JP2005068534A - Welded steel pipe suitable for hydroforming and barring and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a welded steel pipe suited for hydroforming and barring and having a TS of at least 400 MPa and to provide a method for producing the same. <P>SOLUTION: The welded steel pipe has a composition containing 0.01 to less than 0.05% C, at most 1.5% Si, 0.2 to 3.5% Mn, less than 0.002% S, at most 0.1% Al, and at most 0.01% N, with the balance being Fe and unavoidable impurities, has a structure entirely comprising a bainitic-ferrite structure or comprising 1 to 40 vol.% polygonal ferrite structure and the balance of bainitic-ferrite structure and has a texture where the inverse strength ratio in the (110) face of ferrite in a cross section vertical to the longitudinal direction is 2.0 or larger or the inverse ratio of the (200) face of ferrite in a cross section vertical to the peripheral direction is 2.0 or larger. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a welded steel pipe suitable for use as a structural member or underbody member of an automobile, in particular, workability in hydroforming (hydroforming property) and workability in burring in which the hole is expanded by punching (burring property). ) Regarding improvements.

  Hollow members having various cross-sectional shapes have been used as structural members for automobiles. Conventionally, as a method for producing such hollow members, parts formed by press working of steel plates are joined together by spot welding. The manufacturing method has been adopted.

    However, recently, a hollow member for a structural member of an automobile is required to have a higher shock absorption capability at the time of a collision, and a material with higher strength has been used. For this reason, it is becoming increasingly difficult to produce a member having no molding defects and having excellent shape and dimensional accuracy by the conventional press molding method.

  Recently, hydroforming has attracted attention as a new molding method for solving such problems. Hydroforming is a molding method in which a high-pressure liquid is injected into a steel pipe to form a member with a desired shape, and the cross-sectional dimension of the steel pipe can be changed by pipe expansion processing, etc. -An excellent molding method that can increase rigidity.

  By the way, as a steel pipe used for this hydroforming, an ERW steel pipe made of a low and medium carbon steel sheet of C: 0.10 to 0.20 mass%, which is easily obtained and inexpensive, is often used. However, when hydroforming is performed on an electric resistance welded steel pipe made of such a low and medium carbon steel plate, there is a problem that sufficient pipe expansion cannot be performed due to poor workability of the electric resistance welded steel pipe itself.

  In order to improve the workability of the electric resistance welded steel pipe, it is conceivable to use an extremely low carbon steel sheet with a significantly reduced carbon content as a material. However, in the case of an ultra-low carbon ERW steel pipe, the hydroforming property is good, but the crystal grains in the vicinity of the seam are coarsened and softened by the welding heat at the time of manufacturing the steel pipe, so the deformation at the time of pipe expansion is near the seam. However, there is a problem that the high ductility of the material cannot be fully exhibited. For this reason, there is a strong demand for welded steel pipes having material characteristics and seam quality that can sufficiently withstand hydroforming.

In response to this request, Patent Document 1 states that in mass%, C: 0.05 to 0.2%, Si: 1.0% or less, Mn: 1.5% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% Hereinafter, N: 0.01% or less, or further, Group A and / or B (Group A: Cr: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, B: One or more of 0.01% or less, Group B: Ca: 0.02% or less, REM: One or two of 0.02% or less) And a welded steel pipe having a composition composed of the remaining Fe and inevitable impurities, the tensile strength (hereinafter referred to as TS) is 400 MPa or more, the product of n value and r value, and n × r is 0.22 or more. A welded steel pipe excellent in hydroforming properties characterized by the above has been proposed. This welded steel pipe is a drawn steel pipe having the above composition as a raw steel pipe, and after subjecting the raw steel pipe to heat treatment or soaking, the cumulative diameter reduction ratio is 35% or more and the rolling finish temperature is 500 to 900 ° C. It is manufactured by rolling (preferably the cumulative diameter reduction rate in the temperature range below the Ar ₃ transformation point is 20% or more).
JP 2003-49245 A

On the other hand, in recent years, not only hydroforming but also welded steel pipes, which are excellent in burring properties, such as burring, which is difficult to crack even after severe processing, such as burring, which is expanded after punching by punching. It has come to be required. However, although the welded steel pipe described in Patent Document 1 is excellent in hydroforming properties, it is insufficient in terms of burring properties.

  Accordingly, an object of the present invention is to provide a welded steel pipe of TS400 MPa or more that is excellent in burring property in addition to hydroforming property, together with its appropriate manufacturing method.

  In order to achieve the above-mentioned object, the present inventors diligently studied on the basis of the welded steel pipe described in Patent Document 1, and as a result, reduced C and S and the second phase in the structure (: other than the ferrite phase) It has been found that the local elongation increases due to the reduction of the fraction of (phase) and sufficient burring property can be secured, and that the strength decrease due to the reduction of C can be compensated by increasing the amount of Mn and Si, and TS ≧ 400 MPa can be secured, Made the present invention.

  That is, the present invention provides, in mass%, C: 0.01% or more and less than 0.05%, Si: 1.5% or less, Mn: 0.2 to 3.5%, S: less than 0.002%, Al: 0.1% or less, N: 0.01% or less. And having a composition consisting of the remainder Fe and inevitable impurities, the entire structure is bainitic ferrite structure, or polygonal ferrite structure: 1 to 40% by volume, and the remainder consists of bainitic ferrite structure, and The inverse strength ratio of the ferrite (110) plane in the cross section perpendicular to the longitudinal direction (C cross section) is 2.0 or more, or the inverse strength of the ferrite (200) plane in the cross section perpendicular to the circumferential direction (L cross section). It is a welded steel pipe having a tensile strength of 400 MPa or more, excellent in hydroforming properties and burring properties, characterized by having a texture with a ratio of 2.0 or more.

  In the present invention, in addition to the above composition, Group A: Cr: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less, B: One or more of 0.01% or less, and / or Group B: Ca: 0.02% or less, REM: One or two of 0.02% or less may be contained.

  Further, the present invention is, in mass%, C: 0.01% or more and less than 0.05%, Si: 1.5% or less, Mn: 0.2 to 3.5%, P: 0.1% or less, S: less than 0.002%, Al: 0.1% or less, N: 0.01% or less, or further, Group A: Cr: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less, B: welded steel pipe having a composition containing one or more of 0.01% or less and / or group B: Ca: 0.02% or less, REM: one or two of 0.02% or less Hydroforming, characterized by subjecting the raw steel pipe to heat treatment or soaking, and then subjecting the raw steel pipe to drawing rolling with a cumulative diameter reduction ratio of 30% or more and a rolling end temperature of 650 to 900 ° C. This is a method for producing a welded steel pipe having a tensile strength of 400 MPa or more, which is excellent in heat resistance and burring properties. In this production method, it is preferable to cool the steel pipe after drawing to 400 ° C. or less at an average cooling rate of 3 ° C./s or more.

  According to the present invention, an appropriate rolling texture can be formed, and high r value, high n value, and high ductility can be achieved without reducing the strength, so that excellent hydroforming properties can be obtained, and Since the fraction of the second phase is reduced and the local elongation is improved, excellent burring properties can be obtained.

  First, the reason why the composition of the steel used in the present invention is defined as described above will be described. Hereinafter, the mass% related to the composition is simply referred to as%.

C: 0.01% or more and less than 0.05% C is an element that contributes to an increase in the strength of the steel. However, if it is contained in an amount of 0.05% or more, the fraction of the second phase increases, the local elongation decreases, and the burring properties deteriorate. On the other hand, when the content is less than 0.01%, it is difficult to ensure a desired tensile strength. For this reason, C was made into the range of 0.01% or more and less than 0.05%.

Si: 1.5% or less
Si is an element that increases the strength of the steel, and is preferably contained in an amount of 0.01% or more depending on the desired strength. However, if excessively contained, the surface property is significantly deteriorated and the hydroforming property is lowered. Therefore, in the present invention, Si is set to 1.5% or less. In addition, Preferably it is 1% or less.

Mn: 0.2-3.5%
Mn is an element that improves the strength without deteriorating the surface properties and weldability, and is 0.2% or more in order to ensure a desired strength within a low C content range. On the other hand, if the content exceeds 3.5%, the critical expansion ratio at the time of hydroforming is lowered, and the hydroforming property is lowered. For this reason, Mn was limited to 3.5% or less. In addition, Preferably, it is 0.2 to 2%.

S: Less than 0.002% S is present in steel as a non-metallic inclusion, and cracks are likely to occur at the periphery of the hole when burring after hydroforming is small, starting from this non-metallic inclusion. Therefore, the burring property is deteriorated. In order to prevent the deterioration of the burring property, S needs to be limited to less than 0.002%. Further, from the viewpoint of further improving the burring property, S is preferably made 0.001% or less. In addition, since the non-metallic inclusion of S becomes a starting point at which the steel pipe breaks (bursts) at the time of hydroforming and lowers the hydroforming property, in Patent Document 1, the S amount is 0.01% or less in order to prevent this, For more severe burring, it is necessary to make the upper limit of the S amount much lower than that of Patent Document 1 as defined in the present invention.

Al: 0.1% or less
Al is a useful element that acts as a deoxidizer and suppresses coarsening of crystal grains, and is preferably contained in an amount of 0.01% or more. However, if the content exceeds 0.1%, the amount of oxide inclusions that tend to be the starting point of cracking during burring increases and burring properties deteriorate, so Al was made 0.1% or less.

  In the present invention, the material steel pipe is first subjected to heat treatment or soaking. The temperature condition of the heat treatment applied to the raw steel pipe is not particularly limited, but is preferably 700 to 1100 ° C. in order to satisfy the drawing rolling conditions described later. However, if the raw steel pipe is manufactured warmly or hotly and sufficient temperature is maintained during the drawing rolling, it is sufficient to perform soaking to equalize the pipe temperature distribution. is there. Needless to say, heat treatment is performed when the temperature of the material steel pipe is low.

N: 0.01% or less N is an element that combines with Al to refine crystal grains. For this purpose, N is preferably contained in an amount of 0.001% or more, but if contained over 0.01%, ductility is deteriorated. For this reason, N was made into 0.01% or less.

Group A: Cr: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less, B: 0.01% or less 2 or more types
Cr, Ti, Nb, Cu, Ni, Mo, and B are useful elements that can improve strength without impairing ductility, and can be selected and contained as necessary. Such an effect is noticeable when the content of Cr, Ti, Nb, Cu, Ni, and Mo is 0.01% or more, and when B is 0.0001% or more, 0.1% for Cr and 0.05% for Ti and Nb. %, Cu, Ni, Mo 1%, B exceeding 0.01%, the effect is saturated, the effect commensurate with the content can not be expected, economically disadvantageous, Reduces ductility, weldability, hot workability and cold workability of steel. Therefore, Cr: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less, and B: 0.01% or less are preferable.

Group B: Ca: 0.02% or less, REM: One or two of 0.02% or less
Ca and REM are elements that have a function of improving the hydroforming property by making the form of non-metallic inclusions spherical, and can be selected and contained as necessary. Such an effect becomes remarkable when both Ca and REM are contained in an amount of 0.0020% or more. On the other hand, if the content exceeds 0.02%, the amount of inclusions becomes excessive and the cleanliness is lowered. For this reason, it is preferable that both Ca and REM be 0.02% or less. When both Ca and REM are used in combination, the total amount is preferably 0.03% or less.

  The balance other than the above components is Fe and inevitable impurities. Among inevitable impurities, P is allowed to be 0.05% or less, and O is allowed to be 0.01% or less.

  Next, in the welded steel pipe of the present invention, the whole structure is a bainitic ferrite structure, or a structure in which a portion having a volume ratio of 1 to 40% is replaced with a polygonal ferrite structure. By adopting this structure, the second phase fraction is reduced, the local elongation is improved, and the burring property is excellent. From the viewpoint of balance between strength and ductility, the average crystal grain size of the polygonal ferrite structure is preferably 5 μm or less. Here, the structure of the steel pipe was identified by observing, with an optical microscope, a structure obtained by corroding a section perpendicular to the longitudinal direction (C section) with nital, and comparing it with a standard structure. The volume ratio and the average crystal grain size related to the tissue are measured using an image analyzer.

  Further, in the welded steel pipe of the present invention, from the viewpoint of increasing the r value and the n value, the inverse strength ratio of the ferrite (110) plane in the cross section perpendicular to the longitudinal direction is 2.0 or more, or further in the circumferential direction. The ferrite had a texture with an inverse strength ratio of 2.0 or more on the (200) plane of ferrite in a vertical section. Thereby, the outstanding hydroforming property is obtained. Here, the inverse intensity ratio is obtained by irradiating the measurement sample with X-rays, measuring the diffraction intensity from each crystal plane obtained, and taking the ratio with the diffraction intensity of the sample with a random crystal orientation.

  The welded steel pipe of the present invention having the above composition, structure and texture has a high strength of TS400 MPa or more and is excellent in hydroforming properties and burring properties. The hydroforming property is evaluated by measuring the limit tube expansion rate by performing a free bulge test with axial compression using a hydroforming machine (for example, one shown in FIG. 2 of Patent Document 1), and evaluating the value. The Here, limit tube expansion rate = (dmax−d0) / d0 (× 100%); dmax: maximum outer diameter at burst (rupture) (= periphery length of burst portion / circumference ratio), d0: outside before test Diameter. On the other hand, the burring property is evaluated by a hole expansion test as shown in FIG. That is, a hole 2 having an initial diameter Φ 0 = 10 mm is punched into a test piece 1 cut out from a tube after hydroforming and flattened, and a punch 3 is pushed into the hole 2 from below to expand the hole diameter. When the crack generated in the hole penetrates the plate thickness, the push-in of the punch 3 is stopped, the diameter Φ of the expanded hole 2 is measured, and the hole expansion rate is expressed by the formula: (Φ−Φ0) / Φ0 (× 100%) Is calculated and evaluated with that value.

  Next, the manufacturing method of this invention welded steel pipe is demonstrated.

  In the present invention, a welded steel pipe having the above-described composition is used as a raw steel pipe, but the means for manufacturing the raw steel pipe is not particularly limited. The steel strip is cold or warm or hot rolled or bent to form an open tube, and both edges of the open tube are heated to the melting point or higher by induction heating and squeeze-rolled into a squeeze roll. Electric resistance welding, or both edges of the open pipe are heated to a solid pressure welding temperature range below the melting point using induction heating and subjected to abutting pressure welding with a squeeze roll, or a forge welding method, etc. Either can be used suitably. The steel strip used for the production of the raw steel pipe is a slab formed by continuous casting or ingot-bundling after melting the steel having the above composition, and the slab is formed into a hot-rolled steel sheet by hot rolling. In addition, a hot-rolled steel sheet or a cold-rolled steel sheet that is a cold-rolled steel sheet by cold rolling-annealing can be suitably used.

  Further, after the above heat treatment or soaking, drawing rolling is performed at a cumulative diameter reduction ratio of 30% or more and a rolling end temperature of 650 to 900 ° C.

  In this drawing rolling, if the cumulative diameter reduction ratio is less than 30%, the desired structure and texture cannot be obtained, and hydroforming properties and burring properties are deteriorated. Therefore, the cumulative diameter reduction ratio is 30% or more. did. The upper limit of the cumulative diameter reduction rate is not particularly limited, but is preferably about 95% from the viewpoint of reducing uneven thickness and productivity.

  In addition, when the rolling end temperature is less than 650 ° C. or exceeds 900 ° C., the desired structure and texture cannot be obtained, and the hydroforming property and burring property are deteriorated. ˜900 ° C.

  The above-described drawing rolling can be efficiently performed by using a drawing rolling device called a reducer in which a plurality of perforated rolling mills are arranged in tandem.

  Furthermore, in the present invention, from the viewpoint of further improving the strength and ductility by refining the structure, the steel pipe after drawing and rolling is cooled to a cooling stop temperature provided at a temperature range of 400 ° C. or lower at an average cooling rate of 3 ° C./s or higher. Cooling is preferred. The average cooling rate here is an average cooling rate between the rolling end temperature and the cooling stop temperature. Such post-rolling cooling can be easily carried out by disposing a device for spraying water spray or mist on the exit side of the reducer, and setting and operating under appropriate conditions.

  A steel plate (hot rolled steel plate or annealed cold-rolled steel plate) having the composition shown in Table 1 is roll-formed into an open tube under the conditions shown in Table 2, and then both ends in the pipe circumferential direction are heated by induction heating and subjected to butt joining. Thus, a welded steel pipe was obtained.

  These welded steel pipes were subjected to drawing rolling under the conditions shown in Table 2 as raw steel pipes (elementary pipes) to obtain product steel pipes (product pipes) having the sizes shown in Table 2. The following investigation was conducted on the obtained product steel pipe.

  -A JIS No. 11 tensile test specimen was collected and subjected to a tensile test to measure the yield strength YS, tensile strength TS, and elongation El.

  -JIS No. 12 A tensile test specimens were collected (the test direction was parallel to the longitudinal direction), the tensile test was performed, and the n value and the r value were measured by the following equations.

n = ln (σ ₁₀ / σ ₅ ) / ln (ε ₁₀ / ε ₅ )
Here, σ and ε are the true stress and true strain, and the subscript is the% value of the nominal strain at the measurement stage.

r = ln (W ₀ / W _x ) / ln (t ₀ / t _x ) ≈ln (W ₀ / W _x ) / ln (L _x W _x / (L ₀ W ₀ ))
Here, t, W, and L are the plate thickness, plate width, gauge length, and subscript are the% values of the nominal strain at the measurement stage, and x = 6 to 7% and L ₀ = 2 mm.

  -A free bulge test with axial compression was performed using the hydroforming machine described above, and the critical tube expansion rate was measured.

  ・ After hydroforming to a pipe expansion ratio of 40%, the hole expansion ratio was measured by performing the hole expansion test described above.

  -The organization and texture were investigated by the method described above.

  Table 3 shows the results obtained from the above investigation. Table 3 also shows YR (= YS / TS).

  In all of the examples of the present invention, desired structures and textures are obtained, TS is 400 MPa, and excellent hydroforming properties and burring properties are provided. In addition, the structure of the welded steel pipe of the present invention example was finer than the comparative example and the fraction of the second phase was lower as shown in FIG.

It is explanatory drawing which shows the outline | summary of a hole expansion test. (a) is a copy of an optical micrograph showing the structure of a welded steel pipe of Invention Example No. 1 and (b) of Comparative Example No. 13.

Explanation of symbols

1 Specimen 2 Hole 3 Punch

Claims (4)

  In mass%, C: 0.01% or more and less than 0.05%, Si: 1.5% or less, Mn: 0.2 to 3.5%, S: less than 0.002%, Al: 0.1% or less, N: 0.01% or less, the remainder Fe and inevitable A cross section perpendicular to the longitudinal direction, having a composition composed of mechanical impurities, the entire structure being bainitic ferrite structure or polygonal ferrite structure: 1 to 40% by volume and the remainder being bainitic ferrite structure It has a texture in which the (110) plane inverse strength ratio of ferrite is 2.0 or more, or the ferrite (200) plane inverse strength ratio is 2.0 or more in a cross section perpendicular to the circumferential direction. A welded steel pipe with a tensile strength of 400 MPa or more with excellent hydroforming and burring properties.   In addition to the above composition, Group A: Cr: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less, B: 0.01% 2. One or more of the following and / or Group B: Ca: 0.02% or less, REM: One or two of 0.02% or less, A welded steel pipe with a tensile strength of 400 MPa or more with excellent hydroforming and burring properties.   In mass%, C: 0.01% or more and less than 0.05%, Si: 1.5% or less, Mn: 0.2 to 3.5%, P: 0.1% or less, S: less than 0.002%, Al: 0.1% or less, N: 0.01% or less Including or in addition, Group A: Cr: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less, B: 0.01% or less A welded steel pipe having a composition containing one or more of them and / or group B: Ca: 0.02% or less, REM: 0.02% or less, as a material steel pipe, It is excellent in hydroforming and burring characteristics, characterized by subjecting the steel tube to heat treatment or soaking, and then subjecting it to drawing rolling with a cumulative diameter reduction ratio of 30% or more and a rolling end temperature of 650 to 900 ° C. A method for manufacturing welded steel pipes with a tensile strength of 400 MPa or more.   The steel pipe after drawing rolling is cooled to 400 ° C or less at an average cooling rate of 3 ° C / s or more, and the welded steel pipe having a tensile strength of 400 MPa or more excellent in hydroforming properties and burring properties. Production method.

Images