℃以下為宜。The cooling stop temperature is 450～650℃ Based on the thickness center temperature of the hot-rolled steel sheet, if the cooling stop temperature is lower than 450°C, the average aspect ratio of the toughened iron will exceed 0.8. As a result, the yield ratio will easily exceed 95%. On the other hand, if the cooling stop temperature exceeds 650°C, it is higher than the transformation start temperature of the toughened iron, and therefore the area ratio of the toughened iron cannot be maintained at 70% or more. The temperature at which the cooling stops is preferably set at 480°C or higher, and preferably set at 620°C or lower.

接下來，將冷卻工序後的熱軋鋼板進行捲取，然後，實施予以放冷的捲取工序。Next, the hot-rolled steel sheet after the cooling step is coiled, and then, a coiling step for cooling is performed.

捲取工序中，基於鋼管的素材(也就是熱軋鋼板)的鋼板組織的觀點考量，係在450～650℃之捲取溫度的狀態下進行捲取為宜。如果捲取溫度低於450℃的話，變韌鐵的平均長寬比將會超過0.8，其結果，有時候降伏比將會超過95%。另一方面，如果捲取溫度超過650℃的話，溫度是高於變韌鐵的變態開始溫度，因而有時候無法將變韌鐵的面積率維持在70%以上。捲取溫度，較好是設定在480～620℃。In the coiling process, based on the viewpoint of the steel sheet structure of the material of the steel pipe (that is, the hot-rolled steel sheet), the coiling should be carried out at a coiling temperature of 450 to 650°C. If the coiling temperature is lower than 450°C, the average aspect ratio of the toughened iron will exceed 0.8. As a result, the yield ratio will sometimes exceed 95%. On the other hand, if the coiling temperature exceeds 650°C, the temperature is higher than the transformation start temperature of the toughened iron, and therefore sometimes the area ratio of the toughened iron cannot be maintained above 70%. The coiling temperature is preferably set at 480 to 620°C.

接下來，係對於捲取工序後的熱軋鋼板，實施室溫下的滾壓成形工序。室溫下的滾壓成形工序，係藉由對於熱軋鋼板實施室溫下的滾壓成形加工，而成形為圓筒狀之尚未封合的中空管，再將鋼管素材的兩端(亦即，尚未封合的中空管之互相對接的部分)進行電縫焊接，然後，以相對於焊接後的圓型鋼管的鋼管外表面的圓周長度之0.2～0.5%的縮徑率，來進行縮徑輥軋。Next, the hot-rolled steel sheet after the coiling step is subjected to a roll forming step at room temperature. The roll forming process at room temperature is a process of rolling a hot-rolled steel sheet at room temperature to form a cylindrical unsealed hollow tube, and then the both ends of the steel pipe material (also That is, the parts of the hollow pipes that are not yet sealed are welded by electric seam, and then the diameter reduction ratio is 0.2 to 0.5% relative to the circumferential length of the outer surface of the steel pipe of the round steel pipe after welding. Reduced diameter rolling.

縮徑輥軋時的縮徑率為0.2～0.5% 如果縮徑輥軋時的縮徑率低於0.2%的話，在上述之本發明的鋼管的鋼素材中，因為塑性變形所導致的殘留應力的降低不足。其結果，在鋼管外表面的管軸方向的殘留應力會超過250MPa。此外，因加工度不足而導致降伏比低於85%。另一方面，如果縮徑輥軋時的縮徑率超過0.5%的話，將會因為加工硬化而導致降伏比超過95%。其結果，就無法獲得所期望的塑性變形性能(亦即，耐挫曲性能)。又，如果上述的殘留應力超過250MPa的話，耐挫曲性能也會降低。The diameter reduction rate during the diameter reduction rolling is 0.2～0.5% If the diameter reduction ratio during diameter reduction rolling is less than 0.2%, in the steel material of the steel pipe of the present invention described above, the reduction in residual stress due to plastic deformation is insufficient. As a result, the residual stress in the pipe axis direction on the outer surface of the steel pipe exceeds 250 MPa. In addition, the yield ratio is lower than 85% due to insufficient processing. On the other hand, if the diameter reduction ratio during reduction rolling exceeds 0.5%, the yield ratio will exceed 95% due to work hardening. As a result, the desired plastic deformation performance (that is, buckling resistance) cannot be obtained. In addition, if the above-mentioned residual stress exceeds 250 MPa, the buckling resistance will also decrease.

藉由以上所述的工序，來製造出本發明的電焊鋼管。根據本發明，係可製造出：管軸方向的拉伸強度為590MPa以上、0.2%降伏強度為450MPa以上、降伏比為85～95%，且在-30℃溫度時之夏比衝擊試驗的衝擊吸收能量為70J以上，在鋼管外表面的管軸方向的殘留應力為250MPa以下的電焊鋼管。如此一來，得以很容易製造出具有高強度、高韌性、最佳的降伏比及優異的耐挫曲性能之電焊鋼管。這種電焊鋼管，可特別地適用於作為構造物的基礎使用的鋼管樁，因此，非常具有產業上可利用性的效果。Through the above-mentioned steps, the electric resistance welded steel pipe of the present invention is manufactured. According to the present invention, the system can be manufactured: the tensile strength in the tube axis direction is 590MPa or more, the 0.2% yield strength is 450MPa or more, the yield ratio is 85-95%, and the impact of Charpy impact test at -30℃ An electric resistance welded steel pipe with an absorbed energy of 70 J or more and a residual stress in the pipe axis direction on the outer surface of the steel pipe of 250 MPa or less. In this way, it is easy to manufacture electric welded steel pipes with high strength, high toughness, best yield ratio and excellent buckling resistance. This kind of electric-welded steel pipe is particularly suitable for steel pipe piles used as the foundation of structures, and therefore has a very effective industrial applicability.

其次，說明本發明的鋼管樁。Next, the steel pipe pile of the present invention will be explained.

本發明的鋼管樁，板厚度是16mm以下，外徑是300mm以上且700mm以下，係由具有上述組成分及鋼組織的電焊鋼管所構成的。藉由將電焊鋼管的組成分及鋼組織限定為上述的條件，可獲得：管軸方向的拉伸強度為590MPa以上、0.2%降伏強度為450MPa以上、降伏比為85～95%，-30℃溫度時的夏比衝擊試驗的衝擊吸收能量為70J以上、鋼管外表面的管軸方向的殘留應力為250MPa以下之鋼管樁。本發明的鋼管樁，係被打樁植入地下，必要時可以在打樁的途中，就在打樁的現場利用焊接或螺紋接頭之類的連接手段，進行施工來將鋼管樁與鋼管樁彼此連接成長條型的鋼管樁。根據本發明的鋼管樁，因為具有上述的特性，能夠降低在進行打樁時發生挫曲等的問題之虞慮。 [實施例]The steel pipe pile of the present invention has a plate thickness of 16 mm or less, and an outer diameter of 300 mm or more and 700 mm or less, and is composed of an electric welded steel pipe having the above-mentioned composition and steel structure. By limiting the composition and steel structure of the electric welded steel pipe to the above conditions, it is possible to obtain: the axial tensile strength of the pipe is 590MPa or more, the 0.2% yield strength is 450MPa or more, the yield ratio is 85-95%, -30℃ A steel pipe pile in which the impact absorption energy of the Charpy impact test at temperature is 70 J or more and the residual stress in the pipe axis direction on the outer surface of the steel pipe is 250 MPa or less The steel pipe pile of the present invention is piled into the ground. If necessary, welding or threaded joints can be used to connect the steel pipe piles and the steel pipe piles to each other in a long strip when necessary. -Shaped steel pipe piles. According to the steel pipe pile of the present invention, since it has the above-mentioned characteristics, it is possible to reduce the possibility of problems such as buckling during pile driving. [Example]

以下，將佐以實施例更詳細地說明本發明。此外，本發明並不侷限在以下所述的實施例。Hereinafter, the present invention will be explained in more detail with examples. In addition, the present invention is not limited to the embodiments described below.

首先，利用轉爐來熔製具有表1所示的組成分的熔鋼，再以連續鑄造法鑄造成胚材(鋼素材：厚度250mm)。將製得的胚材依據表2-1、表2-2所示的製造條件，實施熱軋工序、冷卻工序、捲取工序、及室溫下的滾壓成形工序，而製造出具有表2-1、表2-2所示的外徑及板厚度的電焊鋼管。此外，室溫下的滾壓成形工序，係將尚未封合的中空管之互相對接的部分進行電縫焊接。First, molten steel having the composition shown in Table 1 was melted using a converter, and then cast into a blank (steel material: thickness 250 mm) by a continuous casting method. According to the manufacturing conditions shown in Table 2-1 and Table 2-2, the prepared blanks were subjected to the hot rolling process, cooling process, coiling process, and roll forming process at room temperature, to produce -1. Electric welded steel pipes with the outer diameter and plate thickness shown in Table 2-2. In addition, the roll forming process at room temperature is to electrically seam weld the butt parts of the hollow tubes that have not been sealed.

從製得的電焊鋼管中採取試驗片，並且利用以下所示的方法，實施了組織觀察、拉伸試驗、夏比衝擊試驗、測定殘留應力、構件壓縮試驗。A test piece was taken from the produced electric welded steel pipe, and the structure observation, tensile test, Charpy impact test, residual stress measurement, and member compression test were performed by the methods shown below.

[組織觀察] 組織觀察用的試驗片，係採取：將電縫焊接部視為0°時之位在圓周方向90°位置的管軸方向的剖面來作為觀察面，經過研磨之後，以硝酸醇溶液進行蝕刻而製作成組織觀察用的試驗片。組織觀察，係使用光學顯微鏡(倍率為1000倍)或掃描型電子顯微鏡(SEM；倍率為1000倍)，來觀察從電焊鋼管的外表面起算之位於板厚度t的1/4t深度位置處的組織，並且進行攝影。從所獲得的光學顯微鏡的影像或SEM影像來求出變韌鐵的面積率。變韌鐵的面積率，係先進行觀察5個視野以上，然後計算出各視野所取得的數值的平均值，來當作變韌鐵的面積率。[Organization Observation] The test piece for structure observation is taken: the cross section of the pipe axis direction at 90° in the circumferential direction when the electric seam welded part is regarded as 0° is used as the observation surface, after polishing, it is etched with a nitric alcohol solution. A test piece for tissue observation was made. Microstructure observation, using an optical microscope (magnification 1000 times) or scanning electron microscope (SEM; magnification 1000 times), to observe the structure at a depth of 1/4t of the plate thickness t from the outer surface of the electric welded steel pipe , And take pictures. The area ratio of the toughened iron is obtained from the obtained optical microscope image or SEM image. The area ratio of the toughened iron is observed in five or more fields of view, and then the average value of the values obtained in each field is calculated as the area ratio of the toughened iron.

又，變韌鐵的平均有效粒徑(平均圓當量直徑)，係使用SEM/EBSD法來進行測定的。有效粒徑的計算方法，係先求出相鄰的結晶粒之間的方位差，將被方位差為15°以上的境界所圍繞的領域視為有效結晶粒時，就以與該有效結晶粒相等面積的圓的直徑，作為變韌鐵的有效粒徑。再計算出所獲得的有效粒徑的算術平均值，作為平均圓當量直徑。測定領域的大小是500μm×500μm、測定間隔的大小是0.5μm。此外，在進行結晶粒徑的解析時，將有效粒徑為2.0μm以下的結晶粒視為測定時的雜訊而從解析對象中排除。In addition, the average effective particle size (average equivalent circle diameter) of the toughened iron was measured using the SEM/EBSD method. The effective particle size calculation method is to first obtain the azimuth difference between adjacent crystal grains. When the area surrounded by the boundary with the azimuth difference of 15° or more is regarded as the effective crystal grain, it is regarded as the effective crystal grain. The diameter of a circle of equal area is used as the effective particle size of toughened iron. Then calculate the arithmetic average of the effective particle size obtained as the average circle equivalent diameter. The size of the measurement area is 500 μm×500 μm, and the size of the measurement interval is 0.5 μm. In addition, when analyzing the crystal grain size, crystal grains with an effective grain size of 2.0 μm or less are regarded as noise during measurement and excluded from the analysis target.

又，變韌鐵的平均長寬比，係對於以上述方法所測定到的各有效結晶粒的板厚度方向的長度、管軸方向的長度進行測定，再分別計算其平均值而算出變韌鐵的平均長寬比。板厚度方向的長度、管軸方向的長度，係分別採用各有效結晶粒中的板厚度方向、管軸方向上的最大長度。In addition, the average aspect ratio of the toughened iron is measured by measuring the length of each effective crystal grain in the plate thickness direction and the length of the tube axis direction measured by the above method, and then calculating the average value to calculate the toughened iron The average aspect ratio. The length in the plate thickness direction and the length in the tube axis direction are the maximum lengths in the plate thickness direction and the tube axis direction in each effective crystal grain.

[拉伸試驗] 拉伸試驗的拉伸試驗片之採取方法，係將所製得的電焊鋼管的電縫焊接部視為0°時之在圓周方向90°的位置處，以拉伸方向與管軸方向保持平行的方式，採取了日本工業規格JIS 5號的拉伸試驗片。然後，依據日本工業規格JIS Z 2241的規定，實施了拉伸試驗。進行測定0.2%降伏強度(降伏強度YS)、拉伸強度TS，然後計算出以(0.2%降伏強度)/(拉伸強度)來定義的降伏比。[Stretching test] The method of taking the tensile test piece for the tensile test is to regard the electric seam welded part of the produced electric welded steel pipe as 0° at a position of 90° in the circumferential direction, and keep the tensile direction parallel to the pipe axis direction The method adopted Japanese Industrial Standards JIS No. 5 tensile test piece. Then, a tensile test was performed in accordance with the Japanese Industrial Standards JIS Z 2241. The 0.2% yield strength (YS) and tensile strength TS are measured, and then the yield ratio defined by (0.2% yield strength)/(tensile strength) is calculated.

[夏比衝擊試驗] 夏比衝擊試驗，係將所製得的電焊鋼管的電縫焊接部視為0°時之在圓周方向90°的位置處，從板厚度t/2的位置，以試驗片長邊方向與管軸方向保持平行的方式，採取出V型凹槽夏比衝擊試驗片。然後，依據日本工業規格JIS Z 2242的規定，實施了試驗溫度為-30℃時的夏比衝擊試驗，因而求出夏比衝擊試驗吸收能(J)。此外，試驗片的個數是各3個，並且計算出其平均值來作為夏比衝擊試驗吸收能(J)。[Charpy Impact Test] The Charpy impact test is based on the electric seam welded part of the produced electric welded steel pipe at a position of 90° in the circumferential direction at 0°, from the position of the plate thickness t/2, with the length of the test piece and the pipe axis The direction is kept parallel, and the V-groove Charpy impact test piece is taken. Then, in accordance with the Japanese Industrial Standards JIS Z 2242, a Charpy impact test at a test temperature of -30°C was performed, and the Charpy impact test absorption energy (J) was obtained. In addition, the number of test pieces was three each, and the average value was calculated as the Charpy impact test absorption energy (J).

[殘留應力的測定] 殘留應力，係使用PULSTEC公司製造的型號μ-X360的殘留應力測定裝置，利用X射線繞射cosα法來進行了測定。殘留應力的測定位置，係選定在所製得的電焊鋼管之管長度的中央位置的外表面，將電縫焊接部視為0°時，選定位於90°的位置、180°的位置、270°的位置之三個部位。將所測得的三個部位的測定值的平均值作為殘留應力。此外，以管軸方向來作為測定應力的方向。[Measurement of residual stress] The residual stress was measured by the X-ray diffraction cosα method using a model μ-X360 residual stress measuring device manufactured by PULSTEC. The measurement position of the residual stress is selected on the outer surface of the center position of the tube length of the produced electric welded steel pipe. When the electric seam welded part is regarded as 0°, select the position at 90°, 180°, 270° The three parts of the position. The average value of the measured values of the three measured locations was taken as the residual stress. In addition, the direction of the tube axis was used as the direction of stress measurement.

[構件壓縮試驗] 在本發明中，係進行構件壓縮試驗，求出挫曲強度比σcr/σy(此處，σcr是挫曲應力度、σy是材料降伏強度)來評判其作為鋼管樁使用的性能。如果挫曲強度比大於減少係數R = 0.8＋2.5×t/r(此處，t是板厚度、r是半徑)的話，就可判斷為：其係具有充分的作為鋼管樁的重要性能之一的挫曲強度。[Component compression test] In the present invention, a member compression test is performed to determine the buckling strength ratio σcr/σy (here, σcr is the buckling stress and σy is the yield strength of the material) to evaluate its performance as a steel pipe pile. If the buckling strength ratio is greater than the reduction factor R = 0.8+2.5×t/r (here, t is the plate thickness and r is the radius), it can be judged that it has sufficient important performance as a steel pipe pile The strength of buckling.

將所獲得的結果分別顯示在表3-1、表3-2中。The obtained results are shown in Table 3-1 and Table 3-2 respectively.

如表1～表3-2所示，落在本發明的範圍內的電焊鋼管都是管軸方向的拉伸強度為590MPa以上、0.2%降伏強度為450MPa以上、降伏比為85～95%、-30℃溫度時的夏比衝擊試驗的衝擊吸收能量為70J以上、管外表面的管軸方向的殘留應力為250MPa以下。此外，也得知具有這些特性的電焊鋼管，也具有作為鋼管樁的重要的性能之充分的挫曲強度。As shown in Table 1 to Table 3-2, all electric welded steel pipes falling within the scope of the present invention have a tensile strength of 590 MPa or more in the tube axis direction, a 0.2% yield strength of 450 MPa or more, and a yield ratio of 85 to 95%. The impact absorption energy of the Charpy impact test at a temperature of -30°C is 70 J or more, and the residual stress in the tube axis direction on the outer surface of the tube is 250 MPa or less. In addition, it is also known that an electric resistance welded steel pipe having these characteristics also has sufficient buckling strength as an important performance of a steel pipe pile.

另一方面，在組成分、鋼組織及製造條件的其中任何一項，落在本發明的範圍外的鋼管，係在管軸方向的拉伸強度、0.2%降伏強度、降伏比、-30℃溫度時的夏比衝擊試驗的衝擊吸收能量、管外表面的管軸方向的殘留應力之中，至少有其中一項以上，無法獲得本發明想要獲得的數值。On the other hand, in terms of composition, steel structure and manufacturing conditions, steel pipes falling outside the scope of the present invention have tensile strength in the pipe axis direction, 0.2% yield strength, yield ratio, and -30°C At least one of the impact absorption energy of the Charpy impact test at temperature and the residual stress in the tube axis direction on the outer surface of the tube is at least one of them, and the value intended by the present invention cannot be obtained.

由以上的說明可知，藉由將電焊鋼管的組成分、鋼組織、及製造條件控制在本發明的範圍內，可以提供很適合作為鋼管樁用之具有最佳的降伏比及高耐挫曲性能，並且兼顧高強度及高韌性的電焊鋼管。From the above description, by controlling the composition, steel structure, and manufacturing conditions of the electric welded steel pipe within the scope of the present invention, it is possible to provide a steel pipe pile with the best yield ratio and high buckling resistance. , And give consideration to high strength and high toughness electric welded steel pipe.

Claims (6)

一種電焊鋼管，其係具有母材部以及在管軸方向上的焊接部之電焊鋼管， 母材部的組成分，以質量%計，係含有 C：0.020～0.11%、 Si：0.60%以下、 Mn：0.50～1.70%、 P：0.030%以下、 S：0.015%以下、 Al：0.010～0.060%、 Nb：0.010～0.080%、 V：0.001～0.060%、 Ti：0.010～0.050%、 N：0.006%以下，其餘部分由Fe及不可避免的雜質所組成， 假設前述母材部的板厚度為t時，位於從前述電焊鋼管的外表面起算之板厚度t的1/4t深度位置處的鋼組織， 以面積率計，係含有變韌鐵為70%以上； 前述變韌鐵的平均有效粒徑，以平均圓當量直徑計，係10.0μm以下；且前述變韌鐵的平均長寬比，係0.1～0.8； 管軸方向的拉伸強度，係590MPa以上；0.2%降伏強度，係450MPa以上；降伏比，係85～95%； 以前述母材部中的管軸方向當作試驗片長邊方向時之在-30℃溫度時之夏比衝擊試驗的衝擊吸收能量，係70J以上； 前述母材部中的鋼管外表面之在管軸方向上的殘留應力，係250MPa以下。An electric welded steel pipe, which is an electric welded steel pipe with a base material part and a welded part in the pipe axis direction, The composition of the base metal part, in mass%, contains C: 0.020～0.11%, Si: 0.60% or less, Mn: 0.50～1.70%, P: 0.030% or less, S: 0.015% or less, Al: 0.010～0.060%, Nb: 0.010～0.080%, V: 0.001～0.060%, Ti: 0.010～0.050%, N: 0.006% or less, the rest is composed of Fe and inevitable impurities, Assuming that the plate thickness of the base material portion is t, the steel structure at a depth of 1/4t of the plate thickness t calculated from the outer surface of the electric resistance welded steel pipe, In terms of area ratio, the system contains more than 70% toughened iron; The average effective particle size of the aforementioned toughened iron is 10.0μm or less in terms of average equivalent circle diameter; and the average aspect ratio of the aforementioned toughened iron is 0.1-0.8; The tensile strength in the axial direction of the pipe is above 590MPa; the yield strength of 0.2% is above 450MPa; the yield ratio is 85-95%; The impact absorption energy of the Charpy impact test at a temperature of -30°C when the tube axis direction in the aforementioned base material portion is taken as the long side direction of the test piece is 70J or more; The residual stress in the pipe axis direction on the outer surface of the steel pipe in the aforementioned base material portion is 250 MPa or less. 如請求項1所述的電焊鋼管，其中，除了前述組成分之外，以質量%計，還含有B：0.008%以下。The electric resistance welded steel pipe according to claim 1, which further contains B: 0.008% or less in mass% in addition to the aforementioned composition. 如請求項1或請求項2所述的電焊鋼管，其中，除了前述組成分之外，以質量%計，還含有從Cr：0.01～1.0%、Mo：0.01～1.0%、Cu：0.01～0.50%、Ni：0.01～1.0%、Ca：0.0005～0.010%之中所選出的一種或兩種以上。The electric welded steel pipe according to claim 1 or claim 2, which, in addition to the aforementioned composition, contains, in terms of mass %, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, and Cu: 0.01 to 0.50 %, Ni: 0.01~1.0%, Ca: 0.0005~0.010% selected from one or two or more. 一種電焊鋼管的製造方法，其係對於鋼素材依序地實施熱軋工序、冷卻工序而製作成熱軋鋼板，然後，對於該熱軋鋼板實施室溫下的滾壓成形工序而製作成電焊鋼管， 前述鋼素材，係具有如請求項1至請求項3中的任一項所記載的組成分； 前述熱軋工序，係對於前述鋼素材進行加熱到1100～1280℃的溫度之後，再以粗輥軋結束溫度為850～1150℃、最終輥軋結束溫度為750～850℃、且在進行粗輥軋與最終輥軋時之在930℃以下的合計軋縮率為65%以上的條件，來實施粗輥軋及最終輥軋而製作成熱軋鋼板的工序； 前述冷卻工序，係對於前述熱軋鋼板，以板厚度中心溫度為準，以開始冷卻至停止冷卻為止的平均冷卻速度為15～35℃/s、冷卻停止溫度為450～650℃的條件，來進行冷卻的工序； 前述室溫下的滾壓成形工序，係先對於將前述熱軋鋼板實施了滾壓成形加工後的鋼管素材進行焊接，再以對於焊接後的鋼管外表面的圓周長度而言的縮徑率為0.2～0.5%的條件，來實施縮徑輥軋。A method for manufacturing an electric welded steel pipe, which is a hot-rolled steel sheet by sequentially performing a hot rolling process and a cooling process on a steel material, and then performing a roll forming process at room temperature on the hot-rolled steel sheet to produce an electric welded steel pipe , The aforementioned steel material has the composition as recorded in any one of Claim 1 to Claim 3; The hot rolling process is to heat the steel material to a temperature of 1100 to 1280°C, and then to finish the rough rolling at a temperature of 850 to 1150°C and the final temperature to finish rolling at 750 to 850°C. The process of performing rough rolling and final rolling to produce a hot-rolled steel sheet under the condition that the total reduction ratio at 930°C or less during rolling and final rolling is 65% or more; The aforementioned cooling step is based on the temperature of the center of the thickness of the hot-rolled steel sheet, with an average cooling rate of 15 to 35°C/s and a cooling stop temperature of 450 to 650°C. Carry out the cooling process; The aforementioned roll forming process at room temperature is to first weld the steel pipe material after the aforementioned hot-rolled steel sheet has been roll-formed, and then take the diameter reduction rate relative to the circumferential length of the welded steel pipe outer surface Under the condition of 0.2 to 0.5%, reduce diameter rolling is performed. 一種電焊鋼管的製造方法，其係對於具有：如請求項1至請求項3中的任一項所記載的組成分，假設板厚度為t時，位於從外表面起算之板厚度t的1/4t深度位置處的鋼組織，以面積率計，係含有變韌鐵為70%以上；前述變韌鐵的平均有效粒徑，以平均圓當量直徑計，係10.0μm以下；且前述變韌鐵的平均長寬比，係0.1～0.8的熱軋鋼板，實施室溫下的滾壓成形工序而製作成電焊鋼管， 前述室溫下的滾壓成形工序，係先對於將前述熱軋鋼板實施了滾壓成形加工後的鋼管素材進行焊接，再以對於焊接後的鋼管外表面的圓周長度而言的縮徑率為0.2～0.5%的條件，來實施縮徑輥軋。A method for manufacturing an electric welded steel pipe, which is based on the composition described in any one of claim 1 to claim 3, assuming that the plate thickness is t, which is located at 1/th of the plate thickness t calculated from the outer surface The steel structure at a depth of 4t, in terms of area ratio, contains more than 70% of toughened iron; the average effective particle size of the aforementioned toughened iron, in terms of average circle equivalent diameter, is less than 10.0μm; and the aforementioned toughened iron The average aspect ratio is 0.1-0.8 hot-rolled steel sheet, which is subjected to the roll forming process at room temperature to produce electric welded steel pipe. The aforementioned roll forming process at room temperature is to first weld the steel pipe material after the aforementioned hot-rolled steel sheet has been roll-formed, and then take the diameter reduction rate relative to the circumferential length of the welded steel pipe outer surface Under the condition of 0.2 to 0.5%, reduce diameter rolling is performed. 一種鋼管樁，其係使用如請求項1至請求項3中的任一項所記載的電焊鋼管。A steel pipe pile using the electric welded steel pipe described in any one of claim 1 to claim 3.