JP2014222160A - Method of estimating tensile characteristic of steel plate after subjected to bending working, in direction orthogonal to working direction - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which is suitable for estimating a pipe axial-direction tensile characteristic of a steel pipe based on a steel plate being a base stock, and accurately estimates the tensile characteristic of the steel plate after subjected to bending working, in a direction orthogonal to the bending working direction based on a tensile characteristic of the steel plate.SOLUTION: The method of estimating a tensile characteristic of a steel plate after subjected to bending working, in a direction orthogonal to a working is provided. In the method, a stress-strain relationship obtained from a tensile strength test piece which is collected from the steel plate after subjected to the bending working, in the direction orthogonal to the working direction is approximated as a Swift equation: σ=C(ε+δ)^n. True stress when C: true strain and being a coefficient in the equation is 1, n:work-hardening coefficient, and δ: a change of a correction term before and after the bending working are represented as a function of the residual strain by machining from the plurality of Swift equations :σ=C(ε+δ)^n which are obtained at each bending working in which a residual strain amount by machining is changed, and the tensile characteristic in the direction orthogonal to the direction of the working direction after the bending working by an arbitrary working strain amount is estimated from the tensile characteristic of the steel plate.

Description

  The present invention relates to a method for estimating a tensile property in a direction perpendicular to a processing direction after bending from a tensile property before bending of a steel sheet, and is suitable for use in estimating the tensile property in the axial direction of a UOE steel pipe. About.

Conventionally, circumferential characteristics have been emphasized from the viewpoint of high-pressure transportation of oil and natural gas.However, in recent years, deformation of steel pipes due to earthquakes and ground deformation in frozen regions has been considered, and axial characteristics (especially Deformation characteristics, low YR, etc.) are required.

  When bending steel plates, the tensile properties, especially the yield point, change before and after forming due to the effects of work hardening and the Bauschinger effect, so when using press-bend steel pipes for buildings or manufacturing UOE steel pipes It has been studied to estimate the mechanical properties after bending from those of the material steel plate.

  Patent Document 1 estimates the mechanical properties after bending with respect to a method for evaluating the influence of the degree of work and the tempering treatment on the mechanical properties in bending of a steel sheet and comparing it with the design required performance value of the building. In order to judge the necessity of SR treatment and to estimate the mechanical properties of the steel plate after SR treatment, mechanical properties (0.2% proof stress, tensile strength, yield ratio) and carbon equivalent (Ceq) of the steel plate Multiple regression analysis was performed by positioning the working degree (t / D = plate thickness / bending diameter) and the SR treatment temperature (° C.) as influential factors on the mechanical properties of the steel sheet after bending and SR treatment. It is described that the mechanical properties of the steel sheet after bending and SR treatment are estimated from the results.

  Non-Patent Document 1 is based on the steels of APIX 60 and 65 for the purpose of clarifying the influence of repeated processing on plastic behavior in order to accurately estimate the strength change of a steel pipe in consideration of work hardening and the Bauschinger effect. Describes the results of examining the strength change when reverse pre-strain smaller than the yield elongation was applied using the sampled bar-shaped specimens, or when tensile and compression processing of minute strains were alternately applied.

  By the way, in recent years, in the manufacture of UOE steel pipes, target values of mechanical properties have been set not only in the pipe circumferential direction but also in the pipe axis direction, and orthogonal to the bending direction after bending by the U and O presses. It is necessary to estimate the material properties in the direction.

  However, the change in mechanical properties from the steel sheet due to bending is considered to be different between the bending direction and the orthogonal direction of bending, so that the mechanical properties of the steel sheet are different between the rolling direction and the direction perpendicular to the rolling direction. Later, it is presumed that the influence evaluation method described in Patent Document 1 that estimates the material properties in the bending direction cannot be used.

  In addition, since the multiple regression analysis is used in the influence evaluation method described in Patent Document 1, the change in the estimated value is represented by a monotonous increase (or decrease), which reflects the stress decrease due to the Bausinger effect in the low strain region. Therefore, it is assumed that the estimation accuracy is lowered, particularly the estimation accuracy of the yield point change is lowered.

  Therefore, the present invention provides a method for accurately estimating the tensile property in the direction perpendicular to the bending direction from the tensile property of the steel plate after bending, which is suitable for estimating the tensile property in the tube axis direction of the steel pipe from the raw steel plate. The purpose is to do.

The object of the present invention can be achieved by the following means.
1. A method for estimating tensile properties in a direction orthogonal to a processing direction after bending a steel sheet, and a stress-strain relationship by a tensile test piece taken from a direction orthogonal to the processing direction after bending the steel sheet : Approximated by σ = C (ε + δ) ^ n, and a plurality of Swift's equations obtained for each bending process with different amounts of processing strain: σ = C (ε + δ) ^ n, C: a true strain Is the true stress when n is 1, n is the work hardening coefficient, and δ is the change in the correction term before and after bending as a function of the amount of processing strain, orthogonal to the processing direction after bending with any amount of processing strain. A method for estimating a tensile property in a direction orthogonal to a processing direction after bending a steel plate, wherein the tensile property in a direction to be performed is estimated from the tensile property of the steel plate.
2. Swift equation after bending: C: true stress when true strain is 1 in σ = C (ε + δ) ^ n, n: work hardening coefficient (2), δ: correction term (3) The method for estimating tensile properties in a direction orthogonal to the processing direction after bending the steel sheet according to 1, wherein
n / n ₀ = a * λ _p + b (1)
C / C ₀ = c * (n / n ₀ ) + d (2)
δ = e × exp (f × λ _p ) (3)
In each equation, λ _p is the amount of strain introduced during machining, in equations (1) and (2), n _{0 and} C ₀ are n and C before bending _{, and in} equations (1) to (3), a, b and c , D, e, f are constants.
3. 3. The method for estimating tensile properties in a direction perpendicular to the processing direction after bending the steel sheet according to 1 or 2, wherein the steel sheet has a bainite-based structure and is of APIX65 grade or higher.

  In the present invention, the stress-strain relationship after bending is approximated by the Swift equation: σ = C (ε + δ) ^ n, and the coefficient of the approximate expression is formulated as a function of the amount of processing strain. It is possible to accurately estimate the tensile properties with respect to, improve the production efficiency of UOE steel pipe, and is extremely useful in industry.

The figure explaining a 4-point bending test method. It is a figure which shows the sampling procedure of a tension test piece from a bending test piece, (a) is a top view, (b) is a side view. Prestrain amount lambda _p and _n / n 0 _{(n 0:} prestrain before hardening _{coefficient, n:} pre work hardening coefficient after strain) diagram illustrating a relationship. shows the relationship between true strain and n / _{n 0} is the stress ratio when the 1 _(C / C _0). The figure which shows the relationship between correction | amendment term (delta) and the amount of predistortion (lambda) _p . For X70 grade steel pipes, the pipe axis direction tensile properties (YS measured value) of the steel pipes and the tube axis direction tensile properties (YS estimated values) estimated by the present invention based on the L direction tensile properties of the steel pipe steel plate The figure which shows a correlation.

  The present invention approximates the stress-strain relationship by a tensile test piece taken from a direction orthogonal to the processing direction after bending the material steel plate by the Swift equation: σ = C (ε + δ) ^ n, and the amount of processing strain (pre -Strain, also referred to as pre-strain amount) From a plurality of Swift formulas obtained for each bending process: σ = C (ε + δ) ^ n, C: a true stress when the true strain is 1 , N: work hardening coefficient, and δ: change in the correction term before and after bending as a function of the amount of processing strain, the tensile properties after bending of any amount of processing strain can be calculated with high accuracy from the tensile properties of the material steel plate It is characterized by estimating.

  This will be specifically described below.

[STEP1]
Tensile test using tensile test specimens sampled from the orthogonal direction of the bending direction (L direction of the raw steel sheet) after introducing processing strain into the bending specimens taken with the C direction of the raw steel sheet as the longitudinal direction in a 4-point bending test Thus, the stress-strain relationship after bending is obtained. The tensile test is performed for each bending process with different processing strains, and the stress-strain relationship is obtained for each.

  FIG. 1 is a diagram for explaining a four-point bending test, in which fulcrums 2a and 2b are disposed above a bending test piece 1 having strain gauges 3a to 3c attached to upper and lower surfaces, and fulcrums 2c and 2d are disposed below. Alternatively, the fulcrums 2c and 2d are raised and lowered to perform bending, and the load is unloaded when an arbitrary amount of processing strain is reached.

  FIG. 2 shows the sampling position of the tensile test piece. During bending, the inner surface is compressed in the circumferential direction and the outer surface is tensile strained. In order to investigate changes in characteristics due to differences in the strain direction, the bending test piece 1 after bending is orthogonal to the bending direction. In this way, two tensile test pieces 4a to 4d each having a diameter of 6 mm were sampled from the positions of 1/4 t (t is the plate thickness (mm)) and 3/4 t (t is the plate thickness (mm)). The tensile test pieces 4a and 4c were sampled from the position on the tension side (outer surface side) by bending, and the tensile test pieces 4b and 4d were sampled from the position on the compression side (inner surface side) by bending.

  Moreover, the stress-strain relationship before pre-strain is obtained by a tensile test using a tensile test piece taken from the L direction of the raw steel plate. The tensile test before and after bending is performed at room temperature.

[STEP2]
Each of the stress-strain relationships determined for each material steel plate and the amount of processing strain is approximated by the Swift equation: σ = C (ε + δ) ^ n, and the coefficients C, n, and δ of the equation are calculated. Formulate the quantity relationship. In the Swift equation: σ = C (ε + δ) ^ n, C: true stress when true strain is 1, n: work hardening coefficient, δ: correction term.

  In the case of the n-th power hardening formula (σ = Cε ^ n), for steel materials having a small work hardening coefficient such as bainite steel, the consistency with the stress-strain curve in the low strain region is deteriorated. The term δ improves accuracy in the low strain region.

The equation representing the stress-strain relationship in the L direction (the tube axis direction after pipe making) of the material steel plate is represented by equation (1), and the stress-strain relationship in the tube axis direction after bending is calculated for each processing strain amount (2). Represented by a formula.
σ = C ₀ (ε + δ) ^ n ₀ (1)
σ = C (ε + δ) ^ n (2)
The coefficient C _{0 in} equation (1) plots the true stress-true strain relationship for any four points between 6% strain and strain after the end of Luders elongation when Lueders elongation is present in the initial tensile properties. The true stress when the true strain is 1 is obtained. The coefficient C after processing of the equation (2) is also obtained by the same method.

In the initial tensile properties, the coefficient n _{0 in the} formula (1) is the value corresponding to the logarithm of the pre-strain amount at any four points between 6% strain and the strain after the end of Luders elongation when there is Luders elongation. It is obtained from the slope of a straight line when the logarithmic relationship of stress is plotted. The coefficient n after processing of equation (2) can also be obtained by the same method.

The result of having calculated | required the coefficient change of the stress-strain relational expression at the time of changing the amount of processing strains about FIGS. 3-5 to the X65 grade grade steel plate which becomes a structure mainly composed of bainite is shown. The test steels were sample A to E shown in Table 1 showing the tensile properties of the raw steel plates. Since samples A to E are bainite-based structures, n ₀ <0.15. sample F and G are for examining the estimation accuracy when the results obtained for samples A to E are applied to steels having different structures (notably different in n ₀ ), and the ferrite-bainite structure having a bainite content of less than 90%. The steel is n ₀ > 0.15. In the present invention, the bainite-based structure is 90% or more of bainite, and the remaining structure is not particularly defined.

FIG. 3 shows the relationship between n / n ₀ (n ₀ : work hardening coefficient before pre-strain _, n: work hardening coefficient after pre-strain) and the pre-strain amount λ _p , and n / n ₀ regardless of the sampling position. = -11.529 λ _p +0.9929. FIG. 4 shows the relationship between the stress ratio (C / C ₀ ) and n / n ₀ when the true strain is 1, and is arranged as C / C ₀ = 0.1394 (n / n ₀ ) +0.8586. .

FIG. 5 shows the relationship between the correction term δ and the processing strain amount λ _p . Since the value of δ tends to converge to zero as the processing strain amount λ _p increases, the relationship of δ to the processing strain amount λ _p is approximated by equation (3).
δ = e × exp (f × λ _p ) (3)
From the figure, the coefficients of equation (3) are e = 0.0106 and f = -95.4.

From the above results, the equation for estimating 0.5% YS of the test steels (samples A to E) in Table 1 according to the processing strain amount λ _p can be expressed by equation (4).
σ = C {ln (1 + 0.005) + δ} ^ n
= [C ₀ {0.139 (-11.529λ _p +0.9929) +0.8586}] {ln (1 + 0.005) + 0.0106exp (−95.4λ _p )} ^ {n ₀ (−11.529λ _p +0.9929)} (4)
The estimation accuracy in the above calculation formula has a high accuracy of ± 20 MPa.

In addition, sample F and G in Table 1 are ferrite-bainite structure and n ₀ > 0.15, and therefore, when the above formula is applied, the low strain exhibits a large work hardening after reaching the plastic region with low stress. The consistency in the region deteriorates and is estimated to be about 30 to 40 MPa higher. In the present invention, since the tensile property is expressed by the Swift equation: σ = C (ε + δ) ^ n, the estimation accuracy is improved when the work hardening index is within a certain range, that is, when the main structure is the same.

In the case of a steel sheet in which n ₀ > 0.15 in a ferrite-bainite structure as in sample F, assuming that the correction term δ = 0, the stress-strain relationship is represented by the n-th power hardening equation (σ = Cε ^ n), and the estimated accuracy Will improve.

  The case where the present invention is applied and the tensile properties in the tube axis direction of a UOE steel pipe manufactured by an arbitrary bending process are estimated from the tensile properties of the raw steel sheet will be specifically described.

According to the present invention, when estimating the pipe axis direction tensile characteristics before and after forming a UOE steel pipe, the amount of strain received by the steel pipe forming is defined on the inner surface side and the outer surface side of the tube.
The process of forming a UOE steel pipe consists of a U press, an O press, a contracted tube, and an expanded tube, and undergoes repeated deformation. When evaluating the pipe axis direction characteristics of the UOE steel pipe, the test piece sampling position is 3 o'clock from the weld line, and is not affected by the U press. In the present invention, for the sake of simplification, it is assumed that the contraction tube is negligibly small and can be ignored.

When the above assumption is applied, the amount of strain that is incurred before forming the steel pipe is as follows: λ _O is the strain amount introduced before the O press (= tube diameter / tube thickness ratio), and λ _E is the tube expansion strain, The inner surface side is expressed by equation (5), and the outer surface side is expressed by equation (6).
Internal strain = −λ _O + λ _E (5)
Strain amount on the outer surface side = λ _O + λ _E (6)
Accordingly, tensile properties of Kangaimen side can be expressed by the sum of the strain amount lambda _p. On the other hand, since the strain direction changes on the inside, the Bausinger effect must be considered. Several Bausinger effects have been reported when reverse prestrain is applied to an arbitrary amount of prestrain (for example, Non-Patent Document 2), and a specific Bausinger effect coefficient B with reference to these values. Can be estimated.

The tensile properties of the inner surface of the tube can be estimated by the expression (7) using the Bauschinger effect coefficient B.
σ = C {ln (1 + 0.005) + δ} ^ n
= B [C ₀ {0.139 (-11.529λ _p +0.9929) +0.8586}] {ln (1 + 0.005) + 0.0106exp (−95.4λ _p )} ^ {n ₀ (−11. 529λ _p +0.9929)} (7)
At this time, λ _p = λ _O.

  Fig. 6 shows the pipe axial direction tensile properties (YS measured value) of steel pipes with the same main structure and different strength levels within the X70 grade, and the L direction tensile properties of the steel pipe steel plates. The result of having compared the pipe axis direction tensile property (YS estimated value) estimated by (7) Formula is shown. In the case of a UOE steel pipe, the L direction of the material steel plate is the pipe axis direction of the steel pipe.

X70 during the production of grades of steel pipe, strain amount introduced in each step and _{λ O = 0.025, λ E =} 0.01. The Bausinger effect coefficient B at this time is about 0.9 from Non-Patent Document 1.

  From FIG. 6, the estimation accuracy is −30 to +20 MPa, which has a higher accuracy than the YS estimation accuracy (± 50 MPa) in the estimation method of Patent Document 1, and has different strength levels as long as the main structure is the same. In this case, it is possible to estimate the tensile property in the pipe axis direction with high accuracy.

  According to the present invention, for a steel sheet for UOE steel pipe (material steel sheet) whose main structure is bainite, the tube shaft after forming a UOE steel pipe from the strain amount in each process during pipe making and the tensile properties of the steel pipe steel plate It is possible to accurately estimate the directional tensile properties. In addition, since the change in tensile properties before and after the manufacture of the steel pipe can be accurately estimated, the required properties for the material steel plate are clarified, and it is possible to select appropriate manufacturing conditions.

1 Bending test pieces 2a, 2b, 2c, 2d Support points 3a, 3b, 3c Strain gauges 4a, 4b, 4c, 4d Tensile test pieces

Japanese Patent No. 3721517

Takada, Sugie, “Effects of repetitive machining on the plastic behavior of steel sheets for line pipes”, Kawasaki Steel Technical Report, October 1974 p. 14-22 T. T. et al. Takeda, Y. et al. Nasu, E .; Shiratori: Bull. Of Yamagata Univ. Eng. Vol. 17, no. 1, 1982

Claims (3)

  A method for estimating tensile properties in a direction orthogonal to a processing direction after bending a steel sheet, and a stress-strain relationship by a tensile test piece taken from a direction orthogonal to the processing direction after bending the steel sheet : Approximated by σ = C (ε + δ) ^ n, and a plurality of Swift's equations obtained for each bending process with different amounts of processing strain: σ = C (ε + δ) ^ n, C: a true strain Is the true stress when n is 1, n is the work hardening coefficient, and δ is the change in the correction term before and after bending as a function of the amount of processing strain, orthogonal to the processing direction after bending with any amount of processing strain. A method for estimating a tensile property in a direction orthogonal to a processing direction after bending a steel plate, wherein the tensile property in a direction to be performed is estimated from the tensile property of the steel plate. Swift equation after bending: C: true stress when true strain is 1 in σ = C (ε + δ) ^ n, n: work hardening coefficient (2), δ: correction term The method for estimating tensile properties in a direction orthogonal to the processing direction after bending the steel sheet according to claim 1, wherein
n / n ₀ = a * λ _p + b (1)
C / C ₀ = c * (n / n ₀ ) + d (2)
δ = e × exp (f × λ _p ) (3)
In each equation, λ _p is the amount of strain introduced during machining, in equations (1) and (2), n _{0 and} C ₀ are n and C before bending _{, and in} equations (1) to (3), a, b and c , D, e, f are constants.   3. The method for estimating tensile properties in a direction perpendicular to the processing direction after bending the steel sheet according to claim 1, wherein the steel sheet has a bainite-based structure and is of APIX65 grade or higher.

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