JP2004083986A - Steel having excellent fatigue property and steel part produced from the steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel which has excellent fatigue properties, and is suitable for a steel part having a part in which tensile stress is produced in an open face not parallel to Mn sulfide inclusions elongated by rolling or forging, and to provide a steel part produced from the steel. <P>SOLUTION: The steel having excellent fatigue properties comprises, by mass, ≤0.01% (inclusive of 0%) S and >0.01 to 0.10% Al. The average value of the ratio (L/D) between the length (L) and the width (D) of Mn sulfide inclusions observed in the L-cross section of the steel is ≤4.5. The steel part is produced from the steel. The steel part has an open face not parallel to the elongating direction of Mn sulfide inclusions elongated by compressive working such as rolling and forging, and has a part in which tensile stress is produced in the open face. <P>COPYRIGHT: (C)2004,JPO

Description

【００４３】
これらの試験片を用いて、引張試験および小野式回転曲げ疲労試験を行ない、耐久比（疲労限界／引張強さ）を求めた。
以上の結果を表２に示す。
【００４４】
【表２】

【００４５】
試番１〜８は本発明鋼であり、いずれも十分な耐久比をもっている。一方、比較鋼の試番９はＭｇ、Ｃａ、Ｚｒ、Ｔｅ、ＲＥＭが本発明範囲に満たないため、また、試番１１はＳが本発明範囲を超えているため、Ｍｎ硫化物系介在物の平均Ｌ／Ｄが本発明範囲を超えて大きくなり、耐久比が劣化した例である。試番１０は、化学成分は本発明範囲内であるが、圧延での減面率が大きすぎ、Ｍｎ硫化物系介在物の平均Ｌ／Ｄが本発明範囲を超えて大きくなったため、耐久比が劣化した例である。また、試番１２はＡｌが本発明範囲よりも少ないため、結晶粒径が粗大化し、耐久比が劣化した例である。
【００４６】
【実施例２】
さらに、本発明鋼の優位性を確認するため、実際の部品による試験を実施した。表３に示す１０種類の化学組成の鋼を溶製し、分塊圧延した鋼片を１１７０℃に加熱し、熱間圧延し、８８０℃で仕上げて、４５ｍｍφの棒鋼とした。該棒鋼の中心付近のＬ断面を鏡面研磨し、直径の中心付近にある１００個程度のＭｎ硫化物系介在物を光学顕微鏡で撮影し、その写真を画像処理装置で読み取り、長さ（Ｌ）と幅（Ｄ）の比（Ｌ／Ｄ）を求め、それらの平均値を計算した。次いで、この棒鋼を加工して、図４に示す、棒鋼の長手方向に沿ったコモンレールを、長さ３２０ｍｍ、本管レール穴径７ｍｍφ、５つの分岐穴の径１ｍｍφ（コモンレール１）、長さ３１０ｍｍ、本管レール穴径６．５ｍｍφ、５つの分岐穴の径１．４ｍｍφ（コモンレール２）、長さ３００ｍｍ、本管レール穴径６ｍｍφ、５つの分岐穴の径１．９ｍｍφ（コモンレール３）の３種類作製した。これらのコモンレールを８３０〜９５０℃に加熱後、焼入れ、５３０〜６６０℃で焼戻した。
【００４７】
【表３】

【００４８】
その後、これらのコモンレールの１つの本管レール穴と５つの分岐穴をすべてシールし、残る１つの本管レール穴から、ポンプで周期的に圧力を変化させるように軽油を圧入した。疲労限界に対応するものとして１０^７回の繰り返す周期的圧力変化に（亀裂が入らずに）耐える軽油の最大の圧力（ＭＰａ）をコモンレールの疲労限特性値として評価した。
以上の結果を表４に示す。
【００４９】
【表４】

【００５０】
試番１３〜１８は本発明鋼であり、いずれもコモンレールの疲労限特性値が高い。一方、比較鋼の試番１９はＭｇ、Ｃａ、Ｚｒ、Ｔｅ、ＲＥＭが本発明範囲に満たないため、Ｍｎ硫化物系介在物の平均Ｌ／Ｄが本発明範囲を超えて大きくなり、Ｍｎ硫化物系介在物が疲労破壊の起点となり、疲労限特性値が劣化した例である。試番２０はＺｒが本発明範囲を超えて、クラスター状介在物が疲労破壊の起点となり疲労限特性値が劣化した例である。試番２１はＳが本発明範囲を超えたために疲労限特性値が劣化した例である。試番２２はＡｌ添加量が本発明範囲よりも少ないため、結晶粒が粗大化し、疲労限特性値が劣化した例である。
【００５１】
【発明の効果】
以上の結果から、本発明によれば、鋼の成分を最適にすること等によりＭｎ硫化物系介在物の平均Ｌ／Ｄが適正値以下に制御され、圧延や鍛造などの圧縮加工によって伸長したＭｎ硫化物系介在物に平行でない開放面を有し、かつ該開放面に引張応力が生じる部分を有する鋼部品に適する優れた疲労特性を有する鋼と、該鋼から作製した部品が提供され、産業上の効果が極めて顕著な発明であるといえる。
【図面の簡単な説明】
【図１】Ｍｎ硫化物系介在物の平均Ｌ／Ｄと耐久比との関係を表わす図である。
【図２】Ｓ含有量と耐久比との関係を表わす図である。
【図３】Ｍｎ硫化物系介在物の存在状態を表す図で、（ａ）は部品においてＭｎ硫化物系介在物の伸長した方向と開放面とが直角な場合を表わす図で、（ｂ）は部品においてＭｎ硫化物系介在物の伸長した方向と開放面とが斜角な場合を表わす図で、（ｃ）は部品においてＭｎ硫化物系介在物の伸長した方向と開放面とが平行な場合を表わす図である。
【図４】コモンレールの概略縦断面を示し、Ｍｎ硫化物系介在物の伸長方向を示す図である。
【符号の説明】
１　鋼材
２　開放面
３　負荷応力の方向
４　伸長したＭｎ硫化物系介在物
５　本管レール穴
６　分岐穴
７　コモンレール長さ
８　本管レール穴径
９　分岐穴径[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel having excellent fatigue properties used for parts such as automobiles and industrial machines, and a steel part produced from the steel, and more particularly, to a steel that has been elongated by compression processing such as rolling or forging. The present invention relates to a steel having excellent fatigue characteristics suitable for a steel part having a portion where a tensile stress is generated on an open surface that is not parallel to the direction of extension of Mn sulfide-based inclusions, and a steel part made from the steel.
[0002]
[Prior art]
In general, a steel material is deformed when a stress equal to or higher than the yield stress is applied. However, even if the stress is equal to or lower than the yield stress, the steel material is broken when repeatedly applied, so-called a phenomenon called metal fatigue. . Since many parts in which steel is used are in such a stress environment, it is extremely important to improve the fatigue strength of the steel. The most orthodox method of increasing fatigue strength is to increase the strength of steel.In recent years, however, it has become clear that inclusions in steel have a negative effect on fatigue strength. The effects of the above have been studied in detail, and control techniques have been developed.
[0003]
For example, Japanese Patent Application Laid-Open No. 4-168247 discloses a technique for improving rolling contact fatigue by including 60% or more of oxide-based inclusions having an aspect ratio of 3.0 or more after steel material processing. Further, Japanese Patent Application Laid-Open No. 2-270935 discloses a technique for improving the fatigue strength of case hardening steel for gears by reducing the number of oxide-based inclusions and nitride-based inclusions having a diameter of 20 μm or more to 14 or less per 1 g of steel. Is disclosed. JP-A-5-25586 and JP-A-7-188853 control MnS of carburizing steel. The former is a technique for improving fatigue strength by adding a specific amount of Ca to shorten the length of MnS, and the latter is a technique for adding Mg to shorten the length of MnS to reduce fatigue strength. It is a technology that improves and has achieved certain results.
[0004]
[Problems to be solved by the invention]
However, in recent years, there is a need for steel materials having more excellent fatigue strength because it is necessary to obtain the same strength even if the size of the steel material is reduced in order to reduce the weight of parts. For example, in a part recently used for a diesel engine, particularly a part such as a common rail, a Mn sulfide-based inclusion 4 elongated by machining is formed by a branch hole 6 as shown in FIG. Since it intersects with the open surface 2 in a direction substantially perpendicular to the open surface, and a repeated tensile stress is applied to the open surface in the circumferential direction of the branch hole, even if the steel material is strengthened at all, the elongated inclusions may become the fracture starting points. And a fatigue strength commensurate with the tensile strength cannot be obtained.
[0005]
The present invention relates to a steel having excellent fatigue properties suitable for a steel part having a portion where a tensile stress is generated on an open surface that is not parallel to the direction of elongation of Mn sulfide-based inclusions elongated by a compression process such as rolling or forging, and the steel. It is an object of the present invention to provide steel parts manufactured from
[0006]
[Means for Solving the Problems]
The present inventors have conducted extensive experiments and conducted extensive studies. As a result, an appropriate amount of Mg or the like, which reduces the S content of steel and reduces the aspect ratio (length / width) of Mn sulfide-based inclusions, and further, Al The inventors have found that the above problem can be solved by adding and controlling the average aspect ratio to a certain value or less, and completed the present invention.
[0007]
The Mn sulfide-based inclusions include MnS as a main component in addition to pure MnS and MnS precipitated with an oxide as a nucleus, and sulfides such as Fe, Ca, Ti, Zr, Mg, and REM contain MnS. Inclusions that form a solid solution with or combine with MnS, and elements other than S, such as MnTe, form compounds with Mn and form solid compounds with MnS or coexist with MnS. In the chemical formula, (Mn, X) (S, Y) (where, X: a sulfide-forming element other than Mn, Y: an element that binds to Mn other than S) It can be expressed as
[0008]
The gist of the present invention is as follows.
(1) In mass%,
S: 0.01% or less (including 0%),
Al: more than 0.01% to 0.1%,
Containing
Mg: 0.0002-0.01%, and Mg / S ≧ 0.05,
Ca: 0.0005 to 0.01%, and Ca / S ≧ 0.05,
Zr: 0.0005 to 0.02%, and Zr / S ≧ 0.05;
Te: 0.0002-0.005%, and Te / S ≧ 0.05,
REM: 0.0005-0.01%, and REM / S ≧ 0.05
And the average value of the ratio (L / D) of the length (L) to the width (D) of the Mn sulfide-based inclusions observed in the L section of the steel is 4 A steel having excellent fatigue properties, characterized by being not more than 0.5.
[0009]
(2) steel in mass%
C: 0.3-0.5%,
Si: 0.5% or less,
Mn: 0.3-1.5%,
P: 0.035% or less (including 0%)
The steel according to (1), which has excellent fatigue characteristics.
[0010]
(3) In mass%,
S: 0.02% or less (including 0%),
Al: more than 0.01% to 0.1%,
Containing
Cr: 0.4 to 1.5%,
Mo: 0.1% to 1.5%,
V: more than 0.1% to 0.6%
Containing one or more of the following,
further,
Mg: 0.0002-0.01%, and Mg / S ≧ 0.05,
Ca: 0.0005 to 0.01%, and Ca / S ≧ 0.05,
Zr: 0.0005 to 0.02%, and Zr / S ≧ 0.05;
Te: 0.0002-0.005%, and Te / S ≧ 0.05,
REM: 0.0005-0.01%, and REM / S ≧ 0.05
And the average value of the ratio (L / D) of the length (L) to the width (D) of the Mn sulfide-based inclusions observed in the L section of the steel is 4 A steel having excellent fatigue properties, characterized by being not more than 0.5.
[0011]
(4) steel in mass%
C: 0.3-0.5%,
Si: 0.5% or less,
Mn: 0.3-1.5%,
P: 0.035% or less (including 0%)
The steel according to the above (3), which has excellent fatigue properties.
[0012]
(5) A steel part produced from the steel according to any one of the above (1) to (4), which is parallel to the elongation direction of the Mn sulfide-based inclusion elongated by a compression process such as rolling or forging. A steel part, characterized by having an open surface which is not satisfactorily, and having a portion where a tensile stress is generated on the open surface.
[0013]
(6) The steel part according to the above (5), wherein the steel part is a common rail.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
First, based on the experimental results that triggered the present invention, the form of the Mn sulfide-based inclusion was changed in the L section of the steel (cross section in the stretching direction by compression processing such as rolling or forging). The reason why the average value of the ratio (L / D) of the length (L) to the width (D) is set to 4.5 or less will be described.
[0015]
A steel ingot containing components of 0.4% C and 0.006% S in mass% was cut into various sizes in order to adjust the reduction in area, and then cut under ordinary conditions. Hot rolling was performed to produce steel plates having a thickness of 15 to 40 mm and a width of 80 mm having Mn sulfide-based inclusions having various L / D shapes. From the vicinity of the center of the thickness of each steel plate, 14 mm square in three directions of L direction (parallel to the stretching direction by rolling), C direction (perpendicular to the left direction), and X direction (45 degrees to the left direction). , And 80 mm long test pieces of square bars were collected.
[0016]
L / D of the Mn sulfide-based inclusion was measured as follows. A mirror bar is polished in a thickness direction cross section (a cross section perpendicular to the longitudinal direction of the square bar) of the L cross section of the square bar sampled in the C direction, and about 100 Mn sulfides near the center of the cross section The system inclusion was photographed with an optical microscope, and the photograph was read by an image processing device to determine the L / D, and then the average value of 100 L / Ds was calculated. This value was used as a representative value of the average L / D of the square bars sampled from the same steel plate in the L and X directions.
[0017]
After quenching these square bars from a temperature of 850 ° C. to 960 ° C. and tempering them at a temperature of 520 ° C. to 660 ° C., they are machined into tensile test pieces and Ono-type rotating bending fatigue test pieces, and a tensile test and Ono-type rotating A bending fatigue test was performed to examine a durability ratio (fatigue limit / tensile strength).
[0018]
As shown in FIG. 1, the durability ratio of the test pieces taken in the C direction and the X direction greatly depends on the average L / D of the Mn sulfide-based inclusions, and when the average L / D exceeds 4.5, the durability ratio increases. Was found to be greatly reduced. On the other hand, the durability ratio of the test piece taken in the L direction is hardly affected by the average L / D value.
[0019]
Although the details of this reason are still unknown, sulfides such as Mn sulfide-based inclusions become elongated by hot working, and when they intersect at an angle with the surface (open face) of the test piece, a notch is formed. It is presumed that the action occurs and acts as an action like a surface scratch. In the Ono-type rotating bending fatigue test, since tensile stress acts on the surface of the test piece, it is presumed that the fatigue strength is deteriorated if there is a notch on the surface of the test piece.
[0020]
From the above experimental results, in the present invention, the average value of the ratio (L / D) of the length (L) to the width (D) of the Mn sulfide-based inclusion was set to 4.5 or less.
[0021]
The average L / D is controlled by the addition of an element that lowers the aspect ratio (L / D) of Mn sulfide-based inclusions such as Mg described below, and the area reduction from the steel ingot or slab.
[0022]
Here, the method of measuring the size of the Mn sulfide-based inclusions is as follows. The vicinity of the center of the steel is mirror-polished in the L section, and about 100 Mn sulfide-based inclusions near the center of the diameter or the thickness are optically polished. A photograph is taken with a microscope, and the photograph is read by an image processing device, and an average L / D is obtained.
[0023]
Next, reasons for limiting the amount of S will be described.
As a group of test steels, a steel ingot containing C: 0.42%, S: 0.001% to 0.025% by mass%, and other components adjusted was melted, hot-rolled, and L-section In Example 1, a steel bar having a diameter of 85 mmφ having Mn sulfide-based inclusions having an average L / D in the range of about 2.5 to 3.5 was produced. In addition, as test group 2, C: 0.42%, Cr: 1%, Mo: 0.55%, V: 0.41%, S: 0.001% to 0.025% by mass% Then, a steel ingot with other components adjusted is melted, hot-rolled, and Mn sulfide having an average L / D form in the range of about 2.5 to 3.5 in the L section in the same manner as the test steel 1 group. A steel bar with a diameter of 85 mmφ having a physical inclusion was produced. Here, the method of measuring the size of the Mn sulfide-based inclusions is as follows. The vicinity of the center of the diameter of each steel bar is mirror-polished in the L section, and about 100 Mn sulfide-based inclusions near the center of the diameter are measured with an optical microscope. , And the photograph was read by an image processing device to determine the average L / D. From these steel bars, a tensile test specimen and an Ono-type rotating bending fatigue test specimen were sampled in the C direction (perpendicular to the stretching direction by rolling). Next, these are quenched from 900 ° C. to 960 ° C. and tempered at 580 ° C. to 660 ° C., and then subjected to a tensile test and an Ono-type rotary bending fatigue test to obtain an endurance ratio (fatigue limit / tensile strength). Was examined.
[0024]
FIG. 2 shows the experimental results of the durability ratio of the test steel 1 group and the test steel 2 group. In the case of the test steel group 1, the S content exceeds 0.01%, and the durability ratio decreases significantly. From this experimental result, S is limited to 0.01% or less (including 0%). On the other hand, in the case of the test steel group 2, the durability ratio is significantly deteriorated from the S content exceeding 0.02%. Although the reason is unknown at present, in the case of steel containing elements such as Cr, Mo, and V, it is estimated that the fatigue strength can be maintained at a certain value up to an S content of 0.02%. Therefore, the amount of S in the case of steel containing these elements is limited to 0.02% or less (including 0%). Preferably it is 0.01% or less.
[0025]
Next, reasons for limiting other chemical components will be described.
Al: more than 0.01% to 0.1%,
Al is an essential element as a deoxidizing element, and has the function of forming AlN and preventing the crystal grains from becoming coarse. The coarsening of the crystal grains not only causes the deterioration of toughness, but also the generation of coarse grains in a part of the part is not preferable because the mechanical properties become non-uniform. Al must be at least 0.01% or more. On the other hand, if it exceeds 0.1%, nozzle clogging during continuous casting becomes severe, so the upper limit is made 0.10%.
[0026]
Mg: 0.0002-0.01%, and Mg / S ≧ 0.05
Ca: 0.0005 to 0.01%, and Ca / S ≧ 0.05
Zr: 0.0005-0.02%, and Zr / S ≧ 0.05
Te: 0.0002 to 0.005%, and Te / S ≧ 0.05
REM: 0.0005-0.01%, and REM / S ≧ 0.05
In the steel of the present invention, one or more of Mg, Ca, Zr, Te, and REM are added. These are useful elements that function to lower the aspect ratio (L / D) of Mn sulfide-based inclusions.
[0027]
When the Mg content is less than 0.0002% and when the Mg content is Mg / S <0.05 in terms of mass% relative to the S content, the effect of reducing the aspect ratio is not observed, while when it exceeds 0.01%, Cluster-like inclusions are formed and become the starting point of fatigue fracture. Therefore, Mg is set to 0.0002 to 0.01% and Mg / S ≧ 0.05. When the Ca content is less than 0.0005% and when the Ca content is Ca / S <0.05 by mass% relative to the S content, there is no effect of lowering the aspect ratio, while when it exceeds 0.01%, the cluster form is reduced. Inclusions form and serve as starting points for fatigue failure. Therefore, Ca is 0.0005 to 0.01%, and Ca / S ≧ 0.05. When Zr is less than 0.0005% and Zr amount is such that Zr / S <0.05 in mass% ratio to S amount, there is no effect of decreasing the aspect ratio, and when it exceeds 0.02%, cluster-like inclusions are formed. It becomes the starting point of fatigue failure. Therefore, Zr is set to 0.0005 to 0.02% and Zr / S ≧ 0.05. When Te is less than 0.0002% and the amount of Te satisfies Te / S <0.05 in terms of mass% with respect to the amount of S, there is no effect of lowering the aspect ratio. Manufacturing becomes difficult. Therefore, Te is set to 0.0002 to 0.005% and Te / S ≧ 0.05. When the REM is less than 0.0005% and the REM amount is REM / S <0.05 in terms of mass% relative to the S amount, there is no effect of lowering the aspect ratio. The REM content is 0.0005 to 0.01% and REM / S ≧ 0.05 in consideration of economy because the effect of reduction is reduced and the element is an expensive element.
[0028]
C: 0.3-0.5%
C is the most basic element that affects fatigue strength, toughness, and ductility, not to mention static strength. If C is less than 0.3%, the static strength and fatigue strength are insufficient, and if more than 0.5%, the toughness deteriorates. Therefore, C is set to 0.3 to 0.5%.
[0029]
Si: not more than 0.5% Si is an important element having a solid solution strengthening ability next to C, but is also an element that significantly deteriorates toughness and workability. If it exceeds 0.5%, the toughness significantly deteriorates. Therefore, Si is limited to 0.5% or less.
[0030]
Mn: 0.3-1.5%
Mn is an important element for improving the hardenability and securing the hardness up to the core of the component even when the cooling rate is insufficient. If it is less than 0.3%, the required strength cannot be secured. If it exceeds 1.5%, toughness and workability deteriorate, so Mn is set to 0.3 to 1.5%.
[0031]
P: 0.035% or less (including 0%)
P is the element which is most easily segregated at the grain boundary among the elements added to steel and makes the grain boundary brittle. In the present invention, it is preferable to reduce as much as possible. In particular, if it exceeds 0.035%, grain boundary destruction becomes significant. Therefore, P is limited to 0.035% or less. When used in an environment where delayed fracture is a problem, the content is preferably limited to 0.015% or less.
[0032]
Cr: 0.4 to 1.5%
Cr, like Mn, is a useful element for improving the hardenability of steel. When Mn alone is insufficient in hardenability, it is a useful additive element for securing hardness up to the core of a part. If it is less than 0.4%, the effect is small, and if it exceeds 1.5%, the effect per added amount is reduced, and it is more advantageous to obtain the effect by adding another element. Therefore, Cr is set to 0.4% to 1.5%.
[0033]
Mo: 0.1% to 1.5%
Mo is an element that causes the so-called secondary hardening that hardens steel during tempering by precipitating Mo carbonitride finely. It is an element that increases the fatigue strength by precipitation hardening and, like Mn and Cr, improves the hardenability. If the amount is less than 0.1%, a sufficient effect cannot be obtained, and if the amount is more than 1.5%, the dissolved carbide remains at the time of quenching heat treatment, and the toughness is deteriorated. Therefore, Mo is set to 0.1% to 1.5%.
[0034]
V: more than 0.1% to 0.6%
V, like Mo, is an element that causes so-called secondary hardening that hardens steel during tempering by precipitating fine carbonitrides. It is an element that brings about refinement of crystal grains by fine carbonitrides, thereby increasing the toughness and further improving the fatigue strength due to precipitation hardening. If the amount is less than 0.1%, a sufficient effect cannot be obtained. If the amount is more than 0.6%, the dissolved carbide remains at the time of quenching heat treatment, and the toughness is rather deteriorated. Therefore, V is set to more than 0.1% to 0.6%.
[0035]
Since the steel of the present invention is used after being subjected to a quenching / tempering treatment in a part manufacturing process, parameters such as a heating temperature and a finishing temperature among the rolling conditions do not significantly affect the strength. It is recommended that the quenching solution temperature during the component manufacturing process be 830 ° C. or higher. However, particularly when a large amount of V or Mo is added, it is preferable that the carbonitride of V or Mo is solution-hardened. The solution temperature is preferably 880 ° C. or higher. Although the tempering temperature is recommended to be 450 ° C. or higher, it is preferable to set the tempering temperature to 550 ° C. or higher when the toughness is to be improved by high temperature tempering.
[0036]
Next, a steel part having excellent fatigue properties manufactured from the steel of the present invention will be described with reference to FIGS. 3 (a), 3 (b) and 3 (c). In the steel of the steel part 1, Mn sulfide-based inclusions 4 elongated by processing such as rolling and forging exist. FIGS. 3A and 3B show a case where the elongated Mn sulfide-based inclusions 4 intersect at an angle that is not parallel to the open surface 2 (the surface of the part or the inner surface of the hole). However, by applying the steel of the present invention to a part having a portion where a tensile stress 3 is generated on such an open surface 2, a steel part having excellent fatigue characteristics can be obtained. On the other hand, as shown in FIG. 3C, even when a tensile stress 3 is generated on the open surface 2, the Mn sulfide-based inclusion 4 is oriented in a direction parallel to the open surface 2. No improvement in fatigue properties can be expected even when the steel of the present invention is applied.
The above effects are as described above.
[0037]
Next, a common rail used in a diesel engine will be described with reference to FIG. 4 as an example of the steel part. The common rail is a part located between the pump and the injector for pumping light oil as fuel, and is a pipe-shaped container for accumulating light oil. The main pipe rail hole 5 is a main pipe of the common rail, and has a role of accumulating light oil. Several branch holes 6 are formed in the main rail hole 5 and open perpendicularly to the hole, through which light oil is pumped to each injector. When the light oil is periodically pumped with the operation of the engine, the pressure of the light oil in the common rail is also periodically increased. A circumferential tensile stress is periodically generated in the main rail hole 5 and the branch hole 6. Since the common rail is usually manufactured along the longitudinal direction of the rolled steel bar, the Mn sulfide-based inclusions 4 elongated by rolling intersect the inner surface of the branch hole 6 at a substantially right angle. Since the branch hole is an open surface, the portion is exactly in the situation shown in FIG. By applying the steel of the present invention to such a common rail, the fatigue characteristics are significantly improved.
[0038]
Hereinafter, the present invention will be described in more detail with reference to Examples. However, these Examples are not intended to limit the present invention. Are included in the target range.
[0039]
Embodiment 1
Steels of 11 kinds of chemical compositions shown in Table 1 were melted, cut from the steel ingots into steel ingots of various sizes in order to adjust the area reduction rate, and a slab rolled into 1200 ° C. Heating, hot rolling, finishing at 900 ℃, to produce steel plates of various thicknesses of thickness 15-35mm, width 80mm, from the vicinity of the thickness center of the steel plate in the C direction 15mm square, length 80mm Square bars were collected.
[0040]
The cross section in the thickness direction (the cross section perpendicular to the longitudinal direction of the square bar) of the L cross section of the steel bar of each square bar is mirror-polished to remove about 100 Mn sulfide-based inclusions near the center of the cross section. The photograph was taken with an optical microscope, the photograph was read by an image processing apparatus, the ratio (L / D) of the length (L) and the width (D) was obtained, and the average value thereof was calculated.
[0041]
After heating these rods to 850-960 ° C, they were quenched and tempered at 530-660 ° C. Thereafter, a tensile test piece and an Ono-type rotating bending fatigue test piece were prepared from these square bars.
[0042]
[Table 1]

[0043]
Using these test pieces, a tensile test and an Ono-type rotary bending fatigue test were performed, and a durability ratio (fatigue limit / tensile strength) was determined.
Table 2 shows the above results.
[0044]
[Table 2]

[0045]
Test Nos. 1 to 8 are steels of the present invention, all of which have a sufficient durability ratio. On the other hand, the sample No. 9 of the comparative steel had Mn, Ca, Zr, Te, and REM less than the range of the present invention, and the sample No. 11 had S exceeding the range of the present invention. This is an example in which the average L / D exceeds the range of the present invention and the durability ratio is deteriorated. In Test No. 10, although the chemical components were within the range of the present invention, the reduction ratio of rolling was too large, and the average L / D of the Mn sulfide-based inclusions exceeded the range of the present invention. Is an example in which is deteriorated. Test No. 12 is an example in which Al is smaller than the range of the present invention, so that the crystal grain size is coarsened and the durability ratio is deteriorated.
[0046]
Embodiment 2
Further, in order to confirm the superiority of the steel of the present invention, tests were performed on actual parts. Steels having ten chemical compositions shown in Table 3 were smelted and slab-rolled steel slabs were heated to 1170 ° C, hot-rolled, and finished at 880 ° C to obtain 45 mmφ steel bars. The L section near the center of the steel bar is mirror-polished, and about 100 Mn sulfide-based inclusions near the center of the diameter are photographed with an optical microscope, and the photograph is read by an image processing device to obtain a length (L). The ratio (L / D) of the width and the width (D) was determined, and the average value thereof was calculated. Next, this bar was processed to form a common rail along the longitudinal direction of the bar shown in FIG. 4 having a length of 320 mm, a main rail hole diameter of 7 mmφ, and five branch holes of 1 mmφ (common rail 1) and a length of 310 mm. , Main rail hole diameter 6.5mmφ, diameter of 5 branch holes 1.4mmφ (common rail 2), length 300mm, main rail hole diameter 6mmφ, diameter of 5 branch holes 1.9mmφ (common rail 3) 3 Kinds were made. After heating these common rails to 830 to 950 ° C, they were quenched and tempered at 530 to 660 ° C.
[0047]
[Table 3]

[0048]
Thereafter, one main rail hole and the five branch holes of these common rails were all sealed, and light oil was injected from the remaining one main rail hole so that the pressure was periodically changed by a pump. It was assessed periodically pressure changes Repeating 10 ⁷ times as corresponding to the fatigue limit the maximum pressure of the gas oil (cracks not to enter) withstand (MPa) as the fatigue limit characteristic value of the common rail.
Table 4 shows the above results.
[0049]
[Table 4]

[0050]
Test Nos. 13 to 18 are the steels of the present invention, all of which have a high fatigue limit characteristic value of the common rail. On the other hand, in sample No. 19 of the comparative steel, Mg, Ca, Zr, Te, and REM were below the range of the present invention. This is an example in which a material-based inclusion becomes a starting point of fatigue fracture, and the fatigue limit characteristic value is deteriorated. Test No. 20 is an example in which Zr exceeds the range of the present invention, and the cluster-like inclusions become the starting point of fatigue fracture, and the fatigue limit characteristic value deteriorates. Test No. 21 is an example in which the fatigue limit characteristic value deteriorated because S exceeded the range of the present invention. Test No. 22 is an example in which the amount of Al added was smaller than the range of the present invention, so that the crystal grains became coarse and the fatigue limit characteristic value deteriorated.
[0051]
【The invention's effect】
From the above results, according to the present invention, the average L / D of the Mn sulfide-based inclusions was controlled to an appropriate value or less by optimizing the components of the steel, etc., and the steel was elongated by compression processing such as rolling or forging. A steel having excellent fatigue characteristics suitable for a steel part having an open surface that is not parallel to the Mn sulfide-based inclusion and having a part where a tensile stress occurs in the open surface, and a part manufactured from the steel are provided. It can be said that the invention has an extremely remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the average L / D of Mn sulfide-based inclusions and the durability ratio.
FIG. 2 is a diagram showing a relationship between an S content and a durability ratio.
3A and 3B are diagrams showing a state of existence of Mn sulfide-based inclusions, and FIG. 3A is a diagram showing a case where an extending direction of Mn sulfide-based inclusions is perpendicular to an open surface in a part; FIG. 4 is a view showing a case where the direction in which the Mn sulfide-based inclusions extend and the open surface in the part are at an oblique angle, and FIG. It is a figure showing a case.
FIG. 4 is a diagram showing a schematic vertical cross section of a common rail and showing an extending direction of Mn sulfide-based inclusions.
[Explanation of symbols]
Reference Signs List 1 steel material 2 open surface 3 direction of applied stress 4 elongated Mn sulfide-based inclusion 5 main rail hole 6 branch hole 7 common rail length 8 main rail diameter 9 branch hole diameter

Claims (6)

In mass%,
S: 0.01% or less (including 0%),
Al: more than 0.01% to 0.1%
Containing
Mg: 0.0002-0.01%, and Mg / S ≧ 0.05,
Ca: 0.0005 to 0.01%, and Ca / S ≧ 0.05,
Zr: 0.0005 to 0.02%, and Zr / S ≧ 0.05;
Te: 0.0002-0.005%, and Te / S ≧ 0.05,
REM: 0.0005-0.01%, and REM / S ≧ 0.05
And the average value of the ratio (L / D) of the length (L) to the width (D) of the Mn sulfide-based inclusions observed in the L section of the steel is 4 A steel having excellent fatigue properties, characterized by being not more than 0.5. Steel, in mass%,
C: 0.3-0.5%,
Si: 0.5% or less,
Mn: 0.3-1.5%,
P: 0.035% or less (including 0%)
The steel according to claim 1, wherein the steel has excellent fatigue properties. In mass%,
S: 0.02% or less (including 0%),
Al: more than 0.01% to 0.1%
Containing
Cr: 0.4 to 1.5%,
Mo: 0.1% to 1.5%,
V: more than 0.1% to 0.6%
Containing one or more of the following,
further,
Mg: 0.0002-0.01%, and Mg / S ≧ 0.05,
Ca: 0.0005 to 0.01%, and Ca / S ≧ 0.05,
Zr: 0.0005 to 0.02%, and Zr / S ≧ 0.05;
Te: 0.0002-0.005%, and Te / S ≧ 0.05,
REM: 0.0005-0.01%, and REM / S ≧ 0.05
And the average value of the ratio (L / D) of the length (L) to the width (D) of the Mn sulfide-based inclusions observed in the L section of the steel is 4 A steel having excellent fatigue properties, characterized by being not more than 0.5. Steel, in mass%,
C: 0.3-0.5%,
Si: 0.5% or less,
Mn: 0.3-1.5%
P: 0.035% or less (including 0%)
4. The steel according to claim 3, wherein the steel has excellent fatigue properties. A steel part made from the steel according to any one of claims 1 to 4, wherein the steel part has an open surface that is not parallel to the direction of elongation of the Mn sulfide-based inclusions elongated by a compression process such as rolling or forging. And a portion having a tensile stress on the open surface. The steel part according to claim 5, wherein the steel part is a common rail.

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