JP2009287108A - Steel superior in fatigue characteristics for common rail, and common rail - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a common rail, which is superior in fatigue strength while securing machinability, and to provide a common rail made from the steel. <P>SOLUTION: The steel for the common rail comprises, by mass%, 0.3 to 0.5% C, 0.05 to 0.5% Si, 0.3 to 1.5% Mn, 0.01 to 0.1% Al, 0.003 to 0.02% Ti, 0.4 to 1.5% Cr, 0.1 to 1.5% Mo, 0.003 to 0.015% N, 0.0003 to 0.01% REM and 0.01 to 0.03% Bi, 0.0015% or less S, 0.035% or less P and 0.003% or less O, and the balance substantially Fe with unavoidable impurities. The common rail is made from the steel, contains sulfide-based inclusions of which the mean value of ratios of the lengths L to the widths D is 4.5 or less in the longitudinal cross-section, and has each of sulfide-based inclusions, nitride-based inclusions and oxide-based inclusions finely dispersed therein. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a common rail steel having excellent fatigue characteristics used as a fuel injection system component for automobiles and the like, and a common rail made of the steel.

  The common rail is used, for example, as a part of a fuel injection system for a diesel engine (see, for example, Patent Document 1 and Patent Document 2), a pipe-shaped hollow main body for accumulating high-pressure fuel, and a through-hole that intersects the hollow main body It has a pipe connection part that flows in and out of high-pressure fuel.

  In the common rail, the pipe connection part is repeatedly subjected to high stress by high-pressure fuel and easily causes fatigue failure, but the extension direction of sulfide inclusions extended by hot working of hot rolling or hot forging and pipe connection One reason is considered to be that the direction of the tensile stress generated in the part intersects.

  In general, a steel material is deformed when a stress higher than the yield stress is applied, but a phenomenon called so-called metal fatigue is known in which a steel material is destroyed when repeatedly applied even if the stress is lower than the yield stress. Since many parts in which steel materials are used are under such a stress environment, it is extremely important to improve the fatigue strength of the steel materials. The most orthodox method for improving fatigue strength is to increase the strength of steel, but in recent years it has become clear that inclusions in steel have an adverse effect on fatigue strength, and inclusions, especially sulfides such as MnS The effects of the type, size, density, etc. of inclusions have been studied in detail, and their control technology has been developed.

  As a common rail, for example, the above-mentioned Patent Document 1 discloses a technique for improving the fatigue characteristics of a product by reducing the ratio of the length and width of Mn sulfide inclusions to a certain value or less. Patent Document 2 discloses a technique for improving machinability by using TiN to precipitate MnS around it to refine sulfide inclusions, thereby miniaturizing Bi metal inclusions. Has been.

Japanese Patent No. 3934511 JP 2005-154886 A

  However, in recent years, further improvement in fatigue strength has been demanded in order to increase the service life or weight of components. Then, this invention makes it a subject to provide the steel for common rails which was excellent in fatigue strength, and the common rail which consists of the steel, ensuring machinability.

In recent years, desulfurization technology of hot metal and molten steel has been developed, and S in the product can be stably suppressed to an extremely low concentration of 0.0015% or less by mass%. In other words, not only the sulfide inclusions but also the nitride inclusions such as TiN and the oxide inclusions such as Al ₂ O ₃ have been adversely affected to improve the performance of the common rail. The two inventions described above do not provide a solution for improving product performance by controlling nitride inclusions such as TiN and oxide inclusions such as Al ₂ O ₃ .
As a result of intensive studies, the inventors have added an appropriate amount of REM during the melting of common rail steel,
(1) The average particle size of sulfide inclusions can be reduced to reduce the aspect ratio (length / width) (2) The average particle size of nitride inclusions can be reduced (3) Oxide inclusions The inventors have found that the three effects of reducing the average particle size of the particles can be expressed simultaneously, and have found that the above problems can be solved, thereby completing the present invention.
The sulfide inclusions are mainly pure MnS and (Mn, REM) S, but MnS, (Mn, REM) S, or (Mn, REM) precipitated with oxide as a nucleus. It contains inclusions containing S as a main component, and sulfides such as Fe, Ca, Ti, Zr, Mg, etc. coexist in solid solution with or bonded to MnS. Further, the nitride-based inclusion refers to TiN or AlN. The oxide inclusions are mainly Al ₂ O ₃ and (Al, REM) ₂ O _3, but some inclusions are so-called oxy such as (Al, REM) (O, S). Some also took the form of sulfide. Since S in these inclusions was sufficiently low in concentration as compared with O, it was herein referred to as an oxide inclusion without being distinguished from those not containing S.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.3-0.5%
Si: 0.05 to 0.5%,
Mn: 0.3 to 1.5%,
Al: 0.01 to 0.1%,
Ti: 0.003 to 0.02%,
Cr: 0.4 to 1.5%,
Mo: 0.1 to 1.5%,
N: 0.003 to 0.015%,
REM: 0.0003 to 0.01%
Bi: 0.01-0.03%
Containing
S: 0.0015% or less,
P: 0.035% or less,
O: limited to 0.003% or less,
A steel for common rails characterized in that the balance substantially consists of Fe and inevitable impurities.

(2) Furthermore, in mass%,
Mg: 0.0002 to 0.01%,
Ca: 0.0005 to 0.01%,
Zr: 0.0005 to 0.02%,
Te: 0.0002 to 0.005%
The common rail steel according to (1) above, which contains one or more of the above.

(3) Made of the steel described in (1) or (2) above, the average value of the ratio (L / D) of length (L) to width (D) is 4.5 or less and equivalent to a circle in the L cross section 5 × 10 ¹ to 1 × 10 ⁴ pieces / mm ^{2 of} sulfide inclusions having a diameter of 0.01 to 5.0 μm are dispersed, and 5 × 10 ^{1 of} sulfide inclusions having an equivalent circle diameter exceeding 5.0 μm. pieces / mm ² or less, the nitride inclusions equivalent circle diameter 0.002~0.5μm is 1 × 10 ³ ~1 × 10 ⁶ cells / mm ² were dispersed nitride exceeding circle equivalent diameter 0.5μm -based inclusions is not more 2 × 10 ² pieces / mm ² or less, further oxide inclusions equivalent circle diameter 0.01~5.0μm is 5 × 10 ¹ ~1 × 10 ⁴ pieces / mm ² dispersed The common rail is characterized in that the number of oxide inclusions having an equivalent circle diameter exceeding 5.0 μm is 5 × 10 ¹ pieces / mm ² or less.

  The L cross section refers to a cross section in the stretching direction by a compression process such as rolling or forging.

  According to the present invention, it is possible to obtain a steel for common rail that secures machinability and is excellent in fatigue strength and durability, and since this steel has good machinability, it becomes easy to manufacture the common rail, Further, the manufactured common rail has excellent fatigue characteristics, and has a remarkable effect that it can be a long-life common rail.

First, by adding an appropriate amount of REM found by the present inventors as a result of intensive studies,
(1) The average particle size of sulfide inclusions can be reduced to reduce the aspect ratio (length / width) (2) The average particle size of nitride inclusions can be reduced (3) Oxide inclusions Three reasons that the average particle diameter of the inclusions can be reduced and the reason for the definition regarding the form and distribution of the inclusions of the present invention will be described.

  Two molten steels containing 0.4% C, 1.0% Mn, 0.001% S, 0.05% Al, 0.01% Ti, 0.001% O by mass% Divided into crucibles, 0.0035% REM was added to only one and solidified in the mold. The two steel ingots thus obtained were hot-rolled under normal conditions to produce a steel plate having a thickness of 15 to 40 mm and a width of 80 mm.

The dispersion state of the precipitate was measured as follows. For each steel plate, a 20 mm square test piece is collected in the L direction (stretching direction by hot rolling) from the vicinity of the center of the thickness, and the L cross section of this test piece is 1000 to 100,000 using a scanning electron microscope (SEM). Observe over a region of at least 1000 μm ² at double magnification, measure particles of the target size, convert to equivalent circle diameter, and change the number per unit area (mm ² ) from the observed number and test area Converted. In addition, unless otherwise indicated, the particle diameter as described in this invention shall point out an equivalent circle diameter.

  Analyze the composition of the target sulfide inclusions, nitride inclusions and oxide inclusions using the attached wavelength dispersive spectroscopy (WDS), and determine the average composition of the oxides. Asked. At this time, in the case where Fe was detected as the analysis value of the oxide composition, the average composition of the oxide was determined by excluding Fe from the analysis value.

(1) Sulfide inclusions The average value of the ratio (L / D) of the length (L) to the width (D) of sulfide inclusions at a level containing REM is 2.8, the level not containing REM The average value of L / D was 3.7, indicating that the L / D of sulfide inclusions containing REM was reduced. REM has a large affinity with S, and it is considered that REM-S harder than MnS was formed.

  It is presumed that sulfide inclusions become elongated due to hot working, and when this intersects the tensile stress direction at a certain angle, a notch action occurs, which acts as if it is a surface defect. In the present invention, when the average L / D of sulfide inclusions exceeds 4.5, the durability ratio is greatly reduced. Therefore, the average value of L / D of sulfide inclusions is set to 4.5 or less.

  The average L / D is controlled not only by addition of REM, but also by addition of Mg, Ca, and Zr, which will be described later, as well as by the area reduction rate from the steel ingot and slab.

  Moreover, as a result of investigating the average particle diameter of sulfide inclusions, it was found that many fine sulfide inclusions containing REM were found. The particle diameter was 0.01 to 5.0 μm in terms of equivalent circle diameter.

  The phenomenon of reducing the average particle size of sulfide inclusions by adding REM is under investigation, but the following mechanism is presumed. That is, REM-S having a melting point higher than that of MnS, which is a main component when REM is not added, is present in the molten steel stage, so that aggregation due to collision between sulfide inclusions is less likely to occur, and as a result, sulfide-based Inclusions are refined to a particle size of 0.01 to 5.0 μm. In addition, in the level which does not add REM, the coarse sulfide type inclusion exceeding 5.0 micrometers remained large compared with the level which added REM. The composition of the sulfide inclusions is almost pure MnS, which is presumed to be agglomerated and coarsened in the molten steel.

Sulfide inclusions are inclusions that are inevitably generated in the desulfurization treatment. A particle size exceeding 5.0 μm is not preferable because it is disadvantageous for fatigue life even if it is in a small amount regardless of L / D, and if it exceeds 5 × 10 ¹ particles / mm ² , the effect becomes large. Therefore, the upper limit is 5 × 10 ¹ pieces / mm ² . The smaller the particle size of 0.01 to 5.0 μm, the more advantageous the fatigue strength. However, when the particle size is less than 5 × 10 ¹ particles / mm ² , the effect is almost saturated, and further reduction leads to an increase in cost. Absent. Therefore, the lower limit is 5 × 10 ¹ pieces / mm ² . On the other hand, if it exceeds 1 × 10 ⁴ pieces / mm ² , the effect on the fatigue life is increased, which is not preferable. Therefore, the upper limit is set to 1 × 10 ⁴ pieces / mm ² .

(2) Nitride inclusions Nitride inclusions precipitated in the steel were observed in detail. The frequency of nitride inclusions, particularly TiN, which precipitates fine oxides as nucleation sites. I found it high. As such a minute oxide, it is an oxide containing REM and further containing at least one of Ti and Al (hereinafter referred to as Ti-Al-REM oxide even if Ti or Al is not contained). I found. The particle size including nitride inclusions was 0.002 to 0.5 μm.

  That is, the presence of Ti-Al-REM oxide in steel increases the number of sites where nitride inclusions precipitate, compared to the case where these oxides do not exist, and the number of nitride inclusions deposited. Will increase. As a result, the nitride inclusions are refined to a particle size of 0.002 to 0.5 μm. In addition, in the level where REM was not added, a large amount of coarse nitride inclusions exceeding 0.5 μm remained as compared with the level where REM was added.

Nitride inclusions larger than 0.5 μm are present in excess of 2 × 10 ² pieces / mm ² , which adversely affects fatigue strength. Therefore, the upper limit is set to 2 × 10 ² pieces / mm ² . On the contrary, if it is a fine particle of 0.002 to 0.5 μm as observed in the present invention, the effect of suppressing the coarsening of crystal grains can be expressed by an appropriate number of dispersions. However, if it is less than 1 × 10 ³ pieces / mm ² , the effect cannot be obtained. Therefore, the lower limit is set to 1 × 10 ³ pieces / mm ² . On the other hand, when it exceeds 1 × 10 ⁶ pieces / mm ² , the fatigue strength and the toughness of the base material are adversely affected. Therefore, the upper limit is set to 1 × 10 ⁶ pieces / mm ² .

Oxide inclusions When REM is not added, oxide inclusions containing Al ₂ O ₃ as a main component are generated. Al ₂ O ₃ has a large cohesive force and tends to form cluster-like coarse oxide inclusions. However, by adding REM having a stronger deoxidizing power than Al, a part of Al ₂ O ₃ is reduced, and an oxide inclusion of (REM, Al) ₂ O ₃ is generated. The particle size of the oxide inclusions observed in this experiment was 0.01 to 5.0 μm at the REM addition level, but coarse oxide inclusions exceeding 5.0 μm at the level where REM was not added. A large amount of the product remained in comparison with the level to which REM was added. Although the mechanism is unknown, the oxide inclusions containing REM tend to be finely dispersed as compared with the oxide inclusions not containing REM, which advantageously works to improve the fatigue strength of the common rail.

Oxide inclusions are inclusions that are inevitably generated in the desulfurization treatment. If the particle diameter exceeds 5.0 μm, even a small amount is disadvantageous for the fatigue life, and if it exceeds 5 × 10 ¹ particles / mm ² , the influence is increased, which is not preferable. Therefore, the upper limit is 5 × 10 ¹ pieces / mm ² . The smaller the particle size of 0.01 to 5.0 μm, the more advantageous the fatigue strength. However, when the particle size is less than 5 × 10 ¹ particles / mm ² , the effect is almost saturated, and further reduction leads to an increase in cost. Absent. Therefore, the lower limit is 5 × 10 ¹ pieces / mm ² . On the other hand, if it exceeds 1 × 10 ⁴ pieces / mm ² , the effect on the fatigue life is increased, which is not preferable. Therefore, the upper limit is set to 1 × 10 ⁴ pieces / mm ² .

Next, the reason for defining the steel component composition of the present invention will be described.
All units are mass%.

C: 0.3-0.5%
C is the most basic element that affects not only static strength but also fatigue strength, toughness, and ductility. When C is less than 0.3%, static strength and fatigue strength are insufficient. Therefore, the lower limit is set to 0.3%. On the other hand, if it exceeds 0.5%, the toughness deteriorates. Therefore, the upper limit is 0.5%.

Si: 0.05-0.5%
Si is an important element having the highest solid solution strengthening ability after C. If Si is less than 0.05%, sufficient strength cannot be obtained. Therefore, the lower limit is made 0.05%. Further, if it exceeds 0.5%, it is also an element that significantly deteriorates toughness and workability. Therefore, the upper limit is 0.5%.

Mn: 0.3 to 1.5%
Mn is an important element for improving the hardenability and ensuring the hardness of the part even when the cooling rate is insufficient. If Mn is less than 0.3%, the required strength cannot be ensured. Therefore, the lower limit is set to 0.3%. Moreover, when it exceeds 1.5%, toughness and workability will deteriorate. Therefore, the upper limit is 1.5%.

Al: 0.01 to 0.1%
Al is an essential element used for deoxidation purposes, and has an effect of suppressing the coarsening of crystal grains by generating AlN. If the Al content is less than 0.01%, this effect is difficult to obtain. The coarsening of the crystal grains not only deteriorates the toughness, but the generation of coarse grains in a part of the part makes the mechanical properties non-uniform. Not desirable. Therefore, the lower limit is made 0.01%. On the other hand, if it exceeds 0.1%, nozzle clogging is likely to occur during casting and oxide inclusions remaining in the steel tend to deteriorate the performance. Therefore, the upper limit is made 0.1%.

Ti: 0.003-0.02%
Ti is an element that forms a nitride like Al, but has excellent thermal stability and maintains the effect of suppressing grain coarsening up to a higher temperature. If Ti is less than 0.003%, this effect is difficult to obtain, so the lower limit is made 0.003%. On the other hand, if it exceeds 0.02%, the effect of refining nitride due to the addition of REM described in the present invention is saturated, TiN itself tends to grow coarsely, and causes a reduction in fatigue strength. Therefore, the upper limit is made 0.02%.

Cr: 0.4 to 1.5%
Cr, like Mn, is a useful element that improves the hardenability of steel. If it is less than 0.4%, this effect cannot be sufficiently obtained. Therefore, the lower limit is set to 0.4%. On the other hand, if it exceeds 1.5%, the effect is almost saturated. Therefore, the upper limit is 1.5%.

Mo: 0.1 to 1.5%
Mo is an element that causes so-called secondary hardening by hardening the steel during tempering by finely precipitating the carbonitride, and is effective in improving fatigue strength. In addition, the effect of improving hardenability is great. If it is less than 0.1%, a sufficient effect cannot be obtained. Therefore, the lower limit is 0.1%. On the other hand, if it exceeds 1.5%, undissolved carbide tends to remain during the quenching heat treatment, and the toughness is deteriorated. Therefore, the upper limit is 1.5%.

N: 0.003 to 0.015%
N produces nitride inclusions such as TiN and AlN, and exhibits the effect of suppressing grain coarsening. If it is less than 0.003%, this effect cannot be sufficiently obtained. Therefore, the lower limit is made 0.003%. On the other hand, if it exceeds 0.015%, the nitride inclusions become coarse and cause a decrease in fatigue strength. Moreover, it reduces hot ductility and becomes a factor of surface defects during casting or rolling. Therefore, the upper limit is made 0.015%. From the viewpoint of steel material cleanliness, it is more desirable to be 0.01% or less.

REM: 0.0003 to 0.01%
REM is the most important element in the present invention, and has the effect of reducing the aspect ratio of sulfide inclusions, the refinement of nitride inclusions, and the refinement of oxide inclusions. If it is less than 0.0003%, these effects cannot be sufficiently obtained. Therefore, the lower limit is set to 0.0003%, but 0.001% or more is more desirable. On the other hand, if it exceeds 0.01%, the cleanliness of the steel is lowered and the toughness of the base material is deteriorated. Therefore, the upper limit is made 0.01%. In addition, although REM represents rare earth elements, such as La and Ce here, arbitrary 1 type or 2 types or more of them can be used.

Bi: 0.01-0.03%
Bi has the effect of improving the machinability of the steel material. In the present invention, since fatigue strength is aimed at and S is suppressed to a low level as described later, MnS having an effect of improving machinability is not sufficiently generated. The common rail often requires an elongated hole for fuel injection, and it is essential to ensure machinability by adding Bi. In order to obtain sufficient machinability, if less than 0.01%, there is no effect, and the lower limit is made 0.01%. On the other hand, if it exceeds 0.03%, the machinability can be further improved, but the hot ductility is extremely deteriorated and the production becomes difficult. Therefore, the upper limit is made 0.03%.

S: 0.0015% or less In the present invention, low-level control of S is very important. If S exceeds 0.0015%, the effect of modifying sulfide inclusions by REM is not sufficiently exhibited and it becomes easy to coarsen, so the performance of the common rail is deteriorated. Limited to 0.0015% or less. More preferably, it is 0.0010% or less.

P: 0.035% or less Among elements added to steel, P is an element that is most easily segregated at the grain boundary and weakens the grain boundary. In the present invention, it is desirable to reduce as much as possible. In particular, if it exceeds 0.035%, grain boundary fracture becomes remarkable, so the upper limit of P is made 0.035%. It is further desirable that the content be 0.015% or less.

O: 0.003% or less O is an element contained in molten steel and is mainly deoxidized by Al or REM. If O exceeds 0.003%, the cleanliness of the steel is deteriorated and the toughness of the base material is deteriorated. Therefore, the upper limit is made 0.003%. From the viewpoint of steel material cleanliness, 0.001% or less is more desirable.

  In the present invention, one or more of the following elements may be added to the molten steel in order to develop the characteristics required for the product.

Mg: 0.0002 to 0.01%
Mg, like REM, modifies Al ₂ O ₃ and has the effect of suppressing oxide inclusion coarsening. It also acts on sulfide inclusions and has the effect of reducing the aspect ratio. However, if the content is less than 0.0002%, these effects cannot be seen, so the lower limit is made 0.0002%. On the other hand, if it exceeds 0.01%, coarse cluster-like oxide inclusions mainly composed of MgO are formed, which is not preferable as a base point of fatigue fracture. Therefore, the upper limit is made 0.01%.

Ca: 0.0005 to 0.01%
Ca, like REM, modifies Al ₂ O ₃ and has the effect of suppressing oxide inclusion coarsening. It also acts on sulfide inclusions and has the effect of reducing the aspect ratio. However, since these effects are not seen at less than 0.0005%, the lower limit is made 0.0005%. On the other hand, if it exceeds 0.01%, an oxide-based inclusion having CaO-Al ₂ O ₃ as a main component and having high extensibility as well as a sulfide-based inclusion is formed, which is not preferable as a starting point for fatigue fracture. . Therefore, the upper limit is made 0.01%.

Zr: 0.0005 to 0.02%
Zr acts on sulfide inclusions and has the effect of reducing the aspect ratio. However, since these effects are not seen at less than 0.0005%, the lower limit is made 0.0005%. However, if it exceeds 0.02%, hard clustered oxide inclusions are generated, which adversely affects the cleanliness and fatigue strength of the steel. Therefore, the upper limit is made 0.02%.

Te: 0.0002 to 0.005%
Te exists mainly in the form of a compound of MnTe and exists in the vicinity of MnS and contributes to a decrease in aspect ratio.
However, if the content is less than 0.0002%, these effects cannot be seen, so the lower limit is made 0.0002%. On the other hand, the hot ductility is greatly deteriorated, and if it exceeds 0.005%, it is difficult to produce by continuous casting because the surface flaw and breakout of the slab frequently occur. Therefore, the upper limit is made 0.005%.

  In addition to the above components, V, Nb, Ni and the like can be added as long as the effects of the present invention are not impaired.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  A steel material was prototyped with the chemical components shown in Table 1 (the balance being Fe and inevitable impurities). 1-10 are invention steels and 11-32 are comparative steels. The prototype steel was melted in the converter, and deoxidized with Si and Mn at the time of steel leaving the converter, and deoxidation and desulfurization with other elements and other components were adjusted during vacuum degassing with ladle refining. Note that Bi, Ca, Mg, and Te were added within 5 minutes before the start of continuous casting in consideration of the high vapor pressure. This molten steel is cast into a 200 mm thick slab by continuous casting, once cooled to room temperature, heated to 1170 ° C. and hot-rolled to obtain a steel plate with a plate thickness of 15 to 35 mm and a width of 80 mm, near the thickness center of the steel plate A square bar of 15 mm square and 80 mm length was collected in the C direction.

  Further, an inclusion observation sample was taken in a thickness direction cross section (a cross section perpendicular to the longitudinal direction of the square bar) of the L cross sections of the steel bars of each square bar.

The size and number of inclusions were examined using a scanning electron microscope (SEM). For sulfide inclusions, nitride inclusions exceeding 0.5 μm, and oxide inclusions, a photograph was taken over an area of at least 10,000 μm ² at a magnification of about 5000 times, and each inclusion was based on this photograph. The size (converted to the equivalent circle diameter) and the number were measured and converted from the test area to the number per unit area (mm ² ). Furthermore, regarding the sulfide type inclusions of 0.01 to 5.0 μm, each L / D was measured, and an average value (arithmetic average) was obtained. Since the nitride inclusions of 0.002 to 0.5 μm were finely dispersed in a large amount, the magnification was set to about 20000 times, and a photograph was taken over an area of at least 250 μm ² or more. The size (converted to the equivalent circle diameter) and the number were measured and converted from the test area to the number per unit area (mm ² ).

  As for L / D, about 100 sulfide inclusions in the vicinity of the center of the cross section were photographed, the photograph was read with an image processing device, each L / D was obtained, and an average value thereof was calculated.

  The dispersion state of the precipitate is analyzed for the composition using the attached wavelength dispersion spectroscopy (WDS) for at least 30 of the sulfide inclusions, nitride inclusions, and oxide inclusions of interest. Then, the average composition of the oxide was determined. At this time, in the case where Fe of ground iron was detected in the analytical value of the oxide composition, the average composition of the oxide was determined by excluding Fe from the analytical value.

At the same time, each square bar was heated to 850 to 960 ° C, quenched, and tempered at 530 to 660 ° C. Thereafter, tensile test pieces and Ono-type rotary bending fatigue test pieces were produced from these square bars, and the durability ratio (fatigue limit / tensile strength) was determined. Further, a machinability test piece was prepared from the same square bar and subjected to a drill drilling test. The drilling test is a method for evaluating machinability at the highest cutting speed (so-called VL1000, unit: m / min) that can cut to a cumulative hole depth of 1000 mm. The larger the value, the better the machinability. Indicates.
The results are shown in Table 2.

  Steel types 1 to 10 are steels of the present invention common rail, satisfy the scope of the present invention, and all have a sufficient durability ratio. On the other hand, 11 to 26 and 28 to 30 of the comparative example steel do not satisfy the scope of the present invention, and do not have a sufficient durability ratio or machinability as the steel of the present invention. It became difficult.

  Comparative Examples 11 and 12 relate to Mn. Comparative Example 11 is an example in which the durability ratio deteriorates because the strength deteriorates because Mn is too small. Further, Comparative Example 12 is an example in which Mn was excessive, so that the toughness was deteriorated and the durability ratio was not satisfied.

  Comparative Examples 13 and 14 are related to S, both of which are excessive, so that the sulfide inclusions are excessive and the durability ratio is deteriorated.

  Comparative Examples 15 and 16 relate to Al. Comparative Example 15 is an example in which since Al was too small, a sufficient amount of 0.002 to 0.5 μm nitride inclusions could not be generated, and the durability ratio deteriorated. Further, Comparative Example 16 is an example in which since Al was excessive, cluster-like oxide inclusions exceeding 5.0 μm were excessively generated in the steel to deteriorate the durability ratio.

  Comparative Examples 17 and 18 relate to Ti. Comparative Example 17 is an example in which since Ti was excessive, a sufficient amount of nitride inclusions of 0.002 to 0.5 μm could not be generated, and the durability ratio deteriorated. Further, Comparative Example 18 is an example in which Ti was excessive, so that nitride inclusions exceeding 0.5 μm were excessively generated in the steel and the durability ratio was deteriorated.

  Since Comparative Examples 19 and 20 are related to O and both are excessive, oxide-based inclusions exceeding 0.5 μm are excessive and the durability ratio is deteriorated.

  Comparative Examples 21 and 22 relate to N. Comparative Example 21 is an example in which N was too small to generate a sufficient amount of nitride inclusions of 0.002 to 0.5 μm and the durability ratio deteriorated. Further, Comparative Example 22 is an example in which N is excessive, so that nitride inclusions exceeding 0.5 μm are excessively generated in the steel and the durability ratio is deteriorated. Moreover, surface flaws are generated in the slab, and this is also considered to be caused by N being a problem.

  Comparative Examples 23, 24, and 25 relate to REM. In Comparative Example 23, REM is too small and does not contain an element having an effect of suppressing the expansion of sulfide inclusions such as Mg and Ca. Therefore, L / D exceeds 4.5, and the sulfide inclusions are fine. This is an example in which the durability ratio is greatly deteriorated because neither the effect of refining, the effect of refining nitride inclusions, or the effect of refining oxide inclusions can be obtained. In Comparative Example 24, although the elongation of sulfide inclusions can be suppressed by addition of Zr, since a sufficient amount of Ti-Al-REM oxide serving as a nucleation site of nitride inclusions does not exist, nitride inclusions are present. This is an example in which the durability ratio was deteriorated because the fine dispersion of the object was not achieved and the oxide inclusions were not refined. In Comparative Example 25, since REM was excessive, oxide inclusions exceeding 5.0 μm were excessively generated to deteriorate the durability ratio.

  Comparative Examples 26 and 27 relate to Bi. Comparative Example 26 is an example in which Bi was insufficient and a sufficient durability ratio was obtained, but sufficient machinability was not obtained. Further, Comparative Steel No. 27 is an example in which Bi was excessive and a large crack was generated in the block rolling stage, so that a sample could not be collected.

  The comparative steels 28, 29 and 30 have chemical components within the scope of the present invention, but the reduction in rolling area is too large, and the average L / L of sulfide inclusions having a particle diameter of 0.01 to 5.0 μm This is an example in which the durability ratio deteriorated because D exceeded the range of the present invention.

Claims (3)

% By mass
C: 0.3-0.5%
Si: 0.05 to 0.5%,
Mn: 0.3 to 1.5%,
Al: 0.01 to 0.1%,
Ti: 0.003 to 0.02%,
Cr: 0.4 to 1.5%,
Mo: 0.1 to 1.5%,
N: 0.003 to 0.015%,
REM: 0.0003 to 0.01%
Bi: 0.01-0.03%
Containing
S: 0.0015% or less,
P: 0.035% or less,
O: limited to 0.003% or less,
A steel for common rails characterized in that the balance substantially consists of Fe and inevitable impurities. Furthermore, in mass%,
Mg: 0.0002 to 0.01%,
Ca: 0.0005 to 0.01%,
Zr: 0.0005 to 0.02%,
Te: 0.0002 to 0.005%
The common rail steel according to claim 1, wherein one or more of them are contained. It consists of steel of Claim 1 or 2, Comprising: In the L section, the average value of ratio (L / D) of length (L) and width (D) is 4.5 or less, and equivalent circle diameter 0.01-5 sulfide inclusions is .0μm is 5 × 10 ¹ ~1 × 10 ⁴ cells / mm ² were dispersed, sulfide inclusions exceeding a circle equivalent diameter 5.0μm is 5 × 10 ¹ cells / mm ² or less Yes, nitride inclusions having an equivalent circle diameter of 0.002 to 0.5 μm are dispersed 1 × 10 ³ to 1 × 10 ⁶ pieces / mm ² , and nitride inclusions having an equivalent circle diameter of more than 0.5 μm are 2 × 10 and ² or / mm ² or less, further oxide inclusions equivalent circle diameter 0.01~5.0μm is 5 × 10 ¹ to 1 × 10 ⁴ pieces / mm ² were dispersed, equivalent circle diameter 5. A common rail characterized in that oxide inclusions exceeding 0 μm are 5 × 10 ¹ pieces / mm ² or less.