CN105008572A - Strength member and manufacturing method therefor - Google Patents

Strength member and manufacturing method therefor Download PDF

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Publication number
CN105008572A
CN105008572A CN201480012993.2A CN201480012993A CN105008572A CN 105008572 A CN105008572 A CN 105008572A CN 201480012993 A CN201480012993 A CN 201480012993A CN 105008572 A CN105008572 A CN 105008572A
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Prior art keywords
strength member
temperature
point
spring
manufacture method
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黑川真平
铃木健
柴入纮介
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NHK Spring Co Ltd
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NHK Spring Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
    • C21D7/06—Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Heat Treatment Of Articles (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

Provided is a strength member wherein, by reducing the mean dislocation density in an arbitrary cross-section, settling resistance and yield strength are substantially improved without reducing cost advantages or adding substantial process changes. By mass, said strength member contains 0.5-0.7% carbon, 1.0-2.0% silicon, 0.1-1.0% manganese, 0.1-1.0% chromium, up to 0.035% phosphorus, and up to 0.035% sulfur, with the remainder comprising iron and unavoidable impurities. By area, bainite constitutes at least 65% of this strength member, and the mean dislocation density in an arbitrary cross-section is no more than 2.0*1016 m-2.

Description

Strength member and manufacture method thereof
Technical field
The present invention relates to resistance to decline (of resistance to へ り), the strength member of yield strength excellence and manufacture method thereof.
Background technology
Such as, as the material of the strength member of car engine valve spring and so on, widely used the Si-Cr steel oil tempering silk with tempered martensite from the viewpoint of resistance to fatigue, resistance to decline in the past.On the other hand, in patent documentation 1, propose following technology: by forming the tissue using the fine bainite of ductility excellence as main body after disc spring, thus give the plastix strain larger than tempered martensite and do not reduce resistance to fatigue.In the art, by reducing the dislocation desity harmful to resistance to decline, and utilize strain aging to carry out pinned dislocation effectively, thus improve resistance to decline.In addition, giving larger plastix strain by carrying out halting process, being therefore endowed larger compressive residual stress at wire internal, resistance to decline and resistance to fatigue can also being improved simultaneously.And then, in the above-described techniques, cheap material can be used, therefore can reduce this advantage of manufacturing cost in addition.
Prior art document
Patent documentation
Patent documentation 1: Japanese Unexamined Patent Publication 2012-111992 publication.
Summary of the invention
the problem that invention will solve
But, in recent years, more seek the low burnup of automobile than ever, to spring, resistance to decline that bolt equal strength part requirement is higher, yield strength.
Therefore, the present invention carries out in view of the foregoing, therefore, its object is to provide following strength member and manufacture method thereof, and described strength member significantly can improve resistance to decline and yield strength and harmless cost advantage and without the need to significantly changing operation.
for the scheme of dealing with problems
The present inventor etc. conduct in-depth research to solve above-mentioned problem, found that: carry out tempering by the martensite generated utilizing water-cooled in means of isothermal quenching, make it resolve into ferrite and cementite and reduce dislocation, thus resistance to decline is significantly improved.Also find in addition: while the dislocation in martensite reduces, tissue can soften sharp, usually can become the reason that fatigue strength is reduced, but by the main body of tissue is made fine bainite, can not occur to reduce by hardness the fatigue strength caused and reduce.On the other hand, the raising of resistance to decline of the present invention and the rising of yield strength are accompanied, therefore, it is possible to be applied to screwed part, the track rods etc. such as the bolt requiring high-yield strength.
Strength member of the present invention completes based on above-mentioned opinion, it is characterized in that, it has: in mass % for C:0.5 ~ 0.7%, Si:1.0 ~ 2.0%, Mn:0.1 ~ 1.0%, Cr:0.1 ~ 1.0%, below P:0.035%, below S:0.035%, surplus comprise the composition of iron and inevitable impurity; And have the tissue of the bainite of more than 65% in area ratio, the average dislocation density of arbitrary section is 2.0 × 10 16m -2below.
In addition, the feature of the manufacture method of strength member of the present invention is, it possesses following operation: be the molding procedure of article shape by wire forming, and described wire rod has in mass % for C:0.5 ~ 0.7%, Si:1.0 ~ 2.0%, Mn:0.1 ~ 1.0%, Cr:0.1 ~ 1.0%, below P:0.035%, below S:0.035%, surplus comprise the composition of iron and inevitable impurity; After the temperature of Ac3 point ~ (Ac3 point+250 DEG C) carries out austenitizing, cool with the speed of more than 20 DEG C/sec, keep more than 400 seconds in the temperature of (Ms point-20 DEG C) ~ (Ms point+60 DEG C), be then cooled to the heat treatment step of room temperature; And, by the tempering process that the aforementioned articles after thermal treatment carries out keeping the temperature of 350 ~ 450 DEG C.Herein, Ac3 point refers to that material is adding the boundary temperature hankering being transformed into austenite one phase territory from ferrite+austenitic two phase regions, and Ms point refers to and starts to generate martensitic temperature in cooling.It should be noted that, when strength member is spring, expect to possess the shot peening operation to goods projection pill.
The present invention is not limited to spring, can also to be applied to as screwed part, the track rods etc. such as bolt all strength members of desired strength.
the effect of invention
According to the present invention, by reducing the average dislocation density of arbitrary section, can obtain and significantly can improve resistance to decline and yield strength and harmless cost advantage and without the need to significantly changing the effects such as operation.
Accompanying drawing explanation
Fig. 1 is the figure of the operation showing manufacture method of the present invention.
Fig. 2 is the figure of relation of tempering temperature in display embodiments of the invention and average dislocation density.
Fig. 3 is the figure of relation of tempering temperature in display embodiments of the invention and remaining shear strain.
Fig. 4 is the figure of relation of the inner hardness of tempering temperature in display embodiments of the invention and spring wire material.
Embodiment
First, the restriction reason for the chemical composition of the steel used in the present invention is described.It should be noted that, in the following description, " % " expression " quality % ".
C：0.5~0.7%
C is the important element for guaranteeing expectation strength, needs containing more than 0.5% to obtain this effect.But when C concentration becomes excessive, the ratio as the retained austenite of soft phase excessively increases and is difficult to obtain expectation strength, therefore suppresses below 0.7%.
Si：1.0~2.0%
Si is the element contributing to solution strengthening, needs containing more than 1.0% to obtain expectation strength.But Si amount is for time excessive, and soft retained austenite ratio uprises, and causes the reduction of intensity on the contrary, therefore suppresses below 2.0%.
Mn：0.1~1.0%
Mn adds as the deoxidant element in refining, on the other hand, being can improve the hardenability of steel and the element of easy raising intensity, therefore, needing containing more than 0.1% to obtain expectation strength.On the other hand, when content is excessive, easily produces segregation and processibility reduces, therefore suppress below 1.0%.
Cr：0.1~1.0%
Cr can improve the hardenability of steel and the element of easy raising intensity.In addition, make the effect that pearlitic transformation postpones in addition, stably can obtain bainite structure (suppression pearlitic structure) during cooling after austenitizing heating, therefore, need containing more than 0.1%.But, excessive containing Cr time, easily produce ferrous-carbide, therefore suppress below 1.0%.
P, below S:0.035%
P and S encourages the element of intercrystalline failure that grain boundary segregation causes, therefore expect that its content is low as far as possible, but it expends smelting cost for inevitable impurity to make it reduce, and therefore the upper limit is set to 0.035%.The content of P and S is preferably less than 0.01%.
Then, in a organized way in the restriction reason of area ratio etc. of bainite be described.
Bainite: 65% refers to upper bainite: make the existing steel carrying out austenitizing below about 550 DEG C and exceed in the temperature province of martensitic transformation starting temperature, isothermal phase change to occur and the metal structure obtained, it is made up of bainite ferrite and ferrous-carbide.Dislocation desity as the bainite ferrite of matrix is high, and in addition, ferrous-carbide has precipitating reinforcing effect, therefore, even if hardness reduces because of the minimizing of the dislocation in martensite, also can improve intensity with bainite structure.
Manufacturing method according to the invention, about bainite structure, the steel carrying out austenitizing are carried out isothermal maintenance near Ms point, therefore, it is possible to obtain the structure of fine precipitation ferrous-carbide in fine bainite ferrite substrate, the reduction of grain-boundary strength is few, even if be high strength, the reduction of ductility toughness is also little.Therefore, even if give large plastix strain, also can not produce defects such as the harmful be full of cracks of resistance to fatigue, can dislocation desity be reduced.Like this, bainite is indispensable tissue for acquisition high strength and high ductibility, and its area ratio is more high more preferred, in order to obtain the high-strength high-tractility of expectation, needs to be more than 65%.
On the other hand, the non-transformed austenite in isothermal maintenance is by becoming martensite, retained austenite being cooled to room temperature afterwards.The tissue of bainite area ratio less than 65% represents that isothermal holding time is short, and the degree of enrichment of the C in non-transformed austenite is in this stage little, and therefore by cooling thereafter, martensite ratio uprises.Therefore, when bainite area ratio is less than 65%, martensite becomes many and can obtain high strength, but otch susceptibility ( ) significantly uprise, therefore cannot give larger plastix strain, resistance to decline does not improve.
It should be noted that, retained austenite is soft, and the shear strain therefore produced because of processing easily remains.Therefore, the amount of retained austenite becomes the index of the amount of remaining shear strain, and its amount, for time excessive, can make resistance to decline reduce.From this viewpoint, the area ratio of retained austenite is expected to suppress below 6.5%.
In addition, have the intensity that can tolerate necessary load-carrying in order to ensure goods, the Vickers' hardness at the center of the arbitrary cross section of goods is expected for more than 450HV.On the other hand, hardness excessive high time, usual elongation diminishes, and the otch susceptibility of steel self increases, and cannot give large plastix strain, therefore expects for below 650HV.
Then, for the manufacture method of strength member of the present invention, be described for spring.Fig. 1 (A) is the figure of the manufacture method of display embodiment, and Fig. 1 (B) is the figure of the manufacture method shown in the past.Spring can manufacture as follows: for the steel of above-mentioned chemical composition, such as after disc spring operation, as required after the seat grinding step both ends of the surface of spring being carried out to grinding, after the temperature of Ac3 point ~ (Ac3 point+250 DEG C) carries out austenitizing, cool with the speed of more than 20 DEG C/sec, more than 400 seconds are kept in the temperature of (Ms point-20 DEG C) ~ (Ms point+60 DEG C), then room temperature is cooled to the speed of cooling of more than 20 DEG C/sec, after above-mentioned heat treatment step, tempering is carried out at 350 ~ 450 DEG C, after shot peening operation, carry out halting treatment process as required, thus manufacture.For be heated to Ac3 point above before structure of steel, be not particularly limited.Such as, can by carried out forge hot, Wire Drawing bar steel be used as starting material.Below, be described for each operation, describe as required and limit reason.
Disc spring operation
It is the cold rolling operation being formed into expectation coil shape.Forming method utilizes the method using spring forming machine (coiling machine), the method etc. using mandrel.It should be noted that, as the spring outside whisker, the spring arbitrarily such as leaf spring, torsion bar, stabilizer can be applied to.
Seat surface grinding step
This operation is carried out as required, is make the both ends of the surface of spring become right-angle plane relative to the axle core of spring and carry out the operation of grinding.
Heat treatment step
After the spring after disc spring is carried out austenitizing, carry out isothermal maintenance, cool thereafter, thus terminate heat treatment step.For the structure of steel carried out before austenitizing, be not particularly limited.Such as, can by carried out forge hot, Wire Drawing bar steel be used as starting material.The temperature of austenitizing needs for Ac3 point ~ (Ac3 point+250 DEG C).Below Ac3 point, can directly become raw-material organizational composition and not carry out austenitizing.In addition, when exceeding (Ac3 point+250 DEG C), old austenite particle diameter easily becomes coarsening, likely causes the reduction of ductility.
After austenitizing, being cooled to the speed of the temperature of carrying out isothermal maintenance, The faster the better, needs to carry out with the speed of cooling of more than 20 DEG C/sec, is preferably more than 50 DEG C/sec.When speed of cooling is less than 20 DEG C/sec, in process of cooling, generate perlite, the bainite of 65 more than area % cannot be obtained.The temperature of carrying out isothermal maintenance needs for (Ms point-20 DEG C) ~ (Ms point+60 DEG C), and it is very important controlling factor as the manufacture method for realizing spring steel of the present invention and spring.When carrying out temperature deficiency (the Ms point-20 DEG C) of isothermal maintenance, the martensite volume that the phase transformation initial stage generates is many and hinder the raising of ductility, in addition, cannot obtain the bainite of 65 more than area %.On the other hand, when the temperature of carrying out isothermal maintenance exceedes (Ms point+60 DEG C), bainite generation coarsening, therefore tensile strength reduces, cannot obtain the intensity that can tolerate load-carrying as spring.Further, by carrying out isothermal maintenance near Ms point as described above, fine bainite can be separated out.By making fine bainite separate out, austenite remains in fine space, can make fine retained austenite grain.
In austenite, bainite is separated out by carrying out isothermal maintenance.The time that isothermal keeps needs to be more than 400 seconds, and it is also very important controlling factor as manufacture method of the present invention.During the deficiency of time 400 seconds that isothermal keeps, carrying out of bainitic transformation is insufficient, and therefore bainite ratio area occupation ratio that is little, bainite becomes less than 65%.It should be noted that, even if the time of carrying out isothermal maintenance is long, the bayesian scale of construction amount of reaching capacity generated, causes the increase of production cost, therefore expects to be within 3 hours.
In order to obtain uniform tissue, The faster the better for the speed of cooling after isothermal keeps, and is preferably the speed of cooling of more than 20 DEG C/sec, is more preferably more than 50 DEG C/sec.Specifically, can be oil cooling, water-cooled.
Tempering process
After heat treatment step, carry out the tempering process being carried out keeping the temperature of 350 ~ 450 DEG C by spring.When tempering temperature is less than 350 DEG C, martensitic decomposition becomes insufficient, and the reduction of dislocation becomes insufficient.In addition, when tempering temperature is more than 450 DEG C, the inner hardness of spring significantly reduces, intensity and fatigue strength reduce.In order to suppress the extreme reduction of the inner hardness of spring, it is less than 400 DEG C that tempering temperature is expected.It is 25 ~ 60 minutes that tempering time is expected.When tempering time was less than 25 minutes, tempering becomes insufficient, and in addition, tempering time, more than 60 minutes, is uneconomic.
Shot peening operation
In shot peening, make the pill impact spring formed by metal, sand etc., thus give compressive residual stress to its surface, make the resistance to fatigue of spring significantly improve thus.In the present invention, on the basis of the compressive residual stress utilizing common shot peening to obtain, the processing of retained austenite is utilized to respond to martensitic transformation and form higher dark compressive residual stress.The pill used in shot peening can use the high rigidity particle etc. of cutting wire, steel ball, FeCrB system etc.In addition, compressive residual stress can be adjusted by the actual effect of pill or average ball equivalent diameter, velocity of projection, projection time and multistage mode of delivery.
To halt treatment process
Process of halting optionally carries out in order to the decline amount (set deformation volume) when making elastic limit significantly improve by applying plastix strain and reduce use.Now, by 200 ~ 300 DEG C of process of carrying out halting (heat halt process), resistance to decline can be improved further.In addition, process by halting and make retained austenite that processing induced transformation occur, thus can expect that it becomes the higher martensite of intensity.Thus, be endowed high compressive residual stress due to the volumetric expansion of accompanying with phase transformation, resistance to fatigue can be improved further.
Embodiment
[the 1st embodiment]
Use the Si-Cr steel oil tempering wire rod (diameter: 4.1mm) comprising the representative chemical composition that table 1 is recorded, after utilizing coiling machine cold dish spring established practice shaped, heat-treat (means of isothermal quenching).About thermal treatment, spring process furnace kept 12 minutes the temperature of 830 DEG C and carries out austenitizing, then carrying out water-cooled, remaining in temperature in the salt bath of 300 DEG C and keep 40 minutes, cool thereafter.
[table 1]
。
Then, tempering is carried out for spring with the temperature shown in table 2.Tempering time is set to 60 minutes.Then, shot peening use ball equivalent diameter is the steel pill of 0.1 ~ 1.0mm.And then, after spring being heated to 200 ~ 300 DEG C, carry out process of halting.For gained spring, investigate each character as shown below.
[table 2]
。
[differentiation of phase]
The differentiation of phase by flooding the several seconds and using tissue thereafter to carry out as follows in the nital liquid of 3% by sample.First, bainite is easily corroded by nital, therefore in optical microscope photograph, is shown as black or grey, and on the other hand, the erosion resistance of retained austenite to nital is high, therefore in opticmicroscope, is shown as white.Utilize this characteristic, image procossing is carried out to optical microscope photograph, thus obtain the overall ratio of bainite (black and grey portion) ratio and retained austenite (white portion).Retained austenite ratio by for polishing ( ) sample use X-ray diffraction method obtain.It should be noted that, in table 2, about the residue tissue of bainite and retained austenite, No.1 and No.2 is martensite, No.3 ~ No.7 is ferrite and cementite.
[Vickers' hardness at center]
In the cross section of sample, 5 place's mensuration are carried out to the Vickers' hardness of central part, obtains its mean value.
[average dislocation density]
Average dislocation density ρ calculates as follows: by document (material と プ ロ セ ス: Japanese Iron Steel Association Hui Talk drills Theory collected works (materials and process: Nippon Steel association speech collection of thesis) 17(3), 396-399 page " X Line inflection The utilizes translocation Wei Mi Du Evaluation value method (utilizing the evaluation method of the dislocation desity of X-ray diffraction) ") as a reference, utilize the following mathematical expression 1 illustrated to obtain strain stress, thus calculate.
[mathematical expression 1]
ρ=14.4ε 2/b 2
Herein, b is Burgers vector (=2.5 × 10 -10m).In addition, strain stress calculates as follows: in the cross section of sample, for central part, the collimator X-ray diffraction device of 0.3mm (the D8 DISCOVER that Bruker company manufactures) is utilized to measure the diffraction peak of ferritic (110), (211), (220), use the peak width at half height β at each peak, according to the relation of following mathematical expression 2, β cos θ/λ and the sin θ/λ of each diffraction peak are marked and drawed into the longitudinal axis and the transverse axis of figure, obtain slope 2 ε of their curve of approximation, thus calculate.
[mathematical expression 2]
βcosθ/λ=0.9/D+2εsinθ/λ
Herein, θ is the value of the half of X-ray diffraction peak position 2 θ, λ is the K α of the pipe ball used as X-ray occurring source 1the wavelength of ray, D are crystallite dimension.
[remaining shear strain]
Remaining shear strain is the index of the resistance to decline representing spring, and value is lower, represents that resistance to decline is more excellent.In the decline test of spring, the mode that sample reaches 1050Mpa with the maximum shear stress is applied load-carrying and compresses fixing, is immersed in the silicone oil of 165 DEG C.From starting dipping, after 24 hours, sample to be taken out from silicone oil, removal load-carrying after reaching room temperature.Decline amount is obtained as follows: before and after above-mentioned decline test, measure load-carrying when compressing the spring into specified altitude, substituted in following mathematical expression 3 by this load-carrying reduction Δ P, obtain remaining shear strain.
[mathematical expression 3]
γ={8D/(πd 3G}}×ΔP
Herein, D is average line loop diameter, d is wire diameter, G is modulus of traverse elasticity (=78,500MPa).
The result measured as mentioned above is merged and is recorded in table 2, and the relation of measured value and tempering temperature is shown in Fig. 2 ~ Fig. 4.Can confirm as shown in Figure 2: when tempering temperature is more than 350 DEG C, average dislocation density sharply reduces, and reaches 2.0 × 10 16m -2below.Accompany therewith, can confirm as shown in Figure 3: when tempering temperature is more than 350 DEG C, remaining shear strain also suddenly swashs reduction, reaches 6.7 × 10 -4below.Remaining shear strain becomes the index of resistance to decline, and the less then resistance to decline of remaining shear strain is higher.In addition, can confirm as shown in Figure 4: when tempering temperature is more than 400 DEG C, the inner hardness of spring sharply reduces.
Can confirm as above: be 2.0 × 10 by making average dislocation density 16m -2below, remaining shear strain is made to be 6.7 × 10 -4below, resistance to decline can be improved.
[the 2nd embodiment]
The Si-Cr steel cold pull wire (diameter: 6.0mm) comprising the representative chemical composition that table 1 is recorded is cut into specified dimension, carries out head forging and thread rolling to after being shaped to bolt, heat-treat (means of isothermal quenching).In thermal treatment, bolt process furnace kept 12 minutes the temperature of 830 DEG C and carries out austenitizing, then carrying out water-cooled, remaining in temperature in the salt bath of 300 DEG C and keep 40 minutes, cool thereafter.
Then, tempering is carried out for bolt with the temperature shown in table 3.Tempering time is set to 60 minutes.For gained bolt, utilize the method identical with embodiment 1 to investigate inner hardness, average dislocation density and bainite area ratio, utilize stretching test machine determination tensile strength and 0.2% yield strength.Their result is merged and is shown in table 3.
[table 3]
。
As shown in table 3ly can to confirm like that: in the bolt of example of the present invention, be 2.0 × 10 by making average dislocation density 16m -2below, high yield ratio can be obtained.
industry applications
The present invention to can be applicable to as the screwed part such as spring, bolt, the track rods etc. such as whisker, leaf spring, torsion bar, stabilizer the strength member of desired strength.

Claims (6)

1. strength member, it is characterized in that, it has: in mass % for C:0.5 ~ 0.7%, Si:1.0 ~ 2.0%, Mn:0.1 ~ 1.0%, Cr:0.1 ~ 1.0%, below P:0.035%, below S:0.035%, surplus comprise the composition of iron and inevitable impurity; And have the tissue of the bainite of more than 65% in area ratio, the average dislocation density of arbitrary section is 2.0 × 10 16m -2below.
2. strength member according to claim 1, is characterized in that, the Vickers' hardness at center is 450 ~ 650HV.
3. the manufacture method of strength member, is characterized in that, it possesses following operation:
Be the molding procedure of article shape by wire forming, described wire rod has in mass % for C:0.5 ~ 0.7%, Si:1.0 ~ 2.0%, Mn:0.1 ~ 1.0%, Cr:0.1 ~ 1.0%, below P:0.035%, below S:0.035%, surplus comprise the composition of iron and inevitable impurity;
After the temperature of Ac3 point ~ (Ac3 point+250 DEG C) carries out austenitizing, cool with the speed of more than 20 DEG C/sec, keep more than 400 seconds in the temperature of (Ms point-20 DEG C) ~ (Ms point+60 DEG C), be then cooled to the heat treatment step of room temperature; And
By the tempering process of the described goods after thermal treatment the temperature maintenance of 350 ~ 450 DEG C.
4. the manufacture method of strength member according to claim 3, is characterized in that, it possesses: in the rear shot peening operation to described goods projection pill of described tempering process.
5. the manufacture method of the strength member according to claim 3 or 4, is characterized in that, speed of cooling when being cooled to room temperature is set to more than 20 DEG C/sec.
6. the manufacture method of strength member according to claim 4, is characterized in that, it possesses: the treatment process of halting of described goods being given to permanent strain after described shot peening operation.
CN201480012993.2A 2013-03-08 2014-03-07 Strength member and manufacturing method therefor Pending CN105008572A (en)

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EP2966186A4 (en) 2016-11-23

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