JP4912013B2 - Manufacturing method of press-bend cold-formed circular steel pipe with excellent earthquake resistance - Google Patents

Description

  The present invention produces a cold-formed circular steel pipe having a steel pipe tensile strength of 490 MPa class or higher by applying the press bend cold-forming method without performing heat treatment after the steel sheet is produced into a steel pipe by cold forming. In particular, the present invention relates to a useful method for manufacturing a press-bend cold-formed circular steel pipe that is excellent in earthquake resistance and can be suitably used for a building structure.

  A circular steel pipe used for a pillar of a building structure is required to have a yield ratio YR (= yield stress YS / tensile strength TS) of 85% or less from the viewpoint of seismic safety. On the other hand, as a method of manufacturing a steel pipe by cold forming, in addition to a UOE forming method (Uing press-Oing press-expander method) applied to a steel pipe for line pipes, a press bend cold forming method (hereinafter, simply “ The “press bend method” is sometimes used.

  Among the above forming methods, the UOE forming method enables high-efficiency and high-precision processing, but due to the limit of equipment capacity, the steel sheet thickness t is less than 40 mm, and t / D (D: outer diameter of steel pipe). Limited to less than 0.05. On the other hand, the press bend method is a method in which a part of a steel sheet is stamped and bent (straight line portion), and the stamping position is sequentially moved to form a circular shape, which is a method with high processing capability. Therefore, it is used for forming a steel pipe that is used for a pillar of a building structure and requires a strong work such as a thickness of 0.05 to 0.10 with a steel plate thickness t of 40 mm or more. The press bend method is applied.

  In such a press bend method, when bending is performed such that t / D is 0.05 or more, the yield ratio YR increases greatly and often exceeds 85%. The steel pipe (after pipe making) had to be subjected to annealing (Stress Relieving: SR treatment) for the purpose of removing residual stress, resulting in higher costs, longer construction period and lower productivity.

  Further, in a method in which heat treatment is not performed after cold forming, in a steel pipe having a small workability (t / D) (for example, less than 0.05), even if the yield ratio YR can be secured to 85% or less, the workability (t / D When D) increases (for example, 0.05 or more), it is a fact that a steel pipe having a yield ratio YR of 85% or less cannot be manufactured.

Various technologies have been proposed so far as technologies relating to cold-formed steel pipes and steel plates applied to such steel pipes. For example, in Patent Document 1, as a technique for producing a steel plate used for a 590 MPa class low yield ratio steel pipe for construction, a method of directly quenching from Ar ₃ temperature or less after hot rolling and thereafter not tempering, or Ac ₁ A steel sheet having a tensile strength of 680 MPa or more and a yield ratio YR of 80% or less is manufactured by reheating to a temperature range of to Ac ₃ and quenching and thereafter not tempering. It is disclosed that the tensile strength TS of a steel pipe is adjusted to an appropriate range by heat treatment at a temperature of Ac ₁ or less after forming.

However, this technique is based on the premise that the steel pipe is heat-treated at a temperature of Ac ₁ or less after cold forming. The yield ratio YR at the steel plate stage is low but the toughness is low, and the tensile strength TS is high. Therefore, the heat treatment after pipe making becomes indispensable, and the problems of high cost, long construction period, and low productivity remain unresolved.

In Patent Document 2, as a method of manufacturing a low yield ratio steel pipe for construction, a steel sheet that has been air-cooled or water-cooled after hot rolling is cold-worked in a range of workability (t / D) of 0.10 or less (10% or less). A method has been proposed in which a pipe is formed by molding, and the steel pipe is reheated to a temperature of 700 to 850 ° C. to normalize it. In Patent Document 3, a steel sheet is heated to a temperature of Ac ₃ or higher, rapidly cooled to room temperature, then heated to a two-phase temperature range of Ac _{1 to} Ac ₃ , and then air-cooled steel sheet is formed, and further, 500 to 600 is manufactured. A method of reheating to a temperature range of ° C. is disclosed. These techniques are also techniques based on the premise that heat treatment is performed after cold forming (after pipe making), as in Patent Document 1, and there are problems of high cost, long construction period, and low productivity. is there.

On the other hand, as a method of not performing heat treatment after forming the steel pipe, for example, a technique such as Patent Document 4 is also proposed. In this technique, after hot rolling, it is reheated to Ac _{3 to} 1000 ° C. and quenched, and then 700 By reheating to a temperature of ˜850 ° C. and quenching, and further tempering at Ac ₁ point or less, the yield ratio YR of the steel sheet is YR (%) ≦ 80−0.8 × [(t / D) × 100], and using this steel plate, a steel pipe is manufactured by cold forming in the range of t / D ≦ 0.10, whereby the plate thickness is 100 mm or less and the YR in the pipe axis direction is 80%. An architectural low yield ratio 600 MPa class steel pipe is obtained as follows.

In Patent Document 5, after hot rolling, quenching is performed from a temperature of 750 ° C. or higher to room temperature, followed by reheating to a temperature of 700 to 850 ° C. and further tempering at Ac ₁ point or less. , The yield ratio YR of the steel sheet is controlled to YR (%) ≦ 80−0.8 × [(t / D) × 100], and a method of manufacturing a steel pipe by cold forming the steel sheet is disclosed.

Furthermore, Patent Document 6, air cooled after performing rolling finishing temperature as _{Ar 3 + 120 ℃ ~Ar 3 +} 20 ℃, quenching from Ar ₃ -20 ℃ ~Ar ₃ -100 ℃ to 200 ° C. or less, further Ar _By tempering at a temperature of ₃ or less, the yield ratio YR of the steel sheet is controlled to YR (%) ≦ 80−0.8 × [(t / D) × 100], and the steel sheet is cold-formed to form a steel pipe A method is disclosed.

  The techniques of the above Patent Documents 4 to 6 relate to a 590 MPa class steel pipe, but are only intended to reduce the yield ratio YR, and the steel pipe standard yield stress YS and tensile strength TS are applied to the steel sheet as they are. Is. Therefore, it cannot be said that appropriate yield stress YS and tensile strength TS are achieved in the steel sheet. For these reasons, the yield stress YS and the tensile strength TS greatly deviate from the optimum values for the steel pipe after forming into the steel pipe and greatly exceed. If the strength of the steel pipe becomes too high, the earthquake resistance (yield ratio YR, toughness) of the steel pipe column deteriorates and the yield stress YS and tensile strength TS at the steel plate stage are not controlled. Fluctuations in the yield stress YS and the tensile strength TS after forming become large, the uniform plastic deformability of the building structure is lowered, and a problem arises in the earthquake resistance as the structure. In order to avoid such problems, after all, heat treatment after forming the steel pipe is required.

  In the case of a steel sheet in which only the yield ratio YR is reduced to 80-0.8 [(t / D) × 100] (%) (for example, YR is about 72% when t / D is 0.1%). Even if a steel pipe with a small workability (t / D) can be made, when the bending work with a workability (t / D) of 0.1 is performed, the yield ratio YR rises so much that the yield ratio In many cases, the upper limit (85%) of YR is far exceeded, and as a result, heat treatment of the steel pipe is required. Therefore, it is impossible to reliably obtain a steel pipe that does not require heat treatment while still being cold formed only by specifying the yield ratio of the steel sheet to this level.

In addition to the above technique, as a technique including steel pipe forming conditions, for example, in Patent Document 7, after hot rolling at a temperature of Ar ₃ or higher, accelerated cooling to Ar ₃ −50 ° C. ± 30 ° C. After holding for 150 seconds, accelerated cooling to 400 to 600 ° C. at a cooling rate of 1 to 40 ° C./second is performed, and the obtained steel plate is piped and further expanded to make a steel pipe that does not require heat treatment after pipe making. A manufacturing method has been proposed. However, this technology assumes the production of steel pipes for line pipes, and the thickness of the steel sheet is limited to 50 mm, and a pipe expansion process with a pipe expansion ratio of 0.8% or more is necessary after pipe formation. Is.

Further, Patent Document 8 proposes a method for producing a 490 MPa class steel pipe that does not require heat treatment after pipe making by rolling a steel sheet after completion of rolling at a temperature of 780 ° C. or higher and then air-cooled steel sheet by the UOE method. Yes. However, this technology also assumes the production of steel pipes for line pipes, and the UOE method has a steel plate thickness of up to about 50 mm and cannot be applied to steel plates with a steel plate thickness of 50 to 100 mm. .
JP, 2005-163159, A Claims etc. Japanese Patent Application Laid-Open No. 6-128641 JP, 2004-300461, A Claims etc. Japanese Patent Laid-Open No. 7-109521 JP, 6-264144, A Claims etc. JP, 6-264143, A Claims etc. Japanese Patent Application Laid-Open No. 10-31081 JP, 5-156357, A Claim etc.

  The present invention has been made under such circumstances. The purpose of the present invention is to perform SR treatment after forming a thick steel plate having a thickness of 50 mm or more into a steel pipe by the press bend method. Another object of the present invention is to provide a useful method for producing a cold formed round steel pipe having a low yield ratio of 490 MPa class or more capable of exhibiting predetermined mechanical properties.

The production method of the present invention that can achieve the above-mentioned object includes C: 0.02 to 0.20% (meaning of mass%, the same shall apply hereinafter), Si: 0.05 to 0.5%, Mn: 0.00. 50 to 2.0%, Al: 0.01 to 0.1% and N: 0.002 to 0.007%, respectively, P: 0.02% or less (excluding 0%) and S : 0.008% or less (excluding 0%), respectively, and using a steel slab whose balance is Fe and inevitable impurities, heat treatment of any of (1) to (3) below is performed By manufacturing thick steel plate
Round steel pipe thickness is t (mm), outer diameter is D (mm), steel pipe product standard yield strength is YS ₀ (MPa) or more, tensile strength is TS ₀ (MPa) or more, yield ratio is 85% In the following, the yield strength in the steel sheet is (YS ₀ -980 t / D) to (YS ₀ -980 t / D + 120) (MPa), and the tensile strength of the steel sheet is (TS ₀ -560 t / D) to (TS _{0 -560t / D + 100) (} MPa), yield ratio of the steel sheet (75-82t / D) (%) or less, so that the hardness at a depth of 1mm from the steel plate front and back surfaces (Vickers hardness) is HV140~200 To control each
The steel sheet has a gist in that a circular steel pipe is formed by press vent cold forming and no subsequent heat treatment is performed.

  (1) After heating and rolling a steel slab to 1000 to 1250 ° C, air cooling from a temperature of 800 ° C or higher, reheating to a temperature of 850 ° C or higher, and 200 ° C or lower at a cooling rate of 1 to 50 ° C / sec. After cooling to 700 to 850 ° C. in a two-phase region temperature, cooling to 200 ° C. or less at a cooling rate of 1 to 50 ° C./second, further heating to a temperature of 500 to 700 ° C. and then air cooling. .

  (2) After heating and rolling the steel slab to 1000 to 1250 ° C., the steel slab is cooled from a temperature of 800 ° C. or more to 200 ° C. or less at a cooling rate of 1 to 50 ° C./second, and subsequently in a two-phase region of 700 to 850 ° C. After reheating to temperature, it is cooled to 200 ° C. or lower at a cooling rate of 1 to 50 ° C./second, further heated to a temperature of 500 to 700 ° C. and then air-cooled.

  (3) After heating and rolling the steel slab to 1000 to 1250 ° C, the steel slab is cooled from a two-phase region temperature of 650 to 800 ° C to 200 ° C or less at a cooling rate of 1 to 50 ° C / second, and further 500 to 700 ° C. After heating to the temperature of, cool with air.

  The steel slab to be used in the present invention further includes (a) Cu: 1% or less (not including 0%), Ni: 1.5% or less (not including 0%), Cr: 1%, if necessary. 1 or more selected from the group consisting of the following (not including 0%) and Mo: 1% or less (not including 0%), (b) Nb: 0.05% or less (not including 0%), ( c) V: 0.1% or less (excluding 0%), (d) B: 0.003% or less (not including 0%), (e) Ca: 0.005% or less (including 0%) And / or rare earth elements: 0.05% or less (not including 0%), (f) Ti: 0.025% or less (not including 0%), etc. are also effective. The properties of the steel pipe can be further improved depending on the components contained.

  According to the present invention, by appropriately adjusting the chemical composition of the steel sheet and using a thick steel sheet that has been subjected to appropriate heat treatment, a low yield ratio can be obtained without forming the steel pipe by the press bend method and without performing SR treatment. Thus, a cold-formed circular steel pipe of 490 MPa or more can be obtained, and this steel pipe can be suitably used for a virtue structure that requires earthquake resistance.

  Circular steel pipe columns for building structures are produced by press bending using a thick steel plate as a raw material and applied by the press bending method. 0.5 or more), the increase in the yield ratio YR after pipe making becomes large. Therefore, in order to reduce the yield ratio YR, SR processing is generally performed after cold forming (tube making). . However, performing heat treatment after pipe production has problems in terms of cost and productivity. For these reasons, the present inventors examined a method that can omit the SR treatment after pipe making from various angles. In particular, the above heat treatment also has the effect of stabilizing the steel pipe quality after cold forming, so the knowledge that it is important to stabilize the quality after pipe making in order to omit the heat treatment after pipe making Based on the study.

  According to the study by the present inventors, the amount of change in the mechanical properties (yield stress YS, tensile strength TS, and yield ratio YR) after cold forming is grasped by simply measuring the amount of change in tensile strain. (Predicted) turned out to be impossible. For example, the amount of change in yield stress YS and tensile strength TS after tensile pre-strain is described in the WES standard (WES2808-2003) of the Japan Welding Association, but the amount of change after pipe making is described in the WES standard. There are circumstances that cannot be predicted. Such circumstances include the following three points (a) to (c).

  (A) In the WES standard, the direction of tensile pre-strain and the direction of tensile test after pre-strain are the same direction, but the direction of bending strain in pipe making and the direction of tensile test after pipe making are perpendicular directions.

  (B) Since the tensile pre-strain in the WES standard is uniformly applied to the entire plate thickness, the tensile test after pre-strain is a test with a uniform pre-strained material, but the strain in bending is the highest. Since the strain on the surface is the largest and the strain distribution is inclined in the plate thickness direction, the tensile property of the material having the pre-strain distribution in the cross-sectional direction of the test piece is evaluated in the tensile test after bending.

  (C) In the WES standard, ideal tensile properties after uniform tensile pre-strain, but in actual bending, the strain greatly differs between the part where the press bending is embossed and the part where the embossing is not performed. Uniform pre-strain in the direction.

  Thus, the amount of change in mechanical properties (yield stress YS, tensile strength TS, and yield ratio YR) after press bending differs in the Hazinger effect due to the different pre-strain directions and the strain distribution in the plate thickness direction. This is different from the amount of change in mechanical properties (yield stress YS, tensile strength TS and yield ratio YR) after ideal tensile pre-strain, such as the influence of mold bending strain. It cannot be predicted accurately.

  According to the WES standard, when the pre-strain amount is ε, the yield stress YS variation ΔYS and the tensile strength TS variation ΔTS are given by ΔYS = 4400ε and ΔTS = 800ε, respectively. However, tensile strain and bending strain are fundamentally different concepts and cannot be made to correspond to each other. That is, there is no tensile strain amount, and the amount of change in the mechanical properties of the steel pipe after forming cannot be accurately evaluated without considering the requirement of the degree of bending (t / D).

  Therefore, the present inventors conducted various experiments on the influence of the difference in the pre-strain direction and the amount of change in the tensile performance after actual bending. In the experimental result in which the direction of the tensile pre-strain is a right-angle direction, the yield stress YS change amount (ΔYS) is less than half despite the same pre-strain. The reason why such a situation occurs is apparently due to a new phenomenon such as the Hausinger effect, and it has been found that this cannot be predicted with a predistortion type such as the WES standard.

  In addition, when an unknown element called bending is added, it has been found that the work hardening by bending is larger than the work hardening by pulling. That is, the result is that the change amount (ΔTS) of the tensile strength TS is about 1.5 times larger than the change amount predicted by the WES standard. Such a result was considered to be caused by an increase in the amount of plastic deformation in press bending.

  If heat treatment is not performed after bending, the tensile properties of steel pipes cannot be secured stably unless the amount of change in tensile properties after bending is accurately predicted. It is difficult to produce steel pipes by predicting mechanical characteristics.

  The amount of change in mechanical properties (the ΔYS, ΔTS, ΔYR) and the degree of work (t) when the present inventors are press-bending-molded various steel plates (those satisfying the chemical components defined in the present invention). / D) was investigated (see Examples below for tensile test conditions). The results are shown in FIGS. 1 to 3 (t / D on the x-axis and ΔYS, ΔTS or ΔYR on the y-axis). That is, FIG. 1 shows the relationship between the working degree (t / D) and the yield stress change amount ΔYS after forming, FIG. 2 shows the relationship between the working degree (t / D) and the tensile strength change amount ΔTS after forming, 3 is a graph showing the relationship between the working degree (t / D) and the yield ratio change amount ΔYR after forming.

Assuming that the thickness of the steel sheet is t and the outer diameter of the steel pipe is D, the amount of change (ΔYS, ΔTS, ΔYR) corresponding to the degree of bending work (t / D) is calculated by the least square method. When (t / D) is in the range of 0.05 to 0.1, it has been found that the following relationships (1) to (3) are established (see FIGS. 1 to 3).
ΔYS = 980 (t / D) +40 (MPa) (1)
ΔTS = 560 (t / D) (MPa) (2)
ΔYR = 82 (t / D) +8 (%) (3)

From these relational expressions, it is possible to predict the mechanical properties after forming the steel pipe according to the bending degree of the steel pipe, so by setting the target quality in the steel plate and controlling the quality of the steel plate It becomes possible to accurately manage the quality of the steel pipe. That is, when the lower limit of the yield stress YS of the steel pipe product standard is YS ₀ (MPa), the lower limit of the tensile strength TS is TS ₀ (MPa), and the upper limit of the yield ratio YR is 85%, the target yield ratio YR is If it is 83%, the target yield stress YS in the steel pipe is YS ₀ (MPa) +100 (MPa) from the balance between the yield stress YS and the tensile strength TS in the steel plate standard of 490 to 550 MPa class. The target tensile strength TS can be set as TS ₀ +50 (MPa).

Furthermore, if the target YS range of the steel plate is 120 MPa and the target TS range is 100 MPa in consideration of the variation in the mechanical properties of the steel plate itself and the mechanical properties after forming into the steel pipe, the target for the steel plate to be used The mechanical characteristics managed as follows must be in the range of the following formulas (4) to (6).
YS: (YS ₀ -980 t / D) to (YS ₀ -980 t / D + 120) (MPa) (4)
TS: (TS ₀ −560 t / D) to (TS ₀ −560 t / D + 100) (MPa) (5)
YR: (75-82t / D) [ie (83-82 (t / D) -8)]% or less (6)

  Regardless of the steel pipe standard, clarifying and managing the target performance at the steel sheet stage stabilizes the mechanical properties of the steel pipe after cold forming, and the heat treatment can be omitted after pipe making. That is, in order to omit the heat treatment after pipe making and stabilize the quality after pipe making, the amount of change in mechanical properties after pipe making is predicted with high accuracy, and based on this prediction, at the steel plate stage. Creating the right quality is extremely important.

  In addition, since the crack generate | occur | produces at the time of press bending if the hardness of the steel plate front and back before pipe making is too high, it is necessary to make the cross-sectional hardness at 1 mm from the front and back of the steel plate 200 or less in terms of Vickers hardness HV. is there. However, if the front and back surface hardness is too low, the tensile strength TS of the steel pipe after press bending cannot be ensured to be 490 MPa or higher, so the front and back surface hardness at the steel plate stage needs to be 140 HV or higher.

  Next, the reason for limiting the chemical component composition in the steel slab for obtaining the above steel plate in the present invention will be described. As described above, the steel slab to be used in the present invention is C: 0.02 to 0.20%, Si: 0.05 to 0.5%, Mn: 0.50 to 2.0%, Al: 0 0.01 to 0.1% and N: 0.002 to 0.007%, respectively, P: 0.02% or less (excluding 0%) and S: 0.008% or less (0% The reasons for limiting the ranges of these elements are as follows.

[C: 0.02 to 0.20%]
C is an element effective for increasing the strength, but if it is contained excessively, a hardened structure such as a martensite structure is generated, so that the vicinity of the surface after quenching becomes hard and bending workability deteriorates. Since it causes deterioration of weldability and toughness, the upper limit of the C content is set to 0.20%. However, when the C content is less than 0.02%, insufficient strength (less than 490 MPa in tensile strength) occurs. In addition, the minimum with preferable C content is 0.05%, and a preferable upper limit is 0.16%.

[Si: 0.05 to 0.5%]
Si needs to be contained in an amount of 0.05% or more for deoxidation, but if it exceeds 0.5% and it is contained excessively, the weldability is lowered. For these reasons, the Si content needs to be 0.05 to 0.5%. In addition, the minimum with preferable Si content is 0.1%, and a preferable upper limit is 0.4%.

[Mn: 0.50 to 2.0%]
Mn is effective as an element that increases both strength and toughness. In order to exhibit such an effect, it is necessary to contain 0.50% or more of Mn. However, if Mn is contained excessively, weldability deteriorates, so the upper limit is made 2.0%. In addition, the minimum with preferable Mn content is 0.8%, and a preferable upper limit is 1.6%.

[Al: 0.01 to 0.1%]
Al needs to be contained at least 0.01% for deoxidation, but if it is contained excessively, nonmetallic inclusions increase and toughness decreases, so it is necessary to make it 0.1% or less. . In addition, the minimum with preferable Al content is 0.02%, and a preferable upper limit is 0.05%.

[N: 0.002 to 0.007%]
Since N is inevitably mixed during steelmaking and it is difficult to completely remove N, the lower limit was made 0.002%. Further, if the N content is excessive, the toughness deteriorates due to strain aging after cold bending, so it is necessary to make it 0.007% or less. In addition, the upper limit with preferable N content is 0.006%.

[P: 0.02% or less (excluding 0%)]
P is an impurity inevitably mixed in, but if its content is excessive, the toughness of the steel sheet is deteriorated, so it is necessary to suppress it to 0.02% or less. The P content is preferably suppressed to 0.015% or less.

[S: 0.008% or less (excluding 0%)]
S is also an impurity that is inevitably mixed in, but if its content is excessive, the performance in the thickness direction of the steel sheet is deteriorated, and inclusions of MnS are generated in the center of the thickness of the sheet, and from the interface during bending processing. Therefore, it is necessary to suppress it to 0.008% or less. The S content is preferably suppressed to 0.006% or less.

  In the steel slab which is the subject of the present invention, in addition to the above components, it is composed of Fe and unavoidable impurities, but may contain trace components (allowable components) that are inevitably mixed for melting (for example, , Co, Mg, Zr, etc.), such steel slabs are also included in the scope of the present invention. Further, the steel slab to be used in the present invention may further include (a) Cu: 1% or less (not including 0%), Ni: 1.5% or less (not including 0%), Cr if necessary. 1% or less (not including 0%) and Mo: 1% or more selected from the group consisting of 1% or less (not including 0%), (b) Nb: 0.05% or less (not including 0%) ), (C) V: 0.1% or less (not including 0%), (d) B: 0.003% or less (not including 0%), (e) Ca: 0.005% or less (0 %) And / or rare earth elements: 0.05% or less (not including 0%), (f) Ti: 0.025% or less (not including 0%), etc. are also effective. However, the reasons for limiting the range when these components are contained are as follows.

[Cu: 1% or less (not including 0%), Ni: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), and Mo: 1% or less (0% 1 or more selected from the group consisting of
These elements are effective elements for improving the strength of the steel sheet. However, if the content is excessive, the weldability is deteriorated. Therefore, Cu, Cr and Mo need to be 1% or less, and Ni needs to be 1.5% or less.

[Nb: 0.05% or less (excluding 0%)]
Nb is dissolved and rolled during heating, thereby delaying the recrystallization of austenite, introducing strain into the austenite, and improving the strength and toughness of the steel sheet after cooling to room temperature. Such an effect increases as the content thereof increases, but if it is excessively contained, the HAZ (heat affected zone) toughness of the welded portion deteriorates. For these reasons, when Nb is contained, the content is preferably up to about 0.05%. A more preferable upper limit of the Nb content is about 0.025%.

[V: 0.1% or less (excluding 0%)]
V is an element effective for improving the strength and toughness of the steel sheet, but if its content is excessive, the HAZ toughness of the welded portion is deteriorated. For these reasons, when V is contained, the content is preferably up to about 0.1%.

[B: 0.003% or less (excluding 0%)]
B is an element effective for significantly improving the strength of the steel sheet with a small content. However, since the weldability deteriorates when the content is excessive, it is preferable to be up to about 0.003%.

[Ca: 0.005% or less (not including 0%) and / or rare earth element: 0.05% or less (not including 0%)]
Ca and rare earth elements (hereinafter abbreviated as “REM”) are effective elements for controlling the morphology of MnS inclusions and improving the properties in the thickness direction of the steel sheet. Such an effect increases as its content increases, but if it is excessively contained, coarse inclusions are generated and cause cracks. For these reasons, when Ca or REM is contained, it is preferable that Ca is 0.005% or less and REM is 0.05% or less. Note that REM can use any of scandium (Sc), yttrium (Y) and lanthanoid series rare earth elements [for example, cerium (Ce), lanthanum (La), etc.] belonging to Group 3 of the periodic table.

[Ti: 0.025% or less (excluding 0%)]
Ti has the effect of improving the HAZ toughness of the welded joint. Such an effect increases as the content increases, but even if Ti is excessively contained, the effect is saturated, so the upper limit is made 0.025%. In addition, the minimum with preferable Ti content is 0.005%, and a more preferable upper limit is 0.020%.

  In order to produce a cold-formed round steel pipe, it is sufficient to cold-form the round steel pipe by a press vent method using a steel sheet that satisfies the above requirements. It is necessary to perform any one of the following heat treatments (1) to (3) using a satisfactory steel slab.

  (1) After heating and rolling a steel slab to 1000 to 1250 ° C, air cooling from a temperature of 800 ° C or higher, reheating to a temperature of 850 ° C or higher, and 200 ° C or lower at a cooling rate of 1 to 50 ° C / sec. After cooling to 700 to 850 ° C. in a two-phase region temperature, cooling to 200 ° C. or less at a cooling rate of 1 to 50 ° C./second, further heating to a temperature of 500 to 700 ° C. and then air cooling. .

  (2) After heating and rolling the steel slab to 1000 to 1250 ° C., the steel slab is cooled from a temperature of 800 ° C. or more to 200 ° C. or less at a cooling rate of 1 to 50 ° C./second, and subsequently in a two-phase region of 700 to 850 ° C. After reheating to temperature, it is cooled to 200 ° C. or lower at a cooling rate of 1 to 50 ° C./second, further heated to a temperature of 500 to 700 ° C. and then air-cooled.

  (3) After heating and rolling the steel slab to 1000 to 1250 ° C, the steel slab is cooled from a two-phase region temperature of 650 to 800 ° C to 200 ° C or less at a cooling rate of 1 to 50 ° C / second, and further 500 to 700 ° C. After heating to the temperature of, cool with air.

In the heat treatment (1), after normal slab heating, rolling is completed at a temperature of 800 ° C. or higher (Ar ₃ or higher), and this is air-cooled steel plate. Is what makes it. First, the steel sheet after rolling is reheated to a temperature of 850 ° C. or higher (Ar ₃ or higher), completely austenitized, and then cooled to 200 ° C. or lower at a cooling rate of 1 to 50 ° C./second to become a base. An organization is formed. At this time, the slower the cooling rate, the more soft structure is formed, and the yield stress YS, the tensile strength TS, and the yield ratio YR are all lower. Subsequently, by reheating to a two-phase temperature of 700 to 850 ° C., a part of the structure is austenitized together with the softening of the base structure. And in order to harden the austenitic structure | tissue by subsequent cooling, it cools and quenches to 200 degrees C or less with the cooling rate of 1-50 degrees C / sec. Heat treatment at this stage is the most important step in producing a low yield ratio YR steel, and a hard phase and a soft phase can be formed by this treatment. If the two-phase region temperature (700 to 850 ° C.) is deviated during the reheating until the cooling, the target hard phase and soft phase are not formed. Further, by heating to a temperature of 500 to 700 ° C. (Ac ₁ ) and then air cooling (tempering heat treatment), the residual stress in the steel sheet structure is relaxed and the hardened structure is recovered, and the target steel sheet characteristics (yield stress YS, tensile stress) are recovered. Strength TS and yield ratio YR) can be ensured.

In the heat treatment of (2) above, after normal slab heating, the rolling was finished at a temperature of 800 ° C. or higher (Ar ₃ or higher), and cooled from 800 ° C. or higher to 200 ° C. or lower by water cooling (direct quenching DQ or accelerated cooling). A steel plate is used, and a steel plate with a low yield ratio YR is made by online two-time heat treatment. First, the base steel is formed by cooling the rolled steel sheet from the austenite state (Ar ₃ or higher) to 200 ° C. or lower. At this time, the slower the cooling rate, the more soft structure is formed, and the yield stress YS, the tensile strength TS, and the yield ratio YR are all lower. Subsequently, by heating to a two-phase region temperature of 700 to 850 ° C., a part of the structure is austenitized together with the softening of the base structure. And in order to harden the austenitic structure | tissue by subsequent cooling, it cools and quenches to 200 degrees C or less with the cooling rate of 1-50 degrees C / sec. Similar to the heat treatment of (1) above, the heat treatment at this stage is the most important step in producing the low yield ratio YR steel, and a hard phase and a soft phase can be formed by this treatment. If the two-phase region temperature (700 to 850 ° C.) is deviated during the reheating until the cooling, the target hard phase and soft phase are not formed. Further, by heating to a temperature of 500 to 700 ° C. (Ac ₁ ) and then air cooling (tempering heat treatment), the residual stress in the steel sheet structure is relaxed and the hardened structure is recovered, and the target steel sheet characteristics (yield stress YS, tensile stress) are recovered. Strength TS and yield ratio YR) can be ensured.

The heat treatment of the above (3) is performed after performing normal slab heating and rolling, and using a steel plate cooled from a two-phase region temperature of 650 to 800 ° C. to a temperature of 200 ° C. or less by water cooling (direct quenching DQ or accelerated cooling), A steel plate with a low yield ratio YR is made by online tempering heat treatment. First, rolling may be completed in an austenite temperature range of 800 ° C. or higher, or partially in a two-phase temperature range (Ar ₃ or lower). It is important that the subsequent cooling start temperature is a two-phase region temperature of 650 to 800 ° C, and the cooling to 650 to 800 ° C after completion of rolling may be either air cooling or water cooling. In this state of the two-phase region temperature, a part has already transformed into a soft phase of ferrite, and the rest has a two-phase structure of an austenite state before transformation. Cooling from 650 to 800 ° C. requires water cooling (direct quenching DQ or accelerated cooling), and this cooling makes the austenite structure part a hardened structure, so that it is 1 to 50 ° C./second up to a temperature of 200 ° C. or less. Cool with water at a cooling rate of. Heat treatment at this stage is the most important step in producing a low yield ratio YR steel, and a hard phase and a soft phase can be formed by this treatment. Furthermore, by heating to a temperature of 500 to 700 ° C. (Ac ₁ ) and then air cooling (tempering heat treatment), the residual stress in the steel sheet structure is relaxed and the hardened structure is recovered, and the target steel sheet characteristics (yield stress YS, Tensile strength TS and yield ratio YR) can be ensured.

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.

  Using various steel slabs (steel types) having the chemical composition shown in Table 1 below, rolling and heat treatment were performed so as to obtain predetermined steel sheet characteristics, and various steel sheets were manufactured. The manufacturing conditions (rolling and heat treatment conditions) at this time were in accordance with one of the following methods.

[Production conditions]
(A) QQ′T: After heating the steel slab to 1100 ° C., the steel sheet that has been hot-rolled at 900 ° C. is air-cooled, then reheated to 900 ° C. and quenched (Q), and then reheated to 780 ° C. Heating and quenching (Q ′) were performed, and tempering (T) was performed at 550 ° C. (heat treatment according to the present invention).

  (B) DQQ'T: After the steel slab was heated to 1100 ° C., the steel plate that had been hot-rolled at 900 ° C. was cooled to room temperature by direct quenching (DQ) and then reheated to 780 ° C. and quenched (Q ') And tempering (T) was performed at 550 ° C (heat treatment according to the present invention).

  (C) CR-DQ′T: After heating the steel slab to 1100 ° C., controlled rolling (CR) is performed so that the final finish rolling temperature is 800 ° C. or higher up to a predetermined plate thickness, and then air-cooled to 700 After the temperature reached 0 ° C., it was water-cooled (DQ ′) to room temperature and tempered (T) at 550 ° C. (heat treatment according to the present invention).

  (D) DQT: After the steel slab was heated to 1100 ° C., the steel plate that had been hot-rolled at 900 ° C. was cooled to room temperature by direct quenching (DQ) and then tempered (T) at 600 ° C. (Comparison Heat treatment by example).

  (E) DQT: After the steel slab was heated to 1100 ° C., the steel plate that had been hot-rolled at 900 ° C. was cooled to room temperature by direct quenching (DQ) and then tempered (T) at 600 ° C. (Comparison Heat treatment by example).

  (F) TMCP (thermal processing control): After heating the steel slab to 1100 ° C., the steel sheet that has been hot-rolled so that the final rolling temperature is 850 ° C. is then cooled to 500 ° C. by accelerated cooling, and then Air-cooled to room temperature.

  About each obtained steel plate, the workability (t / D) was changed and the steel pipe was produced by press bend cold forming. At this time, the presence or absence of cracking during press bending of the steel pipe was also investigated. In either case, no heat treatment was performed after forming the steel pipe.

  While measuring the mechanical properties (yield stress YS, tensile strength TS and yield ratio YR) of the steel plate, mechanical properties (yield stress YS, tensile strength TS, yield ratio YR) in the tube axis direction (L direction) of the steel pipe And toughness). In either case, heat treatment is not performed after forming into a steel pipe. The evaluation method of mechanical properties (steel plate and steel pipe) and the toughness evaluation method of steel pipe are as follows.

[Mechanical property evaluation method]
JIS Z 2201 No. 4 from t / 4 part (t is the plate thickness) of the steel plate to the L direction (rolling direction) and parallel to the tube axis of the outer t / 4 part of the steel pipe (corresponding to the main rolling direction of the steel plate) A specimen was taken and subjected to a tensile test in accordance with JIS Z 2241. The mechanical properties of the steel sheet (yield stress YS, tensile strength TS, yield ratio [yield stress point / tensile strength × 100%: YR]), steel pipe The mechanical properties (yield point YP, tensile strength TS, yield ratio [yield stress YS / tensile strength × 100%: YR]) were measured. Moreover, about the steel plate, the Vickers hardness in the position of 1 mm below cross-section direction front and back was measured with the load of 10 N / mm < ² >.

[Toughness evaluation method]
A specimen of JIS Z 2202 No. 4 was taken from the outer t / 4 part of the steel pipe in the direction parallel to the pipe axis (main rolling direction of the steel sheet), and Charpy impact test was conducted in accordance with JIS Z 2242. (VTrs) was measured.

  The mechanical properties (actually measured values) of the steel sheet were measured as follows: plate thickness, manufacturing conditions, appropriate range of steel sheet [range of the above formulas (4) to (6): calculated value), and Vickers hardness (HV) of 1 mm below the front and back surfaces of the steel sheet. And in Table 2 below. In addition, the mechanical properties (measured values) of the steel pipe, along with the mechanical properties (standard value) of the steel pipe, the degree of work (t / D), the impact properties (toughness value) of the steel pipe, and the presence or absence of cracks during bending of the steel sheet, Shown in Table 3 below.

  From these results, it can be considered as follows. First, regarding the chemical components of the steel slab, the steel types A to C and G to Q satisfy the chemical composition range defined in the present invention, and the steel types D, E, F, R, and S are defined in the present invention. It is out of the chemical composition range.

  Of these, steel type D has an excessive C content, and with this steel type, even if the manufacturing conditions are appropriate, the steel sheet is outside the proper performance range, and the yield ratio in the steel pipe YR exceeds 85% (Experiment No. 11). In addition, the toughness is poor and cracking occurs when bending into a steel pipe.

  Steel type E has a reduced Mn content, and with this steel type, the tensile strength TS of the steel sheet is lowered even if the production conditions are appropriate, and the proper performance range is deviated. The tensile strength TS after forming the steel pipe is insufficient (Experiment No. 12).

  Steel type F has an excessive S content, and in the case of using this steel type, the toughness after forming the steel pipe is low and cracking occurs (Experiment No. 13). Steel type R has an excessive N content, and in the case of using this steel type, the toughness after forming the steel pipe is low and cracking occurs (Experiment No. 25). Steel type S has an excessive Mn content. With this steel type, the yield stress YS and tensile strength TS of the steel sheet increase, and the yield ratio YS after forming the steel pipe exceeds 85%. The toughness is low and cracking occurs (Experiment No. 26). Moreover, when the hardness of 1 mm below the steel plate surface is high, it turns out that it is in the condition where it is easy to generate | occur | produce a crack when bent to a steel pipe (experiment No.11,25,26).

  Experiment No. Nos. 5, 6, and 9 are those in which the chemical composition of the steel slab satisfies the range specified by the present invention, but the manufacturing conditions are outside the range specified by the present invention, and the mechanical properties of the steel sheet Is out of the proper performance range, and the mechanical properties of the steel pipe are out of the standard value. Moreover, when the hardness of 1 mm below the steel plate surface is high, it turns out that it is in the condition where it is easy to generate | occur | produce a crack when bent to a steel pipe (experiment No.11,25,26).

  In contrast, Experiment No. 1 to 4, 7, 8, 10, 14 to 24 satisfy both the chemical component composition and the production conditions of the steel slab within the range specified in the present invention, and the mechanical properties of the steel sheet are appropriate. It can be seen that a steel pipe satisfying the standard value in the mechanical properties is obtained.

It is a graph which shows the relationship between a workability (t / D) and the yield stress variation | change_quantity (DELTA) YS after shaping | molding. It is a graph which shows the relationship between a workability (t / D) and tensile strength change amount (DELTA) TS after shaping | molding. It is a graph which shows the relationship between a workability (t / D) and the yield ratio variation | change_quantity (DELTA) YR after shaping | molding.

Claims (7)

C: 0.02 to 0.20% (meaning of mass%, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 0.50 to 2.0%, Al: 0.01 to 0. 1% and N: 0.002 to 0.007%, respectively, P: 0.02% or less (not including 0%) and S: 0.008% or less (not including 0%), respectively By using a steel slab composed of Fe and unavoidable impurities in the balance, and performing a heat treatment of any of the following (1) to (3) to produce a thick steel plate,
Round steel pipe thickness is t (mm), outer diameter is D (mm), steel pipe product standard yield strength is YS ₀ (MPa) or more, tensile strength is TS ₀ (MPa) or more, yield ratio is 85% In the following, the yield strength in the steel sheet is (YS ₀ -980 t / D) to (YS ₀ -980 t / D + 120) (MPa), and the tensile strength of the steel sheet is (TS ₀ -560 t / D) to (TS _{0 -560t / D + 100) (} MPa), yield ratio of the steel sheet (75-82t / D) (%) or less, hardness at a depth of 1mm from the steel plate front and back surfaces are the respective control so that HV140～200,
A method for producing a pressed bend cold-formed circular steel pipe having a tensile strength of 490 MPa or more and excellent in earthquake resistance, characterized in that the steel sheet is formed into a circular steel pipe by press-bending cold forming and no subsequent heat treatment is performed.
(1) After heating and rolling a steel slab to 1000 to 1250 ° C, air cooling from a temperature of 800 ° C or higher, reheating to a temperature of 850 ° C or higher, and 200 ° C or lower at a cooling rate of 1 to 50 ° C / sec. After cooling to 700 to 850 ° C. in a two-phase region temperature, cooling to 200 ° C. or less at a cooling rate of 1 to 50 ° C./second, further heating to a temperature of 500 to 700 ° C. and then air cooling. .
(2) After heating and rolling the steel slab to 1000 to 1250 ° C., the steel slab is cooled from a temperature of 800 ° C. or more to 200 ° C. or less at a cooling rate of 1 to 50 ° C./second, and subsequently in a two-phase region of 700 to 850 ° C. After reheating to a temperature, it is cooled to 200 ° C. or lower at a cooling rate of 1 to 50 ° C./second, further heated to a temperature of 500 to 700 ° C. and then air-cooled.
(3) After heating and rolling the steel slab to 1000 to 1250 ° C, the steel slab is cooled from a two-phase region temperature of 650 to 800 ° C to 200 ° C or less at a cooling rate of 1 to 50 ° C / second, and further 500 to 700 ° C. After heating to the temperature of   Steel slab is further Cu: 1% or less (not including 0%), Ni: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), and Mo: 1% The manufacturing method according to claim 1, comprising one or more selected from the group consisting of the following (excluding 0%).   The manufacturing method according to claim 1 or 2, wherein the steel slab further contains Nb: 0.05% or less (not including 0%).   The manufacturing method according to any one of claims 1 to 3, wherein the steel slab further contains V: 0.1% or less (not including 0%).   The manufacturing method according to any one of claims 1 to 4, wherein the steel slab further contains B: 0.003% or less (not including 0%).   The steel slab further contains Ca: 0.005% or less (not including 0%) and / or rare earth elements: 0.05% or less (not including 0%). The manufacturing method of crab.   The manufacturing method according to any one of claims 1 to 6, wherein the steel slab further contains Ti: 0.025% or less (not including 0%).

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