JP2013185164A - Method for manufacturing steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel tube stably satisfying a target value of a mechanical property.SOLUTION: When a steel tube is manufactured by applying cold forming to a steel sheet, a method for manufacturing the steel tube includes: calculating a predicted value Δσof a mechanical property variation of a steel material caused by tube forming; obtaining a predictable accuracy setting value S; and selecting a combination of a steel sheet, a method for manufacturing the same, and tube forming conditions, so as to satisfy the following formula (i): σ-Δσ+S≤σ≤σ-Δσ-S, and, thereby forming the steel tube. Here, in the formula (i), σis an upper limit of a target value of a mechanical property after tube forming, σis a lower limit of the target value of the mechanical property after tube forming, and σis the mechanical property of the steel sheet.

Description

  The present invention relates to a method of manufacturing a steel pipe, and more particularly to a method of manufacturing a steel pipe manufactured by cold forming of a steel plate.

  Steel pipes made of steel plates are used for building structures and the like. The mechanical properties of the steel pipe are different from those of the original steel sheet due to the influence of the processing history during pipe making or the influence of the heat history when heat treatment is performed after processing. Therefore, in order to keep the mechanical properties of the steel pipe within a desired range, it is necessary to consider the mechanical properties of the original steel plate and the amount of change in mechanical properties when manufacturing the steel pipe from the steel plate. However, the sizes of steel pipes used for building structures are diverse, and every time mass production of steel pipes of a predetermined size is started, the amount of change in mechanical properties when manufacturing steel pipes from steel sheets is confirmed. There is a problem that it takes time and cost to determine the required range of mechanical properties.

  As a technique related to a manufacturing method that takes into account the amount of change in mechanical properties when manufacturing a steel pipe from a steel plate, Patent Document 1 proposes a technique related to a method for manufacturing a press bend cold-formed circular steel pipe. According to Patent Document 1, it is said that after forming a thick steel plate into a circular steel pipe, a steel pipe having predetermined mechanical characteristics is obtained.

JP 2007-270304 A

  Even using the technique disclosed in Patent Document 1, it is difficult to obtain a steel pipe that stably satisfies the target value of mechanical properties under various production conditions.

  An object of the present invention is to provide a method of manufacturing a steel pipe that stably satisfies a target value of mechanical properties in order to solve such problems of the prior art.

  The inventors of the present invention have intensively studied the accuracy of predicting the amount of change in mechanical properties particularly when manufacturing a steel pipe from a steel sheet, and have investigated the conditions for improving the accuracy of prediction, thereby completing the present invention. The gist of the present invention resides in a method of manufacturing a steel pipe shown in the following (1) to (3).

(1) When manufacturing a steel pipe by cold forming a steel plate,
Calculate the predicted value Δσ _p of the change in mechanical properties of steel due to pipe making,
A prediction accuracy setting value S is obtained,
A method of manufacturing a steel pipe, comprising selecting a steel sheet satisfying the following formula (i), a manufacturing condition thereof, and a combination of pipe manufacturing conditions for pipe manufacturing.
σ _l −Δσ _p + S ≦ σ _s ≦ σ _u −Δσ _p −S (i)
In addition, the meaning of each symbol in (i) Formula is as follows.
σ _u : upper limit of target value of mechanical properties after pipe making σ _l : lower limit of target value of mechanical properties after pipe making σ _s : mechanical properties of steel sheet

(2) Using a parameter including three or more kinds selected from the carbon content of the steel sheet, the plate thickness and mechanical properties, the tempering temperature, the bending strain, and the stress-relieving heat treatment temperature for the predicted value Δσ _p of the mechanical property change amount. The method for manufacturing a steel pipe according to (1), wherein the calculation is performed as follows.

(3) The method of manufacturing a steel pipe according to (2), wherein the predicted value Δσ _p of the mechanical property change amount is calculated by the following equation (ii).
Δσ _p = {k ₁ (1 + k ₂ · C) (1 + k ₃ · t + k ₄ · t ² ) (1 + k ₅ · T _t ) (1 + k ₆ · T _r ) (ε + k ₇ · ε ² ) + k ₈ · T _r + k ₉ · T _r ² } (1 + k ₁₀ · σ _s ) (ii)
In addition, the meaning of each symbol in the formula (ii) is as follows.
C: Carbon content (mass%) of the base steel sheet
t: Thickness (mm) of base steel plate
σ _s : mechanical properties of base steel sheet T _t : tempering temperature (° C)
ε: Bending strain of 1/4 t (%)
T _r : Stress relief heat treatment temperature (° C.)
k _{1 to} k ₁₀ : coefficients

The mechanical properties in the present invention refer to the yield strength YS (N / mm ² ) and the tensile strength TS (N / mm ² ), and the mechanical property change amount refers to the yield strength change amount ΔYS (N / mm). ² ) and the change in tensile strength ΔTS (N / mm ² ).

  The steel pipe manufactured by the present invention satisfies the target value of the mechanical properties of the steel pipe. In addition, by using the method of manufacturing a steel pipe according to the present invention, every time mass production of a steel pipe of a predetermined size is started, the amount of change in mechanical properties when manufacturing the steel pipe from the steel plate is confirmed, and the mechanical properties of the steel plate are confirmed. There is no need to define the required range, and time and cost can be omitted.

The manufacturing method of the steel pipe which concerns on this invention manufactures a steel pipe from a steel plate by cold forming. At this time, if the steel sheet is cold-worked, the working strain increases according to the working conditions, and the mechanical properties change. The mechanical properties in the present invention include yield strength YS (N / mm ² ), tensile strength TS (N / mm ² ), yield ratio YR (%), elongation (%), Charpy absorbed energy (J), etc. Point to. If the amount of change in mechanical properties according to the processing conditions is known in advance, a steel pipe having the target mechanical properties is produced by cold working using a steel plate having mechanical properties that are low by the amount of change. Can be manufactured.

At this time, since the actual measurement value Δσ _m of the mechanical property change amount varies depending on the chemical composition, size, pipe making conditions, etc. of the base steel plate, it is necessary to measure the time and cost every time mass production of the steel pipe is started. This is not preferable because it leads to waste. Therefore, the predicted value Δσ _p of the mechanical property change amount is used.

However, when the predicted value Δσ _p of the mechanical property change amount is used, if the prediction accuracy is low and the error is large, the mechanical properties of the obtained steel pipe may be out of the upper limit and lower limit of the target value. Therefore, it is necessary to consider the prediction accuracy setting value S. The prediction accuracy setting value S is set to a value equal to or larger than the maximum value of the absolute value of the prediction error with reference to the accuracy confirmation result of the prediction formula described later.

In the method for manufacturing a steel pipe according to the present invention, when the upper limit of the target value of the mechanical properties of the steel pipe after pipe making is σ _u , the lower limit is σ _l and the mechanical properties of the steel sheet before pipe making is σ _s , A steel plate that satisfies the following formula (i) and a combination of its manufacturing conditions and pipe making conditions are selected and piped.
σ _l −Δσ _p + S ≦ σ _s ≦ σ _u −Δσ _p −S (i)

  Thereby, the steel pipe which satisfies the target range of a mechanical property can be obtained reliably. Hereinafter, each requirement of the present invention will be described in more detail.

1. Predicted value of mechanical property change amount Predicted value Δσ _p of the mechanical property change amount when manufacturing a steel pipe from a steel sheet can be calculated from various parameters. Above all, the carbon content, sheet thickness and mechanical properties of the steel sheet, the tempering temperature, which is the manufacturing condition of the steel sheet, and the bending strain and stress relief heat treatment temperature, which are the pipe manufacturing conditions, have a great influence on the calculation of the predicted value Δσ _p. .

A plurality of these parameters were used, the number of parameters used in the prediction formula was changed, the predicted value Δσ _p was calculated, and the prediction accuracy was confirmed. The prediction accuracy was evaluated using the absolute value of the difference between the predicted value Δσ _p of the mechanical property change amount and the actually measured value Δσ _m . Table 1 shows the results. In the calculation of the predicted value Δσ _p in Table 1 and the evaluation of the prediction accuracy, data on a steel pipe having a tensile strength of 440 N / mm ² class is used. As shown in Table 1, the prediction accuracy error when the prediction value Δσ _p is calculated using three or more of these parameters is the prediction accuracy error when the prediction value Δσ _p is calculated using two parameters. It can be seen that there is a marked improvement compared to.

In the method for producing a steel pipe of the present invention, the predicted value Δσ _p of the mechanical property change amount when producing a steel pipe from a steel plate is determined by the carbon content of the steel plate, the plate thickness and the mechanical property, the tempering temperature during the production of the steel plate, In addition, it is preferable to calculate using parameters including three or more selected from bending strain and stress relief heat treatment temperature, which are pipe manufacturing conditions. It is more preferable to calculate using parameters including four or more selected from the above, more preferable to calculate using parameters including five or more, and to calculate using parameters including all six types. Most preferred.

The predicted value Δσ _p of the mechanical property change amount is preferably calculated by the following equation (ii).
Δσ _p = {k ₁ (1 + k ₂ · C) (1 + k ₃ · t + k ₄ · t ² ) (1 + k ₅ · T _t ) (1 + k ₆ · T _r ) (ε + k ₇ · ε ² ) + k ₈ · T _r + k ₉ · T _r ² } (1 + k ₁₀ · σ _s ) (ii)
C is the carbon content (% by mass) of the steel sheet, t is the thickness (mm) of the steel sheet, σ _s is the mechanical property of the steel sheet, T _t is the tempering temperature (° C.), and ε is the bending strain of 1/4 t. (%), T _r is the stress relief heat treatment temperature (° C.), and k _{1 to} k ₁₀ are coefficients.

When the carbon content C, the plate thickness t, the mechanical property σ _s , the tempering temperature T _t , the bending strain ε, and the stress relief heat treatment temperature _{Tr in} the formula (ii) are not selected as parameters, numerical values are used. Enter 0. In addition, in the parameters of the tempering temperature and the stress removal heat treatment temperature, the heat treatment temperature is inputted when the operation is carried out, and the numerical value 0 is inputted when there is none.

The value of the bending strain ε is preferably represented by a value at ¼t. When the center of the plate thickness is the neutral axis, the bending strain amount ε of 1/4 t of the steel pipe (diameter D, plate thickness t) is expressed by equation (iii). Although the position of the neutral axis is not exactly the center of the plate thickness but slightly inside, the detailed calculation of the position of the neutral axis is not necessary from the viewpoint of the accuracy of the prediction formula in the present invention.
ε = t / {2 (D−t)} (iii)

k _{1 to} k ₁₀ are coefficients, and these coefficients may be determined by using the least square method so that the prediction accuracy is improved using, for example, known data relating to the manufacture of steel pipes.

2. Prediction accuracy setting value The prediction accuracy setting value S should be set to a value that is equal to or greater than the maximum absolute value of the error with reference to the accuracy check result of the prediction formula. The provided value is preferable. For example, 40% of the maximum absolute value of the prediction error is set as a margin, and the prediction accuracy setting value S is set. In the example of the number of parameters 6 in Table 1, since the maximum value of the prediction error is 14 N / mm ² , the prediction accuracy setting value S is set to 14 N / mm ² with a margin of about 40%, and 20 N / mm ² Just do it. The margin is more preferably 70% of the maximum absolute value of the prediction error, and more preferably 100%.

3. Chemical composition of steel Although there is no restriction | limiting in particular about the chemical composition of steel used as the raw material of the steel pipe of this invention, It is preferable that a chemical composition contains the following elements as a basic element, and the remainder is Fe and an impurity. Furthermore, you may contain Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, Mg, REM, Sn, etc. as arbitrary addition elements for improving an intensity | strength, toughness, etc. In addition, “%” of the content of each element in the following chemical composition means “mass%”.

C: 0.03-0.19%
C is an element that increases the strength of steel. In order to obtain this effect, the C content is preferably 0.03% or more. However, if the C content exceeds 0.19%, the toughness is reduced and weld cracks are likely to occur. Therefore, the C content is preferably 0.03 to 0.19%. The C content is more preferably 0.06% or more, and more preferably 0.16% or less.

Si: 0.01-0.60%
Si is an element having a deoxidizing action and increasing the strength of steel. In order to obtain this effect, the Si content is preferably 0.01% or more. However, if the Si content exceeds 0.60%, the toughness of the base material and the weld heat affected zone deteriorates. Therefore, the Si content is preferably 0.01 to 0.60%. The Si content is more preferably 0.05% or more. Further, the Si content is more preferably 0.45% or less, and further preferably 0.30% or less.

Mn: 0.5 to 2.0%
Mn has the effect of increasing the strength and toughness of steel. In order to obtain this effect, the Mn content is preferably 0.5% or more. However, if the content exceeds 2.0%, weld cracks are likely to occur. For this reason, the content of Mn is preferably 0.5 to 2.0%. The content of Mn is more preferably 1.0% or more, and further preferably 1.2% or more. Further, the Mn content is more preferably 1.7% or less, and even more preferably 1.6% or less.

P: 0.04% or less P is an element that inevitably exists as an impurity in steel materials and deteriorates toughness. Therefore, the P content is preferably 0.04% or less. The P content is more preferably 0.02% or less, and still more preferably 0.01% or less.

S: 0.04% or less S is an element that inevitably exists as an impurity in steel and is harmful to toughness. Therefore, the S content is preferably 0.04% or less. The S content is more preferably 0.01% or less, further preferably 0.005% or less, and further preferably 0.003% or less.

  In addition, an impurity means the component mixed by various factors of raw materials such as ores and scraps and manufacturing processes, and is allowed within a range that does not adversely affect the present invention.

4). Manufacturing method When manufacturing steel pipes, it is first necessary to prepare steel sheets. A steel plate can be manufactured by performing, for example, the following heating and rolling using a steel piece or a steel ingot having the chemical composition described above. In addition, each following temperature means the surface temperature in the representative position (for example, center part) of a to-be-rolled material.

  The heating temperature before rolling is preferably 950 ° C. or higher, and more preferably 1050 ° C. or higher in order to easily perform hot rolling of the steel material. If heating before rolling is performed at this temperature, effects such as promotion of solid solution of carbonitride are obtained, and strength and toughness are improved. However, if the heating temperature is too high, the austenite crystal grains may become coarse and the toughness may deteriorate. Therefore, the heating temperature is preferably 1250 ° C. or lower, more preferably 1150 ° C. or lower.

  Rolling is preferably performed under the condition that the total rolling reduction in a temperature range of 900 ° C. or lower is 20% or more. Thereby, the structure of the product can be refined, and it becomes easy to ensure good toughness. The total rolling reduction in the temperature range of 900 ° C. or lower is more preferably 40% or higher. Here, “the total rolling reduction in a temperature range of 900 ° C. or less” is {(thickness when reaching 900 ° C.) − (Rolling thickness)} / (thickness when reaching 900 ° C.) X100 (%) is meant. Furthermore, the rolling finishing temperature is preferably 700 ° C. or higher. Thereby, a rolling load can be made small and it becomes easy to ensure a favorable shape. The rolling finishing temperature is more preferably 740 ° C. or higher, and further preferably 780 ° C. or higher.

  Applying accelerated cooling after rolling has the effect of improving strength and toughness, so accelerated cooling may be applied. If the water cooling start temperature is too high, the yield ratio may be too high, and if the water cooling start temperature is too low, the effect of improving the strength and toughness cannot be obtained. Therefore, the desirable range of the water cooling start temperature is 650 to 780 ° C. On the other hand, if the water cooling stop temperature is too high, the effect of improving the strength and toughness cannot be obtained. Therefore, the upper limit of the water cooling stop temperature is preferably 550 ° C. or less, and more preferably 300 ° C. or less.

  The steel sheet after rolling and cooling may be further heat treated to adjust strength and toughness.

  When performing reheating quenching (Q heat treatment), if the heating temperature is too high, the toughness may be reduced, and if the heating temperature is too low, the effect of increasing the strength may not be obtained. It is preferable to set it as ℃ or less and it is preferable to set it as 800 ℃ or more.

  By performing reheating and quenching (L heat treatment) to the two-phase region, the yield ratio can be particularly reduced. The heating temperature needs to be adjusted to a two-phase region of austenite and ferrite.

  By performing tempering (T heat treatment), toughness may be particularly improved. If the tempering temperature is too low, the effect of improving the toughness may not be obtained, and if the temperature is too high, the strength decreases greatly, so the desirable range of the tempering temperature is 350 to 650 ° C. The tempering temperature range is more preferably 380 to 500 ° C.

  A steel pipe is obtained by cold working the steel plate obtained by the above method into a tubular shape and welding the seam. The pipe making may be performed by a press bend pipe or a UOE pipe. In the press bend method, a steel plate is pressed against a mold and bent, and the pressing position is sequentially moved to form a cylindrical shape and welded. Depending on the size, two thick steel plates may be formed into a semi-cylindrical shape and welded by two seams. In the UOE pipe manufacturing method, U-pressing is performed on a rolled steel sheet (raw material) from which the edge portion has been removed to form a U shape. Further, an O press is performed to form an O shape to form a cylindrical shape. The joints that are the ends are abutted, and tack welding, inner surface welding, and outer surface welding are performed, and further, pipe expansion is performed as necessary.

Here, as a welding method, a generally known method such as CO ₂ welding or submerged arc welding may be used. After pipe making, a stress removal heat treatment (SR treatment) may be further performed.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steel No. having the C content shown in Table 2. Steel pieces having a thickness of 1 to 7 and a thickness of 300 mm were produced by a continuous casting method. The contents of Si and Mn in the steel slab are adjusted to 0.20 to 0.30% and 1.35 to 1.55%, respectively, and the contents of P and S present as impurities are each 0.015% or less, It was suppressed to 0.005% or less. In addition, in order to improve strength and the like, 0.5% or less of Cu, 0.5% or less of Ni, 0.5% or less of Cr, 0.3% or less of Mo, 0.06% or less of V, 0.03% or less Nb, 0.03% or less Ti, 0.002% or less B, 0.003% or less Ca, 0.06% or less Al, 0.006% or less N It was.

This steel slab was heated under the conditions shown in Table 2, hot-rolled, and some steel slabs were subjected to accelerated cooling and heat treatment to produce steel sheets. For each steel plate obtained, a tensile test at room temperature was conducted using a round bar tensile test piece (JIS No. 4) taken from a 1/4 position in the thickness direction so that the axis of the test piece was parallel to the rolling direction. The yield strength YS _s (0.2% yield strength) and the tensile strength TS _s were determined.

On the other hand, the steel pipe diameter-thickness ratio (D / t) and the stress removal heat treatment temperature (Tr) described in Tables 3 and 4 are used to predict the amount of change in yield strength and tensile strength when producing a steel pipe (ΔYS _p , ΔTS _p ), the predicted yield strength and tensile strength of the steel pipe were calculated using the above equation (ii) and the coefficient table of Table 5. All six parameters were used to calculate the predicted values. The bending strain amount ε was calculated by the formula (iii). The prediction accuracy setting value S is set to 20 (≈14 × 1.4) with a margin of about 40% from the result of the parameter number 6 in Table 1 (maximum value of prediction error 14).

Further, from the position in the plate thickness direction 1/4 of the base part of the steel pipe made by press-bending under the conditions shown in Tables 3 and 4, the specimen axis is taken so as to be parallel to the longitudinal direction of the steel pipe (rolling direction). Using the round bar tensile test piece (JIS No. 4), a tensile test was performed at room temperature, and yield strength YS _m (0.2% yield strength) and tensile strength TS _m were determined. The lower limit on the target value of yield strength, respectively _YS u: 620N / mm ² and _YS l: a 440 N / mm ^2, upper and lower limits of the target value of the tensile strength, respectively _TS u: 740N / mm ² and TS _l: was 590N / mm ^2.

If the equation (ii) is used, the amount of change in mechanical properties can be predicted. By using this predicted value Δσ _p, as shown in Tables 3 and 4, if the mechanical properties after pipe making are predicted in advance and the steel plate as a material, its manufacturing conditions, and pipe making conditions are determined, the target and It is possible to manufacture a steel pipe that satisfies the mechanical characteristics.

  Steel No. No. 1 made using the steel plate No. 1 1-6, test No. using a steel plate produced by direct quenching (DQ). 1 to 3, only those with a stress relief heat treatment temperature of 600 ° C. satisfy the above formula (i), and a steel plate manufactured by reheating and quenching to the austenite region and reheating and quenching to the two-phase region (QL) Test No. used In Nos. 4 to 6, the formula (i) was satisfied when the stress relief heat treatment temperatures were 550 ° C and 600 ° C. And test No. which does not satisfy (i) Formula. 1, 2 and 4 failed to obtain both the target yield strength and tensile strength.

  Similarly, test no. As shown in 7-13, if the conditions which satisfy | fill a formula (i) are found, the steel pipe should just be manufactured with the manufacturing conditions and pipe making conditions of the steel plate and steel plates.

  As described above, according to the present invention, a steel pipe having the required mechanical characteristics can be manufactured by predicting the mechanical characteristics after pipe manufacturing in advance and determining an appropriate combination of steel plate and manufacturing conditions.

  According to the present invention, a steel pipe that satisfies the target value of mechanical properties can be stably obtained, so that it can be safely used for a building structure or the like. The steel pipe according to the present invention can be obtained with less time and cost.

Claims (3)

When manufacturing steel pipes by cold forming steel sheets,
Calculate the predicted value Δσ _p of the change in mechanical properties of steel due to pipe making,
A prediction accuracy setting value S is obtained,
A steel pipe manufacturing method characterized by selecting a steel sheet that satisfies the following formula (i), a manufacturing method thereof, and a combination of pipe manufacturing conditions and manufacturing the steel pipe.
σ _l −Δσ _p + S ≦ σ _s ≦ σ _u −Δσ _p −S (i)
In addition, the meaning of each symbol in (i) Formula is as follows.
σ _u : upper limit of target value of mechanical properties after pipe making σ _l : lower limit of target value of mechanical properties after pipe making σ _s : mechanical properties of steel sheet The predicted value Δσ _p of the mechanical property change amount is calculated using parameters including three or more kinds selected from the carbon content of the steel plate, the plate thickness and mechanical properties, the tempering temperature, the bending strain, and the stress relief heat treatment temperature. The manufacturing method of the steel pipe of Claim 1 characterized by the above-mentioned. The method for manufacturing a steel pipe according to claim 2, wherein the predicted value Δσ _p of the mechanical property change amount is calculated by the following equation (ii).
Δσ _p = {k ₁ (1 + k ₂ · C) (1 + k ₃ · t + k ₄ · t ² ) (1 + k ₅ · T _t ) (1 + k ₆ · T _r ) (ε + k ₇ · ε ² ) + k ₈ · T _r + k ₉ · T _r ² } (1 + k ₁₀ · σ _s ) (ii)
In addition, the meaning of each symbol in the formula (ii) is as follows.
C: Carbon content (mass%) of the base steel sheet
t: Thickness (mm) of base steel plate
σ _s : mechanical properties of base steel sheet T _t : tempering temperature (° C)
ε: Bending strain of 1/4 t (%)
T _r : Stress relief heat treatment temperature (° C.)
k _{1 to} k ₁₀ : coefficients