JP2019045441A - Method for determining cooling rate of steel pipe and manufacturing method of steel pipe using the same - Google Patents

Abstract

To provide a method for determining a cooling rate capable of obtaining a composition high in martensite ratio in the heat treatment of a steel pipe.SOLUTION: The method is a method for determining a cooling rate of a steel pipe, and includes: a step (Step S1) of cooling a test piece with a predetermined magnitude of stress burdened, at a plurality of cooling rates and measuring a martensite transformation starting temperature for each cooling rate; a step (Step S2) of cooling the test piece by the plurality of cooling rates with no stress burdened and measuring the martensite transformation starting temperature for each cooling rate; and a step (Step S3) of determining that the minimum cooling rate in which a difference with the martensite transformation starting temperature measured with no stress burdened is equal to or less than a predetermined threshold value is the lower limit cooling rate.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method of determining a cooling rate of a steel pipe and a method of manufacturing a steel pipe using the same.

Heat treatment of steel materials has long been widely performed for the purpose of imparting desired characteristics and performance to machine parts and steel products, and the essence is the adjustment of metal structure. Quenching, which is one of heat treatment, heats the material to be heat treated to a temperature above Ac ₃ point (austenite transformation end temperature) to austenite, and then quenches to form a martensitic structure.

  Japanese Patent No. 5252131 describes a method of quenching a steel pipe which can reduce residual stress by non-water-cooling a pipe end which is likely to be a base point of cracking. Japanese Patent No. 6047947 describes a seamless steel pipe excellent in sour resistance, whose surface layer hardness is adjusted by heat treatment.

  The heat treatment of the steel pipe often employs continuous cooling in which a cooling zone is passed while conveying the steel pipe heated by the heating furnace at a predetermined speed. In addition to continuous cooling, there is also a mode in which cooling is performed by stopping at a fixed position, and a mode in which cooling is performed by immersing in a cooling tank. However, when the material to be heat treated is long, the installation space of equipment becomes large. Continuous cooling is also advantageous from the viewpoint of operation efficiency.

  In heat treatment of a steel pipe by continuous cooling, it is necessary to pay attention to circumferential uniform cooling. Uneven cooling in the circumferential direction causes bending of the steel pipe during cooling. This bending becomes larger as the steel pipe becomes longer, which causes undesirable situations such as trouble in line conveyance and addition of a bending correction process. In order to uniformly cool in the circumferential direction, it is preferable to cool the steel pipe from the outer surface. While uniformization of outer surface water cooling can be achieved relatively easily by means such as evenly arranging cooling nozzles in the annular main pipe, in addition to the fact that internal water cooling has many technical problems, the equipment configuration becomes complicated. Is the reason.

  In the case of water cooling from the outer surface, there is a concern that the martensite rate may decrease due to the insufficient cooling rate of the inner surface of the steel pipe. In the design of the cooling equipment, equipment specifications are determined such that the cooling rate of the inner surface of the steel pipe at which the cooling rate is the smallest is equal to or higher than the upper critical cooling rate (the lower limit cooling rate for securing a martensitic structure). In addition, if the facility has an excessive cooling capacity, the cost increases, so it is preferable to set the capacity to the minimum necessary.

Patent No. 5252131 Patent No. 6047947

K. Hase et al. "Bainite formation influenced by large stress", Material Science and Technology, December 2004, Vol. 20, pp. 1499-1505

  The heat treatment conditions are determined based on a continuous cooling transformation diagram (CCT) of the target steel material. CCT is usually created by a test method called Fourmaster test. Specifically, the test pieces are cooled at various cooling rates, and the transformation point, hardness, and structure are examined to obtain the relationship between the cooling rate and the final structure. In the heat treatment aiming at a martensitic structure, the above-mentioned upper critical cooling rate is important information.

  However, as a result of investigations by the present inventors, it was found that in heat treatment of a steel pipe, a structure having a high martensite ratio may not be obtained even when cooled at the upper critical cooling rate.

  An object of the present invention is to provide a method capable of determining a cooling rate capable of obtaining a structure having a high martensite ratio in heat treatment of a steel pipe, and a manufacturing method capable of obtaining a steel pipe having a structure having a high martensite ratio. To provide.

  A method according to an embodiment of the present invention is a method of determining a cooling rate of a steel pipe, wherein a test piece loaded with a predetermined stress is cooled at a plurality of cooling rates to start martensitic transformation at each cooling rate. The steps of measuring the temperature, cooling the specimen at a plurality of cooling rates without applying stress, and measuring the martensitic transformation start temperature for each cooling rate, and the martensitic transformation measured by applying stress. Determining a minimum cooling rate at which the difference between the starting temperature and the martensitic transformation start temperature measured without applying stress is less than or equal to a predetermined threshold as the lower limit cooling rate.

In the method of manufacturing a steel pipe according to one embodiment of the present invention, a raw pipe at a temperature of Ac ₃ or more is cooled so that the cooling rate of the portion where the cooling rate is the smallest is equal to or higher than the lower limit cooling rate determined by the above method. Providing the step of

  According to the present invention, in heat treatment of a steel pipe, it is possible to determine a cooling rate at which a structure having a high martensite rate can be obtained. By performing heat treatment using this cooling rate, a steel pipe having a structure with a high martensite ratio can be obtained.

FIG. 1 is a flow diagram of a method of determining the cooling rate of a steel pipe according to an embodiment of the present invention. FIG. 2A is a view schematically showing an example of the configuration of an apparatus used to measure the Ms point. FIG. 2B is an enlarged view of the periphery of a test piece of the device of FIG. 2A. FIG. 3 is an example of a test pattern when measuring the Ms point. FIG. 4 is a flow chart showing a method of manufacturing a steel pipe according to an embodiment of the present invention. FIG. 5 is a block diagram showing an example of a functional configuration of the heat treatment line. FIG. 6 is a CCT of a steel having a predetermined chemical composition, which was created by Fourmaster test. FIG. 7 is a diagram in which the Ms point is added to the cooling curve. FIG. 8 is a diagram schematically showing a state in which the point Ms has risen. FIG. 9 is a schematic view of a cooling test of a flat plate test piece. FIG. 10 is a model of a flat plate used in numerical analysis. FIG. 11 shows temperature histories at 5 mm, 20 mm and 35 mm positions from the cooling surface. FIG. 12 is a graph showing a time change of stress in the sheet width direction σ _x and martensite volume fraction ξ _M generated during cooling, with 35 mm from the cooling surface of the flat plate test piece as an evaluation point. FIG. 13 is a model of a steel pipe used for numerical analysis. FIG. 14 is a graph showing the time change of circumferential stress σ _x and martensite volume fraction ξ _M generated during cooling, with 35 mm from the cooling surface of the steel pipe as an evaluation point. FIG. 15A is a graph showing the relationship between temperature and elongation when the cooling rate is 1 ° C./sec. FIG. 15B is a graph showing the relationship between temperature and elongation when the cooling rate is 2 ° C./sec. FIG. 15C is a graph showing the relationship between temperature and elongation when the cooling rate is 3 ° C./sec. FIG. 15D is a graph showing the relationship between temperature and elongation when the cooling rate is 4 ° C./sec. FIG. 15E is a graph showing the relationship between temperature and elongation when the cooling rate is 5 ° C./sec. FIG. 16 is a figure for demonstrating the adjustment method of the test piece for hardness and structure | tissue investigation. FIG. 17 is a graph showing the relationship between applied stress and hardness. FIG. 18A is a micrograph of a tissue at an applied stress of 0 MPa when the cooling rate is 2 ° C./sec. FIG. 18B is a photomicrograph of the tissue when the applied stress is 100 MPa and the cooling rate is 2 ° C./sec. FIG. 18C is a micrograph of the tissue at an applied stress of 0 MPa when the cooling rate is 4 ° C./sec. FIG. 18D is a micrograph of the tissue at an applied stress of 100 MPa when the cooling rate is 4 ° C./sec.

  The present inventors investigated the relationship between the heat treatment condition of the steel material and the structure. As a result, it was found that even with a cooling rate at which a martensitic structure can be obtained in a flat plate specimen, a structure having a high martensite ratio may not be obtained in a steel pipe.

  Flat specimens are unconstrained around their circumference and can expand and contract relatively freely during heat treatment. In contrast, in a steel pipe having a closed cross-sectional structure, high stress is generated during heat treatment due to restraint in the circumferential direction. The inventors proceeded further investigation by predicting that the stress during heat treatment affects the structure. As a result, it was clarified that the martensitic transformation start temperature (hereinafter referred to as "Ms point") rises due to stress during heat treatment near the upper critical cooling rate.

  With regard to the effect of stress on tissue, research results have been reported that stress loading under isothermal transformation increases the bainite transformation rate (K. Hase et al. "Bainite formation influenced by large stress", Material Science and Technology , December 2004, Vol. 20, pp. 1499-1505). However, the fact that the martensitic transformation start temperature rises due to the heat treatment stress in the vicinity of the upper critical cooling rate is a new finding not found in the past.

  The present invention has been completed based on the above findings. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or corresponding parts in the drawings have the same reference characters allotted and description thereof will not be repeated. The dimensional ratios among the components shown in the drawings do not necessarily indicate the actual dimensional ratios.

[Determination method of cooling rate of steel pipe]
FIG. 1 is a flow diagram of a method of determining the cooling rate of a steel pipe according to an embodiment of the present invention. In this method, a test piece loaded with a stress of a predetermined size is cooled at a plurality of cooling rates to measure an Ms point for each cooling rate (step S1), and a plurality of test pieces are not loaded. Step of measuring the Ms point for each cooling rate by cooling at a cooling rate (step S2), the difference between the Ms point measured by applying stress, and the Ms point measured without applying stress And determining a minimum cooling rate which is equal to or less than a predetermined threshold as the lower limit cooling rate (step S3). Each step will be described in detail below.

  The test pieces used in each measurement of step S1 and step S2 are test pieces having the same chemical composition as the steel pipe to be heat treated. When the steel pipe to be heat-treated is subjected to pre-heat treatment (for example, normalizing), it is preferable to subject the test piece to the same pre-heat treatment.

  FIG. 2A is a view schematically showing the configuration of an apparatus 20 which is an example of an apparatus used for measuring the Ms point in steps S1 and S2. FIG. 2B is an enlarged view of the vicinity of the test piece S of the device 20 (FIGS. 2A and 2B are views as seen from directions orthogonal to each other). The apparatus 20 includes a chamber 21, holding shafts 22 and 23, chucks 24 and 25, a load cell 26, a cooling nozzle 27, a thermocouple 28, and a differential transformer extensometer 29.

  The test piece S is disposed in the chamber 21 in a state where both ends in the length direction (z direction) are gripped by the chucks 24 and 25. The chucks 24 and 25 are connected to the holding shafts 22 and 23, respectively. The holding shaft 22 is configured to be movable in parallel with the axial direction by a hydraulic servo cylinder (not shown). By moving the holding shaft 22, a predetermined stress can be applied in the longitudinal direction (z direction) of the test piece S. The holding shaft 23 is connected to the load cell 26 and can measure the load applied to the test piece S. The differential transformer type extensometer 29 can measure the elongation in the length direction (z direction) of the test piece S.

  The test piece S is electrically heated via a wire (not shown). The test piece S is also cooled by blowing a refrigerant (for example, He gas) from the cooling nozzle 27. The temperature of the test piece S is measured by a thermocouple 28. With this configuration, the test strip S can be given a predetermined temperature history. During the heat treatment, in order to prevent the oxidation of the test piece S, the atmosphere in the chamber 21 is replaced with an inert gas (for example, argon gas).

  According to the configuration of the apparatus 20, a load can be applied to the test piece S during heat treatment while giving the test piece S a predetermined temperature history. Stress is converted by dividing the load by the cross-sectional area. The temperature and the load can be controlled, for example, in accordance with a test pattern program in accordance with proportional-integral-differential (PID) control. In addition, the apparatus 20 is an illustration, Comprising: The structure of the apparatus used by this embodiment is not limited to this.

  The Ms point can be determined, for example, from the relationship between the temperature of the test piece and the elongation. The specimen during cooling thermally shrinks as the temperature decreases until the Ms point, whereas it expands due to transformation expansion when martensitic transformation starts (that is, passes the Ms point). In this embodiment, in the temperature-elongation curve, a temperature at which the elongation becomes a minimum value is defined as the Ms point (Ms point is strictly a temperature at which the slope of linear expansion of austenite starts to change. However, the elongation is There is no large difference even if it is defined as the temperature which becomes the minimum value, and in this embodiment, there is no problem even if it is defined in this way, since the relative amount of change of the Ms point due to the applied stress is compared.

FIG. 3 is an example of a test pattern when measuring the Ms point. In this example, the test piece is austenitized by holding it at a temperature Ta of Ac ₃ point or more for a predetermined time, and then cooled at a cooling rate C. The cooling rate C is an average cooling rate from the Ac ₁ point to the martensitic transformation end temperature (Mf point). The cooling rate C is preferably kept constant. In the cooling process, when the temperature of the test piece reaches the temperature Tb, a load is applied to the test piece to generate a stress σa.

The temperature Ta is not particularly limited as long as it is Ac ₃ point or higher, but it is preferable to match the holding temperature in the actual heat treatment. The temperature Ta is, for example, Ac ₃ + 50 ° C. to Ac ₃ + 100 ° C.

  The temperature Tb is not particularly limited as long as it is lower than the temperature Ta and higher than the Ms point. However, in an actual heat treatment of a steel pipe, since stress is generated from a point of time when the temperature is lowered to some extent, it is preferable to set in consideration of this. On the other hand, in the vicinity of the upper critical cooling rate, the stress during heat treatment raises the Ms point, so if the temperature Tb is set too low, there is a possibility that the Ms point at that stress can not be measured accurately. Assuming that the bainite transformation start temperature is Bs, the temperature Tb is preferably Ms point to Bs point.

  The stress σa applied to the test piece is not particularly limited, but is preferably set in consideration of the stress generated in the heat treatment of an actual steel pipe. In the vicinity of the upper critical cooling rate, the Ms point tends to rise as the stress σa increases. If the stress σa is too large or too small, it may deviate from the actual situation. The stress σa depends on the chemical composition and dimensions of the steel pipe, but is preferably 10 to 200 MPa, and more preferably 50 to 150 MPa.

In step S1, the above measurement is performed while changing the cooling rate C to measure the Ms point for each cooling rate. That step _{S1, C 2, C 2, ···} , a different cooling rate _{C n,} measuring the Ms point of the test piece loaded with stress σa is cooled at a rate _{C 1} (step S1-1 and), and the step of measuring the Ms point of the test piece loaded with stress σa is cooled at a rate C ₂ (step S1-2), · · ·, a test piece loaded with stress σa is cooled at a rate C _n And (step S1-n) of measuring the Ms point (see FIG. 1). However, n is an integer of 2 or more.

  n is preferably large. That is, it is preferable to measure the Ms point with more cooling rates. In addition, the measurement interval of the cooling rate is preferably fine. The measurement interval is preferably 2 ° C./second or less, more preferably 1 ° C./second or less.

In step S2, measurement similar to step S1 is performed without applying stress. In other words, step S2 is a step of measuring the cooling to the Ms point of the test piece without load stress at the rate C ₁ (step S2-1), cooled specimen without load stress at the rate C ₂ A step of measuring the Ms point (step S2-2),..., A step of cooling the test piece at a speed C _n without applying stress and measuring the Ms point (step S2-n) Includes (see Figure 1).

Here, the Ms point of the test piece loaded with stress .sigma.a measured by cooling at a rate _{C 1 Ms (σa, C 1} ), a test piece loaded with stress .sigma.a measured by cooling at a rate C ₂ Ms (σa, C ₂ ),..., Ms (σa, C _n ) measured by cooling a test piece loaded with stress σa at a speed C _n is Ms (σa, C _n ). Similarly, Ms (0, C ₁ ) measured by cooling the test piece at a speed C ₁ without applying stress, and measured by cooling a test piece at a speed C ₂ without applying stress. Let Ms (0, C ₂ ),..., Ms point measured by cooling the test piece at a speed C _n without applying stress be Ms (0, C _n ).

In step S3, the minimum cooling rate at which the difference between the Ms point measured with stress applied and the Ms point measured without stress applied falls below a predetermined threshold is determined as the lower limit cooling rate. Specifically, Ms (σa, _{C k)} obtained in steps S1 and S2 and Ms (σa, _{C k)} the difference between, calculates _{Ms (σa, C k) -Ms} (0, C k) _{, Ms (σa, C k)} -Ms (σa, C k) is determined as the lower limit the cooling rate a minimum cooling rate _{C k} equal to or less than a predetermined threshold value T _{threshold the.} However, k is an integer of 1 or more and n or less.

Ms _(σa, C k) as -Ms (0, _{C k)} is small, it means that the influence of stress is reduced. The threshold T _threshold for determining the lower limit cooling rate may be, for example, 20 ° C., although depending on the magnitude of the stress σ a loaded in step S1. The threshold T _threshold is preferably 10 ° C., more preferably 5 ° C.

  The method of determining the cooling rate according to an embodiment of the present invention has been described above. Although FIG. 1 illustrates step S1, step S2, and step S3 in this order, the order of step S1 and step S2 may be interchanged. In addition, even if each sub-step of step S1 and each sub-step of step S2 are alternately performed as in step S1-1, step S2-1, step S1-2, step S2-2,. Good. Or you may implement step S3 for every substep like step S1-1, step S2-1, step S3, step S1-2, step S2-2, step S3, and so on.

[Method of manufacturing steel pipe]
Next, a method of manufacturing a steel pipe using the lower limit cooling rate determined by the above method will be described. Hereinafter, in the description of the method of manufacturing a steel pipe, the steel pipe to be subjected to the heat treatment will be referred to as a “element pipe”. Moreover, the term "steel pipe" is used in the sense of a steel pipe which is manufactured by heat treatment, in distinction from "main pipe".

FIG. 4 is a flow chart showing a method of manufacturing a steel pipe according to an embodiment of the present invention. In the method of manufacturing a steel pipe according to the present embodiment, a step of heating the raw pipe to a temperature of Ac ₃ or more (step S4) and a step of cooling the heated raw pipe at a cooling rate of the lower limit cooling rate or more (step S5) And have.

The raw pipe is heated to a temperature of Ac ₃ or more to cause austenite transformation (step S4). The heating temperature is, for example, Ac ₃ + 50 ° C. to Ac ₃ + 100 ° C. If the heating temperature is too high, the austenite grains coarsen and the product performance decreases.

The heated raw pipe is cooled at a cooling rate equal to or higher than the lower limit cooling rate (step S5). Here, the cooling rate of the portion where the cooling rate is the smallest is made to be equal to or higher than the lower limit cooling rate. If there is a portion where the cooling rate is less than the lower limit cooling rate, a structure with a high martensite rate can not be obtained in that portion. The cooling rate is an average cooling rate from the Ac ₁ point to the martensitic transformation end temperature (Mf point).

  When the raw pipe is cooled from the outer surface, the part where the cooling rate is the smallest is the inner surface of the raw pipe (it may be a part slightly inside the inner surface of the raw pipe, but it does not change much). When the hollow shell is cooled from the inner surface, the portion with the lowest cooling rate is the outer surface of the hollow shell. When the hollow shell is cooled from the inner and outer surfaces, the portion with the lowest cooling rate is the thick central portion of the hollow shell.

  In any case, the larger the thickness of the blank, the larger the difference in the cooling rate in the blank. Therefore, in order to make the cooling rate of the portion where the cooling rate is the smallest the cooling rate equal to or lower than the lower limit, it is necessary to increase the cooling capacity by increasing the amount of refrigerant as the thickness of the steel pipe increases. The necessary cooling capacity can be determined, for example, by numerical calculation.

  On the other hand, excessively increasing the cooling rate is not preferable because the equipment cost increases. The upper limit of the cooling rate is preferably the lower limit cooling rate + 5 ° C / second. That is, it is preferable that the cooling rate of the portion where the cooling rate is the smallest be equal to or lower than the lower limit cooling rate + 5 ° C / second.

  FIG. 5 is a block diagram showing a functional configuration of the heat treatment line 100 which is an example of the heat treatment line. The heat treatment line 100 includes a hardening device 60 and a tempering device 70. The hardening device 60 includes a heating device 61 and a cooling device 62. Conveying rollers 80 (conveying devices) are disposed between the respective devices.

  The conveyance roller 80 sequentially conveys the raw pipe from the heating device 61 to the cooling device 62 and from the cooling device 62 to the tempering device 70. The raw pipe is heated by the heating device 61 and cooled by the cooling device 62. The blank is then heated again by the tempering device 70.

According to the configuration of the heat treatment line 100, after heating the raw pipe to Ac ₃ points or more by the heating device 61, the raw pipe can be quenched by cooling the raw pipe by the cooling device 62. Furthermore, the raw pipe can be tempered by heating the raw pipe to a predetermined temperature by the tempering device 70. The tempered tube is, for example, cooled by a cooling device (not shown) and then transported to a flaw detection device or the like.

  According to the configuration of the heat treatment line 100, the heat treatment of hardening and tempering can be continuously performed on the raw pipe. However, quenching and tempering may not be performed continuously. In this case, the heat treatment line 100 may not include the tempering device 70.

  The cooling device 62 includes a plurality of cooling rings, although the detailed configuration is not shown. Each of the plurality of cooling rings includes a plurality of nozzles, and is configured such that the refrigerant can be sprayed from each of the plurality of nozzles to the outer surface of the raw pipe passing inside the cooling ring. The amount of the refrigerant can be controlled for each cooling ring, and the raw pipe can be cooled at an optimum speed by adjusting the amount of refrigerant for each cooling ring and the transport speed of the raw pipe.

Hereinabove, the method for manufacturing a steel pipe according to the embodiment of the present invention has been described. In the above example, the heat treatment (reheat hardening) in which the produced raw pipe is heated and then quenched is described. However, instead of this, heat treatment (direct quenching) may be carried out to quench the high temperature raw pipe immediately after hot working. In this case, the cooling may be started from the temperature of ₃ or more of Ac.

[Effect of this embodiment]
FIG. 6 is a CCT of a steel having a predetermined chemical composition, which was created by Fourmaster test. In FIG. 6, F represents ferrite, B represents bainite, and M represents martensite. It can be seen from FIG. 6 that, in this steel, if it is cooled at a cooling rate of 2 ° C./sec or more, a structure of only martensite can be obtained. That is, the upper critical cooling rate of this steel is about 2 ° C./sec.

  FIG. 7 is a diagram in which Ms points measured by the method of the present embodiment are added to the cooling curve. In the figure, white circles indicate Ms points measured by applying a stress of 100 MPa, and solid circles indicate Ms points measured without applying a stress. It can be seen from FIG. 7 that, at a cooling rate of 2 ° C./sec, which is the upper critical rate, the stress causes the Ms point to rise by about 80 ° C.

  FIG. 8 is a diagram schematically showing a state in which the point Ms has risen. The rise of the Ms point shifts the bainite nose to the short side. Therefore, when the Ms point rises, bainite mixes even at the same cooling rate, and a structure with a high martensite rate can not be obtained.

  Description will be continued with reference to FIG. 7 again. It can be seen from FIG. 7 that as the cooling rate is increased, the rise in Ms point due to stress decreases. Further, in this steel, when the cooling rate is set to 4 ° C./sec or more, it can be seen that the rise of the Ms point due to the stress almost disappears (specific numerical values will be described in detail in the examples). From this result, it is understood that, in the chemical composition of this steel, in order to obtain a structure having a high martensite ratio by heat treatment of a steel pipe, the cooling rate should be 4 ° C./sec or more. That is, the lower limit cooling rate can be determined as 4 ° C./second.

  The effects of the present embodiment have been described above. According to the present embodiment, in the heat treatment of a steel pipe, it is possible to determine the cooling rate at which a structure having a high martensite rate can be obtained. By performing heat treatment using this cooling rate, a steel pipe having a structure with a high martensite ratio can be obtained.

  Hereinafter, the present invention will be more specifically described based on examples. Note that this embodiment does not limit the present invention.

[Cooling test of flat plate specimen]
A flat plate test piece 70 mm wide × 100 mm long × 40 mm thick was prepared, heated to 950 ° C., and then subjected to a cooling test in which water was cooled from one side. FIG. 9 is a schematic view of a cooling test of a flat plate test piece. The temperature during cooling was measured by a thermocouple embedded at a position of 5 mm, 20 mm and 35 mm from the cooling surface and a thermocouple welded to the surface opposite to the cooling surface.

The cooling rate at a position 35 mm from the cooling surface was 1.9 ° C./sec. The cooling rate was calculated from the time required for cooling from the Ac ₁ point (706 ° C.) to the martensitic transformation end temperature (Mf point, 150 ° C.). The temperature of the surface opposite to the cooling surface (40 mm from the cooling surface) was almost the same as the temperature measurement result by the thermocouple embedded 35 mm from the cooling surface. It is considered that this is because the cooling effect by the radiation from the outermost surface to the air atmosphere acts.

  From the result of measurement of the hardness of the flat plate specimen after cooling, it was confirmed that a martensitic structure was obtained over the entire range in the thickness direction.

[Numerical analysis of flat plate specimens]
Numerical analysis by the finite element method (FEM) was carried out under the conditions simulating the above-mentioned cooling test. FIG. 10 is a model of a flat plate used in numerical analysis. From the symmetry of the test piece and the symmetry of the cooling condition, a 1/2 region was defined as the analysis region. A generalized plane strain temperature-deformation coupled analysis element was applied to give cooling heat transfer equivalent to actual steel pipe cooling to the lower end of the model.

  FIG. 11 shows temperature histories at 5 mm, 20 mm and 35 mm positions from the cooling surface. In FIG. 11, the solid line is the measured value in the above-described cooling test, and the broken line is the calculated value obtained from the numerical analysis. It can be confirmed that the result of the numerical analysis corresponds well to the measured value.

FIG. 12 is a graph showing the time change of the stress in the sheet width direction σ _x and the martensite volume fraction 発 生_M generated during cooling, with the evaluation point 35 mm from the cooling surface of the flat plate test piece. Positive stress represents tensile stress and negative stress represents compressive stress. At the evaluation point, first, tensile stress is generated due to thermal contraction (peaks around 50 seconds of elapsed time). Next, a compressive stress occurs due to thermal contraction of the surface opposite to the cooling surface (valley near 190 seconds of elapsed time). After that, the transformation expansion occurs in advance on the cooling surface side, whereby a tensile stress is applied to the evaluation point (a peak near an elapsed time of 250 seconds). After that, the stress turns to the compression direction by the transformation expansion of the evaluation point itself. Although the maximum tensile stress immediately after the start of transformation expansion is 114 MPa, the stress decreases as the martensitic transformation at the evaluation point progresses, and σ _x becomes 0 when ξ _M becomes 60%.

[Cooling test of steel pipe]
Next, a steel pipe having an outer diameter of 426 mm and a thickness of 40 mm was produced from the same steel material as the flat plate test piece and heated to 950 ° C., and then a cooling test was performed in which water was cooled from the outer surface. The cooling rate at a position of 35 mm from the cooling surface (the outer surface of the steel pipe) was 2.9 ° C./sec. This was a value larger than the cooling rate of 1.9 ° C./sec at the 35 mm position in the cooling test on the flat plate specimen (this difference was the single nozzle cooling in the cooling test of the flat plate specimen) On the other hand, it is considered to be one of the causes that the cooling test of the steel pipe was cooling with a plurality of nozzles.) However, in the measurement of the hardness of the steel pipe after cooling, it was found that the hardness was lower than the target value in the region inside the 30 mm position from the cooling surface, and a structure having a high martensite ratio was not obtained.

[Numerical analysis of steel pipe]
The numerical analysis by FEM was conducted also about the cooling test of the steel pipe. FIG. 13 is a model of a steel pipe used for numerical analysis. As in the case of the flat plate test piece, a 1/2 area is defined as an analysis area from the symmetry of the test piece and the symmetry of the cooling condition. Using a model in which a symmetric boundary condition is given to a symmetry plane (with constraint) and a model without a boundary condition (without constraint), the generated stress with and without the constraint is compared.

FIG. 14 is a graph showing the time change of the circumferential stress σ _x and the martensite volume fraction ξ _M generated during cooling, with the evaluation point 35 mm from the cooling surface of the steel pipe (the outer surface of the steel pipe). In the model with restraint, a high tensile stress of 414 MPa was generated immediately before the onset of transformation. On the other hand, in the model without restraint, the maximum tensile stress is 163 MPa, which is a value close to the maximum tensile stress of the flat plate test piece.

  From this result, in the steel pipe of closed cross-sectional shape, high stress is generated during heat treatment due to the restraint in the circumferential direction as compared with the plate-like test piece which can expand and contract relatively freely without being restricted. I know what to do.

[Investigation of influence of applied stress]
Next, using the apparatus having the configuration shown in FIG. 2A and FIG. 2B, the influence of the applied stress on the transformation point, hardness and structure was investigated. A rectangular test piece having a width W of 20 mm, a thickness t of 1.2 mm, and a length L of 200 mm (see FIGS. 2A and 2B) was manufactured from the same steel material as the flat plate test piece or the steel pipe described above. The test piece was heated to 940 ° C. at 10 ° C./sec by electric heating and held for 300 seconds for austenitization, and then cooled using He gas as a refrigerant. Stress was applied when reaching 500 ° C. in the cooling process. The cooling rate was varied from 1 to 5 ° C./sec at 1 ° C./sec intervals. The applied stress was varied from 25 to 100 MPa at 25 MPa intervals. Distance between scores of differential transformer type extensometer G. L was 14.5 mm.

  FIGS. 15A-15E are graphs showing the relationship between temperature and elongation when the cooling rate is 1 ° C./s, 2 ° C./s, 3 ° C./s, 4 ° C./s, and 5 ° C./s, respectively. is there. In these graphs, the temperature at which the elongation becomes a minimum value is taken as the Ms point. The relationship between the cooling rate, the applied stress, and the Ms point is shown in Table 1.

  As shown in FIGS. 15A to 15D and Table 1, when the cooling rate is 1 ° C./sec, the Ms point rises in proportion to the applied stress. As the cooling rate increases, this fluctuation decreases and converges to the original Ms point of 300.degree.

  The cause is considered as follows. While diffusion transformation occurs with a slight driving force and time course, non-diffusion transformation such as martensitic transformation does not occur until the driving force exceeds a certain magnitude. This driving force (interface energy and elastic strain energy required to initiate transformation, energy required for plastic deformation occurring in matrix phases in and around martensite, etc.) is compensated by mechanical external force, and the Ms point rises It is considered to be

  Then, the hardness and structure of the test piece after cooling were investigated. FIG. 16 is a figure for demonstrating the adjustment method of the test piece for hardness and structure | tissue investigation. The central portion in the length direction of the test piece was cut out, and this was further divided into two, and hardness measurement and tissue observation were performed with a cross section in the length direction x thickness direction as a measurement surface / observation surface. This is because a decarburized layer was formed on the surface of the test piece, and the surface measurement did not give accurate results.

  FIG. 17 is a graph showing the relationship between applied stress and hardness. When the cooling rate is 1 ° C./sec, the mixed structure of bainite and martensite is obtained, and the hardness is low. When the cooling rate is 2 ° C./sec or more, it exceeds HV700, but the harder the cooling rate is, the harder it is. In addition, at all cooling rates, the hardness tends to decrease as the applied stress increases.

  FIG. 18A and FIG. 18B are photomicrographs of the tissue at a cooling rate of 2 ° C./sec, respectively with no stress applied and with an applied stress of 100 MPa. In the vicinity of the upper critical cooling rate, the bainite structure (the part appearing black in the photograph) increases due to stress loading. 18C and 18D, on the other hand, are photomicrographs of the tissue with a cooling rate of 4 ° C./s and with no stress applied and with an applied stress of 100 MPa, respectively. can not see.

  From these results, it can be seen that, in the vicinity of the upper critical cooling rate, stress is applied to raise the Ms point, and the bainite nose is shifted to the short time side to decrease the martensite ratio accordingly. In addition, it is understood that the increase of the Ms point and the contamination of bainite thereby can be reduced by increasing the cooling rate.

  FIG. 7 is a diagram (hereinafter referred to as “stress applied CCT”) in which the Ms point obtained by the above measurement is added to the cooling curve. As described above, white circles indicate Ms points measured by applying a stress of 100 MPa, and solid circles indicate Ms points measured without applying a stress. From FIG. 7 and Table 1, it can be seen that if the cooling rate is 4 ° C./sec or more, the rise in Ms point due to stress can be made 10 ° C. or less.

  As described above, by creating the stress load CCT, it is possible to improve the prediction accuracy of the cooling rate necessary for the heat treatment of the steel pipe. In the steel materials investigated this time, the stress dependency of the Ms point disappears almost at 4 ° C / sec or more, but the situation also changes if the target steel materials change. Therefore, it is necessary to create a stress load CCT for each target steel material. This is similar to accumulating the data of the conventional unloaded CCT, but by accumulating the stress applied CCT for each steel material, knowing the critical cooling rate under the stress action which has not been studied conventionally. Can. By this, appropriate cooling conditions can be determined.

  The embodiment of the present invention has been described above. The embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the embodiment described above, and the embodiment described above can be appropriately modified and implemented without departing from the scope of the invention.

Claims (3)

A method of determining the cooling rate of a steel pipe,
Cooling a test piece loaded with a predetermined magnitude of stress at a plurality of cooling rates and measuring a martensitic transformation start temperature for each cooling rate;
Cooling the specimen at a plurality of cooling rates without applying stress and measuring the martensitic transformation start temperature for each cooling rate;
Determine the minimum cooling rate at which the difference between the martensitic transformation start temperature measured with stress applied and the martensitic transformation start temperature measured without stress applied is less than the predetermined threshold as the lower limit cooling rate And a step of Ac steel tube comprising a step of cooling a raw pipe at a temperature of ₃ or more points so that the cooling rate of the portion where the cooling rate is the smallest is equal to or higher than the lower limit cooling rate determined by the method of claim 1 Production method. A method of manufacturing a steel pipe according to claim 2, wherein
The manufacturing method, wherein the cooling rate of the portion where the cooling rate is the smallest is not more than the lower limit cooling rate + 5 ° C determined by the method according to claim 1.