KR101096992B1 - 780MPa CLASS LOW YIELD RATIO CIRCULAR STEEL FOR CONSTRUCTION STRUCTURE EXCELLENT IN EARTHQUAKE-PROOF PERFORMANCE, AND PROCESS FOR PRODUCING THE SAME - Google Patents

Abstract

The steel pipe of the present invention satisfies the requirements of the following (A) to (C) while adjusting the chemical component composition while satisfying a predetermined relational expression.

(A) The average Vickers hardness Hv of the center portion excluding the surface layer portion up to 2 mm deep from each of the front and back surfaces of the steel pipe is 230 to 310,

(B) In the microstructure of the steel pipe, the fraction of bainitic ferrite phase is 80 area% or more, and the fraction of martensite phase is 5 area% or less,

(C) The average Vickers hardness Hv of the surface layer part from each of the front and back surfaces of a steel pipe to 2 mm in depth is 1.3 times or less of the average Vickers hardness Hv of the said center part.

With such a configuration, in the steel pipe of the highest strength class: 780 MPa class, which is located in the most high-strength class in building steel frame, both high strength and resistance yield ratio are achieved, and the steel pipe outer surface side caused by bending during steel pipe forming is achieved. By reducing the hardness, it is possible to contribute to the improvement of the shock resistance by securing ductility and also improving crack resistance by welding.

Description

780MPa CLASS LOW YIELD RATIO CIRCULAR STEEL FOR CONSTRUCTION STRUCTURE EXCELLENT IN EARTHQUAKE-PROOF PERFORMANCE, AND PROCESS FOR PRODUCING THE SAME}

The present invention relates to a circular steel pipe, and a method of manufacturing the same for building steel frame use, which is mainly required for earthquake resistance, in particular a high-strength resistance ratio circular steel pipe having a tensile strength of 780 MPa or more (780 MPa class), 90% or less yield, and such a circular shape A useful method for manufacturing steel pipes.

In order to ensure the earthquake resistance of a building structure, the steel for building uses the upper limit of the yield ratio YR expressed by the ratio (YS / TS) of the yield stress YS and the tensile strength TS under the idea of absorbing seismic energy by plastic deformation after elastic deformation. This is prescribed.

Since the circular steel pipe applied to the above-described building structure is formed by press bending of steel sheet or the like, a material change occurs due to work hardening, and the yield ratio YR and the hardness of the front and back surfaces of the steel pipe increase. In particular, the outer surface side of the circular steel pipe has a large increase in hardness and a tensile stress field as compared with the sheet thickness center portion, so the ductility is lowered.

That is, in the case of deformation under load in the event of a major earthquake, cracks are likely to occur from the outer surface side, and circular steel pipes have inherent problems that do not occur in four-sided box columns. In particular, when an accessory mold or the like is welded to a circular steel pipe, a decrease in ductility of the circular steel pipe surface (outer surface) due to hardening of the heat affected zone HAZ is a problem.

By the way, as a method of manufacturing a steel pipe by cold forming, in addition to the UOE molding method (Uing press-Oing press-expander method) applied to the steel pipe for line pipes, the press-bending cold forming method (henceforth a "press bending method") is called. In some cases) is basically employed. In the above forming method, when the steel sheet is thick (for example, plate thickness: more than 30 mm), and a strong bending process is required, the press bending method is adopted.

In the press bending method, a part of the steel sheet (straight line) is embossed and bent, and the embossing position is sequentially moved to be molded in a circular shape, which is a method having high processing capability. In the case of forming a round steel pipe by such a press bending method, the outer surface hardening becomes particularly remarkable in a round steel pipe, but as a method of reducing such hardness, stress relieving (hereinafter referred to as "SR heat treatment") may be called. Is known.

However, in the case of a 780 MPa class steel pipe, if a conventional steel sheet having a tensile strength of TS: 780 MPa or more is applied on the premise of SR heat treatment, a very hard structure such as martensite or lower bainite is contained in the metal structure due to the large amount of alloying elements added. It is difficult to secure the properties of the resistive ratio YR (hereinafter sometimes referred to as `` low YR characteristic '') as well as to secure the steel plate base material toughness even after SR heat treatment. , The hardness of the steel pipe surface is still hard. On the other hand, when SR heat treatment is made high in order to reduce the hardness of the steel pipe surface, the hardness of the steel pipe thickness center part also falls, and it was difficult to ensure tensile strength TS: 780 MPa or more which is a required strength as a circular steel pipe.

In addition, the demand for building materials is important to ensure high mechanical properties such as high strength and resistance to yield ratio, as well as high heat input welding characteristics and good weldability for reducing construction costs, and therefore, alloying elements cannot be added unnecessarily.

As a technique regarding the steel pipe as described above, various techniques have been proposed until now. For example, Japanese Patent Laid-Open No. 2007-270304 proposes a method for producing a press-bending cold-formed round steel pipe of 490 MPa or more. This technique is useful as a technique of a 490 MPa-class round steel pipe, but since the Vickers hardness Hv at the surface layer portion up to 1 mm from each of the front and back surfaces of the steel sheet is about 140 to 200, and the hardness at the center of the sheet thickness is further lowered, it is 780 MPa or more. Not applicable to tensile strength TS.

In addition, Japanese Laid-Open Patent Publication No. 2003-003229 discloses a thick steel sheet having a main structure of ferrite and a fraction of the hard second phase of 10 to 70%. In this technique, the structure cannot stably secure the tensile strength TS: 780 MPa or more. About a method of manufacturing the same, and provisions ensure only stability of the hard of the "cooling-stop temperature is below 500 ℃" is difficult, and rolling is in the organization control point end temperature: Ar over ₃ transformation point, and cooling rates: 5 ℃ / sec Below, the uniform metal structure and hardness cannot be obtained stably in the plate thickness direction.

Japanese Laid-Open Patent Publication No. 2006-283187 discloses a rolling finish temperature of Ar ₃ using a steel material in which chemical composition is appropriately adjusted. When the hot rolling is carried out to a temperature range above the transformation point, and then quenched to 300 ° C. or lower from the temperature range above the Ar ₃ transformation point, and then reheated to a temperature range of Ac ₁ to Ac ₁ + 150 ° C., the heating rate to the reheating temperature is increased. A method for producing high strength and high toughness steel is proposed in which the residence time in the temperature range of 1 ° C / sec or less and Ac ₁ to Ac ₁ + 150 ° C is within 90 seconds.

However, in this technique, the hardness distribution in the metal structure and the plate thickness direction is not taken into consideration, and when press bending is performed, hardening of the outer surface side hardness cannot be suppressed, and good seismic resistance when the circular steel pipe is used is not exhibited. It is not expected. Moreover, in the manufacturing method, since the rapid heating and the residence time in the two-phase region are short, there is a problem that a uniform structure cannot be obtained in the plate thickness direction.

On the other hand, Japanese Patent Laid-Open No. 2005-68519 proposes a method for producing a high strength thick steel sheet for a building structure having excellent superheat input HAZ toughness. This technique is a technique for securing good HAZ toughness when superheated welding is used as a building structure. However, the steel sheet targeted by this technique is basically of low strength (700 MPa or less), and since the hardness distribution in the sheet thickness direction is not considered, and the C content is relatively high and the rolling temperature is also set, press bending In the round steel pipe after shaping | molding by the method, the hardness of the outer surface side becomes high and favorable seismic resistance cannot be exhibited.

In addition, Japanese Patent Laid-Open No. 2003-293075 proposes a high-strength steel pipe material having a low surface hardness and yield ratio after tube manufacture. Although this technique makes the strength of a steel pipe material 780 MPa class, it is difficult to obtain the strength of 780 MPa or more stably based on the component system. Moreover, in the manufacturing method, it is not prescribed at all about the two-phase reverse quenching temperature, and a uniform metal structure and hardness cannot be obtained in a plate | board thickness direction.

Japanese Patent Laid-Open No. 5-148544 discloses a method for producing a high strength high toughness steel sheet having a uniform hardness distribution in the plate thickness direction. In this technique, by applying a special manufacturing method, such as water cooling once during rolling, re-rolling, and rolling again, fine processed ferrite is generated in the surface layer to reduce surface hardness, thereby achieving uniformity in hardness distribution in the plate thickness direction. It is.

However, in this technique, there is a possibility that the surface layer portion may be softer than the inside of the sheet thickness, and manufacturing management on the mass production surface for obtaining a stable material is difficult. In addition, in this technique, the hardness after processing into a round steel pipe is not considered.

The present invention has been made under such a situation, and its object is to achieve both high strength and resistive ratio for the steel pipe of the tensile strength TS: 780MPa class, which is located in the most high strength class in building steel applications, and bend during forming the steel pipe. It is to provide a circular steel pipe that can contribute to the improvement of the seismic resistance by securing ductility by reducing the hardness of the outer surface side of the steel pipe caused by the and also improve the crack resistance by welding, and a useful method for producing such a circular steel pipe .

Circular steel pipe of the present invention was able to achieve the above object C: 0.01 ~ 0.06% (meaning of mass%, the same below), Si: 0.10 ~ 0.40%, Mn: 1.60 ~ 2.50%, Al: 0.025 ~ 0.090%, Cu: 0.15-0.70%, Ni: 0.90-1.60%, Cr: 0.50-1.35%, Mo: 0.10-0.30%, Ti: 0.008-0.025%, B: 0.0005-0.0025%, N: 0.0030-0.0060% and Ca : 0.0005 to 0.0040%, respectively, the remainder is composed of Fe and inevitable impurities, P: 0.012% or less of the inevitable impurities, S: 0.005% or less and O: 0.0040% or less, respectively, The PCM value represented by Formula 1 is 0.30% or less, and satisfy | fills the requirements of following (A)-(C).

PCM value = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + ( (B) × 5)

(However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu, Ni, Contents (mass%) of Cr, Mo, V, and B are shown.)

(A) The average Vickers hardness Hv of the center portion excluding the surface layer portion up to 2 mm deep from each of the front and back surfaces of the steel pipe is 230 to 310,

(B) In the microstructure of the steel pipe, the fraction of bainitic ferrite phase is 80 area% or more, and the fraction of martensite phase is 5 area% or less,

(C) The average Vickers hardness Hv of the surface layer part from each of the front and back surfaces of a steel pipe to 2 mm in depth is 1.3 times or less of the average Vickers hardness Hv of the said center part.

 In manufacturing such a round steel pipe, after heating the slab which consists of said chemical component to 950-1200 degreeC, it hot-rolls by making finish rolling temperature into the range of 800-930 degreeC, and predetermined | prescribed plate | board thickness. Subsequently, the cooling rate at the position of t / 4 (t: sheet thickness) is 2 to 25 ° C / sec, and water is cooled until the surface temperature is 350 ° C or less, and then the temperature is 700 to 900 ° C. It is good to make it reheat and to harden, to temper at a temperature range of 450-700 degreeC, and to make a steel plate, and to shape | mold into a round steel pipe by the press bending method using the obtained steel plate.

According to the present invention, by appropriately adjusting the chemical composition of the steel pipe, appropriately controlling the area fraction of each phase in the microstructure, and by appropriately adjusting the hardness distribution in the thickness direction, both high strength and resistance ratio of 780 MPa or more are achieved. In addition to this, by reducing the hardness of the outer surface of the steel pipe caused by bending during steel pipe forming, securing ductility, and improving the crack resistance by welding, it was possible to realize a circular steel pipe that can contribute to the improvement of the shock resistance. .

MEANS TO SOLVE THE PROBLEM The present inventors examined from various angles in order to achieve | achieve both high strength of 780 Mpa or more, and resistance ratio, and to reduce hardening of the outer side of the round steel pipe resulting from the work hardening at the time of press bending work. As a result, it is important to first make the fraction (area fraction) of the bainitic ferrite phase to be 80% or more, and to set the area fraction of the martensite phase to 5% or less as the basic microstructure of the steel pipe (i.e. steel sheet) [above (B) Requirements]. Here, the bainitic ferrite phase is a phase of low C bainite tissue transformed at a lower temperature than ferrite, and includes granular bainitic ferrite tissue, broad upper bainite tissue and lower bainite tissue, and the like. It does not include polygonal ferrite tissues or grain boundary ferrite tissues (for example, Collection of Steel Bainite-1): The Japan Steel Association's Bainite Investigation and Research Society, 1992). The martensite phase also includes MA (Martensite-Austenite Constituent).

The low C bainitic ferrite structure has less carbide and less cooling rate dependence, so the hardness uniformity in the plate thickness direction of the steel sheet is high, and the dislocation density is higher than that of conventional polygonal ferrite, so that the amount of work hardening against deformation distortion Becomes small. For this reason, after making into a steel pipe, it contributes to the uniformity of the hardness distribution of the thickness direction. If the area fraction of the bainitic ferrite becomes less than 80% and the area fraction of the hard phase such as martensite increases, the hardness of the outer surface side of the circular steel pipe rises, the deformability deteriorates, and the elongation at break decreases. For this reason, the area fraction of bainitic ferrite needs to be at least 80% or more, preferably 85% or more.

On the other hand, in the martensitic phase, it is necessary to suppress the area fraction to 5% or less from the viewpoint of securing steel pipe (steel plate) toughness. In other words, if the area fraction of martensite phase exceeds 5%, the hard martensite becomes a starting point of breakdown, and a defect that remarkably deteriorates toughness occurs. In addition, although the microstructure of the circular steel pipe of this invention should just be controlled as mentioned above, the remainder may contain a bainite phase, a ferrite phase, etc. as a remainder.

In order to make it a micro structure as mentioned above, although manufacturing conditions need to be controlled appropriately, it is necessary to control chemical composition of a steel plate suitably as a premise. As the basic direction, reduction of the surface hardness of the round steel pipe by reducing the content of C, and production of bainitic ferrite by appropriate addition of Cr, in order to maintain high strength and resistance ratio under the premise thereof, and the bay It is effective to utilize strengthening by solid solution of Cu and Ni in nitic ferrite and strengthening mechanism of quenched structure by B.

An effective means for improving the strength of the steel sheet is to increase the amount of alloying elements. In particular, in order to achieve a high strength of 780 MPa, it is necessary to increase the amount of the alloying element to be relatively large and to use various reinforcing mechanisms accordingly. However, such an increase in the alloying elements will lead to deterioration of the weldability of crack resistance or the mechanical properties of the welded joint. MEANS TO SOLVE THE PROBLEM The present inventors discovered that by adding an appropriate alloying element and optimizing its content, both high strength and low YR characteristics can be made compatible, and work hardening by bending can be reduced.

By satisfying each of the above requirements (microstructure and chemical composition), the hardness distribution in the sheet thickness direction is uniformed, the work hardening amount is stabilized, and the area up to 2 mm below the outer surface of the round steel pipe (up to 2 mm deep from the steel sheet surface). Ratio of the Vickers hardness Hv between the surface layer portion) and the sheet thickness direction center portion (t / 2 portion (t: sheet thickness)) can be suppressed, and the seismic resistance as a round steel pipe can be improved.

From the above point of view, the chemical composition of the circular steel pipe of the present invention is determined, but the reason for the range limitation of each element including the alloy components (C, Cr, Ni, and B) described above will be described. In the present invention, as described above, C: 0.01 to 0.06%, Si: 0.10 to 0.40%, Mn: 1.60 to 2.50%, Al: 0.025 to 0.090%, Cu: 0.15 to 0.70%, Ni: 0.90 to 1.60%, Cr: 0.50 to 1.35%, Mo: 0.10 to 0.30%, Ti: 0.008 to 0.025%, B: 0.0005 to 0.0025%, N: 0.0030 to 0.0060%, and Ca: 0.0005 to 0.0040%, respectively, It is necessary to control the PCM value represented by Formula 1 to an appropriate range, but the reason for limiting the range of these elements is as follows.

[C: 0.01-0.06%]

C has the effect of increasing the strength of the steel sheet, and is an important element for controlling hardness, and is also an element that degrades weldability such as crack resistance. If C content is less than 0.01%, required base material (steel plate) strength cannot be ensured. However, when C content exceeds 0.06%, the hardness distribution of a plate thickness direction will become large by transformation of martensite of a surface layer part. In addition, island-like martensite (including a mixed phase of martensite and austenite (M-A phase)) is excessively generated, HAZ becomes excessively hard, cracks are likely to occur, and it becomes a breakdown point during an earthquake. Moreover, the minimum with preferable C content is 0.02%, and a preferable upper limit is 0.05%.

[Si: 0.10 to 0.40%]

Si is an effective element for improving the strength of steel pipes. In order to exhibit such a strengthening mechanism, it is necessary to contain Si 0.10% or more. However, when the Si content is excessive, the base metal toughness, the HAZ toughness and the weldability deteriorate, so it is 0.40% or less. Moreover, the minimum with preferable Si content is 0.15%, and a preferable upper limit is 0.35%.

[Mn: 1.60-2.50%]

Mn is effective as an element that improves hardenability and increases strength and toughness together. In order to exhibit such an effect, it is necessary to contain Mn 1.60% or more. However, when Mn is excessively contained, the toughness deteriorates, so the upper limit is made 2.50%. Moreover, the minimum with preferable Mn content is 1.80%, and a preferable upper limit is 2.20%.

[Al: 0.025-0.090%]

Al is an element necessary for securing the hardenability of B by deoxidation and free nitrogen fixation. In order to exhibit these effects, it is necessary to contain 0.025% or more. However, when excessively contained, coarse inclusions of aluminum are formed and the base metal toughness is lowered. Therefore, it is necessary to make it 0.090% or less. Moreover, the minimum with preferable Al content is 0.035%, and a preferable upper limit is 0.080%.

[Cu: 0.15-0.70%]

Cu is an element useful for improving the base material strength by solid solution strengthening. In order to exert such an effect, it is necessary to contain Cu 0.15% or more. However, when the Cu content is excessive, Cu cracks may occur at the time of gas cutting. Therefore, the Cu content needs to be 0.70% or less. Moreover, the minimum with preferable Cu content is 0.25%, and a preferable upper limit is 0.65%.

[Ni: 0.90-1.60%]

Ni is an effective element for improving the base metal toughness and HAZ toughness, improving the hardenability, improving the strength, and preventing Cu cracks and weld cracks. In order to exhibit such an effect, it is necessary to contain Ni 0.90% or more. However, when the Ni content becomes excessive, scale flaws tend to occur, so it is necessary to make it 1.60% or less. Moreover, the minimum with preferable Ni content is 1.10%, and a preferable upper limit is 1.35%.

[Cr: 0.50-1.35%]

Cr is an effective element for improving hardenability and improving strength. In order to exhibit such an effect, it is necessary to contain Cr 0.50% or more. However, when the Cr content is excessive, the weld cracking property deteriorates, and therefore it is necessary to make it 1.35% or less. Moreover, the minimum with preferable Cr content is 0.60%, and a preferable upper limit is 1.25%.

[Mo: 0.10-0.30%]

Mo is an element which improves hardenability and improves strength, and is an element which is easy to produce carbide. In order to exhibit the hardenability improvement effect by Mo, it is necessary to contain Mo 0.10% or more. However, when the Mo content is excessive, the hardenability becomes excessive and the weld cracking properties deteriorate, and therefore it is necessary to make it 0.30% or less. Moreover, the minimum with preferable Mo content is 0.15%, and a preferable upper limit is 0.25%.

[Ti: 0.008 ~ 0.025%]

Ti is an element which forms N and nitride (TiN), prevents coarsening of austenite grains (γ grains) during heating before hot rolling, and is effective in improving toughness. It is also effective to secure the hardenability of B by fixing N. In order to exhibit such an effect, it is necessary to contain Ti 0.008% or more. However, when Ti content becomes excess, since TiN coarsens and base metal toughness deteriorates, it is necessary to be 0.025% or less. Moreover, the minimum with preferable Ti content is 0.010%, and a preferable upper limit is 0.018%.

[B: 0.0005-0.0025%]

Free B exists in the gamma grain boundary, and is an effective element for improving hardenability and improving base material strength. If the content of B is less than 0.0005%, the effect of improving the base metal strength is small, and the tensile strength: 780 MPa or more cannot be ensured. However, when the B content becomes excessive, inclusions are generated and the base metal toughness deteriorates. Therefore, the B content needs to be 0.0025% or less. Moreover, the minimum with preferable B content is 0.0008%, and a preferable upper limit is 0.0020%.

[N: 0.0030-0.0060%]

N is an element which is effective in forming TiN, preventing coarsening of γ grains during heating before hot rolling, and improving base material toughness and HAZ toughness. If the content of N is less than 0.0030%, the heating? Grains become coarse and the toughness deteriorates due to the lack of TiN, so it is necessary to contain 0.0030% or more. Moreover, when N content becomes excess and exceeds 0.0060%, the toughness of a steel pipe will deteriorate by embrittlement by bending work. Moreover, the minimum with preferable N content is 0.0035%, and a preferable upper limit is 0.0055%.

[Ca: 0.0005-0.0040%]

Ca is an element effective for harmlessness of the weld cracking property by spheroidization of MnS. In order to exert such an effect, it is necessary to contain Ca 0.0005% or more. However, when the Ca content is excessively greater than 0.0040%, the inclusions are coarsened to degrade the base metal toughness. Moreover, the minimum with preferable Ca content is 0.0015%, and a preferable upper limit is 0.0030%.

[PCM value: 0.30% or less]

The PCM value represented by Equation 1 is the most general requirement as an index for preventing low temperature cracking due to welding construction. In order to prevent a welding crack, it is necessary to make PCM value 0.30% or less. PCM value becomes like this. Preferably it is 0.28% or less.

In the circular steel pipe of the present invention, in addition to the above components, the circular steel pipe includes Fe and inevitable impurities (for example, P, S, O, etc.), but may also include trace components (allowable components) that are inevitably incorporated in a solvent. As such, such as Zr, H, etc., such a round steel pipe is also included in the scope of the present invention. However, about P, S, O, etc. as an unavoidable impurity, it is necessary to suppress in the following ranges respectively from the following viewpoint.

[P: 0.012% or less]

P, which is an unavoidable impurity, adversely affects the toughness of the base metal and the welded part. In order not to cause such a problem, it is necessary to suppress the content to 0.012% or less, preferably 0.010% or less.

[S: 0.005% or less]

Since S forms MnS and degrades weld cracking property, S is as few as possible. From this viewpoint, S content needs to be suppressed to 0.005% or less, Preferably it is good to set it as 0.003% or less.

[O: 0.0040% or less]

O combines with various elements to form oxides. The oxide is coarsened in some cases, causing deterioration of the base metal toughness. From this point of view, the O content needs to be 0.0040% or less. If the content is excessively higher than this, the oxide becomes coarse. Preferably, the content is suppressed to 0.0030% or less.

In the circular steel pipe of the present invention, it is also necessary that the average Vickers hardness Hv of the center portion excluding the surface layer part up to 2 mm deep from each of the front and back surfaces (steel surface constituting the steel pipe) of the steel pipe is 230 to 310 [above (A) ) Requirements. This Vickers hardness Hv has a correlation with tensile strength TS, and in order to obtain desired tensile strength TS and yield ratio YR, it is also necessary that the average Vickers hardness Hv of the center part of steel pipe thickness is 230-310. The average Vickers hardness Hv at this time is continuously measuring the hardness from the position of the depth of 4 mm from the surface of the steel pipe thickness section to the position of 4 mm from the rear side in the back side direction, and averaged the value. If the average Vickers hardness Hv at the center of the steel pipe thickness is less than 230, the resistivity ratio YR can be ensured, but the tensile strength TS is less than 780 MPa, which does not satisfy the strength. Moreover, when the average Vickers hardness Hv of the steel pipe thickness center part exceeds 310, tensile strength will become large too much and yield ratio YR will also become high.

In the circular steel pipe of the present invention, it is also necessary that the average Vickers hardness Hv of the surface layer portion from each of the front and back surfaces of the steel sheet to a depth of 2 mm is 1.3 times or less the average Vickers hardness Hv of the central portion (requirement of (C) above). The average Vickers hardness Hv of this surface layer part is an average value of four points of the position of 1 mm and 2 mm in depth from the surface, and the position of 1 mm and 2 mm in depth from the back surface.

If the ratio of the hardness of the surface layer portion to the center portion of the steel pipe thickness exceeds 1.3 times, the plastic deformation ability of the surface layer portion decreases, so that the ductility of the surface layer portion cannot be followed when the tensile stress due to the heavy load during the earthquake causes a crack from the surface. There is a risk. In addition, in the case of an attached non-metallic welding, there is a possibility that the HAZ hardened portion of the weld becomes a starting point of crack generation, brittle cracks propagate and propagate the low ductility low toughness portion of the surface layer portion, and brittle fracture of the circular steel pipe may occur. The value of this ratio is preferably 1.25 times or less.

In order to manufacture the round steel pipe of this invention, after heating the slab which consists of the above chemical components to 950-1200 degreeC, it hot-rolls by making finish rolling temperature into the range of 800-930 degreeC, and makes it into predetermined plate | board thickness. Subsequently, the cooling rate at the position of t / 4 (t: sheet thickness) is 2 to 25 ° C / sec, and water is cooled until the surface temperature is 350 ° C or less, and then the temperature is in the range of 700 to 900 ° C. After reheating and quenching, tempering at a temperature in the range of 450 to 700 ° C. to form a steel sheet, the obtained steel sheet may be formed into a round steel pipe by a press bending method. The conditions for each process are as follows. .

[Heat cast to 950 ~ 1200 ℃]

This heating temperature greatly influences the structure control before hot rolling. When the heating temperature is less than 950 ° C, the rolling final pass (finishing rolling) temperature is less than 800 ° C, and ferrite precipitates from the surface before water cooling, and the base metal strength of 780 MPa or more cannot be secured, and the hardness distribution in the plate thickness direction is increased. Not uniform On the other hand, when heating temperature exceeds 1200 degreeC, base material toughness will deteriorate by coarsening of (gamma) particle size.

[Hot rolling is carried out with the finish rolling temperature in the range of 800 to 930 ° C. to a predetermined sheet thickness.]

Control cooling presupposes the structure control before it, and for that purpose, it is necessary to manage rolling end temperature (finish rolling temperature) and cooling start temperature in control rolling. If the finish rolling temperature is less than 800 ° C, ferrite is precipitated before cooling starts, and desired strength cannot be obtained. Moreover, when finish rolling temperature exceeds 930 degreeC, the structure before cooling will coarsen, a base material toughness will deteriorate, and the hardness distribution of a plate thickness direction will become large. The finishing rolling temperature is preferably made lower than 900 ° C.

[Cooling rate at position of t / 4 (t: sheet thickness) is 2-25 ° C / sec]

Cooling process (DQ) after rolling is an important process for structure control. If the cooling rate at this time is less than 2 degree-C / sec, it will become impossible to ensure 80% or more of an area fraction of bainitic ferrite (bainite) which is a desired structure. The larger the cooling rate, the finer the bainitic ferrite structure becomes, and the toughness is improved. However, when the cooling rate exceeds 25 ° C / sec, martensite (including MA), which is a harmful tissue, increases in the tissue near the surface. In addition to deterioration of the base metal toughness, ductility (elongation performance) is lowered because the strength is excessive and the surface is hardened. On the other hand, t / 4 (t: sheet thickness) is used as the position for measuring the cooling rate because the steel sheet exhibits average performance.

[Cooling stop temperature: surface temperature of steel sheet is 350 ° C. or less]

The strength of the martensite or lower bainite changes depending on the cooling stop temperature. When the cooling stop temperature exceeds 350 ° C, the low-temperature transformation structure at the sheet thickness center portion decreases, the strength decreases, and the distribution of hardness in the transformation structure and the plate thickness direction becomes uneven in the plate thickness direction. In order to transform uniformly in a plate | board thickness direction, cooling stop temperature needs to be 350 degrees C or less.

[Temperature: Reheated in the range of 700 ~ 900 ° C to quench.

In order to obtain a soft phase and a hard phase composite structure that realizes a low YR characteristic, heating to a temperature in the two-phase region between Ac ₁ and Ac ₃ is an effective means. The temperature for this is 700-900 degreeC, and a part is made into soft structure by tempering by heating to the temperature of a biphase, and part is reverse-transformed into an austenite phase and becomes a hard structure by subsequent cooling. By this two-phase temperature control, the area fraction and hardness of a hard phase can be changed and YS, TS, and YR can be controlled. When the reheating temperature is less than 700 ° C, strength of 780 MPa or more cannot be secured. If the reheating temperature exceeds 900 ° C., the strength is high but no low YR above 85% can be achieved. After reheating at 700-900 degreeC, one part reverse-transforms to austenite, and the austenite phase transforms into a hard phase as it is by quenching (water cooling) after that. On the other hand, since the hard and soft phases are very fine, it is difficult to distinguish them by an optical microscope, and the entire composite structure in which these hard and soft phases are combined is a bainitic ferrite (bainite) phase.

[Tempering (T) in the temperature range of 450 ~ 700 ℃]

The tempering treatment lowers the strength but is effective in lowering the yield ratio YR, improving toughness, and lowering the hardness of the surface portion. In that case, when tempering temperature is 450-700 degreeC, excessive fall of strength can be suppressed, appropriate yield ratio YR and toughness can be obtained, and surface hardness can be reduced. If tempering temperature is less than 450 degreeC, toughness improvement and surface hardness fall will not be enough. On the other hand, when tempering temperature exceeds 700 degreeC, desired intensity | strength (TS, YS) cannot be obtained.

[Formed into round steel pipe by press bending method]

Finally, the steel sheet is cold bent by the press bending method to obtain a steel pipe. As described above, circular steel pipes are manufactured by the UOE molding method if the sheet thickness is 30 mm or less as applied to the line pipe, but in circular steel pipes for building structures, when the sheet thickness is thick and the strength is high, the press bending method ( That is, it is necessary to shape | mold to round steel pipe by press bending process. In the application of this method, since the strong processing of D / t: 10-20 is performed, the surface bending process deformation is large and the work hardening of a surface becomes large. Therefore, a round steel pipe with low surface hardness can be manufactured by press-bending using the steel plate manufactured as mentioned above.

[Heat treatment of round steel pipe]

After forming into a round steel pipe, SR heat treatment may or may not be performed. According to the method of the present invention, since the high strength, the YR is low, and the hardness distribution uniformity in the steel pipe thickness direction is excellent, the SR heat treatment is not necessarily performed. However, when a strong bending process of about D / t ≦ 15 is performed, since YR may exceed 90%, SR heat treatment can be performed, and the heat treatment temperature is in the range of 350 to 650 ° C. There is no YR reduction effect below 350 degreeC. On the other hand, when it exceeds 650 degreeC, the fall of YR and TS is large, and the intensity | strength of 780 Mpa or more cannot be ensured.

Hereinafter, although an Example demonstrates this invention more concretely, this invention is not restrict | limited to the following example of course, Of course, it is possible to add and implement in the range which may be suitable for the meaning of the previous and the later. They are all included in the technical scope of the present invention.

Example

Example 1

After the steel of the chemical composition shown in Tables 1 and 2 was melted by a conventional solvent method to form a steel strip, hot rolling, accelerated cooling (cooling after rolling), two-phase reverse quenching, The steel sheet was manufactured by tempering. Using the obtained steel plate, it formed into the round steel pipe by the press bending method. In addition, Table 1, 2 also showed the PCM value prescribed | regulated by the said Formula (1). The manufacturing conditions at this time are as follows.

[Production conditions]

River No. About 1-22, 24-35, and 37-60, after heating a slab to 1150 +/- 50 DEG C, hot rolling is carried out with the finish rolling temperature (surface temperature) in the range of 900 +/- 30 DEG C, and the plate thickness is 60 mm. Next, the cooling rate at the position of t / 4 (t: sheet thickness) was controlled to 5-25 degrees C / sec, and the surface temperature at the time of cooling stop was 250 degrees C or less. Further, the quenching treatment was performed at a two-phase inverse heat treatment temperature of 700 to 850 ° C, tempered at a temperature range of 450 to 650 ° C to form a steel sheet, and then formed into a round steel pipe by a press bending method using the obtained steel sheet. The bending degree at this time is D / t of 10 (t / D = 0.1) when the diameter of the round steel pipe is D (mm) and the steel sheet thickness is t (mm).

Meanwhile, the river No. About 62-64, the following conditions were changed and the steel plate was produced and shape | molded by the round steel pipe. River No. 62 quenched by making two-phase reverse heat processing temperature into 930 degreeC among the said conditions, and made tempering temperature into 400 degreeC. River No. 63 rolled the finishing rolling temperature to 750 degreeC among the said conditions, and made the reheating (Q ') temperature after cooling into 680 degreeC. River No. 64 remained the same as having performed the two-phase reverse heat processing among the said conditions, and did not perform subsequent tempering. Molding from these steel sheets into round steel pipes was performed using steel no. It carried out by D / t = 10 similarly to 1-22, 24-35, and 37-60.

For each obtained round steel pipe, the microstructure (area fraction of each phase) and hardness of the steel pipe were evaluated by the following method, and the materials (yield strength YS, tensile strength TS, yield ratio YR and toughness v E- ₂₀ ) and weldability Was evaluated by the following method.

[Measurement method of microstructure and hardness]

By measuring the area fractions of the bainitic ferrite phase and martensite phase by image analysis of the microstructure, the Vickers hardness (Hv ₀ ) and the Vickers hardness Hv ₁ in the center portion of the steel sheet were measured (load: 98N), and the hardness The ratio (Hv ₀ / Hv ₁ ) was obtained. The hardness Hv ₀ and the hardness Hv ₁ at this time were measured at intervals of 2 mm in the thickness direction, and the average value thereof was obtained (for example, the Vickers hardness Hv ₀ at the surface layer portion was an average value of hardness from each of the front and rear surfaces to a depth of 2 mm. Becomes).

[Evaluation method of yield strength YS, tensile strength TS]

JIS Z 2201 No. 4 specimens are taken from the outer surface side of the round steel pipe in the tube axis direction (t corresponds to the main rolling direction of the steel sheet) in the t / 4 portion of the steel sheet (t is the thickness of the sheet). The tensile test was carried out, and the yield stress YS (upward yield point YP or 0.2% yield strength sigma _0.2 ), tensile strength TS, and yield ratio YR (yield stress YS / tensile strength TS) of the steel pipe were measured. The acceptance criterion is an average value in two times, and the yield stress YS: 630 MPa or more, the tensile strength TS: 780 to 930 MPa, and the yield ratio YR: 90% or less.

[Toughness Evaluation Method]

JIS Z 2204 notch impact specimens are taken from the outer surface side of the round steel pipe in the tube axis direction (t is the thickness of the steel plate constituting the steel pipe) in the t / 4 portion (t is the thickness of the steel plate constituting the steel pipe). The Charpy impact test was carried out in accordance with 2242 (average value of three tests), and the average absorbed energy vE- ₂₀ at temperature: -20 ° C was measured. This average absorption energy vE- ₂₀ made 47J or more pass.

[Welding property (welding crack property)]

In accordance with the maximum hardness test of the welded heat affected zone (HAZ) specified in JIS Z 3101, the weld bead is placed on the outer surface side of the circular steel pipe, the presence of surface cracks by the penetration test, and the internal cracks by the ultrasonic test. Investigation was made.

The micro composition and hardness distribution (hardness and hardness ratio in the center of the steel sheet) of the steel sheet are shown in Tables 3 and 4 below to evaluate the materials (yield stress YS, tensile strength TS, yield ratio YR and toughness vE- ₂₀ ) and weldability. It shows in Tables 5 and 6. In addition, in Tables 5 and 6, the maximum hardness (Hv) of the HAZ is shown as “welding property”.

From these results, it can consider as follows. First of all, the river No. 1-22 and 24-32 (Tables 1, 3, 5) satisfy | fill the requirements prescribed | regulated by this invention, and satisfy | fill the target value in all the characteristics (general evaluation: (circle)).

In contrast, steel No. 33-35, 37-60, and 62-64 (Table 2, 4, 6) do not satisfy | fill any one of the requirements prescribed | regulated by this invention, and at least one required characteristic is deteriorated (general evaluation × ).

[Example 2]

Steel No. shown in Table 1 above. The steel plate was manufactured by the various manufacturing conditions (DQ-Q'-T) shown to following Table 7 using the thing of 1-11 (thing whose chemical component composition satisfy | fills the range prescribed | regulated by this invention) (experiment No. 1-20). Using the obtained steel plate (plate thickness: 60 mm), it formed into the round steel pipe by the press bending method. About the obtained round steel pipe, material (yield stress YS, tensile strength TS, yield ratio YR, and toughness vE- ₂₀ ) and weldability were evaluated like Example.

In addition, experiment No. 12, 13 is the steel sheet heating temperature is outside the range specified in the present invention, Experiment No. 14 and 15 are those whose finish rolling temperature is out of the range prescribed by the present invention. 15, 16 is the cooling rate is out of the range specified in the present invention, Experiment No. 17 indicates that the cooling stop temperature is out of the range defined by the present invention. 18 and 19 are quenching temperature (heating temperature at the time of quenching) out of the range prescribed | regulated by this invention, Experiment No. 20 and 21 respectively indicate that the tempering temperature is out of the range defined by the present invention.

As is apparent from these results, it can be seen that, in order to obtain a round steel pipe that satisfies the requirements specified in the present invention, manufacturing conditions must also be appropriately controlled.

Claims (2)

Round steel pipe, C: 0.01 to 0.06% (meaning of mass%, the same below), Si: 0.10 to 0.40%, Mn: 1.60 to 2.50%, Al: 0.025 to 0.090%, Cu: 0.15 to 0.70%, Ni: 0.90 to 1.60% , Cr: 0.50 to 1.35%, Mo: 0.10 to 0.30%, Ti: 0.008 to 0.025%, B: 0.0005 to 0.0025%, N: 0.0030 to 0.0060% and Ca: 0.0005 to 0.0040%, respectively, and the balance is iron And inevitable impurities, Among the unavoidable impurities, P: 0.012% or less, S: 0.005% or less and O: 0.0040% or less, respectively, The PCM value represented by the following Equation 1 is 0.30% or less, In addition, a round steel pipe satisfying the requirements of the following (A) to (C). [Equation 1] PCM value = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + ( (B) × 5) (However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu, Ni, Contents (mass%) of Cr, Mo, V, and B are shown.) (A) The average Vickers hardness Hv of the center portion excluding the surface layer portion up to 2 mm deep from each of the front and back surfaces of the steel pipe is 230 to 310, (B) In the microstructure of the steel pipe, the fraction of bainitic ferrite phase is 80 area% or more, and the fraction of martensite phase is 5 area% or less, (C) The average Vickers hardness Hv of the surface layer part from each of the front and back surfaces of a steel pipe to 2 mm in depth is 1.3 times or less of the average Vickers hardness Hv of the said center part. A method for producing the circular steel pipe according to claim 1, wherein the slab made of the chemical component is heated to 950 to 1200 ° C, followed by hot rolling with a finish rolling temperature in the range of 800 to 930 ° C. It is set as the plate | board thickness of and then water-cooled until the cooling rate in the position of t / 4 (t: plate | board thickness) is 2-25 degreeC / sec, and surface temperature becomes 350 degrees C or less, and then temperature: 700- A method for producing a round steel pipe, which is reheated in a range of 900 ° C. to be quenched, tempered at a temperature in the range of 450 ° C. to 700 ° C. to form a steel sheet, and formed into a round steel pipe by a press bending method using the obtained steel sheet.