KR100799421B1 - Cold-formed steel pipe and tube having excellent in weldability with 490MPa-class of low yield ratio, and manufacturing process thereof - Google Patents

Abstract

The cold-formed steel tube of the present invention has a predetermined chemical composition, and the microstructure of the steel sheet is 4 to 70 area% of polygonal ferrite phase, 20 area% or less of polygonal ferrite phase, and 5 area% or less, When the aspect ratio (longer diameter / shorter diameter) is 4.0 or less, the martensite phase and the remainder are composed of bainite phase, and when the sheet thickness is t (mm) and the outer cold bending diameter is d (mm), t / d is 10. It has a cold forming part which is below%. With such a configuration, a cold-formed steel tube having a resistivity ratio can be obtained at a tensile strength of 490 MPa or more without performing SR treatment.

Description

Cold-formed steel pipe and tube having excellent in weldability with 490MPa-class of low yield ratio, and manufacturing process

(Technology)

The present invention relates to a cold-formed steel tube having a resistance to wear ratio tensile strength of 490 MPa class and a method of manufacturing the same, in particular, to a 490 MPa grade cold sheet suitable for use in a CFT structure. It relates to a shaped steel pipe and a method useful for producing such cold formed steel pipe.

(Background)

Building structures are required to have excellent shock resistance and fire resistance, and in order to build a structure of the above-mentioned CFT structure having excellent shock resistance, a cold formed steel pipe exhibiting excellent weldability with high strength and resistance ratio is required.

As a technique related to cold forming steel pipes satisfying such required characteristics, various techniques have been proposed so far. For example, Japanese Patent Application Laid-Open No. Hei 6-128641 discloses an air-tight or water-cooled steel sheet made of 600MPa class and 800MPa class, which is t / D (t: sheet thickness, D: steel pipe outer diameter). In the technique of manufacturing steel pipe by cold forming in the range of ≤ 10% and controlling the yield ratio (YR) ≤ 80-0.8 x t / D, then normalizing the steel sheet in the temperature range of 750 to 850 ° C. Is disclosed.

칭(quenching)하고, 또한 Ac₁점 이하의 온도범위까지 템퍼링(tempering)처리를 행하여 상기 t/D≤10%의 범위에서 관 형태로 냉간성형하고, 그 후 500~600℃의 온도범위에서 아닐링(annealing)하는 것에 관하여 개시되어 있다.Further, Patent Publication No. 2529042 discloses a 590 MPa-grade resistive steel pipe, subject to rolling finish temperature: (Ar ₃ -20 ° C) to (Ar ₃ + 120 ° C), followed by rolling the steel sheet (Ar _3- ). Air cooled to 100 ° C.) to (Ar ₃ -120 ° C.), and then directly to room temperature at this temperature.

Quenching and tempering to a temperature range of Ac ₁ or less, cold forming in the form of a tube in the range of t / D ≦ 10%, and then in a temperature range of 500 to 600 ° C. Disclosed about annealing.

칭 혹은

칭ㆍ템퍼링을 행하고, 상기 t/D≤10%의 범위에서 냉간성형을 행하여 강관을 제작하고, 그 후 650~750℃의 온도범위로 재가열하여 노멀라이징하는 것에 대하여 개시되어 있다.In addition, Japanese Patent Application Laid-open No. Hei 7-233416 discloses a 590 MPa-grade resistive steel pipe, which is reheated at a temperature of ₃ Ac or more.

Ching or

It is disclosed that the steel pipe is manufactured by cold forming and tempering, cold forming in the range of t / D? 10%, and then reheating and normalizing to a temperature range of 650 to 750 ° C.

Although each of the above techniques is for a cold-formed steel pipe having a 590MPa-grade resistive ratio, among them, Japanese Unexamined Patent Publication No. Hei 6-128641 discloses normalizing after cold forming, and Patent No. 2529042 discloses a rolling line. It relates to air-cooling to two phases, and both cause a decrease in productivity in rolling, which is undesirable from an economical point of view.

The technique disclosed in Japanese Patent Laid-Open No. 7-233416 includes a material such as Cu and Ni as an essential component in a steel material, which causes a problem of high material cost. In addition, this technique aims to improve the strength of steel pipes by strengthening the precipitation by adding Cu. However, in the heat treatment process, the temperature of the outer surface and the inner surface is non-uniform, resulting in uneven deposition of Cu. You can expect that enough.

칭)(DQ)에 비해

칭 그대로의 강도가 낮아지므로, 그것을 보충하기 위해 합금원소를 증량할 필요가 있어, 그 결과 용접성이 열화하게 된다.All of the above-listed technologies have a problem in terms of cost and productivity since it is necessary to perform heat treatment after cold forming for the purpose of reducing yield ratio. In addition, when the so-called Delay DQ method of Patent No. 2529042 is applied, direct quenching at a temperature of at least ₃ Ar points (

(DQ)

Since the hardness as it is lowered, it is necessary to increase the alloying element to replenish it, resulting in deterioration of weldability.

칭하고, 이어서 700~850℃의 온도로 재가열하여

칭하여 Ac₁점 이하에서 템퍼링 처리를 행하며, 또한 YR(%)≤80-0.8×t/D로 제어한 강판을 이용하여 t/D≤10%의 범위에서 냉간성형으로 강관을 제작하는 것으로, 이에 의하면 두께:100㎜ 이하, 관축방향의 YR이 80% 이하인 건축용 저항복비 600MPa급 강관을 얻을 수 있다.For this reason, a technique such as Japanese Patent Laid-Open No. 7-109521 has also been proposed as a method of not performing heat treatment after cold forming. This technology is reheated to Ac ₃ ~ 1000 ℃ after hot rolling

Then reheated to a temperature of 700-850 ° C.

As referred performs a tempering treatment at less than Ac ₁ point, and that the steel pipe produced by cold forming in a range of YR (%) ≤80-0.8 × t / t and / D≤10% using a steel plate controlled to D, The According to the present invention, a 600 MPa class steel pipe having a thickness of 100 mm or less and a YR in the tube axis direction of 80% or less can be obtained.

칭, 강관의 인성개선과 용접, 응력제거 처리 등에 의한 연화를 방지하기 위한 템퍼링을 필수공정으로 하는데, 생산성과 비용이라는 관점에서 약간의 문제가 남아있다. 게다가, 이 기술로는 강도확보라고 하는 관점에서는 합금원소의 증량이 필요하기 때문에 용접성이 자연 문제가 된다.This technology is aimed at 600MPa-grade resistive steel pipe, which is reheated to bainize the structure after rolling.

Tempering to prevent softening by toughening, welding, and stress relief treatment of steel pipes and steel pipes is an essential process, but some problems remain in terms of productivity and cost. In addition, the weldability is a natural problem because the technique requires an increase in the alloying element from the viewpoint of securing strength.

On the other hand, as a method of manufacturing a 490 MPa-class resistive high tensile strength steel sheet, a technique such as Japanese Patent Application Laid-Open No. 55-115921 is also proposed. The technique is rolled so that the cumulative reduction ratio at 900 ° C. or lower is 50% or more, the rolling is finished at Ar ₃ or higher and cooled to Ac ₁ or lower, and then reheated in the range of 730 to 850 ° C. or lower. It is air cooling.

칭(Q′)처리한 것에 비해 강도가 낮아지게 되므로, 탄소당량 Ceq(JIS)가 0.40% 이하에서 용접성이 양호한 것은 32㎜정도까지의 판두께에 적용할 수 있지만(예컨데 표 1의 강 No. 1, 2, 4~6), 냉간성형용 두꺼운 강관에 적용하고자 할 때에는 탄소당량 Ceq를 대폭 높일 필요가 있어(예컨데 표 1의 No.3), 이에 따라 용접성이 열화하고 예열이 필요하게 된다. 게다가, 오스테나이트 미재결정역(未再結晶域)(약 900℃~Ar₃점)에서의 압하율을 크게하기 위해, 건축용 강에 요구되는 음향이방성이 작아야 한다는 요건을 만족하지 못하게 된다.In this technique, at a two-phase temperature (above Ac ₁ point, below Ac ₃ point)

Since the strength is lower than that of the Q (Q ') treatment, a good weldability at a carbon equivalent of Ceq (JIS) of 0.40% or less can be applied to a sheet thickness up to about 32 mm (for example, steel No. 1 in Table 1). 1, 2, 4 to 6), when it is applied to the cold forming thick steel pipe, it is necessary to significantly increase the carbon equivalent Ceq (for example, No. 3 in Table 1), which results in deterioration of weldability and preheating. In addition, in order to increase the reduction ratio in the austenite non-recrystallization zone (approximately 900 DEG C to ₃ Ar points), the requirement that the acoustic anisotropy required for the steel for construction be small is not satisfied.

(Tasks to be solved by the invention)

However, due to the revision of the new seismic design method (1981), in the construction field, a design concept that allows plastic deformation of steel materials in the event of a major earthquake, absorbs energy from earthquakes, and prevents the collapse of structures. Adopted mainly on high-rise buildings, the resistance ratio was required as a characteristic for steel materials.

In cold-formed steel pipes applied to concrete-filled steel pipe columns, when a cold cold bend of t / D: 5 to 10% is applied, a yield stress of 2.5 to 5% is given to t / 4. As a result, even if the steel has a tensile strength of 490 MPa, the target yield ratio (yield point / tensile strength) of 85% or less cannot be secured. In such a case, the stress relieving (SR treatment) for the purpose of removing residual stress after cold forming must be performed, resulting in high cost, prolonged air, and reduced productivity.

The present invention has been made under such circumstances, and its object is to provide a cold-formed steel tube having a resistance ratio at tensile strength of 490 MPa or more and a useful method for producing such cold-formed steel tube without SR treatment.

(Means to solve the task)

The 490MPa-grade resistance ratio ratio cold-formed steel pipe of the present invention that can achieve the above object, C: 0.07 ~ 0.18% (mass%, below), Si: 0.05 ~ 1.0%, Mn: 0.7 ~ 1.7%, Ti: 0.002 ˜0.025%, sol. In addition to containing Al: 0.005 to 0.1% and N: 0.001 to 0.008%, Cr: 0.6% or less (including 0%), Mo: 0.5% or less (including 0%) and V: 0.08% or less (including 0%) 1 or 2 or more types selected from the group which consists of C-), and the ratio of Mn content [Mn] and C content [C] satisfy | fills [Mn] / [C] <= 15, and is represented by following formula (1) While the carbon equivalent Ceq value is in the range of 0.34 to 0.42%, the A value represented by the following formula (2) satisfies 1.1 to 2.6, and the balance has a chemical composition consisting of Fe and an unavoidable impurity. Essentially composed phase structure is 4 to 70 area% polygonal ferrite phase, 20 area% or less pseudopolygonal ferrite phase, and 5 area% or less, aspect ratio (long diameter / short diameter) (Iv)) is 4.0 or less island-like martensite phase, the balance is made of bainite phase, the sheet thickness is obtained as t (mm) steel sheet, the steel When the outside cold bending diameter of a pipe is d (mm), it is a steel pipe which makes it possible to obtain the cold forming part whose t / d is 10% or less.

Cep = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 15 ... (1)

However, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] are contents of C, Si, Mn, Ni, Cr, Mo, and V (mass%), respectively. ).

A = (2.16 {Cr} +1) × (3.0 {Mo} +1) × (1.75 {V} +1) ‥‥ (2)

However, {Cr}, {Mo} and {V} represent high capacities (mass%) in the steel sheets of Cr, Mo and V, respectively.

In the cold-formed steel pipe of the present invention, if necessary, (a) Cu: 0.5% or less (does not contain 0%) and / or Ni: 3.0% or less (does not contain 0%), (b) Nb: 0.015% It is also effective to contain less than (0%), (c) Ca: 0.003% or less (0%), (d) rare earth elements: 0.02% or less (0%) Therefore, the characteristic of a steel pipe can be improved further by these contained components.

칭하고, 상기 t/D가 10% 이하의 범위에서 냉간성형되도록 하는 것이 바람직하다.In producing the cold-formed steel pipe as described above, the steel piece having the chemical composition specified in the present invention is heated to a temperature range of 950 ~ 1250 ℃, the austenite unrecrystallization temperature Aγ shown in the following formula (3) After the rolling is completed to form a steel sheet with a cumulative reduction ratio of 60% or less (including 0%) at or below (° C), a cooling rate of 4 to 100 ° C / sec. From a temperature of Ar ₃ transformation point to 450 ° C or less. Accelerated cooling, then reheated to a temperature range of 730 ~ 830 ℃

It is preferable that the t / D be cold formed in the range of 10% or less.

Aγ (° C.) = 887 + 467 [C] + (6445 [Nb] -644√ [Nb]) + (732 [V] -230√ [V]) + 890 [Ti] +363 [Al] -357 [ Si] ‥‥ (3)

However, [C], [Nb], [V], [Ti], [Al], and [Si] represent the contents (mass%) of C, Nb, V, Ti, Al, and Si, respectively.

칭한 후, 상기 강판을 500℃ 이하로 템퍼링을 행한다, (2)상기 압연을 종료한 후, 가속냉각하기 전에 온라인레벨러 교정을 행한다, (3) 강판온도를 400℃ 이하로 하여 냉간성형한다, 등의 조건을 부가하는 것이 바람직하다.In this manufacturing method, (1) it is reheated in the temperature range of 730-830 degreeC,

After tempering, the steel sheet is tempered to 500 ° C. or lower. (2) After completion of the rolling, online leveler calibration is performed before accelerated cooling. (3) Cold forming is performed at a steel sheet temperature of 400 ° C. or lower. It is preferable to add the condition of.

(The best form to carry out invention)

MEANS TO SOLVE THE PROBLEM When the plate | board thickness is t (mm) and outer cold bending diameter is d (mm), the present inventors have a cold-formed site | part where t / d is 10% or less, and it is excellent in weldability as a steel pipe whose tensile strength is 490 MPa or more, In addition, the chemical composition and microstructure were examined in detail to achieve the yield ratio below the target of 85%.

As a result, in order to reduce the yield ratio of the steel pipe, it is important to lower the yield ratio at the steel sheet stage or more in advance in the steel pipe, and to increase uniform elongation δu (plastic stretching to the maximum load). I found out.

In order to make both the resistance ratio and tensile strength at the steel sheet stage coexist, the bainite phase (B) in which the microstructure becomes a hard phase and the polygonal ferrite phase (polygonal ferrite phase: αP) in the soft phase coexist. It was found that it is effective to control the area fraction of the polygonal ferrite phase (αP) to 40 to 70%. It has also been found that the uniform stretching δ u can be increased by polygonalizing the ferrite phase.

In the steel sheet step (before cold forming), when martensite is formed in an island shape, that is, in an island shape, the yield point is reduced and the yield ratio after cold bending is also lowered. Island phase martensite is composed of a mixture of martensite phase and austenite phase (residual austenite phase) (Martensite-Austenite constitute: MA phase), but residual austenite phase γR present in island-like martensite By this cold bending, transformation into processed organic martensite can further increase uniform stretching? U. In addition, in the case where a cold formed steel pipe is subjected to cold bending to form such a structure, the retained austenite phase in the structure disappears and is present as a transformed martensite phase.

Although the cold-formed steel pipe of this invention needs to control microstructure appropriately from the said viewpoint, the reason which limited the range (area fraction) of each phase in this structure is as follows.

[Polygonal ferrite phase αP: 40 ~ 70 area%]

In order to lower the yield ratio, it is effective to produce a polygonalized ferrite (αP) having a small dislocation density in the microstructure after transformation, and to reduce the yield ratio in advance at the steel sheet stage, the area fraction is reduced to 40 It needs to be controlled in the range of ˜70%. When the area fraction of polygonalized ferrite (αP) exceeds 70%, it becomes difficult to secure the target strength in thick materials. On the other hand, when the area fraction of the polygonal ferrite phase (αP) is less than 40%, the yield ratio exceeds the target value (85%).

In order to satisfy the area fraction condition of the polygonal ferrite phase, it is necessary to satisfy the composition conditions specified in the present invention (including those satisfying the above formulas (1) and (2)) and satisfy the following manufacturing conditions. have.

[Polygonal ferrite phase αq: 20 area% or less]

The pseudopolygonal ferrite phase αq having a high dislocation density increases the strength and prevents the movement of the movable potential, thereby increasing the yield ratio. Therefore, the smaller the better, the better it is and the area fraction is 20% or less. I need to do that. Preferably it is about 15% or less.

The pseudopolygonal ferrite phase does not grow into a hexagon (defined by JIS G0551), does not round, and has a jagged tooth shape. Therefore, the pseudo polygonal ferrite phase and the polygonal ferrite phase can be clearly distinguished by observing under a microscope.

[M-A of island shape martensite phase: 5 area% or less]

In the steel sheet step, the martensite phase (M) or martensite-austenite mixed phase (MA phase) has a local island shape (C) in the unmodified austenite and the portion of the alloy element segregation does not undergo bainite transformation. It is a martensite phase (M) and a retained austenite phase (γR) as an island. Among them, the martensitic phase (M) increases the tensile strength effectively acts to reduce the yield ratio. In addition, since residual austenite (γR) expresses the processed organic transformation by the processing strain from the outside, it effectively acts to increase the uniform stretching δu. Therefore, in order to further promote the reduction in yield ratio and the increase in uniform elongation δu, the cold-formed steel pipe is formed of island-like martensite phase (M-A: martensite phase M after transformation from residual austenite). The area fraction of the island-like martensite phase (M-A) is preferably about 5% or less. If the area fraction of the island-like martensite phase (M-A) exceeds 5%, the toughness deteriorates. This area fraction is preferably about 4% or less.

[Aspect ratio of island shape martensite phase (M-A): 4.0 or less]

Even if the area fraction of the island-like martensite phase (M-A) is 5% or less, if the aspect ratio (longer diameter / shorter diameter) exceeds 4.0 in the shape, the uniform elongation δu does not increase and the toughness also deteriorates. In addition, since the MA phase is formed at the old austenite grain boundary, controlling the aspect ratio to 4.0 or less results in less overall body drawing of the old austenite grain, and the formation of the rolled aggregate structure is extremely small. Therefore, it is possible to reduce the acoustic anisotropy of the seam welded portion of the steel pipe (corresponding to the no-drilling portion of the bend).

In the cold-formed steel pipe of the present invention, when the plate thickness is t (mm) and the outer cold bending diameter is d (mm), the cold-formed portion having t / d is 10% or less, but this t / d is 10% In cold processing exceeding, the yield ratio on the tensile strain side exceeds 85% after processing, so that hot- or warm-forming or stress-relieving annealing after molding (SR treatment) is necessary to suppress the increase in yield ratio. Done. Therefore, the t / d needs to be 10% or less. Preferably this t / d is made into 7.5% or less. The processing method for achieving this t / d is not limited to press bending molding, but for example, roll bending, compression press, spinning and the like can also be applied. In addition, the bending temperature can be allowed not only at room temperature but also to a temperature that does not damage the steel sheet material of the present invention (about 400 ° C.). In addition, the cold-formed steel pipe of the present invention includes both circular and rectangular cross-sectional shapes. In addition, the outer cold bending diameter d means the diameter of curvature in the cold-formed (bending) portion, the outer cold bending diameter d is equal to the steel pipe outer diameter D when the cross-sectional shape of the steel pipe is circular.

칭온도를 730~830℃ 정도로 하는 것이 유효하다(이 조건에 대해서는 후술한다).In the cold-formed steel pipe of the present invention, in order to control the quantitative ratio of the ferrite (αP) of the microstructure in the same manner as described above (area fraction 40 to 70%), the ferrite nose of the transformation curve can be shifted in a short time. Specifically, the ratio of [Mn] [Mn] to C content [C] ([Mn] / [C]) is 15 or less, and the two-phase separation of C in two-phase (α + γ region) temperature holding. It is effective to facilitate the two-phase separation. In addition, in order to exhibit such an effect,

It is effective to set the calling temperature at about 730 ° C to 830 ° C (this condition will be described later).

Ferrite soft nitridation and hardening of cementite are added to ferrite in the coexistence state of austenite and ferrite in the presence of a secondary segregation element in ferrite and maintained at two-phase temperature. It is thought that it is effective to diffuse negative segregation elements into unmodified austenite, and then bainite transformation to concentrate the alloy elements discharged in the bainite transformation process into cementite.

칭하는 것이 유효하다(이 조건에 대해서는 후술한다).As the negative segregation element in ferrite, it is noted that the action of Cr, Mo, and V is large, and the alloy element can be separated into two phases by controlling the amount specified in Equation (2) to 1.1 to 2.6 at these high capacities. have. Further, the above-mentioned (2) where a prescribed amount of the slabs in order to control the 1.1 ~ 2.6 by rapid cooling of the temperature at more than Ar ₃ transformation point after rolling end as heated to a temperature range of 950 ~ 1250 ℃, nitride precipitation of the respective elements Precipitation avoidance in the temperature range, Cr, Mo and V in the solid-state state, two phase

It is effective to call it (this condition is mentioned later).

Lowering the yield point and increasing the uniform elongation δu are effective for lowering the dislocation density of ferrite by growing the grains by polygonal processing without ferrite transformation.

In order to reduce the yield ratio and secure the toughness of the cold-formed steel pipe, it is necessary to form a more isotropic martensite phase. From this point of view, the aspect ratio must be 4.0 or less. . Moreover, since the rolling assembly structure is also reduced by lowering the aspect ratio, it is also effective in reducing acoustic anisotropy at seam welded portions of steel pipes (corresponding to no cuts in short cuts).

As a means of suppressing the flattening of the martensite phase or the mixed layer with austenite in the steel sheet step, the cumulative reduction ratio of the austenite unrecrystallization temperature Aγ or less shown in the above formula (3) is 60% or less. Finishing the rolling with (containing 0%) is effective for suppressing flattening of the old austenite grains and the grain boundary precipitated martensite phase or mixed austenite phase. In addition, it is preferable to make the end temperature of rolling at this time into the austenite unrecrystallization temperature A (gamma) or more from a viewpoint of suppressing the flattening of old austenite grain. In addition, in order to control the structure fraction of martensite as described above, it is necessary to replace C in the carbon equivalent formula with the alloying elements (particularly Cr, No, V, etc.) of the present invention.

In the cold-formed steel pipe of the present invention, the microstructure is basically made of bainite other than the above (remaining part), but for this purpose, after precipitation of polygonal ferrite αP of the present invention, accelerated cooling is performed immediately so as not to transform the pearlite. It is good.

Therefore, in the cold-formed steel pipe of the present invention, it is also necessary to have good weldability, but in order to do so, it is possible to suppress martensite or bainiteization in the weld heat affected zone (HAZ) by adding B, thereby preventing crack resistance and HAZ toughness. It will be possible to improve. In addition, TiN is generated by addition of Ti, and the toughness is improved by exerting the refining action of the former austenite grains in the base material and HAZ.

Next, the reason for limiting the chemical composition of the cold formed steel pipe of the present invention will be described. First, as described above, the present invention provides C: 0.07 to 0.18%, Si: 0.05 to 1.0%, and Mn: 0.7 to 1.7% (with the ratio [Mn] / [of Mn content [Mn] and C content [C]). C] ≤15), Ti: 0.002 to 0.025%, sol.Al: 0.005 to 0.1%, and N: 0.001 to 0.008%, Cr: 0.6% or less (including 0%), Mo: 0.5% or less ( 0%) and V: 0.08% or less (including 0%), and one or two or more selected from the group consisting of, and the value specified in the following formulas (1) and (2) in an appropriate range You need to control it. The reason for the range limitation of these elements is as follows.

[C: 0.07 ~ 0.18%]

C is the cheapest element and is effective for increasing the strength. However, if excessively contained, weldability is significantly lowered, the upper limit of the content is made 0.18%. However, when the C content is less than 0.07%, the strength is insufficient, and in order to supplement it, an alloy element is required to be added. However, an excessive addition of these alloy elements causes an increase in yield ratio, which is not preferable. In order to secure the target strength (tensile strength of 490 MPa or more) while suppressing the increase in yield ratio, it is necessary to contain C at least 0.07% or more. Moreover, from a viewpoint of coexistence of base material strength and weld HAZ toughness, the minimum with preferable C content is 0.08%, and a preferable upper limit is 0.16%.

[Si: 0.05 ~ 1.0%]

Si needs to be contained in an amount of 0.05% or more for deoxidation. However, an excessive amount of Si contained in excess of 1.0% lowers the weldability and the HAZ toughness. Therefore, Si content needs to be 0.05 to 1.0%. Moreover, the minimum with preferable Si content is 0.10%, and a preferable upper limit is 0.9%.

[Mn: 0.7 to 1.7%, provided that the ratio [Mn] / [C] ≦ 15 of the Mn content [Mn] and the C content [C])]

Mn is an effective element that increases strength and toughness together. In order to exert such an effect, it is necessary to contain Mn 0.7% or more. However, when Mn is excessively contained, the weldability and the HAZ toughness are significantly deteriorated, so the upper limit is made 1.7%. Moreover, the minimum with preferable Mn content is 1.0%, and a preferable upper limit is 1.6%.

In addition, it is necessary to adjust Mn content to an appropriate range with respect to C content. The ratio [Mn] / [C] of Mn content [Mn] and C content [C] is the portion (nose) protruding from the ferrite transformation curve in the continuous cooling transformation curve (CCT curve) and the isothermal transformation curve (TTT curve). Since the ratio [Mn] / [C] exceeds 15, the ferrite nose retreats to the long side, and the two-phase structure (α) in equilibrium in the two-phase inverse heat treatment (Q ') is obtained. It is inefficient because the holding period for + γ) is long and it is restricted in production. Therefore, the ratio [Mn] / [C] needs to be 15 or less.

[Ti: 0.002 ~ 0.025%]

칭) 후의 Q′처리에서도 역변태 오스테나이트로부터 TiN을 페라이트변태핵(核)으로 하여 폴리고날페라이트의 석출을 촉진시키고, 항복비저감, 균일연신δu의 증대에도 효과가 있다. 이러한 효과를 발휘시키기 위해서는, Ti함유량은 0.002% 이 상으로 할 필요가 있다. 그렇지만, Ti를 과잉 함유시켜도 그 효과가 포화하므로, 그 상한을 0.025%로 한다. 또한, Ti 함유량의 바람직한 하한은 0.008%이고, 바람직한 상한은 0.015%이다.Ti exists as fine TiN in steel at the time of slab heating, and it has an effect which prevents coarsening of a heating austenite grain. It is possible to ensure good toughness and ultrasonic acoustic anisotropy by the combined effect of moderate austenite (γ) recrystallization temperature zone rolling, followed by γ recrystallization temperature Aγ reverse rolling and fine TiN production. Ti is also directly quenched (

In the Q 'treatment after fermentation, TiN is used as a ferrite transformation nucleus from reverse transformation austenite to promote precipitation of polygonal ferrite, and it is also effective in reducing yield ratio and increasing uniform stretching δu. In order to exhibit such an effect, Ti content must be 0.002% or more. However, since the effect is saturated even if it contains Ti excessively, the upper limit is made into 0.025%. Moreover, the minimum with preferable Ti content is 0.008%, and a preferable upper limit is 0.015%.

[sol. Al: 0.005 ~ 0.1%]

Al needs to be contained at least 0.005% for deoxidation. However, when Al is excessively contained, the nonmetallic inclusions increase and the toughness decreases. Therefore, Al should be 0.1% or less. The minimum with preferable Al content is 0.01%, and a preferable upper limit is 0.06%.

[N: 0.001 ~ 0.008%]

N reacts with Ti to form TiN and is an effective element for preventing coarsening of austenite during heating. In order to exhibit such an effect, it is necessary to contain at least 0.001% or more. However, when excessively contained, the toughness of the welded joint deteriorates, and therefore it is necessary to make it 0.008% or less. The minimum with preferable N content is 0.002%, and a preferable upper limit is 0.006%.

(Cr: 0.6% or less (including 0%), Mo: 0.5% or less (including 0%) and V: 0.08% or less (including 0%) Quantity that satisfies (2)

Cr, Mo, and V are elements that improve strength, but when precipitated with a compound, the yield ratio is increased by precipitation strengthening, and the toughness is deteriorated. In order to secure high strength and high toughness while keeping the yield ratio low, it is effective to segregate the segregation in cementite and the ferrite in solid solution. For this reason, the content of Cr, Mo and V is 0.6% or less, 0.5% or less and 0.08% or less (all 0% are included), and the high dose is 1.1 to 2.6 in the value A defined in the above formula (2). You need to control it from within. If this component element amount and A value exceed an upper limit, weldability will be impaired. In addition, when A value is less than 1.1, the yield ratio after steel pipe forming cannot satisfy a target value. Each element is preferably Cr: 0.3% or less, Mo: 0.3% or less, and V: 0.06% or less. Moreover, the preferable range of A value is about 1.05-2.0.

[Ceq: 0.34 ~ 0.42%]

In the formula (1), the carbon equivalent Ceq is an index indicating the hardenability of the HAZ (for example, JIS G 3106), in order to reduce the weld cracking sensitivity and to set the crack prevention preheating temperature in the y-type weld crack test to 25 ° C. or lower. It is necessary to make the Cep value 0.42% or less. On the other hand, in order to secure the tensile strength of 490 MPa or more, the Ceq value needs to be 0.34% or more. The minimum with preferable Ceq value is 0.36%, and a preferable upper limit is 0.40%. In addition, in addition to the basic components C, Si, Mn, Cr, Mo, and V, the component (Ni) contained in the formula (1) is also included in the formula in the above formula (1). It is good to calculate content by the value of Formula (1) in consideration of content, and when not containing, it is good to calculate, without considering these content.

In the cold-formed steel pipe of the present invention, in addition to the above components, there may be Fe and inevitable impurities, but a trace component (allowable component) inevitably mixed in solvent may be included (for example, P, S, O, and B≤0.0005). %, Etc.), such steel slab is also included in the scope of the present invention. In addition, in the cold-formed steel pipe of the present invention, if necessary, (a) Cu: 0.5% or less (does not contain 0%) and / or Ni: 3.0% or less (does not contain 0%), (b) Nb: 0.015% or less (0% not included), (c) Ca: 0.003% or less (0% not included), (d) Rare earth elements: 0.02% or less (0% not included), etc. It is also effective, but the reason for the limitation when containing these components is as follows.

[Cu: 0.5% or less (without 0%) and / or Ni: 3.0% or less (without 0%)]

Since these elements are not only expensive but increase yield ratio, it is preferable to avoid their addition as much as possible. However, in a thick steel sheet, since it has the effect of suppressing the decrease in strength of the sheet thickness center part, it may be added in a small amount. When adding these elements, it is necessary to contain 0.5% of Cu and 3.0% of Ni as an upper limit. The upper limit with preferable Cu content is 0.3%, and the more preferable upper limit of Ni is 1.5%.

[Nb: 0.015% or less (without 0%)]

칭성 향상원소인 Nb를 함유시킨 강에서는 약 2상조직의 베이나이트 량이 증가하고, 연질의 페라이트가 생성되기 어렵게 된다. 그 결과, 항복비가 상승하게 된다. 이러한 이유로, Nb를 함유시킨 경우에는 0.015%정도까지로 하는 것이 바람직하다. Nb 함유량의 보다 바람직한 상한은 0.010%이다.Nb is known as an element that improves both strength and toughness, but when accelerated cooling is performed after hot rolling,

In the steel containing Nb, which is a nicking enhancement element, the amount of bainite in the biphasic structure is increased, and soft ferrite is hardly produced. As a result, the yield ratio increases. For this reason, when it contains Nb, it is preferable to set it as about 0.015%. The upper limit with more preferable Nb content is 0.010%.

[Ca: 0.005% or less (not including 0%)]

Ca is an element effective for reducing the anisotropy, which has a spherical effect of nonmetallic inclusions, but when contained in an amount exceeding 0.005%, the toughness deteriorates due to the increase of inclusions. Therefore, when it contains Ca, it is preferable to set it as 0.005% or less. The minimum with preferable Ca content is 0.0005%, and a more preferable upper limit is 0.003%.

[Rare earth element: 0.02% or less (0% not included)]

Rare earth element (hereinafter referred to as REM) is an oxysulfide element that suppresses austenite abnormal growth in the presence of TiN and improves toughness of HAZ, but when it is contained in excess of 0.02%, steel cleanliness is increased. Deteriorates, causing internal defects. In order to exhibit the effect by REM, it is preferable to contain 0.002% or more, and a more preferable upper limit is 0.01%. In addition, as REM, all scankanium (Sc), itrium (Y), and a lanthanoid series rare earth element which belong to the 3rd genus of a periodic table can be used.

칭)도 포함) 등의 공정을 거치므로써, 상기와 같은 화학성분조성 및 조직을 갖는 강관을 제조하는 것이 바람직하고, 따라서 제조방법에 대해서는 특히 한정하는 것은 아니지만(후술 실시예의 실험 No. 42~46), 본 발명 방법에 따라 제조하는 것이 바람직하다. 다음으로, 본 발명의 제조방법으로 규정하는 각 요건에 대하여 설명한다.In order to manufacture the cold-formed steel pipe of the present invention, processes such as heating, hot rolling, cooling, and heat treatment are basically performed using steel pieces in a slab manufactured by a casting method or an ingot method. Controlled cooling after hot or hot rolling (accelerated cooling or direct quenching)

It is preferable to manufacture a steel pipe having the chemical composition and structure as described above, and therefore, the manufacturing method is not particularly limited (Experiment Nos. 42 to 46 in Examples below). ), Is preferably prepared according to the method of the present invention. Next, each requirement prescribed | regulated by the manufacturing method of this invention is demonstrated.

[Heating temperature of steel strip: 950 ~ 1250 ℃]

When the heating temperature of the steel piece is set to a temperature of more than 1250 ° C, the austenite grains of the steel piece rapidly generate lip growth, and the structure after transformation becomes coarse bainite structure, which significantly reduces the toughness of the steel sheet. On the other hand, when the heating temperature is less than 950 ° C, the cumulative reduction in pressure under (γ-recrystallization temperature Aγ-50 ° C) becomes large, resulting in excessive atomization of the old austenite grains, yield point YP, 0.2% yield strength α 0.2 and yield ratio YR are greatly increased. For this reason, the heating temperature of the steel piece needs to be in the range of 950-1250 degreeC. Preferably this heating temperature is 1000 degreeC or more and 1150 degrees C or less.

[The cumulative reduction in pressure at γ or less of recrystallization temperature Aγ is 60% or less]

As described above, in order to suppress the flattening of the martensite phase or the mixed phase with austenite in the steel sheet step, the cumulative reduction ratio at the? Microcrystallization temperature Aγ or lower needs to be 60% or less. If the cumulative reduction ratio exceeds 60%, the excessive austenite grains become excessively fined and the yield ratio increases. In addition, said "rolling reduction rate" is calculated | required by {(t1-t2) / t1} * 100 (%), when the thickness of the steel plate before and after rolling is called t1 (mm) and t2 (mm), respectively.

[After the end of the rolling, it is cooled at a cooling rate of 4 ~ 100 ℃ / sec. Up to 450 ℃ or less at a temperature above the Ar ₃ transformation point]

In order to achieve uniform dispersion of C and solid solution of Cr, Mo, and V in the microstructure of the steel sheet, and to secure the strength, it is necessary to accelerately cool the Ar ₃ transformation point to 450 ° C or lower after rolling. The cooling start temperature is lower than the Ar ₃ transformation point, the cooling stop temperature is higher than 450 ° C, or the cooling rate is 4 ° C / sec. If less than 1, the transformation of transformation is insufficient (that is, the second phase of the complex tissue is difficult to be bainite) and all employment of Cr, Mo, and V is not achieved. About the upper limit of the cooling rate at this time, it is 100 degreeC / sec. From a viewpoint of the limit of the cooling capacity of a cooling medium. It is necessary to make the following. Further, Ar ₃ transformation point of the present invention employing a value that is calculated by the following formula (4).

Ar ₃ transformation point = 910-310 [C] -80 [Mn] -20 [Cu] -15 [Cr] -55 [Ni] -80 [Mo] +0.35 (t-8) ‥ (4)

Where t is the steel plate thickness

칭]Reheat to a temperature range of 730 ~ 830 ℃

Ching]

칭하는 것은, 역변태 오스테나이트에서

칭으로 주상(主相)인 베이나이트 조직과 섬형상 마르텐사이트상을 석출시키기 위해서이 다.By maintaining the accelerated-cooled steel sheet at a two-phase (α + γ) temperature, once dispersed C is separated into two phases by polygonal ferrite and reverse transformation austenite produced at the time of accelerated cooling, (Iii) Segregation (concentration lower than average concentration (addition amount)) and regular segregation of austenite (concentration higher than average concentration (addition amount)) were produced. In addition, for each element of Cr, Mo, and V once solid-dissolved by accelerated cooling, in this two-phase maintenance, the side segregation into polygonal ferrite and the regular segregation into austenite are generated, and the yield ratio is reduced. It was possible to solve the opposite problem of securing high strength. When the holding temperature in the two-phase zone is less than 730 ° C and exceeds 830 ° C, the amount of reverse transformation austenite and polygonal ferrite of too small, respectively, increases the yield ratio at the steel sheet stage and yields after cold forming steel pipe. The rain cannot satisfy the target yield ratio. In addition, after maintaining at the 2-phase temperature

It is called inverse transformation austenite

This is to deposit the main phase bainite structure and island martensite phase.

[Cold forming in the range of t / d 10% or less when the plate thickness is t (mm) and the outer cold bending diameter is d (mm).

The cold-formed steel pipe of the present invention has a cold-formed portion where t / d is 10% or less so that the yield ratio on the tensile strain side becomes 85% or less after processing, and the range where t / d is 10% or less for forming such a portion. Cold forming at

칭한 후, 상기 강판을 500℃ 이하로 템퍼링을 행한다, (2)상기 압연을 종료한 후, 가속냉각하기 전에 온라인레벨러 교정을 행한다, (3) 강판온도를 400℃ 이하로 하여 냉간성형한다, 등의 조건을 부가하는 것이 바람직한데, 이들 요건을 규정하는 이유는 다음과 같다.The production method of the present invention, if necessary (1) reheated to a temperature range of 730 ~ 830 ℃

After tempering, the steel sheet is tempered to 500 ° C. or lower. (2) After completion of the rolling, online leveler calibration is performed before accelerated cooling. (3) Cold forming is performed at a steel sheet temperature of 400 ° C. or lower. It is preferable to add the conditions of. The reason for specifying these requirements is as follows.

칭한 후, 상기 강판을 500℃ 이하로 템퍼링에서 행한다]Reheat to a temperature range of 730 ~ 830 ℃

The steel sheet is then tempered to 500 ° C. or lower.

칭 자체에서 생성된 베이나이트 조직 중의 C가 확산ㆍ응집되어 펄라이트를 생성시키므로, 강도가 저하하게 된다. 따라서, 템퍼링 온도는 500℃ 이하로 하는데, 바람직하게는 480℃ 이하로 하는 것이 좋다.In order to eliminate the residual stress of the two-phase quenched steel sheet, it is also effective to selectively temper at 500 ° C or lower. If the tempering temperature at this time exceeds 500 ℃,

Since C in the bainite structure produced by the quenching itself diffuses and aggregates to generate pearlite, the strength is lowered. Therefore, the tempering temperature is 500 ° C. or less, preferably 480 ° C. or less.

[Online leveler calibration is performed after the rolling is finished and before accelerated cooling]

칭) 전의 열간교정에 의해 평탄도가 양호하게 되므로, 앞ㆍ뒷끝단부에 대한 균일냉각이 가능하게 되고, 기계적 성질이 안정되며 회수율(收率)이 향상된다. 따라서, 압연종료 후에 직접담금질 전에서의 온라인레벨러 교정(on-line leveller 矯正)을 행하는 것이 효과적이다.After finishing rolling, direct quenching is performed even in the case of flatness failure at the front and rear ends of the rolled steel sheet.

Flatness is improved by the hot calibration before it, so that uniform cooling to the front and rear ends is possible, the mechanical properties are stabilized, and the recovery rate is improved. Therefore, it is effective to perform on-line leveler correction before direct quenching after the end of rolling.

[Cold forming at steel sheet temperature below 400 ℃]

Although the bending temperature (molding temperature) can be allowed not only to room temperature but also to a temperature that does not damage the steel sheet material of the present invention (about 400 ° C.), molding deterioration such as spring back during cold forming In order to alleviate the factor, it is also effective to selectively mold (warm molding) at 400 ° C. or less, where the microstructure does not change and the dislocation density can be reduced. If the molding temperature at this time exceeds 400 ° C, C diffuses and a part of bainite, the main phase, begins to change to pearlite, resulting in a decrease in strength. The preferable temperature of this formation temperature is 300 degrees C or less.

Hereinafter, an Example is given and this invention is demonstrated more concretely. The present invention is not limited by the following examples in the first place, and of course, modifications can be made within a range suitable for the purpose of the preceding and later stages, and all of them belong to the technical scope of the present invention.

[ Example ]

Steels of the chemical composition shown in the following Tables 1 and 2 were dissolved in a conventional solvent method, and all the treatments shown below were performed (types 1 to 3) to prepare steel sheets. Tables 1 and 2 also show the values of the carbon equivalent Ceq, the values of [Mn] / [C], and γ recrystallization temperature defined by the above formula (1).

[Process Procedure]

Type 1: After normal heating and hot rolling, direct quenching (DQ) is performed, followed by quenching (Q ') or 500 ° C. or lower after heat treatment holding at two-phase temperature (Ac ₁ or higher, Ac ₃ or lower). Accelerated cooling to.

Type 2: After completion of rolling, after complete cooling to less than ₃ points of Ar to an air cooling degree, accelerated cooling or direct quenching (DQ ') was performed at two-phase station temperature (above Ar ₁ point and below Ar ₃ point).

Type 3: After hot rolling, accelerated cooling was performed to maintain the two-phase temperature, and then accelerated cooling or direct quenching (DQ) was performed again.

Subsequently, some of them were subjected to tempering without tempering (T) at a temperature of less than Ac ₁ point. The production conditions at this time are shown in Tables 3 to 5 together with the values of the above Formula (2) and the Ar ₃ transformation point.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

TABLE 5

About each obtained steel plate, cold press molding was performed by changing t / d and the steel pipe was produced. The mechanical properties of the steel plate (yield point YP, tensile strength TS, uniform elongation δu) and the type of microstructure, as well as the mechanical properties (yield point, tensile strength TSk, yield ratio YR and toughness) in the tube axis direction (L direction) ) And Cr, Mo, V high capacity, microstructure was measured, and the material was evaluated based on the following criteria.

[High capacity of Cr, Mo, V]

Regarding the high capacities of Cr, Mo, and V of the steel pipe, the amount of each element precipitated as the addition amount-precipitate of each element was calculated. About the element amounts of Cr, Mo, and V deposited as precipitates, the amount of precipitated elements was measured by the electrolytic extraction residue method in the cross section parallel to the surface of the outer t / 4 portion of the steel pipe.

[Material evaluation criteria]

As the material evaluation criteria, the tensile strength in the tube axis direction of the steel pipe was set to TS: 490 MPa or more, yield ratio YR: 85% or less, and wavefront transition temperature (vTrs):-20 ° C or less.

The evaluation method of mechanical properties (steel plate and steel pipe), the toughness evaluation method of steel pipe, and the microstructure measurement method are as follows.

[Evaluation Method of Mechanical Characteristics]

Test specimen of JIS Z 2201 4 in t / 4 part of steel plate (t is plate thickness) in 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 sheet). Tensile tests were conducted in accordance with JIS Z 2241, and mechanical properties of the steel sheet (yield point YP, tensile strength TS, uniform elongation δu), mechanical properties of steel pipe (yield point YP, tensile strength TS, yield ratio (yield point / tensile strength × 100%: YR)) was measured.

[Toughness Evaluation Method]

Take the JIS Z 2202 No. 4 test piece in the direction parallel to the tube axis (main rolling direction of the steel plate) at the outer t / 4 part of the steel pipe, and carry out the Charpy impact test in accordance with JIS Z 2242, and the wavefront transition temperature ( vTrs) was measured.

[Microstructure Measurement Method]

In the steel sheet step, the microstructure of the t / 4 part in the main rolling direction of the steel sheet was observed with an optical microscope, and with respect to the residual austenite γR present, the steel sheet t / 4 part was electropolished at 50 to 100 µm. X-ray diffraction was performed to confirm the presence of residual austenite γR from the highest intensity ratio between the α-Fe (200) plane and the γ-Fe (200) plane. An image of the microstructure obtained by nitrial etching is analyzed by analyzing the image of the outer t / 4 part parallel to the tube axis of the steel pipe (corresponding to the main rolling direction of the steel sheet) and the t / 4 part of the main rolling direction of the steel sheet. Polygonal ferrite αp, Polygonal ferrite αq), their area fraction, the area fraction of bainite (B), the area fraction of pearlite (P), and the like were measured. The island-like martensite phase (M-A phase) was analyzed by analyzing a photograph of a microstructure etched with 1/4 part of the rolling direction plate thickness surface with a reper reagent, and measured the area fraction and the aspect ratio.

The weldability (welding crack resistance and HAZ toughness) of the steel pipe that satisfies the material standard was evaluated by the following method.

[Welding Crack Resistance]

In accordance with the y-type welding crack test method specified in JIS Z 3158, carbon dioxide gas welding was performed with a heat input amount of 1.7 KJ / mm, and the root crack prevention preheating temperature was measured. 25 degreeC or less was made into the pass.

[HAZ Toughness]

Perform seam welding of double-sided submerged arc welding (SAW) with a heat input of 7 KJ / mm (improved X), and take a Charpy impact test piece (JIS Z 2204 4) in the direction perpendicular to the tube axis at the outer t / 4 part, and take 0 ° C. The average impact absorption energy vE0 at was obtained (average of three tests). The average vE0 was 47J or more as the pass.

The weldability test results are shown in Tables 6 to 8 together with mechanical properties (steel plates and pipes) and microstructures. From these results, the following facts were found. First, experiment No. 1 is the ash as it is by the control rolling of V single additive steel. Since Ceq exceeded the range prescribed | regulated by this invention, the welding crack prevention preheating temperature became 50 degreeC, and HAZ toughness also fell.

Experiment No. 2 is an accelerated cooling 450 degreeC stop material of Nb addition steel, and since polygonal ferrite was not produced | generated in a microstructure, yield ratio YR after cold bending did not satisfy 85% or less of the target value.

칭(Q′)한 것으로, 폴리고날페라이트상의 면적분율이 본 발명에서 규정하는 범위보다 작아졌으므로, 냉간굽힘 후의 항복비 YR이 목표치인 85% 이하를 만족 하지 못하게 되었다.Experiment No. The third one was accelerated cooling 450 ℃ of Nb single additive steel after stopping at two-phase temperature

In the case of Q (Q '), since the area fraction of polygonal ferrite phase is smaller than the range prescribed by the present invention, the yield ratio YR after cold bending does not satisfy the target value of 85% or less.

Experiment No. Although the value (A value) of said Formula (2) exists in the range prescribed | regulated by this invention, the thing of 14 has more C content than the range prescribed | regulated by this invention, and the base material and HAZ toughness fell.

Experiment No. 15, Mn content became more than the range prescribed | regulated by this invention, and the yield ratio YR after cold bending did not satisfy 85% or less which is a target value.

Experiment No. 18 is C content and Mn content less than the range prescribed | regulated by this invention, and tensile strength TS after cold bending did not satisfy more than 490 Mpa which is a target value.

Experiment No. 22, Ti content became more than the range prescribed | regulated by this invention, and the yield ratio YR after cold bending did not satisfy 85% or less which is a target value.

Experiment No. 24, Mo content became more than the range prescribed | regulated by this invention, and HAZ toughness did not satisfy 47J or more which is a target value.

Experiment No. As for 29, Si content becomes larger than the range prescribed | regulated by this invention, and the island-like martensite fraction of a steel pipe becomes larger than the range prescribed | regulated by this invention, so that the yield ratio YR after cold bending may not satisfy 85% or less of a target value. It became.

Experiment No. The Cu content of 32 is more than the range prescribed | regulated by this invention, and the welding crack prevention preheating temperature does not satisfy 25 degrees C or less of the target. In addition, the wavefront transition temperature vTrs did not meet the target value of −20 ° C. or lower and the HAZ toughness of 47 J or higher.

Experiment No. As for 35, Nb content became more than the range prescribed | regulated by this invention, and the yield ratio YR after cold bending did not satisfy 85% or less which is a target value.

Experiment No. 38, Ca and REM content exceeded the range prescribed | regulated by this invention, and the wavefront transition temperature vTrs after cold bending did not satisfy -20 degrees C or less.

Experiment No. The heating temperature of 51 became 1300 degreeC, and the wavefront transition temperature vTrs after cold bending did not satisfy -20 degreeC or less.

Experiment No. 54, heating temperature became 900 degreeC, and the cumulative reduction ratio became 100%, and the yield ratio YR after cold bending did not satisfy 85% or less of the target value. In addition, experiment No. As for 55, the cumulative reduction ratio became 80%, and the yield ratio YR after cold bending did not satisfy 85% or less of the target value.

Experiment No. The accelerated cooling start temperature after rolling was 760 degreeC, and the polygonal ferrite fraction became 80 area%, and the tensile strength TS fell.

Experiment No. In 60, the accelerated cooling stop temperature after rolling was 580 ° C, and bainite was reduced during the accelerated cooling, and pearlite was produced. Thus, tensile strength TS decreased.

Experiment No. The 61 had an accelerated cooling rate after rolling of 1.5 deg. C / sec, and the bainite was reduced during the accelerated cooling, and pearlite was produced. Thus, the tensile strength TS decreased.

칭 전의 재가열온도가 850℃로 되고, 폴리고날페라이트분율이 35면적%로 되어, 냉간굽힘 후의 항복비 YR이 목표치인 85% 이하를 만족하지 못하게 되었다.Experiment No. 63 is,

The reheating temperature before quenching became 850 degreeC, and the polygonal ferrite fraction became 35 area%, and the yield ratio YR after cold bending did not satisfy 85% or less of the target value.

칭 전의 재가열온도가 Ac₁ 직하의 700℃로 되고,

칭 전의 재가열 승온시에 생성된 펄라이트가 남아있게 되어, 인장강도 TS가 저하하였다.Experiment No. 66 is,

The reheating temperature before quenching is 700 ° C. directly below Ac ₁ ,

The pearlite produced at the time of reheating before quenching remained, and the tensile strength TS fell.

Experiment No. The tempering temperature of 68 became 600 degreeC, the polygonal ferrite fraction became 80 area%, the tensile strength TS fell, and the yield ratio YR after cold bending did not satisfy 85% or less of the target value.

Experiment No. In 70, t / d at the time of cold forming became 15%, and the yield ratio YR after cold bending did not satisfy 85% or less of the target value.

In contrast, all of the requirements defined in the present invention (experiments No. 4 to 13, 16, 17, 19 to 21, 23, 25 to 28, 30, 31, 34, 36, 37, 39 to 50, 52, 53, 56, 57, 59, 62, 64, 65, 67, 69, 71) met the target values for all characteristics.

칭(Q′)한 것, 실험 No. 39는 또한 템퍼링처리(T)를 행한 것이다.In addition, experiment No. The manufacturing point in 4-71 is as follows. That is, experiment No. 4 to 38 is a two-phase temperature after the end of rolling the steel composition of the chemical composition shown in Table 1, 2

Q (Q '), experiment No. 39 is also a tempering process (T).

Experiment No. 40 was stopped by accelerated cooling 450 degreeC after completion | finish of rolling, and experiment No. 41 is also a tempering process (T).

Experiment No. 42 is obtained by completely quenching (air cooling) and quenching (DQ ') directly at two-phase temperature after completion of rolling. 43 is also a tempering process (T).

Experiment No. 44 is accelerated-cooled to more than ₁ point of Ar and less than ₃ points of Ar after completion | finish of rolling, and it hold | maintains by air cooling for 60 second, and produces polygonal ferrite (alpha) P, and then directly quenched.

Experiment No. 45, after the end of rolling, accelerated cooling to more than ₁ point of Ar and less than ₃ points of Ar, and then maintained at 2-phase temperature in an on-line maintenance furnace to produce polygonal ferrite αP, and then directly It was quenched and experiment No. 46 is also a tempering process (T).

Experiment no. 47 and 48 change t / d to 7.5, 5 (%) within the range prescribed | regulated by this invention. Experiment No. 49 is the result of having a plate | board thickness of 40 mm. Experiment No. 50 is press-bended after heating up at 400 degreeC.

Experiment No. 51-54 is a chemical component prescribed | regulated by this invention, and changes heating temperature in the range of 900-1300 degreeC. Experiment No. 55-58 are the chemical components prescribed | regulated by this invention, and change the accelerated cooling start temperature after rolling.

Experiment No. 59 and 60 are chemical components prescribed | regulated by this invention, and change in accelerated cooling stop temperature. Experiment No. 60-62 are the chemical components prescribed | regulated by this invention, and change the accelerated cooling rate after rolling.

칭할 때의 가열온도(Q′)를 변화시킨 것이다. 실험 No. 67, 68은, 본 발명에서 규정하는 화학성분으로, 템퍼링 온도(T)를 변화시킨 것이다.Experiment No. 63-66 is a chemical component prescribed | regulated by this invention,

In this case, the heating temperature Q 'was changed. Experiment No. 67 and 68 are chemical components prescribed | regulated by this invention, and change tempering temperature (T).

Experiment No. 69 is a chemical component specified in the present invention, which is an on-line leveler calibration before accelerated cooling. Experiment No. 70 is a chemical component prescribed | regulated by this invention, and made t / d of cold bending outside the range prescribed | regulated by this invention. Experiment No. 71 is a component prescribed | regulated by this invention, and made bending temperature into 400 degreeC.

TABLE 6

TABLE 7

TABLE 8

According to the present invention, by appropriately adjusting the chemical composition of the steel sheet and appropriately controlling the volume fraction of each phase in the microstructure, a 490 MPa grade cold-formed steel pipe can be obtained at a resistance ratio without performing SR treatment. The cold-formed steel pipe obtained in this manner can be suitably used in a building having a CFT structure by appropriately controlling the manufacturing conditions thereof.

Claims (9)

As cold forming steel pipe, C: 0.07 to 0.18%, (mass%, all same below) Si: 0.05-1.0%, Mn: 0.7-1.7%, Ti: 0.002 ~ 0.025%, sol.Al: 0.005 to 0.1% and N: 0.001% to 0.008%, respectively, At least one selected from the group consisting of Cr: 0.6% or less (including 0%), Mo: 0.5% or less (including 0%), and V: 0.08% or less (including 0%), The ratio of Mn content [Mn] and C content [C] [Mn] / [C] ≤15 Satisfy The carbon equivalent Ceq value shown by the following formula (1) is in the range of 0.34 to 0.42%, A value represented by the following formula (2) satisfies 1.1 to 2.6, The microstructure is 40 to 70 area% polygonal ferrite phase, 20 area% or less pseudopolygonal ferrite phase, and 5 area% or less and has an aspect ratio (long diameter / short diameter). Island) martensite phase of less than 4.0), the balance consists of bainite phase, Cold-formed steel pipe obtained from a steel plate having a sheet thickness of t (mm) and having a cold-formed portion having a t / d of 10% or less when the outer cold bending diameter of the steel pipe is d (mm). . Cep = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 15 ... (1) However, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] are contents of C, Si, Mn, Ni, Cr, Mo, and V (mass%), respectively. ). A = (2.16 {Cr} +1) × (3.0 {Mo} +1) × (1.75 {V} +1) ‥‥ (2) However, {Cr}, {Mo}, and {V} represent high capacities (mass%) of Cr, Mo, and V in the steel sheet, respectively. The cold formed steel tube according to claim 1, further comprising at least one of Cu: 0.5% or less (not including 0%) and Ni: 3.0% or less (not including 0%). The cold formed steel pipe according to claim 1, further comprising Nb: 0.015% or less (not including 0%). 2. The cold formed steel pipe according to claim 1, further comprising Ca: 0.005% or less (not including 0%). The cold formed steel pipe according to claim 1, further comprising a rare earth element: 0.02% or less (not including 0%). The method for manufacturing a cold formed steel pipe of claim 1, The steel piece is heated to a temperature range of 950-1250 ° C .; The steel sheet is rolled to form a steel sheet such that the cumulative reduction ratio of the austenite unrecrystallization temperature Aγ (° C.) or less shown in the following formula (3) is 60% or less (including 0%); . To Ar ₃ transformation point or the cooling rate of 4 ~ 100 ℃ / sec at a temperature up to or less than 450 ℃ accelerated cooling the steel plate and; After reheating the accelerated cooled steel sheet to a temperature range of 730 ~ 830 ℃

Called;

A cold forming steel pipe manufacturing method, wherein said steel sheet is cold-formed in the range of t / d of 10% or less. Aγ (° C.) = 887 + 467 [C] + (6445 [Nb] -644√ [Nb]) + (732 [V] -230√ [V]) + 890 [Ti] +363 [Al] -357 [ Si] ‥‥ (3) However, [C], [Nb], [V], [Ti], [Al], and [Si] represent the contents (mass%) of C, Nb, V, Ti, Al, and Si, respectively. The method of claim 6, wherein the reheating to a temperature range of 730 ~ 830 ℃

The cold-formed steel pipe manufacturing method characterized by tempering below 500 degreeC with respect to the said steel plate. 7. The method for manufacturing a cold formed steel pipe according to claim 6, wherein an on-line leveler calibration is performed on the steel sheet after the rolling is completed before accelerated cooling. 7. The method for manufacturing a cold formed steel pipe according to claim 6, wherein the cold forming is performed at a steel sheet temperature of 400 deg.