阅读说明：本技术 燃料喷射管用钢管及使用其的燃料喷射管 (Steel pipe for fuel injection pipe and fuel injection pipe using same ) 是由 牧野泰三 山崎正弘 大村朋彦 荒井勇次 远藤修 芹泽直树 增田辰也 于 2020-02-13 设计创作，主要内容包括：一种燃料喷射管用钢管,其中,钢管的化学组成以质量％计为C：0.17～0.27％、Si：0.05～0.40％、Mn：0.30～2.00％、P：0.020％以下、S：0.0100％以下、O：0.0040％以下、Ca：0.0010％以下、Al：0.005～0.060％、N：0.0020～0.0080％、Ti：0.005～0.015％、Nb：0.015～0.045％、Cr：0～1.00％、Mo：0～1.00％、Cu：0～0.50％、Ni：0～0.50％、V：0～0.15％、余量：Fe和杂质,金相组织实质上包含回火马氏体、或者回火马氏体和回火贝氏体,硬度为350～460HV1,基于CoKα特征X射线衍射的(211)衍射面的晶面间距为以下,且半值宽度为1.200°以下,直径为50nm以上的渗碳体的个数密度为20/μm～2以下。(A steel pipe for a fuel injection pipe, wherein the chemical composition of the steel pipe is, in mass%, C: 0.17 to 0.27%, Si: 0.05 to 0.40%, Mn: 0.30-2.00%, P: 0.020% or less, S: 0.0100% or less, O: 0.0040% or less, Ca: 0.0010% or less, Al: 0.005-0.060%, N: 0.0020 to 0.0080%, Ti: 0.005-0.015%, Nb: 0.015-0.045%, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-0.50%, Ni: 0-0.50%, V: 0 to 0.15% and the balance: fe and impurities, the metallurgical structure substantially comprises tempered martensite or tempered martensite and tempered bainite, the hardness is 350-460 HV1, and the interplanar spacing of the (211) diffraction plane based on CoKa characteristic X-ray diffraction is The number density of cementite particles having a half-value width of 1.200 DEG or less and a diameter of 50nm or more is 20/mum 2 The following.)

1. A steel pipe for a fuel injection pipe, wherein,

the chemical composition of the steel pipe is calculated by mass percent

C：0.17～0.27％、

Si：0.05～0.40％、

Mn：0.30～2.00％、

P: less than 0.020%,

S: less than 0.0100%,

O: less than 0.0040 percent,

Ca: less than 0.0010 percent,

Al：0.005～0.060％、

N：0.0020～0.0080％、

Ti：0.005～0.015％、

Nb：0.015～0.045％、

Cr：0～1.00％、

Mo：0～1.00％、

Cu：0～0.50％、

Ni：0～0.50％、

V：0～0.15％、

And the balance: fe and impurities in the iron-based alloy, and the impurities,

the metallographic structure of the central part of the wall thickness of the steel pipe comprises tempered martensite or tempered martensite and tempered bainite, the total area ratio of the tempered martensite to the tempered bainite is more than 95%,

the hardness of the central part of the wall thickness of the steel pipe is 350-460 HV1,

the interplanar spacing of the (211) diffraction plane based on the CoK α characteristic X-ray diffraction is 1.Wherein the half-value width of the (211) diffraction plane is 1.200 DEG or less,

the number density of cementite with diameter of 50nm or more is 20/mum²The following.

2. The steel pipe for a fuel injection pipe according to claim 1, wherein a chemical composition of the steel pipe contains, in mass%, a chemical component selected from the group consisting of

Cr：0.03～1.00％、

Mo：0.03～1.00％、

Cu：0.01～0.50％、

Ni: 0.01 to 0.50%, and

v: 0.02-0.15% of more than 1.

3. A fuel injection pipe using the steel pipe for a fuel injection pipe according to claim 1 or 2.

Technical Field

The present invention relates to a steel pipe for a fuel injection pipe and a fuel injection pipe using the same.

Background

As measures against future energy depletion, there are being actively developed activities for promoting energy saving activities, resource circulation activities, and technical developments for achieving these goals. In particular, in recent years, as a global measure, reduction in global warming has been strongly demanded for preventing global warmingCO produced by combustion of fuel₂The amount of discharge of (c).

As CO₂As an internal combustion engine with a small emission amount, a diesel engine used for automobiles and the like can be mentioned. However, despite the CO of diesel engines₂The amount of emissions is small, but there is a problem of generation of black smoke. Black smoke is produced when the injected fuel is deficient in oxygen. That is, partial thermal decomposition of the fuel causes dehydrogenation reaction, and a black smoke precursor substance is generated, and the precursor substance is thermally decomposed again, aggregated, and combined into black smoke. The black smoke thus generated may cause air pollution and may adversely affect the human body.

The amount of the black smoke generated can be reduced by increasing the pressure of fuel injected into the combustion chamber of the diesel engine. However, for this purpose, the steel pipe for fuel injection is required to have high fatigue strength. The following techniques are disclosed for such a fuel injection pipe or a steel pipe for a fuel injection pipe.

Patent document 1 discloses a method for manufacturing a fuel injection steel pipe for a diesel engine, in which the inner surface of a hot-rolled seamless steel pipe blank is ground and polished by shot blasting, and then cold-drawn. By adopting this manufacturing method, the depth of flaws (irregularities, scab marks, micro cracks, etc.) on the inner surface of the steel pipe can be controlled to 0.10mm or less, and it is considered that the steel pipe for fuel injection can be made stronger.

Patent document 2 discloses a steel pipe for a fuel injection pipe, in which the maximum diameter of nonmetallic inclusions present at least up to 20 μm deep on the inner surface of the steel pipe is 20 μm or less, and the tensile strength is 500MPa or more.

Patent document 3 discloses a steel pipe for a fuel injection pipe, wherein the tensile strength is 900N/mm²As described above, the maximum diameter of the nonmetallic inclusions present at least on the inner surface of the steel pipe up to 20 μm deep is 20 μm or less.

Patent document 3 discloses a technique of manufacturing a steel pipe using a steel material excluding coarse inclusions of a-, B-, and C-series by reducing S, designing a casting method, and reducing Ca, adjusting the steel material to a target diameter by cold working, and then achieving a tensile strength of 900MPa or more by quenching and tempering, and in examples, a critical internal pressure of 260 to 285MPa is achieved.

Patent document 4 discloses a steel pipe for a fuel injection pipe having a tensile strength of 800MPa or more, preferably 900MPa or more and excellent internal pressure fatigue resistance, and a fuel injection pipe using the same.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 9-57329

Patent document 2: international publication No. 2007/119734

Patent document 3: international publication No. 2009/008281

Patent document 4: international publication No. 2015/129617

Non-patent document

Non-patent document 1: zhongshan Yingjie, Yuanyuan Guangxong, Gangcun-Man, Fuji Benbo Ji, Fujing Qing, prediction of fatigue strength of spot welded joint of thin plate for automobile by microminiature test piece, Material, No. 10, p.1136-1142, No. 53, No. 10, 2004-2004

Non-patent document 2: bowuliang, heat treatment, 41(2001), p.164

Disclosure of Invention

Problems to be solved by the invention

Although the steel pipe for fuel injection manufactured by the method disclosed in patent document 1 has high strength, the fatigue life commensurate with the strength of the steel pipe material cannot be obtained. If the strength of the steel pipe material becomes high, the pressure applied to the inside of the steel pipe can be increased. However, when pressure is applied to the inside of the steel pipe, the critical internal pressure (hereinafter referred to as "critical internal pressure") at which the inner surface of the steel pipe is not broken by fatigue depends not only on the strength of the steel pipe material. That is, even if the strength of the steel pipe material is increased, the desired critical internal pressure cannot be obtained. Since steel pipes are susceptible to fatigue when used under high internal pressure, the fatigue life is also shortened.

The steel pipe for a fuel injection pipe disclosed in patent documents 2 and 3 has advantages of long fatigue life and high reliability. However, the critical internal pressure of the steel pipe disclosed in patent document 2 is 255MPa or less, and the critical internal pressure is 260 to 285MPa in patent document 3. In particular, in the automobile industry, further improvement of internal pressure is required, and development of a fuel injection pipe having a tensile strength of 800MPa or more and a critical internal pressure of more than 270MPa, and particularly a fuel injection pipe having a tensile strength of 900MPa or more and a critical internal pressure of more than 300MPa is required. Although the critical internal pressure generally tends to increase slightly depending on the tensile strength of the fuel injection pipe, it is not easy to stably secure a high critical internal pressure particularly in a high-strength fuel injection pipe of 800MPa or more, in consideration of various factors involved.

The steel pipe for a fuel injection pipe disclosed in patent document 4 has a Tensile Strength (TS) of 800MPa or more, preferably 900MPa or more, and has a high critical internal pressure characteristic, and therefore has extremely high reliability. However, in recent years, a steel pipe for a fuel injection pipe is required to have higher strength, for example, 1100MPa or more.

Therefore, the present inventors have found that the strength of a steel pipe for a fuel injection pipe disclosed in patent document 4 is improved, and as a result, the hydrogen embrittlement resistance of the steel pipe is remarkably reduced. In order to ensure higher reliability, even when high strength is provided, it is required to suppress embrittlement due to hydrogen entering in the production process.

An object of the present invention is to solve the above-described problems and to provide a steel pipe for a fuel injection pipe having high strength and excellent hydrogen embrittlement resistance, and a fuel injection pipe using the same.

Means for solving the problems

The present invention has been made to solve the above-described problems, and the subject of the present invention is the following steel pipe for a fuel injection pipe and the fuel injection pipe using the same.

(1) A steel pipe for a fuel injection pipe, wherein,

the chemical composition of the steel pipe is calculated by mass percent

C：0.17～0.27％、

Si：0.05～0.40％、

Mn：0.30～2.00％、

P: less than 0.020%,

S: less than 0.0100%,

O: less than 0.0040 percent,

Ca: less than 0.0010 percent,

Al：0.005～0.060％、

N：0.0020～0.0080％、

Ti：0.005～0.015％、

Nb：0.015～0.045％、

Cr：0～1.00％、

Mo：0～1.00％、

Cu：0～0.50％、

Ni：0～0.50％、

V：0～0.15％、

And the balance: fe and impurities in the iron-based alloy, and the impurities,

the metallographic structure of the central part of the wall thickness of the steel pipe comprises tempered martensite or tempered martensite and tempered bainite, the total area ratio of the tempered martensite to the tempered bainite is more than 95%,

the hardness of the central part of the wall thickness of the steel pipe is 350-460 HV1,

the interplanar spacing of the (211) diffraction plane based on CoKa characteristic X-ray diffraction isWherein the half-value width of the (211) diffraction plane is 1.200 DEG or less,

the number density of cementite with diameter of 50nm or more is 20/mum²The following.

(2) The steel pipe for a fuel injection pipe as set forth in the above (1), wherein the chemical composition of the steel pipe contains a chemical component selected from the group consisting of

Cr：0.03～1.00％、

Mo：0.03～1.00％、

Cu：0.01～0.50％、

Ni: 0.01 to 0.50%, and

v: 0.02-0.15% of more than 1.

(3) A fuel injection pipe using the steel pipe for a fuel injection pipe according to the above (1) or (2).

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a steel pipe for a fuel injection pipe having a tensile strength of 1100MPa or more and excellent hydrogen embrittlement resistance can be obtained.

Drawings

FIG. 1 is a view showing the shape of a test piece used in a hydrogen charging constant load test.

Detailed Description

The present inventors have made extensive studies to solve the above-mentioned problems, and as a result, have obtained the following findings.

In order to ensure a predetermined strength, it is necessary to increase the hardness by making the metallographic structure substantially a single phase of tempered martensite or a complex phase structure including tempered martensite and tempered bainite.

On the other hand, it was found that when the hardness is too large, the hydrogen embrittlement resistance is remarkably decreased. Therefore, the hardness of steel needs to be controlled within a predetermined range.

Further, it has been found that hydrogen embrittlement cannot be suppressed in some cases by merely reducing the hardness of steel. This is considered to be because the strain and dislocation of the crystal lattice caused by the solid-solution C absorb hydrogen to deteriorate the hydrogen embrittlement resistance. As a result of investigation using various steel materials, it was found that excellent hydrogen embrittlement resistance can be ensured by reducing lattice strain and dislocation, specifically, by controlling the interplanar spacing and half width of the (211) diffraction plane to be equal to or less than predetermined values.

In addition, the presence of coarse carbides in the steel provides a starting point and propagation path for hydrogen cracks. In addition, it is difficult to secure fine carbides that trap hydrogen, and an effect of suppressing absorption of hydrogen into lattice strain and dislocations cannot be obtained.

The present invention has been made based on the above findings. Hereinafter, each element of the present invention will be described in detail.

1. Chemical composition

The reasons for limiting the elements are as follows. In the following description, "%" as to the content means "% by mass".

C：0.17～0.27％

C is an element effective for inexpensively improving the strength of steel. In order to secure a desired tensile strength, the C content needs to be 0.17% or more. However, if the C content exceeds 0.27%, the workability is deteriorated. Therefore, the C content is set to 0.17 to 0.27%. The C content is preferably 0.20% or more. The C content is preferably 0.25% or less, and more preferably 0.23% or less.

Si：0.05～0.40％

Si is an element having not only a deoxidizing effect but also an effect of improving the hardenability of steel to improve strength. In order to reliably obtain these effects, the Si content needs to be 0.05% or more. However, if the Si content exceeds 0.40%, the toughness is lowered. Therefore, the Si content is set to 0.05 to 0.40%. The Si content is preferably 0.15% or more, and preferably 0.35% or less.

Mn：0.30～2.00％

Mn is an element effective not only for deoxidation but also for improving the hardenability of steel to improve strength and toughness. However, if the content is less than 0.30%, sufficient strength cannot be obtained, while if it exceeds 2.00%, MnS coarsens, and elongation occurs during hot rolling, and conversely, toughness decreases. Therefore, the Mn content is set to 0.30 to 2.00%. The Mn content is preferably 0.40% or more, more preferably 0.50% or more. The Mn content is preferably 1.70% or less, and more preferably 1.50% or less.

P: 0.020% or less

P is an element inevitably present as an impurity in the steel. When the content exceeds 0.020%, not only hot workability is deteriorated, but also toughness is remarkably deteriorated due to grain boundary segregation. Therefore, the P content needs to be 0.020% or less. The lower the content of P, the better, it is preferably 0.015% or less, and more preferably 0.012% or less. However, since an excessive decrease increases the production cost, the lower limit is preferably set to 0.005%.

S: 0.0100% or less

S is an element inevitably present in steel as an impurity, as is P. If the content exceeds 0.0100%, the steel segregates at grain boundaries, and sulfide-based inclusions are formed, resulting in a decrease in fatigue strength. Therefore, the S content needs to be 0.0100% or less. The lower the S content, the better, it is preferably 0.0050% or less, and more preferably 0.0035% or less. However, since an excessive decrease leads to an increase in manufacturing cost, the lower limit thereof is preferably set to 0.0005%.

O: 0.0040% or less

O forms a coarse oxide, and thus the critical internal pressure is likely to decrease. From this viewpoint, the O content needs to be 0.0040% or less. The lower the content of O, the better, the more preferably 0.0035% or less, more preferably 0.0025% or less, and still more preferably 0.0015% or less. However, since an excessive decrease leads to an increase in manufacturing cost, the lower limit thereof is preferably set to 0.0005%.

Ca: 0.0010% or less

Ca acts to agglomerate silicate inclusions (group C of JIS G0555), and when the Ca content exceeds 0.0010%, coarse C-based inclusions are formed, and the critical internal pressure is lowered. Therefore, the Ca content is set to 0.0010% or less. The Ca content is preferably 0.0007% or less, more preferably 0.0003% or less. Since Ca contamination of facilities involved in steel refining can be eliminated if the facilities do not perform Ca treatment for a long period of time, the Ca content in steel can be substantially 0%.

Al：0.005～0.060％

Al is an element effective for deoxidizing steel, and is also an element having an effect of improving toughness and workability of steel. In order to obtain these effects, it is necessary to contain 0.005% or more of Al. On the other hand, if the Al content exceeds 0.060%, inclusions are likely to be generated, and particularly in Ti-containing steel, the risk of generating Ti — Al composite inclusions increases. Therefore, the Al content is set to 0.005-0.060%. The Al content is preferably 0.008% or more, and more preferably 0.010% or more. The Al content is preferably 0.050% or less, and more preferably 0.040% or less. In the present invention, the Al content means the content of acid-soluble Al (sol. Al).

N：0.0020～0.0080％

N is an element inevitably present as an impurity in steel. However, in the present invention, it is necessary to leave 0.0020% or more of N in order to prevent coarsening of crystal grains by utilizing the pinning effect of TiN. On the other hand, when the N content exceeds 0.0080%, the risk of producing large Ti — Al composite inclusions becomes high. Therefore, the N content is set to 0.0020 to 0.0080%. The N content is preferably 0.0025% or more, and more preferably 0.0027% or more. The N content is preferably 0.0065% or less, and more preferably 0.0050% or less.

Ti：0.005～0.015％

Ti is an element that contributes to preventing coarsening of crystal grains by precipitating as fine particles of TiN or the like. In order to obtain this effect, the Ti content needs to be 0.005% or more.

Here, when the internal pressure fatigue test is performed using the sample, the fatigue crack is generated and propagated from the high-stress inner surface, and is broken while reaching the outer surface. In this case, the starting point portion may or may not have inclusions.

When the starting point is free of inclusions, a flat fracture morphology, referred to as a faceted fracture, is recognized there. This is formed by the propagation of cracks generated in the grain unit to several grains around it in a shear type called mode ii. When the facet fracture grows to a critical value, the propagation mode changes to an open type called mode I, reaching damage. The growth of the notch-like fracture depends on the prior austenite grain diameter (hereinafter referred to as "prior γ grain diameter") which is a size unit in which the initial crack is generated, and is promoted when the prior γ grain diameter is large. This means that the fatigue strength of the matrix structure is lowered when the primary γ particle diameter is coarse, even if the inclusions do not serve as a starting point.

When the Ti content is high, it is observed from the fracture of the steel pipe subjected to the internal pressure fatigue test that a plurality of Al having a diameter of 20 μm or less are crosslinked to a film-like thin layer containing Ti as a main component₂O₃Complex inclusions (hereinafter referred to as Ti-Al complex inclusions) in the form of inclusions. Particularly when the Ti content exceeds 0.015%, there is a risk of generating large Ti — Al composite inclusions. The large Ti — Al composite inclusions risk causing a reduction in the life under very high internal pressure conditions. Therefore, the Ti content needs to be 0.015% or less.

In order to prevent coarsening of the primary γ particles, the Ti content is preferably 0.006% or more, more preferably 0.007% or more. In addition, the Ti content is preferably 0.013% or less, more preferably 0.012% or less, from the viewpoint of preventing formation of Ti — Al composite inclusions.

Nb：0.015～0.045％

Nb is finely dispersed in steel as carbide or carbonitride, and strongly binds grain boundaries, thereby contributing to refinement of the structure and having an effect of increasing the critical internal pressure. In addition, fine dispersion of Nb carbides or carbonitrides improves the toughness of the steel. To achieve these objects, 0.015% or more of Nb needs to be contained. On the other hand, when the Nb content exceeds 0.045%, carbides and carbonitrides coarsen, and the toughness is rather lowered. Therefore, the content of Nb is set to 0.015 to 0.045%. The Nb content is preferably 0.018% or more, and more preferably 0.020% or more. The Nb content is preferably 0.040% or less, and more preferably 0.035% or less.

Cr：0～1.00％

Cr is an element having an effect of improving hardenability and wear resistance, and therefore may be contained as necessary. However, since the toughness and cold workability are reduced when the Cr content exceeds 1.00%, the Cr content is 1.00% or less when included. The Cr content is preferably 0.80% or less. When the above-described effects are desired, the Cr content is preferably 0.03% or more, more preferably 0.05% or more, still more preferably 0.20% or more, and still more preferably 0.30% or more.

Mo：0～1.00％

Mo is an element that contributes to securing high strength by improving hardenability and enhancing temper softening resistance. Therefore, Mo may be contained as necessary. However, even if the Mo content exceeds 1.00%, the effect is saturated, and as a result, the alloy cost increases. Therefore, when contained, the Mo content is 1.00% or less. The Mo content is preferably 0.45% or less. When the above-described effects are desired, the Mo content is preferably 0.03% or more, and more preferably 0.08% or more.

Cu：0～0.50％

Cu is an element having an effect of improving strength and toughness by improving hardenability of steel. Therefore, Cu may be contained as necessary. However, the effect is saturated even if the Cu content exceeds 0.50%, resulting in an increase in alloy cost. Therefore, when contained, the Cu content is set to 0.50% or less. The Cu content is preferably 0.40% or less, and more preferably 0.35% or less. When the above-described effects are desired, the Cu content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.05% or more.

Ni：0～0.50％

Ni is an element having an effect of improving strength and toughness by improving hardenability of steel. Therefore, Ni may be contained as necessary. However, even if the Ni content exceeds 0.50%, the effect is saturated, resulting in an increase in alloy cost. Therefore, when contained, the Ni content is 0.50% or less. The Ni content is preferably 0.40% or less, more preferably 0.35% or less. When the above-described effects are desired, the Ni content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.08% or more.

V：0～0.15％

V is an element that precipitates as fine carbide (VC) during tempering to improve temper softening resistance, realizes high-temperature tempering, and contributes to high strength and high toughness of steel. Therefore, V may be contained as necessary. However, since the toughness is rather lowered when the V content exceeds 0.15%, the V content is 0.15% or less when contained. The V content is preferably 0.12% or less, more preferably 0.10% or less. When the above-described effects are desired, the V content is preferably 0.02% or more, and more preferably 0.04% or more.

In the chemical composition of the steel pipe for a fuel injection pipe of the present invention, the balance is Fe and impurities. Here, the "impurities" are components which are allowed to fall within a range not adversely affecting the present invention and are mixed in by raw materials such as ores and scraps and various factors of a manufacturing process in industrial steel manufacturing.

2. Metallographic structure

The metallographic structure at the center of the wall thickness of the steel pipe for a fuel injection pipe according to the present invention is substantially composed of a tempered martensite structure or a mixed structure of tempered martensite and tempered bainite. Specifically, the metallographic structure contains tempered martensite or tempered martensite and tempered bainite, and the total area ratio of the tempered martensite and the tempered bainite is 95% or more.

If a ferrite/pearlite structure is present in the structure, even if damage at the origin of inclusions is solved, damage is generated from a ferrite phase having a low local hardness, and the critical internal pressure expected from the macro hardness and the tensile strength cannot be obtained. In addition, it is difficult to ensure high tensile strength in a structure containing no tempered martensite or a ferrite/pearlite structure.

In addition, as described above, in order to suppress hydrogen embrittlement caused by absorption of hydrogen by lattice strain and dislocations, it is necessary to reduce lattice strain and dislocations. Specifically, the interplanar spacing of the (211) diffraction plane based on the CoK α characteristic X-ray diffraction is set asHereinafter, the half width of the (211) diffraction plane is set to 1.200 ° or less. The measurement was carried out using an X-ray diffraction apparatus under the conditions of CoK.alpha.characteristic X-ray, tube voltage 30kV and tube current 100 mA.

The reason why the (211) diffraction plane is focused is that carbon atoms intrude between the (001) planes when carbon is dissolved in a solid solution to widen the interplanar spacing, and the change in the (001) plane does not geometrically affect the (211) plane itself. The interplanar spacing d was calculated from the diffraction angle (angle of peak, 2 θ) using the bragg equation of the following formula.

λ＝2×d×sinθ

Wherein λ is the wavelength of the diffracted X-ray, and in CoKa ray is

The diffraction angle was corrected by confirming whether the diffraction peak position of a specific plane was deviated from the reference position using the Si standard plate. In addition, by using LaB₆(lanthanum hexaboride) as a standard sample and correcting the half width by measuring the width of the device in advanceAnd (4) degree.

In addition, in order to ensure hydrogen embrittlement resistance, it is necessary to disperse fine cementite serving as a hydrogen trapping site. On the other hand, when the amount of coarse cementite increases, not only the coarse cementite itself becomes a starting point and a propagation path of a hydrogen crack, but also it is difficult to secure fine cementite. Therefore, the number density of coarse cementite particles having a diameter of 50nm or more is set to 20/μm²The following.

In the present invention, the number density of coarse cementite is measured by Transmission Electron Microscope (TEM) observation. Specifically, a thin film having a thickness of 100nm was formed from the central part of the steel pipe wall, and observed by TEM at a magnification of 30000 times. Then, cementite having a diameter of 50nm or more was determined. The major axis and the minor axis were measured by approximating each crystal grain to an ellipse, and the average value of these was taken as the diameter of the cementite. Then, the number density is determined by dividing the number of the determined coarse cementite by the field area.

3. Mechanical Properties

The steel pipe for a fuel injection pipe according to the present invention has a hardness of 350 to 460HV1 at the center of the wall thickness. If the above hardness is less than 350HV1, it is difficult to obtain sufficient strength and critical internal pressure. On the other hand, when the hardness exceeds 460HV1, hydrogen embrittlement resistance is significantly reduced. "HV 1" is a "hardness symbol" in the vickers hardness test with a test force of 9.8N (1kgf) (see JIS Z2244: 2009).

By setting the hardness of the central portion of the wall thickness to 350HV1 or more, a tensile strength of 1100MPa or more and a critical internal pressure of 350MPa or more can be obtained. By setting the critical internal pressure to 350MPa or more, safety against fracture fatigue can be ensured. When the tensile strength of 1200MPa or more is desired, the hardness is preferably 400HV1 or more.

In the present invention, the critical internal pressure means that the minimum internal pressure is 18MPa in the internal pressure fatigue test and the repeated internal pressure fluctuation is given as a sine wave with time, even if the number of repetitions reaches 10⁷The highest internal pressure (MPa) at which no damage (leakage) occurs next time. Specifically, the maximum internal pressure is on the vertical axis and the weight of damage is on the horizontal axisOn the S-N diagram of the multiple times, the minimum value of the maximum internal pressure at which the damage will occur is equal to even 10⁷The median value of the maximum value that is not damaged next time is set as the critical internal pressure.

4. Size of

The size of the steel pipe for a fuel injection pipe according to the present invention is not particularly limited. However, the fuel injection pipe generally requires a certain capacity to reduce internal pressure variation during use. Therefore, the inner diameter of the steel pipe for a fuel injection pipe according to the present invention is preferably 2.5mm or more, more preferably 3.0mm or more. Further, since the fuel injection pipe needs to withstand a high internal pressure, the thickness of the steel pipe is preferably 1.5mm or more, more preferably 2.0mm or more. On the other hand, if the outer diameter of the steel pipe is too large, bending and the like become difficult. Therefore, the outer diameter of the steel pipe is preferably 20mm or less, more preferably 10mm or less.

In order to withstand high internal pressure, it is desirable that the steel pipe have a larger inner diameter and a larger wall thickness. If the inner diameter of the steel pipe is constant, the outer diameter of the steel pipe increases as the wall thickness increases. That is, in order to withstand a high internal pressure, it is desirable that the larger the inner diameter of the steel pipe, the larger the outer diameter of the steel pipe. In order to obtain a sufficient critical internal pressure of the steel pipe for a fuel injection pipe, it is desirable that the outer diameter and the inner diameter of the steel pipe satisfy the following formula (i).

D/d≥1.5···(i)

Wherein D in the above formula (i) is the outer diameter (mm) of the steel pipe for a fuel injection pipe, and D is the inner diameter (mm).

The ratio D/D of the outer diameter to the inner diameter of the steel pipe is preferably 2.0 or more. On the other hand, the upper limit of D/D is not particularly limited, but when the value is too large, bending becomes difficult, and therefore, it is preferably 3.0 or less, more preferably 2.8 or less.

5. Manufacturing method

The method for producing a steel pipe for a fuel injection pipe according to the present invention is not particularly limited, and for example, in the case of producing a seamless steel pipe, a steel ingot in which inclusions are suppressed in advance is prepared by the following method, a raw pipe is produced from the steel ingot by a method such as mannesmann tube making, cold-worked into a desired dimensional shape, and then heat-treated.

In order to suppress the formation of inclusions, it is preferable to adjust the chemical composition as described above and increase the sectional area of the cast slab at the time of casting. This is because large inclusions float up during the period until solidification after casting. The sectional area of the cast slab at the time of casting is desirably 200000mm²The above. Further, by reducing the casting speed, light nonmetallic inclusions can be floated as slag to reduce the nonmetallic inclusions themselves in the steel. For example, the continuous casting may be carried out at a casting speed of 0.3 to 0.7 m/min.

A strip billet for pipe is prepared from the thus obtained cast slab by a method such as blooming. Then, for example, piercing rolling and drawing rolling are performed by a mannesmann mandrel mill tube making method, and sizing rolling is performed by a drawing and reducing mill or the like to be processed into a predetermined hot tube making size. Next, the cold drawing process is repeated several times to obtain a prescribed cold worked dimension. Cold drawing may be facilitated by performing a stress relief anneal prior to or during cold drawing. Other tube-making methods such as a mandrel pipe-making method may also be used.

In this way, after the final cold drawing process is performed, heat treatment of quenching and tempering is performed in order to satisfy the mechanical properties of the target fuel injection pipe.

In the quenching treatment, it is preferable to heat to Ac₃Quenching at a temperature of +30 ℃ or higher. By setting the heating temperature to Ac₃The transformation point is +30 ℃ or higher, the formation of quenched martensite is sufficient, and a desired tensile strength can be obtained. In addition, in order to reduce the number of coarse cementite and finely disperse the coarse cementite, it is necessary to completely dissolve carbon during heating. This is because if the heating temperature is less than Ac₃The transformation point +30 c, carbon may not be completely solid-dissolved. On the other hand, the heating temperature is preferably 1150 ℃ or lower. This is because when the heating temperature exceeds 1150 ℃, coarsening of γ grains is likely to occur. The heating temperature is more preferably 1000 ℃ or higher.

Ac₃The phase transition point is calculated according to the following equation described in non-patent document 2.

Ac₃(℃)＝912-230.5C+31.6Si-20.4Mn-39.8Cu-18.1Ni-14.8Cr+16.8Mo

In the above formula, the symbol of an element indicates the content (% by mass) of each element contained in the steel material, and if not, 0 is substituted.

The heating method in quenching is not particularly limited, but when the heating is not in a protective atmosphere, the scale formed on the surface of the steel pipe increases by heating at a high temperature for a long time, and the dimensional accuracy and the surface properties deteriorate, and therefore, when the heating is performed in a furnace such as a step furnace, the holding time is preferably short, such as about 10 to 20 minutes. From the viewpoint of suppressing the scale, an atmosphere having a low oxidizing ability or a non-oxidizing reducing atmosphere is preferable as the heating atmosphere.

It is preferable to use a high-frequency induction heating method or a direct energization heating method as the heating method because heating can be maintained for a short time and scale formation on the surface of the steel pipe can be suppressed to a minimum. In this case, the holding time at the heating temperature is preferably 1s or less. In addition, in order to completely dissolve carbon in a solid solution during heating, the heating rate is preferably relatively low, and is preferably set to 20 to 80 ℃/s.

In order to stably and reliably obtain a desired strength, the cooling rate in the temperature range of 500 to 800 ℃ is preferably 50 ℃/s or more, more preferably 100 ℃/s or more, and still more preferably 125 ℃/s or more. As the cooling method, quenching treatment such as water quenching is preferably employed.

The steel pipe cooled to normal temperature by quenching is hard and brittle by itself. Further, lattice strain and dislocation caused by solid-solution C may deteriorate hydrogen embrittlement resistance. Therefore, tempering is preferably performed. However, when the temperature of tempering exceeds 450 ℃, the hardness is significantly reduced, and it becomes difficult to obtain desired strength. On the other hand, if the tempering temperature is less than 250 ℃, there is a risk that tempering is liable to be insufficient, toughness and workability are insufficient, and reduction of lattice strain and dislocation is insufficient. Therefore, the tempering temperature is preferably 250 to 450 ℃, more preferably 300 to 420 ℃. The holding time at the tempering temperature is not particularly limited, and is usually about 10 to 120 minutes. After the tempering, the bending can be corrected by a leveler or the like as appropriate.

The steel pipe for a fuel injection pipe of the present invention can be manufactured into a high-pressure fuel injection pipe by forming a connection head at both end portions thereof, for example.

Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

Examples

11 steel blanks having the chemical compositions shown in table 1 were produced. The casting speed in the continuous casting was set to 0.5 m/min, and the sectional area of the cast slab was set to 200000mm²The above.

[ Table 1]

A strip billet for pipe making was produced from the above steel billet, piercing-rolling and drawing-rolling were carried out by the Mannesmann mandrel pipe making method, sizing-rolling was carried out by a stretch reducer, and hot working was carried out to produce a pipe having a diameter of 34mm in outside diameter and 4.5mm in wall thickness. To draw the hot worked tube blank, the top end of the tube blank is first reduced and coated with a lubricant. Then, drawing is performed using a drawing die and a mandrel, softening annealing is performed as necessary, the pipe diameter is gradually reduced, and a steel pipe having an outer diameter of 8.0mm and an inner diameter of 4.0mm is processed.

[ Table 2]

Then, quenching and tempering treatment was performed under the conditions shown in table 2, and descaling and smoothing treatment was performed on the inner and outer surfaces. In this case, the quenching treatment was carried out under the conditions of high-frequency heating to 1100 ℃ at the temperature increase rate shown in Table 2, rapid cooling at a rate of 50 ℃/s or more (holding time 1s or less) in test Nos. 1 to 8 and 11 to 19 in Table 2, holding at 1000 ℃ and 1100 ℃ for 10 minutes, respectively, in test Nos. 9 and 10, and then water cooling at a rate of 50 ℃/s or more. The tempering treatment is carried out under the condition of keeping at 150-640 ℃ for 10 minutes and then naturally cooling. Specific tempering temperatures are also shown in table 2.

The obtained steel pipe was subjected to a tensile test using test piece No. 11 specified in JIS Z2241 (2011) to determine the tensile strength. If the steel pipe does not have a sufficiently long straight portion, a small test piece in the shape of a thin dumbbell as shown in non-patent document 1 can be cut out and subjected to a tensile test.

Further, a sample for tissue observation was taken out from each steel pipe, and a cross section perpendicular to the pipe axis direction was mechanically polished. Polishing was performed with sandpaper and a grinding wheel, and a nital-ethanol etching solution was used, thereby confirming a single phase of substantially tempered martensite or a mixed structure of tempered martensite and tempered bainite. That is, the total area ratio of tempered martensite and tempered bainite in all structures is 95% or more.

Further, the interplanar spacing and half-value width of the (211) diffraction plane were measured using an X-ray diffraction apparatus. The measurement conditions were CoK α characteristic X-ray, tube voltage 30kV, and tube current 100 mA. Correcting the diffraction angle by confirming whether the diffraction peak position of a specific plane deviates from the reference position using a Si standard plate, and correcting the diffraction angle by using LaB₆Lanthanum hexaboride (lanthanum hexaboride) was used as a standard sample and the width of the device was measured in advance to correct the half width.

In addition, preliminary cementite observation was performed using a Scanning Electron Microscope (SEM). As a result, in test No.7, a granular comparison which is considered to be cementite was observed, and no other comparison was observed. For this reason, in addition to test No.7, tests nos. 4, 5 and 8, which are representative of other samples, were observed using TEM.

Specifically, a thin film having a thickness of 100nm was formed from the central part of the steel pipe wall, and observed by TEM at a magnification of 30000 times, thereby specifying cementite having a diameter of 50nm or more. The major axis and the minor axis were measured by approximating each crystal grain to an ellipse, and the average value of these was taken as the diameter of the cementite. Then, the number density is determined by dividing the number of the determined coarse cementite by the field area.

Although the tests 1 to 3, 6, and 9 to 19 were not subjected to TEM observation, since the heating conditions during quenching were the same as those of the tests 4 and 8, it is considered that the number density of coarse cementite was in the range equivalent to the values of the tests 4 and 8, and was at least lower than the value of the test 5 in which the temperature rise rate was high during quenching.

Next, vickers hardness was measured in the central portion of the thickness of the steel pipe according to JIS Z2244 (2009). The test force was set to 9.8N (1 kgf).

The internal pressure fatigue test was carried out according to the following procedure. First, each steel pipe was cut into a length of 200mm, and subjected to pipe end processing to prepare a test piece of a jet pipe for internal pressure fatigue test. In the fatigue test, one end face of a sample is sealed, a pressure medium, i.e., hydraulic oil, is sealed in the sample from the other end face, and the internal pressure of the sealed portion is repeatedly varied in a sine wave form with respect to time within a range from the maximum internal pressure to the minimum 18 MPa. The frequency of the internal pressure fluctuation was set to 8 Hz. The number of repetitions as a result of the internal pressure fatigue test was set to 10⁷The maximum internal pressure at which breakage (leakage) does not occur next time was evaluated as the critical internal pressure.

In order to evaluate the hydrogen embrittlement resistance, the amount of diffusible hydrogen was measured by the following method. First, each steel pipe was cut into a length of 10mm, placed in a quadrupole hydrogen analyzer, and then heated from room temperature to 300 ℃, and the total amount of released hydrogen was measured to determine the amount of diffusible hydrogen C₀。

Next, a test piece of the shape shown in fig. 1 was prepared, and a hydrogen charging constant load test was performed in which a test load was continuously applied for up to 200 hours while charging hydrogen. The dimensions shown in fig. 1 are expressed in mm. The hydrogen charge was performed by immersing the test piece in an electrolyte and applying a potential as a cathode. The amount of hydrogen entering the electrolyte was adjusted by adjusting the concentration and current density of the electrolyte using a saline solution to which ammonium thiocyanate was added.

The test load was determined by the following procedure. The maximum stress was determined by performing elasto-plastic FEM analysis of the actual injection pipe using the stress-strain curve obtained by the steel pipe tensile test as input data. Next, the test piece was subjected to elasto-plastic analysis, and the load at which the maximum stress generated in the notch portion of the test piece was the same as the actual injection pipe was determined and used as the test load.

When the test piece is broken or not broken after 200 hours in the hydrogen charging constant load test, the test piece is taken out, and the amount of diffusible hydrogen is measured and taken as the critical diffusible hydrogen amount C_th. Wherein, C in the case of non-fracture_thIs a measured value or more. Critical diffusible hydrogen quantity C_thDivided by the amount of diffusible hydrogen C₀Obtained C_th/C₀This is referred to as hydrogen resistance safety factor and is used as an evaluation index. In the present invention, it is judged from the conventional data that there is a risk of hydrogen embrittlement when the hydrogen-resistant safety factor is 2.0 or less, and there is no risk of hydrogen embrittlement when it exceeds 2.0.

These results are also shown in Table 2.

As shown in Table 2, test Nos. 3 to 5, 8 and 11 to 15 satisfying the limitations of the present invention have high critical internal pressure and excellent hydrogen embrittlement resistance. On the other hand, test nos. 1, 2, 6, 7, 9, 10 and 16 to 19 are comparative examples which do not satisfy any of the limitations of the present invention.

Specifically, in test nos. 1 and 2, the tempering temperature was high, and the hardness was lowered, so that the critical internal pressure was also poor. On the other hand, in test nos. 6, 9 and 10, the tempering temperature was low, and the lattice strain could not be reduced, and in test nos. 6 and 10, the hardness was too large, and as a result, the hydrogen embrittlement resistance was deteriorated. In test No.7, the temperature rise rate during quenching was high, so that carbon could not be completely dissolved in a solid solution, and the number density of coarse carbides was too high, resulting in deterioration of hydrogen embrittlement resistance.

Further, test No.16 had too low a C content, and as a result, the hardness was low, and the critical internal pressure was also poor. Test No.17 had too high Ti content, test No.18 had low Ti and Nb contents, and test No.19 did not contain Nb, and therefore, the results were that the critical internal pressures were poor.

Industrial applicability

According to the present invention, a steel pipe for a fuel injection pipe having a tensile strength of 1100MPa or more and excellent hydrogen embrittlement resistance can be obtained. Therefore, the steel pipe for a fuel injection pipe according to the present invention can be suitably used as a fuel injection pipe for an automobile in particular.