WO2011118841A1 - High-strength electrical-resistance-welded steel pipe and manufacturing method therefor - Google Patents

Abstract

The disclosed high-strength electrical-resistance-welded steel pipe is suitable for use as an automobile shock-absorbing member. Said steel pipe contains, by mass, between 0.05% and 0.20% carbon, between 0.5% and 2.0% silicon, between 1.0% and 3.0% manganese, at most 0.1% phosphorus, at most 0.01% sulfur, between 0.01% and 0.1% aluminum, and at most 0.005% nitrogen, with the remainder comprising iron and unavoidable impurities. Said steel pipe has a two-phase structure comprising a ferrite phase and a martensite phase, with the volume fraction of the martensite phase between 20% and 60%. The disclosed steel pipe has a tensile strength (TS) of at least 1,180 MPa, an axial elongation (El) of at least 10%, and a yield ratio of less than 90%. After a coating baking process in which the steel pipe is prestressed by 2% and then heat-treated at 170°C for 10 minutes, the amount of strength increase (BH amount) is at least 100 MPa and the yield ratio increases to at least 90%.

Description

High-strength ERW steel pipe and manufacturing method thereof

INDUSTRIAL APPLICABILITY The present invention is a high-strength electric steel pipe suitable for a crash member of an automobile such as a door impact beam, a cross member, a pillar, or the like. The present invention relates to a resistance welded steel tube, 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, particularly for the purpose of ensuring the safety of passengers, the automobile body is designed to absorb impact energy at the time of collision. For example, a door impact beam, which is an impact absorbing member, is subjected to a quenching treatment as shown in, for example, Patent Document 1 to obtain a martensitic structure (martensitic structure). high strength steel pipes having a desired high strength have been applied.

The technique described in Patent Document 1 is as follows: C: 0.15 to 0.22%, Mn: 1.5% or less, Si: 0.5% or less, Ti: 0.04% or less, B: 0.0003 1 to 2 or more of Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less A high strength steel pipe for machine structure is obtained by quenching a steel pipe containing steel to obtain a high strength steel pipe for machine structure with a tensile strength of 120 kgf / mm ² or more (electric resistance welded steel tube structural use). is there. According to the technology described in Patent Document 1, 10% applicable as a door impact bar (door impact beam) and a core for bumper, which is a steel pipe for reinforcing an automobile. A high-strength steel pipe having the above-described good elongation and a tensile strength of 120 kgf / mm ² or more can be obtained by a single heat treatment.
Further, as steel sheets having a tensile strength of 120 kgf / mm ² or more, Patent Documents 2 to 7 describe techniques relating to high-strength cold-rolled steel sheets having a tensile strength of 900 MPa or more used for automobile structural members. . Each of these steel sheets has a two-phase structure of a ferrite phase and a martensite phase, or a structure including a bainite phase and a retained austenite phase, and defines an upper limit value of the area ratio of the bainite phase and the retained austenite phase. By adopting such a structure, the steel sheet has both workability and high strength.

Japanese Patent Laid-Open No. 3-122219 JP 2010-255094 A JP 2010-126787 A JP 2009-242816 A JP 2009-203550 A JP 2007-100114 A JP 2005-163055 A

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.
Further, in the techniques described in Patent Documents 2 to 5, since the cooling rate after heating and holding during annealing is slow, carbides precipitate, the amount of solid solution C of ferrite becomes insufficient, and prestraining-paint baking treatment There is a problem in that the amount of increase in strength (BH amount) due to is small, and the BH amount: 100 MPa or more cannot be secured.
Further, in the technique described in Patent Document 6, the cooling rate from heating and holding during annealing to the start of water quenching is not considered, for example, the time from the production line layout to the start of water quenching. When the cooling rate is long and slow, the distribution of C amount between ferrite and austenite proceeds, and the amount of solid solution C remaining in the ferrite that is considered to contribute to the BH characteristics becomes insufficient. Therefore, Patent Document 6 does not describe or guarantee that the amount of BH is 100 MPa or more.
Moreover, in the technique described in Patent Document 7, the cooling rate at the time of finish annealing is as slow as 550 ° C./min at the maximum shown in the examples, and only about 8% elongation is obtained. Further, the elongation is low overall and is 11% at the maximum. Therefore, when the steel sheet manufactured by the technique described in Patent Document 7 is processed into an electric-resistance-welded steel pipe, the elongation is further reduced and the elongation of 10% or more is ensured as a steel pipe when considering the processing strain during pipe making. There is a problem that you can not.
In view of such demands, the present invention provides a high-strength electric resistance welded steel pipe that has excellent workability and that can ensure excellent shock absorption characteristics that are suitable for automobile shock absorbing members and a method for manufacturing the same. The purpose is to provide.

In addition, “high strength” here refers to a case where the tensile strength TS is 1180 MPa or more. In addition, “excellent workability” here refers to a tube axis direction (tube axis direction) obtained by a tensile test using a JIS No. 12 tensile test piece (GL: 50 mm) defined in JIS standards. The elongation El of the direction is 10% or more, preferably 12% or more, and the yield ratio in the tube axis direction (= (0.2% proof stress / tensile strength) × 100 (%)). Is less than 90%. In addition, “excellent shock absorption characteristics” here means that the tube was subjected to 2% pre-strain and further subjected to heat treatment (baking finishing) at 170 ° C. × 10 min. The difference in strength between 0.2% proof stress and 2% pre-strain strength is a strength increase (BH (hardening value)) of 100 MPa or more, and the yield ratio in the tube axis direction is 90%. This is the case. 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 the electric resistance welded steel pipe while maintaining high strength. As a result, a steel sheet (cold rolled steel sheet) having a dual phase structure composed of ferrite and martensite, excellent workability, and desired bake hardenability. ) Was adopted as a steel pipe material, and a pipe making method was adopted that would allow pipe making without reducing the excellent workability of the material as much as possible, resulting in 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 strength, thereby improving the proof stress and excellent shock absorption characteristics as a member It was found that it 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: A composition comprising 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, the balance composed of Fe and inevitable impurities, and 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%, tensile strength TS is 1180 MPa or more, tube axis direction elongation El is 10% or more, and yield ratio is less than 90%. The pre-strain: 2%, and after applying the heat treatment at 170 ° C. for 10 minutes, the strength increase (BH amount) after the baking process is 100 MPa or more and the yield ratio is 90% or more. Strength ERW steel pipe.

(2) In (1), in addition to the above-mentioned composition, in addition, by 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: 0.0050% or less. A high-strength ERW steel pipe characterized by that.
(3) In (1) or (2), in addition to the above composition, the composition further contains, by mass%, Ca: 0.0050% or less and REM: 0.0050% or less. Strength ERW steel pipe.

(4) A hot rolling process (hot rolling process) for hot rolling a steel material into the steel material, and pickling the hot rolled sheet (pickling) cold rolling (cold rolling process) that is subjected to cold rolling (cold rolling sheet) and then subjected to annealing treatment (cold rolling process). The steel pipe material is made into a steel pipe material by subjecting it to an annealing process to make a rolled and annealed sheet, and then the steel pipe material is continuously formed on the steel pipe material to form a substantially cylindrical shape. Open pi pe), and when a tube production process for forming an electric resistance welded pipe by electrowelding the open pipe is performed to obtain an electric resistance welded steel pipe, the steel material in mass%, C: 0.05 ~ 0.20%, Si: 0.5 ~ 2.0%, Mn: 1.0 ~ 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 ~ A steel material having a composition comprising 0.1%, N: 0.005% or less, the balance being Fe and inevitable impurities, and the hot rolling process is performed at a finish rolling temperature of Ar ₃ transformation point or higher Then, a hot rolling with a coiling temperature of 500 to 700 ° C. is performed to form a hot rolled sheet, and the annealing process is performed on the cold rolled sheet with an Ac ₁ transformation point to an Ac ₃ transformation point. Temperature in the two-phase temperature range Each time after heating and soaking, it was cooled to a temperature in the range of 600 to 750 ° C. at an average cooling rate of 10 ° C./s or more (defined as average cooling rate 1), From the temperature range of 600 to 750 ° C. to room temperature, on average, it is cooled at a cooling rate of 500 ° C./s or higher (defined as average cooling rate 2), and then the temperature range is 150 to 300 ° C. A re-tempering treatment (tempering treatment) is performed to form a cold-rolled annealed plate, the forming is a cage roll type roll forming, the ERW steel pipe has a tensile strength TS of 1180 MPa or more, a pipe shaft Elongation El is 10% or more, yield ratio is less than 90%, pre-strain: 2%, and then heat treatment of 170 ° C x 10 min. ) Is a steel pipe having a yield ratio of 90% or higher and a yield ratio of 90 MPa or higher.

(5) In (4), in addition to the above-mentioned composition, in addition, by 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: 0.0050% or less. A method for producing a high-strength ERW steel pipe.
(6) In (4) or (5), in addition to the above composition, the composition further contains, by mass%, Ca: 0.0050% or less and REM: 0.0050% or less. Manufacturing method of 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 a door impact beam, and other automotive shock absorbing members such as cross members and pillars that require particularly workability, as well as automotive framework members such as automobile frame members. 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 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 effect of strengthening steel, and in the present invention, it is necessary to contain 0.05% or more in order to secure a desired strength. On the other hand, the content exceeding 0.20% lowers weldability. Therefore, in the present invention, C is limited to a range of 0.05 to 0.20%. Note that the content is preferably 0.08 to 0.18%.

Si: 0.5 to 2.0%
Si acts as a deoxidizer, solidifies to strengthen steel, and further promotes the formation of ferrite, and is an important element for ensuring excellent workability. Moreover, Si can achieve a desired high strength by suppressing the martensite phase fraction to a low level by solid solution strengthening of 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 chemical conversion treatability is lowered. Therefore, in the present invention, Si is limited to the range of 0.5 to 2.0%. Note that the content is preferably 1.0 to 1.8%.

Mn: 1.0 to 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 secure a desired strength, Mn is contained in an amount of 1.0% or more in the present invention. Need. On the other hand, if the content exceeds 3.0%, segregation is promoted, slab cracks are likely to occur during casting, and the martensite phase is excessively increased to reduce workability. For this reason, Mn was limited to the range of 1.0 to 3.0%. Note that the content is preferably 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 raises 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 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 Was 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 deoxidizing agent, and in order to obtain such an effect, the content of 0.01% or more is required. 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 an impurity as much as possible. However, excessive reduction increases the refining cost. For this reason, N was limited to 0.005% or less with substantially no adverse effect. 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: One or two selected from 0.0050% or less 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 having an action of increasing the strength of steel and improving corrosion resistance, and can be contained as necessary. Such an effect is recognized with a content of 0.05% or more. However, 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. More preferably, it is 0.08 to 0.5%.

Ni: 1.0% or less Ni 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 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. More preferably, it is 0.08 to 0.5%.

Cr: 0.5% or less Cr is an element having an 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 with a content of 0.05% or more, but if it exceeds 0.5%, the workability is lowered. For this reason, when it contains, it is preferable to limit Cr to 0.5% or less. More preferably, it is 0.05 to 0.4%.

Mo: 0.5% or less Mo is an element having an action of increasing the strength of steel by precipitation strengthening in addition to improving hardenability, and can be contained as necessary. Such an effect is recognized with a content of 0.05% or more. However, when the content 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. More preferably, the content 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 when 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. More preferably, the content 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. More preferably, the content 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. More preferably, the content 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 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. More preferably, the content 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 morphological control of sulfide inclusions. Can be contained depending on the case. Such an effect is recognized when the content of Ca and REM is 0.0020% or more. However, when the content exceeds 0.0050%, the amount of inclusions is excessively increased and the cleanness of the steel is decreased. For this reason, when it contains, it is preferable to limit both Ca and REM to 0.0050% or less. More preferably, the content is 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 has a two-phase structure including a martensite phase with a volume ratio of 20 to 60% and the remaining ferrite phase. By setting it as such a structure | tissue, it becomes possible to combine desired high intensity | strength, the outstanding workability, and the outstanding paint bake hardenability.

If the martensite phase is less than 20% by volume, it becomes a structure 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. The volume ratio is preferably 40 to 55%.

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, and rolling may be difficult. On the other hand, when the heating exceeds 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 higher than the Ar ₃ transformation point of the steel strip as the material to be rolled, and the finish rolling is taken up at a winding temperature of 500 to 700 ° C.
When the finish rolling finish temperature is less than the Ar ₃ transformation point, the finish rolling is a rolling at two-phase region (α + γ), and a mixed grain structure in which extremely coarse crystal grains and fine crystal grains are mixed. (Mixed grain structure). For this reason, even if it performs a cold rolling process-annealing process after that, favorable workability cannot be ensured, or rough skin arises at the time of processing, such as press forming (press forming) and bending work (bending work). For this reason, the finish rolling finishing temperature of the hot rolling is limited to the Ar ₃ transformation point or higher. In addition, when the coiling temperature is less than 500 ° C., a hard phase is generated during cooling, so that a rolling load at the time of cold rolling is increased and productivity is decreased. On the other hand, when the temperature is higher than 700 ° C., non-transformed austenite is transformed into pearlite, so that workability is deteriorated. For this reason, the coiling temperature is limited to a range of 500 to 700 ° C. In addition, Preferably it is 650 degrees C or less.

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, and is kept soaked, and then the average cooling rate of 10 to a temperature in the range of 600 to 750 ° C. After cooling at ℃ / s or higher (average cooling rate 1), rapid cooling treatment (average cooling rate 2) is performed at a cooling rate of 500 ° C / s or higher on average from a temperature in the range of 600 to 750 ° C to room temperature. Next, a tempering treatment is performed to reheat to a temperature range of 150 to 300 ° C. to obtain a cold-rolled annealed plate. In order to stably secure desired high strength and BH characteristics, the cooling rate (average cooling rate 1) from the heating and holding to the rapid cooling start temperature is preferably 15 ° C./s or more, The average cooling rate (average cooling rate 2) is preferably 800 ° C./s or more. More preferably, it is 1000 ° C./s or more. More preferably, it is 1100 degrees C / s or more.

When the temperature at which heating and soaking is maintained is out of the two-phase temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point, the desired tissue fraction is obtained by rapid cooling thereafter. The (ferrite + martensite) structure cannot be secured. Further, when the cooling rate from the heating holding temperature to the rapid cooling start temperature (average cooling rate 1) is less than 10 ° C./s, the distribution of the C amount of ferrite and austenite proceeds, and the solid content in the ferrite that is considered to contribute to the BH characteristics is increased. Since the amount of dissolved C is reduced, desired BH characteristics cannot be obtained. On the other hand, when 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 ensured. In addition, it is desirable that the soaking time at the above-described temperature is 30 seconds or longer.

Further, when the cooling rate from the temperature in the range of 600 to 750 ° C. to room temperature (average cooling rate 2) is less than 500 ° C./s on average, the amount of martensite transformation is small, and the desired structure fraction (ferrite + (Martensite) structure cannot be obtained and desired high strength cannot be ensured. In addition, the amount of dissolved C in ferrite that is considered to contribute to BH characteristics decreases, so the desired BH amount is 100 MPa or more. Cannot be obtained. In addition, the cooling rate of the rapid cooling treatment is an average between the rapid cooling start temperature and 200 ° C.
In addition, although it does not specifically limit about the method of a rapid cooling process, From the viewpoint of suppressing the material dispersion | variation in the steel plate width direction and a longitudinal direction, it is preferable to set it as the cooling using jet flow water (jet flow water).
Furthermore, in the annealing process of the present invention, a tempering treatment (tempering treatment) for reheating to a temperature range of 150 to 300 ° C. is performed after the rapid cooling treatment for the purpose of further improving the toughness. If the tempering temperature is less than 150 ° C., the toughness improving effect cannot be expected.

On the other hand, when it exceeds 300 ° C., the ductility is lowered due to low-temperature tempering brittleness. For this reason, the temperature range of reheating was limited to 150 to 300 ° C.
The obtained cold-rolled annealed sheet may be further subjected to temper rolling if necessary. At this time, it is preferable that the rolling reduction of skinpass rolling is 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. The pipe making process is a process in which a steel pipe material is continuously formed to form a substantially cylindrical open pipe, and the open pipe is electro-welded to form an electric-welded pipe.
In the present invention, the forming in the pipe making process is a roll forming method using 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. In addition, among the roll forming by the cage roll method, the CBR method (Chance free Bulge Roll method) is preferable. In molding by this method, distortion applied to the band plate during molding can be minimized, and deterioration of material properties due to work hardening can be suppressed.

An example of an ERW steel pipe manufacturing facility employing CBR roll forming is shown in FIG. In CBR roll forming, both edge portions of the strip 1 are pre-formed with an edge bend roll 2 and then a center bend roll 3 and a cage roll 4 are used to form a central portion of the strip. Was bent and formed into a vertically long oval figured tube, and then four bends in the circumferential direction of the pipe were once over-bended by a fin pass roll 5. This is a forming method of forming a circular element pipe by performing stretch forming of the tube side part and bending and returning of the overbend part (bend and return forming) by reducing the diameter. Kawasaki Steel Technical Report, vol.32 (2000), see pp49 ~ 53). Note that the roll forming method of the CBR method has less strain applied to the material (band plate) than the conventional BD (breakdown) method, and further, variation in strain applied in the tube circumferential direction. Is small. While pressing the circular element tube thus obtained with a squeeze roll 7, the butt portion is joined by welding means (high-frequency resistance welding) 6, and an electric-welded steel pipe 8 and To do.

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. Therefore, it is possible to minimize distortion applied during pipe making, suppress work hardening, have excellent workability, and further ensure excellent shock absorption characteristics after becoming a member. Possible to produce high strength ERW steel pipe.

The obtained high-strength ERW steel pipe has a tensile strength TS of 1180 MPa or more, an elongation El in the pipe axis direction of 10% or more, a yield ratio of less than 90%, and a pre-strain: 2%, and then 170 ° C. × 10 min. This is a steel pipe having an increase in strength (BH amount) of 100 MPa or more and a yield ratio of 90% or more after the coating baking process for performing the heat treatment.
If the elongation in the tube axis direction of the electric resistance welded steel pipe 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 if 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.
Moreover, if the BH amount of the ERW steel pipe after the paint baking process is less than 100 MPa, the energy that can be absorbed at the time of collision decreases, and the function as an impact member cannot be satisfied. Preferably, the amount of BH 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. Therefore, it can be set within the range of 100 to 130 MPa. Further, if the yield ratio of the ERW steel pipe is less than 90%, the energy that can be absorbed at the time of collision decreases, and the function as an impact member cannot be satisfied.

In the present invention, the heat treatment condition of the coating baking process is a heat treatment of 170 ° C. × 10 min. This condition is the lowest heat treatment condition that provides an increase in strength (BH amount) of 100 MPa or more after the coating baking process. There are other preferable conditions, and the ERW steel pipe of the present invention has a strength increase amount (BH amount) after the paint baking process of 100 MPa or more. As a heat treatment condition for obtaining a strength increase amount (BH amount) of 100 MPa or more after the coating baking process, it is preferable that the heating temperature is in the range of 170 to 250 ° C. and the holding time is in the range of 10 to 30 minutes. If the heating temperature is less than 170 ° C., the solid solution C necessary to bring about a desired increase in strength does not diffuse until dislocations are sufficiently fixed, so that the amount of increase in strength after the desired baking treatment ( BH amount) cannot be secured. On the other hand, if the temperature exceeds 250 ° C. and the temperature is excessively high, productivity is lowered and the material may be deteriorated because it may be heated to a blue-hot brittle region.

In addition, when the holding time is as short as less than 10 min, the diffusion time is insufficient, and the required amount of solute C cannot reach dislocation, so that the amount of increase in strength after the desired baking process (BH amount) ) Cannot be secured. On the other hand, when the holding time is longer than 30 min, the productivity is lowered. Preferably it is 25 min or less.

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 under the conditions shown in Table 2 to form hot rolled sheets (thickness 2.4 to 3.0 mm), and then pickled and cold rolled on the hot rolled sheets. A cold-rolled step of forming a cold-rolled plate and an annealing step under the conditions shown in Table 2 were applied to the cold-rolled plate to obtain a cold-rolled annealed plate (plate thickness 1.2 to 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) Tissue observation
From the obtained steel pipe material, a specimen for structure observation was collected, the cross section in the rolling direction was polished, corroded using a nital liquid, and a scanning electron microscope (magnification: 2000 times). Observe the tissue using 10 images, image more than 10 fields of view, use an image analyzer to identify the type of tissue such as ferrite and martensite, and calculate the fraction of each phase (volume fraction) did.

(2) Tensile test
In accordance with JIS Z 2201, the JIS No. 12 tensile test piece (marking distance: 50 mm) is collected from the obtained steel pipe material in accordance with the JIS Z 2201 standard, and conforms to the JIS Z 2241 standard. Then, a tensile test was performed, 0.2% yield strength YS (MPa), tensile strength TS (MPa), and elongation El (%) were obtained, yield ratio YR was calculated, and strength and workability were evaluated.

The obtained results are shown in Table 3.
The obtained steel pipe material was formed by CBR roll forming to obtain a substantially cylindrical open pipe. Next, while pressing the butt portion with a squeeze roll, the butt portion is electro-welded by high-frequency resistance welding, and an ERW steel pipe (size: outer diameter 48.6 mmφ × wall thickness 1.2 to 1.8 mm) did. For some steel pipes, the forming in the pipe making process is 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 A specimen for microstructural observation is collected from the obtained steel pipe, 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). 10 images or more, and using an image analyzer, identify the type of tissue such as ferrite and martensite, and calculate the average tissue fraction (volume ratio) of each phase over 10 fields of view. did.

(2) Tensile test JIS No. 12 tensile test piece (marking distance: 50 mm) was sampled from the obtained steel pipe so that the tensile direction was the pipe axis direction in accordance with the provisions of JIS Z 2201. In accordance with the provisions of 2241, a tensile test is carried out, 0.2% yield strength YS (MPa), tensile strength TS (MPa), elongation El (%) are obtained, yield ratio YR is calculated, strength and processing Sex was evaluated.

(3) Paint baking test From the obtained steel pipe, in accordance with the provisions of JIS Z 2201, a JIS No. 12 tensile test piece was sampled so that the tensile direction was the pipe axis direction. A coating baking process was performed, in which a tensile strain was applied and a heat treatment was performed at 170 ° C. for 10 minutes. In addition, the tensile test piece was extract | collected from each pipe circumferential direction position (Electric seam part is 0 degree, a total of 11 positions at a 30 degree pitch in the circumferential direction. Excluding ERW part).

Then, a tensile test is performed on the treated test piece to obtain 0.2% proof stress YS and tensile strength TS after the paint baking process, and the yield ratio after the paint baking process (= (YS / TS) × 100). (%)) Was calculated. Further, as shown in FIG. 2, 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. In addition, YS and TS calculated | required the arithmetic average of the value in each position of the circumferential direction.

Table 3 shows the obtained results.

In all of the examples of the present invention, the tensile strength TS: high strength of 1180 MPa or more, the elongation El in the tube axis direction is 10% or more, and the yield ratio in the tube axis direction (= (0.2% proof stress / tensile strength). ) X 100 (%)) is less than 90%, pipe with excellent workability and 2% or more pre-strain and heat treatment (paint baking process) at 170 ° C x 10 min. The electric resistance welded steel pipe has an excellent impact absorption characteristic with an axial yield ratio of 90% or more and a BH amount of 100 MPa or more. Further, 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.

On the other hand, in the comparative examples that are out of the scope of the present invention, the strength is insufficient, the workability is lowered, or the BH amount is insufficient.
In addition, the influence of paint baking treatment conditions was further investigated.
Steel pipe No. shown in Table 2 In accordance with the provisions of JIS Z 2201, the JIS No. 12 tensile test piece was sampled from No. 1 (invention example) so that the tensile direction would be the tube axis direction, and then 2% tensile strain was applied as a pre-strain. Then, a coating baking process was performed in which a heat treatment was performed by changing the heating temperature and holding time in the range of 100 to 250 ° C. × 5 to 30 min. In addition, the tensile test piece was extract | collected from each pipe circumferential direction position (Electric seam part is 0 degree, a total of 11 positions at a 30 degree pitch in the circumferential direction. Excluding ERW part). Then, a tensile test is performed on the paint baking processed test piece to obtain 0.2% proof stress YS and tensile strength TS after the paint baking process, and the yield ratio after the paint baking process (= (YS / TS) × 100 (%)) was calculated. Further, as shown in FIG. 2, 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. In addition, YS and TS calculated | required the arithmetic average of the value in each position of the circumferential direction. Table 4 shows the obtained results.

When the heating temperature of the heat treatment is less than 170 ° C., which is a condition outside the range of the preferred paint baking treatment, a BH amount of 100 MPa or more is used unless an excessively long paint baking treatment is performed without considering a decrease in productivity. Is not secured stably. In addition, the excessively long coating baking time here means time exceeding 30 minutes. Further, even when the heating temperature is 170 ° C. or higher, if the holding time is 5 minutes which is less than 10 minutes, a BH amount of 100 MPa or more may not be secured, and the desired BH amount cannot be secured stably. .

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 pipe 9 Cutting machine 10 Open pipe

Claims (6)

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: a composition containing 0.005% or less, the balance consisting of Fe and inevitable impurities, and a two-phase structure consisting of a ferrite phase and a martensite phase, wherein the martensite phase is 20 to 60% by volume With 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%, a pre-strain: 2%, and a heat treatment of 170 ° C. × 10 min. A high-strength electric resistance welded steel pipe with a subsequent increase in strength (BH amount) of 100 MPa or more and a yield ratio of 90% or more.
2. In addition to the above-mentioned composition, further, by 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. ERW steel pipe.
3. The high-strength ERW steel pipe according to claim 1 or 2, further comprising, in addition to the above composition, Ca: 0.0050% or less and REM: 0.0050% or less in terms of mass%.
4. 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. When the electric pipe is subjected to a pipe forming process to form an electric resistance welded pipe by electro-welding the open pipe, 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: 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 at a finish rolling end temperature of Ar ₃ transformation point or higher and a coiling temperature of 500 to 700 ° C. to obtain a hot rolled sheet,
   The annealing step is maintained at a two-phase temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point, and after that, the average cooling rate is 10 ° C./s or more to a temperature in the range of 600 to 750 ° C. After cooling at a cooling rate of 600 ° C. to 750 ° C., quenching from room temperature to room temperature at a cooling rate of 500 ° C./s or higher, and then performing a process of maintaining soaking at a temperature range of 150 ° C. to 300 ° C. ,
   The molding is a cage roll type roll molding,
   The ERW steel pipe has a tensile strength TS of 1180 MPa or more, an elongation E1 in the pipe axis direction of 10% or more, a yield ratio of less than 90%, a pre-strain: 2%, and then heat treatment at 170 ° C. × 10 min. A method for producing a high-strength ERW steel pipe, which is a steel pipe having a strength increase amount (BH amount) of 100 MPa or more and a yield ratio of 90% or more after paint baking.
5. In addition to the above-mentioned composition, further, by 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: 0.0050% or less, the composition containing one or more selected from the group consisting of two or more. A method for manufacturing ERW steel pipes.
6. The method for producing a high-strength electric resistance welded steel pipe according to claim 4 or 5, wherein, in addition to the composition, the composition further contains, by mass%, Ca: 0.0050% or less and REM: 0.0050% or less.

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