CN104968821A - Electric-resistance-welded steel pipe - Google Patents

Electric-resistance-welded steel pipe Download PDF

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Publication number
CN104968821A
CN104968821A CN201480007099.6A CN201480007099A CN104968821A CN 104968821 A CN104968821 A CN 104968821A CN 201480007099 A CN201480007099 A CN 201480007099A CN 104968821 A CN104968821 A CN 104968821A
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steel pipe
welded steel
steel
fatigue
amount
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CN104968821B (en
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荒谷昌利
冈部能知
丰田俊介
河端良和
堀弘道
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JFE Steel Corp
JFE Engineering Corp
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NKK Corp
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    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/009—Pearlite

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Abstract

Provided is an electric-resistance-welded steel pipe having excellent fatigue strength. The electric-resistance-welded steel pipe has a composition which contains, in terms of mass%, 0.35-0.55% C, 0.01-1.0% Si, 1.0-3.0% Mn, up to 0.02% P, up to 0.01% S, up to 0.005% Al, up to 0.0050% N, and 0.1-0.5% Cr, with the remainder comprising Fe and unavoidable impurities, and has a metallographic structure which comprises pearlite, ferrite, and bainite, the areal proportion of the pearlite being 85% or more and the sum of the areal proportion of the ferrite and the areal proportion (including zero) of the bainite being 15% or less, and in which the prior-austenite grain diameter is 25 [mu]m or larger.

Description

Electricresistance welded steel pipe
Technical field
The present invention relates to the electricresistance welded steel pipe (electric resistance welded steel pipe) that fatigue characteristic (fatigue characteristic) are excellent.
Background technology
In automotive industry, in order to the lightness that gets both (weight saving) and rigidity (stiffnessproperty), in the past, be developed with the cavitation (hollowing) employing the drivetrain parts (driving part) that the transmission shaft (drive shaft) of bar steel (bar steel) etc. is representative.As one of material that such cavitation uses, propose weldless steel tube (seamlesssteel pipe), such as, Patent Document 1 discloses a kind of hollow actuating shaft (hollow driveaxis), its with by the weldless steel tube of steel composition control in desired scope for material, have the cold-workability (cold workability) that the numbering of the autstenitic grain size after quenching (austenite grain size number) is the excellence of more than 9 concurrently, hardening capacity (hardenability), toughness (toughness) and torsional fatigue strength (torsion fatigue strength) (below sometimes referred to as fatigue strength), the fatigue lifetime (fatigue life) of playing stably.
But there are the following problems in its manufacture method for weldless steel tube: because of surface take off charcoal (surfacedecarburization), surface spots is large, must carry out the problem of grinding, grinding by effects on surface to obtain sufficient resistent fatigue characteristic; The problem of whirl (rotated object) may not be applicable to because of bias partially thick (unevenness and eccentricity inthickness).
On the other hand, have studied electricresistance welded steel pipe few for the problems referred to above is used for transmission shaft purposes.Such as, Patent Document 2 discloses with by the electricresistance welded steel pipe of steel composition control in desired scope for material, to resistance welding portion (weld of ERW) and neighbouring enforcement quenching thereof, temper and hardening treatment of carrying out (hardening treatment), thus improve the technology of the intensity of steel pipe itself.
Prior art document
Patent documentation
Patent documentation 1: International Publication WO2006/104023 publication
Patent documentation 2: Japanese Unexamined Patent Publication 2002-356742 publication
Summary of the invention
But, though electricresistance welded steel pipe is dimensional precision excellence compared with weldless steel tube, but for the purposes that transmission shaft etc. requires very high dimensional precision, need to utilize cold-drawn to process (colddrawing) and improve dimensional precision (dimension accuracy).In this case, need to carry out normalizing (normalizing) after cold-drawn processing.Its reason utilizes normalizing to eliminate following problem: because the impact of processing strain (processing strain) causes toughness to decline under (1) cold-drawn appearance; (2) electric resistance welding portion passes through the thermal history (thermalhistory) of anxious hot and chilling when welding and hardens, local hardening; (3) exist on the junction surface of electric resistance welding carbon concentration low be called as the problems such as the thin layer of white layer (white layer).
If carry out normalizing, then the toughness due to electricresistance welded steel pipe is low, so there is the danger of brittle rupture (brittle failure) under having practical service environment.In addition, during for transmission shaft, due to load shear-stress repeatedly (shearing stress), stress in bending (bending stress), so electric resistance welding portion and near portion produce stress raisers (stressconcentration), have the life-span short, occur fatigure failure (fatigue breaking) danger.Therefore, normalizing treatment is of crucial importance when electricresistance welded steel pipe being used for transmission shaft, is the process large to the properties influence of the steel pipe becoming end article simultaneously.
During using high carbon steel as the material of electricresistance welded steel pipe, deviation due to the speed of cooling after normalizing causes metal structure (metallic structure), and great changes will take place, is transformed into martensite (martensite) from ferrite (ferrite), perlite (pearlite).Therefore, also can generate martensitic stucture, thus using high carbon steel as the material of electricresistance welded steel pipe time, disclosed in patent documentation 1, patent documentation 2, from the viewpoint of guaranteeing that toughness temper is required operation, have the problem causing manufacturing cost to rise.
The object of the invention is to, in order to solve above-mentioned problem, even if when providing a kind of material using high carbon steel as electricresistance welded steel pipe, the metal structure after normalizing and tensile strength are not vulnerable to the impact of the speed of cooling after normalizing yet, can guarantee the electricresistance welded steel pipe of stable fatigue strength.
The present inventor etc. are in order to solve above-mentioned problem, conduct in-depth research, found that by the Al amount in steel is controlled in proper range, thus metal structure after normalizing and tensile strength are not vulnerable to the impact of the speed of cooling after normalizing, can guarantee stable fatigue strength.In addition, by original austenite particle diameter (primary austenite grain size) is controlled in proper range, even if be ferritic steel, the perlitic steel of the tensile strength with equal extent, (1) intensity of perlite itself also increases, (2) expanding resistance of fatigue crack (fatigue crack propagationresistance) also increases, and obtains higher fatigue strength.
The present inventor etc. will using steel standard SAE1541 (0.42%C-1.5%Mn-0.0035%N) for basal component and change Al amount hot-rolled steel sheet (coiling temperature 650 DEG C) as material, after being made into electricresistance welded steel pipe (external diameter 89mm, wall thickness 4.7mm) by rolling and forming and high frequency resistance welding (high-frequency resistance welding), make undergauge wroughtsteel pipe (external diameter 45mm, wall thickness 4.5mm) through pyrocondensation footpath rolling (hotreducing).Thereafter, after utilizing cold-drawn to be processed into cold drawing steel tube (external diameter 40mm, wall thickness 4.0mm), carry out normalizing (keeping 920 DEG C × 10 minutes, speed of cooling 0.5 ~ 3.0 DEG C/s after soaking) and make goods steel pipe.
The relation of the speed of cooling of normalizing shown in Fig. 1 and HV hardness (Vickers hardness).When known Al amount is less than 0.005%, in the scope that speed of cooling is wide in range, obtain almost constant HV hardness, on the other hand, when Al amount is more than 0.007%, HV hardness is large by the impact of speed of cooling, and when speed of cooling is slow, HV hardness sharply declines.
Al shown in Fig. 2 measures the relation with sheet interlayer spacing (lamellar spacing), and Al shown in Fig. 3 measures the relation with original austenite particle diameter, and Al shown in Fig. 4 measures the relation with torsional fatigue strength.Should illustrate, the speed of cooling of normalizing is 1 DEG C/s.Known along with Al amount minimizing original austenite grain coarsening, torsional fatigue strength rises thereupon.When Al amount is less than 0.005%, its effect reaches capacity, and torsional fatigue strength is also stablized.
Fig. 5 represents that the fracture portion after to fatigue test (fatigue test) carries out the result of cross-section (cross-section observation), the figure of Fig. 5 (a) to be the figure of the fatigue crack growth situation representing 0.03%Al material, Fig. 5 (b) be fatigue crack growth situation representing 0.003%Al material.The extensions path (propagasion route) of crackle is represented with white line.Known fatigue cracking is pressed with the exterior side of pipe for starting point, and the mode thereafter through soft proeutectoid ferrite (pro-eutectoidferrite) carries out crack propagation.In addition, the pearlitic grain (being equivalent to original austenite grain) on the surface that deduction is surrounded by proeutectoid ferrite is large, crackle expansion in zigzag (in a zig-zag manner) while significantly wriggling thereupon, therefore crack growth resistance increases, and fatigue strength improves.
Reason for the result obtaining Fig. 1, Fig. 2 and Fig. 3 is considered as follows.Namely, Al amount is few, the amount of the aluminium nitride (aluminum nitride) that the stage before normalizing separates out is just few, and the pinning effect (pinning effect) that therefore aluminium nitride produces reduces, and promotes the austenitic grain growing in normalizing process.Because perlite, ferrite are phase change location with original austenite crystal prevention, if so original austenite particle diameter becomes large, grain boundary area tails off, then phase change location also reduces, and ferritic point of rate reduces.If be particularly because Al measures the austenite crystal growth that the pinning effect produced by the aluminium nitride of separating out before normalizing (AlN) at most suppresses in normalizing process at the slow regional observation of speed of cooling to being measured the difference of hardness caused by Al in Fig. 1, the final pearlitic sheet interlayer spacing generated is wide simultaneously, so hardness declines.The slippage of this hardness is obvious especially in the low speed of cooling territory of not easily hardening, and depends on Al amount in steel (separating out AlN amount) strongly.When Al amount is less than 0.005%, the precipitation of aluminium nitride (AlN) is few, also dissolves in normalizing process even if separate out, and therefore pinning effect disappears, and austenite crystal easily carries out grain growing, and pearlitic sheet interlayer spacing narrows.And the change caused by speed of cooling is also little.
Relation for austenite particle diameter and sheet interlayer spacing and intensity is considered as follows.That is, if austenite particle diameter is large, pearlitic phase change location (being mainly austenite grain boundary) reduces, and therefore pearlitic transformation temperature declines.Its result, from perlite equilibrium phase change temperature (pearlite equilibriumtransformation temperature) to the temperature head of phase transformation starting point, namely condensate depression (degreeof undercooling) rises, thus make sheet interlayer spacing narrowing, according to the relation of all the time known sheet interlayer spacing and pearlitic intensity, think that pearlitic intensity rises.Result is thought because perlite intensity rises, and makes fatigue cracking not easily through pearlitic structure, and crackle is avoided perlite and expands in zigzag, and therefore expanding resistance of fatigue crack increases, and causes fatigue strength to rise.
The present invention furthers investigate further above-mentioned viewpoint and completes, and its purport is as follows.
[1] a kind of electricresistance welded steel pipe, become to be grouped in mass % containing C:0.35 ~ 0.55%, Si:0.01 ~ 1.0%, Mn:1.0 ~ 3.0%, below P:0.02%, below S:0.01%, below Al:0.005%, below N:0.0050%, Cr:0.1 ~ 0.5%, remainder is made up of Fe and inevitable impurity, metal structure is by perlite, ferrite and bainite are formed, above-mentioned pearlitic Line Integral rate is more than 85%, the Line Integral rate (comprising 0) of above-mentioned ferritic Line Integral rate and above-mentioned bainite add up to less than 15%, original austenite particle diameter is more than 25 μm.
The electricresistance welded steel pipe of the excellent in fatigue characteristics [2] according to above-mentioned [1], it is characterized in that, except mentioned component composition, in mass % further containing more than a kind that is selected from Ti:0.005 ~ 0.1%, B:0.0003 ~ 0.0050%, below Mo:2%, below W:2%, below Nb:0.1%, below V:0.1%, below Ni:2%, below Cu:2%, below Ca:0.02%, below REM:0.02.
According to the present invention, obtain the electricresistance welded steel pipe of the resistent fatigue characteristic possessed needed for transmission shaft.
Accompanying drawing explanation
Fig. 1 is the figure of the relation of speed of cooling when normalizing is described and HV hardness.
Fig. 2 is the figure of the relation representing Al amount and sheet interlayer spacing in steel.
Fig. 3 is the figure of the relation representing Al amount and original austenite particle diameter in steel.
Fig. 4 is the figure of the relation representing Al amount and torsional fatigue strength in steel.
Fig. 5 is the figure of the expansion movement that fatigue cracking is described.((a) is 0.03%Al material, and (b) is 0.003%Al material)
Embodiment
Below the restriction reason of each integrant of the present invention is described.
1. become to be grouped into
First, the reason that the one-tenth of regulation steel of the present invention is grouped into is described.Should illustrate, composition % all represents quality %.
C：0.35～0.55％
C lower than 0.35% time, can not get sufficient intensity, can not get require resistent fatigue characteristic.On the other hand, if more than 0.55%, then because weldability is deteriorated, so can not get stable electric resistance welding quality.Therefore, C amount is the scope of 0.35 ~ 0.55%.Be preferably the scope of 0.40 ~ 0.45%.
Si：0.01～1.0％
Si adds in order to deoxidation sometimes, lower than 0.01% time can not get sufficient deoxidation effect (deoxidation effect).Meanwhile, Si is also solution strengthening element (solute strengtheningelements), need containing more than 0.01% in order to obtain this effect.On the other hand, if more than 1.0%, then the hardening capacity of steel pipe declines.Si amount is the scope of 0.01 ~ 1.0%.Be preferably 0.1 ~ 0.4%.
Mn：1.0～3.0％
Mn promotes pearlitic transformation, and improves the element of hardening capacity, need add more than 1.0% to obtain this effect.On the other hand, if more than 3.0%, reduce electric resistance welding quality (welding quality of ERW), remained austenite content (amount of residualaustenite) increases in addition, and resistent fatigue characteristic declines.Mn amount is the scope of 1.0 ~ 3.0%.Be preferably the scope of 1.4 ~ 2.0%.
Below P:0.02%
In the present invention, P is inevitable impurity, and the upper limit of its amount is less than 0.02%.P has the trend of segregation portion (segregation part) denseization formed when casting continuously, also has residual in the hot-rolled steel sheet of tube material.Upsetting (upset) is implemented owing to making the edge of steel band (edges) dock when electric resistance welding, so the segregation part of P denseization is exposed from the outside surface of pipe and internal surface sometimes, when giving secondary processing (secondary processing) such as flat processing (flattening forming) to this part, there is the danger cracked.Therefore, less than 0.01% is preferably.
Below S:0.01%
In the present invention, S is inevitable impurity, and the upper limit of its amount is less than 0.01%.If the amount of S at most material toughness decline, the Mn in addition also in steel is combined and forms MnS.It becomes the inclusion of the length extended along long side direction in hot-rolled process, and processibility, toughness are declined.Therefore, be preferably less than 0.005%, be more preferably less than 0.003%.
Below Al:0.005%
Al be in the present invention realize desired by original austenite particle diameter and the important element of torsional fatigue strength that accompanies with it, if increased containing the Al more than 0.005%, AlN amount of precipitation, pinning effect is played in normalizing process, therefore austenitic grain growing is suppressed, and can not get desired austenite particle diameter.Therefore, Al amount is less than 0.005%.Be preferably less than 0.003%.
Below N:0.0050%
N is combined with Al and forms AlN, contributing to the element of the austenitic grain growing suppressed in normalizing process, being required to be less than 0.0050% to suppress this effect.Should illustrate, be preferably less than 0.0035%.
Cr：0.1～0.5％
Cr is the element that pearlitic transformation temperature is declined, and causes pearlitic sheet interlayer spacing narrowing by it, and pearlitic intensity rises, so torsional fatigue strength rises.Need containing more than 0.1% in order to play this effect.On the other hand, if containing the Cr more than 0.5%, then form oxide compound, it remains in resistance welding portion, so electric resistance welding (weldability of ERW) may deterioration.Therefore, Cr amount is the scope of 0.1 ~ 0.5%.Should illustrate, be preferably the scope of 0.15 ~ 0.30%.
Be more than basic chemical composition of the present invention, for the object improving intensity, fatigue strength further, can also containing more than a kind in Ti, B, Mo, W, Nb, V, Ni, Cu, Ca, REM shown below.
Ti：0.005～0.1％
Ti has the effect N in steel being fixed as TiN.But, lower than 0.005% time, cannot give full play to the ability of fix N, on the other hand, if more than 0.1%, the processibility of steel and toughness decline.During containing Ti, Ti amount is preferably the scope of 0.005 ~ 0.1%.Be more preferably the scope of 0.01 ~ 0.04%.
B：0.0003～0.0050％
B is the element improving hardening capacity.Lower than 0.0003% time, cannot give full play to hardening capacity improve effect.On the other hand, even if containing the B more than 0.0050%, its effect also reaches capacity, and promotes that transgranular fracture (intergranular fracture) makes resistent fatigue characteristic deterioration at grain boundary segregation.During containing B, B amount is preferably the scope of 0.0003 ~ 0.0050%.Be more preferably the scope of 0.0010 ~ 0.0040%.
Below Mo:2%
Mo improves the element of hardening capacity, for the intensity improving steel thus to improve fatigue strength effective.In order to obtain this effect, preferably containing more than 0.001%.But if containing the Mo more than 2%, then processibility obviously declines.During containing Mo, Mo amount is preferably less than 2%.Be more preferably the scope of 0.001 ~ 0.5%.
Below W:2%
Effective in the intensity that W improves steel forming carbide.In order to obtain this effect, preferably containing more than 0.001%.But if containing the W more than 2%, then unnecessary Carbide Precipitation, makes resistent fatigue characteristic decline, processibility (workability) declines.During containing W, W amount is preferably less than 2%.Be more preferably the scope of 0.001 ~ 0.5%.
Below Nb:0.1%
Nb is the element improving hardening capacity, in addition contributes to forming carbide and making intensity increase.In order to obtain this effect, preferably containing more than 0.001%.But even if containing the Nb more than 0.1%, this effect is also saturated, processibility declines.During containing Nb, Nb amount is preferably less than 0.1%.Be more preferably the scope of 0.001 ~ 0.04%.
Below V:0.1%
V be improve steel forming carbide intensity in effectively and there is the element of anti-temper softening (temper softening resistance).In order to obtain this effect, preferably containing more than 0.001%.But during containing V more than 0.1%, this effect is saturated, processibility declines.During containing V, V amount is preferably less than 0.1%.Be more preferably the scope of 0.001 ~ 0.5%.
Below Ni:2%
Ni improves the element of hardening capacity, for the intensity improving steel thus to improve fatigue strength effective.In order to obtain this effect, preferably containing more than 0.001%.But if containing the Ni more than 2%, then processibility obviously declines.During containing Ni, Ni amount is preferably less than 2%.Be more preferably the scope of 0.001 ~ 0.5%.
Below Cu:2%
Cu improves the element of hardening capacity, for the intensity improving steel thus to improve fatigue strength effective.In order to obtain this effect, preferably containing more than 0.001%.But if containing the Cu more than 2%, then processibility obviously declines.During containing Cu, Cu amount is preferably less than 2%.Be more preferably the scope of 0.001 ~ 0.5%.
Below Ca:0.02%, below REM:0.02%
Ca, REM are all the effective elements of crackle starting point to reducing when the form of non-metallic inclusion (non-metal inclusion) is spherical, the fatigure failure be endowed under the such environment for use of cyclic stress (cyclic stress), can selectivity contain as required.Such effect presents when Ca, REM are the content of more than 0.0020%.On the other hand, if containing Ca, the REM more than 0.02%, then inclusion amount is too much, and degree of cleaning (cleaning level) decline.Therefore, time containing Ca, REM, Ca, REM are preferably less than 0.02% respectively.And during with both Ca, REM, be preferably less than 0.03% to add up to gauge.
Remainder in steel composition of the present invention beyond mentioned component is Fe and inevitable impurity.
2. about metal structure
In the present invention, form perlite and count more than 85% with Line Integral rate (area ratio), the Line Integral rate (comprising 0) of ferritic Line Integral rate and bainite add up to less than 15% metal structure.
Expanded by zigzag to play fatigue cracking as mentioned above and improve expanding resistance of fatigue crack, improve the such effect of fatigue strength, Main Tissues is perlite, and its Line Integral rate is required to be more than 85%.On the other hand, though if soft ferritic Line Integral rate with for hard but the total not playing the Line Integral rate (comprising 0) of the bainite (bainite) of the effect identical with perlite more than 15%, then the raising effect of fatigue strength reduces.Therefore, pearlitic Line Integral rate is more than 85%, the Line Integral rate (comprising 0) of ferritic Line Integral rate and bainite add up to less than 15%.
Original austenite particle diameter is more than 25 μm
The perlite particle diameter on the surface surrounded by ferrite lamellae is large, and the deflection of fatigue cracking is just large, and crack growth resistance just becomes large.Because the crystal boundary of ferrite at original austenite generates, so the crystal grain of original austenite is larger, the perlite particle diameter on surface is larger.In order to improve crack growth resistance, original austenite particle diameter is required to be more than 25 μm, and during lower than 25 μm, the rising of expanding resistance of fatigue crack is insufficient.
Should illustrate, according to known theory all the time, the narrower pearlitic intensity of pearlitic sheet interlayer spacing is larger.In order to improve pearlitic intensity, make the not through perlite of fatigue cracking and get around perlite, sheet interlayer spacing is preferably below 170nm.Be more preferably below 150nm.
Embodiment 1
The steel billet of the composition of steel shown in his-and-hers watches 1 (quality %) carries out hot rolling and obtains steel band, after being made into electricresistance welded steel pipe (external diameter 89mm, wall thickness 4.7mm) by rolling and forming (roll forming) and high frequency resistance welding, make the steel pipe (external diameter 45mm, wall thickness 4.5mm) of undergauge rolling through the rolling of pyrocondensation footpath.Thereafter, after being processed into cold drawing steel tube (cold drawnsteel tube) (external diameter 40mm, wall thickness 4.0mm) by cold-drawn, carry out normalizing (keeping 920 DEG C × 10 minutes, speed of cooling 0.5 ~ 3.0 DEG C/s after soaking) and make goods steel pipe.
Take vertically tension test sheet (tensile specimen) (JIS12 test film) from goods steel pipe, measure tensile strength.In addition, the pipe circumferential direction cross section of steel pipe is exposed to the corrosion of austenite grain boundary, carry out the mensuration of austenite particle diameter.The mensuration opticmicroscope (optical microscope) of particle diameter carries out photograph taking to 10 visual fields under multiplying power 400 times, carry out the mensuration of particle diameter based on method of section (method of section), its mean value is representatively worth.
In addition, about the mensuration of pearlitic sheet interlayer spacing, equally nital corrosion (nital corrosion treatment) is carried out to pipe circumferential direction cross section, by scanning electronic microscope (electron scanning microscope) under multiplying power 20000 times, after selecting 10 visual fields to carry out photograph taking from the layer of cementite (cementite) relative to the visual field that paper stands vertically as far as possible, measure sheet interlayer spacing with method of section, its mean value is representatively worth.
Under being the condition of sinusoidal wave (sine wave), stress ratio R=﹣ 1 (symmetrical cycle), torsional fatigue test is carried out to these steel pipes at frequency (frequency) 3Hz, waveform (wave shape), obtain fatigue strength σ w.Should illustrate, σ w is that multiplicity reaches the stress also do not ruptured for 2,000,000 times.These evaluating characteristics results are shown in table 2 and table 3.
Should illustrate, about the evaluation of strength stability (strength stability), if the deviation (difference of maxima and minima) of the tensile strength TS of the speed of cooling of normalizing when the range of 0.5 ~ 3.0 DEG C/s for 50MPa within, then be evaluated as good (zero), if more than 50MPa, be then evaluated as bad (×).
From table 2, table 3, the strength variance that whole electricresistance welded steel pipes of example of the present invention all cause for the change of the speed of cooling by normalizing is little, strength stability is excellent, stablize thereupon, and sheet interlayer spacing is narrow, original austenite particle diameter is large, and therefore expanding resistance of fatigue crack is high, has stable and high torsional fatigue strength.
On the other hand, when aluminum amount exceedes the high aluminum amount material of the scope of the invention, the region tensile strength slow in the speed of cooling of normalizing is low, and torsional fatigue strength is also low thereupon.In addition, in the region that speed of cooling is high, although the difference of tensile strength and example of the present invention diminishes, torsional fatigue strength is lower than the present invention example.Think that this is caused by the difference of original austenite particle diameter and pearlitic intensity difference.
Should illustrate, the tube material making electricresistance welded steel pipe in the present embodiment is hot-rolled steel sheet, but the present invention is not limited thereto, and also can be to use cold-rolled steel strip as the mode of tube material.In addition, can be do not carry out the rolling of pyrocondensation footpath and using the mode of common electricresistance welded steel pipe as cold-drawn pipe.

Claims (2)

1. an electricresistance welded steel pipe, become to be grouped in mass % containing C:0.35 ~ 0.55%, Si:0.01 ~ 1.0%, Mn:1.0 ~ 3.0%, below P:0.02%, below S:0.01%, below Al:0.005%, below N:0.0050%, Cr:0.1 ~ 0.5%, remainder is made up of Fe and inevitable impurity, metal structure is by perlite, ferrite and bainite are formed, described pearlitic Line Integral rate is more than 85%, the Line Integral rate of described ferritic Line Integral rate and described bainite add up to less than 15%, original austenite particle diameter is more than 25 μm, wherein, the Line Integral rate of described bainite comprises 0.
2. an electricresistance welded steel pipe, except described one-tenth is grouped into, in mass % further containing more than a kind that is selected from Ti:0.005 ~ 0.1%, B:0.0003 ~ 0.0050%, below Mo:2%, below W:2%, below Nb:0.1%, below V:0.1%, below Ni:2%, below Cu:2%, below Ca:0.02%, below REM:0.02.
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