JP2012126974A - High strength steel artifact superior in notch fatigue strength, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel artifact consisting of ultrahigh-strength low-alloy TRIP steel (TBF steel) with high notch fatigue strength, manufactured by controlling a component addition amount of chemical composition and heat treatment condition without depending on a forging temperature, a forging processing rate and the like.SOLUTION: The high strength steel artifact superior in notch fatigue strength includes 0.15 to 0.25% C, ≤2.5% Si (not including 0%), 0.5 to 2% Mn, 0.5 to 1.5% Cr, ≤0.5% Mo, ≤0.1% Nb, and remainder being Fe and unescapable impurities, and has a carbon equivalent (Ceq) of 0.65% or larger and less than 0.75%, which is determined by following formula. Moreover, in the metal structure of the high strength steel artifact, a mother phase includes not less than 65% of a Russ-like bainitic ferrite by volume rate to all the structure, and 5% or less in total of polygonal ferrite and granular bainitic ferrite by volume rate to all the structure, and a second phase includes 5 to 20% of remained austenite by volume rate to all the structure and 10% or less of martensite by volume rate to all the structure. The carbon equivalent is determined by the formula: Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14.

Description

  The present invention relates to a high-strength steel processed product excellent in notch fatigue strength and a method for producing the same. More specifically, the metal structure is mainly composed of lath-shaped bainitic ferrite, retained austenite, and martensite, and is high. High-strength processed products, high-strength forged products, high-pressure fuel pipes, and their high-strength products with excellent notch fatigue strength made of ultra-high strength low-alloy TRIP steel (TBF steel) with high yield strength and tensile strength It relates to a manufacturing method.

  The “high-strength forged product” of the present invention typically includes, for example, a near net shape forged product, and further forges a primary forged product as well as a primary forged product (cold, warm forged, etc.) ) Precision forged products such as secondary forged products and tertiary forged products obtained by further processing, final products obtained by processing the forged products into complex shapes, common rails for accumulator fuel injection systems mounted on diesel engines, All fuel injection pipes and the like are also included.

  Forged products in industrial technical fields such as automobiles, electrical machinery, and machines are generally manufactured by performing various forgings (processing) at different heating temperatures and then tempering treatments (heat treatments) such as quenching and tempering. For example, when an automobile is taken as an example, a hot forged product (pressurizing temperature 1100 to 1300 ° C.) or warm is used for a common shaft for a pressure accumulation fuel injection system mounted on a crankshaft, a connecting rod, a transmission gear, a diesel engine, etc. Forged products (pressurizing temperature 600 to 800 ° C.) are widely used for pinion gears, gears, steering shafts, valve lifters and the like, and cold forged products (pressurized at room temperature) are widely used.

In recent years, in order to ensure weight reduction and collision safety of automobile bodies, application of formable ultra-high strength low alloy TRIP steel (TBF steel) with transformation-induced plasticity of retained austenite has been studied.
For example, in Patent Document 1, the tensile strength is 600 MPa class by adopting a unique heat treatment in which both annealing and forging are performed at a two-phase region temperature of ferrite and austenite and then austempering is performed at a predetermined temperature. In the above-described high strength region, a technique related to a method for producing a high-strength forged product excellent in the balance between elongation and strength-drawing characteristics is disclosed in Patent Document 2, and after making tempered bainite or martensite separately, ferrite is generally used. A high-strength forged product with an excellent balance between elongation and strength-drawing characteristics is produced by adopting a method in which both annealing and forging are performed at a two-phase temperature of austenite and austenite, and then austempering is performed at a predetermined temperature. Further, in Patent Document 3, after heating to the temperature range of the two-phase region, forging is performed in the two-phase region, Sutenpa processing by the applied, it is possible to reduce the temperature at the time of forging, are able to produce with a workability and excellent stretch flangeability high strength forgings techniques are disclosed.

However, when manufacturing a forged product obtained by these methods, the problems described below may occur.
Since the forged product generates heat according to its processing rate, the part temperature during forging may vary depending on the part. For example, when forging is performed at a high temperature (near Ac3 point), if the processing rate is high, the calorific value increases, and coalescence and growth of austenite occur. Therefore, coarse residual austenite is generated after heat treatment, It is conceivable to deteriorate the characteristics (problems at high temperature forging). On the other hand, when forging is performed on the low temperature side (near Ac1 point), since a sufficient calorific value cannot be secured if the processing rate is low, a large amount of unstable retained austenite is generated. It is conceivable that hard martensite is generated and the impact characteristics are deteriorated (problems at low temperature forging). Therefore, if the temperature and the processing rate of the forged product are different, partially coarse retained austenite and unstable austenite are likely to be generated, and it is difficult to obtain stable and excellent impact resistance characteristics as a whole forged product.

On the other hand, in Patent Document 4, one or more of Nb, Ti, and V are added at the time of hot-rolled steel material production, and an appropriate amount of Al is added, and annealing is generally performed at a two-phase region temperature of ferrite and austenite. By adopting a heat treatment of austempering at a predetermined temperature after performing both forging, it is excellent in the balance of elongation and strength-drawing characteristics regardless of the forging temperature and the forging rate, and the tensile strength is 600 MPa or more. Disclosed is a technology capable of manufacturing high-strength steel products with excellent impact resistance and high-pressure fuel piping (particularly fuel injection pipes for diesel engines and common rails for diesel engines with high strength and excellent impact resistance). Has been.
The invention disclosed in Patent Document 4 is excellent in that it has a special effect that cannot be obtained by the techniques disclosed in Patent Documents 1 to 3, and its ultra-high strength low alloy TRIP steel (TBF steel) is It is expected to contribute greatly by reducing the weight of automobile bodies and ensuring collision safety. However, this ultra-high strength low alloy TRIP steel (TBF steel) has a higher yield strength and tensile strength because fine bainite ferrite and polygonal ferrite coexist in the matrix together with the lath structure of bainite ferrite. In order to obtain a complete TBF steel for achieving the above, high hardenability is required.
In addition, the ultra-high strength low alloy TRIP steel (TBF steel) disclosed in Patent Document 5 has high fatigue strength in addition to excellent cold forgeability, so it is used for an accumulator fuel injection system mounted on a diesel engine. Application to various automobile parts such as common rails and fuel injection pipes can be expected. However, in order to make this possible, in addition to improving the hardenability of TBF steel, it is necessary to improve notch fatigue strength. However, the TBF steel having this high hardenability and high notch fatigue strength is in an undeveloped situation.

Japanese Patent Laid-Open No. 2004-292876 JP 2005-120397 A JP 2004-285430 A JP 2007-231353 A JP 2010-106353 A

  The present invention has been made in view of the above-described situation, and by controlling the component addition amount of chemical composition and heat treatment conditions regardless of the forging temperature, the forging rate, etc., ultra-high strength having high notch fatigue strength An object of the present invention is to provide a high-strength steel processed product made of low-alloy TRIP steel (TBF steel) and a method for manufacturing the same.

The present inventors made ultra-high strength low alloy TRIP steel (TBF steel) having high notch fatigue strength by controlling the component addition amount of chemical composition and heat treatment conditions regardless of forging temperature, forging rate, etc. In order to establish a high-strength steel processed product and to establish these manufacturing methods, ultra-high-strength low-alloy TRIP steel (TBF steel) having a matrix structure of lath bainitic ferrite, retained austenite, and martensite We made a prototype and investigated its notch fatigue characteristics.
As a result, in the C—Si—Mn-based TBF steel, the matrix structure is mainly composed of lath-shaped baini by containing appropriate amounts of Cr, Mo, Nb and B and setting the carbon equivalent (Ceq) to an appropriate value. Superb notch fatigue strength, consisting of tick ferrite and containing a small amount of polygonal ferrite and granular bainitic ferrite, and the second phase structure has fine microstructure of retained austenite and martensite. It has been found that high strength low alloy TRIP steel (TBF steel) can be obtained.

  That is, the processed product made of high-strength steel excellent in notch fatigue strength according to the present invention is C: 0.15-0.25%, Si: 2.5% or less (excluding 0%), Mn: 0 0.5-2%, Cr: 0.5-1.5%, Mo: 0.5% or less, Nb: 0.1% or less, and a carbon equivalent (Ceq) defined by the following formula 1 Is 0.65% or more and less than 0.75%, and consists of the balance Fe and unavoidable impurities, and the metal structure is composed of lath bainitic ferrite with a volume ratio of 65% or more with respect to the entire structure. In addition, the total amount of polygonal ferrite and granular bainitic ferrite is 5% or less by volume with respect to the entire structure, and the second phase structure contains residual austenite with a volume ratio of 5 to 20% with respect to the entire structure. Contain 10% or less of the site by volume ratio of the entire tissue The one in which the features.

[Formula 1]
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14

  The high-strength steel processed product excellent in notch fatigue strength may further contain B: 0.0025% or less as another element.

  The high-strength steel processed product is not only a primary forged product such as a near net shape forged product, but also a secondary forged product obtained by further forging (cold, warm forging, etc.) the primary forged product, In addition to precision forged products such as tertiary forged products, and final products obtained by processing the forged products into complex shapes, high-pressure fuel pipes such as common rails for accumulator fuel injection systems and fuel injection pipes mounted on diesel engines Is mentioned.

  The method for producing a high-strength steel processed product excellent in notch fatigue strength according to the present invention uses a steel material that satisfies the above-mentioned composition, and holds the steel material for a predetermined time in a temperature range of Ac3 or higher, And after forging in the temperature range, cooling to 300 to 450 ° C. (preferably 325 to 425 ° C.) at a predetermined average cooling rate, and holding for 100 to 2000 seconds in the temperature range, To do.

  In addition, the present invention uses a steel material satisfying the above component composition as a method for producing the high-pressure fuel pipe, holds the steel material in a temperature range of Ac3 point or higher for a predetermined time, and performs forging in the temperature range. After being applied, it is cooled to 300 to 450 ° C. (preferably 325 to 425 ° C.) at a predetermined average cooling rate, passed through a step of holding for 100 to 2000 seconds in the temperature range, then cooled to room temperature, and then gun drilled It is characterized by performing drilling in the tube axis direction by the method, tube drawing processing for rolling in the tube axis direction, cutting processing, terminal processing, and machining.

  In the present invention, the preferable heat treatment condition is that the holding time in the temperature range of the Ac3 point or higher is 1 second or longer and the average cooling rate is 1 ° C./s or higher.

  The present invention employs a predetermined heat treatment using a steel material containing an appropriate amount of Cr, Mo, Nb, and B and having a carbon equivalent (Ceq) set to an appropriate value in order to improve hardenability and notch fatigue strength. Thus, the parent phase structure is mainly composed of lath-shaped bainitic ferrite, and contains a small amount of polygonal ferrite and granular bainitic ferrite, and the second phase structure has a fine structure composed of fine retained austenite and martensite. An ultra-high strength low alloy TRIP steel (TBF steel) having a metal structure and excellent notch fatigue strength can be obtained, and regardless of heating temperature, processing rate (forging rate, rolling rate, etc.), etc. A high-strength steel processed product having high notch fatigue strength with excellent hardenability can be provided.

In the first embodiment of the present invention and showing, as a function of the hardenability defined by the following equation 2 (? F _i), tensile strength (TS), a change in the notch tensile strength (TS _N). FIG. 6 is a graph showing a change in notch strength ratio (NSR = TS _N / TS) as a function of hardenability (Πf _i ) in Example 1; FIG. 6 is a graph showing the change in the fatigue limit of a smooth test piece and a test piece having a notch as a function of hardenability (Πf _i ) in Example 1; FIG. 6 is also a diagram showing the relationship between the hardenability (Πf _i ) function and the notch sensitivity q of the fatigue limit in Example 1. As a function of carbon equivalent (Ceq) also in the first embodiment, is a graph showing changes in the fatigue strength (FL) and notched fatigue strength (FL _N). Similarly, the steel type No. 1 in Example 1 was used. 5 is a diagram showing a metallographic structure (micrograph) of the test steel of No. 5, wherein (a) is a matrix structure (green phase) and residual austenite (red phase) of lath-like haenitic ferrite, and (b) is martensite. (Yellow-green phase) and retained austenite (black phase) are shown.

[Formula 2]
Πf _i = (1 + 0.64Si) × (1 + 4.10Mn) × (1 + 2.83P) × (1−0.62S) × (1 + 2.33Cr) × (1 + 0.52Ni) × (1 + 3.14Mo) × (1 + 0. 27Cu) × (1 + 1.5 (0.9-C))
However, the last term is effective only for B-containing steel.

  In the present invention, the value of the carbon equivalent (Ceq) defined by the above formula 1 is limited to 0.65% or more and less than 0.75%. On the other hand, at 0.75% or more, the hardenability becomes excessive, and the yield stress and the tensile strength become excessively high.

In the present invention, the contents of Cr, Mo, and Nb are specified to the above values in order to improve the notch fatigue strength, for the reason described below.
That is, Cr is useful as a steel strengthening element and is an element effective not only for stabilizing retained austenite (γR) and securing a predetermined amount, but also for improving the hardenability of steel. However, in order to sufficiently exhibit the effect of improving hardenability, it is necessary to contain 0.5 to 1.5% of Cr. The reason is that if the Cr content is less than 0.5%, the hardenability of the steel cannot be improved sufficiently. On the other hand, if it exceeds 1.5%, the hardenability increases, but the carbon concentration of the retained austenite is low. This is because it becomes stable.
Mo is an element effective for improving the hardenability of steel, but it is necessary to contain 0.5% or less in order to fully exhibit the effect. Nb has the effect of maintaining the microstructure of the steel and improving the impact resistance, but it is necessary to contain it in an amount of 0.1% or less in order to fully exhibit the effect.

  In the present invention, in order to improve the notch fatigue strength, it is necessary to control other components as follows.

-C: 0.15-0.25%
C is an essential element for securing high strength and securing retained austenite. More specifically, it is effective to secure C in austenite and leave stable retained austenite even at room temperature to improve ductility and impact resistance, but if it is less than 0.15%, the effect is sufficiently obtained. On the other hand, when the addition amount is increased, the amount of retained austenite is increased and high ductility and impact resistance characteristics are obtained. However, if it exceeds 0.25%, not only the effect is saturated, but also defects due to center segregation and the like occur and impact resistance is deteriorated, so the upper limit was limited to 0.25%.

・ Si: 2.5% or less (excluding 0%)
Since Si is an oxide-generating element, if it is excessively contained, the impact resistance is deteriorated, so the addition amount is set to 2.5% or less.

Mn: 0.5-2%
Mn is an element necessary for stabilizing austenite and obtaining a specified amount of retained austenite. In order to effectively exhibit such an action, it is necessary to add 0.5% or more (preferably 0.7% or more, more preferably 1% or more). However, if excessively added, adverse effects such as slab cracking occur, so the content was made 2% or less.

-B: 0.0025% or less B is an element effective for improving the hardenability of steel like Cr, Mo and the like, but has an effect of not reducing the carbon concentration of retained austenite. Further, 0.0025% or less is preferable in order to increase the hardenability without reducing the notch fatigue strength and to keep the cost low. Unlike other additive components, B does not enter the crystal grains and precipitates at the grain boundaries. Therefore, the additive with B is more excellent in workability such as rolling than the additive with the other components.

  In the present invention, the metal structure is defined as described above for the reasons described below.

-Matrix structure: Lath-like bainitic ferrite is 65% or more in volume ratio with respect to the entire structure, and polygonal ferrite and granular bainitic ferrite in total are 5% or less in volume ratio with respect to the entire structure. In order to improve notch fatigue strength, impact resistance and internal pressure fatigue resistance of excellent high-strength steel products, the volume ratio of the lath bainitic ferrite to the entire structure must be 65% or more. . The total volume ratio of polygonal ferrite and granular bainitic ferrite was set to 5% or less because the toughness is reduced when it exceeds 5%.

Second phase structure: residual austenite is 5 to 20% in volume ratio with respect to the entire structure, martensite is 10% or less in volume ratio with respect to the entire structure. And a small amount of polygonal ferrite and granular bainitic ferrite, and the second phase structure is composed of fine retained austenite and martensite. Of these, retained austenite is effective in improving the total elongation, and also effective in improving the impact resistance by becoming crack resistance due to plastic-induced martensitic transformation. If the rate is less than 5%, the above effect cannot be sufficiently exhibited. On the other hand, if it exceeds 20%, the C concentration in the retained austenite becomes low and becomes unstable retained austenite. Therefore, the volume ratio with respect to the whole structure was set to 5 to 20%. In addition, since martensite becomes a starting point of fracture at the interface with the parent phase, the volume ratio with respect to the entire structure is set to 10% or less.

  Next, the method for producing a processed product made of high-strength steel according to the present invention uses a steel material that satisfies the above component composition, and holds the steel material in a temperature range of Ac3 point or higher for a predetermined time, preferably 1 second or more. After performing plastic working in the region, it is cooled to 300 to 450 ° C. (325 to 425 ° C.) at a predetermined average cooling rate, preferably an average cooling rate of 1 ° C./s or more, and 100 to 2000 seconds ( (Preferably 1000 seconds) is included. The reason why the heat treatment conditions are specified is as follows.

  First, holding a steel material at a temperature range of Ac3 or higher for 1 second or more is achieved by setting the heating temperature to a temperature ranging from about two phases to an austenite single phase and fine lath bainitic ferrite (matrix structure) and This is because a second phase structure can be obtained. This is because if the heating temperature is less than the Ac3 point, fine lath bainitic ferrite and the second phase structure are not satisfactorily precipitated. The holding time in the above temperature range is preferably 1 second or longer because, for example, when high-frequency heating is employed as the heating means, the temperature can be held instantaneously in the temperature range of the Ac3 point or higher. The upper limit is not particularly limited, but is about 30 minutes considering productivity.

  Examples of the plastic working include forging, extruding, drilling, or pipe-drawing by roll forming. Conditions in these processes are not particularly limited, and may be performed by a commonly performed method.

The present invention also defines a method for manufacturing a fuel injection pipe for a diesel engine or a common rail for a diesel engine by adopting the above manufacturing conditions.
As a method of manufacturing a fuel injection pipe for a diesel engine, a steel material satisfying the above component composition is used, and after passing through a step of heating and holding at a temperature of 1200 ° C. or higher, a step of performing a hot extrusion process, After holding at a temperature range of Ac3 or higher for a predetermined time, preferably 1 second or longer, and performing warm extrusion in the temperature range, 300 ° C at a predetermined average cooling rate, preferably an average cooling rate of 1 ° C / s or higher. After cooling to ~ 450 ° C (preferably 325 to 425 ° C) and holding in this temperature range for 100 to 2000 seconds (preferably 500 to 1500 seconds), it is cooled to room temperature, and then by a gun drilling method. It is possible to employ a method of performing drilling in the tube axis direction, tube drawing processing for rolling in the tube axis direction, cutting processing, terminal processing, and bending processing.

  In addition, as a method for manufacturing a common rail for a diesel engine, a steel material satisfying a specified composition is used, and the steel material is held at a temperature range of Ac3 point or higher for a predetermined time, preferably 1 second or more. And then forging at a predetermined average cooling rate, preferably 300 to 450 ° C. (preferably 325 to 425 ° C.) at an average cooling rate of 1 ° C./s or more, and 100 to 2000 in this temperature range. After passing through the process of hold | maintaining for 2 seconds (preferably 500-1500 second), it cools to normal temperature, Then, the method of performing the drilling process of the pipe axis direction by a gun drilling method, a cutting process, and machining can be employ | adopted.

  In addition, in the method of manufacturing the diesel engine fuel injection pipe or the diesel engine common rail, after performing hot working, it may be cooled to a temperature range of Ac3 or higher, but the cooling method is not particularly limited. Further, it is desirable that the cooling to room temperature is performed promptly after the step of holding for 100 to 2000 seconds. Furthermore, the cooling method for cooling to room temperature after hot working is not particularly limited.

  Examples of steel materials used in the above production method include billets and hot rolled round bars, etc., but these are melted into steel that satisfies the target components in the usual manner, and then processed as hot slabs. Or what was obtained by performing hot processing after heating once again what was cooled to room temperature should just be used.

  Hereinafter, based on an Example, this invention is demonstrated more concretely. However, the present invention is not limited by the following examples, and all modifications and implementations within the scope not departing from the spirit are included in the technical scope of the present invention.

For the test steel, steel type Nos. Having the composition shown in Table 1 were used. 1 to 6 billets (units in the table are% by mass, remaining Fe and inevitable impurities) were each reheated to 1250 ° C., hot-rolled, pickled, and machined. Six types of round steel bars having a diameter of 13 mm were used. Steel type no. 1 is a basic steel (base steel). 2 and 3 are conventional steels and steel types No. 4 to 6 are steels of the present invention.
Next, from these hot-rolled round steel bars, smooth materials and notches (stress concentration Kt = 1.7) for tensile test (parallel portion diameter 3 mm) and fatigue test (parallel portion diameter 3 mm) are processed, After the matrix phase was made into lath-shaped bainitic ferrite, γ region annealing (held at 900 ° C. for 1200 seconds) was followed by austempering treatment at 400 ° C. for 500 seconds.
Steel type No. in this example. The results of investigating notch tensile properties and notch fatigue strength properties of TBF steels 1 to 6 in the following manner are shown in Table 2 and FIGS. Furthermore, the steel type No. of this example. 4-6, steel type No. is a representative example. FIG. 6 shows the metal structure (micrograph) of the test piece of No. 5 (steel of the present invention), and Table 3 shows the volume ratio of the structure. The green and red phases in FIG. 6 (a) show the matrix structure of lath bainitic ferrite and retained austenite, respectively, the yellowish green phase in FIG. 6 (b) shows martensite, and the black phase shows retained austenite. .

・ Notched tensile properties:
The tensile test uses the above-mentioned tensile test piece, and uses a hard type universal tester (Shimadzu Autograph AG-10TD, manufactured by Shimadzu Corporation) as the tester, and details the initial yield behavior (0.2% yield strength). In order to investigate, a strain gauge (gauge length 10 mm, manufactured by Kyowa Denki Co., Ltd.) was attached to the parallel part of the test piece. The test temperature was 25 ° C. and the crosshead speed was 1 mm / min. The results are shown in Table 2 and FIGS. 1 and 2 show the changes in tensile strength (TS), notch tensile strength (TS _N ), and notch strength ratio (NSR = TS _N / TS) as a function of hardenability (Πf _i ).
・ Notch fatigue strength characteristics:
For the fatigue test, the fatigue test piece was used, and a multi-axis load fatigue tester (PMF-10 manufactured by Tokyo Shiki Seisakusho Co., Ltd.) was used as the tester. The test temperature was 25 ° C. and the stress ratio R = 0.1. The frequency was 80 Hz. The results are shown in FIG. 3, FIG. 4, and FIG. 3 and 4 show the change in fatigue limit of smooth and notched specimens and notch susceptibility factors as a function of hardenability (Πf _i ). FIG. 5 shows the change in fatigue strength (FL) and notch fatigue strength (FL _N ) as a function of carbon equivalent (Ceq).
-Observation of metal structure:
The microstructure of each test piece was observed with a light microscope (400 times or 1000 times magnification) and a scanning electron microscope (SEM: 1000 times or 4000 times), saturation magnetization method (heat treatment). , Vol. 136, (1996), P.322), measurement of residual austenite amount, measurement of C concentration in austenite by X-ray, transmission electron microscope (TEM: magnification: 10,000 times), FE / SEM-EBSP with step interval of 100 nm. The tissue analysis by was performed and the tissue was identified. Table 3 shows the volume ratio of the tissues examined for each of the test pieces thus obtained.

From these results, it can be considered as follows.
(1). As apparent from Table 2 showing the Vickers hardness and tensile properties of each TBF steel, the steel type No. 1-6, steel grade No. It can be seen that the 4-6 TBF steels of the present invention all have a Vickers hardness of HV338-385 and increase with increasing hardenability. In addition, as is clear from FIG. 1 and FIG. 2 showing the notch tensile properties of each steel, the tensile strength, notch tensile strength, and notch strength ratio increase with hardenability as well as Vickers hardness. No. The present invention steels 4 to 6 have high tensile strength, notch tensile strength and notch strength ratio.
Steel type No. In the TBF steel of the present invention of 4 to 6, the fatigue limit (FL) of the test piece having a smooth test piece and a notch increases as the hardenability increases or the Cr and / or Mo content increases. However, the hardenability dependence of the fatigue limit (FL _N ) of the notched material was greater than that of the smooth specimen (FIG. 3). It was also found that the resulting notch sensitivity q decreases with increasing hardenability (FIG. 4). The notch sensitivity q is a value obtained by the following formula 3. At that time, a stress concentration factor of 1.7 was used.

[Formula 3]
q = (Kf-1) / (Kt-1)
Kt: Stress concentration coefficient Kf: Fatigue notch coefficient (= FL _N / FL)

Steel type No. In the TBF steels 1 to 6, no difference was observed in the fatigue limit (FL) of the smooth specimen, but the fatigue limit (FL _N ) of the notched specimen has a carbon equivalent (Ceq) of 0. When it is 65 or more, it shows a tendency to increase.

(2). Steel type no. The steels of the present invention (TBF steel) shown in FIGS. As shown in FIG. 6, the matrix structure is mainly composed of lath-shaped bainitic ferrite, and has a small amount of polygonal ferrite and granular bainitic ferrite. The phase structure was composed of fine retained austenite and martensite, the formation of proeutectoid ferrite was suppressed, and the crystal grains were refined. Steel type No. with high hardenability (Πf _i ). In the steels 4 to 6 of the present invention (TBF steel), the hardness (Hv) increased due to the formation of an island-like martensite structure in the austempering treatment at an Ms temperature (martensite start temperature) or higher.

  Steel type No. in Table 1 The steel billet having the composition shown in No. 4 is heated and held at a temperature of 1200 ° C. and subjected to hot extrusion, and then cooled to 940 ° C. A round bar is formed by extruding, the round bar is cooled to 325 ° C. at a cooling rate of 4 ° C./s, held in the temperature range for 1800 seconds, then cooled to room temperature at a predetermined cooling rate, and then a gun drill. Drilled in the direction of the tube axis by machining to form a fuel injection tube element tube, and the element tube is subjected to a predetermined tube expansion process to produce a fuel with an outer diameter of 8.0 mm, an inner diameter of 3.0 mm, and a wall thickness of 2.5 mm A steel pipe for an injection pipe is obtained, cut into a desired length, a threaded part such as a nut is inserted, a terminal process is performed to press-mold the connection head, and a bending process is further applied to a fuel injection pipe for a diesel engine. Got.

  Steel type No. in Table 1 The billet made of the present invention having the components shown in 5 is held at a temperature of 950 ° C. for 1 second or more, forged in the temperature range, and subsequently cooled to 300 ° C. at a cooling rate of 1 ° C./s, Austempering treatment is performed by holding at temperature for 2000 seconds, then drilled in the direction of the tube axis by a gun drilling method in the cold, and then machined cold to obtain an outer diameter of 30 mm, an inner diameter of 8 mm, and a wall thickness of 12 mm. For the common rail.

  The fuel injection pipe for the diesel engine of the second embodiment and the common rail for the diesel engine of the third embodiment are both high in strength, can obtain high notch fatigue strength, and can be reduced in size and weight. It was confirmed.

  The present invention employs a predetermined heat treatment using a steel material containing an appropriate amount of Cr, Mo, Nb, and B and having a carbon equivalent (Ceq) set to an appropriate value in order to improve hardenability and notch fatigue strength. Thus, the parent phase structure is mainly composed of lath-shaped bainitic ferrite, and contains a small amount of polygonal ferrite and granular bainitic ferrite, and the second phase structure has a fine structure composed of fine retained austenite and martensite. An ultra-high strength low alloy TRIP steel (TBF steel) having a metal structure and excellent notch fatigue strength can be obtained, and regardless of heating temperature, processing rate (forging rate, rolling rate, etc.), etc. Since it can provide high strength steel products with high notch fatigue strength with excellent hardenability, it can be applied to various automotive parts such as fuel injection pipes for diesel engines and common rails. Use can be more expected.

Claims (8)

C: 0.15-0.25%, Si: 2.5% or less (not including 0%), Mn: 0.5-2%, Cr: 0.5-1.5%, Mo: 0.00. 5% or less, Nb: 0.1% or less, and the carbon equivalent (Ceq) defined by the following formula is 0.65% or more and less than 0.75%, and consists of the balance Fe and inevitable impurities, Further, the metal structure has a lath-like bainitic ferrite with a volume ratio of 65% or more with respect to the whole structure, and a total of polygonal ferrite and granular bainitic ferrite with a volume ratio of 5 with respect to the whole structure. 2% or less, the second phase structure contains residual austenite in a volume ratio of 5 to 20% with respect to the entire structure, and martensite with a volume ratio of 10% or less with respect to the entire structure. High strength steel processed product.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14   Furthermore, B: The processed product made from high strength steel excellent in the notch fatigue strength of Claim 1 containing 0.0025% or less.   The processed product made of high-strength steel excellent in notch fatigue strength according to claim 1, wherein the processed product is a forged product.   The processed product made of high-strength steel excellent in notch fatigue strength according to claim 1 or 2, wherein the processed product is a high-pressure fuel pipe.   The processed product made of high-strength steel according to claim 4, wherein the high-pressure fuel pipe is a fuel injection pipe for a diesel engine or a common rail for a diesel engine and has excellent notch fatigue strength.   It is a method of manufacturing the high-strength steel processed product according to any one of claims 1 to 5, wherein a steel material satisfying the composition of claim 1 is used, and the steel material is at a temperature of Ac3 point or higher. After maintaining for a predetermined time in the region and forging in the temperature range, it is cooled to 300 to 450 ° C. (preferably 325 to 425 ° C.) at a predetermined average cooling rate, and is maintained in the temperature range for 100 to 2000 seconds. The manufacturing method of the high strength steel processed goods excellent in the notch fatigue strength characterized by including a process.   A method for producing a high-pressure fuel pipe according to claim 6, wherein a steel material satisfying the component composition according to claim 1 is used, and the steel material is held in a temperature range of Ac3 point or higher for a predetermined time, After forging in the temperature range, it is cooled to 300 to 450 ° C. (preferably 325 to 425 ° C.) at a predetermined average cooling rate, and after being kept in the temperature range for 100 to 2000 seconds, it is cooled to room temperature. After that, it is excellent in notch fatigue strength characterized by performing drilling in the tube axis direction by gun drilling method, tube drawing process rolling in the tube axis direction, cutting processing, terminal processing, and machining. A method for manufacturing high-strength steel products.   The high strength with excellent notch fatigue strength according to claim 6 or 7, wherein the holding time in the temperature range of the Ac3 point or higher is 1 second or longer, and the average cooling rate is 1 ° C / s or higher. Manufacturing method for steel products.