JP5732999B2 - High-strength ERW steel pipe and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength electric resistance welded steel pipe suitable for automobile impact members such as door impact beams, cross members, and pillars, and more particularly to a high-strength electric resistance welded steel pipe excellent in both workability and shock absorption characteristics. .

  In recent years, for the purpose of improving the safety of automobiles, in particular, ensuring the safety of passengers, automobile bodies have been provided with shock absorbing members for absorbing impact energy at the time of collision. For example, as shown in Patent Document 1, a high-strength steel pipe having a martensite structure and a desired high strength is applied to a door impact beam that is a structural member.

The technology described in Patent Document 1 includes C: 0.15-0.22%, Mn: 1.5% or less, Si: 0.5% or less, Ti: 0.04% or less, B: 0.0003-0.0035%, N: 0.0080% or less, or even, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, subjected to quenching treatment in one or steel pipe containing two or more, tensile strength: 120 kgf / mm ² or more mechanical structures This is a method for producing a high-strength steel pipe for machine structure, in which a high-strength steel pipe is obtained. According to the technology described in Patent Document 1, it has a good elongation of 10% or more and can be applied as a core material for door impact bars (bump impact beams) and bumpers, which are steel tubes for automobile reinforcement, and has a tensile strength. It is said that a high-strength steel pipe of 120 kgf / mm ² or more can be obtained by a single heat treatment.

Japanese Patent Laid-Open No. 3-122219

However, the technique described in Patent Document 1 does not cause a big problem in the case of a door impact beam that is used in a straight pipe without performing special processing, but requires complicated processing into various shapes. In other automobile impact absorbing members such as cross members, pillars, and the like, the steel pipe used is required to have a further excellent workability in addition to high strength.
In view of such demands, the present invention has excellent workability, and in particular, even when flattening a steel pipe, there is no occurrence of cracks in the vicinity of the electric-welded welded part, and further, automobile impact absorption is achieved. An object of the present invention is to provide a high-strength electric resistance welded steel pipe suitable for a member and capable of ensuring excellent shock absorption characteristics and a method for producing the same.

  Here, “high strength” refers to a case where the tensile strength TS is 1180 MPa or more. The term “excellent workability” here means that the elongation El in the tube axis direction obtained by a tensile test using a JIS No. 12 tensile test piece (GL: 50 mm) specified by the JIS standard is 10% or more. Preferably, it is 12% or more and the yield ratio (= (0.2% proof stress / tensile strength) × 100 (%)) in the tube axis direction is less than 90%. The “excellent shock absorption property” here refers to 0.2% proof stress after applying a pre-strain of 2% or more to the pipe and further heat treatment (paint baking treatment) at 170 ° C for 20 minutes. It is assumed that the strength increase amount (BH amount), which is the difference from the strength after imparting strain, is 100 MPa or more and the yield ratio in the tube axis direction is 90% or more. The amount of BH is defined in FIG.

  In order to achieve the above-mentioned object, the present inventors diligently studied various measures for improving the workability of an electric resistance welded steel pipe while maintaining high strength. As a result, a steel pipe material (cold-rolled steel sheet) that has a two-phase structure consisting of ferrite and martensite, has excellent workability, and has the desired paint bake hardenability is reduced as much as possible. By adopting a pipe making method that can make pipes without making them, we have come up with an ERW steel pipe with excellent workability. And after giving the desired member shape to such an ERW steel pipe, heat resistance (paint baking process) is applied to increase the yield strength. It has been found that absorption characteristics can be secured.

The present invention has been completed based on such findings and further investigations. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.05 to 0.20%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005 And a composition composed of the balance Fe and inevitable impurities, and a two-phase structure composed of a ferrite phase and a martensite phase, the structure having a martensite phase volume ratio of 20 to 60%, and tensile Excellent processability with strength TS of 1180 MPa or more, tube axis elongation El of 10% or more, and yield ratio of less than 90%, and pre-strain: 2%, and then baking with 170 ° C x 20 min heat treatment The strength increase after treatment (BH amount) is 100MPa or more and the yield ratio is 90% or more. It has excellent shock absorption characteristics, and the inner bead height of the ERW weld is -0.1 to 0.1mm. A high-strength ERW steel pipe characterized by being.

(2) In (1), in addition to the above composition, in addition to mass, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti : 0.05% or less, W: 0.05% or less, B: A composition containing one or more selected from 0.0050% or less.
(3) In (1) or (2), in addition to the above composition, the composition further contains, by mass%, one or two selected from Ca: 0.0050% or less and REM: 0.0050% or less A high-strength ERW steel pipe characterized by

(4) A hot rolling process in which the steel material is hot-rolled into a hot-rolled sheet, and the hot-rolled sheet is pickled, and then cold-rolled into a cold-rolled sheet. An annealing process is performed on the cold-rolled sheet to form a cold-rolled annealed sheet to obtain a steel pipe material, and then the steel pipe material is continuously formed on the steel pipe material to form a substantially cylindrical shape. After the open pipe is electro-welded to form an electric-welded pipe, a pipe-forming step of cutting the weld bead of the electric-welded welded portion formed by the electric-resistance welding is performed to obtain an electric-welded steel pipe In this case, the steel material is, in mass%, C: 0.05 to 0.20%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%. , N: include 0.005% or less, in a steel material having a composition the balance being Fe and unavoidable impurities, said hot-rolled process, finish rolling temperature is Ar ₃ transformation point or higher, A hot rolling process in which the coiling temperature is 500 to 650 ° C. is used as a hot rolled sheet, and the annealing process is performed on the cold rolled sheet in a two-phase temperature range of Ac ₁ transformation point to Ac ₃ transformation point. After heating and soaking, the sample is subjected to a rapid cooling treatment in which the cooling is performed at an average cooling rate of 500 ° C./s from a temperature in the range of 600 to 750 ° C. to room temperature, and then a temperature of 150 to 300 ° C. A process of tempering to reheat to a range and making it a cold-rolled annealed plate, the forming is a cage roll type roll forming, and the inner bead height is -0.1 to 0.1 mm by cutting the weld bead, The ERW steel pipe has a two-phase structure composed of a ferrite phase and a martensite phase, the martensite phase has a structure with a volume ratio of 20 to 60%, a tensile strength TS of 1180 MPa or more, and a pipe axis direction Excellent workability with an elongation El of 10% or more and a yield ratio of less than 90% and pre-strain: 2% after applying 1% It is a steel pipe with excellent shock absorption characteristics that yield strength increase (BH amount) after paint baking treatment with heat treatment of 70 ℃ x 20min is 100MPa or more and yield ratio is 90% or more. A method for producing a high-strength ERW steel pipe.

(5) In (4), in addition to the above composition, in addition to mass, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti : 0.05% or less, W: 0.05% or less, and B: A composition containing one or more selected from 0.0050% or less.
(6) In (4) or (5), in addition to the above composition, the composition further contains one or two selected from Ca: 0.0050% or less and REM: 0.0050% or less by mass% A method for producing a high-strength ERW steel pipe.

  ADVANTAGE OF THE INVENTION According to this invention, it is the high intensity | strength electro-sewing which has the outstanding workability suitable for the impact-absorbing member of a motor vehicle, and can ensure the outstanding impact-absorbing characteristic, after shape | molding in a real member shape. Steel pipes can be manufactured inexpensively and easily, and have remarkable industrial effects. The high-strength electric resistance welded steel pipe according to the present invention is not limited to the door impact beam, but other automobile shock absorbing members such as cross members and pillars that require workability in particular, and automobile members such as automobile frame members in general. There is also an effect that it can be applied as a material for automobile parts.

It is explanatory drawing which shows typically an example of the electric-resistance-welded steel pipe manufacturing equipment which employ | adopted the CBR type roll forming method suitable for implementation of this invention. It is sectional drawing which shows the ERW welding part vicinity in a cross section perpendicular | vertical to a pipe | tube longitudinal direction, (a) shows the state before bead cutting, (b) shows the state after bead cutting. It is explanatory drawing which shows typically an example of an inner surface bead cutting device. It is explanatory drawing which shows typically the execution point of a flat work test. It is explanatory drawing which shows typically the definition of the strength increase amount (BH amount) after a paint baking process.

First, the reasons for limiting the composition of the high-strength ERW steel pipe according to the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.05-0.20%
C is an element having an action of strengthening steel, and in the present invention, it is necessary to contain 0.05% or more in order to ensure a desired strength. On the other hand, if the content exceeds 0.20%, weldability decreases. For this reason, in this invention, C was limited to 0.05 to 0.20% of range. In addition, Preferably it is 0.08 to 0.18%.

Si: 0.5-2.0%
Si acts as a deoxidizer and has the effect of solid solution to strengthen the steel and promote the formation of ferrite, and is an important element for securing excellent workability. In addition, Si can achieve a desired high strength by suppressing the martensite phase fraction to a low level by solid-solution strengthening the ferrite phase. In order to acquire such an effect, 0.5% or more of content is required. On the other hand, if the content exceeds 2.0%, a large amount of Si oxide is formed on the surface of the steel sheet, and the chemical conversion processability is lowered. For this reason, in this invention, Si was limited to 0.5 to 2.0% of range. In addition, Preferably it is 1.0 to 1.8%.

Mn: 1.0-3.0%
Mn is an element that improves the hardenability, facilitates the formation of a martensite phase, and increases the strength of the steel. In order to ensure the desired strength, Mn is required to be contained in an amount of 1.0% or more. On the other hand, if the content exceeds 3.0%, segregation is promoted, slab cracking is likely to occur during casting, and the martensite phase is excessively increased to deteriorate workability. For this reason, Mn was limited to the range of 1.0 to 3.0%. In addition, Preferably it is 1.5 to 2.5%.

P: 0.1% or less P is preferably reduced as much as possible as an impurity in the present invention in order to avoid adverse effects on workability, but excessive reduction increases the refining cost. For this reason, P was limited to 0.1% or less, which has substantially no adverse effect. In addition, Preferably it is 0.05% or less.
S: 0.01% or less S is an impurity in the present invention, as in the case of P, and is preferably reduced as much as possible in order to avoid adverse effects on workability. However, excessive reduction increases the refining cost, so the upper limit is 0.01. %. In addition, Preferably it is 0.005% or less.

Al: 0.01 to 0.1%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it needs to be contained in an amount of 0.01% or more. On the other hand, even if the content exceeds 0.1%, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, Al was limited to the range of 0.01 to 0.1%. In addition, Preferably it is 0.01 to 0.08%.

N: 0.005% or less N is an element that strengthens steel and lowers formability, and is preferably reduced as much as possible as impurities, but excessive reduction raises the refining cost. For this reason, N was limited to 0.005% or less with substantially no adverse effects. In addition, Preferably it is 0.004% or less.
The above components are basic components. In addition to the basic composition, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti: 0.05% or less, W: 0.05% or less, B: one or more selected from 0.0050% or less, and / or Ca: 0.0050% or less, REM: 0.0050% or less One or two selected from among them can be selected and contained.

Cu, Ni, Cr, Mo, Nb, Ti, W, and B are all elements that increase the strength of the steel, and can be selected as necessary and contained in one or more.
Cu: 1.0% or less
Cu is an element that has the effect of increasing the strength of steel and improving the corrosion resistance, and can be contained if necessary. Such an effect is recognized at a content of 0.05% or more, but a content exceeding 1.0% reduces the hot workability. For this reason, when it contains, it is preferable to limit Cu to 1.0% or less. In addition, More preferably, it is 0.08 to 0.5%.

Ni: 1.0% or less
Ni is an element having an action of increasing the strength of steel and improving the corrosion resistance, and can be contained as necessary. Such an effect is recognized at a content of 0.05% or more, but Ni is an expensive element, and a large content exceeding 1.0% raises the material cost. For this reason, when it contains, it is preferable to limit Ni to 1.0% or less. In addition, More preferably, it is 0.08 to 0.5%.

Cr: 0.5% or less
Cr is an element that has the effect of increasing the strength of steel through improving hardenability and improving corrosion resistance, and can be contained as necessary. Such an effect is recognized at a content of 0.05% or more, but if it exceeds 0.5%, the workability is lowered. For this reason, when contained, Cr is preferably limited to 0.5% or less. In addition, More preferably, it is 0.05 to 0.4%.

Mo: 0.5% or less
Mo is an element that has the effect of increasing the strength of steel by precipitation strengthening in addition to improving hardenability, and can be contained if necessary. Such an effect is recognized at a content of 0.05% or more, but when it exceeds 0.5%, the ductility is lowered and the material cost is increased. For this reason, when it contains, it is preferable to limit Mo to 0.5% or less. In addition, More preferably, it is 0.1 to 0.4%.

Nb: 0.05% or less
Nb is an element having an effect of refining crystal grains and increasing the strength of steel through precipitation strengthening, and can be contained as necessary. Such an effect is recognized at a content of 0.005% or more, but if it exceeds 0.05%, the ductility is lowered. For this reason, when it contains, it is preferable to limit Nb to 0.05% or less. In addition, More preferably, it is 0.008 to 0.03%.

Ti: 0.05% or less
Ti is an element having an effect of refining crystal grains and increasing the strength of steel through precipitation strengthening, and can be contained as necessary. Such an effect is recognized at a content of 0.005% or more, but if it exceeds 0.05%, the ductility is lowered. For this reason, when it contains, it is preferable to limit Ti to 0.05% or less. In addition, More preferably, it is 0.008 to 0.03%.

W: 0.05% or less W is an element having an action of increasing the strength of steel through precipitation strengthening and can be contained as necessary. Such an effect is recognized at a content of 0.01% or more, but if it exceeds 0.05%, the ductility is lowered. For this reason, when it contains, it is preferable to limit W to 0.05% or less. In addition, More preferably, it is 0.01 to 0.03%.

B: 0.0050% or less B is an element having an action of adjusting the martensite fraction within a predetermined range through improvement of hardenability and increasing the strength of the steel, and can be contained as necessary. Such an effect is recognized with a content of 0.0005% or more, but a content exceeding 0.0050% is economically disadvantageous because the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, when it contains, it is preferable to limit B to 0.0050% or less. In addition, More preferably, it is 0.001 to 0.003%.

Ca: 0.0050% or less, REM: 0.0050% or less
Both Ca and REM are elements having an effect of improving ductility through the form control of sulfide inclusions, and can be contained as necessary. Such an effect is observed when the content of Ca and REM is 0.0020% or more. However, when the content exceeds 0.0050%, the amount of inclusions increases too much, and the cleanliness of the steel decreases. For this reason, when contained, both Ca and REM are preferably limited to 0.0050% or less. More preferably, both are 0.0020 to 0.0040%.

The balance other than the above components is Fe and inevitable impurities.
Next, the reason for limiting the structure of the steel pipe of the present invention will be described.
The steel pipe of the present invention includes a martensite phase having a volume ratio of 20 to 60%, and has a two-phase structure composed of the remaining ferrite phase. By setting it as such a structure | tissue, it becomes possible to have desired high intensity | strength, the outstanding workability, and the outstanding paint bake hardenability.

If the martensite phase is less than 20% by volume, the structure is mainly composed of a ferrite phase, and a desired high strength cannot be achieved. On the other hand, when the amount of martensite phase exceeds 60% by volume, it becomes a structure mainly composed of martensite and desired workability cannot be ensured. For this reason, the structure fraction of the martensite phase is limited to a range of 20 to 60% by volume. Preferably, the volume ratio is 40 to 55%.
Next, in the steel pipe of the present invention, it is necessary to adjust the inner bead height of the ERW weld to -0.1 to 0.1 mm. When the inner surface bead height exceeds 0.1 mm, for example, about 0.2 mm, when flattening is performed on the high-strength steel pipe having the above composition and structure, a crack is generated from the inner surface bead portion. This is presumably because the high-strength steel pipe having the composition and structure described above has a large hardness difference between the base metal part and the welded part (bead part and heat-affected zone), and deformation concentrates on the heat-affected zone. If a crack is generated in the steel pipe, the shock absorbing ability is deteriorated when it is used as a member. For this reason, the generation of cracks during flattening must be prevented. If the inner bead height is 0.1 mm or less, cracking from the inner surface can be suppressed even if flattening is performed. If the inner bead height is less than -0.1 mm, considering the case where tube expansion processing is performed thereafter, the inner-bead cutting portion becomes thicker during the tube expansion processing, which may cause breakage. For this reason, in the present invention, the inner bead height is limited to -0.1 to 0.1 mm. In addition, what is necessary is just to cut the outer surface bead height smoothly like a normal ERW steel pipe.
Below, the preferable manufacturing method of this invention steel pipe is demonstrated.

In the present invention, the steel material is subjected to a hot rolling process, a cold rolling process, and an annealing process to obtain a steel pipe material, and then the steel pipe material is subjected to a pipe making process to obtain an electric-welded steel pipe.
Although the manufacturing method of the steel raw material to be used is not particularly limited, the molten steel having the above-described composition is melted by a conventional melting method such as a converter, and steel such as a slab by a continuous casting method or an ingot-rolling method. It is preferable to use a raw material.

The obtained steel material is then subjected to a hot rolling process that is hot-rolled to form a hot-rolled sheet.
The obtained steel material may be reheated after cooling, or when the steel material retains a predetermined amount of heat, it may be directly sent and hot rolled without reheating. In the case of reheating, the heating temperature is preferably 1000 to 1250 ° C. If the heating temperature at the time of reheating is less than 1000 ° C., the deformation resistance is high and the load applied to the rolling mill becomes too large, which may make rolling difficult. On the other hand, when heating is performed at a temperature exceeding 1250 ° C., the coarsening of crystal grains proceeds and the ductility and the like are significantly reduced.

Hot rolling consists of rough rolling and finish rolling. The rough rolling conditions are not particularly limited as long as a sheet bar having a predetermined size and shape can be obtained. In the finish rolling, the finish rolling finish temperature is set to be higher than the Ar ₃ transformation point of the steel strip that is the material to be rolled, and the finish rolling is taken up at a winding temperature of 500 to 650 ° C.
When the finish rolling finish temperature is less than the Ar ₃ transformation point, the finish rolling is a (α + γ) two-phase region rolling, resulting in a mixed grain structure in which extremely coarse crystal grains and fine crystal grains are mixed. For this reason, even if it performs a cold rolling process-annealing process after that, favorable workability cannot be ensured, or rough skin arises in processes, such as press molding and bending. For this reason, the finish rolling finishing temperature of hot rolling is limited to the Ar ₃ transformation point or higher. On the other hand, when the coiling temperature is less than 500 ° C., a hard phase is generated during cooling, so that a rolling load during cold rolling increases and productivity decreases. On the other hand, when the temperature is higher than 650 ° C., untransformed austenite is transformed into pearlite, so that workability is deteriorated. For this reason, the coiling temperature was limited to a range of 500 to 650 ° C.

Next, the hot-rolled sheet obtained through the hot-rolling process is subjected to pickling treatment, and then subjected to a cold-rolling process in which cold rolling is performed to obtain a cold-rolled sheet. Conditions for the cold rolling process such as cold rolling reduction are not particularly specified.
The obtained cold-rolled sheet is then subjected to an annealing process to form a cold-rolled annealed sheet.
In the present invention, the annealing step is an important step in securing desired workability and desired paint bake hardenability (BH property). The annealing process preferably uses a continuous annealing line.

In the annealing process, the cold-rolled sheet is heated to a temperature in the two-phase temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point, held soaked, and then averaged from a temperature in the range of 600 to 750 ° C. to room temperature. Then, a rapid cooling process for cooling at a cooling rate of 500 ° C./s or more is performed, and then a tempering process for reheating to a temperature range of 150 to 300 ° C. is performed to form a cold-rolled annealed plate. In order to stably secure a desired high strength, the average cooling rate in the rapid cooling treatment is preferably set to 800 ° C./s or more. More preferably, it is 1000 ° C./s or more.

If the temperature for heating and soaking is out of the two-phase temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point, the desired structure fraction (ferrite + martensite) structure can be secured by the subsequent rapid cooling. Disappear. On the other hand, if the rapid cooling start temperature is out of the range of 750 ° C. to 600 ° C., it becomes impossible to obtain a (ferrite + martensite) structure having a desired structure fraction. If the quenching start temperature is higher than 750 ° C, the ductility is lowered, and if it is lower than 600 ° C, a desired high strength cannot be secured. In addition, it is desirable that the soaking time at the above-mentioned temperature is 30 s or longer.

Further, when the cooling rate from the temperature in the range of 600 to 750 ° C. to room temperature is less than 500 ° C./s on average, the amount of martensite transformation is small and a desired structure fraction (ferrite + martensite) structure is obtained. The desired high strength cannot be ensured. The cooling rate is an average between the quenching start temperature and 200 ° C.
In addition, although it does not specifically limit about the method of a rapid-cooling process, From a viewpoint of suppressing the material dispersion | variation in a steel plate width direction and a longitudinal direction, it is preferable to set it as cooling using jet water.
Furthermore, in the annealing step of the present invention, after the rapid cooling treatment, a tempering treatment is performed in which reheating is performed to a temperature range of 150 to 300 ° C. for the purpose of further improving toughness. If the tempering temperature is less than 150 ° C, the effect of improving toughness cannot be expected.

On the other hand, when it exceeds 300 ° C., ductility is lowered due to low temperature temper brittleness. For this reason, the temperature range of reheating was limited to 150-300 degreeC.
The obtained cold-rolled annealed sheet may be further subjected to temper rolling as necessary. At that time, the temper reduction ratio is preferably 0.2% or more and 1.0% or less. If the temper reduction ratio is less than 0.2%, the shape correction effect cannot be obtained. On the other hand, if it exceeds 1.0%, the deterioration of elongation becomes remarkable.

The cold-rolled annealed plate (cold-rolled annealed steel strip) that has undergone the above-described process is used as a steel pipe material, and then the steel pipe material is subjected to a pipe making process to obtain an electric-welded steel pipe. In the pipe making process, the steel pipe material is continuously formed into a substantially cylindrical open pipe, the open pipe is electro-welded to form an electric-welded pipe, and the weld bead of the electric-welded welded portion of the electric-welded pipe is cut. It is a process to do.
In the present invention, the forming in the pipe making process is roll forming by a cage roll method. Roll forming by the cage roll method refers to roll forming of a method in which small rolls called cage rolls are lined up on the tube outer surface side and formed smoothly. Of the roll forming by the cage roll method, CBR roll forming is preferred. In molding by this method, distortion applied to the strip during molding can be minimized, and deterioration of material properties due to work hardening can be suppressed.

An example of the production equipment of the ERW steel pipe which employ | adopted CBR type roll forming is shown in FIG. In CBR roll forming, both edge portions of the strip 1 are pre-formed by the edge bend roll 2, and then the center portion of the strip is bent by the center bend roll 3 and the cage roll 4, so that the oblong oblong shape is formed. After making the raw pipe, and then overbending the four places in the pipe circumferential direction with the fin pass roll 5, the pipe side part is stretched and the overbend part is bent back by compressing the diameter. This is a method of forming a pipe (see Kawasaki Steel Technical Report, vol. 32 (2000), pp 49-53). The roll forming method of the CBR method has less strain applied to the material (strip) than the conventional BD (breakdown) method, and further, variation in strain applied in the tube circumferential direction is small. It has characteristics. While joining the circular element pipe obtained in this way with the squeeze roll 7, the butt portion is joined by the welding means (high frequency resistance welding) 6, and bead cutting is further performed on the inner and outer surfaces to obtain an electric resistance steel pipe 8. .
FIG. 2 is a cross-sectional view in the vicinity of the welded portion of a cross section perpendicular to the longitudinal direction of the steel pipe after ERW welding, (a) is a state before bead cutting, and (b) is a state after bead cutting. Indicates. Immediately after the electric-welding welding, as shown in FIG. 2A, a weld bead is formed on the inner and outer surfaces of the steel pipe 8, that is, an inner bead 11 is formed on the inner surface of the steel pipe 8, and an outer bead 12 is formed on the outer surface of the steel pipe 8. Usually, the inner bead 11 and the outer bead 12 are cut to lower the bead height as shown in FIG.
For example, as shown in FIG. 3, the cutting of the inner surface bead includes a bar-like bar 13 that can be inserted into the steel pipe 8, and a support member 15 that is provided at the tip of the bar 13 and to which a cutting tool 14 is attached. Perform with a cutting machine. In the cutting apparatus shown in FIG. 3, the inner surface bead is continuously cut by pressing the tip of the cutting tool 14 against the weld bead on the inner surface of the steel pipe on the downstream side of the welding point 16 of the traveling steel pipe 8.
In this internal bead cutting, the internal bead height after cutting is set to -0.1 to 0.1 mm. This is because, as described above, if the inner bead height is out of the range of -0.1 to 0.1 mm, cracks occur in the vicinity of the inner bead when the obtained steel pipe is flattened. Here, as shown in FIG. 2 (b), the inner surface bead height refers to the height h of the inner surface bead from the arc of the inner peripheral surface in a cross section perpendicular to the pipe longitudinal direction of the steel pipe.

  Using a steel plate (steel pipe material) having high strength, excellent workability, and excellent paint bake hardenability obtained by the manufacturing method as described above, pipes are manufactured in the pipe forming process as described above. With this, the strain applied during pipe making can be reduced to a minimum, work hardening can be suppressed, and excellent workability can be obtained. A high-strength ERW steel pipe capable of ensuring excellent shock absorption characteristics can be manufactured.

The obtained high-strength ERW steel pipe has a tensile strength TS of 1180 MPa or more, an elongation El in the tube axis direction of 10% or more, a yield ratio of less than 90%, and pre-strain: 2%, then 170 ° C x 20 min This steel pipe has a strength increase amount (BH amount) of 100 MPa or more and a yield ratio of 90% or more after the coating baking process is performed.
When the elongation of the electric resistance steel pipe in the pipe axis direction is less than 10%, the workability as a pipe is lowered, and it becomes difficult to form a desired shape. The elongation is preferably 12% or more, and when the yield ratio of the ERW steel pipe exceeds 90%, the workability as a pipe is lowered, and it becomes difficult to form a desired shape. The yield ratio is preferably 85% or less. Further, if the BH amount of the electric resistance welded pipe is less than 100 MPa, the energy that can be absorbed at the time of collision is reduced, and the function as an impact member cannot be satisfied. Preferably, the BH amount is 110 MPa or more. In addition, in the pipe making process adopted in the production of the electric resistance welded steel pipe of the present invention, strain applied during pipe making can be reduced to a minimum, and further, variation in strain applied in the pipe circumferential direction is reduced. In the ERW steel pipe of the present invention, the variation in the BH amount at each position in the pipe circumferential direction (difference between the maximum value and the minimum value) is small, and the BH amount at each position in the pipe circumferential direction excluding the ERW is uniform. And within the range of 100 to 130 MPa.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. These slabs (steel materials) are subjected to a hot rolling process with the conditions shown in Table 2 to form hot rolled sheets, and then pickled, and cold rolled with the conditions shown in Table 2 to the hot rolled sheets. The cold-rolled step and the annealing step under the conditions shown in Table 2 were applied to the cold-rolled plate to obtain a cold-rolled annealed plate having a thickness of 1.8 mm, which was a steel pipe material. A specimen was collected from the obtained steel pipe material, and a structure observation and a tensile test were performed. The test method was as follows.

(1) Microstructure observation From the obtained steel pipe material, a specimen for microstructural observation is collected, the cross section in the rolling direction is polished, corroded using a nital liquid, and the structure is obtained using a scanning electron microscope (magnification: 2000 times). Were observed, 10 or more fields of view were imaged, and using an image analysis device, the type of structure such as ferrite and martensite was identified, and the structure fraction (volume ratio) of each phase was calculated.

(2) Tensile test In accordance with the provisions of JIS Z 2201, a JIS No. 12 tensile test piece (marking distance: 50 mm) was collected from the obtained steel pipe material so that the tensile direction was the rolling direction. In accordance with the provisions of 2241, a tensile test is conducted to obtain 0.2% proof stress YS (MPa), tensile strength TS (MPa), and elongation El (%), and calculate the yield ratio YR to determine the strength and workability. evaluated.

Table 4 shows the obtained results.
Subsequently, the obtained steel pipe material was subjected to a pipe making process shown in Table 3 to obtain an electric resistance steel pipe (size: outer diameter 48.6 mmφ × wall thickness 1.8 mm). In the pipe making process, the steel pipe material is formed by CBR roll forming to form a substantially cylindrical open pipe, and then the butt is pressed by high-frequency resistance welding while pressing the butt with a squeeze roll. Welded. After ERW welding, the inner and outer surfaces were subjected to bead cutting. In this bead cutting, the inner bead height was changed in the range of -0.2 to 0.2 mm. In some steel pipes, the forming in the pipe making process was formed by the BD method.

The obtained ERW steel pipe was subjected to a structure observation, a tensile test, and a paint baking treatment test to evaluate the structure, tensile characteristics, and paint bake hardening characteristics. The test method was as follows.
(1) Microstructure observation From the obtained steel pipe, a specimen for microstructural observation is collected, the cross section in the axial direction of the pipe is polished, corroded using a nital liquid, and the structure is obtained using a scanning electron microscope (magnification: 2000 times). Were observed, 10 or more fields of view were imaged, and using an image analysis device, the type of structure such as ferrite and martensite was identified, and the structure fraction (volume ratio) of each phase was calculated.

(2) Tensile test In accordance with the provisions of JIS Z 2201, a JIS No. 12 tensile test piece (mark distance: 50 mm) was collected from the obtained steel pipe so that the tensile direction was the pipe axis direction. In accordance with the provisions of 2241, a tensile test is conducted to obtain 0.2% proof stress YS (MPa), tensile strength TS (MPa), and elongation El (%), and calculate the yield ratio YR to determine the strength and workability. evaluated.

(3) Flattening test From the obtained steel pipe, a flattened test piece having a length of 500 mm was cut out and subjected to a flattening test. As shown in FIG. 4, the collected flattened test piece 20 is 7/8 × D (D is the test piece) so that the weld bead position 21 of the test piece 20 is 90 ° from the flattening direction. The outer surface was flattened to the outer diameter before processing), and the presence or absence of cracks in the bead portion was visually inspected after the flat processing. The flat workability was evaluated with the case where cracks were recognized as bad (x) and the case where cracks were not observed as good (◯).
(4) Pipe expansion test Using the 6-segment segment expansion machine, the obtained steel pipe was expanded to expand the outer diameter to 1.05 x D (D is the outer diameter before processing the test piece). The presence or absence of cracks was visually inspected, and the pipe expansion workability was evaluated with the cracks occurring as bad (X) and the cracks not occurring as good (O).
(5) Paint baking test From the obtained steel pipe, in accordance with the provisions of JIS Z 2201, JIS No. 12 tensile test specimens were collected so that the tensile direction would be the pipe axis direction. A paint baking process was performed, in which a tensile strain was applied and a heat treatment was performed at 170 ° C. for 20 minutes. The tensile test specimens were collected from each position in the circumferential direction of the tube (the position of ERW was 0 °, and a total of 11 positions at a pitch of 30 ° in the circumferential direction, excluding the ERW).

  Then, a tensile test is performed on the treated specimen to obtain a 0.2% proof stress YS and a tensile strength TS after the paint baking process, and a yield ratio (= (YS / TS) × 100 (%) after the paint baking process. )) Was calculated. Further, as shown in FIG. 5, the coating bake hardening amount (BH amount) was calculated as a difference between the 0.2% proof stress after the coating baking treatment and the strength after applying the 2% tensile strain. As for the amount of BH, the maximum value and the minimum value at each position in the circumferential direction were obtained. For YS and TS, the arithmetic average of the values at each position in the circumferential direction was obtained.

  The results obtained are shown in Table 5.

In all of the examples of the present invention, the tensile strength TS: high strength of 1180 MPa or more, and the elongation El in the tube axis direction is 10%.
Excellent workability with a yield ratio (= (0.2% proof stress / tensile strength) x 100 (%)) in the tube axis direction of less than 90%, plus 2% or more pre-strain In addition, after heat treatment (paint baking treatment) at 170 ° C x 20 min, the electrode has an excellent shock absorption characteristic with a yield ratio in the tube axis direction of 90% or more and a BH amount of 100 MPa or more. It is a sewn steel pipe. Furthermore, in the example of the present invention, there is little variation in the BH amount at each position in the circumferential direction, and all are within the range of 100 to 130 MPa. Furthermore, in the present invention, even when flattening is performed, cracks are not observed in the bead part, and flatness is excellent. Further, even if pipe expansion is performed, cracks are not observed in the bead part. It is a steel pipe with excellent properties.

  On the other hand, the comparative examples that are outside the scope of the present invention are insufficient in strength, workability is reduced, impact absorbability is lowered, flat workability is inferior, or pipe workability is poor. Inferior.

DESCRIPTION OF SYMBOLS 1 Strip 2 Edge bend roll 3 Center bend roll 4 Cage roll 5 Fin pass roll 6 Welding means 7 Squeeze roll 8 Electric resistance welded tube 9 Cutting machine
10 open tube
11 inner bead
12 External bead
13 bar
14 bytes
15 Support member
20 Flat processing specimen
21 Weld bead position

Claims (6)

% By mass
C: 0.05-0.20%, Si: 0.5-2.0%,
Mn: 1.0 to 3.0%, P: 0.1% or less,
S: 0.01% or less, Al: 0.01 to 0.1%,
N: 0.005% or less, a composition composed of the balance Fe and inevitable impurities, and a two-phase structure composed of a ferrite phase and a martensite phase, the martensite phase having a volume ratio of 20 to 60%. Excellent workability with tensile strength TS of 1180 MPa or more, tube axis elongation El of 10% or more, yield ratio of less than 90%, and pre-strain: 2%, and then heat treatment at 170 ° C x 20 min. It has excellent shock absorption characteristics such that the amount of increase in strength (BH amount) after coating baking treatment is 100MPa or more and the yield ratio is 90% or more, and the inner bead height of ERW welds is -0.1 ~ High-strength ERW steel pipe characterized by being 0.1mm.   In addition to the above composition, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti: 0.05% or less, W: 0.05 The high-strength electric resistance welded steel pipe according to claim 1, wherein the composition contains one or more selected from B: 0.005% or less and B: 0.0050% or less.   The composition according to claim 1 or 2, further comprising one or two kinds selected from Ca: 0.0050% or less and REM: 0.0050% or less in mass% in addition to the composition. High strength ERW steel pipe as described. A hot rolling process in which the steel material is hot-rolled into a hot-rolled sheet on the steel material, a cold-rolling process in which the hot-rolled sheet is subjected to a pickling treatment, and then cold-rolled into a cold-rolled sheet, An annealing process is performed on the cold-rolled sheet to form a cold-rolled annealed sheet to obtain a steel pipe material, and then the steel pipe material is continuously formed on the steel pipe material to form a substantially cylindrical open pipe. And, after the open pipe is electro-welded to form an electro-sewn pipe, a pipe forming step of cutting the weld bead of the electro-sealed welded portion formed by the electro-resistance welding is performed to form an electro-welded steel pipe. Steel material in mass%
C: 0.05-0.20%, Si: 0.5-2.0%,
Mn: 1.0 to 3.0%, P: 0.1% or less,
S: 0.01% or less, Al: 0.01 to 0.1%,
N: A steel material containing 0.005% or less and having the balance Fe and inevitable impurities,
The hot rolling step is a step of performing hot rolling with a finish rolling end temperature of Ar ₃ transformation point or higher and a coiling temperature of 500 to 650 ° C. to form a hot rolled plate, and the annealing step is the cold rolled plate. In addition, after heating and soaking to a temperature in the two-phase temperature range of Ac ₁ transformation point to Ac ₃ transformation point, from the temperature range of 600 to 750 ° C. to room temperature, the average is 500 ° C./s or more. It is subjected to a rapid cooling process that cools at a cooling rate, and then a tempering process that is reheated to a temperature range of 150 to 300 ° C. to form a cold-rolled annealed sheet, and the molding is a cage roll type roll molding, The inner bead height is set to -0.1 to 0.1 mm by cutting the weld bead, and the ERW steel pipe is a two-phase structure composed of a ferrite phase and a martensite phase, and the martensite phase is 20 to 60% by volume. It has a certain structure, tensile strength TS is 1180 MPa or more, tube axis direction elongation El is 10% or more, and yield ratio is 90 Excellent workability of less than 10% and pre-strain: 2%, and then the yield strength increase (BH amount) after coating baking treatment with heat treatment of 170 ℃ x 20min is 100MPa or more, and the yield ratio is 90 %. A method for producing a high-strength ERW steel pipe, characterized by being a steel pipe having an excellent impact absorption characteristic of at least%.   In addition to the above composition, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti: 0.05% or less, W: 0.05 % Or less, B: It is set as the composition containing 1 type, or 2 or more types chosen from below 0.0050%, The manufacturing method of the high intensity | strength ERW steel pipe of Claim 4 characterized by the above-mentioned.   The composition according to claim 4 or 5, further comprising, in addition to the above composition, one or two kinds selected from Ca: 0.0050% or less and REM: 0.0050% or less in terms of mass%. The manufacturing method of the high intensity | strength ERW steel pipe of description.

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