JP2009161811A - Steel sheet for low yield ratio high strength steel pipe, method for producing the same, and low yield ratio high strength steel pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for a low yield ratio high strength steel pipe which can stably attain the yield stress YS of ≥650 MPa and the yield ratio YR of ≤90% in the base metal of a steel pipe subjected to cold forming, to provide a method for producing the same, and to provide a low yield ratio high strength steel pipe obtained by subjecting the steel sheet to cold forming. <P>SOLUTION: Disclosed is a steel sheet for a low yield ratio high strength steel pipe having a componential composition comprising, by mass, 0.03 to 0.10% C, 0.05 to 0.50% Si, 1.4 to 3.0% Mn, ≤0.02% P, ≤0.0050% S, ≤0.1% Al and ≤0.0070% N, and further comprising one or two kinds selected from 0.1 to 1.0% Cu and 0.1 to 2.0% Ni, and having a steel structure composed of a mixed structure consisting of bainite having a Vickers hardness Hv of 200 to 330 and insular martensite having a volume fraction of 5 to 20% and a Vickers hardness Hv of 450 to 650. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a steel plate for steel pipes having high strength, low yield ratio and high toughness used for building structures and the like, a manufacturing method thereof, and a low yield ratio high strength steel pipe using the steel plate. Here, the high strength in the present invention is a yield stress YS of 650 MPa or more, a low yield ratio is a yield ratio YR of 90% or less, and a high toughness is a Charpy absorbed energy vE _{0 at} 0 ° C. of 100 J or more. Means that. The steel plate in the present invention means a thick steel plate having a thickness of 9 mm or more.

  In recent years, with the increase in size and span of building structures and the like, the steel materials used for them are being made thicker and stronger. In addition, these steel materials are required to have a high allowable stress and a low yield ratio from the viewpoint of ensuring the safety of steel structures against earthquakes.

  If the yield ratio of the steel material is lowered, even if a load higher than the yield stress is applied, the stress allowed until failure increases and the uniform elongation also increases, so that the steel material has excellent plastic deformability. However, in a high-strength steel material having a tensile strength TS exceeding 780 MPa, a large amount of alloy components are added to ensure strength, so that the yield ratio increases and ductility and toughness tend to decrease accordingly.

  In recent years, the use of steel pipe columns having a circular cross section has been expanded from the viewpoint of emphasizing the design of building structures and from the viewpoint of increasing the degree of freedom in structural design, such as beam mounting. As a method of manufacturing a circular steel pipe, a method of manufacturing by a centrifugal casting method and a method of manufacturing a thick steel plate by cold forming are generally used, but the steel tube obtained by the latter method is more important in terms of earthquake resistance. In addition to its excellent fracture toughness characteristics, it is also advantageous in terms of productivity, dimensional accuracy, and weldability.

  Further, in the case of a method of manufacturing a steel sheet by cold forming, as an index of the amount of strain that the steel sheet undergoes during pipe forming, generally, when the thickness of the material is t and the outer diameter of the steel pipe is D, (t / D) A tube thickness ratio represented by 100 (%) is used. As the value of (t / D) increases, the work hardening amount increases and the yield ratio YR in the steel pipe axial direction increases, resulting in a decrease in earthquake resistance. Therefore, in order to obtain a steel pipe having a low yield ratio, it is necessary to sufficiently reduce the yield ratio of the material steel plate before pipe making in consideration of the work hardening amount by cold forming.

  Conventionally, steel sheets having a low yield ratio and high strength generally have a microstructure in which ferrite is the main component and bainite or martensite is dispersed as a hard second phase. Therefore, when the steel sheet has a large volume fraction of ferrite, the base material properties of the steel pipe after warm forming are low yield with high yield stress YS of 650 MPa or more and yield ratio YR of 90% or less. There was a drawback that it was difficult to make the ratio.

Therefore, in order to stably obtain high strength steel pipes with low yield ratio and high toughness, development of steel pipe materials (steel plates) with lower yield ratio, high strength and high toughness is required. Various proposals have been made. For example, Patent Document 1 discloses that a steel pipe material thick steel plate containing 1 to 15% of bainite and martensite in a ferrite structure having an average particle diameter of 20 μm or more is cold-formed to form a steel pipe. Techniques for achieving strength and low yield ratio are described. Patent Document 2 describes a technique in which cold bending workability is improved while maintaining a yield stress of 650 MPa or more by setting the area ratio of the bainite structure to 90% or more. Patent Document 3 describes a technique for achieving a high strength and a low yield ratio even after cold forming by controlling the chemical composition within an appropriate range and setting the martensite ratio of the material to 80% or more. ing. Furthermore, Patent Document 4 and Patent Document 5 describe a technique for achieving high strength and a low yield ratio by reheating and normalizing a steel pipe after cold forming.
JP 2007-039811 A JP 2007-100190 A JP 2006-283117 A Japanese Patent Laid-Open No. 06-128641 Japanese Patent Laid-Open No. 06-049541

  However, in the technique described in Patent Document 1, since the volume fraction of ferrite is high, it is difficult to stably achieve a yield stress YS: 650 MPa or more after pipe forming. Further, in the techniques described in Patent Document 2 and Patent Document 3, it is difficult to achieve a yield ratio YR: 90% or less after pipe forming by work hardening by cold forming during pipe forming, There is concern about a decrease in toughness. Furthermore, in the techniques described in Patent Document 4 and Patent Document 5, since it is necessary to normalize the steel pipe after cold forming at a high temperature, the surface properties of the steel pipe are reduced and the manufacturing cost is increased. There are problems such as extension of lead time.

  Therefore, an object of the present invention is for a high toughness low yield ratio high strength steel pipe that can stably achieve a yield stress YS of a base metal in a cold formed steel pipe of 650 MPa or more and a yield ratio YR of 90% or less. An object of the present invention is to provide a steel plate and a manufacturing method thereof, and a low yield ratio high strength steel pipe obtained by cold forming the steel plate.

  In order to achieve the above-mentioned problems, the inventors have studied various factors affecting the strength, yield ratio and toughness of steel pipes manufactured by cold forming, and the prior art is brittle and reduces the ductility and toughness of the base metal. Focusing on the island martensite that was not actively used, we further researched it. As a result, in steel pipes manufactured by cold forming, in order to stably achieve the base material yield stress YS: 650 MPa or more and the yield ratio YR: 90% or less, the component composition of the steel plate used as the material of the steel pipe is appropriate. We found that it is important to control the volume ratio and hardness of each steel sheet within the proper range by controlling the volume ratio and hardness of the steel sheet to a microstructure consisting of a mixed structure of bainite and island martensite. . In addition, in order to obtain a steel sheet having the above microstructure, after the hot rolling of the steel material whose component composition is controlled within an appropriate range, a cooling treatment is performed in which the cooling rate and the cooling stop temperature are optimized. It has been found that it is important to perform a reheating process in which the temperature rate and the reheating temperature are optimized. The present invention has been made by further studying the above findings.

  That is, the present invention is C: 0.03-0.10 mass%, Si: 0.05-0.50 mass%, Mn: 1.4-3.0 mass%, P: 0.02 mass% or less, S: 0 .0050 mass% or less, Al: 0.1 mass% or less, N: 0.0070 mass% or less, and Cu: 0.1 to 1.0 mass%, Ni: 0.1 to 2.0 mass% Bainite containing seeds or two kinds, the balance being composed of Fe and inevitable impurities, Vickers hardness Hv of 200 to 330, volume fraction of 5 to 20% and Vickers hardness Hv of 450 A steel plate for a low-yield-ratio high-strength steel pipe, characterized by having a microstructure composed of a mixed structure with 650 island martensite.

  In addition to the above component composition, the steel sheet of the present invention further includes Cr: 1.0 mass% or less, Mo: 1.0 mass% or less, Nb: 0.1 mass% or less, V: 0.2 mass% or less, Ti: 0 0.03 mass% or less and B: 0.005 mass% or less, or one or more selected from Ca, 0.005 mass% or less, REM: 0.02 mass% or less, and Mg: 0.005 mass% or less 1 type or 2 types or more chosen from among these are contained.

Further, the present invention is a steel slab having the above component composition is heated to 1000 to 1250 ° C., after completion of the hot rolling at 800 ° C. or higher temperatures, Ar ₃ transformation point -50 ° C. from Ar ₃ transformation point or more of the temperature -Ar ₃ transformation point -cooled to a temperature of 250 ° C. at a cooling rate of 5-100 ° C./sec, and then reheated to a temperature of 600 ° C.-Ac ₁ transformation point at a heating rate of 0.5 ° C./ses or higher. And a method of manufacturing a steel plate for a low yield ratio high strength steel pipe, characterized by air cooling.

  The production method of the present invention is characterized in that the air-cooled steel sheet is further tempered at a temperature of 400 to 600 ° C.

Further, the present invention relates to the above steel plate or the steel plate obtained by the above method with the following formula (1):
t / D × 100 ≦ 10 (%) (1)
Where t: plate thickness (mm), D: steel pipe outer diameter (mm)
The steel pipe is cold formed so as to satisfy the above relationship, and has a yield strength YS of 650 MPa or more and a yield ratio YR of 90% or less.

  According to the present invention, since the yield stress YS of the steel pipe base material formed by cold forming is 650 MPa or more, a low yield ratio high strength steel pipe having a yield ratio of 90% or less can be stably produced. This can greatly contribute to the increase in the size of steel structures, the improvement of earthquake resistance, and the improvement of construction efficiency.

First, the microstructure which is a feature of the steel plate for low yield ratio and high strength steel pipe of the present invention will be described.
The microstructure of the steel sheet of the present invention is characterized by mainly containing bainite and containing island martensite as a hard second phase. Here, the bainite needs to have a Vickers hardness Hv in a range of 200 to 330. If the bainite hardness Hv is less than 200, it is not possible to stably obtain a yield stress of the steel pipe base material after cold forming: 650 MPa or more. On the other hand, if the Hv exceeds 330, the steel pipe base material after cold forming is obtained. This is because the ductility and toughness of the steel deteriorate. Preferably, the hardness of bainite is in the range of Hv: 210-320.

  On the other hand, the island-shaped martensite needs to have a Vickers hardness Hv in the range of 450 to 650 and a fraction of 5 to 20% in terms of volume fraction. Insular martensite is harder than the parent phase (bainite) because C is concentrated, and the tensile strength TS of the base material is increased, while the dislocation density is high. This is an indispensable structure for realizing both high strength and low yield ratio characteristics. However, if the hardness Hv of the island martensite is less than 450, it is difficult to make the yield ratio of the steel pipe base material after cold forming 90% or less. On the other hand, when Hv exceeds 650, the ductility and toughness of the steel pipe base material after cold forming are lowered. Therefore, the hardness Hv of the island martensite is in the range of 450 to 650. Preferably, it is the range of Hv: 470-630.

  Furthermore, if the fraction of island martensite is less than 5% in terms of volume fraction, the steel pipe base material after pipe forming cannot have the characteristics of having both high strength and low yield ratio, while 20% If it exceeds, ductility and low-temperature toughness of the steel pipe base material will decrease. Therefore, the fraction of island martensite is in the range of 5 to 20% in volume fraction. Preferably, it is 6 to 18% of range.

  The volume fraction of the island-like martensite is obtained by subjecting a thickness cross section in the rolling direction of the steel sheet sample to repeller corrosion (JOURNAL OF METALS, March, 1980, p. 38-39) and a magnification of 1000 times using an optical microscope. It can be obtained by analyzing the image of the tissue photograph observed and photographed, obtaining the area fraction, and regarding this as the volume fraction.

Moreover, the hardness of bainite and island-like martensite is the load of 9.8 × 10 ⁻³ N by using a micro Vickers hardness meter to determine the hardness of each phase at 1/2 t of the rolling section of the steel sheet sample. Under the conditions of (1 gf) to 2.94 × 10 ⁻¹ N (30 gf), preferably 4.9 × 10 ⁻² N (5 gf), at least 10 grains of each phase are measured, and the average value is determined for each phase. Say it. Under these conditions, errors due to test conditions can be ignored.

In addition, when the structure | tissue of a bainite and an island-like martensite is very fine and cannot measure using a micro Vickers hardness meter, you may use the nanoindentation method (indentation method). In this case, the indentation hardness is a load of 4.9 × 10 ⁻⁴ N (50 mgf) to 4.9 × 10 ⁻³ N (500 mgf), preferably 9.8 × 10 ⁻⁴ N (100 mgf). H _IT is measured, and from this value, the following equation (2);
Hv = 0.91H _IT (2)
Here, H _IT : It can convert into Vickers hardness Hv using the indentation hardness by a nanoindentation method.

  In the microstructure of the steel sheet of the present invention, the balance other than the bainite and island martensite is made of pearlite and cementite. However, since these tissues cause a decrease in strength, the volume fraction is preferably 5% or less. This is because if the volume fraction is 5% or less, the adverse effect on the steel sheet strength can be almost ignored.

Next, the component composition that the steel sheet of the present invention should have will be described.
C: 0.03-0.10 mass%
C has a large effect of increasing the strength of steel, and is an element necessary for ensuring the strength necessary for a structural steel material. If C is less than 0.03 mass%, the island-like martensite volume fraction cannot be 5% or more and the hardness of the island-like martensite Hv is 450 or more. Therefore, the yield strength of the steel pipe base material after cold forming is 650 MPa or more. And a yield ratio of 90% or less cannot be achieved. On the other hand, if C exceeds 0.10 mass%, it becomes impossible to make the volume fraction of island martensite 20% or less and the hardness of island martensite Hv650 or less, and toughness of the steel pipe base material and the welded portion Reduce or degrade weld crack resistance. Therefore, C is set to a range of 0.03 to 0.10 mass%. Preferably, it is the range of 0.04-0.09 mass%.

Si: 0.05-0.50 mass%
Si is an element added as a deoxidizer and for increasing the strength of steel, and it is necessary to add 0.05 mass% or more. However, if added over 0.50 mass%, the toughness of the steel pipe base material and the welded portion is lowered, and the weldability is also lowered. Therefore, Si is set to a range of 0.05 to 0.50 mass%. Preferably, it is 0.10 to 0.45 mass%.

Mn: 1.4 to 3.0 mass%
Mn increases the strength of steel and stabilizes austenite. Therefore, it promotes the formation of island martensite, yield stress of 650 MPa or more and yield ratio of 90% or less of the steel pipe base material after cold forming. It is an element necessary to achieve this. In order to obtain such an effect, it is necessary to add 1.4 mass% or more of Mn. On the other hand, if added over 3.0 mass%, the toughness of the base material and the toughness of the weld heat affected zone are lowered. Therefore, Mn is in the range of 1.4 to 3.0 mass%. Preferably, it is 1.5 to 2.8 mass%.

P: 0.02 mass% or less P has an effect of increasing the strength of steel, but is also a harmful impurity element that lowers toughness. Therefore, it is desirable to reduce it as much as possible. In particular, when P exceeds 0.02 mass%, this effect becomes significant, so the upper limit is made 0.02 mass%. On the other hand, excessive reduction leads to an increase in refining costs, so the lower limit is preferably about 0.005 mass%.

S: 0.0050 mass% or less S is a harmful impurity element that lowers the low-temperature toughness of the steel material, and is desirably reduced as much as possible. In particular, when S is contained in an amount exceeding 0.0050 mass%, the above-described adverse effect becomes remarkable. Therefore, S is set to 0.0050 mass% or less.

Al: 0.1 mass% or less Al is an element added as a deoxidizer, and is most commonly used in a molten steel deoxidation process of high-strength steel. Further, N in the steel is fixed as AlN, which contributes to improvement of the toughness of the steel pipe base material. Such an effect is recognized by addition of Al: 0.005 mass% or more. However, if added in excess of 0.1 mass%, the toughness of the base material is reduced and mixed with the weld metal during welding, so that the toughness of the welded portion is reduced. Therefore, Al is 0.1 mass% or less. Preferably, it is the range of 0.01-0.07 mass%.

N: 0.0070 mass% or less N is an element contained in steel as an unavoidable impurity. In particular, when it exceeds 0.0070 mass%, the toughness of the base material and the welded portion decreases. . Therefore, N is set to 0.0070 mass% or less.

In addition to the said component, the steel plate of this invention needs to contain further 1 type or 2 types chosen from Cu and Ni in the following range.
Cu: 0.1 to 1.0 mass%
Cu has a large solid solution strengthening ability and contributes to an increase in strength of the base material. Moreover, since austenite is stabilized and hardenability is improved by preferentially concentrating in austenite, it contributes to the stable formation of island martensite. Therefore, Cu is an important element in achieving both high strength and a low yield ratio. In order to obtain such an effect, it is necessary to add Cu by 0.1 mass% or more. However, if it exceeds 1.0 mass%, it causes hot brittleness and deteriorates the surface properties of the steel sheet. Therefore, Cu is added in the range of 0.1 to 1.0 mass%. Preferably, it is the range of 0.2-0.7 mass%.

Ni: 0.1 to 2.0 mass%
Ni, like Cu, is an important element in the present invention, which stabilizes austenite and enhances hardenability by preferentially concentrating in austenite and contributes to stable formation of island martensite. It is a useful component for achieving both strength and low yield ratio. Moreover, it is a component which has the effect of strengthening a base material by solid solution strengthening and improves low-temperature toughness. In order to obtain such an effect, it is necessary to add 0.1 mass% or more of Ni. However, even if added over 2.0 mass%, the effect is saturated, an effect commensurate with the amount added cannot be obtained, and the raw material cost only rises. Therefore, Ni is set to a range of 0.1 to 2.0 mass%. Preferably it is the range of 0.2-1.7 mass%.

In order to improve the strength of the steel, the steel plate of the present invention can be added with one or more selected from Cr, Mo, V, Nb, Ti and B in addition to the above basic components.
Cr: 1.0 mass% or less Cr is an effective component for improving the strength of steel. To obtain this effect, it is preferable to add 0.05 mass% or more. However, since addition exceeding 1.0 mass% reduces the toughness of a base material and a welding part, it is preferable to set it as 1.0 mass% or less.

Mo: 1.0 mass% or less Mo is an effective component for improving the strength of steel. To obtain this effect, it is preferable to add 0.05 mass% or more. However, since addition exceeding 1.0 mass% reduces the toughness of a base material and a welding part, it is preferable to set it as 1.0 mass% or less.

V: 0.2 mass% or less V is an effective component for improving the strength of steel. To obtain this effect, it is preferable to add 0.01 mass% or more. However, since addition exceeding 0.2 mass% reduces the toughness of a base material and a welding part, it is preferable to set it as 0.2 mass% or less.

Nb: 0.1 mass% or less Nb is an effective component for improving the strength of steel. To obtain this effect, 0.005 mass% or more is preferably added. However, since addition exceeding 0.1 mass% reduces the toughness of a base material and a welding part, it is preferable to set it as 0.1 mass% or less.

Ti: 0.03 mass% or less Ti has a strong affinity for N, and precipitates as TiN during solidification, thereby contributing to increasing the toughness of the weld. However, if it exceeds 0.03 mass%, the toughness of the base material decreases, so 0.03 mass% or less is preferably added.

B: 0.005 mass% or less B has an effect of increasing the strength of steel through the improvement of hardenability. However, addition exceeding 0.005 mass% remarkably increases the hardenability and brings about a decrease in the toughness and ductility of the base material. Therefore, B is preferably added in an amount of 0.005 mass% or less.

The steel plate of the present invention can further contain one or more selected from Ca, REM, and Mg in addition to the above components for the purpose of improving toughness.
Ca: 0.005 mass% or less Ca has an effect of improving toughness through refinement of crystal grains. In order to obtain this effect, 0.001 mass% or more is preferably added. However, even if added over 0.005 mass%, the effect is saturated. Therefore, Ca is preferably added in an amount of 0.005 mass% or less.

REM: 0.02 mass% or less REM, like Ca, has the effect of improving toughness through crystal grain refinement. In order to obtain this effect, 0.002 mass% or more is preferably added. However, the effect is saturated even if added over 0.02 mass%. Therefore, it is preferable to add REM 0.02 mass% or less.

Mg: 0.005 mass% or less Mg, like Ca, has an effect of improving toughness through refinement of crystal grains. In order to obtain this effect, 0.001 mass% or more is preferably added. However, even if added over 0.005 mass%, the effect is saturated. Therefore, Mg is preferably added in an amount of 0.005 mass% or less.

Furthermore, in the steel plate of the present invention, the above component has the following formula (3):
Ceq (mass%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (3)
Here, each element symbol is the content of each element (mass%)
It is preferable to contain so that the carbon equivalent Ceq defined by may exceed 0.47 mass%. This is because by making Ceq more than 0.47 mass%, the yield stress YS of the steel pipe base material after cold forming can be more stably increased to a high strength of 650 MPa or more.

  In the steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

Next, the manufacturing method of the steel plate of this invention is demonstrated. In addition, the temperature in subsequent description is a temperature of 1/2 sheet thickness of a steel plate.
Heating temperature of steel material: 1000 to 1250 ° C. Steel having the above-described composition is melted using conventional methods such as converters, electric furnaces, vacuum melting furnaces, etc., and is continuously cast or ingot-bundled. A steel material (slab) is formed by the method, and this steel material is heated to a temperature of 1000 to 1250 ° C. and hot-rolled. Here, if the heating temperature is less than 1000 ° C., the deformation resistance in hot rolling becomes high, and the amount of reduction per pass cannot be increased, so the number of rolling passes increases and the rolling efficiency decreases, There is a possibility that casting defects existing in the slab cannot be crimped. On the other hand, when the heating temperature exceeds 1250 ° C., the amount of scale generated during heating increases and surface traces are likely to occur, and the care load on the steel sheet surface after rolling increases. Therefore, the heating temperature of the steel material is in the range of 1000 to 1250 ° C.

Hot rolling The heated steel material is then hot rolled to a steel plate having a thickness of 9 mm or more. At this time, the hot rolling requires the rolling end temperature to be 800 ° C. or higher. If rolling end temperature is less than 800 degreeC, since a deformation resistance is high and a rolling load increases, the burden of a rolling mill will increase. There is no restriction | limiting in particular about other hot rolling conditions, What is necessary is just to be able to satisfy | fill predetermined plate | board thickness and shape. In addition, in the case of a very thick steel plate having a plate thickness exceeding 80 mm, it is desirable to perform rolling at a rolling reduction of 15% or more for at least one pass or more in order to press-bond zaku defects inside the steel material. Moreover, in the case of an extremely thick steel plate, it is necessary to wait in the middle of rolling in order to lower the rolling temperature, which hinders productivity. Also from this viewpoint, the rolling end temperature is set to 800 ° C. or higher.

Steel sheet has finished cooling conditions hot rolling after the hot rolling is from Ar ₃ transformation point or more of the temperature cooled to stop temperature of (Ar ₃ transformation point -250 ° C. to Ar ₃ transformation point -50 ° C.), 5 to 100 It is necessary to perform accelerated cooling at an average cooling rate of ° C / sec. In the production method of the present invention, the cooling stop temperature is a particularly important control factor. When the cooling stop temperature becomes lower than (Ar ₃ transformation point −250 ° C.), the bainite Vickers hardness satisfies Hv 330 or less. In addition, the amount of retained austenite at the time of cooling stop is insufficient, and the fraction of island-like martensite generated from the retained austenite during subsequent reheating and air cooling cannot be made 5% or more in volume fraction. As a result, the yield ratio of the steel pipe base material after cold forming cannot be made 90% or less. On the other hand, when the cooling stop temperature becomes higher than (Ar ₃ transformation point −50 ° C.), not only does it not satisfy the Vickers hardness Hv of 200 or higher of bainite, but also the diffusion of C to the residual austenite does not proceed, so the Vickers hardness Hv450 The above island-like martensite is not generated, and the yield stress of the steel pipe base material after cold forming cannot be realized at 650 MPa or more and a yield ratio of 90% or less. In addition, when the cooling rate after the hot rolling is less than 5 ° C./sec, the microstructure after cooling becomes a structure mainly composed of ferrite. Therefore, the yield stress of the steel pipe base material obtained by cold forming is set to 650 MPa or more. I can't do that. On the other hand, when the cooling rate exceeds 100 ° C./sec, temperature unevenness occurs due to the position in the steel sheet, and uniform temperature control becomes difficult, so that the variation in material becomes large.

Reheating treatment After the accelerated cooling is finished, the steel plate is temporarily stopped from cooling, and then reheated to a temperature range of 600 ° C to Ac ₁ transformation point at a temperature rising rate of 0.5 ° C / sec or more, and then air-cooled. It is necessary to apply.
This is because, as described above, after the end of hot rolling, the temperature range from the Ar ₃ transformation point or higher to the temperature of (Ar ₃ transformation point −250 ° C. to Ar ₃ transformation point −50 ° C.) is 5 to 100 ° C. / When accelerated cooling is performed in sec, the steel sheet structure immediately after cooling is a microstructure in which retained austenite is finely dispersed in a bainite-based structure. However, after that, by reheating to a temperature range from 600 ° C. to Ac ₁ transformation point at a rate of 0.5 ° C./sec or more and air cooling, C diffuses and concentrates in finely dispersed residual austenite. In this way, island-like martensite is generated, and the microstructure of the present invention, which is composed of a mixed structure of bainite and island-like martensite, is obtained. As a result, the steel pipe preform after warm forming can have a yield stress of 650 MPa or more and a yield ratio of 90% or less.

When the heating rate in the reheating is less than 0.5 ° C./sec, pearlite transformation occurs, so that island martensite is not generated, and the yield stress of the steel pipe base material after cold forming is 650 MPa or more, or The yield ratio of the steel pipe base material after the hot forming cannot be made 90% or less. Moreover, since the time to reheat to the target temperature becomes long, manufacturing efficiency falls. In addition, when the reheating temperature is less than 600 ° C., the concentration of C in the retained austenite is delayed, so that island martensite having a volume fraction of 5% or more and a Vickers hardness of Hv450 or more is not generated. The yield stress of the steel pipe base material cannot be 650 MPa or more and the yield ratio cannot be 90% or less. On the other hand, when the reheating temperature exceeds the Ac ₁ transformation point, the bainite is softened, the bainite Vickers hardness Hv does not become 200 or more, and the yield stress of the steel pipe base material after cold forming may be 650 MPa or more. become unable. In addition, in order to promote the spreading | diffusion of C to a retained austenite, it is preferable that the said reheating temperature shall be 100 degreeC or more higher than a cooling stop temperature. The holding time at the reheating temperature is preferably about 15 minutes or less in order not to impair productivity. As a reheating method, heating by an atmospheric furnace, heating by a gas flame, induction heating, or the like can be used, but in consideration of economy, controllability, etc., induction heating is preferable.

The Ar ₃ transformation point and the Ac ₁ transformation point both have a correlation with the component composition, and can be obtained by the following formulas (4) and (5).
Ar ₃ transformation point (° C.) = 868-396C + 25Si-68Mn-21Cu-36Ni-25Cr-30Mo (4)
Ac ₁ transformation point (° C.) = 751-27C + 18Si-12Mn-23Cu-23Ni + 24Cr + 23Mo-40V-6Ti + 233Nb-169Al-895B (5)
Here, each said element symbol means each content (mass%).

  In addition, after the steel plate of this invention is cooled to room temperature after the said reheating process, it may further reheat to the temperature of 400-600 degreeC, and may give a tempering process. This tempering treatment can further improve toughness. By this tempering process, the microstructure becomes bainite and tempered island martensite, but if the hardness of the island martensite is sufficiently higher than the parent phase bainite, high strength and low yield are achieved. The characteristic which combines the ratio can be obtained. In order to obtain such a tempering effect, it is preferable to reheat to 400 ° C. or higher. However, heating exceeding 600 ° C. causes a decrease in strength. Therefore, the temperature of the tempering treatment is preferably in the range of 400 to 600 ° C.

Next, the manufacturing method of the low yield ratio high strength steel pipe using the steel plate of this invention is demonstrated.
The low yield ratio high strength steel pipe of the present invention is obtained by cold forming the steel sheet obtained as described above, and the thickness t (mm) and the steel pipe outer diameter D (mm) are expressed by the following formula (1):
t / D × 100 ≦ 10 (%) (1)
It is characterized by the fact that the steel pipe has a dimension that satisfies the relational expression. The reason why the above formula (1) needs to be satisfied is that when t / D × 100 exceeds 10%, the steel plate of the material is work-hardened, the ductility and toughness are reduced, and the yield ratio is 90% or less. It is because it becomes impossible.

  As described above, a steel material that satisfies the above component composition is made into a steel sheet that satisfies the above hot rolling conditions, cooling conditions and reheating, air cooling conditions, or further satisfies the above tempering conditions to form a steel sheet. A steel sheet having a microstructure composed of a mixed structure of bainite and island martensite having a fraction can be obtained. And by cold-forming this steel sheet into a steel pipe, a low-yield-ratio high-strength steel pipe having the characteristics that the steel pipe base material has a yield stress of 650 MPa or more and a yield ratio of 90% or less can be obtained.

Steel with the composition shown in Table 1 is melted by a converter and ladle refining, and the steel is continuously cast into a steel material, then hot rolled under the conditions shown in Table 2 and accelerated cooled. Then, after reheating and holding for a certain period of time, it was air-cooled to obtain a steel plate having a thickness of 12 to 45 mm. Furthermore, about some steel plates, the tempering process was performed after that. A V-notch impact test piece was taken from the position of the steel sheet obtained as described above in accordance with the provisions of JIS Z2202 (1998), and Charpy impact was performed according to the provisions of JIS Z2242 (1998). The test was carried out, the absorbed energy (vE ₀ ) at 0 ° C. was measured, and the toughness of the base material was evaluated.
In addition, the obtained steel sheet was subjected to repeller corrosion in the rolling direction cross section of each steel sheet, the microstructure was observed at a magnification of 1000 times using an optical microscope, and the structure of the structure was examined. The volume fraction of martensite was obtained. Further, the hardness Hv of bainite and island martensite was measured by a micro Vickers hardness meter and a nanoindentation method (indentation method).

  Next, the steel sheet was cold-formed into a steel pipe having the dimensions (t / D) shown in Table 3, and then a JIS No. 12B tensile test piece was taken from the steel pipe in parallel with the pipe axis direction, and JIS Z2241 ( 1998), a tensile test was performed, the yield stress YS, the tensile strength TS, and the elongation El were measured, and the yield ratio YR was determined.

The measurement results are shown in Table 3. Inventive steel sheets suitable for the present invention are obtained by cold forming the steel sheets, both having a microstructure consisting of bainite and island martensite, and having a high toughness with Charpy absorbed energy vE ₀ exceeding 100 J. The steel pipe base metal has the characteristics of high strength, high ductility and low yield ratio with yield stress YS of 650 MPa or more, tensile strength TS of 780 MPa or more, yield ratio YR of 90% or less, and total elongation El of 16% or more. It will have.
On the other hand, the steel sheet of the comparative example that deviates from the conditions of the present invention has a microstructure that does not satisfy the conditions of the present invention, and as a result, the toughness of the steel sheet is inferior, or the yield stress of the base material after pipe forming One or more characteristics of YS and yield ratio YR do not satisfy the target characteristics.

  Since the low yield ratio high strength steel pipe of the present invention has high strength and low yield ratio and high toughness, it can be suitably used for building pipes, bridges, steel towers, offshore structures, and the like.

Claims (7)

C: 0.03-0.10 mass%,
Si: 0.05-0.50 mass%,
Mn: 1.4 to 3.0 mass%,
P: 0.02 mass% or less,
S: 0.0050 mass% or less,
Al: 0.1 mass% or less,
N: 0.0070 mass% or less,
It contains one or two of Cu: 0.1 to 1.0 mass%, Ni: 0.1 to 2.0 mass%, and the balance is composed of Fe and inevitable impurities, and has a Vickers hardness. Low yield ratio high, characterized by having a microstructure comprising a mixed structure of bainite having a Hv of 200 to 330 and an island martensite having a volume fraction of 5 to 20% and a Vickers hardness Hv of 450 to 650 Steel sheet for strength steel pipe. In addition to the above component composition, Cr: 1.0 mass% or less, Mo: 1.0 mass% or less, Nb: 0.1 mass% or less, V: 0.2 mass% or less, Ti: 0.03 mass% or less, and B: The steel plate for low yield ratio and high strength steel pipe according to claim 1, comprising one or more selected from 0.005 mass% or less. In addition to the above component composition, it further contains one or more selected from Ca: 0.005 mass% or less, REM: 0.02 mass% or less, and Mg: 0.005 mass% or less. The steel plate for low yield ratio and high strength steel pipes according to claim 1 or 2. The steel slab having the component composition according to any one of claims 1 to 3 is heated to 1000 to 1250 ° C, and after hot rolling is finished at a temperature of 800 ° C or higher, the temperature from Ar ₃ transformation point or higher to Ar ₃ transformation point −50 ° C. to Ar ₃ transformation point—cooled at a cooling rate of 5 to 100 ° C./sec to a temperature of 250 ° C., then 0.5 ° C./ses or more to a temperature of 600 ° C. to Ac ₁ transformation point A method for producing a steel plate for a low-yield-ratio high-strength steel pipe, which is reheated at a rate of temperature rise and air-cooled. The method for producing a steel plate for a low yield ratio high strength steel pipe according to claim 4, wherein the steel plate after air cooling is further tempered at a temperature of 400 to 600 ° C. A steel pipe obtained by cold forming the steel sheet according to any one of claims 1 to 3 so that a thickness t (mm) and a steel pipe outer diameter D (mm) satisfy a relationship of the following formula (1): A low-yield-ratio high-strength steel pipe having a yield stress YS of 650 MPa or more and a yield ratio YR of 90% or less.
T / D × 100 ≦ 10 (%) (1)
Where t: plate thickness (mm), D: steel pipe outer diameter (mm) The low yield ratio high strength steel pipe according to claim 6, wherein the steel pipe steel sheet is obtained by the method according to claim 4 or 5.