CN1811000A - High strength spring steel having excellent hydrogen embrittlement resistance - Google Patents
High strength spring steel having excellent hydrogen embrittlement resistance Download PDFInfo
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Abstract
The present invention provides a high strength steel used for spring steel that has excellent hydrogen embrittlement resistance. The high strength steel which spring steel having excellent hydrogen embrittlement resistance comprises 0.20 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, higher than 0% and not higher than 1.5% of Al, 0.15% or less P, 0.02% or less S, and balance of iron and inevitable impurities and the structure includes: 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 10% or less (may be 0%) in total content of ferrite and pearlite in the proportion of area to the entire structure, and also the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher and the steel tensile strength is 1860 MPa or higher.
Description
Technical field
The present invention relates to a kind of high-strength spring steel of hydrogen-embrittlement resistance excellence, relate to particularly that to be suppressed at tensile strength be the high-strength spring steel that becomes the hydrogen embrittlement of problem and spontaneous crackle, delayed fracture in the above spring steel of 1860MPa.
Background technology
The chemical ingredients of spring steel that is used for the high-strength spring of the valve spring of motor car engine, the bearing spring of suspension etc., be defined as JIS G 3565～3567 and JIS G 4801, to satisfying after the steel hot rolling that this one-tenth is grouped into, implement Wire Drawing and reach specified line footpath, this is after oil tempering is carried out spring processing (cold dish is around spring) after handling, or to after the rolled stock enforcement Wire Drawing, heat and the formation spring, carry out Q-tempering (heat coiling spring) thereafter, make spring thus.
Nowadays, be accompanied by the lightness of automobile etc., spring also requires high strength more, though being the above spring of 1800MPa, tensile strength realizes, but follow so high strength of spring, if, problem then occurs such as the so-called delayed fracture of unexpected disruptive through long-time.
In order to address this problem, for example in the patent documentation 1, point out in basic composition, to add the alloying element of so-called Cr, V, Ni, Cu, B, Nb, improve erosion resistance thus, make the crystal grain granular to improve anti-delayed fracture.And, in the patent documentation 2, point out by in basic composition, containing Ni, Cr, Cu, V, thereby promote the toughness of the steel after the Q-tempering, and improve erosion resistance, thereby anti-elastic sag resistance and hydrogen embrittlement resistance are promoted.
In patent documentation 3, as hydrogen trap sites (trap site), make contain V, Mo, Ti, Nb and Zr any more than a kind or 2 kinds oxide compound, carbide and nitride and at least a kind of existence of its any compound precipitate more than 2 kinds, improve hydrogen embrittlement resistance thus.Specifically, the median size that makes these precipitates is that 0.05 μ m is above and below 1.0 μ m, and makes averaged particles be spaced apart 3～30 times of median size, improves hydrogen embrittlement resistance thus.
But, employed alloying element costliness in these technology is restricted with the high-strength spring steel of the anti-delayed fracture excellence of offering of lower priced articles.In addition, if this contains some alloying element in a large number, then there is the problem that is difficult to recycle.
In the patent documentation 4, do not need to point out the interpolation of above-mentioned alloying element,, just can improve hydrogen embrittlement resistance by the control tissue.Specifically, tissue makes old austenitic crystallization particle diameter granular based on martensite or bainite, and limits the thick not number of molten carbide, improves the hydrogen embrittlement resistance of high strength spring steel wire thus.But, even control the form of separating out of precipitate, also all there is limitation in the lifting of hydrogen occlusion capacity, is difficult to realize more excellent hydrogen embrittlement resistance.
The flat 10-183302 communique of patent documentation 1 Te Open
No. 3064672 communique of patent documentation 2 Te Xu
Patent documentation 1 Te Open 2001-288539 communique
Patent documentation 1 Te Open 2004-143482 communique
Summary of the invention
The present invention carries out in view of the above problems, and its purpose is that it is the above high strength of 1860MPa that a kind of tensile strength is provided, the high-strength spring steel that hydrogen embrittlement resistance significantly improves.
High-strength spring steel of the present invention is characterized in that, satisfies C:0.20～0.60% (meaning of quality %, in one-tenth is grouped into below identical); Si:1.0～3.0%; Mn:1.0～3.5%; Al:1.5% following (not containing 0%); Below the P:0.15%; Below the S:0.02%; Remainder is iron and unavoidable impurities,
Calculate by area occupation ratio with respect to whole tissue,
Residual austenite (residual austenite) is more than 1%,
Bainite ferrite (bainitic ferrite) and martensite add up to more than 80%,
Ferrite (ferrite) and perlite (pearlite) add up to below 10% and (contain 0%),
And the mean axis of above-mentioned residual austenite crystal grain is more than 5 than (major axis/minor axis), and tensile strength is 1860MPa above (hereinafter referred to as " steel 1 of the present invention ") in addition.
Another high-strength spring steel of the present invention is characterized in that, satisfies C:0.20～0.60%; Si:1.0～3.0%; Mn:1.0～3.5%; Al:0.5% following (not containing 0%); Below the P:0.15%; Below the S:0.02%; Remainder is iron and unavoidable impurities,
Calculate by area occupation ratio with respect to whole tissue,
Residual austenite is more than 1%,
Bainite ferrite and martensite add up to more than 80%,
Ferrite and perlite add up to below 10% and (contain 0%),
And the mean axis of above-mentioned residual austenite crystal grain is more than 5 than (major axis/minor axis), and tensile strength is 1860MPa above (hereinafter referred to as " steel 2 of the present invention ") in addition.
And high-strength spring steel of the present invention can also contain Nb:0.1% following (not containing 0%) and/or Mo:1.0% following (not containing 0%) and Cu:2% following (not containing 0%) and/or Ni:5% following (not containing 0%).
According to the present invention, the hydrogen of invading from the outside is by innoxious, and the tensile strength that hydrogen embrittlement resistance has been enhanced is the above high-strength spring steel of 1860MPa, it does not add expensive element, also can make to high productivity, the automobile of giving birth to as utmost point difficult labours such as the being used for delayed fracture spring of parts can be provided at an easy rate.Also have, high-strength spring steel of the present invention is compared with currently available products, so because the few recirculation of alloying element is also excellent.
Embodiment
As High Strength Steel, always, the general tempered martensite steel that is adopted and the situation of martensite+ferritic steel, the delayed fracture that hydrogen causes is considered to be in hydrogen such as old austenite grain boundary and gathers and form hole etc., this part produces as starting point, in order to reduce the susceptibility of delayed fracture, as existing technology as mentioned above, make as the carbide of the trap sites of hydrogen etc. and evenly and fine disperse, and reducing the diffustivity hydrogen concentration, this is used as general solution and adopts.But, disperse as the carbide of the trap sites of hydrogen etc. so in a large number, because trapping ability is limited, so can not suppress the delayed fracture that hydrogen causes fully.
Therefore present inventors based on to the taking into full account of the environment for use of spring steel, in order to realize the hydrogen embrittlement resistance (anti-delayed fracture) of height, and discuss to the concrete method of improvement.
Its result, draw as drawing a conclusion, thereby in order to improve hydrogen embrittlement resistance by the starting point that reduces intercrystalline failure, the parent phase of spring steel, be not as the general martensite single phase structure that adopts of High Strength Steel, and preferably bainite ferrite is main body " bainite ferrite and martensitic two phase constitutions ".Under the situation of above-mentioned martensite single phase structure, separate out at grain boundary carbide (for example membranaceous cementite etc.), intercrystalline failure is easy to generate, with respect to this, if bainite ferrite is main body " bainite ferrite and martensitic two phase constitutions ", this bainite ferrite is different with general (polygon) ferrite so, be tabular ferrite dislocation desity height, same with the monophasic situation of martensite, can easily improve and organize all intensity, in addition, because hydrogen is captured in a large number on this dislocation, so also can improve hydrogen embrittlement resistance.And, exist by making this bainite ferrite and residual austenite described later, also have the advantage of generation of the carbide of the starting point that can prevent to become intercrystalline failure.
And, improve in order to make the hydrogen trapping ability, thereby, find that also the residual austenite of formation lath-shaped is very effective by realizing the innoxious raising hydrogen embrittlement resistance of hydrogen.In the past, residual austenite was considered to and can causes detrimentally affect to hydrogen embrittlement resistance and fatigue.But discuss as can be known by present inventors, existing residual austenite is micron-sized bulk, the residual austenite of this form can be caused detrimentally affect to hydrogen embrittlement resistance and fatigue, but, if with the morphology Control of this residual austenite is the strip of submicron level, the original hydrogen occlusion function that has of residual austenite is brought into play so, and occlusion captures hydrogen in large quantities, can improve hydrogen embrittlement resistance significantly.
Below, describe in detail in the present invention about each reason of organizing of regulation.
＜bainite ferrite (BF)+martensite (M): more than 80% 〉
In the present invention, form bainite ferrite and martensitic two phase constitutions (bainite ferrite is a main body).As mentioned above, bainite ferrite is organized as hard, can obtain high strength easily.And, the dislocation desity height of parent phase, hydrogen is captured in a large number on this dislocation, and its result also has the TRIP steel than other, the advantage of can occlusion a large amount of hydrogen.In addition, on the border of the bainite ferrite of lath-shaped, the residual austenite of the lath-shaped of the present invention's regulation generates easily, also has the advantage that can access very excellent extension.In order to bring into play this effect effectively, in area occupation ratio, establish bainite ferrite and martensite and add up to more than 80% with respect to whole tissue, be preferably more than 85%, more preferably more than 90%.Also have, its upper limit can determine to obtain according to the balance with its hetero-organization (residual austenite), and during tissue (ferrite etc.) beyond not containing residual austenite, its upper limit is controlled at 99%.
So-called above-mentioned bainite ferrite means it is tabular ferrite, and the bottom tissue that dislocation desity is high with dislocation-free or have the polygon ferrite of few bottom tissue, by the SEM observation, is clearly distinguished as following quilt.
The area occupation ratio of bainite ferrite tissue is following and try to achieve.Promptly, cut out can observe after the cross section of the position of the radius 1/2 of test portion (bar-shaped), corrode with nital, SEM (ScanningElectron Microscope is passed through in mensuration zone (about 50 * 50 μ m) to the position arbitrarily in this plane, scan the type electron microscope) observe (multiplying power: 1500 times), and calculate.
Bainite ferrite shows that in the SEM photo Dark grey is (as SEM, also have and separately to distinguish bainite ferrite and residual austenite, martensitic situation), but, the polygon ferrite is black in the SEM photo, does not comprise residual austenite and martensite in inside with polygonal shape.
The SEM that the present invention uses, be " possess EBSP (Electron Back ScatteringPattern) detector high resolving power type FE-SEM (Field Emission type ScanningElectron Microscope; Philips corporate system; XL30S-FEG) ", has in the SEM viewing area advantage of resolving by the EBSP detector.The EBSP method is made a simple explanation here, EBSP shines specimen surface with electron beam, the reflection electronic that produces when resolving thus and the Kikuchi style that obtains, determine the instrument of the crystal orientation of electron beam incident position with this, electron beam is carried out 2 dimension scannings at specimen surface, measure the crystal orientation of each institute's determining deviation, the orientation that just can measure specimen surface distributes.Utilize this EBSP to observe, can have following advantage, promptly can be identified in and be judged as homologue and the different thickness of slab direction of crystal orientation difference tissue in the common microscopic examination according to aberration.
＜residual austenite (residual γ, γ R): more than 1% 〉
Residual austenite, not only useful as always known to the raising of full extension, but also be the tissue that helps very much the raising of hydrogen embrittlement resistance, therefore make it exist more than 1% in the present invention.Be preferably more than 2%, more preferably more than 3%.Also have,, then can not guarantee the high strength expected, so recommend its upper limit to be made as 20% if above-mentioned residual austenite exists in large quantities.More preferably below 15%.
And as mentioned above as can be known, if residual austenite is a lath-shaped, the hydrogen trapping ability is compared with carbide and is become big utterly so, particularly its shape with mean axis than (major axis/minor axis) 5 when above, it is innoxious that what is called is implemented essence by the hydrogen of atomospheric corrosion intrusion, can improve hydrogen embrittlement resistance significantly.The mean axis ratio of above-mentioned residual austenite is preferably more than 10, advances any and is preferably more than 15.
From the viewpoint of the stability of above-mentioned residual austenite, recommend the C concentration (C in the residual austenite
γ R) be more than 0.8%.And, if this C
γ RBe controlled at more than 0.8%, then also can improve extension etc. effectively.Be preferably more than 1.0%, more preferably more than 1.2%.Though preferred described C γ R is high,, the upper limit that general consideration may be adjusted in the actually operating probably is 1.6%.
According to above-mentioned FE-SEM/EBSP method, above-mentioned residual austenite is just as the observed zone of FCC phase (face-centered cubic lattice).An object lesson as the EBSP measurement, the same with the situation of above-mentioned bainite ferrite and martensitic observation, the mensuration zone (about 50 * 50 μ m) that can list with the position arbitrarily in the cross section of the position of the radius 1/2 of test portion (bar-shaped) is an object, and 0.1 μ m measures with measuring interval.Also have, when being ground to this mensuration face, carry out electrolytic polishing for the phase transformation that prevents residual austenite.Then, use above-mentioned " FE-SEM that possesses the EBSP detector ", the sample irradiating electron beam in being arranged on the SEM lens barrel.Take the EBSP image that is projected on the screen with high sensitivity camera (Dage-MTI Inc. makes VE-1000-SIT), put into computer with graphic form.Carry out image analysis with computer then, by with both having known that the figure that crystal system [being FCC (face-centered cubic lattice) mutually during residual austenite] simulation obtains compared, the FCC of decision is made coloured picture mutually.Obtain the area occupation ratio that is illustrated the zone of (mapping) like this, it is decided to be " area occupation ratio of residual austenite ".What in addition, the hardware of above-mentioned parsing and software used is the OIM of TexSEM Laboratories Inc. (Orientation Imaging Microscopy
TM) system.
Also have, the measurement of above-mentioned mean axis ratio is to observe (1.5 ten thousand times of multiplying powers) with TEM (Transmission ElectronMicroscope), measurement is in optional 3 visuals field, the major axis and the minor axis of the residual austenite crystal grain that exists are obtained axial ratio, with its mean value as the mean axis ratio.
＜ferrite (F)+perlite (P): (contain 0%) below 10% 〉
Spring steel of the present invention, can be only (promptly by above-mentioned tissue, the mixed structure of bainite ferrite+martensite and residual austenite) constitutes, but in the scope of not damaging effect of the present invention, the ferrite that also can have as its hetero-organization (also has, at this what is called " ferrite ", the meaning is the polygon ferrite, i.e. dislocation-free density or its few ferrite) and perlite.These are inevitable remaining and tissues of obtaining in manufacturing processed of the present invention, but few more good more, are suppressed at below 10% in the present invention.Be preferably and be lower than 5%, more preferably be lower than 3%.
The present invention, as mentioned above, particularly the main points with the control metal structure are feature, still, so that easily make the lifting of hydrogen embrittlement resistance and high strength, need the one-tenth of steel to be grouped into to satisfy as described below in order to form this tissue.
<C：0.20～0.60％>
C is in order to ensure the high strength more than the 1860MPa, and guarantees the element of necessity of residual austenite.In detail, making austenite comprise sufficient C content in mutually, is the mutually residual important element of austenite that at room temperature makes expectation, and it is contained more than 0.20%.Be preferably more than 0.25%.But if C content is excessive, then because toughness reduces, and hydrogen embrittlement resistance becomes and reduces easily, so to suppress be below 0.60%.Be preferably below 0.5%.
<Si：1.0～3.0％>
Si suppresses residual austenite to decompose effectively, the important element of carbide Cheng Sheng, and, still to the effective substitutional solid solution strengthening element of the hardization of material.In order to bring into play such effect effectively, need contain more than 1.0%.Be preferably more than 1.2%, more preferably more than 1.5%.But if Si content is excessive, then because toughness reduces, hydrogen embrittlement resistance becomes and reduces easily, so be suppressed at below 3.0%.Be preferably below 2.7%, more preferably below 2.5%.
<Mn：1.0～3.5％>
Mn makes stabilization of austenite, the necessary element of the residual austenite that obtains expecting.In order to bring into play this effect effectively, wish it is contained more than 1.0%.Be preferably more than 1.2%, more preferably more than 1.5%.On the other hand, if Mn content is excessive, then because segregation becomes significantly, the easy deterioration of processibility, so with 3.5% as higher limit.Be preferably below 3.2%, more preferably below 3.0%.
＜Al:1.5% following (not containing 0%)〉(situation of steel 1 of the present invention)
＜Al:0.5% following (not containing 0%)〉(situation of steel 2 of the present invention)
For deoxidation Al can add to more than 0.01%.Al not only has desoxydatoin in addition, also has the element that improves the erosion resistance effect and improve hydrogen-embrittlement resistance.
As the mechanism of above-mentioned erosion resistance raising effect, specifically, be commonly considered as improving mother metal himself erosion resistance and the effect of the generation rust that produces by atomospheric corrosion, still, the effect of inferring the latter's generation rust particularly is big.Its reason is, above-mentioned generation rust has excellent protectiveness than common iron rust is careful, so atomospheric corrosion is suppressed, the hydrogen amount that its result produces this atomospheric corrosion reduces, thus hydrogen embrittlement, and promptly delayed fracture is suppressed effectively.
Also have, in the mechanism of the hydrogen embrittlement resistance castering action of Al, it is agnogenio in detail, but infer is at steel surface because denseization of Al, thereby make the intrusion of hydrogen in steel become difficulty and since in the steel velocity of diffusion of hydrogen descend and to make hydrogen move difficulty, be difficult to produce hydrogen embrittlement.But also can think, because the interpolation of Al makes the stability of lath-shaped residual austenite increase, help to improve hydrogen-embrittlement resistance.
For the erosion resistance effect of bringing into play Al effectively and the effect that improves hydrogen-embrittlement resistance, the Al amount can be made as more than 0.02%, be preferably more than 0.2%, more preferably more than 0.5%.
But,, guarantee processibility in order to realize increase, the gigantism of inclusiones such as inhibited oxidation aluminium, and guarantee the generation of tiny residual austenite, and suppress increase, need be suppressed at the Al amount below 1.5% with the cost that contains the corrosion that the Al inclusion is a cause and suppress to make.Viewpoint from making is preferably the A3 point is adjusted to below 1000 ℃.
On the other hand, as mentioned above, owing to increase Al content, then inclusion such as aluminum oxide increases, and so the delayed fracture deterioration in characteristics is in order fully to suppress the inclusion of described aluminum oxide etc., obtain the more excellent steel of delayed fracture characteristic, then need the Al amount is suppressed at below 0.5%.Be preferably below 0.3%, more preferably below 0.1%.
＜P:0.15% is following 〉
P is because be to encourage to produce the element of intercrystalline failure by grain boundary segregation, so wish that its content is low, its upper limit is made as 0.15%.Preferably be suppressed to below 0.1%, more preferably below 0.05%.
＜S:0.02% is following 〉
S is because be the element that encourages the hydrogen absorption of spring steel under corrosive environment, so wish that its content is low, its upper limit is made as 0.02%.Be preferably below 0.01%.
Regulation contains element as mentioned above in the present invention, the remainder composition comes down to Fe, but in steel, the unavoidable impurities that is introduced into as situation according to raw material, resource, producing apparatus etc., much less allow to contain the N (nitrogen) below 0.01%, in that described effect of the present invention is not caused in the dysgenic scope, as described below, also can make it contain other elements in addition energetically.
＜Nb:0.1% following (not containing 0%) and/or Mo:1.0% following (not containing 0%) 〉
Nb is to the strength enhancing of spring steel and the very effective element of grain refining, particularly by with the compound interpolation of following Mo, this effect is brought into play fully.In order to bring into play such effect, recommend Nb to contain promising more than 0.005% (more preferably more than 0.01%).But, Nb is excessively contained, this effect is saturated, is waste economically, therefore is suppressed at below 0.1%.
Mo has the stabilization of austenite of making and guarantees residual austenite, suppresses hydrogen and invades and make effect that hydrogen embrittlement resistance promotes and the effect that improves the hardenability of spring steel.In addition, also have of the effect of reinforcement crystal boundary with the generation of inhibition hydrogen embrittlement.In order to bring into play such effect effectively, it is promising more than 0.005% (more preferably more than 0.01%) to recommend that Mo is contained.But even Mo content surplus, above-mentioned effect is saturated, is waste economically, so be suppressed at below 1.0%.
＜Cu:2% following (not containing 0%) and/or Ni:5% following (not containing 0%) 〉
By Cu and/or Ni are contained, can suppress to become the generation of hydrogen of the reason of hydrogen embrittlement fully, and suppress the intrusion of the hydrogen that takes place to spring steel.Its result, by with the synergistic effect that improves by the hydrogen trapping ability of the spring steel due to the above-mentioned organizational controls, the diffustivity hydrogen concentration in the spring steel can be reduced to harmless level fully.
Specifically, Cu, Ni have the erosion resistance that makes steel self and improve, and suppress fully because the effect that the hydrogen due to the corrosion of spring steel takes place.And these elements also have among the rust that promotion generates in atmosphere, because of the so-called ferric oxide with protectiveness stable on the thermodynamics: the effect of the generation of alpha-feooh.Promote that by the generation that realizes this rust the hydrogen that can suppress to take place can improve the hydrogen embrittlement resistance under abominable corrosive environment fully to the intrusion of spring steel.Particularly by making Cu, Ni coexistence, this effect is easy to performance.
In order to bring into play above-mentioned effect,, preferably be made as more than 0.03% in that Cu is contained sometimes.More preferably more than 0.1%.And, in that Ni is contained sometimes, preferably be made as more than 0.03%, more preferably more than 0.1%.
Also have, if make excessive the containing of element of any, then processibility reduces, and therefore preferably be controlled to be: the situation of Cu is (more preferably below 1.5%) below 2%; The situation of Ni is (more preferably below 3%) below 5%.
＜Cr:2% following (not containing 0%) 〉
Cr can damage the deformable energy hardly and improves hardenability, easily reaches high-intensity useful element.In order to bring into play such effect fully, it is contained more than 0.1%, but if excessively contain, then cementite is easy to generate, residual austenite is difficult to remnants, so preferably add in the scope below 2%.
＜Ti and/or V: add up to 0.003～1.0% 〉
Ti and above-mentioned Cu, Ni are same, have the generation facilitation effect of protectiveness rust.This protectiveness rust particularly suppresses the erosion resistance (as a result of being hydrogen embrittlement resistance) that generates is caused the generation of dysgenic β-FeOOH under chloride environment, have the effect that is highly profitable.The formation of such protectiveness rust is particularly promoted by compound interpolation Ti and V.Ti also is an element of paying very excellent erosion resistance, also has the advantage that purifies steel concurrently.
Also having V, with the Ti coexistence, except that having the effect that improves hydrogen embrittlement resistance, also is strength enhancing, the grain refining effective elements to spring steel as mentioned above.
In order to give full play to the effect of above-mentioned Ti and/or V, preferably make its total contain (more preferably more than 0.01%) more than 0.003%.Particularly from improving the viewpoint of hydrogen embrittlement resistance, preferably add Ti and surpass 0.03%, more preferably adding Ti is more than 0.05%.Therefore on the other hand, excessively add Ti, effect also is a state of saturation, be preferred economically, also have, as if excessive interpolation V, then carbonitride to separate out change many, cause the reduction of processibility and hydrogen embrittlement resistance.So Ti and/or V are recommended in the scope that adds up to below 1.0% and add.More preferably add up to below 0.8%.
<Zr：0.003～1.0％>
Zr is strength enhancing, the grain refining effective elements to spring steel, with the Ti coexistence, has the effect that improves hydrogen embrittlement resistance.In order to bring into play such effect effectively, Zr is contained more than 0.003%.On the other hand, if Zr excessively contains, then carbonitride separate out excessively, processibility and hydrogen embrittlement resistance reduce, so can add in the scope below 1.0%.
<B：0.0002～0.01％>
B is the strength enhancing effective elements to spring steel, preferably makes it contain (more preferably more than 0.0005%) more than 0.0002%.On the other hand, if B is excessive contains, hot workability deterioration then is so preferably be suppressed at (more preferably below 0.005%) below 0.01%.
In spring steel of the present invention, except that steel disc, ingot casting, also comprise by these being carried out the wire rod that hot rolling obtains and further implementing the resulting drawing material of Wire Drawing.
The present invention not regulation creates conditions, but in order to form the above-mentioned tissue that can improve hydrogen embrittlement resistance and intensity simultaneously, heat-treats with following main points after being recommended in wire drawing.Promptly be recommended as, with drawing material at A
3Point～(A
3Point+100 ℃) temperature (T1) heating keeps 10～1800 seconds (t1) afterwards, and the average cooling rate above with 3 ℃/s is cooled to the temperature (T2) that (Ms point-50 ℃)～Bs order, in this temperature field heating maintenance 60～3600 seconds (t2).
If above-mentioned T1 surpasses (A
3Point+100 ℃), or t1 surpasses 1800 seconds, then can cause austenitic crystal grain-growth, organizes thickization, so be not preferred.On the other hand, if above-mentioned T1 is lower than A
3Point then can not get the bainite ferrite tissue of stipulating.And when above-mentioned t1 was lower than 10 seconds, austenitizing can not fully carry out, and cementite and other alloy carbides are remaining, so be not preferred.Above-mentioned T1 is preferably A
3More than the point and at (A
3Point+50 ℃) below, above-mentioned t1 is preferably more than 30 seconds and below 1500 seconds, more preferably more than 60 seconds and below 1200 seconds.
Then cool off, in the present invention, recommend to be cooled to (Ms point-50 ℃) above and temperature Bs point below, heat maintenance 60～3600 seconds at this temperature field with the above average cooling rate of 3 ℃/s.
As mentioned above, the average cooling rate above with 3 ℃/s carries out, and is because of the bainite ferrite tissue of guaranteeing to expect, and avoids according to the present invention not being the generation of preferred pearlitic structure.Preferred this average cooling rate be big, be recommended as more than the 10 ℃/s (more preferably 20 ℃/more than the s).
Secondly, by making its constant temperature phase transformation being chilled to more than (Ms point-50 ℃) and after the temperature below the Bs point, can import the tissue of regulation.If heating keeps temperature to surpass the Bs point, then do not generate in a large number for preferred perlite according to the present invention, can not guarantee the bainite ferrite tissue of stipulating.On the other hand, if heating keeps temperature to be lower than (Ms point-50 ℃), then the area occupation ratio of residual austenite tails off.
And if the heating hold-time surpasses 3600 seconds, then residual austenite is decomposed, and cementite generates, and can not bring into play desired characteristics.On the other hand, the heating hold-time is lower than 60 seconds, because the diffusion of C is insufficient, so can't form residual austenite, also can not get desired characteristics this moment.The heating hold-time, be preferably more than 100 seconds and below 3000 seconds, more preferably more than 180 seconds and below 2400 seconds.
Spring steel of the present invention by will be by the resulting wire rod wire drawing of hot rolling, be implemented above-mentioned thermal treatment (austempering processing) and is obtained.
Before described wire drawing, usually, also can carry out softening annealing, peeling, patenting processing etc.After spring forms, as carried out, also can implement to remove stress annealing, two shot peening, low-temperature annealing, cold halting handled etc.
With the spring steel that the present invention obtains, high strength and having excellent hydrogen embrittlement resistance, and, because the fatigue characteristic that also possess all the time to be pursued, so be used for the manufacturing of the spring that adopted in for example automotive field, industrial machinery field etc.Particularly, spring of the restoring means of the machinery of the bearing spring of the most suitable valve spring that is used in motor car engine, suspension, clutch spring, brake spring etc. etc.
Below, enumerates embodiment and be described more specifically the present invention, but the present invention is not subjected to the restriction of following embodiment certainly, before can meeting, in the scope of aim described later, implement suitable change, these any all be contained in technical scope of the present invention.
Embodiment
The one-tenth that melting is put down in writing by table 1 is grouped into after the steel of the No.A～P that is constituted, by forging the steel billet of making 115mm side, is rolling to after the 12.5mm, and Wire Drawing reaches 12mm.This drawing material is cut to length 300mm heat-treats (modified) afterwards.This thermal treatment is at (A
3Point+30 ℃) after heating kept 5 minutes, be cooled to To ℃ of table 2,, after this, obtain the spring steel of naturally cooling in this temperature (To ℃) maintenance t as shown in table 2 second with 10 ℃/s.
With following main points, metal structure, tensile strength (TS), extension [complete extend (E1)], hydrogen embrittlement resistance, the fatigue characteristic of each spring steel that obtains have thus been investigated respectively.
[observation of metal structure]
The mensuration zone of position (about 50 μ m * 50 μ m arbitrarily in the cross section of the position of the radius 1/2 of the test portion that obtains, measuring interval is 0.1 μ m), observe, take pictures, follow aforesaid method, measured the area occupation ratio of bainite ferrite (BF) and martensite (M), the area occupation ratio of residual austenite (residual γ).Then, in optional 2 open countries are looked, measure equally, try to achieve mean value.And, deduct the area occupied rate of these tissues, in the hope of other tissue.In addition, follow aforesaid method, measured the mean axis ratio of residual austenite crystal grain.
[mensuration of tensile strength]
From above-mentioned each spring steel, by mechanical workout, make the tension test sheet of diameter 8mm, carry out stretching experiment with this experiment slice and measured tensile strength (TS).
[evaluation of hydrogen embrittlement resistance]
From above-mentioned each spring steel,, be made into the delayed fracture test film (diameter of parallel portion is 8mm, and the diameter of notch part is 6mm) that has the ring-type breach by mechanical workout.Then, carry out tension test in the tensile load mode in 5% salt, with respect to above-mentioned TS, the ratio of the tensile strength in this test is that being evaluated as more than 0.4 has excellent hydrogen embrittlement resistance.
In addition, 4 pliability tests of hydrogen filling in the steel grade of a part, have also been carried out.In detail, will cut out the long narrow test film of 65mm * 8mm from each above-mentioned steel exactly, impregnated in (0.5mol/H
2SO
4+ 0.01mol/KSCN) in the solution, carry out the cathode hydrogen filling, with 1 hour not the maximum stress of fracture measure as boundary rupture stress (DFL).Then, try to achieve the ratio (DFL leads) of the DFL of the experiment No.1 (grade of steel A) corresponding to table 2.
[evaluations of fatigue characteristic]
In addition, in order to investigate fatigue characteristic, carried out following fatigue test as spring necessity.Promptly, above-mentioned spring steel is rolling to after the 8.0mm, further Wire Drawing is to 4.6mm, the sample that employing is formed by OT (Oil Tempered) line, village's formula rotary bending fatigue test in carrying out, measure limit of fatigue, this limit of fatigue is divided by by above-mentioned tensile strength and tries to achieve the limit of fatigue ratio.So, this limit of fatigue is than being the excellent in fatigue characteristics that is evaluated as more than 0.30.Also have, this fatigue test is to be more than the 1860MPa to be that object carries out with above-mentioned tensile strength.This is because the low then relative fatigue characteristic of tensile strength are also guaranteed.
Its result is recorded in table 2 in the lump.
Table 1
| The steel grade numbering | Chemical ingredients is formed ※(mass％) | Ac3 (℃) | Bs (℃) | Ms (℃) | ||||||||||
| C | Si | Mn | P | S | Al | Nb | Mo | Cu | Ni | Other | ||||
| A | 0.41 | 2.03 | 2.97 | 0.03 | 0.003 | 0.033 | - | - | - | - | - | 815.9 | 452 | 269 |
| B | 0.38 | 2.01 | 2.52 | 0.03 | 0.003 | 0.031 | 0.05 | - | - | - | - | 832.5 | 500.6 | 297.7 |
| C | 0.40 | 1.98 | 2.30 | 0.03 | 0.002 | 0.033 | - | 0.3 | - | - | - | 844.8 | 490.1 | 289.2 |
| D | 0.40 | 2.00 | 2.03 | 0.02 | 0.003 | 0.032 | - | - | 0.3 | 0.3 | 826.4 | 528 | 299 | |
| E | 0.35 | 1.48 | 2.49 | 0.02 | 0.002 | 0.032 | - | - | - | - | Ti：0.05 | 828.2 | 511 | 313 |
| F | 0.29 | 1.98 | 1.99 | 0.03 | 0.003 | 0.033 | - | - | - | - | Ti：0.08，B：0.003 | 895.7 | 573 | 358 |
| G | 0.51 | 2.01 | 2.47 | 0.02 | 0.003 | 0.032 | - | - | - | - | Cr：0.05 | 807.6 | 470 | 237 |
| H | 0.15 | 2.52 | 3.02 | 0.02 | 0.003 | 0.031 | 0.05 | 0.5 | - | - | - | 895.6 | 476 | 380 |
| I | 0.41 | 0.49 | 2.48 | 0.02 | 0.002 | 0.033 | - | 0.5 | - | - | - | 770.5 | 455 | 274 |
| J | 0.70 | 2.01 | 2.01 | 0.033 | 0.013 | 0.003 | - | - | - | - | - | 794.0 | 460.1 | 162.9 |
| K | 0.40 | 2.01 | 2.52 | 0.03 | 0.003 | 0.052 | - | - | - | - | - | 837.7 | 495.2 | 288.2 |
| L | 0.40 | 1.98 | 2.49 | 0.03 | 0.002 | 0.341 | 0.05 | - | - | - | - | 952.8 | 497.9 | 289.2 |
| M | 0.42 | 2.00 | 2.50 | 0.03 | 0.002 | 0.422 | - | 0.3 | - | - | - | 992.1 | 466.7 | 273.1 |
| N | 0.39 | 1.99 | 2.51 | 0.03 | 0.002 | 0.531 | 0.05 | 0.2 | - | - | - | 1036.6 | 482.2 | 289.1 |
| O | 0.42 | 2.00 | 2.55 | 0.03 | 0.002 | 0.71 | 0.05 | 0.2 | - | - | - | 1102.6 | 470.5 | 273.6 |
| P | 0.40 | 2.01 | 2.60 | 0.03 | 0.002 | 1.59 | 0.05 | 0.2 | - | - | - | 1456.8 | 471.4 | 281.4 |
Remainder is iron and unavoidable impurities
Table 2
| Experiment No. | The steel grade numbering | To | t | Residual γ | The mean axis ratio of residual γ crystal grain | BF+M | F | TS | The delayed fracture strength ratio | The limit of fatigue ratio | The DFL ratio |
| ℃ | sec | Area % | - | Area % | Area % | MPa | - | - | |||
| 1 | A | 260 | 1200 | 9 | ○ | 91 | 0 | 1920 | 0.45 | 0.31 | 1.00 |
| 2 | B | 260 | 1200 | 10 | ○ | 90 | 0 | 1930 | 0.49 | 0.30 | - |
| 3 | C | 260 | 1200 | 11 | ○ | 89 | 0 | 1960 | 0.46 | 0.33 | - |
| 4 | D | 280 | 1800 | 10 | ○ | 90 | 0 | 1930 | 0.47 | 0.35 | - |
| 5 | E | 300 | 1200 | 7 | ○ | 93 | 0 | 1990 | 0.48 | 0.33 | - |
| 6 | F | 320 | 600 | 6 | ○ | 94 | 0 | 1890 | 0.50 | 0.33 | - |
| 7 | G | 280 | 2400 | 12 | ○ | 88 | 0 | 2020 | 0.44 | 0.32 | - |
| 8 | H | 300 | 1800 | 2 | ○ | 96 | 0 | 1570 | 0.51 | - | - |
| 9 | I | 300 | 1200 | 1 | × | 99 | 0 | 1440 | 0.35 | - | - |
| 10 | J | 300 | 1200 | 14 | ○ | 86 | 0 | 2100 | 0.31 | 0.21 | - |
| 11 | A | 480 | 1200 | 0 ※ | × | 0 ※ | 35 ※ | 1210 | 0.72 | - | - |
| 12 | A | 200 | 1200 | 1 | × | 99 | 0 | 1910 | 0.27 | 0.25 | - |
| 13 | A | 350 | 7200 | 1 | × | 99 | 0 | 1380 | 0.33 | - | - |
| 14 | A | 320 | 10 | 1 | × | 99 | 0 | 1820 | 0.29 | 0.27 | - |
| 15 | K | 280 | 1200 | 9 | ○ | 91 | 0 | 1924 | 0.45 | 0.31 | 1.24 |
| 16 | L | 280 | 1200 | 9 | ○ | 91 | 0 | 1930 | 0.46 | 0.32 | 1.56 |
| 17 | M | 280 | 1200 | 10 | ○ | 90 | 0 | 1939 | 0.47 | 0.32 | 1.66 |
| 18 | N | 280 | 1200 | 10 | ○ | 90 | 0 | 1945 | 0.48 | 0.33 | 1.71 |
| 19 | O | 280 | 1200 | 11 | ○ | 89 | 0 | 1950 | 0.49 | 0.32 | 1.78 |
| 20 | P | 280 | 1200 | 13 | × | 65 | 22 | 1329 | 0.29 | - | 0.73 |
※ remainder perlite
Can carry out following investigation (in addition, following No. represents the experiment No. in the 2nd) according to table 1,2.
Satisfy No.1～7,15～19 of the prerequisite of the present invention's regulation, demonstrate the above high strength of 1860MPa, and having excellent hydrogen embrittlement resistance under abominable corrosive environment.Particularly, from No.15～19 as can be known, it demonstrates more excellent hydrogen embrittlement resistance.
With respect to this, No.8～14,20 of the regulation of discontented unabridged version invention have following shortcoming respectively.
That is, No.8 is because C content is very few, so can not guarantee the intensity that the present invention stipulates.Also have, No.9 is because Si contains quantity not sufficient, so can not guarantee the intensity that the present invention stipulates.
No.10 is the example that adopts the excessive steel grade J of C content, but separates out because of carbide, so hydrogen embrittlement resistance and fatigue characteristic deterioration.
Also have No.11～14, the steel that adopt the one-tenth that satisfies the present invention's regulation to be grouped into, but because the not condition manufacturing to recommend, so can not guarantee desirable tissue.
That is, No.11 is because the austempering treatment temp is too high, thus can not guarantee bainite ferrite, and martensite and residual austenite, and can't guarantee high strength.
No.12 is because the austempering treatment temp is too low, also have No.13 long because of the austempering treatment time, No.14 is too short because of the austempering treatment time in addition, so any residual γ all is polygon, and the result of formation hydrogen embrittlement resistance deterioration.
No.20 is because be higher than the Al content of stipulating as the present invention 1, so can guarantee the residual austenite of specified amount, but this residual austenite is less than the mean axis ratio of unabridged version invention regulation, and do not become the parent phase of expectation, in addition, because also generated the inclusion of AlN etc., so the hydrogen embrittlement resistance deterioration.
Claims (4)
1, a kind of high-strength spring steel that has excellent hydrogen embrittlement resistance is made of following composition, in quality %, satisfies
C：0.20～0.60％，
Si：1.0～3.0％，
Mn：1.0～3.5％，
Below the Al:1.5%, but do not contain 0%,
Below the P:0.15%,
Below the S:0.02%,
Remainder is iron and unavoidable impurities, it is characterized in that,
Calculate by area occupation ratio with respect to whole tissue,
Residual austenite is more than 1%,
Bainite ferrite and martensite adds up to more than 80%,
Ferrite and perlite add up to below 10% and comprise 0%,
And, the mean axis ratio of above-mentioned residual austenite crystal grain, promptly major axis/minor axis is more than 5, in addition, tensile strength is more than the 1860MPa.
2, a kind of high-strength spring steel that has excellent hydrogen embrittlement resistance is made of following composition, in quality %, satisfies
C：0.20～0.60％，
Si：1.0～3.0％，
Mn：1.0～3.5％，
Below the Al:0.5%, but do not contain 0%,
Below the P:0.15%,
Below the S:0.02%,
Remainder is iron and unavoidable impurities, it is characterized in that,
Calculate by area occupation ratio with respect to whole tissue,
Residual austenite is more than 1%,
Bainite ferrite and martensite adds up to more than 80%,
Ferrite and perlite add up to below 10% and comprise 0%,
And, the mean axis ratio of above-mentioned residual austenite crystal grain, promptly major axis/minor axis is more than 5, tensile strength is more than the 1860MPa in addition.
3, according to the high-strength spring steel of claim 1 or 2 records, it is characterized in that,
In quality %, also contain
Nb:0.1% is following but do not contain 0%, and/or
Mo:1.0% is following but do not contain 0%.
4, according to the high-strength spring steel of each record of claim 1～3, it is characterized in that,
In quality %, also contain
Cu:2% is following but do not contain 0%, and/or
Ni:5% is following but do not contain 0%.
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