JP7769218B2 - Spiral steel pipe and its manufacturing method - Google Patents

Description

The present invention relates to spiral steel pipes.

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

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

Patent Document 1 proposes a spiral steel pipe containing 0.08% or more of Nb, Ti, and V, aiming to achieve high strength through precipitation strengthening by carbides and improved 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.

Patent Document 2 proposes a spiral steel pipe in which Mneq (Mneq = Mn + 0.15Si + 0.8Cr + 0.5Mo) is introduced as an index representing the hardenability of retained austenite, and the hardenability of the steel plate is moderately improved by adjusting this to 1.5 to 3.5%. The structure of this steel pipe also ensures strength and toughness by combining bainitic ferrite as the main phase with 2% or more of martensite at the center of the plate thickness.

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

Japanese Patent Application Laid-Open No. 2014-5519 JP 2015-193887 A JP 2016-47956 A

[0003] When manufacturing a steel sheet for spiral steel pipes (hereinafter sometimes simply referred to as a steel sheet ), fluctuations in the coiling temperature (fluctuations in the coiling temperature) may occur during the hot rolling process, resulting in large quality variations in the steel sheet in the longitudinal direction. Furthermore, even in the coil after coiling, the temperature history of the steel sheet differs between the front end (the innermost winding part of the coil), the middle part, and the rear end (the outermost part of the coil), further increasing the problem of quality variations.

Traditionally, materials have been designed to achieve a single bainite phase by including alloying elements such as Ni and Mo, and to meet required specifications even when variations are 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 quality variations are taken into account, many areas do not meet the standard values. In other words, product yield decreases, hindering productivity.

On the other hand, the technologies 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. To obtain such a structure, it is difficult to control the manufacturing conditions of the steel sheet , particularly the temperature history after coiling, resulting in a problem of large quality variations within the steel sheet . Furthermore, since the steel sheet contains martensite, the material design emphasizes strength, making it difficult to ensure toughness. Therefore, when considering variations in strength and toughness (Charpy absorbed energy), this results in a problem of low productivity.

In order to solve this problem, the present inventors re-analyzed and examined the strength and toughness of conventional steel sheets containing Ni and Mo. As a result, it was found that although the conventional steel sheets have an average strength of about 670 MPa, the average absorbed energy in Charpy tests (hereinafter simply referred to as absorbed energy) is about 70 J, and when the variation is taken into consideration, the margin for absorbed energy is small.

The objective of this invention is to satisfy the required specifications by improving toughness in 570 MPa-class spiral steel pipes, while allowing for a certain degree of strength reduction by reducing the Ni and Mo content.

The inventors 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 eliminating martensite, which deteriorates toughness, and by using a structure consisting mainly of or consisting of a single phase of bainitic ferrite. Furthermore, to compensate for the decrease in toughness caused by the partial formation of polygonal ferrite and pearlite due to fluctuations in manufacturing conditions (for example, fluctuations in coiling temperature), they came up with the idea of refining the bainitic ferrite crystal grains, which led to the creation of the present invention. The gist of the present invention is as follows.

[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: 0.050-0.100%,
Si: 0 to 0.55%,
Mn: 0.50-2.00%,
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]
Further, the components are, in mass %,
O: 0 to 0.0050%,
Ca: 0-0.0050%,
B: 0 to 0.0020%,
Contains one or more elements selected from the group consisting of V: 0 to 0.060% and 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]
A method for producing a steel plate to be used for a spiral steel pipe according to any one of the above [1] to [4], characterized in that, when producing a steel plate having the chemical composition according to the above [1] or [2] by hot rolling, the heating temperature before hot rolling is 1150°C or higher, the overall reduction rate of finish rolling in hot rolling is 30% or higher, the final stage finish rolling temperature 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.

The present invention has the advantage of improving quality variation in 570 MPa-class spiral steel pipes without using expensive alloying elements such as Ni and Mo.

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, the case where no content is contained (0%) may be included.
The steel pipe base material refers to the steel plate ( steel plate wound in a spiral shape) that makes up the spiral steel pipe, and refers to the steel plate other than the welded part (the part not affected by the welding).
The center in the plate thickness direction of a steel pipe base material (hereinafter sometimes simply referred to as "center") refers to a region within a distance range of 1/4 of the plate thickness from the center of the plate thickness toward both surfaces in the plate thickness direction cross section of the steel plate constituting the steel pipe base material. In other words, it refers to a region in the plate thickness direction cross section of the steel plate whose width is within 1/2 of the plate thickness from the center of the plate thickness.

<Chemical composition of steel plate>
C: 0.050-0.100%
C is effective in improving the strength of steel, and in order 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. Therefore, 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.

Si: 0.55% or less Si is an element necessary for deoxidation. A high Si content makes it easier to form island martensite, significantly deteriorating low-temperature toughness, so the Si content is preferably 0.55% or less. More preferably, it is 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 since reducing Si is expensive, the lower limit may be set to 0.01%.

Mn: 0.50-2.00%
Mn acts as an element for improving hardenability, and to obtain this effect, it is preferable to include 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.

Al: 0.035% or less. Al is typically used as a deoxidizer and is an element contained in steel. Since a high Al content increases Al-based non-metallic inclusions, reduces the cleanliness of the steel, and deteriorates its toughness, the Al content is preferably 0.035% or less. More preferably, the Al content is 0.033 % or less, 0.031 % or less, 0.030 % or less, 0.028 % or less, or 0.026 % or less. Since deoxidation can also be achieved with Si or Ti, the inclusion of Al is not essential. Therefore, the lower limit of Al is preferably 0%, but since reducing Al is expensive, the lower limit may be set to 0.001%.

Nb: 0.010-0.080%
Nb is an element that forms carbides and nitrides and improves the strength of steel sheets through precipitation strengthening. Therefore, the Nb content should 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 form, and toughness decreases. From the viewpoint of strength and toughness, the Nb content should be 0.080% or less, preferably 0.075% or less, 0.070% or less, 0.065% or less, or 0.060% or less.

Ti: 0.020% or less Like Nb, Ti is an element that forms carbides and nitrides and improves steel sheet strength through precipitation strengthening. On the other hand, if the Ti content is too high, Ti oxides aggregate and coarsen, resulting in a decrease in 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 its lower limit is preferably 0%. However, there is no need to intentionally remove Ti, and its lower limit may be set to 0.001%, 0.003%, or 0.005%.

Nb + Ti: 0.080% or less As mentioned above, both Nb and Ti form carbides and nitrides to improve the strength of the steel sheet , but if the amount is too large, hard phases such as coarse carbonitrides are formed, resulting in a decrease in toughness. Therefore, the total amount 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%.

P: 0.035% or less S: 0.035% or less Both P and S are impurities and are elements 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 the content of these elements, the better. It is advisable to set both P and S to 0.035% or less. Preferably, each of them is set to 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 to 0.001%.

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 of 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 benefits. These elements are described below.

N: 0.0045% or less N is an element that bonds with Ti, Nb, and Al to form nitrides. If the N content is high, the dissolved N reduces toughness, 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 not be contained, and its lower limit may be 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.

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 as low as possible, and is preferably 0.0050% or less, and more preferably 0.0030% or less.

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 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 the spiral steel pipe, and a suitable Ca content is 0.0050% or less, 0.0045% or less, or 0.0040% or less. On the other hand, 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.

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 a spiral steel pipe, and the preferred B content is 0.0020% or less.

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

Nb + Ti + V: 0.080% or less Like Ti, Nb forms carbides and nitrides to improve the strength of the steel sheet , but too much V 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%.

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 contained in the base material of spiral steel pipes, and the preferred Mg content is 0.0100% or less.

In addition to the elements that may be contained above, the remainder is Fe and impurities. Here, "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 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 preferred ranges mentioned above.

Furthermore, while the present invention was developed on the premise that it does not contain the expensive elements Ni, Mo, Cu, and Cr, these elements may be contained as unavoidable impurities. Of course, even if these elements are contained, the structural structure specified in this invention can be achieved and the problem can be solved. Even in this case, it falls within the technical scope of this invention. If Ni, Mo, Cu, and Cr are contained, the following should be done.

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 a spiral steel pipe, and even if it is contained, the Ni content should be 0.05% or less.

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 a spiral steel pipe, and even if it is contained, the Mo content should be 0.05% or less.

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 a spiral steel pipe, and even if it is contained, the Cr content should be 0.05% or less.

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

<Central organization>
The structure at the center of the steel pipe base material in the thickness direction is preferably a bainitic ferrite single phase (100% by 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 (within the 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 by 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, and 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%.

On the other hand, the steel pipe base material of the present invention does not contain martensite or bainite. This is because hard phases such as martensite and bainite not only degrade toughness, but also make temperature control during manufacturing difficult, reducing productivity and increasing quality variation.

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 and Repera corrosion. 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, and the average area ratio of each structure is calculated.

<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 generated. 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. The lower limit need not be particularly limited.

Bainitic ferrite grains can be measured using electron backscatter diffraction (EBSD). A test specimen 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. Points where the angular difference between adjacent measurement points is 15° or more are considered to be grain boundaries, and the area surrounded by these grain boundaries is considered to be a grain. The diameter of a circle with an area equivalent to that of a single grain is considered to be 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.

<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. The lower limit is not particularly limited, but it is preferably about 5 mm thicker than the thickness of a spiral steel pipe that is normally used.

<Tensile strength, energy absorption amount in Charpy impact test>
Test specimens for the tensile strength, yield strength, and Charby impact tests are taken so as to include the center of the steel plate in the thickness direction.
The tensile strength and yield strength are measured by a uniaxial tensile strength test in accordance with JIS Z2241: 2011. The tensile strength and yield strength are each obtained by performing a tensile test three times on a test piece taken from the same steel plate and arithmetically averaging the measured values.
The Charpy impact test is performed in accordance with JIS Z2242:2018 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.
In the case of a 570 MPa-class spiral steel pipe, it is sufficient if the tensile strength (TS) is 570 MPa or more, the yield strength (YS) is 450 MPa or more, and the Charpy impact value (Cv) at -5°C is 47 J or more. Considering the quality variation of steel sheets , it is sufficient if the above characteristic values are satisfied within a range of, for example, up to three times the standard deviation σ of the characteristic values.

<Manufacturing method>
A method for manufacturing a steel plate for a spiral steel pipe according to one embodiment of the present invention will be described. Steel having a predetermined chemical composition is melted and cast to produce a steel billet (e.g., slab). The method for melting and casting the steel billet is not particularly limited. That is, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out to adjust the chemical composition to the above-mentioned one, 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 chemical composition can be controlled within the chemical composition range of the present invention.

The cast slab is heated to a specified temperature before hot rolling begins. 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 special cooling. The heating temperature of the cast slab before hot rolling should be 1150°C or higher to promote the dissolution of Nb. On the other hand, if the temperature is too high, energy costs will increase, so it is best to keep it below 1350°C.

A conventional hot rolling method can be used for the hot rolling. Finish rolling is usually performed in a multi-stage (e.g., 4- to 6-stage) continuous rolling process. The overall reduction ratio of the finish rolling performed in this multi-stage continuous rolling process is preferably 30% or more, more preferably 40% or more, 50% or more, or even 60% or more. Furthermore, the reduction ratio may be higher in the earlier stages (upstream) than in the later stages (downstream), and may be lower in the later stages (downstream). To refine the bainitic ferrite grains in the steel sheet structure, the finish rolling temperature in the final stage is preferably 850°C or higher, more preferably 900°C or higher. That is, by completing rolling in the two-phase region, the austenite structure can be refined, ultimately reducing the grain size of the bainitic ferrite.

After the completion of finish rolling, the steel sheet is cooled to a temperature in the temperature range of 650°C 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 of 1 to 10°C/sec that is slower than the cooling rate of the first-stage cooling (second-stage cooling). The first-stage cooling should begin 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 of 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 of 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.

By controlling the cooling in this way, the steel sheet is kept in a temperature range near the transformation temperature, and the austenite in the steel sheet can be transformed into vatinic ferrite.
After cooling, the finished hot-rolled steel sheet is wound into a coil. The winding temperature at this time is preferably 620°C or lower. As mentioned above, the winding temperature is prone to fluctuation (fluctuation), and the winding temperature tends to be particularly high at the tip of the steel sheet . The tip of the steel sheet is located at the innermost part of the coil and is difficult to cool. In addition, if the coiling temperature is high, the tip of the steel sheet is slowly cooled and turns into ferrite, making it difficult to achieve strength. Therefore, the coiling temperature is more preferably 610°C or lower, or 600°C or lower. 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 higher. More preferably, it is 450°C or higher, 500°C or higher, or even 550°C or higher.

There are no particular restrictions on the method for manufacturing spiral steel pipes from hot-rolled steel sheets. Existing manufacturing methods can be applied.

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 objectives of the present invention.

Steel material with the chemical composition shown in Table 1 was melted, and the refined molten steel was formed into slabs using a continuous casting method. Steel strips with a thickness of 25 mm were produced under the manufacturing conditions shown in Table 2.

Next, test pieces were taken from the front, middle, and rear ends of the steel plates obtained, 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 in the plate thickness direction could be observed. For the tensile tests and Charpy impact tests, ten test pieces were taken from different positions from each part.

Observation and testing methods were performed according to the methods described above. The results are shown in Table 3. As shown in Table 3, one embodiment of the present invention, even without containing Ni, Mo, Cu, or Cr, was able to 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 taking into account variation (3σ: σ is the standard deviation). Examples of conventional steel sheets containing added Mo and Ni are listed as reference examples.

The present invention can be used for spiral steel pipes. For example, it can be used for spiral steel pipes for civil engineering and construction.

Claims (5)

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. Further, the components are, in mass %,
O: 0 to 0.0050%,
Ca: 0-0.0050%,
Cr: 0-0.05 %,
Cu: 0-0.05 %,
B: 0 to 0.0020%,
Contains one or more elements selected from the group consisting of V: 0 to 0.060% and 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. The spiral steel pipe according to claim 1, wherein the thickness of the steel pipe base material is 5 to 25 mm. A spiral steel pipe according to any one of claims 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. A method for producing a steel plate to be used for a spiral steel pipe according to claim 1, wherein, when a steel plate having the chemical composition according to claim 1 or 2 is produced by hot rolling, the heating temperature before hot rolling is 1150°C or higher, the reduction ratio of the entire finish rolling in the hot rolling is 30% or higher, the final stage finish rolling temperature 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 of 1 to 10°C/ sec which is slower than the cooling rate of the first-stage cooling, and the steel is coiled at 620°C or lower.