JP2008255394A - Steel pipe having excellent workability, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive steel pipe having satisfactory workability, and to provide a method for producing the same. <P>SOLUTION: In a steel pipe produced by subjecting a steel pipe as a pipe stock to cold pipe drawing and thereafter heating the same in the temperature range from an Ac1 transformation point -70°C to an Ac1 transformation point, in the whole region of the steel pipe including a seam weld zone, the fraction of ferrite is controlled to ≥60%, average crystal grain size is controlled to ≥10 μm, the average value of an aspect ratio is controlled to 1.0 to <5.0, and a specified texture is formed so as to obtain a steel pipe in which rL and rC are increased, and workability is improved. Further, In the method for producing the same, the pipe stock is subjected to cold pipe drawing so as to be a specified reduction of area and a specified reduction of thickness, is heated in the temperature range of an Ac1 transformation point -70°C to an Ac1 transformation point for 30 s, and is thereafter cooled. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a structural steel pipe formed by drawing, bending, hydroforming or the like, a steel pipe excellent in workability suitable for piping, and the like, and a method for manufacturing the same.

  For the purpose of reducing the manufacturing cost of automobiles by omitting processes and reducing the number of parts, a hydroforming technique for manufacturing parts having a complicated shape from a steel pipe has been disclosed (see Patent Document 1). In order to fully utilize the merits of such hydroforming technology, a steel pipe having a high r value (Rankford value), which is an index of plastic anisotropy, is desirable, and a steel pipe for hydroforming and a method for manufacturing the same are disclosed. (See Patent Documents 2 to 6).

  However, these steel pipes are manufactured by using a steel pipe as a raw pipe, heating the raw pipe and then reducing the diameter at a relatively high temperature to control the texture of the steel pipe and increasing the r value. For this reason, there is a problem that the manufacturing equipment becomes expensive and the manufacturing cost increases.

  On the other hand, a method of heat-treating a steel pipe after pipe forming using a cold rolled steel sheet having a high r value in a rolling direction and a direction orthogonal to the rolling direction is disclosed (see Patent Document 7). However, in this method, when manufacturing a steel pipe, the r value between the part melted at the seam weld and the part transformed to austenite by being reheated to the Ac1 transformation point or higher is lowered. There is a problem that it is inferior. Furthermore, this method has a problem that the manufacturing cost of the steel pipe also increases because the manufacturing cost of the cold-rolled steel sheet as the material is high.

  On the other hand, the tensile strength is 350 MPa or more, the r values in the axial direction and the circumferential direction of the steel pipe are both 1.3 or more, and the n value “n” and the tensile strength “TS [MPa]” in the axial direction of the steel pipe are TS + 3285 × As a steel pipe excellent in workability satisfying the relationship of n> 1082, and a method for producing such a steel pipe, a method of heating after forming a cold-rolled steel sheet as a raw material is disclosed (see Patent Document 8). ).

However, in this method, when the steel pipe is manufactured, the r value between the portion melted at the seam welded portion and the portion transformed to austenite by being reheated to the Ac1 transformation point or higher by simply heating the steel pipe. There is a problem that improvement is not recognized and workability is inferior. Furthermore, this method is a method of manufacturing a steel pipe using a cold rolled steel sheet as a raw material, and controlling the texture of the steel pipe by heating the steel pipe to increase the r value. As a result, there is a problem that the manufacturing cost of the steel pipe increases.
JP-A-10-175026 JP 2001-348643 A JP 2001-348647 A JP 2001-348648 A JP 2002-20841 A JP 2002-115029 A JP 2002-115780 A JP 2004-68040 A

  This invention is proposed in view of such a conventional situation, and it aims at providing the steel pipe which has cheap and favorable workability, and its manufacturing method.

The gist of the present invention aimed at solving the above problems is as follows.
A steel pipe made by using a steel pipe as a raw pipe, and after the pipe is cold-drawn and heated in a temperature range from Ac1 transformation point to 70 ° C. to Ac1 transformation point, the entire steel pipe including a seam welded portion The ferrite fraction is 60% or more, the average crystal grain size is 10 μm or more, the average aspect ratio is 1.0 or more and less than 5.0, and the axial r value (rL) of the steel pipe is 1. A steel pipe excellent in workability, wherein the steel pipe has an r value (rC) of 1.2 or more in the circumferential direction of the steel pipe. The steel pipe is used as a base pipe, and the {111} X-ray reflection surface random intensity ratio of the plate surface at a 1/2 plate thickness of the steel pipe is 2.0 or more and 7.0 or less, and the {110} X-ray reflection surface random intensity ratio is 1.0 to 5.0, {100} X-ray reflecting surface random intensity ratio is 3.0 or less, the ferrite fraction is 60% or more in the entire steel pipe including the seam weld, and the average grain size is Excellent in workability, characterized by being 10 μm or more, r value (rL) in the axial direction of the steel pipe being 1.2 or more, and r value (rC) in the circumferential direction of the steel pipe being 1.2 or more. Steel pipe. The steel pipe having excellent workability as described in (1) or (2) above, wherein the difference between the maximum value and the minimum value of the hardness of the steel pipe including the seam welded portion of the steel pipe is Hv50 or less. The tube is cold-drawn so that the area reduction rate is 10% or more and 60% or less and the thickness reduction rate is 1% or more, and it is 30 seconds or more in the temperature range from Ac1 transformation point to 70 ° C to Ac1 transformation point. A method for producing a steel pipe excellent in workability, characterized by cooling after heating. The process for producing a steel pipe excellent in workability as described in (4) above, wherein a step of heating and cooling to a temperature equal to or higher than the Ac3 transformation point is added before the pipe is cold drawn. Method.

  As described above, according to the present invention, it is possible to obtain a steel pipe that is inexpensive and excellent in workability, and structural steel pipes, pipes, etc. for processing such steel pipes by drawing, bending, hydroforming, etc. By applying to the above, the safety can be remarkably improved, the manufacturing cost can be reduced by omitting the process and reducing the number of parts, and the resources can be effectively used.

Hereinafter, the steel pipe excellent in workability of the present invention and the manufacturing method thereof will be described in detail.
The inventors of the present invention have made a steel pipe as a raw pipe and made an expensive cold-rolled steel sheet as a raw material without requiring an expensive manufacturing facility for immediately reducing the diameter after heating the raw pipe. The steel pipe which can be manufactured cheaply and has excellent workability in the entire area of the steel pipe including the seam welded portion and a manufacturing method thereof have been intensively studied.

  As a result, a steel pipe was used as a raw pipe, and the pipe was cold-drawn so that the area reduction rate was 10% or more and 60% or less, and the thickness reduction rate was 1% or more, and from Ac1 transformation point -70 ° C. After heating for 30 seconds or more in the temperature range of the Ac1 transformation point and cooling, the r value (rL) in the axial direction of the steel pipe and the r value (rC) in the circumferential direction of the steel pipe in the entire steel pipe including the seam welded portion are obtained. As a result, it was found that the workability such as bending and hydroforming was improved, and the present invention was completed.

  That is, the steel pipe excellent in workability of the present invention is manufactured by using a steel pipe as a raw pipe, and after the pipe is cold-drawn, it is heated in a temperature range from Ac1 transformation point to 70 ° C to Ac1 transformation point. A steel pipe having a ferrite fraction of 60% or more, an average crystal grain size of 10 μm or more, and an average aspect ratio of 1.0 or more and less than 5.0, The r value (rL) in the axial direction of the steel pipe is 1.2 or more, and the r value (rC) in the circumferential direction of the steel pipe is 1.2 or more.

  In addition, the steel pipe excellent in workability of the present invention has a steel pipe as a raw pipe, and the {111} X-ray reflecting surface random strength ratio of the plate surface at a 1/2 plate thickness of the steel pipe is 2.0 or more and 7.0 or less. , {110} X-ray reflection surface random intensity ratio is 1.0 or more and 5.0 or less, {100} X-ray reflection surface random intensity ratio is 3.0 or less, and ferrite throughout the steel pipe including seam welds. Is 60% or more, the average grain size is 10 μm or more, the r value (rL) in the axial direction of the steel pipe is 1.2 or more, and the r value (rC) in the circumferential direction of the steel pipe is 1. It is characterized by being 2 or more.

  Specifically, in a steel pipe that is manufactured by using a steel pipe as a raw pipe, after the pipe is cold-drawn and then heated in the temperature range from the Ac1 transformation point to -70 ° C. to the Ac1 transformation point, bending and hydroforming In order to improve the workability such as, it is preferable to set rL and rC to 1.2 or more in the entire steel pipe including the seam weld as in the present invention. In general, it is known that workability improves as the r value increases. When rL and rC increase in the entire circumferential direction including the seam weld, if rL and rC are 1.2 or more, it is possible to sufficiently ensure workability such as bending and hydroforming. . The r value (rD) in the 45 ° direction does not have a significant effect on workability, and is not particularly limited. However, under such steel pipe manufacturing conditions, rD is less than 1.2.

  Regarding the measurement method of rL of steel pipe, first, in accordance with JIS Z 2201, a No. 12 arc-shaped test piece is taken with the axial direction of the steel pipe as the longitudinal direction, and the marks are marked and the distance between the marks is measured. Measure. Next, after applying a strain gauge to the center of the parallel part of the test piece in the width direction, an extensometer was attached and a tensile strain of 10% was applied with a tensile tester. RL was calculated from the strain change in the width direction.

  Regarding the measurement method of rC of steel pipe, first, cut the steel pipe and make it flat with a press etc., sample the JIS Z 2201 No. 13B test piece with the circumferential direction as the longitudinal direction, and mark the test piece parallel part Marking was performed, and the gauge distance and the thickness and width of the parallel part of the test piece were measured. Next, an extensometer was attached to the test piece, 10% tensile strain was applied with a tensile tester, and rC was calculated from the plate width of the test piece before and after introduction of the tensile strain and the distance between the gauge points.

  The steel pipe structure of the present invention preferably has a ferrite fraction of 60% or more and an average crystal grain size of 10 μm or more throughout the steel pipe including the seam weld. If the ferrite fraction and the average crystal grain size are out of the specified range, it will be difficult to obtain a good r value. In addition, when the average crystal grain size of ferrite is 100 μm or more, there are cases where it becomes a problem such as rough skin during molding. Therefore, the average crystal grain size of ferrite is more preferably less than 100 μm.

  The ferrite fraction and the average crystal grain size were measured in the range of the thickness 3/8 to 5/8 of the cut surface (L cross section) parallel to the axial direction of the steel pipe. The ferrite fraction may be measured by a point calculation method or the like, and the average crystal grain size of the ferrite may be measured by an intercept method or the like. In order to reduce the measurement error, it is necessary to measure a region where 100 or more crystal grains exist. Etching is preferably nital.

  Furthermore, the steel pipe structure of the present invention preferably has an average aspect ratio of ferrite of 1.0 or more and less than 5.0. The average value of the aspect ratio of ferrite is 1.0, where the long side and the short side are the same. On the other hand, when the average value of the aspect ratio of the ferrite is 5.0 or more, recrystallization due to the heat treatment after the tube drawing process is not formed, and the processed ferrite formed by the tube drawing process remains, so a good r value is obtained. I can't get it. Further, the average aspect ratio of the ferrite is more preferably 1.0 or more and less than 4.0, and still more preferably 1.0 or more and less than 3.0.

  The aspect ratio mentioned here is the same as the degree of expansion measured by the method of JIS G 0552. That is, in the case of the present invention, a crystal cut by a line segment of a certain length perpendicular to the axial direction within the range of the plate thickness of 3/8 to 5/8 on the cutting plane (L cross section) parallel to the axial direction of the steel pipe. It is given by dividing the number of grains by the number of grains cut by a line segment of the same length as described above parallel to the rolling direction.

  In order to increase the r value of the steel pipe as in the present invention, it is necessary to control the texture. That is, in order to increase rC and rL and obtain excellent workability of the steel pipe, the {111} X-ray reflecting surface random intensity ratio of the plate surface at 1/2 the thickness of the steel pipe as in the present invention is set. 2.0 or more and 7.0 or less, {110} X-ray reflection surface random intensity ratio is 1.0 or more and 5.0 or less, and {100} X-ray reflection surface random intensity ratio is 3.0 or less. preferable. If these random intensity ratios are out of the specified range, it becomes difficult to set rC and rL to 1.2 or more, respectively. In addition, when the random intensity ratio is increased only in a specific orientation, the random intensity ratio in other orientations is lowered, so the upper limit of the random intensity ratio in each orientation is defined.

The X-ray random intensity ratio in each direction is measured by X-ray diffraction. Specifically, an arc-shaped test piece is cut out from a steel pipe and made into a flat plate shape by a press or the like, and then X-ray analysis is performed. In addition, when an arc-shaped test piece is formed into a flat plate shape, it should be performed with as low a strain as possible in order to avoid the influence of crystal rotation due to processing of the test piece, and the upper limit of the strain amount introduced by processing should be 10% or less. preferable.
The plate-like sample thus obtained is polished to the vicinity of the center of the plate thickness by mechanical polishing or chemical polishing, and finished to a mirror surface by buffing, and at the same time the strain is removed by electrolytic polishing or chemical polishing. Adjust the thickness center layer to be the measurement surface.
In addition, when a segregation band is observed in the plate thickness center layer of the sample, a place without the segregation band may be measured in the range of 3/8 to 5/8 of the plate thickness. Furthermore, when X-ray measurement is difficult, the measurement may be performed by the EBSP method.

In processing such as bending and hydroforming of a steel pipe, deterioration of ductility due to a local increase in hardness of the steel pipe significantly reduces workability. For this reason, it is necessary to make the material uniform throughout the steel pipe including the seam welded portion of the steel pipe.
In this invention, it is preferable that the difference of the maximum value of hardness and the minimum value is Hv50 or less in the whole steel pipe including the seam weld part of a steel pipe. In particular, in a process such as severe hydroforming, if this difference exceeds Hv50, breakage may occur due to insufficient ductility at a location with high hardness. Therefore, the difference between the maximum value and the minimum value of the hardness of the entire steel pipe including the seam welded portion of the steel pipe is preferably Hv50 or less, more preferably Hv30 or less.
In addition, about the hardness measurement of the steel pipe, the circumferential direction whole area including the plate | board thickness 3 / 8-5 / 8 of the cut surface (C cross section) of the steel pipe and a seam weld part was measured with the Vickers hardness meter. .

Next, the manufacturing method of the steel pipe excellent in workability of this invention is demonstrated.
The method for producing a steel pipe excellent in workability according to the present invention comprises using a steel pipe as a raw pipe, and extending the cold pipe in a cold manner so that the area reduction ratio is 10% or more and 60% or less and the thickness reduction ratio is 1% or more. The tube is processed, heated in the temperature range from Ac1 transformation point -70 ° C. to Ac1 transformation point for 30 seconds or more, and then cooled.

Specifically, when the area reduction ratio of the tube is less than 10%, sufficient crystal rotation does not occur, and the recrystallized texture after heat treatment cannot be controlled to a specific orientation, so the r values in the C and L directions are high. Don't be. On the other hand, if the area reduction ratio of the blank tube exceeds 60%, it tends to break during cold working. Therefore, it is preferable that the area reduction rate of the raw tube is 10% or more and 60% or less.
The area reduction rate is a value represented by {(cross-sectional area before tube drawing-cross-sectional area after tube drawing) / (cross-sectional area before tube drawing)} × 100 (%).

When the thinning ratio is less than 1%, the recrystallized texture after the heat treatment cannot be controlled to a specific orientation, so the r values in the C and L directions do not increase. Therefore, the thinning rate is preferably 1% or more. In order to make this thinning rate 1% or more, a method of controlling the plate thickness by inserting a plug into the steel pipe at the time of cold drawing. It is desirable to use
The thickness reduction rate is a value represented by {(thickness before tube drawing−thickness after tube drawing) / (thickness before tube drawing)} × 100 (%).

  After cold drawing, if the heating temperature is less than Ac1 transformation point -70 ° C., recrystallization does not proceed sufficiently, so the target recrystallization texture and ferrite structure cannot be obtained, and the C and L directions are not obtained. The r value does not increase. On the other hand, when the heating temperature exceeds the Ac1 transformation point, austenite transformation occurs, the target texture and ferrite structure cannot be obtained, and the r values in the C and L directions do not increase. Therefore, the heating temperature of the steel pipe is preferably in the temperature range from Ac1 transformation point -70 ° C to Ac1 transformation point.

  Further, when the steel pipe is heated in the temperature range from Ac1 transformation point -70 ° C to Ac1 transformation point, if the heating time is less than 30 seconds, recrystallization does not proceed sufficiently, and a specific recrystallization texture and Since the target ferrite structure cannot be obtained, the r values in the C and L directions do not increase. Therefore, the heating time of the steel pipe is preferably 30 seconds or longer. In addition, cooling after heating a steel pipe may be either air cooling or water cooling.

In the present invention, a step of heating and cooling to a temperature equal to or higher than the Ac3 transformation point may be added before the cold drawing. In this case, the steel pipe texture including the seam welded portion is randomized by heating and cooling to a temperature equal to or higher than the Ac3 transformation point before cold drawing, and the subsequent cold drawing. Higher r-values can be obtained by tube processing and heat treatment. On the other hand, when the heating temperature before cold drawing is less than the Ac3 transformation point, the texture is not sufficiently randomized.
In addition, by heating and cooling to a temperature above the Ac3 transformation point before cold drawing, the material of the steel pipe including the seam weld is homogenized, and the subsequent cold drawing is performed. By heat treatment, the material can be homogenized throughout the steel pipe including the seam weld. On the other hand, when the heating temperature before cold drawing is less than the Ac3 transformation point, the materials of the seam welded part and the base material part are inhomogeneous.

  In the present invention, the steel component of the steel pipe is not particularly specified, but the effect of the present invention described above can be obtained regardless of the steel component as long as it is used for ordinary structural steel pipes and automotive steel plates. be able to. Further, when the Ac1 transformation point differs depending on the steel component, it is necessary to measure the Ac1 transformation point in advance when setting the heat treatment conditions. This Ac1 transformation point can be measured by a four master test or the like.

  Hereinafter, the effects of the present invention will be made clearer by examples. In addition, this invention is not limited to a following example, In the range which does not change the summary, it can change suitably and can implement.

In this example, a mother pipe before cold working having each steel component shown in Table 1 was prepared so that the steel pipe size was 63.5φ × 2.2 mmt, and the pipe was drawn and then heat-treated. Table 2 shows the manufacturing conditions and mechanical properties of each steel pipe.
In addition, the heat treatment before drawing was performed with a 950 ° C. heating normal.
The workability of the steel pipe was evaluated by the following method. That is, a scribed circle of 10 mmφ was transferred to the steel pipe, and the inner pressure and the axial push amount were controlled to perform the overhang forming in the circumferential direction. Then, the axial strain and the circumferential strain of the portion showing the maximum tube expansion rate immediately before the burst (tube expansion rate = maximum circumferential length after molding / circumferential length of the mother tube) were measured. The ratio ρ of the two strains and the maximum pipe expansion ratio are plotted, and the pipe expansion ratio Re at which ρ = −0.5 is used as the formability index of the hydroform.

As can be seen from Table 2, no. The steel pipes 1 to 7 (examples of the present invention) all have a good texture and an r value, have a high maximum tube expansion ratio during hydroforming, and have good workability.
In contrast, no. The steel pipes (comparative examples) of 8 to 13 are inferior in workability because the conditions to be provided are not appropriate.
Specifically, no. Since the steel pipe No. 8 has a low area reduction rate, the r value is low and the maximum pipe expansion rate is low.
No. The steel pipe No. 9 was broken at the time of drawing because the area reduction rate was too high.
No. Since the steel pipe of No. 10 has a low thickness reduction rate, the rL value is low, so the maximum tube expansion rate is low, and there is no normal heat treatment before tube drawing, so the difference between the maximum and minimum hardness values is large.
No. Since the steel tube No. 11 has a low heat treatment temperature, the average crystal grain size of ferrite is small and the average value of the aspect ratio of ferrite is large, so the r value is low and the maximum tube expansion ratio is low.
No. Since the steel pipe No. 12 has a high heat treatment temperature, the r value is low and the maximum pipe expansion rate is low.
No. Since the steel pipe No. 13 has a short heat treatment time, the ferrite fraction is low and the r value is low, so the maximum pipe expansion ratio is low.

  Since the steel pipe of the present invention is inexpensive and excellent in workability, it can be widely applied to structural steel pipes and pipes that are processed by drawing, bending, hydroforming and the like.

Claims (5)

  A steel pipe made by using a steel pipe as a raw pipe, and after the pipe is cold-drawn and heated in a temperature range from Ac1 transformation point to 70 ° C. to Ac1 transformation point, the entire steel pipe including a seam welded portion The ferrite fraction is 60% or more, the average crystal grain size is 10 μm or more, the average aspect ratio is 1.0 or more and less than 5.0, and the axial r value (rL) of the steel pipe is 1. A steel pipe excellent in workability, wherein the steel pipe has an r value (rC) of 1.2 or more in the circumferential direction of the steel pipe.   The steel pipe is used as a base pipe, and the {111} X-ray reflection surface random intensity ratio of the plate surface at a 1/2 plate thickness of the steel pipe is 2.0 or more and 7.0 or less, and the {110} X-ray reflection surface random intensity ratio is 1.0 to 5.0, {100} X-ray reflecting surface random intensity ratio is 3.0 or less, the ferrite fraction is 60% or more in the entire steel pipe including the seam weld, and the average grain size is Excellent in workability, characterized by being 10 μm or more, r value (rL) in the axial direction of the steel pipe being 1.2 or more, and r value (rC) in the circumferential direction of the steel pipe being 1.2 or more. Steel pipe.   The steel pipe excellent in workability according to claim 1 or 2, wherein the difference between the maximum value and the minimum value of hardness in the entire steel pipe including the seam welded portion of the steel pipe is Hv50 or less.   The tube is cold-drawn so that the area reduction rate is 10% or more and 60% or less and the thickness reduction rate is 1% or more, and it is 30 seconds or more in the temperature range from Ac1 transformation point to 70 ° C to Ac1 transformation point. A method for producing a steel pipe excellent in workability, characterized by cooling after heating.   The method for producing a steel pipe with excellent workability according to claim 4, further comprising a step of heating and cooling the raw pipe to a temperature equal to or higher than the Ac3 transformation point before the pipe is cold drawn. .