JP7034861B2 - Steel sheets and pipes for circular steel pipes with high strength, low yield ratio and excellent weldability, and their manufacturing methods - Google Patents

Description

The present invention relates to steel sheets and pipes for circular steel pipes having high strength, low yield ratio and excellent weldability, and methods for manufacturing them.

As a steel plate for welded structures such as civil engineering, construction, and bridges, a high-strength steel plate having a tensile strength of 550 MPa or more is used. When welding with a small heat input and a short bead length, such as temporary welding or welding of hanging tools, the weld heat-affected zone tends to harden. If the weld heat affected zone is hard, there is a risk of low temperature cracking and delayed fracture, so preheating is performed. Preheating requires time and cost, so it is required to reduce it. Therefore, the steel sheet is required to have excellent weld cracking resistance and weld hardening resistance even if the preheating is reduced.

Further, the circular steel pipe used for a building structure is required to exhibit a low YR with a yield ratio YR (= yield strength YS / tensile strength TS) of 85% or less from the viewpoint of seismic safety.

Patent Documents 1 to 3 are mentioned with respect to a steel pipe having a low yield ratio and a steel plate used for manufacturing the steel pipe. In the following, the steel plate used for manufacturing steel pipes may be referred to as a master plate.

Patent Document 1 describes cold forming with a low yield ratio of 490 MPa class or more, which can exhibit predetermined mechanical properties without performing SR (Stress Relief) treatment after forming a thick steel sheet into a steel pipe. A method for manufacturing a circular steel pipe is shown. Specifically, in Patent Document 1, a steel slab having a predetermined chemical composition is used, and an appropriate heat treatment is performed to produce a thick steel plate, whereby the thickness of the circular steel pipe is t (mm) and the outer diameter is D. When (mm), the yield strength of the steel pipe product standard is YS ₀ (MPa) or more, the tensile strength is TS ₀ (MPa) or more, and the yield ratio is 85% or less, the yield strength of the steel sheet is (YS ₀ ). -980t / D) to (YS _0-980t / D + 120) (MPa), the tensile strength of the steel sheet is (TS _0-560t / D) to (TS _0-560t / D + 100) (MPa), and the yield ratio of the steel sheet is It was shown that the hardness was controlled to be (75-82t / D)% or less and the hardness at a depth of 1 mm from the front and back surfaces of the steel sheet was HV140 to 200, respectively, and the steel sheet was pressed-bend cold-formed into a circular steel pipe. ing.

Patent Document 2 discloses a method for manufacturing a master plate for a building round pillar having a low yield ratio of 80% or less and high toughness without SR treatment. Specifically, in% by weight, C: 0.06 to 0.17%, Si: 0.06 to 0.50%, Mn: 0.5 to 1.6%, P: 0.015% or less, S: 0.005% or less, Al: 0.07% or less, N: 0.006% or less, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.8%, Cr A steel piece containing one or more kinds selected from the group consisting of: 0.05 to 0.5% and Nb: 0.005 to 0.05% is heated to 1000 ° C. or higher and then hot-rolled. After that, it has been shown that the mixture is rapidly cooled from a temperature of ₃ points or more to room temperature, rapidly cooled again from a temperature range of 740 to 780 ° C., and then rebaked in a temperature range of 400 ° C. or higher and 550 ° C. or lower.

In Patent Document 3, C: 0.01 to 0.20%, Si: 0.5% or less, Mn: 0.5 to 1.6%, P: 0.03% or less, S: 0 in terms of weight ratio. Steel containing 0.01% or less, Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0.006% or less, and the balance consisting of iron and unavoidable impurities at 900 to 1200 ° C. After reheating to the temperature range of 900 ° C or lower and rolling so that the cumulative rolling reduction of 900 ° C or lower is 30% or more, quenching from a temperature of 750 ° C or higher to room temperature immediately is performed, and the temperature range is 700 to 850 ° C. Is reheated and hardened, tempered in a temperature range below the Ac ₁ transformation point, and yield ratio (YR) ≤80-0.8 × t / D (t: plate thickness, D: steel pipe outer diameter). It has been shown that a steel pipe with a low yield ratio of 600 N / ^mm for construction is manufactured by cold forming using a steel plate controlled by. That is, the steel pipe is manufactured without performing SR.

Japanese Unexamined Patent Publication No. 2007-270304 Japanese Unexamined Patent Publication No. 9-165622 Japanese Unexamined Patent Publication No. 6-264144

As in Patent Document 1, if the steel pipe is bent without SR, the characteristics of the steel pipe vary greatly. Specifically, it seems that variations in YP of about 80 MPa and variations in TS of about 15 MPa occur regardless of the ratio of the plate thickness t / outer diameter D of the steel pipe. In particular, when the plate thickness is 70 mm or more, the amount of strain introduced is large at the test piece sampling position for characteristic evaluation, and it is difficult to realize low YR. Further, in Patent Document 1, it seems that a thick steel sheet having a strength of 55 kg class, a low YR, and a low carbon equivalent Ceq, that is, weldability, cannot be realized in a thick region having a plate thickness of 70 mm or more.

In Patent Document 2, a large amount of alloying elements are added in order to secure the strength, and as a result, it is considered that the Ceq is high and it is difficult to secure the weldability. Alternatively, it is considered that the YR of the steel sheet becomes high due to the addition of the alloying element, and when SR is applied, the hardness of the soft phase does not decrease and the YR remains high. Further, in Patent Document 2, when the plate thickness is increased, the ratio of the hard phase is reduced and the YR of the original plate is considered to be increased. In addition, the cooling rate may decrease due to the thickening and the strength may decrease due to the SR treatment, which may not satisfy the mechanical properties.

According to Patent Document 3, a steel sheet having a thickness of more than 70 mm can achieve a strength of 55 kg class and a low YR. Conceivable. Further, in Patent Document 3, if SR is performed in order to suppress variations in the characteristics of steel pipes, it is considered that the strength may decrease and the mechanical characteristics may not be satisfied.

The present invention has been made in view of such a situation, and an object thereof is for a circular steel pipe showing excellent weldability with high strength and a low yield ratio even in a thick range of a steel plate having a thickness of 70 mm or more. It is an object of the present invention to provide steel sheets, circular steel pipes having high strength and excellent weldability at a low yield ratio, and methods for producing them.

Aspect 1 has a component composition of
C: 0.125 to 0.170% by mass,
Si: 0.10 to 0.60% by mass,
Mn: 0.90 to 1.60% by mass,
P: More than 0% by mass, 0.015% by mass or less,
S: More than 0% by mass, 0.008% by mass or less,
Al: 0.010 to 0.080% by mass,
N: 0.0010 to 0.0065% by mass, and the balance is a steel sheet for circular steel pipes composed of iron and unavoidable impurities.
The plate thickness is 70 mm or more,
The carbon equivalent Ceq represented by the following formula (1) is 0.42% by mass or less,
The weld crack susceptibility composition Pcm represented by the following formula (2) satisfies 0.15 to 0.27% by mass, and the hardenable multiple DI represented by the following formula (3) satisfies 1.05 or more.
The metal structure at the 1/4 position of the plate thickness consists of a hard phase and a soft phase, the fraction of the hard phase is 15 to 27 area%, the rest is the soft phase, and the hardness Hv (3 gf) of the soft phase is 149 to. 180, the hardness Hv (3 gf) of the hard phase is 260 to 330, and
A steel sheet for circular steel pipes having a yield ratio of 73% or less, a yield strength YS satisfying the following formula (4), and a tensile strength TS satisfying the following formula (5).
Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (1)
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (2)
When the amount of Mn is 1.20% by mass or more,
DI = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200) × [B] +1)
If the amount of Mn is less than 1.20% by mass,
DI = 1.16 × ([C] / 10) ^0.5 × (0.7 × [Si] +1) × (3.33 × [Mn] +1) × (0.35 × [Cu] +1) × (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200 x [B] +1) … (3)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] in the above formulas (1) to (3) are The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B shown in mass% are shown respectively, and the elements not contained are set to zero.
YS = (385-840 x t / D) to (710-940 x t / D) x 0.73 (MPa) ... (4)
TS = (540-170 × t / D) to (710-940 × t / D) (MPa) ... (5)
However, D in the above formulas (4) and (5) indicates the outer diameter (mm) of the circular steel pipe, and t indicates the plate thickness (mm) of the circular steel pipe.

Aspect 2 is further described.
Cu: More than 0% by mass, 0.35% by mass or less,
Ni: More than 0% by mass, 0.35% by mass or less,
Cr: More than 0% by mass, 0.35% by mass or less,
Mo: More than 0% by mass, 0.35% by mass or less,
V: More than 0% by mass, 0.050% by mass or less,
B: Circular according to aspect 1 containing one or more elements selected from the group consisting of more than 0% by mass, 0.0030% by mass or less, and Ti: more than 0.005% by mass, 0.030% by mass or less. It is a steel plate for steel pipes.

Aspect 3 is the steel sheet for circular steel pipe according to Aspect 1 or 2, further comprising Ca: more than 0% by mass and 0.0030% by mass or less.

Aspect 4 is a circular steel pipe formed of the steel plate for circular steel pipe according to any one of aspects 1 to 3.

Aspect 5 is the method for manufacturing a steel sheet for a circular steel pipe according to any one of Aspects 1 to 3.
The steel pieces are heated to 950 to 1250 ° C., hot rolling is performed under the conditions that the cumulative rolling reduction rate in the temperature range of 860 to 1000 ° C. is 30% or more and the finish rolling temperature is 840 to 960 ° C., and then the average cooling is performed. Cool from a cooling start temperature of 800 ° C or higher to a cooling stop temperature of 500 ° C or lower at a speed of 2 to 30 ° C / s.
Next, the hot-rolled material is reheated at a quenching temperature of 735 to 850 ° C. and a quenching heating time of 5 to 60 minutes, and then quenched. Then, if the tempering temperature: DI is less than 1.4, the temperature is 400 ° C. or higher. , 600 ° C. or higher, DI is 1.4 or higher, 400 ° C. or higher, 650 ° C. or lower, tempering time: 5 to 60 minutes. This is a method for manufacturing a steel sheet for circular steel pipes.

Aspect 6 is a method for manufacturing a circular steel pipe according to Aspect 4, wherein the circular steel pipe is manufactured.
Using the steel sheet for circular steel pipe according to any one of aspects 1 to 3,
Bending process to satisfy D / t = 10 to 30 (D is the outer diameter of the circular steel pipe (mm), t is the plate thickness of the circular steel pipe (mm)).
This is a method for manufacturing a circular steel pipe, which comprises a step of performing heat treatment at a temperature of 450 ° C. or higher and 650 ° C. or lower in this order.

According to the present invention, it is possible to provide a steel sheet for circular steel pipes and a circular steel pipe having high strength, a low yield ratio and excellent weldability even in a region where the thickness of the steel sheet is thick, and a method for producing the same.

FIG. 1 is a graph showing t / D and ΔTS1 (difference between TS of steel pipe and TS of steel plate) when steel pipe is processed. FIG. 2 is a graph showing t / D and ΔTS2 (difference between the maximum TS of the steel pipe and the minimum TS of the steel pipe) when the steel pipe is machined. FIG. 3 is a graph showing t / D and ΔYS (difference between YS of steel pipe and YS of steel plate) when steel pipe is processed. FIG. 4 is a graph showing the relationship between the hardness of the soft phase and the yield ratio YR.

When the thickness of the steel plate is as thick as, for example, more than about 50 mm, the circular steel pipe is manufactured by bending with a press bend. Dislocations are introduced into the steel sheet due to the strain generated by this bending process. The dislocations introduced increase as the plate thickness increases. Therefore, it is important to control the above dislocations in order to achieve strength of 55 kg class or higher and a low yield ratio in thick-walled steel pipes.

Further, when processing is performed on a thicker steel plate having a plate thickness of 70 mm or more under severe conditions where the value of D / t (D: diameter of steel pipe, t: plate thickness of steel pipe) is small, the pressing position of the press is reached. The variation in characteristics increases accordingly. Therefore, in the present invention, heat treatment (SR) after manufacturing the steel pipe is indispensable, and the characteristics of the steel pipe can be stably obtained by carrying out SR.

As a steel sheet manufacturing process for achieving a low YR, a DQ (direct quenching) -Q'(two-phase region quenching) -T (tempering) process after hot rolling is effective. However, it is difficult to obtain high strength by this process. When the thickness of the original plate is as thick as 70 mm or more, it is difficult to obtain particularly high strength.

As a result of repeated studies in consideration of these points, in order to realize high strength and low YR of steel pipes, it is necessary to determine the characteristic range of steel sheets in consideration of bending during steel pipe manufacturing and SR. It was found that in order to secure the characteristics of this steel sheet, a hard phase and a soft phase should be present in the structure of the steel sheet, and the hardness and ratio of each phase should be controlled within an appropriate range. In the present invention, the DQ-Q'-T process is adopted after hot rolling in order to realize low YR, but even if the temperature becomes high during the two-phase region quenching, dislocations remain and the hardness of the soft phase is appropriate. It is necessary to control DI in order to keep the range within a certain range, it is important to control the alloying elements in order to keep the ratio of the hardness of the hard phase and the hard phase in an appropriate range, and further, the steel plate and the steel pipe It was found that it is necessary to control the above alloying elements on the premise that the carbon equivalent Ceq is suppressed in order to secure the weldability of the above.

Hereinafter, the plate thickness, characteristics, structure, composition and manufacturing method thereof, and the circular steel pipe and its manufacturing method of the steel sheet for circular steel pipe of the present invention will be described in order. In the following, the steel plate for circular steel pipe and the circular steel pipe may be simply referred to as a steel plate and a steel pipe, respectively.

1. 1. Plate thickness The steel plate of the present invention is premised on a plate thickness of 70 mm or more. The plate thickness can be further 80 mm or more, and further 85 mm or more. On the other hand, in consideration of the characteristics required for the steel sheet of the present invention and the manufacturing conditions for realizing the characteristics, the upper limit of the plate thickness is about 100 mm.

2. 2. Characteristic

(1) Yield ratio YR, yield strength YS, and tensile strength TS
Yield ratio YR: 73% or less, yield strength YS: within the range of the following formula (4), tensile strength TS: within the range of the following formula (5) YS = (385-840 × t / D) to (710-940 × t / D) × 0.73 (MPa) ... (4)
TS = (540-170 × t / D) to (710-940 × t / D) (MPa) ... (5)
However, D in the above formulas (4) and (5) indicates the outer diameter (mm) of the circular steel pipe, and t indicates the plate thickness (mm) of the circular steel pipe.

In the present invention, it was aimed to obtain YS ≧ 385 MPa, 550 MPa ≦ TS ≦ 670 MPa, and YR ≦ 85% as the characteristics of the steel pipe. Then, as described above, the target characteristic range of the original plate, that is, the characteristics of the steel plate was set as described above in consideration of the bending process and SR at the time of manufacturing the steel pipe.

As a result of conducting an experiment to determine the characteristic range of the original plate, the following was found. That is, as t / D increases, dislocation introduction during bending increases. It was found that most of the strain was recovered by the subsequent SR processing, but the strain remained depending on the t / D value, and the yield ratio became high. Therefore, it is necessary to set the target value of the strength of the original plate according to t / D. The D / t is in the range of 10 to 30, and the t / D is in the range of 0.033 to 0.10.

The range of YS and TS of the steel sheet is based on the following derivation. As a result of estimating the minimum value of the amount of change in characteristics when a steel pipe was manufactured from a steel plate, the following results were obtained. In addition, as shown below, the target value was set in consideration of the characteristic change depending on the pushing position.

FIG. 1 is a graph showing t / D and ΔTS1 (difference between TS of steel pipe and TS of steel plate) when steel pipe is processed. The "TS of steel pipe" here is the minimum value at that time, although the characteristics vary depending on the pushing position when bent. Further, FIG. 2 is a graph showing t / D and ΔTS2 (difference between the maximum TS of the steel pipe and the minimum TS of the steel pipe) when the steel pipe is machined. Since the variation is applied only on the upper limit side, regarding the TS range of the steel sheet, (550-ΔTS1 (max)) to (670-ΔTS1 (min) -ΔTS2), FIGS. 1 and 2 show (550-10-170 × t). / D) to (670 + 20-340 × t / D + 20-600 × t / D) (MPa), that is, (540-170 × t / D) to (710-940 × t / D) (MPa) were derived.

FIG. 3 is a graph showing t / D and ΔYS (difference between YS of steel pipe and YS of steel plate) when steel pipe is processed. Based on this graph, the range of YS of the steel sheet was also determined in the same manner as the TS of the steel sheet.

By setting the YR to 73% or less regardless of t / D, the target steel pipe characteristics can be stably realized. YR is preferably 70% or less. The lower the YR, the more preferable, and the lower limit thereof is not particularly limited, but the lower limit is approximately 50%.

The sampling position of the test piece used for the above-mentioned characteristic evaluation of the steel sheet is the position of the steel sheet corresponding to the position of 1/4 of the plate thickness from the outer surface side of the steel pipe, that is, the plate from the surface of the steel plate that comes into contact with the punch during bending for manufacturing the steel pipe. The position should be 3/4 of the thickness. However, the characteristics and structure of the steel plate (original plate) are symmetrical with respect to the center of the plate thickness, and even if evaluated at a position of 1/4 of the plate thickness from the surface of the steel plate, the values are the same. Therefore, in the present invention, the above-mentioned characteristics and structure are evaluated at a position of 1/4 of the plate thickness from the surface of the steel plate regardless of the front and back surfaces.

(2) Weldability The steel sheet of the present invention is also required to have weldability required for manufacturing steel pipes. Further, the steel pipe is also required to have weldability required for manufacturing a welded structure. In the present invention, it was evaluated that the weldability was excellent by setting the Ceq described later to 0.42% by mass or less.

3. 3. Structure The metal structure of the steel sheet of the present invention consists of a hard phase and a soft phase at a position of 1/4 of the plate thickness. Further, the fraction of the hard phase is 15 to 27 area%, and the balance is the soft phase. Further, the hardness Hv (3 gf) of the soft phase is 149 to 180, and the hardness Hv (3 gf) of the hard phase is 260 to 330. Hereinafter, this organization will be described.

In order to achieve the above-mentioned tensile properties in the low Ceq component range, it is necessary to control the hardness and ratio of each of the hard phase and the soft phase.

In the present invention, the "hard phase" is a phase formed by quenching and tempering a reverse-transformed austenite portion generated during heating in a two-phase region. It is a phase in which carbon and alloying elements are concentrated during reverse transformation and cementite is aggregated. Specifically, it refers to the structure of any one or more of tempered bainite and tempered martensite having a high carbon concentration.

Further, in the present invention, the "soft phase" means that bainite and pseudopolygonal ferrite produced by heating, hot rolling and cooling (accelerated cooling) after hot rolling are subjected to subsequent reheating, that is, during two-phase region heating. , A structure that is tempered at high temperature without reverse transformation to austenite and contains cell-like dislocations. Pseudopolygonal ferrite has a very small amount of carbon solid solution, so it is a phase in which cementite does not exist in the microstructure observation. Bainite is tempered at high temperatures, producing very coarse carbides. When measuring hardness, avoid coarse carbides.

In the present invention, since tempering is performed at 400 ° C. or higher in the steel sheet manufacturing process, the hard embrittled structure such as MA disappears and does not exist.

In order to ensure the desired properties of the steel sheet, especially high tensile strength and low yield ratio, it is necessary to optimize the fraction of the hard phase in the entire structure of the steel. By increasing the fraction of the hard phase to 15 area% or more, high tensile strength and low yield ratio can be ensured. The fraction of the hard phase is preferably 17.0 area% or more, more preferably 18.0 area% or more. On the other hand, if the fraction of the hard phase is too high, the hardness of the hard phase tends to decrease. Therefore, the fraction of the hard phase is set to 27 area% or less. It is preferably 26.0 area% or less, more preferably 25.0 area% or less.

Furthermore, by optimizing the hardness of each phase, high tensile strength and low yield ratio can be ensured more reliably. In particular, a low yield ratio can be easily realized by controlling the hardness range of the soft phase. The hardness Hv (3 gf) of the soft phase is 149 or more. The hardness Hv (3 gf) of the soft phase is preferably 152 or more. The hardness Hv (3 gf) of the soft phase is 180 or less, preferably 175 or less. Since the hardness of the soft phase is determined at the time of quenching, the hardness of the soft phase corresponding to the high dislocation density is controlled by DI. By increasing the DI, the hardness of the soft phase can be increased and the high strength of the steel sheet can be achieved.

High strength can be realized by setting the hardness Hv (3 gf) of the hard phase to 260 or more. The hardness Hv (3 gf) of the hard phase is preferably 270 or more, more preferably 280 or more. On the other hand, if the hardness of the hard phase is too high, the phase becomes very brittle and the toughness of the base metal decreases. Therefore, the hardness Hv (3 gf) of the hard phase is set to 330 or less. The hardness Hv (3 gf) of the hard phase is preferably 320 or less. The hardness of the hard phase can be achieved by optimizing the amount of alloying elements. In particular, it is better to strictly control the amount of C.

4. Component composition In the present invention, after satisfying the range of each component described later, carbon equivalent Ceq, weld crack susceptibility composition Pcm, and hardenability multiple DI, which are the parameters represented by the following formulas (1) to (3), are used. Must be within each range. Hereinafter, each parameter will be described.

Carbon equivalent Ceq represented by the following formula (1) is 0.42% by mass or less Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4+ [V] / 14 ... (1)
However, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] in the above formula (1) are C, Si and Mn represented by mass%, respectively. , Ni, Cr, Mo and V are shown, and the content of elements not contained is set to zero.

Ceq is called carbon equivalent. In the present invention, Ceq is set to 0.42% by mass or less from the viewpoint of ensuring the weldability of the steel plate and the steel pipe. Ceq is preferably 0.40% by mass or less. From the viewpoint of ensuring weldability, the lower the Ceq is, the more preferable it is, so the lower limit is not particularly limited. The lower limit of Ceq can be, for example, about 0.35% by mass. In the present invention, Ceq is reduced as described above from the viewpoint of ensuring weldability. However, when the Ceq is made small, the strength becomes low. Therefore, in the present invention, it is necessary to keep the Ceq value small and to achieve high strength.

Weld crack susceptibility composition Pcm represented by the following formula (2) is 0.15 to 0.27% by mass.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] in the above formula (2) are mass%, respectively. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B shown in the above are shown, and the content of the elements not contained is set to zero.

Pcm is called the weld crack sensitive composition. This Pcm is set to 0.27% by mass or less in order to stably suppress welding cracks even in a thick steel sheet having a large degree of restraint. It is preferably 0.26% by mass or less. The lower the value, the more preferable, but considering the component composition specified in the present invention, the lower limit is about 0.20% by mass.

When the hardenable multiple DI represented by the following formula (3) is 1.05 or more and the Mn amount is 1.20% by mass or more,
DI = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200) × [B] +1)
If the amount of Mn is less than 1.20% by mass,
DI = 1.16 × ([C] / 10) ^0.5 × (0.7 × [Si] +1) × (3.33 × [Mn] +1) × (0.35 × [Cu] +1) × (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200 x [B] +1) … (3)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] in the above formula (3) are C represented by mass%, respectively. , Si, Mn, Cu, Ni, Cr, Mo and V are shown, and the content of elements not contained is set to zero.

DI is called a hardenability multiple and is an index showing hardenability. As described above, in order to increase the hardness of the soft phase in the metal structure of the steel sheet and achieve the high strength of the steel sheet, the DI is set to 1.05 or more. DI is preferably 1.10 or more, more preferably 1.20 or more. The upper limit of DI is not particularly limited, but is about 2.00.

Next, the range of each component will be described.

C: 0.125 to 0.170% by mass
C is an element that contributes to high strength, and on the other hand, is also an element that deteriorates weldability. In the present invention, the amount of C is set to 0.125% by mass or more in order to achieve high strength while keeping the hardness of the hard phase in a desired structure, particularly a metal structure, within a predetermined range. The amount of C is preferably 0.130% by mass or more. When the amount of C is large, it becomes easy to secure the strength, but the weld crack resistance is deteriorated. In the present invention, the amount of C is 0.170% by mass or less from the viewpoint of ensuring weld crack resistance. The amount of C is preferably 0.165% by mass or less.

Si: 0.10 to 0.60% by mass
Si is an element effective as a deoxidizing material and also an element effective for improving the strength of the base material. Therefore, the amount of Si is set to 0.10% by mass or more. The amount of Si is preferably 0.15% by mass or more, and more preferably 0.20% by mass or more. On the other hand, from the viewpoint of ensuring weldability, the amount of Si is set to 0.60% by mass or less. The amount of Si is preferably 0.55% by mass or less, more preferably 0.50% by mass or less, still more preferably 0.45% by mass or less.

Mn: 0.90 to 1.60% by mass
Mn is an element that stabilizes austenite and lowers the transformation temperature, thereby contributing to the improvement of hardenability. In order to exert this effect, Mn is contained in an amount of 0.90% by mass or more. The amount of Mn is preferably 1.00% by mass or more. On the other hand, if Mn is excessively contained, MnS becomes coarse and the toughness of the base metal deteriorates. Therefore, the upper limit of the amount of Mn is set to 1.60% by mass. The amount of Mn is preferably 1.55% by mass or less.

P: More than 0% by mass, 0.015% by mass or less P, which is an unavoidable impurity, adversely affects the toughness of the base metal and the weld. Therefore, it is necessary to suppress the P content to 0.015% by mass or less. It is preferably 0.010% by mass or less. Industrially, it is difficult to set the amount of P to 0% by mass, and the lower limit is about 0.002% by mass.

S: More than 0% by mass, 0.008% by mass or less S is preferably less because it forms MnS and deteriorates impact characteristics and base metal elongation. Therefore, the S content needs to be suppressed to 0.008% by mass or less. The amount of S is preferably 0.005% by mass or less, more preferably 0.0030% by mass or less. Industrially, it is difficult to set the amount of S to 0% by mass, and the lower limit is about 0.001% by mass.

Al: 0.010 to 0.080 mass%
Al is an element required for deoxidation. In order to exert this effect, the amount of Al is 0.010% by mass or more. The amount of Al is preferably 0.015% by mass or more, and more preferably 0.020% by mass or more. On the other hand, if Al is excessively contained, coarse alumina-based inclusions are formed and the impact characteristics are deteriorated. Therefore, the upper limit of the amount of Al is set to 0.080% by mass. The amount of Al is preferably 0.070% by mass or less, more preferably 0.060% by mass or less.

N: 0.0010 to 0.0065% by mass
N is an element that produces AlN, prevents coarsening of γ grains during heating before hot rolling and during welding, and is effective in improving base metal toughness and HAZ toughness. In order to exert this effect, N is contained in an amount of 0.0010% by mass or more. The amount of N is preferably 0.0020% by mass or more. On the other hand, if N is excessively contained, the toughness of the base metal deteriorates due to the increase in the solid solution N. Therefore, the amount of N is set to 0.0065% by mass or less. The amount of N is preferably 0.0060% by mass or less.

The basic components of the steel sheet of the present invention are as described above, and the balance is iron and unavoidable impurities. Inevitable impurities are elements that are brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like. Inevitable impurities include, for example, O, Sb and the like, as well as Nb of 0.005% by mass or less. Nb is not added because it significantly increases the hardness of the soft phase, and even if it is contained, the amount thereof is about the above-mentioned unavoidable impurities. Further, even when Ti is not added, Ti may be contained as an unavoidable impurity in an amount of 0.005% by mass or less. It should be noted that, for example, P and S, which usually have a smaller content, are preferable, and therefore are unavoidable impurities, but there are elements whose composition range is separately defined as described above. Therefore, the above-mentioned "unavoidable impurities" in the present specification mean those excluding the elements whose composition range is separately defined.

The steel sheet of the present invention may contain the above elements in the component composition, and the parameters represented by the formulas (1) to (3) may be within a predetermined range. The selective elements described below may not be contained, but by containing them together with the above elements as needed, it is possible to more easily achieve high strength and contribute to improvement of base metal toughness and the like. Hereinafter, the selected element will be described.

Cu: More than 0% by mass, 0.35% by mass or less,
Ni: More than 0% by mass, 0.35% by mass or less,
Cr: More than 0% by mass, 0.35% by mass or less,
Mo: More than 0% by mass, 0.35% by mass or less,
V: More than 0% by mass, 0.050% by mass or less,
B: One or more elements selected from the group consisting of more than 0% by mass and 0.0030% by mass or less, and Ti: more than 0.005% by mass and 0.030% by mass or less.

Cu, Ni, Cr, Mo, V, and B are all effective elements for improving hardenability and improving the strength and toughness of the base metal without significantly adversely affecting weldability and HAZ toughness. .. Ti is an element that combines with N to form TiN, prevents the coarsening of austenite grains, that is, γ grains, during heating before hot rolling, and contributes to the improvement of the toughness of the base metal. It also has the effect of fixing N in the steel to prevent deterioration of the toughness of the base metal due to the solid solution N.

In order to exert these effects, when Cu, Ni, Cr, and Mo are contained, the content of each element is preferably more than 0% by mass, more preferably 0.05% by mass or more, still more preferably. It is 0.10% by mass or more. V is preferably more than 0% by mass, more preferably 0.005% by mass or more, still more preferably 0.010% by mass or more. B is preferably more than 0% by mass, more preferably 0.0005% by mass or more. Ti is preferably more than 0.005% by mass.

On the other hand, if these elements are excessively contained, the raw material cost will increase. Therefore, the amount of Cu, Ni, Cr and Mo is preferably 0.35% by mass or less, more preferably 0.30. It is less than mass%. V is preferably 0.050% by mass or less. B is preferably 0.0030% by mass or less. The Ti is preferably 0.030% by mass or less.

Ca: More than 0% by mass, 0.0030% by mass or less Ca contributes to the spheroidization of MnS and is an effective element for improving the toughness of the base metal and the ductility in the plate thickness direction. In order to exert the effect, the Ca amount is preferably more than 0% by mass, more preferably 0.0005% by mass or more. On the other hand, when the amount of Ca is excessive, inclusions become coarse and cause cracking. Therefore, the amount of Ca is preferably 0.0030% by mass or less.

5. Circular steel pipe The present invention also includes a circular steel pipe obtained by using the above steel plate. Examples of the circular steel pipe include a cold circular steel pipe manufactured by the method described later. As described above, the characteristics required for the circular steel pipe are YS ≧ 385 MPa, 550 MPa ≦ TS ≦ 670 MPa, and YR ≦ 85%. The range of the outer diameter D of the circular steel pipe is 700 to 3000 mm, and the range of the plate thickness t of the circular steel pipe is 70 mm or more and 100 mm or less. The plate thickness t of the circular steel pipe is substantially the same as the plate thickness of the steel plate used for manufacturing the circular steel pipe. In the present invention, as described above, D / t is in the range of 10 to 30, and t / D is in the range of 0.033 to 0.10.

6. Method for Manufacturing Steel Sheet Next, a method for manufacturing a steel sheet according to the present invention will be described. In this method, a steel piece satisfying the above-mentioned composition is heated to 950 to 1250 ° C., and then hot rolling is performed. After performing under the condition of 960 ° C., the material is cooled from a cooling start temperature of 800 ° C. or higher to a cooling stop temperature of 500 ° C. or lower at an average cooling rate of 2 to 30 ° C./s, and then the hot rolled material is subjected to quenching temperature: Reheating at 735 to 850 ° C., quenching heating time: 5 to 60 minutes, and then quenching. After that, tempering temperature: 400 ° C. or higher, less than 600 ° C., DI is 1.4 when DI is less than 1.4. In the above cases, tempering is performed under the conditions of 400 ° C. or higher, 650 ° C. or lower, and tempering time: 5 to 60 minutes. The reason for defining these manufacturing conditions will be described.

[Heating temperature: 950 to 1250 ° C]
When the heating temperature is low, the elements are difficult to dissolve in solid solution, and in particular, the carbides do not re-dissolve and become coarse in rolling and subsequent heat treatment. Therefore, the heating temperature was set to 950 ° C. or higher. It is preferably 990 ° C. or higher. On the other hand, if the heating temperature is too high, γ becomes coarse, the hardenability becomes high, and the fraction of the soft phase decreases. Therefore, the heating temperature is set to 1250 ° C. or lower. It is preferably 1200 ° C. or lower.

[Cumulative reduction rate in the temperature range of 860 to 1000 ° C: 30% or more]
In order to obtain the properties required by the present invention, it is important to homogenize the structure. For that purpose, it is necessary to repeatedly recrystallize the austenite grains by rolling in the recrystallization temperature range. Therefore, the cumulative reduction rate in the temperature range of 860 to 1000 ° C. is set to 30% or more. The cumulative reduction rate is preferably 35% or more. The upper limit of the cumulative reduction rate is approximately 70%.

[Finishing rolling temperature (FRT): 840 to 960 ° C]
When the finish rolling temperature becomes low, austenite does not recrystallize during hot rolling, that is, rolling in a so-called unrecrystallized region. Since the acoustic anisotropy increases when the rolling reduction in the unrecrystallized region is increased, the lower limit of the finish rolling temperature is set to 840 ° C. The finish rolling temperature is preferably 860 ° C. or higher. The finish rolling temperature is 960 ° C. or lower. It is preferably 900 ° C. or lower.

[Average cooling rate: 2 to 30 ° C / s]
After the hot rolling, the mixture is cooled from the following cooling start temperature to the cooling stop temperature at an average cooling rate of 2 to 30 ° C./s. If the average cooling rate during cooling after hot rolling is slow, the hardness of the soft phase and the fraction of the hard phase decrease, and the yield strength becomes insufficient. Therefore, the average cooling rate is set to 2 ° C./s or higher. It is preferably 3 ° C./s or higher. On the other hand, if the average cooling rate is excessively high, the fraction of the hard phase increases more than necessary and the yield ratio increases. Therefore, the average cooling rate is set to 30 ° C./s or less. It is preferably 20 ° C./s or less.

[Cooling start temperature (SCT): 800 ° C or higher]
When the cooling start temperature at the average cooling rate is lower than 800 ° C. at the surface temperature of the steel sheet, soft polygonal ferrite is formed, which causes a decrease in the strength of the base metal. Therefore, cooling at the above average cooling rate starts from a temperature of 800 ° C. or higher. The cooling start temperature is preferably 840 ° C. or higher. The upper limit of the cooling start temperature is not particularly limited, and is approximately the same as the finish rolling temperature.

[Cooling shutdown temperature (FCT): 500 ° C or less]
When cooling at the average cooling rate is stopped in a temperature range higher than 500 ° C., the transformation is not completed, the hardness of the hard phase decreases, and the strength, particularly the yield strength, is insufficient. Therefore, the cooling shutdown temperature is set to 500 ° C. or lower. The cooling shutdown temperature is preferably 400 ° C. or lower, and may be further 200 ° C. or lower. The lower limit of the cooling stop temperature is not particularly limited, and cooling may be performed at the above average cooling rate to near room temperature.

When the cooling stop temperature is higher than room temperature, air cooling may be performed from the cooling stop temperature to room temperature.

After cooling to the room temperature, the mixture is reheated to the following quenching temperature, then quenched, and then tempered. Hereinafter, each condition will be described.

[Quenching temperature: 735-850 ° C]
[Quenching heating time: 5 to 60 minutes]
The quenching temperature corresponds to the temperature in the two-phase region. This reheating may be referred to as two-phase region heating. When the quenching temperature is low, the reverse transformation fraction is insufficient, and the hard phase fraction is insufficient to achieve high strength. Therefore, the quenching temperature is set to 735 ° C. or higher. It is preferably 750 ° C. or higher. On the other hand, when the quenching temperature is high, the reverse transformation fraction increases, but the component concentration of the portion to be the hard phase is insufficient, and the hardness of the hard phase decreases. Therefore, the quenching temperature is set to 850 ° C. or lower. It is preferably 830 ° C or lower, and may be further 800 ° C or lower. Further, if the quenching heating time, that is, the holding time at the quenching temperature is short, the elements are difficult to concentrate and the hardness cannot be secured. Therefore, the quenching and heating time is set to 5 minutes or more. It is preferably 10 minutes or more. On the other hand, if the quenching and heating time is long, the productivity is lowered, so the time is set to 60 minutes or less.

After the holding, quenching is performed to about room temperature. Quenching can be water quenching, oil quenching, or the like. Alloy elements such as C, Mn, and Mo are concentrated in the austenite that has been reverse-transformed by the two-phase region heating, and is in a state of high hardenability. Therefore, even if the average cooling rate at the time of quenching is slow, the hard phase can be obtained, but in order to surely increase the hardness of the hard phase, it is preferable to cool the quenching at an average cooling rate of 1 ° C./s or more.

[Tempering temperature: 400 ° C or higher, less than 600 ° C when DI is less than 1.4,
When DI is 1.4 or more, 400 ° C or higher, 650 ° C or lower]
[Tempering time: 5 to 60 minutes]
In the present invention, in order to secure high strength, the range of the tempering temperature, particularly the upper limit of the tempering temperature is set according to DI. When the tempering temperature is low, the hard phase becomes too hard and the strength becomes higher than necessary. Therefore, the lower limit of the tempering temperature is 400 ° C. or higher regardless of DI. The tempering temperature is preferably 450 ° C. or higher, more preferably 490 ° C. or higher, regardless of DI. On the other hand, when the tempering temperature becomes high, the hardness of the hard phase decreases, the strength tends to be insufficient, the hardness ratio of the soft phase and the hard phase decreases, and the yield ratio increases. Therefore, the tempering temperature is set to less than 600 ° C. when the DI is less than 1.4, and 650 ° C. or lower when the DI is 1.4 or more. The tempering temperature is preferably 580 ° C. or lower when the DI is less than 1.4, and preferably 610 ° C. or lower when the DI is 1.4 or more. In addition, in order to obtain the effect of tempering, the tempering time is set to 5 minutes or more. It is preferably 10 minutes or more. On the other hand, from the viewpoint of productivity, the tempering time is 60 minutes or less.

7. Manufacturing method of circular steel pipe

The method for manufacturing a circular steel pipe of the present invention will be described. In this method, the steel plate is used and bent so as to be in the range of D / t = 10 to 30 (however, D: outer diameter of circular steel pipe (mm), t: plate thickness of circular steel pipe (mm)). This includes a step and a step of performing heat treatment (SR) at a temperature of 450 ° C. or higher and 650 ° C. or lower in this order. In the present invention, cold bending is performed as the bending. As a method of the bending process, a press bend method can be mentioned. The heat treatment (SR) is performed to remove the strain introduced in the bending process.

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited by the following examples, and can be carried out with appropriate modifications to the extent that it can meet the above-mentioned and later-described intent, and both of them are the technical scope of the present invention. Included in.

Steel pieces (slabs) satisfying the composition shown in Table 1 were obtained by a conventional method. Blanks in Table 1 mean that the component was not detected. After heating the steel pieces to the heating temperatures shown in Tables 2-1 and 2-2, hot rolling and cooling immediately after hot rolling are performed under the conditions shown in Tables 2-1 and 2-2. rice field. The "reduction rate" in Tables 2-1 and 2-2 indicates the cumulative reduction rate in the temperature range of 860 to 1000 ° C. In addition, air cooling was performed from the cooling shutdown temperature to room temperature. Then, it was reheated from room temperature to the quenching temperature shown in Table 2-1 and Table 2-2, and after maintaining the quenching heating time shown in Table 2-1 and Table 2-2 at this quenching temperature, quenching was performed. Next, tempering was performed under the conditions shown in Tables 2-1 and 2-2 to obtain steel sheets having the plate thickness shown in Tables 2-1 and 2-2 as the finish thickness.

The heating temperature at the time of quenching and tempering is the temperature at the center of the plate thickness of the steel plate, and is calculated from the atmosphere temperature in the furnace of the heat treatment furnace and the in-fire time by the difference method, or when an experimental furnace is used, the same plate. It is the temperature actually measured by inserting a thermocouple into a thick dummy material. The surface temperature of the steel sheet was measured as the temperature during heating-hot rolling-cooling before quenching / tempering. In addition, in the heating in a general heating furnace, the temperature at the center of the thickness of the steel plate is almost the same as the surface temperature.

The metallographic structure of the obtained steel sheet was evaluated as follows, and a tensile test was carried out as follows to evaluate the tensile properties. In this example, in each of the examples, the characteristics were evaluated on the assumption that the obtained steel pipe was D / t = 10, that is, t / D = 0.1.

[Observation of metallographic structure]
The metallographic structure was observed as follows.
(1) A sample was taken from the above steel sheet so that a sheet thickness cross section including the front and back surfaces of the steel sheet, which was parallel to the rolling direction (axial direction of the steel pipe) and perpendicular to the surface of the steel sheet, could be observed.
(2) The observation surface was mirror-finished by polishing with wet emery polishing paper (# 150 to # 1000) or by a polishing method having the same function (polishing with a polishing agent such as diamond slurry). ..
(3) The polished sample was corroded with a 3% nital solution according to the purpose to reveal a hard phase and a soft phase.
(4) Each phase fraction was calculated from the exposed structure at the site of 1/4 of the plate thickness t.

<Measuring method of hardness of soft phase and hard phase>
The hardness Hv (3 gf) of each phase of the soft phase and the hard phase was measured using the above corroded sample and using a Micro Vickers hardness tester. The measured load was 0.03N. The hardness of the soft phase was measured by measuring the hardness of the portion where cementite was not present, and the hardness of the hard phase was measured by measuring the hardness of the portion where cementite was agglomerated. This measurement was performed at least 5 points for each phase at a position of 1/4 of the plate thickness.

[Tensile test (evaluation of tensile properties)]
A round bar tensile test piece (same as JIS No. 4 test piece) is collected from a position of 1/4 of the plate thickness in the rolling direction, that is, in the pipe axis direction (L direction), and a tensile test is performed in the same manner as JIS Z 2201, and yield is performed. The strength YS and the tensile strength TS were measured, and the yield ratio YR was determined. Then, those having a yield strength of 301 to 450 MPa, a tensile strength of 523 to 616 MPa, and a yield ratio of 73% or less were evaluated as exhibiting high strength and low yield ratio. In Tables 3-1 and 3-2 below, the yield strength, tensile strength, and yield ratio are shown as YS, TS, and YR, respectively. These results are shown in Table 3-1 and Table 3-2. In addition, in Table 3-1 and Table 3-2, the UE (uniform elongation) obtained by the above tensile test is also shown.

Further, Test No. using the steel grade E in Table 1 Using 28 steel sheets, cold bending was performed under the conditions of D / t = 10 and D / t = 15, and then SR treatment was performed at the temperatures shown in Table 4 to obtain steel pipes. Then, using the steel pipe, a tensile test was conducted under the condition of using a round bar test piece JIS No. 4 (JIS Z 2201) similar to that of a steel plate. The test piece sampling position is 1/4 of the plate thickness from the outer surface side of the steel pipe. The results are shown in Table 4.

Consider the results in Tables 1-4.

Test No. Since 1 to 61 satisfy the component composition specified in the present invention and the steel sheet is manufactured under the specified conditions, the obtained steel sheet has a desired structure, and high strength and low yield ratio can be realized. .. Also. Test No. Since the steel plates 1 to 61 have a small Ceq, it can be seen that excellent weldability can be exhibited when a circular steel pipe is manufactured using the steel plates.

On the other hand, the test No. In Nos. 62 to 80, since at least one of the component composition and the manufacturing method is out of the specified range, the desired metal structure cannot be obtained, and at least one of the strength, the yield ratio, and the weldability is inferior. ..

Of these, No. In 62 to 65, 70, 71, and 77 to 79, the hardness of the soft phase was increased and the YR was increased because Nb was added. In addition, No. The Ceq of 71 is high, and weldability cannot be ensured.

No. In 66 and 67, the DI is below the specified range, and No. Since the tempering temperature of 67 was high, the TS was low in both cases.

No. In 68, 69 and 72-76, the tempering temperature was higher than the upper limit set for DI, so that the TS was low in all of them.

No. In No. 80, since the quenching temperature was low and the amount of reverse transformation to the gamma region was small, the fraction of the hard phase was small and high strength could not be ensured.

As an example, the above No. As a result of manufacturing a steel pipe using 28 steel plates, the obtained steel pipe satisfied the target YS ≧ 385 MPa, 550 ≦ TS ≦ 670 MPa, and YR ≦ 85% as shown in Table 4. It can be seen that if the steel sheet of the present invention is used, a steel pipe having desired characteristics can be obtained. Further, since the obtained steel pipe uses a steel plate having a small Ceq, excellent weldability can be exhibited, for example, when manufacturing a building structure.

FIG. 4 shows a graph showing the relationship between the hardness of the soft phase and the yield ratio YR, which was created using the data obtained in this example. From FIG. 4, it can be seen that YR 73% or less can be realized by setting the hardness Hv (3 gf) of the soft phase within the range of 149 to 180. In addition, the comparative example (■) showing the low YR surrounded by the elliptical broken line in FIG. 4 is an example inferior to other characteristics such as low strength. In addition, No. Reference numeral 80 is an example of insufficient strength as described above.

The steel sheet for circular steel pipes of the present invention exhibits high strength, low yield ratio, and excellent weldability, and is therefore suitable for manufacturing circular steel pipes having a low yield ratio and a tensile strength of 550 MPa class or more. The circular steel pipe is particularly excellent in earthquake resistance and can be suitably used for building structures.

Claims (6)

Ingredient composition,
C: 0.125 to 0.170% by mass,
Si: 0.10 to 0.60% by mass,
Mn: 0.90 to 1.60% by mass,
P: More than 0% by mass, 0.015% by mass or less,
S: More than 0% by mass, 0.008% by mass or less,
Al: 0.010 to 0.080% by mass,
N: 0.0010 to 0.0065% by mass, and the balance is a steel sheet for circular steel pipes composed of iron and unavoidable impurities.
The plate thickness is 70 mm or more,
The carbon equivalent Ceq represented by the following formula (1) is 0.42% by mass or less,
The weld crack susceptibility composition Pcm represented by the following formula (2) satisfies 0.15 to 0.27% by mass, and the hardenable multiple DI represented by the following formula (3) satisfies 1.05 or more.
The metal structure at the 1/4 position of the plate thickness is composed of tempered bainite, which is a hard phase, or tempered bainite, which is a soft phase , and pseudopolygonal ferrite, which are one or more structures of tempered martensite, and the fraction of the hard phase. Is 15 to 27 area%, the balance is the soft phase, the hardness Hv (3 gf) of the soft phase is 149 to 180, and the hardness Hv (3 gf) of the hard phase is 260 to 330.
A steel sheet for circular steel pipes having a yield ratio of 73% or less, a yield strength YS satisfying the following formula (4), and a tensile strength TS satisfying the following formula (5).
Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (1)
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (2)
When the amount of Mn is 1.20% by mass or more,
DI = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200) × [B] +1)
If the amount of Mn is less than 1.20% by mass,
DI = 1.16 × ([C] / 10) ^0.5 × (0.7 × [Si] +1) × (3.33 × [Mn] +1) × (0.35 × [Cu] +1) × (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200 x [B] +1) … (3)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] in the above formulas (1) to (3) are The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B shown in mass% are shown respectively, and the elements not contained are set to zero.
YS = (385-840 x t / D) to (710-940 x t / D) x 0.73 (MPa) ... (4)
TS = (540-170 × t / D) to (710-940 × t / D) (MPa) ... (5)
However, D in the above formulas (4) and (5) indicates the outer diameter (mm) of the circular steel pipe, t indicates the plate thickness (mm) of the circular steel pipe, and t / D is 0.033 to 0.10. It is within the range . In addition,
Cu: More than 0% by mass, 0.35% by mass or less,
Ni: More than 0% by mass, 0.35% by mass or less,
Cr: More than 0% by mass, 0.35% by mass or less,
Mo: More than 0% by mass, 0.35% by mass or less,
V: More than 0% by mass, 0.050% by mass or less,
B: The first aspect of claim 1 comprising one or more elements selected from the group consisting of more than 0% by mass, 0.0030% by mass or less, and Ti: more than 0.005% by mass, 0.030% by mass or less. Steel plate for circular steel pipe. The steel sheet for circular steel pipe according to claim 1 or 2, further comprising Ca: more than 0% by mass and 0.0030% by mass or less. A circular steel pipe formed of the steel plate for circular steel pipe according to any one of claims 1 to 3. The method for manufacturing a steel sheet for a circular steel pipe according to any one of claims 1 to 3.
The steel pieces are heated to 950 to 1250 ° C., hot rolling is performed under the conditions that the cumulative rolling reduction in the temperature range of 860 to 1000 ° C. is 30% or more and the finish rolling temperature is 840 to 960 ° C., and then the average cooling is performed. Cool from a cooling start temperature of 800 ° C or higher to a cooling stop temperature of 500 ° C or lower at a speed of 2 to 30 ° C / s.
Next, the hot-rolled material is reheated at a quenching temperature of 735 to 850 ° C. and a quenching heating time of 5 to 60 minutes, and then quenched. Then, if the tempering temperature: DI is less than 1.4, the temperature is 400 ° C. or higher. A method for manufacturing a steel sheet for a circular steel pipe, which is tempered under the conditions of less than 600 ° C., 400 ° C. or higher when DI is 1.4 or higher, 650 ° C. or lower, and tempering time: 5 to 60 minutes. The method for manufacturing a circular steel pipe according to claim 4.
Using the steel sheet for circular steel pipe according to any one of claims 1 to 3,
Bending process to satisfy D / t = 10 to 30 (D is the outer diameter of the circular steel pipe (mm), t is the plate thickness of the circular steel pipe (mm)).
A method for manufacturing a circular steel pipe, which comprises a step of performing heat treatment at a temperature of 450 ° C. or higher and 650 ° C. or lower in this order.

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