JP4974331B2 - Steel high-strength processed product excellent in impact resistance and strength-ductility balance and manufacturing method thereof, and fuel injection pipe for diesel engine and common rail manufacturing method excellent in high strength, impact resistance and internal pressure fatigue characteristics - Google Patents

Description

  The present invention relates to a steel high-strength processed product excellent in impact resistance and strength-ductility balance, and a manufacturing method thereof, and a fuel injection pipe for a diesel engine and a common rail excellent in impact strength and internal pressure fatigue properties. Regarding the manufacturing method, in detail, in a high strength region where the tensile strength is 600 MPa or more, a steel high-strength processed product and a high-pressure fuel pipe (especially high strength and impact resistance properties) having excellent strength-ductility balance and excellent impact resistance properties. Further, the present invention relates to a diesel engine fuel injection pipe excellent in internal pressure fatigue resistance, a common rail for a pressure accumulation fuel injection system mounted on a diesel engine, and a manufacturing method thereof.

  The “forged product” of the present invention typically includes, for example, a near net shape forged product, and the primary forged product as well as the primary forged product is further forged (cold, warm forged, etc.). Also included are precision forged products such as secondary forged products and tertiary forged products obtained in this manner, and final products obtained by processing the forged products into complex shapes.

  The use of forged products is increasing in industrial technical fields such as automobiles, electrical machinery and machinery. Forged products are generally manufactured by performing various forgings (processing) with different heating temperatures, followed by tempering (heat treatment) such as quenching and tempering. For example, in the case of automobiles, crankshafts, connecting rods, and transmissions. Hot forged products (pressurizing temperature 1100 to 1300 ° C) and warm forged products (pressurizing temperature 600 to 800 ° C) are commonly used for common rails for accumulator fuel injection systems mounted on gears and diesel engines, respectively. ing. In addition, cold forged products (pressurized at room temperature) are widely used for pinion gears, gears, steering shafts, valve lifters, and the like.

  In recent years, the weight reduction of the car body has been actively promoted for the purpose of improving the fuel efficiency of automobiles, but on the other hand, there is a tendency to strengthen the laws and regulations for collision safety of automobiles, and the strength of forged parts is also increased. At the same time, high impact resistance is required. Therefore, application of TRIP (Transformation Induced Plasticity) steel as a steel material used for such a forged product is being studied.

  By adopting a unique heat treatment in Patent Document 1 that employs an austempering treatment at a predetermined temperature after performing both annealing and forging at two-phase region temperatures of ferrite and austenite. Discloses that a high-strength forged product having an excellent balance between elongation and strength-drawing characteristics can be obtained in a high-strength region of 600 MPa or more.

  Also in Patent Document 2, after making tempered bainite or tempered martensite separately, both annealing and forging are generally performed at a two-phase region temperature of ferrite and austenite, and then austempering is performed at a predetermined temperature. Thus, it is disclosed that a high-strength forged product having an excellent balance between elongation and strength-drawing characteristics can be obtained.

Furthermore, in Patent Document 3, after heating to the temperature range of the two-phase region, forging processing is performed in the two-phase region, and then the prescribed austempering treatment is performed, whereby the temperature during forging processing can be lowered and excellent. It is disclosed that a high-strength forged product having stretch flangeability and workability can be manufactured.
Japanese Patent Laid-Open No. 2004-292876 JP 2005-120397 A JP 2004-285430 A

  The forged product obtained by the method of Patent Document 1 and the like has an excellent balance between elongation and strength-drawing characteristics, and has a tensile strength of 600 MPa or more. However, when producing a forged product by the forging method, the following problems may occur.

  When manufacturing a forged product, heat is generated in accordance with the processing rate. Therefore, when the heating temperature and the processing rate are different between parts, the part temperature may be different between parts. At this time, when heating to high temperature (near Ac3 point) or high temperature and high processing rate (forging, etc.) is performed, the calorific value becomes large and coalescence / growth of austenite occurs. It is conceivable that coarse retained austenite is formed and the impact characteristics are deteriorated (problem in the case of high temperature and high processing rate). On the other hand, when heating is performed at a low temperature (near Ac1 point) or when processing at a low temperature and a low processing rate (forging, etc.) is performed, a sufficient calorific value cannot be secured, and a large amount of unstable retained austenite is generated. It is conceivable that after heat treatment, hard martensite that becomes the starting point of fracture is generated and the impact resistance is deteriorated (problem in the case of low temperature and low processing rate).

  Therefore, if the temperature and the processing rate are different between parts of the part, the coarse retained austenite and unstable retained austenite are likely to be generated partially, and it is difficult to realize stable and excellent impact resistance characteristics as a whole part. .

  Further, when applied to the diesel engine common rail and the diesel engine fuel injection pipe, high internal pressure is repeatedly applied, and therefore, an internal pressure fatigue resistance is also required.

  The present invention has been made in view of the above problems, and its purpose is not depending on the heating temperature and processing rate, but the tensile strength is 600 MPa or more, and it has an excellent strength-ductility balance and has impact resistance. Providing excellent steel high-strength processed products, high-pressure fuel piping (especially diesel engine fuel injection pipes and diesel engine common rails with high strength, excellent impact resistance and internal pressure fatigue resistance), and methods for producing these There is to do.

The steel high-strength processed product according to the present invention is
C: 0.1 to 0.7% (meaning mass%, the same applies to chemical components below),
Si: 1.5% or less (excluding 0%),
Mn: 0.5-3%,
Al: 0.2 to 1.5%,
One or more selected from the group consisting of Nb, Ti and V: 0.01 to 0.1% in total;
P: 0.05% or less (excluding 0%),
S: 0.02% or less (excluding 0%)
And the balance consists of iron and inevitable impurities,
The metal structure is
Ferrite: 50% or more (meaning area% with respect to the whole structure, the same applies to the structure below), the second phase structure is composed of retained austenite and bainite and / or martensite,
Retained austenite: 10-30%,
Martensite: 5% or less (including 0%)
It has features where

The steel high-strength processed product is still another element,
(1) 1% or less in total of Cr and / or Mo (excluding 0%)
(2) Ni: 0.5% or less (not including 0%) and / or Cu: 0.5% or less (not including 0%)
(3) Ca: 0.003% or less (not including 0%) and / or REM: 0.003% or less (not including 0%)
(4) B: 0.003% or less (excluding 0%)
May be included.

  Examples of the processed product include a forged product. Moreover, high-pressure fuel piping is mentioned as said processed goods, As this high-pressure fuel piping, the fuel injection pipe for diesel engines or the common rail for diesel engines is mentioned specifically ,.

The present invention also defines a method for producing the steel high-strength processed product,
(I) Use a steel material that satisfies the above composition,
After holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C) and performing plastic working (working c) in this temperature range, 300 at an average cooling temperature of 3 ° C / s or more. The process which cools to -500 degreeC and hold | maintains for 60 to 3600 second in this temperature range (process C1)
It has the characteristic in including. The present invention also provides
(II) Use a steel material that satisfies the above composition,
After holding for 10 seconds or more in the temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.), it is cooled to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and 60 to 3600 in this temperature range. Step for holding second (step C2)
A method having features is also defined where

  In the above method, at least one type of processing (processing b) selected from the group consisting of hot plastic processing, warm plastic processing, cold plastic processing and cutting processing is performed prior to step C1 or step C2. One or more steps B may be included, and step A of heating to 1200 ° C. or higher may be included before step C1 or step C2.

  Examples of the process c include a forging process, an extrusion process, a perforating process, or a tube extending process by roll forming. Examples of the plastic processing in the processing b include forging processing, extrusion processing, drilling processing, or pipe drawing processing by roll forming, and examples of the cutting processing in the processing b include drill processing.

The present invention also defines the following method as a method of manufacturing the fuel injection pipe for the diesel engine or a method of manufacturing the common rail for the diesel engine.
(1) A method for producing the fuel injection pipe for a diesel engine,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
Step of performing hot extrusion, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and performing the warm extrusion process in the temperature range, 3 ° C / s After cooling to 300 to 500 ° C. at the above average cooling temperature and holding for 60 to 3600 seconds in this temperature range, cooling to room temperature, then drilling in the tube axis direction by a gun drilling method, radial direction and And / or a method characterized by sequentially performing a drawing process, a cutting process, a terminal process, and a bending process for rolling in the axial direction of the pipe.
(2) A method of manufacturing the common rail for a diesel engine,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
Step of performing hot extrusion, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and performing the warm extrusion process in the temperature range, 3 ° C / s After cooling to 300 to 500 ° C. at the above average cooling temperature and holding for 60 to 3600 seconds in this temperature range, cooling to room temperature, then drilling in the tube axis direction by a gun drilling method, radial direction and And / or a method characterized by sequentially performing a drawing process, a cutting process, a machining process, and an assembling process for rolling in the axial direction of the pipe.
(3) A method of manufacturing the diesel engine fuel injection pipe,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
A process of hot extrusion,
A process of drilling in the tube axis direction by a gun drilling method, and holding at a temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C) for 10 seconds or more, and warm roll forming (roll reduce) in the temperature range After performing the tube drawing process to roll in the radial direction and the tube axis direction, after cooling to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and passing through the step of holding for 60 to 3600 seconds in the temperature range, A method characterized by sequentially performing a tube drawing process, a cutting process, a terminal process, and a bending process in which the steel sheet is cooled to room temperature and then cold rolled in the radial direction and / or the tube axis direction.
(4) A method for producing a common rail for the diesel engine,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
A process of hot extrusion,
Drilling in the direction of the pipe axis by a drilling method, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and warm roll forming (roll reduce) in the temperature range After performing the tube drawing process to roll in the radial direction and the tube axis direction, after cooling to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and passing through the step of holding for 60 to 3600 seconds in the temperature range, A method characterized by sequentially performing a tube drawing process, a cutting process, a machining process, and an assembling process in which the steel sheet is cooled to room temperature and then rolled in the radial direction and / or the tube axis direction in the cold.
(5) A method for producing the fuel injection pipe for the diesel engine,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
Mannesmann-type drilling process,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
A tube drawing process for rolling in the radial direction and / or the tube axis direction in the cold, and 3 ° C / s or more after holding for 10 seconds or more in the temperature range of (Ac1 point -30 ° C) to (Ac3 point + 100 ° C) It is characterized by the fact that it is cooled to 300 to 500 ° C. at an average cooling temperature of, and maintained at 60 to 3600 seconds in this temperature range, then cooled to room temperature, and then cut processing, terminal processing, and bending processing are sequentially performed. Having a method.
(6) A method of manufacturing the common rail for a diesel engine,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
Mannesmann-type drilling process,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
A tube drawing process for rolling in the radial direction and / or the tube axis direction in the cold, and 3 ° C / s or more after holding for 10 seconds or more in the temperature range of (Ac1 point -30 ° C) to (Ac3 point + 100 ° C) The product is characterized in that it is cooled to 300 to 500 ° C. at an average cooling temperature of, and maintained at 60 to 3600 seconds in this temperature range, then cooled to room temperature, and then sequentially subjected to cutting, machining, and assembly. Having a method.
(7) A method of manufacturing the common rail for a diesel engine,
Using a steel material that satisfies the above composition,
Rolling in the radial and axial directions hot or warm,
Rolling in the radial and axial directions cold,
Drilling in the direction of the pipe axis by a drilling method,
A tube drawing process of rolling in a radial direction and a tube axis direction by hot roll forming (roll reduction) at a temperature of 1200 ° C. or higher, and a temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) for 10 seconds. After being held above, it is cooled to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and after being held for 60 to 3600 seconds in this temperature range, it is cooled to room temperature, and then cut and machined And a method characterized by sequentially performing assembly processing.
(8) A method of manufacturing the common rail for a diesel engine,
Using a steel material that satisfies the above composition,
Rolling in the radial and axial directions hot or warm,
Drilling in the direction of the tube axis by gun drilling,
Cutting process,
Machining process,
Heat holding at 1200 ° C. or higher, and holding at a temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) for 10 seconds or more, to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or higher. A method characterized by cooling and holding for 60 to 3600 seconds in the temperature range, cooling to room temperature, and then performing assembly processing.
(9) A method of manufacturing the common rail for a diesel engine,
Using a steel material that satisfies the above composition,
Rolling in the radial and axial directions hot or warm,
Rolling in the radial and axial directions cold,
Drilling in the direction of the pipe axis by a drilling method,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher; a cutting process;
Machining step, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), cooling to 300-500 ° C at an average cooling temperature of 3 ° C / s or more, A method having a feature in that after passing through a step of holding for 3 to 3600 seconds in a region, it is cooled to room temperature and then assembled.
(10) A method of manufacturing the common rail for a diesel engine,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
A process of performing hot working (especially forging),
(Ac1 point-30 ° C) to (Ac3 point + 100 ° C) held for 10 seconds or more, forged in this temperature range, cooled to 300-500 ° C at an average cooling temperature of 3 ° C / s or more, A method characterized by performing machining after a step of holding in a temperature range for 60 to 3600 seconds and a step of drilling in a tube axis direction by a gun drilling method.

  In the manufacture of the fuel injection pipe for a diesel engine, after the bending process, which is the final process, an auto-frettage process may be further performed. Further, in the production of the diesel engine common rail, after the assembly process, which is the final process, or after the machining process, which is the final process, an auto-frettage process may be further performed.

  Using a steel material containing one or more selected from the group consisting of Nb, Ti and V and an appropriate amount of Al, annealing at austenite single-phase region and generally two-phase region temperatures of ferrite and austenite (and plastic working such as forging) After performing the heat treatment by austempering at a predetermined temperature and securing a specified structure, the tensile strength is 600 MPa or more regardless of the heating temperature and the processing rate (forging rate, rolling rate, etc.) Steel processed products with excellent strength-elongation balance and impact resistance characteristics, high-pressure fuel pipes (particularly for diesel engine fuel injection pipes and diesel engines with high strength, impact resistance characteristics and internal pressure fatigue resistance characteristics) Common rail).

  The present inventors, regardless of heating temperature and processing rate, have a tensile strength of 600 MPa or more, a steel high-strength processed product that has excellent strength-ductility balance and excellent impact resistance, and a high-pressure fuel pipe (particularly, Diesel engine fuel injection pipe and diesel engine common rail) with high strength, excellent impact resistance and internal pressure fatigue resistance) and coarseness when processing at high temperature and high processing rate to establish these manufacturing methods From the viewpoints of preventing the formation of new retained austenite and preventing the formation of martensite when processing at low temperature and low processing rate.

as a result,
(I) In order to prevent austenite from coalescence / growth and to austenite coarsening when processing at high temperature and high processing rate, the steel material is selected from the group consisting of Nb, Ti and V 1 What is necessary is just to contain a specified amount of seeds or more,
(Ii) In order to prevent martensite formation when processing at a low temperature and a low processing rate, an appropriate amount of Al should be contained.
And the present invention has been conceived. Hereinafter, these points will be described in detail.

  First, in the present invention, as described in (i) above, the steel material contains at least one selected from the group consisting of Nb, Ti and V, and carbides and carbonitrides of these elements can be finely precipitated in the manufacturing process. For example, the carbide or the like suppresses coalescence and growth of the austenite so that the structure of the entire processed product becomes uniform and fine, and the retained austenite can be uniformly and finely dispersed after the heat treatment. Precipitation strengthening with the above elements is also effective for further strengthening parts. In order to fully exhibit these effects, it is necessary to contain 0.01% or more in total of at least one selected from the group consisting of Nb, Ti and V, preferably 0.02% or more in total. is there. On the other hand, since the effect is saturated and economically wasteful even if the above elements are contained excessively, one or more selected from the group consisting of Nb, Ti and V is 0.1% or less in total. To do.

  In the present invention, an appropriate amount of Al is contained to prevent martensite formation when processing at a low temperature and a low processing rate. In order to prevent the formation of martensite, it is effective to improve the C concentration in austenite, and in order to improve the C concentration in austenite, the action of expanding the solid solubility limit of C in austenite rather than Si. It was found that it is effective to use strong Al. Al has a higher ferrite stabilizing ability than Si, and by positively containing Al, the transformation start is relatively faster than in the case of adding Si, and C is contained in austenite by heating and holding (forging, etc.) for a very short time. Tends to be concentrated. Therefore, austenite can be more stabilized regardless of heating and processing conditions. As a result, the C concentration distribution of austenite shifts to a high concentration side, and the amount of retained austenite is increased, resulting in high impact resistance. It is considered to be obtained.

  In order to fully exhibit the said effect, it is necessary to make Al 0.2% or more, Preferably it is 0.5% or more. On the other hand, if the Al content exceeds 1.5%, the Ac3 transformation point of the steel must be raised and the heating temperature needs to be increased, which is not preferable in actual operation. Preferably it is 1.2% or less.

  As described above, one or more selected from the group consisting of Nb, Ti and V and an appropriate amount of Al are included when producing a hot-rolled steel material, and as described later, an austenite single-phase region and generally two phases of ferrite and austenite. By adopting a heat treatment that performs both annealing (and plastic processing such as forging) at a range temperature and then austempering at a predetermined temperature, the metal structure specified as described below, and thus desired characteristics, can be easily secured. be able to.

<Ferrite: 50% or more in space factor (area%) with respect to the entire structure>
In order to improve the ductility of a high-strength processed product having a tensile strength of 600 MPa or more, the presence of ferrite is important, and in order to improve the ductility by ferrite, the space factor of ferrite is 50% or more (preferably 60% or more). However, if ferrite is generated excessively, it will be difficult to secure the required strength, and many voids will be generated at the interface between the ferrite and the second phase structure, which will be described later. 90% is preferable. More preferably, it is 80% or less. In the present invention, “ferrite” refers to polygonal ferrite and bainitic ferrite. Here, the ferrite includes all of those generated during plastic working such as forging, those generated during austempering, and those generated during the cooling process.

<Second phase structure: retained austenite, and bainite and / or martensite Residual austenite (γ _R ): 10-30% in space factor (area%) with respect to the entire structure
Martensite: 5% or less (including 0%) in space factor (area%) for all tissues>
The processed product of the present invention contains residual austenite as a second phase structure and bainite and / or martensite as a metal structure together with the ferrite. Of these, retained austenite is effective in improving the total elongation, and is also effective in improving the impact resistance properties by providing crack resistance due to plasticity-induced martensitic transformation. In order to fully exhibit this effect, it is necessary to make the space factor of retained austenite 10% or more. Preferably it is 15% or more. On the other hand, if the amount of retained austenite exceeds 30%, the C concentration in the retained austenite becomes low and becomes unstable retained austenite, so that the above effect cannot be fully exhibited. Therefore, in the present invention, the space factor of retained austenite is set to 30% or less. Preferably it is 25% or less.

  The C concentration (CγR) in the retained austenite is preferably 0.8% or more. That is, the C concentration in the retained austenite has a great influence on the plasticity-induced martensitic transformation and is controlled to 0.8% or more (preferably 1% or more, more preferably 1.2% or more) to improve ductility. It is also effective for improving impact resistance. The higher the C concentration in the retained austenite, the better. However, in practical operation, the upper limit that can be adjusted is considered to be approximately 1.6%.

  Further, as described above, bainite and / or martensite may be included as the second phase structure, but these may inevitably remain in the production process of the present invention. Among these, when martensite is present, the interface between the martensite and the parent phase serves as a starting point of fracture, so that the space factor of martensite with respect to the entire structure is 5% or less (preferably 3% or less, more preferably 1 % Or less).

  In the present invention, it is necessary to control the other components as follows in order to easily form the metal structure and efficiently improve the impact resistance, strength, and strength-ductility balance.

<C: 0.1 to 0.7%>
C is an element necessary for securing high strength and retaining retained austenite. More specifically, it is effective for securing C in the austenite and leaving stable retained austenite even at room temperature to improve ductility and impact resistance. In order to fully exhibit the effect, the C content is preferably 0.1% or more. Particularly, when the C content is 0.2% or more, the amount of retained austenite increases and C is concentrated in the retained austenite. It becomes easy to obtain high ductility and impact resistance. However, if the amount of C exceeds 0.7%, not only the effect is saturated, but also defects such as center segregation occur and impact resistance characteristics deteriorate, so the upper limit of C amount is set to 0.7% . Preferably it is 0.5% or less.

<Si: 1.5% or less (excluding 0%)>
Si is an oxide-generating element, and if it is contained excessively, the impact resistance is deteriorated. Therefore, the Si content is set to 1.5% or less. Preferably it is 1.2% or less. The steel product of the present invention is premised on the addition of Al that exhibits the same effect as Si, but from the viewpoint of increasing the amount of retained austenite due to solid solution strengthening by adding Si and suppressing precipitation of carbides, You may make it contain 0.5% or more.

<Mn: 0.5 to 3%>
Mn is an element necessary for stabilizing austenite and obtaining a specified amount of retained austenite. In order to exhibit such an action effectively, the amount of Mn needs to be 0.5% or more (preferably 0.7% or more, more preferably 1% or more). However, if the amount of Mn is excessive, adverse effects on production such as cracking of the cast slab will occur, so the content is made 3% or less. Preferably it is 2.5% or less, More preferably, it is 2% or less.

<P: 0.05% or less (excluding 0%)>
<S: 0.02% or less (excluding 0%)>
P and S are elements inevitably mixed in the steel. When the content of S is high, inclusions (MnS) increase and the impact resistance of steel products accompanied by plastic working such as forged parts is reduced. . In the present invention, from the viewpoint of ensuring impact resistance, P is 0.05% or less and S is 0.02% or less.

  The contained elements specified in the present invention are as described above, and the balance is iron and inevitable impurities, and as the inevitable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. . Further, it is possible to further contain the following elements.

<Cr and / or Mo in total 1% or less (excluding 0%)>
These elements are useful elements for strengthening steel and are effective in stabilizing retained austenite and securing a predetermined amount. In order to exhibit such an action effectively, it is preferable to contain 0.05% or more (more preferably 0.1% or more) of Cr and / or Mo in total. However, even if the above elements are added excessively, the above effect is saturated, which is economically useless. Therefore, Cr and / or Mo should be 1% or less (more preferably 0.8% or less) in total.

<Ni: 0.5% or less (not including 0%) and / or Cu: 0.5% or less (not including 0%)>
These elements, like Cr and Mo, are useful elements for strengthening steel, and are effective elements for stabilizing retained austenite and ensuring a specified amount. In order to effectively exhibit such an action, when Ni is contained, 0.05% or more (more preferably 0.1% or more), and when Cu is contained, 0.05% or more (more preferably 0%). .1% or more). However, even if Ni or Cu exceeds 0.5%, the above effect is saturated and is economically useless. More preferably, Ni: 0.4% or less, Cu: 0.4% or less.

<Ca: 0.003% or less (not including 0%) and / or REM: 0.003% or less (not including 0%)>
Ca and REM (rare earth elements) are elements that control the form of sulfide in steel and are effective in improving workability. Here, examples of rare earth elements used in the present invention include Sc, Y, lanthanoid series elements, and the like. In order to effectively exhibit the above action, it is desirable that the content is 0.0003% or more (more preferably 0.0005% or more) when Ca or REM is contained. However, even if both Ca and REM are added in excess of 0.003%, the above effect is saturated, which is economically useless. More preferably, it is 0.0025% or less.

<B: 0.003% or less (excluding 0%)>
B has an effect of improving hardenability and increasing strength in a small amount. In order to effectively exhibit such an action, the B content is preferably set to 0.0005% or more. However, if B is added excessively, the grain boundary becomes brittle, and cracks are generated by processing such as casting and rolling, so the upper limit is preferably made 0.003%. More preferably, it is 0.002% or less.

  As described above, in order to obtain the above metal structure by containing a specified amount of one or more selected from the group consisting of Nb, Ti and V in the steel material and containing an appropriate amount of Al, a steel product is manufactured by the following method. It is necessary to manufacture high-strength processed products.

That is, the manufacturing method of the present invention uses a steel material satisfying the above component composition,
After holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C) and performing plastic working (working c) in this temperature range, 300 at an average cooling temperature of 3 ° C / s or more. After cooling to ~ 500 ° C and holding in the temperature range for 60 to 3600 seconds (step C1) or (Ac1 point-30 ° C) to (Ac3 point + 100 ° C) for 10 seconds or more, 3 A step of cooling to 300 to 500 ° C. at an average cooling temperature of at least ° C./s and holding for 60 to 3600 seconds in the temperature range (step C2),
However, the reason for defining the conditions is as follows.

<Hold for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C)>
As described above, ferrite is generated and a fine second phase structure can be obtained by setting the heating temperature to a temperature ranging from approximately two phases to austenite single phase. In general, the lower the forging temperature and the hot working temperature, the easier it is to produce a fine second phase structure. Therefore, the lower limit was defined as (Ac1 point −30 ° C.). On the other hand, when the heating temperature exceeds (Ac3 point + 100 ° C.), especially when the processing rate is high, the effect of preventing the coalescence / growth of austenite by Nb, Ti, V is not sufficiently generated, and austenite becomes coarse. An upper limit was defined.

  The heating time in the above temperature range is 10 seconds or longer (preferably 30 seconds or longer). Thereby, a uniform structure can be obtained. The upper limit is not particularly limited, but is about 3600 seconds in consideration of productivity.

  In addition, after soaking at a temperature of Ac3 point or higher, desired ferrite may be generated through a temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) in the cooling process.

  The processing temperature of the processing c in the step C1 does not have to be exactly the same as the above holding temperature, and can be changed within the temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.).

  Examples of the processing c include forging processing, extrusion processing, perforating processing, or tube drawing processing by roll forming. Specifically, forging processing, extrusion processing, or roll processing using a die, a roll, or the like heated to the above temperature range. Examples of the tube-drawing process by molding are given, but the conditions in these processes are not particularly limited, and may be performed by a commonly performed method.

  The lower limit of the processing rate (forging amount or rolling amount) of the processing c is preferably 10%. This is because by setting the processing rate of the processing c to 10% or more, the second phase structure can be refined and the characteristics can be further improved. Preferably it is 20% or more, more preferably 30% or more. The upper limit is not particularly limited, but if the processing rate is increased, the workability is lowered, and it is necessary to increase the capacity of the press and the rolling mill, and the production scale becomes too large. In addition, problems such as easy cracking occur when processed into parts. Considering these points, the upper limit of the processing rate of the processing c is approximately 150%, and more preferably 120% or less.

<Cool to 300-500 ° C. at an average cooling temperature of 3 ° C./s or more and hold in this temperature range for 60-3600 seconds>
In the present invention, after maintaining for 10 seconds or more in the temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) as described above, the average cooling rate is 3 ° C./s or more. It is for suppressing. Preferably it is 5 degrees C / s or more, More preferably, it is 10 degrees C / s or more. The upper limit of the average cooling rate is not particularly limited, and the upper limit is preferably as large as possible. However, the average cooling rate is appropriately appropriately controlled in relation to the actual operation level.

  In the case of the present invention, as described above, in particular, after heating to the temperature of the austenite state, by cooling at an average cooling rate of 3 ° C./s or higher to around 400 ° C., the retained austenite is finer than that of conventional TRIP steel. In this way, excellent internal pressure fatigue resistance can be ensured by making the steel processed product in which the retained austenite is fine and dispersed in this way. The reason is considered as follows.

  That is, when a load is applied to the obtained steel work product and the tensile stress generated inside increases locally, so-called stress concentration occurs, but when the stress concentration exceeds a certain limit, the stress concentration is generated. Fine retained austenite present in the part is transformed into fine and hard martensite (induced martensite transformation). In this way, when the place where the tensile stress acts is suddenly transformed into martensite, the place expands, so that the tensile stress is offset by the compressive stress generated in the vicinity, and the tensile stress is relaxed. It is thought that fatigue characteristics can be secured.

  In this invention, it cools to 300-500 degreeC with the said average cooling rate, and hold | maintains for 60 to 3600 second (austemper process) in this temperature range. As described above, the austempering temperature is set to 300 to 500 ° C. when the temperature is lower than 300 ° C., the diffusion of C is slow and a specified amount of retained austenite cannot be obtained. Preferably it is 350 degreeC or more. On the other hand, when the temperature exceeds 500 ° C., cementite precipitates, so C concentration does not occur in austenite, and a specified amount of retained austenite cannot be obtained. Preferably it is 450 degrees C or less.

  Also, the austempering time was set to 60 to 3600 seconds because when C is less than 60 seconds, the concentration of C is insufficient and a predetermined amount of retained austenite is not generated, and unstable retained austenite is transformed into martensite. It is. Preferably it is 100 seconds or more. On the other hand, if it exceeds 3600 seconds, the generated retained austenite is decomposed. Preferably it is 3000 seconds or less.

  In order to avoid decomposition of the retained austenite, it is desirable that the cooling after the austempering is promptly performed while taking care not to heat the austempering temperature or higher.

  In the manufacturing method of the present invention, one or more types of processing (processing b) selected from the group consisting of hot plastic processing, warm plastic processing, cold plastic processing, and cutting processing are performed before the step C1 or the step C2. 1 step or more may be included. Examples of the plastic processing in the processing b include forging processing, extrusion processing, drilling processing, or tube-drawing processing by roll forming, and examples of the cutting processing in the processing b include drill processing. The conditions in are not particularly limited, and may be performed by a conventional method.

  Moreover, it is recommended that the process A heated to 1200 degreeC or more is included before the said process C1 or the process C2. By heating at this temperature, Ti, Nb, V carbides and carbonitrides precipitated in the melting process are sufficiently re-dissolved, and fine carbides, carbonitrides and nitrides are precipitated during subsequent cooling. Can do. As a result, the austenite coarsening can be sufficiently suppressed in the process B or the like. On the other hand, the higher the heating and holding temperature, the better. Therefore, an upper limit is not particularly defined, but it is desirable to appropriately control appropriately in relation to the actual operation level.

As a method of the present invention, using a steel material such as a billet,
-Step A-Step C1 or Step C2 (Description is omitted for steps after Step C1 or Step C2. The same applies hereinafter)
-Process B-Process C1 or Process C2
-Step A-Step B-Step C1 or Step C2
-Process B-Process A-Process C1 or Process C2
Step B-Step A-Step B (In this step B, the same processing as the first step B may be performed, or a different processing may be performed)-Step C1 or Step C2
And the like. Here, when the process B is hot working, the process may also serve as the process A.

A method for manufacturing a fuel injection pipe for a diesel engine or a common rail for a diesel engine using the manufacturing conditions is also defined.
(1) As a method of manufacturing a fuel injection pipe for a diesel engine,
Using steel that satisfies the specified composition,
Heating and holding at 1200 ° C. or higher,
Step of performing hot extrusion, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and performing the warm extrusion process in the temperature range, 3 ° C / s After cooling to 300 to 500 ° C. at the above average cooling temperature and holding for 60 to 3600 seconds in this temperature range, cooling to room temperature, then drilling in the axial direction by gun drilling, radial and / or Alternatively, there may be mentioned a method of sequentially performing a tube drawing process, a cutting process, a terminal process, and a bending process for rolling in the tube axis direction.
(2) As a method for producing a common rail for a diesel engine, the same conditions as in the method for producing a fuel injection pipe for a diesel engine are adopted,
Using steel that satisfies the specified composition,
Heating and holding at 1200 ° C. or higher,
Step of performing hot extrusion, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and performing the warm extrusion process in the temperature range, 3 ° C / s After cooling to 300 to 500 ° C. at the above average cooling temperature and holding for 60 to 3600 seconds in this temperature range, cooling to room temperature, then drilling in the tube axis direction by a gun drilling method, radial direction and Examples thereof include a method of sequentially performing a tube drawing process, a cutting process, a machining process, and an assembly process for rolling in the tube axis direction.

In the above methods (1) and (2), after hot extrusion, cooling may be performed to a temperature range of (Ac1 point −30 ° C.) to (Ac3 point + 100 ° C.). It is not limited. Moreover, after passing through the process hold | maintained for 60-3600 second, it is desirable to cool to normal temperature rapidly as above-mentioned.
(3) As a method of manufacturing the fuel injection pipe for diesel engines,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
A process of hot extrusion,
A process of drilling in the tube axis direction by a gun drilling method, and holding at a temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C) for 10 seconds or more, and warm roll forming (roll reduce) in the temperature range After performing the tube drawing process to roll in the radial direction and the tube axis direction, after cooling to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and passing through the step of holding for 60 to 3600 seconds in the temperature range, Examples include a method of sequentially performing a tube drawing process, a cutting process, a terminal process, and a bending process in which the steel sheet is cooled to room temperature and then rolled in the radial direction and / or the tube axis direction.
(4) As a method of manufacturing the diesel engine common rail, the same conditions as the method of manufacturing the diesel engine fuel injection pipe are adopted,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
A process of hot extrusion,
Drilling in the direction of the pipe axis by a drilling method, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and warm roll forming (roll reduce) in the temperature range After performing the tube drawing process to roll in the radial direction and the tube axis direction, after cooling to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and passing through the step of holding for 60 to 3600 seconds in the temperature range, There is a method of performing a tube drawing process, a cutting process, a machining process, and an assembling process in order of cooling to room temperature and then cold rolling in the radial direction and / or the tube axis direction.

In the methods (3) and (4), after hot extrusion is performed, cooling is performed and then drilling is performed in the tube axis direction by a drill (gun drill) processing method. However, the cooling method is not particularly limited.
(5) As a method of manufacturing the fuel injection pipe for diesel engines,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
Mannesmann-type drilling process,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
A tube drawing process for rolling in the radial direction and / or the tube axis direction in the cold, and 3 ° C / s or more after holding for 10 seconds or more in the temperature range of (Ac1 point -30 ° C) to (Ac3 point + 100 ° C) The method of cooling to 300-500 degreeC with the average cooling temperature of this, passing through the process hold | maintained for 60 to 3600 second in this temperature range, cooling to normal temperature, and performing the cutting process, a terminal process, and a bending process in order after that is mentioned. It is done.
(6) As a method of manufacturing the diesel engine common rail, the same conditions as the method of manufacturing the diesel engine fuel injection pipe are adopted,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
Mannesmann-type drilling process,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
A tube drawing process for rolling in the radial direction and / or the tube axis direction in the cold, and 3 ° C / s or more after holding for 10 seconds or more in the temperature range of (Ac1 point -30 ° C) to (Ac3 point + 100 ° C) The method of cooling to 300 to 500 ° C. at an average cooling temperature of, passing through the step of holding for 60 to 3600 seconds in the temperature range, cooling to room temperature, and then sequentially performing cutting, machining, and assembly processing. It is done.

In the methods (5) and (6), after the tube is drawn by hot roll forming, it is cooled to room temperature, but the cooling method is not particularly limited.
(7) As a method for manufacturing the diesel engine common rail,
Using a steel material that satisfies the above composition,
Rolling in the radial and axial directions hot or warm,
Rolling in the radial and axial directions cold,
Drilling in the direction of the pipe axis by a drilling method,
A tube drawing process of rolling in a radial direction and a tube axis direction by hot roll forming (roll reduction) at a temperature of 1200 ° C. or higher, and a temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) for 10 seconds. After being held above, it is cooled to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and after being held for 60 to 3600 seconds in this temperature range, it is cooled to room temperature, and then cut and machined And a method of sequentially performing assembly processing.
(8) As a method for manufacturing the diesel engine common rail,
Using a steel material that satisfies the above composition,
Rolling in the radial and axial directions hot or warm,
Drilling in the direction of the tube axis by gun drilling,
Cutting process,
Machining process,
Heat holding at 1200 ° C. or higher, and holding at a temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) for 10 seconds or more, to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or higher. There is a method in which after cooling and holding in the temperature range for 60 to 3600 seconds, cooling to room temperature and then assembling.
(9) As a method for manufacturing the diesel engine common rail,
Using a steel material that satisfies the above composition,
Rolling in the radial and axial directions hot or warm,
Rolling in the radial and axial directions cold,
Drilling in the direction of the pipe axis by a drilling method,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
Cutting process,
Machining step, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), cooling to 300-500 ° C at an average cooling temperature of 3 ° C / s or more, After passing through the process which hold | maintains for 60 to 3600 second in an area | region, the method of cooling to normal temperature and performing an assembly process after that is mentioned.
(10) As a method for manufacturing the diesel engine common rail,
Using a steel material that satisfies the above composition,
Heating and holding at 1200 ° C. or higher,
A process of performing hot working (especially forging),
(Ac1 point-30 ° C) to (Ac3 point + 100 ° C) held for 10 seconds or more, forged in this temperature range, cooled to 300-500 ° C at an average cooling temperature of 3 ° C / s or more, Examples thereof include a method characterized by performing machining after a step of holding in a temperature range for 60 to 3600 seconds and a step of drilling in the tube axis direction by a gun drilling method.

  In the above method (10), after hot working, after cooling, it is heated to a temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), but the cooling method is not particularly limited. .

  In the manufacture of the fuel injection pipe for a diesel engine, after the bending process, which is the final process, an auto-frettage process may be further performed. Further, in the production of the diesel engine common rail, after the assembly process, which is the final process, or after the machining process, which is the final process, an auto-frettage process may be further performed. This auto-frettage process is a method in which only the inner peripheral surface is plastically processed by applying an internal pressure, and the internal-pressure fatigue resistance can be enhanced by this auto-fretage process.

  Examples of the steel material used in the above method include billets and hot-rolled steel sheets, etc., but these are prepared as a slab by melting steel that satisfies the target components as usual, or processed while hot, Or what was obtained by performing hot processing after heating once again to what was cooled to room temperature should just be used.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

[Example 1]
First, test steel slabs of steel type symbols A to Y having the composition shown in Table 1 (the units in the table are mass%, the balance: iron and unavoidable impurities) are manufactured by continuous casting. After reheating to a temperature range of ° C. (step A), hot rolling (processing b) was performed until the plate thickness became 13 mm (step B). Then, after pickling, it machined and produced the test piece for forging of 10 mm x 10 mm x 80 mm.

  Next, the test piece was heated at the forging temperature shown in Table 2 for 20 minutes, and forged (process c) was performed using a mold heated to the same temperature as the heating temperature of each test piece. Compression forging strain (same as the processing rate in Table 2) was applied.

  Thereafter, the austempering treatment was performed by cooling to the austempering temperature shown in Table 2 at an average cooling rate of 10 ° C./s and maintaining the temperature for the time shown in Table 2.

For each forged material thus obtained, tensile strength (TS: MPa), elongation (El:%), Charpy impact value (CIV: J / cm ² ), and space factor (area ratio) of each structure Were measured in the following manner.

<Measurement of tensile strength and elongation>
Using a JIS No. 4 test piece (gauge length 20 mm, parallel portion length 22 mm, width 6 mm, thickness 1.2 mm) taken from 1/4 thickness with respect to the forging direction of the forged material, tensile strength (TS), Elongation (El) was measured. The test conditions are crosshead speed: 1 mm / min and measurement temperature: RT.

<Charpy impact test>
Charpy impact using a JIS No. 4 Charpy impact test piece (length 55 mm, height 10 mm, width 2.5 mm) having a V-notch with a depth of 2 mm taken from 1/4 thickness with respect to the forging direction of the forged material. The value CIV was measured. Test conditions are 5 m / s and measurement temperature: RT.

And the CIV is the case of 100 J / cm ² than "excellent very impact resistance (◎)", a case of 80 J / cm ² or more 100 J / cm ² or less "excellent impact resistance (○)", 80 J The case of less than / cm ² was evaluated as “inferior in impact resistance (×)”.

<Observation of metal structure>
Cut out a test piece from 1/4 thickness with respect to the forging direction of the forged material, and apply nital corrosion and repeller corrosion to the test piece.
Observation with an optical microscope (magnification: 400 times or 1000 times) and observation with a scanning electron microscope (SEM, magnification: 1000 times or 4000 times),
・ Measurement of retained austenite amount by saturation magnetization method (heat treatment, Vol. 136, (1996), P.322),
・ Transmission electron microscope (TEM: magnification 10,000 times), structure analysis by FE / SEM-EBSP with a step interval of 100 nm and space factor (area%) of ferrite, residual γ, martensite and bainite in the structure Asked.

  These measurement results are also shown in Table 2.

  The following alphabets can be considered from Tables 1 and 2 (note that the following alphabet indicates the steel type symbol in Tables 1 and 2, and the following No. indicates the material No. in Table 2).

First, no. 2 , 4-6 , 9-10 , 13-14 , 17-18 , 21-22 , 25 , 28-36 are all obtained by the method specified in the present invention, satisfying the specified component composition. In addition, since it has a specified structure, it has excellent tensile strength and elongation characteristics, as well as excellent impact resistance characteristics.

  Of these, No. Nos. 4 to 6 are examples in which forging was performed by using steel type C satisfying the component composition defined in the present invention, forging temperature was varied in the range of 700 to 900 ° C., and the processing rate during forging was varied in the range of 20 to 100%. However, even when forging is performed at any of the above processing rates, a forged product having excellent tensile strength and elongation characteristics and excellent impact resistance characteristics is obtained.

  No. 9-10 is an example in which the austempering temperature after forging was changed in the range of 350-450 ° C. using steel type C satisfying the component composition defined in the present invention, and the austempering treatment was performed at any of the above temperatures. Also in this case, a forged product having excellent tensile strength and elongation characteristics and excellent impact resistance characteristics is obtained.

  No. 13-14 is an example in which the austempering time after forging was changed in the range of 60-3000 sec using steel type C satisfying the component composition defined in the present invention, but when the austempering treatment was performed in any of the above times In addition, a forged product having excellent tensile strength and elongation characteristics and excellent impact resistance characteristics is obtained.

  On the other hand, the following examples that do not satisfy any of the requirements defined in the present invention have the following problems.

  First, no. No. 1 is an example using a steel type A with a small amount of C, a specified amount of residual γ cannot be secured, and is inferior in strength and impact resistance.

  No. 3 uses steel type C that satisfies the component composition defined in the present invention, but the forging temperature is lower than (Ac1 point-30 ° C.) and the processing rate is small, so that sufficient residual γ cannot be secured, Inferior in strength and impact resistance.

  No. No. 7 uses steel type C that satisfies the component composition defined in the present invention, but because the forging temperature exceeds (Ac3 + 100 ° C.) and the processing rate is too high, a large amount of martensite is generated, resulting in impact resistance characteristics. Decreased.

  No. No. 8 uses steel type C that satisfies the component composition defined in the present invention, but because the austempering temperature is too low, a large amount of martensite is generated, and the impact resistance is lowered.

  No. No. 11 uses steel type C that satisfies the component composition defined in the present invention, but the austempering temperature is too high, so that sufficient residual γ cannot be secured and the strength and impact resistance are poor.

  No. No. 12 uses steel type C that satisfies the component composition defined in the present invention, but since the austempering time is too short, sufficient residual γ cannot be secured, and the strength and impact resistance are poor.

  No. No. 15 uses steel type C that satisfies the component composition defined in the present invention, but since the austempering time is too long, the residual γ cannot be secured and the impact resistance is poor.

  No. No. 16 uses steel type D having an excessive amount of C, so that both residual γ and martensite are excessive and inferior in impact resistance.

  No. No. 19 uses steel type G having an excessive amount of Si, so that martensite is generated excessively and impact resistance is deteriorated.

  No. No. 20 uses steel type H in which the amount of Mn is insufficient, so that martensite is generated excessively and impact resistance characteristics are deteriorated.

  No. Since No. 23 uses steel type K with an excessive amount of Mn, segregation of Mn occurred and the impact resistance characteristics deteriorated.

  No. No. 26 uses steel type N with an excessive amount of Al, so that sufficient residual γ cannot be secured, and the impact resistance characteristics deteriorated.

  No. No. 27 uses a steel type O that does not contain any of Ti, Nb, and V. No. 37 was insufficient in the impact resistance properties because the total of one or more selected from the group consisting of Nb, Ti and V was insufficient, resulting in coarse residual γ of crystal grains.

[Example 2]
A billet made of steel type B was heated and held at 1250 ° C. (step A), subjected to hot extrusion (step B), cooled to 950 ° C., held at that temperature for 10 seconds or more, and then warm extrusion The round bar was cooled to 300 ° C. at a cooling rate of 10 ° C./s, held at this temperature for 240 seconds or longer (step C1), and then cooled to room temperature. After that, drilling is performed in the direction of the tube axis by a gun drilling method, and then the tube is stretched to make the product dimensions of an outer diameter of 8.0 mm, a wall thickness of 2.5 mm, and an inner diameter of 3.0 mm. Then, after inserting a screw part such as a nut, a terminal process for press-molding the connection head was performed, and a bending process was further performed to obtain a fuel injection pipe.

[Example 3]
A billet made of steel type C was heated and held at 1250 ° C. (step A), subjected to hot extrusion (step B), and then cooled to room temperature. Thereafter, drilling is performed in the tube axis direction by a gun drilling method (step B ′), followed by hot roll forming after holding at 950 ° C. for 10 seconds or more, and then 400 ° C. at an average cooling rate of 5 ° C./s. Then, the austempering treatment was performed to maintain the temperature for 120 seconds (step C1). After that, the tube is cold-drawn to obtain a product with an outer diameter of 8.0 mm, a wall thickness of 2.5 mm, and an inner diameter of 3.0 mm, and then subjected to cutting, terminal processing and bending to produce fuel. A jet tube was obtained.

[Example 4]
A billet made of steel type C was heated and held at 1250 ° C. (step A), subjected to drilling by the Mannesmann method (step B), held at 950 ° C. for 10 seconds or more, and then subjected to hot roll forming, The sample was cooled to 500 ° C. at an average cooling rate of 10 ° C./s and subjected to austempering treatment for 180 seconds at that temperature (step C1). Further, the tube was cold-drawn to obtain an outer diameter of 8.0 mm, a wall thickness of 2.5 mm, and an inner diameter of 3.0 mm, and then subjected to cutting, terminal processing and bending to obtain a fuel injection pipe. .

[Example 5]
A billet made of steel type C is heated and held at 1250 ° C. (step A), subjected to drilling by a Mannesmann method (step B), and then hot-rolled at a temperature of 1200 ° C. or higher (steps A and B ′). Thereafter, the tube was drawn in the cold (step B ″). Next, after heating and holding at 950 ° C. for 10 seconds or more, the tube was cooled to 400 ° C. at an average cooling temperature of 3 ° C./s or more. Austempering treatment was performed for 70 seconds (step C2), and the tube was cold-drawn to obtain an outer diameter of 32 mm, an inner diameter of 10 mm, and a wall thickness of 11 mm. A common rail was obtained by performing machining such as drilling a sheet surface and a φ3 mm branch hole and assembly processing such as welding a screw sleeve around the periphery of the branch hole.

[Example 6]
After hot extrusion using a billet made of steel type C (process B), cold drilling is performed in the tube axis direction by gun drilling (process B '), and hot roll forming (processing) at 1200 ° C or higher. b) (step A, B ″), hold at 950 ° C. for 10 seconds or more, and then cool the raw tube to 400 ° C. at an average cooling temperature of 3 ° C./s or more, Austempering treatment was performed for 2 seconds (step C2), and the tube was cold-drawn to obtain an outer diameter of 30 mm, an inner diameter of 9 mm, and a wall thickness of 10.5 mm. A common rail was obtained by performing machining such as drilling of a surface and a φ3 mm branch hole, assembly of a retainer having a screw sleeve around the branch hole, and the like.

[Example 7]
The billet made of steel type C was cold-rolled and then drilled in the tube axis direction by a gun drilling method (step B). Then, after hot roll forming was performed at 1200 ° C. or higher (steps A and B ′), the tube was held at 950 ° C. for 10 seconds or longer, and the tube was cooled to 400 ° C. at an average cooling temperature of 3 ° C./s or higher. Thereafter, an austempering treatment was performed for 120 seconds at the temperature (step C2). Further, the tube was cold-drawn to obtain an outer diameter of 32 mm, an inner diameter of 10 mm, and a wall thickness of 11 mm, and this was sequentially subjected to cutting, machining, and assembly to obtain a common rail.

[Example 8]
A billet made of steel type C is subjected to hot or warm extrusion, and then cold drilled in the direction of the tube axis by a gun drilling method, followed by cold drawing to give an outer diameter of 32 mm, an inner diameter of 10 mm, meat The tube was cut to a thickness of 11 mm (Step B) and machined. Next, the tube is heated to 1200 ° C. or higher (step A), held at 950 ° C. for 10 seconds or longer, then cooled to 400 ° C. at an average cooling temperature of 3 ° C./s or higher, and held at that temperature for 120 seconds. (Step C2). This was further assembled to obtain a common rail.

[Example 9]
After heating the steel material consisting of steel type C to 1200 ° C or higher (step A), after performing hot extrusion (step B), cutting to the desired length, then holding at 950 ° C for 10 seconds or longer, After hot forging into a shape having a large number of bosses with an outer diameter of 32 mm and φ18 mm, it is cooled to 400 ° C. at an average cooling temperature of 3 ° C./s or higher, and then subjected to austempering treatment for 120 seconds. (Step C1). Next, the tube was cooled to room temperature, and a tube hole having an inner diameter of 10 mm was formed in the tube axis direction by a gun drilling method. Thereafter, machining was performed such as drilling a M16 external screw on the outer periphery of the boss, a conical sheet surface on the top of the boss, and a φ3 mm branch hole to obtain a common rail.

[Example 10]
After heating the steel material made of steel type C to 1200 ° C or higher (step A), forging (step B) and holding at 950 ° C for 10 seconds or longer, many bosses with an outer diameter of 32 mm and φ18 mm of the main body After hot forging into the shape, the steel was cooled to 400 ° C. at an average cooling temperature of 3 ° C./s or higher, and then subjected to austempering treatment for 120 seconds at that temperature (step C1). Next, the tube was cooled to room temperature, and a tube hole having an inner diameter of 10 mm was formed in the tube axis direction by a long drilling method. Thereafter, machining was performed such as drilling a M16 external screw on the outer periphery of the boss, a conical sheet surface on the top of the boss, and a φ3 mm branch hole to obtain a common rail.

[Example 11]
The billet made of steel type B was heated to 1200 ° C. or higher (step A) and then cooled to room temperature. This is drilled in the tube axis direction by a gun drilling method (step B), then heated to 950 ° C., held at that temperature for 10 seconds or more, then subjected to hot roll forming, and at a cooling rate of 10 ° C./s. After cooling to 300 ° C., the temperature was maintained for 240 seconds or longer (step C1), and then cooled to room temperature. Next, after extending the tube to obtain a product with an outer diameter of 8.0 mm, a wall thickness of 2.5 mm, and an inner diameter of 3.0 mm, this is subjected to cutting processing, terminal processing, and further bending processing to produce a fuel injection tube. Got.

[Example 12]
The billet made of steel type B was heated to 1200 ° C. or higher (step A) and then cooled to room temperature. This was drilled in the tube axis direction by a gun drilling method (step B), then heated to 950 ° C., held at that temperature for 10 seconds or more, and then cooled to 300 ° C. at a cooling rate of 10 ° C./s. For 240 seconds or longer (step C2), and then cooled to room temperature. After that, after extending the tube to make the product dimensions of an outer diameter of 8.0 mm, a wall thickness of 2.5 mm, and an inner diameter of 3.0 mm, the fuel injection tube is subjected to cutting, terminal processing and further bending. Got.

[Example 13]
After cutting a steel material made of steel type C to a desired length, it is warm forged (process B), heated to 1200 ° C or higher (process A), and held at 950 ° C for 10 seconds or longer. The austempering treatment is performed by hot forging into a shape having a large number of bosses with an outer diameter of 32 mm and φ18 mm of the main body, and then cooling to 400 ° C. at an average cooling temperature of 3 ° C./s or more, and holding at that temperature for 120 seconds. (Step C1). Next, the tube was cooled to room temperature, and a tube hole with an inner diameter of 9 mm was formed in the tube axis direction by a long drilling method. Thereafter, machining was performed such as drilling a M16 external screw on the outer periphery of the boss, a conical sheet surface on the top of the boss, and a φ3 mm branch hole to obtain a common rail.

[Example 14]
After performing a warm roll forming process on a steel pipe made of steel type C (step B), this is heated and held at 1250 ° C. (step A), then held at 950 ° C. for 10 seconds or more, and then hot extrusion is performed. And then cooled to 300 ° C. at a cooling rate of 10 ° C./s, held at that temperature for 240 seconds or more (step C1), and then cooled to room temperature. After that, after extending the tube to obtain a product with an outer diameter of 8.0 mm, a wall thickness of 2.5 mm, and an inner diameter of 3.0 mm, this is subjected to cutting, terminal processing and bending to form a fuel injection tube. Obtained.

[Example 15]
After drilling in the tube axis direction by a gun drilling method using a steel bar made of steel type B (step B), it is heated to 950 ° C., held at that temperature for 10 seconds or more, and then 300 ° C. at a cooling rate of 10 ° C./s. It was cooled to 0 ° C., kept at that temperature for 240 seconds or longer (step C2), and then cooled to room temperature. After that, after extending the tube to make the product dimensions of an outer diameter of 8.0 mm, a wall thickness of 2.5 mm, and an inner diameter of 3.0 mm, the fuel injection tube is subjected to cutting, terminal processing and further bending. Got.

[Example 16]
After drilling in the tube axis direction by the Mannesmann method with a steel bar made of steel type C (Step B), heating to 950 ° C. and holding at that temperature for 10 seconds or more, then performing hot extrusion molding, Then, after cooling to 300 ° C. at a cooling rate of 10 ° C./s, the temperature was maintained for 240 seconds or longer (step C1), and then cooled to room temperature. After that, after drawing the tube, the product dimensions are 6.35mm in outer diameter, 2mm in thickness, 2.35m in inner diameter, and then cut, end processed, and further bent to obtain a fuel injection tube. It was.

  As a result of examining the internal pressure fatigue limit of each of the fuel injection pipes of Examples 2 to 4, 11, 12, 14 to 16 and the common rails of Examples 5 to 10 and 13 using an internal pressure repeated fatigue test machine, Both the injection pipe and the common rail did not break even when an internal pressure exceeding 2500 Bar was repeatedly applied 10 million times or more, and exhibited excellent internal pressure fatigue resistance.

  The fuel injection pipes of Examples 2 to 4, 11, 12, 14 to 16 and the common rails of Examples 5 to 10 and 13 are filled with high-pressure water or high-pressure oil after the final process. The internal pressure fatigue resistance was further improved by performing the cottage treatment process.

Claims (27)

C: 0.1 to 0.7% (meaning mass%, the same applies to chemical components below),
Si: 1.5% or less (excluding 0%),
Mn: 0.5-3%,
Al: 0.5 to 1.5%,
One or more selected from the group consisting of Nb, Ti and V: 0.01 to 0.1% in total;
P: 0.05% or less (excluding 0%),
S: 0.02% or less (excluding 0%)
And the balance consists of iron and inevitable impurities,
The metal structure is
Ferrite: 50% or more (meaning area% with respect to the whole structure, the same applies to the structure below), the second phase structure is composed of retained austenite and bainite and / or martensite,
Retained austenite: 10-30%,
Martensite (including 0%) 5% or less,
Bainite: 3% or less (including 0%)
Steel high-strength processed product with excellent impact resistance and strength-ductility balance characterized by satisfying Furthermore,
Ni: 0.5% or less (not including 0%) and / or Cu: 0.5% or less (not including 0%)
A steel high-strength processed product according to claim 1 comprising: Furthermore,
Ca: 0.003% or less (not including 0%) and / or REM: 0.003% or less (not including 0%)
A steel high-strength processed product according to claim 1 or 2 comprising: The steel high-strength processed product according to any one of claims 1 to 3 , further comprising B: 0.003% or less (not including 0%). The steel high-strength processed product according to any one of claims 1 to 4 , wherein the processed product is a forged product. The steel high-strength processed product according to any one of claims 1 to 4 , wherein the processed product is a high-pressure fuel pipe. The high-pressure fuel pipe is a diesel engine fuel injection pipe with high strength, excellent impact resistance and internal pressure fatigue characteristics, or a diesel engine common rail with high strength, excellent impact resistance and internal pressure fatigue characteristics. 6. A steel high-strength processed product according to 6. A method for producing a steel high-strength processed product according to any one of claims 1 to 7 ,
Use a steel material satisfying the component composition according to any one of claims 1 to 4 ,
After holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C) and performing plastic working (working c) in this temperature range, 300 at an average cooling temperature of 3 ° C / s or more. The process which cools to -500 degreeC and hold | maintains for 60 to 3600 second in this temperature range (process C1)
A method for producing a steel high-strength processed product excellent in impact resistance characteristics and strength-ductility balance. A method for producing a steel high-strength processed product according to any one of claims 1 to 7 ,
Use a steel material satisfying the component composition according to any one of claims 1 to 4 ,
After holding for 10 seconds or more in the temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.), it is cooled to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and 60 to 3600 in this temperature range. Step for holding second (step C2)
A method for producing a steel high-strength processed product excellent in impact resistance characteristics and strength-ductility balance. Prior to Step C1 or Step C2, Step B is performed by performing one or more types of processing (processing b) selected from the group consisting of hot plastic processing, warm plastic processing, cold plastic processing, and cutting processing. The manufacturing method of Claim 8 or 9 including the process or more. Wherein before step C1 or to the step C2, the production method according to any one of claims 8 to 10 including a step A of heat above 1200 ° C.. The processing c is forging, extrusion method according to claim 8, 10 or 11 is Shinkan machining by drilling, or roll forming. The manufacturing method according to any one of claims 10 to 12 , wherein the plastic processing in the processing b is forging processing, extrusion processing, perforating processing, or tube drawing processing by roll forming. Cutting in the machining b The production method according to any one of claims 10 to 12, which is a drilling. A method for producing a fuel injection pipe for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Heating and holding at 1200 ° C. or higher,
Step of performing hot extrusion, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and performing the warm extrusion process in the temperature range, 3 ° C / s After cooling to 300 to 500 ° C. at the above average cooling temperature and holding for 60 to 3600 seconds in this temperature range, cooling to room temperature, then drilling in the tube axis direction by a gun drilling method, radial direction and A fuel injection pipe for a diesel engine excellent in high strength, impact resistance and internal pressure fatigue characteristics, characterized by sequentially performing tube drawing, cutting, terminal processing, and bending in the axial direction of the tube. Production method. A method for producing a common rail for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Heating and holding at 1200 ° C. or higher,
Step of performing hot extrusion, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and performing the warm extrusion process in the temperature range, 3 ° C / s After cooling to 300 to 500 ° C. at the above average cooling temperature and holding for 60 to 3600 seconds in this temperature range, cooling to room temperature, then drilling in the tube axis direction by a gun drilling method, radial direction and A method for producing a common rail for a diesel engine excellent in high strength, impact resistance and internal pressure fatigue characteristics, characterized by sequentially performing tube drawing, cutting, machining, and assembly that are rolled in the direction of the pipe axis . A method for producing a fuel injection pipe for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Heating and holding at 1200 ° C. or higher,
A process of hot extrusion,
A process of drilling in the tube axis direction by a gun drilling method, and holding at a temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C) for 10 seconds or more, and warm roll forming (roll reduce) in the temperature range After performing the tube drawing process to roll in the radial direction and the tube axis direction, after cooling to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and passing through the step of holding for 60 to 3600 seconds in the temperature range, It is cooled to room temperature and then rolled in the radial direction and / or the tube axis direction in the cold, and then it is sequentially subjected to pipe drawing, cutting, terminal processing, and bending, and it has high strength and impact resistance and resistance. A method of manufacturing a fuel injection pipe for a diesel engine having excellent internal pressure fatigue characteristics. A method for producing a common rail for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Heating and holding at 1200 ° C. or higher,
A process of hot extrusion,
Drilling in the direction of the pipe axis by a drilling method, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), and warm roll forming (roll reduce) in the temperature range After performing the tube drawing process to roll in the radial direction and the tube axis direction, after cooling to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and passing through the step of holding for 60 to 3600 seconds in the temperature range, It is cooled to room temperature and then rolled in the radial direction and / or tube axis direction in the cold, and then it is subjected to pipe drawing, cutting, machining, and assembly in order, and it has high strength and impact resistance and resistance to heat. A common rail manufacturing method for diesel engines with excellent internal pressure fatigue characteristics. A method for producing a fuel injection pipe for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Heating and holding at 1200 ° C. or higher,
Mannesmann-type drilling process,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
A tube drawing process for rolling in the radial direction and / or the tube axis direction in the cold, and 3 ° C / s or more after holding for 10 seconds or more in the temperature range of (Ac1 point -30 ° C) to (Ac3 point + 100 ° C) It is cooled to 300 to 500 ° C. at an average cooling temperature of, and after passing through a step of holding for 60 to 3600 seconds in the temperature range, it is cooled to room temperature, and then cutting, terminal processing, and bending are sequentially performed. The manufacturing method of the fuel-injection pipe for diesel engines excellent in the high intensity | strength and impact-resistant characteristic and internal-pressure fatigue | exhaustion-proof characteristic to make. A method for producing a common rail for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Heating and holding at 1200 ° C. or higher,
Mannesmann-type drilling process,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
A tube drawing process for rolling in the radial direction and / or the tube axis direction in the cold, and 3 ° C / s or more after holding for 10 seconds or more in the temperature range of (Ac1 point -30 ° C) to (Ac3 point + 100 ° C) It is cooled to 300 to 500 ° C. at an average cooling temperature of, and after passing through a step of holding for 60 to 3600 seconds in the temperature range, it is cooled to room temperature, and then cutting, machining, and assembly are sequentially performed. The manufacturing method of the common rail for diesel engines which is excellent in high strength, impact resistance and internal pressure fatigue resistance. A method for producing a common rail for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Rolling in the radial and axial directions hot or warm,
Rolling in the radial and axial directions cold,
Drilling in the direction of the pipe axis by a drilling method,
A tube drawing process of rolling in a radial direction and a tube axis direction by hot roll forming (roll reduction) at a temperature of 1200 ° C. or higher, and a temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) for 10 seconds. After being held above, it is cooled to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or more, and after being held for 60 to 3600 seconds in this temperature range, it is cooled to room temperature, and then cut and machined And a method of manufacturing a common rail for a diesel engine excellent in high strength, impact resistance and internal pressure fatigue characteristics, characterized by sequentially performing assembly processing. A method for producing a common rail for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Rolling in the radial and axial directions hot or warm,
Drilling in the direction of the tube axis by gun drilling,
Cutting process,
Machining process,
Heat holding at 1200 ° C. or higher, and holding at a temperature range of (Ac1 point−30 ° C.) to (Ac3 point + 100 ° C.) for 10 seconds or more, to 300 to 500 ° C. at an average cooling temperature of 3 ° C./s or higher. A diesel engine excellent in high strength, impact resistance and internal pressure fatigue characteristics, characterized by cooling and holding for 60 to 3600 seconds in the temperature range, cooling to room temperature, and then assembling. For manufacturing common rails. A method for producing a common rail for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Rolling in the radial and axial directions hot or warm,
Rolling in the radial and axial directions cold,
Drilling in the direction of the pipe axis by a drilling method,
A tube drawing process for rolling in a radial direction and a pipe axis direction by hot roll forming (roll reduce) at a temperature of 1200 ° C. or higher;
Cutting process,
Machining step, and holding for 10 seconds or more in the temperature range of (Ac1 point-30 ° C) to (Ac3 point + 100 ° C), cooling to 300-500 ° C at an average cooling temperature of 3 ° C / s or more, A method for producing a common rail for a diesel engine excellent in high strength, impact resistance and internal pressure fatigue characteristics, characterized in that after passing through the step of holding for 3600 seconds in the region, it is cooled to room temperature and then assembled. . A method for producing a common rail for a diesel engine according to claim 7 ,
Using a steel material that satisfies the component composition according to any one of claims 1 to 4 ,
Heating and holding at 1200 ° C. or higher,
A process of hot working,
(Ac1 point-30 ° C) to (Ac3 point + 100 ° C) held for 10 seconds or more, forged in this temperature range, cooled to 300-500 ° C at an average cooling temperature of 3 ° C / s or more, It has excellent strength and impact resistance and internal pressure fatigue characteristics, characterized by being machined after a step of holding in the temperature range for 60 to 3600 seconds and a step of drilling in the tube axis direction by a gun drilling method. A method of manufacturing a common rail for a diesel engine. The manufacturing method according to claim 24 , wherein the hot working is forging. The manufacturing method according to claim 15, 17, or 19 , further subjected to auto-fretting treatment after the bending process as a final process. The manufacturing method according to any one of claims 16 , 18 and 20 to 25 , further subjected to an autofretting process after the assembly process which is the final process or the machining which is the final process.