JP2007291416A - High tension steel pipe for automobile high pressure piping - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tension steel pipe for automobile high pressure piping composed of a welded steel pipe or an electric resistance welded steel pipe having low alloy steel and excellent inner pressure fatigue characteristic and high tensile strength. <P>SOLUTION: The high tension steel pipe for automobile high pressure piping, composed of the welded steel pipe or the electric resistance welded steel pipe uses the TRIP (low alloy transformation induced plasticity steel sheet) as a material which is manufactured by applying rapid-cooling treatment with an AM steel (annealed martensite steel: 0.1-0.6 wt.% C, 1.5 wt.% Si, and 1.5 wt.% Mn) in the low alloy high tension steel after solid-solution treatment, and further, an austempering treatment is applied after heating. Alternatively, the high tension steel pipe for automobile high pressure piping, composed of the welded steel pipe or the electric resistance welded steel pipe uses the TRIP steel sheet as a material which is manufactured by rapidly cooling with a BF steel (bainitic ferrite steel: 0.1-0.6 wt.% C, 1.5 wt.% Si, and 1.5 wt.% Mn) to the austempering treatment temperature after solid-solution treatment, and the austempering treatment is applied at this temperature. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-tensile steel pipe for automobile high-pressure piping suitable for a high-pressure pipe for fluid that fluctuates at a high pressure, such as a fuel pipe for automobiles and a brake pipe. In addition, the present invention relates to a welded steel pipe or an ERW steel pipe having a high tensile strength made of a material capable of obtaining excellent welding quality.

High-strength steel pipes with a tensile strength of 490 N / mm class ^{2 to} 600 N / mm class ² and stainless steel double-wound steel pipes are generally used for fluid pipes that undergo severe pressure fluctuations at high pressures such as automobile fuel pipes and brake pipes. (See Patent Documents 1-3). However, materials exhibiting high tensile strength are generally high-carbon, high-alloy tempered steel materials that are hard and extremely difficult to roll-form, and because they are high alloys, use austenitic materials that utilize work-hardening properties. Even so, it was impossible to obtain a tensile strength of 70 kg / mm ² or more.

Further, in recent years, direct internal combustion engines have been required to have a high internal pressure (injection pressure) of 12 to 20 MPa or more as a countermeasure against global warming due to CO ₂ , fuel consumption measures, and environmental measures. It was feared that steel pipes could not withstand internal pressure fatigue or vibration fatigue caused by high pressure fluid in the gasoline engine and diesel engine.

On the other hand, among conventional methods of manufacturing steel pipes, it is advantageous to use soft materials with low carbon content in terms of formability and weldability in the production of ERW steel pipes, especially low frequency resistance. This is conspicuous in welding, direct current resistance welding, etc., but the carbon material used for the production of welded pipes is also used in other methods such as high frequency induction welding, TIG welding, laser welding, plasma welding, etc. The upper limit of the content is about 0.6 wt% in terms of welding quality, and it is impossible to obtain a tensile strength of 80 to 180 kg / mm ² without curing heat treatment below this carbon content, and a conventional austenitic material Even if the work-hardening properties of the material are utilized, the tensile strength is limited to about 70 kg / mm ² at most.
JP 2000-5813 A JP 2000-5814 A JP-A-5-33103

The present invention has been made to solve the above-mentioned conventional problems, and has the characteristics of a low-alloy and high-strength steel (with a tensile strength of 80 to 180 kg / mm ² ) and excellent resistance to repeated internal pressure fatigue resistance. In the case of a direct-injection gasoline engine, a high-tensile steel pipe for automobile high-pressure piping is provided, which is made of a welded steel pipe or an ERW steel pipe having a high tensile strength capable of ensuring an internal pressure cyclic fatigue limit stress exceeding 12 to 20 MPa. It is intended.

The high-tensile steel pipe for automobile high-pressure piping according to the present invention is a low-alloy high-strength steel AM steel (annealed martensite steel: C0.1-0.6 wt%, Si1.5 wt%, Mn1.5 wt%), and solidified. It is characterized by comprising a welded steel pipe or an ERW steel pipe made of a TRIP steel sheet (low alloy transformation-induced plastic-type strength steel sheet) that has been subjected to rapid cooling after the treatment, and further subjected to austempering after heating.
Here, the solution treatment is a heat treatment in which heating is held at 950 ° C., preferably for 1200 seconds, and then rapidly cooled, and the austempering treatment is performed in 325 ° C. after holding at 780 ° C. for 60-3000 seconds, preferably 1000 seconds. It is characterized in that it is a heat treatment which is rapidly cooled to 0 ° C. and kept at this temperature for 60 to 3000 seconds, preferably 1000 seconds.

Further, another high-tensile steel pipe for automobile high-pressure piping according to the present invention is a low-alloy high-tensile steel BF steel (Bainitec ferrite steel: C0.1-0.6 wt%, Si1.5 wt%, Mn1.5 wt% ), Rapidly cooled to the austempering temperature after solution treatment, and made of a welded steel pipe or an ERW steel pipe made of TRIP steel sheet (low alloy transformation-induced plastic type strength steel sheet) that has been austempered at that temperature. It is characterized by.
Here, the solution treatment is a heat treatment which is preferably heated and maintained at 950 ° C. for 1200 seconds and then rapidly cooled to the austempering temperature, and the austemper treatment is preferably performed at 325 to 475 ° C. for 60 to 3000 seconds. The heat treatment is held for 1000 seconds.

  Furthermore, the present invention is a low alloy high strength steel AM steel (annealed martensite steel: C 0.1 to 0.6 wt%, Si 1.5 wt%, Mn 1.5 wt%), or BF steel (Bainitec ferrite steel). : CIP 0.1 to 0.6 wt%, Si 1.5 wt%, Mn 1.5 wt%) TRIP steel plate (low alloy transformation-induced plastic mold strength steel plate) as it is rolled or subjected to normal softening heat treatment A high-strength steel pipe for automobile high-pressure piping, which is made of a welded steel pipe or ERW steel pipe having 5 to 25 wt% of retained austenite as a material.

Furthermore, the above-described high-tensile steel pipe for automobile high-pressure piping according to the present invention comprises a welded steel pipe or an ERW steel pipe having a wall thickness of t2.5 mm or less and an outer diameter of 12.7 mm or less. A high-strength steel welded steel pipe for automobile high-pressure piping, which is made of a TRIP steel electric-welded steel pipe or welded steel pipe having an outer diameter of t2.5 mm or less and an outer diameter of 12.7 mm or less, has a retained austenite of 5 to 25 wt% and a tensile strength of 80 to 180 kg / mm ^2. It is characterized by this.

  Furthermore, the high-strength steel pipe for automobile high-pressure piping made of the TRIP steel according to the present invention is obtained by any one of high-frequency induction welding, high-frequency resistance welding, low-frequency resistance welding, DC resistance welding, TIG welding, plasma welding, and laser welding. These high-strength steel pipes for automobile high-pressure piping made of TRIP steel are used for high-pressure fluid piping such as fuel piping and brake piping.

  The high-strength steel pipe for automobile high-pressure pipes made of TRIP steel welded steel pipe or ERW steel pipe according to the present invention has a high plastic deformability and becomes a work-induced martensite structure by plastic working. It is possible to improve the internal pressure fatigue resistance by preventing the progress of cracks generated from the notch tip derived from soot and non-metallic inclusions, etc. The high-pressure fuel pipe of a direct injection gasoline engine has an internal pressure repetition of 12-20 MPa or more. It has an excellent effect of being able to withstand vibration fatigue caused by vibrations from the engine and the vehicle body.

The low-alloy transformation-induced plastic type strength steel in the present invention was developed for the purpose of reducing the weight of press-formed parts of passenger car undercarriage in recent years and utilizes strain-induced transformation (TRIP) of retained austenite (γ _R ). Ferrite (α _f ) + bainite (α _b ) + γ _R composite structure steel [TRIP type Dual-Phase steel, TDP steel] and bainitec ferrite (α _bf ) + γ _R steel [ TRIP type bainite steel, TB steel].
Here, the transformation-induced plasticity is a large elongation accompanying the transformation of an austenite (γ) layer that exists in a scientifically unstable state into martensite by the addition of mechanical energy.
That is, the TRIP steel is a steel obtained by applying a specific heat treatment to a limited plastic steel to obtain a metal structure in which retained austenite and bainite structure are mixed around the grain boundary of the α layer. The characteristics of TRIP steel having such a metal structure include high plastic deformability and high strength and hardness because it becomes a martensite structure by processing.

The high-tensile steel pipe for automobile high-pressure piping according to the present invention is made of a low alloy transformation-induced plastic type strength steel having 5 to 25 wt% of retained austenite having such characteristics, so that the workability is in the middle of pipe making. The inner surface of the flow path is a tube having a depth of 20 μm or less. Further, when the tube is drawn, the reduction can be made large, so that the number of times of drawing can be reduced, and further, if the same reduction is performed, the processing can be performed with a small drawing machine with a small die and a small drawing force.
In addition, since the austenite (γ) structure is improved in both hardness and tensile strength due to the precipitation of work-induced martensite, internal pressure fatigue resistance, sheet surface scratch resistance, bending shape stability, Excellent vibration fatigue characteristics.
Furthermore, the low alloy transformation induced plasticity strength steel has the property (TRIP phenomenon) that the austenite in the locally deformed part transforms into hard martensite and strengthens that part (TRIP phenomenon). In the case of high-strength piping made of mold strength steel, even if vibration fatigue or internal pressure fatigue progresses, the fatigue portion is strengthened by the above characteristics and a resistance force is generated to prevent the tube from being broken. Therefore, the conventional STKM material of JIS G 3445 Compared with STS material of JIS G 3455, SUS material of JIS G 3459, etc., it has a longer life.
In the present invention, the retained austenite of the low alloy transformation-induced plastic-type strength steel is limited to 5 to 25 wt% because the transformation amount from retained austenite to martensite is less than 5 wt% when exposed to high stress. This is because the strength is not sufficiently increased, and on the other hand, if it exceeds 25 wt%, it is difficult to secure a desired strength. As the austempering treatment conditions in the present invention, the treatment temperature was set to 325 to 475 ° C. The amount of retained austenite is insufficient when the temperature is less than 325 ° C., and the amount of retained austenite is precipitated when the temperature exceeds 475 ° C. This is because there is a shortage. Further, the reason why the austempering time is set to 60 to 3000 seconds is that the amount of retained austenite is insufficient when it is less than 60 seconds, while the amount of retained austenite is decomposed and insufficient when it exceeds 3000 seconds.

When manufacturing high-strength steel pipes for automobile high-pressure pipes consisting of welded steel pipes or ERW steel pipes made of TRIP steel according to the present invention, AM steel (annealed martensite steel: C 0.1-0.6 wt%, Si 1.5 wt%, In the case of Mn 1.5 wt%), the TRIP steel plate that has been subjected to rapid cooling treatment after solid solution treatment and further subjected to austemper treatment after heating is subjected to high frequency induction welding, high frequency resistance welding, low frequency resistance welding, direct current resistance welding, TIG welding. It can be manufactured by welding by any one of plasma welding and laser welding.
In the case of BF steel (Bainitec ferrite steel: C0.1-0.6 wt%, Si1.5 wt%, Mn1.5 wt%), after solid solution treatment, it is rapidly cooled to the austempering treatment temperature. The tempered TRIP steel sheet can be manufactured by welding by any one of high frequency induction welding, high frequency resistance welding, low frequency resistance welding, DC resistance welding, TIG welding, plasma welding, and laser welding.

The high-strength steel pipe for automobile high-pressure piping of the present invention is a thin-walled electric-welded steel pipe or welded steel pipe having a wall thickness t of 2.5 mm or less and an outer diameter of 12.7 mm or less made of TRIP steel such as AM steel or BF steel. Therefore, it is possible to easily and inexpensively provide fluid piping that undergoes severe pressure fluctuations at high pressure, such as automobile fuel piping, brake piping, power steering piping, and direct-injection gasoline engine injection piping. The coil material of thin-walled steel strips such as AM steel and BF steel for thin-walled electric-welded steel pipes or welded steel pipes has a very smooth surface due to the transfer of a mirror-rolled rolling roll as an original characteristic of the coil material, There are advantages such that the surface layer structure becomes dense due to surface reduction and surface quenching during rolling, and defects such as non-metallic inclusions and wrinkles are extremely less than seamless steel pipes.
Thus, high-strength steel pipes made of thin TRIP steel that are welded or electro-welded with extremely few surface defects are high pressure pipes such as automobile fuel pipes, brake pipes, power steering pipes, and direct injection gasoline engine injection pipes. When it is used as a pipe for fluids whose pressure fluctuates drastically, processing strain is applied to the inside and outside of the bending part by bending the pipe, resulting in work-induced martensite regardless of compression and tension. Improve strength. At the same time, in the case of a direct-injection gasoline engine, when it is used at an engine injection pressure of 12 to 20 MPa or more, processing-induced martensite is generated on the entire inner surface of the pipe, which contributes to improvement of the internal pressure fatigue strength. Not too long.

  On the other hand, although it is a welded steel pipe or ERW steel pipe made from a coil material of a thin steel strip with very few surface defects, there should be non-metallic inclusions or notches on the inner surface. Even when tensile stress due to repeated internal pressure exceeding 12 to 20 MPa is concentrated on the tip of the crack and the crack is about to progress, work-induced martensite is concentrated on the crack tip and hardens to prevent the progress of the crack. The high-strength steel pipe for automobile high-pressure piping having a high internal pressure fatigue limit stress can be obtained by combining the characteristics that can be obtained and the effect that the work hardening martensite is caused by the bending strain and the inner surface becomes a compressive stress field. Can be provided.

Using a TRIP steel plate (steel strength 460 MPa) having a thickness of 0.6 mm and a coil width of 41.2 mm made of AM steel (annealed martensite steel: C 0.20 wt%, Si 1.5 wt%, Mn 1.5 wt%) and having low DC characteristics As shown in FIG. 1, a heat pattern of solid solution treatment and austempering treatment of AM steel is manufactured with a frequency resistance welder at a pipe making speed of 60 m / min. After the ERW steel pipe was held at 950 ° C. for 1000 seconds and subjected to solid solution treatment, it was subjected to oil quenching treatment to room temperature, followed by holding at 780 ° C. for 1000 seconds and reheating, and then rapidly cooling to 400 ° C. Thereafter, the temperature was maintained at the temperature for 1000 seconds, and an austempering treatment was performed to obtain an ERW steel pipe made of TRIP steel having a strength of 1150 MPa and an austenite volume ratio of 21%.
The obtained ERW steel pipe was subjected to a spread test and a bending test in accordance with JASO M 101 “Definition of Metal Pipes for Automotive Piping”. As a result, the spread test showed a 1.4D (1.2D or more) bending test. However, no adhesion abnormality was found in the weld. In other words, in this example, the TRIP steel sheet was piped without being subjected to austempering treatment, and was subjected to austempering treatment after pipe making, and the plate thickness (thickness) was as thin as 0.6 mm. The effect of giving complete work-induced martensite was obtained.

A TRIP steel plate with a plate thickness of 1.0 mm and a coil width of 39.1 mm made of BF steel (Bainitec ferrite steel: C 0.35 wt%, Si 1.5 wt%, Mn 1.5 wt%) is shown in FIG. As shown in the heat pattern of the treatment and the austempering treatment, after holding the solution at 950 ° C. for 1200 seconds and performing the solution treatment, the solution is rapidly cooled to about 400 ° C. and held at the temperature for 500 seconds to reach the austempering treatment and normal temperature After the oil quench treatment, the austempered steel plate (steel strength: 1120 MPa) was used to make a pipe at a pipe making speed of 4.2 m / min with a TIG welder, and the strength was 1010 MPa and the austenite volume ratio of 21% TRIP steel. A welded steel pipe (tube diameter 1/2 inch) was obtained.
The obtained welded steel pipe was subjected to the same pipe expansion test and bending test as in Example 1. As a result, the pipe expansion test showed 1.3D (1.2D or more). No adhesion abnormality was confirmed. In this example, the austempering treatment was performed in the steel plate state, but the austenite treatment was not performed after the pipe making, so the portion of the bonding heat affected zone of about 2 mm disappeared from the retained austenite, Since the work-induced martensite did not occur during the tensile test, the apparent tensile strength was lowered, but there was no problem because the result was almost the same as that of Example 1 by performing the heat treatment again.

Using a TRIP steel plate (steel strength 460 MPa) having a plate thickness of 2.5 mm and a coil width of 39.5 mm made of AM steel (annealed martensite steel: C 0.20 wt%, Si 1.5 wt%, Mn 1.5 wt%) and having low DC characteristics An electric resistance welded steel pipe (tube diameter: 12.5 mm) is manufactured at a pipe making speed of 45 m / min with a frequency resistance welder, and then this electric resistance welded steel pipe is extended with a plug die to obtain an outer diameter of 9 mm and a wall thickness of 2.5 mm. This electric thick steel pipe was held at 950 ° C. for 1200 seconds and subjected to a solid solution treatment, then subjected to an oil quench treatment to room temperature, then reheated to 780 ° C. and held for 2400 seconds, After abruptly cooling to 400 ° C., an austempering treatment was performed to hold the temperature at this temperature for 1800 seconds to obtain an ERW thick-walled steel pipe made of TRIP steel having a strength of 1100 MPa and an austenite volume ratio of 20%.
As a result of conducting an internal pressure fatigue test and a vibration bending fatigue test with a maximum pressure of 100 MPa on the obtained electric-welded thick steel pipe, both tests showed excellent characteristics.

The high-strength steel pipe for automobile high-pressure piping made of the TRIP steel welded steel pipe or ERW steel pipe of the present invention has high plastic deformability and becomes a work-induced martensite structure by plastic working, and the internal flaw generated by internal pressure or external force. In addition, it is possible to improve the internal pressure fatigue resistance by preventing the progress of cracks generated from the notch tips derived from non-metallic inclusions, etc., and fuel pipes for automobiles, brake pipes, power steering pipes, direct injection type gasoline engine injections In the case of a pipe, it has an excellent effect that it can withstand a high spray pressure of 12 to 20 MPa or more.
Therefore, the high-tensile steel pipe for automobile high-pressure piping according to the present invention greatly contributes to recent warming due to CO ₂ , fuel consumption measures, environmental measures, and the like.

It is a figure which shows an example of the heat pattern of the solution treatment and the austemper process of AM steel in this invention. It is a figure which similarly shows an example of the heat pattern of the solution treatment of BF steel, and an austemper process.

Claims (11)

  Low alloy high strength steel AM steel (annealed martensite steel: C0.1-0.6wt%, Si1.5wt%, Mn1.5wt%), rapid cooling after solution treatment, and further austempering after heating A high-tensile steel pipe for automobile high-pressure piping, which is made of a welded steel pipe or an electric-welded steel pipe made of a TRIP steel sheet (low alloy transformation-induced plastic-type strength steel sheet) to which is applied.   2. The high-strength steel pipe for high-pressure automobile piping according to claim 1, wherein the solution treatment is a heat treatment of heating and holding at 950 ° C. and then rapidly cooling. 3.   The said austempering process is a heat treatment which hold | maintains at 780 degreeC for 60-3000 second, then rapidly cools to 325-475 degreeC, and hold | maintains at this temperature for 60-3000 second, The high pressure piping for motor vehicles of Claim 1 characterized by the above-mentioned. High tensile steel pipe.   BF steel (Bainitec ferritic steel: C0.1-0.6 wt%, Si1.5 wt%, Mn1.5 wt%), a low-alloy high-strength steel, is rapidly cooled to the austempering temperature after solution treatment. A high-tensile steel pipe for automobile high-pressure piping, consisting of a welded steel pipe or an ERW steel pipe made of a TRIP steel sheet (low alloy transformation-induced plastic-type strength steel sheet) that has been subjected to austempering treatment.   5. The high-tensile steel pipe for high-pressure automobile piping according to claim 4, wherein the solution treatment is a heat treatment in which the solution is heated to 950 ° C. and then rapidly cooled to an austempering temperature.   6. The high-tensile steel pipe for high-pressure automobile piping according to claim 4, wherein the austempering treatment is a heat treatment for heating and holding at 325 to 475 ° C. 6.   Low alloy high strength steel AM steel (annealed martensite steel: C 0.1-0.6 wt%, Si 1.5 wt%, Mn 1.5 wt%), or BF steel (Bainitec ferrite steel: C0.1-0. 6% by weight, 1.5% Si, 1.5% Mn) TRIP steel sheet (low alloy transformation induced plastic strength steel sheet), which is made of a steel sheet that has been rolled or subjected to normal softening heat treatment, and A high-tensile steel pipe for automobile high-pressure piping, which is a welded steel pipe or ERW steel pipe having 5 to 25 wt% residual austenite.   The high-tensile steel pipe for automobile high-pressure piping according to any one of claims 1 to 7, comprising a welded steel pipe or an electric resistance steel pipe having a wall thickness of t2.5 mm or less and an outer diameter of 12.7 mm or less. The high-strength steel welded steel pipe for automobile high-pressure piping composed of TRIP steel electric-welded steel pipe or welded steel pipe having a wall thickness t of 2.5 mm or less and an outer diameter of 12.7 mm or less is a retained austenite of 5 to 25 wt%, a tensile strength of 80 to 180 kg / high tensile steel pipe for automotive high-pressure pipe according to any one of claims 1 to 8, characterized in that it has mm ^2.   The high-strength steel pipe for automobile high-pressure piping made of TRIP steel is welded by any one of high-frequency induction welding, high-frequency resistance welding, low-frequency resistance welding, DC resistance welding, TIG welding, plasma welding, and laser welding. The high-tensile steel pipe for automobile high-pressure piping according to any one of 1 to 9. The high-pressure steel pipe for high-pressure piping made of TRIP steel is used for high-pressure fluid piping such as fuel piping and brake piping. High tensile steel pipe.

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