Spiral steel pipe and its manufacturing method

Abstract

To satisfy a required specification by improving toughness, while allowing some degree of strength lowering on the premise to reduce Ni and Mo, in a 570 MPa class spiral steel pipe.SOLUTION: By forming a texture that does not contain martensite, which deteriorates toughness, and has a bainitic ferrite single phase or main phase (90% or more), which improves toughness while maintaining a certain level of strength. Furthermore, in order to compensate for the deterioration in toughness due to the formation of pearlite and polygonal ferrite in some parts due to fluctuations in manufacturing conditions, the crystal grains of bainitic ferrite were made finer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a spiral steel pipe. [Background technology]

[0002] Spiral steel pipes have traditionally been used in the civil engineering and construction fields, where design guidelines were revised following the Great East Japan Earthquake, resulting in increased demand for high-strength steel pipes, particularly 570 MPa-class steel pipes. The main specifications for 570MPa-class steel pipe are a tensile strength (TS) of 570MPa or more, a yield strength (YS) of 450MPa or more, and a Charpy impact value (Cv) of 47J or more at -5°C. To meet these specifications, performance has traditionally been ensured by reducing carbon (C), adding alloy elements (such as Ni and Mo) that have a high hardenability-improving effect, and creating a bainite single-phase structure.

[0003] On the other hand, in the civil engineering and construction sector, there is a strong market demand for lower prices, and there is a need to lower the price (cost) while still meeting the strength and toughness specifications.To meet this demand, steel pipes that reduce the amount of expensive alloying elements (such as Ni and Mo) have been proposed as a technology to be applied.

[0004] Patent Document 1 proposes a spiral steel pipe containing 0.08% or more of Nb, Ti, and V, aiming to increase strength through precipitation strengthening by carbides and improve toughness through grain refinement by nitrides. The structure of this steel pipe is a combination of bainitic ferrite as the main phase and martensite, ensuring strength and toughness.

[0005] Patent Document 2 introduces Mneq (Mneq = Mn + 0.15Si + 0.8Cr + 0.5Mo) as an index representing the hardenability of the retained austenite, and by making this 1.5 to 3.5%, the steel can be made board A spiral steel pipe with moderately improved hardenability has been proposed. The structure of this steel pipe also ensures strength and toughness by combining bainitic ferrite as the main phase with 2% or more martensite at the center of the plate thickness.

[0006] Patent Document 3 proposes a spiral steel pipe with a relatively high carbon (C) content of 0.10 to 0.20% to increase strength. The structure of this steel pipe also has bainitic ferrite as the main phase, with martensite, bainite, and pearlite in a content of 10% or more, ensuring strength and toughness. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-5519 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-193887 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-47956 Summary of the Invention [Problem to be solved by the invention]

[0008] Steel for spiral steel pipes board (Hereafter, simply steel board When manufacturing steel sheets, fluctuations in the coiling temperature may occur during the hot rolling process, and this may cause problems such as the temperature fluctuations of the steel sheets. board Furthermore, the quality of the steel in the coil after winding becomes unstable. board The temperature history differs at the tip (innermost winding of the coil), middle part, and rear end (outermost part of the coil), which further increases the problem of quality variations.

[0009] In the past, materials were designed to satisfy the required specifications by incorporating alloying elements such as Ni and Mo to achieve a single bainite phase, even when variations were taken into account. However, if expensive elements such as Ni and Mo are reduced to reduce costs, the single bainite phase cannot be maintained, reducing strength and toughness, and when considering variations in quality, many parts do not meet the standard values. In other words, product yield decreases and productivity is hindered.

[0010] On the other hand, the techniques proposed in Patent Documents 1 to 3, which reduce alloying elements such as Ni and Mo, combine bainitic ferrite as the main phase with hard phases such as martensite. board It is difficult to control the manufacturing conditions, especially the temperature history after coiling, and as a result, the steel board The problem is that there is a large variation in the quality of the steel. Furthermore, because it contains martensite, the material design prioritizes strength, and it is actually difficult to ensure toughness. Therefore, when considering variations in strength and toughness (Charpy absorbed energy), there is a problem that productivity is low as a result.

[0011] To solve this problem, the present inventors have developed a method for manufacturing a steel containing Ni and Mo. board The strength and toughness of the conventional steel were analyzed and examined again. board Although the average strength is about 670 MPa, the average absorbed energy in the Charpy test (hereinafter simply referred to as absorbed energy) is about 70 J, and it was found that there is little margin for absorbed energy when the variation is taken into account.

[0012] Therefore, the present invention aims to satisfy the required specifications by improving toughness in a 570 MPa-class spiral steel pipe, while allowing for a certain degree of strength reduction by reducing Ni and Mo. [Means for solving the problem]

[0013] The present inventors have conducted extensive research and development to solve the above-mentioned problems. As a result, they came up with the idea that toughness can be improved while maintaining a certain level of strength by forming a structure that is a single phase or mainly composed of bainitic ferrite, without including martensite, which deteriorates toughness. Furthermore, they came up with the idea of ​​refining the bainitic ferrite grains to compensate for the decrease in toughness caused by the partial formation of polygonal ferrite or pearlite due to fluctuations in manufacturing conditions (for example, fluctuations in coiling temperature), and have thus achieved the present invention. The gist of the present invention is as follows.

[0014] [1] Spiral-wound steel board A spiral steel pipe in which the width direction end faces are welded together, and the composition of the steel pipe base material is, in mass%, C: 0.050~0.100%, Si: 0 to 0.55% Mn: 0.50 to 2.00%, P: 0.035% or less, S: 0.035% or less, Al: 0 to 0.035%, Nb: 0.010~0.080%, Ti: 0 to 0.020% N: 0 to 0.0045% Nb+Ti: 0.080% or less, The balance is Fe and impurities. The structure at the center of the steel pipe base material in the plate thickness direction is such that the total area ratio of polygonal ferrite and pearlite is 10% or less, and the remainder is bainitic ferrite, A spiral steel pipe characterized in that the average crystal grain size of the bainitic ferrite is 15 μm or less. [2] Furthermore, the components are, in mass %, O: 0 to 0.0050%, Ca: 0 to 0.0050%, B: 0~0.0020%, V: 0 to 0.060%, and Contains one or more elements selected from the group consisting of Mg: 0 to 0.0100%, The spiral steel pipe according to [1], wherein when V is contained, Nb+Ti+V: 0.08% or less. [3] The spiral steel pipe according to [1] or [2], wherein the thickness of the steel pipe base material is 6 to 25 mm. [4] The spiral steel pipe according to any one of [1] to [3], wherein the tensile strength of the steel pipe base material is 570 MPa or more, and the energy absorption amount in a Charpy impact test at -5°C is 47 J or more. [5] Steel having the composition described in [1] or [2] above board When manufacturing by hot rolling, the heating temperature before hot rolling is 1150°C or higher, the reduction rate of the entire finish rolling in hot rolling is 30% or higher, the final stage finish rolling temperature is 850°C or higher, and after the end of finish rolling, the first stage cooling is performed to a temperature in the temperature range of 650°C to 660°C at a cooling rate of 5 to 20°C / second, and thereafter, the cooling rate to 620°C is slower than the first stage cooling and is 1 to 10°C / second. Seconds The steel used for the spiral steel pipe according to any one of the above [1] to [4], characterized in that a second stage of cooling is performed at a cooling rate of 620°C or less, and the steel is coiled at a temperature of 620°C or less. board Manufacturing method. [Effects of the Invention]

[0015] The present invention has the effect of improving quality variations in a 570 MPa class spiral steel pipe without using expensive alloy elements such as Ni and Mo. DETAILED DESCRIPTION OF THE INVENTION

[0016] An example of an embodiment of the present invention will be described below. Unless otherwise specified, "%" regarding the content of an element means mass % and "%" regarding the content of a structure means area %. Furthermore, unless a lower limit is particularly specified, it may include the case where no content is contained (0%). The steel pipe base material is the steel that makes up the spiral steel pipe. board (Spiral wound steel board ) and the steel other than the welded part (the part not affected by the welding) board Refers to... The center of the steel pipe base material in the thickness direction (hereinafter, sometimes simply referred to as "center") is the steel that constitutes the steel pipe base material. board In the cross section of the plate thickness direction, it refers to the area within a distance range of 1 / 4 of the plate thickness from the center of the plate thickness to both surfaces. board In a cross section in the thickness direction of a plate, this refers to the area whose width is within half of the plate thickness, centered at the center of the plate thickness.

[0017] <Chemical composition of steel plate> C: 0.050 to 0.100% C is effective in improving the strength of steel, and to obtain the desired strength, it is advisable to add 0.05% or more, preferably 0.055% or more, 0.060% or more, or 0.065% or more. If the C content is too high, hardenability improves and martensite is more likely to form, reducing the toughness of the base material, so the C content is preferably 0.100% or less. Preferably, it is 0.098% or less, 0.096% or less, 0.094% or less, 0.092% or less, 0.090% or less, or 0.088% or less.

[0018] Si: 0.55% or less Silicon is an element necessary for deoxidation. A high Si content makes it easier for island martensite to form, significantly deteriorating low-temperature toughness, so the Si content is preferably 0.55% or less. More preferably, it should be 0.50% or less, 0.40% or less, 0.35% or less, 0.33% or less, or 0.30% or less. Since deoxidation can also be achieved with Al or Ti, the inclusion of Si is not essential. Therefore, the lower limit of Si is preferably 0%, but because reducing Si is expensive, the lower limit may be set to 0.01%.

[0019] Mn: 0.50 to 2.00% Mn acts as an element for improving hardenability, and to obtain this effect, it is preferable to add 0.50% or more. More preferably, it is 0.60% or more, 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. A high Mn content increases the hardenability of the steel, degrading HAZ toughness and weldability, and further promotes center segregation in continuously cast steel billets, degrading the low-temperature toughness of the base material. Therefore, the Mn content is preferably 2.00% or less. More preferably, it is 1.90% or less, 1.80% or less, 1.75% or less, 1.70% or less, 1.65% or less, or 1.60% or less.

[0020] Al: 0.035% or less Al is normally used as a deoxidizer and is an element contained in steel. If the Al content is too high, the amount of Al-based non-metallic inclusions increases, the cleanliness of the steel decreases, and toughness deteriorates, so it is preferable to keep the Al content to 0.035% or less, and more preferably 0. 0 33% or less, 0. 0 31% or less, 0. 0 30% or less, 0. 0 28% or less, or 0. 0 It is recommended that the Al content be 26% or less. Since deoxidation can be achieved with Si and Ti, the Al content is not essential. Therefore, the lower limit of Al is preferably 0%, but since reducing Al is expensive, the lower limit may be set at 0.001%.

[0021] Nb: 0.010 to 0.080% Nb forms carbides and nitrides, and strengthens the steel by precipitation strengthening. board Nb is an element that improves strength. Therefore, it is recommended that the Nb content be 0.010% or more, preferably 0.015% or more, or 0.020% or more. On the other hand, if the Nb content is too high, carbonitrides are more likely to be formed, and toughness decreases. From the viewpoint of strength and toughness, it is recommended that the Nb content be 0.080% or less, preferably 0.075% or less, 0.070% or less, 0.065% or less, or 0.060% or less.

[0022] Ti: 0.020% or less Like Nb, Ti also forms carbides and nitrides, strengthening the steel through precipitation strengthening. board Ti is an element that improves strength. On the other hand, if the Ti content is too high, Ti oxides aggregate and coarsen, reducing toughness. Therefore, it is preferable to set the Ti content to 0.020% or less, 0.018% or less, or 0.016% or less. Ti does not necessarily need to be contained, and the lower limit is preferably 0%. However, there is no need to intentionally remove Ti, and the lower limit may be set to 0.001%, 0.003%, or 0.005%.

[0023] Nb+Ti: 0.080% or less As mentioned above, both Nb and Ti form carbides and nitrides, and boardAlthough Nb improves strength, too much Nb and Ti forms hard phases such as coarse carbonitrides, which reduces toughness. Therefore, the total content of Nb and Ti should be 0.080% or less, preferably 0.078% or less, 0.076% or less, 0.074% or less, 0.072% or less, 0.070% or less, 0.068% or less, 0.066% or less, 0.064% or less, 0.062% or less, 0.060% or less, 0.058% or less, 0.056% or less, 0.054% or less, 0.052% or less, or 0.050% or less. The lower limit of the total amount of Nb+Ti does not need to be particularly limited, but since there is no need to specifically remove Nb and Ti, the lower limit may be set to 0.001%, 0.003%, 0.005%, 0.007%, 0.008%, 0.009%, or 0.010%.

[0024] P:0.035% or less S: 0.035% or less Both P and S are impurities that not only deteriorate the toughness of the steel pipe base material, but also the toughness of the welded portion of a spiral steel pipe. The lower their contents, the better. Both P and S should be 0.035% or less. Preferably, each should be 0.030% or less, 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less. It is preferable to have no P or S (0%), but because reducing P or S requires significant equipment costs, the lower limit may be set at 0.001%.

[0025] In one embodiment of the present invention, in addition to the above elements, the remainder is Fe and impurities. By containing the above elements in the specified amounts, a 570 MPa-class spiral steel pipe with a good balance between strength and toughness can be obtained. Furthermore, in addition to these elements, the spiral steel pipe of this embodiment may contain one or more elements selected from the group consisting of N, O, Ca, B, V, and Mg in place of a portion of the Fe. These elements do not necessarily need to be contained, but their inclusion can provide additional effects. These elements will be explained below.

[0026] N: 0.0045% or less N is an element that bonds with Ti, Nb, and Al to form nitrides. A high N content reduces toughness due to the presence of dissolved N, so the N content is preferably 0.0045% or less. More preferably, it is 0.040% or less, 0.038% or less, 0.036% or less, or 0.035% or less. N is not an essential element and may be omitted, with the lower limit even being 0%. However, because nitrides such as TiN act as pinning particles and have a certain toughness-improving effect, the N content should be 0.0001% or more, preferably 0.0005% or more.

[0027] O: 0.0050% or less O is an element that forms pinning particles. However, since the inclusion of O reduces the cleanliness of the steel, the content is preferably small, preferably 0.0050% or less, and more preferably 0.0030% or less.

[0028] Ca:0.0050% or less Ca is an element that controls the morphology of sulfide-based inclusions and improves low-temperature toughness. If the Ca content is too high, CaO-CaS may form large clusters or inclusions, which may adversely affect toughness. Ca does not necessarily need to be contained in the base material of a spiral steel pipe, and a suitable Ca content is 0.0050% or less, 0.0045% or less, or 0.0040% or less. However, to obtain a certain effect, Ca may be contained in an amount of 0.0001% or more, 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0007% or more.

[0029] B: 0.0020% or less B is an element that is effective in improving the hardenability of the base material and suppressing the formation of grain boundary ferrite. As the B content increases, the effect saturates. B does not necessarily need to be contained in the base material of spiral steel pipes, and the preferred B content is 0.0020% or less.

[0030] V:0.060% or less V is an element that improves the strength of the base material. If the V content is too high, the yield ratio may increase due to precipitation hardening. V does not necessarily need to be contained in the base material of spiral steel pipes, and the suitable V content is 0.060% or less.

[0031] Nb+Ti+V:0.080% or less V, like Nb and Ti, forms carbides and nitrides and board Although V improves strength, if it is too much, it forms hard phases such as coarse carbonitrides, resulting in a decrease in toughness. Therefore, when V is contained, the total amount of Nb, Ti, and V is set to 0.080% or less, preferably 0.078% or less, 0.076% or less, 0.074% or less, 0.072% or less, 0.070% or less, 0.068% or less, 0.066% or less, 0.064% or less, 0.062% or less, 0.060% or less, 0.058% or less, 0.056% or less, 0.054% or less, 0.052% or less, or 0.050% or less. The lower limit of the total amount of Nb+Ti+V does not need to be particularly limited, but since there is no need to specifically remove Nb, Ti, and V, the lower limit may be set to 0.001%, 0.003%, 0.005%, 0.007%, 0.008%, 0.009%, or 0.010%.

[0032] Mg: 0.0100% or less Mg is an element that forms inclusions such as MgAl2O4 and MgS. MgAl2O4 precipitates on TiN. These inclusions act as pinning particles, suppressing the coarsening of austenite grains in the HAZ, refining the microstructure, and improving low-temperature toughness. As the Mg content increases, the effect saturates. Mg does not necessarily need to be included in the base material of spiral steel pipes, and the preferred Mg content is 0.0100% or less.

[0033] In addition to the above elements, the remainder is Fe and impurities. Here, the term "impurities" refers to elements that are inevitably and unintentionally mixed in during the industrial production of steel, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention. Specific examples of impurities include P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H. Of these, it is preferable to control the amounts of P and S so that they fall within the above-mentioned preferred ranges.

[0034] Furthermore, although the present invention is based on the premise that the expensive elements Ni, Mo, Cu, and Cr are not contained, these elements may be contained as unavoidable impurities. Of course, even if these elements are contained, the structural composition defined by the present invention can be achieved and the problem can be solved. Even in this case, it is within the technical scope of the present invention. If Ni, Mo, Cu, and Cr are contained, the following procedure is recommended.

[0035] Ni: 0.05% or less Ni is an element that can improve the strength of the base material without reducing toughness. As the Ni content increases, the effect saturates. Ni does not necessarily need to be contained in the base material of spiral steel pipes, and even if it is contained, the Ni content should be kept below 0.05%.

[0036] Mo: 0.05% or less Mo is an element that can improve the strength of the base material. If the Mo content is too high, the effect saturates and the toughness decreases. Mo does not necessarily need to be contained in the base material of spiral steel pipes, and even if it is contained, the Mo content should be 0.05% or less.

[0037] Cr:0.05% or less Cr is an element that can improve the strength of the base material. As the Cr content increases, the effect saturates. Cr does not necessarily need to be contained in the base material of spiral steel pipes, and even if it is contained, the Cr content should be 0.05% or less.

[0038] Cu:0.05% or less Copper is an element that can improve the strength of the base material. As the amount of copper increases, the effect saturates. Cu does not necessarily need to be contained in the base material of spiral steel pipes, and even if it is contained, the amount of copper should be 0.05% or less.

[0039] <Central Organization> The structure of the steel pipe base material at the center in the thickness direction is preferably a bainitic ferrite single phase (100% area fraction). Bainitic ferrite is a structure that can ensure appropriate strength and toughness. In an embodiment that satisfies the composition specifications of the present invention, even when considering variations within the coil (steel plate), a tensile strength of 570 MPa or more and a Charpy absorbed energy of 47 J or more at -5°C can be ensured. The bainitic ferrite should be 90% or more in area fraction, and preferably 91% or more, 92% or more, 93% or more, 94% or more, 95% or more, 96% or more, 97% or more, 98% or more, 99% or more, and 99.95%, 99.96%, 99.97%, 99.98%, or 99.99%. Other than bainitic ferrite, the total area ratio of pearlite and polygonal ferrite is preferably 10% or less, preferably 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less. There is no particular lower limit for the total content of pearlite and polygonal ferrite, but the remainder other than bainitic ferrite should be at least one of pearlite and polygonal ferrite. In other words, the remainder other than pearlite or polygonal ferrite is bainitic ferrite. From the viewpoint of toughness, it is desirable to suppress pearlite as much as possible, and the area ratio of pearlite is preferably 5% or less, 4% or less, 3% or less, 2% or less, 1% or less, 0.5% or less, 0.3% or less, 0.1% or less, or 0%.

[0040] On the other hand, the steel pipe base material according to the present invention does not contain martensite or bainite, because hard phases such as martensite and bainite not only deteriorate toughness but also make temperature control during manufacturing difficult, thereby worsening productivity and increasing quality variations.

[0041] The area ratio of the structure is measured as follows: A test piece is taken from the center of the steel pipe base material, polished, and then subjected to nital etching and Repera etching. The exposed structure is measured using an optical microscope in 10 fields of view, focusing on the structure observed within an area of ​​1000 μm x 1000 μm. The obtained images are analyzed to calculate the average area ratio of each structure.

[0042] <Average grain size of bainitic ferrite is 15 μm or less> Furthermore, by refining the grain size of bainitic ferrite, toughness can be further improved. This ensures toughness even when pearlite or polygonal ferrite is formed. The average equivalent circle diameter of bainitic ferrite is preferably 15 μm or less, and more preferably 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, or 10 μm or less. There is no particular lower limit.

[0043] Bainitic ferrite grains can be measured using electron backscatter diffraction (EBSD). A test piece taken from the center of the steel pipe base material is analyzed using EBSD in 20 fields of view within an area of ​​500 μm x 500 μm. The grain boundaries are identified as points where the angle difference between adjacent measurement points is 15° or more, and the area surrounded by these boundaries is considered to be a grain. The diameter of a circle with an area equivalent to that of a single grain is defined as the grain size of that grain (circle-equivalent grain size). The average grain size (average circle-equivalent grain size) can be calculated by arithmetically averaging the obtained grain sizes.

[0044] <Thickness> The thickness of the spiral steel pipe according to one embodiment of the present invention (thickness of the steel pipe base material) is not particularly limited. However, assuming that the spiral steel pipe is manufactured using a normal hot rolling process, it is preferably 25 mm or less in view of the coiling temperature and the temperature history after coiling. There is no particular lower limit, but it is preferably about 5 mm thicker than the thickness of a spiral steel pipe that is normally used.

[0045] <Tensile strength, energy absorption amount in Charpy impact test> The specimens for tensile strength, yield strength, and Charby impact tests were made of steel. board The specimen is taken so that the center of the plate in the thickness direction is included. Tensile strength and yield strength are measured in a uniaxial tensile strength test in accordance with JIS Z2241:2011. board Tensile tests are performed three times on test pieces taken from each sample, and the measured values ​​are arithmetically averaged to obtain the respective values. The Charpy impact test conforms to JIS Z2242:2018, and is performed at -5°C to measure the absorbed energy. The absorbed energy is obtained by performing the Charpy impact test three times and arithmetically averaging the measured values. For 570MPa-class spiral steel pipe, the tensile strength (TS) must be 570MPa or more, the yield strength (YS) must be 450MPa or more, and the Charpy impact value (Cv) at -5℃ must be 47J or more. board Considering the quality variations, it is sufficient to satisfy the above characteristic values ​​within a range of, for example, up to three times the standard deviation σ of the characteristic values.

[0046] <Manufacturing method> Steel for spiral steel pipe according to one embodiment of the present invention board The manufacturing method of the present invention will be described below. Steel having a predetermined composition is melted and cast to produce a steel billet (slab, etc.). The method for melting and casting the steel billet is not particularly limited. That is, after melting in a blast furnace, electric furnace, etc., various secondary smelting processes may be carried out to adjust the composition to the above-mentioned composition, and then the steel may be cast by a method such as ordinary continuous casting or thin slab casting. In this case, scrap may be used as the raw material as long as the composition can be controlled within the composition range of the present invention.

[0047] The cast slab is heated to a predetermined temperature before starting hot rolling. In the case of continuous casting, it may be cooled to a low temperature once and then reheated before hot rolling, or it may be heated and hot rolled immediately after continuous casting without any cooling. The heating temperature of the cast slab before hot rolling is preferably 1150°C or higher from the viewpoint of promoting the solid solution of Nb. On the other hand, if the temperature is too high, the energy cost increases, so it is preferably 1350°C or lower.

[0048] For hot rolling, a normal hot rolling method can be adopted. Finish rolling is usually carried out by multi-stage (for example, 4 to 6 stages, etc.) continuous rolling. The reduction rate of the entire finish rolling carried out by this multi-stage continuous rolling is preferably 30% or more, more preferably 40% or more, 50% or more, or 60% or more. Furthermore, it is preferable that the reduction rate be higher on the front stage (upstream side) than on the rear stage (downstream side), and that the reduction rate be lower on the rear stage (downstream side). Steel board In order to refine the bainitic ferrite grains in the structure, the final stage finish rolling temperature is preferably set to 850°C or higher, and more preferably 900°C or higher. That is, by completing the rolling in the two-phase region, the austenite structure can be refined, and the final grain size of the bainitic ferrite can be reduced.

[0049] After the finish rolling, the material is cooled to a temperature in the range of 650 to 660°C at a cooling rate of 5 to 20°C / sec (first stage cooling), and then cooled to 620°C at a cooling rate slower than the first stage cooling and 1 to 10°C / sec. Seconds The first stage cooling is preferably started immediately after the completion of finish rolling, and the cooling start temperature is 800°C or higher, preferably 850°C or higher, or 900°C or higher. The upper limit of the cooling rate for the first stage cooling can be 20°C / sec, 18°C / sec, 16°C / sec, 14°C / sec, 12°C / sec, or 10°C / sec, and the lower limit can be 5°C / sec, 6°C / sec, or 7°C / sec. The upper limit of the cooling rate for the second stage cooling can be 10°C / sec, 9°C / sec, 8°C / sec, 7°C / sec, 6°C / sec, or 5°C / sec, and the lower limit can be 1°C / sec, 2°C / sec, or 3°C / sec. After the first stage of cooling, the material may be cooled by holding it in the temperature range of 620°C to 650°C for 0.5 to 20 seconds. The cooling rate after holding is not particularly limited. It may be a cooling rate of 5 to 20°C / second, similar to the first stage.

[0050] These cooling controls allow the steel board By keeping the temperature in the range near the transformation temperature of boardThe austenite in the steel can be transformed into vatinic ferrite. After cooling, the hot-rolled steel sheet is wound into a coil. The winding temperature is preferably 620°C or less. As mentioned above, the winding temperature is prone to fluctuations, especially in the case of steel. board The coiling temperature at the tip tends to be higher. board The tip is located at the innermost part of the coil and is difficult to cool. If the winding temperature is high, the steel board The tip portion is slowly cooled and turns into ferrite, making it difficult to obtain strength. Therefore, the coiling temperature is more preferably set to 610°C or less, or 600°C or less. On the other hand, if the coiling temperature is too low, hard phases such as bainite increase, deteriorating toughness. Therefore, the coiling temperature is preferably 400°C or more. It is even more preferably set to 450°C or more, 500°C or more, or even 550°C or more.

[0051] There are no particular limitations on the method for manufacturing a spiral steel pipe from a hot-rolled steel sheet, and any existing manufacturing method may be used. [Example]

[0052] An example of the present invention will be described. The conditions in the example are one embodiment adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one embodiment. The present invention can adopt various embodiments as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0053] Steel materials having the chemical compositions shown in Table 1 were melted, and the refined molten steel was formed into slabs by continuous casting. These were then manufactured under the manufacturing conditions shown in Table 2 to produce steel strips having a thickness of 25 mm.

[0054] Next, the resulting steel board Test pieces were taken from the front, middle, and rear ends of the steel sheet, and microstructure observation, tensile tests, and Charpy impact tests were performed. Three test pieces for microstructure observation were taken from different positions from each part so that the center of the sheet thickness direction could be observed. Ten test pieces were taken from different positions from each part for the tensile tests and Charpy impact tests.

[0055] The observation and testing methods were performed according to the methods described above. The results are shown in Table 3. As shown in Table 3, it was shown that the steel sheet according to one embodiment of the present invention can satisfy the requirements of a tensile strength of 570 MPa or more, a yield strength of 450 MPa or more, and a Charpy absorbed energy at -5°C of 47 J or more, even when taking into account variations (3σ: σ is standard deviation), without containing Ni, Mo, Cu, or Cr. Note that examples of conventional steel sheets containing added Mo or Ni are shown as reference examples.

[0056] [Table 1]

[0057] [Table 2]

[0058] [Table 3] [Industrial Applicability]

[0059] The present invention can be used for spiral steel pipes, for example, spiral steel pipes for civil engineering and construction.

Claims

1. A spiral steel pipe in which the widthwise end faces of spirally wound steel plates are welded together, and the composition of the steel pipe base material is, in mass%, C: 0.05-0.10%, Si: 0 to 0.55%, Mn: 0.50-2.00%, P: 0.035% or less, S: 0.035% or less, Al: 0-0.030%, Nb: 0.010-0.080%, Ti: 0 to 0.020%, N: 0 to 0.0045%; Nb + Ti: 0.08% or less, The balance is Fe and impurities, The structure at the center of the steel pipe base material in the plate thickness direction is such that the total area ratio of polygonal ferrite and pearlite is 10% or less, and the remainder is bainitic ferrite, A spiral steel pipe characterized in that the average crystal grain size of the bainitic ferrite is 15 μm or less.

2. Further, the components are, in mass %, O: 0 to 0.0050%, Ca: 0-0.0050%, Cr: 0 to 0.05%, Cu: 0 to 0.05%, B: 0 to 0.0020%, V: 0 to 0.060%, and Contains one or more elements selected from the group consisting of Mg: 0 to 0.0100%, 2. The spiral steel pipe according to claim 1, wherein when V is contained, the total amount of Nb + Ti + V is 0.08% or less.

3. 3. The spiral steel pipe according to claim 1, wherein the thickness of the steel pipe base material is 5 to 25 mm.

4. 4. The spiral steel pipe according to claim 1, wherein the steel pipe base material has a tensile strength of 570 MPa or more and an energy absorption amount in a Charpy impact test at -5°C of 47 J or more.

5. A method for manufacturing a steel plate to be used for a spiral steel pipe according to any one of claims 1 to 4, characterized in that when manufacturing a steel plate having the composition according to claim 1 or 2 by hot rolling, the heating temperature before hot rolling is 1150°C or higher, the reduction rate of the entire finish rolling in the hot rolling is 30% or higher, the finish rolling temperature in the final stage is 850°C or higher, first-stage cooling is performed after the end of finish rolling to a temperature in the temperature range of 650°C to 660°C at a cooling rate of 5 to 20°C / sec, and thereafter second-stage cooling is performed to 620°C at a cooling rate that is slower than the cooling rate of the first-stage cooling and is 1 to 10°C / sec, and the steel is coiled at 620°C or lower.

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